# 06 - 77 Cancer Cell Biology

### 77 Cancer Cell Biology

obstacle. For example, when a tumor-suppressor gene is inactivated, 
some downstream component of the pathway is likely to be activated, 
thereby presenting a realistic target. Alternatively, the mutational inac­
tivation of a DNA repair pathway might create a unique dependence 
on the repair pathways that remain intact. An example of this is pro­
vided by PARP-1 inhibitors, which have been successfully used to treat 
patients whose tumors have inactivating mutations of genes involved in 
DNA repair processes, such as BRCA1. Patterns of global gene expres­
sion can be used to help unravel such pathways and are already being 
used to predict drug sensitivities and provide prognostic information 
in addition to that provided by DNA sequence analysis. Evaluation of 
proteomic and metabolomic patterns may also prove useful for this 
purpose.

■
■THE FUTURE
A revolution in cancer genetics has occurred in the past 30 years. Most 
types of cancer are now understood at the DNA sequence level, and 
this accomplishment has led to an increasingly refined understanding 
of tumorigenesis. Cancer gene mutations have proven to be reliable 
biomarkers for cancer detection and monitoring as well as for inform­
ing therapeutics through precision medicine approaches. Gene-based 
tests are already standard of care for patients with certain tumor types, 
such as colorectal and lung cancers, and the utility of these tests will 
undoubtedly be expanded in the coming years as new therapies and 
ways of predicting responses to therapies are developed. While effec­
tive treatment of advanced cancers remains difficult, the early promise 
shown by immune-based therapies notwithstanding, it is expected that 
breakthroughs in these areas will continue to emerge and be applicable 
to an ever-increasing number of cancers. Moreover, with the hoped-for 
advances in diagnostics, particularly in the earlier detection of cancers, 
the new and old therapies for cancer can be expected to have a much 
greater impact on reducing cancer deaths.
PART 4
Oncology and Hematology
Acknowledgments
The authors gratefully acknowledge the past contributions of Pat J. 
Morin, Jeff Trent, and Francis Collins to earlier versions of this chapter.
■
■FURTHER READING
Bunz F: Principles of Cancer Genetics, 3rd ed. Dordrecht, Springer, 
2022.
Le DT et al: PD-1 Blockade in tumors with mismatch-repair deficiency. 
N Engl J Med 372:2509, 2015.
Vogelstein B, Kinzler KW: The path to cancer—three strikes and 
you’re out. N Engl J Med 373:1895, 2015.
Vogelstein B et al: Cancer genome landscapes. Science 339:1546, 
2013.
Jeffrey W. Clark, Dan L. Longo

Cancer Cell Biology
■
■CANCER CELL BIOLOGY
Cancers are characterized by unregulated cell division, avoidance of 
cell death, tissue invasion, and the ability to spread to other areas of the 
body (metastasize). A neoplasm is benign when it grows in an unregu­
lated fashion without tissue invasion or metastasizing. The presence 
of unregulated growth, tissue invasion, and the ability to metastasize 
is characteristic of malignant neoplasms. Cancers are named based on 
their tissue of origin: those derived from epithelial tissue are called car­
cinomas, those derived from mesenchymal tissues are sarcomas, and 

those derived from hematopoietic tissue are leukemias, lymphomas, 
and plasma cell dyscrasias (including multiple myeloma).
Cancers arise as a consequence of genetic alterations, the vast 
majority of which begin in a single cell and therefore are monoclonal 
in origin. However, because a wide variety of genetic and epigenetic 
changes can occur in different cells within malignant tumors over 
time, as well as varied responses with their microenvironments and the 
biology of the patient, most cancers are characterized by plasticity and 
marked heterogeneity in the populations of cells and their composite 
behavior. In addition, extrinsic factors in the cancer environment (e.g., 
the stroma, infiltrating cells, various cell-to-cell interactions, spatial 
orientation, secreted factors, and availability of oxygen and nutrients) 
vary in different areas within the tumor or different metastases, com­
pounding this heterogeneity. This heterogeneity significantly compli­
cates the treatment of most cancers because it is likely that there are 
subsets of cells that will be resistant to therapy for a variety of reasons 
and will therefore survive and proliferate even if the majority of cells 
are killed.
A few cancers appear to, at least initially, be primarily driven by an 
alteration in a dominant gene that produces uncontrolled cell prolif­
eration. Examples include chronic myeloid leukemia (abl), about half 
of melanomas (braf), Burkitt’s lymphoma (c-myc), and subsets of lung 
adenocarcinomas (egfr, alk, ros1, met, ret, braf, and ntrk). Genes that 
can promote cell growth when altered are often called oncogenes. They 
were first identified as critical elements of viruses that cause animal 
tumors; it was subsequently found that the viral genes had normal 
counterparts with important functions in the cell and had been cap­
tured and mutated by viruses as they passed from host to host.
However, most human cancers are characterized by a multiple-step 
process involving many genetic abnormalities, each of which contrib­
utes to the loss of control of cell proliferation and differentiation and 
the acquisition of capabilities, such as tissue invasion, the ability to 
metastasize, angiogenesis (development of new blood vessels required 
for tumor growth), and alteration of the extracellular environment. 
These properties are not found in the normal adult cell from which 
the tumor is derived. Indeed, normal cells have a large number of 
safeguards against DNA damage (including multiple DNA repair and 
extensive DNA damage response mechanisms), uncontrolled prolif­
eration, and invasion. Many cancers go through recognizable steps of 
progressively more abnormal phenotypes: hyperplasia, to adenoma, to 
dysplasia, to carcinoma in situ, to invasive cancer with the ability to 
metastasize (Table 77-1). For most cancers, these changes occur over a 
prolonged period of time, usually many years.
In most organs, only primitive undifferentiated cells are capable of 
proliferating and cells lose the capacity to proliferate as they differenti­
ate and acquire functional capabilities. The expansion of the primitive 
cells (stem cells) is linked to some functional need in the host, such as 
normal turnover of tissues or regeneration after acute injury, through 
receptors that receive signals from cells and other factors in the local 
tissue microenvironment or through hormonal and other influences 
delivered by the vascular supply. In the absence of such signals, the 
cells are at rest or quiescent (out of the cell cycle but capable of being 
activated to reenter the cell cycle). The signals that induce quiescence 
in primitive cells as well as those that keep the cells at rest are com­
plex, including the process for quiescent entry, maintenance, and exit. 
Although much has been learned, including the importance of notch 
signaling, the STING pathway, other quiescent factors, and transcrip­
tional, posttranscriptional, and epigenetic regulation in quiescent 
entry and maintenance, overall control of the process within the body 
remains incompletely understood. These signals must be, at least in 
part, environmental, based on the observations that a regenerating liver 
stops growing when it has replaced the portion that has been surgically 
removed after partial hepatectomy and regenerating bone marrow 
stops growing when the peripheral blood counts return to normal. 
Cancer cells clearly have lost responsiveness to such controls and do 
not recognize when they have overgrown the niche normally occupied 
by the organ from which they are derived. A better understanding of 
these mechanisms of growth regulation in the context of organ homeo­
stasis continues to evolve.

TABLE 77-1  Phenotypic Characteristics of Malignant Cells
Deregulated cell proliferation: Loss of function of negative growth regulators 
(tumor suppressor genes, i.e., Rb, p53) and increased action of positive growth 
regulators (oncogenes, i.e., Ras, Myc). Leads to aberrant cell cycle control and 
includes loss of normal checkpoint responses.
Failure to differentiate: Arrest at a stage before terminal differentiation. 
May retain stem cell properties. (Frequently observed in leukemias due to 
transcriptional repression of developmental programs by the gene products of 
chromosomal translocations.)
Loss of normal apoptosis pathways: Inactivation of p53, increases in Bcl-2 
(antiapoptotic) family members. This defect enhances the survival of cells with 
oncogenic mutations and genetic instability and allows clonal expansion and 
diversification within the tumor without activation of physiologic cell death 
pathways.
Genetic instability: Defects in DNA repair pathways leading to either single 
nucleotide or oligonucleotide mutations (as in microsatellite instability, MIN) or, 
more commonly, chromosomal instability (CIN) leading to aneuploidy (abnormal 
number of chromosomes in a cell). Caused by loss of function of a number of 
proteins including p53, BRCA1/2, mismatch repair genes, DNA repair enzymes, 
and the spindle checkpoint. Leads to accumulation of a variety of mutations in 
different cells within the tumor and heterogeneity.
Loss of replicative senescence: Normal cells stop dividing in vitro after 25–50 
population doublings. Arrest is mediated by the Rb, p16INK4a, and p53 pathways. 
While most cells remain arrested, genetic and epigenetic changes in a subset 
of cells allow further replication, leading to telomere loss, with crisis leading 
to death of many cells. Cells that survive often harbor gross chromosomal 
abnormalities and the ability to continue to proliferate. These cells express 
telomerase, which maintains telomeres and is important for ongoing growth of 
these cells. Relevance to human in vivo cancer remains uncertain. Many human 
cancers express telomerase.
Nonresponsiveness to external growth-inhibiting signals: Cancer cells have 
lost responsiveness to signals normally present to stop proliferating when they 
have overgrown the niche normally occupied by the organ from which they are 
derived. Our understanding about this mechanism of growth regulation remains 
limited.
Increased angiogenesis: Due to increased gene expression of proangiogenic 
factors (VEGF, FGF, IL-8, angiopoietin) by tumor or stromal cells, or loss of 
negative regulators (endostatin, tumstatin, thrombospondin).
Invasion: Cell mobility and ability to move through extracellular matrix and into 
other tissues or organs. Loss of cell-cell contacts (gap junctions, cadherins) and 
increased production of matrix metalloproteinases (MMPs). Can take the form 
of epithelial-to-mesenchymal transition (EMT), with anchored epithelial cells 
becoming more like motile fibroblasts.
Metastasis: Spread of tumor cells to lymph nodes or distant tissue sites. Limited 
by the ability of tumor cells to migrate out of initial site and to survive in a foreign 
environment, including evading the immune system (see below).
Evasion of the immune system: Downregulation of MHC class I and II molecules; 
induction of T-cell tolerance; inhibition of normal dendritic cell and/or T-cell 
function; antigenic loss variants and clonal heterogeneity; increase in regulatory 
T cells.
Shift in cell metabolism: Complex changes including alterations due to 
tumor stress such as hypoxia and energy generation shifts from oxidative 
phosphorylation to aerobic glycolysis generate building blocks for malignant cell 
production and proliferation.
Complex interactions with the extracellular environment around the 
cancer cells: Induction of changes as well as complex interactions with the 
extracellular environment around cancer cells, including modifications to the 
extracellular matrix, vasculature, chemokines, mesenchymal stromal cells, 
fibroblasts, immune cells, other hematopoietic cells, platelets, nerves, and 
potentially infectious agents impacting many of the above processes.
Abbreviations: FGF, fibroblast growth factor; IL, interleukin; MHC, major 
histocompatibility complex; VEGF, vascular endothelial growth factor.
■
■DIFFERENCES BETWEEN PEDIATRIC AND ADULT 
CANCERS
The underlying importance of genetic mutations and other molecular 
changes is similar for pediatric and adult cancers. However, some 
important differences exist. Childhood cancers have a different epi­
demiology (e.g., they do not have the same extent of environmental 
or lifestyle risk factors), are much less frequent, and have a different 
spectrum of frequency beginning primarily during embryogenesis 
in mesodermal (e.g., sarcomas or hematologic malignancies such as 

acute lymphocytic leukemia [ALL] or lymphomas) or ectodermal (e.g., 
neuronal including brain and spinal) tissues. They generally have fewer 
genetic changes and lower mutational burdens than adult cancers. 
Likely due to a number of factors including the nature of the cancers 
(e.g., less genetic complexity than adult cancers) and the ability of 
children to tolerate more intense chemotherapy regimens, childhood 
cancers generally tend to be much more responsive to chemotherapy 
than adult cancers with significantly higher rates of cure.

■
■CANCER AS AN ORGAN THAT IGNORES ITS NICHE
The fundamental cellular defects that create a malignant neoplasm act 
at the cellular level, and some of these are cell autonomous. However, 
that is not the entire story. Cancers consist of both malignant cells as 
well as other cells, blood vessels, extracellular matrix, and signaling 
and other molecules in the cancer microenvironment. They behave as 
organs that have lost their specialized function and stopped respond­
ing to signals that would limit their growth in tightly regulated normal 
tissue homeostasis. Most human cancers usually become clinically 
detectable when a primary mass is approximately 1 cm in diameter—
such a mass consists of about 109 cells. Often, patients present with 
tumors that are approximately 1010 cells. Although it varies by type 
of cancer and where the primary tumor and metastases are located, 
a lethal tumor burden is usually about 1012–1013 cells. If all malignant 
cells were dividing without any cell death at the time of diagnosis, most 
patients would reach a lethal tumor burden in a very short time. How­
ever, human tumors grow by Gompertzian kinetics—this means that 
not every daughter cell produced by a cell division is actively dividing. 
In addition, the overall growth rate of a tumor depends on differences 
between growth rates of different cells within the tumor and rate of 
cell loss. The growth fraction of a tumor declines with time, largely 
due to factors in the microenvironment and accumulation of genetic 
damage over time. The growth fraction of the first malignant cell is 
100%, and by the time a patient presents for medical care, the growth 
fraction is estimated to be <5%, although the fraction varies between 
different types of cancers and even different cancers of the same type 
in different individuals. This fraction is often similar to the growth 
fraction of normal bone marrow and normal intestinal epithelium, the 
most highly proliferative normal tissues in the human body, a fact that 
may explain the dose-limiting toxicities to these tissues of agents that 
target dividing cells.
CHAPTER 77
Cancer Cell Biology
The implication of these data is that the tumor is slowing its own 
growth over time. How does it do this? The tumor cells have multiple 
genetic lesions that tend to promote proliferation, yet by the time the 
tumor is clinically detectable, its capacity for proliferation has declined. 
Better understanding of how a tumor slows its own growth would 
provide important clues for better cancer treatment. A number of fac­
tors, including those in the tumor microenvironment, are known to 
contribute to the decreased proliferation of tumor cells over time in 
the patient. For example, normal cells and other factors in the micro­
environment can contribute to slowing down the growth of cancer 
cells. Some cancer cells are hypoxemic and have inadequate supply of 
nutrients and energy. Some have sustained too much genetic damage to 
complete the cell cycle but have lost the capacity to undergo apoptosis 
and therefore survive but do not proliferate. However, an important 
subset is not actively dividing but retains the capacity to divide and can 
start dividing again under certain conditions such as when the tumor 
mass is reduced by treatments leading to improved conditions in the 
tumor microenvironment favorable for cell proliferation.
Just as the bone marrow increases its rate of proliferation in response 
to bone marrow–damaging agents, the tumor also seems to sense when 
tumor cell numbers have been reduced and can respond by increasing 
growth rate. However, the critical difference is that the marrow stops 
growing when it has reached its production goals, whereas tumors do 
not.
The ultimate structure and organization of an organ are based on 
a number of factors including growth, migration, elimination, and 
death of various cells; communication between cells to establish the 
correct architecture; competition between cells; and the composition 
of the extracellular matrix that is produced. In addition to normal cells

stopping proliferation in an organ when that is appropriate, normal 
tissues have various mechanisms for eliminating cells in both the pro­
cess of development as well as ongoing homeostasis of an organ. These 
include mechanical processes based on a number of factors including 
cell size, cell shape, and topology between cells that can determine cell 
fate as well as an active process of cell extrusion, which plays a major 
role in the elimination of both cells that are no longer needed by the 
organ and cells that are damaged and potentially dangerous (such as 
those with mutations that might be precursors for malignancy). The 
process of cell extrusion may depend on cell cycle arrest in the S phase; 
aberrations in this process may contribute to the metastatic process. A 
variety of processes, including major alterations in cell cycle control, 
apoptosis and other mechanisms of cell death, and uncontrolled cell 
signaling, all contribute to defects in appropriate cell extrusion contrib­
uting to the development of cancer.

Additional tumor cell vulnerabilities are likely to be detected when 
we learn more about how normal cells respond to “stop” signals from 
their environment, and why and how tumor cells and tissues fail to 
heed such signals.
■
■CELL CYCLE CHECKPOINTS
The cell division cycle consists of four phases—G1 (growth and 
preparation for DNA synthesis), S (DNA synthesis), G2 (preparation 
to divide), and M (mitosis, cell division). Cells can also exit the cell 
cycle and be quiescent (G0). Progression of a cell through the cell cycle 
is tightly regulated at a number of checkpoints (especially at the G1/S 
boundary, the G2/M boundary, and during M [spindle checkpoint]) 
by an array of genes that are targeted by specific genetic alterations in 
cancer. These checkpoints are quality-control features; at G1, the check­
point does not allow cells to proceed that are not ready for genome 
replication; at G2/M, the cell assesses whether the genome has been 
appropriately duplicated and is ready to divide. Critical proteins in 
these control processes that are frequently mutated or otherwise inacti­
vated in cancers are called tumor-suppressor genes because when they 
function normally, they inhibit the development or growth of cancer 
cells. Examples include p53 and Rb (discussed below).
PART 4
Oncology and Hematology
In the first phase, G1, preparations are made to replicate the genetic 
material. The cell stops before entering the DNA synthesis phase, or 
S phase, to take inventory. Are we ready to replicate our DNA? Is the 
DNA repair machinery in place to fix any mutations that are detected? 
Are the DNA replicating enzymes available? Is there an adequate 
supply of nucleotides? Is there sufficient energy to proceed? The reti­
noblastoma protein, Rb, plays a central role in placing a brake on the 
process until the cell is ready. When the cell determines that it is pre­
pared to move ahead, sequential activation of cyclin-dependent kinases 
(CDKs) results in the inactivation of the brake, Rb, by phosphorylation. 
Phosphorylated Rb releases the S phase–regulating transcription factor, 
E2F/DP1, and genes required for S-phase progression are expressed. If 
the cell determines that it is unready to move ahead with DNA replica­
tion, a number of inhibitors are capable of blocking the action of the 
CDKs, including p21Cip2/Waf1, p16Ink4a, and p27Kip1. Nearly every 
cancer has one or more defects in the G1 checkpoint that permit pro­
gression to S phase despite abnormalities in DNA repair machinery or 
other deficiencies that would affect normal DNA synthesis.
At the end of the G2 phase and before the M phase, after the cell has 
exactly duplicated its DNA content, a second inventory is taken at the 
G2 checkpoint. Have all of the chromosomes been fully duplicated? 
Were all segments of DNA copied only once? Has all damaged DNA 
been repaired? Do we have the right number of chromosomes and the 
right amount of DNA? If so, the cell proceeds to G2, in which the cell 
prepares for division by synthesizing mitotic spindle and other proteins 
needed to produce two daughter cells. If DNA damage is detected, the 
p53 pathway is normally activated. Called the guardian of the genome, 
p53 is a transcription factor that is normally present in the cell in very 
low levels. This level is generally regulated through its rapid turnover. 
Normally, p53 is bound to mdm2, a ubiquitin ligase that both inhibits 
p53 transcriptional activation and also targets p53 for degradation in 
the proteasome. When DNA damage is sensed, the ATM (ataxiatelangiectasia mutated) pathway is activated; ATM phosphorylates 

1. DNA DAMAGE CHECKPOINT
2. ONCOGENE CHECKPOINT
myc, E2F, EIA
ATM/ATR
p53
mdm2
chk1/chk2
Induction of P14ARF
P
mdm2
P14ARF
mdm2
P
P
Transcriptional
activation of p53responsive genes
P
P
p53 Tetramer
FIGURE 77-1  Induction of p53 by the DNA damage and oncogene checkpoints. 
In response to noxious stimuli, p53 and mdm2 are phosphorylated by the ataxiatelangiectasia mutated (ATM) and related ATR serine/threonine kinases, as well 
as the immediate downstream checkpoint kinases, Chk1 and Chk2. This causes 
dissociation of p53 from mdm2, leading to increased p53 protein levels and 
transcription of genes leading to cell cycle arrest (p21Cip1/Waf1) or apoptosis (e.g., 
the proapoptotic Bcl-2 family members Noxa and Puma). Inducers of p53 include 
hypoxemia, DNA damage (caused by ultraviolet radiation, gamma irradiation, or 
chemotherapy), ribonucleotide depletion, and telomere shortening. A second 
mechanism of p53 induction is activated by oncogenes such as Myc, which promote 
aberrant G1/S transition. This pathway is regulated by a second product of the Ink4a 
locus, p14ARF (p19 in mice), which is encoded by an alternative reading frame (ARF) 
of the same stretch of DNA that codes for p16Ink4a. Levels of ARF are upregulated by 
Myc and E2F, and ARF binds to mdm2 and rescues p53 from its inhibitory effect. This 
oncogene checkpoint leads to the death or senescence (an irreversible arrest in G1 
of the cell cycle) of renegade cells that attempt to enter S phase without appropriate 
physiologic signals. Senescent cells have been identified in patients whose 
premalignant lesions harbor activated oncogenes, for instance, dysplastic nevi 
that encode an activated form of BRAF (see below), demonstrating that induction 
of senescence is a protective mechanism that operates in humans to prevent the 
outgrowth of neoplastic cells.
mdm2, releasing it from its inhibitory bond to p53. p53 then stops 
cell cycle progression, directs the synthesis of repair enzymes, or if the 
damage is too great, initiates apoptosis (programmed cell death) of the 
cell to prevent the propagation of a damaged cell (Fig. 77-1).
A second method of activating p53 involves the induction of 
p14ARF by hyperproliferative signals from oncogenes. p14ARF com­
petes with p53 for binding to mdm2, allowing p53 to escape the effects 
of mdm2 and accumulate in the cell. p53 then stops cell cycle progres­
sion by activating CDK inhibitors such as p21 and/or initiating the 
apoptosis pathway. Not surprisingly given its critical role in control­
ling cell cycle progression, mutations in the gene for p53 on chromo­
some 17p are among the most frequent mutations in human cancers, 
although percentages vary between different cancers. Most commonly 
these mutations are acquired in the malignant tissue in one allele 
and the second allele is inactivated (such as by deletion or epigenetic 
silencing), leaving the cell unprotected from DNA-damaging agents or 
activated oncogenes.
Some environmental exposures produce signature mutations in p53; 
for example, aflatoxin exposure leads to mutation of arginine to serine 
at codon 249 and leads to hepatocellular carcinoma. In rare instances, 
p53 mutations are in the germline (Li-Fraumeni syndrome) and pro­
duce a familial cancer syndrome. Another mechanism for inactivation 
of p53 in malignant cells is due to alterations in regulators such as 
overexpression of the inhibitory mdm2 protein. Whether inactivated 
by mutation or inhibited by regulatory factors, absence of normal p53 
function leads to chromosomal instability and accumulation of DNA 
damage including acquisition of properties that give the abnormal cell 
a proliferative and survival advantage. Like Rb dysfunction, most can­
cers have mechanisms that disable the p53 pathway. Indeed, the impor­
tance of p53 and Rb in the development of cancer is underscored by the 
neoplastic transformation mechanism of human papillomavirus. This 
virus has two main oncogenes, E6 and E7. E6 acts to increase the rapid

turnover of p53, and E7 acts to inhibit Rb function; inhibition of these 
two targets is required for transformation of epithelial cells by the virus.
Another cell cycle checkpoint exists when the cell is undergoing 
division (M phase); this is the spindle checkpoint, which acts to ensure 
that there is proper attachment of chromosomes to the mitotic spindle 
before progression through the cell cycle can occur. If the spindle 
apparatus does not properly align the chromosomes for division, if the 
chromosome number is abnormal (i.e., greater or less than 4n), or if 
the centromeres are not properly paired with their duplicated partners, 
then the cell initiates a cell death pathway to prevent the production 
of aneuploid progeny (having an altered number of chromosomes). 
Abnormalities in the spindle checkpoint facilitate the development 
of aneuploidy, which is frequently found in cancers. In some tumors, 
aneuploidy is a predominant genetic feature.
In other tumors, a defect in the cells’ ability to repair errors in the 
DNA, such as due to mutations in genes coding for the proteins critical 
for mismatched DNA repair, is the primary genetic lesion. Cancer cells 
can have defects in any of several DNA repair pathways in addition to 
mismatch repair, including deficient interstrand cross-link, doublestrand breaks (homologous recombination or nonhomologous end 
joining repair), single-strand breaks, base excision, nucleotide excision, 
and translesional synthesis.
In general, tumors have either defects in chromosome number or 
defective DNA repair pathways but not both. Defects that lead to can­
cer include abnormal cell cycle checkpoints, inadequate DNA repair, 
and failure to preserve genome integrity leading to DNA damage. 
These defects and the stress of the resultant increased DNA damage 
make cancer cells more vulnerable to additional DNA damage, which 
can be exploited by chemotherapy, radiation therapy, targeted therapy, 
and immunotherapy—the major systemic therapeutic approaches 
effective against cancer.
Alternatively, research is ongoing in an attempt to therapeutically 
restore the defects in cell cycle regulation and DNA repair that charac­
terize cancer, although this remains a challenging problem because it is 
much more difficult to restore normal biologic function than to inhibit 
abnormal function of proteins driving cell proliferation, such as occurs 
with activated oncogenes. Newer approaches to gene editing (e.g., 
clustered regularly interspaced short palindromic repeats [CRISPR]) 
and subsequent modifications to this approach should eventually make 
gene editing more clinically feasible.
■
■CELLULAR SENESCENCE
The irreversible cessation of growth of normal cells while the cells 
remain viable has been termed cellular senescence. Senescence is 
important for several processes involved in normal development and 
homeostasis including embryogenesis and wound healing. It is also an 
important component of host mechanisms to prevent tumorigenesis by 
preventing replication of abnormal cells as well as other mechanisms 
including secreted substances that can stimulate an immune response 
against the abnormal senescent cell. However, paradoxically, senescent 
cells in tumors can also stimulate tumorigenesis and malignant pro­
gression, in part by other secreted substances that stimulate a harmful 
inflammatory response. It was initially identified by the fact that when 
normal cells are placed in culture in vitro, most are not capable of sus­
tained growth. They quickly reach a point where they either undergo 
cell death due to excessive DNA damage or other factors or they 
become senescent. Fibroblasts are an exception to this rule. When they 
are cultured, fibroblasts may divide 30–50 times and then they undergo 
what has been termed a “crisis” during which the majority of cells stop 
dividing (usually due to an increase in p21 expression, a CDK inhibi­
tor). This form of senescence is termed replicative senescence. Many 
other cells die, and a small fraction emerge that have acquired genetic 
and epigenetic changes that permit their uncontrolled growth. Among 
the cellular changes during in vitro propagation is telomere shorten­
ing. DNA polymerase is unable to replicate the tips of chromosomes, 
resulting in the loss of DNA at the specialized ends of chromosomes 
(called telomeres) with each replication cycle. At birth, human telo­
meres are 15- to 20-kb pairs long and are composed of tandem repeats 
of a six-nucleotide sequence (TTAGGG) that associate with specialized 

telomere-binding proteins to form a T-loop structure that protects the 
ends of chromosomes from being mistakenly recognized as damaged. 
The loss of telomeric repeats with each cell division cycle causes grad­
ual telomere shortening, leading to growth arrest when one or more 
critically short telomeres trigger a p53-regulated DNA-damage check­
point response. Cell death usually ensues when the unprotected ends of 
chromosomes lead to chromosome fusions or other catastrophic DNA 
rearrangements. Cells with certain abnormalities, such as those with 
nonfunctional pRb and p53, can bypass this growth arrest. The ability 
to bypass telomere-based growth limitations is thought to be a critical 
step in the evolution of most malignancies. This occurs by reactivation 
of telomerase expression in cancer cells. Telomerase is an enzyme that 
adds TTAGGG repeats onto the 3′ ends of chromosomes. It contains 
a catalytic subunit with reverse transcriptase activity (hTERT) and an 
RNA component that provides the template for telomere extension. 
Most normal somatic cells do not express sufficient telomerase to 
prevent telomere attrition with each cell division. Exceptions include 
stem cells (such as those found in hematopoietic tissues, gut and 
skin epithelium, and germ cells) that require extensive cell division 
to maintain tissue homeostasis. More than 90% of human cancers 
express high levels of telomerase that prevent telomere shortening to 
critical levels and allow indefinite cell proliferation. In vitro experi­
ments indicate that inhibition of telomerase activity leads to tumor cell 
apoptosis. Major efforts are underway to develop methods to inhibit 
telomerase activity in cancer cells. For example, the protein component 
of telomerase (hTERT) may act as one of the most widely expressed 
tumor-associated antigens and can be targeted by vaccine approaches. 
However, a caveat to targeting telomerase for anticancer treatment is 
the potential for inhibiting its activity in certain normal cells (such as 
stem cells) required for maintaining the normal physiologic state.

CHAPTER 77
Cancer Cell Biology
Although most of the functions of telomerase relate to cell division, 
it also has several other effects including interfering with the differenti­
ated functions of at least certain stem cells. However, the impact on dif­
ferentiated function of normal nonstem cells is less clear. The picture is 
further complicated by the fact that rare genetic defects in the telom­
erase enzyme seem to cause dyskeratosis congenita (characterized by 
abnormalities in various rapidly dividing cells in the body including 
skin, nails, oral mucosa, hair, and bone marrow with increased risk 
for leukemia and certain other cancers). This can be associated with 
a number of other abnormalities including pulmonary fibrosis, bone 
marrow failure (aplastic anemia), or liver fibrosis. However, paradoxi­
cally, defects in nutrient absorption in the gastrointestinal tract, a site 
that should be highly sensitive to defective cell proliferation, are not 
seen. Much remains to be learned about how telomere shortening 
and telomere maintenance are related to human illness in general and 
cancer in particular.
A variety of other stresses on cells (both environmental and intrin­
sic including radiation, chemotherapy, reactive oxygen species, and 
oncogenic mutations) can also lead to senescence, primarily those that 
induce DNA damage similar to that seen in cells with shortened telo­
meres. This is termed replicative senescence.
■
■SIGNAL TRANSDUCTION PATHWAYS IN CANCER 
CELLS
Signals that affect cell behavior come from adjacent cells, the stroma in 
which the cells are located, hormonal signals that originate remotely, 
and the cells themselves (autocrine signaling). These signals generally 
exert their influence on the receiving cell through activation of signal 
transduction pathways that have as their end result the induction of 
activated transcription factors that mediate a change in cell behavior 
or function or the acquisition of effector machinery to accomplish a 
new task. Although signal transduction pathways can lead to a wide 
variety of outcomes, many such pathways rely on cascades of signals 
that sequentially activate different proteins or glycoproteins and lipids 
or glycolipids, and the activation steps often involve the addition or 
removal of one or more phosphate groups on a downstream target.
Other chemical changes can result from signal transduction path­
ways, but reversible phosphorylation and dephosphorylation play a 
major role. Proteins that add phosphate groups to other molecules

(proteins, lipids, or nucleic acids) are called kinases. Two major classes 
of kinases involved in signal transduction pathways important for can­
cer cells are tyrosine kinases that phosphorylate tyrosine and serine/
threonine kinases that phosphorylate serine/threonine either directly 
or indirectly. However, some kinases can phosphorylate both, such 
as the MEK kinases that can phosphorylate both threonine and tyro­
sine. Phosphatases (protein tyrosine phosphatases and protein serine/
threonine phosphatases) remove the phosphate groups to reverse the 
kinase activity.

Various kinases play critical roles in signal transduction pathways 
important for malignant cells. These include a number of recep­
tor tyrosine kinases (RTKs) as well as various protein kinases (both 
tyrosine and serine/threonine kinases) downstream of receptors that 
transmit the signals within the cell (Fig. 77-2). Two important signal­
ing pathways are the RAS-RAF-MEK-ERK pathway and the phospha­
tidylinositol-3-kinase (PI3K) pathway (Fig. 77-2). Although pathways 
are depicted as distinct, complex interactions between pathways occur 
within cells.
Normally, kinase activity is short-lived and reversed by protein 
phosphatases. However, in many human cancers, RTKs or compo­
nents of their downstream pathways are activated by mutation, gene 
PART 4
Oncology and Hematology
PI3K
inhibitors
PIP2
RAS
Grb2/mSOS
PI3K
PIP3
PDK1
AKT
Multiple
targets
Everolimus
mTOR
Protein
synthesis
p70S6k
ERK1/2
Activated
transcription
factors
ECM
Integrin
receptor
Cytoskeleton
FAK
c-Src
Activated
kinases
STAT
Midostaurin
JAK inhibitors
JAK
PKC
Multiple
targets
PLC-γ
Ca2+
Tamoxifen
SERMS
PIP2
DAG
FIGURE 77-2  Therapeutic targeting of signal transduction pathways in cancer cells. Three major signal transduction pathways are activated by receptor tyrosine kinases 
(RTKs). 1. The protooncogene Ras is activated by the Grb2/mSOS guanine nucleotide exchange factor, which induces an association with Raf and activation of downstream 
kinases (MEK and ERK1/2). 2. Activated PI3K phosphorylates the membrane lipid PIP2 to generate PIP3, which acts as a membrane-docking site for a number of cellular 
proteins including the serine/threonine kinases PDK1 and Akt. PDK1 has numerous cellular targets, including Akt and mTOR. Akt phosphorylates target proteins that promote 
resistance to apoptosis and enhance cell cycle progression, while mTOR and its target p70S6K upregulate protein synthesis to potentiate cell growth. 3. Activation of PLC-γ 
leads the formation of diacylglycerol (DAG) and increased intracellular calcium, with activation of multiple isoforms of PKC and other enzymes regulated by the calcium/
calmodulin system. Other important signaling pathways involve non-RTKs that are activated by cytokine or integrin receptors. Janus kinases (JAK) phosphorylate STAT 
(signal transducer and activator of transcription) transcription factors, which translocate to the nucleus and activate target genes. Integrin receptors mediate cellular 
interactions with the extracellular matrix (ECM), inducing activation of FAK (focal adhesion kinase) and c-Src, which activate multiple downstream pathways, including 
modulation of the cell cytoskeleton. Many activated kinases and transcription factors migrate into the nucleus, where they regulate gene transcription, thus completing the 
path from extracellular signals, such as growth factors, to a change in cell phenotype, such as induction of differentiation or cell proliferation. The nuclear targets of these 
processes include transcription factors (e.g., Myc, AP-1, and serum response factor) and the cell cycle machinery (cyclin-dependent kinases [CDKs] and cyclins). Inhibitors 
of many of these pathways have been developed for the treatment of human cancers. Examples of inhibitors that are either approved or are currently being evaluated in 
clinical trials are shown in purple type.

amplification, or chromosomal translocations to have enhanced and/
or prolonged activity. Because these pathways are important in regulat­
ing proliferation, survival, migration, and angiogenesis, they have been 
identified as important targets for cancer therapeutics.
Inhibition of kinase activity is effective in the treatment of a number 
of neoplasms. Lung cancers with mutations in the epidermal growth 
factor receptor are highly responsive to osimertinib as well as other 
inhibitors (Table 77-2). Inhibitors have been developed to treat lung 
cancers with other tyrosine kinase–activating mutations (including 
anaplastic lymphoma kinase [ALK], ROS1, NTRK, MET, HER2, and 
RET). BRAF (a serine/threonine kinase) inhibitors are highly effective 
in melanomas and thyroid cancers and are also used in combination 
with other agents for lung and colon cancers as well as other solid 
tumors with BRAF V600E mutations. Targeting the MEK protein 
(which phosphorylates both threonine and tyrosine residues) down­
stream of BRAF also has activity against BRAF mutant melanomas, 
and combined inhibition of BRAF and MEK is more effective than 
either alone with activity that extends to BRAF-mutant lung cancer. 
Janus kinase (JAK) inhibitors are active in myeloproliferative syn­
dromes in which JAK2 activation is a pathogenetic event. Imatinib 
(which targets a number of tyrosine kinases) is an effective agent in 
Ligand
RTK
Monoclonal antibody
Under
investigation
Tyrosine kinase
inhibitors
Raf
kinase
inhibitors
RASC
inhibitors
Raf
GAP
MEK
inhibitors
MEK
ERK
inhibitors
Multiple
cytoplasmic
targets
AP-1 (Jun/Fos)
Serum response factor
MYC
Cyclin D1
CDK/cyclin
  complexes
CDK H/b
inhibitors
Cell cycle
regulation
Nucleus
Estrogen
receptor

TABLE 77-2  Some FDA-Approved Molecularly Targeted Agents for the Treatment of Cancer
DRUG
MOLECULAR TARGET
DISEASE
MECHANISM OF ACTION
All-trans retinoic acid
PML-RARα oncogene
Acute promyelocytic leukemia M3 AML, 
t(15;17)
Imatinib, dasatinib, nilotinib, 
ponatinib, bosutinib
Bcr-Abl, c-Abl, c-Kit, 
PDGFR-α/β
Chronic myeloid leukemia, GIST
Blocks ATP binding to tyrosine kinase active site
Ripretinib
c-Kit, PDGFR-α
GIST
Inhibits tyrosine kinase activity
Asciminib
Bcr-Abl
Chronic myeloid leukemia
Allosteric inhibitor of BCR-ABL
Sunitinib
c-Kit, VEGFR-2, PDGFR-β, 
Flt-3
GIST, RCC, PNET
Inhibits activated c-Kit and PDGFR in GIST; inhibits VEGFR in 
RCC and probably in PNET
Sorafenib
RAF, VEGFR-2, PDGFR-α/β, 
Flt-3, c-Kit
RCC, hepatocellular carcinoma (HCC), 
differentiated thyroid cancer, desmoid
Regorafenib
VEGFR1–3, TIE-2, FGFR1, KIT, 
RET, PDGFR
Colorectal cancer, GIST, HCC
Competitive inhibitor ATP binding site of tyrosine kinase domain 
multiple kinases including VEGFR
Larotrectinib, entrectinib
NTRK
Cancers with NTRK mutation
Competitive inhibitor of ATP binding site of the tyrosine kinase 
domain of NTRK
Axitinib
VEGFR1–3
RCC
Competitive inhibitor ATP binding site of tyrosine kinase domain 
VEGF receptors
Erlotinib
EGFR
NSCLC, pancreatic cancer
Competitive inhibitor of the ATP-binding site of the EGFR
Afitinib
EGFR (and other HER family)
NSCLC
Irreversible inhibitor of ATP-binding site of HER family members
Osimertinib
EGFR (T790M)
NSCLC
Inhibits EGFR mutations including T790M mutant NSCLC
Dacomitinib
EGFR
NSCLC (exon19 deletion/exon 21 L858R)
Inhibits EGFR mutant lung cancer 
Mobocertinib/EGFR/NSCLC/Tumors with Exon20 insertion 
mutations
Erdafitinib, pemigatinib, 
futibatinib, infigratinib
FGFR2, FGFR3
Urothelial (erdafitinib), myeloid/
lymphoid neoplasms (pemigatinib) 
cholangiocarcinoma (pemigatinib, 
futibatinib)
Lapatinib, tucatinib, niratinib
HER2/neu
Breast cancer, CRC (tucatinib + 
trastuzumab)
Crizotinib, ceritinib, alectinib, 
brigatinib, lorlatinib
ALK
NSCLC ALK+ large cell lymphoma, 
inflammatory myofibroblastic tumors 
(crizotinib)
Crizotinib, entrectinib 
repotrectinib
ROS1
NSCLC
Inhibitor of ROS1 tyrosine kinase
Palbociclib, ribociclib, 
abemaciclib
CDK4/6
Breast
Inhibitor of CDK4/6
Bortezomib, carfilzomib, 
ixazomib
Proteasome
Multiple myeloma
Inhibits proteolytic degradation of multiple cellular proteins
Vemurafenib, dabrafenib 
Encorafenib
BRAF V600E
Melanoma lung cancer, CRC (combined 
with Cetuximab)
Trametinib, Cobimetinib, 
binimetinib
MEK
Melanoma
Inhibitor of serine-threonine kinase domain of MEK
Cabozantinib
RET, MET, VEGFR
MTC, RCC
Competitive inhibitor of ATP-binding site of tyrosine kinase 
domain of multiple kinases, including VEGFR2 and RET
Capmatinib, tepotinib
MET
NSCLC with MET exon14 deletions
 
Selpercatinib, vandetinib, 
pralsetinib
RET
NSCLC, MTC, RET fusion thyroid cancer, 
RET fusion positive solid tumors
Temsirolimus
mTOR
RCC
Competitive inhibitor of mTOR serine-threonine kinase
Everolimus
mTOR
RCC, PNET
Binds to immunophilin FK binding protein-12, which forms a 
complex that inhibits mTOR kinase
Vorinostat, romidepsin, 
belinostat
HDAC
CTCL/PTL
HDAC inhibitor, epigenetic modulation
Panobinostat
HDAC
MM
HDAC inhibitor, epigenetic modulation
Ruxolitinib
JAK-1, 2
Myelofibrosis
Competitive inhibitor of tyrosine kinase
Vismodegib
Hedgehog pathway
Basel cell cancer (skin)
Inhibits smoothened in hedgehog pathway
Lenvatinib
Multikinase inhibitor 
(VEGFR, FGFR, PGFR-α, 
others)
RCC, thyroid cancer, HCC
Competitive inhibitor of ATP-binding site of tyrosine kinase 
domain of multiple kinases
Olaparib, rucaparib, 
niraparib, talazoparib
PARP
BRCA mutant ovarian, breast, prostate, 
pancreas cancers; not all agents 
approved for all cancers
Venetoclax
BCL-2
CLL (with 17p deletion)
Inhibits BCL-2 and enhances apoptosis
Ibrutinib, acalabrutinib 
pirtobrutinib, zanubrutinib
Bruton tyrosine kinase (BTK) CLL, MCL, MZL, SLL, WM
Inhibitor of BTK

Inhibits transcriptional repression by PML-RARα 
Adagrasib, Solorasib/KRAS12C/NSCLC/Inhibits KRAS12C
Targets VEGFR pathways in RCC and HCC. Possible activity 
against BRAF in thyroid cancer
CHAPTER 77
Inhibits tyrosine kinase of FGFR
Cancer Cell Biology
Competitive inhibitor of the ATP-binding site of HER2
Inhibitor of ALK tyrosine kinase
Inhibitor of serine-threonine kinase domain of V600E mutant 
of BRAF
Inhibitor of RET, VEGFR1, VEGFR2 tyrosine kinases
Inhibits PARP and DNA repair
(Continued)

TABLE 77-2  Some FDA-Approved Molecularly Targeted Agents for the Treatment of Cancer
DRUG
MOLECULAR TARGET
DISEASE
MECHANISM OF ACTION
Ivosidenib, olutasidenib
IDH1
AML, MDS, cholangiocarcinoma
IDH1 inhibitor
Gilteritinib, quizartinib
FLT3
AML
FLT3 inhibitor
Idelalisib
PI3K-delta
CLL
Inhibits PI3k-delta, preventing proliferation and inducing 
apoptosis
Alpelisib
PIK3CA
Breast cancer with a PIK3CA mutation
Inhibits PIK3CA
Belzutifan
Hif-2α
HIF-1α-associated RCC, pancreatic 
neuroendocrine, CNS hemangioblastoma
Capivasertib
AKT
Breast cancer
Inhibits AKT
Umbralisib
PI3K-delta, CK1-epsilon
MZL, FL
Inhibits PI3K-delta and CK1-epsilon
Selinexor
Exportin-1
MM, DLBCL
Induces apoptosis of cells
Tazemetostat
EZH2
FL, epithelioid sarcoma
Inhibits EZH2
Monoclonal Antibodies
Trastuzumab, margetuximab
HER2/neu (ERBB2)
Breast cancer, gastric or GEJ cancer
Binds HER2 on tumor cell surface and induces receptor 
internalization
Pertuzumab
HER2/neu (ERBB2)
Breast cancer
Binds HER2 on tumor cell surface at distinct site from 
trastuzumab and prevents binding to other receptors
Cetuximab
EGFR
Colon cancer, squamous cell carcinoma of the 
head and neck
PART 4
Oncology and Hematology
Panitumumab
EGFR
Colon cancer
Similar to cetuximab but fully humanized rather than 
chimeric
Necitumumab
EGFR
Squamous NSCLC
Binds EGFR
Rituximab
CD20
B-cell lymphomas and leukemias that express 
CD20
Alemtuzumab
CD52
Chronic lymphocytic leukemia and CD52expressing lymphoid tumors
Bevacizumab
VEGF
Colorectal, lung cancers, RCC, glioblastoma
Inhibits angiogenesis by high-affinity binding to VEGF
Ziv-aflibercept
VEGFA, VEGFB, PLGF
Colorectal cancers
Inhibits angiogenesis by high-affinity binding to VEGFA, 
VEGFB, and PLGF
Ramucirumab
VEGFR
Gastric, colorectal, lung cancers
Inhibits angiogenesis by binding to VEGFR
Ipilimumab
CTLA-4
Melanoma, HCC, MSI-high colorectal cancer
Blocks CTLA-4, preventing interaction with CD80/86 and 
T-cell inhibition
Nivolumab, pembrolizumab, 
dostarlimab-gxly, toripalimab, 
retifanlimab-dlwr, 
cemiplimab-rwlc
PD-1
Melanoma, head and neck cancer, NSCLC, 
SCLC, Hodgkin’s disease, urothelial cancer, 
RCC, HCC, gastric cancer, esophageal 
cancer, cholangiocarcinoma, MSI-high 
cancers, endometrial cancer, cervical 
cancer, cutaneous squamous cell carcinoma, 
basal cell carcinoma, breast cancer, 
nasopharyngeal cancer, Merkel cell tumor
Atezolizumab, durvalumab, 
avelumab
PD-L1
NSCLC, urothelial cancer, SCLC (durvalumab), 
HCC (atezolizumab), Merkel cell cancer 
(avelumab)
Relatlimab
LAG3
Melanoma (combined with nivolumab)
Blocks LAG3 interaction with MHCII and other ligands 
inhibiting immune activation
Denosumab
Rank ligand
Breast, prostate
Inhibits Rank ligand, primary signal for bone removal
Dinutuximab
Glycolipid GD2
Neuroblastoma (pediatric)
Immune-mediated attack on GD2-expressing cells
Daratumumab, Isatuximab
CD38
MM
Binds to CD38 on MM cells causing apoptosis by 
antibody-dependent or compliment-mediated cytotoxicity
Elotuzumab
SLAMF7
MM
Activating NK cells to kill MM cells
Olaratumab
PDGFRα
Soft tissue sarcomas
Blocks PDGFRα activity
Naxitamab
GD2
Neuroblastoma
Immune-mediated antitumor effect
Bispecific Antibodies
Blinatumomab
CD19 and CD3
Ph-relapsed precursor B-cell ALL
Binds CD19 on ALL cells and CD3 on T cells; immune 
attack on CD19-expressing cells
Glofitamab-gxbm, epcoritamabbysp, mosunetuzumab-axgb
CD20 and CD3
DLBCL, FL
Binds CD20 on DLBCL or FL and CD3 on T cells, immune 
attack on CD20-expressing cells
Teclistamab-cqyv, 
elranatamab-bcmm
B-cell maturation 
antigen (BCMA) and 
CD3
MM
Binds BCMA on MM cells and CD3 on T cells
Talquetamab
CD3 and GPRC5D
MM
Binds CD3 T cells and GPRC5D-expressing MM cells

(Continued)
Inhibits Hif-2α
Binds extracellular domain of EGFR and blocks binding 
of EGF and TGF-α; induces receptor internalization. 
Potentiates the efficacy of chemotherapy and 
radiotherapy
Multiple potential mechanisms, including direct induction 
of tumor cell apoptosis and immune mechanisms
Immune mechanisms
Blocks PD-1, preventing interaction with PD-L1 and T-cell 
inhibition
Blocks PD-L1, preventing interaction with PD-1 and T-cell 
inhibition
(Continued)

TABLE 77-2  Some FDA-Approved Molecularly Targeted Agents for the Treatment of Cancer
DRUG
MOLECULAR TARGET
DISEASE
MECHANISM OF ACTION
Amivantamab-vmjw
EGFR and MET
NSCLC
Targets EGFR exon 20 insertion mutations by also inhibiting 
MET
Tebentafusp-tebn
GP100 and CD3
Uveal melanoma
Binds GP100 on melanoma cells and CD3 on T cells
Antibody-Chemotherapy Conjugates
Brentuximab vedotin
CD30
Hodgkin’s disease, anaplastic lymphoma Delivers chemotherapeutic agent MMAE to CD30-expressing 
Ado-trastuzumab emtansine
HER2
Breast cancer
Delivers chemotherapeutic agent emtansine to 

HER2-expressing breast cancer cells
Fam-trastuzumab
HER2
Breast, NSCLC, and gastric cancers
Delivers chemotherapeutic agent deruxtecan to 

HER2-expressing breast cancer cells
Sacituzumab govitecan
Trop2
Breast, urothelial cancers
Delivers chemotherapy to Trop2-expressing cells
Enfortumab-vedotin
Nectin-4
Urothelial cancers
Delivers chemotherapeutic agent MMAE to 

Nectin-4-expressing cells
Polatuzumab-vedotin
CD79b
DLBCL or high-grade BCL
Delivers MMAE chemotherapy to B-cell lymphomas
Loncastuximab tesirine-lpyl
CD19
DLBCL
Delivers chemotherapy to CD19 expressing cells
Mirvetuximab 
soravtansine-gynx
Folate receptor alpha
Ovarian, fallopian, peritoneal cancers
Delivers chemotherapy to folate receptor alpha tumors
Tisotumab vedotin-tftv
Tissue factor (TF)
Cervical cancer
Delivers chemotherapy to TF-positive cells
Gemtuzumab ozogamicin
CD33
Pediatric CD33+ AML
Delivery of chemotherapy to CD33+ cells
CAR-T Cells and Tumor-Infiltrating Lymphocyte (TIL)
Tisagenlecleucel, 
axicabtagene ciloleucel, 
brexucabtagene autoleucel, 
lisocabtagene maraleucel
CD19
ALL (tisagenlecleucel), DLBCL/highgrade BCL (axicabtagene ciloleucel), 
B-cell precursor ALL (brexucabtagene), 
large BCL (lisocabtagene maraleucel)
Ciltacabtagene autoleucel
Idecabtagene vicleucel
BCMA
MM
Targets T cells to protein on surface of MM cells
Lifileucel
Melanoma antigens
Melanoma
Tumor-infiltrating lymphocyte therapy
Abbreviations: ALL, acute lymphocytic leukemia; AML, acute myeloid leukemia; BCL, B-cell lymphoma; CAR-T, chimeric antigen receptor T cells; CLL, chronic lymphocytic 
leukemia; CRC, colorectal cancer; CTCL, cutaneous T cell lymphoma; DLBCL, diffuse large B-cell lymphoma; EGFR, epidermal growth factor receptor; FDA, U.S. Food and Drug 
Administration; FGFR, fibroblast growth factor receptor; FL, follicular lymphoma; Flt-3, fms-like tyrosine kinase-3; GEJ, gastroesophageal junction; GIST, gastrointestinal stromal 
tumor; HDAC, histone deacetylases; MCL, mantle cell lymphoma; MM, multiple myeloma; MSI, microsatellite instability; MMAE, monomethyl auristatin E; MTC, medullary thyroid 
cancer; mTOR, mammalian target of rapamycin; MZL, mantle zone lymphoma; NK, natural killer; NSCLC, non-small-cell lung cancer; PARP, poly-ADP ribose polymerase; PDGFR, 
platelet-derived growth factor receptor; PLGF, placenta growth factor; PML-RARα, promyelocytic leukemia–retinoic acid receptor-alpha; PNET, pancreatic neuroendocrine 
tumors; PTL, peripheral T-cell lymphoma; RCC, renal cell cancer; t(15;17), translocation between chromosomes 15 and 17; SCLC, small-cell lung cancer; SLL, small lymphocytic 
lymphoma; TGF-α, transforming growth factor-alpha; VEGFR, vascular endothelial growth factor receptor; WM, Waldenström’s macroglobulinemia.
Note: The pace of drug discovery is rapid and this list is not comprehensive.
tumors that have translocations of the c-Abl and BCR gene (such as 
chronic myeloid leukemia), mutant c-Kit (gastrointestinal stromal cell 
tumors), or mutant platelet-derived growth factor receptor (PDGFRα; 
gastrointestinal stromal tumors). Second-generation inhibitors of 
BCR-Abl, dasatinib and nilotinib, are even more effective, and the 
third-generation agent bosutinib has activity in some patients who 
have progressed on other inhibitors, while the third-generation agent 
ponatinib has activity against the T315I mutation, which is resistant 
to the other agents. Although almost all tyrosine kinase inhibitors are 
not entirely selective for one protein, certain inhibitors have significant 
activity against a broad number of proteins. These include sorafenib, 
regorafenib, cabozantinib, sunitinib, and lenvatinib. These have shown 
antitumor activity in various malignancies, including renal cell cancer 
(RCC) (sorafenib, sunitinib, cabozantinib, lenvatinib), hepatocellular 
carcinoma (sorafenib, regorafenib, lenvatinib), gastrointestinal stromal 
tumor (GIST) (sunitinib, regorafenib), thyroid cancer (sorafenib, cabo­
zantinib, lenvatinib), colorectal cancer (regorafenib), and pancreatic 
neuroendocrine tumors (sunitinib).
Inhibitors of the PI3K pathway also have been approved for cancer 
therapy. The PI3K family includes three classes and several isoforms 
within each class. Inhibitors against different isoforms have proved 
effective against different types of malignancies, with inhibitors of the 
delta isoform (either specifically or also with inhibition of other iso­
forms; e.g., idelalisib) having activity against lymphoid malignancies 
(CLL), whereas the specific inhibitor of a mutation in the alpha isoform 
(alpelisib) has activity against breast cancers with this mutation. Inhibi­
tors of mammalian target of rapamycin (mTOR; which is downstream 
of PI3K; e.g., everolimus, temsirolimus) are active in RCC; welldifferentiated nonfunctional neuroendocrine tumors of pancreatic, 

(Continued)
tumor cells
CHAPTER 77
Targeted T cells to protein on surface of malignant cells
Cancer Cell Biology
gastrointestinal, or lung origin; and breast cancer. Additional inhibitors 
of the PI3K pathway and other phospholipid signaling pathways such 
as the phospholipase C-gamma pathway, which are involved in a large 
number of cellular processes important in cancer development and 
progression, are being evaluated.
The list of active agents and treatment indications is growing rapidly 
(Table 77-2). These agents have ushered in a new era of personalized 
therapy. For many cancers, it is now standard for tumor biopsies to 
be assessed for specific molecular changes that predict response and 
to have clinical decision-making guided by those results. This is now 
an important component of choosing therapy for metastatic lung, gas­
troesophageal, melanoma, breast, and colorectal cancers as well as in 
adjuvant therapy for breast cancer. This list will continue to evolve as 
both new agents against existing targets are developed and new targets 
are discovered.
An alternative approach to testing samples directly from tumors is to 
test blood for the presence of mutations or amplification in circulating 
tumor DNA, which has the significant advantage of being noninvasive. 
As cancers grow, some of the cells die, break apart, and release cellular 
contents, including DNA, into the circulation. Sensitive methods have 
been developed to detect this DNA and to identify mutations and other 
DNA changes in the malignant cells. This has the potential advantage 
over tumor biopsies of sampling all of the tumor and not being lim­
ited to one site that may not be representative of the overall tumor 
heterogeneity. Distinct metastatic lesions may have different genetic 
abnormalities that will not be detected in a biopsy of a single site. 
In addition to identifying potential changes that can be targeted for 
therapy, there is also the potential for monitoring a patient’s response to 
therapy, identifying resistance mechanisms to therapy earlier, detecting

disease recurrence before it can be detected by tumor markers or scans, 
monitoring bodily fluids in addition to blood, and possibly providing a 
means of earlier initial detection of cancer if sufficiently sensitive and 
specific detection methods can be developed. Optimizing the sensitiv­
ity and specificity of these tests is essential for their potential utility in 
patient care. Research is ongoing to determine if other cellular compo­
nents specific to cancer cells (e.g., mRNA, proteins from mutant genes, 
or other protein modifications found in cancer cells) might also be 
useful for diagnosis or monitoring therapeutic response.

However, none of these targeted therapies has yet been curative by 
themselves for any malignancy, although prolonged periods of disease 
control lasting many years frequently occur in chronic myeloid leuke­
mia (CML), including a >80% survival rate at 10 years, and antibodies 
to HER2 have increased survival for breast cancer patients in combina­
tion with chemotherapy. The reasons for the failure of targeted agents 
to cure are not completely defined, although resistance to the treat­
ment ultimately develops in most patients. In some tumors, resistance 
to kinase inhibitors is related to proliferation of cells with a mutation 
in the target kinase that inhibits drug binding. Many of these kinase 
inhibitors act as competitive inhibitors of the ATP-binding pocket. 
ATP is the phosphate donor in these phosphorylation reactions. For 
example, mutation in the critical BCR-ABL kinase in the ATP-binding 
pocket (such as the threonine to isoleucine change at codon 315 
[T315I]) can prevent imatinib binding. Other resistance mechanisms 
include alterations in other signal transduction pathways to bypass 
the inhibited pathway. As resistance mechanisms continue to be bet­
ter defined, rational strategies to overcome resistance are emerging. 
In addition, many kinase inhibitors are less specific for an oncogenic 
target than was hoped, and toxicities related to off-target inhibition of 
kinases limit the use of the agent at a dose that would optimally inhibit 
the cancer-relevant kinase.
PART 4
Oncology and Hematology
Antibodies against protein targets more highly expressed on malig­
nant than normal cells can also be used to deliver highly toxic com­
pounds relatively specifically to cancer cells. Examples of protein targets 
for currently approved antibody-drug conjugates include CD30 for 
Hodgkin’s and anaplastic lymphomas; HER2 on breast cancer; CD33 on 
acute myeloid leukemias; CD22 on B-cell acute lymphocytic and hairy 
cell leukemias; and CD79b on diffuse large B-cell lymphomas.
Another strategy to enhance the antitumor effects of targeted agents 
is to use them in rational combinations with each other as well as with 
chemotherapy or immunotherapy agents that kill cells in ways dis­
tinct from agents targeting specific mutant or overexpressed proteins. 
Combinations of trastuzumab (a monoclonal antibody that targets the 
HER2 receptor [member of the EGFR family]) with chemotherapy 
have significant activity against breast, gastric, and esophageal cancers 
that have high levels of expression of the HER2 protein. The activity of 
trastuzumab and chemotherapy can be enhanced further by combina­
tions with another targeted monoclonal antibody (pertuzumab), which 
prevents dimerization of the HER2 receptor with other HER family 
members including HER3, or in some cases with immunotherapy, 
such as combinations of chemotherapy, trastuzumab, and the immu­
notherapeutic agent pembrolizumab against HER2-positive gastric and 
esophageal cancers.
Although targeted therapies have not yet resulted in cures when 
used alone, their use in the adjuvant setting and when combined with 
other effective treatments has substantially increased the fraction of 
patients cured. For example, the addition of rituximab, an anti-CD20 
antibody, to combination chemotherapy in patients with diffuse 
large B-cell lymphoma improves cure rates by ~15%. The addition of 
trastuzumab, an antibody to HER2, to combination chemotherapy in 
the adjuvant treatment of HER2-positive breast cancer significantly 
improves overall survival.
A major effort continues to develop targeted therapies for mutations 
in the ras family of genes, which play a critical role in transmitting 
signals through a number of downstream signaling pathways includ­
ing the MAP (mitogen-activated protein) kinase and PI3K pathways. 
Mutations in ras are the most common mutations in oncogenes in 
cancers (especially kras) but have proved to be very difficult targets for 
a number of reasons related to the structure of RAS proteins as well 

as mechanisms of activation and inactivation (active when bound to 
guanosine triphosphate [GTP] and inactive when bound to guanosine 
diphosphate [GDP]). RAS proteins are not kinases but bind directly 
to the BRAF serine/threonine kinase with preferential binding when 
RAS is in the active GTP bound state. Agents that target one of the 
mutant forms of KRAS (12C), which is the most common RAS muta­
tion in lung cancer and is also found in a subset of other cancers, have 
sufficient antitumor activity to now be approved for the treatment of 
these lung cancers. They are under active study (often in combina­
tion with other agents) for the treatment of other cancers that have 
the KRAS12C mutation. Agents targeting other mutations in the RAS 
genes (especially other KRAS-mutant proteins) are also being evalu­
ated in clinical trials.
One strategy for new drug development is to take advantage of socalled oncogene addiction. This situation (Fig. 77-3) is created when 
a tumor cell develops an activating mutation in an oncogene that 
becomes a dominant pathway for survival and growth with reduced 
contributions from other pathways, even when there may be abnor­
malities in those pathways. This dependency on a single pathway 
creates a cell that is vulnerable to inhibitors of that oncogene pathway. 
For example, cells harboring mutations in BRAF are sensitive to MEK 
inhibitors that inhibit signaling via the BRAF pathway.
Proteins critical for transcription of other proteins essential for 
malignant cell survival or proliferation provide another potential target 
for treating cancers. The transcription factor nuclear factor (NF)-κB is 
a heterodimer composed of p65 and p50 subunits that associate with 
an inhibitor, IκB, in the cell cytoplasm. In response to growth factor 
or cytokine signaling, a multisubunit kinase called IKK (IκB-kinase) 
phosphorylates IκB and directs its degradation by the ubiquitin/
proteasome system. NF-κB, free of its inhibitor, translocates to the 
nucleus and activates target genes, many of which promote the survival 
of tumor cells. One of the mechanisms by which novel drugs called 
proteasome inhibitors are thought to produce an anticancer effect is by 
blocking the proteolysis of IκB, thereby preventing NF-κB activation. 
For reasons that have not been fully elucidated, this has a differential 
toxicity effect on tumor, as compared to normal cells. Although this 
mechanism appears to be an important aspect of the antitumor effects 
of proteasome inhibitors, other effects involving the inhibition of the 
degradation of multiple cellular proteins important in malignant cell 
survival or proliferation also play a role.
Proteasome inhibitors (e.g., bortezomib, carfilzomib, ixazomib) 
have activity in patients with multiple myeloma, including partial 
and complete remissions. Inhibitors of IKK are also in development, 
with the hope of more selectively blocking the degradation of IκB, 
thus “locking” NF-κB in an inhibitory complex and rendering the 
cancer cell more susceptible to apoptosis-inducing agents. Many other 
transcription factors are activated by phosphorylation, which can be 
prevented by tyrosine or serine/threonine kinase inhibitors, a number 
of which are currently in clinical trials.
Estrogen receptors (ERs) and androgen receptors (ARs), members 
of the steroid hormone family of nuclear receptors, are targets of inhi­
bition by drugs used to treat breast and prostate cancers, respectively. 
Selective estrogen receptor modulators (SERMs) have been developed 
as a treatment approach for ER-positive breast cancer. Tamoxifen, a 
partial agonist and antagonist of ER function, is frequently used in 
breast cancer, can mediate tumor regression in metastatic breast cancer, 
and can prevent disease recurrence in the adjuvant setting. Tamoxifen 
binds to the ER and modulates its transcriptional activity, inhibiting 
activity in the breast but promoting activity in bone but unfortunately 
also in uterine epithelium, leading to a small increased risk of uterine 
cancer. Attempts have been made to develop SERMs that would have 
antiestrogenic effects in both breast and uterus while maintaining 
protective effects on bone. However, none of these to date has been an 
improvement over tamoxifen. Aromatase inhibitors, which block the 
conversion of androgens to estrogens in breast and subcutaneous fat 
tissues, have demonstrated improved clinical efficacy compared with 
tamoxifen in postmenopausal women and are often used as first-line 
therapy in postmenopausal patients with ER-positive disease. They are 
occasionally used in premenopausal patients with ER-positive disease

Normal cell
Base
excision
repair
Tumor cell
BRCA1, 2
nonmutated
Normal cell
Base
excision
repair
Tumor cell
BRCA1, 2
mutated
FIGURE 77-3  Synthetic lethality. Genes are said to have a synthetic lethal relationship when mutation of either gene alone is tolerated by the cell, but mutation of both genes 
leads to lethality, as originally noted by Bridges and later named by Dobzhansky. Thus, mutant gene a and gene b have a synthetic lethal relationship, implying that the loss of 
one gene makes the cell dependent on the function of the other gene. In cancer cells, loss of function of a DNA repair gene like BRCA1, which repairs double-strand breaks, 
makes the cell dependent on base excision repair mediated in part by PARP. If the PARP gene product is inhibited, the cell attempts to repair the break using the error-prone 
nonhomologous end-joining method, which results in tumor cell death. High-throughput screens can now be performed using isogenic cell line pairs in which one cell line 
has a defined defect in a DNA repair pathway. Compounds can be identified that selectively kill the mutant cell line; targets of these compounds have a synthetic lethal 
relationship to the repair pathway and are potentially important targets for future therapeutics.
in combination with ovarian suppression approaches such as lutein­
izing hormone–releasing hormone (LHRH) agonists.
A number of approaches have been developed for blocking andro­
gen stimulation of prostate cancer, including decreasing production by 
the testicles (e.g., orchiectomy, LHRH agonists or antagonists), directly 
blocking actions of androgen (a number of agents have been developed 
to do this), or blocking production by inhibiting the enzyme CYP17, 
which is central in production of androgens from cholesterol.
■
■CANCER-SPECIFIC GENETIC CHANGES AND 
SYNTHETIC LETHALITY
The concepts of oncogene addiction and synthetic lethality have 
spurred new drug development targeting oncogene- and tumorsuppressor pathways. As discussed earlier in this chapter and outlined 
in Fig. 77-3, cancer cells can become dependent upon signaling 
pathways containing activated oncogenes; this can effect proliferation 
(i.e., mutated KRAS, BRAF, overexpressed MYC, or activated tyrosine 
kinases). Additional genetic changes in malignant cells or unique 
features of tumors including defects in DNA repair (e.g., loss of 
BRCA1 or BRCA2 gene function), modifications in cell cycle control 
(e.g., changes in protein levels or mutations in cyclins and CDKs), 
enhanced survival mechanisms (overexpression of Bcl-2 or NF-κB), 
altered cell metabolism (such as occurs when mutant KRAS enhances 
glucose uptake and aerobic glycolysis), tumor-stromal interactions, 
and angiogenesis (e.g., production of vascular endothelial growth fac­
tor [VEGF] in response to HIF-2α in RCC) can also be successfully 
exploited to relatively specifically target cancers. However, resistance 
to inhibition of specific oncogenic pathways almost always eventually 
develops. In addition, targeting defects in tumor-suppressor genes 
has been much more difficult, both because the target of mutation is 

+ PARP inhibition
PARP
PARP
PARP
PARP
PARP
PARP
PARP
PARP
Homologous
double strand
break repair
No cell
killing
CHAPTER 77
Homologous
double strand
break repair
Selective
tumor
cell killing
–
Cancer Cell Biology
often deleted and because it is much more difficult to restore normal 
function than to inhibit abnormal function of a protein. Synthetic 
lethality occurs when loss of function in either of two or more genes 
individually has limited effects on cell survival but loss of function 
in both (or more) genes leads to cell death. In the case of oncogeneaddicted pathways, identifying genes that have a synthetic lethal rela­
tionship with the activated pathway may allow enhanced cell killing 
and decreased resistance by targeting those genes or their proteins. 
In the case of mutant tumor-suppressor genes, identifying genes that 
have a synthetic lethal relationship to those mutated pathways may 
allow targeting by inhibiting proteins required uniquely by those cells 
for survival or proliferation (Fig. 77-3). This is a much more tractable 
approach than attempting to repair normal function of the mutant 
suppressor gene itself. Examples of synthetic lethality with clinical 
impact have been identified. For instance, cells with mutations in the 
BRCA1 or BRCA2 tumor-suppressor genes (e.g., a subset of breast and 
ovarian cancers) are unable to repair DNA damage by homologous 
recombination. Poly-ADP ribose polymerase (PARP) is a family of 
proteins important for single-strand break (SSB) DNA repair. PARP 
inhibition results in selective killing of cancer cells that have lost 
BRCA1 or BRCA2 function. A number of PARP inhibitors have been 
approved for treatment of ovarian, breast, and pancreatic cancers with 
BRCA mutations and are likely to have activity in other tumors with 
defective DNA repair mechanisms. The concept of synthetic lethality 
provides a framework for genetic screens to identify other synthetic 
lethal combinations involving known tumor-suppressor genes and 
development of novel therapeutic agents to target dependent path­
ways. Other unique aspects of malignant tumors, including those 
outlined elsewhere in the chapter, may also be vulnerable to synthetic 
lethal interactions.

■
■EPIGENETIC INFLUENCES ON CANCER GENE 
TRANSCRIPTION
Chromatin structure regulates the hierarchical order of sequential 
gene transcription that governs differentiation and tissue homeostasis. 
Disruption of chromatin remodeling (the process of modifying chro­
matin structure to control exposure of specific genes to transcriptional 
proteins, thereby controlling the expression of those genes) leads to 
aberrant gene expression that can significantly alter the biology of 
cells including inducing proliferation or migration of cells. Epigenetic 
changes are those that alter the pattern of gene expression that persist 
across at least one cell division but are not caused by changes in the 
DNA code. These include alterations of chromatin structure mediated 
by methylation of cytosine residues of DNA (primarily in context of 
CpG dinucleotides in somatic cells), modification of histones by alter­
ing acetylation or methylation, or changes in higher-order chromo­
some structure (Fig. 77-4). Appropriate control of DNA methylation 
is essential for normal cell function and development, and both altered 
methylation and hypomethylation of histones occur in cancers. Hyper­
methylation of DNA promoter regions is a common mechanism by 
which tumor-suppressor loci are epigenetically silenced in cancer cells. 
Thus, one allele of a tumor-suppressor gene may be inactivated by 
mutation or deletion, while expression of the other allele is epigeneti­
cally silenced, usually by methylation, leading to loss of gene function. 
Aberrant hypomethylation is also frequently found in a number of 
cancers consistent with the dysregulated pattern of gene transcription 
that is a hallmark of cancer cells, with some genes being inappropri­
ately turned off while others are inappropriately turned on. Specific 
changes in DNA methylation in cancer cells provide a potentially more 
sensitive and specific approach to utilizing circulating tumor DNA to 
identify the presence of cancer than utilizing only DNA mutational 
analysis.

PART 4
Oncology and Hematology
Acetylation of the amino terminus of the core histones H3 and H4 
induces an open chromatin conformation that promotes transcription 
HDAC
MeCP
Nucleosomes
CpG Island in
promoter region
HAT: histone acetyl transferase
HDAC: histone deacetylase
    :unmethylated CpG
    :methylated CpG
DNMT: DNA methyltransferase
MeCP: methylcytosine binding protein
Co-activator
complex
HAT
HAT
Tc
factor
Tc
factor
Tc
factor
“Open” chromatin configuration
permits binding of multiple
sequence-specific transcription
factors that cooperatively promote
gene expression.
Nucleosomes
Nucleosomes
FIGURE 77-4  Epigenetic regulation of gene expression in cancer cells. Tumor-suppressor genes are often epigenetically silenced in cancer cells. In the upper portion, a CpG 
island within the promoter and enhancer regions of the gene has been methylated, resulting in the recruitment of methyl-cytosine binding proteins (MeCP) and complexes 
with histone deacetylase (HDAC) activity. Chromatin is in a condensed, nonpermissive conformation that inhibits transcription. Clinical trials are under way utilizing the 
combination of demethylating agents such as 5-aza-2′-deoxycytidine plus HDAC inhibitors, which together confer an open, permissive chromatin structure (lower portion). 
Transcription factors bind to specific DNA sequences in promoter regions and, through protein-protein interactions, recruit coactivator complexes containing histone acetyl 
transferase (HAT) activity. This enhances transcription initiation by RNA polymerase II and associated general transcription factors. The expression of the tumor-suppressor 
gene commences, with phenotypic changes that may include growth arrest, differentiation, or apoptosis.

initiation. Histone acetylases are components of coactivator complexes 
recruited to promoter/enhancer regions by sequence-specific transcrip­
tion factors during the activation of genes (Fig. 77-4). Histone deacety­
lases (HDACs; multiple HDACs are encoded in the human genome) are 
recruited to genes by transcriptional repressors and prevent the initia­
tion of gene transcription. Methylated cytosine residues in promoter 
regions become associated with methyl cytosine–binding proteins that 
recruit protein complexes with HDAC activity. The balance between 
permissive and inhibitory chromatin structure is therefore largely 
determined by the activity of transcription factors in modulating the 
“histone code” and the methylation status of the genetic regulatory 
elements of genes. The pattern of gene transcription is aberrant in all 
human cancers, and in many cases, epigenetic events are responsible. 
Epigenetic events play a critical role in carcinogenesis (e.g., long-lasting 
changes in methylation induced by smoking) and are found in prema­
lignant lesions. Unlike genetic events that alter DNA primary structure 
(e.g., deletions), epigenetic changes are potentially reversible and 
appear amenable to therapeutic intervention. In certain human can­
cers, including a subset of pancreatic cancers and multiple myeloma, 
the p16Ink4a promoter is inactivated by methylation, thus permitting 
the unchecked activity of CDK4/cyclin D and rendering pRb nonfunc­
tional. In sporadic forms of renal, breast, and colon cancer, the von 
Hippel–Lindau (VHL), breast cancer 1 (BRCA1), and serine/threonine 
kinase 11 (STK11) genes, respectively, can be epigenetically silenced. 
Other targeted genes include the p15Ink4b CDK inhibitor, glutathioneS-transferase (which detoxifies reactive oxygen species [ROS]), and 
the E-cadherin molecule (important for junction formation between 
epithelial cells). Epigenetic silencing can affect genes involved in DNA 
repair, thus predisposing to further genetic damage. Examples include 
MLH1 (mutL homologue in sporadic colon cancers that have micro­
satellite instability) and MSH2 in a subset of hereditary nonpolyposis 
colon cancer patients who have a mutation in the 3′ end of epithelial 
cell adhesion molecule (EPCAM). These are critical genes involved in 
No transcription
Differentiation arrested
Deregulated proliferation
DNMT
HDAC
MeCP
Nucleosomes
Treatment:
5-aza-2'-deoxycytidine
HDAC inhibitors
Active transcription
of tumor
suppressor genes
RNA
polymerase II
and general
transcription
machinery

repair of mismatched bases that occur during DNA synthesis, and their 
silencing can lead to mutations in the DNA.
Human leukemias often have chromosomal translocations that code 
for novel fusion proteins with activities that alter chromatin structure 
by interacting with HDACs or histone acetyl transferases (HATs). 
For example, the promyelocytic leukemia–retinoic acid receptor α 
(PML-RARα) fusion protein, generated by the t(15;17) translocation 
observed in most cases of acute promyelocytic leukemia (APL), binds 
to promoters containing retinoic acid response elements and recruits 
HDACs to these promoters, effectively inhibiting gene expression. 
This arrests differentiation at the promyelocyte stage and promotes 
tumor cell proliferation and survival. Treatment with pharmacologic 
doses of all-trans retinoic acid (ATRA), the ligand for RARα, results 
in the release of HDAC activity and the recruitment of coactivators, 
which overcome the differentiation block. This induced differentiation 
of APL cells has improved treatment of these patients but also has led 
to a novel treatment toxicity when newly differentiated tumor cells 
infiltrate the lungs. ATRA represents a treatment paradigm for the 
reversal of epigenetic changes in cancer. Other leukemia-associated 
fusion proteins, such as Tel-acute myeloid leukemia (AML1), AML1eight-twenty-one (ETO), and the MLL fusion proteins seen in acute 
myeloid leukemia (AML) and acute lymphocytic leukemia, also lead to 
repression through the HDAC complex. Therefore, efforts are ongoing 
to determine the structural basis for interactions between transloca­
tion fusion proteins and chromatin-remodeling proteins and to use 
this information to rationally design small molecules that will disrupt 
specific protein-protein associations, although this has proven to be 
technically difficult. Several drugs that block the enzymatic activity 
of HDACs (HDAC inhibitors [HDACis]) are approved for cancer 
treatment, and others are being tested. HDACis have demonstrated 
sufficient antitumor activity against cutaneous T-cell lymphoma (vori­
nostat, romidepsin), peripheral T-cell lymphoma (romidepsin, belino­
stat), and multiple myeloma (panobinostat) to be approved by the U.S. 
Food and Drug Administration (FDA).
HDACis have also demonstrated antitumor activity in clinical stud­
ies against some solid tumors, and additional studies are ongoing. 
HDACis may target cancer cells via a number of mechanisms including 
both epigenetic modulation via histone acetylation and effects on other 
proteins that are acetylated. The pleiotropic effects of some HDACis 
include enhancement of apoptosis by upregulation of a number of pro­
teins that enhance apoptosis including death receptors (DR4/5, FAS, 
and their ligands) and downregulation of proteins that inhibit apopto­
sis (e.g., X-linked inhibitor of apoptosis [XIAP]); upregulation of pro­
teins that inhibit cell cycle progression (e.g., p21Cip1/Waf1); inhibition 
of DNA repair and generation of ROS leading to increased DNA dam­
age; and disruption of the chaperone protein HSP90. Efforts are also 
under way to modulate other epigenetic processes such as reversing the 
hypermethylation of CpG islands that characterizes many malignan­
cies. Drugs that induce DNA demethylation, such as 5-aza-2-deoxy­
cytidine, can lead to reexpression of silenced genes in cancer cells with 
restoration of function, and 5-aza-2-deoxycytidine is approved for 
use in myelodysplastic syndrome. However, 5-aza-2-deoxycytidine 
has limited aqueous solubility and is myelosuppressive, limiting its 
usefulness. Other inhibitors of DNA methyltransferases are in devel­
opment. In ongoing clinical trials, inhibitors of DNA methylation are 
being combined with HDACis, with the idea that reversing coexisting 
epigenetic changes will reverse the deregulated patterns of gene tran­
scription in cancer cells.
Epigenetic gene regulation can also occur via microRNAs or long 
noncoding RNAs (lncRNA). MicroRNAs (miRNA) are short (average 
22 nucleotides in length) single strand RNA molecules that regulate 
gene expression after transcription by specifically binding to and 
inhibiting the translation or promoting the degradation of mRNA 
transcripts. It is estimated that >1000 miRNAs are encoded in the 
human genome. Each tissue has a distinctive repertoire of miRNA 
expression, and this pattern is altered in specific ways in cancers. 
miRNA’s are involved in controlling multiple aspects of cell biology 
through modulating protein expression (primarily by down regula­
tion) and thus are also  involved in multiple aspects of cancer biology. 

Specific correlations between expression of different miRNA molecules 
and tumor biology and clinical behavior are continuing to emerge. 
Therapies targeting miRNAs are not currently at hand but represent an 
ongoing area of treatment development. LncRNAs are longer than 200 
nucleotides and comprise the largest group of noncoding RNAs. Some 
of them have been shown to play important roles in gene regulation. 
The potential for altering these RNAs for therapeutic benefit is an area 
of active investigation. In addition to epigenetic changes, mutations in 
genes (such as enhancer and promoter regions) involved in controlling 
expression of other genes important in cancer cell biology can also 
lead to enhanced or decreased expression of the protein products of 
these genes.

APOPTOSIS AND OTHER MECHANISMS OF 
CELL DEATH
Tissue homeostasis requires a balance between the death of aged, 
terminally differentiated cells or severely damaged cells and their 
renewal by proliferation of committed progenitors. Genetic damage to 
growth-regulating genes of stem cells could lead to catastrophic results 
for the host as a whole. Thus, in normal cells, the genetic events caus­
ing activation of oncogenes or loss of tumor suppressors, which would 
be predicted to lead to unregulated cell proliferation unless corrected, 
also usually activate signal transduction pathways that block aberrant 
cell proliferation. These pathways can lead to forms of programmed 
cell death including apoptosis or autophagy (degradation of proteins 
and organelles by lysosomal proteases) or irreversible growth arrest 
(senescence). A number of other regulated cell death processes have 
been identified, including: pyroptosis, a caspase-1-dependent process 
leading to cleavage of gasdermins with subsequent formation of pores 
in the plasma membrane; ferroptosis (iron and reactive oxygen species 
dependent); and necroptosis (caspase-independent regulated cell death 
involving breakdown of cellular components and cell rupture, leading 
to inflammation and damage to surrounding tissues), which also play 
roles in tissue homeostasis and cell death. However, the exact roles 
they play in the fate of cancer cells and tissues are still being elucidated. 
Much as a panoply of intra- and extracellular signals impinge upon 
the core cell cycle machinery to regulate cell division, so too these 
signals are transmitted to a core enzymatic machinery that regulates 
cell death and survival. Cancer cells have developed mechanisms that 
either inhibit these processes to prevent cell death or utilize them to 
enhance survival.
CHAPTER 77
Cancer Cell Biology
Apoptosis is a tightly regulated process induced by two main path­
ways (Fig. 77-5). The extrinsic pathway of apoptosis is activated by 
cross-linking members of the tumor necrosis factor (TNF) receptor 
superfamily, such as CD95 (Fas) and death receptors DR4 and DR5, 
by their ligands, Fas ligand or TRAIL (TNF-related apoptosis-inducing 
ligand), respectively. This induces the association of FADD (Fas-

associated death domain) and procaspase-8 to death domain motifs 
of the receptors. Caspase-8 is activated and then cleaves and activates 
effector caspases-3 and -7, which then target cellular constituents 
(including caspase-activated DNase, cytoskeletal proteins, and a num­
ber of regulatory proteins), inducing the morphologic appearance 
characteristic of apoptosis, which pathologists term karyorrhexis (liter­
ally “nucleus bursting”).
The intrinsic pathway of apoptosis is initiated by the release of 
cytochrome c and SMAC (second mitochondrial activator of cas­
pases) from the mitochondrial intermembrane space in response to a 
variety of noxious stimuli, including DNA damage, loss of adherence 
to the extracellular matrix (ECM), oncogene-induced proliferation, 
and growth factor deprivation. Upon release into the cytoplasm, 
cytochrome c associates with dATP, procaspase-9, and the adaptor 
protein APAF-1, leading to the sequential activation of caspase-9 and 
effector caspases. SMAC binds to and blocks the function of inhibitor 
of apoptosis proteins (IAP), negative regulators of caspase activation. 
The release of apoptosis-inducing proteins from the mitochondria 
is regulated by pro- and antiapoptotic members of the Bcl-2 family. 
Antiapoptotic members (e.g., Bcl-2, Bcl-XL, and Mcl-1) associate with 
the mitochondrial outer membrane via their carboxyl termini, expos­
ing to the cytoplasm a hydrophobic binding pocket composed of Bcl-2

Trail

DR4 or
DR5
FADD
Caspase 8
Pro-caspase 9
Cyt c
APAF-1
dATP 

SMAC
IAP
BH3-only
proteins
Intermembrane
space
Bak

BcI2
Matrix
Bax
PART 4
Oncology and Hematology
Outer
membrane
Mitochondrion
FIGURE 77-5  Therapeutic strategies to overcome aberrant survival pathways in cancer cells. 1. The extrinsic pathway of apoptosis can be selectively induced in 
cancer cells by TRAIL (the ligand for death receptors 4 and 5) or by agonistic monoclonal antibodies. 2. Inhibition of antiapoptotic Bcl-2 family members with antisense 
oligonucleotides or inhibitors of the BH3-binding pocket will promote formation of Bak- or Bax-induced pores in the mitochondrial outer membrane. 3. Epigenetic silencing 
of APAF-1, caspase-8, and other proteins can be overcome using demethylating agents and inhibitors of histone deacetylases. 4. Inhibitor of apoptosis proteins (IAP) 
blocks activation of caspases; small-molecule inhibitors of IAP function (mimicking SMAC action) should lower the threshold for apoptosis. 5. Signal transduction pathways 
originating with activation of receptor tyrosine kinase receptors (RTKs) or cytokine receptors promote survival of cancer cells by a number of mechanisms. Inhibiting 
receptor function with monoclonal antibodies, such as trastuzumab or cetuximab, or inhibiting kinase activity with small-molecule inhibitors can block the pathway. 6. The 
Akt kinase phosphorylates many regulators of apoptosis to promote cell survival; inhibitors of Akt may render tumor cells more sensitive to apoptosis-inducing signals; 
however, the possibility of toxicity to normal cells may limit the therapeutic value of these agents. 7 and 8. Activation of the transcription factor NF-κB (composed of p65 
and p50 subunits) occurs when its inhibitor, IκB, is phosphorylated by IκB-kinase (IKK), with subsequent degradation of IκB by the proteasome. Inhibition of IKK activity 
should selectively block the activation of NF-κB target genes, many of which promote cell survival. Inhibitors of proteasome function are U.S. Food and Drug Administration 
approved and may work in part by preventing destruction of IκB, thus blocking NF-κB nuclear localization. NF-κB is unlikely to be the only target for proteasome inhibitors.
homology (BH) domains 1, 2, and 3 that is crucial for their activity. 
Perturbations of normal physiologic processes in specific cellular 
compartments lead to the activation of BH3-only proapoptotic family 
members (e.g., Bad, Bim, Bid, Puma, Noxa, and others) that can alter 
the conformation of the outer-membrane proteins Bax and Bak, which 
then oligomerize to form pores in the mitochondrial outer membrane 
resulting in cytochrome c release. If proteins composed only by BH3 
domains are sequestered by Bcl-2, Bcl-XL, or Mcl-1, pores do not 
form and apoptosis-inducing proteins are not released from the mito­
chondria. The ratio of levels of antiapoptotic Bcl-2 family members 
and the levels of proapoptotic BH3-only proteins at the mitochondrial 
membrane determines the activation state of the intrinsic pathway. The 
mitochondrion must therefore be recognized not only as an organelle 
with vital roles in intermediary metabolism and oxidative phosphory­
lation but also as a central regulatory structure of the apoptotic process. 
The evolution of tumor cells to a more malignant phenotype requires 
the acquisition of genetic changes that subvert apoptosis pathways and 
promote cancer cell survival and resistance to anticancer therapies.

GF
RTK

PI3K

Mdm2
AKT
Cytokine
receptor
Effector
caspases
BAD
Caspase

FKHR

Substrate
cleavage

IKK
IκB
p65
p50
Cytoskeletal
disruption

Proteasome
NF-κB
genes activated
DNA degradation
Chromatin condensation
Lamin cleavage
Nucleus
Death-inducing signals
• DNA damage
• Oncogene-induced proliferation
• Loss of attachment to ECM
• Chemotherapy, radiation therapy
However, this means that cancer cells may be more vulnerable than 
normal cells to therapeutic interventions that target the apoptosis 
pathways that cancer cells depend upon. For instance, overexpression 
of Bcl-2 as a result of the t(14;18) translocation contributes to follicular 
lymphoma, and it is highly expressed in many lymphoid malignancies 
including chronic lymphocytic leukemia (CLL). Upregulation of Bcl-2 
expression is also observed in other cancers including prostate, breast 
and lung cancers, and melanoma. Targeting of antiapoptotic Bcl-2 
family members has been accomplished by the identification of several 
low-molecular-weight compounds that bind to the hydrophobic pock­
ets of either Bcl-2 or Bcl-XL and block their ability to associate with 
death-inducing BH3-only proteins. An oral BH3 mimetic inhibitor of 
BCL-2, venetoclax, is approved for use in patients with refractory CLL 
with 17p deletion, and is active in AML. Preclinical studies targeting 
death receptors DR4 and -5 have demonstrated that recombinant, 
soluble, human TRAIL or humanized monoclonal antibodies with 
agonist activity against DR4 or -5 can induce apoptosis of tumor cells 
while sparing normal cells. The mechanisms for this selectivity may

include expression of decoy receptors or elevated levels of intracellular 
inhibitors (such as FLIP, which competes with caspase-8 for FADD) 
by normal cells but not tumor cells. Synergy has been shown between 
TRAIL-induced apoptosis and chemotherapeutic agents in some pre­
clinical studies. However, studies have not yet shown significant clini­
cal activity of approaches targeting the TRAIL pathway.
Many of the signal transduction pathways perturbed in cancer pro­
mote tumor cell survival (Fig. 77-5). These include activation of the 
PI3K/Akt pathway, increased levels of the NF-κB transcription factor, 
and epigenetic silencing of genes such as APAF-1 (apoptosis protease 
activating factor-1 involved in activating caspase-9 and essential for 
apoptosis) and caspase-8. Each of these pathways is a target for thera­
peutic agents that, in addition to affecting cancer cell proliferation or 
gene expression, may render cancer cells more susceptible to apoptosis, 
thus promoting synergy when combined with other chemotherapeutic 
agents.
Some tumor cells resist drug-induced apoptosis indirectly by elimi­
nating the noxious stimulus-inducing apoptosis through expression 
of one or more members of the ABC (ATP-binding cassette proteins) 
family of ATP-dependent efflux pumps that mediate the multidrug 
resistance (MDR) phenotype. The prototype member of this family, 
P-glycoprotein (PGP), spans the plasma membrane 12 times and has 
two ATP-binding sites. Hydrophobic drugs (e.g., anthracyclines and 
vinca alkaloids) are recognized by PGP as they enter the cell and are 
pumped out. Numerous clinical studies have failed to demonstrate that 
drug resistance can be overcome using inhibitors of PGP. However, 
ABC transporters have different substrate specificities, and inhibition 
of a single family member may not be sufficient to overcome the MDR 
phenotype. Efforts to reverse PGP-mediated drug resistance continue.
In addition to its role in cell death, autophagy can also serve as a 
homeostatic mechanism to promote cell survival by recycling cellular 
components to provide necessary energy. The mechanisms that control 
the balance between enhancing survival versus leading to cell death are 
still not fully understood. Autophagy appears to play conflicting roles 
in the development and survival of cancer. Early in the carcinogenic 
process, it can act as a tumor suppressor by preventing the cell from 
accumulating abnormal proteins and organelles. However, in estab­
lished tumors, it may serve as a mechanism of survival for cancer cells 
when they are stressed by damage such as from chemotherapy. Preclin­
ical studies have indicated that inhibition of this process can enhance 
the sensitivity of cancer cells to chemotherapy or radiation therapy, and 
ongoing trials are evaluating inhibitors of autophagy in combination 
with chemotherapy and/or radiation therapy. Better understanding of 
the factors that control the survival-promoting versus death-inducing 
aspects of autophagy is required in order to know how to best manipu­
late it for therapeutic benefit.
■
■METASTASIS
The metastatic process accounts for the vast majority of deaths from 
solid tumors, and therefore, an understanding of this process is critical 
for improvements in survival from cancer. The biology of metastasis 
is complex and requires multiple steps. The initial step involves cell 
migration and invasion through the ECM. The three major features 
of tissue invasion are cell adhesion to the basement membrane, local 
proteolysis of the membrane, and movement of the cell through the 
rent in the membrane and the ECM. Cells that lose contact with the 
ECM normally undergo programmed cell death (anoikis-apoptosis 
induced by the loss of contact), and this process has to be suppressed 
in cells that metastasize. Another process important for many, but not 
necessarily all, metastasizing epithelial cancer cells is epithelial mes­
enchymal transition (EMT). This is a process by which cells lose their 
epithelial properties and gain mesenchymal properties. This normally 
occurs during the developmental process in embryos, allowing cells to 
migrate to their appropriate destinations in the embryo. It also occurs 
in wound healing, tissue regeneration, and fibrotic reactions, but in all 
of these processes, cells stop proliferating when the process is complete. 
Malignant cells that metastasize often undergo EMT as an important 
step in that process but retain the capacity for unregulated prolifera­
tion. However, there is evidence that not all metastasizing cancer cells 

require EMT, and the exact role of EMT in different metastasizing 
cancer cells continues to be elucidated. Malignant cells that gain access 
to the circulation must then repeat those steps at a remote site, find a 
hospitable niche in a foreign tissue, avoid detection and elimination by 
host defenses including the immune system, and induce the growth of 
new blood vessels. Some metastatic cells occur as oligoclonal clusters, 
which appear to be more potent in establishing metastasis than single 
cells, perhaps, in part, through differential and cooperative effects 
in evading host defenses. The rate-limiting step for metastasis is the 
ability for tumor cells to survive and expand in the novel microenvi­
ronment of the metastatic site, and multiple host-tumor interactions 
determine the ultimate outcome (Fig. 77-6).

As is true for cells in primary cancers, there is significant hetero­
geneity as well as plasticity in metastatic cancer cells. In addition to 
actively dividing cells, a population of quiescent cells are present that 
can evade the immune system as well as chemotherapy targeting divid­
ing cells. The processes that keep metastatic cancer cells quiescent as 
well as lead them to divide are complex, as discussed in the introduc­
tory section. Efforts to inhibit growth of metastatic cells by modulating 
these pathways are being explored. Few drugs have been developed to 
attempt to directly target the process of metastasis, in part because the 
specifics of the critical steps in the process that would be potentially 
good targets for drugs are still being identified. However, a number 
of potential targets are known. HER2 can enhance the metastatic 
potential of breast cancer cells, and as discussed above, the monoclonal 
antibody trastuzumab, which targets HER2, improves survival in the 
adjuvant setting for HER2-positive breast cancer patients. A number 
of other potential targets that increase metastatic potential of cells in 
preclinical studies include HIF-1 and -2, transcription factors induced 
by hypoxia within tumors, growth factors (e.g., cMET and VEGFR), 
oncogenes (e.g., SRC), adhesion molecules (e.g., focal adhesion kinase 
[FAK]), ECM proteins (e.g., matrix metalloproteinases 1 and 2), and 
inflammatory molecules (e.g., COX-2).
CHAPTER 77
Cancer Cell Biology
The metastatic phenotype is likely restricted to a fraction of tumor 
cells (Fig. 77-6). A number of genetic and epigenetic changes are 
required for tumor cells to be able to metastasize, including activation 
of metastatic-promoting genes and inhibition of genes that suppress 
the metastatic ability. Given the role of microRNAs in controlling gene 
expression (see epigenetic section) including those critical to the meta­
static process, efforts are under way to modulate these to try to inhibit 
metastasis. Cells with metastatic capability frequently express chemo­
kine receptors that are likely important in the metastatic process. A 
number of candidate metastasis-suppressor genes have been identified, 
including genes coding for proteins that enhance apoptosis, suppress 
cell division, are involved in the interactions of cells with each other 
or the ECM, or suppress cell migration. The loss of function of these 
genes enhances metastasis. Gene expression profiling is being used to 
study the metastatic process and other properties of tumor cells that 
may predict susceptibilities.
An example of the ability of malignant cells to survive and grow in 
a novel microenvironment is bone metastases. Bone metastases can be 
extremely painful, cause fractures of weight-bearing bones, can lead to 
hypercalcemia, and are a major cause of morbidity for cancer patients. 
Osteoclasts and their monocyte-derived precursors express the sur­
face receptor RANK (receptor activator of NF-κB), which is required 
for terminal differentiation and activation of osteoclasts. Osteoblasts 
and other stromal cells express RANK ligand (RANKL), as both a 
membrane-bound and soluble cytokine. Osteoprotegerin (OPG), a 
soluble receptor for RANKL produced by stromal cells, acts as a decoy 
receptor to inhibit RANK activation. The relative balance of RANKL 
and OPG determines the activation state of RANK on osteoclasts. Bone 
modulation and resorption by osteoclasts is an important component 
of the establishment and progression of metastases in bone. Many 
tumors increase osteoclast activity by secretion of substances such as 
parathyroid hormone (PTH), PTH-related peptide, interleukin (IL) 1, 
or Mip1 that perturb the homeostatic balance of bone remodeling by 
increasing RANK signaling. One example is multiple myeloma, where 
tumor cell–stromal cell interactions activate osteoclasts and inhibit 
osteoblasts, leading to the development of multiple lytic bone lesions.

Basement
membrane
Normal epithelial
cells
Cytokeratin
Adherens
junction
E-cadherin
Tumor cell
TGF-β
receptor
TGF-β
N-Cadherin
Snail
Twist
HGF
New integrin
expression
N-Cadherin
C-Met
PART 4
Oncology and Hematology
FIGURE 77-6  Oncogene signaling pathways are activated during tumor progression and promote metastatic potential. This figure shows a cancer cell that has undergone 
epithelial to mesenchymal transition (EMT) under the influence of several environmental signals. Critical components include activated transforming growth factor beta 
(TGF-β) and the hepatocyte growth factor (HGF)/c-Met pathways, as well as changes in the expression of adhesion molecules that mediate cell-cell and cell–extracellular 
matrix interactions. Important changes in gene expression are mediated by the Snail and Twist family of transcriptional repressors (whose expression is induced by the 
oncogenic pathways), leading to reduced expression of E-cadherin, a key component of adherens junctions between epithelial cells. This, in conjunction with upregulation 
of N-cadherin, a change in the pattern of expression of integrins (which mediate cell–extracellular matrix associations that are important for cell motility), and a switch in 
intermediate filament expression from cytokeratin to vimentin, results in the phenotypic change from adherent highly organized epithelial cells to motile and invasive cells 
with a fibroblast or mesenchymal morphology. EMT is thought to be an important step leading to metastasis in some human cancers. Host stromal cells, including tumorassociated fibroblasts and macrophages, play an important role in modulating tumor cell behavior through secretion of growth factors and proangiogenic cytokines, and 
matrix metalloproteinases that degrade the basement membrane. VEGF-A, -C, and -D are produced by tumor cells and stromal cells in response to hypoxemia or oncogenic 
signals and induce production of new blood vessels and lymphatic channels through which tumor cells metastasize to lymph nodes or tissues.
Inhibition of RANKL by an antibody (denosumab) can prevent fur­
ther bone destruction. Bisphosphonates are also effective inhibitors 
of osteoclast function that are used in the treatment of cancer patients 
with bone metastases.
■
■CANCER STEM CELLS
Normal tissues have stem cells capable of self-renewal and repairing 
damaged tissue, whereas the majority of cells in normal tissues do not 
have this capacity. Similarly, only a small proportion of the cells within 
a tumor are capable of initiating colonies in vitro or forming tumors 
at high efficiency when injected into immunocompromised NOD/
SCID mice. For example, AML and CML have a small population of 
cells (estimated to be <1%) that have properties of stem cells, such 
as unlimited self-renewal and the capacity to cause leukemia when 
serially transplanted in mice. These cells have an undifferentiated 
phenotype (Thy1–CD34+CD38– and do not express other differentia­
tion markers) and resemble normal stem cells in many ways but are 
no longer under homeostatic control (Fig. 77-7). Solid tumors may 
also contain a population of stem cells. It is not yet known how often 
cancers may originate within a stem cell population, although a body 
of evidence argues that stem cells are likely involved in the develop­
ment of the majority of cancers. Cancer stem cells, like their normal 
counterparts, have unlimited proliferative capacity and paradoxically 
traverse the cell cycle at a slow rate; cancer growth occurs largely due 
to expansion of the stem cell pool, the unregulated proliferation of an 

Lamina propria
Tumor-associated
fibroblast
New lymph vessel
MMP
Cytokines
growth
factors
Tumor-associated
macrophage
Invasion
New blood vessel
VEGF-A
HOST STROMAL CELLS
amplifying population, and failure of apoptosis pathways (Fig. 77-7). 
Slow cell cycle progression and high levels of expression of antiapop­
totic Bcl-2 family members and drug efflux pumps of the MDR family 
render cancer stem cells less vulnerable to cancer chemotherapy or 
radiation therapy.
Implicit in the cancer stem cell hypothesis is the idea that failure to 
cure most human cancers is due to the fact that current therapeutic 
agents are not very effective in killing stem cells. Efforts are ongoing 
to identify and isolate cancer stem cells from different types of malig­
nancies, which should allow determination of the aberrant signaling 
pathways that distinguish these cells from normal tissue stem cells. 
These would serve as potential therapeutic targets. Evidence that cells 
with stem cell properties can arise from other epithelial cells within 
the cancer by processes such as epithelial mesenchymal transition also 
implies that it is essential to treat all of the cancer cells, and not just 
those with current stem cell–like properties, in order to eliminate the 
self-renewing cancer cell population. The exact nature of cancer stem 
cells remains an area of investigation. One of the unanswered questions 
is the exact origin of cancer stem cells for the different cancers.
PLASTICITY AND RESISTANCE
Cancer cells, and especially stem cells, have the capacity for significant 
plasticity, allowing them to alter multiple aspects of cell biology in 
response to external factors (e.g., chemotherapy, radiation therapy, 
inflammation, immune response). In addition, heterogeneity between

NORMAL TISSUE
CANCER
Stem Cells
Stem cell niche
Paracrine signals
Polarized division
Daughter
cell
Stem
cell
Transit-amplifying cells
Exponential growth
Regulated activation of
differentiation program
Loss of self-renewal
capacity
Multilineage differentiation
Growth arrest
Maintenance of tissue
architecture and homeostasis
FIGURE 77-7  Cancer stem cells play a critical role in the initiation, progression, and resistance to therapy of malignant neoplasms. In normal tissues (left), homeostasis is 
maintained by asymmetric division of stem cells, leading to one progeny cell that will differentiate and one cell that will maintain the stem cell pool. This occurs within highly 
specific niches unique to each tissue, such as in close apposition to osteoblasts in bone marrow, or at the base of crypts in the colon. Here, paracrine signals from stromal 
cells, such as sonic hedgehog or Notch ligands, as well as upregulation of β-catenin and telomerase, help to maintain stem cell features of unlimited self-renewal while 
preventing differentiation or cell death. This occurs in part through upregulation of the transcriptional repressor Bmi-1 and inhibition of the p16Ink4a/Arf and p53 pathways. 
Daughter cells leave the stem cell niche and enter a proliferative phase (referred to as transit-amplifying) for a specified number of cell divisions, during which time a 
developmental program is activated, eventually giving rise to fully differentiated cells that have lost proliferative potential. Cell renewal equals cell death, and homeostasis 
is maintained. In this hierarchical system, only stem cells are long-lived. The hypothesis is that cancers harbor stem cells that make up a small fraction (i.e., 0.001–1%) of 
all cancer cells. These cells share several features with normal stem cells, including an undifferentiated phenotype, unlimited self-renewal potential, and a capacity for 
some degree of differentiation; however, due to initiating mutations (mutations are indicated by lightning bolts), they are no longer regulated by environmental cues. The 
cancer stem cell pool is expanded, and rapidly proliferating progeny, through additional mutations, may attain stem cell properties, although most of this population is 
thought to have a limited proliferative capacity. Differentiation programs are dysfunctional due to reprogramming of the pattern of gene transcription by oncogenic signaling 
pathways. Within the cancer transit-amplifying population, genomic instability generates aneuploidy and clonal heterogeneity as cells attain a fully malignant phenotype 
with metastatic potential. The cancer stem cell hypothesis has led to the idea that current cancer therapies may be effective at killing the bulk of tumor cells but do not 
kill tumor stem cells, leading to a regrowth of tumors that is manifested as tumor recurrence or disease progression. Research is in progress to identify unique molecular 
features of cancer stem cells that can lead to their direct targeting by novel therapeutic agents.
the different clones of cells within the tumor population and their 
interactions with each other and the tumor microenvironment pro­
vides the tumor with the capacity for significant plasticity in dealing 
with both internal and external stresses. Thus, a major problem in can­
cer therapy is that malignancies have a wide spectrum of mechanisms 
for both initial and adaptive resistance to treatments. These include 
inhibiting drug delivery to the cancer cells, blocking drug uptake 
and retention, increasing drug metabolism, altering levels of target 
proteins making them less sensitive to drugs, acquiring mutations in 
target proteins making them no longer sensitive to the drug, modify­
ing metabolism and cell signaling pathways, using alternate signaling 
pathways, adjusting the cell replication process including mechanisms 
by which the cell deals with DNA damage, inhibiting apoptosis, and 
evading the immune system. Thus, most metastatic cancers (except 
those curable with chemotherapy such as germ cell tumors) eventually 
become resistant to the therapy being utilized. Overcoming resistance 
is a major area of research.
■
■CANCER METABOLISM
One of the distinguishing characteristics of cancer cells is that they have 
altered metabolism as compared with normal cells in supporting sur­
vival, their high rates of proliferation, and ability to metastasize. Com­
plicating studies evaluating metabolic differences between normal and 
malignant cells is that there is heterogeneity in metabolism between 
different cells within a cancer. Malignant cells must focus a significant 
fraction of their energy resources into synthesis of proteins and other 
molecules (building blocks required for the production of new cells) 
while still maintaining sufficient ATP production to survive and grow. 

Differentiation
Cancer Stem Cells
Altered or expanded
stem cell niche
Initiating mutations
Transit-amplifying cells
Exponential growth
Altered transcription
program
Differentiation arrest
Genetic instability
Secondary mutations
Limited self-renewal capacity
Partial differentiation
No growth arrest
CHAPTER 77
Loss of tissue architecture 
and homeostasis control
Cancer Cell Biology
Although normal proliferating cells also have similar needs, there are 
differences in how cancer cells metabolize glucose and a number of 
other compounds including the amino acid glutamine as compared to 
normal cells in part because of genetic and epigenetic changes within 
cancer cells but also likely due to differences in the environments of 
cancer and normal cells. Many cancer cells utilize aerobic glycolysis 
(the Warburg effect) (Fig. 77-8) to metabolize glucose, leading to 
increased lactic acid production, whereas normal cells utilize oxidative 
phosphorylation in mitochondria under aerobic conditions, a much 
more efficient process for generating ATP for energy utilization but 
one that does not produce the same level of building blocks needed for 
new cells. One consequence is increased glucose uptake and utilization 
by cancer cells, a fact utilized in fluorodeoxyglucose (FDG)-positron 
emission tomography (PET) scanning to detect tumors. A number of 
proteins in cancer cells, including cMYC, HIF1, RAS, p53, pRB, and 
AKT, are involved in modulating glycolytic processes and controlling 
the Warburg effect. Although these pathways overall remain difficult to 
target therapeutically, some progress has been made in targeting HIF1 
and the RAS pathways with inhibitors approved to treat cancers with 
mutations in HIF1α or KRAS12C. In addition, both the PI3K pathway 
with signaling through mTOR and the AMP-activated kinase (AMPK) 
pathway that inhibits mTORC1 (a protein complex that includes 
mTOR) are important in controlling the glycolytic process and thus 
provide potential targets for inhibiting this process. An inhibitor of 
mTOR is approved for use against RCC (temsirolimus), and another 
inhibitor (everolimus) has activity against breast and neuroendocrine 
cancer and RCC. Other mTOR inhibitors are in trials, and modulators 
of AMPK are being investigated. The inefficient utilization of glucose

Differentiated tissue
Tumor
Proliferative
tissue
or
+O2
–O2
+/–O2
Glucose
Glucose
Glucose
Pyruvate
Pyruvate
Pyruvate
O2
O2
Lactate
Lactate
Lactate
CO2
Anaerobic
glycolysis
2 mol ATP/
mol glucose
Oxidative
phosphorylation
–36 mol ATP/
mol glucose
FIGURE 77-8  Warburg versus oxidative phosphorylation. In most normal tissues, the vast majority of cells are differentiated and dedicated to a particular function within 
the organ in which they reside. The metabolic needs are mainly for energy and not for building blocks for new cells. In these tissues, ATP is generated by oxidative 
phosphorylation that efficiently generates about 36 molecules of ATP for each molecule of glucose metabolized. By contrast, proliferative tumor tissues, especially in the 
setting of hypoxia, a typical condition within tumors, use aerobic glycolysis to generate energy for cell survival and generation of building blocks for new cells.
PART 4
Oncology and Hematology
by malignant cells also leads to a need for alternative metabolic path­
ways for other compounds as well, one of which is glutamine. Similar 
to glucose, this provides both a source for structural molecules as well 
as energy production. Similarly to glucose, glutamine is also ineffi­
ciently utilized by cancer cells. Cancer cells can also take up nutrients 
released by surrounding cells and tissues, increasing the complexity of 
successfully therapeutically inhibiting metabolism in cancer.
Mutations in genes involved in the metabolic process occur in a 
number of cancers. Among the most frequently found to date are muta­
tions in isocitrate dehydrogenases 1 and 2 (IDH1 and IDH2). These 
have been most commonly seen in gliomas, AMLs, and intrahepatic 
cholangiocarcinomas. These mutations lead to the production of an 
oncometabolite (2-hydroxyglutarate [2HG]) instead of the normal 
product α-ketoglutarate. Although the exact mechanisms of oncogen­
esis by 2HG are still being elucidated, α-ketoglutarate is a key cofactor 
for a number of dioxygenases involved in controlling DNA methyla­
tion. 2HG can act as a competitive inhibitor for α-ketoglutarate, lead­
ing to alterations in methylation status (primarily hypermethylation) of 
genes (leading to epigenetic changes) that can have profound effects on 
a number of cellular processes including differentiation. Inhibitors of 
mutant IDH1 and/or IDH2 are approved for treating IDH mutant AML 
and cholangiocarcinoma; and a dual IDH1/2 inhibitor is approved for 
treatment of low-grade gliomas and astrocytomas with IDH mutations.
Much needs to be learned about the specific differences in metabo­
lism between cancer cells and normal cells in order to develop more 
effective approaches to using these differences therapeutically; how­
ever, even with the currently limited state of knowledge, modulators of 
metabolism are being tested clinically. One of these is the antidiabetic 
agent metformin, both alone and in combination with chemotherapeu­
tic agents. Metformin inhibits gluconeogenesis and may have direct 
effects on tumor cells by activating AMPK, a serine/threonine protein 
kinase that is downstream of the LKB1 tumor suppressor, and thus 
inhibiting mTOR complex 1 (mTORC1). This leads to decreased pro­
tein synthesis and proliferation. Studies to date have not yet established 
metformin to have a clear role as an anticancer agent.
■
■TUMOR MICROENVIRONMENT, ANGIOGENESIS, 
AND IMMUNE EVASION
Tumors consist not only of malignant cells but also of a complex 
microenvironment including many other types of cells (including 
lymphocytes, macrophages, myeloid cells; other inflammatory cells; 

5%
85%
CO2
Aerobic
glycolysis
(Warburg effect)
–4 mol ATP/mol glucose
vascular cells, lymphatic endothelial cells, nerve cells, fibroblasts, and 
fat cells), ECM, stroma, secreted factors (including growth factors and 
hormones), reactive oxygen and nitrogen species, mechanical factors, 
blood vessels, and lymphatics. There is extensive cross-talk between the 
cells with each other, the ECM, and the various secreted factors within 
the tumor microenvironment. This microenvironment is not static but 
rather is dynamic and continually evolving. Both the complexity and 
dynamic nature of the microenvironment enhance the difficulty of 
treating tumors. The microenvironment is involved in altered tumor 
metabolism, tumor maintenance, growth, phenotypic plasticity, metas­
tasis, and immune escape, and can contribute to resistance to antican­
cer therapies through a number of mechanisms. These include immune 
evasion by a variety of mechanisms including suppression of effector 
T cells, increase in regulatory T cells, induction of an inflammatory 
environment, and altered vasculature that inhibits effector T-cell access 
to malignant cells. Similarly, it contributes to drug resistance through 
multiple mechanisms, alteration in metabolic pathways including 
creating a hypoxic and acidic environment, vascular and mechanical 
factors that limit drug access to malignant cells, various secreted factors 
that inhibit apoptosis or stimulate survival pathways, and generation of 
ROS that enhance drug resistance. Multiple additional mechanisms are 
also involved in enhancing resistance to immune-mediated anticancer 
effects and anticancer drug therapy.
■
■OBESITY AND CANCER
Significant evidence links obesity and the increased risk of devel­
oping certain cancers including postmenopausal breast, colorectal, 
ovarian, endometrial, esophageal, gallbladder, thyroid, and kidney 
cancers, among others. Less certain are the mechanisms responsible 
for this risk. As outlined above, cancers arise in an environment with 
multiple factors, many of which can stimulate cell proliferation. Obe­
sity impacts a variety of factors including hormonal factors, altered 
metabolism (especially adipose metabolism), and mediators of inflam­
matory response that all can impact the development of malignancy. 
Obesity is associated with a number of hormonal changes including 
high insulin, glucagon, and leptin levels that can stimulate growth of 
cells. It also leads to insulin resistance, which may contribute to cancer 
cell development, in part by increasing insulin-like growth factor-1 
(IGF-1) levels. Obesity also leads to alterations in adipose, including 
fatty acid, metabolism with production of compounds important for 
energy metabolism as well as for membrane function within cells that

may contribute to carcinogenic process. Obesity 
contributes to an inflammatory environment in 
a variety of ways including increased levels of 
inflammatory proteins such as IL-6 and TNF-α. 
In terms of impact on survival with cancer, data 
primarily from breast cancer suggest that obesity 
is associated with decreased survival likely due, at 
least in part, to the impact of obesity on hormonal 
factors in development of certain breast cancers, 
although this may be limited to subsets of breast 
cancer patients. Some studies have suggested, para­
doxically, that obesity may be associated with 
improved survival in some patients such as those 
with advanced-stage colorectal cancer. Further­
more, immune checkpoint inhibitor therapy has 
appeared to be more effect in obese patients. 
Clearly, the biology of the association between obe­
sity and cancer and its impact on disease outcome 
is complex, and additional studies are necessary to 
better define the mechanisms involved.
Vascular mimicry—
tumor cells as
part of vessel wall
Tumor
■
■MECHANISMS OF TUMOR VESSEL 
FORMATION
One of the critical elements of tumor cell pro­
liferation is delivery of oxygen, nutrients, and 
circulating factors important for growth and sur­
vival. Thus, a critical element in growth of pri­
mary tumors and formation of metastatic sites 
is the angiogenic switch: the ability of the tumor 
to promote the formation of new blood vessels, 
including the recruitment of vascular endothelial 
cells (ECs). The angiogenic switch is a phase in 
tumor development when the dynamic balance of 
pro- and antiangiogenic factors is tipped in favor of 
vessel formation by the effects of the tumor on its 
immediate environment. Stimuli for tumor angio­
genesis include hypoxemia, inflammation, and 
genetic lesions in oncogenes or tumor suppressors 
that alter tumor cell gene expression. Angiogenesis 
consists of several steps, including the stimula­
tion of ECs by growth factors, degradation of the 
ECM by proteases, proliferation and migration of 
ECs into the tumor, and the eventual formation 
of new capillary tubes. Tumors use a number of 
mechanisms to promote vascularization, subvert­
ing normal angiogenic processes for this purpose 
(Fig. 77-9). Primary or metastatic tumor cells 
sometimes arise in proximity to host blood vessels 
and grow around these vessels, parasitizing nutri­
ents by co-opting the local blood supply. However, 
most tumor blood vessels arise by the process of 
sprouting, in which tumors secrete trophic angio­
genic molecules, the most potent being vascular 
endothelial growth factors (VEGFs), that induce the proliferation and 
migration of host ECs into the tumor. Sprouting in normal and patho­
genic angiogenesis is regulated by three families of transmembrane 
RTKs expressed on ECs and their ligands (VEGFs, angiopoietins, and 
ephrins; Fig. 77-10), which are produced by tumor cells, inflammatory 
cells, or stromal cells in the tumor microenvironment.
FIGURE 77-9  Tumor angiogenesis is a complex process involving many different cell types that must 
proliferate, migrate, invade, and differentiate in response to signals from the tumor microenvironment. 
Endothelial cells (ECs) sprout from host vessels in response to VEGF, bFGF, Ang2, and other proangiogenic 
stimuli. Sprouting is stimulated by VEGF/VEGFR2, Ang2/Tie-2, and integrin/extracellular matrix (ECM) 
interactions. Bone marrow–derived circulating endothelial precursors (CEPs) migrate to the tumor in 
response to VEGF and differentiate into ECs, while hematopoietic stem cells differentiate into leukocytes, 
including tumor-associated macrophages that secrete angiogenic growth factors and produce matrix 
metalloproteinases (MMPs) that remodel the ECM and release bound growth factors. Tumor cells 
themselves may directly form parts of vascular channels within tumors. The pattern of vessel formation is 
haphazard: vessels are tortuous, dilated, leaky, and branch in random ways. This leads to uneven blood flow 
within the tumor, with areas of acidosis and hypoxemia (which stimulate release of angiogenic factors) and 
high intratumoral pressures that inhibit delivery of therapeutic agents.
Central to the angiogenic response are hypoxia-inducible factors 
(HIFs; especially 1 and 2), which are transcription factors that normally, 
in response to hypoxia, stimulate the transcription of a large number of 
genes responsive to hypoxia, including genes involved in metabolism as 
well as angiogenesis. HIF1 has a bigger role in stimulating metabolism 
(glycogenesis), whereas HIF2 plays a bigger role in angiogenesis. HIF 
protein function can also be enhanced in a number of ways in cancer 
not involving hypoxia, including mutations in the von Hippel–Lindau 
tumor suppressor gene (an E3 ubiquitin ligase that controls HIF levels 
by targeting it for degradation), such as occurs in some RCCs. Among 

CEP contributes newly
differentiated EC
HSC-derived
macrophage
Tumor
Leaky
vessels
Tumor
Region of
hypoxemia
100 µm
High
intratumoral
pressure
Dilated
leaky
   tumor
         vessel
VEGF
VEGF
VEGFR2
VEGFR2
Destabilization
Tie 2
Tie 2
Ang2
ανβ 2
ανβ 3
ανβ 5
α5β 1
α5β 1
ECM
Tie 2
New
sprout
Follows VEGF gradient to tumor
CHAPTER 77
Migrate to tumor
Host blood vessel
VEGFR2
CEP
CD133
Cancer Cell Biology
Bone marrow–derived cells
(from hemangioblast)
VEGFR1
HSC
c-kit
Tumor cells
Host EC
Tumor EC
Circulating endothelial
precursors (CEP)
Hematopoietic cell–derived
leukocytes (HSC)
the genes stimulated by HIF are VEGF and VEGF receptors. VEGFs 
and their receptors are required for embryonic vasculogenesis (devel­
opment of new blood vessels when none preexist) and normal (wound 
healing, corpus luteum formation) and pathologic angiogenesis (tumor 
angiogenesis, inflammatory conditions such as rheumatoid arthritis). 
VEGF-A is a heparin-binding glycoprotein with at least four isoforms 
(splice variants) that regulates blood vessel formation by binding to 
the RTKs VEGFR1 and VEGFR2, which are expressed on all ECs in 
addition to a subset of hematopoietic cells (Fig. 77-9). VEGFR2 plays a 
more direct role in regulating EC proliferation, migration, and survival, 
whereas VEGFR1 appears to have more nuanced functions with a less 
direct role in stimulating EC processes in the normal adult (even act­
ing as a decoy protein for VEGFA to decrease binding to VEGFR2) but 
with important effects during embryogenesis and on tumor angiogen­
esis. Tumor vessels may be more dependent on VEGFR signaling for 
growth and survival than normal ECs.

Endothelial cell–“specific” ligand/receptor complexes
PIGF
VEGF-A
VEGF-B
VEGF-C
Ang-1 Ang-2
Ephrins
Extracellular
matrix
bFGF
PDGF
Kinase
domain
αvβ3
Matrix
(attachment)
EPHB4
(Arteryvein
differentiation,
vessel
remodeling)
VEGFR1
VEGFR2
VEGFR3
(Endothelial cells)
(Lymphatics)
Tie-2
(Blood vessel
stabilization
and
remodeling)
Downstream pathways
Ras/MAPK
Pl3K/AKT
Rho/Rac/cdc42
NFκB
PART 4
Oncology and Hematology
Endothelial cell proliferation, migration, survival
FIGURE 77-10  Critical molecular determinants of endothelial cell biology. Angiogenic endothelium expresses 
a number of receptors not found on resting endothelium. These include receptor tyrosine kinases (RTKs) and 
integrins that bind to the extracellular matrix and mediate endothelial cell (EC) adhesion, migration, and invasion. 
ECs also express RTKs (i.e., the fibroblast growth factor [FGF] and platelet-derived growth factor [PDGF] receptors) 
that are found on many other cell types. Critical functions mediated by activated RTK include proliferation, 
migration, and enhanced survival of endothelial cells, as well as regulation of the recruitment of perivascular 
cells and bloodborne circulating endothelial precursors and hematopoietic stem cells to the tumor. Intracellular 
signaling via EC-specific RTK utilizes molecular pathways that may be targets for future antiangiogenic therapies.
While VEGF signaling is a critical initiator of angiogenesis, this is 
a complex process regulated by additional signaling pathways (Fig. 
77-10). The angiopoietin, Ang1, produced by stromal cells, binds to 
the EC RTK Tie-2 and promotes the interaction of ECs with the ECM 
and perivascular cells, such as pericytes and smooth-muscle cells, to 
form tight, nonleaky vessels. PDGF and basic fibroblast growth fac­
tor (bFGF) help to recruit these perivascular cells. Ang1 is required 
for maintaining the quiescence and stability of mature blood vessels 
and prevents the vascular permeability normally induced by VEGF 
and inflammatory cytokines. For tumor cell–derived VEGF to initiate 
sprouting from host vessels, the stability conferred by the Ang1/Tie2 
pathway must be perturbed; this occurs by the secretion of Ang2 by 
ECs that are undergoing active remodeling. Ang2 binds to Tie2 and is 
a competitive inhibitor of Ang1 action: under the influence of Ang2, 
preexisting blood vessels become more responsive to remodeling sig­
nals, with less adherence of ECs to stroma and associated perivascular 
cells and more responsiveness to VEGF. Therefore, Ang2 is required at 
early stages of tumor angiogenesis for destabilizing the vasculature by 
making host ECs more sensitive to angiogenic signals. In the presence 
of Ang2, there is no stabilization by the Ang1/Tie2 interaction, and 
tumor blood vessels are leaky, hemorrhagic, and have poor association 
of ECs with underlying stroma. Sprouting tumor ECs express high 
levels of the transmembrane protein ephrin-B2 and its receptor, the 
RTK EPH, whose signaling appears to work with the angiopoietins 
during vessel remodeling. During embryogenesis, EPH receptors are 
expressed on the endothelium of primordial venous vessels while the 
transmembrane ligand ephrin-B2 is expressed by cells of primordial 
arteries; the reciprocal expression may regulate differentiation and pat­
terning of the vasculature.
A number of additional ubiquitously expressed host molecules play 
critical roles in normal and pathologic angiogenesis. Proangiogenic 

cytokines, chemokines, and growth factors 
secreted by stromal cells or inflammatory 
cells make important contributions to neo­
vascularization, including bFGF, transform­
ing growth factor-β (TGF-β), TNF-α, and 
IL-8. In contrast to normal endothelium, 
angiogenic endothelium overexpresses spe­
cific members of the integrin family of ECMbinding proteins that mediate EC adhesion, 
migration, and survival. Specifically, expres­
sion of integrins αvβ3, αvβ5, and α5β1 
mediates spreading and migration of ECs and 
is required for angiogenesis induced by VEGF 
and bFGF, which in turn can upregulate EC 
integrin expression. The αvβ3 integrin physi­
cally associates with VEGFR2 in the plasma 
membrane and promotes signal transduction 
from each receptor to promote EC prolifera­
tion (via focal adhesion kinase, src, PI3K, and 
other pathways) and survival (by inhibition of 
p53 and increasing the Bcl-2/Bax expression 
ratio). In addition, αvβ3 forms cell-surface 
complexes with matrix metalloproteinases 
(MMPs), zinc-requiring proteases that cleave 
ECM proteins, leading to enhanced EC 
migration and the release of heparin-binding 
growth factors, including VEGF and bFGF. 
EC adhesion molecules can be upregulated 
(i.e., by VEGF, TNF-α) or downregulated 
(by TGF-β); this, together with chaotic blood 
flow, explains poor leukocyte-endothelial 
interactions in tumor blood vessels and may 
help tumor cells avoid immune surveillance.
Generalized
growth factor
receptors
FGF
receptor
PDGF
receptor
(Recruitment
of smoothmuscle cells
and pericytes)
Tumor blood vessels are not normal; they 
have chaotic architecture and blood flow. 
Due to an imbalance of angiogenic regulators 
such as VEGFs and angiopoietins (see below), 
tumor vessels are tortuous and dilated with 
an uneven diameter, excessive branching, and shunting. Tumor blood 
flow is variable, with areas of hypoxemia and acidosis leading to the 
selection of cancer cell variants that are resistant to hypoxemia-induced 
apoptosis (often involving the loss of p53 expression). Tumor vessel 
walls have numerous openings, widened interendothelial junctions, 
and discontinuous or absent basement membrane. This contributes to 
the high permeability of these vessels and, together with lack of func­
tional intratumoral lymphatics, causes increased interstitial pressure 
within the tumor (which also interferes with the delivery of therapeutics 
to the tumor; Figs. 77-9, 77-10, and 77-11). Tumor blood vessels have a 
deficit of perivascular cells such as pericytes and smooth-muscle cells 
that normally regulate flow in response to tissue metabolic needs. 
Unlike normal blood vessels, the vascular lining of tumor vessels is not 
a homogeneous layer of ECs but often consists of a mosaic of ECs and 
tumor cells, which, because of their plasticity, can upregulate expres­
sion of genes normally only seen in ECs under hypoxic conditions. 
These cancer cell–derived vascular channels, which may be lined by 
ECM secreted by the tumor cells, are referred to as vascular mimicry. 
During tumor angiogenesis, ECs are highly proliferative and express a 
number of plasma membrane proteins that are characteristic of acti­
vated endothelium, including growth factor receptors and adhesion 
molecules such as integrins. These abnormalities in tumor vascula­
ture provide potential differential sensitivities from normal vessels to 
approaches inhibiting the process, allowing for the use of antiangio­
genic agents in cancer treatment.
Lymphatic vessels also exist within tumors. Development of tumor 
lymphatics is associated with expression of VEGFR3 and its ligands 
VEGF-C and VEGF-D. The role of these vessels in tumor cell metas­
tasis to regional lymph nodes remains to be determined. However, 
VEGF-C levels correlate significantly with metastasis to regional lymph 
nodes in lung, prostate, and colorectal cancers.

A. Normal blood vessel
Low IP
Normoxic
Physiologic pH
Hierarchical
branching
Even blood
distribution
Lumen
EC
BM
BM
Pericytes
Tight junctions between EC
Well-formed BM
Pericyte coverage
Normal permeability
C. Treatment with bevacizumab (Early)
D. Treatment with bevacizumab (Late)
Low IP
Less hypoxemia
Less acidosis
Normalization
of vessels
Improved
blood flow
Lumen
EC
BM
Pericytes
More efficient delivery of
chemotherapy and oxygen
Reduced permeability
Death of EC due to loss of VEGF
survival signals (plus chemotherapy
or radiotherapy)
Apoptosis of tumor due to starvation
and/or effects of chemotherapy
FIGURE 77-11  Normalization of tumor blood vessels due to inhibition of VEGF signaling. A. Blood vessels in normal tissues exhibit a regular hierarchical branching 
pattern that delivers blood to tissues in a spatially and temporally efficient manner to meet the metabolic needs of the tissue (top). At the microscopic level, tight junctions 
are maintained between endothelial cells (ECs), which are adherent to a thick and evenly distributed basement membrane (BM). Pericytes form a surrounding layer that 
provides trophic signals to the EC and helps maintain proper vessel tone. Vascular permeability is regulated, interstitial fluid pressure (IP) is low, and oxygen tension and 
pH are physiologic. B. Tumors have abnormal vessels with tortuous branching and dilated, irregular interconnecting branches, causing uneven blood flow with areas of 
hypoxemia and acidosis. This harsh environment selects genetic events that result in resistant tumor variants, such as the loss of p53. High levels of VEGF (secreted by 
tumor cells) disrupt gap junction communication, tight junctions, and adherens junctions between EC via src-mediated phosphorylation of proteins such as connexin 43, 
zonula occludens-1, VE-cadherin, and α/β-catenins. Tumor vessels have thin, irregular BM, and pericytes are sparse or absent. Together, these molecular abnormalities 
result in a vasculature that is permeable to serum macromolecules, leading to high tumor interstitial pressure, which can prevent the delivery of drugs to the tumor cells. 
This is made worse by the binding and activation of platelets at sites of exposed BM, with release of stored VEGF and microvessel clot formation, creating more abnormal 
blood flow and regions of hypoxemia. C. In experimental systems, treatment with bevacizumab or blocking antibodies to VEGFR2 leads to changes in the tumor vasculature 
that have been termed vessel normalization. During the first week of treatment, abnormal vessels are eliminated or pruned (dotted lines), leaving a more normal branching 
pattern. ECs partially regain features such as cell-cell junctions, adherence to a more normal BM, and pericyte coverage. These changes lead to a decrease in vascular 
permeability, reduced interstitial pressure, and a transient increase in blood flow within the tumor. Note that in murine models, this normalization period lasts only for ~5–6 
days. D. After continued anti-VEGF/VEGFR therapy (which is often combined with chemo- or radiotherapy), ECs die, leading to tumor cell death (either due to direct effects 
of the chemotherapy or lack of blood flow).
■
■ANTIANGIOGENIC THERAPY
Angiogenesis inhibitors function by targeting the critical molecular 
pathways involved in EC proliferation, migration, and/or survival, 
many of which are highly expressed in the activated endothelium in 
tumors. Inhibition of growth factor and adhesion-dependent signaling 

B. Tumor blood vessel
High IP
High VEGF
Hypoxemia
Acidosis
Tortuous
vessels
Haphazard blood
flow
Lumen
EC
Tumor cells
Loss of EC junction complexes
Irregular or no BM
Absent (or few) pericyte
Increased permeability
CHAPTER 77
Collapse
of tumor
vasculature
Cancer Cell Biology
Lumen
EC
BM
Tumor cells
pathways can induce EC apoptosis with concomitant inhibition of 
tumor growth. Different types of tumors can use distinct combinations 
of molecular mechanisms to activate the angiogenic switch. Therefore, 
it is doubtful that a single antiangiogenic strategy will suffice for all 
human cancers; rather, a number of agents or combinations of agents

Ang 1
Ang 2
Novel
inhibitors
Anti-VEGF
MoAb
VEGF
VEGFR2
Kinase
domain
Tie2
receptor
Enhanced
binding to ECM,
vessel stabilization
Specific
kinase
inhibitors
Proliferation
survival
migration
Anti-integrin
MoAb,
RGD peptides
αvβ3
αvβ5
α5β1
Nucleus
Microtubules
Extracellular
matrix (ECM)
2-Methoxy estradiol
MMPs
(invasion,
growth factor
release)
MMP inhibitors
PART 4
Oncology and Hematology
FIGURE 77-12  Knowledge of the molecular events governing tumor angiogenesis has led to a number of therapeutic strategies to block tumor blood vessel formation. 
The successful therapeutic targeting of VEGF and its receptors VEGFR is described in the text. Other endothelial cell (EC)–specific receptor tyrosine kinase pathways (e.g., 
angiopoietin/Tie2 and ephrin/EPH) are likely targets for the future. Ligation of the αvβ3 integrin is required for EC survival. Integrins are also required for EC migration and 
are important regulators of matrix metalloproteinase (MMP) activity, which modulates EC movement through the ECM as well as release of bound growth factors. Targeting 
of integrins includes development of blocking antibodies, small peptide inhibitors of integrin signaling, and arg-gly-asp–containing peptides that prevent integrin:ECM 
binding. Peptides derived from normal proteins by proteolytic cleavage, including endostatin and tumstatin, inhibit angiogenesis by mechanisms that include interfering 
with integrin function. Signal transduction pathways that are dysregulated in tumor cells indirectly regulate EC function. Inhibition of EGF-family receptors, whose signaling 
activity is upregulated in a number of human cancers (e.g., breast, colon, and lung cancers), results in downregulation of VEGF and IL-8, while increasing expression of the 
antiangiogenic protein thrombospondin-1. The Ras/MAPK, PI3K/Akt, and Src kinase pathways constitute important antitumor targets that also regulate the proliferation 
and survival of tumor-derived EC. The discovery that ECs from normal tissues express tissue-specific “vascular addressins” on their cell surface suggests that targeting 
specific EC subsets may be possible.
will be needed, depending on distinct programs of angiogenesis used 
by different human cancers. Despite this, experimental data indicate 
that for some tumor types, blockade of a single growth factor (e.g., 
VEGF) may inhibit tumor-induced vascular growth.
Bevacizumab, an antibody that binds circulating VEGF, modestly 
potentiates the effects of a number of different types of active chemo­
therapeutic regimens used to treat a variety of different tumor types 
including colon, lung, ovarian, and cervical cancers. It also has some 
activity in combination with immunotherapy against RCCs and alone 
for glioblastomas. Other protein inhibitors of the VEGF signaling path­
way approved for anticancer therapy include ramucirumab (a mono­
clonal antibody directed against VEGFR2, approved for use against 
gastric/gastroesophageal, colon, and lung cancers) and ziv-aflibercept 
(a recombinant protein inhibitor of VEGF, approved for colorectal 
cancer). Hypertension is the most common side effect of inhibitors of 
VEGF (or its receptors) but can be treated with antihypertensive agents 
and uncommonly requires discontinuation of therapy. Rare but seri­
ous potential risks include arterial thromboembolic events, including 
stroke and myocardial infarction, hemorrhage, bowel perforation, and 
inhibition of wound healing.
Several small-molecule inhibitors (SMIs) that target VEGF RTK 
activity but are also inhibitory to other kinases have also been approved 
to treat certain cancers. Sunitinib (see above and Table 77-2) has activ­
ity directed against mutant c-Kit receptors (approved for GIST), but 
also targets VEGFR and PDGFR, and has antitumor activity against 
pancreatic neuroendocrine and metastatic RCCs, presumably on 
the basis of its antiangiogenic activity. Similarly, sorafenib, originally 
developed as a Raf kinase inhibitor but with potent activity against 
VEGFR and PDGFR, has activity against RCC, differentiated thyroid 
and hepatocellular cancers, and desmoid tumors. A closely related mol­
ecule to sorafenib, regorafenib, has activity against colorectal cancer, 

Stromal cell
Novel
inhibitors
EPH receptor
Ephrin-B2
Endothelial cell
GIST, gastric, and hepatocellular cancers. Other inhibitors of the VEGF 
pathway approved for the treatment of various cancers include axitinib, 
pazopanib, lenvatinib, and cabozantinib.
Antiangiogenic agents have been particularly effective against RCC 
for which angiogenic factors are important for its development and 
growth. The modest success in targeting tumor angiogenesis against 
most other cancers has led to enhanced enthusiasm for the develop­
ment of drugs that target other aspects of the angiogenic process; 
some of these therapeutic approaches are outlined in Fig. 77-12. An 
inhibitor of HIF2-α has sufficient antitumor activity against RCC, 
pancreatic neuroendocrine tumors, and hemangioblastomas develop­
ing in patients with germline VHL mutations to be approved for these 
indications.
Evidence of enhanced activity has been seen when anti-VEGF 
agents are used in combination with immunomodulators including 
immune checkpoint inhibitors. Examples of approved combinations 
include durvalumab plus bevacizumab for HCC and lenvatinib plus 
pembrolizumab for endometrial cancer and RCC.
■
■EVASION OF THE IMMUNE SYSTEM BY CANCERS
The immune system plays a critical role in maintaining organismal 
integrity including by defending against pathogens as well as prevent­
ing and limiting the growth of cancers. There is a complex interaction 
between cancer and the host from the development of the first malig­
nant cell to the establishment of a clinical cancer and its subsequent 
growth, invasion, and metastasis. The immune system plays a critical 
role in the prevention of cancer development. This is exemplified by the 
increased risk for cancer development in individuals who are signifi­
cantly immunosuppressed, such as by inherited defects in mechanisms 
important for immune function, the immunosuppression necessary to 
maintain allogeneic organ transplants, and immunosuppression seen

from certain infections such as human immunodeficiency virus. It also 
plays a critical role in inhibiting the process of cells metastasizing as 
well as growth of metastatic cells at the sites of metastasis.
There are two components of the immune system. The first is 
innate immunity (present in the organism and not dependent on prior 
exposure to a specific antigen, such as those present in a pathogen or 
malignant cell), which tends to be general and not specific and a stimu­
lus is not remembered if encountered again. The second is the adaptive 
immune component, which depends on the innate immune process for 
activation and provides the specificity to the response with significant 
expansion of cells to target the specific antigens present on the patho­
gen or malignant cell and memory of the encounter such that exposure 
to the same stimulus elicits an even more rapid and vigorous response. 
Thus, while the innate process provides the first line of defense, the 
adaptive process is necessary for the specificity of response and provid­
ing memory to more rapidly attack cells should the pathogen infection 
recur or the malignant cells grow. The immune system has to be tightly 
regulated to allow for clearance of unwanted antigens while preventing 
an immune-mediated attack on the self. (See Chap. 360 for details on 
the function of the immune system.)
Not surprisingly, since cancers arise from normal cells within the 
body that have a variety of processes to prevent harm or destruction 
by the immune system, they have a variety of mechanisms that allow 
them to evade detection and elimination by the immune system. These 
include downregulation of cell surface proteins involved in immune 
recognition (including MHC proteins and tumor-specific antigens), 
expression of other cell surface proteins that inhibit immune func­
tion (including members of the B7 family of proteins such as PD-L1), 
secretion of proteins and other molecules that are immunosuppressive 
such as TGF-β, recruitment and expansion of immunosuppressive 
cells such as regulatory T cells (which are important for maintaining 
tolerance against self-antigens), induction of T-cell tolerance, and 
downregulation of death receptors. Due to the marked heterogeneity 
of cells within a cancer, as well as the complexity and dynamic changes 
in the tumor microenvironment, a variety of immune-suppressive 
mechanisms are continuously occurring and changing. In addition, 
the inflammatory effects of some of the immune mediator cells in the 
tumor microenvironment (including tissue-associated macrophages 
and myeloid-derived suppressor cells) can suppress effector T-cell 
responses against the tumor as well as stimulate inflammation that can 
enhance tumor growth.
There are marked differences in the way different malignancies 
respond to current immunotherapeutic approaches. For example, mel­
anomas, RCC, Merkel cell carcinomas, cancers with defects in DNA 
repair associated with microsatellite instability with accumulation of 
gene mutations, and lymphomas (including Hodgkin’s) respond well to 
current immunotherapeutic approaches, whereas microsatellite-stable 
pancreatic and colon cancers do not. While there is not a complete 
understanding of why these differences exist and many factors both 
within the cancer cells and in the microenvironment may play a 
role, several factors have been identified that appear to be important. 
These include the number of mutations present in the tumor (tumor 
mutational burden), presence of increased neoantigens, expression of 
immune checkpoint proteins (e.g., PD-L1 for anti-PD-1 or anti-PD-L1 
therapy), density of tumor-infiltrating lymphocytes, and host genetic 
factors. One of these (PD-L1 expression by the tumor) has sufficient 
predictive value for certain tumors (e.g., non-small-cell lung cancer or 
gastroesophageal cancers) to be used in making treatment decisions 
regarding the use of antibodies targeting PD-1 or PD-L1. However, nei­
ther PD-L1 expression nor any other marker can predict responsive­
ness of most tumors to immunotherapy. Better biomarkers that define 
potential responsiveness of specific cancers to immunotherapy are 
badly needed. A major area of research is to try to identify approaches 
that would convert cancers that are not responsive to immunotherapy 
to being responsive.
Immunotherapy approaches to treat cancer can be divided into 
those aimed at activating the immune response and those designed to 
release the brakes that prevent an effective immune response against 
tumors. Releasing the brakes is also important for maintaining the 

effectiveness of approaches that activate the immune response since, 
given the normally tight regulation of immune function, activation 
induces changes in the braking system to prevent the immune system 
from damaging normal tissues. Approaches at activating the immune 
response against cancer including using immunostimulatory mol­
ecules such as interferons, IL-2, and especially monoclonal antibod­
ies have had success in treating a number of different cancers. For 
example, antibodies that target molecules highly expressed on certain 
cancers, such as CD20 on malignant B cells or HER2 on a variety of 
cancers including breast and gastroesophageal cancers, which acti­
vate the immune response locally against those malignancies, have 
proven highly effective.

A more direct approach to enhance the activity of T cells directed 
against specific tumors involves isolating T cells from patients and 
reengineering the cells to express chimeric antigen receptors (CAR-T) 
that recognize antigens present on the cells of that individual’s tumor. 
The most commonly used approach to date has been to engineer the 
cells to express receptors targeting the CD19 antigen on ALL, dif­
fuse large B-cell lymphoma (DLBCL) cells, follicular lymphoma, and 
mantle cell lymphoma. These have been shown to have significant 
antitumor activity in the treatment of patients with ALL and DLBCL, 
including durable remissions in patients refractory to standard thera­
pies, and are approved for these malignancies. In addition, anti-B-cell 
maturation antigen (BCMA) CAR-T therapies have been approved for 
the treatment of multiple myeloma.
CHAPTER 77
However, there have also been significant issues with toxicity 
including cytokine release syndrome, organ toxicity felt to be due to 
inadvertent targeting of antigens present in the organ, neurotoxicity, 
and potentially an increased risk for subsequent development of T-cell 
malignancies. These patients often require aggressive supportive care 
by individuals experienced in the delivery of CAR-T therapy. In addi­
tion, as is true for most anticancer therapies, mechanisms of resistance 
have developed, most commonly the outgrowth of tumor cells no 
longer expressing the antigen. Mechanisms for preventing the devel­
opment of resistant cells are being explored, including combinations 
targeting different antigens. In addition to potentially preventing or 
overcoming resistance to the targeting of a single antigen, this could 
potentially increase efficacy and better reflects the normal immune 
response to pathogens or cancers in targeting multiple different anti­
gens. CAR-T therapies are undergoing clinical investigation against 
other hematologic malignancies and solid tumors. Approaches to 
develop allogeneic CAR-T therapies are also being explored with the 
aim of having an off-the-shelf product that could be used in a number 
of patients rather than generating each treatment specifically for one 
recipient.
Cancer Cell Biology
Another approach utilizing lymphocytes to treat cancer involves 
utilization of autologous tumor-derived T cells. Tumor-infiltrating 
lymphocyte therapy in combination with IL-2 is now approved for 
the treatment of melanoma. Given previously demonstrated efficacy 
against other cancers, such as RCC, tumor-infiltrating lymphocyte 
therapy may eventually be approved for other cancers as well. However, 
technical issues of getting adequate expansion of the cells may limit 
this approach.
The immune response against cancers may also be able to be 
enhanced through targeting of proteins or cells (e.g., regulatory T cells) 
involved in normal homeostatic control to prevent autoimmune 
damage to the host but that malignant cells and their stroma can 
also utilize to inhibit the immune response directed against them. A 
component of this process includes a number of immune checkpoints 
that involve interaction of proteins on the surface of effector T cells 
with proteins on self-cells (or cancer cells that arise from normal cells) 
that inhibit activation of the T cells. By inhibiting the binding of the 
proteins involved in this process, the brake on the effector T cells are 
released and they can be activated. The presence of neoantigens (e.g., 
mutant proteins) enhances the activation of the T cells against cancer 
cells as compared to normal cells. Sufficient clinical antitumor activity 
has been seen for monoclonal antibodies targeting various proteins 
involved in this process, including CTLA-4, PD-1, PD-L1, and LAG3 
(others continue to be explored), for them to be approved. These are

Tumor cells
Elaboration of 
immunosuppressive
cytokines
TGF-β
Interleukin-4
Interleukin-6
Interleukin-10
Immunosuppressive
immune cells
PART 4
Oncology and Hematology
T regulatory cells
CD11+ granulocytes
Macrophages
FIGURE 77-13  Tumor-host interactions that suppress the immune response to the tumor.
co-inhibitory molecules that are expressed on the surface of cancer 
cells, and/or cells of the immune system, and/or stromal cells and are 
involved in inhibiting the immune response against both normal cells 
(their normal protective mechanism for the host) and also cancer cells 
that use this inhibitory process to evade immune-mediated cell death 
(Figs. 77-13 and 77-14). This approach has had clinical activity against 
a wide variety of cancers. A monoclonal antibody directed against 
CTLA-4 is approved for the treatment of melanoma and several other 
malignancies, and antibodies targeting PD-1 or PD-L1 are approved 
for use against melanoma, RCC, lung cancer (both non-small-cell lung 
and small-cell lung), head and neck cancer, nasopharyngeal cancer, 
urothelial cancer, cervical cancer, endometrial cancer, hepatocellular 
carcinoma, gastric cancer, esophageal cancer, cutaneous squamous cell 
carcinoma, basal cell carcinoma, Merkel cell cancer, primary B-cell 
mediastinal lymphoma, Hodgkin’s lymphoma, and in a cancer-agnostic 
PD-L1
PD-L1
Cancer cells
CD28
CD80/86
MHC
-
CD80/86
CTLA-4
Tumor
antigens (TA)
Antigen-presenting
cell/dendritic cell
FIGURE 77-14  Inhibition of T-cell activation against cancer cells by engagement of co-inhibitory molecules including PD-1, PD-L1, and CTLA-4 and reversal of this 
inhibition by antibodies against these proteins. The red ovals in the T cell indicate inhibitory signals, and the green oval indicates stimulatory signals.

T-cell inactivation
Induction of CTLA-4
Induction of PD-1
Cell signaling disruption
Class I MHC loss
in tumor cells
STAT-3 signaling
loss in T cells
Generation of
indoleamine 2,
3-dioxygenase
Degradation of
T-cell receptor
ζ chain
approach, cancers with high microsatellite instability (MSI) or high 
tumor mutational burden (TMB).
They continue to be evaluated against other malignancies as well. 
The combination of anti-CTLA-4 and anti-PD-1 antibodies has been 
approved for treatment of a number of malignancies, including mela­
noma, RCC, HCC, NSCLC, mesothelioma, and MSI-high metastatic 
colorectal cancers. Specific determinants of response to immune 
checkpoint inhibitors are still being defined, but in addition to high 
PD-L1 expression, the presence of increased neoantigens in the tumor, 
such as seen in patients with MSI-high and TMB-high cancers, may be 
one important determinant of better responses.
A number of other proteins are involved in controlling the immune 
response (both ones that enhance activity [e.g., CD27 and CD40] as 
well as ones involved in inhibiting response [e.g., TIM-3, TIGIT]). 
Antibodies have been developed to modulate function of these 
Anti-PD-L1
antibodies
Anti-PD-1
antibodies
PD-1
+
–
PD-1
+
+
MHC
T-cell
receptor
T cell
receptor
–
+
Cancer cell
T cell
Anti-CTLA-4
antibodies