The hallmarks of cancer

The hallmarks of cancer

Cancer cells are able to proliferate in an uncontrolled fashion; - their ability to divide and spread is unbounded. Cancer cell growth destroys first the tissue from which they arise and - eventually the person in which they are present. In order to survive, divide, invade and spread, cancer cells have to acquire a number of characteristics. No one charac - teristic is su ffi cient and not all characteristics ar e absolutely necessary . These features, based on articles by Hanahan and Weinberg, are given in Summary box 12.1 . Establish an autonomous lineage Cells develop independence from the normal signals that control supply and demand. The healing of a wound is a physiological process; the cellular response is exquisitely coordinated so that - proliferation occurs when it is needed and ceases when it is no - longer required. The whole process is controlled by a series of - Maurice Hugh Frederick Wilkins , 1916–2004, of Summary box 12.1 Features of malignant transformation /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF signals telling cells when, and when not, to divide. Cancer cells escape from this normal system of checks and balances: they grow and proliferate in the absence of external stimuli and regardless of signals telling them to desist. Oncogenes are genes with the potential to cause cancer if mutated and expressed at high levels; they are key factors in carcinogenesis. Most oncogenes are normally inv olved in physiological processes, i.e. cell growth, but if mutated they can predispose a cell to cancer and in concert with other onco genes can enable cancer cell survival and development of an established tumour. The implication is that we all carry the seeds of our own destruction: genetic sequences that, through mutation, can turn into active oncogenes and thereby cause malignant transformation. Indeed, through study of the timing of key genetic events in adult cancer development, a handful of cells develop the founder mutations of cancer development several decades prior to diagnosis. If the individual is unfor tunate enough to accumulate further mutations in key driver genes, that cell becomes malignant and, if it proliferates, a can cer may develop. Only very rarely is a single mutation su ffi cient to cause cancer; multiple mutations are usually required. Colo rectal cancer pro vides the classical example of how multiple mutations are necessary for the complete transformation from normal cell to malignant cell. V ogelstein and his colleagues identified the genes implicated and also postula ted that it is necessary to have mutations in all the relevant genes; they also noted that these mutations must be acquired in a specific sequence for malignant transformation to occur. Obtain replicative immortality According to the Hayflick hypothesis, normal cells are permit ted to undergo only a finite number of divisions. For humans this number is between 40 and 60. The limitation is imposed by the progressive shortening of the end of the chr omosome (the telomere) that occurs each time a cell divides; eventually the lineage will die out. Cancer cells utilise the enzyme telomerase to rebuild the telomere at each cell division, such that there is Bert Vogelstein , b.1949, molecular biologist, Johns Hopkins Hospital, Baltimore, MD, USA. Leonard Hayflick , b.1928, while working at the Wistar Institute in Philadelphia in 1962, he noted that normal mammalian cells growing in culture had a limited, rather than an indefinite, capacity for self-replication. cancer cell hence develops immortality . Evade apoptosis Apoptosis, taken from the Greek for ‘leaf fall’, is a form of programmed cell death that occurs as the direct result of inter - nal cellular events instructing the cell to die. Unlike necrosis, which is a form of traumatic cell death resulting from acute cellular injury , apoptosis is an orderly and internally driven process. The cell dismantles itself neatly for disposal ( Figure 12.1 ) . There is minimal inflammatory response. Apoptosis is a physiological process. Cells that are redundant normally die by apoptosis and this is an important self-regulatory mechanism in growth and development, i.e . cells in the web space of the embryo die by apoptosis, or lymphocytes that could react to self. Genes, such as p53, that can activate apoptosis function as tumour suppressor genes. Mutation in such genes causes a loss of this inhibitory function, which will contribute to malignant transformation as apoptosis is evaded; this means that the wrong cells can be in the wrong places at the wrong times. - - - - Acquire angiogenic competence A mass of cancer cells cannot, in the absence of a blood supply , grow beyond a diameter of about 1 /uni00A0 mm. This places a severe restriction on the capabilities of the tumour (note - that the word tumour means swelling and does not mean the lesion is malignant, although ‘tumour’ is often taken by patients to be synonymous with cancer). It cannot grow much larger or spread widely within the body . If, however, the mass of cancer cells is able to attract or to construct a blood supply then it is able to quit its dormant state and behave in a far

Establish an autonomous lineage Resist signals that inhibit growth Sustain proliferative signalling Obtain replicative immortality Evade apoptosis Acquire angiogenic competence Acquire ability to invade, disseminate and implant Evocation of in /f_l ammation Evade detection/elimination Loss of specialist cell function Develop ability to change energy metabolism AB AB MC AB MN Figure 12.1 Electron micrograph of apoptotic bodies (AB) engulfed by a macrophage. Note the macrophage nucleus (MN) and macrophage cytoplasm (MC).

vessels is termed angiogenesis and is a key feature of malignant transformation. Acquire ability to invade Cancer cells have no respect for the structure of normal tissues. They acquire the ability to breach the basement membrane and gain direct access to blood and lymph vessels. Cancer cells use three main mechanisms to facilitate invasion: (i) cause a rise in the interstitial pressure within a tissue; (ii) secrete enzymes that dissolve extracellular matrix; and (iii) become mobile. Unrestrained proliferation and a lack of contact inhibition enable cancer cells to exert pressure directly on the surrounding tissue and push beyond the normal limits. They secrete collagenases and proteases that chemically dissolve any extracellular boundaries that would otherwise limit their spread through tissues and, by modulating the expression of cell surface molecules called integrins, are able to detach them selves from the extracellular matrix. The abnormal integrins associated with malignancy can also transmit signals from the environment to the cytoplasm and nucleus of the cancer cells (‘outside-in signalling’) and these signals can induce increased motility . These processes are similar to those involved in normal development, i.e. in the migration of the neural crest or the formation of the heart. Epithelial cells behave as if they were mesenchymal cells and the process is termed epithelial– mesenchymal transition (EMT). EMT is a crucial step in malignant transformation and many of the genes and proteins implicated in the formation of cancer control processes are involved in EMT , e.g. Src, Ras, integrins, Wnt / β -catenin, Notch. Acquire ability to disseminate and implant Once cancer cells gain access to vascular and lymphovascular spaces, they can be readily distributed systemically throughout the body . This is not, of itself, su ffi cient to cause tumours to develop at distant sites. The cells also need to acquire the ability to implant. As Paget pointed out over a century ago, there is a crucial relationship here between the seed (the tumour cell) and the soil (the distant tissue). Most of the cancer cells discharged into the circulation probably do not form viable metastases. Circulating cancer cells can be identified in patients who never develop clinical evidence of metastatic disease; presumably these cells die if they cannot implant or they are destroyed by the patient’s immune system. Cancer can spread as individual cells or cell clumps that migrate and implant. Whether spread occurs in groups or as individual cells there is still the problem of crossing the vascular endothelium (and basement membrane) to gain access to the Stephen Paget , 1855–1926, surgeon, The West London Hospital, London, UK. Paget’s ‘seed and soil’ hypothesis is contained in his paper ‘The distribution of secondary growths in cancer of the breast’, published in the Lancet in 1889. Paul Ehrlich , 1854–1915, Professor of Hygiene, the University of Berlin, and later Director of the Institute for Infectious Diseases, Berlin, Germany . In 1908, he shared the Nobel Prize in Physiology or Medicine with Elie Metchniko ff Zoology at Odessa in Russia, and later worked at the Pasteur Institute in Paris, France. Sir Frank McFarlane Burnet , 1899–1985, Australian virologist, Walter and Eliza Hall Institute, Melbourne, Australia. Burnett shared the 1960 Nobel Prize in Physiology or Medicine with Sir Peter Brian Medawar , 1915–1987, Jodrell Professor of Zoology , University College, London, UK, ‘for their discovery of acquired immunological tolerance’. Lewis Thomas , 1913–1993, American pathologist and immunologist, who became President of the Sloan Kettering Memorial Institute, New Y ork, NY , USA. tissues by exploiting, and subverting, the normal inflammatory response. By expressing inflammatory cytokines, cancer cells can deceiv e the endothelium of the host tissue into becoming activated and allowing cancer cells access to the extravascular space. Activated endothelium expresses receptors that bind to integrins and selectins on the surface of cells, allowing the can - cer cells to move across the endothelial barrier. Tumour-related inflammation A malignancy can provoke an inflammatory response and the cytokines and other factors produced as a result of that response may act to promote and sustain malignant transformation. Growth factors, mutagenic reactive oxygen species, angiogenic factors and anti-apoptotic factors may all be produced as part of an inflammatory process and all may contribute to the progression of a cancer. - Evade detection/elimination Although derived from normal cells (‘self ’) cancer cells are, in terms of their genetic make-up, behaviour and character - istics, foreign (‘not self ’). As such, they ought to provoke an immune response and be eliminated. It is entirely possible that malignant transformation is a more frequent event than the emergence of clinical cancer. T he possible role of the immune system in eliminating nascent cancers was proposed by Paul Ehrlich in 1909 and revisited by both Sir Frank McFarlane Burnet and Lewis Thomas in the late 1950s. Cancer cells, or at least those that give rise to clinical disease, appear to gain the ability to escape detection by the immune system. This may be through suppressing expression of tumour-associated antigens or it may be through actively co-opting one part of the immune system to help the tumour escape detection by other parts of the immune surveillance system. This hallmark has been exploited in recent years in the development of T-cell checkpoint inhibitors, which ‘take the brakes’ o ff the immune system to re-enable T-cell killing of cancer cells, e.g. in renal cell carcinoma, lung cancer and melanoma. Loss of specialist cell function Cancer cells are geared to excessive proliferation. They do not need to develop or retain those specialised functions that prior to malignant transformation were their physiological function. These cells can therefore a ff ord to repress or permanently lose those genes that control such functions. The longer term disadvantage to this process is that cancer cells are vulnerable to external stressors, which may , in part, explain why some cancer treatments work. , 1845–1916, ‘in recognition of his work on immunity’. Metchniko ff was Professor of Blood flow in malignant tumours is often sporadic and unre liable. As a result, cancer cells may have to spend prolonged periods in low-oxygen states (i.e. relative hypoxia). Compared with the corresponding normal cells, some cancer cells may be better able to surviv e in hypoxic conditions. This ability may enable tumours to grow and develop despite an impoverished blood supply . Cancer cells can alter their metabolism even when oxygen is abundant; they break down glucose but do not, as normal cells would do, send the resulting pyruvate to the mitochondria for conversion, in an oxygen-dependent process, to carbon dioxide. This is the phenomenon of aerobic glycolysis, or the Warburg e ff ect, and leads to the production of lactate. In an act of symbiosis, lactate-producing cancer cells may provide lactate for adjacent cancer cells, which are then able to use it, via the citric acid cycle, for energy production. This cooperation is similar to that which occurs in skeletal muscle during exercise. The hallmarks of cancer

Cancer cells are able to proliferate in an uncontrolled fashion; - their ability to divide and spread is unbounded. Cancer cell growth destroys first the tissue from which they arise and - eventually the person in which they are present. In order to survive, divide, invade and spread, cancer cells have to acquire a number of characteristics. No one charac - teristic is su ffi cient and not all characteristics ar e absolutely necessary . These features, based on articles by Hanahan and Weinberg, are given in Summary box 12.1 . Establish an autonomous lineage Cells develop independence from the normal signals that control supply and demand. The healing of a wound is a physiological process; the cellular response is exquisitely coordinated so that - proliferation occurs when it is needed and ceases when it is no - longer required. The whole process is controlled by a series of - Maurice Hugh Frederick Wilkins , 1916–2004, of Summary box 12.1 Features of malignant transformation /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF signals telling cells when, and when not, to divide. Cancer cells escape from this normal system of checks and balances: they grow and proliferate in the absence of external stimuli and regardless of signals telling them to desist. Oncogenes are genes with the potential to cause cancer if mutated and expressed at high levels; they are key factors in carcinogenesis. Most oncogenes are normally inv olved in physiological processes, i.e. cell growth, but if mutated they can predispose a cell to cancer and in concert with other onco genes can enable cancer cell survival and development of an established tumour. The implication is that we all carry the seeds of our own destruction: genetic sequences that, through mutation, can turn into active oncogenes and thereby cause malignant transformation. Indeed, through study of the timing of key genetic events in adult cancer development, a handful of cells develop the founder mutations of cancer development several decades prior to diagnosis. If the individual is unfor tunate enough to accumulate further mutations in key driver genes, that cell becomes malignant and, if it proliferates, a can cer may develop. Only very rarely is a single mutation su ffi cient to cause cancer; multiple mutations are usually required. Colo rectal cancer pro vides the classical example of how multiple mutations are necessary for the complete transformation from normal cell to malignant cell. V ogelstein and his colleagues identified the genes implicated and also postula ted that it is necessary to have mutations in all the relevant genes; they also noted that these mutations must be acquired in a specific sequence for malignant transformation to occur. Obtain replicative immortality According to the Hayflick hypothesis, normal cells are permit ted to undergo only a finite number of divisions. For humans this number is between 40 and 60. The limitation is imposed by the progressive shortening of the end of the chr omosome (the telomere) that occurs each time a cell divides; eventually the lineage will die out. Cancer cells utilise the enzyme telomerase to rebuild the telomere at each cell division, such that there is Bert Vogelstein , b.1949, molecular biologist, Johns Hopkins Hospital, Baltimore, MD, USA. Leonard Hayflick , b.1928, while working at the Wistar Institute in Philadelphia in 1962, he noted that normal mammalian cells growing in culture had a limited, rather than an indefinite, capacity for self-replication. cancer cell hence develops immortality . Evade apoptosis Apoptosis, taken from the Greek for ‘leaf fall’, is a form of programmed cell death that occurs as the direct result of inter - nal cellular events instructing the cell to die. Unlike necrosis, which is a form of traumatic cell death resulting from acute cellular injury , apoptosis is an orderly and internally driven process. The cell dismantles itself neatly for disposal ( Figure 12.1 ) . There is minimal inflammatory response. Apoptosis is a physiological process. Cells that are redundant normally die by apoptosis and this is an important self-regulatory mechanism in growth and development, i.e . cells in the web space of the embryo die by apoptosis, or lymphocytes that could react to self. Genes, such as p53, that can activate apoptosis function as tumour suppressor genes. Mutation in such genes causes a loss of this inhibitory function, which will contribute to malignant transformation as apoptosis is evaded; this means that the wrong cells can be in the wrong places at the wrong times. - - - - Acquire angiogenic competence A mass of cancer cells cannot, in the absence of a blood supply , grow beyond a diameter of about 1 /uni00A0 mm. This places a severe restriction on the capabilities of the tumour (note - that the word tumour means swelling and does not mean the lesion is malignant, although ‘tumour’ is often taken by patients to be synonymous with cancer). It cannot grow much larger or spread widely within the body . If, however, the mass of cancer cells is able to attract or to construct a blood supply then it is able to quit its dormant state and behave in a far

Establish an autonomous lineage Resist signals that inhibit growth Sustain proliferative signalling Obtain replicative immortality Evade apoptosis Acquire angiogenic competence Acquire ability to invade, disseminate and implant Evocation of in /f_l ammation Evade detection/elimination Loss of specialist cell function Develop ability to change energy metabolism AB AB MC AB MN Figure 12.1 Electron micrograph of apoptotic bodies (AB) engulfed by a macrophage. Note the macrophage nucleus (MN) and macrophage cytoplasm (MC).

vessels is termed angiogenesis and is a key feature of malignant transformation. Acquire ability to invade Cancer cells have no respect for the structure of normal tissues. They acquire the ability to breach the basement membrane and gain direct access to blood and lymph vessels. Cancer cells use three main mechanisms to facilitate invasion: (i) cause a rise in the interstitial pressure within a tissue; (ii) secrete enzymes that dissolve extracellular matrix; and (iii) become mobile. Unrestrained proliferation and a lack of contact inhibition enable cancer cells to exert pressure directly on the surrounding tissue and push beyond the normal limits. They secrete collagenases and proteases that chemically dissolve any extracellular boundaries that would otherwise limit their spread through tissues and, by modulating the expression of cell surface molecules called integrins, are able to detach them selves from the extracellular matrix. The abnormal integrins associated with malignancy can also transmit signals from the environment to the cytoplasm and nucleus of the cancer cells (‘outside-in signalling’) and these signals can induce increased motility . These processes are similar to those involved in normal development, i.e. in the migration of the neural crest or the formation of the heart. Epithelial cells behave as if they were mesenchymal cells and the process is termed epithelial– mesenchymal transition (EMT). EMT is a crucial step in malignant transformation and many of the genes and proteins implicated in the formation of cancer control processes are involved in EMT , e.g. Src, Ras, integrins, Wnt / β -catenin, Notch. Acquire ability to disseminate and implant Once cancer cells gain access to vascular and lymphovascular spaces, they can be readily distributed systemically throughout the body . This is not, of itself, su ffi cient to cause tumours to develop at distant sites. The cells also need to acquire the ability to implant. As Paget pointed out over a century ago, there is a crucial relationship here between the seed (the tumour cell) and the soil (the distant tissue). Most of the cancer cells discharged into the circulation probably do not form viable metastases. Circulating cancer cells can be identified in patients who never develop clinical evidence of metastatic disease; presumably these cells die if they cannot implant or they are destroyed by the patient’s immune system. Cancer can spread as individual cells or cell clumps that migrate and implant. Whether spread occurs in groups or as individual cells there is still the problem of crossing the vascular endothelium (and basement membrane) to gain access to the Stephen Paget , 1855–1926, surgeon, The West London Hospital, London, UK. Paget’s ‘seed and soil’ hypothesis is contained in his paper ‘The distribution of secondary growths in cancer of the breast’, published in the Lancet in 1889. Paul Ehrlich , 1854–1915, Professor of Hygiene, the University of Berlin, and later Director of the Institute for Infectious Diseases, Berlin, Germany . In 1908, he shared the Nobel Prize in Physiology or Medicine with Elie Metchniko ff Zoology at Odessa in Russia, and later worked at the Pasteur Institute in Paris, France. Sir Frank McFarlane Burnet , 1899–1985, Australian virologist, Walter and Eliza Hall Institute, Melbourne, Australia. Burnett shared the 1960 Nobel Prize in Physiology or Medicine with Sir Peter Brian Medawar , 1915–1987, Jodrell Professor of Zoology , University College, London, UK, ‘for their discovery of acquired immunological tolerance’. Lewis Thomas , 1913–1993, American pathologist and immunologist, who became President of the Sloan Kettering Memorial Institute, New Y ork, NY , USA. tissues by exploiting, and subverting, the normal inflammatory response. By expressing inflammatory cytokines, cancer cells can deceiv e the endothelium of the host tissue into becoming activated and allowing cancer cells access to the extravascular space. Activated endothelium expresses receptors that bind to integrins and selectins on the surface of cells, allowing the can - cer cells to move across the endothelial barrier. Tumour-related inflammation A malignancy can provoke an inflammatory response and the cytokines and other factors produced as a result of that response may act to promote and sustain malignant transformation. Growth factors, mutagenic reactive oxygen species, angiogenic factors and anti-apoptotic factors may all be produced as part of an inflammatory process and all may contribute to the progression of a cancer. - Evade detection/elimination Although derived from normal cells (‘self ’) cancer cells are, in terms of their genetic make-up, behaviour and character - istics, foreign (‘not self ’). As such, they ought to provoke an immune response and be eliminated. It is entirely possible that malignant transformation is a more frequent event than the emergence of clinical cancer. T he possible role of the immune system in eliminating nascent cancers was proposed by Paul Ehrlich in 1909 and revisited by both Sir Frank McFarlane Burnet and Lewis Thomas in the late 1950s. Cancer cells, or at least those that give rise to clinical disease, appear to gain the ability to escape detection by the immune system. This may be through suppressing expression of tumour-associated antigens or it may be through actively co-opting one part of the immune system to help the tumour escape detection by other parts of the immune surveillance system. This hallmark has been exploited in recent years in the development of T-cell checkpoint inhibitors, which ‘take the brakes’ o ff the immune system to re-enable T-cell killing of cancer cells, e.g. in renal cell carcinoma, lung cancer and melanoma. Loss of specialist cell function Cancer cells are geared to excessive proliferation. They do not need to develop or retain those specialised functions that prior to malignant transformation were their physiological function. These cells can therefore a ff ord to repress or permanently lose those genes that control such functions. The longer term disadvantage to this process is that cancer cells are vulnerable to external stressors, which may , in part, explain why some cancer treatments work. , 1845–1916, ‘in recognition of his work on immunity’. Metchniko ff was Professor of Blood flow in malignant tumours is often sporadic and unre liable. As a result, cancer cells may have to spend prolonged periods in low-oxygen states (i.e. relative hypoxia). Compared with the corresponding normal cells, some cancer cells may be better able to surviv e in hypoxic conditions. This ability may enable tumours to grow and develop despite an impoverished blood supply . Cancer cells can alter their metabolism even when oxygen is abundant; they break down glucose but do not, as normal cells would do, send the resulting pyruvate to the mitochondria for conversion, in an oxygen-dependent process, to carbon dioxide. This is the phenomenon of aerobic glycolysis, or the Warburg e ff ect, and leads to the production of lactate. In an act of symbiosis, lactate-producing cancer cells may provide lactate for adjacent cancer cells, which are then able to use it, via the citric acid cycle, for energy production. This cooperation is similar to that which occurs in skeletal muscle during exercise. The hallmarks of cancer

Cancer cells are able to proliferate in an uncontrolled fashion; - their ability to divide and spread is unbounded. Cancer cell growth destroys first the tissue from which they arise and - eventually the person in which they are present. In order to survive, divide, invade and spread, cancer cells have to acquire a number of characteristics. No one charac - teristic is su ffi cient and not all characteristics ar e absolutely necessary . These features, based on articles by Hanahan and Weinberg, are given in Summary box 12.1 . Establish an autonomous lineage Cells develop independence from the normal signals that control supply and demand. The healing of a wound is a physiological process; the cellular response is exquisitely coordinated so that - proliferation occurs when it is needed and ceases when it is no - longer required. The whole process is controlled by a series of - Maurice Hugh Frederick Wilkins , 1916–2004, of Summary box 12.1 Features of malignant transformation /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF signals telling cells when, and when not, to divide. Cancer cells escape from this normal system of checks and balances: they grow and proliferate in the absence of external stimuli and regardless of signals telling them to desist. Oncogenes are genes with the potential to cause cancer if mutated and expressed at high levels; they are key factors in carcinogenesis. Most oncogenes are normally inv olved in physiological processes, i.e. cell growth, but if mutated they can predispose a cell to cancer and in concert with other onco genes can enable cancer cell survival and development of an established tumour. The implication is that we all carry the seeds of our own destruction: genetic sequences that, through mutation, can turn into active oncogenes and thereby cause malignant transformation. Indeed, through study of the timing of key genetic events in adult cancer development, a handful of cells develop the founder mutations of cancer development several decades prior to diagnosis. If the individual is unfor tunate enough to accumulate further mutations in key driver genes, that cell becomes malignant and, if it proliferates, a can cer may develop. Only very rarely is a single mutation su ffi cient to cause cancer; multiple mutations are usually required. Colo rectal cancer pro vides the classical example of how multiple mutations are necessary for the complete transformation from normal cell to malignant cell. V ogelstein and his colleagues identified the genes implicated and also postula ted that it is necessary to have mutations in all the relevant genes; they also noted that these mutations must be acquired in a specific sequence for malignant transformation to occur. Obtain replicative immortality According to the Hayflick hypothesis, normal cells are permit ted to undergo only a finite number of divisions. For humans this number is between 40 and 60. The limitation is imposed by the progressive shortening of the end of the chr omosome (the telomere) that occurs each time a cell divides; eventually the lineage will die out. Cancer cells utilise the enzyme telomerase to rebuild the telomere at each cell division, such that there is Bert Vogelstein , b.1949, molecular biologist, Johns Hopkins Hospital, Baltimore, MD, USA. Leonard Hayflick , b.1928, while working at the Wistar Institute in Philadelphia in 1962, he noted that normal mammalian cells growing in culture had a limited, rather than an indefinite, capacity for self-replication. cancer cell hence develops immortality . Evade apoptosis Apoptosis, taken from the Greek for ‘leaf fall’, is a form of programmed cell death that occurs as the direct result of inter - nal cellular events instructing the cell to die. Unlike necrosis, which is a form of traumatic cell death resulting from acute cellular injury , apoptosis is an orderly and internally driven process. The cell dismantles itself neatly for disposal ( Figure 12.1 ) . There is minimal inflammatory response. Apoptosis is a physiological process. Cells that are redundant normally die by apoptosis and this is an important self-regulatory mechanism in growth and development, i.e . cells in the web space of the embryo die by apoptosis, or lymphocytes that could react to self. Genes, such as p53, that can activate apoptosis function as tumour suppressor genes. Mutation in such genes causes a loss of this inhibitory function, which will contribute to malignant transformation as apoptosis is evaded; this means that the wrong cells can be in the wrong places at the wrong times. - - - - Acquire angiogenic competence A mass of cancer cells cannot, in the absence of a blood supply , grow beyond a diameter of about 1 /uni00A0 mm. This places a severe restriction on the capabilities of the tumour (note - that the word tumour means swelling and does not mean the lesion is malignant, although ‘tumour’ is often taken by patients to be synonymous with cancer). It cannot grow much larger or spread widely within the body . If, however, the mass of cancer cells is able to attract or to construct a blood supply then it is able to quit its dormant state and behave in a far

Establish an autonomous lineage Resist signals that inhibit growth Sustain proliferative signalling Obtain replicative immortality Evade apoptosis Acquire angiogenic competence Acquire ability to invade, disseminate and implant Evocation of in /f_l ammation Evade detection/elimination Loss of specialist cell function Develop ability to change energy metabolism AB AB MC AB MN Figure 12.1 Electron micrograph of apoptotic bodies (AB) engulfed by a macrophage. Note the macrophage nucleus (MN) and macrophage cytoplasm (MC).

vessels is termed angiogenesis and is a key feature of malignant transformation. Acquire ability to invade Cancer cells have no respect for the structure of normal tissues. They acquire the ability to breach the basement membrane and gain direct access to blood and lymph vessels. Cancer cells use three main mechanisms to facilitate invasion: (i) cause a rise in the interstitial pressure within a tissue; (ii) secrete enzymes that dissolve extracellular matrix; and (iii) become mobile. Unrestrained proliferation and a lack of contact inhibition enable cancer cells to exert pressure directly on the surrounding tissue and push beyond the normal limits. They secrete collagenases and proteases that chemically dissolve any extracellular boundaries that would otherwise limit their spread through tissues and, by modulating the expression of cell surface molecules called integrins, are able to detach them selves from the extracellular matrix. The abnormal integrins associated with malignancy can also transmit signals from the environment to the cytoplasm and nucleus of the cancer cells (‘outside-in signalling’) and these signals can induce increased motility . These processes are similar to those involved in normal development, i.e. in the migration of the neural crest or the formation of the heart. Epithelial cells behave as if they were mesenchymal cells and the process is termed epithelial– mesenchymal transition (EMT). EMT is a crucial step in malignant transformation and many of the genes and proteins implicated in the formation of cancer control processes are involved in EMT , e.g. Src, Ras, integrins, Wnt / β -catenin, Notch. Acquire ability to disseminate and implant Once cancer cells gain access to vascular and lymphovascular spaces, they can be readily distributed systemically throughout the body . This is not, of itself, su ffi cient to cause tumours to develop at distant sites. The cells also need to acquire the ability to implant. As Paget pointed out over a century ago, there is a crucial relationship here between the seed (the tumour cell) and the soil (the distant tissue). Most of the cancer cells discharged into the circulation probably do not form viable metastases. Circulating cancer cells can be identified in patients who never develop clinical evidence of metastatic disease; presumably these cells die if they cannot implant or they are destroyed by the patient’s immune system. Cancer can spread as individual cells or cell clumps that migrate and implant. Whether spread occurs in groups or as individual cells there is still the problem of crossing the vascular endothelium (and basement membrane) to gain access to the Stephen Paget , 1855–1926, surgeon, The West London Hospital, London, UK. Paget’s ‘seed and soil’ hypothesis is contained in his paper ‘The distribution of secondary growths in cancer of the breast’, published in the Lancet in 1889. Paul Ehrlich , 1854–1915, Professor of Hygiene, the University of Berlin, and later Director of the Institute for Infectious Diseases, Berlin, Germany . In 1908, he shared the Nobel Prize in Physiology or Medicine with Elie Metchniko ff Zoology at Odessa in Russia, and later worked at the Pasteur Institute in Paris, France. Sir Frank McFarlane Burnet , 1899–1985, Australian virologist, Walter and Eliza Hall Institute, Melbourne, Australia. Burnett shared the 1960 Nobel Prize in Physiology or Medicine with Sir Peter Brian Medawar , 1915–1987, Jodrell Professor of Zoology , University College, London, UK, ‘for their discovery of acquired immunological tolerance’. Lewis Thomas , 1913–1993, American pathologist and immunologist, who became President of the Sloan Kettering Memorial Institute, New Y ork, NY , USA. tissues by exploiting, and subverting, the normal inflammatory response. By expressing inflammatory cytokines, cancer cells can deceiv e the endothelium of the host tissue into becoming activated and allowing cancer cells access to the extravascular space. Activated endothelium expresses receptors that bind to integrins and selectins on the surface of cells, allowing the can - cer cells to move across the endothelial barrier. Tumour-related inflammation A malignancy can provoke an inflammatory response and the cytokines and other factors produced as a result of that response may act to promote and sustain malignant transformation. Growth factors, mutagenic reactive oxygen species, angiogenic factors and anti-apoptotic factors may all be produced as part of an inflammatory process and all may contribute to the progression of a cancer. - Evade detection/elimination Although derived from normal cells (‘self ’) cancer cells are, in terms of their genetic make-up, behaviour and character - istics, foreign (‘not self ’). As such, they ought to provoke an immune response and be eliminated. It is entirely possible that malignant transformation is a more frequent event than the emergence of clinical cancer. T he possible role of the immune system in eliminating nascent cancers was proposed by Paul Ehrlich in 1909 and revisited by both Sir Frank McFarlane Burnet and Lewis Thomas in the late 1950s. Cancer cells, or at least those that give rise to clinical disease, appear to gain the ability to escape detection by the immune system. This may be through suppressing expression of tumour-associated antigens or it may be through actively co-opting one part of the immune system to help the tumour escape detection by other parts of the immune surveillance system. This hallmark has been exploited in recent years in the development of T-cell checkpoint inhibitors, which ‘take the brakes’ o ff the immune system to re-enable T-cell killing of cancer cells, e.g. in renal cell carcinoma, lung cancer and melanoma. Loss of specialist cell function Cancer cells are geared to excessive proliferation. They do not need to develop or retain those specialised functions that prior to malignant transformation were their physiological function. These cells can therefore a ff ord to repress or permanently lose those genes that control such functions. The longer term disadvantage to this process is that cancer cells are vulnerable to external stressors, which may , in part, explain why some cancer treatments work. , 1845–1916, ‘in recognition of his work on immunity’. Metchniko ff was Professor of Blood flow in malignant tumours is often sporadic and unre liable. As a result, cancer cells may have to spend prolonged periods in low-oxygen states (i.e. relative hypoxia). Compared with the corresponding normal cells, some cancer cells may be better able to surviv e in hypoxic conditions. This ability may enable tumours to grow and develop despite an impoverished blood supply . Cancer cells can alter their metabolism even when oxygen is abundant; they break down glucose but do not, as normal cells would do, send the resulting pyruvate to the mitochondria for conversion, in an oxygen-dependent process, to carbon dioxide. This is the phenomenon of aerobic glycolysis, or the Warburg e ff ect, and leads to the production of lactate. In an act of symbiosis, lactate-producing cancer cells may provide lactate for adjacent cancer cells, which are then able to use it, via the citric acid cycle, for energy production. This cooperation is similar to that which occurs in skeletal muscle during exercise.