- Email: [email protected]
Biology of cancer
CANCER BIOLOGY AND IMAGING
Biology of cancer
Oncogenes are derived from mutated versions of normal cellular genes (called proto-oncogenes) that control cell proliferation, survival and spread. In normal cells, the expression of proto-oncogenes is very tightly regulated to avoid uncontrolled cell growth. In cancer, activating mutations of proto-oncogenes are responsible for uncontrolled cell division, enhanced survival (even in the face of anti-cancer treatment) and dissemination. Oncogenes are described as being phenotypically dominant e a single mutated copy of a proto-oncogene is sufficient to promote cancer e and are generally not associated with inherited cancer syndromes. Two exceptions to this rule are mutations in the ret proto-oncogene that are associated with multiple endocrine neoplasia (MEN) syndromes (types 2A and 2B) and germline mutations in H-ras that can cause Costello’s syndrome (high birth weight, cardiomyopathy and predisposition to cancers). Oncogenes can be activated in three ways to cause cancers (Figure 2). Tumour suppressor genes (TSG) are normal cellular genes whose function involves inhibition of cell proliferation and survival. They are frequently involved in controlling cell cycle progression and apoptosis. TSG are phenotypically recessive e the function of both copies must be lost in order to promote cancer e and are responsible for inherited cancer syndromes (see Genetic predisposition to cancer in Medicine 2012; 40(1)).
Kevin J Harrington
Abstract Cancer is caused by aberrant patterns of gene expression. Most common cancers are caused by acquired mutations in somatic cells. In contrast, specific germline mutations can account for rare hereditary cancer syndromes. In general, the genes affected in cancers can be divided into two groups: oncogenes and tumour suppressor genes. Oncogenes undergo activation and are phenotypically dominant, while tumour suppressor genes undergo inactivation and are phenotypically recessive. Oncogenic activation can occur by specific point mutations within the sequence of a gene, by amplification of the number of copies of the gene or by translocation of DNA to a site where transcription is more active or where the formation of a new fusion gene generates a protein with enhanced biological activity. Tumour suppressor genes are inactivated by mutations that destroy the function of the protein encoded by the gene. The biological behaviour of cancer can be considered in terms of eight specific hallmarks and two additional socalled enabling characteristics. Improved understanding of the mechanistic basis of these processes has resulted in rapid progress in diagnosis, treatment and prognostication in cancer medicine.
Keywords angiogenesis; apoptosis; cancer; enabling characteristics; hallmarks; metastasis; mutation; oncogene; tumour suppressor gene
The structure of DNA DNA consists of a double helix composed of a deoxyribose sugarphosphate backbone and four bases (adenine, guanine, thymine and cytosine). The four bases form hydrogen bonds with specific bases on the opposite strand. A binds with T and C binds with G. deoxyR, deoxyribose; p, phosphate; a, adenine; t, thymine; c, cytosine; g, guanine.
Introduction Cancer is a genetic disease that occurs when the information in cellular DNA becomes corrupted, leading to abnormal patterns of gene expression. As a result, the effects of normal genes that control normal cellular functions, such as growth, survival and spread, are enhanced and those of genes that suppress these effects are repressed. The main mechanism by which this corruption of the genetic code occurs is through the accumulation of mutations, although there is increasing recognition of the role of non-mutational (epigenetic) changes in the process. Aberrant gene expression leads to a number of key changes in fundamental biological processes within cancer cells e the so-called hallmarks and enabling characteristics of cancer1,2 (Figure 1).
3’
5’
deoxyR
T
A
P
deoxyR
deoxyR
P G
C
P
deoxyR
deoxyR
Cancer genes
P
C
G
P
Cancer is driven by two classes of genes (oncogenes and tumour suppressor genes), each of which provides an essential function in normal cells.
deoxyR
deoxyR
P A
MEDICINE 39:12
deoxyR
5’
T
Kevin J Harrington PhD FRCP FRCR is a Reader in Biological Cancer Therapies at The Institute of Cancer Research, Targeted Therapy Laboratory, Division of Cancer Biology, London, UK and a Consultant Clinical Oncologist at The Royal Marsden Hospital, London. His research interests include gene and virotherapy of cancer and targeted radiation sensitisation of cancer.
3’ Figure 1
689
Ó 2011 Elsevier Ltd. All rights reserved.
CANCER BIOLOGY AND IMAGING
Oncogenic activation via three pathways
Growth factor independence
Normal gene expression leads to formation of normal mRNA and expression of a normal protein in normal amounts. a Specific mutations in the sequence of the DNA code lead to alterations in the amino acid sequence of the protein, giving it enhanced activity. b Increased numbers of normal copies of the gene (amplification) result in the formation of increased am ounts of normal protein. c Translocation of part of the DNA from one chromosomal location to another can result in the generation of a fusion protein with enhanced biological activity.
Growth factor independence can lead to sustained signalling in pathways that control essential biological functions, such as growth, apoptosis, angiogenesis, invasion and DNA damage repair. EGF binds to extracellular domain
Intracellular domain undergoes phosphorylation
Cell membrane P-
Intracellular domain
Normal protein in normal amount
Phosphorylated receptor activates signal transduction pathways
Normal mRNA
Abnormal DNA
Normal DNA a
-P
b
c
Angiogenesis
Invasion
Anti-apoptosis
Gene transcription
Proliferation DNA repair
mRNA Figure 3
Protein Mutant protein Amplified normal protein
Fusion protein
Growth factor independence A general scheme for the function of growth factor receptors and their ligands in promoting cell growth (and other effects) is shown in Figure 3. In this case, binding of epidermal growth factor (the cognate ligand) to its specific ligand-binding domain on the extracellular component of the epidermal growth factor receptor (EGFR) leads to a signal being passed from the membrane to the nucleus via a cascade of intermediary messengers, such that ligand binding on the cell surface alters the behaviour of the cell. Under normal circumstances, activation of growth factor receptors is very tightly controlled e as is the synthesis and release of the ligands that stimulate them. Cancer cells frequently usurp normal growth factor signalling pathways and use them to promote unrestrained cell division.3 Cancer cells exploit three main strategies for achieving selfsufficiency in growth factors: they manufacture and release growth factors which stimulate their own receptors (autocrine signalling) and those of their immediate neighbours (paracrine signalling); they alter the number, structure or function of the growth factor receptors on their surface, such that they are more likely to send a growth signal to the nucleus (even in the absence of the cognate ligand); and they deregulate the signalling pathway downstream of the growth factor receptor so that it is permanently turned on (constitutively active).
Figure 2
Hallmarks of cancer and enabling characteristics In 2000, Hanahan and Weinberg described six key changes that occur in cancer (growth factor independence, evading growth suppressors, avoiding apoptosis, maintaining replicative potential, angiogenesis and invasion/metastasis); these can be seen as largely responsible for driving malignant behaviour.1 Recently, they have updated their description to include two additional emerging hallmarks (re-programming energy metabolism and evading immune destruction) and two enabling characteristics (genomic instability and inflammation) (Table 1).2 The role played by each of these processes will be reviewed briefly below.
Hallmarks of cancer and enabling characteristics Hallmarks of cancer C Growth factor independence or self-sufficiency C Insensitivity to anti-growth signals C Avoidance of programmed cell death (apoptosis) C Ability to recruit a dedicated blood supply C Immortalization by reactivation of telomerase C Ability to invade adjacent normal tissues and metastasize to distant sites C Reprogrammed energy metabolism C Evading immune destruction
Insensitivity to anti-growth signals Several normal anti-growth signals counteract the positively acting growth signals described above. Anti-growth signals work either by forcing cells into quiescence (G0 stage of the cell cycle) or by inducing their terminal differentiation such that they are permanently unable to re-enter the cell cycle. Anti-growth signalling is mediated by ligands (e.g. transforming growth factor beta, TGF-b) that act on cellular receptors (e.g. TGF-b receptor) and send signals to the nucleus via second messengers. These pathways are mainly involved in controlling the cell cycle clock and mediate their effects through proteins that include
Enabling characteristics of cancer Genomic instability C Inflammation C
Table 1
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Ó 2011 Elsevier Ltd. All rights reserved.
CANCER BIOLOGY AND IMAGING
retinoblastoma protein (Rb), cyclins, cyclin-dependent kinases (CDK) and their inhibitors (CDKi). Abnormalities in anti-growth signalling pathways are extremely common in cancer and play a role in helping cancer cells to progress through the cell cycle. Therefore, loss of Rb and members of the CDKi family, and overexpression of certain cyclins and CDK have been shown to occur in a large number of tumour types.
have switched off their apoptotic pathway are more likely to be intrinsically resistant to anti-cancer treatments. In fact, the use of these treatments may promote the accumulation of other mutations that may have a negative influence on the biology of the disease. Sustained angiogenesis In normal tissues, the growth of new blood vessels (angiogenesis) is held very tightly in check by a balance between positive (proangiogenic) and negative (anti-angiogenic) signals (see Table 2). The growth of cancer deposits is intimately related to their ability to secure a blood supply. A small cluster of cancer cells can grow to 60e100 mm by deriving a supply of oxygen and nutrients by direct diffusion, but beyond this size the fledgling tumour must acquire its own dedicated blood supply. Cancers acquire the ability to grow a new blood supply by subverting the balance between pro- and anti-angiogenic factors. Essentially, cancers switch to an ‘angiogenic phenotype’ by upregulating production of pro-angiogenic proteins, such as vascular endothelial growth factor (VEGF), and/ or by downregulating production of anti-angiogenic proteins, such as thrombospondin-1.
Avoidance of apoptosis Normal cells continually audit their viability by assessing the balance of survival (anti-apoptotic) and death (pro-apoptotic) signals that they receive. In normal cells, DNA damage leads to a block in proliferation (cell cycle arrest) while the potential for repair is assessed. If the level of damage exceeds the capacity for repair, the balance of anti- and pro-apoptotic signals tips and the cell undergoes programmed cell death (apoptosis). This prevents maintenance of DNA damage and avoids the risk that mutations will be passed to the progeny of cell division. As such, this mechanism represents a very powerful barrier to the development of cancer. Loss of normal apoptotic pathway signalling is an extremely common event in cancer. Indeed, two of the best-known cancerassociated genes (p53 (TSG) and bcl-2 (oncogene)) are intimately involved in apoptosis. The two main mechanisms of apoptotic signalling (intrinsic and extrinsic pathways) are illustrated in a simplified form in Figure 4. Cancer cells are able to evade apoptosis through an ability to ignore signals sent through the extrinsic pathway, or by re-setting the balance of intracellular proand anti-apoptotic molecules in favour of inhibition of apoptosis. By circumventing apoptosis, cancer cells can sustain DNA damage without it causing cell death (unless the damage is to a gene that is absolutely necessary for cell survival). Therefore, cancer cells that
Cellular immortalization Normal somatic cells can undergo only a finite number of cell divisions (Hayflick limit) before they enter a period of permanent growth arrest, known as replicative senescence. This process occurs as a result of the cells’ inability to replicate the ends of their chromosomes (the telomeres) fully at each division. Therefore, over time the telomeres get progressively shorter, effectively acting as molecular clocks that count down the cells’ lifespan. In contrast, stem cells and malignant cells have acquired immortality by maintaining the length of their telomeres. In most tumours, this occurs through upregulation of the enzyme telomerase, but in 10e15% of cases a different mechanism e the alternative lengthening of the telomeres (ALT) e is responsible. Telomerase enzymatic activity involves a large number of proteins but its two main components are an RNA template (hTR) and a reverse transcriptase enzyme (hTERT); the reverse transcriptase uses the hTR RNA template as a guide in the resynthesis of the DNA sequence of the telomere. Therefore, tumours that have reactivated the expression of telomerase are
Normal apoptotic signalling pathways Cells can undergo programmed cell death in response to activation of either the intrinsic or extrinsic apoptotic pathway. Cancers frequently subvert these pathways to allow them to survive signals that would lead to the death of normal cells. EXTRINSIC PATHWAY Death ligand
Death receptor
Extrinsic pathway signalling molecule (caspase 8)
Mitochondrion
Intrinsic pathway signalling molecule (cytochrome C)
Caspase 3
Cell membrane
Pro- and anti-angiogenic factors Pro-angiogenic C Vascular endothelial growth factor (VEGF) C Basic fibroblast growth factor (bFGF) C Acidic fibroblast growth factor (aFGF) C Transforming growth factors a and b (TGF-a, TGF-b) C Platelet-derived growth factor (PDGF) C Tumour necrosis factor a (TNF-a)
Pro-apoptotic signal Anti-apoptotic signal
Damage sensor (p53)
Anti-angiogenic C Angiostatin C Endostatin C Thrombospondin-1 and -2 (TSP-1, TSP-2) C Interleukins (IL-1b, IL-12, IL-18) C Anti-thrombin III
Genotoxic insult
INTRINSIC PATHWAY
Common final pathway to cell death
Table 2
Figure 4
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Ó 2011 Elsevier Ltd. All rights reserved.
CANCER BIOLOGY AND IMAGING
Reprogrammed energy metabolism In their updated review, Hanahan and Weinberg have designated reprogrammed energy metabolism as an emerging hallmark of cancer. This hallmark recognizes the fact that the chronic, uncontrolled cell proliferation in cancer requires a reconfiguration of the way in which cancer cells metabolize glucose. Normal cells process glucose initially in the cytoplasm by glycolysis, to yield pyruvate, and then in the mitochondria by oxidative phosphorylation, to generate carbon dioxide and water. In contrast, even under oxygenated conditions, cancer cells tend to switch their metabolism to preferential use of glycolysis with generation of lactate (the so-called Warburg effect). As yet, the reasons for this change are not clear, but the fact that it is driven by mutations in key oncogenes and tumour suppressor genes suggest that it is an important underlying principle of cancer biology.
Invasion and metastasis of cancer cells Invasion and metas tasis of cancer cells results from upregulated expression of molecules that allow cells to digest the extracellular matrix around them, migrate and intravasate into blood vessels and then take up residence in distant organs. The sites of distant metastasis can be determined by the expression of specific chemokine receptors by cancer cells that allow them to home in on suitable sites to establish secondary deposits. Patient with breast cancer Tumour cell expressing specific chemokine receptor Lymph node
Tumour cell invasion, migration and intravasation
Evading immune destruction An unresolved issue regarding tumour formation and maintenance is the role of the immune system. According to the theory of immune surveillance, the immune system mounts a constant vigil against the emergence of pre-malignant and frankly malignant cells. The most often cited evidence for this effect comes from the observation that chronic immune suppression is associated with a marked increase in specific cancers, especially those of viral origin. There is also evidence that the immune system presents a significant barrier to non-virally induced cancers in immunocompetent patients. Thus, the occurrence of tumours can be perceived as a failure of the immune system to recognize, reject and destroy tumour cells that express altered self antigens. As part of this process, it is thought that selection of less immunogenic cancer cells (through immuno-editing) and active recruitment of immunosuppressive components of the immune system [e.g. regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSCs)] to some cancers allows tumours to develop and spread without becoming targets for immune clearance.
Lung
Liver Tumour cell invades lymphatic or blood vessel Bone Metastasis specifically to tissues expressing high levels of cognate ligand (chemokine)
Brain
Figure 5
able to re-build the parts of their telomeres that they lose with each round of cell division and thereby avoid being sidelined into replicative senescence. Invasion and metastasis (Figure 5) Distant metastases cause 90% of cancer deaths. Invasion and metastasis involves careful orchestration of a series of complex biological processes: detachment from immediate neighbours and stroma at the local site enzymatic digestion of the extracellular matrix followed by specific directional motility penetration (intravasation) of blood or lymphatic vessels and tumour embolization survival in the circulation until arrival at the metastatic site, which may be chosen on the basis of provision of a favourable supply of appropriate growth factors adherence of the metastasis to the endothelium of blood vessels at its destination and extravasation from the vessel proliferation and invasion of the new location and recruitment of a new blood supply. One of the key processes underlying invasion and metastasis of epithelial tumours is the epithelial-to-mesenchymal transition (EMT). This multifaceted programme can be engaged transiently or stably by invading cancer cells. The patterns of metastasis of different cancers to specific organs (e.g. breast cancer to liver, bone and brain; lung cancer to brain and adrenal gland) are not random, but appear to be driven by expression of chemokine receptors by tumour cells that allow them to ‘seek’ a suitable environment in which to establish a colony.4
MEDICINE 39:12
Enabling characteristics of cancer As part of the biology underpinning cancer development, two key enabling characteristics have recently been defined: genomic instability and inflammation. The first relates to the state in which cancer cells lose control of the integrity of their genetic material and acquire an increasing repertoire of mutational changes that progressively alter their biology and promote the hallmarks of cancer. The second enabling characteristic describes the common situation in which pre-malignant and frankly malignant lesions excite an inflammatory state, through the recruitment and activation of components of the immune system that promote and support tumour growth and spread. A
FURTHER READING 1 Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000; 100: 57e70. 2 Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell 2001; 144: 646e74. 3 Rogers SJ, Harrington KJ, Rhys Evans P, O-Charoenrat P, Eccles SA. Biological significance of c-erbB family oncogenes in head and neck cancer. Cancer Metastasis Rev 2005; 24: 47e69. 4 Muller A, Homey B, Soto H, et al. Involvement of chemokine receptors in breast cancer metastasis. Nature 2001; 410: 50e6.
692
Ó 2011 Elsevier Ltd. All rights reserved.
Biology of cancer
Oncogenes are derived from mutated versions of normal cellular genes (called proto-oncogenes) that control cell proliferation, survival and spread. In normal cells, the expression of proto-oncogenes is very tightly regulated to avoid uncontrolled cell growth. In cancer, activating mutations of proto-oncogenes are responsible for uncontrolled cell division, enhanced survival (even in the face of anti-cancer treatment) and dissemination. Oncogenes are described as being phenotypically dominant e a single mutated copy of a proto-oncogene is sufficient to promote cancer e and are generally not associated with inherited cancer syndromes. Two exceptions to this rule are mutations in the ret proto-oncogene that are associated with multiple endocrine neoplasia (MEN) syndromes (types 2A and 2B) and germline mutations in H-ras that can cause Costello’s syndrome (high birth weight, cardiomyopathy and predisposition to cancers). Oncogenes can be activated in three ways to cause cancers (Figure 2). Tumour suppressor genes (TSG) are normal cellular genes whose function involves inhibition of cell proliferation and survival. They are frequently involved in controlling cell cycle progression and apoptosis. TSG are phenotypically recessive e the function of both copies must be lost in order to promote cancer e and are responsible for inherited cancer syndromes (see Genetic predisposition to cancer in Medicine 2012; 40(1)).
Kevin J Harrington
Abstract Cancer is caused by aberrant patterns of gene expression. Most common cancers are caused by acquired mutations in somatic cells. In contrast, specific germline mutations can account for rare hereditary cancer syndromes. In general, the genes affected in cancers can be divided into two groups: oncogenes and tumour suppressor genes. Oncogenes undergo activation and are phenotypically dominant, while tumour suppressor genes undergo inactivation and are phenotypically recessive. Oncogenic activation can occur by specific point mutations within the sequence of a gene, by amplification of the number of copies of the gene or by translocation of DNA to a site where transcription is more active or where the formation of a new fusion gene generates a protein with enhanced biological activity. Tumour suppressor genes are inactivated by mutations that destroy the function of the protein encoded by the gene. The biological behaviour of cancer can be considered in terms of eight specific hallmarks and two additional socalled enabling characteristics. Improved understanding of the mechanistic basis of these processes has resulted in rapid progress in diagnosis, treatment and prognostication in cancer medicine.
Keywords angiogenesis; apoptosis; cancer; enabling characteristics; hallmarks; metastasis; mutation; oncogene; tumour suppressor gene
The structure of DNA DNA consists of a double helix composed of a deoxyribose sugarphosphate backbone and four bases (adenine, guanine, thymine and cytosine). The four bases form hydrogen bonds with specific bases on the opposite strand. A binds with T and C binds with G. deoxyR, deoxyribose; p, phosphate; a, adenine; t, thymine; c, cytosine; g, guanine.
Introduction Cancer is a genetic disease that occurs when the information in cellular DNA becomes corrupted, leading to abnormal patterns of gene expression. As a result, the effects of normal genes that control normal cellular functions, such as growth, survival and spread, are enhanced and those of genes that suppress these effects are repressed. The main mechanism by which this corruption of the genetic code occurs is through the accumulation of mutations, although there is increasing recognition of the role of non-mutational (epigenetic) changes in the process. Aberrant gene expression leads to a number of key changes in fundamental biological processes within cancer cells e the so-called hallmarks and enabling characteristics of cancer1,2 (Figure 1).
3’
5’
deoxyR
T
A
P
deoxyR
deoxyR
P G
C
P
deoxyR
deoxyR
Cancer genes
P
C
G
P
Cancer is driven by two classes of genes (oncogenes and tumour suppressor genes), each of which provides an essential function in normal cells.
deoxyR
deoxyR
P A
MEDICINE 39:12
deoxyR
5’
T
Kevin J Harrington PhD FRCP FRCR is a Reader in Biological Cancer Therapies at The Institute of Cancer Research, Targeted Therapy Laboratory, Division of Cancer Biology, London, UK and a Consultant Clinical Oncologist at The Royal Marsden Hospital, London. His research interests include gene and virotherapy of cancer and targeted radiation sensitisation of cancer.
3’ Figure 1
689
Ó 2011 Elsevier Ltd. All rights reserved.
CANCER BIOLOGY AND IMAGING
Oncogenic activation via three pathways
Growth factor independence
Normal gene expression leads to formation of normal mRNA and expression of a normal protein in normal amounts. a Specific mutations in the sequence of the DNA code lead to alterations in the amino acid sequence of the protein, giving it enhanced activity. b Increased numbers of normal copies of the gene (amplification) result in the formation of increased am ounts of normal protein. c Translocation of part of the DNA from one chromosomal location to another can result in the generation of a fusion protein with enhanced biological activity.
Growth factor independence can lead to sustained signalling in pathways that control essential biological functions, such as growth, apoptosis, angiogenesis, invasion and DNA damage repair. EGF binds to extracellular domain
Intracellular domain undergoes phosphorylation
Cell membrane P-
Intracellular domain
Normal protein in normal amount
Phosphorylated receptor activates signal transduction pathways
Normal mRNA
Abnormal DNA
Normal DNA a
-P
b
c
Angiogenesis
Invasion
Anti-apoptosis
Gene transcription
Proliferation DNA repair
mRNA Figure 3
Protein Mutant protein Amplified normal protein
Fusion protein
Growth factor independence A general scheme for the function of growth factor receptors and their ligands in promoting cell growth (and other effects) is shown in Figure 3. In this case, binding of epidermal growth factor (the cognate ligand) to its specific ligand-binding domain on the extracellular component of the epidermal growth factor receptor (EGFR) leads to a signal being passed from the membrane to the nucleus via a cascade of intermediary messengers, such that ligand binding on the cell surface alters the behaviour of the cell. Under normal circumstances, activation of growth factor receptors is very tightly controlled e as is the synthesis and release of the ligands that stimulate them. Cancer cells frequently usurp normal growth factor signalling pathways and use them to promote unrestrained cell division.3 Cancer cells exploit three main strategies for achieving selfsufficiency in growth factors: they manufacture and release growth factors which stimulate their own receptors (autocrine signalling) and those of their immediate neighbours (paracrine signalling); they alter the number, structure or function of the growth factor receptors on their surface, such that they are more likely to send a growth signal to the nucleus (even in the absence of the cognate ligand); and they deregulate the signalling pathway downstream of the growth factor receptor so that it is permanently turned on (constitutively active).
Figure 2
Hallmarks of cancer and enabling characteristics In 2000, Hanahan and Weinberg described six key changes that occur in cancer (growth factor independence, evading growth suppressors, avoiding apoptosis, maintaining replicative potential, angiogenesis and invasion/metastasis); these can be seen as largely responsible for driving malignant behaviour.1 Recently, they have updated their description to include two additional emerging hallmarks (re-programming energy metabolism and evading immune destruction) and two enabling characteristics (genomic instability and inflammation) (Table 1).2 The role played by each of these processes will be reviewed briefly below.
Hallmarks of cancer and enabling characteristics Hallmarks of cancer C Growth factor independence or self-sufficiency C Insensitivity to anti-growth signals C Avoidance of programmed cell death (apoptosis) C Ability to recruit a dedicated blood supply C Immortalization by reactivation of telomerase C Ability to invade adjacent normal tissues and metastasize to distant sites C Reprogrammed energy metabolism C Evading immune destruction
Insensitivity to anti-growth signals Several normal anti-growth signals counteract the positively acting growth signals described above. Anti-growth signals work either by forcing cells into quiescence (G0 stage of the cell cycle) or by inducing their terminal differentiation such that they are permanently unable to re-enter the cell cycle. Anti-growth signalling is mediated by ligands (e.g. transforming growth factor beta, TGF-b) that act on cellular receptors (e.g. TGF-b receptor) and send signals to the nucleus via second messengers. These pathways are mainly involved in controlling the cell cycle clock and mediate their effects through proteins that include
Enabling characteristics of cancer Genomic instability C Inflammation C
Table 1
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Ó 2011 Elsevier Ltd. All rights reserved.
CANCER BIOLOGY AND IMAGING
retinoblastoma protein (Rb), cyclins, cyclin-dependent kinases (CDK) and their inhibitors (CDKi). Abnormalities in anti-growth signalling pathways are extremely common in cancer and play a role in helping cancer cells to progress through the cell cycle. Therefore, loss of Rb and members of the CDKi family, and overexpression of certain cyclins and CDK have been shown to occur in a large number of tumour types.
have switched off their apoptotic pathway are more likely to be intrinsically resistant to anti-cancer treatments. In fact, the use of these treatments may promote the accumulation of other mutations that may have a negative influence on the biology of the disease. Sustained angiogenesis In normal tissues, the growth of new blood vessels (angiogenesis) is held very tightly in check by a balance between positive (proangiogenic) and negative (anti-angiogenic) signals (see Table 2). The growth of cancer deposits is intimately related to their ability to secure a blood supply. A small cluster of cancer cells can grow to 60e100 mm by deriving a supply of oxygen and nutrients by direct diffusion, but beyond this size the fledgling tumour must acquire its own dedicated blood supply. Cancers acquire the ability to grow a new blood supply by subverting the balance between pro- and anti-angiogenic factors. Essentially, cancers switch to an ‘angiogenic phenotype’ by upregulating production of pro-angiogenic proteins, such as vascular endothelial growth factor (VEGF), and/ or by downregulating production of anti-angiogenic proteins, such as thrombospondin-1.
Avoidance of apoptosis Normal cells continually audit their viability by assessing the balance of survival (anti-apoptotic) and death (pro-apoptotic) signals that they receive. In normal cells, DNA damage leads to a block in proliferation (cell cycle arrest) while the potential for repair is assessed. If the level of damage exceeds the capacity for repair, the balance of anti- and pro-apoptotic signals tips and the cell undergoes programmed cell death (apoptosis). This prevents maintenance of DNA damage and avoids the risk that mutations will be passed to the progeny of cell division. As such, this mechanism represents a very powerful barrier to the development of cancer. Loss of normal apoptotic pathway signalling is an extremely common event in cancer. Indeed, two of the best-known cancerassociated genes (p53 (TSG) and bcl-2 (oncogene)) are intimately involved in apoptosis. The two main mechanisms of apoptotic signalling (intrinsic and extrinsic pathways) are illustrated in a simplified form in Figure 4. Cancer cells are able to evade apoptosis through an ability to ignore signals sent through the extrinsic pathway, or by re-setting the balance of intracellular proand anti-apoptotic molecules in favour of inhibition of apoptosis. By circumventing apoptosis, cancer cells can sustain DNA damage without it causing cell death (unless the damage is to a gene that is absolutely necessary for cell survival). Therefore, cancer cells that
Cellular immortalization Normal somatic cells can undergo only a finite number of cell divisions (Hayflick limit) before they enter a period of permanent growth arrest, known as replicative senescence. This process occurs as a result of the cells’ inability to replicate the ends of their chromosomes (the telomeres) fully at each division. Therefore, over time the telomeres get progressively shorter, effectively acting as molecular clocks that count down the cells’ lifespan. In contrast, stem cells and malignant cells have acquired immortality by maintaining the length of their telomeres. In most tumours, this occurs through upregulation of the enzyme telomerase, but in 10e15% of cases a different mechanism e the alternative lengthening of the telomeres (ALT) e is responsible. Telomerase enzymatic activity involves a large number of proteins but its two main components are an RNA template (hTR) and a reverse transcriptase enzyme (hTERT); the reverse transcriptase uses the hTR RNA template as a guide in the resynthesis of the DNA sequence of the telomere. Therefore, tumours that have reactivated the expression of telomerase are
Normal apoptotic signalling pathways Cells can undergo programmed cell death in response to activation of either the intrinsic or extrinsic apoptotic pathway. Cancers frequently subvert these pathways to allow them to survive signals that would lead to the death of normal cells. EXTRINSIC PATHWAY Death ligand
Death receptor
Extrinsic pathway signalling molecule (caspase 8)
Mitochondrion
Intrinsic pathway signalling molecule (cytochrome C)
Caspase 3
Cell membrane
Pro- and anti-angiogenic factors Pro-angiogenic C Vascular endothelial growth factor (VEGF) C Basic fibroblast growth factor (bFGF) C Acidic fibroblast growth factor (aFGF) C Transforming growth factors a and b (TGF-a, TGF-b) C Platelet-derived growth factor (PDGF) C Tumour necrosis factor a (TNF-a)
Pro-apoptotic signal Anti-apoptotic signal
Damage sensor (p53)
Anti-angiogenic C Angiostatin C Endostatin C Thrombospondin-1 and -2 (TSP-1, TSP-2) C Interleukins (IL-1b, IL-12, IL-18) C Anti-thrombin III
Genotoxic insult
INTRINSIC PATHWAY
Common final pathway to cell death
Table 2
Figure 4
MEDICINE 39:12
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Ó 2011 Elsevier Ltd. All rights reserved.
CANCER BIOLOGY AND IMAGING
Reprogrammed energy metabolism In their updated review, Hanahan and Weinberg have designated reprogrammed energy metabolism as an emerging hallmark of cancer. This hallmark recognizes the fact that the chronic, uncontrolled cell proliferation in cancer requires a reconfiguration of the way in which cancer cells metabolize glucose. Normal cells process glucose initially in the cytoplasm by glycolysis, to yield pyruvate, and then in the mitochondria by oxidative phosphorylation, to generate carbon dioxide and water. In contrast, even under oxygenated conditions, cancer cells tend to switch their metabolism to preferential use of glycolysis with generation of lactate (the so-called Warburg effect). As yet, the reasons for this change are not clear, but the fact that it is driven by mutations in key oncogenes and tumour suppressor genes suggest that it is an important underlying principle of cancer biology.
Invasion and metastasis of cancer cells Invasion and metas tasis of cancer cells results from upregulated expression of molecules that allow cells to digest the extracellular matrix around them, migrate and intravasate into blood vessels and then take up residence in distant organs. The sites of distant metastasis can be determined by the expression of specific chemokine receptors by cancer cells that allow them to home in on suitable sites to establish secondary deposits. Patient with breast cancer Tumour cell expressing specific chemokine receptor Lymph node
Tumour cell invasion, migration and intravasation
Evading immune destruction An unresolved issue regarding tumour formation and maintenance is the role of the immune system. According to the theory of immune surveillance, the immune system mounts a constant vigil against the emergence of pre-malignant and frankly malignant cells. The most often cited evidence for this effect comes from the observation that chronic immune suppression is associated with a marked increase in specific cancers, especially those of viral origin. There is also evidence that the immune system presents a significant barrier to non-virally induced cancers in immunocompetent patients. Thus, the occurrence of tumours can be perceived as a failure of the immune system to recognize, reject and destroy tumour cells that express altered self antigens. As part of this process, it is thought that selection of less immunogenic cancer cells (through immuno-editing) and active recruitment of immunosuppressive components of the immune system [e.g. regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSCs)] to some cancers allows tumours to develop and spread without becoming targets for immune clearance.
Lung
Liver Tumour cell invades lymphatic or blood vessel Bone Metastasis specifically to tissues expressing high levels of cognate ligand (chemokine)
Brain
Figure 5
able to re-build the parts of their telomeres that they lose with each round of cell division and thereby avoid being sidelined into replicative senescence. Invasion and metastasis (Figure 5) Distant metastases cause 90% of cancer deaths. Invasion and metastasis involves careful orchestration of a series of complex biological processes: detachment from immediate neighbours and stroma at the local site enzymatic digestion of the extracellular matrix followed by specific directional motility penetration (intravasation) of blood or lymphatic vessels and tumour embolization survival in the circulation until arrival at the metastatic site, which may be chosen on the basis of provision of a favourable supply of appropriate growth factors adherence of the metastasis to the endothelium of blood vessels at its destination and extravasation from the vessel proliferation and invasion of the new location and recruitment of a new blood supply. One of the key processes underlying invasion and metastasis of epithelial tumours is the epithelial-to-mesenchymal transition (EMT). This multifaceted programme can be engaged transiently or stably by invading cancer cells. The patterns of metastasis of different cancers to specific organs (e.g. breast cancer to liver, bone and brain; lung cancer to brain and adrenal gland) are not random, but appear to be driven by expression of chemokine receptors by tumour cells that allow them to ‘seek’ a suitable environment in which to establish a colony.4
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Enabling characteristics of cancer As part of the biology underpinning cancer development, two key enabling characteristics have recently been defined: genomic instability and inflammation. The first relates to the state in which cancer cells lose control of the integrity of their genetic material and acquire an increasing repertoire of mutational changes that progressively alter their biology and promote the hallmarks of cancer. The second enabling characteristic describes the common situation in which pre-malignant and frankly malignant lesions excite an inflammatory state, through the recruitment and activation of components of the immune system that promote and support tumour growth and spread. A
FURTHER READING 1 Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000; 100: 57e70. 2 Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell 2001; 144: 646e74. 3 Rogers SJ, Harrington KJ, Rhys Evans P, O-Charoenrat P, Eccles SA. Biological significance of c-erbB family oncogenes in head and neck cancer. Cancer Metastasis Rev 2005; 24: 47e69. 4 Muller A, Homey B, Soto H, et al. Involvement of chemokine receptors in breast cancer metastasis. Nature 2001; 410: 50e6.
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