Apoptosis & Cancer Ppt Powerpoint Presentation slides Anonymous Medical Slideshow APOPTOSIS AND ITS RELATION TO CANCER

Apoptosis & Cancer

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Slide 1 :

Apoptosis & Cancer ?? ???????

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Initiation of apoptosis In principle, there are two alternative pathways that initiate apoptosis: one is mediated by death receptors on the cell surface — sometimes referred to as the 'extrinsic pathway'; the other is mediated by mitochondria — referred to as the 'instrinsic pathway'. In both pathways, cysteine aspartyl-specific proteases (caspases) are activated that cleave cellular substrates, and this leads to the biochemical and morphological changes that are characteristic of apoptosis. Death receptors are members of the tumour-necrosis factor (TNF) receptor superfamily and comprise a subfamily that is characterized by an intracellular domain — the death domain. Death receptors are activated by their natural ligands, the TNF family. When ligands bind to their respective death receptors — such as CD95, TRAIL-R1 (TNF-related apoptosis-inducing ligand-R1) or TRAIL-R2 — the death domains attract the intracellular adaptor protein FADD (Fas-associated death domain protein, also known as MORT1), which, in turn, recruits the inactive proforms of certain members of the caspase protease family. The caspases that are recruited to this death-inducing signalling complex (DISC) — caspase-8 and caspase-10 — function as 'initiator' caspases. At the DISC, procaspase-8 and procaspase-10 are cleaved and yield active initiator caspases. In some cells — known as type I cells — the amount of active caspase-8 formed at the DISC is sufficient to initiate apoptosis directly, but in type II cells, the amount is too small and mitochondria are used as 'amplifiers' of the apoptotic signal. Activation of mitochondria is mediated by the BCL2 family member BID. BID is cleaved by active caspase-8 and translocates to the mitochondria.

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The two main apoptic signalling pathway Apoptosis can be initiated by two alternative pathways: either through death receptors on the cell surface (extrinsic pathway) or through mitochondria (intrinsic pathway). In both pathways, induction of apoptosis leads to activation of an initiator caspase: caspase-8 and possibly caspase-10 for the extrinsic pathway; and caspase-9, which is activated at the apoptosome, for the intrinsic pathway. The initiator caspases then activate executioner caspases. Active executioner caspases cleave the death substrates, which eventually results in apoptosis. There is crosstalk between these two pathways. For example, cleavage of the BCL2-family member BID by caspase-8 activates the mitochondrial pathway after apoptosis induction through death receptors, and can be used to amplify the apoptotic signal.

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Death receptos and ligands Ligands are shown at the top, receptors at the bottom. Death receptors and death ligands are grouped in a box. DcR3 (decoy receptor 3) acts as a decoy receptor for CD95L (dotted line). The other molecules outside the box can bind to death receptors or ligands as indicated, but have not been shown to transmit an apoptotic signal. The death domain is shown as a pink box.

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Apoptosis signalling through death receptors Binding of death ligands (CD95L is used here as an example) to their receptor leads to the formation of the death-inducing signalling complex (DISC). In the DISC, the initiator procaspase-8 is recruited by FADD (FAS-associated death domain protein) and is activated by autocatalytic cleavage. Death-receptor-mediated apoptosis can be inhibited at several levels by anti-apoptotic proteins: CD95L can be prevented from binding to CD95 by soluble 'decoy' receptors, such as soluble CD95 (sCD95) or DcR3 (decoy receptor 3). FLICE-inhibitory proteins (FLIPs) bind to the DISC and prevent the activation of caspase-8; and inhibitors of apoptosis proteins (IAPs) bind to and inhibit caspases. FLIPL and FLIPS refer to long and short forms of FLIP, respectively.

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Mitochondria & the BCL2 family Death initiated at the mitochondrial level is regulated by the members of the BCL2 family. BCL2 family members can be divided into anti-apoptotic (BCL2, BCL-XL, BCL-w, MCL1, A1/BFL1, BOO/DIVA, NR-13) and pro-apoptotic proteins (BAX, BAK, BOK/MTD, BCL-XS, BID, BAD, BIK/NBK, BLK, HRK/DP5, BIM/BOD, NIP3, NIX, NOXA, PUMA, BMF). Most anti-apoptotic members contain the BCL2 homology (BH) domains 1, 2 and 4, whereas the BH3 domain seems to be crucial for apoptosis induction. The pro-apoptotic members can be subdivided into the BAX subfamily (BAX, BAK, BOK) and the BH3-only proteins (for example, BID, BAD and BIM). After activation by an apoptotic stimulus, mitochondria release cytochrome c, AIF (apoptosis inducing factor) and other apoptogenic factors from the intermembrane space to the cytosol. Concomitantly, the mitochondrial transmembrane potential drops. According to one model, mitochondrial membrane permeabilization involves the permeability transition pore complex (PTPC), a multiprotein complex that consists of the adenine nucleotide translocator (ANT) of the inner membrane, the voltage-dependent anion channel of the outer membrane and various other proteins. BCL2 proteins might interact with the PTPC and regulate its permeability. According to another model, BH3-only proteins serve as 'death sensors' in the cytosol or cytoskeleton. Following a death signal, they interact with members of the BAX subfamily. After this interaction, BAX proteins undergo a conformational change, insert into the mitochondrial membrane, oligomerize and form protein-permeable channels. Anti-apoptotic BCL2 proteins inhibit the conformational change or the oligomerization of BAX and BAK. The localization of the pro-apoptotic BCL2 family member BAD is regulated by phosphorylation. Only non-phosphorylated BAD is capable of antagonizing anti-apoptotic BCL2 or BCL-XL on the mitochondrial membrane. BAD phosphorylation results in its redistribution to the cytosol and its sequestration by 14-3-3 proteins.

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Apoptosis signalling through mitochondria Chemotherapy, irradiation and other stimuli can initiate apoptosis through the mitochondrial (intrinsic) pathway. Pro-apoptotic BCL2 family proteins — for example, BAX, BID, BAD and BIM — are important mediators of these signals. Activation of mitochondria leads to the release of cytochrome c (Cyt c) into the cytosol, where it binds apoptotic protease activating factor 1 (APAF1) to form the apoptosome. At the apoptosome, the initiator caspase-9 is activated. Apoptosis through mitochondria can be inhibited on different levels by anti-apoptotic proteins, including the anti-apoptotic BCL2 family members BCL2 and BCL-XL and inhibitors of apoptosis proteins (IAPs), which are regulated by SMAC/DIABLO (second mitochondria-derived activator of caspase/direct IAP binding protein with low pI). Another way is through survival signals, such as growth factors and cytokines, that activate the phosphatidylinositol 3-kinase (PI3K) pathway. PI3K activates AKT, which phosphorylates and inactivates the pro-apoptotic BCL2-family member BAD.

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Execution of apoptosis Once the initiator caspases are activated, they cleave and activate 'executioner' caspases, mainly caspase-3, caspase-6 and caspase-7. The active executioner caspases then cleave each other and, in this way, an amplifying proteolytic cascade of caspase activation is started. Eventually, the active executioner caspases cleave cellular substrates — the 'death substrates' — which leads to characteristic biochemical and morphological changes. Cleavage of nuclear LAMINS is involved in chromatin condensation and nuclear shrinkage. Cleavage of the inhibitor of the DNase CAD (caspase-activated deoxyribonuclease, DFF40), ICAD (also known as DNA fragmentation factor, 45 kDa; DFF45), causes the release of the endonuclease, which travels to the nucleus to fragment DNA. Cleavage of cytoskeletal proteins such as actin, plectin, Rho kinase 1 (ROCK1) and gelsolin leads to cell fragmentation, blebbing and the formation of apoptotic bodies. After exposure of 'eat me' signals (for example, exposure of phosphatidylserine and changes in surface sugars), the remains of the dying cell are engulfed by phagocytes. Besides these prototypic caspase-dependent apoptosis pathways, there are also molecularly less-well-defined cell-death pathways that do not require caspase activation. These pathways share some, but not all, the characteristics of apoptotic classical pathways. Therefore, they cannot be readily classified as apoptosis or necrosis and have been called 'necrotic-like' or 'apoptotic-like' cell death or paraptosis.

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Regulation of apoptosis The apoptotic self-destruction machinery is tightly controlled. Various proteins regulate the apoptotic process at different levels. FLIPs (FADD-like interleukin-1 -converting enzyme-like protease (FLICE/caspase-8)-inhibitory proteins) interfere with the initiation of apoptosis directly at the level of death receptors. Two splice variants — a long form (FLIPL) and a short form (FLIPS) — have been identified in human cells. Both forms share structural homology with procaspase-8, but lack its catalytic site. This structure allows them to bind to the DISC, thereby inhibiting the processing and activation of the initiator caspase-8. The members of the BCL2 family, which regulate apoptosis at the mitochondrial level, are an important class of regulatory proteins. They can be divided into anti-apoptotic and pro-apoptotic proteins according to their function. BCL2 family proteins influence the permeability of the mitochondrial membrane. The IAPs (inhibitor of apoptosis proteins) constitute a third class of regulatory proteins. IAPs bind to and inhibit caspases. They might also function as ubiquitin ligases, promoting the degradation of the caspases that they bind. IAPs are characterized by a domain termed the baculoviral IAP repeat (BIR). Nine IAP family members — including XIAP (hILP, MIHA, ILP-1), cIAP1 (MIHB, HIAP-2), cIAP2 (HIAP-1, MIHC, API2), NAIP, ML-IAP, ILP2, livin (KIAP), apollon and survivin — have been identified in human cells. However, not all BIR-containing proteins have been shown to suppress apoptosis, and some of them might also have functions other than caspase inhibition. IAPs are inhibited by a protein named SMAC/DIABLO (second mitochondria-derived activator of caspase/direct IAP binding protein with low pI), which is released from mitochondria along with cytochrome c during apoptosis and promotes caspase activation by binding to, and inhibiting, IAPs.

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Physiological growth contol and apoptosis In cells and tissues of multicellular organisms, potent physiological mechanisms govern cell proliferation and homeostasis. Many of these growth-control mechanisms are linked to apoptosis: excessive proliferation or growth at inappropriate sites induces apoptosis in the affected cells. Tumours can proliferate beyond these constraints, which limit growth in normal tissue. Therefore, resistance of tumour cells to apoptosis is an essential feature of cancer development. This assumption is confirmed by the finding that deregulated proliferation alone is not sufficient for tumour formation, but leads to cell death: overexpression of growth-promoting oncogenes — such as c-MYC, E1A or E2F1 — sensitizes cells to apoptosis23. Besides the expression of proteins that promote cell proliferation, tumour progression requires the expression of anti-apoptotic proteins or the inactivation of essential pro-apoptotic proteins. The molecular connections between cell cycle and cell death are not entirely clear, but the p53 pathway seems to be involved. Proliferative signals induce ARF, the product encoded by an alternative reading frame within the CDKN2A tumour-suppressor gene locus, which also encodes the cyclin-dependent kinase inhibitor INK4A. ARF interacts with the ubiquitin ligase MDM2, and prevents it from binding p53 and targeting it for destruction in the proteasome. Upregulation of p53 leads to cell-cycle arrest and apoptosis.

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P53 and apoptosis in tumors p53 is a key element in apoptosis induction in tumour cells. p53 is inhibited by MDM2, a ubiquitin ligase that targets p53 for destruction by the proteasome. MDM2 is inactivated by binding to ARF. Cellular stress, including that induced by chemotherapy or irradiation, activates p53 either directly, by inhibition of MDM2, or indirectly by activation of ARF. ARF can also be induced by proliferative oncogenes such as RAS. Active p53 transactivates pro-apoptotic genes — including BAX, NOXA, CD95 and TRAIL-R1 — to promote apoptosis. TRAIL-R1, tumour-necrosis-factor-related apoptosis-inducing ligand receptor 1.

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Physiological growth contol and apoptosis The relationship between proliferation and cell death might also reflect the fact that cells require survival signals. Lack of these signals triggers apoptosis — a phenomenon called 'death by neglect'. Survival signals include growth factors, cytokines, hormones and other stimuli, such as signals given by adhesion molecules. In general, survival signals are mediated by means of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway. Depending on the stimulus, further mechanisms must be present that deliver anti-apoptotic survival signals. Anoikis is a special case of death by neglect and is triggered by inadequate or inappropriate cell–matrix contacts. Binding of INTEGRINS to the extracellular matrix conveys survival signals by activating the PI3K/AKT pathway. Anoikis involves the pro-apoptotic BCL2 family proteins BIM and BMF. In healthy cells, these proteins bind to the cytoskeleton, but after the cell has detached from the extracellular matrix, BIM and BMF are released and interact with the anti-apoptotic protein BCL2. Resistance to anoikis might facilitate metastasis by allowing cells to survive following detachment from the matrix in their tissue of origin and travelling to distant sites.

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Survial signalling through PI3K/AKT

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Physiological growth contol and apoptosis Normal diploid cells have a limited replicative potential, and this is another means by which excessive proliferation is controlled. After progressing through 60–70 divisions, cells cease to proliferate — a state called senescence — and die. The finite number of divisions is determined by the length of the telomeres at the chromosome ends, which shorten during each cell cycle. Once a critically short length is reached, the sensors for DNA damage are triggered and induce cell-cycle arrest or apoptosis. Again, p53 seems to be important for this response to telomere erosion but, although p53 deficiency temporarily rescues cells from apoptosis, telomere loss ultimately results in a genetic catastrophy, triggering p53-independent apoptosis. In tumour cells, telomeres are stabilized by expression of telomerase or a poorly characterized mechanism that is known as alternate lengthening of telomeres (ALT).

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Apoptosis induction by the immune system If cells manage to circumvent the built-in constraints to unlimited proliferation, the organism has to rely on the immune system as a watch-dog against tumour initiation — a concept called immunosurveillance. The main effector cells against tumours are cytotoxic T cells of the ADAPTIVE IMMUNE SYSTEM and natural killer (NK) cells of the INNATE IMMUNE SYSTEM. T cells and NK cells use two main mechanisms to kill tumour cells: the granule exocytosis pathway and the CD95L pathway. In the calcium-dependent granule exocytosis pathway, lymphocytes secrete a membrane permeability protein called perforin and proteolytic enzymes known as granzymes from cytotoxic granules towards the target cell. In the presence of calcium, perforin polymerizes and initiates ill-defined changes in the target-cell membrane that allow granzymes to pass into the cell. Granzymes are neutral serine proteases that can activate caspases in the target cell. In addition, granzyme B might directly cleave the BCL2 family member BID to activate the mitochondrial death pathway. In the CD95L pathway, which is calcium independent, the lymphocyte exhibits the death ligand CD95L on the cell surface and triggers apoptosis through the CD95 receptor on the target cell. Resistance of tumour cells to these effector mechanisms not only leads to escape of the tumours from immunosurveillance, but might also markedly influence the efficacy of immunotherapy.

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Therapeutic induction of apoptosis Cancer treatment by chemotherapy and -irradiation kills target cells primarily by the induction of apoptosis. However, few tumours are sensitive to these therapies, and the development of resistance to therapy is an important clinical problem. Patients who have a tumour relapse usually present with tumours that are more resistant to therapy than the primary tumour. Failure to activate the apoptotic programme represents an important mode of drug resistance in tumour cells. Anticancer drugs are classified as DNA-damaging agents, ANTIMETABOLITES, mitotic inhibitors, nucleotide analogues or inhibitors of TOPOISOMERASES. Treatment with these agents or with -irradiation causes cellular stress and finally cell death. A key element in stress-induced apoptosis is p53. Rapid induction of p53 function is achieved in response to most forms of stress through post-translational mechanisms. p53 can be stabilized and activated through the inactivation of MDM2, either by ARF, as discussed above, or by direct phosphorylation of MDM2. In addition, many post-translational modifications of p53 have been shown to enhance its transcriptional activity in response to stress, including phosphorylation, SUMOYLATION and acetylation. The transcriptional activity of p53 is important for its pro-apoptotic function. p53 can induce the expression of proteins involved in the mitochondrial pathway — such as BAX, NOXA, PUMA and p53AIP1 — and in the death receptor pathway — such as CD95, TRAIL-R1 and TRAIL-R2. Moreover, transcriptionally independent activities of p53 mediate some of its pro-apoptotic effects, including protein–protein interactions, direct effects in the mitochondria and relocalization of death receptors to the cell surface.

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Therapeutic induction of apoptosis Another stress pathway that is activated in response to chemotherapy is the stress-activated protein kinase (SAPK, also known as JUN-N-terminal kinase or JNK) pathway. SAPKs, which are members of the mitogen-activated protein kinase family, can regulate the activity of AP-1 transcription factors. Known pro-apoptotic target genes for AP-1 are CD95L and TNF- . Moreover, oxidative stress triggered by the production of reactive oxygen intermediates and glutathione depletion can also induce CD95L expression. The best-defined mechanism by which therapy-induced cellular stress eventually leads to the death of tumour cells — particularly liver tumour cells — involves the CD95 system. Chemotherapeutic drugs (for example, the nucleotide analogue 5-fluoruracil, 5-FU) induce CD95 by a transcriptionally regulated, p53-dependent mechanism. They also engage the SAPK/JNK pathway, which eventually leads to upregulation of CD95L. Upregulation of CD95 and CD95L then allows the cells to either commit suicide or kill neighbouring cells. Clearly, this is not the only pathway of chemotherapy-induced cell death. Many drugs seem to initiate the mitochondrial pathway directly. Moreover, cell death might not even require caspase activation. It is questionable whether a single predominant effector pathway of chemotherapy can be identified at all. Probably, the pathway engaged depends on the stress stimulus, the cell type, the tumour environment and many other factors. However, because chemotherapy and irradiation exert their effects primarily by apoptosis induction, it is conceivable that modulation of the key elements of apoptosis signalling directly influences therapy-induced tumour-cell death.

Slide 18 :

Expression of anti-apoptotic proteins Tumour cells can acquire resistance to apoptosis by various mechanisms that interfere at different levels of apoptosis signalling. One mechanism is the overexpression of anti-apoptotic genes. A common feature of follicular B-cell lymphoma is the chromosomal translocation t(14;18), which couples the BCL2 gene to the immunoglobulin heavy chain locus, leading to enhanced BCL2 expression. BCL2 cooperates with the oncoprotein c-MYC or, in acute promyelocytic leukaemia, the promyelocytic leukaemia–retinoic-acid-receptor- (PML–RAR ) fusion protein, thereby contributing to tumorigenesis. Some studies have shown a correlation between high levels of BCL2 expression and the severity of malignancy of human tumours. Moreover, it has been shown in in vitro and in vivo models that BCL2 expression confers resistance to many kinds of chemotherapeutic drugs and irradiation. In some types of tumours, a high level of BCL2 expression is associated with a poor response to chemotherapy and seems to be predictive of shorter, disease-free survival. The tumour-associated viruses Epstein–Barr virus (EBV) and human herpesvirus 8 (HHV8 or Kaposi's sarcoma-associated herpesvirus) encode proteins that are homologues of BCL2. Both proteins — BHRF1 from EBV and KSbcl-2 (vBcl-2) from HHV8 — have an anti-apoptotic function and enhance survival of the infected cells. In this way, they might contribute to tumour formation after virus infection, and to resistance of these tumours to therapy.

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Expression of anti-apoptotic proteins In addition, other anti-apoptotic BCL2 family members also seem to be involved in resistance of tumours to apoptosis. For example, BCL-XL can confer resistance to multiple apoptosis-inducing pathways in cell lines and seems to be upregulated by a constitutively active mutant epidermal growth factor receptor (EGFR) in vitro. MCL1 (myeloid cell leukaemia sequence 1) can also render cell lines resistant to chemotherapy. In some leukaemia patients, MCL1 expression was increased at the time of relapse, which indicates that some anticancer drugs might select for leukaemia cells that have elevated MCL1 levels. Human melanomas and a murine B-cell lymphoma cell line were shown to express high levels of FLIP, which interferes with apoptosis induction at the level of the death receptors. Moreover, in EBV-positive Burkitt's lymphoma cell lines, an increased FLIP:caspase-8 ratio was correlated with resistance to CD95-mediated apoptosis93. Viral analogues of FLIP, called viral FLIPs (v-FLIPs), are encoded by some tumorigenic viruses, including HHV8. In cells that are latently infected with HHV8, v-FLIP is expressed at low levels, but its expression is increased in advanced Kaposi's sarcomas or on serum withdrawal from lymphoma cells in culture. Therefore, v-FLIPs might contribute to the persistence and oncogenicity of v-FLIP-encoding viruses. Although FLIP expression prevents apoptosis induction through death receptors, it does not inhibit cell death induced by perforin/granzyme, chemotherapeutic drugs or g-irradiation. Nevertheless, it mediates the immune escape of tumours in mouse models. Tumours with high expression levels of FLIP were shown to escape from T-cell-mediated immunity in vivo, despite the presence of the perforin/granzyme pathway, so tumour cells with elevated FLIP levels seem to have a selective advantage. FLIP overexpression also prevents rejection of tumours by perforin-deficient NK cells.

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Expression of anti-apoptotic proteins Another mechanism by which tumours interfere with death-receptor-mediated apoptosis might be the expression of soluble receptors that act as decoys for death ligands. To date, two distinct soluble receptors — soluble CD95 (sCD95) and decoy receptor 3 (DcR3) — have been shown to competitively inhibit CD95 signalling. sCD95 is expressed in various malignancies, and elevated levels can be found in the sera of cancer patients. High sCD95 serum levels were associated with poor prognosis in melanoma patients. DcR3 binds to CD95L and the TNF family member LIGHT (a cytokine that is homologous to lymphotoxins, exhibits inducible expression and competes with herpes simplex virus (HSV) glycoprotein D for herpesvirus entry mediator (HVEM), a receptor expressed by T cells) and inhibits CD95L-induced apoptosis. It is genetically amplified in several lung and colon carcinomas and is overexpressed in several adenocarcinomas, glioma cell lines and glioblastomas. Ectopic expression of DcR3 in a rat glioma model resulted in decreased immune-cell infiltration, which indicates that DcR3 is involved in immune evasion of malignant glioma.

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Expression of anti-apoptotic proteins Expression of the IAP-family protein survivin is highly tumour specific. It is found in most human tumours but not in normal adult tissues. In neuroblastoma, expression correlates with a more aggressive and unfavourable disease. But although survivin has a BIR domain, it is not clear whether it directly acts as an apoptosis inhibitor, for example by binding to caspase-9 or interacting with SMAC/DIABLO. Survivin might also be necessary for completion of the cell cycle. Nevertheless, overexpression of survivin counteracts apoptosis in some settings: in transgenic mice that express survivin in the skin, its anti-apoptotic function was more prominent than its role in cell division. Survivin inhibited UVB-induced apoptosis in vitro and in vivo, whereas it did not affect CD95-induced cell death. Expression of a non-phosphorylatable mutant of survivin induces cytochrome c release and cell death. In xenograft tumour models, this mutant suppressed tumour growth and reduced intraperitoneal tumour dissemination.

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Expression of anti-apoptotic proteins Another IAP family member, cIAP2, is affected by the translocation t(11;18)(q21;q21) that is found in about 50% of marginal cell lymphomas of the mucosa-associated lymphoid tissue (MALT). This indicates a role for cIAP2 in the development of MALT lymphoma. ML-IAP is expressed at high levels in melanoma cell lines, but not in primary melanocytes. Melanoma cell lines that express ML-IAP are significantly more resistant to drug-induced apoptosis than those that do not express ML-IAP. Finally, tumour cells resist killing by cytotoxic lymphocytes not only by blocking the death-receptor pathway, but also by interfering with the perforin/granzyme pathway. Expression of the serine protease inhibitor PI-9/SPI-6, which inhibits granzyme B, results in the resistance of tumour cells to cytotoxic lymphocytes, leading to immune escape.

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Inactivation of pro-apoptotic genes. Besides overexpression of anti-apoptotic genes, tumours can acquire apoptosis resistance by downregulating or mutating pro-apoptotic molecules. In certain types of cancer, the pro-apoptotic BCL2 family member BAX is mutated. Frameshift mutations that lead to loss of expression, and mutations in the BH domains that result in loss of functions, are common. Tumour cell lines with frameshift mutations are more resistant to apoptosis. Reduced BAX expression is associated with a poor response rate to chemotherapy and shorter survival in some situations. Several studies in mice have confirmed the function of Bax as a tumour suppressor. In a transgenic mouse tumour, Bax expression is induced by p53, resulting in slow tumour growth and a high percentage of apoptotic cells. In Bax-deficient mice, however, tumour growth is accelerated and apoptosis decreases, indicating that Bax is required for a full p53-mediated response. In a different study, induction of Bax expression in an inducible cell line restored sensitivity to apoptosis and significantly reduced tumour growth in severe combined immunodeficient (SCID) mice. Moreover, others showed that inactivation of wild-type Bax confers a strong advantage during clonal evolution of the tumour. Injection of clones with either wild-type or mutant Bax into nude mice led to outgrowth of tumours that did not express Bax in both situations.

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Inactivation of pro-apoptotic genes. Metastatic melanomas have found another way to escape mitochondria-dependent apoptosis. These tumours often do not express APAF1, which forms an integral part of the apoptosome, and the APAF1 locus shows a high rate of allelic loss. The remaining allele is transcriptionally inactivated by gene methylation. APAF1-negative melanomas fail to respond to chemotherapy — a situation that is commonly found in this type of tumour. A similar strategy has been reported for neuroblastomas in which the N-MYC oncogene has been amplified. In these tumours, the gene for the initiator caspase-8 is frequently inactivated by gene deletion or methylation. Caspase-8-deficient neuroblastoma cells are resistant to death-receptor- and DOXORUBICIN-mediated apoptosis. Moreover, death receptors are downregulated or inactivated in many tumours. The expression of the death receptor CD95 is reduced in some tumour cells — for example, in hepatocellular carcinomas, neoplastic colon epithelium, melanomas and other tumours — compared with their normal counterparts. Loss of CD95, probably by downregulation of transcription, might contribute to chemoresistance and immune evasion. Oncogenic RAS seems to downregulate CD95, and in hepatocellular carcinomas loss of CD95 expression is accompanied by p53 aberrations.

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Inactivation of pro-apoptotic genes. Several CD95 gene mutations have been reported in primary samples of myeloma and T-cell leukaemia. The mutations include point mutations in the cytoplasmic death domain of CD95 and a deletion that leads to a truncated form of the death receptor. These mutated forms of CD95 might interfere in a dominant-negative way with apoptosis induction by CD95. In families with germ-line CD95 mutations, which usually result in autoimmune lymphoproliferative syndrome (ALPS), the risk of developing lymphomas is increased. Deletions and mutations of the death receptors TRAIL-R1 and TRAIL-R2 have also been observed in tumours. The frequent deletion of the chromosomal region 8p21-22 in head and neck cancer and in non-small-cell lung cancers affects the TRAIL-R2 gene. Mutations have been found in the ectodomain or the death domain of TRAIL-R1 or TRAIL-R2. Further mutations result in truncated forms of these TRAIL receptors or other anti-apoptotic forms. Finally, reduced expression of the pro-apoptotic protein XAF1 (XIAP-associated factor 1) has been observed in various cancer cell lines. XAF1 binds to XIAP and antagonizes its anti-apoptotic function at the level of the caspases.

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Alterations of the p53 pathway As p53 has a central function in apoptosis induction, alterations of the p53 pathway influence the sensitivity of tumours to apoptosis. Tumours that are deficient in Trp53 (the gene that encodes p53 in mice) in immunocompromised mice and cell lineages from transgenic mice that express mutant Trp53 showed a poor response to -irradiation or chemotherapy. Specific mutations in TP53 (the gene that encodes p53 in humans) have been linked to primary resistance to doxorubicin treatment and early relapse in patients with breast cancer141. In cancer cell lines, the specific disruption of the TP53 gene conferred resistance to 5-FU, but greater sensitivity to adriamycin or radiation in vitro142. Mutations of CDKN2A, which encodes ARF (as well as INK4A), are almost as widespread in tumours as are TP53 mutations. Lymphomas from Trp53-knockout mice and from Cdkn2a-knockout mice are highly invasive, display apoptotic defects and are markedly resistant to chemotherapy in vitro and in vivo. In about 70% of breast cancers, wild-type TP53 is expressed but fails to suppress tumour growth. This might be explained by a lack of the ASPP (apoptosis stimulating protein of p53) family of proteins. ASPP proteins interact with p53 and specifically enhance the DNA-binding and transactivation function of p53 on the promoters of proapoptotic genes in vivo. In this way, they stimulate apoptosis induction by p53 and do not affect proliferation. ASPP expression is frequently downregulated in breast carcinomas that express wild-type TP53, resulting in p53 unresponsiveness.

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Altered survival signalling Most tumours are independent of the survival signals that protect normal cells from death by neglect. This is achieved by alterations in the PI3K/AKT pathway. Oncogenes such as RAS or BCR–ABL can increase PI3K activity. The catalytic subunit of PI3K has been shown to be amplified in ovarian cancer. PTEN, the cellular antagonist of PI3K, is frequently deleted in advanced tumours, and a significant rate of PTEN mutations can be found in various cancer types. Moreover, AKT, the serine/threonine kinase that mediates survival signals, is overexpressed in several malignancies. All of these alterations lead to a 'constitutively active' survival signalling pathway that enhances the insensitivity of tumour cells to apoptosis induction.

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Further Mechanism Resistance to chemotherapy can also be attributed to the presence of a molecular transporter that actively expels chemotherapeutic drugs from the tumour cells. The two transporters that are commonly found to confer chemoresistance in cancer are the MDR1 gene products P-glycoprotein and MRP (multidrug resistance-associated protein). P-glycoprotein protects cells not only from chemotherapy-induced apoptosis, but also from other caspase-dependent death stimuli such as CD95L, TNF and UV irradiation. However, it does not confer resistance to the perforin/granzyme pathway. An important factor influencing apoptosis of tumour cells is the transcription factor nuclear factor B (NF- B). Normally, NF- B remains sequestered in an inactive state by the cytoplasmic inhibitor of NF- B (I B) proteins. However, a variety of external stimuli — including cytokines, pathogens, stress and chemotherapeutic agents — can lead to activation of NF- B by phosphorylation, ubiquitylation, and the subsequent degradation of I B. The DNA-binding subunits of NF- B migrate into the nucleus and activate expression of target genes. Depending on the stimulus and the cellular context, NF- B can activate pro-apoptotic genes, such as those encoding CD95, CD95L and TRAIL receptors, and anti-apoptotic genes, such as those encoding IAPs and BCL-XL. Genes encoding NF- B or I B proteins are amplified or translocated in human cancer157. In Hodgkin's disease cells, constitutive activity of NF- B has been observed. The extracellular matrix might also contribute to drug resistance in vivo. Small-cell lung cancer is surrounded by an extensive stroma of extracellular matrix, and adhesion of the cancer cells to the extracellular matrix suppresses chemotherapy-induced apoptosis through integrin signalling. Furthermore, in myeloma, constitutive activation of STAT3 signalling upregulates BCL-XL and so confers resistance to apoptosis.

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Summary Apoptosis is a multi-step, multi-pathway cell-death programme that is inherent in every cell of the body. In cancer, the apoptosis:cell-division ratio is altered, which results in a net gain of malignant tissue. Apoptosis can be initiated either through the death-receptor or the mitochondrial pathway. Caspases that cleave cellular substrates leading to characteristic biochemical and morphological changes are activated in both pathways. The apoptotic process is tightly controlled by various proteins. There are also other caspase-independent types of cell death. Many physiological growth-control mechanisms that govern cell proliferation and tissue homeostasis are linked to apoptosis. Therefore, resistance of tumour cells to apoptosis might be an essential feature of cancer development. Immune cells (T cells & natural killer cells) can kill tumour cells using the granule exocytosis pathway or the death-receptor pathway. Apoptosis resistance of tumour cells might lead to escape from immunosurveillance and might influence the efficacy of immunotherapy. Cancer treatment by chemotherapy and -irradiation kills target cells primarily by inducing apoptosis. Therefore, modulation of the key elements of apoptosis signalling directly influences therapy-induced tumour-cell death. Tumour cells can acquire resistance to apoptosis by the expression of anti-apoptotic proteins or by the downregulation or mutation of pro-apoptotic proteins. Alterations of the p53 pathway also influence the sensitivity of tumour cells to apoptosis. Moreover, most tumours are independent of survival signals because they have upregulated the phosphatidylinositol 3-kinase (PI3K)/AKT pathway.

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Grossary ADAPTIVE IMMUNE SYSTEM Adaptive immunity — also known as specific or acquired immunity — is mediated by antigen-specific lymphocytes and antibodies; it is highly antigen specific and includes the development of immunological memory. ANTIMETABOLITES Antimetabolites (for example, methotrexate) block specific metabolic pathways by competitive binding to the substrate-binding site of enzymes that are involved in metabolism. DOXORUBICIN A chemotherapeutic drug that induces DNA strand breaks, which initiate apoptosis. INNATE IMMUNE SYSTEM The innate immune system includes phagocytes, natural killer cells, the complement system and other non-specific components. It protects against infections using mechanisms that exist before infection, providing a rapid response to microbes that is essentially the same regardless of the type of infection. INTEGRINS A large family of heterodimeric transmembrane proteins that promote adhesion of cells to the extracellular matrix or to other cells. LAMINS A group of intermediate-filament proteins that form the fibrous network (nuclear lamina) on the inner surface of the nuclear envelope. RNA INTERFERENCE (RNAi). Use of double-stranded RNA to target specific mRNAs for degradation, resulting in sequence-specific post-transcriptional gene silencing.

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Specific mutations in TP53 (the gene that encodes p53 in humans) have beenIn cancer cell lines, the more Specific mutations in TP53 (the gene that encodes p53 in humans) have beenIn cancer cell lines, the specific disruption of the TP53 gene conferred … less

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