Apoptosis in Cancer
The correlation between apoptosis and cancer was first considered when the oncogene Bcl-2 was identified and characterized in leukemia and follicular lymphoma (1). It was later found that Bcl-2 is an anti-apoptotic agent that operates in the outer membrane of the mitochondrion. The majority of human cancer cells are able to evade death by employing an anti-apoptotic strategy, suggesting that the ability for normal cells to undergo apoptosis is an important mechanism in the prevention of cancer. By evading apoptosis, neoplastic cells create an opportunity to accumulate genetic mutations, instabilities, and resistance (2). Apoptosis can also function to remove cancer cells as it is utilized in several anti-cancer therapies. Apoptosis may be induced directly via direct toxicity or indirectly by induction of mechanisms upstream of apoptosis. For more about therapies, see Cancer Treatments Targeting Apoptosis.
1) Disruption in the balance of pro-apoptotic and anti-apoptotic factors
Apoptosis is triggered when the balance between anti-apoptotic and pro-apoptotic proteins shift to favor cell death. However, carcinogenesis can occur when pro-apoptotic proteins are underexpressed and/or anti-apoptotic proteins are overexpressed (3).
Anti-apoptotic and pro-apoptotic factors in the Bcl-2 family:
Oncogenic Bcl-2 arises when portions of human chromosome 14 and 18 are exchanged. The mutated bcl-2 gene was found to protect cells from apoptosis in tissue culture and also to cause drug resistance, preventing apoptosis even in drug and toxin treated cells. A combination of apoptosis inhibition by mutant bcl-2 and growth stimulation by oncogene activation of another gene can lead to rapid development of cancers. For example, Bcl-2 and myc overexpression can lead to aggressive B-cell malignancies including lymphoma (9). Bcl-2 belongs toa family of proteins consisting of anti-apoptotic and pro-apoptotic factors (3). Members of this family control the flow of molecules through channels in the mitochondrial membrane. They affect mitochondrial proteins such as cytochrome c and can trigger their release into the cytosol (10). The anti-apoptotic members close mitochondrial channels, sequestering cytochrome c in the mitochondrion while the pro-apoptotic members hold open the channels, allowing cytochrome c to leave and activate apoptosis. Alternatively, anti-apoptotic members can inhibit Apaf-1 (Apoptotic protease activating factor 1) (5). In cancer, the pro-apoptotic Bcl-2 proteins are often downregulated and the anti-apoptotic proteins are upregulated (3). For example, overexpression of Bcl-2 inhibits apoptosis and is found in neuroblastoma, glioblastoma, and breast cancer (4). Overexpression of Bcl-XL or the impaired function of Bax contribute to drug resistance in tumors (2).
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Table 8.3.1 Bcl-2 family of proteins that control the activation of apoptosis.
2) Defects and mutations in p53
The p53 tumor suppressor gene is the most commonly mutated gene in human cancers and defective forms are present in the majority of tumors for all types of cancer (3, 9). When cells are in a state of metabolic disorder or genetic damage, p53 may induce cell cycle arrest, DNA repair, or apoptosis. These functions reduce the risk of transformation and proliferation of genetically damaged genes. The apoptotic pathway is known to be induced by hypoxia or oncogenic mitogenes. Experiments studying cells expressing oncogenic myc proteins showed that the role of p53 in initiating apoptosis is vital in preventing tumor growth progression (9.) Alteration of p53 is a common anti-apoptotic strategy in cancer cells. Silencing of p53 mutations can restore a cell’s susceptibility to apoptosis (3). See more about p53 in Chapter 3. p53 induces apoptosis through two major pathways: p53-independent and p53-dependent (5) shown in Figure 8.3.1.
p53 activates Bax and inhibits anti-apoptotic proteins. The ATM-p53 pathway is found to activate and overexpress KILLER/DR5 resulting in apoptosis (5). KILLER/DR5 is a member of the TNF-related apoptosis-inducing ligand (TRAIL) receptors (5). Humans have four TRAIL receptor types (5):
The p53-independent pathway mainly consists of Fas/APO1 part of the tumor necrosis factor receptor family (5). The Fas/APO1 receptor contains an extracellular cysteine-rich domain and an intracellular death domain (5). When the Fas/APO1 receptor binds its FasL ligand, trimerization of the receptor occurs and the death domain recruits FADD (5). FADD has a death receptor domain that recruits caspase 8 to its death-inducing signaling complex resulting in the activation of a caspase cascade (5). p53 increases the number of Fas/APO1 receptors but apoptosis through this receptor remains independent of p53 (5). The IGF-RP3 gene blocks prosurvival signals by binding to IGF-1 (5). p53 is known to target IGF-RP3 to induce apoptosis, however, apoptosis by IGF-RP3 can still occur in the absence of p53 (5). Any sort of mutational inactivation or reduction in p53 can result in apoptosis inhibition and carcinogenesis.
3) Increased expression of IAPs
The inhibitor of apoptosis proteins (IAP) are another group of anti-apoptotic proteins. They are characterized by a conserved baculovirus IAP repeat (BIR) domain (3) and their ability to inhibit caspase activity by binding to their active sites via the IAP BIR domains (3). This interaction results in caspase degradation or segregation from their substrates (3). Abnormal expression of IAPs has been reported in pancreatic cancer where cIAP-2 is overexpressed (6). Upregulation of IAPs are not only involved in tumor progression but also drug resistance (3). It is found that upregulation of IAPs in melanoma and lymphoma result in cisplatin and camptotecin resistance (3). The most commonly overexpressed IAP is Survivin (3), contributing to many cancer malignancies (3).
There are eight IAPs in humans, of which four (c-IAP1, c-IAP2, ML-IAP and XIAP) impact apoptotic pathways directly. C-IAP1 and c-IAP2 are implicated in the extrinsic apoptotic pathway; by ubiquitinating RIP-1 (a death-domain containing kinase responsible for binding tumor necrosis factor receptor 1, TNFR1), which inhibits RIP-1 binding to FADD and caspase-8, thus preventing apoptosis from being induced. XIAP, however is implicated in the intrinsic pathway of apoptosis, by binding caspase 3, 7 and 9, thus inhibiting their function (11).
XIAP and c-IAP1 overexpression has been observed in cancer cell lines, making IAPs attractive putative targets for cancer therapeutics. Depending upon the IAP in question, overexpression of these proteins can be associated with poor prognosis. For example, c-IAP protein overexpression has been associated with resistance to treatment (e.g. in cervical cancer and myeloma). Fusion proteins between c-IAP2 and MALT1 have been detected in mucosa-associated lymphoid tissues, which have been discussed by Almagro et al. to be the result of gene translocation. The described fusion protein subsequently acts as a pro-oncogene, inducing constitutive activation of NF-kB and its downstream pathways. XIAP in non-small cell lung cancer cases has however shown good prognosis, due to an association with high sensitivity to chemotherapeutic agents (cytarabine and nucleoside analogues). ML-IAP overexpression is specific to melanomas, in which it has been observed to have an anti-apoptotic effect, an association with increased expression of MITF (Microphthalmia-associated transcription factor) oncogene, and poor prognosis (11).
Current mechanisms for targeting IAP proteins include; ‘Smac-derived peptides, small molecule antagonists and antisense oligonucleotides’ (). So-called monovalent IAP antagonists confer one binding domain that mimics that of Smac (an IAP protein inhibitor), whereas bivalent antagonists confer two Smac-mimicking binding motifs, which further enhances the antagonistic effect by being able to bind multiple domains simultaneously. c-IAP1 antagonists have been discovered to function by inducing a change in c-IAP conformation, leading to the dimerization of a c-IAP RING domain, which leads to c-IAP autiubiquitination and subsequently the c-IAP being degraded at the proteasome. Combination therapy of IAP antagonists with TNFa has been observed to result in apoptotic cell death, yet in the case of FADD or caspase 8 being rendered dysfunctional, the treated cells will enter necrosis (11).
4) Reduction in Caspase Activity
There are two major groups of caspases: the initiators, which activate the apoptosis pathway, and the effectors that carry out cleavage of cellular components (3). Caspases are the most important players in apoptosis (3). Downregulation of caspases can result in evasion of cell death and the dowregulation of specific caspases is often associated with specific cancer phenotypes. In colorectal cancer, caspase-9 is frequently downregulated (3). Levels of caspase-3 mRNA in breast, ovarian, and cervical cancers are almost undetectable (3). Restoration of caspase-3 activity in these cells can restore their susceptibility to apoptosis inducers (3). In some cases such as choriocarcinoma, there is dual downregulation of caspase 8 and caspase 10 (7).
This reduction in caspase activity also has implications that exceed a cell's attempted initation of apoptosis. While apoptosis is classically considered a "cellular suicide" or pre-programmed cellular death pathway, it can also be triggered via extracellular mechanisms that are independent of the cell's recognition of its own dysfunction. The apoptotic caspase pathway is also harnessed by the immune system to kill target cells. In fact, caspase-dependent cell death is one of the central mechanisms by which Cytotoxic T Lymphocytes and Natural Killer cells kill target cells, such as in cancer; this is a mechanism that centers around Granzyme B. Granzyme B is a serine protease that is contained in granules within the CTL or NK cell, and is the most abundant in the granzyme family. It is released into target cells upon recognition through a pathway that also requires a protein called perforin. Upon entry into the cell, Granzyme B can cleave its particularly narrow range of substrates, the caspases. This cleavage results in caspase activation, and subsequent cleavage of other caspases to initiate the apoptotic pathway. the narrow specificity of Granzyme B means that it must work through the caspase pathway, and thus cells what have down-regulated their caspases will have an inhibited response. Thus, cells with decreased caspase levels not only have a reduction in their ability to initiate their own death, but also have an increased tolerance to immune recognition(12).
5) Impaired Death Receptor Signals
The extrinsic pathway is laden with death receptors and their ligands. These receptors share the common characteristic of death domains which recruit factors that trigger cell death. There are four major ways in which cells evade the extrinsic pathway:
Decoy receptors can bind to the same ligands as death receptors but lack a death domain (3). Loss of any death receptors or their functions contribute to carcinogenesis. For example, the loss of Fas and dysregulation of FasL, DR4, and DR5 are responsible for cervical carcinogenesis (8). Therefore, we find that cancer cells adapt several mechanisms to evade cell death.
A video that covers some parts of apoptosis in cancer for review: http://www.youtube.com/watch?v=8VSgOeJy4dQ
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