Identification of a Novel Anti-apoptotic E3 Ubiquitin Ligase That Ubiquitinates Antagonists of Inhibitor of Apoptosis Proteins SMAC, HtrA2, and ARTS (original) (raw)
Cell, 2005
members of the family promote the release of apoptogenic proteins from mitochondria. The fate of a cell un-and Xiaodong Wang* Howard Hughes Medical Institute and der apoptotic stimulation is determined by the balance between the pro-and antiapoptotic members of the Department of Biochemistry University of Texas Southwestern Medical Center Bcl-2 family. Therefore, studying the regulation of such balance will be critically important for our understand-at Dallas Dallas, Texas 75390 ing of apoptosis regulation. Among the antiapoptotic members of the Bcl-2 family proteins, Mcl-1 is unique in that it is an early-response Summary gene that can be rapidly induced and turned over (Kozopas et al., 1993; Yang et al., 1995; Yang et al., 1996). The elimination of Mcl-1, an anti-apoptotic Bcl-2 fam-This property enables Mcl-1 to function at an early step ily member, is an early and required step for DNA in a signaling cascade, consisting of Bcl-2 family prodamage-induced apoptosis. The degradation of Mcl-1 teins, and provides an acute protective function against can be blocked by proteasome inhibitors, suggesting apoptosis induced by a variety of stimuli, including a role for the ubiquitin proteasome pathway in apopto-DNA damage, adenoviral infection, growth factors' sis. Here, we demonstrate that Mcl-1 is ubiquinated at withdrawal, and treatment of cytotoxic agents (Cucofive lysines. Biochemical fractionation of cell extracts nati et al., 2003; Derouet et al., 2004; Huang et al., 2000; allowed us to identify a 482 kDa HECT-domain-contain-Le Gouill et al., 2004; Nijhawan et al., 2003; Piret et al., ing ubiquitin ligase named Mule (Mcl-1 ubiquitin ligase 2004; Zhang et al., 2002; Zhou et al., 1997). Consis-E3) that is both required and sufficient for the polytently, disappearance of Mcl-1 is associated with the ubiquitination of Mcl-1. Mule also contains a region onset of apoptosis and is achieved by the combination similar to the Bcl-2 homology region 3 (BH3) domain of synthesis blockage and continuous degradation (Cuthat allows Mule to specifically interact with Mcl-1. conati et al., 2003; Nijhawan et al., 2003). Elimination of Mule expression by RNA interference The degradation of Mcl-1 in HeLa cells can be stabilizes Mcl-1 protein, resulting in an attenuation of blocked by proteasome inhibitors, suggesting a role for the apoptosis induced by DNA-damage agents. Thus, the ubiquitin proteasome pathway in apoptosis up-Mule is a unique BH3-containing E3 ubiquitin ligase stream of Bcl-2 family of proteins (Derouet et al., 2004; apical to Bcl-2 family proteins during DNA damage-Nencioni et al., 2004; Cuconati et al., 2003; Nijhawan et induced apoptosis. al., 2003). In the current report, we demonstrate that Mcl-1 is polyubiquitinated in vivo and in vitro. Using Introduction HeLa cell extracts as a source, we established a cellfree assay for Mcl-1 ubiquitination. Biochemical frac-Apoptosis is a form of cell death orchestrated by chains tionation of HeLa cell extracts allowed us to identify of biochemical reactions. Cells undergoing apoptosis and purify a 482 kDa E3 ubiquitin ligase that mediated show characteristic morphological features such as the polyubiquitination of Mcl-1. This protein turned out condensation of cytoplasmic and nuclear contents, to be a novel homologous to E6-AP carboxyl terminus blebbing of plasma membranes, fragmentation of nu-(HECT)-domain-containing ubiquitin ligase. We named clei, and ultimately breakdown into membrane bound this protein Mule, for Mcl-1 ubiquitin ligase E3. apoptotic bodies that are rapidly phagocytized (Kerr et al., 1972). Results Mitochondria play an important role in regulating apoptosis induced by intracellular damaging signals Ubiquitination of Mcl-1 In Vivo and In Vitro such as DNA damage in mammalian cells (Danial and Proteins targeted for proteasome degradation are usu-Korsmeyer, 2004). Apoptotic stimuli exert their effects ally modified by a polyubiquitin chain (Hershko and on mitochondria to cause the release of proapoptotic Ciechanover, 1998). To test whether Mcl-1 is ubiqfactors like cytochrome c and Smac/Diablo. These facuitinated, we treated HeLa cells expressing Flag-Mcl-1 tors either directly activate caspases, a group of intrawith a proteasome inhibitor MG132 to block the procellular cysteine proteases that execute apoptosis by teasome activity and performed immunoprecipitation cleaving their substrates or releasing caspase inhibition analysis for Flag-Mcl-1 to test for the accumulation of imposed by the inhibitor-of-apoptosis proteins (IAPs; higher-molecular-weight forms of Mcl-1 that could be Du et al., 2000; Liu et al., 1996; Verhagen et al., 2000). ubiquitin modified. As shown in Figure 1A, higher-Mitochondrial response to apoptotic stimuli is regumolecular-weight protein bands recognized by antilated by the pro-and antiapoptotic Bcl-2 family of pro-Mcl-1 antibody were indeed accumulated upon MG132 teins (Gross et al., 1999; Martinou and Green, 2001). treatment (Figure 1A, lanes 3 and 4). To rule out a possi-Antiapoptotic proteins such as Bcl-2, Bcl-xL, and Mcl-1 bility that these higher-molecular-weight bands were protect mitochondrial integrity, while the proapoptotic from Mcl-1-associated protein rather than Mcl-1 itself, the protein mixture was first heated in the presence of 1% SDS and 5 mM DTT to disassociate protein-protein
PloS one, 2011
Here we identified an evolutionarily highly conserved and ubiquitously expressed protein (C9orf82) that shows structural similarities to the death effector domain of apoptosis-related proteins. RNAi knockdown of C9orf82 induced apoptosis in A-549 and MCF7/casp3-10b lung and breast carcinoma cells, respectively, but not in cells lacking caspase-3, caspase-10 or both. Apoptosis was associated with activated caspases-3, -8, -9 and -10, and inactivation of caspases 10 or 3 was sufficient to block apoptosis in this pathway. Apoptosis upon knockdown of C9orf82 was associated with increased caspase-10 expression and activation, which was required for the generation of an 11 kDa tBid fragment and activation of Caspase-9. These data suggest that C9orf82 functions as an anti-apoptotic protein that modulates a caspase-10 dependent mitochondrial caspase-3/9 feedback amplification loop. We designate this ubiquitously expressed and evolutionarily conserved anti-apoptotic protein Conserved Anti-Ap...
… International Journal of …, 2011
ARTS (Sept4 i2) is a mitochondrial pro-apoptotic tumor suppressor protein. In response to apoptotic signals, ARTS translocates to the cytosol where it promotes caspase activation through caspase de-repression and proteasome mediated degradation of X-linked Inhibitor of Apoptosis Protein (XIAP). Here we show that XIAP regulates the levels of ARTS by serving as its ubiquitin ligase, thereby providing a potential feedback mechanism to protect against unwanted apoptosis. Using both in vitro and in vivo ubiquitination assays we found that ARTS is directly ubiquitinated by XIAP. Moreover, we found that XIAP-induced ubiquitination and degradation is prevented by removal of the first four amino acids in the N-terminus of ARTS, which contains a single lysine residue at position 3. Thus, this lysine at position 3 is a likely target for ubiquitination by XIAP. Importantly, although the stabilized ARTS lacking its first 4 residues binds XIAP as well as the full length ARTS, it is more potent in promoting apoptosis than the full length ARTS. This suggests that increased stability of ARTS has a significant effect on its ability to induce apoptosis. Collectively, our data reveal a mutual regulatory mechanism by which ARTS and XIAP control each other's levels through the ubiquitin proteasome system.
Journal of Biological Chemistry, 2001
To identify human proteins that bind to the Smac and caspase-9 binding pocket on the baculoviral inhibitor of apoptosis protein (IAP) repeat 3 (BIR3) domain of human XIAP, we used BIR3 as an affinity reagent, followed by elution with the BIR3 binding peptide AVPIA, microsequencing, and mass spectrometry. The mature serine protease Omi (also known as HtrA2) was identified as a mitochondrial direct BIR3-binding protein and a caspase activator. Like mature Smac (also known as Diablo), mature Omi contains a conserved IAP-binding motif (AVPS) at its N terminus, which is exposed after processing of its N-terminal mitochondrial targeting sequence upon import into the mitochondria. Mature Omi is released together with mature Smac from the mitochondria into the cytosol upon disruption of the outer mitochondrial membrane during apoptosis. Finally, mature Omi can induce apoptosis in human cells in a caspase-independent manner through its protease activity and in a caspase-dependent manner via its ability to disrupt caspase-IAP interaction. Our results provide clear evidence for the involvement of a mitochondrial serine protease in the apoptotic pathway, emphasizing the critical role of the mitochondria in cell death.
Regulation of the Proapoptotic ARTS Protein by Ubiquitin-mediated Degradation
Journal of Biological Chemistry, 2005
ARTS is a mitochondrial protein that promotes apoptosis induced by a variety of proapoptotic stimulators. ARTS induces apoptosis, at least in part, through binding to and antagonizing IAPs (inhibitors of apoptosis proteins). As a result of ARTS binding to IAPs, caspase inhibition is removed and apoptosis can be executed. Here we show that high cellular levels of ARTS protein sensitize cells toward apoptosis. Accordingly, in healthy cells ARTS levels are kept low through constant ubiquitin-mediated degradation. Upon proapoptotic stimuli, the ubiquitination process is inhibited, resulting in increased levels of ARTS. Increased ARTS in turn leads to a decrease of Bcl-2 and Bcl-xL protein levels, cytochrome c release from mitochondria and apoptosis.
Regulation of Apoptosis by Inhibitors of Apoptosis (IAPs)
2013
Inhibitors of Apoptosis (IAPs) are a family of proteins with various biological functions including regulation of innate immunity and inflammation, cell proliferation, cell migration and apoptosis. They are characterized by the presence of at least one N-terminal baculoviral IAP repeat (BIR) domain involved in protein-protein interaction. Most of them also contain a C-terminal RING domain conferring an E3-ubiquitin ligase activity. In drosophila, IAPs are essential to ensure cell survival, preventing the uncontrolled activation of the apoptotic protease caspases. In mammals, IAPs can also regulate apoptosis through controlling caspase activity and caspase-activating platform formation. Mammalian IAPs, mainly X-linked IAP (XIAP) and cellular IAPs (cIAPs) appeared to be important determinants of the response of cells to endogenous or exogenous cellular injuries, able to convert the survival signal into a cell death-inducing signal. This review highlights the role of IAP in regulating apoptosis in Drosophila and Mammals.
Regulation of apoptosis by XIAP ubiquitin-ligase activity
Genes & Development, 2008
Because IAPs are frequently overexpressed in human tumors, they have become major pharmacological targets for developing new cancer therapeutics. However, the precise physiological function of individual mammalian IAPs and their role as E3 ubiquitin-ligases in situ remain largely obscure. Here, we investigated the function of XIAP ubiquitin-ligase activity by inactivating the RING motif via gene targeting in the mouse. Removing the RING stabilized XIAP in apoptotic thymocytes, demonstrating that XIAP ubiquitin-ligase activity is a major determinant of XIAP protein stability. Surprisingly, the increased amounts of "XIAP-BIR-only" protein did not lead to attenuated but rather increased caspase activity and apoptosis. ⌬RING embryonic stem cells and fibroblasts had elevated caspase-3 enzyme activity, and XIAP ⌬RING embryonic fibroblasts were strongly sensitized to TNF-␣-induced apoptosis. Similar results were obtained with XIAP deficient mice. Furthermore, deletion of the RING also improved the survival of mice in the Eµ-Myc lymphoma model. This demonstrates a physiological requirement of XIAP ubiquitin-ligase activity for the inhibition of caspases and for tumor suppression in vivo.
Ubiquitylation in apoptosis: a post-translational modification at the edge of life and death
Nature Reviews Molecular Cell Biology, 2011
Apoptosis is mediated by the assembly of signalling complexes that culminates in the activation of a cell death programme. It is evident that these complexes are subject to substantial post-translational regulation through modification by the 76-amino-acid protein ubiquitin. The ubiquitylation of constituent components in the apoptotic pathway often destabilizes them by targeting them for proteasomal degradation. However, just as importantly, the ubiquitin chain-mediate d assembl y of apoptotic signalling complexes illuminates how ubiquityl ation can have non-degradative functions. Recent progress provides insight into how these seemingly disparate outcomes of ubiquitylatio n in apoptosis are mediated. This Review addresses the intersection of these two exciting fields: apoptosis and the ubiquitin-proteasom e system (UPS). We first describe the biochemistry of ubiquitylation and the various forms of ubiquitin modification, ranging from monoubiquitylation to linkage-specific polyubiquitin chain formation. This is followed by an introduction to the apoptotic pathways. We then discuss how apoptotic pathways are regulated by various components of the UPS, including ubiquitin E3 ligases and deubiquitinases (DUBs), before describing how the deregulation of ubiquitylation, and subsequently of apoptosis, can result in human diseases, such as cancer.
Targeting Inhibitors of Apoptosis Proteins (IAPs) For New Breast Cancer Therapeutics
Journal of Mammary Gland Biology and Neoplasia, 2012
Apoptosis resistance is a hallmark of human cancer. Research in the last two decades has identified key regulators of apoptosis, including inhibitor of apoptosis proteins (IAPs). These critical apoptosis regulators have been targeted for the development of new cancer therapeutics. In this article, we will discuss three members of IAP proteins, namely XIAP, cIAP1 and cIAP2, as cancer therapeutic targets and the progress made in developing new cancer therapeutic agents to target these IAP proteins. Apoptosis and human cancer Apoptosis, or programmed cell-death, is a fundamental cellular process to remove damaged or unwanted cells in multiple cellular organisms. Improper regulation of apoptosis is therefore linked in many human diseases, including cancer, autoimmune diseases, inflammation and neurological diseases. 1-3 In fact, defective regulation of apoptosis is a hallmark of human cancer. 4 Basic apoptosis pathways Apoptosis is a tightly regulated process. Several major apoptosis pathways have been identified and characterized in the last two decades, although these pathways often have extensive cross-talks. The intrinsic and extrinsic apoptosis pathways are two of the best studied (Figure 1). 5 The intrinsic, or mitochondria, apoptotic pathway integrates a variety of cell stress signals and is initiated by permeabilization of the outer membrane of mitochondria and loss of mitochondrial potential. On the molecular level, the intrinsic pathway involves the translocation and oligomerization of Bax or Bak, members of the Bcl-2 family proteins, which forms a pore in the outer member of mitochondria and leads to the release of proapoptotic molecules such as cytochrome C. Upon its release from mitochondria into cytosol, cytochrome C, together with dATP, Apaf-1 and procaspase-9, forms the apoptosome, which processes the procaspase-9 zymogen into the active form of caspase-9. Caspase-9 then cleaves and activates caspase-3,-6 and-7, which leads to further processing of downstream cell-death substrates, and ultimately apoptosis. The extrinsic, or death-receptor, apoptotic pathway, is initiated by the binding of death ligands such as Fas/Apo-1, TNF-alpha, Apo2L/TRAIL, and Apo3L ligands to their cognate cell-surface receptors, FasR, TNFR1, DR4/DR5 and DR3, respectively. The binding of these Phone:
Review: Mitochondrial apoptotic pathways
BIOCELL, 2005
Apoptosis or programmed cell death (PCD) is a physiological process characteristic of pluricellular organisms leading to self-destruction of the cell. It is therefore involved in development, homeostasis and host defense. However, a significant difference has been shown between mammalian cell apoptosis and non-mammalian cell apoptosis: mitochondria are implicated only in the former. Execution of PCD includes the release of several proapoptotic proteins from the intermembrane space of mitochondria. They could exert their actions through a caspase dependent as well as a caspase independent way. On the other hand, regulation of PCD is mainly given by the Bcl-2 family members, which are in turn essentially regulated by activation of death receptors and/or DNA damage. Nowadays, execution of apoptosis is better known than its regulation. Nevertheless, we are still far of a complete understanding of the apoptotic process.
Cell, 2000
cytochrome c complex then recruits the initiator caspase of this pathway, procaspase-9 and induces its autoactivation (Li et al., 1997b; Zou et al., 1999). Caspase-9 in turn activates downstream caspases including caspase-3, -6, and -7 (Li et al., 1997b; Srinivasula et al., ). University of Texas Southwestern Proteins of the Bcl-2 family are major regulators of Medical Center at Dallas the mitochondria-initiated caspase activation pathway Dallas, Texas 75235 (reviewed by Adams and Cory, 1998). The anti-apoptotic members of this family, including Bcl-2 and Bcl-X L , preserve mitochondrial integrity and prevent the release of Summary cytochrome c in the presence of apoptotic stimuli (Kluck et al., 1997; Yang et al., 1997). Conversely, the proapo-We report here the identification of a novel protein, ptotic members of this family such as Bad, Bax, Bid, Smac, which promotes caspase activation in the cytoand Bim move from other cellular compartments to mitochrome c/Apaf-1/caspase-9 pathway. Smac promotes chondria in response to apoptotic stimuli and promote caspase-9 activation by binding to inhibitor of apocytochrome c release (reviewed by Gross et al., 1999). Inhibitors of apoptosis proteins, IAPs, are another ptosis proteins, IAPs, and removing their inhibitory family of proteins that regulate the cytochrome c/Apaf-1 activity. Smac is normally a mitochondrial protein but caspase activating pathway (reviewed by Deveraux is released into the cytosol when cells undergo apoand Reed, 1999). Initially identified in the genome of a ptosis. Mitochondrial import and cleavage of its signal baculovirus as suppressors of apoptosis in host cells, peptide are required for Smac to gain its apoptotic endogenous IAPs have been found in a variety of organactivity. Overexpression of Smac increases cells' senisms including seven in mammals so far (Crook et al., sitivity to apoptotic stimuli. Smac is the second mito-1993; Deveraux and Reed, 1999). The antiapoptotic acchondrial protein, along with cytochrome c, that protivity of IAPs has been attributed to the conserved bacumotes apoptosis by activating caspases. lovirus IAP repeat (BIR) domain (Takahashi et al., 1998). Three human IAPs, XIAP, c-IAP-1, and c-IAP-2 have Introduction been shown to bind procaspase-9 and prevent its activation (Deveraux et al., 1998). These IAPs can also di-One of the key regulatory steps for apoptosis is the rectly bind and inhibit active caspases (Deveraux et al., activation of caspases, the intracellular cysteine prote-1998). In Drosophila, the anti-apoptotic activity of IAPs ases that cleave substrates after aspartic acid residues is countered by Reaper, Hid, and Grim (Vucic et al., (reviewed by Thornberry and Lezebnik, 1998). Existing 1998; McCarthy and Dixit, 1998; Goyal et al., 2000). as inactive zymogens in living cells, apoptotic caspases Mammalian homologs of Reaper, Hid, and Grim have become activated during apoptosis either through autonot been identified. catalysis or cleavage by other caspases. Active caspases Despite significant progress in dissecting the cytothen cleave many important intracellular substrates, chrome c-mediated apoptotic pathway, several puzleading to the characteristic morphological changes aszling experimental observations remain to be explained. sociated with apoptotic cells. These changes include First of all, certain types of cells are responsive to michromatin condensation, DNA fragmentation into nucleocroinjected cytochrome c while others are not (Li et somal fragments, nuclear membrane break down, exteral., 1997a). Second, healthy neurons do not respond nalization of phosphatidylserine, and formation of apoto microinjected cytochrome c unless they have been ptotic bodies that are readily taken up by phagocytosis subjected to NGF withdrawal for a certain period of time, gaining the status of "competent to die" (Deshmukh and (reviewed by Thornberry and Lezebnik, 1998).
Caspase-9-induced Mitochondrial Disruption through Cleavage of Anti-apoptotic BCL-2 Family Members
Journal of Biological Chemistry, 2007
Mitochondrial disruption during apoptosis results in the release of cytochrome c that forms apoptosomes with Apaf-1 and caspase-9. Activation of caspase-9 by dimerization in apoptosomes then triggers a caspase signaling cascade. In addition, other apoptosis signaling molecules released from the mitochondrion, such as apoptosis-inducing factor and endonuclease G, may induce caspase-9-independent apoptosis. To determine the signaling events induced by caspase-9, we used chemically induced dimerization for specific activation of caspase-9. We observed that caspase-9 dimerization resulted in the loss of mitochondrial membrane potential and the cleavage of anti-apoptotic Bcl-2, Bcl-xL, and Mcl-1. Moreover, cleavage-resistant Bcl-2, Bcl-xL, or Mcl-1 potently inhibited caspase-9-dependent loss of mitochondrial membrane potential and the release of cytochrome c. Our data suggest that a caspase-9 signaling cascade induces feedback disruption of the mitochondrion through cleavage of anti-apoptotic Bcl-2, Bcl-xL, and Mcl-1. In mitochondrion-dependent apoptosis, exposure of cells to various stress signals triggers the disruption of the mitochondrion, leading to the release of cytochrome c into the cytosol (1-6). The formation of apoptosome containing cytochrome c, Apaf-1, and caspase-9 results in the activation of caspase-9 in apoptosome through dimerization (7-11). Caspase-9 then triggers a caspase signaling cascade to induce apoptosis. In addition to cytochrome c, the disruption of the mitochondrion also leads to the release of other factors. Apoptosis-inducing factor and endonuclease G released from the mitochondrion may induce caspase-9-independent apoptosis (12-15). The Bcl-2 family proteins are important for regulating the integrity of the mitochondrion. Bcl-2 family members share the Bcl-2 homology (BH) 3 domains and can be divided into three subfamilies (16-18), including the anti-apoptotic subfamily
PubMed, 2012
Cell death regulation is vital for maintenance of homeostasis and proper development of multicellular organisms. Inhibitor of apoptosis (IAP) proteins are implicated in multiple ways in cell death regulation, ranging from inhibition of apoptosis and necrosis to the regulation of cell cycle and inflammation. Due to their prominent ability to control cell death and elevated expression in a variety of cancer cell types, IAP proteins are attractive targets for the development of novel anti-cancer treatments. The most widely used strategy for targeting IAP proteins is based on mimicking the natural IAP antagonist, SMAC/DIABLO. IAP antagonists are currently being tested in humans and they were designed for anti-cancer therapy but they could potentially also be considered for treatments of the immune system disorders. In this manuscript we will review the functional roles of IAP proteins, specifically of c-IAP1, c-IAP2, ML-IAP and XIAP, and evaluate IAP targeting strategies for disease treatments. This article is part of a Special Issue entitled "Apoptosis: Four Decades Later".
The role of mitochondrial factors in apoptosis: a Russian roulette with more than one bullet
Cell Death and Differentiation, 2002
Mitochondria are`life-essential' organelles for the production of metabolic energy in the form of ATP. Paradoxically mitochondria also play a key role in controlling the pathways that lead to cell death. This latter role of mitochondria is more than just a`loss of function' resulting in an energy deficit but is an active process involving different mitochondrial proteins. Cytochrome c was the first characterised mitochondrial factor shown to be released from the mitochondrial intermembrane space and to be actively implicated in apoptotic cell death. Since then, other mitochondrial proteins, such as AIF, Smac/DIABLO, endonuclease G and Omi/HtrA2, were found to undergo release during apoptosis and have been implicated in various aspects of the cell death process. Members of the Bcl-2 protein family control the integrity and response of mitochondria to apoptotic signals. The molecular mechanism by which mitochondrial intermembrane space proteins are released and the regulation of mitochondrial homeostasis by Bcl-2 proteins is still elusive. This review summarises and evaluates the current knowledge concerning the complex role of released mitochondrial proteins in the apoptotic process.
Substrates of IAP Ubiquitin Ligases Identified with a Designed Orthogonal E3 Ligase, the NEDDylator
Molecular Cell, 2013
Inhibitors of Apoptosis Protein (IAPs) are guardian ubiquitin ligases that keep classic proapoptotic proteins in check. Systematic identification of additional IAP substrates is challenged by the heterogeneity and sheer number of ubiquitinated proteins (>5,000). Here we report a powerful catalytic tagging tool, the NEDDylator, which fuses a NEDD8 E2conjugating enzyme, Ubc12, to the ubiquitin ligase, XIAP or cIAP1. This permits transfer of the rare ubiquitin homolog NEDD8 to the ubiquitin E3 substrates, allowing them to be efficiently purified for LC-MS/MS identification. We have identified >50 potential IAP substrates of both cytosolic and mitochondrial origin that bear hallmark N-terminal IAP binding motifs. These substrates include the recently discovered protein phosphatase PGAM5, which we show is proteolytically processed, accumulates in cytosol during apoptosis, and sensitizes cells to death. These studies reveal mechanisms and antagonistic partners for specific IAPs, and provide a powerful technology for labeling binding partners in transient protein-protein complexes.
Apoptosis in cancer: Key molecular signaling pathways and therapy targets
Acta Oncologica, 2009
Apoptosis is a physiological process vital for embryologic development and the maintenance of homeostasis in multicellular organisms, but it is also involved in a wide range of pathological processes, including cancer. In mammalian cells, apoptosis has been divided into two major pathways: the extrinsic pathway, activated by proapoptotic receptor signals at the cellular surface, and the intrinsic pathway, which involves the disruption of mitochondrial membrane integrity. Although many of the proteins vital for apoptosis have been identified, the molecular pathways of cellular death still remain to be elucidated. This review provides references concerning the apoptotic molecules, their interactions, the mechanisms involved in apoptosis resistance, and also the modulation of apoptosis for the treatment of cancer.
Journal of Biological Chemistry, 2004
Smac/DIABLO, HtrA2/Omi, and caspase-9 play key roles in the initiation of apoptosis. The inhibitor of apoptosis proteins (IAPs) are believed to bind to the N-terminal IAP binding motifs of the mature (proteolytically processed) forms of Smac, HtrA2, and caspase-9. However, we show here that BRUCE/Apollon, a 528-kDa IAP whose degradation promotes apoptosis, associates with their precursors as well as the mature forms by binding to regions in addition to the IAP binding motif. Through these associations, BRUCE promotes the degradation of Smac and inhibits the activity of caspase-9 but not the effector caspase, caspase-3. In response to apoptotic stimuli, BRUCE is degraded by proteasomes and/or cleaved by caspases and HtrA2 depending on the specific stimulus and the cell type. These results suggest that the ability of BRUCE to antagonize both the precursor and mature forms of Smac and caspase-9 is an important mechanism for the prevention of apoptosis under normal conditions. In response to apoptotic stimuli, caspases are activated to initiate irreversible proteolytic cascades that result in the destruction of key cellular components and rapid cell death (1, 2). Activation of these cascades can be triggered by the release of cytochrome c from mitochondria (3). Cytochrome c then binds to Apaf-1 and activates the initiator caspase-9, which cleaves and activates the effector caspases, such as caspases-3 and-7. The activity of caspases can be inhibited by the inhibitor of apoptosis protein (IAP) 1 family of proteins, which are defined by the presence of one to three tandem baculoviral IAP repeats (BIRs) (3, 4). On the other hand, the actions of IAP proteins are opposed by certain pro-apoptotic factors, especially Smac/DIA-BLO and HtrA2/Omi. These proteins contain the specific IAP