Cloning and characterization of an inhibitor of apoptosis protein (IAP) from Bombyx mori.

We cloned a novel inhibitor of apoptosis protein (IAP) family member, BmIAP, from Bombyx mori BmN cells. BmIAP contains two baculoviral IAP repeat (BIR) domains followed by a RING domain. BmIAP shares striking amino acid sequence similarity with lepidopteran IAPs, SfIAP and TnIAP, and with two baculoviral IAPs, CpIAP and OpIAP, suggesting evolutionary conservation. BmIAP blocks programmed cell death (apoptosis) in Spodoptera frugiperda Sf-21 cells induced by p35 deficient Autographa californica nucleopolyhedrovirus (AcMNPV). This anti-apoptotic function requires both the BIR domains and RING domain of BmIAP. In mammalian cells, BmIAP inhibits Bax induced but not Fas induced apoptosis. Further biochemical data suggest that BmIAP is a specific inhibitor of mammalian caspase-9, an initiator caspase in the mitochondria/cytochrome-c pathway, but not the downstream effector proteases, caspase-3 and caspase-7. These results suggest that suppression of apoptosis by lepidopteran IAPs in insect cells may involve inhibition of an upstream initiator caspase in the conserved mitochondria/cytochrome-c pathway for apoptosis.

TRAF1 is a substrate of caspases activated during tumor necrosis factor receptor-alpha-induced apoptosis.

TRAF family proteins are signal-transducing adapter proteins that interact with the cytosolic domains of tumor necrosis factor (TNF) family receptors. Here we show that TRAF1 (but not TRAF2-6) is cleaved by certain caspases in vitro and during TNF-alpha- and Fas-induced apoptosis in vivo. (160)LEVD(163) was identified as the caspase cleavage site within TRAF1, generating two distinct fragments. Significant enhancement of TNF receptor-1 (CD120a)- and, to a lesser extent, Fas (CD95)-mediated apoptosis was observed when overexpressing the C-terminal TRAF1 fragment in HEK293T and HT1080 cells. The same fragment was capable of potently suppressing TNF receptor-1- and TRAF2-mediated nuclear factor-kappaB activation in reporter gene assays, providing a potential mechanism for the enhancement of TNF-mediated apoptosis. Cell death induced by other death receptor-independent stimuli such as cisplatin, staurosporine, and UV irradiation did not result in cleavage of TRAF1, and overexpression of the C-terminal TRAF1 fragment did not enhance cell death in these cases. TRAF1 cleavage was markedly reduced in cells that contain little procaspase-8 protein, suggesting that this apical protease in the TNF/Fas death receptor pathway is largely responsible. These data identify TRAF1 as a specific target of caspases activated during TNF- and Fas-induced apoptosis and illustrate differences in the repertoire of protease substrates cleaved during activation of different apoptotic pathways.

Antiapoptotic proteins. The bcl-2 and inhibitor of apoptosis protein families.

The balance between pro- and antiapoptotic proteins can determine cellular fate. In this regard, the Bcl-2 and IAP protein families have evolved as highly conserved regulators of cell death. A further testament to their critical roles in maintaining balance between cell life and death may be the increasing implication of Bcl-2 and TAP proteins in the pathologies of human diseases. Although much has been learned about these families of proteins, future studies of the Bcl-2 and IAP families are sure to hold more exciting discoveries and will continue to reveal new strategies for combating human diseases.

Caspase-3 is essential for procaspase-9 processing and cisplatin-induced apoptosis of MCF-7 breast cancer cells.

In this study, we sought to investigate in more detail the role of caspase-3 in apoptotic processes in cultured cells and in cell-free extracts of breast cancer cells. We present evidence that apoptosis of caspase-3-deficient MCF-7 breast cancer cells is defective in response to cisplatin treatment, as determined by chromatin condensation, nuclear fragmentation, DNA fragmentation, and release of cytochrome c from the mitochondria. Reconstitution of MCF-7 cells by stable transfection of CASP-3 cDNA restores all these defects and results in an extensive apoptosis after cisplatin treatment. We further show that in extracts from caspase-3-deficient MCF-7 cells, procaspase-9 processing is strongly impaired after stimulation with either cytochrome c or recombinant caspase-8. Reconstitution of MCF-7 cell extracts with procaspase-3 corrects this defect, resulting in an efficient and complete processing of procaspase-9. Together, our data define caspase-3 as an important integrator of the apoptotic process in MCF-7 breast cancer cells and reveal an essential function of caspase-3 for procaspase-9 processing.

Changes in intramitochondrial and cytosolic pH: early events that modulate caspase activation during apoptosis.

Mitochondria trigger apoptosis by releasing caspase activators, including cytochrome c (cytC). Here we show, using a pH-sensitive green fluorescent protein (GFP), that mitochondria-dependent apoptotic stimuli (such as Bax, staurosporine and ultraviolet irradiation) induce rapid, Bcl-2-inhibitable mitochondrial alkalinization and cytosol acidification, followed by cytC release, caspase activation and mitochondrial swelling and depolarization. These events are not induced by mitochondria-independent apoptotic stimuli, such as Fas. Activation of cytosolic caspases by cytC in vitro is minimal at neutral pH, but maximal at acidic pH, indicating that mitochondria-induced acidification of the cytosol may be important for caspase activation; this finding is supported by results obtained from cells using protonophores. Cytosol acidification and cytC release are suppressed by oligomycin, a FoF1-ATPase/H +-pump inhibitor, but not by caspase inhibitors. Ectopic expression of Bax in wild-type, but not FoF1/H+-pump-deficient, yeast cells similarly results in mitochondrial matrix alkalinization, cytosol acidification and cell death. These findings indicate that mitochondria-mediated alteration of intracellular pH may be an early event that regulates caspase activation in the mitochondrial pathway for apoptosis.

Evolutionary conservation of apoptosis mechanisms: lepidopteran and baculoviral inhibitor of apoptosis proteins are inhibitors of mammalian caspase-9.

We cloned a new inhibitor of apoptosis protein (IAP) homolog, SfIAP, from Spodoptera frugiperda Sf-21 cells, a host of insect baculoviruses. SfIAP contains two baculovirus IAP repeat domains followed by a RING domain. SfIAP has striking amino acid sequence similarity with baculoviral IAPs, CpIAP and OpIAP, suggesting that baculoviral IAPs may be host-derived genes. SfIAP and baculoviral CpIAP inhibit Bax but not Fas-induced apoptosis in human cells. Their apoptosis-suppressing activity in mammalian cells requires both baculovirus IAP repeat and RING domains. Further biochemical data suggest that SfIAP and CpIAP are specific inhibitors of mammalian caspase-9, the pinnacle caspase in the mitochondria/cytochrome c pathway for apoptosis, but are not inhibitors of downstream caspase-3 and caspase-7. Thus the mechanisms by which insect and baculoviral IAPs suppress apoptosis may involve inhibition of an insect caspase-9 homologue. Peptides representing the IAP-binding domain of the Drosophila cell death protein Grim abrogated human caspase suppression by SfIAP and CpIAP, implying evolutionary conservation of the functions of IAPs and their inhibitors.

Cleavage of human inhibitor of apoptosis protein XIAP results in fragments with distinct specificities for caspases.

Several human inhibitor of apoptosis (IAP) family proteins function by directly inhibiting specific caspases in a mechanism that does not require IAP cleavage. In this study, however, we demonstrate that endogenous XIAP is cleaved into two fragments during apoptosis induced by the tumor necrosis factor family member Fas (CD95). The two fragments produced comprise the baculoviral inhibitory repeat (BIR) 1 and 2 domains (BIR1-2) and the BIR3 and RING (BIR3-Ring) domains of XIAP. Overexpression of the BIR1-2 fragment inhibits Fas-induced apoptosis, albeit at significantly reduced efficiency compared with full-length XIAP. In contrast, overexpression of the BIR3-Ring fragment results in a slight enhancement of Fas-directed apoptosis. Thus, cleavage of XIAP may be one mechanism by which cell death programs circumvent the anti-apoptotic barrier posed by XIAP. Interestingly, ectopic expression of the BIR3-Ring fragment resulted in nearly complete protection from Bax-induced apoptosis. Use of purified recombinant proteins revealed that BIR3-Ring is a specific inhibitor of caspase-9 whereas BIR1-2 is specific for caspases 3 and 7. Therefore XIAP possesses two different caspase inhibitory activities which can be attributed to distinct domains within XIAP. These data may provide an explanation for why IAPs have evolved with multiple BIR domains.

Release of caspase-9 from mitochondria during neuronal apoptosis and cerebral ischemia.

Caspase-9 is critical for cytochrome c (cyto-c)-dependent apoptosis and normal brain development. We determined that this apical protease in the cyto-c pathway for apoptosis resides inside mitochondria in several types of cells, including cardiomyocytes and many neurons. Caspase-9 is released from isolated mitochondria on treatment with Ca2+ or Bax, stimuli implicated in ischemic neuronal cell death that are known to induce cyto-c release from mitochondria. In neuronal cell culture models, apoptosis-inducing agents trigger translocation of caspase-9 from mitochondria to the nucleus, which is inhibitable by Bcl-2. Similarly, in an animal model of transient global cerebral ischemia, caspase-9 release from mitochondria and accumulation in nuclei was observed in hippocampal and other vulnerable neurons exhibiting early postischemic changes preceding apoptosis. Loss of mitochondrial barrier function during neuronal damage from ischemia or other insults therefore may play an important role in making certain caspases available to participate in apoptosis.

Caspase-9 can be activated without proteolytic processing.

The recombinant form of the proapoptotic caspase-9 purified following expression in Escherichia coli is processed at Asp315, but largely inactive; however, when added to cytosolic extracts of human 293 cells it is activated 2000-fold in the presence of cytochrome c and dATP. Thus, the characteristic activities of caspase-9 are context-dependent, and its activation may not recapitulate conventional caspase activation mechanisms. To explore this hypothesis we produced recombinant forms of procaspase-9 containing mutations that disabled one or both of the interdomain processing sites of the zymogen. These mutants were able to activate downstream caspases, but only in the presence of cytosolic factors. The mutant with both processing sites abolished had 10% of the activity of wild-type, and was able to support apoptosis, with equal vigor to wild-type, when transiently expressed in 293 cells. Thus caspase-9 has an unusually active zymogen that does not require proteolytic processing, but instead is dependent on cytosolic factors for expression of its activity.

Endogenous inhibitors of caspases.

Caspases are cysteine proteases that are specific for aspastic acid residues. These enzymes have been extensively characterized as integral and highly conserved components of a variety of cell death programs. Cowpox and several insect viruses have evolved mechanisms that counter host cell suicide by encoding proteins that directly inhibit caspases-thereby allowing propagation of viral progeny within the host cell. It has only recently been elucidated, however, that endogenous cellular inhibitors of the caspases exist. To date five members of the inhibitor of apoptosis (IAP) family of proteins has been identified in humans and at least three of these have been shown directly to inhibit specific caspases. Thus, members of the IAP family of proteins are the only endogenous inhibitors of caspases known in mammals. Here we discuss the caspase and IAP families of proteins and review the data concerning their relationship.

IAPs block apoptotic events induced by caspase-8 and cytochrome c by direct inhibition of distinct caspases.

Inhibitor of apoptosis (IAP) gene products play an evolutionarily conserved role in regulating programmed cell death in diverse species ranging from insects to humans. Human XIAP, cIAP1 and cIAP2 are direct inhibitors of at least two members of the caspase family of cell death proteases: caspase-3 and caspase-7. Here we compared the mechanism by which IAPs interfere with activation of caspase-3 and other effector caspases in cytosolic extracts where caspase activation was initiated by caspase-8, a proximal protease activated by ligation of TNF-family receptors, or by cytochrome c, which is released from mitochondria into the cytosol during apoptosis. These studies demonstrate that XIAP, cIAP1 and cIAP2 can prevent the proteolytic processing of pro-caspases -3, -6 and -7 by blocking the cytochrome c-induced activation of pro-caspase-9. In contrast, these IAP family proteins did not prevent caspase-8-induced proteolytic activation of pro-caspase-3; however, they subsequently inhibited active caspase-3 directly, thus blocking downstream apoptotic events such as further activation of caspases. These findings demonstrate that IAPs can suppress different apoptotic pathways by inhibiting distinct caspases and identify pro-caspase-9 as a new target for IAP-mediated inhibition of apoptosis.

X-linked IAP is a direct inhibitor of cell-death proteases.

The inhibitor-of-apoptosis (IAP) family of genes has an evolutionarily conserved role in regulating programmed cell death in animals ranging from insects to humans. Ectopic expression of human IAP proteins can suppress cell death induced by a variety of stimuli, but the mechanism of this inhibition was previously unknown. Here we show that human X-chromosome-linked IAP directly inhibits at least two members of the caspase family of cell-death proteases, caspase-3 and caspase-7. As the caspases are highly conserved throughout the animal kingdom and are the principal effectors of apoptosis, our findings suggest how IAPs might inhibit cell death, providing evidence for a mechanism of action for these mammalian cell-death suppressors.

The c-IAP-1 and c-IAP-2 proteins are direct inhibitors of specific caspases.

The inhibitor of apoptosis (IAP) family of proteins are highly conserved through evolution. However, the mechanisms by which these proteins interfere with apoptotic cell death have been enigmatic. Recently, we showed that one of the human IAP family proteins, XIAP, can bind to and potently inhibit specific cell death proteases (caspases) that function in the distal portions of the proteolytic cascades involved in apoptosis. In this study, we investigated three of the other known members of the human IAP family, c-IAP-1, c-IAP-2 and NAIP. Similarly to XIAP, in vitro binding experiments indicated that c-IAP-1 and c-IAP-2 bound specifically to the terminal effector cell death proteases, caspases-3 and -7, but not to the proximal protease caspase-8, caspases-1 or -6. In contrast, NAIP failed to bind tightly to any of these proteases. Recombinant c-IAP-1 and c-IAP-2 also inhibited the activity of caspases-3 and -7 in vitro, with estimated Kis of <=0.1 microM, whereas NAIP did not. The BIR domain-containing region of c-IAP-1 and c-IAP-2 was sufficient for inhibition of these caspases, though proteins that retained the RING domain were somewhat more potent. Utilizing a cell-free system in which caspases were activated in cytosolic extracts by addition of cytochrome c, c-IAP-1 and c-IAP-2 inhibited both the generation of caspase activities and proteolytic processing of pro-caspase-3. Similar results were obtained in intact cells when c-IAP-1 and c-IAP-2 were overexpressed by gene transfection, and apoptosis was induced by the anticancer drug, etoposide. Cleavage of c-IAP-1 or c-IAP-2 was not observed when interacting with the caspases, implying a different mechanism from the baculovirus p35 protein, the broad spectrum suicide inactivator of caspases. Taken together, these findings suggest that c-IAP-1 and c-IAP-2 function similarly to XIAP by inhibiting the distal cell death proteases, caspases-3 and -7, whereas NAIP presumably inhibits apoptosis via other targets.