title: XIAP activity dictates Apaf-1 dependency for caspase 9 activation.
abstract: The current model for the intrinsic apoptotic pathway holds that mitochondrial activation of caspases in response to cytotoxic drugs requires both Apaf-1-induced dimerization of procaspase 9 and Smac/Diablo-mediated sequestration of inhibitors of apoptosis proteins (IAPs). Here, we showed that either pathway can independently promote caspase 9 activation in response to apoptotic stimuli. In drug-treated Apaf-1(-/-) primary myoblasts, but not fibroblasts, Smac/Diablo accumulates in the cytosol and sequesters X-linked IAP (XIAP), which is expressed at lower levels in myoblasts than in fibroblasts. Consequently, caspase 9 activation proceeds in Apaf-1(-/-) myoblasts; concomitant ablation of Apaf-1 and Smac is required to prevent caspase 9 activation and the onset of apoptosis. Conversely, in stimulated Apaf-1(-/-) fibroblasts, the ratio of XIAP to Smac/Diablo is high compared to that for myoblasts and procaspase 9 is not activated. Suppressing XIAP with exogenous Smac/Diablo or a pharmacological inhibitor can still induce caspase 9 in drug-treated Apaf-1-null fibroblasts. Thus, caspase 9 activation in response to intrinsic apoptotic stimuli can be uncoupled from Apaf-1 in vivo by XIAP antagonists.
authors.affiliation: Division of Cell & Molecular Biology, Toronto General Research Institute-University Health Network, Toronto, Ontario, Canada M5G 2M1.
authors.forename: Andrew T
authors.initials: AT
authors.lastname: Ho
authors.forename: Qin H
authors.initials: QH
authors.lastname: Li
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Okada
authors.forename: Tak W
authors.initials: TW
authors.lastname: Mak
authors.forename: Eldad
authors.initials: E
authors.lastname: Zacksenhaus
mesh_terms: D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins; D053477:Apoptotic Protease-Activating Factor 1 / Q000172:deficiency / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D053453:Caspase 9 / Q000378:metabolism*; D002478:Cells, Cultured; D003433:Crosses, Genetic; D004789:Enzyme Activation; D005260:Female; D005347:Fibroblasts / Q000166:cytology / Q000201:enzymology; D006579:Heterozygote; D008297:Male; D051379:Mice; D051336:Mitochondrial Membranes / Q000378:metabolism; D024101:Mitochondrial Proteins / Q000172:deficiency / Q000378:metabolism; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D032446:Myoblasts / Q000166:cytology / Q000201:enzymology; D010539:Permeability; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D051017:Apoptosis Regulatory Proteins; D053477:Apoptotic Protease-Activating Factor 1; D002352:Carrier Proteins; C499605:Diablo protein, mouse; D024101:Mitochondrial Proteins; D051636:X-Linked Inhibitor of Apoptosis Protein; D053453:Caspase 9