title: Bax/Bak-dependent, Drp1-independent Targeting of X-linked Inhibitor of Apoptosis Protein (XIAP) into Inner Mitochondrial Compartments Counteracts Smac/DIABLO-dependent Effector Caspase Activation.
abstract: Efficient apoptosis requires Bax/Bak-mediated mitochondrial outer membrane permeabilization (MOMP), which releases death-promoting proteins cytochrome c and Smac to the cytosol, which activate apoptosis and inhibit X-linked inhibitor of apoptosis protein (XIAP) suppression of executioner caspases, respectively. We recently identified that in response to Bcl-2 homology domain 3 (BH3)-only proteins and mitochondrial depolarization, XIAP can permeabilize and enter mitochondria. Consequently, XIAP E3 ligase activity recruits endolysosomes into mitochondria, resulting in Smac degradation. Here, we explored mitochondrial XIAP action within the intrinsic apoptosis signaling pathway. Mechanistically, we demonstrate that mitochondrial XIAP entry requires Bax or Bak and is antagonized by pro-survival Bcl-2 proteins. Moreover, intramitochondrial Smac degradation by XIAP occurs independently of Drp1-regulated cytochrome c release. Importantly, mitochondrial XIAP actions are activated cell-intrinsically by typical apoptosis inducers TNF and staurosporine, and XIAP overexpression reduces the lag time between the administration of an apoptotic stimuli and the onset of mitochondrial permeabilization. To elucidate the role of mitochondrial XIAP action during apoptosis, we integrated our findings within a mathematical model of intrinsic apoptosis signaling. Simulations suggest that moderate increases of XIAP, combined with mitochondrial XIAP preconditioning, would reduce MOMP signaling. To test this scenario, we pre-activated XIAP at mitochondria via mitochondrial depolarization or by artificially targeting XIAP to the intermembrane space. Both approaches resulted in suppression of TNF-mediated caspase activation. Taken together, we propose that XIAP enters mitochondria through a novel mode of mitochondrial permeabilization and through Smac degradation can compete with canonical MOMP to act as an anti-apoptotic tuning mechanism, reducing the mitochondrial contribution to the cellular apoptosis capacity.
authors.affiliation: From the Lysosomal Systems Biology, German Cancer Research Center, Bioquant, University of Heidelberg, 69120 Heidelberg, Germany a.brady@dkfz.de.
authors.forename: Anne
authors.initials: A
authors.lastname: Hamacher-Brady
authors.affiliation: Bioquant, University of Heidelberg, 69120 Heidelberg, Germany the Systems Biology of Cell Death Mechanisms, German Cancer Research Center, the Department of Surgery, Heidelberg University Hospital, and n.brady@dkfz.de.
authors.forename: Nathan Ryan
authors.initials: NR
authors.lastname: Brady
mesh_terms: D000818:Animals; D051017:Apoptosis Regulatory Proteins; D015153:Blotting, Western; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D020169:Caspases / Q000378:metabolism*; D002478:Cells, Cultured; D045304:Cytochromes c / Q000378:metabolism; D034281:Dynamins; D004622:Embryo, Mammalian / Q000166:cytology; D004789:Enzyme Activation; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000235:genetics / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D061986:MCF-7 Cells; D018345:Mice, Knockout; D008856:Microscopy, Fluorescence; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D051336:Mitochondrial Membranes / Q000378:metabolism; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism*; D010539:Permeability / Q000187:drug effects; D021381:Protein Transport; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000235:genetics / Q000378:metabolism*; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000235:genetics / Q000378:metabolism*; D051028:bcl-2-Associated X Protein / Q000235:genetics / Q000378:metabolism*
chemical_list: D051017:Apoptosis Regulatory Proteins; C412403:DIABLO protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008869:Microtubule-Associated Proteins; D024101:Mitochondrial Proteins; D051636:X-Linked Inhibitor of Apoptosis Protein; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D045304:Cytochromes c; D020169:Caspases; D020558:GTP Phosphohydrolases; C110768:DNM1L protein, human; D034281:Dynamins
keywords: E3 ubiquitin ligase; X-linked inhibitor of apoptosis protein (XIAP); cell death; dynamin-related protein 1 (Drp1); mitochondria; mitochondrial apoptosis; mitochondrial outer membrane permeabilization (MOMP); tumor necrosis factor (TNF)