Omi/HtrA2 catalytic cleavage of inhibitor of apoptosis (IAP) irreversibly inactivates IAPs and facilitates caspase activity in apoptosis

Abstract

Omi/HtrA2 is a mitochondrial serine protease that is released into the cytosol during apoptosis to antagonize inhibitors of apoptosis (IAPs) and contribute to caspase-independent cell death. Here, we demonstrate that Omi/HtrA2 directly cleaves various IAPs in vitro, and the cleavage efficiency is determined by its IAP-binding motif, AVPS. Cleavage of IAPs such as c-IAP1 substantially reduces its ability to inhibit and ubiquitylate caspases. In contrast to the stoichiometric anti-IAP activity by Smac/DIABLO, Omi/HtrA2 cleavage of c-IAP1 is catalytic and irreversible, thereby more efficiently inactivating IAPs and promoting caspase activity. Elimination of endogenous Omi by RNA interference abolishes c-IAP1 cleavage and desensitizes cells to apoptosis induced by TRAIL. In addition, overexpression of cleavage-site mutant c-IAP1 makes cells more resistant to TRAIL-induced caspase activation. This IAP cleavage by Omi is independent of caspase. Taken together, these results indicate that unlike Smac/DIABLO, Omi/HtrA2's catalytic cleavage of IAPs is a key mechanism for it to irreversibly inactivate IAPs and promote apoptosis.

Intramitochondrial serine protease activity of Omi/HtrA2 is required for caspase-independent cell death of human neutrophils

“…The Omi/HtrA2-mediated proteolytic cleavage might inactivate and remove antiapoptotic molecules, as has been shown for IAPs, 8,9 or it might be required for activation of precursor proteins whose function might be necessary for the caspase-independent neutrophil cell death. Since this type of cell death has been suggested to be mediated by the mitochondria-derived reactive oxygen species (ROS), 5 it is logical to suppose that a mitochondrial antioxidant system, which normally inactivates an excess of ROS, is destroyed in this experimental setting.…”

“…However, it has been shown that in 293 cells transfected with the active site mutant Omi/HtrA2, which had no protease activity, caspase activation and cell death were markedly diminished, whereas transfection of the wild-type protein induced both events and resulted in significant reduction in the amount of XIAP in the transfected cells, which appeared to be a substrate of Omi/ HtrA2. 8,9 Obviously, the way Omi/HtrA2 participates in cell death may differ in various cell types.…”

Major apoptotic mechanisms and genes involved in apoptosis

“…It was reported that SMAC is overexpressed in several types of solid tumors such as colon, stomach, prostate, ovary, and lung cancers [73]. Likewise, HtrA2/Omi delivery is triggered via apoptotic induction and then, it binds to IAPs through its IAP-binding motif (IBM) [74]. XAF1, the other potent IAP antagonist, binds to BIR domains of IAPs such as XIAP, cIAP1, and cIAP2 and by this way, promotes apoptosis [75].…”

The adaptor protein TRADD activates distinct mechanisms of apoptosis from the nucleus and the cytoplasm

“…8 In addition, the mitochondrial serine protease Omi/HtrA2 has been shown to facilitate caspase-9 activation by disrupting inhibitor of apoptosis protein (IAP) inhibition of caspase-9 9-11 and degrading IAPs. 12,13 TNF signals through two receptors, tumor necrosis factor receptor (TNF-R) 1 and TNF-R2. Signaling through TNF-R1 14,15 activates both prosurvival signals including NFkB and Jun N-terminal kinase as well as proapoptotic signals.…”