Caspase-9 cleavage, do you need it?

Abstract

Caspase-9, which is activated by association with the Apaf-1 (apoptotic protease-activating factor-1) apoptosome complex, cleaves and activates the downstream effector caspases-3 and -7, thereby executing the caspase-cascade and cell-death programme. Although caspase-9 does not need to be cleaved to be active, apoptotic cell death is always accompanied by autocatalytic cleavage and by further downstream effector caspase-dependent cleavage of caspase-9. In this issue of the Biochemical Journal, Denault and co-workers evaluate the role of caspase-3-dependent cleavage of caspase-9 and conclude that this mechanism mainly serves to enhance apoptosis by alleviating XIAP (X-linked inhibitor of apoptosis) inhibition of the apical caspase.Key words: apoptotic protease-activating factor-1 (Apaf-1), apoptosome, caspase-3, caspase-9, second mitochondrial activator of caspases (Smac), X-linked inhibitor of apoptosis (XIAP). ACTIVATING THE CASPASE CASCADE AND THE ROLE OF XIAPCaspases are present in healthy cells as zymogens, which are usually activated by proteolytic cleavage of an interlinker peptide sequence that allows rearrangement of peptide loops to form a fully functional active site. Caspases-3 and -7 are the apical effector caspases and are activated by either caspase-9 or caspase-8. The inappropriate activation of this essentially irreversible caspase cascade requires an independent and tightly regulated failsafe mechanism. To this end, the cell has evolved altogether different mechanisms for activating the initiator caspases that are responsible for cleaving and activating procaspases-3 and -7. In the case of the intrinsic pathway, which can be triggered by chemical-or radiation-induced damage, the execution phase of cell death involves an efflux of cytochrome c from the mitochondria, which binds to and activates Apaf-1 (apoptotic proteaseactivating factor-1). This large (∼ 130-140 kDa) protein is a mammalian homologue of CED-4, an essential protein involved in Caenorhabditis elegans programmed cell death. Apaf-1 oligomerizes to form the seven-spoked, wheel-like Apaf-1 apoptosome complex, which recruits caspase-9 to form the active holoenzyme caspase-activating apoptosome complex. This complex then efficiently recruits and directly cleaves procaspase-3 or -7 with high efficiency [1]. However, caspase-9 [like caspase-8 in the DISC (death-inducing signalling complex)] is an unusual caspase in that, in addition to its CARD (caspase-recruiting domain), it has a long interlinker peptide separating the small and large subunit domains. Crystallographic studies show that this flexible linker peptide allows formation of the active-site pocket in the zymogen, but further conformational changes are required to fully activate its active site [2]. The mechanism for this is controversial: one theory suggests that the apoptosome activates caspase-9 by facilitating dimerization [3], whereas an alternative hypothesis suggests that binding to the apoptosome is sufficient to induce the necessary conformational changes to activate cas...

The effect of environmental micropollutant (DEET) on the expression of cell cycle and apoptosis regulatory proteins in human cells

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Caspase-3 cleaves XIAP in a positive feedback loop to sensitize melanoma cells to TRAIL-induced apoptosis

“…Liberated caspase-3 then cleaves XIAP in a positive regulatory feedback loop, thereby quickly enhancing the proteasomal XIAP turnover, amplifying its autocatalytic activation and consequently synergistically executing apoptosis via two different pathways. Despite from execution of cell death via cleavage of death substrates, activated caspase-3 may now also trigger final activation of caspase-9 by cleaving XIAP bound to caspase-9 and/or by cleavage of caspase-9 itself, thereby enhancing its catalytic activity (Twiddy and Cain, 2007) and alleviating XIAP binding (Denault et al, 2007). Consequently, this positive amplification loop requires both cytochrome c and Smac release, although Smac appears to be the primary trigger.…”

Caspase 8 and caspase 9 gene polymorphisms and susceptibility to gastric cancer

“…CASP9 is involved in activation through the apoptosomedriven intrinsic pathway [16]. The release of apoptogenic proteins, caused by perturbation of the mitochondria, results in the activation of caspases which are responsible for most of the biochemical and morphological changes observed during apoptosis.…”