The protein structures that shape caspase activity, specificity, activation and inhibition

Abstract

The death morphology commonly known as apoptosis results from a post-translational pathway driven largely by specific limited proteolysis. In the last decade the structural basis for apoptosis regulation has moved from nothing to 'quite good', and we now know the fundamental structures of examples from the initiator phase, the pre-mitochondrial regulator phase, the executioner phase, inhibitors and their antagonists, and even the structures of some substrates. The field is as well advanced as the best known of proteolytic pathways, the coagulation cascade. Fundamentally new mechanisms in protease regulation have been disclosed. Structural evidence suggests that caspases have an unusual catalytic mechanism, and that they are activated by apparently unrelated events, depending on which position in the apoptotic pathway they occupy. Some naturally occurring caspase inhibitors have adopted classic inhibition strategies, but other have revealed completely novel mechanisms. All of the structural and mechanistic information can, and is, being applied to drive therapeutic strategies to combat overactivation of apoptosis in degenerative disease, and underactivation in neoplasia. We present a comprehensive review of the caspases, their regulators and inhibitors from a structural and mechanistic point of view, and with an aim to consolidate the many threads that define the rapid growth of this field.

Thirty-eight-negative kinase 1 (TNK1) facilitates TNFα-induced apoptosis by blocking NF-κB activation

“…Caspase 9 mediates activation of apoptosis via the intrinsic or mitochondrial pathway that responds to stress or genotoxic damage (Fuentes-Prior and Salvesen, 2004). We observed an increase in caspase 9 cleavage in TNK1-expressing cells upon treatment with TNFa ( Figure 5c).…”

“…In contrast, staurosporine that activates the mitochondrial apoptotic pathway induced substantial mitochondrial disintegration already in the absence of TNFa (Figure 5d). Permeabilization of the mitochondrial outer membrane during intrinsic apoptosis leads to the release of cytochrome c and second mitochondria-derived activator of caspases (SMAC) from the mitochondria into the TNK1 blocks TNFa-induced NF-jB activation N Azoitei et al cytosol (Fuentes-Prior and Salvesen, 2004;Green and Kroemer, 2004). Expression of TNK1 and/or incubation of cells with TNFa did not result in an appreciable increase in cytosolic cytochrome c or SMAC (Figure 5e).…”

Methods for the assessment of mitochondrial membrane permeabilization in apoptosis

“…The extrinsic and intrinsic pathways converge on the common degradation phase mediated by executioner caspases-3, -6 and -7 [25], but are interconnected also upstream, at the integration/decision phase, at least in some models of apoptosis. Indeed, caspase-8 induces the proteolytic maturation of the BH3-only protein Bid, which, in its truncated form (tBid) translocates to mitochondria and favors MMP [21,26,27].…”

MCF-7 breast carcinoma cells do not express caspase-3

“…Apoptosis, a fundamental process essential for normal tissue homeostasis and development, is closely associated with the activation of a class of aspartate-specific cysteinedependent proteases, called caspases, that lead to the demise of the cell via limited proteolysis of a multitude of cellular substrates [1,2]. Caspases are expressed as inactive zymogens that become activated upon cleavage by other caspases in a so-called caspase activation cascade, or by mere oligomerization instigated by the formation of large multi-protein complexes such as the death-inducing signaling complex (DISC) or the apoptosome [3,4].…”