Excerpt
Apoptosis, a fundamental process essential for normal tissue homeostasis and development, is closely associated with the activation of a class of aspartate-specific cysteine-dependent 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]. Whereas DISC formation occurs via the so-called extrinsic death pathway that is instigated by activation of members of the death receptor family such as CD95, tumor necrosis factor receptor (TNF-R) or the receptors of the TNF-R-related apoptosis-inducing ligand (TRAIL), the apoptosome is formed following activation of mitochondria and is hence termed the intrinsic or mitochondrial death pathway. Based on their order in cell death pathways, caspases can be divided into initiator (caspase-2, -8, -9, and -10) and effector (caspase-3, -6, and -7) caspases. Among them, caspase-3, a member of the latter group, is absolutely crucial for apoptosis induction, as this enzyme is not only activated downstream of both the extrinsic and intrinsic death pathway, it is also responsible for the cleavage of the majority of substrates known so far [
1,
5]. More importantly, with the proteolysis of discrete substrates, caspase-3 evokes some of the typical morphological and biochemical alterations associated with apoptosis. For instance, whereas the caspase-3-mediated cleavages of α-fodrin, gelsolin, rho-associated kinase-1 (ROCK-1) and p21-activated kinase 2 (PAK2) contribute to membrane blebbing, cleavage of the inhibitor of the caspase-activated DNase (ICAD) leads to the typical DNA fragmentation pattern observed in apoptosis [
1]. Furthermore, with the cleavage-mediated activation of the calcium-independent phospholipase A2 and subsequent production of the chemotactic phospholipid lysophosphatidylcholine, caspase-3 appears to be also responsible for the generation of so-called “eat-me” signals that induce migration of phagocytes to the site of apoptotic cell death [
6]. Thus, caspase-3 not only instigates and pursues the demise of a cell, but, in addition, makes sure that the corpse is properly disposed, a function crucial for avoiding inflammatory processes. …
