Calcium binding of ARC mediates regulation of caspase 8 and cell death

Apoptosis or programmed cell death is genetically controlled and plays a central role in normal development and tissue homeostasis, including development of the nervous system and regulation of the immune system (8, 25, 35). Dysregulated apoptosis has been implicated in the pathogenesis of cancer and autoimmune, neurodegenerative, and cardiovascular diseases (28). The cell death machinery that is conserved throughout evolution is composed of activators, inhibitors, and effectors (14). The effector arm of the cell death pathway consists of a family of cysteinyl aspartate-specific proteases called caspases (2). Data suggest that apoptotic cell death can be brought about by the loss of Ca2+ homeostatic control, but it can also be finely tuned positively or negatively by more subtle changes in Ca2+ distribution within intracellular compartments (29). While protein kinases such as AKT and ERK have been reported to modulate caspase activity through phosphorylation (1, 7), there have been few regulatory molecules directly linking cytotoxic Ca2+ signaling to caspase activity. Based on sequence similarities, three prominent interaction motifs involved in apoptosis are recognized. The death domain superfamily consists of death domain (DD), death effector domain (DED), and caspase recruitment domain (CARD) families (16, 26). In recent years, a number of CARD-containing proteins have been identified and participate in various signaling pathways during apoptosis and NF-κB activation. ARC is a CARD protein that selectively interacts with the initiator caspase 8 and significantly attenuates death receptor-induced apoptosis (22). Recently, ARC was also found capable of blocking caspase-independent events such as hypoxia-induced cytochrome c release and hydrogen peroxide (H2O2)-induced necrotic cell death (10, 27). We also described the protective role of ARC during hypoxia of hippocampal neurons (17). In addition, ARC is known to be phosphorylated by protein kinase CK2, modulating the subcellular localization of ARC (23). While increasing evidence suggests an inhibitory role for ARC in the diverse cell death processes, the precise mechanism by which ARC interferes with caspase-dependent and caspase-independent cell death has not been defined yet. In the present study, we postulate that ARC is a Ca2+-binding CARD protein that modulates activation of caspase 8.