Requirement of biphasic calcium release from the endoplasmic reticulum for Fas-mediated apoptosis

Fas receptor is a member of the tumor necrosis factor-α family of death receptors that mediate physiologic apoptotic signaling. To investigate the molecular mechanisms regulating calcium mobilization during Fas-mediated apoptosis, we have analyzed the sequential steps leading to altered calcium homeostasis and cell death in response to activation of the Fas receptor. We show that Fas-mediated apoptosis requires endoplasmic reticulum-mediated calcium release in a mechanism dependent on phospholipase C-γ1 (PLC-γ1) activation and Ca²⁺ release from inositol 1,4,5-trisphosphate receptor (IP₃R) channels. The kinetics of Ca²⁺ release were biphasic, demonstrating a rapid elevation caused by PLC-γ1 activation and a delayed and sustained increase caused by cytochrome c binding to IP ₃R. Blocking either phase of Ca²⁺ mobilization was cytoprotective, highlighting PLC-γ1 and IP₃R as possible therapeutic targets for disorders associated with Fas signaling.