A Caspase-activated Factor (CAF) Induces Mitochondrial Membrane Depolarization and Cytochrome c Release by a Nonproteolytic Mechanism

It is well established that apoptosis is accompanied by activation of procaspases and by mitochondrial changes, such as decrease in mitochondrial transmembrane potential (ΔΨ(m)) and release of cytochrome c. We analyzed the causal relationship between activated caspases and these mitochondrial phenomena. Purified recombinant caspase-1, -11, -3, -6, -7, and -8 were incubated with mitochondria in the presence or absence of additional cellular components, after which ΔΨ(m) was determined. At lower caspase concentrations, only caspase-8 was able to activate a cytosolic factor, termed caspase-activated factor (CAF), which resulted in decrease in ΔΨ(m) and release of cytochrome c. Both CAF-mediated activities could not be blocked by protease inhibitors, including oligopeptide caspase inhibitors. CAF-induced cytochrome c release, but not decrease of ΔΨ(m), was blocked in mitochondria from cells overexpressing Bcl-2. CAF is apparently involved in decrease of ΔΨ(m) and release of cytochrome c, whereas Bcl-2 only prevents the latter. Hence, CAF may form the link between death domain receptor–dependent activation of procaspase-8 and the mitochondrial events studied.