Bax and Bak promote apoptosis by modulating endoplasmic reticular and mitochondrial Ca²⁺ stores

Alterations in intracellular Ca²⁺ homeostasis and cytochrome c release from mitochondria have been implicated in the regulation of apoptosis, but the relationship between these events remains unclear. Here we report that enforced expression of either Bax or Bak via adenoviral gene delivery results in the accumulation of the proteins in the endoplasmic reticulum (ER) and mitochondria, resulting in early caspase-independent BCL-2-sensitive release of the ER Ca²⁺ pool and subsequent Ca²⁺ accumulation in mitochondria. The inhibition of ER-to-mitochondrial Ca²⁺ transport with a specific inhibitor of mitochondrial Ca²⁺ uptake attenuates cytochrome c release and downstream biochemical events associated with apoptosis. Bax and Bak also directly sensitize mitochondria to cytochrome c release induced by immediate emptying of ER Ca²⁺ pool. Our results demonstrate that the effects of the "multidomain" proapoptotic BCL-2 family members Bak and Bax involve direct effects on the endoplasmic reticular Ca²⁺ pool with subsequent sensitization of mitochondria to calcium-mediated fluxes and cytochrome c release. These effects modulate the kinetics of cytochrome c release and apoptosis.