Bax-mediated Ca2+ mobilization promotes cytochrome c release during apoptosis

Previous studies have demonstrated that Ca2+ is released from the endoplasmic reticulum (ER) in some models of apoptosis, but the mechanisms involved and the functional significance remain obscure. We confirmed that apoptosis induced by some (but not all) proapoptotic stimuli was associated with caspase-independent, BCL-2-sensitive emptying of the ER Ca2+ pool in human PC-3 prostate cancer cells. This mobilization of ER Ca2+ was associated with a concomitant increase in mitochondrial Ca2+ levels, and neither ER Ca2+ mobilization nor mitochondrial Ca2+ uptake occurred in Bax-null DU-145 cells. Importantly, restoration of DU145 Bax expression via adenoviral gene transfer restored ER Ca2+ release and mitochondrial Ca2+ uptake and dramatically accelerated the kinetics of staurosporine-induced cytochrome c release, demonstrating a requirement for Bax expression in this model system. In addition, an inhibitor of the mitochondrial Ca2+ uniporter (RU-360) attenuated mitochondrial Ca2+ uptake, cytochrome c release, and DNA fragmentation, directly implicating the mitochondrial Ca2+ changes in cell death. Together, our data demonstrate that Bax-mediated alterations in ER and mitochondrial Ca2+ levels serve as important upstream signals for cytochrome c release in some examples of apoptosis.