Bax-mediated Ca2+ Mobilization Promotes Cytochrome c Release during Apoptosis

Abstract

Previous studies have demonstrated that Ca(2+) 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 Ca(2+) pool in human PC-3 prostate cancer cells. This mobilization of ER Ca(2+) was associated with a concomitant increase in mitochondrial Ca(2+) levels, and neither ER Ca(2+) mobilization nor mitochondrial Ca(2+) uptake occurred in Bax-null DU-145 cells. Importantly, restoration of DU-145 Bax expression via adenoviral gene transfer restored ER Ca(2+) release and mitochondrial Ca(2+) 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 Ca(2+) uniporter (RU-360) attenuated mitochondrial Ca(2+) uptake, cytochrome c release, and DNA fragmentation, directly implicating the mitochondrial Ca(2+) changes in cell death. Together, our data demonstrate that Bax-mediated alterations in ER and mitochondrial Ca(2+) levels serve as important upstream signals for cytochrome c release in some examples of apoptosis.