Bcl-2 inhibits Bax translocation from cytosol to mitochondria during drug-induced apoptosis of human tumor cells

The pro-apoptotic protein, Bax, has been reported to translocate from cytosol to mitochondria following exposure of cells to apoptotic stresses including cytokine withdrawal and treatment with glucocorticoids and cytotoxic drugs. These observations, coupled with reports showing that Bax causes the release of mitochondrial cytochrome c, implicate Bax as a central mediator of the apoptotic process. In this report we demonstrate by subcellular fractionation a significant shift in Bax localization from cytosol to cellular membranes in two human tumor cell lines exposed to staurosporine or etoposide. Immunofluorescence studies confirmed that Bax specifically relocalized to the mitochondria. This redistribution of Bax occurred in concert with, or just prior to, proteolytic processing of procaspase-3, activation of DEVD-specific cleavage activity and degradation of poly(ADP-ribose) polymerase. However, Bax membrane translocation was independent of caspase activity as determined using the broad-range caspase inhibitor z-VAD-fmk. High level overexpression of the anti-apoptotic protein Bcl-2 prevented Bax redistribution to the mitochondria, caspase activation and apoptosis following exposure to staurosporine or etoposide. These data confirm the role of Bax in mitochondrial cytochrome c release, and indicate that prevention of Bax translocation to the mitochondrial membrane represents a novel mechanism by which Bcl-2 inhibits drug-induced apoptosis.