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 cytos ol to mitochondria following exposure of cells to apoptotic stresses includ ing cytokine withdrawal and treatment with glucocorticoids and cytotoxic dr ugs. These observations, coupled with reports showing that Bax causes the r elease of mitochondrial cytochrome c, implicate Bax as a central mediator o f the apoptotic process. In this report we demonstrate by subcellular fract ionation a significant shift in Bax localization from cytosol to cellular m embranes in two human tumor cell lines exposed to staurosporine or etoposid e. Immunofluorescence studies confirmed that Bax specifically relocalized t o 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 ov erexpression of the anti-apoptotic protein Bcl-2 prevented Bax redistributi on to the mitochondria, caspase activation and apoptosis following exposure to staurosporine or etoposide, These data confirm the role of Bax in mitoc hondrial cytochrome c release, and indicate that prevention of Bax transloc ation to the mitochondrial membrane represents a novel mechanism by which B cl-2 inhibits drug-induced apoptosis.