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Previously we reported that proteasome inhibitors were able to overcome Bcl-2-mediated protection from apoptosis. Here we show that inhibition of the proteasome activity in Bcl-2-overexpressing cells accumulates the proapoptotic Bax protein to mitochondria/cytoplasm, where it interacts to Bcl-2 protein. This event was followed by release of mitochondrial cytochrome c into the cytosol and activation of caspase-mediated apoptosis. In contrast, proteasome inhibition did not induce any apparent changes in Bcl-2 protein levels. In addition, treatment with a proteasome inhibitor increased levels of ubiquitinated forms of Bax protein, without any effects on Bax mRNA expression. We also established a cell-free Bax degradation assay in which an in vitro-translated, (35)S-labeled Bax protein can be degraded by a tumor cell protein extract, inhibitable by addition of a proteasome inhibitor or depletion of the proteasome or ATP. The Bax degradation activity can be reconstituted in the proteasome-depleted supernatant by addition of a purified 20S proteasome or proteasome-enriched fraction. Finally, by using tissue samples of human prostate adenocarcinoma, we demonstrated that increased levels of Bax degradation correlated well with decreased levels of Bax protein and increased Gleason scores of prostate cancer. Our studies strongly suggest that ubiquitin/proteasome-mediated Bax degradation is a novel survival mechanism in human cancer cells and that selective targeting of this pathway should provide a unique approach for treatment of human cancers, especially those overexpressing Bcl-2.

Bax degradation by the ubiquitin/proteasome-dependent pathway: Involvement in tumor survival and progression