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

Abstract

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 mitochondria͞cytoplasm, 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 ubiquitin͞proteasome-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.

Down-regulation of the Mixed-lineage Dual Leucine Zipper-bearing Kinase by Heat Shock Protein 70 and Its Co-chaperone CHIP

“…Recent work has also established that the proteasome system plays a pivotal role in the regulation of apoptosis by modulating the levels of proteins that have either pro-apoptotic or anti-apoptotic properties (63). Especially important among these are the tumor suppressor p53 (64), the Bcl-2 family member Bax (65), and the inhibitor of apoptosis (IAP) proteins (66). It is clear from our data that activation of both exogenous and endogenous DLK serves as an essential trigger mechanism for its proteasomal-dependent proteolysis.…”

Regulation of death receptor signaling by the ubiquitin system

“…111 Although the cytosolic form of Bax appears to be stable, the activated mitochondrial form is reported to be unstable. Enhanced degradation of mitochondrial Bax is reported in prostate cancers 112 and in malignant B cells, and the therapeutic proteasome inhibitor Velcade has been shown to stabilize Bax and sensitize CLL cells to TRAIL-induced apoptosis. 113 The ubiquitination site of Bax has been mapped to the alpha-5 helix, that when deleted, stabilized Bax protein levels.…”

Bcl‐2 family proteins: The sentinels of the mitochondrial apoptosis pathway

“…While BH3 mimetics have great potential as ammunition in the fight against cancer, these compounds still require functionally active adapter proteins, Bax or Bak. Cases where chemotherapeutic drug treatment has resulted in chemo resistance mutations in the instability tract in the bax gene (88) or cancer types such as prostate adenocarcinoma which are highly associated with increased proteasomal degradation of Bax protein (89), BH3 mimetics will render little help. Therefore, understanding how Bax/Bak activation results in apoptosis are of added significance.…”