Study of Mechanisms of an Anti-Apoptotic Proteins, BI-1 in Prostate Cancer

Dysregulation of apoptosis is an important mechanism underlying the resistance to radio- or chemotherapy of androgen-insensitive prostate cancer. BI-1 and BAR were identified as inhibitors of a pro-apoptotic Bcl-2 family member, Bax, identified by a functional yeast screen. Both BI-1 and BAR are membrane proteins located at endoplasmic reticulum ER. The importance of BI-1 in prostate cancer is underscored by its specific up-regulation in malignant cells. Furthermore, reduction of BI-1 expression by small interference RNA siRNA causes significant increase of spontaneous cell death in prostate carcinoma cells. This research project is about a regulatory protein network involving BI-1, BAR, Bap31, and Bcl-2. Using a coimmunoprecipitation assays, we demonstrated that BAR is bound to BI-1, Bap31, and Bcl-2, forming a multi-protein complex at endoplasmic reticulum. BAR is a multifunctional protein with a coiled-coiled domain, a SAM domain, and a predicted E3 ubiquitin ligase RING domain. The RING domain is proved to be essential for the E3 ubiquitin ligase activity of BAR. Acting as an E3 ubiquitin ligase, BAR specifically reduces the protein level of BI-1, but not Bap31 or Bcl-2. On the other hand, Bap31 was found to specifically increase the stability of BAR by decreasing proteosome-dependent degradation of BAR. A regulatory hierarchy seems to exist that Bap31 regulates the protein stability of BAR, an E3 ubiquitin ligase, which in turn modulates the stability of BI-1 in the protein complex.