Novel roles for Bcl2 family proteins at the endoplasmic reticulum

The Bcl2 family proteins are central regulators of apoptosis; the primary form of physiological cell death. Furthermore, inappropriate activation or silencing of Bcl2 family members has been associated with cancer development and treatment resistance. These proteins therefore represent attractive therapeutic targets, and, in consequence, an increased understanding of the roles played by Bcl2 proteins will be important for the development and implementation of targeted therapies. Although best characterized with respect to their role at the mitochondria, it is now evident that Bcl2 proteins also function at the endoplasmic reticulum (ER). The focus of this thesis is on the role of Bcl2 proteins at the ER, specifically with respect to cell death initiated by the ER localized proteins Bik and p20Bap31. Bik and p20Bap31 were previously shown to initiate similar proapoptotic pathways, characterized by an early release of ER calcium stores, and inhibitable by Bcl2. The results of the current study are two-fold: first, using ER-targeted Bak and Bcl2 (Bakb5 and Bcl2b5) in a Bak/Bax deficient background, I have shown that Bik can disrupt an interaction between Bak and Bcl2 at the ER. Furthermore, Bik could overcome the protective effect of Bcl2b5 with respect to Bakb5. This finding provides the first direct evidence for regulation of cell death via binary interactions between Bcl2 family members at the ER. The second part of this study was designed to determine the roles of Bax/Bak and ER-restricted Bcl2 in the p20Bap31-initiated pathway. Using E1A/DNp53 transformed wild-type and Bax/Bak double knockout baby mouse kidney epithelial cells, I have shown that ectopic expression of p20Bap31, but not of Bik, can initiate a paraptosis-like form of non-apoptotic, Bax/Bak independent, cell death. Of particular importance, cell death could be delayed by Bcl2b5, in the absence of Bax/Bak; pointing to a novel, Bax/Bak independent, prosurvival role for Bcl2 at the ER. In summary, this study demonstrates the ability of Bcl2 family members to regulate cell death through binary interactions at the ER, and of Bcl2 to inhibit cell death in a Bax/Bak independent manner. Furthermore, a novel non-apoptotic cell death pathway initiated by p20Bap31 expression is identified.