Bax is a pro-death tumor suppressor in the Bcl-2 family, and is frequently mutated in microsatellite instable tumors, especially Hereditary... Show moreBax is a pro-death tumor suppressor in the Bcl-2 family, and is frequently mutated in microsatellite instable tumors, especially Hereditary Non-Polyposis Colorectal Cancer (HNPCC). The loss of apoptotic Bax contributes to tumor development and chemoresistance. We recently uncovered that the combination of a Bax microsatellite mutation with a specific alternative splicing generated a unique Bax isoform (BaxΔ2) in Bax-negative cells. Similar to the prototype Baxα, BaxΔ2 is a potent pro-apoptotic molecule. However, the pro-apoptotic mechanism, therapeutic implication, and tumor tissue distribution of BaxΔ2 protein remain elusive. In this thesis research, we isolated and analyzed isogenic sub-cell lines that represent different Bax microsatellite statuses from colorectal cancer cells. We found that the colon cancer cells harboring Bax microsatellite G7/G7 alleles produced low levels of endogenous BaxΔ2 transcripts and proteins. BaxΔ2-positive cells were selectively sensitive to a subgroup of chemotherapeutics in comparison with BaxΔ2-negative cells. Different from other Bax isoforms, which mostly act through targeting mitochondria, BaxΔ2 recruited caspase-8 into the aggregates for activation, and consequently induced cell death independent of the mitochondrial pathway. Furthermore, the distribution of BaxΔ2 protein was mostly found in well-differentiated epithelial cells in primary colon tumor tissues or in primary squamous buccal cells, which contain Bax G7 mutation. However, not all cells harboring the Bax G7 mutation had a detectable level of BaxΔ2 proteins. These data suggest that, similar to Baxα, BaxΔ2 protein is pro-apoptotic, but not toxic to normal cells; expression of BaxΔ2 protein restores apoptotic program in Bax negative cells via a non-classical signaling pathway. Importantly, BaxΔ2 may provide a selective chemotherapeutic advantage for certain Bax-negative colon tumors. Ph.D. in Biology, July 2014 Show less
