Abstract 1713: BAX and BAK are preferentially activated by BIM and BID, respectively, affecting clinical chemotherapy response.

Apoptosis is a highly regulated form of cell death that is essential for maintenance of homeostasis and is controlled by the BCL-2 family of proteins. Two key members of this family, the effectors BAX and BAK, can be activated by BIM or BID, which triggers their oligomerization and permeabilization of the mitochondrial membrane, a crucial event during apoptosis. Despite their essential and distinct roles in maintaining homeostasis, BIM and BID are considered to be functionally redundant in their ability to activate BAX and BAK. However, by separately examining the activation of BAX and BAK by BIM and BID we find that these apoptosis-regulating proteins exhibit differing interaction preferences. Specifically, BIM preferentially activates BAX while BID preferentially activates BAK in murine and human cells. The preferential activation is evident when treating cells with BIM and BID BH3 peptides or recombinant, full-length proteins. Calculated EC50 values demonstrate a nearly twenty-fold decrease in the BID BH3 concentration required to activate BAK rather than BAX (0.54μM for BAK versus 9.3μM for BAX) and a three-fold decrease in the BIM BH3 concentration required to activate BAX rather than BAK (6.7μM for BAK versus 2.3μM for BAX). Based on these findings, we hypothesized that cells lacking BAK would be resistant to commonly used topoisomerase inhibitors including topotecan, etoposide and doxorubicin which can induce apoptosis via activation of BID. We confirm that BAK-/-, but not BAX-/-, cells are in fact resistant to these agents but not chemotherapies that induce apoptosis independently of BID activation including the alkylating agent cisplatin, the microtubule inhibitor paclitaxel, or the nucleoside analogue gemcitabine. Topoisomerase inhibitors are frequently used to treat ovarian cancer patients who have failed initial therapy with carboplatin and paclitaxel, yet clinical responses to these second-line therapies are varied. Importantly, we found that 23.8% of ovarian primary tumors possess a heterozygous or homozygous loss of BAK1, which encodes BAK. These patients exhibit an vastly inferior overall survival when treated with topoisomerase inhibitors in the clinic (p=0.0048) while loss of BAX had no effect on survival (p=0.9534). In addition, BAK1 loss had no effect on response to treatment with agents that were not topoisomerase inhibitors including carboplatin and paclitaxel. Thus, BIM and BID have non-overlapping roles in the induction of apoptosis via BAX and BAK which affect clinical responses to chemotherapy. Citation Format: Kristopher A. Sarosiek, John Bachman, Joan Montero, Luv Patel, Joshua J. Sims, Xiaoke Chi, David W. Andrews, Peter Sorger, Anthony Letai. BAX and BAK are preferentially activated by BIM and BID, respectively, affecting clinical chemotherapy response. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1713. doi:10.1158/1538-7445.AM2013-1713