Introduction and hypothesis: Defects in apoptosis not only provide cancer cells with an intrinsic survival advantage, but also confer inherent resistance to chemotherapeutic drugs. Microtubule inhibitors such as vinblastine and Taxol are important antitumor agents. Bcl-xL is the most important protein regulating cellular sensitivity to drugs and its phosphorylation appears to be a universal response to microtubule inhibitors. We hypothesize that phosphorylation of Bcl-xL is a key event in apoptosis induced by microtubule inhibitors and that phosphorylation alters its interaction with other proteins to promote apoptosis. Approach and Results: We previously identified Ser62 as the major site of phosphorylation in Bcl-xL induced by microtubule inhibitors. Phospho-defective (Ser62\#8594;Ala) Bcl-xL and phospho-mimic (Ser62-Asp) Bcl-xL mutants were generated and stably or transiently expressed in KB-3 cells to probe the functional significance of phosphorylation. Individual mutants were readily expressed in cells at comparable levels and without adverse effects. However, when transient co-transfections with myc-Bax were performed, wild-type and phospho-defective Bcl-xL protected cells from Bax-induced cell death, but phospho-mimic Bcl-xL had a greatly reduced capacity to do so. Co-immunoprecipitation revealed that phosphorylation caused wild-type Bcl-xL to release bound Bax, whereas phospho-defective Bcl-xL retained the ability to bind Bax. Further, phospho-mimic Bcl-xL had a reduced ability to bind Bax. Conclusion: These studies provide novel mechanistic insight into the role of Bcl-xL phosphorylation and suggest that phosphorylation plays a pro-apoptotic role at least in part by disabling the ability of Bcl-xL to sequester Bax. Further studies aim to elucidate whether phosphorylation modulates binding of other apoptotic regulatory proteins to Bcl-xL, and to determine whether Bcl-2 phosphorylation plays an analogous role.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3263.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO