Abstract 334: Mcl-1 protects prostate cancer cells from chemotherapy-induced DNA damage

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Mcl-1 (myeloid cell leukemia sequence 1 [Bcl2-related]) is predominantly localized in the cytoplasm and protects cells from apoptosis by blocking mitochondrial outer membrane permeabilization. There is evidence that nuclear localization of Mcl-1 has pleiotropic effects independent of apoptosis, including functional roles in DNA damage response. Mcl-1 is highly expressed in castration-resistant prostate cancer (CRPC), resulting in resistance to apoptosis and association with poor prognosis. The role of nuclear Mcl-1 in mediating sensitivity to chemotherapy-induced DNA damage and apoptotic cell death in prostate cancer (PCa) is not known. The novel antimitotic agent ENMD-1198 is a more stable and potent derivative of 2-methoxyestradiol (2ME2). We confirmed that 1198 was a more potent inhibitor of androgen-dependent LNCaP and castration-resistant DU145 and PC3 PCa cell viability compared to 2ME2 and induced apoptotic cell death in docetaxel-resistant PC3 cells that overexpress the multi-drug resistance protein (MDR). The ability of 1198 to increase the proteasomal-mediated degradation of Mcl-1 correlated with enhanced apoptotic cell death and shRNA knockdown of Mcl-1 further sensitized LNCaP and PC3 cells to 1198-mediated cell death. We then showed that the combination of 1198 with betulinic acid (BA, a plant-derived small molecule that can increase apoptosis specifically in cancer but not in normal cells) reduced nuclear Mcl-1, increased a pro-death Mcl-1 isoform, increased gamma histone 2AX (marker of DNA damage), and enhanced apoptotic cell death in human PC3 CRPC cells. In the TRAMP mouse model of PCa, the 1198 (75 mg/kg) + BA (10 mg/kg) combination significantly reduced PCa weights compared to 1198, BA alone, and vehicle controls. However, the reduced PCa weights between the 1198 + BA combination and BA alone was more associated with decreased Mcl-1 and increased gamma-H2AX (DNA damage) immunostaining compared to cleaved caspase-3 (apoptosis), CD31(angiogenesis), or Ki67 (proliferation). Furthermore, our results showed that blocking the DNA damage response signaling pathway with an ATM kinase (KU55933) or DNA-PK (NU7441) inhibitor lowered apoptotic cell death induced by the 1198 + BA combination in PC3 cells. Overall, our results demonstrate that nuclear Mcl-1 has an important role in protecting PCa cells from DNA damage and that agents such as 1198 and BA that reduce total/nuclear Mcl-1 and increase pro-death Mcl-1 isoforms will sensitize PCa cells to antimitotic DNA damaging agents. Citation Format: Carlos Perez-Stable, Teresita Reiner, Alicia de las Pozas, Ricardo Parrondo. Mcl-1 protects prostate cancer cells from chemotherapy-induced DNA damage. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 334. doi:10.1158/1538-7445.AM2014-334