Abstract 3522: Both mitochondrial function and composition of BCL2 family proteins determines sensitivity to Cisplatin in ovarian cancer cells and are promising targets to overcome Cisplatin resistance in ovarian cancer

Mitochondria are critical target structures of platinum drugs. To study whether mitochondrial activities of Cisplatin are critical for its cytotoxic function we studied mitochondrial mass, mitochondrial function and composition of pro- and anti-apoptotic BCL2 family proteins in three resistant and three sensitive high-grade serous carcinoma (HGSC) cell lines. Sensitive cell lines differed from resistant cells by higher mitochondrial mass (MM), higher levels of mitochondrial reactive oxygen species (mtROS) and higher basal oxygen consumption rate (OCR) levels as well as a higher ratio of pro- vs. anti-apoptotic BCL2 family proteins. Cell death upon Cisplatin in sensitive cells was dependent on induction of reactive oxygen species (ROS), loss of mitochondrial membrane potential (MMP), induction of BAX-BAK pores and caspases. Genetically alteration of the BCL2 family protein ratio by siRNA mediated knockdown of pro-apoptotic NOXA reduced acute response in sensitive cell lines, whereas knockdown of anti-apoptotic BCLw rendered resistant cells sensitive. In analogy, pharmacological inhibition of anti-apoptotic proteins by ABT737 also completely re-sensitized Cisplatin resistant cell lines. Cisplatin exposure increased MM, mtROS and OCR in both sensitive and insensitive HGSC cell lines. However, these parameters reached higher peak levels in cell lines sensitive to induction of cell death by Cisplatin. Knockdown of PGC-1α (the master regulator of mitochondrial biogenesis) partially rescues sensitive cells from apoptosis. In addition, pharmacological inhibition of ATP synthase by Oligomycin A blocks MMP and mtROS induction by Cisplatin and inhibits Cisplatin induced cell death. This supports the hypothesis that induction of mitochondria leading to increased release of mtROS contributes to cell death by Cisplatin. We finally tested whether other modulators of mitochondrial function restore sensitivity to Cisplatin in resistant cells. Whereas Complex-I and Complex-III inhibitors did not affect cytotoxicity of Cisplatin, the iron chelator VLX600 had significant synergistic activity with Cisplatin in resistant cancer cells. VLX600 inhibits oxygen consumption almost completely in combination with Cisplatin whereas both compounds had no significant effect on survival when used as monotherapy. In conclusion, cytotoxicity of Cisplatin in HGSC cell lines depends on efficient induction of MM and mtROS. Resistance can efficiently be targeted by modulating the mitochondrial function by VLX600. These observations support the view that mitochondria are attractive targets for increasing Cisplatin activity on cancer cells. Citation Format: Markus Kleih, Simon Heine, Kathrin Bopple, Meng Dong, Heiko van der Kuip, Walter E. Aulitzky. Both mitochondrial function and composition of BCL2 family proteins determines sensitivity to Cisplatin in ovarian cancer cells and are promising targets to overcome Cisplatin resistance in ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3522.