Abstract 1255: Mifepristone causes endoplasmic reticulum stress, triggers the unfolded protein response, and increases autophagic flux in ovarian cancer cells

Mifepristone (MF) is a synthetic steroid that blocks the growth of ovarian cancer cells causing cytostasis or lethality depending on its concentration. Unbiased genomic and proteomic screenings using ovarian cancer cell lines of different genetic backgrounds and sensitivities to platinum led to the identification of two key genes up-regulated by MF and that are involved in the unfolded protein response (UPR): the master chaperone of the endoplasmic reticulum (ER), glucose regulated protein (GRP) 78, and the CCAAT/enhancer binding protein homologous transcription factor (CHOP); both proteins are known for balancing ER stress in a ‘yin’ (CHOP; pro-cell death factor) ‘yang’ (GRP78; pro-survival factor) manner. GRP78 and CHOP were up-regulated in ovarian cancer cells regardless of p53 status and platinum sensitivity. Further studies revealed that MF caused expansion of the ER and triggered the UPR. The three UPR associated signaling pathways, PERK, IRE1α, and ATF6, were activated by MF. While low-dose MF caused cytostasis with compensatory UPR activation, high-dose MF induced lethality with concurrent UPR activation, up-regulation of pro-apoptotic factor Bim, down-regulation of anti-apoptotic factor Bcl-2, and cleavage of PARP, a downstream target for executer caspases. MF-induced lethality but not cytostasis was abrogated by the ER stress inhibitor salubrinal, indicating that ER stress is required when MF causes cell death but not when its action is limited to cell cycle arrest. Because ER stress involves the accumulation of misfolded/unfolded proteins that alter ER homeostasis, the UPR engages compensatory mechanisms including short-term attenuation of the rate of protein translation, and degradation of surplus proteins by the ubiquitin proteasome system (UPS) and/or autophagy. Contrary to our expectation, MF-induced UPR associated with short-term increase rather than decrease, in protein translation rate. Moreover, the accumulation of poly-ubiquitinated proteins-a surrogate marker of the activity of the UPS-was slightly decreased by MF, suggesting that ER-associated protein degradation (ERAD) may be operating at a faster pace. Finally, MF increased the accumulation of LC3-II, which results from the fusion of LC3-I and phosphatidylethanolamine along the process of autophagosome formation. MF-induced LC3-II accumulation also augmented in the presence of the lysosomal degradation inhibitor bafilomycin A1, indicating that MF increases autophagic flux. In MF-treated cells, the induction of autophagy may counterbalance and alleviate ER expansion induced by ER stress. The stimulation by MF of ER stress, UPR, and autophagy offers a therapeutic opportunity for utilizing this compound to sensitize ovarian cancer cells to aggravators of the ER and/or inhibitors of autophagy. Citation Format: Lei Zhang, Alicia Goyeneche, Rekha Srinivasan, Carlos Telleria. Mifepristone causes endoplasmic reticulum stress, triggers the unfolded protein response, and increases autophagic flux in ovarian cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1255. doi:10.1158/1538-7445.AM2015-1255