Abstract 4809: Bardoxolone-methyl suppresses both androgen receptor and its splice-variant ARv7 in prostate cancer cells to enhance the anti-cancer efficacy of enzalutamide

Prostate cancer (PCa) is a leading cause of morbidity and mortality in elderly men in the United States. Despite androgen deprivation therapy (ADT), the recurrence of castration resistant prostate cancer (CRPC) remains a significant challenge. Augmented signaling via the full-length androgen receptor (AR) and the constitutively active AR splice variants, especially ARv7, is associated with ADT-resistance. Therefore, a proper understanding of augmented AR signaling and adjunct strategies to suppress both AR and ARv7 levels are critically needed. ADT-induced oxidative stress (Ox-stress) can facilitate CRPC outgrowth. We previously documented that the Ox-stress induced transcription factor, Nrf2 can suppress AR expression and AR-transactivation function in PCa cells. Recently, we also showed that the Nrf2-inducing natural compound, sulforaphane (SFN) can decrease both AR and ARv7 levels in PCa cells, and co-exposure to SFN enhanced the efficacy of anti-androgens. In this study, we examined whether a synthetic triterpenoid drug, Bardoxolone-methyl (CDDO-me), a potent enhancer of Nrf2, can similarly decrease AR and ARv7 expression. Exposure to nanomolar (nM) concentrations of CDDO-me rapidly downregulated AR levels in the androgen-dependent LNCaP cells and androgen-independent C4-2B cells. Exposure to CDDO-me also suppressed both AR and ARv7 levels in the CRPC line, CWR22Rv1. A biphasic effect of CDDO-me on reactive oxygen species (ROS) levels was observed. Pre-exposure to the antioxidant, n-acetyl cysteine (NAC) blocked the AR-suppressive effect of CDDO-me. Molecular mechanistic studies using actinomycin-D (transcription inhibitor), cycloheximide (translation inhibitor), and MG132 (proteasomal inhibitor) suggested that CDDO-me may decrease AR gene expression and increase degradation of AR and ARv7 proteins. Furthermore, exposure to CDDO-me increased proteasomal activity in PCa cells and suppressed the levels of several AR splicing factors, i.e. the heteronuclear ribonucleoproteins A1 and H1 (hnRNP-A1 and hnRNP-H1). Co-exposure to CDDO-me increased the anti-cancer efficacy of enzalutamide (ENZ) as evident by decreased cell-viability (MTT), migration (scratch-wound) and colony forming unit (CFU) assays. Therefore, adjunct treatment with low-dose CDDO-me may be an effective strategy to suppress AR and ARv7 levels and augment the efficacy of anti-androgens. Most importantly, since CDDO-me is in late-stage clinical trials for chronic kidney disease (CKD), its repositioning as a potent AR suppressive agent may be of significant translational value in PCa patients undergoing ADT. Citation Format: Namrata Khurana, Partha Chandra, Hogyoung Kim, Asim B Abdel-Mageed, Suresh Sikka, Debasis Mondal. Bardoxolone-methyl suppresses both androgen receptor and its splice-variant ARv7 in prostate cancer cells to enhance the anti-cancer efficacy of enzalutamide [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 4809.