Abstract 1023: Combining all-trans retinoic acid therapy with androgen receptor N-terminal domain inhibitors for the treatment of castration-resistant prostate cancer
Authors: Leung Jacky K. , Sadar Marianne D. 
Published: August 2019

Abstract
Androgen receptor (AR) signaling plays an essential role in all stages of prostate cancer. Androgen-deprivation therapy is generally effective for advanced prostate cancer until progression to lethal metastatic castration-resistant prostate cancer (mCRPC). Most CRPC continues to be driven by AR signaling. AR transcriptional activity requires a functional N-terminal domain (NTD). Transactivation of AR is mediated by ligand-independent activation by cross-talk with signal transduction pathways targeting the AR NTD, gain-of-function mutations in AR ligand-binding domain (LBD), or expression of truncated AR splice variants that lack LBD (e.g., AR-V7). The intrinsically disordered AR NTD is essential for its transcriptional activity. It harbors six putative binding sites for Pin1, a proline isomerase that regulates protein conformation at specific phosphorylated-Ser/Thr-Pro motifs. All-trans retinoic acid (ATRA) is a validated and potent Pin1 inhibitor. Since conformational changes within the AR NTD are required for transactivation, perturbation of its structure may be a promising approach to block its activity. The purpose of this study was to assess a therapy that combined ATRA with antagonists of AR NTD, ralaniten (EPI-002) and its analogues. We hypothesized that targeting Pin1 activity with ATRA would disrupt the AR NTD and enhance inhibition by EPI compounds which bind to Tau-5 in the NTD. Using reporter gene assays, proliferation assays, and cell cycle analysis by flow cytometry, we tested ATRA in isolation and in combination with EPI in androgen-sensitive (LNCaP) and androgen-independent (LN95) prostate cancer cell lines. We found that treatment with ATRA decreased the transcriptional activity of AR and androgen-induced expression of PSA. ATRA also attenuated the transcriptional activity of AR-V7 and androgen-independent growth of LN95 cells expressing both full-length AR and AR-V7. Co-immunoprecipitation studies confirmed interactions between Pin1 and specific regions of AR NTD. In combination, ATRA had synergistic interaction with EPI compounds and lowered the effective inhibitory concentrations for blocking AR transcriptional activity. Furthermore, combinations decreased cell cycle progression of LN95 cells through S-phase, leading to accumulation of cells in G1 and induction of senescence. These preclinical findings showed that ATRA enhanced the potency of AR NTD inhibitors and support a novel therapeutic strategy for CRPC. Future studies will determine the in vivo antitumor effect of ATRA in combination with EPI compounds on the growth of CRPC xenografts.
Citation Format: Jacky K. Leung, Marianne D. Sadar. Combining all-trans retinoic acid therapy with androgen receptor N-terminal domain inhibitors for the treatment of castration-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1023.