Abstract 4678: A novel nuclear transporter for androgen receptor and AR-variant-7 in castration resistant prostate cancer: Ideal therapeutic target

Abstract

Abstract Introduction: The dismal outcome of androgen receptor (AR)-targeted therapies in Castration-resistant prostate cancer (CRPC) has been largely attributed to the activation of AR-variants in tumor cells. Although the nuclear carrier of full-length AR (ARFL) is known, the mechanism underlying the nuclear translocation of AR-variants (ARv; which do not have androgen-binding domain) under castration conditions is not completely understood. Understanding such a mechanism could form the basis of future therapies to treat CRPC disease. Objective: We determined the mechanism involved in the nuclear transport of LBD-devoid AR-variants under CRPC conditions. We for the first time show that “cFLIP” acts a (i) shuttling protein between cytoplasm and nucleus of cells, and (ii) conduit for AR-FL and ARv7 in CRPC cells. Results: We show that (i) cFLIP-expressing CRPC cells exhibit higher proliferative index than cFLIP-deficient counterparts and (ii) cFLIP expression is high in prostatic tissues of castrated (ARR2.IκBMyc) transgenic mice. The significance of cFLIP as an upstream of AR could be ascertained from the finding that suppressing cFLIP decreased PSA levels and transcriptional activities of AR-FL and ARv7 in CRPC cells. These data prompted to further investigate the correlation between cFLIP, AR-FL and ARv7. We show that cFLIP forms a complex with ARv-7 and AR-FL in CRPC cells. We also show that cFLIP/ARv7 complex levels are high in nuclei than in cytoplasm of CRPC cells suggesting the possibility of cFLIP as a protein carrier. Using transfections with cFLIP-Long, cFLIP-short-variant and mutants (cFLIPL-435mt/472mt/439mt), two sequences on C-terminus of cFLIP were identified for nuclear translocation of AR. These data were also validated in PC3 cells (in which ARv7 was ectopically expressed). We show that β-catenin is required for complex stabilization and AR-nuclear translocation activity of cFLIP. Previously, cFLIP was identified as an androgen-responsive gene. However, we show that cFLIP expression is independent of androgen in CRPC cells Finally, we established the significance of cFLIP/ARv7 complex as a therapeutic target and showed that simultaneous targeting of ARv and cFLIP (using nanoparticle-loaded cFLIP-siRNA + ARv7-siRNA) significantly inhibited CRPC-type tumor growth and improved the outcome of docetaxel therapy in mouse model. We also identified a novel natural inhibitor of cFLIP/ARv. Conclusions: We identified novel (i) transporter of AR and (ii) approach for treating CRPC in men. Citation Format: Aijaz Parray, Hifzur R. Siddique, Alyssa Langfald, Pooja Singh, Mikihiko Naito, Robert Matusik, Ingo Schmitz, Shahriar Koochekpour, Badrinath R. Konety, Mohammad Saleem. A novel nuclear transporter for androgen receptor and AR-variant-7 in castration resistant prostate cancer: Ideal therapeutic target. [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 4678. doi:10.1158/1538-7445.AM2015-4678