Abstract CN06-03: Androgen receptor splice variants dimerize to regulate gene expression and promote castration-resistant cell growth

Recurrence with lethal castration-resistant prostate cancer after androgen deprivation therapy remains the major challenge in treatment of advanced prostate cancer. Significant advances in our understanding of continued androgen receptor (AR) signaling in castration-resistant prostate cancer have led to the development and FDA approval of two next-generation androgen-directed therapies, the androgen biosynthesis inhibitor abiraterone and the potent AR antagonist enzalutamide. These drugs heralded a new era of prostate cancer therapy. However, some patients present with therapy-resistant disease, and most initial responders develop acquired resistance within months of therapy initiation. The resistance is typically accompanied by increased prostate-specific antigen, indicating reactivated AR signaling. Accumulating evidences indicate that prostate tumors can adapt to these androgen-directed therapies, including abiraterone and enzalutamide, by signaling through constitutively-active alternative splicing variants of AR (AR-Vs) that lack the ligand-binding domain. AR-V7 and ARv567es are two major AR-Vs expressed in clinical specimens. They localize primarily to the nucleus, and regulate target-gene expression in a ligand-independent manner. However, little is known about how they control gene expression. In the present study, we used the bimolecular fluorescence complementation (BiFC) and bioluminescence resonance energy transfer (BRET) assays to show that AR-V7 and ARv567es not only homodimerize and heterodimerize with each other but also heterodimerize with the full-length androgen receptor (AR-FL). The heterodimerization of AR-V and AR-FL was mediated by N- and C-terminal interactions and by the DNA-binding domain of each molecule, whereas AR-V homodimerization was mediated only by DNA-binding domain interactions. Significantly, AR-V dimerization was required to transactivate target genes and to confer castration-resistant cell growth. It is of note that the heterodimerization between AR-V7 and AR-FL or between ARv567es and AR-FL was independent of androgen and cannot be modulated by antiandrogens such as enzalutamide alone. However, androgen inhibited the heterodimerization, and this effect was blocked by enzalutamide. The data indicate that AR-V7 and ARv567es may compete with AR-FL for dimerizing with AR-FL. Since AR-V7 and ARv567es localize constitutively to the nucleus, their heterodimerization with AR-FL facilitated AR-FL nuclear localization in the absence of androgen. In the nucleus, AR-V co-occupied the promoter of a canonical AR target with AR-FL in a mutually-dependent manner. Thus, the data suggest that AR-V7 and ARv567es can activate the AR-FL in an androgen-independent manner. Since AR-Vs are often co-expressed with AR-FL in biological contexts, it is conceivable that the ability of AR-Vs to heterodimerize with and activate AR-FL in an androgen-independent manner could be equally important as their AR-FL-independent activity to castration resistance. Our results elucidate the mechanism by which AR-Vs mediate gene regulation and provide a pivotal pathway for rational drug design to disrupt AR-V signaling, as a rational strategy for effective treatment of advanced prostate cancer. Citation Format: Yang Zhan, Duo Xu, Yanfeng Qi, Bo Cao, Shanshan Bai, Oliver Sartor, Erik K. Flemington, Haitao Zhang, Yan Dong. Androgen receptor splice variants dimerize to regulate gene expression and promote castration-resistant cell growth. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr CN06-03.