Abstract A13: Androgen receptor functions as an adaptor protein for recruitment of protein phosphatase 1a to chromatin and CDK9 mobilization

Abstract

Abstract The androgen receptor (AR) plays a central role in prostate cancer (PCa) development and progression. AR activity can be modulated by phosphorylation at multiple sites, with S81 phosphorylation mediated by CDK9 playing a particularly important role that may be due to enhancing recruitment of the coactivator p300. The CDK9/cyclin T complex (P-TEFb) is recruited to enhancer/promoter sites by BRD4, where it phosphorylates NELF and RNA polymerase 2 to stimulate transcriptional elongation. A large fraction of P-TEFb is held in an inactive state by association with a 7SK RNA complex, and can be released from this complex through dephosphorylation of CDK9 by protein phosphatase 1 (PP1). The major isoform of the PP1 catalytic subunit, PP1a, associates with a large number of regulatory proteins that target it to specific sites. PNUTS is a regulatory protein targeting PP1a to chromatin, but additional mechanisms that may target PP1a to chromatin to mediate CDK9 dephosphorylation and drive transcriptional elongation remain to be identified. We reported previously that AR could interact with PP1a, and that PP1a could enhance AR activity by dephosphorylating S650 in the AR hinge region and thereby prevent its nuclear export. In this study we show by chromatin immunoprecipitation that AR also recruits PP1a to androgen-stimulated enhancers, and show that PP1a inhibition with tautomycin prevents P-TEFb recruitment to AR-regulated genes and CDK9-mediated phosphorylation of RNA polymerase 2. We further show that PP1a binds to the AR ligand binding domain independently of ligand, and that this binding is negatively regulated by the androgen induced AR N/C-terminal interaction. Significantly, the AR N/C interaction impairs AR recruitment of coactivators prior to chromatin binding, but is disrupted on chromatin to enhance coactivator recruitment, which would also facilitate recruitment of PP1a. These findings indicate that AR functions as a PP1a regulatory protein targeting PP1a to chromatin, and thereby identify a novel mechanism for the selective recruitment of PP1a to a specific subset of genes where it can drive transcriptional elongation, and may also regulate mRNA splicing. Citation Format: Xiaming Liu, Yanfei Gao, Changmeng Cai, Steven Balk, Shaoyong Chen. Androgen receptor functions as an adaptor protein for recruitment of protein phosphatase 1a to chromatin and CDK9 mobilization. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr A13.