Abstract IA8: Mechanisms regulating distinct AR transcriptional programs in PCa

Abstract

Abstract Androgen receptor (AR) plays a central role in prostate cancer (PCa), and androgen deprivation is still the standard systemic therapy used for metastatic disease, but patients invariably relapse with a more aggressive form of PCa (castration resistant/recurrent PCa, CRPC). Studies in clinical samples and xenograft models indicate that increased AR gene expression plays a major role in the progression to CRPC. Previous studies showed that the agonist-liganded AR could suppress AR gene expression, so that AR mRNA levels increase acutely after castration, but the molecular basis for this negative regulation was not known. We have found that the agonist-liganded AR represses AR gene transcription through binding to a conserved site in the second intron of the AR gene (AR binding site 2, ARBS2). In the absence of androgen, ARBS2 is associated with FOXA1, OCT1, GATA2, and H3K4 mono- and dimethylation that are further increased in cells adapted to androgen deprivation, consistent with this site functioning as an enhancer that contributes to increased AR gene expression in CRPC. Significantly, the agonist liganded AR recruits lysine specific demethylase 1 (LSD1) to ARBS2, which then represses AR gene transcription through demethylation of the enhancer associated H3K4me1,2 marks. In contrast to this LSD1 mediated H3K4 demethylation and transcriptional repression, LSD1 associated with AR stimulated genes functions as a coactivator through demethylation of repressive H3K9me1,2 marks. The molecular basis for these distinct functions of LSD1 on AR stimulated versus AR repressed may be related to histone phosphorylation, but this or other mechanisms remain to be established and are under investigation. The rapid androgen mediated downregulation of the androgen synthetic enzymes AKR1C3 and HSD17B6 is similarly LSD1 dependent, indicating that the agonist liganded AR directly mediates a physiological intracellular negative feedback loop to regulate AR activity. Further analysis of gene expression in androgen starved versus androgen stimulated PCa cells revealed LSD1 dependent androgen repression of multiple genes mediating DNA synthesis and cell cycle progression, versus androgen stimulated expression of genes mediating synthesis of lipids, amino acids, and other metabolic processes. This profile is consistent with AR function in normal prostate epithelium to drive terminal differentiation and synthesis of seminal fluid, and may provide a molecular basis for the biphasic response to androgen stimulation whereby PCa cells proliferate in response to low levels of androgen but are growth arrested at high concentrations. Significantly, expression of AR repressed genes is generally increased in CRPC, indicating that an unintended consequence of androgen deprivation may be to increase expression of a subset of genes driving tumor growth. Taken together, these findings elucidate a mechanism that contributes to increased AR gene expression and restored AR activity in CRPC, and identify a suppressor element and transcriptional repressor function for the agonist liganded AR. The distinct mechanisms of AR and LSD1 action on enhancer versus suppressor elements may make it possible to selectively augment AR transcriptional repressor function and thereby prevent or delay the emergence of CRPC. Citation Format: Changmeng Cai, Housheng He, Myles Brown, Steven Balk. Mechanisms regulating distinct AR transcriptional programs in PCa [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr IA8.