Abstract 1514: A novel approach for targeting androgen receptor signaling axis in prostate cancer

Abstract Background and Objectives: The androgen receptor (AR) is a ligand-activated transcription factor (TF) that plays critical oncogenic roles. Medical castration therapy is the mainstay treatment for advanced PrCa, but the AR signaling axis frequently remains active in castration-resistant PrCa (CRPC). Despite advances in AR targeting with abiraterone and enzalutamide, disease progression re-occurs in the form of CRPCs that still express AR-dependent genes. This highlights the need for novel approaches to block AR signaling and CRPC growth. The GATA family of TFs contains six members in mammals, all of which bind a consensus DNA sequence (A/T)GATA(A/G) to regulate gene expression. GATA2 is the predominant family member in prostate luminal epithelial cells. In a search for TFs that control AR expression, we found that GATA2 induces AR gene expression as well as promotes the recruitment of coactivators to the AR transcriptional complex. We found that GATA2 protein expression was strongly correlated with AR protein expression (Spearman's correlation coefficient: 0.323, p-value <0.0001) in our PrCa specimens (n=383). The intensity of GATA2 immunostaining in our BCM patient cohort positively correlated with Gleason score (R=0.37, p-value <0.01) and was a significant predictor of biochemical recurrence (p-value <0.001). We also demonstrated that GATA2 directly promotes expression of both full-length and AR splice-variant, resulting in a strong positive correlation between GATA2 and AR expression in both PrCa cell lines and patient specimens. Conversely, GATA2 expression is repressed by androgen and AR, suggesting a negative feedback regulatory loop that, upon androgen deprivation, derepresses GATA2 to contribute to AR overexpression in CRPC. Simultaneously, GATA2 is necessary for optimal transcriptional activity of both full-length and AR splice-variants. Using ChIP-Seq we found that GATA2 colocalizes with AR and Forkhead box protein A1 (FOXA1) on chromatin to enhance recruitment of steroid receptor coactivators (SRC1, SRC2, and SRC3) and formation of the transcriptional holocomplex. Lastly, we identified a GATA2 small molecule inhibitor (SMI) that can suppress the expression and transcriptional function of both full-length and splice-variant AR and exerted potent anticancer activity against PrCa cell lines. We propose that the inhibition of GATA2 is a ‘first-in-field’ approach to target AR expression and function, including ligand-independent AR, for the treatment of CRPC. Citation Format: Salma Kaochar, Christopher Foley, Cristian Coarfa, Nicholas Mitsiades. A novel approach for targeting androgen receptor signaling axis in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1514. doi:10.1158/1538-7445.AM2017-1514