Abstract 3131: Androgen receptor promotes ligand-independent prostate cancer progression through c-Myc upregulation

Prostate cancer is the most common cancer in men in the United States and the second leading cause of cancer death. The androgen receptor (AR) is the central signaling pathway in prostate cancer. Because of this, androgen deprivation therapy (ADT), which involves reducing levels of androgen ligands or interfering with binding of ligands to AR, has been the major therapeutic focus for the past 70 years. However, in many patients there is no benefit from ADT, and all prostate cancers eventually progress. At progression on ADT, the AR is ubiquitously expressed. For this reason, we contend that AR-dependent mechanisms are critical for prostate cancer progression after ADT. Indeed, recent work demonstrates that the AR can function independently of ligands to promote prostate cancer cell survival. However, specific ligand-independent AR target genes that account for this effect were not well-characterized. We show here that c-Myc, which is a key mediator of ligand-independent prostate cancer progression, is a key ligand-independent AR target gene. Using microarray analysis, we found that c-Myc and AR expression levels strongly correlated with each other in tumors from patients with castration-resistant prostate cancer (CRPC) progressing despite ADT. We confirmed that AR directly regulates c-Myc transcription in a ligand-independent manner, that AR and c-Myc suppression reduces ligand-independent prostate cancer cell growth, and that ectopic expression of c-Myc attenuates the anti-tumor activity of AR suppression. Importantly, treatment with the bromodomain inhibitor JQ1 suppressed c-Myc function and suppressed ligand-independent prostate cancer cell survival. Our results define a new link between two critical proteins in prostate cancer - AR and c-Myc - and demonstrate the potential of AR and c-Myc-directed therapies to improve prostate cancer control. Citation Format: Lina Gao, Jacob Schwartzman, Angela Gibbs, Richard Kleinschmidt, Beth Wilmot, Daniel Bottomly, Ilsa Coleman, Peter Nelson, Shannon McWeeney, Joshi J. Alumkal. Androgen receptor promotes ligand-independent prostate cancer progression through c-Myc upregulation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3131. doi:10.1158/1538-7445.AM2013-3131