Abstract 1234: Galeterone-induced degradation of the androgen receptor involves inhibition of deubiquitinating enzymes

Abstract

Abstract Galeterone is a highly selective oral small molecule drug candidate that disrupts androgen receptor (AR) signaling through degradation of the AR, is a potent CYP17 lyase inhibitor, and possesses AR antagonist activity. Galeterone-induced AR degradation was observed in models having either full-length AR or known constitutively active truncated forms of the AR receptor that lack the ligand binding domain (LBD), AR-V7 and AR567es, suggesting the LBD of the AR is not required for galeterone-dependent AR degradation. Further, galeterone-induced AR degradation activity is blocked by co-administration of the proteasome inhibitor MG132. Along these lines, galeterone-induced AR degradation can be blocked by selective knock-down of the E3 ligases, Mdm2 and CHIP. We utilized a series of biochemical and cell-based in vitro methodologies to further elucidate and characterize additional signaling molecules participating in the proteasomal-dependent mechanism of galeterone-induced AR degradation. We screened a panel of 22 deubiquitinating enzymes (DUBs) in vitro and demonstrated that galeterone selectively inhibited enzymatic activity of two of the DUBs, USP12 and USP46. Separately, we used surface plasmon resonance to demonstrate a dose-dependent direct binding of galeterone to each of USP12 and USP46, alone or when pre-complexed with UAF1. USP12 is a co-activator of the AR, and selective knock-down of this DUB has been shown to increase AR degradation. USP12/USP46 have also been linked to regulation of phosphatases (PHLPPs) through ubiquitination. PHLPPs dephosphorylate AKT, providing an important regulatory mechanism for controlling the PI3K/AKT pathway. It is known that galeterone induces an increase in pAKT and pMdm2, the latter being a substrate of activated AKT. This suggests that inhibition of USP12/USP46 may regulate pAKT levels through enhanced degradation of PHLPPs via increased ubiquitination. These data support a differentiating mechanism of galeterone from other AR targeting agents whereby inhibition of USP12/USP46 leads to enhanced AR degradation. Citation Format: Daniel T. Dransfield, Nivedita Namdev, Douglas B. Jacoby, Karen Ferrante. Galeterone-induced degradation of the androgen receptor involves inhibition of deubiquitinating enzymes. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1234.