Repression of androgen receptor mediated transcription by the ErbB-3 binding protein, Ebp1

Abstract

Members of the ErbB family of receptors have been implicated in regulation of androgen receptor (AR) activity. Ebp1, an ErbB-3 binding protein recently cloned in our laboratory, possesses an LXXLL motif important in mediating interactions with nuclear hormone receptors. Therefore, we sought to determine if Ebp1 could bind AR and influence AR transcriptional activation potential. We demonstrate in this study that Ebp1 bound to AR in vitro and in vivo, and that this binding was increased by androgen treatment. The C terminal 79 amino acids of Ebp1 were sufficient to bind AR. The N terminal domain of AR was responsible for binding Ebp1. Ligand-mediated transcriptional activation of both artificial and natural AR regulated promoters was inhibited by ectopic expression of ebp1 in transient transfection systems. Ebp1 deletion mutants that either lacked the C terminal AR binding region or had a mutated LXXLL motif failed to inhibit AR activated transcription. PSA expression from its endogenous promoter was also decreased in LNCaP prostate cancer cells overexpressing Ebp1. The growth of AR positive LNCaP cells was inhibited by ectopic expression of ebp1, but mutants that failed to repress transcription did not inhibit cell growth. These studies suggest that Ebp1 may play a role in the function of the AR and provide a link between ErbB receptors and the AR.

Specificity and heregulin regulation of Ebp1 (ErbB3 binding protein 1) mediated repression of androgen receptor signalling

“…We have previously established that Ebp1, a protein cloned in our laboratory via its interactions with the ErbB3 receptor, inhibits AR-mediated transcription and growth of the AR positive LNCaP cell line (Zhang et al, 2002). In this report, we confirm and extend our findings by demonstrating that Ebp1 is capable of inhibiting receptor transactivation independent of cell type or AR target promoter.…”

“…We previously demonstrated that Ebp1 inhibited transactivation of the artificial ARE 2 DS promoter in COS cells transfected with wildtype AR, and the PSA promoter in LNCaP cells that harbor a mutant AR (Zhang et al, 2002). We were interested in determining if Ebp1 represses wild-type AR in prostate cancer cell lines.…”

“…We had previously demonstrated that Ebp1 represses both exogenous and endogenous PSA promoter activity in LNCaP cells (Zhang et al, 2002). We therefore tested the ability of Ebp1 to repress the MMTV-luc and probasin native androgen responsive promoters.…”

“…Ectopic expression of ebp1 inhibits ligand-mediated transcriptional activation of both artificial and natural AR regulated promoters in COS cells transfected with wild-type AR and in LNCaP cells that express a mutant AR. The transcription of the endogenous PSA gene is also decreased in LNCaP cells stably transfected with Ebp1 (Zhang et al, 2002). However, the effect of Ebp1 on transactivation of wild-type AR in prostate cancer cells was not established.…”

“…Our laboratory has recently demonstrated that a protein Ebp1, isolated by its binding to HRG's cognate receptor ErbB3 , binds AR in vitro and in vivo (Zhang et al, 2002). Ebp1 is expressed in both normal prostate epithelial cells and in the prostate cancer cell lines LNCaP, DU-145 and PC-3 (Xia et al, 2001b).…”