Androgen receptor (AR) plays a central role in normal prostate growth and in prostate cancer development. The goal of this study was to identify negative regulatory factor(s) that contribute to proliferation inhibition of prostate cancer cells. The biphasic proliferation pattern of LNCaP human prostate cancer cells at low versus high androgen concentrations was utilized to examine regulatory factors involved in AR-mediated regulation of E2F1 gene transcription. E2F1 is a key cell cycle regulatory transcription factor and an important proliferation stimulator of prostate cancer cells. LNCaP cells are known to be stimulated by ≤ 1 nM androgen, while the cells are arrested at G1/S at high androgen (≥10 nM). Quantitative RT-PCR revealed increased expression of E2F1 and E2F1 target genes (such as cyclin A and the antiapoptotic protein Bcl2) in LNCaP cells treated with 0.1nM R1881 (a synthetic androgen). This up regulation was markedly reduced at the 50 nM R1881 treatment condition. The E2F1 promoter activity showed a similar biphasic regulation by low versus high androgen. Functional analysis of deleted E2F1 promoter fragments identified an androgen responsive region (ARR) in the E2F1 upstream promoter. Chromatin immunoprecipitation (ChIP) of LNCaP cells treated with 0.1nM R1881 showed co-recruitment of AR and the p160 coactivator SRC3 to the ARR in parallel to E2F1 gene induction and increased cell proliferation. AR and SRC3 occupancy at ARR was abrogated by 50 nM R1881 treatment, in parallel to de-induction of E2F1 gene expression. Proliferation inhibition at 50 nM R1881 and loss of AR occupancy at ARR was prevented by SRC3 over expression. Reciprocally, proliferation stimulation by 0.1nM R1881 was lost in SRC3-silenced cells. In further novel findings, we show that the corepressor protein Alien (also known as TRIP15) strongly interacted with AR at 50 nM R1881, while interaction of AR with Alien was much weaker at 0.1 nM R1881. Androgen dose-dependent differential interaction between AR and Alien was demonstrated by co-immunoprecipitation and GST pull down assays of LNCaP cell nuclear extracts. Alien was absent constitutively from ARR in the E2F1 promoter at all conditions (i.e. LNCaP cells treated with vehicle, 0.1 nM R1881 or 50 nM R1881). This suggests that Alien interacts with AR in the nucleoplasm, not in a chromatin environment of the E2F1 promoter. Stable knockdown of Alien prevented proliferation inhibition of LNCaP cells at 50 nM R1881. We conclude that the corepressor Alien/TRIP15 negatively regulates proliferation of prostate cancer cells by sequestering AR in the nucleoplasm, which in turn leads to interference with E2F1 gene induction by androgen-activated AR. In ongoing studies we are utilizing a xenograft tumor model in immune-compromised mice to validate the inhibitory role of the corepressor Alien/TRIP15 in the proliferation of human prostate cancer cells in vivo.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1252.