Prolonged Androgen Receptor Loading Onto Chromatin and the Efficient Recruitment of p160 Coactivators Contribute to Androgen-Independent Growth of Prostate Cancer Cells

BACKGROUND. Growth of most ablation-resistant prostate cancers (CaPs) is dependent on androgen receptor (AR) activity in chromatin, but cancer cells in these tumors have acquired altered AR activation. It is unclear how the aberrantly activated AR loads onto regulatory regions of AR-targeted genes. The purpose of this study was to assess the AR chromatin loading in an androgen-depleted environment. METHODS. The expression of PSA in androgen-resistant CaP cells was determined using RT-PCR and Western blot analysis. In order to investigate the binding of the AR to the PSA gene regulatory regions, chromatin immunoprecipitation (ChIP) was performed in the androgen-independent cds2 cell line in the presence or absence of androgens. In addition, we examined the involvement of p160 coactivators in the chromatin loading of the AR. RESULTS. It was found that constitutive activation of PSA expression was the result of sustained occupancy by the AR at the regulatory region of this gene. This stable AR loading was not blocked by the AR antagonist bicalutamide. Furthermore, androgen-resistant CaP cells highly expressed both AR and the p160 coactivators and the AR was able to recruit TIF2. Downregulation of TIF2 using short hairpin RNA disrupted the AR loading to the PSA enhancer and subsequently inhibited AR activity. CONCLUSION. Prolonged AR localization to the regulatory regions of AR targeted genes and the recruitment of p160 coactivators are a potential mechanism leading to androgen-independent activation of the AR. Disruption of AR chromatin loading could therefore become an important therapeutic target for this disease. Prostate 68: 1816-1826, 2008, (C) 2008 Wiley-Liss. Inc.