Androgens repress Bcl-2 expression via activation of the retinoblastoma (RB) protein in prostate cancer cells

The oncogene Bcl- 2 is upregulated frequently in prostate tumors following androgen ablation therapy, and Bcl- 2 overexpression may contribute to the androgen- refractory relapse of the disease. However, the molecular mechanism underlying androgenic regulation of Bcl- 2 in prostate cancer cells is understood poorly. In this study, we demonstrated that no androgen response element ( ARE) was identified in the androgen- regulated region of the P1 promoter of Bcl- 2 gene, whereas, we provided evidence that the androgenic effect is mediated by E2F1 protein through a putative E2F- binding site in the promoter. We further demonstrated that retinoblastoma ( RB) protein plays a critical role in androgen regulation of Bcl- 2. The phosphorylation levels of RB at serine residues 780 and 795 were decreased in LNCaP cells treated with androgens. Ectopic expression of a constitutively active form of RB inhibited expression of Bcl- 2. Knockdown of endogenous RB protein by an Rb small inference RNA ( siRNA) induced an increase in Bcl- 2 levels. Most importantly, the effect of androgens on Bcl- 2 was abolished completely by specific inhibition of RB function with a mutated E1A. Finally, androgen treatment of LNCaP cells upregulated specifically levels of the cyclin-dependent kinase inhibitors ( CDKIs) p15INK4B and p27KIP1. Ectopic expression of p15INK4B and/ or p27KIP1 inhibited Bcl- 2 expression. Knockdown of endogenous p15INK4B or p27KIP1 protein with a pool of siRNAs diminished androgen- induced downregulation of Bcl- 2 expression. Therefore, our data indicate that androgens suppress Bcl- 2 expression through negatively modulating activities of the E2F site in the Bcl- 2 promoter by activating the CDKI- RB axis.