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.

We thank Drs LM Boxer, RJ Kelm, JR Nevins, and Z Zacksenhaus for plasmids. This work was supported in part by a grant (DK60920) from the National Institutes of Health and a grant from the TJ

Martell Foundation (DJT), and a developmental award of the Mayo Prostate Cancer SPORE (CA91956) funded by the National Cancer Institute (HH).

Departments of Urology and Biochemistry/Molecular Biology, Mayo Clinic/Foundation, Rochester, 55905, MN, USA

Haojie Huang, Ofelia L Zegarra-Moro, Douglas Benson & Donald J Tindall

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