The molecular mechanism underlying the actions of estrogens in normal prostate physiology and prostate cancer development remains unclear. In the present study we tested the hypothesis that estrogens modulate androgen-dependent events in prostate cells by examining the effects of 17β-estradiol (E2) on androgen-responsive genes (ARGs) in the androgenresponsive LNCaP cells. We found that LNCaP cells express estrogen receptor-β (ER-β) as the major form of ER and ER treatment with E2 led to an increase in cell growth. The proliferative effect of E2 correlated with induction of several ARGs by E2. Interestingly, some other ARGs did not respond to E2. Consistent with involvement of ER-β, the induction of both cell growth and ARG mRNA levels by E2 was attenuated by the pure antiestrogen ICI 182,780. Moreover, we found ER-β small interfering RNA attenuated induction of ARG mRNAs by E2. However, the effect of E2 on ARG mRNA appeared also to require the androgen receptor and to be mediated through activation of the extracellular-signal regulated kinase (ERK) pathway. These results provide mechanistic evidence supporting a direct effect of estrogen, mediated through ER-β- and ERK-dependent pathways, on specific molecular targets in human prostate cancer cells. © 2006 Wiley-Liss, Inc.