Androgen receptor (AR) plays an important role in normal prostate function as well as in the etiology of prostate cancer. Activation of AR is dictated by hormone binding and by interactions with coregulators. Several of these coregulators are known targets of Ras-related signals. Recent evidence suggests that Ras activation may play a causal role in the progression of prostate cancer toward a more malignant and hormone-insensitive phenotype. In the present study, we used a transcription factor-transcription factor interaction array method to identify the zinc finger protein Ras-responsive element binding protein (RREB-1) as a partner and coregulator of AR. In LNCaP prostate cancer cells, RREB-1 was found to be present in a complex with endogenous AR as determined by coimmunoprecipitation, glutathione S-transferase pull down, and immunofluorescence analyses. RREB-1 bound to the prostate-specific antigen (PSA) promoter as assessed by chromatin immunoprecipitation. Transient expression of RREB-1 down-regulated AR-mediated promoter activity and suppressed expression of PSA protein. The repressor activity of RREB-1 was significantly attenuated by cotransfection of activated Ras. Moreover, expression of the dominant-negative N-17-Ras or, alternatively, use of the MAPK kinase inhibitor PD98059 [2-(2-amino-3-methyoxyphenyl)-4H-1-benzopyran-4-one] abolished the effect of Ras in attenuating RREB-1-mediated repression. Furthermore, inhibition of RREB-1 expression by RNA interference enhanced the effect of Ras on PSA promoter activity and PSA expression. In addition, activation of the Ras pathway depleted AR from the RREB-1/AR complex. Collectively, our data for the first time identify RREB-1 as a repressor of AR and further implicate the Ras/MAPK kinase pathway as a likely antagonist of the inhibitory effects of RREB-1 on androgenic signaling.