The Zinc Finger Protein Ras-Responsive Element Binding Protein-1 Is a Coregulator of the Androgen Receptor: Implications for the Role of the Ras Pathway in Enhancing Androgenic Signaling in Prostate Cancer

Abstract

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.

RREB1 Transcription Factor Splice Variants in Urologic Cancer

“…Despite its implication in Ras, Ral, p16, p53, and androgen receptor 10,11,14,19,20 signaling pathways, sparse data exist regarding the nature, expression, or function of RREB1 mRNA and protein in cancer cells. This is the first study to address this gap by describing RREB1 expression in human cancers, as well as the first to examine isoform expression in human tissues and cell lines.…”

“…Mukhopadhyay et al 20 reported that RREB1 binds AR and inhibits its function as a transcription factor. In addition, RREB1 binding and inhibition of AR activity was relieved by activated Ras signaling.…”

“…[25][26][27][28][29][30] In prostate cancer, RREB1 binds the PSA promoter only in association with AR to repress transcription. 20 In summary, the current literature on RREB1 suggests con-text-dependent phenotypes that may suppress or promote carcinogenesis and tumor progression.…”

“…Genes induced by RREB1 include those encoding calcitonin, 11 FSH, 12 MT-IIA, 13 p53, 14 and secretin 15 ; genes repressed by RREB1 include those encoding angiotensinogen, 16 HLA-G, 17 hZIP1, 18 p16, 19 and PSA. 20 RREB1 has also been found to bind nuclear proteins, such as CtBP, 21 NeuroD, 15 and androgen receptor (AR). 20 The initial study of RREB1 found that the gene product bound the calcitonin promoter in medullary thyroid carcinomas in response to Ras.…”

“…20 RREB1 has also been found to bind nuclear proteins, such as CtBP, 21 NeuroD, 15 and androgen receptor (AR). 20 The initial study of RREB1 found that the gene product bound the calcitonin promoter in medullary thyroid carcinomas in response to Ras. 11 In bladder cancer, the tumor suppressor p16 is commonly lost as an early event in tumorigenesis, and RREB1 was found to bind and repress transcription of the Cdkn2a locus.…”