Pax6 Represses Androgen Receptor-Mediated Transactivation by Inhibiting Recruitment of the Coactivator SPBP

Abstract

The androgen receptor (AR) has a central role in development and maintenance of the male reproductive system and in the etiology of prostate cancer. The transcription factor Pax6 has recently been reported to act as a repressor of AR and to be hypermethylated in prostate cancer cells. SPBP is a transcriptional regulator that previously has been shown to enhance the activity of Pax6. In this study we have identified SPBP to act as a transcriptional coactivator of AR. We also show that Pax6 inhibits SPBP-mediated enhancement of AR activity on the AR target gene probasin promoter, a repression that was partly reversed by increased expression of SPBP. Enhanced expression of Pax6 reduced the amount of SPBP associated with the probasin promoter when assayed by ChIP in HeLa cells. We mapped the interaction between both AR and SPBP, and AR and Pax6 to the DNA-binding domains of the involved proteins. Further binding studies revealed that Pax6 and SPBP compete for binding to AR. These results suggest that Pax6 represses AR activity by displacing and/or inhibiting recruitment of coactivators to AR target promoters. Understanding the mechanism for inhibition of AR coactivators can give rise to molecular targeted drugs for treatment of prostate cancer.

Identification of two independent nucleosome-binding domains in the transcriptional co-activator SPBP

“…However, some ePHD/ADD-containing proteins are associated with transcriptional activation, such as the MLL family and trithorax homologues which are associated with establishment of H3K4 methylation (reviewed in [48]). We have found SPBP to act as a transcriptional co-activator of several sequence-specific transcription factors [5,6,8], whereas RAI1 acted more like a transcriptional repressor [8]. This may indicate that these homologous proteins have distinct roles and are associated with distinct protein complexes.…”

“…As described previously, the cysteine-rich C-terminal end of SPBP can be viewed as a PHD-like region (zinc ligands [5][6][7][8][9][10][11][12] with an additionally N-terminal zinc-finger (zinc ligands 1-4) [8]. Such extended PHD finger-like structures are also present in the C-terminus of RAI1, and in eight other groups of proteins: (i) the MLL (mixed lineage leukaemia) proteins MLL, MLL2 and MLL 4; (ii) the trithorax homologues AF10, AF17 and MLL6; (iii) enhancer of polycomb family members BRD1/BRL and BR140; (iv) the JADE family members JADE 1, JADE 3 and JADE 4; (v) Jumonji family transcription factors and demethylases including GASC-1 (JMJD2C); (vi) the chromatinassociated protein ATRX; (vii) the demethyl DNA transferases DNMT3L, DNMT3A and DNMT3B; and (viii) the SET domain protein histone methyltransferase NSD1 ( Figure 4A).…”

“…SPBP was originally found to be involved in transcriptional activation of the MMP3 (matrix metalloprotease 3) promoter via a specific DNA sequence [3]. Later it was shown to act as a transcriptional coregulator that enhances [4][5][6] or represses [7] the transcriptional activity of certain transcription factors or cofactors such as Sp1 (specificity protein 1), Ets1 (E twenty-six 1), Pax6 (paired box protein 6), SNURF (small nuclear RING-finger)/RNF4, c-Jun, AR (androgen receptor) and ERα (oestrogen receptor α). The ePHD/ADD domain is shown to interact with the BRCT6 domain of the checkpoint mediator TopBP1 (DNA topoisomerase II binding protein 1) [8].…”

A Phylogenetic Study of SPBP and RAI1: Evolutionary Conservation of Chromatin Binding Modules

“…All these regions are localized in the C-terminal half of the protein, while most of the N-terminal part has been identified as a transcriptional activation domain, suggesting that it has strong capability to interact with other proteins involved in transcription. Interaction of SPBP with the ligand activated transcription factors Androgen receptor and Estrogen receptor is known [23,24]. Interestingly, a yeast two-hybrid screen acknowledged the RNA polymerase II subunit Rpb7, the RNA binding proteins hnRNPH/F, the SWI/SNF complex protein ARID1B and the chromatin modulator G9a as putative interaction partners of SPBP (our unpublished results).…”

“…Later, SPBP is shown to act as a transcriptional coactivator enhancing the transcriptional activity of certain transcription factors and cofactors [21,23]. SPBP is also shown to repress the transcriptional activity of estrogen receptor α (ERα) [24].…”