The second largest subunit of RNA polymerase II interacts with and enhances transactivation of androgen receptor

Androgen receptor coregulators: recruitment via the coactivator binding groove

Androgens are key regulators of male sexual differentiation and essential for development and maintenance of male reproductive tissues. The androgens testosterone and dihydrotestosterone mediate their effect by binding to, and activation of the androgen receptor (AR). Upon activation, the AR is able to recognize specific DNA sequences in gene promoters and enhancers from where it recruits coregulators to orchestrate chromatin remodeling and transcription regulation. The number of proteins that bind to the AR has surpassed 200 and many of them enhance (coactivator) or repress (corepressor) its transactivating capacity. For most of these coregulators, their AR binding interface and their exact mode of action still needs to be elucidated, but for some of the more classical coactivators and corepressors, we gained insight in their working mechanisms. Of particular interest are specific sequences (LxxLL and FxxLF-like motifs) in a subset of coactivators that interact with the AR via a coactivator binding groove in the ligand-binding domain. As compared to other steroid receptors, the conformation of the AR coactivator binding pocket is unique and preferentially binds FxxLF-like motifs. This predisposition is expected to contribute to the regulation of specific sets of target genes via recruitment of selected coregulators. This review provides an overview of these (inter)actions with a focus on the unique characteristics of the AR coactivator binding groove.

The chromatin remodeling factor SRCAP modulates expression of prostate specific antigen and cellular proliferation in prostate cancer cells

The SNF2-related CBP activator protein (SRCAP) serves as a coactivator for several nuclear receptors including the androgen receptor (AR). SRCAP is an ATPase that is the core subunit of a large multiprotein complex and was shown to incorporate the histone variant H2A.Z into nucleosomes. In this report, we demonstrate that SRCAP is expressed in the epithelium of normal prostate and in prostate carcinoma cells, and is associated with AR in the nucleus. Using transient transfection assays we demonstrate that SRCAP activates hormone-dependent transcription of the androgen responsive, prostate specific antigen (PSA)-Luciferase reporter gene in human prostate cells. The in vivo occupancy of SRCAP at the endogenous PSA promoter is demonstrated using chromatin immunoprecipitation assays. ShRNA mediated knockdown of SRCAP resulted in decreased H2A.Z binding at the enhancer region of the PSA promoter and decreased expression of PSA in prostate cancer cells. Furthermore, inhibition of SRCAP expression significantly inhibited androgen dependent prostate cancer cell growth. These data identify SRCAP as a physiologically relevant mediator of PSA expression, and demonstrate that SRCAP plays a role in prostate cancer cell proliferation.

Androgen receptor (AR) coregulators: a diversity of functions converging on and regulating the AR transcriptional complex

Androgens, acting through the androgen receptor (AR), are responsible for the development of the male phenotype during embryogenesis, the achievement of sexual maturation at puberty, and the maintenance of male reproductive function and behavior in adulthood. In addition, androgens affect a wide variety of nonreproductive tissues. Moreover, aberrant androgen action plays a critical role in multiple pathologies, including prostate cancer and androgen insensitivity syndromes. The formation of a productive AR transcriptional complex requires the functional and structural interaction of the AR with its coregulators. In the last decade, an overwhelming and ever increasing number of proteins have been proposed to possess AR coactivating or corepressing characteristics. Intriguingly, a vast diversity of functions has been ascribed to these proteins, indicating that a multitude of cellular functions and signals converge on the AR to regulate its function. The current review aims to provide an overview of the AR coregulator proteins identified to date and to propose a classification of these AR coregulator proteins according to the function(s) ascribed to them. Taken together, this approach will increase our understanding of the cellular pathways that converge on the AR to ensure an appropriate transcriptional response to androgens.

The coregulator Alien

Alien has characteristics of a corepressor for selected members of the nuclear hormone receptor (NHR) superfamily and also for transcription factors involved in cell cycle regulation and DNA repair. Alien mediates gene silencing and represses the transactivation of specific NHRs and other transcription factors to modulate hormone response and cell proliferation. Alien is a highly conserved protein and is expressed in a wide variety of tissues. Knockout of the gene encoding Alien in mice is embryonic lethal at a very early stage, indicating an important evolutionary role in multicellular organisms. From a mechanistic perspective, the corepressor function of Alien is in part mediated by histone deacetylase (HDAC) activity. In addition, Alien seems to modulate nucleosome assembly activity. This suggests that Alien is acting on chromatin not only through recruitment of histone-modifying activities, but also through enhancing nucleosome assembly.

The role of the general transcription factor IIF in androgen receptor-dependent transcription

The androgen receptor (AR) is a member of the steroid receptor subfamily of nuclear receptors and is important for normal male sexual differentiation and fertility. The major transactivation function of the AR, termed activation function 1 (AF1), is modular in structure and has been mapped to the N terminus of the protein. To understand better the mechanisms whereby the AR activates transcription, we have established a novel cell-free transcription assay. This is based on the use of a dual reporter gene template, containing promoter proximal and distal G-less cassettes, which result in different size transcripts that can be easily detected and quantified. The promoter proximal transcript gives an indication of transcription initiation and promoter escape, whereas the relative levels of the distal transcript indicate elongation efficiency. The AR-AF1-Lex protein enhanced production of both transcripts whereas, in the absence of DNA binding, the AF1 domain squelched both initiation and elongation. Mutations in the transactivation domain that impaired transactivation and/or binding of the general transcription factor IIF (TFIIF) were found to reduce the ability of AR-AF1 to squelch transcription. Addition of recombinant TFIIF reversed squelching of the promoter-proximal but not the -distal G-less transcript, whereas addition of TATA-binding protein failed to reverse squelching of either transcript. Taken together, these results demonstrate that the AR N-terminal transactivation function, AF1, has the potential to regulate transcription at both the level of initiation and elongation, and that interactions with TFIIF are important during preinitiation complex assembly/open complex formation and/or promoter escape.

Actin monomer enhances supervillin-modulated androgen receptor transactivation

Actin-binding protein, supervillin, has been identified as an androgen receptor (AR) coregulator. Although actin has been suggested to participate in transcription regulation, the mechanism is not clear. Here we demonstrate signals involved in the cytoskeleton dynamic can modulate the coregulator function of supervillin. Three actin isoforms cooperate with supervillin in additive manner to further enhance AR transactivation. Latrunculin B toxin, an actin chelator, reduces the availability of monomer actin and attenuates supervillin function. Rac, the small G-protein kinase, is well studied in reorganization of cytoskeleton. The overexpression of constitutive-active Rac triggers the membrane ruffling site and reduces the coregulator activity of supervillin. Together, the availability of actin monomer affects supervillin-modulated AR transactivation.