Identification and characterization of ART-27, a novel coactivator for the androgen receptor N terminus

Structure and function of steroid receptor AF1 transactivation domains: induction of active conformations

Steroid hormones are important endocrine signalling molecules controlling reproduction, development, metabolism, salt balance and specialized cellular responses, such as inflammation and immunity. They are lipophilic in character and act by binding to intracellular receptor proteins. These receptors function as ligand-activated transcription factors, switching on or off networks of genes in response to a specific hormone signal. The receptor proteins have a conserved domain organization, comprising a C-terminal LBD (ligand-binding domain), a hinge region, a central DBD (DNA-binding domain) and a highly variable NTD (N-terminal domain). The NTD is structurally flexible and contains surfaces for both activation and repression of gene transcription, and the strength of the transactivation response has been correlated with protein length. Recent evidence supports a structural and functional model for the NTD that involves induced folding, possibly involving alpha-helix structure, in response to protein-protein interactions and structure-stabilizing solutes.

UXT is a novel centrosomal protein essential for cell viability

Ubiquitously expressed transcript (UXT) is a prefoldinlike protein that has been suggested to be involved in human tumorigenesis. Here, we have found that UXT is overexpressed in a number of human tumor tissues but not in the matching normal tissues. We demonstrate that UXT is located in human centrosomes and is associated with gamma-tubulin. In addition, overexpression of UXT disrupts centrosome structure. Furthermore, abrogation of UXT protein expression by small interfering RNA knockdown leads to cell death. Together, our findings suggest that UXT is a component of centrosome and is essential for cell viability. We propose that UXT may facilitate transformation by corrupting regulated centrosome functions.

Androgen Receptor Mutations Identified in Prostate Cancer and Androgen Insensitivity Syndrome Display Aberrant ART-27 Coactivator Function

The transcriptional activity of the androgen receptor (AR) is modulated by interactions with coregulatory molecules. It has been proposed that aberrant interactions between AR and its coregulators may contribute to diseases related to AR activity, such as prostate cancer and androgen insensitivity syndrome (AIS); however, evidence linking abnormal receptor-cofactor interactions to disease is scant. ART-27 is a recently identified AR N-terminal coactivator that is associated with AR-mediated growth inhibition. Here we analyze a number of naturally occurring AR mutations identified in prostate cancer and AIS for their ability to affect AR response to ART-27. Although the vast majority of AR mutations appeared capable of increased activation in response to ART-27, an AR mutation identified in prostate cancer (AR P340L) and AIS (AR E2K) show reduced transcriptional responses to ART-27, whereas their response to the p160 class of coactivators was not diminished. Relative to the wild-type receptor, less ART-27 protein associated with the AR E2K substitution, consistent with reduced transcriptional response. Surprisingly, more ART-27 associated with AR P340L, despite the fact that the mutation decreased transcriptional activation in response to ART-27. Our findings suggest that aberrant AR-coactivator association interferes with normal ART-27 coactivator function, resulting in suppression of AR activity, and may contribute to the pathogenesis of diseases related to alterations in AR activity, such as prostate cancer and AIS.

Role of TGF-beta2 in the human hair cycle

Male pattern baldness is the result of premature entry into catagen due to androgens. In order to prevent hair loss, it is important to understand two critical steps, i.e., the induction mechanism of premature entry and the regression process of catagen. At the initiation, dihydrotestosterone (DHT) stimulates synthesis of transforming growth factor-beta2 (TGF-beta2) in dermal papilla cells. TGF-beta2 suppresses proliferation of epithelial cells and stimulates synthesis of certain caspases. Then TGF-beta2 triggers the intrinsic caspase network and subsequently epithelial cells are eliminated through apoptotic cell death. TGF-beta antagonists are effective in preventing catagen-like morphological changes and in promoting elongation of hair follicles in vivo and in vitro. These lines of evidence strongly suggest the presence of a "catagen cascade" in male pattern baldness, involving: (1) the conversion of testosterone to DHT by type II 5-alpha-reductase; (2) the synthesis of TGF-beta2 in dermal papilla cells; and (3) the activation of the intrinsic caspase network. These sequential events contribute to the shortening of the human hair cycle.

Recruitment of beta-catenin by wild-type or mutant androgen receptors correlates with ligand-stimulated growth of prostate cancer cells

Prostate cancers respond to treatments that suppress androgen receptor (AR) function, with bicalutamide, flutamide, and cyproterone acetate (CPA) being AR antagonists in clinical use. As CPA has substantial agonist activity, it was examined to identify AR coactivator/corepressor interactions that may mediate androgen-stimulated prostate cancer growth. The CPA-liganded AR was coactivated by steroid receptor coactivator-1 (SRC-1) but did not mediate N-C terminal interactions or recruit beta-catenin, indicating a nonagonist conformation. Nonetheless, CPA did not enhance AR interaction with nuclear receptor corepressor, whereas the AR antagonist RU486 (mifepristone) strongly stimulated AR-nuclear receptor corepressor binding. The role of coactivators was further assessed with a T877A AR mutation, found in LNCaP prostate cancer cells, which converts hydroxyflutamide (HF, the active flutamide metabolite) into an agonist that stimulates LNCaP cell growth. The HF and CPA-liganded T877A ARs were coactivated by SRC-1, but only the HF-liganded T877A AR was coactivated by beta-catenin. L-39, a novel AR antagonist that transcriptionally activates the T877A AR, but still inhibits LNCaP growth, similarly mediated recruitment of SRC-1 and not beta-catenin. In contrast, beta-catenin coactivated a bicalutamide-responsive mutant AR (W741C) isolated from a bicalutamide-stimulated LNCaP subline, further implicating beta-catenin recruitment in AR-stimulated growth. Androgen-stimulated prostate-specific antigen gene expression in LNCaP cells could be modulated by beta-catenin, and endogenous c-myc expression was repressed by dihydrotestosterone, but not CPA. These results indicate that interactions between AR and beta-catenin contribute to prostate cell growth in vivo, although specific growth promoting genes positively regulated by AR recruitment of beta-catenin remain to be identified.

Oxandrolone blocks glucocorticoid signaling in an androgen receptor-dependent manner

The anabolic steroid oxandrolone is increasingly used to preserve or restore muscle mass in those with HIV infection or serious burns. These effects are mediated, in part, by the androgen receptor (AR). Anti-glucocorticoid effects have also been reported for some anabolic steroids, and the goal of our studies was to determine whether oxandrolone had a similar mechanism of action. Studies with in vitro translated glucocorticoid receptor (GR), however, showed no inhibition of cortisol binding by oxandrolone. Conversely, experiments in cell culture systems demonstrated significant antagonism of cortisol-induced transcriptional activation by oxandrolone in cells expressing both the AR and GR. Inhibition was not overcome by increased cortisol concentration, and no inhibition by oxandrolone was observed in cells expressing GR alone, confirming that non-competitive mechanisms were involved. AR-dependent repression of transcriptional activation by oxandrolone was also observed with the synthetic glucocorticoids dexamethasone and methylprednisolone. Furthermore, the AR antagonists 2-hydroxyflutamide and DDE also repressed GR transactivation in an AR-dependent manner. A mutant AR lacking a functional nuclear localization signal (AR(4RKM)) was active in oxandrolone-mediated repression of GR even though oxandrolone-bound AR(4RKM) failed to enter the nucleus and did not affect nuclear import of GR. These data indicate a novel action of oxandrolone to suppress glucocorticoid action via crosstalk between AR and GR.

ART-27, an Androgen Receptor Coactivator Regulated in Prostate Development and Cancer

Androgen receptor trapped clone-27 (ART-27) is a newly described transcriptional coactivator that binds to the N terminus of the androgen receptor (AR). Given the vital importance of AR signaling in prostate growth and differentiation, we investigated the role of ART-27 in these processes. Immunohistochemical studies indicate that ART-27 protein is expressed in differentiated epithelial cells of adult human prostate and breast tissue. In prostate, ART-27 is abundant in AR-positive prostate luminal epithelial cells, in contrast to the stroma, where cells express AR but not ART-27. The use of a rat model of androgen depletion/reconstitution indicates that ART-27 expression is associated with the elaboration of differentiated prostate epithelial cells. Interestingly, regulated expression of ART-27 in the androgen-sensitive LNCaP prostate cancer cell line inhibits androgen-mediated cellular proliferation and enhances androgen-mediated transcription of the prostate-specific antigen (PSA) gene. Consistent with a growth suppressive function, we show that ART-27 expression levels are negligible in human prostate cancer. Importantly, examination of ART-27 protein expression in early fetal prostate development demonstrates that ART-27 is detected only when the developing prostate gland has proceeded from a solid mass of undifferentiated cells to a stage in which differentiated luminal epithelial cells are evident. Thus, ART-27 is an AR cofactor shown to be subject to both cell type and developmental regulation in humans. Overall, the results suggest that decreased levels of ART-27 protein in prostate cancer tissue may occur as a result of de-differentiation, and indicate that ART-27 is likely to regulate a subset of AR-responsive genes important to prostate growth suppression and differentiation.

ARA67/PAT1 functions as a repressor to suppress androgen receptor transactivation

The androgen receptor (AR) may recruit multiple coregulators for proper or optimal transactivation. Here we report the identification and characterization of ARA67/PAT1 as an AR coregulator from a prostate cDNA library. ARA67/PAT1 was screened out as an AR N terminus interacting protein. Interaction mapping shows that the cooperation of multiple domains within ARA67/PAT1 may be required for the maximal interaction with AR. ARA67/PAT1 functions as a repressor with better suppressive effects on AR compared to glucocorticoid receptor and estrogen receptor. Further mechanism dissection reveals that the interrupted AR cytoplasmic-nuclear shuttling may play a major role in ARA67/PAT1 mediated suppression on AR. Together, these results suggest that ARA67/PAT1 may function as a novel repressor that can modulate AR function in prostate cancer.

The mRNA Export Factor Human Gle1 Interacts with the Nuclear Pore Complex Protein Nup155

The protein Gle1 is required for export of mRNAs from the nucleus to the cytoplasm in both lower and higher eukaryotic cells. In human (h) cells, shuttling of hGle1 between the nucleus and cytoplasm is essential for bulk mRNA export. To date, no hGle1-interacting proteins have been reported and the mechanism by which hGle1 interacts with the nuclear pore complex (NPC) and mediates export is unknown. To identify proteins that can interact with hGle1, a genome-wide yeast two-hybrid screen was performed. Three potential hGle1-interacting partners were isolated, including clones encoding the C-terminal region of the NPC protein hNup155. This interaction between hGle1 and full-length hNup155 was confirmed in vitro, and deletion analysis identified the N-terminal 29 residues of hGle1 as the hNup155-binding domain. Experiments in HeLa cells confirmed that the nuclear rim localization of the major hGle1 protein variant (hGle1B) was dependent on the presence of these 29 N-terminal residues. This suggests that this domain of hGle1 is necessary for targeting to the NPC. This work also characterizes the first domain in hNup155, a 177 C-terminal amino acid span that binds to hGle1. The mutual interaction between hGle1 and the symmetrically distributed nuclear pore protein Nup155 suggests a model in which hGle1's association with hNup155 may represent a step in the Gle1-mediated mRNA export pathway.

E2F6 negatively regulates BRCA1 in human cancer cells without methylation of histone H3 on lysine 9

E2F6 contains a DNA binding domain that is very similar to that of the other members of the E2F family of transcriptional regulators. However, E2F6 cannot bind to all promoters that contain consensus E2F-binding sites. Therefore, we used a combination of chromatin immunoprecipitation and genomic microarrays to identify promoters bound by E2F6 in human cells. Although most of the identified promoters were bound by multiple E2F family members, one promoter was bound only by E2F6. To determine which of the newly identified promoters were regulated by E2F6, we reduced the level of E2F6 by using RNA interference technology. We found that mRNA transcribed from promoters bound by E2F6 was increased after reduction of the amount of E2F6 protein in the cell. Interestingly, many of the E2F6-regulated genes encoded functions involved in tumor suppression and the maintenance of chromatin structure. Specifically, our results suggest that E2F6 represses transcription of the brca1, ctip, art27, hp1alpha, and the rbap48 genes. E2F6 has been postulated to mediate transcriptional repression by recruiting a histone H3 methyltransferase to the DNA. However, we found that the E2F6-regulated promoters did not contain histone H3 methylated at lysine 9. To determine the mechanism by which E2F6 regulates transcription, we performed chromatin immunoprecipitation before and after the introduction of small inhibitory ribonucleic acids specific to E2F6. We found that depletion of E2F6 resulted in the recruitment of E2F1 to the target promoters. In summary, we have identified 48 endogenous target genes of E2F6 and have shown that E2F6 can repress target promoters in a manner that does not require histone H3 methylation at lysine 9.

Isolation and identification of L-dopa decarboxylase as a protein that binds to and enhances transcriptional activity of the androgen receptor using the repressed transactivator yeast two-hybrid system

The AR (androgen receptor) is a ligand-regulated transcription factor, which belongs to the steroid receptor family and plays an essential role in growth and development of the prostate. Transcriptional activity of steroid receptors is modulated by interaction with co-regulator proteins and yeast two-hybrid analysis is commonly used to identify these steroid receptor-interacting proteins. However, a limitation of conventional two-hybrid systems for detecting AR protein partners has been that they only allow for analysis of the ligand- and DNA-binding domains of the receptor, as its NTD (N-terminal domain) possesses intrinsic transactivation activity. To identify AR N-terminus-interacting proteins, its NTD was used in the RTA (repressed transactivator) system, which is specifically designed for transactivator bait proteins and was shown to be suitable for two-hybrid analysis with the AR NTD. DDC (L-dopa decarboxylase) was detected multiple times as a novel AR-interacting protein, which was subsequently confirmed in vitro and in vivo. Furthermore, transient transfection of DDC in prostate cancer cells strongly enhanced ligand-dependent AR transcriptional activity, an effect that was antagonized using high concentrations of the anti-androgen bicalutamide. Glucocorticoid receptor activity was also strongly enhanced with DDC co-transfection, while oestrogen receptor activity was only mildly affected. Together, our data demonstrate that DDC interacts with AR to enhance steroid receptor transactivation, which may have important implications in prostate cancer progression.

p54nrb acts as a transcriptional coactivator for activation function 1 of the human androgen receptor

The androgen receptor (AR) has two transactivation functions that have been mapped to the N- and C-terminal domains and designated as activation function-1 (AF-1) and AF-2, respectively. While the molecular basis for AF-2 function has been well studied, little is known about AF-1 coregulators. Therefore, we attempted to identify AF-1-interacting proteins from HEK293 cells by biochemical purification followed by mass fingerprinting by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). Purified AF-1 region-interacting proteins were found to contain nuclear RNA-binding protein p54(nrb), polypyrimidine tract-binding protein-associated splicing factor (PSF), paraspeckle protein 1 (PSP1), and PSP2, which are assumed to be involved in pre-mRNA processing. p54(nrb) interacted with AR via the A/B domain in a ligand-dependent manner. Reflecting the physical interaction between p54(nrb) and the AR A/B domain, AR AF-1 function was potentiated by p54(nrb). Our results suggest that p54(nrb) functions as a coactivator of AR that potentiates transcription, and presumably splicing as well.

Molecular action of androgens

The androgen receptor (AR), which mediates androgen action in the cell, belongs to the superfamily of nuclear receptors, a large group of transcription factors. Recent studies have described how the AR acts on specific target genes. The receptor's specificity of action depends on its regulation at different levels: expression in the cells, ligand binding and DNA-specific sequence recognition by structurally conserved domains and regulation by transcriptional factors in an integrated response. We propose, here, an overview of recent works on the molecular regulation of androgen-dependent genes by AR.

The androgen receptor interacts with multiple regions of the large subunit of general transcription factor TFIIF

The androgen receptor (AR) is a ligand-activated transcription factor that regulates genes important for male development and reproductive function. The main determinants for the transactivation function lie within the structurally distinct amino-terminal domain. Previously we identified an interaction between the AR-transactivation domain (amino acids 142-485) and the general transcription factor TFIIF (McEwan, I. J., and Gustafsson, J.-A. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8485-8490). We have now mapped the binding sites for the AR-transactivation domain within the RAP74 subunit of TFIIF. Both the amino-terminal 136 amino acids and the carboxyl-terminal 155 amino acids of RAP74 interacted with the AR-transactivation domain and were able to rescue basal transcription after squelching by the AR polypeptide. Competition experiments demonstrated that the AR could interact with the holo-TFIIF protein and that the carboxyl terminus of RAP74 represented the principal receptor-binding site. Point mutations within AR-transactivation domain distinguished the binding sites for RAP74 and the p160 coactivator SRC-1a and identified a single copy of a six amino acid repeat motif as being important for RAP74 binding. These data indicate that the AR-transactivation domain can potentially make multiple protein-protein interactions with coactivators and components of the general transcriptional machinery in order to regulate target gene expression.

Activation function-1 domain of androgen receptor contributes to the interaction between subnuclear splicing factor compartment and nuclear receptor compartment. Identification of the p102 U5 small nuclear ribonucleoprotein particle-binding protein as a coactivator for the receptor

In the androgen receptor (AR), most of its transactivation activity is mediated via the activation function-1 (AF-1). By employing yeast two-hybrid assay, we isolated a cDNA sequence encoding a protein binding to AR-AF-1. This protein, named ANT-1 (AR N-terminal domain transactivating protein-1), enhanced the ligand-independent autonomous AF-1 transactivation function of AR or glucocorticoid receptor but did not enhance that of estrogen receptor alpha. In contrast, the ANT-1 did not enhance any ligand-dependent AF-2 activities. Furthermore, the ligand-independent interaction between AR-AF-1 and ANT-1 was confirmed in vivo and in vitro. The ANT-1 sequence was identical to that of a protein that binds to U5 small nuclear ribonucleoprotein particle, a human homologue of yeast splicing factor Prp6p, involved in spliceosome. ANT-1 was compartmentalized into 20-40 coarse splicing factor compartment speckles against the background of the diffuse reticular distribution. AR colocalized with ANT-1 only in the diffusely distributed area, whereas the ANT-1 speckles were spatially distinct from but surrounded by the AR compartments. The active gene transcription has been shown to couple simultaneously with pre-mRNA processing at the periphery of the splicing factor compartment. The molecular interaction between two spatially distinct subnuclear compartments mediated by ANT-1 may therefore recruit AR into the transcription-splicing-coupling machinery.