Identification of two transcription activation units in the N-terminal domain of the human androgen receptor

The cochaperone Bag-1L enhances androgen receptor action via interaction with the NH2-terminal region of the receptor

Members of the Bag-1 family of cochaperones regulate diverse cellular processes including the action of steroid hormone receptors. The largest member of this family, Bag-1L, enhances the transactivation function of the androgen receptor. This occurs primarily through interaction with the NH(2) and COOH termini of the receptor. At the NH(2) terminus of the receptor, Bag-1L interacts with a region termed tau 5. Bag-1M, a naturally occurring variant of Bag-1L that binds to tau 5 but is defective in the COOH-terminal interaction, is less efficient in enhancing the transactivation function of the receptor. Surface plasmon resonance and transfection studies showed that the molecular chaperone Hsp70 contributes to the binding of Bag-1L to tau 5 and to the regulation of the transactivation function of the androgen receptor. Chromatin immunoprecipitation studies demonstrated that the androgen receptor, Hsp70, and Bag-1L are all targeted to the androgen response elements of the gene that encodes prostate-specific antigen. These studies demonstrate the regulation of transcriptional activity of androgen receptor by a molecular chaperone-cochaperone complex.

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.

Protein inhibitor of activated signal transducer and activator of transcription 1 interacts with the N-terminal domain of mineralocorticoid receptor and represses its transcriptional activity: implication of small ubiquitin-related modifier 1 modification

Molecular mechanisms underlying mineralocorticoid receptor (MR)-mediated gene expression are not fully understood but seem to largely depend upon interactions with specific coregulators. To identify novel human MR (hMR) molecular partners, yeast two-hybrid screenings performed using the N-terminal domain as bait, allowed us to isolate protein inhibitor of activated signal transducer and activator of transcription (PIAS)1 and PIASxbeta, described as SUMO (small ubiquitin-related modifier) E3-ligases. Specific interaction between PIAS1 and hMR was confirmed by glutathione-S-transferase pull-down experiments and N-terminal subdomains responsible for physical contacts were delineated. Transient transfections demonstrated that PIAS1 is a corepressor of aldosterone-activated MR transactivation but has no significant effect on human glucocorticoid receptor transactivation. The agonist or antagonist nature of the bound ligand also determines PIAS1 corepressive action. We provided evidence that PIAS1 conjugated SUMO-1 to hMR both in vitro and in vivo. Deciphering the unique sumoylation pattern of hMR, which possesses five consensus SUMO-1 binding sites, by combinatorial lysine substitutions, revealed a major impact of sumoylation on hMR properties. Using a murine mammary tumor virus promoter, PIAS1 action was independent of sumoylation whereas with glucocorticoid response element promoter, PIAS1 corepressive action depended on hMR sumoylation status. Taken together, our results identify a novel function for PIAS1 which interacts with the N-terminal domain of hMR and represses its ligand-dependent transcriptional activity, at least in part, through SUMO modifications.

Implications of a polyglutamine tract in the function of the human androgen receptor

The androgen receptor (AR) is a ligand-dependent transcription factor and belongs to the nuclear receptor family. The AR gene contains a long polymorphic CAG repeat, coding for a polyglutamine tract. In the full size AR, the deletion of the polyglutamine tract results in an increase in the transactivation through canonical AREs. However, this effect is clearly dependent on the response elements, since it is not observed on selective elements. In our assays, a deletion of the repeat positively affected the interactions of the ligand-binding domain with the amino-terminal domain as well as the recruitment of the p160 coactivator SRC-1e to the amino-terminal domain of the AR. This is reflected by an enhanced coactivation of the AR by SRC-1e.

Identification of a highly conserved domain in the androgen receptor that suppresses the DNA-binding domain-DNA interactions

The androgen receptor (AR) is a ligand-regulated and sequence-specific transcription factor that activates or represses expression of target genes. Here, we show that the N terminus of AR contains an inhibitory domain located in an 81-amino acid segment lying upstream of the DNA-binding domain (DBD). The inhibitory domain interacted directly with DBD and repressed DBD binding to the androgen response element. Mutations of the conserved amino acid residues (K520E and R538E) within the inhibitory domain decreased its inhibiting ability in vitro and increased AR trans-activation in vivo. These data demonstrate the existence of a novel inhibitory domain in the N-terminal part of AR, which might play important roles in the regulation of AR trans-activation.

Anti-androgenic properties of Compound A, an analog of a non-steroidal plant compound

We investigated the interactions between Compound A (CpdA), an analog of a hydroxyphenyl aziridine precursor found in an African shrub, and the androgen receptor (AR). CpdA represses androgen-induced activation of both specific and non-specific androgen DNA response elements. While a similar effect was obtained for the progesterone receptor (PR) via a non-specific hormone response element, CpdA had no effect on the actions of the glucocorticoid and mineralocorticoid receptors. CpdA represses the ligand-dependent interaction between the NH(2)- and COOH-terminal domains of the AR, similar to well-characterised anti-androgens. CpdA also interferes with the interaction of steroid receptor co-activator 1 (SRC1) with the activation domain AF2 but not with AF1. However, CpdA does not compete with androgen for binding to the AR. These results demonstrate that CpdA elicits anti-androgenic actions by a mechanism other than competitive binding for the AR.

Dual function of an amino-terminal amphipatic helix in androgen receptor-mediated transactivation through specific and nonspecific response elements

Steroid receptors are transcription factors that, upon binding to their response elements, regulate the expression of several target genes via direct protein interactions with transcriptional coactivators. For the androgen receptor, additional interactions between the amino- and carboxyl-terminal regions have been reported. The first amino acids of the amino-terminal domain are necessary for this amino/carboxyl-terminal interaction. Deletion of a FQNLF core sequence in this region blunts the interaction, as does a G21E mutation. We investigated the effect of the aforementioned mutations in the context of the full size androgen receptor on a series of selective and nonselective androgen response elements. Strikingly, the FQNLF deletion strongly reduced the androgen receptor capacity to transactivate through nonselective motifs but did not affect its activity on selective elements. Although the G21E mutation strongly impairs the amino/carboxyl-terminal interaction, it does not significantly influence androgen receptor activity on either selective or nonselective elements. Surprisingly, this mutation leads to an increased binding of the amino-terminal domain to the glutamine-rich region of the steroid receptor coactivator-1 of the p160 family. Taken together, these data suggest that the amino-terminal amino acids of the androgen receptor play a key role in determining its transcriptional activity by modulating the interaction with the ligand-binding domain as well as interaction with p160 coactivators.

A novel inducible transactivation domain in the androgen receptor: implications for PRK in prostate cancer

In addition to the classical activation by ligands, nuclear receptor activity is also regulated by ligand-independent signalling. Here, we unravel a novel signal transduction pathway that links the RhoA effector protein kinase C-related kinase PRK1 to the transcriptional activation of the androgen receptor (AR). Stimulation of the PRK signalling cascade results in a ligand-dependent superactivation of AR. We show that AR and PRK1 interact both in vivo and in vitro. The transactivation unit 5 (TAU-5) located in the N-terminus of AR suffices for activation by PRK1. Thus, TAU-5 defines a novel, signal-inducible transactivation domain. Furthermore, PRK1 promotes a functional complex of AR with the co-activator TIF-2. Importantly, PRK signalling also stimulates AR activity in the presence of adrenal androgens, which are still present in prostate tumour patients subjected to testicular androgen ablation therapy. Moreover, PRK1 activates AR even in the presence of the AR antagonist cyproterone acetate that is used in the clinical management of prostate cancer. Since prostate tumours strongly overexpress PRK1, our data support a model in which AR activity is controlled by PRK signalling.

AKT-independent protection of prostate cancer cells from apoptosis mediated through complex formation between the androgen receptor and FKHR

Recent studies suggested that the protection of cell apoptosis by AKT involves phosphorylation and inhibition of FKHR and related FOXO forkhead transcription factors and that androgens provide an AKT-independent cell survival signal in prostate cancer cells. Here, we report receptor-dependent repression of FKHR function by androgens in prostate cancer cells. Transcriptional analysis demonstrated that activation of the androgen receptor caused an inhibition of both wild-type FKHR and a mutant in which all three known AKT sites were mutated to alanines, showing that the repression is AKT independent. In vivo and in vitro coprecipitation studies demonstrated that the repression is mediated through protein-protein interaction between FKHR and the androgen receptor. Mapping analysis localized the interacting domains to the carboxyl terminus between amino acids 350 and 655 of FKHR and to the amino-terminal A/B region and the ligand binding domain of the receptor. Further analysis demonstrated that the activated androgen receptor blocked FKHR's DNA binding activity and impaired its ability to induce Fas ligand expression and prostate cancer cell apoptosis and cell cycle arrest. These studies identify a new mechanism for androgen-mediated prostate cancer cell survival that appears to be independent of the activity of the receptor on androgen response element-mediated transcription and establish FKHR and related FOXO forkhead proteins as important nuclear targets for both AKT-dependent and -independent survival signals in prostate cancer cells.

Antiandrogens: selective androgen receptor modulators

Antiandrogens can efficiently block androgen receptor (AR) mediated gene expression, and are therefore useful tools in the treatment of androgen dependent prostate cancer. Antiandrogens are either complete or partial inhibitors of AR activity, depending on the nature of the compound. As compared to androgens, antiandrogens induce a different AR conformation, thereby influencing the recruitment of co-regulators (coactivators and corepressors). This ligand-selective modulation of AR activity is affected by an AR mutation (Thr877Ala substitution) found in prostate cancer. In contrast to the wild-type AR, the mutant AR conformation induced by cyproterone acetate (CPA) and hydroxyflutamide (OHF) is comparable to that induced by androgens. As a consequence, this might affect recruitment of co-regulators, thereby allowing CPA and OHF to act as strong agonists on the mutant AR.

Characterization of the two coactivator-interacting surfaces of the androgen receptor and their relative role in transcriptional control

The androgen receptor interacts with the p160 coactivators via two surfaces, one in the ligand binding domain and one in the amino-terminal domain. The ligand binding domain interacts with the nuclear receptor signature motifs, whereas the amino-terminal domain has a high affinity for a specific glutamine-rich region in the p160s. We here describe the implication of two conserved motifs in the latter interaction. The amino-terminal domain of the androgen receptor is a very strong activation domain constituent of Tau5, which is mainly active in the absence of the ligand binding domain, and Tau1, which is only active in the presence of the ligand binding domain. Both domains are, however, implicated in the recruitment of the p160s. Mutation analysis of the p160s has shown that the relative contribution of the two recruitment mechanisms via the signature motifs or via the glutamine-rich region depend on the nature of the enhancers tested. We propose, therefore, that the androgen receptor-coactivator complex has several alternative conformations, depending partially on the context of the enhancer.

Amino acids 3-13 and amino acids in and flanking the 23FxxLF27 motif modulate the interaction between the N-terminal and ligand-binding domain of the androgen receptor

The N-terminal domain (NTD) and the ligand-binding domain (LBD) of the androgen receptor (AR) exhibit a ligand-dependent interaction (N/C interaction). Amino acids 3-36 in the NTD (AR3-36) play a dominant role in this interaction. Previously, it has been shown that a PhixxPhiPhi motif in AR3-36, 23FxxLF27, is essential for LBD interaction. We demonstrate in the current study that AR3-36 can be subdivided into two functionally distinct fragments: AR3-13 and AR16-36. AR3-13 does not directly interact with the AR LBD, but rather contributes to the transactivation function of the AR.NTD-AR.LBD complex. AR16-36, encompassing the 23FxxLF27 motif, is predicted to fold into a long amphipathic alpha-helix. A second PhixxPhiPhi candidate protein interaction motif within the helical structure, 30VREVI34, shows no affinity to the LBD. Within AR16-36, amino acid residues in and flanking the 23FxxLF27 motif are demonstrated to modulate N/C interaction. Substitution of Q24 and N25 by alanine residues enhances N/C interaction. Substitution of amino acids flanking the 23FxxLF27 motif by alanines are inhibitory to LBD interaction.

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.

Ligand-independent activation of the androgen receptor by interleukin-6 and the role of steroid receptor coactivator-1 in prostate cancer cells

The androgen receptor (AR) can be activated in the absence of androgens by interleukin-6 (IL-6) in human prostate cancer cells. The events involved in ligand-independent activation of the AR are unknown, but have been suggested to involve phosphorylation of the AR itself or a receptor-associated protein. Steroid receptor coactivator-1 (SRC-1) has been shown to interact with the human AR and to modulate ligand-dependent AR transactivation and is regulated by phosphorylation by MAPK. To date, no one has examined the role of SRC-1 in ligand-independent activation of the AR by IL-6 or other signaling pathways known to activate the full-length receptor. This study addressed this and has revealed the following. 1) SRC-1 similarly enhanced ligand-independent activation of the AR by IL-6 to the same magnitude as that obtained via ligand-dependent activation. 2) Androgen and IL-6 stimulated the MAPK pathway. 3) MAPK was required for both ligand-dependent and ligand-independent activation of the AR. 4) Phosphorylation of SRC-1 by MAPK was required for optimal ligand-independent activation of the AR by IL-6. 5) Protein-protein interaction between endogenous AR and SRC-1 was dependent upon treatment of LNCaP cells with IL-6 or R1881. 6) Protein-protein interaction between the AR N-terminal domain and SRC-1 was independent of MAPK. 7) Ligand-independent activation of the AR did not occur by a mechanism of overexpression of either solely wild-type SRC-1 or mutant SRC-1 that mimics its phosphorylated form.

Conformational analysis of the androgen receptor amino-terminal domain involved in transactivation. Influence of structure-stabilizing solutes and protein-protein interactions

The androgen receptor (AR) is a member of the nuclear receptor superfamily. Sequences within the large amino-terminal domain of the receptor have been shown to be important for transactivation and protein-protein interactions; however, little is known about the structure and folding of this region. In the present study we show that a 344-amino acid polypeptide representing the main determinants for transactivation has the propensity to form alpha-helical structure and that mutations which disrupt putative helical regions alter conformation. Folding of the AR was observed in the presence of the helix-stabilizing solvent trifluoroethanol and the natural osmolyte trimethylamine N-oxide (TMAO). TMAO resulted in the movement of two tryptophan residues to a less solvent-exposed environment and the formation of secondary/tertiary structure resistant to protease cleavage. Critically, binding to the RAP74 subunit of the general transcription factor TFIIF resulted in extensive protease resistance, consistent with induced folding of the receptor transactivation domain. These data indicate that this region of the AR is structurally flexible and folds into a stable conformation upon interactions with a component of the general transcription machinery.

Androgen receptor (AR) coregulators: an overview

The biological action of androgens is mediated through the androgen receptor (AR). Androgen-bound AR functions as a transcription factor to regulate genes involved in an array of physiological processes, most notably male sexual differentiation and maturation, and the maintenance of spermatogenesis. The transcriptional activity of AR is affected by coregulators that influence a number of functional properties of AR, including ligand selectivity and DNA binding capacity. As the promoter of target genes, coregulators participate in DNA modification, either directly through modification of histones or indirectly by the recruitment of chromatin-modifying complexes, as well as functioning in the recruitment of the basal transcriptional machinery. Aberrant coregulator activity due to mutation or altered expression levels may be a contributing factor in the progression of diseases related to AR activity, such as prostate cancer. AR demonstrates distinct differences in its interaction with coregulators from other steroid receptors due to differences in the functional interaction between AR domains, possibly resulting in alterations in the dynamic interactions between coregulator complexes.

The presence of both the amino- and carboxyl-terminal domains in the AR is essential for the completion of a transcriptionally active form with coactivators and intranuclear compartmentalization common to the steroid hormone receptors: a three-dimensional imaging study

To clarify the physiological significance of the intranuclear speckled distribution, or foci formation, of liganded steroid receptors, the subnuclear distribution of green (GFP), yellow (YFP), and cyan (CFP) fluorescent protein-tagged receptors and coactivators was investigated. The foci formation of 5 alpha-dihydrotestosterone (DHT)-bound AR-GFP in COS7 cells was abolished by the cotransfection of a CBP Delta (118-2393) fragment eliciting a dominant negative effect on the transactivation capacity of the AR. The N-terminal AR fragment (AR-AF-1-YFP), which has a strong constitutive transactivation function, formed foci without DHT, whereas the C-terminal AR fragment (AR-AF-2-CFP), which has a quite low transactivation function, was distributed homogeneously even in the presence of DHT. The reporter gene assay showed a synergism between the transactivation functions of AR-AF-1 and AR-AF-2. This synergism was not reflected by the above two-dimensional imaging. In contrast, a three-dimensional imaging method clearly showed a difference in the intranuclear spatial distribution. The DHT-bound wild-type AR-GFP alone or AR-AF-1-YFP plus DHT-bound AR-AF-2-CFP was distributed as approximately 300 discrete spots in one nucleus, whereas AR-AF-1-YFP alone was distributed as one volume in a reticular pattern. Furthermore, not only AR but also the glucocorticoid receptor-YFP, ER alpha -GFP, and YFP-tagged SRC-1, TIF2, and CBP were found to be accumulated in identical spots in the presence of ligand. All of the above results indicate that CBP is one of the factors essential for foci formation of the AR, and may propose the hypothesis that transcriptionally activated steroid receptors, regardless of the type of receptor, are transferred to common compartments (foci) and form a complex with coactivators, and this process is essential to full transactivation.

Activation of the androgen receptor N-terminal domain by interleukin-6 via MAPK and STAT3 signal transduction pathways

The androgen receptor (AR) is a ligand-activated transcription factor that mediates the biological responses of androgens. However, non-androgenic pathways have also been shown to activate the AR. The mechanism of cross-talk between the interleukin-6 (IL-6) and AR signal transduction pathways was investigated in LNCaP human prostate cancer cells. IL-6 induced several androgen-response element-driven reporters that are dependent upon the AR, increased the phosphorylation of mitogen-activated protein kinase (MAPK), and activated the AR N-terminal domain (NTD). Inhibitors to MAPK and JAK decreased the IL-6-induced phosphorylation of MAPK and activation of the AR NTD. Immunoprecipitation and transactivation studies showed a direct interaction between amino acids 234-558 of the AR NTD and STAT3 following IL-6 treatment of LNCaP cells. These results demonstrate that activation of the human AR NTD by IL-6 was mediated through MAPK and STAT3 signal transduction pathways in LNCaP prostate cancer cells.

Domain interactions between coregulator ARA(70) and the androgen receptor (AR)

The coregulator function of AR-associated protein 70 (ARA(70)) was investigated to further characterize its interaction with the AR. Using a yeast two-hybrid assay, androgen-dependent binding of ARA(70) deletion mutants to the AR ligand-binding domain (LBD) was strongest with ARA(70) amino acids 321-441 of the 614 amino acid ARA(70) protein. Mutations adjacent to or within an FxxLF motif in this 120-amino acid region abolished androgen-dependent binding to the AR-LBD both in yeast and in glutathione-S-transferase affinity matrix assays. Yeast one-hybrid assays revealed an intrinsic ARA(70) transcriptional activation domain within amino acids 296-441. In yeast assays the ARA(70) domains for transcriptional activation and for binding to the AR-LBD were inhibited by the C-terminal region of ARA(70). Full-length ARA(70) increased androgen-dependent AR transactivation in transient cotransfection assays using a mouse mammary tumor virus-luciferase reporter in CV1 cells. ARA(70) also increased constitutive transcriptional activity of an AR NH(2)-terminal-DNA binding domain fragment and bound this region in glutathione-S-transferase affinity matrix assays. Binding was independent of the ARA(70) FxxLF motif. The results identify an ARA(70) motif required for androgen-dependent interaction with the AR-LBD and demonstrate that ARA(70) can interact with the NH(2)-terminal and carboxyl-terminal regions of AR.

Cyclin D1: mechanism and consequence of androgen receptor co-repressor activity

Androgen receptor regulation is pivotal for prostate growth and development. Activation of the receptor is dictated by association with androgen (ligand) and through interaction with co-activators and co-repressors. We have shown previously that cyclin D1 functions as a co-repressor to inhibit ligand-dependent androgen receptor activation. We demonstrate that cyclin D1 directly binds the N terminus of the androgen receptor and that this interaction is independent of ligand. Furthermore, we show that the interaction occurs in the nucleus and does not require the LXXLL motif of cyclin D1. Although two distinct transactivation domains exist in the N terminus (AF-1 and AF-5), the data shown support the hypothesis that cyclin D1 targets the AF-1 transactivation function. The constitutively active AF-5 domain was refractory to cyclin D1 inhibition. By contrast, cyclin D1 completely abolished androgen receptor activity, even in the presence of potent androgen receptor co-activators. This action of cyclin D1 at least partially required de-acetylase activity. Finally, we show that transient, ectopic expression of cyclin D1 results in reduced cell cycle progression in androgen-dependent LNCaP cells independent of CDK4 association. Collectively, our data support a model wherein cyclin D1 has a mitogenic (CDK4-dependent) function and an anti-mitogenic function (dependent on regulation of the AF-1 domain) that can collectively control the rate of androgen-dependent cellular proliferation. These findings provide insight into the non-cell cycle functions of cyclin D1 and provide the impetus to study its pleiotropic effects in androgen-dependent cells, especially prostatic adenocarcinomas.

c-Jun potentiates the functional interaction between the amino and carboxyl termini of the androgen receptor

The transactivation functions of the human androgen receptor (hAR) are regulated by several accessory factors that can be either positive or negative. One factor that has been previously shown to mediate hAR transactivation is the proto-oncoprotein c-Jun. The positive effect is a primary one, can be exerted by both endogenous and exogenous c-Jun, and requires multiple regions of c-Jun. However, the exact mechanism by which c-Jun exerts its enhancing function is unknown. In this study, we have used a mammalian two-hybrid system to ask if c-Jun influences the ligand-dependent amino- to carboxyl-terminal (N-to-C) interaction of hAR, which is thought to be responsible for the homodimerization of this receptor. Our results show that c-Jun enhances both hAR N-to-C terminal interaction and DNA binding in vitro. We have also tested a panel of c-Jun and c-Fos mutants for their activities on the N-to-C interaction, and the data demonstrate that the activities of these mutants parallel their activities on hAR transactivation. A mutation in the hAR activation function-2 (AF-2) abrogates N-to-C interaction, DNA binding, and transactivation, and these activities are not rescued by exogenous c-Jun. Interestingly, the p160 coactivator TIF2 can stimulate hAR N-to-C interaction, a finding consistent with the effect on hAR transactivation. These data strongly suggest that the hAR N-to-C interaction is the target of c-Jun action, and this activity requires a functional receptor AF-2.

Androgen regulation of the cell-cell adhesion molecule-1 (Ceacam1) gene

Previous studies have established that the cell-cell adhesion molecule-1 (CEACAM1, previously known as C-CAM1) functions as a tumor suppressor in prostate cancer and is involved in the regulation of prostate growth and differentiation. However, the molecular mechanism that modulates CEACAM1 expression in the prostate is not well defined. Since the growth of prostate epithelial cells is androgen-regulated, we investigated the effects of androgen and the androgen receptor (AR) on CEACAM1 expression. Transient transfection experiments showed that the AR can enhance the Ceacam1 promoter activity in a ligand-dependent manner and that the regulatory element resides within a relatively short (-249 to -194 bp) segment of the 5'-flanking region of the Ceacam1 gene. This androgen regulation is likely through direct AR-promoter binding because a mutant AR defective in DNA binding failed to upregulate reporter gene expression. Furthermore, electrophoretic mobility shift assays demonstrated that the AR specifically binds to this sequence, and mutation analysis of the potential ARE sequences revealed a region within the sequence that was required for the AR to activate the Ceacam1 gene. Therefore, the regulation of Ceacam1 gene expression by androgen may be one of the mechanisms by which androgen regulates prostatic function.

Repression of androgen-regulated gene expression by dominant negative androgen receptors

The androgen receptor (AR) is a ligand-dependent transcription activator responsible for male sexual development. In order to specifically inhibit the AR pathway, dominant negative ARs were constructed by inactivation of the major transactivation domains of the wild type AR and fusing this mutant (AR122) to the Krüppel-associated box (KRAB) repressor domain and/or histone deacetylase (HDAC1). The HDAC1-KRAB-AR122 protein was the most successful dominant negative AR, capable of repressing the wild type AR ninefold when co-expressed at a 1:1 plasmid ratio. A maximal repression of 41-fold was achieved when HDAC1-KRAB-AR122 was cotransfected with the wild type AR at a 4:1 plasmid ratio. HDAC1-KRAB-AR122 repressed transcription in a ligand-dependent manner since it inhibited a constitutively active AR mutant (AR5) only in the presence of agonists. High concentrations of partial agonists such as RU486, cyproterone acetate, and estradiol were also capable of triggering repression by HDAC1-KRAB-AR122. The potent dominant negative AR proteins might prove useful tools to inhibit AR function in vitro and in vivo.

p53 represses androgen-induced transactivation of prostate-specific antigen by disrupting hAR amino- to carboxyl-terminal interaction

Prostate-specific antigen (PSA) is highly overexpressed in prostate cancer. One important regulator of PSA expression is the androgen receptor (AR), the nuclear receptor that mediates the biological actions of androgens. AR is able to up-regulate PSA expression by directly binding and activating the promoter of this gene. We provide evidence here that that this AR activity is repressed by the tumor suppressor protein p53. p53 appears to exert its inhibition of human AR (hAR) by disrupting its amino- to carboxyl-terminal (N-to-C) interaction, which is thought to be responsible for the homodimerization of this receptor. Consistent with this, p53 is also able to block hAR DNA binding in vitro. Our previous data have shown that c-Jun can mediate hAR transactivation, and this appears to result from a positive effect on hAR N-to-C interaction and DNA binding. Interestingly, c-Jun is able to relieve the negative effects of p53 on hAR transactivation, N-to-C interaction, and DNA binding, demonstrating antagonistic activities of these two proteins. Importantly, a p53 mutation found in metastatic prostate cancer severely disrupts the p53 negative activity on hAR, suggesting that the inability of p53 mutants to down-regulate hAR is, in part, responsible for the metastatic phenotype.

Relationship between expansion of the CAG repeat in exon 1 of the androgen receptor gene and idiopathic male infertility

OBJECTIVE

To determine whether expansion of CAG repeats in exon 1 of the androgen receptor is correlated with impaired spermatogenesis in patients with male idiopathic infertility.

DESIGN

A retrospective study.

SETTING

Medical school in Besançon, France.

PARTICIPANT(S)

Thirty-seven infertile patients with azoospermia or oligospermia and 50 fertile controls.

INTERVENTION(S)

History, physical, hormonal assays, semen analysis, and collection of blood samples in order to study the androgen receptor's gene.

MAIN OUTCOME MEASURE(S)

Blood samples were collected from each infertile patient and control. The length of the CAG repeat segment was evaluated by using polymerase chain reaction (PCR) electrophoresis in exon 1 and PCR single-strand conformation polymorphism in exons 2-8.

RESULT(S)

The mean length of the CAG repeats was significantly different between infertile and fertile patients (23.91 +/- 0.5 vs. 22.20 +/- 0.4). No mutation was detected in exons 2-8 of the androgen receptor gene in infertile patients.

CONCLUSION(S)

Expansion of the CAG repeat segment of the androgen receptor is correlated with male idiopathic infertility. The number of CAG repeats may therefore have a modulatory effect on normal androgen receptor function.

DNA binding-independent transcriptional activation by the androgen receptor through triggering of coactivators

Androgens have critical roles in the development and maintenance of the male reproductive system and are important for progression of prostate cancer. The effects of androgens are mediated by the androgen receptor (AR), which is a ligand-modulated transcription factor that belongs to the nuclear receptor superfamily. In the presence of androgens, AR binds to androgen response elements in the vicinity of androgen receptor target genes and activates transcription. In addition, liganded AR can interfere with the activity of other transcription factors, such as activator protein-1 and nuclear factor kappaB, for which DNA binding by AR is not necessary. In this study, we describe a novel ligand-dependent transactivation function for AR that is independent of its DNA binding ability. AR dramatically increased the intrinsic transcriptional activity of the nuclear receptor coactivators glucocorticoid receptor-interacting protein-1 (GRIP1), cAMP response element-binding protein-binding protein, and p300 that are tethered to DNA. This "triggering" phenomenon required both similar and distinctly different regions of AR compared with those needed for ligand-dependent transactivation from androgen-responsive elements. Furthermore, the domains of GRIP1 required for triggering by AR are different from those required when GRIP1 serves as a coactivator for AR at androgen-responsive promoters. These data suggest that triggering may constitute an important part of the mechanism by which AR regulates transcription.

Analysis of glucocorticoid and androgen receptor gene fusions delineates domains required for transcriptional specificity

Androgen receptor (AR) and glucocorticoid receptor (GR) influence distinct physiologic responses in steroid-responsive cells despite their shared ability to selectively bind in vitro to the same canonical DNA sequence (TGTTCT). While the DNA-binding domains (DBDs) of these receptors are highly conserved, the amino N-terminal domain (NTD) and hormone-binding domain (HBD) are evolutionarily divergent. To determine the relative contribution of these functional domains to steroid-specific effects in vivo, we constructed a panel of AR/GR gene fusions by interchanging the NTD, DBD, and HBD regions of each receptor and measured transcriptional regulatory activities in transfected kidney and prostate cell lines. We found that GR was approximately 10-fold more active than AR when tested with the mouse mammary tumor virus promoter, and that this difference in activity was primarily owing to sequence divergence in the NTDs. We also tested transcriptional activation of the androgen-dependent rat probasin promoter, and in this case, AR was at least twofold more active than GR. Analysis of the chimeric receptors revealed that this difference mapped to the DBD region of the two receptors. Transcriptional repression functions of the wild-type and chimeric receptors were measured using an activator protein 1 (AP-1) transrepression assay and identified the GR HBD as a more potent transrepressor of AP-1 transcriptional activation than the AR HBD. Taken together, our analyses reveal that evolutionary sequence divergence between AR and GR functional domains results in unique promoter-specific activities within biologic systems in which both AR and GR are normally expressed.

A novel mutation (N233K) in the transactivating domain and the N756S mutation in the ligand binding domain of the androgen receptor gene are associated with male infertility

OBJECTIVE

Resistance to androgens has been suggested as a possible cause of male infertility. This hypothesis is based mainly on binding studies in genital skin fibroblasts but the molecular evidence is sparse.

DESIGN

Molecular studies of the androgen receptor gene were performed in 10 azoo- or oligozoospermic men, presenting with clinical signs of low androgen activity-poor virilization and high serum LH despite elevated testosterone levels, but without genital malformations.

PATIENTS

Ten men with serum LH >10 IU/l and testosterone >30 nmol/l as well as a low sperm concentration < 20 x 106/ml.

MEASUREMENTS

Genomic DNA was prepared from peripheral leucocytes and PCR-amplification of the coding region of androgen receptor was performed, followed by direct sequencing. Identified mutations were reconstructed by site-directed mutagenesis and the functional properties of the mutants were analysed, using transient expression in COS-1 cells and subsequent transactivation assays. Hormone binding assays were performed in genital skin fibroblasts from the patients.

RESULTS

Two of the 10 men were shown to have a mutation in the androgen receptor gene. Subject 1, who presented with azoospermia, serum testosterone (T) 50 nmol/l and LH 20 IU/l, had a mutation in exon 1, changing amino acid asparagine 233 to lysine (N233K). In fibroblasts cultured from genital skin, the receptor affinity for 5alpha-dihydrotestosterone (DHT) was normal as compared to healthy controls, but the receptor-hormone complex was thermolabile at 42 degrees C. Subject 2 exhibited severe oligozoospermia and a similar endocrine pattern (T = 50 nmol/l and LH = 25 IU/l). He had a mutation in exon 5 changing asparagine 756 to serine (N756S). The affinity for DHT in cultured genital fibroblasts from this patient was reduced. Transactivation was abnormal for both mutants, N233K reaching 46% and N756S 38% of wild type activity when stimulated with 10 nmol/l DHT.

CONCLUSIONS

Androgen receptor mutations may affect sperm production without resulting in genital malformations. Thus, in infertile men with a clinical presentation of poor androgen activity and an endocrine profile compatible with androgen resistance, mutations in the androgen receptor should be taken into consideration.

Prostate cancer and the androgen receptor: strategies for the development of novel therapeutics

The early demonstrations that prostate cancer was hormone-sensitive initiated a therapeutic strategy of hormone ablation that is still in use today. Although chemical or surgical castration reduces androgen stimulation of the androgen receptor (AR) and produces tumor regression, little survival benefit is achieved. Patients with metastatic cancer eventually relapse as their tumors progress to hormone independence. The AR is a member of the steroid receptor family; however, it manifests many unique features including: N-terminal, C-terminal interactions and antiparallel dimerization, unique N-terminal domains for co-factor recruitment, AR-specific co-activators and upstream promoter/enhancer response elements that amplify AR-mediated responses. The AR is regulated by phosphorylation and cross-talk with several signaling pathways, including MAP kinases, PKA and PKC. Non-genomic effects of AR to regulate transcription factors elk-1 and -2 have also been demonstrated. These unique features suggest mechanisms by which novel therapeutics might target and influence AR-mediated actions. Progress in this direction has been realized with the recent synthesis of non-steroidal androgen agonists that may have tissue-selective effects.

Response to treatment in patients with partial androgen insensitivity due to mutations in the DNA-binding domain of the androgen receptor

The androgen insensitivity syndrome is a disorder caused by deficient function of the androgen receptor, characterized by varying degrees of undermasculinization in karyotypic males. We have identified four mutations in the androgen receptor gene, in the region encoding the DNA-binding domain of the protein. Two mutations, R607X and R615G, were found in patients with complete insensitivity to androgens, whereas the other two, S578T and A596T, were found in patients with partial insensitivity. The functional consequences of the three missense mutations were assayed in vitro after transient expression of the receptors in COS cells. All mutants showed normal androgen binding but abnormal abilities to stimulate transcription of an androgen-responsive reporter gene. R615G abolished transactivation whereas S578T and A596T were partially malfunctional. The function of A596T, but not of S578T, was normalized at high androgen concentrations in vitro, reflecting the in vivo situation. Thus, patients with specific mutations in the DNA-binding domain of the androgen receptor may benefit from androgen treatment.

Covalent modification of the androgen receptor by small ubiquitin-like modifier 1 (SUMO-1)

Modification by SUMO-1 is proposed to play a role in protein targeting and/or stability. The SUMO-1-conjugating enzyme Ubc9 interacts with androgen receptor (AR), a ligand-activated transcription factor belonging to the steroid receptor superfamily. We show here that AR is covalently modified by SUMO-1 (sumoylated) in an androgen-enhanced fashion and identify the principal acceptor site in the N-terminal domain of AR. Substitutions of sumoylated Lys residues enhanced transcriptional activity of AR without influencing its transrepressing activity. Interestingly, the same Lys residues form the cores of the recently described transcriptional synergy control motifs in AR [Iñiguez-Lluhi, J. A. & Pearce, D. (2000) Mol. Cell. Biol. 20, 6040-6050]. These motifs, which match perfectly with the sumoylation consensus sequence, are also present in the N-terminal domains of glucocorticoid, mineralocorticoid, and progesterone receptor. Taken together, our data suggest that reversible sumoylation is a mechanism for regulation of steroid receptor function.

Thyroid receptor activator molecule, TRAM-1, is an androgen receptor coactivator

An androgen receptor (AR) interacting protein was isolated from a HeLa cell complementary DNA library by two-hybrid screening in yeast using the AR DNA and ligand binding domains [amino acids (aa) 481-919] as bait. AR binding of the protein in yeast was dependent on the presence of testosterone or dihydrotestosterone (DHT). The isolated protein is identical to thyroid receptor activator molecule TRAM-1 but lacking aa 1-458. TRAM-1 is a steroid receptor coactivator-3 (SRC-3) subtype. In affinity matrix assays, 35S-labeled TRAM-1 bound the GST-AR ligand binding domain (aa 624-919) and GST-AR N-terminal and DNA binding domains (aa 1-660), but not the GST-AR DNA binding domain (aa 544-634) alone. Coexpression of TRAM-1 increased DHT-dependent AR transactivation 5-fold and constitutive activity of AR (aa 1-660) N-terminal and DNA-binding domains increased 9-fold. Full-length TRAM-1 (aa 1-1424) and the partial (aa 459-1424) were AR and GR coactivators as was SRC-1. In human testis, immunostaining of SRC-3 colocalized with AR in nuclei of Sertoli cells and peritubular myoid cells, indicating it could function as an AR coactivator in these cells. SRC-3 was also present in nuclei of spermatogenic cells where AR was not expressed, suggesting it might also be a coactivator with other nuclear receptors that regulate spermatogenesis.

The size of the CAG repeat in exon 1 of the androgen receptor gene shows no significant relationship to impaired spermatogenesis in an infertile Caucasoid sample of German origin

The androgen receptor (AR) gene, located on the X-chromosome at Xq11-12, contains in exon 1 a polymorphic CAG repeat which codes for a polyglutamine tract. Contractions of the CAG repeat are said to be related to prostate cancer. In contrast, sizeable expansion of the CAG repeat can cause spinal and bulbar muscular atrophy (SBMA). In infertile patients of Chinese origin and in a Melbourne multinational population impaired sperm production has been postulated to be related to moderate expansions of the polyglutamine tract. In a study of a Swedish population of infertile patients these findings could not be corroborated. The aim of our investigation was to examine the correlation between the length of the CAG repeat and impaired sperm production in an infertile Caucasoid patient sample of German ethnic origin. We found no statistically significant relationship between the size of the CAG repeat or polyglutamine tract and idiopathic impaired sperm production in the population studied. The variability of the results by various investigators may be attributed to different ethnic origins and hence different genetic modifiers of the populations studied and/or to the high probability that these infertile males may represent a heterogeneous group with respect to the causes of defective spermatogenesis.

FHL2, a novel tissue-specific coactivator of the androgen receptor

The control of target gene expression by nuclear receptors requires the recruitment of multiple cofactors. However, the exact mechanisms by which nuclear receptor-cofactor interactions result in tissue-specific gene regulation are unclear. Here we characterize a novel tissue-specific coactivator for the androgen receptor (AR), which is identical to a previously reported protein FHL2/DRAL with unknown function. In the adult, FHL2 is expressed in the myocardium of the heart and in the epithelial cells of the prostate, where it colocalizes with the AR in the nucleus. FHL2 contains a strong, autonomous transactivation function and binds specifically to the AR in vitro and in vivo. In an agonist- and AF-2-dependent manner FHL2 selectively increases the transcriptional activity of the AR, but not that of any other nuclear receptor. In addition, the transcription of the prostate-specific AR target gene probasin is coactivated by FHL2. Taken together, our data demonstrate that FHL2 is the first LIM-only coactivator of the AR with a unique tissue-specific expression pattern.

Protein inhibitor of activated STAT-1 (signal transducer and activator of transcription-1) is a nuclear receptor coregulator expressed in human testis

An androgen receptor (AR) interacting protein was isolated from a HeLa cell cDNA library by two-hybrid screening in yeast using the AR DNA+ligand binding domains as bait. The protein has sequence identity with human protein inhibitor of activated signal transducer and activator of transcription (PIAS1) and human Gu RNA helicase II binding protein (GBP). Binding of PIAS1 to human AR DNA+ligand binding domains was androgen dependent in the yeast liquid beta-galactosidase assay. Activation of binding by dihydrotestosterone was greater than testosterone > estradiol > progesterone. PIAS1 binding to full-length human AR in a reversed yeast two hybrid system was also androgen dependent. [35S] PIAS1 bound a glutathione S-transferase-AR-DNA binding domain (amino acids 544-634) fusion protein in affinity matrix assays. In transient cotransfection assays using CV1 cells with full-length human AR and a mouse mammary tumor virus luciferase reporter vector, there was an androgen-dependent 3- to 5-fold greater increase in luciferase activity with PIAS1 over that obtained with an equal amount of control antisense cDNA or mutant PIAS1. Constitutive transcriptional activity of the AR N-terminal+DNA binding domain was increased 6-fold by PIAS1. PIAS1 also enhanced glucocorticoid receptor transactivation in response to dexamethasone but inhibited progesterone-induced progesterone receptor transactivation in the same assay system. mRNA for PIAS1 was highly expressed in testis of human, monkey, rat, and mouse. In rat testis the onset of PIAS1 mRNA expression coincided with the initiation of spermatogenesis between 25-30 days of age. Immunostaining of human and mouse testis with PIAS1-specific antiserum demonstrated coexpression of PIAS1 with AR in Sertoli cells and Leydig cells. In addition, PIAS1 was expressed in spermatogenic cells. The results suggest that PIAS1 functions in testis as a nuclear receptor transcriptional coregulator and may have a role in AR initiation and maintenance of spermatogenesis.

Androgen insensitivity

The androgen receptor (AR) protein regulates transcription of certain genes. Usually, this activity depends upon a central DNA-binding domain that permits the binding of androgen-AR complexes to regulatory DNA sequences near or in a target gene. The AR also has a C-terminal androgen-binding domain (ABD) and an N-terminal modulatory domain. These domains interact among themselves and with coregulatory, nonreceptor proteins to determine vector control over a gene's transcription rate. The precise roles of these proteins are active research areas. Severe X-linked androgen receptor gene (AR) mutations cause complete androgen insensitivity, mild ones impair virilization with or without infertility, and moderate ones sometimes yield a wide phenotypic spectrum among sibs. Different expressivity may reflect variability of AR-interactive proteins. The family history must identify heterozygous XX females with sparse, delayed, or asymmetric pubic/axillary hair or delayed menarche and infertile XY maternal aunts or uncles. Mutation type and density vary along the length of the AR. N-terminal polyglutamine tract expansion limits AR transactivation, causing a form of mild androgen insensitivity. Analysis of ABD mutations that do not impair androgen binding or impair it selectively will illuminate its intradomain properties. For partial androgen insensitivity and mild androgen insensitivity, pharmacotherapy with certain androgens or other steroids may overcome some dysfunction of certain mutant ARs. Experience with this approach is limited; outcomes have been generally disappointing.

The AF1 and AF2 domains of the androgen receptor interact with distinct regions of SRC1

The androgen receptor is unusual among nuclear receptors in that most, if not all, of its activity is mediated via the constitutive activation function in the N terminus. Here we demonstrate that p160 coactivators such as SRC1 (steroid receptor coactivator 1) interact directly with the N terminus in a ligand-independent manner via a conserved glutamine-rich region between residues 1053 and 1123. Although SRC1 is capable of interacting with the ligand-binding domain by means of LXXLL motifs, this interaction is not essential since an SRC1 mutant with no functional LXXLL motifs retains its ability to potentiate androgen receptor activity. In contrast, mutants lacking the glutamine-rich region are inactive, indicating that this region is both necessary and sufficient for recruitment of SRC1 to the androgen receptor. This recruitment is in direct contrast to the recruitment of SRC1 to the estrogen receptor, which requires interaction with the ligand-binding domain.

Analysis of exon 1 mutations in the androgen receptor gene

Eleven mutations in exon 1 of the androgen receptor gene (AR) have been identified in 15 individuals with Androgen Insensitivity syndrome (AIS). Nine of the mutations yield a stop codon directly, or due to a frameshift, in individuals with complete AIS (CAIS). One individual with CAIS had three different mutations in exon 1: one is nominally silent (Glu 211; GAG 995 GAA); two are missense (Pro 390 Arg and Glu 443 Arg). Five unrelated individuals with either CAIS, partial AIS (PAIS) or mild AIS (MAIS) had GAG 995 GAA as their only alteration. This report almost doubles the number of exon 1 mutations stored in the AR Mutation Database, reinforces their highly predominant nonsense character, and identifies Pro 390 and/or Gln 443 as residues that are probably necessary for one or more specific functions of the AR's N-terminal transactivation domain.

The androgen receptor amino-terminal domain plays a key role in p160 coactivator-stimulated gene transcription

Steroid receptors are conditional transcription factors that, upon binding to their response elements, regulate the expression of target genes via direct protein interactions with transcriptional coactivators. We have analyzed the functional interactions between the androgen receptor (AR) and 160-kDa nuclear receptor coactivators. Upon overexpression in mammalian cells, these coactivators enhance the transcriptional activity of both the amino-terminal domain (NTD) and the ligand-binding domain (LBD) of the AR. The coactivator activity for the LBD is strictly ligand-controlled and depends on the nature of the DNA-binding domain to which it is fused. We demonstrate that the NTD physically interacts with coactivators and with the LBD and that this interaction, like the functional interaction between the LBD and p160 coactivators, relies on the activation function 2 (AF2) core domain. The mutation of a highly conserved lysine residue in the predicted helix 3 of the LBD (K720A), however, blunts the functional interaction with coactivators but not with the NTD. Moreover, this mutation does not affect the transcriptional activity of the full-size AR. A mutation in the NTD of activation function AF1a (I182A/L183A), which dramatically impairs the activity of the AR, has no effect on the intrinsic transcriptional activity of the NTD but interferes with the cooperation between the NTD and the LBD. Finally, p160 proteins in which the three LXXLL motifs are mutated retain most of their coactivator activity for the full-size AR, although they are no longer functional for the isolated LBD. Together, these data suggest that in the native AR the efficient recruitment of coactivators requires a functional association of the NTD with the LBD and that the binding of coactivators occurs primarily through the NTD.

Isolation and characterization of ARA160 as the first androgen receptor N-terminal-associated coactivator in human prostate cells

The androgen receptor (AR) is a member of the steroid receptor superfamily that may require coactivators for proper or maximal transactivation. Using a purified AR N-terminal peptide as a probe to screen the human testis expression library, we identified an androgen-enhanced AR N-terminal-associated protein ARA160, which consists of 1,093 amino acids with an apparent molecular mass of 160 kDa. Sequence comparison in GenBank(TM) reveals that ARA160 shares an identical sequence with a HIV-1 TATA element modulatory factor, TMF. The far-Western blotting and co-immunoprecipitation assays demonstrate that the AR can interact directly with ARA160/TMF. Affinity gel pull-down and mammalian two-hybrid assays further suggest androgen can enhance significantly the interaction between AR and ARA160. Transient transfection assays demonstrated that ARA160 might function as a coactivator for AR-mediated transactivation in human prostate cancer PC-3 cells. Our data further suggest that this AR N-terminal coactivator can function cooperatively with AR C-terminal coactivator, ARA70, in PC-3 cells. Together, our data demonstrate that ARA160 might represent the first identified androgen-enhanced N-terminal coactivator for the AR.

Specific androgen receptor activation by an artificial coactivator

Transcription activation of steroid receptors, such as the androgen receptor (AR), is mediated by coactivators, which bridge the receptor to the preinitiation complex. To develop a tool for studying the role of the AR in normal development and disease, we constructed artificial coactivators consisting of the transcription activation domains of VP16 or p65/RelA and the AR hinge and ligand-binding domain (ARLBD), which has been shown to interact with the AR N-terminal domain. The artificial VP16-ARLBD and ARLBD-p65 coactivators interacted with the AR N terminus and wild-type AR in an androgen-dependent and androgen-specific manner. VP16-ARLBD and ARLBD-p65 enhanced the AR transactivity up to 4- and 13-fold, respectively, without affecting the expression of the AR protein. The coactivators did not enhance the transcription activity of the progesterone receptor (PR) or the glucocorticoid receptor (GR), showing their specificity for the AR. In addition, to construct PR- and GR-specific coactivators, the VP16 activation domain was fused to the PR and GR hinge/ligand-binding domain. Although VP16-PRLBD and VP16-GRLBD interacted with the C-terminal portion of steroid receptor coactivator-1, they did not enhance the transcription activity of their receptor. The presented strategy of directing activation domains or other protein activities into the DNA-bound AR complex provides a novel means of manipulating AR function in vitro and in vivo.

Mechanisms of androgen receptor activation and function

Androgens play a crucial role in several stages of male development and in the maintenance of the male phenotype. Androgens act in their target cells via an interaction with the androgen receptor, resulting in direct regulation of gene expression. The androgen receptor is a phosphoprotein and modulation of the phosphorylation status of the receptor influences ligand-binding and consequently transcription activation of androgen responsive genes. Androgen binding induces a conformational change in the ligand-binding domain, accompanied by additional receptor phosphorylation. Subsequently the liganded androgen receptor interacts with specific androgen response elements in the regulatory regions of androgen target genes, resulting in stimulation of gene expression. Anti-androgens induce a different conformational change of the ligand-binding domain, which does not or only partially result in stimulation of transactivation. Interestingly, different anti-androgens can induce different inactive conformations of the androgen receptor ligand-binding domain. Recent evidence strongly supports a ligand dependent functional interaction between the ligand-binding domain and the NH2-terminal transactivating domain of the androgen receptor. Two regions in the NH2-terminal domain are involved in this interaction, whereas in the ligand-binding domain the AF-2 AD core region is involved.

Androgen-independent induction of prostate-specific antigen gene expression via cross-talk between the androgen receptor and protein kinase A signal transduction pathways

Transcription of the prostate-specific antigen (PSA) gene escapes regulation by androgens in advanced prostate cancer. To determine the molecular mechanism(s) of androgen-independent regulation of the PSA gene, the possibility that the androgen receptor (AR) is activated in the absence of androgen by stimulation of protein kinase A (PKA) was investigated. Activation of PKA by forskolin resulted in elevated expression of the PSA gene in androgen-depleted LNCaP cells, an effect that was blocked by the antiandrogen, bicalutamide. Further evidence that induction of PSA gene expression was dependent on AR was obtained from experiments using PC3 cells devoid of AR. Neither PSA, PB, nor ARR3 androgen-responsive reporters could be induced by activation of PKA in the absence of transfected AR. In addition, when nuclear AR from forskolin-treated LNCaP cells was incubated with oligonucleotides encoding an androgen response element of the PSA promoter and examined by electromobility shift assay, an increase in AR-androgen response element complex formation was observed. Lastly, cotransfection of an expression vector for a chimeric protein encoding the amino-terminal domain of the human AR linked to Gal4 and a 5xGal4UAS reporter gene construct resulted in activation of the amino-terminal domain of the AR by stimulation of PKA activity. These results demonstrate androgen-independent induction of PSA gene expression in prostate cancer cells by an AR-dependent pathway.

Interaction of the putative androgen receptor-specific coactivator ARA70/ELE1alpha with multiple steroid receptors and identification of an internally deleted ELE1beta isoform

Steroid-regulated gene transcription requires the coordinate physical and functional interaction of hormone receptors, basal transcription factors, and transcriptional coactivators. In this context ARA70, previously called RFG and ELE1, has been described as a putative coactivator that specifically enhances the activity of the androgen receptor (AR) but not that of the glucocorticoid receptor (GR), the progesterone receptor, or the estrogen receptor (ER). Here we describe the cloning of the cDNA for ELE1/ARA70 by RT-PCR from RNA derived from different cell lines (HeLa, DU-145, and LNCaP). In accordance with the previously described sequence, we obtained a 1845-bp PCR product for the HeLa and the LNCaP RNA. Starting from T-47D RNA, however, an 860-bp PCR product was obtained. This shorter variant results from an internal 985-bp deletion and is called ELE1beta; accordingly, the longer isoform is referred to as ELE1alpha. The deduced amino acid sequence of ELE1alpha, but not that of ELE1beta, differs at specific positions from the one previously published by others, suggesting that these two proteins are encoded by different nonallelic genes. ELE1alpha is expressed in the three prostate-derived cell lines examined (PC-3, DU-145, and LNCaP), and this expression is not altered by androgen treatment. Of all rat tissues examined, ELE1alpha expression is highest in the testis. This is also the only tissue in which we could demonstrate ELE1beta expression. Both ELE1alpha and ELE1beta interact in vitro with the AR, but also with the GR and the ER, in a ligand-independent way. Overexpression of either ELE1 isoform in DU-145, HeLa, or COS cells had only minor effects on the transcriptional activity of the human AR. ELE1alpha has no intrinsic transcription activation domain or histone acetyltransferase activity, but it does interact with another histone acetyltransferase, p/CAF, and the basal transcription factor TFIIB. The interaction with the AR occurs through the ligand-binding domain and involves the region corresponding to the predicted helix 3. Mutation in this domain of leucine 712 to arginine greatly reduces the affinity of the AR for ELE1alpha but has only moderate effects on its transcriptional activity. Taken together, we have identified two isoforms of the putative coactivator ARA70/ELE1 that may act as a bridging factor between steroid receptors and components of the transcription initiation complex but which lack some fundamental properties of a classic nuclear receptor coactivator. Further experiments will be required to highlight the in vivo role of ELE1 in nuclear receptor functioning.

Multiple receptor domains interact to permit, or restrict, androgen-specific gene activation

A critical problem within transcription factor families is how diverse regulatory programs are directed by highly related members. Androgen and glucocorticoid receptors (AR, GR) recognize a consensus DNA hormone response element (HRE), but they activate target genes with precise specificity, largely dependent on the promoter and cell context. We have assessed the role of different receptor domains in hormone-specific response by testing chimeras of AR and GR for their ability to activate the androgen-specific enhancer of the mouse sex-limited protein (Slp) gene. Although all of the mutant receptors activated simple HREs, only a few activated the androgen-specific element. One component shared by receptors functional on the AR-specific target was the AR DNA binding domain. Activation was not due to differential DNA affinity but rather to the AR DNA binding domain escaping suppression directed at the GR DNA binding domain in this enhancer context. A further mechanism increasing specific activation was cooperation of receptors at multiple and weak HREs, which was accentuated in the presence of both the AR N terminus and ligand binding domain. These domains together increased recognition of weak HREs, as demonstrated by in vitro DNase I footprinting and transactivation of mutant enhancers. Further, AR N-terminal subdomains reported to interact directly with the ligand binding domain relieved an inhibitory effect imposed by that domain. Therefore, functions intrinsic to AR augment steroid-specific gene activation, by evading negative regulation operating on the domains of other receptors and by enhancing cooperativity through intra- and inter-receptor domain interactions. These subtle distinctions in AR and GR behavior enforce transcriptional specificity established by the context of nonreceptor factors.

Differential transactivation by the androgen receptor in prostate cancer cells

BACKGROUND

The purpose of this study was to determine the contribution of different transactivating regions of the androgen receptor (AR) to the induction of androgen-regulated promoters in poorly (PC3 cells) and well-differentiated (LNCaP cells) prostate cancer cell lines.

METHODS

PC3 and LNCaP cells were co-transfected with plasmids expressing full-length AR or deletion mutants together with luciferase reporters linked to the probasin (PB) and PSA promoters; as well as to ARR3tk, a PB-derived recombinant promoter.

RESULTS

Androgen induction of the ARR3tk promoter in the presence of AR was 8- to 10-fold higher than that seen with the PB promoter. Activation of ARR3tk was greatest with an androgen-independent construct in which the first 231 amino acids and the ligand binding domain had been removed, indicating that this promoter is more responsive to activating functions in the N-terminal domain than in the ligand binding domain. By comparison, induction of the PB promoter was greatest with the full-length AR, which suggests that the ligand binding domain also makes a major contribution to the activation of this promoter. In similar analyses with the PSA promoter, AR regions required for promoter induction was dependent on the host cell type. In PC3 cells, the predominant AR transactivation function was androgen-independent and resided in the N-terminal domain, whereas in LNCaP cells, the highest level of induction was androgen dependent and also required participation of the ligand binding domain.

CONCLUSIONS

Our results indicate that the relative utilization of transactivating functions in N-terminal and ligand binding domains of the AR is promoter and cell specific.

Functional interactions of the AF-2 activation domain core region of the human androgen receptor with the amino-terminal domain and with the transcriptional coactivator TIF2 (transcriptional intermediary factor2)

Previous studies in yeast and mammalian cells showed a functional interaction between the amino-terminal domain and the carboxy-terminal, ligand-binding domain (LBD) of the human androgen receptor (AR). In the present study, the AR subdomains involved in this in vivo interaction were determined in more detail. Cotransfection experiments in Chinese hamster ovary (CHO) cells and two-hybrid experiments in yeast revealed that two regions in the NH2-terminal domain are involved in the functional interaction with the LBD: an interacting domain at the very NH2 terminus, located between amino acid residues 3 and 36, and a second domain, essential for transactivation, located between residues 370 and 494. Substitution of glutamic acid by glutamine at position 888 (E888Q) in the AF-2 activation domain (AD) core region in the LBD, markedly decreased the interaction with the NH2-terminal domain. This mutation neither influenced hormone binding nor LBD homodimerization, suggesting a role of the AF-2 AD core region in the functional interaction between the NH2-terminal domain and the LBD. The AF-2 AD core region was also involved in the interaction with the coactivator TIF2 (transcriptional intermediary factor 2), as the E888Q mutation decreased the stimulatory effect of TIF2 on AR AF-2 activity. Cotransfection of TIF2 and the AR NH2-terminal domain expression vectors did not result in synergy between both factors in the induction of AR AF-2 activity. TIF2 highly induced AR AF-2 activity on a complex promoter [mouse mammary tumor virus (MMTV)], but it was hardly active on a minimal promoter (GRE-TATA). In contrast, the AR NH2-terminal domain induced AR AF-2 activity on both promoter constructs. These data indicate that both the AR NH2-terminal domain and the coactivator TIF2 functionally interact, either directly or indirectly, with the AF-2 AD core region in the AR-LBD, but the level of transcriptional response induced by TIF2 depends on the promoter context.

Intermolecular NH2-/carboxyl-terminal interactions in androgen receptor dimerization revealed by mutations that cause androgen insensitivity

Structural alignment of the human androgen receptor dimer was investigated by introducing steroid binding domain mutations that cause partial or complete androgen insensitivity into fusion proteins containing the full-length androgen receptor or the steroid binding domain. Most of the mutants had unchanged apparent equilibrium androgen binding affinity and increased dissociation rates of [3H]methyltrienolone and required increased dihydrotestosterone concentrations for transcriptional activation. In a 2-hybrid protein interaction assay in mammalian cells, the steroid binding domain interacts with an NH2-terminal-DNA binding domain fragment and with the full-length androgen receptor at physiological androgen concentrations in a dose-dependent manner. However, mutations at Val-889 and Arg-752 disrupt the NH2-/carboxyl-terminal interaction when introduced into the steroid binding domain fragment but not when present in the full-length androgen receptor. The N-C bimolecular interaction reduces the dissociation rate of bound androgen and slows the degradation rate of the carboxyl-terminal steroid binding domain fragment. The results suggest that steroid binding domain residues Val-889 and Arg-752 are critical to the NH2-/carboxyl-terminal interaction and that an intermolecular N-C interaction occurs during receptor dimerization that results in an antiparallel arrangement of androgen receptor monomers.