RAC3, a steroid/nuclear receptor-associated coactivator that is related to SRC-1 and TIF2

Differential induction of androgen receptor transactivation by different androgen receptor coactivators in human prostate cancer DU145 cells

Recently identified androgen receptor (AR) coactivators were used in this study to determine whether the specificity of sex hormones and antiandrogens could be modulated at the coactivator level. We found that ARA70 is the best coactivator to confer the androgenic activity on 17beta-estradiol. Only ARA70 and ARA55 could increase significantly the androgenic activity of hydroxyflutamide, a widely used antiand rogen for the treatment of prostate cancer. None of the AR coactivators we tested could significantly confer androgenic activity on progesterone and glucocorticoid at their physiological concentrations (1-10nM). We also found that ARA70, ARA55, and ARA54, but not steroid receptor coactivator-1 (SRC-1) and Rb, could significantly enhance the delta5-androstenediol-mediated AR transactivation. Furthermore, in comparing the relative specificity of these coactivators to AR in DU145 cells, our results suggested that ARA70 has a relatively higher specificity and that SRC-1 can enhance almost equally well many other steroid receptors. Finally, our data demonstrated that AR itself and some select AR coactivators such as ARA70 or ARA54 could, respectively, interact with CBP and p300/CBP-associated factors that have histone acetyl-transferase activity for assisting chromatin remodeling. Together, our data suggest that the specificity of sex hormones and antiandrogens can be modulated by some selective AR coactivators. These findings may not only help us to better understand the specificity of the sex hormones and antiandrogens, but also facilitate the development of better antiandrogens to fight the androgen-related diseases, such as prostate cancer.

NRIF3 is a novel coactivator mediating functional specificity of nuclear hormone receptors

Many nuclear receptors are capable of recognizing similar DNA elements. The molecular event(s) underlying the functional specificities of these receptors (in regulating the expression of their native target genes) is a very important issue that remains poorly understood. Here we report the cloning and analysis of a novel nuclear receptor coactivator (designated NRIF3) that exhibits a distinct receptor specificity. Fluorescence microscopy shows that NRIF3 localizes to the cell nucleus. The yeast two-hybrid and/or in vitro binding assays indicated that NRIF3 specifically interacts with the thyroid hormone receptor (TR) and retinoid X receptor (RXR) in a ligand-dependent fashion but does not bind to the retinoic acid receptor, vitamin D receptor, progesterone receptor, glucocorticoid receptor, or estrogen receptor. Functional experiments showed that NRIF3 significantly potentiates TR- and RXR-mediated transactivation in vivo but has little effect on other examined nuclear receptors. Domain and mutagenesis analyses indicated that a novel C-terminal domain in NRIF3 plays an essential role in its specific interaction with liganded TR and RXR while the N-terminal LXXLL motif plays a minor role in allowing optimum interaction. Computer modeling and subsequent experimental analysis suggested that the C-terminal domain of NRIF3 directly mediates interaction with liganded receptors through an LXXIL (a variant of the canonical LXXLL) module while the other part of the NRIF3 protein may still play a role in conferring its receptor specificity. Identification of a coactivator with such a unique receptor specificity may provide new insight into the molecular mechanism(s) of receptor-mediated transcriptional activation as well as the functional specificities of nuclear receptors.

Amplification and overexpression of peroxisome proliferator-activated receptor binding protein (PBP/PPARBP) gene in breast cancer

Peroxisome proliferator-activated receptor binding protein (PBP), a nuclear receptor coactivator, interacts with estrogen receptor alpha (ERalpha) in the absence of estrogen. This interaction was enhanced in the presence of estrogen but was reduced in the presence of antiestrogen, tamoxifen. Transfection of PBP in CV-1 cells resulted in enhancement of estrogen-dependent transcription, indicating that PBP serves as a coactivator in ER signaling. To examine whether overexpression of PBP plays a role in breast cancer because of its coactivator function in ER signaling, we determined the levels of PBP expression in breast tumors. High levels of PBP expression were detected in approximately 50% of primary breast cancers and breast cancer cell lines by ribonuclease protection analysis, in situ hybridization, and immunoperoxidase staining. Fluorescence in situ hybridization of human chromosomes revealed that the PBP gene is located on chromosome 17q12, a region that is amplified in some breast cancers. We found PBP gene amplification in approximately 24% (6/25) of breast tumors and approximately 30% (2/6) of breast cancer cell lines, implying that PBP gene overexpression can occur independent of gene amplification. This gene comprises 17 exons that, together, span >37 kilobases. The 5'-flanking region of 2.5 kilobase pairs inserted into a luciferase reporter vector revealed that the promoter activity in CV-1 cells increased by deletion of nucleotides from -2,500 to -273. The -273 to +1 region, which exhibited high promoter activity, contains a typical CCAT box and multiple cis-elements such as C/EBPbeta, YY1, c-Ets-1, AP1, AP2, and NFkappaB binding sites. These observations, in particular PBP gene amplification, suggest that PBP, by its ability to function as ERalpha coactivator, might play a role in mammary epithelial differentiation and in breast carcinogenesis.

Structure and chromosomal locations of mouse steroid receptor coactivator gene family

The newly recognized steroid receptor coactivators (SRC-1, SRC-2, and SRC-3) belong to a homologous gene family and are important transcriptional mediators for nuclear receptors. Through fluorescence in situ hybridization, we have mapped the mouse SRC-1, SRC-2, and SRC-3 genes to chromosomal locations 12A2-A3, 1A3-A5, and 2H2-H4, respectively. By screening a mouse genomic DNA library, performing long-range polymerase chain reaction and sequencing, we have cloned and characterized the mouse SRC-3 gene. The SRC-3 gene contains 19 exons and spans more than 38 kilobases (kb). Intron sizes are variable. Intron 1 (13.5 kb) and intron 15 (4.6 kb) contribute to almost half the total length of the gene. Among 20 exons identified, exon 10 is the largest (869 bp) and encodes the receptor interaction domain. The start and stop codons for translation are in exon 2 and 20, respectively. The relationship between SRC-3 gene structure and its functional protein domains suggests that many functional domains or subdomains are encoded by individual exons. The correlation between gene structure and alternative splice variants is also discussed. In summary, we have defined the structure of mouse SRC-3 gene and found that the genes in the SRC family are located in different mouse chromosomes. This information is important for developing valuable animal models harboring multiple disruptions of the SRC gene family to study their biological functions.

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.

A novel cofactor for p300 that regulates the p53 response

The ability of p53 to function as a transcription factor is instrumental in facilitating the response to cellular stress, and p300/CBP proteins, which act as coactivators for diverse transcription factors, participate in regulating p53 activity. We report a novel cofactor for p300 that facilitates the p53 response by augmenting p53-dependent transcription and apoptosis. JMY and p300 associate in physiological conditions, and, during the cellular stress response, the p300/JMY complex is recruited to activated p53. The bax gene is efficiently activated by JMY, and protein isoforms that arise through alternative splicing alter the functional outcome of the p53 response. The results provide compelling evidence that the p300/JMY coactivator complex plays a central role in facilitating the p53 response.

Activating signal cointegrator 1, a novel transcription coactivator of nuclear receptors, and its cytosolic localization under conditions of serum deprivation

Activating signal cointegrator 1 (ASC-1) harbors an autonomous transactivation domain that contains a putative zinc finger motif which provides binding sites for basal transcription factors TBP and TFIIA, transcription integrators steroid receptor coactivator 1 (SRC-1) and CBP-p300, and nuclear receptors, as demonstrated by the glutathione S-transferase pull-down assays and the yeast two-hybrid tests. The ASC-1 binding sites involve the hinge domain but not the C-terminal AF2 core domain of nuclear receptors. Nonetheless, ASC-1 appears to require the AF2-dependent factors to function (i.e., CBP-p300 and SRC-1), as suggested by the ability of ASC-1 to coactivate nuclear receptors, either alone or in cooperation with SRC-1 and p300, as well as its inability to coactivate a mutant receptor lacking the AF2 core domain. By using indirect immunofluorescence, we further show that ASC-1, a nuclear protein, is localized to the cytoplasm under conditions of serum deprivation but is retained in the nucleus when it is serum starved in the presence of ligand or coexpressed CBP or SRC-1. These results suggest that ASC-1 is a novel coactivator molecule of nuclear receptors which functions in conjunction with CBP-p300 and SRC-1 and may play an important role in establishing distinct coactivator complexes under different cellular conditions.

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.

Hormone-independent transcriptional activation and coactivator binding by novel orphan nuclear receptor ERR3

Orphan nuclear receptors share sequence homology with members of the nuclear receptor superfamily, but ligands are unknown or unnecessary. A novel orphan receptor, estrogen receptor-related protein 3 (ERR3), was identified by yeast two-hybrid screening, using the transcriptional coactivator glucocorticoid receptor interacting protein 1 (GRIP1) as bait. The putative full-length mouse ERR3 contains 458 amino acids and is closely related to two known orphan receptors ERR1 and ERR2. All the ERR family members share an almost identical DNA-binding domain, which has 68% amino acid identity with that of estrogen receptor. ERR3 bound specifically to an estrogen response element and activated reporter genes controlled by estrogen response elements, both in yeast and in mammalian cells, in the absence of any added ligand. A conserved AF-2 activation domain located in the hormone-binding domain of ERR3 was primarily responsible for transcriptional activation. The ERR3 AF-2 domain bound GRIP1 in a ligand-independent manner both in vitro and in vivo, through the LXXLL motifs of GRIP1, and GRIP1 functioned as a transcriptional coactivator for ERR3 in both yeast and mammalian cells. Expression of ERR3 in adult mouse was restricted; highest expression was observed in heart, kidney, and brain. In the mouse embryo no expression was observed at day 7, and highest expression occurred around the 11-15 day stages. Although ERR3 is much more closely related to ERR2 than to ERR1, the expression pattern for ERR3 was similar to that of ERR1 and distinct from that for ERR2, suggesting a unique role for ERR3 in development.

Mechanisms that mediate negative regulation of the thyroid-stimulating hormone alpha gene by the thyroid hormone receptor

A group of transcriptional cofactors for nuclear hormone receptors, referred to as corepressors (CoRs) and coactivators (CoAs), has been shown to induce transcriptional silencing and hormone-induced activation, respectively, of genes that contain positive hormone response elements. Transcriptional silencing by CoRs involves the recruitment of histone deacetylases (HDACs), whereas ligand-dependent activation is associated with the recruitment of CoAs, which possess or recruit histone acetyltransferases (HATs). In a reciprocal manner, negatively regulated genes are stimulated by nuclear receptors in the absence of ligand and are repressed in response to ligand binding to receptors. We show here that negative regulation of the thyroid-stimulating hormone alpha (TSHalpha) promoter by the thyroid hormone receptor (TR) involves a novel mechanism in which the recruitment of CoRs by TR is associated with transcriptional stimulation and histone acetylation. Expression of excess HDAC reverses the stimulation mediated by the TR.CoR complex, consistent with a pivotal role for acetylation in this event. Addition of the ligand, 3,5,3'-triiodothyronine (T3), induces transcriptional repression of the TSHalpha promoter and is associated with the loss of histone acetylation. T3-dependent repression is blocked by phosphorylation of cAMP response element binding protein, or by inhibition of HDAC, indicating that receptor action is subverted by maneuvers that stimulate histone acetylation of the target gene. We propose that negative regulation of a subset of genes by TR involves the active exchange of CoRs and CoAs with intrinsic promoter regulatory elements that normally strongly induce histone acetylation and transcriptional activation.

Identification of mouse TRAP100: a transcriptional coregulatory factor for thyroid hormone and vitamin D receptors

Nuclear hormone receptors (NRs) regulate transcription in part by recruiting distinct transcriptional coregulatory complexes to target gene promoters. The thyroid hormone receptor (TR) was recently purified from thyroid hormone-cultured HeLa cells in association with a complex of novel nuclear proteins termed TRAPs (thyroid hormone receptor-associated proteins) ranging in size from 20 to 240 kDa. The TRAP complex markedly enhances TR-mediated transcription in vitro, suggesting a coactivator role for one or more of the TRAP components. Here we present the mouse cDNA for the 100-kDa component of the TRAP complex (mTRAP100). The mTRAP100 protein contains seven LxxLL motifs thought to be potential binding surfaces for liganded NRs, yet surprisingly fails to interact with TR and other NRs in vitro. By contrast, mTRAP100 coprecipitates in vivo with another component of the TRAP complex (TRAP220), which directly contacts TR and the vitamin D receptor in a ligand-dependent manner. Our findings thus suggest that TRAP100 is targeted to NRs in association with TRAP complexes specifically containing TRAP220. Transient overexpression of mTRAP100 in mammalian cells further enhances ligand-dependent transcription by both TR and the vitamin D receptor, revealing a functional role for mTRAP100 in NR-mediated transactivation. The presence of an intrinsic mTRAP100 transactivation function is suggested by the ability of mTRAP100 to activate transcription constitutively when tethered to the GAL4 DNA-binding domain. Collectively, these findings suggest that TRAP100, in concert with other TRAPs, plays an important functional role in mediating transactivation by specific NRs.

Nuclear receptor coregulators: cellular and molecular biology

Nuclear receptor coregulators are coactivators or corepressors that are required by nuclear receptors for efficient transcripitonal regulation. In this context, we define coactivators, broadly, as molecules that interact with nuclear receptors and enhance their transactivation. Analogously, we refer to nuclear receptor corepressors as factors that interact with nuclear receptors and lower the transcription rate at their target genes. Most coregulators are, by definition, rate limiting for nuclear receptor activation and repression, but do not significantly alter basal transcription. Recent data have indicated multiple modes of action of coregulators, including direct interactions with basal transcription factors and covalent modification of histones and other proteins. Reflecting this functional diversity, many coregulators exist in distinct steady state precomplexes, which are thought to associate in promoter-specific configurations. In addition, these factors may function as molecular gates to enable integration of diverse signal transduction pathways at nuclear receptor-regulated promoters. This review will summarize selected aspects of our current knowledge of the cellular and molecular biology of nuclear receptor coregulators.

Production and characterization of monoclonal antibodies to the steroid receptor coactivator AIB1

The steroid coactivator AIB1 (Ampified In Breast cancer 1) is a member of the SRC-1 family of transactivation coactivators that is amplified in about 7% of breast tumor samples. Hybridomas were established that secreted monoclonal antibodies (MAbs) to a recombinant glutathione-S-transferase (GST) fusion protein expressing the steroid receptor interacting domain of AIB1 (GST-AIB.T1:a.a. 605-1294). Four MAbs from these hybridomas were characterized and designated AX15.1, AX15.2, AX15.3, and AX15.4. The MAbs were shown to be specific to AIB1 and did not cross-react with two similar coactivators SRC-1 and TIF2 as shown in Western blot analysis and enzyme-linked immunoadsorbent assay (ELISA). Western blot analysis using the four MAbs showed specific recognition of AIB1 protein as a 160 kDa band in lysates from cell lines containing AIB1 gene-amplification. The MAbs immunoprecipitated in vitro-translated AIB1 and cellular AIB1 from metabolically labeled cells. The results show that these newly described MAbs are useful in studies of AIB1 function and expression.

P/CAF associates with cyclin D1 and potentiates its activation of the estrogen receptor

Cyclin D1 is overexpressed in a significant percentage of human breast cancers, particularly in those that also express the estrogen receptor (ER). We and others have demonstrated previously that experimentally overexpressed cyclin D1 can associate with the ER and stimulate its transcriptional functions in the absence of estrogen. This effect is separable from the established function of cyclin D1 as a regulator of cyclin-dependent kinases. Here, we demonstrate that cyclin D1 can also interact with the histone acetyltransferase, p300/CREB-binding protein-associated protein (P/CAF), thereby facilitating an association between P/CAF and the ER. Ectopic expression of P/CAF potentiates cyclin D1-stimulated ER activity in a dose-dependent manner. This effect is largely dependent on the acetyltransferase activity of P/CAF. These results suggest that cyclin D1 may trigger the activation of the ER through the recruitment of P/CAF, by providing histone acetyltransferase activity and, potentially, links to additional P/CAF-associated transcriptional coactivators.

Molecular determinants of the estrogen receptor-coactivator interface

Transcriptional activation by the estrogen receptor is mediated through its interaction with coactivator proteins upon ligand binding. By systematic mutagenesis, we have identified a group of conserved hydrophobic residues in the ligand binding domain that are required for binding the p160 family of coactivators. Together with helix 12 and lysine 366 at the C-terminal end of helix 3, they form a hydrophobic groove that accommodates an LXXLL motif, which is essential for mediating coactivator binding to the receptor. Furthermore, we demonstrated that the high-affinity binding of motif 2, conserved in the p160 family, is due to the presence of three basic residues N terminal to the core LXXLL motif. The recruitment of p160 coactivators to the estrogen receptor is therefore likely to depend not only on the LXXLL motif making hydrophobic interactions with the docking surface on the receptor, but also on adjacent basic residues, which may be involved in the recognition of charged residues on the receptor to allow the initial docking of the motif.

Role of the essential yeast protein PSU1 in p6anscriptional enhancement by the ligand-dependent activation function AF-2 of nuclear receptors

Nuclear receptors (NRs) can function as ligandinducible transregulators in both mammalian and yeast cells, indicating that important features of transcriptional control have been conserved throughout evolution. We report here the isolation and characterization of an essential yeast protein of unknown function, PSU1, which exhibits properties expected for a co-activator/mediator of the ligand-dependent activation function AF-2 present in the ligand-binding domain (LBD, region E) of NRs. PSU1 interacts in a ligand-dependent manner with the LBD of several NRs, including retinoic acid (RARalpha), retinoid X (RXRalpha), thyroid hormone (TRalpha), vitamin D3 (VDR) and oestrogen (ERalpha) receptors. Importantly, both in yeast and in vitro, these interactions require the integrity of the AF-2 activating domain. When tethered to a heterologous DNA-binding domain, PSU1 can activate transcription on its own. By using yeast reporter cells that express PSU1 conditionally, we show that PSU1 is required for transactivation by the AF-2 of ERalpha. Taken together these data suggest that in yeast, PSU1 is involved in ligand-dependent transactivation by NRs. Sequence analysis revealed that in addition to a highly conserved motif found in a family of MutT-related proteins, PSU1 contains several alpha-helical leucine-rich motifs sharing the consensus sequence LLxPhiL (x, any amino acid; Phi, hydrophobic amino acid) in regions that elicit either transactivation or NR-binding activity.

EM-652 (SCH 57068), a third generation SERM acting as pure antiestrogen in the mammary gland and endometrium

Breast cancer is the most frequent cancer in women while it is the second cause of cancer death. Estrogens are well recognized to play the predominant role in breast cancer development and growth and much efforts have been devoted to the blockade of estrogen formation and action. The most widely used therapy of breast cancer which has shown benefits at all stages of the disease is the use of the antiestrogen Tamoxifen. This compound, however, possesses mixed agonist and antagonist activity and major efforts have been devoted to the development of compounds having pure antiestrogenic activity in the mammary gland and endometrium. Such a compound would avoid the problem of stimulation of the endometrium and the risk of endometrial carcinoma. We have thus synthesized an orally active non-steroidal antiestrogen, EM-652 (SCH 57068) and the prodrug EM-800 (SCH57050) which are the most potent of the known antiestrogens. EM-652 is the compound having the highest affinity for the estrogen receptor, including estradiol. It has higher affinity for the ER than ICI 182780, hydroxytamoxifen, raloxifene, droloxifene and hydroxytoremifene. EM-652 has the most potent inhibitory activity on both ER alpha and ER beta compared to any of the other antiestrogens tested. An important aspect of EM-652 is that it inhibits both the AF1 and AF2 functions of both ER alpha and ER beta while the inhibitory action of hydroxytamoxifen is limited to AF2, the ligand-dependent function of the estrogen receptors. AF1 activity is constitutive, ligand-independent and is responsible for mediation of the activity of growth factors and of the ras oncogene and MAP-kinase pathway. EM-652 inhibits Ras-induced transcriptional activity of ER alpha and ER beta and blocks SRC-1-stimulated activity of the two receptors. EM-652 was also found to block the recruitment of SRC-1 at AF1 of ER beta, this ligand-independent activation of AF1 being closely related to phosphorylation of the steroid receptors by protein kinase. Most importantly, the antiestrogen hydroxytamoxifen has no inhibitory effect on the SRC-1-induced ER beta activity while the pure antiestrogen EM-652 completely abolishes this effect, thus strengthening the need to use pure antiestrogens in breast cancer therapy in order to control all known aspects of ER-regulated gene expression. In fact, the absence of blockade of AF2 by hydroxytamoxifen could explain why the benefits of tamoxifen observed up to 5 years become negative at longer time intervals and why resistance develops to tamoxifen. EM-800, the prodrug of EM-652, has been shown to prevent the development of dimethylbenz(a)anthracene (DMBA)-induced mammary carcinoma in the rat, a well-recognized model of human breast cancer. It is of interest that the addition of dehydroepiandrosterone, a precursor of androgens, to EM-800, led to complete inhibition of tumor development in this model. Not only the development, but also the growth of established DMBA-induced mammary carcinoma was inhibited by treatment with EM-800. An inhibitory effect was also observed when medroxyprogesterone was added to treatment with EM-800. Uterine size was reduced to castration levels in the groups of animals treated with EM-800. An almost complete disappearance of estrogen receptors was observed in the uterus, vaginum and tumors in nude mice treated with EM-800. EM-652 was the most potent antiestrogen to inhibit the growth of human breast cancer ZR-75-1, MCF-7 and T-47D cells in vitro when compared with ICI 182780, ICI 164384, hydroxytamoxifen, and droloxifene. Moreover, EM-652 and EM-800 have no stimulatory effect on the basal levels of cell proliferation in the absence of E2 while hydroxytamoxifen and droloxifene had a stimulatory effect on the basal growth of T-47D and ZR-75-1 cells. EM-652 was also the most potent inhibitor of the percentage of cycling cancer cells. (ABSTRACT TRUNCATED)

Regulation of androgen receptor messenger ribonucleic acid expression in the developing rat forebrain

By postnatal day 10 (PND-10), males express more androgen receptor (AR) messenger RNA (mRNA) than females in the principal portion of the bed nucleus of the stria terminalis (BSTpr) and medial preoptic area (MPO), but not in the ventromedial hypothalamus. The development of these region-specific sex differences in AR mRNA expression may be critical for the organization of male-typical neural circuitry and may represent the onset of sex differences in the sensitivity of the rat brain to the actions of androgens. In this study, we used a 35S-labeled riboprobe and in situ hybridization to address whether postnatal testosterone exposure is important for the up-regulation of AR mRNA content in the developing rat forebrain. In the BSTpr and the MPO of PND-10 rats, males gonadectomized on PND-0 or PND-5 had lower levels of AR mRNA compared with intact or sham-operated control males. Daily replacement of testosterone to animals gonadectomized on PND-0 maintained AR mRNA content in the BSTpr and the MPO at levels equal to those in intact males. In contrast, there was no effect of gonadectomy or testosterone replacement on AR mRNA expression in the ventromedial hypothalamus. Thus, the postnatal hormonal environment may permit the development of region-specific sex differences in AR mRNA. Significant alterations in AR mRNA expression in the BSTpr and MPO in PND-10 male rats were induced by gonadectomy as late as PND-8. Males gonadectomized on PND-8 had levels of AR mRNA significantly lower than those in intact males, but significantly higher than those in intact females. Further, when animals were gonadectomized on PND-0 and given testosterone on PND-8 and PND-9, levels of AR mRNA were also intermediate between those found in intact males and intact females. The exact time course for transcriptional regulation of AR mRNA in the developing rat brain is unknown. However, others have shown significant regulation of AR mRNA within hours of hormone treatment, so that 2 days of hormone withdrawal or replacement are probably sufficient to achieve new steady state levels of message. Moreover, sexually dimorphic neuronal loss has been documented to peak in hypothalamic cell groups during the first postnatal week. Thus, it is likely that changes in the number of AR mRNA-expressing cells as well as the amount of AR mRNA expression per cell are responsible for the development of male-typical AR mRNA content.

Nuclear receptor coactivators: multiple enzymes, multiple complexes, multiple functions

Nuclear receptors are ligand-inducible transcription factors which mediate the physiological effects of steroid, thyroid and retinoid hormones. By regulating the assembly of a transcriptional preinitiation complex at the promoter of target genes, they enhance the expression of these genes in response to hormone. Recent evidence suggests that nuclear receptors act in part by recruiting multiple coregulator proteins which may have specific functions during transcriptional initiation. Liganded receptors recruit members of the SRC family, a group of structurally and functionally related transcriptional coactivators. Receptors also interact with the transcriptional cointegrators p300 and CBP, which are proposed to integrate diverse afferent signals at hormone-regulated promoters. p300/CBP and members of the SRC coactivator family have intrinsic histone acetyltransferase activity which is believed to disrupt the nucleosomal structure at these promoters. Other nuclear receptor coactivators include a member of the SWI/SNF complex, BRG-1, which couples ATP hydrolysis to chromatin remodelling, and the E3 ubiquitin-protein ligases E6-AP and RPF-1. Finally, nuclear receptor coactivators appear to be organized into preformed subcomplexes, an arrangement that may facilitate their efficient assembly into diverse higher order configurations.

SMRTe, a silencing mediator for retinoid and thyroid hormone receptors-extended isoform that is more related to the nuclear receptor corepressor

SMRT (silencing mediator for retinoid and thyroid hormone receptors) and N-CoR (nuclear receptor copressor) mediate transcriptional repression of important regulators that are involved in many signaling pathways. SMRT and N-CoR are related proteins that form complexes with mSin3A/B and histone deacetylases to induce local chromatin condensation and transcriptional repression. However, SMRT is substantially smaller than N-CoR, lacking an N-terminal domain of approximately 1,000 aa that are present in N-CoR. Here, we report the identification of SMRT-extended (SMRTe), which contains an N-terminal sequence that shows striking similarity with N-CoR. As in N-CoR, this SMRTe-N-terminal domain also represses basal transcription. We find that SMRTe expression is regulated during cell cycle progression and SMRTe transcripts are present in many embryonic tissues. These data redefine a structurally and functionally more related nuclear receptor corepressor family and suggest an additional role for SMRTe in the regulation of cycle-specific gene expression in diverse signaling pathways.

Cloning and characterization of human prostate coactivator ARA54, a novel protein that associates with the androgen receptor

Androgen receptor (AR) is a member of the steroid receptor superfamily that may require coactivators for proper or maximal transactivation. Using a yeast two-hybrid screening followed by mammalian cell analyses, we identified a novel ligand-dependent AR-associated protein, ARA54, which consists of 474 amino acids with a molecular mass of 54 kDa. We demonstrated that ARA54 might function as a preferential coactivator for AR-mediated transactivation in human prostate cancer DU145 cells. Interestingly, our data also showed that ARA54 could significantly enhance the transcriptional activity of LNCaP mutant AR (ARt877a) but not wild type AR or another mutant AR (ARe708k) in the presence of 10 nM 17beta-estradiol or 1 microM hydroxyflutamide. These results imply that both ARA54 and the positions of the AR mutation (877 versus 708) might contribute to the specificity of AR-mediated transactivation. Our findings further demonstrated that the C-terminal domain of ARA54 can serve as a dominant negative inhibitor and exogenous full-length ARA54 can reverse this squelching effect on AR transcriptional activity. Co-expression of ARA54 with other AR coactivators, such as ARA70 or SRC-1, showed additive stimulation of AR-mediated transactivation, which indicates that these cofactors may function individually as AR coactivators to induce AR target gene expression. Through our findings, we have identified and characterized a novel AR coactivator, ARA54, which may play an important role in the AR signaling pathway in human prostate.

Cloning and characterization of androgen receptor coactivator, ARA55, in human prostate

Androgen receptor (AR) is a hormone-activated transcriptional factor that can bind to androgen response elements and that regulates the transcription of target genes via a mechanism that presumably involves cofactors. We report here the cloning of a novel AR coactivator ARA55 using a yeast two-hybrid system. ARA55 consists of 444 amino acids with the predicted molecular mass of 55 kDa and its sequence shows very high homology to mouse hic5, a TGF-beta1-inducible gene. Yeast and mammalian two-hybrid systems and co-immunoprecipitation assays all prove ARA55 can bind to AR in a ligand-dependent manner. Transient transfection assay in prostate cancer DU145 cells further demonstrates that ARA55 can enhance AR transcriptional activity in the presence of 1 nM dihydrotestosterone or its antagonists such as 100 nM 17beta-estradiol or 1 microM hydroxyflutamide. Our data also suggest the C-terminal half of ARA55, which includes three LIM motifs, is sufficient to interact with AR. Northern blot and polymerase chain reaction quantitation showed ARA55 can be expressed differently in normal prostate and prostate tumor cells. Together, our data suggests that ARA55 may play very important roles in the progression of prostate cancer by the modulation of AR transactivation.

p300 interacts with the N- and C-terminal part of PPARgamma2 in a ligand-independent and -dependent manner, respectively

The nuclear peroxisome proliferator-activated receptor gamma (PPARgamma) activates the transcription of multiple genes involved in intra- and extracellular lipid metabolism. Several cofactors are crucial for the stimulation or the silencing of nuclear receptor transcriptional activities. The two homologous cofactors p300 and CREB-binding protein (CBP) have been shown to co-activate the ligand-dependent transcriptional activities of several nuclear receptors as well as the ligand-independent transcriptional activity of the androgen receptor. We show here that the interaction between p300/CBP and PPARgamma is complex and involves multiple domains in each protein. p300/CBP not only bind in a ligand-dependent manner to the DEF region of PPARgamma but also bind directly in a ligand-independent manner to a region in the AB domain localized between residue 31 to 99. In transfection experiments, p300/CBP could thereby enhance the transcriptional activities of both the activating function (AF)-1 and AF-2 domains. p300/CBP displays itself at least two docking sites for PPARgamma located in its N terminus (between residues 1 and 113 for CBP) and in the middle of the protein (between residues 1099 and 1460).

Nuclear factor I-mediated repression of the mouse mammary tumor virus promoter is abrogated by the coactivators p300/CBP and SRC-1

To better understand the function of nuclear factor I (NFI) proteins in transcription, we have used transient transfection assays to assess transcriptional modulation by NFI proteins on the NFI-dependent mouse mammary tumor virus (MMTV) promoter. Expression of NFI-C or NFI-X, but not NFI-A or NFI-B proteins, represses glucocorticoid induction of the MMTV promoter in HeLa cells. Repression is DNA binding-independent as a deletion construct expressing the NH2-terminal 160 residues of NFI-C represses but does not bind DNA. Repression by NFI-C is cell type-dependent and occurs in HeLa and COS-1 cells but not 293 or JEG-3 cells. NFI-C does not repress progesterone induction of the MMTV promoter in HeLa cells, suggesting that progesterone induction of the promoter differs mechanistically from glucocorticoid induction. NFI-C-mediated repression is alleviated by overexpression of glucocorticoid receptor (GR), suggesting that NFI-C represses the MMTV promoter by preventing GR function. However, repression by NFI-C occurs with only a subset of glucocorticoid-responsive promoters, as the chimeric NFIGREbeta-gal promoter that is activated by GR is not repressed by NFI-C. Since the coactivator proteins p300/CBP, SRC-1A, and RAC3 had previously been shown to function at steroid hormone-responsive promoters, we asked whether they could influence NFI-C-mediated repression of MMTV expression. Expression of p300/CBP or SRC-1A alleviates repression by NFI-C, whereas RAC3 has no effect. This abrogation of NFI-C-mediated repression by p300/CBP and SRC-1A suggests that repression by NFI-C may occur by interference with coactivator function at the MMTV promoter.

Thyroid hormone receptor-associated proteins and general positive cofactors mediate thyroid hormone receptor function in the absence of the TATA box-binding protein-associated factors of TFIID

Coactivators previously implicated in ligand-dependent activation functions by thyroid hormone receptor (TR) include p300 and CREB-binding protein (CBP), the steroid receptor coactivator-1 (SRC-1)-related family of proteins, and the multicomponent TR-associated protein (TRAP) complex. Here we show that two positive cofactors (PC2 and PC4) derived from the upstream stimulatory activity (USA) cofactor fraction act synergistically to mediate thyroid hormone (T3)-dependent activation either by TR or by a TR-TRAP complex in an in vitro system reconstituted with purified factors and DNA templates. Significantly, the TRAP-mediated enhancement of activation by TR does not require the TATA box-binding protein-associated factors of TFIID. Furthermore, neither the pleiotropic coactivators CBP and p300 nor members of the SRC-1 family were detected in either the TR-TRAP complex or the other components of the in vitro assay system. These results show that activation by TR at the level of naked DNA templates is enhanced by cooperative functions of the TRAP coactivators and the general coactivators PC2 and PC4, and they further indicate a potential functional redundancy between TRAPs and TATA box-binding protein-associated factors in TFIID. In conjunction with earlier studies on other nuclear receptor-interacting cofactors, the present study also suggests a multistep pathway, involving distinct sets of cofactors, for activation of hormone responsive genes.

Inhibition of hSP-B promoter in respiratory epithelial cells by a dominant negative retinoic acid receptor

Retinoic acid (RA) receptors (RARs) belong to the nuclear hormone receptor superfamily and play important roles in lung differentiation, growth, and gene regulation. Surfactant protein (SP) B is a small hydrophobic protein synthesized and secreted by respiratory epithelial cells in the lung. Expression of the SP-B gene is modulated at the transcriptional and posttranscriptional levels. In the present work, immunohistochemical staining revealed that RAR-alpha is present on day 14.5 of gestation in the fetal mouse lung. To assess whether RAR is required for SP-B gene transcription, a dominant negative mutant human (h) RAR-alpha403 was generated. The hRAR-alpha403 mutant was transcribed and translated into the truncated protein product by reticulocyte lysate in vitro. The mutant retained DNA binding activity in the presence of retinoid X receptor-gamma to an RA response element in the hSP-B promoter. When transiently transfected into pulmonary adenocarcinoma epithelial cells (H441 cells), the mutant hRAR-alpha403 was readily detected in the cell nucleus. Cotransfection of the mutant hRAR-alpha403 repressed activity of the hSP-B promoter and inhibited RA-induced surfactant proprotein B production in H441 cells, supporting the concept that RAR is required for hSP-B gene transcription in vitro.

Recent advances in understanding thyroid hormone receptor coregulators

Thyroid hormones (L-triiodothyronine, T3; L-tetraiodothyronine, T4) regulate normal cellular growth and development, and general metabolism as well. Their various actions are mediated by the thyroid hormone receptor, a ligand-dependent transcriptional factor belonging to the nuclear hormone receptor superfamily. The recent discovery of coregulators (coactivators, corepressors, and cointegrators) has greatly enhanced our understanding of thyroid hormone receptor functions. Hence we review and discuss, in brief, the potential role of thyroid hormone receptor coregulators involved in diverse cellular activities.

Mouse steroid receptor coactivator-1 is not essential for peroxisome proliferator-activated receptor alpha-regulated gene expression

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors, and it is assumed that the biological effects of these receptors depend on interactions with recently identified coactivators, including steroid receptor coactivator-1 (SRC-1). We assessed the in vivo function of SRC-1 on the PPARalpha-regulated gene expression in liver by generating mice in which the SRC-1 gene was inactivated by gene targeting. The homozygous (SRC-1(-/-)) mice were viable and fertile and exhibited no detectable gross phenotypic defects. When challenged with a PPARalpha ligand, such as ciprofibrate or Wy-14,643, the SRC-1(-/-) mice displayed typical pleiotropic responses, including hepatomegaly, peroxisome proliferation in hepatocytes, and increased mRNA and protein levels of genes that are regulated by PPARalpha. These alterations were indistinguishable from those exhibited by SRC-1(+/+) wild-type mice fed either ciprofibrate- or Wy-14, 643-containing diets. These results indicate that SRC-1 is not essential for PPARalpha-mediated transcriptional activation in vivo and suggest redundancy in nuclear receptor coactivators.

An additional region of coactivator GRIP1 required for interaction with the hormone-binding domains of a subset of nuclear receptors

Transcriptional coactivators of the p160 family (SRC-1, GRIP1, and p/CIP) associate with DNA-bound nuclear receptors (NRs) and help the NRs to recruit an active transcription initiation complex to the promoters of target genes. Previous studies have demonstrated the importance of the NR interaction domain (NID) of p160 proteins containing three NR box motifs (LXXLL) for the interaction with the hormone-binding domains of NRs. Here we report that, in addition to NID, another region of coactivator GRIP1 (amino acids 1011-1121), called the auxiliary NID (NIDaux), is required in vitro and in vivo for efficient interaction with a subset of NRs, including the glucocorticoid receptor (GR), androgen receptor, and retinoic acid receptor alpha. A second group of NRs, which includes the progesterone receptor, retinoid X receptor alpha, thyroid hormone receptor beta1, and vitamin D receptor, required only NID for efficient interaction. For binding to GR, the NID and NIDaux of GRIP1 must act in cis, but deletion of up to 144 amino acids between the two regions did not reduce binding efficiency. Amino acids 1011-1121 of GRIP1 also contain a p300 interaction domain, but mutational analysis indicated that the p300 interaction function within this region is separable from the ability to contribute to GR hormone-binding domain binding. SRC-1 lacks an NIDaux activity equivalent to that in GRIP1.

The Angelman syndrome-associated protein, E6-AP, is a coactivator for the nuclear hormone receptor superfamily

In this study, we found that the E6-associated protein (E6-AP/UBE3A) directly interacts with and coactivates the transcriptional activity of the human progesterone receptor (PR) in a hormone-dependent manner. E6-AP also coactivates the hormone-dependent transcriptional activities of the other members of the nuclear hormone receptor superfamily. Previously, it was shown that E6-AP serves the role of a ubiquitin-protein ligase (E3) in the presence of the E6 protein from human papillomavirus types 16 and 18. Our data show that the ubiquitin-protein ligase function of E6-AP is dispensable for its ability to coactivate nuclear hormone receptors, showing that E6-AP possesses two separable independent functions, as both a coactivator and a ubiquitin-protein ligase. Disruption of the maternal copy of E6-AP is correlated with Angelman syndrome (AS), a genetic neurological disorder characterized by severe mental retardation, seizures, speech impairment, and other symptoms. However, the exact mechanism by which the defective E6-AP gene causes AS remains unknown. To correlate the E6-AP coactivator function and ubiquitin-protein ligase functions with the AS phenotype, we expressed mutant forms of E6-AP isolated from AS patients and assessed the ability of each of these mutant proteins to coactivate PR or provide ubiquitin-protein ligase activity. This analysis revealed that in the majority of the AS patients examined, the ubiquitin-protein ligase function of E6-AP was defective whereas the coactivator function was intact. This finding suggests that the AS phenotype results from a defect in the ubiquitin-proteosome protein degradation pathway.

Histone acetylases and deacetylases in cell proliferation

There are several enzymes, acetylases and deacetylases, that can regulate transcription by modifying the acetylation state of histones or other promoter-bound transcription factors. Some of these enzymes are present in multisubunit complexes. Recent efforts to understand the biological role of these enzymes reveals their involvement in cell-cycle regulation and differentiation. Furthermore, accumulating evidence suggests that deregulation of acetylase and deacetylase activity plays a causative role in the generation of cancer.

CBP-dependent and independent enhancing activity of steroid receptor coactivator-1 in thyroid hormone receptor-mediated transactivation

Full-length of steroid receptor coactivator-1 (F-SRC-1) has been shown to interact with thyroid hormone receptors (TRs) in a ligand-dependent manner and to stimulate receptor-dependent transcription. To identify functional domains of F-SRC-1, several internal deletion mutants of F-SRC-1 were constructed. Although in vitro pull down assay with TR showed interaction of all of these mutants with TR, lack of mid legion (amino acids 398-1172) lost enhancing activity of TR-mediated transcription in a transient transfection assay. However, F-SRC-1 mutant lacking CBP-interacting domain still preserved enhancing activity. Surprisingly, F-SRC-1 mutants also increased basal level of viral promoter activity depending upon their deleted region. Yeast activation function assay revealed that these F-SRC-1 mutants had intrinsic activation function when bound to DNA. Analyses of small fragments of F-SRC-1 identified three separable activation domains. In vitro binding assay showed that TBP and TFIIB bound to C-terminal half of F-SRC-1. These results suggest that F-SRC-1 can function via both CBP-dependent and independent manners using various sets of activation domains and that direct interactions between F-SRC-1 and TBP or TFIIB may not be important for CBP-independent transcription.

Segregation of steroid receptor coactivator-1 from steroid receptors in mammary epithelium

Steroid receptor coactivator-1 (SRC-1) family members interact with steroid receptors, including estrogen receptor alpha (ERalpha) and progesterone receptor (PR), to enhance ligand-dependent transcription. However, the expression of ERalpha and SRC-1 was found to be segregated in distinct subsets of cells within the epithelium of the estrogen-responsive rat mammary gland. This finding was in contrast to the finding for the stroma, where significant numbers of cells coexpressed ERalpha and SRC-1. Treatment of animals with estrogen induced PR expression in the ERalpha-expressing mammary epithelial cells in the absence of detectable SRC-1 and did not affect the segregated pattern of SRC-1 and ERalpha expression. PR was neither expressed nor induced by estrogen treatment in stroma, despite the coexpression of ERalpha and SRC-1. These results suggest that SRC-1 is not necessary for ERalpha-mediated induction of PR in mammary epithelial cells and is also not sufficient for PR induction in stromal cells expressing both ERalpha and SRC-1. Furthermore, the expression of SRC-1 in a subpopulation of mammary epithelial cells distinct from those expressing ERalpha or PR raises the possibility that SRC-1 has cell type-specific functions other than simply to act as coactivator for ERalpha or PR in the mammary epithelium.

Functional diversity of vertebrate ARNT proteins: identification of ARNT2 as the predominant form of ARNT in the marine teleost, Fundulus heteroclitus

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a member of the bHLH/PAS protein superfamily. ARNT dimerizes with several PAS superfamily members, including the ligand-activated aryl hydrocarbon receptor (AHR), forming a complex that alters transcription by binding specific elements within the promoters of target genes. Two genes encode different forms of the protein in rodents: ARNT1, which is widely expressed, and ARNT2, which is limited to the brain and kidneys of adults and specific neural and branchial tissues of embryos. In an effort to characterize aryl hydrocarbon signaling mechanisms in Fundulus heteroclitus, a marine teleost that can develop heritable xenobiotic resistance, we have isolated a liver cDNA encoding an ARNT homolog. The protein exhibits AHR-dependent DNA binding capability typical of other vertebrate ARNTs. Unexpectedly, phylogenetic analysis reveals that the cDNA encodes an ARNT2. This is the only detectable ARNT sequence in Fundulus liver, gill, ovary, and brain, suggesting that ARNT2 is the predominant form of ARNT in this species. Also surprising is the relative lack of sequence identity with another fish ARNT protein, rainbow trout ARNTb, which we show forms a distinct branch outside the ARNT1 and ARNT2 clades in phylogenetic analyses. Functional diversity of ARNT proteins in fish may have important implications for the assessment of aryl hydrocarbon effects on natural populations. The increasing use of fish models in developmental and toxicological studies underscores the importance of identifying taxon-specific roles of ARNT proteins and their potential dimeric partners in the PAS superfamily.

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.

The orphan nuclear receptor SHP inhibits agonist-dependent transcriptional activity of estrogen receptors ERalpha and ERbeta

SHP (short heterodimer partner) is an unusual orphan nuclear receptor that contains a putative ligand-binding domain but lacks a conserved DNA-binding domain. Although no conventional receptor function has yet been identified, SHP has been proposed to act as a negative regulator of nuclear receptor signaling pathways, because it interacts with and inhibits DNA binding and transcriptional activity of various nonsteroid receptors, including thyroid hormone and retinoid receptors. We show here that SHP interacts directly with agonist-bound estrogen receptors, ERalpha and ERbeta, and inhibits ER-mediated transcriptional activation. SHP specifically targets the ligand-regulated activation domain AF-2 and competes for binding of coactivators such as TIF2. Thus, SHP may represent a new category of negative coregulators for ligand-activated nuclear receptors. SHP mRNA is widely expressed in rat tissues including certain estrogen target tissues, and subcellular localization studies demonstrate that SHP is a nuclear protein, suggesting a biological significance of the SHP interactions with ERs. Taken together, these results identify ERs as novel SHP targets and suggest that competition for coactivator-binding is a novel mechanism by which SHP may inhibit nuclear receptor activation.

Functional analysis of a novel estrogen receptor-beta isoform

A new level of complexity has recently been added to estrogen signaling with the identification of a second estrogen receptor, ERbeta. By screening a rat prostate cDNA library, we detected ERbeta as well as a novel isoform that we termed ERbeta2. ERbeta2 contains an in-frame inserted exon of 54 nucleotides that results in the predicted insertion of 18 amino acids within the ERbeta hormone-binding domain. We also have evidence for the expression of both ERbeta1 and ERbeta2 in human cell lines. Competition ligand binding analysis of bacterially expressed fusion proteins revealed an 8-fold lower affinity of ERbeta2 for 17beta-estradiol (E2) [dissociation constant (Kd approximately 8 nM)] as compared with ERbeta1 (Kd approximately 1 nM). In vitro transcribed and translated ERbeta1 and ERbeta2 bind specifically to a consensus estrogen responsive element in a gel mobility shift assay. Furthermore, we show heterodimerization of ERbeta1 and ERbeta2 with each other as well as with ERalpha. In affinity interaction assays for proteins that associate specifically with the hormone-binding domain of these receptors, we demonstrate that the steroid receptor coactivator SRC-1 interacts in an estrogen-dependent manner with ERalpha and ERbeta1, but not with ERbeta2. In cotransfection experiments with expression plasmids for ERalpha, ERbeta1, and ERbeta2 and an estrogen-responsive element-containing luciferase reporter, the dose response of ERbeta1 to E2 was similar to that of ERalpha although the maximal stimulation was approximately 50%. In contrast, ERbeta2 required 100- to 1000-fold greater E2 concentrations for maximal activation. Thus, ERbeta2 adds yet another facet to the possible cellular responses to estrogen.

The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen

Ligand-dependent activation of transcription by nuclear receptors (NRs) is mediated by interactions with coactivators. Receptor agonists promote coactivator binding, and antagonists block coactivator binding. Here we report the crystal structure of the human estrogen receptor alpha (hER alpha) ligand-binding domain (LBD) bound to both the agonist diethylstilbestrol (DES) and a peptide derived from the NR box II region of the coactivator GRIP1 and the crystal structure of the hER alpha LBD bound to the selective antagonist 4-hydroxytamoxifen (OHT). In the DES-LBD-peptide complex, the peptide binds as a short alpha helix to a hydrophobic groove on the surface of the LBD. In the OHT-LBD complex, helix 12 occludes the coactivator recognition groove by mimicking the interactions of the NR box peptide with the LBD. These structures reveal the two distinct mechanisms by which structural features of OHT promote this "autoinhibitory" helix 12 conformation.

A novel natural mutation in the thyroid hormone receptor defines a dual functional domain that exchanges nuclear receptor corepressors and coactivators

In a patient with severe resistance to thyroid hormone (RTH), we found a novel mutation (leucine to serine in codon 454, L454S) of the thyroid hormone receptor beta. This mutation is in the ligand-dependent transactivation domain that has been shown to interact with transcriptional coactivators (CoAs). The mutant protein binds T3, but its ability to activate transcription of a positively regulated gene (TRE-tk-Luc), and to repress a negatively regulated gene (TSHalpha-Luc), is markedly impaired. As anticipated from its location, the L454S mutant interacts weakly with CoAs, such as SRC1 and glucocorticoid receptor interacting protein 1 (GRIP1) in gel mobility shift assays and in mammalian two-hybrid assays, even in the presence of the maximal dose of T3. In contrast, in the absence of T3, the L454S mutant interacts much more strongly with nuclear receptor corepressor (NCoR) than does the wild-type receptor, and the T3-dependent release of NCoR is markedly impaired. By comparison, the NCoR interaction and T3-dependent dissociation of an adjacent AF-2 domain mutant (E457A) are normal. These findings reveal that the Leu 454 is involved directly, or indirectly, in the release of corepressors (CoRs) as well as in the recruitment of CoAs. The strong interaction with NCoR at a physiological concentration of T3 results in constitutive activation of the TSH genes as well as constitutive silencing of positively regulated genes. When the dominant negative effect was examined among various mutants, it correlated surprisingly well with the potency of NCoR binding but not with the degree of impairment in CoA binding. These findings suggest that the defective release of NCoRs, along with retained dimerization and DNA binding, are critical features for the inhibitory action of mutant thyroid hormone receptors. These studies also suggest that helix 12 of the thyroid hormone receptor acts as a dual functional domain. After the binding of T3, its conformation changes, causing the disruption of CoR binding and the recruitment of CoAs.

SRC-1 and GRIP1 coactivate transcription with hepatocyte nuclear factor 4

Hepatocyte nuclear factor-4 (HNF4), a member of the nuclear receptor superfamily, plays an important role in tissue-specific gene expression, including genes involved in hepatic glucose metabolism. In this study, we show that SRC-1 and GRIP1, which act as coactivators for various nuclear receptors, associate with HNF4 in vivo and enhance its transactivation potential. The AF-2 domain of HNF4 is required for this interaction and for the potentiation of transcriptional activity by these coactivators. p300 can also serve as a coactivator with HNF4, and it synergizes with SRC-1 to further augment the activity of HNF4. HNF4 is also a key regulator of the expression of hepatocyte nuclear factor-1 (HNF1). The overexpression of SRC-1 or GRIP1 enhances expression from a HNF1 gene promoter-reporter in HepG2 hepatoma cells, and this requires an intact HNF4-binding site in the HNF1 gene promoter. Type 1 maturity onset diabetes of young (MODY), which is characterized by abnormal glucose-mediated insulin secretion, is caused by mutations of the HNF4 gene. A mutation of the HNF4-binding site in the HNF1 gene promoter has also been associated with MODY. Thus, HNF4 is involved in the regulation of glucose homeostasis at several levels and along with the SRC-1, GRIP1, and p300 may play an important role in the pathophysiology of non-insulin-dependent diabetes mellitus.

Bcl3, an IkappaB protein, as a novel transcription coactivator of the retinoid X receptor

We have recently shown that the IkappaB protein IkappaBbeta interacted with the retinoid X receptor (RXR) and inhibited the 9-cis-retinoic acid (RA)-dependent transactivations (Na, S.-Y., Kim, H.-J., Lee, S.-K., Choi, H.-S., Na, D. S., Lee, M.-O., Chung, M., Moore, D. D., and Lee, J. W. (1998) J. Biol. Chem. 6, 3212-3215). Herein, we show that a distinct IkappaB protein Bcl3 also interacts with RXR, as shown in the yeast two-hybrid tests and glutathione S-transferase pull-down assays. The Bcl3 interaction involved two distinct subregions of RXR, i.e. constitutive interactions of the N-terminal ABC domains and 9-cis-RA-dependent interactions of the C-terminal DEF domains. In contrast to IkappaBbeta, Bcl3 did not interact with the AF2 domain of RXR. Bcl3 specifically interacted with the general transcription factors TFIIB, TBP, and TFIIA but not with TFIIEalpha in the GST pull-down assays. TBP and TFIIA, however, were not able to interact with IkappaBbeta. Accordingly, Bcl3 coactivated the 9-cis-RA-induced transactivations of RXR, in contrast to the inhibitory actions of IkappaBbeta. In addition, coexpression of SRC-1 but not p300 further stimulated the Bcl3-mediated enhancement of the 9-cis-RA-induced transactivations of RXR. These results suggest that distinct IkappaB proteins differentially modulate the 9-cis-RA-induced transactivations of RXR in vivo.

Ligand-independent recruitment of steroid receptor coactivators to estrogen receptor by cyclin D1

The estrogen receptor (ER) is an important regulator of growth and differentiation of breast epithelium. Transactivation by ER depends on a leucine-rich motif, which constitutes a ligand-regulated binding site for steroid receptor coactivators (SRCs). Cyclin D1 is frequently amplified in breast cancer and can activate ER through direct binding. We show here that cyclin D1 also interacts in a ligand-independent fashion with coactivators of the SRC-1 family through a motif that resembles the leucine-rich coactivator binding motif of nuclear receptors. By acting as a bridging factor between ER and SRCs, cyclin D1 can recruit SRC-family coactivators to ER in the absence of ligand. A cyclin D1 mutant that binds to ER but fails to recruit coactivators preferentially interferes with ER activation in breast cancer cells that have high levels of cyclin D1. These data support that cyclin D1 contributes significantly to ER activation in breast cancers in which the protein is overexpressed. Our present results reveal a novel route of coactivator recruitment to ER and establish a direct role for cyclin D1 in regulation of transcription.

Determinants of coactivator LXXLL motif specificity in nuclear receptor transcriptional activation

Ligand-dependent activation of gene transcription by nuclear receptors is dependent on the recruitment of coactivators, including a family of related NCoA/SRC factors, via a region containing three helical domains sharing an LXXLL core consensus sequence, referred to as LXDs. In this manuscript, we report receptor-specific differential utilization of LXXLL-containing motifs of the NCoA-1/SRC-1 coactivator. Whereas a single LXD is sufficient for activation by the estrogen receptor, different combinations of two, appropriately spaced, LXDs are required for actions of the thyroid hormone, retinoic acid, peroxisome proliferator-activated, or progesterone receptors. The specificity of LXD usage in the cell appears to be dictated, at least in part, by specific amino acids carboxy-terminal to the core LXXLL motif that may make differential contacts with helices 1 and 3 (or 3') in receptor ligand-binding domains. Intriguingly, distinct carboxy-terminal amino acids are required for PPARgamma activation in response to different ligands. Related LXXLL-containing motifs in NCoA-1/SRC-1 are also required for a functional interaction with CBP, potentially interacting with a hydrophobic binding pocket. Together, these data suggest that the LXXLL-containing motifs have evolved to serve overlapping roles that are likely to permit both receptor-specific and ligand-specific assembly of a coactivator complex, and that these recognition motifs underlie the recruitment of coactivator complexes required for nuclear receptor function.

Changes in the structure of the ligand or substitutions to AF2 residues in the estrogen receptor make independent contributions to coactivator sensitivity by SRC-1

The estrogen receptor (ER) is a ligand-inducible transcription factor which depends, in part, upon the C-terminal activation function (AF2) in order to regulate the expression of target genes. AF2 residues fold into an amphipathic alpha-helix on helix 12 of the ER, with hydrophobic and acidic faces. It is believed that AF2 mediates the gene regulatory activities of ligand-activated ER by interacting with coactivator proteins. We have analyzed the contribution of acidic AF2 residues to the process of ER coactivation by the steroid receptor coactivator, SRC-1. In HeLa cells, SRC-1 coexpression was found to restore transcriptional potency to otherwise inert complexes of wild type ER and 4-hydroxyestratrien-17beta-ol. SRC-1 coexpression also enhanced transcriptional activity of reporter genes induced by an ER mutant with neutral replacements to acidic AF2 residues, in response to E2 or 4-hydroxyestratrien-17beta-ol. By contrast, ER complexes from ICI164,384-treated HeLa cells were both transcriptionally inactive and coactivator insensitive. It is concluded that changes to the structure of the ligand or substitutions to acidic residues in the AF2 region of the receptor contribute independently to the control of coactivator sensitivity in ER.

The aryl hydrocarbon receptor: a comparative perspective

The aryl hydrocarbon receptor (Ah receptor or AHR) is a ligand-activated transcription factor involved in the regulation of several genes, including those for xenobiotic-metabolizing enzymes such as cytochrome P450 1A and 1B forms. Ligands for the AHR include a variety of aromatic hydrocarbons, including the chlorinated dioxins and related halogenated aromatic hydrocarbons whose toxicity occurs through activation of the AHR. The AHR and its dimerization partner ARNT are members of the emerging bHLH-PAS family of transcriptional regulatory proteins. In this review, our current understanding of the AHR signal transduction pathway in non-mammalian and other non-traditional species is summarized, with an emphasis on similarities and differences in comparison to the AHR pathway in rodents and humans. Evidence and prospects for the presence of a functional AHR in early vertebrates and invertebrates are also examined. An overview of the bHLH-PAS family is presented in relation to the diversity of bHLH-PAS proteins and the functional and evolutionary relationships of the AHR and ARNT to the other members of this family. Finally, some of the most promising directions for future research on the comparative biochemistry and molecular biology of the AHR and ARNT are discussed.

Structure and specificity of nuclear receptor-coactivator interactions

Combinatorial regulation of transcription implies flexible yet precise assembly of multiprotein regulatory complexes in response to signals. Biochemical and crystallographic analyses revealed that hormone binding leads to the formation of a hydrophobic groove within the ligand binding domain (LBD) of the thyroid hormone receptor that interacts with an LxxLL motif-containing alpha-helix from GRIP1, a coactivator. Residues immediately adjacent to the motif modulate the affinity of the interaction; the motif and the adjacent sequences are employed to different extents in binding to different receptors. Such interactions of amphipathic alpha-helices with hydrophobic grooves define protein interfaces in other regulatory complexes as well. We suggest that these common structural elements impart flexibility to combinatorial regulation, whereas side chains at the interface impart specificity.