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

Estrogen receptor isoform-specific regulation of endogenous gene expression in human osteoblastic cell lines expressing either ER? or ER?

Estrogen (17beta-estradiol, E2) plays pivotal roles in the function and maintenance of the skeleton, including the bone-forming osteoblasts (OBs). The functions of E2 are largely mediated through two distinct estrogen receptor isoforms, ERalpha and ERbeta, both of which are expressed in OBs. The level of each isoform dominates at early or late stages of OB differentiation. To date, only a limited comparison between the transcriptional targets of ERalpha and ERbeta on endogenous gene expression has been reported. We have developed new stable cell lines, which contain doxycycline (Dox)-inducible ERalpha and ERbeta, in the U2OS human osteosarcoma to determine the global transcriptional profile of ERalpha- and ERbeta-regulation of endogenous gene expression. The U2OS-ERalpha and U2OS-ERbeta cell lines were treated with Dox and either vehicle control or E2 for 24 h. Gene expression analysis was performed using a microarray containing approximately 6,800 full-length genes. We detected 63 genes that were regulated solely by ERalpha and 59 genes that were only regulated solely by ERbeta. Of the ERalpha-regulated genes, 81% were upregulated and 19% were inhibited. Similarly 76% of the ERbeta-regulated genes were upregulated whereas 24% were inhibited by E2. Surprisingly, only 17 genes were induced by both ERalpha and ERbeta. Real-time PCR was employed to confirm the expression of a selected number of genes. The regulation of a number of known E2-responsive genes in human OBs, as well as many interesting novel genes, is shown. The distinct patterns of E2-dependent gene regulation in the U2OS cells by ERalpha and ERbeta shown here suggest that during OB differentiation, when either isoform dominates, a unique pattern of gene responses will occur, partially due to the differentiation state and the ER isoform present.

Retinoid receptors and their coregulators

Retinoids regulate gene transcription by binding to the nuclear receptors, the retinoic acid (RA) receptors (RARs), and the retinoid X receptors (RXRs). RARs and RXRs are ligand-activated transcription factors for the regulation of RA-responsive genes. The actions of RARs and RXRs on gene transcription require a highly coordinated interaction with a large number of coactivators and corepressors. This review focuses on our current understanding of these coregulators known to act in concert with RARs and RXRs. The mechanisms of action of these coregulators are beginning to be uncovered and include the modification of chromatin and the recruitment of basal transcription factors. Challenges remain to understand the specificity of action of RARs and RXRs and the formation of specific transcription complexes consisting of the receptors, coregulators, and other unknown factors.

Review of the in vivo functions of the p160 steroid receptor coactivator family

The p160 steroid receptor coactivator (SRC) gene family contains three homologous members, which serve as transcriptional coactivators for nuclear receptors and certain other transcription factors. These coactivators interact with ligand-bound nuclear receptors to recruit histone acetyltransferases and methyltransferases to specific enhancer/promotor regions, which facilitates chromatin remodeling, assembly of general transcription factors, and transcription of target genes. This minireview summarizes our current knowledge about the molecular structures, molecular mechanisms, temporal and spatial expression patterns, and biological functions of the SRC family. In particular, this article highlights the roles of SRC-1 (NCoA-1), SRC-2 (GRIP1, TIF2, or NCoA-2) and SRC-3 (p/CIP, RAC3, ACTR, AIB1, or TRAM-1) in development, organ function, endocrine regulation, and nuclear receptor function, which are defined by characterization of the genetically manipulated animal models. Furthermore, this article also reviews our current understanding of the role of SRC-3 in breast cancer and discusses possible mechanisms for functional specificity and redundancy among SRC family members.

Multiple mechanisms control brain aromatase activity at the genomic and non-genomic level

Evidence has recently accumulated indicating that aromatase activity in the preoptic area is modulated in parallel by both slow (hours to days) genomic and rapid (minutes to hours) non-genomic mechanisms. We review here these two types of control mechanisms and their potential contribution to various aspects of brain physiology in quail. High levels of aromatase mRNA, protein and activity (AA) are present in the preoptic area of this species where the transcription of aromatase is controlled mainly by steroids. Estrogens acting in synergy with androgens play a key role in this control and both androgen and estrogen receptors (ER; alpha and beta subtypes) are present in the preoptic area even if they are not necessarily co-localized in the same cells as aromatase. Steroids have more pronounced effects on aromatase transcription in males than in females and this sex difference could be caused, in part, by a sexually differentiated expression of the steroid receptor coactivator 1 in this area. The changes in aromatase concentration presumably control seasonal variations as well as sex differences in brain estrogen production. Aromatase activity in hypothalamic homogenates is also rapidly (within minutes) down-regulated by exposure to conditions that enhance protein phosphorylation such as the presence of high concentrations of calcium, magnesium and ATP. Similarly, pharmacological manipulations such as treatment with thapsigargin or stimulation of various neurotransmitter receptors (alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA)) leading to enhanced intracellular calcium concentrations depress within minutes the aromatase activity measured in quail preoptic explants. The effects of receptor stimulation are presumably direct: electrophysiological data confirm the presence of these receptors in the membrane of aromatase-expressing cells. Inhibitors of protein kinases interfere with these processes and Western blotting experiments on brain aromatase purified by immunoprecipitation confirm that the phosphorylations regulating aromatase activity directly affect the enzyme rather than another regulatory protein. Accordingly, several phosphorylation consensus sites are present on the deduced amino acid sequence of the recently cloned quail aromatase. Fast changes in the local availability of estrogens in the brain can thus be caused by aromatase phosphorylation so that estrogen could rapidly regulate neuronal physiology and behavior. The rapid as well as slower processes of local estrogen production in the brain thus match well with the genomic and non-genomic actions of steroids in the brain. These two processes potentially provide sufficient temporal variation in the bio-availability of estrogens to support the entire range of established effects for this steroid.

Expression of AR associated protein 55 (ARA55) and androgen receptor in prostate cancer

BACKGROUND

Androgen receptor (AR) transcription is modulated by several cofactors such as AR associated proteins (ARA) including ARA70, ARA54, and ARA55. ARA55 increases AR transcription and alters ligand specificity. We hypothesized that ARA55 might play an important role in prostate cancer development or progression. We evaluated the messenger RNA (mRNA) expression of ARA55 in prostate cancer tissues, and analyzed the relation between ARA55 expression and clinical characteristics.

METHODS

A total of 30 prostate cancer specimens (20 previously untreated prostate cancers and 10 recurrent, hormone-refractory prostate cancers (HRPC)) and 5 benign prostatic hypertrophy (BPH) tissue samples were examined. mRNA expression of ARA55 and AR were analyzed by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) using real time PCR.

RESULTS

ARA55 expression was identified in all tissue samples of previously untreated prostate cancer, HRPC and BPH. ARA55 expression level in HRPC specimens was significantly lower than that in previously untreated prostate cancer (P = 0.02) or BPH (P = 0.005) samples using quantitative PCR. On the other hand, higher ARA55 expression was associated with shorter recurrence-free survival (P = 0.02) and overall survival (P = 0.01) in HRPC patients. AR expression was also revealed in all specimens of both prostate cancer and BPH. AR expression level in HRPC samples was significantly higher than that in previously untreated prostate cancer (P = 0.001) and BPH (P = 0.01) samples.

CONCLUSIONS

ARA55 may be associated with prostate cancer development and progression. ARA55 expression level in HRPC specimens was significantly lower than that in previously untreated prostate cancer or BPH specimens. On the contrary, our results suggested that a higher ARA55 expression level may result in unfavorable recurrence-free survival and overall survival in HRPC patients. The role of ARA55 may differ between prostate cancer development and the process of progression to a hormone-refractory state. These data not only help to understand the molecular mechanism of prostate cancer development or recurrence, but may also lead to a therapeutic strategy for recurrent prostate cancer that is refractory to hormonal treatment.

Subcellular localization and mechanisms of nucleocytoplasmic trafficking of steroid receptor coactivator-1

Steroid hormone receptors are ligand-stimulated transcription factors that modulate gene transcription by recruiting coregulators to gene promoters. Subcellular localization and dynamic movements of transcription factors have been shown to be one of the major means of regulating their transcriptional activity. In the present report we describe the subcellular localization and the dynamics of intracellular trafficking of steroid receptor coactivator 1 (SRC-1). After its synthesis in the cytoplasm, SRC-1 is imported into the nucleus, where it activates transcription and is subsequently exported back to the cytoplasm. In both the nucleus and cytoplasm, SRC-1 is localized in speckles. The characterization of SRC-1 nuclear localization sequence reveals that it is a classic bipartite signal localized in the N-terminal region of the protein, between amino acids 18 and 36. This sequence is highly conserved within the other members of the p160 family. Additionally, SRC-1 nuclear export is inhibited by leptomycin B. The region involved in its nuclear export is localized between amino acids 990 and 1038. It is an unusually large domain differing from the classic leucine-rich NES sequences. Thus SRC-1 nuclear export involves either an alternate type of NES or is dependent on the interaction of SRC-1 with a protein, which is exported through the crm1/exportin pathway. Overall, the intracellular trafficking of SRC-1 might be a mechanism to regulate the termination of hormone action, the interaction with other signaling pathways in the cytoplasm and its degradation.

Dominant-negative nuclear receptor corepressor relieves transcriptional inhibition of retinoic acid receptor but does not alter the agonist/antagonist activities of the tamoxifen-bound estrogen receptor

Repression of the transcriptional activities of the estrogen receptor (ER) is a main goal in the treatment of breast cancer. The antiestrogen tamoxifen is an effective therapy for breast cancer patients because it inhibits estrogen-stimulated gene expression and cell proliferation. Previous studies have implicated a complex containing the nuclear receptor corepressor (N-CoR) in the mechanism by which tamoxifen represses ER-mediated transcriptional activity. In the present study a truncated N-CoR construct was used to inhibit endogenous N-CoR activity in an ER-positive breast cancer cell line. This dominant-negative N-CoR was successful in relieving repression conferred by the unliganded retinoic acid receptor, but it failed to affect the transcriptional activity of the ER in the presence of tamoxifen. Correspondingly, the histone acetylation levels of nucleosomes on endogenous estrogen-responsive genes were unaltered in cells expressing the N-CoR dominant-negative, regardless of ligand. In addition, in vitro cell proliferation and in vivo tumor growth were unchanged in cells that express dominant-negative N-CoR. In conclusion, these results may reveal that N-CoR affects tamoxifen-liganded ER in a manner distinct from its influence on retinoic acid receptor-mediated transcriptional activity or that corepressors other than N-CoR may be involved in the ability of tamoxifen to repress estrogen-responsive transcription and tumor growth.

Full activation of estrogen receptor alpha activation function-1 induces proliferation of breast cancer cells

The effects of estrogen and anti-estrogen are mediated through the estrogen receptors (ER) alpha and beta, which function as ligand-induced transcriptional factors. Recently, one of the phthalate esters, n-butylbenzyl phthalate (BBP), has been shown to induce estrogen receptor-mediated responses. By using the truncated types of ER mutants, we revealed that activation function-1 (AF-1) activity was necessary for the BBP-dependent transactivation function of ERalpha. AF-1 is also known to be responsible for the partial agonistic activity of tamoxifen. Whereas tamoxifen exhibits an anti-estrogenic effect on proliferation of the MCF-7 breast cancer cell line, BBP showed an estrogenic effect on MCF-7 to stimulate proliferation. In vivo and in vitro binding assays revealed that whereas 4-hydroxytamoxifen (OHT) induced binding of ERalpha to both an AF-1 coactivator complex (p68/p72 and p300) and corepressor complexes (N-CoR/SMRT), BBP selectively enhanced the binding to the AF-1 coactivators. We also showed that the transcriptional activity of OHT-bound ERalpha was modulated by the ratio between the AF-1 coactivator and corepressor complexes. Expression of a dominant-negative type of N-CoR inhibited the interaction between OHT-bound ERalpha and N-CoR/SMRT and enhanced the transcriptional activity of OHT-bound ERalpha. Furthermore, the cell growth of MCF-7 stably expressing the dominant-negative type of N-CoR was enhanced by the addition of OHT. These results indicated that fully activated AF-1 induces the stimulation of breast cancer growth and that the ratio between AF-1 coactivators and corepressors plays a key role to prevent proliferation of tumor by tamoxifen.

Ligand-independent interactions of p160/steroid receptor coactivators and CREB-binding protein (CBP) with estrogen receptor-alpha: regulation by phosphorylation sites in the A/B region depends on other receptor domains

Estrogen receptor (ER)alpha and ERbeta are transcription factors that can be activated by both ligand binding and growth factor signaling. Estradiol increases ER activity in part by enhancing interactions between its carboxy-terminal, ligand-binding domain (LBD) and the p160/SRC (steroid receptor coactivator) and p300/CBP (cAMP response element binding protein (CREB) binding protein) families of coactivators. In the absence of ligand and the LBD, these cofactors can also interact with the amino-terminal (A/B) domain of ERs in vitro. SRC-1 also enhances the ligand-independent activity of the full-length receptor. Both ligand-independent and estradiol-induced ER activity are increased by phosphorylation at specific serine (Ser) residues in the A/B domain (Ser104/106 and Ser118 in ERalpha). In the case of ERbeta, phosphorylation enhances the ligand-independent recruitment and action of SRC-1. We show here that unliganded ERalpha can activate endogenous gene expression in MCF-7 cells, and that this activation is mediated in part by a p160 coactivator. In transfected HeLa cells, we show that the full-length ERalpha can interact physically and functionally with p160/SRCs and CBP in the absence of ligand and that mutation of Ser104/106/118 affects these interactions. Accordingly, ERalpha dephosphorylation decreases its ligand-independent interaction with SRC-1 and CBP in vitro. In HeLa cells, both Ser104/106 and Ser118 impinge on SRC-1 action by two mechanisms: 1) a seemingly indirect effect on SRC-1 recruitment that requires other receptor domains in addition to the A/B, consistent with our finding that the ligand-independent interaction between the A/B and the LBD and its enhancement by SRC-1 depend in part on Ser104/106/118; and 2) an effect on SRC-1 coactivation that can be observed in the absence of the LBD. Ser104/106/118 can also affect coactivation by a subset of coactivators in the presence of hormone, albeit to a lesser extent than in its absence. Altogether, our observations suggest that the enhancement of ERalpha activity by p160/SRCs and CBP can be regulated by phosphorylation and stress the importance of using full-length receptors to assess the role of the activation function-1 in cofactor recruitment.

Regulation of expression of thyroid hormone receptor isoforms and coactivators in liver and heart by thyroid hormone

Autoregulation of thyroid hormone (TH) receptors (TRs) is a mechanism whereby a cell can regulate its responsiveness to TH. Nuclear coactivators (NCoAs) modulate TH action and may also be important for regulation of TR expression. We have determined the effect of TH withdrawal and treatment on the expression of different isoforms of TR as well as expression of the NCoAs SRC-1, TIF-2 and SRC-3 using quantitative real time polymerase chain reaction. In order to identify the effect that each TR isoform exerts over the expression of the other, NCoA and TR transcripts were measured in liver and heart tissue from wild type mice or mice with deletion of either TR isoform or SRC-1 genes. In liver, regulation of TR beta1 and TR alpha2 subtype expression is inversely related to TH levels and the regulation of TR beta expression is, in part, controlled by TR alpha. In the heart, the opposite is the case, regulation of TR alpha2 and TR beta1 isoform expression is directly related to TH levels and this regulation is primarily controlled by TR alpha. Although NCoAs are, in general, increased in response to hypothyroidism or in states of TH resistance, SRC-1 specifically does not regulate TR isoform expression. We have demonstrated that TR isoforms and NCoAs are autoregulated transcription factors with tissue specificity.

Thyroid hormone receptor-specific interactions with steroid receptor coactivator-1 in the pituitary

Steroid receptor coactivator-1 (SRC-1) is a transcription cofactor that enhances the hormone-dependent action mediated by the thyroid hormone (TH) receptor (TR) as well as other nuclear receptors. However, it is not known whether the SRC-1-mediated activation of TH-regulated gene transcription is TR isoform specific in the pituitary. We generated mice that were deficient in TRalpha and SRC-1 (TRalpha(0/0)SRC-1(-/-)), as well in TRbeta and SRC-1 (TRbeta(-/-)SRC-1(-/-)), and thyroid function tests and effects of TH deprivation and TH treatment were compared with wild-type mice or mice with deletion of either TRs or SRC-1 alone. We have shown that 1) TRbeta(-/-)SRC-1(-/-) mice demonstrate more severe TH resistance than either the SRC-1(-/-) or TRbeta(-/-) mice; the additive effect indicates that SRC-1 has an independent role in TH action over that of TRbeta; 2) SRC-1 facilitates TRbeta and TRalpha-mediated down-regulation of TSH, as TRalpha(0/0)SRC-1(-/-) mice demonstrate TH resistance rather than hypersensitivity as seen in TRalpha(0/0)mice; and 3) a compensatory increase in SRC-1 expression is associated with the TH hypersensitivity seen in TRalpha-deficient animals. We conclude that SRC-1 action in the pituitary mediates TH action via specific TR subtypes.

Activation and binding of peroxisome proliferator-activated receptor gamma by synthetic cannabinoid ajulemic acid

Ajulemic acid (AJA) is a synthetic analog of the tetrahydrocannabinol (THC) metabolite THC-11-oic acid; THC is a major active ingredient of the drug marijuana derived from the plant cannabis. AJA has potent analgesic and anti-inflammatory activity without the psychotropic action of THC. Unlike the nonsteroidal anti-inflammatory drugs, AJA is not ulcerogenic at therapeutic doses, making it a promising anti-inflammatory drug. However, the mechanism of AJA action remains unknown. Here we report that AJA binds directly and specifically to the peroxisome proliferator-activated receptor gamma (PPARgamma), a pharmacologically important member of the nuclear receptor superfamily. Functional assay indicates that AJA activates the transcriptional activity of both human and mouse PPARgamma at pharmacological concentrations. Activation of PPARgamma by AJA requires the AF-2 helix of the receptor, suggesting that AJA activates PPARgamma through the ligand-dependent AF-2 function. AJA binding consistently enables PPARgamma to recruit nuclear receptor coactivators. In addition, we show that AJA inhibits interleukin-8 promoter activity in a PPARgamma-dependent manner, suggesting a link between the anti-inflammatory action of AJA and the activation of PPARgamma. Finally, we find that AJA treatment induces differentiation of 3T3 L1 fibroblasts into adipocytes, a process mediated by PPARgamma. Together, these data indicate that PPARgamma may be a molecular target for AJA, providing a potential mechanism for the anti-inflammatory action of AJA, and possibly other cannabinoids. These studies also implicate other potential therapeutic actions of AJA through PPARgamma activation in multiple signaling pathways.

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.

Ligand and coactivator identity determines the requirement of the charge clamp for coactivation of the peroxisome proliferator-activated receptor gamma

The activation function 2 (AF-2)-dependent recruitment of coactivator is essential for gene activation by nuclear receptors. We show that the peroxisome proliferator-activated receptor gamma (PPARgamma) (NR1C3) coactivator-1 (PGC-1) requires both the intact AF-2 domain of PPARgamma and the LXXLL domain of PGC-1 for ligand-dependent and ligand-independent interaction and coactivation. Although the AF-2 domain of PPARgamma is absolutely required for PGC-1-mediated coactivation, this coactivator displayed a unique lack of requirement for the charge clamp of the ligand-binding domain of the receptor that is thought to be essential for LXXLL motif recognition. The mutation of a single serine residue adjacent to the core LXXLL motif of PGC-1 led to restoration of the typical charge clamp requirement. Thus, the unique structural features of the PGC-1 LXXLL motif appear to mediate an atypical mode of interaction with PPARgamma. Unexpectedly, we discovered that various ligands display variability in terms of their requirement for the charge clamp of PPARgamma for coactivation by PGC-1. This ligand-selective variable requirement for the charge clamp was coactivator-specific. Thus, distinct structural determinants, which may be unique for a particular ligand, are utilized by the receptor to recognize the coactivator. Our data suggest that even subtle differences in ligand structure are perceived by the receptor and translated into a unique display of the coactivator-binding surface of the ligand-binding domain, allowing for differential recognition of coactivators that may underlie distinct pharmacological profiles observed for ligands of a particular nuclear receptor.

Role of the estrogen receptor coactivator AIB1 (SRC-3) and HER-2/neu in tamoxifen resistance in breast cancer

BACKGROUND

AIB1 (SRC-3) is an estrogen receptor (ER) coactivator that, when overexpressed in cultured cells, can reduce the antagonist activity of tamoxifen-bound ERs. Signaling through the HER-2 receptor pathway activates AIB1 by phosphorylation. To determine whether high AIB1 expression alone or together with HER-2 reduces the effectiveness of tamoxifen in breast cancer patients, we quantified expression of AIB1 and HER-2 in tumors from breast cancer patients with long-term clinical follow-up who received either no adjuvant therapy or adjuvant tamoxifen therapy after breast cancer surgery.

METHODS

AIB1 and HER-2 protein levels in tumors from 316 breast cancer patients were determined using western blot analysis. Molecular variables (e.g., expression of AIB1, ER, progesterone receptor, p53, Bcl-2), tumor characteristics, and patient outcome were assessed using Spearman rank correlation. Disease-free survival (DFS) curves were derived from Kaplan-Meier estimates, and the curves were compared by log-rank tests. The effect of AIB1 on DFS adjusted for other prognostic factors was assessed by multivariable analysis using the Cox proportional hazards model. All statistical tests were two-sided.

RESULTS

High AIB1 expression in patients not receiving adjuvant tamoxifen therapy was associated with better prognosis and longer DFS (P =.018, log-rank test). In contrast, for patients who did receive tamoxifen therapy, high AIB1 expression was associated with worse DFS (P =.049, log-rank test), which is indicative of tamoxifen resistance. The test for interaction between AIB1 expression and tamoxifen therapy was statistically significant (P =.004). When expression of AIB1 and HER-2 were considered together, patients whose tumors expressed high levels of both AIB1 and HER-2 had worse outcomes with tamoxifen therapy than all other patients combined (P =.002, log-rank test).

CONCLUSIONS

The antitumor activity of tamoxifen in patients with breast cancer may be determined, in part, by tumor levels of AIB1 and HER-2. Thus, AIB1 may be an important diagnostic and therapeutic target.

Inactivation of the nuclear receptor coactivator RAP250 in mice results in placental vascular dysfunction

Coactivators constitute a diverse group of proteins that are essential for optimal transcriptional activity of nuclear receptors. In the past few years many coactivators have been identified but it is still unclear whether these proteins interact indiscriminately with all nuclear receptors and whether there is some redundancy in their functions. We have previously cloned and characterized RAP250 (ASC-2/PRIP/TRBP/NRC), an LXXLL-containing coactivator for nuclear receptors. In order to study its biological role, Rap250 null mice were generated by gene targeting. Here we show that genetic disruption of Rap250 results in embryonic lethality at embryonic day (E) 13.5. Histological examination of placentas revealed a dramatically reduced spongiotrophoblast layer, a collapse of blood vessels in the region bordering the spongiotrophoblast, and labyrinthine layers in placentas from Rap250(-/-) embryos. These findings suggest that the lethality of Rap250(-/-) embryos is the result of obstructed placental blood circulation. Moreover, the transcriptional activity of PPAR gamma is reduced in fibroblasts derived from Rap250(-/-) embryos, suggesting that RAP250 is an essential coactivator for this nuclear receptor in the placenta. Our results demonstrate that RAP250 is necessary for placental development and thus essential for embryonic development.

Structural modeling of ataxin-3 reveals distant homology to adaptins

Spinocerebellar ataxia type 3 (SCA3) is a polyglutamine disorder caused by a CAG repeat expansion in the coding region of a gene encoding ataxin-3, a protein of yet unknown function. Based on a comprehensive computational analysis, we propose a structural model and structure-based functions for ataxin-3. Our predictive strategy comprises the compilation of multiple sequence and structure alignments of carefully selected proteins related to ataxin-3. These alignments are consistent with additional information on sequence motifs, secondary structure, and domain architectures. The application of complementary methods revealed the homology of ataxin-3 to ENTH and VHS domain proteins involved in membrane trafficking and regulatory adaptor functions. We modeled the structure of ataxin-3 using the adaptin AP180 as a template and assessed the reliability of the model by comparison with known sequence and structural features. We could further infer potential functions of ataxin-3 in agreement with known experimental data. Our database searches also identified an as yet uncharacterized family of proteins, which we named josephins because of their pronounced homology to the Josephin domain of ataxin-3.

SRC-1 null mice exhibit moderate motor dysfunction and delayed development of cerebellar Purkinje cells

Hormones and nuclear receptors (NRs) play important roles in brain development and function. The recently identified steroid receptor coactivator (SRC) family contains three homologous members that can enhance transcriptional activities of NRs and certain non-NR transcription factors. To study the role of SRC-1 in brain development and function, we examined the spatial and temporal expression patterns of SRC-1 and characterized the phenotypes of brain development and function in SRC-1 knock-out (SRC-1(-)/-) mice. In the adult mouse brain, SRC-1 is highly expressed in the olfactory bulb, hippocampus, piriform cortex, amygdala, hypothalamus, cerebellum, and brainstem. Multiple behavioral tests revealed that SRC-1(-)/- mice exhibit normal hippocampal function but moderate motor dysfunction. The behavior phenotypes correlate with the spatial distribution of the SRC family members. In most brain structures where SRC-1 is expressed, SRC-2 is expressed at lower levels; however, SRC-3 mRNA is detectable only in the hippocampus. In the adult cerebellum, Purkinje cells (PCs) preferentially express SRC-1 over SRC-2, but SRC-2 mRNA is slightly elevated in the SRC-1(-)/- PCs. During embryonic development, SRC-1 is expressed in the cerebellar primordium. SRC-2 is expressed in PCs after postnatal day (P) 10. Time course analysis revealed that the precursors of SRC-1(-)/- PCs were generated approximately 2 d later than wild-type precursor cells. A further delay in SRC-1(-)/- PC maturation was detected at the neonatal stage. The morphology and number of SRC-1(-)/- PCs were equivalent to wild type by P10; this timing correlated with the early expression of SRC-2 in the SRC-1(-)/- PCs. These results demonstrate that the relative levels of SRC expression are region specific, and the degree of overlapping expression may influence their functional redundancy. Disruption of SRC-1 specifically delays the PC development and maturation in early stages and results in moderate motor dysfunction in adulthood.

Sex differences in the distribution of the steroid receptor coactivator SRC-1 in the song control nuclei of male and female canaries

The steroid receptor coactivator SRC-1 modulates ligand-dependent transactivation of several nuclear receptors, including the receptors for sex steroid hormones. The distribution of SRC-1 transcripts was analyzed here by in situ hybridization in coronal sections through the brain of male and female canaries. A broad but heterogeneous distribution of SRC-1 transcripts was observed with high numbers of densely labeled cells being present in many steroid-sensitive areas including the medial preoptic nucleus, several hypothalamic nuclei, five song control nuclei (HVc, the lateral and medial portion of the magnocellular nucleus of the anterior neostriatum, area X and the nucleus uvaeformis) and several catecholaminergic areas (area ventralis of Tsai, substantia nigra, locus coeruleus). The volume of two song control nuclei, HVc and area X were reconstructed based on the boundaries of the cell groups exhibiting a denser SRC-1 expression as compared to the surrounding areas. Sex differences in the expression of SRC-1 were also detected in several song control nuclei. In particular, the volume of HVc based on the high density of SRC-1 expression was significantly larger in males than in females. The effect of steroids on the song control system could be, at least in part, indirect and result from a modulation by steroids of the catecholaminergic inputs to the song control nuclei. The presence of the steroid receptor coactivator SRC-1 in the telencephalic song control nuclei and in the catecholaminergic cell groups that innervate the song system supports the idea that SRC-1 expression could play an active role in the control of singing behavior by modulating estrogen and androgen receptor action at both locations.

Natural structural variants of the nuclear receptor farnesoid X receptor affect transcriptional activation

The Farnesoid X receptor (FXR) is a member of the nuclear hormone receptor superfamily that has been shown to play an important role in bile acid and cholesterol homeostasis. Here we identify four murine FXR transcripts, derived from a single gene, that encode four isoforms, FXRalpha1, FXRalpha2, FXRbeta1, and FXRbeta2. FXRalpha and FXRbeta differ at their amino terminus, and FXRalpha1 and FXRbeta1 have a four-amino acid residue insertion in the hinge region immediately adjacent to the DNA binding domain. Real time PCR and 5'-rapid amplification of cDNA ends followed by Southern blotting reveal that these four transcripts are expressed differentially in liver, intestine, kidney, adrenals, stomach, fat, and heart. Electrophoretic mobility shift assays demonstrate that FXRalpha2 and FXRbeta2 bind to FXR response elements with a higher affinity as compared with FXRalpha1 and FXRbeta1, suggesting that the four-amino acid insert may affect FXR function. Consistent with this idea, the results of transient transfection experiments demonstrate that the four FXR isoforms differentially transactivated a number of promoter-reporter genes; activation of an ileal bile acid-binding protein promoter-reporter gene varied 20-fold depending on the FXR isoform; the rank order of activation was FXRbeta2 > FXRalpha2 FXRalpha1 = FXRbeta1. In contrast, SHP reporter or BSEP reporter genes were activated to similar degrees by each of the FXR isoforms. Finally, NIH3T3 cells were stably infected with individual murine FXR isoforms, and the cells were treated with FXR ligands. The endogenous ileal bile acid-binding protein gene was activated by the four FXR isoforms with the same rank order as seen in transfections. This effect was gene-specific, since induction of bile salt export pump mRNA was independent of the FXR isoform. These observations suggest that there are four distinct murine FXR isoforms that differentially regulate gene expression in numerous tissues in vivo.

Molecular communication between androgen receptor and general transcription machinery

The androgen-androgen receptor (AR) signaling pathway plays a key role in proper development and function of male reproductive organs. Like other transcriptional regulators, AR may communicate with the general transcription machinery on the core promoter to exert its function as a transcriptional modulator. The molecular communication between AR and the general transcription machinery may be achieved either by the direct protein-protein interaction between AR and the general transcription machinery or by the indirect interaction mediated by coregulators. Analyses of AR-mediated transcription suggest that the orchestrated interaction of AR with the transcription factors IIF (TFIIF) and IIH (TFIIH), and positive transcription elongation factor b (P-TEFb), may increase efficiency of transcriptional elongation from the androgen target genes, such as prostate specific antigen (PSA). Based on studies so far, AR may regulate transcription not by enhanced assembly of preinitiation transcription complex but by regulating promoter clearance and elongation stage of transcription.

SRC-1 null mice exhibit moderate motor dysfunction and delayed development of cerebellar Purkinje cells

Hormones and nuclear receptors (NRs) play important roles in brain development and function. The recently identified steroid receptor coactivator (SRC) family contains three homologous members that can enhance transcriptional activities of NRs and certain non-NR transcription factors. To study the role of SRC-1 in brain development and function, we examined the spatial and temporal expression patterns of SRC-1 and characterized the phenotypes of brain development and function in SRC-1 knock-out (SRC-1(-)/-) mice. In the adult mouse brain, SRC-1 is highly expressed in the olfactory bulb, hippocampus, piriform cortex, amygdala, hypothalamus, cerebellum, and brainstem. Multiple behavioral tests revealed that SRC-1(-)/- mice exhibit normal hippocampal function but moderate motor dysfunction. The behavior phenotypes correlate with the spatial distribution of the SRC family members. In most brain structures where SRC-1 is expressed, SRC-2 is expressed at lower levels; however, SRC-3 mRNA is detectable only in the hippocampus. In the adult cerebellum, Purkinje cells (PCs) preferentially express SRC-1 over SRC-2, but SRC-2 mRNA is slightly elevated in the SRC-1(-)/- PCs. During embryonic development, SRC-1 is expressed in the cerebellar primordium. SRC-2 is expressed in PCs after postnatal day (P) 10. Time course analysis revealed that the precursors of SRC-1(-)/- PCs were generated approximately 2 d later than wild-type precursor cells. A further delay in SRC-1(-)/- PC maturation was detected at the neonatal stage. The morphology and number of SRC-1(-)/- PCs were equivalent to wild type by P10; this timing correlated with the early expression of SRC-2 in the SRC-1(-)/- PCs. These results demonstrate that the relative levels of SRC expression are region specific, and the degree of overlapping expression may influence their functional redundancy. Disruption of SRC-1 specifically delays the PC development and maturation in early stages and results in moderate motor dysfunction in adulthood.

Nuclear receptor superfamily: Principles of signaling

Nuclear receptors (NRs) comprise a family of 49 members that share a common structural organization and act as ligand-inducible transcription factors with major (patho)physiological impact. For some NRs (“orphan receptors”), cognate ligands have not yet been identified or may not exist. The principles of DNA recognition and ligand binding are well understood from both biochemical and crystal structure analyses. The 3D structures of several DNA-binding domains (DBDs),in complexes with a variety of cognate response elements, and multiple ligand-binding domains (LBDs), in the absence (apoLBD)and presence (holoLBD) of agonist, have been established and reveal canonical structural organization. Agonist binding induces a structural transition in the LBD whose most striking feature is the relocation of helix H12, which is required for establishing a coactivator complex, through interaction with members of the p160 family (SRC1, TIF2, AIB1) and/or the TRAP/DRIP complex. The p160-dependent coactivator complex is a multiprotein complex that comprises histone acetyltransferases (HATs), such as CBP,methyltransferases, such as CARM1, and other enzymes (SUMO ligase,etc.). The agonist-dependent recruitment of the HAT complex results in chromatin modification in the environment of the target gene promoters, which is requisite to, or may in some cases be sufficient for, transcription activation. In the absence of ligands, or in the presence of some antagonists, certain NRs are bound to distinct multiprotein complexes through the interaction with corepressors, such as NCoR and SMRT. Corepressor complexes comprise histone deacetylases (HDACs) that have the capacity to condense chromatin over target gene promoters. Ligands have been designed that selectively modulate the interaction between NRs and their coregulators. Both HATs and HDACs can also modify the acetylation status of nonhistone proteins, but the significance in the context of NR signaling is unclear. NRs communicate with other intracellular signaling pathways on a mutual basis, and their functionality may be altered, positively or negatively, by post-translational modification. The majority of NRs act as retinoid X receptor (RXR) heterodimers in which RXR cannot a priori respond autonomously to its cognate ligand to activate target gene transcription. This RXR subordination allows signaling pathway identity for the RXR partner. The corresponding mechanism is understood and reveals cell and NR selectivity, indicating that RXR can, under certain conditions, act autonomously. NRs are regulators of cell life and death,and NR malfunction can be at the basis of both disease and therapy, as is impressively documented in the case of acute promyelocytic leukemia. Recently, several pathways have been uncovered that link NR action with cell proliferation and apoptosis.

Alternate surfaces of transcriptional coregulator GRIP1 function in different glucocorticoid receptor activation and repression contexts

Members of the mammalian p160 family, such as GRIP1, are known as glucocorticoid receptor (GR) coactivators; at certain glucocorticoid response elements (GREs), however, GRIP1 acts as a GR corepressor. We characterized functional interactions of GR and GRIP1 in a repression complex where GR tethers to DNA-bound activator protein-1 (AP-1), as at the human collagenase-3 gene, and tested whether the identified interactions were similar or different at other response elements. At the AP-1 tethering GRE, we mapped the GRIP1 corepressor activity to a domain distinct from the two known GRIP1 activation domains; it exhibited intrinsic GR-independent repression potential when recruited to DNA via Gal4 DNA-binding domain. Interestingly, neither the domain nor the activity was detected in the other two p160 family members, SRC1 and RAC3. The same GRIP1 corepression domain was required for GR-mediated repression at the nuclear factor-kappaB (NF-kappaB) tethering GRE of the human IL-8 gene. In contrast, at the osteocalcin gene GRE, where GR represses transcription by binding to a DNA site overlapping the TATA box, both GRIP1 and SRC1 corepressed, and the GRIP1-specific repression domain was dispensable. Thus, in a single cell type, GR and GRIP1 conferred one mode of activation and two modes of repression by selectively engaging distinct surfaces of GRIP1 in a response element-specific manner.

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.

Molecular structure and biological function of the cancer-amplified nuclear receptor coactivator SRC-3/AIB1

Nuclear hormone receptors are ligand-dependent transcription factors that require coactivators to regulate target gene expression. The steroid receptor coactivator-3 (SRC-3), also known as p/CIP, RAC3, AIB1, ACTR and TRAM-1, is a cancer-amplified coactivator in the SRC gene family that also contains SRC-1 and TIF2/GRIP1. SRC-3 interacts with nuclear receptors and certain other transcription factors, recruits histone acetyltransferases and methyltransferases for chromatin remodeling and facilitates target gene transcription. Accumulated results from both ex vivo and animal model studies indicate that SRC-3 plays important roles in many biological processes involving cell proliferation, cell migration, cell differentiation, somatic growth, sexual maturation, female reproductive function, vasoprotection and breast cancer. This article summarizes our current knowledge about SRC-3 under the following topics: molecular cloning and characterization; molecular structure and functional mechanisms; SRC-3 as a molecular target of growth factors and cytokines; organization and expression of the SRC-3 gene; generation and characterization of SRC-3 knockout mice; role of SRC-3 in the vasoprotective effects of estrogen; role of SRC-3 in cell migration, proliferation and cancers.

Steroid receptor coactivator SRC-1 exhibits high expression in steroid-sensitive brain areas regulating reproductive behaviors in the quail brain

The steroid receptor coactivator SRC-1 modulates ligand-dependent transactivation of several nuclear receptors, including the receptors for sex steroid hormones. Reducing the expression of SRC-1 by injection of specific antisense oligonucleotides markedly inhibits the effects of estrogens of the sexual differentiation of brain and behavior in rats and inhibits the activation of female sexual behavior in adult female rats. SRC-1 thus appears to be involved in both the development and activation of sexual behavior. In the Japanese quail brain, we amplified by RT-PCR a 3,411-bp fragment extending from the HLH domain to the activating domain-2 of the protein. The quail SRC-1 is closely related to the mammalian (m) SRC-1 and contains a high proportion of GC nucleotides (62.5%). Its amino acid sequence presents 70% identity with mammalian SRC-1 and contains the three conserved LXXLL boxes involved in the interaction with nuclear receptors. In both males and females, RT-PCR demonstrates a similarly high level of expression in the telencephalon, diencephalon, optic lobes, brain stem, spinal cord, pituitary, liver, kidney, adrenal gland, heart, lung, gonads and gonoducts. Males express significantly higher levels of SRC-1 in the preoptic area-hypothalamus than females. In both sexes, lower levels of expression are observed in the cerebellum and muscles. In situ hybridization utilizing a mixture of four digoxigenin-labeled oligonucleotides confirms at the cellular level the widespread distribution of SRC-1 mRNA in the brain and a particularly dense expression in steroid-sensitive areas that play a key role in the control of male sexual behavior. These data confirm the presence and describe for the first time the SRC-1 distribution in the brain of an avian species. They confirm its broad, nearly ubiquitous, distribution in the entire body including the brain as could be expected for a coactivator that regulates to the action of many nuclear receptors. However this distribution is heterogeneous in the brain and sexually differentiated in at least some areas. The very dense expression of SRC-1 in limbic and mesencephalic nuclei that are associated with the control of male sexual behavior is consistent with the notion that this coactivator plays a significant role in the activation of this behavior.

A role for TGF-beta in estrogen and retinoid mediated regulation of the nuclear receptor coactivator AIB1 in MCF-7 breast cancer cells

AIB1 (amplified in breast cancer 1) is a nuclear receptor coactivator gene amplified and overexpressed in breast cancer. However, the mechanisms by which AIB1 is regulated are unclear. Here we show that 17beta-estradiol represses AIB1 mRNA and protein expression in MCF-7 human breast cancer cells primarily by suppressing AIB1 gene transcription. Estrogen levels present in fetal calf serum are sufficient to maintain AIB1 mRNA and protein at low basal levels, and this repression is reversed by the addition of antiestrogens or all-trans retinoic acid. Interestingly, cycloheximide inhibition experiments revealed that secondary protein synthesis was necessary to induce AIB1 expression by antiestrogens and retinoids. Experiments with TGF-beta and TGF-beta blocking antibodies demonstrated that this growth factor modulates AIB1 expression and showed that the antiestrogen and retinoid induction of AIB1 gene expression is mediated at least in part through TGF-beta. These data reveal a mechanism of estrogen-induced down-modulation of the overall hormone sensitivity of cells through feedback inhibition of coactivator gene expression. These data also suggest that antiestrogens can shift the sensitivity of cells to non-estrogenic proliferative signaling by increasing cellular levels of AIB1. This effect may play a role in breast cancer progression and resistance to drug treatment.

NRC-interacting factor 1 is a novel cotransducer that interacts with and regulates the activity of the nuclear hormone receptor coactivator NRC

We previously reported the cloning and characterization of a novel nuclear hormone receptor transcriptional coactivator, which we refer to as NRC. NRC is a 2,063-amino-acid nuclear protein which contains a potent N-terminal activation domain and several C-terminal modules which interact with CBP and ligand-bound nuclear hormone receptors as well as c-Fos and c-Jun. In this study we sought to clone and identify novel factors that interact with NRC to modulate its transcriptional activity. Here we describe the cloning and characterization of a novel protein we refer to as NIF-1 (NRC-interacting factor 1). NIF-1 was cloned from rat pituitary and human cell lines and was found to interact in vivo and in vitro with NRC. NIF-1 is a 1,342-amino-acid nuclear protein containing a number of conserved domains, including six Cys-2/His-2 zinc fingers, an N-terminal stretch of acidic amino acids, and a C-terminal leucine zipper-like motif. Zinc fingers 1 to 3 are potential DNA-binding BED finger domains recently proposed to play a role in altering local chromatin architecture. We mapped the interaction domains of NRC and NIF-1. Although NIF-1 does not directly interact with nuclear receptors, it markedly enhances ligand-dependent transcriptional activation by nuclear hormone receptors in vivo as well as activation by c-Fos and c-Jun. These results, and the finding that NIF-1 interacts with NRC in vivo, suggest that NIF-1 functions to regulate transcriptional activation through NRC. We suggest that NIF-1, and factors which associate with coactivators but not receptors, be referred to as cotransducers, which act in vivo either as part of a coactivator complex or downstream of a coactivator complex to modulate transcriptional activity. Our findings suggest that NIF-1 may be a functional component of an NRC complex and acts as a regulator or cotransducer of NRC function.

Microtubule-dependent subcellular redistribution of the transcriptional coactivator p/CIP

The transcriptional coactivator p/CIP is a member of a family of nuclear receptor coactivator/steroid receptor coactivator (NCoA/SRC) proteins that mediate the transcriptional activities of nuclear hormone receptors. We have found that p/CIP is predominantly cytoplasmic in a large proportion of cells in various tissues of the developing mouse and in a number of established cell lines. In mouse embryonic fibroblasts, serum deprivation results in the redistribution of p/CIP to the cytoplasmic compartment and stimulation with growth factors or tumor-promoting phorbol esters promotes p/CIP shuttling into the nucleus. Cytoplasmic accumulation of p/CIP is also cell cycle dependent, occurring predominantly during the S and late M phases. Leptomycin B (LMB) treatment results in a marked nuclear accumulation, suggesting that p/CIP undergoes dynamic nuclear export as well as import. We have identified a strong nuclear import signal in the N terminus of p/CIP and two leucine-rich motifs in the C terminus that resemble CRM-1-dependent nuclear export sequences. When fused to green fluorescent protein, the nuclear export sequence region is cytoplasmic and is retained in the nucleus in an LMB-dependent manner. Disruption of the leucine-rich motifs prevents cytoplasmic accumulation. Furthermore, we demonstrate that cytoplasmic p/CIP associates with tubulin and that an intact microtubule network is required for intracellular shuttling of p/CIP. Immunoaffinity purification of p/CIP from nuclear and cytosolic extracts revealed that only nuclear p/CIP complexes possess histone acetyltransferase activity. Collectively, these results suggest that cellular compartmentalization of NCoA/SRC proteins could potentially regulate nuclear hormone receptor-mediated events as well as integrating signals in response to different environmental cues.

Identification of protein arginine methyltransferase 2 as a coactivator for estrogen receptor alpha

In an attempt to isolate cofactors capable of influencing estrogen receptor alpha (ERalpha) transcriptional activity, we used yeast two-hybrid screening and identified protein arginine methyltransferase 2 (PRMT2) as a new ERalpha-binding protein. PRMT2 interacted directly with three ERalpha regions including AF-1, DNA binding domain, and hormone binding domain in a ligand-independent fashion. The ERalpha-interacting region on PRMT2 has been mapped to a region encompassing amino acids 133-275. PRMT2 also binds to ERbeta, PR, TRbeta, RARalpha, PPARgamma, and RXRalpha in a ligand-independent manner. PRMT2 enhanced both ERalpha AF-1 and AF-2 transcriptional activity, and the potential methyltransferase activity of PRMT2 appeared pivotal for its coactivator function. In addition, PRMT2 enhanced PR, PPARgamma, and RARalpha-mediated transactivation. Although PRMT2 was found to interact with two other coactivators, the steroid receptor coactivator-1 (SRC-1) and the peroxisome proliferator-activated receptor-interacting protein (PRIP), no synergistic enhancement of ERalpha transcriptional activity was observed when PRMT2 was coexpressed with either PRIP or SRC-1. In this respect PRMT2 differs from coactivators PRMT1 and CARM1 (coactivator-associated arginine methyltransferase). These results suggest that PRMT2 is a novel ERalpha coactivator.

The function of TIF2/GRIP1 in mouse reproduction is distinct from those of SRC-1 and p/CIP

Human TIF2 (hTIF2) is a member of the p160 family of nuclear receptor coactivators, which includes SRC-1 and p/CIP. Although the functions of hTIF2 and of its mouse homolog (GRIP1 or mTIF2) have been clearly established in vitro, their physiological role remains elusive. Here, we have generated mice lacking mTIF2/GRIP1 and examined their phenotype with a particular emphasis on reproductive functions. TIF2(-/-) mice are viable, but the fertility of both sexes is impaired. Male hypofertility is due to defects in both spermiogenesis (teratozoospermia) and age-dependent testicular degeneration, and TIF2 expression appears to be essential for adhesion of Sertoli cells to germ cells. Female hypofertility is due to a placental hypoplasia that most probably reflects a requirement for maternal TIF2 in decidua stromal cells that face the developing placenta. We conclude that TIF2 plays a critical role in mouse reproductive functions, whereas previous reports have not revealed serious fertility impairment in SRC-1(-/-) or p/CIP(-/-) mutants. Thus, even though the three p160 coactivators exhibit strong sequence homology and similar activity in assays in vitro, they play distinct physiological roles in vivo, as their genetic eliminations result in distinct pathologies.

A structural model of the constitutive androstane receptor defines novel interactions that mediate ligand-independent activity

Unlike classical nuclear receptors that require ligand for transcriptional activity, the constitutive androstane receptor (CAR) is active in the absence of ligand. To determine the molecular contacts that underlie this constitutive activity, we created a three-dimensional model of CAR and verified critical structural features by mutational analysis. We found that the same motifs that facilitate ligand-dependent activity in classical receptors also mediated constitutive activity in CAR. This raises a critical question: how are these motifs maintained in an active conformation in unliganded CAR? The model identified several novel interactions that account for this activity. First, CAR possesses a short loop between helix 11 and the transactivation domain (helix 12), as well as a short carboxy-terminal helix. Together, these features favor ligand-independent docking of the transactivation domain in a position that is characteristic of ligand-activated receptors. Second, this active conformation is further stabilized by a charge-charge interaction that anchors the carboxy-terminal activation domain to helix 4. Mutational analysis of these interactions provides direct experimental support for this model. We also show that ligand-mediated repression of constitutive activity reflects both a displacement of coactivator and a recruitment of corepressor. Our data demonstrate that CAR utilizes the same conserved structural motifs and coregulator proteins as originally defined for classical nuclear receptors. Despite these remarkable similarities, our model demonstrates how a few critical changes in CAR can dramatically reverse the transcriptional activity of this protein.

Targeting of SWI/SNF chromatin remodelling complexes to estrogen-responsive genes

SWI/SNF complexes are ATP-dependent chromatin remodelling enzymes that have been implicated in the regulation of gene expression in yeast and higher eukaryotes. BRG1, a catalytic subunit in the mammalian SWI/SNF complex, is required for transcriptional activation by the estrogen receptor, but the mechanisms by which the complex is recruited to estrogen target genes are unknown. Here, we have identified an interaction between the estrogen receptor and BAF57, a subunit present only in mammalian SWI/SNF complexes, which is stimulated by estrogen and requires both a functional hormone-binding domain and the DNA-binding region of the receptor. We also found an additional interaction between the p160 family of coactivators and BAF57 and demonstrate that the ability of p160 coactivators to potentiate transcription by the estrogen receptor is dependent on BAF57 in transfected cells. Moreover, chromatin immunoprecipitation assays demonstrated that BAF57 is recruited to the estrogen-responsive promoter, pS2, in a ligand-dependent manner. These results suggest that one of the mechanisms for recruiting SWI/SNF complexes to estrogen target genes is by means of BAF57.

Direct interactions between corepressors and coactivators permit the integration of nuclear receptor-mediated repression and activation

The unliganded thyroid hormone receptor beta (TRbeta) represses the basal transcriptional activity of target genes, in part through interactions with the nuclear receptor corepressor (N-CoR). In this study we have identified a rather unexpected interaction between N-CoR and the nuclear receptor coactivator ACTR. We have demonstrated in vitro and in intact cells that N-CoR directly associates with ACTR and that the interaction surfaces on N-CoR and ACTR are distinct from those required for TR binding. The significance of this finding was demonstrated by showing that N-CoR facilitates an interaction between unliganded-TRbeta and ACTR. One possible consequence of the formation of the trimeric complex of N-CoR/ACTR/unliganded-TR is that N-CoR may raise the local concentration of ACTR at target gene promoters. In support of this hypothesis it was demonstrated that the presence of N-CoR can enhance TRbeta-mediated transcriptional activation. It is proposed, therefore, that TRbeta- mediated activation and repression are integrally linked in a manner that is not predicted by the current models of nuclear receptor action.

Minireview: nuclear receptor coactivators--an update

Nuclear receptors (NRs) regulate the expression of target genes in response to activation by steroid hormones and other ligands, as well as a variety of other signaling pathways. NR coactivators are defined as cellular factors recruited by activated NRs that complement their function as mediators of the cellular response to endocrine signals. In this review, we will focus upon advances in our understanding of the function of coactivators as their characterization has progressed from mechanistic studies to an exploration of their biological roles in living animals.

Effects of selective estrogen receptor modulators (SERMs) on coactivator nuclear receptor (NR) box binding to estrogen receptors

Coactivators are required for activation of target genes by nuclear receptors. A well-studied class of coactivators, the p160 proteins, use short nuclear receptor interaction domains (NR boxes) to bind to the activated ligand-binding domain of a nuclear receptor. To investigate how selective estrogen receptor modulators (SERMs) affect NR box recruitment, we compared the recruitment of p160 NR box peptides to the estrogen receptor (ER)alpha and ER beta in the presence of 17beta-estradiol (E2), 4-OH tamoxifen (4-OH Tam), LY 117018 (a raloxifene analog), and ICI 182780 (ICI, an ER antagonist). Our coactivator interaction assay utilizes time-resolved fluorescence technology to assess the binding of the 10 NR boxes derived from the three known p160 coactivators (SRC-1, -2, -3) to the ER subtypes in the presence of each ligand. The SERMs we studied did not increase NR box binding to either ER alpha or ER beta, but instead were potent antagonists decreasing estradiol-dependent NR box binding. We also demonstrated inverse agonism for all of the SERMs tested as they dose-dependently decreased hormone-independent NR box binding to ER beta. Therefore, the SERMs studied behave as antagonists of ER alpha and ER beta NR box binding and do not increase coactivator NR box binding to either ER subtype. In addition, we examined the preference of E2-bound ER alpha and ER beta for various naturally occurring NR boxes including the 10 SRC boxes as well as the motifs from PGC-1, TRBP, TRAP220, and CBP. Interestingly, a clear preferential pattern of interaction was noted that was receptor specific.

Progesterone receptors - animal models and cell signaling in breast cancer: Role of steroid receptor coactivators and corepressors of progesterone receptors in breast cancer

Progesterone, an ovarian steroid hormone, plays a key role in the development and function of the mammary gland, as it also does in the uterus and the ovary. The action of progesterone is mediated through its intracellular cognate receptor, the progesterone receptor (PR), which functions as a transcription factor that regulates gene expression. As with other nuclear receptors, coregulators (coactivators and corepressors) recruited by the liganded or unliganded PR, either to enhance or to suppress transcription activity, modulate the function of the PR. Mutation or aberrant expression of the coregulators might thus affect the normal function of the PR and hence disrupt the normal development of the mammary gland, which may lead to breast cancer.

Interaction of transcriptional intermediary factor 2 nuclear receptor box peptides with the coactivator binding site of estrogen receptor alpha

The activation function 2/ligand-dependent interaction between nuclear receptors and their coregulators is mediated by a short consensus motif, the so-called nuclear receptor (NR) box. Nuclear receptors exhibit distinct preferences for such motifs depending both on the bound ligand and on the NR box sequence. To better understand the structural basis of motif recognition, we characterized the interaction between estrogen receptor alpha and the NR box regions of the p160 coactivator TIF2. We have determined the crystal structures of complexes between the ligand-binding domain of estrogen receptor alpha and 12-mer peptides from the Box B2 and Box B3 regions of TIF2. Surprisingly, the Box B3 module displays an unexpected binding mode that is distinct from the canonical LXXLL interaction observed in other ligand-binding domain/NR box crystal structures. The peptide is shifted along the coactivator binding site in such a way that the interaction motif becomes LXXYL rather than the classical LXXLL. However, analysis of the binding properties of wild type NR box peptides, as well as mutant peptides designed to probe the Box B3 orientation, suggests that the Box B3 peptide primarily adopts the "classical" LXXLL orientation in solution. These results highlight the potential difficulties in interpretation of protein-protein interactions based on co-crystal structures using short peptide motifs.

Androgen receptor-interacting protein 3 and other PIAS proteins cooperate with glucocorticoid receptor-interacting protein 1 in steroid receptor-dependent signaling

Androgen receptor (AR)-interacting protein 3 (ARIP3/PIASxalpha) is a coregulator capable of modulating transcriptional activity of various steroid receptors. We have characterized functional regions of ARIP3 and studied its interaction with the glucocorticoid receptor (GR)-interacting protein 1 (GRIP1). We find that the potential zinc-binding domain is critical for ARIP3 to function as a coactivator; the deletion of amino acids 347-418 or the mutation of the conserved cysteines 385 and 388 to serines converts ARIP3 to a transcriptional repressor from AR-dependent minimal promoters and abolishes its ability to activate GR. By contrast, mutations in the two LXXLL motifs of ARIP3 have relatively minor effects on its ability to regulate AR or GR function. ARIP3 is able to interact with different regions of GRIP1, but the strongest interaction is detected with the C-terminal region (amino acids 1122-1462) of GRIP1. The interaction of ARIP3 with the latter GRIP1 domain or full-length GRIP1 and the ability of ARIP3 to cooperate with GRIP1 in the regulation of AR- or GR-dependent transcription are dependent on the ARIP3 zinc-binding region. We also find a strong synergism between GRIP1 and two other PIAS family members, Miz1 and PIAS1. Taken together, our results suggest that PIAS proteins and GRIP1 interact functionally in transcriptional regulation.

Selective alteration of gene expression in response to natural and synthetic retinoids

BACKGROUND

Retinoids are very potent inducers of cellular differentiation and apoptosis, and are efficient anti-tumoral agents. Synthetic retinoids are designed to restrict their toxicity and side effects, mostly by increasing their selectivity toward each isotype of retinoic acids receptors (RARalpha,beta, gamma and RXRalpha, beta, gamma). We however previously showed that retinoids displayed very different abilities to activate retinoid-inducible reporter genes, and that these differential properties were correlated to the ability of a given ligand to promote SRC-1 recruitment by DNA-bound RXR:RAR heterodimers. This suggested that gene-selective modulation could be achieved by structurally distinct retinoids.

RESULTS

Using the differential display mRNA technique, we identified several genes on the basis of their differential induction by natural or synthetic retinoids in human cervix adenocarcinoma cells. Furthermore, this differential ability to regulate promoter activities was also observed in murine P19 cells for the RARbeta2 and CRABPII gene, showing conclusively that retinoid structure has a dramatic impact on the regulation of endogenous genes.

CONCLUSIONS

Our findings therefore show that some degree of selective induction or repression of gene expression may be achieved when using appropriately designed ligands for retinoic acid receptors, extending the concept of selective modulators from estrogen and peroxisome proliferator activated receptors to the class of retinoid receptors.

Regulation of SRC-3 (pCIP/ACTR/AIB-1/RAC-3/TRAM-1) Coactivator activity by I kappa B kinase

In the past few years, many nuclear receptor coactivators have been identified and shown to be an integral part of receptor action. The most frequently studied of these coactivators are members of the steroid receptor coactivator (SRC) family, SRC-1, TIF2/GRIP1/SRC-2, and pCIP/ACTR/AIB-1/RAC-3/TRAM-1/SRC-3. In this report, we describe the biochemical purification of SRC-1 and SRC-3 protein complexes and the subsequent identification of their associated proteins by mass spectrometry. Surprisingly, we found association of SRC-3, but not SRC-1, with the I kappa B kinase (IKK). IKK is known to be responsible for the degradation of I kappa B and the subsequent activation of NF-kappa B. Since NF-kappa B plays a key role in host immunity and inflammatory responses, we therefore investigated the significance of the SRC-3-IKK complex. We demonstrated that SRC-3 was able to enhance NF-kappa B-mediated gene expression in concert with IKK. In addition, we showed that SRC-3 was phosphorylated by the IKK complex in vitro. Furthermore, elevated SRC-3 phosphorylation in vivo and translocation of SRC-3 from cytoplasm to nucleus in response to tumor necrosis factor alpha occurred in cells, suggesting control of subcellular localization of SRC-3 by phosphorylation. Finally, the hypothesis that SRC-3 is involved in NF-kappa B-mediated gene expression is further supported by the reduced expression of interferon regulatory factor 1, a well-known NF-kappa B target gene, in the spleens of SRC-3 null mutant mice. Taken together, our results not only reveal the IKK-mediated phosphorylation of SRC-3 to be a regulated event that plays an important role but also substantiate the role of SRC-3 in multiple signaling pathways.

ERAP140, a conserved tissue-specific nuclear receptor coactivator

We report here the identification and characterization of a novel nuclear receptor coactivator, ERAP140. ERAP140 was isolated in a screen for ER alpha-interacting proteins using the ER alpha ligand binding domain as a probe. The ERAP140 protein shares no sequence and has little structural homology with other nuclear receptor cofactors. However, homologues of ERAP140 have been identified in mouse, Drosophila, and Caenorhabditis elegans. The expression of ERAP140 is cell and tissue type specific and is most abundant in the brain, where its expression is restricted to neurons. In addition to interacting with ER alpha, ERAP140 also binds ER beta, TR beta, PPAR gamma, and RAR alpha. ERAP140 interacts with ER alpha via a noncanonical interaction motif. The ER alpha-ERAP140 association can be competed by coactivator NR boxes, indicating ERAP140 binds ER alpha on a surface similar to that of other coactivators. ERAP140 can enhance the transcriptional activities of nuclear receptors with which it interacts. In vivo, ERAP140 is recruited by estrogen-bound ER alpha to the promoter region of endogenous ER alpha target genes. Furthermore, the E(2)-induced recruitment of ERAP140 to the promoter follows a cyclic pattern similar to that of other coactivators. Our results suggest that ERAP140 represents a distinct class of nuclear receptor coactivators that mediates receptor signaling in specific target tissues.

Vitamin D analogue-specific recruitment of vitamin D receptor coactivators

Synthetic ligands for the vitamin D receptor (VDR) are potential therapeutic agents for metabolic, neoplastic, and autoimmune disorders. Some of these ligands have similar or more potent antiproliferative, yet reduced hypercalcemic actions, than calcitriol. However, the mechanisms for these differential actions have not been clearly defined. We hypothesized that these gene- and tissue-specific effects may relate to ligand-directed selective recruitment of transcriptional coactivators. To identify key elements in ligand structure that facilitate VDR-coactivator interactions, the current studies assessed the ability of the VDR to recruit the coactivators GRIP1 and RAC3 following activation by a series of 20-R- and 20-S (20-epi)-modified analogues. The strength of VDR-coactivator interactions was ligand-specific and did not always correlate with ligand-receptor binding affinity. In general, the 20-epi analogues enhanced these interactions, whereas the 20-R-modified analogues were less effective than calcitriol. The 16-ene,23-yne modification and fluorinated substituents to the side-chain attenuated interaction with coactivators. The enhanced ability of the VDR to recruit GRIP1 following activation by the 20-epi analogues was consistent with potentiation of 20-epi analogue-induced transactivation of the osteocalcin gene promoter by GRIP1. Overall, the structure of the ligand side-chain as well as its orientation seemed to affect the avidity of coactivator binding. These results suggest that selective recruitment of coactivators may contribute to gene- and tissue-specific effects of vitamin D analogues.

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.

Requirement of helix 1 and the AF-2 domain of the thyroid hormone receptor for coactivation by PGC-1

Although PGC-1 (peroxisome proliferator-activated receptor-gamma coactivator-1) has been previously shown to enhance thyroid hormone receptor (TR)/retinoid X receptor-mediated ucp-1 gene expression in a ligand-induced manner in rat fibroblast cells, the precise mechanism of PGC-1 modulation of TR function has yet to be determined. In this study, we show that PGC-1 can potentiate TR-mediated transactivation of reporter genes driven by natural thyroid hormone response elements both in a ligand-dependent and ligand-independent manner and that the extent of coactivation is a function of the thyroid hormone response element examined. Our data also show that PGC-1 stimulation of TR activity in terms of Gal4 DNA-binding domain fusion is strictly ligand-dependent. In addition, an E457A AF-2 mutation had no effect on the ligand-induced PGC-1 enhancement of TR activity, indicating that the conserved charged residue in AF-2 is not essential for this PGC-1 function. Furthermore, GST pull-down and mammalian two-hybrid assays demonstrated that the PGC-1 LXXLL motif is required for ligand-induced PGC-1/TR interaction. This agonist-dependent PGC-1/TR interaction also requires both helix 1 and the AF-2 region of the TR ligand-binding domain. Taken together, these results support the notion that PGC-1 is a bona fide TR coactivator and that PGC-1 modulates TR activity via a mechanism different from that utilized with peroxisome proliferator activator receptor-gamma.

Mechanisms of action of estrogen and progesterone

Estrogen and progesterone are steroid hormones that play a pivotal role in the regulation of mammalian reproduction. One primary action of these hormones is to regulate the development and function of the uterus. These hormones act by regulating the transcription of specific genes in the uterus. The actions of these hormones are mediated by their specific hormone receptors. These receptors are nuclear transcription factors, whose transcriptional regulatory activity is mediated by the binding of the specific steroid to these molecules. Once these receptors bind hormone, they can bind to specific cis-acting sequences in the promoter region of responsive genes and regulate transcription of these genes. In the regulation of transcription, these receptors interact with specific cofactors to activate the transcriptional machinery. A second gene family, the Steroid Receptor Coactivator (SRC) family, has been identified that serves to modulate the transcriptional activity of the hormone receptors. To date, three members of the SRC family have been identified. During the last decade, gene targeting technology has been used to identify the role of these receptors in the regulation of reproduction and uterine biology.

Formation of the androgen receptor transcription complex

Androgen receptor (AR) is required for sexual differentiation and is implicated in the development of prostate cancer. Here we describe distinct functions for cofactor proteins and gene regulatory elements in the assembly of AR-mediated transcription complexes. The formation of an activation complex involves AR, coactivators, and RNA polymerase II recruitment to both the enhancer and promoter, whereas the formation of a repression complex involves factors bound only at the promoter and not the enhancer. These results suggest a model for the functional coordination between the promoter and enhancer in which communication between these elements is established through shared coactivators in the AR transcription complex.

Combinatorial control of gene expression by nuclear receptors and coregulators

The nuclear receptor (NR) superfamily of transcription factors regulates gene expression in response to endocrine signaling, and recruitment of coregulators affords these receptors considerable functional flexibility. We will place historical aspects of NR research in context with current opinions on their mechanism of signal transduction, and we will speculate upon future trends in the field.