Steroid hormone receptors: many actors in search of a plot

Localization of glucocorticoid hormone receptors in mitochondria of human cells

Glucocorticoid hormones regulate the transcription of nuclear genes by way of their cognate receptors. In addition, these hormones also modulate mitochondrial gene transcription by mechanisms which are as yet poorly understood. Using immunofluorescence labeling and confocal laser scanning microscopy we show that the glucocorticoid receptor of HeLa and Hep-2 cells is specifically enriched at the sites of the mitochondria which were visualized by labeling with the vital dye CMX and antibodies against cytochrome oxidase subunit I. Immunogold electron microscopy demonstrated that the receptor was located within the inner space of the mitochondria. Immunoblotting experiments also revealed the presence of glucocorticoid receptor in mitochondria isolated from HeLa and Hep-2 cells. Finally, living HeLa cells expressing green fluorescent-glucocorticoid receptor fusion protein revealed a distinct mitochondrial GFP fluorescence. Our results support the concept of a receptor-mediated direct action of steroid hormones on mitochondrial gene transcription.

Retinoic acid redifferentiation therapy for thyroid cancer

For the treatment of differentiated thyroid cancer, surgery, radioiodide therapy, and thyrotropin-suppressive thyroxine application represent established therapeutic measures of proven efficiency, affording a good prognosis for this disease. However, in up to 30% of the cases, dedifferentiation is observed, giving rise to tumors that are refractory to conventional treatment. Eventually, this may lead to the most malignant human tumor, anaplastic thyroid carcinoma, with a life expectancy of only a few months after diagnosis. Among novel approaches for the treatment of dedifferentiated thyroid carcinomas, retinoic acid redifferentiation therapy was evaluated in several in vitro and in vivo studies. Cell culture experiments in thyroid carcinoma lines show that RA treatment affects thyroid specific functions (type I 5'-deiodinase, sodium/iodide-symporter), cell-cell or cell-matrix interaction (intercellular adhesion molecule-1, E-cadherin), differentiation markers (alkaline phosphatase, CD97), growth, and tumorigenicity. The observed changes, which involve multiple parameters that characterize a mature, functional thyrocyte, may be interpreted as partial redifferentiation. In clinical pilot studies, about 40% of the patients responded to RA application with an increased radioiodide uptake. In an evaluation of 20 RA-treated patients with well-documented data sets, 8 exhibited a decrease (4) or stabilization (4) in tumor size and/or in serum thyroglobulin levels in addition to enhanced radioiodide transport. This indicates that these patients with a long history of unresponsiveness to other treatment may have experienced an actual therapeutic benefit. These data suggest that RA redifferentiation therapy, considering especially its comparatively mild side effects, may soon represent an alternative therapeutic approach to otherwise untreatable thyroid tumors.

Prediction of interaction partners for orphan nuclear receptors by prior-based protein sequence profiles

We present a prior-based profile method for the prediction of protein-protein interaction partners that is here applied to the nuclear receptor superfamily. In this method, the diagnostic features are locally encoded in the physicochemical properties of residues in the interaction surface that are conserved in all proteins belonging to the defining set. The procedure models the positional variation based on that observed in the defining set and a prior-based substitution matrix derived from over 20,000 highly conserved positions in a set of 147 functional protein families. The method clusters sets of nuclear receptors known to interact with retinoid X receptor or corepressor proteins with predictive sets of receptors in C. elegans and higher metazoans. The method effectively reduces the search space of all possible interactions and yields experimentally testable predictions. Applications of this novel approach extend to interaction prediction problems in general, particularly to those that are not amenable to analysis by the rigid-body approximation.

Glucocorticoid stimulation of corticotropin-releasing hormone gene expression requires a cyclic adenosine 3',5'-monophosphate regulatory element in human primary placental cytotrophoblast cells

Production of placental CRH, which is identical to the peptide synthesized and secreted in the hypothalamus, has been linked to human parturition. Glucocorticoids stimulate placental CRH secretion and messenger ribonucleic acid expression, in contrast to their inhibition of CRH synthesis in the hypothalamus. A positive feedforward loop involving glucocorticoid-CRH-ACTH-glucocorticoid is thought to drive the exponential increase in placental CRH leading to delivery. Tissue-specific effects of glucocorticoids on CRH expression are therefore of interest. Using human primary placental cells, we investigated the mechanism by which glucocorticoids stimulate placental CRH gene expression. Nuclear run-on transcription shows that in human placental cells glucocorticoids up-regulate transcription of human CRH (hCRH). Using transient transfection assays we demonstrate that dexamethasone up-regulates both basal and cAMP-stimulated hCRH promoter activity, correlating well with the increase in endogenous CRH peptide levels. Through mutagenesis and deletion analyses we show that dexamethasone stimulation of hCRH gene transcription requires a functional cAMP regulatory element (CRE); this CRE is adequate to confer dexamethasone stimulation upon a heterologous promoter, and electrophoretic mobility shift assay studies show that a placental nuclear protein specifically binds to the hCRH CRE.

A role for the Hsp40 Ydj1 in repression of basal steroid receptor activity in yeast

In addition to its roles in translocation of preproteins across membranes, Ydj1 facilitates the maturation of Hsp90 substrates, including mammalian steroid receptors, which activate transcription in yeast in a hormone-dependent manner. To better understand Ydj1's function, we have constructed and analyzed an array of Ydj1 mutants in vivo. Both the glucocorticoid receptor and the estrogen receptor exhibited elevated activity in the absence of hormone in all ydj1 mutant strains, indicating a strict requirement for Ydj1 activity in hormonal control. Glucocorticoid receptor containing a mutation in the carboxy-terminal transcriptional activation domain, AF-2, retained elevated basal activity, while mutation of the amino-terminal transactivation domain, AF-1, eliminated the elevated basal activity observed in ydj1 mutant strains. This result indicates that the source of activity is AF-1, which is normally repressed by the carboxy-terminal hormone binding domain in the absence of hormone. Chimeric proteins containing the hormone binding domain of the estrogen or glucocorticoid receptor fused to heterologous activation and DNA binding domains also exhibited elevated activity in the absence of hormone. Thus, Ydj1 mutants appear to increase basal receptor activity by altering the ability of the hormone binding domain of the receptor to repress nearby activation domains. We propose that Ydj1 functions to present steroid receptors to the Hsp90 pathway for folding and hormonal control. In the presence of Ydj1 mutants that fail to bind substrate efficiently, some receptor escapes the Hsp90 pathway, resulting in constitutive activity.

Expression of glucocorticoid receptor beta in lymphocytes of patients with glucocorticoid-resistant ulcerative colitis

BACKGROUND & AIMS

Recently, the glucocorticoid receptor beta (hGRbeta) was suggested to play a role as a dominant negative regulator for determining glucocorticoid response. The aim of this study was to clarify whether reverse-transcription polymerase chain reaction (RT-PCR) analysis of hGRbeta messenger RNA (mRNA) can predict the response to glucocorticoids in patients with ulcerative colitis.

METHODS

Total RNA obtained from peripheral blood mononuclear cells (PBMCs) of 23 patients with ulcerative colitis and 20 healthy volunteers was reverse transcribed; the resulting complementary DNA was amplified using specific primers for hGRalpha and hGRbeta. Protein expression of hGR in PBMCs was confirmed by immunoprecipitation-Western blot analysis.

RESULTS

The expression of hGRalpha mRNA (477 base pairs) was detected in all patients and all healthy volunteers. In contrast, a hGRbeta mRNA (366 base pairs) was detected in 1 (9.1%) of 11 glucocorticoid-sensitive patients, 10 (83.3%) of 12 glucocorticoid-resistant patients, and 2 (10%) of 20 healthy volunteers. The positive rate of hGRbeta mRNA in the resistant group was significantly higher than that in the sensitive group (P = 0.0019). The hGRbeta band could be detected by immunoprecipitation-Western blotting in hGRbeta mRNA-positive patients.

CONCLUSIONS

The results show that the expression of hGRbeta mRNA in PBMCs examined by RT-PCR may serve as a novel predictor of glucocorticoid response in ulcerative colitis.

Glucocorticoid receptors in the euryhaline teleost Anguilla anguilla

To determine the importance of glucocorticoids in the salt water adaptation of European yellow eel we have evaluated the concentration, affinity and physical properties of glucocorticoid receptors (GR) in gill from both sea water- (SW) and freshwater-adapted (FW) animals. Using ligand binding techniques we demonstrated that high affinity GR were present in both cytosolic and nuclear fractions obtained from whole gill. Isoelectric focusing (IEF) of branchial GR indicated the presence of two distinct species, with pI values of 6.1 and 6.7. The form at pI 6.7 sedimented with a Svedberg constant of 4S on glycerol density gradients while the pI 6. 1 sedimented in fractions corresponding to 9S. Treatment of the pI 6. 1 form with urea (4 M) resulted in generation of the form with pI 6. 7. The evidence thus suggested that the oligomeric urea-sensitive form (pI 6.1) contained a form of GR which, as a monomer, focused at pI 6.7. IEF revealed the same concentrations of the pI 6.7 form in both SW and FW. However, there was significantly more (3-fold) pI 6. 1 isoform in FW than in SW, and this form decreased gradually during the course of seawater transfer. A transient increase of the nuclear-bound GR was also observed during SW adaptation. The balance between these forms could represent a dynamic parameter with important implications regarding GR function and gill responses to cortisol in salt water adaptation in teleosts.

Ligand structure-dependent differences in activation of estrogen receptor alpha in human HepG2 liver and U2 osteogenic cancer cell lines

Differences in ligand-activation of estrogen receptor alpha (ER(alpha)) were investigated in human HepG2 liver carcinoma and U2 osteogenic sarcoma cells transfected with wild-type ER (ER-wt) and variants expressing only activation function 1 (ERAF1) or AF2 (ER-AF2). The estrogen-responsive C3-luc construct containing the complement C3 gene promoter linked to a bacterial luciferase reporter gene was used to determine ligand-induced wild-type or variant ER activation. The quality pattern of ER-dependent responses was similar in both cell lines for a series of weakly estrogenic hydroxy and dihydroxyaromatic compounds including p-octylphenol, p-nonylphenol, 2',4',6'-trichloro-4-biphenylol, 2',3',4', 5'-tetrachloro-4-biphenylol, bisphenol A and 2, 2'-bis(p-hydroxyphenyl)-1,1,1-trichloroethane. However, some significant quantitative differences in these compounds were also observed. The weakly estrogenic pesticide, kepone, and the phytoestrogens, resveratrol (a trihydroxystilbene) and naringen (a flavanone), induced distinctly different patterns of responses; induction by these compounds was not observed in either cell line cotransfected with ER-wt or ER-AF2. In contrast, naringen activated ER-AF1 in HepG2 cells and resveratrol activated ER-AF1 in U2 cells. In HepG2 cells cotreated with E2 plus the estrogenic compounds, only BPA and resveratrol exhibited ER(alpha) antagonist activity. Structure-dependent differences in ER(alpha) activation and inhibition are consistent with the increasingly complex patterns of ER action in various tissues and indicate that the estrogenic activity of an individual compound can only be determined by using an extensive testing protocol.

The retinoblastoma-interacting zinc-finger protein RIZ is a downstream effector of estrogen action

Co-immunoprecipitation experiments in cell extract from cultured cells or target tissues indicated that estrogen receptor was complexed with the retinoblastoma binding protein RIZ in a ligand-dependent manner. Mapping of interaction sites indicated that in both proteins the same regions and motifs responsible for the interaction of transcriptional co-activator and nuclear receptors were involved. In cultured cells, estradiol induced a redistribution of RIZ protein within the nucleus and in the cytoplasm. A similar effect was produced in vivo, in prepuberal rat endometrium, by administration of a physiological dose of estradiol. Therefore, RIZ protein could be a specific effector of estrogen action downstream of the hormone-receptor interaction, presumably involved in proliferation control.

Conformational changes in the human estrogen receptor observed by (19)F NMR

The (19)F NMR spectra of the 5F-Trp labeled glutathione-S-transferase fusion protein with residues 282-595 of the human estrogen receptor show that there is a distinct conformational change in the protein when estradiol is added to the unliganded protein. Our studies show the empty receptor to have more conformational flexibility than the liganded form. This study shows the applicability of (19)F NMR to study conformational change in large protein systems.

Estrogen receptor alpha gene polymorphisms and endometrial cancer risk

Since the estrogen receptor alpha (ER) is an important mediator of hormonal responses such as proliferation in estrogen-sensitive tissues, we hypothesized that polymorphisms in the ER gene could be functional and associated with endometrial cancer risk. We performed a population-based case-control study in Sweden, focusing on restriction fragment length polymorphisms for XbaI and PvuII and an upstream TA repeat polymorphism. In the main analysis, 154 cases and 205 controls who never used hormone replacement therapy took part and we calculated age-adjusted and multivariate odds ratios (OR) and 95% confidence intervals (CI) using unconditional logistic regression. The XbaI X allele appeared to confer a reduced risk for endometrial cancer. The multivariate OR for the XX genotype was 0.52 (95% CI 0.21-1.29) compared to the xx genotype and there were suggestions of decreasing risk with increasing number of X alleles (P for trend = 0.07). The PvuII PP genotype was also associated with a non-significantly decreased risk for endometrial cancer (multivariate OR 0.70, 95% CI 0.34-1.44) compared with the pp genotype (P for trend = 0.43). The multivariate OR for two short TA (<19 repeats) alleles versus two long alleles was 1.54 (95% CI 0. 73-3.27) and there were suggestions of increasing risk with increasing number of short alleles (P for trend = 0.26). We observed the same pattern of results in an expanded group of subjects, which included women who had used hormone replacement (in total 288 cases and 392 controls). Our data suggest that variants of the ER gene may be associated with an altered risk of endometrial cancer.

Expression of the glucocorticoid receptor in early and late passage C-6 glioma cells and in normal astrocytes derived from aged mouse cerebral hemispheres

The presence of the glucocorticoid receptor in early and late passage C-6 glioma cells 2B clone and in astrocytes derived from aged mouse cerebral hemispheres has been documented by immunoblotting and/or immunofluorescence labelling. All cell types studied express the glucocorticoid receptor of molecular weight 97 KDa. In addition, in astrocytes derived from aged mouse cerebral hemispheres a smaller molecular weight polypeptide reacting with anti-glucocorticoid receptor antibody was also demonstrated. No difference in the amount of the 97 KDa glucocorticoid receptor between early and late C-6 2B cells was observed, whereas the astrocytes from aged cerebral hemispheres contained considerably reduced amounts of the glucocorticoid receptor compared to C-6 2B cells. Late passage C-6 2B cells were immunofluorescence labelled with the anti-glucocorticoid antibody, the receptor being almost exclusively present in the cytoplasm, with particular concentration in the perinuclear region. The presence of glucocorticoid receptor of molecular weight 97 KDa in glial cells corroborates and expands the existing data based on radioligand binding and immunocytochemical studies. These cell populations can be exploited as a model system for the study of the effects of glucocorticoids on senescence and brain aging.

Subtle shifts in the ratio between pro- and antiapoptotic molecules after activation of corticosteroid receptors decide neuronal fate

Glucocorticoid receptor (GR) activation induces apoptosis of granule cells in the hippocampus. In contrast, neuroprotection is seen after mineralocorticoid receptor (MR) activation. To date there is no in vivo evidence for direct interactions between corticosteroids and any of the key regulatory molecules of programmed cell death. In this report, we show that the opposing actions of MR and GR on neuronal survival result from their ability to differentially influence the expression of members of the bcl-2 gene family; specifically, in the rat hippocampus, activation of GR induces cell death by increasing the ratio of the proapoptotic molecule Bax relative to the antiapoptotic molecules Bcl-2 or Bcl-x(L); the opposite effect is observed after stimulation of MR. The same results were obtained in both young and aged animals; however, older subjects (which were more susceptible to GR-mediated apoptosis) tended to express the antiapoptotic genes more robustly. Using a loss-of-function mouse model, we corroborated the observations made in the rat, demonstrating Bax to be essential in the GR-mediated cell death-signaling cascade. In addition, we show that GR activation increases and MR activation decreases levels of the tumor suppressor protein p53 (a direct transcriptional regulator of bax and bcl-2 genes), thus providing new information on the early genetic events linking corticosteroid receptors with apoptosis in the nervous system.

Regulation of the trans-activation potential of STAT5 through its DNA-binding activity and interactions with heterologous transcription factors

Extracellular hormones, growth factors and cytokines relay their effects on the transcription of genes through the recognition of specific receptors and intracellular signalling molecules. Signal transducers and activators of transcription (STATs) have been recognized as crucial intracellular signalling molecules. The cytokine receptor-associated Janus kinases (JAKs) convert the latent monomeric form of the STAT molecules to the activated dimeric form through tyrosine phosphorylation. The dimers bind to specific DNA response elements and are able to induce transcription. This induction requires the full-length form of the STAT molecules. Negative regulatory potential is exerted by the short form of the molecule, which lacks the trans-activation domain. This short form is activated and dimerized, but dephosphorylation is impaired. The short form of STAT occupies the DNA-binding sites in a stable fashion and acts as a strong suppressor of wild-type action. Positive enhancement of STAT5 trans-activation potential is provided by the glucocorticoid receptor. Ligand activation of this receptor causes the formation of a complex with STAT5 and deviation to the STAT5 DNA-binding site. An additional regulatory loop is provided by the reactivation of the short form of STAT5 through glucocorticoid receptor association. Conversely, classical glucocorticoid-responsive genes are negatively affected by STAT5 activation.

A tissue-specific coactivator of steroid receptors, identified in a functional genetic screen

Steroid receptors mediate responses to lipophilic hormones in a tissue- and ligand-specific manner. To identify nonreceptor proteins that confer specificity or regulate steroid signaling, we screened a human cDNA library in a steroid-responsive yeast strain. One of the identified cDNAs, isolated in the screen as ligand effect modulator 6, showed no homology to yeast or Caenorhabditis elegans proteins but high similarity to the recently described mouse coactivator PGC-1 and was accordingly termed hPGC-1. The hPGC-1 DNA encodes a nuclear protein that is expressed in a tissue-specific manner and carries novel motifs for transcriptional regulators. The expression of hPGC-1 in mammalian cells enhanced potently the transcriptional response to several steroids in a receptor-specific manner. hPGC-1-mediated enhancement required the receptor hormone-binding domain and was dependent on agonist ligands. Functional analysis of hPGC-1 revealed two domains that interact with steroid receptors in a hormone-dependent manner, a potent transcriptional activation function, and a putative dimerization domain. Our findings suggest a regulatory function for hPGC-1 as a tissue-specific coactivator for a subset of nuclear receptors.

5'-heterogeneity of glucocorticoid receptor messenger RNA is tissue specific: differential regulation of variant transcripts by early-life events

Glucocorticoid receptor (GR) gene expression is regulated in a complex tissue-specific manner, notably by early-life environmental events that program tissue GR levels. We have identified and characterized several new rat GR mRNAs. All encode a common protein, but differ in their 5'-leader sequences as a consequence of alternate splicing of, potentially, 11 different exon 1 sequences. Most are located in a 3-kb CpG island, upstream of exon 2, that exhibits substantial promoter activity in transfected cells. Ribonuclease (RNase) protection analysis demonstrated significant levels of six alternate exons 1 in vivo in rat, with differences between liver, hippocampus, and thymus reflecting tissue-specific differences in promoter activity. Two of the alternate exons 1 (exons 1(6) and 1(10)) were expressed in all tissues examined, together present in 77-87% of total GR mRNA. The remaining GR transcripts contained tissue-specific alternate first exons. Importantly, tissue-specific first exon usage was altered by perinatal environmental manipulations. Postnatal handling, which permanently increases GR in the hippocampus, causing attenuation of stress responses, selectively elevated GR mRNA containing the hippocampus-specific exon 1(7). Prenatal glucocorticoid exposure, which increases hepatic GR expression and produces adult hyperglycemia, decreased the proportion of hepatic GR mRNA containing the predominant exon 1(10), suggesting an increase in a minor exon 1 variant. Such tissue specificity of promoter usage allows differential GR regulation and programming.

Molecular chaperones activate the Drosophila ecdysone receptor, an RXR heterodimer

The steroid hormone 20-hydroxyecdysone coordinates the stages of Drosophila development by activating a nuclear receptor heterodimer consisting of the ecdysone receptor, EcR, and the Drosophila RXR receptor, USP. We show that EcR/USP DNA binding activity requires activation by a chaperone heterocomplex like that required for activation of the vertebrate steroid receptors, but not previously shown to be required for activation of RXR heterodimers. Six proteins normally present in the chaperone complex were individually purified and shown to be sufficient for this activation. We also show that two of the six (Hsp90 and Hsc70) are required in vivo for ecdysone receptor activity, and that EcR is the primary target of the chaperone complex.

The retinoblastoma-interacting zinc-finger protein RIZ is a downstream effector of estrogen action

Co-immunoprecipitation experiments in cell extract from cultured cells or target tissues indicated that estrogen receptor was complexed with the retinoblastoma binding protein RIZ in a ligand-dependent manner. Mapping of interaction sites indicated that in both proteins the same regions and motifs responsible for the interaction of transcriptional co-activator and nuclear receptors were involved. In cultured cells, estradiol induced a redistribution of RIZ protein within the nucleus and in the cytoplasm. A similar effect was produced in vivo, in prepuberal rat endometrium, by administration of a physiological dose of estradiol. Therefore, RIZ protein could be a specific effector of estrogen action downstream of the hormone-receptor interaction, presumably involved in proliferation control.

Peroxisome proliferator-activated receptors: insight into multiple cellular functions

Peroxisome proliferator-activated receptors, PPARs, (NR1C) are nuclear hormone receptors implicated in energy homeostasis. Upon activation, these ligand-inducible transcription factors stimulate gene expression by binding to the promoter of target genes. The different structural domains of PPARs are presented in terms of activation mechanisms, namely ligand binding, phosphorylation, and cofactor interaction. The specificity of ligands, such as fatty acids, eicosanoids, fibrates and thiazolidinediones (TZD), is described for each of the three PPAR isotypes, alpha (NR1C1), beta (NR1C2) and gamma (NR1C3), so as the differential tissue distribution of these isotypes. Finally, general and specific functions of the PPAR isotypes are discussed, namely their implication in the control of inflammatory responses, cell proliferation and differentiation, the roles of PPARalpha in fatty acid catabolism and of PPARgamma in adipogenesis.

Truncated estrogen receptor product-1 suppresses estrogen receptor transactivation by dimerization with estrogen receptors alpha and beta

The estrogen receptor (ER) is a ligand-activated transcription factor that acts as a homodimer. Truncated estrogen receptor product-1 (TERP-1) is a pituitary-specific, estrogen-induced, isoform of rat ERalpha that is transcribed from a unique start site and contains only the C-terminal region of the full-length receptor. TERP-1 does not affect transcription directly but suppresses ligand-activated ERalpha and ERbeta activity. Because TERP-1 contains a dimerization domain and part of the coactivator binding pocket, we hypothesized that it modulates ER function by direct interactions with full-length ER or the steroid receptor coactivator, SRC-1. Localization studies demonstrate that TERP-1 is present in the cytoplasm and nucleus of transfected cells and colocalizes with nuclear ER. Protein binding studies show that TERP-1 forms heterodimers with both ERalpha and ERbeta and inhibits ERalpha binding to its cognate DNA response element. TERP-1 also binds SRC-1, and increasing levels of SRC-1 decrease the TERP-1-ERalpha interactions, in agreement with the rescue of TERP-1-suppressed ERalpha transcriptional activity by SRC-1. Mutational analysis of TERP-1 and ERalpha in the activation helix and the AF-2 dimerization helix indicates that TERP-1 acts predominantly through dimerization with ERalpha. Therefore, TERP-1 suppression of ER transcription occurs primarily by formation of inactive heterodimers and secondarily by competition for coactivators.

Steroid/nuclear receptor coactivators

In higher eukaryotes, steroids/thyroid hormones and many lipophilic compounds regulate cellular physiology through binding to the steroid/nuclear receptor proteins. Steroid/nuclear receptors are ligand-dependent transcriptional activators that can stimulate gene expression. This transcriptional activation plays a pivotal role in hormone-regulated physiological and pharmacological responses. In recent years, several steroid/nuclear receptor cofactors have been identified and found to interact with the receptor and modulate its transcriptional activity. Among these cofactors, a family of three co-activators has been the focus of intense studies. Although gaps remain, progress has been made in understanding how a given co-activator interacts with the receptor and promotes transcriptional activation. We are beginning to understand co-activator action; for instance, several studies have established the molecular basis of antagonism by anti-hormones and the connection of co-activators with human cancers.

Glucocorticoid receptor alpha and beta isoforms are not mutated in bipolar affective disorder

The periodically hyperactive hypothalamic-pituitary-adrenal (HPA) axis in bipolar affective disorders, as well as the reported changes in the binding characteristics of the glucocorticoid receptor (GR), suggest the possible involvement of the GR in the aetiopathology of this disease. This was investigated by screening the coding sequences of both GR isoforms, GRalpha and GRbeta, for the presence of mutations. As a genetic predisposition has been implicated, we included in this study bipolar patients who were siblings. By RT-PCR of peripheral blood mononuclear cells from patients suffering from bipolar illness, using primers spanning the whole length of the GRalpha and GRbeta coding region and subsequent agarose gel electrophoresis, heteroduplex and sequence analyses, no GR mutations could be detected. Since glucocorticoid receptor activity can be modulated by agents other than the respective ligand (eg by growth factors, cytokines and stress signals), our results favor derangements in the modulation of GR activity by such agents and not in the primary structure of the receptor as aetiopathologic factors of bipolar disease.

Human ERRgamma, a third member of the estrogen receptor-related receptor (ERR) subfamily of orphan nuclear receptors: tissue-specific isoforms are expressed during development and in the adult

The nuclear receptor protein superfamily is a large group of transcription factors involved in many aspects of animal development, tissue differentiation, and homeostasis in the higher eukaryotes. A subfamily of receptors, ERRalpha and beta (estrogen receptor-related receptor alpha and beta), closely related to the ER, were among the first orphan nuclear receptors identified. These receptors can bind DNA as monomers and are thought to activate transcription constitutively, unaffected by beta-estradiol. Studies of the expression patterns of ERRalpha and gene disruption experiments of ERRbeta indicate that they play an important role in the development and differentiation of specific tissues in the mouse. In this work we demonstrate the existence in humans of a third member of this subfamily of receptors, termed ERRgamma, which is highly expressed in a number of diverse fetal and adult tissues including brain, kidney, pancreas, and placenta. The ERRgamma mRNA is highly alternatively spliced at the 5'-end, giving rise to a number of tissue-specific RNA species, some of which code for protein isoforms differing in the N-terminal region. Like ERRalpha and beta, ERRgamma binds as a monomer to an ERRE. A GAL4-ERRgamma fusion protein activates transcription in a ligand-independent manner in transfected HEK293 cells to a greater degree than either the GAL4-ERRalpha or -beta fusion proteins.

Specific inhibition of glucocorticoid-induced thymocyte apoptosis by substance P

Glucocorticoids (GC) are strong inducers of thymocyte apoptosis. In the present study we looked into the possibility that the neuropeptide substance P (SP) might serve as an antagonist to GC-induced apoptosis. Indeed, SP inhibited hydrocortisone (HC)-induced apoptosis of CD4+CD8+ thymocytes in mice, both in vivo and in vitro. It also inhibited HC-induced apoptosis in the T cell hybridoma line 2B4.11, which is sensitive to GC. The inhibitory effect was complete if SP was given with HC or within 1 h after it; partial inhibitory effect could be seen at 2 h and no effect at 3 h. The presence of the SP antagonist nullified SP effect. The effect was specific to both components of the system (i.e., HC as apoptosis inducer and SP as its inhibitor), as judged from comparison to three other apoptosis-inducing means (irradiation, thymic epithelial cells, or retinoic acid), and to two other neuropeptides (somatostatin and vasoactive intestinal peptide). SP/HC antagonism was further demonstrated in two relevant molecular events: 1) HC augmented GC receptor production in our cell system and this was inhibited by SP; and 2) HC reduced the expression of the transcription factor NF-kappaB, SP increased it and when both were present, SP effect dominated. On the other hand, the level of IkappaB (NF-kappaB inhibitory molecule) was decreased by SP, preserved at a relatively high level with HC, and when both SP and HC were present, SP effect dominated. The intensity of SP effect, both in vivo and in vitro, its specificity, its inhibition by SP antagonist, as well as the previously documented presence of SP and its receptor in the thymus suggest that SP might be a physiological antagonist of the potent thymocyte apoptosis induced by GC.

E1A-mediated repression of progesterone receptor-dependent transactivation involves inhibition of the assembly of a multisubunit coactivation complex

The steroid hormone progesterone acts via high-affinity nuclear receptors that interact with specific DNA sequences located near the promoter of the hormone-responsive gene. Recent studies suggested that the hormone-occupied progesterone receptor (PR) mediates gene activation by recruiting a cellular coregulatory factor, termed coactivator, to the target promoter. The identity and mechanism of action of the coactivator(s) that regulates transcriptional activity of PR are currently under investigation. Here we provide evidence that the hormone-occupied PR forms a multisubunit receptor-coactivator complex containing two previously described coactivators, CREB-binding protein (CBP) and steroid receptor coactivator 1 (SRC-1, a member of the p160 family of coactivators), in nuclear extracts of human breast tumor T47D cells. The association of CBP and SRC-1/p160 with the receptor complex is entirely hormone dependent. Both CBP and SRC-1/p160 possess intrinsic histone acetyltransferase (HAT) activity, and it has been recently proposed that these coactivators function by modulating chromatin structure at the promoter of the target gene. Interestingly, addition of purified CBP to the nuclear extracts of T47D cells markedly stimulated progesterone- and PR-dependent transcription from a nucleosome-free, progesterone response element (PRE)-linked reporter DNA template. Furthermore, depletion of SRC-1/p160 by immunoprecipitation from these transcriptional extracts also significantly impaired PR-mediated RNA synthesis from a naked PRE-linked DNA template. These results strongly implied that CBP and SRC-1/p160 facilitate receptor-mediated transcription in these cell extracts through mechanisms other than chromatin remodeling. We also observed that the adenoviral oncoprotein E1A, which interacts directly with CBP, repressed PR-mediated transactivation when added to the nuclear extracts of T47D cells. Supplementation with purified CBP overcame this inhibition, indicating that the inhibitory effect of E1A is indeed due to a blockade of CBP function. Most importantly, we noted that binding of E1A to CBP prevented the assembly of a coactivation complex containing PR, CBP, and SRC-1/p160, presumably by disrupting the interaction between CBP and SRC-1/p160. These results strongly suggested that E1A repressed receptor-mediated transcription by blocking the formation or recruitment of coactivation complexes. Collectively, our results support the hypothesis that the assembly of a multisubunit coactivation complex containing PR, CBP, and SRC-1/p160 is a critical regulatory step during hormone-dependent gene activation by PR and that the fully assembled complex has the ability to control transcription through mechanisms that are independent of the histone-modifying activities of its component coactivators.

Mouse Zac1, a transcriptional coactivator and repressor for nuclear receptors

Transcriptional activation by nuclear hormone receptors is mediated by the 160-kDa family of nuclear receptor coactivators. These coactivators associate with DNA-bound nuclear receptors and transmit activating signals to the transcription machinery through two activation domains. In screening for mammalian proteins that bind the C-terminal activation domain of the nuclear receptor coactivator GRIP1, we identified a new variant of mouse Zac1 which we call mZac1b. Zac1 was previously discovered as a putative transcriptional activator involved in regulation of apoptosis and the cell cycle. In yeast two-hybrid assays and in vitro, mZac1b bound to GRIP1, to CREB-binding protein (CBP) and p300 (which are coactivators for nuclear receptors and other transcriptional activators), and to nuclear receptors themselves in a hormone-independent manner. In transient-transfection assays mZac1b exhibited a transcriptional activation activity when fused with the Gal4 DNA binding domain, and it enhanced transcriptional activation by the Gal4 DNA binding domain fused to GRIP1 or CBP fragments. More importantly, mZac1b was a powerful coactivator for the hormone-dependent activity of nuclear receptors, including androgen, estrogen, glucocorticoid, and thyroid hormone receptors. However, with some reporter genes and in some cell lines mZac1b acted as a repressor rather than a coactivator of nuclear receptor activity. Thus, mZac1b can interact with nuclear receptors and their coactivators and play both positive and negative roles in regulating nuclear receptor function.

Cloning of a mouse glucocorticoid modulatory element binding protein, a new member of the KDWK family

A mouse cDNA that encodes a nuclear DNA binding protein was identified by yeast two-hybrid screening using the activation domain 2 of the nuclear receptor coactivator TIF2 as a bait. BLAST analysis revealed that the identified cDNA encodes a KDWK domain and contains sequences almost identical to three tryptic peptides of rat GMEB-1 which together with the GMEB-2 heterodimeric partner binds to the GME/CRE sequence (glucocorticoid modulatory element) of the tyrosine aminotransferase (TAT) promoter. Mouse GMEB-1 is ubiquitously expressed in all the tissues examined. In vitro translated mGMEB-1 bound specifically to GME oligonucleotides, either alone or as a heterodimer with rGMEB-2. Transient transfection experiments with TAT promoter reporter genes suggest a potential role for mGMEB-1 as a transcriptional regulator of the TAT promoter.

Expression of the early-late gene encoding the nuclear receptor HR3 suggests its involvement in regulating the vitellogenic response to ecdysone in the adult mosquito

The insect steroid hormone, 20-hydroxyecdysone (20E), is a key factor controlling critical developmental events of embryogenesis, larval molting, metamorphosis, and, in some insects, reproduction. We are interested in understanding the molecular basis of the steroid hormone ecdysone action in insect egg development. The yellow fever mosquito, Aedes aegypti, in addition to being an important vector of human diseases, represents an outstanding model for studying molecular mechanisms underlying egg maturation due to stringently controlled, blood meal-activated reproductive events in this insect. To elucidate the genetic regulatory hierarchy controlling the reproductive ecdysone response, we have investigated ecdysone-regulated gene expression in vitellogenic mosquito ovaries and fat bodies. We have previously demonstrated the conservation of a primary ecdysone-triggered regulatory hierarchy, implicated in development of immature stages of Drosophila, represented by the ecdysone receptor/Ultraspiracle complex and an early gene E75 during the reproductive ecdysone response (Wang, S.-F., Miura, K., Miksicek, R.J., Segraves, W.A., Raikhel, A.S., 1998. DNA binding and transactivation characteristics of the mosquito ecdysone receptor - Ultraspiracle complex. J. Biol. Chem. 273, 27531-27540; Pierceall, W. E., Li, C., Biran, A., Miura, K., Raikhel, A.S., Segraves, W.A., 1999. E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction. Mol. Cell. Endocrinol. 150, 73-89). The present paper demonstrates that conservation of the factors involved in the ecdysone-responsive genetic hierarchy regulating female reproduction extends beyond the early genes. Here, we identify AHR3, a highly conserved homologue of the Drosophila HR3 early-late ecdysone-inducible gene in the mosquito. We show that AHR3 is expressed in both vitellogenic tissues of the female mosquito, the fat body and the ovary. The expression of AHR3 correlates with the ecdysteroid titer, reaching a peak at 24 h after a blood meal. Moreover, in vitro fat body culture experiments demonstrate that the kinetics and dose response of AHR3 to 20-hydroxyecdysone (20E), an active ecdysteroid in the mosquito, is similar to those of the late vitellogenic genes rather than the early E75 gene. However, as shown for other early and early-late genes, the 20E activation of AHR3 is not inhibited by the presence of cycloheximide, a protein synthesis inhibitor. Taken together, these findings strongly suggest AHR3 involvement in regulating the vitellogenic response to ecdysone in the adult mosquito.

Effect of steroid on hyperoxia-induced ICAM-1 expression in pulmonary endothelial cells

Intercellular adhesion molecule-1 (ICAM-1) of the vascular endothelium plays a key role in the development of pulmonary oxygen toxicity. We studied the effect of steroid on hyperoxia-induced ICAM-1 expression using cultured endothelial cells in vitro. Human pulmonary artery endothelial cells (HPAECs) were cultured to confluence, and then the monolayers were exposed to either control (21% O(2)-5% CO(2)) or hyperoxic (90% O(2)-5% CO(2)) conditions with and without a synthetic glucocorticoid, methylprednisolone (MP). MP reduced hyperoxia-induced ICAM-1 and ICAM-1 mRNA expression in a dose-dependent manner. Neutrophil adhesion to hyperoxia-exposed endothelial cells was also inhibited by MP treatment. In addition, MP attenuated hyperoxia-induced H(2)O(2) production in HPAECs as assessed by flow cytometry. An electrophoretic mobility shift assay demonstrated that hyperoxia activated nuclear factor-kappaB (NF-kappaB) but not activator protein-1 (AP-1) and that MP attenuated hyperoxia-induced NF-kappaB activation dose dependently. With Western immunoblot analysis, IkappaB-alpha expression was decreased by hyperoxia and increased by MP treatment. These results suggest that MP downregulates hyperoxia-induced ICAM-1 expression by inhibiting NF-kappaB activation via increased IkappaB-alpha expression.

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

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

Investigating the function of follicular subpopulations during Drosophila oogenesis through hormone-dependent enhancer-targeted cell ablation

Although it is known that the establishment of polarity during Drosophila oogenesis is initiated by signalling from the oocyte to the overlying follicle cells, much less is understood about the role of specific follicular subpopulations. One powerful approach for addressing this question, toxigenic cell ablation of specific subpopulations, has not previously been applicable to studying follicular subpopulations because many of the genes and Gal4 enhancer trap insertions that are expressed in the ovary are also expressed at earlier times in development. To overcome this problem, we have utilized a fusion protein between Gal4 and the human estrogen receptor to achieve hormone-dependent, tissue-specific gene expression of UAS-linked transgenes in flies. We used this system to study the role of the polar subpopulations of follicle cells during oogenesis by expressing within them a modified form of diphtheria toxin that causes cell death. Our results confirmed previous functions ascribed to these cells, and also demonstrated a previously undescribed role for the border cells in facilitating the migration of the anterior Fasciclin III-expressing polar pair cells to the edge of the oocyte.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Resistance to thyroid hormone (RTH) syndrome reveals novel determinants regulating interaction of T3 receptor with corepressor

Thyroid hormone receptors (T3Rs) both repress and activate gene transcription by interacting with auxiliary factors denoted corepressors and coactivators. Resistance to thyroid hormone (RTH) syndrome in humans is manifested as a failure to respond properly to elevated circulating thyroid hormone. RTH syndrome has been mapped to T3Rbeta mutations that alter the transcriptional properties of the receptor, resulting in a dominant negative phenotype. We report here a characterization of a series of RTH mutant T3Rs that exhibit unusual interactions with corepressor. Two mutations in receptor helix 11 (delta430, delta432) greatly enhance the ability of the mutant receptors to bind to corepressor. A distinct mutation, V264D, in an 'omega loop' region of the receptor, impairs corepressor release but does not fully eliminate the ability to recruit coactivator. These mutations reveal novel determinants that regulate the interaction of the T3R with important ancillary cofactors, and that are disrupted in a human endocrine disease.

Coregulator small nuclear RING finger protein (SNURF) enhances Sp1- and steroid receptor-mediated transcription by different mechanisms

The small nuclear RING finger protein SNURF is not only a coactivator in steroid receptor-dependent transcription but also activates transcription from steroid-independent promoters. In this work, we show that SNURF, via the RING finger domain, enhances protein binding to Sp1 elements/GC boxes and interacts and cooperates with Sp1 in transcriptional activation. The activation of androgen receptor (AR) function requires regions other than the RING finger of SNURF, and SNURF does not influence binding of AR to cognate DNA elements. The zinc finger region (ZFR) together with the hinge region of AR are sufficient for contacting SNURF. The nuclear localization signal in the boundary between ZFR and the hinge region participates in the association of AR with SNURF, and a receptor mutant lacking the C-terminal part of the bipartite nuclear localization signal shows attenuated response to coexpressed SNURF. Some AR ZFR point mutations observed in patients with partial androgen insensitivity syndrome or male breast cancer impair the interaction of AR with SNURF and also render AR refractory to the transcription-activating effect of SNURF. Collectively, SNURF modulates the transcriptional activities of androgen receptor and Sp1 via different domains, and it may act as a functional link between steroid- and Sp1-regulated transcription.

Differential interaction of steroid hormone receptors with LXXLL motifs in SRC-1a depends on residues flanking the motif

Steroid hormones induce the transcriptional activity of their cognate receptors by recruiting a variety of cofactors. One of these, steroid receptor co-activator-1 (SRC-1) interacts with the ligand binding domains of a number of different receptors by means of LXXLL motifs. We have investigated the relative interaction of four such motifs in SRC-1a using a yeast two-hybrid assay. We demonstrate that ERalpha, ERbeta and ERbeta2 preferentially interact with motif 2 while GR, AR, PPARalpha and PPARgamma preferentially interact with motif 4. We show that the interactions depend not only on the LXXLL motif itself but also on residues flanking the motif.

Evaluation of apoptosis and the glucocorticoid receptor in the cartilage growth plate and metaphyseal bone cells of rats after high-dose treatment with corticosterone

A connection has been suggested between glucocorticoid-induced osteopenia and an increase in the apoptosis of bone cells, and between the dimerization of the glucocorticoid receptor (GR) and the development of apoptosis. On this basis, a study has been carried out on the relationships between the occurrence of apoptotic cells and their detectable GR content, and between apoptosis frequency and changes in histomorphometric variables, in the growth plate and secondary spongiosa of rat long bones after the high-dose (10 mg/day) administration of corticosterone (CORT) and after recovery. The main results of the CORT treatment were: a significant increase in apoptotic osteoblasts, and a concomitant decrease in the histomorphometric variables of bone formation, with a reversal of both values during recovery; a nonsignificant increase in the apoptosis of osteoclasts, without changes in the histomorphometric variables of bone resorption; a significant increase in apoptotic terminal hypertrophic chondrocytes; the presence of GR in all types of skeletal cells in control rats, with different (cytoplasmic and/or nuclear) immunohistochemical detection in the same type of cell; a decrease in GR detection in proliferative chondrocytes and osteocytes in CORT and recovery groups, and in the maturative/hypertrophic chondrocytes of the recovery group; a fall in growth cartilage width, possibly due to the reduced proliferation of proliferative chondrocytes and increased apoptosis in terminal hypertrophic chondrocytes. In conclusion, pharmacological doses of CORT reduce bone formation by increasing osteoblast apoptosis; they reduce growth cartilage width, probably by inhibiting chondrocyte proliferation and increasing the apoptosis of terminal hypertrophic chondrocytes, and they reduce osteocyte GR. Although these effects appear to be mediated by the presence of GR in all skeletal cells, no precise correlation between GR immunohistochemical detection and apoptosis induction has been found.

The DNA binding-independent function of the glucocorticoid receptor mediates repression of AP-1-dependent genes in skin

The glucocorticoid receptor (GR) mediates the biological effects of glucocorticoids (GCs) through activation or repression of gene expression, either by DNA binding or via interaction with other transcription factors, such as AP-1. Work in tissue culture cells on the regulation of AP-1-dependent genes, such as collagenase (MMP-13) and stromelysin (MMP-3) has suggested that the antitumor and antiinflammatory activity of GCs is mediated, at least in part, by GR-mediated downmodulation of AP-1. Here, we have identified phorbol ester-induced expression of MMP-3 and MMP-13 in mouse skin as the first example of an in vivo system to measure negative interference between AP-1 and GR in the animal. Cell type-specific induction of these genes by tumor promoters is abolished by GCs. Importantly, this is also the case in GR(dim) mice expressing a DNA binding-defective mutant version of GR. In contrast, the newly identified target genes in skin, plasma glutathione peroxidase and HSP-27, were induced by GC in wild-type, but not in GR(dim) mice. Thus, these data suggest that the DNA binding-independent function of the GR is dispensable for repression of AP-1 activity in vivo and responsible for the antitumor promoting activity of GCs.

Transcriptional anti-repression. Thyroid hormone receptor beta-2 recruits SMRT corepressor but interferes with subsequent assembly of a functional corepressor complex

Thyroid hormone receptors (T3Rs) are hormone-regulated transcription factors. Different T3R isoforms are expressed in a tissue-specific and developmentally regulated manner. The T3Ralpha-1, beta-0, and beta-1 isoforms typically repress target gene expression in the absence of hormone and activate transcription in the presence of hormone. Intriguingly, however, the T3Rbeta-2 isoform fails to repress, and instead is able to activate transcription in both the absence and presence of hormone. We investigated the molecular mechanism behind this absence of repression by T3Rbeta-2. Repression by T3Ralpha-1, beta-0, and beta-1 is mediated by the ability of these isoforms to physically recruit a SMRT/N-CoR corepressor complex. We determined that the unliganded T3Rbeta-2 also recruits the SMRT corepressor; in contrast to the alpha-1, beta-0, and beta-1 isoforms, however, the T3Rbeta-2 protein interacts not only with the C-terminal "receptor-interaction domain" of SMRT, but also makes additional contacts with the N-terminal "silencing domain" of the SMRT corepressor. These additional, T3Rbeta-2-specific contacts interfere with the subsequent association of SMRT with mSin3, a crucial second subunit of the corepressor holo-complex. Our results suggest that T3Rbeta-2 regulates transcription through a novel anti-repression mechanism, recruiting SMRT, but preventing the subsequent formation of a functional corepressor complex.

Activity of the Nurr1 carboxyl-terminal domain depends on cell type and integrity of the activation function 2

Nurr1, a member of the nuclear hormone receptor superfamily, was recently demonstrated to be of critical importance in the developing central nervous system, where it is required for the generation of midbrain dopamine cells. Nuclear receptors encompass a transcriptional activation function (activation function 2; AF2) within their carboxyl-terminal domains important for ligand-induced transcriptional activation. Since a Nurr1 ligand remains to be identified, the role of the Nurr1 AF2 region in transcriptional activation is unclear. However, here we show that the Nurr1 AF2 contributes to constitutive activation independent of exogenously added ligands in human embryo kidney 293 cells and in neural cell lines. Extensive mutagenesis indicated a crucial role of the AF2 core region for transactivation but also identified unique features differing from previously characterized receptors. In addition, Nurr1 did not appear to interact with, and was not stimulated by, several previously identified coactivators such as the steroid receptor coactivator 1. In contrast, adenovirus protein E1A, stably expressed in 293 cells, was shown to contribute to AF2-dependent activation. Finally, while the AF2 core of RXR is required for ligand-induced transcriptional activation by Nurr1-RXR heterodimers, the functional integrity of Nurr1 AF2 core is not critical. These results establish that the ligand binding domain of Nurr1 has intrinsic capacity for transcriptional activation depending on cell type and mode of DNA binding. Furthermore, these results are consistent with the possibility that gene expression in the central nervous system can be modulated by an as yet unidentified ligand interacting with the ligand binding domain of Nurr1.

Convergence of two repressors through heterodimer formation of androgen receptor and testicular orphan receptor-4: a unique signaling pathway in the steroid receptor superfamily

The androgen receptor (AR) binds to androgen response elements and regulates target genes via a mechanism involving coregulators. Here we demonstrate that the AR can interact with the testicular orphan receptor-4 (TR4) and function as a repressor to down-regulate the TR4 target genes by preventing the TR4 binding to its target DNA. Interestingly, the heterodimerization of AR and TR4 also allows TR4 to repress AR target gene expression. Simultaneous exposure to both receptors therefore could result in bidirectional suppression of their target genes. Together, these data demonstrate that the coupling of two different receptors, through the heterodimerization of AR and TR4, is a unique signaling pathway in the steroid receptor superfamily, which may facilitate further understanding of the complicated androgen action in prostate cancer or libido.

A selective group of dopaminergic neurons express Nurr1 in the adult mouse brain

Nurr1, an orphan receptor of the nuclear receptor superfamily, is widely expressed in the central nervous system (CNS) including brain regions where dopaminergic neurons are abundant. Recent analyses of Nurr1 null mutant mice have shown that Nurr1 is essential for the development and survival of midbrain dopaminergic neurons. However, other dopaminergic neuronal populations do not seem to be affected by ablation of the Nurr1 gene. The purpose of the present study was to investigate the degree of co-existence of Nurr1 mRNA and tyrosine hydroxylase (TH) immunoreactivity in the brain of adult mice to better characterize the selective effects of Nurr1 on catecholaminergic neurons. Our results indicate that the majority of TH-immunoreactive neurons in the substantia nigra (SN; 96%), ventral tegmental area (VTA; 95%), retrorubral field (91%), olfactory bulb (85%), linear nucleus raphe (91%) and central grey (61%) express Nurr1. In contrast, dopaminergic cells of the paraventricular and periventricular hypothalamic nucleus showed only a few Nurr1/TH double labeled neurons, while TH-immunoreactive neurons in the arcuate nucleus and zona incerta did not express Nurr1 mRNA. Nurr1 expression was also excluded from (nor)adrenergic neurons of the brainstem. In conclusion, Nurr1 transcripts were not found in all CNS catecholaminergic neurons. Nurr1 expression was confined to periglomerular and midbrain dopaminergic neurons. These results suggest that within the adult mouse brain, Nurr1 may participate in dopaminergic functions of the olfactory bulb and midbrain.

The nuclear-receptor interacting protein (RIP) 140 binds to the human glucocorticoid receptor and modulates hormone-dependent transactivation

The glucocorticoid receptor (GR) regulates target gene expression in response to corticosteroid hormones. We have investigated the mechanism of transcriptional activation by the GR by studying the role of the receptor interacting protein RIP140. Both in vivo and in vitro protein-protein interaction assays revealed a ligand-dependent interaction between the GR and RIP140. The ligand binding domain of the GR was sufficient for this interaction, while both the N- and C-terminal regions of RIP140 bound to the receptor. In a yeast transactivation assay RIP140 and SRC-1, a member of the steroid receptor coactivator family of proteins, both enhanced the transactivation activity of a GR protein (GRA-1) in which the potent N-terminal tau1 transactivation domain has been deleted. In contrast, in COS-7 cells increasing amounts of RIP140 significantly inhibited GRdeltatau1 function. In cotransfection studies in COS-7 cells, RIP140 also inhibited receptor activity in presence of both SRC-1 and the coactivator protein CBP together. Thus, in yeast cells a stimulation of receptor activity was observed, while in mammalian cells RIP140 repressed GR function. Taken together, these data suggest that, (1) RIP140 is a target protein for the GR and (2) RIP140 can modulate the transactivation activity of the receptor.

Molecular determinants of nuclear receptor-corepressor interaction

Retinoic acid and thyroid hormone receptors can act alternatively as ligand-independent repressors or ligand-dependent activators, based on an exchange of N-CoR or SMRT-containing corepressor complexes for coactivator complexes in response to ligands. We provide evidence that the molecular basis of N-CoR recruitment is similar to that of coactivator recruitment, involving cooperative binding of two helical interaction motifs within the N-CoR carboxyl terminus to both subunits of a RAR-RXR heterodimer. The N-CoR and SMRT nuclear receptor interaction motifs exhibit a consensus sequence of LXX I/H I XXX I/L, representing an extended helix compared to the coactivator LXXLL helix, which is able to interact with specific residues in the same receptor pocket required for coactivator binding. We propose a model in which discrimination of the different lengths of the coactivator and corepressor interaction helices by the nuclear receptor AF2 motif provides the molecular basis for the exchange of coactivators for corepressors, with ligand-dependent formation of the charge clamp that stabilizes LXXLL binding sterically inhibiting interaction of the extended corepressor helix.

Identification of a mineralocorticoid/glucocorticoid response element in the human Na/K ATPase alpha1 gene promoter

Sodium-potassium ATPase (Na/K ATPase) is a major target of mineralocorticoids. Both aldosterone and glucocorticoids activate the human Na/K ATPase alpha1 and beta1 genes transcriptionally. The mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) have been shown to bind the glucocorticoid response element (GRE); however, a specific element responsible for the activation of the MR is not known. Sequence analysis of the putative regulatory region of the Na/K ATPase alpha1 gene revealed the presence of a hormone response element that allows the MR to interact with it, at least as well as if not better than the GR. This response element is designated MRE/GRE. In this investigation, we demonstrated the MR and GR induced gene expression in COS-1 cells by cotransfecting with respective expression plasmids (RshMR and RshGR) along with a luciferase reporter. The synthetic MRE/GRE linked to a neutral promoter was activated by MR (6-fold); however, the GR induced a lower level of expression (3.8-fold), suggesting that the element may be preferably MR responsive. Mutations in the synthetic MRE/GRE could not induce the expression with MR, whereas GR had a small effect. Electrophoretic mobility shift analyses demonstrated a direct interaction of MR and GR with the MRE/GRE that was supershifted by an antiMR antibody and the complex was partially cleared by an antiGR antibody, respectively, whereas nonimmune serum had no effect. Footprinting analyses of the promoter region showed that a portion of the DNA containing this element is protected by recombinant MR and GR. Thus these data confirm that this MRE/GRE interacts with both MR and GR but interaction with receptors may be more MR-responsive than response elements previously described.

Effect of ligand and DNA binding on the interaction between human transcription intermediary factor 1alpha and estrogen receptors

Hormonal regulation of gene activity is mediated by nuclear receptors acting as ligand-activated transcription factors. To achieve efficient regulation of gene expression, these receptors must interact with different type of molecules: 1) the steroid hormone, 2) the DNA response element, and 3) various proteins acting as transcriptional cofactors. In the present study, we have investigated how ligand and DNA binding influence the in vitro interaction between estrogen receptors (ERs) and the transcription intermediary factor hTIF1alpha (human transcriptional intermediary factor 1alpha). We first optimized conditions for the coactivator-dependent receptor ligand assay to lower ED50, and we then analyzed the ability of various natural and synthetic estrogens to allow the binding of the two types of proteins. Results were compared with the respective affinities of these ligands for the receptor. We then developed a protein-protein-DNA assay allowing the quantification of cofactor-ER-estrogen response element (ERE) complex formation in the presence of ligand and used measurements of fluorescence anisotropy to define the equilibrium binding parameters of the interaction. We demonstrated that the leucine-charged domain of hTIF1alpha is sufficient to interact with ERE-bound ERalpha in a ligand-dependent manner and showed that binding of ERalpha onto DNA does not significantly affect its hormone-dependent association with TIF1alpha. Finally, we show that, mainly in the absence of hormone, hTIF1alpha interacts better with ERbeta than with ERalpha independently of the presence of ERE.

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

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

Increased induction of osteopetrosis, but unaltered lymphomagenicity, by murine leukemia virus SL3-3 after mutation of a nuclear factor 1 site in the enhancer

SL3-3 is a murine leukemia virus which is only weakly bone pathogenic but highly T-cell lymphomagenic. A major pathogenic determinant is the transcriptional enhancer comprising several transcription factor binding sites, among which are three identical sites for nuclear factor 1 (NF1). We have investigated the pathogenic properties of NF1 site enhancer mutants of SL3-3. Two different mutants carrying a 3-bp mutation either in all three NF1 sites or in the central site alone were constructed and assayed in inbred NMRI mice. The wild type and both mutants induced lymphomas in all mice, with a mean latency period of 9 weeks. However, there was a considerable difference in osteopetrosis induction. Wild-type SL3-3 induced osteopetrosis in 11% of the mice (2 of 19), and the triple NF1 site mutant induced osteopetrosis in none of the mice (0 of 19), whereas the single NF1 site mutant induced osteopetrosis in 56% (10 of 18) of the mice, as determined by X-ray analysis. A detailed histological examination of the femurs of the mice was carried out and found to support this diagnosis. Thus, the NF1 sites of SL3-3 are major determinants of osteopetrosis induction, without determining lymphomagenesis. This conclusion was further supported by evaluation of the bone pathogenicity of other SL3-3 enhancer variants, the lymphomagenicity of which had been examined previously. This evaluation furthermore strongly indicated that the core sites, a second group of transcription factor binding sites in the viral enhancer, are necessary for the osteopetrosis induction potential of SL3-3.