Characterization of the DNA-binding and dimerization properties of the nuclear orphan receptor germ cell nuclear factor

The orphan receptor germ cell nuclear factor (GCNF) is a member of the superfamily of nuclear receptors. During development, GCNF exhibits a restricted brain-specific expression pattern, whereas GCNF expression in the adult is germ cell specific. Therefore, the receptor may participate in the regulation of neurogenesis and reproductive functions. No natural GCNF target gene has yet been identified, but recent data demonstrate specific and high-affinity binding of GCNF either to the direct repeat DNA element AGGTCAAGGTCA (DR0) or to extended half-sites, such as TCAAGGTCA. In this study, we show that murine GCNF (mGCNF) can bind as a homodimer to extended half-sites, thus describing a novel property within the nuclear receptor superfamily. Homodimeric binding to extended half-sites requires the presence of a dimerization function within the mGCNF DNA-binding domain (DBD) and a novel dimerization surface encompassing the putative helix 3 and the helix 12 region of the mGCNF ligand-binding domain (LBD). In addition, the mGCNF LBD has the potential to adopt different conformations with distinct dimerization properties. The helix 12 region of the mGCNF LBD not only regulates the switch between these dimerization conformations but also dictates the DNA-binding behavior and transcriptional properties of the different dimerization conformations. In summary, our findings describe unique DNA-binding and dimerization properties of a nuclear receptor and suggest a novel mechanism that allows mGCNF to modulate target gene activity.

Synergism between a half-site and an imperfect estrogen-responsive element, and cooperation with COUP-TFI are required for estrogen receptor (ER) to achieve a maximal estrogen-stimulation of rainbow trout ER gene

In all oviparous, liver represents one of the main E2-target tissues where estrogen receptor (ER) constitutes the key mediator of estrogen action. The rainbow trout estrogen receptor (rtER) gene expression is markedly up-regulated by estrogens and the sequences responsible for this autoregulation have been located in a 0.2 kb upstream transcription start site within - 40/- 248 enhancer region. Absence of interference with steroid hormone receptors and tissue-specific factors and a conserved basal transcriptional machinery between yeast and higher eukaryotes, make yeast a simple assay system that will enable determination of important cis-acting regulatory sequences within rtER gene promoter and identification of transcription factors implicated in the regulation of this gene. Deletion analysis allowed to show a synergistic effect between an imperfect estrogen-responsive element (ERE) and a consensus half-ERE to achieve a high hormone-dependent transcriptional activation of the rtER gene promoter in the presence of stably expressed rtER. As in mammalian cells, here we observed a positive regulation of the rtER gene promoter by the chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) through enhancing autoregulation. Using a point mutation COUP-TFI mutant unable to bind DNA demonstrates that enhancement of rtER gene autoregulation requires the interaction of COUP-TFI to the DNA. Moreover, this enhancement of transcriptional activation by COUP-TFI requires specifically the AF-1 transactivation function of ER and can be observed in the presence of E2 or 4-hydroxytamoxifen but not ICI 164384. Thus, this paper describes the reconstitution of a hormone-responsive transcription unit in yeast in which the regulation of rtER gene promoter could be enhanced by the participation of cis-elements and/or trans-acting factors, such as ER itself or COUP-TF.

Regulation of the mdm2 oncogene by thyroid hormone receptor

The mdm2 gene is positively regulated by p53 through a p53-responsive DNA element in the first intron of the mdm2 gene. mdm2 binds p53, thereby abrogating the ability of p53 to activate the mdm2 gene, and thus forming an autoregulatory loop of mdm2 gene regulation. Although the mdm2 gene is thought to act as an oncogene by blocking the activity of p53, recent studies indicate that mdm2 can act independently of p53 and block the G1 cell cycle arrest mediated by members of the retinoblastoma gene family and can activate E2F1/DP1 and the cyclin A gene promoter. In addition, factors other than p53 have recently been shown to regulate the mdm2 gene. In this article, we report that thyroid hormone (T3) receptors (T3Rs), but not the closely related members of the nuclear thyroid hormone/retinoid receptor gene family (retinoic acid receptor, vitamin D receptor, peroxisome proliferation activation receptor, or retinoid X receptor), regulate mdm2 through the same intron sequences that are modulated by p53. Chicken ovalbumin upstream promoter transcription factor I, an orphan nuclear receptor which normally acts as a transcriptional repressor, also activates mdm2 through the same intron region of the mdm2 gene. Two T3R-responsive DNA elements were identified and further mapped to sequences within each of the p53 binding sites of the mdm2 intron. A 10-amino-acid sequence in the N-terminal region of T3Ralpha that is important for transactivation and interaction with TFIIB was also found to be important for activation of the mdm2 gene response element. T3 was found to stimulate the endogenous mdm2 gene in GH4C1 cells. These cells are known to express T3Rs, and T3 is known to stimulate replication of these cells via an effect in the G1 phase of the cell cycle. Our findings, which indicate that T3Rs can regulate the mdm2 gene independently of p53, provide an explanation for certain known effects of T3 and T3Rs on cell proliferation. In addition, these findings provide further evidence for p53-independent regulation of mdm2 which could lead to the development of tumors from cells that express low levels of p53 or that express p53 mutants defective in binding to and activating the mdm2 gene.

COUP-TFI expression in human adrenocortical adenomas: possible role in steroidogenesis

Chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) is an orphan nuclear receptor essential for neurogenesis, organogenesis, and cell fate determination. CYP17 gene transcription has recently been shown to be activated by SF-1 (steroidogenic factor-1) binding to a cyclic AMP-responsive sequence within the promoter region of the gene, and inhibited by COUP-TF binding to the sequence. Thus, COUP-TF and SF-1 act as a transcriptional repressor and activator, respectively, of CYP17 gene expression. Transcriptional repression by COUP-TFI is mediated by corepressors, N-CoR (nuclear receptor-corepressor) and SMRT (silencing mediator for retinoid and thyroid hormone-receptor), whereas transcriptional activation by SF-1 is mediated by coactivator SRC-1 (steroid receptor coactivator-1). We therefore examined the expression of COUP-TFI, SF-1, SRC-1, N-CoR, and SMRT in a variety of adrenocortical adenomas and compared the results with CYP17 mRNA levels. We found significantly high COUP-TFI mRNA expression in nonfunctional adenomas (n=8: 220+/-16%; normal 96+/-4%), a deoxycorticosterone-producing adenoma (n=1: 200%), and a pre-clinical Cushing's adenoma (n=1: 280%), intermediate COUP-TFI expression in cortisol-producing adenomas (n=8: 63+/-5%), and low COUP-TFI expression in aldosterone-producing adenomas (n=8: 49+/-4%). In contrast to COUP-TFI, SF-1 mRNA expression did not vary significantly among adrenals. We did not detect the expected negative correlation between COUP-TFI and CYP17 mRNA levels in adrenocortical adenomas. High COUP-TFI expression was associatedwith a nonfunctioning phenotype. Interestingly, the pattern of COUP-TFI expression was similar to the profile of N-CoR expression, but not of SMRT expression. These results indicate that COUP-TFI and N-CoR may play a role in steroidogenesis by human adrenocortical adenomas.

All-trans-retinoic acid upregulates the expression of COUP-TFI in early-somite mouse embryos cultured in vitro

Exposure of embryos to an excess of retinoic acid (RA) modifies the spatio-temporal pattern of expression of developmental genes. RA regulates the expression of target genes through binding of the retinoid nuclear receptors (RARs and RXRs), as heterodimers, to regulatory cis-acting elements. COUP-TF factors, which are able to dimerize with the RXRs and to compete with the retinoid receptors for their DNA binding sites, are suspected to modulate the retinoid signal transduction pathway. Therefore, COUP-TF factors may be involved in the regulation of the expression of developmental genes and/or in the modifications induced by an excess of RA in the expression of these genes. The aim of this work is to assess whether RA-induced modifications in the expression of Krox-20 and Hox genes correlate with alterations of the expression of COUP-TF genes. In addition to spatial modifications in the expression patterns of Krox-20 and Hox genes, we report here an upregulation of the expression level of COUP-TFI after RA exposure. However, this abnormality did not spatially overlap with the modifications observed in the expression of Krox-20 and Hox genes. These data suggest an involvement of COUP-TFI in the generation of RA-induced abnormalities, but do not support the hypothesis of an involvement of this factor in the regulation of the expression of Hox or Krox-20 genes.

Transcriptional regulation of mitochondrial HMG-CoA synthase in the control of ketogenesis

Mitochondrial and cytosolic HMG-CoA synthases are encoded by two different genes. Control of ketogenesis is exerted by transcriptional regulation of mitochondrial HMG-CoA synthase. Fasting, cAMP, and fatty acids increase its transcriptional rate, while refeeding and insulin repress it. Fatty acids increase transcription through peroxisomal proliferator regulatory element (PPRE), to which peroxisome proliferator activated receptor (PPAR) can bind. Other transcription factors such as chicken ovalbumin upstream promoter transcription factor (COUP-TF) and hepatocyte nuclear factor 4 (HNF-4) compete for the PPRE site, modulating the response of PPAR.

Stat 5b and the orphan nuclear receptors regulate expression of the alpha2-macroglobulin (alpha2M) gene in rat ovarian granulosa cells

Alpha2-macroglobulin (alpha2M) is a serine protease inhibitor and cytokine inactivator associated with inflammation and tissue remodeling. The gene encoding this protein is selectively induced in the rat corpus luteum by the luteotropic hormone and cytokine, PRL. The promoter of the alpha2M gene contains two regulatory regions that bind a diverse set of transcription factors and confer functional activity in ovarian granulosa-luteal cells. The PRL response element (PRLRE) binds PRL-activated (tyrosine-phosphorylated) signal transducers and activators of transcription (Stat 5b and Stat 5a). 5'-Deletion of the Stat-binding sites or mutation of either one or both of these sites within the context of the intact promoter abolished PRL inducibility of alpha2M promoter-reporter constructs in granulosa-luteal cells. Cotransfection with a vector expressing a dominant negative, truncated form of Stat 5b abolished PRL-induced activation of a2M transgenes. 5'-Deletion of the Stat-binding sites abolished all promoter-reporter activity in response to PRL. Internal deletion of a second functional domain 3' of the PRLRE also abolished PRL inducibility and markedly reduced basal activity, indicating that functional interactions between these two regions might occur. The 3'-region was shown to bind orphan members of the nuclear receptor superfamily, steroidogenic factor 1 (SF-1) and chicken ovalbumin upstream promoter-transcription factor (COUP-TF) and has been called the orphan receptor response element (ORRE). When site-specific mutations were made in either the SF-1 -binding site or the two COUP-TF direct repeat (DR1 and DR2) binding sites in the context of the intact promoter, specific changes in the functional activity of this novel region of the alpha2M promoter were observed. Mutation of the SF-1 site drastically reduced basal activity of the alpha2M promoter. Mutation of the COUP-TF sites caused the basal activity of the alpha2M promoter to increase markedly. Neither mutation altered the PRL inducibility of these constructs. Lastly, differentiation of cultured granulosa cells was required for functional activity of both the PRLRE and the ORRE. Collectively, these results document for the first time that Stat 5b, SF-1, and COUP-TF each exert specific effects on the function of the alpha2M promoter: basal activity is controlled by the balance of SF-1 (positive) and COUP-TF (negative) activities and PRL inducibility is mediated by activation of Stat 5b. These results add alpha2M to the list of nonsteroidal genes regulated by SF-1 in the gonads and provide the first evidence that COUP-TF has a specific role in regulating ovarian gene activity. In addition, the ORRE and PRLRE act independently of, rather than synergistically with, each other to regulate basal and PRL-induced expression of alpha2M in ovarian luteal cells.

Cellular components interact with adenovirus type 5 minimal DNA packaging domains

Adenovirus type 5 DNA packaging is initiated from the left end of the viral genome and depends on the presence of a cis-acting packaging domain located between nucleotides 194 and 380. Multiple redundant packaging elements (termed A repeats I through VII [AI through AVII]) are contained within this domain and display differential abilities to support DNA packaging in vivo. The functionally most important repeats, AI, AII, AV, and AVI, follow a bipartite consensus motif exhibiting AT-rich and CG-rich core sequences. Results from previous mutational analyses defined a fragment containing AV, AVI, and AVII as a minimal packaging domain in vivo, which supports a functional independence of the respective cis-acting sequences. Here we describe multimeric versions of individual packaging elements as minimal packaging domains that can confer viability and packaging activity to viruses carrying gross truncations within their left end. These mutant viruses directly rate the functional role that different packaging elements play relative to each other. The A repeats are likely to be binding sites for limiting, trans-acting packaging factors of cellular and/or viral origin. We report here the characterization of two cellular binding activities interacting with all of the minimal packaging domains in vitro, an unknown binding activity termed P-complex, and the transcription factor chicken ovalbumin upstream promoter transcription factor. The binding of both activities is dependent on the integrity of the AT-rich, but not the CG-rich, consensus half site. In the case of P-complex, binding affinity for different minimal packaging domains in vitro correlates well with their abilities to support DNA packaging in vivo. Interestingly, P-complex interacts not only with packaging elements but also with the left terminus of the viral genome, the core origin of replication. Our data implicate cellular factors as components of the viral packaging machinery. The dual binding specificity of P-complex for packaging and replication sequences may further suggest a direct involvement of left-end replication sequences in viral DNA encapsidation.

The murine Dax-1 promoter is stimulated by SF-1 (steroidogenic factor-1) and inhibited by COUP-TF (chicken ovalbumin upstream promoter-transcription factor) via a composite nuclear receptor-regulatory element

The Dax-1 gene encodes a protein that is structurally related to members of the orphan nuclear receptor superfamily. Dax-1 is coexpressed with another orphan nuclear receptor, steroidogenic factor-1 (SF-1), in the adrenal, gonads, hypothalamus, and pituitary gland. Mutations in Dax-1 cause adrenal hypoplasia congenita, a disorder that is characterized by adrenal insufficiency and hypogonadotropic hypogonadism. These developmental and endocrine abnormalities are similar to those caused by disruption of the murine Ftz-F1 gene (which encodes SF-1), suggesting that these nuclear receptors act along the same developmental cascade. Cloning of the murine Dax-1 gene revealed a candidate SF-1-binding site in the Dax-1 promoter. In transient expression assays in SF-1-deficient JEG-3 cells, SF-1 stimulated expression of the Dax-1 promoter. However, deletion or mutation of the consensus SF-1-binding site did not eliminate SF-1 stimulation. Further analyses revealed the presence of a cryptic SF-1 site that creates an imperfect direct repeat of the SF-1 element. When linked to the minimal thymidine kinase promoter, each of the isolated SF-1 sites was sufficient to mediate transcriptional regulation by SF-1. Mutation of both SF-1 sites eliminated SF-1 binding and stimulation of the Dax-1 promoter. Unexpectedly, mutation of either half of the composite SF-1 sites increased basal activity in JEG-3 cells, suggesting interaction of a repressor protein. Gel shift analyses of the composite response element revealed an additional complex that was not supershifted by SF-1 antibodies. This complex was eliminated by mutation of either half-site, and it was supershifted by antibodies against chicken ovalbumin upstream promoter-transcription factor (COUP-TF). We propose that Dax-1 is stimulated by SF-1, and that SF-1 and COUP-TF provide antagonistic pathways that converge upon a common regulatory site.

Nuclear receptor involvement in the regulation of rat cytochrome P450 3A23 expression

Many genes of the cytochrome P450 3A (CYP3A) subfamily, including several human and rat isoforms, are inducible by glucocorticoids. In the rat CYP3A23 gene, a 110-base pair segment of the proximal 5'-flanking region mediates dexamethasone activation. Three binding sites (DexRE-1, DexRE-2, and Site A), identified by DNase I footprinting analysis, were characterized for their relative contribution to both basal activity and dexamethasone inducibility. Site-directed mutagenesis of DexRE-1 (-144 to -169) and DexRE-2 (-118 to -136) demonstrated that each contained a core imperfect AGGTCA direct repeat, which comprised a consensus nuclear receptor binding site, and was essential for dexamethasone responsiveness but was not required for basal activity. Competition gel shift and supershift analyses revealed that both sites can bind the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor. Site A (-85 to -110) was shown to be important for both basal activity and dexamethasone responsiveness. Point mutants displayed a reduced (2-3-fold) induction response, compared with 15-fold for wild-type, which was accompanied by a 40-60% drop in basal activity. Site A was shown to bind the liver-enriched nuclear receptor hepatocyte nuclear factor 4. Our studies demonstrate that the mechanism mediating glucocorticoid-inducible transcriptional activity of CYP3A23 involves multiple binding sites for members of the nuclear receptor superfamily.

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) modulates expression of the Purkinje cell protein-2 gene. A potential role for COUP-TF in repressing premature thyroid hormone action in the developing brain

The cerebellar Purkinje cell-specific PCP-2 gene is transcriptionally activated by thyroid hormone during the 2nd and 3rd weeks of postnatal life in the rat. In contrast, thyroid hormone has no detectable effects on PCP-2 expression in the fetal rat. We now present data that suggest that the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor (COUP-TF) represses triiodothyronine (T3)-dependent transcriptional activation of PCP-2 in the immature Purkinje cell. Gel shift assays show that the PCP-2 A1TRE and adjoining sequences (-295/-199 region) bind to rat and mouse brain nucleoproteins in a developmentally regulated fashion and that one of these nucleoproteins could be the orphan nucleoprotein COUP-TF. In support of this hypothesis, in vitro translated COUP-TF binds to the -295/-199 region and COUP-TF represses T3-dependent activation of the PCP-2 promoter in transient transfection analyses. Finally, immunohistochemical studies reveal that COUP-TF is specifically expressed in the immature fetal and early neonatal Purkinje cell and that this expression diminishes coincident with thyroid hormone induction of PCP-2 expression. Our findings are consistent with the hypothesis that the presence or absence of inhibitory proteins bound to the thyroid hormone response element of T3-responsive genes governs the responsivity of these genes to thyroid hormone during brain development.

Seven-up, the Drosophila homolog of the COUP-TF orphan receptors, controls cell proliferation in the insect kidney

Morphogenesis of the insect kidney, the Malpighian tubules, is controlled in Drosophila by a single large cell, the tip cell. It has been postulated that this cell sends out a mitogenic signal that induces the division of neighboring cells. The signal and the molecules that receive it have remained elusive. We show that the COUP-TF-related nuclear orphan receptor Seven-up is a key component that becomes induced in response to mitogenic EGF receptor signaling activity emanating from the tip cell. Seven-up in turn is capable of regulating the transcription of cell cycle regulators.

Functional interactions between nuclear receptors recognizing a common sequence element, the direct repeat motif spaced by one nucleotide (DR-1)

Direct repeat motifs composed of two hexamer half-sites spaced by a single nucleotide (DR-1) are recognized by several members of the nuclear hormone receptor superfamily. We examined, by means of gene transfection assays, the interplay between the DR-1-binding nuclear receptors commonly expressed in liver, peroxisome proliferator-activated receptor alpha (PPARalpha), hepatocyte nuclear factor-4 (HNF-4), and chicken ovalbumin upstream transcription factor I (COUP-TFI). Both PPARalpha and HNF-4 efficiently bound to the acyl-CoA oxidase gene enhancer element, but PPARalpha exhibited much stronger transactivation than HNF-4. As a result, HNF-4 suppressed the gene-activating function of PPARalpha, when they were expressed together, due to competition for a common binding site. On the other hand, HNF-4, but not PPARalpha, effectively bound to the apolipoprotein CIII gene element, and activated gene transcription. PPARalpha had no effect even when co-expressed with HNF-4. COUP-TFI bound to both elements, and suppressed the gene activation by PPARalpha and HNF-4. Thus, these nuclear receptors have individual functions in gene regulation, and exhibit complex compound effects when they co-exist.

Expression of retinoid X receptors and COUP-TFI in a human salivary gland adenocarcinoma cell line

The growth of the adenocarcinoma cell line derived from human salivary gland (HSG) is regulated by all-trans-retinoic acid (t-RA), which binds to its specific receptor, retinoic acid receptors (RARs), located in the nucleus, and thereby transactivates target genes. In this study, we examined the binding characteristics of the nuclear extract of HSG cells to the retinoic acid response element (RARE) compared with those of in vitro translated RAR alpha and retinoid X receptor alpha (RXR alpha), a heterodimeric partner of RAR alpha. Gel shift analysis using anti-RAR alpha and anti-RXR alpha antibodies revealed that the translated RAR alpha bound to RARE as a heterodimer with RXR alpha. In contrast, the binding of the nuclear extract of HSG cells to RARE showed a heterogeneous pattern, suggesting the existence of several species of RXRs as well as RARs in the nuclei of HSG cells. We therefore tried to clone these putative RXRs by the polymerase chain reaction using degenerated oligonucleotide primers conserved across the RXR family. The DNA sequencing of the recombinant clones revealed the expression of RXR alpha and RXR beta. In addition, chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI), which is also an RXR family member, was cloned. To evaluate the transcriptional activity of RARs and RXRs endogenously expressed in HSG cells, we performed a transient transfection analysis. When HSG cells were transfected with a luciferase reporter plasmid containing two repeats of either the RARE of the RAR beta gene or that of cellular retinol-binding protein II gene, positioned upstream of a thymidine kinase promoter fused to the luciferase sequence, a 2-3-fold induction of luciferase activity was observed in both cases. These results suggest that RARs and RXRs endogenously expressed in HSG cells were transcriptionally active in vivo. Thus, our findings showed that RXR alpha, RXR beta, and COUP-TFI are expressed in HSG cells and suggest that these molecules function as heterodimeric partners of RARs and (or) competitive repressors for RAREs and are involved in cellular responses mediated by retinoids.

TR4 orphan receptor crosstalks to chicken ovalbumin upstream protein-transcription factor and thyroid hormone receptor to induce the transcriptional activity of the human immunodeficiency virus type 1 long-terminal repeat

Here we investigate the roles of human testicular orphan receptors, TR2 and TR4, on the gene regulation of the long-terminal repeat of the human immunodeficiency virus type 1 (HIV-LTR). In gel-retardation assays, a palindromic element at the 5'-end of HIV-LTR,5'-AGGGGTCAGATATCCACTGACCTTT-3',showed high affinity to TR2 and TR4 with an equilibrium dissociation constant (Kd) of 1.11 +/- 0.48 (n = 3) and 0.52 +/- 0.12 nM (n = 3), respectively. Interestingly, each half-site of the palindromic element is sufficient to compete with the binding of the labeled palindromic element to TR2 or TR4 with an equilibrium inhibition constant (ki) around 10 nM. However, the transiently expressed TR2 or TR4 in Chinese hamster ovary (CHO) cells or Japanese quail muscle myoblasts (QM7) cells showed no activity in regulating the transcriptional activity of the chloramphenicol acetyltransferase (CAT) reporter gene inserted downstream of the HIV-LTR promoter. Although both TR2 and TR4 showed no effect on CAT activity by itself, our data showed only the TR4 could crosstalk to the chicken ovalbumin upstream protein-transcription factor (COUP-TF1) and thyroid hormone receptor (TR alpha 1), and potentiated the transcriptional activity of HIV-LTR on the CAT reporter gene regulated by COUP-TF1 and TR alpha 1. These results indicate that TR4, but not TR2, may couple to other nuclear receptors in the upregulation of the HIV replication.

Chicken ovalbumin upstream promoter-transcription factor interacts with estrogen receptor, binds to estrogen response elements and half-sites, and inhibits estrogen-induced gene expression

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) was identified as a low abundance protein in bovine uterus that co-purified with estrogen receptor (ER) in a ligand-independent manner and was separated from the ER by its lower retention on estrogen response element (ERE)-Sepharose. In gel mobility shift assays, COUP-TF bound as an apparent dimer to ERE and ERE half-sites. COUP-TF bound to an ERE half-site with high affinity, Kd = 1.24 nM. In contrast, ER did not bind a single ERE half-site. None of the class II nuclear receptors analyzed, i.e. retinoic acid receptor, retinoid X receptor, thyroid receptor, peroxisome proliferator-activated receptor, or vitamin D receptor, were constituents of the COUP-TF.DNA binding complex detected in gel mobility shift assays. Direct interaction of COUP-TF with ER was indicated by GST "pull-down" and co-immunoprecipitation assays. The nature of the ER ligand influenced COUP-TF-ERE half-site binding. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), COUP-TF-half-site interaction decreased. Conversely, COUP-TF transcribed and translated in vitro enhanced the ERE binding of purified estradiol (E2)-liganded ER but not 4-OHT-liganded ER. Co-transfection of ER-expressing MCF-7 human breast cancer cells with an expression vector for COUP-TFI resulted in a dose-dependent inhibition of E2-induced expression of a luciferase reporter gene under the control of three tandem copies of EREc38. The ability of COUP-TF to bind specifically to EREs and half-sites, to interact with ER, and to inhibit E2-induced gene expression suggests COUP-TF regulates ER action by both direct DNA binding competition and through protein-protein interactions.

A role for estrogen-related receptor alpha in the control of mitochondrial fatty acid beta-oxidation during brown adipocyte differentiation

Little is known about the factors involved in the brown adipocyte gene regulatory program. In contrast to the white adipocyte, the brown adipocyte is characterized by abundant mitochondria and high level expression of mitochondrial fatty acid beta-oxidation enzymes. Previous studies in transgenic mice have shown that the brown adipose-enriched expression of a key beta-oxidation enzyme, medium chain acyl-coenzyme A dehydrogenase (MCAD), requires cis-acting elements located within the proximal promoter region of the MCAD gene. The levels of mRNA encoding MCAD and several other beta-oxidation cycle enzymes were coordinately induced during differentiation of brown adipocytes in culture. Expression of transgenes comprised of MCAD gene promoter fragments fused to chloramphenicol acetyltransferase reporters in differentiating brown adipocytes revealed that a known nuclear receptor response element (NRRE-1) was required for the transcriptional induction of the MCAD gene during brown adipocyte differentiation. Electrophoretic mobility shift assays and antibody recognition studies identified distinct brown adipocyte differentiation stage-specific, NRRE-1-protein complexes; the orphan nuclear receptors, chicken ovalbumin upstream promoter transcription factors I and II, were identified as major the NRRE-1 binding proteins in the pre-adipocyte, whereas the estrogen-related receptor alpha (ERRalpha) bound NRRE-1 in extracts prepared from differentiated brown adipocytes. DNA binding studies performed with a series of NRRE-1 mutant probes indicated that ERRalpha was capable of binding two distinct sites within NRRE-1, each of which conform to the known ERRalpha monomeric binding consensus. The expression of ERRalpha paralleled NRRE-1 binding activities and MCAD expression during brown adipocyte differentiation, cardiac development, and among a variety of adult mouse tissues. These results identify a new class of ERRalpha target genes and implicate ERRalpha and chicken ovalbumin upstream promoter transcription factor in the control of a pivotal metabolic pathway during brown adipocyte differentiation.

COUP-TF and Sp1 interact and cooperate in the transcriptional activation of the human immunodeficiency virus type 1 long terminal repeat in human microglial cells

We have recently reported that chicken ovalbumin upstream promoter transcription factor (COUP-TF) activates human immunodeficiency virus type 1 (HIV-1) gene transcription in glial and neuronal cells. Here, we have examined the role of COUP-TF in microglial cells, the major target cells for HIV-1 infection in brain. We show that COUP-TF activates gene expression from both the lymphotropic LAI and the macrophage-tropic JR-FL HIV-1 strains. Although COUP-TF binds to the -352/-320 nuclear receptor responsive element of the long terminal repeat, it functions as a transcriptional activator by acting on the -68/+29 minimal promoter. This region is a direct target of transcription factors Sp1 and Sp3. We report the discovery and features of a physical and functional interplay between COUP-TF and Sp1. Our cotransfection experiments provide evidence for a functional synergism between Sp1 and COUP-TF leading to enhanced transcriptional activity of the HIV-1 long terminal repeat through the Sp1 element. In contrast, Sp3 functions as a repressor of Sp1- or COUP-TF-induced activation. We further demonstrate that COUP-TF and Sp1 are capable of physically interacting, via the DNA-binding domain of COUP-TF, in vitro and in the cell. These findings reveal how the novel interplay of Sp1 and COUP-TF families of transcription factors regulate HIV-1 gene expression.

Differential regulation of the pre-C and pregenomic promoters of human hepatitis B virus by members of the nuclear receptor superfamily

Synthesis of the pre-C and pregenomic RNAs of human hepatitis B virus (HBV) is directed by two overlapping yet separate promoters (X. Yu and J. E. Mertz, J. Virol. 70:8719-8726, 1996). Previously, we reported the identification of a binding site for the nuclear receptor hepatocyte nuclear factor 4 (HNF4) spanning the TATA box-like sequence of the pre-C promoter. This HNF4-binding site consists of an imperfect direct repeat of the consensus half-site sequence 5'-AGGTCA-3' separated by one nucleotide; i.e., it is a DR1 hormone response element (HRE). We show here that other receptors, including chicken ovalbumin upstream promoter transcription factor 1 (COUP-TF1), human testicular receptor 2 (TR2), and peroxisome proliferator-activated receptors (PPARs) as heterodimers with retinoid X receptors (RXRs), can also specifically bind this DR1 HRE. Synthesis of the pre-C and pregenomic RNAs was affected both in transfected hepatoma cells and in a cell-free transcription system by the binding of factors to this DR1 HRE. Interestingly, whereas some members of the hormone receptor superfamily differentially repressed synthesis of the pre-C RNA (e.g., HNF4 and TR2) or activated synthesis of the pregenomic RNA (e.g., PPARgamma-RXRalpha), other members (e.g., COUP-TF1) coordinately repressed synthesis of both the pre-C and pregenomic RNAs. Thus, HBV likely regulates its expression and replication in part via this DR1 HRE. These findings indicate that appropriate ligands to nuclear receptors may be useful in the treatment of HBV infection.

Chicken ovalbumin upstream-promoter transcription factor (COUP-TF) could act as a transcriptional activator or repressor of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene

The chicken ovalbumin upstream-promoter transcription factor (COUP-TF) has a dual effect on the regulation of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene. COUP-TF could act as a transcriptional activator or repressor of this gene through different DNA sequences. COUP-TF induces expression of a reporter gene linked to the mitochondrial HMG-CoA synthase gene promoter in human hepatoma HepG2 cells, but represses it in a Leydig tumour cell line (R2C); in both these cell lines the expression of the mitochondrial HMG-CoA synthase gene mimics that of liver and testis. The activation is promoted by a fragment of the gene from coordinates -62 to +28, which contains a GC box and a TATA box, and where no COUP-TF binding site was observed by in vitro DNA binding studies. On the other hand, the COUP-TF inhibitory effect is mainly due to repression of peroxisome-proliferator-activated receptor-dependent activation of the gene, interacting with the region from -104 to -92. To our knowledge this work represents the second example of a target gene for COUP-TF I that could be either activated or repressed by the action of this receptor through different DNA sequences of the same gene.

The nuclear orphan receptors COUP-TF and ARP-1 positively regulate the trout estrogen receptor gene through enhancing autoregulation

The rainbow trout estrogen receptor (rtER) is a positively autoregulated gene in liver cells. In a previous report, we showed that upregulation is mediated by an estrogen response element (ERE) located in the proximal promoter of the gene and that a half binding site for nuclear receptors (5'-TGACCT-3') located 15 bp upstream of the ERE is involved in the magnitude of the estrogen response. We now report that the human orphan receptor COUP-TF and a COUP-TF-like protein from trout liver are able to bind to the consensus half-site. When cotransfected with the rtER gene proximal promoter, COUP-TF had no regulatory functions on its own. Interestingly, COUP-TF enhanced rtER transactivation properties in the presence of estradiol in a dose-dependent manner when cotransfected with the rtER gene promoter. Unliganded retinoid receptor heterodimers had the same helper function as COUP-TF in the presence of estradiol but were switched to repressors when the ligand all-trans-retinoic acid was added. Mutation of the consensus half-site only slightly reduced COUP-TF helper function, suggesting that it actually results from a complex mechanism that probably involves both DNA binding of COUP-TF to the promoter and protein-protein interaction with another transcription factor bound to the promoter. Nevertheless, a DNA-binding-defective mutant of COUP-TF was also defective in ER helper function. Competition footprinting analysis suggested that COUP-TF actually establishes contacts with the consensus upstream half-site and the downstream ERE half-site that would form a DR-24-like response element. Interaction of COUP-TF with the DR-24 element was confirmed in footprinting assays by using nuclear extracts from Saccharomyces cerevisiae expressing COUP-TF. Finally, interaction of COUP-TF with mutants of the rtER gene promoter showed that COUP-TF recognizes the ERE when the upstream half-site is mutated. These data show that COUP-TF may activate transcription through interaction with other nuclear receptors. This cross-talk between liganded nuclear receptors and orphan receptors is likely to modulate the spectrum of action of a particular ligand-receptor complex and may participate in the cell-type specificity of the ligand effect.

Null mutation of mCOUP-TFI results in defects in morphogenesis of the glossopharyngeal ganglion, axonal projection, and arborization

The COUP-TFs are orphan members of the steroid/thyroid hormone receptor superfamily. Multiple COUP-TF members have been cloned and they share a high degree of sequence homology between species as divergent as Drosophila and humans, suggesting a conservation of function through evolution. The COUP-TFs are highly expressed in the developing nervous systems of several species examined, indicating their possible involvement in neuronal development and differentiation. In the mouse, there are two very homologous COUP-TF genes (I and II) and their expression patterns overlap extensively. To study the physiological function of mCOUP-TFI, a gene-targeting approach was undertaken. We report here that mCOUP-TFI null animals die perinataly. Mutant embryos display an altered morphogenesis of the ninth cranial ganglion and nerve. The aberrant formation of the ninth ganglion is most possibly attributable to extra cell death in the neuronal precursor cell population. In addition, at midgestation, aberrant nerve projection and arborization were oberved in several other regions of mutant embryos. These results indicate that mCOUP-TFI is required for proper fetal development and is essential for postnatal development. Furthermore, mCOUP-TFI possesses vital physiological functions that are distinct from mCOUP-TFII despite of their high degree of homology and extensive overlapping expression patterns.

Gene silencing by chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) is mediated by transcriptional corepressors, nuclear receptor-corepressor (N-CoR) and silencing mediator for retinoic acid receptor and thyroid hormone receptor (SMRT)

Chicken ovalbumin upstream promoter-transcription factors (COUP-TFs) are orphan receptors that belong to the steroid/thyroid hormone receptor (TR) superfamily and can repress the transcriptional activity of several target genes; however, the precise mechanism of this repression is unknown. Transfection of a Gal4 DNA-binding domain fused to the putative ligand-binding domain of COUP-TFI (Gal4-COUP-TFI) significantly represses the basal transcriptional activity of a reporter gene containing Gal4-binding sites. Cotransfection of COUP-TFI can relieve the Gal4-COUP-TFI repression in a dose-dependent manner. In contrast, COUP-TFI delta35, which lacks the repressor domain (the C-terminal 35 amino acids), fails to relieve this repression. This finding suggests that the repressor domain of COUP-TFI may squelch a limiting amount of corepressor in HeLa cells. In addition, increasing concentrations of TRbeta also can relieve the COUP-TFI repression in a hormone-sensitive manner. Similarly, overexpression of increasing concentration of COUP-TFI, but not COUP-TFI delta35, can squelch the silencing activity of the unliganded TRbeta. Collectively, these results indicate that COUP-TFI and TRbeta share a common corepressor(s) for their silencing activity. To determine which corepressor is involved in the COUP-TF-silencing activity, we used a yeast two-hybrid and in vitro GST pull-down assays to demonstrate that COUP-TFI can interact with the fragment of N-CoR (nuclear receptor-corepressor) encoding amino acids 921-2453 and the fragments of SMRT (silencing mediator for retinoic acid receptor and TR) encoding amino acids 29-564 and 565-1289, respectively. Interestingly, the fragment of SMRT encoding amino acids 1192-1495, which strongly interacts with TRbeta, interacts very weakly with COUP-TFI. Furthermore, overexpression of N-CoR or SMRT potentiates the silencing activity of COUP-TFI and can relieve the COUP-TFI-mediated squelching of Gal4-COUP-TFI activity. Therefore, our studies indicate that N-CoR and SMRT act as corepressors for the COUP-TFI silencing activity.

Multiple orphan nuclear receptors converge to regulate rat P450c17 gene transcription: novel mechanisms for orphan nuclear receptor action

The orphan nuclear receptor steroidogenic factor-1 (SF-1) plays a key role in regulating the expression of the rat P450c17 gene in testicular Leydig and in adrenocortical cells. Other DNA sequences, not bound by SF-1, are also involved in transcriptional regulation of the rat P450c17 gene in both cell types. The region from -447/-399 or from -447/-419 increased both basal and cAMP-induced transcription, and the region from -418/-399 increased basal transcription to a greater extent than the intact -447/-399 DNA. The -447/-399 DNA sequence contains three imperfect copies of the orphan nuclear receptor-binding motif, AGGTCA, and at least three known orphan nuclear receptors, chicken ovalbumin upstream promoter transcription factor (COUP-TF), SF-1, and an early response gene induced by nerve growth factor (NGFI-B), bind to -447/-399 DNA. The AGGTCA triad is bound by one set of nuclear proteins when these three elements are colinear and is bound by a different set of proteins when these elements are separated. When the elements are separated, COUP-TF no longer binds, and the region -418/-399 is bound by a protein that greatly stimulates basal transcription. The region -447/-419 is bound by two different proteins that mediate both basal and cAMP-stimulated transcription. We call the protein binding to -418/-399 steroidogenic factor inducer of transcription-1 (StF-IT-1), and one of the proteins binding to -447/-419, StF-IT-2. SF-1 binds to a second AGGTCA element in the -447/-419 region. StF-IT-1 and StF-IT-2 are both found in Leydig and adrenal cells, and transcriptional regulation is similar in both cell types. SF-1 and NGF-IB may increase transcription by displacing COUP-TF (a transcriptional repressor) because these proteins share DNA-binding domains. However, neither SF-1 nor NGF-IB alone, binding as monomers, increases transcription. Rather, these proteins must interact with another DNA-binding protein, e.g. StF-IT-2, to increase transcription. StF-IT-2 also requires interaction with SF-1 (or NGF-IB) bound to DNA and cannot increase transcription by itself. This mechanism of action is different from the mechanism by which SF-1 regulates transcription from the -84/-55 region of the rat P450c17 gene. Thus, we have defined a novel mechanism of action for orphan nuclear receptors that bind to DNA as monomers.

Chicken ovalbumin upstream promoter transcription factors act as auxiliary cofactors for hepatocyte nuclear factor 4 and enhance hepatic gene expression

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) strongly inhibit transcriptional activation mediated by nuclear hormone receptors, including hepatocyte nuclear factor 4 (HNF-4). COUP-TFs repress HNF-4-dependent gene expression by competition with HNF-4 for common binding sites found in several regulatory regions. Here we show that promoters, such as the HNF-1 promoter, which are recognized by HNF-4 but not by COUP-TFs are activated by COUP-TFI and COUP-TFII in conjunction with HNF-4 more than 100-fold above basal levels, as opposed to about 8-fold activation by HNF-4 alone. This enhancement was strictly dependent on an intact HNF-4 E domain. In vitro and in vivo evidence suggests that COUP-TFs enhance HNF-4 activity by a mechanism that involves their physical interaction with the amino acid 227 to 271 region of HNF-4. Our results indicate that in certain promoters, COUP-TFs act as auxiliary cofactors for HNF-4, orienting the HNF-4 activation domain in a more efficient configuration to achieve enhanced transcriptional activity. These findings provide new insights into the regulatory functions of COUP-TFs, suggesting their involvement in the initial activation and subsequent high-level expression of hepatic regulators, as well as in the positive and negative modulation of downstream target genes.