A review on the role of NR2F1-AS1 in the development of cancer

NR2F1-AS1 is a natural antisense transcript with prominent roles in the carcinogenesis. It acts as an oncogene in almost all types of cancers except for cervical and colorectal cancers. It can act as a molecular sponge for miR-17, miR-371a-3p, miR-363, miR-29a-3p, miR-493-5p, miR-190a, miR-140, miR-642a, miR-363, miR-493-5p, miR-483-3p, miR-485-5p, miR-146a-5p, miR-877-5p, miR-338-3 P and miR-423-5p to influence expression of several cancer-related genes. Thus, the sponging role of NR2F1-AS1 is the most appreciated route of its contribution in the carcinogenesis. In addition, NR2F1-AS1 affects activity of IGF-1/IGF-1R/ERK, PI3K/AKT/GSK-3β and Hedgehog pathways. The current narrative review aims at summarization of the results of studies that highlighted the role of NR2F1-AS1 in the carcinogenesis.

The COUP-TFs compose a family of functionally related transcription factors

The chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are members of the steroid/thyroid hormone receptor superfamily and function in transcriptional regulation of a wide variety of genes. The COUP-TFs purified from HeLa nuclear extract by COUP-affinity chromatography are composed of multiple M(r) forms. The Low M(r) COUP-TFs (43,000, 44,000, 46,000, and 47,000 M(r)) produce a relatively fast migrating complex (C1) with DNA in electrophoresis mobility shift assays, while the high M(r) forms (66,000, 68,000, 72,000, and 74,000 M(r)) produce a slower migrating (C2) complex. The high M(r) COUP-TFs were purified by gel filtration chromatography and independently formed the C2 DNA complex, probably acting as dimers. The high M(r) forms are indistinguishable from the low M(r) COUP-TFs in DNA binding and in enhancement of in vitro transcription from the ovalbumin promoter. The finding of multiple COUP-TF forms led us to clone a second low M(r) COUP-TF, "COUP-TF2." The COUP-TF2 sequence has very strong homology with COUP-TF1. The N-termini of COUP-TF1 and COUP-TF2 are least similar, but both contain glutamine-rich and proline-rich motifs, putative activation domains.

Dynamic expression of COUP-TFI and COUP-TFII during development and functional maturation of the mouse inner ear

COUP-TFs (chicken ovalbumin upstream promoter-transcription factors) are orphan members of the nuclear receptor superfamily of ligand-activated receptors for which their ligands have not been identified. The two mammalian proteins, COUP-TFI and COUP-TFII, share 80% sequence identity and regulate many aspects of mammalian development and differentiation. In this report, we systemically examined the temporal and spatial expression of COUP-TFI and COUP-TFII transcripts and, for the first time, their protein during development and functional maturation of the cochlea. Both COUP-TFI and COUP-TFII were expressed early in the developing otic vesicle. COUP-TFI expression correlated with the differentiation of hair cells and support cells in the organ of Corti, whereas COUP-TFII expression was down-regulated with differentiation of the organ of Corti. Furthermore, we report for the first time, that the generally nuclear COUP-TFI receptor protein was localized in the cytoplasm of maturing hair cells and pillar cells. Collectively, although COUP-TFI and COUP-TFII are homologues, the expression of each orphan receptor has a restricted and dynamic expression during cochlea development particularly during patterning and differentiation of the cochlear structures.

A neural-specific splicing event generates an active form of the Wiskott-Aldrich syndrome protein

Actin polymerization is required for cellular events such as podosome, lamellipode or filopode formation in migrating cells, and members of the Wiskott-Aldrich syndrome protein (WASP) family have essential roles in regulating actin dynamics at the cell leading edge. However, WASP proteins need first to be activated in order to be able to target actin polymerization. Here, we show the occurrence of a neural-specific splicing event, which is favoured by the nuclear orphan receptor chicken ovalbumin upstream promoter-transcription factor I, and generates a truncated WASP protein deleted of exon 2-encoded amino acids. This deletion relocates the protein to the plasma membrane and induces the formation of actin-rich podosome-like structures that also contain paxillin and vinculin. Furthermore, expression of the truncated protein in PC12 cells, as well as in primary neurons, stimulates neuritogenesis. These data underscore the importance of the neural-specific splicing of WASP RNA during development.

Chicken ovalbumin upstream promoter-transcription factor members repress retinoic acid-induced Cdx1 expression

It is well established that Hox genes are key players in specifying positional identity along the anterior-posterior axis and are targets of diverse transcription factors implicated in axial patterning. Members of the CDX family (CDX1, -2, and -4) are among such effectors of Hox expression as pertains to vertebral patterning in the mouse. Cdx members are themselves targets of signaling molecules that are also implicated in axial patterning, including retinoic acid (RA) and certain members of Wnt and fibroblast growth factor families. In this regard, we have previously shown that, in the mouse, Cdx1 is directly regulated by RA at the late primitive streak stage (embryonic day (E) 7.5) through a RA response element in the proximal Cdx1 promoter. At E8.5, Cdx1 expression remains essentially limited to the posterior embryo. RA, however, is excluded from the caudal embryo at this later stage, but is found in a more anterior domain encompassing the prospective trunk region. These observations suggest the existence of a repressor mechanism that prevents expression of Cdx1 in these anterior domains of retinoid signaling at E8.5. In the present study, we present evidence suggesting that chicken ovalbumin upstream promoter-transcription factor (COUP-TF) members antagonize RA-induced Cdx1 expression by competing with retinoid X receptor-retinoic acid receptor heterodimers for binding to the Cdx1 RA response element. Consistent with this, in situ hybridization analysis revealed that COUP-TFs are highly expressed in the anterior embryo in domains where Cdx1 transcripts are excluded. Together with other data, these findings suggest a model by which COUP-TF expression is induced by RA in the trunk region as a negative feedback mechanism to restrict Cdx1 expression to the caudal embryo.

Ubc9 and Protein Inhibitor of Activated STAT 1 Activate Chicken Ovalbumin Upstream Promoter-Transcription Factor I-mediated Human CYP11B2 Gene Transcription

Aldosterone synthase (CYP11B2) is involved in the final steps of aldosterone biosynthesis and expressed exclusively in the adrenal zona glomerulosa cells. Using an electrophoretic mobility shift assay, we demonstrate that COUP-TFI binds to the -129/-114 element (Ad5) of human CYP11B2 promoter. Transient transfection in H295R adrenal cells demonstrated that COUP-TFI enhanced CYP11B2 reporter activity. However, the reporter construct with mutated Ad5 sequences showed reduced basal and COUP-TFI-enhanced activity, suggesting that binding of COUP-TFI to Ad5 is important for CYP11B2 transactivation. To elucidate molecular mechanisms of COUP-TFI-mediated activity, we subsequently screened for COUP-TFI-interacting proteins from a human adrenal cDNA library using a yeast two-hybrid system and identified Ubc9 and PIAS1, which have small ubiquitin-related modifier-1 (SUMO-1) conjugase and ligase activities, respectively. The coimmunoprecipitation assays confirmed that COUP-TFI forms a complex with Ubc9 and PIAS1 in mammalian cells. Immunohistochemistry showed that Ubc9 and PIAS1 are markedly expressed in rat adrenal glomerulosa cells. Coexpression of Ubc9 and PIAS1 synergistically enhanced the COUP-TFI-mediated CYP11B2 reporter activity, indicating that both proteins function as coactivators of COUP-TFI. However, sumoylation-defective mutants, Ubc9 (C93S) and PIAS1 (C351S), continued to function as coactivators of COUP-TFI, indicating that sumoylation activity are separable from coactivator ability. In addition, chromatin immunoprecipitation assays demonstrated that ectopically expressed COUP-TFI, Ubc9, and PIAS1 were recruited to an endogenous CYP11B2 promoter. Moreover, reduction of Ubc9 or PIAS1 protein levels by small interfering RNA inhibited the CYP11B2 transactivation by COUP-TFI. Our data support a physiological role of Ubc9 and PIAS1 as transcriptional coactivators in COUP-TFI-mediated CYP11B2 transcription.

Peroxisome proliferator-activated receptor gamma coactivator-1alpha, as a transcription amplifier, is not essential for basal and hormone-induced phosphoenolpyruvate carboxykinase gene expression

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the initial step in hepatic gluconeogenesis. In the fasted state, PEPCK gene expression is activated by glucagon (via cAMP) and glucocorticoids. Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) plays an important role in energy homeostasis and is considered to be a key regulator of hepatic gluconeogenesis in response to fasting. It is not clear whether PGC-1alpha is obligatory for the activation of the transcription program of gluconeogenic genes, or whether it amplifies an existing process. H4IIE hepatoma cells were used to address this key point. These cells respond appropriately to all of the hormones involved in the regulation of gluconeogenic genes, yet they are devoid of PGC-1alpha. Also, these hormone responses occur in the absence of ongoing protein synthesis, so the necessary complement of transcription factors exists in untreated cells. However, exogenous expression of PGC-1alpha in these cells does enhance basal and hormone-induced expression of the PEPCK and glucose-6-phosphatase genes. Mutational analyses of the PEPCK gene promoter reveal that one element in the PEPCK gene promoter, glucocorticoid accessory factor 3, which binds chicken ovalbumin upstream promoter-transcription factor, is of particular importance. Taken together, these data suggest that, under chronic fasting conditions, i.e. when high levels of cAMP and glucocorticoids induce PGC-1alpha expression, this coactivator markedly amplifies PEPCK gene expression and gluconeogenesis.

The pig CYP2E1 promoter is activated by COUP-TF1 and HNF-1 and is inhibited by androstenone

Functional analysis of the pig cytochrome P4502E1 (CYP2E1) promoter identified two major activating elements. One corresponded to the hepatic nuclear factor 1 (HNF-1) consensus binding sequence at nucleotides -128/-98 and the other was located in the region -292/-266. The binding of proteins in pig liver nuclear extracts to a synthetic double-stranded oligonucleotide corresponding to this more distal activating sequence was studied by electrophoretic mobility shift assay. The minimum protein binding sequence was identified as TGTTCTGACCTCTGGG. Gel super-shift assays identified the protein binding to this site as chick ovalbumin upstream promoter transcription factor 1 (COUP-TF1). Androstenone inhibited promoter activity in transfection experiments only with constructs which included the COUP-TF1 binding site. Androstenone inhibited COUP-TF1 binding to synthetic oligonucleotides but did not affect HNF-1 binding. The results offer an explanation for the inhibition of CYP2E1 protein expression by androstenone in isolated pig hepatocytes and may be relevant to the low expression of hepatic CYP2E1 in those pigs which accumulate high levels of androstenone in vivo.

Chicken ovalbumin upstream promoter-transcription factor is a negative regulator of steroidogenesis in bovine adrenal glomerulosa cells

The octapeptide hormone, angiotensin II (AngII) and ACTH stimulate mineralocorticoid biosynthesis in the zona glomerulosa of the adrenal cortex in part by promoting the transcription of the gene coding for the steroidogenic acute regulatory (StAR) protein. We have examined whether chicken ovalbumin upstream promoter-transcription factor (COUP-TF), a member of the orphan nuclear receptor family of transcription factors, is involved in this transcriptional regulation. We analyzed COUP-TF and StAR mRNA and protein levels in bovine adrenal glomerulosa cells in primary culture. COUP-TF protein was readily detectable in nonstimulated cells, and AngII markedly reduced its expression in a time- and concentration-dependent manner (IC50 = 1 nm), to 46 +/- 4.4% of control levels after 6 h, (n = 3; P < 0.01). This repression was paralleled by a marked decrease in COUP-TF mRNA levels, reaching 18 +/- 8.8% of controls (n = 3, P < 0.01) after 6 h and by a 20-fold increase in aldosterone output. In bovine glomerulosa cells overexpressing COUP-TFI and -II, the induction of StAR mRNA and protein elicited by AngII was completely suppressed to control levels, and the aldosterone response was significantly reduced (from 4.8 +/- 1.1-fold the basal value in mock-infected cells to 1.9 +/- 0.5-fold and 2.2 +/- 0.7-fold in COUP-TFI- and COUP-TFII-expressing cells, respectively; n = 3; P < 0.01 for both differences). Finally, by using electrophoretic mobility shift assays and chromatin immunoprecipitation, we have shown a direct interaction between COUP-TF and the proximal StAR promoter. These results suggest that COUP-TF exerts a tonic inhibition on steroidogenesis by repressing StAR protein expression and that activators of aldosterone biosynthesis lift this inhibition in part by repressing COUP-TF levels.

Chicken ovalbumin upstream promoter-transcription factor I represses the transcriptional activity of the human muscle glycogen phosphorylase promoter in C2C12 cells

The responsiveness of the 1.13 kb proximal human muscle glycogen phosphorylase (MGP) gene promoter to the chicken ovalbumin upstream promoter-transcription factor (COUP-TF) repressor, known to be ablated during muscle cell differentiation, was examined. Constitutive expression of COUP-TFI repressed the activity of the promoter in C2C12 muscle cells and sequential deletion analysis mapped the sensitive region between nucleotides -362 and -185, which included a putative consensus COUP-TF binding half-site at -198/-193. Mutation of this site abolished transcriptional response to COUP-TFI of the -362 construct. A -209/-180 probe bound in vitro to COUP-TFI and to protein extracts from proliferating but not fusing myoblasts. Thus, COUP-TF may be involved in repression of the human MGP gene promoter at the myoblast stage.

The nuclear orphan receptor COUP-TFI is important for differentiation of oligodendrocytes

We report here that a member of the nuclear hormone receptor superfamily, chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TFI), plays a critical role in glial cell development and subsequent central nervous system myelination. We demonstrate that COUP-TF1 is expressed in cells of oligodendrocyte lineage. Furthermore, we demonstrate that COUP-TFI null mutant mice exhibit delayed axon myelination and increased dysmyelination in the central nervous system. Using in vitro differentiation assays, we show that these myelination defects are due to delays in oligodendrocyte differentiation. Finally, in situ hybridization and transfection analysis suggests that COUP-TFI acts as an upstream regulator of SCIP/Oct-6/Tst-1, a transcription factor involved in axon myelination. Taken together, these results suggest that COUP-TFI is an important regulator of oligodendrocyte differentiation.

Repression of gonadotropin-releasing hormone (GnRH) gene expression by melatonin may involve transcription factors COUP-TFI and C/EBP beta binding at the GnRH enhancer

Melatonin activates membrane-bound G-protein-coupled receptors mt1 and MT2, but may also bind a family of orphan nuclear receptors, including RORalpha and RZRbeta, representing another potential molecular mechanism of melatonin action. Recently, we demonstrated that melatonin downregulates gonadotropin-releasing hormone (GnRH) gene expression in the GT1-7 cell line, specifically at the level of the neuron-specific GnRH gene enhancer. In this study, we have examined the region located at -1736/-1728 of the GnRH enhancer shown to be involved in the repression of GnRH by melatonin. This region includes hexameric consensus binding sites for orphan nuclear receptors, including ROR/RZR and COUP-TFI, as well as other defined consensus binding sites for AP-1 and C/EBP. Using electrophoretic mobility shift analysis (EMSA), we have demonstrated that GT1-7 nuclear proteins bind specifically to this region of the GnRH enhancer to form 4 complexes. EMSA antibody supershift analysis indicates that the transcription factors COUP-TFI and C/EBP beta bind two specific complexes, but RORalpha, Oct-1, Pbx-1, c-fos, or c-jun antibodies failed to produce any detectable supershifts. These results provide the first evidence that melatonin may mediate its direct neuroendocrine control of GnRH gene expression through transcription factor binding at specific regions of the GnRH enhancer.

Gene Silencing by Nuclear Orphan Receptors

Nuclear orphan receptors represent a large and diverse subgroup in the nuclear receptor superfamily. Although putative ligands for these orphan members remain to be identified, some of these receptors possess intrinsic activating, inhibitory, or dual regulatory functions in development, differentiation, homeostasis, and reproduction. In particular, gene-silencing events elicited by chicken ovalbumin upstream promoter-transcription factors (COUP-TFs); dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1 (DAX-1); germ cell nuclear factor (GCNF); short heterodimer partner (SHP); and testicular receptors 2 and 4 (TR2 and TR4) are among the best characterized. These orphan receptors are critical in controlling basal activities or hormonal responsiveness of numerous target genes. They employ multiple and distinct mechanisms to mediate target gene repression. Complex cross-talk exists between these orphan receptors at their cognate DNA binding elements and an array of steroid?nonsteroid hormone receptors, other transcriptional activators, coactivators and corepressors, histone modification enzyme complexes, and components of basal transcriptional components. Therefore, perturbation induced by these orphan receptors at multiple levels, including DNA binding activities, receptor homo- or heterodimerization, recruitment of cofactor proteins, communication with general transcriptional machinery, and changes at histone acetylation status and chromatin structures, may contribute to silencing of target gene expression in a specific promoter or cell-type context. Moreover, the findings derived from gene-targeting studies have demonstrated the significance of these orphan receptors' function in physiologic settings. Thus, COUP-TFs, DAX-1, GCNF, SHP, and TR2 and 4 are known to be required for multiple physiologic and biologic functions, including neurogenesis and development of the heart and vascular system steroidogenesis and sex determination, gametogenesis and embryonic development, and cholesterol?lipid homeostasis.

Repression of the luteinizing hormone receptor gene promoter by cross talk among EAR3/COUP-TFI, Sp1/Sp3, and TFIIB

Transcription of luteinizing hormone receptor (LHR) gene is activated by Sp1/Sp3 at two Sp1 sites and is repressed by nuclear orphan receptors EAR2 and EAR3 through a direct-repeat (DR) motif. To elucidate the mechanism of the orphan receptor-mediated gene repression, we explored the functional connection between the orphan receptors and Sp1/Sp3 complex, and its impact on the basal transcription machinery. The Sp1(I) site was identified as critical for the repression since its mutation reduced the inhibition by EAR2 and abolished the inhibition by EAR3. Cotransfection analyses in SL2 cells showed that both Sp1 and Sp3 were required for this process since EAR3 displayed a complete Sp1/Sp3-dependent inhibitory effect. Functional cooperation between Sp1 and DR domains was further supported by mutual recruitment of EAR3 and Sp1/Sp3 bound to their cognate sites. Deletion of EAR3 N-terminal and DNA-binding domains that reduced its interaction with Sp1 impaired its inhibitory effect on human LHR (hLHR) gene transcription. Furthermore, we demonstrate interaction of TFIIB with Sp1/Sp3 at the Sp1(I) site besides its association with EAR3 and the TATA-less core promoter region. Such interaction relied on Sp1 site-bound Sp1/Sp3 complex and adaptor protein(s) present in the JAR nuclear extracts. We further demonstrated that EAR3 specifically decreased association of TFIIB to the Sp1(I) site without interfering on its interaction with the hLHR core promoter. The C-terminal region of EAR3, which did not participate in its interaction with Sp1, was required for its inhibitory function and may affect the association of TFIIB with Sp1. Moreover, perturbation of the association of TFIIB with Sp1 by EAR3 was reflected in the reduced recruitment of RNA polymerase II to the promoter. Overexpression of TFIIB counteracted the inhibitory effect of EAR3 and activated hLHR gene transcription in an Sp1 site-dependent manner. These findings therefore indicate that TFIIB is a key component in the regulatory control of EAR3 and Sp1/Sp3 on the initiation complex. Such cross talk among EAR3, TFIIB, and Sp1/Sp3 reveals repression of hLHR gene transcription by nuclear orphan receptors is achieved via perturbation of communication between Sp1/Sp3 at the Sp1-1 site and the basal transcription initiator complex.

Constructing the mammalian neocortex: the role of intrinsic factors

The mammalian neocortex is subdivided into regions that are specialised for the processing of particular forms of information. These regions are distinct in terms of their cytoarchitecture, electrophysiology, and connectivity. How this regional diversity is generated through development is currently a topic of considerable interest and has centered upon two main issues. First, to what extent are these regions prespecified by intrinsic genetic mechanisms? Second, what is the influence of extrinsic activity in transmitting signals that ultimately shape functional regions? Historically, experimental evidence has tended to emphasise the role of extrinsic influences, but the identification and analysis of several genes that are expressed asymmetrically in the developing neocortex have tempered this viewpoint. We review current literature from the standpoint that intrinsic influences act early in neocortical development to generate molecular patterning whose main role is the guidance of long-range projections from the dorsal thalamus. Extrinsic influences appear to generate receptive fields for peripheral input, the summation of which determines the areal extent of particular neocortical region.

Molecular regionalization of the neocortex is disrupted in Fgf8 hypomorphic mutants

The neocortex is divided into multiple areas with specific architecture, molecular identity and pattern of connectivity with the dorsal thalamus. Gradients of transcription factor expression in the cortical primordium regulate molecular regionalization and potentially the patterning of thalamic projections. We show that reduction of Fgf8 levels in hypomorphic mouse mutants shifts early gradients of gene expression rostrally, thereby modifying the molecular identity of rostral cortical progenitors. This shift correlates with a reduction in the size of a molecularly defined rostral neocortical domain and a corresponding rostral expansion of more caudal regions. Despite these molecular changes, the topography of projections between the dorsal thalamus and rostral neocortex in mutant neonates appears the same as the topography of wild-type littermates. Overall, our study demonstrates the role of endogenous Fgf8 in regulating early gradients of transcription factors in cortical progenitor cells and in molecular regionalization of the cortical plate.

Recruitment of Tat to heterochromatin protein HP1 via interaction with CTIP2 inhibits human immunodeficiency virus type 1 replication in microglial cells

The Tat protein of human immunodeficiency virus type 1 (HIV-1) plays a key role as inducer of viral gene expression. We report that Tat function can be potently inhibited in human microglial cells by the recently described nuclear receptor cofactor chicken ovalbumin upstream promoter transcription factor-interacting protein 2 (CTIP2). Overexpression of CTIP2 leads to repression of HIV-1 replication, as a result of inhibition of Tat-mediated transactivation. In contrast, the related CTIP1 was unable to affect Tat function and viral replication. Using confocal microscopy to visualize Tat subcellular distribution in the presence of the CTIPs, we found that overexpression of CTIP2, and not of CTIP1, leads to disruption of Tat nuclear localization and recruitment of Tat within CTIP2-induced nuclear ball-like structures. In addition, our studies demonstrate that CTIP2 colocalizes and associates with the heterochromatin-associated protein HP1alpha. The CTIP2 protein harbors two Tat and HP1 interaction interfaces, the 145-434 and the 717-813 domains. CTIP2 and HP1alpha associate with Tat to form a three-protein complex in which the 145-434 CTIP2 domain interacts with the N-terminal region of Tat, while the 717-813 domain binds to HP1. The importance of this Tat binding interface and of Tat subnuclear relocation was confirmed by analysis of CTIP2 deletion mutants. Our findings suggest that inhibition of HIV-1 expression by CTIP2 correlates with recruitment of Tat within CTIP2-induced structures and relocalization within inactive regions of the chromatin via formation of the Tat-CTIP2-HP1alpha complex. These data highlight a new mechanism of Tat inactivation through subnuclear relocalization that may ultimately lead to inhibition of viral pathogenesis.

Mechanism of a transcriptional cross talk between transforming growth factor-beta-regulated Smad3 and Smad4 proteins and orphan nuclear receptor hepatocyte nuclear factor-4

We have shown previously that the transforming growth factor-beta (TGFbeta)-regulated Sma-Mad (Smad) protein 3 and Smad4 proteins transactivate the apolipoprotein C-III promoter in hepatic cells via a hormone response element that binds the nuclear receptor hepatocyte nuclear factor 4 (HNF-4). In the present study, we show that Smad3 and Smad4 but not Smad2 physically interact with HNF-4 via their Mad homology 1 domains both in vitro and in vivo. The synergistic transactivation of target promoters by Smads and HNF-4 was shown to depend on the specific promoter context and did not require an intact beta-hairpin/DNA binding domain of the Smads. Using glutathione S-transferase interaction assays, we established that two regions of HNF-4, the N-terminal activation function 1 (AF-1) domain (aa 1-24) and the C-terminal F domain (aa 388-455) can mediate physical Smad3/HNF-4 interactions in vitro. In vivo, Smad3 and Smad4 proteins enhanced the transactivation function of various GAL4-HNF-4 fusion proteins via the AF-1 and the adjacent DNA binding domain, whereas a single tyrosine to alanine substitution in AF-1 abolished coactivation by Smads. The findings suggest that the transcriptional cross talk between the TGFbeta-regulated Smads and HNF-4 is mediated by specific functional domains in the two types of transcription factors. Furthermore, the specificity of this interaction for certain target promoters may play an important role in various hepatocyte functions, which are regulated by TGFbeta and the Smads.

Distinct modes of regulation of transcription of hepatitis B virus by the nuclear receptors HNF4alpha and COUP-TF1

To study the effects of the nuclear receptors (NRs) HNF4alpha and COUP-TF1 on the life cycle of hepatitis B virus (HBV), the human hepatoma cell line Huh7 was transiently cotransfected with plasmids containing the HBV genome and encoding these two NRs. Overexpression of HNF4alpha and COUP-TF1 led to a 9-fold increase and a 7- to 10-fold decrease, respectively, in viral DNA synthesis. These two NRs also exhibited distinct modes of regulation of viral transcription. Overexpression of HNF4alpha led to a more-than-10-fold increase in synthesis of the pregenomic RNA but to only a 2- to 3-fold increase in synthesis of the pre-C and S RNAs. Moreover, the NR response element within the pre-C promoter, NRRE(preC,) played the major role in activation of pregenomic RNA synthesis by HNF4alpha. On the other hand, overexpression of COUP-TF1 led to an over-10-fold repression of synthesis of both pre-C and pregenomic RNAs mediated through either NRRE(preC) or NRRE(enhI). HNF4alpha and COUP-TF1 antagonized each other's effects on synthesis of pregenomic RNA and viral DNA when they were co-overexpressed. A naturally occurring HBV variant which allows for binding by HNF4alpha but not COUP-TF1 in its NRRE(preC) exhibited significantly higher levels of synthesis of pregenomic RNA and viral DNA than wild-type HBV in coexpression experiments. Last, deletion analysis revealed that non-NRRE sequences located within both the C and pre-S1 regions are also essential for maximum activation of the pregenomic promoter by HNF4alpha but not for repression by COUP-TF1. Thus, HNF4alpha and COUP-TF1 function through different mechanisms to regulate expression of the HBV genes.

Control of human carnitine palmitoyltransferase II gene transcription by peroxisome proliferator-activated receptor through a partially conserved peroxisome proliferator-responsive element

The expression of several genes involved in fatty acid metabolism is regulated by peroxisome proliferator-activated receptors (PPARs). To gain more insight into the control of carnitine palmitoyltransferase (CPT) gene expression, we examined the transcriptional regulation of the human CPT II gene. We show that the 5'-flanking region of this gene is transcriptionally active and binds PPARalpha in vivo in a chromatin immunoprecipitation assay. In addition, we characterized the peroxisome proliferator-responsive element (PPRE) in the proximal promoter of the CPT II gene, which appears to be a novel PPRE. The sequence of this PPRE contains one half-site which is a perfect consensus sequence (TGACCT) but no clearly recognizable second half-site (CAGCAC); this part of the sequence contains only one match to the consensus, which seems to be irrelevant for the binding of PPARalpha. As expected, other members of the nuclear receptor superfamily also bind to this element and repress the activation mediated by PPARalpha, thus showing that the interplay between several nuclear receptors may regulate the entry of fatty acids into the mitochondria, a crucial step in their metabolism.

Transcriptional regulation of Na+/H+ exchanger expression in the intact mouse

We examined regulation of the Na+/H+ exchanger (NHE1 isoform) in the developing mouse. We generated transgenic mice with the Na+/H+ exchanger promoter directing expression of the beta-Galactosidase reporter. We found that expression of the Na+/H+ exchanger was maximum in the heart and liver of 12-day-old embryonic mice. Similar results were found in mice using the green fluorescent protein reporter driven by the Na+/H+ exchanger promoter. Detailed examination of the myocardium revealed that the GFP reporter protein was expressed in the cytoplasm of cardiomyocyte cells. We examined NHE1 protein expression in transgenic mice lacking the transcription factors AP-2alpha or the transcription factor COUP-TF1. Eighteen-day-old AP-2alpha heterozygote mice show no large changes in NHE1 expression in heart, lung, liver, kidney and brain. In contrast, 18-day-old embryos from AP-2alpha null mice showed a large increase in Na+/H+ exchanger protein expression in the brain. NHE1 protein levels in COUP-TF1 knockout embryos did not differ from wild type embryos. The results suggest that AP-2alpha and COUP-TF1 are not critical to NHE1 expression in the late stage embryo and that other related transcription factors may function in regulation of the Na+/H+ exchanger.

Activation of the MAP kinase pathway induces chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) expression in human breast cancer cell lines

Growth factors are essential for cellular growth and differentiation in both normal and malignant human breast epithelial cells. In the present study we investigated the effect of epidermal growth factor (EGF), transforming growth factor alpha (TGFalpha) and phorbol myristate acetate (PMA) on chicken ovalbumin upstream promoter-transcription factor (COUP-TF) expression in human breast cancer cells. The orphan receptors COUP-TFI and COUP-TFII are members of the nuclear receptor superfamily. The high degree of evolutionary conservation of these proteins strongly argues for an important biological function. COUP-TF expression was highest in SK-BR3 cells (approximately 130 amol/ micro g total RNA), while the lowest COUP-TF expression was observed in MCF-7 cells (3.5 amol/ micro g total RNA). While treatment of EGF, TGFalpha and PMA induced expression of COUP-TFII, COUP-TFI did not respond to these agents. Oncostatin M (OSM) is known to exert an antiproliferative effect in breast cancer cells. Treatment of MCF-7 cells with OSM resulted in an approximately 90% reduction of COUP-TFII mRNA expression. In SK-BR3 cells, treatment with the MEK inhibitor UO126 resulted in a profound suppression of endogenous COUP-TFII expression. Furthermore, cotreatment with UO126 prevented induction of COUP-TFII expression by EGF in MCF-7 cells. In conclusion, our data provide evidence, for the first time, that mitogenic substances which activate the MAP kinase pathway, can induce COUP-TFII expression. Our results strongly suggest that an active MAP kinase pathway is essential for COUP-TFII expression in human breast cancer cells.

A complex deoxyribonucleic acid response element in the rat Ca(2+)/calmodulin-dependent protein kinase IV gene 5'-flanking region mediates thyroid hormone induction and chicken ovalbumin upstream promoter transcription factor 1 repression

Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is regulated by T(3) in a time- and concentration-dependent manner in the developing rat brain and plays an important role in neuronal-specific gene regulation. T(3) treatment, but not retinoic acid (RA), stimulated endogenous CaMKIV mRNA 5-fold in mouse embryonic stem (ES) cells differentiated into neurons. We localized a region -750 to -700 in the CaMKIV gene 5'-flanking region that conferred T(3) responsiveness and bound thyroid hormone receptor (TR), retinoic acid receptor (RAR), and chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1). T(3) and RA treatment stimulated the CaMKIV hormone response element. Cotransfection of a COUP-TF1 expression vector repressed the T(3) response and augmented the RA response. Mutational analysis identified three half-sites arranged in a direct repeat (AB) and overlapping inverted repeat (BC), required for functional induction and receptor binding. TR and RAR bound predominantly to the BC portion of the element and COUP-TF1 to the AB region, with a close correlation of binding and functional studies. COUP-TF1 binding did not influence TR/retinoid X receptor binding but modestly augmented RAR/retinoid X receptor binding. A single element confers T(3) and COUP-TF1 regulation of CaMKIV expression.

A COUP-TF/Svp homolog is highly expressed during vitellogenesis in the mosquito Aedes aegypti

In the mosquito Aedes aegypti, vitellogenesis is activated via an ecdysteroid hormonal cascade initiated by a blood meal. The functional ecdysone receptor is a heterodimer composed of the ecdysone receptor (EcR) and ultraspiracle, the homolog of the retinoid X receptor. The precise tuning of this hormonal response requires participation of both positive and negative transcriptional regulators. In Drosophila, Svp, a homolog of chicken ovalbumin upstream promoter transcription factor (COUP-TF), inhibits ecdysone receptor complex-mediated transactivation in vitro and in vivo. Here we report the cloning and characterization of the Svp homolog in mosquito Aedes aegypti, AaSvp. It possesses a high degree of amino acid sequence similarity to the members of the COUP-TF/Svp subfamily. AaSvp transcripts and protein are present in the fat body at high levels from the state of arrest to about 60 h post blood meal. AaSvp binds strongly to a variety of direct repeats of the sequence AGGTCA, but weakly to inverted repeats such as hsp27 EcRE. Transient transfection assays in Drosophila S2 cells showed that AaSvp was able to repress 20-hydroxyecdysone (20E)-dependent transactivation mediated by the mosquito ecdysteroid receptor complex. These data suggest that AaSvp negatively regulates the 20E signaling in the fat body during mosquito vitellogenesis.

Regulation of human Clara cell 10 kD protein expression by chicken ovalbumin upstream promoter transcription factors (COUP-TFs)

Clara cell 10 kD protein (CC10) is expressed specifically in a portion of nonciliated airway epithelial cells. The molecular mechanisms that determine its high specificity are not clear. Transcription factors implicated in the regulation of CC10 in rodents do not show the same level of cell specificity. We report here that a 3.3 kb human CC10 DNA fragment, containing the 5' flanking region and promoter, directs lacZ reporter expression in a small portion of Clara cells of the airway epithelia of transgenic mice, indicating the requirement of additional regulatory elements for expression. Addition of an intron containing a transcription enhancer from the human cytokeratin 18 gene greatly enhances the level of transgene expression and broadens epithelial specificity. To gain insight into the mechanisms underlying the cell specificity of human CC10 expression, we performed a promoter analysis of the CC10 gene and a yeast one-hybrid screening to identify factors that regulate the promoter. We have found that chicken ovalbumin upstream promoter transcription factors (COUP-TFs) interact with a proximal promoter region and confirmed the interaction by gel-shift assays. Cotransfection analyses with reporter constructs in cultured cells indicated that COUP-TFs inhibit human CC10 expression. These experiments suggest that COUP-TFs may play a pivotal role in cell specificity of the human CC10 gene by inhibiting its expression in nonpermissive cells.

Formation of an hER alpha-COUP-TFI complex enhances hER alpha AF-1 through Ser118 phosphorylation by MAPK

The enhancement of the human estrogen receptor alpha (hER alpha, NR3A1) activity by the orphan nuclear receptor COUP-TFI is found to depend on the establishment of a tight hER alpha-COUP-TFI complex. Formation of this complex seems to involve dynamic mechanisms different from those allowing hER alpha homodimerization. Although the hER alpha-COUP-TFI complex is present in all cells tested, the transcriptional cooperation between the two nuclear receptors is restricted to cell lines permissive to hER alpha activation function 1 (AF-1). In these cells, the physical interaction between COUP-TFI and hER alpha increases the affinity of hER alpha for ERK2/p42(MAPK), resulting in an enhanced phosphorylation state of the hER alpha Ser118. hER alpha thus acquires a strengthened AF-1 activity due to its hyperphosphorylation. These data indicate an alternative interaction process between nuclear receptors and demonstrate a novel protein intercommunication pathway that modulates hER alpha AF-1.

Orphan receptor chicken ovalbumin upstream promoter transcription factors inhibit steroid factor-1, upstream stimulatory factor, and activator protein-1 activation of ovine follicle-stimulating hormone receptor expression via composite cis-elements

The FSH receptor (FSHR) is selectively expressed in the granulosa and Sertoli cells in a development-dependent manner. Little is known regarding how the regulatory factors balance expression of this gene in ovarian cycles or spermatogenic stages. We have used the ovine FSHR promoter as a model system and identified a third regulatory element (RE-3) located at -197 to -171 of the strongest promoter. Gel mobility shift and antibody supershift assays demonstrated that nuclear factors c-Fos/c-Jun, steroidogenic factor-1 (SF-1), upstream stimulatory factor-1/2 (USF-1/2), and chicken ovalbumin upstream promoter transcription factor-1/2 (COUP-TFI/II) potentially bound to RE-3. We have also extended our previous observations by showing that a sequence containing an E-box was not only bound by USF proteins but also recognized by COUP-TF orphan receptors. Functional studies demonstrated that USF-1/2, c-Fos/c-Jun, and SF-1 were activators, whereas COUP-TFs were repressors. Our studies indicated that RE-3 mediated SF-1 activation as well as phorbol 12-myristate 13-acetate stimulation, whereas COUP-TFs inhibited AP-1, USFs, and SF-1 activation. We also demonstrated that both COUP-TF-binding sites in the core promoter were required for the bipartite elements to oppose their competitor binding. These data suggest a mechanism by which positive and negative regulators compete for the common regulatory elements, providing antagonistic pathways that might govern the expression of FSHR in gonadal cells.

Multiple phosphorylation events control chicken ovalbumin upstream promoter transcription factor I orphan nuclear receptor activity

Chicken ovalbumin upstream promoter transcription factor I (COUP-TFI) is an orphan member of the nuclear hormone receptor superfamily that comprises key regulators of many biological functions, such as embryonic development, metabolism, homeostasis, and reproduction. Although COUP-TFI can both actively silence gene transcription and antagonize the functions of various other nuclear receptors, the COUP-TFI orphan receptor also acts as a transcriptional activator in certain contexts. Moreover, COUP-TFI has recently been shown to serve as an accessory factor for some ligand-bound nuclear receptors, suggesting that it may modulate, both negatively and positively, a wide range of hormonal responses. In the absence of any identified cognate ligand, the mechanisms involved in the regulation of COUP-TFI activity remain unclear. The elucidation of several putative phosphorylation sites for MAPKs, PKC, and casein kinase II within the sequence of this orphan receptor led us to investigate phosphorylation events regulating the various COUP-TFI functions. After showing that COUP-TFI is phosphorylated in vivo, we provide evidence that in vivo inhibition of either MAPK or PKC signaling pathway leads to a specific and pronounced decrease in COUP-TFI-dependent transcriptional activation of the vitronectin gene promoter. Focusing on the molecular mechanisms underlying the MAPK- and PKC-mediated regulation of COUP-TFI activity, we show that COUP-TFI can be directly targeted by PKC and MAPK. These phosphorylation events differentially modulate COUP-TFI functions: PKC-mediated phosphorylation enhances COUP-TFI affinity for DNA and MAPK-mediated phosphorylation positively regulates the transactivation function of COUP-TFI, possibly through enhancing specific coactivator recruitment. These data provide evidence that COUP-TFI is likely to integrate distinct signaling pathways and raise the possibility that multiple extracellular signals influence biological processes controlled by COUP-TFI.

Regulation of aromatase promoter activity in human breast tissue by nuclear receptors

Using the yeast one-hybrid approach to screen a human breast tissue hybrid cDNA expression library, we have found that four orphan/nuclear receptors, ERRalpha-1, EAR-2, COUP-TFI (EAR-3), and RARgamma, bind to the silencer (S1) region of the human aromatase gene. S1 down regulates promoters I.3 and II of the human aromatase gene. In this study, the interaction of EAR-2, COUP-TFI, and RARgamma with S1 was confirmed by DNA mobility shift analysis. In contrast to the findings that ERRalpha-1 behaves as a positive regulatory factor, these three nuclear receptors were found, by mammalian cell transfection experiments, to act as negative regulatory factors by binding to S1. Furthermore, the negative action of these three nuclear receptors could override the positive effect of ERRalpha-1. RT-PCR analysis of 11 cell lines and 55 human breast tumor specimens has shown that these nuclear receptors are expressed in human breast tissue. Since EAR-2, COUP-TFI, and RARgamma are expressed at high levels, it is likely that S1 is a negative regulatory element that suppresses aromatase promoters I.3 and II in normal breast tissue. In cancer tissue, S1 may function as a positive element since ERRalpha-1 is expressed, but EAR-2 and RARgamma are only present in a small number of tumor specimens. This hypothesis is sustained by the finding that there is a weak inverse correlation between the expression of COUP-TFI and that of aromatase in breast tumor tissue. Our studies have revealed that estrogen receptor alpha (ERalpha) can also bind to S1, in a ligand-dependent manner. By binding to S1, ERalpha down-regulates the aromatase promoter activity. These results demonstrate that nuclear receptors play important roles in modulating aromatase expression in human breast tissue.

Induction of chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI ) gene expression is mediated by ETS factor binding sites

Chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI, or NR2F1) is an orphan nuclear receptor that plays a major role in the development of the nervous system. We show here that three ETS response elements in the COUP-TFI promoter mediate its transcription. A reporter gene containing these ETS binding sites is activated by Ets-1, while the same reporter with point mutations on all three ETS response elements is not. We also show that Ets-1 binds to these response elements and that other ETS factors also transactivate the COUP-TFI promoter. In addition, COUP-TFI is coexpressed with some ETS factors in the mouse embryo. These results indicate that members of the ETS family can activate COUP-TFI gene expression.

A unique downstream estrogen responsive unit mediates estrogen induction of proteinase inhibitor-9, a cellular inhibitor of IL-1beta- converting enzyme (caspase 1)

Recently, proteinase inhibitor 9 (PI-9) was identified as the first endogenous inhibitor of caspase 1 (IL-1beta-converting enzyme). The regulation of PI-9 expression, therefore, has great importance in the control of inflammatory processes. We reported that PI-9 mRNA and protein are rapidly and directly induced by estrogen in human liver cells. Using transient transfections to assay PI-9 promoter truncations and mutations, we demonstrate that this strong estrogen induction is mediated by a unique downstream estrogen responsive unit (ERU) approximately 200 nucleotides downstream of the transcription start site. Using primers flanking the ERU in chromatin immunoprecipitation assays, we demonstrate estrogen-dependent binding of ER to the cellular PI-9 promoter. The ERU consists of an imperfect estrogen response element (ERE) palindrome immediately adjacent to a direct repeat containing two consensus ERE half-sites separated by 13 nucleotides (DR13). In transient transfections, all four of the ERE half-sites in the imperfect ERE and in the DR13 were important for estrogen inducibility. Transfected chicken ovalbumin upstream transcription factor I and II down-regulated estrogen-mediated expression from the ERU. EMSAs using purified recombinant human ERalpha demonstrate high-affinity binding of two ER complexes to the ERU. Further EMSAs showed that one ER dimer binds to an isolated DR13, supporting the view that one ER dimer binds to the imperfect ERE and one ER dimer binds to DR13. Deoxyribonuclease I footprinting showed that purified ER protected all four of the half-sites in the ERU. Our finding that a direct repeat can function with an imperfect ERE palindrome to confer estrogen inducibility on a native gene extends the repertoire of DNA sequences able to function as EREs.

EAR2 and EAR3/COUP-TFI regulate transcription of the rat LH receptor

Our previous studies demonstrated regulation of the human LH receptor (hLHR) promoter by nuclear orphan receptors EAR2, EAR3/COUP-TFI (repression), and TR4 (activation) through a direct-repeat motif (hDR). The current studies investigated the differential binding of orphan receptors to rat (rLHR) and hLHR promoters, and their modulation of rLHR gene transcription in rat granulosa cells. The rLHR DR with one nucleotide difference from hDR at its core sequence mediated inhibition of the rLHR transcription, to which EAR2 and EAR3/COUP-TFI but not TR4 bound. The A/C mismatch was responsible for the lack of TR4 binding and function, but had no effect on EAR2 and EAR3/COUP-TFI. EAR2 and EAR3/COUP-TF bound to the rLHR DR with lower affinity than to the hDR, and exhibited lesser inhibitory capacity. This difference resulted from the lack of a guanine in the rDR, which is present 3' next to the hDR core. These studies have identified sequence-specific requirements for the binding of EAR2, EAR3/COUP-TFI, and TR4 to the DRs that explain their differential regulation of rat and human LHR genes. In addition, hCG treatment significantly reduced the inhibition of rLHR gene in granulosa cells and also decreased EAR2 and EAR3/COUP-TFI protein levels. These results indicate that hormonally regulated expression of EAR2 and EAR3/COUP-TFI contributes to gonadotropin-induced derepression of LHR promoter activity in granulosa cells.

Retinoic acid prevents experimental Cushing syndrome

Cushing syndrome is caused by an excess of adrenocorticotropic hormone (ACTH) production by neuroendocrine tumors, which subsequently results in chronic glucocorticoid excess. We found that retinoic acid inhibits the transcriptional activity of AP-1 and the orphan receptors Nur77 and Nurr1 in ACTH-secreting tumor cells. Retinoic acid treatment resulted in reduced pro-opiomelanocortin transcription and ACTH production. ACTH inhibition was also observed in human pituitary ACTH-secreting tumor cells and a small-cell lung cancer cell line, but not in normal cells. This correlated with the expression of the orphan receptor COUP-TFI, which was found in normal corticotrophs but not in pituitary Cushing tumors. COUP-TFI expression in ACTH-secreting tumor cells blocked retinoic acid action. Retinoic acid also inhibited cell proliferation and, after prolonged treatment, increased caspase-3 activity and induced cell death in ACTH-secreting cells. In adrenal cortex cells, retinoic acid inhibited corticosterone production and cell proliferation. The antiproliferative action and the inhibition of ACTH and corticosterone produced by retinoic acid were confirmed in vivo in experimental ACTH-secreting tumors in nude mice. Thus, we conclude that the effects of retinoic acid combine in vivo to reverse the endocrine alterations and symptoms observed in experimental Cushing syndrome.

Molecular cloning and characterization of human nonsteroidal anti-inflammatory drug-activated gene promoter. Basal transcription is mediated by Sp1 and Sp3

Nonsteroidal anti-inflammatory drug-activated gene (NAG-1) is known to be associated with anti-tumorigenic activity and belongs to the transforming growth factor-beta superfamily. In the present study, we cloned the promoter region (-3500 to +41) and investigated the transcriptional regulatory mechanisms of the basal expression of the human NAG-1 gene. Several potential transcription factor-binding sites in this region were identified. Based on the results from clones of nested deletions, the construct between -133 and +41 base pairs contains three Sp1-binding sites (Sp1-A, Sp1-B, and Sp1-C), which confer basal transcription specific activity of NAG-1 expression. When the Sp1-C site was mutated (GG to TT), a 60-80% decrease in promoter activity was observed in HCT-116 cells. Gel shift, co-transfection, and chromatin immunoprecipitation assays showed that the Sp transcription factors bind to the Sp1-binding sites and transactivate NAG-1 expression. In addition, chicken ovalbumin upstream promoter-transcription factor 1 can interact with the C-terminal region of Sp1 and Sp3 proteins and induce NAG-1 promoter activity through Sp1 and Sp3 transcription factors. These results identify the critical regulatory regions for the human NAG-1 basal promoter. Furthermore, the results suggest that the level of expression of the NAG-1 gene will depend on the availability of Sp proteins and on co-factors such as chicken ovalbumin upstream promoter-transcription factor 1.

COUP-TFI: an intrinsic factor for early regionalization of the neocortex

Regionalization of the cerebral cortex is thought to involve two phases: an early regionalization phase and a later refinement phase. It has been shown that early regionalization of the neocortex does not require thalamic inputs and is regulated by intrinsic factors. Recently, two such intrinsic factors, Pax6 and Emx2, have been identified. In this study, we identified COUP-TFI as a regulatory factor for early neocortical regionalization. The spatial and temporal expression pattern of COUP-TFI suggested a role in specification of the neocortex and in maintaining cortical identity. Altered region-specific expression of marker genes in the cortex as well as miswired area-specific connections between the cortex and the thalamus in COUP-TFI null mice indicate COUP-TFI plays a critical role in regulating early regionalization. Our results substantiate that COUP-TFI, an intrinsic factor, may work in concert with Pax6 and Emx2 to specify neocortical identity.

Spot 14 protein interacts and co-operates with chicken ovalbumin upstream promoter-transcription factor 1 in the transcription of the L-type pyruvate kinase gene through a specificity protein 1 (Sp1) binding site

In hepatocytes, the amount of the Spot 14 (S14) protein is closely related to the full expression of enzymes involved in the glycolytic and lipogenic pathways. In the present study we address the role played by this protein in the control of transcription of the L-type pyruvate kinase (L-PK) gene in primary hepatocytes. We show that human S14, which by itself does not bind to the L-PK promoter, physically interacts with the human chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1) and induces the switch of this factor from a repressor to an activator. However, the enhancing activity of S14 and COUP-TF1 depends on the presence of a proximal GC-rich box (the L0 element) that specifically binds nuclear proteins from the livers of rats fed a glucose-rich diet. Moreover, the L0 element, which strongly binds dephosphorylated specificity protein 1 (Sp1), loses all affinity when this factor is phosphorylated by cAMP-dependent protein kinase. Mutations that affect binding of Sp1 and nuclear proteins to the L0 box also decrease basal transcription and impair glucose responsiveness of the promoter. These results therefore shed light on the mechanism by which the S14 protein, whose concentration rapidly rises after glucose intake, contributes to the full activity of the L-PK promoter.

Friend of GATA 2 physically interacts with chicken ovalbumin upstream promoter-TF2 (COUP-TF2) and COUP-TF3 and represses COUP-TF2-dependent activation of the atrial natriuretic factor promoter

Friend of GATA (FOG)-2 is a multi-zinc finger transcriptional corepressor protein that binds specifically to GATA4. Gene targeting studies have demonstrated that FOG-2 is required for normal cardiac morphogenesis, including the development of the coronary vasculature, left ventricular compact zone, and heart valves. To better understand the molecular mechanisms by which FOG-2 regulates these cardiac developmental programs, we screened a mouse day 11 embryo library using a yeast two-hybrid interaction trap with the fifth and sixth zinc fingers of FOG-2 as bait. Using this approach, we isolated clones encoding the orphan nuclear receptors chicken ovalbumin upstream promoter-transcription factor (COUP-TF) 2 and COUP-TF3. COUP-TF2-null embryos die during embryonic development with defective angiogenesis and cardiac defects, a pattern that partly resembles the FOG-2-null phenotype. The interaction between COUP-TF2 and FOG-2 in mammalian cells was confirmed by co-immunoprecipitation of these proteins from transfected COS-7 cells. The sites of binding interaction between COUP-TF2 and FOG-2 were mapped to zinc fingers 5 and 6 and fingers 7 and 8 of FOG-2 and to the carboxyl terminus of the COUP-TF proteins. Binding to COUP-TF2 was specific because FOG-2 did not interact with the ligand-binding domains of retinoid X receptor alpha, glucocorticoid receptor, and peroxisome proliferating antigen receptor gamma, which are related to the COUP-TF proteins. Full-length FOG-2 markedly enhanced transcriptional repression by GAL4-COUP-TF2(117-414), but not by a COUP-TF2 repression domain mutant. Moreover, FOG-2 repressed COUP-TF2dependent synergistic activation of the atrial natriuretic factor promoter by both GATA4 and the FOG-2-independent mutant GATA4-E215K. Taken together, these findings suggest that FOG-2 functions as a corepressor for both GATA and COUP-TF proteins.

A role for the COUP-TF-related gene seven-up in the diversification of cardioblast identities in the dorsal vessel of Drosophila

The Drosophila gene tinman is essential for dorsal vessel (heart) formation and is structurally and functionally conserved in vertebrates. In the mature embryonic dorsal vessel, tinman is expressed in four of the six pairs of cardioblasts in each segment. We provide evidence that seven-up, which is homologous to the vertebrate COUP-TF transcription factor and is expressed in the non-Tinman-expressing cardioblasts, represses tinman in these cells. Loss of function seven-up mutations derepress tinman expression in these cardioblasts while ectopic expression of seven-up represses tinman in the cardioblasts that normally express it. These changes are correlated with alterations in the expression of additional molecular markers for each of these two types of cardioblasts, such as the novel T-box-containing gene Tb66F2 and the potassium channel-encoding gene sur. These observations suggest that seven-up has a role in diversifying cardioblast identities within each segment. We also describe the tinman cis sequences that mediate tinman repression by seven-up and examine whether Seven-up can bind these sequences to directly inhibit tinman.

COUP-TFII Is Up-regulated in Adenovirus Type 12 Tumorigenic Cells and Is a Repressor of MHC Class I Transcription

Down-regulation of the MHC class I enhancer in tumorigenic Ad12 cells is associated with strong binding of COUP-TF and negligible binding of activator NF-kappaB. By comparison, in nontumorigenic Ad5 cells, class I expression is high due to negligible binding of COUP-TF and strong binding of NF-kappaB. Here, we show that COUP-TFII, but not COUP-TFI, is expressed in Ad12-transformed cells. The dramatically stronger DNA binding of COUP-TFII to the class I enhancer in Ad12- compared to Ad5-transformed cells correlates with higher COUP-TFII promoter activity and higher levels of COUP-TFII mRNA and protein. Significantly, NF-kappaB p50/p52 double-knockout cells enabled us to demonstrate directly that COUP-TFII can completely repress both nonactivated and NF-kappaB-activated MHC class I transcription.

Regulation of laminin and COUP-TF expression in extraembryonic endodermal cells

Laminin expression and the subsequent deposition of a basement membrane by primitive endoderm cells is necessary for early mammalian development. We demonstrate that the transcription factors COUP-TF I and II are up-regulated in primitive endoderm cells faster than LAMB1 and LAMC1, and that either COUP-TF is sufficient to induce expression of these laminin genes.

Interplay between liganded and orphan nuclear receptors controls reproductive pathways

Nuclear receptors are transcription factors that belong to an evolutionary ancient superfamily. These proteins, which are even present in primitive metazoans, are implicated in all levels of cell fate: proliferation, differentiation, and apoptosis. Some of these nuclear receptors behave as ligand-inducible transcription factors, as they have acquired during evolution the ability to bind ligands. This is the case for some proteins that recognize small hydrophobic signaling molecules, and particularly the estrogen receptor (ER or NR3A1), which regulates the target gene's transcription rate under estrogen binding. It is now known that the ER alone regulates the transcription of many genes, such as those implicated in reproductive functions. However, this ER-mediated signaling pathway could be modulated by other transcription factors. Our work has established that two other orphan nuclear receptors (SF-1 or NR5A1 and the COUP-TFs, NR2F1 and NR2F2) can enhance two ER-regulated genes implicated in salmonid reproductive functions: the ER gene itself, and the sGTHIIbeta gene. Moreover, some xenoestrogens could disturb these regulations. Therefore, our data contribute to the concept that interplay between nuclear receptors is an important event for the transcriptional regulation of genes controlling cellular functions.

COUP-TFI and COUP-TFII regulate expression of the NHE through a nuclear hormone responsive element with enhancer activity

The chicken ovalbumin upstream promoter-transcription factors (COUP-TFs) are orphan receptors involved in regulation of embryonic development and neuronal cell fate determination. We identified a target of COUP-TF involved in cell proliferation and cell differentiation. Using reporter assays, footprint analysis, and electrophoretic mobility shift assays, we showed that a nuclear hormone-responsive element located at -841/-800 nt of the mouse Na(+)/H(+) exchanger (NHE) promoter binds COUP-TF with enhancer activity. Mutation at -829/-824 nt (and secondarily at -837/-833) prevents COUP binding and activation of the NHE promoter. In vivo expression of COUP isoforms in NIH 3T3 or CV1 cells transactivates from the nuclear hormone-responsive element and from the entire NHE1 promoter. Transactivation is greater for COUP-TFII, is increased for either COUP isoform by the presence of high serum concentrations, and is greatly reduced by mutations preventing COUP binding. In vivo COUP expression in NIH 3T3 cells results in increased synthesis of NHE. Expression of COUP-TFII induced by either retinoic acid or dimethyl sulfoxide in differentiating P19 cells increases NHE expression. The results show that COUP-TF regulates expression of the NHE and provide a mechanism that may be important in physiological and pathological situations linked to its upregulation.

COUP-TF1 antagonizes Nkx2.5-mediated activation of the calreticulin gene during cardiac development

Calreticulin, a Ca(2+) binding chaperone of the endoplasmic reticulum, is also highly expressed in the embryonic heart, and knockout of the calreticulin gene is lethal during embryogenesis because of impaired cardiac development. The protein is down-regulated after birth, and elevated expression of calreticulin in newborn hearts is associated with severe cardiac pathology and death. Here we show that the transcription factor Nkx2.5 activates expression of the calreticulin gene in the heart. Binding of chicken ovalbumin upstream promoter-transcription factor 1 to the Nkx2.5 binding site suppresses transcription from the calreticulin promoter. Nkx2.5 and chicken ovalbumin upstream promoter-transcription factor 1 play antagonistic roles in regulating the expression of calreticulin during cardiac development. These studies indicate that cardiac-specific transcription factor Nkx2.5 plays a central role in activating calreticulin expression and that there is a cooperation between chicken ovalbumin upstream promoter-transcription factor 1 and Nkx2.5 at the calreticulin promoter.

COUP-TFI (chicken ovalbumin upstream promoter-transcription factor I) regulates cell migration and axogenesis in differentiating P19 embryonal carcinoma cells

The developmental expression patterns of the nuclear orphan receptors COUP-TFs (chicken ovalbumin upstream promoter-transcription factors) have been correlated to neurogenesis in several animal species. Nevertheless, the role of COUP-TFs in neurogenesis remains unknown. We have studied the functional involvement of COUP-TFI in retinoic acid (RA)-induced neuronal differentiation of P19 embryonal carcinoma cells through two complementary approaches: 1) deregulated expression of COUP-TFI, and 2) inactivation of endogenous COUP-TFs by means of a dominant-negative COUP-TFI mutant. Low levels of wild-type (wt)COUP-TFI transgene expression did not inhibit neural cell fate and primarily enhanced neuron outgrowth from RA-treated P19 aggregates. In contrast, high COUP-TFI expression impeded the neuronal differentiation of P19 cells induced with RA, resulting in cell cultures lacking neurons. This morphological effect was correlated to an elevated level of E-cadherin mRNA. The dominant-negative COUP-TFI mutant induced cell packing after RA treatment and inhibited neurite extension and neuron outgrowth from aggregates. A RGD peptide interference assay indicated that endogenous COUP-TFs could favor migration of neurons through an integrin-dependent mechanism. Accordingly, vitronectin mRNA levels were shown to be up-regulated by COUP-TFI by RT-PCR analysis, and COUP-TFI stimulated the mouse vitronectin promoter activity in transient transfection assays. Taken together, these data indicate that COUP-TFI is not simply a global repressor of retinoid functions, but shows a high selectivity for regulating genes involved in cellular adhesion and migration processes that are particularly important for neuronal differentiation.

Synergistic activation of the Atlantic salmon hepatocyte nuclear factor (HNF) 1 promoter by the orphan nuclear receptors HNF4 and chicken ovalbumin upstream promoter transcription factor I (COUP-TFI)

Hepatocyte nuclear factor 1 (HNF1) is a liver-enriched transcription factor that plays an important role in transcriptional networks involved in liver function. The promoters of mammalian HNF1 genes contains a single binding site for another liver-enriched transcription factor, the nuclear hormone receptor HNF4. A transcriptional hierarchy involving HNF4-mediated activation of the HNF1 promoter has been proposed to be of crucial importance in maintaining the differentiated hepatocyte phenotype. Here we present evidence that the Atlantic salmon HNF1 promoter contains three nuclear-hormone-receptor-binding sequences. Gel-shift assays showed that these motifs are recognized with different affinities by HNF4 and the orphan nuclear receptors chicken ovalbumin upstream promoter transcription factors COUP-TFI and COUP-TFII. In hepatoma cells, the site showing highest affinity for HNF4 appears to be crucial for promoter activity. Transfection experiments in non-hepatic cells indicated that the salmon HNF1 promoter was activated by both HNF4 and COUP-TFs. We also identified a promoter fragment encompassing the two more distal nuclear-hormone-binding sites that was activated by HNF4, unaffected by COUP-TF and showed a strong synergistic activation by HNF4/COUP-TF. Results are presented detailing these interactions in relation to the salmon HNF1 promoter architecture.

Differential expression of COUP-TFI, CHL1, and two novel genes in developing neocortex identified by differential display PCR

Genes that control the specification and differentiation of the functionally specialized areas of the mammalian neocortex are likely expressed across the developing neocortex in graded or restricted patterns. To search for such genes we have performed a PCR-based differential display screen using RNAs from rostral neocortex, which included the primary motor area, and caudal neocortex, which included the primary visual area, of embryonic day 16 rats. We identified 82 differentially expressed gene fragments. Secondary screening by in situ hybridization confirmed that five fragments, representing four genes, are differentially expressed across developing rat neocortex. Two of the genes, chick ovalbumin upstream transcription factor I (COUP-TFI) and close homolog of L1 (CHL1), have been cloned previously, but their differential expression in cortex has not been reported. Sequences from the other two fragments suggest that they represent novel genes. The expression patterns include graded, restricted, and discontinuous expression with abrupt borders that might correlate with those of areas. The differential expression patterns of all four genes are established before the arrival of thalamocortical afferents, suggesting that they are independent of thalamic influence, and could direct or reflect arealization. In addition, COUP-TFI and CHL1 exhibit dynamic expression patterns that undergo substantial changes after thalamocortical afferents invade the cortical plate, suggesting that thalamic axons may influence their later expression. Postnatally, COUP-TFI is most prominently expressed in layer 4, in both rats and mice, and CHL1 is expressed in layer 5. COUP-TFI expression in cortex, and in ventral telencephalon and dorsal thalamus, suggests several possible causes for the loss of layer 4 neurons and the reduced thalamocortical projection reported in COUP-TFI knock-out mice.

Molecular basis for treating endometriosis with aromatase inhibitors

Although treatment of one unusually aggressive case of postmenopausal endometriosis with an aromatase inhibitor has been strikingly successful, large clinical trials are required to establish whether aromatase inhibitors will have a significant role in the medical management of endometriosis. Introduction of aromatase inhibitors into the treatment of endometriosis underscores the importance of basic research leading to the development of novel strategies in reproductive disorders. It was shown earlier that aromatase activity was not detectable in normal endometrium. Aromatase, however, is expressed inappropriately in endometriosis and stimulated by prostaglandin E2. Aromatase activity gives rise to local biosynthesis of oestrogen, which, in turn, stimulates prostaglandin E2 production, thus establishing a positive feedback cycle. This favours accumulation of oestrogen and prostaglandins in endometriosis, which is an inflammatory disorder dependent on oestrogen for growth.

The retinoid X receptor response element in the human aldehyde dehydrogenase 2 promoter is antagonized by the chicken ovalbumin upstream promoter family of orphan receptors

Two tandem sites in the aldehyde dehydrogenase 2 promoter (designated FP330-5' and FP330-3') that bind members of the nuclear receptor superfamily were recently identified. Antibodies against apolipoprotein regulatory protein (ARP-1) altered DNA-protein interactions in electrophoretic mobility shift assays using oligonucleotides representing either promoter site and rat liver or cultured cell nuclear extracts. In vitro-translated chicken ovalbumin upstream promoter transcription factor (COUP-TFI), ARP-1, or ErbA-related protein 2 (Ear2) bound both sites. In addition, ARP-1/RXR, COUP-TFI/RXR, and ARP-1/COUP-TFI heterodimers bound the FP330-3' site. Mutagenesis of the FP330-3' site indicated that a DR-1 element was the preferred binding site for these factors. Transfected expression plasmids for these factors suppressed basal expression of reporter constructs containing the FP330-3' sites and the induction of the reporter by RXRalpha plus retinoic acid. Mutation of the two sites increased activity of a construct driven by 600 bp of the ALDH2 promoter in cell lines expressing COUP-TFs. The ALDH2 FP330-3' site appears to represent a complex nuclear receptor response element that is activated by RXRs and HNF-4 but repressed by members of the COUP-TF family.