Control of transcription initiation in vitro requires binding of a transcription factor to the distal promoter of the ovalbumin gene

Recent progress on the role and molecular mechanism of chicken ovalbumin upstream promoter-transcription factor II in cancer

Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) is an orphan receptor that regulates the expression of genes involved in development and homeostasis. COUP-TFII is also dysregulated in cancer, where it plays important roles in oncogenesis and malignant progression. Recent studies have also investigated altered microRNA-mediated regulation of COUP-TFII in cancer. Although many investigators have studied the expression and clinical significance of COUP-TFII in several cancer types, there remain many controversies regarding its role in these diseases. In this review, we will describe the functions and underlying molecular mechanisms of COUP-TFII in several cancers, especially colorectal, gastric, breast, and prostate cancer; additionally, we will briefly summarize what is known about microRNA-mediated regulation of COUP-TFII.

Activation of COUP-TFI by a Novel Diindolylmethane Derivative

Chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) is an orphan receptor and member of the nuclear receptor superfamily. Among a series of methylene substituted diindolylmethanes (C-DIMs) containing substituted phenyl and heteroaromatic groups, we identified 1,1-bis(3'-indolyl)-1-(4-pyridyl)-methane (DIM-C-Pyr-4) as an activator of COUP-TFI. Structure activity studies with structurally diverse heteroaromatic C-DIMs showed that the pyridyl substituted compound was active and the 4-pyridyl substituent was more potent than the 2- or 3-pyridyl analogs in transactivation assays in breast cancer cells. The DIM-C-Pyr-4 activated chimeric GAL4-COUP-TFI constructs containing full length, C- or N-terminal deletions, and transactivation was inhibited by phosphatidylinositol-3-kinase and protein kinase A inhibitors. However, DIM-C-Pyr-4 also induced transactivation and interactions of COUP-TFI and steroid receptor coactivators-1 and -2 in mammalian two-hybrid assays, and ligand-induced interactions of the C-terminal region of COUP-TFI were not affected by kinase inhibitors. We also showed that DIM-C-Pyr-4 activated COUP-TFI-dependent early growth response 1 (Egr-1) expression and this response primarily involved COUP-TFI interactions with Sp3 and to a lesser extent Sp1 bound to the proximal region of the Egr-1 promoter. Modeling studies showed interactions of DIM-C-Pyr-4 within the ligand binding domain of COUP-TFI. This report is the first to identify a COUP-TFI agonist and demonstrate activation of COUP-TFI-dependent Egr-1 expression.

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.

The pleiotropic transcriptional regulator COUP-TFI plays multiple roles in neural development and disease

Transcription factors are expressed in a dynamic fashion both in time and space during brain development, and exert their roles by activating a cascade of multiple target genes. This implies that understanding the precise function of a transcription factor becomes a challenging task. In this review, we will focus on COUP-TFI (or NR2F1), a nuclear receptor belonging to the superfamily of the steroid/thyroid hormone receptors, and considered to be one of the major transcriptional regulators orchestrating cortical arealization, cell-type specification and maturation. Recent data have unraveled the multi-faceted functions of COUP-TFI in the development of several mouse brain structures, including the neocortex, hippocampus and ganglionic eminences. Despite NR2F1 mutations and deletions in humans have been linked to a complex neurodevelopmental disease mainly associated to optic atrophy and intellectual disability, its role during the formation of the retina and optic nerve remains unclear. In light of its major influence in cortical development, we predict that its haploinsufficiency might be the cause of other cognitive diseases, not identified so far. Mouse models offer a unique opportunity of dissecting COUP-TFI function in different regions during brain assembly; hence, the importance of comparing and discussing common points linking mouse models to human patients' symptoms.

Overexpression of COUP‑TFII suppresses proliferation and metastasis of human gastric cancer cells

The abnormal expression of the chicken ovalbumin upstream promoter transcription factor 2 (COUP-TFII) is associated with numerous forms of cancer, including gastric, prostate, colon and lung cancer. However, previous studies investigating the association between COUP-TFII expression and the occurrence, recurrence, invasion and metastasis of gastric cancer are limited in number. In the present study, it was revealed that the expression of COUP-TFII is significantly reduced in gastric carcinoma tissues compared with normal gastric mucosa cells (GES-1). In addition, the expression of COUP-TFII was also reduced in gastric cancer cell lines compared with GES-1 cells. Furthermore, it was revealed that ectopic expression of COUP-TFII was able to suppress the proliferation, migration and invasion of gastric cells, as well as inhibit hepatic metastasis, in vivo. In addition, it was demonstrated that COUP-TFII knockdown was able to promote the proliferation, migration and invasion of GES-1 cells in vitro. Furthermore, database analysis suggested that COUP-TFII expression in patients with gastric cancer is correlated with clinical stage classification and increased expression levels of COUP-TFII improved overall survival rates in patients with gastric cancer. The results of the present study suggest that COUP-TFII functions as a significant regulatory suppressor of gastric cancer growth and metastasis, and suggests that COUP-TFII may serve as a novel diagnostic and prognostic biomarker for gastric cancer metastasis.

Transcriptional Factors Mediating Retinoic Acid Signals in the Control of Energy Metabolism

Retinoic acid (RA), an active metabolite of vitamin A (VA), is important for many physiological processes including energy metabolism. This is mainly achieved through RA-regulated gene expression in metabolically active cells. RA regulates gene expression mainly through the activation of two subfamilies in the nuclear receptor superfamily, retinoic acid receptors (RARs) and retinoid X receptors (RXRs). RAR/RXR heterodimers or RXR/RXR homodimers bind to RA response element in the promoters of RA target genes and regulate their expressions upon ligand binding. The development of metabolic diseases such as obesity and type 2 diabetes is often associated with profound changes in the expressions of genes involved in glucose and lipid metabolism in metabolically active cells. RA regulates some of these gene expressions. Recently, in vivo and in vitro studies have demonstrated that status and metabolism of VA regulate macronutrient metabolism. Some studies have shown that, in addition to RARs and RXRs, hepatocyte nuclear factor 4α, chicken ovalbumin upstream promoter-transcription factor II, and peroxisome proliferator activated receptor β/δ may function as transcriptional factors mediating RA response. Herein, we summarize current progresses regarding the VA metabolism and the role of nuclear receptors in mediating RA signals, with an emphasis on their implication in energy metabolism.

The role of the orphan nuclear receptor COUP-TFII in tumorigenesis

The chicken ovalbumin upstream promoter transcription factors (COUP-TFs), members of the nuclear receptor superfamily, consist of two highly homologous subtypes, COUP-TFI (EAR-3, NR2F1) and COUP-TFII (ARP-1, NR2F2). They are referred to as orphan receptors because the COUP-TF ligands have yet to be identified. Since the discovery of COUP-TFs in 1986, extensive studies have demonstrated their crucial functions in a variety of developmental processes, such as organogenesis, angiogenesis, and metabolic homeostasis. Recently, emerging evidence has highlighted that COUP-TFs, specifically COUP-TFII, play important roles in tumorigenesis. In this review, we will discuss the critical functions of COUP-TFII in the development of the tumor microenvironment, the progression of various cancers, and its underlying mechanisms.

Regulatory potential of COUP-TFs in development: stem/progenitor cells

The formation of complex organisms is highly dependent on the differentiation of specialized mature cells from common stem/progenitor cells. The orphan nuclear receptors chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are broadly, but not ubiquitously, expressed in multiple tissues throughout embryonic development and COUP-TFs are indispensible for proper organogenesis. Recently, growing evidence suggests a critical role of COUP-TFs in multiple aspects of stem/progenitor cell biology. In this review, we highlight the progress of COUP-TFs function and its underlying mechanism in driving stem/progenitor cell self-renewal, lineage specification, differentiation, maintenance, and cell identity in diverse tissue types. These studies provide novel insights into future clinical utilities of COUP-TFs in stem cell based therapies and in the management of diseases.

Nuclear receptors in bone physiology and diseases

During the last decade, our view on the skeleton as a mere solid physical support structure has been transformed, as bone emerged as a dynamic, constantly remodeling tissue with systemic regulatory functions including those of an endocrine organ. Reflecting this remarkable functional complexity, distinct classes of humoral and intracellular regulatory factors have been shown to control vital processes in the bone. Among these regulators, nuclear receptors (NRs) play fundamental roles in bone development, growth, and maintenance. NRs are DNA-binding transcription factors that act as intracellular transducers of the respective ligand signaling pathways through modulation of expression of specific sets of cognate target genes. Aberrant NR signaling caused by receptor or ligand deficiency may profoundly affect bone health and compromise skeletal functions. Ligand dependency of NR action underlies a major strategy of therapeutic intervention to correct aberrant NR signaling, and significant efforts have been made to design novel synthetic NR ligands with enhanced beneficial properties and reduced potential negative side effects. As an example, estrogen deficiency causes bone loss and leads to development of osteoporosis, the most prevalent skeletal disorder in postmenopausal women. Since administration of natural estrogens for the treatment of osteoporosis often associates with undesirable side effects, several synthetic estrogen receptor ligands have been developed with higher therapeutic efficacy and specificity. This review presents current progress in our understanding of the roles of various nuclear receptor-mediated signaling pathways in bone physiology and disease, and in development of advanced NR ligands for treatment of common skeletal disorders.

Characterization of a cis-acting element involved in cell-specific expression of the zebrafish brain aromatase gene

The cytochrome P450 Aromatase is the key enzyme catalyzing the conversion of androgens into estrogens. In zebrafish, the brain aromatase is encoded by cyp19b. Expression of cyp19b is restricted to radial glial cells bordering forebrain ventricles and is strongly stimulated by estrogens during development. At the promoter level, we have previously shown that an estrogen responsive element (ERE) is required for induction by estrogens. Here, we investigated the role of ERE flanking regions in the control of cell-specific expression. First, we show that a 20 bp length motif, named G x RE (glial x responsive element), acts in synergy with the ERE to mediate the estrogenic induction specifically in glial cells. Second, we demonstrate that, in vitro, this sequence binds factors exclusively present in glial or neuro-glial cells and is able to confer a glial specificity to an artificial estrogen-dependent gene. Taken together, these results contribute to the understanding of the molecular mechanisms allowing cyp19b regulation by estrogens and allowed to identify a promoter sequence involved in the strong estrogen inducibility of cyp19b which is specific for glial cells. The exceptional aromatase activity measured in the brain of teleost fish could rely on such mechanisms.

Seven-up facilitates insect counter-defense by suppressing cathepsin B expression

When challenged by the dietary soybean cysteine protease inhibitor scN, the cowpea bruchid (Callosobruchus maculatus) adapts to the inhibitory effects by readjusting the transcriptome of its digestive system, including the specific activation of a cathepsin B-like cysteine protease CmCatB. To understand the transcriptional regulation of CmCatB, we cloned a portion of its promoter and demonstrated its activity in Drosophila cells using a chloramphenicol acetyltransferase reporter system. EMSAs detected differential DNA-binding activity between nuclear extracts of scN-adapted and -unadapted midguts. Two tandem chicken ovalbumin upstream promoter (COUP) elements were identified in the CmCatB promoter that specifically interacted with a protein factor unique to nuclear extracts of unadapted insect guts, where CmCatB expression was repressed. Seven-up (Svp) is a COUP-TF-related transcription factor that interacted with the COUP responsive element. Polyclonal anti-(mosquito Svp) serum abolished the specific DNA-binding activity in cowpea bruchid midgut extracts, suggesting that the protein factor is an Svp homolog. Subsequent cloning of a cowpea bruchid Svp (CmSvp) indicated that it shares a high degree of amino acid sequence similarity with COUP-TF/Svp orphan nuclear receptor family members from varied species. The protein was more abundant in scN-unadapted insect guts than scN-adapted guts, consistent with the observed DNA-binding activity. Furthermore, CmCatB expression was repressed when CmSvp was transiently expressed in Drosophila cells, most likely through COUP binding. These findings indicate that CmSvp may contribute to insect counter-defense, in part by inhibiting CmCatB expression under normal growth conditions, but releasing the inhibition when insects are challenged by dietary protease inhibitors.

Protein-DNA Array-based Identification of Transcription Factor Activities Regulated by Interaction with the Glucocorticoid Receptor

The glucocorticoid receptor (GR) regulates gene expression by binding specific sequence elements within the promoters of target genes or by cross-talk with other transcription factors (TFs). For some TFs, interaction with the GR results in alteration of DNA binding and transcriptional regulation. We used a protein-DNA array, a system that facilitates simultaneous profiling of the activities of multiple transcription factors, to systematically examine the potential cross-talk of GRalpha with 149 TFs. Using this array, we identified several TFs, including IRF, E47, and COUP-TF, whose DNA binding activities were modulated by GRalpha. We then confirmed these results with in vitro electrophoretic mobility shift assays and in vivo reporter assays. In this study, IRF and E47 were identified as participants in GRalpha cross-talk for the first time. This new finding expands our understanding of the functional role of GRalpha in the context of gene expression regulation.

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.

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.

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.

The affinity and activity of the multiple hormone response element in the proximal promoter of the human oxytocin gene

In vivo there appears to be a marked association between oestrogen levels and the expression of the oxytocin (OT) gene in most tissues. Transfection and DNA-protein binding experiments using high levels of either oestrogen receptor (ER)alpha or ERbeta imply a direct interaction of these transcription factors with the multiple hormone response element (HRE) at approximately -160 from the transcription start site of the OT gene in most species. In an extensive set of experiments, we show, using both transfection and protein-DNA binding, that low to moderate amounts of either oestrogen receptor, while being able to interact directly with a classic oestrogen response element (ERE) fail to interact with the human OT -160 HRE. Instead, this element, similar to its bovine counterpart, has a high affinity for the orphan receptors steroidogenic factor 1 and chicken ovalbumin upstream promoter transcription factor. Second, the human and bovine OT promoter can be made artificially responsive towards oestrogen in a cotransfection system over-expressing ERalpha or ERbeta, but not in cells expressing natural levels of these steroid receptors. Interestingly, nuclear extracts from both ERalpha-positive MCF7 cells and ERalpha-negative MDA-MB231 cells both contain a transcription factor which binds specifically to both the hOT-HRE element and to a classic ERE, and which has orphan receptor-like binding properties rather than those of an oestrogen receptor. Together, these and other results suggest that oestrogen action in vivo on the OT gene in all species is more likely to involve a DNA-independent mechanism than classic direct interactions with dimeric oestrogen receptors.

An upstream element containing an ETS binding site is crucial for transcription of the human presenilin-1 gene

Deletion mapping of the human presenilin-1 (PS1) promoter delineated the most active fragment from -118 to +178 in relation to the transcription start site mapped in this study, in both human neuroblastoma SK-N-SH and hepatoma HepG2 cells. 5' deletions revealed that a crucial element controlling over 90% of the promoter activity in these cell lines is located between -22 and -6. A mutation altering only two nucleotides of the ETS consensus sequence present at -12 (GGAA to TTAA) has a similar effect. Electrophoretic mobility shift assays showed that a set of specific complexes between nuclear factors and the PS1 promoter are eliminated by this point mutation, as well as by competition with an ETS consensus oligonucleotide. Competition experiments in DNase I footprinting correlated with electrophoretic mobility shift assays and showed that only one of several footprints over the PS1 promoter is eliminated by competition with an ETS consensus oligonucleotide. It extends from -14 to -6 and surrounds the ETS motif present at -12. Thus, a crucial ETS element is present at -12 and binds a protein(s) recognizing specifically the ETS consensus motif. At least one such complex is eliminated by preincubating the nuclear extract with an antibody with broad cross-reactivity with Ets-1 and Ets-2 proteins, thus confirming that an ETS transcription factor(s) recognizes the -12 motif. Several Sp1 binding motifs at positions -70, -55, and +20 surround this ETS element. Competition DNase I footprinting showed that Sp1-like nuclear factors recognize specifically these sites in both cell lines. Furthermore, a combination of 5' and 3' deletions indicated the presence of positive promoter elements between -96 and -35 as well as between +6 and +42. Thus, transfection and footprinting assays correlate to suggest that Sp1 transcription factor(s) bind at several sites upstream and downstream from the initiation site and activate the transcription of the PS1 promoter. Sequences downstream from the transcription initiation site also contain major control elements. 3' deletions from +178 to +107 decreased promoter activity by 80%. However, further deletion to +42 increased promoter activity by 3-4-fold. Collectively, these data indicate that sequences upstream and downstream from the transcription start site each control over 80% of the promoter activity. Hence, this suggests that protein-protein interactions between factors recognizing downstream and upstream sequences are involved.

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): expression during mouse embryogenesis

Members of the steroid/thyroid hormone receptor superfamily such as TR, RAR, RXR and VDR are known to play important roles in regulation of gene expression during development, differentiation and homeostasis. COUP-TFs are orphan members of this superfamily of nuclear receptors and have been shown to negatively regulate the ability of these nuclear receptors to transactivate target genes. Two different mechanisms are implicated in this repression. First, COUP-TFs bind to AGGTCA direct repeats and palindromes with various spacings, which include response elements for TR, RAR, RXR and VDR, allowing for direct competition of COUP-TFs for the response elements. Second, COUP-TFs can heterodimerize with RXRs, the essential cofactor for effective binding of VDR, TRs and RARs to their cognate response elements. The physiological significance of this negative effect of COUP-TF on the activity of these receptors has been analyzed. Detection of COUP-TF transcripts during mouse development reveal discrete spatial and temporal expression domains consistent with COUP-TFs being involved in regulation of gene expression during embryogenesis. Transcripts are localized within discrete regions of the central and peripheral nervous system including the inner ear. In addition, COUP-TFs are found in many tissues including testes, ovary, prostate, skin, kidney, lung, stomach, intestine, pancreas and salivary gland. Some of these expression domains colocalize with those of TR, RAR, and RXR. The simultaneous expression of these genes raise the possibility that COUP-TFs can act as negative regulatory factors during development and differentiation.

COUP-TF an orphan member of the steroid/thyroid hormone receptor superfamily

COUP-TFs are orphan members of the steroid/thyroid hormone receptor superfamily. They are transcription factors that are highly conserved across species and have a wide spectrum of binding specificity. In tissue cultures, COUP-TFs repress the transactivation function of several other members of the steroid/thyroid hormone receptor superfamily, and they also negatively regulate the transcription from promoters of many other genes. The expression profile of COUP-TFs in several species suggests that they play an important role in development and differentiation.

Ear3/COUP-TF binds most tightly to a response element with tandem repeat separated by one nucleotide

Ear3/COUP-TF belongs to the steroid/thyroid hormone receptor superfamily of nuclear transcription factors. Previous studies have identified six target response elements to which the factors of the family bind with various affinities. They are palindromic pair of hexameric half site (Thyroid hormone Response Element; TRE, Estrogen Response Element; ERE, Glucocorticoid Response Element; GRE) or directly repeated half sites [Direct Repeat (DR) 3, DR4, DR5]. In this report, these six elements and three additional putative elements, DR0, DR1, and DR2, which are directly repeated half sites separated by 0, 1 and 2 nucleotides, respectively, were tested for binding to Ear3/COUP-TF protein employing gel retardation method. Among the palindromic series of response elements, TRE showed the highest affinity, ERE showed second highest, and no binding was observed for GRE. Among six direct repeat response elements, DR1 showed the highest affinity. Comparison of binding affinities between DR1 and TRE revealed that the DR1 has the highest affinity of all the nine response elements tested. The consensus sequence of the DR1 element resembles well the COUP sequence found in the upstream promoter of chicken ovalbumin gene, which has been shown to serve as a natural response element for Ear3/COUP-TF. Retinoid X receptor (RXR), another member of the receptor family, reportedly utilizes DR1 sequence as its response element, suggesting that Ear3/COUP-TF could regulate the expression of the same subsets of genes as RXR does.

Identification of a novel transcription factor, ACF, in cultured avian fibroblast cells that interacts with a Marek's disease virus late gene promoter

Interactions between factors in duck and chick embryo fibroblast (DEF and CEF, respectively) nuclear extracts and the Marek's disease virus (MDV) gp57-65 gene promoter were investigated. Results of in vitro transcription and gel mobility-shift assays indicated that multiple cellular factors interact with 5'-flanking sequences of the MDV gp57-65 gene. One sequence-specific DNA binding activity (termed ACF for avian cell factor(s)) was identified by interaction of DEF and CEF nuclear extract proteins with a particular site (nucleotides -193 to -177) in the MDV gp57-65 gene promoter. Binding of ACF to its apparent recognition sequence, contained within the 17-bp oligonucleotide 5'-CTAGTTTACTTGTTTGT-3' (ACF-12), was highly sequence-specific. Radiolabeled ACF-12 oligonucleotide bound significant ACF protein in the presence of a 400-fold molar excess of unlabeled nonspecific competitor DNA. A similar amount of specific competitor completely abolished ACF binding to probe DNA. Deletion of the ACF binding site from MDV gp57-65 gene promoters linked to a chloramphenicol acetyltransferase (CAT) reporter gene reduced expression of CAT activity by twofold relative to that seen with a gp57-65 promoter-CAT construct containing an intact ACF binding site. Transfection inhibition assays using double-stranded ACF binding site competitors reduced steady-state levels of gp57-65 mRNA in MDV infected cells by over twofold relative to those in control infected cells. Introduction of a similar amount of nonspecific double-stranded oligonucleotide had no adverse effect on gp57-65 mRNA levels. These data suggest that ACF is important for efficient expression of gp57-65.

Regulatory elements of the pro-opiomelanocortin gene: pituitary specificity and glucocorticoid repression

A short 543-bp fragment o f the pro-opiomelanocortin gene is sufficient for pituitary-specific expression and, in the anterior pituitary gland, for repression of pro-opiomelanocortin transcription by glucocorticoids. Within this 5'-flanking fragment of the gene, multiple regulatory elements contribute to tissue-specific expression and a single glucocorticoid receptor binding site acts as a "negative glucocorticoid response element." The current model for glucocorticoid repression depends on the mutually exclusive binding of the glucocorticoid receptor and of a positive transcription factor, the chicken ovalbumin upstream promoter element transcription factor, to overlapping DNA sequences within the negative glucocorticoid response element.

COUP transcription factor is a member of the steroid receptor superfamily

The COUP (chicken ovalbumin upstream promoter) transcription factor (COUP-TF) exists in a number of different tissues and is essential for expression of the chicken ovalbumin gene. It binds to the ovalbumin promoter and, in conjunction with a second protein (S300-II), stimulates initiation of transcription in vitro. COUP-TF also binds specifically to the rat insulin promoter element, although the two binding sites share little sequence similarity. Here we report the isolation of a human complementary DNA clone encoding COUP-TF. Comparison of the amino-acid sequence of COUP-TF with known sequences reveals that it is a member of the steroid/thyroid hormone/vitamin receptor superfamily. Consequently, it is the first member of this family that has been shown to function in a cell-free transcription system. We conclude that this superfamily of gene regulators contains proteins which bind and activate distal promoter elements of eukaryotic genes.

Glucocorticoid repression of pro-opiomelanocortin gene transcription

Transcription of the pro-opiomelanocortin (POMC) gene is repressed by glucocorticoids in the anterior pituitary gland. We have defined an element within the POMC promoter which is responsible for this regulatory feedback. This element, the "negative glucocorticoid response element" (nGRE), was localized in the proximal region of the POMC promoter and it contains a binding site for the glucocorticoid receptor. Receptor binding to the nGRE was correlated to hormone-dependent repression by using promoter mutagenesis. The nGRE was also shown to contain a binding site for a nuclear protein of the COUP family of transcription factors. Since the binding sites for COUP and the glucocorticoid receptor overlap, glucocorticoid-dependent repression of POMC transcription may result from mutually exclusive binding of these two nuclear transcription factors.

Characterization of the trans-activation-responsive element of the parvovirus H-1 P38 promoter

The parvovirus early protein NS1 positively regulates the expression of the P38 promoter for the viral capsid protein gene. We have examined the trans-activation of P38 by NS1 by using fusions of P38 to the reporter gene, chloramphenicol acetyltransferase (cat). Maximal trans-activation requires a small 5' cis element (tar) between -137 and -116. The tar element has activity in both orientations when 5' to the P38 promoter, but no activity has been detected 3' to the promoter. The wild-type P38 has a biphasic response to NS1 depending on the dosage of the NS1-expressing plasmid. Promoters lacking the tar also have a biphasic response that is reduced about 10-fold, and they can be inhibited by larger doses of the NS1 plasmid. Heterologous promoters from other viruses and the Harvey-ras oncogene promoter are inhibited by NS1. Truncated and internally deleted versions of NS1 lose the trans-activation, but some of them retain the inhibitory properties. Thus transactivation can be uncoupled from inhibition. The tar element has shown no activity with the heterologous simian virus 40 early promoter. In contrast, the P38 promoter responds to a heterologous enhancer, but the enhanced promoter loses activity to trans-activation by NS1. In summary, the P38 tar element has some of the properties of an enhancer with a high preference for a 5' position and a stringent requirement for the P38 promoter.

Interactions between a DNA-binding transcription factor (COUP) and a non-DNA binding factor (S300-II)

We have identified previously two transcription factors, COUP (chicken ovalbumin upstream promoter) and S300-II, from HeLa cell nuclear extracts. In this paper, the purine base and the phosphate backbone contact sites for the COUP transcription factor were defined. These studies indicate that the COUP box transcription factor interacts with specific base residues in the major groove of the DNA helix. In addition, we have purified the S300-II factor over 100,000-fold. The polypeptide possessing functional transcriptional activity has been identified by SDS-PAGE followed by gel-slice elution and a renaturation assay. It is absolutely required for in vitro function of the ovalbumin promoter. In addition, S300-II stimulates transcription from the MMTV and lysozyme promoters. Kinetic studies probing the interaction of S300-II with COUP factor suggest that it may stabilize COUP-promoter complexes by slowing their rate of dissociation.