Regulation of the human P450scc gene by steroidogenic factor 1 is mediated by CBP/p300

Transcriptional activation of thymidylate synthase by 17beta-estradiol in MCF-7 human breast cancer cells

Thymidylate synthase (TS) catalyzes methylation of deoxyuridine phosphate to give deoxythymidine phosphate, and 17beta-estradiol (E2) induces TS gene expression in MCF-7 human breast cancer cells. Analysis of the TS gene promoter showed that E2-responsiveness required the -229 to -140 promoter region containing a G-rich sequence and CACCC box. Subsequent mutational analysis of this region indicated that only the G-rich motif (-150 to -142) was required for E2 action. Results of gel mobility shift and in vitro DNA footprinting assays showed that both estrogen receptor alpha (ERalpha) and Sp1 proteins were required for hormone-induced trans-activation that involved ERalpha/Sp1 binding to the G-rich site in which only Sp1 protein bound DNA. Both proteins also interacted in Drosophila cells in functional assays, confirming the transcriptional activation of TS-involved ERalpha/Sp1, and this adds to the increasing number of genes that are activated through this pathway in breast cancer cells.

Impaired steroidogenic factor 1 (NR5A1) activity in mutant Y1 mouse adrenocortical tumor cells

Mutants isolated from the Y1 mouse adrenocortical tumor cell line (clones 10r-9 and 10r-6) are resistant to ACTH because they fail to express the melanocortin-2 receptor (MC2R). In this study, we show that a luciferase reporter plasmid driven by 1,800 bp of the proximal promoter region of the MC2R was expressed poorly in the mutant cells compared with parent Y1 cells. The differential expression of the MC2R in parent and mutant cells resulted from impaired activity of the orphan nuclear receptor NR5A1 (SF1) on the promoter as determined by 5'-deletion analysis. Furthermore, the activity of an SF1 expression plasmid on an SF1-dependent reporter plasmid was compromised in mutant clones. The site-specific DNA binding properties of SF1 from parent and mutant cells did not differ as determined in electrophoretic mobility shift assays, and the addition of the activation domain of VP16 to the amino terminus of SF1 restored the transcriptional activity of the protein. In addition, the levels of SF1 and other cofactors including WT1, CBP/p300, and steroid receptor coactivator 1 did not differ appreciably between parent and mutant cells. Taken together, these results suggest that ACTH resistance in the mutant clones resulted from a defect that affected the activation properties of SF1 rather than its DNA binding activity. Consistent with the observed impairment in SF1 function, other SF1-dependent genes, including Cyp11b1 and steroidogenic acute regulatory protein (StAR), were poorly expressed and global steroidogenesis, as evidenced by the metabolism of 22(R)-hydroxycholesterol to steroid products, was impaired. Interestingly, MC2R, Cyp11a, Cyp11b1, and StAR transcripts were not affected to the same degree, suggesting that each of these genes may have a different absolute requirement for SF1. These mutants thus provide an experimental paradigm to identify factors that influence SF1 function and to evaluate the relative importance of SF1 in the expression of genes essential for adrenal steroidogenesis.

Synergy of SF1 and RAR in activation of Oct-3/4 promoter

The Oct-3/4 transcription factor is expressed in the earliest stages of embryogenesis, and is thus likely to play an important role in regulation of initial decisions in development. For the first time, we have shown that SF1 and Oct-3/4 are co-expressed in embryonal carcinoma (EC) P19 cells, and their expression is down-regulated with very similar kinetics following retinoic acid (RA) induced differentiation of these cells, suggesting a functional relationship between the two. Previously, we have shown that the Oct-3/4 promoter harbors an RA-responsive element, RAREoct, which functions in EC cells as a binding site for positive regulators of transcription, such as RAR and RXR. In this study we have identified in the Oct-3/4 promoter two novel SF1-binding sites: SF1(a) and SF1(b). The proximal site, SF1(a), is located within the RAREoct, and the distal site, SF1(b), is located between nucleotide -193 and -209 of the Oct-3/4 promoter. Both sites contribute to activation of Oct-3/4 promoter in EC cells, with SF1(a) playing a more crucial role. SF1, and its isoforms ELP2 and ELP3 bind to both SF1 sites and activate the Oct-3/4 promoter. This activation depends on the presence of SF1 DNA-binding domain. Thus, Oct-3/4 is the first EC-specific gene reported that is regulated by SF1. Interestingly, SF1 and RAR form a novel complex on the RAREoct sequence that synergistically activate the Oct-3/4 promoter. Both RARE and SF1 cis regulatory elements, as well as the SF1 DNA-binding domain, are needed for this synergism. SF1 and Oct-3/4 transcription factors play a role in the same developmental regulatory cascade.

Synergistic activation of the inhibin alpha-promoter by steroidogenic factor-1 and cyclic adenosine 3',5'-monophosphate

The inhibin alpha-subunit gene is expressed in the ovary, testis, adrenal, and pituitary. Because this pattern of expression corresponds to that of the orphan nuclear receptor, steroidogenic factor-1 (SF-1), we hypothesized that the inhibin alpha promoter might be regulated by SF-1. Expression of exogenous SF-1, in an SF-1 deficient cell line, caused modest stimulation of the inhibin alpha promoter. However, activation of the cAMP pathway, which is known to regulate inhibin alpha expression, greatly enhanced the actions of SF-1. Coexpression of SF-1 with the catalytic subunit of cAMP-dependent protein kinase A caused greater than 250-fold stimulation, whereas only 4- or 7-fold stimulation was seen by the SF-1 or protein kinase A pathway alone. Synergistic stimulation by SF-1 and the cAMP pathway was also seen in GRMO2 granulosa cells, which express endogenous SF-1. Deletion and site-directed mutagenesis localized a novel SF-1 regulatory element (TCA GGGCCA; -137 to -129) adjacent to a variant cAMP-response element (CRE; -120 to -114). The synergistic property of SF-1 and cAMP stimulation was inherent within this composite inhibin alpha fragment (-146 and -112), as it was transferable to heterologous promoters. Mutations in either the CRE or the SF-1 regulatory element completely eliminated synergistic activation by these pathways. The binding of SF-1 and CRE binding protein (CREB) to the inhibin alpha regulatory elements was relatively weak in gel mobility shift assays, consistent with their deviation from consensus binding sites. However, SF-1 was found to interact with CREB using an assay in which epitope-tagged SF-1 was expressed in cells and used to pull down in vitro translated CREB. Expression of CREB binding protein (CBP), a coactivator that interacts with SF-1 and CREB, further enhanced transcription by these pathways. Stimulation by the SF-1 and cAMP pathways was associated with increased histone H4 acetylation, suggesting that chromatin remodeling accompanies their actions. We propose a model in which direct interactions of SF-1, CREB, and associated coactivators like CBP induce strongly cooperative transactivation by pathways that individually have relatively weak effects on transcription.

Function of steroidogenic factor 1 domains in nuclear localization, transactivation, and interaction with transcription factor TFIIB and c-Jun

Normal endocrine development and function require nuclear hormone receptor SF-1 (steroidogenic factor 1). To understand the molecular mechanism of SF-1 action, we have investigated its domain function by mutagenesis and functional analyses. Our mutant studies show that the putative AF2 (activation function 2) helix located at the C-terminal end is indispensable for gene activation. SF-1 does not have an N-terminal AF1 domain. Instead, it contains a unique FP region, composed of the Ftz-F1 box and the proline cluster, after the zinc finger motif. The FP region interacts with transcription factor IIB (TFIIB) in vitro. This interaction requires residues 178-201 of TFIIB, a domain capable of binding several transcription factors. The FP region also mediates physical interaction with c-Jun, and this interaction greatly enhances SF-1 activity. The putative SF-1 ligand, 25-hydroxycholesterol, has no effects on these bindings. In addition, the Ftz-F1 box contains a bipartite nuclear localization signal (NLS). Removing the basic residues at either end of the key nuclear localization sequence NLS2.2 abolishes the nuclear transport. Expression of mutants containing only the FP region or lacking the AF2 domain blocks wild-type SF-1 activity in cells. By contrast, the mutant having a truncated nuclear localization signal lacks this dominant negative effect. These results delineate the importance of the FP and AF2 regions in nuclear localization, protein-protein interaction, and transcriptional activation.

Activator protein-2 mediates transcriptional activation of the CYP11A1 gene by interaction with Sp1 rather than binding to DNA

The ovine P45 side chain cleavage (CYP11A1) enzyme gene, which catalyzes the initial enzymatic step in steroid hormone biosynthesis is transcriptionally regulated in cultured steroidogenic human trophoblastic JEG-3 cells. The ovine CYP11A1 promoter contains two GC-rich footprinted regions referred to as ovine footprints 5 (OF5) and OF3, which are well conserved among the CYP11A1 promoters of different species. These GC-rich sequences resemble activator protein-2 (AP-2)/Sp1 binding sites and were previously implicated in basal and cAMP-regulated activity of the bovine and ovine CYP11A1 promoters. In the current studies, AP-2 induced the ovine CYP11A1 promoter 4.5-fold in JEG-3 cells with full induction requiring the previously defined cAMP-responsive elements. Point mutation of OF3 abolished induction by AP-2, and OF3 was sufficient for induction by AP-2 when linked to a heterologous promoter. AP-2 induction of the CYP11A1 promoter required the basic region (N165-N278) and the carboxy terminus of AP-2 (N413-N437). In the course of investigating the mechanisms by which OF5 and OF3 regulated CYP11A1 transcription, we found that OF5 and OF3 bound Sp1 and Sp3 in JEG-3 cells. AP-2 did not bind OF5 or OF3 directly but rather formed a multiprotein complex with Sp1 in JEG-3 cells. AP-2 associated directly with Sp1 in vitro requiring the AP-2 basic region and the Sp1 carboxy terminus. AP-2 induced Sp1/Sp3 activity independently of AP-2 binding to DNA using a GAL4 paradigm. The Sp1 and Sp3 transactivation domains were linked to the DNA-binding domain of GAL4, and their activity was assessed using a luciferase reporter gene containing only the GAL4 DNA-binding sites linked to the minimal TATA site. AP-2 induced Sp1/ Sp3-GAL4 activity 3- to 4-fold, requiring both the amino and extreme carboxy terminus of AP-2. We conclude that AP-2 can bind to and stimulate Sp1 activity and induces the ovine CYP11A1 promoter through conserved Sp1/Sp3-binding sites in JEG-3 cells. The induction of Sp1 activity by AP-2 may contribute to the induction of other genes that bind Sp1.

Steroidogenic factor-1 mediates cyclic 3',5'-adenosine monophosphate regulation of the high density lipoprotein receptor

The high density lipoprotein (HDL) receptor mediates the uptake of cholesterol and cholesteryl esters, substrates for steroidogenesis, from an HDL particle in the adrenal gland and gonads. We report here that treatment of rat luteal cells with 1 mM (Bu)2cAMP for 24 h dramatically induced (118-fold) HDL receptor messenger RNA levels. The rat HDL receptor promoter contains a steroidogenic factor-1 (SF-1)-binding site (SFBd; 5'-TCAAGGCC-3') through which SF-1 protein binds and activates transcription of this gene in both human HTB9 bladder carcinoma and mouse Y1 tumor cells, an effect that is enhanced by cAMP. These observations demonstrate that this motif is required for both basal and cAMP-induced regulation of the HDL receptor gene. Cotransfection studies in Kin 8 cells, a Y1 cell line resistant to cAMP activation as a result of a mutation in the protein kinase A (PKA) regulatory subunit, showed that a functional PKA is required for cAMP induction of HDL receptor gene transcription. Deleting the activation function-2 domain (amino acids 448-461) or mutating Ser430, a potential consensus phosphorylation site for PKA in the SF-1 protein, decreased both basal and cAMP-induced activation of the HDL receptor promoter. These data suggest that these regions within the SF-1 protein are required for both basal and cAMP-induced regulation of the HDL receptor gene. The mediation of cAMP responsiveness of the HDL receptor gene by SF-1 suggests how important this trans-acting factor is in steroid hormone synthesis by assuring that all required elements (substrate and enzymes) are present when they are needed for maximal steroid production.

p300/CBP is required for transcriptional induction by interleukin-4 and interacts with Stat6

Interleukin-4 (IL-4) induces tyrosine phosphorylation of the latent transcription factor Stat6, which mediates the transcriptional responses of IL-4. The transactivation domain of Stat6 has recently been mapped to the C-terminal region of Stat6. We have investigated the mechanism by which Stat6, through its transactivation domain, induces transcription. Previous studies have shown that diverse regulated transcription factors interact with coactivators such as p300 and CBP. We report that Stat6 used the interaction with p300/CBP to exert its stimulatory effects. Overexpression of p300/CBP increased IL-4-induced transcription of Stat6 activated reporter genes. The requirement of p300/CBP for Stat6-mediated transactivation is shown by coexpression of the adenovirus E1A protein. E1A repressed the IL-4-induced reporter gene activity, while mutants of E1A, which do not interact with p300/CBP, failed to block the IL-4-induced response. In addition, we found that the minimal transactivation domain of Stat6, when fused to the GAL4 DNA-binding domain, was repressed by E1A, whereas the fusion protein p300-VP16 increased the transcriptional activity. In two-hybrid protein interaction assays in mammalian cells, we mapped the interaction domain of CBP to a C-terminal region between amino acids 1850 and 2176, a region distinct from the interaction domain of CBP with Stat1, Stat2 or Stat5. Finally, we show that antibodies raised against p300 coimmunoprecipitated Stat6 and p300 from transfected COS7 cells and antibodies against Stat6 coimmunprecipitated endogenous Stat6 and CBP from Ba/F3 cells. Our data suggest that the transactivation domain of Stat6 makes contact with the basal transcription machinery by binding to p300/CBP.

Steroidogenic factor-1: its role in endocrine organ development and differentiation

The cloning of the first steroid hormone receptor over a decade ago provided vital insight into the mechanisms by which steroid hormones activate gene transcription. When bound by hormone, these receptors function as ligand-dependent transcription factors by binding to unique response elements in the promoter of specific target genes. Over 60 receptors have now been characterized in this superfamily of steroid receptors. Many receptors known as orphan receptors have been cloned by homology and have no known ligands but appear to be mediators of endocrine function in the adult and in many cases are essential developmental regulators in endocrine organogenesis. One such receptor is steroidogenic factor-1 (SF-1). While initially cloned as a transcriptional regulator of the various steroidogenic enzyme genes in the adrenal and gonad, it has become clear through genetic ablation experiments in mice that SF-1 is an essential factor in adrenal and gonadal development and for the proper functioning of the hypothalamic-pituitary-gonadal axis. In addition, these studies have revealed that SF-1 is necessary for the formation of the ventromedial nucleus of the hypothalamus. While we have learned much since the initial cloning of SF-1, the mechanisms by which SF-1 regulates these various developmental programs remain elusive. This article focuses on the characterization of SF-1 and its emerging role in endocrine homeostasis. Specific attention is placed on the mechanisms of action of this unique member of the nuclear receptor superfamily.

Steroidogenic factor 1 (SF-1) is essential for endocrine development and function

Steroidogenic factor 1 (SF-1), an orphan nuclear receptor, initially was isolated as a key regulator of the tissue-specific expression of the cytochrome P450 steroid hydroxylases. Thereafter, analyses of sites of SF-1 expression during mouse embryological development hinted at considerably expanded roles for SF-1, roles that were strikingly confirmed through the analyses of SF-1 knockout mice. These SF-1 knockout mice exhibited adrenal and gonadal agenesis, associated with male-to-female sex reversal of their internal and external genitalia and death from adrenocortical insufficiency. These findings showed unequivocally that SF-1 is essential for the embryonic survival of the primary steroidogenic organs. SF-1 knockout mice also had impaired pituitary expression of gonadotropins and agenesis of the ventromedial hypothalamic nucleus (VMH), establishing that SF-1 regulates reproductive function at all three levels of the hypothalamic-pituitary gonadal axis. This article reviews the experiments that have defined these essential roles of SF-1 in endocrine development and highlights important areas for future studies.

Molecular cloning and expression of the SF-1/Ad4BP gene in the frog, Rana rugosa

SF-1/Ad4BP is a transcriptional factor that was originally found to be a mammalian homologue of the Drosophila Ftz-F1 (fushi tarazu factor 1) (), and transcribed from a gene designated the Ftz-F1 gene (). Ftz-F1 gene-deficient mice lack adrenal glands and gonads. Besides mammals, however, the SF-1/Ad4BP cDNA has only been isolated to date in fish and birds. To understand its role(s) for adrenal and gonadal development in vertebrates, cloning of this gene in animals other than mammals is required. In this study, we succeeded to isolate frog (Rana rugosa) SF-1/Ad4BP cDNA from a testis lambdagt10 cDNA library. It encoded a protein of 468 amino acids, and its open reading frame (ORF) shared 70% similarity with that of chicken OR2.1 (a SF-1/Ad4BP homologue) and 62% with bovine SF-1/Ad4BP. SF-1/Ad4BP mRNA was expressed in the testes, brains, adrenals/kidneys and spleens, but not ovaries, of adult frogs. In addition, we also cloned the 5'-untranslated region (4.6kb) of the SF-1/Ad4BP gene with exons I and II. Genomic structure analysis revealed that frog SF-1/Ad4BP was also transcribed from the same gene as that of mammals. However, many Ftz-F1-related proteins have been reported so far. The Ftz-F1 gene does not encode all of those Ftz-F1-related proteins. Thus, the name of Ftz-F1 is not adequate for the gene coding SF-1/Ad4BP. Here, we propose the use of SF-1/Ad4BP instead of Ftz-F1 for the gene that encodes SF-1/Ad4BP in vertebrates.

The roles of steroidogenic factor 1 in endocrine development and function

The orphan nuclear receptor steroidogenic factor 1 (SF-1) was initially isolated as a key regulator of the cytochrome P450 steroid hydroxylases in adrenocortical and gonadal cells. Subsequent analyses of SF-1 knockout mice have expanded considerably our understanding of the roles that SF-1 plays in endocrine development. These SF-1 knockout mice lacked adrenal glands and gonads, with consequent male-to-female sex reversal of their internal and external genitalia. Thus, SF-1 is essential for the embryonic survival of the primary steroidogenic organs. They further exhibited impaired gonadotrope function and agenesis of the ventromedial hypothalamic nucleus, establishing that SF-1 contributes to reproductive function at all three levels of the hypothalamic-pituitary-gonadal axis. This report reviews experiments that have defined these critical roles of SF-1 in endocrine development, and highlights areas of ongoing investigation.

Porcine steroidogenic factor-1 gene (pSF-1) expression and analysis of embryonic pig gonads during sexual differentiation

The porcine steroidogenic factor-1 gene (pSF-1) was cloned using a combination of genomic and RT-PCR based cloning methods. pSF-1 consists of an open reading frame of 1383 nt corresponding to a deduced amino acid sequence of 461 aa, similar to bovine and human SF-1. Sequence homologies between pSF-1 and human, bovine and mouse molecules indicate strong evolutionary conservation at both the nt and aa levels. Northern analysis of pSF-1 expression in adult steroidogenic tissues correlated with porcine steroidogenic acute regulatory protein gene (pStAR) and porcine side chain cleavage (pP450scc) gene expression. Notably, pSF-1 expression was readily detected in neonatal testes, absent at 3 weeks of age, and again readily detected at 3 months and in adult testes. pSF-1 expression was weak but detectable in placental tissues at various times of gestation, and was correlated with pStAR and pP450scc expression, indicating classical steroidogenesis in this organ. In developing gonads from 6-12 weeks of gestation, i.e. during the time of sex differentiation in the pig, Northern analysis demonstrated increasing expression of PSF-1 in fetal testes and no expression in ovaries. This expression pattern was paralleled for pStAR, pP450scc, and porcine Müllerian inhibitory substance (pMIS), consistent with pSF-1 involvement in both steroid and protein hormone secretions of the developing testes during sex differentiation. Porcine SRY HMG-box related gene-9 (pSOX-9) expression also paralleled that of pSF-1 in developing testes. In contrast, DSS-AHC critical region on the X chromosome, gene 1 (pDAX-1) was expressed predominantly in the developing ovaries, indicating a possible reciprocal regulation of pSF-1 and pDAX-1 genes in developing pig testes and ovaries.