The orphan receptor SF-1 binds to the COUP-like element in the promoter of the actively transcribed oxytocin gene

Activation of the black seabream (Acanthopagrus schlegeli) somatolactin-alpha gene promoter by Pit-1c in the Hepa-T1 cell-line

Somatolactin (SL) is a pituitary hormone of the growth hormone (GH) gene family found only in fish. To understand the regulation of this hormone at the level of gene transcription, we obtained a SLalpha gene from black seabream (bsb), with its 5' flanking promoter region carrying several putative transcription factors including seven binding sites for pituitary-specific transcription factor 1 (Pit-1). To study the actions of Pit-1 on this gene promoter, we cloned three variants of bsbPit-1 (Pit-1a, Pit-1b and Pit-1c) derived from alternative splicing of mRNA or differential transcription start sites from black seabream pituitary. The deduced amino acid sequences of these Pit-1s contained 371 amino acids (aa), 333 and 311aa for the three Pit-1 variants, Pit-1a, Pit-1b and Pit-1c, respectively, with diverse regions of Pit-1 located at the transactivation domain. The actions of bsbPit-1 variants on the bsbSL gene promoter were investigated using a co-transfection assay, with a reporter gene using a transient expression assay in Hepa-T1 cells. The N-terminus truncated isoform bsbPit-1c showed the highest level of activity on SLalpha gene promoter activation in Hepa-T1 cells; however, neither Pit-1a nor Pit-1b activated the bsbSL gene promoter in the same study.

Transcriptional activation of the oxytocin promoter by oestrogens uses a novel non-classical mechanism of oestrogen receptor action

Transcriptional activation of the gene coding for the neuropeptide hormone oxytocin by oestrogens does not follow the classical model of oestrogen receptor action. The oxytocin promoter does not contain an oestrogen response element (ERE), but instead a high-affinity binding site for nuclear orphan receptors. In the present study, the oestrogen-dependent up-regulation of the bovine oxytocin promoter is investigated in MDA-MB 231 cells. Control by oestrogen is shown to be dependent on the integrity of the nuclear orphan receptor binding site and the presence of ligand-activated oestrogen receptor, but independent of oestrogen receptor binding to DNA. Partial agonists tamoxifen and raloxifen and the pure antagonist ICI 182 780 all show agonistic activities on transcription, while exhibiting normal binding affinities to oestrogen receptor (ER)alpha. Nuclear orphan receptors oestrogen receptor-related receptor alpha (ERRalpha) and germ cell nuclear factor (GCNF) are expressed to significant levels in MDA-MB 231 cells. Binding of ERRalpha to the oxytocin promoter binding site can be demonstrated, suggesting the involvement of this nuclear orphan receptor in oestrogen-dependent up-regulation. The oestrogenic stimulation of the oxytocin promoter apparently is dependent on the stimulation of the transcriptional activity of this nuclear orphan receptor by ERK-1/ERK-2 mitogen-activated protein kinases (MAP kinases). This novel nonclassical mechanism of oestrogen action most probably is not restricted to the regulation of neuropeptide hormone expression, but may further contribute to the multitude of tissue-specific effects of oestrogenic substances.

SF-1 a key player in the development and differentiation of steroidogenic tissues

Since its discovery in the early 1990s, the orphan nuclear receptor SF-1 has been attributed a central role in the development and differentiation of steroidogenic tissues. SF-1 controls the expression of all the steroidogenic enzymes and cholesterol transporters required for steroidogenesis as well as the expression of steroidogenesis-stimulating hormones and their cognate receptors. SF-1 is also an essential regulator of genes involved in the sex determination cascade. The study of SF-1 null mice and of human mutants has been of great value to demonstrate the essential role of this factor in vivo, although the complete adrenal and gonadal agenesis in knock-out animals has impeded studies of its function as a transcriptional regulator. In particular, the role of SF-1 in the hormonal responsiveness of steroidogenic genes promoters is still a subject of debate. This extensive review takes into account recent data obtained from SF-1 haploinsufficient mice, pituitary-specific knock-outs and from transgenic mice experiments carried out with SF-1 target gene promoters. It also summarizes the pros and cons regarding the presumed role of SF-1 in cAMP signalling.

DAX1 and its network partners: exploring complexity in development

DAX1 encoded by NR0B1, when mutated, is responsible for X-linked adrenal hypoplasia congenita (AHC). AHC is due to failure of the adrenal cortex to develop normally and is fatal if untreated. When duplicated, this gene is associated with an XY sex-reversed phenotype. DAX1 expression is present during development of the steroidogenic hypothalamic-pituitary-adrenal-gonadal (HPAG) axis and persists into adult life. Despite recognition of the crucial role for DAX1, its function remains largely undefined. The phenotypes of patients and animal models are complex and not always in agreement. Investigations using cell lines have proved difficult to interpret, possibly reflecting cell line choices and their limited characterization. We will review the efforts of our group and others to identify appropriate cell lines for optimizing ex vivo analysis of NR0B1 function throughout development. We will examine the role of DAX1 and its network partners in development of the hypothalamic-pituitary-adrenal/gonadal axis (HPAG) using a variety of different types of investigations, including those in model organisms. This network analysis will help us to understand normal and abnormal development of the HPAG. In addition, these studies permit identification of candidate genes for human inborn errors of HPAG development.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Expression and regulation of transcripts encoding two members of the NR5A nuclear receptor subfamily of orphan nuclear receptors, steroidogenic factor-1 and NR5A2, in equine ovarian cells during the ovulatory process

Steroidogenic factor-1 (SF-1, NR5A1a) is a member of the NR5A nuclear receptor subfamily and has been implicated as a key transcriptional regulator of all ovarian steroidogenic genes in vitro. To establish links between the expression of SF-1 and that of the steroidogenic genes in vivo, the objectives of this study were to clone equine SF-1 and examine the regulation of its messenger RNA (mRNA) in follicular cells during human CG (hCG)-induced ovulation. The equine SF-1 primary transcript was cloned by a combination of RT-PCR techniques. Results showed that the transcript was composed of a 5'-untranslated region (UTR) of 161 bp, an open reading frame (ORF) of 1386 bp that encodes a highly-conserved 461-amino acid protein, and a 3'-UTR of 518 bp. The cloning of SF-1 also led to the unexpected and serendipitous isolation of the highly-related orphan nuclear receptor NR5A2, which was shown to include a 5'-UTR of 243 bp, an ORF of 1488 bp, and a 3'-UTR of 1358 bp. The NR5A2 ORF encodes a 495-amino acid protein that is 60% identical to SF-1, including 99%-similar DNA-binding domains. Northern blot analysis revealed that SF-1 and NR5A2 were expressed in all major steroidogenic tissues, with the exception that NR5A2 was not present in the adrenal. Interestingly, NR5A2 was found to be, by far, the major NR5A subfamily member expressed in the preovulatory follicle and the corpus luteum. Using a semiquantitative RT-PCR/Southern blotting approach, the regulation of SF-1 and NR5A2 mRNAs in vivo was studied in equine follicular cells obtained from preovulatory follicles isolated between 0 and 39 h post hCG. Results showed that the theca interna was the predominant site of SF-1 mRNA expression in the follicle, and that hCG caused a significant decrease in SF-1 levels between 12-39 h in theca interna and between 24-39 h post hCG in granulosa cells (P < 0.05). In contrast, the granulosa cell layer was the predominant, if not the sole, site of NR5A2 mRNA expression in the follicle. Importantly, NR5A2 was much more highly expressed in granulosa cells than SF-1. The administration of hCG caused a significant decrease in NR5A2 transcripts in granulosa cells at 30, 36, and 39 h post hCG (P < 0.05). Thus, this study is the first to report the concomitant regulation of SF-1 in theca interna and granulosa cells throughout the ovulation/luteinization process, and to demonstrate the novel expression and hormonal regulation of NR5A2 in ovarian cells. Based on the marked expression of NR5A2 in equine granulosa and luteal cells and on mounting evidence of a functional redundancy between SF-1 and NR5A2 in other species, it is proposed that NR5A2 may play a key role in the regulation of gonadal steroidogenic gene expression.

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.

Ku autoimmune antigen is involved in placental regulation of rat P450c17 gene transcription

The steroidogenic enzyme P450c17 (17alpha hydroxylase/C17,20 lyase) regulates a key branchpoint in steroidogenesis, as its activity directs the steroid biosynthetic pathways toward glucocorticoid or sex hormone synthesis. Expression of the P450c17 gene is transcriptionally regulated in steroidogenic tissues by cAMP. We showed that DNA between -84 and -55 in the rat P450c17 gene was bound uniquely by steroidogenic factor-1 (SF-1), which regulated both basal and cAMP-stimulated transcription in mouse adrenocortical and Leydig cells. SF-1 gene ablation experiments in mice indicate that SF-1 is not mandatory for placental steroidogenesis. We studied P450c17 gene regulation in the placenta using human placental JEG-3 trophoblast cells. Transfection of reporter luciferase gene constructs containing serial deletions of the 5' flanking region of the rat P450c17 gene showed that DNA between -98 and +13 mediated basal and cAMP-regulated transcription in placental JEG-3 cells, as it did in adrenal and Leydig cells. DNase footprints further identified a region between -88 and the TATA box that was bound by protein. Transfection of luciferase reporter constructs containing -84 to -55 of the rat P450c17 DNA ligated to the minimal promoter of the thymidine kinase gene showed that this DNA increased both basal and cAMP-simulated luciferase activity. Gel mobility shift assays identified two DNA-protein complexes with JEG-3 cell nuclear extracts that were different from complexes formed with MA-10 cell extracts and did not involve SF-1. Mutational analysis of the -84/-55 DNA showed that JEG-3 nuclear proteins bound to a site containing, but not identical to, the SF-1 sequence. One complex involved Ku autoimmune antigen, which bound to DNA sequence specifically. Overexpression of Ku antigen in MA-10 cells stimulated rat P450c17 gene transcription, thus demonstrating a biologic effect of Ku. Ku also bound to a similar region of the human P450c17 gene, and the DNA region to which Ku bound was transcriptionally active in JEG-3 cells. Ku was also found in extracts from rat placenta and bound to the -84/-55 rat P450c17 DNA. These data demonstrate a role of Ku in regulating P450c17 gene expression. These data further indicate that although human P450c17 is not normally expressed in the placenta, factors that could activate this gene are indeed present.

Hypothalamic transcription factors and the regulation of the hypothalamo-neurohypophysial system

The transcription factors that confer high level expression and regulate the genes encoding neurohypophysial hormones are largely unknown. A number of different approaches have been taken to identify these factors and to elucidate molecular mechanisms of physiological gene regulation. In this chapter two transcription factor families are considered: homeodomain proteins and nuclear receptors. Their identification in the hypothalamus and actions on the OT gene are addressed here.

The molecular basis of oxytocin and oxytocin receptor gene expression in reproductive tissues

Transcriptional regulation of the oxytocin and oxytocin receptor genes underly to a large degree the highly specific and often transient physiologies associated with this peptide hormone system. Using a variety of homologous transcription assays we have endeavoured to identify and characterize the cis and trans elements responsible for the regulation in vivo of the oxytocin peptide gene and the gene for the oxytocin receptor. The bovine ovarian granulosa cell model is a primary culture system where under stimulation by insulin or IGF-I and LH the endogenous oxytocin gene is massively upregulated. We have identified a proximal response element at -160, which in vivo binds the competing nuclear receptors, SF1 and COUP-TF. Additionally ovarian specific transcription factors bind at two additional sites in the distal promoter region. For the bovine oxytocin receptor gene, we have taken advantage of the high endogenous expression of the receptor in the endometrium of the estrous cycle. Using a combination of primary cell culture techniques and in vitro binding of nuclear protein extracts from tissues expressing the receptor in vivo, we have shown there to be a combination of constitutive and inhibitory elements controlling oxytocin receptor gene expression. Similar results were obtained for the human oxytocin receptor gene. At birth there may additionally be a specific stimulatory effect on transcription in the myometrium.

Transgenic and transcriptional studies on neurosecretory cell gene expression

1. Studies of the regulation of neurosecretory cell gene expression suffer from the lack of suitable cell lines. Two approaches have been used to overcome this deficit: transfection of neuropeptide genes into heterologous cell lines and generation of transgenic animals. 2. Studies with heterologous cell lines have revealed the potential involvement of nuclear hormone receptors, POU proteins, and fos/jun/ATF family members in the regulation of the vasopressin and oxytocin genes. Although limited in their scope, these studies have contributed greatly to the dissection of basic properties of elements in the vasopressin and oxytocin gene promoters. 3. Transgenic mice, and more recently rats, have been used to elucidate genomic regions governing cell specificity and physiological regulation of neurosecretory gene expression. The genes encoding the neuropeptides vasopressin and oxytocin have been used in many transgenic studies, due to the well-defined expression patterns and physiology of the endogenous neuropeptides. Cell-specific and physiologically regulated expression of these transgenes has been achieved, demonstrating the action of putative repressor elements and regulation of the expression of one gene by sequences present in the other gene. 4. Appropriate expression and translation of transgenes have resulted in the production of several useful systems. Expression of oncogene sequences in gonadotropin-releasing hormone neurons has allowed the development of cell lines from the resulting tumors, overproduction of corticotropin-releasing factor has produced animal models of anxiety and obesity, and directed ectopic expression of growth hormone has generated a potentially useful rat model of dwarfism. These and other animal models of human disease will provide important avenues for the development of therapeutic strategies.

Alterations in the chromatin structure of the distal promoter region of the bovine oxytocin gene correlate with ovarian expression

The mechanisms regulating the expression of the neuropeptide hormone gene oxytocin have not yet been elucidated in detail. The binding of the orphan receptor Ad4BP, the bovine homolog of steroidogenic factor-1 (SF-1), which is correlated with in vivo oxytocin transcription in the luteinizing granulosa cells of the bovine corpus luteum, is not sufficient to explain the transcriptional up-regulation in these cells. Therefore, we started experiments to identify other regions of the oxytocin locus that are involved in gene activation. The study presented here is the very first investigation of DNA methylation and chromatin structure in the distal promoter region of the bovine oxytocin gene. We show that this region is tissue-specifically hypomethylated in bovine granulosa cells. Upon stimulation of the cells with the adenylate cyclase-activator forskolin, a DNase I-hypersensitive site is induced in the distal promoter region. Additionally, we find binding of a monomeric nuclear orphan receptor directly within the region of inducible DNase I sensitivity; this factor is not identical to Ad4BP/SF-1. This study identifies a region in the bovine oxytocin distal promoter where tissue-specific changes in DNA methylation and chromatin structure correlate with high induction of oxytocin gene transcription, and suggests that the binding of transcription factors to this region may be important for the up-regulation of oxytocin gene expression.

The testis and the adrenal are (transcriptionally) the same

We have been studying the transcriptional regulation of the rat P450c17 gene in both adrenocortical and Leydig cells, to assess which DNA sequences are required for its basal and hormonally stimulated transcription. Comparing the transcriptional regulation in both of these cell types enables us to demonstrate whether specific nuclear factors required for transcriptional regulation of the rat P450c17 gene are tissue-specifically expressed, and whether the same cis-acting DNA elements in the gene are required for transcriptional regulation in both of these two different steroidogenic tissues. Using such an approach, we previously demonstrated that the transcriptional regulation of the rat P450scc gene uses different cis-acting DNA sequences in steroidogenic versus neural tissues, and requires the expression of tissue-specific nuclear factors that are unique to neural tissue. However, in studying the transcriptional regulation of the rat P450c17 gene in cultured mouse adrenocortical Y-1 and mouse Leydig MA-10 cells, we have shown that identical DNA sequences necessary for basal and cAMP-stimulated transcriptional regulation in these two cell types, and that identical nuclear factors from Y-1 and from MA-10 cells bind to these sequences. We have identified four transcriptionally active regions within 500 bp of the transcription initiation start site that are important for basal and/or cAMP-stimulated transcriptional regulation of this gene in Y-1 and MA-10 cells. This paper will discuss two of these regions in greater detail. By studying the regulation of the rat P450c17 gene, we have identified two new members of the orphan nuclear receptor gene family and have discovered new alternative mechanisms by which orphan nuclear receptors activate gene transcription in both mouse adrenocortical Y-1 and Leydig MA-10 cells.

Transcriptional regulation of the genes encoding the cytochrome P-450 steroid hydroxylases

Steroid hormone biosynthesis requires the concerted action of a related group of cytochrome P-450 steroid hydroxylases. In recent years considerable effort has been directed toward defining the molecular basis for the cell-selective expression of these genes and their transcriptional regulation by trophic hormones. The orphan nuclear receptor SF-1, acting through a conserved element found in the proximal promoter regions of all steroid hydroxylase genes, seems to be a major, but not exclusive, determinant of cell-selective gene expression. In contrast, the coordinate responses of the steroid hydroxylases to trophic hormones apparently involves an interplay of multiple proteins that collectively lead to a synchronous induction of gene expression. In some instances these interactions apparently involve transcription factors that also contribute to the cell-selective expression of these genes.