Tissue-specific gene expression in the pituitary: the glycoprotein hormone alpha-subunit gene is regulated by a gonadotrope-specific protein

[Clinical and molecular characteristics of patients with 46,XY DSD due to NR5A1 gene mutations]

Steroidogenic factor 1 (SF1, NR5A1) is a nuclear receptor that regulates multiple genes involved in adrenal and gonadal development, steroidogenesis, and the reproductive axis. Human mutations in SF1 were initially found in patients with severe gonadal dysgenesis and primary adrenal failure. However, more recent case reports have suggested that heterozygous mutations in SF1 may also be found in patients with 46,XY partial gonadal dysgenesis and underandrogenization but normal adrenal function. We have analyzed the gene encoding SF1 (NR5A1) in a cohort of 310 Russian patients with 46,XY disorders of sex development (DSD). Heterozygous SF1 variants were found in 36 out of 310 (11.6%) of cases, among them 15 were not previously described. We have not found any phenotype-genotype correlations and any clinical and laboratory markers that would allow to suspect this type of before conducting molecular genetic analysis.

Clinical Applications of Gonadotropins in the Male

The pituitary gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) play a pivotal role in reproduction. The synthesis and secretion of gonadotropins are regulated by complex interactions among several endocrine, paracrine, and autocrine factors of diverse chemical structure. In men, LH regulates the synthesis of androgens by the Leydig cells, whereas FSH promotes Sertoli cell function and thereby influences spermatogenesis. Gonadotropins are complex molecules composed of two subunits, the α- and β-subunit, that are noncovalently associated. Gonadotropins are decorated with glycans that regulate several functions of the protein including folding, heterodimerization, stability, transport, conformational maturation, efficiency of heterodimer secretion, metabolic fate, interaction with their cognate receptor, and selective activation of signaling pathways. A number of congenital and acquired abnormalities lead to gonadotropin deficiency and hypogonadotropic hypogonadism, a condition amenable to treatment with exogenous gonadotropins. Several natural and recombinant preparations of gonadotropins are currently available for therapeutic purposes. The difference between natural and the currently available recombinant preparations (which are massively produced in Chinese hamster ovary cells for commercial purposes) mainly lies in the abundance of some of the carbohydrates that conform the complex glycans attached to the protein core. Whereas administration of exogenous gonadotropins in patients with isolated congenital hypogonadotropic hypogonadism is a well recognized therapeutic approach, their role in treating men with normogonadotropic idiopathic infertility is still controversial. This chapter concentrates on the main structural and functional features of the gonadotropin hormones and how basic concepts have been translated into the clinical arena to guide therapy for gonadotropin deficit in males.

Glucocorticoids induce human glycoprotein hormone alpha-subunit gene expression in the gonadotrope

The human glycoprotein hormone alpha-subunit (alphaGSU) gene is transcriptionally regulated by glucocorticoids in a cell type-specific fashion. In direct contrast to repression of alphaGSU by glucocorticoids in placenta, glucocorticoid receptor (GR) modulation in the pituitary is little understood. We show that glucocorticoids stimulate the alphaGSU promoter in immortalized pituitary gonadotrope-derived LbetaT2 cells, whereas estrogens, androgens, and progestins have no significant effect. Moreover, GR acts in a dose-dependent manner at physiological concentrations of glucocorticoids. Transient transfection of GR with dexamethasone (Dex) treatment further stimulates the alphaGSU promoter, but this induction is severely diminished using a receptor mutated in the DNA-binding domain. Truncation and cis mutations demonstrate that glucocorticoid response element 2 (GRE2) and cAMP-response element 2 (CRE2) within -168 bp of the human alphaGSU promoter are critical for induction. Moreover, dominant-negative CRE-binding protein markedly inhibits basal but also Dex induction of alphaGSU promoter activity. Additionally, GR specifically binds to GRE2 in the human alphaGSU promoter in vitro and to the 5' region of the endogenous mouse alphaGSU gene in vivo. Furthermore, overexpression of the homeobox factor, Distal-less 3 that regulates this gene in placental cells through a site partially overlapping GRE2, blocks Dex induction of alphaGSU in gonadotrope cells, indicating that placenta-specific expression of Dlx3 may interfere with GR, resulting in repression in placental cells vs. induction in gonadotrope cells. These results demonstrate the stimulatory role played by glucocorticoids in alphaGSU gene expression in the pituitary gonadotrope, in contrast to repression in placental cells, and highlight the tissue-specific nature of steroid hormone action.

Fanconi anemia A is a nucleocytoplasmic shuttling molecule required for gonadotropin-releasing hormone (GnRH) transduction of the GnRH receptor

GnRH binds its cognate G protein-coupled GnRH receptor (GnRHR) located on pituitary gonadotropes and drives expression of gonadotropin hormones. There are two gonadotropin hormones, comprised of a common alpha- and hormone-specific beta-subunit, which are required for gonadal function. Recently we identified that Fanconi anemia a (Fanca), a DNA damage repair gene, is differentially expressed within the LbetaT2 gonadotrope cell line in response to stimulation with GnRH. FANCA is mutated in more than 60% of cases of Fanconi anemia (FA), a rare genetically heterogeneous autosomal recessive disorder characterized by bone marrow failure, endocrine tissue cancer susceptibility, and infertility. Here we show that induction of FANCA protein is mediated by the GnRHR and that the protein constitutively adopts a nucleocytoplasmic intracellular distribution pattern. Using inhibitors to block nuclear import and export and a GnRHR antagonist, we demonstrated that GnRH induces nuclear accumulation of FANCA and green fluorescent protein (GFP)-FANCA before exporting back to the cytoplasm using the nuclear export receptor CRM1. Using FANCA point mutations that locate GFP-FANCA to the cytoplasm (H1110P) or functionally uncouple GFP-FANCA (Q1128E) from the wild-type nucleocytoplasmic distribution pattern, we demonstrated that wild-type FANCA was required for GnRH-induced activation of gonadotrope cell markers. Cotransfection of H1110P and Q1128E blocked GnRH activation of the alphaGsu and GnRHR but not the beta-subunit gene promoters. We conclude that nucleocytoplasmic shuttling of FANCA is required for GnRH transduction of the alphaGSU and GnRHR gene promoters and propose that FANCA functions as a GnRH-induced signal transducer.

Induction of GH, PRL, and TSHβ mRNA by transfection of Pit-1 in a human pituitary adenoma-derived cell line

The functional development of pituitary cells depends on the expression of a combination of transcription factors and co-factors. Pituitary-specific transcription factor-1 (Pit-1) is required for the expression of growth hormone (GH), prolactin (PRL), and the thyroid-stimulating hormone beta subunit (TSH beta) and acts synergistically with the estrogen receptor (ER) and GATA-binding protein 2 (GATA-2) to induce PRL and TSH beta expression, respectively. The glycoprotein hormone alpha subunit (alpha SU) is the first hormone to be expressed during pituitary development. In addition to being expressed in follicle-stimulating hormone, luteinizing hormone (LH), and TSH cells, alpha SU is reported to co-localize with GH in pituitary cells. These findings have led to the suggestion that the expression of Pit-1 in cells of the alpha SU-based gonadotropin cell lineage might also lead to the expression of GH. In this study, we transfected HP 75 cells (derived from a human non-functioning pituitary adenoma that expressed alpha SU and LH beta) with Pit-1 by using an adenovirus FLAG-Pit-1 construct. Most of the transfected cells expressed GH mRNA, with fewer cells expressing PRL and TSH beta mRNA. The HP 75 cells expressed the genes for ER and GATA-2, thus allowing their expression of GH, PRL, and TSH beta mRNA in response to Pit-1. These results support the hypothesis that GH can be induced in cells that possess an active alpha SU gene and shed light on the basic molecular mechanism that drives the development of GH, PRL, and TSH beta expression in the alpha SU-based gonadotroph lineage.

Transcriptional activity of the 5' upstream region of the porcine glycoprotein hormone alpha subunit gene

Gene expression of the porcine glycoprotein hormone alpha subunit (p-alphaGSU) was examined in LbetaT2 cells, which were established from the anterior pituitary lobe of the immortalized transgenic mouse and produce alphaGSU, and in CHO cells cloned from Chinese hamster ovaries. Expression of the reporter gene fused with p-alphaGSU gene upstream in LbetaT2 cells showed that the distal regions -540/-240 and -798/-541 are important for the activation of gene expression. In contrast, the transcriptional activity of the distal region of p-alphaGSU gene was repressed in CHO cells. The region -540/-240 contains an adequate enhancer, called pituitary glycoprotein hormone basal element, whereas the region -798/-541 has no distinguished element. Transfection of the expression vector containing cDNA of a pan-pituitary activator, Ptx1, whose putative binding sites are present scatted in the distal region of the p-alphaGSU gene, revealed unexpectedly that this factor significantly suppressed the expression of p-alphaGSU gene in LbetaT2 cells, indicating that Ptx1 is unrelated to the upregulation in the region -798/-541. Thus, this study demonstrated for the first time that the distal region -798/-541of the p-alphaGSU gene is indispensable for prominent expression of this gene in which an as yet unidentified factor may participate.

Multiple and overlapping combinatorial codes orchestrate hormonal responsiveness and dictate cell-specific expression of the genes encoding luteinizing hormone

Normal reproductive function in mammals requires precise control of LH synthesis and secretion by gonadotropes of the anterior pituitary. Synthesis of LH requires expression of two genes [alpha-glycoprotein subunit (alphaGSU) and LHbeta] located on different chromosomes. Hormones from the hypothalamus and gonads modulate transcription of both genes as well as secretion of the biologically active LH heterodimer. In males and females, the transcriptional tone of the genes encoding alphaGSU and LHbeta reflects dynamic integration of a positive signal provided by GnRH from hypothalamic neurons and negative signals emanating from gonadal steroids. Although alphaGSU and LHbeta genes respond transcriptionally in the same manner to changes in hormonal input, different combinations of regulatory elements orchestrate their response. These hormone-responsive regulatory elements are also integral members of much larger combinatorial codes responsible for targeting expression of alphaGSU and LHbeta genes to gonadotropes. In this review, we will profile the genomic landscape of the promoter-regulatory region of both genes, depicting elements and factors that contribute to gonadotrope-specific expression and hormonal regulation. Within this context, we will highlight the different combinatorial codes that control transcriptional responses, particularly those that mediate the opposing effects of GnRH and one of the sex steroids, androgens. We will use this framework to suggest that GnRH and androgens attain the same transcriptional endpoint through combinatorial codes unique to alphaGSU and LHbeta. This parallelism permits the dynamic and coordinate regulation of two genes that encode a single hormone.

Transcriptional control during mammalian anterior pituitary development

The mammalian anterior pituitary gland is a compound endocrine organ that regulates reproductive development and fitness, growth, metabolic homeostasis, the response to stress, and lactation, by actions on target organs such as the gonads, the liver, the thyroid, the adrenals, and the mammary gland. The protein and peptide hormones that control these physiological parameters are secreted by specialized pituitary cell types that derive from a common origin in the early ectoderm. Collectively, the broad physiological importance of the pituitary gland, its intriguing organogenesis, and the clinical and agricultural significance of its actions, have established pituitary development as an excellent model system for the study of the gene-regulatory cascades that guide vertebrate cell determination and differentiation. We review the transcriptional pathways that regulate the commitment of the individual pituitary cell lineages and that subsequently modulate trophic hormone gene activity in the differentiated cells of the mature gland.

Steroidogenic factor-1 and the gonadotrope-specific element enhance basal and pituitary adenylate cyclase-activating polypeptide-stimulated transcription of the human glycoprotein hormone alpha-subunit gene in gonadotropes

In the anterior pituitary, expression of the common glycoprotein hormone alpha-subunit (alphaGSU) is mediated in part by multiple response elements residing in the distal promoter (-435 bp). One such site is the gonadotrope-specific element (GSE), which is bound by the orphan nuclear receptor steroidogenic factor-1 (SF-1) and confers pituitary adenylate cyclase-activating polypeptide (PACAP)-stimulated alphaGSU expression. Here we investigated the functional importance of the GSE and SF-1 phosphorylation in both basal and stimulated alphaGSU transcription. Mutation of the GSE reduced basal and PACAP-stimulated alphaGSU promoter activity in the alphaT3-1 gonadotrope cell line. Overexpression of wild-type SF-1, but not an S203A mutant form of SF-1, enhanced basal and PACAP-stimulated alphaGSU promoter activity. The effect of PACAP on alphaGSU promoter activity was inhibited after overexpression of MAPK phosphatase. Helix assembly of the SF-1 ligand-binding domain was stimulated by PACAP in vitro via a MAPK-dependent pathway, as determined using a mammalian two-hybrid assay. PACAP quickly activated MAPK (within 5 min) and also resulted in elevated levels of phospho-cAMP response element-binding protein and phospho-SF-1, as judged by a specific antiphospho-S203 antibody; this effect was blocked by the MAPK kinase inhibitor, UO126. Collectively, these data demonstrate that SF-1 binds to the GSE and activates both basal and PACAP-stimulated alphaGSU transcription, which is further increased by phosphorylation at Ser203 via MAPK. These data suggest strongly that the induction of alphaGSU gene expression by peptide hormone signaling is coupled directly to the posttranslational status of SF-1.

Pituitary tumorigenesis targeted by the ovine follicle-stimulating hormone beta-subunit gene regulatory region in transgenic mice

Targeted tumorigenesis in transgenic mice has been a powerful tool for the study of gene expression and oncogenesis, as well as for the production of differentiated immortal cell lines from rare cell types. Follicle-stimulating hormone (FSH) is secreted by the gonadotrope cells of the anterior pituitary gland and plays a pivotal role in mammalian reproduction. Here we have used the regulatory region of the ovine FSH beta gene to direct expression of the SV40 T antigen oncogene to gonadotrope cells in the pituitary of transgenic mice. Two of five transgenic mouse lines bearing this fusion gene rapidly developed pituitary tumors, with appearance of adenomatous foci as early as 6 weeks of age, resulting in death by 12 weeks of age in both genders. Histologic examination of tumor development over time revealed that increases in cell proliferation and dysplasia were accompanied by decreases in synthesis of pituitary hormones, indicating dedifferentiation of the pituitary cells. Histological features observed in these tumors were in agreement with this rapid transformation of cell phenotype. Tumors were multifocal in origin, and the most highly transformed cell types observed consisted of giant pale basophilic cells with enormous hyperploid nuclei associated with infiltrating neuronal-like cells, which were very abundant at later stages of tumor development. Mitotic indices were much higher in transgenic than wild-type pituitaries, as expected. Morphologic analysis of the gonads of these transgenic mice showed no major developmental differences, as compared to wild-type littermates, however the length of the seminiferous tubules in transgenic males was greater than age-matched wild-type animals. Despite this phenotype difference, both genders were fertile, with normal sperm development observed in males and normal estrous cycle stages in females. Moreover, while 8 -- 10-week-old transgenic males had much lower blood levels of FSH than littermates, transgenic female FSH levels were the same as those of wild-type females. These animals offer a unique and potentially useful model of organ-specific tumorigenesis, where a multistage pathway of tumor development is evident, both histologically and temporally. Study of such models will advance our knowledge on the physiological and molecular mechanisms involved in gene expression as well as tumor formation.

Expression of the mouse gonadotropin-releasing hormone receptor gene in alpha T3-1 gonadotrope cells is stimulated by cyclic 3',5'-adenosine monophosphate and protein kinase A, and is modulated by Steroidogenic factor-1 and Nur77

Regulation of GnRH receptor (GnRHR) expression levels in the pituitary is a crucial control point in reproduction. The promoter of the mouse GnRHR gene contains nuclear receptor half-sites (NRS) at -244/-236 and -15/-7 relative to the translation start site. Although binding of steroidogenic factor-1 (SF-1) to the -244/-236NRS is implicated in mediating basal and gonadotrope-specific expression, no function or protein-DNA interactions have previously been described for the -15/-7NRS. We report that levels of the endogenous GnRHR mRNA in alpha T3-1 cells are stimulated by forskolin and 8-bromo-cAMP. We also show that the orphan nuclear receptor Nur77 is expressed in alpha T3-1 cells, and that both SF-1 and Nur77 bind to the -15/-7NRS and -244/-236NRS in vitro. We show that the activity of the proximal (-579/+1) mouse GnRHR promoter is up-regulated by protein kinase A, via a mechanism that is modulated by SF-1, both positively and negatively, through binding to the -244/-236NRS or the -15/-7NRS, respectively. Nur77 appears to be capable of acting as a negative regulator of this response, via the -15/-7NRS. Furthermore, we show that forskolin up-regulates SF-1 mRNA levels in alpha T3-1 cells, indicating that the levels of SF-1 play a role in modulating the protein kinase A response.

Extracellular signal-regulated kinase and c-Src, but not Jun N-terminal kinase, are involved in basal and gonadotropin-releasing hormone-stimulated activity of the glycoprotein hormone alpha-subunit promoter

Addition of a GnRH agonist (GnRH-A) to alphaT3-1 cells stimulates different MAPK cascades: ERK, Jun N-terminal kinase (JNK), and p38. Activation of JNK, ERK, and p38 shows a unique fold activation ratio of 25:12:2, which might encode signal specificity. ERK is translocated to the nucleus within 20 min with a peak at 120 min of GnRH-A stimulation. We used the human alpha-subunit promoter linked to chloramphenicol acetyl transferase (alphaCAT) to examine the role of ERK, JNK, and c-Src, which is implicated in MAPK activation, in basal and GnRH-stimulated alphaCAT. Addition of GnRH-A resulted in a 3-fold increase in alphaCAT, whereas the Ca(2+) ionophore ionomycin and the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA) had no effect. Addition of GnRH-A and TPA, but not GnRH-A and ionomycin, produced a synergistic response, whereas removal of Ca(2+), but not down-regulation of TPA-sensitive PKCs, abolished GnRH-A-stimulated alphaCAT. Thus, regulation of alpha-promoter activity by GnRH is Ca(2+) dependent and is further augmented by PKC. Cotransfection of alphaCAT and constitutively active or dominant negative plasmids of ERK and JNK cascade members, or the use of the ERK inhibitor PD98059, revealed that ERK, but not JNK, is involved in basal and GnRH-A-stimulated alphaCAT. Because c-Src participates in MAPK activation by GnRH, we also studied its role. Cotransfection of alphaCAT and the dominant negative form of c-Src or incubation with the c-Src inhibitor PP1 reduced GnRH-A-stimulated alphaCAT. The 5'-deletion analysis revealed that the -846/-420 region participated in basal alpha-transcription. In addition, the -346/-156 region containing the pituitary glycoprotein hormone basal element, alpha-basal elements, glycoprotein-specific element, and upstream response element is involved in basal and GnRH-A-stimulated alphaCAT. ERK contribution to GnRH maps to -346/-280 containing the pituitary glycoprotein hormone basal element and alpha-basal elements 1/2. Surprisingly, although c-Src is involved in GnRH-A-stimulated ERK, its involvement is mapped to another region (-280/-180) containing the glycoprotein-specific element. Thus, ERK and c-Src but not JNK are involved in basal and GnRH-A-stimulated-alphaCAT, whereas c-Src contribution is independent of ERK activation.

Embryonic expression of the luteinizing hormone beta gene appears to be coupled to the transient appearance of p8, a high mobility group-related transcription factor

A comparison between two pituitary-derived cell lines (alpha T3-1 and L beta T2) that represent gonadotropes at early and late stages of development, respectively, was performed to further elucidate the genomic repertoire required for gonadotrope specification and luteinizing hormone beta (LH beta) gene expression. One isolated clone that displayed higher expression levels in L beta T2 cells encodes p8, a high mobility group-like protein with mitogenic potential that is up-regulated in response to proapoptotic stimuli and in some developing tissues. To test the functional significance of this factor in developing gonadotropes, a knockdown of p8 in L beta T2 cells was generated. The loss of p8 mRNA correlated with loss of endogenous LH beta mRNA and the loss of activity of a transfected LH beta promoter-driven reporter, even upon treatment with gonadotropin-releasing hormone. In addition, expression of p8 mRNA in developing mouse pituitary glands mirrored its expression in the gonadotrope-derived cell lines and coincided with the first detectable appearance of LH beta mRNA. In contrast, p8 mRNA was undetectable in the pituitary glands of normal adults. Taken together, our data indicate that p8 is a stage-specific component of the gonadotrope transcriptome that may play a functional role in the initiation of LH beta gene expression during embryonic cellular differentiation.

Neuron-restricted expression of the rat gonadotropin-releasing hormone gene is conferred by a cell-specific protein complex that binds repeated CAATT elements

GnRH gene expression is restricted to a tiny population of neurons scattered throughout the mediobasal hypothalamus. The combination of a 300-bp enhancer and the 173-bp promoter from the rat GnRH gene can confer this narrow specificity in transgenic mice and in transfections of hypothalamic GT1-7 cells. In the present study, we identify repeated CAATT elements in the 3' region of the rat GnRH enhancer that bind a tissue-restricted protein complex and play a significant role in cell-restricted expression of the GnRH gene. Deletions of multiple repeats demonstrate their importance in transcriptional activity. In fact, even mutation of a single repeat reduces expression. This reduction can be compensated by the conserved GnRH promoter, which also contains such elements and binds this protein complex. In Southwestern analysis, three proteins from GT1-7 nuclear extract bind to the CAATT element, and these proteins are not found in NIH3T3 cells. This cell-specific protein complex has properties of the Q50 homeodomain family of transcription factors and binds to as many as seven binding sites in the enhancer and promoter to play a key role in GnRH gene expression in the hypothalamus.

Mechanism of gonadotropin gene expression. Identification of a novel negative regulatory element at the transcription start site of the glycoprotein hormone alpha-subunit gene

Regulation of the glycoprotein hormone alpha-subunit (GPHalpha) gene has been studied extensively in pituitary and placental cell lines, but little is known of the transcriptional regulators important for its ectopic expression. To investigate the molecular basis for ectopic expression, it was critical to define cis-regulatory elements and their cognate trans-acting factors that modulate promoter activity in epithelial cell types that do not normally express GPH. DNA-mediated transient expression of promoter-reporter constructs was used to identify a novel negative regulatory element located at the GPHalpha gene transcription start site. Truncation or site-directed mutagenesis of this element produced up to a 10-fold increase in promoter activity. Electrophoretic mobility shift analysis detected a protein that binds specifically to a DNA motif encompassing the cap site. Based on competitive DNA binding studies with mutated oligonucleotides, it was determined that bases from -5 to -2 and +4 to +11 are critical for protein binding. The DNA sequence flanking the transcription start site from -9 to +11 is an imperfect palindrome; consequently, this motif is referred to as the cap site diad element (CSDE) and the cognate factor as the cap site-binding protein (CSBP). CSBP activity was present at different levels in nuclear extracts prepared from a variety of cell types. Significantly, the ratio of activities exhibited by the GPHalpha promoter with a mutated CSDE compared with the promoter with a wild-type CSDE was dependent on the transfected cell line and its content of CSBP. These results indicate that a negative regulatory element centered at the GPHalpha gene cap site and its cognate DNA-binding protein make a significant contribution to the production of alpha-subunit in a variety of tumor tissues. A detailed understanding of this cis/trans pair may further suggest a mechanism to explain, at least in part, how this gene becomes activated in nonendocrine tumors.

CCAAT/enhancer binding protein beta regulates expression of the cystatin-related epididymal spermatogenic (Cres) gene

The CRES protein is a member of the cystatin superfamily of cysteine protease inhibitors with restricted expression in stage-specific germ cells, proximal caput epididymidis, and anterior pituitary gonadotroph cells. To elucidate the molecular mechanisms regulating the highly restricted expression of the cres gene, we have sequenced 1.6 kilobases of mouse cres 5' flanking sequence and performed studies to examine the cres gene promoter. Two putative CCAAT/enhancer binding protein (C/EBP) transcription factor binding motifs exist within the first 135 base pairs of cres promoter. Furthermore, our studies demonstrate that cres mRNA levels are dramatically reduced in the epididymides of C/EBP beta-deficient mice. These data suggest that the C/EBP family of transcription factors, in particular C/EBP beta, plays a role in the regulation of cres gene expression. In support of this finding, Northern blot analysis showed that C/EBP beta is the predominant C/EBP family member expressed in the L beta T2 gonadotroph cell line and the proximal caput epididymidis. Also, gel shift and supershift assays demonstrated that C/EBP beta protein in nuclear extracts from L beta T2 gonadotroph cells and epididymal cells bound to the two C/EBP sites in the cres promoter. Finally, to test the in vivo function of the C/EBP sites in cres gene expression, transfection studies were performed in L beta T2 gonadotroph cells and two heterologous cell systems. These experiments showed a significant reduction of cres transactivation when either C/EBP sites were mutated, and no transC/EBP activation of the cres promoter when both C/EBP sites were mutated. Taken together, these studies demonstrate that the C/EBP beta transcription factor is necessary for high levels of cres gene expression in the proximal caput epididymidis and anterior pituitary gonadotroph cells.

Tilapia glycoprotein hormone alpha subunit: cDNA cloning and hypothalamic regulation

The cDNA encoding the glycoprotein alpha (GPalpha) subunit of tilapia (Oreochromis mossambicus) was partially cloned using RACE-polymerase chain reaction (PCR) technique. The amplified cDNA was found to be 583 bases long, and to consist of a portion of the signal peptide, the full sequence encoding the mature peptide (94 amino acids) and the 3' untranslated region. Northern blot analysis revealed a single band of approximately 600 bp. Alignment of the deduced amino acids of the mature protein showed that the tilapia GPalpha subunit shares more than 80% identity with that of other perciform fish (i.e. striped bass, sea bream and yellowfin porgy) and less than 70% with that of more taxonomically remote fish and other vertebrates. Exposure of dispersed tilapia pituitary cells to salmon gonadotropin-releasing hormone (sGnRH) elevated GPalpha mRNA levels via both PKC and cAMP-protein kinase A (PKA) pathways. The transcript levels were also regulated by pituitary adenylate cyclase activating polypeptide (PACAP) and neuropeptide Y (NPY), both acting through PKC and PKA pathways. Moreover, a combined treatment of PACAP or NPY with GnRH seems to have an additive effect on the GPalpha subunit gene transcription. These results suggest that in tilapia the expression of GPalpha subunit is regulated by GnRH mainly via PKC and PKA pathways. Furthermore, PACAP and NPY can elevate the GnRH-stimulated GPalpha subunit transcription and can directly affect the subunit mRNA levels, via the same transduction pathways.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

cAMP response element-binding protein-binding protein mediates thyrotropin-releasing hormone signaling on thyrotropin subunit genes

Transcription of pituitary alpha-glycoprotein hormone subunit (alpha-GSU) and thyrotropin beta subunit (TSH-beta) genes is stimulated by thyrotropin-releasing hormone (TRH). Since cAMP response element-binding protein (CREB)-binding protein (CBP) integrates a number of cell signaling pathways, we investigated whether CBP is important for TRH stimulation of the TSH subunit genes. Cotransfection of E1A in GH(3) cells completely blocked TRH stimulation of the TSH subunit genes, suggesting that CBP is a key factor for TRH signaling in the pituitary. CBP and Pit-1 acted synergistically in TRH stimulation of the TSH-beta promoter, and amino acids 1-450 of CBP were sufficient for the TRH effect. In contrast, on the human alpha-GSU promoter, CREB and P-Lim mediated TRH signaling. Intriguingly, CREB was phosphorylated upon TRH stimulation, leading to CBP recruitment to the alpha-GSU promoter. CBP also interacted with P-Lim in a TRH-dependent manner, suggesting that P-Lim is an important factor for non-cAMP response element-mediated TRH stimulation of this promoter. Distinct domains of CBP were required for TRH signaling by CREB and P-Lim on the alpha-GSU promoter, amino acids 450-700 and 1-450, respectively. Thus, the amino terminus of CBP plays a critical role in TRH signaling in the anterior pituitary via both Pit-1-dependent and -independent pathways, yielding differential regulation of pituitary gene products.

Sequence analysis of the human glycoprotein hormone alpha-subunit gene 5'-flanking DNA and identification of a potential regulatory element as an alu repetitive sequence

The nucleotide sequence of the human glycoprotein hormone alpha-subunit (GPHalpha) gene 5'-flanking DNA was determined from -1637 to +49 relative to the cap site (+1). Comparison of the upstream sequence of the human gene with those of rhesus and mouse demonstrates regions with variable identity. When the 1.7 kb fragment was used to drive the expression of chloramphenicol acetyltransferase (CAT) in transiently transfected HeLa cells, it was found that CAT activity was elevated about 3-fold when the fragment was truncated from -1637 to -846, suggesting the presence of a negative regulatory element in the distal 5'-flanking DNA. This overlaps an Alu repetitive sequence (ARS) located between nucleotides -1330 and -1007. Gel mobility shift and DNase protection analyses identified a protein binding site centered around -1100 in the ARS second monomer. The GPHalpha upstream ARS was cloned in both orientations in positions upstream and downstream from the bacterial CAT gene under control of the herpes simplex virus thymidine kinase (tk) promoter. DNA-mediated transient transfection of these plasmids revealed a marked inhibition (79-82%) of CAT production by the ARS when it was cloned upstream from the tk promoter and in the same orientation as that found in the GPHalpha 5'-flanking DNA. Smaller decreases (29-57%) were produced by the ARS cloned upstream from the tk promoter in the reverse orientation. In marked contrast, the Alu repetitive element had little or no effect when cloned in either orientation downstream from the tk-CAT gene. Introduction of a second ARS downstream from the CAT reporter gene in vectors already containing an ARS upstream from the tk promoter significantly reduced the strong negative effect elicited by the upstream repetitive element. When compared to the Blur 8 Alu element, the GPHalpha upstream ARS differs markedly with respect to its effect on tk-CAT expression in transient assays and as a substrate for DNA binding proteins present in HeLa nuclear extracts. Together, the transient expression results demonstrate that ARS elements can influence expression of nearby class II promoters. The extent of this effect depends on element position and orientation, cell type, the particular ARS (e.g., GPHalpha or Blur 8), and whether copies were present both upstream and downstream from the transcription unit.

Cellular responses to sodium butyrate exhibit the dominance of one parental phenotype in somatic cell hybrids

The glycoprotein hormone alpha-subunit (GPHalpha) gene is inducible by sodium butyrate (NaBtr) in nontrophoblastic tumor cell lines such as HeLa (cervical carcinoma) but not in trophoblastic tumor cell lines such as JEG-3 (choriocarcinoma). The studies summarized in this report examined the ability of NaBtr to induce GPHalpha expression in somatic cell hybrids between HeLa SR3(hyg) and JEG-3(neo). The hybrid cells, pooled clones resistant to both hygromycin B and G418 sulfate, have been named JELA and were indistinguishable from the SR3 parent with regard to induction of the GPHalpha gene. The effects of NaBtr on cell proliferation were also similar in HeLa and JELA but different from those in JEG-3. The GPHalpha gene could be induced by NaBtr in the JEG-3 parent only when they were simultaneously treated with cycloheximide (CHX). The ability of NaBtr to induce GPHalpha in CHX-treated JEG-3 cells occurred concomitantly with a change in the electrophoretic mobility of enhancer binding proteins as determined in gel shift assays. The DNA-protein complexes generated between a trophoblast specific element (TSE) and nuclear proteins in HeLa SR3 and JELA migrated significantly more slowly than the complex generated by JEG-3 nuclear proteins. However, when nuclear extracts were prepared from CHX-treated JEG-3 cells, the complex generated with the TSE oligonucleotide migrated more slowly than the complex from untreated JEG-3 cells and coincident with the complexes produced with nuclear extracts from HeLa SR3 and JELA cells. Together, these data demonstrate that inducibility of the GPHalpha gene by NaBtr in JELA cell hybrids resembles that of the HeLa SR3 parent and that its inducibility in the JEG-3 parent parallels the status of an enhancer binding protein (TSEB) as judged from changes in electrophoretic mobility. The results are consistent with a model in which the status of TSEB has a profound influence on the gene's response to NaBtr.

The equine luteinizing hormone beta-subunit promoter contains two functional steroidogenic factor-1 response elements

The requirements for basal expression of the LH beta-subunit promoter in pituitary gonadotropes are largely unknown. We have used the equine (e) LHbeta subunit promoter as a model to unravel the combinatorial code required for gonadotrope expression. Through the use of 5'-deletion mutagenesis, a region between -185 and -100 of the eLHbeta promoter was shown to play a critical role in maintaining basal promoter activity in alphaT3-1 and LbetaT2 cells. This region encompasses the steroidogenic factor-1 (SF-1) binding site that has been reported to have a functional role in expression of the LHbeta promoter in other species. We have also identified an additional SF-1 site at -55 to -48. Binding of SF-1 to both sites was confirmed by electrophoretic mobility shift assays. Mutations within these sites, either individually or in combination, did not attenuate basal activity of the eLHbeta promoter in alphaT3-1 cells, but did diminish promoter activity in LbetaT2 cells. Interestingly, cotransfection with an expression vector encoding SF-1 induced eLHbeta promoter activity, and this induction was abrogated by mutations within the SF-1 sites in alphaT3-1 cells. Block replacement mutagenesis was performed on the -185/-100 region of the eLHbeta promoter to identify DNA response elements responsible for maintaining basal promoter activity. From this analysis, two regions emerged as being important: a distal 31-bp segment (-181 to -150) and an element located immediately 3' to the distal SF-1 site (-119 to -106). It is hypothesized that these two regions as well as the SF-1 sites represent regulatory elements that contribute to a combinatorial code involved in targeting expression of the eLHbeta promoter to gonadotropes.

Influence of a cap site element on tissue-restricted expression of the glycoprotein hormone alpha-subunit gene

Little is known of the transcriptional regulators important for expression of the glycoprotein hormone alpha-subunit (GPHalpha) gene in nonendocrine tumors, which secrete free alpha-subunit at an incidence of 25-80%. Consequently, attempts were made to define cis-regulatory elements and their cognate trans-acting factors that modulate promoter activity in epithelial cell types that do not normally express the glycoprotein hormones. DNA-mediated transient expression of promoter-reporter constructs was used to identify a novel negative regulatory element located at the GPHalpha gene transcription start site. Mutagenesis of this element produced a 2- to 10-fold increase in promoter activity, depending on the particular mutation and the transfected tumor cell line. Electrophoretic mobility shift analysis detected a protein that binds specifically to a DNA motif encompassing the cap site. It was present at different levels in a variety of cell types. Significantly, the degree to which activity of the wild-type promoter was suppressed relative to that of the mutant promoter was proportional to the level of cap site binding protein in the collection of cell lines examined. These results indicate that a negative regulatory element centered at the GPHalpha gene cap site and its cognate DNA-binding protein make a significant contribution to the production of alpha-subunit in a variety of tumor tissues. A detailed understanding of this cis/trans pair may further suggest a mechanism to explain, at least in part, how this gene becomes activated in nonendocrine tumors.

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 interacts with a gonadotrope-specific element within the first exon of the human gonadotropin-releasing hormone receptor gene to mediate gonadotrope-specific expression

GnRH plays a pivotal role in regulating human reproductive functions. This hypothalamic peptide interacts with its receptor (GnRHR) on the pituitary gonadotropes to trigger the secretion of gonadotropins, which, in turn, regulates the release of sex steroids from the gonads. In light of the importance of GnRHR, the molecular mechanisms underlying the transcriptional regulation of the human GnRHR (hGnRHR) gene become a key issue in understanding human reproduction. In this report, the possible involvement of steriodogenic factor-1 (SF-1) as a key cell-specific regulator for hGnRHR gene expression was examined. By the transient luciferase reporter gene assays, the wild-type promoter, containing 2.3 kb ofthe hGnRHR gene 5'-flanking region relative to the ATG codon, was able to drive a 3.6 +/- 0.2-fold (P < 0.05) increase in luciferase activity in the mouse alphaT3-1 gonadotropes. Subsequent deletion analysis indicated that the most proximal 173 bp within the first exon of the gene, although not a promoter itself, contains a critical regulatory element(s) essential for the basal expression of the hGnRHR gene. The functional roles of the putative gonadotrope-specific elements (GSE; consensus 5'-CTG(A)/(T)CCTTG-3') residing at positions -5, -134, and -396 were studied by site-directed mutagenesis, and it was found that only the mutation at position -134 significantly reduced the promoter activity (80% reduction; P < 0.05). The attenuation effect of this GSE mutant was cell specific, as it was restricted to alphaT3-1 cells, but not to COS-7 and human ovarian adenocarcinoma (SKOV-3) cells. Competitive mobility shift assays using either alphaT3-1 nuclear extract or recombinant SF-1 protein clearly indicated that SF-1 is able to interact specifically with this GSE element positioned at -134. Using a SF-1 antibody that completely abrogated complex formation in the gel shift assays, the involvement of endogenous nuclear SF-1 was further evidenced. By competitive gel shift assays using oligoprimers with 2-bp scanning mutations, the sequences essential for the interaction with SF-1 were identified (5'-TTG(A)/(T)CCCTG-3', underlined sequences were important). To study the in vivo function of SF-1, vector directing expression of sense or antisense SF-1 messenger RNA (mRNA) was cotransfected with the hGnRHR promoter-luciferase construct into alphaT3-1, SKOV-3, and COS-7 cells. Overexpression of the SF-1 mRNA was able to enhance promoter activities in all of the cells tested. On the contrary, expression of the antisense SF-1 mRNA reduced the hGnRHR promoter activity only in alphaT3-1 cells, not in COS-7 or SKOV-3 cells. In summary, the data reported here provide conclusive evidence that SF-1 interacts with the GSE motif at position -134 within the first exon of the hGnRHR gene to mediate its cell-specific expression.

An upstream regulator of the glycoprotein hormone alpha-subunit gene mediates pituitary cell type activation and repression by different mechanisms

Targeting of alpha-subunit gene expression within the pituitary is influenced by an upstream regulatory region that directs high level expression to thyrotropes and gonadotropes of transgenic mice. The same region also enhanced the activity of the proximal promoter in transfections of pituitary-derived alpha-TSH and alpha-T3 cells. We have localized the activating sequences to a 125-bp region that contains consensus sites for factors that also play a role in proximal promoter activity. Proteins present in alpha-TSH and alpha-T3 cells as well as those from GH3 somatotrope-derived cells interact with this region. The upstream area inhibited proximal alpha-promoter activity by 80% when transfected into GH3 cells. Repression in GH3 cells was mediated through a different mechanism than enhancement, as supported by the following evidence. Reversing the orientation of the area resulted in a loss of proximal promoter activation in alpha-TSH and alpha-T3 cells but did not relieve repression in GH3 cells. Mutation of proximal sites shown to be important for activation had no effect on repression. Finally, bidirectional deletional analysis revealed that multiple elements are involved in activation and repression and, together with the DNA binding studies, suggests that these processes may be mediated through closely juxtaposed or even overlapping elements, thus perhaps defining a new class of bifunctional gene regulatory sequence.

Structural characterization of the chicken SF-1/Ad4BP gene

SF-1/Ad4BP was identified as a master regulator controlling steroidogenic P-450 genes and belongs to the steroid hormone receptor superfamily. It is expressed in the adrenal cortex, gonads, and pituitary gonadotroph. Targeted disruption of the mouse SF-1/Ad4BP gene showed that it plays a critical role in the development of the steroidogenic tissues and pituitary gonadotroph. We have recently cloned the chicken SF-1/Ad4BP cDNA and have now cloned the chicken SF-1/Ad4BP gene and analyzed its promoter activity. This gene consists of seven exons as well as mammalian counterparts and spans about 15 kb. In mice, the gene encodes another protein, ELP, but we could not find the open reading frame of ELP in the chicken SF-1/Ad4BP gene. The promoter of this gene included five putative cis elements (E, CCAAT, GC and TATA boxes and a GA-rich element), although no TATA box has been found in mammalian counterparts. The E and CCAAT boxes moderately affected promoter activity and the GA-rich element and TATA box were essential for the expression of the chicken SF-1/Ad4BP gene.

Activation of the p38 mitogen-activated protein kinase pathway by gonadotropin-releasing hormone

Previous studies have shown that interaction of GnRH with its serpentine, G protein-coupled receptor results in activation of the extracellular signal regulated protein kinase (ERK) and the Jun N-terminal protein kinase (JNK) pathways in pituitary gonadotropes. In the present study, we examined GnRH-stimulated activation of an additional member of the mitogen-activated protein kinase (MAPK) superfamily, p38 MAPK GnRH treatment of alphaT3-1 cells resulted in tyrosine phosphorylation of several intracellular proteins. Separation of phosphorylated proteins by ion exchange chromatography suggested that GnRH receptor stimulation can activate the p38 MAPK pathway. Immunoprecipitation studies using a phospho-tyrosine antibody resulted in increased amounts of immunoprecipitable p38 MAPK from alphaT3-1 cells treated with GnRH. Immunoblot analysis of whole cell lysates using a phospho-specific antibody directed against dual phosphorylated p38 kinase revealed that GnRH-induced phosphorylation of p38 kinase was dose and time dependent and was correlated with increased p38 kinase activity in vitro. Activation of p38 kinase was blocked by chronic phorbol ester treatment, which depletes protein kinase C isozymes alpha and epsilon. Overexpression of p38 MAPK and an activated form of MAPK kinase 6 resulted in activation of c-jun and c-fos reporter genes, but did not alter the expression of the glycoprotein hormone alpha-subunit reporter. Inhibition of p38 activity with SB203580 resulted in attenuation of GnRH-induced c-fos reporter gene expression, but was not sufficient to reduce GnRH-induced c-jun or glycoprotein hormone alpha-subunit promoter activity. These studies provide evidence that the GnRH signaling pathway in alphaT3-1 cells includes protein kinase C-dependent activation of the p38 MAPK pathway. GnRH integration of c-fos promoter activity may include regulation by p38 MAPK.

Identification and characterization of the gonadotropin-releasing hormone response elements in the mouse gonadotropin-releasing hormone receptor gene

The response of the pituitary gonadotrope to gonadotropin-releasing hormone (GnRH) correlates directly with the concentration of GnRH receptors (GnRHR) on the cell surface, which is mediated in part at the level of GnRHR gene expression. Several hormones have been implicated in this regulation, most notably GnRH itself. Despite these observations and the central role that GnRH is known to play in reproductive development and function, the molecular mechanism(s) by which GnRH regulates transcription of the GnRHR gene has not been well elucidated. Previous studies in this laboratory have identified and partially characterized the promoter region of the mouse GnRHR gene and demonstrated that the regulatory elements for tissue-specific expression as well as for GnRH regulation are present within the 1.2-kilobase 5'-flanking sequence. By using deletion and mutational analysis as well as functional transfection studies in the murine gonadotrope-derived alphaT3-1 cell line, we have localized GnRH responsiveness of the mouse GnRHR gene to two DNA sequences at -276/-269 (designated Sequence Underlying Responsiveness to GnRH-2 (SURG-2), which contains the consensus sequence for the activating protein-1-binding site) and -292/-285 (a novel element designated SURG-1), and demonstrated that this response is mediated via protein kinase C. By using the electrophoretic mobility shift assay, we further demonstrate that a member(s) of the Fos/Jun heterodimer superfamily is responsible in part for the DNA-protein complexes formed on SURG-2, using alphaT3-1 nuclear extracts. These data define a bipartite GnRH response element in the mouse GnRHR 5'-flanking sequence and suggest that the activating protein-1 complex plays a central role in conferring GnRH responsiveness to the murine GnRHR gene.

Activation of the glycoprotein hormone alpha-subunit gene promoter in thyrotropes

Pituitary expression of the glycoprotein hormone alpha-subunit gene localizes to thyrotropes and gonadotropes. Factors that activate the alpha-subunit promoter exclusively in thyrotropes have not been described. In the current studies, alpha-subunit promoter activity was compared in alphaTSH thyrotropic and alphaT3 gonadotropic cells. Deletional analysis revealed a more important contribution of the -453/-381 region in thyrotropic cells and the -341/-297 region in gonadotropic cells, while other deletions had similar effects. Mutational analysis revealed some regions that were functionally similar, while others were active only in thyrotropic cells, such as the regions - 434/ - 421, - 398/ - 385, - 376/- 364 and - 363/- 351, that decreased activity by approximately 2-fold each. Combined mutation of the regions - 434/ - 421, - 398/ - 385 and - 376/ - 364 decreased activity by 5-fold in thyrotropic cells and not at all in gonadotropic cells. Cotransfection with Ptx1 in CV-1 cells resulted in a strong stimulation of alpha-subunit promoter activity, that was significantly diminished with mutation of the - 398/ - 385 region that disrupts the Ptx1 binding site, suggesting that this factor may play a role in thyrotrope-specific activation. This analysis provides basic information important to identify common and unique factors contributing to the cell-specific expression of the alpha-subunit gene.

Functional interactions of an upstream enhancer of the mouse glycoprotein hormone alpha-subunit gene with proximal promoter sequences

Transcription of the glycoprotein hormone alpha-subunit gene in the pituitary is governed by different promoter elements in thyrotropes and gonadotropes. We recently identified an upstream enhancer that directs a high level of cell type specific expression in transgenic mice and stimulates proximal promoter activity in cultured alphaTSH and alphaT3 cells. To assess the contribution of promoter sequences that functionally interact with the enhancer, we mutated two proximal elements shown to be important in both thyrotrope and gonadotrope cells. Disruption of the pituitary glycoprotein hormone basal element (PGBE), which binds a LIM homeodomain protein, resulted in a decrease in basal promoter activity in both alphaTSH and alphaT3 cells. Enhancer function was completely abolished by the PGBE site mutation in alphaT3 gonadotropes, whereas some stimulatory activity remained in alphaTSH thyrotropes. Mutation of the gonadotrope specific element (GSE), which binds SF1 and is important for basal activity in gonadotropes and TRH response in thyrotropes, resulted in declines in basal and enhanced promoter activity only in alphaT3 cells and not in alphaTSH cells. Despite this decrease in enhanced activity, the GSE mutated promoter still retained some enhancer stimulated activity, suggesting that the PGBE site still functionally interacts in the absence of an intact GSE. This mutation had no effect in alphaTSH cells. These data suggest that although the enhancer works in both cell types it exhibits cell type specific functional characteristics.

Steroidogenic factor-1 and early growth response protein 1 act through two composite DNA binding sites to regulate luteinizing hormone beta-subunit gene expression

Recent in vivo and in vitro studies have implicated the orphan nuclear receptor, steroidogenic factor-1 (SF-1), and the early growth response protein 1 (Egr-1) in the transcriptional regulation of the luteinizing hormone beta-subunit (LHbeta) gene. We have previously demonstrated the ability of SF-1 to bind to and transactivate the rat LHbeta gene promoter acting at a consensus gonadotrope-specific element (GSE) located at position -127. We have now identified a second functional GSE site at position -59. In addition, based on electrophoretic mobility shift assay, in vitro translated Egr-1 is shown to bind to two putative Egr-1 binding sites (positions -112 and -50), which appear to be paired with the identified GSE sites. By transient transfection assay in pituitary-derived GH3 cells, it was seen that Egr-1 increases promoter activity of region -207/+5 of the rat LHbeta gene promoter through action at both Egr-1 sites. Furthermore, LHbeta gene promoter activity is markedly augmented in the presence of both factors together relative to activity in the presence of SF-1 or Egr-1 alone (150-fold versus 14-fold and 12-fold, respectively). These data define two composite SF-1-Egr-1 response-elements in the proximal LHbeta gene promoter and suggest that SF-1 and Egr-1 act synergistically to increase expression of the LHbeta gene in the gonadotrope.

Discrete stages of anterior pituitary differentiation recapitulated in immortalized cell lines

Targeted expression of oncogenes in transgenic mice can immortalize specific cell types to serve as valuable cultured model systems. Utilizing promoter regions from pituitary genes activated in the gonadotrope/thyrotrope lineage at discrete stages of development, we have demonstrated that targeted oncogene expression in transgenic mice can produce cell lines representing sequential stages of differentiation. Each cell line expresses a specific subset of the genes denoting differentiated function such as the subunits of the glycoprotein hormones, hormone receptors, and transcriptional regulatory proteins. These model systems have allowed detailed investigations into molecular and cellular mechanisms otherwise inaccessible in vivo or in complex primary pituitary cell cultures.

Identification of cis-acting deoxyribonucleic acid elements that mediate gonadotropin-releasing hormone stimulation of the rat luteinizing hormone beta-subunit gene

GnRH plays a critical role in reproductive development and function by regulating the biosynthesis and secretion of the pituitary gonadotropins, LH and FSH. Although it is known that GnRH induces gonadotropin subunit gene transcription, the mechanism by which this occurs has not been elucidated. Studies have been hindered by the lack of available cell lines that express the LH and FSH subunit genes and respond to GnRH. We have transfected the rat pituitary GH3 cell line with the rat GnRH receptor complementary DNA. These cells, when cotransfected with regulatory regions of the LH or FSH subunit genes fused to a luciferase reporter gene, respond to GnRH with an increase in promoter activity comparable to that seen in primary rat pituitary cells. In this study, we have used this cell model to identify cis-acting elements of the LHbeta gene that mediate stimulation by GnRH. Analysis of a series of 5'-deletion and internal deletion constructs has revealed two regions of the rat LHbeta gene promoter involved in mediating the response to GnRH, region A (-490/-352) and region B (-207/-82). Fusion of region A upstream of a heterologous minimal promoter linked to the luciferase gene conferred GnRH responsiveness to the promoter, whereas region B did not. However, the presence of both regions A and B conferred a greater GnRH response than region A alone. Electrophoretic mobility shift assay revealed the presence of a protein(s) binding to region A using GH3 as well as alphaT3-1 nuclear extracts. Thus, region A (-490/-352) confers GnRH responsiveness to the LHbeta subunit gene and binds to a protein(s) present in pituitary cell lines. DNA sequences in region B (-207/-82) also contribute to GnRH responsiveness. The identification of putative GnRH response elements in the rat LHbeta gene promoter will aid in elucidation of the mechanisms of regulation of gene expression by GnRH.

Cell-specific expression of the mouse glycoprotein hormone alpha-subunit gene requires multiple interacting DNA elements in transgenic mice and cultured cells

The glycoprotein hormone alpha-subunit gene is expressed and differentially regulated in pituitary gonadotropes and thyrotropes. Previous gene expression studies suggested that cell specificity may be regulated by distinct DNA elements. We have identified an enhancer region between -4.6 and -3.7 kb that is critical for high level expression in both gonadotrope and thyrotrope cells of transgenic mice. Fusion of the enhancer to -341/+43 mouse alpha-subunit promoter results in appropriate pituitary cell specificity and transgene expression levels that are similar to levels observed with the intact -4.6 kb/+43 construct. Deletion of sequences between -341 and -297 resuited in a loss of high level expression and cell specificity, exhibited by ectopic transgene activation in GH-, ACTH-, and PRL-producing pituitary cells as well as in other peripheral tissues. Consistent with these results, transient cell transfection studies demonstrated that the enhancer stimulated activity of a -341/+43 alpha-promoter in both alphaTSH and alphaT3 cells, but it did not enhance alpha-promoter activity significantly in CV-1 cells. Removal of sequences between -341 and -297 allowed the enhancer to function in heterologous cells. Loss of high level expression and cell specificity may be due to loss of sequences required for binding of the LIM homeoproteins or the alpha-basal element 1. These data demonstrate that the enhancer requires participation by both proximal and distal sequences for high level expression and suggests that sequences from -341 to -297 are critical for restricting expression to the anterior pituitary.

The tripartite basal enhancer of the gonadotropin-releasing hormone (GnRH) receptor gene promoter regulates cell-specific expression through a novel GnRH receptor activating sequence

The molecular mechanisms regulating restricted expression of GnRH receptor and gonadotropin subunit genes to gonadotrope cells have been the focus of intense interest. Using deletion and mutational analysis we have identified a tripartite enhancer that regulates cell-specific expression of the GnRH receptor gene in the gonadotrope-derived alphaT3-1 cell line. Individual elements of this enhancer include binding sites for steroidogenic factor-1; activator protein 1 (AP-1); and a novel element referred to as the GnRH receptor activating sequence (GRAS). Mutation of each element alone results in loss of approximately 60% of promoter activity. Combinatorial mutations of any two elements decreases promoter activity by approximately 80%. Finally, mutation of all three elements reduces promoter activity to a level not different from promoterless vector. Using 2-bp mutations, we have defined the functional requirements for transcriptional activation by GRAS. The core motif of GRAS is at -391 to -380 bp relative to the start site of translation and has the sequence 5'-CTAGTCACAACA-3'. Three copies of GRAS or GRAS with a 2-bp mutation (muGRAS) were cloned into a luciferase expression vector immediately upstream of the thymidine kinase minimal promoter (TK) and tested for expression in alphaT3-1 cells. When compared with TK promoter alone, activity of 3xGRAS-TKLUC was increased by more than 5-fold while activity of 3xmuGRAS-TKLUC was unchanged. When 3xGRAS-TKLUC was transfected into a variety of nongo-nadotrope cell lines, it did not increase activity of the TK promoter. We propose that basal activity of the GnRH receptor gene is regulated by a tripartite enhancer, and the key component of this enhancer is an element, GRAS, that activates transcription in a cell-specific fashion.

Development and differentiation of pituitary cells

Classically, it was thought that the adenohypophyseal gland originated from the oral ectoderm. Its development has been the object of numerous studies over many years. However, several questions are still raised about its origin, differentiation, and commitment. The adenohypophyseal gland could originate from the anterior ridge of the neural plate. Glandular adenohypophyseal cells are committed very early in embryonic life. Interactions between adenohypophyseal presumptive territory and neighboring tissues can exist very soon, as early as at the open neural stage. The expression of a given phenotype by the committed cells seems to be controlled by a number of differentiation and/or transcription factors. In view of all these studies, performed with the use of different in vivo and in vitro models, classical concepts of the embryology of the adenohypophyseal gland need to be reevaluated. Indeed, many questions remain unanswered concerning the molecular mechanisms of known and unknown factors controlling development of the adenohypophyseal gland.

Transcriptional repression of the alpha-subunit gene by androgen receptor occurs independently of DNA binding but requires the DNA-binding and ligand-binding domains of the receptor

The pituitary glycoprotein hormones LH and FSH regulate the reproductive cycle and are sensitive to feedback by gonadal steroids. The common alpha-subunit shared by these hormones is transcriptionally repressed by androgen receptor (AR) in the presence of its ligand dihydrotestosterone. This identifies at least one mechanism that contributes to AR-dependent suppression of gonadotropin synthesis. Repression of alpha-subunit transcription by AR requires only the sequences within the first 480 bp of the promoter. While this region contains a high-affinity binding site for AR, this element does not mediate the suppressive effects of androgens. Instead, two other elements within the promoter-regulatory region (alpha-basal element and cAMP-regulatory element), which are important for expression of the alpha-subunit gene in gonadotropes, mediate the effects of AR. This suggests that AR inhibits activity of the alpha-subunit promoter by interfering with the transcriptional properties of the proteins that bind to alpha-basal element and the cAMP-regulatory elements. Furthermore, transfection analysis of various mutant ARs identified both the DNA-binding and ligand-binding domains of the receptor as critical for repression. Comparisons with the MMTV promoter revealed distinct structural requirements that underlie the transactivation and transrepression properties of AR.

Pituitary cytokine and growth factor expression and action

The complex range of pituitary regulatory mechanisms reviewed here underlies the critical function of the pituitary in sustaining all higher life forms. Thus, the ultimate net secretion of pituitary hormones is determined by signal integration from all three tiers of pituitary control. It is clear from our current knowledge that the trophic hormone cells of the anterior pituitary are uniquely specialized to respond to these signals. Unravelling their diversity and complexity will shed light upon the normal function of the master gland. Understanding these control mechanisms will lead to novel diagnosis and therapy of disordered pituitary function (357).

The cAMP response elements of the alpha subunit gene bind similar proteins in trophoblasts and gonadotropes but have distinct functional sequence requirements

The alpha subunit gene encodes a common subunit shared by all glycoprotein hormones. This single copy gene is expressed in pituitary gonadotropes and thyrotropes of all mammals and in placental trophoblasts of primates and horses. Tandem cAMP response elements (CREs) in the promoter of the human gene are key mediators of this pattern of cell-specific expression. Replacing the palindromic CREs with non-primate variant CREs significantly attenuated activity in trophoblasts but not in gonadotropes. Furthermore, proteins binding the palindromic CRE cross-reacted with antibodies for CREB, CREM, ATF1, ATF2, and c-Jun, while proteins binding the variant CRE cross-reacted only with ATF2 and c-Jun antibodies. The data suggest that ATF2 and c-Jun can activate transcription through the CREs in gonadotropes but not in trophoblasts. Additional analyses indicated that while promoters with either palindromic or variant CREs have similar overall activity in gonadotropes, the variant CREs make a much smaller contribution to promoter activity than their palindromic counterparts. The weaker contribution of the variant CREs is compensated by the activity of two upstream elements present in the promoter. This compensation probably occurs through an indirect mechanism, as the binding affinity of proteins to the CRE is not influenced by the presence of these upstream elements.

The proximal 350 bp of 5'-flanking sequence of the human α-subunit glycoprotein hormone gene functions in the pituitary gland, but not the placenta, in transgenic mice

To understand better the minimal DNA sequence requirements for regulated expression of the human α-subunit glycoprotein hormone gene (Hα), two lines of transgenic mice were constructed that contained a fusion gene (Hα-350CAT) consisting of only 350 bp of 5'-flanking sequence of Hα linked to the bacterial gene encoding chloramphenicol acetyltransferase (CAT). CAT activity was detectable in pituitary, but not in brain, heart, kidney, liver, lung, pancreas, or spleen in transgenic mice. Gonadectomy increased (p<0.05) CAT activity in the pituitaries of males (6.5±1.4% conversion/μg protein; mean ± SEM) and females (14.5±4.2) compared to intact males (1.2±0.3) and females (6.7±1.0). In addition, administration of a GnRH antagonist (antide; 60 μg/injection; one injection every other day) for 10 d to gonadectomized animals decreased (p<0.05) CAT activity in males (3.5±0.8) and females (2.9±0.5) compared to gonadectomized animals that received saline. Antide also reduced (p<0.05) serum concentrations of luteinizing hormone in gonadectomized males and females compared to gonadectomized animals that received saline. Surprisingly, CAT activity in the placenta of Hα-350CAT transgenic mice was not detectable (<3 SD above the mean of CAT activity in placenta from nontransgenic mice;n=77). Thus, expression of the human α-subunit promoter in the placenta of transgenic mice appears to require DNA sequences upstream of the proximal 350 bp of 5'-flanking sequence, whereas the proximal 350 bp of the human α-subunit gene contains sufficient DNA sequence to target pituitary-specific expression and confer responsiveness to gonadal hormones and GnRH.

Immortalization of pituitary cells at discrete stages of development by directed oncogenesis in transgenic mice

Targeted expression of oncogenes in transgenic mice can immortalize specific cell types to serve as valuable cultured model systems. Utilizing promoter regions from a set of genes expressed at specific stages of differentiation in a given cell lineage, we demonstrate that targeted oncogenesis can produce cell lines representing sequential stages of development, in essence allowing both spatial and temporal immortalization. Our strategy was based on our production of a committed but immature pituitary gonadotrope cell line by directing expression of the oncogene SV40 T antigen using a gonadotrope-specific region of the human glycoprotein hormone alpha-subunit gene in transgenic mice. These cells synthesize alpha-subunit and gonadotropin-releasing hormone (GnRH) receptor, yet are not fully differentiated in that they do not synthesize the beta-subunits of luteinizing hormone (LH) or follicle-stimulating hormone (FSH). This observation lead to the hypothesis that targeting oncogenesis with promoters that are activated earlier or later in development might immortalize cells that were more primitive or more differentiated, respectively. To test this hypothesis, we used an LHbeta promoter to immortalize a cell that represents a subsequent stage of gonadotrope differentiation (expression of alpha-subunit, GnRH receptor, and LH beta-subunit but not FSH beta-subunit). Conversely, targeting oncogenesis with a longer fragment of the human alpha-subunit gene (which is activated earlier in development) resulted in the immortalization of a progenitor cell that is more primitive, expressing only the alpha-subunit gene. Interestingly, this transgene also immortalized cells of the thyrotrope lineage that express both alpha- and beta-subunits of thyroid-stimulating hormone and the transcription factor GHF-1 (Pit-1). Thus, targeted tumorigenesis immortalizes mammalian cells at specific stages of differentiation and allows the production of a series of cultured cell lines representing sequential stages of differentiation in a given cell lineage.

A steroidogenic factor-1 binding site is required for activity of the luteinizing hormone beta subunit promoter in gonadotropes of transgenic mice

Analysis of luteinizing hormone (LH) beta subunit promoters from a broad range of species including teleosts and humans revealed strict conservation of a sequence homologous to the steroidogenic factor-1 (SF-1) regulatory element of cytochrome P-450 steroid hydroxylase genes. Interaction between SF-1 and this putative response element in the bovine LH beta promoter was confirmed by electrophoretic mobility shift assays. Furthermore, cotransfection of alpha T3-1 cells with an expression vector encoding SF-1 induced binding site-dependent transcription from the bovine LH beta promoter. Physiological significance of the LH beta SF-1 consensus sequence was established using transgenic mice containing either the wild type bovine promoter or a promoter with a site-specific mutation of this site. Mutation of the SF-1 binding site nearly eliminated promoter activity, and the mutant transgene remained inactive following induction of gonadotropin-releasing hormone accomplished by castrating male and female mice. Thus, increases of gonadotropin-releasing hormone within a physiological range did not compensate for the loss of the SF-1 binding site. Together, these findings indicate that the SF-1 binding site is a key regulator of LH beta promoter activity in vivo and implicate SF-1 as at least one of the transcription factors that acts through this site.

Stimulation of luteinizing hormone beta gene promoter activity by the orphan nuclear receptor, steroidogenic factor-1

The orphan nuclear receptor, steroidogenic factor-1 (SF-1), is expressed in the pituitary and in the gonadotrope precursor cell line, alphaT3-1, where it is believed to enhance expression of the common gonadotropin alpha-subunit gene through transactivation of the gonadotrope-specific element (GSE). Sequence analysis of the rat luteinizing hormone beta-subunit (LH beta) gene promoter revealed the presence of a consensus GSE at -127 to -119 (TGACCTTGT). We have demonstrated the ability of SF-1 to bind specifically to this putative GSE sequence by electrophoretic mobility shift assay, utilizing both alphaT3-1 nuclear extracts and in vitro translated SF-1. In addition, mutation of the putative LHbeta-GSE (TGAAATTGT) eliminated specific DNA binding. To examine the ability of SF-1 to enhance LHbeta promoter activity, CV-1 cells, which lack endogenous SF-1, were cotransfected with an SF-1-containing expression vector and an LHbeta-luciferase reporter construct. When cotransfected with -209/+5 of the LHbeta promoter, SF-1 increased luciferase activity by 56-fold. SF-1 responsiveness was markedly diminished with loss of the putative GSE region in deletion constructs and in the presence of a two base pair mutation, analogous to the mutation which eliminated DNA binding. Finally, the LHbeta-GSE was able to confer SF-1 responsiveness on a heterologous minimal growth hormone promoter, GH50 (57-fold). We conclude that SF-1 both binds to and transactivates the rat LHbeta promoter. These data suggest that SF-1 may participate in the expression of the LHbeta gene by the gonadotrope.