An Otx-related homeodomain protein binds an LHbeta promoter element important for activation during gonadotrope maturation

Chromatin status and transcription factor binding to gonadotropin promoters in gonadotrope cell lines

BACKGROUND

Proper expression of key reproductive hormones from gonadotrope cells of the pituitary is required for pubertal onset and reproduction. To further our understanding of the molecular events taking place during embryonic development, leading to expression of the glycoproteins luteinizing hormone (LH) and follicle-stimulating hormone (FSH), we characterized chromatin structure changes, imparted mainly by histone modifications, in model gonadotrope cell lines.

METHODS

We evaluated chromatin status and gene expression profiles by chromatin immunoprecipitation assays, DNase sensitivity assay, and RNA sequencing in three developmentally staged gonadotrope cell lines, αT1-1 (progenitor, expressing Cga), αT3-1 (immature, expressing Cga and Gnrhr), and LβT2 (mature, expressing Cga, Gnrhr, Lhb, and Fshb), to assess changes in chromatin status and transcription factor access of gonadotrope-specific genes.

RESULTS

We found the common mRNA α-subunit of LH and FSH, called Cga, to have an open chromatin conformation in all three cell lines. In contrast, chromatin status of Gnrhr is open only in αT3-1 and LβT2 cells. Lhb begins to open in LβT2 cells and was further opened by activin treatment. Histone H3 modifications associated with active chromatin were high on Gnrhr in αT3-1 and LβT2, and Lhb in LβT2 cells, while H3 modifications associated with repressed chromatin were low on Gnrhr, Lhb, and Fshb in LβT2 cells. Finally, chromatin status correlates with the progressive access of LHX3 to Cga and Gnrhr, followed by PITX1 binding to the Lhb promoter.

CONCLUSION

Our data show the gonadotrope-specific genes Cga, Gnrhr, Lhb, and Fshb are not only controlled by developmental transcription factors, but also by epigenetic mechanisms that include the modulation of chromatin structure, and histone modifications.

Homeodomain Proteins SIX3 and SIX6 Regulate Gonadotrope-specific Genes During Pituitary Development

Sine oculis-related homeobox 3 (SIX3) and SIX6, 2 closely related homeodomain transcription factors, are involved in development of the mammalian neuroendocrine system and mutations of Six6 adversely affect fertility in mice. We show that both small interfering RNA knockdown in gonadotrope cell lines and knockout of Six6 in both embryonic and adult male mice (Six6 knockout) support roles for SIX3 and SIX6 in transcriptional regulation in gonadotrope gene expression and that SIX3 and SIX6 can functionally compensate for each other. Six3 and Six6 expression patterns in gonadotrope cell lines reflect the timing of the expression of pituitary markers they regulate. Six3 is expressed in an immature gonadotrope cell line and represses transcription of the early lineage-specific pituitary genes, GnRH receptor (GnRHR) and the common α-subunit (Cga), whereas Six6 is expressed in a mature gonadotrope cell line and represses the specific β-subunits of LH and FSH (LHb and FSHb) that are expressed later in development. We show that SIX6 repression requires interaction with transducin-like enhancer of split corepressor proteins and competition for DNA-binding sites with the transcriptional activator pituitary homeobox 1. Our studies also suggest that estradiol and circadian rhythm regulate pituitary expression of Six6 and Six3 in adult females but not in males. In summary, SIX3 and SIX6 play distinct but compensatory roles in regulating transcription of gonadotrope-specific genes as gonadotrope cells differentiate.

Constitutively active FOXO1 diminishes activin induction of Fshb transcription in immortalized gonadotropes

In the present study, we investigate whether the FOXO1 transcription factor modulates activin signaling in pituitary gonadotropes. Our studies show that overexpression of constitutively active FOXO1 decreases activin induction of murine Fshb gene expression in immortalized LβT2 cells. We demonstrate that FOXO1 suppression of activin induction maps to the −304/−95 region of the Fshb promoter containing multiple activin response elements and that the suppression requires the FOXO1 DNA-binding domain (DBD). FOXO1 binds weakly to the −125/−91 region of the Fshb promoter in a gel-shift assay. Since this region of the promoter contains a composite SMAD/FOXL2 binding element necessary for activin induction of Fshb transcription, it is possible that FOXO1 DNA binding interferes with SMAD and/or FOXL2 function. In addition, our studies demonstrate that FOXO1 directly interacts with SMAD3/4 but not SMAD2 in a FOXO1 DBD-dependent manner. Moreover, we show that SMAD3/4 induction of Fshb-luc and activin induction of a multimerized SMAD-binding element-luc are suppressed by FOXO1 in a DBD-dependent manner. These results suggest that FOXO1 binding to the proximal Fshb promoter as well as FOXO1 interaction with SMAD3/4 proteins may result in decreased activin induction of Fshb in gonadotropes.

Fox tales: regulation of gonadotropin gene expression by forkhead transcription factors

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are produced by pituitary gonadotrope cells and are required for steroidogenesis, the maturation of ovarian follicles, ovulation, and spermatogenesis. Synthesis of LH and FSH is tightly regulated by a complex network of signaling pathways activated by hormones including gonadotropin-releasing hormone, activin and sex steroids. Members of the forkhead box (FOX) transcription factor family have been shown to act as important regulators of development, homeostasis and reproduction. In this review, we focus on the role of four specific FOX factors (FOXD1, FOXL2, FOXO1 and FOXP3) in gonadotropin hormone production and discuss our current understanding of the molecular function of these factors derived from studies in mouse genetic and cell culture models.

Forkhead box O1 is a repressor of basal and GnRH-induced Fshb transcription in gonadotropes

Synthesis of the gonadotropin β-subunits is tightly controlled by a complex network of hormonal signaling pathways that may be modulated by metabolic cues. Recently, we reported that insulin regulates FOXO1 phosphorylation and cellular localization in pituitary gonadotropes and that FOXO1 overexpression inhibits Lhb transcription. In the current study, we investigated whether FOXO1 modulates Fshb synthesis. Here, we demonstrate that FOXO1 represses basal and GnRH-induced Fshb transcription in LβT2 cells. In addition, we show that PI3K inhibition, which increases FOXO1 nuclear localization, results in decreased Fshb mRNA levels in murine primary pituitary cells. FOXO1 also decreases transcription from the human FSHB promoter, suggesting that FOXO1 regulation of FSHB transcription may be conserved between rodents and humans. Although the FOXO1 DNA-binding domain is necessary for suppression of Fshb, we do not observe direct binding of FOXO1 to the Fshb promoter, suggesting that FOXO1 exerts its effect through protein-protein interactions with transcription factors required for Fshb synthesis. FOXO1 suppression of basal Fshb transcription may involve PITX1 because PITX1 interacts with FOXO1, FOXO1 repression maps to the proximal Fshb promoter containing a PITX1-binding site, PITX1 induction of Fshb or a PITX1 binding element in CV-1 cells is decreased by FOXO1, and FOXO1 suppresses Pitx1 mRNA and protein levels. GnRH induction of an Fshb promoter containing a deletion at -50/-41 or -30/-21 is not repressed by FOXO1, suggesting that these two regions may be involved in FOXO1 suppression of GnRH-induced Fshb synthesis. In summary, our data demonstrate that FOXO1 can negatively regulate Fshb transcription and suggest that FOXO1 may relay metabolic hormonal signals to modulate gonadotropin production.

Msx1 homeodomain protein represses the αGSU and GnRH receptor genes during gonadotrope development

Multiple homeodomain transcription factors are crucial for pituitary organogenesis and cellular differentiation. A homeodomain repressor, Msx1, is expressed from the ventral aspect of the developing anterior pituitary and implicated in gonadotrope differentiation. Here, we find that Msx1 represses transcription of lineage-specific pituitary genes such as the common α-glycoprotein subunit (αGSU) and GnRH receptor (GnRHR) promoters in the mouse gonadotrope-derived cell lines, αT3-1 and LβT2. Repression of the mouse GnRHR promoter by Msx1 is mediated through a consensus-binding motif in the downstream activin regulatory element (DARE). Truncation and mutation analyses of the human αGSU promoter map Msx1 repression to a site at -114, located at the junctional regulatory element (JRE). Dlx activators are closely related to the Msx repressors, acting through the same elements, and Dlx3 and Dlx2 act as transcriptional activators for GnRHR and αGSU, respectively. Small interfering RNA knockdown of Msx1 in αT3-1 cells increases endogenous αGSU and GnRHR mRNA expression. Msx1 gene expression reaches its maximal expression at the rostral edge at e13.5. The subsequent decline in Msx1 expression specifically coincides with the onset of expression of both αGSU and GnRHR. The expression levels of both αGSU and GnRHR in Msx1-null mice at e18.5 are higher compared with wild type, further confirming a role for Msx1 in the repression of αGSU and GnRHR. In summary, Msx1 functions as a negative regulator early in pituitary development by repressing the gonadotrope-specific αGSU and GnRHR genes, but a temporal decline in Msx1 expression alleviates this repression allowing induction of GnRHR and αGSU, thus serving to time the onset of gonadotrope-specific gene program.

FOXO1 transcription factor inhibits luteinizing hormone β gene expression in pituitary gonadotrope cells

Synthesis of luteinizing hormone (LH) is tightly controlled by a complex network of hormonal signaling pathways that can be modulated by metabolic cues, such as insulin. One group of candidate genes that may be regulated by insulin signaling in pituitary gonadotrope cells is the FOXO subfamily of forkhead transcription factors. In this study we investigated whether FOXO1 is expressed in gonadotropes and if it can modulate LH β-subunit (Lhb) gene expression. We demonstrated that FOXO1 is expressed in murine gonadotrope cells and that insulin signaling increased FOXO1 phosphorylation and cytoplasmic localization in a PI3K-dependent manner. We also showed that FOXO1 repressed basal transcription and gonadotropin-releasing hormone (GnRH) induction of both the murine and human LHB genes in LβT2 cells, suggesting that FOXO1 regulation of LHB transcription may be conserved between rodents and humans. Although we did not detect FOXO1 binding to the proximal Lhb promoter, the FOXO1 DNA binding domain was necessary for the suppression, suggesting that FOXO1 exerts its effect through protein-protein interactions with transcription factors/cofactors required for Lhb gene expression. FOXO1 repression mapped to the proximal Lhb promoter containing steroidogenic factor 1 (SF1), pituitary homeobox 1 (PTX1), and early growth response protein 1 (EGR1) binding elements. Additionally, FOXO1 blocked induction of the Lhb promoter with overexpressed SF1, PTX1, and EGR1, indicating that FOXO1 repression occurs via these transcription factors but not through regulation of their promoters. In summary, we demonstrate that FOXO1 phosphorylation and cellular localization is regulated by insulin signaling in gonadotropes and that FOXO1 inhibits Lhb transcription. Our study also suggests that FOXO1 may play an important role in controlling LH levels in response to metabolic cues.

Stress levels of glucocorticoids inhibit LHβ-subunit gene expression in gonadotrope cells

Increased glucocorticoid secretion is a common response to stress and has been implicated as a mediator of reproductive suppression upon the pituitary gland. We utilized complementary in vitro and in vivo approaches in the mouse to investigate the role of glucocorticoids as a stress-induced intermediate capable of gonadotrope suppression. Repeated daily restraint stress lengthened the ovulatory cycle of female mice and acutely reduced GnRH-induced LH secretion and synthesis of LH β-subunit (LHβ) mRNA, coincident with increased circulating glucocorticoids. Administration of a stress level of glucocorticoid, in the absence of stress, blunted LH secretion in ovariectomized female mice, demonstrating direct impairment of reproductive function by glucocorticoids. Supporting a pituitary action, glucocorticoid receptor (GR) is expressed in mouse gonadotropes and treatment with glucocorticoids reduces GnRH-induced LHβ expression in immortalized mouse gonadotrope cells. Analyses revealed that glucocorticoid repression localizes to a region of the LHβ proximal promoter, which contains early growth response factor 1 (Egr1) and steroidogenic factor 1 sites critical for GnRH induction. GR is recruited to this promoter region in the presence of GnRH, but not by dexamethasone alone, confirming the necessity of the GnRH response for GR repression. In lieu of GnRH, Egr1 induction is sufficient for glucocorticoid repression of LHβ expression, which occurs via GR acting in a DNA- and dimerization-independent manner. Collectively, these results expose the gonadotrope as an important neuroendocrine site impaired during stress, by revealing a molecular mechanism involving Egr1 as a critical integrator of complex formation on the LHβ promoter during GnRH induction and GR repression.

Runt-related transcription factors impair activin induction of the follicle-stimulating hormone {beta}-subunit gene

Synthesis of the FSH beta-subunit (FSHbeta) is critical for normal reproduction in mammals, and its expression within the pituitary gonadotrope is tightly regulated by activin. Here we show that Runt-related (RUNX) proteins, transcriptional regulators known to interact with TGFbeta signaling pathways, suppress activin induction of FSHbeta gene expression. Runx2 is expressed within the murine pituitary gland and dramatically represses activin-induced FSHbeta promoter activity, without affecting basal expression in LbetaT2 cells, an immortalized mouse gonadotrope cell line. This repressive effect is specific, because RUNX2 induces LHbeta transcription (with or without activin) and does not interfere with GnRH induction of either gonadotropin beta-subunit gene. Analysis of the murine FSHbeta promoter by transfection and gel shift assays reveals that RUNX2 repression localizes to a Runx-binding element at -159/-153, which is adjacent to a previously recognized region critical for activin induction. Mutation of this -153 activin-response element or, indeed, any of the five activin-responsive regions prevents activin induction and, in fact, RUNX2 suppression, instead converting RUNX2 to an activator of the FSHbeta gene. Although the Runx-binding element is required for RUNX2-mediated repression of FSHbeta induction by either activin or Smad3, confirming a functional role of this novel site, protein interactions in addition to those between RUNX2 and Smads are necessary to account for full repression of activin induction. In summary, the present study provides evidence for Runx2-mediated repression of activin-induced FSHbeta gene expression and reveals the context dependence of Runx2 action in hormonal regulation of the gonadotropin genes.

Hormones in synergy: regulation of the pituitary gonadotropin genes

The precise interplay of hormonal influences that governs gonadotropin hormone production by the pituitary includes endocrine, paracrine and autocrine actions of hypothalamic gonadotropin-releasing hormone (GnRH), activin and steroids. However, most studies of hormonal regulation of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in the pituitary gonadotrope have been limited to analyses of the isolated actions of individual hormones. LHbeta and FSHbeta subunits have distinct patterns of expression during the menstrual/estrous cycle as a result of the integration of activin, GnRH, and steroid hormone action. In this review, we focus on studies that delineate the interplay among these hormones in the regulation of LHbeta and FSHbeta gene expression in gonadotrope cells and discuss how signaling cross-talk contributes to differential expression. We also discuss how recent technological advances will help identify additional factors involved in the differential hormonal regulation of LH and FSH.

Progesterone Inhibits basal and gonadotropin-releasing hormone induction of luteinizing hormone beta-subunit gene expression

LH and FSH play critical roles in mammalian reproduction by mediating steroidogenesis and gametogenesis in the gonad. Gonadal steroid hormone feedback to the hypothalamus and pituitary influences production of the gonadotropins. We previously demonstrated that progesterone differentially regulates the expression of the LH and FSH beta-subunits at the level of the gonadotrope: FSHbeta transcription is induced, whereas LHbeta is repressed. In this study, we investigated the mechanism of progesterone repression of LHbeta gene expression using immortalized gonadotrope-derived LbetaT2 cells. The progesterone suppression of both basal and GnRH-induced LHbeta gene expression occurs in a hormone- and receptor-dependent manner. Chromatin immunoprecipitation demonstrates that the hormone-bound progesterone receptor (PR) is recruited to the endogenous mouse LHbeta promoter. In addition, suppression requires both the amino-terminal and DNA-binding regions of PR. Furthermore, progesterone suppression does not require direct PR binding to the promoter, and, thus, PR is likely recruited to the promoter via indirect binding through other transcription factors. These data demonstrate that the molecular mechanism for progesterone action on the LHbeta promoter is distinct from FSHbeta, which involves direct PR binding to the promoter to produce activation. It also differs from androgen repression of LHbeta gene expression in that, rather than Sp1 or steroidogenic factor-1 elements, it requires elements within -300/-250 and -200/-150 that also contribute to basal expression of the LHbeta promoter. Altogether, our data indicate that progesterone feedback at the level of the pituitary gonadotrope is likely to play a key role in differential production of the gonadotropin genes.

Conservation of mechanisms mediating gonadotrophin-releasing hormone 1 stimulation of human luteinizing hormone beta subunit transcription

Gonadotrophin-releasing hormone (GNRH1) regulates pituitary luteinizing hormone (LH). Previous studies have delineated a mechanism for GNRH1-induced LHbeta subunit gene (Lhb) transcription, the rate-limiting step in LH production. GNRH1 induces expression of early growth response 1 (EGR1), which interacts with steroidogenic factor 1 (SF1) and paired-like homeodomain transcription factor 1 (PITX1) to regulate Lhb promoter activity. Though the cis-elements for these factors are conserved across species, regulation of human LHB transcription has not been thoroughly investigated. We therefore characterized LHB transcriptional regulation by GNRH1 using promoter-reporter analyses in LbetaT2 cells. GNRH1 stimulated LHB transcription via an extracellular signal-regulated kinase 1/2 pathway. EGR1 bound to two binding sites on the LHB promoter and this binding was increased by GNRH1. Mutation of either site or knockdown of endogenous EGR1 decreased basal and/or GNRH1-regulated promoter activity. The human LHB promoter also contains low and high affinity SF1 binding sites. Mutation of these elements or depletion of endogenous SF1 impaired basal and ligand-induced transcription. Knockdown of PITX1 or PITX2 isoforms impaired GNRH1 induction, and endogenous PITX1 bound to the candidate PITX binding site on the LHB promoter. Thus, the mechanism described for GNRH1 regulation of Lhb in other species is largely conserved for human LHB. We also uncover a previously unappreciated role for PITX2 isoforms in this process.

NeuroD1 and Mash1 temporally regulate GnRH receptor gene expression in immortalized mouse gonadotrope cells

Accurate spatial and temporal expression of gonadotrope-specific genes, such as the gonadotropin-releasing hormone receptor (GnRHR) gene, is critical for gonadotrope maturation. Herein, we show that a specific E-box in the mouse GnRHR promoter binds two group A basic-helix-loop-helix (bHLH) transcription factors. Mutation of this E-box decreases expression in mouse gonadotrope-derived alphaT3-1 and LbetaT2 cell lines. Microarray and western blots show that the bHLH transcription factor NeuroD1 is strongly expressed in the gonadotrope progenitor, alphaT3-1, whereas Mash1 is strongly expressed in the more mature gonadotrope, LbetaT2. Over-expression of NeuroD1 or Mash1 increases expression of the GnRHR gene or a multimer of the E-box and this increase is lost upon mutation of the E-box. Electrophoretic mobility shift assays reveal that the GnRHR E-box binds NeuroD1 from alphaT3-1 cells, but binds Mash1 from LbetaT2 cells. The sequential binding of different members of the group A bHLH transcription factor family to mouse GnRHR E-box 3 as the gonadotrope differentiates may represent a mechanism necessary for proper spatial and temporal expression of the GnRHR during gonadotrope development.

Paired-like homeodomain transcription factors 1 and 2 regulate follicle-stimulating hormone beta-subunit transcription through a conserved cis-element

Paired-like homeodomain transcription factors (PITX) regulate the activity of pituitary hormone-encoding genes. Here, we examined mechanisms through which the family of PITX proteins control murine FSH beta-subunit (Fshb) transcription. We observed that endogenous PITX1 and PITX2 isoforms from murine LbetaT2 gonadotrope cells could bind a highly conserved proximal cis-element. Transfection of PITX1 or PITX2C in heterologous cells stimulated both murine and human Fshb/FSHB promoter-reporter activities, and in both cases, mutation of the critical cis-element abrogated these effects. In homologous LbetaT2 cells, the same mutation decreased basal reporter activity and greatly reduced activin A-stimulated transcription from murine and human promoter-reporters. Transfecting dominant-negative forms of PITX1 or PITX2C or knocking down PITX1 or -2 expression by RNA interference in LbetaT2 cells inhibited murine Fshb transcription, confirming roles for endogenous PITX proteins. Both PITX1 and PITX2C interacted with Smad3 (an effector of the activin signaling cascade in these cells) in coprecipitation experiments, and the PITX binding site mutation greatly inhibited Smad2/3/4-stimulated Fshb transcription. In summary, both PITX1 and PITX2C regulate murine and human Fshb/FSHB transcription through a conserved cis-element in the proximal promoter. Furthermore, the data indicate both common and distinct mechanisms of PITX1 and PITX2C action.

Novel forms of Paired-like homeodomain transcription factor 2 (PITX2): generation by alternative translation initiation and mRNA splicing

BACKGROUND

Members of the Paired-like homeodomain transcription factor (PITX) gene family, particularly PITX1 and PITX2, play important roles in normal development and in differentiated cell functions. Three major isoforms of PITX2 were previously reported to be produced through both alternative mRNA splicing (PITX2A and PITX2B) and alternative promoter usage (PITX2C). The proteins derived from these mRNAs contain identical homeodomain and carboxyl termini. Differences in the amino-termini of the proteins may confer functional differences in some contexts.

RESULTS

Here, we report the identification of two novel PITX2 isoforms. First, we demonstrate that the Pitx2c mRNA generates two protein products, PITX2Calpha and PITX2Cbeta, via alternative translation initiation. Second, we identified a novel mRNA splice variant, Pitx2b2, which uses the same 5' splice donor in intron 2 as Pitx2b (hereafter referred to as Pitx2b1), but employs an alternative 3' splice acceptor, leading to an in-frame deletion of 39 base pairs relative to Pitx2b1. Pitx2b2 mRNA is expressed in both murine and human pituitary. The data show that in a murine gonadotrope cell line and adult murine pituitary what was previously thought to be PITX2B1 is actually PITX2Cbeta, or perhaps PITX2B2. PITX2B1 is expressed at lower levels than previously thought. PITX2Cbeta and PITX2B2 activate gonadotrope-specific gene promoter-reporters similarly to known PITX2 isoforms.

CONCLUSION

We have identified and characterized two novel isoforms of PITX2, generated by alternative translation initiation (PITX2Cbeta) and alternative mRNA splicing (PITX2B2). These proteins show similar DNA binding and trans-activation functions as other PITX2 isoforms in vitro, though their conservation across species suggests that they may play distinct, as yet unidentified, roles in vivo.

Molecular physiology of pituitary development: signaling and transcriptional networks

The pituitary gland is a central endocrine organ regulating basic physiological functions, including growth, the stress response, reproduction, metabolic homeostasis, and lactation. Distinct hormone-producing cell types in the anterior pituitary arise from a common ectodermal primordium during development by extrinsic and intrinsic mechanisms, providing a powerful model system for elucidating general principles in mammalian organogenesis. The central purpose of this review is to inspect the integrated signaling and transcriptional events that affect precursor proliferation, cell lineage commitment, terminal differentiation, and physiological regulation by hypothalamic tropic factors.

Activin and glucocorticoids synergistically activate follicle-stimulating hormone beta-subunit gene expression in the immortalized LbetaT2 gonadotrope cell line

FSH is produced by the pituitary gonadotrope to regulate gametogenesis. Production of the beta-subunit of FSH is the rate-limiting step in FSH synthesis, and a number of peptide and steroid hormones within the reproductive axis have been found to regulate transcription of the FSH beta-subunit gene. Although both activin and glucocorticoids are notable regulators of FSHbeta by themselves, we find that cotreatment results in a synergistic interaction on the mouse FSHbeta promoter at the level of the gonadotrope using transient transfection of a reporter gene into the LbetaT2 immortalized gonadotrope-derived cell line. This synergistic interaction is specific to FSHbeta, because only additive effects of these two hormones are observed on LH beta-subunit, GnRH receptor, and mouse mammary tumor virus gene expression. Components of both activin and glucocorticoid signaling are found to be necessary for synergy, and there are specific cis elements on the mouse FSHbeta promoter that contribute to the synergistic response as well. We also identify novel activin-responsive regions in the mouse FSHbeta promoter and find that the -120 site can bind Smad2/3 in vitro. In addition, the glucocorticoid receptor and Smad3 are sufficient to confer a striking synergy with glucocorticoids on the mouse FSHbeta promoter. Our studies provide the first evidence of a synergistic interaction between activin and glucocorticoids within the gonadotrope cell and demonstrate that this synergy can occur directly at the level of the mouse FSHbeta promoter.

Androgens, progestins, and glucocorticoids induce follicle-stimulating hormone beta-subunit gene expression at the level of the gonadotrope

FSH is produced by the pituitary gonadotrope to regulate gametogenesis. Steroid hormones, including androgens, progestins, and glucocorticoids, have all been shown to stimulate expression of the FSHbeta subunit in primary pituitary cells and rodent models. Understanding the molecular mechanisms of steroid induction of FSHbeta has been difficult due to the heterogeneity of the anterior pituitary. Immortalized LbetaT2 cells are a model of a mature gonadotrope cell and express the endogenous steroid receptor for each of the three hormones. Transient transfection of each receptor, along with ligand treatment, stimulates the mouse FSHbeta promoter, but induction is severely diminished using receptors that lack the ability to bind DNA, indicating that induction is likely through direct DNA binding. All three steroid hormones act within the first 500 bp of the FSHbeta promoter where six putative hormone response elements exist. The -381 site is critical for FSHbeta induction by all three steroid hormones, whereas the -197 and -139 sites contribute to maximal induction. Interestingly, the -273 and -230 sites are also necessary for androgen and progestin induction of FSHbeta, but not for glucocorticoid induction. Additionally, we find that all three receptors bind the endogenous FSHbeta promoter, in vivo, and specifically bind the -381 site in vitro, suggesting that the binding of the receptors to this element is critical for the induction of FSHbeta by these 3-keto steroid hormones. Our data indicate that androgens, glucocorticoids, and progestins act via their receptors to directly activate FSHbeta gene expression in the pituitary gonadotrope.

Activin regulates luteinizing hormone beta-subunit gene expression through Smad-binding and homeobox elements

LH beta-subunit (LHbeta), which is essential for ovulation and reproductive fitness, is synthesized specifically in pituitary gonadotropes. In this study, we show that LHbeta gene expression is induced by activin in mouse primary pituitary cells if the cells are treated within 24 h after dispersion in culture. Furthermore, male mice deficient in Smad3, and therefore in activin signaling, have lower expression of both LHbeta and FSHbeta mRNAs compared with their wild-type littermates. Using the LbetaT2 immortalized mouse gonadotrope cell line that endogenously expresses LH, we identify specific elements in the regulatory region of the rat LHbeta gene necessary for its induction by activin. Activin responsiveness is conferred by a promoter-proximal region located -121/-86 from the transcriptional start site. Maximal LHbeta induction by activin requires a homeobox element (HB) and a 5'-early growth response (Egr) site found in this region of the promoter. Juxtaposed to the HB are three Smad-binding elements (SBEs), which are essential for LHbeta induction. Interestingly, two of the SBEs are also critical for basal expression of the LHbeta gene. We demonstrate that Smad proteins are necessary and sufficient for activin induction of the LHbeta gene. Furthermore, Smad proteins can bind one of the identified SBEs. In addition to binding this SBE, Smad proteins interact with pituitary homeobox 1 (Ptx-1) and orthodenticle homeobox 1 (Otx-1), which can bind the HB located close to the Smad-binding site. Thus, activin induction of LHbeta gene expression requires a combination of several transcription factors, both basal and activin induced, as well as cooperation between multiple DNA elements.

Mouse GnRH receptor gene expression is mediated by the LHX3 homeodomain protein

Appropriate expression of GnRH receptor (GnRHR) is necessary for the correct regulation of the gonadotropins, LH and FSH, by GnRH. GnRHR is primarily expressed in the gonadotrope cells of the anterior pituitary, and a number of regulatory elements important for both basal and hormonal regulation of the gene have been identified. Using the gonadotrope-derived cell line, alpha T3-1, that endogenously expresses GnRHR, we have identified an ATTA element located at -298 relative to the transcriptional start site that is essential for basal expression of the GnRHR gene. LHX3, a member of the LIM homeodomain family, binds the -298 ATTA site in vitro as well as to the endogenous GnRHR promoter in vivo. Additionally, LHX3 specifically activates through this -298 ATTA site in transient transfection assays. LHX3 is essential for pituitary development and has been implicated in the regulation of a number of pituitary specific genes; however, this is the first report identifying its role in the regulation of GnRHR.

Essential role of the homeodomain for pituitary homeobox 1 activation of mouse gonadotropin-releasing hormone receptor gene expression through interactions with c-Jun and DNA

The gonadotropin-releasing hormone receptor (GnRHR) is expressed primarily in the gonadotropes of the anterior pituitary. Pituitary homeobox 1 (Pitx-1) has been shown to activate pituitary-specific gene expression by direct DNA binding and/or protein-protein interaction with other transcription factors. We hypothesized that Pitx-1 might also dictate tissue-specific expression of the mouse GnRHR (mGnRHR) gene in a similar manner. Pitx-1 activated the mGnRHR gene promoter, and transactivation was localized to sequences between -308 and -264. Pitx-1 bound to this region only with low affinity. This region includes an activating protein 1 (AP-1) site, which was previously shown to be important for mGnRHR gene expression. Further characterization indicated that an intact AP-1 site was required for full Pitx-1 responsiveness. Furthermore, Pitx-1 and AP-1 were synergistic in the activation of the mGnRHR gene promoter. A Pitx-1 homeodomain (HD) point mutation, which eliminated DNA binding ability, caused only a partial reduction of transactivation, whereas deletion of the HD completely prevented transactivation. Pitx-1 interacted directly with c-Jun, and the HD was sufficient for this interaction. While the point mutation in the Pitx-1 HD did not affect interaction with c-Jun, deletion of the HD eliminated the interaction. Taken together, our studies indicate that Pitx-1 can direct transactivation of the mGnRHR gene, in part by DNA binding and in part by an action of Pitx-1 as a cofactor for AP-1, augmenting AP-1 activity through a novel protein-protein interaction between c-Jun and the HD of Pitx-1.

Activin regulation of the follicle-stimulating hormone beta-subunit gene involves Smads and the TALE homeodomain proteins Pbx1 and Prep1

FSH is critical for normal reproductive function in both males and females. Activin, a member of the TGFbeta family of growth factors, is an important regulator of FSH expression, but little is known about the molecular mechanisms through which it acts. We used transient transfections into the immortalized gonadotrope cell line LbetaT2 to identify three regions (at -973/-962, -167, and -134) of the ovine FSH beta-subunit gene that are required for full activin response. All three regions contain homology to consensus binding sites for Smad proteins, the intracellular mediators of TGFbeta family signaling. Mutation of the distal site reduces activin responsiveness, whereas mutation of either proximal site profoundly disrupts activin regulation of the FSHbeta gene. These sites specifically bind LbetaT2 nuclear proteins in EMSAs, and the -973/-962 site binds Smad4 protein. Interestingly, the protein complex binding to the -134 site contains Smad4 in association with the homeodomain proteins Pbx1 and Prep1. Using glutathione S-transferase interaction assays, we demonstrate that Pbx1 and Prep1 interact with Smads 2 and 3 as well. The two proximal activin response elements are well conserved across species, and Pbx1 and Prep1 proteins bind to the mouse gene in vivo. Furthermore, mutation of either proximal site abrogates activin responsiveness of a mouse FSHbeta reporter gene as well, confirming their functional conservation. Our studies provide a basis for understanding activin regulation of FSHbeta gene expression and identify Pbx1 and Prep1 as Smad partners and novel mediators of activin action.

Androgen regulates follicle-stimulating hormone beta gene expression in an activin-dependent manner in immortalized gonadotropes

Little is known about the molecular mechanisms of androgen regulation of the FSHbeta gene; however, studies suggest that it consists of a complex feedback loop that involves multiple mechanisms acting at both the level of the hypothalamus and the pituitary. In the present study, we address androgen regulation of the FSHbeta gene in immortalized gonadotrope cells and investigate the roles of activin and GnRH in androgen action. Using transient transfection assays in the FSHbeta-expressing mouse gonadotrope cell line, LbetaT2, we demonstrate that androgens stimulate expression of an ovine FSHbeta reporter gene in a dose-dependent manner. Mutation of either of two conserved androgen response elements at -245/-231 and -153/-139 within the proximal region of the ovine FSHbeta gene promoter abolishes this stimulation, and androgen receptor binds directly to the -244 ARE in vitro. Androgen induction of the FSHbeta reporter gene is also dependent upon the activin autocrine loop present in the LbetaT2 cells, as well as an activin-response element at -138/-124 of the FSHbeta gene. However, activin regulation of other genes remains unaffected by androgens. In addition, androgens stimulate expression of a mouse GnRH receptor reporter gene, and thus may indirectly augment the response of the FSHbeta gene to GnRH. Taken together, these data demonstrate that, in mouse gonadotropes, androgens act directly on the ovine FSHbeta gene to stimulate expression by a mechanism that is dependent upon activin, as well as acting indirectly, potentially through a second mechanism that may be dependent upon induction of GnRH receptor.

Nuclear factor Y and steroidogenic factor 1 physically and functionally interact to contribute to cell-specific expression of the mouse Follicle-stimulating hormone-beta gene

FSH is a heterodimeric glycoprotein hormone secreted from the gonadotrope cell population of the anterior pituitary. Despite its crucial role in mammalian reproduction, very little is known about regulation of the FSH beta-subunit gene at the molecular level. In this report, we examine the basis for cell-specific expression of FSH beta using the mouse L beta T2 and alpha T3-1 gonadotrope-derived cell lines. Characterization of the hormonal content of L beta T2 and alpha T3-1 cells at the protein level classifies these cells as relatively mature and immature gonadotropes, respectively. We studied L beta T2 cell-specific expression of FSH beta using 398 bp of the mouse FSH beta regulatory region linked to a luciferase reporter gene in transient transfection assays. This mouse FSH beta promoter can direct reporter gene expression specifically to L beta T2 cells when compared with other pituitary- and non-pituitary-derived cell lines, including alpha T3-1 cells. Furthermore, it is induced by activin, and interruption of the autocrine activin loop in L beta T2 cells by the addition of follistatin reduces its expression. Truncation analysis indicates that several regions of the promoter are involved in this specificity and that these can be dissociated from activin regulation. We identify binding sites for the orphan nuclear receptor steroidogenic factor-1 and the heterotrimeric transcription factor nuclear factor Y and show that these elements functionally interact to regulate FSH beta gene expression in an L beta T2 cell-specific manner. Moreover, steroidogenic factor-1 and nuclear factor Y are shown to physically interact with each other. This study is the first to demonstrate the presence of basal FSH beta protein in L beta T2 cells and to identify specific elements within the FSH beta promoter that contribute to basal and cell-specific expression of the gene.