Differential gonadotropin-releasing hormone stimulation of rat luteinizing hormone subunit gene transcription by calcium influx and mitogen-activated protein kinase-signaling pathways

Deletion of Gαq/11 or Gαs Proteins in Gonadotropes Differentially Affects Gonadotropin Production and Secretion in Mice

Gonadotropin-releasing hormone (GnRH) regulates gonadal function via its stimulatory effects on gonadotropin production by pituitary gonadotrope cells. GnRH is released from the hypothalamus in pulses and GnRH pulse frequency differentially regulates follicle-stimulating hormone (FSH) and luteinizing hormone (LH) synthesis and secretion. The GnRH receptor (GnRHR) is a G protein-coupled receptor that canonically activates Gα q/11-dependent signaling on ligand binding. However, the receptor can also couple to Gα s and in vitro data suggest that toggling between different G proteins may contribute to GnRH pulse frequency decoding. For example, as we show here, knockdown of Gα s impairs GnRH-stimulated FSH synthesis at low- but not high-pulse frequency in a model gonadotrope-derived cell line. We next used a Cre-lox conditional knockout approach to interrogate the relative roles of Gα q/11 and Gα s proteins in gonadotrope function in mice. Gonadotrope-specific Gα q/11 knockouts exhibit hypogonadotropic hypogonadism and infertility, akin to the phenotypes seen in GnRH- or GnRHR-deficient mice. In contrast, under standard conditions, gonadotrope-specific Gα s knockouts produce gonadotropins at normal levels and are fertile. However, the LH surge amplitude is blunted in Gα s knockout females and postgonadectomy increases in FSH and LH are reduced both in males and females. These data suggest that GnRH may signal principally via Gα q/11 to stimulate gonadotropin production, but that Gα s plays important roles in gonadotrope function in vivo when GnRH secretion is enhanced.

The role of mitogen-activated protein kinase-extracellular receptor kinase pathway in female fertility outcomes: a focus on pituitary gonadotropins regulation

Mammalian reproduction systems are largely regulated by the secretion of two gonadotropins, that is, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The main action of LH and FSH on the ovary is to stimulate secretion of estradiol and progesterone, which play an important role in the ovarian function and reproductive cycle control. FSH and LH secretions are strictly controlled by the gonadotropin-releasing hormone (GnRH), which is secreted from the hypothalamus into the pituitary vascular system. Maintaining normal secretion of LH and FSH is dependent on pulsatile secretion of GnRH. Extracellular signal-regulated kinase (ERK) proteins, as the main components of mitogen-activated protein kinase (MAPK) signaling pathways, are involved in the primary regulation of GnRH-stimulated transcription of the gonadotropins' α subunit in the pituitary cells. However, GnRH-stimulated expression of the β subunit has not yet been reported. Furthermore, GnRH-mediated stimulation of ERK1 and ERK2 leads to several important events such as cell proliferation and differentiation. In this review, we briefly introduce the relationship between ERK signaling and gonadotropin secretion, and its importance in female infertility.

Gonadotropin regulation by pulsatile GnRH: Signaling and gene expression

The precise orchestration of hormonal regulation at all levels of the hypothalamic-pituitary-gonadal axis is essential for normal reproductive function and fertility. The pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by pituitary gonadotropes. GnRH acts by binding to its high affinity seven-transmembrane receptor (GnRHR) on the cell surface of anterior pituitary gonadotropes. Different signaling cascades and transcriptional mechanisms are activated, depending on the variation in GnRH pulse frequency, to stimulate the synthesis and release of FSH and LH. While changes in GnRH pulse frequency may explain some of the differential regulation of FSH and LH, other factors, such as activin, inhibin and sex steroids, also contribute to gonadotropin production. In this review, we focus on the transcriptional regulation of the gonadotropin subunit genes and the signaling pathways activated by pulsatile GnRH.

Differential roles of PKC isoforms (PKCs) in GnRH stimulation of MAPK phosphorylation in gonadotrope derived cells

The role of protein kinase C (PKC) isoforms (PKCs) in GnRH-stimulated MAPK [ERK1/2, JNK1/2 and p38) phosphorylation was examined in gonadotrope derived cells. GnRH induced a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2 and p38MAPK. Gonadotropes express conventional PKCα and PKCβII, novel PKCδ, PKCε and PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein (GFP)-PKCs constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs) has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in ERK1/2, JNK1/2 and p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in MAPKs phosphorylation may be explained by persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane. Thus, we have identified the PKCs involved in GnRH stimulated MAPKs phosphorylation in gonadotrope derived cells. Once activated, the MAPKs will mediate the transcription of the gonadotropin subunits and GnRH receptor genes.

Differential roles of PKC isoforms (PKCs) and Ca2+ in GnRH and phorbol 12-myristate 13-acetate (PMA) stimulation of p38MAPK phosphorylation in immortalized gonadotrope cells

We examined the role of PKCs and Ca²⁺ in GnRH-stimulated p38MAPK phosphorylation in the gonadotrope derived αT3-1 and LβT2 cell lines. GnRH induced a slow and rapid increase in p38MAPK phosphorylation in αT3-1 and LβT2 cells respectively, while PMA gave a slow response. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs), has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in p38MAPK phosphorylation may be explained by differential localization of the PKCs. Basal, GnRH- and PMA- stimulation of p38MAPK phosphorylation in αT3-1 cells is mediated by Ca²⁺ influx via voltage-gated Ca²⁺ channels and Ca²⁺ mobilization, while in the differentiated LβT2 gonadotrope cells it is mediated only by Ca²⁺ mobilization. p38MAPK resides in the cell membrane and is relocated to the nucleus by GnRH (∼5 min). Thus, we have identified the PKCs and the Ca²⁺ pools involved in GnRH stimulated p38MAPK phosphorylation.

Functional Role of Gonadotrope Plasticity and Network Organization

Gonadotrope cells of the anterior pituitary are characterized by their ability to mount a cyclical pattern of gonadotropin secretion to regulate gonadal function and fertility. Recent in vitro and in vivo evidence suggests that gonadotropes exhibit dramatic remodeling of the actin cytoskeleton following gonadotropin-releasing hormone (GnRH) exposure. GnRH engagement of actin is critical for gonadotrope function on multiple levels. First, GnRH-induced cell movements lead to spatial repositioning of the in vivo gonadotrope network toward vascular endothelium, presumably to access the bloodstream for effective hormone release. Interestingly, these plasticity changes can be modified depending on the physiological status of the organism. Additionally, GnRH-induced actin assembly appears to be fundamental to gonadotrope signaling at the level of extracellular signal-regulated kinase (ERK) activation, which is a well-known regulator of luteinizing hormone (LH) β-subunit synthesis. Last, GnRH-induced cell membrane projections are capable of concentrating LHβ-containing vesicles and disruption of the actin cytoskeleton reduces LH secretion. Taken together, gonadotrope network positioning and LH synthesis and secretion are linked to GnRH engagement of the actin cytoskeleton. In this review, we will cover the dynamics and organization of the in vivo gonadotrope cell network and the mechanisms of GnRH-induced actin-remodeling events important in ERK activation and subsequently hormone secretion.

GnRH Induces ERK-Dependent Bleb Formation in Gonadotrope Cells, Involving Recruitment of Members of a GnRH Receptor-Associated Signalosome to the Blebs

We have previously described a signaling complex (signalosome) associated with the GnRH receptor (GnRHR). We now report that GnRH induces bleb formation in the gonadotrope-derived LβT2 cells. The blebs appear within ~2 min at a turnover rate of ~2-3 blebs/min and last for at least 90 min. Formation of the blebs requires active ERK1/2 and RhoA-ROCK but not active c-Src. Although the following ligands stimulate ERK1/2 in LβT2 cells: EGF > GnRH > PMA > cyclic adenosine monophosphate (cAMP), they produced little or no effect on bleb formation as compared to the robust effect of GnRH (GnRH > PMA > cAMP > EGF), indicating that ERK1/2 is required but not sufficient for bleb formation possibly due to compartmentalization. Members of the above mentioned signalosome are recruited to the blebs, some during bleb formation (GnRHR, c-Src, ERK1/2, focal adhesion kinase, paxillin, and tubulin), and some during bleb retraction (vinculin), while F-actin decorates the blebs during retraction. Fluorescence intensity measurements for the above proteins across the cells showed higher intensity in the blebs vs. intracellular area. Moreover, GnRH induces blebs in primary cultures of rat pituitary cells and isolated mouse gonadotropes in an ERK1/2-dependent manner. The novel signalosome-bleb pathway suggests that as with the signalosome, the blebs are apparently involved in cell migration. Hence, we have extended the potential candidates which are involved in the blebs life cycle in general and for the GnRHR in particular.

Role of pituitary adenylate cyclase-activating polypeptide in modulating hypothalamus-pituitary neuroendocrine functions in mouse cell models

Pituitary adenylate cyclase-activating polypeptide (PACAP) was originally identified as a hypothalamic activator of cyclic adenosine monophosphate production in pituitary cells. PACAP and its receptor are expressed not only in the central nervous system, but also in peripheral organs, and function to stimulate pituitary hormone synthesis and secretion as both a hypothalamic-pituitary-releasing factor and an autocrine-paracrine factor within the pituitary. PACAP stimulates the expression of the gonadotrophin α, luteinising hormone (LH) β and follicle-stimulating hormone (FSH) β subunits, as well as the gonadotrophin-releasing hormone (GnRH) receptor and its own PACAP type I receptor (PAC1R) in gonadotrophin-secreting pituitary cells. In turn, GnRH, which is known to be a crucial component of gonadotrophin secretion, stimulates the expression of PACAP and PAC1R in gonadotrophs. In addition, PAC1R and PACAP modulate the functions of GnRH-producing neurones in the hypothalamus. This review summarises the current understanding of the possible roles of PACAP and PAC1R in modulating hypothalamus and pituitary neuroendocrine cells in the mouse models.

Molecular mechanisms of gonadotropin-releasing hormone signaling: integrating cyclic nucleotides into the network

Gonadotropin-releasing hormone (GnRH) is the primary regulator of mammalian reproductive function in both males and females. It acts via G-protein coupled receptors on gonadotropes to stimulate synthesis and secretion of the gonadotropin hormones luteinizing hormone and follicle-stimulating hormone. These receptors couple primarily via G-proteins of the Gq/ll family, driving activation of phospholipases C and mediating GnRH effects on gonadotropin synthesis and secretion. There is also good evidence that GnRH causes activation of other heterotrimeric G-proteins (Gs and Gi) with consequent effects on cyclic AMP production, as well as for effects on the soluble and particulate guanylyl cyclases that generate cGMP. Here we provide an overview of these pathways. We emphasize mechanisms underpinning pulsatile hormone signaling and the possible interplay of GnRH and autocrine or paracrine regulatory mechanisms in control of cyclic nucleotide signaling.

GnRH pulse frequency-dependent stimulation of FSHβ transcription is mediated via activation of PKA and CREB

Expression of pituitary FSH and LH, under the control of pulsatile GnRH, is essential for fertility. cAMP response element-binding protein (CREB) has been implicated in the regulation of FSHβ gene expression, but the molecular mechanisms by which pulsatile GnRH regulates CREB activation remain poorly understood. We hypothesized that CREB is activated by a distinct signaling pathway in response to pulsatile GnRH in a frequency-dependent manner to dictate the FSHβ transcriptional response. GnRH stimulation of CREB phosphorylation (pCREB) in the gonadotrope-derived LβT2 cell line was attenuated by a protein kinase A (PKA) inhibitor, H89. A dominant negative PKA (DNPKA) reduced GnRH-stimulated pCREB and markedly decreased GnRH stimulation of FSHβ mRNA and FSHβLUC activity, but had little effect on LHβLUC activity, indicating relative specificity of this pathway. In perifusion studies, FSHβ mRNA levels and FSHβLUC activities were increased by pulsatile GnRH, with significantly greater increases at low compared with high pulse frequencies. DNPKA markedly reduced these GnRH-stimulated FSHβ responses at both low and high pulse frequencies. Correlating with FSHβ activation, both PKA activity and levels of pCREB were increased to a greater extent by low compared with high GnRH pulse frequencies, and the induction of pCREB was also attenuated by overexpression of DNPKA at both low and high pulse frequencies. Taken together, these data indicate that a PKA-mediated signaling pathway mediates GnRH activation of CREB at low-pulse frequencies, playing a significant role in the decoding of the hypothalamic GnRH signal to result in frequency-dependent FSHβ activation.

GnRH regulation of Jun and Atf3 requires calcium, calcineurin, and NFAT

GnRH binds to its receptor on gonadotropes and activates multiple members of the MAPK signaling family that in turn regulates the expression of several immediate early genes (IEGs) including Jun, Fos, Atf3, and Egr1. These IEGs confer hormonal responsiveness to gonadotrope-specific genes including Gnrhr, Cga, Fshb, and Lhb. In this study we tested the hypothesis that GnRH specifically regulates the accumulation of Jun and Atf3 mRNA through a pathway that includes intracellular Ca²⁺, calcineurin, and nuclear factor of activated T cells (NFAT). Our results indicate that pretreatment of murine LβT2 cells with 1, 2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)-ester, a Ca²⁺ chelator, reduced the expression of all the IEGs to varying degrees, whereas treatment with thapsigargin, an intracellular Ca²⁺ protein pump inhibitor, increased the expression of the IEG. Furthermore, cyclosporin A, a calcineurin-specific inhibitor, reduced the ability of GnRH to regulate accumulation of Jun and Atf3 mRNA and to a lesser extent Fos. In contrast, Egr1 mRNA was unaffected. NFATs are transcription factors regulated by calcineurin and were detected in LβT2 cells. GnRH increased luciferase activity of an NFAT-dependent promoter reporter that was dependent on intracellular Ca²⁺ and calcineurin activity. Additionally, although small interfering RNA specific for Nfat4 only marginally reduced GnRH regulation of Jun, Fos, and Atf3 mRNA accumulation, activity of an activator protein-1-responsive reporter construct was reduced by 48%. Together these data suggest that calcineurin and NFAT are new members of the gonadotrope transcriptional network that confer hormonal responsiveness to several key genes required for gonadotropin synthesis and secretion.

Extracellular Signal-Regulated Kinase (ERK) Activation and Mitogen-Activated Protein Kinase Phosphatase 1 Induction by Pulsatile Gonadotropin-Releasing Hormone in Pituitary Gonadotrophs

The frequency of gonadotropin-releasing hormone (GnRH) pulse secreted from the hypothalamus differently regulates the expressions of gonadotropin subunit genes, luteinizing hormone β (LHβ) and follicle-stimulating hormone β (FSHβ), in the pituitary gonadotrophs. FSHβ is preferentially stimulated at slower GnRH pulse frequencies, whereas LHβ is preferentially stimulated at more rapid pulse frequencies. Several signaling pathways are activated, including mitogen-activated protein kinase (MAPK), protein kinase C, calcium influx, and calcium-calmodulin kinases, and these may be preferentially regulated under certain conditions. Previous studies demonstrated that MAPK pathways, especially the extracellular signal-regulated kinase (ERK), play an essential role for induction of gonadotropin subunit gene expression by GnRH, whereas, MAPK phosphatases (MKPs) inactivate MAPKs through dephosphorylation of threonine and/or tyrosine residues. MKPs are also induced by GnRH, and potential feedback regulation between MAPK signaling and MKPs within the GnRH signaling pathway is evident in gonadotrophs. In this paper, we reviewed and mainly focused on our observations of the pattern of ERK activation and the induction of MKP by different frequencies of GnRH stimulation.

Gonadotropin-releasing hormone/gonadotropin-releasing hormone receptor signaling in the placenta

PURPOSE OF REVIEW

This review summarizes our current understanding of the role of gonadotropin-releasing hormone (GnRH)/GnRH receptor (GnRHR) signaling at the maternal-fetal interface.

RECENT FINDINGS

Several isoforms of GnRH and GnRHR are described. The hypothalamic decapeptide, GnRH-I, binds to the anterior pituitary and induces the synthesis and secretion of luteinizing hormone and follicle-stimulating hormone. It is also found in extrahypothalamic sites. A second isoform, GnRH-II, acts both in the hypothalamus and other organ systems, including placenta, breast, endometrium, and ovary. Although several putative isoforms of GnRHR have been identified, it is clear that, in humans, both GnRH-I and GnRH-II signal through a single receptor, GnRHR-I. GnRH-I, GnRH-II, and GnRHR-I mRNA and protein have been identified in placenta and regulate the β-subunit of human chorionic gonadotropin production, which is essential for the maintenance of early pregnancy. They may also play a role in the autocrine/paracrine regulation of trophoblast invasion through extracellular matrix remodeling.

SUMMARY

GnRH-I and GnRH-II have multiple extrapituitary roles. In placenta, they bind to GnRHR-I to stimulate the production of β-subunit of human chorionic gonadotropin. They may also play a role in trophoblast invasion. A better understanding of the molecular mechanisms involved in GnRH/GnRHR signaling at the maternal-fetal interface may identify novel roles for GnRH agonists/antagonists in the prevention or treatment of hormonally mediated diseases.

Secretoneurin stimulates the production and release of luteinizing hormone in mouse L{beta}T2 gonadotropin cells

Secretoneurin (SN) is a functional secretogranin II (SgII)-derived peptide that stimulates luteinizing hormone (LH) production and its release in the goldfish. However, the effects of SN on the pituitary of mammalian species and the underlying mechanisms remain poorly understood. To study SN in mammals, we adopted the mouse LβT2 gonadotropin cell line that has characteristics consistent with normal pituitary gonadotrophs. Using radioimmunoassay and real-time RT-PCR, we demonstrated that static treatment with SN induced a significant increment of LH release and production in LβT2 cells in vitro. We found that GnRH increased cellular SgII mRNA level and total SN-immunoreactive protein release into the culture medium. We also report that SN activated the extracellular signal-regulated kinases (ERK) in either 10-min acute stimulation or 3-h chronic treatment. The SN-induced ERK activation was significantly blocked by pharmacological inhibition of MAPK kinase (MEK) with PD-98059 and protein kinase C (PKC) with bisindolylmaleimide. SN also increased the total cyclic adenosine monophosphate (cAMP) levels similarly to GnRH. However, SN did not activate the GnRH receptor. These data indicate that SN activates the protein kinase A (PKA) and cAMP-induced ERK signaling pathways in the LH-secreting mouse LβT2 pituitary cell line.

Induction of dual-specificity phosphatase 1 (DUSP1) by pulsatile gonadotropin-releasing hormone stimulation: role for gonadotropin subunit expression in mouse pituitary LbetaT2 cells

In pituitary gonadotrophs, GnRH induces expression of the mitogen-activated protein kinases (MAPK3/1) dephosphorylating enzyme, dual-specificity phosphatase 1 (DUSP1). Here we examined DUSP1 expression levels following pulsatile GnRH stimulation of the LbetaT2 gonadotroph cells. DUSP1 expression was increased more prominently following high-frequency (every 30 min) GnRH pulse stimulation (7.02- ± 1.47-fold) than low-frequency (every 120 min) GnRH pulses (2.68- ± 0.09-fold). With high-frequency GnRH pulses, DUSP1 expression increased by 2.89- ± 0.32-fold 2 h after GnRH pulse initiation (four 5-min pulses). DUSP1 expression was not induced following lower frequency GnRH pulses, even when the GnRH concentration was increased. Under high-frequency conditions, MAPK3/1 phosphorylation was observed 10 min after the GnRH pulse and decreased to basal levels after 25 min. However, MAPK3/1 dephosphorylation did not occur concurrently with DUSP1 expression. Overexpression of MAP3K1, a kinase upstream of MAPK3/1, increased both the Lhb and the Fshb subunit promoter activities, which could be completely inhibited by cotransfection with DUSP1-expressing vectors. Serum response factor (Srf) promoter activities induced by MAP3K1 were also prevented by DUSP1 overexpression, confirming that MAPK3/1 has an important role in gonadotropin subunit gene expression. Both high- and low-frequency GnRH pulse stimulation failed to increase the Lhb and Fshb subunit gonadotropin gene expression levels upon DUSP1 overexpression. Our study demonstrates that DUSP1 is specifically expressed following high-frequency GnRH pulses and that this effect may participate in the differential regulation of gonadotropin subunit expression in association with MAPK3/1 phosphorylation.

Gonadotropin-releasing hormone-I-mediated activation of progesterone receptor contributes to gonadotropin alpha-subunit expression in mouse gonadotrophs

In pituitary cells, cross talk between GnRH-I and the progesterone receptor accentuates gonadotropin production. We show that GnRH-I activates a progesterone response element (PRE)-driven luciferase reporter gene at 8 h and gonadotropin alpha-subunit (gsu alpha) gene expression at 24 h in two mouse gonadotrope cell lines, alpha T3-1 and L beta T2. In alpha T3-1 cells, progesterone had an additive effect on GnRH-I-induced PRE-luciferase reporter gene activity but not on GSU alpha mRNA levels. However, progesterone had no synergistic effect on the GnRH-I-induced expression of these genes in L beta T2 cells. Up-regulation of the PRE-luciferase reporter gene by GnRH-I was attenuated by pretreatment with protein kinase A (H89) and protein kinase C (GF109203X) inhibitors in both cell lines, whereas only GF109203X inhibited GnRH-I-induced GSU alpha mRNA levels. Most important, in both cell lines within the same time frame, knockdown of progesterone receptor levels by small interfering RNA reduced GnRH-I activation of GSU alpha mRNA levels by approximately 40%. We conclude that the effect of GnRH-I on gsu alpha expression in both alpha T3-1 and L beta T2 cells is mediated by ligand-independent activation of progesterone receptor and that this contributes to the self-priming effect of GnRH-I in the pituitary.

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.

Regulation of Lhb and Egr1 gene expression by GNRH pulses in rat pituitaries is both c-Jun N-terminal kinase (JNK)- and extracellular signal-regulated kinase (ERK)-dependent

Pulsatile GNRH regulates the gonadotropin subunit genes in a differential manner, with faster frequencies favoring Lhb gene expression and slower frequencies favoring Fshb. Early growth response 1 (EGR1) is critical for Lhb gene transcription. We examined GNRH regulation of EGR1 and its two corepressors, Ngfi-A-binding proteins 1 and 2 (NAB1 and NAB2), both in vivo and in cultured rat pituitary cells. In rats, fast GNRH pulses (every 30 min) stably induced Egr1 primary transcript (PT) and mRNA 2-fold (P < 0.05) for 1-24 h. In contrast, slow GNRH pulses (every 240 min) increased Egr1 PT at 24 h (6-fold; P < 0.05) but increased Egr1 mRNA 4- to 5-fold between 4 and 24 h. Both GNRH pulse frequencies increased EGR1 protein 3- to 4-fold. In cultured rat pituitary cells, GNRH pulses (every 60 min) increased Egr1 (PT, 2.5- to 3-fold; mRNA, 1.5- to 2-fold; P < 0.05). GNRH pulses had little effect on Nab1/2 PT/mRNAs either in vivo or in vitro. We also examined specific intracellular signaling cascades activated by GNRH. Inhibitors of mitogen-activated protein kinase 8/9 (MAPK8/9 [also known as JNK]; SP600125) and MAP Kinase Kinase 1 (MAP2K1 [also known as MEK1]; PD98059) either blunted or totally suppressed the GNRH induction of Lhb PT and Egr1 PT/mRNA, whereas the MAPK14 (also known as p38) inhibitor SB203580 did not. In summary, pulsatile GNRH stimulates Egr1 gene expression and protein in vivo but not in a frequency-dependent manner. Additionally, GNRH-induced Egr1 gene expression is mediated by MAPK8/9 and MAPK1/3, and both are critical for Lhb gene transcription.

Induction of dual specificity phosphatase 1 (DUSP1) by gonadotropin-releasing hormone (GnRH) and the role for gonadotropin subunit gene expression in mouse pituitary gonadotroph L beta T2 cells

We examined the expression of dual specificity phosphatase 1 (DUSP1) by gonadotropin-releasing hormone (GnRH) stimulation and investigated the role of DUSP1 on gonadotropin gene expression using LbetaT2 gonadotroph cell line. DUSP1 expression was markedly increased 60 min after GnRH stimulation, and mitogen-activated protein kinase 3/1 (MAPK3/1) activation was gradually decreased after 60 min. GnRH-induced MAPK3/1 activation was completely inhibited by U0126, a MEK inhibitor, whereas GnRH-induced DUSP1 expression was partially inhibited by U0126. GnRH-induced DUSP1 induction was inhibited by triptolide, a diterpenoid triepoxide. In contrast, this compound potentiated MAPK3/1 activation. U0126 prevented GnRH-stimulated gonadotropin subunit promoter activation dose dependently, and 10 muM of U0126 reduced the effects of GnRH on the Lhb and Fshb promoters to 79.15% and 55.66%, respectively. GnRH-stimulated activation of Lhb and Fshb promoters as well as serum response factor (Srf) promoters were almost completely inhibited by triptolide, suggesting that this component had a nonspecific effect to the cells. Dusp1 siRNA reduced the expression of DUSP1 and augmented MAPK3/1 phosphorylation, but it did not increase of gonadotropin promoters. By overexpression of DUSP1, both GnRH-stimulated Lhb and Fshb promoters were significantly reduced. We have previously shown that insulin-like growth factor 1 (IGF1) increases MAPK3/1 but does not activate gonadotropin subunit promoters. IGF1 failed to induce DUSP1 expression. In addition, under pulsatile GnRH stimulation, DUSP1 expression was observed following high-frequency GnRH pulses but not following low-frequency pulses. Our study demonstrated that DUSP1, induced by GnRH, functions not only as an MAPK3/1-inactivating phosphatase but also as an important mediator in gonadotropin subunit gene expression regulation.

Gonadotropin-releasing hormone-mediated phosphorylation of estrogen receptor-alpha contributes to fosB expression in mouse gonadotrophs

Estrogen receptors (ERs) are activated by their ligands as well as signaling pathways that alter ER phosphorylation in response to peptide hormones and growth factors. In pituitary gonadotrophs, GnRHs act via the type I GnRH receptor (GnRHR). Both GnRH subtypes (GnRH-I and -II) activate an estrogen response element (ERE)-driven luciferase reporter gene in LbetaT2 mouse pituitary cells, and GnRH-I is most potent in this regard. Moreover, antide (a GnRH antagonist) and a GnRHR small interfering RNA (siRNA) abrogate this effect, whereas an ERalpha antagonist (ICI 182,780) does not. The ERalpha in LbetaT2 cells is phosphorylated at Ser(118) in the nucleus and at Ser(167) in both nucleus and cytoplasm after GnRH treatments and coincided with increased ERalpha binding to its coactivator, the p300/cAMP response element-binding protein-associated factor (PCAF). Moreover, siRNA-mediated knockdown of PCAF levels attenuated GnRH-induced ERE-luciferase transactivation in these cells. Most importantly, both GnRH subtypes robustly up-regulated expression of the immediate early response gene, fosB, whereas cotreatment with ERalpha siRNA or PCAF siRNA attenuated this effect. This appears to occur at the transcriptional level because corecruitment of ERalpha and PCAF to an ERE within the endogenous fosB promoter was increased by GnRH treatments, as shown by chromatin immunoprecipitation assays. These data demonstrate that GnRH-mediated phosphorylation of ERalpha in mouse LbetaT2 pituitary cells results in its rapid association with PCAF and the transcriptional activation of fosB, and we demonstrate that this in turn likely activates other genes in pituitary cells including the FSH beta-subunit gene.

Estradiol-17beta inhibits gonadotropin-releasing hormone-induced Ca2+ in gonadotropes to regulate negative feedback on luteinizing hormone release

In pituitary gonadotropes, estrogens have biphasic actions to cause an initial negative feedback followed by a positive feedback on LH secretion, but the mechanisms involved are not clearly understood. To investigate the feedback effects of estrogen, we used mixed ovine pituitary cell cultures (48-72 h), which were treated with 10(-9) M estradiol-17beta (E(2)) or vehicle followed by a pulse of 10(-9) M GnRH. Medium was collected for LH assay and cells extracted to determine activation of MAPK (phosphorylated ERK-1/2). E(2) treatment for 5 min reduced GnRH-induced LH release and caused phosphorylation of ERK-1/2. E(2) alone also caused phosphorylation of ERK-1/2, similar to the response evoked by GnRH alone. GnRH increased cytoplasmic intracellular free calcium concentration ([Ca(2+)](i)) and this was abolished by 2 min pretreatment with E(2) or E-bovine serum albumen conjugate. Blockade of Ca(2+) channels with nifedipine had no effect on the initial peak of GnRH-induced increase in [Ca(2+)](i) but reduced its duration by 27 +/- 6%. Depletion of intracellular Ca(2+) stores with thapsigargin prevented GnRH-induced increase in [Ca(2+)](i). Thapsigargin (10(-7) M) or nifedipine (10(-5) M) pretreatment (15 min) of cells lowered GnRH-induced LH secretion by 30 +/- 6 and 50% +/- 4%, respectively. We conclude that inhibition of the GnRH-induced increase in [Ca(2+)](i) in gonadotropes by E(2) is a likely mechanism for the negative feedback effect of E(2) on LH secretion involving a rapid nongenomic effect of E(2). Activation of the MAPK pathway by E(2) may be the mechanism for the time-delayed positive feedback effect on LH secretion at the level of the gonadotrope.

A preformed signaling complex mediates GnRH-activated ERK phosphorylation of paxillin and FAK at focal adhesions in L beta T2 gonadotrope cells

Most receptor tyrosine kinases and G protein-coupled receptors (GPCRs) operate via a limited number of MAPK cascades but still exert diverse functions, and therefore signal specificity remains an enigma. Also, most GPCR ligands utilize families of receptors for mediation of diverse biological actions; however, the mammalian type I GnRH receptor (GnRHR) seems to be the sole receptor mediating GnRH-induced gonadotropin synthesis and release. Signaling complexes associated with GPCRs may thus provide the means for signal specificity. Here we describe a signaling complex associated with the GnRHR, which is a unique GPCR lacking a C-terminal tail. Unlike other GPCRs, this signaling complex is preformed, and exposure of L beta T2 gonadotropes to GnRH induces its dynamic rearrangement. The signaling complex includes c-Src, protein kinase C delta, -epsilon, and -alpha, Ras, MAPK kinase 1/2, ERK1/2, tubulin, focal adhesion kinase (FAK), paxillin, vinculin, caveolin-1, kinase suppressor of Ras-1, and the GnRHR. Exposure to GnRH (5 min) causes MAPK kinase 1/2, ERK1/2, tubulin, vinculin, and the GnRHR to detach from c-Src, but they reassociate within 30 min. On the other hand, FAK, paxillin, the protein kinase Cs, and caveolin-1 stay bound to c-Src, whereas kinase suppressor of Ras-1 appears in the complex only 30 min after GnRH stimulation. GnRH was found to activate ERK1/2 in the complex in a c-Src-dependent manner, and the activated ERK1/2 subsequently phosphorylates FAK and paxillin. In parallel, caveolin-1, FAK, vinculin, and paxillin are phosphorylated on Tyr residues apparently by GnRH-activated c-Src. Receptor tyrosine kinases and GPCRs translocate ERK1/2 to the nucleus to phosphorylate and activate transcription factors. We therefore propose that the role of the multiprotein signaling complex is to sequester a cytosolic pool of activated ERK1/2 to phosphorylate FAK and paxillin at focal adhesions.

GnRH stimulates expression of PACAP in the pituitary gonadotropes via both the PKA and PKC signaling systems

Recent studies have demonstrated a clear role for pituitary adenylate cyclase-activating polypeptide (PACAP) in the regulation of gonadotropin biosynthesis and secretion, both alone and in conjunction with GnRH. First defined as a hypothalamic releasing factor, PACAP subsequently has been identified in the gonadotrope subpopulation of the anterior pituitary gland, suggesting that PACAP may act as an autocrine-paracrine factor in this tissue. In initial studies, we determined that GnRH markedly stimulated endogenous PACAP mRNA levels and promoter-reporter activity in the mature gonadotrope cell line, LbetaT2. GnRH-stimulated rat PACAP promoter activity was blunted with deletion from position -915 to -402 and eliminated with further truncation to position -77 relative to the transcriptional start site. Site-directed mutagenesis demonstrated a functional requirement for a cAMP response element (CRE)-like site at position -205 and an activating protein-1 (AP-1)-like site at position -275, both of which bound CRE binding protein and AP-1 family members on EMSA. Treatment with pharmacological activators or inhibitors of second messenger signaling pathways implicated the protein kinase A, protein kinase C, and MAPK pathways in the GnRH response. In support of these in vitro data, we demonstrate that JunB binds to the rat PACAP gene promoter by chromatin immunoprecipitation assay and that small interfering RNA knockdown of JunB, cFos, and CRE binding protein factors blunts PACAP expression. In summary, these results further elucidate the complex functional interactions between PACAP and GnRH in the anterior pituitary. Specifically, these studies demonstrate that GnRH-stimulated PACAP gene expression is mediated via multiple signaling pathways acting on CRE/AP-1 sites in the proximal gene promoter. Because both PACAP and GnRH regulate gonadotropin biosynthesis and secretion, these results provide important insight into the critical fine tuning of gonadotrope function and, thereby, the maintenance of normal reproductive function.

Regulation of intracellular signaling cascades by GNRH pulse frequency in the rat pituitary: roles for CaMK II, ERK, and JNK activation

Pulsatile GnRH (GNRH) differentially regulates LH and FSH subunit genes, with faster frequencies favoring Lhb transcription and slower favoring Fshb. Various intracellular pathways mediate the effects of GNRH, including CaMK II (CAMK2), ERK, and JNK. We examined whether activation of these pathways is regulated by GNRH pulse frequency in vivo. GNRH-deficient rats received GNRH pulses (25 ng i.v. every 30 or 240 min for 8 h, vehicle to controls). Pituitaries were collected 5 min after the last pulse, bisected, and one half processed for RNA (to measure beta subunit primary transcripts [PTs]) and the other for protein. Phosphorylated CAMK2 (phospho-CAMK2), ERK (mitogen-activated protein kinase 1/3 [MAPK1/3], also known as p42 ERK2 and p44 ERK1, respectively), and JNK (MAPK8/9, also known as p46 JNK1 and p54 JNK2, respectively) were determined by Western blotting. The 30-min pulses maximally stimulated Lhb PT (8-fold), whereas 240 min was optimal for Fshb PT (3-fold increase). Both GNRH pulse frequencies increased phospho-CAMK2 4-fold. Activation of MAPK1/3 was stimulated by both 30- and 240-min pulses, but phosphorylation of MAPK3 was significantly greater following slower GNRH pulses (240 min: 4-fold, 30 min: 2-fold). MAPK8/9 activation was unchanged by pulsatile GNRH in this paradigm, but as previous results showed that GNRH-induced activation of MAPK8/9 is delayed, 5 min after GNRH may not be optimal to observe MAPK8/9 activation. These data show that CAMK2 is activated by GNRH, but not in a frequency-dependant manner, whereas MAPK3 is maximally stimulated by slow-frequency GNRH pulses. Thus, the ERK response to slow pulse frequency is part of the mechanisms mediating Fhb transcriptional responses to GNRH.

Extracellular signal-regulated kinase mediates gonadotropin subunit gene expression and LH release responses to endogenous gonadotropin-releasing hormones in goldfish

The possible involvement of extracellular signal-regulated kinase (ERK) in mediating the stimulatory actions of two endogenous goldfish gonadotropin-releasing hormones (salmon (s)GnRH and chicken (c)GnRH-II) on gonadotropin synthesis and secretion was examined. Western blot analysis revealed the presence of ERK and phosphorylated (p)ERK in goldfish brain, pituitary, liver, ovary, testis and muscle tissue extracts, as well as extracts of dispersed goldfish pituitary cells and HeLa cells. Interestingly, a third ERK-like immunoreactive band of higher molecular mass was detected in goldfish tissue and pituitary cell extracts in addition to the ERK1-p44- and ERK2-p42-like immunoreactive bands. Incubation of primary cultures of goldfish pituitary cells with either a PKC-activating 4beta-phorbol ester (TPA) or a synthetic diacylglycerol, but not a 4alpha-phorbol ester, elevated the ratio of pERK/total (t)ERK for all three ERK isoforms. The stimulatory effects of TPA were attenuated by the PKC inhibitor GF109203X and the MEK inhibitor PD98059. sGnRH and cGnRH-II also elevated the ratio of pERK/tERK for all three ERK isoforms, in a time-, dose- and PD98059-dependent manner. In addition, treatment with PD98059 reduced the sGnRH-, cGnRH-II- and TPA-induced increases in gonadotropin subunit mRNA levels in Northern blot studies and sGnRH- and cGnRH-II-elicited LH release in cell column perifusion studies with goldfish pituitary cells. These results indicate that GnRH and PKC can activate ERK through MEK in goldfish pituitary cells. More importantly, the present study suggests that GnRH-induced gonadotropin subunit gene expression and LH release involve MEK/ERK signaling in goldfish.

Pulsatile gonadotropin-releasing hormone stimulation of gonadotropin subunit transcription in rat pituitaries: evidence for the involvement of Jun N-terminal kinase but not p38

We investigated whether Jun N-terminal kinase (JNK) and p38 mediate gonadotropin subunit transcriptional responses to pulsatile GnRH in normal rat pituitaries. A single pulse of GnRH or vehicle was given to female rats in vivo, pituitaries collected, and phosphorylated JNK and p38 measured. GnRH stimulated an increase in JNK phosphorylation within 5 min, which peaked 15 min after GnRH (3-fold). GnRH also increased p38 phosphorylation 2.3-fold 15 min after stimulus. Rat pituitary cells were given 60-min pulses of GnRH or media plus the JNK inhibitor SP600125 (SP, 20 microM), p38 inhibitor SB203580 (20 microM), or vehicle. In vehicle-treated groups, GnRH pulses increased LHbeta and FSHbeta primary transcript (PT) levels 3-fold. SP suppressed both basal and GnRH-induced increases in FSHbeta PT by half, but the magnitude of responses to GnRH was unchanged. In contrast, SP had no effect on basal LHbeta PT but suppressed the stimulatory response to GnRH. SB203580 had no effect on the actions of GnRH on either LH or FSHbeta PTs. Lbeta-T2 cells were transfected with dominant/negative expression vectors for MAPK kinase (MKK)-4 and/or MKK-7 plus a rat LHbeta promoter-luciferase construct. GnRH stimulated a 50-fold increase in LHbeta promoter activity, and the combination of MKK-4 and -7 dominant/negatives suppressed the response by 80%. Thus, JNK (but not p38) regulates both LHbeta and FSHbeta transcription in a differential manner. For LHbeta, JNK is essential in mediating responses to pulsatile GnRH. JNK also regulates FSHbeta transcription (i.e. maintaining basal expression) but does not play a role in responses to GnRH.

Luteinizing hormone beta promoter stimulation by adenylyl cyclase and cooperation with gonadotropin-releasing hormone 1 in transgenic mice and LBetaT2 Cells

Rat luteinizing hormone beta (Lhb) gene transcription is stimulated by hypothalamic gonadotropin-releasing hormone 1 (GnRH1), and this response may be modulated by other signaling pathways such as cAMP. Here we characterize the ability of cAMP, alone or with GnRH1, to stimulate Lhb gene transcription in mouse pituitary and clonal gonadotroph cells. Both cAMP and pituitary adenylyl cyclase-activating peptide increase GnRH1 stimulation of luciferase activity in pituitaries of mice expressing the rat Lhb-luciferase transgene, suggesting cAMP and GnRH1 pathways interact in vivo. cAMP stimulation of the Lhb-luciferase transgene was similar between females in metestrus and proestrus, but GnRH1 stimulation was greater at proestrus. Additive effects with combined treatments were observed at metestrus and proestrus. Elevated intracellular cAMP stimulated Lhb promoter activity in LbetaT2 clonal gonadotroph cells, alone and with GnRH1. In LbetaT2 cells, cAMP stimulation of the Lhb promoter was eliminated by inhibition of protein kinase A (PKA); GnRH1 stimulation was partially suppressed by either PKA or protein kinase C inhibitors. Only the proximal GnRH1-responsive region of the promoter was required for cAMP stimulation, and mutation of the 3' NR5A1 site diminished the response. Regulation of primary mRNA transcripts from the endogenous Lhb gene by cAMP and GnRH1 correlated with results from the Lhb-luciferase transgene or transfected promoter. Occupancy of the endogenous promoter by EGR1 was increased by GnRH1 with or without forskolin, but forskolin alone had little effect. Thus, cAMP stimulation of Lhb promoter activity, and enhancement of GnRH1 stimulation, occurs in multiple physiological states independent of steroid status, via a PKA-dependent mechanism.

Neuroendocrine plasticity in the anterior pituitary: gonadotropin-releasing hormone-mediated movement in vitro and in vivo

The secretion of LH is cued by the hypothalamic neuropeptide, GnRH. After delivery to the anterior pituitary gland via the hypothalamic-pituitary portal vasculature, GnRH binds to specific high-affinity receptors on the surface of gonadotrope cells and stimulates synthesis and secretion of the gonadotropins, FSH, and LH. In the current study, GnRH caused acute and dramatic changes in cellular morphology in the gonadotrope-derived alphaT3-1 cell line, which appeared to be mediated by engagement of the actin cytoskeleton; disruption of actin with jasplakinolide abrogated cell movement and GnRH-induced activation of ERK. In live murine pituitary slices infected with an adenovirus-containing Rous sarcoma virus-green fluorescent protein, selected cells responded to GnRH by altering their cellular movements characterized by both formation and extension of cell processes and, surprisingly, spatial repositioning. Consistent with the latter observation, GnRH stimulation increased the migration of dissociated pituitary cells in transwell chambers. Our data using live pituitary slices are a striking example of neuropeptide-evoked movements of cells outside the central nervous system and in a mature peripheral endocrine organ. These findings call for a fundamental change in the current dogma of simple passive diffusion of LH from gonadotropes to capillaries in the pituitary gland.

Reciprocal cross talk between gonadotropin-releasing hormone (GnRH) and prostaglandin receptors regulates GnRH receptor expression and differential gonadotropin secretion

The asynchronous secretion of gonadotrope LH and FSH under the control of GnRH is crucial for ovarian cyclicity but the underlying mechanism is not fully resolved. Because prostaglandins (PG) are autocrine regulators in many tissues, we determined whether they have this role in gonadotropes. We first demonstrated that GnRH stimulates PG synthesis by induction of cyclooxygenase-2, via the protein kinase C/c-Src/phosphatidylinositol 3'-kinase/MAPK pathway in the LbetaT2 gonadotrope cell line. We then demonstrated that PGF(2alpha) and PGI2, but not PGE2 inhibited GnRH receptor expression by inhibition of phosphoinositide turnover. PGF(2alpha), but not PGI2 or PGE2, reduced GnRH-induction of LHbeta gene expression, but not the alpha-gonadotropin subunit or the FSHbeta subunit genes. The prostanoid receptors EP1, EP2, FP, and IP were expressed in rat gonadotropes. Incubations of rat pituitaries with PGF(2alpha), but not PGI2 or PGE2, inhibited GnRH-induced LH secretion, whereas the cyclooxygenase inhibitor, indomethacin, stimulated GnRH-induced LH secretion. None of these treatments had any effect on GnRH-induced FSH secretion. The findings have thus elaborated a novel GnRH signaling pathway mediated by PGF(2alpha)-FP and PGI2-IP, which acts through an autocrine/paracrine modality to limit autoregulation of the GnRH receptor and differentially inhibit LH and FSH release. These findings provide a mechanism for asynchronous LH and FSH secretions and suggest the use of combination therapies of GnRH and prostanoid analogs to treat infertility, diseases with unbalanced LH and FSH secretion and in hormone-dependent diseases such as prostatic cancer.

Steroid receptor coactivator-3 is required for progesterone receptor trans-activation of target genes in response to gonadotropin-releasing hormone treatment of pituitary cells

Regulation of gonadotropin production involves interplay between steroids and neuropeptides, and we have examined the effects of gonadotropin-releasing hormones (GnRH I and GnRH II) on progesterone receptor (PR) activation in alphaT3-1 pituitary cells. Treatment with GnRHs activated a progester-one response element (PRE)-luciferase reporter gene, and this was blocked by protein kinase C and protein kinase A inhibitors but not by RU486. Treatment with GnRHs phosphorylated the PR at Ser(294) and increased PR translocation to the nucleus within 1 h. Interactions between the PR and several coactivators were examined, and treatment with GnRHs specifically induced PR-steroid receptor coactivator-3 (SRC-3) interactions within 8 h. In chromatin immunoprecipitation assays, recruitment of PR and SRC-3 by the PREs of the luciferase reporter gene or the gonadotopin alpha-subunit gene promoter was also increased by GnRHs within 8 h, while progesterone-induced recruitment of PR to the PREs occurred in association with much less SRC-3. A small interfering RNA knockdown of type I GnRH receptor levels reduced PR activation by GnRHs, while progesterone-dependent PR activation was unaffected. Moreover, small interfering RNA knockdown of SRC-3 abolished PRE-luciferase trans-activation by the PR in response to GnRHs. Collectively, these data indicate that PR activation by GnRHs in alphaT3-1 cells is type I GnRH receptor-mediated and that trans-activation of PR-responsive genes requires SRC-3 in this context.

Mechanisms for pulsatile regulation of the gonadotropin subunit genes by GNRH1

The frequency of gonadotropin-releasing hormone (GNRH1, or GnRH) pulses secreted from the hypothalamus determine the ratios of the gonadotropin subunit genes luteinizing hormone beta (Lhb), follicle-stimulating hormone beta (Fshb) and the common alpha-glycoprotein subunit gene (Cga) transcribed in the anterior pituitaries of mammals. Fshb is preferentially transcribed at slower GNRH1 pulse frequencies, whereas Lhb and Cga are preferentially transcribed at more rapid pulse frequencies. Producing the gonadotropins in the correct proportions is critical for normal fertility. Currently, there is no definitive explanation for how GNRH1 pulses differentially activate gonadotropin subunit gene transcription. Several pathways may contribute to this regulation. For example, GNRH1-regulated GNRH1-receptor concentrations may lead to variable signaling pathway activation. Several signaling pathways are activated by GnRH, including mitogen-activated protein kinase, protein kinase C, calcium influx, and calcium-calmodulin kinase, and these may be preferentially regulated under certain conditions. In addition, some signaling proteins feed back to downregulate their own levels. Other arms of gonadotroph signaling appear to be regulated by synthesis, modification, and degradation of either transcription factors or regulatory proteins. Finally, the dynamic binding of proteins to the chromatin, and how that might be regulated by chromatin-modifying proteins, is addressed. Oscillations in expression, modification, and chromatin binding of the proteins involved in gonadotropin gene expression are likely a link between GNRH1 pulsatility and differential gonadotropin transcription.

Gonadotropin-releasing hormone pulse frequency-dependent activation of extracellular signal-regulated kinase pathways in perifused LbetaT2 cells

The pattern of GnRH release is associated with differential synthesis and release of LH and FSH. Using a perifusion system, we previously reported that stimulation of the LbetaT2 cell line with varying GnRH pulse frequencies resulted in differential stimulation of LHbeta and FSHbeta gene transcription, analogous to previous observations in primary gonadotropes. In the present study, we investigated the patterns of MAPK activation by GnRH and the role of MAPK in mediating the frequency-dependent effects. In static culture, ERK activation in LbetaT2 cells stimulated with continuous GnRH (10 nM) was maximal by 10 min and persisted for up to 6 h, with a return to basal levels by 20 h. In contrast, stimulation with continuous GnRH (10 nM) in perifused cells resulted in a more sustained activation of ERK. To investigate the effects of GnRH pulse frequency on ERK activation, perifused LbetaT2 cells were stimulated with pulsatile GnRH at a frequency of one pulse every 30 min or one pulse every 2 h for 20 h (10 nM, 5 min/pulse). After the final GnRH pulse, cells were lysed at frequent intervals and levels of ERK phosphorylation were measured. Under high-frequency conditions, ERK activation was maximal 10 min after the GnRH pulse and returned to baseline levels by 20 min. In contrast, under lower GnRH pulse frequency conditions, ERK activation occurred more rapidly and activation was more sustained, with a slower rate of ERK dephosphorylation. These changes resulted in different levels of nuclear phosphorylated ERK. Blockade of ERK activation abolished GnRH-dependent activation of LHbeta and FSHbeta transcription at both high and low pulse frequencies. These results demonstrate that in perifused LbetaT2 cells, distinct patterns of ERK activation/inactivation are regulated by GnRH pulse frequency, and the difference in ERK activation may be important for GnRH pulse frequency-dependent differential stimulation of LHbeta and FSHbeta gene expression.

Molecular biology of gonadotropin-releasing hormone (GnRH)-I, GnRH-II, and their receptors in humans

In human beings, two forms of GnRH, termed GnRH-I and GnRH-II, encoded by separate genes have been identified. Although these hormones share comparable cDNA and genomic structures, their tissue distribution and regulation of gene expression are significantly dissimilar. The actions of GnRH are mediated by the GnRH receptor, which belongs to a member of the rhodopsin-like G protein-coupled receptor superfamily. However, to date, only one conventional GnRH receptor subtype (type I GnRH receptor) uniquely lacking a carboxyl-terminal tail has been found in the human body. Studies on the transcriptional regulation of the human GnRH receptor gene have indicated that tissue-specific gene expression is mediated by differential promoter usage in various cell types. Functionally, there is growing evidence showing that both GnRH-I and GnRH-II are potentially important autocrine and/or paracrine regulators in some extrapituitary compartments. Recent cloning of a second GnRH receptor subtype (type II GnRH receptor) in nonhuman primates revealed that it is structurally and functionally distinct from the mammalian type I receptor. However, the human type II receptor gene homolog carries a frameshift and a premature stop codon, suggesting that a full-length type II receptor does not exist in humans.

Testosterone stimulates follicle-stimulating hormone beta transcription via activation of extracellular signal-regulated kinase: evidence in rat pituitary cells

This study investigated whether estradiol (E2) or testosterone (T) activate extracellular signal-regulated kinase (ERK) and calcium/calmodulin-dependent kinase II (Ca/CaMK II), as indicated by enzyme phosphorylation in rat pituitaries. In vivo studies used adult female rats given E2, T, or empty silastic capsules (vehicle controls). Twenty-four hours later, the rats were given a single pulse of GnRH (300 ng) or BSA-saline (to controls) and killed 5 min later. GnRH stimulated a two- to three-fold rise in activated Ca/CaMK II, and E2 and T had no effect on Ca/CaMK II activation. In contrast, both GnRH and T stimulated threefold increases in ERK activity, with additive effects seen following the combination of GnRH+T. E2 had no effect on ERK activity. In alpha T3 clonal gonadotrope cells, dihydrotestosterone did not activate ERK alone but enhanced and prolonged the ERK responses to GnRH, demonstrating direct effects on the gonadotrope. Thus, the ERK response to GnRH plus androgen was enhanced in both rat pituitary and alpha T3 cells. In vitro studies with cultured rat pituitary cells examined the effect of GnRH+/-T in the presence of the mitogen-activated protein (MAP) kinase kinase inhibitor, PD-098059 (PD). Results showed that PD suppressed ERK activational and FSH beta transcriptional responses to T. These findings suggest that one site of T regulation of FSH beta transcription is through the selective stimulation of the ERK pathway.

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.

Differential regulation of gonadotropins and glycoprotein hormone alpha-subunit by IGF-I in anterior pituitary cells from male rats

IGF-I has been demonstrated to stimulate basal and GnRH-induced gonadotropin release. IGF-I also elicites alpha-subunit secretion in human pituitary tumor cells. The aims of this study were to evaluate both the effect of IGF-I on gonadotropin LH-beta and FSH-beta mRNA levels and glycoprotein alpha-subunit gene expression in cultured rat anterior pituitary cells. The exposure of pituitary cells to recombinant human IGF-I (rhlGF-I; 2 microg/ml) for 72 h markedly stimulated basal LH and FSH release whereas their mRNA levels remained unmodified. IGF-I elicited a-subunit release from pituitary cells (p < 0.01) and augmented its mRNA levels. Exposure to IGF-I consistently reduced GH release from pituitary cells. This study shows that the gonadotropin-releasing effects of IGF-I are not paralleled by changes in their mRNAs whereas IGF-I stimulates not only alpha-subunit release but also its mRNA levels. This study provides the first observation of alpha-subunit regulation by IGF-I in normal pituitary cells, where a differential regulation between release and synthesis for gonadotropin a-and 1-subunits is also shown.

Small nuclear RING finger protein stimulates the rat luteinizing hormone-beta promoter by interacting with Sp1 and steroidogenic factor-1 and protects from androgen suppression

GnRH controls expression of the LH subunit genes, alpha and LHbeta, with the LHbeta subunit regulated most dramatically. Two enhancer regions, distal and proximal, on the rat LHbeta gene promoter cooperate for full basal expression and GnRH stimulation. It has been hypothesized that the transcription factors binding to these regions, Sp1, Egr-1, and steroidogenic factor 1 (SF-1), may interact directly or indirectly via a coactivator. One such coactivator may be small nuclear RING finger protein (SNURF), which is expressed in pituitary tissue and the LbetaT2 gonadotrope cell line. In transfection experiments in LbetaT2 cells, SNURF stimulated basal expression of LHbeta and increased overall GnRH stimulation. SNURF specifically stimulated LHbeta, with no effect on the alpha-subunit promoter. SNURF interacts with Sp1 and SF-1, but not Egr-1, in pull-down experiments. Point mutations or deletions of SNURF functional domains demonstrated that Sp1 and SF-1 interactions with SNURF are required for SNURF stimulatory effects on the LHbeta promoter. Endogenous SNURF is associated with the LHbeta promoter on native chromatin, suggesting that it plays a physiological role in LHbeta gene expression. SNURF also binds the androgen receptor, and SNURF overexpression overcomes androgen suppression of GnRH-stimulated LHbeta but not alphasubunit promoter activity. SNURF mutations that disrupt Sp1 or SF-1 binding eliminate rescue by SNURF. We conclude that SNURF may mediate interactions between the distal and proximal GnRH response regions of the LHbeta promoter to stimulate transcription and can also protect the promoter from androgen suppression.

Acute regulation of translation initiation by gonadotropin-releasing hormone in the gonadotrope cell line LbetaT2

The hypothalamic neuropeptide hormone GnRH is the central regulator of reproductive function. GnRH stimulates the synthesis and release of the gonadotropins LH and FSH by the gonadotropes of the anterior pituitary through activation of the G-protein-coupled GnRH receptor. In this study, we investigated the role of translational control of hormone synthesis by the GnRH receptor in the novel gonadotrope cell line LbetaT2. Using immunohistochemical and RIA studies with this model, we show that acute GnRH-induced synthesis and secretion of LH are dependent upon new protein synthesis but not new mRNA synthesis. We examined the response to GnRH and found that activation of cap-dependent translation occurs within 4 h. LHbeta promoter activity was also examined, and we found no increases in LHbeta promoter activity after 6 h of GnRH stimulation. Additionally, we show that increased phosphorylation of translation initiation proteins, 4E-binding protein 1, eukaryotic initiation factor 4E, and eukaryotic initiation factor 4G, occur in a dose- and time-dependent manner in response to GnRH stimulation. Quantitative luminescent image analysis of Western blots shows that 10 nm GnRH is sufficient to cause a maximal increase in factor phosphorylation, and maximal responses occur within 30 min of stimulation. Further, we demonstrate that the MAPK kinase inhibitor, PD 98059, abolishes the GnRH-mediated stimulation of a cap-dependent translation reporter. More specifically, we demonstrate that PD 98059 abolishes the GnRH-mediated stimulation of a downstream target of the ERK pathway, MAPK-interacting kinase. Based on these findings, we conclude that acute GnRH stimulation of LbetaT2 cells increases translation initiation through ERK signaling. This may contribute to the acute increases in LHbeta subunit production.

Gonadotropin-releasing hormone stimulation of gonadotropin subunit transcription: evidence for the involvement of calcium/calmodulin-dependent kinase II (Ca/CAMK II) activation in rat pituitaries

The intracellular pathways mediating GnRH regulation of gonadotropin subunit transcription remain to be fully characterized, and the present study examined whether calcium/calmodulin-dependent kinase II (Ca/CAMK II) plays a role in the rat pituitary. Preliminary studies demonstrated that a single pulse of GnRH given to adult rats stimulated a transient 2.5-fold rise in Ca/CAMK II activity (as determined by an increase in Ca/CAMK II phosphorylation), with peak values at 5 min, returning to basal 45 min after the pulse. Further studies examined the alpha, LHbeta, and FSHbeta transcriptional responses to GnRH or Bay K 8644+KCl (BK+KCl) pulses in vitro in the absence or presence of the Ca/CAMK II-specific inhibitor, KN-93. Gonadotropin subunit transcription was assessed by measuring primary transcripts (PTs) by quantitative RT-PCR. In time-course studies, both GnRH and BK+KCl pulses given alone increased all three subunit PTs after 6 h (2- to 4-fold). PT responses to GnRH increased over time (3- to 8-fold over basal at 24 h), although BK+KCl was ineffective after 24 h. KN-93 reduced the LHbeta and FSHbeta transcriptional responses to GnRH by 50-60% and completely suppressed the alphaPT response. In contrast, KN-93 showed no inhibitory effects on basal transcriptional activity or LH or FSH secretion. In fact, KN-93 tended to increase basal alpha, LHbeta, and FSHbeta PT levels and enhance LH secretory responses to GnRH. These results reveal that Ca/CAMK II plays a central role in the transmission of pulsatile GnRH signals from the plasma membrane to the rat alpha, LHbeta, and FSHbeta subunit genes.

The calcium component of gonadotropin-releasing hormone-stimulated luteinizing hormone subunit gene transcription is mediated by calcium/calmodulin-dependent protein kinase type II

Calcium influx plays a critical role in GnRH regulation of rat LH subunit gene transcription, but the site(s) of action are undefined. We investigated the potential of GnRH acting through calcium to activate calcium/calmodulin-dependent protein kinase type II (Ca/CaMK II) in mouse gonadotrope-derived LbetaT2 cells. GnRH stimulated Ca/CaMK II beta subunit activity 3-fold 2 min after treatment and returned to control values by 45 min. The Ca/CaMK II response to GnRH was blocked by administration of the Ca/CaMK II-specific inhibitor, KN-93. The calcium channel activator Bay K 8644 stimulated a 3-fold increase in Ca/CaMK II activity, similar to GnRH. Blocking calcium influx with nimodipine or depleting intracellular calcium storage pools with thapsigargin each resulted in a partial suppression of GnRH-induced activation of Ca/CaMK II, and in combination, completely suppressed the Ca/CaMK II response to GnRH. KN-93 and nimodipine also suppressed alpha-subunit and LHbeta promoter responses to GnRH by 40-60%. LHbeta promoter constructs containing either proximal or proximal and distal GnRH-responsive regions were sensitive to inhibition. These data show for the first time that Ca/CaMK II activation plays an important role in the transmission of GnRH signals from the plasma membrane to the LH subunit genes.

Regulation of gonadotropin alpha subunit gene expression by dopamine D(2) receptor agonist in clonal mouse gonadotroph alphaT3-1 cells

Pituitary prolactin biosynthesis is negatively regulated by hypothalamic dopamine through D(2) receptors in pituitary lactotrophs, but little is known about the direct effect of dopamine on gonadotrophs. In this study, the clonal gonadotroph-derived cell line, alphaT3-1, was used to examine whether gene expression of the pituitary gonadotropin alpha subunit, stimulated with GnRH or pituitary adenylate cyclase-activating polypeptide (PACAP), was controlled by dopamine D(2) receptor. Western blotting and reverse transcription-polymerase chain reaction analysis demonstrated the presence of dopamine D(2) receptors in alphaT3-1 cells. Both GnRH and PACAP increased alpha subunit gene expression. GnRH-induced alpha subunit gene expression was not affected by quinpirol, a specific dopamine D(2) receptor agonist. In contrast, PACAP-induced gene expression was significantly lower in the presence of quinpirol. The roles of extracellular signal-regulated kinase (ERK) and cAMP in the expression of the alpha subunit gene were examined. GnRH activated ERK, but PACAP did not, and the activation was not inhibited by quinpirol. GnRH-induced alpha subunit gene expression was completely inhibited by an ERK inhibitor, PD098059. Cyclic AMP accumulation in alphaT3-1 cells was increased by treatment with PACAP, and quinpirol inhibited this effect. GnRH did not affect cAMP production in these cells. These results suggest that in alphaT3-1 cells, dopamine D(2) receptors negatively regulate pituitary alpha subunit gene expression in association with the cAMP-dependent pathway, but not with the ERK pathway.

c-Jun N-terminal kinase activation of activator protein-1 underlies homologous regulation of the gonadotropin-releasing hormone receptor gene in alpha T3-1 cells

Reproductive function is dependent on the interaction between GnRH and its cognate receptor found on gonadotrope cells of the anterior pituitary gland. GnRH activation of the GnRH receptor (GnRHR) is a potent stimulus for increased expression of multiple genes including the gene encoding the GnRHR itself. Thus, homologous regulation of the GnRHR is an important mechanism underlying gonadotrope sensitivity to GnRH. Previously, we have found that GnRH induction of GnRHR gene expression in alpha T3-1 cells is partially mediated by protein kinase C activation of a canonical activator protein-1 (AP-1) element. In contrast, protein kinase A and a cAMP response element-like element have been implicated in mediating the GnRH response of the GnRHR gene using a heterologous cell model (GGH(3)). Herein we find that selective removal of the canonical AP-1 site leads to a loss of GnRH regulation of the GnRHR promoter in transgenic mice. Thus, an intact AP-1 element is necessary for GnRH responsiveness of the GnRHR gene both in vitro and in vivo. Based on in vitro analyses, GnRH appeared to enhance the interaction of JunD, FosB, and c-Fos at the GnRHR AP-1 element. Although enhanced binding of cFos reflected an increase in gene expression, GnRH appeared to regulate both FosB and JunD at a posttranslational level. Neither overexpression of a constitutively active Raf-kinase nor pharmacological blockade of GnRH-induced ERK activation eliminated the GnRH response of the GnRHR promoter. GnRH responsiveness was, however, lost in alpha T3-1 cells that stably express a dominant-negative c-Jun N-terminal kinase (JNK) kinase, suggesting a critical role for JNK in mediating GnRH regulation of the GnRHR gene. Consistent with this possibility, we find that the ability of forskolin and membrane-permeable forms of cAMP to inhibit the GnRH response of the GnRHR promoter is associated with a loss of both JNK activation and GnRH-mediated recruitment of the primary AP-1-binding components.

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.

Different signaling pathways control acute induction versus long-term repression of LHbeta transcription by GnRH

GnRH regulates pituitary gonadotropin gene expression through GnRH receptor activation of the protein kinase C (PKC) and calcium signaling cascades. The pulsatile pattern of GnRH release is crucial for induction of LHbeta-subunit (LHbeta) gene expression; however, continuous prolonged GnRH exposure leads to repression of LHbeta gene transcription. Although in part, long-term repression may be due to receptor down-regulation, the molecular mechanisms of this differential regulation of LHbeta transcription are unknown. Using transfection into the LH-secreting immortalized mouse gonadotrope cell line (LbetaT4), we have demonstrated that LHbeta gene transcription is increased by acute activation (6 h) of GnRH receptor or PKC but not calcium influx; in contrast long-term activation (24 h) of GnRH receptor, PKC, or calcium influx each repress LHbeta transcription. Whereas blockade of PKC prevented the acute action of GnRH and unmasked an acute repression of LHbeta transcription by calcium, it did not prevent long-term repression by GnRH or calcium. Removal of calcium resulted in potentiation of acute GnRH and PKC induction of LHbeta gene expression but prevented long-term repression by GnRH and reduced long-term repression by either calcium or 12-O-tetradecanoyl-phorbol-13-acetate (TPA). We conclude that GnRH uses PKC for acute induction, and calcium signaling is responsible for long-term repression of LHbeta gene expression by GnRH. Furthermore, analysis of the responsiveness of truncated and mutated LHbeta promoter regions demonstrated that not only do acute induction and long-term repression use different signaling systems, but they also use different target sequences for regulating the LHbeta gene.

Regulation of human glycoprotein hormone alpha-subunit gene transcription in LbetaT2 gonadotropes by protein kinase C and extracellular signal-regulated kinase 1/2

Transcriptional activation of the human glycoprotein hormone alpha-subunit (alphaGSU) promoter in response to GnRH and phorbol-12-myristate-13-acetate (PMA) has been well characterized in alphaT3-1 gonadotropes but not investigated in the more differentiated LbetaT2 clonal gonadotrope. We have evaluated alphaGSU transcription in the more mature LbetaT2 cell line, using deletion and heterologous constructs of the alphaGSU promoter linked to a luciferase reporter gene. Basal alphaGSU-promoter activity was significantly less in LbetaT2 cells than in alphaT3-1 cells, but stimulation of transfected cells with GnRH and PMA resulted in similar increases in alphaGSU-promoter activity. Deletional analysis of the human alphaGSU promoter in LbetaT2 cells indicated that sequences between -398 and -244 and between -244 and -195 base pairs (bp) were involved in regulating basal alphaGSU-promoter transcription, whereas the region between -244 and -195 bp regulated PMA-stimulated promoter activity. Deletion of this promoter region containing a steroidogenic factor-1 (SF-1) binding site abolished basal and PMA-stimulated transcription. Site-directed mutagenesis of the SF-1 binding site resulted in a significant attenuation of basal and PMA-stimulated alphaGSU transcription. Pretreatment of LbetaT2 cells with a mitogen-activated protein kinase kinase-specific inhibitor, U0126, abolished the PMA-stimulated increase in MAPK activity and significantly reduced basal and PMA-stimulated promoter activity. Electrophoretic mobility shift assays for SF-1 and GATA revealed that PMA failed to affect SF-1 binding but enhanced GATA binding to a consensus GATA oligonucleotide, an effect that was blocked with U0126 pretreatment, suggesting that GATA may mediate ERK activation of alphaGSU transcription. Our data suggests that, in the mature LbetaT2 gonadotrope cell line, two regions of the human alphaGSU promoter regulate basal transcription and that SF-1 is involved in mediating basal and PMA-stimulated promoter activity. Furthermore, PKC-stimulated transcription partially relies on ERK acting on elements downstream of -244 bp of the human alphaGSU promoter.

Activation of MAPK cascades by GnRH: ERK and Jun N-terminal kinase are involved in basal and GnRH-stimulated activity of the glycoprotein hormone LHbeta-subunit promoter

The role of ERK and Jun N-terminal kinase (JNK) in basal- and GnRH-stimulated LHbeta-promoter activity was examined in the gonadotroph cell line LbetaT-2. GnRH agonist (GnRH-A) stimulates the MAPK cascades ERK, JNK, and p38MAPK, with a peak at 7 min for ERK and at 60 min for JNK and p38MAPK. The rat glycoprotein hormone LHbeta-subunit promoter, linked to the chloramphenicol acetyl transferase (CAT) reporter gene, was used to follow its activation. Addition of GnRH-A (10 nM) to LbetaT-2 cells resulted in a 6-fold increase in LHbeta-CAT activity at 8 h, which was markedly reduced by a GnRH antagonist. The PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA), but not the Ca(2+) ionophore ionomycin, stimulated LHbeta-CAT activity. Addition of GnRH-A and TPA together did not produce an additive response. Down-regulation of PKC, but not removal of Ca(2+), abolished the GnRH-A and the TPA response. Cotransfection of the LHbeta-promoter and the constitutively active form of Raf-1 stimulated basal and GnRH-A-induced LHbeta-CAT activity. The dominant negative forms of the ERK cascade members Ras, Raf-1, and MAPK/ERK kinase (MEK) markedly reduced basal and GnRH-A-induced LHbeta-CAT activity, Similar results were obtained with the MEK inhibitor PD 098059. Cotransfection of the LHbeta-promoter and the constitutively active CDC42 stimulated basal and GnRH-A-induced LHbeta-CAT activity. The dominant negative forms of the JNK cascade members Rac, CDC42, and SEK markedly diminished basal and GnRH-A-induced LHbeta-CAT activity. Interestingly, the constitutively active form of c-Src stimulated the basal and the GnRH-A response, whereas the dominant negative form of c-Src, or the c-Src inhibitor PP1 diminished basal and the GnRH-A response. We conclude that ERK and JNK are involved in basal and GnRH-A stimulation of LHbeta-CAT activity. c-Src participates also in LHbeta-promoter activation by a mechanism which might be linked to ERK and JNK activation.

Gonadotropin subunit transcriptional responses to calcium signals in the rat: evidence for regulation by pulse frequency

Alterations in the frequency of calcium influx signals to rat pituitary cells can regulate the expression of gonadotropin subunit mRNAs in a differential manner, producing effects that are similar to those previously found for GnRH. The present study was conducted to investigate whether this reflects a transcriptional response to calcium pulse frequency, as determined by alterations in primary transcript (PT) expression. Perifused rat pituitary cells were given pulses of the calcium channel-activator Bay K 8644 (BK; with 10 mM KCl in the injectate) for 6 h. The response to alterations in pulse dose was examined by giving pulses of 1, 3, or 10 microM BK at 60-min intervals. Maximal increases in LHbeta and FSHbeta PTs were obtained with the 3-microM BK pulse dose and with the 10-microM dose for alpha. To investigate the effect of calcium pulse frequency, 3-microM BK pulses were given at intervals of 15, 60, or 180 min. Alpha PT was selectively stimulated by 15-min pulses and LHbeta by 15- and 60-min pulses of BK. In contrast, FSHbeta PT was maximally stimulated by the slower, 180-min pulse interval. These findings reveal that pulsatile increases in intracellular calcium stimulate alpha, LHbeta, and FSHbeta transcription in a differential manner. Thus, intermittent changes in intracellular calcium appear to be important in the transmission of GnRH pulse signals from the plasma membrane to the gene, and they may mediate the differential actions of pulse frequency on gonadotropin subunit gene expression.

Androgen suppression of GnRH-stimulated rat LHbeta gene transcription occurs through Sp1 sites in the distal GnRH-responsive promoter region

Steroids may regulate LH subunit gene transcription by modulating hypothalamic GnRH pulse patterns or by acting at the pituitary gonadotrope to alter promoter activity. We tested direct pituitary effects of the androgen dihydrotestosterone (DHT) to modulate the rat LHbeta promoter in transfected LbetaT2 clonal gonadotrope cells and in pituitaries of transgenic mice expressing LHbeta-luciferase. The LHbeta promoter (-617 to +44 bp)-luciferase construct was stimulated in LbetaT2 cells 7- to 10-fold by GnRH. Androgen treatment had little effect on basal promoter activity but suppressed GnRH stimulation by approximately 75%. GnRH stimulation of LHbeta was also suppressed by DHT in isolated pituitary cells from male or female mice with functional nuclear ARs, but not in male littermates with mutant AR. GnRH stimulation of the LHbeta promoter requires interactions between a complex distal response element containing two specificity protein-1 (Sp1) binding sites and a CArG box, and a proximal element with two bipartite binding sites for steroidogenic factor-1 and early growth response protein-1 (Egr-1). DHT effectively suppressed promoter constructs with an intact distal response element. The distal response element does not bind AR, but AR reduces Sp1 binding to this region. Glutathione-S-transferase pull-down studies demonstrated direct interactions of AR with Sp1, which requires the DNA-binding domain of AR, and weaker interactions with Egr-1. We conclude that androgen suppression of the rat LHbeta promoter occurs primarily through direct interaction of AR with Sp1, with some possible role through binding to Egr-1. These interactions result in interference with GnRH-stimulated gene transcription by reducing cooperation between the distal and proximal GnRH response elements.

Steroid-independent activation of ER by GnRH in gonadotrope pituitary cells

In the rat pituitary gland the mechanism responsible for ERalpha regulation has not been fully elucidated. Using transient transfection assays in alphaT3-1 cells, a cell line of gonadotrope origin, we show that GnRH stimulates estrogen response element-containing promoters in an estrogen-independent manner. This effect was strictly ER and GnRH receptor dependent, as no activation of the reporter gene was observed in presence of the anti-estrogen ICI 182,780 or a GnRH antagonist. These data suggest that the GnRH-triggered signaling pathway results in 17beta-estradiol-independent trans-activation of the ERalpha in alphaT3-1 cells. Furthermore, an additive activation was achieved when cells were treated with both GnRH and 17beta-estradiol. In primary pituitary cells, GnRH alone (100 nM) did not cause a significant stimulation of reporter gene activity, presumingly due to the low amount of gonadotropes. Interestingly, the combination of 17beta-estradiol and GnRH resulted in a significant increase in ERalpha trans-activation compared with that in cells treated with 17beta-estradiol alone. This enhancement was prevented by ICI 182,780, showing an ERalpha requirement. Moreover, we show that the effects of GnRH on ERalpha transcriptional activity in gonadotrope cell lines are mediated by the PKC/MAPK pathway. In conclusion, our data demonstrate that GnRH is an important signal in the regulation of ERalpha trans-activation in gonadotrope cells.