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

PACAP: A regulator of mammalian reproductive function

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an ancestral molecule that was isolated from sheep hypothalamic extracts based on its action to stimulate cAMP production by pituitary cell cultures. PACAP is one of a number of ligands that coordinate with GnRH to control reproduction. While initially viewed as a hypothalamic releasing factor, PACAP and its receptors are widely distributed, and there is growing evidence that PACAP functions as a paracrine/autocrine regulator in the CNS, pituitary, gonads and placenta, among other tissues. This review will summarize current knowledge concerning the expression and function of PACAP in the hypothalamic-pituitary-gonadal axis with special emphasis on its role in pituitary function in the fetus and newborn.

Signaling pathways and promoter regions that mediate pituitary adenylate cyclase activating polypeptide (PACAP) self-regulation in gonadotrophs

Pituitary adenylate cyclase-activating polypeptide (PACAP) is thought to play a role in the development and regulation of gonadotrophs. PACAP levels are very high in the rodent fetal pituitary, and decline substantially and rapidly at birth, followed by a significant rise in FSHβ and GnRH-R expression. Because there is evidence that PACAP stimulates its own transcription, we propose that this self-regulation is interrupted around the time of birth. To begin to examine the mechanisms for PACAP self-regulation, we used two well-established gonadotroph cell lines, αT3-1 cells and the more mature LβT2 cells which were transfected with a PACAP promoter-reporter construct As in vivo, the basal PACAP transcription level is significantly lower in the more mature LβT2 cells in which basal cAMP signaling is also much reduced. The PACAP promoter was stimulated by PACAP in both cell lines. Treatment with inhibitors of second messenger pathways implicated PKA, PKC and MAPK in PACAP transcription. Three regions of the PACAP promoter were found to confer inhibition or stimulation of PACAP transcription. By inhibiting cAMP response element binding (CREB) activity and mutating a proximal CREB binding site, we found that CREB is essential for promoter activation. Finally, overexpression of PACAP receptor HOP1 isoform, to increase the level in LβT2 cells to that of αT3-1 cells and simulate the E19 pituitary, increased PACAP- stimulated sensitivity and significantly altered downstream gene transcription. These results provide novel insight into the feed-forward regulation of PACAP expression that may help initiate gonadotroph function at birth.

PACAP induces FSHβ gene expression via EPAC

Gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are heterodimers of a common α subunit and unique β subunits. Regulation of their levels, primarily by GnRH, is critical for reproductive function. Several other hormones modulate gonadotropin expression, either independently or by modifying the responsiveness to GnRH. Pituitary adenylate cyclase activating peptide (PACAP) is one such hormone. Four-hour treatment of female mouse primary pituitary cells by either GnRH or PACAP induced FSHβ expression, while 24-h treatment repressed FSHβ. Both PACAP and GnRH caused FSH secretion into the medium. In the gonadotropes, PACAP activates primarily Gαs and increases concentration of cAMP, while GnRH primarily functions via Gαq and increases calcium concentration. Herein, we compared PACAP and GnRH signaling pathways that lead to the induction of FSHβ expression. Interestingly, constitutively active Gαs represses LHβ and induces FSHβ expression, while Gαq induces both β-subunits. We determined that FSHβ induction by PACAP requires functional EPAC, a cAMP sensor protein that serves as a guanine exchange factors for small G proteins that then bridges cAMP signaling to MAPK pathway. We further demonstrate that in addition to the prototypical small G protein Ras, two members of the Rho subfamily, Rac and CDC42 are also necessary for PACAP induction of FSHβ, likely via activation of p38 MAPK that leads to induction of cFOS, a critical transcription factor that is necessary and sufficient for FSHβ induction. Therefore, PACAP-induced cAMP pathway leads to MAPK activation that stimulates cFOS induction, to induce the expression of FSHβ subunit and increase FSH concentration.

Regulatory Architecture of the LβT2 Gonadotrope Cell Underlying the Response to Gonadotropin-Releasing Hormone

The LβT2 mouse pituitary cell line has many characteristics of a mature gonadotrope and is a widely used model system for studying the developmental processes and the response to gonadotropin-releasing hormone (GnRH). The global epigenetic landscape, which contributes to cell-specific gene regulatory mechanisms, and the single-cell transcriptome response variation of LβT2 cells have not been previously investigated. Here, we integrate the transcriptome and genome-wide chromatin accessibility state of LβT2 cells during GnRH stimulation. In addition, we examine cell-to-cell variability in the transcriptional response to GnRH using Gel bead-in-Emulsion Drop-seq technology. Analysis of a bulk RNA-seq data set obtained 45 min after exposure to either GnRH or vehicle identified 112 transcripts that were regulated >4-fold by GnRH (FDR < 0.05). The top regulated transcripts constitute, as determined by Bayesian massive public data integration analysis, a human pituitary-relevant coordinated gene program. Chromatin accessibility [assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq)] data sets generated from GnRH-treated LβT2 cells identified more than 58,000 open chromatin regions, some containing notches consistent with bound transcription factor footprints. The study of the most prominent open regions showed that 75% were in transcriptionally active promoters or introns, supporting their involvement in active transcription. Lhb, Cga, and Egr1 showed significantly open chromatin over their promoters. While Fshb was closed over its promoter, several discrete significantly open regions were found at -40 to -90 kb, which may represent novel upstream enhancers. Chromatin accessibility determined by ATAC-seq was associated with high levels of gene expression determined by RNA-seq. We obtained high-quality single-cell Gel bead-in-Emulsion Drop-seq transcriptome data, with an average of >4,000 expressed genes/cell, from 1,992 vehicle- and 1,889 GnRH-treated cells. While the individual cell expression patterns showed high cell-to-cell variation, representing both biological and measurement variation, the average expression patterns correlated well with bulk RNA-seq data. Computational assignment of each cell to its precise cell cycle phase showed that the response to GnRH was unaffected by cell cycle. To our knowledge, this study represents the first genome-wide epigenetic and single-cell transcriptomic characterization of this important gonadotrope model. The data have been deposited publicly and should provide a resource for hypothesis generation and further study.

GnRH Stimulates Peptidylarginine Deiminase Catalyzed Histone Citrullination in Gonadotrope Cells

Peptidylarginine deiminase (PAD) enzymes convert histone tail arginine residues to citrulline resulting in chromatin decondensation. Our previous work found that PAD isoforms are expressed in female reproductive tissues in an estrous cycle-dependent fashion, but their role in the anterior pituitary gland is unknown. Thus, we investigated PAD expression and function in gonadotrope cells. The gonadotrope-derived LβT2 cell line strongly expresses PAD2 at the protein level compared with other PAD isoforms. Consistent with this, PAD2 protein expression is highest during the estrous phase of the estrous cycle and colocalizes with the LH β-subunit in the mouse pituitary. Using the GnRH agonist buserelin (GnRHa), studies in LβT2 and mouse primary gonadotrope cells revealed that 30 minutes of stimulation caused distinct puncta of PAD2 to localize in the nucleus. Once in the nucleus, GnRHa stimulated PAD2 citrullinates histone H3 tail arginine residues at sites 2, 8, and 17 within 30 minutes; however, this effect and PAD2 nuclear localization was blunted by incubation of the cells with the pan-PAD inhibitor, biphenyl-benzimidazole-Cl-amidine. Given that PAD2 citrullinates histones in gonadotropes, we next analyzed the functional consequence of PAD2 inhibition on gene expression. Our results show that GnRHa stimulates an increase in LHβ and FSHβ mRNA and that this response is significantly reduced in the presence of the PAD inhibitor biphenyl-benzimidazole-Cl-amidine. Overall, our data suggest that GnRHa stimulates PAD2-catalyzed histone citrullination in gonadotropes to epigenetically regulate gonadotropin gene expression.

A new role for wilms tumor protein 1: differential activities of + KTS and -KTS variants to regulate LHβ transcription

Luteinizing hormone (LH) is synthesized and secreted throughout the reproductive cycle from gonadotrope cells in the anterior pituitary, and is required for steroidogenesis and ovulation. LH contains an α-subunit common with FSH, and a unique LHβ subunit that defines biological activity. Basal LHβ transcription is low and stimulated by hypothalamic GnRH, which induces synthesis of early growth response protein-1 (Egr1), and stimulates binding of transcription factors Egr1 and steroidogenic factor-1 (SF1) on the promoter. WT1 (Wilms tumor protein1) is a zinc finger transcription factor with an essential role in urogenital system development, and which regulates several reproductive genes via interactions with SF1 or binding to GC-rich elements such as Egr1 binding sites. We investigated a potential role for WT1 in LHβ transcription in clonal mouse gonadotrope LβT2 cells. WT1 was present in LβT2 and mouse pituitary cells, and protein bound to the endogenous LHβ promoter. Interestingly, mRNAs for WT1(+KTS), which contains a three amino-acid insertion between the 3rd and 4th zinc fingers, and the WT1 (-KTS) variant were both expressed at significant levels. WT1 mRNAs and protein were decreased approximately 50% by GnRH treatment, under conditions where Egr1 mRNA and protein, and LHβ transcription, were stimulated. Decreasing expression of mRNA for WT1 (-KTS) decreased stimulation of LHβ and Egr1 by GnRH, whereas decreasing both WT1 (-KTS) and (+KTS) increased endogenous LHβ transcription, and prevented LHβ but not Egr1 stimulation by GnRH, suggesting differing biological activities for the WT1 isoforms. Overexpression of WT1 showed that WT1(-KTS) enhanced LHβ promoter GnRH stimulation 2-to-3-fold and required the 3'Egr1 site, but WT1(+KTS) repressed both basal and GnRH-stimulated LHβ promoter activity by approximately 70%. Our data suggest that WT1 can modulate LHβ transcription, with differential roles for the two WT1 variants; WT1 (-KTS) enhances and WT1 (+KTS) suppresses transcription.

Growth differentiation factor 9 (GDF9) forms an incoherent feed-forward loop modulating follicle-stimulating hormone β-subunit (FSHβ) gene expression

Gonadotropin-releasing hormone (GnRH) is secreted in brief pulses from the hypothalamus and regulates follicle-stimulating hormone β-subunit (FSHβ) gene expression in pituitary gonadotropes in a frequency-sensitive manner. The mechanisms underlying its preferential and paradoxical induction of FSHβ by low frequency GnRH pulses are incompletely understood. Here, we identify growth differentiation factor 9 (GDF9) as a GnRH-suppressed autocrine inducer of FSHβ gene expression. GDF9 gene transcription and expression were preferentially decreased by high frequency GnRH pulses. GnRH regulation of GDF9 was concentration-dependent and involved ERK and PKA. GDF9 knockdown or immunoneutralization reduced FSHβ mRNA expression. Conversely, exogenous GDF9 induced FSHβ expression in immortalized gonadotropes and in mouse primary pituitary cells. GDF9 exposure increased FSH secretion in rat primary pituitary cells. GDF9 induced Smad2/3 phosphorylation, which was impeded by ALK5 knockdown and by activin receptor-like kinase (ALK) receptor inhibitor SB-505124, which also suppressed FSHβ expression. Smad2/3 knockdown indicated that FSHβ induction by GDF9 involved Smad2 and Smad3. FSHβ mRNA induction by GDF9 and GnRH was synergistic. We hypothesized that GDF9 contributes to a regulatory loop that tunes the GnRH frequency-response characteristics of the FSHβ gene. To test this, we determined the effects of GDF9 knockdown on FSHβ induction at different GnRH pulse frequencies using a parallel perifusion system. Reduction of GDF9 shifted the characteristic pattern of GnRH pulse frequency sensitivity. These results identify GDF9 as contributing to an incoherent feed-forward loop, comprising both intracellular and secreted components, that regulates FSHβ expression in response to activation of cell surface GnRH receptors.

PACAP modulates GnRH signaling in gonadotropes

Hypothalamic gonadotropin-releasing hormone is known to be critical for normal gonadotropin biosynthesis and secretion by the gonadotrope cells of the anterior pituitary gland. Additional regulation is provided by gonadal steroid feedback as well as by intrapituitary factors, such as activin and follistatin. Less well-appreciated is the role of pituitary adenylate-cyclase activating polypeptide (PACAP) as both a hypothalamic-pituitary releasing factor as well as an autocrine-paracrine factor within the pituitary. PACAP regulates gonadotropin expression alone and through modulation of GnRH responsiveness achieved by increases in GnRH receptor expression and interactions at the level of intracellular signaling pathways. In addition to direct effects on the gonadotrope, PACAP stimulates follistatin secretion by the folliculostellate cells and thereby contributes to differential expression of the gonadotropin subunits. Conversely, GnRH augments the ability of PACAP to regulate gonadotrope function by increasing pituitary PACAP and PACAP receptor expression. This review will summarize the current understanding of the mechanisms by which PACAP modulates gonadotrope function, with a focus on interactions with GnRH.

Homer1 alternative splicing is regulated by gonadotropin-releasing hormone and modulates gonadotropin gene expression

Hypothalamic gonadotropin-releasing hormone (GnRH) plays a critical role in reproductive physiology by regulating follicle-stimulating hormone (FSH) and luteinizing hormone (LH) gene expression in the pituitary. Analysis of gonadotrope deep-sequencing data identified a global regulation of pre-mRNA splicing by GnRH. Homer1, a gene encoding a postsynaptic density scaffolding protein, was selected for further study. Homer1 expresses a short splice form, Homer1a, and more-abundant long transcripts Homer1b/c. GnRH induced a modest increase in Homer1b/c expression and a dramatic increase in the Homer1a splice form. G protein knockdown studies suggested that the Homer1 induction, but not the regulated splicing, was Gαq/11 dependent. Phosphorylation of the splicing regulator SRp20 was found to be induced by GnRH. SRp20 depletion attenuated the GnRH-induced increase in the Homer1a-to-Homer1b/c ratio and modulated the effects of GnRH on FSHβ and LHβ expression. Homer1 gene knockdown resulted in increased GnRH-induced FSHβ and LHβ transcript levels. Furthermore, splice-form-specific reduction of Homer1b/c increased both FSHβ and LHβ mRNA induction, whereas reduction of Homer1a had the opposite effect on FSHβ induction. These results indicate that the regulation of Homer1 splicing by GnRH contributes to gonadotropin gene control.

GATA augments GNRH-mediated increases in Adcyap1 gene expression in pituitary gonadotrope cells

Pituitary adenylate cyclase-activating polypeptide 1 (PACAP or ADCYAP1) regulates gonadotropin biosynthesis and secretion, both alone and in conjunction with GNRH. Initially identified as a hypothalamic-releasing factor, ADCYAP1 subsequently has been identified in pituitary gonadotropes, suggesting it may act as an autocrine-paracrine factor in this tissue. GNRH has been shown to increase pituitary Adcyap1 gene expression through the interaction of CREB and jun/fos with CRE/AP1 cis-elements in the proximal promoter. In these studies, we were interested in identifying additional transcription factors and cognate cis-elements which regulate Adcyap1 gene promoter activity and chose to focus on the GATA family of transcription factors known to be critical for both pituitary cell differentiation and gonadotropin subunit expression. By transient transfection and electrophoretic mobility shift assay analysis, we demonstrate that GATA2 and GATA4 stimulate Adcyap1 promoter activity via a GATA cis-element located at position -191 in the rat Adcyap1 gene promoter. Furthermore, we show that addition of GATA2 or GATA4 significantly augments GNRH-mediated stimulation of Adcyap1 gene promoter activity in the gonadotrope LβT2 cell line. Conversely, blunting GATA expression with specific siRNA inhibits the ability of GNRH to stimulate ADCYAP1 mRNA levels in these cells. These data demonstrate a complex interaction between GNRH and GATA on ADCYAP1 expression, providing important new insights into the regulation of gonadotrope function.

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.

Androgen receptor drives transcription of rat PACAP in gonadotrope cells

Gonadotropin expression is precisely regulated within the hypothalamic-pituitary-gonadal axis through the complex interaction of neuropeptides, gonadal steroids. and both gonadal- and pituitary-derived peptides. In the anterior pituitary gland, the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) modulates gonadotropin biosynthesis and secretion, acting both alone and in conjunction with GnRH. Steroid hormone feedback also influences gonadotropin expression via both direct and indirect mechanisms. Evidence from nonpituitary tissues suggests that PACAP may be a target for gonadal steroid regulation. In the present study, we show that androgen markedly stimulates rat (r) PACAP promoter-reporter activity in the LβT2 mature mouse gonadotrope cell line. 5'-Serial deletion analysis of reporter constructs identifies 2 regions of androgen responsiveness located at (-915 to -818) and (-308 to -242) of the rPACAP promoter. Androgen receptor (AR) binds directly to DNA cis-elements in each of these regions in vitro. Site-directed mutagenesis of 3 conserved hormone response element half-sites straddling the (-308 to -242) region dramatically blunts androgen-dependent PACAP promoter activity and prevents AR binding at the mutated promoter element. Chromatin immunoprecipitation demonstrates that endogenous AR binds the homologous region on mouse chromatin in LβT2 cells in both the presence and absence of androgen. These data demonstrate that androgen stimulates PACAP gene expression in the pituitary gonadotrope via direct binding of AR to a specific cluster of evolutionarily conserved hormone response elements in the proximal rPACAP gene promoter. Thus, androgen regulation of pituitary PACAP expression may provide an additional layer of control over gonadotropin expression within the hypothalamic-pituitary-gonadal axis.

AMP-activated protein kinase is a key intermediary in GnRH-stimulated LHβ gene transcription

GnRH regulation of pituitary gonadotropin gene transcription is critical for fertility, and metabolic dysregulation is associated with reproductive disorders and altered hypothalamic-pituitary responses. Here, we examined signaling pathways in gonadotropes through which GnRH modulates gonadotropin levels, and potential common signaling pathways with insulin. Using LβT2 cells, we show that GnRH rapidly (5 minutes) triggers activating phosphorylation of AMP-activated protein kinase (AMPK) up to 5-fold; this stimulation is enhanced by insulin through increased total AMPKα levels and activity. GnRH also stimulated c-Jun N-terminal kinase (JNK) and ERK activation, whereas insulin alone stimulated Akt. Inhibition of AMPK activity by compound C, or diminishing AMPK levels by small interfering RNA against AMPKα, prevented GnRH-stimulated transcription of the endogenous LHβ gene and transfected LHβ promoter. Egr-1 (early growth response-1), a transcription factor required for LHβ expression, is synthesized in response to GnRH, and compound C prevents this induction. However, overexpression of Egr-1 in the presence of compound C did not restore GnRH stimulation of LHβ, suggesting that AMPK stimulation of transcription also occurs through additional mechanisms or signaling pathways. One such pathway may be JNK activation, because GnRH stimulation of JNK activity and LHβ transcription occurs more slowly than stimulation of AMPK activity, and AMPK inhibition by compound C or small interfering RNA also prevented GnRH-stimulated JNK phosphorylation. Finally, in primary mouse pituitary cells, GnRH also stimulates AMPK, and AMPK inhibition suppresses GnRH-stimulated LHβ transcription. These studies indicate a novel role for AMPK in GnRH-stimulated transcription in pituitary gonadotropes and a potential common mechanism for GnRH and metabolic modulation of fertility.

GnRH decreases adiponectin expression in pituitary gonadotropes via the calcium and PKA pathways

As endocrinologically active cells, adipocytes are capable of secreting various adipocytokines such as leptin, resistin, and adiponectin to impact metabolic function. Although adipocytes remain to be the primary site of synthesis and secretion, there is now growing evidence that supports the presence of adiponectin and its receptors within the hypothalamic-pituitary-gonadal axis, providing a possible link between obesity and abnormal reproductive physiology. It has been demonstrated that adiponectin may reduce gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus as well as modulate gonadal steroid hormone production. Furthermore, prior data indicate that adiponectin may play a role in decreasing luteinizing hormone secretion from pituitary gonadotropes. We aimed to identify the hormonal regulators of adiponectin and its receptors, AdipoR1 and AdipoR2, in pituitary gonadotropes using immortalized gonadotropic LβT2 cells and primary rat pituitary cells. Our study shows significant alterations in adiponectin expression across the estrous cycle. In addition, we present a novel finding that GnRH suppresses pituitary adiponectin expression via the calcium and protein kinase A intracellular pathways in both cultured rat primary pituitary cells and the LβT2 gonadotrope cell line. The GnRH did not alter expression of the adiponectin receptors, AdipoR1 and AdipoR2, in cultured gonadotropes. Expression of the adiponectin receptors, AdipoR1 and AdipoR2, was not altered by GnRH in cell culture but in vivo or in vitro. Our data suggest that gonadotrope function may be modulated by GnRH-mediated changes in adiponectin expression.

β-catenin regulates GnRH-induced FSHβ gene expression

The regulation of gonadotropin synthesis by GnRH plays an essential role in the neuroendocrine control of reproduction. The known signaling mechanisms involved in gonadotropin synthesis have been expanding. For example, involvement of β-catenin in LHβ induction by GnRH has been discovered. We examined the role of β-catenin in FSHβ gene expression in LβT2 gonadotrope cells. GnRH caused a sustained increase in nuclear β-catenin levels, which was significantly reduced by c-Jun N-terminal kinase (JNK) inhibition. Small interfering RNA-mediated knockdown of β-catenin mRNA demonstrated that induction of FSHβ mRNA by GnRH depended on β-catenin and that regulation of FSHβ by β-catenin occurred independently of the JNK-c-jun pathway. β-Catenin depletion had no impact on FSHβ mRNA stability. In LβT2 cells transfected with FSHβ promoter luciferase fusion constructs, GnRH responsiveness was conferred by the proximal promoter (-944/-1) and was markedly decreased by β-catenin knockdown. However, none of the T-cell factor/lymphoid enhancer factor binding sites in that region were required for promoter activation by GnRH. Chromatin immunoprecipitation further corroborated the absence of direct interaction between β-catenin and the 1.8-kb FSHβ promoter. To elucidate the mechanism for the β-catenin effect, we analyzed approximately 1 billion reads of next-generation RNA sequencing β-catenin knockdown assays and selected the nuclear cofactor breast cancer metastasis-suppressor 1-like (Brms1L) as one candidate for further study. Subsequent experiments confirmed that Brms1L mRNA expression was decreased by β-catenin knockdown as well as by JNK inhibition. Furthermore, knockdown of Brms1L significantly attenuated GnRH-induced FSHβ expression. Thus, our findings indicate that the expression of Brms1L depends on β-catenin activity and contributes to FSHβ induction by GnRH.

CREB binding protein (CBP) activation is required for luteinizing hormone beta expression and normal fertility in mice

Normal function of the hypothalamic-pituitary-gonadal axis is dependent on gonadotropin-releasing hormone (GNRH)-stimulated synthesis and secretion of luteinizing hormone (LH) from the pituitary gonadotroph. While the transcriptional coactivator CREB binding protein (CBP) is known to interact with Egr-1, the major mediator of GNRH action on the Lhb gene, the role of CBP in Lhb gene expression has yet to be characterized. We show that in the LβT2 gonadotroph cell line, overexpression of CBP augmented the response to GNRH and that knockdown of CBP eliminated GNRH responsiveness. While GNRH-mediated phosphorylation of CBP at Ser436 increased the interaction with Egr-1 on the Lhb promoter, loss of this phosphorylation site eliminated GNRH-mediated Lhb expression in LβT2 cells. In vivo, loss of CBP phosphorylation at Ser436 rendered female mice subfertile. S436A knock-in mice had disrupted estrous cyclicity and reduced responsiveness to GNRH. Our results show that GNRH-mediated phosphorylation of CBP at Ser436 is required for Egr-1 to activate Lhb expression and is a requirement for normal fertility in female mice. As CBP can be phosphorylated by other factors, such as insulin, our studies suggest that CBP may act as a key regulator of Lhb expression in the gonadotroph by integrating homeostatic information with GNRH signaling.

Targeted pituitary overexpression of pituitary adenylate-cyclase activating polypeptide alters postnatal sexual maturation in male mice

The neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) is present in high concentrations within the hypothalamus, suggesting that it may be a hypophysiotropic factor, whereas pituitary expression suggests a paracrine function. PACAP stimulates gonadotropin secretion and enhances GnRH responsiveness. PACAP increases gonadotropin α-subunit (αGSU), lengthens LHβ, but reduces FSHβ mRNA levels in adult pituitary cell cultures in part by increasing follistatin. PACAP stimulates LH secretion in rats; however, acceptance of PACAP as a regulator of reproduction has been limited by a paucity of in vivo studies. We created a transgenic mouse model of pituitary PACAP overexpression using the αGSU subunit promoter. Real-time PCR was used to evaluate PACAP, follistatin, GnRH receptor, and the gonadotropin subunit mRNA in male transgenic and wild-type mice of various ages. Transgenic mice had greater than 1000-fold higher levels of pituitary PACAP mRNA; and immunocytochemistry, Western blot, and ELISA analyses confirmed high peptide levels. FSH, LH, and testosterone levels were significantly suppressed, and the timing of puberty was substantially delayed in PACAP transgenic mice in which gonadotropin subunit and GnRH receptor mRNA levels were reduced and pituitary follistatin expression was increased. Microarray analyses revealed 1229 of 45102 probes were significantly (P < 0.01) different in pituitaries from PACAP transgenic mice, of which 83 genes were at least 2-fold different. Genes involved in small molecule biochemistry, cancer, and reproductive system diseases were the top associated networks. The GnRH signaling pathway was the top canonical pathway affected by pituitary PACAP excess. These experiments provide the first evidence that PACAP affects gonadotropin expression and sexual maturation in vivo.

Possible involvement of PACAP and PACAP type 1 receptor in GnRH-induced FSH β-subunit gene expression

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor, PACAP type 1 receptor (PAC1-R) play an important role in the induction of pituitary gonadotropins. In this present study, we examined whether the PAC1-R was involved in the action of gonadotropin-releasing hormone (GnRH) on gonadotropin FSHβ subunit expression. In a static culture, GnRH stimulation significantly increased PAC1-R expression as well as PACAP gene expression in the gonadotroph cell line, LβT2. Stimulation with low frequency GnRH pulses, which preferentially increase FSHβ, increased the expression of both the PAC1-R and the PACAP genes to a greater extent than did high frequency pulses. In the determination of transcriptional activity, the GnRH antagonist, cetrotide inhibited GnRH-induced FSHβ promoter activity completely, but PACAP6-38, a PACAP antagonist, had no effect on GnRH-induced FSHβ promoter activity. As expected, PACAP-induced FSHβ promoter activity was significantly prevented by PACAP6-38, but was not affected by cetrotide. PACAP6-38, however, significantly prevented GnRH-increased FSHβ mRNA expression. These observations suggest that GnRH-induced FSHβ gene expression is stimulated partially through PAC1-R by gonadotrophs producing PACAP or PAC1-R.

GnRH-induced PACAP and PAC1 receptor expression in pituitary gonadotrophs: a possible role in the regulation of gonadotropin subunit gene expression

We examined the expression of pituitary adenylate cyclase-activating polypeptide (PACAP) and the PACAP type 1 receptor (PAC1-R) mRNA following gonadotropin-releasing hormone (GnRH) stimulation using the gonadotroph cell line LbetaT2. GnRH stimulation increased PACAP and PAC1-R mRNA expression in a static culture. Increase in the cell surface density of the PAC1-R following transfection with PAC1-R expression vectors significantly increased gonadotropin LHbeta and FSHbeta subunit promoter activities following 100 nM PACAP stimulation. In addition, increasing concentrations of PACAP stimulation augmented the promoter activities for both LHbeta and FSHbeta in PAC-1R overexpressing cells. In the cells with PAC1-R, the effect of GnRH was further potentiated in the presence of PACAP from 5.31+/-0.93 to 9.89+/-0.38-fold for LHbeta and for FSHbeta subunit, respectively; from 2.58+/-0.31-fold by GnRH alone to 10.90+/-2.79-fold with PACAP. The combination treatment with GnRH and PACAP did not augment the ERK phosphorylation induced by GnRH alone. PACAP expectedly increased cAMP accumulation and this effect was significantly attenuated in the presence of GnRH. PACAP gene expression was more prominent following lower frequency GnRH pulses (every 120 min) in a perifused culture. Our results suggest that PACAP and PAC1-R are produced locally within the gonadotrophs following GnRH stimulation. They subsequently affect the gonadotrophs in an autocrine manner and modulate the GnRH pulse-dependent specific regulation of gonadotropin subunits.

Signaling responses to pulsatile gonadotropin-releasing hormone in LbetaT2 gonadotrope cells

The hypothalamic neuropeptide gonadotropin-releasing hormone (GnRH) is secreted in a pulsatile fashion by hypothalamic neurons, and alterations in pulse frequency and amplitude differentially regulate gonadotropin synthesis and release. In this study, we investigated the kinetics of G(s) and G(q) signaling in response to continuous or pulsatile GnRH using fluorescence resonance energy transfer reporters in live mouse LbetaT2 gonadotrope cells. cAMP and protein kinase A-dependent reporters showed a rapid but transient increase in fluorescence resonance energy transfer signal with increasing doses of constant GnRH, and in contrast diacylglycerol (DAG) and calcium reporters showed a rapid and sustained signal. Multiple pulses of GnRH caused multiple pulses of cAMP and protein kinase A activation without desensitization, but the DAG and calcium reporters were rapidly desensitized resulting in inhibition of calcium and DAG responses. At the transcriptional level, both a cAMP-dependent cAMP-response element reporter and a DAG/calcium-dependent AP-1 reporter showed a pulse frequency-dependent increase in luciferase activity. However, constant GnRH stimulation gave very little cAMP-response element activation but very strong AP-1 activation. Based on these data, we propose that both the GnRH-R-G(s) and G(q) pathways are responsive to pulses of GnRH, but only the G(q) pathway is responsive to constant GnRH. Furthermore, the G(q) pathway is subject to desensitization with multiple GnRH pulses, but the G(s) pathway is not.

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.

Novel actions of estrogen to promote proliferation: integration of cytoplasmic and nuclear pathways

Both steroids and growth factors stimulate proliferation of steroid-dependent tumor cells, and interaction between these signaling pathways occurs at several levels. Steroid receptors are classified as ligand-activated transcription factors, and steps by which they activate target gene transcription are well understood. Several steroid responses have now been functionally linked to other intracellular signaling pathways, including c-Src or tyrosine kinase receptors. Steroids such as 17beta-estradiol (E2), via binding to cytoplasmic or membrane-associated receptors, were also shown to rapidly activate intracellular signaling cascades such as ERK, PI3K and STATs. These E2-stimulated phosphorylations can then contribute to altered tumor cell function. ER-positive breast cancer cells, in which proliferation is stimulated by E2 and suppressed by antiestrogens, have been of particular interest in dissecting nuclear and cytoplasmic roles of estrogen receptors (ER). In some cell contexts, ER interacts directly with the intracellular tyrosine kinase c-Src and other cytoplasmic signaling and adaptor molecules, such as Shc, PI3K, MNAR, and p130 Cas. Although the hierarchy among these associations is not known, it is clear that c-Src plays a fundamental role in both growth factor and E2-stimulated cell growth, and this may also require other growth factor receptors such as those for EGF or IGF-1. STAT transcription factors represent one pathway to integrate E2 cytoplasmic and nuclear signaling. STAT5 is phosphorylated in the cytoplasm at an activating tyrosine in response to E2 or EGF, and then is translocated to the nucleus to stimulate target gene transcription. E2 stimulates recruitment of STAT5 and ER to the promoter of several proliferative genes, and STAT5 knockdown prevents recruitment of either protein to these promoters. STAT5 activation by E2 in breast cancer cells requires c-Src and EGF receptor, and inhibition of c-Src or EGFR, or knockdown of STAT5, prevents E2 stimulation of several genes and breast cancer cell proliferation. Hyperactivation of the growth factor receptor-c-Src pathway can in some contexts decrease growth responses to E2, or render cells and tumors resistant to suppressive actions of endocrine therapies. Crosstalk between growth factors and steroids in both the cytoplasm and nucleus may thus have a profound impact on complex biological processes such as cell growth, and may play a significant role in the treatment of steroid-dependent breast cancers.

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.

Decreased BRCA1 confers tamoxifen resistance in breast cancer cells by altering estrogen receptor-coregulator interactions

The breast cancer susceptibility gene 1 (BRCA1) is mutated in approximately 50% of hereditary breast cancers, and its expression is decreased in 30-40% of sporadic breast cancers, suggesting a general role in breast cancer development. BRCA1 physically and functionally interacts with estrogen receptor-alpha (ERalpha) and several transcriptional regulators. We investigated the relationship between cellular BRCA1 levels and tamoxifen sensitivity. Decreasing BRCA1 expression in breast cancer cells by small interfering RNA alleviated tamoxifen-mediated growth inhibition and abolished tamoxifen suppression of several endogenous ER-targeted genes. ER-stimulated transcription and cytoplasmic signaling was increased without detectable changes in ER or ER coregulator expression. Co-immunoprecipitation studies showed that with BRCA1 knockdown, tamoxifen-bound ERalpha was inappropriately associated with coactivators, and not effectively with corepressors. Chromatin immunoprecipitation studies demonstrated that with tamoxifen, BRCA1 knockdown did not change ERalpha promoter occupancy, but resulted in increased coactivator and decreased corepressor recruitment onto the endogenous cyclin D1 promoter. Our results suggest that decreased BRCA1 levels modify ERalpha-mediated transcription and regulation of cell proliferation in part by altering ERalpha-coregulator association. In the presence of tamoxifen, decreased BRCA1 expression results in increased coactivator and decreased corepressor recruitment on ER-regulated gene promoters.

Proteasome regulation of dynamic transcription factor occupancy on the GnRH-stimulated luteinizing hormone beta-subunit promoter

GnRH is the main modulator of LH secretion and transcription of the LH subunit genes in pituitary gonadotropes. The LHbeta gene is preferentially transcribed during pulsatile GnRH stimuli of one pulse/30 min and is thus carefully controlled by specific signaling pathways and transcription factors. We now show that GnRH-stimulated LHbeta transcription is also influenced by the ubiquitin-proteasome system. GnRH-stimulated activity of an LHbeta reporter gene was prevented by proteasome inhibitors MG-132 and lactacystin. Inhibition was not rescued by overexpression of two key transcription factors for LHbeta, early growth response-1 (Egr-1) and steroidogenic factor-1 (SF-1). Increased endogenous LHbeta transcription after GnRH treatment was also prevented by MG-132, as measured by primary transcript assays. To investigate possible mechanisms of LHbeta transcriptional inhibition by proteasome blockade, we employed chromatin immunoprecipitation to measure LHbeta promoter occupancy by transcription factors. Without GnRH, binding was low and unorganized. With GnRH, Egr-1 and SF-1 associations were stimulated, cyclic, and coincidental, with a period of approximately 30 min. MG-132 disrupted GnRH-induced Egr-1 and SF-1 binding and prevented phosphorylated RNA polymerase II association with the LHbeta promoter. Egr-1, but not SF-1, protein was induced by GnRH and accumulated with MG-132. Egr-1 and SF-1 were ubiquitinated in gonadotropes and ubiquitinated forms of these factors associated with the LHbeta promoter, suggesting their degradation may be key for LHbeta proteasome-dependent transcription. Together, these results demonstrate that degradation via the proteasome is vital to GnRH-stimulated LHbeta expression, and this occurs in part by allowing proper transcription factor associations with the LHbeta promoter.

Welcoming beta-catenin to the gonadotropin-releasing hormone transcriptional network in gonadotropes

GnRH binds its G-coupled protein receptor, GnRHR, on pituitary gonadotropes and stimulates transcription of Cga, Lhb, and Fshb. These three genes encode two heterodimeric glycoprotein hormones, LH and FSH, that act as gonadotropins by regulating gametogenesis and steroidogenesis in both the testes and ovary. GnRH also regulates transcription of Gnrhr. Thus, regulated expression of Cga, Lhb, Fshb, and Gnrhr provides a genomic signature unique to functional gonadotropes. Steadily increasing evidence now indicates that GnRH regulates transcription of its four signature genes indirectly through a hierarchical transcriptional network that includes distinct subclasses of DNA-binding proteins that comprise the immediate early gene (IEG) family. These IEGs, in turn, confer hormonal responsiveness to the four signature genes. Although the IEGs confer responsiveness to GnRH, they cannot act alone. Instead, additional DNA-binding proteins, including the orphan nuclear receptor steroidogenic factor 1, act permissively to allow the four signature genes to respond to GnRH-induced changes in IEG levels. Emerging new findings now indicate that beta-catenin, a transcriptional coactivator and member of the canonical WNT signaling pathway, also plays an essential role in transducing the GnRH signal by interacting with multiple DNA-binding proteins in gonadotropes. Herein we propose that these interactions with beta-catenin define a multicomponent transcriptional network required for regulated expression of the four signature genes of the gonadotrope, Cga, Lhb, Fshb, and Gnrhr.