Studies of gonadotropin-releasing hormone (GnRH) action using GnRH receptor-expressing pituitary cell lines

Unraveling Hypothalamus-Pituitary dysregulation: Hypergonadotropism in F1 progeny due to prenatal exposure to hexavalent chromium

The endocrine disruptor hexavalent chromium [Cr(VI)] is a proven reproductive toxicant. We recently demonstrated that prenatal Cr(VI) exposure causes testicular resistance to gonadotropins, resulting in hypergonadotropic hypoandrogenism in F1 rats. However, the mechanism driving hypergonadotropism in F1 rats exposed to Cr(VI) prenatally remains an enigma. Therefore, we hypothesized that 'Prenatal Cr(VI) exposure may disrupt steroid hormones-mediated negative feedback regulation of the hypothalamic GnRH, and its receptor in the pituitary of F1 rats, leading to hypergonadotropism.' We administered potassium dichromate (50, 100, or 200 mg/L) to pregnant rats through drinking water between days 9 and 14, and their male F1 offspring were euthanized at 60 days of age. Prenatal Cr(VI) exposure in F1 rats resulted in the accumulation of Cr in the hypothalamus and pituitary. Western blot detected decreased hypothalamic GnRH, Kisspeptin1, and its receptor GPR54, along with diminished ERα, AR, aromatase, and 5α reductase, and GnRH regulatory transcription factors Pit-1 and GATA-4 proteins. Immunohistochemical studies revealed increased immunopositivity of GnRH receptor, AR, 5α reductase, ERα, ERβ, and aromatase proteins in the pituitary, whereas decreased Kisspeptin1, GPR54, and inhibin β. Our findings imply that Cr(VI) exposure during the prenatal period disrupts the hypothalamic Kisspeptin-GPR54-Pit-1/GATA4-GnRH network, boosting the pituitary GnRH receptor. We conclude that prenatal exposure to Cr(VI) alters GnRH expression in the hypothalamus and its receptor in the pituitary of F1 progeny through interfering with the negative feedback effect of androgens and estrogens.

Impact of Ovariectomy on the Anterior Pituitary Gland in Female Rats

Ovariectomy (OVX) causes a depletion of circulating estradiol (E2) and influences hypothalamic kisspeptin neurons, which govern gonadotropin-releasing hormone (GnRH) release and ultimately gonadotropin secretion. In this study, we examined the changes induced by OVX on the anterior pituitary gland in female rats. OVX significantly increased the mRNA expression of gonadotropin α, luteinizing hormone (LH) β, and follicle-stimulating hormone (FSH) β subunits within the pituitary gland compared with control (sham-operated) rats, and this was completely suppressed by E2 supplementation. High-dose dihydrotestosterone supplementation also prevented the OVX-induced increase in the expression of the three gonadotropin subunits. GnRH receptor mRNA expression within the pituitary was significantly increased in OVX rats, and this increase was completely inhibited by E2 supplementation. The mRNA expression of the receptors for adenylate cyclase-activating polypeptide and kisspeptin was unchanged by OVX. Although the mRNA levels of inhibin α, βA, and βB subunits within the pituitary gland were not modulated by OVX, follistatin gene expression within the pituitary gland was increased by OVX, and this increase was completely inhibited by E2 supplementation after OVX. In experiments using a pituitary gonadotroph cell model (LβT2 cells), follistatin itself did not modulate the mRNA expression of gonadotropin LHβ and FSHβ subunits, and the GnRH-induced increase in the expression of these genes was slightly inhibited in the presence of follistatin. Our current observations suggest that OVX induces several characteristic changes in the pituitary gland of rats.

Epigenetic repression of gonadotropin gene expression via a GnRH-mediated DNA delivery system

The reproductive axis is activated by gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gonadotropes to secrete hormones that drive gonadal function and steroidogenesis. Thus repression of this axis, which is conserved across mammals and sexes, can reduce steroid levels and/or prevent reproduction. Steroid-dependent pathologies, including various cancers, are commonly treated with GnRH super-analogs which have long-term side-effects, while humane solutions for controlling reproduction in domestic and wild animal populations are lacking. GnRH-conjugated toxins are undergoing clinical trials for GnRHR-expressing cancer cells, and have been examined for gonadotrope ablation in animals, but showed low and/or transient effects and administration of toxins has many potential complications. Here we exploit GnRH targeting to gonadotropes to deliver DNA encoding an effector that induces gonadotropin gene repressive epigenetic modifications which are perpetuated over time. Several layers of specificity are endowed through targeting to GnRHR-expressing cells and due to local cleavage of the peptide packaging the DNA; the DNA-encoded effector is expressed and directed to the target genes by the DNA binding domain of a highly specific transcription factor. This design has multiple advantages over existing methods of shutting down the reproductive axis, and its modular design should allow adaptation for broad applications.

Roles of Gonadotropin Receptors in Sexual Development of Medaka

The gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are secreted from the pituitary and bind to the FSH receptor (FSHR) and LH receptor (LHR) to regulate gonadal development in vertebrates. Previously, using fshr-knockout (KO) medaka (Oryzias latipes), we demonstrated that FSH regulates ovarian development by elevating estrogen levels. However, the lhr-KO phenotype in medaka is poorly characterized. Here, we generated lhr-KO medaka using the transcription activator-like effector nuclease (TALEN) technique. We analyzed its phenotype and that of fshr-KO, lhr;fshr double-heterozygotes (double-hetero), and double-KO fish. All genetically male medaka displayed normal testes and were fertile, whereas fshr-KO and double-KO genetically female fish displayed small ovaries containing many early pre-vitellogenic oocytes and were infertile. Although lhr-KO genetically female fish had normal ovaries with full-grown oocytes, ovulation did not occur. Levels of 17α,20β-dihydroxy-4-pregnen-3-one, which is required for meiotic maturation of oocytes and sperm maturation in teleost fish, were significantly decreased in all KO female medaka ovaries except for double-heteros. Further, 17β-estradiol levels in fshr-KO and double-KO ovaries were significantly lower than those in double-heteros. These findings indicate that LH is necessary for oocyte maturation and FSH is necessary for follicle development, but that neither are essential for spermatogenesis in medaka.

Psychological aspects of infertility. A systematic review

Objective

Fertility may be defined as a capacity to conceive and produce offspring. Infertility is characterized by failure to establish a clinical pregnancy after 12 months of regular and unprotected sexual intercourse. Infertility concerns an estimated 8–12% of the global population, and is associated with factors including time of unwanted non-conception, age of female partner and number of diseases impacting fertility. Unexplained infertility is described as idiopathic. This study aimed to analyse and evaluate the influence of mental disorders, often considered as reasons for idiopathic infertility, on female and male fertility, including stress, depression, sleep and eating disorders, and addictions.

Methods

This systematic review comprised a search of MEDLINE, Cochrane and PubMed databases for relevant articles that were analysed by two independent reviewers.

Results

A total of 106 articles published between 1955–2019 were included. Mental disorders modify endocrine gland and immune system functioning at both the tissue and cellular level, and are negatively associated with female and male fertility.

Conclusion

Mental disorders may negatively impact female and male fertility. Further studies are required to explain the exact role and contribution of mental disorders to fertility.

Electron Paramagnetic Resonance Gives Evidence for the Presence of Type 1 Gonadotropin-Releasing Hormone Receptor (GnRH-R) in Subdomains of Lipid Rafts

This study investigated the effect of type 1 gonadotropin releasing hormone receptor (GnRH-R) localization within lipid rafts on the properties of plasma membrane (PM) nanodomain structure. Confocal microscopy revealed colocalization of PM-localized GnRH-R with GM₁-enriched raft-like PM subdomains. Electron paramagnetic resonance spectroscopy (EPR) of a membrane-partitioned spin probe was then used to study PM fluidity of immortalized pituitary gonadotrope cell line αT3-1 and HEK-293 cells stably expressing GnRH-R and compared it with their corresponding controls (αT4 and HEK-293 cells). Computer-assisted interpretation of EPR spectra revealed three modes of spin probe movement reflecting the properties of three types of PM nanodomains. Domains with an intermediate order parameter (domain 2) were the most affected by the presence of the GnRH-Rs, which increased PM ordering (order parameter (S)) and rotational mobility of PM lipids (decreased rotational correlation time (τc)). Depletion of cholesterol by methyl-β-cyclodextrin (methyl-β-CD) inhibited agonist-induced GnRH-R internalization and intracellular Ca²⁺ activity and resulted in an overall reduction in PM order; an observation further supported by molecular dynamics (MD) simulations of model membrane systems. This study provides evidence that GnRH-R PM localization may be related to a subdomain of lipid rafts that has lower PM ordering, suggesting lateral heterogeneity within lipid raft domains.

Troxerutin protects against DHT-induced polycystic ovary syndrome in rats

The exact pathogenesis of polycystic ovary syndrome (PCOS), the most common neuroendocrine disorder in women of reproductive age, has not been fully elucidated. Recent studies suggested that chronic inflammation and neurotransmitter disorder involved in the progress of PCOS. Troxerutin, a natural flavonoid, was reported to possess neuroprotective effect in several disease models by inhibiting inflammation or enhancing neurotrophic factor. In this study, we investigated the possible protective effect and mechanism of troxerutin in a dihydrotestosterone (DHT)-induced rat model of PCOS. The PCOS rat models were treated with troxerutin at a dose of 150 mg/kg or 300 mg/kg for up to 4 weeks. Results showed that 300 mg/kg troxerutin significantly decreased the body weight gain and improved the pathological changes of ovary induced by DHT. Meanwhile, the elevated gonadotrophin-releasing hormone (GnRH), gonadotrophin and testosterone in the serum of PCOS rats were reduced with the treatment of troxerutin. The expression of kisspeptin and NKB in arcuate nucleus and their receptors kiss1r and NK3r in GnRH positive neurons of median eminence were markedly decreased in troxerutin-treated rats. Of note, the GnRH inhibitory regulator GABA and stimulatory regulator glutamate were also restored to the normal level by troxerutin. The present study indicated that troxerutin may exhibit a protective effect in PCOS rat model via regulating neurotransmitter release.

Plasticity of Anterior Pituitary Gonadotrope Cells Facilitates the Pre-Ovulatory LH Surge

Gonadotropes cells located in the anterior pituitary gland are critical for reproductive fitness. A rapid surge in the serum concentration of luteinizing hormone (LH) secreted by anterior pituitary gonadotropes is essential for stimulating ovulation and is thus required for a successful pregnancy. To meet the requirements to mount the LH surge, gonadotrope cells display plasticity at the cellular, molecular and morphological level. First, gonadotrope cells heighten their sensitivity to an increasing frequency of hypothalamic GnRH pulses by dynamically elevating the expression of the GnRH receptor (GnRHR). Following ligand binding, GnRH initiates highly organized intracellular signaling cascades that ultimately promote the synthesis of LH and the trafficking of LH vesicles to the cell periphery. Lastly, gonadotrope cells display morphological plasticity, where there is directed mobilization of cytoskeletal processes towards vascular elements to facilitate rapid LH secretion into peripheral circulation. This mini review discusses the functional and organizational plasticity in gonadotrope cells including changes in sensitivity to GnRH, composition of the GnRHR signaling platform within the plasma membrane, and changes in cellular morphology. Ultimately, multimodal plasticity changes elicited by gonadotropes are critical for the generation of the LH surge, which is required for ovulation.

GnRH Antagonists Produce Differential Modulation of the Signaling Pathways Mediated by GnRH Receptors

Commercial gonadotropin-releasing hormone (GnRH) antagonists differ by 1-2 amino acids and are used to inhibit gonadotropin production during assisted reproduction technologies (ART). In this study, potencies of three GnRH antagonists, Cetrorelix, Ganirelix and Teverelix, in inhibiting GnRH-mediated intracellular signaling, were compared in vitro. GnRH receptor (GnRHR)-transfected HEK293 and neuroblastoma-derived SH-SY5Y cell lines, as well as mouse pituitary LβT2 cells endogenously expressing the murine GnRHR, were treated with GnRH in the presence or absence of the antagonist. We evaluated intracellular calcium (Ca²⁺) and cAMP increases, cAMP-responsive element binding-protein (CREB) and extracellular-regulated kinase 1 and 2 (ERK1/2) phosphorylation, β-catenin activation and mouse luteinizing-hormone β-encoding gene (Lhb) transcription by bioluminescence resonance energy transfer (BRET), Western blotting, immunostaining and real-time PCR as appropriate. The kinetics of GnRH-induced Ca²⁺ rapid increase revealed dose-response accumulation with potency (EC50) of 23 nM in transfected HEK293 cells, transfected SH-SY5Y and LβT2 cells. Cetrorelix inhibited the 3 × EC₅₀ GnRH-activated calcium signaling at concentrations of 1 nM-1 µM, demonstrating higher potency than Ganirelix and Teverelix, whose inhibitory doses fell within the 100 nM-1 µM range in both transfected HEK293 and SH-SY5Y cells in vitro. In transfected SH-SY5Y, Cetrorelix was also significantly more potent than other antagonists in reducing GnRH-mediated cAMP accumulation. All antagonists inhibited pERK1/2 and pCREB activation at similar doses, in LβT2 and transfected HEK293 cells treated with 100 nM GnRH. Although immunostainings suggested that Teverelix could be less effective than Cetrorelix and Ganirelix in inhibiting 1 µM GnRH-induced β-catenin activation, Lhb gene expression increase occurring upon LβT2 cell treatment by 1 µM GnRH was similarly inhibited by all antagonists. To conclude, this study has demonstrated Cetrorelix-, Ganirelix- and Teverelix-specific biased effects at the intracellular level, not affecting the efficacy of antagonists in inhibiting Lhb gene transcription.

Cytogenetic, Genomic, and Functional Characterization of Pituitary Gonadotrope Cell Lines

LβT2 and αT3-1 are important, widely studied cell line models for the pituitary gonadotropes that were generated by targeted tumorigenesis in transgenic mice. LβT2 cells are more mature gonadotrope precursors than αT3-1 cells. Microsatellite authentication patterns, chromosomal characteristics, and their intercellular variation have not been reported. We performed microsatellite and cytogenetic analysis of both cell types at early passage numbers. Short tandem repeat (STR) profiling was consistent with a mixed C57BL/6J × BALB/cJ genetic background, with distinct patterns for each cell type. Spectral karyotyping in αT3-1 cells revealed cell-to-cell variation in chromosome composition and pseudodiploidy. In LβT2 cells, chromosome counting and karyotyping demonstrated pseudotriploidy and high chromosomal variation among cells. Chromosome copy number variation was confirmed by single-cell DNA sequencing. Chromosomal compositions were consistent with a male sex for αT3-1 and a female sex for LβT2 cells. Among LβT2 stocks used in multiple laboratories, we detected two genetically similar but distinguishable lines via STR authentication, LβT2a and LβT2b. The two lines differed in morphological appearance, with LβT2a having significantly smaller cell and nucleus areas. Analysis of immediate early gene and gonadotropin subunit gene expression revealed variations in basal expression and responses to continuous and pulsatile GnRH stimulation. LβT2a showed higher basal levels of Egr1, Fos, and Lhb but lower Fos induction. Fshb induction reached significance only in LβT2b cells. Our study highlights the heterogeneity in gonadotrope cell line genomes and provides reference STR authentication patterns that can be monitored to improve experimental reproducibility and facilitate comparisons of results within and across laboratories.

GnRH-A Key Regulator of FSH

The hypothalamic decapeptide, GnRH, is the gatekeeper of mammalian reproductive development and function. Activation of specific, high-affinity cell surface receptors (GnRH receptors) on gonadotropes by GnRH triggers signal transduction cascades to stimulate the coordinated synthesis and secretion of the pituitary gonadotropins FSH and LH. These hormones direct gonadal steroidogenesis and gametogenesis, making their tightly regulated production and secretion essential for normal sexual maturation and reproductive health. FSH and LH are glycoprotein heterodimers comprised of a common α-subunit and a unique β-subunit (FSHβ and LHβ, respectively), which determines the biological specificity of the gonadotropins. The unique β-subunit is the rate-limiting step for the production of the mature gonadotropins. Therefore, FSH synthesis is regulated at the transcriptional level by Fshb gene expression. The overarching goal of this review is to expand our understanding of the mechanisms and pathways underlying the carefully orchestrated control of FSH synthesis and secretion by GnRH, focusing on the transcriptional regulation of the Fshb gene. Identification of these regulatory mechanisms is not only fundamental to our understanding of normal reproductive function but will also provide a context for the elucidation of the pathophysiology of reproductive disorders and infertility to lead to potential new therapeutic approaches.

Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling

Reproduction and fertility are regulated via hormones of the hypothalamic-pituitary-gonadal (HPG) axis. Control of this reproductive axis occurs at all levels, including the brain and pituitary, and allows for the promotion or inhibition of gonadal sex steroid secretion and function. In addition to guiding proper gonadal development and function, gonadal sex steroids also act in negative- and positive-feedback loops to regulate reproductive circuitry in the brain, including kisspeptin neurones, thereby modulating overall HPG axis status. Additional regulation is also provided by sex steroids made within the brain, including neuroprogestins. Furthermore, because reproduction and survival need to be coordinated and balanced, the HPG axis is able to modulate (and be modulated by) stress hormone signalling, including cortiscosterone, from the hypothalamic-pituitary-adrenal (HPA) axis. This review covers recent data related to the neural, hormonal and stress regulation of the HPG axis and emerging interactions between the HPG and HPA axes, focusing on actions at the level of the brain and pituitary.

Molecular regulation of follicle-stimulating hormone synthesis, secretion and action

Follicle-stimulating hormone (FSH) plays fundamental roles in male and female fertility. FSH is a heterodimeric glycoprotein expressed by gonadotrophs in the anterior pituitary. The hormone-specific FSHβ-subunit is non-covalently associated with the common α-subunit that is also present in the luteinizing hormone (LH), another gonadotrophic hormone secreted by gonadotrophs and thyroid-stimulating hormone (TSH) secreted by thyrotrophs. Several decades of research led to the purification, structural characterization and physiological regulation of FSH in a variety of species including humans. With the advent of molecular tools, availability of immortalized gonadotroph cell lines and genetically modified mouse models, our knowledge on molecular mechanisms of FSH regulation has tremendously expanded. Several key players that regulate FSH synthesis, sorting, secretion and action in gonads and extragonadal tissues have been identified in a physiological setting. Novel post-transcriptional and post-translational regulatory mechanisms have also been identified that provide additional layers of regulation mediating FSH homeostasis. Recombinant human FSH analogs hold promise for a variety of clinical applications, whereas blocking antibodies against FSH may prove efficacious for preventing age-dependent bone loss and adiposity. It is anticipated that several exciting new discoveries uncovering all aspects of FSH biology will soon be forthcoming.

Regulation of reproduction via tight control of gonadotropin hormone levels

Mammalian reproduction is controlled by the hypothalamic-pituitary-gonadal axis. GnRH from the hypothalamus regulates synthesis and secretion of gonadotropins, LH and FSH, which then control steroidogenesis and gametogenesis. In females, serum LH and FSH levels exhibit rhythmic changes throughout the menstrual or estrous cycle that are correlated with pulse frequency of GnRH. Lack of gonadotropins leads to infertility or amenorrhea. Dysfunctions in the tightly controlled ratio due to levels slightly outside the normal range occur in a larger number of women and are correlated with polycystic ovaries and premature ovarian failure. Since the etiology of these disorders is largely unknown, studies in cell and mouse models may provide novel candidates for investigations in human population. Hence, understanding the mechanisms whereby GnRH regulates gonadotropin hormone levels will provide insight into the physiology and pathophysiology of the reproductive system. This review discusses recent advances in our understanding of GnRH regulation of gonadotropin synthesis.

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.

The potential regulatory mechanisms of the gonadotropin-releasing hormone in gonadotropin transcriptions identified with bioinformatics analyses

BACKGROUND

The regulation of gonadotropin synthesis and release by gonadotropin-releasing hormone (GnRH) plays an essential role in the neuroendocrine control of reproduction. However, the mechanisms underlying gonadotropin regulation by GnRH pulse frequency and amplitude are still ambiguous. This study aimed to explore the molecular mechanisms and biological pathways associated with gonadotropin synthesis by GnRH pulse frequencies and amplitudes.

METHODS

Using GSE63251 datasets downloaded from the Gene Expression Omnibus (GEO), differentially expressed genes (DEGs) were screened by comparing the RNA expression from the GnRH pulse group, the GnRH tonic group and the control group. Pathway enrichment analyses of DEGs was performed, followed by protein-protein interaction (PPI) network construction. Furthermore, sub-network modules were constructed by ClusterONE and GO function and pathways analysed by DAVID. In addition, the relationship between the metabolic pathways and the GnRH pathway was verified in vitro.

RESULTS

In total, 531 common DEGs were identified in GnRH groups, including 290 up-regulated and 241 down-regulated genes. DEGs predominantly enriched in 16 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, including 11 up-regulated pathways (signallingsignallingmetabolic pathways, signallingand GnRH signalling pathway) and 5 down-regulated pathways (type II diabetes mellitus). Moreover, FBJ osteosarcoma oncogene (FOS) and jun proto-oncogene (JUN) had higher connectivity degrees in the PPI network. Three modules in the PPI were identified with ClusterONE. The genes in module 1 were significantly enriched in five pathways, including signallingthe insulin resistance and GnRH signalling pathway. The genes in modules 2 and 3 were mainly enriched in metabolic pathways and steroid hormone biosynthesis, respectively. Finally, knockdown leptin receptor (LEPR) and insulin receptor (INSR) reversed the GnRH-modulated metabolic related-gene expression.

CONCLUSIONS

The present study revealed the involvement of GnRH in the regulation of gonadotropin biosynthesis and metabolism in the maintenance of reproduction, achieved by bioinformatics analyses. This, indicates that the GnRH signalling pathway played a central linkings role in reproductive function and metabolic balance. In addition, the present study identified the difference response between GnRH pulse and GnRH tone, indicated that abnormal GnRH pulse and amplitude may cause disease, which may provide an improved understanding of the GnRH pathway and a new insight for disease diagnosis and treatment.

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.

Interactions between Two Different G Protein-Coupled Receptors in Reproductive Hormone-Producing Cells: The Role of PACAP and Its Receptor PAC1R

Gonadotropin-releasing hormone (GnRH) and gonadotropins are indispensable hormones for maintaining female reproductive functions. In a similar manner to other endocrine hormones, GnRH and gonadotropins are controlled by their principle regulators. Although it has been previously established that GnRH regulates the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—both gonadotropins—from pituitary gonadotrophs, it has recently become clear that hypothalamic GnRH is under the control of hypothalamic kisspeptin. Prolactin, which is also known as luteotropic hormone and is released from pituitary lactotrophs, stimulates milk production in mammals. Prolactin is also regulated by hypothalamic factors, and it is thought that prolactin synthesis and release are principally under inhibitory control by dopamine through the dopamine D2 receptor. In addition, although it remains unknown whether it is a physiological regulator, thyrotropin-releasing hormone (TRH) is a strong secretagogue for prolactin. Thus, GnRH, LH and FSH, and prolactin are mainly regulated by hypothalamic kisspeptin, GnRH, and TRH, respectively. However, the synthesis and release of these hormones is also modulated by other neuropeptides in the hypothalamus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic peptide that was first isolated from sheep hypothalamic extracts based on its ability to stimulate cAMP production in anterior pituitary cells. PACAP acts on GnRH neurons and pituitary gonadotrophs and lactotrophs, resulting in the modulation of their hormone producing/secreting functions. Furthermore, the presence of the PACAP type 1 receptor (PAC1R) has been demonstrated in these cells. We have examined how PACAP and PAC1R affect GnRH- and pituitary hormone-secreting cells and interact with their principle regulators. In this review, we describe our understanding of the role of PACAP and PAC1R in the regulation of GnRH neurons, gonadotrophs, and lactotrophs, which are regulated mainly by kisspeptin, GnRH, and TRH, respectively.

Dynamin Is Required for GnRH Signaling to L-Type Calcium Channels and Activation of ERK

We have shown that GnRH-mediated engagement of the cytoskeleton induces cell movement and is necessary for ERK activation. It also has previously been established that a dominant negative form of the mechano-GTPase dynamin (K44A) attenuates GnRH activation of ERK. At present, it is not clear at what level these cellular events might be linked. To explore this, we used live cell imaging in the gonadotrope-derived αT3-1 cell line to determine that dynamin-green fluorescent protein accumulated in GnRH-induced lamellipodia and plasma membrane protrusions. Coincident with translocation of dynamin-green fluorescent protein to the plasma membrane, we demonstrated that dynamin colocalizes with the actin cytoskeleton and the actin binding protein, cortactin at the leading edge of the plasma membrane. We next wanted to assess the physiological significance of these findings by inhibiting dynamin GTPase activity using dynasore. We find that dynasore suppresses activation of ERK, but not c-Jun N-terminal kinase, after exposure to GnRH agonist. Furthermore, exposure of αT3-1 cells to dynasore inhibited GnRH-induced cyto-architectural rearrangements. Recently it has been discovered that GnRH induced Ca(2+) influx via the L-type Ca(2+) channels requires an intact cytoskeleton to mediate ERK phosphorylation. Interestingly, not only does dynasore attenuate GnRH-mediated actin reorganization, it also suppresses Ca(2+) influx through L-type Ca(2+) channels visualized in living cells using total internal reflection fluorescence microscopy. Collectively, our data suggest that GnRH-induced membrane remodeling events are mediated in part by the association of dynamin and cortactin engaging the actin cytoskeleton, which then regulates Ca(2+) influx via L-type channels to facilitate ERK phosphorylation.

Rapid communication: A microRNA-132/212 pathway mediates GnRH activation of FSH expression

GnRH plays a key role in the vertebrate reproductive system by stimulating biosynthesis and secretion of pituitary gonadotropins. However, the potential involvement of microRNAs (miRNAs) on this activation has still to be explored. In this study, we investigated the role of miRNA-132 and miRNA-212, two tandemly expressed miRNAs that target the same transcripts, on GnRH-induced FSH expression. We first showed that the GnRH stimulation of FSH secretion was reduced and Fshb mRNA abolished by blocking miR-132/212 action in rat pituitary cells. In mouse LβT2 gonadotrope cells, the GnRH stimulation of Fshb mRNA was also demonstrated to be dependent on miR-132/212 and reproduced by overexpressing one or both miRNAs. We then showed that the miR-132/212-mediated action of GnRH involved a posttranscriptional decrease of sirtuin 1 (SIRT1) deacetylase. The lower level of SIRT1 deacetylase correlated with an increase in the acetylated form of Forkhead Box O1 (FOXO1), a transcriptional repressor of Fshb. Interestingly, we show that the acetylated mimicking mutant of FOXO1 was localized outside the nucleus, thus alleviating its repressive effect on Fshb transcription. Overall, we demonstrate that the GnRH stimulation of Fshb expression is dependent on miR-132/212 and involves a SIRT1-FOXO1 pathway. This is the first demonstration of an obligatory microRNA pathway in the GnRH-regulated expression of a gonadotropin gene.

KISS1R signals independently of Gαq/11 and triggers LH secretion via the β-arrestin pathway in the male mouse

Hypothalamic GnRH is the master regulator of the neuroendocrine reproductive axis, and its secretion is regulated by many factors. Among these is kisspeptin (Kp), a potent trigger of GnRH secretion. Kp signals via the Kp receptor (KISS1R), a Gαq/11-coupled 7-transmembrane-spanning receptor. Until this study, it was understood that KISS1R mediates GnRH secretion via the Gαq/11-coupled pathway in an ERK1/2-dependent manner. We recently demonstrated that KISS1R also signals independently of Gαq/11 via β-arrestin and that this pathway also mediates ERK1/2 activation. Because GnRH secretion is ERK1/2-dependent, we hypothesized that KISS1R regulates GnRH secretion via both the Gαq/11- and β-arrestin-coupled pathways. To test this hypothesis, we measured LH secretion, a surrogate marker of GnRH secretion, in mice lacking either β-arrestin-1 or β-arrestin-2. Results revealed that Kp-dependent LH secretion was significantly diminished relative to wild-type mice (P < .001), thus supporting that β-arrestin mediates Kp-induced GnRH secretion. Based on this, we hypothesized that Gαq/11-uncoupled KISS1R mutants, like L148S, will display Gαq/11-independent signaling. To test this hypothesis, L148S was expressed in HEK 293 cells. and results confirmed that, although strongly uncoupled from Gαq/11, L148S retained the ability to trigger significant Kp-dependent ERK1/2 phosphorylation (P < .05). Furthermore, using mouse embryonic fibroblasts lacking β-arrestin-1 and -2, we demonstrated that L148S-mediated ERK1/2 phosphorylation is β-arrestin-dependent. Overall, we conclude that KISS1R signals via Gαq/11 and β-arrestin to regulate GnRH secretion. This novel and important finding could explain why patients bearing some types of Gαq/11-uncoupled KISS1R mutants display partial gonadotropic deficiency and even a reversal of the condition, idiopathic hypogonadotropic hypogonadism.

Roles of binding elements, FOXL2 domains, and interactions with cJUN and SMADs in regulation of FSHβ

We previously identified FOXL2 as a critical component in FSHβ gene transcription. Here, we show that mice deficient in FOXL2 have lower levels of gonadotropin gene expression and fewer LH- and FSH-containing cells, but the same level of other pituitary hormones compared to wild-type littermates, highlighting a role of FOXL2 in the pituitary gonadotrope. Further, we investigate the function of FOXL2 in the gonadotrope cell and determine which domains of the FOXL2 protein are necessary for induction of FSHβ transcription. There is a stronger induction of FSHβ reporter transcription by truncated FOXL2 proteins, but no induction with the mutant lacking the forkhead domain. Specifically, FOXL2 plays a role in activin induction of FSHβ, functioning in concert with activin-induced SMAD proteins. Activin acts through multiple promoter elements to induce FSHβ expression, some of which bind FOXL2. Each of these FOXL2-binding sites is either juxtaposed or overlapping with a SMAD-binding element. We determined that FOXL2 and SMAD4 proteins form a higher order complex on the most proximal FOXL2 site. Surprisingly, two other sites important for activin induction bind neither SMADs nor FOXL2, suggesting additional factors at work. Furthermore, we show that FOXL2 plays a role in synergistic induction of FSHβ by GnRH and activin through interactions with the cJUN component of the AP1 complex that is necessary for GnRH responsiveness. Collectively, our results demonstrate the necessity of FOXL2 for proper FSH production in mice and implicate FOXL2 in integration of transcription factors at the level of the FSHβ promoter.

Influence of stress-induced intermediates on gonadotropin gene expression in gonadotrope cells

Despite extensive investigation, a comprehensive understanding of the mechanisms whereby stress impacts fertility remains elusive. Since the 1930s, when Hans Selye popularized studying adaptations to stress (Selye, 1937), we have learned that compensatory mechanisms involve a complex interplay of neural and hormonal processes that allow various body functions to adjust to stress, in a coordinated manner. In terms of reproduction, the adjustment to a stressor interferes with integrated functioning at multiple levels of regulation--the hypothalamus, anterior pituitary gland, gonads, and neural centers coordinating behavior. Various mediators are postulated to participate in reproductive suppression. These include catecholamines, cytokines, prostaglandins, endogenous opioid peptides, and hormones of the hypothalamic-pituitary-adrenal axis. This review focuses on one class of mediators, the glucocorticoids, and provides our views on the relevance and mode of action of this inhibitory intermediate within the anterior pituitary gonadotrope, as a potential cellular site whereby glucocorticoids contribute to stress-induced reproductive suppression.

Interaction of gonadal steroids and gonadotropin-releasing hormone on pituitary adenylate cyclase-activating polypeptide (PACAP) and PACAP receptor expression in cultured rat anterior pituitary cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are expressed in the hypothalamus, the gonadotrope cells of the anterior pituitary gland, and the gonads, forming an autocrine-paracrine system in these tissues. Within the pituitary, PACAP functions either alone or synergistically with gonadotropin-releasing hormone (GnRH) to stimulate gonadotropin gene expression and secretion. Our goal was to define the hormonal regulation of pituitary PACAP and PACAP receptor (PAC1) gene expression by dihydrotestosterone (DHT), estradiol, and progesterone alone or in conjunction with GnRH. Treatment of adult male rat pituitary cell cultures with DHT or progesterone augmented GnRH-mediated increase in PACAP messenger RNA (mRNA) levels, but neither had an effect when present alone. Conversely, estradiol treatment blunted PACAP gene expression but did not alter GnRH effects on PACAP expression. Expression of PACAP receptor mRNA was decreased by GnRH treatment, minimally increased by DHT treatment, but not altered by the addition of estradiol or progesterone. DHT and GnRH together blunted PACAP receptor gene expression. Taken together, these results suggest that the activity of the intrapituitary PACAP-PAC1 system is regulated via the complex interaction of gonadal steroids and hypothalamic GnRH.

Association of obesity with hormonal imbalance in infertility: a cross-sectional study in north Indian women

Hormones play an important role in the development and regulation of reproductive function and the menstrual cycle of women. Extremes of body weight tend to affect the homeostasis of the hypothalamo-pituitary-gonadal axis. This cross-sectional study was carried out in 113 women (57 with primary infertility and 56 with secondary infertility) in the age group 20-35 years, presenting for hormonal evaluation of infertility in a tertiary care hospital. After preliminary clinical evaluation, anthropometric indices (height, weight, BMI, waist circumference and waist hip ratio) were measured in all subjects. Fasting blood sample drawn on second/third day of menstrual cycle was analysed for serum luteinizing hormone, follicle stimulating hormone (FSH), prolactin and thyroid stimulating hormone (TSH). Serum FSH levels showed a significant positive correlation with indicators of central obesity (waist circumference and waist hip ratio in both the study groups). In primary infertility, significant positive correlation was also observed between serum FSH levels and other markers of obesity like body weight, hip circumference and BMI. In secondary infertility, serum prolactin and serum TSH levels demonstrated a significant positive correlation with body weight and BMI. Obesity is associated with hormonal derangements which are responsible for infertility. In overweight women with infertility, weight loss should be considered as a first line treatment.

Phosphorylation of ATF2 and interaction with NFY induces c-Jun in the gonadotrope

Induction of c-Jun and c-Fos, partners that comprise the AP1 transcription factor, is critical for GnRH regulation of FSHβ gene expression. The signaling pathways that are necessary for regulation of AP1 in the gonadotrope cell are not known. Here, we investigate the mechanism of c-Jun induction by GnRH, the sole regulator of c-Jun in the gonadotrope. We identify that GnRH phosphorylates ATF2 via p38 and JNK, the same pathways responsible for GnRH induction of c-Jun. Upon phosphorylation, ATF2 binds the CRE element within the c-Jun proximal promoter and interacts with NFY. Functional ATF2 is necessary for both GnRH induction of c-Jun and FSHβ. Taken together, these studies elucidate the specificity of c-Jun induction by GnRH in the gonadotrope by demonstrating GnRH activation of the p38 and JNK signaling pathways that lead to phosphorylation of ATF2, providing critical insight into GnRH regulation of its target gene, the gonadotropin subunit FSHβ.

GnRH increases c-Fos half-life contributing to higher FSHβ induction

GnRH is a potent hypothalamic regulator of gonadotropin hormones, LH and FSH, which are both expressed within the pituitary gonadotrope and are necessary for the stimulation of gametogenesis and steroidogenesis in the gonads. Differential regulation of LH and FSH, which is essential for reproductive fitness, is achieved, in part, through the varying of GnRH pulse frequency. However, the mechanism controlling the increase in FSH during the periods of low GnRH has not been elucidated. Here, we uncover another level of regulation by GnRH that contributes to differential expression of the gonadotropins and may play an important role for the generation of the secondary rise of FSH that stimulates folliculogenesis. GnRH stimulates LHβ and FSHβ subunit transcription via induction of the immediate early genes, Egr1 and c-Fos, respectively. Here, we determined that GnRH induces rapidly both Egr1 and c-Fos, but specifically decreases the rate of c-Fos degradation. In particular, GnRH modulates the rate of c-Fos protein turnover by inducing c-Fos phosphorylation through the ERK1/2 pathway. This extends the half-life of c-Fos, which is normally rapidly degraded. Confirming the role of phosphorylation in promoting increased protein activity, we show that a c-Fos mutant that cannot be phosphorylated by GnRH induces lower expression of the FHSβ promoter than wild-type c-Fos. Our studies expand upon the role of GnRH in the regulation of gonadotropin gene expression by highlighting the role of c-Fos posttranslational modification that may cause higher levels of FSH during the time of low GnRH pulse frequency to stimulate follicular growth.

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

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

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

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

G protein-coupled receptors involved in GnRH regulation: molecular insights from human disease

In the past two decades, an increasing body of evidence has demonstrated that several G protein-coupled receptor (GPCR)-ligand pairs are critical for normal human reproductive development and function. Patients harboring genetic insults in either the receptors or their cognate ligands have presented with reproductive disorders characterized by varying degrees of GnRH deficiency. These disorders include idiopathic hypogonadotropic hypogonadism (IHH) and Kallmann Syndrome (KS). Conversely, mutations in some of these ligand-receptor pairs have been associated with accelerated reproductive maturation, manifested as central precocious puberty (CPP). To date, a series of elegant studies have characterized four GPCRs that play important roles in the neuroendocrine control of human reproductive development and function: GnRHR, KISS1R, PROKR2 and NK3R. Furthermore, these studies provide insights into the mechanisms by which mutations in these receptors give rise to reproductive disease phenotypes. This report will review mutations identified in GPCRs involved in the neuroendocrine control of the human reproductive axis with the aims of elucidating structure-function relationships of these GPCRs and identifying correlations between these structure-function relationships and the genotypic-phenotypic characterization of the patients.

Gonadotropin-releasing hormone pulse sensitivity of follicle-stimulating hormone-beta gene is mediated by differential expression of positive regulatory activator protein 1 factors and corepressors SKIL and TGIF1

Gonadotropin synthesis and release is dependent on pulsatile stimulation by the hypothalamic neuropeptide GnRH. Generally, slow GnRH pulses promote FSH production, whereas rapid pulses favor LH, but the molecular mechanism underlying this pulse sensitivity is poorly understood. In this study, we developed and tested a model for FSHβ regulation in mouse LβT2 gonadotropes. By mining a previous microarray data set, we found that mRNA for positive regulators of Fshb expression, such as Fos and Jun, were up-regulated at slower pulse frequencies than a number of potential negative regulators, such as the corepressors Skil, Crem, and Tgif1. These latter corepressors reduced Fshb promoter activity whether driven by transfection of individual transcription factors or by treatment with GnRH and activin. Overexpression of binding or phosphorylation-defective ski-oncogene-like protein (SKIL) and TG interacting factor (TGIF1) mutants, however, failed to repress Fshb promoter activity. Knockdown of the endogenous repressors SKIL and TGIF1, but not cAMP response element-modulator, increased Fshb promoter activity driven by constant GnRH or activin. Chromatin immunoprecipitation analysis showed that FOS, SKIL, and TGIF1 occupy the FSHβ promoter in a cyclical manner after GnRH stimulation. Overexpression of corepressors SKIL or TGIF1 repressed induction of the Fshb promoter at the slow GnRH pulse frequency but had little effect at the fast pulse frequency. In contrast, knockdown of endogenous SKIL or TGIF1 selectively increased Fshb mRNA at the fast GnRH pulse frequency. Therefore, we propose a potential mechanism by which production of gonadotropin Fshb is modulated by positive transcription factors and negative corepressors with different pulse sensitivities.

Gonadotropin-releasing hormone induces miR-132 and miR-212 to regulate cellular morphology and migration in immortalized LbetaT2 pituitary gonadotrope cells

GnRH is central to the regulation of reproductive function. It acts on pituitary gonadotropes to stimulate LH and FSH synthesis and secretion. We had previously presented evidence for translational control of LHβ synthesis; therefore we investigated whether micro-RNAs might play a role in GnRH regulation in LβT2 cells. We show here that GnRH strongly induces the AK006051 gene transcript that encodes two micro-RNAs, miR-132 and miR-212, within the first intron. We show furthermore that the AK006051 promoter region is highly GnRH responsive. We verify that the p250Rho GTPase activating protein (GAP) is a target of miR-132/212 and show that GnRH treatment leads to a decrease in mRNA and protein expression. This reduction is blocked by an anti-miR to miR-132/212 and mimicked by a pre-miR-132. GnRH inhibits p250RhoGAP expression through a miR-132/212 response element within the 3'-untranslated region. The loss of p250RhoGAP expression leads to activation of Rac and marked increases in both the number and length of neurite-like processes extending from the cell. Knockdown of p250RhoGAP by small interfering RNA induces the same morphological changes observed with GnRH treatment. In addition, loss of p250RhoGAP causes an increase in cellular motility. Our findings suggest a novel pathway regulating long-term changes in cellular motility and process formation via the GnRH induction of miR-132/212 with the subsequent down-regulation of p250RhoGAP.

GnRH induces the c-Fos gene via phosphorylation of SRF by the calcium/calmodulin kinase II pathway

Despite extensive studies on GnRH regulation of the gonadotropin subunit genes, very little is known about mechanism of induction of intermediary immediate early genes, such as c-Fos, that are direct targets of GnRH signaling and that upon induction, activate transcription of gonadotropin genes. Although c-Fos is induced by a variety of stimuli in other cell types, in the gonadotropes, only GnRH induces c-Fos and through it FSHβ. Thus, understanding the specificity of c-Fos induction by GnRH will provide insight into GnRH regulation of FSHβ gene expression. GnRH induction of c-Fos in LβT2 cells requires the serum response factor (SRF)-binding site, but not the Ets/ELK1 site. This is in contrast to c-Fos induction by growth factors in other cells, which activate c-Fos transcription via phosphorylation of ELK1 and require the ELK1-binding site. The SRF site alone is sufficient for induction by GnRH, whereas induction by 12-tetradecanoylphorbol-13-acetate (TPA) requires both the ELK1 and SRF sites. Although ELK1 site is not required, upon GnRH stimulation, ELK1 interacts with SRF and is recruited to the SRF site. GnRH phosphorylates ELK1 through ERK1/2 and p38 MAPK, which correlates with the signaling pathways necessary for c-Fos and FSHβ induction. GnRH also causes phosphorylation of SRF through calmodulin-dependent kinase II (CamKII), which leads to increased binding to its site. CamKII activation is sufficient for phosphorylation of SRF and for induction of the c-Fos gene through the SRF site. Thus, GnRH uses a combination of growth factor signaling and the CamKII pathway to induce c-Fos to regulate FSHβ gene expression in gonadotrope cells.

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

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

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.

Frequency-dependent regulation of follicle-stimulating hormone beta by pulsatile gonadotropin-releasing hormone is mediated by functional antagonism of bZIP transcription factors

Oscillatory synthesis and secretion of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), under the control of pulsatile hypothalamic gonadotropin-releasing hormone (GnRH), is essential for normal reproductive development and fertility. The molecular mechanisms by which various patterns of pulsatile GnRH regulate gonadotrope responsiveness remain poorly understood. In contrast to the alpha and LH beta subunit genes, FSH beta subunit transcription is preferentially stimulated at low rather than high frequencies of pulsatile GnRH. In this study, mutation of a cyclic AMP response element (CRE) within the FSH beta promoter resulted in the loss of preferential GnRH stimulation at low pulse frequencies. We hypothesized that high GnRH pulse frequencies might stimulate a transcriptional repressor(s) to attenuate the action of CRE binding protein (CREB) and show that inducible cAMP early repressor (ICER) fulfills such a role. ICER was not detected under basal conditions, but pulsatile GnRH stimulated ICER to a greater extent at high than at low pulse frequencies. ICER binds to the FSH beta CRE site to reduce CREB occupation and abrogates both maximal GnRH stimulation and GnRH pulse frequency-dependent effects on FSH beta transcription. These data suggest that ICER production antagonizes the stimulatory action of CREB to attenuate FSH beta transcription at high GnRH pulse frequencies, thereby playing a critical role in regulating cyclic reproductive function.

Insulin augments gonadotropin-releasing hormone induction of translation in LbetaT2 cells

The integrated signaling of insulin and gonadotropin-releasing hormone in the pituitary gonadotropes may have a profound bearing on reproductive function, although the cross-receptor signaling mechanisms are unclear. We demonstrate that the insulin receptor is constitutively localized to non-caveolar lipid raft microdomains in the pituitary gonadotrope cell line LbetaT2. The localization to rafts is consistent with similar localization of the GnRH receptor. Insulin receptor phosphorylation occurs in raft domains and activates the downstream signaling targets Insulin Receptor Substrate1 and Akt/Protein Kinase B. Although insulin alone does not strongly activate the extracellular signal-regulated kinase second messenger cascade, co-stimulation potentiates the phosphorylation of the extracellular signal-regulated kinase by gonadotropin-releasing hormone. The co-stimulatory effect of insulin and gonadotropin-releasing hormone is also evident in increased activation of cap-dependent translation. In contrast, co-stimulation attenuates Akt/Protein Kinase B activation. Our results show that both gonadotropin-releasing hormone and insulin are capable of mutually altering their respective regulatory signaling cascades. We suggest that this provides a mechanism to integrate neuropeptide and energy homeostatic signals to modulate reproductive function.

Aging attenuates the pituitary response to gonadotropin-releasing hormone

CONTEXT

Complex changes in GnRH secretion occur with aging in women, but little is known about the effect of aging on the pituitary per se.

OBJECTIVE

The aim of the study was to determine whether pituitary responsiveness to GnRH is attenuated with aging.

DESIGN AND SETTING

A GnRH antagonist and graded doses of GnRH were used to isolate pituitary responsiveness in Clinical Research Center studies at an academic medical center.

SUBJECTS

Subjects were healthy postmenopausal women (PMW) aged 48-57 yr (n = 10) or 70-77 yr (n= 9).

INTERVENTIONS

A suppressive dose of the NAL-GLU GnRH antagonist (150 microg/kg sc) was administered and was followed by GnRH doses of 25, 75, 250, or 750 ng/kg iv every 4 h.

RESULTS

The LH response to GnRH was attenuated with aging (P = 0.05) with an interaction between age and dose (P = 0.01) such that the LH amplitude was less in older PMW at the higher doses (250 ng/kg, 50 +/- 9 vs. 29 +/- 4.9 IU/liter, for young and old PMW, respectively, P = 0.02; and 750 ng/kg, 97.7 +/- 11 vs. 70.2 +/- 9.3 IU/liter, P = 0.002), but not the lower doses of GnRH. The FSH response to GnRH was also attenuated with aging in PMW (P = 0.005).

CONCLUSIONS

In studies that isolated the pituitary from endogenous GnRH stimulation, aging attenuated the LH and FSH responses to exogenous GnRH in PMW. These studies indicate that the pituitary plays a role in the decline in gonadotropin levels with aging, further supporting the potential contribution of age-associated changes in both hypothalamic and pituitary function to reproductive senescence.

Expression of a gonadotropin-releasing hormone receptor-simian virus 40 T-antigen transgene has sex-specific effects on the reproductive axis

The GnRH receptor (GnRHR) responds to pulsatile GnRH signals to coordinate pituitary gonadotropin synthesis and secretion. Previously, a 1.2-kb fragment of the 5'-flanking region isolated from the mouse GnRHR gene was shown to target expression to pituitary gonadotropes in vivo. The 1.2-kb gene promoter fused to the simian virus 40 large T antigen (TAg) was used to generate transgenic mice that form gonadotrope-derived pituitary tumors at 4-5 months of age. Transgenic female mice have hypogonadotropic hypogonadism, infantile gonads, and are infertile throughout their life span, whereas males remain reproductively intact until their tumors become large. We hypothesized that the targeted TAg expression causes a sex-specific disruption of the reproductive axis at the level of the pituitary gland. To test this hypothesis, we characterized the pituitary gonadotropin beta-subunit and TAg expression patterns, and measured plasma gonadotropin and gonadal steroid levels in female and male mice before and after pituitary tumor development. TAg expression was observed in transgenic females and males 15 d of age, before tumor development. Interestingly, and in contrast to the transgenic males, pituitary LH beta and FSH beta subunit protein levels, and plasma LH and FSH levels, were reduced in transgenic females. Reproductive organs in transgenic female mice remained underdeveloped but were normal in transgenic males. We conclude that the expression of the TAg transgene driven by the GnRHR gene promoter results in female-specific infertility due to disruption of gonadotropin production and secretion even before tumor development.

The mechanism responsible for the supraphysiologic gonadotropin surge in females treated with gonadotropin-releasing hormone (GnRH) agonist and primed with GnRH antagonist

OBJECTIVE

To elucidate the physiologic mechanism responsible for the supraphysiologic gonadotropin release from the pituitary induced by gonadotropin-releasing hormone (GnRH) agonist in female rats primed with GnRH antagonist.

DESIGN

Controlled experimental intervention.

SETTING

Government research facility.

ANIMAL(S)

Forty 8-week-old Sprague-Dawley rats.

INTERVENTION(S)

Forty oophorectomized rats were randomized into four treatment groups of 10: group A, control vehicles; group B, GnRH agonist (leuprolide acetate; 1.7 microg/kg twice a day) on day 4; group C, GnRH antagonist (Nal-Lys; 3 mg/kg each day) days 1 to 4; or group D, GnRH antagonist (Nal-Lys; 3 mg/kg each day) days 1 to 4 plus GnRH agonist (1.7 microg/kg twice a day) on day 4.

MAIN OUTCOME MEASURE(S)

Immunohistochemical methods, Northern and in situ hybridization to quantitate pituitary follicle-stimulating hormone beta (FSH-beta), luteinizing hormone beta (LH-beta), and GnRH receptor (GnRH-R) messenger RNA (mRNA), and receptor protein levels in all treatment groups.

RESULT(S)

Treatment with GnRH antagonist was associated with increased storage of gonadotropin in the pituitary for FSH-beta and LH-beta, but mRNA levels were unchanged. The GnRH-R mRNA decreased after GnRH-agonist treatment but remained stable in the GnRH-antagonist treatment groups. Levels of GnRH-R were decreased after GnRH-antagonist treatment.

CONCLUSION(S)

These data indicate that the in vivo mechanism responsible for the exaggerated release of gonadotropins in rats primed with GnRH antagonist and treated with GnRH agonist was an increase in releasable gonadotropin pools coupled with a reduction in GnRH-R, but receptor function was preserved.

Activator protein-1 and smad proteins synergistically regulate human follicle-stimulating hormone beta-promoter activity

GnRH1 stimulates the synthesis and secretion of FSH and LH from the anterior pituitary gland. The molecular mechanisms through which GnRH1 produces these effects in humans have not been determined. Here, we examined transcriptional regulation of the human FSHbeta (FSHB) subunit using reporter assays in immortalized murine gonadotrope cells. GnRH1 dose and time dependently stimulated FSHB promoter activity, with peak stimulation occurring at 8 h. GnRH1 rapidly stimulated various MAPK cascades, though the ERK1/2 and p38 pathways appeared to be most critical for FSHB induction. Indeed, constitutively active forms of both Raf1 kinase and MAP2K6 (MKK6) were sufficient to stimulate reporter activity. GnRH1 stimulated activator protein-1 (AP-1) (FosB, c-fos, JunB, and cJun) synthesis and complex formation, the latter of which bound to a conserved cis-element within -120 bp of the transcription start site. A second, lower affinity, site was mapped more proximally. Mutations of both cis-elements diminished GnRH1-stimulated promoter activity, though disruption of the higher affinity site had a more dramatic effect. A dominant-negative Fos protein dose dependently inhibited GnRH1-stimulated FSHB transcription, confirming a role for endogenous AP-1 proteins. MAPK kinase 1 (MEK1) and p38 inhibitors significantly attenuated GnRH1-stimulated c-fos, FosB, and JunB synthesis, suggesting a mechanism whereby the ERK1/2 and p38 signaling pathways regulate FSHB transcription. Activins and inhibins potently regulate FSH synthesis in rodents, but their roles in FSH regulation in humans are less clear. Activin A, though weak on its own, synergized with GnRH1 to stimulate human FSHB promoter activity. In contrast, activin A partially inhibited GnRH1-stimulated LHbeta subunit (LHB) transcription. The GnRH1 and activin A signaling pathways appear to converge at the level of the high-affinity AP-1 site. Fos and Jun proteins synergistically regulate reporter activity through this element, and their effects are potentiated by coexpression of either Smad2 or Smad3, effectors in the activin signaling cascade. In summary, GnRH1 and activin A synergistically regulate human FSHB subunit transcription. The combined actions of AP-1 and Smad proteins acting through a conserved AP-1 element provide a candidate mechanism for this effect. The ability of activins to potentiate selectively the effects of GnRH1 on FSHB expression suggests a model for preferential increases in FSH secretion at the luteal-follicular transition of the menstrual cycle.

GNRH induces the unfolded protein response in the LbetaT2 pituitary gonadotrope cell line

The neuropeptide GNRH 1 stimulates the secretion of the reproductive hormone LH in pituitary gonadotropes. Other secretory cell types depend on the unfolded protein response (UPR) pathway to regulate protein synthesis and protect against endoplasmic reticulum (ER) stress in response to differentiation or secretory stimuli. This study investigated the role of the UPR in GNRH action within the LbetaT2 gonadotrope model. Cells were treated with GNRH, and the activation of UPR signaling components and general translational status was examined. The ER-resident stress sensors, Atf6, Eif2ak3, and Ern1, are all present, and GNRH stimulation results in the phosphorylation of eukaryotic translation initiation factor 2A kinase 3 and its downstream effector, eukaryotic translation initiation factor 2A. Additionally, activation of the UPR was confirmed both in LbetaT2 as well as mouse primary pituitary cells through identifying GNRH-induced splicing of Xbp1 mRNA, a transcription factor activated by splicing by the ER stress sensor, ER to nucleus signaling 1. Ribosome profiling revealed that GNRH stimulation caused a transient attenuation in translation, a hallmark of the UPR, remodeling ribosomes from actively translating polysomes to translationally inefficient ribonucleoprotein complexes and monosomes. The transient attenuation of specific mRNAs was also observed. Overall, the results show that GNRH activates components of the UPR pathway, and this pathway may play an important physiological role in adapting the ER of gonadotropes to the burden of their secretory demand.

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.

A mathematical model for the actions of activin, inhibin, and follistatin on pituitary gonadotrophs

The timed secretion of the luteinizing hormone (LH) and follicle stimulating hormone (FSH) from pituitary gonadotrophs during the estrous cycle is crucial for normal reproductive functioning. The release of LH and FSH is stimulated by gonadotropin releasing hormone (GnRH) secreted by hypothalamic GnRH neurons. It is controlled by the frequency of the GnRH signal that varies during the estrous cycle. Curiously, the secretion of LH and FSH is differentially regulated by the frequency of GnRH pulses. LH secretion increases as the frequency increases within a physiological range, and FSH secretion shows a biphasic response, with a peak at a lower frequency. There is considerable experimental evidence that one key factor in these differential responses is the autocrine/paracrine actions of the pituitary polypeptides activin and follistatin. Based on these data, we develop a mathematical model that incorporates the dynamics of these polypeptides. We show that a model that incorporates the actions of activin and follistatin is sufficient to generate the differential responses of LH and FSH secretion to changes in the frequency of GnRH pulses. In addition, it shows that the actions of these polypeptides, along with the ovarian polypeptide inhibin and the estrogen-mediated variations in the frequency of GnRH pulses, are sufficient to account for the time courses of LH and FSH plasma levels during the rat estrous cycle. That is, a single peak of LH on the afternoon of proestrus and a double peak of FSH on proestrus and early estrus. We also use the model to identify which regulation pathways are indispensable for the differential regulation of LH and FSH and their time courses during the estrous cycle. We conclude that the actions of activin, inhibin, and follistatin are consistent with LH/FSH secretion patterns, and likely complement other factors in the production of the characteristic secretion patterns in female rats.

Increased basal and pulsatile secretion of FSH and LH in young men with 47,XXY or 46,XX karyotypes

OBJECTIVE

The regulation of normal sexual maturation and reproductive function is dependent on a precise hormonal regulation at hypothalamic, pituitary, and gonadal levels. The aim of this study was to investigate the neuroendocrine integrity of the pituitary-gonadal axis in patients with primary testicular failure due to supernumerary X chromosomes.

DESIGN

Cross-sectional study.

METHODS

In this study, 7 untreated patients with primary gonadal insufficiency due to SRY-positive 46,XX (n=4) and 46,XXY karyotypes (n=3) aged 18.8 years and 25 age-matched healthy controls participated. Reproductive hormones, testicular size, and overnight LH and FSH serum profiles and overnight urine LH and FSH excretion were determined.

RESULTS

Basal LH and FSH secretion was elevated 6.3- and 25.4-fold respectively in the patients and the amount of LH and FSH secreted per burst were 2.0- and 6.6-fold elevated. We found significantly more LH but not FSH peaks per 24 h, as estimated by the Weibull lambda analysis. There was no difference between approximate entropy ratios or Weibull gamma analyses indicating comparable orderliness and regularity of LH and FSH secretion. Overnight urinary LH and FSH excretion was significantly elevated in patients compared with controls and correlated significantly with calculated total overnight LH and FSH secretion respectively, thus validating deconvolution.

CONCLUSION

In this group of patients with severe hypergonadotropic hypogonadism due to a supernumerary X chromosome, higher basal, pulsatile, and total LH and FSH secretion were associated with significantly more LH peaks per 24 h in comparison with healthy controls. Thus, our data indicate that in patients with Klinefelter syndrome and XX male karyotypes the entire hypothalamic-pituitary-gonadal axis has undergone functional changes.

Gonadotropin-releasing hormone analog structural determinants of selectivity for inhibition of cell growth: support for the concept of ligand-induced selective signaling

GnRH and its receptor are expressed in human reproductive tract cancers, and direct antiproliferative effects of GnRH analogs have been demonstrated in cancer cell lines. The intracellular signaling responsible for this effect differs from that mediating pituitary gonadotropin secretion. The GnRH structure-activity relationship is different for the two effects. Here we report a structure-activity relationship study of GnRH agonist antiproliferative action in model cell systems of rat and human GnRH receptors stably expressed in HEK293 cells. GnRH II was more potent than GnRH I in inhibiting cell growth in the cell lines. In contrast, GnRH I was more potent than GnRH II in stimulating inositol phosphate production, the signaling pathway in gonadotropes. The different residues in GnRH II (His(5), Trp(7), Tyr(8)) were introduced singly or in pairs into GnRH I. Tyr(5) replacement by His(5) produced the highest increase in the antiproliferative potency of GnRH I. Tyr(8) substitution of Arg(8) produced the most selective analog, with very poor inositol phosphate generation but high antiproliferative potency. In nude mice bearing tumors of the HEK293 cell line, GnRH II and an antagonist administration was ineffective in inhibiting tumor growth, but D-amino acid stabilized analogs (D-Lys(6) and D-Arg(6)) ablated tumor growth. Docking of GnRH I and GnRH II to the human GnRH receptor molecular model revealed that Arg(8) of GnRH I makes contact with Asp(302), whereas Tyr(8) of GnRH II appears to make different contacts, suggesting these residues stabilize different receptor conformations mediating differential intracellular signaling and effects on gonadotropin and cell growth. These findings provide the basis for the development of selective GnRH analog cancer therapeutics that directly target tumor cells or inhibit pituitary gonadotropins or do both.

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.

Diversity of actions of GnRHs mediated by ligand-induced selective signaling

Geoffrey Wingfield Harris' demonstration of hypothalamic hormones regulating pituitary function led to their structural identification and therapeutic utilization in a wide spectrum of diseases. Amongst these, Gonadotropin Releasing Hormone (GnRH) and its analogs are widely employed in modulating gonadotropin and sex steroid secretion to treat infertility, precocious puberty and many hormone-dependent diseases including endometriosis, uterine fibroids and prostatic cancer. While these effects are all mediated via modulation of the pituitary gonadotrope GnRH receptor and the G(q) signaling pathway, it has become increasingly apparent that GnRH regulates many extrapituitary cells in the nervous system and periphery. This review focuses on two such examples, namely GnRH analog effects on reproductive behaviors and GnRH analog effects on the inhibition of cancer cell growth. For both effects the relative activities of a range of GnRH analogs is distinctly different from their effects on the pituitary gonadotrope and different signaling pathways are utilized. As there is only a single functional GnRH receptor type in man we have proposed that the GnRH receptor can assume different conformations which have different selectivity for GnRH analogs and intracellular signaling proteins complexes. This ligand-induced selective-signaling recruits certain pathways while by-passing others and has implications in developing more selective GnRH analogs for highly specific therapeutic intervention.

p38 mitogen-activated protein kinase is critical for synergistic induction of the FSH(beta) gene by gonadotropin-releasing hormone and activin through augmentation of c-Fos induction and Smad phosphorylation

GnRH and activin independently and synergistically activate transcription of the FSH beta-subunit gene, the subunit that provides specificity and is the limiting factor in the synthesis of the mature hormone. This synergistic interaction, as determined by two-way ANOVA, is specific for FSHbeta and may, therefore, contribute to differential expression of the two gonadotropin hormones, which is critical for the reproductive cycle. We find that the cross-talk between the GnRH and activin signaling pathways occurs at the level of p38 MAPK, because the synergy is dependent on p38 MAPK activity, which is activated by GnRH, and activin cotreatment augments p38 activation by GnRH. Both the Smad and activator protein-1 binding sites on the FSHbeta promoter are necessary and sufficient for synergy. After cotreatment, Smad 3 proteins are more highly phosphorylated on the activin-receptor signaling-dependent residues on the C terminus than with activin treatment alone, and c-Fos is more highly expressed than with GnRH treatment alone. Inhibition of p38 by either of two different inhibitors or a dominant-negative p38 kinase abrogates synergy on FSHbeta expression, reduces c-Fos induction by GnRH, and prevents the further increase in c-Fos levels that occurs with cotreatment. Additionally, p38 is necessary for maximal Smad 3 C-terminal phosphorylation by activin treatment alone and for the further increase caused by cotreatment. Thus, p38 is the pivotal signaling molecule that integrates GnRH and activin interaction on the FSHbeta promoter through higher induction of c-Fos and elevated Smad phosphorylation.