Novel aspects of GnRH-induced intracellular signaling and secretion in pituitary gonadotrophs

Exploring Hormone Therapy Effects on Reproduction and Health in Transgender Individuals

Transgender individuals often face elevated mental health challenges due to gender dysphoria, but gender-affirming treatments such as surgery and hormone therapy have been linked to significant improvements in mental well-being. The potential influence of time and circadian rhythms on these treatments is prevalent. The intricate interplay between hormones, clock genes, and fertility is profound, acknowledging the complexity of reproductive health in transgender individuals. Furthermore, risks associated with gender-affirming hormonal therapy and potential complications of puberty suppression emphasize the importance of ongoing surveillance for these patients and the need of fertility preservation and family-building options for transgender individuals. This narrative review delves into the intricate landscape of hormone therapy for transgender individuals, shedding light on its impact on bone, cardiovascular, and overall health. It explores how hormone therapy affects bone maintenance and cardiovascular risk factors, outlining the complex interplay of testosterone and estrogen. It also underscores the necessity for further research, especially regarding the long-term effects of transgender hormones. This project emphasizes the critical role of healthcare providers, particularly obstetricians and gynecologists, in providing affirming care, calling for comprehensive understanding and integration of transgender treatments. This review will contribute to a better understanding of the impact of hormone therapy on reproductive health and overall well-being in transgender individuals. It will provide valuable insights for healthcare providers, policymakers, and transgender individuals themselves, informing decision-making regarding hormone therapy and fertility preservation options. Additionally, by identifying research gaps, this review will guide future studies to address the evolving healthcare needs of transgender individuals. This project represents a critical step toward addressing the complex healthcare needs of this population. By synthesizing existing knowledge and highlighting areas for further investigation, this review aims to improve the quality of care and support provided to transgender individuals, ultimately enhancing their reproductive health and overall well-being.

The role of kisspeptin in the pathogenesis of a polycystic ovary syndrome

Hypothalamic-pituitary gonadal (HPG) axis is responsible for the development and regulation of the female reproductive system. In polycystic ovary syndrome (PCOS), there is a disturbance in the HPG axis. Kisspeptin, a neuropeptide produced by the KISS1 gene, plays a vital role in the regulation of HPG axis by binding with its receptors KISS1R/GPR54, and stimulates gonadotropin secretion from the hypothalamus into pituitary to release luteinizing hormone (LH) and follicle stimulating hormone (FSH). Polymorphisms or mutations in the KISS1 gene can cause disturbance in the kisspeptin signaling pathway and is thought to disrupt HPG axis. Altered signaling of kisspeptin can cause abnormal secretion of GnRH pulse, which leads to increased LH/FSH ratio, thereby affecting androgen levels and ovulation. The increased levels of androgen worsen the symptoms of PCOS. In the present article, we review the molecular physiology and pathology of kisspeptin and how it is responsible for the development of PCOS. The goal of this review article is to provide an overview and metabolic profile of kisspeptin in PCOS patients and the expression of kisspeptin in PCOS animal models. In the present article, we also review the molecular physiology and pathology of kisspeptin and how it is responsible for the development of PCOS.

Mathematical models in GnRH research

Mathematical modelling is an indispensable tool in modern biosciences, enabling quantitative analysis and integration of biological data, transparent formulation of our understanding of complex biological systems, and efficient experimental design based on model predictions. This review article provides an overview of the impact that mathematical models had on GnRH research. Indeed, over the last 20 years mathematical modelling has been used to describe and explore the physiology of the GnRH neuron, the mechanisms underlying GnRH pulsatile secretion, and GnRH signalling to the pituitary. Importantly, these models have contributed to GnRH research via novel hypotheses and predictions regarding the bursting behaviour of the GnRH neuron, the role of kisspeptin neurons in the emergence of pulsatile GnRH dynamics, and the decoding of GnRH signals by biochemical signalling networks. We envisage that with the advent of novel experimental technologies, mathematical modelling will have an even greater role to play in our endeavour to understand the complex spatiotemporal dynamics underlying the reproductive neuroendocrine system.

Transgender medicine - puberty suppression

Puberty suppression is the reversible first step of endocrine medical treatment in transgender youth, and allows for two very important aspects of transgender management. Firstly, it buys the patient, family and their medical team time to fully evaluate the presence and persistence of gender dysphoria. Secondly, it successfully prevents the development of cis-gender unwanted secondary sexual characteristics. The latter, when present, almost certainly increase the burden of psychological co-morbidity for any transgender person. This management is modelled from treatment of gonadotropin-dependent precious puberty, with use of GnRH agonists at its core. With the increasing number of transgender youth treated, and the changing demographics of patients seeking medical care, providers are faced with the decision to start puberty blockade at younger ages than previous decades. This article will review the rationale behind puberty blockade for transgender children, the providers' options for achieving this goal, the emerging literature for potential adverse effects on such an approach, as well as identify directions of potential future research.

Gonadotropin-releasing hormone signaling: An information theoretic approach

Gonadotropin-releasing hormone (GnRH) is a peptide hormone that mediates central control of reproduction, acting via G-protein coupled receptors that are primarily G_q coupled and mediate GnRH effects on the synthesis and secretion of luteinizing hormone and follicle-stimulating hormone. A great deal is known about the GnRH receptor signaling network but GnRH is secreted in short pulses and much less is known about how gonadotropes decode this pulsatile signal. Similarly, single cell measures reveal considerable cell-cell heterogeneity in responses to GnRH but the impact of this variability on signaling is largely unknown. Ordinary differential equation-based mathematical models have been used to explore the decoding of pulse dynamics and information theory-derived statistical measures are increasingly used to address the influence of cell-cell variability on the amount of information transferred by signaling pathways. Here, we describe both approaches for GnRH signaling, with emphasis on novel insights gained from the information theoretic approach and on the fundamental question of why GnRH is secreted in pulses.

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

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

Exploring Dynamics and Noise in Gonadotropin-Releasing Hormone (GnRH) Signaling

Gonadotropin-releasing hormone (GnRH) acts via G-protein coupled receptors on pituitary gonadotropes. These are G_q-coupled receptors that mediate acute effects of GnRH on the exocytotic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the chronic regulation of their synthesis. FSH and LH control steroidogenesis and gametogenesis in the gonads so GnRH mediates control of reproduction by the central nervous system. GnRH is secreted in short pulses and the effects of GnRH on its target cells are dependent on the dynamics of these pulses. Here we provide a brief overview of the signaling network activated by GnRH with emphasis on the use of high content imaging for their examination. We also describe computational approaches that we have used to simulate GnRH signaling in order to explore dynamics, noise, and information transfer in this system.

Mathematical modeling of gonadotropin-releasing hormone signaling

Gonadotropin-releasing hormone (GnRH) acts via G-protein coupled receptors on pituitary gonadotropes to control reproduction. These are G_q-coupled receptors that mediate acute effects of GnRH on the exocytotic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the chronic regulation of their synthesis. GnRH is secreted in short pulses and GnRH effects on its target cells are dependent upon the dynamics of these pulses. Here we overview GnRH receptors and their signaling network, placing emphasis on pulsatile signaling, and how mechanistic mathematical models and an information theoretic approach have helped further this field.

Information Transfer via Gonadotropin-Releasing Hormone Receptors to ERK and NFAT: Sensing GnRH and Sensing Dynamics

Information theoretic approaches can be used to quantify information transfer via cell signaling networks. In this study, we do so for gonadotropin-releasing hormone (GnRH) activation of extracellular signal-regulated kinase (ERK) and nuclear factor of activated T cells (NFAT) in large numbers of individual fixed LβT2 and HeLa cells. Information transfer, measured by mutual information between GnRH and ERK or NFAT, was <1 bit (despite 3-bit system inputs). It was increased by sensing both ERK and NFAT, but the increase was <50%. In live cells, information transfer via GnRH receptors to NFAT was also <1 bit and was increased by consideration of response trajectory, but the increase was <10%. GnRH secretion is pulsatile, so we explored information gained by sensing a second pulse, developing a model of GnRH signaling to NFAT with variability introduced by allowing effectors to fluctuate. Simulations revealed that when cell–cell variability reflects rapidly fluctuating effector levels, additional information is gained by sensing two GnRH pulses, but where it is due to slowly fluctuating effectors, responses in one pulse are predictive of those in another, so little information is gained from sensing both. Wet laboratory experiments revealed that the latter scenario holds true for GnRH signaling; within the timescale of our experiments (1 to 2 hours), cell–cell variability in the NFAT pathway remains relatively constant, so trajectories are reproducible from pulse to pulse. Accordingly, joint sensing, sensing of response trajectories, and sensing of repeated pulses can all increase information transfer via GnRH receptors, but in each case the increase is small.

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

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

Intrinsic and Regulated Gonadotropin-Releasing Hormone Receptor Gene Transcription in Mammalian Pituitary Gonadotrophs

The hypothalamic decapeptide gonadotropin-releasing hormone (GnRH), acting via its receptors (GnRHRs) expressed in pituitary gonadotrophs, represents a critical molecule in control of reproductive functions in all vertebrate species. GnRH-activated receptors regulate synthesis of gonadotropins in a frequency-dependent manner. The number of GnRHRs on the plasma membrane determines the responsiveness of gonadotrophs to GnRH and varies in relation to age, sex, and physiological status. This is achieved by a complex control that operates at transcriptional, translational, and posttranslational levels. This review aims to overview the mechanisms of GnRHR gene (Gnrhr) transcription in mammalian gonadotrophs. In general, Gnrhr exhibits basal and regulated transcription activities. Basal Gnrhr transcription appears to be an intrinsic property of native and immortalized gonadotrophs that secures the presence of a sufficient number GnRHRs to preserve their functionality independently of the status of regulated transcription. On the other hand, regulated transcription modulates GnRHR expression during development, reproductive cycle, and aging. GnRH is crucial for regulated Gnrhr transcription in native gonadotrophs but is ineffective in immortalized gonadotrophs. In rat and mouse, both basal and GnRH-induced Gnrhr transcription rely primarily on the protein kinase C signaling pathway, with subsequent activation of mitogen-activated protein kinases. Continuous GnRH application, after a transient stimulation, shuts off regulated but not basal transcription, suggesting that different branches of this signaling pathway control transcription. Pituitary adenylate cyclase-activating polypeptide, but not activins, contributes to the regulated transcription utilizing the protein kinase A signaling pathway, whereas a mechanisms by which steroid hormones modulate Gnrhr transcription has not been well characterized.

The relationship between basal and regulated Gnrhr expression in rodent pituitary gonadotrophs

Hypothalamic GnRH together with gonadal steroids and activins/inhibin regulate its receptor gene (Gnrhr) expression in vivo, which leads to crucial changes in GnRHR numbers on the plasma membrane. This is accompanied by alterations in the gonadotroph sensitivity and responsiveness during physiologically relevant situations. Here we investigated basal and GnRH-regulated Gnrhr expression in rodent pituitary gonadotrophs in vitro. In pituitary cells from adult animals cultured in the absence of GnRH and steroid hormones, the Gnrhr expression was progressively reduced but not completely abolished. The basal Gnrhr expression was also operative in LβT2 immortalized gonadotrophs never exposed to GnRH. In both cell types, basal transcription was sufficient for the expression of functional GnRHRs. Continuous application of GnRH transiently elevated the Gnrhr expression in cultured pituitary cells followed by a sustained fall without affecting basal transcription. Both basal and regulated Gnrhr transcriptions were dependent on the protein kinase C signaling pathway. The GnRH-regulated Gnrhr expression was not operative in embryonal pituitary and LβT2 cells and was established neonatally, the sex-specific response patterns were formed at the juvenile-peripubertal stage and there was a strong correlation between basal and regulated gene expression during development. Thus, the age-dependent basal and regulated Gnrhr transcription could account for the initial blockade and subsequent activation of the reproductive system during development.

Gonadotropin inhibitory hormone (GnIH) as a regulator of gonadotropes

Gonadotropin inhibitory hormone (GnIH) has emerged as a negative regulator of gonadotrope function in a range of species. In rodents, such as rats and mice, GnIH exerts influence upon GnRH cells within the brain. In other species, however, the peptide is secreted into hypophysial portal blood to act on pituitary gonadotropes. In particular, a series of studies in sheep have demonstrated potent actions at the level of the pituitary gland to counteract the function of GnRH in terms of the synthesis and secretion of gonadotropins. This review focuses on the action of GnIH at the level of the gonadotrope.

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.

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.

Female-specific induction of rat pituitary dentin matrix protein-1 by GnRH

Hypothalamic GnRH is the primary regulator of reproduction in vertebrates, acting via the G protein-coupled GnRH receptor (GnRHR) in pituitary gonadotrophs to control synthesis and release of gonadotropins. To identify elements of the GnRHR-coupled gene network, GnRH was applied in a pulsatile manner for 6 hours to a mixed population of perifused pituitary cells from cycling females, mRNA was extracted, and RNA sequencing analysis was performed. This revealed 83 candidate-regulated genes, including a large number coding for secreted proteins. Most notably, GnRH induces a greater than 600-fold increase in expression of dentin matrix protein-1 (Dmp1), one of five members of the small integrin-binding ligand N-linked glycoprotein gene family. The Dmp1 response is mediated by the GnRHR, not elicited by other hypothalamic releasing factors, and is approximately 20-fold smaller in adult male pituitary cells. The sex-dependent Dmp1 response is established during the peripubertal period and independent of the developmental pattern of Gnrhr expression. In vitro, GnRH-induced expression of this gene is coupled with release of DMP1 in extracellular medium through the regulated secretory pathway. In vivo, pituitary Dmp1 expression in identified gonadotrophs is elevated after ovulation. Cell signaling studies revealed that the GnRH induction of Dmp1 is mediated by the protein kinase C signaling pathway and reflects opposing roles of ERK1/2 and p38 MAPK; in addition, the response is facilitated by progesterone. These results establish that DMP1 is a novel secretory protein of female rat gonadotrophs, the synthesis and release of which are controlled by the hypothalamus through the GnRHR signaling pathway. This advance raises intriguing questions about the intrapituitary and downstream effects of this new player in GnRH signaling.

Differential signaling of the GnRH receptor in pituitary gonadotrope cell lines and prostate cancer cell lines

The GnRH receptor (GnRHR) mediates the pituitary functions of GnRH, as well as its anti-proliferative effects in sex hormone-dependent cancer cells. Here we compare the signaling of GnRHR in pituitary gonadotrope cell lines vs. prostate cancer cell lines. We first noticed that the expression level of PKCα, PKCβII and PKCε is much higher in αT3-1 and LβT2 gonadotrope cell lines vs. LNCaP and DU-145 cell lines, while the opposite is seen for PKCδ. Activation of PKCα, PKCβII and PKCε by GnRH is relatively transient in αT3-1 and LβT2 gonadotrope cell lines and more prolonged in LNCaP and DU-145 cell lines. On the otherhand, the activation and re-distribution of the above PKCs by PMA was similar for both gonadotrope cell lines and prostate cancer cell lines. Activation of ERK1/2 by GnRH and PMA was robust in the gonadotrope cell lines, with a smaller effect observed in the prostate cancer cell lines. The Ca(2+) ionophore A23187 stimulated ERK1/2 in gonadotrope cell lines but not in prostate cancer cell lines. GnRH, PMA and A23187 stimulated JNK activity in gonadotrope cell lines, with a more sustained effect in prostate cancer cell lines. Sustained activation of p38 was observed for PMA and A23187 in Du-145 cells, while p38 activation by GnRH, PMA and A23187 in LβT2 cells was transient. Thus, differential expression and re-distribution of PKCs by GnRH and the transient vs. the more sustained nature of the activation of the PKC-MAPK cascade by GnRH in gonadotrope cell lines vs. prostate cancer cell lines respectively, may provide the mechanistic basis for the cell context-dependent differential biological responses observed in GnRH interaction with pituitary gonadotropes vs. prostate cancer cells.

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

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

Gonadotrophin-releasing hormone signalling downstream of calmodulin

Gonadotrophin-releasing hormone (GnRH) regulates reproduction via binding a G-protein coupled receptor on the surface of the gonadotroph, through which it transmits signals, mostly via the mitogen-activated protein (MAPK) cascade, to increase synthesis of the gonadotrophin hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Activation of the MAPK cascade requires an elevation in cytosolic Ca(2+) levels, which is a result of both calcium influx and mobilisation from intracellular stores. However, Ca(2+) also transmits signals via an MAPK-independent pathway, through binding calmodulin (CaM), which is then able to bind a number of proteins to impart diverse downstream effects. Although the ability of GnRH to activate CaM was recognised over 20 years ago, only recently have some of the downstream effects been elucidated. GnRH was shown to activate the CaM-dependent phosphatase, calcineurin, which targets gonadotrophin gene expression both directly and indirectly via transcription factors such as nuclear factor of activated T-cells and Nur77, the Transducer of Regulated CREB (TORC) co-activators and also the prolyl isomerase, Pin1. Gonadotrophin gene expression is also regulated by GnRH-induced CaM-dependent kinases (CaMKs); CaMKI is able to derepress the histone deacetylase-inhibition of β-subunit gene expression, whereas CaMKII appears to be essential for the GnRH-activation of all three subunit genes. Asides from activating gonadotrophin gene expression, GnRH also exerts additional effects on gonadotroph function, some of which clearly occur via CaM, including the proliferation of immature gonadotrophs, which is dependent on calcineurin. In this review, we summarise these pathways, and discuss the additional functions that have been proposed for CaM with respect to modifying GnRH-induced signalling pathways via the regulation of the small GTP-binding protein, Gem, and/or the regulator of G-protein signalling protein 2.

Differential involvement of phosphoinositide 3-kinase in gonadotrophin-releasing hormone actions in gonadotrophs and somatotrophs of goldfish, Carassius auratus

In goldfish, two endogenous gonadotrophin-releasing hormones (GnRHs) [salmon (s)GnRH and chicken (c)GnRH-II] control maturational gonadotrophin-II [lutenising hormone (LH)] and growth hormone (GH) secretion via Ca(2+)-dependent intracellular signalling pathways. We investigated the involvement of phosphoinositide 3-kinase (PI3K) in GnRH-evoked LH and GH release and associated intracellular Ca(2+) increases ([Ca(2+)](i) ) in goldfish gonadotrophs and somatotrophs. Immunoreactive PI3K p85α, the predominant regulatory subunit for class IA PI3Ks, was detected in goldfish pituitary tissue extracts and both endogenous GnRH isoforms increased phosphorylation of PI3K p85α in excised pituitary fragments. sGnRH- and cGnRH-II-elicited LH release responses from primary cultures of pituitary cells and [Ca(2+)](i) increases in identified gonadotrophs were significantly reduced in the presence of PI3K inhibitors wortmannin (100 nm) and LY294002 (10 μm). Unexpectedly, wortmannin and LY294002 inhibited GnRH-evoked GH release but only attenuated the [Ca(2+)](i) response in identified somatotrophs to cGnRH-II, and not sGnRH. On the other hand, Ca(2+) ionophore-evoked LH and GH secretion remained unaltered in the presence of the PI3K inhibitors, suggesting that general decreases in the releasable hormone pool or sensitivity to [Ca(2+)](i) changes did not underlie the ability of wortmannin and LY294002 to reduce the actions of GnRH. These results provide the first evidence for the presence and involvement of PI3K in GnRH-induced LH and GH release in any primary pituitary cell system. In gonadotrophs, the inhibitory action of PI3K on both sGnRH and cGnRH-II involves the attenuation of their evoked [Ca(2+)](i); in contrast, GnRH isoform-specific effects occur in somatotrophs.

The relationship between pulsatile GnRH secretion and cAMP production in immortalized GnRH neurons

In perifused immortalized GnRH neurons (GT1-7), simultaneous measurements of GnRH and cAMP revealed that the secretory profiles for both GnRH and cAMP are pulsatile. An analysis of GnRH and cAMP pulses in 16 independent experiments revealed that 25% of pulses coincide. Inversion of the peak and nadir levels was found in 33% and random relationship between GnRH and cAMP found in 42% of analyzed pulses. The random relation between GnRH and cAMP pulse resets to synchronous after an inverse relation between pulses occurred during the major GnRH release, indicating that GnRH acts as a switching mechanism to synchronize cAMP and GnRH release in perifused GT1-7 neurons. Activation of GnRH receptors with increasing agonist concentrations caused a biphasic change in cAMP levels. Low nanomolar concentrations increased cAMP production, but at high concentrations the initial increase was followed by a rapid decline to below the basal level. Blockade of the GnRH receptors by peptide and nonpeptide antagonists generated monotonic nonpulsatile increases in both GnRH and cAMP production. These findings indicate that cAMP positively regulates GnRH secretion but does not participate in the mechanism of pulsatile GnRH release.

Using automated imaging to interrogate gonadotrophin-releasing hormone receptor trafficking and function

Gonadotrophin-releasing hormone (GnRH) acts via seven transmembrane receptors on gonadotrophs to stimulate gonadotrophin synthesis and secretion, and thereby mediates central control of reproduction. Type I mammalian GnRHR are unique, in that they lack C-terminal tails. This is thought to underlie their resistance to rapid homologous desensitisation as well as their slow rate of internalisation and inability to provoke G-protein-independent (arrestin-mediated) signalling. More recently it has been discovered that the vast majority of human GnRHR are actually intracellular, in spite of the fact that they are activated at the cell surface by a membrane impermeant peptide hormone. This apparently reflects inefficient exit from the endoplasmic reticulum and again, the absence of the C-tail likely contributes to their intracellular localisation. This review is intended to cover some of these novel aspects of GnRHR biology, focusing on ways that we have used automated fluorescence microscopy (high content imaging) to explore GnRHR localisation and trafficking as well as spatial and temporal aspects of GnRH signalling via the Ca(2+)/calmodulin/calcineurin/NFAT and Raf/MEK/ERK pathways.

Effect of RF-amide-related peptide-3 on luteinizing hormone and follicle-stimulating hormone synthesis and secretion in ovine pituitary gonadotropes

GnRH provides the primary stimulus for the reproductive axis, but original work also revealed the existence of a gonadotropin-inhibitory hormone (GnIH) in birds. In mammals, GnIH properties are displayed by a hypothalamic dodecapeptide, which is a member of the RF-amide family, namely RF-amide-related peptide (RFRP)-3. This peptide inhibits GnRH-stimulated gonadotropin secretion from ovine pituitary cells in culture, but it is not known whether there are effects on gonadotropin synthesis. The aim of the present study was to determine the effects of RFRP-3 on the expression of genes for beta-subunits of the gonadotropins in ovine pituitary cells from gonadectomized ewes and rams. Cells in primary culture were given GnRH or vehicle pulses every 8 h for 24 h with and without RFRP-3 treatment. GnRH stimulated LH and FSH secretion, which was reduced by RFRP-3. Quantitative real-time PCR revealed increased expression of LHbeta and FSHbeta subunit genes after GnRH treatment and a specific reduction in expression after RFRP-3 treatment. There was no effect on the expression of GH, proopiomelanocortin, or prolactin genes. Western blotting showed that GnRH stimulated phosphorylation of ERK (phospho-ERK-1/2), and this effect was abolished by RFRP-3. We conclude that RFRP-3 acts on the pituitary gonadotropes to inhibit synthesis of the gonadotropins, and this effect may be mediated by a reduction in the GnRH-stimulated second messenger phospho-ERK-1/2.

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.

GnRH pulsatility, the pituitary response and reproductive dysfunction

GnRH plays an essential role in neuroendocrine control of reproductive function. In mammals, the pattern of gonadotropin secretion includes both pulse and surge phases, which are regulated independently. The pulsatile release of GnRH and LH plays an important role in the development of sexual function and in the normal regulation of the menstrual cycle. The importance of GnRH pulsatility was established in a series of classic studies. Fertility is impaired when GnRH pulsatility is inhibited by chronic malnutrition, excessive caloric expenditure, or aging. A number of reproductive disorders in women with including hypogonadotropic hypogonadism, hypothlamic amenorrhea, hyperprolactinemia and polycystic ovary syndrome (PCOS) are also associated with disruption of the normal pulsatile GnRH secretion. Despite these findings, the molecular mechanisms of this pulsatile GnRH regulation are not well understood. Here, we review recent studies about GnRH pulsatility, signaling and transcriptional response, and its implications for disease.

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.

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.

Rapid in vivo effects of estradiol-17beta in ovine pituitary gonadotropes are displayed by phosphorylation of extracellularly regulated kinase, serine/threonine kinase, and 3',5'-cyclic adenosine 5'-monophosphate-responsive element-binding protein

We have determined the time course of phosphorylation of MAPK/ERK, cAMP-responsive element-binding protein (CREB), and serine/threonine kinase (Akt) in ovine pituitary gonadotropes after in vivo injection (iv) of either 25 mug estradiol-17beta (E17beta) or vehicle. In ovariectomized ewes, E17beta increased the number of gonadotropes expressing phosphorylated (p)ERK-1/2 and pCREB immunoreactivity (-IR) within 90 min, as assessed by immunohistochemistry. By Western blot, we also showed that pERK-1/2, pCREB, and pAkt (ser 473) proteins were up-regulated by E17beta. In ovariectomized, hypothalamo-pituitary-disconnected animals, gonadotrope function was restored with hourly GnRH pulses (iv), and E17beta injection (iv) reduced LH response within 1 h. Immunohistochemistry showed that E17beta increased pERK-1/2-IR in gonadotropes within 15 min and peak response at 60 min. The number of cells immunoreactive for pCREB was greater in E17beta-treated animals than in vehicle-injected controls at 60 and 90 min. Western blot revealed a pERK-1/2 response within 15 min and pCREB response at 30 min. Up-regulation of pAkt occurred within 60 min of E17beta treatment. Thus, rapid effects of E17beta on gonadotropes involve phosphorylation of second messenger proteins with a time course that may relate to the rapid negative feedback effect to reduce responsiveness to GnRH.

Short-term effects of IGF-I and estradiol on LH secretion from female rat gonadotrophs

OBJECTIVE

Growth factors and ovarian steroids modulate LH-secretion from pituitary gonadotrophs. Our previous studies demonstrated that long-term IGF-I treatment enhanced LH-secretion from female rat pituitary cells and estradiol facilitated this effect. The effects of estradiol on LH secretion are time-dependent. Short-term treatment inhibited, long-term treatment enhanced GnRH-induced LH-secretion in serum-containing medium. Here we tested the short-term actions of IGF-I and its interaction with estradiol and whether IGF-I is a prerequisite for the negative effect of short-term estradiol treatment in female rat pituitary cells.

DESIGN

Pituitary cells were incubated with a series of increasing concentrations of estradiol (1 pM, 10 pM, 50 pM, 100 pM, 500 pM, 1 nM, 10 nM and 100 nM) for 4 h, IGF-I (10 pM, 100 pM, 1 nM and 10 nM) for 4 h and 14 h and their combinations for 4h in serum-free medium, and then stimulated with 1 nM GnRH during the last 3h of incubation. To clarify the role of IGF-I, cells were incubated simultaneously with estradiol, IGF-I and antibody against IGF-I. LH was measured by radioimmunoassay.

RESULTS

Short-term IGF-I treatment did not modify basal or GnRH-induced LH-secretion. Short-term treatment with estradiol did not affect basal or GnRH-induced LH-secretion in serum-free medium. The addition of 100 pM IGF-I to serum-free medium established the negative effect of estradiol short-term treatment on GnRH-induced LH-secretion. The addition of IGF-I antibody fully abolished the negative effect of estradiol.

CONCLUSIONS

In conclusion, effects of IGF-I on LH-secretion in female rat pituitary cells require long-term treatment. The negative effect of estradiol short-term treatment on GnRH-induced LH-secretion is dependent on serum-containing medium or the addition of 100 pM IGF-I to serum-free medium.

Protein kinase C cross-talk with gonadotrope progesterone receptor is involved in GnRH-induced LH secretion

In the absence of progesterone (P), the anti-P at the receptor RU486 reduces basal and GnRH-stimulated LH secretion both in vivo and in vitro, demonstrating the existence of a ligand-independent activation of progesterone receptor (LIAPR). The aim of the present study was to determine which component of the intracellular LH secretory pathway activated by GnRH is responsible for LIAPR. To do this, anterior pituitary dispersed cells from female rats in proestrus, cultured in the presence of 17beta-estradiol, were incubated with activators or inhibitors of PKC, cAMP-PKA signalling pathways or intracellular calcium (Ca2+) traffic, in the presence or absence of RU486. Results showed that RU486 reduced both GnRH- and the PKC activator PMA-induced LH secretion. In GnRH-stimulated cells incubated with the PKC inhibitor BIS-I or treated with PMA "overnight", RU486 had no effect on reduced LH secretion, nor on stimulated LH secretion elicited by the Ca2+ ionophore ionomycin. Moreover, when GnRH- or PMA-treated cells were co-incubated with 1 microM of the L-type Ca2+ channel blocker nifedipine or the intracellular Ca2+ chelator BAPTA-AM, RU486 potentiated the expected inhibition of these drugs on LH secretion. Activation (forskolin, 8-Br-cAMP) or inhibition (MDL-12,330A) of the cAMP-PKA signalling cascade affected neither the GnRH- and PMA-induced increase of LH secretion nor the reduction of LH secretion due to RU486. Taken together, the data point to the existence of a Ca2+ -independent PKC-PR cross-talk mechanism as part of the intracellular signalling of GnRH-stimulated LH secretion.

Dependence of GnRH-induced phosphorylation of CREB and BAD on EGF receptor transactivation in GT1-7 neuronal cells

The hypothalamic neuropeptide, gonadotropin releasing hormone (GnRH), is a primary regulatory factor in the neuroendocrine control of reproduction. The GnRH decapeptide is released in an episodic manner from hypothalamic GnRH neurons, which are known to express GnRH receptors. Here we examined the signaling pathways by which autocrine GnRH stimulation generates cell survival and proliferative signals in hypothalamic GT1-7 cells. Both GnRH and epidermal growth factor (EGF) caused rapid phosphorylation of cyclic AMP response element binding protein (CREB) and BAD. The selective epidermal growth factor receptor (EGF-R) antagonist, AG1478, attenuates the phosphorylation of these proteins by GnRH and EGF. Inhibition of PKC and Src abolished the stimulatory effects of GnRH, but not that of EGF, consistent with a critical role of these signaling molecules upstream of the EGF-R. All of these effects of GnRH were mimicked by phorbol 12 myristate 13-acetate (PMA). Consistent with the prosurvival and mitogenic effects of phosphoinositide 3-kinase/Akt (P13-K/Akt) downstream of the EGF-R, inhibition of P13-K diminished the activation of these proteins following stimulation with GnRH, EGF, and PMA. Overexpression of dominant negative Akt attenuated agonist-induced phosphorylation of BAD, but not that of ERK1/2 and CREB. Moreover, overexpression of wild-type RSK-1 resulted in enhanced basal as well as agonist-induced phosphorylation of CREB and BAD, indicating a critical role of RSK-1 in activating cytosolic as well as nuclear proteins. These data reveal novel signaling mechanisms of GnRH-induced phosphorylation of CREB and BAD in GT1-7 neurons through transactivation of the EGF-R.

Endothelin-induced, Long Lasting, and Ca2+ Influx-independent Blockade of Intrinsic Secretion in Pituitary Cells by Gz Subunits

The G protein-coupled receptors in excitable cells have prominent roles in controlling Ca2+-triggered secretion by modulating voltage-gated Ca2+ influx. In pituitary lactotrophs, spontaneous voltage-gated Ca2+ influx is sufficient to maintain prolactin release high. Here we show that endothelin in picomolar concentrations can interrupt such release for several hours downstream of spontaneous and high K+-stimulated voltage-gated Ca2+ influx. This action occurred through the Gz signaling pathway; the adenylyl cyclase-signaling cascade could mediate sustained inhibition of secretion, whereas rapid inhibition also occurred at elevated cAMP levels regardless of the status of phospholipase C, tyrosine kinases, and protein kinase C. In a nanomolar concentration range, endothelin also inhibited voltage-gated Ca2+ influx through the G i/o signaling pathway. Thus, the coupling of seven-transmembrane domain endothelin receptors to Gz proteins provided a pathway that effectively blocked hormone secretion distal to Ca2+ entry, whereas the cross-coupling to G i/o proteins reinforced such inhibition by simultaneously reducing the pacemaking activity.

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

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

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

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

Gonadotropin-Releasing Hormone (GnRH) Receptor Expression and Membrane Signaling in Early Embryonic GnRH Neurons: Role in Pulsatile Neurosecretion

The characteristic pulsatile secretion of GnRH from hypothalamic neurons is dependent on an autocrine interaction between GnRH and its receptors expressed in GnRH-producing neurons. The ontogeny and function of this autoregulatory process were investigated in studies on the properties of GnRH neurons derived from the olfactory placode of the fetal rat. An analysis of immunocytochemically identified, laser-captured fetal rat hypothalamic GnRH neurons, and olfactory placode-derived GnRH neurons identified by differential interference contrast microscopy, demonstrated coexpression of mRNAs encoding GnRH and its type I receptor. Both placode-derived and immortalized GnRH neurons (GT1-7 cells) exhibited spontaneous electrical activity that was stimulated by GnRH agonist treatment. This evoked response, as well as basal neuronal firing, was abolished by treatment with a GnRH antagonist. GnRH stimulation elicited biphasic intracellular calcium ([Ca2+]i) responses, and both basal and GnRH-stimulated [Ca2+]i levels were reduced by antagonist treatment. Perifused cultures released GnRH in a pulsatile manner that was highly dependent on extracellular Ca2+. The amplitude of GnRH pulses was increased by GnRH agonist stimulation and was diminished during GnRH antagonist treatment. These findings demonstrate that expression of GnRH receptor, GnRH-dependent activation of Ca2+ signaling, and autocrine regulation of GnRH release are characteristics of early fetal GnRH neurons and could provide a mechanism for gene expression and regulated GnRH secretion during embryonic migration.

Matrix metalloproteinases in reproductive endocrinology

One of the most common mechanisms for transactivation of epidermal growth factor receptor (EGF-R) by G protein-coupled receptors (GPCRs) is through the release of local EGF-like ligands from transmembrane precursors by the proteolytic action of matrix metalloproteinases (MMPs). These enzymes are crucial factors in the normal physiology of the reproductive system and also participate in neuroendocrine regulation through mediation of gonadotropin-releasing hormone (GnRH) action. Recent studies by Roelle et al. showed that GnRH-induced activation of the EGF-R and extracellular signal-regulated kinases 1 and 2 (ERK1/2) in pituitary gonadotrophs occurs through ectodomain shedding of heparin binding-EGF (HB-EGF) by MMP2 and MMP9, indicating a crucial role for MMPs in GnRH signalling.

Neuropeptide-induced Transactivation of a Neuronal Epidermal Growth Factor Receptor Is Mediated by Metalloprotease-dependent Formation of Heparin-binding Epidermal Growth Factor

Numerous external stimuli, including G protein-coupled receptor agonists, cytokines, growth factors, and steroids activate mitogen-activated protein kinases (MAPKs) through phosphorylation of the epidermal growth factor receptor (EGF-R). In immortalized hypothalamic neurons (GT1-7 cells), agonist binding to the gonadotropin-releasing hormone receptor (GnRH-R) causes phosphorylation of MAPKs that is mediated by protein kinase C (PKC)-dependent transactivation of the EGF-R. An analysis of the mechanisms involved in this process showed that GnRH stimulation of GT1-7 cells causes release/shedding of the soluble ligand, heparin binding epidermal growth factor (HB-EGF), as a consequence of metalloprotease activation. GnRH-induced phosphorylation of the EGF-R and, subsequently, of Shc, ERK1/2, and its dependent protein, p90RSK-1 (p90 ribosomal S6 kinase 1 or RSK-1), was abolished by metalloprotease inhibition. Similarly, blockade of the effect of HB-EGF with the selective inhibitor CRM197 or a neutralizing antibody attenuated signals generated by GnRH and phorbol 12-myristate 13-acetate, but not those stimulated by EGF. In contrast, phosphorylation of the EGF-R, Shc, and ERK1/2 by EGF and HB-EGF was independent of PKC and metalloprotease activity. The signaling characteristics of HB-EGF closely resembled those of GnRH and EGF in terms of the phosphorylation of EGF-R, Shc, ERK1/2, and RSK-1 as well as the nuclear translocation of RSK-1. However, neither the selective Src kinase inhibitor PP2 nor the overexpression of negative regulatory Src kinase and dominant negative Pyk2 had any effect on HB-EGF-induced responses. In contrast to GT1-7 cells, human embryonic kidney 293 cells expressing the GnRH-R did not exhibit metalloprotease induction and EGF-R transactivation during GnRH stimulation. These data indicate that the GnRH-induced transactivation of the EGF-R and the subsequent ERK1/2 phosphorylation result from ectodomain shedding of HBEGF through PKC-dependent activation of metalloprotease(s) in neuronal GT1-7 cells.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

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

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

The role of estrogen-dependent progesterone receptor in protein kinase C-mediated LH secretion and GnRH self-priming in rat anterior pituitary glands

The aim of the present study was to explore the involvement of pituitary progesterone receptor (PR) in PKC-mediated LH secretion and LHRH self-priming and the role of the estrogen (E) environment. Eight randomly selected hemipituitaries from adult female rats in proestrus or from 2 weeks ovariectomized (OVX) rats were incubated, in the absence of progesterone (P), over 3 h in Dulbecco's modified Eagle's medium (DMEM). In the first experiment, hemipituitaries were incubated continuously with: medium alone, GnRH (10 nM), the PKC stimulator PMA (100 nM), the PKC inhibitor staurosporine (100 nM), the antiprogestin at the receptor RU486 (10 nM), LHRH+staurosporine, GnRH+RU486 or PMA+RU486. In the second experiment, hemipituitaries were incubated, one h apart, with GnRH to determine the GnRH self-priming and this was compared with the priming effect of PMA. Also, the effect of staurosporine and RU486 during the induction period (1st h) on GnRH and PMA priming was evaluated. Medium was aspirated at the end of each h to determine LH accumulation and to evaluate GnRH self-priming. Both GnRH and PMA stimulated LH secretion. Staurosporine and RU486 reduced basal and GnRH-stimulated LH secretion, and RU486 reduced PMA-stimulated LH secretion from proestrus pituitaries. The stimulating effect of GnRH and PMA on LH secretion and the inhibitory action of staurosporine and RU486 on basal or stimulated LH secretion were significantly reduced in OVX-rats. Both GnRH and PMA induced GnRH priming. Staurosporine during the induction h reduced GnRH self-priming while RU486 reduced both GnRH self-potentiation and PMA priming. The magnitude of these inhibitory effects was blunted in OVX-rats. These results showed that PKC signaling pathway in the gonadotrope mediates, at least in part, basal and GnRH-stimulated LH secretion and GnRH self-priming. Also, the results are suggestive of an interaction of PKC signaling pathway with E-dependent PR in a ligand-independent activation manner in the gonadotrope.

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

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

Activation and nuclear translocation of PKCdelta, Pyk2 and ERK1/2 by gonadotropin releasing hormone in HEK293 cells

The mechanism of agonist-induced activation of Pyk2 and its relationship with ERK1/2 phosphorylation was analyzed in HEK293 cells stably expressing the gonadotropin releasing hormone (GnRH) receptor. GnRH stimulation caused rapid and sustained phosphorylation of ERK1/2 and Pyk2 that was accompanied by their nuclear translocation. Pyk2 was also localized on cell membranes and at focal adhesions. Dominant negative Pyk2 (PKM) had no effect on GnRH-induced ERK1/2 phosphorylation and c-fos expression. These actions of GnRH on ERK1/2 and Pyk2 were mimicked by activation of protein kinase C (PKC) and were abolished by its inhibition. GnRH caused translocation of PKCalpha and delta, but not of epsilon, iota and lambda, to the cell membrane, as well as phosphorylation of Raf at Ser338, a major site in the activation of MEK/ERK1/2. Stimulation of HEK293 cells by EGF caused marked ERK1/2 phosphorylation that was attenuated by the selective EGFR receptor (EGF-R) kinase inhibitor, AG1478. However, GnRH-induced ERK1/2 activation was independent of EGF-R activation. These results indicate that activation of PKC is responsible for GnRH-induced phosphorylation of both ERK1/2 and Pyk2, and that Pyk2 activation does not contribute to GnRH signaling. Moreover, GnRH-induced phosphorylation of ERK1/2 and expression of c-fos in HEK293 cells is independent of Src and EGF-R transactivation, and is mediated through the PKC/Raf/MEK cascade.

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

Dependence of gonadotropin-releasing hormone-induced neuronal MAPK signaling on epidermal growth factor receptor transactivation

The hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH), utilizes multiple signaling pathways to activate extracellularly regulated mitogen-activated protein kinases (ERK1/2) in normal and immortalized pituitary gonadotrophs and transfected cells expressing the GnRH receptor. In immortalized hypothalamic GnRH neurons (GT1-7 cells), which also express GnRH receptors, GnRH, epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) caused marked phosphorylation of ERK1/2. This action of GnRH and PMA, but not that of EGF, was primarily dependent on activation of protein kinase C (PKC), and the ERK1/2 responses to all three agents were abolished by the selective EGF receptor kinase inhibitor, AG1478. Consistent with this, both GnRH and EGF increased tyrosine phosphorylation of the EGF receptor. GnRH and PMA, but not EGF, caused rapid phosphorylation of the proline-rich tyrosine kinase, Pyk2, at Tyr(402). This was reduced by Ca(2+) chelation and inhibition of PKC, but not by AG1478. GnRH stimulation caused translocation of PKC alpha and -epsilon to the cell membrane and enhanced the association of Src with PKC alpha and PKC epsilon, Pyk2, and the EGF receptor. The Src inhibitor, PP2, the C-terminal Src kinase (Csk), and dominant-negative Pyk2 attenuated ERK1/2 activation by GnRH and PMA but not by EGF. These findings indicate that Src and Pyk2 act upstream of the EGF receptor to mediate its transactivation, which is essential for GnRH-induced ERK1/2 phosphorylation in hypothalamic GnRH neurons.

In vivo modulation of follicle-stimulating hormone release and beta subunit gene expression by activin A and the GnRH agonist buserelin in female rats

The effects of separate and simultaneous recombinant bovine (rb) activin A and buserelin administration on the FSH release and pituitary FSH beta subunit gene expression in vivo were examined in ovariectomised, estradiol pretreated rats. The animals received a single injection of either rb activin A (50 ng), buserelin (1 micro g) or activin/buserelin (50 ng+1 micro g/0.1 ml PBS) into the jugular vein and were killed 30 min, 1, 3 and 5h later. Activin A stimulated FSH release and effect appeared 1h after injection (168% increase of controls) reaching a maximum at 3h (437% of controls). Activin A and buserelin exerted their effects with a distinct time courses: activin's stimulation was not so rapid when compared with buserelin. The simultaneous administration of rb activin A and buserelin amplified FSH release (118, 309, 1006 and 779% of controls). The low dose of activin A was sufficient to elevate FSH beta mRNA level as early as 3 and 5h after administration (170 and 140%, respectively). Activin plus buserelin stimulation resulted in a higher (340 and 360% of controls) FSH beta gene expression than after their separate administration. These results suggest that activin and buserelin may act independently and synergistically in the regulation of FSH release and beta subunit mRNA level.

3',5'-cyclic adenosine monophosphate augments intracellular Ca2+ concentration and gonadotropin-releasing hormone (GnRH) release in immortalized GnRH neurons in an Na+ -dependent manner

In GT1-7 cells, cAMP increases the intracellular Ca2+ concentration ([Ca2+](i)) through activation of the voltage-gated Ca2+ channels, thereby facilitating GnRH release. To activate these channels, the membrane potential must be depolarized. In the present study we hypothesize that cAMP depolarizes the cells by increasing the membrane Na+ permeability, as in the case of somatotrophs and pancreatic beta-cells. To examine this, we analyzed [Ca2+](i) and [Na+](i) in GT1-7 cells by an intracellular ion-imaging technique along with cAMP assay by RIA. Forskolin, a direct activator of adenylyl cyclase, increased [Ca2+](i) and [Na+](i) via cAMP formation. The forskolin-induced increase in [Ca2+](i) depended on the presence of Ca2+ and Na+ in the extracellular solution. This response was blocked by the voltage-gated Ca2+ channel blocker, nifedipine; the nonselective cation channel blocker, gadolinium (Gd3+); and the cyclic nucleotide-gated channel blocker, l-cis-diltiazem. In contrast, the forskolin-induced increase in [Na+](i) depended only on extracellular Na+, not on Ca2+. Gd3+ and l-cis-diltiazem also blocked the increase in [Na+](i). Furthermore, the forskolin-induced increase in GnRH release was blunted in both low Ca2+ and low Na+ media. The results indicate that cAMP increases the membrane Na+ permeability, probably through nonselective cation channels on GT1-7 cells, thereby promoting GnRH release.

Effects of progesterone (P) and antiprogestin RU486 on LH and FSH release by incubated pituitaries from rats treated with the SERM LY11701 8-HCl and/or recombinant human FSH

The aim of the present study was to investigate the role of the estrogen (ES) background on the effects of P or its antagonist RU486 on basal and LHRH-stimulated LH and FSH secretion. To do this, pituitaries collected from: intact rats in proestrus; rats injected with the ES antagonist LY11701 8-HCl; rats injected with recombinant-human FSH (r-hFSH) to stimulate ovarian hormonogenesis; and rats injected with both LY11701 8-HCl and r-hFSH were incubated with or without LHRH (10 nM) in the presence of P (100 nM) or RU486 (10 nM). RU486 decreased basal and LHRH-stimulated release of LH and FSH and LHRH self-priming in pituitaries from control rats, while P increased both pituitary responsiveness and LHRH self-priming. These effects were absent in pituitaries from rats treated either with the ES antagonist or r-hFSH, which, in the absence of P or RU486 in the incubation medium, reduced gonadotropin release. Because r-hFSH did not increase E2 serum concentration significantly, the putative FSH-dependent ovarian non-steroidal gonadotropin surge inhibiting factor (GnSIF) might be the hormonal cause of the reduced secretion of LH and FSH. Combined treatment with LY117018-HCl and r-hFSH had additive inhibitory effects on gonadotropin release. These results indicate that ES-inducible P receptor (PR) in the pituitary can be activated in a ligand-independent manner by intracellular messengers giving rise to enhanced basal and LHRH-stimulated gonadotropin secretion. The results also suggested that the r-hFSH-stimulated ovarian bioactive entity GnSIF and RU486 may share a similar mechanism of action involving pituitary PR.

Transcriptional activation of the ovine follicle-stimulating hormone-beta gene by gonadotropin-releasing hormone involves multiple signal transduction pathways

GnRH regulates gonadotrope cells through GnRH receptor activation of the PKC-, MAPK-, and calcium-activated signaling cascades. Due to the paucity of homologous model systems expressing FSHbeta, little is known about the specific mechanisms involved in transcriptional regulation of this gene by GnRH. Previous studies from our laboratory demonstrated that the gonadotrope-derived LbetaT2 cell line expresses FSHbeta mRNA. In the present study we characterized the mechanisms involved in GnRH regulation of the FSHbeta promoter using this cell model. Using transfection assays, we show that GnRH regulation of the ovine FSHbeta promoter involves at least two elements, present between -4152/-2878 and -2550/-1089 bp, in association with one or several elements within the proximal region of the promoter. Surprisingly, the two activating protein-1 sites previously shown to be involved in the FSHbeta response to GnRH in heterologous cells do not play a role in GnRH responsiveness in the gonadotrope cell model. Here we demonstrate that calcium influx itself is not sufficient to confer the response, but it is necessary for both 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and GnRH induction of the FSHbeta gene. Moreover, we show that GnRH regulation of FSHbeta gene expression is mediated by PKC and establish the presence of multiple PKC isozymes in LbetaT2 cells. Interestingly, GnRH and TPA induce activity of the FSHbeta promoter through different, although possibly overlapping, pools of PKC isoforms. This is further supported by the use of a MAPK inhibitor, which abolishes the induction of FSHbeta by GnRH, but not by TPA. In conclusion, we have demonstrated that calcium, PKC, and MAPK signaling systems are all involved in the induction of FSHbeta gene expression by GnRH in the LbetaT2 mouse gonadotrope cell model.