Mechanism of GnRH receptor signaling on gonadotropin release and gene expression in pituitary gonadotrophs

PTPN5 promotes follicle-stimulating hormone secretion through regulating intracellular calcium homeostasis

Protein tyrosine phosphatase non-receptor type 5 (PTPN5), also called striatal-enriched protein tyrosine phosphatase (STEP), is highly expressed in neurons of the basal ganglia, hippocampus, cortex, and related structures, also in the pituitary. Gonadotropins are the key regulator of the reproduction in mammals. In this study, PTPN5 is detected to express in murine pituitary in a developmental manner. Moreover, the expression of PTPN5 in the pituitary is heavily reduced after ovary removal. Follicle-stimulating hormone (FSH) secretion in gonadotropes is regulated by PTPN5 via binding GnRH to GnRH-R. Two parallel signaling pathways, Gs-protein kinase A (PKA)-PTPN5 and Gq-phospholipases C (PLC)-p38 MAPK-PTPN5, cooperatively regulate GnRH-induced FSH secretion. We also show that influx of Ca²⁺ activates the Ca²⁺ -dependent phosphatase calcineurin, leading to the phosphorylation and activation of PTPN5. The intracellular release of Ca²⁺ is reduced via TC2153. In conclusion, blocking or knocking out of PTPN5 reduces the release of FSH in whole pituitary. Mechanically, PTPN5 regulates gonadotropes' function through regulating intracellular calcium homeostasis.

New perspectives on PPAR, VDR and FXRα as new actors in testicular pathophysiology

The incidence of reproductive disorders is constantly increasing and affects 15% of couples, with male's abnormalities diagnosed in almost half of the cases. The male gonads exert two major functions of the testis with the productions of gametes (exocrine function) and of sexual hormones (endocrine function). In the last decades, next to steroid receptors such as estrogen and androgen receptors, the involvement of other members of the nuclear receptor superfamily have been described such as Steroidogenic factor-1 (SF-1), Nerve growth factor IB (NGFIB), Liver-X-Receptorα (LXRα) and Dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1 (DAX-1). The purpose of this review is to highlight the emerging roles of some members of the nuclear receptor superfamily among which the vitamin-D Receptor (VDR), Peroxisome Proliferator-Activated Receptor (PPAR), Farnesoid-X-Receptor-α (FXRα). We discuss how these receptors could participate to explain male fertility disorders; and their potential to be use as biomarkers or therapeutic targets for management of fertility disorders.

Cholesterol: A Gatekeeper of Male Fertility?

Cholesterol is essential for mammalian cell functions and integrity. It is an important structural component maintaining the permeability and fluidity of the cell membrane. The balance between synthesis and catabolism of cholesterol should be tightly regulated to ensure normal cellular processes. Male reproductive function has been demonstrated to be dependent on cholesterol homeostasis. Here we review data highlighting the impacts of cholesterol homeostasis on male fertility and the molecular mechanisms implicated through the signaling pathways of some nuclear receptors.

Activins and Inhibins: Roles in Development, Physiology, and Disease

Since their original discovery as regulators of follicle-stimulating hormone (FSH) secretion and erythropoiesis, the TGF-β family members activin and inhibin have been shown to participate in a variety of biological processes, from the earliest stages of embryonic development to highly specialized functions in terminally differentiated cells and tissues. Herein, we present the history, structures, signaling mechanisms, regulation, and biological processes in which activins and inhibins participate, including several recently discovered biological activities and functional antagonists. The potential therapeutic relevance of these advances is also discussed.

GnRH Regulates Gonadotropin Gene Expression Through NADPH/Dual Oxidase-Derived Reactive Oxygen Species

The appropriate control of synthesis and secretion of the gonadotropin hormones LH and FSH by pituitary gonadotropes is essential for the regulation of reproduction. The hypothalamic neuropeptide GnRH is the central regulator of both processes, coordinating secretion with transcription and translation of the gonadotropin hormone subunit genes. The MAPK family of second messengers is strongly induced in gonadotropes upon GnRH stimulation, and multiple pathways activate these kinases. Intracellular reactive oxygen species participate in signaling cascades that target MAPKs, but also participate in signaling events indicative of cell stress. The NADPH oxidase (NOX)/dual oxidase (DUOX) family is a major enzymatic source of intracellular reactive oxygen, and we show that GnRH stimulation of mouse primary pituitary cells and the LβT2 gonadotrope cell line elevates intracellular reactive oxygen via NOX/DUOX activity. Mouse pituitary and LβT2 cells abundantly express NOX/DUOX and cofactor mRNAs. Pharmacological inhibition of NOX/DUOX activity diminishes GnRH-stimulated activation of MAPKs, immediate-early gene expression, and gonadotropin subunit gene expression. Inhibitor studies implicate the calcium-activated DUOX family as a major, but not exclusive, participant in GnRH signaling. Knockdown of DUOX2 in LβT2 cells reduces GnRH-induced Fshb, but not Lhb mRNA levels, suggesting differential sensitivity to DUOX activity. Finally, GnRH pulse-stimulated FSH and LH secretion are suppressed by inhibition of NOX/DUOX activity. These results indicate that reactive oxygen is a potent signaling intermediate produced in response to GnRH stimulation and further suggest that reactive oxygen derived from other sources may influence the gonadotrope response to GnRH stimulation.

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

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.

Castration-induced modifications of GnRH-elicited [Ca2+](i) signaling patterns in male mouse pituitary gonadotrophs in situ: studies in the acute pituitary slice preparation

Gonadotropin-releasing hormone (GnRH) binds to pituitary gonadotroph receptors and initiates [Ca(2+)](i) signals and gonadotropin secretion. Here, we recorded GnRH-induced Ca(2+) signals in acute pituitary slices from both intact and castrated male mice 15 and 45 days after orchiectomy (GnX). Cells responding with "noncanonical" sequences of Ca(2+) signaling to increasing GnRH concentrations ([GnRH]; oscillatory responses at a given [GnRH] and transient responses at both lower and higher concentrations) were augmented significantly in the castrated mice. Also, 15 days after GnX the number and size of gonadotrophs were augmented, confirming earlier anatomical studies. Hypertrophied gonadotrophs after 15 days after GnX tended to display GnRH-induced Ca(2+) responses of greater amplitude. Furthermore, median effective dose (ED50) for GnRH decreased from 0.17 nM (control) to ~0.07 nM after GnX, suggesting increased GnRH responsiveness of the gonadotroph population. The progression of Ca(2+) response patterns reported in control male rat gonadotrophs (oscillations declining and spike-plateau responses dominating at increasing [GnRH]) was less conspicuous in mouse gonadotrophs in situ. Also, GnX-induced alterations in rat gonadotrophs (persistence of Ca(2+) oscillations even at [GnRH] >100 nM) were not mirrored by mouse gonadotrophs in situ. Contrary to observations in intact and 15-day castrated mice, after 45 days of GnX the hump component diminished and oscillations were augmented with increasing [GnRH], but Ca(2+) response patterns of gonadotrophs in situ remained virtually unchanged in response to [GnRH]s >1 nM, suggesting dose discrimination failure at high [GnRH]s. This study underscores the notion that GnRH responsiveness and the effects of testosterone deficiency may not be equal in pituitary gonadotrophs across species.

Polymorphisms associated with egg number at 300 days of age in chickens

We looked for variations that could be associated with chicken egg number at 300 days of age (EN300) in seven genes of the hypothalamic-pituitary-gonadal axis, including gonadotrophin-releasing hormone-I (GnRH-I), GnRH receptor (GnRHR), neuropeptide Y (NPY), dopamine D2 receptor (DRD2), vasoactive intestinal polypeptide (VIP), VIP receptor-1 (VIPR-1), prolactin (PRL), and the QTL region between 87 and 105 cM of the Z chromosome. Ten mutations in the seven genes were chosen to do marker-trait association analyses in a population comprising 1310 chickens, which were obtained from a company located in Guangdong Province of China. The C1704887T of VIPR-1 was found to have a highly significant association with EN300. The T5841629C of DRD2 and the C1715301T of VIPR-1 were significantly associated with EN300. A highly significant association was also found between the C1704887T-C1715301T haplotypes of VIPR-1 and EN300. H1H3 had the highest EN300. Four PCR-RFLP variations in the candidate QTL region were selected to investigate their genetic effects on EN300. The haplotypes of T32742468C-G32742603A in this region showed a highly significant association with EN300. Bioinformatics analyses showed that both T32742468C and G32742603A were located in intron 1 of the SH3-domain GRB2-like 2 (SH3GL2) gene. We conclude that five SNPs, including C1704887T and C1715301T of VIPR-1, T5841629C of DRD2, and T32742468C and G32742603A of SH3GL2, would be useful as markers for breeding to increase chicken EN300.

Identification of estradiol/ERα-regulated genes in the mouse pituitary

Estrogen acts to prime the pituitary prior to the GnRH-induced LH surge by undiscovered mechanisms. This study aimed to identify the key components that mediate estrogen action in priming the pituitary. RNA extracted from the pituitaries of metestrous (low estrogen) and proestrus (high estrogen) stage mice, as well as from ovariectomized wild-type and estrogen receptor α (ERα) knockout mice treated with 17β-estradiol (E(2)) or vehicle, was used for gene expression microarray. Microarray data were then aggregated, built into a functional electronic database, and used for further characterization of E(2)/ERα-regulated genes. These data were used to compile a list of genes representing diverse biological pathways that are regulated by E(2) via an ERα-mediated pathway in the pituitary. This approach substantiates ERα regulation of membrane potential regulators and intracellular vesicle transporters, among others, but not the basic components of secretory machinery. Subsequent characterization of six selected genes (Cacna1a, Cacna1g, Cited1, Abep1, Opn3, and Kcne2) confirmed not only ERα dependency for their pituitary expression but also the significance of their expression in regulating GnRH-induced LH secretion. In conclusion, findings from this study suggest that estrogen primes the pituitary via ERα by equipping pituitary cells with critical cellular components that potentiate LH release on subsequent GnRH stimulations.

The synthesis and secretion of inhibins

Inhibin A and B, dimeric glycoproteins comprising an α- and β((A/B))-subunit, negatively regulate follicle stimulating hormone (FSH) synthesis by the pituitary. The expression of α- and β-subunits within Sertoli cells of the testis and granulosa cells of the ovary is controlled by a range of transcription factors, including CREB, SP-1, Smads, and GATA factors. The inhibin α- and β-subunits are synthesized as precursor molecules consisting of an N-terminal propeptide and a C-terminal mature domain. Recently, we showed that hydrophobic residues within the propeptides of the α- and β-subunits interact noncovalently with their mature domains, maintaining the molecules in a conformation competent for dimerization. Dimeric precursors are cleaved by proprotein convertases and mature inhibins are secreted from the cell noncovalently associated with their propeptides. Propeptides may increase the half-life of inhibin A and B in circulation, but they are readily displaced in the presence of the high-affinity receptors, betaglycan, and ActRII.

Functional diversity of signaling pathways through G protein-coupled receptor heterodimerization with a species-specific orphan receptor subtype

Gonadotropin-releasing hormones (GnRHs) play pivotal roles in control of reproduction via a hypothalamic-pituitary-periphery endocrine system and nervous systems of not only vertebrates but also invertebrates. GnRHs trigger several signal transduction cascades via GnRH receptors (GnRHRs), members of the G protein-coupled receptor (GPCR) family. Recently, six GnRHs (tunicate GnRH [tGnRH]-3 to tGnRH-8) and four GnRHRs (Ciona intestinalis [Ci]-GnRHR1 to GnRHR-4), including a species-specific paralog, Ci-GnRHR4 (R4) regarded as an orphan receptor or nonfunctional receptor, were identified in the protochordate, C. intestinalis, which lacks the hypothalamic-pituitary system. Here, we show novel functional modulation of GnRH signaling pathways via GPCR heterodimerization. Immunohistochemical analysis showed colocalization of R1 and R4 in test cells of the ascidian ovary. The native R1-R4 heterodimerization was detected in the Ciona ovary by coimmunoprecipitation analysis. The heterodimerization in HEK293 cells cotransfected with R1 and R4 was also observed by coimmunoprecipitation and fluorescent energy transfer analyses. Binding assay revealed that R4 had no affinity for tGnRHs, and the heterodimerization did not alter the binding affinity of R1 to the ligands. The R1-R4 elicited 10-fold more potent Ca2+ mobilization than R1 exclusively by tGnRH-6, although R1-mediated cyclic AMP production was not affected by any of tGnRHs via the R1-R4 heterodimer. Moreover, the R1-R4 heterodimer potentiated translocation of both Ca2+-dependent protein kinase C-alpha (PKCalpha) by tGnRH-6 and Ca2+-independent PKCzeta by tGnRH-5 and tGnRH-6, eventually leading to the upregulation of extracellular signal-regulated kinase (ERK) phosphorylation compared with R1 alone. These results provide evidence that the species-specific GnRHR orphan paralog, R4, serves as an endogenous modulator for the fine-tuning of activation of PKC subtype-selective signal transduction via heterodimerization with R1 and that the species-specific GPCR heterodimerization, in concert with multiplication of tGnRHs and Ci-GnRHRs, participates in functional evolution of neuropeptidergic GnRH signaling pathways highly conserved throughout the animal kingdom.

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

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

Type I gonadotropin-releasing hormone receptor mediates the antiproliferative effects of GnRH-II on prostate cancer cells

BACKGROUND

GnRH-II has been shown to exert a strong antiproliferative action on tumors of the female reproductive system. The data so far reported on the effects of GnRH-II on prostate cancer growth are controversial. Moreover, it is still unclear through which receptor [type I or type II GnRH-receptor (GnRH-R)] GnRH-II might modulate cancer cell proliferation.

OBJECTIVE

The objective of this work was to investigate whether GnRH-II might affect the proliferation of prostate cancer cells and to identify the GnRH-R through which the peptide might exert its activity.

DESIGN

We investigated the effects of GnRH-II on prostate cancer cell proliferation. We then transfected PC3 cells with a small interfering RNA targeted to type I GnRH-R. After receptor silencing we evaluated the effects of GnRH-II on cell proliferation and on forskolin-induced intracellular cAMP accumulation. Similar experiments were performed by silencing type II GnRH-R.

RESULTS

GnRH-II exerted an antiproliferative activity on prostate cancer cells. Transfection of PC3 cells with a type I GnRH-R small interfering RNA resulted in a significant decrease of the expression of this receptor. After type I GnRH-R silencing: 1) the antiproliferative effect of GnRH-II was completely abrogated; and 2) GnRH-II lost its capacity to counteract the forskolin-induced cAMP accumulation. On the contrary, type II GnRH-R silencing did not counteract the antiproliferative effect of GnRH-II.

CONCLUSIONS

GnRH-II exerts a specific and significant antiproliferative action on prostate cancer cells. This antitumor effect is mediated by the activation of type I (but not of type II) GnRH-R and by its coupled cAMP intracellular signaling pathway.

GnRH-regulated expression of Jun and JUN target genes in gonadotropes requires a functional interaction between TCF/LEF family members and beta-catenin

GnRH regulates gonadotrope function through a complex transcriptional network that includes three members of the immediate early gene family: Egr1, Jun, and Atf3. These DNA-binding proteins act alone or in pairs to confer hormonal responsiveness to Cga, Lhb, Fshb, and Gnrhr. Herein we suggest that the transcriptional response of Jun requires a functional interaction between the T-cell factor (TCF)/lymphoid enhancer factor (LEF) family of DNA-binding proteins and beta-catenin (officially CTNNB1), a coactivator of TCF/LEF. Supporting data include demonstration that GnRH increases activity of TOPflash, a TCF/LEF-dependent luciferase reporter, in LbetaT2 cells, a gonadotrope-derived cell line. Additional cotransfection experiments indicate that a dominant-negative form of TCF7L2 (TCFDN) that binds DNA, but not beta-catenin, blocks GnRH induction of TOPflash. Overexpression of AXIN, an inhibitor of beta-catenin, also reduces GnRH stimulation of TOPflash. Transduction of LbetaT2 cells with TCFDN adenoviruses diminishes GnRH stimulation of Jun mRNA without altering expression of Egr1 and Atf3, two other immediate early genes that confer GnRH responsiveness. Reduction of beta-catenin in LbetaT2 cells, through stable expression of short hairpin RNA, also selectively compromises GnRH regulation of Jun expression and levels of JUN protein. Finally, overexpression of TCFDN attenuates GnRH regulation of Cga promoter activity, a known downstream target of JUN. Together, these results indicate that GnRH regulation of Jun transcription requires a functional interaction between TCF/LEF and beta-catenin and that alteration of either impacts expression of JUN downstream targets such as Cga.

Luteinizing Hormone-Releasing Hormone I (LHRH-I) and Its Metabolite in Peripheral Tissues

Luteinizing hormone-releasing hormone (LHRH) was first isolated in the mammalian hypothalamus and shown to be the primary regulator of the reproductive system through its initiation of pituitary gonadotropin release. Since its discovery, this form of LHRH (LHRH-I) has been shown to be one of many structural variants with a variety of roles in both the brain and peripheral tissues. Enormous interest has been focused on LHRH-I and LHRH-II and their cognate receptors as targets for designing therapies to treat cancers of the reproductive system. LHRH-I is processed by a zinc metalloendopeptidase EC 3.4.24.15 (EP24.15) that cleaves the hormone at the fifth and sixth bond of the decapeptide (Tyr5-Gly6) to form LHRH-( 1 – 5 ). We have previously reported that the autoregulation of LHRH gene expression can also be mediated by its processed peptide, LHRH-( 1 – 5 ). Furthermore, LHRH-( 1 – 5 ) has also been shown to be involved in cell proliferation. This review will focus on the possible roles of LHRH and its processed peptide, LHRH-( 1 – 5 ), in non-hypothalamic tissues.

Fanconi anemia A is a nucleocytoplasmic shuttling molecule required for gonadotropin-releasing hormone (GnRH) transduction of the GnRH receptor

GnRH binds its cognate G protein-coupled GnRH receptor (GnRHR) located on pituitary gonadotropes and drives expression of gonadotropin hormones. There are two gonadotropin hormones, comprised of a common alpha- and hormone-specific beta-subunit, which are required for gonadal function. Recently we identified that Fanconi anemia a (Fanca), a DNA damage repair gene, is differentially expressed within the LbetaT2 gonadotrope cell line in response to stimulation with GnRH. FANCA is mutated in more than 60% of cases of Fanconi anemia (FA), a rare genetically heterogeneous autosomal recessive disorder characterized by bone marrow failure, endocrine tissue cancer susceptibility, and infertility. Here we show that induction of FANCA protein is mediated by the GnRHR and that the protein constitutively adopts a nucleocytoplasmic intracellular distribution pattern. Using inhibitors to block nuclear import and export and a GnRHR antagonist, we demonstrated that GnRH induces nuclear accumulation of FANCA and green fluorescent protein (GFP)-FANCA before exporting back to the cytoplasm using the nuclear export receptor CRM1. Using FANCA point mutations that locate GFP-FANCA to the cytoplasm (H1110P) or functionally uncouple GFP-FANCA (Q1128E) from the wild-type nucleocytoplasmic distribution pattern, we demonstrated that wild-type FANCA was required for GnRH-induced activation of gonadotrope cell markers. Cotransfection of H1110P and Q1128E blocked GnRH activation of the alphaGsu and GnRHR but not the beta-subunit gene promoters. We conclude that nucleocytoplasmic shuttling of FANCA is required for GnRH transduction of the alphaGSU and GnRHR gene promoters and propose that FANCA functions as a GnRH-induced signal transducer.

Activation of mitogen-activated protein kinase (MAPK) by GnRH is cell-context dependent

The interaction of GnRH with its cognate receptor (GnRHR) in pituitary gonadotropes includes activation of Gq/G11 and phospholipase Cbeta (PLCbeta), which generates the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which are required for Ca2+ mobilization and PKC isoforms activation. Activation of PKC in pituitary gonadotropes leads to the activation of the major members of the mitogen-activated protein kinase superfamily (MAPK), namely: extracellular signal-regulated kinase (ERK), jun-N-terminal Kinase (JNK) and p38MAPK. The above pathways mediate GnRH-induced gonadotropin release and synthesis. Here we summarise the diverse mechanisms utilized by GnRH to activate the MAPK members and show that they depend on "cell-context".

Gonadotropin-releasing hormone in apoptosis of prostate cancer cells

GnRH and its analogs (GnRH-a) are used extensively for the treatment of prostate cancer and other hormone-dependent diseases via the desensitization of pituitary gonadotropes, which consequently leads to the inhibition of gonadotropins, gonadal steroids and tumor growth. The actions of GnRH-a are mediated by the GnRH receptor (GnRHR) that is expressed in both the pituitary and extrapituitary sites, including normal tissues and tumors. Several studies have provided evidence that besides its pituitary effects, GnRH-a may exert direct anti-proliferative and apoptotic effects in tumor cells. These effects are mediated by the GnRHRs via signal transduction mechanisms that are distinct from the classical pituitary mechanisms. Here we describe the direct effects of GnRH-a on prostate cancer and other types of cancer. Interestingly, androgen ablation by GnRH-a is the main treatment for hormone-dependent prostate cancer. However, most of these tumors become eventually hormone-refractory, and are no longer sensitive to the GnRH-a-mediated reduction in androgen levels. Hence, the ability of GnRH-a to induce direct effects such as apoptosis may have large implications regarding the clinical use of GnRH-a. Therefore, an understanding of the cellular mechanisms involved in GnRH-a action may lead to better therapeutic modalities for the treatment of advanced prostate cancer and other malignancies.

Genomics view of gonadotrope signaling circuits

Gonadotropin-releasing hormone (GnRH) binds to the pituitary GnRH receptor to activate signal transduction cascades that ultimately modulate gonadotropin biosynthesis. Comprehensive studies of the GnRH-activated gene program in the LbetaT2 gonadotrope cell line have greatly increased our knowledge of the number of early and intermediate gene transcripts that are modulated by GnRH. Among the classes of gene induced are several whose protein products provide feedback to various levels of signaling pathways, suggesting that gene induction forms an integral component of signal transduction and contributes to longer-timescale feedback and feedforward loops. High-throughput quantitative genomic studies, mathematical modeling and biochemical studies are beginning to delineate the organization and function of the signal-decoding and logic circuit modules of the gonadotrope's signal transduction network.

Gonadotropin-releasing hormone regulates expression of the DNA damage repair gene, Fanconi anemia A, in pituitary gonadotroph cells

Gonadal function is critically dependant on regulated secretion of the gonadotropin hormones from anterior pituitary gonadotroph cells. Gonadotropin biosynthesis and release is triggered by the binding of hypothalamic GnRH to GnRH receptor expressed on the gonadotroph cell surface. The repertoire of regulatory molecules involved in this process are still being defined. We used the mouse L beta T2 gonadotroph cell line, which expresses both gonadotropin hormones, as a model to investigate GnRH regulation of gene expression and differential display reverse transcription-polymerase chain reaction (RT-PCR) to identify and isolate hormonally induced changes. This approach identified Fanconi anemia a (Fanca), a gene implicated in DNA damage repair, as a differentially expressed transcript. Mutations in Fanca account for the majority of cases of Fanconi anemia (FA), a recessively inherited disease identified by congenital defects, bone marrow failure, infertility, and cancer susceptibility. We confirmed expression and hormonal regulation of Fanca mRNA by quantitative RT-PCR, which showed that GnRH induced a rapid, transient increase in Fanca mRNA. Fanca protein was also acutely upregulated after GnRH treatment of L beta T2 cells. In addition, Fanca gene expression was confined to mature pituitary gonadotrophs and adult mouse pituitary and was not expressed in the immature alpha T3-1 gonadotroph cell line. Thus, this study extends the expression profile of Fanca into a highly specialized endocrine cell and demonstrates hormonal regulation of expression of the Fanca locus. We suggest that this regulatory mechanism may have a crucial role in the GnRH-response mechanism of mature gonadotrophs and perhaps the etiology of FA.

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.

Extracellular signal-regulated kinase and c-Src, but not Jun N-terminal kinase, are involved in basal and gonadotropin-releasing hormone-stimulated activity of the glycoprotein hormone alpha-subunit promoter

Addition of a GnRH agonist (GnRH-A) to alphaT3-1 cells stimulates different MAPK cascades: ERK, Jun N-terminal kinase (JNK), and p38. Activation of JNK, ERK, and p38 shows a unique fold activation ratio of 25:12:2, which might encode signal specificity. ERK is translocated to the nucleus within 20 min with a peak at 120 min of GnRH-A stimulation. We used the human alpha-subunit promoter linked to chloramphenicol acetyl transferase (alphaCAT) to examine the role of ERK, JNK, and c-Src, which is implicated in MAPK activation, in basal and GnRH-stimulated alphaCAT. Addition of GnRH-A resulted in a 3-fold increase in alphaCAT, whereas the Ca(2+) ionophore ionomycin and the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA) had no effect. Addition of GnRH-A and TPA, but not GnRH-A and ionomycin, produced a synergistic response, whereas removal of Ca(2+), but not down-regulation of TPA-sensitive PKCs, abolished GnRH-A-stimulated alphaCAT. Thus, regulation of alpha-promoter activity by GnRH is Ca(2+) dependent and is further augmented by PKC. Cotransfection of alphaCAT and constitutively active or dominant negative plasmids of ERK and JNK cascade members, or the use of the ERK inhibitor PD98059, revealed that ERK, but not JNK, is involved in basal and GnRH-A-stimulated alphaCAT. Because c-Src participates in MAPK activation by GnRH, we also studied its role. Cotransfection of alphaCAT and the dominant negative form of c-Src or incubation with the c-Src inhibitor PP1 reduced GnRH-A-stimulated alphaCAT. The 5'-deletion analysis revealed that the -846/-420 region participated in basal alpha-transcription. In addition, the -346/-156 region containing the pituitary glycoprotein hormone basal element, alpha-basal elements, glycoprotein-specific element, and upstream response element is involved in basal and GnRH-A-stimulated alphaCAT. ERK contribution to GnRH maps to -346/-280 containing the pituitary glycoprotein hormone basal element and alpha-basal elements 1/2. Surprisingly, although c-Src is involved in GnRH-A-stimulated ERK, its involvement is mapped to another region (-280/-180) containing the glycoprotein-specific element. Thus, ERK and c-Src but not JNK are involved in basal and GnRH-A-stimulated-alphaCAT, whereas c-Src contribution is independent of ERK activation.

Gonadotropin-releasing hormone-induced stimulation of the rat secretogranin II promoter involves activation of CREB

To investigate the events involved in regulation of the secretogranin II (SgII) gene, luciferase reporter constructs were transfected into gonadotrope-derived, alphaT3-1 cells. DNA between -91 and -60 relative to the transcription start site was found to be required for GnRH induced SgII reporter gene activation. This region contains a consensus cAMP response element (CRE) and disruption of this CRE reduced GnRH responsiveness of the SgII promoter. CREB was shown to bind to the SgII CRE and transfection studies with a dominant-negative CREB mutant provided evidence that CREB is required for GnRH responsiveness of the SgII promoter. An expression vector for an inhibitor of the cAMP-dependent protein kinase was found to reduce the ability of cAMP or GnRH to activate the SgII-luciferase reporter gene. These studies offer evidence that GnRH-induced activation of the SgII promoter in the alphaT3-1 cell line requires cAMP-dependent protein kinase activity and a functional CRE within the 5'-flanking region of the gene.

Ubiquitination and proteasomal degradation of endogenous and exogenous inositol 1,4,5-trisphosphate receptors in alpha T3-1 anterior pituitary cells

In alphaT3-1 mouse anterior pituitary gonadotropes, chronic activation of gonadotropin-releasing hormone (GnRH) receptors causes inositol 1,4,5-trisphosphate (InsP(3)) receptor down-regulation (Willars, G. B., Royall, J. E., Nahorski, S. R., El-Gehani, F., Everest, H. and McArdle, C. A. (2001) J. Biol. Chem. 276, 3123-3129). In the current study, we sought to define the mechanism behind this adaptive response. We show that GnRH induces a rapid and dramatic increase in InsP(3) receptor polyubiquitination and that proteasome inhibitors block InsP(3) receptor down-regulation and cause the accumulation of polyubiquitinated receptors. Thus, the ubiquitin/proteasome pathway is active in alphaT3-1 cells, and GnRH regulates the levels of InsP(3) receptors via this mechanism. Given these findings and further characterization of this system, we also examined the possibility that alphaT3-1 cells could be used to examine the ubiquitination of exogenous InsP(3) receptors introduced by cDNA transfection. This was found to be the case, since exogenous wild-type InsP(3) receptors, but not binding-defective mutant receptors, were polyubiquitinated in a GnRH-dependent manner, and agents that inhibited the polyubiquitination of endogenous receptors also inhibited the polyubiquitination of exogenous receptors. Further, we used this system to determine whether phosphorylation was involved in triggering InsP(3) receptor polyubiquitination. This was not the case, since mutation of serine residues 1588 and 1755 (the predominant phosphorylation sites in the type I receptor) did not inhibit polyubiquitination. In total, these data show that the ubiquitin/proteasome pathway is active in anterior pituitary cells, that this pathway targets both endogenous and exogenous InsP(3) receptors in GnRH-stimulated alphaT3-1 cells, and that, in contrast to the situation for many other substrates, phosphorylation does not trigger InsP(3) receptor polyubiquitination.

Involvement of both G(q/11) and G(s) proteins in gonadotropin-releasing hormone receptor-mediated signaling in L beta T2 cells

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of the pituitary gonadotropins. GnRH acts through a plasma membrane receptor that is a member of the G protein-coupled receptor (GPCR) family. These receptors interact with heterotrimeric G proteins to initiate downstream signaling. In this study, we have investigated which G proteins are involved in GnRH receptor-mediated signaling in L beta T2 pituitary gonadotrope cells. We have shown previously that GnRH activates ERK and induces the c-fos and LH beta genes in these cells. Signaling via the G(i) subfamily of G proteins was excluded, as neither ERK activation nor c-Fos and LH beta induction was impaired by treatment with pertussis toxin or a cell-permeable peptide that sequesters G beta gamma-subunits. GnRH signaling was partially mimicked by adenoviral expression of a constitutively active mutant of G alpha(q) (Q209L) and was blocked by a cell-permeable peptide that uncouples G alpha(q) from GPCRs. Furthermore, chronic activation of G alpha(q) signaling induced a state of GnRH resistance. A cell-permeable peptide that uncouples G alpha(s) from receptors was also able to inhibit ERK, c-Fos, and LH beta, indicating that both G(q/11) and G(s) proteins are involved in signaling. Consistent with this, GnRH caused GTP loading on G(s) and G(q/11) and increased intracellular cAMP. Artificial elevation of cAMP with forskolin activated ERK and caused a partial induction of c-Fos. Finally, treatment of G alpha(q) (Q209L)-infected cells with forskolin enhanced the induction of c-Fos showing that the two pathways are independent and additive. Taken together, these results indicate that the GnRH receptor activates both G(q) and G(s) signaling to regulate gene expression in L beta T2 cells.