Activation of MAP kinase associated with the priming effect of LHRH

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

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

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

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

Encoding and decoding mechanisms of pulsatile hormone secretion

Ultradian pulsatile hormone secretion underlies the activity of most neuroendocrine systems, including the hypothalamic-pituitary adrenal (HPA) and gonadal (HPG) axes, and this pulsatile mode of signalling permits the encoding of information through both amplitude and frequency modulation. In the HPA axis, glucocorticoid pulse amplitude increases in anticipation of waking, and, in the HPG axis, changing gonadotrophin-releasing hormone pulse frequency is the primary means by which the body alters its reproductive status during development (i.e. puberty). The prevalence of hormone pulsatility raises two crucial questions: how are ultradian pulses encoded (or generated) by these systems, and how are these pulses decoded (or interpreted) at their target sites? We have looked at mechanisms within the HPA axis responsible for encoding the pulsatile mode of glucocorticoid signalling that we observe in vivo. We review evidence regarding the 'hypothalamic pulse generator' hypothesis, and describe an alternative model for pulse generation, which involves steroid feedback-dependent endogenous rhythmic activity throughout the HPA axis. We consider the decoding of hormone pulsatility by taking the HPG axis as a model system and focussing on molecular mechanisms of frequency decoding by pituitary gonadotrophs.

Gonadotropin-releasing hormone receptor system: modulatory role in aging and neurodegeneration

Receptors for hormones of the hypothalamic-pituitary-gonadal axis are expressed throughout the brain. Age-related decline in gonadal reproductive hormones cause imbalances of this axis and many hormones in this axis have been functionally linked to neurodegenerative pathophysiology. Gonadotropin-releasing hormone (GnRH) plays a vital role in both central and peripheral reproductive regulation. GnRH has historically been known as a pituitary hormone; however, in the past few years, interest has been raised in GnRH actions at non-pituitary peripheral targets. GnRH ligands and receptors are found throughout the brain where they may act to control multiple higher functions such as learning and memory function and feeding behavior. The actions of GnRH in mammals are mediated by the activation of a unique rhodopsin-like G protein-coupled receptor that does not possess a cytoplasmic carboxyl terminal sequence. Activation of this receptor appears to mediate a wide variety of signaling mechanisms that show diversity in different tissues. Epidemiological support for a role of GnRH in central functions is evidenced by a reduction in neurodegenerative disease after GnRH agonist therapy. It has previously been considered that these effects were not via direct GnRH action in the brain, however recent data has pointed to a direct central action of these ligands outside the pituitary. We have therefore summarized the evidence supporting a central direct role of GnRH ligands and receptors in controlling central nervous physiology and pathophysiology.

GnRH signaling, the gonadotrope and endocrine control of fertility

Mammalian reproductive cycles are controlled by an intricate interplay between the hypothalamus, pituitary and gonads. Central to the function of this axis is the ability of the pituitary gonadotrope to appropriately respond to stimulation by gonadotropin-releasing hormone (GnRH). This review focuses on the role of cell signaling and in particular, mitogen-activated protein kinase (MAPK) activities regulated by GnRH that are necessary for normal fertility. Recently, new mouse models making use of conditional gene deletion have shed new light on the relationships between GnRH signaling and fertility in both male and female mice. Within the reproductive axis, GnRH signaling is initiated through discrete membrane compartments in which the receptor resides leading to the activation of the extracellular signal-regulated kinases (ERKs 1/2). As defined by gonadotrope-derived cellular models, the ERKs appear to play a central role in the regulation of a cohort of immediate early genes that regulate the expression of late genes that, in part, define the differentiated character of the gonadotrope. Recent data would suggest that in vivo, conditional, pituitary-specific disruption of ERK signaling by GnRH leads to a gender-specific perturbation of fertility. Double ERK knockout in the anterior pituitary leads to female infertility due to LH biosynthesis deficiency and a failure in ovulation. In contrast, male mice are modestly LH deficient; however, this does not have an appreciable impact on fertility.

Pulsatile and sustained gonadotropin-releasing hormone (GnRH) receptor signaling: does the ERK signaling pathway decode GnRH pulse frequency?

Gonadotropin-releasing hormone (GnRH) acts via G-protein-coupled receptors on gonadotrophs to stimulate synthesis and secretion of luteinizing hormone and follicle-stimulating hormone. It is secreted in pulses, and its effects depend on pulse frequency, but decoding mechanisms are unknown. Here we have used an extracellular signal regulated kinase-green fluorescent protein (ERK2-GFP) reporter to monitor GnRH signaling. GnRH caused dose-dependent ERK2-GFP translocation to the nucleus, providing a live-cell readout for activation. Pulsatile GnRH caused dose- and frequency-dependent ERK2-GFP translocation. These responses were rapid and transient, showed only digital tracking, and did not desensitize under any condition tested (dose, frequency, and receptor number varied). We also tested for the effects of cycloheximide (to prevent induction of nuclear-inducible MAPK phosphatases) and used GFP fusions containing ERK mutations (D319N, which prevents docking domain-dependent binding to MAPK phosphatases, and K52R, which prevents catalytic activity). These manipulations had little or no effect on the translocation responses, arguing against a role for MAPK phosphatases or ERK-mediated feedback in shaping ERK activation during pulsatile stimulation. GnRH also caused dose- and frequency-dependent activation of the alpha-gonadotropin subunit-, luteinizing hormone beta-, and follicle-stimulating hormone beta- luciferase reporters, and the latter response was inhibited by ERK1/2 knockdown. Moreover, GnRH caused frequency-dependent activation of an Egr1-luciferase reporter, but the response was proportional to cumulative pulse duration. Our data suggest that frequency decoding is not due to negative feedback shaping ERK signaling in this model.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Facilitation or inhibition of the oestradiol-induced gonadotrophin surge in the immature female rat by progesterone: effects on pituitary responsiveness to gonadotrophin-releasing hormone (GnRH), GnRH self-priming and pituitary mRNAs for the progesterone receptor A and B isoforms

Progesterone can either facilitate or inhibit the oestradiol (E(2))-induced gonadotrophin surge. We have previously developed immature female rat models to characterise and investigate the mechanisms of progesterone inhibition or facilitation. The aim of the present study was to determine the role of pituitary responsiveness to gonadotrophin-releasing hormone (GnRH) and GnRH self-priming under conditions of progesterone-facilitation and progesterone-inhibition, and whether the underlying mechanisms reflect changes in mRNAs encoding the A and B isoforms of the progesterone receptor (PR) in the pituitary gland. Pituitary responsiveness to GnRH, determined by measuring the luteinising hormone (LH) response to one i.v. injection of GnRH, was decreased by 60-80% (P < 0.001) in the progesterone-inhibition model. GnRH self-priming, estimated as the increment in the LH response to the second of two injections of GnRH separated by 60 min, was also significantly reduced (P < 0.05) in this model. In the progesterone-facilitation model, the LH response to GnRH injection was increased 2.5-3-fold (P < 0.05), an effect suppressed by the progesterone receptor antagonist, mifepristone. Progesterone-facilitation of LH release and increased pituitary responsiveness to GnRH were blocked by sheep anti-GnRH serum injected i.v. immediately after insertion of progesterone implants. The PR-B mRNA isoform, measured by solution hybridisation/RNase protection assay, was the predominant form in the pituitary of the immature female rat. PR-B was increased by E(2) and decreased by progesterone in both models. Thus, in immature female rats, progesterone-inhibition of the E(2)-induced LH surge is due to significant reduction in pituitary responsiveness to GnRH as well as in the magnitude of GnRH self-priming. Progesterone-facilitation of the E(2)-induced LH surge is due to increased pituitary responsiveness to GnRH, which is mediated by PR, and depends on endogenous GnRH release. The differences between progesterone-facilitation and progesterone-inhibition are not due to differences in regulation of pituitary PR-B mRNA.

The luteinizing hormone-releasing hormone inhibits the anti-apoptotic activity of insulin-like growth factor-1 in pituitary alphaT3 cells by protein kinase Calpha-mediated negative regulation of Akt

The luteinizing hormone-releasing hormone (LHRH) receptor is a G protein-coupled receptor involved in the synthesis and release of pituitary gonadotropins and in the proliferation and apoptosis of pituitary cells. Insulin-like growth factor-1 receptor (IGF-1R) is a tyrosine kinase receptor that has a mitogenic effect on pituitary cells. In this study, we used the alphaT3 gonadotrope cell line as a model to characterize the IGF-1R signaling pathways and to investigate whether this receptor interacts with the LHRH cascade. We found that IGF-1 activated the IGF-1R, insulin receptor substrate (IRS)-1, phosphatidylinositol 3-kinase, and Akt in a time-dependent manner in alphaT3 cells. The MAPK (ERK1/2, p38, and JNK) pathways were only weakly activated by IGF-1. In contrast, LHRH strongly stimulated the MAPK pathways but had no effect on Akt activation. Cotreatment with IGF-1 and LHRH had various effects on these signaling pathways. 1) It strongly increased IGF-1-induced tyrosine phosphorylation of IRS-1 and IRS-1-associated phosphatidylinositol 3-kinase through activation of the epidermal growth factor receptor. 2) It had an additive effect on ERK1/2 activation without modifying the phosphorylation of p38 and JNK1/2. 3) It strongly reduced IGF-1 activation of Akt. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays and cell cycle analysis revealed that, in addition to having an additive effect on ERK1/2 activation, cotreatment with IGF-1 and LHRH also had an additive effect on cell proliferation. The LHRH-induced inhibition of Akt stimulated by IGF-1 was completely blocked by Safingol, a protein kinase C (PKC) alpha-specific inhibitor, and by a dominant negative form of PKCalpha. Finally, we showed that the inhibitory effect of LHRH on IGF-1-induced PKCalpha-mediated Akt activation was associated with a marked reduction in Bad phosphorylation and a substantial decrease in the ability of IGF-1 to rescue alphaT3 cells from apoptosis induced by serum starvation. Our results demonstrate for the first time that several interactions take place between IGF-1 and LHRH receptors in gonadotrope cells.

No Evidence for Pituitary Priming to Gonadotropin-Releasing Hormone in Relation to Luteinizing Hormone (LH) Secretion Prior to the Preovulatory LH Surge in Ewes1

The purpose of this study was to determine the occurrence of and the regulatory mechanisms involved in priming of the pituitary to GnRH before the preovulatory LH surge in sheep. Experiment 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36 h after induced luteolysis in groups Lut and Foll, respectively. After 4 h, ewes were administered either saline (n = 5) or 250 ng (n = 8) or 10 microg (n = 8) of GnRH. Five ewes per treatment group were killed 1 h later, while remaining animals were blood sampled for a further 7 h. Experiment 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2 h before GnRH (10 microg) treatment until 7 h after. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LHbeta mRNA levels were similar between groups. Mean FSHbeta mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH(+ve)/secretogranin II(-ve) (SgII(-ve)) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming before onset of the preovulatory LH surge. Constitutive release of LH(+ve)/SgII(-ve) granules may maintain basal LH levels while smaller sized, presumably mature granules may be preferentially released after GnRH stimulation.

Structure of the GnRH receptor-stimulated signaling network: insights from genomics

The GnRH receptor influences gene expression in the gonadotrope through activating signaling cascades that modulate transcription factor expression and activity. A longstanding question in neuroendocrinology is how instructions received at the membrane in the form of the pattern of receptor stimulation are processed into specific biosynthetic changes at each gonadotropin promoter. Signal transduction from the membrane to preformed transcription factors relies on recognition of altered conformations. Signal transduction through the layers of the gene network also requires the biosynthesis of new transcription factors. The signal processing of this system depends on its molecular connectivity map and its feedback and feed-forward loops. Review of signal transduction, gene control, and genomic studies provide evidence of key loops that cross between cellular and nuclear compartments. Genomic studies suggest that the signal transduction and gene network form a continuum. We propose that information transfer in the gonadotrope depends on robust signaling modules that serve to integrate events at different time scales across cytoplasmic and nuclear compartments.

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.

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

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

An early growth response protein (Egr) 1 cis-element is required for gonadotropin-releasing hormone-induced mitogen-activated protein kinase phosphatase 2 gene expression

In pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) activates all three major mitogen-activated protein kinase (MAPK) cascades. The MAPKs play key roles in transcriptional activation of GnRH-responsive genes. MAPK phosphatases (MKPs) are dual specificity protein phosphatases involved in feedback regulation of MAPK activity. Previous studies indicate that GnRH activates MKP-2 expression in gonadotropes, dependent upon activation of multiple MAPKs and discrete Ca(2+) signals. To further understand the transcriptional mechanism(s) of MKP-2 induction by GnRH, we studied the activity of a 198-nucleotide MKP-2 proximal promoter region that supports GnRH responsiveness in reporter gene assays. Functional analysis of the MKP-2 promoter confirmed a requirement for the protein kinase C-extracellular signal-regulated kinase (ERK) pathway and VGCC-derived Ca(2+) signals in transcriptional activation of the MKP-2 gene. However, the inhibitory effect of thapsigargin on MKP-2 protein expression previously identified was not mediated at the level of promoter activation, suggesting a distinct mechanism for the action of thapsigargin-sensitive Ca(2+) signals. MGRE (MKP-2 GnRH response element) within the MKP-2 promoter mediated promoter activation through the protein kinase C-ERK pathway. The zinc finger transcription factor Egr-1 was identified in the MGRE-binding complex. Egr-1/MGRE binding was induced by GnRH in an ERK-dependent manner. Transcriptional activity of Egr-1 protein was enhanced by GnRH treatment. In addition, overexpression of the Egr-interacting protein, NAB1, resulted in increased GnRH-stimulated MKP-2 gene transcription. Consistent with the putative role of Egr-1 in MKP-2 promoter regulation, Egr-1 protein expression closely correlated with the expression of MKP-2 protein in alpha T3-1 cells. Together, these data suggest that Egr-1 may be a key factor in mediating GnRH-dependent transcriptional activation of the MKP-2 gene.

Intracellular signaling pathways mediated by the gonadotropin-releasing hormone (GnRH) receptor

The hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator of the reproductive system, triggering the synthesis and release of LH and FSH in the pituitary. GnRH transmits its signal via two specific serpentine receptors that belong to the large group of G-protein coupled receptors (GPCRs). Here we review the intracellular signaling pathways mediated by the GnRH receptor (GnRHR). In pituitary-derived alpha T3-1 cells, a widely used model for GnRH action, GnRHR signaling includes activation of mitogen-activated protein kinase (MAPK) cascades, which provide an important link for the transmission of signals from the cell surface to the nucleus and play a role in the regulation of gonadotropin transcription. Activation of ERK--one of the MAPK cascades--by GnRH in these cells depends mainly on the phosphorylation of Raf1 by PKC, supported by a pathway involving c-Src, dynamin, and Ras. On the other hand, the activation of JNK, another MAPK cascade, involves PKC, c-Src, CDC42/Rac1, and probably MEKK1. The GnRHR is also expressed in non-pituitary cells and was found to be involved in the inhibition of cell proliferation in certain cells. Therefore, GnRHR represents a potential target for GnRH-analogs used for cancer treatment. Interestingly, the signaling mechanism of the GnRHR in other cell types significantly differs from that in pituitary cells. Studies conducted in GnRHR-expressing COS7 cells have shown that GnRHR transmits its signals mainly via Gi, EGF receptor, c-Src, and is not dependent on PKC. Understanding the signaling mechanisms elicited by GnRHR can shed light on the mechanism of action of GnRH in pituitary and extra-pituitary tissues.

Cyclic adenosine 3',5'-monophosphate (cAMP) and cAMP responsive element-binding protein are involved in the transcriptional regulation of gonadotropin-releasing hormone (GnRH) receptor by GnRH and mitogen-activated protein kinase signal transduction pathway in GGH(3) cells

Stimulation of mouse GnRH receptor promoter by a GnRH agonist (Buserelin), or by a cAMP analogue, significantly increased reporter (luciferase) activity. Overexpression of Raf-1, ERK1, or ERK2 partially blocked Buserelin-stimulated luciferase activity. In contrast, treatment with a mitogen-activated protein kinase (MAPK) kinase inhibitor (PD 98059) activated basal and Buserelin-stimulated luciferase activity in a dose-dependent manner. Transient transfection of the deleted cAMP response element expression vector followed by pretreatment with PD98059 prior to Buserelin stimulation showed that the transcriptional response was decreased compared to wild-type promoter. A gel-mobility shift assay using a probe containing the cAMP response element showed the presence of two specific protein-DNA complexes that contain one or more members of the cAMP responsive element-binding (CREB) protein family. These results suggest that cAMP and CREB participate in the GnRH activation of GnRH receptor promoter activity and that the MAPK cascade is involved in the negative regulation of basal and GnRH-stimulated GnRH receptor transcriptional activity.

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

Gonadotropin releasing hormone (GnRH), the first key hormone of reproduction, is synthesized and secreted from the hypothalamus in a pulsatile manner and stimulates pituitary gonadotrophs (5-10% of the pituitary cells) to synthesize and release gonadotropin luteinizing hormone (LH) and follicle stimulating hormone (FSH). Gonadotrophs consist of 60% multihormonal cells (LH+FSH) and 18% LH- and 22% FSH-containing cells. LH and FSH, members of the glycoprotein hormone family, stimulate spermatogenesis, folliculogenesis, and ovulation. Although GnRH plays a pivotal role in gonadotropin synthesis and release, other factors such as gonadal steroids and gonadal peptides exert positive and negative feedback mechanisms, which affect GnRH actions. GnRH actions include activation of phosphoinositide turnover as well as phospholipase D and A2, mobilization and influx of Ca2+, activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). A complex crosstalk between the above messenger molecules mediates the diverse actions of GnRH. Understanding the signaling mechanisms involved in GnRH actions is the basis for our understanding of basic reproductive functions in general and gonadotropin synthesis and release in particular.

GnRH receptor signaling in tilapia pituitary cells: role of mitogen-activated protein kinase (MAPK)

The role of mitogen-activated protein kinase (MAPK, also known as extracellular signal regulated kinase; ERK) stimulation in gonadotropin-releasing hormone (GnRH) signaling was investigated in cultured pituitary cells of tilapia hybrids (Oreochromis niloticus x O. aureus). Exposure of the cells to salmon GnRH (sGnRH) resulted in a dose- and time-dependent elevation in ERK levels. The PKC activator, 1-O-tetradecanoyl phorbol-13-acetate (TPA) increased kinase levels, while addition of GnRH had no further effect. However, chronic exposure to TPA resulted in reduction of basal and GnRH-induced ERK elevation. When PKC was inhibited by GF109203X, the GnRH-elevated ERK levels were totally abolished. The role of MAPK activation on GPalpha, FSHbeta and LHbeta gene expression was determined by administration of MAPK-kinase (MEK) inhibitor (PD98059; PD). This inhibitor completely blocked GnRH-induced increases in ERK activity. Furthermore, it suppressed GPalpha and LHbeta mRNA responses to GnRH, but had no effect on FSHbeta transcript levels. PD also decreased basal LHbeta mRNA levels. These results indicate that in tilapia pituitary cells, GnRH activates MAPK cascade in a PKC-dependent manner. ERK is involved in GnRH elevation of GPalpha and LHbeta, but not in FSHbeta genes transcription.

Role of dynamin, Src, and Ras in the protein kinase C-mediated activation of ERK by gonadotropin-releasing hormone

G-protein-coupled receptors are a large group of integral membranal receptors, which in response to ligand binding initiate diverse downstream signaling. Here we studied the gonadotropin-releasing hormone (GnRH) receptor, which uses Gq for its downstream signaling. We show that extracellular signal-regulated kinase (ERK) activation is fully dependent on protein kinase C (PKC), but only partially dependent on Src, dynamin, and Ras. Receptor tyrosine kinases, FAK, Gbetagamma, and beta-arrestin, which were implicated in some G-protein-coupled receptor signaling to MAPK cascades, do not play a role in the GnRH to ERK pathway. Our results suggest that the activation of ERK by GnRH involves two distinct signaling pathways, which converge at the level of Raf-1. The main pathway involves a direct activation of Raf-1 by PKC, and this step is partially dependent on a second pathway consisting of Ras activation, which occurs in a dynamin-dependent manner, downstream of Src.

Activation of mitogen-activated protein kinase phosphatase 2 by gonadotropin-releasing hormone

The aim of these studies was to identify the signaling mechanism(s) that contribute to GnRH-induced expression of MAPK phosphatase (MKP)-2, a dual specificity phosphatase that selectively inactivates MAPKs. GnRH receptor activation induced MKP-2 expression in both clonal (alphaT3-1) and primary gonadotropes. Activation of PKC isozymes was sufficient and required for MKP-2 induction. Inhibition of the extracellular signal-regulated kinase (ERK) or c-Jun N-terminal kinase (JNK) but not the p38 MAPK cascade was sufficient to block GnRH-induced MKP-2 expression. Induction of MKP-2 by GnRH was dependent on elevation in intracellular Ca(2+). Inhibition of Ca(2+) influx through L-type voltage-gated calcium channels blocked GnRH-induced MKP-2 expression. Depletion of intracellular Ca(2+) stores with thapsigargin blocked MKP-2 activation by GnRH independent of ERK and JNK activity. These results support the conclusion that MKP-2 induction by GnRH occurs via MAPK-dependent and -independent pathways. One mechanism requires GnRH-induced ERK and JNK activation, while a second MAPK-independent pathway requires a thapsigargin-sensitive calcium signal.

Stimulation of mitogen-activated protein kinase by gonadotropin-releasing hormone in human granulosa-luteal cells

The present study investigated the activation of mitogen-activated protein kinases (MAPKs) by a GnRH agonist (GnRHa) in human granulosa-luteal cells (hGLCs). The phosphorylation state of p44 and p42 MAPK was examined using antibodies that distinguish phospho-p44/42 MAPK (Thr(202)/Tyr(204)) from total p44/42 MAPK (activated plus inactivated). Activation of MAPK by GnRHa was observed within 5 min and was sustained for 60 min after treatment. GnRHa stimulated MAPK activation in a dose-dependent manner, with maximum stimulation (6.7-fold over basal levels) at 10(-7) M. Pretreatment with a protein kinase C (PKC) inhibitor, GF109203X, completely blocked GnRHa-induced MAPK activation. In addition, pretreatment with a PKC activator, phorbol-12-myristate 13-acetate, potentiated GnRH-induced MAPK activation. These results indicate that GnRHa stimulates MAPK activation through a PKC-dependent pathway in hGLCs, possibly coupled to G(q)alpha protein. MAPK activation was also observed in response to 8-bromo-cAMP or cholera toxin, but not pertussis toxin. Forskolin (50 microM) substantially stimulated a rapid cAMP accumulation, whereas GnRHa (10(-7) M) or pertussis toxin (100 mg/ml) did not affect basal intracellular cAMP levels. Cotreatment of GnRHa (10(-7) M) did not attenuate forskolin- or hCG-stimulated cAMP accumulation. These results suggest that the GnRH receptor is probably not coupled to G(s)alpha or G(i)alpha in hGLCs. Finally, GnRHa (10(-7) M) stimulated a significant increase in Elk-1 phosphorylation and c-fos messenger RNA expression, as revealed by an in vitro kinase assay and Northern blot analysis, respectively. These results clearly demonstrate that GnRH activates the MAPK cascade through a PKC-dependent pathway in the human ovary.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

Activation of the luteinizing hormone beta promoter by gonadotropin-releasing hormone requires c-Jun NH2-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by gonadotropin-releasing hormone (GnRH) in the gonadotrope cell line LbetaT2 was investigated. Treatment with gonadotropin-releasing hormone agonist (GnRHa) activates extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK). Activation of ERK by GnRHa occurred within 5 min, and declined thereafter, whereas activation of JNK by GnRHa occurred with a different time frame, i.e. it was detectable at 5 min, reached a plateau at 30 min, and declined thereafter. GnRHa-induced ERK activation was dependent on protein kinase C or extracellular and intracellular Ca(2+), whereas GnRHa-induced JNK activation was not dependent on protein kinase C or on extracellular or intracellular Ca(2+). To determine whether a mitogen-activated protein kinase family cascade regulates rat luteinizing hormone beta (LHbeta) promoter activity, we transfected the rat LHbeta (-156 to +7)-luciferase construct into LbetaT2 cells. GnRH activated the rat LHbeta promoter activity in a time-dependent manner. Neither treatment with a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, PD98059, nor cotransfection with a catalytically inactive form of a mitogen-activated protein kinase construct inhibited the induction of the rat LHbeta promoter by GnRH. Furthermore, cotransfection with a dominant negative Ets had no effect on the response of the rat LHbeta promoter to GnRH. On the other hand, cotransfection with either dominant negative JNK or dominant negative c-Jun significantly inhibited the induction of the rat LHbeta promoter by GnRH. In addition, GnRH did not induce either the rat LHbeta promoter activity in LbetaT2 cells transfected stably with dominant negative c-Jun. These results suggest that GnRHa differentially activates ERK and JNK, and a JNK cascade is necessary to elicit the rat LHbeta promoter activity in a c-Jun-dependent mechanism in LbetaT2 cells.

Divergent signaling pathways requiring discrete calcium signals mediate concurrent activation of two mitogen-activated protein kinases by gonadotropin-releasing hormone

Receptors coupled to heterotrimeric G proteins are linked to activation of mitogen-activated protein kinases (MAPKs) via receptor- and cell-specific mechanisms. We have demonstrated recently that gonadotropin-releasing hormone (GnRH) receptor occupancy results in activation of extracellular signal-regulated kinase (ERK) through a mechanism requiring calcium influx through L-type calcium channels in alphaT3-1 cells and primary rat gonadotropes. Further studies were undertaken to explore the signaling mechanisms by which the GnRH receptor is coupled to activation of another member of the MAPK family, c-Jun N-terminal kinase (JNK). GnRH induces activation of the JNK cascade in a dose-, time-, and receptor-dependent manner in clonal alphaT3-1 cells and primary rat pituitary gonadotrophs. Coexpression of dominant negative Cdc42 and kinase-defective p21-activated kinase 1 and MAPK kinase 7 with JNK and ERK indicated that specific activation of JNK by GnRH appears to involve these signaling molecules. Unlike ERK activation, GnRH-stimulated JNK activity does not require activation of protein kinase C and is not blocked after chelation of extracellular calcium with EGTA. GnRH-induced JNK activity was reduced after treatment with the intracellular calcium chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester), whereas activation of ERK was not affected. Chelation of intracellular calcium also reduced GnRH-induced activation of JNK in rat pituitary cells in primary culture. GnRH-induced induction and activation of the JNK target c-Jun was inhibited after chelation of intracellular calcium, whereas induction of c-Fos, a known target of ERK, was unaffected. Therefore, although activation of ERK by GnRH requires a specific influx of calcium through L-type calcium channels, JNK activation is independent of extracellular calcium but sensitive to chelation of intracellular calcium. Our results provide novel evidence that GnRH activates two MAPK superfamily members via strikingly divergent signaling pathways with differential sensitivity to activation of protein kinase C and mobilization of discrete pools of calcium.

Activation of MAPK cascades by G-protein-coupled receptors: the case of gonadotropin-releasing hormone receptor

G-protein-coupled receptors (GPCRs) are a large group of integral membrane receptors that transmit signals from a diverse array of external stimuli, including neurotransmitters, hormones, phospholipids, photons, odorants and taste ligands. In response to ligand binding, the GPCRs initiate diverse downstream signaling pathways through four groups of G proteins and other interacting proteins. Key components in GPCR-induced intracellular signaling are four groups of mitogen-activated protein kinase (MAPK) cascades: extracellular signal-related kinase (ERK), Jun N-terminal kinase (JNK), p38MAPK and big MAPK (BMK). The hallmark of MAPK signaling is the stimulation-dependent nuclear translocation of the involved kinases, which regulate gene expression and the cytoplasmic acute response to mitogenic, stress-related, apoptotic and survival stimuli. A special type of GPCR is the gonadotropin-releasing hormone (GnRH) receptor, which uses primarily the Gq protein for its downstream signaling. GnRH activates all four MAPK cascades by a PKC-dependent mechanism. Common signaling molecules, including the tyrosine kinase c-SRC and the small GTPases CDC42, RAC and RAS, are implicated in various aspects of the GnRH-MAPK pathways. Thus, the activation of MAPK cascades by GnRH opens a new vista in the understanding of the transcriptional regulation of genes encoding gonadotropins. However, additional studies on cell lines and whole animals are required to understand GnRH signaling in the context of other hormones during the reproductive cycle of mouse and human.

Calcium influx through L-type channels is required for selective activation of extracellular signal-regulated kinase by gonadotropin-releasing hormone

The hypothalamic decapeptide gonadotropin-releasing hormone stimulates mobilization of two discrete pools of calcium in clonal (alphaT3-1) and primary pituitary gonadotropes. A multidisciplinary approach was implemented to investigate the effects of discrete calcium fluctuations on the signaling pathways linking the gonadotropin-releasing hormone receptor to activation of mitogen-activated protein kinases and immediate early genes. Blockade of calcium influx through nifedipine-sensitive voltage-gated calcium channels reduced buserelin-induced activation of extracellular signal-regulated kinase (ERK) and c-Fos while activation of c-Jun N-terminal kinase and c-Jun was unaffected. Inhibition of buserelin-stimulated ERK activity by nifedipine was also observed in rat pituitary cells in primary culture. Direct activation of alphaT3-1 cell L-type calcium channels with the agonist Bay-K 8644 resulted in phosphorylation of ERK and induction of c-Fos. However, simple voltage-induced channel activation did not produce a sufficient calcium signal, since depolarization with 35 mM KCl failed to induce activation of ERK. Depletion of intracellular calcium stores with thapsigargin did not affect buserelin-induced ERK activation. An inhibitor of protein kinase C decreased calcium influx through nifedipine-sensitive calcium channels and phosphorylation of ERK induced by buserelin. Pharmacological inhibition of protein kinase C did not block Bay-K 8644-induced ERK activation. These observations suggest that calcium influx through L-type channels is required for GnRH-induced activation of ERK and c-Fos and that the influence of calcium lies downstream of protein kinase C.

Potential involvement of calcium, CaM kinase II, and MAP kinases in PCB-stimulated insulin release from RINm5F cells

Polychlorinated biphenyls (PCBs) are environmental contaminants that induce release of insulin in rat insulinoma cells, RINm5F (Fischer et al., Life Sci. (1996) 59, 2041-2049). In the present study the mechanisms of this effect were investigated using noncytotoxic concentrations (10 microg/ml) of a PCB mixture, Aroclor-1254, and the pure PCB congeners 2,2',4,4'-tetrachlorobiphenyl and 2,2',4,4',5, 5'-hexachlorobiphenyl. Treatment of RINm5F cells with each of these agents resulted in a rapid increase in intracellular free calcium. The presence of extracellular calcium was required for PCB-induced insulin release because removal of calcium from the medium attenuated the effect. In addition, pretreatment of RINm5F cells with the calcium channel blocker verapamil also blocked PCB-induced insulin release. To determine whether PCB-related insulin release could be associated with the enzyme, calcium/calmodulin-dependent kinase II (CaM kinase II), RINm5F cells were pretreated with the CaM kinase II inhibitor KN-93. PCB-induced insulin release was completely blocked by KN-93. Under similar treatment conditions, PCBs also induced the activity of mitogen-activated protein kinases (MAPK) 1 and 2. However, inhibition of MAPK activation by a specific inhibitor, PD-98059 (10.0 microM) did not prevent insulin release induced by PCBs. The results of the present investigation suggest a role for calcium and CaM kinase II in PCB-induced insulin release. Furthermore, the results suggest that insulin release by PCBs is independent of the activation of MAPKs.

Role of protein kinase C in facilitation of luteinizing hormone (LH)-releasing hormone-induced LH surges by neuropeptide Y

In female rats, neuropeptide Y (NPY) facilitates LHRH-induced LH surges without affecting basal LH release. The signal transduction mechanisms mediating this facilitation are unknown. Here, the involvement of PKC in this process was investigated. Anterior pituitaries (APs) were removed from rats at 1400 h proestrus and perifused in vitro with M199 for 5 h. After an equilibration and baseline period, tissue received hourly 5-minute pulses of the PKC inhibitor GF109203X (GFX), 2.5 microM, followed 15 min later by a 5-minute pulse of LHRH (10(-8) M), NPY (10(-6) M), or phorbol 12-myristate 13-acetate (PMA, 50 nM), or some combination. This regimen was repeated hourly for 3 h. As shown previously, NPY had no effect on basal LH release but greatly facilitated LHRH-induced LH release. Treatment with PMA also facilitated LHRH-induced LH release, to approximately the same degree as NPY. Inhibition of PKC activity with GFX completely prevented NPY's and PMA's facilitation of LH release but did not inhibit LH release stimulated by LHRH alone. Because previous work suggested involvement of both NPY and PKC in alterations of LHRH receptor affinity or number, the in vivo effects of NPY on LHRH binding characteristics were also investigated. Although NPY treatment reliably enhanced LHRH-induced LH and FSH surges in proestrous rats, this action was not accompanied by any detectable change in the affinity or concentration of LHRH receptors in anterior pituitary cell membranes. In summary, we have found that NPY's actions are blocked by PKC inhibition, mimicked by PKC stimulation, and not associated with any overt alterations in LHRH receptor affinity or number. We conclude that PKC activation is required for NPY's facilitation of LHRH-induced LH surges, and that this mechanism likely involves PKC targets other than those which may alter LHRH receptor number or affinity.

Early growth response protein 1 binds to the luteinizing hormone-beta promoter and mediates gonadotropin-releasing hormone-stimulated gene expression

The hypothalamic neuropeptide, GnRH, regulates the synthesis and secretion of LH from pituitary gonadotropes. Furthermore, it has been shown that the LH beta-subunit gene is regulated by the transcription factors steroidogenic factor-1 (SF-1) and early growth response protein 1 (Egr1) in vitro and in vivo. The present study investigated the roles played by Egr1 and SF-1 in regulating activity of the equine LH beta-subunit promoter in the gonadotrope cell line, alpha T3-1, and the importance of these factors and cis-acting elements in regulation of the promoter by GnRH. All four members of the Egr family were found to induce activity of the equine promoter. The region responsible for induction by Egr was localized to the proximal 185 bp of the promoter, which contained two Egr response elements. Coexpression of Egr1 and SF-1 led to a synergistic activation of the equine (e)LH beta promoter. Mutation of any of the Egr or SF-1 response elements attenuated this synergism. Endogenous expression of Egr1 in alpha T3-1 cells was not detectable under basal conditions, but was rapidly induced after GnRH stimulation. Reexamination of the promoter constructs harboring mutant Egr or SF-1 sites indicated that these sites were required for GnRH induction. In fact, mutation of both Egr sites within the eLH beta promoter completely attenuated its induction by GnRH. Thus, GnRH induces expression of Egr1, which subsequently activates the eLH beta promoter. Finally, GnRH not only induced expression of Egr1, but also its corepressor, NGFI-A (Egr1) binding protein (Nab1), which can repress Egr1- induced transcription of the eLH beta promoter.

The role of protein kinases A and C pathways in the regulation of mitogen-activated protein kinase activation in response to gonadotropin-releasing hormone receptor activation

There is convincing evidence that mitogen-activated protein kinase (MAPK) activation is coupled to both receptor tyrosine kinase and G protein-coupled receptors. The presence of the epidermal growth factor (EGF) receptor and the GnRH receptor on the surface of GGH(3)1' cells makes this cell line a good model for the assessment of MAPK activation by receptor tyrosine kinases and G protein-coupled receptors. In this study, to assess the activated and total (i.e. activated plus inactivated) MAPK, the phosphorylation state of p44 and p42 MAPKs was examined using antisera that distinguish phospho-p44/42 MAPK (Thr202/Tyr204) from p44/42 MAPK (phosphorylation state independent). The data show that both EGF (200 ng/ml) and Buserelin (a GnRH agonist; 10 ng/ml) provoke rapid activation of MAPK (within 5 and 15 min, respectively) after binding to their receptors. The role of protein kinase A (PKA) and protein kinase C (PKC) signal transduction pathways in mediating MAPK activation was also assessed. Both phorbol ester (phorbol 12-myristate 13-acetate; 10 ng/ml) and (Bu)2cAMP (1 mM) trigger the phosphorylation of MAPK, suggesting potential roles for PKC and PKA signaling events in MAPK activation in GGH(3)1' cells. Treatment of PKC-depleted cells with Buserelin activated MAPK, suggesting involvement of PKC-independent signal transduction pathways in MAPK activation in response to GnRH. Similarly, treatment of PKC-depleted cells with forskolin (50 microM) or cholera toxin (100 ng/ml) stimulated MAPK activation, whereas pertussis toxin (100 ng/ml) had no measurable effect. To further assess the role of PKA in response to EGF and Buserelin, cells were treated with EGF (200 ng/ml) for 3 min or with Buserelin (10 ng/ml) for 10 min after pretreatment with 3-isobutyl-1-methylxanthine (0.5 mM), forskolin (50 microM), or (Bu)2cAMP (1 mM) for 15 min. The results show that MAPK can be activated in a PKA-dependent manner in GGH(3)1' cells. Consistent with previous reports, the current data support the view that MAPK activation can be achieved via both PKC- and PKA-dependent signaling pathways triggered by the GnRH receptor that couples to G(q/11) and Gs alpha-subunit proteins. In contrast, G(i/o)alpha does not appear to participate in MAPK activation in GGH(3)1' cells.

Homologous regulation of the gonadotropin-releasing hormone receptor gene is partially mediated by protein kinase C activation of an activator protein-1 element

Homologous regulation of GnRH receptor (GnRHR) gene expression is an established mechanism for controlling the sensitivity of gonadotropes to GnRH. We have found that expression of the GnRHR gene in the gonadotrope-derived alpha T3-1 cell line is mediated by a tripartite enhancer that includes a consensus activator protein-1 (AP-1) element, a binding site for SF-1 (steroidogenic factor-1), and an element we have termed GRAS (GnRHR-activating sequence). Further, in transgenic mice, approximately 1900 b.p. of the murine GnRHR gene promoter are sufficient for tissue-specific expression and GnRH responsiveness. The present studies were designed to further delineate the molecular mechanisms underlying GnRH regulation of GnRHR gene expression. Vectors containing 600 bp of the murine GnRHR gene promoter linked to luciferase (LUC) were transiently transfected into alpha T3-1 cells and exposed to treatments for 4 or 6 h. A GnRH-induced, dose-dependent increase in LUC expression of the -600 promoter was observed with maximal induction of LUC noted at 100 nM GnRH. We next tested the ability of GnRH to stimulate expression of vectors containing mutations in each of the components of the tripartite enhancer. GnRH responsiveness was lost in vectors containing mutations in AP-1. Gel mobility shift data revealed binding of fos/jun family members to the AP-1 element of the murine GnRHR promoter. Treatment with GnRH or phorbol-12-myristate-13-acetate (PMA) (100 nM), but not forskolin (10 microM), increased LUC expression, which was blocked by the protein kinase C (PKC) inhibitor, GF109203X (100 nM), and PKC down-regulation (10 nM PMA for 20 h). In addition, a specific MEK1/MEK2 inhibitor, PD98059 (60 microM), reduced the GnRH and PMA responses whereas the L-type voltage-gated calcium channel agonist, +/- BayK 8644 (5 microM), and antagonist, nimodipine (250 nM), had no effect on GnRH responsiveness. Furthermore, treatment of alpha T3-1 cells with 100 nM GnRH stimulated phosphorylation of both p42 and p44 forms of extracellular signal-regulated kinase (ERK), which was completely blocked with 60 microM PD98059. We suggest that GnRH regulation of the GnRHR gene is partially mediated by an ERK-dependent activation of a canonical AP-1 site located in the proximal promoter of the GnRHR gene.

Activation of the p38 mitogen-activated protein kinase pathway by gonadotropin-releasing hormone

Previous studies have shown that interaction of GnRH with its serpentine, G protein-coupled receptor results in activation of the extracellular signal regulated protein kinase (ERK) and the Jun N-terminal protein kinase (JNK) pathways in pituitary gonadotropes. In the present study, we examined GnRH-stimulated activation of an additional member of the mitogen-activated protein kinase (MAPK) superfamily, p38 MAPK GnRH treatment of alphaT3-1 cells resulted in tyrosine phosphorylation of several intracellular proteins. Separation of phosphorylated proteins by ion exchange chromatography suggested that GnRH receptor stimulation can activate the p38 MAPK pathway. Immunoprecipitation studies using a phospho-tyrosine antibody resulted in increased amounts of immunoprecipitable p38 MAPK from alphaT3-1 cells treated with GnRH. Immunoblot analysis of whole cell lysates using a phospho-specific antibody directed against dual phosphorylated p38 kinase revealed that GnRH-induced phosphorylation of p38 kinase was dose and time dependent and was correlated with increased p38 kinase activity in vitro. Activation of p38 kinase was blocked by chronic phorbol ester treatment, which depletes protein kinase C isozymes alpha and epsilon. Overexpression of p38 MAPK and an activated form of MAPK kinase 6 resulted in activation of c-jun and c-fos reporter genes, but did not alter the expression of the glycoprotein hormone alpha-subunit reporter. Inhibition of p38 activity with SB203580 resulted in attenuation of GnRH-induced c-fos reporter gene expression, but was not sufficient to reduce GnRH-induced c-jun or glycoprotein hormone alpha-subunit promoter activity. These studies provide evidence that the GnRH signaling pathway in alphaT3-1 cells includes protein kinase C-dependent activation of the p38 MAPK pathway. GnRH integration of c-fos promoter activity may include regulation by p38 MAPK.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

Transcriptional activation of gonadotropin-releasing hormone (GnRH) receptor gene by GnRH: involvement of multiple signal transduction pathways

Previous studies have shown that GnRH activates transcriptional activity of its own receptor (GnRHR) gene in part through the cAMP signal transduction pathway. In the present study we explored the possible involvement of multiple signal transduction pathways in GnRH regulation of GnRHR gene transcription; these studies relied upon a luciferase reporter gene vector (GnRHR-pXP2) containing a 1226-bp promoter fragment (-1164 to +62, relative to the major transcription start site) of the mouse GnRHR gene in GGH3 cells (GH3 cells stably expressing rat GnRHR). Activation of protein kinase C (PKC) by phorbol myristic acid significantly stimulated GnRHR-luciferase reporter gene (GnRHR-Luc) activity, but did not potentiate the stimulation of GnRHR-Luc activity by the GnRH agonist, buserelin (GnRH-A). Inhibition of PKC by PKC inhibitor (GF 109203X) or depletion of PKC blocked phorbol myristic acid- or GnRH-A-stimulated GnRHR-Luc activity, but did not affect (Bu)2cAMP-stimulated GnRHR-Luc activity. In addition, GnRH-A-stimulated GnRHR-Luc activity was inhibited by preventing external Ca2+ influx with the external Ca2+ chelator EGTA or the Ca2+ ion channel antagonist, D600. Surprisingly, overexpression of the mitogen-activated protein kinase (MAPK) kinase kinase (Raf-1) inhibited GnRHR-Luc activity and partially blocked GnRH-A-stimulated GnRHR-Luc activity. In contrast, inhibition of MAPK activity by MAPK kinase inhibitor (PD 98059) or by overexpression of kinase-deficient MAPKs activated basal and GnRH-A-stimulated GnRHR-Luc activity. These results suggested that PKC- and Ca2+-dependent signal transduction pathways participate in the GnRH activation of GnRHR promoter activity, and that the MAPK cascade is involved in the negative regulation of basal and GnRH-stimulated GnRHR transcriptional activity conferred by the 1226-bp promoter fragment.

Mechanism of GnRH receptor signaling: combinatorial cross-talk of Ca2+ and protein kinase C

Gonadotropin-releasing hormone (GnRH), the first key hormone of reproduction, is synthesized in the hypothalamus and is released in a pulsatile manner to stimulate pituitary gonadotrope-luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and release. Gonadotropes represent only about 10% of pituitary cells and are divided into monohormonal cells (18% LH and 22% FSH cells) and 60% multihormonal (LH + FSH) cells. GnRH binds to a specific seven transmembrane domain receptor which is coupled to Gq and activates sequentially different phospholipases to provide Ca2+ and lipid-derived messenger molecules. Initially, phospholipase C is activated, followed by activation of both phospholipase A2 (PLA2) and phospholipase D (PLD). Generation of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG) lead to mobilization of intracellular pools of Ca2+ and activation of protein kinase C (PKC). Early DAG and Ca2+, derived via enhanced phosphoinositide turnover, might be involved in rapid activation of selective Ca(2+)-dependent, conventional PKC isoforms (cPKC). On the other hand, late DAG, derived from phosphatidic acid (PA) via PLD, may activate Ca(2+)-independent novel PKC isoforms (nPKC). In addition, arachidonic acid (AA) which is liberated by activated PLA2, might also support selective activation of PKC isoforms (PKCs) with or without other cofactors. Differential cross-talk of Ca2+, AA, and selective PKCs might generate a compartmentalized signal transduction cascade to downstream elements which are activated during the neurohormone action. Among those elements is the mitogen-activated protein kinase (MAPK) cascade which is activated by GnRH in a PKC-, Ca(2+)-, and protein tyrosine kinase (PTK)-dependent fashion. Transcriptional regulation can be mediated by the activation of transcription factors such as c-fos by MAPK. Indeed, GnRH activates the expression of both c-jun and c-fos which might participate in gene regulation via the formation of AP-1. The signaling cascade leading to gonadotropin (LH and FSH) gene regulation by GnRH is still not known and might involve the above-mentioned cascades. AA and selective lipoxygenase products such as leukotriene C4 also participate in GnRH action, possibly by cross-talk with PKCs, or by an autocrine/paracrine amplification cycle. A complex combinatorial, spatial and temporal cross-talk of the above messenger molecules seems to mediate the diverse effects elicited by GnRH, the first key hormone of the reproductive cycle.

Mechanism of mitogen-activated protein kinase activation by gonadotropin-releasing hormone in the pituitary of alphaT3-1 cell line: differential roles of calcium and protein kinase C

The mechanism of mitogen-activated protein kinase (MAPK, ERK) stimulation by the GnRH analog [D-Trp6]GnRH (GnRH-a) was investigated in the gonadotroph-derived alphaT3-1 cell line. GnRH-a as well as the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol-13-acetate (TPA) stimulated a sustained response of MAPK activity, whereas epidermal growth factor (EGF) stimulated a transient response. MAPK kinase (MEK) is also activated by GnRH-a, but in a transient manner. GnRH-a and TPA apparently activated mainly the MAPK isoform ERK1, as revealed by Mono-Q fast protein liquid chromatography followed by Western blotting as well as by gel kinase assay. GnRH-a and TPA stimulated the tyrosine phosphorylation of several proteins, and this effect as well as the stimulation of MAPK activity were inhibited by the PKC inhibitor GF 109203X. Similarly, down-regulation of TPA-sensitive PKC subspecies nearly abolished the effect of GnRH-a and TPA on MAPK activity. Furthermore, the protein tyrosine kinase (PTK) inhibitor genistein inhibited protein tyrosine phosphorylation and reduced GnRH-a-stimulated MAPK activity by 50%, suggesting the participation of genistein-sensitive and insensitive pathways in GnRH-a action. Although Ca2+ ionophores have only a marginal stimulatory effect, the removal of Ca2+ markedly reduced MAPK activation by GnRH-a and TPA, but had no effect on GnRH-a and TPA stimulation of protein tyrosine phosphorylation. Interestingly, the removal of Ca2+ also partly inhibited the activation of MAPK by EGF and vanadate/H2O2. Thus, a calcium-dependent component(s) downstream of PKC and PTK might also participate in MAPK activation. Elevation of cAMP by forskolin exerted partial inhibition on EGF, but not on TPA or GnRH-a action, suggesting that MEK activators other than Raf-1 might be involved in GnRH action. We conclude that Ca2+, PTK, and PKC participate in the activation of MAPK by GnRH-a, with Ca2+ being necessary downstream to PKC and PTK.

The mitogen-activated protein kinase pathway in rat islets of Langerhans: studies on the regulation of insulin secretion

The expression of mitogen-activated protein kinases (MAPKs) and MAPK kinases (MEKs) in rat islets of Langerhans and the involvement of MAPKs in regulated insulin secretion were examined. Two major isoforms of both MEK (45 and 46 kDa) and MAPK (42 and 44 kDa) were detected in rat islets and shown to be localized to insulin-secreting beta cells by detection of their expression in the beta cell line MIN6. The tyrosine phosphatase inhibitor sodium pervanadate, and, to a lesser extent, the serine/threonine phosphatase inhibitor okadaic acid, stimulated MAPK phosphorylation, as assessed by a shift in its electrophoretic mobility and by increased phosphotyrosine immunoreactivity of immunoprecipitated MAPK. The increase in MAPK phosphorylation stimulated by sodium pervanadate was not coupled to an increase in MAPK activity, but okadaic acid, either alone or in the presence of sodium pervanadate, caused an increase in myelin basic protein phosphorylation by MAPK. Neither okadaic acid nor sodium pervanadate, either individually or combined, stimulated insulin secretion. 4 beta-phorbol myristate acetate stimulated an increase in phosphorylation of the 42 kDa isoform of MAPK (erk2) in human umbilical vein endothelial cells, but neither it nor glucose affected either the phosphorylation state of islet erk2 or the activities of immunoprecipitated islet MAPKs. These results provide evidence for the presence of a regulated MAPK pathway in adult rat islets, but our data suggest that MAPK activation alone is not a sufficient stimulus for insulin secretion.

Activation of MAP kinase by the LHRH receptor through a dual mechanism involving protein kinase C and a pertussis toxin-sensitive G protein

The LHRH receptor in alpha T3-1 gonadotrope cells was shown to bring about a marked and sustained activation of MAP kinase. This response was prevented by protein kinase C inhibition or down-regulation and could be partially mimicked by phorbol ester. Additional evidence for inhibition of this response by pertussis toxin and partial mimicry by mastoparan (in a pertussis toxin-sensitive manner) provides the first evidence for Gi/Go-mediated signal transduction by the LHRH receptor. This dual mechanism of MAP kinase activation appears to be exceptional amongst the G protein-linked receptors that have been investigated.