Signal transduction of the gonadotropin releasing hormone (GnRH) receptor: cross-talk of calcium, protein kinase C (PKC), and arachidonic acid

C-Src is Activated by the EGF Receptor in a Pathway that Mediates JNK and ERK Activation by Gonadotropin-Releasing Hormone in COS7 Cells

The key participants in G-protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. The prototypical GPCR is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here, we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via Gαi and the EGF receptor, without the involvement of Hb-EGF or PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and PI3K. ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Beside the main pathway, the dissociated Gβγ and β-arrestin may initiate additional (albeit minor) pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other GnRHR-bearing cells, indicating that GnRH can utilize various signaling mechanisms for MAPK activation. The unique pathway elucidated here, in which c-Src and PI3K are sequentially activated downstream of the EGF receptor, may serve as a prototype of signaling mechanisms by GnRHR and additional GPCRs in various cell types.

GnRH suppresses excitability of visual processing neurons in the optic tectum

Animals change their behavior in response to sensory cues in the environment as well as their physiological status. For example, it is generally accepted that their sexual behavior is modulated according to seasonal environmental changes or the individual's maturational/reproductive status, and neuropeptides have been suggested to play important roles in this process. Some behavioral modulation arises from neuropeptide modulation of sensory information processing in the central nervous system, but the neural mechanisms still remain unknown. Here we focused on the neural basis of neuropeptide modulation of visual processing in vertebrates. The terminal nerve neurons that contain gonadotropin-releasing hormone 3 (TN-GnRH3 neurons) are suggested to modulate reproductive behavior and have massive projections to the optic tectum (OT), which plays an important role in visual processing. In the present study, to examine whether GnRH3 modulates retino-tectal neurotransmission in the OT, we analyzed the effect of GnRH3 electrophysiologically and morphologically. We found that field potentials evoked by optic tract fiber stimulation, which represent retino-tectal neurotransmission, were modulated postsynaptically by GnRH3. Whole cell recording from postsynaptic neurons in the retino-tectal pathway suggested that GnRH3 activates large-conductance Ca(2+)-activated K(+) (BK) channels and thereby suppresses membrane excitability. Furthermore, our improved morphological analysis using fluorescently labeled GnRH peptides showed that GnRH receptors are localized mainly around the cell bodies of postsynaptic neurons. Our results indicate that TN-GnRH3 neurons modulate retino-tectal neurotransmission by suppressing the excitability of projection neurons in the OT, which underlies the neuromodulation of behaviorally relevant visual information processing by the neuropeptide GnRH3.

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.

Use of goserelin in the treatment of breast cancer

Gonadotropin-releasing hormone analogs are, alongside tamoxifen, one of the most commonly used drugs in the treatment of pre-/perimenopausal endocrine-responsive breast cancer. Goserelin, as a principal agent of this class of drugs, is mainly investigated in clinical trials. The indirect comparison of goserelin with tamoxifen as a single drug in the adjuvant setting showed similar efficacy. Furthermore, goserelin is as effective as cyclophosphamide, methotrexate and 5-fluorouracil chemotherapy, and total endocrine blockade as a combination of gonadotropin-releasing hormone analog and tamoxifen showed a comparable benefit with anthracycline-containing adjuvant chemotherapy. Goserelin administered after cessation of chemotherapy leads to a further improvement and may be equieffective as tamoxifen or a combination of both. Data concerning taxane-based and dose-dense chemotherapy as well as combination of gonadotropin-releasing hormone analogs with third-generation aromatase inhibitors are still lacking (ongoing suppression of ovarian function, tamoxifen and exemestane, and premenopausal endocrine-responsive chemotherapy trials). Moreover, duration of therapy with gonadotropin-releasing hormone analogs (2-3 years or longer) is still a matter of debate. Palliative endocrine treatment is standard in the first-line therapy of patients without life-threatening disease and endocrine-responsive breast cancer. Treatment decisions depend upon adjuvant endocrine pretreatment. Clinical data regarding ovarian protection by synchronous use of gonadotropin-releasing hormone in young breast cancer patients receiving chemotherapy are incoherent.

High glucose-induced proteome alterations in retinal pigmented epithelium cells and its possible relevance to diabetic retinopathy

Diabetic retinopathy can cause poor vision and blindness. Previous research has shown that high blood glucose weakens retinal capillaries and induces glycoxidation. However, the detailed molecular mechanisms underlying the effects of high blood glucose on development of diabetic retinopathy have yet to be elucidated. In this study, we cultured a retinal pigmented epithelium cell line (ARPE-19) in mannitol-balanced 5.5 mM, 25 mM, and 100 mM d-glucose media, and evaluated protein expression and redox-regulation. We identified 56 proteins that showed significant changes in protein expression, and 33 proteins showing significant changes in thiol reactivity, in response to high glucose concentration. Several proteins that are involved in signal transduction, gene regulation, and transport showed significant changes in expression, whereas proteins involved in metabolism, transport, and cell survival displayed changes in thiol reactivity. Further analyses of clinical plasma specimens confirmed that the proteins lamin B2, PUMA, WTAP, ASGR1, and prohibitin 2 showed type 2 diabetic retinopathy-dependent alterations. In summary, in this study, we used a comprehensive retinal cell-based proteomic approach for the identification of changes in protein expression and redox-associated retinal markers induced by high glucose concentration. Some of the identified proteins have been validated with clinical samples and provide potential targets for the prognosis and diagnosis of diabetic retinopathy.

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

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

Pulsatile GnRH secretion: roles of G protein-coupled receptors, second messengers and ion channels

The pulsatile secretion of GnRH from normal and immortalized hypothalamic GnRH neurons is highly calcium-dependent and is stimulated by cAMP. It is also influenced by agonist activation of the endogenous GnRH receptor (GnRH-R), which couples to multiple G proteins. This autocrine mechanism could serve as a timer to determine the frequency of pulsatile GnRH release by regulating Ca(2+)- and cAMP-dependent signaling and GnRH neuronal firing. The firing of individual and/or bursts of action potentials (APs) in spontaneously active GnRH neurons is followed by afterhyperpolarization (AHP) that lasts from several milliseconds to several seconds. GnRH-induced activation of GnRH neurons causes a significant increase in medium AHP that is partially sensitive to apamin. GnRH-induced modulation of Ca(2+) influx and the consequent changes in AHP current suggest that the GnRH receptors expressed in hypothalamic GnRH neurons are important modulators of their neuronal excitability. The coexistence of multiple regulatory mechanisms could provide a high degree of redundancy in the maintenance of this crucial component of the reproductive process. It is also conceivable that this multifactorial system could reflect the gradation from simple to more complex neuroendocrine control systems for regulating hypothalamo-pituitary function and gonadal activity during the evolution of the GnRH pulse generator.

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.

GnRH pulsatility, the pituitary response and reproductive dysfunction

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

Luteinizing hormone-releasing hormone agonists in premenopausal hormone receptor-positive breast cancer

Ovarian function suppression for the treatment of premenopausal breast cancer was first used in the late 19th century. Traditionally, ovarian function suppression had been accomplished irreversibly via irradiation or surgery, but analogues of the luteinizing hormone-releasing hormone (LH-RH) have emerged as reliable and reversible agents for this purpose, especially the LH-RH agonists. Luteinizing hormone-releasing hormone antagonists are in earlier stages of development in breast cancer and are not currently in clinical use. Luteinizing hormonereleasing hormone agonists act by pituitary desensitization and receptor downregulation, thereby suppressing gonadotrophin release. Limited information is available comparing the efficacies of the depot preparations of various agonists, but pharmacodynamic studies have shown comparable suppressive capabilities on estradiol and luteinizing hormone. At present, only monthly goserelin is Food and Drug Administration-approved for the treatment of estrogen receptor-positive, premenopausal metastatic breast cancer in the United States. Luteinizing hormone-releasing hormone agonists have proven to be as effective as surgical oophorectomy in premenopausal advanced breast cancer. They offer similar outcomes compared with tamoxifen, but the endocrine combination appears to be more effective than LH-RH agonists alone. In the adjuvant setting, LH-RH agonists versus no therapy reduce the annual odds of recurrence and death in women aged>50 years with estrogen receptor-positive tumors. Luteinizing hormone-releasing hormone agonists alone or in combination with tamoxifen have shown disease-free survival rates similar to chemotherapy with CMF (cyclophosphamide/methotrexate/5-fluorouracil). Outcomes of chemotherapy with or without LH-RH agonists are comparable, though a few trials favor the combination in young premenopausal women (aged<40 years). Adjuvant LH-RH agonists with or without tamoxifen might be as efficacious as tamoxifen alone, and the additional benefit from chemotherapy is unclear. Adequately powered studies are now studying the relative merits of combining adjuvant tamoxifen or aromatase inhibitors with ovarian function suppression, the additional benefits of adding ovarian function suppression to chemotherapy, and the need for adjuvant chemotherapy for women treated with combined ovarian function suppression and anti-estrogen therapy.

Loss of Gq/11 family G proteins in the nervous system causes pituitary somatotroph hypoplasia and dwarfism in mice

Heterotrimeric G proteins of the Gq/11 family transduce signals from a variety of neurotransmitter and hormone receptors and have therefore been implicated in various functions of the nervous system. Using the Cre/loxP system, we generated mice which lack the genes coding for the alpha subunits of the two main members of the Gq/11 family, gnaq and gna11, selectively in neuronal and glial precursor cells. Mice with defective gnaq and gna11 genes were morphologically normal, but they died shortly after birth. Mice carrying a single gna11 allele survived the early postnatal period but died within 3 to 6 weeks as anorectic dwarfs. In these mice, postnatal proliferation of pituitary somatotroph cells was strongly impaired, and plasma growth hormone (GH) levels were reduced to 15%. Hypothalamic levels of GH-releasing hormone (GHRH), an important stimulator of somatotroph proliferation, were strongly decreased, and exogenous administration of GHRH restored normal proliferation. The hypothalamic effects of ghrelin, a regulator of GHRH production and food intake, were reduced in these mice, suggesting that an impairment of ghrelin receptor signaling might contribute to GHRH deficiency and abnormal eating behavior. Taken together, our findings show that Gq/11 signaling is required for normal hypothalamic function and that impairment of this signaling pathway causes somatotroph hypoplasia, dwarfism, and anorexia.

Gonadotropin-releasing hormone-desensitized LbetaT2 gonadotrope cells are refractory to acute protein kinase C, cyclic AMP, and calcium-dependent signaling

Sustained exposure of gonadotropes to GnRH causes a pronounced desensitization of gonadotropin release, but the mechanisms involved are poorly understood. It is known that desensitization is associated with decreased GnRH receptor and Gq/11 levels in alphaT3-1 cells, but it is not known whether downstream signaling is impaired. We have shown previously that chronic stimulation of signaling via expression of an active form of Galphaq causes GnRH resistance in LbetaT2 cells. In this study we investigated whether chronic GnRH treatment could down-regulate protein kinase C (PKC), cAMP, or Ca2+-dependent signaling in LbetaT2 cells. We found that chronic GnRH treatment desensitizes cells to acute GnRH stimulation not only by reducing GnRH receptor and Gq/11 expression but also by down-regulating PKC, cAMP, and calcium-dependent signaling. Desensitization was observed for activation of ERK and p38 MAPK and induction of c-fos and LHbeta protein expression. Activation of individual signaling pathways was able to partially mimic the desensitizing effect of GnRH on ERK, p38 MAPK, c-fos, and LHbeta but not on Gq/11. Chronic stimulation with phorbol esters reduced GnRH receptor expression to the same extent as chronic GnRH. Sustained GnRH also desensitized PKC signaling by down-regulating the delta, epsilon, and theta isoforms of PKC. We further show that chronic GnRH treatment causes heterologous desensitization of other Gq-coupled receptors.

c-Src is activated by the epidermal growth factor receptor in a pathway that mediates JNK and ERK activation by gonadotropin-releasing hormone in COS7 cells

Key participants in G protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. A prototypic GPCR that has been widely used to study these signaling mechanisms is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via G alpha i, EGF receptor without the involvement of Hb-EGF, and c-Src, but independently of PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and phosphatidylinositol 3-kinase (PI3K). ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Besides the main pathway, the dissociated G beta gamma and beta-arrestin may initiate additional, albeit minor, pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other cell lines bearing GnRHR, indicating that GnRH can utilize various signaling mechanisms for the activation of MAPK cascades. The unique pathway elucidated here in which c-Src and PI3K are sequentially activated downstream of the EGF receptor may serve as a prototype of signaling mechanisms by GnRHR and by additional GPCRs in various cell types.

Differential regulation of gonadotropin-releasing hormone (GnRH)I and GnRHII messenger ribonucleic acid by gonadal steroids in human granulosa luteal cells

In humans, reproduction was generally believed to be controlled by only one form of GnRH (called mammalian GnRH or GnRHI). However, recently, a second form of GnRH, analogous to chicken GnRHII, was discovered in several tissues, including the human ovary. The regulation and function of GnRHI in the hypothalamus has been well studied. However, the function and regulation of GnRHI, and particularly GnRHII in the ovary, is less well understood. Because gonadal sex steroids are one of the main regulators of reproduction, we investigated, in the present study, the regulation of GnRHI and GnRHII mRNA expression by 17beta-estradiol (E2) and RU486 (a progesterone antagonist) in human granulosa luteal cells (hGLCs). The levels of the mRNA transcripts encoding the two GnRH forms were examined using semiquantitative RT-PCR followed by Southern blot analysis. With time in culture, GnRHI and GnRHII mRNA levels significantly increased, by 120% and 210%, at d 8 and d 1, respectively. The levels remained elevated until the termination of these experiments at d 10. A 24-h treatment of hGLCs with E2 (10(-9) to 10(-7) M) resulted in a dose-dependent decrease and increase in mRNA expression of GnRHI and GnRHII, respectively. E2 (10(-9) M) significantly decreased GnRHI mRNA levels (by 55%) and increased GnRHII mRNA levels (by 294%). Time-course studies demonstrated that E2 (10(-9) M) significantly decreased GnRHI mRNA levels in a time-dependent manner, with maximal inhibition of 77% at 48 h. In contrast, GnRHII mRNA levels significantly increased in a time-dependent fashion, reaching a maximum level of 280% at 24 h. Cotreatment of hGLCs with E2 and tamoxifen (an E2 antagonist) reversed the inhibitory and stimulatory effects of E2 on the mRNA expression of GnRHI and GnRHII, respectively. Time- and dose-dependent treatment with RU486 did not affect GnRHI mRNA levels in hGLCs. In contrast, RU486 treatment significantly increased GnRHII mRNA levels in hGLCs in a time- and dose-dependent fashion, with a maximum increase being observed at 24 h (with 10(-5)M RU486). In summary, the present study demonstrated that the expression of GnRHI and GnRHII at the transcriptional level is differently regulated by E2 and P4 in hGLCs.

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.

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.

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.

Regulation of human gonadotropin-releasing hormone receptor gene expression in placental cells

GnRH has been suggested to regulate hCG secretion in the placenta. In the present study, we report isolation of full-length GnRH receptor (GnRHR) complementary DNA from human placental cells, including a choriocarcinoma cell line (JEG-3), immortalized extravillous trophoblasts (IEVT), and first trimester cytotrophoblast cells in primary culture. Sequence analysis of the placental GnRHR complementary DNA revealed a 100% similarity to its pituitary counterpart. Northern blot analysis using polyadenylated RNA isolated from JEG-3 and IEVT cells revealed a 2.5- and 1.2-kb GnRHR transcripts. Using semiquantitative RT-PCR, regulation ofplacental GnRHR gene expression was examined. In contrast to pituitary gonadotrope alphaT3-1 cells, down-regulation of GnRHR messenger RNA (mRNA) levels was not observed in placental cells after 24 h of 0.1-microM GnRH agonist (GnRHa) treatment. Instead, a 43% (P < 0.01) and 30% (P < 0.05) increase in GnRHR mRNA levels was observed in JEG-3 and IEVT cells, respectively. In addition, 10 microM phorbol ester or forskolin treatments resulted in a significant increase in GnRHR expression in both JEG-3 and IEVT cells. The GnRHa-induced increase in GnRHR expression was shown to be a receptor-mediated process, as cotreatment of GnRH antagonist abolished the effect. It has also been demonstrated that these stimulatory effects on GnRHR gene expression were regulated at least in part at the transcriptional level. Pretreatment of JEG-3 cells with a specific protein kinase C inhibitor (GF109203X), adenylate cyclase inhibitor (SQ22536), or protein kinase A inhibitor [PKI-(14-22) amide, myristylated] reversed GnRHa-induced GnRHR gene expression, suggesting that the placental GnRHR couples to the protein kinase C (PKC) and cAMP/ protein kinase A (PKA) pathways. By Northern blot analysis, we observed a 100% (P < 0.001) increase in hCGbeta mRNA levels after 0.1 microM GnRHa treatment in JEG-3 cells. Again, this effect was prevented in the presence of either protein kinase C inhibitor or adenylate cyclase inhibitor, further supporting the role of the PKC and PKA pathways in GnRHR-coupled signaling in placental cells. In summary, these data strongly support the idea that 1) GnRH plays an autocrine/paracrine role in regulating placental function through a receptor-mediated mechanism; and 2) the placental GnRHR couples to both the PKC and PKA pathways.

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.

Dual intracellular pathways in gonadotropin releasing hormone (GNRH) induced desensitization of luteinizing hormone (LH) secretion

The mechanisms of GnRH-induced desensitization of LH secretion are poorly understood. Protein kinase C (PKC) and protein kinase A (PKA) desensitize some receptors of the 7-membrane type, and the GnRH receptor has consensus phosphorylation sites for PKC in the first and third intracellular loops, and a site for PKA in the first intracellular loop. In the first set of experiments we determined whether synthetic peptides representing the three intracellular loops of the receptor could be phosphorylated in vitro by purified PKC and PKA. As compared with a model substrate peptide for PKC, the third intracellular loop was phosphorylated 74% and the first intracellular loop 21%; PKA-phosphorylated the first intracellular loop peptide 17% as well as a model peptide substrate. In the second set of experiments, we used phorbol 12-myristate 13 acetate (PMA), an established PKC stimulator, and cholera toxin (CTX), established to activate the Gs protein and presumed to activate PKA, to treat cultured rat pituitary cells followed by LH measurements. Treatment with both drugs severely impaired GnRH-stimulated LH secretion whereas neither drug alone reduced LH secretion. Dibutyryl cAMP did not duplicate the effects of cholera toxin suggesting that the CTX action could not be explained by an increase in cAMP. These results suggest that more than one intracellular signaling pathway requires activation in order to induce desensitization; one pathway involves PKC and the other involves a pathway stimulated by cholera toxin, presumably Gs protein, which does not involve PKA.

Activation of gonadotropin-releasing hormone receptors induces a long-term enhancement of excitatory postsynaptic currents mediated by ionotropic glutamate receptors in the rat hippocampus

Whole-cell patch-clamp recordings were made from CA1 pyramidal neurons of the rat hippocampus to study the modulation of gonadotropin-releasing hormone (GnRH) on synaptic transmission mediated by ionotropic glutamate receptors. Leuprolide (10(-9)-10(-7) M), a specific GnRH analog, concentration-dependently elicited a long-lasting potentiation of excitatory postsynaptic currents (EPSCs) mediated by ionotropic glutamate receptors. GnRH receptor-induced synaptic potentiation was blocked by 1 microM [Acetyl-3,4-dehydro-Pro1,D-p-F-Phe2,D-Trp3,6]-LHRH, a specific GnRH receptor antagonist. Furthermore, GnRH receptor-induced synaptic potentiation was associated with the stimulation of protein kinase C (PKC), being considerably attenuated by a potent PKC inhibitor (30 microM H-7). The results suggest a long-term enhanced modulation of GnRH on synaptic transmission mediated by ionotropic glutamate receptors, possibly via the actions of PKC in the hippocampus that is an important integrative system in the regulation of reproductive processes.

Alanine-261 in intracellular loop III of the human gonadotropin-releasing hormone receptor is crucial for G-protein coupling and receptor internalization

Gonadotropin-releasing hormone (GnRH) is a decapeptide that regulates reproductive function via binding to the GnRH receptor, which is a G-protein-coupled receptor (GPCR). For several members of this family, the C-terminal domain of intracellular loop III is important in ligand-mediated coupling to G-proteins; mutations in that region can lead to constitutive activity. A specific alanine residue is involved in certain GPCRs, the equivalent of which is Ala-261 in the GnRH receptor. Mutation of this residue to Leu, Ile, Lys, Glu or Phe in the human GnRH receptor did not result in constitutive activity and instead led to complete uncoupling of the receptor (failure to support GnRH-stimulated inositol phosphate production). When this residue was mutated to Gly, Pro, Ser or Val, inositol phosphate production was still supported. All the mutants retained the ability to bind ligand, and the affinity for ligand, where measured, was unchanged. These results show that Ala-261 cannot be involved in ligand binding but is critical for coupling of the receptor to its cognate G-protein. Coupling is also dependent on the size of the residue in position 261. When the amino acid side chain has a molecular mass of less than 40 Da efficient coupling is still possible, but when its molecular mass exceeds 50 Da the receptor is uncoupled. Internalization studies on the Ala261-->Lys mutant showed a marked decrease in receptor internalization compared with the wild type, indicating that coupling is necessary for effective receptor internalization in the GnRH receptor system. Activation of protein kinase C (with PMA), but not protein kinase A (with forskolin) markedly increased the internalization of the mutant receptor while having a small effect on the wild-type receptor.

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.

Differential activation of protein kinase C delta and epsilon gene expression by gonadotropin-releasing hormone in alphaT3-1 cells. Autoregulation by protein kinase C

The effect of gonadotropin-releasing hormone (GnRH) upon protein kinase C (PKC) delta and PKCepsilon gene expression was investigated in the gonadotroph-derived alphaT3-1 cell line. Stimulation of the cells with a stable analog [D-Trp6]GnRH (GnRH-A) resulted in a rapid elevation of PKCepsilon mRNA levels (1 h), while PKCdelta mRNA levels were elevated only after 24 h of incubation. The rapid elevation of PKCepsilon mRNA by GnRH-A was blocked by pretreatment with a GnRH antagonist or actinomycin D. The PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA), but not the Ca2+ ionophore ionomycin, mimicked the rapid effect of GnRH-A upon PKCepsilon mRNA elevation. Additionally, the rapid stimulatory effect of GnRH-A was blocked by the selective PKC inhibitor GF109203X, by TPA-mediated down-regulation of endogenous PKC, or by Ca2+ removal. Interestingly, serum-starvation (24 h) advanced the stimulation of PKCdelta mRNA levels by GnRH-A and the effect could be detected at 1 h of incubation. The rapid effect of GnRH-A upon PKCdelta mRNA levels in serum-starved cells was mimicked by TPA, but not by ionomycin, and was abolished by down-regulation of PKC or by Ca2+ removal. Preactivation of alphaT3-1 cells with GnRH-A for 1 h followed by removal of ligand and serum resulted in elevation of PKCdelta mRNA levels after 24 h of incubation. Western blot analysis revealed that GnRH-A and TPA stimulated (within 5 min) the activation and some degradation of PKCdelta and PKCepsilon. We conclude that Ca2+ and PKC are involved in GnRH-A elevation of PKCdelta and PKCepsilon mRNA levels, with Ca2+ being necessary but not sufficient, while PKC is both necessary and sufficient to mediate the GnRH-A response. A serum factor masks PKCdelta but not PKCepsilon mRNA elevation by GnRH-A, and its removal exposes preactivation of PKCdelta mRNA by GnRH-A which can be memorized for 24 h. PKCdelta and PKCepsilon gene expression evoked by GnRH-A is autoregulated by PKC, and both isotypes might participate in the neurohormone action.

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.

Regulation of the functional activity of the human dopamine transporter by the arachidonic acid pathway

The role of arachidonic acid was examined in the regulation of dopamine transport in C6 glioma cells stably expressing the human dopamine transporter. Exogenously added arachidonic acid (20-160 microM) stimulated [3H]dopamine uptake when pre-incubated for short times (15-30 min); 160 microM arachidonic acid inhibited following longer pre-exposures (45-60 min). Under the same conditions, only decreases were observed in the binding of the cocaine analog [3H]2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane ([3H]WIN 35,428). The reduction in dopamine transporter activity by arachidonic acid (at 160 microM for 60 min) was caused by a decrease in the Vmax (from 202 to 44 pmol/mg/min) opposed by a smaller reduction in K(m) (from 1.2 to 0.8 microM), whereas the effect of arachidonic acid (at 160 microM for 15 min) on [3H]WIN 35,428 binding was caused by a reduction in the Bmax (from 1.8 to 1.3 pmol/mg) without a change in Kd (7.2 nM). Upon 15-min exposure, melittin, an activator of phospholipase A2, and nordihydroguaiaretic acid, a lipooxygenase inhibitor, both expected to cause enhanced endogenous arachidonic acid, inhibited [3H]dopamine uptake and [3H]WIN 35,428 binding with an IC50 value close to 1 microM, whereas thimerosal, which raises arachidonic acid by inhibiting lipid reacylation, caused similar reductions at the sub-millimolar level. Co-presence of stauroporine (0.3-2 microM), an inhibitor of protein kinase C, had little or no effect on the melittin- or arachidonic acid-induced inhibition of [3H]dopamine uptake. Both the melittin- and arachidonic acid-, but not phorbol 12-myristate 13-acetate-induced inhibition of uptake were counteracted by bovine serum albumin (0.1 and 1 mg/ml) which binds arachidonic acid. The data taken together suggest that the inhibitory effects of arachidonic acid activators and those of protein kinase C activators on dopamine uptake are mediated by separate mechanisms.