Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:2.7.11.13 (protein kinase C)
 49,245 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Pretreatment of pituitary cell cultures with GnRH causes altered gonadotrope responsiveness to LH secretagogues. The precise mechanism by which this occurs is not understood. Because a G protein appears to be activated after GnRH stimulation of the gonadotrope, a role for this moiety in GnRH-stimulated alterations in gonadotrope responsiveness was assessed. We show that 3 h pretreatment of pituitary cell cultures with 10 mM NaF (a G protein activator), resulted in decreased gonadotrope responsiveness to subsequent GnRH treatment (3 h, 100 nM; 34.4 +/- 1.6% vs. 23.4 +/- 1.5% of total cellular LH). NaF-provoked gonadotrope desensitization to GnRH also occurred in the presence of 3 mM EGTA and in cells which had been depleted of protein kinase C. Desensitization to GnRH did not occur in response to pretreatment with (Bu)2cAMP (8 h, 1 mM). In addition, neither GnRH nor NaF stimulated inositol phosphate production above basal levels after the NaF pretreatment. GnRH receptor binding also decreased by 30% with NaF pretreatment. In contrast, 3 h NaF (10 mM) pretreatment enhanced responsiveness of the gonadotrope to the Ca2+ ionophore A23187 in a protein kinase C- and cAMP-dependent manner. Responsiveness to the phorbol ester, phorbol 12-myristate 13-acetate, was also increased, whereas responsiveness to the Ca2+ channel activator maitotoxin was unchanged. These data suggest that G protein activation by NaF provokes gonadotrope desensitization to GnRH stimulation by both decreasing receptor numbers and by uncoupling of the receptors from inositol phosphate production. In addition, a distinct G protein action appears to be involved in sensitizing the gonadotrope to A23187 and phorbol 12-myristate 13-acetate.
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PMID:Sodium fluoride provokes gonadotrope desensitization to gonadotropin-releasing hormone (GnRH) and gonadotrope sensitization to A23187: evidence for multiple G proteins in GnRH action. 131 44

Gonadal steroids act at the pituitary to regulate gonadotropin-releasing hormone (GnRH) receptor number and the responsiveness of gonadotropes to GnRH and can act at post-receptor sites to modulate Ca(2+)-mediated and protein kinase C-mediated signal-transducing pathways. However, such effects have been seen in the mixed cell population of primary cell cultures and may involve indirect effects on cells other than gonadotropes. Here, steroid effects on a recently described gonadotrope-derived cell line (alpha T3-1 cells) have been assessed. In these cells estradiol, progesterone, testosterone and corticosterone all exerted trophic effects. Estradiol increased [3H]thymidine incorporation with an EC50 of 10(-12) to 10(-11) M and this effect was blocked by keoxifene, an estrogen receptor antagonist. Estradiol also reduced binding of [125I]buserelin (EC50 approximately 10(-11) M), an effect which appears to reflect a reduction in GnRH receptor number rather than a change in Kd. Estradiol also shifted the dose-response curve for GnRH-stimulated inositol phosphate (IP) accumulation rightward, increasing the EC50 for this GnRH effect by approximately 20-fold. Accordingly estradiol acts directly upon alpha T3-1 cells not only to reduce GnRH receptor number, but also to reduce the efficiency of coupling of residual GnRH receptors to second messenger generation.
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PMID:Estradiol regulates gonadotropin-releasing hormone receptor number, growth and inositol phosphate production in alpha T3-1 cells. 133 8

Gonadotropin-releasing hormone (GnRH) stimulated the formation of two major metabolites of the 5-lipoxygenase pathway, leukotriene (LT) B4 and LTC4, as well as luteinizing hormone (LH) release in primary cultures of rat anterior pituitary cells. Several lines of evidence suggested the presence of a GnRH-dependent pituitary endocrine system in which LTs act as second messengers for LH release: (i) GnRH-dependent LT formation was observed within 1 min and immediately preceded GnRH-induced LH release, whereas exogenous LTs stimulated LH release at low concentrations; (ii) the dose responses of GnRH-induced LT production and LH release were similar and both effects required the presence of extracellular Ca2+ ions; (iii) GnRH-induced LH release was blocked by up to 45% following the administration of several LT receptor antagonists; (iv) LTE4 action on LH secretion was entirely abolished by LT receptor antagonists; and (v) an activator of protein kinase C acted synergistically with LTE4 to induce LH release. The major source of LT formation in the pituitary cell cultures appeared to be the gonadotrophs, as shown by GnRH receptor desensitization experiments. The results demonstrate the presence of a GnRH-activatable 5-lipoxygenase pathway in anterior pituitary cells and provide strong support for the hypothesis that LTs play a role in LH release in the GnRH signaling pathway.
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PMID:Production of leukotrienes in gonadotropin-releasing hormone-stimulated pituitary cells: potential role in luteinizing hormone release. 165 55

Gonadotropin-releasing hormone (GnRH) stimulates synthesis and release of the pituitary gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH). Other actions of GnRH include gonadotrope sensitization and desensitization as well as stimulation of GnRH receptor synthesis. Gonadotropin release initiated by increased intracellular calcium is a result of calcium mobilization from intracellular stores and influx of extracellular calcium through receptor-operated channels. Increases in intracellular calcium and the presence of both calmodulin and calmodulin binding-proteins in pituitary suggests that formation of Ca(2+)-calmodulin complexes and subsequent alteration of calmodulin-binding protein activity are likely important intermediate steps in the signaling pathway of LH release. Although activation of protein kinase C is not necessary for GnRH-stimulated LH release or gonadotrope desensitization, it appears to be essential for GnRH effects on LH beta gene expression. Therefore gonadotrope responses are apparently mediated by multiple intracellular signaling mechanisms.
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PMID:Regulation of the pituitary gonadotrope by gonadotropin-releasing hormone: multiple intracellular effectors. 190 69

Follicle-stimulating hormone (FSH)-suppressing protein (FSP) or follistatin, a novel gonadal glycoprotein hormone, has been shown to have chronic inhibitory effects on the secretion of both FSH and luteinizing hormone (LH) in response to gonadotropin-releasing hormone (GnRH) in vitro. The present study was designed to investigate the acute effects of bovine FSP on GnRH-stimulated gonadotropin secretion and to examine the potential subcellular sites of this action of FSP using cultured pituitary cells. Anterior pituitaries from adult male Sprague-Dawley rats were enzymatically dispersed and cultured for 48 h, after which the cells were treated with bovine FSP for 6 h, followed by a 4 h stimulation with secretagogues in the continued presence of FSP. Results showed that the 35 kDa form of bovine FSP (0.1-3 nM) dose-dependently suppressed GnRH-stimulated FSH and LH secretion, with inhibition of 38 and 25%, respectively, at 3 nM. In addition, FSP suppressed gonadotropin secretion in response to activators of protein kinase C (phorbol 12-myristate 13-acetate (PMA) and mezerein) and a calcium ionophore (A23187). However, FSP had no effect on gonadotropin secretion evoked by melittin, an activator of phospholipase A2. Furthermore, 35 kDa bovine FSP did not compete with GnRH for GnRH binding sites in a direct competition study and treatment of cultured pituitary cells with FSP (0.1-3 nM) for 10 h did not alter the number of GnRH binding sites on the cell membranes. Finally, similar inhibitory effects on gonadotropin secretion in response to GnRH, PMA and mezerein were obtained with the 31 and 39 kDa forms of bovine FSP, each at a concentration of 1 nM. We conclude from the present study that FSP acutely inhibits GnRH-stimulated gonadotropin secretion in cultured pituitary cells, and that FSP exerts its action beyond the GnRH receptor, possibly by affecting the protein kinase C and/or the calcium-calmodulin systems.
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PMID:Acute inhibitory effect of follicle-stimulating hormone-suppressing protein (FSP) on gonadotropin-releasing hormone-stimulated gonadotropin secretion in cultured rat anterior pituitary cells. 212 65

The neurohormone gonadotropin-releasing hormone (GnRH) is a decapeptide which is synthesized in the hypothalamus and released into the hypophysial portal system in a pulsatile manner. GnRH exerts its effect on the anterior pituitary gonadotrophs where it regulates the secretion and synthesis of gonadotropins (luteinizing hormone and follicle-stimulating hormone) through receptor-mediated actions. The GnRH receptor has been characterized and shown to be coupled to the formation of 'second messengers' which participate in signal transduction mechanisms. GnRH stimulation of luteinizing hormone release is a Ca2(+)-dependent process. G protein, phosphoinositide hydrolysis, protein kinase C as well as arachidonic acid and some of its metabolites were identified as possible mediators in the process.
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PMID:The gonadotropin-releasing hormone receptor: signals involved in gonadotropin secretion and biosynthesis. 217 Feb 60

Gonadotropin-releasing hormone (GnRH) regulates pituitary gonadotropin release by a Ca2+-dependent mechanism involving receptor-mediated phosphoinositide hydrolysis. Previous studies indicate that activation of pituitary protein kinase C (PKC), while not required for acute gonadotropin release in response to GnRH, is likely involved in the chronic regulation of gonadotrope responsiveness. Studies from our laboratory have shown that activation of PKC by phorbol esters produces both the uncoupling of GnRH-stimulated phosphoinositide hydrolysis and the selective enhancement of GnRH agonist binding in pituitary cell cultures. In the present work, we have examined the possibility that these processes are related in mechanism. Dissociation of bound agonist radioligand at 23 degrees C was found to be reduced in the presence of phorbol esters, and ligand bound in the presence of phorbol ester was resistant to displacement by competing ligands at 4 degrees C. However, agonist bound in the presence of phorbol ester was dissociable by subsequently washing cells at pH 3. Receptor photoaffinity labeling studies confirmed that agonist association with membrane component(s) identified as the GnRH receptor was increased in the presence of phorbol ester. These results suggest that, in the presence of a phorbol ester PKC activator, agonist-occupied GnRH receptors remain at the cell surface, but are sequestered in some manner. In other experiments, cell preloaded with [3H]inositol were treated with GnRH agonist ligand and phorbol ester at 4 degrees C to form a pool of sequestered, agonist-occupied receptors, and then displaceable (nonsequestered) agonist was removed by incubation with antagonist ligand. After addition of LiCl and warming to 37 degrees C, [3H]inositol phosphate production (an index of phosphoinositide hydrolysis) in phorbol ester-treated cells was reduced to 67% of vehicle control, although residual specific agonist binding had been increased to greater than 300% of control. The appearance of sequestered receptors and inhibition of [3H]inositol phosphate production had similar phorbol ester concentration dependencies. These results suggest that the same agonist-occupied GnRH receptors sequestered as a result of PKC activation also are preferentially uncoupled from phosphoinositide hydrolysis.
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PMID:Protein kinase C-mediated gonadotropin-releasing hormone receptor sequestration is associated with uncoupling of phosphoinositide hydrolysis. 254 81

We have investigated the stimulation of phospholipase D activity by the gonadotropin-releasing hormone receptor agonist [D-Ala6, des-Gly10]GnRH N-ethylamide (GnRH-A) in preovulatory, cultured granulosa cells. GnRH-A stimulated up to 10-fold accumulation of phosphatidylethanol, produced by phospholipase D phosphatidyl transferase activity when ethanol acts as the phosphatidyl group acceptor. The effect of GnRH-A was concentration dependent (EC50 = 1 nM) and was inhibited by a specific GnRH receptor antagonist. Low GnRH-A concentrations (less than 10 nM) stimulated also accumulation of phosphatidic acid, but at higher concentrations this response was attenuated. Propranolol, which inhibits phosphatidic acid phosphohydrolase, increased both basal and GnRH-A-stimulated production of phosphatidic acid. A protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA, 100 nM), increased up to 30-fold phosphatidylethanol levels. The effects of supramaximal concentrations of GnRH-A (50 nM) and TPA (1 microM) on the accumulation of phosphatidylethanol were additive, suggesting that the two agents may not act via the same mechanism. This is supported by the fact that 1-(5-isoquinolinesulfonyl)-2-methylpiperazine, a protein kinase C inhibitor, inhibited the effect of TPA 50%, but not that of GnRH-A. However, 24 h pretreatment with TPA abolished cellular response to subsequent treatment with either TPA or GnRH-A. The stimulatory action of GnRH on steroidogenesis could be mimicked by elevating endogenous phosphatidic acid levels in granulosa cells. Exogenous phospholipase D (from Streptomyces chromofuscus, 10 IU/ml) significantly increased (2.7-fold) progesterone production by the cells; under the same conditions, GnRH-A and FSH stimulated progesterone production 3- and 2.6-fold, respectively. Similarly, propranolol stimulated progesterone production 2.2-fold. These results suggest that, in granulosa cells, GnRH receptors are coupled to a phospholipase D whose activation may participate in transducing the GnRH signal for accelerated steroidogenesis. Phospholipase D activity can be independently regulated also by protein kinase C. The possible interrelationships between phospholipase D and other phospholipases which may be activated by GnRH in these ovarian cells are discussed.
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PMID:Gonadotropin-releasing hormone activates phospholipase D in ovarian granulosa cells. Possible role in signal transduction. 266 40

Gonadotropin-releasing hormone (GnRH) stimulates release of pituitary gonadotropins by activating specific plasma membrane receptors. In the present studies, we have used activators of the Ca2+- and phospholipid-dependent protein kinase (protein kinase C) to probe the binding characteristics of agonist- or antagonist-occupied GnRH receptors in intact cell cultures, using a radioligand receptor assay. Specific binding of [125I-Tyr5,D-Ser(tBu)6,Pro9,NHEt]GnRH (Buserelin), a high-affinity GnRH agonist, was increased to 180% of control in the presence of 150 nM phorbol 12-myristate 13-acetate (PMA) or 100 nM phorbol 12,13-dibutyrate (PDB), and to 125% of control in the presence of 200 microM 1,2-dioctanoylglycerol, after 20 min at 23 degrees C. The PMA effects were associated with apparent increases in both binding affinity and number of binding sites. The effects of protein kinase C activators on Buserelin binding were concentration- and time-dependent and were not seen with 4 alpha-PMA or 1,2-dioctanoyl-3-Cl-glycerol, neither of which activate protein kinase C. In contrast, PMA had no measurable effects on specific binding of a GnRH receptor antagonist, Ac[D-pCl-Phe1,2,D-Trp3,125I-Tyr5,D-Lys6,D-Ala10]GnRH. When cell cultures were pretreated with 100 nM PDB in the absence of GnRH and then washed to remove the phorbol ester, no effects of prior protein kinase C activation were detected upon subsequent addition of Buserelin. However, when PDB pretreatment was carried out in the presence of 0.3 microM GnRH, residual enhancement of Buserelin binding, but not antagonist binding, was observed at either 23 or 4 degrees C. The radiolabeled agonist activated, and the antagonist blocked, GnRH receptor-mediated luteinizing hormone release and [3H]inositol phosphate production in cells preloaded with [3H]inositol. These findings suggest that the action of protein kinase C on the GnRH receptor, either direct or indirect, requires the receptor to be in an activated (agonist-occupied) state but does not require receptor internalization. The mechanism of these effects on GnRH agonist binding is not known but may involve sequestration of surface receptors, expression of new receptors, and/or modulation of GnRH receptor affinity.
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PMID:Differential sensitivity of agonist- and antagonist-occupied gonadotropin-releasing hormone receptors to protein kinase C activators. A marker for receptor activation. 283 Feb 80

The principal physiological site of action of GnRH is on pituitary gonadotrophs. Although binding sites for this peptide are described in the gonads, placenta, breast and brain, their precise physiological/pharmacological relevance remains to be elucidated. The pituitary action of GnRH can be divided conveniently into an immediate release of LH and FSH within minutes, the synthesis of LH and FSH over a matter of hours (intermediate action), and long-term morphologic changes lasting several days. Most, if not all, of these actions are initiated following interaction with high-affinity (Kd = 0.5 nM) stereospecific receptors on gonadotrophs. GnRH receptor concentration is negatively regulated by testosterone and progesterone, and positively regulated by oestradiol in vivo as well as by the ligand itself. In-vivo and in-vitro GnRH regulates its own receptors depending on its pulsatile or continuous secretion which increase or decrease numbers (up-or down-regulation), respectively. In-vivo change in GnRH receptors probably reflect the extent or pituitary exposure to endogenous GnRH and is an indirect index of hypothalamic GnRH secretion. Up-regulation is not ligand specific since other activators of LH release e.g. depolarization, ionophores, and cAMP derivatives, are also effective in vitro. Down-regulation is specific to the ligand and contributes to 'desensitization' which also includes disruption of the signal transduction mechanism (uncoupling of receptors), and depletion of cellular LH stores. GnRH receptor occupation activates membrane phospholipase C to hydrolyse polyphosphoinositides with the formation of inositol-1-4-5-trisphosphate and diacylglycerol which activates protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Mechanism of GnRH action in gonadotrophs. 283 40


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