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

The ability of gonadotropin releasing hormone (GnRH) to elevate cellular levels of mRNA for beta-subunit of luteinizing hormone (LH) has been examined in monolayer cultures from rat pituitary. Low concentrations of GnRH (100 pM) induced a 6.8-fold increase in LH-beta mRNA, while higher concentrations of GnRH were less effective. The low concentrations of GnRH (100 pM) did not result in altered GnRH receptor levels (92 +/- 12% compared to controls) after 24 h treatment but did increase protein kinase C activity to 249 +/- 16%. The protein kinase C activator, phorbol 12-myristate 13-acetate, at concentrations (2-20 nM) which did not deplete protein kinase C, stimulated LH-beta mRNA levels 2-5-fold after 24 h. Higher concentrations of phorbol 12-myristate 13-acetate, which depleted protein kinase C activity, substantially reduced the ability of 100 pM GnRH to stimulate increases in LH-beta mRNA levels. As previously observed, protein kinase C-depleted cells exhibited normal LH release in response to GnRH stimulation. These studies demonstrate that low concentrations of GnRH may have an important role in regulation of gonadotropin biosynthesis. Furthermore, the results suggest that activation of protein kinase C is sufficient to stimulate increases in LH-beta mRNA levels and that protein kinase C is necessary for normal GnRH stimulation of LH-beta mRNA levels. Accordingly, we postulate that protein kinase C may mediate the action of GnRH on LH-beta mRNA levels.
 ...
PMID:Stimulation of rat luteinizing hormone-beta messenger RNA levels by gonadotropin releasing hormone. Apparent role for protein kinase C. 284 35

Gonadotropin-releasing hormone (GnRH) produces a rapid and concentration-dependent hydrolysis of polyphosphoinositides in rat anterior pituitary cells in culture. Evaluation of the action of the decapeptide by measurement of [3H]-inositol phosphates and of prelabeled phosphoinositides demonstrated an effect on phosphatidylinositol-4,5-bis-phosphate and phosphatidylinositol-4-phosphate earlier than on phosphatidylinositol. The receptor antagonist [D-pGlu1,D-Phe2,D-Trp3,6]-luteinizing hormone-releasing hormone blocked the effect of GnRH on [3H]-inositol phosphate production. Protein kinase C activators attenuated GnRH-induced phosphoinositide hydrolysis, while neither cyclic AMP analogs nor cyclic GMP analogs were effective. These results indicate that phosphoinositide hydrolysis represents an important postreceptor transducing mechanism for GnRH action at the gonadotroph and that protein kinase C (but not cyclic nucleotides) may exert a negative feedback control on GnRH receptor-coupling mechanisms.
 ...
PMID:Gonadotropin-releasing hormone-stimulated phosphoinositide hydrolysis in the anterior pituitary. Modulation by protein kinase C but not by cyclic nucleotides. 285 23

The demonstration that GnRH provokes the accumulation of diacylglycerol and the redistribution of protein kinase C to the membrane fraction in gonadotropes suggests a role for this enzyme as a mediator of GnRH action. In the present work we have investigated the possibility that protein kinase C might mediate GnRH-stimulated receptor down-regulation and desensitization. Pretreatment of pituitary cells for 6 h with GnRH (10(-11) - 10(-6) M) caused a biphasic change in GnRH receptor number [the maximum binding (Bmax) for 125I-buserelin binding was increased by 10(-10) M GnRH and reduced by 10(-7) and 10(-6) M GnRH] and caused desensitization (pretreatment with 10(-9) - 10(-6) M GnRH reduced the proportion of cellular LH released in a subsequent challenge with GnRH). Pretreatment for 6 h with 0.2-200 nM phorbol myristate acetate (a protein kinase C-activating phorbol ester) did not cause desensitization, but at 200 nM, did reduce GnRH receptor number. As a further test of the requirement for protein kinase C for GnRH action, cells were depleted of all measurable protein kinase C (and rendered unresponsive to protein kinase C activators) by prior treatment with a high dose of phorbol myristate acetate (500 nM for 6 h followed by 12 h in plating medium). Depletion of protein kinase C did not alter the ability of GnRH to desensitize gonadotropes or down-regulate its own receptors. The demonstration that the effects of GnRH on receptor number and gonadotrope responsiveness are neither blocked by depletion of protein kinase C nor entirely mimicked by activation of protein kinase C suggests that these effects of the releasing hormone are not solely mediated by this enzyme.
 ...
PMID:Homologous down-regulation of gonadotropin-releasing hormone receptors and desensitization of gonadotropes: lack of dependence on protein kinase C. 285 5

The hypothalamic control of reproductive function is expressed through the receptor-mediated actions of GnRH on the pituitary gonadotroph. GnRH receptors in the pituitary gland exhibit prominent variations in number during the ovarian cycle and after changes in steroid feedback, and are modulated by the rate of GnRH secretion from the hypothalamus. In cultured pituitary cells, GnRH receptors undergo down-regulation during exposure to GnRH agonists, followed by a subsequent elevation of sites that is dependent on protein synthesis. GnRH antagonists do not cause receptor down-regulation, but high-affinity antagonist analogs bind for extended periods to cause receptor occlusion and prolonged inhibition of GnRH action. Analysis of the rat pituitary GnRH receptor by photoaffinity labeling reveals two binding subunits of mol. wt 53,000 and 42,000. The receptor-activated processes leading to gonadotropin secretion are highly calcium-dependent, and are initiated by rapid phospholipid hydrolysis with production of arachidonic acid metabolites, diacylglycerol, and inositol phosphates. The role of protein kinase C in gonadotropin secretion is indicated by the ability of phorbol esters and synthetic diacylglycerols to stimulate LH release, the inhibition of protein kinase C and LH release by retinal, and the redistribution of protein kinase C between cytosol and membrane fractions during stimulation of pituitary gonadotrophs by GnRH. It is likely that the effects of arachidonate metabolites are integrated with those of calcium-calmodulin and calcium, phospholipid-dependent protein kinases during the immediate and sustained phases of GnRH-induced gonadotropin secretion.
 ...
PMID:GnRH receptors and actions in the control of reproductive function. 300 12

GnRH interacts with a plasma membrane receptor to provoke gonadotropin release, as well as regulate numbers of its own receptor and target cell responsiveness. Receptor numbers are altered in different physiological states of the animal. Microaggregation of the GnRH receptor mimics all known actions of the releasing hormone, and therefore is viewed as an early step in the molecular mechanism of hormone action. Internalized hormone is neither necessary nor sufficient for stimulation of known releasing hormone actions. Evidence summarized in the present work suggests that Ca2+ serves a role as a second messenger for GnRH-stimulated gonadotropin release, and that it may be involved in receptor up-regulation in response to the releasing hormone. In the former role, diacylglycerols by their action on protein kinase C, in a fashion independent of the Ca2+ -calmodulin system, may act as a signal amplifier.
 ...
PMID:Mechanism of action of gonadotropin releasing hormone. 301 Aug 23

The stimulation of gonadotropin release from pituitary cell cultures by GnRH has been linked to inositol phospholipid breakdown to diacylglycerols and subsequent activation of protein kinase C as well as Ca2+ mobilization. In order to examine the means of receptor coupling to a phospholipase C-type reaction, we evaluated the role of guanine nucleotides in inositol phospholipid breakdown. In these studies ATP (50 microM) was used for cell permeabilization to allow guanine nucleotides access to the intracellular compartment. Under these conditions GTP and the GTP analog, guanylylimidodiphosphate (GMP-PNP), stimulated a time- and dose-dependent increase in LH release and inositol phosphate accumulation. These actions of GTP and GMP-PNP were not observed unless ATP was included in the treatment media. Other closely related nucleotides and nucleosides alone, or in the presence of ATP, did not elevate LH release above basal levels. We also evaluated the actions of pertussis toxin and cholera toxin on mediating the effect of GTP, GMP-PNP, and GnRH on LH release and inositol phosphate accumulation. After treatment with these agents, no changes were observed in the ability of GnRH, GTP, or GMP-PNP to stimulate either LH release or inositol phosphate accumulation. The additional observation that GnRH-, GTP-, or GMP-PNP-stimulated LH release and inositol phosphate accumulation were blocked by a potent GnRH antagonist suggests that a G protein is functionally associated with the GnRH receptor recognition site.
 ...
PMID:Stimulation of luteinizing hormone (LH) release and phospholipid breakdown by guanosine triphosphate in permeabilized pituitary gonadotropes: antagonist action suggests association of a G protein and gonadotropin-releasing hormone receptor. 302 16

GnRH stimulates LH release from pituitary gonadotropes. Prolonged exposure of these cells to GnRH results in decreased sensitivity to further stimulation by the releasing hormone both in vivo and in vitro. Chelation of extracellular Ca++ with EGTA blocks GnRH-stimulated LH release but does not prevent subsequent desensitization. Desensitization occurs when cells are preincubated in EGTA containing 10(-7) M GnRH for a variety of times (20 min to 12 h) or when cells are preincubated for 3 h in EGTA with 10(-10), 10(-9), or 10(-8) M GnRH. A GnRH antagonist does not cause desensitization to GnRH and blocks desensitization in response to GnRH in the Ca++-free medium. Preincubation in EGTA containing 10(-7) M GnRH for 3 h did not alter sensitivity of cells to sn 1,2 dioctanoylglycerol (a protein kinase C activator), Ca++ ionophore A23187, or veratridine (an activator of endogenous ion channels). These results suggest that desensitization results from occupancy of the GnRH receptor by an agonist and may be uncoupled from LH release.
 ...
PMID:Gonadotropin-releasing hormone-mediated desensitization of cultured rat anterior pituitary cells can be uncoupled from luteinizing hormone release. 308 23

Desensitization of pituitary gonadotropes by exposure to 10 nM gonadotropin-releasing hormone (GnRH) for 6 h severely impaired the luteinizing hormone (LH) response to a second 3-h treatment with GnRH, and reduced the secretory responses to 50 microM arachidonic acid (AA), 100 nM tetradecanoyl phorbol-13-acetate (TPA), and AA + TPA. Pretreatment with AA blocked subsequent responses to AA but not to other secretagogues. Pretreatment with TPA attenuated the LH response to TPA, but not to GnRH, AA, and AA + TPA. After exposure to AA + TPA, all subsequent responses were abolished. Each of the secretagogues reduced GnRH receptor binding, but only GnRH-induced receptor loss and desensitization were reversed by simultaneous incubation with a GnRH antagonist. Similar results were obtained when 16-h pretreatment periods were used, or when the data were normalized for the concomitant reduction of cellular LH content. These findings indicate that GnRH-receptor loss and depletion of LH content are not the sole causes of GnRH-induced desensitization. Receptor uncoupling and impairment of AA- and protein kinase C-dependent pathways may also be involved in this process.
 ...
PMID:Desensitization of pituitary gonadotropes by mediators of LH release. 313 22

The present study was designed to test the hypothesis that there is a functional interaction between the calcium-calmodulin system, which appears to mediate the action of gonadotropin-releasing hormone (GnRH), and activators of protein kinase C, which stimulate luteinizing hormone (LH) release by a mechanism which does not require extracellular calcium. We have examined a diacylglycerol and a phorbol ester, which both activate protein kinase C and stimulate LH release. These compounds show synergistic action with calcium ionophore A23187 as secretogogues. Pimozide (a calmodulin antagonist), methoxyverapamil (a calcium ion channel inhibitor), and Ac[D-pCl-Phe1,2-D-Trp3-D-Lys6-D-Ala10]GnRH (a potent gonadotropin-releasing hormone antagonist) were used to show that the diacylglycerol and phorbol ester can stimulate LH release in a manner that is independent of both Ca2+ and calmodulin and do not work by means of a direct action on the GnRH receptor. These observations, coupled with previously published reports that extracellular Ca2+ mobilization is both necessary and sufficient for initiation and perpetuation of GnRH-stimulated LH release, indicate that activation of protein kinase C by endogenous diacylglycerols may serve as an amplifier of the GnRH-stimulated signal which appears to be mediated independently by the Ca2+-calmodulin system.
 ...
PMID:Diacylglycerols and protein kinase C. Potential amplifying mechanism for Ca2+-mediated gonadotropin-releasing hormone-stimulated luteinizing hormone release. 315 62

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts via type I receptors in the pituitary to stimulate cAMP production. Gonadotropes are likely target cells for PACAP action, and we have recently shown alpha T3-1 cells, a clonal gonadotrope-derived cell line, to be PACAP responsive. Here we have explored the influence of GnRH on PACAP action in alpha T3-1 cells and show that PACAP38-stimulated cAMP production is inhibited by GnRH in both the presence and the absence of a phosphodiesterase inhibitor. This effect appears not to be Ca++ mediated but is mimicked by protein kinase C activation with phorbol 12-myristate 13-acetate. However, GnRH and phorbol 12-myristate 13-acetate do not inhibit binding of [125I]PACAP27 to intact alpha T3-1 cells, nor do they inhibit forskolin- or cholera toxin-stimulated cAMP accumulation, implying that the inhibitory effects are exerted at early stages in the PACAP receptor signaling pathway but distal to receptor occupancy. When cells were preincubated with PACAP38, extensive washing failed to prevent the stimulatory effect of the polypeptide presumably because of the slow rate of receptor-ligand dissociation. However, when the time course of PACAP38-stimulated effects on intracellular cAMP was assessed, the stimulatory effect of PACAP38 could be rapidly reversed by GnRH addition, and the inhibitory effect of GnRH was rapidly be reversed by a GnRH receptor antagonist. The data provide the first demonstration of cross-talk between phospholipase C and adenylate cyclase-activating peptides in gonadotrope-derived cells and establish the potential for hormonal modulation of PACAP action. We suggest that this inhibitory effect of GnRH might enable the releasing hormone to control the kinetics of cAMP signaling in gonadotropes in vivo.
 ...
PMID:Pituitary adenylate cyclase-activating polypeptide effects in pituitary cells: modulation by gonadotropin-releasing hormone in alpha T3-1 cells. 751 5


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>