Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.1.4.3 (phospholipase C)
 18,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The dihydropyridine Ca2+ channel activator BAY K 8644 (1 microM) stimulated basal prolactin secretion from perifused primary cultures of anterior pituitary cells and potentiated the stimulation of prolactin secretion by 1 microM thyrotropin-releasing hormone (TRH) 5-fold over 30 min. This potentiation was mimicked by other dihydropyridine agonists CGP 28392 and (+)-SDZ 202-791 and by (-)-BAY K 8644 (1 microM), but not by (+)-BAY K 8644. The Ca2+ channel antagonist nimodipine, at a concentration sufficient to block BAY K 8644-stimulated 45Ca2+ uptake in GH4C1 anterior pituitary tumor cells, decreased basal prolactin secretion and blocked the enhancement of basal and TRH-stimulated secretion by BAY K 8644. These results suggest that dihydropyridine agonists potentiate TRH-induced secretion through interaction with known stereospecific sites on Ca2+ channels. In GH4C1 cells, BAY K 8644 alone did not affect inositol polyphosphate accumulation, but potentiated TRH-stimulated accumulation of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Accumulation of the Ca(2+)-mobilizing isomer inositol 1,4,5-trisphosphate was not potentiated, suggesting that potentiation of TRH-stimulated hormone secretion by BAY K 8644 does not result from synergistic stimulation of phospholipase C, but may correlate with enhanced inositol trisphosphate-3-kinase activity.
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PMID:Ca2+ channel agonists enhance thyrotropin-releasing hormone-induced inositol phosphates and prolactin secretion. 171 95

A permanent line of cells has been established from the transplantable rat pituitary tumor 7315a. P11 cells have been cloned repeatedly, and after more than 60 passages their growth and characteristics are stable. Results of radioligand binding studies with 125I-lysergic acid diethylamide (125I-LSD) indicate that P11 cells express serotonin-2 (5-HT2) receptors. Analysis of the binding of 125I-LSD to membranes prepared from P11 cells revealed the presence of a single class of high affinity sites (Kd = 1.6 nM; Bmax = 211 fmol/mg of protein). The pharmacological profile of the inhibition of the binding of 125I-LSD by a panel of drugs was consistent with the expected profile of these drugs at 5-HT2 receptors. The affinity of the site for serotonin was in the low micromolar range and was decreased by GTP. Phosphoinositide hydrolysis in P11 cells, measured in the presence of lithium, was stimulated by serotonin. Increasing concentrations of the 5-HT2-selective antagonist ketanserin blocked phosphoinositide hydrolysis stimulated by serotonin, and Schild analysis was consistent with a simple competitive interaction. The Ki for ketanserin derived from Schild analysis was comparable to the Ki for ketanserin at the binding site for 125I-LSD. These results suggest that stimulation of phosphoinositide hydrolysis in P11 cells by serotonin is mediated by 5-HT2 receptors. Pretreatment of P11 cells with pertussis toxin caused ADP-ribosylation of Gi and Go, but did not affect the ability of serotonin to stimulate phosphoinositide hydrolysis. Therefore, the guaninine nucleotide-binding protein involved in the coupling of 5-HT2 receptors to phospholipase C in P11 cells is unlikely to be either Gi or Go. P11 cells expressing 5-HT2 receptors coupled to phosphoinositide hydrolysis will be a useful model system for future studies of the regulation and function of 5-HT2 receptors on cultured cells.
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PMID:Serotonin-2 receptors coupled to phosphoinositide hydrolysis in a clonal cell line. 216 57

Chlordiazepoxide (CDE) has been shown to antagonize the effects of TRH to stimulate the hydrolysis of phosphoinositides and elevate cytoplasmic free calcium in rat pituitary tumor (GH3) cells. Herein, we show that CDE inhibits TRH stimulation of PRL secretion and that the effect of CDE to antagonize TRH action is caused by its ability to compete with TRH for binding to receptors on GH3 cells. We also use CDE to explore whether continued receptor occupancy is required for prolonged stimulation of cellular responses. CDE had no effect on basal PRL secretion, but caused a dose-dependent inhibition of TRH-induced PRL secretion. CDE decreased the affinity of TRH binding to intact GH3 cells without affecting the maximum binding capacity. As shown previously, CDE had no effect on phosphoinositide metabolism, which was monitored because it appears to be a mechanism for signal transduction by TRH, and when added simultaneously with TRH, caused a dose-dependent inhibition of TRH-induced phosphoinositide metabolism. When CDE was added to cells 2.5 or 5 min after TRH, CDE rapidly terminated the stimulation by TRH of phosphoinositide hydrolysis, shown as inhibition of the continued formation of inositol phosphates and inositol, and of the decrease in phosphoinositides. Lastly, when cells were stimulated with 50 nM TRH, then exposed to 100 microM CDE, and finally to 1000 nM TRH, inositol phosphate formation was stimulated, then inhibited, and then restimulated. These data demonstrate that CDE acts as a competitive antagonist of TRH action on GH3 cells by competing with TRH for binding to its receptor and that continued stimulation by TRH of phospholipase C-mediated hydrolysis of phosphoinositides is tightly coupled to receptor occupancy.
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PMID:Evidence for tight coupling of receptor occupancy by thyrotropin-releasing hormone to phospholipase C-mediated phosphoinositide hydrolysis in rat pituitary cells: use of chlordiazepoxide as a competitive antagonist. 301 58

GH3 pituitary tumor cells were labeled to isotopic equilibrium with [3H]inositol. Thyrotropin-releasing hormone (TRH), which has been shown to stimulate inositol phospholipid metabolism in these cells, enhanced the accumulation of [3H]inositol-derived radioactivity in the cell's acid-soluble fraction. Separation of the [3H]inositol metabolites by ion-exchange chromatography revealed that TRH induced a rapid rise in the cellular content of [3H]inositol mono-, bis-, and trisphosphate. The latter two metabolites accumulated in a multiphasic manner with an initial peak 5-10 sec after TRH addition. This was followed by a short-lived decline and a secondary rise which left the metabolite levels elevated for at least 50 min. The GH3 cell [3H]inositol monophosphate and [3H] inositol content also rose in response to TRH, but the latter accumulated with a considerably slower time course than the phosphorylated derivatives. None of these responses could be mimicked by the calcium ionophore A23187. Incubation of GH3 cells with TRH in the presence of lithium led to an enhanced accumulation of [3H]inositol monophosphate and, to a lesser extent, of [3H]inositol bis- and trisphosphate. This accumulation rose in a linear fashion with time for at least 20 min, by which point 50% of the [3H]inositol-containing phospholipids had been depleted. When lithium was added 30 min after TRH, [3H]inositol monophosphate accumulated at the same rate as was found when TRH and lithium were added together, indicating that the TRH-induced phospholipid response in GH3 cells does not desensitize. Under normal conditions, approximately equal amounts of the three [3H]inositol phosphates were formed within 5 sec of TRH addition. However, when TRH was added to cells grown chronically in lithium-containing medium, or to cells incubating at a subphysiological temperature (25 degrees), greater than 90% of the metabolites formed were the bis- or trisphosphates. This indicates that the primary event stimulated by TRH is the breakdown by phospholipase C of phosphatidylinositol 4,5-bisphosphate and, perhaps also, of phosphatidylinositol 4-phosphate.
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PMID:Thyrotropin-releasing hormone-stimulated [3H]inositol metabolism in GH3 pituitary tumor cells. Studies with lithium. 632 43

Transforming growth factor-alpha (TGF alpha) is a growth regulatory peptide expressed largely as a high mol wt species in the anterior pituitary gland. The overall objective of this work was to test the hypothesis that altered expression of TGF alpha may play a role in the tumorigenicity of the GH4C1 cell line. We examined expression of TGF alpha in three related clones of pituitary tumor cells (GH1, GH3, and GH4C1) grown as transplantable tumors, the MtT/W5 tumor from which they were derived, and anterior pituitary glands of Wistar-Furth rats, the source of the MtT/W5 tumor. Wistar-Furth anterior pituitary, MtT/W5, GH1, GH3, and GH4C1 extracts all contained TGF alpha-specific immunoreactivity, which, when examined on sodium dodecyl sulfate-gel transfers, was of high relative mol wt, corresponding to incompletely processed TGF alpha. In neither the anterior pituitary nor the tumors was the fully processed 6-kilodalton TGF alpha form identified, indicating that mature TGF alpha is expressed to only a limited degree in normal and tumor pituitary tissue. We next determined whether a lack of receptors for TGF alpha may account for the MtT/W5 tumor phenotype in vivo. Scatchard analysis of [125I] epidermal growth factor ([125I]EGF) saturation isotherm binding identified a comparable class of sites in both the anterior pituitary gland and GH4C1 transplantable tumors. Specific binding sites were also found in MtT/W5, GH1, and GH3 tumors. Thus, the functional components of a TGF alpha pathway exist in both the anterior pituitary gland and GH4C1 transplantable tumors. We lastly examined whether a TGF alpha pathway plays a functional role in GH4C1 tumor formation. Toward this aim, we isolated TGF alpha-nonresponsive variants by two different selection schemes: one using a TGF alpha-toxin conjugate, and the other using a TGF alpha-inducible morphological phenotype. Each variant had decreased [125I]EGF specific binding and little or no EGF growth inhibitory response in vitro. We also isolated a mutagen-induced revertant from one of the variants based on expression of the TGF alpha-inducible morphological phenotype. These cells were found to have a normal complement of receptors and EGF growth inhibitory response in vitro. GH4C1, the two variants, and the revertant cells were inoculated into Wistar-Furth rats, and their growth observed for 8 weeks. The GH4C1 cells and the revertant formed tumors by 8 weeks, whereas the two variant cells failed to form tumors.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:A transforming growth factor-alpha pathway is expressed in GH4C1 rat pituitary tumors and appears necessary for tumor formation. 801 79

Mastoparan has been reported to induce a wide variety of cellular actions by activating GTP-binding proteins (G proteins) in various cells. Here, we demonstrate that mastoparan is able to stimulate the secretion of PRL from rat anterior pituitary tumor GH3 cells in dose- and time-dependent manners. Mastoparan had no effect on the accumulation of intracellular cAMP; however, it induced a rapid increase in the intracellular Ca2+ concentration in GH3 cells. Extracellular Ca2+ was required for mastoparan-induced PRL secretion, which was inhibited by nifedipine, an L-type Ca2+ channel blocker. Incubation of mastoparan with myo-[3H]inositol-labeled GH3 cells also resulted in the increased formation of inositol phosphates (InsPs) compared with control cells. Neomycin sulfate and U73122, both phospholipase C inhibitors, suppressed mastoparan-induced PRL secretion. Guanosine 5'-1beta-thioldiphosphate (GDPbetaS) encapsulated in GH3 cells by reversible electropermeabilization suppressed the response to mastoparan. However, pretreatment with pertussis toxin had no effect on the stimulation of PRL secretion by mastoparan, and both Mas7 (a highly active analogue of mastoparan) and Mas17 (an inactive analogue) enhanced the secretion of PRL to a similar level to that of mastoparan-induced GH3 cells. In contrast, the substance P-related peptide GPant-2A, a Gq antagonist, inhibited mastoparan-induced PRL release, whereas GPant-2, a G(i/o) antagonist, did not in electropermeabilized GH3 cells. Moreover, a specific G(q/11) antibody against the carboxyl terminus of the G(q/11) alpha-subunit blocked the stimulatory effect of mastoparan on secretion and mastoparan-stimulated InsPs production in digitonin-permeabilized GH3 cells. These results indicate that mastoparan induces the Ca2+-regulated secretion of PRL from GH3 cells by activating G(q/11) and the phospholipase C pathway.
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PMID:Mastoparan-stimulated prolactin secretion in rat pituitary GH3 cells involves activation of Gq/11 proteins. 911 92

The effect of ethanol on maxi Ca2+-activated K+ channels (BK channels) in GH3 pituitary tumor cells was investigated using single-channel recordings and focusing on intracellular signal transduction. In outside-out patches, ethanol caused a transient concentration-dependent increase of BK-channel activity. 30 mm (1.4 per thousand) ethanol significantly increased mean channel open time and channel open probability by 26.3 +/- 9% and 78.8 +/- 10%, respectively; single-channel current amplitude was not affected by ethanol. The augmenting effect of ethanol was blocked in the presence of protein kinase C (PKC) inhibitors staurosporine, bisindolylmaleimide, and PKC (19-31) pseudosubstrate inhibitor as well as by AMP-PNP (5'-adenylylimidodiphosphate), a nonhydrolyzable ATP-analogue, but not by the phospholipase C blocker U-73122. Phosphatase inhibitors microcystin-LR and okadaic acid promoted the ethanol effect. The blocking effect was released at higher concentrations of ethanol (100 mm) suggesting a second site of action or a competition between blockers and ethanol. Our results suggest that the effect of ethanol on BK-channels is mediated by PKC stimulation and phosphorylation of the channels which increases channel activity and hence may influence action potentials duration and hormone secretion.
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PMID:Ethanol activates maxi Ca2+-activated K+ channels of clonal pituitary (GH3) cells. 917 11