Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.1.4.3 (phospholipase C)
18,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

TRH increases cytosolic-free calcium ([Ca2+]i) by activating phospholipase C(PL-C), which induces phosphoinositol hydrolysis, leading to Ca2+ mobilization from inositol trisphosphate (IP3) sensitive stores, and by increasing Ca2+ influx. Increases in [Ca2+]i stimulate PRL secretion. We investigated the effects of U-73122, an aminosteroid inhibitor of PL-C dependent processes, on TRH-stimulated second messenger pathways and on PRL secretion in GH3 rat pituitary cells. [Ca2+]i was monitored by Indo-1 fluorescence, and IP3 and metabolites separated on ion exchange columns. In Ca(2+)-free buffer, [Ca2+]i was 96 +/- 6 nM and increased to 323 +/- 23 nM (P less than 0.001) after TRH (100 nM). U-73122 dose dependently inhibited the TRH effect (IC50 = 967 nM; complete inhibition at 3-5 microM). Subsequent addition of monensin (100 microM) increased [Ca2+]i from 107 +/- 4 to 142 +/- 4 nM (P < 0.001), confirming our previous findings of a non-TRH regulated Ca2+ pool in GH3 cells. Pretreatment (15 sec) with U-73122 partly inhibited the TRH effect on [Ca2+]i; complete suppression occurred with 70 sec of pretreatment. An inactive analog (U-73343) had no inhibitory effect at 5 microM. U-73122 acted noncompetitively, as the mean maximum velocity (expressed as percent increase in [Ca2+]i after TRH) was reduced from 225 to 91 while the Michaelis-Menten constant for TRH was unchanged (15.4 vs. 13.8 nM, n = 3). Of note, U-73122, at 3-5 microM, increased basal [Ca2+]i from 109 +/- 5 to 120 +/- 5 nM (P less than 0.001). In 1.3 mM Ca2+ buffer containing nifedipine (1 microM) and verapamil (50 microM), similar effects of U-73122 (5 microM) were observed on basal and TRH-stimulated [Ca2+]i. IP3, IP2, and IP1 increased to 241 +/- 12%, 148 +/- 23%, and 167 +/- 39% of control, 30 sec after TRH (100 nM); these responses were prevented by 1 microM U-73122. At 5 microM, U-73122 also significantly increased IP3 levels. TRH (100 nM) increased 4-h PRL secretion from 16.3 +/- 1.4 to 27.6 +/- 3.2 ng/well (P less than 0.05). U-73122 (5 microM) increased basal PRL secretion to 35.9 +/- 3.2 ng/well (P less than 0.05), but abolished the TRH effect. In contrast, U-73343 (with Ca2+ channel blockers) did not inhibit the TRH effect on PRL (control: 24.3 +/- 2.1; TRH: 51.0 +/- 6.3 ng/well).(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:U-73122, an aminosteroid phospholipase C antagonist, noncompetitively inhibits thyrotropin-releasing hormone effects in GH3 rat pituitary cells. 139 32

We have reviewed the literature, which supports an important role for dopamine withdrawal in the regulation of PRL secretion. Concentrations of dopamine in the hypophyseal portal circulation are sufficient to occupy the majority of dopamine receptors (1) and tonically suppress PRL secretion (20-26). Brief escapes from dopaminergic regulation associated with the secretion of PRL have been observed (37-41). Therefore, dopamine regulates secretion of PRL both by occupancy of, as well as dissociation from, specific D2 dopamine receptors. The rapid off rate from its receptor (2) is consistent with signals transmitted through brief decreases in dopamine concentration. The removal of dopamine for 10 min results in increases in intracellular cAMP and presumably activation of protein kinase A (39, 138) as well as activation of phospholipase C (137, 138) and protein kinase C (136). The removal of dopamine results directly in the release of PRL (37-41). Furthermore, the brief removal of dopamine results in the long-term potentiation of the PRL-releasing action of TRH (38-40). The potentiating action of dopamine withdrawal appears to be mediated by the activation of protein kinase A since pretreatment with VIP, a hormone that signals via protein kinase A, also potentiates the action of TRH (39). TRH stimulates PRL release via Ca2+/protein kinase C (177-184). The potentiating action of dopamine removal is selective for the Ca2+/protein kinase C pathway since dopamine removal does not potentiate the PRL-secreting action of VIP (38, 87, 92). The action of TRH is potentiated up to 30 min after the return of dopamine and the suppression of PRL to basal levels (38). In Fig. 10, dopamine dissociation from its receptor or VIP association to its receptor are shown separated by a broken line to indicate that by the time the potentiation of the action of TRH is tested, either dopamine is again occupying its receptor or VIP is no longer present. Therefore, the effect of protein kinase A activation is remembered by the lactotroph. We hypothesize that the responsiveness of the cell to TRH is potentiated by the phosphorylation of proteins by protein kinase A. Two potential substrates for protein kinase A are voltage-dependent Ca2+ channels and protein phosphatase inhibitors that would prolong the action of protein kinase C. When TRH occupies its receptor, intracellular Ca2+ levels are increased first from intracellular stores and subsequently by extracellular Ca2+ influx (187-189). Intracellular Ca2+ is mobilized by increased levels of IP3(128). Extracellular Ca2+ enters the lactotroph via voltage-dependent Ca2+ channels (189, 190).(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Dissociation of dopamine from its receptor as a signal in the pleiotropic hypothalamic regulation of prolactin secretion. 161 63

Human decidual tissue synthesizes and secretes a protein that is identical to pituitary prolactin in its chemical, biological and immunological properties. Nevertheless, the factors that regulate the synthesis and release of prolactin from the decidual tissues appear to be different to those regulating the synthesis and release of pituitary prolactin. Studies from our laboratory over the past few years indicate that the synthesis and release of decidual prolactin are regulated, at least in part, by factors released by placenta, fetal membranes and decidua. The placenta releases a 23.5 KMr protein [decidual prolactin-releasing factor (PRL-RF)] that stimulates a rapid release of prolactin within the first few minutes of exposure and a sustained, prolonged, increase in the synthesis and release of prolactin beginning 6-8 h after exposure. The acute release of prolactin in response to PRL-RF is inhibited by decidual prolactin release-inhibitory factor (PRL-IF), a 35-45 K Mr protein that is released by the decidua. The secondary increase in the synthesis and release of prolactin in response to PRF-RF is blocked by lipocortin I, which is synthesized by both the placenta and decidua. IGF-I, insulin and relaxin also stimulate the synthesis and release of prolactin. However, the stimulation in response to these factors does not occur until 24-48 h after exposure. The cellular mechanisms involved in the release of decidual prolactin are as yet unknown. However, recent studies implicate activation of adenylate cyclase, phospholipase C-mediated phosphoinositide hydrolysis and phospholipase A2-mediated arachidonic acid release in the regulation of prolactin release. The finding that the synthesis and release of decidual prolactin are regulated, at least in part, by PRL-RF, IGF-I, insulin, relaxin and lipocortin I strongly suggests that there is novel feedback regulation between the placenta, fetal membranes, and decidua in the regulation of the synthesis and release of decidual prolactin.
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PMID:Regulation of the synthesis and release of decidual prolactin by placental and autocrine/paracrine factors. 183 Dec 61

TRH and phorbol dibutyrate (PDBu) stimulate PRL secretion and synthesis from GH4C1 rat pituitary cells through activation of protein kinase C (PKC). TRH responses are mediated by increases in cellular levels of two PKC activators, Ca2+ and diacylglycerol (DAG), whereas PDBu acts as a DAG analog. We conducted experiments to compare the effects of Ca2+ and PDBu/DAG on alpha-PKC redistribution and to determine to what components of the particulate fraction activated alpha-PKC associates. Subcellular fractionation experiments demonstrated that TRH and PDBu both caused chelator-stable association of alpha-PKC with the particulate fraction. In contrast, Ca2+-mediated association with the particulate fraction was not chelator stable. Immunocytofluorescence experiments also demonstrated that TRH, PDBu, and increased cytosolic Ca2+ (due to ionomycin or K+ depolarization) caused redistribution. The effect of TRH was rapid and transient, similar to TRH stimulation of phospholipase C. The translocated alpha-PKC in the particulate fraction from TRH- or PDBu-treated cultures was not solubilized with Triton X-100. In comparable studies using an immunofluorescence assay, alpha-PKC immunofluorescence remained in detergent-insoluble preparations from TRH- and PDBu-stimulated, but not resting cells. The association of activated alpha-PKC with chelator- and detergent-insoluble material suggested that activated alpha-PKC may be associated with membrane and cytoskeletal components.
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PMID:Activation of alpha-protein kinase C leads to association with detergent-insoluble components of GH4C1 cells. 210 89

The phospholipid platelet-activating factor (PAF) (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) stimulated the accumulation of inositol phosphates in cultures of rat and bovine anterior pituitary cells. In response to PAF, inositol 1,4-bisphosphate showed the largest percent increase of the inositol phosphates in the presence of lithium chloride. PAF induced an increase of inositol 1,4,5-trisphosphate, the biologically active isomer responsible for mobilization of intracellular calcium. A characterization of the PAF response indicated that PAF, but not its biologically inactive enantiomer, induced the accumulation of inositol phosphates in the rat anterior pituitary. Further, the PAF receptor antagonist L652731 reduced PAF stimulation. The ED50 for PAF-induced inositol 1,4-bisphosphate accumulation was 0.4 nM. PAF induced a rapid response that did not persist beyond 20 min. While PAF treatment of anterior pituitary cells did not alter TRH-induced inositol phosphate accumulation, it did prevent a second exposure of PAF from inducing inositol phosphate accumulation. These data suggest that PAF induces a rapid stimulation of phospholipase C causing the hydrolysis of phosphatidylinositol 4,5-bisphosphate and the generation of the second messengers, inositol 1,4,5-trisphosphate and diglyceride, in anterior pituitary tissue. This action is transient probably due to PAF receptor desensitization. The action of PAF on generation of inositol phosphates may account, in part, for PAF-induced secretion of PRL and GH.
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PMID:Platelet activating factor induces inositol phosphate accumulation in cultures of rat and bovine anterior pituitary cells. 216 2

In order to define the roles of cAMP and inositol phosphates (IPx) in the mechanisms governing dopamine (DA)-regulated PRL release, we have carried out studies with enriched lactotrophs from dispersed anterior pituitaries of estrogen-treated rats. Changes in the intracellular levels of cAMP as well as IPx were determined in response to the acute addition or removal of DA. The withdrawal of DA from the incubation medium in cells cultured with DA (500 nM) for 24 h resulted in a rapid and significant increase in cAMP concentration from 1.29 +/- 0.098 to 3.89 +/- 0.199 pmol/dish. On the other hand, the administration of DA for 10 min to cells cultured without it resulted in a significant decrease in intracellular cAMP (from 3.04 +/- 0.208 to 1.62 +/- 0.057 pmol/dish). Similarly, PRL released into the medium was stimulated (95.1 +/- 9%) or inhibited (46.9 +/- 6%) after DA withdrawal or addition, respectively. Cells incubated 24 h with [3H]inositol and DA (500 nM) responded to DA withdrawal with a significant increase in the concentration of [3H]IPx (5148 +/- 199 vs. 8,376 +/- 164 cpm/dish), whereas acute DA administration had no effect on the level of [3H]IPx. The administration of 8-Br-cAMP (0.5 and 2.5 mM) and forskolin (10 microM) for 10 min to cells tonically cultured in the presence of DA had no effect on the intracellular concentration of [3H]IPx, although they decreased the relative proportion of [3H]IP3 fraction from 22.1% to 11.6%, 13.6%, and 9.6%, respectively. The administration of TRH (100 nM), either under tonic DA inhibition or 10 min after the transient removal of DA inhibition, resulted in a similar stimulation of IPx formation (from 5,625 +/- 155 to 21,830 +/- 100 and 24,870 +/- 80 cpm/dish, respectively). However, the release of PRL induced by TRH was potentiated 6-fold (38.2 +/- 2.17 vs. 227 +/- 41 ng/dish) by the transient removal of DA. These findings support the conclusions that: 1) DA receptors in lactotrophs are negatively coupled to adenylate cyclase as seen during the addition or removal of DA. 2) DA receptors are negatively coupled to phospholipase C; however activation is only seen upon the removal of DA. 3) The effect of DA withdrawal on the formation of IPx is not secondary to the activation of adenylate cyclase. 4) The potentiation of the PRL response to TRH after DA withdrawal does not involve increased production of IPx.
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PMID:Effect of dopamine withdrawal on activation of adenylate cyclase and phospholipase C in enriched lactotrophs. 245 18

We investigated the effects of angiotensin peptides on the breakdown of specific membrane phospholipids, the inositol lipids, in anterior pituitary cells in culture, measuring the water-soluble products (inositol phosphates) produced during the cleavage of phosphoinositides by phospholipase C. Both angiotensin II and angiotensin I in the presence of 10 mM LiCl potently increased, in a concentration-dependent manner, total [3H]inositol phosphate and PRL release in cultured rat anterior pituitary cells. The release of LH, TSH, or GH was not significantly enhanced by the peptides. The effect on inositol phosphate accumulation was significant at 0.01 nM, and maximal stimulation (approximately 5-fold increase) occurred at 10 nM, with an ED50 of about 0.3 nM. The stimulatory effects of both angiotensin II and angiotensin I were antagonized by the receptor antagonists saralasin and Sar1,Ile8-angiotensin II. Moreover, 1 microM captopril, an inhibitor of angiotensin-converting enzyme, antagonized the effects of 0.1 and 1 nM angiotensin I, suggesting that the effect of angiotensin I on phosphoinositide breakdown and PRL release is dependent on prior conversion of angiotensin I to angiotensin II. The effect of angiotensin II was very rapid. Fractionation of the water-soluble inositol phosphates showed that angiotensin II significantly increased inositol bisphosphate and inositol triphosphate at 10 sec, whereas inositol monophosphate was increased only after 40 sec. These data indicate that in the pituitary, and presumably in the lactotroph, the binding of angiotensin II to specific membrane receptors provokes increased polyphosphoinositide hydrolysis, leading to increased production of intracellular messengers, i.e. inositol triphosphate and 1,2-diacylglycerol, responsible for the stimulation of PRL release.
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PMID:Angiotensin peptides stimulate phosphoinositide breakdown and prolactin release in anterior pituitary cells in culture. 300 Jul 36

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

The possible role of the phospholipase enzymes in the prolactin stimulation of mitogenesis in Nb2 node lymphoma cells was investigated. Two phospholipase inhibitors including quinacrine and alpha-para-dibromoacetophenone (BPB) were employed. Quinacrine at concentrations of 1-5 microM attenuated the magnitude of the PRL stimulation of cell division; at concentrations of 10 microM and above quinacrine abolished the PRL response. BPB at concentrations of 1-10 microM also inhibited the mitogenic effect of PRL in a concentration response fashion. The polyunsaturated fatty acid arachidonic acid partially reversed the inhibitory effects of these drugs. In further studies, exogenously added phospholipase C at concentrations of 5-50 ng/ml was found to potentiate the mitogenic effect of prolactin when prolactin was employed at a concentration that evoked a half-maximal response. By itself, however, phospholipase C had no effect on the rate of cell division. Phospholipase A2 either by itself or in the presence of prolactin was without effect.
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PMID:Possible involvement of the phospholipases in the mitogenic actions of prolactin (PRL) on Nb2 node lymphoma cells. 310 86

The purpose of these studies was to determine whether increased cellular diacylglycerol could modulate phorbol ester receptor properties, in order to demonstrate that diacylglycerol can interact with and modulate the phorbol ester receptor in intact cells. Treatment of GH4C1 cells with bacterial phospholipase C caused an increase in cellular diacylglycerol. This was accompanied by increased PRL secretion and decreased epidermal growth factor (EGF) binding, two responses that also occur with phorbol ester treatment of GH4C1 cells. Phospholipase C treatment led to decreased apparent affinity for phorbol esters with no change in receptor number when measured in intact cells. This is consistent with increased concentrations of a competitive inhibitor of phorbol ester binding in treated cultures. Phospholipase C treatment caused a change in subcellular distribution of phorbol ester receptors, another response characteristic of phorbol ester treatment. TRH is known to activate endogenous phospholipase C activity in these cells, leading to a transient increase in diacylglycerol levels. TRH treatment also led to a transient change in subcellular distribution of phorbol ester receptors. In addition, a coordinate change in subcellular distribution of protein kinase C was observed. These data suggest that diacylglycerol is an endogenous ligand for the target for phorbol ester action in GH4C1 cells.
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PMID:Increased diacylglycerol content with phospholipase C or hormone treatment: inhibition of phorbol ester binding and induction of phorbol ester-like biological responses. 393 61


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