G protein-mediated inhibition of phospholipase C activity in a solubilized membrane preparation

Correlation of apparent affinity values from H3-receptor binding assays with apparent affinity (pKapp) and intrinsic activity (alpha) from functional bioassays

BACKGROUND AND PURPOSE

Agonist apparent affinities (pK(I)') in histamine H(3)-receptor binding assays were higher than expected from apparent affinity values (pK(app)) estimated in bioassay. Here, we investigate whether the degree of pK(I)' overestimation is related to agonist intrinsic efficacy, by studying the effect of buffer composition on the pK(I)' of ligands with varying intrinsic activity.

EXPERIMENTAL APPROACH

In the guinea-pig ileum bioassay, intrinsic activity (alpha) was determined from the maximal inhibition of the contraction produced by increasing agonist concentration. pK(app) values were estimated using the method of Furchgott. The pK(L) of [(3)H]clobenpropit in guinea-pig cerebral cortex was estimated by saturation analysis in 20 mM HEPES-NaOH buffer (buffer B(0,0,0)), or buffer B(0,0,0) containing 70 mM CaCl(2), 100 mM NaCl and 100 mM KCl (buffer B(0.07,0.1,0.1)). PK(I) values were determined in competition studies in both buffers.

KEY RESULTS

[(3)H]clobenpropit saturation isotherms had n (H) values of unity in both buffers. In buffer B(0.07,0.1,0.1), agonist pK(I)' values were closer to pK(app) values than in buffer B(0,0,0) but were associated with n (H) values <1. A two-site analysis of agonist data in buffer B(0.07, 0.1, 0.1) provided a better fit than a one-site fit and low affinity values (pK(IL)) were comparable to pK(app). Differences between the pK(I)' in buffer B(0,0,0) and pK(IL) values in buffer B(0.07,0.1,0.1) (DeltapK) were correlated with alpha.

CONCLUSIONS AND IMPLICATIONS

H(3)-receptor binding assays conducted in buffer B(0,0,0) and buffer B(0.07,0.1,0.1) can provide a measure of ligand affinity (pK(app)) and intrinsic efficacy. The assay predicts that some ligands previously classified as H(3)-receptor antagonists may possess residual intrinsic efficacy.

Regulatory interactions between the amino terminus of G-protein betagamma subunits and the catalytic domain of phospholipase Cbeta2

We previously identified a 10-amino acid region from the Y domain of phospholipase Cbeta2 (PLCbeta2) that associates with G-protein betagamma subunits (Sankaran, B., Osterhout, J., Wu, D., and Smrcka, A. V. (1998) J. Biol. Chem. 273, 7148-7154). We mapped the site for cross-linking of a synthetic peptide (N20K) corresponding to this Y domain region to Cys(25) within the amino-terminal coiled-coil domain of Gbetagamma (Yoshikawa, D. M., Bresciano, K., Hatwar, M., and Smrcka, A. V. (2001) J. Biol. Chem. 276, 11246-11251). Here, further experiments with a series of variable length cross-linking agents refined the site of N20K binding to within 4.4-6.7 angstroms of Cys(25). A mutant within the amino terminus of the Gbeta subunit, Gbeta(1)(23-27)gamma(2), activated PLCbeta2 more effectively than wild type, with no significant change in the EC(50), indicating that this region is directly involved in the catalytic regulation of PLCbeta2. This mutant was deficient in cross-linking to N20K, suggesting that a binding site for the peptide had been eliminated. Surprisingly, N20K could still inhibit Gbeta(1)(23-27)gamma(2)-dependent activation of PLC, suggesting a second N20K binding site. Competition analysis with a peptide that binds to the Galpha subunit switch II binding surface of Gbetagamma indicates a second N20K binding site at this surface. Furthermore, mutations to the N20K region within the Y-domain of full-length PLCbeta2 inhibited Gbetagamma-dependent regulation of the enzyme, providing further evidence for aGbetagamma binding site within the catalytic domain of PLCbeta2. The data support a model with two modes of PLC binding to Gbetagamma through the catalytic domain, where interactions with the amino-terminal coiled-coil domain are inhibitory, and interactions with the Galpha subunit switch II binding surface are stimulatory.

Phosphatidic acid modulates G protein regulation of phospholipase C-beta1 activity in membranes

Regulation of G protein stimulated phospholipase C-beta1 (PLC-beta1) activity by phosphatidic acid (PA) was determined in membranes. In cerebral cortical membranes, PLC-beta1 is under dual regulation by G protein stimulatory and inhibitory mechanisms. PA stimulated basal activity and was synergistic with G protein activation in increasing PLC-beta1 activity. Lysophosphatidic acid (LPA) also stimulated PLC-beta1 activity, but was less effective then PA. PA stimulation of PLC-beta1 activity was relatively independent of acyl chain length. PA decreased the Ca2+ dependence for G protein stimulation of PLC-beta1 activity. PA modulated the dual G protein regulation of PLC-beta1 activity, increasing stimulatory regulation and reducing inhibitory G protein regulation. The sensitivity to guanosine 5'-[gamma-thio]trisphosphate (GTP-gamma-S) and carbachol stimulation of PLC-beta1 activity was increased by PA. These results demonstrate that PA regulates both basal activity and G protein stimulation of PLC-beta1 activity. The data indicates that PA regulates the PLC-beta1 signaling pathway and thus may have an important role in the modulation of cell activation.

Cholera toxin and extracellular Ca2+ induce adherence of non-piliated Neisseria: evidence for an important role of G-proteins and Rho in the bacteria-cell interaction

In this study, we characterize the interaction between non-piliated (P-) Neisseria gonorrhoeae and human epithelial cells. P- mutants lacking the pilus subunit protein PilE attach at low levels to cells. Although the binding may not lead to heavy inflammatory responses, the interaction between P- Neisseria and host cells most probably play a role in colonization and asymptomatic carriage of the pathogen. Here we show that the adherence of P N. gonorrhoeae is blocked by GDP-beta-S [guanosine 5'-O(thio)diphosphate], a non-hydrolyzable GTP analogue, and by C3 exotoxin, an inhibitor of the small G-protein Rho. G-protein activators such as cholera toxin, that activates Gs, and fluoroaluminate, a general G-protein activator, induced bacterial adherence. Furthermore, increase of the extracellular free [Ca2+] dramatically enhanced adherence of non-piliated Neisseria. The pharynx and the urogenital tract are natural entry sites of the pathogenic Neisseria species, and at both sites the epithelial cells can be exposed to wide variations in Ca2+ concentration. Taken together, these data show the importance of extracellular Ca2+ in the pathogenic Neisseria-host interaction, and reveal a novel function of cholera toxin, namely induction of bacterial adherence.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

Molecular diversity and double regulatory mechanism of activation of phospholipase C in rat brain

Whereas evidence for a G protein-dependent stimulation of phospholipase C (PLC) is abundant, reports on the inhibition of PLC through a G protein-mediated pathway have only recently begun to appear. In the present study, cerebral cortex membranes were chosen since they have a readily measurable Gpp[NH]p and Ca2+-stimulated PLC activity. Nanomolar concentrations of Gpp[NH]p, a hydrolysis-resistant GTP analogue, inhibited basal inositol 1,4,5-trisphosphate (IP3) production, with a maximum inhibition of 25% at 10 nM. Increasing the concentrations of Gpp[NH]p to over 10 nM resulted in a reversal of the inhibitory effect and onset of stimulation of IP3 production. GDPbetaS as a G protein inhibitor and U-73122 as a putative PLC-beta inhibitor had little effect on basal IP3 production at 100 microM and 1 microM, respectively. However, GDPbetaS and U-73122 completely antagonized both the inhibition and the stimulation of IP3 production produced by lower and higher concentrations, respectively, of Gpp[NH]p. Rat cortical membranes expressed a greater amount of PLC-beta1. These data suggest that PLC-beta1 isozymes may be regulated by both inhibitory and stimulatory G protein-mediated mechanisms.

Enhanced type 1alpha metabotropic glutamate receptor-stimulated phosphoinositide signaling after pertussis toxin treatment

The regulation of phosphoinositide hydrolysis by the type 1alpha metabotropic glutamate receptor (mGluR1alpha) was investigated in stably transfected baby hamster kidney (BHK) cells. Incubation of the cells with L-glutamate, quisqualate, and 1-aminocyclopentane-1S, 3R-dicarboxylic acid resulted in a marked accumulation of [3H]inositol monophosphate (InsP1) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] mass in a time- and concentration-dependent manner. Pretreatment of BHK-mGluR1alpha cells with pertussis toxin [ 100 ng/ml, 24 hr] led to a dramatic 12-16-fold increase in the accumulation of [3H]InsP1 and a 2-fold increase in Ins(1,4,5)P3 in the absence of added agonist. Although only very low levels (</=1 microM) of L-glutamate could be detected in medium taken from control and PTX-treated cell monolayers, the PTX-elicited effect on basal [3H]InsP1 was fully reversed by preincubation of cells in the presence of glutamic-pyruvic transaminase and pyruvate, suggesting that an increased sensitivity to endogenous glutamate was responsible for the apparent agonist-independent activation of phosphoinositidase C (PIC) after PTX treatment. Consistent with this hypothesis, in the presence of glutamic-pyruvic transaminase/pyruvate, the maximal [3H]InsP1 response to quisqualate was increased by >/=75%, and the EC50 shifted leftward by 65-fold [-log EC50 values (molar), 7.26 +/- 0.23 versus 5.45 +/- 0.07; n = 4) in PTX-treated compared with control cells. In contrast, antagonist effects on agonist-stimulated [3H]InsP1 responses were similar in control and PTX-treated BHK-mGluR1alpha cells. These changes in the concentration-effect curves for mGluR agonists are consistent with a model in which the receptor associates with PTX-sensitive inhibitory (Gi/o) and PTX-insensitive stimulatory (Gq/11) G proteins that can each influence PIC activity. The present observations are consistent with a dual regulation of mGluR1alpha-mediated PIC activity that could be fundamental in controlling the output of phosphoinositide-derived messengers.

Effect of membrane lipid alteration on the growth, phospholipase C activity and G protein of HT-29 tumor cells

The objective of the present study was to examine the effect of modifying the fatty acid composition of membranes on cell growth and phosphoinositide specific phospholipase C (PLC) activity in HT-29 colon cancer cells. Cells were seeded at a density of 12 x 10(3) cells/cm2 and supplemented with 30 microM of either 18:0, 18:2 (n6) or 18:3 (n3) complexed to bovine serum albumin (BSA) in DMEM medium. Cell growth was followed for 12 days. The 18:0 supplemented cells (control) reached maximum growth at day nine which was greater than either 18:2 (n6) or 18:3 (n3) supplemented cells. There was no difference between the latter two groups in their growth. To investigate the fatty acid incorporation of the supplemented fatty acid and how they may influence composition in the cell membrane, we examined the fatty acid composition of each phospholipid (PL) species. Both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were significantly influenced by the type of fatty acid supplemented. Supplementation with 18:0 resulted in HT-29 cell membranes having more monounsaturated fatty acids than the cells grown in the other fatty acids. Polyunsaturated fatty acid (PUFA) supplementation (both 18:2 and 18:3) resulted in the enrichment of PUFA in the PL fractions. Cells supplemented with 18:3 (n3) had the highest unsaturation index in membrane PE as compared to the other phospholipid species. PLC activity of the membranes was measured using PIP2 as a substrate in the presence of 15 micrograms alamethicin and 42 microM free calcium. The contribution of G protein to the activity of the enzyme was assessed using GTP gamma(S). PLC activity of HT-29 cells was 16% higher in the presence of GTP gamma(S) response. GTP gamma(S)-activated PLC activity of 18:3 (n3) supplemented cells was 81% of those supplemented with either 18:0 or 18:2 (n6) cells. It is concluded that the decrease in cell proliferation with supplementation with 18:3 (n3) may be mediated through its inhibitory effect on PLC, which provides the second messengers for protein kinase C (PKC) activation. PLC may be influenced by an increased unsaturation index of the PE fraction of the HT-29 tumor cell membranes.

Phospholipase C isoforms in vascular smooth muscle and their regulation by G-proteins

1. We sought to reconstitute and characterize G-protein linked phosphatidyl-D-inositol 4,5-bisphosphate (PIP2)-directed phospholipase C (PLC) isoform activity in pig aortic vascular smooth muscle. 2. Six soluble PLC isoforms, namely gamma 1, delta 1 and beta 1 to beta 4 were partially separated by heparin affinity chromatography and were identified by Western blotting using specific antibodies. 3. In separate experiments, PLC activity was measured in the eluted fractions. Four of the partially resolved PLC isoforms gamma 1, beta 4, beta 2 and beta 1, showed corresponding activity using exogenous [3H]-PIP2 as substrate. 4. The isolated soluble PLC isoforms were reconstituted with receptors and guanyl nucleotide regulatory proteins (G-proteins) by addition of plasma membranes, the phospholipids which had been prelabelled with [3H]-myo-inositol. When so reconstituted PLC beta 2, beta 3 and beta 4 were inhibited (40 +/- 9, 47 +/- 12 and 40 +/- 5% respectively n = 12, +/-s.e.mean and each P < 0.05) by the addition of 1 mM guanosine 5'[beta gamma-imido]triphosphate (p[NH]ppG). 5. By contrast, when plasma membranes were preincubated with pertussis toxin to inhibit the activity of G-protein subunits G alpha i/alpha o the activities of PLC beta 2, beta 3 and beta 4 were stimulated (46 +/- 11, 31 +/- 9 and 37 +/- 8% respectively, n = 12, +/- s.e.mean and each P < 0.05) by the addition of p[NH]ppG. 6. Using well resolved fractions containing only PLC beta 3, time-dependent activity in the presence of p[NH]ppG was measurable only with membranes pretreated with pertussis toxin. 7. PLC beta 3 activity, measured with pertussis pretreated membranes, showed a dose-dependent increase in the presence of p[NH]ppG or guanosine 5'-[gamma-thio]triphosphate (GTP[S]). This increase with 10 microM p[NH]ppG or GTP[S] 10% +/- 4 and 12% +/- 5 respectively (both P < 0.05 vs control without GTP analogue +/- s.e.mean, n = 10) was abolished by 50 microM guanosine 5'-[beta-thio]diphosphate (GDP[S]) which also reduced constitutive PLC beta 3 activity by 9% +/- 4. 8. G-protein antibodies were used to neutralize PLC activity. Antibody to G alpha q/alpha 11, added to membrane fractions pretreated with pertussis toxin and assayed with GTP[S], reduced PLC beta 3 activity by 21% +/- 6 P < 0.02, n = 6, but was without effect on non-pertussis pretreated membranes. Antibodies to G alpha i1/alpha i2 had no effect. Antibodies to G-protein beta subunits had no effect on PLC beta 3 activity with pertussis pretreated preparations but activity without pertussis pretreatment was increased by 30% +/- 10, P < 0.03, n = 6. All results were expressed as % change from controls containing rabbit IgG. 9. In conclusion, pig aortic vascular smooth muscle contains six PLC isoforms. Activation of pertussis sensitive G-protein by GTP analogues results in inhibition of PLC beta 3 activity from liberated G-protein beta gamma subunits. Stimulation of PLC beta 3 activity is associated with a G-protein of the G alpha q family acting through the alpha subunit. The results suggest that the G-protein linked PLC beta isoforms in vascular smooth muscle demonstrate dual regulation by an inhibitory pertussis-sensitive pathway and a stimulatory G-protein of the G alpha q family, which is the case for PLC beta 3. This dual regulation is analogous to that of adenyl cyclase.

Thromboxane A2 receptor-mediated signal transduction in rabbit aortic smooth muscle cells

1. 9,11-Epithio-11,12-methenothromboxane A2 (STA2), a stable analogue of thromboxane A2 (TXA2), contracted rabbit aortic smooth muscles (RASM) and accumulated [3H]inositol phosphates in cultured RASM cells. The contraction and phosphoinositide hydrolysis were competitively inhibited by TXA2 receptor antagonists, including ONO NT-126, S-145, SQ29548, KW3635, GR32191B and ONO3708. 2. STA2 inhibited [3H]ONO NT-126 binding in a concentration-dependent manner in membranes derived from cultured aortic smooth muscle cells, but GTP gamma S, a stable GTP analogue, did not affect STA2-induced inhibition of [3H]ONO NT-126 binding. 3. The time course analysis revealed that STA2 rapidly decreased inositol phosphate level and therefter increased. Pertussis toxin did not attenuate but rather increased STA2-induced phosphoinositide hydrolysis. 4. TXA2 receptor stimulation results in at least two signaling pathways in RASM cells: stimulation and inhibition of phosphoinositide hydrolysis.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Imipramine stimulates phospholipase C activity in rat brain

We previously demonstrated that antidepressant drugs (ADs) cause Ca2+ release from inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in cultured neurons of rat frontal cortex. The present study examines the mechanism by which tricyclic ADs activate phospholipase C (PLC) in rat frontal cortex. Using an exogenous substrate to measure PLC activity, we demonstrated that a tricyclic AD, imipramine, stimulated PLC activity of the frontal cortex membrane in a concentration-dependent manner. Two tricyclic ADs, desipramine and amitriptyline, also stimulated PLC activity, while Li+ or pargyline had no effect on PLC activity. Although imipramine did not activate PLC in the membrane in the absence of Ca2+, imipramine synergistically activated PLC in the presence of Ca2+. This result indicates that the mechanism of PLC activation by imipramine is different from its activation by Ca2+. Imipramine stimulated PLC activity in the cytosol of rat frontal cortex as well as in the membrane. Preincubation of the cytosol with anti-PLC-beta 1 antibody prevented the imipramine-mediated activation of PLC. However, preincubation with anti-PLC-gamma 1 or anti-PLC-delta 1 did not prevent activation of PLC. These results suggest that imipramine activates PLC-beta 1 directly without receptor or guanine nucleotide binding protein mediation.

Interactions between adenosine A1- and histamine H1-receptors

The interactions or "cross-talk" between adenosine A1-receptors and receptors coupled to phospholipase C (leading to the hydrolysis of inositol phospholipids) have been well documented in the literature. For example, activating the A1-receptor selectively potentiates the histamine H1-receptor stimulated hydrolysis of inositol phospholipids in guinea-pig cerebral slices. In contrast, when the adenosine receptor is activated in the cerebral cortex of mouse or man the histamine response is selectively inhibited. Our studies have focused on the smooth muscle cell line, DDT1 MF-2, derived from hamster vas deferens. These cells express A1-receptors which, in addition to the expected negative coupling to adenylate cyclase, also stimulate inositol phospholipid hydrolysis and Ca2+ mobilization. These A1-receptors also potentiate histamine H1-receptor responses, i.e. inositol phospholipid hydrolysis and Ca2+ mobilization. The mechanism(s) underlying the potentiation or inhibition of histamine H1-receptor responses by the adenosine A1-receptor remain to be unravelled. One mechanism may involve intracellular "cross-talk" at the G-protein level. This review will discuss how beta gamma subunits from G(i) proteins could be involved in augmenting responses to calcium mobilizing receptors.

Purification of phospholipase C from rat cerebral cortex

Phospholipase C from rat cerebral cortex was purified to homogeneity by use of DEAE Bio-Gel A agarose, hydroxyapatite, and heparin agarose chromatography. The purified phospholipase C (PLC) was purified 622.4-fold and its molecular weight is estimated to be 97,500. We obtained a final specific activity of 3.112 mumol of phosphatidylinositol hydrolyzed/min/mg of protein. It is specific for inositol phospholipids. The purified enzyme has an apparent optimum pH 7.0. Calcium is required for its activity. Western blotting analysis showed that two proteins were recognized by anti-PLC antiserum.