Beta-adrenergic receptor-mediated phospholipase C activation independent of cAMP formation in turkey erythrocyte membranes

Hydrolysis of phosphatidylinositol 4,5-bisphosphate mediates calcium-induced inactivation of TRPV6 channels

TRPV6 is a member of the transient receptor potential superfamily of ion channels that facilitates Ca(2+) absorption in the intestines. These channels display high selectivity for Ca(2+), but in the absence of divalent cations they also conduct monovalent ions. TRPV6 channels have been shown to be inactivated by increased cytoplasmic Ca(2+) concentrations. Here we studied the mechanism of this Ca(2+)-induced inactivation. Monovalent currents through TRPV6 substantially decreased after a 40-s application of Ca(2+), but not Ba(2+). We also show that Ca(2+), but not Ba(2+), influx via TRPV6 induces depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2) or PIP(2)) and the formation of inositol 1,4,5-trisphosphate. Dialysis of DiC(8) PI(4,5)P(2) through the patch pipette inhibited Ca(2+)-dependent inactivation of TRPV6 currents in whole-cell patch clamp experiments. PI(4,5)P(2) also activated TRPV6 currents in excised patches. PI(4)P, the precursor of PI(4,5)P(2), neither activated TRPV6 in excised patches nor had any effect on Ca(2+)-induced inactivation in whole-cell experiments. Conversion of PI(4,5)P(2) to PI(4)P by a rapamycin-inducible PI(4,5)P(2) 5-phosphatase inhibited TRPV6 currents in whole-cell experiments. Inhibiting phosphatidylinositol 4 kinases with wortmannin decreased TRPV6 currents and Ca(2+) entry into TRPV6-expressing cells. We propose that Ca(2+) influx through TRPV6 activates phospholipase C and the resulting depletion of PI(4,5)P(2) contributes to the inactivation of TRPV6.

Protein kinase C phosphorylates RGS2 and modulates its capacity for negative regulation of Galpha 11 signaling

RGS proteins (regulators of G protein signaling) attenuate heterotrimeric G protein signaling by functioning as both GTPase-activating proteins (GAPs) and inhibitors of G protein/effector interaction. RGS2 has been shown to regulate Galpha(q)-mediated inositol lipid signaling. Although purified RGS2 blocks PLC-beta activation by the nonhydrolyzable GTP analog guanosine 5'-O-thiophosphate (GTPgammaS), its capacity to regulate inositol lipid signaling under conditions where GTPase-promoted hydrolysis of GTP is operative has not been fully explored. Utilizing the turkey erythrocyte membrane model of inositol lipid signaling, we investigated regulation by RGS2 of both GTP and GTPgammaS-stimulated Galpha(11) signaling. Different inhibitory potencies of RGS2 were observed under conditions assessing its activity as a GAP versus as an effector antagonist; i.e. RGS2 was a 10-20-fold more potent inhibitor of aluminum fluoride and GTP-stimulated PLC-betat activity than of GTPgammaS-promoted PLC-betat activity. We also examined whether RGS2 was regulated by downstream components of the inositol lipid signaling pathway. RGS2 was phosphorylated by PKC in vitro to a stoichiometry of approximately unity by both a mixture of PKC isozymes and individual calcium and phospholipid-dependent PKC isoforms. Moreover, RGS2 was phosphorylated in intact COS7 cells in response to PKC activation by 4beta-phorbol 12beta-myristate 13alpha-acetate and, to a lesser extent, by the P2Y(2) receptor agonist UTP. In vitro phosphorylation of RGS2 by PKC decreased its capacity to attenuate both GTP and GTPgammaS-stimulated PLC-betat activation, with the extent of attenuation correlating with the level of RGS2 phosphorylation. A phosphorylation-dependent inhibition of RGS2 GAP activity was also observed in proteoliposomes reconstituted with purified P2Y(1) receptor and Galpha(q)betagamma. These results identify for the first time a phosphorylation-induced change in the activity of an RGS protein and suggest a mechanism for potentiation of inositol lipid signaling by PKC.

A beta-2-adrenergic receptor activates adenylate cyclase in human erythrocyte membranes at physiological calcium plasma concentrations

More information is needed about the subtype of the beta-adrenergic receptor coupled to the G-protein-adenylate cyclase (AC) system in human erythrocytes and about the optimal experimental conditions to study this system. In this study we describe the characteristics of spontaneous and beta-agonist-activated AC in human erythrocytes. Human erythrocyte membranes were isolated and AC activity was utilized to assess the quantity of cAMP. Our data show that the subtype beta-2 is the functional beta-adrenergic receptor involved in such activation; this modifies the beta-adrenergic-stimulated activity of AC in human erythrocytes. Isoproterenol in a medium with calcium (1-10 mM, range that includes physiological plasma concentrations) enhances the activation of AC; this effect was blocked by propranolol, but not by atenolol. We conclude that in human erythrocytes subtype beta-2 is the functional beta-adrenergic receptor and that such a response depends to a large extent on Ca(2+) concentrations.

Pertussis Toxin Shows Distinct Early Signalling Events in Platelet-Activating Factor–, Leukotriene B4–, and C5a-Induced Eosinophil Homotypic Aggregation In Vitro and Recruitment In Vivo

The present study was performed to investigate the early signalling events responsible for eosinophil activation in response to platelet-activating factor (PAF ), C5a, and leukotriene B4 (LTB4 ). We evaluated the effect of pertussis toxin (PTX) on eosinophil aggregation in vitro and cutaneous eosinophil recruitment in vivo. Further studies using the protein kinase inhibitors Ro 31-8220 and staurosporine were performed in vitro to assess in more detail the early signalling events induced by these three mediators. Our results show that C5a and LTB4 signal predominantly or exclusively through a PTX-sensitive G protein that is negatively modulated by protein kinase C, possibly at the level of phospholipase C-β. In contrast, PAF activates eosinophils independent of Gi by a mechanism that is abolished by Ro 31-8220, a selective protein kinase C inhibitor. In addition, these results show for the first time that a receptor-operated event on the eosinophil is essential for chemoattractant-induced eosinophil recruitment in vivo.

beta-Adrenergic stimulation induces activation of protein kinase C and inositol 1,4,5-trisphosphate increase in epidermis

Epidermal keratinocytes express beta 2-adrenergic receptors on the cell membrane. The binding of the agonists to the beta 2-adrenergic receptors regulates activation of adenylate cyclase. This transmembrane signaling system has been regarded to be one of the important pathways for the functions of keratinocytes. We previously reported that beta-adrenergic stimulation induced a transient increase of intracellular Ca2+ in normal human epidermal keratinocytes. Thus we investigated the effects of epinephrine on another transmembrane signaling system, the phosphatidyl-inositol signal transduction pathway in pig epidermis. Treatment of pig pure epidermis with epinephrine resulted in a transient increase in inositol 1,4,5-trisphosphate with a peak at 30 s. Epinephrine induced translocation of protein kinase C from cytosol to the membrane fraction. The activation of protein kinase C, translocation of protein kinase C from cytosol to the membrane fraction, was confirmed using the beta-adrenergic agonist isoproterenol. Moreover, the effect of epinephrine on the activation of protein kinase C was inhibited by preincubation with propranolol, a beta-adrenergic antagonist. The increase in inositol 1,4,5-trisphosphate and translocation of protein kinase C by epinephrine are consistent with the view that beta-adrenergic stimulation induces turnover of inositol phospholipid in pig epidermis.

Stimulation of the mitogen-activated protein kinase via the A2A-adenosine receptor in primary human endothelial cells

Adenosine exerts a mitogenic effect on human endothelial cells via stimulation of the A2A-adenosine receptor. This effect can also be elicited by the beta2-adrenergic receptor but is not mimicked by elevation of intracellular cAMP levels. In the present work, we report that stimulation of the A2A-adenosine receptor and of the beta2-adrenergic receptor activates mitogen-activated protein kinase (MAP kinase) in human endothelial cells based on the following criteria: adenosine analogues and beta-adrenergic agonists cause an (i) increase in tyrosine phosphorylation of the p42 isoform and to a lesser extent of the p44 isoform of MAP kinase and (ii) stimulate the phosphorylation of myelin basic protein by MAP kinase; (iii) this is accompanied by a redistribution of the enzyme to the perinuclear region. Pretreatment of the cells with cholera toxin (to down-regulate Gsalpha) abolishes activation of MAP kinase by isoproterenol but not that induced by adenosine analogues. In addition, MAP kinase stimulation via the A2A-adenosine receptor is neither impaired following pretreatment of the cells with pertussis toxin (to block Gi-dependent pathways) nor affected by GF109203X (1 microM; to inhibit typical protein kinase C isoforms) nor by a monoclonal antibody, which blocks epidermal growth factor-dependent signaling. In contrast, MAP kinase activation is blocked by PD 098059, an inhibitor of MAP kinase kinase 1 (MEK1) activation, which also blunts the A2A-adenosine receptor-mediated increase in [3H]thymidine incorporation. Activation of the A2A-adenosine receptor is associated with increased levels of GTP-bound p21(ras). Thus, our experiments define stimulation of MAP kinase as the candidate cellular target mediating the mitogenic action of the A2A-adenosine receptor on primary human endothelial cells; the signaling pathway operates via p21(ras) and MEK1 but is independent of Gi, Gs, and the typical protein kinase C isoforms. This implies an additional G protein which links this prototypical Gs-coupled receptor to the MAP kinase cascade.

Cyclic AMP-induced desensitization of G-protein-regulated phospholipase C in turkey erythrocyte membranes

The interaction of the cyclic AMP and inositol lipid signalling systems was studied in turkey erythrocytes. Elevation of intracellular cyclic AMP concentrations by pretreatment of the cells with forskolin or 8-Br-cAMP resulted in a marked decrease in responsiveness of phospholipase C to G-protein activators in membranes prepared from treated cells. Decreases in responsiveness occurred with a t1/2 of approximately 5 min and were reversible after transfer of desensitized cells to drug-free medium. Pretreatment of the cells with forskolin inhibited inositol phosphate formation in a concentration-dependent manner and addition of the phosphodiesterase inhibitor IBMX 93-isobutyl-1-methylxanthine) during pretreatment increased the capacity of forskolin to desensitize phospholipase C activity. IBMX also produced a similar potentiation of forskolin-stimulated accumulation of cyclic AMP in turkey erythrocytes. Isoproterenol pretreatment of the cells induced, like forskolin, partial inhibition of inositol phosphate generation in response to G-protein activators and to P2y purinoceptor and beta-adrenoceptor agonists. The capacity of isoproterenol to induce desensitization of phospholipase C activity also was increased by the presence of IBMX during pretreatment of the cells. H8 (N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide), an inhibitor of cyclic AMP-regulated protein kinase, completely prevented forskolin-induced desensitization but only partially blocked isoproterenol-induced desensitization. These results indicate that the cyclic AMP signalling cascade has a major inhibitory influence on receptor- and G-protein-activated inositol lipid signaling.

Stimulation of phospholipase C activity by norepinephrine, t-ACPD and bombesin in LA-N-2 cells

The release of [3H]inositol phosphates from myo-[3H]inositol-prelabeled LA-N-2 cells was measured in the presence of beta-adrenoceptor, metabotropic glutamate and bombesin agonists. Norepinephrine and isoproterenol increased the formation of [3H]inositol phosphates in a dose-dependent manner, with an EC50 of 100 microM for norepinephrine and an EC50 of 5 microM for isoproterenol. These stimulations were abolished by propranolol, a beta-adrenoceptor antagonist, with an IC50 in the range of 50-55 microM for both norepinephrine and isoproterenol. The stimulation of [3H]inositol phosphate appearance occurred with varying concentrations of trans-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD), a metabotropic glutamate receptor agonist. This release of [3H] inositol phosphates was blunted by its antagonist, 2-amino-3-phosphonopropionic acid (AP-3). Bombesin and neuromedin-B, a bombesin-like peptide, also increased the appearance of [3H]inositol phosphates. This was blunted by the antagonist [Tyr4, D-Phe12] bombesin. The appearance of [3H]inositol phosphates stimulated by t-ACPD was coupled through a cholera toxin-sensitive G-protein and the bombesin-stimulated appearance of [3H]inositol phosphates was coupled through a pertussis toxin-sensitive G-protein. The norepinephrine-stimulated appearance of [3H]inositol phosphates was toxin insensitive. The stimulation of the [3H]inositol phosphate appearance by these three agonists was protein kinase and Ca2+ independent.

Receptor-induced heterologous desensitization of receptor-regulated phospholipase C

Activation of the P2Y purinoceptor on turkey erythrocytes results in a G11-mediated activation of a phospholipase C-beta isoenzyme and hydrolysis of polyphosphoinositides. The role of the protein kinase C and Ca(2+)-mobilizing arms of the inositol lipid signalling cascade in P2Y purinoceptor-induced desensitization of phospholipase C has been examined using erythrocytes as a model system. Preincubation of intact erythrocytes with either P2Y purinoceptor agonist, ADP beta S, or the protein kinase C-activating phorbol ester, phorbol 12-myristate, 13 acetate (PMA), resulted in a time of preincubation-dependent decrease in guanine nucleotide-, P2Y purinoceptor-, and beta-adrenoceptor-stimulated phospholipase C activities in membranes isolated from these cells. The extent of heterologous desensitization induced by ADP beta S and PMA were additive suggesting that they did not share a common mechanism. A lack of involvement of activation of protein kinase C in P2Y purinoceptor-induced heterologous desensitization was further supported by the observation that although protein kinase C inhibitors or down-regulation of protein kinase C resulted in a loss of PMA-induced desensitization, neither treatment affected the extent of P2Y purinoceptor-induced desensitization. In addition, elevation of intracellular Ca2+ or prevention of its elevation did not induce heterologous desensitization and had no effect on the desensitization induced by ADP beta S. Thus, neither the protein kinase C nor Ca2+ mobilizing arms of the inositol lipid signalling pathway appear to be involved in P2Y purinoceptor promoted heterologous desensitization of phospholipase C. These results are consistent with the existence of a novel feedback pathway for agonist-induced heterologous desensitization of a second messenger generating enzyme. Preincubation of cells with ADP beta S or the beta-adrenoceptor agonist, isoproterenol, followed by rechallenge with each of the receptor agonists revealed that receptor-specific desensitization occurs in addition to heterologous desensitization. Thus, multiple mechanisms account for agonist-induced desensitization of the inositol lipid signalling system of turkey erythrocytes.

Selective coupling of beta 2-adrenergic receptor to hematopoietic-specific G proteins

The coupling of the beta 2-adrenergic receptor (AR) to the alpha subunits of the Gq class of G proteins was investigated in a cotransfection system. COS-7 cells cotransfected with the beta 2-AR cDNA and the G alpha 15 or G alpha 16 cDNA showed marked norepinephrine-induced increases in accumulation of inositol phosphates in a concentration-dependent manner. However, cells cotransfected with the cDNA encoding G alpha q, G alpha 11, or G alpha 14 instead of G alpha 16 gave no ligand-dependent activation of phospholipase C (PLC). The facts that the beta-AR agonist isoprenaline can also induce activation of PLC in cells coexpressing beta 2-AR and G alpha 16 and that the beta 2-AR-specific antagonist propranolol can block norepinephrine-induced activation of PLC in these cotransfected cells further indicate that it is the beta 2-AR that mediates the activation of phospholipase C in these cotransfected cells. To test the possibility of involvement of G beta gamma, a G beta gamma antagonist, G gamma 3 mutant with substitution of a Ser residue for the C-terminal Cys residue, was used because this protein, when expressed in COS-7 cells, can inhibit only G beta gamma-mediated but not G alpha-mediated activation of PLC. The result that the G gamma 3 mutant could not inhibit beta 2-adrenergic receptor-mediated activation of PLC in cells cotransfected with the G alpha 16 cDNA suggests that G beta gamma is unlikely to be a major mediator of beta 2-adrenergic receptor-induced activation of PLC. Thus, we conclude that the beta 2-adrenergic receptor can specifically couple to G alpha 15 and G alpha 16, but not to G alpha q, G alpha 11, or G alpha 14 to activate PLC.

cAMP does not regulate [Ca2+]i in human tracheal epithelial cells in primary culture

Human tracheal epithelial cells in primary culture respond to different receptor agonists with different peak intracellular calcium concentrations. From resting concentration 138 +/- 13 nM, bradykinin (0.1 microM) produces an increase to a maximum of 835 +/- 195 nM, histamine (10 microM) to 352 +/- 51 nM, and ATP (5-500 microM) to more than 1500 nM. Nine of 14 cultures also responded to isoproterenol (10 microM), though with a smaller increase, to 210 +/- 29 nM. A response was observed with isoproterenol, and epinephrine, but not norepinephrine, phenylephrine or methoxamine, was inhibited by propranolol but not phentolamine, and so this appeared to be a beta-adrenergic response. However, no response could be detected to adenosine, prostaglandin E2 or forskolin, agents that activate adenylate cyclase, or to permeant analogs of cAMP (CPT-cAMP or db-cAMP). The intracellular calcium response to isoproterenol did not follow either the time-course or the desensitization pattern of the cAMP response. Thus, this response to isoproterenol is not mediated by cAMP. No relation was demonstrated between cAMP production by other agonists and the response of intracellular calcium. Pretreatment with agents that increase cAMP did not affect the calcium responses to ATP or bradykinin. Thus, cAMP does not regulate intracellular calcium concentration in human tracheal epithelial cells. The variation in peak intracellular calcium responses to various agonists may be explained by the presence of multiple second messengers (other than cAMP), multiple intracellular pools of calcium, or cell heterogeneity. The agonists tested had the same relative potency in cells from patients with cystic fibrosis as in non-cystic fibrosis cells.

Differential effects of P2-purinoceptor antagonists on phospholipase C- and adenylyl cyclase-coupled P2Y-purinoceptors

1. Stimulation of P2Y-purinoceptors on turkey erythrocytes and many other cell types results in activation of phospholipase C. In contrast, we have observed recently that P2Y-purinoceptors on C6 rat glioma cells are not coupled to phospholipase C, but rather, inhibit adenylyl cyclase. 2. In this study we investigated the pharmacological selectivity of the P2-purinoceptor antagonists, suramin, reactive blue 2, and pyridoxal phosphate 6-azophenyl 2',4'-disulphonic acid (PPADS) for phospholipase C- and adenylyl cyclase-coupled P2Y-purinoceptors. 3. In C6 glioma cells, suramin and reactive blue 2 competitively antagonized the inhibitory effect of 2MeSATP on adenylyl cyclase (pKB = 5.4 +/- 0.2 and 7.6 +/- 0.1, respectively), whereas PPADS at concentrations up to 100 microM had no effect. 4. In contrast, in the turkey erythrocyte preparation, PPADS at concentrations up to 30 microM was a competitive antagonist of P2Y-purinoceptor-stimulated phospholipase C activity (pKB = 5.9 +/- 0.1). Suramin and reactive blue 2 produced both a shift to the right of the concentration-effect of 2MeSATP for the activation of phospholipase C and a significant decrease in the maximal inositol phosphate response. 5. Turkey erythrocytes also express a phospholipase C-coupled beta-adrenoceptor. Concentrations of PPADS that competitively inhibited the P2Y-purinoceptor-mediated response had only minimal effects on the activation of phospholipase C by beta-adrenoceptors. In contrast, suramin and reactive blue 2 produced a non-competitive inhibition, characterized by decreases in the maximal response to isoprenaline with no change in the potency of this beta-adrenoceptor agonist. 6. The differential effect of PPADS on P2Y-purinoceptors of C6 glioma cells and turkey erythrocytes adds further support to the idea that different P2Y-purinoceptor subtypes mediate coupling to adenylylcyclic and phospholipase C.

Modulation of the ATP induced [Ca2+]c increase in AS-30D hepatoma cells

1. The regulation of the increase in the cytosolic calcium concentration ([Ca2+]c) induced by extracellular ATP in AS-30D hepatoma cells was studied. 2. Homologous desensitization involving the refilling of intracellular calcium pools and the participation of protein kinase C was found. 3. Isoproterenol, forskolin and dibutyryl-cyclic AMP also induced an increase in [Ca2+]c. 4. Interestingly, synergism was found for isoproterenol or forskolin and ATP. 5. The results suggest that there are two pathways for mobilizing [Ca2+]c in AS-30D hepatoma cells; one is activated by ATP receptors and the other by cyclic AMP.

Mechanisms of multifunctional signalling by G protein-linked receptors

A number of G protein-linked receptors have recently been shown to regulate multiple effector pathways both when expressed endogenously and, more frequently, following transfection into heterologous systems. The mechanisms responsible for such signal bifurcation are beginning to be unravelled. In this Viewpoint article, Graeme Milligan analyses the information on these mechanisms and considers whether they are dependent upon either levels of expression of the receptor or the cell type.

Subunit interactions of GTP-binding proteins

Fluorescence energy transfer [cf. Förster, T. (1948) Ann. Phys. 6, 55-75] was tested for its suitability to study quantitative interactions of subunits of G0 with each other and these subunits or trimeric G0 with the beta 1-adrenoceptor in detergent micelles or after reconstitution into lipid vesicles [according to Feder, D., Im, M.-J., Klein, H. W., Hekman, M., Holzhöfer, A, Dees, C., Levitzki, A., Helmreich, E. J. M. & Pfeuffer, T. (1986) EMBO J. 5, 1509-1514]. For this purpose, alpha 0- and beta gamma-subunits and trimeric G0 purified from bovine brain, the beta gamma-subunits from bovine rod outer segment membranes and the beta 1-adrenoceptor from the turkey erythrocyte were all labelled with either tetramethylrhodamine maleimide or fluorescein isothiocyanate under conditions which leave the labelled proteins functionally intact. In the case of alpha 0- and beta gamma-interactions, specific high-affinity binding interactions (Kd approximately 10 nM) and nonspecific low-affinity binding interactions (Kd approximately 1 microM) could be readily distinguished by comparing fluorescence energy transfer before and after dissociation with 10 microM guanosine 5'-O-[gamma-thio]triphosphate and 10 mM MgCl2 where only low-affinity binding interactions remained. Interactions between alpha 0- and beta gamma-subunits from bovine brain or from bovine retinal transducin did not differ much. The beta gamma-subunits from bovine brain were found to bind with high transfer efficiency and comparable affinities to the hormone-activated and the nonactivated beta 1-receptor reconstituted in lipid vesicles: Kd = 100 +/- 20 and 120 +/- 20 nM, respectively; however, beta gamma-subunits from transducin appeared to bind more weakly to the beta 1-adrenoceptor than beta gamma-subunits from bovine brain. Separated purified homologous alpha 0- and beta gamma-subunits from bovine brain interfered mutually with each other in binding to the beta 1-adrenoceptor presumably because they had a greater affinity for each other than for the receptor. These findings attest to the suitability of fluorescence energy transfer for studying protein-protein interactions of G-proteins and G-protein-linked receptors. Moreover, they supported the previous finding [Kurstjens, N. P., Fröhlich, M., Dees, C., Cantrill, R. C., Hekman, M. & Helmreich, E. J. M. (1991) Eur. J. Biochem. 197, 167-176] that beta gamma-subunits can bind to the nonactivated beta 1-adrenoceptor.