Peroxides of vanadate induce activation of phospholipase D in HL-60 cells. Role of tyrosine phosphorylation

The enhancement by wortmannin of protein kinase C-dependent activation of phospholipase D in vascular endothelial cells

Phosphatidic acid generation by phospholipase D (PLD) activation has been implicated in agonist- and oxidant-mediated endothelial cell signal transduction. We examined the effect of wortmannin on PLD activation in pulmonary artery endothelial and smooth muscle cells in culture. Pretreatment of bovine pulmonary artery endothelial cells (BPAECs) with wortmannin potentiated TPA- (100 nM), ATP- (100 microM), and bradykinin- (1 microM) induced [32P]PEt formation, an index of PLD activation. However, wortmannin by itself had no effect on PLD activity. The potentiating effect of wortmannin on TPA-induced PLD activation was dose- (1-10 microM) and time-dependent (5-30 min) and was inhibited by bisindoylmalemide, an inhibitor of protein kinase C (PKC). Furthermore, down-regulation of PKC by prolonged treatment with TPA (100 nM, 18 h) attenuated the wortmannin effect. This effect of wortmannin was specific for TPA- or agonist-induced PLD activation as no potentiation of [32P]PEt formation was observed with H2O2 (1 mM) or ionomycin (1 microM). The effect of wortmannin was not due to activation of PKC alpha as determined by western blot analysis of PKC alpha in the cytosol and membrane fractions. Also, genistein, an inhibitor of tyrosine kinases, did not attenuate the wortmannin-mediated potentiation of PLD thereby suggesting non-involvement of protein tyrosine phosphorylation. These results indicate that wortmannin potentiates PKC-dependent stimulation of PLD in vascular endothelial cells.

Oxidants as stimulators of signal transduction

Redox (oxidation-reduction) reactions regulate signal transduction. Oxidants such as superoxide, hydrogen peroxide, hydroxyl radicals, and lipid hydroperoxides (i.e., reactive oxygen species) are now realized as signaling molecules under subtoxic conditions. Nitric oxide is also an example of a redox mediator. Reactive oxygen species induce various biological processes such as gene expression by stimulating signal transduction components such as Ca(2+)-signaling and protein phosphorylation. Various oxidants increase cytosolic Ca2+; however, the exact origin of Ca2+ is controversial. Ca2+ may be released from the endoplasmic reticulum, extracellular space, or mitochondria in response to oxidant-influence on Ca2+ pumps, channels, and transporters. Alternatively, oxidants may release Ca2+ from Ca2+ binding proteins. Various oxidants stimulate tyrosine as well as serine/threonine phosphorylation, and direct stimulation of protein kinases and inhibition of protein phosphatases by oxidants have been proposed as mechanisms. The oxidant-stimulation of the effector molecules such as phospholipase A2 as well as the activation of oxidative stress-responsive transcription factors may also depend on the oxidant-mediated activation of Ca(2+)-signaling and/or protein phosphorylation. In addition to the stimulation of signal transduction by oxidants, the observations that ligand-receptor interactions produce reactive oxygen species and that antioxidants block receptor-mediated signal transduction led to a proposal that reactive oxygen species may be second messengers for transcription factor activation, apoptosis, bone resorption, cell growth, and chemotaxis. Physiological significance of the role of biological oxidants in the regulation of signal transduction as well as the mechanisms of the oxidant-stimulation of signal transduction are discussed.

Regulation of Src family tyrosine kinase activities in adherent human neutrophils. Evidence that reactive oxygen intermediates produced by adherent neutrophils increase the activity of the p58c-fgr and p53/56lyn tyrosine kinases

Src family tyrosine kinases have been implicated in the adhesion-dependent activation of neutrophil functions (Yan, S. R., Fumagalli, L., and Berton, G. (1995) J. Inflamm. 45, 297-312; Lowell, C. A., Fumagalli, L., and Berton, G. (1996) J. Cell Biol. 133, 895-910). Because the activity of tyrosine kinases can be affected by oxidants, we investigated whether reactive oxygen intermediates (ROI) produced by adherent neutrophils regulate Src family kinase activities. Inhibition of ROI production by diphenylene iodonium, an inhibitor of NADPH oxidase, or degradation of H2O2 by exogenously added catalase inhibited the adhesion-stimulated activities of p58(c-fgr) and p53/56(lyn). In addition, adhesion-stimulated p58(c-fgr) and p53/56(lyn) activities were greatly reduced in neutrophils from patients with chronic granulomatous disease (CGD) that are deficient in the production of ROI. Exogenously added H2O2 increased p58(c-fgr) and p53/56(lyn) activities in nonadherent neutrophils. Although ROI regulated the activities of p58(c-fgr) and p53/56(lyn), they did not affect the redistribution of the two kinases to a Triton X-100-insoluble, cytoskeletal fraction that occurs in adherent neutrophils. Tyrosine phosphorylation of proteins in adherent, CGD neutrophils was only partially inhibited, suggesting that the full activation of p58(c-fgr) and p53/56(lyn), which depends on endogenously produced ROI, does not represent an absolute requirement for protein tyrosine phosphorylation. The adhesion-stimulated activity of the tyrosine kinase p72(syk) was not affected by catalase in normal neutrophils, and it was comparable in normal and CGD neutrophils. These findings suggest that ROI endogenously produced by adherent neutrophils regulate Src family kinases activity selectively and establish the existence of a cross-talk between reorganization of the cytoskeleton, production of ROI, and Src family tyrosine kinase activities in signaling by adhesion.

Small GTPase-regulated phospholipase D in granulocytes

This review examines the functional role of phospholipase D in the neutrophil. Phospholipase D is emerging as an important component in the signal transduction pathways leading to granulocyte activation. Through the second messenger it produces, phosphatidic acid, phospholipase D plays an active role in the regulation of granulocyte NADPH oxidase activation and granular secretion. Many factors from both the cytosol and the membrane are necessary for maximal phospholipase D activation. This paper will focus on the regulation of phospholipase D by low molecular weight GTP-binding proteins, tyrosine kinases, and protein kinase C.

Regulation of phospholipase D by tyrosine kinases

Activation of phospholipase D (PLD) represents part of an important signalling pathway in mammalian cells. Phospholipase D catalyzed hydrolysis of phospholipids generates phosphatidic acid (PA) which is subsequently metabolized to lyso-PA (LPA) or diacylglycerol (DAG). While DAG is an endogenous activator of protein kinase C (PKC), PA and LPA have been recognized as second messengers as well. Activation of PLD in response to an external stimulus may involve PKC, Ca2+, G-proteins and/or tyrosine kinases. In this review, we will address the role of protein tyrosine phosphorylation in growth factor-, agonist- and oxidant-mediated activation of PLD. Furthermore, a possible link between PKC, Ca2+, G-proteins and tyrosine kinases is discussed to indicate the complexity involved in the regulation of PLD in mammalian cells.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Biochemistry and cell biology of phospholipase D in human neutrophils

Neutrophils play a major role host defense against invading microbes. Recent studies have emphasized the importance of the phospholipase D (PLD) in the signalling cascade leading to neutrophil activation. Phospholipase D catalyzes the hydrolysis of phospholipids to generate phosphatidic acid with secondarily generation of diradylglycerol; both of these products have been implicated as second messengers. Herein, we discuss the regulation and the biochemistry of the receptor-regulated PLD in human neutrophils. In vivo and in vitro studies suggest an activation mode in which initial receptor-linked activation of phospholipase C generates diacylglycerol and inositol trisphosphate. The resulting calcium flux along with the diacylglycerol activate a conventional isoform of protein kinase C (PKC), probably PKC beta 1. This PKC, in turn phosphorylates a plasma membrane component resulting in PLD activation and a second outpouring of diradylglycerol. The small GTP-binding proteins, RhoA and ARF, also participate in this process, and synergize with a 50 kDa cytosolic regulatory factor.

Pervanadate stimulation of wortmannin-sensitive and -resistant 2-deoxyglucose transport in adipocytes

Pervanadate mimics several distinct insulin effects, including stimulation of hexose uptake in the in vitro system, and reduces the blood glucose level in streptozotocin-treated diabetic rats. It has been proposed that pervanadate induces insulin-like effects mediated through autophosphorylation and activation of insulin receptor (IR) even in the absence of insulin by inhibiting protein tyrosine phosphatases. This study focused on the mechanism of pervanadate action on hexose uptake. Both insulin (100 nM) and pervanadate (100 microM), a protein tyrosine phosphatase inhibitor, induced a marked increase in the phosphorylation at tyrosine residues of IR and insulin receptor substrate 1 (IRS-1) and in 2-deoxyglucose uptake in 3T3-L1 adipocytes. Wortmannin (1 microM), a specific phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor, inhibited the increased 2-deoxyglucose uptake by insulin completely but that by pervanadate only partially. On the other hand, both insulin- and pervanadate-stimulated PI 3-kinase activities were inhibited completely by wortmannin (100 nM), suggesting that the pervanadate-induced wortmannin-resistant effect on hexose uptake may be mediated through a PI 3-kinase-independent pathway. This pervanadate-induced wortmannin-resistant effect was abolished by ST-638, a specific tyrosine kinase inhibitor. These data suggest that at least two distinct tyrosine phosphorylation pathways may be involved in the insulin-like effect of pervanadate.

Endogenous reactive oxygen intermediates activate tyrosine kinases in human neutrophils

In response to invading microorganisms, neutrophils produce large amounts of superoxide and other reactive oxygen intermediates (ROI) by assembly and activation of a multicomponent enzyme complex, the NADPH oxidase. While fulfilling a microbicidal role, ROI have also been postulated to serve as signaling molecules, because activation of the NADPH oxidase was found to be associated with increased tyrosine phosphorylation (Fialkow, L., Chan, C. K., Grinstein, S., and Downey, G.P. (1993) J. Biol. Chem. 268, 17131-17137). The mechanism whereby ROI induces phosphotyrosine accumulation was investigated using electroporated neutrophils stimulated with guanosine 5'-O-3-thiotriphosphate in order to bypass membrane receptors. In vitro immune complex assays and immunoblotting were used to identify five tyrosine kinases present in human neutrophils. Of these, p56/59hck, p72syk, and p77btk were activated during production of ROI. Interestingly, the in vitro autophosphorylation activities of p53/56lyn and p59fgr were found to decline with ROI production. The mode of regulation of p56/59hck was explored in detail. Oxidizing agents were unable to activate p56/59hck in vitro and, once activated in situ, reducing agents failed to inactivate it, suggesting that the effects of ROI are indirect. Tyrosine phosphorylation of p56/59hck paralleled its activation, and dephosphorylation in vitro reversed the stimulation. We therefore conclude that tyrosine phosphorylation is central to the regulation of p56/59hck and likely also of p72syk, which is similarly phosphorylated upon activation of the oxidase. Because ROI have been shown to reduce the activity of tyrosine phosphatases, we suggest that this inhibition allows constitutively active kinases to auto/transphosphorylate on stimulatory tyrosine residues, leading to an increase in their catalytic activity. Enhanced phosphotyrosine accumulation would then result from the combined effects of increased phosphorylation with decreased dephosphorylation.

Peroxovanadium compounds: biological actions and mechanism of insulin-mimesis

When used alone, both vanadate and hydrogen peroxide (H2O2) are weakly insulin-mimetic, while in combination they are strongly synergistic due to the formation of aqueous peroxovanadium species pV(aq). Administration of these pV(aq) species leads to activation of the insulin receptor tyrosine kinase (IRK), autophosphorylation at tyrosine residues and inhibition of phosphotyrosine phosphatases (PTPs). We therefore undertook to synthesize a series of peroxovanadium (pV) compounds containing one or two peroxo anions, an oxo anion and an ancillary ligand in the inner co-ordination sphere of vanadium, whose properties and insulin-mimetic potencies could be assessed. These pV compounds were shown to be the most potent inhibitors of PTPs yet described. Their PTP inhibitory potency correlated with their capacity to stimulate IRK activity. Some pV compounds showed much greater potency as inhibitors of insulin receptor (IR) dephosphorylation than epidermal growth factor receptor (EGFR) dephosphorylation, implying relative specificity as PTP inhibitors. Replacement of vanadium with either molybdenum or tungsten resulted in equally potent inhibition of IR dephosphorylation. However IRK activation was reduced by greater than 80% suggesting that these compounds did not access intracellular PTPs. The insulin-like activity of these pV compounds were demonstrable in vivo. Intra venous (i.v.) administration of bpV(pic) and bpV(phen) resulted in the lowering of plasma glucose concentrations in normal rats in a dose dependent manner. The greater potency of bpV(pic) compared to bpV(phen) was explicable, in part, by the capacity of the former but not the latter to act on skeletal muscle as well as liver. Finally administration of bpV(phen) and insulin led to a synergism, where tyrosine phosphorylation of the IR beta-subunit increased by 20-fold and led to the appearance of four insulin-dependent in vivo substrates. The insulin-mimetic properties of the pV compounds raises the possibility for their use as insulin replacements in the management of diabetes mellitus.

Roles of protein-tyrosine phosphatases in Stat1 alpha-mediated cell signaling

Different Stat proteins are activated through phosphorylation of unique tyrosine residues in response to different cytokines and growth factors. Interferon-gamma activates Stat1 molecules that form homodimers and bind cognate DNA elements. Here we show that treatment of permeabilized cells with 200-500 microM peroxo-derivatives of vanadium, molybdenum, and tungsten results in the accumulation of constitutively phosphorylated Stat1 alpha molecules. In contrast, treatment of permeabilized cells with orthovanadate, vanadyl sulfate, molybdate, and tungstate at the same range of concentrations does not result in the accumulation of activated Stat1 alpha molecules in the absence of ligand. However, these compounds inhibit the inactivation of interferon-gamma-induced DNA-binding activity of Stat1 alpha. A 4-6-h exposure of the permeabilized cells to orthovanadate, molybdate, and tungstate, but not vanadyl sulfate, results in a ligand-independent activation of Stat1 alpha, which is blocked by the inhibition or depletion of NADPH oxidase activity in the cells, indicating that NADPH oxidase-catalyzed superoxide formation is required for the bioconversion of these metal oxides to the corresponding peroxo-compounds. Interestingly, ligand-independent Stat1 alpha activation by peroxo-derivatives of these transition metals does not require Jak1, Jak2, or Tyk2 kinase activity, suggesting that other kinases can phosphorylate Stat1 alpha on tyrosine 701.

Immunoprecipitation of a phospholipase D activity with antiphosphotyrosine antibodies

When granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated human neutrophils were challenged with the chemotactic factor fMet-Leu-Phe, it was possible to detect a time-dependent increase in the hydrolytic (as measured by the production of phosphatidic acid, PA) and the transphosphatidylation (as measured by the production of phosphatidylethanol, PEt) activities of phospholipase D in intact cells prelabeled with a radioactive fatty acid. Both activities were inhibited by preincubation of cells with genistein. Appropriate conditions were developed to test the PLD transphosphatidylation activity against exogenous phosphatidylcholine (PCho) in an in vitro system. As in intact cells, increased PLD activity could be detected in cell lysates obtained from fMet-Leu-Phe-treated cells compared with controls. When lysates were immunoprecipitated with antiphosphotyrosine antibodies, a PLD activity was found only in immune complexes that were prepared from fMet-Leu-Phe-treated cells. Conversely, no activity was found in lysates immunoprecipitated with an irrelevant antibody (GTPase-activating protein, GAP) that nevertheless was able to recognize a tyrosylphosphorylated form of GAP, as demonstrated by western blotting. These data suggest that a PCho-PLD, or a tightly bound protein, is tyrosine phosphorylated during cell activation.

Mastoparan-induced phosphatidylcholine hydrolysis by phospholipase D activation in human astrocytoma cells

1. The effect of mastoparan on phosphatidylcholine hydrolysis was examined in 1321N1 human astrocytoma cells. Mastoparan (3-30 microM) caused an accumulation of diacylglycerol (DG) and phosphatidic acd (PA) accompanied by choline release in a concentration- and time-dependent manner. 2. In the presence of 2% n-butanol, mastoparan (3-100 microM) induced phosphatidylbutanol (PBut) accumulation in a concentration- and time-dependent manner, suggesting that mastoparan activates phospholipase D (PLD). Propranolol (30-300 microM), a phosphatidate phosphohydrolase inhibitor, inhibited DG accumulation induced by mastoparan, supporting this idea. 3. Depletion of extracellular free calcium ion did not alter the effect of mastoparan on PLD activity. 4. A protein kinase C (PKC) inhibitor, calphostin C (1 microM), did not inhibit mastoparan-induce PLD activation but the ability of mastoparan to stimulate phospholipase D activity was decreased in the PKC down regulated cells. 5. PLD activity stimulated by mastoparan was not prevented by pretreatment of the cells with pertussis toxin (PT) or C3 ADP-ribosyltransferase. Furthermore, guanine nucleotides did not affect PLD activity stimulation by mastoparan in membrane preparations. 6. Mastoparan stimulated PLD in several cell lines such as RBL-2H3, RBL-1, HL-60, P388, endothelial cells, as well as 1321N1 human astrocytoma cells. 7. These results suggest that mastoparan induces phosphatidylcholine (PC) hydrolysis by activation of PLD, not by activation of phosphatidylcholine-specific phospholipase C (PC-PLC); mastoparan-induced PLD activation is not mediated by G proteins.

Regulation of phospholipase D by protein kinase C in human neutrophils. Conventional isoforms of protein kinase C phosphorylate a phospholipase D-related component in the plasma membrane

In a variety of intact cells, phorbol esters are known to activate phospholipase D. In a cell-free system consisting of plasma membrane and cytosol from human neutrophils, phorbol esters activated phospholipase D in an adenosine nucleotide triphosphate-dependent manner. ATP gamma S (adenosine 5'-O-(thiotriphosphate)) was 2-3-fold more effective than ATP, while ADP and AppNHp (adenyl-5'-yl imidodiphosphate) were ineffective, and activation was blocked by the kinase inhibitor staurosporine. In cytosol deplete of protein kinase C by chromatography on threnoine-Sepharose, phorbol ester-dependent activation was lost, but was restored upon addition of purified rat brain protein kinase C. The target for phosphorylation was shown to be the plasma membrane plasma membrane was phosphorylated using ATP gamma S/phorbol 12,13-dibutyrate and protein kinase C and was reisolated to remove activators. Upon adding nucleotide-depleted cytosol, activator-independent phospholipase D activity was seen. Using this prephosphorylation protocol, PKC-dependent activation of plasma membranes was found to require micromolar calcium, implicating a conventional protein kinase C. Using recombinant isoforms of protein kinase C, only the conventional isoforms showed significant activation, with the following rank order of potency: beta 1 > alpha > gamma; the beta 2, delta, epsilon, eta, and sigma isoforms showed little or no activity. Thus, conventional isoform(s) of protein kinase C activate neutrophil phospholipase D by phosphorylating a target protein located in the plasma membrane.

Tyrosine kinase-mediated signal transduction pathways and the actions of polypeptide growth factors and G-protein-coupled agonists in smooth muscle

This synopsis focuses on the role that tyrosine kinase pathways may play in the acute regulation of smooth muscle contractility by receptor-kinase-activating growth factors, such as epidermal growth factor-urogastrone (EGF-URO) and by G-protein-coupled agonists, such as angiotensin-II. Growth factor-activated response paradigms that modulate smooth muscle contractility are summarized and the parallels between the actions of G-protein-coupled agonists and growth factors in these response systems are pointed out. A possible dynamic interplay between tyrosine kinase and tyrosine phosphatase activities to modulate tissue tension is also hypothesized. Finally, a model is proposed, wherein an intermediary tyrosine kinase pathway is suggested as a point of convergence for the regulation of smooth muscle contractility by agonists as diverse as EGF-URO and angiotensin-II.

Alpha 2-adrenoceptor-mediated vasoconstriction requires a tyrosine kinase

alpha 2-adrenoceptor-mediated contractions of the rabbit saphenous vein were previously found to be inhibited by wortmannin, a protein kinase inhibitor which blocks receptor-dependent phospholipase D activation. Since other studies have indicated that receptor-dependent phospholipase D activation required activity of a tyrosine kinase, we examined the influence of several tyrosine kinase inhibitors on both alpha 2-adrenoceptor-mediated contractions of rabbit saphenous vein and alpha 1-adrenoceptor-mediated contractions of rabbit aorta. Methyl 2,5-dihydroxycinnamate, genistein and erbstatin each caused non-competitive inhibition of rabbit saphenous vein contractions elicited by the alpha 2-adrenoceptor-selective agonist 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline (UK14304), yielding complete inhibition at 100 microM and IC50 values of 15, 35 and 40 microM respectively. By contrast, phenylephrine-induced dose-response curves in rabbit aorta were largely unaffected by tyrosine kinase inhibitors at 50 microM. In a separate analysis of intracellular Ca(2+)-dependent and extracellular Ca(2+)-dependent alpha 1-adrenoceptor responses of rabbit aorta, genistein (50 microM) did partially reduce the initial intracellular Ca(2+)-dependent response, but did not reduce maximal response. Methyl 2,5-dihydroxycinnamate (25 microM) had no effect on intracellular or extracellular Ca2+ responses in rabbit aorta. High K(+)-induced contractions of both rabbit saphenous vein and aorta were unaffected by up to 100 microM of the tyrosine kinase inhibitors. These results indicate an obligatory requirement for tyrosine kinase activity in alpha 2-adrenoceptor-mediated but not alpha 1-adrenoceptor-mediated vasoconstriction.

Involvement of protein kinase C activation in alpha 2-adrenoceptor-mediated contractions of rabbit saphenous vein

The role of protein kinase C alpha 2-adrenoceptor-induced contractions of rabbit saphenous vein was investigated. Contractions induced by the alpha 2-adrenoceptor-selective agonist 5-bromo-6-[2-imidazolin-2-ylamino]-quinoline (UK14304) were inhibited by prior treatment with pertussis toxin and by Ca2+ removal, confirming a Gi/Go-dependent coupling pathway which was highly dependent upon Ca2+ influx. Protein kinase C inhibitors calphostin-C and staurosporine each caused a non-competitive inhibition of UK14304 response. Down-regulation of protein kinase C by pretreatment with tetradecanoylphorbol acetate reduced UK14304 response by almost 90% with no effect on contractions induced by elevated KCl. The ineffectiveness of L-type Ca2+ channel blockers and the absence of stimulated 45Ca2+ uptake or efflux by UK14304 indicated that phospholipid-derived products were most likely responsible for protein kinase C activation. alpha 2-Adrenoceptor stimulation failed to increase [3H]myoinositol phosphate formation, but caused a significant increase in the formation of both [32P]phosphatidic acid and diacylglycerol, indicating the possible activation of phospholipase D activity. These results suggest that protein kinase C is important for the vasoconstriction induced by alpha 2-adrenoceptors and that diacylglycerol derived from receptor-initiated phospholipase D activity may provide protein kinase C stimulation.

Tyrosine phosphorylation-dependent stimulation of amyloid precursor protein secretion by the m3 muscarinic acetylcholine receptor

Stimulation of m1 and m3 muscarinic acetylcholine receptors, which are coupled to phosphoinositide hydrolysis and protein kinase C activation, has been shown to increase the release of soluble amyloid precursor protein derivatives (APPs). The effect is mimicked by phorbol esters, which directly activate protein kinase C. Using human embryonic kidney cells expressing individual muscarinic receptor subtypes, we found that stimulation of APPs release by the muscarinic agonist carbachol was only partially reduced by a specific inhibitor of protein kinase C (the bisindolylmaleimide GF 109203X), while the response to phorbol 12-myristate 13-acetate (PMA) was abolished. The increase in APPs release elicited by carbachol and PMA was accompanied by elevated tyrosine phosphorylation of several proteins and reduced by tyrosine kinase inhibitors; GF 109203X significantly reduced the stimulation of tyrosine phosphorylation by carbachol and PMA. Inhibition of protein tyrosine phosphatases by vanadyl hydroperoxide markedly increased cellular tyrosine phosphorylation and enhanced APPs release as effectively as PMA and carbachol. Direct phosphorylation of amyloid precursor protein on tyrosine residues following treatment with carbachol, PMA, or vanadyl hydroperoxide was not observed. The results implicate both tyrosine phosphorylation and protein kinase C-dependent mechanisms in the regulation of APPs release by G protein-coupled receptors, and suggest that carbachol and PMA increase APPs release from human embryonic kidney cells expressing m3 muscarinic receptors via partially divergent pathways that converge at a tyrosine phosphorylation-dependent step.

Cholinergic activation of phospholipase D in lacrimal gland acini is independent of protein kinase C and calcium

To determine if rat lacrimal gland acini contain phospholipase D (PLD) activity, we took advantage of PLD's unique ability, in the presence of ethanol, to catalyze a transphosphatidylation reaction to produce phosphatidylethanol (PEth). Lacrimal gland acini were labeled for 3 h with [14C]stearic acid, preincubated for 20 min in the presence of 2% ethanol, and incubated for 20 min with or without agonists. Total cellular lipids were then extracted and analyzed by thin-layer chromatography, and the radioactivity was determined by liquid scintillation counting. Carbachol (1 mM), a cholinergic agonist, stimulated the production of both [14C]PEth and [14C]phosphatidic acid ([14C]PA) twofold. This effect was completely blocked by the muscarinic antagonist atropine (10 microM). [14C]PEth accumulation was also stimulated twofold by the active phorbol esters, 4 beta-phorbol 12-myristate 13-acetate and 4 beta-phorbol 12,13-dibutyrate at 1 microM. Ionomycin (1 microM), a Ca2+ ionophore, also stimulated the production of [14C]PEth twofold. In contrast to carbachol, neither phorbol esters nor ionomycin stimulated [14C]PA production. Neither [14C]PEth nor [14C]PA production was altered by epinephrine (1 mM), a nonselective adrenergic agonist, or phenylephrine (0.1 and 1 mM), a specific alpha 1-adrenergic agonist. We concluded that PLD activity, modulated by muscarinic receptors, protein kinase C, and Ca2+, but not by adrenergic receptors, is present in rat lacrimal gland acini. We also concluded that cholinergic activation of PLD appears to be independent of PKC and Ca2+.

Interleukin-11 induces phosphatidic acid formation and activates MAP kinase in mouse 3T3-L1 cells

Interleukin-11 (IL-11) stimulated [3H]phosphatidic acid (PA) formation in [3H]arachidonic acid (AA) prelabelled quiescent mouse 3T3-L1 cells. When IL-11 stimulated 3T3-L1 cells were incubated with NaF, a phosphatidic acid phosphohydrolase (PAP) inhibitor, increased PA formation was observed. In the presence of ethanol, phosphatidylethanol accumulated at the expense of PA. These results indicated that the formation of PA upon IL-11 stimulation was a result of phospholipase D (PLD) activation. Endogenous accumulation of PA by NaF treatment or exogenously added PA enhanced tyrosine phosphorylation of two proteins of 44 KDa (p44) and 47 KDa (p47) whereas tyrosine phosphorylation of other proteins was not affected. Among various PA species, dipalmitoyl PA was found to be most effective in enhancing tyrosine phosphorylation of these proteins. p44 and p47 cross reacted with anti-MAP kinase monoclonal antibody (MoAb) in both immunoprecipitation and western blot analysis. Lysates from IL-11-induced or PA-induced cells stimulated phosphorylation of a synthetic peptide substrate for MAP kinase, indicating the activation of MAP kinase in the induced cells. These studies suggest that one of the cellular signalling mechanisms of IL-11 in 3T3-L1 cells involves the activation of phospholipase D to produce the second messenger PA. The increased level of PA enhances tyrosine phosphorylation of p44 and p47 which belong to the members of MAP kinase family and thus transduces some of the mitogenic signals of IL-11 in this cell line.

Mechanisms of phospholipase D stimulation by m3 muscarinic acetylcholine receptors. Evidence for involvement of tyrosine phosphorylation

In human embryonic kidney cells stably expressing the human m3 muscarinic acetylcholine receptor (mAChR) subtype, agonist (carbachol) activation stimulated phospholipase C, increased cytoplasmic calcium concentration, induced tyrosine phosphorylation of various cellular proteins and activated phospholipase D. Bypassing membrane receptors, phospholipase D was activated in these cells by direct activation of protein kinase C by phorbol esters, by direct activation of GTP-binding proteins by A1F4- and a stable GTP analogue (in permeabilized cells), by increasing cytoplasmic calcium concentration with the calcium ionophore A23187 and also apparently by tyrosine phosphorylation. In order to identify possible mechanisms by which the m3 mAChR couples to phospholipase D, various inhibitors of protein kinase C, tyrosine kinases and calcium-dependent events were studied. Prevention of an agonist-induced increase in cytoplasmic calcium concentration did not alter the mAChR-induced phospholipase D stimulation. The protein kinase C inhibitors, calphostin C and staurosporine, efficiently prevented phospholipase D activation by phorbol 12-myristate 13-acetate but only partially inhibited the activation induced by the mAChR agonist. Additionally, down-regulation of protein kinase C by prolonged exposure to phorbol 12-myristate 13-acetate abrogated phospholipase D activation by this effector but had only minor or no effects on the response to the mAChR agonist and direct activators of GTP-binding proteins. In contrast, the tyrosine kinase inhibitor genistein abolished the carbachol-induced and A1F4(-)-induced phospholipase D activation but had no effect on enzyme activation by phorbol 12-myristate 13-acetate. The data indicate that phospholipase D in m3 mAChR-expressing human embryonic kidney cells can be activated by various different mechanisms, i.e. receptor agonists, GTP-binding proteins, protein kinase C-dependent and calcium-dependent events and tyrosine phosphorylation. The coupling of m3 mAChR to phospholipase D appears to be largely independent of concomitant phospholipase C activation with subsequent increase in cytoplasmic calcium concentration and protein kinase C activity. The data instead suggest the involvement of an essential protein tyrosine phosphorylation mechanism in phopsholipase D activation by the m3 mAChR and heterotrimeric GTP-binding proteins.

Regulation of vascular and gastric smooth muscle contractility by pervanadate

1. The contractile actions of vanadate (VO4) and pervanadate (PV, peroxide(s) of vanadate) were studied in rat gastric longitudinal muscle strips and in aortic rings. The roles of extracellular sodium and calcium were evaluated and the potential effects of nerve-released agonists were considered. The possibility that these responses were due to the potentiation of tyrosine kinase activity, as a result of PV-mediated tyrosine phosphatase inhibition was explored with the use of tyrosine kinase inhibitors (genistein, tyrphostin) and by Western blot analysis of phosphotyrosyl proteins in PV-treated tissues. The ability of PV to mimic the action of the tyrosine kinase receptor-associated agonist, epidermal growth factor-urogastrone (EGF-Uro), in the gastric preparation was also studied. 2. PV caused concentration-dependent contractions in both gastric and aorta-derived tissues, with a potency that was 1 to 2 orders of magnitude greater than that of VO4. 3. Although repeated exposure of gastric and aortic tissues to a fixed concentration of VO4 caused reproducible contractions in both tissues, repeated exposure of gastric tissue to PV caused an increased contractile response plateauing after 3 exposures. In contrast, a single exposure of aortic tissue to PV (20 microM) caused a prolonged desensitization of the tissue to the subsequent contractile actions of PV or other agonists. 4. The contractile responses to PV were unaffected in both preparations by tetrodotoxin, atropine, yohimbine and phenoxybenzamine; and in the aortic preparation, the responses to VO4 and PV were the same in the presence or absence of a functional endothelium. 5. PV-induced contractions in both tissues were observed in the absence of extracellular sodium but required extracellular calcium and were attenuated by 1 micro M nifedipine.6. In the gastric preparation, the characteristics of the contractile actions of PV paralleled those of EGF-Uro in terms of (1) inhibition by genistein, (2) inhibition by indomethacin and (3) a requirement for extracellular calcium. These response characteristics differed from those of other contractile agonists such as carbachol.7. In both the gastric and aortic preparations genistein was able to inhibit PV-induced contractions selectively without causing comparable inhibition of KCI-induced contractions. Tyrphostin (AG18) also selectively blocked PV-induced contractions in the gastric, but not in the aortic preparation.8. In both the gastric and aortic tissue, in step with an increased contractile response, PV caused increases in tissue phosphotyrosyl protein content, as detected by Western blot analysis using a monoclonal antiphosphotyrosine antibody; the increases in phosphotyrosyl protein content were reduced when tissues were treated with PV at the same time as a tyrosine kinase inhibitor.9 PV, at sub-contractile concentrations, potentiated the contractile action of angiotensin II in both the gastric and aorta tissue.10 We conclude that the growth factor-mimetic agent, PV, is a much more potent contractile agonist than V04 in both vascular and gastric smooth muscle tissue. PV can cause enhanced tissue phosphotyrosyl protein content most likely via the inhibition of tissue protein tyrosine phosphatases. The contractile actions of PV, which require extracelullar calcium and are independent of extracellular sodium, would appear not to be due either to Na+/Ca2" exchange, promoted by Na+/K+-ATPase inhibition or to the inhibition of Ca2+-ATPase and might be best explained by the ability of PV, via tyrosine phosphatase inhibition, to potentiate a tyrosine kinase pathway linked to calcium entry and to the contractile process.

Neutrophil "priming" induced by orthovanadate: evidence of a role for tyrosine phosphorylation

The mechanism of neutrophil "priming" is unknown. In this study the level of tyrosine phosphorylation within intact neutrophils, using orthovanadate, have been manipulated. It has been demonstrated that this procedure both increased tyrosine phosphorylation of a number of protein substrates, including a prominent band at 74 kDa, and also primed the neutrophil oxidase response with a time and orthovanadate concentration-dependency, which were consistent with a role for tyrosine phosphorylation. No effect of orthovanadate on cytosolic-free Ca2+ concentration or actin polymerization was detected. Inhibition of tyrosine phosphorylation by genistein prevented "priming" by orthovanadate. This data thus provided evidence of a role for tyrosine phosphorylation in neutrophil "priming".

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of phospholipase D in CHO cells transfected with the human epidermal growth factor (EGF) receptor: differential effects of protein kinase C activation and EGF

Multiple intracellular signal transduction pathways, including phospholipases A2 and D, can be activated by epidermal growth factor (EGF) in both a protein kinase C (PKC)-dependent and -independent manner. We investigated the activation of phospholipase D (PLD) by a PKC activator, phorbol myristate acetate (PMA) and by EGF in CHO cells transfected with the full-length EGF receptor. In cells labelled with arachidonic acid or linoleic acid, PMA activated a PLD, determined by formation of the transphosphatidylation product phosphatidylethanol in the presence of ethanol. A basal PLD activity was seen in linoleic acid-labelled cells but not in cells labelled with arachidonic acid. This basal activity was augmented by the protein phosphotyrosine phosphatase inhibitor vanadate and reduced by tyrosine kinase inhibition and was contributed to by PKC, as activity could not be elicited following prolonged exposure to phorbol ester, known to down-regulate some PKC isoforms. By contrast, EGF failed to stimulate formation of phosphatidylethanol in cells labelled with either fatty acid species. It is proposed that in the basal condition PKC-dependent PLD activation and protein tyrosine kinase phosphorylation are linked (possibly by a phospholipase C (PLC)-mediated formation of diacylglycerol); EGF which activated a phospholipase A2 (PLA2) but which failed to elicit PLC activation in these cells is without further effect on PLD.

4-Hydroxynonenal, a metabolite of lipid peroxidation, activates phospholipase D in vascular endothelial cells

We have examined the activation of phospholipase D (PLD) in bovine pulmonary artery endothelial cells (BPAEC) treated with 4-hydroxynonenal (4-HNE). Treatment of BPAEC labelled with [32P] orthophosphate (5 h for minimal phospholipid labelling) and [3H] myristic acid (24 h) with 4-HNE in the presence of 0.5% ethanol resulted in the formation of [3H] phosphatidylethanol (PEt) and [3H] phosphatidic acid (PA) with very little accumulation of [32P] PEt. The formation of [3H] PEt, as opposed to [32P] PEt, suggests that PEt synthesis was not through de novo pathway but rather through the PLD mechanism. 4-Hydroxynonenal-induced PLD activation was dose and time dependent, and was not associated with cytotoxicity as determined by [3H] deoxyglucose release. The formation of PEt was not affected by chelation of either extracellular Ca2+ with EGTA (5 mM, 30 min) or intracellular Ca2+ with BAPTA-AM (25 microM, 30 min). Treatment of BPAEC with either staurosporine (10 microM, 15 min), a protein kinase C (PKC) inhibitor, or down regulation of PKC by chronic 12-0-tetradecanoylphorbol-13-acetate (TPA) treatment (100 nM, 18 h) had no effect on 4-HNE-induced PLD activation. These results indicate that PLD activation by 4-HNE is independent of PKC activity. We also examined the specificity of nonylaldehyde derivatives and hydroxyalkenals on PLD activation. In addition to 4-HNE, 4-hydroxyoctenal and 4-hydroxyhexenal also stimulated [32P] PEt formation. Among the various nonylaldehydes examined, only trans-2-nonenal and trans-2-cis 6-nonadienal exhibited PLD activation, suggesting the requirement of a trans double bond at carbon 2 and a hydroxyl group at carbon 4. However, in contrast to 4-HNE-induced PLD activation of BPAEC monolayers, treatment of 105,000 x g membranes with 4-HNE had no effect on PLD catalyzed hydrolysis of [2-14C] oleoyl phosphatidylcholine. These data provide evidence that 4-HNE, a metabolite of membrane lipid peroxidation, may be involved in endothelial cell signal transduction, through the activation of phospholipase D and the generation of second messengers like phosphatidic acid and diacylglycerol.

Distinct mechanisms of phospholipase D activation and attenuation utilized by different mitogens in NIH-3T3 fibroblasts

The activation of phospholipase D (PLD) by platelet-derived growth factor (PDGF), prostaglandin F2 alpha and 12-O-tetradecanoylphorbol 13-acetate (TPA) was studied in NIH-3T3 fibroblasts. PLD activation was determined by measuring the production of both [3H]phosphatidic acid and [3H]phosphatidylpropanol (products of the PLD-catalyzed hydrolysis and transphosphatidylation reactions, respectively), in cells that were metabolically pre-labeled with [3H]oleic acid. All mitogens caused a rapid (within 2 min) activation of PLD. Activation of PLD by prostaglandin F2 alpha and PDGF was transient and declined to near basal levels by 15 min and 55 min, respectively. In contrast, TPA-induced activation of PLD was sustained for at least 60 min of incubation. A combination of maximally effective concentrations of PDGF and TPA stimulated PLD activity in a non-additive manner, while the effect of prostaglandin F2 alpha was additional to that of either PDGF or TPA. The protein kinase inhibitor staurosporine inhibited PLD activation by PDGF or TPA with almost identical dose/response curves. In contrast, staurosporine potentiated prostaglandin-F2 alpha-induced PLD activation. The specific protein kinase C inhibitor GF109203X (a bisindolylmaleimide) inhibited PLD activation by prostaglandin F2 alpha and PDGF at concentrations higher than those required for inhibition of PLD activation induced by TPA. Depletion of cellular protein kinase C abolished PLD activation by all three mitogens without affecting in vitro activity of membrane-bound PLD. The distinct kinetics of PLD activation and its differential susceptibility to protein kinase inhibitors suggest the existence of agonist-specific activation and/or inactivation mechanisms. The results indicate also that protein kinase C participates in the mechanism of PLD activation via PDGF, while the effect of prostaglandin F2 alpha involves a pathway independent of protein kinase C.

Rat brain cytosol contains a factor which reconstitutes guanine-nucleotide-binding-protein-regulated phospholipase-D activation in HL60 cells previously permeabilized with streptolysin O

We report that guanosine 5'-[gamma-thio]triphosphate (GTP[S]) can stimulate phospholipase D (PLD) in HL60 cells acutely permeabilized with streptolysin O. The ability of GTP[S] to stimulate PLD is impaired if the cells are previously permeabilized such that the majority of the cytosol has leaked out. Rat brain and HL60 cytosols were both found to restore GTP[S]-stimulated PLD activity in a reconstitution assay consisting of previously permeabilized HL60 cells. Rat brain cytosol was fractionated on heparin agarose and assayed for reconstitution of GTP[S]-stimulated PLD activity. The active fractions were pooled, concentrated and chromatographed on gel filtration to assess its molecular mass. The molecular mass of the reconstituting factor was found to be 16 kDa. Reconstitution by the cytosolic factor was dependent on GTP[S]. Ca2+ (pCa 5), MgATP and MgCl2 enhanced GTP[S]-dependent reconstitution of PLD activity in the previously permeabilized HL60 cells. These results demonstrate the presence in rat brain cytosol of a factor which is an activator of GTP[S]-stimulated PLD.

Endothelin-1 stimulated phospholipase D in A10 vascular smooth muscle derived cells is dependent on tyrosine kinase. Evidence for involvement in stimulation of mitogenesis

The mechanism whereby endothelin stimulates mitogenesis of vascular smooth muscle cells is not understood. Here we show that endothelin-1 stimulates phospholipase D by a protein kinase C and tyrosine kinase dependent mechanism, and present evidence that implicate the phosphatidic acid formed by phospholipase D in the mitogenic response.

Phospholipase D and cell signaling

Phospholipase D, which hydrolyzes phospholipids (primarily phosphatidylcholine) to generate phosphatidic acid, has emerged as a critical component in cellular signal transduction. Research during the past year has confirmed and extended the view that phosphatidic acid and its dephosphorylated product, sn-1,2-diacylglycerol, are important intracellular second messengers and that the coupling of phospholipase D to specific receptors occurs through multiple mechanisms involving protein kinase C, protein tyrosine kinase, Ca2+ and GTP-binding proteins.