Activation of phospholipase D in rabbit neutrophils by fMet-Leu-Phe is mediated by a pertussis toxin-sensitive GTP-binding protein that may be distinct from a phospholipase C-regulating protein

Phospholipase A(2)-mediated fusion of neutrophil-derived membranes is augmented by phosphatidic acid

Phosphatidic acid (PA), the product of phospholipase D (PLD) metabolism, is not only an important second messenger in neutrophil signal transduction but PA generation increases membrane fusogenicity. Following neutrophil stimulation, PA formation can be detected in azurophil, specific, and plasma membrane vesicle subcellular fractions, suggesting a potential role for PA formation in granule-plasma membrane fusion. Neutrophil stimulation also activates phospholipase A(2) (PLA(2)) and the release of arachidonic acid. In vitro fusion of plasma membrane vesicles and specific granules with complex liposomes were dependent on PLA(2) (<10 microM Ca(2+)) while the presence of PA in the liposomes augmented the effects of PLA(2). Azurophil granules were extremely resistant to fusion (no fusion at 12 mM Ca(2+) even in the presence of PLA(2)). However, in the presence of both PA and PLA(2) fusion could be detected at <5 microM Ca(2+), suggesting a direct role for phospholipid metabolism in neutrophil degranulation.

Aluminum fluoride inhibits phospholipase D activation by a GTP-binding protein-independent mechanism

Aluminum fluoride (AlF4-) inhibited guanine nucleotide-activated phospholipase D (PLD) in rat submandibular gland cell-free lysates in a concentration-dependent response. This effect was consistent in permeabilized cells with endogenous phospholipid PLD substrates. Inhibition was not caused by either fluoride or aluminum alone and was reversed by aluminum chelation. Inhibition of PLD by aluminum fluoride was not mediated by cAMP, phosphatases 1, 2A or 2B, or phosphatidate phosphohydrolase. AlF4- had a similar inhibitory effect on rArf-stimulated PLD, but did not block the translocation of Arf from cytosol to membranes, indicating a post-GTP-binding-protein site of action. Oleate-sensitive PLD, which is not guanine nucleotide-dependent, was also inhibited by AlF4-, supporting a G protein-independent mechanism of action. A submandibular Golgi-enriched membrane preparation had high PLD activity which was also potently inhibited by AlF4-, leading to speculation that the known fluoride inhibition of Golgi vesicle transport may be PLD-mediated. It is proposed that aluminum fluoride inhibits different forms of PLD by a mechanism that is independent of GTP-binding proteins and that acts via a membrane-associated target which may be the enzyme itself.

Upregulation of macrophage plasma membrane and nuclear phospholipase D activity on ligation of the alpha2-macroglobulin signaling receptor: involvement of heterotrimeric and monomeric G proteins

The effect of ligating the alpha2-macroglobulin signaling receptor (alpha2MSR) with receptor-recognized forms of alpha2M (alpha2M*) was studied with respect to phospholipase D (PLD) activity in murine macrophages, their plasma membranes, and nuclei. PLD activity in plasma membranes and nuclei increased linearly up to a ligand concentration of about 100 pM of either alpha2M* or a cloned and expressed receptor binding fragment (RBF). The RBF binding site mutant K1370A, which binds with high affinity to alpha2MSR, also increased nuclear PLD activity comparable to RBF and alpha2M*. Phorbol dibutyrate caused a two- to threefold stimulation of membrane and nuclear PLD activity, whereas PLD activity was nearly abolished by downregulation of protein kinase C; prior treatment with staurosporin, genestein, cyclosporin A, actinomycin D; or chelation of intracellular Ca2+. In permeabilized macrophages, isolated plasma membranes, and nuclei, GTP-gamma-S increased alpha2M*-stimulated PLD activity via a pertussis toxin-insensitive G protein and this effect was abolished on preincubation with GDP-beta-S. Incubation of plasma membranes with polyclonal antibody against sARFII, or the addition of cytosol which was immunoprecipitated with antibody against sARFII, greatly reduced alpha2M*-stimulated PLD activity in the presence of GTP-gamma-S. Preincubation of plasma membranes with GDP-beta-S prior to the addition of GTP-gamma-S and recombinant ARF1 significantly inhibited alpha2M*-stimulation of PLD activity. Nuclear PLD activity was maximally stimulated in the presence of both GTP-gamma-S and rARF1, whereas plasma membrane PLD activity was maximally stimulated in the presence of rARF1, GTP-gamma-S, RhoA, and ATP. In contrast, nuclear PLD activity was not affected by RhoA either alone or in combination with GTP-gamma-S or ATP.

Fodrin inhibits phospholipases A2, C, and D by decreasing polyphosphoinositide cell content

Brain fodrin inhibited in a dose dependent manner the GTPgammaS-stimulated cytosolic PLA2 (cPLA2), PLC, and PLD activities in differentiated HL-60 cells permeabilized with streptolysin O. cPLA2 and PLD were inhibited by the same concentrations of fodrin (IC50=1.5-2 nM) but PLC was inhibited by lower concentrations (IC50=0.3 nM). Moreover, the rates of inhibition were different between the phospholipases. Spectrin, which shares 50% homology with fodrin, had similar effects on the three phospholipases. However, using cytosol-depleted cells or recombinant PLD1, we showed that fodrin was not a direct inhibitor. Studying the potential mechanisms of these inhibitions, we demonstrated that a major decrease in membrane phosphatidylinositol 4-monophosphate (PtdIns(4)P) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) amounts was induced by fodrin. Exogenous PtdIns(4,5)P2 partly reversed fodrin inhibition of GTPgammaS-stimulated phospholipase C activity. Hence, inhibition of PLC, cPLA2, and PLD activities observed with fodrin could be related to the decrease of PtdIns(4,5)P2, substrate of PLC, a cofactor of PLD and an enhancer of cPLA2 activity.

Activation of phospholipase D by ADP-ribosylation factor in rat submandibular acinar cells

The hydrolysis of membrane phosphatidylcholine by the enzyme phospholipase D is a key initial step in the intracellular release of the signalling molecules phosphatidic acid, diacylglycerol and arachidonic acid. Guanine nucleotide-dependent pathway leading to PLD activation were investigated in enzymatically dispersed rat submandibular acinar cells. Guanosine 5'-O-[gamma-thio]triphosphate (GTP gamma S) caused the time- and concentration-dependent stimulation of PLD in permeabilized cells. This effect was lost in prepermeabilized cells, from which cytosolic components had been allowed to leak, but was restored when endogenous cytosol, or cytosol from platelets, was added back to such cells. PLD was also activated in cytosol-depleted cells by GTP gamma S in combination with purified ARF (ADP-ribosylation factor), a low M(r) guanine nucleotide-binding protein of the ras superfamily. Additional evidence for the involvement of ARF in PLD activation was the inhibition of carbachol- or GTP gamma S-induced stimulation of the enzyme by brefeldin A, a blocker of ARF activation; and the observed translocation of ARF from cytosol to membrane on GTP gamma S treatment in permeabilized cells. The heterotrimeric G-protein stimulator, AlFn, also activated PLD, and this response, too, was inhibited by brefeldin A, suggesting the downstream involvement of ARF in coupling AlFn action to phospholipase D elevation. PLD activation caused by both GTP gamma S and AlFn was only partially reduced after treatment of cells with U73122, a demonstrated inhibitor of phospholipase C in the Gq-coupled phosphoinositide signal-transduction pathway. It is therefore proposed that in rat submandibular mucous acinar cells, a guanine nucleotide-regulated PLD activation pathway exists that involves the sequential actions of a G heterotrimeric protein and ARF. It is further suggested that this pathway is independent of the Gq/PLC/phosphatidylinositol signal transduction system.

Protein tyrosine kinase involvement in the production of superoxide anion by neutrophils exposed to Aroclor 1242, a mixture of polychlorinated biphenyls

Neutrophils produce superoxide anion (O2-) when exposed in vitro to Aroclor 1242, a mixture of polychlorinated biphenyls (PCBs). The mechanism for this effect shares some similarities with the mechanism by which the physiologic agonist f-Met-Leu-Phe (fMLP) activates neutrophils. Since production of O2- in response to fMLP involves GTP-binding proteins and protein tyrosine kinases (PTKs), the current study was undertaken to determine whether these signalling pathways are involved in PCB-induced neutrophil activation. Neutrophils exposed to Aroclor 1242 or fMLP produced significant O2-. Pretreatment of intact neutrophils with pertussis toxin or cholera toxin or exposure of permeabilized cells to GDPbetaS significantly inhibited O2- production in fMLP-treated neutrophils but did not alter the response to Aroclor 1242. Pretreatment with genistein, an inhibitor of PTKs, significantly inhibited O2- production in both Aroclor 1242- and fMLP-treated neutrophils; however, daidzein, a structural analogue of genistein which lacks activity against PTKs, was without effect. Exposure of neutrophils to Aroclor 1242 resulted in an increase within 1 min in tyrosine phosphorylation of proteins in the 40 and 60 kDa molecular mass ranges which persisted for up to 10 min. Similar results were obtained with 2,2',4,4'-tetrachlorobiphenyl (2,2',4,4'-TCB), a PCB congener that stimulates O2- production. In contrast, 3,3',4,4',5-pentachlorobiphenyl (3,3',4,4',5-PeCB), a congener that does not generate O2-, caused only a transient increase in tyrosine phosphorylation of proteins in the 40 kDa range with no effect on 60 kDa proteins. These data suggest that Aroclor 1242 activates neutrophils to produce O2- by a mechanism that requires tyrosine kinase activity; however, heterotrimeric G-proteins are not likely to be involved.

Arf and RhoA regulate both the cytosolic and the membrane-bound phospholipase D from human placenta

In this paper we demonstrate for the first time that human placenta contains a cytosolic phospholipase D (PLD) activity. This activity had a pH optimum of 7.0 and was stimulated by PIP2 and inhibited by oleate. Furthermore, cytosolic PLD was stimulated by 30 microM GTP gamma S (6-14-fold) and by the small G proteins 1 microM mArf3 (2-fold) and 0.37 nM RhoA (2-fold). This is the first report to show RhoA activation of a cytosolic PLD. The activation by mArf3 was maintained after partial purification on DEAE Sepharose of the enzyme. We have previously reported the existence of a membrane-bound PLD from human placenta, which is stimulated by PIP2, but not by oleate (Vinggaard, A. M. & Hansen, H. S. (1995) Biochim. Biophys. Acta 1258, 169-176). Here we show that oleic acid and alpha-linolenic acid both dose-dependently inhibited solubilized membrane PLD (65% inhibition at 4 mM), whereas stearic acid (4 mM) had no effect. Thus, the presence of double bonds in the fatty acid is important for the inhibitory effect. Furthermore, placental membrane PLD was activated by 30 microM GTP gamma S (4-fold) and by mArf3 (1 microM) and RhoA (0.37 nM) by a factor of 3 and 2, respectively. The solubilized membrane phospholipase D was partially purified to a basal specific activity of 25-37 nmol/min/mg. This preparation was devoid of endogenous RhoA and Arf and could not be stimulated by GTP gamma S. However, mArf3 (1 microM) still activated this partially purified membrane PLD, whereas RhoA (0.37 nM) was not able to activate this PLD fraction. In conclusion, our results suggest that the human placenta contains a PLD that is located both in the cytosol and the membranes, and that is activated by PIP2, mArf3 and RhoA but inhibited by oleate.

Pharmacological characterization of metabotropic glutamate receptors coupled to phospholipase D in the rat hippocampus

1. Phospholipase D (PLD) is the key enzyme in a signal transduction pathway leading to the formation of the second messengers phosphatidic acid and diacylglycerol. In order to define the pharmacological profile of PLD-coupled metabotropic glutamate receptors (mGluRs), PLD activity was measured in slices of adult rat brain in the presence of mGluR agonists or antagonists. Activation of the phospholipase C (PLC) pathway by the same agents was also examined. 2. The mGluR-selective agonist (1S,3R)-l-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced a concentration-dependent (10-300 microM) activation of PLD in the hippocampus, neocortex, and striatum, but not in the cerebellum. The effect was particularly evident in hippocampal slices, which were thus used for all subsequent experiments. 3. The rank order of potencies for agonists stimulating the PLD response was: quisqualate > ibotenate > (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine > (1S,3R)-ACPD > L-cysteine sulphinic acid > L-aspartate > L-glutamate. L-(+)-2-Amino-4-phosphonobutyric acid and the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate failed to activate PLD. (RS)-3,5-dihydroxyphenylglycine (100300 microM), an agonist of mGluRs of the first group, stimulated PLC but inhibited the PLD response elicited by 100 microM (1S,3R)-ACPD. 4. (+)-alpha-Methyl-4-carboxyphenylglycine (0.1-1 mM), a competitive antagonist of mGluRs of the first and second group, elicited a significant PLD response. L-(+)-2-Amino-3-phosphonopropionic acid (1 mM), an antagonist of mGluRs of the first group, inhibited the 100 microM (1S,3R)-ACPD-induced PLC response but produced a robust stimulation of PLD. 5. 12-O-Tetradecanoylphorbol 13-acetic acid and phorbol 12,13-dibutyrate (PDBu), activators of protein kinase C, at 1 microM had a stimulatory effect on mGluRs linked to PLD but depressed (1S,3R)-ACPD-induced phosphoinositide hydrolysis. The protein kinase C inhibitor, staurosporine (1 and 10 microM) reduced PLD activation induced by 1 microM PDBu but not by 100 microM (1S,3R)-ACPD. 6. Our results suggest that PLD-linked mGluRs in rat hippocampus may be distinct from any known mGluR subtype coupled to PLC or adenylyl cyclase. Moreover, they indicate that independent mGluRs coupled to the PLC and PLD pathways exist and that mGluR agonists can stimulate PLD through a PKC-independent mechanism.

Synergistic activation of rat brain phospholipase D by ADP-ribosylation factor and rhoA p21, and its inhibition by Clostridium botulinum C3 exoenzyme

An activator of rat brain phospholipase D (PLD) that is distinct from the already identified PLD activator, ADP-ribosylation factor (ARF), was partially purified from bovine brain cytosol by a series of chromatographic steps. The partially purified preparation contained a 22-kDa substrate for Clostridium botulinum C3 exoenzyme ADP-ribosyltransferase, which strongly reacted with anti-rhoA p21 antibody, but not with anti-rac1 p21 or anti-cdc42Hs p21 antibody. Treatment of the partially purified PLD-activating factor with both C3 exoenzyme and NAD significantly inhibited the PLD-stimulating activity. These results suggest that rhoA p21 is, at least in part, responsible for the PLD-stimulating activity in the preparation. Recombinant isoprenylated rhoA p21 expressed in and purified from Sf9 cells activated rat brain PLD in a concentration- and GTP gamma S (guanosine 5'-O-(3-thiotriphosphate))-dependent manner. In contrast, recombinant non-isoprenylated rhoA p21 (fused to glutathione S-transferase) expressed in Escherichia coli failed to activate the PLD. This difference cannot be explained by a lower affinity of non-isoprenylated rhoA p21 for GTP gamma S, as the rates of [35S]GTP gamma S binding were very similar for both recombinant preparations and the GTP gamma S-bound form of non-isoprenylated rhoA p21 did not induce PLD activation. Interestingly, recombinant isoprenylated rhoA p21 and ARF synergistically activated rat brain PLD; a similar pattern was seen with the partially purified PLD-activating factor. The synergistic activation was inhibited by C3 exoenzyme-catalyzed ADP-ribosylation of recombinant isoprenylated rhoA p21 in a NAD-dependent manner. Inhibition correlated with the extent of ADP-ribosylation. These findings suggest that rhoA p21 regulates rat brain PLD in concert with ARF, and that isoprenylation of rhoA p21 is essential for PLD regulation in vitro.

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.

Purinergic agonist and G protein stimulation of phospholipase D in rat liver plasma membranes. Independence from phospholipase C activation

Hormonal regulation of phospholipase D (PLD) was studied in isolated rat liver plasma membranes. Purinergic agents and a submaximal concentration of guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of GTP, synergistically stimulate phosphatidylethanol formation, a measure of PLD activity. The rank order of efficacy for stimulation of PLD activity in the presence of 0.2 microM GTP gamma S was beta, gamma-methylene-ATP > adenosine 5'-0-(3-thiotriphosphate) = ATP = ADP = 2-methylthio-ATP > alpha, beta-methylene-ATP = UTP. This pattern of activation does not conform to the series at known P2 receptors. GTP gamma S stimulated PLD activity in a dose-dependent manner, and the GTP gamma S dose-response curve for phosphatidylethanol formation was shifted to the left by an analog of ATP. Activation of PLD by purinergic agents in the presence of GTP gamma S supports the involvement of a purinergic receptor of the P2 class and a GTP-binding protein. Purinergic agents competitively inhibited [35S]adenosine 5'-0-(3-thiotriphosphate) binding to plasma membranes in the rank order adenosine 5'-0'(3-thiotriphosphate) > ATP > alpha,beta-methylene-ATP = UTP >> beta, gamma-methylene-ATP = ADP. Stimulation of phosphoinositide phospholipase C (PI-PLC) by purinergic agents, as measured by release of radioactivity from endogenously myo[3H]inositol-labeled plasma membranes, occurred in the order alpha, beta-methylene-ATP >> 2-methylthio-ATP. Beta, gamma-methylene-ATP had little effect on PI-PLC activity. Different dose-response relationships for agonist-stimulation of PI-PLC and PLD indicate that activation of PI-PLC is not involved in stimulation of PLD in rat liver plasma membranes, and suggest that purinergic activation of PLD occurs via a pathway involving a G protein and a heretofore uncharacterized P2 receptor.

Low molecular weight GTP-binding proteins in HL-60 granulocytes. Assessment of the role of ARF and of a 50-kDa cytosolic protein in phospholipase D activation

Phospholipase D (PLD) activation by guanine nucleotides requires protein cofactors in both the plasma membrane and the cytosol. HL-60 cytosol was fractionated by ammonium sulfate and gel-permeation chromatography. Two cytosolic protein fractions were found to reconstitute the GTP gamma S (guanosine 5'-3-O-(thio)triphosphate)-stimulated PLD in a reconstitution assay consisting of 3H-labeled HL-60 membranes and eluted column fractions. The major peak of reconstituting activity was in the region of 50 kDa, and a second discrete peak of PLD reconstitution activity was observed in the region of 18 kDa. Rho GDP/GTP exchange inhibitor, Rho GDI, comigrated with Rac2 and RhoA, but not Rac1. RhoA and Rac2 were entirely complexed with Rho GDI and eluted with an apparent molecular mass of 43 kDa by gel filtration chromatography. The partial overlap between cytosolic Rac2 and RhoA with the 50-kDa peak of reconstituting activity was not consistent with the participation of cytosolic Rho-related GTPases in the activation of PLD by guanine nucleotides. However, recombinant Rho GDI, which inhibits nucleotide exchange on the Rho family of small GTP-binding proteins, reduced GTP gamma S-stimulated PLD activity in HL-60 homogenates. The stimulatory exchange factor, Smg GDS, which is active on Rho and Rac, could be partially separated from the PLD-stimulating factor(s) by gel-permeation chromatography. Moreover, recombinant Smg GDS failed to stimulate GTP-dependent PLD activity. Cytosolic ADP-ribosylation factor (ARF) was exclusively located in the 18-kDa peak of reconstitution activity. Faint amounts of membrane-bound ARF were also detected using the monoclonal antibody 1D9. The effects of the 50-kDa and 18-kDa PLD-inducing factors on the salt-extracted PLD activity were synergistic. The weak stimulatory effect of ARF alone suggested that the GTP gamma S-stimulated PLD activity is dependent on the presence of another protein(s), presumably ARF-regulatory proteins. We propose that a membrane-bound GTP-binding protein, possibly ARF, may be involved in the activation of PLD when combined with the component(s) of the 50-kDa fraction.

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.

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)

Receptor- and phorbol ester-mediated phospholipase D activation in rat parotid involves two different pathways

We have investigated phospholipase D (PLD) activation in rat parotid acini prelabeled with [14C]stearic acid. In the presence of 2% ethanol, muscarinic and alpha-adrenergic agonists stimulated the formation of [14C]phosphatidylethanol as a result of a PLD activity. The calcium ionophore, ionomycin, and the phorbol esters, 4 beta-phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu), also stimulated phosphatidylethanol accumulation, but 1-oleyl-2-acetyl-sn-glycerol (OAG), a permeant analogue of diacylglycerol did not. Chelerythrine and staurosporine, two inhibitors of protein kinase C, failed to affect any response. These results suggest that protein kinase C was not involved in the regulation of PLD activity. A difference between PLD regulation by PMA and receptor-mediated agonists was observed with regard to the extracellular calcium requirement. Our results strongly suggest that PLD activation in parotid acini involved different pathways: a calcium-dependent pathway activated by receptor-mediated agonists and a calcium-independent pathway activated by phorbol esters. Moreover, we observed that PLD activation did not result in any change in phosphatidic acid level. We propose that the phosphatidyl transferase activity of PLD reflected a metabolic pathway which may allow a base-exchange reaction in parotid gland.

Increased fMet-Leu-Phe receptor expression and altered superoxide production of neutrophil granulocytes in septic and posttraumatic patients

Activation of neutrophils by various inflammatory stimuli has been shown to play a pivotal role in septic and posttraumatic tissue injury. To further elucidate the mechanisms modulating the oxidative metabolism, we assessed superoxide production induced by N-formylmethionyl-leucyl-phenylalanine (FMLP) and phorbol myristate acetate and the expression of FMLP receptors of human neutrophils on several days during sepsis and after trauma. Neutrophils of septic patients isolated on days 0-4 after the diagnosis of sepsis showed a significant, more than twofold increase in specific binding of [3H]FMLP at 1, 120, and 240 nM. Scatchard plot analyses revealed that this increase in specific binding was due to an increase in the number of low- and high-affinity FMLP receptors with no changes in receptor affinity. On days 5-10 after the onset of sepsis the up-regulation of FMLP receptors on circulating neutrophils was followed by receptor down-regulation. Likewise, neutrophils from patients with trauma that was not complicated by sepsis bound significantly more [3H]FMLP than neutrophils from volunteers. However, the increase in FMLP receptors was less than that in septic neutrophils and returned earlier to normal. In accordance with the up-regulation of FMLP receptors, neutrophils obtained from patients with sepsis or after trauma on days 1-4 and days 1-2, respectively, produced significantly more superoxide anion upon stimulation with FMLP. However, after stimulation with phorbol myristate acetate, a receptor-independent activator of protein kinase C, these cells released less superoxide anion than controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ release from inositol 1,4,5-trisphosphate-sensitive Ca2+ store by antidepressant drugs in cultured neurons of rat frontal cortex

The ability of antidepressant drugs (ADs) to increase the concentration of intracellular Ca2+ ([Ca2+]i) was examined in primary cultured neurons from rat frontal cortices using the Ca(2+)-sensitive fluorescent indicator fura-2. Amitriptyline, imipramine, desipramine, and mianserin elicited transient increases in [Ca2+]i in a concentration-dependent manner (100 microM to 1 mM). These four AD-induced [Ca2+]i increases were not altered by the absence of external Ca2+ or by the presence of La3+ (30 microM), suggesting that these ADs provoked intracellular Ca2+ mobilization rather than Ca2+ influx. All four ADs increased inositol 1,4,5-trisphosphate (IP3) contents by 20-60% in the cultured cells. The potency of the IP3 production by these ADs closely correlated with the AD-induced [Ca2+]i responses. Pretreatment with neomycin, an inhibitor of IP3 generation, significantly inhibited amitriptyline- and imipramine-induced [Ca2+]i increases. In addition, by initially perfusing with bradykinin (10 microM) or acetylcholine (10 microM), which can stimulate the IP3 generation and mobilize the intracellular Ca2+, the amitriptyline responses were decreased by 76% and 69%, respectively. The amitriptyline-induced [Ca2+]i increases were unaffected by treatment with pertussis toxin. We conclude that high concentrations of amitriptyline and three other ADs mobilize Ca2+ from IP3-sensitive Ca2+ stores and that the responses are pertussis toxin-insensitive. However, it seems unlikely that the effects requiring high concentrations of ADs are related to the therapeutic action.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

New pathways of phagocyte activation: the coupling of receptor-linked phospholipase D and the role of tyrosine kinase in primed neutrophils

Protein kinase C (PKC) appears to have a central role in the O2- response of neutrophils following stimulation of membrane receptors. The second messenger, diacylglycerol (DG), that activates PKC is derived from membrane phospholipids via activation of phosphatidylinositol 4,5-bisphosphate (PIP2)-phospholipase C (PLC) and phospholipase D (PLD), with the latter pathway being more prominent in primed cells. In resting cells receptor coupling to PLD is through a G-protein. Priming brings a cytoplasmic tyrosine kinase into the transducer sequence which, through protein phosphorylation, increases the efficiency of coupling between membrane receptors and PLD. Phosphatidic acid (PA), the initial product of the PLD pathway, also appears to act as a second messenger by directly activating the NADPH oxidase responsible for generating O2-. Interconversion of PA and DG by phosphatidate phosphohydrolase and DG kinase determines which of these second messengers has the dominant role.

Activation of phospholipase D by endothelin-1 and other pharmacological agents in rabbit iris sphincter smooth muscle

The stimulation of phospholipase D (PLD) activity by endothelin-1 (ET1) was investigated in rabbit iris sphincter prelabelled with [3H]myristic acid. In the presence of 0.5% ethanol, ET1 caused a time- and dose-dependent increase in the production of [3H]phosphatidylethanol ([3H]PEt). Within 30 s the peptide increased PEt formation by 30% and after 5 min increased it by 140%. The EC50 value for ET1-stimulated PEt formation was found to be 30 nM. This value is appreciably lower than the EC50 we previously obtained for ET1-induced inositol trisphosphate production (45 nM), but considerably higher than that for arachidonic acid release (1 nM). PEt formation was significantly stimulated by prostaglandin F20, phorbol 12,13-dibutyrate (PDBu), chloroform, A23187 and A1F4-, but it was not affected by carbachol or the platelet-activating factor. PDBu-stimulated PEt formation was blocked by staurosporine and it was not potentiated by A23187. Staurosporine had no effect on ET1-stimulated PEt formation. Our data indicate that ET1 stimulation of PLD occurs independently of protein kinase C activation, phospholipase C activation and intracellular Ca2+ mobilization, and phospholipase A2 activation. In this tissue the ET1 receptor is probably coupled to the three phospholipases through several G-proteins, and this appears to be species and receptor type specific.

Pertussis toxin-insensitive G protein mediates carbachol activation of phospholipase D in rat pheochromocytoma PC12 cells

In the present study, an activation mechanism for phospholipase D (PLD) in [3H]palmitic acid-labeled pheochromocytoma PC12 cells in response to carbachol (CCh) was investigated. PLD activity was assessed by measuring the formation of [3H]phosphatidylethanol ([3H]PEt), the specific marker of PLD activity, in the presence of 0.5% (vol/vol) ethanol. CCh caused a rapid accumulation of [3H]-PEt, which reached a plateau within 1 min, in a concentration-dependent manner. The [3H]PEt formation by CCh was completely antagonized by atropine, demonstrating that the CCh effect was mediated by the muscarinic acetylcholine receptor (mAChR). A tumor promoter, phorbol 12-myristate 13-acetate (PMA), also caused an increase in [3H]-PEt content, which reached a plateau at 30-60 min after exposure, but an inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate, did not. Although a protein kinase C (PKC) inhibitor, staurosporine (5 microM), blocked PMA-induced [3H]PEt formation by 77%, it had no effect on the CCh-induced formation. These results suggest that mAChR-induced PLD activation is independent of PKC, whereas PLD activation by PMA is mediated by PKC. NaF, a common GTP-binding protein (G protein) activator, and a stable analogue of GTP, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), also stimulated [3H]PEt formation in intact and digitonin-permeabilized cells, respectively. GTP, UTP, and CTP were without effect. Furthermore, guanosine 5'-O-(2-thiodiphosphate) significantly inhibited CCh- and GTP gamma S-induced [3H]PEt formation in permeabilized cells but did not inhibit the formation by PMA, and staurosporine (5 microM) had no effect on [3H]PEt formation by GTP gamma S.(ABSTRACT TRUNCATED AT 250 WORDS)