Phospholipase D: regulation and functional significance

Regulation of phospholipase D isoenzymes by transforming Ras and atypical protein kinase C-iota

The activation of phospholipase D (PLD) by transforming Ras is well documented. Although two distinct PLD isoforms, PLD1 and PLD2, have been cloned from mammalian cells, it has remained unclear whether both isoenzymes are activated by Ras and, if this is the case, whether they are stimulated by a common mechanism. In the present study we show that expression of transforming Ras in HC11 mouse mammary epithelial cells enhanced the activity of endogenous PLD. Co-expression of Ras with either PLD1b or PLD2 resulted in elevated activities of both PLD isoenzymes in HC11 cells, indicating that transforming Ras was capable of activating both PLD isoforms in vivo. Ras-induced activation of PLD was resistant to the protein kinase C (PKC) inhibitor GF109203X, which preferentially affects conventional- and novel-type PKCs, but sensitive to Ro-31-8220, which inhibits atypical PKCs more effectively. Co-transfection of atypical PKC-iota with either PLD1b or PLD2 led to a selective activation of PLD2 by PKC-iota, whereas PLD1b was not affected. PLD1b, however, was found to be a potent activator of PKC-iota, whereas PLD2 was less effective in this respect. The data suggest that PKC-iota acts upstream of PLD2 and that PLD1b is implicated in the activation of PKC-iota. The data are discussed as indicating a putative signalling cascade comprising Ras-->PLD1b-->PKC-iota-->PLD2. Evidence for the implication of this pathway in the transcriptional regulation of cyclin D1 is also presented.

Bradykinin and phorbol ester but not 5-HT2B receptor activation stimulate phospholipase D activity in the rat stomach fundus

1. Serotonin has been implicated as a mediator involved in migraine headache, an effect that may involve central 5-HT2B receptor activation. 5-HT2B receptor signal transduction in controversial. 2. Rat stomach fundus contraction to serotonin has been used as a model for 5-HT2B receptor activation. Serotonin-induced contractility involves intracellular calcium release and activation of protein kinase C without stimulation of phosphoinositide (PI) hydrolysis. 3. Since phospholipase D (PLD) activation results in phosphatidic acid production, which can release intracellular calcium and provide diacylglycerol for PKC activation, the purpose of this study was to determine whether the 5-HT2B receptor coupled to PLD activation using the rat stomach fundus as a model system. 4. Using phosphatidylethanol production to measure PLD activity, both bradykinin (0.01-1 microM) and phorbol dibutyrate (PDBu, 1 microM) stimulated PLD activity in rat stomach fundal strips, indicating that this tissue possesses an active PLD system. 5. Under identical conditions, 5-hydroxytryptamine (5-HT) failed to stimulate PLD activity over a concentration range (1 nM-1 microM) documented to induce 5-HT2B receptor-mediated contraction in rat stomach fundus. Thus, the 5-HT2B contractile receptor in rat stomach fundus is not coupled to PLD activation, whereas both bradykinin and phorbol ester do couple to PLD.

Activation of muscarinic receptors in porcine airway smooth muscle elicits a transient increase in phospholipase D activity

Phospholipase D (PLD) is a phosphodiesterase that catalyses hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. In the presence of ethanol, PLD also catalyses the formation of phosphatidylethanol, which is a unique characteristic of this enzyme. Muscarinic receptor-induced changes in the activity of PLD were investigated in porcine tracheal smooth muscle by measuring the formation of [3H]phosphatidic acid ([3H]PA) and [3H]phosphatidylethanol ([3H]PEth) after labeling the muscle strips with [3H]palmitic acid. The cholinergic receptor agonist acetylcholine (Ach) significantly but transiently increased formation of both [3H]PA and [3H]PEth in a concentration-dependent manner (>105-400% vs. controls in the presence of 10(-6) to 10(-4) M Ach) when pretreated with 100 mM ethanol. The Ach receptor-mediated increase in PLD activity was inhibited by atropine (10(-6) M), indicating that activation of PLD occurred via muscarinic receptors. Activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) increased PLD activity that was effectively blocked by the PKC inhibitors calphostin C (10(-8) to 10(-6) M) and GFX (10(-8) to 10(-6) M). Ach-induced increases in PLD activity were also significantly, but incompletely, inhibited by both GFX and calphostin C. From the present data, we conclude that in tracheal smooth muscle, muscarinic acetylcholine receptor-induced PLD activation is transient in nature and coupled to these receptors via PKC. However, PKC activation is not solely responsible for Ach-induced activation of PLD in porcine tracheal smooth muscle.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Abnormal content of n-6 and n-3 long-chain unsaturated fatty acids in the phosphoglycerides and cholesterol esters of parahippocampal cortex from Alzheimer's disease patients and its relationship to acetyl CoA content

The long-chain fatty acid composition of cholesterol esters, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI) from parahippocampal cortex of Alzheimer's disease (AD) patients and control subjects was examined. In general the PC fraction contained less polyunsaturated long-chain fatty acids than did PE, PS or PI. Of the n-6 polyunsaturated long-chain fatty acids, PI contained the greatest incorporation of these acids followed by PE. There were significant differences between controls and AD patients in total n-6 EFAs. Arachidonic acid (C20:4n-6) was the predominant fatty acid of this family found to be present. In AD, PE and PS showed a deficit of adrenic acid (C22:4n-6) content and PE also contained less arachidonic acid. In AD subjects, the cholesterol esters contained significantly less n-3 polyunsaturated fatty acids with, specifically, a reduction in alpha-linolenic acid. Acetyl CoA content of hippocampal cortex was greater in AD patients than in control subjects indicating either an increased extent of oxidative metabolism or a failure to utilise acetyl CoA for anabolic processes. Abnormal magnitude of oxidative processes could give rise to the biosynthesis of PE and PS species containing less n-6 polyunsaturated fatty acids than occurs in control subjects.

Induction of cytosolic phospholipase A2 activity by phosphatidic acid and diglycerides in permeabilized human neutrophils: interrelationship between phospholipases D and A2

Relationships between phospholipases are poorly understood, but phosphatidic acid (PA) and diglycerides (DGs), produced by phospholipase D (PLD) and phosphatidate phosphohydrolase actions, might function as second messengers coupling cell stimulation to cellular responses. This study investigates the role of PLD-mediated PA and DG formation in inducing phospholipase A2 (PLA2) activity in intact human neutrophils (PMNs) and in PMNs permeabilized with Staphylococcus aureus alpha-toxin. PMNs were labelled with [3H]arachidonic acid (AA) to assess AA release and metabolism and diacylglycerol formation, or with [3H]1-O-hexadecyl-2-lyso-glycerophosphatidylcholine for the determination of platelet-activating factor (PAF), PA and alkylacylglycerol production. In intact PMNs primed with tumour necrosis factor alpha before stimulation with N-formyl-Met-Leu-Phe, AA release and metabolism and PAF formation increased in parallel with enhanced PA and DG formation, and inhibition of PA and DG production led to a decrease in both AA release and PAF accumulation. In alpha-toxin-permeabilized PMNs, AA release and PAF production result from the specific activation of cytosolic PLA2 (cPLA2). In this system, PA and DG formation were always present when cPLA2 activation occurred; blocking PA and DG production inhibited AA release and PAF accumulation. Adding either PA or DG back to permeabilized cells (with endogenous PA and DG formation blocked) led to a partial restoration of AA release and PAF formation; a combination of PA and DGs reconstituted full cPLA2 activity. These results strongly suggest that products of PLD participate in activating cPLA2 in PMNs.

Characterization of phospholipase D activation by muscarinic receptors in human neuroblastoma SH-SY5Y cells

The cholinergic regulation of phospholipase D activity was studied in SH-SY5Y human neuroblastoma cells with phosphatidylethanol formation as a specific marker for the enzyme activity. The muscarinic antagonists, hexahydrosiladifenidol and pirenzepine, inhibited carbachol-induced phosphatidylethanol formation in a concentration-dependent manner and the inhibitory constants indicated that muscarinic M1 receptors are responsible for the major part of the phospholipase D activation. The mechanism of receptor-mediated phospholipase D activation varies between different cell types and receptors. In SH-SY5Y cells, the carbachol-induced phospholipase D activity was inhibited by protein kinase C inhibitors. Since both phospholipases D and C are activated by muscarinic stimulation in SH-SY5Y cells, most of the phospholipase D activation is probably secondary to the protein kinase C activation that follows phospholipase C-mediated increase in diacylglycerols. Other kinases may be involved in the regulation since also a tyrosine kinase inhibitor decreased the phosphatidylethanol formation. Stimulation of G-protein(s) and increase in the intracellular Ca2+ concentration activated phospholipase D and may be additional mechanisms for the muscarinic regulation of phospholipase D in SH-SY5Y cells. Propranolol, an inhibitor of phosphatidic acid phosphohydrolase, increased the carbachol-induced formation of phosphatidic acid at the expense of 1,2-diacylglycerol. This indicates that phospholipase D contributes to the formation of 1,2-diacylglycerol after carbachol stimulation in SH-SY5Y cells.

Nuclear translocation of RhoA mediates the mitogen-induced activation of phospholipase D involved in nuclear envelope signal transduction

In this paper we demonstrate for the first time a mitogen-induced activation of a nuclear acting phosphatidylcholine-phospholipase D (PLD) which is mediated, at least in part, by the translocation of RhoA to the nucleus. Addition of alpha-thrombin to quiescent IIC9 cells results in an increase in PLD activity in IIC9 nuclei. This is indicated by an increase in the alpha-thrombin-induced production of nuclear phosphatidylethanol in quiescent cells incubated in the presence of ethanol as well as an increase in PLD activity in isolated nuclei. Consistent with our previous report (Wright, T. M., Willenberger, S., and Raben, D. M. (1992) Biochem. J. 285, 395-400), the presence of ethanol decreases the alpha-thrombin-induced production of phosphatidic acid without affecting the induced increase in nuclear diglyceride, indicating that the increase in nuclear PLD activity is responsible for the effect on phosphatidic acid, but not that on diglyceride. Our data further demonstrate that RhoA mediates the activation of nuclear PLD. RhoA translocates to the nucleus in response to alpha-thrombin. Additionally, PLD activity in nuclei isolated from alpha-thrombin-treated cells is reduced in a concentration-dependent fashion by incubation with RhoGDI and restored by the addition of prenylated RhoA in the presence of guanosine 5'-3-O-(thio)triphosphate. Western blot analysis indicates that this RhoGDI treatment results in the extraction of RhoA from the nuclear envelope. These data support a role for a RhoA-mediated activation of PLD in our recently described hypothesis, which proposes that a signal transduction cascade exists in the nuclear envelope and represents a novel signal transduction cascade that we have termed NEST (nuclear envelope signal transduction).

Inhibition of phospholipase D activity by fodrin. An active role for the cytoskeleton

Phospholipase D (PLD) is a major enzyme implicated in important cellular processes such as secretion and proliferation. The knowledge of its regulation is essential to understand the control of these phenomena. Several proteins activating PLD have been described in the last years. In this report, we chromatographed bovine brain cytosolic proteins to identify fodrin, the non-erythroid spectrin, as the first described inhibitor of PLD. A cytosolic fraction with an inhibitory effect on PLD activity loses its capacity after immunoprecipitation of fodrin. Moreover, at 1 nM, purified fodrin blocks fully and quickly PLD activity, whatever the stimuli used. In contrast, fodrin has no effect on adenylate cyclase activity. Fodrin-analogous proteins like dimeric or tetrameric erythroid spectrin have the same inhibitory effect on PLD, at higher concentrations. Other cytoskeletal proteins, actin and vimentin, are inefficient on PLD inhibition. The mechanisms implicated in PLD modulation such as post-translational modifications of fodrin and the role of small G-proteins on the cytoskeleton regulation are discussed. In conclusion, this study reveals that fodrin is involved in the control of PLD activity, suggesting that the cytoskeleton could have an active role in control of secretion and proliferation.

Regulation of phospholipase D activity in neuroblastoma cells

The regulation of phospholipase D was studied in human neuroblastoma cells using phosphatidylethanol as a marker of the enzyme activity. Carbachol induced phospholipase D activity in SH-SY5Y cells. Muscarinic antagonists inhibited the response with potencies suggesting that muscarinic M1 receptors are responsible for the activation. In permeabilized SH-SY5Y cells, both the carbachol- and GTP gamma S-induced Peth formation was inhibited by GDP beta S, indicating that both responses are mediated via a G-protein. The protein kinase C inhibitors, bisindolylmaleimide and staurosporine significantly inhibited the carbachol-induced Peth formation whereas H7 had no effect. Thus, the cholinergic activation of phospholipase D in SH-SY5Y cells is probably mediated via a direct receptor-G-protein coupling but an involvement of protein kinase C cannot be excluded. Calmidazolium, a calmodulin antagonist, induced an increase in phosphatidylethanol formation in both SH-SY5Y and IMR-32 cells. This effect was inhibited by genistein and tyrphostin, indicating a tyrosine kinase dependent pathway for phospholipase D activation in neuroblastoma cells.

Differential regulation of receptor-stimulated cyclic adenosine monophosphate production by polyvalent cations in MC3T3-E1 osteoblasts

Extracellular cations have paradoxical trophic and toxic effects on osteoblast function. In an effort to explain these divergent actions, we investigated in MC3T3-E1 osteoblasts if polyvalent cations differentially modulate the agonist-stimulated cyclic adenosine monophosphate (cAMP) pathway, an important regulator of osteoblastic function. We found that a panel of cations, including gadolinium, aluminum, calcium, and neomycin, inhibited prostaglandin E1 (PGE)-stimulated cAMP accumulation but paradoxically potentiated parathyroid hormone (PTH)-stimulated cAMP production. In contrast, these cations had no effect on forskolin- or cholera toxin-induced increases in cAMP, suggesting actions proximal to adenylate cyclase and possible modulation of receptor interactions with G proteins. Phorbol 12-myristate 13-acetated (PMA) mimicked the effects of cations on PGE1- and PTH-stimulated cAMP accumulation in MC3T3-E1 cells, respectively, diminishing and augmenting the responses. Moreover, down-regulation of protein kinase C (PKC) by overnight treatment with PMA prevented gadolinium (Gd3+) from attenuating PGE1- and enhancing PTH-stimulated cAMP production, indicating involvement of PKC-dependent pathways. Cations, however, activated signal transduction pathways not coupled to phosphatidylinositol-specific phospholipase C (PI-PLC), since there was no corresponding increase in inositol phosphate formation or intracellular calcium concentrations. In addition, pertussis toxin treatment failed to prevent Gd(3+)-mediated suppression of PGE1-stimulated cAMP, suggesting actions independent of Gm. Thus, polyvalent cations may either stimulate or inhibit hormone-mediated cAMP accumulation in osteoblasts. These differential actions provide a potential explanation for the paradoxical trophic and toxic effects of cations on osteoblast function that occur in vivo under different hormonal conditions.

Phospholipase D in platelets and megakaryocytic cells

Phospholipase D (PLD) is stimulated in platelets by various agents. Phosphatidylcholine is the major substrate for PLD. This enzymatic pathway generates phosphatidic acid selectively. Guanine nucleotides also stimulate PLD in platelet membranes. Furthermore, tyrosine kinase may also be involved in platelet PLD regulation. It appears that multiple signals acting sequentially or in parallel converge on PLD. Among others, PLD has been proposed to play a role in platelet secretion and PLA2 regulation. PLD is also present in platelet percursor megakaryocytric cells and can be activated by platelet agonists. In these cells both PKC and G-proteins (e.g. Rho) may regulate PLD activity. The significance of PLD in megakaryocytes awaits investigation. These recent developments offer new avenues of research to further elucidate the biochemistry of platelet and megakaryocyte function.

Muscarinic regulation of phospholipase D and its role in arachidonic acid release in rat submandibular acinar cells

The characteristics of muscarinic cholinergic-induced phospholipase D (PLD) activation, and the involvement of the enzyme in the release of arachidonic acid were examined in rat submandibular acinar cells. Carbachol produced a dose-related activation of PLD to around fivefold control values at 100 microM agonist concentration. This was associated with the appearance of free choline, phosphatidic acid and arachidonic acid, indicating that the PLD substrate was phosphatidylcholine. The response to carbachol was inhibited by 60% by U73122, a blocker of a phospholipase C (PLC) specific to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], suggesting that the cleavage of phosphatidylcholine by PLD was, at least in part, secondary to agonist-coupled hydrolysis of PtdIns(4,5)P2 by PLC. Consistent with this, PLD was also activated to levels comparable to those induced by carbachol, by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), and the Ca2+ mobilizer, thapsigargin, two agents that respectively mimic the activation of protein kinase C (PKC) by diacylglycerol and the elevation of cytosolic Ca2+ by inositol 1,4,5-triphosphate [Ins(1,4,5)P3] in the phosphoinositide effect. The cell-permeant Ca2+ chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM) abolished the thapsigargin-induced activation of PLD and inhibited the responses of PLD to carbachol and TPA by 60%. The PKC inhibitor, Ro-31-8220, also inhibited the activation of PLD by carbacol and TPA to a level of approximately double control values, but had no effect on the thapsigargin-induced elevation of PLD. A role for both the PKC-associated and Ca(2+)-mobilizing arms of the PtdIns(4,5)P2-PLC pathway in PLD regulation is thus suggested. Pretreatment of cells with the phosphatidate phosphohydrolase blocker, propranolol, significantly enhanced the carbachol-induced elevation of phosphatidic acid, but decreased agonist-stimulated production of diacylglycerol and arachidonic acid, indicating that phosphatidlycholine was the likely source of arachidonic acid. We therefore propose that, in submandibular mucous acinar cells, muscarinic activation of the PtdIns(4,5)P2-PLC pathway regulates phosphatidylcholine-specific PLD through both the PKC- and Ca(2+)-mobilizing arms of the phosphoinositide response, and that diacylglycerol, derived from phosphatidylcholine via phosphatidic acid, is a source of free arachidonic acid.

Rapid and persistent desensitization of m3 muscarinic acetylcholine receptor-stimulated phospholipase D. Concomitant sensitization of phospholipase C

Activation of muscarinic acetylcholine receptors (mAChR) in human embryonic kidney (HEK) cells stably expressing the human m3 subtype leads to stimulation of both phospholipase C (PLC) and D (PLD). mAChR-stimulated PLD was turned off after 2 min of receptor activation with either the full (carbachol) or partial agonist (pilocarpine) and remained completely suppressed for at least 4 h. Partial recovery was observed 24 h after agonist removal. This rapid arrest of PLD response was not due to a loss of cell surface receptors and was also not caused by negative feedback due to concomitant activation of protein kinase C, tyrosine phosphorylation, increase in cytosolic calcium, or activation of Gi proteins. Furthermore, PLD stimulation by directly activated protein kinase C and GTP-binding proteins was unaltered in carbachol-pretreated cells. Finally, neither prevention of PLD stimulation during carbachol pretreatment by genistein nor inhibition of protein synthesis by cycloheximide, added before or after carbachol challenge, resulted in recovery of mAChR-stimulated PLD. The short term carbachol pretreatment nearly completely abolished agonist-induced binding of guanosine 5'-O-(3-thiotriphosphate) to membranes or permeabilized adherent cells. Full recovery of this response was achieved after 4 h. Similar to transfected m3 mAChR, PLD stimulation by endogenously expressed purinergic receptors was also fully blunted after 2 min of agonist (ATP) treatment. Preexposure of HEK cells to either receptor agonist partially, but not completely, reduced PLD stimulation by the other agonist. In contrast to desensitization of PLD stimulation, 2 min of carbachol treatment led to a sensitization, by up to 2-fold, of mAChR-stimulated inositol phosphate formation. This supersensitivity was also observed with pilocarpine, which acted as a full agonist on PLC. On the basis of these results, we conclude that the m3 mAChR stimulates PLD and PLC in HEK cells with distinct efficiencies and with very distinct durations of each response. The rapid and long lasting desensitization of the PLD response is apparently not due to a loss of cell surface receptors or PLD activation by GTP-binding proteins, but it may involve, at least initially, an uncoupling of receptors from GTP-binding proteins and most likely a loss of an as yet undefined essential transducing component.

Guanosine 5'-[gamma-thio]triphosphate induces membrane localization of cytosol-independent phospholipase D activity in a cell-free system from U937 promonocytic leucocytes

Activation of phospholipase D (PLD) in phagocytic leucocytes requires protein components present in both the plasma membrane and the cytosol, but the catalytic and regulatory factors are not fully defined. We have characterized the effect of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) on the subcellular requirements for reconstitution of PLD activity, using a cell-free system from U937 human promonocytic leucocytes. Incubation of permeabilized cells with 100 microM GTP[S] resulted in a membrane-localized PLD activity which was independent of added cytosol. The PLD activity of membranes from GTP[S]-treated cells was 7-fold greater than the basal activity of control membranes, and could be further augmented by the addition of ATP. This was the first demonstration of a stable agonist-regulated PLD activity in membranes from phagocytic leucocytes which was quantitatively comparable with that seen in a fully reconstituted system. Cytosol from GTP[S]-treated cells had a decreased capacity to support PLD activation, consistent with GTP[S]-induced depletion of a factor essential for reconstitution of PLD activity. Incubation of isolated membrane and cytosol with GTP[S] also resulted in a cytosol-independent PLD activity in the re-isolated membranes. The effect of GTP[S] could be mimicked by guanosine 5'-[beta gamma-imido]triphosphate, but not by aluminium fluoride, consistent with the involvement of a low-molecular-mass GTP-binding protein(s). Incubation of isolated subcellular fractions with GTP[S], followed by removal of unbound nucleotide, suggested that at least one of the GTP-binding proteins involved in the membrane localization of PLD activity was itself present in the membrane fraction. These data were consistent with a model in which activation of GTP-binding protein(s) resulted in the stable assembly of an active PLD signalling complex at the membrane surface.

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.