Protein kinase C alpha mediates phospholipase D activation by nucleotides and phorbol ester in Madin-Darby canine kidney cells. Stimulation of phospholipase D is independent of activation of polyphosphoinositide-specific phospholipase C and phospholipase A2

Regulation of angiotensin II-induced neuromodulation by MARCKS in brain neurons

Angiotensin II (Ang II) exerts chronic stimulatory actions on tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), and the norepinephrine transporter (NET), in part, by influencing the transcription of their genes. These neuromodulatory actions of Ang II involve Ras-Raf-MAP kinase signal transduction pathways (Lu, D., H. Yang, and M.K. Raizada. 1997. J. Cell Biol. 135:1609-1617). In this study, we present evidence to demonstrate participation of another signaling pathway in these neuronal actions of Ang II. It involves activation of protein kinase C (PKC)beta subtype and phosphorylation and redistribution of myristoylated alanine-rich C kinase substrate (MARCKS) in neurites. Ang II caused a dramatic redistribution of MARCKS from neuronal varicosities to neurites. This was accompanied by a time-dependent stimulation of its phosphorylation, that was mediated by the angiotensin type 1 receptor subtype (AT1). Incubation of neurons with PKCbeta subtype specific antisense oligonucleotide (AON) significantly attenuated both redistribution and phosphorylation of MARCKS. Furthermore, depletion of MARCKS by MARCKS-AON treatment of neurons resulted in a significant decrease in Ang II-stimulated accumulation of TH and DbetaH immunoreactivities and [3H]NE uptake activity in synaptosomes. In contrast, mRNA levels of TH, DbetaH, and NET were not influenced by MARKS-AON treatment. MARCKS pep148-165, which contains PKC phosphorylation sites, inhibited Ang II stimulation of MARCKS phosphorylation and reduced the amount of TH, DbetaH, and [3H]NE uptake in neuronal synaptosomes. These observations demonstrate that phosphorylation of MARCKS by PKCbeta and its redistribution from varicosities to neurites is important in Ang II-induced synaptic accumulation of TH, DbetaH, and NE. They suggest that a coordinated stimulation of transcription of TH, DbetaH, and NET, mediated by Ras-Raf-MAP kinase followed by their transport mediated by PKCbeta-MARCKS pathway are key in persistent stimulation of Ang II's neuromodulatory actions.

Phospholipase D: a novel major player in signal transduction

The role of the mammalian phospholipase D (PLD) in the control of key cellular responses has been recognised for a long time, but only recently have there been the reagents to properly study this very important enzyme in the signalling pathways, linking cell agonists with intracellular targets. With the recent cloning of PLD isoenzymes, their association with low-molecular-weight G proteins, protein kinase C and tyrosine kinases, the availability of antibodies and an understanding of the role of PLD product, phosphatidic acid (PA), in cell physiology, the field is gaining momentum. In this review, we will explore the molecular properties of mammalian PLD and its gene(s), the complexity of this enzyme regulation and the myriad physiological roles for PLD and PA and related metabolic products, with particular emphasis on a role in the activation of NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.

Phorbol ester stimulation of phosphatidylcholine synthesis requires expression of both protein kinase C-alpha and phospholipase D

The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates both the synthesis and phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho). Here, attached and suspended NIH 3T3 fibroblasts as well as variants of the MCF-7 human breast carcinoma cell line expressing PKC-alpha and a PtdCho-specific PLD activity at widely different levels were used to determine the possible role of PKC-alpha, PtdCho hydrolysis, and choline uptake in the mediation of PMA effect on PtdCho synthesis. In wild-type MCF-7 cells, which express both PKC-alpha and PLD activities at very low levels, PMA had little effects on the uptake or incorporation [14C]choline into PtdCho. In multidrug resistant MCF-7/MDR1 cells, which highly express PKC-alpha but lack the PtdCho-specific PLD activity, 100-nM PMA had relatively small stimulatory effects on the uptake of [14C]choline (approximately 1.5-fold) and [14C]PtdCho synthesis (1.5- to 2-fold). In NIH 3T3 fibroblasts and MCF-7/PKC-alpha cells, both expressing PKC-alpha and PLD activities at high levels, 10-100-nM PMA enhanced [14C]choline uptake only slightly (1.7- to 2.2-fold), while it had much greater (approximately 4-9-fold) stimulatory effects on PtdCho synthesis. PMA significantly enhanced the formation of phosphatidic acid (PtdOH) in MCF-7/PKC-alpha cells (2.8-fold increase), but not in MCF-7/MDR1 cells (1.4-fold increase), while in both cell lines it had only small (1.3-1.5-fold) stimulatory effects on 1,2-diacylglycerol (1, 2-DAG) formation. In suspended NIH 3T3 cells, 200-300-mM ethanol blocked the stimulatory effect of PMA on PtdOH formation without affecting PtdCho synthesis indicating that neither PtdOH nor 1,2-DAG derived from it is a mediator of PMA effect on PtdCho synthesis. In attached NIH 3T3 cells, dimethylbenz[a]anthracene enhanced phosphocholine formation and, thus, choline uptake without increasing PtdCho synthesis or modifying the effect of PMA. While the results indicate that the stimulatory effect of PMA on PtdCho synthesis requires the expression of both PKC-alpha and a PtdCho-specific PLD, they do not support a role for 1,2-DAG, PtdOH or choline in the mediation of PMA effect.

Involvement of phosphatidate phosphohydrolase in arachidonic acid mobilization in human amnionic WISH cells

Prostaglandins are known to play a central role in the initiation of labor in humans, and amnionic cells constitute a major source of these compounds. Prostaglandin synthesis and release by amnion cells in response to hormones and ligands takes place after a characteristic 4-5 h lag. However, we report herein that free arachidonic acid (AA), the metabolic precursor of prostaglandins, can be induced at much shorter times (1 h) in human amnionic WISH cells by phorbol 12-myristate 13-acetate (PMA) through activation of protein kinase Calpha (PKCalpha). WISH cells were found to possess both cytosolic group IV phospholipase A2 (cPLA2) and Group VI Ca2+-independent phospholipase A2 (iPLA2). Of these, the cPLA2 was found to be the likely mediator of AA mobilization in PMA-activated WISH cells. PMA also activates phospholipase D (PLD) in these cells and ethanol, a compound that inhibits PLD-mediated phosphatidic acid (PA) formation, blocked AA release. Moreover, prevention of PA dephosphorylation by the PA phosphohydrolase inhibitors propranolol and bromoenol lactone, resulted in inhibition of AA release by PMA-treated WISH cells. Collectively, these data suggest that activation of cPLA2 and attendant AA release by phorbol esters in WISH cells requires prior generation of DAG by phosphatidate phosphohydrolase.

RANTES Activation of Phospholipase D in Jurkat T Cells: Requirement of GTP-Binding Proteins ARF and RhoA

The chemokine RANTES is a potent agonist of T cell activation. In an investigation of signal-transduction events activated by this chemokine, we have shown that RANTES stimulates dose-dependent phospholipase D (PLD) activity in Jurkat cells. Equilibrium-binding analyses using 125I-labeled RANTES indicated the presence of a receptor for RANTES on these cells, which has a Kd of 0.1 nM, is expressed at approximately 600 sites per cell, and a binding specificity that was not comparable with that of any of the known chemokine receptors, since 125I-labeled RANTES was displaced by macrophage-inflammatory protein-1β (but not macrophage-inflammatory protein-1α), monocyte-chemotactic protein-1 (MCP-1), MCP-3, MCP-4, and eotaxin. RANTES-induced PLD activation was augmented by GTPγS, but not GDPβS, and inhibited by the protein kinase C inhibitor bisindolylmaleimide, as well as the fungal metabolite brefeldin A, and C3 exoenzyme (Clostridium botulinum), implicating the activation of RhoA. RANTES also induced GTP-GDP exchange of immunoprecipitated RhoA. RANTES-stimulated PLD activity was dependent on an ADP-ribosylation factor(s), as assessed by inhibition studies using a synthetic inhibitory peptide of the N-terminal 16 amino acids of ADP-ribosylation factor 1. These studies indicate the potential existence of a novel receptor-mediated mechanism for activation of T cells by the chemokine RANTES.

Involvement of phospholipase D and protein kinase C in phorbol ester and fatty acid stimulated turnover of phosphatidylcholine and phosphatidylethanolamine in neural cells

Hydrolysis of phosphatidylcholine (PtdCho) can provide lipid second messengers involved in sustained signal transduction. Four neural-derived cell lines (C6 rat glioma; N1E-115 mouse and SK-N-MC and SK-N-SH human neuroblastoma) express different protein kinase C (PKC) isoforms and differentially respond to 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA)-stimulation of PtdCho synthesis. We examined involvement of PLD and PKC in the hydrolysis and resynthesis of PtdCho and phosphatidylethanolamine stimulated by beta-TPA, bryostatin (a non-phorbol PKC activator) and oleic acid (18:1n-9) in the four cell lines. beta-TPA or bryostatin produced similar enhancement of [3H]Cho incorporation, loss of stimulated synthesis after down regulation of PKC, and activation of PLD. In C6 cells, staurosporine (STS) and bis-indolylmaleimide (BIM) only partially inhibited basal and beta-TPA-stimulated PLD activity measured as choline or ethanolamine release; phosphatidylbutanol formation after prelabeling with [9,10-3H]18:1n-9, [9,10-3H]myristic acid (14:0), [1-14C]eicosapentaenoic acid (20:5n-3) or 1-O-[alkyl-1', 2-3H]-sn-glyceryl-3-phosphorylcholine gave similar results. STS at >200 nM activated PLD in the presence or absence of beta-TPA. In SK-N-SH cells where PtdCho synthesis was stimulated by beta-TPA or bryostatin, no effect of these agents on PLD was observed. 18:1n-9 stimulated PtdCho synthesis and, to a lesser extent, hydrolysis by PLD both with and without beta-TPA present. Fatty acids had no effect on PKC activities and down regulation of PKC with beta-TPA enhanced fatty acid stimulation of PtdCho synthesis. Thus, activation of PLD hydrolysis preceding resynthesis is involved in the stimulatory effects of beta-TPA on PtdCho synthesis in some but not all of these neural derived cells. Further, PLD hydrolysis of PtdCho and PtdEtn appear to have differing aspects of regulation. Fatty acid regulation of PtdCho synthesis occurs independent of PKC activation. Accordingly, regulation of membrane phospholipid degradation and resynthesis in association with lipid second messenger generation can involve a complex interplay of PLD, PKC, and fatty acids. (c) 1998 Elsevier Science B.V.

Stimulation of phospholipase D via alpha1-adrenergic receptors in Madin-Darby canine kidney cells is independent of PKCalpha and -epsilon activation

We have demonstrated previously that protein kinase Calpha (PKCalpha) plays a key role in regulating phospholipase D (PLD) activation by nucleotides and the phorbol ester phorbol-12-myristate-13-acetate in Madin-Darby canine kidney (MDCK-D1) cells. In the current work, we investigated PLD activation in MDCK-D1 cells triggered by the adrenergic receptor agonist epinephrine and its mechanism of activation. Epinephrine, acting through the alpha1-adrenergic receptor subtype, promoted transient translocation of PKCalpha and more prolonged translocation of PKCepsilon to the membrane fraction, indicating activation of these two isoforms. In addition, epinephrine promoted activation of PLD, as shown by a sustained accumulation of phosphatidylethanol. All of these events were blocked by pretreatment of cells with the alpha1-adrenergic antagonist prazosin. D609, an inhibitor of phosphatidylcholine hydrolysis, blocked translocation of PKCalpha and PKCepsilon but did not inhibit PLD activation. Unlike results with PMA, or with the P2 purinergic receptor agonist ATP, epinephrine-stimulated PLD activity was not inhibited in MDCK-D1 cells in which PKCalpha expression is attenuated by an antisense cDNA construct or in cells in which PKC activity was inhibited by 1 microM GF 109203X. However, PLD activation by epinephrine was abolished by concomitant incubation of cells with the calcium chelator EGTA. These data, together with previous results, are consistent with the hypothesis that in MDCK-D1 cells, epinephrine acting on alpha1-adrenergic receptors, promotes a rapid increase in cytosolic Ca2+ that promotes activation of PLD through an as-yet poorly defined mechanism. The data demonstrate that different types of G protein-linked receptors that activate PLD can mediate this activation in either a PKC activation-dependent or -independent manner within a single cell type.

Receptor-mediated endocytosis in kidney proximal tubules: recent advances and hypothesis

Preparation of kidney proximal tubules in suspension allows the study of receptor-mediated endocytosis, protein reabsorption, and traffic of endosomal vesicles. The study of tubular protein transport in vitro coupled with that of the function of endosomal preparation offers a unique opportunity to investigate a receptor-mediated endocytosis pathway under physiological and pathological conditions. We assume that receptor-mediated endocytosis of albumin in kidney proximal tubules in situ and in vitro can be regulated, on the one hand, by the components of the acidification machinery (V-type H+-ATPase, Cl(-)-channel and Na+/H+-exchanger), giving rise to formation and dissipation of a proton gradient in endosomal vesicles, and, on the other hand, by small GTPases of the ADP-ribosylation factor (Arf)-family. In this paper we thus analyze the recent advances of the studies of cellular and molecular mechanisms underlying the identification, localization, and function of the acidification machinery (V-type H+-ATPase, Cl(-)-channel) as well as Arf-family small GTPases and phospholipase D in the endocytotic pathway of kidney proximal tubules. Also, we explore the possible functional interaction between the acidification machinery and Arf-family small GTPases. Finally, we propose the hypothesis of the regulation of translocation of Arf-family small GTPases by an endosomal acidification process and its role during receptor-mediated endocytosis in kidney proximal tubules. The results of this study will not only enhance our understanding of the receptor-mediated endocytosis pathway in kidney proximal tubules under physiological conditions but will also have important implications with respect to the functional consequences under some pathological circumstances. Furthermore, it may suggest novel targets and approaches in the prevention and treatment of various diseases (cystic fibrosis, Dent's disease, diabetes and autosomal dominant polycystic kidney disease).

Bradykinin-stimulated arachidonic acid release from MDCK cells is not protein kinase C dependent

Bradykinin (BK)-induced release of arachidonic acid (AA) from Madin-Darby canine kidney (MDCK) D1 cells was investigated. Phorbol 12-myristate 13-acetate (PMA) caused a synergistic increase in BK- and A-23187-induced release of AA but alone had no effect on this release. Inhibition of protein kinase C (PKC) with bisindolmaleimide I (BIS) abolished the synergistic effects of PMA but did not affect AA release caused by BK or A-23187 alone. Downregulation of PKC with 100 nM PMA resulted in a reduction of AA release induced by BK or A-23187 addition, which corresponded to a decrease in cytoplasmic phospholipase A2 (cPLA2) activity as measured in cell extracts. Although Western blotting revealed no differences in cPLA2 expression as a result of PMA treatment, phosphorylation of the enzyme, as assessed by phosphoserine content, was significantly reduced in PKC-depleted cells. These results imply that, with PKC downregulation, subsequent BK stimulation results in a Ca(2+)-dependent translocation of a less phosphorylated, less active form of cPLA2. Any stimulation of PKC by BK addition did not appear as a significant event in onset responses leading to AA release. On the other hand, inhibition of the mitogen-activated protein kinase (MAPK) cascade with the MAPK kinase inhibitor, PD-98059, significantly decreased BK-induced release of AA, a finding that, with our other results, points to the existence of a PKC-independent route for stimulation of MAPK and the propagation of onset responses.

Expression of protein kinase C-beta promotes the stimulatory effect of phorbol ester on phosphatidylethanolamine synthesis

Stimulation of phosphatidylethanolamine (PtdEtn) synthesis by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) has reportedly been found only in hepatocytes expressing the alpha-, betaII-, epsilon-, and zeta-PKC isozymes. In contrast, stimulation of phosphatidylcholine synthesis by PKC activators, known to be mediated by PKC-alpha, is widespread in mammalian cells. In this work, various cell lines exhibiting characteristic differences in their PKC systems were used to determine the role of specific PKC isozymes in the mediation of PMA effect on PtdEtn synthesis. In NIH 3T3 fibroblasts, which express high levels of PKC-alpha but none of the beta (betaI or betaII) isoforms, PMA did not stimulate PtEtn synthesis. In contrast, in Rat-6 fibroblasts overexpressing PKC-betaI, 10-100 nM PMA considerably (1.7- to 2.6-fold) enhanced PtdEtn synthesis. In wild-type or multidrug resistant MCF-7 human breast carcinoma cells, which express PKC-alpha and PKC-betaII (to varying extents) but not PKC-betaI, PMA had only small or no effects on PtdEtn synthesis. In contrast, in MCF-7 cells overexpressing PKC-alpha, and as a consequence also expressing the betaI- and betaII-PKC isoforms, PMA effectively stimulated the synthesis of PtdEtn. Finally, in HL60 human leukemia cells, which contains PKC-betaII as the major PKC isoform, PMA again stimulated PtdEtn synthesis. The results establish that while stimulation of PtdEtn synthesis by PMA occurs only in selected cell lines, this phenomenon is not restricted to hepatocytes. Furthermore, the data indicate that expression of either PKC-betaI or PKC-betaII, but not PKC-alpha, correlates with the effect of PMA on PtdEtn synthesis. Overall, these observations strongly suggest that regulation of PtdEtn and PtdCho synthesis by PMA involves separate PKC isozymes, i.e., PKC-beta and PKC-alpha, respectively.

Antisense oligodeoxynucleotide to PKC-delta blocks alpha 1-adrenergic activation of Na-K-2Cl cotransport

A role for protein kinase C (PKC)-delta and -zeta isotypes in alpha 1-adrenergic regulation of human tracheal epithelial Na-K-2Cl cotransport was studied with the use of isotype-specific PKC inhibitors and antisense oligodeoxy-nucleotides to PKC-delta or -zeta mRNA. Rottlerin, a PKC-delta inhibitor, blocked 72% of basolateral-to-apical, bumetanide-sensitive 36Cl flux in nystatin-permeabilized cell monolayers stimulated with methoxamine, an alpha 1-adrenergic agonist, with a 50% inhibitory concentration of 2.3 microM. Methoxamine increased PKC activity in cytosol and a particulate fraction; the response was insensitive to PKC-alpha and -beta II isotype-specific inhibitors, but was blocked by general PKC inhibitors and rottlerin. Rottlerin also inhibited methoxamine-induced PKC activity in immune complexes of PKC-delta, but not PKC-zeta. At the subcellular level, methoxamine selectively elevated cytosolic PKC-delta activity and particulate PKC-zeta activity. Pretreatment of cell monolayers with antisense oligodeoxynucleotide to PKC-delta for 48 h reduced the amount of whole cell and cytosolic PKC-delta, diminished whole cell and cytosolic PKC-delta activity, and blocked methoxamine-stimulated Na-K-2Cl cotransport. Sense oligodeoxynucleotide to PKC-delta and antisense oligodeoxynucleotide to PKC-zeta did not alter methoxamine-induced cotransport activity. These results demonstrate the selective activation of Na-K-2Cl cotransport by cytosolic PKC-delta.

Alkyl lysophospholipids inhibit phorbol ester-stimulated phospholipase D activity and DNA synthesis in fibroblasts

The antineoplastic alkyl lysophospholipids (ALP) 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) and 1-S-hexadecylthio-2-methoxymethyl-2-deoxy-rac-glycero-3-phosphocho line (BM41.440) were found to alter phospholipase D (PLD)-mediated phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) hydrolysis in NIH 3T3 fibroblasts. After a shorter (50 min) treatment, 2.5-7.5 microg/ml concentrations of ALP stimulated PtdCho, but not PtdEtn, hydrolysis 2-4-fold. At the same time, 7.5-25 microg/ml concentrations of ALP significantly inhibited the larger (5.8-6.5-fold) stimulatory effects of phorbol 12-myristate 13-acetate (PMA) on both PtdCho and PtdEtn hydrolysis. When a brief (30 min) exposure of cells to 1-2.5 microg/ml concentrations of BM 41.440 was followed by incubation of washed cells for 3-16 h prior to the assay of PLD activity or DNA synthesis, the treated cells exhibited no increased PtdCho hydrolysis, while their responses to the stimulatory PMA effects on both PLD activity and DNA synthesis were strongly reduced. The results suggest that the PLD and protein kinase C systems may be important cellular targets of ALP actions.

Protection against methoxyacetic-acid-induced spermatocyte apoptosis with calcium channel blockers in cultured rat seminiferous tubules: possible mechanisms

A calcium-mediated mechanism underlying spermatocyte apoptosis induced by 2-methoxyethanol (2-ME) has been previously proposed. This hypothesis was tested in vitro in the present study using cultured juvenile (25 days old) and adult rat seminiferous tubules (JRST and ARST, respectively) with methoxyacetic acid (MAA, the active metabolite of 2-ME). In JRST, spermatocyte degeneration was morphologically obvious 19 hr after a 5-hr exposure to 5 mM MAA. The lesion was unaffected by the presence or absence of extratubular Ca2+. However, MAA-induced cell death was significantly prevented by cotreatment with the dihydropyridines (DHP) nifedipine (50 microM) and nicardipine (20 microM), as well as verapamil (50 microM) and TMB-8 (50 microM), all of which are able to inhibit calcium movement through plasma membranes. However, neither ryanodine, dantrolene, nor cyclosporin A and ruthenium red, which inhibit Ca2+ mobilization from intracellular stores (endoplasmic reticulum and mitochondria), affected the MAA-induced cell death. Inhibition of calcium mobilization through IP3-sensitive pathways by blocking the product of IP3 with manoalide, neomycin, and U73122 did not block the MAA-induced lesion. The protective effects of 50 microM nifedipine and 50 microM TMB-8 were also observed in ARSTs treated with 10 mM MAA for 5 hr. However, when rat testicular sections were immunohistochemically stained with monoclonal antibodies specific for the alpha 1 (the DHP receptor) or the alpha 2 subunits of DHP-sensitive calcium channels, no positive staining was found. Finally, in an attempt to see whether the intracellular free calcium concentrations ([Ca2+]i) in germ cells were increased after the MAA treatment, intact seminiferous tubules were loaded with indo-1 and were measured using laser-scanning confocal microscopy. No detectable increase in the signal in MA A-sensitive spermatocytes was observed, while a 34-54% increase in the signal could be detected in the same cell types when tubules were exposed to 10 microM of the calcium ionophore 4-bromo-A23187 for 5 min. Collectively, these data suggest that the protective effect of calcium channel blockers against the MAA-induced spermatocyte apoptosis is probably not through their blocking effect on DHP-sensitive calcium channels. We postulate alternate mechanisms based on stabilization of cells membranes, or interactions with calmodulin or protein kinase C.

Metal ion stimulation of phospholipase D-like activity of isolated rat intestinal mitochondria

Presence of phospholipase D-like (PLD) activity in the intestinal mitochondria was identified using endogenous phospholipids as substrate. The enzyme had a pH optimum of 6.5, did not show trans-phosphatidylation activity in the presence of ethanol or butanol, and the product formed was phosphatidic acid (PA). This was confirmed by separation of reaction products by high-performance liquid chromatography and analysis of composition of the PA formed which gave phosphate/fatty acid ratio of 1:2 PLD-like activity was further confirmed by the formation of ethanolamine and choline as products of enzyme action. This activity was stimulated by various metal ions; when stimulated by Mg2+ and Ba2+, it hydrolyzed both phosphatidylcholine and phosphatidylethanolamine, and when stimulated by Ca2+, it preferentially hydrolyzed phosphatidylethanolamine. There was no requirement for sodium oleate for the PLD-like activity in mitochondria. These results suggest that intestinal mitochondria have an active PLD-like enzyme which differs in certain properties from phospholipase D from other tissues.

Inhibition of GTP gamma S-dependent phospholipase D and Rho membrane association by calphostin is independent of protein kinase C catalytic activity

We studied the relationships between the activation of phospholipase D (PLD) by guanine nucleotides and phorbol esters in permeabilized U937 promonocytes and in solubilized extracts prepared from U937 cell membranes. Treatment of permeabilized cells with phorbol myristate acetate (PMA) strongly potentiated GTP gamma S-dependent PLD activity at free Ca2+ < 100 nM. In the absence of GTP gamma S, PMA stimulated only minor PLD activity. This suggested synergistic interaction between regulatory G-proteins and a protein kinase C (PKC) family kinase. The potential role of PKC was evaluated by testing two mechanistically distinct PKC inhibitors, bisindolylmaleimide (BIM) and calphostin. BIM inhibits PKC enzymes via competition with ATP for binding to the catalytic domain, while calphostin competes with PMA or diglyceride for binding to the regulatory domain. The ability of PMA to potentiate the GTP gamma S-dependent PLD was not inhibited by BIM. In contrast, calphostin strongly inhibited the GTP gamma S-dependent PLD activity, both in the presence and absence of PMA as a potentiating agent. Calphostin also produced complete inhibition of a GTP gamma S-dependent PLD activity, present in solubilized membrane extracts, which was assayed using phospholipid vesicles of defined composition. Treatment of reconstituted membrane/cytosol mixtures with calphostin also produced complete inhibition of the GTP gamma S-induced translocation of Rho A from cytosol to membrane. In contrast to its effects on the U937 cell PLD, calphostin did not inhibit the activity of purified PLD from cabbage. These results suggest that the assembly of active RhoA/PLD signaling complexes on membranes involves a phorbol ester/calphostin-binding protein, but is not dependent on PKC-type catalytic activity.

Regulation of phospholipase D in L6 skeletal muscle myoblasts. Role of protein kinase c and relationship to protein synthesis

The addition of vasopressin or 12-O-tetradecanoylphorbol-13-acetate (TPA) to prelabeled L6 myoblasts elicited increases in [14C]ethanolamine release, suggesting the activation of phospholipase D activity or activities. While the effects of both agonists on intracellular release were rapid and transient, when extracellular release of [14C]ethanolamine was measured, the effect of vasopressin was again rapid and transient, whereas that of TPA was delayed but sustained. Effects of both agonists on intra- and extracellular release were inhibited by the protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC down-regulation by preincubation with TPA. The formation of phosphatidylbutanol elicited by vasopressin and TPA mirrored their effects on extracellular [14C]ethanolamine release in that the former was transient, whereas the latter was sustained. Responses to both agonists were abolished by PKC down-regulation. When protein synthesis was examined, the stimulation of translation by TPA and transcription by vasopressin were inhibited by Ro-31-8220. In contrast, down-regulation of PKC inhibited the synthesis response to TPA but not vasopressin. Furthermore, following down-regulation, the effect of vasopressin was still blocked by the PKC inhibitors, Ro-31-8220 and bisindolylmaleimide. Analysis of PKC isoforms in L6 cells showed the presence of alpha, epsilon, delta, mu, iota, and zeta. Down-regulation removed both cytosolic (alpha) and membrane-bound (epsilon and delta) isoforms. Thus, the elevation of phospholipase D activity or activities induced by both TPA and vasopressin and the stimulation of translation by TPA involves PKC-alpha, -epsilon, and/or -delta. In contrast, the increase in transcription elicited by vasopressin involves mu, iota, and/or zeta. Hence, although phospholipase D may be linked to increases in translation elicited by TPA, it is not involved in the stimulation of transcription by vasopressin.

Dependence of intracellular signaling and neurosecretion on phospholipase D activation in immortalized gonadotropin-releasing hormone neurons

The excitability of gonadotropin-releasing hormone (GnRH) neurons is essential for episodic neuropeptide release, but the mechanism by which electrical activity controls GnRH secretion is not well characterized. The role of phospholipase D (PLD) in mediating the activity-dependent secretory pathway was investigated in immortalized GT1 neurons, which both secrete GnRH and express GnRH receptors. Activation of these Ca2+-mobilizing receptors was associated with transient hyperpolarization of GT1 cells, followed by sustained firing of action potentials. This was accompanied by an increase in PLD activity, as indicated by elevated phosphatidylethanol (PEt) production. GnRH-induced PEt production was reduced by inhibition of phospholipase C-dependent phosphoinositide hydrolysis by U73122 and neomycin, suggesting that signaling from phospholipase C led to activation of PLD. The intermediate role of protein kinase C (PKC) in this process was indicated by the ability of phorbol 12-myristate 13-acetate to induce time- and dose-dependent increases in PEt and diacylglycerol, but not inositol trisphosphate, and by reduction of GnRH-induced PEt accumulation in PKC-depleted cells. Consistent with the role of action potential-driven Ca2+ entry in this process, agonist-induced PLD activity was also reduced by nifedipine and low extracellular Ca2+. Inhibition of the PLD pathway by ethanol and propranolol reduced diacylglycerol production and caused a concomitant fall in GnRH release. These data indicate that voltage-gated Ca2+ entry and PKC act in an independent but cooperative manner to regulate PLD activity, which contributes to the secretory response in GT1 cells. Thus, the electrical activity of the GnRH-secreting neuron participates in the functional coupling between GnRH receptors and PLD pathway.

Antisense oligonucleotides targeting human protein kinase C-alpha inhibit phorbol ester-induced reduction of bradykinin-evoked calcium mobilization in A549 cells

Regulation of the bradykinin-evoked increase in intracellular Ca2+ concentration by protein kinase C (PKC)-alpha was investigated in A549 human lung carcinoma cells. Bradykinin, a potent and selective kinin B2 receptor agonist, induces calcium mobilization in a concentration-dependent fashion in this cell line. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent activator of PKC, is known to reduce the amplitude of agonist-induced calcium mobilization in various cell lines. Because PKC-alpha is a major PKC isozyme in A549 cells, we investigated whether this isozyme plays a role in this process. A 20-mer phosphorothioate oligonucleotide targeting the 3'-untranslated region of the human PKC-alpha mRNA, which contains 2'-methoxyethyl modifications incorporated into the 5' and 3' segments of the oligonucleotide, was used to assess the putative role of PKC-alpha in the receptor regulation. ISIS 9606 reduced PKC-alpha mRNA for > or = 72 hr after the initial treatment and the reduction was concentration dependent, whereas the mismatch control, ISIS 13009, had no effect. Concentrations of ISIS 9606 of 150 nM specifically reduced the level of immunoreactive PKC-alpha protein by 66.3 +/- 2.5% at 72 hr after treatment, without an effect on immunoreactive PKC-delta protein. This reduction in PKC-alpha was sufficient to inhibit the reduction of bradykinin-induced calcium mobilization by TPA. This finding is corroborated by the use of staurosporine, a nonselective PKC inhibitor, that prevented the effect of TPA. These results suggest that PKC-alpha is involved in kinin B2 receptor regulation by phorbol esters in A549 cells.

Granulocyte-Macrophage Colony-Stimulating Factor–Activated Signaling Pathways in Human Neutrophils. I. Tyrosine Phosphorylation-Dependent Stimulation of Phosphatidylinositol 3-Kinase and Inhibition by Phorbol Esters

Phosphatidylinositol 3-kinase (PI3-kinase) is a cytosolic enzyme that plays key roles in mediating signaling through many receptors. The heterodimeric form of PI3-kinase is made up of a regulatory subunit, p85, and a catalytic subunit, p110. Although granulocyte-macrophage colony-stimulating factor (GM-CSF) has been shown to activate PI3-kinase, the mechanisms by which this activation is mediated and regulated are incompletely understood. Here we show that treatment of human neutrophils with GM-CSF induced both time- and concentration-dependent increases in the level of tyrosine phosphorylation of p85. The ability of GM-CSF to activate PI3-kinase was abolished by pretreating the cells with erbstatin, a tyrosine kinase inhibitor. The simultaneous treatment of the cells with GM-CSF and phorbol esters such as phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu) significantly inhibited both the tyrosine phosphorylation of p85 and the activation of PI3-kinase. The inhibitory effects of phorbol esters were not induced by their inactive analogues and they were selective to the stimulation of tyrosine phosphorylation of p85 since phorbol esters did not alter the enhancement of the pattern of tyrosine phosphorylation of other cellular proteins, including that of Jak2 induced by GM-CSF. However, PMA significantly inhibited the in situ tyrosine phosphorylation and the activation of lyn observed in response to GM-CSF. The results suggest that the activation of PI3-kinase by GM-CSF is mediated by the tyrosine phosphorylation of p85 and that this activation is downregulated by PKC possibly via the inhibition of lyn.

Cell signalling through guanine-nucleotide-binding regulatory proteins (G proteins) and phospholipases

Phospholipases are important enzymes in cell signal transduction since they hydrolyze membrane phospholipids to generate signalling molecules. Heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins) play a major role in their regulation by a variety of agonists that activate receptors with seven membrane-spanning domains. Phospholipases of the C type, which hydrolyze inositol phospholipids to yield inositol trisphosphate and diacylglycerol, are regulated by the alpha and betagamma subunits of certain heterotrimeric G proteins as well as by receptor-associated and non-receptor-associated tyrosine kinases. Phospholipases of the D type, which hydrolyze phosphatidylcholine to phosphatidic acid, are regulated by members of the ADP-ribosylation factor and Rho subfamilies of small G proteins, and by protein kinase C and other factors. This review presents recent information concerning the molecular details of G protein regulation of these phospholipases.

Phospholipase D hydrolysis of plasmalogen and diacyl ethanolamine phosphoglycerides by protein kinase C dependent and independent mechanisms

Ethanolamine phosphoglycerides (EPG) are potential sources of lipid second messengers in signal transduction pathways. We investigated EPG turnover, including both 1-alkenyl-2-acyl- (plasmalogen) and diacyl-classes, in response to stimulation of protein kinase C (PKC) by phorbol ester (4 beta-12-O-tetradecanoylphorbol-13-acetate (TPA)) in cultured C6 rat glioma cells. Release of ethanolamine to the medium from EPG prelabeled with [14C]ethanolamine indicated that initial (< 60 min) TPA-stimulated hydrolysis of EPG was predominantly by phospholipase D (PLD). Effects of TPA on PLD activity specifically with EPG was confirmed using trans-phosphatidylation by incubating cells prelabeled with [14C]eicosapentaenoic acid (20:5n-3) with 100 nM TPA and 1% butanol. Analysis of acid-labile phosphatidylbutanol and remaining EPG showed utilization of both plasmalogen and non-plasmalogen EPG. Staurosporine (STS) inhibited PKC at 200-500 nM but stimulated PLD activity 2-fold at > or = 1 microM. However, STS did not eliminate all TPA-stimulated PLD activity, even when PKC was > 98% inhibited. Bis-indolylmaleimide (BIM) fully inhibited PKC activity but had no independent effects on PLD and did not completely inhibit TPA- or bryostatin-stimulated PLD activity. Down-regulation of PKC by chronic exposure to TPA eliminated stimulation of PLD by TPA but not by STS. Thus, PLD hydrolysis of both plasmalogen and diacyl-EPG is a source of potential lipid second messengers in C6 glioma cells. PLD is stimulated by activation of PKC and by PKC-independent action of STS. Further, the possibility that TPA may also elicit responses through a mechanism independent of PKC activity is suggested.

P2-purinoceptors utilize multiple signalling pathways in MDCK-D1 cells

1. Madin-Darby canine kidney (MDCK) cells are a widely used model system for the study of epithelial cells. We have utilized a clonal variant, MDCK-D1, to examine signalling by P2-purinoceptors. 2. Several lines of evidence that lead us to conclude that MDCK-D1 cells co-express P2a- and P2y-purinoceptors and that both subtypes are linked to the release of arachidonic acid and metabolites (AA) include: (a) relative potency of nucleotide analogues in promoting AA release; (b) blockade by the antagonist suramin of response to the P2Y-selective agonist, 2-methylthio ATP (2-MT-ATP), but not to the P2a-selective agonist, UTP; and (c) additivity of response to 2-MT-ATP and UTP. AA release is a consequence of activation of phospholipase A2 (PLA2), most likely the 85 kDa cytosolic PLA2. 3. Treatment of MDCK-D1 cells with ATP, but not UTP, increases inositol 1,4,5-trisphosphate formation while both UTP and ATP increase phosphatidylcholine hydrolysis, ATP, UTP, and 2-MT-ATP can also stimulate phospholipase D activity. 4. Purine nucleotides increase cellular cAMP levels in MDCK-D1 cells in a manner that depends, at least in part, on activation of cyclooxygenase, since cAMP generation stimulated by ATP or UTP is inhibited by treatment of cells with indomethacin. Because cyclooxygenase-derived PGE2 can bind to prostaglandin receptors and stimulate synthesis of cAMP, nucleotides may raise cAMP in an autocrine or paracrine fashion. 5. Taken together, these results indicate that MDCK-D1 cells co-express P2a and P2y-purinoceptors and that these receptors utilize several mechanisms to regulate cell function, including activation of multiple phospholipases and autocrine/paracrine action of products.

Protein kinase Calpha is a major mediator of the stimulatory effect of phorbol ester on phospholipase D-mediated hydrolysis of phosphatidylethanolamine

Stimulation of phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho) by phorbol 12-myristate 13-acetate (PMA) has been shown to be mediated by the alpha- and betaI-isoforms of protein kinase C (PKC). To determine the role of various PKC isozymes in the regulation of PLD-mediated phosphatidylethanolamine (PtdEtn) hydrolysis, MCF-7 human breast carcinoma cells overexpressing the alpha- and theta-isoforms, and R6 rat fibroblasts overexpressing the alpha-, betaI-, and epsilon-isoforms were used. In the vector control MCF-7 cells, which contain low levels of PKC-alpha, PMA (100 nM) had only small effects on the hydrolysis of PtdEtn (1.1-1.35-fold) and PtdCho (1.15-1.6-fold). Stable expression of PKC-alpha in MCF-7 cells, which was accompanied by increased levels of the betaI- and theta-isoforms as well, greatly enhanced both PMA-induced PLD-mediated formation of phosphatidylethanol (approximately 5-fold) and the hydrolysis of PtdEtn (2.5-2.9-fold) and PtdCho (5.5-7.2-fold). The effects of PMA on the hydrolysis of PtdEtn (and PtdCho) in MCF-7/PKC-alpha cells were significantly inhibited by 0.5-3 microM concentrations of Gö 6976, a selective inhibitor of the conventional PKC subfamily. Stable expression of PKC-alpha in R6 fibroblasts enhanced, at a shorter (10 min) incubation time, the effects of PMA on the hydrolysis of both PtdEtn and, to a lesser extent, PtdCho. In contrast, stable expression of PKC-betaI in R6 fibroblasts, which originally did not contain this enzyme, enhanced the effects of PMA only on PtdCho, but not PtdEtn, hydrolysis. Overexpression of either PKC-theta in MCF-7 cells or PKC-epsilon in R6 and NIH 3T3 fibroblasts had no detectable effects on PMA-induced hydrolysis of PtdEtn. Collectively, the results suggest that PKC-alpha has a major role in the mediation of phorbol ester action on PtdEtn hydrolysis, while PtdCho hydrolysis may be regulated by both the alpha and betaI isoforms.

Effects of phorbol ester on phospholipase D and mitogen-activated protein kinase activities in T-lymphocyte cell lines

The effects of phorbol 12-myristate 13-acetate (PMA) on the activities of phospholipase D (PLD3), mitogen-activated protein kinase (ERK), and c-Jun N-terminal kinase (JNK) were studied in Jurkat, a human T cell line, and EL4, a murine T-cell line. PMA treatment rapidly activated PLD in Jurkat, as detected either in intact or broken cells. In contrast, PMA did not stimulate PLD activity in EL4 cells. PLD activity was not detected in membranes prepared from EL4 cells. Jurkat, but not EL4, expresses a 120-kDa protein recognized by an anti-PLD antibody. In both Jurkat and EL4 cells, PMA caused activation of ERKs. Incubation of EL4 cells with bacterial PLD increased phosphatidic acid levels, but did not activate ERK. In both EL4 and Jurkat cells, co-stimulation with PMA and ionomycin stimulated JNK activity. These results show that activation of PLD is not required for activation of ERKs or JNKs by PMA in T-cell lines. Thus, while PLD activity is expressed in some T-cell lines, the role of this enzyme and its products in T-cell activation remain to be elucidated.

Role of protein kinase C alpha, Arf, and cytoplasmic calcium transients in phospholipase D activation by sodium fluoride in osteoblast-like cells

The effect of fluoride on phospholipase D (PLD) activation was studied using in vitro culture of Saos-2, MG-63 osteosarcoma cells, and normal osteoblast-like cells derived from human bone explants. Millimolar concentrations of NaF induced a significant accumulation of phosphatidylethanol (PEt) in Saos-2 cells but not in MG-63 and normal osteoblast-like cells. PLD activation was evident at 15 mM and concentration-dependent up to 50 mM. This stimulation was inhibited by deferoxamine, a chelator of Al3+, suggesting that PLD activation involves fluoride-sensitive G proteins. A good correlation was found between the levels of intracellular free Ca2+ and the activation of PLD. The time courses of the two responses were nearly identical. The ability of NaF to induce both responses was largely dependent on the presence of extracellular calcium. The calcium ionophore A23187 reproduced the effect of NaF, and this effect was antagonized by EGTA, suggesting that PLD activation was, at least in part, a calcium-regulated event. Phorbol 12-myristate 13-acetate (PMA) also stimulated PLD activity in human bone cells. Protein kinase C alpha (PKC alpha) and epsilon were expressed in Saos-2 cells. Acute pretreatment of cells with PMA reduced concomitantly the amounts of PKC alpha, but not of PKC epsilon, and the subsequent activation of PLD elicited by PKC activators. The PLD response to NaF was not attenuated but rather enhanced by down-regulation of PKC alpha. Therefore, PKC-alpha-induced PLD activation is unlikely to mediate the effect of NaF. Moreover, PMA and NaF showed a supraadditive effect on PLD activation in Saos-2 cells. This stimulation, in contrast to NaF alone, was not reduced by EGTA. Hence, mobilization of calcium by NaF cannot account for the enhanced PLD activation in response to PMA stimulation. Membrane Arf and RhoA contents were assessed by Western immunoblot analyses. Membranes derived from NaF-stimulated Saos-2 cells contained more Arf and RhoA when compared with membranes derived from control or PMA-stimulated cells. Translocation of the small GTPases was calcium-independent. We conclude that PLD activation by NaF in Saos-2 cells includes a fluoride-sensitive G protein, increases in the levels of intracellular calcium, and Arf/RhoA redistribution to membranes. The results also indicate that the NaF-induced Arf/RhoA translocation exerts in concert with PMA-activated PKC alpha a synergistic effect on the activation of PLD in Saos-2 cells.

Role of PLA2, PLC, and PLD in bradykinin-induced release of arachidonic acid in MDCK cells

The role of cytosolic phospholipase A2 (cPLA2), phosphatidylcholine-specific phospholipase C (PC-PLC) and phospholipase D (PLD) in the bradykinin (BK)-stimulated release of arachidonic acid (AA) was examined in Madin-Darby canine kidney (MDCK) cells. Release of AA, phosphorylcholine, choline, and phosphatidic acid (PA) or the transphosphatidylation product, phosphatidylethanol, was detected after 1 min of BK stimulation. A role for PC-PLC was confirmed with D609, which reduced BK-stimulated AA by 70%. Ethanol (EtOH), which blunts PA formation, diminished BK-stimulated AA release by 50%. Together, D609 and EtOH inhibited this release almost completely. Evidence indicated that diacylglycerol and PA can enhance PLA2 activity when added to cytosol extracts. The enzyme responsible for AA release was characterized as cPLA2, since PLA2 activity assayed in cell extracts was largely inhibited by an antibody to this enzyme. The membrane fraction PLA2 activity increased significantly in BK-stimulated cells. We conclude that BK signaling in MDCK cells is mediated by the lipid products of PC-PLC and PLD, increasing cPLA2 activity, possibly by causing perturbations in the bilayer structure of its substrate, by a direct effect on the enzyme or by activation of protein kinases such as protein kinase C.

Possible involvement of phospholipase D and protein kinase C in vascular growth induced by elevated glucose concentration

Hyperglycemia is believed to be a major cause of diabetic vascular complications. To elucidate the effect of hyperglycemia on vascular response, we studied hyperproliferation, hypertrophy, and the natriuretic peptide response of vascular smooth muscle cells under high-glucose conditions. We observed that cells cultured in high glucose (22.2 mmol/L) showed hyper-proliferation and hypertrophy and that natriuretic peptide receptor responses were suppressed compared with cells cultured in normal glucose (5.6 mmol/L). We also examined phospholipase D and protein kinase C activities and found that in high-glucose conditions such activities are higher than in cells cultured in normal glucose. The activation of phospholipase D was not prevented by coincubation with 1 mumol/L protein kinase C(19-36), a specific protein kinase C inhibitor, but the activation of protein kinase C was. Protein kinase C(19-36) also markedly attenuated vascular hyperproliferation and hypertrophy as well as glucose-induced suppression of natriuretic peptide receptor response. These results show that hyperglycemia may be linked to vascular hyperproliferation, hypertrophy, and a suppressed natriuretic peptide receptor response, which are caused by increased phospholipase D and protein kinase C activities.

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.

Phospholipase D-derived products in the regulation of 12-O-tetradecanoylphorbol-13-acetate-stimulated prostaglandin synthesis in madin-darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA) in the presence of ethanol synthesize phosphatidylethanol (PEt) instead of phosphatidic acid (PA) and diglyceride (DG). We have used ethanol to block the production of phospholipase D (PLD)-derived PA and DG (from PA hydrolysis) to study their role in signal transduction. In MDCK cells, TPA-stimulated prostaglandin E2 (PGE2) synthesis was inhibited by ethanol at concentrations which inhibit PA and DG formation. In addition, TPA elicited a prolonged increase in PGE2 synthesis that is dependent upon continuous activation of PLD. The TPA-stimulated translocation of protein kinase Calpha (PKCalpha) from cytosol to membrane was unaffected by ethanol. This suggests that PLD-derived products act downstream of PKC in TPA-stimulated prostaglandin synthesis. The calcium ionophore, A23187, did not activate PLD, and PGE2 synthesis in response to A23187 was unaffected by ethanol. TPA increased prostaglandin endoperoxide H synthase (PGHS) activity and increased the amount of immunodetectable prostaglandin endoperoxide H synthase 2 (PGHS-2). A23187 did not induce PGHS-2 and A23187-stimulated PGE2 synthesis appears to be due to the constitutively expressed PGHS-1. Blocking the formation of PLD-derived products, PA and DG, inhibited the induction of PGHS-2 by TPA. These results indicate that prolonged PGE2 synthesis in response to TPA is due to the continuous induction of PGHS-2, which is dependent upon PLD activation. In contrast, induction of PGHS-2 by epidermal growth factor was not affected by ethanol. Epidermal growth factor did not induce PKCalpha translocation nor activate PLD. Taken together, these data suggest that PLD-derived PA or DG act as second messengers in the induction of PGHS-2 by PKC-dependent pathways. The demonstration that inhibition of TPA-induced PA formation inhibits Raf-1 translocation in MDCK cells (Ghosh, S., Strum, J. C., Sciorra, V. A., Daniel, L. W. , and Bell, R. M. (1996) J. Biol. Chem. 271, 8472-8480) suggests that PA is the active PLD metabolite in TPA-stimulated signaling.

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.

Raf-1 kinase possesses distinct binding domains for phosphatidylserine and phosphatidic acid. Phosphatidic acid regulates the translocation of Raf-1 in 12-O-tetradecanoylphorbol-13-acetate-stimulated Madin-Darby canine kidney cells

Previous studies demonstrated that the cysteine-rich amino-terminal domain of Raf-1 kinase interacts selectively with phosphatidylserine (Ghosh, S., Xie, W. Q., Quest, A. F. G., Mabrouk, G. M., Strum, J. C., and Bell, R. M. (1994) J. Biol. Chem. 269, 10000-10007). Further analysis showed that full-length Raf-1 bound to both phosphatidylserine and phosphatidic acid (PA). Specifically, a carboxyl-terminal domain of Raf-1 kinase (RafC; residues 295 648 of human Raf-1) interacted strongly with phosphatidic acid. The binding of RafC to PA displayed positive cooperativity with Hill numbers between 3.3 and 6.2; the apparent Kd ranged from 4.9 +/- 0.6 to 7.8 +/- 0.9 mol % PA. The interaction of RafC with PA displayed a pH dependence distinct from the interaction between the cysteine-rich domain of Raf-1 and PA. Also, the RafC-PA interaction was unaffected at high ionic strength. Of all the lipids tested, only PA and cardiolipin exhibited high affinity binding; other acidic lipids were either ineffective or weakly effective. By deletion mutagenesis, the PA binding site within RafC was narrowed down to a 35-amino acid segment between residues 389 and 423. RafC did not bind phosphatidyl alcohols; also, inhibition of PA formation in Madin-Darby canine kidney cells by treatment with 1% ethanol significantly reduced the translocation of Raf-1 from the cytosol to the membrane following stimulation with 12-O-tetradecanoylphorbol-13-acetate. These results suggest a potential role of the lipid second messenger, PA, in the regulation of translocation and subsequent activation of Raf-1 in vivo.

Protein kinase C isozymes and substrates

As most cells express more than one type of protein kinase C (PKC), it has been difficult to establish the role of individual PKCs in cellular functions. Isozyme differences in cofactor requirements and subcellular location, in addition to variability in expression of PKCs and substrates among various cell types, are all involved in determining the effects of PKC activation. Recent identification of cellular PKC-targeting proteins and of isozyme-selective functions has provided new insight into the roles of individual PKCs in cellular processes.

Expression of the nucleoside triphosphate pyrophosphohydrolase PC-1 is induced by basic fibroblast growth factor (bFGF) and modulated by activation of the protein kinase A and C pathways in osteoblast-like osteosarcoma cells

The closely related cytokines bFGF and aFGF regulate the function of bone cells and mineralization. Osteoblasts express PPi-generating nucleoside triphosphate pyrophosphohydrolase (NTPPPH)/nucleotide phosphodiesterase I activity. bFGF and aFGF (10 ng/ml) up-regulated NTPPPH in human SaOS-2 and U2OS osteosarcoma cells, which express osteoblast-like features in culture. The induction was selective as alkaline phosphatase activity was down-regulated and specific as insulin-like growth factor-1 (IGF-1) and interleukin-1 beta (IL-1 beta) were not active. Furthermore, IL-1 beta but not IGF-1 inhibited bFGF-induced up-regulation of NTPPPH. The induced NTPPPH remained predominantly associated with cells. bFGF can induce signaling through pathways including protein kinase A (PKA) and protein kinase C (PKC)-mediated transduction. An activator of the PKA pathway (8-bromo cyclic adenosine monophosphate [cAMP]) induced NTPPPH. Furthermore, pretreatment with the PKC activator phorbol myristate acetate (PMA) (80 nM) markedly increased subsequent NTPPPH induction by both bFGF and cAMP. The PMA effect was associated with morphologic changes characterized by long, thin intercellular extensions. PKC desensitization also potentially contributed to this effect because the PKC inhibitors staurosporine and H-7 enhanced bFGF-induced and cAMP-induced NTPPPH expression in the absence of morphologic changes. We observed that bFGF induced expression of PC-1, a member of the NTPPPH gene family. The majority of NTPPPH activity was depleted by immunoadsorption using a monoclonal antibody to native human PC-1. bFGF- and aFGF-induced production of PC-1/NTPPPH in osteoblastoid cells may contribute to the effects of FGFs on bone metabolism.

The incorporation of arachidonic acid into triacylglycerol in P388D1 macrophage-like cells

When 388D1 cells are incubated in media containing 10 microM [3H]arachidonic acid (delta4Ach), the label is rapidly incorporated into phospholipids and triacylglycerol. However, incorporation of [3H]delta4Ach into phospholipids clearly precedes incorporation into triacylglycerol, indicating that the phospholipid pool constitutes the primary metabolic fate of the delta4Ach via a remodelling pathway. In contrast, [3H]delta4Ach is incorporated into triacylglycerol almost exclusively via de novo synthesis, as evidenced by studies using propranolol, a phosphatidate phosphohydrolase inhibitor. This compound induced a time-dependent and concentration-dependent increase in the levels of [3H]delta4Ach-containing phosphatidate that is directly correlated with a decrease in the levels of [3H]delta4Ach-containing triacylglycerol. In addition, no change in the levels of [3H]delta4Ach-containing diacylglycerol and monoacylglycerol were apparent along the time course of fatty acid incorporation into triacylglycerol. However, a sharp and transient accumulation of cell-associated free [3H]delta4Ach was detected shortly after exposure of the cells to the radioactive fatty acid. Collectively, the results reported herein suggest that free delta4Ach availability determines the patterns of incorporation and distribution of this fatty acid among the various lipid classes of P338D1 cells.

P2 purinergic receptor agonists enhance cAMP production in Madin-Darby canine kidney epithelial cells via an autocrine/paracrine mechanism

Mechanisms of cross-talk between different classes of signaling molecules are inadequately understood. We have used clonal Madin-Darby canine kidney (MDCK-D1) epithelial cells as a model system to investigate the effects of extracellular nucleotides (e.g. ATP, UTP), which promote increase in activity of several phospholipases, on cAMP production. In contrast to observations in some other cell systems, ATP and UTP, acting via P2 purinergic receptors, stimulated cAMP production in MDCK-D1 cells. At maximally effective concentrations, ATP and UTP were not additive with the beta-adrenergic receptor agonist isoproterenol, but were synergistic with forskolin in increasing cAMP production, indicating that G alpha s is activated by these nucleotides. Additionally, we found that (a) nucleotide-induced increases in cAMP were blocked by indomethacin, a cyclooxygenase inhibitor, (b) arachidonic acid increased cellular cAMP levels in an indomethacin-sensitive fashion, and (c) PGE2, the major metabolite of arachidonic acid, stimulated cAMP formation. Overall, our results suggest a mechanism by which extracellular nucleotides stimulate release of arachidonic acid which is metabolized to PGE2 which, in turn, acts in an autocrine/paracrine fashion via prostaglandin receptors to activate G alpha s and increase cAMP. Based on the ability of extracellular nucleotides to stimulate the formation and release of prostaglandins in MDCK-D1 epithelial and other cells, we hypothesize that receptor-mediated prostaglandin release may be a general mechanism that regulates cAMP formation in many types of cells.

Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms

Phosphatidylcholine (PtdCho) can provide lipid second messengers involved in signal transduction pathways. As a measure of phospholipid turnover in response to extracellular stimulation, we investigated differential enhancement of [3H]choline incorporation into PtdCho by phorbol esters. In C6 rat glioma and SK-N-SH human neuroblastoma cells, [3H]PtdCho synthesis was 2-4 fold stimulated by beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA) when [3H]choline was incubated simultaneously with, or 15 min prior to, beta-TPA treatment. By contrast, in N1E-115 mouse and SK-N-MC human neuroblastoma cells, phorbol esters had no appreciable effect on [3H]choline incorporation; however, in all cells, 200 microM oleic acid enhanced PtdCho synthesis, indicating a stimulable process. Alterations by thymeleatoxin (TMT), an activator of conventional PKC isoforms (alpha, beta and gamma), were similar to beta-TPA. We investigated whether expression of specific PKC isoforms might correlate with these effects of phorbol esters on PtdCho synthesis. All cell lines bound phorbol esters, had PKC activity that was translocated by phorbol esters and differentially expressed isoforms of PKC. Northern and western blot analyses, using specific cDNA and antibodies for PKC-alpha, -beta, -gamma, -delta, -epsilon, and -zeta, revealed that expression of alpha-isoform predominated in C6 and SK-N-SH cells. In contrast, TPA-responsive beta-isoform predominated in SK-N-MC cells. gamma-PKC was not detected in any cells and only in C6 cells was PKC-delta present and translocated by beta-TPA treatment. PKC-epsilon was not detected in SK-N-MC cell lines but translocated with TPA treatment in the other three cell lines. PKC-zeta was present in all cells but was unaltered by TPA treatment. Accordingly, stimulation of PtdCho turnover by phorbol esters correlated only with expression of PKC-alpha; presence of PKC-beta alone was insufficient for a TPA response.

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.

Fluid shear stress stimulates mitogen-activated protein kinase in endothelial cells

Local alterations in the hemodynamic environment regulate endothelial cell function, but the signal-transduction mechanisms involved in this process remain unclear. Because mitogen-activated protein (MAP) kinases have been shown to be activated by physical forces, we measured the phosphorylation and enzyme activity of MAP kinase to identify the signal events involved in the endothelial cell response to fluid shear stress. Flow at physiological shear stress (3.5 to 117 dynes/cm2) activated 42-kD and 44-kD MAP kinases present in cultured bovine aortic endothelial cells, with maximal effect at 12 dynes/cm2. Activation of a G protein was necessary, as demonstrated by complete inhibition by the nonhydrolyzable GDP analog GDP-beta S. Activation of protein kinase C (PKC) was required, as shown by inhibiting PKC with staurosporine or downregulating PKC with phorbol 12,13-dibutyrate. Both Ca(2+)-dependent and -independent PKC activity, measured by translocation and substrate phosphorylation, increased in response to flow. However, MAP kinase activation was not dependent on Ca2+ mobilization, since Ca2+ chelation had no inhibitory effect. On the basis of these findings, it is proposed that flow activates two signal-transduction pathways in endothelial cells. One pathway is Ca2+ dependent and involves activation of phospholipase C and increases in intracellular Ca2+. A new pathway, described in the present study, is Ca2+ independent and involves a G protein and increases in PKC and MAP kinase activity.

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.

Activation of phospholipase D by prostaglandin F2 alpha in rat luteal cells and effects of inhibitors of arachidonic acid metabolism

In rat luteal cells labeled with [3H]oleic acid, PGF2 alpha-stimulated phospholipase D (PLD) activation was investigated. The PLD activity was detected by measuring the accumulation of [3H]phosphatidylethanol (PtdEt) in the presence of ethanol. PGF2 alpha stimulated PtdEt accumulation at concentrations of more than 100 nM in the presence of ethanol. However, PtdEt accumulation did not change in the absence of ethanol. PGF2 alpha (1 microM) increased PtdEt accumulation after 1 min, and the accumulation reached a plateau by 2-3 min. These results indicate that PGF2 alpha activates PLD in rat luteal cells. U-73122, a phospholipase C (PLC) inhibitor, and staurosporine, a protein kinase C (PKC) inhibitor, did not inhibit PGF2 alpha-stimulated [3H]PtdEt accumulation. These results suggest that PGF2 alpha-induced PLD activation is different from PLC-PKC systems. We reported previously that PGF2 alpha stimulated the release of arachidonic acid. The effects of indomethacin, nordihydroguaiaretic acid (NDGA), and 5,8,11,14-eicosatetraynoic acid (ETYA), inhibitors of arachidonic acid metabolism, on PGF2 alpha-stimulated PtdEt accumulation were examined. Pretreatment with indomethacin enhanced PGF2 alpha-induced PtdEt accumulation. In contrast, pretreatment with NDGA and ETYA inhibited PGF2 alpha-induced PtdEt accumulation. It is suggested that PGF2 alpha-stimulated PLD activation is mediated via lipoxygenase products.

Differential regulation of phospholipase D and phospholipase C by protein kinase C-beta and -delta in liver macrophages

We have studied activation of phospholipase (PL) C and PLD in liver macrophages labelled with [3H]arachidonic acid. Zymosan, phorbol 12-myristate 13-acetate (PMA), A23187 and fluoride but not arachidonic acid or lipopolysaccharide (LPS) induce an activation of PLD ([3H]phosphatidylethanol (PEt) accumulation). An activation of PLC ([3H]diacylglycerol (DAG) accumulation) is measured with zymosan, PMA and fluoride but not with A23187, LPS or arachidonic acid whereas inositol phosphates are formed with zymosan, only. Removal of extracellular calcium reduces the formation of [3H]PEt and [3H]DAG while pretreatment of the cells with dexamethasone reduces [3H]PEt formation, only. PMA- and zymosan-induced activation of PLD and PMA-induced activation of PLC both seem to be mediated by protein kinase (PK) C-beta whereas zymosan-induced activation of PLC is negatively controlled by PKC-delta. We could furthermore present evidence that the release of [3H]arachidonic acid in these cells occurs independent of an activation of PLD.

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.

Preferential inhibition of phorbol ester-induced hydrolysis of phosphatidylethanolamine by N-acetylsphingosine in NIH 3T3 fibroblasts

It has been reported that in rat fibroblasts cell-permeable ceramide analogs inhibit agonist-induced phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho). Here we demonstrate that relatively short (30 min) treatments of NIH 3T3 fibroblasts with 15-60 microM concentrations of N-acetylsphingosine result in preferential, although not exclusive, inhibition of phorbol 12-myristate 13-acetate-induced PLD-mediated hydrolysis of phosphatidylethanolamine (PtdEtn). The results suggest that in different cell types the PtdEtn- and PtdCho-hydrolyzing PLD activities are differentially sensitive to the inhibitory effect of ceramide.

A phospholipase D-mediated pathway for generating diacylglycerol in nuclei from Madin-Darby canine kidney cells

Many receptors, in response to their specific ligands, trigger activation of phospholipase D (PLD), resulting in the production of phosphatidic acid which, in turn, is acted upon by a specific phosphatase, phosphatidate phosphohydrolase, to produce diacylglycerol. We report here that isolated nuclei from Madin-Darby canine kidneys (MDCK)-D1 cells exhibit a PLD activity that is enhanced by the presence of ATP. PLD activity was measured in the presence of ethanol, by quantitating the production of phosphatidylethanol. Non-phosphorylating ATP analogs were unable to substitute for ATP in activating PLD, indicating that ATP acts as a phosphoryl group donor in a kinase-mediated phosphorylation reaction. The protein kinase C inhibitors chelerythrine and calphostin completely suppressed the ATP-induced nuclear PLD, implicating protein kinase C as the kinase involved in ATP-dependent PLD activity in nuclei from MDCK-D1 cells. In the absence of ethanol, phosphatidic acid was detected in ATP-treated nuclei. Accumulation of phosphatidic acid preceded or closely paralleled that of diacylglycerol, suggesting a precursor-product relationship. Consistent with those results, we detected phosphatidate phosphohydrolase activity in MDCK-D1 cell nuclei. Measurements of phosphatidic acid and diacylglycerol levels at increasing amounts of ethanol demonstrated that PLD and phosphatidate phosphohydrolase are responsible for generating the majority of the diacylglycerol accumulating in MDCK-D1 cell nuclei. The ability of nuclei to generate diacylglycerol from the concerted action of those two enzymes provides a means to regulate nuclear lipid synthesis as well as protein kinase C activity.

Signal transduction pathways leading to arachidonic acid release from neutrophilic HL-60 cells. The involvement of G protein, protein kinase C and phospholipase A2

Arachidonic acid release from undifferentiated and neutrophilic HL-60 cells was studied. In neutrophilic cells it was stimulated by N-formyl-Met-Leu-Phe and mastoparan by a mechanism involving Gi protein and phospholipase C and was largely dependent on diacyglycerol lipase. Maximum release from both cell types was achieved with fluoride and required cellular energy. Inhibitor studies suggest that arachidonic acid release by fluoride stimulation leads to phospholipase A2 activation with signal transduction involving phospholipase C and protein kinase C. Only neutrophilic cells responded to phorbol ester if Ca(2+)-ionophore was simultaneously present but this effect was abolished by extended treatment with phorbol ester. Thus, protein kinase C plays a major role in highly stimulated neutrophilic cells. These cells are differently equipped with protein kinase C isoenzymes compared with undifferentiated cells. In contrast, both cell types contain similar levels of type II and cytosolic phospholipases A2, the former being by far the more prevalent.

Evidence that ceramide selectively inhibits protein kinase C-alpha translocation and modulates bradykinin activation of phospholipase D

Sphingomyelinase (SMase) treatment (0.1 unit/ml for up to 30 min) of mouse epidermal (HEL-37) or human skin fibroblast (SF 3155) cells preincubated with [3H]serine to label the sphingomyelin pool caused the accumulation of labeled ceramide but not sphingosine or ceramide 1-phosphate. Incubation of HEL-37 cells with dioctanoylglycerol (diC8) or SF 3155 cells with bradykinin caused translocation of calcium/phosphatidylserine-dependent protein kinase C (PKC) activity to particulate material. In both cell lines the translocation was blocked by SMase treatment of the cells or by incubation with the cell-permeable ceramide analogue N-acetylsphingosine (C2-Cer). Western blot analysis indicated that treatment of HEL-37 cells with diC8 or SF 3155 cells with bradykinin resulted in the translocation of both PKC-alpha and PKC-espilon to particulate material. Treatment with SMase or C2-Cer specifically blocked the translocation of PKC-alpha but not that of PKC-epsilon. Pretreatment of cells with SMase or C2-Cer also inhibited the activation of phospholipase D activity induced by either diC8 (HEL-37 cells) or bradykinin (SF 3155 cells). The data provide strong evidence that ceramide can negatively regulate the translocation of PKC-alpha but not PKC-epsilon and further suggest that PKC-alpha may be involved in regulating phospholipase D activity.