Phospholipase D in cell signalling and its relationship to phospholipase C

Enhancement in Phospholipase D Activity as a New Proposed Molecular Mechanism of Haloperidol-Induced Neurotoxicity

Membrane phospholipase D (PLD) is associated with numerous neuronal functions, such as axonal growth, synaptogenesis, formation of secretory vesicles, neurodegeneration, and apoptosis. PLD acts mainly on phosphatidylcholine, from which phosphatidic acid (PA) and choline are formed. In turn, PA is a key element of the PLD-dependent secondary messenger system. Changes in PLD activity are associated with the mechanism of action of olanzapine, an atypical antipsychotic. The aim of the present study was to assess the effect of short-term administration of the first-generation antipsychotic drugs haloperidol, chlorpromazine, and fluphenazine on membrane PLD activity in the rat brain. Animals were sacrificed for a time equal to the half-life of the antipsychotic drug in the brain, then the membranes in which PLD activity was determined were isolated from the tissue. The results indicate that only haloperidol in a higher dose increases the activity of phospholipase D. Such a mechanism of action of haloperidol has not been described previously. Induction of PLD activity by haloperidol may be related to its mechanism of cytotoxicity. The finding could justify the use of PLD inhibitors as protective drugs against the cytotoxicity of first-generation antipsychotic drugs like haloperidol.

A Novel Anti-Inflammatory Role of Omega-3 PUFAs in Prevention and Treatment of Atherosclerosis and Vascular Cognitive Impairment and Dementia

Atherosclerosis is an inflammatory chronic disease affecting arterial vessels and leading to vascular diseases, such as stroke and myocardial infarction. The relationship between atherosclerosis and risk of neurodegeneration has been established, in particular with vascular cognitive impairment and dementia (VCID). Systemic atherosclerosis increases the risk of VCID by inducing cerebral infarction, or through systemic or local inflammatory factors that underlie both atherosclerosis and cognition. Omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) are involved in inflammatory processes, but with opposite roles. Specifically, omega-3 PUFAs exert anti-inflammatory properties by competing with omega-6 PUFAs and displacing arachidonic acid in membrane phospholipids, decreasing the production of pro-inflammatory eicosanoids. Experimental studies and some clinical trials have demonstrated that omega-3 PUFA supplementation may reduce the risk of different phenotypes of atherosclerosis and cardiovascular disease. This review describes the link between atherosclerosis, VCID and inflammation, as well as how omega-3 PUFA supplementation may be useful to prevent and treat inflammatory-related diseases.

Priming by tumor necrosis factor-α of human neutrophil NADPH-oxidase activity induced by anti-proteinase-3 or anti-myeloperoxidase antibodies

Anti-proteinase-3 (anti-PR3) or anti-myeloperoxidase (anti-MPO) antibodies are capable of activating human neutrophils primed by TNF-alpha in vitro. We described previously the involvement of FcgammaRIIa and beta(2) integrins in this neutrophil activation. In the literature, the requirement of TNF priming has been attributed to an effect of TNF-alpha on the expression of PR3 or MPO on the cell surface. Under our experimental conditions, TNF-alpha (2 ng/ml) increased the binding of the antibody against PR3, whereas binding of the antibody against MPO could hardly be detected, not even after TNF-alpha treatment. The aim of this study was to consider (an)other(s) role(s) for TNF-alpha in facilitating the NADPH-oxidase activation by these antibodies. We demonstrate the early mobilization of the secretory vesicles as a result of TNF-induced increase in intracellular-free calcium ions, the parallel colocalization of gp91(phox), the main component of the NADPH oxidase with beta(2) integrins and FcgammaRIIa on the neutrophil surface, and the FcgammaRIIa clustering upon TNF priming. TNF-alpha also induced redistribution of FcgammaRIIa to the cytoskeleton in a dose- and time-dependent manner. Moreover, blocking CD18 MHM23 antibody, cytochalasin B, and D609 (an inhibitor of phosphatidylcholine phospholipase C) inhibited this redistribution and the respiratory burst in TNF-treated neutrophils exposed to anti-PR3 or anti-MPO antibodies. Our results indicate direct effects of TNF-alpha in facilitating neutrophil activation by these antibodies and further support the importance of cytoskeletal rearrangements in this priming process.

The differential regulation of phosphatidylinositol 4-phosphate 5-kinases and phospholipase D1 by ADP-ribosylation factors 1 and 6

Phosphatidylinositol 4-phosphate 5-kinases [PtdIns4P5Ks] synthesise the majority of cellular phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and phospholipase D1 (PLD1) synthesises large amounts of phosphatidic acid (PtdOH). The activities of PtdIns4P5Ks and PLDs are thought to be coupled during cell signalling in order to support large simultaneous increases in both PtdIns(4,5)P(2) and PtdOH, since PtdOH activates PtdIns4P5Ks and PLD1 requires PtdIns(4,5)P(2) as a cofactor. However, little is known about the control of such a system. Membrane recruitment of ADP-ribosylation factors (Arfs) activates both PtdIns4P5Ks and PLDs, but it is not known if each enzyme is controlled in series by different Arfs or in parallel by a single form. We show through pull-down and vesicle sedimentation interaction assays that PtdIns4P5K activation may be facilitated by Arf-enhanced membrane association. However PtdIns4P5Ks discriminate poorly between near homogeneously myristoylated Arf1 and Arf6 although examples of all three known active isoforms (mouse alpha>beta, gamma) respond to these G-proteins. Conversely PLD1 genuinely prefers Arf1 and so the two lipid metabolising enzymes are differentially controlled. We propose that isoform selective Arf/PLD interaction and not Arf/PtdIns4P5K will be the critical trigger in the formation of distinct, optimal triples of Arf/PLDs/PtdIns4P5Ks and be the principle regulator of any coupled increases in the signalling lipids PtdIns(4,5)P(2) and PtdOH.

Atrial Natriuretic Peptide Effects on Intracellular pH Changes and ROS Production in HEPG2 Cells: Role of p38 MAPK and Phospholipase D

AIMS

The present study was performed to evaluate Atrial Natriuretic Peptide (ANP) effects on intracellular pH, phospholipase D and ROS production and the possible relationship among them in HepG2 cells. Cancer extracellular microenvironment is more acidic than normal tissues and the activation of NHE-1, the only system able to regulate pHi homeostasis in this condition, can represent an important event in cell proliferation and malignant transformation.

METHODS

The ANP effects on pHi were evaluated by fluorescence spectrometry. The effects on p38 MAPK and ROS production were evaluated by immunoblots and analysis of DCF-DA fluorescence, respectively. RT-PCR analysis and Western blotting were used to determine the ANP effect on mRNA NHE-1 expression and protein levels. PLD-catalyzed conversion of phosphatidylcholine to phosphatydilethanol (PetOH), in the presence of ethanol, was monitored by thin layer chromatography.

RESULTS

A significant pHi decrease was observed in ANP-treated HepG2 cells and this effect was paralleled by the enhancement of PLD activity and ROS production. The ANP effect on pHi was coupled to an increased p38 MAPK phosphorylation and a down-regulation of mRNA NHE-1 expression and protein levels. Moreover, the relationship between PLD and ROS production was demonstrated by calphostin-c, a potent inhibitor of PLD. At the same time, all assessed ANP-effects were mediated by NPR-C receptors.

CONCLUSION

Our results indicate that ANP recruits a signal pathway associated with p38 MAPK, NHE-1 and PLD responsible for ROS production, suggesting a possible role for ANP as novel modulator of ROS generation in HepG2 cells.

Oxidant-induced pHi/Ca2+ changes in rat aortic smooth muscle cells. The role of atrial natriuretic peptide

The aim of this study was to investigate the effects of oxidative stress on PLD activity, [Ca2+]i and pHi levels and the possible relationship among them. Moreover, since atrial natriuretic peptide (ANP) protects against oxidant-induced injury, we investigated the potential protective role of the hormone in rat aortic smooth muscle (RASM) cells exposed to oxidative stress. Water-soluble 2,2'-Azobis (2-amidinopropane) dihydrochloride (AAPH) was used as free radical generating system, since it generates peroxyl radicals with defined reaction and the half time of peroxyl radicals is longer than other ROS. A significant increase of PLD activity was related to a significant decrease in pHi, while [Ca2+]i levels showed an increase followed by a decrease after cell exposure to AAPH. [Ca2+]i changes and pHi fall induced by AAPH were prevented by cadmium which inhibits a plasma membrane Ca2+ ATPase coupled to Ca2+/H+ exchanger, that operates the efflux of Ca2+ coupled to H+ influx. The involvement of PLD in pHi and [Ca2+]i changes was confirmed by calphostin-c treatment, a potent inhibitor of PLD, which abolished all AAPH-induced effects. Pretreatment of RASM cells with pharmacological concentrations of ANP attenuated the AAPH effects on PLD activity as well as [Ca2+]i and pHi changes, while no effects were observed with physiological ANP concentrations, suggesting a possible role of the hormone as defensive effector against early events of the oxidative stress.

MAP kinase signaling in diverse effects of ethanol

Chronic ethanol abuse is associated with liver injury, neurotoxicity, hypertension, cardiomyopathy, modulation of immune responses and increased risk for cancer, whereas moderate alcohol consumption exerts protective effect on coronary heart disease. However, the signal transduction mechanisms underlying these processes are not well understood. Emerging evidences highlight a central role for mitogen activated protein kinase (MAPK) family in several of these effects of ethanol. MAPK signaling cascade plays an essential role in the initiation of cellular processes such as proliferation, differentiation, development, apoptosis, stress and inflammatory responses. Modulation of MAPK signaling pathway by ethanol is distinctive, depending on the cell type; acute or chronic; normal or transformed cell phenotype and on the type of agonist stimulating the MAPK. Acute exposure to ethanol results in modest activation of p42/44 MAPK in hepatocytes, astrocytes, and vascular smooth muscle cells. Acute ethanol exposure also results in potentiation or prolonged activation of p42/44MAPK in an agonist selective manner. Acute ethanol treatment also inhibits serum stimulated p42/44 MAPK activation and DNA synthesis in vascular smooth muscle cells. Chronic ethanol treatment causes decreased activation of p42/44 MAPK and inhibition of growth factor stimulated p42/44 MAPK activation and these effects of ethanol are correlated to suppression of DNA synthesis, impaired synaptic plasticity and neurotoxicity. In contrast, chronic ethanol treatment causes potentiation of endotoxin stimulated p42/44 MAPK and p38 MAPK signaling in Kupffer cells leading to increased synthesis of tumor necrosis factor. Acute exposure to ethanol activates pro-apoptotic JNK pathway and anti-apoptotic p42/44 MAPK pathway. Apoptosis caused by chronic ethanol treatment may be due to ethanol potentiation of TNF induced activation of p38 MAPK. Ethanol induced activation of MAPK signaling is also involved in collagen expression in stellate cells. Ethanol did not potentiate serum stimulated or Gi-protein dependent activation of p42/44 MAPK in normal hepatocytes but did so in embryonic liver cells and transformed hepatocytes leading to enhanced DNA synthesis. Ethanol has a 'triangular effect' on MAPK that involve direct effects of ethanol, its metabolically derived mediators and oxidative stress. Acetaldehyde, phosphatidylethanol, fatty acid ethyl ester and oxidative stress, mediate some of the effects seen after ethanol alone whereas ethanol modulation of agonist stimulated MAPK signaling appears to be mediated by phosphatidylethanol. Nuclear MAPKs are also affected by ethanol. Ethanol modulation of nuclear p42/44 MAPK occurs by both nuclear translocation of p42/44 MAPK and its activation in the nucleus. Of interest is the observation that ethanol caused selective acetylation of Lys 9 of histone 3 in the hepatocyte nucleus. It is plausible that ethanol modulation of cross talk between phosphorylation and acetylations of histone may regulate chromatin remodeling. Taken together, these recent developments place MAPK in a pivotal position in relation to cellular actions of ethanol. Furthermore, they offer promising insights into the specificity of ethanol effects and pharmacological modulation of MAPK signaling. Such molecular signaling approaches have the potential to provide mechanism-based therapy for the management of deleterious effects of ethanol or for exploiting its beneficial effects.

Differential sensitivity of human monocytes and macrophages to ANP: a role of intracellular pH on reactive oxygen species production through the phospholipase involvement

Atrial natriuretic peptide (ANP), a cardiovascular hormone, elicits different biological actions in the immune system. The aim of the present work was to study the effect of ANP on the intracellular pH (pHi) of human monocytes and macrophages and to investigate whether pHi changes could play a role on phospholipase activities and reactive oxygen species (ROS) production. Human macrophages isolated by peripheral blood mononuclear cells and THP-1 monocytes, which were shown to express all three natriuretic peptide receptors (NPR-A, NPR-B, and NPR-C), were treated with physiological concentrations of ANP. A significant decrease of pHi was observed in ANP-treated macrophages with respect to untreated cells; this effect was paralleled by enhanced phospholipase activity and ROS production. Moreover, all assessed ANP effects seem to be mediated by the NPR-C. In contrast, no significant effect on pHi was observed in THP-1 monocytes treated with ANP. Treatment of macrophages or THP-1 monocytes with 5-(N-ethyl-N-isopropyl)amiloride, a specific Na(+)/H(+) antiport inhibitor, decreases pHi in macrophages and monocytes. Our results indicate that only macrophages respond to ANP in terms of pHi and ROS production, through diacylglycerol and phosphatidic acid involvement, pointing to ANP as a new modulator of ROS production in macrophages.

Essential fatty acids and the brain

OBJECTIVE

To review the role of essential fatty acids in brain membrane function and in the genesis of psychiatric disease.

METHOD

Medline databases were searched for published articles with links among the following key words: essential fatty acids, omega-3 fatty acids, docosahexanoic acid, eicosapentanoic acid, arachidonic acid, neurotransmission, phospholipase A2, depression, schizophrenia, mental performance, attention-deficit hyperactivity disorder, and Alzheimer's disease. Biochemistry textbooks were consulted on the role of fatty acids in membrane function, neurotransmission, and eicosanoid formation. The 3-dimensional structures of fatty acids were obtained from the Web site of the Biochemistry Department, University of Arizona (2001).

RESULTS

The fatty acid composition of neuronal cell membrane phospholipids reflects their intake in the diet. The degree of a fatty acid's desaturation determines its 3-dimensional structure and, thus, membrane fluidity and function. The ratio between omega-3 and omega-6 polyunsaturated fatty acids (PUFAs), in particular, influences various aspects of serotoninergic and catecholaminergic neurotransmission, as shown by studies in animal models. Phospholipase A2 (PLA2) hydrolyzes fatty acids from membrane phospholipids: liberated omega-6 PUFAs are metabolized to prostaglandins with a higher inflammatory potential, compared with those generated from the omega-3 family. Thus the activity of PLA2 coupled with membrane fatty acid composition may play a central role in the development of neuronal dysfunction. Intervention trials in human subjects show that omega-3 fatty acids have possible positive effects in the treatment of various psychiatric disorders, but more data are needed to make conclusive directives in this regard.

CONCLUSION

The ratio of membrane omega-3 to omega-6 PUFAs can be modulated by dietary intake. This ratio influences neurotransmission and prostaglandin formation, processes that are vital in the maintenance of normal brain function.

Phospholipase D regulates calcium oscillation frequency and nuclear factor-kappaB activity in histamine- stimulated human endothelial cells

Histamine stimulates [Ca(2+)](i) oscillations in human aortic endothelial cells (HAEC), the frequency of which regulates the activity of nuclear factor-kappaB (NF-kappaB). This study was performed to determine whether phospholipase D (PLD) is involved in this signaling pathway. At a concentration of 1 microM, which stimulates [Ca(2+)](i) oscillations in this cell type, histamine initiated a twofold increase in [(32)P]phosphatidybutanol (PBt), an index of PLD activity as early as 5 min after stimulation. During established [Ca(2+)](i) oscillations induced by 1 microM histamine, 0.3% n-butanol, which "functionally" redirects phosphatidic acid formed by PLD to PBt, decreased [Ca(2+)](i) oscillation frequency by approximately 50% and produced a similar reduction in NF-kappaB activity. In the presence of the inositol 1,4,5-trisphosphate receptor blocker xestospongin C, which itself decreases the frequency of histamine-stimulated [Ca(2+)](i) oscillations, n-butanol produced a further decrease in oscillation frequency that was not associated with an additional reduction in NF-kappaB activity. This study shows that activation of PLD by histamine regulates [Ca(2+)](i) oscillation frequency and NF-kappaB activity in HAEC.

Lysophosphatidic acid activates the 70-kDa S6 kinase via the lipoxygenase pathway

Many hormones are known to activate the 70-kDa S6 kinase (p70(S6K)). The signalling pathways mediating p70(S6K) activation are only partially characterized. We investigate, in this report, the mechanisms by which lysophosphatidic acid (LPA) activates p70(S6K). We observed that p70(S6K) activation was conventional, in that it was sensitive to both rapamycin and PI3 kinase inhibition. p70(S6K) activation appeared to be caused by the activation of several phospholipase pathways. LPA was an effective stimulus of phospholipase C induced intracellular calcium mobilization, which appeared to participate in p70(S6K) activation. Similarly, the effect of LPA on p70(S6K) activity was antagonized by butan-1-ol but not butan-2-ol suggesting the involvement of agonist stimulated phospholipase D activity. Further, antagonism of the phospholipase A(2) and lipoxygenase pathways attenuated p70(S6K) activation indicating a novel mechanism of p70(S6K) regulation. We conclude that in Swiss 3T3 cells LPA coordinates activation of several phospholipases to regulate p70(S6K).

Metabolic fate of [14C]-ethanol into endothelial cell phospholipids including platelet-activating factor, sphingomyelin and phosphatidylethanol

The metabolic fate of ethanol into the phospholipid pool of calf pulmonary artery endothelial cells was studied. [14C]-ethanol was incorporated into various endothelial cell phospholipids including phosphatidylethanol (PEth), which may represent a substantial fraction in microdomains of membrane phospholipids. The incorporation into phospholipids was reduced in the presence of pyrazole and cyanamide, inhibitors of ethanol metabolism. Wortmannin, the phosphatidylinositol 3-kinase inhibitor, increased [14C]-PEth formation. [3H]-acetate was also incorporated into endothelial cell phospholipids but in a different pattern. Distribution of [3H]-acetate and [14C]-ethanol into the fatty acyl moiety versus the glycerophosphoryl backbone of the phospholipids was also different. Stimulation of the endothelial cells with ATP increased [3H]-acetate incorporation into platelet-activating factor (PAF) and ethanol decreased it. Ethanol exposure increased ATP-stimulated [3H]-acetate incorporation into sphingomyelin. However, ATP had no effect on the incorporation of [14C]-ethanol into any phospholipids. The results suggest that the two precursors contribute to a separate acetate pool and that the sphingomyelin cycle may be sensitized in ethanol-treated cells. Thus, metabolic conversions of ethanol into lipids and the effect of ethanol on specific lipid mediators, e.g PAF, PEth and sphingomyelin, may be critical determinants in the altered responses of the endothelium in alcoholism.

A HPLC-fluorescence detection method for determination of cardiac phospholipase D activity in vitro

A nonradioactive assay for the investigation of phospholipase D (PLD) activity in cardiac membranes has been developed. A fluorescent derivative of phosphatidylcholine [2-decanoyl-1-(O-(11-(4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3proprionyl)amino) undecyl) sn-glycero-3-phosphocholine] was utilized as substrate in an in vitro PLD-catalyzed transphosphatidylation reaction utilizing ethanol as second substrate. Unreacted phosphatidylcholine and the products of phospholipase activity (PEtOH, phosphatidylethanol; PA, phosphatidic acid; DAG, diacylglycerol) were separated by a binary gradient HPLC system and detected by fluorometry. The detection limit of this assay is approximately 0.6 pmol PEtOH. The reaction proceeded at a linear rate for up to 45 min and increased linearly with increasing amounts of rat cardiac membrane protein in a range of 0.625 microg up to at least 25 microg. In the presence of potassium fluoride, formation of fluorescent PA increased at the expense of DAG generation, demonstrating the presence of PA phosphohydrolase activity in rat cardiac membranes. PEtOH formation was unchanged in the presence of the PA phosphohydrolase inhibitor, indicating that the phosphatidylalcohol is not subject to further metabolism by this enzyme. Activation of protein kinase C by phorbol ester significantly increased PLD activity in cardiac membranes. This assay proved to be sensitive for accurate and rapid assessment of PLD activity in cardiac membranes permitting further characterization of the regulation of PLD signal transduction in the heart.

Inositol 1,4,5-Trisphosphate-independent Ca2+Mobilization Triggered by a Lipid Factor Isolated from Vitreous Body

A complex phospholipid from bovine vitreous body with a strong Ca(2+)-mobilizing activity has been recently isolated to homogeneity by our group. In this work, a sequential analysis of its transmembrane signaling pathway has been undertaken to characterize the intracellular mechanisms responsible for the Ca(2+) rise. The results show that this phospholipid induces, in a dose-dependent manner (ED(50) of around 0.25 microgram/ml), a Ca(2+) mobilization from inositol 1,4,5-trisphosphate-insensitive intracellular stores, with no participation of extracellular Ca(2+). Upon repeated administration, it shows no signs of autologous desensitization, does not induce heterologous desensitization of the L-alpha-lysophosphatidic acid (LPA) receptor but is desensitized by the previous administration of LPA. The Ca(2+)-mobilizing activity requires a membrane protein, is blocked after preincubation of the cells with pertussis toxin and phorbol esters, as well as by U73122 (an inhibitor of phospholipases C/D), R59022 (a diacylglycerol kinase inhibitor), and D609 (which inhibits phosphatidylcholine-specific phospholipase C). Upon administration of this phospholipid, the intracellular levels of phosphatidic acid (PA) rise with a time course that parallels that of the Ca(2+) mobilization, suggesting that PA could be responsible for this Ca(2+) signal. Exposure to AACOCF(3) (a specific inhibitor of phospholipase A(2)) does not modify the Ca(2+) rise, ruling out the possibility that the PA generated could be further converted to LPA by the action of phospholipase A(2). Based on the experimental data obtained, a signaling pathway involving a phosphatidylcholine-specific phospholipase C coupled to diacylglycerol kinase is proposed. This compound may represent a new class of bioactive lipids with a putative role in the physiology of the vitreous body.

Phosphatidic acid increases inositol-1,4,5,-trisphosphate and [Ca2+]i levels in neonatal rat cardiomyocytes

Phosphatidic acid (PA), which can be synthesized de novo, or as a product of phosphatidylcholine hydrolysis and/or phosphorylation of 1,2-diacylglycerol (DAG), mediates diverse cellular functions in various cell types, including cardiomyocytes. We set out to characterize the effect of PA on intracellular free calcium ([Ca2+]i) and inositol-1,4,5-trisphosphate (IP(3)) levels in primary cultures of neonatal rat cardiomyocytes. Addition of PA led to rapid, concentration and time dependent increases in both IP(3) and [Ca2+]i levels in adherent cells. There was strong correlation in the concentration-response relationships between IP(3) and [Ca2+]i increases evoked by PA. Incubation with the sarcoplasmic reticulum (SR) Ca2+ pump inhibitor, cyclopiazonic acid (CPA), significantly attenuated the PA evoked [Ca2+]i increase but had no significant effect on IP(3) accumulation. The phospholipase C (PLC) inhibitor, D-609, attenuated both IP(3) and [Ca2+]i elevations evoked by PA whereas staurosporine (STS), a potent and non-selective PKC inhibitor, had no significant effect on either. Another PLC inhibitor, U73122, but not its inactive analog, U73343, also inhibited PA evoked increases in [Ca2+]i. Depletion of extracellular calcium attenuated both basal and PA evoked increases in [Ca2+]i. The PLA(2) inhibitors, bromophenylacyl-bromide (BPB) and CDP-choline, had no effect on PA evoked [Ca2+]i responses. Neither the DAG analog, dioctanoylglycerol, nor the DAG kinase inhibitor, R59949, affected PA evoked changes in [Ca2+]i. Taken together, these data indicate that PA, in a manner independent of PKC, DAG, or PLA(2), may enhance Ca2+ release from IP(3) sensitive SR Ca(2+) stores via activation of PLC in neonatal rat cardiomyocytes.

Expression of phospholipase D isoforms in mammalian cells

Two mammalian isoforms of phospholipase D, PLD1 and PLD2, have recently been characterized at the molecular level. Effects of physiologic agonists on PLD activity in intact cells, as characterized in earlier studies, have generally not been attributed to specific PLD isoforms. Recent work has established that expression of PLD1 and PLD2 varies within tissues and between cell lines. A single cell type can express one, both, or neither isoform, although most cells co-express PLD1 and PLD2. Lymphocytes often lack expression of one or both isoforms of PLD. Relative levels of PLD mRNA expression vary considerably between established cell lines. Expression of transcripts for both PLD1 and PLD2 can be regulated at the transcriptional level by growth and differentiation factors in cultured cells. Thus, it is apparent that the known mammalian PLD isoforms are subject to regulation at the transcriptional level. The available data do not conclusively establish whether PLD1 and PLD2 are the only isoforms responsible for agonist-mediated PLD activation. Further studies of the regulation of expression of PLD isoforms should provide insight into the roles of PLD1 and PLD2 in physiologic responses, and may suggest whether additional forms of PLD remain to be characterized.

Phosphatidic acid as the biosynthetic precursor of the endocannabinoid 2-arachidonoylglycerol in intact mouse neuroblastoma cells stimulated with ionomycin

In mouse neuroblastoma N18TG2 cells prelabeled with [3H]arachidonic acid ([3H]AA) the biosynthesis of 2-arachidonoylglycerol (2-AG) is induced by ionomycin in a fashion sensitive to an inhibitor of diacylglycerol (DAG) lipase, RHC 80267, but not to four different phospholipase C (PLC) blockers. Pulse experiments with [3H]AA showed that ionomycin stimulation leads to the sequential formation of [3H]phosphatidic acid ([3H]PA), [3H]DAG, and [3H]2-AG. [3H]2-AG biosynthesis in N18TG2 cells prelabeled with [3H]AA was counteracted by propranolol and N-ethylmaleimide, two inhibitors of the Mg2+/Ca2(+)-dependent brain PA phosphohydrolase. Pretreatment of cells with exogenous phospholipase D (PLD) led to a strong potentiation of ionomycin-induced [3H]2-AG formation. These data indicate that DAG precursors for 2-AG in intact N18TG2 cells are obtained from the hydrolysis of PA and not through the activation of PLC. The presence of 2% ethanol during ionomycin stimulation failed to elicit the synthesis of [3H]phosphatidylethanol and did not counteract the formation of [3H]PA, thus arguing against the activation of PLD by the Ca2+ ionophore. Selective inhibitors of secretory phospholipase A2 and the acyl-CoA acylase inhibitor thimerosal significantly reduced [3H]2-AG biosynthesis. The implications of these latter findings, and of the PA-dependent pathways of 2-AG formation described here, are discussed.

The transition from a pharmacophore-guided approach to a receptor-guided approach in the design of potent protein kinase C ligands

The pharmacophore-guided approach used in the first phase of the design of novel protein kinase C (PKC) ligands was based on the study of the geometry of bioequivalent pharmacophores present in diacylglycerol (DAG) and in the more potent phorbol ester tumor promoters. A number of potent DAG lactones were generated by this approach, in which the glycerol backbone was constrained into various heterocyclic rings to reduce the entropic penalty associated with DAG binding. Based on the information provided by X-ray and NMR structures of the cysteine-rich, C1 phorbol ester/DAG binding domain, the DAG lactones were further modified to optimize their interaction with a group of highly conserved hydrophobic amino acids along the rim of the C1 domain. This receptor-guided approach culminated with the synthesis of a series of "super DAG" molecules that can bind to PKC with low nanomolar affinities. These compounds provide insight into the basis for PKC ligand specificity. Moreover, some of the compounds reviewed herein show potential utility as antitumor agents.

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.

Binding of GM-CSF to adherent neutrophils activates phospholipase D

When the hematopoietic growth factor granulocyte-macrophage colony-stimulating factor was incubated with neutrophils adherent to plastic tissue culture plates or plates coated with extracellular matrix proteins, a rapid (3 min) but transient formation of phosphatidic acid was observed. This stimulation was dependent on the dose of GM-CSF, with an EC50 of 140 pM, and was further enhanced (up to 350%) with the PA phosphatase inhibitor propranolol in a dose-dependent manner. Conversely, GM-CSF was unable to trigger any PA formation in neutrophils maintained in suspension, even in the presence of soluble fibronectin. However, GM-CSF did prime the cells for enhanced PA formation in the presence of a secondary stimulus (fMet-Leu-Phe or PAF). GM-CSF also caused a time-dependent stimulation of diacylglycerol formation in adherent, but not suspended, cells and elicited a time-dependent stimulation of phosphatidylethanol formation, with a concomitant decrease in the formation of PA only at early (< 7 min) times. These observations were consistent with a rapid activation of the enzyme phospholipase D in adherent cells stimulated with GM-CSF. Additional data indicated that the source of DAG was PLD coexisting with PLC, especially at later times ( > 7 min) of stimulation with GM-CSF. Finally, the formation of PA and PEt, and to a minor extent, DAG, were inhibited by the protein tyrosine kinase inhibitor erbstatin in conditions in which tyrosine phosphorylation occurred. Taken together the data indicate that GM-CSF rapidly activates PLD in adherent cells, which is responsible for the generation of PA. Thus, PLD activation is an early event in neutrophil signal transduction following exposure of adherent cells to GM-CSF.

Adenosine and preconditioning revisited

1. Myocardial tolerance against infarction is substantially increased by exposing myocytes to 3-10 min transient ischaemia. In this phenomenon, termed 'preconditioning', the adenosine receptor is one of the redundant triggers and the best characterized factor in the cardioprotective mechanism. 2. An increase in interstitial adenosine during preconditioning is thought to be derived primarily from hydrolysis of 5'-AMP in the myocyte by cytosolic 5'-nucleotidase, although a contribution of ectosolic 5'-nucleotidase remains controversial. Adenosine production during ischaemia is substantially suppressed in the preconditioned myocardium, probably due to a decrease in ATP utilization. 3. The adenosine receptor needs to be activated not only at the time of preconditioning ischemia, but also during ischaemic insult for the preconditioning to be cardioprotective. However, the extent of cardioprotection afforded by preconditioning is primarily determined by the interstitial adenosine level achieved during preconditioning ischaemia, not by the level during sustained ischaemia. These data suggest that a post-receptor mechanism downstream of the adenosine receptor may be up-regulated after preconditioning. 4. Studies in vitro suggest that the subtypes of adenosine receptor relevant to preconditioning against infarction are A1 and A3, the activation of which appears to provide additive protection. The functional interrelationship between these subtypes in vivo remains unknown. 5. An important step downstream of adenosine receptor activation is protein kinase C (PKC), which facilitates opening of ATP-sensitive potassium (KATP) channels, probably leading to enhancement of myocardial tolerance. However, activation of other protein kinases, such as tyrosine kinase, may also be important in preconditioning, depending on the animal species and preconditioning protocols. The PKC isoform and location of KATP channels (i.e. sarcolemmal vs mitochondrial KATP) that induce anti-infarct tolerance in myocytes remain to be identified.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Phorbol ester-induced production of prostaglandin E2 from phosphatidylcholine through the activation of phospholipase D in UMR-106 cells

To determine the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on phospholipase D (PLD) activity in osteoblast-like UMR-106 cells, we used cells prelabeled with [3H] myristic acid or [3H] arachidonic acid, which were preferentially incorporated to phosphatidylcholine. The treatment of [3H] myristate-labeled cells with TPA in the presence of 1% ethanol caused a dose-dependent formation of [3H] phosphatidylethanol (PEt), a product specific to PLD, suggesting an activation of this enzyme. Pretreatment of the cells with protein kinase C (PKC) inhibitors (GF109203X, staurosporine or H-7) abolished the TPA-dependent formation of PEt. The PEt formation in response to TPA treatment was not observed after the pretreatment of the cells with TPA to downregulate PKC. These results suggest the involvement of PKC in the TPA-induced activation of PLD. With [3H] arachidonate-labeled cells, TPA treatment in the absence of ethanol resulted in the liberation of [3H] arachidonic acid, which was gradually converted to prostaglandin E2 (PGE2), but the accumulations of [3H] phosphatidic acid (PA) and [3H] diacylglycerol (DAG) were very small and temporary. In contrast, PA was linearly accumulated following TPA treatment, when the cells were pretreated with an inhibitor of phosphatidate phosphohydrolase (PAP), propranolol, with no accumulation of either DAG or arachidonic acid. The TPA treatment of the cells pretreated with a DAG lipase inhibitor, RHC-80267, caused the generation of DAG after a lag period of approximately 5 min, with a very small and temporary accumulation of PA. The TPA treatment of cells pretreated with a cyclooxygenase (COX) inhibitor, indomethacin, blocked the PGE2 production. The TPA-induced PGE2 production was not affected by the pretreatment of cells with a phospholipase A2 inhibitor, p-bromophenacylbromide, or with a phospholipase C inhibitor, D-609. TPA also stimulated PGE2 production in osteoblastic cells that were enzymatically isolated from adult rat calvaria, and the experiments with lipid metabolizing enzyme inhibitors gave the same profile of inhibition of TPA-induced PGE2 production as was observed in UMR-106 cells. These results suggest that PA formed as a consequence of the activation of PLD by TPA is rapidly converted to arachidonic acid via a PAP/DAG lipase pathway, followed by a gradual conversion of arachidonic acid to PGE2 by COX in both UMR-106 cells and isolated adult osteoblastic cells, and that neither phospholipase A2 nor phospholipase C is involved in the TPA-induced PGE2 production. To the best of our knowledge, this is the first report that shows that the activation of PKC in osteoblastic cells leads to the production of PGE2 via a PLD/PAP/DAG lipase/COX pathway.

Increased levels of methylated intermediates of phosphatidylcholine lead to enhanced phospholipase D activity

Previous work from this laboratory and others has shown that neurotransmitters can activate phospholipase D. Unlike the phospholipase C that specifically hydrolyzes inositol-containing phospholipids, phospholipase D in neuronal tissue specifically hydrolyzes phosphatidylcholine. One route for the synthesis of phosphatidylcholine, is via methylation of phosphatidylethanolamine. Using an in vitro assay, we have previously shown that methylated intermediates are also good substrates for phospholipase D (1). In this manuscript we demonstrate that these intermediates are also substrates in the intact PC12 cells. Cells incubated with methyl and dimethylethanolamine incorporate more [3H]palmitic acid into the corresponding phospholipid, phosphatidyl-N-methylethanolamine and phosphatidyl-N,N-dimethylethanolamine. In these cells bradykinin causes a greater increase in [3H]phosphatidylethanol production. Elevated levels of [3H]phosphatidylcholine do not enhance bradykinin-stimulated [3H]phosphatidylethanol production, therefore, this effect is specific for the methylated intermediates. Finally, this effect is not due to some generalized enhancement of receptor coupling because incubation of the cells with methylethanolamine does not lead to an increase in bradykinin stimulated inositol phosphate production.

Activation of a Plasma Membrane–Associated Neutral Sphingomyelinase and Concomitant Ceramide Accumulation During IgG-Dependent Phagocytosis in Human Polymorphonuclear Leukocytes

The sphingomyelin cycle, which plays an important role in regulation of cell growth, differentiation, and apoptosis, involves the formation of ceramide by the action of a membrane-associated, Mg2+-dependent, neutral sphingomyelinase and/or a lysosomal acid sphingomyelinase. In human polymorphonuclear leukocytes (PMNs), ceramide production correlates with and plays a role in the regulation of functional responses such as oxidant release and Fcγ receptor-mediated phagocytosis. To increase our understanding of the sphingomyelin cycle in human PMNs, the cellular location of neutral and acid sphingomyelinases was investigated in resting, formylmethionylleucylphenylalanine (FMLP)-activated, and FMLP-activated PMNs engaged in phagocytosis. In resting PMNs, a Mg2+-dependent, neutral sphingomyelinase was the predominant activity and was localized to the plasma membrane fractions along with the majority of ceramide. Upon FMLP-activation, there was a 1.9-fold increase in this neutral, Mg2+-dependent sphingomyelinase activity, which increased to 2.7-fold subsequent to phagocytosis of IgG opsonized targets. This increase in sphingomyelinase activity was restricted to the plasma membrane fractions, which were also the site of increased ceramide levels. Phospholipase D (PLD) activity, which is a target of ceramide action and is required for phagocytosis, was also found primarily in the plasma membrane fractions of FMLP-activated and phagocytosing PMNs. Our findings indicate that in human PMNs engaged in phagocytosis, the sphingomyelin cycle is restricted to the plasma membrane where intracellular targets of ceramide action, such as PLD, are localized.

Activation of a Plasma Membrane–Associated Neutral Sphingomyelinase and Concomitant Ceramide Accumulation During IgG-Dependent Phagocytosis in Human Polymorphonuclear Leukocytes

The sphingomyelin cycle, which plays an important role in regulation of cell growth, differentiation, and apoptosis, involves the formation of ceramide by the action of a membrane-associated, Mg2+-dependent, neutral sphingomyelinase and/or a lysosomal acid sphingomyelinase. In human polymorphonuclear leukocytes (PMNs), ceramide production correlates with and plays a role in the regulation of functional responses such as oxidant release and Fcγ receptor-mediated phagocytosis. To increase our understanding of the sphingomyelin cycle in human PMNs, the cellular location of neutral and acid sphingomyelinases was investigated in resting, formylmethionylleucylphenylalanine (FMLP)-activated, and FMLP-activated PMNs engaged in phagocytosis. In resting PMNs, a Mg2+-dependent, neutral sphingomyelinase was the predominant activity and was localized to the plasma membrane fractions along with the majority of ceramide. Upon FMLP-activation, there was a 1.9-fold increase in this neutral, Mg2+-dependent sphingomyelinase activity, which increased to 2.7-fold subsequent to phagocytosis of IgG opsonized targets. This increase in sphingomyelinase activity was restricted to the plasma membrane fractions, which were also the site of increased ceramide levels. Phospholipase D (PLD) activity, which is a target of ceramide action and is required for phagocytosis, was also found primarily in the plasma membrane fractions of FMLP-activated and phagocytosing PMNs. Our findings indicate that in human PMNs engaged in phagocytosis, the sphingomyelin cycle is restricted to the plasma membrane where intracellular targets of ceramide action, such as PLD, are localized.

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.

Lipid ozonation products activate phospholipases A2, C, and D

Ozone exposure, in vitro, has been shown to activate phospholipases A2 (PLA2), C (PLC), and D (PLD) in airway epithelial cells. However, because of its high reactivity, ozone cannot penetrate far into the air/lung tissue interface. It has been proposed that ozone reacts with unsaturated fatty acids (UFA) in the epithelial lining fluid (ELF) and cell membranes to generate a cascade of lipid ozonation products (LOP) that mediate ozone-induced toxicity. To test this hypothesis, we exposed cultured human bronchial epithelial cells (BEAS-2B) to LOP (1-100 microM) produced from the ozonation of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) and measured the activity of PLA2, PLC, and PLD. The PLA2 isoform responsible for arachidonic acid release (AA) in stimulated cultures was also characterized. Activation of PLA2, PLC, and PLD by three oxidants, hydrogen peroxide (H2O2), tert-butyl hydroperoxide (t-BOOH) and 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) also was measured and compared to that of LOP. The derivatives of ozonized POPC at the sn-2 residue, 9-oxononanoyl (PC-ALD), 9-hydroxy-9-hydroperoxynonanoyl (PC-HHP), and 8-(-5-octyl-1,2,4-trioxolan-3-yl-) octanoyl (POPC-OZ) selectively activated PLA2 in a dose-dependent fashion. Cytosolic PLA2 (cPLA2) measured in the cytosolic fraction of stimulated cell lysates was found to be the predominant isoform responsible for AA release. PLC activation was exclusively induced by the hydroxyhydroperoxide derivatives. PC-HHP and the 9-carbon hydroxyhydroperoxide (HHP-C9) increased PLC activity. PLD activity also was induced by LOP generated from POPC. Incubation of cultures with H2O2 alone did not stimulate PLC; however, in the presence of the aldehyde, nonanal, a 62 +/- 2% increase in PLC activity was found, suggesting that the increase in activity was due to the formation of the intermediate HHP-C9. t-BOOH, and AAPH also failed to induce PLA2 activation, but did activate PLC, under conditions of exposure identical to that of LOP. Only t-BOOH activated PLD. These results suggest that biologically relevant concentrations of LOP activate PLA2, PLC, and PLD in the airway epithelial cell, a primary target to ozone exposure. The activation of these phospholipases may play a role in the development of lung inflammation during ozone exposure.

Primed neutrophils require phosphatidic acid for maximal receptor-activated elastase release

BACKGROUND

Priming of neutrophils (PMNs) for protease release is believed to be central to the pathogenesis of PMN-mediated tissue injury observed in ARDS/MOF. Defining the intracellular signaling pathways involved with this excessive protease release may aid in establishing future therapies for ARDS. Phospholipase D (PLD) production of phosphatidic acid (PA) is thought to be pivotal in reactive oxygen species generation but its role in degranulation (i. e., protease release) remains unclear. Our hypothesis was that primed neutrophils require PLD production of PA for maximal activated release of elastase.

METHODS

Isolated human PMNs were incubated with a well described antagonist of PA production, ethanol (ETOH, 100-1000 mg/dL), and then primed (PAF, 200 nM) followed by activation (fMLP, 1 microM). To mimic fMLP receptor-dependent activation, PMNs were primed and then directly activated with exogenous dioctanoyl l-alpha-phosphatidic acid (PA8, 0.5-200 microM). To confirm the importance of PA in elastase release, PA8 was given to primed-activated PMNs after ethanol pretreatment in an attempt to recover the maximal response. Elastase release was measured by the cleavage of AAPV-pNA.

RESULTS

PA blockade with ETOH attenuated PAF-primed/fMLP-activated PMN elastase release in a dose-dependent manner. Exogenous PA8 reproduced maximally primed-activated PMN elastase release, and furthermore, PA8 was able to restore maximal elastase release following ethanol attenuation.

CONCLUSIONS

Elastase release from PAF-primed/ fMLP-activated neutrophils is dependent on PA production. Thus, PA production appears to be involved in both oxidant-dependent and independent mechanisms of neutrophil cytotoxicity and may be a potential therapeutic target in the treatment of hyperinflammatory diseases such as ARDS/MOF.

Tyrosine kinases and calcium dependent activation of endothelial cell phospholipase D by diperoxovanadate

Reactive oxygen species (ROS) mediated modulation of signal transduction pathways represent an important mechanism of cell injury and barrier dysfunction leading to the development of vascular disorders. Towards understanding the role of ROS in vascular dysfunction, we investigated the effect of diperoxovanadate (DPV), derived from mixing hydrogen peroxide and vanadate, on the activation of phospholipase D (PLD) in bovine pulmonary artery endothelial cells (BPAECs). Addition of DPV to BPAECs in the presence of .05% butanol resulted in an accumulation of [32P] phosphatidylbutanol (PBt) in a dose- and time-dependent manner. DPV also caused an increase in tyrosine phosphorylation of several protein bands (Mr 20-200 kD), as determined by Western blot analysis with antiphosphotyrosine antibodies. The DPV-induced [32P] PBt-accumulation was inhibited by putative tyrosine kinase inhibitors such as genistein, herbimycin, tyrphostin and by chelation of Ca2+ with either EGTA or BAPTA, however, pretreatment of BPAECs with the inhibitor PKC bisindolylmaleimide showed minimal inhibition. Also down-regulation of PKC alpha and epsilon, the major isotypes of PKC in BPAECs, by TPA (100 nM, 18 h) did not attenuate the DPV-induced PLD activation. The effects of putative tyrosine kinase and PKC inhibitors were specific as determined by comparing [32P] PBt formation between DPV and TPA. In addition to tyrosine kinase inhibitors, antioxidants such as N-acetylcysteine and pyrrolidine dithiocarbamate also attenuated DPV-induced protein tyrosine phosphorylation and PLD stimulation. These results suggest that oxidation, prevented by reduction with thiol compounds, is involved in DPV-dependent protein tyrosine phosphorylation and PLD activation.

Neuromedin B activates phospholipase D through both PKC-dependent and PKC-independent mechanisms

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca2+]i at 0.2 nM, in [3H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 microM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca2+-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca2+]i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca2+]i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

Phospholipase D hydrolyzes short-chain analogs of phosphatidylcholine in the absence of detergent

Phospholipase D is an important enzyme in signal transduction in neuronal tissue. A variety of assays have been used to measure phospholipase D activity in vitro. The most typical measure of phospholipase D activity is the production of phosphatidylethanol in the presence of ethanol. Phosphatidylethanol is a product of transphosphatidylation activity that is considered a unique property of phospholipase D. To support transphosphatidylation activity, high concentrations of ethanol may be required. Furthermore, most assays in the literature utilize a detergent. These extreme conditions, detergent and ethanol, may alter phospholipase D and hinder the study of its regulation. In this manuscript we describe an assay that eliminates these potentially confounding conditions. It utilizes high specific activity [3H]butanol as a nucleophilic receptor. This eliminates the need for high concentrations of alcohol. The substrate is an analog of phosphatidylcholine that contains short-chain fatty acids, 1,2-dioctanoyl-sn-glycero-3-phosphocholine. Phospholipase D readily hydrolyzes this substrate in the absence of detergent. This novel assay should be useful in the further characterization of phospholipase D.

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.

Phosphatidylinositol- and phosphatidylcholine-dependent phospholipases C are involved in the mechanism of action of atrial natriuretic factor in cultured rat aortic smooth muscle cells

We have investigated the involvement of specific phospholipase systems and their possible mutual relationship with the mechanism by which atrial natriuretic factor (ANF) increases phosphatidate (PA) and diacylglycerol (DAG) in rat aortic smooth muscle cells (RASMC), one of the major targets of this hormone. Our results indicate that ANF initially stimulates a phosphatidylinositol-dependent phospholipase C (PI-PLC) with a significant increase of DAG, enriched in arachidonate, and inositol trisphosphate (IP3) and then a phosphatidylcholine-dependent phospholipase C (PC-PLC) with formation of DAG, enriched in myristate, and phosphocholine (Pcho). Moreover, ANF stimulates PA formation at an intermediate stage between early and late DAG formation. The transphosphatidylation reaction, as well as its labeling ratio, demonstrate that phosphatidylcholine-dependent phospholipase D (PC-PLD) is not involved. Our experiments with R59022, a DAG kinase (DAGK) inhibitor, indicate that such an increase may be due to the phosphorylation of DAG derived from phosphatidylinositol (PI) hydrolysis. Our results show that phorbol 12-myristate 13 acetate (PMA) plays a significant role in late DAG formation and that Pcho is released concomitantly, suggesting there is a relationship between the two phospholipase Cs (PLCs) that occurs through a protein kinase C (PKC) translocation from cytosol to the plasma membrane. These findings are confirmed by the use of PKC inhibitors calphostin, H7, and staurosporine. The involvement of membrane phospholipid hydrolysis and the ensuing production of second messengers might explain the vasorelaxant effect of ANF.

Changes in cellular and plasma membrane phospholipid composition after lipopolysaccharide stimulation of human neutrophils, studied by 31P NMR

Lipopolysaccharide (endotoxin, LPS) exerts potent proinflammatory effects on neutrophils which may involve membrane phospholipid metabolism. The cellular and plasma membrane phospholipid composition of resting neutrophils and those stimulated with 50 microg ml(-1) LPS were studied by 31P NMR and chemical analysis. A rapid new method for plasma membrane purification was employed, involving the direct lysis of cytoplasts. Chemical analyses showed that, although total cellular phospholipid content did not change with LPS stimulation, there was twice the amount of phospholipid present in plasma membranes isolated from stimulated cells, resulting in a lowered cholesterol/phospholipid ratio. Since internal membranes have lower cholesterol content this result is consistent with an origin from insertion of these membranes (most probably from the endoplasmic reticulum) into the plasma membrane, thereby increasing its fluidity. The individual phospholipid classes of both cells and membranes were quantified by 31P-NMR spectroscopy after dissolution in sodium cholate without prior extraction of lipids, allowing partial resolution of the major phospholipid classes and ether-linked phospholipids. Ether-linked lipids were distinguished from diacyl phospholipids by hydrolysis of lipid extracts with HCl and phospholipase A1, There was a significant increase in phosphatidylserine in both cells and plasma membranes after stimulation, with a decrease in the phosphatidylethanolamine (diacyl and plasmalogen) content in the cells. Plasma membranes from stimulated cells exhibited a significant decrease in a phospholipid tentatively identified as 2-arachidonoyl-1-alkyl-sn-glycero-3-phosphocholine, a precursor of the lipid inflammatory mediator, platelet-activating factor. This report is the first to elaborate the changes in phospholipid composition in human neutrophils as a whole, and in plasma membranes separated from them, before and after stimulation by the physiological activator, LPS.

Studies on ether-phospholipids of vascular smooth muscle cells. Identification of a rapid Ca(2+)-dependent hydrolysis of alkyl-phosphatidylethanolamine promoted by endothelin-l

We have investigated the metabolism of 1-O-[3H]octadecyl-sn-glycero-3-phosphocholine ([3H]lyso PAF) and [3H] myristic acid in secondary cultures of aortic smooth muscle cells (SMC) to characterize the origin of second messengers generated upon stimulation with endothelin-l (ET-l). When cells were labelled with [3H]lyso PAF, we observed a transfer of the label from phosphatidylcholine (PC) to phosphatidylethanolamine (PE) In contrast, incubation with [3H]lyso PAF labelled mainly alkyl-subclasses while [3H]myristate was associated with diacyl-subclasses. Using these specific labelling procedures, we have shown that ET-l induced a strong hydrolysis of PE. This hydrolysis was specific for alkyl-PE with a maximum after 5 s of stimulation. We have also observed an extracellular Ca(2+)-dependent increase in diglyceride (DG), phosphatidic acid (PA) and mainly triglyceride (TG) concomitant to alkyl-PE hydrolysis. Thus, alkyl-DG generated from alkyl-PE appears to be a major product in ET-l stimulation of SMC. These results suggest a new level of complexity in the signal transduction cascade involving a specificity for phospholipid subclasses.

Phospholipase D activity in the intestinal mitochondria: activation by oxygen free radicals

A prominent feature of cell damage caused by oxidative stress is morphological and functional changes in the mitochondria. The present study looked at the effect of free radical exposure on intestinal mitochondrial lipids. Free radical exposure did not alter neutral lipids, but among the phospholipids, phosphatidylethanolamine (PE) content was decreased on exposure to superoxide anion, generated by xanthine-xanthine oxidase or menadione with a concomitant increase in the level of phosphatidic acid (PA), suggesting activation of phospholipase D (PLD). This enzyme did not show transphosphatidylation 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 HPLC. This alteration in mitochondrial phospholipid was abolished by the presence of superoxide dismutase. Exposure to H2O2 did not have any significant effect. Activation of PLD by free radicals was further confirmed by quantitation of ethanolamine released from PE. Absence of any change in the content of lysophospholipid or diglyceride following exposure of mitochondria to superoxide ruled out the involvement of phospholipase A2 or C in the altered lipid composition. Moreover, inclusion of phospholipase A2 inhibitors, chlorpromazine, or p-bromophenacyl bromide did not prevent the generation of PA on exposure to free radicals. These findings suggest that superoxide anion stimulates intestinal mitochondrial PLD resulting in PE degradation and PA formation. These alterations in mitochondrial lipids may play a role in causing the functional alteration seen in oxidative stress.

Type of dietary fiber, not fat, alters phospholipase D and ornithine decarboxylase activities in the rat large intestine

We investigated the effect of dietary fatty acid composition (n-6 vs. n-3) and fiber (highly fermentable vs. less fermentable) on the activities of phospholipase D (PLD) and ornithine decarboxylase (ODC) in the rat large intestine (cecum and proximal and distal colon). Twenty-four Sprague-Dawley rats (215-270 g) ate synthetic diets with 2% safflower oil plus 21.5% safflower or fish oil and 10% cellulose or guar gum for four weeks. Cecal bile acids and free fatty acids were higher in rats fed guar gum than in rats fed cellulose. Rats fed fish oil had more proximal colonic mucosal and cecal bile acids than those fed safflower oil. PLD activity was 23% lower in the proximal colon of rats fed guar gum than in those fed cellulose, but the mucosal weight was not different. ODC activity was lower but cecal mucosal wet weight was higher in the cecum of the rats fed guar gum than in the cecum of the rats fed cellulose. The activities of PLD and ODC are affected by dietary fiber and may not be accurate markers for tissue growth in the colonic mucosa.

Inhibition of phospholipase D by a protein factor from bovine brain cytosol. Partial purification and characterization of the inhibition mechanism

A specific protein inhibitor of partially purified bovine brain phospholipase D (PLD) was identified from bovine brain cytosol. The PLD inhibitor has been enriched through several chromatographic steps and characterized with respect to size and mechanism of inhibition. The inhibitor showed an apparent molecular mass of 30 kDa by Superose 12 gel exclusion chromatography and inhibited PLD activity with an IC50 of 7 nM. The inhibitor had neither proteolytic activity nor phospholipid-hydrolyzing activity. Because phosphatidylinositol 4,5-bisphosphate (PIP2), which is included in substrate vesicles, is an essential cofactor for PLD, we examined whether the inhibition might be mediated by sequestration of PIP2. PIP2 hydrolysis by phospholipase C (PLC)-beta1 was not affected by the inhibitor and the inhibitor did not bind to substrate vesicles containing PIP2. In contrast, a PH domain derived from PLC-delta1, which could bind to PIP2, showed a nearly identical inhibition of both PLC-beta1 and PLD activities. Thus, the PLD inhibition by the inhibitor is due to the specific interaction with not PIP2 but PLD. The suppression of PLD activity by the inhibitor was largely eliminated by the addition of ADP-ribosylation factor (ARF) and GTPgammaS. We propose that the inhibitor plays a negative role in regulation of PLD activity by PIP2 and ARF.

Phospholipase D plays a role in ischemic preconditioning in rabbit heart

BACKGROUND

Activation of protein kinase C (PKC) is thought to be a critical step in ischemic preconditioning. Many receptor agonists activate PKC via stimulation of phospholipase C (PLC), which degrades membrane phospholipids to diacylglycerol (DAG), an important PKC cofactor. However, adenosine receptors, critical components of the prototypical preconditioning pathway, are not thought to couple to PLC in the cardiomyocyte. We therefore tested whether ischemic preconditioning or adenosine might instead activate phospholipase D (PLD) to produce DAG.

METHODS AND RESULTS

PLD activity was measured in isolated rabbit hearts. Ischemic injury was evaluated in either isolated rabbit hearts or dispersed myocytes. PLD activity doubled from a control level of 74.8 +/- 10.0 to 140.0 +/- 11.5 mumol.min-1.g-1 (P < .025) after two 5-minute periods of global ischemia separated by 5 minutes of reperfusion. A similar increase was noted after the heart had been exposed to (R)-N6-(2-phenylisopropyl)-adenosine [(R)-PIA] for 20 minutes. When sodium oleate, which activates PLD, was administered to isolated hearts before a 30-minute coronary occlusion, infarct size (15.6 +/- 2.0% of the risk zone) was significantly smaller than in untreated hearts (30.4 +/- 2.2%; P < .01). Exposure to sodium oleate significantly prolonged the rate of isolated myocyte survival during simulated ischemia. Propranolol 100 mumol/L, which blocks DAG production from metabolites produced by PLD catalysis, completely abolished the protective effects of both metabolic preconditioning and (R)-PIA exposure in myocytes.

CONCLUSIONS

We conclude that PLD stimulation is involved in the protection of ischemic preconditioning in the rabbit heart.

The influence of acute ethanol ingestion on phospholipase D activity in rat pancreas. An in vitro and in vivo study

CONCLUSION

Since phosphatidic acid (PA), a product of phospholipase D(PLD), is known as a second messenger probably involved in cell proliferation and differentiation, our results potentially suggest a new mechanism for pancreatic tissue injury after ethanol ingestion.

BACKGROUND

The mechanisms by which ethanol causes pancreatic injury are still not clear. In vitro studies have suggested a relationship of PLD to ethanol metabolism. This study was undertaken to establish the involvement of PLD in ethanol metabolism in isolated pancreatic acini and to determine the influence of acute ethanol ingestion on PLD activity in pancreas and pancreatic growth after cerulein (Ce) infusion.

METHODS

Dispersed pancreatic acini prelabeled with 3H myristic acid were incubated with 500 pM Ce in the presence of different concentrations of ethanol; then labeled PA and phosphatidylethanol (PEt) production were measured under the same experimental conditions. For in vivo study, male rats were infused with Ce (0.25 microgram/kg/h) or saline; 1 h before infusion, animals were treated with 40% ethanol (5 g/kg p.o.) or saline, respectively. After 1, 2, and 48 h of Ce infusion, rats were killed; dispersed pancreatic acini were then prepared and PLD activity was measured. Pancreatic weight, protein, RNA, and DNA content were also established.

RESULTS

The production of PEt in vitro after Ce stimulation was significantly elevated with 1% ethanol in the medium. In the presence of different concentrations of ethanol (0.5-2%), a significant inhibition of PA accumulation in in vitro experiments was observed. The decrease of PA accumulation with ethanol was parallel to the increase of PEt production under the same experimental conditions. PLD activity was significantly elevated after 1 and 2 h of Ce infusion (116 and 105%, respectively), reaching control value after 48 h. Acute ethanol ingestion significantly diminished PLD activity after 1 and 2 h. After 48 h of Ce infusion, a significant increase in pancreatic weight, protein, RNA, and DNA content in pancreatic tissue was found. Ethanol was not able to influence pancreatic weight, proteins and RNA content. However, it had the potency to diminish DNA content after 48 h of Ce infusion.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

Regulation of phospholipase D by tyrosine kinases

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

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.

Phospholipase D activation by P2Z-purinoceptor agonists in human lymphocytes is dependent on bivalent cation influx

The role of bivalent cations in ATP-stimulated phospholipase D (PLD) activity was investigated in human leukaemic lymphocytes. Cells were labelled with [3H]oleic acid and incubated with extracellular ATP or benzoylbenzoic ATP in the presence of 1 mM Ca2+ and butanol, and PLD activity was assayed by the accumulation of [3H]phosphatidylbutanol ([3H]PBut). ATP stimulated PLD activity in a dose-dependent manner, and the inhibitory effects of suramin, oxidized ATP and extracellular Mg2+ suggested that the effect of ATP was mediated by P2Z purinoceptors known to be present on lymphocytes. Thapsigargin increased cytosolic [Ca2+] but did not stimulate PLD activity, whereas preloading cells with a Ca2+ chelator reduced cytosolic [Ca2+] and, paradoxically, potentiated ATP-stimulated [3H]PBut accumulation. ATP-stimulated [3H]PBut formation was supported by both Ba2+ and Sr2+ when they were substituted for extracellular Ca2+. Addition of EGTA to block bivalent cation influx inhibited the majority of ATP-stimulated PLD activity. Furthermore ATP-stimulated PLD activity showed a linear relationship to extracellular [Ba2+], and ATP-induced 133Ba2+ influx also had a linear dependence on extracellular [Ba2+]. These results suggest that ATP stimulates PLD activity in direct proportion to the influx of bivalent cations through the P2Z-purinoceptor ion channel and that this PLD activity is insensitive to changes in bulk cytosolic [Ca2+].

Sodium fluoride inhibits the antisecretory effect of peptide YY and its analog in rabbit jejunum

The antisecretory peptide YY (PYY) inhibits jejunal secretion through and inhibitory protein (Gi), whereas sodium fluoride (NaF) is a potent activator of G-proteins. This work was conducted to characterize the role of NaF in the antisecretory effect of PYY. For this purpose, electrogenic chloride secretion was assessed by measuring the in vitro variations in short-circuit current (delta Isc) due to alterations in ionic transport, using Ussing chambers Results: 1) NaF induced a transient increase in Isc at concentrations exceeding 5 mM. 2) 2 mM NaF inhibited the antisecretory effect of 0.1 microM PYY and of its analog P915. 3) stimulation of secretion by forskolin and dbcAMP was halved in the presence of 2 mM NaF. 4) Inhibition of protein kinase C by 0.1 mM bisindolylmaleimide caused a sustained increase in Isc in the presence of 5 mM NaF. In conclusion, these results confirm that PYY inhibits electrogenic chloride secretion and show that NaF stimulates it, and suggest that NaF reduces PYY-induced inhibition via a G-dependent and a G-independent functional pathway.