Is phosphatidic acid a calcium ionophore under neurohumoral control?

Calcium in the action of growth factors

The proliferation of cells in vivo and in culture is regulated by polypeptide growth factors, such as epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). Binding of growth factors to their specific cell-surface receptors initiates a cascade of biochemical events in the cell which ultimately leads to DNA synthesis and cell division. Immediate consequences of receptor activation include tyrosine-specific protein phosphorylations, a sustained increase in cytoplasmic pH and a transient rise in cytoplasmic free Ca2+. The PDGF-induced Ca2+ signal is due to Ca2+ release from intracellular stores, whereas EGF seems to activate a voltage-independent Ca2+ channel in the plasma membrane. Monoclonal antibodies to the EGF receptor that stimulate the tyrosine-specific protein kinase fail to raise [Ca2+]i and are not mitogenic for quiescent cells. These results suggest that activation of the EGF receptor tyrosine kinase is not sufficient for the induction of a Ca2+ signal, and that the rise in [Ca2+]i is indispensable for cell proliferation.

Modification of intracellular free calcium in cultured A10 vascular smooth muscle cells by exogenous phosphatidic acid

Exogenous phosphatidic acid (PA) was observed to produce a concentration-dependent increase in [Ca(2+)](i) in cultured A10 vascular smooth muscle cells. Preincubation of cells with sarcoplasmic reticulum Ca(2+)-ATPase inhibitors (cyclopiazonic acid and thapsigargin), a phospholipase C inhibitor (2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate), inositol 1,4,5-trisphosphate receptor antagonists (2-aminoethoxydiphenyl borate and xestospongin), and an activator of protein kinase C (PKC) (phorbol 12-myristate 13-acetate) depressed the PA-evoked increase in [Ca(2+)](i). Although EGTA, an extracellular Ca(2+) chelator, decreased the PA-induced increase in [Ca(2+)](i), sarcolemmal Ca(2+)-channel blockers (verapamil or diltiazem) did not alter the action of PA. On the other hand, inhibitors of PKC (bisindolylmaleimide I) and G(i)-protein (pertussis toxin) potentiated the increase in [Ca(2+)](i) evoked by PA significantly. These results suggest that the PA-induced increase in [Ca(2+)](i) in vascular smooth muscle cells may occur upon the activation of phospholipase C and the subsequent release of Ca(2+) from the inositol 1,4,5-trisphosphate-sensitive Ca(2+) pool in the sarcoplasmic reticulum. This action of PA may be mediated through the involvement of PKC.

Modulation of membrane curvature by phosphatidic acid and lysophosphatidic acid

The local generation of phosphatidic acid plays a key role in the regulation of intracellular membrane transport through mechanisms which are largely unknown. Phosphatidic acid may recruit and activate downstream effectors, or change the biophysical properties of the membrane and directly induce membrane bending and/or destabilization. To evaluate these possibilities, we determined the phase properties of phosphatidic acid and lysophosphatidic acid at physiological conditions of pH and ion concentrations. In single-lipid systems, unsaturated phosphatidic acid behaved as a cylindrical, bilayer-preferring lipid at cytosolic conditions (37 degrees C, pH 7.2, 0.5 mM free Mg2+), but acquired a type-II shape at typical intra-Golgi conditions, a mildly acidic pH and submillimolar free Ca2+ (pH 6.6-5.9, 0.3 mM Ca2+). Lysophosphatidic acid formed type-I lipid micelles in the absence of divalent cations, but anhydrous cation-lysophosphatidic acid bilayer complexes in their presence. These data suggest a similar molecular shape for phosphatidic acid and lysophosphatidic acid at cytosolic conditions; however, experiments in mixed-lipid systems indicate that their shape is not identical. Lysophosphatidic acid stabilized the bilayer phase of unsaturated phosphatidylethanolamine, while the opposite effect was observed in the presence of phosphatidic acid. These results support the hypothesis that a conversion of lysophosphatidic acid into phosphatidic acid by endophilin or BARS (50 kDa brefeldin A ribosylated substrate) may induce negative spontaneous monolayer curvature and regulate endocytic and Golgi membrane fission. Alternative models for the regulation of membrane fission based on the strong dependence of the molecular shape of (lyso)phosphatidic acid on pH and divalent cations are also discussed.

Involvement of phospholipase D in store-operated calcium influx in vascular smooth muscle cells

In non-excitable cells, sustained intracellular Ca2+ increase critically depends on influx of extracellular Ca2+. Such Ca2+ influx is thought to occur by a 'store-operated' mechanism, i.e. the signal for Ca2+ entry is believed to result from the initial release of Ca2+ from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Here we show that the depletion of cellular Ca2+ stores by thapsigargin or bradykinin is functionally linked to a phosphoinositide-specific phospholipase D (PLD) activity in cultured vascular smooth muscle cells (VSMC), and that phosphatidic acid formed via PLD enhances sustained calcium entry in this cell type. These results suggest a regulatory role for PLD in store-operated Ca2+ entry in VSMC.

Phosphatidic acid induces calcium influx in neutrophils via verapamil-sensitive calcium channels

Phosphatidic acid (PA) induces a biphasic Ca(2+) mobilization response in human neutrophils. The initial increase is due to the mobilization of Ca(2+) from intracellular stores, whereas the secondary increase is due to the influx of Ca(2+) from extracellular sources. The present investigation characterizes PA-induced Ca(2+) influx in neutrophils. Depolarization of neutrophils by 50 mM KCl enhanced PA-induced Ca(2+) influx, whereas verapamil, a Ca(2+) channel blocker, attenuated this response in a dose-dependent manner. These observations suggest that PA-induced Ca(2+) influx is mediated via verapamil-sensitive Ca(2+) channels. Stimulation of neutrophils with exogenous PA results in accumulation of endogenously generated PA with a time course similar to the effects of exogenous PA on Ca(2+) influx. Ethanol inhibited the accumulation of endogenous PA and calcium mobilization, indicating that activation of membrane phospholipase D plays a role in PA-mediated Ca(2+) influx. The results of this study suggest that exogenously added PA stimulates the generation of intracellular PA, which then mediates Ca(2+) influx through verapamil-sensitive Ca(2+) channels.

Phosphatidic acid-induced elevation of intracellular Ca2+ is mediated by RhoA and H2O2 in Rat-2 fibroblasts

We have investigated possible roles of RhoA and H2O2 in the elevation of intracellular Ca2+ ([Ca2+]i) by phosphatidic acid (PA) in Rat-2 fibroblasts. PA induced a transient elevation of [Ca2+]i in the presence or absence of EGTA. Lysophosphatidic acid (LPA) also increased [Ca2+]i, but the sustained Ca2+ response was inhibited by EGTA. LPA stimulated the production of inositol phosphates, but PA did not. In the presence of EGTA, preincubation with LPA completely blocked the subsequent elevation of [Ca2+]i by PA, but not vice versa. PA stimulated the translocation of RhoA to the particulate fraction as did LPA. Scrape loading of C3 transferase inhibited the transient Ca2+ response to PA, but not to LPA, suggesting an essential role of RhoA in the elevation of [Ca2+]i by PA. H2O2 also induced a transient increase of [Ca2+]i as did PA. H2O2 scavengers, catalase and N-acetyl-L-cysteine, completely blocked the rise of [Ca2+]i stimulated by PA, but not by LPA. Furthermore, preincubation with PA blocked the subsequent Ca2+ response to H2O2, and the incubation with H2O2 also blocked the PA-induced rise of [Ca2+]i. Thus, it was suggested that PA stimulated Ca2+ release from PA-sensitive, but not inositol 1,4,5-trisphosphate-sensitive, Ca2+ stores by the activation of RhoA and intracellular H2O2.

Activation of intestinal mitochondrial phospholipase D by polyamines and monoamines

Intestinal mitochondria have a phospholipase D (PLD) activity which was stimulated by polyamines and monoamines resulting in the formation of phosphatidic acid (PA) from endogenous phospholipids. When stimulated by polyamines, mitochondrial PLD utilized endogenous phosphatidylethanolamine (PE) as substrate whereas stimulated by monoamines, both PE and phosphatidylcholine (PC) were hydrolysed. Stimulation of PA formation by spermine was enhanced by the presence of calcium. Since polyamines are known to alter the calcium transport by mitochondria and PA is known to possess an ionophore effect, stimulation of PA formation in mitochondria by polyamines suggests that polyamine-induced alteration in calcium homeostasis might involve a PA related mechanism.

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.

Gene cloning and characterization of CDP-diacylglycerol synthase from rat brain

A cDNA encoded a 462-amino acid protein, which showed CDP-diacylglycerol synthase (CDS) activity was cloned for the first time as the vertebrate enzyme molecule from rat brain cDNA library. The deduced molecular mass of this rat CDS was 53 kDa, and putative primary structure included several possible membrane- spanning regions. At the amino acid sequence level, rat CDS shared 55.5%, 31. 7%, and 20.9% identity with already known Drosophila, Saccharomyces cerevisiae, and Escherichia coli CDS, respectively. This rat CDS preferred 1-stearoyl-2-arachidonoyl phosphatidic acid as a substrate, and its activity was strongly inhibited by phosphatidylglycerol 4, 5-bisphosphate. By immunoblotting analysis of COS cells overexpressed with the epitope-tagged for rat CDS, a 60-kDa band was detected. By epitope-tag immunocytochemistry, the CDS protein was mainly localized in close association with the membrane of the endoplasmic reticulum of the transfected cells. The intense mRNA expression of CDS was localized in the cerebellar Purkinje cells, the pineal body, and the inner segment of photoreceptor cells. Additionally, very intense expression was detected in postmitotic spermatocytes and spermatids.

Exogenous phosphatidic acid with saturated short-chain fatty acyl groups induces superoxide anion release from guinea pig peritoneal polymorphonuclear leukocytes by three different mechanisms

Treatment of suspensions of guinea pig peritoneal polymorphonuclear leukocytes (PMN) with four species of phosphatidate (PA) containing short-chain fatty acids induced sustained superoxide anion (O2-) production after a lag time. The rank order of efficiency of these PAs in triggering O2- production was PA8:0 [1,2-dioctanoyl-sn-glycerol-3-phosphate (GP)] > PA10:0 (1,2-didecanoyl-GP) > PA6:0 (1,2-dicaproyl-GP) > > PA12:0 (1,2-dilauroyl-GP). The O2- release from PMN stimulated with PA10:0 or PA12:0, but not with PA6:0 or PA8:0, was lowered by the addition of 1 mM extracellular Ca2+. Studies with various inhibitors showed that the mechanism of multiphasic O2- production induced by PA8:0 depended on its concentration: 1 and 3 microM PA8:0 induced O2- production constantly after a lag time through a protein kinase-dependent mechanism that was inhibited by 100 nM staurosporine. With concentrations of PA of 10 microM or more, an additional mechanism that was independent of protein kinase became operative and predominant over the protein kinase-dependent one. This protein kinase-independent mechanism was inhibited selectively by 80 microM TMB-8. Concentrations of 30, 60 and 100 microM PA first elicited transient O2- production via another protein kinase-dependent mechanism that was more sensitive to H-7 than to staurosporine, and then sustained O2- production, mainly driven by the protein kinase-independent mechanism. Metabolism of exogenously added [14C]PA8:0 in intact PMN was examined in the presence and absence of propranolol. Results suggest that PA itself is more important rather than its degradation products such as diacylglycerol, in inducing O2- production via three different mechanisms described above.

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.

Phosphatidic acid: a lipid messenger involved in intracellular and extracellular signalling

Generated during the initial phases of cell signalling, phosphatidic acid has been implicated as a messenger involved in the activation of cellular kinases and phospholipases as well as certain proto-oncogene products and low-molecular-weight G-proteins. Although many of the reported effects of phosphatidic acid can be attributed to metabolites generated by cellular hydrolases, the parent compound clearly possesses important biological activities. However, instead of acting as a ubiquitous second messenger mediating signalling events shared by a wide variety of cells, in many systems the phospholipid seems to function specifically, regulating unique functions confined to specialized groupings of cells. One such function is neutrophil superoxide generation, which is induced when phosphatidic acid, generated by activated phospholipase D (PLD), facilitates the interaction of a cytoplasmic low-molecular-weight G-protein with dormant, membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Positioned on the outer surface of the plasma membrane of triggering cells, phosphatidic acid potentially mediates the "juxtacrine" stimulation of cells in direct contact. This review critically evaluates the known biological effects of phosphatidic acid as opposed to functions induced by its metabolites and addresses the mechanisms by which these effects are specifically induced by this phospholipid.

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.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.

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.

Phosphatidic acid activation of protein kinase C in LA-N-1 neuroblastoma cells

Phosphatidic acid (PA), a hydrolytic product of phospholipase D activity, stimulated cytosolic protein kinase C (PKC) activity when LA-N-1 neuroblastoma cells in culture were treated with PA, without translocating the enzyme to the membrane. Treatment of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) translocated and activated PKC in a dogmatic manner. Partially purified PKC activity derived from LA-N-1 neuroblastoma cells was stimulated by PA alone or in the presence of phosphatidylserine or TPA, without affecting [3H]phorbol dibutyrate binding, indicating that the site of action of PA was different from the phorbol ester or diacylglycerol binding site. These results suggest an unorthodox pattern of PKC stimulation mediated by PA which appears to be yet another mode of PA signal transduction.

Phosphatidic acid and lysophosphatidic acid induce haptotactic migration of human monocytes

The present study was aimed at defining the chemotactic activity of phosphatidic acid, which is rapidly produced by phagocytes in response to chemotactic agonists. Exogenously added phosphatidic acid induced human monocyte directional migration across polycarbonate filters with an efficacy (number of cell migrated) comparable to that of "classical" chemotactic factors. In lipid specificity studies, activity of phosphatidic acid decreased with increasing acyl chain length but was restored by introducing unsaturation in the acyl chain with the most active form being the natural occurring 18:0,20:4-phosphatidic acid. Lysophosphatidic acid was also active in inducing monocyte migration. No other phospholipid and lysophospholipid tested was effective in this response. Monocyte migration was regulated by a gradient of phosphatidic acid and lysophosphatidic acid bound to the polycarbonate filter, in the absence of detectable soluble chemoattractant. Migration was also observed if phospholipids were bound to fibronectin-coated polycarbonate filters. Thus, phosphatidic acid and lysophosphatidic acid, similarly to other physiological chemoattractants (e.g. C5a and interleukin-8), induce cell migration by an haptotactic mechanism. Phosphatidic acid caused a rapid increase of filamentous actin and, at higher concentrations, induced a rise of intracellular calcium concentration. Monocyte migration to phosphatidic acid and lysophosphatidic acid, but not to diacylglycerol, was inhibited in a concentration-dependent manner by Bordetella pertussis toxin, while cholera toxin was ineffective. In the chemotactic assay, phosphatidic acid and lysophosphatidic acid induced a complete homologous desensitization and only partially cross-desensitized one with each other, or with diacyl-glycerol and monocyte chemotactic protein-1. Suramine inhibited monocyte chemotaxis with a different efficiency phosphatidic acid > lysophosphatidic acid" diacyl-glycerol On the contrary, monocyte chemotactic protein-1-induced chemotaxis was not affected by the drug. Collectively, these data show that phosphatidic acid induces haptotactic migration of monocytes that is at least in part receptor-mediated. These results support a role for phosphatidic acid and lysophosphatidic acid in the regulation of leukocyte accumulation into tissues.

Purification and characterization of novel plasma membrane phosphatidate phosphohydrolase from rat liver

An N-ethylmaleimide-insensitive phosphatidate phosphohydrolase, which also hydrolyzes lysophosphatidate, was isolated from the plasma membranes of rat liver. The specific activity of an anionic form of the enzyme (53 kDa, pI < 4) was increased 2700-fold. A cationic form of enzyme (51 kDa, pI = 9) was purified to homogeneity, but the -fold purification was low because the activity of the highly purified enzyme was unstable. Immunoprecipitating antibodies raised against the homogeneous protein confirmed the identity of the cationic protein as the phosphohydrolase and were used to identify the anionic enzyme. Both forms are integral membrane glycoproteins that were converted to 28-kDa proteins upon treatment with N-glycanase F. Treatment of the anionic form with neuraminidase allowed it to be purified in the same manner as the cationic enzyme and yielded an immunoreactive protein with a molecular mass identical to the cationic protein. Thus, the two ionic forms most likely represent different sialated states of protein. An immunoreactive 51-53-kDa protein was detected in rat liver, heart, kidney, skeletal muscle, testis, and brain. Little immunoreactive 51-53-kDa protein was detected in rat thymus, spleen, adipose, or lung tissue. This work provides the tools for determining the regulation and function of the phosphatidate phosphohydrolase in signal transduction and cell activation.

Dependence of stimulus-transcription coupling on phospholipase D in agonist-stimulated pituitary cells

Stimulation of phospholipase D activity is frequently observed during agonist activation of Ca(2+)-mobilizing receptors, but the cellular functions of this signaling pathway are not well defined. Pituitary gonadotrophs express Ca(2+)-mobilizing receptors for gonadotropin-releasing hormone (GnRH) and endothelin (ET), activation of which stimulates luteinizing hormone secretion and transient expression of c-fos. In pituitary cells and alpha T3-1 gonadotrophs, GnRH action was associated with both initial and sustained diacylglycerol (DG) production, whereas ET-1 induced only a transient DG response. Also, phospholipase D activity, estimated by the production of phosphatidylethanol from phosphatidylcholine in the presence of ethanol, was stimulated by GnRH but not ET-1. Such formation of phosphatidylethanol at the expense of phosphatidic acid (PA) during GnRH-induced activation of phospholipase D significantly reduced the production of PA, DG, and cytidine diphosphate diacylglycerol. Inhibition of PA-phosphohydrolase activity by propranolol also decreased GnRH-induced DG production and, in contrast to ethanol, increased PA and cytidine diphosphate diacylglycerol levels. The fall in DG production caused by ethanol and propranolol was accompanied by inhibition of GnRH-induced c-fos expression, whereas agonist-induced luteinizing hormone release was not affected. In contrast to their inhibitory actions on GnRH-induced early gene expression, neither ethanol nor propranolol affected ET-1-induced c-fos expression, or GnRH- and ET-1-induced inositol trisphosphate/Ca2+ signaling. These findings demonstrate that phospholipase D participates in stimulus-transcription but not stimulus-secretion coupling, and indicate that DG is the primary signal for this action.

Effect of carbachol on phospholipase C-mediated phosphatidylinositol 4,5-bisphosphate hydrolysis, and its modulation by isoproterenol in rabbit corneal epithelial cells

The effects of carbachol (CCh) on phospholipase C(PLC)-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and its modulation by isoproterenol were investigated in SV40-adenovirus transformed rabbit corneal epithelial cells (RCEC). When examined under light microscope, these cells exhibited a cobblestone-like appearance typical of the corneal epithelial cells grown in primary culture. Addition of CCh (0.1 mM) for 30 min to RCEC, prelabeled with 32Pi, decreased the radioactivity in phosphatidylinositol 4-phosphate and PIP2 by 15 and 27%, respectively, and concomitantly increased the radioactivity in phosphatidylinositol and phosphatidic acid by 14 and 38%, respectively. When the concentration of CCh was increased to 1 mM, the changes in radioactivity were even more pronounced. Addition of CCh (0.1 mM) to the cells, prelabeled with myo[3H]inositol, increased the accumulation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3) by 115%, indicating stimulation of PLC-mediated PIP2 hydrolysis. Similar increases were also observed in [3H]InsP1 and [3H]InsP2. The effects of CCh on inositol phosphate accumulation were time- and dose-dependent, and were inhibited by atropine (10 microM), suggesting that the observed effects of CCh were mediated by activation of muscarinic cholinergic receptors. The effects of CCh were antagonized more potently by 4-diphenylacetoxy N-methyl-piperidine than by pirenzepine, indicating that the muscarinic receptors involved in PLC activation are probably of M3 type. By Western immunoblotting analysis with various anti-PLC antibodies, the RCEC were shown to contain PLC gamma 1 and PLC delta 1 in the soluble fraction and PLC beta 1 in the microsomal fraction. Addition of isoproterenol to RCEC, increased cAMP both in a time- and dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Time-course changes in content and fatty acid composition of phosphatidic acid from rat thymocytes during concanavalin A stimulation

Several studies have shown the potential role of phosphatidic acid (PA) as a second messenger in different cell types. Thus, PA has been shown to mimic physiological agonists leading to various cellular responses, such as neurotransmitter and hormone release, cell proliferation by modulating DNA or RNA synthesis, the expression of several proto-oncogenes and growth factors, and the stimulation of enzyme activities such as phospholipase C (PLC), protein kinases and cyclic AMP (cAMP) phosphodiesterase. Stimulation of [3H]arachidonate-labelled rat thymocytes with the mitogen lectin concanavalin A (con A) resulted in enhanced production of radiolabelled PA after only 5 min of activation. The radiolabelled PA increase corresponded to a real increase in PA mass as determined by GLC quantification of its fatty acid content. In the presence of ethanol (0.5%), formation of phosphatidylethanol was not observed after 5 min of con A activation. Pretreatment of cells with R 59022 (10 microM), a diacylglycerol (DAG) kinase inhibitor, showed an inhibition in the formation of radiolabelled PA and in PA mass. These results suggest that the PLC-DAG kinase may be the pathway for PA synthesis in the first minutes of mitogenic thymocyte activation. A detailed analysis of the fatty acid composition showed that the relative amount of unsaturated fatty acids was increased in PA from stimulated cells concomitantly with a decrease in saturated ones; in particular, arachidonic acid was increased approximately 2-fold only 2 min after con A addition whereas palmitic acid was decreased for the whole period investigated (20 min). These changes favour the hydolysis of phosphoinositides rather than phosphatidylcholines by PLC. As PA remains a minor phospholipid, these changes are unlikely to affect cell membrane fluidity; but PA being now well recognized as a potential second messenger, its increased content as well as its increased unsaturation in the fatty acyl moiety might modulate several signalling pathways or the activity of enzymes such as cyclic nucleotide phosphodiesterase, controlling in this way the cellular level of cAMP, a negative regulator of blastic transformation.

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

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

Activation of diacylglycerol kinase by carbachol in guinea pig taenia coli

Changes in diacylglycerol kinase (DG kinase) activity in carbachol (CCh)-stimulated guinea pig taenia coli were investigated. In a mixed micellar assay system, added 1,2-dioctanoyl-sn-glycerol (diC8) and endogenous DG were competitively bound to common DG kinase isozymes from guinea pig taenia coli and phosphorylated, suggesting that diC8 is useful as a probe of agonist effects on DG kinase activity. In phosphorus-32 ([32P]Pi)- and diC8-prelabeled guinea pig taenia coli, diC8 was phosphorylated by DG kinase to [32P]dioctanoyl-phosphatidic acid ([32P]diC8-PA). CCh increased the accumulation of both [32P]diC8-PA and endogenous [32P]phosphatidic acid ([32P]PA) in a time- and dose-dependent manner (0.1-100 microM CCh). CCh-induced increases in [32P]diC8-PA and [32P]PA were inhibited by 1 microM atropine and 3 microM DG kinase inhibitor (R59022). These findings indicated the activation of DG kinase by muscarinic receptor stimulation in guinea pig taenia coli. Therefore, DG kinase activation may play an important role in CCh-induced PA formation. CCh-induced [32P]diC8-PA and [32P]PA accumulation was dependent on intracellular calcium concentrations. However, a KCl-induced increase in intracellular calcium, without receptor stimulation, was ineffective. Moreover, treatment with phorbol ester also increased accumulation of both PA species in KCl-treated tissues. These findings suggest that muscarinic receptor mediated activation of DG kinase may require both an increase in intracellular calcium and PKC activation in guinea pig taenia coli.

Activation of phospholipase D in human platelets by collagen and thrombin and its relationship to platelet aggregation

Stimulation of phospholipase D after activation of cell surface receptors has been reported in many cell types. We have investigated the mechanism of activation of this enzyme by collagen in the human platelet by assaying the release of [3H]methylcholine from [3H]methylphosphatidylcholine. Results from these studies suggest that phospholipase D activity is regulated by reversible phosphorylation. Phospholipase D activity was stimulated when platelet-rich plasma was preincubated with collagen and was not inhibited by aspirin. Among various aggregating agents tested, collagen and thrombin but not ADP activated phospholipase D activity (2- to 3-fold). The addition of sphingosine inhibited phospholipase D activity. Preincubation of platelet-rich plasma with sphingosine inhibited collagen- and thrombin-induced platelet aggregation and the release of ATP. The inhibitory effect of sphingosine on collagen- and thrombin- induced platelet aggregation and release of ATP was dose-dependent. The functional significance of phospholipase D activation was also tested by examining the effect of the product, phosphatidic acid, on collagen-induced platelet aggregation and release of ATP. Platelet shape change and the reversibility of platelet aggregation resulted by the addition of phosphatidic acid to platelet-rich plasma. Furthermore, the simultaneous addition of phosphatidic acid and collagen shortened the latency period but had no effect on platelet aggregation. Two platelet proteins (47 kDa and 22 kDa) increased in phosphorylation after the addition of 1 microM phosphatidic acid which did not cause platelet aggregation. These results suggest that collagen stimulates phospholipase D activity which plays a secondary role in platelet aggregation and the release reaction.

Non-genomic signal transduction pathway of vitamin D in muscle

The secosteroid hormone 1,25(OH)2-vitamin D3 rapidly activates voltage-dependent Ca2+ channels of the L-type in skeletal and cardiac muscle cells by a non-genomic mechanism which involves guanine nucleotide binding (G) protein-medicated stimulation of the adenylate cyclase/cAMP/protein kinase A messenger system. Modifications in calmodulin intracellular distribution induced by PKA-dependent membrane protein phosphorylation may participate in the fast regulation of muscle Ca2+ influx by 1,25(OH)2D3. The protein kinase C pathway also plays a role modulating 1,25(OH)2D3 signal transduction in muscle by cross-talk with the PKA system. The hormone sequentially activates phospholipases C and D providing diacylglycerol for PKC activation and inositol triphosphate for intracellular Ca2+ mobilization. In addition, 1,25(OH)2D3 rapidly stimulates phospholipase A2 generating arachidonic acid for the eicosanoid pathway. Specificity of hormone effects suggests that binding to a muscle membrane-bound receptor mediates these events.

Angiotensin II stimulates NADH and NADPH oxidase activity in cultured vascular smooth muscle cells

The signaling pathways involved in the long-term metabolic effects of angiotensin II (Ang II) in vascular smooth muscle cells are incompletely understood but include the generation of molecules likely to affect oxidase activity. We examined the ability of Ang II to stimulate superoxide anion formation and investigated the identity of the oxidases responsible for its production. Treatment of vascular smooth muscle cells with Ang II for 4 to 6 hours caused a 2.7 +/- 0.4-fold increase in intracellular superoxide anion formation as detected by lucigenin assay. This superoxide appeared to result from activation of both the NADPH and NADH oxidases. NADPH oxidase activity increased from 3.23 +/- 0.61 to 11.80 +/- 1.72 nmol O2-/min per milligram protein after 4 hours of Ang II, whereas NADH oxidase activity increased from 16.76 +/- 2.13 to 45.00 +/- 4.57 nmol O2-/min per milligram protein. The NADPH oxidase activity was stimulated by exogenous phosphatidic and arachidonic acids and was partially inhibited by the specific inhibitor diphenylene iodinium. NADH oxidase activity was increased by arachidonic and linoleic acids, was insensitive to exogenous phosphatidic acid, and was inhibited by high concentrations of quinacrine. Both of these oxidases appear to reside in the plasma membrane, on the basis of migration of the activity after cellular fractionation and their apparent insensitivity to the mitochondrial poison KCN. These observations suggest that Ang II specifically activates enzyme systems that promote superoxide generation and raise the possibility that these pathways function as second messengers for long-term responses, such as hypertrophy or hyperplasia.

Activation of phospholipase D by guanosine 5'[gamma-thio]triphosphate and AlF4- in bovine corneal epithelial cells

We have investigated the regulation of phospholipase D (PLD) by guanine nucleotides and AlF4- in bovine corneal epithelial cells (BCEC) prelabeled with [3H]myristic acid. In the presence of ethanol, AlF4- increased the production of [3H]PA and [3H]PET indicating activation of PLD in these cells. The effects of AlF4- were time- and dose-dependent. Addition of guanosine 5[gamma-thio]triphosphate (GTP gamma S), to streptolysin O-permeabilized cells also resulted in increased accumulation of [3H]PA and [3H]PEt. Other guanine and adenine nucleotides were ineffective, and guanosine thiodiphosphate inhibited the GTP gamma S-induced activation of PLD. Direct addition of GTP gamma S to microsomal fraction prepared from [3H]myristate-labeled BCEC resulted in increased formation of [3H]PEt in a time- and dose-dependent manner. The activation of PLD by GTP gamma S in the microsomal fraction was absolutely dependent on the presence of Ca2+ > 0.5 microM. Addition of Ca2+ (10-100 microM) alone dose-dependently stimulated the PLD activity. Treatment of the microsomal fraction with phorbol esters had no effect on the ability of GTP gamma S to stimulate PLD. Addition of isoproterenol to BCEC resulted in several-fold stimulation of cAMP, but it had no effect on basal or PDBu-induced stimulation of PLD. Taken together, the data suggest that a GTP-binding protein is involved in regulation of PLD in BCEC, and that maximal stimulation of PLD probably results from an interaction between Ca2+, PKC and G-protein in BCEC.

Phospholipases and melatonin signal transduction in the ovine pars tuberalis

The potential role of phospholipases in mediating melatonin-dependent inhibition of adenylyl cyclase was investigated in pars tuberalis (PT) cultures. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulated the release of choline metabolites and increased the transphosphatidylation reaction. The calcium ionophore A23187 stimulated the release of arachidonic acid from cultures. These observations demonstrate phospholipase A and D activities within PT. Phosphatidic acid inhibited forskolin-stimulated production of cyclic AMP both in PT cells and in membrane preparations. This indicates that melatonin could inhibit adenylyl cyclase by increasing phosphatidic acid levels through activation of cellular phospholipases. Melatonin did not stimulate the release of arachidonic acid or choline from PT cultures, nor did it increase intracellular levels of hydrophobic second messengers or stimulate transphosphatidylation. Therefore melatonin does not stimulate phospholipase A and D pathways in PT cells. However, these pathways are present in the PT and their activation could potentially modulate the cellular actions of melatonin.

Phosphatidic acid promotes phosphoinositide metabolism and DNA synthesis in cultured cortical astrocytes

The addition of exogenous phosphatidic acid (PA) to cultured cortical astrocytes prelabelled with [3H]inositol resulted in the accumulation of intracellular [3H]inositol phosphates (IP) in a concentration-dependent (EC50 = 20 microM) manner. Analysis of the individual IPs formed following a PA challenge revealed a rapid but transient generation of [3H]inositol trisphosphate (IP3) indicating the involvement of phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown in this response a fact which was confirmed when the recovery of radiolabel in membrane phosphoinositides was assessed. PA's ability to stimulate IP3 accumulation was found to be dependent upon its acyl-chain length. Dioleoyl-PA (C18:1) was equally as effective as PA from egg yolk lecithin in this respect whilst dipalmitoyl-PA (C16:0) was less so and dimyristoyl-PA (C14:0) and dilauroyl-PA (C12:0) were without effect. In subconfluent, serum-deprived cultures, PA was found to increase DNA synthesis following a 48 h exposure period. This effect was observed over the same concentration range used to measure phosphoinositide breakdown and was found to be mediated by the activation of protein kinase C. As with its effect on phosphoinositide metabolism, PA's ability to promote DNA synthesis was correlated with its acyl-chain length. These data show that PA is capable of stimulating both phosphoinositide metabolism and DNA synthesis in cultured astrocytes possibly via the activation of specific membrane receptors. However, the precise relationship between these events remains to be elucidated.

Activation of actin polymerization by phosphatidic acid derived from phosphatidylcholine in IIC9 fibroblasts

alpha-Thrombin induced a change in the cell morphology of IIC9 fibroblasts from a semiround to an elongated form, accompanied by an increase in stress fibers. Incubation of the cells with phospholipase D (PLD) from Streptomyces chromofuscus and exogenous phosphatidic acid (PA) caused similar morphological changes, whereas platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA) induced different changes, e.g., disruption of stress fibers and cell rounding. alpha-Thrombin, PDGF, and exogenous PLD increased PA by 20-40%, and PMA produced a smaller increase. alpha-Thrombin and exogenous PLD produced rapid increases in the amount of filamentous actin (F-actin) that were sustained for at least 60 min. However, PDGF produced a transient increase of F-actin at 1 min and PMA caused no significant change. Dioctanoylglycerol was ineffective except at 50 micrograms/ml. Phospholipase C from Bacillus cereus, which increased diacylglycerol (DAG) but not PA, did not change F-actin content. Down-regulation of protein kinase C (PKC) did not block actin polymerization induced by alpha-thrombin. H-7 was also ineffective. Exogenous PA activated actin polymerization with a significant effect at 0.01 microgram/ml and a maximal increase at 1 microgram/ml. No other phospholipids tested, including polyphosphoinositides, significantly activated actin polymerization. PDGF partially inhibited PA-induced actin polymerization after an initial increase at 1 min. PMA completely or largely blocked actin polymerization induced by PA or PLD. These results show that PC-derived PA, but not DAG or PKC, activates actin polymerization in IIC9 fibroblasts, and indicate that PDGF and PMA have inhibitory effects on PA-induced actin polymerization.

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

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

Phospholipase D treatment enhances gonadotrophin receptor-coupled adenylate cyclase activity in isolated bovine luteal cells

LH-stimulated adenylate cyclase activity in membrane preparations of bovine luteal cells could be enhanced by treating the cells with either phospholipase D or its hydrolysis product, phosphatidic acid. Similar augmentary effects were also produced following treatment of the cells with EGF. Moreover, EGF could stimulate the formation of [3H]phosphatidic acid in [3H]myristic acid preloaded cells, suggesting that EGF is able to activate cellular phospholipase D. Also, PMA was able to increase the phosphatidic acid formation with a parallel increase in the adenylate cyclase activity. We propose, therefore, that phosphatidic acid may act as an intracellular second messenger linking EGF-mediated activation of phospholipase D with the sensitization of LH receptor-coupled adenylate cyclase signalling system.

Association between leukotriene B4-induced phospholipase D activation and degranulation of human neutrophils

We have explored the role of phospholipase D (PLD) activation in leukotriene B4 (LTB4)-induced Ca2+ mobilization and degranulation of human neutrophils. Stimulation of [3H]alkyl-acyl-phosphatidylcholine-labeled neutrophils with LTB4 resulted in a rapid accumulation of [3H]alkyl-phosphatidic acid (PA) as well as a somewhat slower accumulation of [3H]alkyl-diglyceride (DG). In the presence of ethanol, PLD catalyzed a transphosphatidylation reaction in which LTB4 increased [3H]alkyl-phosphatidylethanol formation and simultaneously decreased LTB4-induced PA and DG accumulation. This pattern of lipid metabolism is consistent with the conclusion that LTB4 stimulates PLD activity in human neutrophils. Additional studies in which the extracellular and intracellular concentrations of Ca2+ were varied indicated that maximal LTB4-induced PLD activation was dependent upon Ca2+ and potentiated by inhibitors of protein kinase C. The time-course and concentration-response curves for LTB4-induced PLD activation were different from those for LTB4-induced Ca2+ mobilization, as measured by fura-2 fluorescence. On the other hand, the concentration-response curve for LTB4-induced PLD activation was similar to that for LTB4-induced degranulation. Preincubation of the cells with ethanol inhibited LTB4-induced PA and DG accumulation, as well as degranulation, suggesting that one or both of these metabolites were important for this response. In contrast, ethanol had no effect on LTB4-induced Ca2+ mobilization. Propranolol, an inhibitor of phosphatidate phosphohydrolase, abolished DG accumulation in response to LTB4 but had no effect on degranulation, suggesting that PA is more important than DG as a mediator of degranulation. Taken collectively, these data indicate that LTB4-induced activation of PLD in human neutrophils is mediated by a Ca(2+)-dependent mechanism, but not by protein kinase C. In addition, PLD activation in these cells may induce degranulation, but not Ca2+ mobilization.

Regulation of diacylglycerol metabolism by vasoconstrictor hormones in intact small arteries

The initiation of receptor-mediated small artery contraction is dependent on inositol 1,4,5-trisphosphate-stimulated release of stored calcium. The role of the other product of inositol lipid hydrolysis, 1,2-diacylglycerol, in maintaining contraction remains controversial. Therefore, we have determined the contractile response of rat subcutaneous small arteries (< 300 microns i.d.), when mounted as ring preparations in a myograph, to noradrenaline, angiotensin II, KCl-induced membrane depolarization, and a cell-permeable diglyceride, dioctanoylglycerol. In parallel experiments, the conversion of this diglyceride to dioctanoylphosphatidate was studied in 32P-labeled vessels. Dioctanoylglycerol produced a slow-onset sustained contraction that was dependent on extracellular calcium. This was accompanied by the generation of the lipid dioctanoylphosphatidate. Noradrenaline and KCl induced rapid-onset sustained contractions and increased the production of dioctanoylphosphatidate (75% and 91%, respectively). In addition, dioctanoylglycerol levels were reduced (41%) after noradrenaline stimulation, suggesting activation of diacylglycerol kinase. In contrast, the contractile response to angiotensin II was transient, and this agonist did not significantly affect the conversion of dioctanoylglycerol to phosphatidate. Noradrenaline markedly increased (fourfold) the formation of endogenous phosphatidate, whereas endogenous 1,2-diacylglycerol was increased (47%) with angiotensin II. These results demonstrate that phosphatidate formation is regulated by vasoconstrictor hormones during receptor-mediated contraction, independent of diglyceride mass. Modulation of the levels of lipid second messengers downstream from phospholipid hydrolysis may represent a mechanism by which agonists that act through the same signaling system produce different contractile responses.

Enhancement of phospholipase A2 activation by phosphatidic acid endogenously formed through phospholipase D action in rat peritoneal mast cell

Contribution of phosphatidic acid (PA) generated by activated phospholipase (PL) D to PLA2 activation was studied in rat peritoneal mast cells. Exogenous didecanoyl PA induced arachidonate liberation in the permeabilized cells which was inhibited by p-bromophenacyl bromide. Upon exposure of the cells to ethanol in a high enough concentration to prevent PA formation, A23187-induced arachidonate liberation was suppressed by 50% and the rest was completely inhibited by p-bromophenacyl bromide. In contrast, propranolol, which enhanced PA accumulation, significantly increased the arachidonate liberation. These results suggest that A23187-induced PLA2 activation may be potentiated, at least in part, by PA generated through PLD action.

Phosphatidylinositol metabolism in hypertrophic rat heart

The accumulation of inositol 1,4,5-trisphosphate (IP3) after hormonal stimulation has a physiological role, possibly by alteration of Ca2+ levels in cardiac myocyte. However, this accumulation has not been studied under pathophysiological conditions. In this report, we examine phosphatidylinositol metabolism during cellular response to norepinephrine in pressure-overloaded hypertrophic rat heart. After stimulation with norepinephrine, the accumulations of IP3 and diacylglyceride significantly increased in isolated myocytes from stroke-prone spontaneously hypertensive rat (SHRSP) heart, indicating phosphatidylinositol-specific phospholipase C activity increased in SHRSP heart cells. Protein kinase C activity was also enhanced in SHRSP, with a marked increase in particulate activity. We determined the intracellular calcium concentration and found it to be higher in SHRSP than in Wistar-Kyoto (WKY) rats at 30-40 weeks of age. Ca2+ influx was also elevated in SHRSP stimulated by norepinephrine. In SHRSP heart, cytosolic Ca2+ concentration may rise quickly in response to some stimuli, such as alpha 1-adrenergic stimulation, which is shown to be one of the pathways that increases cytosolic Ca2+ levels in hypertrophied rat heart. These data suggest that a part of the phosphatidylinositol-turnover pathway, such as the phosphatidylinositol 4,5-bisphosphate-IP3-Ca2+ pathway or the diacylglyceride-protein kinase C pathway, may play an important role in the development of hypertrophy in SHRSP heart.

Effect of (1-34) parathyroid hormone-related peptide on the composition and turnover of phospholipids in syncytiotrophoblast brush border and basal plasma membranes of human placenta

The effect of parathyroid hormone-related peptide on the lipid composition and the turnover of phosphoinositides was studied in brush border and basal plasma membranes of human placenta syncytiotrophoblasts. Lipid composition of the two polar membranes differed markedly with respect to the cholesterol/phospholipid ratio (0.57 +/- 0.04 and 0.91 +/- 0.05 in basal plasma membranes and brush border membranes, respectively). Sphingomyelin was the major phospholipid in both membranes. Except for the phosphoinositide-phosphatidylserine complex which was higher in basal plasma membranes, the phospholipid composition was comparable in the brush border membrane and basal plasma membranes. Incubation of the tissue with 10(-8) M parathyroid hormone-related peptide (1-34) resulted in a significant increase in the phosphatidylinositol phosphate content of the two membranes and in the phosphatidylinositol biphosphate concentration in the basal plasma membranes. Finally, when the tissue was preincubated with [3H]myo-inositol in the presence of 10(-8) M parathyroid hormone-related peptide (1-34), the hormone significantly stimulated the inositol phosphate release by the two membranes. These results demonstrate that: (1) in the placental syncytiotrophoblast, as found in other transport epithelia, the lipid composition of the polar membranes is different; (2) parathyroid hormone-related peptide stimulates the phosphoinositide turnover in both membranes.

Phosphatidic acid stimulates inositol 1,4,5-trisphosphate production in adult cardiac myocytes

The cellular content of phosphatidic acid can increase in response to several agonists either by phosphorylation of diacylglycerol after phospholipase C-catalyzed hydrolysis of phospholipids or directly through activation of phospholipase D. Although previous findings indicated that the generation of phosphatidic acid was exclusively a means of regulation of the cellular concentration of diacylglycerol, more recent studies have indicated that phosphatidic acid may also directly regulate several cellular functions. Accordingly, the present study was performed to assess whether phosphatidic acid could stimulate cardiac phospholipase C in intact adult rabbit ventricular myocytes. The mass of inositol 1,4,5-trisphosphate [Ins (1,4,5)P3] was determined by a specific and sensitive binding protein assay and by direct mass measurement using anion exchange chromatography for separation of selected inositol phosphates and gas chromatography and mass spectrometry for quantification of inositol monophosphate (IP1), inositol bisphosphate (IP2), inositol trisphosphate (IP3), and inositol tetrakisphosphate (IP4). Phosphatidic acid (10(-9)-10(-6) M) elicited a rapid concentration-dependent increase in Ins (1,4,5)P3 accumulation, with the peak fourfold to fivefold increase at 30 seconds of stimulation; the concentration required for 50% of maximal stimulation was 4.4 x 10(-8) M. The time course of individual inositol phosphates indicated a successive increase in the mass of IP3, IP4, IP2, and IP1 in response to stimulation with phosphatidic acid. The production of Ins (1,4,5)P3 in response to phosphatidic acid was not altered in the absence of extracellular calcium or in the presence of extracellular EGTA (10(-3) M). Thus, these findings indicate that phosphatidic acid is a potent activator of inositol phosphate production in adult ventricular myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Bradykinin stimulates phospholipase D in primary cultures of guinea-pig tracheal smooth muscle

Conditions were established for the primary culture of guinea-pig tracheal smooth muscle cells, the identity of which was confirmed by the presence of smooth muscle alpha-actin by western blotting. Cells were preincubated with [3H]palmitate which was incorporated, almost exclusively, into phosphatidylcholine. When these cells were stimulated by either bradykinin or phorbol 12-myristate 13-acetate (PMA), in the presence of butan-1-ol, the non-metabolizable product [3H]phosphatidylbutanol ([3H]PtdBut) accumulated by virtue of the phosphatidyltransferase activity of phospholipase D. The activation of phospholipase D by bradykinin was inhibited by 86 +/- 11% (N = 3 experiments) in the presence of the protein kinase C inhibitor, staurosporine (1 microM) and by 88 +/- 11% (N = 3 experiments) in cells that had been chronically treated with PMA to down-regulate their protein kinase C. PMA-stimulated phospholipase D was similarly affected (92 +/- 2% inhibited by staurosporine, 87 +/- 6% inhibited by protein kinase C down-regulation). Removal of extracellular Ca2+ markedly reduced the bradykinin-stimulated phospholipase D response (by 73 +/- 10%, N = 3 experiments) but had only a limited effect upon PMA-stimulated phospholipase D activity (by 23 +/- 6%, N = 3 experiments). [AIF4](-)-stimulation of the cells also resulted in the activation of phospholipase D, indicating the involvement of a G-protein. However, this was not Gi since pertussis-toxin pretreatment of the cells failed to abolish either bradykinin-stimulated inositol (1,4,5)trisphosphate formation or [3H]PtdBut accumulation. Western blotting revealed the presence of Gq/G11 which couples to the inositol lipid-directed phospholipase C. Indomethacin (10 microM) was without effect upon bradykinin-stimulated phospholipase D activity, suggesting that the bradykinin effects were not mediated indirectly by cyclooxygenase products. The role of phospholipase D activation in tracheal smooth muscle may be to, indirectly, produce diacylglycerol for the activation of protein kinase C which has been implicated in sustained contraction. However, the immediate product of phospholipase D, phosphatidate, has been proposed to have a number of second messenger roles and may itself, by an undefined mechanism, be involved in the sustained contraction of airway smooth muscle.

Phospholipase D: regulation and functional significance

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

Substance P induces a rise in intracellular calcium concentration in human T lymphocytes in vitro: evidence of a receptor-independent mechanism

The neuropeptide substance P (SP) has been shown to play an important role as a mediator of neurogenic inflammation. Moreover, in vitro SP is capable of modulating the activity of lymphocytes, monocytes and polymorphonuclear cells. We have examined one of the early events that occur after addition of SP to human peripheral blood mononuclear cells (PBMC). Addition of 10(-6)-10(-4) M SP to human peripheral blood mononuclear cells results in a dose-dependent rise in intracellular calcium concentration as determined by FACS analysis. We show that the effect of SP cannot be attenuated by the SP receptor antagonist [D-Pro4,D-Trp7,9]-SP(4-11), indicating that the response is not mediated via a SP receptor. Amphiphilic peptides like SP appear to have the capacity to insert themselves into the cell membrane and interact directly with intracellular proteins. This hypothesis is supported by the fact that the amphiphilic analogue of SP, [D-Pro2,D-Phe7,D-Trp9]-SP, is capable of inducing a calcium response in our system, although it is known as an SP receptor antagonist. Functionally, we show that SP increases the proliferative response of T cells induced by suboptimal concentrations of the mitogen PHA. These data provide evidence of a potential role of SP in the regulation of lymphocyte activation.