Recent advances in our understanding of the biochemical interactions between platelet-activating factor and arachidonic acid

In Vivo Cellular Phosphatidylcholine Kinetics of CD15+ Leucocytes and CD3+ T-Lymphocytes in Adults with Acute Respiratory Distress Syndrome

Mammalian cell membranes composed of a mixture of glycerophospholipids, the relative composition of individual phospholipids and the dynamic flux vary between cells. In addition to their structural role, membrane phospholipids are involved in cellular signalling and immunomodulatory functions. In this study, we investigate the molecular membrane composition and dynamic flux of phosphatidylcholines in CD15+ leucocytes and CD3+ lymphocytes extracted from patients with acute respiratory distress syndrome (ARDS). We identified compositional variations between these cell types, where CD15+ cells had relatively higher quantities of alkyl-acyl PC species and CD3+ cells contained more arachidonoyl-PC species. There was a significant loss of arachidonoyl-PC in CD3+ cells in ARDS patients. Moreover, there were significant changes in PC composition and the methyl-D9 enrichment of individual molecular species in CD15+ cells from ARDS patients. This is the first study to perform an in vivo assessment of membrane composition and dynamic changes in immunological cells from ARDS patients.

Interactions between platelet activating factor and eicosanoids during endotoxic shock in anaesthetized pigs

The effects of platelet activating factor (PAF) on eicosanoid release during endotoxic shock was investigated in anaesthetized pigs receiving 5 μg kg⁻¹ Escherichia coli endotoxin (LPS) into the superior mesenteric artery over a 60 min period, by measuring plasma levels of a variety of mediators. Fifteen of the 31 animals infused with LPS and not treated with BN 52021, a PAF receptor antagonist, died within 30 min after the commencement of LPS infusion (non-survivors), while the other 16 survived the experimental period of 3 h, though in a state of shock (survivors). No alterations were observed in plasma concentrations of eicosanoids in the non-survivors. A significant, though transient, increase in eicosanoid concentrations occurred only in the survivors. Treatment with BN 52021 (4 mg kg-1, i.v.) injected 5 min prior to LPS infusion, failed to exert any effect on the survival rate. However, pretreatment with BN 52021 prevented circulatory collapse in the survivors and reduced the concentration of cyclooxygenase enzyme products, without affecting LTB₄ release. Exogenous administration of PAF (0.01 μg kg⁻¹) caused hypotension and increased TXB₂ levels although 6-keto PGF_1α and LTB₄ concentrations were unchanged. The data suggest that prostanoid formation may be secondary to PAF release in circulatory collapse evoked by LPS infusion in survivors, and give further support to the suggestion that PAF prostanoid interaction is important during endotoxic shock. However, their role in early death seems to be negligible, indicating the importance of other mediators.

Lipid mediators and vector infection: Trypanosoma rangeli inhibits Rhodnius prolixus hemocyte phagocytosis by modulation of phospholipase A2 and PAF-acetylhydrolase activities

In this work we investigated the effects of Trypanosoma rangeli infection through a blood meal on the hemocyte phagocytosis in experiments using the 5th instar larvae of Rhodnius prolixus. Hemocyte phagocytic activity was strongly blocked by oral infection with the parasites. In contrast, hemocyte phagocytosis inhibition caused by T. rangeli infection was rescued by exogenous arachidonic acid (20 microg/insect) or platelet activating factor (PAF; 1 microg/insect) applied by hemocelic injection. Following the oral infection with the protozoan we observed significant attenuation of phospholipase A2 (PLA2) activities in R. prolixus hemocytes (cytosolic PLA2: cPLA2, secreted PLA2: sPLA2 and Ca+2-independent PLA2: iPLA2) and enhancement of sPLA2 activities in cell-free hemolymph. At the same time, the PAF-acetyl hydrolase (PAF-AH) activity in the cell-free hemolymph increased considerably. Our results suggest that T. rangeli infection depresses eicosanoid and insect PAF analogous (iPAF) pathways giving support to the role of PLA2 in the regulation of arachidonic acid and iPAF biosynthesis and of PAF-AH by reducing the concentration of iPAF in R. prolixus. This illustrates the ability of T. rangeli to modulate the immune responses of R. prolixus to favor its own multiplication in the hemolymph.

Blockades of phospholipase A(2) and platelet-activating factor receptors reduce the hemocyte phagocytosis in Rhodnius prolixus: in vitro experiments

The hemocytes phagocytosis in response to microorganisms may play an important role in the cellular immune responses of insects. Here, we have evaluated the effects of the platelet-activating factor (PAF) and eicosanoids in the phagocytosis of hemocyte monolayers of Rhodnius prolixus to the yeast Saccharomyces cerevisiae. Experiments showed that the phagocytosis of yeast cells by Rhodnius hemocytes is very efficient in both controls and cells treated with PAF and arachidonic acid. Phagocytosis of yeast particles is significantly blocked when the specific phopholipase A(2) inhibitor, dexamethasone, is applied on the hemocytes. By contrast, dexamethasone-pretreated hemocyte monolayers exhibit a drastic increase in the quantity of yeast cell-hemocyte internalization when the cells are treated by arachidonic acid. In addition, phagocytosis presents significant reduction in hemocyte monolayers treated with a specific PAF receptor antagonist, WEB 2086. Nevertheless, inhibition of phagocytosis with WEB 2086 is counteracted by the treatment of the hemocyte monolayers with PAF. In conclusion, phagocytosis of yeast cells by hemocytes is related to the activation of PAF receptors and eicosanoid pathways in the bloodsucking bug, R. prolixus.

WEB 2086, a platelet-activating factor antagonist, inhibits prophenoloxidase-activating system and hemocyte microaggregation reactions induced by Trypanosoma rangeli infection in Rhodnius prolixus hemolymph

The effects of the triazolodiazepine WEB 2086, a platelet-activating factor (PAF) antagonist, on hemocyte microaggregation and prophenoloxidase (proPO)-activating system in the hemolymph, hemocoelic infection and mortality in fifth-instar larvae of Rhodnius prolixus inoculated with Trypanosoma rangeli were investigated. Hemocoelic injection of short T. rangeli epimastigotes (1x10(4) parasites/insect) in R. prolixus that were previously fed with blood containing 1muM of WEB 2086 resulted in (i) reduced hemocyte microaggregations as well as an attenuated proPO system in the hemolymph and (ii) greater parasitemia and mortality among the insects. In vitro assays using hemolymph from insects previously fed with blood containing WEB 2086 exhibited attenuated hemocyte microaggregations when T. rangeli was employed as the inducer of the reaction, and this effect was not counteracted by PAF treatment. In vitro assays using hemolymph from insects previously fed with blood, regardless of WEB 2086 presence increased the PO activity when incubated with the parasites. However, the PO activity was drastically inhibited when hemolymph from insects fed with blood, whether or not it contained WEB 2086, was incubated with fat body homogenates from insects fed with blood containing WEB 2086. The addition of PAF did not enhance the PO activity. These analyses did not reveal any PAF influence on WEB 2086 effects in the two defense reactions.

The effect of dietary supplementation with n-3 polyunsaturated fatty acids on the generation of platelet-activating factor and leukotriene B4 in hypoxic-ischemic brain in young mice

Platelet-activating factor (PAF), leukotriene B(4) (LTB(4)) and other cytokines have been indicated to be responsible for the neuronal damage in hypoxic-ischemic brain. Diets in omega-3 (n-3) fatty acids appear to have an antiinflammatory effect, which is thought to be due to decrease in active prostaglandins and leukotrienes production after incorporation of these fatty acids into cell membrane phospholipids. We investigated the effect of dietary supplementation with n-3 fatty acids on endogenous PAF and LTB(4) biosynthesis in hypoxic-ischemic brain of young mice. Young mice were randomly divided into four groups: Group 1 mice were fed standard chow (n-3 polyunsaturated fatty acids free); Group 2 and Group 3 mice were given standard diet supplemented with 10% by weight of fish oil, as source of n-3 polyunsaturated fatty acids, for 3 and 6 weeks, respectively. Group 4 mice served as control. We injured the right cerebral hemisphere of young mice by ligating the right common carotid artery and exposing the mice to 8% oxygen for 60 min. Approximately 10-fold increase in PAF concentration was determined in hypoxic-ischemic brain tissue of Group 1 mice. Tissue concentration of PAF showed a profound decline in Group 3 mice compared to Groups 1 and 2 (P<0.01, P<0.05, respectively). LTB(4) was also significantly elevated in the brain of Group 1 mice when compared to the brain of control mice (P<0.001). A striking decline was observed in the concentration of LTB(4) in both Group 2 and Group 3 mice compared to Group 1 mice (P<0.05, P<0.01, respectively). The present study shows that n-3 fatty-acid-enriched diet inhibits endogenous PAF and LTB(4) generation in hypoxic-ischemic brain tissue; however it demonstrates that 6 weeks of dietary supplementation with n-3 fatty acids results in a significant decrease in tissue level of PAF in the brain.

Inhibition of platelet-activating factor synthesis in human neutrophils and platelets by propionyl-L-carnitine

Propionyl-L-carnitine (PrC) has been shown to exert beneficial effects in the treatment of myocardial and peripheral ischemia in man. These conditions are associated with the activation of circulating neutrophils and platelets. To determine whether PrC could affect the synthesis of lipid mediators known to influence neutrophil and platelet functions, we explored the effects of PrC on the synthesis of platelet-activating factor (PAF) and arachidonic acid (AA) metabolites. Preincubation (90 min) of human neutrophils with PrC (0.1-100 microM) inhibited the synthesis of PAF and of a PAF analog (1-alkyl-1'enyl-2-acetyl-sn-glycero-3-phosphoethanolamine: AEGPE) induced in vitro by the calcium ionophore A23187. In contrast, concentrations of PrC up to 100 microM did not influence the uptake of exogenous AA or the A23187-induced release of AA and eicosanoids from neutrophils in vitro. PrC (1 microM) also inhibited PAF synthesis from human platelets stimulated in vitro with thrombin, but had no effect on thrombin-induced aggregation. Oral administration of PrC (2 g/day for two weeks) to five normal volunteers resulted in a significant inhibition of PAF and AEGPE synthesis by neutrophils stimulated with A23187 ex vivo, with no effect on AA or eicosanoid release. These data indicate that PrC selectively inhibits in vitro and ex vivo PAF synthesis from human neutrophils and platelets without influencing AA metabolism or eicosanoid release. This effect of PrC might represent an additional mechanism by which this molecule can exert protective effects in tissue ischemia and in other inflammatory diseases associated with neutrophil and platelet activation.

Subcellular localization of enzyme activities involved in the metabolism of platelet-activating factor in rainbow trout leukocytes

The subcellular distribution of an alkyllyso-GPC: acetyl-CoA acetyltransferase (EC 2.3.1.67) and transacylase, two important enzyme activities involved in the remodeling pathway for the biosynthesis of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) have been examined in leukocytes isolated from the pronephros of the rainbow trout, Oncorhynchus mykiss. Contrary to mammalian systems, in which the acetyltransferase is localized to intracellular membranes, the subcellular distribution of an acetyltransferase activity in rainbow trout leukocytes was localized to the plasma membrane. Analysis of the acetyltransferase products by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC) confirmed synthesis of two subclasses of PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and 1-acyl-2-acetyl-sn-glycero-3-phosphocholine. The transacylase activity in this study was detected in membrane fractions in two domains of the intermediate density region which also contained the NADH dehydrogenase activity, a marker enzyme for the endoplasmic reticulum. Acylation of lysoPAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) exhibited approximately 95% specificity for omega-3 fatty acids. Acylation patterns were not significantly different in either domain of the endoplasmic reticulum. A model is proposed herein for the metabolism of PAF in rainbow trout leukocytes.

Evidence that 85 kDa phospholipase A2 is not linked to CoA-independent transacylase-mediated production of platelet-activating factor in human monocytes

Platelet-activating factor (PAF) production is carefully controlled in inflammatory cells. The specific removal of arachidonate (AA) from 1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (GPC), thought to be mediated by CoA-independent transacylase (CoA-IT), is required to generate the PAF precursor 1-O-alkyl-2-lyso-GPC in human neutrophils. Exposure of A23187-stimulated human monocytes to the CoA-IT inhibitors SK&F 98625 and SK&F 45905 inhibited PAF formation (IC50s of 10 and 12 microM, respectively), indicating that these cells also need CoA-IT activity for PAF production. Because CoA-IT activity transfers arachidonate to a 2-lyso phospholipid substrate, its activity is obligated to an sn-2 acyl hydrolase to form the 2-lyso phospholipid substrate. SB 203347, an inhibitor of 14 kDa phospholipase A2 (PLA2), and AACOCF3, an inhibitor of 85 kDa PLA2, both inhibited AA release from A23187-stimulated human monocytes. However, AACOCF3 had no effect on A23187-induced PAF formation at concentrations as high as 3 microM. Further, depletion of 85 kDa PLA2 using antisense (SB 7111, 1 microM) had no effect on PAF production, indicating a lack of a role of 85 kDa PLA2 in PAF biosynthesis. Both SB 203347 and the 14 kDa PLA2 inhibitor scalaradial blocked PAF synthesis in monocytes (IC50s of 2 and 0.5 microM, respectively), suggesting a key role of 14 kDa PLA2 in this process. Further, A23187-stimulated monocytes produced two forms of PAF: 80% 1-O-alkyl-2-acetyl-GPC and 20% 1-acyl-2-acetyl-GPC, which were both equally inhibited by SB 203347. In contrast, inhibition of CoA-IT using SK&F 45905 (20 microM) had a greater effect on the production of 1-O-alkyl (-80%) than of 1-acyl (-14%) acetylated material. Finally, treatment of U937 cell membranes with exogenous human recombinant (rh) type II 14 kDa PLA2, but not rh 85 kDa PLA2, induced PAF production. Elimination of membrane CoA-IT activity by heat treatment impaired the ability of 14 kDa PLA2 to induce PAF formation. Taken together, these results suggest that a 14 kDa PLA2-like activity, and not 85 kDa PLA2, is coupled to monocyte CoA-IT-induced PAF production.

Inhibition of PAF synthesis by stimulated human polymorphonuclear leucocytes with cloricromene, an inhibitor of phospholipase A2 activation

1. A phospholipase A2 (PLA2) represents the key enzyme in the remodelling pathway of platelet-activating factor (PAF) synthesis in human polymorphonuclear (PMN) leucocytes. 2. PLA2 activation is also the rate-limiting step for the release of the arachidonic acid utilized for the synthesis of leukotrienes in stimulated leucocytes; however, it is unknown whether the PLA2s involved in the two biosynthetic pathways are identical. 3. Cloricromene (8-monochloro-3-beta-diethylaminoethyl-4-methyl-7-ethoxy- carbonylmethoxy coumarin) is an antithrombotic coumarin derivative which inhibits platelet and leucocyte function and suppresses arachidonic acid liberation by interfering with PLA2 activation. 4. The aim of the present study was to assess whether chloricromene inhibits PAF synthesis by stimulated human polymorphonuclear leucocytes (PMNs). 5. Cloricromene (50-500 microM) inhibited in a concentration-dependent manner the release of PAF, as measured by h.p.l.c. bioassay, from A23187-stimulated PMNs. Significant inhibition (45%) of PAF-release was obtained with 50 microM cloricromene and the IC50 was 85 microM. Mepacrine (500 microM), a non-specific PLA2 inhibitor, strikingly reduced PAF release. 6. The incorporation of [3H]-acetate into [3H]-PAF induced by serum-treated zymosan in human PMNs was also inhibited concentration-dependently by cloricromene, with an IC50 of 105 microM. Mepacrine also suppressed [3H]-acetate incorporation into [3H]-PAF. 7. Cloricromene did not affect the activities of the enzymes involved in PAF-synthesis acetyltransferase or phosphocholine transferase. 8. Our data demonstrate that cloricromene, an inhibitor of PLA2-activation in human leucocytes, reduces the synthesis of PAF by stimulated PMNs. This finding has a twofold implication: the PLA2s (or the mechanisms that regulate their activation) involved in PAF synthesis and arachidonate release in human leucocytes are either identical or else indistinguishable by their sensitivity to cloricromene; the inhibition of PAF release by activated leucocytes may contribute to the antithrombotic and anti-ischaemic activities exerted by cloricromene.

Enzymes of platelet activating factor synthesis in brain

In this review, evidence is summarized for the production of PAF in brain, in response to stimulation associated with pathology. As well, there is a growing literature on the duality of actions of this lipid autocoid upon nervous tissue, indicated by extracellular and intracellular actions and binding sites for PAF in brain. The metabolic routes to PAF can be divided into the de novo and remodelling pathways of synthesis. The de novo route consists of 1-alkyl glycerophosphate acetyltransferase, and the subsequent actions of distinct phosphohydrolase and cholinephosphotransferase activities. This acetyltransferase can be activated by phosphorylation, and inhibited by MgATP and fatty acyl CoA thioesters, inhibitions which have particular relevance to brain ischemia. There is also evidence that the cholinephosphotransferase is controlled by phosphorylation, and regulated by levels of CDP-choline. The remodelling pathway to PAF relies upon the actions of phospholipase A2 or CoA-independent transacylases to generate the 1-alkyl glycerophosphorylcholine, as substrate for a distinct acetyltransferase. Following stimulation, rising intracellular calcium may trigger arachidonate selective cytosolic phospholipase activity which leads to increased PAF synthesis. The 1-alkyl glycerophosphocholine acetyltransferase activity is quite small in brain in comparison with the de novo acetyltransferase activity, and is also controlled by phosphorylation. Evidence has been presented for the actions of both pathways in brain, in response to biologically relevant stimulation pertinent to the disease state.

Platelet-activating factor-induced loss of vascular responsiveness to noradrenaline in pithed rats: involvement of nitric oxide

The role of nitric oxide and cyclo-oxygenase products in the platelet-activating factor (PAF)-induced hyporesponsiveness to noradrenaline was investigated in pithed rats. Infusion of PAF (30 ng/kg/min) for 60 min reduced the mean arterial blood pressure and impaired the pressor responses to noradrenaline (10 ng/kg, 100 ng/kg, 1 microgram/kg). Administration of NG-monomethyl-L-arginine (L-NMMA, 30 mg/kg) restored the reduced MABP and the impaired responses to their original levels. Indomethacin (5 mg/kg) had no significant effect on the PAF-induced hyporesponsiveness. Administration of 30 mg/kg L-NMMA caused hypertension in the PAF vehicle-treated animals and reduced the pressor response to 1 microgram/kg noradrenaline. Administration of 3 mg/kg L-NMMA had no significant effect on the responsiveness to noradrenaline. These results suggest that nitric oxide contributes to the PAF-induced hyporesponsiveness to noradrenaline and that cyclo-oxygenase products do not play a major role in this hyporesponsiveness.

Sea urchins--a new model for PAF research in embryology

This study reports on the effect of PAF on sperm motility, fertilization and early embryo development in the sea urchin Strongylocentrotus intermedius. PAF proved to be the amplifier of sperm motility and fertilizing capability at 10(-8)-10(-9) M and was toxic at higher concentrations. BN 52201 did not counteract the PAF action. The advantages of using sea urchins for PAF research in embryology are discussed.

Effects of non-steroidal anti-inflammatory drugs on polymorphonuclear leukocyte functions in vitro: focus on fenamates

Prostanoid-independent anti-rheumatic effects of non-steroidal anti-inflammatory drugs (NSAIDs) are a matter of debate. The aim of the present study was to compare the effects of chemically different NSAIDs (diclofenac, indomethacin, ketoprofen, paracetamol, piroxicam and four fenamates: flufenamic, meclofenamic, mefenamic and tolfenamic acids) on human polymorphonuclear leukocyte (PMN) functions, i.e. calcium ionophore A23187-triggered degranulation, leukotriene B4 (LTB4) release, platelet-activating factor (PAF) production and migration towards LTB4. The four fenamates caused a dose-dependent inhibition of each of the PMN functions tested. Flufenamic, meclofenamic and tolfenamic acids were about equipotent to inhibit PMN degranulation (IC50S 21-32 microM) and LTB4 release (IC50s 21-25 microM) whereas mefenamic acid achieved similar effects at somewhat higher drug concentrations. Tolfenamic and meclofenamic acids were the most potent fenamates to inhibit PAF synthesis (IC50s 37 and 51 microM) as well as migration towards LTB4 (IC50s 61 and 92 microM). Out of the other NSAIDs, diclofenac (which is chemically related to fenamates) suppressed degranulation as well as LTB4 and PAF production. Indomethacin inhibited LTB4 and PAF synthesis whereas ketoprofen reduced degranulation. The inhibitory effects of the non-fenamate NSAIDs occurred only at drug concentrations far higher than those achieved clinically. Paracetamol and piroxicam (up to 300 microM) did not influence the PMN functions tested. We conclude that NSAIDs with a fenamate structure differ from other NSAIDs by inhibiting PMN functions induced either by receptor-mediated stimulus (LTB4) or calcium ionophore (A23187) at micromolar drug concentrations.

Interactions between synthesis of platelet-activating factor and leukotriene B4 in isolated human polymorphonuclear leukocytes

The aim of the present work was to study interactions between the synthesis of platelet-activating factor (PAF) and leukotriene B4 (LTB4) in human polymorphonuclear leukocytes (PMNs) in vitro. PAF, at nanomolar concentrations, stimulated calcium ionophore A23187-activated PMNs to release LTB4 and 5-hydroxyeicosatetraenoic acid (5-HETE). This seems to be a receptor-mediated process as it was blocked by a PAF receptor antagonist WEB 2086 (IC50 6.6 +/- 3.9 microM). Moreover, LTB4 stimulated the formation of PAF in activated PMNs. WEB 2086 did not, however, alter PMN migration towards either LTB4 or the chemotactic peptide FMLP. This suggests that the enhancement of PAF synthesis in response to LTB4 is a concomitant event rather than a mediating process in LTB4-induced chemotactic movement of PMNs. These effects are implicated in the complex network of interactions between inflammatory mediators that results accumulation and activation of PMNs in the exacerbation of inflammatory processes.

The platelet-activating factor precursor of the injured cornea is selectively implicated in arachidonate and eicosanoid release

The purpose of this study was to isolate the platelet-activating factor (PAF) precursor and other choline phosphoglycerides (GPC) i.e. the alkenylacyl and diacyl lipids from the rabbit cornea, to analyze their fatty acid content and to determine which pool was the most susceptible to arachidonate depletion when activated corneal tissue released arachidonic acid (AA) and metabolites. Rabbit iridal GPC was also analyzed for comparative purposes. The fatty acid methyl esters of the GPC components extracted from the rabbit cornea and iris-ciliary body, isolated by high performance liquid chromatography (HPLC), were determined by capillary gas liquid chromatography. Rabbit corneas were labelled in vivo by intracameral injection of 3H-AA (1 microCi, specific activity = 218 Ci/mmol) and cryogenically injured 18 h later. Corneas were incubated in vitro and the AA and eicosanoids released into the medium were extracted and separated by HPLC. The GPC was extracted from the tissues and the labeling of the three GPC constituents was quantified by liquid scintillation counting. The corneal and iridal PAF precursor represented 4.1 +/- 0.2% and 2.9 +/- 0.2% respectively of total GPC in those tissues. On a mole basis, the alkyl arachidonoyl species constituted 12.7 +/- 0.7% of the corneal and 38 +/- 0.6% of the iridal PAF precursors respectively. The release of AA and prostaglandins by the cornea was linear until 15 min; whereas 12-HETE levels continuously increased until 60 min. All GPC components lost label but 1-O-alkyl-2-arachidonoyl was the most affected, with its labeled content 50% less than the non-injured control.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of arachidonic acid on indices of phospholipase A2 activity in the human neutrophil

The present studies were conducted to understand better the regulation of phospholipase A2 (PLA2)-dependent mobilization of lipid mediators by arachidonic acid (C20:4). After stimulation of human neutrophils, g.l.c./m.s. analysis of non-esterified fatty acids indicated that the quantity of C20:4 increased as a function of time after stimulation, from undetectable quantities to > 800 pmol/10(7) cells. In contrast with C20:4, the quantities of other free fatty acids such as oleic and linoleic were high in resting cells and did not change after stimulation. Some 15% of the C20:4 released from cellular lipids remained cell-associated. To examine the effect of C20:4 on its own release, neutrophils were exposed to [2H8]C20:4, to differentiate it by g.l.c./m.s. from naturally occurring C20:4. In A23187-stimulated neutrophils, low concentrations (5-10 microM) of [2H8]C20:4 added just before A23187 increased the quantity of C20:4 produced by the cell, whereas higher concentrations (30-50 microM) decreased the quantity of C20:4 released from phospholipids. As other measures of PLA2 activity, the effects of C20:4 on production of platelet-activity factor (PAF) and leukotriene B4 (LTB4) were assessed. C20:4 treatment just before stimulation of neutrophils blocked PAF and LTB4 production in a concentration-dependent manner (IC50 10-20 microM). The effect of C20:4 was not blocked by the cyclo-oxygenase inhibitor naproxine (10 microM), nor could it be mimicked by 1 microM LTB4, 5-hydroxyeicosa-6,8,11,14-tetraenoic acid (5HETE), 5-hydroperoxyeicosa-6,8,11,14-tetraenoic acid (5HPETE) or 15-hydroxyeicosa-5,8,11,13-tetraenoic acid (15HETE). The 5-lipoxygenase (5LO) inhibitor zileuton induced a concentration-dependent decrease in PAF, with a maximal effect of a 50% decrease at 10-50 microM. The decrease in PAF by the 5LO inhibitor could not be circumvented by addition of 1 microM 5HETE, 5HPETE and LTB4, and may be attributed to the capacity of zileuton to increase the quantity of C20:4 in A23187-treated neutrophils. The inhibitory effect of C20:4 (20-40 microM) on PAF production could be antagonized by the protein kinase C inhibitor staurosporine (30 nM), but not by inhibitors of protein kinase A, tyrosine kinase or calmodulin kinase II. Taken together, these data demonstrate that C20:4 is selectively released from membrane phospholipids of A23187-stimulated neutrophils, and this C20:4 may play an important role in regulating the mobilization of C20:4 by altering PLA2 activity.

Bombesin stimulates distinct time-dependent changes in the sn-1,2-diradylglycerol molecular species profile from Swiss 3T3 fibroblasts as analysed by 3,5-dinitrobenzoyl derivatization and h.p.l.c. separation

We have developed procedures for the analysis of endogenous diradylglycerol (DRG) molecular species using derivatization with 3,5-dinitrobenzoyl chloride. The introduction of this strong chromatophore enabled us to separate less than 1 nmol of DRG into its three classes (diacylglycerol, alkylacylglycerol and alkenylacylglycerol) using a combination of h.p.l.c. and t.l.c. followed by reversed-phase h.p.l.c. to resolve these classes into their component molecular species. When applied to Swiss 3T3 mouse fibroblasts stimulated with bombesin for 25 s, 5 min or 30 min, subtle time-dependent changes in the DRG patterns were observed, with only certain polyunsaturated 1,2-diacyglycerol species [18:0/20:3(n-9), 18:0/20:4(n-6), 18:0/20:4(n-3), 18:0/20:5(n-3), 18:1(n-9)/20:3(n-9), 18:1(n-9)/20:4(n-6), 16:0/22:6(n-3), 18:0/20:3(n-6) and 16:0/20:5(n-3)] showing significant agonist-stimulated increases. The amounts of the first six species were all raised at 25 s, whereas all except the latter two were elevated at 5 min. By 30 min these last species were also increased but 18:0/20:3(n-9) had returned to basal levels. Overall DRG levels, as measured by total molecular-species peak area, remained effectively constant. No changes in the amount or species profile of 1-alkyl-2-acylglycerol were observed. Comparison of these species with the acyl-chain structure of phospholipids supports the idea that inositol lipids could be the source of DRG at early stimulation times, but phosphatidylcholine appears to be a phospholipase substrate at all times. These results indicate sequential activation of several phospholipases with different substrate specificities and/or access to different phospholipid pools. They also suggest that only polyunsaturated DRGs act as second messengers and that changes in the relative amounts of these species may trigger activation of different proteins and/or isoforms (e.g. the different isoforms of protein kinase C).