Selective deacylation of arachidonate-containing ethanolamine-linked phosphoglycerides in stimulated human neutrophils

Myo-Inositol Moderates Glucose-Induced Effects on Human Placental 13C-Arachidonic Acid Metabolism

Maternal hyperglycemia is associated with disrupted transplacental arachidonic acid (AA) supply and eicosanoid synthesis, which contribute to adverse pregnancy outcomes. Since placental inositol is lowered with increasing glycemia, and since myo-inositol appears a promising intervention for gestational diabetes, we hypothesized that myo-inositol might rectify glucose-induced perturbations in placental AA metabolism. Term placental explants (n = 19) from women who underwent a mid-gestation oral glucose-tolerance-test were cultured with 13C-AA for 48 h in media containing glucose (5, 10 or 17 mM) and myo-inositol (0.3 or 60 µM). Newly synthesized 13C-AA-lipids were quantified by liquid-chromatography-mass-spectrometry. Increasing maternal fasting glycemia was associated with decreased proportions of 13C-AA-phosphatidyl-ethanolamines (PE, PE-P), but increased proportions of 13C-AA-triacylglycerides (TGs) relative to total placental 13C-AA lipids. This suggests altered placental AA compartmentalization towards storage and away from pools utilized for eicosanoid production and fetal AA supply. Compared to controls (5 mM glucose), 10 mM glucose treatment decreased the amount of four 13C-AA-phospholipids and eleven 13C-AA-TGs, whilst 17 mM glucose increased 13C-AA-PC-40:8 and 13C-AA-LPC. Glucose-induced alterations in all 13C-AA lipids (except PE-P-38:4) were attenuated by concurrent 60 µM myo-inositol treatment. Myo-inositol therefore rectifies some glucose-induced effects, but further studies are required to determine if maternal myo-inositol supplementation could reduce AA-associated pregnancy complications.

Markers of monocyte activation revealed by lipidomic profiling of arachidonic acid-containing phospholipids

Stimulated human monocytes undergo an intense trafficking of arachidonic acid (AA) among glycerophospholipidclasses. Using HPLC coupled to electrospray ionization mass spectrometry, we have characterized changes in the levels of AA-containing phospholipid species in human monocytes. In resting cells, AA was found esterified into various molecular species of phosphatidylinositol (PI), choline glycerophospholipids (PCs), and ethanolamine glycerophospholipids (PEs). All major AA-containing PC and PI molecular species decreased in zymosan-stimulated cells; however, no PE molecular species was found to decrease. In contrast, the levels of three AA-containing species increased in zymosan-activated cells compared with resting cells: 1,2-diarachidonyl-glycero-3-phosphoinositol [PI(20:4/20:4)]; 1,2-diarachidonyl-glycero-3-phosphocholine [PC(20:4/20:4)]; and 1-palmitoleoyl-2-arachidonyl-glycero-3-phosphoethanolamine [PE(16:1/20:4)]. PI(20:4/20:4) and PC(20:4/20:4), but not PE(16:1/20:4), also significantly increased when platelet-activating factor or PMA were used instead of zymosan to stimulate the monocytes. Analysis of the pathways involved in the synthesis of these three lipids suggest that PI(20:4/20:4) and PC(20:4/20:4) were produced in a deacylation/reacylation pathway via acyl-CoA synthetase-dependent reactions, whereas PE(16:1/20:4) was generated via a CoA-independent transacylation reaction. Collectively, our results define the increases in PI(20:4/20:4) and PC(20:4/20:4) as lipid metabolic markers of human monocyte activation and establish lipidomics as a powerful tool for cell typing under various experimental conditions.

In vitro generation of superoxide anion by the hemocytes ofMacrobrachium rosenbergii: possible mechanism and pathways

The hemocytes from the giant freshwater prawn Macrobrachium rosenbergii were examined for their ability to generate superoxide anion (O(2) (-)) in vitro upon exposure to various components derived from microbial cell wall components. Among the test molecules, laminarin (a polymer of beta-1, 3 glucans), mannan and LPS from five different bacterial species produced a differential response in terms of stimulated O(2) (-) production in prawn hemocytes, suggesting the ability of the hemocytes to differentiate non-self. This response was almost completely inhibited by superoxide dismutase (SOD) suggesting SOD-inhibitable O(2) (-) generation by prawn hemocytes. Phorbol 12-myristate 13-acetate (PMA) and Ca ionophore led to enhanced O(2) (-) generation by the hemocytes and this suggests the possible role of protein kinase C and Ca(2+) ions in such generation. Cytochemical analysis using nitroblue tetrazolium (NBT)-staining revealed the importance of granular hemocytes in O(2) (-) generation in these prawns. Inhibition of O(2) (-) generation by inhibitors of NADPH-oxidase and phenoloxidase pathways clearly reveal the involvement of two different pathways in non-self stimulated O(2) (-) generation by the prawn hemocytes. These findings demonstrate the importance of O(2) (-) generation and role of possible pathways in hemocyte mediated cellular immune response of a crustacean.

Myeloperoxidase-derived 2-chlorohexadecanal forms Schiff bases with primary amines of ethanolamine glycerophospholipids and lysine

Numerous studies have suggested relationships between myeloperoxidase, inflammation, and atherosclerosis. MPO-derived reactive chlorinating species (RCS) attack membrane plasmalogens releasing alpha-chloro-fatty aldehydes (alpha-Cl-FALDs) including 2-chlorohexadecanal (2-ClHDA). The molecular targets of alpha-Cl-FALDs are not known. The current study demonstrates 2-ClHDA adducts with ethanolamine glycerophospholipids and Fmoc-lysine. Utilizing electrospray ionization mass spectrometry, chlorinated adducts were observed that are apparent Schiff base adducts. Reduction of these Schiff base adducts with sodium cyanoborohydride resulted in a novel, stable adduct produced by the elimination of HCl. NMR further confirmed this structure. 2-ClHDA adducts with ethanolamine glycerophospholipids were also substrates for phospholipase D (PLD). The hydrolysis products were derivatized to pentafluorobenzoyl esters, and further structurally confirmed by GC-MS. Multiple molecular species of 2-ClHDA-N-modified ethanolamine glycerophospholipids were observed in endothelial cells treated with 2-ClHDA. These results show novel Schiff base adducts of alpha-Cl-FALDs with primary amines, which may represent an important fate of alpha-Cl-FALDs.

Analysis of polyunsaturated aminophospholipid molecular species using isotope-tagged derivatives and tandem mass spectrometry/mass spectrometry/mass spectrometry

When aminophospholipids with only saturated and monounsaturated fatty acids esterified to the glycerol backbone were labeled with isotopically enriched N-methylpiperazine acetic acid N-hydroxysuccinimide ester reagents, it was found that they could be readily detected as N-methylpiperazine-amide-tagged aminophospholipids using a precursor scan of the stable isotope reporter ion (m/z 114-117) formed by tandem mass spectrometry/mass spectrometry. However, it was found in the current study that these precursor ion scans are not useful in determining the changes of aminophospholipids with polyunsaturated fatty acids (PUFAs) esterified to the glycerol backbone due to the presence of interfering ions in the reporter ion region. Therefore, a method was developed using tandem mass spectrometry/mass spectrometry/mass spectrometry (MS(3)) to obtain reporter ion ratios that were not distorted by interfering ions present in the collision-induced dissociation spectra of nontagged aminophospholipids with PUFAs. This new MS(3) method for N-methylpiperazine- amide-tagged aminophospholipids was used to examine the fate of diacyl, ether, or plasmalogen glycerophosphoethanolamine (GPEtn) species after exposure of human polymorphonuclear leukocytes to A23187 and granulocyte macrophage-colony-stimulating factor/formyl-methionyl-leucyl-phenylalanine stimuli, which can induce eicosanoid biosynthesis, to follow those GPEtn molecular species which were the source of arachidonic acid released. Upon stimulation of the human polymorphonuclear leukocyte, it was found that the abundant arachidonoyl GPEtn plasmalogen molecular species were uniquely reduced in relative content compared to ether or diacyl species and this subclass of GPEtn may be a source of the arachidonic acid converted to leukotrienes by the 5-lipoxygenase pathway activated in this cell.

Regulation of arachidonic acid availability for eicosanoid production

Mammalian cells have developed specific pathways for the incorporation, remodeling, and release of arachidonic acid. Acyltransferase and transacylase pathways function to regulate the levels of esterified arachidonic acid in specific phospholipid pools. There are several distinct, differentially regulated phospholipases A2in cells that mediate agonist-induced release of arachidonic acid. These pathways are important in controlling cellular levels of free arachidonic acid. Both arachidonic acid and its oxygenated metabolites are potent bioactive mediators that regulate a myriad of physiological and pathophysiological processes.Key words: phospholipase A2, arachidonic acid, eicosanoid, phospholipid.

Differential effects of arachidonoyl trifluoromethyl ketone on arachidonic acid release and lipid mediator biosynthesis by human neutrophils. Evidence for different arachidonate pools

The goal of this study was to determine the effects of a putative specific cytosolic phospholipase A2 inhibitor, arachidonyl trifluoromethyl ketone (AACOCF3), on arachidonic acid (AA) release and lipid mediator biosynthesis by ionophore-stimulated human neutrophils. Initial studies indicated that AACOCF3 at concentrations 0-10 micro m did not affect AA release from neutrophils. In contrast, AACOCF3 potently inhibited leukotriene B4 formation by ionophore-stimulated neutrophils (IC50 approximately 2.5 micro m). Likewise, AACOCF3 significantly inhibited the biosynthesis of platelet activating factor. In cell-free assay systems, 10 micro m AACOCF3 inhibited 5-lipoxygenase and CoA-independent transacylase activities. [3H]AA labeling studies indicated that the specific activities of cell-associated AA mimicked that of leukotriene B4 and PtdCho/PtdIns, while the specific activities of AA released into the supernatant fluid closely mimicked that of PtdEtn. Taken together, these data argue for the existence of segregated pools of arachidonate in human neutrophils. One pool of AA is linked to lipid mediator biosynthesis while another pool provides free AA that is released from cells. Additionally, the data suggest that AACOCF3 is also an inhibitor of CoA-independent transacylase and 5-lipoxygenase. Thus, caution should be exercised in using AACOCF3 as an inhibitor of cytosolic phospholipase A2 in whole cell assays because of the complexity of AA metabolism.

Participation of prostaglandin E2 and platelet-activating factor in thapsigargin-induced production of interleukin-6

Incubation of rat peritoneal macrophages in the presence of thapsigargin increased production of prostaglandin E2, intracellular platelet-activating factor (PAF) and interleukin-6. However, no PAF was detected in the conditioned medium. In the presence of SK&F 98625 (diethyl 7-(3,4,5-triphenyl-2-oxo-2,3-dihydroimidazol-1-yl)heptane phosphonate), a CoA-independent transacylase inhibitor, the thapsigargin-induced increases in the interleukin-6 mRNA level and interleukin-6 production were suppressed in a concentration-dependent manner. This inhibitor also suppressed the production of prostaglandin E2 and intracellular PAF. The PAF receptor antagonists such as E6123 ((S)-(+)-6-(2-chlorophenyl)-3-cyclopropanecarbonyl-8,11-dimethyl-2,3,4,5-tetrahydro-8H-pyrido[4',3':4,5]thieno[3,2-f][1,2,4]triazolo [4,3-a][1,4]diazepine) and L-652,731 (2,5-bis(3,4,5-trimethylphenyl)tetrahydrofuran) partially inhibited the thapsigargin-induced increase in the levels of interleukin-6 mRNA and interleukin-6 protein. The SK&F 98625-induced suppression of interleukin-6 mRNA accumulation and interleukin-6 production was partially restored by addition of exogenous prostaglandin E2. However, exogenous PAF failed to reverse the suppression suggesting that the intracellular PAF does not act in an autocrine mechanism. These findings suggested that the concurrently produced prostaglandin E2 and intracellular PAF participate in the thapsigargin-induced increase in the interleukin-6 mRNA level and interleukin-6 production by rat peritoneal macrophages.

Lipid remodeling in mouse liver and plasma resulting from delta6 fatty acid desaturase inhibition

Electrospray/tandem mass spectrometry was used to quantify lipid remodeling in mouse liver and plasma during inhibition of polyunsaturated fatty acid synthesis by the delta6 fatty acid desaturase inhibitor, SC-26196. SC-26196 caused increases in linoleic acid and corresponding decreases in arachidonic acid and docosahexaenoic acid in select molecular species of phosphatidylcholine, phosphatidylethanolamine, and cholesterol esters but not in phosphatidylserine, phosphatidylinositol, or triglycerides. For linoleic acid-, arachidonic acid-, and docosahexaenoic acid-containing phospholipid species, this difference was, in part, determined by the fatty acid at the sn-1 position, namely, palmitic or stearic acid. An understanding of phospholipid remodeling mediated by delta6 desaturase inhibition should aid in clarifying the contribution of arachidonic acid derived via de novo synthesis or obtained directly in the diet during inflammatory responses.

Involvement of calcium-independent phospholipase A2 in uterine stromal cell phospholipid remodelling

The role of Ca2+-independent phospholipase A2 (iPLA2) in arachidonic (AA) and docosahexaenoic (DHA) acid incorporation and phospholipid remodelling in rat uterine stromal cells (UIII cells) was studied. Incorporation of AA and DHA into UIII cell phospholipids was Ca2+-independent. Bromoenollactone (BEL), a potent inhibitor of iPLA2, reduced lysophosphatidylcholine level and AA incorporation into phospholipids by approximately 20%. DHA incorporation was not affected by BEL, indicating that the pathways for AA and DHA incorporation are partially different. In control cells, the transfer of AA occurred mainly from diacyl-glycerophosphocholine (GroPCho) to alkenylacyl-glycerophosphoethanolamine (GroPEtn) and to a lesser extent from diacyl-GroPCho to diacyl-GroPEtn. [3H]DHA was redistributed from diacyl-GroPCho and alkylacyl-GroPEtn to alkenylacyl-GroPEtn. BEL treatment inhibited completely the redistributrion of AA within diacyl-GroPCho and diacyl -GroPEtn and reduced the [3H]DHA content of diacyl-GroPEtn, indicating that a BEL-sensitive iPLA2 controls the redistribution of polyunsaturated fatty acids to diacyl-GroPEtn. In contrast the redistribution of radioactive AA and DHA to alkenylacyl-GroPEtn was almost insensitive to BEL. The analysis of substrate specificity and BEL sensitivity of iPLA2 activity indicates that UIII cells exhibit at least two isoforms of iPLA2, one of which is BEL-sensitive and quite selective of diacyl species, and another one that is insensitive to BEL and selective for alkenylacyl-GroPEtn. Taken together, these results suggest that several iPLA2 participate independently in the remodelling of UIII cell phospholipids.

Preferential distribution of long chain polyunsaturated fatty acids in phospatidyl ethanolamine fraction of guinea pig alveolar apical membranes

We investigated the fatty acid distribution in guinea pig alveolar apical membranes at different developmental stages. Fatty acid composition of the purified membranes isolated from guinea pig fetuses (at 65 day, term=68 day), neonates (day 1) and adult males was determined. The levels of arachidonic acid (AA) and docosahexaenoic acid (DHA) were higher in the adult guinea pig alveolar apical membrane phosphatidylethanolamine (PE) fraction (9. 3+/-2.2 and 2.9+/-1.0%, respectively) while in other phospholipids (PL) fractions their levels were low or absent (P<0.01). Furthermore, levels of AA and DHA in the PE fraction of apical membrane increased significantly from fetal (6.6+/-3.0 and 0.8+/-0.4%, respectively) to neonatal life (10.3+/-1.5 and 3.0+/-0.8%, respectively). Increase in the level of DHA (almost four-fold) was much more pronounced than that of AA (P<0.05). As for guinea pig alveolar membranes, EPA and AA were mostly present in the PE fraction in pulmonary adenocarcinoma derived cells (A549 cells), a parallel model of type II pneumocytes, with the levels of AA around three-fold greater than that of EPA, Binding of radiolabelled fatty acids to A549 cells showed no significant differences between the maximum uptake achieved for different fatty acids (AA, 1.7+/-0.2, EPA, 2.3+/-0.3, LA, 1.7+/-0.2, OA, 2.0+/-0.2nmol/mg protein, P>0.5). Once the fatty acids were taken up by these cells AA was mostly identifiable in the monoacylglycerol (MAG) fraction, whereas EPA was equally distributed between the MAG and PL fractions. Oleic acid was mainly present in the triglyceride (TAG) fraction whereas LA was evenly distributed between the TAG, MAG, and PL fractions. Our data demonstrate a preferential distribution of AA and DHA in PE fractions of alveolar apical membranes during development.

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.

Regulation and metabolism of arachidonic acid

The metabolism of AA reflects a carefully balanced series of biochemical pathways. The level of free arachidonate in a cells is controlled by de novo synthesis, dietary uptake, and transcellular metabolism. Lysophospholipids are key controlling substrates for a variety of acyl transferase and transacylase reactions, whose combined effect is to remodel cellular membranes placing AA in up to 20 different molecular species of phospholipids. PLA2 enzymes, both cytosolic and secretory, can release AA for subsequent metabolism via lipoxygenase, COX, and cytochrome P450 enzymes into a variety of eicosanoid products. Reactions are often tissue- and cell-specific, and provide a spectrum of inflammatory mediator release in which many of the molecular details remain to be elucidated.

Possible participation of intracellular platelet-activating factor in tumor necrosis factor-alpha production by rat peritoneal macrophages

Stimulation of rat peritoneal macrophages by thapsigargin (46.1 nM) increased levels of tumor necrosis factor-alpha and prostaglandin E2 in the conditioned medium. Platelet-activating factor (PAF) was not detected in the conditioned medium, but the level of cell-associated PAF was increased transiently by thapsigargin. The PAF receptor antagonists such as E6123 ((S)-(+)-6-(2-chlorophenyl)-3-cyclopro-panecarbonyl-8,11-dim ethyl-2,3,4,5-tetrahydro-8 H-pyrido[4',3':4,5]thieno [3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine), L-652,73 1 (2,5-bis(3,4,5-trimethoxyphenyl) tetrahydrofuran) and CV-6209 (2-[N-acetyl-N-(2-methoxy-3-octadecyl-carbamoyloxy propoxycarbonyl)aminomethyl]-1-ethylpyridinium chloride) inhibited thapsigargin-induced production of tumor necrosis factor-alpha. The cyclooxygenase inhibitor indomethacin inhibited prostaglandin E2 production, and further enhanced thapsigargin-induced tumor necrosis factor-alpha production in parallel with further increase in cell-associated PAF production. The enhancement of tumor necrosis factor-alpha production induced by thapsigargin plus indomethacin was also inhibited by E6123, L-652,731 and CV-6209. However, exogenously added PAF up to 100 nM did not stimulate production of tumor necrosis factor-alpha. The level of tumor necrosis factor-alpha mRNA was increased by thapsigargin, but was lowered by the PAF receptor antagonist E6123, suggesting that the inhibition of tumor necrosis factor-alpha production by the PAF receptor antagonist is induced at the level of mRNA for tumor necrosis factor-alpha. These findings suggested that concurrently produced cell-associated PAF in thapsigargin-stimulated macrophages up-regulates production of tumor necrosis factor-alpha by acting as an intracellular signaling molecule and the PAF receptor antagonists might penetrate into the cells and antagonize the action of intracellular PAF.

Purification and characterization from rat kidney membranes of a novel platelet-activating factor (PAF)-dependent transacetylase that catalyzes the hydrolysis of PAF, formation of PAF analogs, and C2-ceramide

We have previously identified two enzyme activities that transfer the acetyl group from platelet-activating factor (PAF) in a CoA-independent manner to lysoplasmalogen or sphingosine in HL-60 cells, endothelial cells, and a variety of rat tissues. These were termed as PAF:lysoplasmalogen (lysophospholipid) transacetylase and PAF:sphingosine transacetylase, respectively. In the present study, we have solubilized and purified this PAF-dependent transacetylase 13,700-fold from rat kidney membranes (mitochondrial plus microsomal membranes) based on the PAF:lysoplasmalogen transacetylase activity. The mitochondria and microsomes were prepared and washed three times, then solubilized with 0.04% Tween 20 at a detergent/protein (w/w) ratio of 0.1. The solubilized fractions from mitochondria and microsomes were combined and subjected to sequential column chromatographies on DEAE-Sepharose, hydroxyapatite, phenyl-Sepharose, and chromatofocusing. The enzyme was further purified by native-polyacrylamide gel electrophoresis (PAGE) and affinity gel matrix in which the competitive inhibitor of the enzyme, 1-O-hexadecyl-2-N-methylcarbamyl-sn-glycero-3-phosphoethanolamine was covalently attached to the CH-Sepharose. On SDS-PAGE, the purified enzyme showed a single homogeneous band with an apparent molecular mass of 40 kDa. The purified enzyme catalyzed transacetylation of the acetyl group not only from PAF to lysoplasmalogen forming plasmalogen analogs of PAF, but also to sphingosine producing N-acetylsphingosine (C2-ceramide). In addition, this enzyme acted as a PAF-acetylhydrolase in the absence of lipid acceptor molecules. These results suggest that PAF-dependent transacetylase is an enzyme that modifies the cellular functions of PAF through generation of other diverse lipid mediators.

Secretory and cytosolic phospholipases A2 are activated during TNF priming of human neutrophils

Cytokines alter neutrophil (PMN) function during inflammation, and Tumor Necrosis Factor (TNF) in vitro primes PMN such that receptor-mediated stimulation causes markedly enhanced release of arachidonic acid. We hypothesized that two Ca(2+)-dependent PLA2's in PMN might be activated during priming of the cell, thus affecting arachidonate release. A low molecular weight, secretory PLA2 was identified by enzymatic activity in the cell free supernates of primed or stimulated PMN, and in PMN disrupted by nitrogen cavitation. The enzymatic activity was calcium-dependent, acid stable, destroyed by dithiothreitol, and blocked by anti-sPLA2 antibodies. TNF caused secretion of sPLA2 and also caused an increase in cell-associated sPLA2 enzymatic activity. Activation and release were maximal with fMLP stimulation of TNF-primed PMN. Neutrophils also contained a cytosolic PLA2 (cPLA2) characterized by enzymatic activity which was calcium dependent, enhanced by dithiothreitol, and blocked by anti-cPLA2 antibody. TNF caused a doubling of cPLA2 enzymatic activity which was associated with phosphorylation of the enzyme as judged by a migration shift on Western blots. Thus, TNF priming of human PMN caused marked increase in fMLP stimulated AA release in parallel to enhanced activity of two different PLA2's.

Leukotriene B4 stimulates the release of arachidonate in human neutrophils via the action of cytosolic phospholipase A2

Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation and is involved in the receptor-mediated activation of a number of leukocyte responses including degranulation, superoxide formation, and chemotaxis. In the present research, stimulation of unprimed polymorphonuclear leukocytes (neutrophils) with LTB4 results in the transient release of arachidonate as measured by mass. This release of arachidonate was maximal at an LTB4 concentration of 50-75 nM and peaked at 45 s after stimulation with LTB4. The transient nature of this release can be attributed, in part, to a fast (< 60 s) metabolism of the added LTB4. Moreover, the inhibition of the reacylation of the released arachidonate with thimerosal results in greater than 4-times as much arachidonate released. Thus, a rapid reacylation of the released arachidonate also contributes to the transient nature of its measured release. Multiple additions of LTB4, which would be expected to more closely resemble the situation in vivo where the cell may come into contact with an environment where LTB4 is in near constant supply, yielded a more sustained release of arachidonate. No release of [3H]arachidonate was observed when using [3H]arachidonate-labeled cells. This indicates that the release of arachidonate as measured by mass is most probably the result of hydrolysis of arachidonate-containing phosphatidylethanolamine within the cell since the radiolabeled arachidonate is almost exclusively incorporated into phosphatidylcholine and phosphatidylinositol pools under the non-equilibrium radiolabeling conditions used. Consistent with the role of cytosolic phospholipase A2 (cPLA2) in the release of arachidonate, potent inhibition of the LTB4-stimulated release was observed with methylarachidonylfluorophosphonate, an inhibitor of cPLA2 (IC50 of 1 microM). The bromoenol lactone of the calcium-independent phosphospholipase A2. failed to affect LTB4-stimulated release of arachidonate in these cells.

Leukotriene B4 and platelet activating factor production in permeabilized human neutrophils: role of cytosolic PLA2 in LTB4 and PAF generation

The specific type of phospholipase A2 (PLA2) involved in formation of leukotriene B4 (LTB4) and platelet activating factor (PAF) in inflammatory cells has been controversial. In a recent report we characterized activation of the 'cytosolic' form of PLA2 (cPLA2) in human neutrophils (PMN) permeabilized with Staphylococcus aureus alpha-toxin under conditions where the secretory form of PLA2 (sPLA2) was inactive. In the current study, generation of both LTB4 and PAF in porated PMN are demonstrated. PMN, prelabeled with [3H]arachidonic acid (3H-AA, to assess AA release and LTB4 production) or with 1-O-[9',10'-3H]hexadecyl-2-lyso-glycero-3-phosphocholine (3H-lyso-PAF, for determination of lyso-PAF and PAF formation), were permeabilized with alpha-toxin in a 'cytoplasmic' buffer supplemented with acetyl CoA. Maximum production of both PAF and LTB4 required addition of 500 nM Ca2+, G-protein activation induced with 10 microM GTP gamma S, and stimulation with the chemotactic peptide, N-formyl-Met-Leu-Phe (FMLP, 1 microM); LTB4 production was confirmed by radioimmunoassay. Removal of acetyl CoA from the system had little effect on LTB4 generation but blocked PAF production with a concomitant increase in lyso-PAF formation LTB4 and PAF production occurred in parallel over time and at differing ATP and Ca2+ concentrations. Further work demonstrated that: (i) maximum production of both inflammatory mediators required a hydrolyzable form of ATP; (ii) blocking phosphorylation with staurosporin inhibited production of both; (iii) the reducing agent, dithiotreitol, had little affect on LTB4 formation but slightly enhanced PAF generation. This study clearly shows that cPLA2 activation can provide precursors for both LTB4 and PAF, that maximum PAF and LTB4 formation occur under conditions that induced optimal cPLA2 activation, that a close coupling between LTB4 and PAF formation exists, and that, after substrate generation, no additional requirements are necessary for LTB4 and PAF generation in the permeabilized PMN system.

Comparison of acceptor and donor substrates in the CoA-independent transacylase reaction in human neutrophils

In human neutrophils (PMN) the ethanolamine-containing phosphoglyceride fraction (PE), principally plasmalogen-linked PE (1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine), is the major store of arachidonic acid (AA). Exogenous AA is initially incorporated into 1-acyl-linked phosphoglycerides and is believed to be transferred into the 1-ether-linked phosphoglycerides via the action of a CoA-independent transacylase (CoA-IT). We have investigated the selectivity for both the "acceptor' lysophospholipids and "donor' AA-containing phospholipid substrates in the CoA-IT reaction. Evidence suggests CoA-IT may also participate in the synthesis of platelet activating factor. The transfer of [3H]AA from endogenously labeled choline-containing phosphoglycerides (PC) to exogenously added alkenyl-lyso-PE (0-50 microM) was examined in saponin-permeabilized PMN. In these "donor' studies, we observed that [3H]AA was transferred from both alkyl- and diacyl-linked PC in a proportional manner. More detailed molecular species analysis showed that [3H]AA was deacylated from all the major AA-containing molecular species in both the alkyl and diacyl subclasses with no selectivity for either subclass. To investigate the "acceptor' selectivity, membrane fractions prelabeled with either [3H]alkyl-arachidonoyl-PE or -PC were utilized as donor substrates. Various unlabeled lysophospholipids (10 microM) were added and the generation of [3H]lyso-PE or -PC was monitored as a measure of CoA-IT activity. Significant subclass preference was observed upon addition of lyso-PE species (1-alkenyl > 1-alkyl > 1-acyl) however, little selectivity was seen with the corresponding lyso-PC species. On the other hand, lysophosphatidylserine, lysophosphatidylinositol, and lysophosphatidic acid all served as poor acceptor substrates in the reaction. These data from PMN are consistent with other evidence that the CoA-IT plays a pivotal role in the enrichment of AA into plasmalogen-linked PE.

Alterations in individual molecular species of human platelet phospholipids during thrombin stimulation: electrospray ionization mass spectrometry-facilitated identification of the boundary conditions for the magnitude and selectivity of thrombin-induced platelet phospholipid hydrolysis

Although the rapid thrombin-induced release of arachidonic acid in human platelets has been known for over 20 years, the amount of arachidonic acid mass mobilized and the source of the released arachidonic acid has remained a subject of intense controversy. Herein, we exploit the analytic power and sensitivity of electrospray ionization mass spectrometry to identify plasmenylethanolamines as the largest source of arachidonic acid mass released during thrombin stimulation and to demonstrate the presence of multiple novel molecular species of plasmenylethanolamines in human platelets. Specifically, 90 s after thrombin stimulation a total of 60.1 nmol of arachidonic acid-containing phospholipids/10(9) platelets was hydrolyzed which included the loss of 31.8 nmol/10(9) platelets from ethanolamine glycerophospholipids (hydrolysis of plasmenylethanolamines represented 63% of the mass lost from the ethanolamine glycerophospholipid pool) but only 10.9 nmol/10(9) platelets from choline glycerophospholipids. Human platelet phosphatidylserine and phosphatidylinositol pools contained similar amounts of arachidonic acid mass in resting platelets (approximately equal to 20 nmol/10(9) platelets), and each pool contributed 8.7 nmol/10(9) platelets after thrombin stimulation. From these results, a lower boundary for the rate of thrombin-induced arachidonic acid mobilization in human platelets can be set at > 60 nmol/10(9) platelets, thereby identifying specific kinetic characteristics and substrate selectivities of the phospholipase(s) activated during platelet stimulation. Collectively, these results underscore the importance of plasmenylethanolamines as the major storage depot of arachidonic acid in resting platelets and as the major source of arachidonic acid mobilized after thrombin stimulation of human platelets.

Genetic analysis of the chromosomal region encoding lysophospholipase L2 of Vibrio cholerae O1

From the Tn5-inserted mutant library of Vibrio cholerae O1, we found a mutant, NF404, which lost the production of both hemolysin and cholera toxin (CT) even though the Tn5-insertion site was out side from the structural genes for hemolysin and CT. Cloning and sequencing analysis of the homologous region from the wild-type strain, revealed that the sequence spanning the coding region of an ORF1 nominated as lypA, encoding a 39.5 kDa protein. Deduced amino acid sequence of the lypA gene had 37.6% identity to the lysophospholipase L2 (EC 3.1.1.5) of Escherichia coli. In the downstream of lypA, a second open reading frame (ORF2) encoding an unknown protein with molecular weight of 19.9 kDa was found. Assaying the lysophospholipase L2 activity in the cell extract of E.coli harboring lypA in an expression vector showed clear increase in the enzymatic activity.

Activation of cytosolic phospholipase A2 in permeabilized human neutrophils

Neutrophils (PMN) contain two types of phospholipase A2 (PLA2), a 14 kDa 'secretory' Type II PLA2 (sPLA2) and an 85 kDa 'cytosolic' PLA2 (cPLA2), that differ in a number of key characteristics: (1) cPLA2 prefers arachidonate (AA) as a substrate but hydrolyzes all phospholipids; sPLA2 is not AA specific but prefers ethanolamine containing phosphoacylglycerols. (2) cPLA2 is active at nM calcium (Ca2+) concentrations; sPLA2 requires microM Ca2+ levels. (3) cPLA2 activity is regulated by phosphorylation; sPLA2 lacks phosphorylation sites. (4) cPLA2 is insensitive to reduction; sPLA2 is inactivated by agents that reduce disulfide bonds. We utilized PMN permeabilized with Staphylococcus aureus alpha-toxin to determine whether one or both forms of PLA2 were activated in porated cells under conditions designed to differentiate between the two enzymes. PMN were labeled with [3H]AA to measure release from phosphatidylcholine and phosphatidylinositol; gas chromatography-mass spectrometry was utilized to determine total AA release (mainly from phosphatidylethanolamine) and to assess oleate and linoleate mass. A combination of 500 nM Ca2+, a guanine nucleotide, and stimulation with n-formyl-met-leu-phe (FMLP) were necessary to induce maximal AA release in permeabilized PMN measured by either method; AA was preferentially released. [3H]AA and AA mass release occurred in parallel over time. A hydrolyzable form of ATP was necessary for maximum AA release and staurosporin inhibited PLA2 activation. Dithiothreitol treatment had little affect on [3H]AA release and metabolism but inhibited AA mass release. Assay of cell supernatants after cofactor addition did not detect sPLA2 activity and the cytosolic buffer utilized did not support activity of recombinant sPLA2. These results strongly suggested that cPLA2 was the enzyme activated in the permeabilized cell model and this is the first report which unambiguously demonstrates AA release in response to activation of a specific type of PLA2 in PMN.

Lipid changes in HL-60 cells on differentiation into macrophages by treatment with a phorbol ester

We studied changes in lipid composition of human promyelocytic leukemia cells (HL-60) on differentiation to the macrophage/monocytic lineage by treatment with the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate. Differentiation was accompanied by: (i) a decrease in the level of phospholipids; (ii) a greater amount of triacylglycerols; (iii) an increase in 1-alk-1'-enyl-2-acyl- and 1-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine and a decrease in 1-alkyl-2-acyl-sn-glycero-3-phosphocholine; and (iv) an increase in the level of arachidonic acid in ethanolamine phospholipids. The increased levels of ether-linked lipids and of arachidonic acid in ethanolamine phospholipids are consistent with an enhanced biosynthesis of platelet-activating factor and eicosanoids, which are particularly important in the macrophage function.

The CoA-independent transacylase in PAF biosynthesis: tissue distribution and molecular species selectivity

Microsomal membranes from six different rat tissues (spleen, lung, kidney, brain, testis, and liver) were found to possess CoA-independent transacylase activity that could both acylate lyso-[3H]PAF (1-[3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine) and then deacylate the 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine product via the transacylation of added exogenous 1-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine. Platelet-activating factor (1-[3H]hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) was produced when acetyl-CoA was added to the spleen microsomes during generation of lyso-[3H]PAF by the transacylases. More extensive studies with subcellular fractions from spleen revealed that, in addition to microsomes, the transacylase activities were also present in the 15,000 x g membrane fraction but not in the cytosol. Analysis of molecular species of 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine before and after addition of 1-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine as the acyl acceptor demonstrated a high selectivity for polyunsaturated fatty acids (> 3 double bonds/acyl group) in both the acylation and deacylation processes that occurred in testicular microsomal membranes. The transfer of acyl groups by the transacylase appeared to be equally effective for either arachidonic or docosapentaenoic(n - 6) fatty acids, whereas linoleic and oleic fatty acids were not transferred from 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine following the addition of 1-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine. Similar experiments with the membrane fraction of undifferentiated HL-60 cells showed that arachidonic acid supplementation of intact cells enhanced both the CoA-independent transacylation of lyso-[3H]PAF and the subsequent deacylation of 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine caused by addition of 1-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine. Differentiation of the HL-60 cells into a neutrophil-like form had no effect on the transacylase activity. Our results indicate the PAF-related transacylase is widely distributed among tissues and, although highly selective for polyunsaturated acyl groups, does not discriminate selectively among the polyunsaturates.

CoA-independent transacylase activity is increased in human neutrophils after treatment with tumor necrosis factor alpha

CoA-independent transacylase (CoA-IT) appears to play a critical role in lipid mediator generation by rapidly moving arachidonate (AA) between phospholipid pools during cell activation. Tumor necrosis factor-alpha (TNF) pretreatment of human neutrophils increases agonist-induced production of inflammatory mediators. The current study tested if the TNF-induced increase in lipid mediator production may be, in part, due to altered CoA-IT activity. Neutrophils were treated with TNF (250 U/ml, 30 min), homogenates prepared, and CoA-IT activity measured by the ability of these homogenates to acylate 1-[3H]alkyl-2-lyso-sn-glycero-3-phosphocholine (GPC). There was an increased CoA-IT activity, from 9.1 +/- 1.1 to 13.7 +/- 1.4 pmol/mg per min in control vs. TNF-treated samples, respectively. Varying the concentration of 1-alkyl-2-lyso-GPC revealed an increased CoA-IT activity in microsomes that was due to an increased Vmax, from 26 to 54 pmol/mg per min. The ability of TNF to increase CoA-IT activity was concentration-dependent, with maximal response observed at 25 U/ml. This effect on CoA-IT appears to be specific, in that TNF treatment of neutrophils had no effect on CoA-dependent acylation of 1-acyl-2-lyso-sn-glycero-3-phosphocholine, using either AA-CoA or linolenoyl-CoA as substrates. In the intact cell, the movement of [3H]AA from other phospholipids into PE in fMLP-stimulated neutrophils was greatly enhanced after TNF treatment, demonstrating a functional consequence of increased CoA-IT activity. In addition, TNF treatment doubled platelet-activating factor production in response to the chemotactic peptide fMLP, as measured by [3H]acetate incorporation, while the response to A23187 remained unchanged. Taken together, these results provide the first evidence of modulation of CoA-IT activity by a proinflammatory cytokine and suggest that one mechanism for augmented lipid mediator formation is through increases in CoA-IT activity.

Incorporation of polyunsaturated fatty acids into phospholipids of hemocytes from the tobacco hornworm, Manduca sexta

We examined the incorporation of four radioactive fatty acids, 18:1n-9, 18:2n-6, 20:4n-6 and 20:5n-3, into cellular lipids of hemocytes from tobacco hornworms, Manduca sexta. Most of the radioactivity associated with 18:1n-9 was recovered from triacylglycerols (TGs), and the radioactivity associated with 18:2n-6 was heavily incorporated into phospholipids (PLs) and TGs. Most of the radioactivity associated with the two eicosanoid-precursor polyunsaturated fatty acids (PUFAs), 20:4n-6 and 20:5n-3, was incorporated into PLs. The incorporated fatty acids were redistributed among the lipid classes during 2 h incubations. The two C20 PUFAs were moved from PLs to TGs. While 18:2n-6 underwent little change, 18:1n-9 was redistributed from TGs to PLs. Within PLs, each of the fatty acids were incorporated into phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PtG) and phosphatidylserine/inositol (PS/PI). The incorporation patterns changed over time, indicating that the incorporated fatty acids were redistributed among the four PL fractions. The radioactivity associated with 18:1n-9 was mostly recovered from the sn-1 position of PC (59%) and PE (83%). Most of the radioactivity associated with 18:2n-6 was found in the sn-2 position of PC (88%) and PE (67%). Over 90% of the radioactivity associated with 20:5n-3 was recovered from the sn-2 position of PC and PE. Incorporation of 20:4n-6 differed from 20:5n-3 because more radioactivity was recovered from the sn-2 position of PC (93%) than PE (69%). These findings are in line with the general background of lipid biochemistry, from which incorporation of 20:4n-6 into PE marks a notable departure: 31% of the radioactivity associated with this acid was recovered from the sn-1 position of PE. These findings indicate that hemocytes from the tobacco hornworm elaborate a fatty acid incorporation system, which exhibits specificity with respect to fatty acid structure and lipid class.

Complete discrimination of docosahexaenoate from arachidonate by 85 kDa cytosolic phospholipase A2 during the hydrolysis of diacyl- and alkenylacylglycerophosphoethanolamine

In our previous report (Shikano, M., Masuzawa, Y. and Yazawa, K. (1993) J. Immunol. 150, 3525-3533), we described that the enrichment of docosahexaenoic acid (DHA, 22:6(n - 3)) reduces both arachidonic acid (AA, 20:4(n - 6)) release and platelet-activating factor (PAF) synthesis in human eosinophilic leukemia cells, Eol-1. Since no DHA release was observed in response to Ca-ionophore stimulation, we presumed that the phospholipase A2 (PLA2) responsible for AA release and PAF synthesis can not hydrolyze the DHA moiety of phospholipids. In the present paper, we examined whether DHA-containing diacyl- and alkenylacylglycerophosphoethanolamine (DHA-diacylGPE and DHA-alkenylacyGPE) are susceptible to the action of AA-preferential 85 kDa cytosolic phospholipase A2 (cPLA2) from rabbit platelets in comparison with AA and eicosapentaenoic acid (EPA, 20:5(n - 3)) derivatives. When diacylGPE was used as a substrate, DHA release was almost negligible under the assay condition that allowed AA and EPA to be liberated at the rates of 4.3 mumol/min per mg protein and 2.5 mumol/min per mg protein, respectively. On the other hand, 14 kDa type II PLA2 hydrolyzed DHA-diacylGPE as well as AA-diacylGPE and EPA-diacylGPE. When DHA-diacylGPE and AA-diacylGPE were mixed at equimolar concentrations, DHA release by cPLA2 was not observed and AA release was reduced to 32% in the case without DHA-diacylGPE. This indicated that DHA-diacylGPE is a poor substrate but possesses the inhibitory activity for cPLA2. cPLA2 does not clearly discriminate between AA-alkenylacylGPE and AA-diacylGPE. As in the case using diacylGPE as a substrate, DHA-alkenylacylGPE was completely discriminated from AA-alkenylacylGPE by cPLA2. The roles of DHA and cPLA2 in the synthesis of lipid mediators will be discussed in relation to the new aspects of the substrate specificity of cPLA2 provided here.

Biosynthesis of platelet-activating factor in cultured mast cells. Involvement of the CoA-independent transacylase demonstrated by analysis of the molecular species of platelet-activating factor

We have recently demonstrated that arachidonate [20:4(5,8,11,14)] was primarily linked to the hexadecyl (16:0) and octadecenyl (18:1) species of alkylacyl derivatives of glycerolphosphocholine (GroPCho). Consistent with the involvement of arachidonate-specific CoA-independent transacylase in the synthesis of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-GroPCho), 16:0 and 18:1 PAF species were formed upon antigen stimulation [Joly, F., Breton, M., Wolf, C., Ninio, E. & Colard, O. (1992) Biochim. Biophys. Acta 1125, 305-312]. In the present work, addition of lyso-PAF to mast cells resulted in PAF production. We analyzed the PAF species formed in the presence of a defined lyso-PAF molecular species in order to differentiate between either direct acetylation or involvement of the membrane precursor. The 18:1 lyso-PAF was more effective than the 16:0 in producing PAF which was composed of 95% 18:1 PAF, the balance being 16:0, indicating that part of the acetylated lyso-PAF originated from the cellular pool of alkyl-arachidonyl-GroPCho in resting cells. Consistent with alkyl-arachidonyl-GroPCho species content and acetyltransferase specificity, similar amounts of 16:0 and 18:1 PAF species were formed when mast cells were stimulated with antigen. Supplemented with 16:0 or 18:1 lyso-PAF, antigen-stimulated mast cells responded by 230% and 125% increase in PAF synthesis, respectively. As expected, the amount of the PAF species corresponding to the added lyso-PAF was increased. More interestingly, addition of 16:0 lyso-PAF almost doubled the amount of 18:1 PAF content as compared to antigen alone, thus indicating that the lyso-PAF formed via the CoA-independent transacylase was significantly used for PAF synthesis, despite a large excess of exogenous lyso-PAF. The CoA-independent transacylase, measured using [3H]lyso-PAF as a substrate in sonicates from antigen-stimulated cells, was decreased concurrently with PAF formation. In conclusion, we show that when lyso-PAF is added to mast cells, a direct acetylation may occur. However, PAF is preferentially synthesized through a mechanism involving the CoA-independent transacylase reaction.

Role of the plasma membrane in signal transduction in human polymorphonuclear leukocytes

To more closely examine the role of the cell surface in transmembrane signal transduction in human neutrophils, sealed right-side-out membrane vesicles free of organellar membrane components were used as models of the plasma membrane. These vesicles, incubated with a fluorescent analogue of the chemotactic peptide fMLP, bound this ligand similarly in extent and kinetics to intact neutrophils. Vesicles responded to this stimulation with a slow increase in internal [Ca++] which was inhibited by EGTA but not by verapamil; the cytosolic Ca++ transient seen in intact cells within 10 sec of stimulation was absent in vesicles. The vesicles also maintained a transmembrane potential (psi) and were depolarized by the K+ ionophore valinomycin. However, unlike intact cells which hyperpolarized and then depolarized in response to fMLP, the vesicles demonstrated only a sustained hyperpolarization. Vesicles also differed from intact cells by not producing superoxide (O2-) in response to fMLP. Finally, fMLP caused dramatic alterations in membrane vesicle lipid metabolism: at early time points (within 5-10 sec), there was a transient production of diacylglycerol (DAG) concomitant with inositol lipid breakdown, with no apparent hydrolysis of non-inositol phospholipids. For up to 5 min after stimulation, there was no increase in the levels of phosphatidic acid or of inositol lipids. Thus, a significant portion of the signalling pathway in neutrophils is located at the cell surface or in the plasma membrane and functions independently of intracellular components. Furthermore, the plasma membrane is intimately involved in events occurring during both the early (DAG generation) and late (slow, prolonged rise in [Ca++]) phases of cellular response. In contrast, several of the responses to fMLP (the Ca++ transient, depolarization, generation of O2-, recycling of lipid metabolites) involve signalling machinery not constitutively resident on the neutrophil surface.

Evidence for different mechanisms involved in the formation of lyso platelet-activating factor and the calcium-dependent release of arachidonic acid from human neutrophils

Recent studies suggest that the first step in platelet-activating factor (PAF) biosynthesis, 1-alkyl-2-lyso-GPC (lyso PAF) formation, may be initiated by the selective transfer of arachidonate from 1-alkyl-2-arachidonoyl-GPC to an acceptor lyso phospholipid by a CoA-independent transacylase activity (CoA-IT). The present study was designed to determine whether the formation of 1-alkyl-2-lyso-GPC and the release of arachidonic acid can occur by different mechanisms. These experiments examined both the formation of 1-[3H]alkyl-2-lyso-GPC from 1-[3H]alkyl-2-acyl-GPC and the release of arachidonic acid from membrane phospholipids as determined by GC/MS in neutrophil homogenates under various conditions. The addition of unlabelled lyso phospholipids to neutrophil homogenates stimulated the time-dependent formation of 1-[3H]alkyl-2-lyso-GPC from 1-[3H]alkyl-2-acyl-GPC. Without exogenous lyso phospholipids, little 1-[3H]alkyl-2-lyso-GPC was formed in this reaction. The activity which catalyzed the formation of 1-[3H]alkyl-2-lyso-GPC had characteristics identical to CoA-IT as indicated by the fact that both reactions were: independent of Ca2+, Mg2+, CoA and CoA fatty acids, located in microsomal fractions, and stable in 10 mM dithiothreitol. In sharp contrast to the aforementioned reaction, addition of lyso phospholipids did not affect the quantity of arachidonic acid released from membrane phospholipids. Furthermore, there was a Ca(2+)-independent release of arachidonic acid from membrane phospholipid that was increased 4 to 5-fold after the addition of 5 mM Ca2+. Finally, Ca(2+)-dependent arachidonic acid release was inhibited by putative phospholipase A2 inhibitors, aristolochic acid and scalaradial, at concentrations where neither the production of 1-[3H]alkyl-2-lyso-GPC nor Ca(2+)-independent arachidonic acid release was altered. Together these data imply that there may be different mechanisms involved in the formation of 1-alkyl-2-lyso-GPC and arachidonic acid from membrane phospholipids.

The hydrolysis of natural phosphatidylethanolamines by phospholipase A2 from rat serum: a degree of selectivity is shown for docosahexaenoate release

The selectivity of phospholipase A2 from serum was evaluated using radioassays and mass analyses of fatty acids liberated from phosphatidylcholine and phosphatidylethanolamine. These natural phospholipid substrates were labelled at the sn-2 position with radioactive oleate, linoleate and arachidonate. The rates of release of fatty acids were compared with their abundance at the sn-2 position of these phospholipid substrates. While there was little or no selectivity in the liberation of these fatty acids from phosphatidylcholine, there was some evidence for a preferential release of arachidonate with respect to linoleate from phosphatidylethanolamine. Mass analyses of free fatty acid products revealed that docosahexaenoate was consistently liberated at levels that exceeded its abundance at the sn-2 position of phosphatidylethanolamine. Three different, natural phosphatidylethanolamines with varying levels of docosahexaenoate showed a 1.2-1.8-fold enrichment of this polyunsaturate in the free fatty acid products compared with its abundance at the sn-2 position. This preference could also be shown when phosphatidylethanolamine was mixed with synthetic phosphatidylcholine as co-sonicated substrates. This preferential release of docosahexaenoate by serum phospholipase A2 is of considerable significance in the nervous system which is enriched in this polyunsaturate. The potential competition between liberated docosahexaenoate and arachidonate may be of fundamental importance in the response of brain to hemorrhage.

Transmembrane signalling and paf-acether biosynthesis

Expression of lyso paf-acether (lyso paf):acetyl-CoA acetyltransferase and its activation above basal levels by specific agonists controls the rate of paf biosynthesis in proinflammatory cells. Acetyltransferase activation in these cells is due to the rapid postranslational modification of an inactive precursor by phosphorylation, most probably catalyzed by a cAMP-dependent kinase. However, the possibility exists that a calcium/calmodulin-dependent kinase can be implicated as well. Unlike murine cultured mast cells, human neutrophils form paf when stimulated with phorbol myristate acetate (PMA) or diacylglycerol. In both cell types, acetyltransferase is activated by PMA. Controversy exists as to whether PMA activates the remodeling pathway, i.e. the activation of phospholipase A2 and acetyltransferase, or the de novo route through CDPcholine cholinephosphotransferase action on alkylacetylglycerol. There is some indication that PKC might regulate paf biosynthesis. The implication of a GTP-regulated protein has also been postulated in signal transduction leading to paf formation in endothelial cells, neutrophils, and mast cells. The topography of paf formation is discussed in light of the subcellular distribution of acetyltransferase in neutrophils and Krebs II cells.

The superoxide-generating oxidase of phagocytic cells. Physiological, molecular and pathological aspects

Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O2-. The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were diassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O2- upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.

Arachidonic acid metabolism during antigen and ionophore activation of the mouse bone marrow derived mast cell

This study has examined the metabolism of arachidonic acid in the mouse bone marrow-derived mast cell (BMMC) during immunologic and nonimmunologic activation. The predominant pools of endogenous arachidonate in the mast cells were found in ethanolamine (46%), choline (39%) and inositol (14%) containing glycerolipids. Initial studies established conditions where equilibrium labelling of these major phospholipids in the BMMC could be reached. Upon challenge, arachidonate was lost from all major phospholipid classes (phosphatidylethanolamine greater than phosphatidylcholine greater than phosphatidylinositol). There was a small but significant increase in the amount of label associated with phosphatidic acid during cell activation. Arachidonate was distributed among 1-acyl, 1-alkyl and 1-alk-1-enyl-linked subclasses of PC and PE. The rank order of loss of labelled arachidonate from the major PE and PC subclasses during antigen and ionophore activation was 1-alk-enyl-2-arachidonoyl-GPE greater than 1-acyl-2-arachidonoyl-GPC greater than 1-acyl-2-arachidonoyl-GPE greater than 1-alkyl-2-arachidonoyl-GPC. Labelled products released into the supernatant fluids and free arachidonic acid within the cell accounted for the bulk of arachidonate lost from phospholipids. Labelled products in the supernatant fluids were composed of LTB4, LTC4, PGD2 and free arachidonic acid. BMMC phospholipids were also labelled for 24 hr with [3H]choline, [3H]myoinositol or [14H]ethanolamine and labelled 2-lyso phospholipids were measured after cell activation. Radioactivity in lysophospholipids from PC, PE and PI increased significantly between 30 s and 2 min after antigen activation and then declined. Taken together, these studies suggest that arachidonate is mobilized predominantly from PE and in particular 1-alk-1-enyl-2-arachidonoyl-GPE by the direct removal of arachidonate from the sn-2 position of the molecule. Most of this arachidonate is then released from cells as eicosanoids or free fatty acid.