Regulation of platelet activating factor synthesis: modulation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine:acetyl-CoA acetyltransferase by phosphorylation and dephosphorylation in rat spleen microsomes

Enzymatic methods for choline-containing water soluble phospholipids based on fluorescence of choline oxidase: Application to lyso-PAF

In this paper we present methods to determine water soluble phospholipids containing choline (wCh-PL). The analytes were hydrolyzed by the enzyme phospholipase D and the choline formed was oxidized by the enzyme Choline Oxidase (ChOx); the fluorescence changes of the ChOx are followed during the enzymatic reaction, avoiding the necessity of an indicating step. Both reactions (hydrolysis and oxidation) can be combined in two different ways: 1) a two-step process (TSP) in which the hydrolysis reaction takes place during an incubation time and then the oxidation reaction is carried out, the analytical signal being provided by the intrinsic fluorescence of ChOx due to tryptophan; 2) a one-step process (OSP) in which both enzymatic reactions are carried out simultaneously in the same test; in this case the analytical signal is provided by the ChOx extrinsic fluorescence due to a fluorescent probe (Ru (II) chelate) linked to the enzyme (ChOx-RuC). The analytical capabilities of these methods were studied using 1,2-dioctanoyl-sn-glycero-3-phosphocholine (C₈PC), a water soluble short alkyl chain Ch-PL as a substrate, and 1-O-hexadecyl-sn-glyceryl-3-phosphorylcholine (lyso-PAF). The analytical features of merit for both analytes using both methods were obtained. The TSP gave a 10-fold sensitivity and lower quantification limit (1.0*10^-5 M for lyso-PAF), but OSP reduced the determination time and permitted to use the same enzyme aliquot for several measurements. Both methods gave similar precision (RSD 7%, n = 5). The TSP was applied to the determination of C₈PC and lyso-PAF in spiked synthetic serum matrix using the standard addition method. The application of this methodology to PLD activity determination is also discussed.

Single-cell lipidomics: characterizing and imaging lipids on the surface of individual Aplysia californica neurons with cluster secondary ion mass spectrometry

Neurons isolated from Aplysia californica , an organism with a well-defined neural network, were imaged with secondary ion mass spectrometry, C(60)-SIMS. A major lipid component of the neuronal membrane was identified as 1-hexadecyl-2-octadecenoyl-sn-glycero-3-phosphocholine [PC(16:0e/18:1)] using tandem mass spectrometry (MS/MS). The assignment was made directly off the sample surface using a C(60)-QSTAR instrument, a prototype instrument that combines an ion source with a commercial electrospray ionization/matrix-assisted laser desorption ionization (ESI/MALDI) mass spectrometer. Normal phase liquid chromatography mass spectrometry (NP-LC-MS) was used to confirm the assignment. Cholesterol and vitamin E were also identified with in situ tandem MS analyses that were compared to reference spectra obtained from purified compounds. In order to improve sensitivity on the single-cell level, the tandem MS spectrum of vitamin E reference material was used to extract and compile all the vitamin E related peaks from the cell image. The mass spectrometry images reveal heterogeneous distributions of intact lipid species, PC(16:0e/18:1), vitamin E, and cholesterol on the surface of a single neuron. The ability to detect these molecules and determine their relative distribution on the single-cell level shows that the C(60)-QSTAR is a potential platform for studying important biochemical processes, such as neuron degeneration.

Enhancement of Acetyl-CoA: 1-O-Alkyl-2-lyso-sn-glycero-3-phosphocholine Acetyltransferase Activity by Hydrogen Peroxide

The synthesis of platelet-activating factor (PAF) by human umbilical vein endothelial cell (HUVEC) in response to H2O2 was significantly increased in a concentration-dependent manner. When HUVEC were pretreated with diethyl maleate, which depletes intracellular glutathione, PAF synthesis was enhanced 3-fold upon 5 mM H2O2-treatment. Intracellular redox was involved in regulating PAF synthesis, since the addition of antioxidants such as N-acetylcysteine, pyrrolidinecarbodithioic acid (PDTC), and Trolox reduced PAF production in H2O2-treated HUVEC. The activity of acetyl-CoA: 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, which is involved in the last step of PAF synthesis, was also activated in H2O2-treated cells. However, exogenous lyso-PAF addition had not effected to acetyltransferase activity. The acetyltransferase activity responded quickly to H2O2-treatment, but the activation was transitory. A tyrosine kinase inhibitor and a calmodulin antagonist blocked acetyltransferase activity in H2O2-stimulated cells, suggesting that tyrosine kinase and calcium/calmodulin-dependent protein kinase are involved in regulating acetyltransferase activity. These observations suggest that H2O2 is one of the modulators of lyso-PAF acetyltransferase activity via a phosphorylation system and platelet-activating factor (PAF) synthesis.

Characterization of acetyl-CoA: lyso-PAF acetyltransferase of human mesangial cells

Platelet activating factor (PAF) is a potent inflammatory mediator produced by various renal cells and it is implicated in renal pathology. The aim of this study is the characterization of remodeling lyso-PAF acetyltransferase, which is activated under inflammatory conditions, in human mesangial cell. Total membranes of mesangial cells were isolated and enzymatic activity and kinetic parameters were determined by trichloroacetic acid precipitation method. The effect of BSA, divalent cations, EDTA, and various chemicals on the activity of lyso-PAF acetyltransferase was also studied. Various detergents were also tested for the solubilization of the enzyme and only glycerol did not affect its activity. Partial purification of solubilized enzyme preparations of human kidney tissue and mesangial cells was performed on anion exchange column chromatography and native-PAGE electrophoresis and two active fractions were detected.

Application of a TCA-precipitation method for the determination of 1-alkyl-sn-glycero-3-phosphate: Acetyl-CoA acetyltransferase in human renal tissue

The activity of 1-alkyl-sn-glycero-3-phosphate:Acetyl-CoA acetyltransferase, which catalyses the first step of the de novo biosynthesis of PAF, was determined and characterised in cortical and medullary human renal tissues. A novel thin-layer chromatographic system as well as a trichloroacetic acid precipitation method, were utilised in order to determine the enzyme's activity. The acetyltransferase activity was associated with the membranous fractions of the renal tissue, it showed an optimum pH of 8.4 and it had a bell-shaped dependence on BSA concentration. One or more disulphide bonds were necessary for the action of acetyltransferase while the enzyme seemed to be independent from divalent cations. Two assay products were extracted from the incubation mixture namely alkylacetylphosphatidic acid, produced by the acetylating action of the acetyltransferase on alkyllyso-phosphatidic acid and alkylacetyl-glycerol, which is produced by the action of a phosphohydrolase on alkylacetylphosphatidic acid. The presence of NaF in the assay mixture resulted to a decreased degradation of alkylacetylphosphatidic acid, as well as to an increased overall product formation. Cortical and medullary acetyltransferases share similar biochemical properties and there is no statistical difference between the two activities.

Acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (lyso-PAF AT) activity in cortical and medullary human renal tissue

Platelet-activating factor (PAF) is one of the most potent inflammatory mediators. It is biosynthesized by either the de novo biosynthesis of glyceryl ether lipids or by remodeling of membrane phospholipids. PAF is synthesized and catabolized by various renal cells and tissues and exerts a wide range of biological activities on renal tissue suggesting a potential role during renal injury. The aim of this study was to identify whether cortex and medulla of human kidney contain the acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (lyso-PAF AT) activity which catalyses the last step of the remodeling biosynthetic route of PAF and is activated in inflammatory conditions. Cortex and medulla were obtained from nephrectomized patients with adenocarcinoma and the enzymatic activity was determined by a trichloroacetic acid precipitation method. Lyso-PAF AT activity was detected in both cortex and medulla and distributed among the membrane subcellular fractions. No statistical differences between the specific activity of cortical and medullary lyso-PAF AT was found. Both cortical and medullary microsomal lyso-PAF ATs share similar biochemical properties indicating common cellular sources.

Overexpression of phospholipid hydroperoxide glutathione peroxidase modulates acetyl-CoA, 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase activity

The synthesis of platelet-activating factor (PAF) by -stimulated RBL-2H3 cells was significantly suppressed by overexpression of phospholipid hydroperoxide glutathione peroxidase (PHGPx). When the cells overexpressing PHGPx (L9 cells) were pretreated with diethyl maleate, which reduces PHGPx activity, PAF synthesis upon stimulation rose to levels seen in mock-transfected cells (S1 cells). Hydroperoxide levels, which are reduced in L9 cells, are involved in regulating PAF synthesis, because the addition of hydroperoxyeicosatetraenoic acid increased PAF production in -stimulated L9 cells to control cell levels. The activity of acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, which is involved in the last step of PAF synthesis, is also reduced in L9 cells. p38 kinase inhibitors block acetyltransferase activity in normal -stimulated cells, suggesting that p38 kinase is involved in regulating acetyltransferase activity. Recombinant active p38 kinase activates acetyltransferase, whereas alkaline phosphatase reverses this, suggesting p38 kinase directly phosphorylates acetyltransferase. p38 kinase phosphorylation is blocked in L9 cells, indicating that high hydroperoxide levels are needed for the activation of p38 kinase. Thus, intracellular hydroperoxide levels participate in regulating p38 kinase phosphorylation, which in turn controls the activation of acetyltransferase and thus the synthesis of PAF. These observations suggest that PHGPx is an important component of the mechanisms regulating inflammation.

MgATP may depress de novo neuronal nuclear PAF generation by promoting the formation of alkylacylglycerophosphate, an inhibitor of alkylglycerophosphate acetyltransferase

MgATP substantially inhibited 1-alkyl-sn-glycero-3-phosphate (AGP) acetyltransferase found in neuronal nuclei. Other nucleotides and the ATP analogue AMP-PNP did not show a comparable inhibition. MgATP inhibition decreased in the presence of bovine serum albumin or the fatty acyl CoA synthetase inhibitor, Triacsin C. MgATP inhibition increased when nuclei were preincubated in 50 mM Tris-HCl (pH 7.4)/1 mM MgCl(2) at 37 degrees C, and preincubations elevated levels of nuclear free fatty acid. Exogenous free fatty acid, added to the acetylation incubations, increased the inhibition seen in the presence of MgATP. Oleoyl CoA, in the absence of MgATP, also inhibited AGP acetylation. These results suggested that MgATP supported the conversion of nuclear free fatty acids to fatty acyl CoA. Fatty acyl CoA may directly inhibit nuclear AGP acetyltransferase, but inhibition brought about by MgATP was competitive for the AGP substrate, suggesting an inhibitor close in structure to AGP. 1-Hexadecyl-2-arachidonoyl-sn-glycero-3-phosphate was identified as a competitive inhibitor for AGP in the acetylation reaction. Neuronal nuclei can convert AGP to 1-alkyl-2-acyl-sn-glycero-3-phosphate (AAcylGP), a reaction dependent upon MgATP and the presence of acetyl CoA or free CoA. This nuclear acylation was increased by free fatty acid addition and was seen using oleoyl CoA in the absence of MgATP. Nuclear AAcylGP formation was inhibited by bovine serum albumin and by Triacsin C. Thus, nuclear AGP acetyltransferase may be regulated by AGP acyltransferase activity and the availability of MgATP, a nucleotide that is rapidly lost during brain ischemia.

Regulation of platelet-activating factor synthesis in human neutrophils by MAP kinases

Human neutrophils (PMN) are potentially a major source of platelet-activating factor (PAF) produced during inflammatory responses. The stimulated synthesis of PAF in PMN is carried out by a phospholipid remodeling pathway involving three enzymes: acetyl-CoA:lyso-PAF acetyltransferase (acetyltransferase), type IV phospholipase A(2) (cPLA(2)) and CoA-independent transacylase (CoA-IT). However, the coordinated actions and the regulatory mechanisms of these enzymes in PAF synthesis are poorly defined. A23187 has been widely used to activate the remodeling pathway, but it has not been shown how closely its actions mimic those of physiological stimuli. Here we address this important problem and compare responses of the three remodeling enzymes and PAF synthesis by intact cells. In both A23187- and N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated PMN, acetyltransferase activation is blocked by SB 203580, a p38 MAP kinase inhibitor, but not by PD 98059, which blocks activation of the ERKs. In contrast, either agent attenuated cPLA(2) activation. Correlating with these results, SB 203580 decreased stimulated PAF formation by 60%, whereas PD 98059 had little effect. However, the combination of both inhibitors decreased PAF formation to control levels. Although a role for CoA-IT in PAF synthesis is recognized, we did not detect activation of the enzyme in stimulated PMN. CoA-IT thus appears to exhibit full activity in resting as well as stimulated cells. We conclude that the calcium ionophore A23187 and the receptor agonist fMLP both act through common pathways to stimulate PAF synthesis, with p38 MAP kinase regulating acetyltransferase and supplementing ERK activation of cPLA(2).

Properties and regulation of microsomal PAF-synthesizing enzymes in rat brain cortex

Platelet-activating factor (PAF) is a phospholipid mediator of long-term potentiation, synaptic plasticity and memory formation as well as of the development of brain damage. In brain, PAF is synthesized by two distinct pathways but their relative contribution to its productions, in various physiological and pathological conditions, is not established. We have further investigated on the properties of the two enzymes that catalyze the last step of the de novo or remodeling pathways in rat brain microsomes, PAF-synthesizing phosphocholinetransferase (PAF-PCT) and lysoPAF acetyltransferase (lysoPAF-AT), respectively. The latter enzyme is fully active at microM Ca2+ concentration, inhibited by MgATP and activated by phosphorylation. Because the reversibility of the reaction catalyzed by PAF-PCT, its direction depends on the ratio [CDP-choline]/[CMP], which is related to the energy charge of the cell. These and other properties indicate that the de novo pathway should mainly contribute to PAF synthesis for maintaining its basal levels under physiological conditions. The remodeling pathway should be more involved in the production of PAF during ischemia. During reperfusion, the overproduction of PAF should be the result of the concomitant activation of both pathways.

Lipid acetylation reactions and the metabolism of platelet-activating factor

In this review properties of lipid acetyltransferase enzymes are outlined. The three activities of interest are lyso PAF acetyltransferase (acetyl CoA: 1-alkyl-sn-glycero-3-phosphocholine acetyltransferase), AGP acetyltransferase (acetyl CoA: 1-alkyl sn-glycero-3-phosphate acetyltransferase) and a transacetylase activity that can transfer acetyl groups from PAF to lipid acceptors in the formation of 1-alkenyl-2-acetyl-sn-glycero-3-phosphoethanolamine and N-acetyl sphingosine (C2 ceramide). This review focuses on the role of acetyltransferases and transacetylases within the metabolism of platelet-activating factor and specifically addresses characteristics of the enzymes, including subcellular localization, substrate selectivity, and enzymatic regulation.

Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase is directly activated by p38 kinase

Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, along with phospholipase A2, is a key regulator of platelet-activating factor biosynthesis via the remodeling pathway. We have now obtained evidence in human neutrophils indicating that this enzyme is regulated by a specific member of the mitogen-activated protein kinases, namely the p38 kinase. We earlier demonstrated that tumor necrosis factor-alpha (TNF-alpha) as well as N-formyl-methionyl-leucyl-phenylalanine treatment leads to increased phosphorylation and activation of p38 kinase in human neutrophils. Strikingly, in the present study these stimuli increased the catalytic activity of acetyltransferase up to 3-fold, whereas 4-phorbol 12-myristate 13-acetate, which activates the extracellular-regulated kinases (ERKs) but not p38 kinase, had no effect. Furthermore, a selective inhibitor of p38 kinase, SB 203580, was able to abolish the TNF-alpha- and N-formyl-methionyl-leucyl-phenylalanine-induced activation of acetyltransferase. The same effect was not observed in the presence of an inhibitor that blocked ERK activation (PD 98059). Complementing the findings in intact cells, we have shown that recombinant, activated p38 kinase added to microsomes in the presence of Mg2+ and ATP increased acetyltransferase activity to the same degree as in microsomes obtained from TNF-alpha-stimulated cells. No activation of acetyltransferase occurred upon treatment of microsomes with either recombinant, activated ERK-1 or ERK-2. Finally, the increases in acetyltransferase activity induced by TNF-alpha could be ablated by treating the microsomes with alkaline phosphatase. Thus acetyltransferase appears to be a downstream target for p38 kinase but not ERKs. These data from whole cells as well as cell-free systems fit a model wherein stimulus-induced acetyltransferase activation is mediated by a phosphorylation event catalyzed directly by p38 kinase.

MgATP has different inhibitory effects on the use of 1-acyl-lysophosphatidylcholine and lyso platelet-activating factor acceptors by neuronal nuclear acetyltransferase activities

The inhibitory effects of MgATP on neuronal nuclear acetyltransferase activities were studied using lyso platelet-activating factor (lyso-PAF, 1-alkyl-sn-glycero-3-phosphocholine) and lysophosphatidylcholine (lyso-PC, 1-acyl-sn-glycero-3-phosphocholine). The nuclear (N1) acetylation of lyso-PC was more profoundly inhibited by MgATP. MgATP did not alter the apparent Km for acetyl-CoA in either acetylation reaction. The inhibitory effects of MgATP were not seen for other nucleotides or MgAMP-PCP. Kinase inhibitors such as staurosporine (1 microM), chelerythrine, and R59022 (diglyceride kinase inhibitor I) did not block the MgATP inhibition of either acetylation. However, the addition of phospholipids to the assays indicated a selective inhibitory effect for PIP (25-50 microM) in the nuclear acetylation of lyso-PAF. When N1 was incubated with [gamma-33P]ATP, phosphatidic acid and PIP were the principal radioactive lipid products. While the extent of MgATP inhibition of lyso-PAF acetylation was similar at different concentrations of lyso-PAF, increasing lyso-PC concentrations greatly decreased the MgATP inhibition seen in lyso-PC acetylations. Nuclear envelopes prepared in the presence of PMSF, and fraction N1 exposed to PMSF, did not show the inhibitory effect of MgATP on lyso-PC acetylation. PMSF (an inhibitor of certain phospholipase and lysophospholipase activities) did not reduce the MgATP inhibition of lyso-PAF acetylation. Arachidonoyl trifluoromethylketone, an inhibitor of cytosolic phospholipases A2 and of lysophospholipase activity associated with cPLA2, also blocked the inhibitory effect of MgATP on lyso-PC acetylation. Using radioactive lyso-PC substrate, fraction N1 produced labeled free fatty acid and phosphatidylcholine. In the presence of acetyl-CoA, the production of radioactive phosphatidylcholine increased almost 6-fold when MgATP was also included in these incubations. In the presence of MgATP and acetyl-CoA, PMSF reduced the levels of radioactive free fatty acid and phosphatidylcholine derived from lyso-PC, while Triacsin C, an inhibitor of acyl CoA synthetase, decreased phosphatidylcholine labeling. These findings suggest that MgATP inhibition of lyso-PC acetylation results from a loss of lyso-PC substrate that is largely mediated by nuclear lysophospholipase, acyl-CoA synthetase and lyso-PC acylation. Thus the neuronal nuclear production of Acyl PAF may be regulated by paths that compete for the lyso-PC substrate. In contrast, the acetylation of lyso-PAF is inhibited by PIP, a product of nuclear PI kinase reactions.

The role of platelet-activating factor-dependent transacetylase in the biosynthesis of 1-acyl-2-acetyl-sn-glycero-3-phosphocholine by stimulated endothelial cells

Acyl analogs of platelet-activating factor (PAF) (1-acyl-2-acetyl-sn-glycero-3-phosphocholine, acylacetyl -GPC) are the predominant products synthesized during thrombin or ionophore A23187-mediated activation of endothelial cells. However, the biosynthetic pathway responsible for the production of acylacetyl-GPC is not well understood. In the present investigation, we have demonstrated that the acyl analogs of PAF are also the major products from calf pulmonary artery endothelial cells in response to a time-dependent stimulation of ATP (10(-3) M), bradykinin (10(-8) M), or ionophore A23187 (2 microM). In addition, we have found that the CoA-independent PAF:acyllyso-GPC transacetylase recently identified by us is concurrently and transiently induced with maximal 4-fold enhancement at 5 min and returned to near basal level by 10 min treatment of endothelial cells with ATP. Acid phosphatase reduces the increased PAF:acyllyso-GPC transacetylase activity from the homogenates of ATP-activated endothelial cells. Reduced PAF:acyllyso-GPC transacetylase activity can be restored by incubating the acid phosphatase-treated homogenates with ATP (5 mM) and Mg2+ (10 mM). Furthermore, okadaic acid, a protein phosphatase 1 and 2A inhibitor, incubated with endothelial cells in a dose-dependent manner (1-100 nM) for 10-min potentiates and sustained the stimulation of PAF:acyllyso-GPC transacetylase activity by ATP. On the other hand, genistein, tyrphostin-25 (inhibitors of tyrosine-specific protein kinase), and calphostin C (an inhibitor of protein kinase C) block the activation of PAF:acyllyso-GPC transacetylase by ATP. These results are consistent with the notion that ATP regulates the transacetylase activity by reversible activation and inactivation via the phosphorylation and dephosphorylation cycle. ATP also augments the activities of alkyllyso-GPC/acyllyso-GPC:acetyl-CoA acetyltransferase. However, the activation of the acetyltransferases precedes that of the transacetylase with peak activation occurring at 1-2 min of the ATP treatment. In addition, sodium vanadate, also an inhibitor of protein phosphatase, stimulates the increase in the incorporation of [3H]acetate into acyl[3H]acetyl-GPC of the ATP-treated endothelial cells. Collectively, our data show that both acetyltransferases and transacetylase participate in and contribute to the biosynthesis of acyl analogs of PAF in a coordinate fashion in endothelial cells.

Neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor are located in the nuclear envelope and show differential losses in activity

Neuronal nuclear fraction N1 was isolated from cerebral cortices of 15-day-old rabbits, and nuclear subfractions prepared, in order to study the location of nuclear lyso platelet-activating factor (lyso-PAF) acetyltransferase and alkylglycerophosphate (AGP) acetyltransferase, and factors that affect the loss of these two nuclear activities. Subfractionation of prelabelled N1 indicated that the nuclear envelope had the highest percentage of the radioactive acetylated products alkylacetylglycerophosphate (AAGP) and PAF, and the distribution of these phospholipids reflected phospholipid distributions in the nuclear subfractions. The majority (95%) of radioactive AAGP and PAF was also recovered in Triton X-100 extracts of prelabelled nuclei, suggesting that these acetylated lipids are located in nuclear membranes rather than in the nuclear matrix/chromatin. Of the nuclear subfractions, the envelope had the highest AGP and lyso-PAF acetyltransferase specific activities which were close to corresponding values seen in the parent N1 fraction. Thus the nuclear AGP and lyso-PAF acetyltransferases were principally localized to the nuclear membranes. Differentials in activity loss were seen for the two acetyltransferase activities. In the nuclear envelope fractions, the lyso-PAF acetyltransferase was the more susceptible to oxidation reactions which could be reversed or blocked by the use of reducing agents. In preincubations, N1 showed greater losses in lyso-PAF acetyltransferase activity than in AGP acetyltransferase activity, losses which were not attributable to oxidation. Addition of cytosolic fraction S3 to preincubations promoted losses for each acetyltransferase in N1, and gave evidence for cytosolic and endogenous nuclear contributions to the activity loss. Addition of okadaic acid to the preincubations did not prevent the decline of either acetyltransferase in intact nuclei, but did diminish the loss of nuclear lyso-PAF acetyltransferase activity promoted by S3 addition, and also blocked the loss of this acetyltransferase seen in preincubations of isolated nuclear envelopes. This suggests that nuclear lyso-PAF acetyltransferase is susceptible to okadaic acid-sensitive nuclear and cytosolic protein phosphatase activities, while AGP acetyltransferase may lose activity by the action of other phosphatases or by other mechanisms within the nucleus.

Microplate chromatography assay for acetyl-CoA: lysoplatelet-activating factor acetyltransferase

Acetyl-CoA:lysoplatelet-activating factor (1-O-alkyl-sn-glycero-3-phosphocholine) acetyltransferase (lysoPAF-AT) (EC 2.3.1.67) is a key enzyme in the biosynthesis of platelet-activating factor (PAF) and has been shown to be activated by various extracellular stimuli. A novel method to determine the enzyme activity is described here, which enables 96 simultaneous assays in a standard 96-well microplate format. The assay is based on the quantification of the incorporation of [3H]acetyl-CoA into PAF in the presence of lysoPAF. The radioactive products are separated from the substrate with a 96-well-formatted chromatography device using a Multiscreen plate (Millipore) prefilled with octyl-silica gel. As little as 1 mg octyl-silica gel was sufficient for the efficient recovery of the radioactive product, resulting in the very low background and thus high sensitivity. The enzyme activity could be measured directly with whole cell lysates from various cells cultured in 96-well microplate scale. This tailor-made microplate chromatography separation step is readily applicable for other kinds of enzyme assays.

Inhibition of lysoPAF acetyltransferase activity by flavonoids

OBJECTIVE AND DESIGN

Several kinds of flavonoids, widely distributed natural products of the vegetable kingdom which possess anti-inflammatory activity, were examined for inhibitory effects on the acetyl-CoA: 1-alkyl-2-lyso-sn-glycero-3-phosphocholine (lysoPAF) acetyltransferase activity.

METHODS

Acetyl-CoA:lysoPAF acetyltransferase activity was determined using homogenates of a rat mucosal-type mastocytoma cell line, RBL-2H3 as an enzyme source. The production of platelet-activating factor (PAF) in rat peripheral white blood cells stimulated with the calcium ionophore A23187 was studied.

RESULTS

Of the flavonoids tested, luteolin and quercetin exhibited significant inhibitory effects (IC50, 45 microM and 80 microM, respectively), whereas other structurally-related flavonoids failed to affect the lysoPAF acetyltransferase activity. Luteolin did not suppress the activity of choline acetyltransferase, suggesting that the inhibition observed here was specific. Luteolin also inhibited the production of PAF in rat peripheral white blood cells.

CONCLUSIONS

These results indicate that luteolin could become a leading compound for developing a novel type of anti-inflammatory, anti-allergic drugs that target lysoPAF acetyltransferase.

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.

Activities of enzymes in platelet activating factor biosynthetic pathways in the gerbil model of cerebral ischemia

The activities of enzymes in platelet activating factor (PAF) biosynthetic pathways were analyzed in hippocampal and cerebral cortical regions of normal and ischemic gerbil brain to assess changes in enzyme activities and potential modulators that could explain the accentuated production of PAF seen in ischemia. Global forebrain ischemia was produced by bilateral carotid artery ligation, and the effectiveness of the ligation was shown by free fatty acid release and ATP depletion. Specific activities of 1-alkyl-2-acetyl-sn-glycerol (AAG) choline phosphotransferase, 1-alkyl-sn-glycero-3-phosphate (AGP) acetyl transferase, and 1-alkyl-sn-glycero-3-phosphocholine (lyso PAF) acetyl transferase in tissue homogenates were in the ratio 4:1:0.1, respectively. Sham-operated and ischemic or ischemic-reperfused tissues showed similar activities for individual enzymes, indicating that enzyme levels or activation states did not change in ischemic or reperfused tissues. However, small metabolites (relevant to ischemia) added to the in vitro assays did modify enzyme activities. Physiological concentrations of MgATP severely inhibited AGP acetyl transferase activity, and this resulted in the ratio of AGP acyl transferase to AGP acetyl transferase activities changing from 48:1 in the presence of 2.5 mM MgATP to 6:1 in the absence of MgATP. This suggests that falling ATP levels in cerebral ischemia may promote the de novo pathway of PAF biosynthesis by releasing inhibition of AGP acetyl transferase. Lyso PAF acetyl transferase was much less active than AGP acetyl transferase and was also inhibited by MgATP. AAG choline phosphotransferase was not inhibited by MgATP but was inhibited by calcium. However the superior specific activity of the choline phosphotransferase in comparison with the AGP acetyl transferase suggested that the lowered choline phosphotransferase activity in the presence of rising intracellular calcium would not seriously compromise the synthesis of PAF by the de novo route. Both acetyl transferase enzymes were also inhibited by oleoyl CoA.

High-density lipoprotein inhibits the synthesis of platelet-activating factor in human vascular endothelial cells

The regulation of platelet-activating factor (PAF) synthesis by serum lipoproteins was investigated in human umbilical vein endothelial cells. High-density lipoprotein (HDL) inhibited PAF synthesis in agonist (thrombin, histamine, and A23187)-stimulated endothelial cells, that was determined by incorporation of [3H]acetate into PAF and by bioassay. The inhibition by HDL was increased in a concentration-dependent manner, but was reversed as the concentration of thrombin increased. HDL did not affect the time course of PAF production. HDL lipids suppressed the PAF production to a lesser extent than HDL. The reduction of PAF accumulation in HDL, did not result from degradation of PAF but inhibition of PAF synthesis, which was mainly mediated via the blockade of acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase activation. HDL did not prevent the release of [3H]arachidonic acid in thrombin-stimulated endothelial cells. The binding of 125I-HDL to endothelial cells and its uptake were not enhanced by thrombin stimulation. These results demonstrate that HDL may inhibit the activation of acetyltransferase by thrombin at the cell surface. This observation may explain a part of mechanism of HDL action.

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.

Plasma platelet-activating factor acetylhydrolase is a secreted phospholipase A2 with a catalytic triad

Platelet-activating factor (PAF) is a potent pro-inflammatory autacoid with diverse physiological and pathological actions. These actions are modulated by PAF acetylhydrolase, which hydrolyzes the sn-2 ester bond to yield the biologically inactive lyso-PAF. In contrast to most secreted phospholipase A2s, plasma PAF acetylhydrolase is calcium-dependent and contains a GXSXG motif that is characteristic of the neutral lipases and serine esterases. In this study we tested whether the serine in this motif is part of the active site of plasma PAF acetylhydrolase and, if so, what the other components of the active site are. Using site-directed mutagenesis, we demonstrated that Ser-273 (of the GXSXG motif), Asp-296, and His-351 are essential for catalysis. These residues were conserved in PAF acetylhydrolase sequences isolated from bovine, dog, mouse, and chicken. The linear orientation and spacing of these catalytic residues are consistent with the alpha/beta hydrolase conformation of other lipases and esterases. In support of this model, analysis of systematic truncations of PAF acetylhydrolase revealed that deletions beyond 54 amino acids from the NH2 terminus and 21 from the COOH terminus resulted in a loss of enzyme activity. These observations demonstrate that although plasma PAF acetylhydrolase is a phospholipase A2 it has structural properties characteristic of the neutral lipases and esterases.

Platelet-activating factor: the biosynthetic and catabolic enzymes

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MgATP inhibits the synthesis of 1-alkyl-2-acetyl-sn-glycero-3-phosphate by microsomal acetyltransferase of immature rabbit cerebral cortex

The activity of 1-alkyl-sn-glycero-3-phosphate (AGP) acetyltransferase was studied using microsomal fractions isolated from cerebral cortices of 15-day-old rabbits. Fraction P3A was isolated using buffered 0.32 M sucrose containing mercaptoethanol, EDTA and NaF. This fraction had specific AGP acetyltransferase activities which were 4.9-times those of microsomal fraction P3B isolated in 0.32 M sucrose alone. This P3B activity was increased 2.4-times after a preincubation in the presence of ATP, MgCl2 and a high-speed supernatant fraction from cerebral cortex. Further, the activities of both P3A and P3B were almost completely eliminated by preincubation in the presence of alkaline phosphatase. Thus an activation of the AGP acetyltransferase by phosphorylation was indicated. While there was little inhibition of the P3A AGP acetyltransferase in the presence of added ATP, the magnesium salt form of ATP (1 mM) was severely inhibitory, bringing about 86% inhibition for P3A and 91% for P3B. The inhibitory effects of MgADP and MgAMP were smaller, and MgATP was a much more effective inhibitor than MgCTP, MgGTP and MgUTP which brought about 20-38% inhibitions of P3A activity at 1 mM concentrations. The effect of MgATP may be of particular relevance to the synthesis of platelet activating factor (PAF) following a period of ischemia in brain. Falling MgATP levels during energy failure could relieve the inhibition of AGP acetyltransferase seen in healthy cells and allow the formation of 1-alkyl-2-acetyl-sn-glycero-3-phosphate, which is the first committed intermediate in the de novo pathway of PAF synthesis.

The potential for platelet-activating factor synthesis in brain: properties of cholinephosphotransferase and 1-alkyl-sn-glycero-3-phosphate acetyltransferase in microsomal fractions of immature rabbit cerebral cortex

The synthesis of platelet-activating factor (PAF) was studied in microsomal fractions of cerebral cortices of 15-day-old rabbits. These included: a total microsomal fraction P3, rough and smooth microsomes, R and S, and microsomal fraction P derived from isolated nerve cell bodies. Cholinephosphotransferase (CPT) generating PAF from alkylacetylglycerol had the highest specific activities in fractions R and P (24 and 6 times the homogenate values, based on membrane phospholipid content). This CPT activity differed from that which synthesized phosphatidylcholine as the latter was sensitive to dithiothreitol inhibition and was more readily inhibited by Triton X-100. As the CPT activity for PAF synthesis relies on the production of alkylacetylglycerol we studied the acetyltransferase which forms 1-alkyl-2-acetyl-sn-glycero-3-phosphate (AAGP). This enzyme had the highest specific activity in fraction R, followed by fractions P3 and P. There was evidence that the acetyltransferase was more active in a phosphorylated form. NaF maximized the recovery of AAGP products in the assays. The pH optimum for acetylation was in a range of 8.0-9.0. Lyso PAF did not inhibit the formation of AAGP and the rates of formation of PAF by acetylation were less than 5% of values for AAGP synthesis. During AAGP formation there was no evidence for subsequent alkylacetylglycerol formation in the absence of NaF, but a small formation of radioactive PAF could be demonstrated from AAGP under the CPT assay conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of the biosynthesis of platelet-activating factor in alveolar macrophages

Activities of enzymes which metabolize lysoplatelet-activating factor (lysoPAF) and platelet-activating factor (PAF) were studied in rabbit alveolar macrophage lysates. Substantial acetyltransferase activity was noted in the presence of 100 microM acetyl-coenzyme A (CoA), and this activity was increased in A23187-stimulated cell lysate. On the other hand, in the absence of exogenous acetyl-CoA, lysoPAF was mainly acylated through a transacylation pathway rather than by acetyltransferase in both control and A23187-stimulated cell lysates. We confirmed that the intracellular concentration of acetyl-CoA is relatively low. The observations suggest that the transacylation system may play an equally important role in the regulation of the availability of lysoPAF in intact cells. Intracellular lysoPAF was also maintained at relatively low levels. Interestingly, large amounts of PAF were produced even in unstimulated cells upon addition of an excess of exogenous lysoPAF, suggesting that generation of an adequate amount of lysoPAF within cells may be sufficient to trigger PAF synthesis in this type of cells.

Kinetic studies of human and rat neutrophil lysoPAF acetyltransferase using lysoPAF and dansyllysoPAF as substrates

Enzyme kinetic studies of lysoPAF acetyltransferase from microsomal preparations of human and rat neutrophils were carried out using lysoPAF or dansyllysoPAF as substrate. With the human enzyme, incomplete conversion of the substrate into the product was observed at 37 degrees C with both substrates. The acetyltransferase was inactivated at 37 degrees C in the absence of substrate with a half-life of 7.5 min. However, the initial rate of product formation under the assay conditions was linear up to 10 min. Both enzymes were optimally active at 40 microM concentration with either substrate, but enzyme activity was inhibited at higher substrate levels. At a constant substrate concentration (40 microM), the Km (microM) and Vmax (nmol product/min/mg protein) values for the human acetyltransferase, with respect to acetyl-CoA were 132 and 23.1, respectively, with lysoPAF as substrate, and 105 and 26.7, respectively, when dansyllysoPAF was used. The Km and Vmax values for the rat enzyme were 105 and 6.5, respectively, with lysoPAF as substrate, and 120 and 5.4, respectively, when dansyllysoPAF was used. Under our standard conditions, lysoPAF required 1 mg of BSA per mL in the assay, whereas full activity of both enzymes was seen with dansyllysoPAF even in the absence of BSA. The results show that dansyllysoPAF can replace lysoPAF in the assay without any significant changes in kinetic parameters.

Phorbol ester stimulates PAF synthesis via the activation of protein kinase C in rat leukocytes

When rat pleural mononuclear leukocytes were stimulated with 1 microM phorbol myristate acetate (PMA), platelet-activating factor (PAF)-like activity was detected in the supernatant and the cellular fractions of the incubation mixture, as measured by rabbit platelet aggregation. C16PAF activity peaked at 30 min in both fractions. Acetyltransferase activity in the microsomal fraction of the stimulated cells also increased rapidly and showed a peak at 10 min. A protein kinase C inhibitor, staurosporine, and an inhibitor of phospholipase A2, p-bromophen-acylbromide, inhibited stimulated PAF formation in both fractions. Staurosporine also inhibited PMA induced acetyltransferase activity. The data suggest that PMA stimulates PAF synthesis by the remodeling pathway in rat pleural cells through activation of both phospholipase A2 and acetyltransferase, and that the acetyltransferase, in turn, may be activated through activation of protein kinase C.

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.

Platelet-activating factor (PAF) stimulates the lysoPAF acetyltransferase in leukocyte-rich plasma: use in PAF antagonist studies

Addition of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to leukocyte-rich plasma from several species resulted in the rapid and pronounced activation of the PAF biosynthetic enzyme acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67). Activation of acetyltransferase by PAF occurred in leukocyte-rich plasma from human, chimpanzee, rhesus monkey, and dog. The neutrophil was indicated to be the major cellular source of the activatable acetyltransferase in leukocyte-rich plasma. The induction of acetyltransferase was substantial with 10 nM PAF, and maximal at 10-30 seconds. Measurable acetyltransferase activation was significantly greater when the PAF-activated cells were separated from the plasma by centrifugation before the acetyltransferase assay. This may be due in part to the removal of the PAF-specific acetylhydrolase present in plasma which can cleave the acetyl group from PAF. Measuring PAF activation of acetyltransferase in leukocyte-rich plasma can be useful to determine the potency of PAF antagonists with neutrophils in plasma compared to isolated neutrophils in aqueous buffer, and as an ex vivo assay to determine the efficacy and plasma concentration equivalents of antagonists administered to whole animals. The PAF antagonist L-659,989 was shown to be 3-5 times more potent in inhibiting PAF induction of acetyltransferase in isolated human neutrophils than in human leukocyte-rich plasma, with IC50 values of 10 nM and 40 nM, respectively. In the ex vivo assay, oral administration of the PAF antagonist L-667,131 to dogs resulted in very substantial inhibition of PAF induction of acetyltransferase in the leukocyte-rich plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of PAF-synthesizing phosphocholinetransferase and evidence for lysoPAF acetyltransferase activity in rat brain

Several reports have indicated that platelet-activating factor (PAF) may play a role in the physiopathology of nervous tissue. We previously have demonstrated the presence, in the microsomal fractions of rat brain, of a phosphocholinetransferase which is able to synthesize PAF by the de novo pathway. The presence of dithiothreitol in the medium increases the rate of PAF biosynthesis, whereas it inhibits the synthesis of long-chain alkylacyl- and diacyl-glycerophosphocholines (GPC), including dioctanoyl-GPC. This and other properties, such as pH dependence and thermal stability, indicate that rat brain may have two distinct enzymes for the synthesis of PAF and other choline phospholipids. The affinity of these enzymes for CDPcholine is similar to that reported for other tissues, the Km being 42 microns and 55 microns with alkylacetylglycerol and dioctanoylglycerol as lipid substrates, respectively. The Vmax values were 3.0 and 2.2 nmol/mg prot/min for PAF and dioctanoyl-GPC, respectively. In addition, it was shown that the microsomal fraction of rat brain contains an acetyltransferase which can convert lysoPAF to PAF. Since it has been reported previously that brain tissue possesses phospholipase A2 activity that can hydrolyze alkylacyl-GPC to lysoPAF, we conclude that brain tissue has all enzymic activities for the synthesis of PAF by the "remodeling pathway". The role of the two routes of PAF biosynthesis in nervous tissue remains to be established.

Transient activation of 1-O-alkyl-sn-glycero-3-phosphocholine: acetyl-CoA acetyltransferase during the incubation of macrophages

The activity of the platelet-activating factor (PAF)-synthesizing enzyme, 1-O-alkyl-sn-glycero-3-phosphocholine (lysoPAF):acetyl-CoA acetyltransferase (EC 2.3.1.67) in alveolar macrophage lysate was found to be elevated after warming the cells to 37 degrees C. Such an increase in enzyme activity was detectable only when intact cells were warmed. The stimulation was transient, reaching a peak at 2 min, and then gradually decreased to the control level. We could not find increased PAF formation in warmed cells which had increased acetyltransferase activity, even though substantial amounts of lysoPAF were shown to be present within cells. In contrast, considerable amounts of PAF were formed after treatment of the cells with exogenous lysoPAF. These results suggest that the activation of acetyltransferase is not sufficient to induce PAF formation and that the increased availability of substrates, especially lysoPAF, in the cells is indispensable for triggering PAF biosynthesis in this type of cells.

Production of platelet-activating factor is a component of the angiotensin II-protein kinase C activation pathway in bovine adrenocortical cells

Lyso-platelet-activating factor (lyso-PAF): acetyl-CoA acetyltransferase (EC 2.3.1.67) enzyme activity was characterized for the first time in bovine adrenocortical tissue. It was found to be associated with the microsomal membrane fraction, in which it exhibited a specific activity of 0.4 nmol/min per mg of protein and catalytic properties similar to those described in other cell types. The adrenocortical acetyltransferase activity was increased by 2-3-fold on incubation of the preparation with purified protein kinase C (PKC) under phosphorylating condition. This activation was optimal after 5 min of incubation and paralleled an increase in PKC-catalysed 32P incorporation into microsomal proteins. Both acetyltransferase activation and protein phosphorylation were dependent on the presence of Ca2+ and phospholipids, and were blocked in the presence of the potent PKC inhibitor H-7. In the intact adrenocortical cell, angiotensin II and a potent phorbol ester (phorbol 12-myristate 13-acetate) were able to rapidly induce an increase in the biosynthesis of PAF, which was mostly released into the extracellular medium. These data suggest that bovine adrenocortical lyso-PAF acetyltransferase may be regulated by a PKC-dependent activation pathway, whereas no evidence for an additional adrenocorticotropin/cyclic AMP-dependent stimulation process was obtained in this cell type. Bovine adrenocortical cell membrane preparations were shown to possess high-affinity PAF-binding sites (Kd approximately 0.5 nM). Altogether, these observations suggest that PAF production and release may play a role in the autocrine or paracrine control of adrenocortical cell activation.

Protein kinase C and cyclic AMP modulate thrombin-induced platelet-activating factor synthesis in human endothelial cells

Stimulation of human endothelial cells (EC) by thrombin elicits a rapid increase of intracellular free Ca2+ [(Ca2+]i), platelet-activating factor (PAF) production and 1-O-alkyl-2-lyso-sn-glycero-3- phosphocholine (lyso-PAF): acetyl-CoA acetyltransferase (EC 2.3.1.67) activity. The treatment of EC with thrombin leads to a 90% decrease in the cytosolic protein kinase C (PKC) activity; this dramatic decline is accompanied by an increase of the enzymatic activity in the particulate fraction. The role of PKC in thrombin-mediated PAF synthesis has been assessed: (1) by the blockade of PKC activity with partially selective inhibitors (palmitoyl-carnitine, sphingosine and H-7); (2) by chronic exposure of EC to phorbol 12-myristate 13-acetate (PMA), which results in down-regulation of PKC. In both cases, a strong inhibition of thrombin-induced PAF production is observed, suggesting obligatory requirement of PKC activity for PAF synthesis. It is suggested that PKC regulates EC phospholipase A2 (PLA2) activity as thrombin-induced arachidonic acid (AA) release is 90% inhibited in PKC-depleted cells. Brief exposure of EC to PMA strongly inhibits thrombin-induced [Ca2+]i rise, acetyltransferase activation and PAF production, suggesting that, in addition to the positive forward action, PKC provides a negative feedback control over membrane signalling pathways involved in the thrombin effect on EC. Forskolin and iloprost, two agents that increase the level of cellular cAMP in EC, are very effective in inhibiting thrombin-evoked cytosolic Ca2+ rise, acetyltransferase activation and PAF production; this suggests that endogenously generated prostacyclin (PGI2) may modulate the synthesis of PAF in human endothelial cells.

C20 polyunsaturated fatty acids and phorbol myristate acetate enhance agonist-stimulated synthesis of 1-radyl-2-acetyl-sn-glycero-3-phosphocholine in vascular endothelial cells

This study has investigated the effect of supplementation of vascular endothelial cells with arachidonate and other polyunsaturated fatty acids on the agonist-stimulated synthesis of platelet activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; 1-alkyl-2-acetyl-GPC). Incubation of calf pulmonary artery endothelial cells for 48 h in medium containing 40 microM arachidonate resulted in a 2-3-fold enhancement of [3H]acetate incorporation into 1-radyl-2[3H]acetyl-GPC in response to either bradykinin or calcium ionophore A23187. The effects of arachidonate supplementation were both dose- and time-dependent, requiring a minimum exogenous arachidonate concentration of 2.5 microM and an incubation time of 4-6 h. Eicosapentaenoate and docosahexaenoate also enhanced the synthesis of 1-radyl-2-[3H]acetyl-GPC, but were less potent than arachidonate; alpha-linolenate, linoleate and oleate were without effect. Although not effective as an agonist, phorbol myristate acetate potentiated A23187- and bradykinin-stimulated synthesis of 1-radyl-2-[3H]acetyl-GPC. The effects of arachidonate supplementation were synergistic with potentiation by phorbol myristate acetate. Sphingosine inhibited agonist-stimulated incorporation of [3H]acetate into 1-radyl-2-[3H]acetyl-GPC both in the presence and absence of PMA. Characterization of the radiolabeled material indicated that the primary product was the acyl analogue of PAF (1-acyl-2-acetyl-GPC) rather than PAF. The results from this study suggest that agonist-stimulated synthesis of 1-radyl-2-acetyl-GPC in vascular endothelial cells is modulated both by cellular fatty acyl composition and activation of protein kinase C. Enrichment of vascular endothelial cells with fatty acids, which are mobilized by agonist-stimulated phospholipase A2, may enhance subsequent deacylation of choline phospholipids and, thus, increase synthesis of both 1-acyl-2-acetyl-GPC and PAF.

Platelet-activating factor and related acetylated lipids as potent biologically active cellular mediators

Platelet-activating factor (PAF or 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is the most potent lipid mediator yet discovered. It is known to stimulate a wide span of biological responses ranging from aggregation and degranulation of platelets and neutrophils to a variety of cellular effects involving the stimulation of chemotaxis; chemokinesis; superoxide formation; protein phosphorylation; activation of protein kinase C, arachidonic acid, and phosphoinositide metabolites; glycogenolysis; and tumor necrosis factor production. Obviously, with such a diversity of biological activities, it is not surprising that PAF has been considered to be a key component in numerous diseases related to hypersensitivity and inflammatory responses. Evidence has also been presented for the role of PAF in physiological processes, particularly those involving reproduction and fetal development. Furthermore, because of its potent hypotensive action, PAF has been implicated as a contributing factor in blood pressure regulation. PAF is produced by two independent enzymatic pathways. The remodeling route involves the structural modification of a membrane lipid (1-alkyl-2-acyl-sn-glycero-3-phosphocholine) by replacement of the acyl moiety with an acetate group. An alternate route is the de novo synthesis of PAF from an O-alkyl analogue of a lysophosphatidic acid that requires a reaction sequence of acetylation, dephosphorylation, and phosphocholine addition steps. Hypersensitivity and other pathophysiological reactions are thought to be caused by activation of the remodeling pathway, whereas the de novo route is believed to be the source of endogenous levels of PAF required for physiological functions. Inactivation of PAF occurs when the acetate group is hydrolyzed by an acetylhydrolase that is present in both extra- and intracellular compartments, although the catalytic activity of the two forms of acetylhydrolase are identical, some of their properties differ. The control of PAF metabolism is very complex, but acetylhydrolase, Ca2+, phosphorylation/dephosphorylation of enzymes, and fatty acids (especially polyunsaturates) appear to be important regulatory factors. Specific PAF receptors have clearly been demonstrated on several different types of cells, and although the mechanism of PAF actions is poorly understood, it appears that the PAF/receptor-induced responses are closely associated with the signal transduction process; both G proteins and adenyl cyclase appear to be involved. Because significant quantities of PAF are often retained within certain cells, the possibility of PAF serving as an intracellular mediator has also been proposed.

Platelet-activating factor (PAF) and its relation to prostaglandins, leukotrienes and other aspects of arachidonate metabolism

This article summarizes some of the previously reported findings regarding a lipid mediator known as platelet-activating factor (PAF), and briefly describes its effects on cells and tissues. The effects of PAF have also been considered in relation to certain products of arachidonate metabolism released in response to PAF.

Lipid metabolism and signal transduction in endothelial cells

Endothelial cells have the capacity to metabolize several important lipids; this includes the ability to store and then metabolize arachidonate, as well as the capacity to synthesize platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine). Arachidonate is predominantly metabolized via cyclooxygenase to PGI2 although the spectrum of prostaglandins may vary depending upon the source of the endothelial cell. Biosynthesis of eicosanoids and PAF are likely to be an important physiologic function of the endothelial cell as these potent lipids appear to have a role in maintaining vascular tone and mediating interactions of the endothelium with circulating inflammatory cells. In addition to production of eicosanoids and PAF, endothelial cells metabolize exogenous arachidonate and arachidonate metabolites and other fatty acids such as linoleate to bioactive compounds (HODEs). There is also evidence that small amounts of arachidonate are metabolized via a lipoxygenase. The physiologic significance of these minor lipid pathways is not known at this time. Production of eicosanoids and PAF is not a constitutive function of the endothelial cell. Lipid biosynthesis by endothelial cells is one component of the early activation response that occurs in response to stimulation with pro-inflammatory and vasoactive hormones or to pathologic agents such as oxidants and bacterial toxins. A central mechanism for activation of the relevant pathways is a rise in cellular calcium concentrations that can be mediated by hormone-receptor-binding or by direct permeabilization of the cell membrane to calcium (Fig. 3). Regulatory mechanisms distal to the calcium signal are unknown, but current evidence suggests that calcium directly or indirectly activates phospholipases that release arachidonate from phospholipids and hydrolyze a specific phospholipid to the immediate precursor of PAF. There is evidence that protein kinase C may, in part, regulate this process, but the role of other potential regulatory components, such as other protein kinases or G-proteins is not known. As noted above, the most direct mechanism for initiation of PAF biosynthesis and arachidonate release would be activation of a phospholipase A2 as shown in Fig. 3. Activation of other phospholipases (e.g. phospholipase C) may contribute to the total amount of arachidonate released, although the magnitude of that contribution is not yet known. In addition to generation of PAF and eicosanoids, activation of endothelial cell phospholipases generates second messengers that are important in intracellular signaling (Fig. 4). Activation of phospholipase C, in response to hormonal stimulation, generates diacylglycerol and inositol phosphates from phosphatidylinositol. Each of these is a potent intracellular second messenger.(ABSTRACT TRUNCATED AT 400 WORDS)