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

Rapid production of platelet-activating factor is induced by protein kinase Cα-mediated phosphorylation of lysophosphatidylcholine acyltransferase 2 protein

Platelet-activating factor (PAF), a potent proinflammatory lipid mediator, is synthesized rapidly in response to extracellular stimuli by the activation of acetyl-CoA:lyso-PAF acetyltransferase (lyso-PAFAT). We have reported previously that lyso-PAFAT activity is enhanced in three distinct ways in mouse macrophages: rapid activation (30 s) after PAF stimulation and minutes to hours after LPS stimulation. Lysophosphatidylcholine acyltransferase 2 (LPCAT2) was later identified as a Ca(2+)-dependent lyso-PAFAT. However, the mechanism of rapid lyso-PAFAT activation within 30 s has not been elucidated. Here we show a new signaling pathway for rapid biosynthesis of PAF that is mediated by phosphorylation of LPCAT2 at Ser-34. Stimulation by either PAF or ATP resulted in PKCα-mediated phosphorylation of LPCAT2 to enhance lyso-PAFAT activity and rapid PAF production. Biochemical analyses showed that the phosphorylation of Ser-34 resulted in augmentation of Vmax with minimal Km change. Our results offer an answer for the previously unknown mechanism of rapid PAF production.

Phosphorylation of lysophosphatidylcholine acyltransferase 2 at Ser34 enhances platelet-activating factor production in endotoxin-stimulated macrophages

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid mediator that elicits various cellular functions under physiological and pathological conditions. We have recently identified two enzymes involved in PAF production: lysophosphatidylcholine acyltransferase-1 (LPCAT1) and LPCAT2. We found that LPCAT2 is highly expressed in inflammatory cells and is activated by lipopolysaccharide (LPS) treatment through Toll-like receptor 4. However, the molecular mechanism for the activation remains elusive. In this study, Phos-tag SDS-PAGE revealed the LPS-induced phosphorylation of LPCAT2. Furthermore, mass spectrometry and mutagenesis analyses identified Ser(34) of LPCAT2 as the phosphorylation site to enhance the catalytic activities. The experiments using inhibitors and siRNA against MAPK cascades demonstrated that LPCAT2 phosphorylation through LPS-TLR4 signaling may directly depend on MAPK-activated protein kinase 2 (MAPKAP kinase 2 or MK2). These findings develop a further understanding of both PAF production and phospholipid remodeling triggered by inflammatory stimuli. Specific inhibition of the PAF biosynthetic activity by phosphorylated LPCAT2 will provide a novel target for the regulation of inflammatory disorders.

Lipid mediators in health and disease: enzymes and receptors as therapeutic targets for the regulation of immunity and inflammation

Prostaglandins, leukotrienes, platelet-activating factor, lysophosphatidic acid, sphingosine 1-phosphate, and endocannabinoids, collectively referred to as lipid mediators, play pivotal roles in immune regulation and self-defense, and in the maintenance of homeostasis in living systems. They are produced by multistep enzymatic pathways, which are initiated by the de-esterification of membrane phospholipids by phospholipase A2s or sphingo-myelinase. Lipid mediators exert their biological effects by binding to cognate receptors, which are members of the G protein-coupled receptor superfamily. The synthesis of the lipid mediators and subsequent induction of receptor activity is tightly regulated under normal physiological conditions, and enzyme and/or receptor dysfunction can lead to a variety of disease conditions. Thus, the manipulation of lipid mediator signaling, through either enzyme inhibitors or receptor antagonists and agonists, has great potential as a therapeutic approach to disease. In this review, I summarize our current state of knowledge of the synthesis of lipid mediators and the function of their cognate receptors, and discuss the effects of genetic or pharmacological ablation of enzyme or receptor function on various pathophysiological processes.

A Single Enzyme Catalyzes Both Platelet-activating Factor Production and Membrane Biogenesis of Inflammatory Cells

Platelet-activating factor (PAF) is a potent proinflammatory lipid mediator eliciting a variety of cellular functions. Lipid mediators, including PAF are produced from membrane phospholipids by enzymatic cascades. Although a G protein-coupled PAF receptor and degradation enzymes have been cloned and characterized, the PAF biosynthetic enzyme, aceyl-CoA:lyso-PAF acetyltransferase, has not been identified. Here, we cloned lyso-PAF acetyltransferase, which is critical in stimulus-dependent formation of PAF. The enzyme is a 60-kDa microsomal protein with three putative membrane-spanning domains. The enzyme was induced by bacterial endotoxin (lipopolysaccharide), which was suppressed by dexamethasone treatment. Surprisingly, the enzyme catalyzed not only biosynthesis of PAF from lyso-PAF but also incorporation of arachidonoyl-CoA to produce PAF precursor membrane glycerophospholipids (lysophosphatidylcholine acyltransferase activity). Under resting conditions, the enzyme prefers arachidonoyl-CoA and contributes to membrane biogenesis. Upon acute inflammatory stimulation with lipopolysaccharide, the activated enzyme utilizes acetyl-CoA more efficiently and produces PAF. Thus, our findings provide a novel concept that a single enzyme catalyzes membrane biogenesis of inflammatory cells while producing a prophlogistic mediator in response to external stimuli.

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.

Role of MSK1 in the signaling pathway leading to VEGF-mediated PAF synthesis in endothelial cells

Vascular endothelial growth factor (VEGF) inflammatory effects require acute platelet-activating factor (PAF) synthesis by endothelial cells (EC). We previously reported that VEGF-mediated PAF synthesis involves the activation of VEGF receptor-2/Neuropilin-1 complex, which is leading to the activation of p38 and p42/44 mitogen-activated protein kinases (MAPKs) and group V secretory phospholipase A(2) (sPLA(2)-V). As the mechanisms regulating sPLA(2)-V remain unknown, we addressed the role of the mitogen- and stress-activated protein kinase-1 (MSK1), which can be rapidly and transiently activated by p38 or p42/44 MAPKs. In native bovine aortic endothelial cells (BAEC), we observed a constitutive protein interaction of MSK1 with p38, p42/44 MAPKs, and sPLA(2)-V. These protein interactions were maintained in BAEC transfected either with the empty vector pCDNA3.1, wild-type MSK1 (MSK1-WT) or N-terminal dead kinase MSK1 mutant (MSK1-D195A). However, in BAEC expressing C-terminal dead kinase MSK1 mutant (MSK1-D565A), the interaction between MSK1 and sPLA(2)-V was reduced by 82% and 90% under basal and VEGF-treated conditions as compared to native BAEC. Treatment with VEGF for 15 min increased basal PAF synthesis in native BAEC, pCDNA3.1, MSK1-WT, and MSK1-D195A by 166%, 139%, 125%, and 82%, respectively. In contrast, PAF synthesis was prevented in cells expressing MSK1-D565A mutant. These results demonstrate the essential role of the C-terminal domain of MSK1 for its constitutive interaction with sPLA(2)-V, which appears essential to support VEGF-mediated PAF synthesis.

Angiopoietins-1 and -2 are both capable of mediating endothelial PAF synthesis: intracellular signalling pathways

Vascular endothelial growth factor (VEGF) is the only angiogenic growth factor capable of inducing an inflammatory response and we have recently demonstrated that its inflammatory effect is mediated by the endothelial synthesis of platelet-activating factor (PAF). Recently discovered, Ang1 and Ang2, upon binding to Tie2 receptor, modulate vascular permeability and integrity, contributing to angiogenesis. Ang1 was initially identified as a Tie2 agonist whereas Ang2 can behave as a context-dependent Tie2 agonist or antagonist. We sought to determine if Ang1 and/or Ang2 could modulate PAF synthesis in bovine aortic endothelial cells (BAEC) and if so, through which intracellular signalling pathways. Herein, we report that Ang1 and Ang2 (1 nM) are both capable of mediating a rapid Tie2 phosphorylation and a rapid, progressive and sustained endothelial PAF synthesis maximal within 4 h (1695% and 851% increase, respectively). Angiopoietin-mediated endothelial PAF synthesis requires the activation of the p38 and p42/44 MAPKs, PI3K intracellular signalling pathways, and a secreted phospholipase A(2) (sPLA(2)-V). Furthermore, angiopoietin-mediated PAF synthesis is partly driven by a relocalization of endogenous VEGF to the cell surface membrane. Our results demonstrate that the angiopoietins constitute another class of angiogenic factors capable of mediating PAF synthesis which may contribute to proinflammatory activities.

Lysophospholipid transacetylase in the regulation of PAF levels in human monocytes and macrophages

The transacetylase (TA), reported to be identical to platelet-activating factor (PAF) acetylhydrolase (II), is a multifunctional enzyme with three catalytic activities: lysophospholipid transacetylase (TA(L)), sphingosine transacetylase (TA(S)), and acetylhydrolase (AH). We report that TA(L) activity participates in the control of PAF levels in monocytes and macrophages and that its regulation differs in these two types of cells. In monocytes, LPS or granulocyte-macrophage colony-stimulating factor (GM-CSF) specifically increased the TA(L) activity. Western blot analysis and enzyme assays on immunoprecipitates revealed that the increased activity can be ascribed to PAF-AH (II) and that both translocation from cytosol to membranes and p38/ERKs-mediated phosphorylation regulate the enzyme activation. Instead, in macrophages differentiated in vitro from monocytes by incubation with FCS, an increase of both TA(L) and AH activities was observed. Moreover, activation of ERKs and p38 MAP kinase was not required for the up-regulation of PAF-AH (II) in differentiated macrophages. The differences observed in macrophages as compared to monocytes can be explained by 1) p38/ERKs-independent phosphorylation of PAF-AH (II) and 2) appearance of plasma PAF-AH in the course of macrophage differentiation.

Inhibition of platelet-activating factor biosynthesis by adenosine and histamine in human neutrophils: involvement of cPLA2alpha and reversal by lyso-PAF

Leukotrienes (LT) and platelet-activating factor (PAF) are important lipid mediators of inflammation. We and others reported previously that autacoids such as adenosine, histamine, prostaglandin E2, and beta-adrenergic agents inhibit LT biosynthesis in activated human polymorphonuclear leukocytes (PMN). In this study, we demonstrate that CGS-21680 (a selective agonist of the adenosine A2A receptor) and histamine also potently inhibit PAF biosynthesis in agonist [formyl Met-Leu-Phe (fMLP)]- and thapsigargin-activated human PMN. The observed inhibitions of PAF biosynthesis were reversed effectively by exogenous 1-O-alkyl-lyso-sn-glyceryl-3-phosphocholine (lyso-PAF), suggesting that these effects of CGS-21680 and histamine implicate the blockade of cytosolic phospholipase A2alpha (cPLA2alpha) activity and lyso-PAF release and that the acetyl-coenzyme A/lyso-PAF acetyl transferase is not inhibited by the autacoids. Accordingly, the cPLA2alpha inhibitor pyrrophenone completely blocked PAF formation, and lyso-PAF similarly prevented this effect of pyrrophenone. The inhibitory effects of CGS-21680 and histamine on PAF biosynthesis were prevented by the protein kinase A inhibitor H-89, supporting roles for the Gs -coupled receptors A2A and H2, respectively, and cyclic adenosine monophosphate in the inhibitory mechanism. The fMLP-induced phosphorylations of p38 and extracellular signal-regulated kinase 1/2 were not altered significantly by the CGS-21680, indicating that inhibition of these kinases is not involved in the inhibitory effect of the adenosine A2A receptor ligand on LT and PAF biosynthesis. These data further emphasize the multiple and potent inhibitory effects of adenosine and histamine on leukocyte functions, in particular, on the biosynthesis of two classes of important lipid mediators and their putative regulatory roles in immune processes in health and diseases.

Lipoteichoic acid-induced nitric oxide production depends on the activation of platelet-activating factor receptor and Jak2

NO production by macrophages in response to lipoteichoic acid (LTA) and a synthetic lipopeptide (Pam3CSK4) was investigated. LTA and Pam3CSK4 induced the production of both TNF-alpha and NO. Inhibitors of platelet-activating factor receptor (PAFR) blocked LTA- or Pam3CSK4-induced production of NO but not TNF-alpha. Jak2 tyrosine kinase inhibition blocked LTA-induced production of NO but not TNF-alpha. PAFR inhibition blocked phosphorylation of Jak2 and STAT1, a key factor for expressing inducible NO synthase. In addition, LTA did not induce IFN-beta expression, and p38 mitogen-activated protein serine kinase was necessary for LTA-induced NO production but not for TNF-alpha production. These findings suggest that Gram-positive bacteria induce NO production using a PAFR signaling pathway to activate STAT1 via Jak2. This PAFR/Jak2/STAT1 signaling pathway resembles the IFN-beta, type I IFNR/Jak/STAT1 pathway described for LPS. Consequently, Gram-positive and Gram-negative bacteria appear to have different but analogous mechanisms for NO production.

Priming effect of lipopolysaccharide on acetyl-coenzyme A:lyso-platelet-activating factor acetyltransferase is MyD88 and TRIF independent

LPS has a priming effect on various stimuli. For instance, LPS priming enhances the production of platelet-activating factor (PAF), a proinflammatory lipid mediator that is induced by PAF itself. Among various enzymes responsible for PAF biosynthesis, acetyl-coenzyme A:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase is one of the enzymes activated by PAF receptor stimulation. In this study we investigated the priming effect of LPS on the acetyltransferase activation by PAF in TLR4-knockout (KO) mice, MyD88-KO mice, and Toll/IL-1R domain-containing adaptor inducing IFN-beta (TRIF)-KO mice. This enzyme was biphasically activated by LPS. Although the first peak occurred within 30 min in wild-type (WT), but not TLR4-KO or MyD88-KO, macrophages, the second phase reached a maximum within hours in WT, MyD88-KO, and TRIF-KO, but not in TLR4-KO, macrophages. Only in the second phase was the increase in acetyltransferase activity upon PAF receptor activation remarkably enhanced in WT, MyD88-KO, and TRIF-KO cells, but not in TLR4-KO cells. These data demonstrated that LPS exerted a priming effect on PAF receptor-mediated acetyltransferase activation through the TLR4-dependent, but MyD88- and TRIF-independent, pathway.

Stress-induced platelet-activating factor synthesis in human neutrophils

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) is a potent inflammatory mediator produced by cells in response to physical or chemical stress. The mechanisms linking cell injury to PAF synthesis are unknown. We used liquid chromatography-tandem mass spectrometry to investigate stress-induced PAF synthesis in human neutrophils. PAF synthesis induced by extracellular pH 5.4 correlated with the activation of a stress-activated kinase, p38 mitogen-activated protein kinase (MAPK), and was blocked by the p38 MAPK inhibitor SB 203580. A key enzyme of PAF synthesis, acetyl-CoA:lysoPAF acetyltransferase, which we have previously shown is a target of p38 MAPK, was also activated in an SB 203580-sensitive fashion. Another MAPK pathway, extracellular signal-regulated kinase-1/2 (ERK-1/2), was also activated. Surprisingly, the pharmacological blockade of the ERK-1/2 pathway with PD 98059 did not block, but rather enhanced, PAF accumulation. Two unexpected actions of PD 98059 may underlie this phenomenon: an augmentation of stress-induced p38 MAPK phosphorylation and an inhibition of PAF catabolism. The latter effect did not appear to be due to a direct inhibition of PAF acetylhydrolase. Finally, similar results were obtained using another form of cellular stress, hypertonic sodium chloride. These data are consistent with a model in which stress-induced PAF accumulation is regulated positively by p38 MAPK and negatively by ERK-1/2. Such a model contrasts with the PAF accumulation induced by other forms of stimulation, which we and others have found is up-regulated by both p38 MAPK and ERK-1/2.

Platelet-activating factor, a pleiotrophic mediator of physiological and pathological processes

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid with diverse pathological and physiological effects. This bioactive phospholipid mediates processes as diverse as wound healing, physiological inflammation, apoptosis, angiogenesis, reproduction and long-term potentiation. Recent progress has demonstrated the participation of MAP kinase signaling pathways as modulators of the two critical enzymes, phospholipase A2 and acetyltransferase, involved in the remodeling pathway of PAF biosynthesis. The unregulated production of structural analogs of PAF by non-specific oxidative reactions has expanded this superfamily of signaling molecules to include "PAF-like" lipids whose mode of action is identical to that of authentic PAF. The action of members of this family is mediated by the PAF receptor, a G protein-coupled membrane-spanning molecule that can engage multiple signaling pathways in various cell types. Inappropriate activation of this signaling pathway is associated with many diseases in which inflammation is thought to be one of the underlying features. Inactivation of all members of the PAF superfamily occurs by a unique class of enzymes, the PAF acetylhydrolases, that have been characterized at the molecular level and that terminate signals initiated by both regulated and unregulated PAF production.

Synthesis of novel lysophosphatidylcholine analogues using serine as chiral template

Four novel lysophosphatidylcholine (lysoPC) analogues, (S)-N-stearoyl-O-phosphocholineserine methyl ester [(S)-1a], (R)-1-lyso-2-stearoylamino-2-deoxy-sn-glycero-3-phosphatidylcholine [(R)-2a], (R)-N-stearoyl-O-phosphocholineserine methyl ester [(R)-1b], and (S)-1-lyso-2-stearoylamino-2-deoxy-sn-glycero-3-phosphatidylcholine [(S)-2b], were synthesized starting from serine as a chiral template. These synthetic compounds exhibited greatly enhanced hyphal transition inhibitory activity in Candida as compared to the natural lysoPC.

Sphingosine 1-phosphate effect on endothelial cell PAF synthesis: Role in cellular migration

Sphingosine 1-phosphate (S1P) and vascular endothelial growth factor (VEGF) are two inflammatory mediators capable of promoting endothelial cell (EC) migration and angiogenesis. As VEGF inflammatory effect is mediated by the synthesis of endothelial platelet-activating factor (PAF) which is also contributing to VEGF chemotactic activity, we wanted to assess if S1P can trigger PAF synthesis in EC and if S1P-induced migration is PAF-dependent. Treatment of bovine aortic EC (BAEC) with S1P (10(-10)-10(-6) M) increased dose- and time-dependently the synthesis of PAF by up to 3.3-fold above the basal level, with a maximal amount of PAF detected at 20 min post-stimulation. This biological response was attenuated by inhibiting p38 mitogen-activated protein kinase (MAPK), cytosolic or secreted phospholipase A(2) (cPLA(2), sPLA(2)) activity, suggesting that p38 MAPK activation by S1P promotes the conversion of membrane phospholipids into PAF through the combined activation of cPLA(2) and sPLA(2). Interestingly, pretreatment of BAEC with extracellular PAF receptor antagonists (BN52021, 10(-5) M and CV3988, 10(-6) M) reduced by up to 42% the cellular migration induced by S1P (10(-6) M). These data demonstrate the capacity of S1P to induce PAF synthesis, which contributes in part to S1P chemotactic activity.

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).

Endothelial cell PAF synthesis following thrombin stimulation utilizes Ca(2+)-independent phospholipase A(2)

Platelet activating factor (PAF) is a potent lipid autocoid that is rapidly synthesized and presented on the surface of endothelial cells following thrombin stimulation. PAF production may occur via de novo synthesis or by the combined direct action of phospholipase A(2) (PLA(2)) and acetyl-CoA:lyso-PAF acetyltransferase or via the remodeling pathway. This study was undertaken to define the role of PLA(2) and plasmalogen phospholipid hydrolysis in PAF synthesis in thrombin-treated human umbilical artery endothelial cells (HUAEC). Basal PLA(2) activity in HUAEC was primarily found to be Ca(2+)-independent (iPLA(2)), membrane-associated, and selective for arachidonylated plasmenylcholine substrate. Thrombin stimulation of HUAEC resulted in a preferential 3-fold increase in membrane-associated iPLA(2) activity utilizing plasmenylcholine substrates with a minimal increase in activity with alkylacyl glycerophospholipids. No change in cystolic iPLA(2) activity in thrombin-stimulated HUAEC was observed. The thrombin-stimulated activation of iPLA(2) and associated hydrolysis of plasmalogen phospholipids was accompanied by increased levels of arachidonic acid (from 1.1 +/- 0.1 to 2.8 +/- 0.1%) and prostacyclin release (from 38 +/- 12 to 512 +/- 24%) as well as an increased level of production of lysoplasmenylcholine (from 0.6 +/- 0.1 to 2.1 +/- 0.3 nmol/mg of protein), lysophosphatidylcholine (from 0.3 +/- 0.1 to 0.6 +/- 0.1 nmol/mg of protein), and PAF (from 790 +/- 108 to 3380 +/- 306 dpm). Inhibition of iPLA(2) with bromoenol lactone resulted in inhibition of iPLA(2) activity, plasmalogen phospholipid hydrolysis, production of choline lysophospholipids, and PAF synthesis. These data indicate that PAF production requires iPLA(2) activation in thrombin-stimulated HUAEC and may occur through the CoA-independent transacylase remodeling pathway rather than as a direct result of the PLA(2)-catalyzed hydrolysis of membrane alkylacyl glycerophosphocholine.

Regulation of VEGF-induced endothelial cell PAF synthesis: role of p42/44 MAPK, p38 MAPK and PI3K pathways

1. Vascular endothelial growth factor (VEGF) is a potent angiogenic and inflammatory mediator. We have recently shown that this latter effect requires the activation of Flk-1 receptor and subsequent endothelial cell (EC) PAF synthesis. However, the intracellular events that regulate EC PAF synthesis upon Flk-1 stimulation by VEGF remain to be elucidated. 2. Using specific inhibitors and Western blot analysis, we herein report that in bovine aortic endothelial cells (BAEC), VEGF induces the synthesis of PAF through the cascade activation of Flk-1 receptor, phospholipase Cgamma (PLCgamma), protein kinase C (PKC) and p42/44 mitogen-activated protein kinases (MAPK). 3. Moreover, we demonstrate that VEGF-mediated PAF synthesis requires the activation of p38 MAPK, likely by directing the conversion of lyso-PAF to PAF. 4. Interestingly, we observed that VEGF also promoted the activation of the phosphatidyl inositol-3-phosphate kinase (PI3K) pathway, and that its blockade potentiated PAF synthesis following a VEGF treatment. Consequently, it appears that the PI3K pathway acts as a negative regulator of EC PAF synthesis. 5. Taken together, these results allow a better understanding of the intracellular events activated upon EC stimulation by VEGF, and shed a new light on the mechanisms by which VEGF induces PAF synthesis.

Phospholipase D is required in the signaling pathway leading to p38 MAPK activation in neutrophil-like HL-60 cells, stimulated by N-formyl-methionyl-leucyl-phenylalanine

Human acute myelogenous leukemia cells (HL-60 cells) can be induced to differentiate to neutrophils by exposure to dibutyryl-cyclic AMP. The differentiation of HL-60 cells allowed the mitogen-activated protein kinases p38 and p44/p42 to be rapidly and transiently activated upon stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP). Western blot analysis using phosphospecific p38 and p44/p42 mitogen-activated protein kinase antibodies showed that increasing concentrations of ethanol or 1-butanol but not 2-butanol (0.05-0.5%) inhibited fMLP-induced p38 activation but did not inhibit p44/p42 activation. These data indicated that activation of phospholipase D (PLD) was required for activation of p38 but not p44/p42. We compared the effect of fMLP with those of tumor necrosis factor alpha (TNF alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF). We found that ethanol did not inhibit p38 phosphorylation upon stimulation with either GM-CSF or TNF alpha. These results suggested that in cells stimulated with fMLP, PLD was upstream of p38. To further test the involvement of PLD, we used antisense inhibition of human PLD1 expression. Treatment with antisense oligonucleotides inhibited p38 but not p44/p42 phosphorylation. These data supported a role for human PLD1 in fMLP-induced p38 activation in neutrophil-like HL-60 cells. In addition, the results obtained with TNF alpha and GM-CSF demonstrated that p38 activation occurred independently of PLD activation.

Platelet-activating factor and related lipid mediators

Platelet-activating factor (PAF) is a phospholipid with potent, diverse physiological actions, particularly as a mediator of inflammation. The synthesis, transport, and degradation of PAF are tightly regulated, and the biochemical basis for many of these processes has been elucidated in recent years. Many of the actions of PAF can be mimicked by structurally related phospholipids that are derived from nonenzymatic oxidation, because such compounds can bind to the PAF receptor. This process circumvents much of the biochemical control and presumably is regulated primarily by the rate of degradation, which is catalyzed by PAF acetylhydrolase. The isolation of cDNA clones encoding most of the key proteins involved in regulating PAF has allowed substantial recent progress and will facilitate studies to determine the structural basis for substrate specificity and the precise role of PAF in physiological events.

Roles of cytosolic phospholipase A(2) and platelet-activating factor receptor in the Ca-induced biosynthesis of PAF

Casein-elicited peritoneal exudate cells (PEC), mainly consisted of neutrophils, were collected from platelet-activating factor receptor-knock-out (PAFR-KO), cytosolic phospholipase A(2) knock-out (cPLA(2)-KO), and wild-type (WT) mice. After stimulation of PEC with calcium ionophore A 23187, PAF levels were measured by radio-ligand binding assay using receptor-rich membrane fraction prepared from the PAF receptor transgenic mice. We found that the level of PAF production by PEC was not different between WT and PAFR-KO mice. On the other hand, cPLA(2)-KO mice were deficient in the PAF production. These results provide the direct evidence while cPLA(2) is essential in the production of PAF, PAF receptor deficiency has little effect on the PAF production.

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.