Activation of the acetyl-coenzyme A:lysoplatelet-activating factor acetyltransferase regulates platelet-activating factor synthesis in human endothelial cells

The lipid biology of sepsis

Sepsis, defined as the dysregulated immune response to an infection leading to organ dysfunction, is one of the leading causes of mortality around the globe. Despite the significant progress in delineating the underlying mechanisms of sepsis pathogenesis, there are currently no effective treatments or specific diagnostic biomarkers in the clinical setting. The perturbation of cell signaling mechanisms, inadequate inflammation resolution, and energy imbalance, all of which are altered during sepsis, are also known to lead to defective lipid metabolism. The use of lipids as biomarkers with high specificity and sensitivity may aid in early diagnosis and guide clinical decision making. In addition, identifying the link between specific lipid signatures and their role in sepsis pathology may lead to novel therapeutics. In this review, we discuss the recent evidence on dysregulated lipid metabolism both in experimental and human sepsis focused on bioactive lipids, fatty acids, and cholesterol as well as the enzymes regulating their levels during sepsis. We highlight not only their potential roles in sepsis pathogenesis but also the possibility of using these respective lipid compounds as diagnostic and prognostic biomarkers of sepsis.

Dual phase regulation of experimental allergic encephalomyelitis by platelet-activating factor

Experimental allergic encephalomyelitis (EAE) serves as a model for multiple sclerosis and is considered to be a CD4⁺ Th1 cell–mediated autoimmune disease. To investigate the role of platelet-activating factor (PAF) in this disease, PAF receptor (PAFR) KO (PAFR-KO) and wild-type (WT) mice, on a C57BL/6 genetic background, were immunized with myelin oligodendrocyte glycoprotein 35–55. The levels of PAF production and PAFR mRNA expression in the spinal cord (SC) correlated with the EAE symptoms. PAFR-KO mice showed lower incidence and less severe symptoms in the chronic phase of EAE than WT mice. However, no difference was observed in T cell proliferation, Th1-cytokine production, or titer of IgG2a between both genotypes. Before onset, as revealed by microarray analysis, mRNAs of inflammatory mediators and their receptors—including IL-6 and CC chemokine receptor 2—were down-regulated in the SC of PAFR-KO mice compared with WT mice. Moreover, in the chronic phase, the severity of inflammation and demyelination in the SC was substantially reduced in PAFR-KO mice. PAFR-KO macrophages reduced phagocytic activity and subsequent production of TNF-α. These results suggest that PAF plays a dual role in EAE pathology in the induction and chronic phases through the T cell–independent pathways.

Regulation of platelet-activating factor synthesis in human monocytes by dipalmitoyl phosphatidylcholine

Platelet-activating factor (PAF) has a major role in inflammatory responses within the lung. This study investigates the effect of pulmonary surfactant on the synthesis of PAF in human monocytic cells. The pulmonary surfactant preparation Curosurf significantly inhibited lipopolysaccharide (LPS)-stimulated PAF biosynthesis (P<0.01) in a human monocytic cell line, Mono mac-6 (MM6), as determined by (3)H PAF scintillation-proximity assay. The inhibitory properties of surfactant were determined to be associated, at least in part, with the 1,2-dipalmitoyl phosphatidylcholine (DPPC) component of surfactant. DPPC alone also inhibited LPS-stimulated PAF biosynthesis in human peripheral blood monocytes. DPPC treatment did not affect LPS-stimulated phospholipase A(2) activity in MM6 cell lysates. However, DPPC significantly inhibited LPS-stimulated coenzyme A (CoA)-independent transacylase and acetyl CoA:lyso-PAF acetyltransferase activity. DPPC treatment of MM6 cells decreased plasma membrane fluidity as demonstrated by electron paramagnetic resonance spectroscopy coupled with spin labeling. Taken together, these findings indicate that pulmonary surfactant, particularly the DPPC component, can inhibit LPS-stimulated PAF production via perturbation of the cell membrane, which inhibits the activity of specific membrane-associated enzymes involved in PAF biosynthesis.

Oxidant stress enhances Lyso-PAF-AcT activity by modifying phospholipase D and phosphatidic acid in aortic endothelial cells

Oxidant stress, as a consequence of selenium (Se) deficiency, alters production of vasoactive compounds including platelet-activating factor (PAF). Recent studies report that enhanced PAF production during Se deficiency is a consequence of increased lyso-PAF:acetyl-coenzyme A acetyltransferase (Lyso-PAF-AcT) activity. To elucidate the mechanism behind increased Lyso-PAF-AcT activity during oxidant stress, phospholipase D (PLD) activity and phosphatidic acid (PA) production were examined. Increased PLD activity and PA production were exhibited in bovine aortic endothelial cells using a Se-deficient model of oxidant stress. The direct effects of PLD and PA on Lyso-PAF-AcT activity were assessed using selective inhibitors and repletion experiments. Following the inhibition of PLD and addition of exogenous PA, Lyso-PAF-AcT activity significantly decreased and increased, respectively. Therefore, Se deficiency enhances Lyso-PAF-AcT activity in part by modifying PLD and PA. This suggests a novel link between Se status and PAF production, providing potential upstream therapeutic targets for PAF regulation under conditions of oxidant stress.

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.

Types of purinoceptors and phospholipase A2 involved in the activation of the platelet-activating factor-dependent transacetylase activity and arachidonate release by ATP in endothelial cells

Acyl analogs of PAF are the major products synthesized during agonist stimulation of endothelial cells. We have previously shown that PAF: 1-acyl-2-lyso-sn-glycero-3-phosphocholine transacetylase in calf pulmonary artery endothelial cells is activated by ATP through protein phosphorylation, and the increase in transacetylase activity by ATP contributes to the biosynthesis of acyl analogs of PAF (J. Biol. Chem. 272, 17431-17437, 1997). To understand the mechanisms(s) by which ATP stimulates acyl analogs of PAF production, we have identified the subtypes of the purinergic receptor that are linked to the activation of two enzymes involved in the generation of acyl analogs of PAF, namely, transacetylase and phospholipase A2. Experiments with transient transfection of the cells with antisense and sense thio-oligonucleotide to cytosolic phospholipase A2 (cPLA2) were also performed to evaluate whether downstream activation of cPLA2 is involved in ATP-receptor mediated induction of arachidonate release and synthesis of radylacetyl-GPC. We found that the P2u/P2Y2 receptor, which recognizes a pyrimidine nucleotide, UTP, as well as purine nucleotides, shows a potency profile of UTP > ATP = ATP gamma S > 2-methylthio-ATP in mediating the activation of PAF: lysophospholipid transacetylase. On the other hand, ADP beta S and 2-methylthio-ATP have similar potencies as ATP but have lower potencies than UTP and ATP gamma S in stimulating the release of arachidonate. These results suggest that both P2u/P2Y2 and P2y/P2Y1 receptor subtypes promote arachidonate release. In addition, transient transfection of endothelial cells with cPLA2 antisense but not the sense thio-oligonucleotide inhibited the stimulation of arachidonate release and [3H]acetate incorporation into radyl[3H]acetyl-GPC. Thus, our data suggest that a receptor-mediated process is involved in the activation of transacetylase for the induced synthesis of acyl analogs of PAF in endothelial cells. Furthermore, it is likely that cPLA2 supplies the lysophospholipids as substrates for the transacetylation reaction.

Roles of plasma platelet-activating factor acetylhydrolase in allergic, inflammatory, and atherosclerotic diseases

Platelet-activating factor (PAF) mediates a variety of physiologic and pathologic events by activating platelets, neutrophils, monocytes, macrophages, and smooth muscle cells. A strongly oxidizing environment induces fragmentation of the polyunsaturated fatty acids of membrane phospholipids, and the resulting oxidized phospholipids are structurally similar to PAF and mimic its biologic actions. The effects of PAF and oxidized phospholipids are abolished by hydrolysis of the sn-2 residue, a reaction catalyzed by PAF acetylhydrolase. Plasma and intracellular forms of PAF acetylhydrolase have been purified and characterized. The plasma form binds with high affinity to lipoproteins in plasma. Furthermore, changes in the activity of this enzyme are associated with various human diseases and animal models of human pathology, suggesting that it may play important roles in their pathogenesis. Studies that have defined the properties of this enzyme and its roles in physiologic and pathologic processes are reviewed. Such studies have provided insight into the functions of PAF and oxidized phospholipids as well as into the etiology of allergic, inflammatory, and atherosclerotic diseases.

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.

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.

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.

Characterization of platelet-activating factor synthesis in glomerular endothelial cell lines

Platelet-activating factor synthesis in two transformed lines of glomerular endothelial cells was characterized and contrasted with platelet-activating factor production in macrovascular-derived endothelial cells as well as with glomerular cells of mesenchymal origin. Platelet-activating factor synthesis was assessed in intact cells and in cell-free preparations. Glomerular endothelial cells constitutively synthesize bio-active alkyl-PAF, and this basal activity can be chronically augmented by various inflammatory and thrombotic agents. In contrast, thrombin-mediated platelet-activating factor formation in bovine pulmonary aortic endothelial cells as well as in glomerular mesangial cells is acute and transient. The potential role of anti-inflammatory prostanoids to function as negative feedback modulators of thrombin- or endothelin-mediated platelet-activating factor synthesis was also investigated, as the synthesis of platelet-activating factor is often associated with the formation of these prostanoids. Indomethacin augmented receptor-mediated platelet-activating factor synthesis while prostanoids of the E and I series reduced agonist-stimulated PAF synthesis. In summary, the unique capacity of glomerular endothelial cells to respond to inflammatory stimuli with sustained platelet-activating factor synthesis is a clear indication of this cell's pivotal role in augmenting the inflammatory response in the limited environment of the glomerulus.