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

cDNA Cloning and Expression of Biologically Active Platelet Activating Factor-Acetylhydrolase (PAF-AH) from Bovine Mammary Gland

Platelet activating factor (PAF)-acetylhydrolase (PAF-AH) is an enzyme that hydrolyzes the acetyl ester at the sn-2 position of PAF, and converts it to the inactive metabolite, lyso PAF. This enzyme is distributed widely in the intracellular as well as the extracellular matrix and is believed to be a defense mechanism that protects the host against the toxic effects of PAF and other biologically active oxidized phospholipids. Purification and expression of cDNA cloning of the intracellular and extracellular types of PAF-AH from several sources from different species have been reported. In this study, the cDNA for PAF-AH was cloned by reverse transcription (RT)-PCR from total RNA of bovine mammary gland. The complete amino acid sequences from the cDNA contains 444 amino acids and was identical to that of the PAF-AH isolated from the bovine spleen cDNA library except for two mismatches of amino acid residues (Thr-247 to Met and Ile-431 to Thr). Recombinant PAF-AH was expressed in HEK 293 cells, which exhibited enzyme activity in the in vitro assay system. Furthermore, recombinant bovine PAF-AH was identified by western blot using human plasma PAF-AH antibody as a monomeric polypeptide with a molecular weight of approximately 43 kDa. This protein can be applied to in vivo models to test its protective role against the deleterious PAF actions.

Anti-apoptotic Actions of the Platelet-activating Factor Acetylhydrolase I α2 Catalytic Subunit

Platelet-activating factor (PAF) is an important mediator of cell loss following diverse pathophysiological challenges, but the manner in which PAF transduces death is not clear. Both PAF receptor-dependent and -independent pathways are implicated. In this study, we show that extracellular PAF can be internalized through PAF receptor-independent mechanisms and can initiate caspase-3-dependent apoptosis when cytosolic concentrations are elevated by approximately 15 pM/cell for 60 min. Reducing cytosolic PAF to less than 10 pM/cell terminates apoptotic signaling. By pharmacological inhibition of PAF acetylhydrolase I and II (PAF-AH) activity and down-regulation of PAF-AH I catalytic subunits by RNA interference, we show that the PAF receptor-independent death pathway is regulated by PAF-AH I and, to a lesser extent, by PAF-AH II. Moreover, the anti-apoptotic actions of PAF-AH I are subunit-specific. PAF-AH I alpha1 regulates intracellular PAF concentrations under normal physiological conditions, but expression is not sufficient to reduce an acute rise in intracellular PAF levels. PAF-AH I alpha2 expression is induced when cells are deprived of serum or exposed to apoptogenic PAF concentrations limiting the duration of pathological cytosolic PAF accumulation. To block PAF receptor-independent death pathway, we screened a panel of PAF antagonists (CV-3988, CV-6209, BN 52021, and FR 49175). BN 52021 and FR 49175 accelerated PAF hydrolysis and inhibited PAF-mediated caspase 3 activation. Both antagonists act indirectly to promote PAF-AH I alpha2 homodimer activity by reducing PAF-AH I alpha1 expression. These findings identify PAF-AH I alpha2 as a potent anti-apoptotic protein and describe a new means of pharmacologically targeting PAF-AH I to inhibit PAF-mediated cell death.

Gene expression alterations during HGF-induced dedifferentiation of a renal tubular epithelial cell line (MDCK) using a novel canine DNA microarray

Hepatocyte growth factor (HGF) elicits a broad spectrum of biological activities, including epithelial cell dedifferentiation. One of the most widely used and best-studied polarized epithelial cell lines is the Madin-Darby canine kidney (MDCK) cell line. Here, we describe and validate the early response of polarized monolayers of MDCK cells stimulated with recombinant HGF using a novel canine DNA microarray designed to query 12,473 gene sequences. In our survey, eight genes previously implicated in the HGF signaling pathway were differentially regulated, demonstrating that the system was responsive to HGF. Also identified were 117 genes not previously known to be involved in the HGF pathway. The results were confirmed by real-time PCR or Western blot analysis for 38 genes. Of particular interest were the large number of differentially regulated genes encoding small GTPases, proteins involved in endoplasmic reticulum translation, proteins involved in the cytoskeleton, the extracellular matrix, and the hematopoietic and prostaglandin systems.

CMP-N-acetylneuraminic acid synthetase from Escherichia coli K1 is a bifunctional enzyme: identification of minimal catalytic domain for synthetase activity and novel functional domain for platelet-activating factor acetylhydrolase activity

Escherichia coli CMP-NeuAc synthetase (EC 2.7.7.43) catalyzes the synthesis of CMP-NeuAc from CTP and NeuAc, which is essential for the formation of capsule polysialylate for strain K1. Alignment of the amino acid sequence of E. coli CMP-NeuAc synthetase with those from other bacterial species revealed that the conserved motifs were located in its N termini, whereas the C terminus appeared to be redundant. Based on this information, a series of deletions from the 3'-end of the CMPNeuAc synthetase coding region was constructed and expressed in E. coli. As a result, the catalytic domain required for CMP-NeuAc synthetase was found to be in the N-terminal half consisting of amino acids 1-229. Using the strategy of tertiary structure prediction based on the homologous search of the secondary structure, the C-terminal half was recognized as an alpha1-subunit of bovine brain platelet-activating factor acetylhydrolase isoform I. The biochemical analyses showed that the C-terminal half consisting of amino acids 228-418 exhibited platelet-activating factor acetylhydrolase activity. The enzyme properties and substrate specificity were similar to that of bovine brain alpha1-subunit. Although its physiological function is still unclear, it has been proposed that the alpha1-subunit-like domain of E. coli may be involved in the traversal of the blood-brain barrier.

Molecular characterization of the constitutive expression of the plasma platelet-activating factor acetylhydrolase gene in macrophages

Plasma platelet-activating factor acetylhydrolase (PAF-AH) is a phospholipase that inactivates platelet-activating factor (PAF) and PAF-like lipids to generate products with little or no biological activity. The levels of circulating PAF-AH correlate with several disease syndromes. We previously reported that mediators of inflammation regulate the expression of the human PAF-AH gene at the transcriptional level. In the present paper, we characterize the constitutive expression of plasma PAF-AH using the mouse gene as a model system, and we report comparative results obtained using human and mouse promoter constructs. We first cloned, sequenced and analysed the promoter region of the murine plasma PAF-AH (mPAF-AH) gene and found that this gene lacks a canonical TATA box. We demonstrated that the cis -elements required for basal transcription are localized within the -316 to -68 bp region. In vitro band-shift and supershift assays showed that Sp1 and Sp3 transcription factors from RAW264.7 and J774A.1 macrophage nuclear extracts bound strongly to a distal GC-rich site within -278/-243 [specificity protein (Sp-A)] and to a proximal TC-rich motif within -150/-114 (Sp-B). In addition, we observed weak binding to a GA-rich site within -110/-82 (Sp-C). The regions containing Sp-B and Sp-C are highly conserved between the human and mouse genes. Forced expression of Sp1 or Sp3 in Sp-lacking Drosophila SL2 cells induced markedly the activity of the exogenous mPAF-AH promoter in a dose-dependent manner, and this induction was dependent on the presence of intact Sp-A and Sp-B. Interestingly, we found that the Sp1- and Sp3-associated DNA-binding activities increased during the maturation of primary human monocytes into macrophages in cell culture. These results demonstrate that Sp1 and Sp3 are key factors that contribute to the basal, constitutive transcription of the plasma PAF-AH gene in macrophages.

Inhibition of Platelet-Activating Factor (PAF) Acetylhydrolase by Methyl Arachidonyl Fluorophosphonate Potentiates PAF Synthesis in Thrombin-Stimulated Human Coronary Artery Endothelial Cells

We have previously demonstrated that thrombin stimulation of endothelial cells results in increased membrane-associated, Ca(2+)-independent phospholipase A2 (iPLA2) activity, accelerated hydrolysis of membrane plasmalogen phospholipids, and production of several biologically active phospholipid metabolites, including prostacyclin and platelet-activating factor (PAF) that is abolished by pretreatment with the iPLA2-selective inhibitor bromoenol lactone. This study was designed to further investigate the role of alternative PLA2 inhibitors, including methyl arachidonyl fluorophosphonate (MAFP, an inhibitor of cytosolic PLA2 isoforms), on phospholipid turnover and PAF production from thrombin-stimulated human coronary artery endothelial cells (HCAECs). Paradoxically, pretreatment of HCAEC with MAFP (5-25 microM) resulted in a significant increase in PAF production in both unstimulated and thrombin-stimulated cells that was found to be a direct result of inhibition of PAF acetylhydrolase (PAF-AH) activity. Pretreatment with MAFP did not significantly inhibit HCAEC PLA2 activity, possibly due to the localization of PLA2 activity in the membrane fraction rather than the cytosol. Bromoenol lactone did not inhibit PAF-AH activity, even at concentrations as high as 20 microM. We conclude that MAFP augments thrombin-stimulated PAF production by inhibition of PAF catabolism without affecting membrane-associated iPLA2 activity.

Association between the Ala379Val variant of the lipoprotein associated phospholipase A2 and risk of myocardial infarction in the north and south of Europe

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) has been identified as a coronary heart disease (CHD) risk predictor. Both its anti-inflammatory role by hydrolysing platelet activating factor, and pro-inflammatory generation of atherogenic mediators may influence CHD risk. We investigated the association of the activity-reducing A379V variant with risk of myocardial infarction (MI) in a large European case-control study, which compared 527 post-MI men with 566 age-matched controls from north and south Europe. Overall, the frequency of the V379 allele was 0.24 (95%CI 0.21-0.26), with no evidence for differences between centres. Homozygosity for the V379 allele was associated with lower risk of MI, (Odds Ratio (OR) 0.56, 95%CI 0.32-0.98), maintained after adjustment for lifestyle factors and levels of inflammatory risk factors (C-reactive protein, fibrinogen, IL-6) (OR 0.46, 0.22-0.93). There was no evidence of heterogeneity of effect between the centres in the north and south of Europe (P-value for interaction=0.80). Since homozygosity for V379 occurs in only 5-6% of subjects, this genotype is not a major determinant of population genetic risk of CHD, but the association of this genotype with low levels of Lp-PLA(2), strongly support the pro-inflammatory causative, and not consequential, role of Lp-PLA(2) in CHD.

Fenofibrate induces HDL-associated PAF-AH but attenuates enzyme activity associated with apoB-containing lipoproteins

Human plasma platelet-activating factor acetylhydrolase (PAF-AH) is an enzyme associated mainly with the apolipoprotein B (apoB)-containing lipoproteins and primarily with LDL. A small proportion of enzymatic activity is also associated with HDL. Plasma paraoxonase 1 (PON1) is an esterase exclusively associated with HDL. The effect of fenofibrate on PAF-AH and PON1 activities in patients with dyslipidemias of Types IIA, IIB, and IV were studied. Fenofibrate reduced plasma PAF-AH activity in all patient groups. In Type IIA patients, this reduction was mainly due to a fall in enzyme activity associated with the dense LDL subspecies, whereas in Type IIB and Type IV patients, it was due to the decrease in PAF-AH activity associated with both the VLDL+IDL and dense LDL subspecies. Drug therapy in Type IIB and Type IV patients significantly increased the HDL-associated PAF-AH activity due to the increase in enzyme activity associated with the HDL-3c subfraction. Fenofibrate did not affect serum PON1 activities toward paraoxon and phenylacetate in either patient group. The fenofibrate-induced elevation of HDL-associated PAF-AH activity in dyslipidemic patients of Type IIB and Type IV, as well as the reduction in enzyme activity associated with atherogenic apoB-containing lipoproteins in all patient groups, may represent a new and important antiatherogenic effect of this potent lipid-modulating agent.

Plasma platelet activating factor-acetylhydrolase (PAF-AH)

The platelet-activating factor-acetylhydrolase (PAF-AH) is an enzyme which catalyzes the hydrolysis of acetyl ester at the sn-2 position of PAF. The family of PAF-AHs consists of two intracellular isoforms (Ib and II), and one secreted isoform (plasma). These PAF-AHs show different biochemical characteristics and molecular structures. Plasma PAF-AH and intracellular isoform, II degrade not only PAF but also oxidatively fragmented phospholipids with potent biological activities. Among these PAF-AHs, plasma PAF-AH has been the target of many clinical studies in inflammatory diseases, such as asthma, sepsis, and vascular diseases, because the plasma PAF-AH activity in the patients with these diseases is altered when compared with normal individuals. Finding a genetic deficiency in the plasma PAF-AH opened the gate in elucidating the protecting role of this enzyme in inflammatory diseases. The most common loss-of-function mutation, V279F, is found in more than 30% of Japanese subjects (4% homozygous, 27% heterozygous). This single nucleotide polymorphism in plasma PAF-AH and the resulting enzymatic deficiency is thought to be a genetic risk factor in various inflammatory diseases in Japanese subjects. Administration of recombinant plasma PAF-AH or transfer of the plasma PAF-AH gene improves pathology in animal models. Therefore, substitution of plasma PAF-AH would be an effective in the treatment of the patients with the inflammatory diseases and a novel clinical approach. In addition, the detection of polymorphisms in the plasma PAF-AH gene and abnormalities in enzyme activity would be beneficial in the diagnosis of the inflammatory diseases.

Platelet-activating factor acetylhydrolase, and not paraoxonase-1, is the oxidized phospholipid hydrolase of high density lipoprotein particles

Paraoxonase-1 (PON1), an high density lipoprotein (HDL)-associated organophosphate triesterase, suppresses atherosclerosis in an unknown way. Purified PON1 protects lipoprotein particles from oxidative modification and hydrolyzes pro-atherogenic oxidized phospholipids and the inflammatory mediator platelet-activating factor (PAF). We find human PON1 acted as a phospholipase A(2) but not as a phospholipase C or D through cleavage of phosphodiester bonds as expected. PON1 requires divalent cations, but EDTA did not block the phospholipase A(2) activity of PON1. In contrast, a serine esterase inhibitor abolished phospholipase activity even though PON1 has no active-site serine residues. PAF acetylhydrolase, an oxidized phospholipid phospholipase A(2), is a serine esterase associated with specific HDL particles. Western blotting did not reveal detectable amounts of PAF acetylhydrolase in PON1 preparations, although very low amounts of PAF acetylhydrolase might still account for PON1 phospholipase A(2) activity. We revised the standard PON1 purification by first depleting HDL of PAF acetylhydrolase to find PON1 purified in this way no longer hydrolyzed oxidized phospholipids or PAF. Serum from a donor with an inactivating mutation in the PAF acetylhydrolase gene did not hydrolyze oxidized phospholipids or PAF, yet displayed full paraoxonase activity. We conclude that PAF acetylhydrolase is the sole phospholipase A(2) of HDL and that PON1 has no phospholipase activity toward PAF or pro-atherogenic oxidized phospholipids.

Chlorophyllase as a serine hydrolase: identification of a putative catalytic triad

Chlorophyllases (Chlases), cloned so far, contain a lipase motif with the active serine residue of the catalytic triad of triglyceride lipases. Inhibitors specific for the catalytic serine residue in serine hydrolases, which include lipases effectively inhibited the activity of the recombinant Chenopodium album Chlase (CaCLH). From this evidence we assumed that the catalytic mechanism of hydrolysis by Chlase might be similar to those of serine hydrolases that have a catalytic triad composed of serine, histidine and aspartic acid in their active site. Thus, we introduced mutations into the putative catalytic residue (Ser162) and conserved amino acid residues (histidine, aspartic acid and cysteine) to generate recombinant CaCLH mutants. The three amino acid residues (Ser162, Asp191 and His262) essential for Chlase activity were identified. These results indicate that Chlase is a serine hydrolase and, by analogy with a plausible catalytic mechanism of serine hydrolases, we proposed a mechanism for hydrolysis catalyzed by Chlase.

Inflammation, bioactive lipids and atherosclerosis: potential roles of a lipoprotein-associated phospholipase A2, platelet activating factor-acetylhydrolase

It is well established that inflammation is an integral feature of atherosclerosis and of the cardiovascular diseases which it underlies. Oxidative stress is also recognized as a key actor in atherogenesis, in which it is closely associated with the inflammatory response and bioactive lipid formation. Several bioactive lipids have been identified in the atherosclerotic plaque, including the potent inflammatory mediator platelet activating factor (PAF), PAF-like lipids, oxidised phospholipids (oxPL) and lysophosphatidylcholine (lyso-PC). Recent evidence has established a central role of two phospholipases (PL) in atherogenesis, the non-pancreatic Type II secretory phospholipase A(2) (sPLA(2)) and the lipoprotein-associated PLA(2)-alternatively termed as PAF-acetylhydrolase (PAF-AH). sPLA(2) is calcium-dependent and hydrolyses the sn-2 acyl group of glycerophospholipids of lipoproteins and cell membranes to produce lyso-PC and free fatty acids. It is also implicated in isoprostane production from oxPL. sPLA(2) is an acute phase reactant, which is upregulated by inflammatory cytokines and may represent a new independent risk factor for coronary heart disease. In contrast to sPLA(2), PAF-AH is calcium-independent and is specific for short acyl groups at the sn-2 position of the phospholipid substrate and with the exception of PAF, can equally hydrolyze oxPL to generate lyso-PC and oxidized fatty acids. Thus PAF-AH plays a key role in the degradation of proinflammatory oxPL and in the generation of lyso-PC and oxidized fatty acids. PAF-AH equally can also hydrolyze short-chain diacylglycerols, triacylglycerols, and acetylated alkanols, and displays a PLA(1) activity. Whereas sPLA(2) may represent a new independent risk factor for coronary artery disease, the potential relevance of PAF-AH to atherosclerosis remains the subject of debate, and recent results suggest that the potential role of the LDL-associated PAF-AH in atherogenesis may be distinct to that of the HDL-associated enzyme. This review is focused on the main structural and catalytic features of plasma PAF-AH, on the association of the enzyme with distinct lipoprotein particle subspecies, on its cellular sources, and finally on the potential significance of this lipoprotein-associated PLA(2) in cardiovascular disease.

High-density lipoprotein cholesterol and triglycerides as therapeutic targets for preventing and treating coronary artery disease

Epidemiologic and clinical trials show that elevated triglycerides and low levels of high-density lipoprotein cholesterol (HDL-C) are independent risk factors for coronary heart disease (CHD). However, adjustment for covariates frequently weakens or abolishes the predictive significance of triglycerides, whereas the evidence for HDL-C is more consistently strong. Data indicate that there is a 2% to 3% decrease in coronary risk for each 1 mg/dL increase in HDL-C, whereas the benefit of triglyceride lowering appears to occur largely in patients with the highest baseline levels. The 2001 National Cholesterol Education Program Adult Treatment Panel III (ATP III) guidelines for detecting and treating high blood cholesterol reflect our improved understanding of triglycerides and HDL as CHD risk factors. However, the guidelines place more emphasis on lowering triglycerides than on raising HDL-C by identifying non-HDL-C (ie, low-density lipoprotein cholesterol [LDL-C] + very-low-density lipoprotein cholesterol [VLDL-C]) as a secondary target of therapy. In clinical practice, VLDL-C is the most readily available measure of atherogenic triglyceride-rich remnant lipoproteins. On the basis of the available epidemiologic and clinical evidence, refinement of the NCEP guidelines to include more emphasis on raising HDL-C levels should be considered. Novel drugs are being developed that have the potential to increase HDL-C concentrations and/or improve the functionality of HDL.

Metabolism of platelet activating factor by intrapulmonary vascular smooth muscle cells. Effect of oxygen on phospholipase A2 protein expression and activities of acetyl-CoA acetyltransferase and cholinephosphotransferase

We have demonstrated that platelet activating factor (PAF) plays an important physiological role in the maintenance of high pulmonary vascular tone in fetal lambs, a role attributable to increased PAF receptor binding (J. Appl. Physiol. 85 (1998) 1079; Am J. Physiol. 278 (2000) H1168). In this study, we examined the possibility that increased PAF synthesis via de novo and remodeling pathways as well as decreased PAF catabolism in hypoxic state of fetal lungs may account for the PAF action in vivo. We investigated effect of oxygen tension on PAF synthesis by ovine fetal intrapulmonary venous (PV) and arterial (PA) smooth muscle cells pulsed with [3H]choline (de novo), or [3H]acetate (remodeling), while PAF catabolism was studied by assay of acetylhydrolase (PAF-Ah) activity. Hypoxia stimulated PAF synthesis by choline incorporation (pmol/10(6)cells) in both PVSMC (1.14+/-0.13 vs 0.53+/-0.05) and PASMC (0.39+/-0.12 vs 0.22+/-0.04). Hypoxia stimulated PAF synthesis via remodeling pathway only in PVSMC (408+/-32 vs 225+/-17) which was 5-fold greater than in PASMC (77+/-15 vs 105+/-24), however, with A23187 in remodeling pathway, PAF synthesis increased 5-fold compared to baseline conditions and then synthesis in hypoxia was greater than in normoxia in both cell types. Phospholipase A2 protein expression was significantly higher in hypoxia in both cells and was approximately 2-fold higher in PVSMC. PAF-Ah activity (nmol lyso-PAF/min/mg protein) was greater in hypoxia vs normoxia in PVSMC (0.81+/-0.24 vs 0.44+/-0.088), but in PASMC activity was less in hypoxia vs normoxia (1.68+/-0.24 vs 3.93+/-0.44). Compared to PVSMC PAF-Ah activity in PASMC was 4-fold higher in hypoxia. Our data demonstrate that (1) PAF synthesis in intrapulmonary SMC of fetal lambs occurs by both de novo studied by choline incorporation and remodeling pathways, the latter being predominant. (2) There is heterogeneity in PAF synthetic and catabolic activities in lung vasculature of fetal lambs. We conclude that increased PAF synthesis in veins by the two synthetic pathways coupled with decreased catabolism will result in a higher venous PAF levels in the hypoxic environment of fetal lungs. We speculate that in vivo, a high PAF level in veins will make more PAF available for binding to its receptors so as to sustain the desired high venous tone in the fetal pulmonary circulation.

Platelet-activating factor acetylhydrolase

Platelet-activating factor (PAF) is one of the most potent lipid mediators and is involved in a variety of physiological events. The acetyl group at the sn-2 position of its glycerol backbone is required for its biological activity, and deacetylation of PAF induces loss of activity. The deacetylation reaction is catalyzed by PAF-acetylhydrolase (PAF-AH). A series of biochemical and enzymological studies have revealed that there are at least three types of PAF-AH in mammals, namely the intracellular type I and II and plasma type. Type I PAF-AH is a G-protein-like complex of two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause type I lissencephaly. Recent studies indicate that LIS1/beta is important in cellular functions such as induction of nuclear movement and control of microtubule organization. Although circumstantial evidence is accumulating supporting the idea that the catalytic subunits are also involved in microtubule function, it is still not known what role PAF plays in the process and whether PAF is a native endogenous substrate of this enzyme. Type II PAF-AH is a single polypeptide and shows significant sequence homology with plasma PAF-AH. Type II PAF-AH is myristoylated at the N-terminus and like other N-myristoylated proteins, is distributed in both the cytosol and membranes. Plasma PAF-AH is also a single polypeptide and exists in association with plasma lipoproteins. Type II PAF-AH as well as plasma PAF-AH may play roles as scavengers of oxidized phospholipids which are thought to be involved in diverse pathological processes, including disorganization of membrane structure and PAF-like proinflammatory actions. In this chapter, author focuses on the structures and possible biological functions of intracellular PAF-AHs.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

PAF, a putative mediator of oral inflammation

PAF, or platelet-activating factor, is a family of structurally related phospholipids (1-O-alkyl/acyl/alkenyl-2-acetyl-sn-glycero-3-phosphocholine) which possesses a wide spectrum of potent pro-inflammatory actions. These phospholipids are synthesized by a diverse array of cells, including neutrophilic polymorphonuclear leukocytes (PMN), platelets, mast cells, monocytes/macrophages, vascular endothelial cells, and lymphocytes. PAF targets these and other cells via specific, G-protein-coupled receptors to initiate intracrine, autocrine, paracrine, and juxtacrine cell activation. Of importance, these unique acetylated phospholipids are frequently synthesized in concert with pro-inflammatory lipid mediators derived from arachidonic acid. Since PAF synergizes with these and other mediators to amplify the inflammatory response, it seems likely that PAF plays an integral, perhaps pivotal, role in acute and chronic inflammatory processes. PAF is present in the mixed saliva of dentate, but not edentulous, human subjects. The levels of PAF in mixed saliva or in gingival crevicular fluid and tissues are significantly increased during oral inflammatory conditions such as periodontitis and mucositis. Interestingly, the levels of salivary PAF correlate with the extent/severity of these oral diseases. These observations suggest that PAF may participate in pathophysiologic events during the course of oral inflammation. The availability of specific PAF receptor antagonists and human recombinant PAF-acetylhydrolase (PAF-AH), a plasma enzyme which rapidly destroys PAF, should provide clinical tools for the investigation of the role of PAF in these and other inflammatory disorders; and perhaps, ultimately, some of these reagents may prove to be therapeutically useful in the treatment and management of these conditions.

Maturational changes in ovine pulmonary metabolism of platelet-activating factor: implications for postnatal adaptation

We recently reported that PAF acetylhydrolase (PAF-Ah) mRNA level and PAF-Ah activity in lamb lungs are up-regulated in the immediate newborn period, thereby facilitating the fall in postnatal PAF levels as well as a fall in pulmonary vascular resistance (B. O. Ibe, F. C. Sardar, and J. U. Raj, Mol Genet Metab 69:46-55, 2000). We have studied hypoxia effects on PAF synthesis and PAF-Ah activity in fetal lamb pulmonary arterial smooth muscle cells (FPASMC) and endothelial cells (FPAEC). We also studied PAF synthesis by platelets, and PAF-Ah activity in plasma of perinatal lambs at different ages. PAF synthesis (means +/- SEM, pmol/10(6) cells) by SMC in baseline was 168 +/- 27 and increased 3-fold on stimulation with A23187. Hypoxia augmented A23187-stimulated PAF synthesis by 30%. In FPAEC, baseline synthesis was 0.54 +/- 0.062 and increased 3-fold to 1.72 +/-.34. Hypoxia had no effect on PAF synthesis by EC. FPASMC produced over 300-fold more PAF than FPAEC. PAF synthesis by platelets was 47.02 +/- 7.1, 63.4 +/- 6.6, 71.5 +/- 9.9, and 62.2 +/- 5.2 for fetal, and newborn lambs <2 h, <1 day, and 6-12 days, old, respectively. PAF synthesis by platelets of <1 day-old lambs was different from that of fetal lambs. PAF-Ah activity (nmol lyso-PAF/min/mg protein) by FPASMC in normoxia was 3.41 +/- 0.38 which was 50% higher than the rate in hypoxia. Activity in FPAEC was 1.75 +/- 0.37 which was not different from hypoxia. PAF-Ah activity in fetal lamb plasma was 47.83 +/- 6.87 which was different from 155.32 +/- 12.10, the activity in plasma of newborn <1 day old. Activity in the other perinatal lambs did not differ from fetal or newborn <1 d. Our data suggest that lower pulmonary vascular PAF synthesis in normoxia together with higher PAF-Ah activity during immediate postnatal period is necessary to ensure rapid catabolism of PAF in vivo so as to facilitate postnatal adaptation of the pulmonary and systemic circulations.

PAF acetylhydrolase gene polymorphisms and asthma severity

This review describes the current understanding of the contributions of genetic alterations in platelet-activating factor (PAF) acetylhydrolase to the pathogenesis of asthma. A variety of in vitro and in vivo studies, performed by multiple laboratories, suggest that the lipid substrates of this enzyme, PAF and oxidised derivatives of phosphatidylcholines, play important roles as causative factors in many diseases including asthma. PAF acetylhydrolase inactivates PAF and oxidatively-fragmented lipids thus providing a mechanism to prevent their pro-inflammatory effects. Since it is a most unusual protein, the biochemical, structural and functional characteristics of PAF acetylhydrolase continue to be unravelled. First, the ability of this enzyme to inactivate pro-inflammatory lipid mediators is modulated by its association with lipoproteins and by its susceptibility to oxidative inactivation. Second, mediators of inflammation, such as the substrates for PAF acetylhydrolase, alter expression of the protein at the transcriptional level. Third, naturally-occurring variants of PAF acetylhydrolase have catalytic properties different from those exhibited by the most common form of this protein. Thus, a variety of factors, including genetics, contribute to determine the biological level of lipid substrates known to act as mediators of asthma and other diseases. Here, I summarise key studies that implicate PAF and related molecules as important mediators in the pathogenesis of asthma. Next, I describe clinical findings that are consistent with a role of PAF acetylhydrolase as a modulator of asthma. Third, I focus on the biochemical effects associated with naturally-occurring mutations and polymorphisms in the PAF acetylhydrolase gene and the incidence of these genetic variations in populations of asthmatic subjects. Finally, I present my views on the future of this emerging field and the potential utility of performing additional studies aimed at further characterising the contribution of PAF acetylhydrolase to the pathogenesis of a complex syndrome generally recognised as a multifactorial and heterogeneous disease.

Evidence for the autocrine induction of capacitation of mammalian spermatozoa

Mammalian spermatozoa require a maturational event after ejaculation that allows them to acquire the capacity for fertilization. This process, known as capacitation, occurs spontaneously in simple defined medium implicating a potential role of autocrine induction. This study shows that the ether phospholipid 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphocholine (PAF) meets the criteria for an autocrine mediator of capacitation. Sperm released PAF after their dilution into capacitation medium and expressed a receptor for PAF on their membranes. PAF stimulated changes in the motility of sperm and enhanced fertilization in vitro. These actions were inhibited by a PAF receptor antagonist (UR-12519) and by extracellular recombinant PAF:acetylhydrolase (an enzyme that degrades PAF to a biologically inert form). Seminal plasma contained an acid-labile PAF:acetylhydrolase, whereas capacitation was inhibited by an acid-labile factor within seminal plasma, implicating this factor as a potential decapacitation factor within seminal plasma. Sperm from a PAF receptor knock-out mouse strain failed to express the receptor and displayed a significantly (p < 0.01) reduced rate of capacitation, as assessed by the spontaneous onset of the acrosome reaction in vitro. When used for in vitro fertilization, sperm from PAF receptor knock-out mice gave a significantly lower rate of fertilization (21.5%) than did wild-type sperm (66.7%). The study shows for the first time the operation of an autocrine loop that induces capacitation in sperm in vitro and shows that this loop acts in concert with other mediators of capacitation to promote efficient fertilization.

Platelet-activating factor acetylhydrolases: broad substrate specificity and lipoprotein binding does not modulate the catalytic properties of the plasma enzyme

Platelet-activating factor acetylhydrolases (PAF-AHs) are a group of enzymes that hydrolyze the sn-2 acetyl ester of PAF (phospholipase A(2) activity) but not phospholipids with two long fatty acyl groups. Our previous studies showed that membrane-bound human plasma PAF-AH (pPAF-AH) accesses its substrate only from the aqueous phase, which raises the possibility that this enzyme can hydrolyze a variety of lipid esters that are partially soluble in the aqueous phase. Here we show that pPAF-AH has broad substrate specificity in that it hydrolyzes short-chain diacylglycerols, triacylglycerols, and acetylated alkanols, and displays phospholipase A(1) activity. On the basis of all of the substrate specificity results, it appears that the minimal structural requirement for a good pPAF-AH substrate is the portion of a glyceride derivative that includes an sn-2 ester and a reasonably hydrophobic chain in the position occupied by the sn-1 chain. In vivo, pPAF-AH is bound to high and low density lipoproteins, and we show that the apparent maximal velocity for this enzyme is not influenced by lipoprotein binding and that the enzyme hydrolyzes tributyroylglycerol as well as the recombinant pPAF-AH does. Broad substrate specificity is also observed for the structurally homologous PAF-AH which occurs intracellularly [PAF-AH(II)] as well as for the PAF-AH from the lower eukaryote Physarum polycephalum although pPAF-AH and PAF-AH(II) tolerate the removal of the sn-3 headgroup better than the PAF-AH from P. polycephalum does. In contrast, the intracellular PAF-AH found in mammalian brain [PAF-AH(Ib) alpha 1/alpha 1 and alpha 2/alpha 2 homodimers] is more selectively operative on compounds with a short acetyl chain although this enzyme also displays significant phospholipase A(1) activity.

Relative sensitivities of plasma lecithin:cholesterol acyltransferase, platelet-activating factor acetylhydrolase, and paraoxonase to in vitro gas-phase cigarette smoke exposure

In order to identify potential atherogenic properties of gas-phase cigarette smoke, we utilized an in vitro exposure model to determine whether the activities of several putative anti-atherogenic enzymes associated with plasma lipoproteins were compromised. Exposure of heparinized human plasma to gas-phase cigarette smoke produced a dose-dependent reduction in the activity of platelet-activating factor acetylhydrolase (PAF-AH). Reductions of nearly 50% in PAF-AH activity were observed following exposure to gas-phase smoke from four cigarettes over an 8-h period. During this time of exposure, lecithin:cholesterol acyltransferase (LCAT) was rendered almost completely inactive (>80%). In contrast, paraoxonase was totally unaffected by cigarette smoke. Supplementation of plasma with 1 mM reduced glutathione was found to protect both PAF-AH and LCAT from cigarette smoke, suggesting that cysteine modifications may have contributed to the inhibition of these two enzymes. To evaluate this possibility, we blocked the free cysteine residues of these enzymes with the reversible thiol-modifying reagent dithiobisnitrobenzoic acid (DTNB). Reversal of the DTNB-cysteine adducts following cigarette smoke exposures revealed that LCAT, but not PAF-AH, was protected. Moreover, high doses (1.0-10 mM) of acrolein and 4-hydroxynonenal, reactive aldehydic species associated with cigarette smoke, completely inhibited plasma LCAT activity, whereas PAF-AH was resistant to such exposures. Taken together, these results indicate a divergence regarding the underlying mechanism of PAF-AH and LCAT inhibition upon exposure to gas-phase cigarette smoke. While LCAT was sensitive to exposure to volatile aldehydic products involving, in part, cysteine and/or active site modifications, the enzyme PAF-AH exhibited an apparent resistance. The latter suggests that the active site of PAF-AH is in a microenvironment that lacks free cysteine residues and/or is shielded from volatile aldehydic combustion products. Based on these results, we propose that cigarette smoke may contribute to atherogenesis by inhibiting the activities of plasma PAF-AH and LCAT, but the nature of this inhibition differs for the enzymes.

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.

The expanding superfamily of phospholipase A(2) enzymes: classification and characterization

The phospholipase A(2) (PLA(2)) superfamily consists of a broad range of enzymes defined by their ability to catalyze the hydrolysis of the middle (sn-2) ester bond of substrate phospholipids. The hydrolysis products of this reaction, free fatty acid and lysophospholipid, have many important downstream roles, and are derived from the activity of a diverse and growing superfamily of PLA(2) enzymes. This review updates the classification of the various PLA(2)'s now described in the literature. Four criteria have been employed to classify these proteins into one of the 11 Groups (I-XI) of PLA(2)'s. First, the enzyme must catalyze the hydrolysis of the sn-2 ester bond of a natural phospholipid substrate, such as long fatty acid chain phospholipids, platelet activating factor, or short fatty acid chain oxidized phospholipids. Second, the complete amino acid sequence of the mature protein must be known. Third, each PLA(2) Group should include all of those enzymes that have readily identifiable sequence homology. If more than one homologous PLA(2) gene exists within a species, then each paralog should be assigned a Subgroup letter, as in the case of Groups IVA, IVB, and IVC PLA(2). Homologs from different species should be classified within the same Subgroup wherever such assignments are possible as is the case with zebra fish and human Group IVA PLA(2) orthologs. The current classification scheme does allow for historical exceptions of the highly homologous Groups I, II, V, and X PLA(2)'s. Fourth, catalytically active splice variants of the same gene are classified as the same Group and Subgroup, but distinguished using Arabic numbers, such as for Group VIA-1 PLA(2) and VIA-2 PLA(2)'s. These four criteria have led to the expansion or realignment of Groups VI, VII and VIII, as well as the addition of Group XI PLA(2) from plants.

Do membrane-bound enzymes access their substrates from the membrane or aqueous phase: interfacial versus non-interfacial enzymes

For membrane-bound enzymes that act on substrates that partition between the membrane and aqueous phases, it is possible to imagine two fundamentally different mechanisms. Interfacial enzymes must access their substrate from the membrane phase, in other words substrate in the membrane binds directly to the active site of the enzyme at the membrane without mixing with substrate molecules in the aqueous phase. On the other hand, non-interfacial enzymes, either bound to membranes or present in the aqueous phase, must access their substrates from the aqueous phase, i.e. substrate in the aqueous phase binds directly to the enzyme without mixing with substrates in the membrane phase. An interfacial mechanism for some enzymes including secreted and cytosolic phospholipase A(2) and phosphoinositide 3'-hydroxykinase was rigorously proven by demonstrating that these enzymes processively hydrolyze many phospholipids without desorbing from the surface of vesicles (scooting mode). The non-interfacial mechanism is more difficult to establish because it cannot be addressed by steady-state kinetics. Using a pre-steady-state method in which the enzymatic velocity is measured during the time it takes for substrate to exchange between vesicles, a non-interfacial mechanism was proven for vesicle-bound plasma platelet activating factor acetylhydrolase. This enzyme prefers more water-soluble phospholipids such as those with sn-2 acetyl or oxidatively truncated fatty acyl chains, and this is readily explained by the mandatory access of substrate from the aqueous phase.

Platelet-activating factor acetylhydrolases in health and disease

The platelet-activating factor (PAF) acetylhydrolases catalyze hydrolysis of the sn-2 ester bond of PAF and related pro-inflammatory phospholipids and thus attenuate their bioactivity. One secreted (plasma) and four intracellular isozymes have been described. The intracellular isozymes are distinguished by differences in primary sequence, tissue localization, subunit composition, and substrate preferences. The most thoroughly characterized intracellular isoform, Ib, is a G-protein-like complex with two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause Miller-Dieker lissencephaly. Isoform II is a single polypeptide that is homologous to the plasma PAF acetylhydrolase and has antioxidant activity in several systems. Plasma PAF acetylhydrolase is also a single polypeptide with a catalytic triad of amino acids that is characteristic of the alpha/beta hydrolases. Deficiency of this enzyme has been associated with a number of pathologies. The most common inactivating mutation, V279F, is found in >30% of randomly surveyed Japanese subjects (4% homozygous, 27% heterozygous). The prevalence of the mutant allele is significantly greater in patients with asthma, stroke, myocardial infarction, brain hemorrhage, and nonfamilial cardiomyopathy. Preclinical studies have demonstrated that recombinant plasma PAF acetylhydrolase can prevent or attenuate pathologic inflammation in a number of animal models. In addition, preliminary clinical results suggest that the recombinant enzyme may have pharmacologic potential in human inflammatory disease as well. These observations underscore the physiological importance of the PAF acetylhydrolases and point toward new approaches for controlling pathologic inflammation.

Role of platelet-activating factor in functional alterations induced by xenoreactive antibodies in porcine endothelial cells

BACKGROUND

Platelet-activating factor (PAF) is a phospholipid mediator of inflammation which has been implicated in rejection. The interaction of anti-alpha-galactosyl natural antibodies (anti-alpha gal Abs) with endothelial cells is the initial step for the development of xenograft rejection. In our study, we stimulated porcine aortic endothelial cells (PAEC) with anti-alpha gal IgG to investigate the synthesis of PAF from PAEC and its biological consequences.

METHODS AND RESULTS

PAF was extracted and chromatographically purified from cultured PAEC stimulated with baboon anti-alpha gal Abs. The Abs induced a dose-dependent synthesis of PAF peaking after 30 min of incubation, and decreasing thereafter. Concomitant cell shape change, motility, and cytoskeleton redistribution were observed. These events were prevented by addition of a panel of PAF-receptor antagonists. An SV40 T-large antigen-immortalized PAEC line was engineered to express PAF acetyl-hydrolase (PAF-AH) cDNA, the major PAF-inactivating enzyme. These transfected cells exposed to anti-alpha gal Abs showed reduced cell contraction and motility compared with empty vector-transfected cells. Moreover, in PAEC stimulated with anti-alpha gal Abs, the synthesis of PAF promoted the adhesion of a monocytic cell line as shown by the inhibitory effect of PAF-receptor antagonists and of PAF-AH expression. Finally, studies on cell monolayer demonstrated an enhanced permeability 48 hr after exposure to anti-alpha gal Abs, and this increase was prevented by PAF-inactivation and by PAF-receptor blockade.

CONCLUSIONS

These results demonstrate that on stimulation with anti-alpha gal Abs, PAEC synthetize PAF which can contribute to several vascular events involved in xenograft rejection.

Platelet activating factor acetylhydrolase decreases lung reperfusion injury

BACKGROUND

Ischemia-reperfusion injury involves free radical production, polymorphonuclear neutrophil chemotaxis/degranulation, and production of proteolytic enzymes, complement components, coagulation factors, and cytokines. Activated polymorphonuclear neutrophils, endothelial cells, and macrophages produce platelet activating factor, which further promotes these inflammatory reactions. The recently cloned plasma form of platelet activating factor-acetylhydrolase (PAF-AH) demonstrates antiinflammatory effects by degrading platelet activating factor. We evaluated the effects of PAF-AH in an isolated perfused rat lung model by adding it to the flush solutions or to the reperfusion blood.

METHODS

Rat lungs were isolated, flushed with EuroCollins (EC) or University of Wisconsin (UW) solution, stored at 4 degrees C for 6 or 12 hours, and reperfused using a cross-circulating syngeneic support rat. During reperfusion, oxygenation, compliance, and capillary filtration coefficient were calculated. There were four groups in the study; group I (control) had no PAF-AH added, group II had PAF-AH added to the flush solution, group III had PAF-AH added to reperfusion blood, and group IV had PAF-AH added to both flush solution and reperfusion blood.

RESULTS

After 6 hours of storage, oxygenation, compliance, and capillary filtration coefficient significantly improved for EC in group IV. For UW, oxygenation improved in group IV whereas compliance improved in groups II, III, and IV. After 12 hours of storage, compliance improved for EC in group IV and capillary filtration coefficient improved in groups III and IV. For UW, oxygenation and compliance improved in groups II and IV, whereas capillary filtration coefficient improved in group IV.

CONCLUSIONS

Addition of PAF-AH to intracellular organ preservation solutions and to the blood reperfusate significantly improves postreperfusion oxygenation and compliance, and reduces lung capillary permeability.

The Ile198Thr and Ala379Val variants of plasmatic PAF-acetylhydrolase impair catalytical activities and are associated with atopy and asthma

The platelet-activating factor (PAF) represents a phospholipid with complex biological functions, including involvement in inflammatory processes. The degrading enzyme PAF acetylhydrolase (PAFAH) represents a candidate for asthma and other atopic diseases. Two loss-of-function mutations of PAFAH are associated with severe asthma in Japanese individuals. Our aim was to look for further PAFAH variants in white populations, their possible association with atopic and asthmatic phenotypes, and their functional importance. We picked up three common variants in the PAFAH gene: Arg92His (exon 4), Ile198Thr (exon 7), and Ala379Val (exon 11). The known loss-of-function mutations were not seen. The variant allele Thr198 was found to be highly associated with total IgE concentrations in an atopic population (P=.009) and with "atopic asthma" in an asthmatic population (P=.008). The variant allele Val379 was found to be highly associated with "specific sensitization" in the atopic population (P=.002) and with "asthma" in the asthmatic population (P=.003). By use of recombinant PAFAH enzymes, the variant Val379 showed increased (14 microM) and Thr198 markedly increased (42 microM) KM values compared to the wild type (7 microM); furthermore, Vmax of Val379 was highly increased (132%). Thr198 and Val379 influence plasmatic PAFAH toward lower substrate affinities and therefore are very likely to prolong the activities of PAF. At the same time, they are associated with an increased risk to develop asthma and atopy. Thus, two PAFAH variants seem to play a key role in atopic and asthmatic processes in Caucasian populations.

Recombinant human platelet-activating factor-acetylhydrolase inhibits airway inflammation and hyperreactivity in mouse asthma model

Numerous in vitro and in vivo studies in both animal models and human asthmatics have implicated platelet-activating factor (PAF) as an important inflammatory mediator in asthma. In a murine asthma model, we examined the anti-inflammatory activities of recombinant human PAF-acetylhydrolase (rPAF-AH), which converts PAF to biologically inactive lyso-PAF. In this model, mice sensitized to OVA by i.p. and intranasal (i.n.) routes are challenged with the allergen by i.n. administration. The OVA challenge elicits an eosinophil infiltration into the lungs with widespread mucus occlusion of the airways and results in bronchial hyperreactivity. The administration of rPAF-AH had a marked effect on late-phase pulmonary inflammation, which included a significant reduction in airway eosinophil infiltration, mucus hypersecretion, and airway hyperreactivity in response to methacholine challenge. These studies demonstrate that elevating plasma levels of PAF-AH through the administration of rPAF-AH is effective in blocking the late-phase pulmonary inflammation that occurs in this murine allergen-challenge asthma model. These results suggest that rPAF-AH may have therapeutic effects in patients with allergic airway inflammation.

Platelet activating factor acetylhydrolase activity in lamb lungs is up-regulated in the immediate newborn period

We recently showed that platelet activating factor (PAF) is an important modulator of pulmonary vasomotor tone in the fetus, with a significant decrease in circulating PAF levels in the immediate newborn period. In this study, we have determined PAF catabolism by PAF acetylhydrolase (PAF-Ah) in lungs of near-term fetal and newborn 2- to 16-h (<1 day) and 6- to 12-day-old lambs. The rate of PAF catabolism by lung homogenate protein from the three groups of lamb lungs was studied at 37 degrees C in 30 mM Tris buffer, pH 7.5, containing 0.01% BSA. Each lung homogenate protein was incubated for 10 min with 50 microM [(3)H]acetyl-PAF at pO(2) <50 Torr (hypoxia) and approximately 100 Torr (normoxia). PAF-Ah activity was quantified as amount of lyso-PAF produced. PAF-Ah activity (means +/- SEM, nmol lyso-PAF/min/mg protein) in fetal lung homogenate was 1.19 +/- 0.14 and 2.46 +/- 0.05 during hypoxia and normoxia, respectively. The corresponding values for the newborns were newborn <1 day, 1.65 +/- 0.26 and 2.95 +/- 0.07 and newborn 6-12 days, 1.25 +/- 0.10 and 2.84 +/- 0.05. In all groups, PAF-Ah activity was higher in normoxia than in hypoxia. During normoxia, PAF-Ah activity in newborn <1 day was significantly higher than the activity in fetus, but similar to the activity in newborn 6- to 12-day-old lamb lungs. These data show a significant up-regulation of PAF-Ah activity in lungs in the immediate newborn period. PAF-Ah gene expression measured by RT-PCR showed a significant up-regulation of the PAF-Ah gene in lungs of lambs <1 day old, suggesting a transcriptional regulation of the PAF-Ah gene in the immediate newborn period. These results suggest that up-regulation of PAF-Ah activity after birth with oxygenation will result in a decrease in circulating PAF levels, thereby facilitating the fall in pulmonary vascular resistance in the immediate newborn period.

Purification and partial characterization of the phospholipase A2 and co-lytic factor from sea anemone (Aiptasia pallida) nematocyst venom

Functional nematocysts of one specific morphological class, the penetrant microbasic mastigophores, were isolated from the sea anemone, Aiptasia pallida. These nematocysts contain a multicomponent venom composed of several proteins, including those with neurotoxic, hemolytic, and lethal activities. Hemolytic activity is produced by at least three synergistic venom proteins. One of these proteins is identified as a phospholipase A2 (EC 3.1.1.4) which exists in two isozymic forms, alpha and beta, with molecular weights of 45,000 and 43,000, respectively. The beta isozyme has been purified to homogeneity. It is a single-chained glycoprotein with an isoelectric point (pI) of 8.8 and represents 70% of the phospholipase activity of the venom. The activity of the beta isozyme is relatively labile and is inactivated by 3.5 M urea or by heating at 45 degrees C. It is most stable at pH 4.0 and loses 50% of its activity at pH values below 3.5 and above 8.0. A second venom protein has also been purified. It is essential for the hemolytic activity of the venom and is termed co-lytic factor (CLF). It is a monomeric glycoprotein having a pI of 4.5. CLF has a molecular weight of approximately 98,000, a sedimentation coefficient of 4.8 S, and is prolate in shape, having a frictional ratio of about 1.6. CLF constitutes about 1.25% of the total venom protein and is assayed by reversing fatty acid inhibition of the venom hemolysis activity.

PAF-acetylhydrolases

Platelet-activating factor acetylhydrolases (PAF-AHs, EC 3.1.1.47) constitute a unique subfamily of phospholipases A(2), specific for short acyl chains in the sn-2 position of the phospholipid. Their primary substrate is the platelet-activating factor, PAF, from which they cleave an acetyl moiety with concomitant release of lysoPAF. However, some acetylhydrolase will also hydrolyze other polar phospholipids with up to 6-carbons long acyl chains in the sn-2 position. PAF-acetylhydrolases are diverse enzymes, and the well-characterized isoforms are serine-dependent hydrolases, which do not require Ca(2+) for activity. Given the existence of two pools of PAF, intra- and extracellular, the acetylhydrolases can be divided into two subclasses: those found in the cytosol and enzymes secreted to blood plasma or other body fluids. Recent crystallographic studies shed new light on the complex structure-function relationships in PAF-AHs.

Membrane-bound plasma platelet activating factor acetylhydrolase acts on substrate in the aqueous phase

Human plasma platelet activating factor acetylhydrolase (pPAF-AH) is a phospholipase A(2) that specifically hydrolyzes the sn-2 ester of platelet activating factor (PAF) and of phospholipids with oxidatively truncated sn-2 fatty acyl chains. pPAF-AH is bound to lipoproteins in vivo, and it binds essentially irreversibly to anionic and zwitterionic phospholipid vesicles in vitro and hydrolyzes PAF and PAF analogues. Substrate hydrolysis also occurs in the absence of vesicles, with a maximum rate reached at the critical micelle concentration. A novel pre-steady-state kinetic analysis with enzyme tightly bound to vesicles and with a substrate that undergoes slow intervesicle exchange establishes that pPAF-AH accesses its substrate from the aqueous phase and thus is not an interfacial enzyme. Such a mechanism readily explains why this enzyme displays dramatic specificity for phospholipids with short sn-2 chains or with medium-length, oxidatively truncated sn-2 chains since a common feature of these lipids is their relatively high water solubility. It also explains why the enzymatic rate drops as the length of the sn-1 chain is increased. pPAF-AH shows broad specificity toward phospholipids with different polar headgroups. Additional results are that PAF undergoes intervesicle exchange on the subminute time scale and it does not undergo transbilayer movement over tens of minutes.

Probing the substrate specificity of the intracellular brain platelet-activating factor acetylhydrolase

Platelet-activating factor acetylhydrolases (PAF-AHs) are unique PLA2s which hydrolyze the sn-2 ester linkage in PAF-like phospholipids with a marked preference for very short acyl chains, typically acetyl. The recent solution of the crystal structure of the alpha(1) catalytic subunit of isoform Ib of bovine brain intracellular PAF-AH at 1.7 A resolution paved the way for a detailed examination of the molecular basis of substrate specificity in this enzyme. The crystal structure suggests that the side chains of Thr103, Leu48 and Leu194 are involved in substrate recognition. Three single site mutants (L48A, T103S and L194A) were overexpressed and their structures were solved to 2.3 A resolution or better by X-ray diffraction methods. Enzyme kinetics showed that, compared with wild-type protein, all three mutants have higher relative activity against phospholipids with sn-2 acyl chains longer than an acetyl. However, for each of the mutants we observed an unexpected and substantial reduction in the V(max) of the reaction. These results are consistent with the model in which residues Leu48, Thr103 and Leu194 indeed contribute to substrate specificity and in addition suggest that the integrity of the specificity pocket is critical for the expression of full catalytic function, thus conferring very high substrate selectivity on the enzyme.

Hepatic regulation of platelet-activating factor acetylhydrolase and lecithin:cholesterol acyltransferase biliary and plasma output in rats exposed to bacterial lipopolysaccharide

Normal rat bile contains secretory platelet-activating factor acetylhydrolase (PAF-AH), the enzyme capable of hydrolyzing the inflammatory mediator platelet-activating factor (PAF), and phospholipids containing oxidized truncated fatty acids. Because lecithin:cholesterol acyltransferase (LCAT) possesses intrinsic PAF-AH-like activity, it also may represent a potential anti-inflammatory enzyme. The behavior of PAF-AH and LCAT in hepatobiliary inflammatory responses in vivo has not been characterized. We therefore investigated the biliary and plasma secretion and pharmacological characteristics of these enzymes in rats subjected to intraportal bacterial endotoxin exposure (lipopolysaccharide [LPS], Escherichia coli, 055:B5). Portal vein LPS infusion (1 mg/kg, bolus) resulted in a maximal 4- to 5-fold increase in bile PAF-AH-specific activity with a gradual decline to baseline by 18 hours. Biliary PAF-AH hydrolyzed also the truncated sn-2-succinoyl and sn-2-glutaroyl analogs of PAF, indicating a broader activity of PAF-AH in bile toward byproducts of glycerophospholipid peroxidation. Plasma PAF-AH activity was not altered 5 hours after LPS injection compared with saline injection, but it was significantly elevated 18 hours after endotoxin exposure. The levels of LCAT in bile were low and declined to nearly undetectable values by 5 hours after cannulation in both control and LPS-exposed rats. Plasma LCAT activity was significantly increased after 5 hours and decreased 18 hours after LPS injection. In summary, hepatic exposure to endotoxin results in a rapid increase in biliary secretion of PAF-AH followed by elevation of LCAT and PAF-AH levels in plasma. We propose that biliary secretion of PAF-AH may be involved in the hepatic response to endotoxic insult by counteracting potential inflammatory damage in the biliary tree and gastrointestinal tract, whereas plasma increases in LCAT and PAF-AH may promote elimination of excess PAF and oxidized phospholipids in the circulation.

Phospholipase A2: its usefulness in laboratory diagnostics

Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of membrane phospholipids. This article reviews the source and structure of PLA2, the involvement of the enzyme in various biological and pathological phenomena, and the usefulness of PLA2 assays in laboratory diagnostics. Of particular importance is the role of PLA2 in the cellular production of mediators of inflammatory response to various stimuli. Assays for PLA2 activity and mass concentration are discussed, and the results of enzyme determinations in plasma from patients with different pathological conditions are presented. The determination of activity and mass concentration in plasma is particularly useful in the diagnosis and prognosis of pancreatitis, multiple organ failure, septic shock, and rheumatoid arthritis. A very important result is the demonstration that PLA2 is an acute phase protein, like CRP. Indeed, there is a close correlation between PLA2 mass concentration and CRP levels in several pathological conditions. Although the determination of C-reactive protein is much easier to perform and is routinely carried out in most clinical laboratories, the assessment of PLA2 activity or mass concentration has to be considered as a reliable approach to obtain a deeper understanding of some pathological conditions and may offer additional information concerning the prognosis of several disorders.

Deficiency of platelet-activating factor acetylhydrolase is a severity factor for asthma

Asthma, a family of airway disorders characterized by airway inflammation, has an increasing incidence worldwide. Platelet-activating factor (PAF) may play a role in the pathophysiology of asthma. Its proinflammatory actions are antagonized by PAF acetylhydrolase. A missense mutation (V279F) in the PAF acetylhydrolase gene results in the complete loss of activity, which occurs in 4% of the Japanese population. We asked if PAF acetylhydrolase deficiency correlates with the incidence and severity of asthma in Japan. We found that the prevalence of PAF acetylhydrolase deficiency is higher in Japanese asthmatics than healthy subjects and that the severity of this syndrome is highest in homozygous-deficient subjects. We conclude that the PAF acetylhydrolase gene is a modulating locus for the severity of asthma.

Molecular basis of the interaction between plasma platelet-activating factor acetylhydrolase and low density lipoprotein

The platelet-activating factor acetylhydrolases are enzymes that were initially characterized by their ability to hydrolyze platelet-activating factor (PAF). In human plasma, PAF acetylhydrolase (EC 3.1.1.47) circulates in a complex with low density lipoproteins (LDL) and high density lipoproteins (HDL). This association defines the physical state of PAF acetylhydrolase, confers a long half-life, and is a major determinant of its catalytic efficiency in vivo. The lipoprotein-associated enzyme accounts for all of the PAF hydrolysis in plasma but only two-thirds of the protein mass. To characterize the enzyme-lipoprotein interaction, we employed site-directed mutagenesis techniques. Two domains within the primary sequence of human PAF acetylhydrolase, tyrosine 205 and residues 115 and 116, were important for its binding to LDL. Mutation or deletion of those sequences prevented the association of the enzyme with lipoproteins. When residues 115 and 116 from human PAF acetylhydrolase were introduced into mouse PAF acetylhydrolase (which normally does not associate with LDL), the mutant mouse PAF acetylhydrolase associated with lipoproteins. To analyze the role of apolipoprotein (apo) B100 in the formation of the PAF acetylhydrolase-LDL complex, we tested the ability of PAF acetylhydrolase to bind to lipoproteins containing truncated forms of apoB. These studies indicated that the carboxyl terminus of apoB plays a key role in the association of PAF acetylhydrolase with LDL. These data on the molecular basis of the PAF acetylhydrolase-LDL association provide a new level of understanding regarding the pathway for the catabolism of PAF in human blood.

Differential regulation of phospholipase A2 (PLA2)-dependent Ca2+ signaling in smooth muscle by cAMP- and cGMP-dependent protein kinases. Inhibitory phosphorylation of PLA2 by cyclic nucleotide-dependent protein kinases

Both cAMP- and cGMP-dependent protein kinases inhibit agonist-stimulated phospholipase C-beta (PLC-beta) activity and inositol 1,4,5-trisphosphate-dependent Ca2+ release in vascular and visceral smooth muscle. In smooth muscle of the intestinal longitudinal layer, however, the initial steps in Ca2+ mobilization involve activation of cytosolic PLA2 (cPLA2) and arachidonic acid (AA)-dependent stimulation of Ca2+ influx. The present study examined whether cAMP- and cGMP-dependent protein kinases are capable of regulating these processes also. Agents that activated cAMP-dependent protein kinase (5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole 3',5'-cyclic monophosphothioate (Sp-isomer) and isoproterenol), cGMP-dependent protein kinase (8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate and Na nitroprusside), or both kinases (vasoactive intestinal peptide and isoproterenol >1 microM) induced phosphorylation of cPLA2 and inhibition of agonist-stimulated cPLA2 activity. Phosphorylation and inhibition of cPLA2 activity by cAMP- and cGMP-dependent protein kinases were blocked by the corresponding selective inhibitors (cAMP-dependent protein kinase, N-[2(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide hydrochloride (H-89) and myristoylated protein kinase inhibitor () amide; cGMP-dependent protein kinase, (8R,9S, 11S)-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8, 11-epoxy-1H,8H,11H,-2,7b,11a-trizadizobenzo(a,g)cycloocta(c, d, e)-trinden-1-one (KT-5823)). In contrast, AA-stimulated Ca2+ influx was inhibited by agents that activated cGMP-dependent protein kinase only; the inhibition was selectively blocked by KT-5823. The study provides the first evidence of inhibitory phosphorylation of cPLA2 in vivo by cAMP- and cGMP-dependent protein kinases. Inhibition of cPLA2 activity and AA-induced Ca2+ influx partly account for the ability of cAMP-dependent protein kinase and/or cGMP-dependent protein kinase to cause relaxation. Their importance resides in their location at the inception of the Ca2+ signaling cascade.

Bovine seminal platelet-activating factor acetylhydrolase: association properties in seminal plasma and with lipoproteins

The enzyme responsible for most of the phospholipase A2 (PLA2) activity present in bovine seminal plasma was recently purified to homogeneity. Sequencing revealed that the enzyme is also a platelet-activating factor acetylhydrolase (PAF-AH) of the serum type with kinetic properties generally similar to its serum homologue. In the present work, we have attempted to clarify its physiological function by studying its association properties in seminal plasma. As was observed previously for its PLA2 activity, its PAF-AH activity was also inhibited by the major proteins of bovine seminal plasma (BSP proteins). Sequential dilution experiments as well as centrifuging semen on Percoll did not reveal detectable association of PAF-AH with spermatozoa. Neither did the enzyme interact with lipid particles reported to be present in bovine seminal plasma. The purified PAF-AH, however, did display lipoprotein association properties in vitro similar to those demonstrated by the serum enzyme in vivo. At pH 7.4, it could associate with both low density lipoproteins and very low density lipoproteins but not with high density lipoproteins. Overall the data presented here indicate that the enzyme is strongly inactivated as a PAF-AH in seminal plasma and that it does not associate with lipid particles or spermatozoa.

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.

LDL-associated phospholipase A does not protect LDL against lipid peroxidation in vitro

The irreversible proteinase inhibitor Pefabloc (4-[2-aminoethyl] benzenesulfonyl fluoride) inactivates LDL-catalyzed hydrolysis of the short-chain fluorescent phospholipid C6-NBD-PC (1-acyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole)-aminocaproyl phosphatidylcholine). The dose-dependence of this inactivation is similar to that obtained previously for the inhibitory effect of Pefabloc on the hydrolysis of platelet activating factor (PAF) by the LDL-associated PAF acetylhydrolase (PAF-AH), in agreement with the notion that the hydrolysis of C6-NBD-PC and PAF is catalyzed by the same enzyme (LDL-associated phospholipase A; LDL-PLA). This conclusion is also supported by the finding that hydrolysis of C6-NBD-PC by LDL becomes inactivated by LDL oxidation only at late stages of the oxidation, similar to the effect of oxidation on the hydrolysis of PAF by the LDL-associated PAF-AH. Under conditions of complete inactivation of this enzyme towards C6-NBD-PC, the kinetics of lipid peroxidation, induced either by copper ions or by the free radical generator AAPH at varying doses of the prooxidant, was similar to that observed when the PLA was active (i.e., in the absence of Pefabloc). Hence, LDL-associated PLA (PAF-AH) does not protect LDL lipids from peroxidation. Similar results were obtained with fractionated LDL in albumin-containing buffer and for non-fractionated serum, in which copper-induced peroxidation was also not influenced by inactivation of the enzyme responsible for hydrolysis of C6-NBD-PC. Phospholipolysis of short chain phospholipids by LDL-PLA may still play a protective role against the toxic effects of oxidized phospholipids by reducing their internalization into cells (Schmitt et al. 1995).

Secretory PAF-acetylhydrolase of the rat hepatobiliary system: characterization and partial purification

Hepatocytes and Kupffer cells in primary culture both secrete plasma-type platelet-activating factor-acetylhydrolase (pPAF-AH) into serum-free culture medium. The rate of secretion of pPAF-AH by Kupffer cells was 20 to 25 times higher than from hepatocytes, and Kupffer cells expressed a higher level of pPAF-AH mRNA than did hepatocytes. Purified liver cell-secreted pPAF-AH exhibited a major protein band of 65-67 kDa on SDS-PAGE; this was the band predominantly labeled when the enzyme catalytic center was reacted with [3H]diisopropylfluorophosphate ([3H]DFP). Rat bile collected from cannulated bile ducts contained significant PAF-AH activity, and bile samples possessed a prominent band at 30-32 kDa, which was the exclusive target for [3H]DFP. Experiments using tunicamycin, an inhibitor of N-linked glycosylation, and endoglycosidase H suggested that pPAF-AH secreted constitutively by cultured hepatocytes and Kupffer cells is glycosylated. The present study supports the notion that hepatic secretion of pPAF-AH into the blood contributes to the regulation of PAF and oxidized phospholipid levels in the circulation, whereas secretion of PAF-AH into the bile may allow hepatic control of these phospholipid signaling molecules in the gastrointestinal tract.

Structure of a microbial homologue of mammalian platelet-activating factor acetylhydrolases: Streptomyces exfoliatus lipase at 1.9 A resolution

BACKGROUND

Neutral lipases are ubiquitous and diverse enzymes. The molecular architecture of the structurally characterized lipases is similar, often despite a lack of detectable homology at the sequence level. Some of the microbial lipases are evolutionarily related to physiologically important mammalian enzymes. For example, limited sequence similarities were recently noted for the Streptomyces exfoliatus lipase (SeL) and two mammalian platelet-activating factor acetylhydrolases (PAF-AHs). The determination of the crystal structure of SeL allowed us to explore the structure-function relationships in this novel family of homologous hydrolases.

RESULTS

The crystal structure of SeL was determined by multiple isomorphous replacement and refined using data to 1.9 A resolution. The molecule exhibits the canonical tertiary fold of an alpha/beta hydrolase. The putative nucleophilic residue, Ser131, is located within a nucleophilic elbow and is hydrogen bonded to His209, which in turn interacts with Asp177. These three residues create a triad that closely resembles the catalytic triads found in the active sites of other neutral lipases. The mainchain amides of Met132 and Phe63 are perfectly positioned to create an oxyanion hole. Unexpectedly, there are no secondary structure elements that could render the active site inaccessible to solvent, like the lids that are commonly found in neutral lipases.

CONCLUSIONS

The crystal structure of SeL reinforces the notion that it is a homologue of the mammalian PAF-AHs. We have used the catalytic triad in SeL to model the active site of the PAF-AHs. Our model is consistent with the site-directed mutagenesis studies of plasma PAF-AH, which implicate Ser273, His351 and Asp296 in the active site. Our study therefore provides direct support for the hypothesis that the plasma and isoform II PAF-AHs are triad-containing alpha/beta hydrolases.

Substrate specificities of neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor: differences in the use of 1-alkyl and 1-acyl lysophospholipid acceptors

The selectivity of alkylglycerophosphate (AGP) acetyltransferase and lyso-platelet-activating factor (lyso-PAF) acetyltransferase was studied in neuronal nuclei isolated from cerebral cortices of 15-day-old rabbits. Specifically, 1-alkyl and 1-acyl analogues were compared as acceptors in these acetylation reactions. A number of observations supported one nuclear activity in the acetylation of AGP and lyso-PA. Lyso-PA was a competitive substrate for AGP, Km values for AGP and lyso-PA were similar, as were acetylation rates measured at individual AGP or lyso-PA concentrations, and the acetylation of both substrates was unaffected by preincubations with protein phosphatase 1 (PP-1). In contrast, there were a number of differences seen in the acetylation of lyso-PAF and lyso-PC. The kinetics for lyso-PC acetylation (as a function of lyso-PC concentration) were not hyperbolic, and lyso-PC was not a competitive substrate for the acetylation of lyso-PAF. Unlike acetylation rates with lyso-PAF, lyso-PC acetylation was not reduced by preincubations with PP-1, and was less susceptible to inhibition particularly at high levels of free fatty acid. In addition, rates of acetylation of lyso-PC were selectively increased by the presence of lyso-PA. When neuronal nuclear envelope fractions (NE) were prepared from N1, the specific acetylation activity with lyso-PAF was significantly lower in NE, while the activities for lyso-PC were comparable in NE and the parent N1 fraction. The results with the acetylation of lyso-PC and lyso-PAF suggest that the lyso-PC acetyltransferase may be in a uniquely sequestered state within the neuronal nucleus. This could explain the smaller inhibition of lyso-PC acetylation by free fatty acid, the maintenance of lyso-PC acetylation during PP-1 preincubations, the non-hyperbolic response to lyso-PC concentrations and the selective preservation of lyso-PC acetylation during NE isolation. This protected status could result from a more internal location for this acetyltransferase within the membranes of the nuclear envelope, or possibly an association of the enzyme with the nuclear matrix that is disrupted with the exposure of N1 to lyso-PA.