Characterization of serum platelet-activating factor (PAF) acetylhydrolase. Correlation between deficiency of serum PAF acetylhydrolase and respiratory symptoms in asthmatic children

Biology of platelet-activating factor acetylhydrolase (PAF-AH, lipoprotein associated phospholipase A2)

INTRODUCTION

This article is focused on platelet-activating factor acetylhydrolase (PAF-AH), a lipoprotein bound, calcium-independent phospholipase A(2) activity also referred to as lipoprotein-associated phospholipase A(2) or PLA(2)G7. PAF-AH catalyzes the removal of the acyl group at the sn-2 position of PAF and truncated phospholipids generated in settings of inflammation and oxidant stress.

DISCUSSION

Here, I discuss current knowledge related to the structural features of this enzyme, including the molecular basis for association with lipoproteins and susceptibility to oxidative inactivation. The circulating form of PAF-AH is constitutively active and its expression is upregulated by mediators of inflammation at the transcriptional level. This mechanism is likely responsible for the observed up-regulation of PAF-AH during atherosclerosis and suggests that increased expression of this enzyme is a physiological response to inflammatory stimuli. Administration of recombinant forms of PAF-AH attenuate inflammation in a variety of experimental models. Conversely, genetic deficiency of PAF-AH in defined human populations increases the severity of atherosclerosis and other syndromes. Recent advances pointing to an interplay among oxidized phospholipid substrates, Lp(a), and PAF-AH could hold the key to a number of unanswered questions.

Lipoprotein-associated phospholipase A2 and risk of venous thrombosis in older adults

Lipoprotein-associated phospholipase A2 (Lp-PLA2) is an enzyme involved in inflammation and platelet function. Inherited deficiency and elevated levels are associated with atherosclerosis. Given potential common etiologies of atherosclerosis and venous thrombosis (VT), we hypothesized that low and high Lp-PLA2 would be associated with VT risk. Lp-PLA(2) mass and activity were measured in baseline samples of Cardiovascular Health Study participants (5,888 men and women age > or =65), excluding 354 reporting pre-baseline VT. The study endpoint was VT unrelated to cancer after 11.6 years follow-up. Hazard ratios were estimated using Cox proportional hazard models, adjusting for age, race, sex, and body-mass index. With 129 cases of VT, there was no association of Lp-PLA2 activity with risk. Adjusted hazard ratios were 1.19 (CI 0.62, 2.29) and 0.87 (CI 0.43, 1.76) for the lowest and highest decile, respectively, compared to the 10-25th percentile. Corresponding hazard ratios for Lp-PLA2 mass were 1.63 (CI 0.79, 3.34) and 1.33 (CI 0.61, 2.87). Results were robust to several definitions of low or high Lp-PLA2. While the association of Lp-PLA(2) levels with arterial disease events implies a role for this enzyme in atherogenesis, our findings suggest that it is not prothrombotic.

Hypothesis: hemolytic transfusion reactions represent an alternative type of anaphylaxis

Classical anaphylaxis is the most severe, and potentially fatal, type of allergic reaction, manifested by hypotension, bronchoconstriction, and vascular permeability. Similarly, a hemolytic transfusion reaction (HTR) is the most feared consequence of blood transfusion. Evidence for the existence of an alternative, IgG-mediated pathway of anaphylaxis may be relevant for explaining the pathophysiology of IgG-mediated-HTRs. The purpose of this review is to summarize the evidence for this alternative pathway of anaphylaxis and to present the hypothesis that an IgG-mediated HTR is one example of this type of anaphylaxis.

Platelet activating factor as a mediator and therapeutic approach in bronchial asthma

Platelet activating factor (PAF) is a potent phospholipid mediator involved in anaphylaxis and chronic inflammatory disorders, including bronchial asthma. PAF is able to act both, directly as a chemotactic factor and indirectly through the release of other inflammatory agents. Apart from its known potent ability to activate platelets, PAF influences other immune and inflammatory cells function involved in asthma, which may be of importance in the pathogenesis of the disease. In addition, PAF administration can mimic some of abnormalities observed in asthma, including bronchoconstriction, bronchial hyper responsiveness, and gas exchange impairment, which may be mediated by leukotrienes acting as secondary mediators of some PAF effects. Therefore, there has been an extensive interest in the role of PAF in human asthma and major efforts have been continued to discover drugs acting thorough inhibition of PAF effects in the disease. Surprisingly, PAF receptor antagonists have not clearly proven their clinical benefits. It may appear that the combined blockage of PAF effects and other mediators involved in asthma is a way to improve clinical efficacy and also an interesting approach to control inflammation in the disease. This review will focus on two main issues: the role of PAF and PAF antagonists in asthma.

Platelet-activating factor, PAF acetylhydrolase, and severe anaphylaxis

BACKGROUND

Platelet-activating factor (PAF) is an important mediator of anaphylaxis in animals, and interventions that block PAF prevent fatal anaphylaxis. The roles of PAF and PAF acetylhydrolase, the enzyme that inactivates PAF, in anaphylaxis in humans have not been reported.

METHODS

We measured serum PAF levels and PAF acetylhydrolase activity in 41 patients with anaphylaxis and in 23 control patients. Serum PAF acetylhydrolase activity was also measured in 9 patients with peanut allergy who had fatal anaphylaxis and compared with that in 26 nonallergic pediatric control patients, 49 nonallergic adult control patients, 63 children with mild peanut allergy, 24 patients with nonfatal anaphylaxis, 10 children who died of nonanaphylactic causes, 15 children with life-threatening asthma, and 19 children with non-life-threatening asthma.

RESULTS

Mean (+/-SD) serum PAF levels were significantly higher in patients with anaphylaxis (805+/-595 pg per milliliter) than in patients in the control groups (127+/-104 pg per milliliter, P<0.001 after log transformation) and were correlated with the severity of anaphylaxis. The proportion of subjects with elevated PAF levels increased from 4% in the control groups to 20% in the group with grade 1 anaphylaxis, 71% in the group with grade 2 anaphylaxis, and 100% in the group with grade 3 anaphylaxis (P<0.001). There was an inverse correlation between PAF levels and PAF acetylhydrolase activity (P<0.001). The proportion of patients with low PAF acetylhydrolase values increased with the severity of anaphylaxis (P<0.001 for all comparisons). Serum PAF acetylhydrolase activity was significantly lower in patients with fatal peanut anaphylaxis than in control patients (P values <0.001 for all comparisons).

CONCLUSIONS

Serum PAF levels were directly correlated and serum PAF acetylhydrolase activity was inversely correlated with the severity of anaphylaxis. PAF acetylhydrolase activity was significantly lower in patients with fatal anaphylactic reactions to peanuts than in patients in any of the control groups. Failure of PAF acetylhydrolase to inactivate PAF may contribute to the severity of anaphylaxis.

Protection against oxidative stress-induced hepatic injury by intracellular type II platelet-activating factor acetylhydrolase by metabolism of oxidized phospholipids in vivo

Membrane phospholipids are susceptible to oxidation, which is involved in various pathological processes such as inflammation, atherogenesis, neurodegeneration, and aging. One enzyme that may help to remove oxidized phospholipids from cells is intracellular type II platelet-activating factor acetylhydrolase (PAF-AH (II)), which hydrolyzes oxidatively fragmented fatty acyl chains attached to phospholipids. Overexpression of PAF-AH (II) in cells or tissues was previously shown to suppress oxidative stress-induced cell death. In this study we investigated the functions of PAF-AH (II) by generating PAF-AH (II)-deficient (Pafah2(-/-)) mice. PAF-AH (II) was predominantly expressed in epithelial cells such as kidney proximal and distal tubules, intestinal column epithelium, and hepatocytes. Although PAF-AH activity was almost abolished in the liver and kidney of Pafah2(-/-) mice, Pafah2(-/-) mice developed normally and were phenotypically indistinguishable from wild-type mice. However, mouse embryonic fibroblasts derived from Pafah2(-/-) mice were more sensitive to tert-butylhydroperoxide treatment than those derived from wild-type mice. When carbon tetrachloride (CCl(4)) was injected into mice, Pafah2(-/-) mice showed a delay in hepatic injury recovery. Moreover, after CCl(4) administration, liver levels of the esterified form of 8-iso-PGF(2alpha), a known in vitro substrate of PAF-AH (II), were higher in Pafah2(-/-) mice than in wild-type mice. These results indicate that PAF-AH (II) is involved in the metabolism of esterified 8-isoprostaglandin F(2alpha) and protects tissue from oxidative stress-induced injury.

Intracellular PAF catabolism by PAF acetylhydrolase counteracts continual PAF synthesis

Stimulated inflammatory cells synthesize platelet-activating factor (PAF), but lysates of these cells show little enhancement in PAF synthase activity. We show that human neutrophils contain intracellular plasma PAF acetylhydrolase (PLA2G7), an enzyme normally secreted by monocytes. The esterase inhibitors methyl arachidonoylfluorophosphonate (MAFP), its linoleoyl homolog, and Pefabloc inhibit plasma PAF acetylhydrolase. All of these inhibitors induced PAF accumulation by quiescent neutrophils and monocytes that was equivalent to agonist stimulation. Agonist stimulation after esterase inhibition did not further increase PAF accumulation. PAF acetylhydrolase activity in intact neutrophils was reduced, but not abolished, by agonist stimulation. Erythrocytes, which do not participate in the acute inflammatory response, inexplicably express the type I PAF acetylhydrolase, whose only known substrate is PAF. Inhibition of this enzyme by MAFP caused PAF accumulation by erythrocytes, which was hemolytic in the absence of PAF acetylhydrolase activity. We propose that PAF is continuously synthesized by a nonselective acyltransferase activity(ies) found even in noninflammatory cells as a component of membrane remodeling, which is then selectively and continually degraded by intracellular PAF acetylhydrolase activity to modulate PAF production.

The Genetic Basis of Abdominal Aortic Aneurysms: A Review

INTRODUCTION

The pathogenesis of abdominal aortic aneurysm (AAA) remains poorly understood, however significant evidence has emerged in recent years to suggest a chronic inflammatory process. Observational studies have highlighted a familial trend towards AAA development among relatives of affected individuals and it is thought that inflammatory genes may influence an individual's susceptibility. Conflicting reports exist over single gene versus multiple gene inheritance patterns in addition to a collection of studies examining individual inflammatory genes. This paper reviews the evidence for a genetic predisposition to aneurysm formation including familial and segregation studies in addition to experimental evidence investigating specific candidate genes.

METHOD

Medline and Pubmed database searches were conducted using the search terms abdominal aortic aneurysm and gene. Papers were reviewed and references manually searched for further relevant publications which were added to the data. Papers were categorised under the headings familial, segregation and candidate gene studies.

RESULTS

A review of 58 papers is presented under sub-headings as above. In the case of the candidate gene section, a brief report of the functional relevance of each gene is included.

CONCLUSION

A summary of the evidence presented is given and the direction of future work in this field is briefly considered.

Clinical aspects of plasma platelet-activating factor-acetylhydrolase

Plasma platelet-activating factor (PAF)-acetylhydrolase (PAF-AH), which is characterized by tight association with plasma lipoproteins, degrades not only PAF but also phospholipids with oxidatively modified short fatty acyl chain esterified at the sn-2 position. Production and accumulation of these phospholipids are associated with the onset of inflammatory diseases and preventive role of this enzyme has been evidenced by many recent studies including prevalence of the genetic deficiency of the enzyme in the patients and therapeutic effects of treatment with recombinant protein or gene transfer. With respect to the atherosclerosis, however, it is not fully cleared whether this enzyme plays an anti-atherogenic role or pro-atherogenic role because plasma PAF-AH also might produce lysophosphatidylcholine (LysoPC) and oxidatively modified nonesterified fatty acids with potent pro-inflammatory and pro-atherogenic bioactivities. These dual roles of plasma PAF-AH might be regulated by the altered distribution of the enzyme between low density lipoprotein (LDL) and high density lipoprotein (HDL) particles because HDL-associated enzymes are considered to contribute to the protection of LDL from oxidative modification. This review focuses on the recent findings which address the role of this enzyme in the human diseases especially including asthma, septic shock and atherosclerosis.

The Val279Phe variant of the lipoprotein-associated phospholipase A2 gene is associated with catalytic activities and cardiovascular disease in Korean men

CONTEXT AND OBJECTIVE

It is unclear whether lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) exerts a pro- or antiatherogenic effect on cardiovascular disease (CVD). We investigated the association between Lp-PLA(2) variant (V279F and A379V) and CVD in Korean men.

DESIGN

CVD patients (n = 532) and healthy controls (n = 670) were genotyped for the Lp-PLA(2) polymorphism (V279F and A379V).

MAIN OUTCOME MEASURES

We calculated odds ratio (OR) on CVD risk and measured anthropometries, lipid profiles, low-density lipoprotein (LDL) particle size, oxidized LDL, lipid peroxides, and Lp-PLA(2) activity.

RESULTS

The presence of the 279F allele was associated with a lower risk of CVD [OR 0.646 (95% confidence interval 0.490-0.850), P = 0.002], and the association still remained after adjustments for age, body mass index, waist circumference, waist to hip ratio, cigarette smoking, and alcohol consumption [OR 0.683 (95% confidence interval 0.512-0.911), P = 0.009]. Lp-PLA(2) activity was lower in CVD patients taking a lipid-lowering drug (31%), those not taking a lipid-lowering drug (26%), and control subjects (23%) with the V/F genotype, compared with those with the V/V genotype. Subjects with the F/F genotype in controls and two CVD patients groups showed no appreciable enzymatic activity. Control subjects with the V/F genotype had larger LDL particle size than those with the V/V genotype. In addition, control subjects carrying the F allele showed lower malondialdehyde concentrations. On the other hand, we found no significant relationship between A379V genotype and CVD risk.

CONCLUSIONS

The association of the F279 loss of function variant with the reduced risk of CVD supports the concept that Lp-PLA(2) plays a proatherogenic and causative role in CVD.

Plasma PAF-acetylhydrolase: an unfulfilled promise?

Plasma Platelet-activating-Factor (PAF)-acetylhydrolase (PAF-AH also named lipoprotein-PLA(2) or PLA(2)G7 gene) is secreted by macrophages, it degrades PAF and oxidation products of phosphatidylcholine produced upon LDL oxidation and/or oxidative stress, and thus is considered as a potentially anti-inflammatory enzyme. Cloning of PAF-AH has sustained tremendous promises towards the use of PAF-AH recombinant protein in clinical situations. The reason for that stems from the numerous animal models of inflammation, atherosclerosis or sepsis, where raising the levels of circulating PAF-AH either through recombinant protein infusion or through the adenoviral gene transfer showed to be beneficial. Unfortunately, neither in human asthma nor in sepsis the recombinant PAF-AH showed sufficient efficacy. One of the most challenging questions nowadays is as to whether PAF-AH is pro- or anti-atherogenic in humans, as PAF-AH may possess a dual pro- and anti-inflammatory role, depending on the concentration and the availability of potential substrates. It is equally possible that the plasma level of PAF-AH is a diagnostic marker of ongoing atherosclerosis.

A single nucleotide polymorphism in the plasma PAF acetylhydrolase gene and risk of atherosclerosis in Japanese patients with peripheral artery occlusive disease

BACKGROUND

Plasma PAF-acetylhydrolase (PAF-AH) gene polymorphisms (G994 --> T in exon 9) and the resulting deficiency of enzyme activity were identified in the Japanese population. The objective of this study was to assess the joint effect of the polymorphism and hypercholesterolemia on risk of atherosclerosis.

METHODS AND RESULTS

We performed a case-control study including 150 patients who underwent operation for peripheral arterial occlusive disease (PAOD) and 158 controls matched for age and sex. Genomic DNA was analyzed for the mutant allele by a specific polymerase-chain reaction. Plasma PAF-AH activity was measured in both groups. The patients with multiple atherosclerotic diseases showed higher levels of PAF-AH activities than the patients with only peripheral artery occlusive disease among normal genotypes. PAOD patients were assessed either with or without polymorphism or hypercholesterolemia in regard to accompanying coronary artery disease or stroke. The prevalence of the polymorphism was significantly more frequent in the patients with PAOD. The plasma PAF-AH activity was correlated with total cholesterol and LDL level, and inversely related with HDL in normal genotype (GG) PAOD patients. However, neither the correlation nor the inverse relation was found in patients with the polymorphism. Patients with both hypercholesterolemia and the polymorphisms revealed a relative risk for other atherosclerotic disease of 11.5 (6.0-40.3) compared with normal genotype and normal lipid level.

CONCLUSION

The plasma PAF-AH gene polymorphism and hypercholesterolemia may interact and increase the risk of atherosclerosis.

Altered lipoprotein subclass distribution and PAF-AH activity in subjects with generalized aggressive periodontitis

In this study, we examined whether the documented increase of plasma triglycerides in patients with generalized aggressive periodontitis (GAgP) is associated with changes in lipoprotein subclass distribution and/or LDL-associated platelet-activating factor acetylhydrolase (PAF-AH) activity. Lipoprotein subclasses were analyzed in whole plasma samples using nuclear magnetic resonance methods. Compared with subjects without periodontitis (NP subjects; n = 12), GAgP subjects (n = 12) had higher plasma levels of large, medium, and small VLDL (35.0 +/- 6.7 vs. 63.1 +/- 9.6 nmol/l; P = 0.025), higher levels of intermediate density lipoprotein (24.8 +/- 11.6 vs. 87.2 +/- 16.6 nmol/l; P = 0.006), lower levels of large LDL (448.3 +/- 48.5 vs. 315.8 +/- 59.4 nmol/l; P = 0.098), and higher levels of small LDL (488.2 +/- 104.2 vs. 946.7 +/- 151.6 nmol/l; P = 0.021). The average size of LDL from NP and GAgP subjects was 21.4 +/- 0.2 and 20.6 +/- 0.3 nm, respectively (P = 0.031). Compared with NP subjects, GAgP subjects had a greater number of circulating LDL particles (961.3 +/- 105.3 vs. 1,349.0 +/- 133.2 nmol/l; P = 0.032). Differences in the plasma levels of large, medium, and small HDL were not statistically significant. NP and GAgP subjects had similar plasma levels of total LDL-associated PAF-AH activity; however, LDL of GAgP subjects contained less PAF-AH activity per microgram of LDL protein (1,458.0 +/- 171.0 and 865.2 +/- 134 pmol/min/microg; P = 0.014). These results indicate that, in general, GAgP subjects have a more atherogenic lipoprotein profile and lower LDL-associated PAF-AH activity than NP subjects. These differences may help explain the increased risk of GAgP subjects for cardiovascular disease.

Modulating effects of cholesterol feeding and simvastatin treatment on platelet-activating factor acetylhydrolase activity and lysophosphatidylcholine concentration

BACKGROUND

Platelet-activating factor acetylhydrolyse (PAF-AH) is an enzyme that degrades PAF and bioactive oxidized lipids. However, it has been reported to be a risk factor for coronary heart disease. The present study examined the effects of cholesterol feeding and simvastatin treatment on plasma PAF-AH activity.

METHODS

Japanese White rabbits (n=22) were fed a diet containing 0.3% cholesterol and 3% corn oil for 1 month, and then divided into two groups that continued to receive this diet with (treated) or without (control) treatment with simvastatin (0.01%) for another 2 months.

RESULTS

Cholesterol feeding increased and simvastatin treatment decreased apolipoprotein (apo) B-containing lipoprotein subfractions as characterized by capillary isotachophoresis, serum levels of total cholesterol, phospholipids, LDL-C, apoE, plasma and LDL-associated PAF-AH (LDL-PAF-AH) activities, and plasma lyso-PC concentration. Cholesterol feeding also increased apoB levels but decreased the LDL-PAF-AH/LDL-C ratio and did not change the plasma PAF-AH/lyso-PC ratio. Simvastatin treatment did not affect apoB levels and only slightly increased the LDL-PAF-AH/LDL-C ratio. Secretion of PAF-AH activity from monocyte-derived macrophages was increased by cholesterol feeding but not affected by simvastatin treatment. These results indicate that PAF-AH activity is increased by cholesterol feeding due to increased secretion of PAF-AH activity from macrophages and that PAF-AH activity is decreased by simvastatin due to increased removal of lipid and enzyme contents of LDL particles.

CONCLUSION

Cholesterol elevation by cholesterol feeding and cholesterol-lowering by simvastatin modulate plasma PAF-AH activity by different mechanisms.

A polymorphism in plasma platelet-activating factor acetylhydrolase is involved in resistance to immunoglobulin treatment in Kawasaki disease

OBJECTIVE

To investigate whether reduced levels of plasma platelet-activating factor acetylhydrolase (PAF-AH) as a result of a genetic polymorphism are involved in the pathogenesis of Kawasaki disease (KD).

STUDY DESIGN

The frequency of a V279F polymorphism (G/T transversion) in the PAF-AH gene was quantified in 76 Japanese children with KD and 112 healthy Japanese adults using the allele-specific polymerase chain reaction (PCR). Associations between genotype, clinical features, and resistance to intravenous immunoglobulin (IVIG) were investigated in the patients with KD. Plasma PAF-AH activity was measured by using [3H]-acetyl-PAF.

RESULTS

There were no significant differences in genotype frequency between patients and controls (P = .51). Compared with the GG (normal genotype) group, significantly more patients in the GT (heterozygous) +TT (homozygous deficient) group required additional IVIG (52% vs 14%, P = .001). The duration of fever and maximum serum C-reactive protein (CRP) levels also were significantly increased in the GT+TT group (P = .012 and .036, respectively), whereas plasma PAF-AH activity was significantly lower (P <.0001).

CONCLUSION

We conclude that the V279F polymorphism in the plasma PAF-AH gene and consequent enzymatic deficiency is one of the factors for IVIG nonresponse in Japanese patients with acute KD.

Regulating inflammation through the anti-inflammatory enzyme platelet-activating factor-acetylhydrolase

Platelet-activating factor (PAF) is one of the most potent lipid mediators involved in inflammatory events. The acetyl group at the sn-2 position of its glycerol backbone is essential for its biological activity. Deacetylation induces the formation of the inactive metabolite lyso-PAF. This deacetylation reaction is catalyzed by PAF-acetylhydrolase (PAF-AH), a calcium independent phospholipase A2 that also degrades a family of PAF-like oxidized phospholipids with short sn-2 residues. Biochemical and enzymological evaluations revealed that at least three types of PAF-AH exist in mammals, namely the intracellular types I and II and a plasma type. Many observations indicate that plasma PAF AH terminates signals by PAF and oxidized PAF-like lipids and thereby regulates inflammatory responses. In this review, we will focus on the potential of PAF-AH as a modulator of diseases of dysregulated inflammation.

Diagnostic Value of the Serum Platelet-Activating Factor Acetylhydrolase Activity in Inflammatory Bowel Disease

Platelet-activating factor acetylhydrolase (PAF-AH) is an enzyme hydrolyzing platelet-activating factor (PAF), a potent inflammatory mediator, but the relationship between this enzyme and inflammatory bowel disease (IBD) is not fully elucidated. The aim of the present study was to examine the usefulness of the serum PAF-AH activity in order to differentiate ulcerative colitis (UC) from Crohn's disease (CD). The serum PAF-AH activity was measured in 57 patients with IBD (39 UC and 18 CD patients) and 13 control subjects by a spectrophotometric method. The serum PAF-AH activity was thus found to be significantly lower in patients with CD (median 265.5 U/l) than in those with UC (355 U/l) or control subjects (374 U/l). This marker at a cutoff level of 386 U/l demonstrated a sensitivity of 46%, a specificity of 100%, and a positive predictive value of 100% regarding its ability to distinguish UC from CD. Moreover, the marker responded inversely to the changes in the disease activity of IBD. These results suggest that measuring the serum PAF-AH activity is a useful diagnostic modality for making a differential diagnosis between UC and CD.

Components of a platelet-activating factor-signaling loop are assembled in the ovine endometrium late in the estrous cycle

Pulsatile release of uterine prostaglandin F(2alpha) (PGF(2alpha)) induces luteolysis in ruminants. Exogenous PAF is well known to cause PGF(2alpha) release from the ovine uterus. This study examines whether the components of a PAF-signaling loop exist in sheep at the time luteolysis is normally initiated. Day 14 of the cycle was the first day the uterus responded to an infusion of PAF, inducing a significant short-term increase in circulating levels of the PGF(2alpha) metabolite. There was a significant increase of PAF concentration (P < 0.001) in the endometrium and PAF release by tissue explants (P < 0.001) from day 10 to day 16 of the cycle. Endometrial tissue PAF receptor mRNA expression was induced (P < 0.01) by estradiol and progesterone treatment of animals, and transcripts were present between days 10 and 16 of the estrous cycle. Western analysis of endometrial tissue showed marked upregulation of PAF receptor protein expression from day 14 of the cycle, and immunolocalization studies showed that the receptor expression was predominantly around the endometrial glands. PAF:acetylhydrolase was primarily located within the lumen of the endometrial glands. The study shows that a PAF-signaling loop was assembled within the ovine endometrium at the time that PGF(2alpha) pulsatility was first observed.

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

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

Platelet-activating factor acetylhydrolase: is it good or bad for you?

PURPOSE OF REVIEW

Although findings obtained from various studies are inconclusive in determining whether plasma platelet-activating factor acetylhydrolase, or lipoprotein-associated phospholipase A2, plays a proatherogenic or antiatherogenic role in atherosclerosis, many recent reviews appear to favor it as a risk factor for coronary artery disease. To provide a contrasting view, this review focuses on the enzyme's antiatherogenic and antiinflammatory properties.

RECENT FINDINGS

A recent report demonstrates that plasma platelet-activating factor acetylhydrolase activity increases in men and women with stable angina or acute coronary syndromes, supporting previously published data that plasma levels of the protein are independently and positively associated with the risk of coronary artery disease. In contrast, at least four lines of evidence indicate that the enzyme has strong antiatherogenic properties: (1) it inhibits the effects of LDL oxidation, (2) genetic deficiency of plasma levels constitutes a risk factor for vascular diseases including atherosclerosis, (3) adenoviral transfer of the protein reduces atherosclerosis in apolipoprotein E-deficient mice, and (4) pretreatment of an electronegative LDL subfraction isolated from hypercholesterolemic human plasma with a recombinant platelet-activating factor acetylhydrolase completely abolishes the proapoptotic effects of the electronegative LDL on vascular endothelial cells. Additionally, treatment with the recombinant product reduced mortality from severe sepsis in a phase IIb clinical trial. In an animal study, transfection of tumor cells with platelet-activating factor acetylhydrolase inhibited tumor growth at the site of implantation.

SUMMARY

Plasma platelet-activating factor acetylhydrolase becomes progressively activated as atherosclerosis progresses, but lines of evidence indicate that the enzyme possesses potent antiatherogenic and antiinflammatory properties. This raises the question of whether increased activity is a cause or a result of atherosclerosis and, consequently, whether inhibiting the enzyme's activities may decelerate or accelerate the progress of the disease.

Functional impairment of two novel mutations detected in lipoprotein-associated phospholipase A2 (Lp-PLA2) deficiency patients

Plasma lipoprotein-associated phospholipase A2 (Lp-PLA2), also known as platelet-activating factor (PAF) acetylhydrolase (PAF-AH), is a member of the serine-dependent class of A2 phospholipases that hydrolyze sn2-ester bonds of fragmented or oxidized phospholipids at sites where atherosclerotic plaques are forming. Most circulating Lp-PLA2 is bound to low-density lipoprotein (LDL) particles in plasma and the rest to high-density lipoprotein (HDL). Deficiency of Lp-PLA2 is a predisposing factor for cardiovascular diseases in the Japanese population. We describe here two novel mutations of the gene encoding Lp-PLA2, InsA191 and I317N in Japanese subjects. The first patient, with partial Lp-PLA2 deficiency, was heterozygous for the InsA191 mutation; macrophages from this patient secreted only half the normal amount of Lp-PLA2 in vitro. The other patient, who showed complete Lp-PLA2 deficiency, was a compound heterozygote for the novel I317N mutation and a common V279F mutation; macrophages from that patient failed to secrete any Lp-PLA2. Measurement of Lp-PLA2 mass, activity and Western blotting verified impaired production and secretion of the enzyme after transfection of mutant construct into COS-7 cells. These results indicated that both novel mutants, InsA191 and I317N, impair function of the Lp-PLA2 gene.

Platelet-activating factor acetylhydrolase gene polymorphism and its activity in Japanese patients with multiple sclerosis

We evaluated the association of the plasma platelet-activating factor acetylhydrolase (PAF-AH) gene polymorphism (G(994)-->T) and PAF-AH activity with susceptibility and severity of multiple sclerosis (MS) in Japanese. DNA was collected from 216 patients with clinically definite MS (65 opticospinal MS (OS-MS) and 151 conventional MS (C-MS)) and from 213 healthy controls. The missense mutation G(994)-->T that disrupts the PAF-AH activity was determined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). No statistically significant difference in the frequency of genotypes and alleles of the plasma PAF-AH polymorphism was observed among OS-MS patients, C-MS patients and healthy controls. However, the missense mutation tended to be associated with the severity of OS-MS, especially in females (GT/TT genotypes; 51.7% in female rapidly progressive OS-MS vs. 26.6% in female controls, p=0.0870). Moreover, PAF-AH activities were significantly lower in MS than in controls, irrespective of clinical subtypes, among those carrying the identical polymorphism in terms of nucleotide position 994 of the PAF-AH gene. These findings suggest that the PAF-AH gene missense mutation has no relation to either susceptibility or severity of C-MS, yet its activity is down-regulated, and that the mutation has no relation with susceptibility of OS-MS, yet it may confer the severity of female OS-MS.

PAF responsiveness in Japanese subjects with plasma PAF acetylhydrolase deficiency

Approximately 4% of the Japanese population genetically lack plasma platelet activating factor acetylhydrolase (PAF-AH) and show a higher prevalence of thromboembolic disease, but whether they are susceptible to another PAF-related disease, asthma, remains controversial. To determine the role of plasma PAF-AH in airway physiology, we performed PAF bronchoprovocation tests in 8 plasma PAF-AH-deficient subjects and 16 control subjects. Serial inhalation of PAF (1-1000 microg/ml) concentration-dependently induced acute bronchoconstriction, but there was no significant difference between PAF-AH-deficient and control subjects (11.7 +/- 4.6% vs. 9.6 +/- 2.8% decrease in forced expiratory volume in 1 s). Transient neutropenia after single inhalation of PAF (1000 microg/ml) showed no significant difference between the groups either in its magnitude (72 +/- 11% vs. 65 +/- 9% decrease) or duration (4.1 +/- 1.0 vs. 3.3 +/- 0.8 min). In conclusion, a lack of plasma PAF-AH activity alone does not augment physiological responses to PAF in the airway.

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.

Platelet-activating factor inhibits the secretion of platelet-activating factor acetylhydrolase by human decidual macrophages

To clarify the role of platelet-activating factor (PAF) in parturition, the effects of PAF on the secretion of PAF-acetylhydrolase (PAF-AH), a PAF-inactivating enzyme, by decidual macrophage populations were examined. The cells were isolated from human decidual tissue by enzymatic digestion, Ficoll-Paque centrifugation, or flow cytometric sorting. The nonhydrolyzable agonist of PAF, carbamyl-PAF (C-PAF), inhibited the secretion of PAF-AH by either decidual cells or flow cytometrically purified decidual macrophages. A specific PAF receptor antagonist, WEB 2086, blocked the C-PAF-induced inhibition. Lyso-PAF, a metabolite of PAF, had no effect on the enzyme secretion. An intracellular calcium channel blocker, bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester, partially blocked the inhibition by C-PAF, whereas extracellular calcium channel blockers, nifedipine and verapamil, were without effect. The inhibitory effect of C-PAF was also partially blocked by protein kinase C (PKC) inhibitors, sphingosine and H-7. A PKC activator, 12-O-tetradecanoylphobol 13-acetate, decreased the secretion of PAF-AH. The decrease was abolished by the addition of sphingosine and H-7. It is suggested that PAF inhibits the PAF-AH secretion by decidual macrophages and that the inhibitory action is mediated by a signal transduction mechanism involving intracellular calcium and PKC.

Altered distribution of plasma PAF-AH between HDLs and other lipoproteins in hyperlipidemia and diabetes mellitus

Platelet-activating factor acetylhydrolase (PAF-AH) is a phospholipase A2 associated with lipoproteins that hydrolyzes platelet-activating factor (PAF) and oxidized phospholipids. We have developed an ELISA for PAF-AH that is more sensitive than previous methods, and have quantified HDL-associated and non-HDL-associated PAF-AH in healthy, hyperlipidemic, and diabetic subjects. In healthy subjects, plasma total PAF-AH concentration was positively correlated with PAF-AH activity and with plasma total cholesterol, triacylglycerol, LDL cholesterol and apolipoprotein B (apoB) concentrations (all P < 0.01). HDL-associated PAF-AH concentration was correlated positively with plasma apoA-I and HDL cholesterol. Subjects with hyperlipidemia (n = 73) and diabetes mellitus (n = 87) had higher HDL-associated PAF-AH concentrations than did controls (P < 0.01). Non-HDL-associated PAF-AH concentration was lower in diabetic subjects than in controls (P < 0.01). Both hyperlipidemic and diabetic subjects had lower ratios of PAF-AH to apoB (P < 0.01) and higher ratios of PAF-AH to apoA-I (P < 0.01) than did controls. Our results show that the distribution of PAF-AH mass between HDLs and LDLs is determined partly by the concentrations of the lipoproteins and partly by the mass of enzyme per lipoprotein particle, which is disturbed in hyperlipidemia and diabetes mellitus.

Activation of protein kinase C inhibits the secretion of platelet-activating factor acetylhydrolase by human decidual macrophages

Background and Aims:  Platelet activating factor (PAF), a potent phospholipid mediator, has been implicated in a number of reproductive processes through ovulation to parturition. To clarify the regulatory mechanism of PAF metabolism in the decidua, we have investigated the effect of activation of protein kinase C (PKC) on the secretion of PAF-acetylhydrolase (PAF-AH), a PAF-inactivating enzyme, by human decidual macrophages. Methods:  Decidual macrophage populations were isolated from human decidua by using enzymic digestion, Ficoll-Paque centrifugation, or flow cytometric sorting. The cells were treated with a PKC activator (TPA), H-7, dibutyryl cyclic adenosine monophosphate (AMP), Bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra (acetoxymethyl)-ester (BAPTA/AM) and/or nifedipine. The activity of PAF-AH secreted in the culture medium was assayed. Results:  The PKC activator, TPA, inhibited the PAF-AH secretion by decidual cells in a dose-dependent manner. The TPA also decreased the enzyme secretion by flow cytometrically purified macrophages. The inhibitory effect of TPA was blocked by a PKC inhibitor, H-7. Protein kinase A (PKA) activation by dibutyryl cyclic AMP was without effect on the enzyme secretion. Calcium channel blockers, BAPTA/AM and nifedipine had no effect on the PAF-AH secretion. Conclusion:  It is suggested that the TPA-induced inhibition of PAF-AH secretion may be mediated, in part, by a PKC-dependent signal transduction, and that activation of PKC may result in the increase in the local concentration of PAF in the decidua because of its inhibitory effect on the PAF-AH secretion by decidual macrophages. (Reprod Med Biol 2003; 2: 121-126).

Regulation and kinetics of platelet-activating factor and leukotriene C4 synthesis by activated human basophils

BACKGROUND

Allergic disease is the result of an interplay of many different cell types, including basophils and mast cells, in combination with various inflammatory lipid mediators, such as platelet-activating factor (PAF) and leukotrienes (LT). LTC4 synthesis by human basophils has been studied quite extensively. However, not much is known about the synthesis of PAF by human basophils.

OBJECTIVE

In this study, we have made a comprehensive comparison between the kinetics of PAF and LTC4 synthesis, in highly purified basophils, activated with different stimuli or with combinations of stimuli.

METHODS

Synthesis of PAF and LTC4 by human basophils was determined with commercially available assay kits. The basophils were activated with C5a, fMLP, PMA, allergen or anti-IgE, in the absence and presence of IL-3 and/or in combination with elevation of cytosolic free Ca2+ by the sarcoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin.

RESULTS

Most stimuli were found to induce both PAF and LTC4 synthesis. PAF synthesis and LTC4 release were enhanced by preincubation of the basophils with IL-3 or by elevation of cytosolic free Ca2+ by thapsigargin. Incubation of human basophils with IL-3 alone or thapsigargin alone did not result in detectable synthesis of PAF and LTC4, whereas the combination of the two resulted in high amounts of PAF and LTC4 synthesis. Depending on the stimulus used, LTC4 release was 5-100-fold higher than PAF synthesis. In addition, PAF, but not LTC4, was transiently detected, probably due to PAF degradation. LTC4 and PAF synthesis was strongly blocked by inhibitors of cytosolic phospholipase A2, indicating that this enzyme is involved in PAF and LTC4 synthesis by activated human basophils.

CONCLUSION

This study provides a first comprehensive comparison of PAF and LTC4 synthesis in highly purified human basophils, stimulated with a variety of stimuli.

Lipoprotein-associated phospholipase A2 (platelet-activating factor acetylhydrolase) and cardiovascular disease

PURPOSE OF REVIEW

Plasma lipoproteins carry a number of highly active enzymes in the circulation. One of these is lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), also known as platelet-activating factor acetylhydrolase. This review addresses the molecular properties of Lp-PLA(2), the controversy surrounding its role in atherosclerosis and the regulation of its plasma levels in humans.

RECENT FINDINGS

Recent reports indicate that the enzyme Lp-PLA(2) found in both LDL and HDL may be independently regulated in these lipoprotein subclasses and have distinct roles in atherogenesis. Seminal findings establishing the response-to-retention hypothesis of atherosclerosis support further the potentially damaging role that in-situ release of LDL-associated oxidative products by Lp-PLA(2) may have in the formation of arterial wall lesions. In the mouse, where Lp-PLA(2) circulates mainly bound to HDL, overexpression leads to reduced atherosclerosis, raising the possibility that the enzyme in HDL may have a protective role. Further evidence for a potential protective role is seen in studies of partial or complete deficiency of the enzyme. In the more general setting of population studies, however, it is clear that Lp-PLA(2) is a positive risk factor for coronary disease and measurements of its mass may contribute to the prediction of coronary heart disease risk, especially in individuals with low LDL cholesterol levels.

SUMMARY

Lp-PLA(2) is an enzyme with potentially multiple risks in atherosclerosis. In humans the weight of evidence suggests that it is a positive risk factor for coronary heart disease - an observation commensurate with its position in the direct pathological sequence leading from formation of oxidized LDL in the artery wall to cellular dysfunction and formation of lesions.

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications. The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases. The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.

Platelet-activating factor and normal or leukaemic haematopoiesis

Platelet-activating factor (PAF), a phospholipid mediator with a wide range of actions on mature leukocytes, acts directly during early human haematopoiesis by affecting the growth of haematopoietic progenitors and indirectly, by modulating cytokine synthesis by bone marrow stromal cells. At this time, its role during leukaemic diseases remains speculative. The lack of membrane PAF receptor (PAF-R) on leukaemic blasts suggest that this receptor represents a marker of mature cells and its membrane induction a consequence of cell maturation. While the couple PAF/PAF-R has been largely studied using B cell lines, few results are available using B cells of patients with haematopoietic malignancies casting some doubts concerning the potential role (if any) of this molecule during leukaemic diseases.

Augmentation of staphylococcal alpha-toxin signaling by the epidermal platelet-activating factor receptor

Staphylococcal alpha-toxin is a cytolytic toxin secreted by many strains of Staphylococcus aureus that has proinflammatory and cytotoxic effects on human keratinocytes. alpha-toxin exerts its effects by forming a transmembrane pore that behaves like an ionophore for ions such as calcium. Because cellular membrane disruption with resultant intracellular calcium mobilization is a potent stimulus for the synthesis for the lipid mediator platelet-activating factor, the ability of alpha-toxin to induce platelet-activating factor production was assessed, and whether the epidermal platelet-activating factor receptor could augment toxin-induced signaling in epithelial cells examined. Treatment of the human keratinocyte-derived cell line HaCaT with alpha-toxin resulted in significant levels of platelet-activating factor, which were approximately 50% of the levels induced by calcium ionophore A23187. alpha-toxin also stimulated arachidonic acid release in HaCaT keratinocytes. Pretreatment of HaCaT cells with platelet-activating factor receptor antagonists, or overexpression of the platelet-activating factor metabolizing enzyme acetylhydrolase II blunted alpha-toxin-induced arachidonic acid release by approximately one-third, suggesting a role for toxin-produced platelet-activating factor in this process. Finally, retroviral-mediated expression of the platelet-activating factor receptor into the platelet-activating factor receptor-negative epithelial cell line KB resulted in an augmentation of alpha-toxin-mediated intracellular calcium mobilization and arachidonic acid release. These studies suggest that alpha-toxin-mediated signaling can be augmented via the epidermal platelet-activating factor receptor.

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.

Association of plasma PAF acetylhydrolase gene polymorphism with IMT of carotid arteries in Japanese type 2 diabetic patients

The aim of this study was to investigate association of a missense mutation in plasma PAF acetylhydrolase (G994T) with intima media thickness (IMT) of the carotid arteries. One hundred and forty Japanese type 2 diabetic patients aged from 40 to 79 years without severe nephropathy were enrolled in this study. The genotype of the patients was determined by allele specific PCR. IMT of the carotid arteries of the subjects was recorded by B-mode ultrasound imaging. The patients were divided into two groups by genotyping, one carrying two wild alleles (wild group), and another carrying one or two mutant alleles (mutant group). Each group was further divided into two subgroups according to age; one subgroup consisted of 40s or 50s, and another consisted of 60s or 70s. The prevalence of the G994T mutation in the subjects was 28.6% (24.3% heterozygote, and 4.3% homozygote). IMT of the elderly patients of the mutant group was significantly greater (0.98 +/- 0.22 mm, n = 26) than of the elderly patients of the wild group (0.87 +/- 0.20 mm, n = 50, P = 0.0292). There was no significant difference in clinical characteristics between the two subgroups. The results of this study indicate that the missense mutation in plasma PAF acetylhydrolase is associated with development of atherosclerosis in the elderly.

Identification of platelet-activating factor acetylhydrolase II in human skin

Platelet-activating factor acetylhydrolases are a family of specialized phospholipase A2 enzymes. They serve an anti-inflammatory function by converting the proinflammatory autocoid, PAF, into biologically inactive lyso-PAF, by the removal of the sn-2 acetyl group of this glycerophospholipid. Similarly, platelet-activating factor acetylhydrolases can also degrade oxidatively modified sn-2 polyunsaturated-fatty-acid-containing phospholipids, which are toxic to cells. Platelet-activating factor acetylhydrolase II is a recently cloned member of this family of specialized phospholipases. Consistent with a potential role of this intracellular enzyme in protecting membrane phospholipids against oxidative stress, platelet-activating factor acetylhydrolase II has been shown to translocate from cytosol to membranes in response to pro-oxidative stressors, and overexpression of this enzyme decreases the cytotoxic effects of these agents. The objective of this study was to assess whether platelet-activating factor acetylhydrolase II is involved in protecting skin against oxidative stress. Platelet-activating factor acetylhydrolase II protein was demonstrated in human skin by immunohistochemistry, with the highest levels of the enzyme found in sebaceous glands and lesser amounts in epidermal keratinocytes. Treatment of epidermal cells with t-butylhydroperoxide or ultraviolet B radiation resulted in platelet-activating factor acetylhydrolase II translocation from cytosol to membranes. To assess the role of this enzyme in epidermal function, a recombinant retroviral strategy was used to overexpress platelet-activating factor acetylhydrolase II in the human keratinocyte-derived cell line HaCaT. Overexpression of platelet-activating factor acetylhydrolase II protected HaCaT cells against apop tosis induced by oxidative stressors t-butylhydroperoxide and ultraviolet B radiation. Similar levels of apoptosis, however, were seen in both control and platelet-activating-factor-acetylhydrolase-II-over expressing HaCaT cells in response to C2 ceramide. These studies demonstrate the presence of platelet-activating factor acetylhydrolase II in a restricted pattern in human skin, and provide evidence that this specialized phospholipase is involved in protecting this organ against oxidative stress through the degradation of oxidatively modified bioactive phospholipids.

Platelet-activating factor receptor

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was named for its potential to induce platelet aggregation, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor, and activates multiple intracellular signaling pathways. In the last decade, we have identified the PAF receptor structures, intracellular signaling mechanisms, and genomic organizations. Recently, we found a single nucleotide polymorphism of the human PAF receptor (A224D) with an allele frequency of 7.8% in Japanese. Cells expressing this receptor exhibited the reduced cellular signaling, although the binding parameters remain unchanged. We have established two different types of genetically altered mice, i.e. PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice provide a novel and specific approach for identifying the pathophysiological and physiological functions of PAF in vivo. This review focuses on phenotypes of these mutant mice and summarizes the previous reports regarding PAF and PAF receptor.

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.

Single nucleotide polymorphism (G994-->T) in the plasma platelet-activating factor-acetylhydrolase gene is associated with graft patency of femoropopliteal bypass

BACKGROUND

Plasma platelet-activating factor-acetylhydrolase (PAF-AH) is known to catalyze platelet-activating factor (PAF). The single nucleotide polymorphism (SNP) of plasma PAF-AH gene (G994 -->T in exon 9) is associated with a decreased level of plasma PAF-AH activity. This study analyzed the risk of the SNP on graft occlusion of femoropopliteal bypass in patients with atherosclerotic occlusive disease.

METHODS

We retrospectively assessed the patency of 50 above-knee femoropopliteal bypass grafting in 50 patients. Genomic DNA was analyzed for the mutant allele. Plasma PAF-AH activity was measured by radioimmunoassay.

RESULTS

The 10-year cumulative primary patency of the bypass was 78.5% in GG (normal genotype) and 50.0% in GT (heterozygous) or TT (homozygous deficient) (P <.05, Kaplan-Meier method). The relative risk of graft failure in GT or TT genotypes was 1.68 (P =.08, Cox proportional hazards model). PAF-AH activity (nmol/min/50 microL) was 1.92 +/- 0.82 in patients with patent grafts and 1.42 +/- 0.47 in those with occluded grafts (mean +/- standard deviation; P <.05, unpaired t test).

CONCLUSIONS

The SNP of plasma PAF-AH was associated with a decreased primary graft patency of above-knee femoropopliteal bypass. The risk of graft failure may increase when patients have the SNP. To confirm the independent risk of graft failure by the SNP, further study is necessary and prospective study should be performed.

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.

Sol Sherry lecture in thrombosis: molecular events in acute inflammation

The inflammatory response is characterized by a multistep molecular interaction between "signaling" cells, such as endothelial cells, and "responding" cells, such as neutrophils and monocytes. In the first step, selectins produced by signaling cells mediate the tethering of responding cells at sites of inflammation. Subsequently, an additional mediator expressed by signaling cells activates the tethered responding cells. Under pathological conditions, the same mechanism is invoked in inappropriate ways: (1) by prolonged presentation of selectins on the cell surface and (2) by the unregulated production of oxidized phospholipids that mimic the normal secondary signaling molecule, platelet-activating factor (PAF). The enzyme PAF acetylhydrolase (PAF-AH) inactivates PAF and oxidized phospholipids and constitutes an "off" switch that suppresses inflammation. Inhibition of normal PAF-AH function or inactivating mutations of the PAF-AH gene can lead to increased susceptibility to inflammatory disease. These studies have relevance to atherosclerosis and thrombosis, because inflammation is a central feature of both.

Receptor-dependent metabolism of platelet-activating factor in murine macrophages

Degradation of platelet-activating factor (PAF) was examined by incubating PAF with macrophages from PAF receptor-deficient mice. The degradation rate was halved as compared with wild-type mice. The reduction of the rate was comparable with the presence of a PAF antagonist WEB 2086 in wild-type cells. PAF was internalized rapidly (t(12) approximately 1 min) into wild-type macrophages. The PAF internalization was inhibited by the treatment of 0.45 m sucrose but was not affected by phorbol 12-myristate 13-acetate, suggesting that PAF internalizes into macrophages with its receptor in a clathrin-dependent manner. Internalized PAF was degraded into lyso-PAF with a half-life of 20 min. Treatment of concanavalin A inhibited the conversion of PAF into lyso-PAF, suggesting that uptake of PAF enhances PAF degradation. Lyso-PAF was subsequently metabolized into 1-alkyl-2-acyl-phosphatidylcholine. In addition, release of PAF acetylhydrolase from macrophages was enhanced when wild-type macrophages were stimulated with PAF but not from macrophages of PAF receptor-deficient mice. Thus, the PAF stimulation of macrophages leads to its degradation through both intracellular and extracellular mechanisms.

Association of a G994 -->T missense mutation in the plasma platelet-activating factor acetylhydrolase gene with risk of abdominal aortic aneurysm in Japanese

OBJECTIVE

To investigate a possible association with plasma platelet activating factor acetylhydrolase (PAF-AH) gene mutation with the risk of abdominal aortic aneurysm (AAA).

SUMMARY BACKGROUND DATA

Plasma platelet activating factor acetylhydrolase is known to catalyze platelet activating factor (PAF), thereby inactivating its inflammatory function. Deficiency of this enzyme is caused by a missense mutation (G994 -->T) in exon 9 of the plasma PAF-AH gene.

METHODS

We did a case-control study including 131 patients (median age 73.4 [range 50-84] years) and 106 controls matched for age and sex. Genomic DNA was analyzed for the mutant allele by a specific polymerase-chain reaction. Plasma PAF-AH activity was measured in both groups.

RESULTS

The frequency of the mutant allele (T allele) in the plasma PAF-AH gene in AAA patients was significantly higher than in control subjects. The association of the missense mutation with AAA was statistically significant and independent of other risk factors. Among AAA patients with normal genomic type, plasma PAF-AH activity was strongly correlated to the plasma concentration of low density lipoprotein cholesterol (LDL-C), while the correlation was not observed among AAA patients with heterozygotes genotype. Patients having AAA with both T allele and hyperlipidemia were more likely to have other atherosclerotic diseases such as ischemic heart disease, stroke and peripheral arterial occlusive diseases than patients with the normal genomic type and normal lipid level.

CONCLUSIONS

The genetic mutation of plasma PAF-AH gene appear to be an independent risk factor for AAA. Our findings need to be confirmed in a larger, prospective study including patients from different populations.

Airway hyperresponsiveness in transgenic mice overexpressing platelet activating factor receptor is mediated by an atropine-sensitive pathway

Platelet activating factor (PAF) is a potent mediator potentially involved in the pathogenesis of inflammatory disorders, including bronchial asthma. Recently, transgenic mice overexpressing the PAF receptor (PAFR) gene have been established, and exhibit bronchial hyperresponsiveness, one of the cardinal features of asthma. To elucidate the molecular and pathophysiologic mechanisms underlying PAF-associated bronchial hyperreactivity, we studied airway responsiveness to methacholine (MCh) and serotonin (5-hydroxytryptamine; 5-HT) in PAFR-transgenic mice. In addition, we examined the role of the muscarinic receptor in PAF-induced responses and the binding activities of the muscarinic receptor. The PAFR-transgenic mice exhibited hyperresponsiveness to MCh and PAF; however, no significant differences in 5-HT responsiveness were observed between the control and PAFR-transgenic mice. The administration of atropine significantly blocked PAF-induced responses in PAFR-transgenic mice. There were no differences between the two phenotypes in the binding activities of muscarinic receptor. Morphometric analyses demonstrated that PAFR overexpression did not affect airway structure. These findings suggest that the muscarinic pathway may have a key role in airway hyperresponsiveness associated with PAFR gene overexpression. More generally, PAFR-transgenic mice may provide appropriate models for study of the molecular mechanisms underlying PAF-associated diseases.

Single nucleotide polymorphism of human platelet-activating factor receptor impairs G-protein activation

Various proinflammatory and vasoactive actions of platelet-activating factor (PAF) are mediated through a specific G-protein-coupled PAF receptor (PAFR). We identified a novel DNA variant in the human PAFR gene, which substitutes an aspartic acid for an alanine residue at position 224 (A224D) in the putative third cytoplasmic loop. This mutation was observed in a Japanese population at an allele frequency of 7.8%. To delineate the functional consequences of this structural alteration, Chinese hamster ovary cells were stably transfected with constructs encoding either wild-type or A224D mutated PAFR. No significant difference was observed in the expression level of the receptor or the affinity to PAF or to an antagonist, WEB2086, between the cells transfected with wild-type and mutant PAFR. Chinese hamster ovary cells expressing A224D mutant PAFR displayed partial but significant reduction of PAF-induced intracellular signals such as calcium mobilization, inositol phosphate production, inhibition of adenylyl cyclase, and chemotaxis. These findings suggest that this variant receptor produced by a naturally occurring mutation exhibits impaired coupling to G-proteins and may be a basis for interindividual variation in PAF-related physiological responses, disease predisposition or phenotypes, and drug responsiveness.

Intraleucocyte platelet-activating factor levels in desmopressin-treated patients with haemophilia A and von Willebrand disease

Despite the intensive clinical use of 1-deamino-8-D-arginine vasopressin (desmopressin; DDAVP) for 20 years, its mechanism of action is still not completely explained. It has been proposed that DDAVP stimulates release of a 'second messenger' which in turn stimulates release of von Willebrand factor (vWF) from endothelial cells. Platelet-activating factor (PAF) and interleukin (IL)-6 were individually proposed to be mediators for haemostatic action. The aim of this study was to investigate cellular-based PAF levels in patients with haemophilia A (HA) and von Willebrand disease (vWD) before and after DDAVP treatment and also to look for any probable relationship between the haemostatic response of DDAVP and cellular PAF activities. In total, 20 patients (11 HA and nine vWD) were enrolled in the study. DDAVP was given subcutaneously as a single dose (0.3 microg kg(-1)). Ten patients responded to DDAVP and were defined as the 'able group' (four mild HA, six type 1 vWD). The remaining 10 patients did not respond to DDAVP and were defined as the 'unable group' (seven severe HA, three type 3 vWD). Released (extracellular) and intracellular (intraleucocyte) PAF levels under the stimulation of specific agents (A23187 and Zymosan) were measured by high-performance liquid chromatography and radioimmunoassay. Extracellular and intracellular PAF activities were not detected without stimulation in healthy children whereas significantly higher PAF levels were found in the patients (extracellular: 37.5 +/- 34.4 ng per 10(7) cells; intracellular: 24.8 +/- 23.5 ng per 10(7) cells; P=0.0001). Intracellular PAF levels obtained from in vitro unstimulated cells were significantly higher in DDAVP-responsive (able) patients in comparison to DDAVP-unresponsive (unable) patients (52.1 +/- 18.5 vs. 28.9 +/- 8.0 ng per 10(7)cells). After in vitro stimulation by A23187, intracellular PAF activities were significantly higher in patients than in controls (209.3 +/- 26.1 vs. 172 +/- 18.1 ng per 10(7) cells). Intracellular PAF levels obtained from in vitro stimulated cells by A23187 were also significantly higher in the 'able' patients in comparison to the 'unable' patients (277 +/- 43.5 vs. 225 +/- 30 ng per 10(7)cells). In conclusion, cellular PAF activities are significantly higher in patients with HA and vWD. We also suggest that PAF, especially intracellular PAF mediates intracellular signalling and may be one of the important mediators for the haemostatic response of DDAVP.