The degradation of platelet-activating factor in serum and its discriminative value in atherosclerotic patients

Modulation of oxidative stress, inflammation, and atherosclerosis by lipoprotein-associated phospholipase A2

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), also known as platelet-activating factor acetylhydrolase (PAF-AH), is a unique member of the phospholipase A(2) superfamily. This enzyme is characterized by its ability to specifically hydrolyze PAF as well as glycerophospholipids containing short, truncated, and/or oxidized fatty acyl groups at the sn-2 position of the glycerol backbone. In humans, Lp-PLA(2) circulates in active form as a complex with low- and high-density lipoproteins. Clinical studies have reported that plasma Lp-PLA(2) activity and mass are strongly associated with atherogenic lipids and vascular risk. These observations led to the hypothesis that Lp-PLA(2) activity and/or mass levels could be used as biomarkers of cardiovascular disease and that inhibition of the activity could offer an attractive therapeutic strategy. Darapladib, a compound that inhibits Lp-PLA(2) activity, is anti-atherogenic in mice and other animals, and it decreases atherosclerotic plaque expansion in humans. However, disagreement continues to exist regarding the validity of Lp-PLA(2) as an independent marker of atherosclerosis and a scientifically justified target for intervention. Circulating Lp-PLA(2) mass and activity are associated with vascular risk, but the strength of the association is reduced after adjustment for basal concentrations of the lipoprotein carriers with which the enzyme associates. Genetic studies in humans harboring an inactivating mutation at this locus indicate that loss of Lp-PLA(2) function is a risk factor for inflammatory and vascular conditions in Japanese cohorts. Consistently, overexpression of Lp-PLA(2) has anti-inflammatory and anti-atherogenic properties in animal models. This thematic review critically discusses results from laboratory and animal studies, analyzes genetic evidence, reviews clinical work demonstrating associations between Lp-PLA(2) and vascular disease, and summarizes results from animal and human clinical trials in which administration of darapladib was tested as a strategy for the management of atherosclerosis.

Smoking and atherosclerotic cardiovascular disease: Part I: atherosclerotic disease process

The normal endothelium inhibits platelet and leukocyte adhesion to the vascular surface maintaining a balance of profibrinolytic and prothrombotic activity. Endothelial function is assessed largely as endothelium-dependent vasomotion, partly based on the assumption that impaired endothelium-dependent vasodilation reflects the alteration of important endothelial functions. Atherosclerotic risk factors, such as hypercholesterolemia, hypertension, diabetes and smoking, are associated with endothelial dysfunction. In the diseased endothelium, the balance between pro- and antithrombotic, pro- and anti-inflammatory, pro- and antiadhesive or pro- and antioxidant effects shifts towards a proinflammatory, prothrombotic, pro-oxidative and proadhesive phenotype of the endothelium. A common mechanism underlying endothelial dysfunction is related to the increased vascular production of reactive oxygen species. Recent studies suggest that inflammation per se, and C-reactive protein in particular, may contribute directly to endothelial dysfunction. The loss of endothelial integrity is a hallmark of atherosclerosis and the causal possible link between each individual risk factor, the development of atherosclerosis and the subsequent clinical events, such as myocardial infarction or stroke.

Lipoprotein-associated phospholipase A2: a new biomarker for cardiovascular risk assessment and potential therapeutic target

Lipoprotein-associated phospholipase (Lp-PL)A2 is a recently described and potentially useful plasma biomarker associated with cardiovascular disease. The enzyme, originally named platelet-activating factor acetylhydrolase (PAF-AH), has two prominent biological activities. First, it inactivates the prominent proinflammatory mediator PAF-AH. Second, Lp-PLA2 hydrolyzes oxidatively modified polyunsaturated fatty acids producing lysophosphatidylcholine (LysoPC) and oxidized nonesterified fatty acids (OxNEFA). OxNEFA have potent monocyte chemotactic activity and LysoPC upregulates inflammatory mediators, including cytokines, adhesion molecules and the chemotactic mediator MCP-1. Whereas the first activity may be considered antiatherogenic, the prevailing consensus is that Lp-PLA2 is positively associated with coronary disease. Initial evidence for this came largely from the West of Scotland Coronary Prevention Study Group (WOSCOPS) in which Lp-PLA2 was compared among 580 cases and 1160 age-matched controls. In addition, the quantitative contribution of Lp-PLA2 to risk assessment was assessed in a substudy of the Atherosclerosis Risk in Communities (ARIC) study. Although positively correlated with disease, the addition of Lp-PLA2 did not appreciably enhance risk prediction beyond the model employing traditional risk factors. Thus, population screening for subclinical disease using Lp-PLA2 does not appear to be warranted. Presently, the most useful application of Lp-PLA2 testing is to adjust individual risk assessment for those patients found to be at borderline risk using traditional models. In this regard, the marker appears to be particularly useful for gauging risk among patients with metabolic syndrome or diabetes. There is observational evidence that Lp-PLA2 may be a useful guide for therapeutic efficacy, but prospective evaluation will be required. Considering the large number of biomarkers currently under evaluation, it is probable that useful additions to existing risk models may be found in combinatorial models.

Platelet-Activating Factor Acetylhydrolase Is Not Associated with Carotid Intima-Media Thickness in Hypercholesterolemic Sicilian Individuals

Background: Atherosclerosis is a complex, chronic disease that usually arises from the converging action of several pathogenic processes, including hypertension, hyperlipidemia, obesity, and the accumulation of oxidized LDL. Platelet-activating factor acetylhydrolase (PAF-AH) is a LDL- and HDL-bound enzyme that hydrolyzes and inactivates PAF and prevents LDL-cholesterol oxidation, thus delaying the onset of atherosclerotic disease.

Methods: We evaluated the relationship between variants of the PAF-AH gene polymorphisms Arg92His, Ile198Thr, and Ala379Val and the presence of carotid atherosclerosis in 190 hypercholesterolemic Sicilian individuals. Carotid artery intima-media wall thickness (IMT) was measured as an indicator of early atherosclerotic disease. The participants were classified according to having normal (≤1 mm) or abnormal (≥1 mm) IMT and were also investigated for physical characteristics and biochemical indices, including PAF-AH activity.

Results: PAF-AH activity and LDL concentrations were significantly correlated in hypercholesterolemic patients, but plasma PAF-AH activity and HDL were not significantly correlated in either IMT group. No significant differences were detected among the PAF-AH gene polymorphisms in both groups after correction for age, sex, body mass index, plasma glucose and lipid concentrations, PAF-AH activity, blood pressure, and smoking habits. The analysis of PAF-AH genotype distribution showed no significant differences in percentage of 92, 198, and 379 genotypes in both IMT groups.

Conclusion: Our data provided no evidence that PAF-AH polymorphisms influence PAF-AH activity and atherosclerosis in hypercholesterolemic Sicilian patients.

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.

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.

Lp-PLA2: an emerging biomarker of coronary heart disease

Cardiovascular disease is the leading cause of death in most industrialized countries. However, the diagnosis and management of coronary heart disease is far from optimal. Lipoprotein-associated phospholipase A2 (Lp-PLA2), also known as platelet-activating factor acetylhydrolase, is an enzyme that hydrolyses oxidized phospholipids and is primarily associated with low-density lipoprotein. Discussed in this review is the accumulating evidence supporting the view that Lp-PLA2 is a potential biomarker of coronary heart disease and plays and an important proinflammatory role in the progression of atherosclerosis. A new ELISA method for the quantitative measurement of Lp-PLA2 mass in human plasma developed by diaDexus, Inc. is presented. Furthermore, potential clinical applications of Lp-PLA2 mass measurements are proposed.

Plasma platelet-activating factor acetylhydrolase deficiency is associated with atherosclerotic occlusive disease in japan

PURPOSE

Plasma platelet-activating factor acetylhydrolase (PAF-AH) is known to catalyze platelet-activating factor, thereby inactivating its inflammatory function. Deficiency of this enzyme is caused by a missense (G(994)-->T) in exon 9 of the plasma PAF-AH gene. In this study, we investigated a possible association of this mutation with the risk of atherosclerotic occlusive disease (AO) in Japanese patients.

METHODS

We studied 104 patients with AO. The control group consisted of 114 subjects matched for age and sex. Plasma PAF-AH activity was measured in the patients with AO.

RESULTS

The prevalence of the mutant genotype (GT + TT ) was significantly more frequent in patients with AO than in control subjects (36.5% vs 23.7%; P <.05). Among the patients with AO, those with the mutant allele had significantly more risk factors of prior stroke or ischemic heart disease than patients with normal genotypes. Plasma PAF-AH activity was higher in patients with AO than in control subjects in normal genotype subgroups.

CONCLUSIONS

The missense (G(994)-->T) in exon 9 of the plasma PAF-AH gene is associated with AO in Japanese people.

Lipoprotein-associated phospholipase A(2), platelet-activating factor acetylhydrolase: a potential new risk factor for coronary artery disease

A specific and robust immunoassay for the lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), platelet-activating factor acetylhydrolase, is described for the first time. The immunoassay was used to evaluate possible links between plasma Lp-PLA(2) levels and atherosclerosis risk amongst susceptible individuals. Such an investigation was important because Lp-PLA(2) participates in the oxidative modification of low density lipoprotein by cleaving oxidised phosphatidylcholines, generating lysophosphatidylcholine and oxidised free fatty acids. The majority of Lp-PLA(2) was found associated with LDL (approximately 80%) and, as expected, enzyme levels were significantly positively correlated to LDL cholesterol. Plasma Lp-PLA(2) levels were significantly elevated in patients with angiographically proven coronary artery disease (CAD) when compared with age-matched controls, even though LDL cholesterol levels did not differ significantly. Indeed, when included in a general linear model with LDL cholesterol and other risk factors, Lp-PLA(2) appeared to be an independent predictor of disease status. We propose, therefore, that plasma Lp-PLA(2) mass should be viewed as a potential novel risk factor for CAD that provides information related to but additional to traditional lipoprotein measurements.

Spectrophotometric assay for serum platelet-activating factor acetylhydrolase activity

We developed a spectrophotometric assay for serum platelet-activating factor acetylhydrolase (PAF-AH, EC 3.1.1.47.) activity using a platelet-activating factor (PAF) analogue with a 4-nitrophenyl group as substrate. PAF-AH hydrolyzes the sn-2 position of the substrate ¿1-myristoyl-2-(p-nitrophenylsuccinyl)phosphatidylcholine, producing p-nitrophenyl succinate. This liberation was spectrophotometrically monitored and the activity determined from the change in absorption. The assay does not require radioisotopes and is applicable to an automatic analyzer. Utilizing this assay with an automatic analyzer, it is possible to measure the activities of thousands of samples in a few hours with excellent precision (CV 0.5%, n=30) and high correlation (r=0.979, n=100) with the results of a conventional radioisotopic assay. The assay should be particularly useful for clinical diagnostics.

Correlations between plasma platelet-activating factor acetylhydrolase (PAF-AH) activity and PAF-AH genotype, age, and atherosclerosis in a Japanese population

Platelet-activating factor acetylhydrolase (PAF-AH), a plasma enzyme that hydrolyzes PAF and oxidized phospholipids, is thought to be involved in protecting cells against oxidative stress. A G(994) (M allele)-->T (m allele) mutation in the plasma PAF-AH gene, which results in a Val(279)-->Phe substitution in the mature protein, leads to a loss of catalytic activity. To elucidate the relationships among PAF-AH enzyme activity, genotype, age, and atherosclerosis, we assayed these parameters in a large Japanese population (n=3932) that consisted of three groups; a control group (healthy individuals; n=1684), a risk-factor group (individuals having at least one conventional risk factor for atherosclerosis; n=1398), and a diseased group (patients who had suffered a myocardial infarction or stroke; n=850). We observed a significantly increased frequency of the m allele in the diseased group as compared with the control or risk-factor groups. Plasma PAF-AH activity increased significantly with age in women in the control group with the MM and Mm genotypes, and in men in the control group with the MM genotype, but not in men with the Mm genotype. In both the risk-factor and diseased groups, however, no correlation was observed between plasma PAF-AH activity and age in subjects with either genotype. These results suggest that in individuals with the MM genotype, plasma PAF-AH activity may be increased in response to stresses induced by PAF and/or oxidized phospholipids that might accumulate with age, but that this response is not evident or reduced in healthy individuals with the m allele, or in subjects with atherosclerotic disease, or having risk factors. Together with our previous findings, the G(994)-->T mutation in the PAF-AH gene may be one of the genetic determinants for atherosclerotic disease in the Japanese population.

Plasma platelet-activating factor acetylhydrolase activity in human immunodeficiency virus infection and the acquired immunodeficiency syndrome

Platelet-activating factor (PAF) acetylhydrolase (PAF-AH) catalyzes the hydrolysis of PAF, a mediator of inflammation, as well as other biologically active oxidized phospholipids. In humans, plasma PAF-AH activity is bound to low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Higher levels of plasma PAF-AH activity have been found in a variety of diseases, and are thought to be a defense mechanism against the toxic effects of PAF and oxidized phospholipids. We studied plasma PAF-AH activity in patients with human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS), a disease characterized by chronic HIV infection and a systemic host response. Plasma PAF-AH activity was significantly greater in AIDS patients compared with control subjects (25.2 +/- 2.0 v 17.0 +/- 0.8 nmol/min/mL, P < .001). The higher levels of plasma PAF-AH activity were found in LDL (28.2 +/- 2.2 v 18.3 +/- 1.0 nmol/min/mL for AIDS v controls, respectively, P = .0005), but not in HDL. Plasma PAF-AH activity in AIDS correlated with circulating interferon alfa (r = .575, P = .005) and plasma triglycerides (r = .556, P < .0025). The presence of secondary infection in AIDS did not significantly change plasma PAF-AH activity. The initiation of a new antiretroviral regimen with either a protease inhibitor or the nucleoside analog lamivudine did not significantly decrease plasma PAF-AH activity, despite successful suppression of HIV RNA levels. Plasma PAF-AH activity may be a sensitive marker of the host response to infection, and the higher levels of plasma and LDL-associated PAF-AH activity in patients with HIV infection and AIDS may be a physiological response to protect the host against oxidative injury from PAF and oxidized phospholipids.

Regulation of platelet-activating factor (PAF) activity in human diseases by phospholipase A2 inhibitors, PAF acetylhydrolases, PAF receptor antagonists and free radical scavengers

The aim of this review is to present recent findings indicating the likely involvement of platelet-activating factor (PAF) in human diseases, and possible ways of alleviating its harmful effects. PAF is a potent proinflammatory mediator and promotes adhesive interactions between leukocytes and endothelial cells, leading to transendothelial migration of leukocytes, by a process of juxtacrine intercellular signalling. This process leads to activation of leukocytes and the release of reactive oxygen radicals, lipid mediators, cytokines and enzymes. These reaction products subsequently contribute to the pathological features of various inflammatory diseases. The reactive oxygen radicals cause low density lipoprotein (LDL) oxidation which mediates the development of atherosclerosis. Oxidized LDL may damage cellular and subcellular membranes, leading to tissue injury and cell death. Among the therapeutic approaches considered are agents that inhibit/degrade proinflammatory mediators and thereby have anti-inflammatory and/or anti-atherogenic potential. These include inhibitors of phospholipase A2 activity, PAF-acetylhydrolases, PAF antagonists and free radical scavengers/antioxidants, the latter protecting against oxidized LDL-induced cytotoxicity.

Plasma platelet-activating factor acetylhydrolase activity is not associated with premature coronary atherosclerosis

Platelet-activating factor acetylhydrolase activity in plasma was compared between 72 subjects with angiographically normal coronary arteries and matched controls with clinically significant obstruction. No difference was seen, and we conclude that variation in plasma platelet-activating factor acetylhydrolase activity is not a risk factor for coronary artery disease.

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.

Identification of the G994--> T missense in exon 9 of the plasma platelet-activating factor acetylhydrolase gene as an independent risk factor for coronary artery disease in Japanese men

Platelet-activating factor (PAF) acetylhydrolase may play important roles in the pathophysiology of thrombosis and atherosclerosis related to its catalytic action in the degradation of PAF and oxidized phospholipids. A missense mutation (G--> T transversion at nucleotide 994) in the plasma PAF acetylhydrolase gene results in a Val--> Phe substitution at amino acid 279 of the mature protein and a consequent loss of catalytic activity. However, the role of a deficiency or low activity of this enzyme caused by the missense mutation in the etiology of coronary artery disease (CAD) has not been determined. The relation between this mutation and the incidence of CAD in the Japanese population is investigated herein. The genotype of plasma PAF acetylhydrolase (MM, normal; Mm, heterozygote; and mm, deficient homozygote) was determined with a polymerase chain reaction (PCR) assay for 454 patients with myocardial infarction (MI) and 602 control subjects. The frequency of the m allele was significantly higher in male patients with MI (odds ratio, 1.8) than in controls, an association that was more marked in a low-risk subgroup (odds ratio, 2.3). In contrast, the m allele was not associated with MI in women. These results indicate that the G994--> T missense mutation in exon 9 of the plasma PAF acetylhydrolase gene is an independent risk factor for CAD in Japanese men, especially low-risk individuals, but not in women.

Effect of platelet activating factor antagonists in different models of thrombosis

Effect of three specific PAF antagonists, SR-27417, BN-50739 and ginkgolide derivative BN-52021 have been evaluated in models of thrombosis in the mouse, rat and cat. Thrombosis in the mouse was induced by intravenous infusion of collagen and adrenaline. In rats it was induced by inserting a metallic wire into the inferior vena cava. In the cat, thrombus formation was assessed in the extracorporeal shunt. All the antagonists offered a dose-dependent protection against pulmonary thromboembolism in mice (1, 3 and 10 mg/kg) and the thrombosis monitored in the extracorporeal shunt in cats (0.3, 1 and 3 mg/kg). In rats, no significant protection was observed with these antagonists even at the highest dose used.

Cholesterol-rich LDL perfused at physiological LDL-cholesterol concentration induces platelet aggregation and PAF-acetylhydrolase activation

The aim of this research was to perform an in vivo study on the relationships between lipid oxide (LP), platelet aggregation and PAF-acetylhydrolase in a model using perfusion of cholesterol-rich LDL media diluted to physiological LDL-cholesterol concentration. Normal rabbits were infused with LDL (d 1.025-1.063 g/ml) extracted from rabbits previously fed either with standard food (I-LDL group), 1% cholesterol food (II-LDL group) or 1% cholesterol plus probucol (IV-LDL group). CU2+ modified II-LDL was also infused (III-LDL group). After dilution as above, LP increased significantly in III- and II-LDL media. After perfusion, LP significantly increased in III- and II-LDL groups as compared to baseline values and to control. Compared to the I-LDL group, PAF-acetylhydrolase and platelet aggregation significantly increased in III- and II-LDL groups. These data indicate the property of cholesterol- rich LDL to activate PAF-acetylhydrolase and enhance platelet aggregation, even when perfused through a medium containing a physiological LDL-cholesterol concentration.