Identification of a domain that mediates association of platelet-activating factor acetylhydrolase with high density lipoprotein

Research Advance of Chinese Medicine in Treating Atherosclerosis: Focus on Lipoprotein-Associated Phospholipase A2

As a serious cardiovascular disease, atherosclerosis (AS) causes chronic inflammation and oxidative stress in the body and poses a threat to human health. Lipoprotein-associated phospholipase A2 (Lp-PLA2) is a member of the phospholipase A2 (PLA2) family, and its elevated levels have been shown to contribute to AS. Lp-PLA2 is closely related to a variety of lipoproteins, and its role in promoting inflammatory responses and oxidative stress in AS is mainly achieved by hydrolyzing oxidized phosphatidylcholine (oxPC) to produce lysophosphatidylcholine (lysoPC). Moreover, macrophage apoptosis within plaque is promoted by localized Lp-PLA2 which also promotes plaque instability. This paper reviews those researches of Chinese medicine in treating AS via reducing Lp-PLA2 levels to guide future experimental studies and clinical applications related to AS.

Lp-PLA2 (Lipoprotein-Associated Phospholipase A2) Deficiency Lowers Cholesterol Levels and Protects Against Atherosclerosis in Rabbits

BACKGROUND

Elevated plasma Lp-PLA₂ (lipoprotein-associated phospholipase A₂) activity is closely associated with an increased risk of cardiovascular events. However, whether and how Lp-PLA₂ is directly involved in the pathogenesis of atherosclerosis is still unclear. To examine the hypothesis that Lp-PLA₂ could be a potential preventative target of atherosclerosis, we generated Lp-PLA₂ knockout rabbits and investigated the pathophysiological functions of Lp-PLA₂.

METHODS

Lp-PLA₂ knockout rabbits were generated using CRISPR/Cas9 system to assess the role of Lp-PLA₂ in plasma lipids regulation and identify its underlying molecular mechanisms. Homozygous knockout rabbits along with wild-type rabbits were fed a cholesterol-rich diet for up to 14 weeks and their atherosclerotic lesions were compared. Moreover, the effects of Lp-PLA₂ deficiency on the key cellular behaviors in atherosclerosis were assessed in vitro.

RESULTS

When rabbits were fed a standard diet, Lp-PLA₂ deficiency led to a significant reduction in plasma lipids. The decreased protein levels of SREBP2 (sterol regulatory element-binding protein 2) and HMGCR (3-hydroxy-3-methylglutaryl coenzyme A reductase) in livers of homozygous knockout rabbits indicated that the cholesterol biosynthetic pathway was impaired with Lp-PLA₂ deficiency. In vitro experiments further demonstrated that intracellular Lp-PLA₂ efficiently enhanced SREBP2-related cholesterol biosynthesis signaling independently of INSIGs (insulin-induced genes). When fed a cholesterol-rich diet, homozygous knockout rabbits exhibited consistently lower level of hypercholesterolemia, and their aortic atherosclerosis lesions were significantly reduced by 60.2% compared with those of wild-type rabbits. The lesions of homozygous knockout rabbits were characterized by reduced macrophages and the expression of inflammatory cytokines. Macrophages of homozygous knockout rabbits were insensitive to M1 polarization and showed reduced DiI-labeled lipoprotein uptake capacity compared with wild-type macrophages. Lp-PLA₂ deficiency also inhibited the adhesion between monocytes and endothelial cells.

CONCLUSIONS

These results demonstrate that Lp-PLA₂ plays a causal role in regulating blood lipid homeostasis and Lp-PLA₂ deficiency protects against dietary cholesterol-induced atherosclerosis in rabbits. Lp-PLA₂ could be a potential target for the prevention of atherosclerosis.

Oxidized phospholipids and lipoprotein-associated phospholipase A2 (Lp-PLA2 ) in atherosclerotic cardiovascular disease: An update

Inflammation and oxidative stress conditions lead to a variety of oxidative modifications of lipoprotein phospholipids implicated in the occurrence and development of atherosclerotic lesions. Lipoprotein-associated phospholipase A2 (Lp-PLA₂ ) is established as an independent risk biomarker of atherosclerosis-related cardiovascular disease (ASCVD) and mediates vascular inflammation through the regulation of lipid metabolism in the blood and in atherosclerotic lesions. Lp-PLA₂ is associated with low- and high-density lipoproteins and Lipoprotein (a) in human plasma and specifically hydrolyzes oxidized phospholipids involved in oxidative stress modification. Several oxidized phospholipids (OxPLs) subspecies can be detoxified through enzymatic degradation by Lp-PLA₂ activation, forming lysophospholipids and oxidized non-esterified fatty acids (OxNEFAs). Lysophospholipids promote the expression of adhesion molecules, stimulate cytokines production (TNF-α, IL-6), and attract macrophages to the arterial intima. The present review article discusses new data on the functional roles of OxPLs and Lp-PLA₂ associated with lipoproteins.

Hydrogen influences HDL-associated enzymes and reduces oxidized phospholipids levels in rats fed with a high-fat diet

AIMS

Oxidized phospholipids (OxPLs) are formed as a result of oxidative stress, which potentially mediate multiple pathological effects. We aimed to evaluate the effects of hydrogen (H₂) on OxPLs in vivo and the underlying mechanism.

MAIN METHODS

Rats were randomly assigned to three groups: control group fed with a chow diet, model group fed with a high-fat diet, and H₂-treated group fed with a high-fat diet and treated by 4% H₂ inhalation for ten weeks. OxPLs in liver and plasma were analyzed by liquid chromatography-mass spectrometry. High-density lipoprotein (HDL) was separated by ultracentrifugation. A proteomic analysis was performed to reveal the alternation of HDL protein composition and he antioxidant capacity of HDL was tested by low-density lipoprotein oxidation experiment. Furthermore, the activity or expression of HDL-associated enzymes were evaluated.

KEY FINDINGS

Inhalation of 4% H₂ decreased the accumulation of OxPLs in rats. In vitro tests revealed that the different concentrations of H₂ did not inhibit the formation of OxPLs mediated by non-enzymatic oxidation. H₂ inhalation altered the components and enhanced the anti-oxidative capacity of HDL in rats fed with a high-fat diet. Further experiments showed that H₂ significantly regulated the activity of lipoprotein-associated phospholipase A₂, paraoxonase-1, and the expression of lecithin:cholesterol acyltransferase.

SIGNIFICANCE

Our findings revealed that H₂ may reduce the OxPLs levels through its influence on HDL-associated enzymes that can act on OxPLs, suggesting that H₂ can be used in alleviating diseases related to lipid peroxidation due to oxidative stress.

Exploratory analysis of genetic variants influencing molecular traits in cerebral cortex of suicide completers

Genetic factors have been implicated in suicidal behavior. It has been suggested that one of the roles of genetic factors in suicide could be represented by the effect of genetic variants on gene expression regulation. Alteration in the expression of genes participating in multiple biological systems in the suicidal brain has been demonstrated, so it is imperative to identify genetic variants that could influence gene expression or its regulatory mechanisms. In this study, we integrated DNA methylation, gene expression, and genotype data from the prefrontal cortex of suicides to identify genetic variants that could be factors in the regulation of gene expression, generally called quantitative trait locus (xQTLs). We identify 6,224 methylation quantitative trait loci and 2,239 expression quantitative trait loci (eQTLs) in the prefrontal cortex of suicide completers. The xQTLs identified influence the expression of genes involved in neurodevelopment and cell organization. Two of the eQTLs identified (rs8065311 and rs1019238) were previously associated with cannabis dependence, highlighting a candidate genetic variant for the increased suicide risk in subjects with substance use disorders. Our findings suggest that genetic variants may regulate gene expression in the prefrontal cortex of suicides through the modulation of promoter and enhancer activity, and to a lesser extent, binding transcription factors.

Association with lipids or detergents is essential for preservation of the active structure of lipoprotein-associated phospholipase A2

Recombinant lipoprotein-associated phospholipase A₂ (rLp-PLA₂) expressed in HEK293 cells has a propensity to form oligomers in the absence of detergents. Dilution of rLp-PLA₂ in the absence of detergents results in irreversible inactivation of the enzyme. The monomeric rLp-PLA₂ may expose its hydrophobic interfacial binding region or substrate binding compartment to water and that may cause structural collapsing of the enzyme. Formation of self-aggregate, complex with binding partners or association with detergent micelles is to block the access of aqueous solvent to the hydrophobic substrate binding site and therefore prevents the structural collapsing. Dilution inactivation of the enzyme can be prevented in the presence of LDL or HDL suggesting that Lp-PLA₂ association with lipoprotein particles (LDL and HDL) is necessary for Lp-PLA₂ to maintain its enzymatic activity in human plasma. Formation of higher affinity complex gave better protection of rLp-PLA₂ structure and activity. The method can be harnessed to detect the interaction between rLp-PLA₂ and components of lipoprotein particles. Apo(a), ApoB 100 and ApoA1 were found to protect the enzyme from inactivation at roughly the similar level (˜80 ± 5%) comparing to human serum albumin control (˜40%). One mg/ml pig brain phospholipid showed 100% protection under the same conditions.

Lipoprotein-associated phospholipase A2: The story continues

Inflammation is thought to play an important role in the pathogenesis of vascular diseases. Lipoprotein-associated phospholipase A2 (Lp-PLA2) mediates vascular inflammation through the regulation of lipid metabolism in blood, thus, it has been extensively investigated to identify its role in vascular inflammation-related diseases, mainly atherosclerosis. Although darapladib, the most advanced Lp-PLA2 inhibitor, failed to meet the primary endpoints of two large phase III trials in atherosclerosis patients cotreated with standard medical care, the research on Lp-PLA2 has not been terminated. Novel pathogenic, epidemiologic, genetic, and crystallographic studies regarding Lp-PLA2 have been reported recently, while novel inhibitors were identified through a fragment-based lead discovery strategy. More strikingly, recent clinical and preclinical studies revealed that Lp-PLA2 inhibition showed promising therapeutic effects in diabetic macular edema and Alzheimer's disease. In this review, we not only summarized the knowledge of Lp-PLA2 established in the past decades but also emphasized new findings in recent years. We hope this review could be valuable for helping researchers acquire a much deeper insight into the nature of Lp-PLA2, identify more potent and selective Lp-PLA2 inhibitors, and discover the potential indications of Lp-PLA2 inhibitors.

The effect of exenatide (a GLP-1 analog) and sitagliptin (a DPP-4 inhibitor) on plasma platelet-activating factor acetylhydrolase (PAF-AH) activity and concentration in normal and fructose-fed rats

Inflammation and oxidative stress are the two processes crucial in atherogenesis. Platelet-activating factor acetylhydrolase (PAF-AH), a plasma lipoprotein-associated enzyme, degrades pro-inflammatory lipids generated within oxidatively modified lipoproteins. Extensive evidence shows that incretin-based drugs, a new class of anti-diabetic agents, can provide cardiovascular protection that cannot be attributed to their glucose-lowering effects. The present study was undertaken to determine whether the antiatherogenic effects of the GLP-1(glucagon-like peptide-1) receptor agonist (exenatide) and DPP-4(dipeptidyl peptidase-4) inhibitors (sitagliptin) may occur via the regulation of platelet-activating factor acetylhydrolase (PAF-AH) activity/mass and inhibition of low-density lipoprotein (LDL) oxidation in the fructose-fed rats. Normal and fructose-fed rats (8 wk) were treated (4 wk) with sitagliptin (5 and 10 mg/kg p.o.) or with exenatide (5 and 10 µg/kg, s.c.). Plasma PAF-AH activity and phosphatidylcholine (PC) concentration were measured colorimetrically. Plasma PAF-AH concentration, oxidized LDL (oxLDL), hexanoyl-Lys adduct (HEL), lyso-PC, apolipoprotein A-I (apoA-I), apoB, platelet-activating factor (PAF), monocyte chemoattractant protein-1 (MCP-1) and endothelin-1 (ET-1) were measured by ELISA. The four-week exenatide (5 µg/kg, sc.) treatment of fructose fed-rats significantly increased plasma PAF-AH activity (+33%, P < 0.001) and decreased the level of circulating oxLDL (-42%, P < 0.05) and MCP-1 (-23%, P < 0.01). These changes were accompanied by the decrease in plasma PC/lyso-PC (-47%, P < 0.001) and apoB/apoA-I ratio (-75%, P < 0.001). The effect of exenatide on enzyme activity was associated with only a minor effect on metabolic parameters and was independent of weight reduction. Exenatide but not sitagliptin inhibits oxidative modification of LDL probably due to favorable effect on plasma PAF-AH activity.

Impact of a non-synonymous Q281R polymorphism on structure of human Lipoprotein-Associated Phospholipase A2 (Lp-PLA2 )

Non-synonymous single nucleotide polymorphisms (nsSNPs) are genetic variations at single base resulting in an amino acid change which have been associated with various complex human diseases. The human Lipoprotein-associated phospholipase A₂ (Lp-PLA₂ ) gene harbours a rare Q281R polymorphism which was previously reported to cause loss of enzymatic function. Lp-PLA₂ is an important enzyme which catalyzes the hydrolysis of polar phospholipids releasing pro-atherogenic and pro-inflammatory mediators involved in the pathogenesis of atherosclerosis. Our current study is aimed at elucidating the structural and functional consequences of Q281R polymorphism on Lp-PLA₂ . The Q281R mutation is classified as deleterious and causes protein instability as deduced from evolutionary, folding free energy changes and Support vector machine (SVM)-based methods. A Q281R mutant structure was deciphered using homology modelling approach and was validated using phi and psi dihedral angles distribution, ERRAT, Verify_3D scores, Protein Structure Analysis (ProSA) energ,y and Z-score. A decreased hydrophobic interactions and weaker substrate binding affinity was observed in the mutant compared to the wild- type (WT) using molecular docking. Further, the mutant displayed enhanced structural flexibility particularly in the low density lipoprotein (LDL) binding domain, decreased solvent accessibility of catalytic residues-Phe274 and Ser273 and increased Cɑ distance between Phe274 and Leu153 and large conformational entropy change as inferred from all-atom molecular dynamics (MD) simulation and essential dynamics (ED) studies. Our results corroborate well with previous experimental studies and thus these aberrations in the Q281R mutant structure may help explain the molecular basis of loss of enzyme activity.

Oxidized phospholipids and lipoprotein-associated phospholipase A2 as important determinants of Lp(a) functionality and pathophysiological role

Lipoprotein(a) [Lp(a)] is composed of a low density lipoprotein (LDL)-like particle to which apolipoprotein (a) [apo(a)] is linked by a single disulfide bridge. Lp(a) is considered a causal risk factor for ischemic cardiovascular disease (CVD) and calcific aortic valve stenosis (CAVS). The evidence for a causal role of Lp(a) in CVD and CAVS is based on data from large epidemiological databases, mendelian randomization studies, and genome-wide association studies. Despite the well-established role of Lp(a) as a causal risk factor for CVD and CAVS, the underlying mechanisms are not well understood. A key role in the Lp(a) functionality may be played by its oxidized phospholipids (OxPL) content. Importantly, most of circulating OxPL are associated with Lp(a); however, the underlying mechanisms leading to this preferential sequestration of OxPL on Lp(a) over the other lipoproteins, are mostly unknown. Several studies support the hypothesis that the risk of Lp(a) is primarily driven by its OxPL content. An important role in Lp(a) functionality may be played by the lipoprotein-associated phospholipase A₂ (Lp-PLA₂), an enzyme that catalyzes the degradation of OxPL and is bound to plasma lipoproteins including Lp(a). The present review article discusses new data on the pathophysiological role of Lp(a) and particularly focuses on the functional role of OxPL and Lp-PLA₂ associated with Lp(a).

Phospholipases: An Overview

Phospholipases are lipolytic enzymes that hydrolyze phospholipid substrates at specific ester bonds. Phospholipases are widespread in nature and play very diverse roles from aggression in snake venom to signal transduction, lipid mediator production, and metabolite digestion in humans. Phospholipases vary considerably in structure, function, regulation, and mode of action. Tremendous advances in understanding the structure and function of phospholipases have occurred in the last decades. This introductory chapter is aimed at providing a general framework of the current understanding of phospholipases and a discussion of their mechanisms of action and emerging biological functions.

Crystal Structure and Atomic Level Analysis of Plasma PAF-AH

The structure of plasma PAF-AH was solved to a resolution of 1.5Å using X-ray crystallography. The enzyme has a classic α/β serine hydrolase fold containing a catalytic triad of Ser273, Asp296, and His351. A hydrophobic patch of the enzyme involving two α-helices (114-126 and 362-369) and neighboring residues have been shown to be essential for lipoprotein particle binding by mutagenesis and mass spectrometry hydrogen/deuterium exchange experiments. An interface-bound model of the enzyme positions the active site above the hydrophobic-hydrophilic interface and is consistent with the known substrate specificity of the enzyme. Several ligand-bound structures of plasma PAF-AH have been solved with organophosphorus compounds and modeled with competitive inhibitors of high affinity and selectivity. This chapter presents an overview of the structure of plasma PAF-AH, molecular details of its functional role, and the interaction of the enzyme with lipoprotein particles.

Plasma PAF-AH (PLA2G7): Biochemical Properties, Association with LDLs and HDLs, and Regulation of Expression

This chapter is focused on the plasma form of PAF-acetylhydrolase (PAF-AH), a lipoprotein-bound, calcium-independent phospholipase A2 activity also referred to as lipoprotein-associated phospholipase A2 and PLA2G7. 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. 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. Several new mechanisms of regulation have been identified in recent years, including effects mediated by PPARs, VEGFR, and the state of cellular differentiation. Moreover, I discuss recent studies describing significant variations in the structure and regulation of PAF-AH from diverse species, which is likely to have important implications for the function of this enzyme in vivo.

Overexpression of porcine lipoprotein-associated phospholipase A2 in swine

Lipoprotein-associated phospholipase A 2 (Lp-PLA2) is associated with the risk of vascular disease. It circulates in human blood predominantly in association with low-density lipoprotein cholesterol (LDL-C) and hydrolyses oxidized phospholipids into pro-inflammatory products. However, in the mouse circulation, it predominantly binds to high-density lipoprotein cholesterol (HDL-C) and exhibits anti-inflammatory properties. To further investigate the effects of Lp-PLA2 in the circulation, we generated over-expressed Lp-PLA2 transgenic swine. The eukaryotic expression plasmid of porcine Lp-PLA2 which driven by EF1α promoter was constructed and generate transgenic swine via SCNT. The expression and activity of Lp-PLA2 in transgenic swine were evaluated, and the total cholesterol (TC), HDL-C, LDL-C and triglyceride (TG) levels in the fasting and fed states were also assessed. Compared with wild-type swine controls, the transgenic swine exhibited elevated Lp-PLA2 mRNA levels and activities, and the activity did not depend on the feeding state. The TC, HDL-C and LDL-C levels were not significantly increased. There was no change in the TG levels in the fasting state between transgenic and control pigs. However, in the fed state, the TG levels of transgenic swine were slightly increased compared with the control pigs and were significantly elevated compared with the fasting state. In addition, inflammatory gene (interleukin [IL]-6, monocyte chemotactic protein [MCP]-1 and tumor necrosis factor [TNF]-α) mRNA levels in peripheral blood mononuclear cells (PBMCs) were significantly increased. The results demonstrated that Lp-PLA2 is associated with triglycerides which may be helpful for understanding the relationship of this protein with cardiovascular disease.

Relationship between platelet activating factor acetylhydrolase activity and apolipoprotein B levels in patients with peanut allergy

Background

Platelet-activating factor (PAF) is a highly potent phospholipid mediator responsible for the life-threatening manifestations of anaphylaxis. PAF acetylhydrolase (PAF-AH) inactivates PAF and protects against severe anaphylaxis whereas deficiency of PAF-AH predisposes to severe or fatal anaphylaxis. Determinants of PAF-AH activity have not been studied in patients with peanut allergy.

Objectives

To determine whether plasma PAF-AH activity in patients with peanut allergy is related to formation of circulating complexes with apolipoprotein B (apoB) the main surface protein on low density lipoprotein particles.

Methods

Plasma PAF-AH activity and apoB concentrations were measured in 63 peanut allergic patients (35 boys, 28 girls, ages 2 – 19 years). ApoB concentration was measured immunoturbidimetrically using goat anti-human apoB. The correlation between PAF-AH activity and apoB concentration was determined.

Results

A positive correlation was found between PAF-AH activity and apoB concentration (r² = 0.59, P < 0.0001).

Conclusion

In peanut allergic patients, PAF-AH activity strongly correlates with apoB concentration, suggesting the presence of circulating PAF-AH- lipoprotein complexes.

Determination of phospholipase activity of PAF acetylhydrolase

This article presents a radiometric assay to determine the enzymatic activity of platelet-activating factor (PAF) acetylhydrolase (PAF-AH), also known as lipoprotein-associated phospholipase A2 and phospholipase A2 group 7A. The method is based on the release of radioactively labeled acetate from sn-2-labeled PAF and separation of substrate and product using reversed-phase column chromatography on octadecyl silica gel cartridges. The assay is fast, convenient, reproducible, sensitive, and inexpensive. The instrumentation required includes standard laboratory equipment and a liquid scintillation counter. The assay is also useful to determine the activity of intracellular PAF-AH (PAF-AH II), provided that a few modifications are included. The enzymatic activity determined using PAF as the substrate is a direct indication of the ability of plasma samples, purified preparations, and cellular and tissue lysates to hydrolyze short- and medium-chain phospholipids that may or may not harbor oxidized functionalities. In addition, the assay can be used to test the suitability of other phospholipids, including species containing oxidized, long-chain sn-2 fatty acyl groups, as PAF-AH substrates. This versatile assay can be used to accurately determine PAF-AH activity in biological samples and preliminarily assess affinity and efficiency of the hydrolysis of potential substrates present in complex mixtures.

Structural basis of specific interactions of Lp-PLA2 with HDL revealed by hydrogen deuterium exchange mass spectrometry

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), specifically Group VIIA PLA(2), is a member of the phospholipase A(2) superfamily and is found mainly associated with LDL and HDL in human plasma. Lp-PLA(2) is considered as a risk factor, a potential biomarker, a target for therapy in the treatment of cardiovascular disease, and evidence suggests that the level of Lp-PLA(2) in plasma is associated with the risk of future cardiovascular and stroke events. The differential location of the enzyme in LDL/HDL lipoproteins has been suggested to affect Lp-PLA(2) function and/or its physiological role and an abnormal distribution of the enzyme may correlate with diseases. Although a mutagenesis study suggested that a surface helix (residues 362-369) mediates the association between Lp-PLA(2) and HDL, the molecular details and mechanism of association has remained unknown. We have now employed hydrogen deuterium exchange mass spectrometry to characterize the interaction between recombinant human Lp-PLA(2) and human HDL. We have found that specific residues 113-120, 192-204, and 360-368 likely mediate HDL binding. In a previous study, we showed that residues 113-120 are important for Lp-PLA(2)-liposome interactions. We now find that residues 192-204 show a decreased deuteration level when Lp-PLA(2) is exposed to apoA-I, but not apoA-II, the most abundant apoproteins in HDL, and additionally, residues 360-368 are only affected by HDL.The results suggest that apoA-I and phospholipid membranes play crucial roles in Lp-PLA(2) localization to HDL.

Specificity of lipoprotein-associated phospholipase A(2) toward oxidized phosphatidylserines: liquid chromatography-electrospray ionization mass spectrometry characterization of products and computer modeling of interactions

Ca(2+)-independent lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) is a member of the phospholipase A(2) superfamily with a distinguishing characteristic of high specificity for oxidatively modified sn-2 fatty acid residues in phospholipids that has been especially well characterized for peroxidized species of phosphatidylcholines (PC). The ability of Lp-PLA(2) to hydrolyze peroxidized species of phosphatidylserine (PS), acting as a recognition signal for clearance of apoptotic cells by professional phagocytes, as well as the products of the reaction has not been investigated. We performed liquid chromatography-electrospray ionization mass spectrometry-based structural characterization of oxygenated, hydrolyzed molecular species of PS-containing linoleic acid in either the sn-2 position (C(18:0)/C(18:2)) or in both sn-1 and sn-2 positions (C(18:2)/C(18:2)), formed in the cytochrome c- and H(2)O(2)-driven enzymatic oxidation reaction. Cytochrome c has been chosen as a catalyst of peroxidation reactions because of its likely involvement in PS oxidation in apoptotic cells. We found that Lp-PLA(2) catalyzed the hydrolysis of both nontruncated and truncated (oxidatively fragmented) species of oxidized PS species, albeit with different efficiencies, and performed detailed characterization of the major reaction products: oxygenated derivatives of linoleic acid as well as nonoxygenated and oxygenated species of lyso-PS. Among linoleic acid products, derivatives oxygenated at the C(9) position, including 9-hydroxyoctadecadienoic acid (9-HODE), a potent ligand of G protein-coupled receptor G2A, were the most abundant. Computer modeling of interactions of Lp-PLA(2) with different PS-oxidized species indicated that they are able to bind in the proximity (<5 Å) of Ser273 and His351 of the catalytic triad. For 9-hydroxy and 9-hydroperoxy derivatives of oxidized PS, the sn-2 ester bond was positioned very close (<3 Å) to the Ser273 residue, a nucleophile directly attacking the sn-2 bond, thus favoring the hydrolysis reaction. We suggest that oxidatively modified free fatty acids and lyso-PS species generated by Lp-PLA(2) may represent important signals facilitating and regulating the execution of apoptotic and phagocytosis programs essential for the control of inflammation.

Lipoprotein-associated phospholipase A(2) bound on high-density lipoprotein is associated with lower risk for cardiac death in stable coronary artery disease patients: a 3-year follow-up

OBJECTIVES

The aim of this study was to examine the prognostic value of lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) associated with high-density lipoprotein (HDL) (HDL-Lp-PLA(2)) in patients with stable coronary artery disease (CAD).

BACKGROUND

Lp-PLA(2) is a novel risk factor for cardiovascular disease. It has been postulated that the role of Lp-PLA(2) in atherosclerosis may depend on the type of lipoprotein with which it is associated.

METHODS

Total plasma Lp-PLA(2) and HDL-Lp-PLA(2) mass and activity, lipids, and C-reactive protein were measured in 524 consecutive patients with stable CAD who were followed for a median of 34 months. The primary endpoint was cardiac death, and the secondary endpoint was hospitalization for acute coronary syndromes, myocardial revascularization, arrhythmic event, or stroke.

RESULTS

Follow-up data were obtained from 477 patients. One hundred twenty-three patients (25.8%) presented with cardiovascular events (24 cardiac deaths, 47 acute coronary syndromes, 28 revascularizations, 22 arrhythmic events, and 2 strokes). Total plasma Lp-PLA(2) mass and activity were predictors of cardiac death (hazard ratio [HR]: 1.013; 95% confidence interval [CI]: 1.005 to 1.021; p = 0.002; and HR: 1.040; 95% CI: 1.005 to 1.076; p = 0.025, respectively) after adjustment for traditional risk factors for CAD. In contrast, HDL-Lp-PLA(2) mass and activity were associated with lower risk for cardiac death (HR: 0.972; 95% CI: 0.952 to 0.993; p = 0.010; and HR: 0.689; 95% CI: 0.496 to 0.957; p = 0.026, respectively) after adjustment for traditional risk factors for CAD.

CONCLUSIONS

Total plasma Lp-PLA(2) is a predictor of cardiac death, while HDL-Lp-PLA(2) is associated with lower risk for cardiac death in patients with stable CAD, independently of other traditional cardiovascular risk factors.

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.

Lipoprotein-associated phospholipase A2 (Lp-PLA2): a review of its role and significance as a cardiovascular biomarker

OBJECTIVE

To conduct a comprehensive, systematic review of studies assessing the significance of lipoprotein-associated phospholipase A2 in cardiovascular diseases (CVDs).

MATERIAL AND METHODS

A review of the literature was performed using the search term "Lipoprotein-associated phospholipase A2 (Lp-PLA2)" and each of the following terms: "cardiovascular risk," "cardiovascular death," "atherosclerotic disease," "coronary events," "transient ischemic attack (TIA)," "stroke," and "heart failure." The searches were performed on Medline, Google Scholar and ClinicalTrials.gov.

RESULTS

The majority of published studies showed a significant association between Lp-PLA2 levels and cardiovascular events after multivariate adjustment. The association was consistent across a wide variety of subjects of both sexes and different ethnic backgrounds.

CONCLUSIONS

The role of Lp-PLA2 as a significant biomarker of vascular inflammation was confirmed, and Lp-PLA2 seems to be closely correlated to cardiovascular events. It may be an important therapeutic target and may have an important role in prevention, risk stratification and personalised medicine.

Dual activity of serum lipoprotein-associated phospholipase A(2) yielding positive and inverse associations with cardiometabolic risk

BACKGROUND

The clinical relevance of serum lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) in populations prone to cardiometabolic risk needs exploration. We determined major covariates of Lp-PLA(2) mass, and its associations with cardiometabolic disorders.

METHODS

In 736 Turkish adults, serum total Lp-PLA(2) mass was determined by immunoassay. Its association with cardiometabolic risk was assessed in three categories. In a second sample of 98 subjects, enzyme protein in high-density lipoprotein (HDL) was also assayed after precipitation.

RESULTS

Significant inverse correlation existed with high triglyceride/low HDL cholesterol dyslipidemia, waist girth, apolipoprotein C-III, homeostatic model assessment, and linear inverse associations in women with lipoprotein (a) and fibrinogen, suggesting that Lp-PLA(2) mass reflected insulin sensitivity and that HDL bound enzyme mass dominated the associations. Among men, positive linear association with total cholesterol suggested additional association with low-density lipoprotein (LDL)-bound enzyme. High (>450 ng/mL) opposed to low (<210 ng/mL) circulating Lp-PLA(2) mass was associated with prevalent and incident coronary heart disease (CHD) in men. One SD increment in Lp-PLA(2) was associated with a 1.64-fold (95% CI 1.00; 2.70) likelihood of CHD, after adjustment for potential confounders. Furthermore, Lp-PLA(2) categories were significantly, independently and inversely associated in men with diabetes only (OR 0.61) and in women with metabolic syndrome only (OR 0.68), for a 1-SD increment.

CONCLUSIONS

Serum total Lp-PLA(2) mass may indicate either elevated or diminished cardiometabolic risk, specific for gender, depending on its partitioning in lipoprotein groups.

Lipoprotein-associated phospholipase A(2) interacts with phospholipid vesicles via a surface-disposed hydrophobic α-helix

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) plays important roles in both the inhibition and promotion of inflammation in human disease. It catalyzes the hydrolytic inactivation of plasma platelet activating factor (PAF) and is also known as PAF acetylhydrolase. High levels of PAF are implicated in a variety of inflammatory diseases such as asthma, necrotizing enterocolitis, and sepsis. Lp-PLA(2) also associates with lipoproteins in human plasma where it hydrolyzes oxidized phospholipids to produce pro-inflammatory lipid mediators that can promote inflammation and the development of atherosclerosis. Lp-PLA(2) plasma levels have recently been identified as a biomarker of vascular inflammation, atherosclerotic vulnerability, and future cardiovascular events. The enzyme is thus a prominent target for the development of inflammation and atherosclerosis-modulating therapeutics. While the crystallographically determined structure of the enzyme is known, the enzyme's mechanism of interaction with PAF and the function-modulating lipids in lipoproteins is unknown. We have employed peptide amide hydrogen-deuterium exchange mass spectrometry (DXMS) to characterize the association of Lp-PLA(2) with dimyristoylphosphatidylcholine (DMPC) vesicles and found that specific residues 113-120 in one of the enzyme's surface-disposed hydrophobic α-helices likely mediate liposome binding.

Noninvasive molecular imaging reveals role of PAF in leukocyte-endothelial interaction in LPS-induced ocular vascular injury

Uveitis is a systemic immune disease and a common cause of blindness. The eye is an ideal organ for light-based imaging of molecular events underlying vascular and immune diseases. The phospholipid platelet-activating factor (PAF) is an important mediator of inflammation, the action of which in endothelial and immune cells in vivo is not well understood. The purpose of this study was to investigate the role of PAF in endothelial injury in uveitis. Here, we use our recently introduced in vivo molecular imaging approach in combination with the PAF inhibitors WEB 2086 (WEB) and ginkgolide B (GB). The differential inhibitory effects of WEB and GB in reducing LPS-induced endothelial injury in the choroid indicate an important role for PAF-like lipids, which might not require the PAF receptor for their signaling. P-selectin glycoprotein ligand-1-mediated rolling of mouse leukocytes on immobilized P-selectin in our autoperfused microflow chamber assay revealed a significant reduction in rolling velocity on the cells' contact with PAF. Rolling cells that came in contact with PAF rapidly assumed morphological signs of cell activation, indicating that activation during rolling does not require integrins. Our results show a key role for PAF in mediating endothelial and leukocyte activation in acute ocular inflammation. Our in vivo molecular imaging provides a detailed view of cellular and molecular events in the complex physiological setting.

Circulating platelet-activating factor is primarily cleared by transport, not intravascular hydrolysis by lipoprotein-associated phospholipase A2/ PAF acetylhydrolase

RATIONALE

The phospholipid platelet-activating factor (PAF) stimulates all cells of the innate immune system and numerous cardiovascular cells. A single enzyme (plasma PAF acetylhydrolase [PAF-AH] or lipoprotein-associated phospholipase [Lp-PL]A(2)) in plasma hydrolyzes PAF, but significant controversy exists whether its action is pro- or antiinflammatory and accordingly whether its inhibition will slow cardiovascular disease.

OBJECTIVE

We sought to define how PAF and related short-chain oxidized phospholipids turnover in vivo and the role of PAF acetylhydrolase/Lp-PLA(2) in this process.

METHODS AND RESULTS

[(3)H-acetyl]PAF was hydrolyzed by murine or human plasma (t(1/2), 3 and 7 minutes, respectively), but injected [(3)H-acetyl]PAF disappeared from murine circulation more quickly (t(1/2), <30 seconds). [(3)H]PAF clearance was unchanged in PAF receptor(-/-) animals, or over the first 2 half-lives in PAF-AH(-/-) animals. [(3)H]PAF turnover was reduced by coinjecting excess unlabeled PAF or an oxidatively truncated phospholipid, and [(3)H]PAF clearance was slowed in hyperlipidemic apolipoprotein (apo)E(-/-) mice with excess circulating oxidatively truncated phospholipids. [(3)H]PAF, fluorescent NBD-PAF, or fluorescent oxidatively truncated phospholipid were primarily accumulated by liver and lung, and were transported into endothelium as intact phospholipids through a common mechanism involving TMEM30a.

CONCLUSIONS

Circulating PAF and oxidized phospholipids are continually and rapidly cleared, and hence continually and rapidly produced. Saturable PAF receptor-independent transport, rather than just intravascular hydrolysis, controls circulating inflammatory and proapoptotic oxidized phospholipid mediators. Intravascular PAF has access to intracellular compartments. Inflammatory and proapoptotic phospholipids may accumulate in the circulation as transport is overwhelmed by substrates in hyperlipidemia.

Lipoprotein-associated phospholipase A2 as a biomarker of coronary heart disease and a therapeutic target

PURPOSE OF REVIEW

Lipoprotein-associated phospholipase A2 (Lp PLA2) is postulated to occupy a key position in the pathogenic sequence leading to formation of complex atherosclerotic lesions. This study reviews evidence supporting its role as a biomarker of vascular disease and as a possible therapeutic target.

RECENT FINDINGS

Evidence continues to build supporting the usefulness of Lp PLA2 as a predictor of coronary events in the general population and in those with established coronary heart disease. Elevated Lp PLA2 is also associated with stroke and heart failure. The crystal structure of Lp PLA2 is now available and offers insight into the links between structure, function and atherogenic properties. Recently completed studies on the efficacy of darapladib, a specific Lp PLA2 inhibitor, show beneficial changes in plaque morphology in animal models and in humans.

SUMMARY

Lp PLA2 is gaining acceptance as a useful biomarker of chronic inflammation and as a predictor of vascular disease. Early results with darapladib offer promise, but not definitive proof, of a potential role for Lp PLA2 inhibition in coronary heart disease prevention.

Dual roles of endogenous platelet-activating factor acetylhydrolase in a murine model of necrotizing enterocolitis

Human preterm infants with necrotizing enterocolitis (NEC) have increased circulating and luminal levels of platelet-activating factor (PAF) and decreased serum PAF-acetylhydrolase (PAF-AH), the enzyme that inactivates PAF. Formula supplemented with recombinant PAF-AH decreases NEC in a neonatal rat model. We hypothesized that endogenous PAF-AH contributes to neonatal intestinal homeostasis and therefore developed PAF-AH mice using standard approaches to study the role of this enzyme in the neonatal NEC model. After exposure to a well-established NEC model, intestinal tissues were evaluated for histology, proinflammatory cytokine mRNA synthesis, and death using standard techniques. We found that mortality rates were significantly lower in PAF-AH pups compared with wild-type controls before 24 h of life but surviving PAF-AH animals were more susceptible to NEC development compared with wild-type controls. Increased NEC incidence was associated with prominent inflammation characterized by elevated intestinal mRNA expression of sPLA2, inducible NOS, and CXCL1. In conclusion, the data support a protective role for endogenous PAF-AH in the development of NEC, and because preterm neonates have endogenous PAF-AH deficiency, this may place them at increased risk for disease.

Novel mechanism for regulation of plasma platelet-activating factor acetylhydrolase expression in mammalian cells

The plasma form of PAF-AH [PAF (platelet-activating factor) acetylhydrolase; also known as LpPLA2 (lipopoprotein-associated phospholipase A2), PLA2G7] catalyses the release of sn-2 fatty acyl residues from PAF, oxidatively fragmented phospholipids, and esterified isoprostanes. The plasma levels of this enzyme vary widely among mammalian species, including mice and humans, but the mechanisms that account for these differences are largely unknown. We investigated the basis for these variations using molecular and biochemical approaches. We identified an N-terminal domain that played key roles in the determination of steady-state expression levels. The mouse N-terminal domain robustly enhanced protein expression levels, possibly owing to its ability to adopt a globular conformation that is absent in the human protein. We investigated the mechanism(s) whereby the N-terminal stretch modulated PAF-AH levels and found that differential expression was not due to variations in the efficiency of transcription, translation, or mRNA stability. Studies designed to evaluate the ability of precursor forms of PAF-AH to mature to fully active proteins indicated that the N-terminal end of human and mouse PAF-AH played important and opposite roles in this process. These domains also modulated the levels of expression of an unrelated polypeptide by affecting the stability of precursor forms of the protein. These studies provide insights that contribute to our understanding of the molecular features and mechanisms that contribute to differential expression of plasma PAF-AH in mammals.

Genome-wide association study of Lp-PLA(2) activity and mass in the Framingham Heart Study

Lipoprotein-associated phospholipase A₂ (Lp-PLA₂) is an emerging risk factor and therapeutic target for cardiovascular disease. The activity and mass of this enzyme are heritable traits, but major genetic determinants have not been explored in a systematic, genome-wide fashion. We carried out a genome-wide association study of Lp-PLA₂ activity and mass in 6,668 Caucasian subjects from the population-based Framingham Heart Study. Clinical data and genotypes from the Affymetrix 550K SNP array were obtained from the open-access Framingham SHARe project. Each polymorphism that passed quality control was tested for associations with Lp-PLA₂ activity and mass using linear mixed models implemented in the R statistical package, accounting for familial correlations, and controlling for age, sex, smoking, lipid-lowering-medication use, and cohort. For Lp-PLA₂ activity, polymorphisms at four independent loci reached genome-wide significance, including the APOE/APOC1 region on chromosome 19 (p = 6×10⁻²⁴); CELSR2/PSRC1 on chromosome 1 (p = 3×10⁻¹⁵); SCARB1 on chromosome 12 (p = 1×10⁻⁸) and ZNF259/BUD13 in the APOA5/APOA1 gene region on chromosome 11 (p = 4×10⁻⁸). All of these remained significant after accounting for associations with LDL cholesterol, HDL cholesterol, or triglycerides. For Lp-PLA₂ mass, 12 SNPs achieved genome-wide significance, all clustering in a region on chromosome 6p12.3 near the PLA2G7 gene. Our analyses demonstrate that genetic polymorphisms may contribute to inter-individual variation in Lp-PLA₂ activity and mass.

Blood levels of lipoprotein-associated phospholipase A₂ (Lp-PLA₂) show a strong association with atherosclerosis in humans. This enzyme is made by certain cells of the immune system, associates with lipoproteins (HDL and LDL), and is thought to be involved in inflammation. Studies have shown that Lp-PLA₂ is a good predictor of cardiovascular disease, independent of HDL and LDL cholesterol levels. This has led to the development of drugs aimed at inhibiting Lp-PLA₂ as a way to treat or prevent cardiovascular disease. The activity and mass of Lp-PLA₂ are heritable traits, but major genetic determinants have not been explored in a systematic fashion. We examined genetic variants across the human genome to identify genes influencing Lp-PLA₂ activity and mass. We studied 6,668 Caucasian subjects from the population-based Framingham Heart Study. Clinical data and genetic data on 550,000 genetic variants were available for association analysis. There was no overlap in the most significantly associated SNPs for activity and mass. We identified four distinct gene regions showing highly significant associations with Lp-PLA₂ activity, all of which are known to include genes involved in cholesterol metabolism. The only locus associated with Lp-PLA₂ mass was a region harboring PLA2G7, the gene that encodes lipoprotein-associated phospholipase A2.

Molecular Model of Plasma PAF Acetylhydrolase-Lipoprotein Association: Insights from the Structure

Plasma platelet-activating factor acetylhydrolase (PAF-AH), also called lipoprotein-associated phospholipase A₂ (Lp-PLA₂), is a group VIIA PLA₂ enzyme that catalyzes the hydrolysis of PAF and certain oxidized phospholipids. Although the role of PAF-AH as a pro- or anti-atherosclerotic enzyme is highly debated, several studies have shown it to be an independent marker of cardiovascular diseases. In humans the majority of plasma PAF-AH is bound to LDL and a smaller portion to HDL; the majority of the enzyme being associated with small dense LDL and VHDL-1 subclasses. Several studies suggest that the anti- or pro-atherosclerotic tendency of PAF-AH might be dependent on the type of lipoprotein it is associated with. Amino acid residues in PAF-AH necessary for binding to LDL and HDL have been identified. However our understanding of the interaction of PAF-AH with LDL and HDL is still incomplete. In this review we present an overview of what is already known about the interaction of PAF-AH with lipoprotein particles, and we pose questions that are yet to be answered. The recently solved crystal structure of PAF-AH, along with functional work done by others is used as a guide to develop a model of interaction of PAF-AH with lipoprotein particles.

Lipid oxidation and peroxidation in CNS health and disease: from molecular mechanisms to therapeutic opportunities

Reactive oxygen species (ROS) are produced at low levels in mammalian cells by various metabolic processes, such as oxidative phosphorylation by the mitochondrial respiratory chain, NAD(P)H oxidases, and arachidonic acid oxidative metabolism. To maintain physiological redox balance, cells have endogenous antioxidant defenses regulated at the transcriptional level by Nrf2/ARE. Oxidative stress results when ROS production exceeds the cell's ability to detoxify ROS. Overproduction of ROS damages cellular components, including lipids, leading to decline in physiological function and cell death. Reaction of ROS with lipids produces oxidized phospholipids, which give rise to 4-hydroxynonenal, 4-oxo-2-nonenal, and acrolein. The brain is susceptible to oxidative damage due to its high lipid content and oxygen consumption. Neurodegenerative diseases (AD, ALS, bipolar disorder, epilepsy, Friedreich's ataxia, HD, MS, NBIA, NPC, PD, peroxisomal disorders, schizophrenia, Wallerian degeneration, Zellweger syndrome) and CNS traumas (stroke, TBI, SCI) are problems of vast clinical importance. Free iron can react with H(2)O(2) via the Fenton reaction, a primary cause of lipid peroxidation, and may be of particular importance for these CNS injuries and disorders. Cholesterol is an important regulator of lipid organization and the precursor for neurosteroid biosynthesis. Atherosclerosis, the major risk factor for ischemic stroke, involves accumulation of oxidized LDL in the arteries, leading to foam cell formation and plaque development. This review will discuss the role of lipid oxidation/peroxidation in various CNS injuries/disorders.

HDL2 can inhibit further oxidative modification of partially oxidized LDL

AIM

To investigate whether HDL(2) can inhibit further oxidative modification of partially oxidized LDL (ox-LDL) by interrupting the chain oxidation reaction after lipid hydroperoxides (LOOH) formation.

METHODS

Following incubation of LDL 400 microg protein/mL phosphate-buffered saline with Cu(2+) for 1.75 h (defined as 0 min), incubation was continued after adding HDL(2) 200 microg protein/mL or HDL(2) 800 microg protein/mL to give both ox-LDL+HDL(2) 200 microg protein/mL or ox-LDL+HDL(2) 800 microg protein/mL. As a control, ox-LDL 200 microg protein/mL and native LDL were prepared. Each sample was subjected to agarose gel electrophoresis and the LOOH in each sample was measured.

RESULTS

When the electrophoretic mobility of native LDL was designated 1, the relative electrophoretic mobility (REM) of ox-LDL increased significantly over time. The REMs of ox-LDL+HDL(2) 800 microg protein/mL from 10 min to 9 h were significantly lower than the REM of ox-LDL at the respective times (p<0.01). LOOH of ox-LDL+HDL(2) 800 microg protein/mL at 1, 3, 6 and 9 h was significantly higher than LOOH in ox-LDL at the respective times (p<0.01). The results of ox-LDL+HDL(2) 200 microg protein/mL were almost the same but to a lesser extent than the results of ox-LDL+HDL(2) 800 microg protein/mL.

CONCLUSION

The present findings suggest that HDL(2) can inhibit further oxidative modification of partially oxidized LDL by interrupting the chain oxidation reaction after LOOH formation in a concentration-dependent manner.

The role of lipoprotein-associated phospholipase A2 in atherosclerosis may depend on its lipoprotein carrier in plasma

Platelet-activating factor (PAF) acetylhydrolase exhibits a Ca(2+)-independent phospholipase A2 activity and degrades PAFas well as oxidized phospholipids (oxPL). Such phospholipids are accumulated in the artery wall and may play key roles in vascular inflammation and atherosclerosis. PAF-acetylhydrolase in plasma is complexed to lipoproteins; thus it is also referred to as lipoprotein-associated phospholipase A2 (Lp-PLA2). Lp-PLA2 is primarily associated with low-density lipoprotein (LDL), whereas a small proportion of circulating enzyme activity is also associated with high-density lipoprotein (HDL). The majority of the LDL-associated Lp-PLA2 (LDL-Lp-PLA2) activity is bound to atherogenic small-dense LDL particles and it is a potential marker of these particles in plasma. The distribution of Lp-PLA2 between LDL and HDL is altered in various types of dyslipidemias. It can be also influenced by the presence of lipoprotein (a) [Lp(a)] when plasma levels of this lipoprotein exceed 30 mg/dl. Several lines of evidence suggest that the role of plasma Lp-PLA2 in atherosclerosis may depend on the type of lipoprotein particle with which this enzyme is associated. In this regard, data from large Caucasian population studies have shown an independent association between the plasma Lp-PLA2 levels (which are mainly influenced by the levels of LDL-Lp-PLA2) and the risk of future cardiovascular events. On the contrary, several lines of evidence suggest that HDL-associated Lp-PLA2 may substantially contribute to the HDL antiatherogenic activities. Recent studies have provided evidence that oxPL are preferentially sequestered on Lp(a) thus subjected to degradation by the Lp(a)-associated Lp-PLA2. These data suggest that Lp(a) may be a potential scavenger of oxPL and provide new insights into the functional role of Lp(a) and the Lp(a)-associated Lp-PLA2 in normal physiology as well as in inflammation and atherosclerosis. The present review is focused on recent advances concerning the Lp-PLA2 structural characteristics, the molecular basis of the enzyme association with distinct lipoprotein subspecies, as well as the potential role of Lp-PLA2 associated with different lipoprotein classes in atherosclerosis and cardiovascular disease.

Functional Consequences of Mutations and Polymorphisms in the Coding Region of the PAF Acetylhydrolase (PAF-AH) Gene

In the past several years a number of alterations in the PAF-AH/PLA₂G7/LpPLA₂ gene have been described. These include inactivating mutations, polymorphisms in the coding region, and other genetic changes located in promoter and intronic regions of the gene. The consequences associated with these genetic variations have been evaluated from different perspectives, including in vitro biochemical and molecular studies and clinical analyses in human subjects. This review highlights the current state of the field and suggests new approaches that can be used to evaluate functional consequences associated with mutations and polymorphisms in the PAF-AH gene.

Current and emerging paradigms in the therapeutic management of atherosclerosis

BACKGROUND

The pathogenesis of atherosclerosis lies in abnormalities in lipoprotein metabolism leading to pathological interactions with vessel walls and the release of inflammatory components, which further aggravate the disease condition.

OBJECTIVE

To elucidate current and emerging trends in drug discovery towards the development of new entities regulating lipoprotein metabolism and inflammatory components to combat the progression of atherosclerosis.

METHODS

Research/review articles in the public domain and press releases were employed.

RESULTS/CONCLUSION

With the recent failure of torcetrapib and succinobucol, drug discovery and development efforts towards the treatment of atherosclerosis have received a big jolt and have been slowed down to a certain extent [corrected]. But this could be a starting point for several new mechanisms that are emerging to discover new drugs to combat the disease.

Phospholipase A2 structure/function, mechanism, and signaling

Tremendous advances in understanding the structure and function of the superfamily of phospholipase A2 (PLA2) enzymes has occurred in the twenty-first century. The superfamily includes 15 groups comprising four main types including the secreted sPLA2, cytosolic cPLA2, calcium-independent iPLA2, and platelet activating factor (PAF) acetyl hydrolase/oxidized lipid lipoprotein associated (Lp)PLA2. We review herein our current understanding of the structure and interaction with substrate phospholipids, which resides in membranes for a representative of each of these main types of PLA2. We will also briefly review the development of inhibitors of these enzymes and their roles in lipid signaling.

Crystal structure of human plasma platelet-activating factor acetylhydrolase: structural implication to lipoprotein binding and catalysis

Human plasma platelet-activating factor (PAF) acetylhydrolase functions by reducing PAF levels as a general anti-inflammatory scavenger and is linked to anaphylactic shock, asthma, and allergic reactions. The enzyme has also been implicated in hydrolytic activities of other pro-inflammatory agents, such as sn-2 oxidatively fragmented phospholipids. This plasma enzyme is tightly bound to low and high density lipoprotein particles and is also referred to as lipoprotein-associated phospholipase A2. The crystal structure of this enzyme has been solved from x-ray diffraction data collected to a resolution of 1.5 angstroms. It has a classic lipase alpha/beta-hydrolase fold, and it contains a catalytic triad of Ser273, His351, and Asp296. Two clusters of hydrophobic residues define the probable interface-binding region, and a prediction is given of how the enzyme is bound to lipoproteins. Additionally, an acidic patch of 10 carboxylate residues and a neighboring basic patch of three residues are suggested to play a role in high density lipoprotein/low density lipoprotein partitioning. A crystal structure is also presented of PAF acetylhydrolase reacted with the organophosphate compound paraoxon via its active site Ser273. The resulting diethyl phosphoryl complex was used to model the tetrahedral intermediate of the substrate PAF to the active site. The model of interface binding begins to explain the known specificity of lipoprotein-bound substrates and how the active site can be both close to the hydrophobic-hydrophilic interface and at the same time be accessible to the aqueous phase.

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.

Phospholipase A2 biochemistry

The phospholipase A(2) (PLA(2)) superfamily consists of many different groups of enzymes that catalyze the hydrolysis of the sn-2 ester bond in a variety of different phospholipids. The products of this reaction, a free fatty acid, and lysophospholipid have many different important physiological roles. There are five main types of PLA(2): the secreted sPLA(2)'s, the cytosolic cPLA(2)'s, the Ca(2+)independent iPLA(2)'s, the PAF acetylhydrolases, and the lysosomal PLA(2)'s. This review focuses on the superfamily of PLA(2) enzymes, and then uses three specific examples of these enzymes to examine the differing biochemistry of the three main types of these enzymes. These three examples are the GIA cobra venom PLA(2), the GIVA cytosolic cPLA(2), and the GVIA Ca(2+)-independent iPLA(2).

Inhibition of lipoprotein-associated phospholipase A2 reduces complex coronary atherosclerotic plaque development

Increased lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) activity is associated with increased risk of cardiac events, but it is not known whether Lp-PLA(2) is a causative agent. Here we show that selective inhibition of Lp-PLA(2) with darapladib reduced development of advanced coronary atherosclerosis in diabetic and hypercholesterolemic swine. Darapladib markedly inhibited plasma and lesion Lp-PLA(2) activity and reduced lesion lysophosphatidylcholine content. Analysis of coronary gene expression showed that darapladib exerted a general anti-inflammatory action, substantially reducing the expression of 24 genes associated with macrophage and T lymphocyte functioning. Darapladib treatment resulted in a considerable decrease in plaque area and, notably, a markedly reduced necrotic core area and reduced medial destruction, resulting in fewer lesions with an unstable phenotype. These data show that selective inhibition of Lp-PLA(2) inhibits progression to advanced coronary atherosclerotic lesions and confirms a crucial role of vascular inflammation independent from hypercholesterolemia in the development of lesions implicated in the pathogenesis of myocardial infarction and stroke.