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

Role of Platelet Activating Factor as a Mediator of Inflammatory Diseases and Preterm Delivery

Nearly 70% of preterm deliveries occur spontaneously, and the clinical pathways involved include preterm labor and preterm premature rupture of membranes. Prediction of preterm delivery is considered crucial due to the significant effects of preterm birth on health and the economy at both the personal and community levels. Although similar inflammatory processes occur in both term and preterm delivery, the premature activation of these processes or exaggerated inflammatory response triggered by infection or sterile factors leads to preterm delivery. Platelet activating factor (PAF) is a phosphoglycerylether lipid mediator of inflammation that is implicated in infections, cancers, and various chronic diseases and disorders including cardiovascular, renal, cerebrovascular, and central nervous system diseases. In gestational tissues, PAF mediates the inflammatory pathways that stimulate the effector mechanisms of labor, including myometrial contraction, cervical dilation, and fetal membrane rupture. Women with preterm labor and preterm premature rupture of membranes have increased levels of PAF in their amniotic fluid. In mice, the intrauterine or intraperitoneal administration of carbamyl PAF activates inflammation in gestational tissues, thereby eliciting preterm delivery. This review summarizes recent research on PAF as an important inflammatory mediator in preterm delivery and in other inflammatory disorders, highlighting its potential value for prediction, intervention, and prevention of these diseases.

New transcriptome and clinical findings of platelet-activating factor in chronic spontaneous urticaria: Pathogenic and treatment relevance

The objective of the study was to assess the pathogenic and treatment relevance of Platelet Activating factor (PAF) in chronic spontaneous urticaria (CSU). The expression and cellular location of PAF receptor (PAFR) and serum levels of PAF and PAF acetylhydrolase (PAF-AH) in patients with moderate/severe CSU (n = 45) and healthy controls (HCs, n = 17) were studied. Skin samples from the active wheal (LS-CSU, 13 samples for qPCR and 33 for immunohistochemistry) and non-lesional skin (NLS-CSU, 13 samples) of CSU patients and HCs (13 samples and 5 for immunohistochemistry) were analyzed. Serum PAF and PAF-AH levels were measured by ELISA and compared between HC (10 samples) and CSU patients (25 samples) and, among them, between those refractory and non-refractory to second-generation H₁ -antihistamines (sgAH). PAFR mRNA expression was significantly higher in LS-CSU versus HCs (p = 0.014). PAFR positive staining in immunohistochemistry was mainly found in the epidermal basal layer in HCs, whereas it was broadly present along the epidermis in LS-CSU samples. Endothelial cells showed PAFR expression exclusively in LS-CSU and NLS-CSU samples. PAFR expression was observed in the nerves of HC, LS-CSU, and NLS-CSU samples. Double PAFR/CD43 expression showed that T-lymphocytes were the main cell type from the wheal inflammatory infiltrate expressing PAFR. A significantly lower PAF-AH/PAF ratio was observed in sgAH non-responders versus responders (6.1 vs. 12.6; p = 0.049). Our study confirms that PAF is a mediator of wheal pathogenesis in CSU. The significantly lower PAF-AH/PAF ratio in sgAH non-responders vs responders suggests that PAF could be a potential biomarker of sgAH refractoriness.

Platelet Activating Factor (PAF): A Mediator of Inflammation

Platelet-activating factor (PAF) is a phospholipid-derived mediator with an established role in multiple inflammatory states. PAF is synthesized and secreted by multiple cell types and is then rapidly hydrolyzed and degraded to an inactive metabolite, lyso-PAF, by the enzyme PAF acetylhydrolase. In addition to its role in platelet aggregation and activation, PAF contributes to allergic and nonallergic inflammatory diseases such as anaphylaxis, sepsis, cardiovascular disease, neurological disease, and malignancy as demonstrated in multiple animal models and, increasingly, in human disease states. Recent research has demonstrated the importance of the PAF pathway in multiple conditions including the prediction of severe pediatric anaphylaxis, effects on blood-brain barrier permeability, effects on reproduction, ocular diseases, and further understanding of its role in cardiovascular risk. Investigation of PAF as both a biomarker and a therapeutic target continues because of the need for directed management of inflammation. Collectively, studies have shown that therapies focused on the PAF pathway have the potential to provide targeted and effective treatments for multiple inflammatory conditions.

Emerging Roles of Platelets in Allergic Asthma

Allergic asthma is a complex chronic inflammatory disease of the airways, driven by Th2 immune responses and characterized by eosinophilic pulmonary inflammation, airway hyperresponsiveness, excessive mucus production, and airway remodeling. Overwhelming evidence from studies in animal models and allergic asthmatic patients suggests that platelets are aberrantly activated and recruited to the lungs. It has been established that platelets can interact with other immune cells and secrete various biochemical mediators to promote allergic sensitization and airway inflammatory response, and platelet deficiency may alleviate the pathological features and symptoms of allergic asthma. However, the comprehensive roles of platelets in allergic asthma have not been fully clarified, leaving attempts to treat allergic asthma with antiplatelet agents questionable. In this review, we summarize the role of platelet activation and pulmonary accumulation in allergic asthma; emphasis is placed on the different interactions between platelets with crucial immune cell types and the contribution of platelet-derived mediators in this context. Furthermore, clinical antiplatelet approaches to treat allergic asthma are discussed. This review provides a clearer understanding of the roles of platelets in the pathogenesis of allergic asthma and could be informative in the development of novel strategies for the treatment of allergic asthma.

Gene expression of oxidative stress markers and lung function: A CARDIA lung study

BACKGROUND

Circulating markers of oxidative stress have been associated with lower lung function. Our objective was to study the association of gene expression levels of oxidative stress pathway genes (ALOX12, ALOX15, ARG2, GSTT1, LPO, MPO, NDUFB3, PLA2G7, and SOD3) and lung function forced expiratory volume in one second (FEV1 ), forced vital capacity (FVC) in Coronary Artery Risk Development in Young Adults study.

METHODS

Lung function was measured using spirometry and the Nanostring platform was used to estimate gene expression levels. Linear regression models were used to study association of lung function measured at year 30, 10-year decline in lung function and gene expression after adjustment for center, smoking, and BMI, measured at year 25.

RESULTS

The 10-year decline of FEV1 was faster in highest NDUFB3 quartile compared to the lowest (difference = -2.09%; p = 0.001) after adjustment for multiple comparisons. The 10-year decline in FEV1 and FVC was nominally slower in highest versus lowest quartile of PLA2G7 (difference = 1.14%; p = 0.02, and difference = 1.06%; p = 0.005, respectively). The other genes in the study were not associated with FEV1 or FVC.

CONCLUSION

Higher gene expression levels in oxidative stress pathway genes are associated with faster 10-year FEV1 decline.

Potential Metabolic Biomarkers in Adult Asthmatics

Asthma is the most common chronic airway inflammation, with multiple phenotypes caused by complicated interactions of genetic, epigenetic, and environmental factors. To date, various determinants have been suggested for asthma pathogenesis by a new technology termed omics, including genomics, transcriptomics, proteomics, and metabolomics. In particular, the systematic analysis of all metabolites in a biological system, such as carbohydrates, amino acids, and lipids, has helped identify a novel pathway related to complex diseases. These metabolites are involved in the regulation of hypermethylation, response to hypoxia, and immune reactions in the pathogenesis of asthma. Among them, lipid metabolism has been suggested to be related to lung dysfunction in mild-to-moderate asthma. Sphingolipid metabolites are an important mediator contributing to airway inflammation in obese asthma and aspirin-exacerbated respiratory disease. Although how these molecular variants impact the disease has not been completely determined, identification of new causative factors may possibly lead to more-personalized and precise pathway-specific approaches for better diagnosis and treatment of asthma. In this review, perspectives of metabolites related to asthma and clinical implications have been highlighted according to various phenotypes.

Structural Modeling of Wild and Mutant Forms of Human Plasma Platelet Activating Factor-Acetyl Hydrolase Enzyme

Purpose

To investigate the structural features of wild and mutant forms of the pPAF-AH enzyme that are responsible for coronary artery disease.

Methods

Mutant variants of human pPAF-AH having either V279F, Q281R, or both were modelled and evaluated for stereo chemical and structural correctness. The 3D coordinates of substrate PAF were retrieved from the PubChem database was solvated and minimized on Discovery Studio, and docked to the wild and mutant enzyme models. The top docked pose complex was refined by MD simulation.

Results

pPAF-AH model comprises of 420 amino acids in a α/β-hydrolase fold that contains a substrate-binding hydrophobic channel with an active site pocket having a catalytic triad of Ser273, Asp296 and His351. Mutations at positions 279 and 281 are opposite one another on the middle of 12 residues long H5 helix that forms the hydrophobic core of the enzyme. V279F causes a tilt on the axis of the mutation bearing helix to avoid steric clashes with the hydrophobic residues on the β-sheets adjacent to it, inducing subtle conformational changes on the H5-β8 loop, β8 sheet, and the loop bearing Asp296. A cascade of conformational changes induces a change in the orientation of His351 resulting in loss of hydrogen bonded interaction with catalytic Ser273. Q281R causes a shortening of H5 and β8, which induces conformational changes of the loops bearing Ser273 and Asp296, respectively. Simultaneous conformational changes of secondary structural elements result in the flipping of His351 causing a break in the catalytic triad. Also, there is a compromise in the substrate-binding area and volume in the mutants resulting in loss of binding to its substrate.

Conclusion

Mutant enzymes show changes at the site of the mutation, secondary motif conformations and global structural conformations that adversely affect the active site, decrease substrate channel volume and decrease stability, thereby affecting enzymatic function.

Human Secretary Phospholipase A2 Mutations and Their Clinical Implications

Phospholipases A2 (PLA₂s) belong to a superfamily of enzymes responsible for hydrolysis of the sn-2 fatty acids of membrane phospholipids to release arachidonic acid. PLA₂s are the rate limiting enzyme for the downstream synthesis of prostaglandins and leukotrienes that are the main mediators of inflammation. The extracellular forms of this enzyme are also called the secretary phospholipase A2 (sPLA₂) and are distributed extensively in most of the tissues in the human body. Their integral role in inflammatory pathways has been the primary reason for the extensive research on this molecule. The catalytic mechanism of sPLA₂ is initiated by a histidine/aspartic acid/calcium complex within the active site. Though they are known to have certain housekeeping functions, certain mutations of sPLA₂ are known to be implicated in causation of certain pathologies leading to diseases such as atherosclerosis, cardiovascular diseases, benign fleck retina, neurodegeneration, and asthma. We present an overview of human sPLA₂ and a comprehensive compilation of the mutations that result in various disease phenotypes. The study not only helps to have a holistic understanding of human sPLA₂ mutations and their clinical implications, but is also a useful platform to initiate research pertaining to structure–function relationship of the mutations to develop effective therapies for management of these diseases.

Effect of acyl and alkyl analogs of platelet-activating factor on inflammatory signaling

Platelet-activating factor (PAF), a bioactive ether phospholipid with significant pro-inflammatory properties, was identified almost half a century ago. Despite extensive study of this autocoid, therapeutic strategies for targeting its signaling components have not been successful, including the recent clinical trials with darapladib, a drug that targets plasma PAF-acetylhydrolase (PAF-AH). We recently provided experimental evidence that the previously unrecognized acyl analog of PAF, which is concomitantly produced along with PAF during biosynthesis, dampens PAF signaling by acting both as a sacrificial substrate for PAF-AH and probably as an endogenous PAF-receptor antagonist/partial agonist. If this is the scenario in vivo, PAF-AH needs to catalyze the selective hydrolysis of alkyl-PAF and not acyl-PAF. Accordingly, different approaches are needed for treating inflammatory diseases in which PAF signaling is implicated. The interplay between acyl-PAF, alkyl-PAF, PAF-AH, and PAF-R is complex, and the outcome of this interplay has not been previously appreciated. In this review, we discuss this interaction based on our recent findings. It is very likely that the relative abundance of acyl and alkyl-PAF and their interactions with PAF-R in the presence of their hydrolyzing enzyme PAF-AH may exert a modulatory effect on PAF signaling during inflammation.

Increased activity of lipoprotein-associated phospholipase A2 in non-severe asthma

BACKGROUND

Given increased risk of cardiovascular events in asthma we hypothesized that lipoprotein-associated phospholipase A₂ (Lp-PLA₂), an enzyme involved in atherosclerosis, is associated with proinflammatory and prothrombotic blood alterations in this disease.

METHODS

In 164 adult asthmatics (63 with severe asthma) we measured plasma Lp-PLA₂ activity using the PLAC test. We determined its relations to inflammation and prothrombotic blood alterations.

RESULTS

In asthma, Lp-PLA₂ was inversely related to the age (β = -0.1 [-0.18 to -0.02]) and was lower in women (n = 122 [74%], 205 [182-242] vs. 243 [203-262] nmol/min/ml, p = 0.001). Interestingly, Lp-PLA₂ correlated negatively with the asthma severity score (β = -0.15 [-0.23 to -0.07]), being 10.3% higher in those with non-severe (mild or moderate) asthma (n = 101, 62%) as compared to the severe disease subtype (224 [191-261] vs. 203 [181-229], p = 0.006 after adjustment for potential confounders). Lp-PLA₂ activity was positively related to the levels of low-density lipoprotein (β = 0.1 [0.02-0.18]), triglycerides (β = 0.11 [0.03-0.19]) and glucose (β = 0.1 [0.02-0.18]) and inversely to the tumor necrosis factor α (β = -0.27 [-0.35 to -0.2]), high sensitivity C-reactive protein (β = -0.1 [-0.19 to -0.02]) and fibrinogen (β = -0.12 [-0.21 to -0.03]), as well as prothrombin (β = -0.16 [-0.24 to -0.08]), and parameters describing thrombin generation potential, such as endogenous thrombin potential (β = -0.14 [-0.21 to -0.06]) and peak thrombin generated (β = -0.2 [-0.28 to -0.12]).

CONCLUSIONS

Elevated Lp-PLA₂ activity in non-severe asthmatics suggests increased atherosclerotic risk in this group. Lower Lp-PLA₂ activity accompanied by its inverse relationship to inflammatory or prothrombotic blood biomarkers observed in turn in severe asthmatics might be related to the pathogenesis of more severe asthma phenotype.

Platelet-Activating Factor (PAF) in Allergic Rhinitis: Clinical and Therapeutic Implications

Platelet-activating factor (PAF) is a lipid mediator involved in several allergic reactions. It is released from multiple cells of the immune system, such as eosinophils, neutrophils, and mast cells, and also exerts its effect on most of them upon specific binding to its receptor, becoming a pleiotropic mediator. PAF is considered a potential relevant mediator in allergic rhinitis, with a key role in nasal congestion and rhinorrhoea due to its effect on vascular permeability. Interestingly, despite its potential relevance as a therapeutic target, no specific PAF inhibitors have been studied in humans. However, rupatadine, a second-generation antihistamine with dual antihistamine and anti-PAF effects has shown promising results by both blocking nasal symptoms and inhibiting mast cell activation induced by PAF, in comparison to antihistamine receptor drugs. In conclusion, the inhibition of PAF may be an interesting approach in the treatment of allergic rhinitis as part of a global strategy directed at blocking as many relevant inflammatory mediators as possible.

Increased platelet activating factor levels in chronic spontaneous urticaria predicts refractoriness to antihistamine treatment: an observational study

Background

Platelet activating factor (PAF) is an endogenous, active phospholipid released from inflammatory cells, platelets, and endothelial cells, and is involved in the regulation of immune responses. Degradation of PAF by PAF acetylhydrolase (PAF-AH) has been shown to be associated with anaphylaxis, asthma, and peanut allergy. The purpose of this study was to investigate relationships among clinical parameters, including urticaria severity and treatment responsiveness, and PAF and PAF-AH levels in sera from patients with chronic spontaneous urticaria (CSU).

Methods

Serum PAF and PAF-AH levels were measured by enzyme-linked immunosorbent assay in 283 CSU patients and 111 age- and sex-matched, healthy normal controls (NCs). Urticaria severity was evaluated by urticaria activity score over 7 days (UAS7). Within 3 months after measuring PAF levels, patients whose urticaria was not controlled by antihistamine treatment were classified as histamine receptor 1 antagonist (H1RA) non-responders.

Results

Serum PAF levels were significantly higher in CSU patients than in NCs (median 4368.9 vs. 3256.4 pg/ml, p = 0.015), while serum PAF-AH levels were significantly lower in CSU patients (105.6 vs. 125.7 ng/ml, p = 0.001). H1RA non-responders had higher levels of PAF in their sera than H1RA responders. A generalized linear model revealed that a higher UAS7 score (odds ratio 1.023, p = 0.024) and a PAF level ≥ 5000 pg/ml (1.409, p < 0.001) were significant predictors of a poor response to H1RA treatment.

Conclusions

Compared with NCs, CSU patients, particularly those with H1RA refractoriness, showed significant increases in serum PAF levels and decreases in PAF-AH. Therapies modulating PAF and PAF-AH levels could be effective in patients with CSU refractory to antihistamines.

Modulation of inflammatory platelet-activating factor (PAF) receptor by the acyl analogue of PAF

Platelet-activating factor (PAF) is a potent inflammatory mediator that exerts its actions via the single PAF receptor (PAF-R). Cells that biosynthesize alkyl-PAF also make abundant amounts of the less potent PAF analogue acyl-PAF, which competes for PAF-R. Both PAF species are degraded by the plasma form of PAF acetylhydrolase (PAF-AH). We examined whether cogenerated acyl-PAF protects alkyl-PAF from systemic degradation by acting as a sacrificial substrate to enhance inflammatory stimulation or as an inhibitor to dampen PAF-R signaling. In ex vivo experiments both PAF species are prothrombotic in isolation, but acyl-PAF reduced the alkyl-PAF-induced stimulation of human platelets that express canonical PAF-R. In Swiss albino mice, alkyl-PAF causes sudden death, but this effect can also be suppressed by simultaneously administering boluses of acyl-PAF. When PAF-AH levels were incrementally elevated, the protective effect of acyl-PAF on alkyl-PAF-induced death was serially decreased. We conclude that, although acyl-PAF in isolation is mildly proinflammatory, in a pathophysiological setting abundant acyl-PAF suppresses the action of alkyl-PAF. These studies provide evidence for a previously unrecognized role for acyl-PAF as an inflammatory set-point modulator that regulates both PAF-R signaling and hydrolysis.

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.

Novel enzymatic method for assaying Lp-PLA2 in serum

BACKGROUND

Measurement of lipoprotein-associated phospholipase A₂ (Lp-PLA₂) can be used as an adjunct to traditional cardiovascular risk factors for identifying individuals at higher risk of cardiovascular events. This can be performed by quantification of the protein concentration using an ELISA platform or by measuring Lp-PLA₂ activity using platelet-activating factor (PAF) analog as substrate. Here, an enzymatic Lp-PLA₂ activity assay method using 1-O-Hexadecyl-2-acetyl-rac-glycero-3-phosphocholine (rac C₁₆ PAF) was developed.

METHODS

The newly revealed substrate specificity of lysoplasmalogen-specific phospholipase D (lysophospholipase D (LysoPLD)) was exploited. Lp-PLA₂ hydrolyzes 1-O-Hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (C₁₆ PAF) to 1-O-Hexadecyl-2-hydroxy-sn-glycero-3-phosphocholine (LysoPAF). LysoPLD acted on LysoPAF, and the hydrolytically released choline was detected by choline oxidase.

RESULTS

Regression analysis of Lp-PLA₂ activity measured by the enzymatic Lp-PLA₂ activity assay vs. two chemical Lp-PLA₂ activity assays, i.e. LpPLA₂ FS and PLAC® test, and ELISA, gave the following correlation coefficients: 0.990, 0.893 and 0.785, respectively (n = 30).

CONCLUSION

Advantages of this enzymatic Lp-PLA₂ activity assay compared with chemical Lp-PLA₂ methods include the following; (i) only requires two reagents enabling a simple two-point linear calibration method with one calibrator (ii) no need for inhibitors of esterase-like activity in serum.

Asthma and the Risk of Rheumatoid Arthritis: An Insight into the Heterogeneity and Phenotypes of Asthma

Asthma is traditionally regarded as a chronic airway disease, and recent literature proves its heterogeneity, based on distinctive clusters or phenotypes of asthma. In defining such asthma clusters, the nature of comorbidity among patients with asthma is poorly understood, by assuming no causal relationship between asthma and other comorbid conditions, including both communicable and noncommunicable diseases. However, emerging evidence suggests that the status of asthma significantly affects the increased susceptibility of the patient to both communicable and noncommunicable diseases. Specifically, the impact of asthma on susceptibility to noncommunicable diseases such as chronic systemic inflammatory diseases (e.g., rheumatoid arthritis), may provide an important insight into asthma as a disease with systemic inflammatory features, a conceptual understanding between asthma and asthma-related comorbidity, and the potential implications on the therapeutic and preventive interventions for patients with asthma. This review discusses the currently under-recognized clinical and immunological phenotypes of asthma; specifically, a higher risk of developing a systemic inflammatory disease such as rheumatoid arthritis and their implications, on the conceptual understanding and management of asthma. Our discussion is divided into three parts: literature summary on the relationship between asthma and the risk of rheumatoid arthritis; potential mechanisms underlying the association; and implications on asthma management and research.

Nuclear localization of platelet-activating factor receptor controls retinal neovascularization

Platelet-activating factor (PAF) is a pleiotropic phospholipid with proinflammatory, procoagulant and angiogenic actions on the vasculature. We and others have reported the presence of PAF receptor (Ptafr) at intracellular sites such as the nucleus. However, mechanisms of localization and physiologic functions of intracellular Ptafr remain poorly understood. We hereby identify the importance of C-terminal motif of the receptor and uncover novel roles of Rab11a GTPase and importin-5 in nuclear translocation of Ptafr in primary human retinal microvascular endothelial cells. Nuclear localization of Ptafr is independent of exogenous PAF stimulation as well as intracellular PAF biosynthesis. Moreover, nuclear Ptafr is responsible for the upregulation of unique set of growth factors, including vascular endothelial growth factor, in vitro and ex vivo. We further corroborate the intracrine PAF signaling, resulting in angiogenesis in vivo, using Ptafr antagonists with distinct plasma membrane permeability. Collectively, our findings show that nuclear Ptafr translocates in an agonist-independent manner, and distinctive functions of Ptafr based on its cellular localization point to another dimension needed for pharmacologic selectivity of drugs.

New risk factors for adult-onset incident asthma. A nested case-control study of host antioxidant defense

RATIONALE

Host antioxidant defense, consisting of enzymatic antioxidant activity and nonenzymatic antioxidant micronutrients, is implicated in asthma pathogenesis. Studies of antioxidant defense and adult incident asthma have either used measures of antioxidants estimated from questionnaires or not considered enzymatic aspects of host defense.

OBJECTIVES

We conducted the first study designed and powered to investigate the association of antioxidant defenses on adult incident asthma.

METHODS

In a nested case-control study, we followed Shanghai women (aged 40-70 years) without prevalent asthma at baseline, over 8 years. Subjects with incident asthma were ascertained prospectively by gold standard testing of symptomatic women and matched to two asymptomatic control subjects.

MEASUREMENTS AND MAIN RESULTS

Baseline urinary F2-isoprostanes, plasma concentrations of antioxidant micronutrients (tocopherols, xanthines, carotenes, and lycopene), and antioxidant enzyme activity (platelet-activating factor acetylhydrolase [PAF-AH] and superoxide dismutase) were measured from samples collected before disease onset. Among 65,372 women, 150 (0.24%) developed asthma. F2-isoprostane levels before asthma onset were not different between cases and control subjects. Doubling of α-tocopherol concentrations and PAF-AH activity was associated with 50 and 37% decreased risk of incident asthma (α-tocopherol: adjusted odds ratio = 0.52; 95% confidence interval, 0.32-0.84; PAF-AH: adjusted odds ratio = 0.63; 95% confidence interval, 0.42-0.93).

CONCLUSIONS

In this prospective study, α-tocopherol, within normal reference ranges, and PAF-AH enzymatic activity were associated with decreased asthma development. These modifiable risk factors may be an effective strategy to test for primary asthma prevention.

Naturally Occurring Missense Mutation in Plasma PAF-AH Among the Japanese Population

A single nucleotide polymorphism in the plasma PAF-AH enzyme, i.e., G994T, which causes the substitution of Val at amino acid 279 with Phe (V279F), has been found in the Japanese population. This enzyme preferentially degrades oxidatively modulated or truncated phospholipids; therefore, it has been suggested that this enzyme may prevent the accumulation of proinflammatory and proatherogenic oxidized phospholipids. This hypothesis is supported by the higher prevalence of the V279F mutation in patients with asthmatic and atherosclerotic diseases, as compared with healthy controls. This mutation is rare in the Caucasian population. The plasma PAF-AH mass and enzyme activity are distributed over a wide range in the plasma and they are positively correlated with low-density lipoprotein (LDL) cholesterol. However, several clinical studies in the Caucasian population have suggested that this enzyme has the opposite role. This enzyme plays an active role in the development and progression of atherosclerosis via proinflammatory and proatherogenic lysophosphatidylcholine and oxidized fatty acids produced through the oxidation of LDL by this enzyme. Thus, plasma PAF-AH is a unique enzyme with dual roles in human inflammatory diseases. In this chapter, on the basis of recent findings we describe the association between a naturally occurring missense mutation in plasma PAF-AH and human diseases especially including atherosclerosis and asthma.

Endocrine regulation of airway contractility is overlooked

Asthma is a prevalent respiratory disorder triggered by a variety of inhaled environmental factors, such as allergens, viruses, and pollutants. Asthma is characterized by an elevated activation of the smooth muscle surrounding the airways, as well as a propensity of the airways to narrow excessively in response to a spasmogen (i.e. contractile agonist), a feature called airway hyperresponsiveness. The level of airway smooth muscle (ASM) activation is putatively controlled by mediators released in its vicinity. In asthma, many mediators that affect ASM contractility originate from inflammatory cells that are mobilized into the airways, such as eosinophils. However, mounting evidence indicates that mediators released by remote organs can also influence the level of activation of ASM, as well as its level of responsiveness to spasmogens and relaxant agonists. These remote mediators are transported through circulating blood to act either directly on ASM or indirectly via the nervous system by tuning the level of cholinergic activation of ASM. Indeed, mediators generated from diverse organs, including the adrenals, pancreas, adipose tissue, gonads, heart, intestines, and stomach, affect the contractility of ASM. Together, these results suggest that, apart from a paracrine mode of regulation, ASM is subjected to an endocrine mode of regulation. The results also imply that defects in organs other than the lungs can contribute to asthma symptoms and severity. In this review, I suggest that the endocrine mode of regulation of ASM contractility is overlooked.

Platelet aggregation unchanged by lipoprotein-associated phospholipase A₂ inhibition: results from an in vitro study and two randomized phase I trials

BACKGROUND

We explored the theorized upregulation of platelet-activating factor (PAF)- mediated biologic responses following lipoprotein-associated phospholipase A2 (Lp-PLA2) inhibition using human platelet aggregation studies in an in vitro experiment and in 2 clinical trials.

METHODS AND RESULTS

Full platelet aggregation concentration response curves were generated in vitro to several platelet agonists in human plasma samples pretreated with rilapladib (selective Lp-PLA2 inhibitor) or vehicle. This was followed by a randomized, double-blind crossover study in healthy adult men (n = 26) employing a single-agonist dose assay of platelet aggregation, after treatment of subjects with 250 mg oral rilapladib or placebo once daily for 14 days. This study was followed by a second randomized, double-blind parallel-group trial in healthy adult men (n = 58) also treated with 250 mg oral rilapladib or placebo once daily for 14 days using a full range of 10 collagen concentrations (0-10 µg/ml) for characterizing EC50 values for platelet aggregation for each subject. Both clinical studies were conducted at the GlaxoSmithKline Medicines Research Unit in the Prince of Wales Hospital, Sydney, Australia. EC50 values derived from multiple agonist concentrations were compared and no pro-aggregant signals were observed during exposure to rilapladib in any of these platelet studies, despite Lp-PLA2 inhibition exceeding 90%. An increase in collagen-mediated aggregation was observed 3 weeks post drug termination in the crossover study (15.4% vs baseline; 95% confidence interval [CI], 3.9-27.0), which was not observed during the treatment phase and was not observed in the parallel-group study employing a more robust EC50 examination.

CONCLUSIONS

Lp-PLA2 inhibition does not enhance platelet aggregation.

TRIAL REGISTRATION

1) Study 1: ClinicalTrials.gov NCT01745458 2) Study 2: ClinicalTrials.gov NCT00387257.

Association of PLA2G7 gene polymorphisms with ischemic stroke in northern Chinese Han population

OBJECTIVE

Human lipoprotein-associated phospholipase A2 (Lp-PLA2), encoded by the PLA2G7 gene, plays an important role in the pathophysiology of inflammation. This study is aimed at evaluating the potential association of V279F and A379V in PLA2G7 gene with ischemic stroke where inflammatory process is involved.

DESIGN AND METHODS

A total of 386 patients with ischemic stroke and 386 healthy controls were included in the study. The single nucleotide polymorphisms, V279F and A379V, were analyzed by the polymerase chain reaction-ligation detection reaction method.

RESULTS

The frequencies of VV+AV genotype, AV genotype and V allele of A379V in the patients with ischemic stroke were significantly higher than those in the controls (P=0.02, P=0.03, P=0.02, respectively). These correlations still remained after adjusting for confounding risk factors of stroke. Furthermore, subgroup analysis showed that a significant association with A379V was found in large-artery atherosclerotic stroke subgroup. In addition, no significant association was observed between V279F and ischemic stroke.

CONCLUSION

The study indicated that the A379V variant in PLA2G7 gene might contribute to ischemic stroke susceptibility in northern Chinese Han population.

The prevalence of platelet activating factor acetylhydrolase single nucleotide polymorphisms in relationship to necrotizing enterocolitis in Northwest Louisiana infants

PURPOSE

Studies documented that platelet activating factor (PAF) and the enzyme platelet activating factor acetylhydrolase (PAFAH) play a very important role in the pathogenesis of neonatal necrotizing enterocolitis (NEC). In this retrospective, case-controlled pilot study, the authors investigated the prevalence of single nucleotide polymorphisms (Ile198Thr and Ala379Val) of the PAFAH gene.

SUBJECTS AND METHODS

We screened 570 blood samples from both Caucasian and African-American preterm infants in the Northwest Louisiana population for the above mentioned PAFAH gene polymorphisms. Out of 570 infants, 36 had stage I or II NEC based on diagnostic coding, the International Classification of Diseases, 9th revision, Clinical Modification, 2009 (ICD-9-CM). The remaining infants without an ICD-9-CM diagnosis of NEC were recruited as control population. The DNA was isolated and restriction fragment length polymorphism microplate polymerase chain reaction assay was performed.

RESULTS

Variants of the PAFAH gene polymorphism (Ile198Thr and Ala379Val) frequencies were not significantly different between the infants with NEC and the control group (P value of 0.26 by either multiple logistic regression analysis or the Cochran-Mantel-Haenszel test).

CONCLUSIONS

This is the first study of its kind in exploring the relationship between NEC and single nucleotide polymorphisms in the coding genes of the enzyme PAFAH. Our preliminary data demonstrated that adjusted for the effect of race, PAFAH polymorphisms (Ile198Thr and Ala379Val) have no significant effect on NEC.

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.

The involvement of phospholipases A2 in asthma and chronic obstructive pulmonary disease

The increased morbidity, mortality, and ineffective treatment associated with the pathogenesis of chronic inflammatory diseases such as asthma and chronic obstructive pulmonary disease (COPD) have generated much research interest. The key role is played by phospholipases from the A2 superfamily: enzymes which are involved in inflammation through participation in pro- and anti-inflammatory mediators production and have an impact on many immunocompetent cells. The 30 members of the A2 superfamily are divided into 7 groups. Their role in asthma and COPD has been studied in vitro and in vivo (animal models, cell cultures, and patients). This paper contains complete and updated information about the involvement of particular enzymes in the etiology and course of asthma and COPD.

The effect of lipoprotein-associated phospholipase A2 deficiency on pulmonary allergic responses in Aspergillus fumigatus sensitized mice

Background

Lipoprotein-associated phospholipase A₂ (Lp-PLA₂)/platelet-activating factor acetylhydrolase (PAF-AH) has been implicated in the pathogenesis of cardiovascular disease. A therapeutic targeting of this enzyme was challenged by the concern that increased circulating platelet activating factor (PAF) may predispose to or increase the severity of the allergic airway response. The aim of this study was to investigate whether Lp-PLA₂ gene deficiency increases the risk of PAF and IgE-mediated inflammatory responses in vitro and in vivo using mouse models.

Methods

Lp-PLA₂-/- mice were generated and back crossed to the C57BL/6 background. PAF-AH activity was measured using a hydrolysis assay in serum and bronchoalveolar lavage (BAL) samples obtained from mice. Aspergillus fumigatus (Af)-specific serum was prepared for passive allergic sensitization of mice in vivo and mast cells in vitro. β- hexosaminidase release was studied in bone marrow derived mast cells sensitized with Af-specific serum or DNP-IgE and challenged with Af or DNP, respectively. Mice were treated with lipopolysaccharide (LPS) and PAF intratracheally and studied 24 hours later. Mice were sensitized either passively or actively against Af and were studied 48 hours after a single intranasal Af challenge. Airway responsiveness to methacholine, inflammatory cell influx in the lung tissue and BAL, immunoglobulin (ELISA) and cytokine (Luminex) profiles were compared between the wild type (WT) and Lp-PLA₂-/- mice.

Results

PAF-AH activity was reduced but not completely abolished in Lp-PLA₂-/- serum or by in vitro treatment of serum samples with a high saturating concentration of the selective Lp-PLA₂ inhibitor, SB-435495. PAF inhalation significantly enhanced airway inflammation of LPS treated WT and Lp-PLA₂-/- mice to a similar extent. Sensitized WT and Lp-PLA₂-/- bone-marrow derived mast cells released β-hexosaminidase following stimulation by allergen or IgE crosslinking to equivalent levels. Wild type and Lp-PLA₂-/- mice responded to passive or active allergic sensitization by significant IgE production, airway inflammation and hyperresponsiveness after Af challenge. BAL cell influx was not different between these strains while IL-4, IL-5, IL-6 and eotaxin release was attenuated in Lp-PLA₂-/- mice. There were no differences in the amount of total IgE levels in the Af sensitized WT and Lp-PLA₂-/- mice.

Conclusions

We conclude that Lp-PLA₂ deficiency in C57BL/6 mice did not result in a heightened airway inflammation or hyperresponsiveness after PAF/LPS treatment or passive or active allergic sensitization and challenge.

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.

Asthma and coagulation

Asthma is a chronic airway disease characterized by paroxysmal airflow obstruction evoked by irritative stimuli on a background of allergic lung inflammation. Currently, there is no cure for asthma, only symptomatic treatment. In recent years, our understanding of the involvement of coagulation and anticoagulant pathways, the fibrinolytic system, and platelets in the pathophysiology of asthma has increased considerably. Asthma is associated with a procoagulant state in the bronchoalveolar space, further aggravated by impaired local activities of the anticoagulant protein C system and fibrinolysis. Protease-activated receptors have been implicated as the molecular link between coagulation and allergic inflammation in asthma. This review summarizes current knowledge of the impact of the disturbed hemostatic balance in the lungs on asthma severity and manifestations and identifies new possible targets for asthma treatment.

Platelet-activating factor induces TLR4 expression in intestinal epithelial cells: implication for the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a leading cause of morbidity and mortality in neonatal intensive care units, however its pathogenesis is not completely understood. We have previously shown that platelet activating factor (PAF), bacteria and TLR4 are all important factors in the development of NEC. Given that Toll-like receptors (TLRs) are expressed at low levels in enterocytes of the mature gastrointestinal tract, but were shown to be aberrantly over-expressed in enterocytes in experimental NEC, we examined the regulation of TLR4 expression and signaling by PAF in intestinal epithelial cells using human and mouse in vitro cell lines, and the ex vivo rat intestinal loop model. In intestinal epithelial cell (IEC) lines, PAF stimulation yielded upregulation of both TLR4 mRNA and protein expression and led to increased IL-8 secretion following stimulation with LPS (in an otherwise LPS minimally responsive cell line). PAF stimulation resulted in increased human TLR4 promoter activation in a dose dependent manner. Western blotting and immunohistochemical analysis showed PAF induced STAT3 phosphorylation and nuclear translocation in IEC, and PAF-induced TLR4 expression was inhibited by STAT3 and NFκB Inhibitors. Our findings provide evidence for a mechanism by which PAF augments inflammation in the intestinal epithelium through abnormal TLR4 upregulation, thereby contributing to the intestinal injury of NEC.

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.

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.

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.

Genetic variants and haplotypes of lipoprotein associated phospholipase A2 and their influence on cardiovascular disease (The Ludwigshafen Risk and Cardiovascular Health Study)

BACKGROUND

There is increasing evidence that lipoprotein-associated phospholipase A2 (LpPLA2) is associated with cardiovascular disease. However, it is still unclear whether LpPLA2 is simply a marker or has a causal role as either a pro- or anti-atherogenic factor.

METHODS

We analyzed the association of five polymorphisms (-1357G>A, -403T>C, Arg92His, Ile198Thr, Ala379Val) and related haplotypes at the PLA2G7 locus with angiographic coronary artery disease (CAD), plasma LpPLA2 activity, and long-term survival in 3234 patients scheduled for coronary angiography.

RESULTS

The promoter variant -403C and His(92) were associated with a decrease and Val(379) with an increase in plasma LpPLA2 activity. Both coding variants revealed a clear gene-dose effect. Interestingly, the rare Thr(198) allele, which was not associated with any change in plasma LpPLA2 activity, was more frequent in subjects without CAD (P = 0.009), with an adjusted odds ratio for CAD of 0.69 (95% CI: 0.49-0.96; P = 0.029). None of the analyzed variants showed any robust association with all-cause or cardiovascular mortality.

CONCLUSION

Irrespective of the significant association between some variants with plasma LpPLA2 activity, it is still unclear whether these polymorphisms or haplotypes are associated with the risk and outcome of cardiovascular disease in Caucasians.

Associations of PLA2G7 gene polymorphisms with plasma lipoprotein-associated phospholipase A2 activity and coronary heart disease in a Chinese Han population: the Beijing atherosclerosis study

The human PLA2G7 gene encodes lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), an emerging risk factor for cardiovascular diseases. In the present study, seven single nucleotide polymorphisms (SNPs) in the PLA2G7 gene were genotyped in 827 patients with coronary heart disease (CHD), of which 512 were patients with myocardial infarction (MI), and 947 age- and gender-matched controls in a Chinese Han population. Plasma Lp-PLA(2) activity was measured in 416 randomly selected controls and 689 randomly selected CHD patients, including 423 MI patients. Lp-PLA(2) activity in CHD and MI cases was significantly higher (233.42+/-57.66 and 234.27+/-59.51 nmol ml(-1) min(-1), respectively) than in controls (211.47+/-58.61 nmol ml(-1) min(-1)). After adjusting for traditional risk factors by logistic regression, the odds ratios for CHD and MI per 1 standard deviation increment of Lp-PLA(2) activity were 1.27 (95% CI, 1.07-1.50) and 1.27 (95% CI, 1.05-1.54), respectively. Both single SNP analysis and haplotype analysis showed that the V279F and I198T polymorphisms were significantly associated with the reduced Lp-PLA(2) activity, but neither was associated with increased CHD risk. Both univariate and multivariate analyses, adjusting effects of conventional factors, indicated that the rs13210554 T allele increased the risk of MI in this Chinese Han population. In summary, an independent association of increased plasma Lp-PLA(2) activity with CHD and MI existed in this Chinese Han Population. Although V279F and I198T mutations significantly decreased the activity of Lp-PLA(2), only the promoter rs13210554 polymorphism was associated with MI. Lp-PLA(2) activity appears to influence the CHD and MI risk in Chinese Han population.

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.

Polyunsaturated fatty acids block platelet-activating factor-induced phosphatidylinositol 3 kinase/Akt-mediated apoptosis in intestinal epithelial cells

We have shown earlier that platelet-activating factor (PAF) causes apoptosis in enterocytes via a mechanism that involves Bax translocation to mitochondria, followed by caspase activation and DNA fragmentation. Herein we report that, in rat small intestinal epithelial cells (IEC-6), these downstream apoptotic effects are mediated by a PAF-induced inhibition of the phosphatidylinositol 3-kinase (PI 3-kinase)/protein kinase B (Akt) signaling pathway. Treatment with PAF results in rapid dephosphorylation of Akt, phosphoinositide-dependent kinase-1, and the YXXM p85 binding motif of several proteins and redistribution of Akt-pleckstrin homology domain-green fluorescent protein, i.e., an in vivo phosphatidylinositol (3,4,5)-trisphosphate sensor, from membrane to cytosol. The proapoptotic effects of PAF were inhibited by both n-3 and n-6 polyunsaturated fatty acids but not by a saturated fatty acid palmitate. Indomethacin, an inhibitor of prostaglandin biosynthesis, did not influence the baseline or PAF-induced apoptosis, but 2-bromopalmitate, an inhibitor of protein palmitoylation, inhibited all of the proapoptotic effects of PAF. Our data strongly suggest that an inhibition of the PI 3-kinase/Akt signaling pathway is the main mechanism of PAF-induced apoptosis in enterocytes and that polyunsaturated fatty acids block this mechanism very early in the signaling cascade independently of any effect on prostaglandin synthesis, and probably directly via an effect on protein palmitoylation.

Clinical correlations of recent developments in the pathogenesis of atopic dermatitis

Atopic dermatitis is a chronic inflammatory skin disease with a steadily increasing prevalence affecting 10-20 of infants and 1-3 of adults globally. It is often the first clinical manifestation of atopic disease preceding asthma and allergic rhinitis. Probably half of the children with atopic dermatitis develop some other form of atopic disease later in life. The pathogenesis involves a complex interplay of factors including genetic predisposition due to altered immune or skin barrier function, interactions with the environment such as food and allergen exposures, and infectious triggers of inflammation. In this review, we summarize the recent advances in understanding the contribution of different factors in the pathophysiology of atopic dermatitis and how insights provide new therapeutic potential for its treatment.

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.

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.

Two new highly polymorphic markers in the 3' UTR region of the PLA2G7 gene

We described two new highly polymorphic markers located 31 bp downstream of the last nucleotide of exon 12 in the 3' UTR region of the gene PLA2G7: 1344 +31TG(n) AG(m). Eight and 14 alleles were observed for the AG and TG repeats, respectively. These two markers have the highest heterozygosity until now reported for PLA2G7 gene.