PAF responsiveness in Japanese subjects with plasma PAF acetylhydrolase deficiency

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.

Lentiviral-mediated RNA interference of lipoprotein-associated phospholipase A2 ameliorates inflammation and atherosclerosis in apolipoprotein E-deficient mice

Lipoprotein associated phospholipase A2 (Lp-PLA2) overexpression is implicated in athero-sclerosis. In the present study, we evaluated the effects of lentiviral-mediated RNA interference (RNAi) of Lp-PLA2 on inflammation and atherosclerosis in apolipoprotein E-deficient mice. Apolipoprotein E-deficient mice were randomly allocated to control and experimental groups, and constrictive collars were used to induce plaque formation. Eight weeks after surgery, the lentiviral-mediated RNAi construct was used to silence expression of Lp-PLA2. Control and experimental lentivirus was transfected directly into carotid plaques or administered systemically. Tissues were collected for analysis 7 weeks after transfection. Inflammatory gene expression in the plasma and atherosclerotic lesions was then determined at the mRNA and protein levels. We observed no differences in body weight and plasma lipid levels at the end of the investigation. However, the expression levels of Lp-PLA2 and pro-inflammatory cytokines were significantly reduced in the RNAi groups, compared to the controls, whereas the plasma concentration of anti-inflammatory cytokines was markedly increased. Moreover, our results demonstrated a significant reduction in plaque area and lipid content, as well as a rise in collagen content following RNAi treatment. Importantly, when comparing the two methods of viral delivery, we found that transluminal local transfection exhibited enhanced improvement of plaque stability as compared to systemic administration. Inhibition of Lp-PLA2 by lentiviral-mediated RNAi ameliorates inflammation and atherosclerosis in apolipoprotein E-deficient mice. In addition, transluminal local delivery of Lp-PLA2 shRNA is superior to systemic administration for stabilizing atherosclerotic plaques.

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.

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.

Effects of V279F in the Lp-PLA(2) gene on markers of oxidative stress and inflammation in Koreans

BACKGROUND

A single nucleotide polymorphism (SNP), V279F, in the lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) gene is known to influence enzyme activity. It is unclear whether Lp-PLA(2) exerts pro- or antiatherogenic effects in humans. We investigated the interplay between V279F, Lp-PLA(2) activity, oxidative stress and inflammation.

METHODS

We genotyped 2914 healthy Koreans (43-79years) for the Lp-PLA(2) V279F and measured anthropometric parameters, lipid profile, fatty acid composition, lipid peroxides, inflammatory markers and Lp-PLA(2) levels.

RESULTS

Lp-PLA(2) activity was 24% lower in V/F subjects (n=641) than in those with the V/V genotype (n=2227). Enzyme activity was undetectable in F/F subjects. Lp-PLA(2) activity was positively correlated with LDL-cholesterol (r=0.134, P<0.001), ox-LDL (r=0.064, P<0.01), 8-epi-PGF(2alpha) (r=0.198, P<0.001), free fatty acid (r=0.082, P<0.001), and fibrinogen (r=0.112, P<0.01) levels. Additionally, ox-LDL, 8-epi-PGF(2alpha), free fatty acid, and fibrinogen levels were positively correlated with hs-CRP. V279F was associated with LDL-cholesterol and arachidonic acid (AA) in serum phospholipid. F/F subjects had lower LDL-cholesterol than V/V subjects (V/V: 120.9+/-0.69, V/F: 119.4+/-1.26, F/F: 109.2+/-4.84mg/dl, P=0.025). A significant association between the F/F genotype and increasing AA in serum phospholipids was found in subjects with high LDL-cholesterol (> or =130mg/dl) (P=0.003) but not in those with low LDL-cholesterol (<130mg/dl). F/F subjects in the high LDL-cholesterol group had CRP concentrations about three times higher than those with V/V or V/F genotypes (V/V: 1.25+/-0.09, V/F: 0.97+/-0.12, F/F: 3.20+/-0.88mg/dl, P<0.001).

CONCLUSIONS

The recessive effects of Lp-PLA(2) V279F on LDL-cholesterol and significant correlations between Lp-PLA(2) activity and LDL-cholesterol, 8-epi-PGF(2alpha) and fibrinogen support a pro-oxidative or pro-atherogenic role for this enzyme. Paradoxically, the combination of the complete deficiency of Lp-PLA(2) activity and high LDL-cholesterol enhanced lipid peroxidation and inflammation.

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.

The proinflammatory mediator Platelet Activating Factor is an effective substrate for human group X secreted phospholipase A2

Platelet Activating Factor (PAF) is a potent mediator of inflammation whose biological activity depends on the acetyl group esterified at the sn-2 position of the molecule. PAF-acetylhydrolase (PAF-AH), a secreted calcium-independent phospholipase A(2), is known to inactivate PAF by formation of lyso-PAF and acetate. However, PAF-AH deficient patients are not susceptible to the biological effects of inhaled PAF in airway inflammation, suggesting that other enzymes may regulate extracellular levels of PAF. We therefore examined the hydrolytic activity of the recently described human group X secreted phospholipase A(2) (hGX sPLA(2)) towards PAF. Among different sPLA(2)s, hGX sPLA(2) has the highest affinity towards phosphatidylcholine (PC), the major phospholipid of cellular membranes and plasma lipoproteins. Our results show that unlike group IIA, group V, and the pancreatic group IB sPLA(2), recombinant hGX sPLA(2) can efficiently hydrolyze PAF. The hydrolysis of PAF by hGX sPLA(2) rises abruptly when the concentration of PAF passes through its critical micelle concentration suggesting that the enzyme undergoes interfacial binding and activation to PAF. In conclusion, our study shows that hGX sPLA(2) may be a novel player in PAF regulation during inflammatory processes.

Clinical aspects of plasma platelet-activating factor-acetylhydrolase

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

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

CONTEXT AND OBJECTIVE

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

DESIGN

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

MAIN OUTCOME MEASURES

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

RESULTS

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

CONCLUSIONS

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

Lipoprotein-associated phospholipase A2 as a target of therapy

PURPOSE OF REVIEW

Considerable discussion continues regarding the precise role that secreted lipoprotein-associated phospholipase A2 (Lp-PLA2), also called platelet-activating factor acetylhydrolase, plays in atherosclerosis. Since interest in this enzyme as a putative drug target has been based primarily upon its association with low-density lipoprotein (LDL) in human plasma, this review will focus on Lp-PLA2 and human coronary heart disease.

RECENT FINDINGS

Recent reports have linked Lp-PLA2 enrichment not only to the most atherogenic of LDL particles but also to the most advanced, rupture-prone, plaques. Electronegative LDL has been shown to be highly enriched in Lp-PLA2; and in advanced atheroma, Lp-PLA2 levels are highly upregulated, colocalizing with macrophages in both the necrotic core and fibrous cap. Lp-PLA2 is well placed, whether on an oxidation susceptible LDL particle or in the highly oxidative environment of an advanced rupture-prone plaque, to hydrolyse oxidized phospholipid and generate significant quantities of the two pro-inflammatory mediators, lysophosphatidylcholine and oxidized nonesterified fatty acid. Several studies have confirmed that Lp-PLA2 is an independent risk factor for cardiovascular events (i.e. myocardial infarction and stroke). Although epidemiology studies consistently support a relationship between plasma Lp-PLA2 levels and susceptibility to coronary heart disease this is not the case for Lp-PLA2 polymorphisms. Two clinical studies have linked the Ala-379-->Val polymorphism with a reduced risk of myocardial infarction, but functional differences between the AA and VV polymorphs have yet to be demonstrated.

SUMMARY

Lp-PLA2 is intimately associated with several aspects of human atherogenesis. Although various lipid-lowering therapies, such as statins, have been shown to reduce plasma levels of Lp-PLA2, none has been studied in terms of its ability to lower the large macrophage-mediated upregulation of Lp-PLA2 within advanced plaques.