Lipolysis of LDL with phospholipase A2 alters the expression of selected apoB-100 epitopes and the interaction of LDL with cells

Molecular basis for the protective effects of low-density lipoprotein receptor-related protein 1 (LRP1)-derived peptides against LDL aggregation

Aggregated LDL is the first ligand reported to interact with the cluster II CR9 domain of low-density lipoprotein receptor-related protein 1 (LRP1). In particular, the C-terminal half of domain CR9, comprising the region Gly¹¹²⁷-Cys¹¹⁴⁰ exclusively recognizes aggregated LDL and it is crucial for aggregated LDL binding. Our aim was to study the effect of the sequence Gly¹¹²⁷-Cys¹¹⁴⁰ (named peptide LP3 and its retro-enantio version, named peptide DP3) on the structural characteristics of sphingomyelinase- (SMase) and phospholipase 2 (PLA₂)-modified LDL particles. Turbidimetry, gel filtration chromatography (GFC) and transmission electronic microscopy (TEM) analysis showed that LP3 and DP3 peptides strongly inhibited SMase- and PLA₂-induced LDL aggregation. Nondenaturing polyacrylamide gradient gel electrophoresis (GGE), agarose gel electrophoresis and high-performance thin-layer chromatography (HPTLC) indicated that LP3 and DP3 prevented SMase-induced alterations in LDL particle size, electric charge and phospholipid content, respectively, but not those induced by PLA₂. Western blot analysis showed that LP3 and DP3 counteracted changes in ApoB-100 conformation induced by the two enzymes. LDL proteomics (LDL trypsin digestion followed by mass spectroscopy) and computational modeling methods evidenced that peptides preserve ApoB-100 conformation due to their electrostatic interactions with a basic region of ApoB-100. These results demonstrate that LRP1-derived peptides are protective against LDL aggregation, even in conditions of extreme lipolysis, through their capacity to bind to ApoB-100 regions critical for ApoB-100 conformational preservation. These results suggests that these LRP1(CR9) derived peptides could be promising tools to prevent LDL aggregation induced by the main proteolytic enzymes acting in the arterial intima.

Targeting inflammation to reduce cardiovascular disease risk: a realistic clinical prospect?

Data from basic science experiments is overwhelmingly supportive of the causal role of immune-inflammatory response(s) at the core of atherosclerosis, and therefore, the theoretical potential to manipulate the inflammatory response to prevent cardiovascular events. However, extrapolation to humans requires care and we still lack definitive evidence to show that interfering in immune-inflammatory processes may safely lessen clinical atherosclerosis. In this review, we discuss key therapeutic targets in the treatment of vascular inflammation, placing basic research in a wider clinical perspective, as well as identifying outstanding questions.

LINKED ARTICLES

This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.

Novel genetic approach to investigate the role of plasma secretory phospholipase A2 (sPLA2)-V isoenzyme in coronary heart disease: modified Mendelian randomization analysis using PLA2G5 expression levels

BACKGROUND

Secretory phospholipase A2 (sPLA2) enzymes are considered to play a role in atherosclerosis. sPLA2 activity encompasses several sPLA2 isoenzymes, including sPLA2-V. Although observational studies show a strong association between elevated sPLA2 activity and CHD, no assay to measure sPLA2-V levels exists, and the only evidence linking the sPLA2-V isoform to atherosclerosis progression comes from animal studies. In the absence of an assay that directly quantifies sPLA2-V levels, we used PLA2G5 mRNA levels in a novel, modified Mendelian randomization approach to investigate the hypothesized causal role of sPLA2-V in coronary heart disease (CHD) pathogenesis.

METHODS AND RESULTS

Using data from the Advanced Study of Aortic Pathology, we identified the single-nucleotide polymorphism in PLA2G5 showing the strongest association with PLA2G5 mRNA expression levels as a proxy for sPLA2-V levels. We tested the association of this SNP with sPLA2 activity and CHD events in 4 prospective and 14 case-control studies with 27 230 events and 70 500 controls. rs525380C>A showed the strongest association with PLA2G5 mRNA expression (P=5.1×10(-6)). There was no association of rs525380C>A with plasma sPLA2 activity (difference in geometric mean of sPLA2 activity per rs525380 A-allele 0.4% (95% confidence intervals [-0.9%, 1.6%]; P=0.56). In meta-analyses, the odds ratio for CHD per A-allele was 1.02 (95% confidence intervals [0.99, 1.04]; P=0.20).

CONCLUSIONS

This novel approach for single-nucleotide polymorphism selection for this modified Mendelian randomization analysis showed no association between rs525380 (the lead single-nucleotide polymorphism for PLA2G5 expression, a surrogate for sPLA2-V levels) and CHD events. The evidence does not support a causal role for sPLA2-V in CHD.

Recent failures in antiatherosclerotic drug development: examples from the thyroxin receptor agonist, the secretory phospholipase A2 antagonist, and the acyl coenzyme A: cholesterol acyltransferase inhibitor programs

PURPOSE OF REVIEW

To review the published data related to the rise and fall of three different therapeutic approaches, which were investigated to lower cardiovascular disease (CVD) risk.

RECENT FINDINGS

CVD remains a major burden of morbidity and mortality, despite therapeutic interventions. Novel strategies to address this residual risk are eagerly awaited, and a number of novel targets for therapy have been identified. Lipids and lipoproteins have been shown to play an eminent role in atherosclerosis progression, and as such, interventions that influence these biomarkers are crucial in CVD risk prevention. In recent years, however, clinical studies investigating the effect of novel lipid-modifying drugs on cardiovascular risk prevention have not always resulted in the anticipated beneficial outcome. Moreover, the development of therapies directed toward bioactive proteins acting at the crossroads of lipids and inflammation has also been disappointing.

SUMMARY

In this review, we will specifically address the rationale, design, and results of the clinical trials investigating the effects of three of the failing therapies: the thyroxin receptor agonist, the secretory phospholipase A2 antagonist, and the acyl coenzyme A:cholesterol acyltransferase inhibitor.

Comparison of the effects of combination atorvastatin (40 mg) + ezetimibe (10 mg) versus atorvastatin (40 mg) alone on secretory phospholipase A2 activity in patients with stable coronary artery disease or coronary artery disease equivalent

Secretory phospholipase A2 (sPLA2) is an enzyme that plays an important role in the pathogenesis of atherosclerosis and of adverse cardiovascular events. It is currently the target of emerging therapeutic agents. Our study was designed to investigate the effect of aggressive lowering of low-density lipoprotein (LDL) cholesterol with ezetimibe and atorvastatin on sPLA2 activity. We randomized 100 patients with stable coronary artery disease (CAD) or CAD equivalent (diabetes, stroke, or peripheral vascular disease) to receive ezetimibe 10 mg/day in association with atorvastatin 40 mg/day (combination therapy group) versus atorvastatin 40 mg/day and placebo (monotherapy group). Patients on statin therapy before inclusion were allowed to enter the study as long as the potency of the statin was lower than atorvastatin 40 mg/day. Lipid profile, high-sensitivity C-reactive protein (hs-CRP), and sPLA activity were measured at baseline and after 8 weeks of therapy. The decrease in LDL cholesterol was more significant in the combination therapy group, but the decrease in hs-CRP was similar. sPLA2 activity significantly decreased in the ezetimibe/atorvastatin group from 29 U/ml (interquartile range 23 to 35) to 26 U/ml (23 to 29, p = 0.001) but remained similar in the placebo/atorvastatin group (23 U/ml, 19 to 32, vs 22 U/ml, 19 to 28, p = NS). In a multivariate stepwise linear regression model, change in sPLA2 correlated with change in hs-CRP (p <0.001), baseline LDL cholesterol level (p = 0.001), body mass index (p = 0.003), diabetes mellitus (p = 0.04) and combination therapy with ezetimibe/atorvastatin (p = 0.05). In conclusion, this study demonstrates that coadministration of ezetimibe and atorvastatin decreases sPLA2 activity.

After FRANCIS: next steps in the clinical evaluation of varespladib methyl

Secretory phospholipase A2 (sPLA2) represents a family of isoenzymes that participate in lipoprotein and inflammatory pathways, mediate atherosclerosis and enhance myocardial ischemic injury. The Fewer Recurrent Acute Coronary Events with Near-term Cardiovascular Inflammatory Suppression (FRANCIS) trial (NCT00743925) was a Phase II trial designed to examine the effects of varespladib methyl, a small-molecule inhibitor of sPLA2, on plasma biomarkers in patients with acute coronary syndrome (ACS) who were treated with atorvastatin 80 mg and standard-of-care daily. Varespladib methyl significantly reduced low-density lipoprotein cholesterol and inflammatory biomarkers in ACS subjects treated with standard-of-care and atorvastatin 80 mg daily. There was a nonsignificant reduction in major adverse cardiovascular events at study completion; however, positive trends remained for unstable angina and myocardial infarction. In order to achieve the widespread use of varespladib methyl in ACS patients, completion of a prospective, randomized placebo-controlled trial in ACS patients and stable coronary artery disease patients with increased sPLA2 activity will be required.

Randomized trial of an inhibitor of secretory phospholipase A2 on atherogenic lipoprotein subclasses in statin-treated patients with coronary heart disease

AIMS

To investigate the effects of secretory phospholipase A2 (sPLA(2)) inhibition on plasma lipoproteins. Secretory phospholipase A2 isoenzymes promote atherosclerosis by mechanisms that include lipoprotein modification, retention, and oxidation.

METHODS AND RESULTS

Phospholipase Levels And Serological Markers of Atherosclerosis II (PLASMA II) is a Phase II, randomized, double-blind, placebo-controlled parallel-arm study of two once-daily doses of the novel sPLA(2) inhibitor, 1-H-indole-3-glyoxamide or varespladib methyl (Anthera Pharmaceuticals, Hayward, CA, USA). One hundred and thirty-five stable coronary heart disease patients were treated with either varespladib methyl 250 mg once daily, varespladib methyl 500 mg once daily, or placebo for 8 weeks. Varespladib methyl treatment resulted in statistically significant dose-dependent reductions that were different from placebo in sPLA(2) concentration, low-density lipoprotein (LDL) cholesterol, and non-high-density lipoprotein (HDL) cholesterol. When compared with placebo, varespladib methyl 500 mg once daily reduced LDL cholesterol by 15% (P < 0.001), non-HDL cholesterol by 15% (P < 0.001), total very LDL (VLDL) particle concentration by 14% (P = 0.022), and small VLDL particle concentration by 24% (P = 0.030). Relative to baseline, varespladib methyl 500 mg once daily reduced total LDL particle concentration (7%, P = 0.002) and small LDL particle concentration (11%, P = 0.014).

CONCLUSION

Reductions in atherogenic lipoproteins suggest that varespladib methyl 500 mg once daily may be an effective anti-atherosclerotic agent. Trial registered at ClinicalTrials.gov, identifier: NCT00525954.

Effects of varespladib methyl on biomarkers and major cardiovascular events in acute coronary syndrome patients

OBJECTIVES

The purpose of this study was to investigate the effects of varespladib on cardiovascular biomarkers in acute coronary syndrome patients.

BACKGROUND

Secretory phospholipase A(2) (sPLA(2)) represents a family of proatherogenic enzymes that hydrolyze lipoprotein phospholipids, increasing their affinity for intimal proteoglycans; contribute to cholesterol loading of macrophages by nonscavenger receptor mediated pathways; and activate inflammatory pathways. In prospective studies, high sPLA(2)-IIA levels predicted major adverse cardiovascular events in acute coronary syndrome (ACS) and stable coronary heart disease patients.

METHODS

This randomized, double-blind, prospective controlled clinical trial (phase 2B) was designed to investigate the effects of sPLA(2) inhibition with varespladib 500 mg daily versus placebo as adjunctive therapy to atorvastatin 80 mg daily on biomarkers (low-density lipoprotein cholesterol [LDL-C], high-sensitivity C-reactive protein [hsCRP], and sPLA(2)-IIA levels), major adverse cardiovascular events (unstable angina, myocardial infarction, death), and safety. In all, 625 ACS subjects were randomized within 96 h of the index event and treated for a minimum of 6 months.

RESULTS

After 8 weeks (primary efficacy end point), varespladib/atorvastatin reduced mean LDL-C levels from baseline by 49.6% compared with 43.4% with placebo/atorvastatin (p = 0.002). Respective 8-week median reductions in sPLA(2)-IIA levels were 82.4% and 15.6% (p < 0.0001), and hsCRP levels were lowered by 75.0% and 71.0% (p = 0.097). At 24 weeks, respective reductions with varespladib and placebo were as follows: LDL-C 43.5% versus 37.6% (p < 0.05), hsCRP 79.8% versus 77.0% (p = 0.02), and sPLA(2)-IIA 78.5% versus 6.4% (p < 0.0001). Major adverse cardiovascular events were not different from placebo 6 months post-randomization (7.3% varespladib vs. 7.7% placebo). No treatment differences in elevated liver function studies on >1 occasion were observed.

CONCLUSIONS

Varespladib therapy effectively reduced LDL-C and inflammatory biomarkers in ACS patients treated with conventional therapy including atorvastatin 80 mg daily. There were no treatment differences in clinical cardiovascular events. (FRANCIS [Fewer Recurrent Acute Coronary Events With Near-Term Cardiovascular Inflammation Suppression]-ACS Trial: A Study of the Safety and Efficacy of A 002 in Subjects With Acute Coronary Syndromes; NCT00743925).

Bioactive products generated by group V sPLA(2) hydrolysis of LDL activate macrophages to secrete pro-inflammatory cytokines

OBJECTIVE

Previous studies have established that hydrolysis of LDL by Group V secretory phospholipase A(2) (GV sPLA(2)) generates a modified particle capable of inducing macrophage foam cell formation. The aim of the present study was to determine whether GV sPLA(2)-hydrolyzed LDL (GV-LDL) produces pro-atherogenic effects in macrophages independent of cholesterol accumulation.

METHODS AND RESULTS

J-774 cells incubated with GV-LDL produced more TNF-alpha and IL-6 compared to cells incubated with control-LDL. Indirect immunofluorescence showed that GV-LDL but not control-LDL induced nuclear translocation of NFkappaB. Inhibitors of NFkappaB activation, effectively blocked cytokine production induced by GV-LDL. Control-LDL and GV-LDL were separated from albumin present in reaction mixtures by ultracentrifugation. The albumin fraction derived from GV-LDL contained 80% of the FFA generated and was more potent than the re-isolated GV-LDL in inducing pro-inflammatory cytokine secretion. Linoleic acid (18:2) and oleic acid (18:1) were the most abundant FFAs generated, whereas newly formed lyso-PCs contained 14:0 (myristic), 16:1 (palmitic), and 18:2 fatty acyl groups. Experiments with synthetic FFA showed that 18:1 induced J-774 cells to secrete TNF-alpha and IL-6.

CONCLUSIONS

These results indicate that in addition to promoting atherosclerotic lipid accumulation in macrophages, GV sPLA(2) hydrolysis of LDL leads to activation of NFkappaB, a key regulator of inflammation.

Lipoprotein modification by secretory phospholipase A(2) enzymes contributes to the initiation and progression of atherosclerosis

PURPOSE OF REVIEW

Secretory phospholipase A2s (sPLA2s) are considered to be important enzymes in the initiation and progression of atherosclerosis. In this review, we discuss the various mechanisms by which the direct action of the sPLA2s on LDL particles in the arterial intima may contribute to atherogenesis.

RECENT FINDINGS

A wealth of evidence, both in vitro and in vivo, supports a role for the sPLA2s in atherogenesis. Very recently, systemic inhibition of sPLA2s was found to reduce measures of arterial inflammation. The mechanisms behind this inhibition, however, are largely unknown. Here, we discuss the consequences of sPLA2 action on LDL in the arterial intima and address the recent findings regarding the effects of the lipolytic products of sPLA2, lysophosphatidylcholine, and fatty acids on intimal cells. LDL modified by sPLA2 can accumulate in the arterial intima both extracellularly and intracellularly. Importantly, the lipolytic products promote atherosclerosis by monocyte/macrophage recruitment, by enhancing the production of proretentive molecules by vascular smooth muscle cells, and by inducing cell death.

SUMMARY

Recent findings on sPLA2s support the idea that the enzymes contribute to human atherogenesis not only as initiating agents but also in maintaining plaque inflammation.

Generation in human plasma of misfolded, aggregation-prone electronegative low density lipoprotein

Human plasma contains small amounts of a low density lipoprotein in which apoprotein is misfolded. Originally identified and isolated by means of anion-exchange chromatography, this component was subsequently described as electronegative low density lipoprotein (LDL)(-), with increased concentrations associated with elevated cardiovascular disease risk. It has been recognized recently as the trigger of LDL amyloidogenesis, which produces aggregates similar to subendothelial droplets observed in vivo in early atherogenesis. Although LDL(-) has been produced in vitro through various manipulations, the mechanisms involved in its generation in vivo remain obscure. By using a more physiological model, we demonstrate spontaneous, sustained and noticeable production of LDL(-) during incubation of unprocessed human plasma at 37 degrees C. In addition to a higher fraction of amyloidogenic LDL(-), LDL purified from incubated plasma contains an increased level of lysophospholipids and free fatty acids; analysis of LDL lipids packing shows their loosening. As a result, during plasma incubation, lipid destabilization and protein misfolding take place, and aggregation-prone particles are generated. All these phenomena can be prevented by inhibiting calcium-dependent secretory phospholipases A2. Our plasma incubation model, without removal of reaction products, effectively shows a lipid-protein interplay in LDL, where lipid destabilization after lipolysis threatens the apoprotein's structure, which misfolds and becomes aggregation-prone.

Effects of 1-H-indole-3-glyoxamide (A-002) on concentration of secretory phospholipase A2 (PLASMA study): a phase II double-blind, randomised, placebo-controlled trial

BACKGROUND

Secretory phospholipase A(2) (sPLA(2)) enzymes, produced and secreted in human blood vessels and hepatocytes, contribute to the development of atherosclerosis through mechanisms that are both dependent and independent of lipoprotein. We examined the effects of an sPLA(2) inhibitor on enzyme concentration and on plasma lipoproteins and inflammatory biomarkers in patients with coronary heart disease.

METHODS

Patients aged 18 years and older with stable coronary heart disease from the USA and Ukraine were eligible for enrolment in this phase II, randomised, double-blind, placebo-controlled, parallel-arm, dose-response study. 393 patients were randomly assigned by computer-generated sequence to receive either placebo (n=79) or one of four doses of an sPLA(2) inhibitor, A-002 (1-H-indole-3-glyoxamide; 50 mg [n=79], 100 mg [n=80], 250 mg [n=78], or 500 mg [n=77] twice daily), for 8 weeks. The primary endpoint was the change in sPLA(2) group IIA (sPLA(2)-IIA) concentration or activity from baseline to week 8. Analysis was by modified intention to treat (ITT). The ITT population consisted of all patients who received one dose of study treatment; data for patients who dropped out before the end of the study were carried forward from last observation. This trial is registered with ClinicalTrials.gov, number NCT00455546.

FINDINGS

All randomised patients received at least one dose and were included in the ITT population. Data for 45 patients were carried forward from last observation (36 in the A-002 group and nine in the placebo group); the main reason for dropout before completion was because of adverse events. 348 patients reached the primary endpoint (A-002 n=278, placebo n=70). Mean sPLA(2)-IIA concentration fell by 86.7%, from 157 pmol/L to 21 [corrected] pmol/L, in the overall active treatment group, and by 4.8%, from 157 pmol/L to 143 [corrected] pmol/L, in the placebo group (p<0.0001 treatment vs placebo). The reductions in sPLA(2)-IIA concentration in the A-002 groups were dose dependent (ranging from 69.2% in the 50 mg group to 95.8% in the 500 mg group) and differed significantly from placebo (p<0.0001 for all doses). In the 500 mg A-002 treatment group, there was one serious adverse event (exacerbation of underlying chronic obstructive pulmonary disease), but the proportion of patients reporting treatment-emergent adverse events did not differ from placebo. The main side-effects of the drug included headache (n=20), nausea (n=17), and diarrhoea (n=12).

INTERPRETATION

The reductions in sPLA(2)-IIA concentration suggest that A-002 might be an effective anti-atherosclerotic agent.

Dimyristoylphosphotidylcholine induces conformational changes in apoB that lowers lipoprotein(a)

Lipoprotein(a) [Lp(a)] is assembled by the binding of apolipoprotein B (apoB) lysine residues on LDL to lysine binding sites in apolipoprotein(a) [apo(a)] and the subsequent formation of a disulphide bond between apoB and apo(a). In this study, we induced changes in apoB conformation by adding phospholipids to LDL and tested the effect of the altered apoB conformation on Lp(a) assembly. The addition of dimyristoylphosphatidylcholine (DMPC) to isolated LDL induced a decrease in the alpha-helical content of apoB and increased the immunoreactivity of the apoB C terminus toward monoclonal antibodies in the region. These conformational changes were associated with a reduction in the ability of the DMPC-modified LDL to form Lp(a) in in vitro assays. Furthermore, administration of DMPC to Lp(a) transgenic mice lead to a significant but transient decrease in Lp(a) levels (18.6% decrease at 2 h, P < 0.001) which coincided with the association of DMPC with LDL in plasma. Our study shows that changes in apoB conformation in the C-terminal region alter the exposure of sequences required for Lp(a) assembly and reduce the formation of Lp(a) both in vitro and in vivo. We conclude that manipulation of LDL surface phospholipids alters Lp(a) levels.

Impact of a treatment with phospholipase A2 on the physicochemical properties of hen egg yolk

Changes in physicochemical properties of egg yolk were investigated after a treatment with phospholipase A 2 (PLA 2), where phospholipids are converted in lyso-phospholipids. Protein solubility and protein denaturation before and after modification by PLA 2 was monitored as well as the functionality of egg yolk by means of interfacial tension. Enzymatic treatment showed a significant impact on the properties of egg yolk with regard to protein solubility and denaturation behavior. To gain a closer insight, egg yolk was separated in its water-soluble fraction called plasma and the insoluble granules. Both fractions were separately modified by PLA 2. The granule fraction shows a higher protein solubility, and the plasma proteins show very high heat stability after modification by PLA 2. Hypotheses regarding related changes in the low-density lipoprotein (LDL) particles are discussed. Results suggest that significant differences in the functional properties of untreated and PLA 2-modified egg yolk do not primarily result from the existence of lyso-phospholipids but from structural changes in egg yolk granules and LDL particles.

Atherogenic properties of LDL particles modified by human group X secreted phospholipase A2 on human endothelial cell function

Increasing evidence suggests that secreted phospholipases A2 (sPLA2s) play an important role in the pathophysiology of atherosclerosis. Among sPLA2s, the human group X (hGX) enzyme has the highest catalytic activity toward phosphatidylcholine, one of the major phospholipid species of cell membranes and low-density lipoprotein (LDL). Our study examined the presence of hGX sPLA2 in human atherosclerotic lesions and investigated the ability of hGX modified LDL to alter human endothelial cell (HUVEC) function. Our results show that hGX sPLA2 is present in human atherosclerotic lesions and that the hydrolysis of LDL by hGX sPLA2 results in a modified particle that induces lipid accumulation in human monocyte-derived macrophages. Acting on endothelial cells, hGX-modified LDL activates the MAP kinase pathway, which leads to increased arachidonic acid release, increased expression of adhesion molecules on the surface of HUVEC, and increased adhesion of monocytes to HUVEC monolayers. Together, our data suggest that LDL modified by hGX, rather than hGX itself may have strong proinflammatory and proatherogenic properties, which could play an important role in the propagation of atherosclerosis.

Interplay between lipoproteins and bee venom phospholipase A2 in relation to their anti-plasmodium toxicity

We previously showed that the in vitro intraerythrocytic development of the malarial agent Plasmodium falciparum is strongly inhibited by secreted phospholipases A(2) (sPLA(2)s) from animal venoms. Inhibition is dependent on enzymatic activity and requires the presence of serum lipoproteins in the parasite culture medium. To evaluate the potential involvement of host lipoproteins and sPLA(2)s in malaria, we investigated the interactions between bee venom phospholipase A(2) (bvPLA(2)), human triglyceride-rich lipoproteins, and infected erythrocytes. Even at high enzyme concentration (100x IC(50)), bvPLA(2) binding to Plasmodium-infected or normal erythrocytes was not detected. On the contrary, tight association with lipoproteins was observed through the formation of buoyant bvPLA(2)/lipoprotein complexes. Direct involvement of the hydrolysis lipid products in toxicity was demonstrated. Arachidonic acid (C20:4), linoleic acid (C18:2), and, to a lesser extent, docosahexaenoic acid (C22:6) appeared as the main actors in toxicity. Minimal oxidation of lipoproteins enhanced toxicity of the lipolyzed particles and induced their interaction with infected or normal erythrocytes. Fresh or oxidized lipolyzed lipoproteins induced the parasite degeneration without host cell membrane disruption, ruling out a possible membranolytic action of fatty acids or peroxidation products in the death process. In conclusion, our data enlighten on the capability of secreted PLA(2)s to exert cytotoxicity via the extracellular generation of toxic lipids, and raise the question of whether such mechanisms could be at play in pathophysiological situations such as malaria.

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

Apolipoprotein B: a clinically important apolipoprotein which assembles atherogenic lipoproteins and promotes the development of atherosclerosis

Apolipoprotein (apo) B exists in two forms apoB100 and apoB48. ApoB100 is present on very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and LDL. ApoB100 assembles VLDL particles in the liver. This process starts by the formation of a pre-VLDL, which is retained in the cell unless converted to the triglyceride-poor VLDL2. VLDL2 is secreted or converted to VLDL1 by a bulk lipidation in the Golgi apparatus. ApoB100 has a central role in the development of atherosclerosis. Two proteoglycan-binding sequences in apoB100 have been identified, which are important for retaining the lipoprotein in the intima of the artery. Retention is essential for the development of the atherosclerotic lesion.

Decrease in pH Strongly Enhances Binding of Native, Proteolyzed, Lipolyzed, and Oxidized Low Density Lipoprotein Particles to Human Aortic Proteoglycans

Binding of low density lipoprotein (LDL) to proteoglycans and modification of LDL are key processes in atherogenesis. Recently, it has been demonstrated that during atherogenesis the extracellular pH of atherosclerotic lesions decreases. We have examined the effect of the decreased pH on the binding of LDL to human aortic proteoglycans. The binding of native, oxidized, proteolyzed (alpha-chymotrypsin-treated), or lipolyzed (sphingomyelinase- or phospholipase A(2)-treated) LDL particles to proteoglycans were measured in microtiter well assays at pH 5.5-7.5. We found that the lower the pH, the higher the amount of binding of LDL to proteoglycans. At the lowest pH tested (pH 5.5), the amounts of proteoglycan-bound native, proteolyzed, sphingomyelinase-, and phospholipase A(2)-treated LDL were 20-, 23-, 30-, and 37-fold higher, respectively, than at pH 7.5. Interestingly, although oxidized LDL failed to bind to proteoglycans at neutral pH, there was significant binding at acidic pH. Binding of native and modified LDL to proteoglycans at pH 5.5 was blocked by 1 m NaCl, indicating that at neutral pH LDL binds to proteoglycans via ionic interactions. Inhibition of this binding by acetylation and cyclohexanedione treatment of LDL showed that the positively charged amino acids of apolipoprotein B-100, lysine, and arginine, respectively, mediated the ionic interaction. Taken together, our results suggest that in areas of atherosclerotic arterial intima where the extracellular pH decreases, retention of LDL by proteoglycans is enhanced, leading to extracellular accumulation of LDL and progression of the disease.

Group V Secretory Phospholipase A2-modified Low Density Lipoprotein Promotes Foam Cell Formation by a SR-A- and CD36-independent Process That Involves Cellular Proteoglycans

Accumulating evidence indicates that secretory phospholipase A2 (sPLA2) enzymes promote atherogenic processes. We have previously showed the presence of Group V sPLA2 (GV sPLA2) in human and mouse atherosclerotic lesions, its hydrolysis of low density lipoprotein (LDL) particles, and the ability of GV sPLA2-modified LDL (GV-LDL) to induce macrophage foam cell formation in vitro. The goal of this study was to investigate the mechanisms involved in macrophage uptake of GV-LDL. Peritoneal macrophages from C57BL/6 mice (wild type (WT)), C57BL/6 mice deficient in LDL receptor (LDLR-/-), or SR-A and CD36 (DKO) were treated with control LDL, GV-LDL, oxidized LDL (ox-LDL) or LDL aggregated by vortexing (vx-LDL). As expected, ox-LDL induced significantly more cholesterol ester accumulation in WT and LDLR-/- compared with DKO macrophages. In contrast, there was no difference in the accumulation of GV-LDL or vx-LDL in the three cell types. 125I-ox-LDL exhibited high affinity, saturable binding to WT cells that was significantly reduced in DKO cells. Vx-LDL and GV-LDL showed low affinity, non-saturable binding that was similar for both cell types, and significantly higher compared with control LDL. GV-LDL degradation in WT and DKO cells was similar. Analyses by confocal microscopy indicated a distinct intracellular distribution of Alexa-568-labeled GV-LDL and Alexa-488-labeled ox-LDL. Uptake of GV-LDL (but not ox-LDL or vx-LDL) was significantly reduced in cells preincubated with heparin or NaClO3, suggesting a role for proteoglycans in GV-LDL uptake. Our data point to a physiological modification of LDL that has the potential to promote macrophage foam cell formation independent of scavenger receptors.

The catalytically active secretory phospholipase A2 type IIA is involved in restenosis development after PTCA in human coronary arteries and generation of atherogenic LDL

Secretory phospholipase A2 type IIA (sPLA2) may actively contribute to atherogenesis, acting either within the arterial wall or in plasma. Proinflammatory eicosanoids and lysophospholipids, generated through hydrolysis of cell membrane phospho-lipids by sPLA2, initiate and prolong the inflammatory process. In the present study we examined the possible involvement of sPLA2 in development of restenosis in patients undergoing percutaneous transluminal coronary angioplasty (PTCA). We also investigated whether serum sPLA2 could catalyze accumulation of lysophosphatidylcholine (LPC) in LDL. Concentrations and catalytic activities of sPLA2 were measured in blood serum of 49 consenting patients immediately before, 1-7 and 180 days after PTCA. All patients had repeat angiograms at 180-day follow-up. Restenosis was registered in 19 patients. Accumulation of LPC in LDL was evaluated by thin-layer chromatography after incubation of blood serum with LDL. Serum sPLA2 concentrations increased in all study patients by day 1 post-PTCA, but the increase was significantly greater and more protracted in patients who developed restenosis. Catalytic activities increased significantly 6 days post-PTCA in patients who developed restenosis, whereas for patients without restenosis there was no change in serum sPLA2 activity throughout the study period in spite of the sPLA2 presence in blood. Incubation of blood serum (6 days post-PTCA) with LDL resulted in accumulation of LPC only for those patients who subsequently developed restenosis. Manoalide, a specific inhibitor of sPLA2, completely blocked the LPC accumulation. The data indicate that elevated serum sPLA2 activity after PTCA is associated with restenosis development and may be involved in atherogenic modification of LDL in blood serum.

Impaired Binding Affinity of Electronegative Low-Density Lipoprotein (LDL) to the LDL Receptor Is Related to Nonesterified Fatty Acids and Lysophosphatidylcholine Content†

The binding characteristics of electropositive [LDL(+)] and electronegative LDL [LDL(-)] subfractions to the LDL receptor (LDLr) were studied. Saturation kinetic studies in cultured human fibroblasts demonstrated that LDL(-) from normolipemic (NL) and familial hypercholesterolemic (FH) subjects had lower binding affinity than their respective LDL(+) fractions (P < 0.05), as indicated by higher dissociation constant (K(D)) values. FH-LDL(+) also showed lower binding affinity (P < 0.05) than NL-LDL(+) (K(D), sorted from lower to higher affinity: NL-LDL(-), 33.0 +/- 24.4 nM; FH-LDL(-), 24.4 +/- 7.1 nM; FH-LDL(+), 16.6 +/- 7.0 nM; NL-LDL(+), 10.9 +/- 5.7 nM). These results were confirmed by binding displacement studies. The impaired affinity binding of LDL(-) could be attributed to altered secondary and tertiary structure of apolipoprotein B, but circular dichroism (CD) and tryptophan fluorescence (TrpF) studies revealed no structural differences between LDL(+) and LDL(-). To ascertain the role of increased nonesterified fatty acids (NEFA) and lysophosphatidylcholine (LPC) content in LDL(-), LDL(+) was enriched in NEFA or hydrolyzed with secretory phospholipase A(2). Modification of LDL gradually decreased the affinity to LDLr in parallel to the increasing content of NEFA and/or LPC. Modified LDLs with a NEFA content similar to that of LDL(-) displayed similar affinity. ApoB structure studies of modified LDLs by CD and TrpF showed no difference compared to LDL(+) or LDL(-). Our results indicate that NEFA loading or phospholipase A(2) lipolysis of LDL leads to changes that affect the affinity of LDL to LDLr with no major effect on apoB structure. Impaired affinity to the LDLr shown by LDL(-) is related to NEFA and/or LPC content rather than to structural differences in apolipoprotein B.

Electronegative low-density lipoprotein

PURPOSE OF REVIEW

The occurrence in blood of an electronegatively charged LDL was described in 1988. During the 1990s reports studying electronegative LDL (LDL(-)) were scant and its atherogenic role controversial. Nevertheless, recent reports have provided new evidence on a putative atherogenic role of LDL(-). This review focuses on and discusses these new findings.

RECENT FINDINGS

In recent years, LDL(-) has been found to be involved in several atherogenic features through its action on cultured endothelial cells. LDL(-) induces the production of chemokines, such as IL-8 and monocyte chemotactic protein 1, and increases tumor necrosis factor-alpha-induced production of vascular cell adhesion molecule 1, with these molecules being involved in early phases of leukocyte recruitment. LDL(-) from familial hypercholesterolemic patients also decreases DNA synthesis and intracellular fibroblast growth factor 2 production, which may contribute to impaired angiogenesis and increased apoptosis. In addition, the preferential association of platelet-activating factor acetylhydrolase with LDL(-) has been reported, suggesting a proinflammatory role of this enzyme in LDL(-).

SUMMARY

Recent findings suggest that LDL(-) could contribute to atherogenesis via several mechanisms, including proinflammatory, proapoptotic and anti-angiogenesis properties. Further studies are required to define the role of LDL(-) in atherogenesis more precisely and to clarify mechanisms involved in endothelial cell activation.

Molecular Mechanism for Changes in Proteoglycan Binding on Compositional Changes of the Core and the Surface of Low-Density Lipoprotein–Containing Human Apolipoprotein B100

OBJECTIVE

The aim of this study was to investigate the molecular mechanism for changes in proteoglycan binding and LDL receptor affinity on two compositional changes in LDL that have been associated with atherosclerosis: cholesterol enrichment of the core and modification by secretory group IIA phospholipase A2 (sPLA2) of the surface.

METHODS AND RESULTS

Transgenic mice expressing recombinant apolipoprotein (apo) B and sPLA2 were generated. Recombinant LDL were isolated and tested for their proteoglycan and LDL receptor-binding activity. The results show site A (residues 3148-3158) in apoB100 becomes functional in sPLA2-modified LDL and that site A acts cooperatively with site B (residues 3359-3369), the primary proteoglycan-binding site in native LDL, in the binding of sPLA2-modified LDL to proteoglycans. Our results also show that cholesterol enrichment of LDL is associated with increased affinity for proteoglycans and for the LDL receptor. This mechanism is likely mediated by a conformational change of site B and is independent of site A in apoB100.

CONCLUSIONS

Site A in apoB100 becomes functional in sPLA2-modified LDL and acts cooperatively with site B resulting in increased proteoglycan-binding activity. The increased binding for proteoglycans of cholesterol-enriched LDL is solely dependent on site B.

Group V sPLA2 hydrolysis of low-density lipoprotein results in spontaneous particle aggregation and promotes macrophage foam cell formation

OBJECTIVE

Secretory phospholipase A2 (sPLA2) enzymes hydrolyze the sn-2 fatty acyl ester bond of phospholipids to produce a free fatty acid and a lysophospholid. Group V sPLA2 is expressed in cultured macrophage cells and has high affinity for phosphatidyl choline-containing substrates. The present study assesses the presence of group V sPLA2 in human and mouse atherosclerotic lesions and its activity toward low-density lipoprotein (LDL) particles.

METHODS AND RESULTS

Group V sPLA2 was detected in human and mouse atherosclerotic lesions by immunohistochemical staining. Electron microscopic analysis showed that mouse group V sPLA2-modified LDL is significantly smaller (mean diameter+/-SEM=25.3+/-0.25 nm) than native LDL (mean diameter+/-SEM=27.7+/-0.29 nm). Hydrolysis by group V sPLA2 induced spontaneous particle aggregation; the extent of aggregation was directly proportional to the degree of LDL hydrolysis. Group V sPLA2 modification of LDL led to enhanced lipid accumulation in cultured mouse peritoneal macrophage cells.

CONCLUSIONS

Group V sPLA2 may play an important role in promoting atherosclerotic lesion development by modifying LDL particles in the arterial wall, thereby enhancing particle aggregation, retention, and macrophage uptake.

Intimal retention of cholesterol derived from apolipoprotein B100- and apolipoprotein B48-containing lipoproteins in carotid arteries of Watanabe heritable hyperlipidemic rabbits

OBJECTIVE

The arterial retention of apolipoprotein (apo) B100- and apoB48-containing lipoproteins was simultaneously determined in a rabbit model of human hypercholesterolemia using 3D confocal microscopy.

METHODS AND RESULTS

Lipoproteins containing apoB100 (LDL) and apoB48 (chylomicron remnants) were differentially conjugated with fluorophores and simultaneously perfused at equivalent concentrations under physiological conditions in situ through carotid vessels of Watanabe heritable hyperlipidemic rabbits and compared with controls. Retention of lipoproteins was defined as the amount remaining after an extensive washout phase. LDL and chylomicron remnants were both retained, primarily within the subendothelial space. Without a concomitant increase in exposure to lipoproteins, we found a marked increase in the retention of cholesterol within the intima of Watanabe heritable hyperlipidemic rabbits compared with controls, specifically because of increased entrapment of apoB48-containing lipoproteins.

CONCLUSIONS

Collectively, our data suggest that hypercholesterolemia induced as a consequence of LDL receptor deficiency differentially influences retention of LDL and of chylomicron remnants.

Atherogenic role of lysophosphatidylcholine in low-density lipoprotein modified by phospholipase A2 and in diabetic patients: protection by nitric oxide donor

The aim of our study was to investigate the atherogenic role of lysophosphatidylcholine (lyso-PC) in low-density lipoprotein (LDL) under diabetic environment. Expression of monocyte chemoattractant protein-1 (MCP-1) mRNA and nuclear factor-kappa B (NF-kappaB)-DNA binding activity were determined in human umbilical vein endothelial cells (HUVEC) incubated with native or glycoxidized LDL, LDL modified by phospholipase A2 (PLA2) and LDL isolated from diabetic patients. Lyso-PC contents in LDL were measured using electrospray ionization-liquid chromatography/mass spectrometry (ESI-LC/MS). Lyso-PC contents were higher in glycoxidized LDL and PLA2-treated LDL compared with native LDL. Glycoxidized LDL and enrichment of lyso-PC by PLA2 treatment resulted in upregulation of MCP-1 mRNA expression through increased NF-kappaB activity in HUVEC. Moreover, LDL isolated from diabetics contained more lyso-PC than that from nondiabetic subjects, and induced higher MCP-1 mRNA expression and NF-kappaB activity in HUVEC. In both in vitro and human studies, palmitoyl- and stearoyl-lyso-PC contents correlated with MCP-1 expression and NF-kappaB activity. Preincubation with 4-ethyl-2-hydroxyimino-5-nitro-3-hexenamide, a NO donor, abrogated increased expression of MCP-1 mRNA and high NF-kappaB activity induced by PLA2-treated LDL and by LDL isolated from diabetic patients. Our results suggest that lyso-PC contents in LDL play an important role in atherogenesis under diabetic condition, which could be prevented by increased availability of vascular NO.

Potent modification of low density lipoprotein by group X secretory phospholipase A2 is linked to macrophage foam cell formation

The deposition of cholesterol ester within foam cells of the artery wall is fundamental to the pathogenesis of atherosclerosis. Modifications of low density lipoprotein (LDL), such as oxidation, are prerequisite events for the formation of foam cells. We demonstrate here that group X secretory phospholipase A2 (sPLA2-X) may be involved in this process. sPLA2-X was found to induce potent hydrolysis of phosphatidylcholine in LDL leading to the production of large amounts of unsaturated fatty acids and lysophosphatidylcholine (lyso-PC), which contrasted with little, if any, lipolytic modification of LDL by the classic types of group IB and IIA secretory PLA2s. Treatment with sPLA2-X caused an increase in the negative charge of LDL with little modification of apolipoprotein B (apoB) in contrast to the excessive aggregation and fragmentation of apoB in oxidized LDL. The sPLA2-X-modified LDL was efficiently incorporated into macrophages to induce the accumulation of cellular cholesterol ester and the formation of non-membrane-bound lipid droplets in the cytoplasm, whereas the extensive accumulation of multilayered structures was found in the cytoplasm in oxidized LDL-treated macrophages. Immunohistochemical analysis revealed marked expression of sPLA2-X in foam cell lesions in the arterial intima of high fat-fed apolipoprotein E-deficient mice. These findings suggest that modification of LDL by sPLA2-X in the arterial vessels is one of the mechanisms responsible for the generation of atherogenic lipoprotein particles as well as the production of various lipid mediators, including unsaturated fatty acids and lyso-PC.

Biological effects of secretory phospholipase A(2) group IIA on lipoproteins and in atherogenesis

Secretory phospholipase A(2) group IIA(sPLA(2) IIA) can be produced and secreted by various cell types either constitutionally or as an acute-phase reactant upon stimulation by proinflammatory cytokines. The enzyme prefers phosphatidylethanolamine and phosphatidylserine as substrates. One important biological function may be the hydrolytic destruction of bacterial membranes. It has been demonstrated, however, that sPLA(2) can also hydrolyse the phospholipid monolayers of high density lipoprotein (HDL) and low density lipoprotein (LDL) in vitro. Secretory phospholipase A(2)-modified LDL show increased affinity to glycosaminoglycans and proteoglycans, a tendency to aggregate, and an enhanced ability to deliver cholesterol to cells. Incubation of cultured macrophages with PLA(2)-treated LDL and HDL is associated with increased intracellular lipid accumulation, resulting in the formation of foam cells. Elevated sPLA(2)(IIA) activity in blood serum leads to an increased clearance of serum cholesterol. Secretory phospholipase A(2)(IIA) can also be detected in the intima, adventitia and media of the atherosclerotic wall not only in developed lesions but also in very early stages of atherosclerosis. The presence of DNA of Chlamydia pneumoniae, herpes simplex virus, and cytomegalovirus was found to be associated with sPLA(2)(IIA) expression and other signs of local inflammation. Thus, sPLA(2)(IIA) appears to be one important link between the lipid and the inflammation hypothesis of atherosclerosis.

The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways

We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.

Structure of low density lipoprotein (LDL) particles: basis for understanding molecular changes in modified LDL

Low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are not well understood. Here, the existing pieces of structural information on LDL particles are combined with computer models of the individual molecular components to give a detailed structural model and visualization of the particles. Strong evidence is presented in favor of interactions between LDL lipid constituents that lead to specific domain formation in the particles. A new three-layer model, which divides the LDL particle into outer surface, interfacial layer, and core, and which is capable of explaining some seemingly contradictory interpretations of molecular interactions in LDL particles, is also presented. A new molecular interaction model for the beta-sheet structure and phosphatidylcholine headgroups is introduced and an overall view of the tertiary structure of apolipoprotein B-100 in the LDL particles is presented. This structural information is also utilized to understand and explain the molecular characteristics and interactions of modified, atherogenic LDL particles.

Phospholipase A2 and small, dense low-density lipoprotein

High levels of small, dense LDL in plasma are associated with increased risk for cardiovascular disease. There are some biochemical characteristics that may render small, dense LDL particles more atherogenic than larger, buoyant LDL particles. First, small, dense LDL particles contain less phospholipids and unesterified cholesterol in their surface monolayer than do large, buoyant LDL particles. This difference in lipid content appears to induce changes in the conformation of apolipoprotein B-100, leading to more exposure of proteoglycan-binding regions. This may be one reason for the high-affinity binding of small, dense LDL to arterial proteoglycans. Reduction of the phospholipid content in the surface monolayer LDL by treatment with secretory phospholipase A2 (sPLA2) forms small, dense LDL with an enhanced tendency to interact with proteoglycans. Circulating levels of sPLA2-IIA appears to be an independent risk factor for coronary artery disease and a predictor of cardiovascular events. In addition, in-vivo studies support the hypothesis that sPLA2 proteins contribute to atherogenesis and its clinical consequences. These data suggest that modification of LDL by sPLA2 in the arterial tissue or in plasma may be a mechanism for the generation of atherogenic lipoprotein particles in vivo, with a high tendency to be entrapped in the arterial extracellular matrix.

Mildly oxidized LDL induces expression of group IIa secretory phospholipase A(2) in human monocyte-derived macrophages

Phospholipase A(2)s (PLA(2)s) constitute a family of enzymes that hydrolyze fatty acids of membrane phospholipids, thus initiating the synthesis of proinflammatory mediators. Various PLA(2)s have been detected in human atherosclerotic arteries (advanced lesions); however, only the secretory group of PLA(2) has been shown to specifically hydrolyze low density lipoprotein (LDL)-associated phospholipids and, as such, may play a potential role in atherogenesis. In the present study, we investigated the expression pattern of group IIa, IV, and V PLA(2)s in human macrophages, which are the key cells involved in the onset and perpetuation of atherosclerosis. Immunohistochemical staining by double labeling showed that the secretory nonpancreatic PLA(2) (snpPLA(2)) is detectable in macrophages in the intima of early atherosclerotic lesions. Reverse transcription-polymerase chain reaction analysis of RNA extracted from human monocytes clearly showed that expression of group IV PLA(2) was enhanced during differentiation into macrophages, with an onset of induction at days 2 to 3 of differentiation. Group V snpPLA(2) was constitutively expressed on differentiation, whereas the detection of group IIa snpPLA(2) was dependent on both differentiation and subsequent stimulation of macrophages. Indeed, the transcription of group IIa snpPLA(2) in macrophages was induced by treatment with minimally modified or mildly oxidized LDL, whereas native, extensively oxidized, or acetylated LDL had no effect. To our knowledge, this is the first report describing induction of group IIa snpPLA(2) expression in human monocyte-derived macrophages. The mRNA levels of cytosolic PLA(2) group IV and snpPLA(2) group V remained unchanged on LDL treatment. Thus, our results show that the expression of distinct PLA(2) enzymes is regulated not only during differentiation of monocytes into macrophages but also on exposure of macrophages to distinct LDL species. Consequently, our results indicate a potential role for both cytosolic and secretory PLA(2) enzymes in inflammation and in macrophage functions related to atherosclerosis, with a specific role for group IIa snpPLA2 in LDL scavenging.

Lipolytic modification of LDL by phospholipase A2 induces particle aggregation in the absence and fusion in the presence of heparin

One of the first events in atherogenesis is modification of low density lipoprotein (LDL) particles in the arterial wall with ensuing formation of aggregated and fused lipid droplets. The accumulating particles are relatively depleted in phosphatidylcholine (PC). Recently, secretory phospholipase A2 (PLA2), an enzyme capable of hydrolyzing LDL PC into fatty acid and lysoPC molecules, has been found in atherosclerotic arteries. There is also evidence that both LDL and PLA2 bind to the glycosaminoglycan (GAG) chains of extracellular proteoglycans in the arterial wall. Here we studied the effect of heparin GAG on the lipolytic modification of LDL by PLA2. Untreated LDL, heparin-treated LDL, and heparin-bound LDL were lipolyzed with bee venom PLA2. In the presence of albumin, lipolysis resulted in aggregation in all 3 preparations of the LDL particles. Lipolysis of untreated LDL did not result in aggregation if albumin was absent from the reaction medium, and the lipolytic products accumulated in the particles rendering them negatively charged. However, heparin-treated and heparin-bound lipolyzed LDL particles aggregated even in the absence of albumin. Importantly, in the presence of albumin, some of the heparin-treated and heparin-bound lipolyzed LDL particles fused, the proportion of fused particles being substantially greater when LDL was bound to heparin during lipolysis. In summary, lipolysis of LDL PC by PLA2 under physiological conditions, which allow transfer of the lipolytic degradation products to albumin, leads to fusion of LDL particles in the presence, but not in the absence, of heparin. Thus, it is possible that within the GAG meshwork of the arterial intima, PLA2-induced modification of LDL is one source of the lipid droplets during atherogenesis.

Phospholipase A2 type II binds to extracellular matrix biglycan: modulation of its activity on LDL by colocalization in glycosaminoglycan matrixes

We recently reported the presence of secretory, nonpancreatic phospholipase A2 type II (snpPLA2; EC 3.1.1.4) in human atherosclerotic arteries (Hurt-Camejo et al, Arterioscler Thromb Vasc Biol. 1997;17:300-309). SnpPLA2 may generate the proinflammatory products lysophospholipids and free fatty acids, thus contributing to atherogenesis when acting on low density lipoproteins (LDLs) retained in the arterial wall. Immunohistochemical studies showed that smooth muscle cells (SMCs) in human arterial tissue are the main sources of snpPLA2. In cultures of human arterial SMCs, snpPLA2 interacts with versican and smaller heparan/chondroitin sulfate proteoglycans (PGs) secreted as soluble components into the medium. In the present study, we investigated the binding of snpPLA2 to extracellular matrix (ECM) PGs produced by SMCs. The results show that snpPLA2 can bind to the ECM at physiological salt concentrations. ECM-bound snpPLA2 was active, hydrolyzing phosphatidylcholine-containing micelles. Soluble chondroitin-6-sulfate at concentrations >1 micromol/L, but not heparin or heparan sulfate, was able to release ECM-bound snpPLA2. The PG mainly involved in the binding of snpPLA2 was identified as biglycan. Perlecan was also present in the ECM synthesized by SMCs, but it contributed less to the binding of snpPLA2. Experiments with immobilized glycosaminoglycans indicated that snpPLA2 hydrolyzed 7-fold more LDL phospholipids when the lipoprotein and the enzyme were colocalized in a matrix with chondroitin-6-sulfate compared with one with heparin. These data suggest that retention of snpPLA2 in ECMs of different composition may modulate the enzymatic activity of snpPLA2 toward LDL. The results presented in this work support the hypothesis of the potential contribution of snpPLA2 to atherosclerosis.

Sphingomyelinase induces aggregation and fusion, but phospholipase A2 only aggregation, of low density lipoprotein (LDL) particles. Two distinct mechanisms leading to increased binding strength of LDL to human aortic proteoglycans

During atherogenesis, low density lipoprotein (LDL) particles bind to extracellular matrix proteoglycans in the arterial wall, become modified, and appear as aggregated and fused particles. Sphingomyelinase (SMase) and phospholipase A2 (PLA2) have been found in the arterial wall, and, moreover, lesional LDL shows signs of hydrolysis of both sphingomyelin and phosphatidylcholine. We have now studied the effects of these two lipolytic modifications on the aggregation and fusion of LDL particles by hydrolyzing the particles with Bacillus cereus SMase or bee venom PLA2. In addition, the binding strengths of the modified LDL to human aortic proteoglycans (PG) were analyzed on an affinity column. We found that SMase induced aggregation and fusion of LDL, but PLA2 induced only aggregation of the particles. In addition, the SMase-induced aggregation and fusion of LDL was promoted by pretreatment of LDL with PLA2. Determination of the binding strengths of the hydrolyzed LDL revealed that mere lipolysis of LDL without aggregation or fusion, either by SMase or PLA2, did not affect the binding of the particles to PG. Aggregation and fusion of lipolyzed LDL particles, however, increased their strength of binding to PG. Active lysine residues in apolipoprotein B-100 (apoB-100) appear to be involved in the binding of LDL to PG, and, in fact, quantitative 13C NMR analysis revealed that, in the fused LDL particles, the number of active lysine residues per apoB-100 moiety was increased. Moreover, aggregation and fusion of LDL increased the number of apoB-100 copies and, consequently, the number of active lysine residues per aggregate or fused particle. Our present findings therefore (i) show that treatment of LDL with SMase and PLA2 generates modified LDL particles, which then bind to human aortic PG with increased strength, and (ii) suggest that SMase- and PLA2-induced aggregation and fusion of LDL are potential mechanisms leading to focal retention of extracellular lipid in the arterial wall.

Hepatic lipase affects both HDL and ApoB-containing lipoprotein levels in the mouse

Transgenic mice were created overproducing a range of human HL (hHL) activities (4-23-fold increase) to further examine the role of hepatic lipase (HL) in lipoprotein metabolism. A 5-fold increase in heparin releasable HL activity was accompanied by moderate (approx. 20%) decreases in plasma total and high density lipoprotein (HDL) cholesterol and phospholipid (PL) but no significant change in triglyceride (TG). A 23-fold increase in HL activity caused a more significant decrease in plasma total and HDL cholesterol, PL and TG (77%, 64%, 60%, and 24% respectively), and a substantial decrease in lipoprotein lipids amongst IDL, LDL and HDL fractions. High levels of HL activity diminished the plasma concentration of apoA-I, A-II and apoE (76%, 48% and 75%, respectively). In contrast, the levels of apoA-IV-containing lipoproteins appear relatively resistant to increased titers of hHL activity. Increased hHL activity was associated with a progressive decrease in the levels and an increase in the density of LpAI and LpB48 particles. The increased rate of disappearance of 125I-labeled human HDL from the plasma of hHL transgenic mice suggests increased clearance of HDL apoproteins in the transgenic mice. The effect of increased HL activity on apoB100-containing lipoproteins was more complex. HL-deficient mice have substantially decreased apoB100-containing low density lipoproteins (LDL) compared to controls. Increased HL activity is associated with a transformation of the lipoprotein density profile from predominantly buoyant (VLDL/IDL) lipoproteins to more dense (LDL) fractions. Increased HL activity from moderate (4-fold) to higher (5-fold) levels decreased the levels of apoB100-containing particles. Thus, at normal to moderately high levels in the mouse, HL promotes the metabolism of both HDL and apoB-containing lipoproteins and thereby acts as a key determinant of plasma levels of both HDL and LDL.