Minimally oxidized LDL is a potent inhibitor of lecithin:cholesterol acyltransferase activity

Children with familial hypercholesterolemia display changes in LDL and HDL function: A cross-sectional study

BACKGROUND

The functional status of lipoprotein particles contributes to atherogenesis. The tendency of plasma low-density lipoprotein (LDL) particles to aggregate and the ability of igh-density lipoprotein (HDL) particles to induce and mediate reverse cholesterol transport associate with high and low risk for cardiovascular disease in adult patients, respectively. However, it is unknown whether children with familial hypercholesterolemia (FH) display lipoprotein function alterations.

HYPOTHESIS

We hypothesized that FH children had disrupted lipoprotein functions.

METHODS

We analyzed LDL aggregation susceptibility and HDL-apoA-I exchange (HAE), and activity of four proteins that regulate lipoprotein metabolism (cholesteryl ester transfer protein, lecithin-cholesterol acyltransferase, phospholipid transfer protein, and paraoxonase-1) in plasma samples derived from children with FH (n = 47) and from normocholesterolemic children (n = 56). Variation in lipoprotein functions was further explored using an nuclear magnetic resonance-based metabolomics profiling approach.

RESULTS

LDL aggregation was higher, and HAE was lower in FH children than in normocholesterolemic children. LDL aggregation associated positively with LDL cholesterol (LDL-C) and negatively with triglycerides, and HAE/apoA-I associated negatively with LDL-C. Generally, the metabolomic profile for LDL aggregation was opposite of that of HAE/apoA-I.

CONCLUSIONS

FH children displayed increased atherogenicity of LDL and disrupted HDL function. These newly observed functional alterations in LDL and HDL add further understanding of the risk for atherosclerotic cardiovascular disease in FH children.

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

AIMS

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

MAIN METHODS

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

KEY FINDINGS

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

SIGNIFICANCE

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

PON-1 haplotype (-108C>T, L55M, and Q192R) modulates the serum levels and activity PONase promoting an atherogenic lipid profile in rheumatoid arthritis patients

INTRODUCTION/OBJECTIVE

Paraoxonase 1 (PON1) promotes antioxidant and antiatherogenic activity related to the hydrolysis of oxidized lipids of low-density lipoproteins. In rheumatoid arthritis (RA) patients, it has been reported that low PON1 activity is related to an impaired lipid profile, increasing cardiovascular risk (CVR). The goal of this study was to analyze the effect of common PON1 polymorphisms and haplotypes on enzymatic activity, PON1 serum levels (PON1s), and lipid parameters related to atherogenic profile in RA patients.

METHODS

A cross-sectional study was carried out on 250 Mexican patients with RA. The lipid profile was determined by colorimetric tests. The PON1 activity (CMPAase) was measured by spectrophotometry. The levels of PON1s were determined by ELISA, and the polymorphisms in the PON-1 gene (-108C>T, L55M, and Q192R) were genotyped by the PCR-RFLP method. The haplotypes were estimated and statistical analysis was performed.

RESULTS

The median of the CMPAase activity and PON1 levels was 13.91 U/mL and 24.75 ng/mL, respectively. The CMPAase activity was significantly lower in carriers of -108TT and 192QQ genotypes (β = - 4.09, P = 0.001 and β = - 3.73, P = 0.002, respectively); moreover, the PON1 levels were lower in 192Q allele carriers (P < 0.01). The TLQ haplotype was associated with CMPAase activity < 13.91 U/mL (OR = 2.29, P < 0.001), as well as with levels of PON1s < 24.75 ng/mL (OR = 1.65, P = 0.017). In this study, the CMPAase activity (< 13.91 U/mL) showed a positive association with lower levels of high-density lipoprotein cholesterol (HDL-c; < 40/50 mg/dL), and with a triglycerides/HDL-c ratio > 3%, and a total cholesterol/HDL-c ratio > 4.5/5%, all representatives of an atherogenic risk lipid profile.

CONCLUSIONS

PON1 polymorphisms modulate the CMPAase activity and PON1 levels in Mexican patients with RA. The CMPAase activity < 13.91 U/mL is associated with an atherogenic lipid profile, independently of inflammation markers and treatment with anti-rheumatic drugs. Key Points •The haplotype TLQ is a marker for low PONase activity in rheumatoid arthritis. •The haplotype TLQ is a marker for low PON1 serum levels in rheumatoid arthritis. •The enzymatic PON1 activity represents the best marker for an atherogenic lipid profile in rheumatoid arthritis, in comparison with PON1 levels. •The haplotype TLQ is a marker of low PON1 activity, levels of PON1s, and atherogenic lipid profile, independent of treatment therapy in rheumatoid arthritis.

Comparative effects of powder, aqueous and methanolic extracts of purslane (Portulaca oleracea L.) on growth performance, antioxidant status, abdominal fat deposition and plasma lipids in broiler chickens

This study was performed to evaluate the comparative effects of dietary supplementation of dried purslane powder (PP), purslane aqueous extract (PAE) and purslane methanolic extract (PME) on performance, antioxidant status, carcass traits and selected plasma lipid parameters in broiler chickens. In total, 420 1-day-old male broiler chicks were divided into seven treatments for 49 days as follows: control (basal diet), basal diets plus 1500 or 3000 mg/kg of PP (PP1500 and PP3000 respectively), basal diets plus 150 or 300 mg/kg of PAE (PAE150 and PAE300 respectively) and basal diets plus 150 or 300 mg/kg of PME (PME150 and PME300 respectively). During the total period of the experiment (0–49 days of the experiment), birds receiving the PP3000 diet had higher (P &lt; 0.05) bodyweight gain and a lower feed conversion ratio compared with those fed other diets. At 24 and 49 days of the experiment, birds receiving the PP3000 diet showed greater (P &lt; 0.05) plasma and liver activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx), and lower (P &lt; 0.05) plasma and liver levels of malondialdehyde compared with other dietary treatments. Additionally, at 24 days of the experiment, birds receiving the PP3000 diet had a greater (P &lt; 0.05) liver catalase activity than those receiving other dietary treatments. In addition, groups receiving the PP1500, PAE300 or PME300 diets showed greater (P &lt; 0.05) plasma and liver activities of superoxide dismutase, catalase and glutathione peroxidase, as well as lower (P &lt; 0.05) plasma and liver levels of malondialdehyde compared with the control group. At 24 days of the experiment, birds receiving the PP1500 or PP3000 diets showed greater (P &lt; 0.05) jejunal activities of superoxide dismutase, catalase and glutathione peroxidase than other groups. At 49 days of the experiment, birds receiving the PP3000 diet showed greater (P &lt; 0.05) jejunal activities of superoxide dismutase and glutathione peroxidase compared with the control group. Additionally, at both 24 and 49 days of the experiment, groups receiving the PP3000 diet had lower (P &lt; 0.05) jejunal levels of malondialdehyde compared with the control group. At 49 days of the experiment, birds receiving the PP3000 diet had a lower (P &lt; 0.05) relative weight of abdominal fat compared with those receiving the other dietary treatments. Moreover, groups that consumed the PP1500, PAE300 or PME300 diets showed lower (P &lt; 0.05) relative weights of abdominal fat compared with the control group. Groups fed PP, PAE or PME treatments showed lower (P &lt; 0.05) plasma levels of triglycerides, total cholesterol and low-density lipoprotein cholesterol, and higher (P &lt; 0.05) plasma levels of high-density lipoprotein cholesterol than the control group at 24 and 49 days of the experiment, with the most pronounced effects observed in those receiving the PP3000 treatment. In conclusion, PP showed more beneficial effects than PAE and PME, and 3000 mg/kg was the best inclusion level of PP in broiler chicken diets.

Apolipoprotein E particle size is increased in Alzheimer's disease

Introduction

Apolipoprotein E4 (apoE4) is the predominant risk factor for late-onset Alzheimer's disease (AD), but the question of which structural differences might explain its effect remains unclear.

Methods

We compared high-density lipoprotein–like apoE particles from 12 AD and 10 control patients using size-exclusion chromatography.

Results

ApoE particles from patients genotyped as ε4/ε4 were 2.2 ± 0.3 times as massive as particles from ε3/ε3 control subjects and 1.4 ± 0.1 times as massive as particles from ε3/ε3 AD patients. The increased particle size was not because of incorporation of amyloid β or apoE proteolysis products. Particles from AD patients genotyped as ε3/ε3 were 1.59 ± 0.27 times as massive as ε3/ε3 control subjects.

Discussion

Increased particle size in AD is affected by APOE genotype and by disease-related differences in assembly or stability. These differences suggest that lipoprotein assembly or stability in AD brain plays an important role in determining apoE4 pathogenicity.

Mechanism and Physiologic Significance of the Suppression of Cholesterol Esterification in Human Interstitial Fluid

Cholesterol esterification in high density lipoproteins (HDLs) by lecithin:cholesterol acyltransferase (LCAT) promotes unesterified cholesterol (UC) transfer from red cell membranes to plasma in vitro. However, it does not explain the transfer of UC from most peripheral cells to interstitial fluid in vivo, as HDLs in afferent peripheral lymph are enriched in UC. Having already reported that the endogenous cholesterol esterification rate (ECER) in lymph is only 5% of that in plasma, we have now explored the underlying mechanism. In peripheral lymph from 20 healthy men, LCAT concentration, LCAT activity (assayed using an optimized substrate), and LCAT specific activity averaged, respectively, 11.8, 10.3, and 84.9% of plasma values. When recombinant human LCAT was added to lymph, the increments in enzyme activity were similar to those when LCAT was added to plasma. Addition of apolipoprotein AI (apo AI), fatty acid-free albumin, Intralipid, or the d < 1.006 g/ml plasma fraction had no effect on ECER. During incubation of lymph plus plasma, the ECER was similar to that observed with buffer plus plasma. When lymph was added to heat-inactivated plasma, the ECER was 11-fold greater than with lymph plus buffer. Addition of discoidal proteoliposomes of apo AI and phosphatidycholine (PC) to lymph increased ECER 10-fold, while addition of apo AI/PC/UC disks did so by only six-fold. We conclude that the low ECER in lymph is due to a property of the HDLs, seemingly substrate inhibition of LCAT by excess cell-derived UC. This is reversed when lymph enters plasma, consequent upon redistribution of UC from lymph HDLs to plasma lipoproteins.

Lecithin-cholesterol acyltransferase in brain: Does oxidative stress influence the 24-hydroxycholesterol esterification?

24-Hydroxycholesterol (24OH-C) is esterified by the enzyme lecithin-cholesterol acyltransferase (LCAT) in the cerebrospinal fluid (CSF). We report here that the level of 24OH-C esters was lower in CSF of patients with amyotrophic lateral sclerosis than in healthy subjects (54% vs 68% of total 24OH-C, p=0.0005; n=8). Similarly, the level of 24OH-C esters in plasma was lower in patients than in controls (62% vs 77% of total 24OH-C; p=0.0076). The enzyme amount in CSF, as measured by densitometry of the protein band revealed by immunoblotting, was about 4-fold higher in patients than in controls (p=0.0085). As differences in the concentration of the LCAT stimulator Apolipoprotein E were not found, we hypothesized that the reduced 24OH-C esterification in CSF of patients might depend on oxidative stress. We actually found that oxidative stress reduced LCAT activity in vitro, and 24OH-C effectively stimulated the enzyme secretion from astrocytoma cells in culture. Enhanced LCAT secretion from astrocytes might represent an adaptive response to the increase of non-esterified 24OH-C percentage, aimed to avoid the accumulation of this neurotoxic compound. The low degree of 24OH-C esterification in CSF or plasma might reflect reduced activity of LCAT during neurodegeneration.

The positional specificity of EXXK motifs within an amphipathic α-helix dictates preferential lysine modification by acrolein: implications for the design of high-density lipoprotein mimetic peptides

Despite the ability of acrolein to damage proteins, factors governing its reactivity with the ε-amino group of lysine are poorly understood. We used a small 26-mer α-helical peptide (ATI-5261) to evaluate the influence of acidic glutamate (E) residues on site-specific lysine modification by acrolein and if this targeting played a major role in inhibiting the cholesterol efflux activity of the peptide. Exposure of ATI-5261 to acrolein resulted in N-(3-formyl-3,4-dehydropiperidino) (FDP)-lysine adducts at positions 5 and 25 and led to a concentration-dependent reduction in cholesterol efflux activity (55 ± 7 and 83 ± 3% decrease with 5:1 and 20:1 acrolein:peptide molar ratios, respectively). Amino acid substitution (K → R) experiments and mass spectrometry revealed neither K5 nor K25 was preferentially modified by acrolein, despite the location of K5 within a putative EXXK motif. Moreover, both lysine residues remained equally reactive when the lipidated peptide was exposed to acrolein. In contrast, placement of EXXK in the center of ATI-5261 resulted in site-specific modification of lysine. The latter was dependent on glutamate, thus establishing that acidic residues facilitate lysine modification and form the molecular basis of the EXXK motif. Preferential targeting of lysine, however, failed to augment the inhibitory effect of the aldehyde. Overall, the inhibitory effects of acrolein on cholesterol efflux activity were largely dependent on the number of lysine residue modifications and cross-linking of α-helical strands that restricted dissociation of the peptide to active forms.

Oxidized LDL: diversity, patterns of recognition, and pathophysiology

Oxidative modification of LDL is known to elicit an array of pro-atherogenic responses, but it is generally underappreciated that oxidized LDL (OxLDL) exists in multiple forms, characterized by different degrees of oxidation and different mixtures of bioactive components. The variable effects of OxLDL reported in the literature can be attributed in large part to the heterogeneous nature of the preparations employed. In this review, we first describe the various subclasses and molecular composition of OxLDL, including the variety of minimally modified LDL preparations. We then describe multiple receptors that recognize various species of OxLDL and discuss the mechanisms responsible for the recognition by specific receptors. Furthermore, we discuss the contentious issues such as the nature of OxLDL in vivo and the physiological oxidizing agents, whether oxidation of LDL is a prerequisite for atherogenesis, whether OxLDL is the major source of lipids in foam cells, whether in some cases it actually induces cholesterol depletion, and finally the Janus-like nature of OxLDL in having both pro- and anti-inflammatory effects. Lastly, we extend our review to discuss the role of LDL oxidation in diseases other than atherosclerosis, including diabetes mellitus, and several autoimmune diseases, such as lupus erythematosus, anti-phospholipid syndrome, and rheumatoid arthritis.

Modification of low-density lipoprotein by myeloperoxidase-derived oxidants and reagent hypochlorous acid

Substantial evidence supports the notion that oxidative processes contribute to the pathogenesis of atherosclerosis and coronary heart disease. The nature of the oxidants that give rise to the elevated levels of oxidised lipids and proteins, and decreased levels of antioxidants, detected in human atherosclerotic lesions are, however, unclear, with multiple species having been invoked. Over the last few years, considerable data have been obtained in support of the hypothesis that oxidants generated by the heme enzyme myeloperoxidase play a key role in oxidation reactions in the artery wall. In this article, the evidence for a role of myeloperoxidase, and oxidants generated therefrom, in the modification of low-density lipoprotein, the major source of lipids in atherosclerotic lesions, is reviewed. Particular emphasis is placed on the reactions of the reactive species generated by this enzyme, the mechanisms and sites of damage, the role of modification of the different components of low-density lipoprotein, and the biological consequences of such oxidation on cell types present in the artery wall and in the circulation, respectively.

Apolipoprotein A-I-dependent cholesterol esterification in patients with rheumatoid arthritis

Growing evidence suggests that atherogenesis is associated with inflammation or defective removal of cholesterol excess from peripheral cells. Apolipoprotein A-I [ApoA-I] activates the enzyme Lecithin-Cholesterol Acyl-Transferase to esterify cell cholesterol for transport to liver. Haptoglobin [Hpt] was previously found able to bind ApoA-I, and suggested to reduce the enzyme activation. The aim of this study was to demonstrate that enhanced levels of Hpt, as present during inflammation, are associated with low enzyme activity and increased thickness of the arterial wall. Enzyme activity and Hpt concentration were analysed in patients with rheumatoid arthritis having the same plasma levels of antioxidants (ascorbate, urate, alpha-tocopherol, retinol) or oxidation markers (nitrotyrosine, lipoperoxide) of healthy subjects. Cholesterol esterification, determined as ratio of cholesteryl esters with cholesterol in high-density lipoproteins, was lower in patients than in controls, and negatively correlated with the intima-media wall thickness of the common carotid. The ratio of Hpt with ApoA-I was negatively correlated with the enzyme activity, while positively correlated with intima-media wall thickness. The results suggest that high Hpt levels might severely impair the enzyme activity, thus contributing to cholesterol accumulation in vascular cells, and lesion formation in the endothelium.

Elevated levels of erythrocyte-conjugated dienes indicate increased lipid peroxidation in schistosomiasis mansoni patients

Schistosoma mansoni causes liver disease by inducing granulomatous inflammation. This favors formation of reactive oxygen species, including superoxide ions, hydrogen peroxide and hydroxyl radicals all of which may induce lipid peroxidation. We have evaluated lipid peroxidation in 18 patients with hepatosplenic schistosomiasis mansoni previously treated with oxamniquine followed by splenectomy, ligature of the left gastric vein and auto-implantation of spleen tissue, by measuring levels of erythrocyte-conjugated dienes and plasma malondialdehyde (MDA). Age-matched, healthy individuals (N = 18) formed the control group. Erythrocyte-conjugated dienes were extracted with dichloromethane/methanol and quantified by UV spectrophotometry, while plasma MDA was measured by reaction with thiobarbituric acid. Patient erythrocytes contained two times more conjugated dienes than control cells (584.5 +/- 67.8 vs 271.7 +/- 20.1 micromol/l, P < 0.001), whereas the increase in plasma MDA concentration (about 10%) was not statistically significant. These elevated conjugated dienes in patients infected by S. mansoni suggest increased lipid peroxidation in cell membranes, although this was not evident when a common marker of oxidative stress, plasma MDA, was measured. Nevertheless, these two markers of lipid peroxidation, circulating MDA and erythrocyte-conjugated dienes, correlated significantly in both patient (r = 0.62; P < 0.01) and control (r = 0.57; P < 0.05) groups. Our data show that patients with schistosomiasis have abnormal lipid peroxidation, with elevated erythrocyte-conjugated dienes implying dysfunctional cell membranes, and also imply that this may be attenuated by the redox capacity of antioxidant agents, which prevent accumulation of plasma MDA.

Serum paraoxonase activity decreases in rheumatoid arthritis

OBJECTIVE

To estimate the alterations of paraoxonase 1 (PON1) and high-density lipoprotein (HDL) in rheumatoid arthritis (RA).

DESIGN AND METHODS

We investigated the serum enzyme activity and concentration of PON1 and their relationship with serum lipids, high-density lipoprotein (HDL) parameters, and acute phase reactants of serum amyloid A (SAA) and C-reactive protein (CRP) in patients with RA.

RESULTS

Serum paraoxonase (PON) activity was significantly decreased in RA patients (n = 64, 131 +/- 53 micro mol/min/L) compared with healthy subjects (n = 155, 164 +/- 59) despite the absence of any difference in serum lipid levels between the two groups. This decrease of serum PON activity in RA patients was found in every genotype (Q/Q, Q/R, R/R) of PON1 at 192 Q/R. There was a different distribution in PON1 Q/R genotypes between RA patients and healthy subjects, and RA patients exhibited less (44%) positive PON1-Q than did the healthy subjects (66%). In a further investigation of age- and gender-matched subgroups of RA (n = 25) and healthy subjects (n = 25), not only serum PON activity, but also lecithin-cholesterol acyltransferase (LCAT) was found to be significantly decreased in RA patients (125 +/- 61 micro mol/min/L, 63.2 +/- 17.2 nmol/ml/hr/37 degrees C) than in healthy subjects (169 +/- 67, 74.7 +/- 19.5), respectively. PON1 and LCAT as well as HDL constituent apolipoprotein (apo) AI and apo AII, were altered significantly in RA patients.

CONCLUSIONS

Acute-phase HDL, which is remodeled structurally and functionally in RA, might be less anti-atherogenic due to the impairment of original HDL function. These alterations of HDL in RA patients may explain in part the reported increase in cardiovascular mortality in patients with RA.

Cholesteryl ester transfer protein and lecithin:cholesterol acyltransferase activities in hispanic and anglo postmenopausal women: associations with total and regional body fat

Reverse cholesterol transport is one process by which high-density lipoprotein (HDL) cholesterol has been hypothesized to play a role in reducing the risk of coronary heart disease. This study was designed to examine cholesteryl ester transfer protein (CETP) and lecithin:cholesterol acyltransferase (LCAT) activities, 2 modulators of reverse cholesterol transport, in Hispanic and Anglo postmenopausal women. The associations between plasma CETP and LCAT activities and body composition were also examined. Of the 199 subjects, 33% were of Hispanic origin and 47% were undergoing hormone replacement therapy (HRT). Body composition was measured by dual-energy x-ray absorptiometry (DXA) and anthropometry. Plasma CETP activity was higher in Hispanic compared to Anglo women, although the difference was eliminated when data were adjusted for abdominal fat. Hispanic women had lower plasma HDL cholesterol concentrations, higher total cholesterol:HDL cholesterol ratios and triglyceride concentrations, and greater susceptibility of low-density lipoprotein (LDL) particles to oxidation. Hispanic women also had a significantly greater relative deposition of body fat on the trunk and intra-abdominally than did Anglo women, even after adjusting for total body fat. There were no significant ethnic differences in LCAT activity. Plasma CETP and LCAT activities were negatively correlated with HDL cholesterol and positively correlated with total cholesterol, LDL cholesterol, and triglycerides, as well as total and regional body composition variables. In conclusion, results suggest a greater risk for coronary heart disease in Hispanic women compared to Anglo women.

Hypochlorous acid-modified low-density lipoprotein inactivates the lysosomal protease cathepsin B: protection by ascorbic and lipoic acids

Unregulated uptake of oxidized LDL by the scavenger receptor(s) of macrophages is thought to be an early event in atherosclerotic lesion development. Accumulation of oxidized LDL within macrophages may result from resistance of the modified LDL to enzymatic hydrolysis or from direct inactivation of lysosomal enzymes by reactive LDL-associated moieties. Since HOCl-modified LDL has been detected in vivo, the effects of HOCI-modified LDL on the activities of the cysteine protease cathepsin B and the aspartyl protease cathepsin D were investigated. LDL (0.5 mg protein/ml), which had been exposed to HOCl (25-200 microM), caused rapid dose-dependent inactivation of cathepsin B, but not of cathepsin D. Exposure of LDL to HOCl results primarily in the formation of LDL-associated chloramines, and the model chloramine N(alpha)-acetyl-lysine chloramine also caused dose-dependent inactivation of cathepsin B. Incubation of HOCl-modified LDL with ascorbic and lipoic acids (25-200 microM) resulted in dose-dependent reduction of LDL-associated chloramines and concomitant protection against cathepsin B inactivation. Thus, the data indicate that HOCl-modified LDL inactivates cathepsin B by a chloramine-dependent mechanism, most likely via oxidation of the enzyme's critical cysteine residue. Furthermore, small molecule antioxidants, such as ascorbic and lipoic acids, may be able to inhibit this potentially pro-atherogenic process by scavenging LDL-associated chloramines.

Detoxification of oxidized LDL by transferring its oxidation product(s) to lecithin:cholesterol acyltransferase

In the present study, we isolated modified LCAT (m-LCAT) by hydroxyapatite column chromatography after incubation of crude LCAT (after DEAE SephadexA-50 column chromatography, penultimate step of LCAT purification) with oxidized LDL (oxLDL) at 37 degrees C for 1 h. The activity was found to be about 30% lower than that of native LCAT (n-LCAT). When activity was determined in the presence of oxLDL, m-LCAT was less inhibited than n-LCAT by oxLDL. Treatments of purified LCAT either at 56 degrees C for 30 min, at 100 degrees C for 10 min, or with 6 mM 5-5' -dithiobis-2-nitrobenzoic acid or 9 mM diisopropyl fluorophosphates (each at 37 degrees C for 30 min) resulted in the loss of its cholesterol-esterifying activity. When examined for their ability to detoxify oxLDL, native LCAT and LCAT treated at 56 degrees C for 30 min were found to detoxify oxLDL. These results indicate that oxidation product(s) of LDL is transferred and bound to LCAT in a way that does not depend on its cholesterol-esterifying activity, but rather on the availability of the sulfhydryl group of cysteine residue and the hydroxyl group of serine residue.

Paraoxonase 1 overexpression in mice and its effect on high-density lipoproteins

Paraoxonase (PON1) is a high-density lipoprotein (HDL)-associated enzyme believed to protect against the early events of atherogenesis by its ability to hydrolyze oxidized phospholipids. A transgenic mouse overexpressing PON1 (mPON1) was developed to address the question of whether overexpression of PON1 is important in protecting HDL function during oxidative stress. Transgenic mice were obtained that have up to a 5-fold increase in mPON1 activity measured as arylesterase activity [52.7 +/- 17.3 U/ml versus 251.7 +/- 25.1 U/ml for wild-type (WT) and mPON1 high expressers, respectively]; this increase in mPON1 activity was reflected by a 5.3-fold increase in relative mass of the enzyme. Excess mPON1 was associated solely with HDL but did not alter HDL composition, size, or charge. Lecithin:cholesterol acyltransferase (LCAT) on HDL is a sensitive indicator of oxidative stress; exposure of plasmas from both WT and mPON1 overexpresser mice to 0.4 mM copper ions for 2 h showed a 30-40% protection of LCAT activity in mPON1 overexpressers compared to WT. Excess mPON1 also inhibited lipid hydroperoxide formation on HDL. These data strongly suggest that overexpression of mPON1 protects HDL integrity and function.

Comparison of two methods for radioiodination on the oxidizability properties of low density lipoprotein

Radiolabeling of low density lipoprotein (LDL) apoB100 with 125I, an oxidative process, is commonly used in lipoprotein investigation. Since 1) LDL is unstable and oxidation-prone, 2) the modification of apoB100 by oxidation increases the negative charge of particles and leads to the uptake of modified LDL through the scavenger receptor pathway, and 3) oxidized LDL is cytotoxic, it is relevant to investigate whether the oxidative stability of LDL is influenced by its labeling with 125I. The aim of this study was to investigate and compare lipid and protein oxidation markers in human LDL after labeling with 125I by two widely adopted methods that use ICl or the chloramide 1,3,4,6-tetrachloro-3alpha,6alpha-diphenylglycoluril as the oxidizing agent. Native LDL served as a common control and sham-iodinated LDL as a handling control for each procedure. The resistance against copper-induced oxidation of 125I-LDL labeled with ICl was similar to that of controls with regard to the lag time and maximal amount of conjugated diene formed, as there were levels of initial conjugated diene, alpha-tocopherol, and tryptophan. However, radioiodination with the chloramide accelerated the onset of the rapid phase of LDL oxidation due to a drastic depletion of alpha-tocopherol and increased conjugated diene content. Measurements of copper-induced LDL oxidizability showed enhanced indices of lipid oxidation. The lag time and the time to maximal diene production were 65% and 30% shorter than controls. This was accompanied by a 50% reduced tryptophan fluorescence. The anionic surface charge of the LDL particle increased moderately with both labeling procedures. The results indicate that labeling of LDL with 125I may oxidize lipids and apoB100 to a variable extent, depending on the nature of the iodinating agent. This is why assessment of the oxidizability properties of 125I-labeled LDL is recommended for reliable biological studies.

Ldl modified by hypochlorous acid is a potent inhibitor of lecithin-cholesterol acyltransferase activity

Modification of low density lipoprotein (LDL) by myeloperoxidase-generated HOCl has been implicated in human atherosclerosis. Incubation of LDL with HOCl generates several reactive intermediates, primarily N-chloramines, which may react with other biomolecules. In this study, we investigated the effects of HOCl-modified LDL on the activity of lecithin-cholesterol acyltransferase (LCAT), an enzyme essential for high density lipoprotein maturation and the antiatherogenic reverse cholesterol transport pathway. We exposed human LDL (0.5 mg protein/mL) to physiological concentrations of HOCl (25 to 200 micromol/L) and characterized the resulting LDL modifications to apolipoprotein B and lipids; the modified LDL was subsequently incubated with apolipoprotein B-depleted plasma (density >1.063 g/mL fraction), which contains functional LCAT. Increasing concentrations of HOCl caused various modifications to LDL, primarily, loss of lysine residues and increases in N-chloramines and electrophoretic mobility, whereas lipid hydroperoxides were only minor products. LCAT activity was extremely sensitive to HOCl-modified LDL and was reduced by 23% and 93% by LDL preincubated with 25 and 100 micromol/L HOCl, respectively. Addition of 200 micromol/L ascorbate or N-acetyl derivatives of cysteine or methionine completely prevented LCAT inactivation by LDL preincubated with </=200 micromol/L HOCl. Protecting the free thiol groups of LCAT with 5,5'-dithio-bis-(2-nitrobenzoic acid) before exposure to HOCl-modified LDL, which inhibits lipid hydroperoxide-mediated inactivation of LCAT, failed to prevent the loss of enzyme activity. Our data indicate that N-chloramines from HOCl-modified LDL mediate the loss of plasma LCAT activity and provide a novel mechanism by which myeloperoxidase-generated HOCl may promote atherogenesis.

Importance of the free sulfhydryl groups of lecithin-cholesterol acyltransferase for its sensitivity to oxidative inactivation

Lecithin-cholesterol acyltransferase (LCAT) of human plasma is known to be highly susceptible to oxidative inactivation, although the mechanism of this inactivation is unknown. We tested the hypothesis that the high sensitivity of the enzyme is due to the derivatization of its two free SH groups flanking the active site pocket. Modification of the SH groups with a reversible inhibitor protected the enzyme against oxidative inactivation. Mutagenesis of either of the cysteines to glycine increased the resistance of the enzyme, which retained 46% of activity in presence of 150 microM Cu(2+), compared to only 27% of the activity retained by the wild type enzyme (WT). Replacement of both the cysteines with glycines resulted in retention of over 65% activity. Cysteine replacement similarly protected the enzyme from inactivation by the oxidized substrate. Chicken LCAT, which has only one cysteine (Cys(26)), was more resistant than the human enzyme. Introduction of an additional cysteine corresponding to the second cysteine in human LCAT (N184C) resulted in increased susceptibility of chicken enzyme (87% loss of activity in presence of 150 microM Cu(2+), compared to 55% loss in WT). Substitution of the lone cysteine with glycine (C26G) resulted in a more resistant enzyme, which lost <40% activity under the same conditions. These results show that the primary targets of the oxidizing agents or the products of oxidation are the SH groups of the enzyme, whose derivatization leads to steric inhibition of the activity.

High density lipoproteins (HDL) and the oxidative hypothesis of atherosclerosis

The oxidative hypothesis of atherosclerosis classically implies a central role for low density lipoprotein (LDL) oxidation. However, new antiatherogenic properties have been recognized for high density lipoproteins (HDL), apart from their ability to reverse cholesterol transport. Indeed, native HDL could protect LDL from oxidation, thereby minimizing the deleterious consequences of this process. Several mechanisms have been suggested to explain this protective role. Two HDL-associated enzymes, paraoxonase and PAF-acetylhydrolase, detoxify oxidized phospholipids produced by lipid peroxidation. In addition, HDL could reduce hydroperoxides to their corresponding hydroxides. It has also been suggested that HDL could inhibit oxidized LDL-induced transduction signals. However, in vivo HDL oxidation in the subendothelial space would favor the atherosclerotic process. Indeed, atherogenic properties of these oxidized HDL partly result from some loss of their cholesterol effluxing capacity and from an inactivation of the lecithin-cholesterol acyltransferase, which is a HDL-associated enzyme involved in reverse cholesterol transport. Finally, oxidized HDL could induce cholesterol accumulation in macrophages. Further in-depth investigation is needed to assess these antagonistic effects and their consequences for the atherosclerotic process.

Role of lecithin-cholesterol acyltransferase in the metabolism of oxidized phospholipids in plasma: studies with platelet-activating factor-acetyl hydrolase-deficient plasma

To determine the relative importance of platelet-activating factor-acetylhydrolase (PAF-AH) and lecithin-cholesterol acyltransferase (LCAT) in the hydrolysis of oxidized phosphatidylcholines (OXPCs) to lyso-phosphatidylcholine (lyso-PC), we studied the formation and metabolism of OXPCs in the plasma of normal and PAF-AH-deficient subjects. Whereas the loss of PC following oxidation was similar in the deficient and normal plasmas, the formation of lyso-PC was significantly lower, and the accumulation of OXPC was higher in the deficient plasma. Isolated LDL from the PAF-AH-deficient subjects was more susceptible to oxidation, and stimulated adhesion molecule synthesis in endothelial cells, more than the normal LDL. Oxidation of 16:0-[1-14C]-18:2 PC, equilibrated with plasma PC, resulted in the accumulation of labeled short- and long-chain OXPCs, in addition to the labeled aqueous products. The formation of the aqueous products decreased by 80%, and the accumulation of short-chain OXPC increased by 110% in the deficient plasma, compared to the normal plasma, showing that PAF-AH is predominantly involved in the hydrolysis of the truncated OXPCs. Labeled sn-2-acyl group from the long-chain OXPC was not only hydrolyzed to free fatty acid, but was preferentially transferred to diacylglycerol, in both the normal and deficient plasmas. In contrast, the acyl group from unoxidized PC was transferred only to cholesterol, showing that the specificity of LCAT is altered by OXPC. It is concluded that, while PAF-AH carries out the hydrolysis of mainly truncated OXPCs, LCAT hydrolyzes and transesterifies the long-chain OXPCs.

A novel lecithin-cholesterol acyltransferase antioxidant activity prevents the formation of oxidized lipids during lipoprotein oxidation

Recent investigations suggest that high-density lipoprotein (HDL) may play an anti-atherogenic role as an antioxidant and inhibit the oxidative modification of low-density lipoprotein (LDL). The antioxidant activity of HDL has been proposed to be associated with several HDL-bound proteins. We have purified one HDL-associated protein, lecithin:cholesterol acyltransferase (LCAT), to apparent homogeneity and have found that LCAT is not only capable of esterifying cholesterol in the plasma, but can also prevent the accumulation of oxidized lipids in LDL. Addition of pure human LCAT to LDL or palmitoyl-linoleoyl phosphatidylcholine/sodium cholate (PLPC) micelles inhibits the oxidation-dependent accumulation of both conjugated dienes and lipid hydroperoxides. LCAT also inhibits the increase of net negative charge that occurs during oxidation of LDL. LCAT has the ability to prevent spontaneous oxidation and Cu2+ and soybean lipoxygenase-catalyzed oxidation of lipids. The antioxidant activity of LCAT appears to be enzymatic, since the enzyme is active for up to 10 h in the presence of mild free-radical generators. The catalytic serine, residue 181, may mediate this activity and act as a reusable proton donor. Chemical modification of the active serine residue with diisopropylfluorophosphate completely inhibits the ability of LCAT to prevent lipid oxidation. Thus, in addition to its well-characterized phospholipase and acyltransferase activities, LCAT can also act as an antioxidant and prevent the accumulation of oxidized lipid in plasma lipoproteins.

Role of group II secretory phospholipase A2 in atherosclerosis: 1. Increased atherogenesis and altered lipoproteins in transgenic mice expressing group IIa phospholipase A2

Some observations have suggested that the extracellular group IIa phospholipase A2 (sPLA2), previously implicated in chronic inflammatory conditions such as arthritis, may contribute to atherosclerosis. We have examined this hypothesis by studying transgenic mice expressing the human enzyme. Compared with nontransgenic littermates, the transgenic mice exhibited dramatically increased atherosclerotic lesions when maintained on a high-fat, high-cholesterol diet. Surprisingly, the transgenic mice also exhibited significant atherosclerotic lesions when maintained on a low-fat chow diet. Immunohistochemical staining indicated that sPLA2 was present in the atherosclerotic lesions of the transgenic mice. On both chow and atherogenic diets, the transgenic mice exhibited decreased levels of HDLs and slightly increased levels of LDLs compared with nontransgenic littermates. These data indicate that group IIa sPLA2 may promote atherogenesis, in part, through its effects on lipoprotein levels. These data also provide a possible mechanism for the observation that there is an increased incidence of coronary artery disease in many chronic inflammatory diseases.

Human serum paraoxonase (PON 1) is inactivated by oxidized low density lipoprotein and preserved by antioxidants

Human serum paraoxonase (PON1) can protect low density lipoprotein (LDL) from oxidation induced by either copper ion or by the free radical generator azo bis amidinopropane hydrochloride (AAPH). During LDL oxidation in both of these systems, a time-dependent inactivation of PON arylesterase activity was observed. Oxidized LDL (Ox-LDL) produced by lipoprotein incubation with either copper ion or with AAPH, indeed inactivated PON arylesterase activity by up to 47% or 58%, respectively. Three possible mechanisms for PON inactivation during LDL oxidation were considered and investigated: copper ion binding to PON, free radical attack on PON, and/or the effect of lipoprotein-associated peroxides on the enzyme. As both residual copper ion and AAPH are present in the Ox-LDL preparations and could independently inactivate the enzyme, the effect of minimally oxidized (Ox-LDL produced by LDL storage in the air) on PON activity was also examined. Oxidized LDL, as well as oxidized palmitoyl arachidonoyl phosphatidylcholine (PAPC), lysophosphatidylcholine (LPC, which is produced during LDL oxidation by phospholipase A2-like activity), and oxidized cholesteryl arachidonate (Ox-CA), were all potent inactivators of PON arylesterase activity (PON activity was inhibited by 35%-61%). PON treatment with Ox-LDL (but not with native LDL), or with oxidized lipids, inhibited its arylesterase activity and also reduced the ability of the enzyme to protect LDL against oxidation. PON Arylesterase activity however was not inhibited when PON was pretreated with the sulfhydryl blocking agent, p-hydroxymercurybenzoate (PHMB). Similarly, on using recombinant PON in which the enzyme's only free sulfhydryl group at the position of cysteine-284 was mutated, no inactivation of the enzyme arylesterase activity by Ox-LDL could be shown. These results suggest that Ox-LDL inactivation of PON involves the interaction of oxidized lipids in Ox-LDL with the PON's free sulfhydryl group. Antioxidants such as the flavonoids glabridin or quercetin, when present during LDL oxidation in the presence of PON, reduced the amount of lipoprotein-associated lipid peroxides and preserved PON activities, including its ability to hydrolyze Ox-LDL cholesteryl linoleate hydroperoxides. We conclude that PON's ability to protect LDL against oxidation is accompanied by inactivation of the enzyme. PON inactivation results from an interaction between the enzyme free sulfhydryl group and oxidized lipids such as oxidized phospholipids, oxidized cholesteryl ester or lysophosphatidylcholine, which are formed during LDL oxidation. The action of antioxidants and PON on LDL during its oxidation can be of special benefit against atherosclerosis since these agents reduce the accumulation of Ox-LDL by a dual effect: i.e. prevention of its formation, and removal of Ox-LDL associated oxidized lipids which are generated during LDL oxidation.

Inhibition of lecithin cholesterol acyltransferase by phosphatidylcholine hydroperoxides

To gain insight into the nature of the lecithin-cholesterol acyltransferase inhibitory factor(s), we separated and collected the oxidation products from oxidized lipoproteins after lipoxygenase treatment. Isolated fractions identified by chemiluminescence, as hydroperoxides of phosphatidylcholine, were found to produce a significant reduction of lecithin-cholesterol acyltransferase activity. The reaction kinetics of lecithin-cholesterol acyltransferase with reconstitued high density lipoproteins were studied in the presence of 0.6 and 1.2 microM hydroperoxides of phosphatidylcholine. No significant changes in the apparent Vmax were observed but a concentration-dependent increase in slope of the reciprocal plots and in the apparent Km values was observed with increasing hydroperoxide concentrations. These results show that the active site of lecithin-cholesterol acyltransferase is not affected by the presence of phosphatidylcholine hydroperoxides. Nevertheless, hydroperoxides of phosphatidylcholine altered the reactivity of lecithin-cholesterol acyltransferase for reconstitued high density lipoproteins suggesting either an alteration of the binding of lecithin-cholesterol acyltransferase to the reconstitued high density lipoproteins or a competitive inhibition mechanism.

Possible induction of renal dysfunction in patients with lecithin:cholesterol acyltransferase deficiency by oxidized phosphatidylcholine in glomeruli

To clarify the causes of renal dysfunction in familial lecithin:cholesterol acyltransferase (LCAT) deficiency, kidney samples from 4 patients with LCAT deficiency (3 homozygotes and 1 heterozygote) were examined immunohistochemically. All of the patients exhibited corneal opacities, anemia, renal dysfunction, deficiencies in plasma high density lipoprotein and LCAT activity and mass, and an increase in the ratio of plasma unesterified cholesterol to esterified cholesterol. Renal lesions began with the deposition of lipidlike structures in the glomerular basement membrane, and these structures accumulated in the mesangium and capillary subendothelium. By electron microscopy, 2 types of distinctive structure were found in glomerular lesions: vacuole structures and cross-striated, membranelike structures. The plasma oxidized phosphatidylcholine (oxPC) -modified low density lipoprotein (LDL) levels in LCAT-deficient subjects were significantly (P<0.01) higher than those in controls (1.30+/-0.82 versus 0.42+/-0.32 ng/5 microg LDL, respectively), and a significant (P<0.01) difference was observed even after adjustment for confounding factors by an analysis of covariance. The patient with the highest plasma oxPC-modified LDL had the most membranelike structures in the glomeruli and showed the greatest renal deterioration from a young age. In glomerular lesions, although there was an abundance of apoB and apoE, oil red O-positive lipids, macrophages, apoA1, and malondialdehyde were scarce. OxPC was found extracellularly in glomerular lesions, and although its distribution differed from that of apolipoproteins, it was quite similar to that of phospholipids. In conclusion, these results indicate that oxPC in plasma and glomeruli is distinctive for patients with LCAT deficiency. Therefore, oxPC may be a factor in the deterioration of kidneys in patients with familial LCAT deficiency.

High density associated enzymes: their role in vascular biology

Enzymes associated with circulating HDL include lecithin: cholesterol acyl transferase, phospholipid transfer protein, cholesterol ester transfer protein, paraoxonase 1 and platelet activating factor acetylhydrolase. Together with lipoprotein lipase and hepatic lipase these enzymes produce important lipoprotein remodeling and modulate their structure and function and therefore their role in artery wall metabolism.

Effects of peroxynitrite on plasma components of the reverse cholesterol transport pathway

Elimination of cholesterol from arterial tissue, crucial in limiting atherogenesis, may be achieved via high-density lipoprotein (HDL)-mediated reverse cholesterol transport (RCT); components of this pathway can be modulated by oxidative stress. Here we have examined the relations between cholesterol efflux, esterification and transfer in human plasma treated with the powerfully reactive nitrogen species, peroxynitrite. Cellular cholesterol efflux to whole plasma, or to peroxynitrite-modified HDL3, was relatively insensitive to peroxynitrite, as was the transfer of esterified cholesterol. However, plasma cholesterol esterification, via lecithin:cholesterol acyltransferase (LCAT), was markedly inhibited, both directly and indirectly, by peroxynitrite treatment, implying inefficient RCT follows HDL sequestration of cellular cholesterol.

Oxidation of high density lipoproteins. I. Formation of methionine sulfoxide in apolipoproteins AI and AII is an early event that accompanies lipid peroxidation and can be enhanced by alpha-tocopherol

The lipids of high density lipoproteins (HDL) are initially oxidized in preference to those in low density lipoprotein when human plasma is exposed to aqueous peroxyl radicals. In this work we report on the relative susceptibility of HDL protein and lipid to oxidation and on the role HDL's alpha-tocopherol (alpha-TOH) plays in modulating protein oxidation. Exposure of isolated HDL to either low fluxes of aqueous peroxyl radicals, Cu2+ ions, or soybean lipoxygenase resulted in the oxidation of apoAI and apoAII during the earliest stages of the reaction, i.e. after consumption of ubiquinol-10 and in the presence of alpha-TOH. Hydro(pero)xides of cholesteryl esters and phospholipids initially accumulated together with specific oxidized forms of apoAI and apoAII, separated by high pressure liquid chromatography. The specific oxidized forms of apoAI were 16 and 32 mass units heavier than those of the native apolipoproteins and contained 1 and 2 methionine sulfoxide residues per protein, respectively. The third methionine residue in apoAI, as well as Trp residues, remained unoxidized during the earliest stages of HDL oxidation examined. Exposure of isolated apoAI to peroxyl radicals, Cu2+, or soybean lipoxygenase resulted in nonspecific (for peroxyl radicals) or no discernible protein oxidation (Cu2+ and soybean lipoxygenase). This indicated that the formation of the specific oxidized forms of apoAI observed with native HDL was not the result of direct reaction of these oxidants with the apolipoprotein. In vitro and in vivo enrichment of HDL with alpha-TOH resulted in a dose-dependent increase in the extent of peroxyl radical-induced formation of HDL cholesteryl ester hydroperoxides (r = 0.96) and cholesteryl ester hydroxides (r = 0. 92), as well as the loss of apoAI (r = 0.96) and apoAII (r = 0.94). alpha-TOH enrichment also enhanced HDL lipid and protein oxidation induced by Cu2+ or soybean lipoxygenase. These results indicate that the earliest stages of HDL oxidation are accompanied by the oxidation of specific methionine residues in apoAI and apoAII and that in the absence of co-antioxidants, alpha-TOH can promote this process.