High-density lipoprotein inhibits the oxidative modification of low-density lipoprotein

Oxidation of heparin-isolated LDL by hemin. The effect of serum components

Oxidation of low-density lipoprotein (LDL) in the artery wall is probably determined by several factors, some of which may include physiological oxidants such as heme and hydrogen peroxide, blood serum components, and the interaction of the lipoprotein with glycosaminoglycans. Glycosaminoglycans form complexes with LDL that increase its susceptibility to oxidation in vitro. To examine the effect of these factors on oxidation of LDL from serum by using heparin and oxidized the resolubilized lipoprotein (Hep-LDL) with hemin and hydrogen peroxide in the presence of apolipoprotein B lipoprotein-deficient serum (BLPDS). Low levels (2.1%) of BLPDS stimulated the oxidation of Hep-LDL by approximately fivefold, and increasing concentrations reduced oxidation to baseline rates. By comparison, the oxidation of native LDL was stimulated to a similar extent at lower concentrations of BLPDS (0.83%) and returned to baseline more rapidly with increasing levels of the serum fraction. Oxidation rates did not change significantly with increasing concentrations of BLPDS alone. Human serum albumin (HSA) at comparable levels produced changes in the oxidation of Hep-LDL similar to those seen with BLPDS. Degradation of heme was accelerated by low levels of BLPDS or HSA in the presence of hydrogen peroxide but not by higher levels, and maximal degradation rates were inhibited by comparatively low levels of butylated hydroxytoluene (35 mumol/L). This antioxidant also effectively inhibited oxidation of Hep-LDL maximally stimulated by BLPDS. The data suggest that serum components, particularly HSA, modulate the peroxidation of both glycosaminoglycan-treated LDL and native LDL by hemin and hydrogen peroxide via mechanisms that may involve oxidative interactions between heme and HSA. This phenomenon may influence oxidation of LDL in vivo, where levels of HSA in regions of the artery wall are comparable with levels that stimulate the oxidation of Hep-LDL in vitro.

Plasma triglycerides and three lipoprotein cholesterol fractions are independent predictors of the extent of coronary atherosclerosis

BACKGROUND

The lipoprotein system has manifold links to atherosclerotic disease. LDL cholesterol is related to lesion formation and growth. The cholesterol of HDLs is indicative of protection against atherosclerosis. The status of triglycerides and of subfractions of high-density lipoproteins as risk factors is less certain. Also, the magnitude of the atherogenic/protective power of these factors is not known.

METHODS AND RESULTS

Five hundred patients (418 men and 82 women) were enrolled in an angiographic study. A total of 1006 coronary lesions with > or = 50% narrowing were recorded as study end points. By extent of atherosclerosis, defined as the number of > or = 50% lesions, the study subjects were allocated to one of four ordered categories with 0, 1 to 3, 4 to 6, or 7 to 10 lesions, respectively. Subfractions of HDL cholesterol were determined by a dual precipitation method. By a polychotomous logistic regression model, it was found that, besides age and sex, LDL cholesterol, HDL2 cholesterol, HDL3 cholesterol, and triglycerides were independently predictive (P < .05) of the extent of coronary atherosclerosis. An increase in age by 10 years was associated with an increase of the odds ratio for falling into a higher-extent category by a factor of 1.64, and the same increase of the odds ratio was obtained by increasing LDL cholesterol by 0.92 mmol/L or triglycerides by 1.01 mmol/L and by decreasing HDL2 cholesterol by 0.20 mmol/L or HDL3 cholesterol by 0.46 mmol/L. The less sensitive coronary end point, presence of atherosclerosis (ie, observation of > or = 1 lesion of > or = 50%) depended significantly on age, sex, LDL cholesterol, and HDL2 cholesterol, but not on HDL3 cholesterol or triglycerides.

CONCLUSIONS

In addition to LDL, HDL2, and HDL3 cholesterol, triglycerides also proved independently predictive of the extent of coronary atherosclerosis.

Measurement of antioxidant activity in lipoproteins using enhanced chemiluminescence

We describe a new assay for antioxidant activity (AOA) in lipoprotein solutions based upon their potential to quench light emission from a glowing horseradish peroxidase-catalyzed enhanced chemiluminescent reaction. By comparison with the quenching activity of the tocopherol-analogue trolox the AOA can be quantified. These measurements suggest that all lipoprotein fractions have significant AOA and that this has a non-linear relationship with lipoprotein concentration increasing significantly on a per particle basis at higher concentrations. Mean AOA in very low density, low density and high density lipoprotein fractions were 39.9 +/- 5.3, 20.3 +/- 4.0 and 5.3 +/- 1.0 mumol of trolox equivalents per litre, respectively, when measured at 1 mg protein/ml. Using known values for the protein content of the lipoprotein fractions, these values correspond to 79.8 +/- 10.7, 10.3 +/- 2.0 and 0.84 +/- 0.15 equivalents per particle. Parallel measurements of light emission and conjugated diene formation suggest that the oxidative stress imposed by the peroxidase-catalyzed reaction leads to lipid peroxidation but only after all AOA has been exhausted. AOA was significantly correlated with the alpha-tocopherol content in 30 lipoprotein samples (r = 0.764). This assay offers a rapid and simple method for investigating the effects of diseases, drugs or dietary manipulation on lipoprotein AOA.

Recombinant apolipoprotein A-I Milano reduces intimal thickening after balloon injury in hypercholesterolemic rabbits

BACKGROUND

Several epidemiological studies have shown an inverse relation between high-density lipoprotein (HDL) cholesterol levels and coronary heart disease. Recently, observational studies have suggested a similar inverse relation between HDL and restenosis after coronary balloon angioplasty. Despite these observations, it is unclear whether this inverse relation reflects a direct vascular protective effect of HDL or apolipoprotein (apo) A-I, the major apolipoprotein component of HDL. Therefore, to determine whether HDL directly influences neointima formation, we investigated the effect of recombinant apo A-I Milano (apo A-I M), a mutant of human apo A-I with Arg-173 to Cys substitution, on intimal thickening after balloon injury in cholesterol-fed rabbits.

METHODS AND RESULTS

Cholesterol feeding was initiated 18 days before injury and continued until the time of death. Eight rabbits received intravenous injections of 40 mg of apo A-I M linked to a phospholipid carrier on alternate days, beginning 5 days before and continuing for 5 days after balloon injury of femoral and iliac arteries. Eight rabbits received the carrier alone, and four received neither apo A-I M nor the carrier. Three weeks after balloon injury, apo A-I M-treated rabbits had significantly reduced intimal thickness compared with the two control groups (mean +/- SD): 0.49 +/- 0.29 versus 1.14 +/- 0.38 mm2 and 1.69 +/- 0.43 mm2, P < .002 by ANOVA). The intima-to-media ratio was also significantly reduced by apo A-I M (0.7 +/- 0.2 versus 1.5 +/- 0.5 and 2.1 +/- 0.1, P < .002 by ANOVA) compared with the two controls. The fraction of intimal lesion covered by macrophages, as identified by immunohistochemistry using macrophage-specific monoclonal antibody, was significantly less in apo A-I M-treated rabbits compared with carrier-treated animals (25.3 +/- 17% versus 59.4 +/- 12.3%, P < .005). Aortic cholesterol content, measured in an additional 10 rabbits, did not differ significantly between apo A-I M-treated animals (n = 5) and carrier-treated controls (n = 5).

CONCLUSIONS

Apo A-I M significantly reduced intimal thickening and macrophage content after balloon injury in cholesterol-fed rabbits without a change in arterial total cholesterol content. Although the precise mechanism of action remains to be defined, these findings are consistent with a direct vascular effect of apo A-I, which could have potential therapeutic implications.

Malondialdehyde-modified high-density lipoprotein cholesterol: plasma removal, tissue distribution and biliary sterol secretion in rats

Evidence has been accumulating for the putative role of chemically or oxidatively altered lipoproteins in accelerating events in the atherogenic process. In this study, the movement of free cholesterol from native high density lipoprotein (HDL) and malondialdehyde (MDA)-modified HDL to the liver for biliary cholesterol secretion and bile acid transformation was examined in vivo. To this end, HDL from normal donor rats was isolated, conjugated with MDA, labelled with [14C]cholesterol and injected i.v. into rats with bile diversion. While the 6 h collection revealed no substantial differences in bile flow, less 14C excretion was recovered in the fresh bile of animals receiving MDA-modified HDL. Bile analysis indicated that a significant decline in labelled bile acid secretion characterized these rats. Compared with controls, MDA-modified HDL also caused an enhanced accumulation of [14C]cholesterol in the liver and the kidneys, with reduced delivery to the sites of steroidogenesis, i.e., the adrenals and testes. No plasma removal differences were noted in the HDL of both groups of rats. Thus, modification of HDL by MDA seems to impair the tissue distribution of its cholesterol moiety, particularly in the liver, where it accumulates at the expense of bile acid transformation.

Effect of cholesterol feeding on the susceptibility of lipoproteins to oxidative modification

In previous studies we have shown that the liver endothelial and Kupffer cells in hypercholesterolemic rabbits are very active in endocytosis of low-density lipoprotein (LDL) and beta-very-low-density lipoprotein (beta-VLDL) (Nenseter et al. (1992) J. Lipid Res. 33, 867-877; Gudmundsen et al. (1993) J. Lipid Res. 34, 589-600). These data raised the question whether subfractions of LDL and beta-VLDL were modified in vivo to forms recognized by the scavenger/oxidized LDL receptors of the non-parenchymal liver cells. The purpose of the present study was to address this question by assessing the effect of cholesterol feeding on the susceptibility of the lipoproteins to oxidative modification in vitro. In addition, the effect of HDL on the lipid peroxidation of LDL was evaluated. LDL and beta-VLDL were isolated from rabbits given a diet supplemented with cholesterol (2% w/w) for 3 weeks. The extent of Cu(2+)-catalyzed oxidation of the lipoproteins was compared with that of LDL from control-fed rabbits. Extent of oxidation assessed by formation of conjugated dienes, lipid peroxides, thiobarbituric acid-reactive substances, relative electrophoretic mobility and uptake of lipoproteins by J774 macrophages suggested that LDL and beta-VLDL from the hypercholesterolemic rabbits were more susceptible to oxidation than LDL from normolipidemic rabbits. HDL protected LDL and beta-VLDL from lipid peroxidation in vitro. Taken together, the increased susceptibility of LDL and beta-VLDL to oxidative modification in vitro, combined with the low levels of alpha-tocopherol, and the reduced ratio of HDL to LDL cholesterol observed in the hypercholesterolemic rabbits, and the protective effect of HDL on the lipid peroxidation of LDL, support the probability that oxidative modification of LDL and beta-VLDL occur in vivo in the hypercholesterolemic rabbits.

High-density lipoprotein stimulates endothelial cell movement by a mechanism distinct from basic fibroblast growth factor

Endothelial cell (EC) migration is a regulatory event in the formation and repair of blood vessels. Although serum contains substantial promigratory activity, the responsible components and especially the role of lipoproteins have not been determined. We examined the effect of plasma high-density lipoprotein (HDL) on the movement of ECs in vitro. Confluent cultures of bovine aortic ECs in serum-free medium were "wounded," and migration was measured after 24 hours. HDL stimulated migration in a concentration-dependent manner with a half-maximal response at 25 to 40 micrograms cholesterol per milliliter and a maximal twofold stimulation at approximately 150 micrograms cholesterol per milliliter. HDL-stimulated migration was not due to cell proliferation, since migration was increased in the presence of hydroxyurea at a concentration that blocked proliferation. At optimal concentrations, HDL was at least as stimulatory as basic fibroblast growth factor (FGF). However, the activity of HDL was not due to contamination by basic FGF, since antibodies to basic FGF did not block HDL-stimulated movement and since the maximum promigratory activities of basic FGF and HDL were additive. These results indicate that HDL and basic FGF may use distinct signaling pathways to initiate EC movement. This possibility was confirmed by results showing that pertussis toxin suppressed basic FGF-stimulated but not HDL-stimulated EC motility and that inhibitors of phospholipase A2, aristolochic acid and ONO-RS-082, also blocked the promigratory activity of basic FGF but had no effect on the activity of HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary atherosclerotic wall thickening and vascular reactivity in humans. Elevated high-density lipoprotein levels ameliorate abnormal vasoconstriction in early atherosclerosis

BACKGROUND

Abnormal vascular reactivity represents a fundamental disturbance in vascular biology with the development of atherosclerosis. Because endothelial vasodilator function plays a pivotal role in controlling vasomotor tone, we hypothesized that atherosclerotic wall thickening might directly interfere with deficient endothelium-mediated dilation and thereby contribute to the abnormal reactivity of atherosclerotic arteries in vivo.

METHODS AND RESULTS

In 26 patients without focal stenoses in the left anterior descending coronary artery, acetylcholine (0.036 to 3.6 micrograms/mL) was infused into the artery to evaluate endothelium-mediated vasodilation. Segmental vasomotor responses to acetylcholine were correlated with the local extent of atherosclerotic wall thickening quantitated by intracoronary ultrasound examination. Seventeen of the patients also underwent cold pressor testing to assess the vasoreactivity to sympathetic activation. The response of coronary artery segments to acetylcholine varied from 35% dilation to 52% constriction and demonstrated a segmental pattern, with dilation and constriction observed in different segments of the same artery. The vasomotor response to acetylcholine was closely correlated with the extent of local atherosclerotic wall thickening (r = -.82, P < .0001). Over the entire range of atherosclerotic wall thickening, segments from patients with elevated high-density lipoprotein (HDL) cholesterol serum levels (> 75th percentile) demonstrated a significantly blunted constrictor response to acetylcholine (P < .01 at the maximal acetylcholine concentration) compared with segments from patients with HDL cholesterol < 75th percentile. The degree of constriction or dilation in response to the acetylcholine infusion correlated with the response to cold pressor testing (r = .75, P < .0001). Again, the cold pressor test-induced constrictor response was significantly (P < .05) blunted in segments from patients with elevated HDL cholesterol serum levels compared with those from patients with HDL cholesterol < 75th percentile despite equal degrees of atherosclerotic wall thickening.

CONCLUSIONS

Coronary vasomotor responses to the endothelium-dependent dilator acetylcholine and to sympathetic stimulation by cold pressor test correlate with local atherosclerotic wall thickening. Thus, the degree of abnormal local vascular reactivity is closely related to the extent of atherosclerotic "plaque load" in human epicardial arteries in vivo. Elevated HDL cholesterol serum levels ameliorate abnormal vasoconstriction at any given extent of atherosclerotic wall thickening, suggesting that HDL cholesterol exerts a beneficial effect on abnormal vascular reactivity, a fundamental functional disturbance associated with coronary atherosclerosis.

The protective role of high-density lipoprotein on oxidized-low-density-lipoprotein-induced U937/endothelial cell interactions

The adherence of monocytes to the endothelium is an early event in atherogenesis. We have investigated this process by examining whether native and oxidized low-density and high-density lipoproteins could modulate this process. Only oxidized low-density lipoprotein caused a significant dose-dependent and time-dependent increase in U937 monocyte-like cell line binding to human endothelial cells, by a process which required de novo protein synthesis. Interestingly, E-selectin, intercellular adhesion molecule-1, vascular cell-adhesion molecule or P-selectin induction was not apparent in this system suggesting the presence of an alternative system for the interaction of endothelial cells with monocyte-like cells in response to oxidized low-density lipoprotein. High-density lipoprotein completely suppressed oxidized low-density-lipoprotein-induced adhesion of U937 cells to the endothelial monolayer, while oxidized high-density lipoprotein did not. These data suggest that the balance between native and oxidized lipoproteins may play a role in the formation of the atherosclerotic lesion by modulating monocyte endothelial interactions.

Effects of phosphatidylserine on the oxidation of low density lipoprotein

1. LDL was incubated in the presence of 1 microM CuSO4 for 18 hr at 37 degrees C. The content of lipoperoxides was found to be approx. 40 nmol MDA equivalents/mg LDL protein. The addition of 50 microM phosphatidylserine (PS) reduced the content of lipoperoxides to 15% of control values. 2. The electrophoretic mobility observed for LDL oxidized in the presence of PS approximated the mobility observed for native LDL. 3. The formation of conjugated dienes was strongly inhibited when LDL was oxidized in the presence of PS. 4. The addition of 50 microM phosphatidylcholine, phosphatidylglycerol and cardiolipin did not alter the extent of LDL oxidation. 5. PS did not inhibit the oxidation of LDL mediated by J774 macrophages in the presence of Ham's F-10 culture medium. Under these conditions, PS was found to be an excellent substrate for oxidation.

In vitro oxidised HDL is recognized by the scavenger receptor of macrophages: implications for its protective role in vivo

To assess the effects of oxidative modification, human HDL was oxidised in vitro for 12 h (Ox-HDL12) and 24 h (Ox-HDL24) under similar conditions to those commonly used for LDL. The procedure resulted in: an increase in thiobarbituric acid reactive substances but with marginal change in electronegativity; protein denaturation accounting for 16% and 45% loss of immunoreactive apoprotein A-I in the Ox-HDL12 and Ox-HDL24 respectively relative to the non-oxidised, native HDL (Nat-HDL); a decrease in the polyunsaturated fatty acids of the triglyceride, cholesterol ester and phospholipid components of the lipoprotein; an increase in the proportion of short chain saturated fatty acids while the monounsaturated fatty acids remained relatively unchanged. Studies with human macrophages demonstrated: a decrease of 16% and 30% in the capacity of the Ox-HDL12 and Ox-HDL24 respectively to efflux intracellular free cholesterol; 125I-Ox-HDL24 uptake and degradation was directly comparable with that of 125I-Ac-LDL; the addition of excess unlabelled Ox-HDL24, Ac-LDL, Ox-LDL24 and Nat-HDL resulted in 74%, 67%, 69% and 19% displacement of the 125I-Ox-HDL24 respectively; fucoidin and dextran sulphate displaced 125I-Ox-HDL by 20% and 40% respectively; intracellular free and esterified cholesterol was increased 2.5-fold and 4-fold respectively relative to Nat-HDL on incubation with Ox-HDL24. These findings suggest that HDL is susceptible to oxidative modification leading to recognition by the scavenger receptor of macrophages and subsequent intracellular cholesterol accumulation. As such, the in vivo protective role of HDL in cardiovascular disease can be reversed in those circumstances in which HDL, like LDL, undergoes oxidative modification.

Tocopherols and carotenes are differently distributed in subfractions of high-density lipoprotein

An apolipoprotein (apo) E-rich and an apo E-poor fraction of high-density lipoprotein (HDL) were isolated from four healthy men by heparin-Sepharose affinity chromatography. On a cholesterol basis, the apo E-poor HDL fraction contained a third more alpha- and gamma-tocopherol and about a third less alpha- and beta-carotene than the apo E-rich HDL fraction. Plasma concentrations of HDL cholesterol were highly correlated with the contribution of the apo E-rich HDL subfraction to total HDL alpha-tocopherol (r = -0.990, P < 0.001).

Increased oxidizability of plasma low density lipoprotein from patients with coronary artery disease

Oxidative modification of lipoproteins may play a crucial role in the pathogenesis of atherosclerosis. This study was designed to examine whether increased lipid peroxides and/or oxidative susceptibility of plasma lipoproteins occur in patients with coronary artery disease. The levels of lipid peroxides, estimated as thiobarbituric acid-reactive substances (TBARS), were significantly greater in the plasma and very low density lipoprotein (VLDL) of symptomatic patients with coronary artery disease than in those of healthy persons, but the TBARS levels of low density lipoprotein (LDL) and high density lipoprotein (HDL) showed insignificant difference between patients and normals. To evaluate the oxidative susceptibility of lipoproteins, we employed in vitro Cu2+ oxidation of lipoproteins monitored by changes in fluorescence, TBARS level, trinitrobenzene sulfonic acid (TNBS) reactivity, apolipoprotein immunoreactivity and agarose gel electrophoretic mobility. While pooled VLDL and LDL of normal controls were oxidized at 5-10 microM Cu2+, pooled VLDL and LDL of patients with coronary artery disease were oxidized at 1-2.5 microM Cu2+, i.e., at relatively lower oxidative stress. At 5 microM Cu2+, VLDL and LDL of patients with coronary artery disease still showed a faster oxidation rate, judged by the rate of fluorescence increase, higher TBARS level, less TNBS reactivity, greater change in apo B immunoreactivity and higher electrophoretic mobility than those of normal controls. However, the difference on the oxidizability of HDL was insignificant for patients vs. normals. In conclusion, we have shown that plasma VLDL and LDL of patients with coronary artery disease are more susceptible to in vitro oxidative modification than those of healthy persons. The data suggest that enhanced oxidizability of plasma lipoproteins may be an important factor influencing the development of coronary artery disease.

Inhibitory effect of high-density lipoprotein on platelet function is mediated by increase in nitric oxide synthase activity in platelets

Although high-density lipoprotein (HDL) has been found to decrease platelet function per se, little is known regarding the mechanism of its platelet inhibitory effect. In this study, we confirmed the inhibitory effect of HDL on platelet aggregation and 14C-serotonin release in thrombin-activated washed human platelets. The inhibition of platelet function was associated with an increase in nitric oxide synthase activity, measured as the conversion of 3H-L-arginine to 3H-L-citrulline as well as nitrite release in the platelet supernates. The inhibition of platelet function by HDL was reversed by preincubation of washed platelets with an inhibitor of nitric oxide synthase, NG-nitro-L-arginine methyl ester (L-NAME), and potentiated by co-incubation with the precursor of nitric oxide, L-arginine. These observations suggest that HDL decreases platelet function by increasing nitric oxide synthase activity in human platelets.

Lipoprotein metabolism and its relationship to atherosclerosis

Lipoproteins are macromolecular complexes that transport cholesterol and triglycerides through the blood stream. The assembly and secretion of lipoproteins occurs in the liver and small intestine, but many modifications and transformations occur in the plasma. Plasma levels of cholesterol and triglycerides are regulated by both environmental effects on lipid metabolism and by genetic factors affecting both apoproteins on the surface of the lipoproteins and enzymes in plasma. Abnormalities of the lipoprotein transport system can increase an individual's risk for developing atherosclerosis.

Apolipoprotein A-I Q[-2]X causing isolated apolipoprotein A-I deficiency in a family with analphalipoproteinemia

We report a Canadian kindred with a novel mutation in the apolipoprotein (apo) A-I gene causing analphalipoproteinemia. The 34-yr-old proband, product of a consanguineous marriage, had bilateral retinopathy, bilateral cataracts, spinocerebellar ataxia, and tendon xanthomata. High density lipoprotein cholesterol (HDL-C) was < 0.1 mM and apoA-I was undetectable. Genomic DNA sequencing of the proband's apoA-I gene identified a nonsense mutation at codon [-2], which we designate as Q[-2]X. This mutation causes a loss of endonuclease digestion sites for both BbvI and Fnu4HI. Genotyping identified four additional homozygotes, four heterozygotes, and two unaffected subjects among the first-degree relatives. Q[-2]X homozygosity causes a selective failure to produce any portion of mature apoA-I, resulting in very low plasma level of HDL. Heterozygosity results in approximately half-normal apoA-I and HDL. Gradient gel electrophoresis and differential electroimmunodiffusion assay revealed that the HDL particles of the homozygotes had peak Stokes diameter of 7.9 nm and contained apoA-II without apoA-I (Lp-AII). Heterozygotes had an additional fraction of HDL3-like particles. Two of the proband's affected sisters had documented premature coronary heart disease. This kindred, the third reported apoA-I gene mutation causing isolated complete apoA-I deficiency, appears to be at significantly increased risk for atherosclerosis.

Serum lipoproteins and hemostatic function in intermittent claudication

Normoglucemic men with intermittent claudication (n = 41), mean age of 63 years, and sex-, age-, body mass index-, and smoking habit-matched controls (n = 75) were compared for plasma lipoprotein and hemostatic variables. The patients had significantly lower levels of large high-density lipoprotein (HDL) particles (HDL2b, HDL2a, and HDL3a) and elevated lipoprotein(a) [Lp(a)] concentrations than the control subjects. Of the hemostatic variables, plasminogen activator inhibitor-1 (PAI-1), plasma antiplasmin, plasma fibrinogen, and urine beta-thromboglobulin concentrations were significantly elevated in patients. In intermittent claudication patients Lp(a) correlated significantly with activation of the coagulation system, ie, with the levels of plasma fibrinogen and urine fibrinopeptide A. No correlations between the values for Lp(a) and PAI-1 or plasma alpha 2-antiplasmin were seen. The PAI-1 activity showed significant univariate correlations to the levels of HDL3b, HDL2b (inverse), and very-low-density lipoprotein triglycerides, of which the positive correlation to HDL3b persisted in multivariate analysis (r = .48, P < .001). Independent characteristics for intermittent claudication estimated by multiple regression analysis were PAI-1, plasma fibrinogen, and HDL3a, with a combined R2 of .36. The intermittent claudication subgroup that was being treated with beta-blockers/thiazides had a higher frequency of coronary heart disease compared with the other patients. In addition, the patients taking beta-blockers/thiazides had elevated triglyceride concentrations, lower HDL cholesterol with a size shift toward smaller particles, and a tendency toward raised PAI-1 and plasma alpha 2-antiplasmin levels compared with the patient group that did not take these medications.

Delayed catabolism of high density lipoprotein apolipoproteins A-I and A-II in human cholesteryl ester transfer protein deficiency

Deficiency of the cholesteryl ester transfer protein (CETP) in humans is characterized by markedly elevated plasma concentrations of HDL cholesterol and apoA-I. To assess the metabolism of HDL apolipoproteins in CETP deficiency, in vivo apolipoprotein kinetic studies were performed using endogenous and exogenous labeling techniques in two unrelated homozygotes with CETP deficiency, one heterozygote, and four control subjects. All study subjects were administered 13C6-labeled phenylalanine by primed constant infusion for up to 16 h. The fractional synthetic rates (FSRs) of apoA-I in two homozygotes with CETP deficiency (0.135, 0.134/d) were found to be significantly lower than those in controls (0.196 +/- 0.041/d, P < 0.01). Delayed apoA-I catabolism was confirmed by an exogenous radiotracer study in one CETP-deficient homozygote, in whom the fractional catabolic rate of 125I-apoA-I was 0.139/d (normal 0.216 +/- 0.018/d). The FSRs of apoA-II were also significantly lower in the homozygous CETP-deficient subjects (0.104, 0.112/d) than in the controls (0.170 +/- 0.023/d, P < 0.01). The production rates of apoA-I and apoA-II were normal in both homozygous CETP-deficient subjects. The turnover of apoA-I and apoA-II was substantially slower in both HDL2 and HDL3 in the CETP-deficient homozygotes than in controls. The kinetics of apoA-I and apoA-II in the CETP-deficient heterozygote were not different from those in controls. These data establish that homozygous CETP deficiency causes markedly delayed catabolism of apoA-I and apoA-II without affecting the production rates of these apolipoproteins.

Malondialdehyde adducts to, and fragmentation of, apolipoprotein B from human plasma

Many recent in vitro experiments support the hypothesis that oxidatively modified low density lipoproteins (LDLs) could participate in atherogenesis. Oxidation of LDLs, especially derivatization by aldehydes originating from peroxidation of fatty acids and fragmentation of apolipoprotein (apo) B-100 which is their major apolipoprotein, probably occurs extravascularly and the presence of oxidized LDLs in the circulation is not well documented. Using electrophoresis and immunodetection techniques, we studied the structure of apo B and the presence of adducts of malondialdehyde (MDA) to this protein in LDLs from plasma of a limited population of five healthy subjects and nine patients with severe atherosclerosis. In the patient-derived LDLs, apo B appeared extensively fragmented, much more so than in those from the healthy subjects, although LDLs were isolated in all cases in the presence of antioxidants, protease inhibitors and antibiotics. Additionally, in all healthy subjects, we found a minor fragment of apo B-100, apo B-74, whereas the complementary peptide, apo B-26, was not detected; thus the presence of this minor form cannot be related to cleavage of apo B-100, either by proteolysis or by oxidation. We also present evidence that MDA adducts are present in circulating apo B and most of its fragments not only in atheromatous patients, but also in healthy subjects. Our results are consistent with the existence of oxidized LDLs in the human circulation. However, the role of non-oxidative phenomena in the structural modifications affecting apo B which are reported here cannot be excluded.

The role of high-density lipoprotein and lipid-soluble antioxidant vitamins in inhibiting low-density lipoprotein oxidation

1. The oxidation of low-density lipoprotein (LDL) is believed to play a central role in atherogenesis. We have compared the effect of antioxidant vitamins and high-density lipoprotein (HDL) on the Cu(2+)-catalysed oxidation of LDL. 2. Antioxidant vitamin supplementation significantly reduced conjugated diene formation but did not affect the formation of lipid peroxides. 3. Conversely, HDL did not affect conjugated diene formation but inhibited the formation of lipid peroxides by up to 90%. 4. The inhibition by HDL of lipid peroxide formation in oxidized LDL was dependent on the concentration of HDL and was not due to HDL chelating Cu2+. 5. Large interindividual variations in the inhibition of lipid peroxide formation by autologous HDL were evident, which were related to the rate of lipid peroxide generation in the LDL. 6. We conclude that HDL is a powerful antioxidant or more probably inhibitor of LDL oxidation in vitro and may play an important role in vivo in preventing atherosclerosis by inhibiting LDL oxidation in the artery wall.

Interaction of oxidized HDLs with J774-A1 macrophages causes intracellular accumulation of unesterified cholesterol

Uptake of modified lipoproteins by resident arterial monocytes/macrophages is believed to be a key event in the formation of foam cells and thus in the early phases of atherosclerosis. Low-density lipoproteins (LDLs) that undergo oxidative changes become suitable for uptake by macrophages through a specific scavenger receptor that leads to cholesteryl ester accumulation. Because the interaction of other oxidized lipoproteins with macrophages has been poorly investigated, we studied the effect of oxidatively modified high-density lipoproteins (HDLs) on the sterol metabolism of J774-A1 macrophages. Unlike native HDLs, oxidized HDLs caused a concentration-dependent accumulation of unesterified cholesterol and decreased [14C]oleate incorporation into steryl esters. Oxidized HDLs also decreased [14C]acetate incorporation into newly synthesized sterols. Cell surface binding of 125I-oxidized HDLs to the macrophages was saturable, with an apparent dissociation constant (Kd) of 0.96 nmol/mL. Both oxidized and acetylated LDLs but not native lipoproteins could compete for binding of 125I-oxidized HDL. The data support the conclusion that the effects elicited by oxidized HDLs on the sterol metabolism of macrophages are significantly different from those of native HDLs. The binding of oxidized HDLs to macrophages occurs at sites that are likely the same as those for modified LDLs. We speculate that, if occurring in vivo, HDL oxidation would generate modified lipoproteins capable of modulating the cholesterol homeostasis of macrophages.

Oxidative tyrosylation of high density lipoprotein by peroxidase enhances cholesterol removal from cultured fibroblasts and macrophage foam cells

Lipoprotein oxidation is thought to play a pivotal role in atherogenesis, yet the underlying reaction mechanisms remain poorly understood. We have explored the possibility that high density lipoprotein (HDL) might be oxidized by peroxidase-generated tyrosyl radical. Exposure of HDL to L-tyrosine, H2O2, and horseradish peroxidase crosslinked its apolipoproteins and strikingly increased protein-associated fluorescence. The reaction required L-tyrosine but was independent of free metal ions; it was blocked by either catalase or the heme poison aminotriazole. Dityrosine and other tyrosine oxidation products were detected in the apolipoproteins of HDL modified by the peroxidase/L-tyrosine/H2O2 system, implicating tyrosyl radical in the reaction pathway. Further evidence suggests that tyrosylated HDL removes cholesterol from cultured cells more effectively than does HDL. Tyrosylated HDL was more potent than HDL at inhibiting cholesterol esterification by the acyl-CoA:cholesterol acyltransferase reaction, stimulating the incorporation of [14C]acetate into [14C]cholesterol, and depleting cholesteryl ester stores in human skin fibroblasts. Moreover, exposure of mouse macrophage foam cells to tyrosylated HDL markedly diminished cholesteryl ester and free cholesterol mass. We have recently found that myeloperoxidase, a heme protein secreted by activated phagocytes, can also convert L-tyrosine to o,o'-dityrosine. This raises the possibility that myeloperoxidase-generated tyrosyl radical may modify HDL, enabling the lipoprotein to protect the artery wall against pathological cholesterol accumulation.

Is paraoxonase related to atherosclerosis

Human serum paraoxonase is responsible for the hydrolysis of organophosphate anticholinesterases, however, whether the enzyme has a physiological role other than the detoxication of insecticides and nerve gases has remained uncertain. Recently, evidence has begun to accumulate of a relationship between the serum activity of paraoxonase and atherosclerosis. Paraoxonase may a fundamental role in lipoprotein metabolism, preventing oxidative changes to low-density lipoprotein which render the particle atherogenic.

High-density lipoprotein subfractions

High-density lipoprotein (HDL) consists of a heterogeneous group of particles defined either by size or by apolipoprotein content. Subfractions of HDL appear to have distinct but interrelated metabolic functions, including facilitation of cholesteryl ester transfer to low- and very-low-density lipoproteins, modulation of triglyceride-rich particle catabolism, and, possibly, removal of cholesterol from peripheral tissues. Like HDL cholesterol, HDL subfractions are widely affected by a variety of factors. Subfractions also are markers for epidemiologic risk for coronary artery disease. Because they provide information about the physiologic processes of cholesterol metabolism, HDL subfractions are emerging as an increasingly important tool in the study of the relationship between lipids and cardiovascular disease.

Increased production rates of LDL are common in individuals with low plasma levels of HDL cholesterol, independent of plasma triglyceride concentrations

Reduced plasma levels of high density lipoprotein (HDL) cholesterol are associated with increased risk for coronary heart disease. Although plasma HDL levels are, in general, inversely related to plasma triglyceride (TG) concentrations, a small proportion of individuals with low HDL cholesterol concentrations have normal plasma TG levels. We wished to determine whether subjects with low plasma levels of HDL cholesterol could be characterized by common abnormalities of lipoprotein metabolism independent of plasma TGs. Therefore, we studied the metabolism of low density lipoprotein (LDL) apolipoprotein B (apo B) and HDL apolipoprotein A-I (apo A-I) in subjects with low plasma HDL cholesterol concentrations with or without hypertriglyceridemia. Nine subjects with low plasma HDL cholesterol levels and normal levels of plasma TGs and LDL cholesterol were studied. Autologous 131I-LDL and 125I-HDL were injected intravenously, and blood samples were collected for 2 weeks. LDL apo B and HDL apo A-I levels were measured by specific radioimmunoassays. Fractional catabolic rates (FCRs, pools per day) and production rates (PRs, milligrams/kilogram.day) for each apolipoprotein were determined. The results were compared with those obtained previously in nine subjects with low plasma HDL cholesterol levels and hypertriglyceridemia and in seven normal subjects. The normal subjects had an HDL apo A-I FCR (mean +/- SD) of 0.21 +/- 0.04. Despite large differences in plasma TG levels, the HDL apo A-I FCRs were similar in the low-HDL, normal-TG group (0.30 +/- 0.09) and the low-HDL, high-TG group (0.33 +/- 0.10), although only the latter value was significantly increased versus control subjects (p < 0.03). Increased apo A-I FCRs were associated with reduced HDL apo A-I levels in both groups of patients. Apo A-I PRs were similar in all groups. In contrast, LDL apo B PR was increased approximately 50% in the low-HDL, normal-TG group (19.3 +/- 6.6; p < 0.01) compared with normal subjects (12.5 +/- 2.6). There was a strong trend toward a greater LDL apo B PR in the low-HDL, high-TG group (17.6 +/- 4.5; p = 0.06 versus normal subjects) as well. LDL apo B FCRs were similar in all three groups. LDL apo B concentrations were also increased in the group with low HDL cholesterol and normal TG levels. Both groups with low HDL cholesterol levels had cholesterol-depleted LDL and HDL particles.(ABSTRACT TRUNCATED AT 400 WORDS)

High density lipoprotein reverses inhibitory effect of oxidized low density lipoprotein on endothelium-dependent arterial relaxation

We have recently reported that oxidized low density lipoprotein (ox-LDL) inhibits endothelium-dependent arterial relaxation through its increased lysophosphatidylcholine (LPC). In this study we examined whether high density lipoprotein (HDL) has any effect on the inhibition of endothelium-dependent relaxation by ox-LDL in isolated strips of rabbit thoracic aorta. Both low density lipoprotein (LDL) and HDL were isolated from normal human plasma, and LDL was oxidized by exposure to copper. Preincubation of arterial strips with ox-LDL (0.1-0.5 mg protein/ml) inhibited endothelium-dependent relaxation to acetylcholine (ACh) in a concentration-dependent manner. HDL (1 mg protein/ml) by itself had no effect on the relaxation to ACh. In the presence of HDL, the inhibition by ox-LDL was markedly reduced. Although synthetic L-alpha-palmitoyl LPC (5 micrograms/ml) completely abolished a relaxation to ACh, the preincubation of arterial strips with HDL completely prevented the LPC-induced inhibition. Moreover, a relaxation to ACh was almost completely recovered when the strips were washed with buffer containing HDL even after LPC-induced inhibition had occurred. HDL markedly reduced the incorporation of [1-14C]palmitate-labeled LPC ([14C]LPC) into cultured bovine aortic endothelial cells and promoted the release of cell-incorporated [14C]LPC into the medium, resulting in a reduction of the remaining [14C]LPC in the cells. Agarose electrophoresis after incubation of a mixture of ox-LDL labeled with [14C]LPC and unlabeled HDL demonstrated a transfer of [14C]LPC from ox-LDL to HDL. These results indicate that HDL reverses the ox-LDL-induced impairment of endothelium-dependent relaxation by removing LPC from ox-LDL and preventing LPC from acting on the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Two patterns of LDL metabolism in normotriglyceridemic patients with hypoalphalipoproteinemia

The objective of this study was to determine whether normotriglyceridemic patients with low levels of high density lipoprotein (HDL) cholesterol have concomitant defects in the metabolism of low density lipoproteins (LDLs). To address this question, measurements of turnover rates of apolipoprotein A-I (apo A-I) and LDL apolipoprotein B (apo B) were made in 36 middle-aged men with low HDL cholesterol (< 40 mg/dL), normal triglyceride (< 250 mg/dL), and normal total cholesterol (< or = 90th percentile) levels. Similar measurements were made in eight hypertriglyceridemic men having low HDL levels. For control, turnover rates of LDL apo B were measured in 24 healthy, normolipidemic men, and apo A-I kinetics were determined in 20 other healthy men with normal HDL cholesterol levels. In all patients with low HDL levels, fractional catabolic rates (FCRs) for apo A-I were increased compared with control subjects; in contrast, input rates for apo A-I in low-HDL patients were similar to control. Hypertriglyceridemic patients had significantly higher FCRs for LDL (0.463 +/- 0.040 pool/day, [mean +/- SEM]) than control subjects (0.328 +/- 0.008 pool/day, p < 0.001). In normolipidemic patients having low HDL, a bimodal pattern of LDL-apo B kinetics was observed. For 23 low-HDL patients, FCRs for LDL apo B averaged 0.450 +/- 0.017 pool/day and were significantly higher than control values. Additionally, in these patients, levels of very low density lipoprotein plus intermediate density lipoprotein (VLDL+IDL) cholesterol and VLDL+IDL apo B were higher than in control subjects (54 +/- 3 versus 32 +/- 3 mg/dL and 25 +/- 2 versus 18 +/- 1 mg/dL, respectively). The remaining 13 low-HDL patients had lower and essentially normal FCRs for LDL (0.300 +/- 0.009 pool/day); these patients also had relatively low levels of cholesterol and apo B in VLDL+IDL. Thus, two patterns of LDL kinetics were present in normotriglyceridemic patients with low HDL levels. One pattern was indistinguishable from that typically present in patients with hypertriglyceridemia, whereas the other was similar to normal control subjects. These two patterns of LDL-apo B kinetics may reflect different mechanisms for the causation of low HDL cholesterol concentrations.

Comparison of supplementation of RRR-alpha-tocopherol and racemic alpha-tocopherol in humans. Effects on lipid levels and lipoprotein susceptibility to oxidation

It has been suggested that alpha-tocopherol, a safe and effective antioxidant, be used in clinical trials to evaluate the ability of antioxidant therapy to inhibit atherosclerosis. Recent reports, however, have raised the possibility that there may be greater enrichment of plasma low density lipoprotein (LDL) in alpha-tocopherol resulting from the use of the naturally occurring RRR-alpha-tocopherol isomer compared with the other isomers present in the synthetic racemic form of alpha-tocopherol. Therefore, we fed equal dosages (1,600 mg/day) of the two forms of vitamin E to 16 men and women for 8 weeks and compared the effects of this supplementation on the susceptibility of isolated lipoproteins to oxidation. Neither form of vitamin E had appreciable effects on lipid or lipoprotein levels. alpha-Tocopherol levels in LDL increased at a similar rate in both groups and were nearly twofold higher than baseline levels by the end of the study. The susceptibility of LDL to oxidation was measured by formation of conjugated dienes, lipid peroxides, and thiobarbituric acid-reactive substances, as well as by macrophage degradation of LDL exposed to oxidizing conditions in vitro. The susceptibility of LDL to oxidation was decreased in both vitamin E groups compared with the baseline value, and this reduction occurred to a similar extent in both vitamin E-supplemented groups. alpha-Tocopherol levels in LDL also strongly correlated with all measures of LDL oxidation. This study demonstrates that, at this dosage, supplementation with either the natural or synthetic form of alpha-tocopherol provided equal antioxidant protection to LDL.

Antioxidative activity of high density lipoproteins in vivo

The present study consists of experimental and clinical investigations. It was shown that a single intravenous injection of a large dose of human HDL3 (200 mg protein) to rabbits with induced hypercholesterolemia (plasma cholesterol 500-700 mg/dl) was accompanied by a significant elevation of plasma HDL and led to a decrease (P < 0.05) of conjugated dienes and trienes by 20-30% after 6 h. Conjugated dienes remained stable for 24 h after HDL administration. In the clinical investigations a weak but statistically significant negative correlation (r = 0.262; P = 0.006) between HDL cholesterol and the content of conjugated dienes in the plasma of a total group of healthy subjects and patients with coronary heart disease (CHD) was found. The data allowed us to conclude that, in addition to other antioxidative systems, HDL also take part in the protection of plasma lipids from peroxidation.

Oxidized LDL and atherogenesis: relation to risk factors for coronary heart disease

According to a new theory, a critical step in atherogenesis is oxidation of low-density lipoprotein (LDL) within the arterial wall. Direct data supporting this theory are limited, but indirect evidence suggests that oxidized LDL plays a role in atherogenesis. An important question is whether the LDL-oxidation hypothesis conforms to what is known about other risk factors for coronary heart disease (CHD), such as hypertension, smoking, low high-density lipoprotein (HDL) levels, and diabetes mellitus. Perhaps a unified theory of atherogenesis could be formulated if these risk factors exert their atherogenic actions in part by promoting, facilitating, or permitting the oxidation of LDL.

Effect of platelet activating factor on the kinetics of LDL oxidation in vitro

Oxidatively modified low-density-lipoprotein (LDL) might contribute to the atherosclerotic process. This study was performed to examine an effect of platelet-activating factor (PAF) and of synthetic PAF analogs on Cu(II) induced oxidation of LDL in vitro: The D- and L-isomers of PAF and analogs with short-chain sn-2-substituents, 1-O-alkyl-2-butyryl-sn-glycero-3-phosphocholine and 1-O-alkyl-2-heptanoyl-sn-glycero-3-phosphocholine, were found to be the most effective inhibitors of LDL oxidation. Oxidation was inhibited completely at PAF concentrations above 100 microM. Lyso-PAF and analogs carrying longer chains at the sn-2 position were less effective. These results thus provide evidence for the involvement of other parameters in LDL oxidation beyond the content of natural antioxidants like vitamin E and beta-carotene.

Inhibition of low density lipoprotein oxidation by thyronines and probucol

Oxidation of low density lipoproteins (LDL) results in increased macrophage uptake of LDL which may contribute to the formation of macrophage-derived foam cells in the early atherosclerotic lesion. In this study we show that thyroxine (T4), its optical antipodes, certain desiodo analogs and probucol inhibited cupric sulfate-catalyzed oxidation of human LDL in a concentration-dependent manner as assessed by measuring the electrophoretic mobility, thiobarbituric acid reactive substances (TBARS) and LDL degradation in mouse macrophages. In Cu(2+)-catalyzed LDL oxidation at 24 hr, the TBARS level was 80 nmol/mg LDL protein/24-hr incubation. The concentrations (microM) of each agent producing 50% inhibition in the formation of oxidized LDL (IC50) for TBARS, electrophoretic mobility and macrophage degradation, respectively, were 1.13, 1.27 and 1.30 for reversed triiodothyronine; 1.33, 1.80 and 1.27 for triiodothyronine; 1.33, 1.37 and 1.37 for racemic thyroxine, DL-T4; 1.10, 1.40 and 1.50 for L-T4; 1.13, 1.33 and 1.23 for D-T4; and 1.47, 1.63 and 1.37 for probucol. No differences in inhibitory potency were observed when rT3, T3, the optical antipodes of T4 and the hydrophobic antioxidant drug probucol were compared. In air-induced LDL oxidation, TBARS was 16.1 nmol/mg LDL protein/6-hr incubation. The IC50 concentrations (microM) for TBARS and diene conjugation, respectively, were 0.187 and 0.336 for D-T4; 0.205 and 0.243 for L-T4 and 1.30 and 3.02 for probucol. With air-induced LDL oxidation conditions, the L-T4 concentrations included the physiological range, and thyroid-binding globulin did not modify the inhibitory effect of the endogenous enantiomer, L-T4. Putative uptake of this stereoisomer into LDL inhibited oxidation of these lipoproteins. Since concentrations of these thyronines which blocked air-induced LDL oxidation were in the physiological range, we conclude that thyronines, like the pharmacological agent probucol, limit the oxidative modification of LDL and thus may serve as natural inhibitors of atherogenesis.

Effects of oleate-rich and linoleate-rich diets on the susceptibility of low density lipoprotein to oxidative modification in mildly hypercholesterolemic subjects

We report the results of feeding oleate- or linoleate-enriched diets for 8 wk to mildly hypercholesterolemic subjects and the resulting alterations in composition and functional properties of their plasma LDL and HDL. LDL isolated from subjects on oleate-enriched diets was less susceptible to copper-mediated oxidation, as measured by conjugated diene and lipid peroxide formation, and less susceptible to LDL-protein modification, as evidenced by reduced LDL macrophage degradation after copper- or endothelial cell-induced oxidation. For all subjects, the percentage of 18:2 in LDL correlated strongly with the extent of conjugated diene formation (r = 0.89, P < 0.01) and macrophage degradation (r = 0.71, P < 0.01). Oxidation of LDL led to initial rapid depletion of unsaturated fatty acids in phospholipids followed by extensive loss of unsaturated fatty acids in cholesteryl esters and triglycerides. Changes in HDL fatty acid composition also occurred. However, HDL from both dietary groups retained its ability to inhibit oxidative modification of LDL. This study demonstrates that alterations in dietary fatty acid composition can effectively alter the fatty acid distribution of LDL and HDL in hypercholesterolemic subjects and that susceptibility to LDL oxidation is altered by these changes. Substitution of monounsaturated (rather than polyunsaturated) fatty acids for saturated fatty acids in the diet might be preferable for the prevention of atherosclerosis.

Influence of the apoA-II gene locus on HDL levels and fatty streak development in mice

Previous studies have shown that distal mouse chromosome 1 contains the apolipoprotein AII (apoAII) gene, encoding the second most abundant apolipoprotein in high density lipoproteins (HDLs), as well as a gene termed Ath-1 that controls aortic fatty streak development and HDL cholesterol levels in response to a high-fat, high-cholesterol diet. We report genetic studies confirming that the genes are distinct. Using molecular markers for mouse chromosome 1, we have further mapped the two genes, and our results indicate that they are separated by a minimum of 2 cM. We also report evidence that in mice on a low-fat chow diet, the apoAII gene locus influences HDL cholesterol levels. Thus, statistical analysis of two sets of recombinant inbred strains revealed concordant segregation patterns of HDL cholesterol levels and the apoAII gene locus. The effect of apoAII expression on HDL cholesterol levels was further tested by using a congenic strain that exhibits increased apoAII synthesis in comparison to the background strain. The results support the concept that increased synthesis of apoAII results in increased HDL cholesterol levels. Unexpectedly, increased expression of apoAII appeared to promote rather than retard aortic fatty streak development.

Cross-linking of apolipoproteins is involved in a loss of the ligand activity of high density lipoprotein upon Cu(2+)-mediated oxidation

A recent study demonstrated that Cu(2+)-mediated oxidation of high density lipoprotein (HDL) resulted in a loss of the capacity to reduce cholesterol from macrophage foam cells [(1991) Proc. Natl. Acad. Sci. USA 88, 6457-6461]. In the present study we characterized the physicochemical properties of oxidized HDL and correlated them with the ligand activity toward the HDL receptor. Among them, the cross-linking of apolipoproteins and an increase in lipid peroxides were characteristic and closely similar to those of tetranitromethane-treated HDL, an abortive ligand for the HDL receptor. Cellular experiments with murine peritoneal macrophages revealed that both the cellular binding activity of HDL and its capacity to enhance cholesterol efflux from macrophage foam cells were markedly reduced upon oxidation. These results suggest that cross-linking of HDL apolipoproteins is involved in the loss of the ligand activity of oxidized HDL.

High density lipoprotein is the major carrier of lipid hydroperoxides in human blood plasma from fasting donors

Analysis of untreated fresh blood plasma from healthy, fasting donors revealed that high density lipoprotein (HDL) particles carry most (approximately 85%) of the detectable oxidized core lipoprotein lipids. Low density lipoprotein (LDL) lipids are relatively peroxide-free. In vitro the mild oxidation of gel-filtered plasma from fasting donors with a low, steady flux of aqueous peroxyl radicals initially caused preferential oxidation of HDL rather than LDL lipids until most ubiquinol-10 present in LDL was consumed. Thereafter, LDL core lipids were oxidized more rapidly. Isolated lipoproteins behaved similarly. Preferential accumulation of lipid hydroperoxides in HDL reflects the lack of antioxidants in most HDL particles compared to LDL, which contained 8-12 alpha-tocopherol and 0.5-1.0 ubiquinol-10 molecules per particle. Cholesteryl ester hydroperoxides (CEOOHs) in HDL and LDL were stable when added to fresh plasma at 37 degrees C for up to 20 hr. Transfer of CEOOHs from HDL to LDL was too slow to have influenced the in vitro plasma oxidation data. Incubation of mildly oxidized LDL and HDL with cultured hepatocytes afforded a linear removal of CEOOHs from LDL (40% loss over 1 hr), whereas a fast-then-slow biphasic removal was observed for HDL. Our data show that HDL is the principal vehicle for circulating plasma lipid hydroperoxides and suggest that HDL lipids may be more rapidly oxidized than those in LDL in vivo. The rapid hepatic clearance of CEOOHs in HDL could imply a possible beneficial role of HDL by attenuating the build-up of oxidized lipids in LDL.

Modified apolipoprotein pattern after irradiation of human high-density lipoproteins by ultraviolet B

The ultraviolet B-induced destruction of tryptophan residues and lipid peroxidation of high-density lipoproteins is accompanied by the immediate and marked structural modification of the apolipoproteins, as revealed by SDS-polyacrylamide gel electrophoresis and immunoblot with specific monoclonal antibodies. Formation of several polymers of apolipoprotein A-I, apolipoprotein A-II or both apolipoproteins occurred, although apolipoprotein A-II did not contain any Trp residue. These results suggest that initial photochemical damage can be transferred via intramacromolecular processes to other sites within the same apolipoprotein and by intermacromolecular reactions from apolipoprotein A-I to other apolipoproteins. In both cases, lipid peroxidation enhances the propagation of the initial photochemical damage. The physiological significance of this work is discussed with respect to the low-light doses required for the alterations of the high-density lipoproteins.

Inhibition by serum components of oxidation and collagen-binding of low-density lipoprotein

Low-density lipoprotein (LDL) is oxidized by cellular and noncellular mechanisms, both leading to an increased binding to collagen. We have investigated the effect of serum on lipid peroxidation, apoprotein oxidation and the binding of oxidized apoprotein to collagen. During noncellular oxidation, lipoprotein-deficient serum strongly inhibited all three processes. The serum fraction of M(r) > 100,000 was equally inhibitory; this effect was not due to alpha 1 or gamma globulins, alpha 2 macroglobulins, haptoglobins or ceruloplasmin. The serum fraction of M(r) 30,000-100,000 stimulated the binding of oxidized apoprotein but the albumin in this fraction inhibited lipid peroxidation and apoprotein oxidation. Serum ultrafiltrate (M(r) < 1000) inhibited lipid and protein oxidation, and binding; the inhibitory effect was abolished by deionization which removed histidine. The effects of lipoprotein-deficient serum and its fractions on cellular oxidation were similar but weaker than those on noncellular oxidation, HDL inhibited noncellular oxidation as well as binding of oxidized apoprotein. VLDL also inhibited oxidation; this could not be accounted for by its content of apo B. If present in vivo, these inhibitory effects would completely suppress both cellular and noncellular oxidation of LDL and its subsequent binding to collagen.

Oxidized HDL are much less cytotoxic to lymphoblastoid cells than oxidized LDL

The possible effect of oxidized HDL was investigated on lymphoblastoid cells, in comparison to the cytotoxic effect of oxidized LDL. Oxidation of HDL was promoted by UV-C irradiation, or by copper ion (5 microM) or the combination of the two treatments. HDL extensively treated by UV-C for 20 h did not exhibit any cytotoxic effect on cultured lymphoblastoid cells even at a concentration of 500 micrograms apolipoprotein A-I/ml. In contrast to UV-treated (2 h) LDL, which were highly cytotoxic (already at a concentration of 100 micrograms apolipoprotein B/ml), HDL treated by copper or copper + UV were oxidized, as shown by TBARS formation and PUFA content decrease, but were slightly cytotoxic.

The influence of medium components on Cu(2+)-dependent oxidation of low-density lipoproteins and its sensitivity to superoxide dismutase

The extent of in vitro Cu(2+)-dependent oxidation of low-density lipoproteins (LDL) has been reported to vary widely depending upon reaction conditions. In this study, the effect of proteins and amino acids on Cu(2+)-induced LDL oxidation was examined. Treatment of LDL with 5 microM CuSO4 for 18 h in either phosphate-buffered saline (PBS) or Ham's F-10 medium resulted in extensive oxidation as determined by the content of thiobarbituric acid reactive substances (TBARS) and by increased lipoprotein electronegativity. In PBS, oxidation was entirely blocked by histidine and the tripeptide, gly-his-lys (GHK). Oxidation was also prevented by bovine serum albumin, but superoxide dismutase (SOD) provided only 20% protection. Both proteins bound similar amounts of Cu2+, but albumin appeared to be a more effective peroxyl radical trap as evidenced by its ability to prevent LDL oxidation induced by 2,2'-azo-bis(2-amidinopropane hydrochloride). In F-10 medium, SOD had marked inhibitory effects, in contrast to PBS. The addition of disulfides to PBS markedly enhanced the ability of SOD to inhibit oxidation. These results indicate that medium components which affect Cu2+ availability influence LDL oxidation and suggest that albumin is ideally suited as a plasma antioxidant to prevent oxidative modification of LDL. Furthermore, in certain instances, the inhibitory effects of SOD may be attributable to effects such as Cu2+ binding rather than dismutation of superoxide.

Lipid peroxidation--a common pathogenetic mechanism?

Lipid peroxidation is considered at present as one of the basic mechanisms involved in reversible and irreversible cell and tissue damage. The current knowledge about the role of peroxidative breakdown of polyunsaturated fatty acids in the pathogenesis of various diseases has been reviewed. Lipid peroxidation leads to degradation of the lipid membrane, interaction of degradation products with intra- and extracellular targets and to the production of new reactive oxygen species during the course of the chain reaction thus leading to damage of cells and tissues. According to our current view lipid peroxidation is implicated in the pathogenesis of cancer, inflammatory processes, atherosclerosis, toxic injury by xenobiotics and ischemic-reperfusion damage.