Fractionation of charge-modified low density lipoproteins by fast protein liquid chromatography

Range of L5 LDL levels in healthy adults and L5's predictive power in patients with hyperlipidemia or coronary artery disease

Electronegative L5 low-density lipoprotein (LDL) level may be a useful biomarker for predicting cardiovascular disease. We determined the range of plasma L5 levels in healthy adults (n = 35) and examined the power of L5 levels to differentiate patients with coronary artery disease (CAD; n = 40) or patients with hyperlipidemia (HLP) without evidence of CAD (n = 35) from healthy adults. The percent L5 in total LDL (L5%) was quantified by using fast-protein liquid chromatography with an anion-exchange column. Receiver operating characteristic curve analysis was performed to determine cut-off values for L5 levels. The mean L5% and plasma concentration of L5 (ie, [L5]) were significantly higher in patients with HLP or CAD than in healthy adults (P < 0.001). The ranges of L5% and [L5] in healthy adults were determined to be <1.6% and <1.7 mg/dL, respectively. In individuals with L5% >1.6%, the odds ratio was 9.636 for HLP or CAD. In individuals with [L5] >1.7 mg/dL, the odds ratio was 17.684 for HLP or CAD. The power of L5% or [L5] to differentiate patients with HLP or CAD from healthy adults was superior to that of the LDL/high-density lipoprotein ratio. The ranges of L5% and [L5] in healthy adults determined here may be clinically useful in preventing and treating cardiovascular disease.

Can gas replace protein function? CO abrogates the oxidative toxicity of myoglobin

Outside their cellular environments, hemoglobin (Hb) and myoglobin (Mb) are known to wreak oxidative damage. Using haptoglobin (Hp) and hemopexin (Hx) the body defends itself against cell-free Hb, yet mechanisms of protection against oxidative harm from Mb are unclear. Mb may be implicated in oxidative damage both within the myocyte and in circulation following rhabdomyolysis. Data from the literature correlate rhabdomyolysis with the induction of Heme Oxygenase-1 (HO-1), suggesting that either the enzyme or its reaction products are involved in oxidative protection. We hypothesized that carbon monoxide (CO), a product, might attenuate Mb damage, especially since CO is a specific ligand for heme iron. Low density lipoprotein (LDL) was chosen as a substrate in circulation and myosin (My) as a myocyte component. Using oxidation targets, LDL and My, the study compared the antioxidant potential of CO in Mb-mediated oxidation with the antioxidant potential of Hp in Hb-mediated oxidation. The main cause of LDL oxidation by Hb was found to be hemin which readily transfers from Hb to LDL. Hp prevented heme transfer by sequestering hemin within the Hp-Hb complex. Hemin barely transferred from Mb to LDL, and oxidation appeared to stem from heme iron redox in the intact Mb. My underwent oxidative crosslinking by Mb both in air and under N₂. These reactions were fully arrested by CO. The data are interpreted to suit several circumstances, some physiological, such as high muscle activity, and some pathological, such as rhabdomyolysis, ischemia/reperfusion and skeletal muscle disuse atrophy. It appear that CO from HO-1 attenuates damage by temporarily binding to deoxy-Mb, until free oxygen exchanges with CO to restore the equilibrium.

Validation of a novel ELISA for measurement of electronegative low-density lipoprotein

BACKGROUND

Oxidative modification of low-density lipoprotein (LDL) plays a key role in the pathogenesis of atherosclerosis. LDL(-) is present in blood plasma of healthy subjects and at higher concentrations in diseases with high cardiovascular risk, such as familial hypercholesterolemia or diabetes.

METHODS

We developed and validated a sandwich ELISA for LDL(-) in human plasma using two monoclonal antibodies against LDL(-) that do not bind to native LDL, extensively copper-oxidized LDL or malondialdehyde-modified LDL. The characteristics of assay performance, such as limits of detection and quantification, accuracy, inter- and intra-assay precision were evaluated. The linearity, interferences and stability tests were also performed.

RESULTS

The calibration range of the assay is 0.625-20.0 mU/L at 1:2000 sample dilution. ELISA validation showed intra- and inter-assay precision and recovery within the required limits for immunoassays. The limits of detection and quantification were 0.423 mU/L and 0.517 mU/L LDL(-), respectively. The intra- and inter-assay coefficient of variation ranged from 9.5% to 11.5% and from 11.3% to 18.9%, respectively. Recovery of LDL(-) ranged from 92.8% to 105.1%.

CONCLUSIONS

This ELISA represents a very practical tool for measuring LDL(-) in human blood for widespread research and clinical sample use.

Oxidative stress-mediated mesangial cell proliferation requires RAC-1/reactive oxygen species production and beta4 integrin expression

Lipid abnormalities and oxidative stress, by stimulating mesangial cell (MC) proliferation, can contribute to the development of diabetes-associated renal disease. In this study we investigated the molecular events elicited by oxidized low density lipoproteins (ox-LDL) in MC. We demonstrate that in MC cultured in the presence of ox-LDL, survival and mitogenic signals on Akt and Erk1/2 MAPK pathways are induced, respectively. Moreover, as shown by the expression of the dominant negative Rac-1 construct, we first report that ox-LDL-mediated cell survival and cell cycle progression depend on Rac-1 GTPase-mediated reactive oxygen species production and on epidermal growth factor receptor transactivation. By silencing Akt and blocking Erk1/2 MAPK pathways, we also demonstrate that these signals are downstream to Rac-1/reactive oxygen species production and epidermal growth factor receptor activation. Finally, by endogenous depletion of beta4 integrin, expressed in MC, we provide evidence that the expression of this adhesion molecule is essential for ox-LDL-mediated MC dysfunction. Our data identify a novel signaling pathway involved in oxidative stress-induced diabetes-associated renal disease and provide the rationale for therapeutically targeting beta4 integrin.

Chronic intermittent hypoxia induces atherosclerosis

RATIONALE

Obstructive sleep apnea, a condition leading to chronic intermittent hypoxia (CIH), is associated with hyperlipidemia, atherosclerosis, and a high cardiovascular risk. A causal link between obstructive sleep apnea and atherosclerosis has not been established.

OBJECTIVES

The objective of the present study was to examine whether CIH may induce atherosclerosis in C57BL/6J mice.

METHODS

Forty male C57BL/6J mice, 8 weeks of age, were fed either a high-cholesterol diet or a regular chow diet and subjected either to CIH or intermittent air (control conditions) for 12 weeks.

MEASUREMENTS AND MAIN RESULTS

Nine of 10 mice simultaneously exposed to CIH and high-cholesterol diet developed atherosclerotic lesions in the aortic origin and descending aorta. In contrast, atherosclerosis was not observed in mice exposed to intermittent air and a high-cholesterol diet or in mice exposed to CIH and a regular diet. A high-cholesterol diet resulted in significant increases in serum total and low-density lipoprotein cholesterol levels and a decrease in high-density lipoprotein cholesterol. Compared with mice exposed to intermittent air and a high-cholesterol diet, combined exposure to CIH and a high-cholesterol diet resulted in marked progression of dyslipidemia with further increases in serum total cholesterol and low-density lipoprotein cholesterol (124 +/- 4 vs. 106 +/- 6 mg/dl; p < 0.05), a twofold increase in serum lipid peroxidation, and up-regulation of an important hepatic enzyme of lipoprotein secretion, stearoyl-coenzyme A desaturase-1.

CONCLUSIONS

CIH causes atherosclerosis in the presence of diet-induced dyslipidemia.

Hyperlipidemia and lipid peroxidation are dependent on the severity of chronic intermittent hypoxia

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH) and associated with dysregulation of lipid metabolisms and atherosclerosis. Causal relationships between OSA and metabolic abnormalities have not been established because of confounding effects of underlying obesity. The goal of the study was to determine if CIH causes lipid peroxidation and dyslipidemia in the absence of obesity and whether the degrees of dyslipidemia and lipid peroxidation depend on the severity of hypoxia. Lean C57BL/6J mice were exposed to CIH for 4 wk with a fractional inspired O2 (FI(O2)) nadir of either 10% (moderate CIH) or 5% (severe CIH). Mice exposed to severe CIH exhibited significant increases in fasting serum levels of total cholesterol (129 +/- 2.9 vs. 113 +/- 2.8 mg/dl in control mice, P < 0.05) and low-density lipoprotein cholesterol (85.7 +/- 8.9 vs. 56.4 +/- 9.7 mg/dl, P < 0.05) in conjunction with a 1.5- to 2-fold increase in lipoprotein secretion, and upregulation of hepatic stearoyl coenzyme A desaturase 1 (SCD-1). Severe CIH also markedly increased lipid peroxidation in the liver (malondialdehyde levels of 94.4 +/- 5.4 vs. 57.4 +/- 5.2 nmol/mg in control mice, P < 0.001). In contrast, moderate CIH did not induce hyperlipidemia or change in hepatic SCD-1 levels but did cause lipid peroxidation in the liver at a reduced level relative to severe CIH. In conclusion, CIH leads to hypercholesterolemia and lipid peroxidation in the absence of obesity, and the degree of metabolic dysregulation is dependent on the severity of the hypoxic stimulus.

p53 Mediates the accelerated onset of senescence of endothelial progenitor cells in diabetes

Adverse metabolic factors, including oxidized small and dense low density lipoprotein (ox-dmLDL) can contribute to the reduced number and the impaired functions of circulating endothelial progenitors (EPC) in diabetic patients. To elucidate the molecular mechanisms involved, EPC from normal donors were cultured in the presence of ox-dmLDL. Under these experimental conditions EPC undergo to senescent-like growth arrest. This effect is associated with Akt activation, p21 expression, p53 accumulation, and retinoblastoma protein dephosphorylation and with a reduced protective effect against oxidative damage. Moreover, depletion of endogenous p53 expression by small interfering RNA demonstrates that the integrity of this pathway is essential for senescence to occur. Activation of the Akt/p53/p21 signaling pathway and accelerated onset of senescence are also detectable in EPC from diabetic patients. Finally, diabetic EPC depleted of endogenous p53 do not undergo to senescence-growth arrest and acquire the ability to form tube-like structures in vitro. These observations identify the activation of the p53 signaling pathway as a crucial event that can contribute to the impaired neovascularization in diabetes.

Relation between insulin resistance and fast-migrating LDL subfraction as characterized by capillary isotachophoresis

The proportion of the electronegative low density lipoprotein [LDL(-)] subfraction, which is atherogenic, is increased in type 2 diabetes but is not reduced by glycemic control. Therefore, we evaluated the ability of a new technique, capillary isotachophoresis (cITP), to quantify charge-based LDL subfractions and examined the relation between insulin resistance and the cITP fast-migrating (f) LDL levels. Seventy-five 10-year-old boys were included. The two cITP LDL subfractions, fLDL and major LDL subfractions, were proportional to the LDL protein content within the range of 0.1-0.8 mg/ml LDL protein. Levels of cITP fLDL were positively correlated with triglyceride (TG) levels and negatively correlated with LDL size. Insulin resistance as assessed by the homeostasis model assessment (HOMA-IR) was positively correlated (P < 0.01) with cITP fLDL levels (r = 0.41). The relation between HOMA-IR and cITP fLDL levels depended on TG levels but was independent of body mass index and LDL size. cITP lipoprotein analysis is an accurate and sensitive method for quantifying charge-based LDL subfractions in human plasma, and insulin resistance is related to cITP fLDL independent of LDL size.

Intermittent hypoxia induces hyperlipidemia in lean mice

Obstructive sleep apnea, a syndrome leading to recurrent intermittent hypoxia (IH), has been associated previously with hypercholesterolemia, independent of underlying obesity. We examined the effects of experimentally induced IH on serum lipid levels and pathways of lipid metabolism in the absence and presence of obesity. Lean C57BL/6J mice and leptin-deficient obese C57BL/6J-Lep(ob) mice were exposed to IH for five days to determine changes in serum lipid profile, liver lipid content, and expression of key hepatic genes of lipid metabolism. In lean mice, exposure to IH increased fasting serum levels of total cholesterol, high-density lipoprotein (HDL) cholesterol, phospholipids (PLs), and triglycerides (TGs), as well as liver TG content. These changes were not observed in obese mice, which had hyperlipidemia and fatty liver at baseline. In lean mice, IH increased sterol regulatory element binding protein 1 (SREBP-1) levels in the liver, increased mRNA and protein levels of stearoyl-coenzyme A desaturase 1 (SCD-1), an important gene of TG and PL biosynthesis controlled by SREBP-1, and increased monounsaturated fatty acid content in serum, which indicated augmented SCD-1 activity. In addition, in lean mice, IH decreased protein levels of scavenger receptor B1, regulating uptake of cholesterol esters and HDL by the liver. We conclude that exposure to IH for five days increases serum cholesterol and PL levels, upregulates pathways of TG and PL biosynthesis, and inhibits pathways of cholesterol uptake in the liver in the lean state but does not exacerbate the pre-existing hyperlipidemia and metabolic disturbances in leptin-deficient obesity.

Electronegative low-density lipoprotein

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Isolation, characterization, and functional assessment of oxidatively modified subfractions of circulating low-density lipoproteins

OBJECTIVE

Current evidence suggests that oxidatively modified human plasma low-density lipoproteins (ox-LDLs) are proatherogenic and cytotoxic to endothelial and vascular smooth muscle cells. The present study describes a method using ion-exchange chromatography that is capable of large-scale subfractionation of LDL for adequate analyses of composition or bioactivities.

METHODS AND RESULTS

LDLs from normolipidemic (N-LDL) and homozygous familial hypercholesterolemic (FH-LDL) subjects were separated into 5 subfractions (L1 through L5) by high-capacity ion-exchange chromatography. The most strongly retained fraction from FH subjects, FH-L5, suppressed DNA synthesis in cultured bovine aortic endothelial cells and stimulated mononuclear cell adhesion to cultured endothelial cells under flow conditions in vitro. L5, which represented 1.1+/-0.2% and 3.7+/-1.7% of the LDL from N-LDL and FH-LDL, respectively, was more triglyceride-rich (17% versus 5%) and cholesteryl ester-poor (23% versus 33%) than were L1 through L4. Electrophoretic mobilities on agarose gels increased from L1 to L5. According to SDS-PAGE, apolipoprotein B-100 in N-LDL fractions L1 through L5 appeared as a single approximately 500-kDa band. In contrast, the fractions isolated from FH-LDL showed substantial fragmentation of the apolipoprotein B-100, including bands between 200 and 116 kDa. Competitive ELISA analyses using a malondialdehyde-specific monoclonal antibody against Cu2+ ox-LDL suggest that FH-L5 is malondialdehyde-modified.

CONCLUSIONS

Relative to N-LDL, FH-LDL contains higher concentrations of a fraction, L5, that exhibits distinctive physicochemical properties and biological activities that may contribute to initiation and progression of atherogenesis in vivo.

Macrophages take up triacylglycerol-rich emulsions at a faster rate upon co-incubation with native and modified LDL: An investigation on the role of natural chylomicrons in atherosclerosis

Chylomicrons play a role in atherosclerosis, however, because the mechanisms involved in the cell uptake of these particles are not fully understood, investigations were carried out using a radioactively labeled protein-free triacylglycerol-rich emulsion incubated with peritoneal macrophages obtained from normal and apoE-knockout mice. Experiments were done in the presence of substances that inhibit several endocytic processes: EDTA for low density lipoprotein receptor, fucoidan for scavenger receptor, cytochalasin B for phagocytosis, and a lipopolysaccharide for lipoprotein lipase. In addition, triacylglycerol-rich emulsions were also prepared in the presence of native or modified radioactively labeled low density lipoprotein particles that are known to accumulate in the arterial intima. Probucol was also used to prevent the possible role played by an antioxidant in triacylglycerol-rich emulsion uptake. We have shown that triacylglycerol-rich emulsion alone is taken up by a coated-pit-dependent mechanism, mediated by macrophage secretion of apolipoprotein E. Furthermore, native, aggregated, acetylated, and moderately macrophage-oxidized low density lipoprotein stimulate the uptake of a triacylglycerol-rich emulsion through several mechanisms such as an actin-dependent pathway, scavenger receptors, and lipolysis mediated by lipoprotein lipase. On the other hand, in spite of the interaction of low density lipoprotein forms with a triacylglycerol-rich emulsion, the cellular triacylglycerol-rich emulsion uptake is impaired by copper-oxidized low density lipoprotein, possibly due to its diminished affinity towards lipoprotein lipase. We have also shown that macrophages take up aggregated low density lipoprotein better than the acetylated or oxidized forms of low density lipoprotein.

Postprandial oxidative stress

Consumption of a meal containing oxidized and oxidizable lipids gives rise to an increased plasma concentration of lipid hydroperoxides, detectable by a sensitive chemiluminescence procedure. This is associated with increased susceptibility of LDL to oxidation, apparently due a structural perturbation at the particle surface brought about by lipid oxidation products. The postprandial modification of LDL is at least partially accounted for by an increase of LDL-, a subfraction containing lipid oxidation products where apoprotein-B-100 (apoB-100) is denatured. Consuming the meal with a suitable source of antioxidants, such as those found in red wine, minimizes this postprandial oxidative stress. The inhibition of peroxidation of lipids present in the meal during digestion is a possible mechanism for the observed protection of LDL. The in vivo oxidatively modified LDL- has numerous features that correspond to the atherogenic minimally modified LDL produced in vitro. These modified particles could account for a relevant link between nutrition and early biological processes that foster the development of atherosclerosis.

Changes in low-density lipoprotein electronegativity and oxidizability after aerobic exercise are related to the increase in associated non-esterified fatty acids

The immediate effects of intense aerobic exercise on the composition and oxidizability of low- (LDL) and high-density lipoproteins (HDL) were studied in 11 male athletes. Plasma parameters known to affect lipoprotein oxidizability were also evaluated. Lipophilic antioxidants, including alpha-tocopherol and carotenoids, paraoxonase and malondialdehyde (MDA) in plasma remained unchanged after exercise. Increases in the concentration of uric acid, bilirubin and ascorbic acid after the race resulted in a significant increase in total antioxidant serum capacity. LDL, but not HDL, increased its "in vitro"-induced susceptibility to oxidation and the proportion of electronegative LDL (LDL-). The ability of HDL to inhibit the oxidation of LDL remained unchanged after exercise. The enhanced oxidizability of LDL was not explained by increments in its aldehyde content or by decrements in antioxidants. The major compositional change in LDL was an increase in non-esterified fatty acid (NEFA) content (from 4.00+/-1.24 to 19.00+/-14.18 mol NEFA/mol apoB). NEFA also increased in plasma and HDL. "In vitro" experiments showed that incubation of LDL with increasing amounts of NEFA induced a concentration-dependent increase in the proportion of LDL-. Moreover, a slightly increased NEFA content in LDL (15-50 mol NEFA/mol apoB) induced higher susceptibility to oxidation. These "in vitro" results concur with those observed in LDL obtained from athletes after exercise, i.e. a concentration of approximately 20 mol NEFA/mol apoB increased LDL oxidizability and LDL- proportion. We conclude that changes in the qualitative characteristics of LDL after exercise were unrelated to oxidative stress, but were related to the increase in LDL-associated NEFA content.

A novel series of 2,6,7-substituted 2,3-dihydro-1,4-benzodioxin and 2,6,7-substituted 1,4-benzodioxin derivatives as lipid peroxidation inhibitors. Structure-activity relationships for high inhibition of human low-density lipoprotein peroxidation

A series of 6- or 7-substituted 2-carboxamido- or 2-(aminomethyl)-1,4-benzodioxin and -2,3-dihydro-1,4-benzodioxin derivatives were synthesized and evaluated to determine the necessary structural requirements for a high inhibition of human low-density lipoprotein copper-induced peroxidation. The most active compounds (21, 25, 28, 36, and 37) were found between 5 and >45 times more active than probucol itself. Due to both their potency and their structural features, compounds 25 and 36 were selected with others for complementary in vitro and in vivo investigations. Both of them exhibit calcium antagonist properties in the same range of potency as flunarizine itself. Compound 36 was also found to have significant hypolipaemic activity in mice at 100 and 300 mg/kg po, while compound 25 proved to be clearly active in a normobar hypoxia test.

Effect of ethyl esterification of phenolic acids on low-density lipoprotein oxidation

Inhibition of copper-induced low-density lipoprotein (LDL) oxidation by phenolic acids and their ethyl esters was investigated. LDL oxidation was evaluated by the hydroperoxide concentration and the chromatographic pattern of apoprotein fractions after fast protein liquid chromatography (FPLC). Antiradical properties against 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical and 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) were also investigated, and lipophilicity determined by thin-layer chromatography. Caffeic acid at 5 microM and sinapic acid at 10 microM protected LDL against oxidation, inhibiting both hydroperoxide formation and the increase of apoprotein negative charge. Ferulic, gallic and p-hydroxy cinnamic acids were ineffective. Ethyl esterification increased the lipophilicity of the five acids, and enhanced the antioxidant properties of caffeic, sinapic and ferulic acids. Ethyl caffeate was protective at 1 microM. In contrast, gallic and p-hydroxy cinnamic ethyl esters were ineffective. Our results indicate that ethyl esterification of phenolic acids increases lipophilicity of their ethyl esters and may enable a better incorporation into the lipid layer of the LDL particle and the exertion of their antioxidant effect in the true site of lipoperoxidation. However, increasing lipophilicity is not the only mechanism able to potentiate preexisting antioxidant properties of molecules, and probably other mechanisms are implicated.

Electronegative LDL from normolipemic subjects induces IL-8 and monocyte chemotactic protein secretion by human endothelial cells

The presence in plasma of an electronegative LDL subfraction [LDL(-)] cytotoxic for endothelial cells (ECs) has been reported. We studied the effect of LDL(-) on the release by ECs of molecules implicated in leukocyte recruitment [interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1)] and in the plasminogen activator inhibitor-1 (PAI-1). LDL(-), isolated by anion-exchange chromatography, differed from nonelectronegative LDL [LDL(+)] in its higher triglyceride, nonesterified fatty acid, apoprotein E and apoprotein C-III, and sialic acid contents. No evidence of extensive oxidation was found in LDL(-); its antioxidant and thiobarbituric acid-reactive substances contents were similar to those of LDL(+). However, conjugated dienes were increased in LDL(-), which suggests that mild oxidation might affect these particles. LDL(-) increased, in a concentration-dependent manner, the release of IL-8 and MCP-1 by ECs and was a stronger inductor of both chemokines than oxidized LDL (oxLDL) or LDL(+). PAI-1 release increased slightly in ECs incubated with both LDL(-) and oxLDL but not with LDL(+). However, no cytotoxic effects of LDL(-) were observed on ECs. Actinomycin D inhibited the release of IL-8 and MCP-1 induced by LDL(-) and oxLDL by up to 80%, indicating that their production is mediated by protein synthesis. Incubation of ECs with N:-acetyl cysteine inhibited production of IL-8 and MCP-1 induced by LDL(-) and oxLDL by >50%. The free radical scavenger butylated hydroxytoluene slightly inhibited the effect of oxLDL but did not modify the effect of LDL(-). An antagonist (BN-50730) of the platelet-activating factor receptor inhibited production of both chemokines by LDL(-) and oxLDL in a concentration-dependent manner. Our results indicate that LDL(-) shows proinflammatory activity on ECs and may contribute to early atherosclerotic events.

Molecular requirements in the recognition of low-density lipoproteins (LDL) by specific platelet membrane receptors

We have demonstrated that platelet low-density lipoprotein (LDL) receptors differ from classic LDL receptors of nucleated cells. Although positively charged Arg and Lys residues of apoprotein B-100 are known to play a key role in LDL recognition by classic LDL receptors, there are no conclusive data on platelet LDL receptors. This study investigated the molecular requirements of LDL particle recognition by platelet LDL receptors. The involvement of lipid and protein fractions was determined by displacement studies of the binding of 125I-LDL to platelets and fibroblasts (used as a classical LDL receptor model). The role of the protein moiety was evaluated by chemically modifying positively charged apoB residues (Lys, Arg, and Tyr) via copper-induced oxidation, cyclohexanedione, and tetranitromethane, respectively. The involvement of the lipid fraction was determined by ligand binding assays using 125I-LDL particles that had previously been delipidated and subjected to apoB solubilization. The degree of particle modification was analyzed by agarose/acrylamide gel electrophoresis and anion exchange chromatography. Modifying the amino acid residues increased particle electronegativity in the following order of potency: CHD-LDL>TNM-LDL>ox-LDL>native LDL. The results obtained by displacement studies in fibroblasts suggested that the gain in the LDL negative charge was the most important factor in the loss of receptor affinity. The chemical models of protein modification used in our study greatly affected LDL binding to the classical fibroblast receptor. In contrast, there was very slight difference in the displacement capacity on platelet 125I-LDL binding, which suggests that the protein fraction does not play a major role in the interaction of LDL with its platelet receptor. On the other hand, whereas modifying the lipid moiety did not alter the ability of solubilized 125I-apoB to interact with the classical fibroblast LDL receptor, platelet LDL receptors were unable to recognize these particles. In conclusion, our results confirm that the protein fraction plays a key role in the fibroblast LDL-receptor recognition process, whereas the lipid fraction appears to have a more relevant role in platelet LDL-receptor recognition.

Effect of physical exercise on lipoprotein(a) and low-density lipoprotein modifications in type 1 and type 2 diabetic patients

To evaluate the effect of physical exercise on blood pressure, the lipid profile, lipoprotein(a) (Lp(a)), and low-density lipoprotein (LDL) modifications in untrained diabetics, 27 diabetic patients (14 type 1 and 13 type 2) under acceptable and stable glycemic control were studied before and after a supervised 3-month physical exercise program. Anthropometric parameters, insulin requirements, blood pressure, the lipid profile, Lp(a), LDL composition, size, and susceptibility to oxidation, and the proportion of electronegative LDL (LDL(-)) were measured. After 3 months of physical exercise, physical fitness improved (maximal O2 consumption [VO2max], 29.6 +/- 6.8 v 33.0 +/- 8.4 mL/kg/min, P < .01). The body mass index (BMI) did not change, but the waist circumference (83.2 +/- 11.8 to 81.4 +/- 11.2 cm, P < .05) decreased significantly. An increase in the subscapular to triceps skinfold ratio (0.91 +/- 0.37 v 1.12 +/- 0.47 cm, P < .01) and midarm muscle circumference ([MMC], 23.1 +/- 3.4 v 24.4 +/- 3.7 cm, P < .001) were observed after exercise. Insulin requirements (0.40 +/- 0.18 v 0.31 +/- 0.19 U/kg/d, P < .05) and diastolic blood pressure (80.2 +/- 10 v 73.8 +/- 5 mm Hg, P < .01) decreased in type 2 diabetic patients. High-density lipoprotein cholesterol (HDL-C) increased in type 1 patients (1.48 +/- 0.45 v1.66 +/- 0.6 mmol/L, P < .05), while LDL cholesterol (LDL-C) decreased in type 2 patients (3.6 +/- 1.0 v3.4 +/- 0.9 mmol/L, P < .01). Although Lp(a) levels did not vary in the whole group, a significant decrease was noted in patients with baseline Lp(a) above 300 mg/L (mean decrease, -13%). A relationship between baseline Lp(a) and the change in Lp(a) (r = -.718, P < .0001) was also observed. After the exercise program, 3 of 4 patients with LDL phenotype B changed to LDL phenotype A, and the proportion of LDL(-) tended to decrease (16.5% +/- 7.4% v 14.0% +/- 5.1%, P = .06). No changes were observed for LDL composition or susceptibility to oxidation. In addition to its known beneficial effects on the classic cardiovascular risk factors, regular physical exercise may reduce the risk of cardiovascular disease in diabetic patients by reducing Lp(a) levels in those with elevated Lp(a) and producing favorable qualitative LDL modifications.

Oxidized-LDL induce apoptosis in HUVEC but not in the endothelial cell line EA.hy 926

We studied the cytotoxic effect of copper-oxidized LDL in human primary human umbilical vein endothelial cells (HUVEC) and the immortalized EA.hy 926 cell line. Copper oxidized LDL (50-200 microg apoB/ml) induced concentration-dependent apoptotic cell death in HUVEC but did not induce apoptosis in EA.hy 926 cells. Only necrotic EA.hy 926 cells were evidenced at all copper oxidized LDL concentrations (25-200 microg apoB/ml), oxidation states (lightly, moderately and extensively copper-oxidized LDL) and incubation periods (4, 8 and 20 h). The different mechanisms of cell death induced by copper-oxidized LDL in EA.hy 926 cells and HUVEC may be related to various factors such as cytokines. In this study, we investigated whether interleukin-8 may be implicated in this process. The interleukin-8 production was increased in EA.hy 926 cells but not in HUVEC incubated with oxidized LDL. This increase in EA.hy 926 cells was associated with necrosis but not apoptosis. Nevertheless, the addition of interleukin-8 to HUVEC did not inhibit apoptosis induced by oxidized LDL. As the lower antioxidant capacity of EA.hy 926 cells results in higher sensitivity to oxidized LDL cytotoxicity (as we previously described), the redox status of cells may also control the form of endothelial cell death. In atherosclerotic lesions, the formation of apoptotic endothelial cells may result in part from the induction by oxidized LDL.

Effect of simvastatin treatment on the electronegative low-density lipoprotein present in patients with heterozygous familial hypercholesterolemia

Most described modifications of low-density lipoprotein (LDL) cholesterol share an increase in its negative electric charge; in fact, an electronegative form of LDL can be identified and isolated from plasma. Although the exact nature of the chemical modification of electronegative LDL is still controversial, its toxicity on endothelial cells has been demonstrated. Statins have protective effects against cardiovascular disease that are independent of their lipid-lowering action and which could be due, at least in part, to the prevention of LDL modification. We evaluated the effect of 6 months of simvastatin therapy (40 mg/day) on electronegative LDL proportion and LDL susceptibility to in vitro induced oxidation in 21 patients with heterozygous familial hypercholesterolemia (FH). Eleven normolipemic subjects were analyzed as a control group. Total cholesterol as well as LDL and very low density lipoprotein cholesterol, triglycerides, and apoprotein B decreased 30% after the first month of therapy, with no further decreases thereafter. LDL susceptibility to oxidation was similar in FH patients and controls and did not change throughout the treatment. Electronegative LDL proportion was 35.1 +/- 9.9% in FH patients and 9.1 +/- 2.4% in control subjects (p <0.0001) but, in contrast to total LDL cholesterol and the rest of lipid parameters, it decreased to 28.6 +/- 9.1% in the third month and to 21.2 +/- 7.7% in the sixth month of therapy. The decrease in these cytotoxic particles may be a relevant mechanism by which simvastatin protects against cardiovascular disease.

The various effects of fractionated oxidized low density lipoproteins on the growth of smooth muscle cells in culture

The effects of fractionated oxidized low density lipoproteins (oxidized LDL) on the growth of vascular smooth muscle cells (VSMC) and their relationship to the formation of lysophosphatidylcholine (lyso-PC) as well as the activation of protein kinase C (PKC) were studied. VSMC were isolated from porcine aorta by explant culture. LDL was isolated from porcine blood by sequential ultracentrifugation and oxidized LDL was obtained by incubating LDL with 5 microM CuSO(4) at 37 degrees C for various lengths of time. Our results showed that LDL oxidized for 12 h and eluted from fast protein liquid chromatography at 43 min inhibited the growth of VSMC, and that LDL oxidized for longer than 48 h and eluted at 48 min stimulated the growth of VSMC. The formation of lyso-PC in the oxidized LDL correlated well with its stimulatory effect, suggesting that lyso-PC is responsible for the mitogenic effect of oxidized LDL. This stimulatory effect of oxidized LDL was inhibited by staurosporine, a PKC inhibitor. Treatment with oxidized LDL increased the activity of membrane PKC, but it decreased that of cytosolic PKC, suggesting the translocation of PKC from cytosol to the membrane in the presence of oxidized LDL. These results suggested that the oxidized LDL-stimulated VSMC growth was mediated by the formation of lyso-PC and the activation of PKC.

Low-density lipoprotein (LDL) oxidizability before and after LDL apheresis

Oxidation of low-density lipoprotein (LDL) plays a major role in the development of atherosclerosis. Hypercholesterolemia has been associated with enhanced in vitro oxidation of LDL, and lipid-lowering therapy reduces LDL oxidizability. In the present study, we investigated whether LDL apheresis performed with different techniques affects in vitro diene formation (lag phase) and modification of apolipoprotein B-100 (apoB). Baseline and posttreatment diene formation was correlated with the baseline pattern of plasma total fatty acids. We then performed a computer-simulation study to test the hypothesis that LDL apheresis-induced changes in LDL oxidizability are related to changes in the mass ratio between freshly produced and older LDL. In 19 patients aged 49+/-7 years with heterozygous familial hypercholesterolemia (FH) regularly treated with either immunoadsorption, heparin-induced LDL precipitation (HELP), or dextran sulfate (DS) adsorption, we determined lipoprotein levels, the lag phase, apoB modification, and the fatty acid pattern in plasma samples drawn at the onset and termination of one LDL apheresis. LDL apheresis significantly decreased total cholesterol, high-density lipoprotein (HDL) cholesterol, LDL cholesterol, and triglycerides by 50.4%, 14.9%, 62.6%, and 33.6%, respectively. The lag phase increased by a significant mean of 9.8%; the charge of apoB was not altered. The lag phase before treatment positively correlated with the baseline concentration of plasma total palmitic, myristic, and oleic acid. The increase in the lag phase during treatment correlated with a high pretreatment concentration of lauric, linoleic, and docosahexanoic acid. The simulation study indicates that a temporary imbalance between two LDL compartments, one representing freshly secreted LDL and the other representing older LDL, could explain the observed increase in the lag phase after LDL apheresis. In conclusion, in patients with heterozygous FH, LDL apheresis performed with different techniques decreases the susceptibility of LDL to oxidation. This decrease may be related to a temporary mass imbalance between freshly produced and older LDL particles. Furthermore, the baseline fatty acid pattern influences pretreatment and posttreatment susceptibility to oxidation.

Separation of liposomes from plasma components using fast protein liquid chromatography

We describe an efficient method for separating liposomes (large unilamellar vesicles, 120-150 nm diameter) from plasma lipoproteins employing fast protein liquid chromatography (FPLC). This method resolves very low density lipoprotein (VLDL), low-density lipoprotein, high-density lipoprotein, and other plasma components. Selective detection of liposomes (large unilamellar vesicles, 120-150 nm diameter) was achieved using either radiolabeled or fluorescent lipid probes. The liposomes were found to coelute with the earliest FPLC-eluting lipoprotein fraction, VLDL. The remaining plasma lipoprotein and protein components eluted at later times and were resolved from liposomes and VLDL. In order to separate VLDL from liposomes, we selectively precipitated the VLDL fraction from plasma using tungstophosphoric acid and magnesium chloride, prior to separation by FPLC. Furthermore, we demonstrate that this technique can be used to separate liposomes from lipoproteins in plasma samples collected after intravenous administration of liposomes to mice. This technique has wide application in studies of liposome stability in blood and, in particular, for the characterization of liposomal drug carrier systems.

Charge heterogeneity of LDL in asymptomatic hypercholesterolemic men is related to lipid parameters and variations in the ApoB and CIII genes

This study was carried out to examine the relationship between the charge on low density lipoproteins (LDLs) and lipid and clinical parameters in 104 asymptomatic dyslipidemic men and to identify biochemical and genetic factors that could contribute to the charge variability of LDL. LDL charge heterogeneity was evaluated by relative electrophoretic mobility (REM) on preformed 0.5% agarose gels and by chromatographic quantification of a minor electronegative LDL subfraction designated LDL(-). The mean REM value for LDL was 0.147+/-0.016 and the mean LDL(-) subfraction percentage was 5.6+/-2.8%. Both were positively correlated with common atherosclerotic risk factors, especially total cholesterol [for REM, r=0.27, P<0.005; for LDL(-), r=0.28, P=0.008] and LDL cholesterol [for REM, r=0.27, P=0.007; for LDL(-), r=0.26, P=0.01)] levels, and REM was positively correlated with triglycerides (r=0.27, P<0.005) and negatively with apoAI levels (r=-0.30, P<0.002). The variations in LDL charge were not due to oxidation, as measured by the lag phase and binding to the LDL receptor. The results of the 2 methods used to measure LDL charge were significantly correlated and had some identical characteristics (eg, association with LDL apoCIII content and plasma triglyceride levels in borderline and IIb dyslipidemic subjects); these methods reflect different specific features of LDL charge. The percentage of LDL(-) was correlated positively with the LDL sialic acid content (P<0.0001), whereas the REM was related to at least 2 distinct chromosomal loci. Multiple logistic analysis showed that individuals carrying minor alleles of BsrDI (P<0.05), apoCIII/SacI (P<0.01), as well as the frequent allele of XbaI (P<0.05) at the apoB and CIII gene loci had high REMs. This result suggests that LDL charge heterogeneity, which is positively correlated with the atherogenic lipid profile, is influenced by both genetic and biochemical factors.

An immunoenzymatic procedure for human apo B-containing particles quantification using monoclonal antibodies

The present report describes a new monoclonal antibody-based enzyme immunoassay (ELISA) for the quantification of apolipoprotein (apo) B-containing particles in plasma. Native low-density lipoprotein (LDL) and a reference serum were utilized to prepare the standard curve. Three different antibodies to apo B-100--4A6E3, 6A10B10, and 2D9--all produced in our laboratory, were examined. The apparent affinity constants of the monoclonal antibodies (MAbs) 4A6E3, 6A10B10, and 2D9 were 2.9 x 10(9), 1.74 x 10(9), and 0.63 x 10(8), respectively. The standard curve was generated for an apo B-LDL range of 0.1 to 4.0 microg/ml. Evaluating the concentration of apo B-containing particles in plasma may allow for a more accurate assessment of the risk of coronary artery disease.

Sialic acid content of LDL in coronary artery disease: no evidence of desialylation in subjects with coronary stenosis and increased levels in subjects with extensive atherosclerosis and acute myocardial infarction: relation between desialylation and in vitro peroxidation

We recently showed that sialic acid content of LDL was not a marker of early cardiovascular disease (Arterioscler Thromb Vasc Biol. 1995;15:334-339). Here, we investigated this parameter in patients with advanced coronary artery disease (CAD). We first examined 100 patients having undergone coronary angiography. The distribution of LDL sialic acid values was very similar in subjects with no coronary stenosis (31.3+/-3.7 nmol/mg LDL protein, mean+/-SD) and those with > or = 75% stenosis in at least one main coronary artery or > or = 50% stenosis in at least two main coronary arteries (32.1+/-5.5 nmol/mg LDL protein). In contrast, LDL sialic acid content was significantly increased in patients with both coronary stenosis and peripheral arterial atherosclerotic lesions compared with those with either no lesion or only one or the other type of lesion. We then examined LDL sialic acid content in 20 patients with acute myocardial infarction. LDL sialic acid content was significantly higher (35.9+/-3.2 nmol/mg LDL protein) than that in the CAD(-) control group. These data suggest that LDL sialic acid content increases with the extension of atherosclerosis and its progression to acute complications. To explain the discordance with Orekhov and coworkers (Atherosclerosis. 1991;86:153-161), who showed that LDL sialic acid content in patients with advanced CAD was lower than that in healthy subjects, we studied the time courses of sialic acid, TBARS, and vitamin E levels in LDL dialyzed in different experimental conditions. A continuous decrease in both sialic acid and vitamin E levels and an increase in TBARS levels were observed in LDL samples containing less than 1 mmol/L EDTA, the intensity and rapidity of which varied with the EDTA concentration in the buffer. Our data support the idea that desialylation may result from in vitro peroxidation of LDL.

Factors affecting events during oxidation of low density lipoprotein: correlation of multiple parameters of oxidation

The present study shows that copper oxidation of LDL is a tightly-ordered process which can be finely controlled by appropriate selection of duration of oxidation and of concentrations of LDL and copper. Oxidation of LDL (0.1-2.0 mg LDL protein/ml) was carried out by copper catalysis (in the ratio of 2.5 microM Cu2+ to 0.1 mg LDL protein/ml) in phosphate-buffered saline, and was monitored by agarose gel electro-phoresis, gas chromatography (GC), anion exchange fast protein liquid chromatography (FPLC), fluorescence spectroscopy and dynamic light scattering. Analysis of the data showed strong cross correlations between many of the parameters of oxidation. Oxidation was more rapid for lower concentrations than for higher concentrations of LDL, despite the same ratio of copper to LDL being employed. Chemical kinetics analysis of the GC data suggested that 7beta-hydroxycholesterol formation occurred as a first order (or pseudo first order) consecutive reaction to the oxidation of linoleate. The first order rate constants for decomposition of linoleate and production of 7beta-hydroxycholesterol correlated closely with the theoretically-calculated times between collision of LDL particles. LDL particle diameter, measured by dynamic light scattering, increased by ca. 50% over 24 h oxidation, suggesting unfolding of apo B-100. Prolonged oxidation of LDL at low concentration suggested that the radical chain reaction was able to propagate, albeit slowly, on cholesterol after all the polyunsaturated fatty acid was consumed. For higher concentrations of LDL, prolonged oxidation resulted in partial aggregation. These findings are applicable to preparing oxidised LDL with different degrees of oxidation, under controlled conditions, for studying its biological properties.

Oxidative degradation of cholesteryl esters in low-density lipoproteins: analysis by liquid chromatography-light scattering and protection by a new synthetic antioxidant, S20478

Cholesteryl esters in the hydrophobic core of low-density lipoprotein (LDL) particles constitute a major molecular target during copper-mediated oxidation. To facilitate the rapid analysis and quantitation of the oxidative degradation of LDL cholesteryl esters, we describe a new approach based on light scattering detection following separation by HPLC. We have applied this approach to the evaluation of the protective capacity of a new synthetic antioxidant, S20478, during oxidation of LDL in the presence of copper ions. HPLC separation of cholesterol and the four major molecular species of cholesteryl esters (C16:0, C18:1, C18:2 and C20:4) of LDL was achieved in a single run of 20 min with high sensitivity (50 ng) and low background. Time course studies of the oxidative modification of LDL (ratio LDL protein: copper, 100 micrograms/mL: 1 microM) revealed that the content of unsaturated cholesteryl esters (C20:4 and C18:2) decreased (-30% and -15%, respectively) within 90 min of copper-mediated oxidation, while only minor degradation (up to 15%) of monounsaturated (C18:1) and saturated (C16:0) esters occurred. At 24 hours of oxidation, only traces (< 5%) of the C20:4 and C18:2 esters were detectable; whereas 52% of the C18:1 ester remained (P < 0.01). Of the saturated esters, only minor proportions (35% or less) underwent oxidative modification. In addition, some 81% of free cholesterol was conserved as the native sterol. The synthetic antioxidant, S20478 (50 microM) was capable of inhibiting the initiation and the propagation of copper-mediated LDL oxidation as determined by the time- and dose-dependent inhibition of the formation of conjugated dienes and thiobarbituric acid-reactive substances, as well as the conservation of the net electrical charge of LDL; indeed S20478 conserved cholesteryl esters in their native form up to 24 hours. However, after prolonged exposure to copper ions (48 hours), only 47% of the unsaturated esters remained (C18:2, P < 0.05). Nonetheless, S20478 (10 microM) was more efficient in inhibiting copper-mediated LDL oxidation as compared to probucol at the same concentration. These findings suggest that S20478 may be of potential interest in a new antioxidant approach to therapeutic stabilisation and regression of atherosclerotic plaques. Moreover, this method should prove useful in the assessment of the integrity of native LDL, and provides a new chemical marker of the degree of LDL oxidation.

Characterization of cholesterol oxidation products formed by oxidative modification of low density lipoprotein

Oxidative modification of LDL is evidenced by alterations in both the protein and lipid components of the particle. Progressive oxidation of the apoprotein is associated with loss of specific amino acids and a gradual increase in electronegativity. Electronegative LDL has been isolated from human plasma (LDL-) by several groups using liquid chromatographic techniques and appears to be oxidized based on increased lipid peroxide levels and cholesterol oxidation products (ChOx). Formation of LDL- also takes place following Cu(2+)-induced oxidation. Cu(2+)-induced oxidation caused a small fraction of the normal unoxidized LDL (n-LDL) to convert to LDL-during the oxidative lag phase while minimal increases in conjugated dienes were apparent. After the lag phase, there was a further increase in LDL-, a rapid accumulation of conjugated dienes, and another more electronegative particle was formed (LDL2-). By the end of the lag phase, approximately 30% and 12% of the total LDL converted to LDL- and LDL2-, respectively. Nearly 40% of the total ChOx formed was present by the end of the lag period, accompanied by small increases in conjugated dienes. The major products accumulating during this time were 7-ketocholesterol, cholesterol-beta-epoxide and 7-alpha-hydroxycholesterol. Accumulation of predominated during the subsequent propagation phase. At the end of propagation phase there was a six fold increase in conjugated dienes and total ChOx increased eight-fold. It appears that a subpopulation of LDL rapidly converts to LDL-, representing a mildly oxidized but oxidant sensitive LDL population. Oxidation of cholesterol accompanies these early events in LDL oxidation with formation of specific ChOx.

Oxidized low-density lipoprotein induces the production of interleukin-8 by endothelial cells

The concentration of interleukin-8 (IL-8) and RANTES was measured in culture supernatants of human EA.hy 926 endothelial cells incubated with oxidized low-density lipoproteins (LDL). Oxidized LDL induced a 3-fold increase in IL-8 production (p < 0.01), whereas RANTES was not detected. Native LDL did not stimulate IL-8 production. IL-8 production in oxidized-LDL-treated cells was mediated by reactive oxygen species, as it was partially inhibited by catalase and completely inhibited by glutathione peroxidase and N-acetylcysteine (p < 0.01).

Characterization and determinants of an electronegatively charged low-density lipoprotein in human plasma

There is evidence that oxidative modification of low density lipoprotein (LDL) is an important step in the atherosclerotic process. Electronegatively charged LDL has been found in the atherosclerotic lesions of rabbit and human and it is supposed to represent an early modification of LDL in the oxidative process in vivo. We describe a chromatographic method for determination of an LDL subfraction (LDL-) percentage from plasma LDL and the investigation of the lipid composition of native LDL and LDL- fractions separated by this method. We also studied the main determinants of the change in plasma LDL- level in a population sample of 45 men during a 6-months follow-up period. The LDL- fraction separated by anion-exchange chromatography had significantly higher triglyceride and free cholesterol and significantly lower phospholipid and total cholesterol contents, and lower alpha-tocopherol to triglyceride ratio than native LDL. In a 6-month follow up of 45 subjects, decreases in the ratio of blood glutathione disulphide to reduced glutathione, in serum LDL cholesterol and in serum n-3 polyunsaturated fatty acids, and an increase in plasma beta-carotene concentration had independent associations with a decrease of LDL-. The linear regression model including only these four variables was able to account for 47% of the variation of LDL-. Our findings suggest that the change in the plasma LDL- percentage measured by an anion-exchange liquid chromatography is predicted by changes in the major determinants of lipid peroxidation.

Oxidative modification of low-density lipoprotein by the human hepatoma cell line HepG2

The human hepatoblastoma cell line HepG2 is a liver model commonly used for lipid metabolism studies. Numerous cell types have been found to oxidize low-density lipoprotein (LDL) but, to our knowledge, the effects of HepG2 cells on LDL have not been investigated. We found that LDL is modified by HepG2 cells through a peroxidative mechanism, as judged by an increase in TBARS content (which was prevented in the presence of the antioxidants vitamin E, 2,6-di-tertbutyl-cresol and probucol), increased degradation by J774 macrophages, decreased internalization by MRC5 fibroblasts, and aggregation of apo B. Aspirin and allopurinol, which inhibit cyclooxygenase and xanthine-oxidase activities, respectively, had no effect on HepG2-induced LDL modification, and neither did catalase, which dismutates hydrogen peroxide; or mannitol, which scavenges hydroxyl radicals. In contrast, superoxide dismutase, a superoxide anion scavenger, and glutamate and threonine, which alter cellular cystine uptake, prevented LDL modifications, as did the removal of cysteine/cystine from the culture medium. Oxidation of LDL by HepG2 cells might thus involve superoxide anion production and/or thiol metabolism.

Practical approaches to low density lipoprotein oxidation: whys, wherefores and pitfalls

The purpose of this review is to bring together the different approaches for studying the oxidation of low density lipoproteins and try to identify some critical factors which will permit greater comparability between laboratories. These issues are discussed both in terms of the variety of exogenous mediators of oxidation applied (transition metal ions, haem proteins, azo initiators, peroxynitrite, cells etc.) and their raisons d'être, as well as the methodologies (formation of conjugated dienes, hydroperoxides, decomposition products of lipid peroxidation, altered surface charge, macrophage uptake) applicable to the different stages of the oxidation and the factors underlying their accurate execution and interpretation.

A cytotoxic electronegative LDL subfraction is present in human plasma

By using fast protein liquid chromatography, we isolated from human plasma a minor electronegative LDL subfraction designated LDL(-). After immunoaffinity chromatography against apolipoprotein (apo)(a) and apo A-I, LDL(-) represented 6.7 +/- 0.9% (mean +/- SD; n = 18) of total LDL. Compared with the major LDL subfraction, designated LDL(+), LDL(-) contained similar amounts of thiobarbituric acid-reactive substances, conjugated dienes, and vitamin E and had a similar lipid/protein ratio and mean density. Moreover, the apo B of LDL(-) was not aggregated and its LDL receptor-binding activity was slightly increased. These results were consistent with the nonoxidized nature of LDL(-). LDL(-) showed increased contents of sialic acid (38.1 +/- 5.2 versus 28.9 +/- 3.3 nmol/mg protein; n = 7; P < .01), apo C-III (1.43 +/- 0.21% versus 0.14 +/- 0.04%; n = 7; P < .01), and apo E (1.64 +/- 0.26% versus 0.10 +/- 0.05%; n = 7; P < .0005). Compared with LDL(+), LDL(-) displayed enhanced cytotoxic effects on cultured human umbilical vein endothelial cells, as shown by lactate dehydrogenase assay (P < .003; n = 6), neutral red uptake (P < .02; n = 6), and morphological studies. We also studied the relationship of LDL(-) to age and plasma lipid levels in 133 subjects. The percentage of contribution of LDL(-) to total plasma LDL correlated with age (P < .05), total cholesterol (P < .05), and LDL cholesterol (P < .003). In conclusion, this study shows that LDL(-), a circulating human plasma LDL, is an electronegative native LDL subfraction with cytotoxic effects on endothelial cells. This subfraction, which correlates positively with common atherosclerotic risk factors, might induce atherogenesis by actively contributing to alteration of the vascular endothelium.

Oxidative modifications of low-density lipoproteins (LDL) by the human endothelial cell line EA.hy 926

Modifications of LDL by the EA.hy 926 cell line were compared to those generated by human umbilical vein endothelial cells (HUVEC). Thiobarbituric acid reactive substances (TBARS) index values (TBARS sample/TBARS cell-free control ratio) were 2.64 +/- 0.18 (m +/- SE, n = 11) and 3.12 +/- 0.24 (n = 11), for HUVEC and EA.hy 926, respectively. The percentage of the most electronegative modified LDL fraction (fraction C), assessed by using an ion-exchange chromatographic method based on fast protein liquid chromatography (FPLC), represented 14 +/- 3% (n = 34) and 22 +/- 13% (n =10) of total modified LDL in HUVEC and EA.hy 926, respectively. LDL modified by both cell lines showed increased agarose electrophoretic mobility and apo B100 fragmentation on SDS-PAGE. None of the results were significantly different between the two cell lines. Superoxide anion production was 0.12 +/- 0.04 (n = 11) and 0.07 +/- 0.01 nmol/min/mg cell protein (n = 11) in HUVEC and EA.hy 926, respectively. Cell-specific effects on LDL were abrogated in cysteine-free medium. Moreover, cell-modified LDL were similarly degraded by J774 macrophage-like cells. We conclude that EA.hy 926 cells are a good model for investigating endothelial cell-induced modifications of LDL. Advantages include ready availability and less individual variability than with HUVEC.

Characteristics of ten charge-differing subfractions isolated from human native low-density lipoproteins (LDL). No evidence of peroxidative modifications

Native plasma low-density lipoproteins (LDL) were fractionated into ten subfractions with increasingly negative charges (LDL-1, the least electronegative, to LDL-10) using an anion-exchange column coupled to a fast protein-liquid chromatography system. Prior to fractionation, contaminating Lp(a) and apo A-I-containing lipoproteins were removed from LDL preparations by immunoaffinity chromatography. No significant difference in thiobarbituric acid-reactive substances, vitamin E or free aminogroup was found among subfractions, and no peptide with a higher molecular weight than apo B was observed on SDS-PAGE. We observed a gradual increase in cholesterol esters and a concomitant decrease in triglycerides from LDL-1 to LDL-7, and a reverse tendency from LDL-8 to LDL-10 (P < 0.01). Free cholesterol increased linearly from LDL-1 to LDL-10 (P < 0.01). LDL-1 to -3 had a homogeneous density profile, while other more electronegative subfractions showed a bimodal distribution with a second, minor peak of slightly higher density. A gradual increase in apolipoprotein C-III content related to LDL electronegativity was observed (P < 0.001). Apolipoprotein E content was also increased in the last two subfractions (P < 0.01). LDL subfractions displayed a similar binding fate on human fibroblasts, with the exception of the most electronegative subfractions [LDL-(9 + 10)], which bound more actively to apo B/E receptors (P < 0.05). This study shows that charge heterogeneity of native LDL is not related to lipid peroxidation or derivatization of free aminogroups of apolipoprotein B. In contrast, the enrichment of LDL in apolipoproteins other than apo B may explain, in part, the difference in their particle charge.

Increase of LDL susceptibility to oxidation occurring after intense, long duration aerobic exercise

The effect of heavy, long duration aerobic exercise on low density lipoprotein (LDL) susceptibility to oxidation and on distribution of LDL subfractions was studied. Six well-trained runners, previously fasted, ran continuously for 4 h. Controlled intake of liquid and food was permitted during exercise. Total plasma and LDL triglyceride increased significantly. LDL susceptibility to oxidation, measured as conjugated dienes formation, was modified significantly (P < or = 0.05) after running (14% reduction in lag phase time, and 8% increase in maximal curve slope). The percentage of electronegative LDL form (named LDLB) also increased significantly (P < or = 0.05) after exercise both basally (from 7.3% to 11%) and after 2h of induced oxidation (from 40.6% to 52.3%). Neither LDL susceptibility to oxidation nor increase of LDLB was statistically associated with food consumed during the race or modifications of triglycerides suggesting that this effect was due to exercise rather than food-related. The pattern of LDL subfractions was type A in all athletes before and after running. The oxidative LDL changes, seen in exercise conditions similar to those of hard training or competition, demonstrated an unfavourable effect of very intense exercise on lipoprotein metabolism.

Chromatographic techniques for the isolation and purification of lipoproteins

Various modes of chromatography are available for lipoprotein separation. Gel permeation and affinity chromatography are used for preparative purposes and to separate lipoproteins according to size and apolipoprotein content, respectively. Development of rigid supports for gel permeation has led to large improvements in speed and resolution. Reversed-phase high-performance liquid chromatography (HPLC) of apolipoproteins offers the best performance in terms of speed and resolution of structural variants. Due to its high speed and superior resolving power, the recently developed technique of capillary electrophoresis should emerge as an important method for lipoprotein analysis.

A method to screen for the antioxidant effect of compounds on low-density lipoprotein (LDL): illustration with flavonoids

We used a recently described anion-exchange chromatographic method (Vedie et al. J Lipid Res 1991;32:1359) to study the protective effect of potential inhibitors of low-density lipoprotein (LDL) oxidation mediated by cupric ion. By way of an example, we studied eight flavonoids (flavone, 3-hydroxyflavone, chrysin, galangin, fisetin, morin, quercetin, and myricetin) as well as three non-flavonoid antioxidants, butylated hydroxytoluene (BHT), probucol, and vitamin C, as reference compounds. Each compound was tested at various concentrations (1-100 microM). For flavonoid concentrations of 10 microM, an index was calculated as the (LDL control-flavonoid)/(LDL control-probucol) ratio, in which each term is expressed as the percentage of the most electronegative LDL fraction (fraction E). If the index is positive, the flavonoid inhibits LDL oxidation. A value > 1 (3-hydroxyflavone and galangin) means greater activity than probucol, whereas a value < 1 means lower activity (fisetin). If the index is around 0 (flavone and chrysin), the flavonoid is inactive. Finally, a negative value reflects possible prooxidant activity (morin, quercetin, and myricetin). Our results show that this chromatographic method can be applied to screening new pharmacological agents for activity against LDL oxidation.

Probucol as an antioxidant and antiatherogenic drug

Recently, interest has increased in the hypothesis that low-density lipoprotein (LDL) modified by oxidation may lead to the initiation and to the development of atherosclerosis. In vitro studies of cellular interactions with LDL have revealed that various cells, including endothelial cells and smooth muscle cells, can oxidize LDL. The biochemical changes in LDL may further enhance its atherogenic potential. In addition to these in vitro studies, there is in vivo evidence for oxidized LDL in atherosclerotic lesions and for circulating antibodies against oxidized LDL. Probucol, 4,4'-(isopropylidenedithio)bis(2,6-di-tert-butylphenol), is a widely used cholesterol-lowering drug. Recently, there has been accumulating evidence for other mechanisms of probucol's antiatherogenic effects apart from cholesterol-lowering action. Attention has especially focused on probucol's antioxidant action in the mechanism of antiatherogenesis. In the present article, we will summarize the antiatherogenic and antioxidant actions of probucol.

The role of lipid peroxidation and antioxidants in oxidative modification of LDL

The purpose of this study is to provide a comprehensive survey on the compositional properties of LDL (e.g., lipid classes, fatty acids, antioxidants) relevant for its susceptibility to oxidation, on the mechanism and kinetics of LDL oxidation, and on the chemical and physico-chemical properties of LDL oxidized by exposure to copper ions. Studies on the occurrence of oxidized LDL in plasma, arteries, and plaques of humans and experimental animals are discussed with particular focus on the use of poly- and monoclonal antibodies for immunochemical demonstration of apolipoprotein B modifications characteristic for lipid peroxidation. Apart from uptake of oxidized LDL by macrophages, studies describing biological effects of heavily or minimally oxidized LDL are only briefly addressed, since several reviews dealing with this subject were recently published. This article is concluded with a section on the role of natural and synthetic antioxidants in protecting LDL against oxidation, as well as some previously unpublished material from our laboratories.