Evidence for a dominant role of lipoxygenase(s) in the oxidation of LDL by mouse peritoneal macrophages

Protection from oxidized LDL-induced leukocyte adhesion to microvascular and macrovascular endothelium in vivo by vitamin C but not by vitamin E

BACKGROUND

The ability of oxidized LDL (oxLDL) to stimulate leukocyte-endothelium interaction is considered to be an important aspect of its proatherogenic action. Using intravital fluorescence microscopy in the dorsal skinfold chamber model in hamsters, we have previously shown that systemic administration of oxLDL stimulates leukocyte adhesion to microvascular endothelium through a mechanism that involves the generation and action of reactive oxygen species (ROS).

METHODS AND RESULTS

Through the combined use of scanning electron microscopy and intravital microscopy in the same animal model, we demonstrate that oxLDL-induced leukocyte adhesion is not confined to the microcirculation but can also be observed on aortic endothelium. OxLDL-induced leukocyte adhesion to both microvascular and macrovascular endothelium was almost entirely prevented by pretreatment of the hamsters with dietary or intravenous vitamin C, which has the capacity to scavenge and neutralize ROS (arterioles: 20.5 +/- 16.4 cells/mm2 [diet] and 16.3 +/- 23.8 cells/mm2 [IV] versus 74.2 +/- 47.5 cells/mm2 [control, P < .01]; aorta: 1.0 +/- 0.4 cells/mm2 [diet] and 1.1 +/- 0.5 cells/mm2 [IV] versus 14.7 +/- 6.0 cells/mm2 [control, P < .01], 15 minutes after oxLDL, n = 7 animals per group). Vitamin C pretreatment also completely prevented oxLDL-induced leukocyte-platelet aggregate formation in the blood-stream but did not affect leukocyte rolling along the microvascular endothelium. No inhibitory effect on any of the studied parameters was observed as a result of pretreatment of the animals with the lipid-soluble antioxidants vitamin E and probucol.

CONCLUSIONS

The protective effects of vitamin C on oxLDL-induced leukocyte adhesion and aggregate formation were seen at vitamin C plasma levels that can easily be reached in humans by diet or supplementation, suggesting that this could be one of the mechanisms by which vitamin C contributes to the well-documented protraction of atherogenesis as observed in large epidemiological surveys.

Effect of verapamil and nifedipine on cholesteryl ester metabolism and low-density lipoprotein oxidation in macrophages

Using mouse macrophage cultures, the effects of verapamil and nifedipine on cholesteryl ester and low-density lipoprotein (LDL) metabolism were studied with special reference to the following parameters: (a) incorporation of [14C]oleate into cholesteryl esters (ChE), (b) contents of total and free cholesterol (FCh), (c) liberation of [14C]oleate from ChE and incorporation of [3H]FCh into ChE, (d) excretion of [3H]Ch from the cells, and (e) LDL oxidation. Verapamil and nifedipine (10-100 microM) were shown to decrease in a dose-dependent manner the incorporation of [14C]oleate into ChE and to increase the concentration of FCh but had no appreciable effect on the concentration of total cholesterol in macrophages cultured in the presence of acetylated LDL. The drugs stimulated the liberation of [14C]oleate from cellular ChE. The pharmacological concentrations (25-75 microM) of verapamil and nifedipine increased the excretion of [3H]FCh from ChE of macrophages in the presence of serum and high-density lipoproteins. The same concentrations of the drugs inhibited both LDL-derived malonyldialdehyde-like products and nitroblue tetrazolium dye reduction in a dose-dependent fashion. The results obtained suggest that verapamil and nifedipine exert their macrophage-mediated antiatherosclerotic effect via reduction of LDL oxidative modification, reduction of intracellular ChE synthesis, stimulation of ChE hydrolysis and cholesterol excretion from the cells.

Pro-oxidant effects of 7-hydroperoxycholest-5-en-3 beta-ol on the copper-initiated oxidation of low density lipoprotein

In low density lipoproteins (LDL) supplemented with aged cholesterol and oxidized in the presence of Cu2+, an increase of the lipid oxidation parameters was observed compared with pure cholesterol-enriched LDL. A compound, identified as 7-hydroperoxycholesterol (7HPC), isolated from aged cholesterol and added to LDL, reproduced the above effects. The results indicate that the pro-oxidant effect of 7HPC is dependent on the hydroperoxy group since the corresponding alcohol derivative, 7 alpha-hydroxycholesterol, had no such effect. These data suggest that among the LDL-associated lipid peroxides, cholesterol peroxides may have important implications in the susceptibility of this lipoprotein to oxidation.

Cardiovascular disease and nutrient antioxidants: role of low-density lipoprotein oxidation

Increasing evidence indicates that oxidative modification of low-density lipoprotein (LDL) is causally related to atherosclerosis. Oxidatively modified LDL (oxLDL), in contrast to native LDL, is taken up avidly by macrophages, leading to formation of lipid-laden foam cells. Foam cells are pathognomonic of the atherosclerotic fatty streak. Modified LDL may also promote atherosclerosis by many other mechanisms, such as recruitment and retention of monocyte-macrophages, T-lymphocytes, and smooth muscle cells in the arterial intima, and cytotoxicity toward endothelial cells and macrophage-derived foam cells. The "oxidation hypothesis of atherosclerosis" is supported by a number of in vivo findings, such as the presence of oxLDL in atherosclerotic lesions, and increased titers of autoantibodies against modified LDL in patients with atherosclerosis. As a corollary of the oxidation hypothesis of atherosclerosis, antioxidants that can inhibit LDL oxidation may act as antiatherogens. This conception is supported by animal studies showing that antioxidants such as probucol, butylated hydroxytoluene, and alpha-tocopherol can slow the progression of atherosclerosis. Epidemiological and clinical data indicate a protective role of dietary antioxidants against cardiovascular disease, including vitamin E, beta-carotene, and vitamin C. Likewise, basic research studies on LDL oxidation have demonstrated a protective role for antioxidants, present either in the aqueous environment of LDL or associated with the lipoprotein itself. More studies are needed to establish the effectiveness and determine the required doses of specific antioxidants to prevent and possibly treat cardiovascular disease.

The effects of free radical scavengers on the oxidation of low-density lipoproteins by macrophages

Oxidised LDL has been implicated in the pathogenesis of atherosclerosis. Macrophages can oxidatively modify low-density lipoprotein (LDL) in vitro. The mechanisms of this oxidation process are presently unclear. In this study, we have investigated the effects of compounds and enzymes widely used to quench or scavenge active oxygen species to try to identify the oxidative species involved in this process. The data obtained suggest that hydrogen peroxide may possibly play a role in LDL oxidation by macrophages, whereas singlet oxygen and hydroxyl radicals may not. The role of superoxide anions was uncertain because copper-zinc superoxide dismutase (Cu/Zn-SOD) and manganese SOD (Mn-SOD), widely used to determine superoxide-dependency in other systems may be unsuitable in this particular system. Cu/Zn-SOD at high concentrations displayed a variability in its effects, sometimes augmenting LDL oxidation and sometimes inhibiting it. In the experiments in which Cu/Zn-SOD augmented LDL oxidation, heat inactivation of the enzyme decreased the augmentation; in the experiments in which Cu/Zn-SOD inhibited LDL oxidation, it retained its inhibitory effect after heat inactivation. Mn-SOD always inhibited modification even after heat inactivation. We have therefore concluded that superoxide involvement in LDL oxidation by macrophages is still uncertain and the uncertainty will remain until a suitable probe is found.

Analysis of plasma cholesterol oxidation products using gas- and high-performance liquid chromatography/mass spectrometry

The application of gas chromatography and high-pressure liquid chromatography/mass spectrometry techniques for analysis of plasma cholesterol oxidation products is described. Cholesterol oxides that are widely identified in biological samples were subjected to gas (GC) and high-pressure liquid chromatographic (HPLC) separations, and their detection and characterization by mass spectrometry (MS) were compared. Analysis of cholesterol oxides from plasma samples revealed distinct advantages for each method according to the specific cholesterol oxide in question. Whereas HPLC/MS analysis of cholesterol oxides provided less resolution and lower sensitivity as compared to GC/MS, a distinct advantage was evident for direct measurements of cholesterol-7-hydroperoxides and 7-ketocholesterol. These two cholesterol oxides are particularly sensitive to storage in solvents, derivatization procedures, and analytical conditions used for GC analysis, which are minimized or avoided using the HPLC/MS conditions described. Analysis of human and rabbit plasma samples identified cholest-5-ene-3 beta, 7 beta-diol (7 beta-hydroxycholesterol); 5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol (cholesterol-5 alpha, 6 alpha-epoxide); 5 alpha-cholestane-3 beta, 5,6 beta-triol (cholestanetriol); 3 beta-hydroxycholest-5-ene-7-one (7-ketocholesterol); and 5,6 beta-epoxy-5 beta-cholestan-3 beta-ol (cholesterol-5 beta,6 beta-epoxide) as commonly occurring components (trivial names indicated in parentheses). The latter two compounds were dramatically increased in hypercholesterolemic samples and were found in approximately equal amounts in the free cholesterol and cholesteryl ester fractions. Although most of the plasma cholesterol oxides are found in the dietary cholesterol, others are not, particularly cholesterol-5 beta,6 beta-epoxide, suggesting that at least some of these compounds are formed by in vivo oxidation of cholesterol. Despite the readily measurable levels of the above cholesterol oxides, as well as other less prominent oxides, there was no evidence of cholesterol-7-hydroperoxides associated with plasma free cholesterol. Although several of the plasma cholesterol oxides may derive from cholesterol-7-hydroperoxides, it appears that the latter are either unstable and decompose in plasma, are metabolized to other cholesterol oxidation products, or break down during their isolation.

Iron induces lipid peroxidation in cultured macrophages, increases their ability to oxidatively modify LDL, and affects their secretory properties

The present study demonstrates for the first time that iron ions can induce lipid peroxidation in intact macrophages without causing cell death. Macrophage lipid peroxidation increases cell-mediated oxidation of LDL, enhances the release of interleukin 1 and inhibits the release of apolipoprotein E from the macrophages. When cultured macrophages were exposed to ferrous ions (50 microM FeSO4) for 4 h at 37 degrees C, cellular lipid peroxidation (measured by analyses of malondialdehyde (MDA), conjugated dienes (CD), and lipid peroxides (PD)) increased 2-4-fold in comparison with non-treated cells. This process was iron-dose dependent, reached its maximum after 4 h of incubation, and was accompanied by 68% and 53% reductions in the content of the cellular linoleic (18:2), and arachidonic acid (20:4), respectively, and by 29% and 36% reductions of cellular vitamin E and vitamin A, respectively. Cell viability (measured by trypan blue exclusion, by [3H]thymidine incorporation into DNA, by analysis of the release of lactate dehydrogenase (LDH) or [3H]adenine), and cell morphology (studied by scanning electron microscopy) were not significantly affected by the iron-induced oxidative stress. Manitol and dimethylthiourea (DMTU), but not catalase or superoxide dismutase (SOD), significantly inhibited iron-induced cellular lipid peroxide formation, suggesting that hydroxyl radical, but not superoxides or hydrogen peroxides, mediated the iron-induced cellular lipid peroxidation. Incubation of LDL (0.2 mg of protein/ml) with oxidized macrophages resulted in LDL lipids peroxidation, as evidenced by an 8-fold increase in the LDL associated MDA in comparison with LDL that was incubated under similar conditions with non-oxidized macrophages. Furthermore, oxidation of LDL by oxidized macrophages in the presence of copper ions (10 microM CuSO4) was 2-fold higher in comparison with oxidation of LDL by non-oxidized macrophages. The release of apolipoprotein E from oxidized macrophages decreased by 50%, whereas macrophage release of beta-glucuronidase and of interleukin-1 beta increased by 83% and by a factor of 6, respectively. This study demonstrates for the first time that iron ions induce oxidation of the cellular polyunsaturated fatty acids in intact macrophages and that this cellular lipid peroxidation can subsequently induce LDL oxidation.

Effects of supplementing with vitamin E on the uptake of low density lipoprotein and the stimulation of cholesteryl ester formation in macrophages

Vitamin E supplementation has been reported to protect low density lipoprotein (LDL) from copper-induced oxidation and macrophage-mediated oxidation. We investigated the effect of in vitro vitamin E enrichment of LDL on the accumulation of [3H]cholesteryl ester (CE)-LDL and stimulation of cholesteryl ester formation in J774 macrophages. Vitamin E supplementation prolonged lag time (2.9-fold) before the initiation of copper-induced LDL oxidation. LDL, preincubated with 5 microM copper or with macrophages in Ham's F10 medium, accumulated in macrophages much more than did native LDL. However, following vitamin E enrichment, LDL accumulation was significantly reduced following oxidative stress. Vitamin E-enriched LDL also reduced the stimulation of cholesteryl ester formation in macrophages. Moreover, vitamin E enrichment of macrophages reduced the ability of the cells to oxidize LDL. The present results indicate that vitamin E supplementation protects LDL against copper-induced and macrophage-mediated oxidation, inhibits oxidation-dependent accumulation of LDL in macrophages, and prevents stimulation of cholesteryl ester formation in macrophages. Additionally we have provided evidence that intra-cellular enrichment with vitamin E prevents oxidative modification of LDL by macrophages.

Oxidative modification of low-density lipoprotein by human polymorphonuclear leucocytes to a form recognised by the lipoprotein scavenger pathway

The oxidative modification of low-density lipoprotein (LDL) results in the formation of cytotoxic and chemotactic lipids which are thought to be of importance in the development of atherosclerotic lesions. In the present study we show that polymorphonuclear leucocytes (PMNs) can modify LDL to a form which is rapidly incorporated by macrophages via a scavenger receptor pathway. Incubation of 125I-labelled LDL with PMNs in Ham's F-10 medium resulted in oxidation as shown by the appearance of thiobarbituric acid-reactive substances, increased electrophoretic mobility of the LDL and increased degradation of the LDL by mouse peritoneal macrophages. The presence of the anti-oxidant butylated hydroxytoluene or the metal ion chelator, EDTA inhibited the PMN-mediated modification. The degradation of 125I-labelled PMN modified LDL by macrophages was competitively inhibited by unlabelled copper-oxidised LDL but not by native LDL, indicating that the degradation was mediated by the scavenger receptor. The oxidative modification of LDL by PMNs could be of pathophysiological importance in inflammation and in the accelerated atherosclerosis seen following cardiac reperfusion injury.

Oxidation of low density lipoprotein by mesangial cells may promote glomerular injury

Low density lipoprotein (LDL) deposition and local oxidation play a key role in the pathogenesis of atherosclerosis and may likewise contribute to glomerular injury. These studies were designed to determine whether cultured human mesangial cells oxidize homologous LDL and to compare the effects of unmodified and oxidized lipoprotein on cell proliferation, viability and eicosanoid production. Cell-mediated lipoprotein oxidation was demonstrated and could be suppressed by oxygen free radical scavengers and inhibitors of arachidonic acid metabolism. When incubated with cells, oxidized LDL (Ox-LDL) at concentrations up to and including 100 micrograms/ml reduced 3H-thymidine incorporation without causing cytotoxicity as assessed by lactate dehydrogenase release. Under the same conditions there was a concentration-dependent increase in the synthesis of prostaglandins E2,6-keto-PGF1 alpha and thromboxane B2. In contrast, unmodified LDL enhanced DNA synthesis at concentrations less than 40 micrograms/ml and had little effect on eicosanoid production. These results demonstrate that exogenous oxidized LDL inhibits mesangial cell proliferation and increases eicosanoid synthesis. Unmodified lipoprotein can be directly oxidized by these cells through mechanisms that involve generation of oxygen free radicals.

Stimulation of low-density lipoprotein oxidation by insulin and insulin like growth factor I

Hyperinsulinemia has been implicated as an independent risk factor for atherosclerosis. We measured the effect of insulin and related hormones on the oxidation of low density lipoproteins (LDL) and superoxide anion production by peripheral blood mononuclear cells (MC). LDL oxidation was measured by the production of thiobarbituric acid reactive substances (TBARS). Insulin and IGF-I at 10(-7) M caused a 33% and 48% increase in TBARS production, respectively. At 10(-6) M the corresponding values were 63% and 67%. Proinsulin and IGF-II at 10(-6) M had no effect. Glucose caused a concentration dependent (up to 10 mM) stimulation of LDL oxidation reaching 85% and 77% at insulin concentrations of 10(-7) M and 10(-6) M, respectively. The stimulatory effect of insulin was confirmed by measurements of other indices of LDL oxidation, i.e. absorbance at 234 nm, trinitrobenzene sulfonic acid reactivity and electrophoretic mobility. Insulin-stimulated LDL oxidation was inhibited by superoxide dismutase (SOD), but insulin had no effect on MC superoxide production. MC were isolated from five subjects before and after a 5 h hyperinsulinemic, euglycemic clamp. Insulin infusion had no effect on TBARS or superoxide production by MC. Our in vitro experiments suggest that high levels of insulin and IGF-I stimulate MC-mediated oxidation of LDL, an effect that is potentially atherogenic.

Oxidation, lipoxygenase, and atherogenesis

Mounting evidence suggests that oxidative processes contribute to the pathogenesis of atherosclerosis and that antioxidants may represent a strategy to complement the lowering of lipids in the therapy of this disease. Although multiple molecular events have been identified in vitro and although it is tempting to ascribe multiple atherogenic properties to oxidized LDL, our understanding of this process remains incomplete. Further research is warranted in several areas. First, it will be important to selectively inhibit different aspects of the process to determine the relative contribution of various biological targets. In this regard pharmacological inhibition of 15-lipoxygenase in vivo in relevant animal models is required to address the question of the contribution of this enzyme to significant oxidative events. The lack of specific inhibitors has made this task more difficult. It will also be important to define the biologically active moiety of oxidized LDL to begin to determine the mechanisms through which it exerts its atherogenic effects. It is likely that alternate protein targets can be identified both downstream and upstream of the oxidative process. Research is only now beginning to elucidate the inflammatory mechanisms that account for the cellular response. Further research into adhesion events, cytokine profiles, and downstream effector molecules of the oxidative process are likely to identify alternate targets for therapeutic intervention.

Fatty acid composition of low-density lipoprotein influences its susceptibility to autoxidation

Low-density lipoprotein (LDL) oxidation was studied using copper or the water-soluble initiator azobis(2-amidinopropane) dihydrochloride (ABAP) to catalyze the reaction. These studies were carried out with purified, native LDLs that had a well-defined composition and which contained different concentrations of polyunsaturated fatty acids (PUFA) and alpha-tocopherol. The LDL was obtained from nonhuman primates fed diets enriched in cholesterol and one of four types of fatty acids: saturated (Sat), monounsaturated (Mono), omega-6 (omega-6FA), or omega-3 (omega-3FA) fatty acids. The PUFA concentration of the LDLs depended upon the diet and had the following order: omega-6FA > Sat approximately Mono approximately omega-3FA. Linoleic acid was the predominant PUFA in all of the LDLs. The rates of oxidation were linearly dependent upon the concentration of PUFA. When ABAP was used to initiate oxidation the lag time was linearly related to the amount of alpha-tocopherol. However, with copper catalysis no linear correlation was evident. If the different enrichments were analyzed independently, it was found that copper-catalyzed oxidation of LDLs enriched with omega-6 and omega-3 fatty acids showed a linear correlation between the lag time and the amount of alpha-tocopherol but that LDLs enriched with Sat or Mono fatty acids did not show a correlation. These results demonstrate that the rate of oxidation is dependent upon PUFA concentration and that the ability of alpha-tocopherol to inhibit oxidation depends upon the lipid environment and the mode of initiation.

The effect of inhibitors of free radical generating-enzymes on low-density lipoprotein oxidation by macrophages

Oxidised low-density lipoprotein (LDL) produced by the action of arterial cells, including macrophages, has been implicated in atherosclerosis. We have investigated the effect of inhibitors of various cellular free-radical generating enzymes on macrophage-mediated LDL oxidation. Xanthine oxidase and nitric oxide synthase are not responsible for LDL modification by resident mouse peritoneal macrophages. Eicosatetraynoic acid, a lipoxygenase inhibitor, produced a dose-dependent irreversible inhibition of macrophage modification of LDL, but at concentrations rather close to those toxic to the cells. Diphenyl and diphenylene iodonium, NADPH oxidase and mitochondrial electron transport inhibitors, inhibited macrophage oxidation of LDL, at concentrations that were not obviously toxic. This suggests that NADPH oxidase, or some other flavin nucleotide-dependent process, may be involved in LDL oxidation by macrophages. Wortmannin and thiopropionic acid dilauryl ester did not inhibit LDL oxidation, suggesting that inhibition of NADPH oxidase may not be the means by which the iodonium compounds inhibit LDL oxidation. Macrophages from C3H/HeJ mice, which lack receptors for lipopolysaccharide, modified LDL normally, suggesting that the inadvertent priming of resident macrophages by traces of lipopolysaccharide bound to LDL was not involved in LDL oxidation.

Oxidative modification of low density lipoproteins by human polymorphonuclear leukocytes

Oxidatively modified low density lipoproteins are thought to play an important role in the generation of macrophage-derived foam cells in early atherosclerotic lesions. Cultured endothelial cells, monocytes, macrophages and smooth muscle cells can modify low density lipoproteins, either by a free radical mechanism or by the action of lipoxygenases. Previous studies demonstrated that activated human polymorphonuclear leukocytes can oxidize low density lipoprotein lipids. Stimulation of the cells with phorbol 12-myristate 13-acetate resulted in an increase both in superoxide anion production and in low density lipoprotein oxidation. The present results show that the oxidative modification of low density lipoproteins by human polymorphonuclear leukocytes can be inhibited by superoxide dismutase but not by the lipoxygenase inhibitor, (5,8,11,14)-eicosatetraynoic acid. The low density lipoproteins oxidized by polymorphonuclear leukocytes were recognized by the scavenger receptor of macrophages (P 388 cell line). It is proposed that the superoxide anion is an important factor in the oxidative modification of low density lipoproteins induced by polymorphonuclear leukocytes, and that under conditions of increased oxidative metabolism in vivo, polymorphonuclear leukocytes can contribute to foam cell formation by a scavenger receptor-dependent process at lesion sites.

Interleukin-1 increases 15-hydroxyeicosatetraenoic acid formation in cultured human endothelial cells

Interleukin 1 (IL-1) induces prostanoid biosynthesis in endothelial cells by promoting cyclooxygenase expression, but little is known about its activity on the biosynthesis of hydroxyeicosatetraenoic acids (HETEs). We studied the effect of human recombinant IL-1 beta on the conversion of arachidonic acid (AA) to 15-HETE, a powerful inhibitor of the biosynthesis of proinflammatory eicosanoids. Cultured human umbilical vein endothelial cells were incubated with or without IL-1 beta prior to the addition of labeled AA. The eicosanoids produced were analyzed by RP-HPLC. Untreated cells produced little amounts of 15-HETE (6 +/- 3 pmol/10(6) cells), but IL-1 beta treated cells increased 15-HETE formation in a dose-dependent manner (4-5-fold at 10 U/ml IL-1). The production of HETEs by IL-1 beta was dependent on protein synthesis. Aspirin inhibited prostanoids, HHT and 11-HETE dose dependently, whereas it was unable to totally inhibit 15-HETE in IL-1 beta-treated cells (50-60%). Nordihydroguaiaretic acid, a general lipoxygenase inhibitor, preferably inhibited 15-HETE formation but also reduced the synthesis of the other eicosanoids in a dose-dependent manner. Indomethacin and ETYA completely suppressed prostanoids, 11-HETE and 15-HETE formation in resting and IL-1 beta-activated cells. Using specific 15-lipoxygenase oligonucleotides and the reverse transcriptase polymerase chain reaction technique, we were unable to evidence detectable 15-lipoxygenase mRNA both in resting and IL-1-activated endothelial cells. Overall, these results provide evidence that in human endothelial cells IL-1 beta increases 15-HETE production. Data strongly suggest that this effect is mediated by cyclooxygenase rather than 15-lipoxygenase activity or expression.

Autoinhibition of murine macrophage-mediated oxidation of low-density lipoprotein by nitric oxide synthesis

Murine peritoneal macrophages treated with gamma-interferon and lipopolysaccharide (activated cells) oxidized low-density lipoprotein (LDL) less readily than unstimulated cells. Activated cells expressed the enzyme nitric oxide synthase, whose activity was measured by the accumulation of nitrite in the culture supernatant. Treatment of activated macrophages with the arginine analogue NG-monomethyl-arginine (NMMA) inhibited nitric oxide synthesis and restored the ability of the cells to oxidize LDL. This treatment had no effect on the ability of unstimulated cells to oxidize LDL. Similarly, LDL oxidation by activated macrophages in arginine-free Ham's F-10 medium was identical to that of unstimulated cells, whereas restoration of arginine to the medium was associated with nitrite secretion and a decline in LDL oxidation by activated cells only. An inverse relationship between nitric oxide synthesis and LDL oxidation was also demonstrated in the presence of diphenylene iodonium, a flavin analogue which is a potent inhibitor of nitric oxide synthase. Thus nitric oxide synthesis appears to mediate the suppression of LDL oxidation which is associated with the activation of mouse macrophages by gamma-interferon and lipopolysaccharide.

A putative role of dietary omega-3 polyunsaturated fatty acids in oxidative modification of low density lipoprotein

Oxidative modification of low density lipoprotein is a cell-mediated process that is believed to increase the atherogenicity of the particle. There is a growing interest in measures that could inhibit this process because of their potential impact on the risk of atherosclerotic disease. It is suggested that diets rich in polyunsaturated fatty acids of the omega-3 series inhibit cell-induced oxidative modification of low density lipoprotein (LDL), thereby contributing to the prevention of atherosclerosis.

Oxygenation of lipoproteins by mammalian lipoxygenases

Oxidative modification converts low-density lipoprotein (LDL) into its atherogenic form and appears to be a necessary precondition for LDL uptake by macrophages during foam cell formation. Cellular lipoxygenases have been implicated in this process. We studied the interaction of purified mammalian lipoxygenases with human LDL in vitro and found that the arachidonate 15-lipoxygenases of rabbit and man are capable of oxygenating lipoproteins as indicated by oxygen uptake and by the formation of thiobarbituric-acid-reactive substances. Furthermore, oxygenated polyenoic fatty acids, such as 13-hydro(pero)xy-9Z,11E-octadecadienoic acid and 15-hydro(pero)xy-5,8,11,13(Z,Z,Z,E)-eicosatetraenoic acid were detected in the lipid compartment of various lipoproteins classes after lipoxygenase treatment. More than 90% of the oxygenated polyenoic fatty acids were found in the ester-lipid fraction, particularly in the cholesterol esters, whereas only small amounts of free hydro(pero)xy polyenoic fatty acids were detected. Lipoxygenase-catalyzed oxygenation of LDL is not restricted to the lipid compartment but also leads to a cooxidative modification of the apoproteins as indicated by changes in the electrophoretic mobility and by the formation of carbonyl derivatives of amino acid side chains. The possible biological significance of lipoxygenase-induced oxidative modification of lipoproteins in the pathogenesis of atherosclerosis is discussed.

New approaches to atherosclerosis: an overview

The mechanisms of action and selected agents for a variety of approaches to the treatment of atherosclerosis have been reviewed. In Table I, each approach is listed according to its primary physiological effect. This is a simplification, of course, and some agents, such as ACAT inhibitors, may have primary effects in all of these categories. As one goes from left to right, the benefit of each physiological effect becomes more speculative. There is no question of the benefit of LDL reduction, but less evidence exists for the clinical benefits of HDL elevation. With regard to direct anti-atherosclerotic effects, most approaches have yet to gather clinical data of any type. Perhaps as a result, the degree of medicinal chemistry effort in each area to date declines as one goes from left to right. This situation is changing rapidly, however. As evidence supporting the HDL hypothesis accumulates and knowledge of how to elevate HDL levels grows, very exciting opportunities for medical advances present themselves. Likewise, the knowledge base for nonlipid intervention is growing and very rapid advances are being achieved with the plaque-imaging techniques needed for evaluating such agents in man. Such results can only lead to greater opportunities for pharmacological intervention. Thus, in the future, much greater research effort will likely be dedicated to HDL elevation and nonlipid approaches. Through these efforts, physicians of the future should be armed with several complementary agents that can reduce the risk of cardiovascular disease in all patient populations.

The participation of nitric oxide in cell free- and its restriction of macrophage-mediated oxidation of low-density lipoprotein

The potential role of nitric oxide radical (NO .) in macrophage-mediated oxidation and conversion of human low density lipoprotein (LDL) to a high-uptake form was examined by exposing LDL to aerobic solutions of either NO . or 3-morpholino-sydnonimine-hydrochloride (SIN-1, a compound that spontaneously forms NO . and superoxide anion radical) or to mouse peritoneal macrophages in the presence and absence of modulators of cellular NO . synthesis. Incubation with NO . alone caused oxidation of LDL's ubiquinol-10 and accumulation of small amounts of lipid hydroperoxides, but failed to form any high-uptake ligand for endocytosis by macrophages and did not alter the LDL particle charge or the integrity of apoB. Exposure of LDL to SIN-1 resulted in complete consumption of all antioxidants and substantial formation of lipid hydroperoxides, but again had little effect on the lipoprotein particle charge or generation of high-uptake form. Preincubation of macrophages with interferon-gamma increased the cells ability to generate reactive nitrogen metabolites. The extent of cell-mediated oxidation of LDL and the generation of high-uptake LDL was substantial in resident cells in which NO . synthesis was barely detectable, depressed in cells active in NO . synthesis and restored when NO . synthesis was suppressed by the arginine analogue, NMMA. These results suggest that, while together with superoxide anion radical, NO . can oxidize LDL, its synthesis is not required for macrophage-mediated oxidation of LDL in vitro; rather it exerts a protective role in preventing oxidative LDL modification by macrophages.

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.

The inhibition of low-density lipoprotein oxidation by 17-beta estradiol

The antioxidant activities of 17-beta-estradiol (E2) and other steroid hormones were studied by determining their effect on copper-catalyzed (cell-free) and mononuclear cell-mediated oxidation of low-density lipoproteins (LDL), as measured by the production of thiobarbituric acid-reactive substances (TBARS). The oxidation of LDL increased linearly with copper concentrations ranging from 0 to 10 mumol/L. E2 at a concentration of 1 mumol/L inhibited LDL oxidation by 37% to 62% at the various concentrations of copper. In a time-course study, E2 at 1 mumol/L delayed the onset of LDL oxidation in the presence of 5 mumol/L copper. E2 (1 mumol/L) inhibited TBARS production catalyzed by 5 mumol/L copper by 54%, compared with 60% inhibition by 1 mumol/L butylated hydroxytoluene (BHT), a known inhibitor of lipid peroxidation. Estriol at 5 mumol/L decreased LDL oxidation by 49%. Dehydroepiandrosterone (DHEA), testosterone, and estrone had no significant effects. E2 was also an effective inhibitor of mononuclear cell (MNC)-mediated oxidation of LDL, but had no effect on superoxide production by these cells. The onset of TBARS formation from cell-mediated LDL oxidation was also delayed by incubation with 1 mumol/L E2. The results indicate that estrogen may protect against atherosclerosis by inhibiting lipoprotein oxidation.

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.

Lipophilic β-blockers inhibit monocyte and endothelial cell-mediated modification of low density lipoproteins

The effects of propranolol, pindolol and metoprolol on the modification of low density lipoprotein (LDL) by U937 monocyte-like cells, endothelial cells and copper ions were studied by determination of the lipid peroxidation product content and measurement of the relative electrophoretic mobility of the particle. Propranolol and pindolol inhibited LDL oxidation by U937 cells in a dose-dependent manner from 10 to 100 microM, whereas metoprolol had no effect. In the case of LDL modification by endothelial cells, all the three beta-blockers were efficient within the same range of concentrations, and the order of potency was propranolol greater than pindolol greater than metoprolol. In vitro oxidation of LDL in the presence of copper ions was also inhibited by propranolol; pindolol and metoprolol had no significant protective effect in this system. These results concerning the inhibitory action of beta-blockers were confirmed by testing the degradation of modified LDL by J774 macrophages. Although the concentrations of the drugs utilized in this study are relatively high, in long-term treatment beta-blockers might accumulate in target tissues, and the protective effect of propranolol against LDL oxidation might be involved in its inhibitory action on atherosclerosis previously reported in animal models.

High-affinity binding sites for 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE) in carcinoma cells

12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE) enhances tumor cell adhesion to endothelial cells [Honn et al. (1988) Proc. Soc. Exp. Biol. Med. 189, 130-135]. The effect is correlated to surface expression of an integrin receptor, GpIIb/IIIa. Here, we describe evidence for high-affinity binding of 12(S)-HETE to Lewis lung carcinoma cells. Scatchard plot analyses indicated a single class of sites with apparent Kd and Bmax values of 0.44 nM and 66,000 sites per cell, respectively. Competition experiments with unlabeled compounds shod d that the binding was reversible and saturable as well as stereo- and regiospecific. The 12(S)-HETE binding, demonstrated here, might be an important step in a series of events controlling surface expression of integrin receptors.

The simultaneous generation of superoxide and nitric oxide can initiate lipid peroxidation in human low density lipoprotein

Oxidation of low density lipoprotein (LDL) has been shown to occur in the artery wall of atherosclerotic lesions in both animal models and human arteries. The oxidant(s) responsible for initiating this process are under intensive investigation and 15-lipoxygenase has been suggested in this context. Another possibility is that nitric oxide and superoxide, generated by cells present in the artery wall, react together to form peroxynitrite which decomposes to form the highly reactive hydroxyl radical. In the present study we have modelled the simultaneous generation of superoxide and nitric oxide by using the sydnonimine, SIN-1 and have investigated its effects on LDL. SIN-1 liberates both superoxide and nitric oxide during autooxidation resulting in the formation of hydroxyl radicals. We have demonstrated that superoxide generated by SIN-1 is not available to take part in a dismutation reaction since it reacts preferentially with nitric oxide. It follows, therefore, that during the autooxidation of SIN-1 little or no superoxide, or perhydroxyl radical will be available to initiate lipid peroxidation. We have shown that SIN-1 is capable of initiating the peroxidation of LDL and also converts the lipoprotein to a more negatively charged form. The SIN-1-dependent peroxidation of LDL is completely inhibited by superoxide dismutase which scavenges superoxide. Neither sodium nitroprusside or S-nitroso-N-acetyl penicillamine, which only produce nitric oxide, are able to modify LDL. These results are consistent with the hypothesis that a product of superoxide and nitric oxide could oxidize lipoproteins in the artery wall and so contribute to the pathogenesis of atherosclerosis in vivo.

Macrophages and oxidized low density lipoproteins in the pathogenesis of atherosclerosis

Oxidized low density lipoprotein (LDL) may play an important role in the pathogenesis of atherosclerosis. Recent evidence strongly suggests that oxidized LDL is present in atherosclerotic lesions in vivo: 1) LDL isolated from human and rabbit lesions (but not from normal intima) resembles oxidized LDL in its physical, chemical and immunological properties; 2) Oxidized LDL and/or oxidation specific lipid-protein adducts can be demonstrated in human and rabbit lesions by immunocytochemical techniques; 3) Human and rabbit serum contains autoantibodies against oxidized LDL and oxidation specific lipid-protein adducts; 4) atherosclerotic lesions contain IgG that recognizes oxidized LDL and 5) antioxidant therapy slows the development of atherosclerotic lesions in rabbits. Atherosclerosis in human and rabbit arteries may be linked to macrophage-induced oxidative modification of LDL mediated by 15-lipoxygenase which leads to an enhanced uptake of LDL in macrophages by way of the scavenger receptor(s). The identification of LDL oxidation as one of the key events in the early pathogenesis of atherosclerosis offers an interesting possibility to reduce atherosclerosis by antioxidants, enzyme inhibitors and other compounds that protect LDL against oxidative damage and/or reduce the subsequent harmful effects of oxidized LDL on various cellular functions.