Modified plasma-derived lipoproteins in human atherosclerotic plaques

Ex vivo culture of human atherosclerotic plaques: A model to study immune cells in atherogenesis

BACKGROUND AND AIMS

The mechanisms that drive atherosclerotic plaque progression and destabilization in humans remain largely unknown. Laboratory models are needed to study these mechanisms under controlled conditions. The aim of this study was to establish a new ex vivo model of human atherosclerotic plaques that preserves the main cell types in plaques and the extracellular components in the context of native cytoarchitecture.

METHODS

Atherosclerotic plaques from carotid arteries of 28 patients undergoing carotid endarterectomy were dissected and cultured. At various time-points, samples were collected and analysed histologically. After enzymatic digestion, single cells were analysed with flow cytometry. Moreover, tissue cytokine production was evaluated.

RESULTS

We optimised the plaque dissection protocol by cutting plaques into circular segments that we cultured on collagen rafts at the medium-air interface, thus keeping them well oxygenated. With this technique, the relative presence of T and B lymphocytes did not change significantly during culture, and the sizes of lymphocyte subsets remained stable after day 4 of culture. Macrophages, smooth muscle cells, and fibroblasts with collagen fibres, as well as T and B lymphocyte subsets and CD16 natural killer cells, remained largely preserved for 19 days of culture, with a continuous production of inflammatory cytokines and chemokines.

CONCLUSIONS

Our new model of ex vivo human atherosclerotic plaques, which preserves the main subsets of immune cells in the context of tissue cytoarchitecture, may be used to investigate important aspects of atherogenesis, in particular, the functions of immune cells under controlled laboratory conditions.

The effect of essential oils of dietary wormwood (Artemisia princeps), with and without added vitamin E, on oxidative stress and some genes involved in cholesterol metabolism

Wormwood (Artemisia princeps) due to the abundance of antioxidant in its essential oils (EO), has been used as a traditional drug and health food in Korea. Oxidative stress plays an important role in the etiology of atherosclerosis thus antioxidative chemicals improves hepatic lipid metabolism partly by reducing oxysterol formation. The antioxidant activity was assessed using two methods, human low-density lipoprotein (LDL) oxidation and the anti-DPPH free radical assays. It was found that the antioxidant activity of EO with vitamin E higher than EO alone. To study mechanisms accounting for the antiatherosclerotic properties of this wormwood EO, we examined the expression of key genes in cholesterol metabolism such as the LDL receptor, the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and sterol regulatory element binding proteins. The induction was increased up to twofold at 0.05 mg/mL of EO treatment in HepG2 cells for 24h. When EO (0.2 mg/mL) was co-incubated with vitamin E, interestingly, the LDL receptor was dramatically induced by 5-6-folds. HMG-CoA reductase did not change. However, treatment with the higher concentration resulted in cytotoxicity. Our data suggest that wormwood EO with vitamin E may be anti-atherogenic due to their inhibition of LDL oxidation and upregulation of the LDL receptor.

Effect of catechin O-methylated metabolites and analogues on human LDL oxidation

The effects of catechin metabolites and methylated analogues on LDL oxidation were studied in vitro using either a water-soluble initiator or copper ions to induce lipid peroxidation. Direct addition of catechin O-methylated analogues to the oxidation mixture led to a clear protective effect during lag phase and for the metabolites during both lag and propagation phases. The structure-activity relationships obtained with these selectively O-methylated compounds allowed determination of catechin active moietie: the catechol B-ring. Based on physical chemical studies, these results suggest that the mechanism implied in the scavenging properties of flavan-3-ols is not only hydrogen transfer, as generally described, but mainly an electronic transfer from the phenolate, and that 3'- and 4'-O-methylcatechin seem, moreover, to act as amphiphilic chain-breaking antioxidants. However, the plasma concentration of flavan-3-ols necessary to protect LDL is far greater than those usually found in human plasma. Therefore, the data do not support a direct physiological relevance of flavan-3-ols as antioxidants in lipid processes. Future research should focus on other effects besides simple antioxidant ones.

Characterization of antioxidants present in bitter tea (Ligustrum pedunculare)

The present study examined the antioxidants present in bitter tea (Ligustrum pedunculare). It was found that the crude glycoside fraction strongly protected human low-density lipoprotein (LDL) from oxidation. Further column chromatography led to purification of eight phenylethanoid or monoterpene glycosides: lipedoside A-I, lipedoside A-II, lipedoside B-I, lipedoside B-III, lipedoside B-V, lipedoside B-VI, osmanthuside B, and anatolioside. It was found that lipedoside A-I, lipedoside A-II, lipedoside B-V, and lipedoside B-VI were protective, whereas the other four compounds did not protect human LDL from Cu(2+)-medicated oxidation. Lipedoside A-I, lipedoside A-II, lipedoside B-V, and lipedoside B-VI also had a scavenging effect on 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH), comparable to that of alpha-tocopherol. The inhibitory effect of these four phenylethanoid or monoterpene glycosides on oxidation of human LDL and alpha-tocopherol was dose-dependent at concentrations of 5-40 microM. The present results demonstrate that bitter tea as a beverage contains effective antioxidants that may have benefits similar to those of green tea in terms of antioxidant activity.

The oxidant stress hypothesis of atherogenesis

Atherosclerosis is the commonest lesion of blood vessels and is responsible for life-threatening events such as myocardial infarction and stroke. In the last two decades a series of excellent studies unraveled biochemical mechanisms that provided the background for a theory of atherogenesis. This theory is centered on foam cells and on free radical-mediated modification of low density lipoprotein (LDL). Foam cells are the main cell type of atherosclerotic lesions and originate from monocytes migrated from blood and from smooth muscle cells of the arterial wall. Foam cells are engulfed of lipids taken from LDL. Paradoxically, accumulation of LDL in developing foam cells does not occur via the classic LDL receptor. Incubation of macrophages with even very high concentrations of LDL does not appreciably increase cholesterol content. Chemically modified LDL easily enter the cells of atherosclerotic plaque via an unregulated receptor, the scavenger receptor. The most studied chemical modification of LDL is that induced by free radicals.

Hawthorn fruit is hypolipidemic in rabbits fed a high cholesterol diet

The present study examined the hypolipidemic activity of hawthorn fruit. New Zealand white rabbits were fed one of three diets, a reference diet with no cholesterol added (NC), a high cholesterol diet (1 g/100 g, HC) and a HC diet supplemented with 2 g/100 g hawthorn fruit powder (HC-H). After 12 wk, serum total cholesterol (TC) and triacylglycerols (TG) were 23.4 and 22.2% lower, respectively, in the hawthorn fruit group compared with the HC rabbits (P < 0.05). Hawthorn supplementation led to 50.6% less cholesterol accumulation in aorta (P < 0.05) and 23-95% greater excretion of neutral and acidic sterols (P < 0.05). Supplementation of hawthorn fruit did not affect the activities of hepatic 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMG-CoA-R) or cholesterol 7alpha-hydroxylase (CH) but it suppressed the activity of intestinal acyl CoA:cholesterol acyltransferase (ACAT, P < 0.05). The results suggest that the mechanism by which hawthorn fruit decreases serum cholesterol involves, at least in part, the inhibition of cholesterol absorption mediated by down-regulation of intestinal ACAT activity.

Theaflavins in black tea and catechins in green tea are equally effective antioxidants

Green tea catechins, including (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC) and (-)-epigallocatechin gallate (EGCG), are oxidized and dimerized during the manufacture of black tea and oolong tea to form orange-red pigments, theaflavins (TF), a mixture of theaflavin (TF1), theaflavin-3-gallate (TF2A), theaflavin-3'-gallate (TF2B) and theaflavin-3,3'-digallate (TF3). The present study was designed to compare the antioxidant activities of individual TF with that of each catechin using human LDL oxidation as a model. All catechins and TF tested inhibited Cu(+2)-mediated LDL oxidation. Analysis of the thiobarbituric acid-reactive substances (TBARS) and conjugated dienes produced during LDL oxidation revealed that the antioxidant activity was in the order: TF3 > ECG > EGCG > or = TF2B > or = TF2A > TF1 > or = EC > EGC. Four TF derivatives also demonstrated a dose-dependent antioxidant activity in Cu(+2)-mediated LDL oxidation at concentrations of 5-40 micromol/L. These results demonstrate that the TF present in black tea possess at least the same antioxidant potency as catechins present in green tea, and that the conversion of catechins to TF during fermentation in making black tea does not alter significantly their free radical-scavenging activity.

Antioxidative activities of phenylethanoid glycosides from Ligustrum purpurascens

Tea and kudingcha (bitter tea) are the two most popular beverages consumed in China. Tea derived from the leaves of Camellia sinensis has been well studied for its various health benefits, but there are very limited data on the biological activities of bitter tea derived from the leaves of Ligustrum purpurascens (LP). The present study was carried out to characterize the antioxidants present in the bitter tea brewed from the leaves of LP. It was found that the crude glycoside fraction possessed strong protection against oxidation of human low-density lipoprotein (LDL). The column chromatographic separation led to the isolatation of five phenylethanoid glycosides, namely, acteoside, ligupurpuroside A, cis-ligupurpuroside B, trans-ligupurpuroside B, and osmanthuside B. When acteoside was heated in the boiling water, it was isomerized to form isoacteoside. Acteoside, isoacteoside, and ligupurpuroside A purified from LP were protective, whereas cis-ligupurpuroside B, trans-ligupurpuroside B, and osmanthuside B exhibited no protection to human LDL from Cu(2+)-medicated oxidation. Acteoside, isoacteoside, and ligupurpuroside A were also effective in preventing the peroxyl free radical-induced oxidation of alpha-tocopherol in human LDL. The antioxidant activities of acteoside, isoacteoside, and ligupurpuroside A were comparable to that observed for a green tea antioxidant, (-)-epicatechin gallate. The inhibitory effect of these three phenylethanoid glycosides on oxidation of human LDL and alpha-tocopherol was dose-dependent at concentrations of 5-40 microM. The present results suggest that the bitter tea beverage derived from LP contains effective antioxidants that may have an equal benefit as a green tea beverage.

Characterization of antioxidants present in hawthorn fruits

Hawthorn fruit extract has been shown to have many health benefits including being cardiovascular protective, hypotensive and hypocholesterolemic. The present study was carried out to characterize further the antioxidants of hawthorn fruit and their effect on the oxidation of human low density lipoprotein (LDL) and alpha-tocopherol. The dry hawthorn fruit was extracted successively with ether, ethyl acetate, butanol and water. The ethyl acetate fraction was only effective in inhibition of Cu(+2)-mediated LDL oxidation. The column chromatographic separation led to isolation of eight pure compounds; namely, ursolic acid, hyperoside, isoquercitrin, epicatechin, chlorogenic acid, quercetin, rutin and protocatechuic acid. All of these phenolic compounds, except ursolic acid, were protective to human LDL from Cu(+2)-mediated LDL oxidation. They were also effective in preventing the peroxy free radical-induced oxidation of alpha-tocopherol in human LDL. The inhibitory effect of these compounds on oxidation of LDL and alpha-tocopherol was dose-dependent at concentrations ranging from 5 to 40 µM. In addition, supplementation of 2% hawthorn fruit powder significantly elevated serum alpha-tocopherol by 18-20% in rats fed a 30% polyunsaturated canola oil diet, as compared with the control. The present results suggest that part of the mechanism for cardiovascular protective effects of hawthorn fruit might also involve the direct protection to human LDL from oxidation or indirect protection via maintaining the concentration of alpha-tocopherol in human LDL.

Interaction between flavonoids and α-tocopherol in human low density lipoprotein

Oxidative modification of low density lipoprotein (LDL) may play an important role in the development of atherosclerosis. Alpha-tocopherol functions as a major antioxidant in human LDL. The present study was to test whether four natural flavonoids (kempferol, morin, myricetin, and quercetin) would protect or regenerate alpha-tocopherol in human LDL. The oxidation of LDL incubated in sodium phosphate buffer (pH 7.4, 10 mM) was initiated by addition of either 5.0 mM CuSO(4) at 37 degrees C or 1.0 mM of 2,2'-azo-bis (2-amidinopropane) dihydrochloride (AAPH) at 40 degrees C. It was found that alpha-tocopherol was completely depleted within 1 hour. Under the same experimental conditions, all four flavonoids demonstrated a dose-dependent protecting activity to alpha-tocopherol in LDL at the concentration ranging from 1 to 20microM. All flavonoids showed a varying protective activity against depletion of alpha-tocopherol in LDL, with kempherol and morin being less effective than myricetin and quercetin. The addition of flavonoids to the incubation mixture after 5 minutes demonstrated a significant regeneration of alpha-tocopherol in human LDL. The protective activity of four flavonoids to LDL is related to the number and location of hydroxyl groups in the B ring as well as the stability in sodium phosphate buffer.

Dietary flavonoids: intake, health effects and bioavailability

Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin. Over 4000 different flavonoids have been described. They may have beneficial health effects because of their antioxidant properties and their inhibitory role in various stages of tumour development in animal studies. An estimation of the total flavonoid intake is difficult, because only limited data on food contents are available. It is estimated that humans ingest a few hundreds of milligram per day. The average intake of the subclasses of flavonols and flavones in The Netherlands was 23 mg/day. The intake of flavonols and flavones was inversely associated with subsequent coronary heart disease in most but not all prospective epidemiological studies. A protective effect of flavonols on cancer was found in only one prospective study. Flavonoids present in foods were considered non-absorbable because they are bound to sugars as beta-glycosides. However, we found that human absorption of the quercetin glycosides from onions (52%) is far better than that of the pure aglycone (24%). Flavonol glycosides might contribute to the antioxidant defences of blood. Dietary flavonols and flavones probably do not explain the cancer-protective effect of vegetables and fruits; a protective effect against cardiovascular disease is not conclusive.

Regeneration of alpha-tocopherol in human low-density lipoprotein by green tea catechin

Oxidative modification of low-density lipoproteins (LDL) may play an important role in the development of atherosclerosis. alpha-Tocopherol functions as a major antioxidant in human LDL. The present study was to test whether green tea catechins (GTC) would protect or regenerate alpha-tocopherol in human LDL. The oxidation of LDL incubated in sodium phosphate buffer (pH 7.4, 10 mM) was initiated by addition of 1.0 mM of 2,2'-azobis(2-amidinopropane) dihydrochloride at 40 degrees C. It was found that alpha-tocopherol was completely depleted within 1 h. Under the same experimental conditions, the longjing GTC extracts demonstrated a dose-dependent protective activity to alpha-tocopherol in LDL at concentrations ranging from 2 to 20 microM. Four pure epicatechin derivatives showed varying protective activity against depletion of alpha-tocopherol in LDL with (-)-epigallocatechin (EGC) and (-)-epigallocatechin gallate (EGCG) being less effective than (-)-epicatechin (EC) and (-)-epicatechin gallate (ECG). The results showed that addition of longjing GTC extracts, EC, ECG, and EGCG at 5, 10, and 15 min to the incubation mixture demonstrated a gradual regeneration of alpha-tocopherol in human LDL.

Plasma metabolites of quercetin and their antioxidant properties

Quercetin is one of the most widely distributed flavonoids present in fruits and vegetables. The present experiments were performed on rats adapted for 3 wk to a semipurified diet supplemented with 0.2% quercetin. The major part of the circulating metabolites of quercetin (91.5%) are glucurono-sulfo conjugates of isorhamnetin (3'-O-methyl quercetin; 89.1 +/- 2.1 microM) and of quercetin (14.7 +/- 1.7 microM); the minor part (8.5%) is constituted by glucuronides of quercetin and its methoxylated forms (9.6 +/- 2.3 microM). Conjugated dienes formation, resulting from Cu2+-catalyzed oxidation of rat very low density lipoproteins + low density lipoproteins (LDL), was effectively inhibited in vitro by conjugated metabolites of quercetin. These metabolites appeared to be four times more potent than trolox in inhibiting LDL oxidation. Moreover, the plasma from rats adapted to a diet containing 0.2% quercetin exhibited a total antioxidant status markedly higher than that of control rats (+60%). This study shows that ubiquitous quercetin is conjugated in vivo, yielding metabolites that exhibit antioxidant properties. Thus the health benefits of flavonoids in foods can be due to the antioxidant properties of their metabolites.

No effect of consumption of green and black tea on plasma lipid and antioxidant levels and on LDL oxidation in smokers

Intake of flavonoids is associated with a reduced cardiovascular risk. Oxidation of LDL is a major step in atherogenesis, and antioxidants may protect LDL from oxidation. Because tea is an important source of flavonoids, which are strong antioxidants, we have assessed in a randomized, placebo-controlled study the effect of consumption of black and green tea and of intake of isolated green tea polyphenols on LDL oxidation ex vivo and on plasma levels of antioxidants and lipids. Healthy male and female smokers (aged 34+/-12 years, 13 to 16 per group) consumed during a 4-week period 6 cups (900 mL) of black or green tea or water per day, or they received as a supplement 3.6 grams of green tea polyphenols per day (equivalent to the consumption of 18 cups of green tea per day). Consumption of black or green tea had no effect on plasma cholesterol and triglycerides, HDL and LDL cholesterol, plasma vitamins C and E, beta-carotene, and uric acid. No differences were found in parameters of LDL oxidation. Intake of green tea polyphenols decreased plasma vitamin E significantly in that group compared with the control group (-11% P=.016) but had no effect on LDL oxidation ex vivo. We conclude that consumption of black or green tea (6 cups per day) has no effect on plasma lipids and no sparing effect on plasma antioxidant vitamins and that intake of a high dose of isolated green tea polyphenols decreases plasma vitamin E. Although tea polyphenols had a potent antioxidant activity on LDL oxidation in vitro, no effect was found on LDL oxidation ex vivo after consumption of green or black tea or intake of a green tea polyphenol isolate.

Plasma lipoproteins support prothrombinase and other procoagulant enzymatic complexes

The prothrombinase complex (factor [F]Xa, FVa, calcium ions, and lipid membrane) converts prothrombin to thrombin (FIIa). To determine whether plasma lipoproteins could provide a physiologically relevant surface, we determined the rates of FIIa production by using purified human coagulation factors, and isolated fasting plasma lipoproteins from healthy donors. In the presence of 5 nmol/L FVa, 5 nmol/L FXa, and 1.4 micromol/L prothrombin, physiological levels of very low density lipoprotein (VLDL) (0.45 to 0.9 mmol/L triglyceride, or 100 to 200 micromol/L phospholipid) yielded rates of 2 to 8 nmol Flla x L(-1) x s(-1) in a donor-dependent manner. Low density lipoprotein (LDL) and high density lipoprotein (HDL) also supported prothrombinase but at much lower rates (< or =1.0 nmol FIIa x L(-1) x s[-1]). For comparison, VLDL at 2 mmol/L triglyceride yielded approximately 50% the activity of 2X10(8) thrombin-activated platelets per milliliter. Although the FIIa production rate was slower on VLDL than on synthetic phosphatidylcholine/phosphatidylsenne vesicles (approximately 50 nmol FIIa x L(-1) x s[-1]), the prothrombin Km values were similar, 0.8 and 0.5 micromol/L, respectively. Extracted VLDL lipids supported rates approaching those of phosphatidylcholine/phosphatidylserine vesicles, indicating the importance of the intact VLDL conformation. However, the presence of VLDL-associated, factor-specific inhibitors was ruled out by titration experiments, suggesting a key role for lipid organization. VLDL also supported FIIa generation in an assay system comprising 0.1 nmol/L FVIIa; 0.55 nmol/L tissue factor; physiological levels of FV, FVIII, FIX, and FX; and prothrombin (3 nmol/L FIIa x L(-1) x s[-1]). These results indicate that isolated human VLDL can support all the components of the extrinsic coagulation pathway, yielding physiologically relevant rates of thrombin generation in a donor-dependent manner. This support is dependent on the intact lipoprotein structure and does not appear to be regulated by specific VLDL-associated inhibitors. Further studies are needed to determine the extent of this activity in vivo.

Bioavailability and health effects of dietary flavonols in man

Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin. Over 4000 different flavonoids have been described, and they are categorized into flavonols, flavones, catechins, flavanones, anthocyanidins, and isoflavonoids. Flavonoids have a variety of biological effects in numerous mammalian cell systems, as well as in vivo. Recently much attention has been paid to their antioxidant properties and to their inhibitory role in various stages of tumour development in animal studies. Quercetin, the major representative of the flavonol subclass, is a strong antioxidant, and prevents oxidation of low density lipoproteins in vitro. Oxidized low density lipoproteins are atherogenic, and are considered to be a crucial intermediate in the formation of atherosclerotic plaques. This agrees with observations in epidemiological studies that the intake of flavonols and flavones was inversely associated with subsequent coronary heart disease. However, no effects of flavonols on cancer were found in these studies. The extent of absorption of flavonoids is an important unsolved problem in judging their many alleged health effects. Flavonoids present in foods were considered non-absorbable because they are bound to sugars as beta-glycosides. Only free flavonoids without a sugar molecule, the so-called aglycones were thought to be able to pass through the gut wall. Hydrolysis only occurs in the colon by microorganisms, which at the same time degrade flavonoids. We performed a study to quantify absorption of various dietary forms of quercetin. To our surprise, the quercetin glycosides from onions were absorbed far better than the pure aglycone. Subsequent pharmacokinetic studies with dietary quercetin glycosides showed marked differences in absorption rate and bioavailability. Absorbed quercetin was eliminated only slowly from the blood. The metabolism of flavonoids has been studied frequently in various animals, but very few data in humans are available. Two major sites of flavonoid metabolism are the liver and the colonic flora. There is evidence for O-methylation, sulfation and glucuronidation of hydroxyl groups in the liver. Bacterial ring fission of flavonoids occurs in the colon. The subsequent degradation products, phenolic acids, can be absorbed and are found in urine of animals. Quantitative data on metabolism are scarce.

Absorption, metabolism and health effects of dietary flavonoids in man

Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin. Over 4,000 different flavonoids have been described, and they are categorized into flavonols, flavones, catechins, flavanones, anthocyanidins and isoflavonoids. Flavonoids have a variety of biological effects in numerous mammalian cell systems, in vitro as well in vivo. Recently, much attention has been paid to their antioxidant properties and to their inhibitory role in various stages of tumour development in animal studies. Quercetin, the major representative of the flavonol subclass, is a strong antioxidant, and prevents oxidation of low density lipoproteins in vitro. Oxidized low density lipoproteins are atherogenic, and are considered to be a crucial intermediate in the formation of atherosclerotic plaques. This agrees with observations in epidemiological studies that the intake of flavonols and flavones was inversely associated with subsequent coronary heart disease. However, no effects of flavonols on cancer were found in these studies. The extent of absorption of flavonoids is an important unsolved problem in judging their many alleged health effects. Flavonoids present in foods were considered non-absorbable because they are bound to sugars as beta-glycosides. Only free flavonoids without a sugar molecule, the so-called aglycones, were thought to be able to pass through the gut wall. Hydrolysis only occurs in the colon by microorganisms, which at the same time degrade flavonoids. We performed a study to quantify absorption of various dietary forms of quercetin. To our surprise, the quercetin glycosides from onions were absorbed far better than the pure aglycone. Subsequent pharmacokinetic studies with dietary quercetin glycosides showed marked differences in absorption rate and bioavailability. Absorbed quercetin was eliminated only slowly from the blood. The metabolism of flavonoids has been studied frequently in various animals, but very few data in humans are available. Two major sites of flavonoid metabolism are the liver and the colonic flora. There is evidence for O-methylation, sulphation and glucuronidation of hydroxyl groups in the liver. Bacterial ring fission of flavonoids occurs in the colon. The subsequent degradation products, phenolic acids, can be absorbed and are found in urine of animals. Quantitative data on metabolism are scarce.

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.

Similarity between naturally occurring modified desialylated, electronegative and aortic low density lipoprotein

The sialic acid content of electronegative low density lipoprotein (LDL) and LDL isolated from human aortic intima was measured. Sialic acid level in electronegative LDL of healthy subjects was 1.7-fold lower than in native LDL. Sialic acid content in electronegative LDL of coronary atherosclerosis patients was 3-fold lower than in native LDL. Lipoproteins isolated from grossly normal human aortic intima and from fatty streaks contained 20-56% less sialic acid as compared to blood plasma LDL. A negative correlation was established between the ability of electronegative and aortic LDL to stimulate lipid accumulation in cells cultured from uninvolved human aortic intima and lipoprotein sialic acid content. The results obtained indicate that electronegative and aortic LDLs have a low sialic acid content, i.e., are desialylated lipoproteins. Considered together with the fact that all known atherogenic LDLs have similar characteristics, our findings suggest that modified LDLs are the same lipoprotein particles subjected to multiple modification.

Preserved antioxidative defense of lipoproteins in renal failure and during hemodialysis

Contact to artificial surfaces during hemodialysis activates leukocytes, which then form oxidized arachidonic acid products and free radicals. This might promote the oxidative modification of low-density lipoproteins (LDL) that play a key role in the initiation of atherosclerosis. Thus, leukocyte activation could specifically contribute to the high mortality from atherosclerotic complications on long-term hemodialysis. Therefore monitored LDL and high-density lipoprotein (HDL) resistance to copper-stimulated oxidation in patients with end-stage renal disease on maintenance hemodialysis with cellulose acetate or polysulfone membranes (n = 12), in patients with chronic renal failure (n = 13) and in healthy controls (n = 12). Six of the dialysis patients were restudied during a single cuprophane dialysis. Circulating leukocytes were reversibly reduced early in hemodialysis with cellulose acetate (minimum, 83.6% +/- 7.4% of baseline values at 30 minutes after dialysis start), polysulfone (minimum, 80.4% +/- 10.5% at 15 minutes; P < 0.05) and cuprophane (minimum, 24.5% +/- 8.5% at 60 minutes; P < 0.0001). Despite the leukocyte activation, LDL oxidation lag time was not shortened in comparison with healthy controls and was even prolonged at the end of cellulose acetate (P < 0.05) and cuprophane (P < 0.05) dialysis. HDL oxidation lag time increased (12.6% +/- 0.9%; P < 0.0001) 15 to 60 minutes after start of hemodialysis and returned to predialysis values thereafter. In patients with chronic renal failure, the lag time of HDL oxidation was significantly prolonged (13.34 minutes +/- 0.9) compared with healthy controls (10.91 +/- 2.0 minutes; P < 0.01) as well as compared with the dialysis patients at baseline (9.9 minutes +/- 1.4; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Supplementation with low doses of vitamin E protects LDL from lipid peroxidation in men and women

There is accumulating evidence that oxidative modification of LDL is an important step in the process of atherogenesis and that antioxidants may protect LDL from oxidation. We and others have previously shown that ingestion of pharmacological doses of the antioxidant D,L-alpha-tocopherol (vitamin E), far above the recommended daily intake (ie, 12 to 15 IU/d for adults), increases the oxidation resistance of LDL. In this study, we ascertained the minimal supplementary dose of vitamin E necessary to protect LDL against oxidation in vitro. Twenty healthy volunteers (10 men and 10 women, aged 21 to 31 years) ingested consecutively 25, 50, 100, 200, 400, and 800 IU/d, D,L-alpha-tocopherol acetate during six 2-week periods. No changes were observed in LDL triglyceride content, fatty acid composition of LDL, or LDL size during the intervention. Concentrations of alpha-tocopherol in plasma and LDL were both 1.2 times the baseline values after the first period (25 IU/d) and 2.6 and 2.2 times, respectively, after the last period (800 IU/d). There was a linear increase in LDL alpha-tocopherol levels up to an intake of 800 IU/d (r = .79, P < .0001) and a good correlation between alpha-tocopherol in plasma and LDL (r = .66, P < .0001). Simultaneously, the resistance of LDL to oxidation was elevated dose-dependently (+28% after the last period) and differed significantly from the baseline resistance time even after ingestion of only 25 IU/d.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

13C-NMR spectroscopy of human atherosclerotic lesions. Relation between fatty acid saturation, cholesteryl ester content, and luminal obstruction

Previous investigations have used 13C-nuclear magnetic resonance (NMR) spectroscopy to demonstrate the similarities between lipoproteins and the mobile lipids of atheroma. In this study, we tested the hypothesis that 13C-NMR changes are related to indices of histological severity. We classified 20 human arteries according to their obstruction ratio (OR), defined as the ratio of the plaque area to the area delimited by the external elastic lamina. In group A, OR was < 40%, and in group B, OR was > 40%. We analyzed at 9.4 T the resonances of unsaturated (UFA) and polyunsaturated (PUFA) carbons, the resonances of the carbons 19 and 21 (C19, C21) of cholesteryl esters (CE), the methine carbon peak of fatty acids (CH2)n, the choline peak from phospholipids (PL), and the glycerol peak from triglyceride (TG). The UFA/PUFA, UFA/(CH2)n, and PUFA/(CH2)n ratios are markers of fatty acid saturation. (C19, C21)/(CH2)n, choline/(CH2)n, and glycerol/(CH2)n are indices of CE, PL, and TG content, respectively. UFA/PUFA in group A is 1.15 +/- 0.34 versus 1.63 +/- 0.32 in group B (P = .005). PUFA/(CH2)n is 0.26 +/- 0.10 in group A versus 0.16 +/- 0.04 in group B (P = .049). C19, C21/(CH2)n in group A is 0.32 +/- 0.15 versus 0.63 +/- 0.23 for group B (P = .003). No significant difference was found in UFA/(CH2)n or in the TG or PL ratios. 13C spectral examination of human atherosclerosis demonstrates decreased resonances for polyunsaturated fatty acyl chains and cholesteryl esters with increasing obstruction.

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.

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

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

Vitamin E, LDL, and endothelium. Brief oral vitamin supplementation prevents oxidized LDL-mediated vascular injury in vitro

In previously reported in vitro studies, we found that heme, a physiologically widespread hydrophobic iron compound, can rapidly generate oxidized low-density lipoprotein (LDL), which then becomes cytotoxic to cultured vascular endothelial cells; both LDL oxidation and endothelial cytotoxicity were inhibited by incubation with exogenous alpha-tocopherol (vitamin E) or ascorbic acid (vitamin C). Seeking relevance to in vivo conditions, we performed a study in which 10 human volunteers were given daily antioxidant supplements of 800 IU of DL-alpha-tocopherol acetate alone or in combination with 1000 mg of ascorbic acid for 2 weeks. LDL resistance to heme oxidation ex vivo, as measured by the lag time for conjugated-diene formation, increased by as much as threefold from a mean +/- SD of 58 +/- 11 to 104 +/- 18 minutes (P < .001); LDL alpha-tocopherol increased from 11 +/- 2 to 26 +/- 6 molecules per LDL particle (P < .001); and most impressively, cytotoxicity to porcine aortic endothelial cells incubated with LDL conditioned with heme plus H2O2 or with copper was completely prevented (cytotoxicity before supplementation was 42 +/- 12%, decreasing after supplementation to 3 +/- 2%, P < .001). These measurements reverted to their presupplement levels within 2 weeks after participants stopped taking antioxidant supplements and were reproduced in 4 subjects taking 800 IU of DL-alpha-tocopherol acetate supplements alone but not in the same subjects taking 1000 mg ascorbic acid supplements alone. In conclusion, oral vitamin E supplementation increases LDL alpha-tocopherol content, increases LDL resistance to oxidation, and decreases the cytotoxicity of oxidized LDL to cultured vascular endothelial cells.

Oxidation of low density lipoprotein by bovine and porcine aortic endothelial cells and porcine endocardial cells in culture

Oxidation of low density lipoprotein (LDL) in atherosclerotic lesions may be involved in converting macrophages into cholesterol-laden foam cells, a major characteristic of atherosclerotic lesions. It has been reported, and is widely believed, that endothelial cells derived from rabbit, pig and human aortas, but not those derived from bovine aortas, are capable of oxidising LDL in vitro. We have re-investigated this subject and found that during a 48-h incubation period bovine aortic endothelial cells (both in primary culture and in subcultures) were capable of consistently modifying LDL, increasing its uptake and degradation by macrophages by more than 4-fold. Incubation of LDL with bovine aortic endothelial cells for only 24 h, however, produced inconsistent modification of the LDL, whereas mouse peritoneal macrophages consistently modified LDL in 24 h. The modification of LDL by bovine aortic endothelial cells was an oxidative process, as the chain-breaking antioxidants, alpha-tocopherol and probucol, completely or greatly inhibited it. Thus, bovine aortic endothelial cells are capable of oxidising LDL but they are slower at doing so than are certain other types of cells. Nitric oxide generated by activated macrophages has very recently been shown to inhibit their oxidation of LDL. We have therefore investigated whether or not the inhibition of the constitutive nitric oxide synthase of bovine or porcine aortic endothelial cells would increase their rate of oxidation of LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro formation of oxidatively-modified and reassembled human low-density lipoproteins: antioxidant effect of albumin

In early atherogenesis, excess plasma lipoproteins accumulate into the arterial lesion-prone areas as modified and reassembled lipoproteins (MRLp) appearing mostly as lipid droplets and vesicles. In the present study we produced such MRLp, in a cell-free system, devoid of any component of extracellular matrix, by subjecting in vitro human low-density lipoproteins (LDL) to autoxidation or copper-induced oxidation, for up to 96 h. As visualized by negative staining electron microscopy, a large number of lipoprotein particles (Lp) were progressively transformed into aggregates (ALp), fused particles (FLp) and vesicles (VLp). These modifications were paralleled by peroxidation of the samples as revealed by chemical analysis of each MRLp fraction isolated by a three-step purification procedure. LDL peroxidation in the above conditions was inhibited by the presence of albumin as assessed by TBARS and lipid analysis, and by the lack of MRLp formation. This protective effect was independent of albumin source (bovine, human, rabbit) and occurs at an albumin/LDL ratio of 1 when Cu2+ was present, and at a ratio of 0.25 in autoxidative conditions. The results show that: (i) in vitro LDL autoxidation or copper-induced peroxidation in a cell-free system can generate modified and reassembled lipoproteins similar to those detected in vivo in the arterial intima at the inception of atherogenesis; (ii) Lp particles appear to be sequentially transformed in self-aggregates, droplets and vesicles; (iii) serum albumin can completely prevent these LDL alterations.

Can anti-oxidants prevent ischaemic heart disease?

Ischaemic heart disease remains a major cause of mortality in developed countries. A number of important risk factors for the development of coronary atherosclerosis have been identified including hypertension, hypercholesterolaemia, insulin resistance and smoking. However, these factors can only partly explain variations in the incidence of ischaemic heart disease either between populations or within populations over time. In addition, population interventions based upon these factors have had little impact in the primary prevention of heart disease. Recent evidence suggests that one of the important mechanisms predisposing to the development of atherosclerosis is oxidation of the cholesterol-rich low-density lipoprotein particle. This modification accelerates its uptake into macrophages, thereby leading to the formation of the cholesterol-laden 'foam cell'. In vitro, low-density lipoprotein oxidation can be prevented by naturally occurring anti-oxidants such as vitamin C, vitamin E and beta-carotene. This article explores the evidence that these dietary anti-oxidants may influence the rate of progression of coronary atherosclerosis in vivo and discusses the need for formal clinical trials of anti-oxidant therapy.

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.

Oxidation of LDL: role in atherogenesis

Oxidation of LDL is proposed to accelerate atherogenesis by the following sequence of events. LDL accumulates in atherosclerotic plaques, presumably due to interaction with intimal proteoglycans. The LDL then undergoes oxidation, and aldehydic products of lipid peroxidation such as HNE or other aldehyde products derived from lipid peroxidation, induce blocking of lysine residues on apo B. This results in its recognition by the scavenger receptor on tissue macrophages at sites in which LDL concentrations are low. At sites in which the LDL concentration is high, modification with such products induces intermolecular cross-linking and particle aggregation. The aggregated, oxidized LDL particles are then phagocytosed by tissue macrophages to induce lipid loading of these cells and the formation of foam cells, a characteristic of the earliest atherosclerotic lesion. By these mechanisms oxidation of LDL accelerates atherogenesis.

Hemin: a possible physiological mediator of low density lipoprotein oxidation and endothelial injury

Oxidized low density lipoprotein (LDL), formed in vivo from presently unknown reactions, may play a role in atherogenesis. In vitro, transition metals such as iron and copper will facilitate LDL oxidation, but these metals are unlikely to exist in free form in normal body fluids. We have explored the possibility that LDL oxidation may be promoted by heme, a physiologically ubiquitous, hydrophobic, iron-containing compound. Indeed, during several-hour incubation, heme caused extensive oxidative modification of LDL; however, such modification requires only minutes in the presence of small amounts of H2O2 or preformed lipid hydroperoxides within the LDL. Oxidative interactions between heme, LDL, and peroxides lead to degradation of the heme ring and consequent release of heme iron, which further accelerates heme degradation. Coupled (evidently iron-catalyzed) heme degradation and LDL oxidation are both effectively inhibited by hydrophobic antioxidants and iron chelators. That such hemin-induced LDL oxidation may be involved in atherogenesis is supported by the finding that LDL oxidized by hemin is extremely cytotoxic to cultured aortic endothelial cells. Overall, these investigations not only lend support to the idea that LDL oxidation by physiological substances such as heme may play a role in the process of atherogenesis but also may have broader implications, as similar oxidative reactions between heme and unsaturated fatty acids may occur consequent to hemorrhagic injury.

Lesion-derived low density lipoprotein and oxidized low density lipoprotein share a lability for aggregation, leading to enhanced macrophage degradation

In this study we assessed whether low density lipoproteins (LDL) isolated from minced aortic atherosclerotic plaques obtained at autopsy (A-LDL) shared structural and functional properties with LDL oxidized by incubation with Cu2+ for 8-18 hours at 20 degrees C (Ox-LDL). Although both A-LDL and Ox-LDL represented monomeric particles about the size of LDL, both differed from LDL in that they showed an increase in electrophoretic mobility relative to LDL, an increase in cholesterol to protein ratio, and an increase in reactivity with a monoclonal antibody that recognizes epitopes on malondialdehyde (MDA)-modified proteins. In addition, both showed an increase in fluorescence at 360 nm excitation, 430 nm emission, an increase in fragmentation of apolipoprotein B with patterns that were quite similar, and an increase in recognition by the scavenger receptor on mouse peritoneal macrophages (MPMs) based on competition of 125I-A-LDL and 125I-Ox-LDL degradation by excess acetylated LDL. In addition, inhibition of degradation by MPMs of 125I-A-LDL and 125I-Ox-LDL by excess unlabeled Ox-LDL and A-LDL were similar. When MDA was added in increasing amounts to labeled LDL and A-LDL, less MDA was required to modify A-LDL than LDl to obtain ligands that were degraded by MPMs to the same degree. Finally, both A-LDL and Ox-LDL but not LDL underwent aggregation (increased metastability) when concentrated to levels exceeding 1 mg protein/ml and showed enhanced macrophage uptake via phagocytosis (inhibition by cytochalasin D). These results demonstrate that A-LDL and Ox-LDL share properties additional to those previously reported, suggesting that oxidation may be a major mode of modification of LDL accumulating in atherosclerotic lesions. This could lead to lipid loading of macrophages induced by phagocytosis of aggregated particles, in addition to unregulated uptake via the scavenger receptor of monomeric particles.

Enhanced uptake and impaired intracellular metabolism of low density lipoprotein complexed with anti-low density lipoprotein antibodies

We have previously shown that incubation of human macrophages with antigen-antibody complexes prepared with native human low density lipoprotein (LDL) and rabbit anti-LDL antibodies (LDL-ICs) results in an increased intracellular accumulation of cholesteryl esters (CEs) and induces a marked increase in the number of LDL receptors. To determine whether the increased CE accumulation in these cells occurred during incubation of the cells with LDL-ICs or whether it was secondary to the uptake of LDL by overexpressed LDL receptors, we incubated human macrophages with LDL-ICs for 22 hours, followed by incubation with native LDL for another 20 hours. We found that about 90% of the accumulated CEs could be accounted for by the first incubation with LDL-ICs. We then proceeded to show that the CEs accumulated during incubation of cells with LDL-ICs was secondary to enhanced uptake and impaired degradation of the LDL complexed with immunoglobulin G (IgG) (LDL-IC), which led to a marked intracellular accumulation of undergraded LDL (levels 199-fold higher than those obtained when the cells were incubated with the same concentration of native LDL not complexed with IgG). We have also shown that not all CEs accumulated in these cells were derived from accumulation of undegraded LDL and that some of them were derived from the reesterification of free cholesterol released during hydrolysis of LDL. LDL-ICs promoted increased CE accumulation and foam cell formation at concentrations as low as 25 micrograms/ml. To determine which receptors were involved in the uptake of LDL-ICs, we performed experiments in which the uptake of LDL-ICs was competitively inhibited with heat-aggregated gamma globulin, native LDL, beta-very low density lipoprotein, or acetylated LDL. Our results demonstrated that LDL-IC uptake was most effectively inhibited by heat-aggregated gamma globulin, partially inhibited by native LDL or by a monoclonal antibody to the LDL receptor, and not inhibited by acetylated LDL or beta-very low density lipoprotein. Thus, we conclude that the majority of LDL-ICs are taken up through Fc gamma receptors. Finally, we investigated whether the increase in LDL receptor expression was dependent on the receptor pathway used by the LDL-ICs, and we were able to demonstrate that when macrophages were incubated with LDL-ICs prepared with F(ab')2 fragments of the anti-LDL antibody, LDL receptor expression was not enhanced. Therefore, we postulate that the uptake of LDL-ICs through Fc gamma receptors results in an uncoupling of the normal regulation of the LDL receptor expression.

Quantitative studies of transfer in vivo of low density, Sf 12-60, and Sf 60-400 lipoproteins between plasma and arterial intima in humans

To assess the potential of various plasma lipoprotein classes to contribute to the lipid content of the arterial intima, influx and efflux of these plasma lipoprotein fractions into and from the intima of human carotid arteries were measured in vivo. While low density lipoprotein (LDL) is known to transfer from plasma into the arterial wall, there is less information on the atherogenic potential of lipoproteins of intermediate density (Sf 12-60) or of very low density (Sf 60-400). Aliquots of the same lipoprotein (LDL, Sf 12-60 lipoprotein particles, or Sf 60-400 lipoprotein particles) iodinated with iodine-125 and iodine-131 were injected intravenously 18-29 hours and 3-6 hours, respectively, before elective surgical removal of atheromatous arterial tissue, and the intimal clearance of lipoproteins, lipoprotein influx, and fractional loss of newly entered lipoproteins were calculated. Intimal clearance of Sf 60-400 particles was not detectable (less than 0.3 microliter x hr-1 x cm-2), whereas the average value for both LDL and Sf 12-60 lipoprotein particles was 0.9 microliter x hr-1 x cm-2. Since the fractional loss of newly entered LDL and Sf 12-60 lipoprotein particles was also similar, the results suggest similar modes of entry and exit for these two particles. However, due to lower plasma concentrations of Sf 12-60 lipoproteins as compared with LDL, the mass influx of cholesterol in the Sf 12-60 particles was on the order of one 10th of that in LDL, and that of apolipoprotein B was about one 20th.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of the cytocidal macrophage phenotype on the degradation of acetylated low density lipoproteins: dual regulation of scavenger receptor activity and of intracellular degradation of endocytosed ligand

The appearance of lipid-laden macrophages is a characteristic feature in the development of the atherosclerotic plaque. The functional status of macrophages located within the intima of atherosclerotic lesions is as yet unknown; nevertheless, macrophages are known to be exceedingly responsive to their environment and can differentiate to different functional states. The objective of this study was to determine the influence of two definable macrophage functional states, namely the IFN-primed state and the cytocidal state, on the capacity of macrophages to bind and degrade lipoproteins. We report that priming of macrophages with IFN-beta or IFN-gamma failed to influence the ability of macrophages to degrade native low density lipoprotein or acetylated low density lipoprotein (AcLDL). However, challenge with stimuli that induce expression of the cytocidal state (poly[I:C] and LPS) resulted in a marked inhibition of the capacity of the cells to degrade both lipoproteins. The poly[I:C]-induced inhibition of 125I-AcLDL degradation was accompanied by a proportional decrease in the binding of the ligand to its receptor which Scatchard analysis revealed was due to a decrease in receptor number rather than a change in receptor affinity for 125I-AcLDL. However, in addition to the down-regulation of receptor activity, the degradation of endocytosed 125I-AcLDL was also suppressed in macrophages that had been exposed to poly[I:C]. This latter observation suggests that the degradation of endocytosed lipid is also regulated at a second, previously unidentified level, independent of the availability of cell surface ligand receptors. We speculate that this down-regulation in the intracellular hydrolysis of endocytosed lipid may account for the observed accumulation of 125I-AcLDL in these cells.

Macrophage degradation of LDL extracted from human aortic plaques: effect of isolation conditions

Several laboratories have recently reported on the structural and functional characteristics of an LDL fraction isolated from atherosclerotic lesions, designated A-LDL. Given the wide variety of tissue sources and isolation conditions that have been employed, we have addressed whether several procedures currently used affect the interaction of A-LDL with macrophages, and, if so, by what mechanisms. We isolated A-LDL from human aortic plaques by ultracentrifugation and gel filtration chromatography. Although some differences in the chromatographic elution profiles on gel filtration were apparent between homogenized and nonhomogenized extracts, A-LDL isolated from the same pool of plaque minces with or without homogenization showed no differences in macrophage degradation or inhibition of this degradation by excess acetyl-LDL. A-LDL isolated from plaques obtained at surgery or at autopsy less than 12 hr after death also showed no major differences in macrophage recognition, suggesting that post-mortem changes were probably not affecting cell recognition. However, A-LDL particles underwent aggregation when subjected to concentration, when stored for periods of 2 weeks or more, or when subjected to vortexing. The aggregated A-LDL was degraded more readily by macrophages than unaggregated A-LDL, and inhibition of degradation of aggregated A-LDL by excess acetyl-LDL was less than for unaggregated A-LDL. Collectively, these studies show that although post-mortem changes and tissue homogenization do not appreciably affect the interaction of A-LDL with macrophages in culture, other isolation and preparation conditions have dramatic effects which could explain some of the diversity of A-LDL metabolism reported in the literature.

The oxidative modification of low-density lipoproteins by macrophages

1. Mouse resident peritoneal macrophages in culture modified human 125I-labelled low-density lipoprotein (LDL) to a form that other macrophages took up about 10 times as fast as unmodified LDL. The modified LDL was toxic to macrophages in the absence of serum. 2. There was a lag phase of about 4-6 h before the LDL was modified so that macrophages took it up faster. A similar time lag was observed when LDL was oxidized by 5 microM-CuSO4 in the absence of cells. 3. LDL modification was maximal when about 1.5 x 10(6) peritoneal cells were plated per 22.6 mm-diam. well. 4. Re-isolated macrophage-modified LDL was also taken up much faster by macrophages, indicating that the increased uptake was due to a change in the LDL particle itself. 5. Micromolar concentrations of iron were required for the modification of LDL by macrophages to take place. The nature of the other components in the culture medium was also important. Macrophages would modify LDL in Ham's F-10 medium but not in Dulbecco's modified Eagle's medium, even when iron was added to it. 6. The macrophage-modified LDL appeared to be taken up almost entirely via the acetyl-LDL receptor. 7. LDL modification by macrophages was inhibited partially by EDTA and desferrioxamine and completely by the general free radical scavengers butylated hydroxytoluene, vitamin E and nordihydroguaiaretic acid. It was also inhibited completely by low concentrations of foetal calf serum and by the anti-atherosclerotic drug probucol. It was not inhibited by the cyclo-oxygenase inhibitors acetylsalicylic acid and indomethacin. 8. Macrophages are a major cellular component of atherosclerotic lesions and the local oxidation of LDL by these cells may contribute to their conversion into cholesterol-laden foam cells in the arterial wall.

Immunology of atherosclerosis: cellular composition and major histocompatibility complex class II antigen expression in aortic intima, fatty streaks, and atherosclerotic plaques in young and aged human specimens

There is evidence that fatty streaks in arteries can transform into atherosclerotic plaques. Mononuclear cells, including both monocytes and lymphocytes, are among the first cells participating in the development of atherosclerosis of experimental animals. To investigate the roles of different cell types in human atherosclerosis, we enumerated and compared the cellular compositions of normal intima, the transition zone (the area between the normal intima and the core of fatty streaks), fatty streaks, and plaques in young (age 16-30 years) and aged (over 60 years) human specimens using double-staining immunofluorescence with a series of monoclonal and polyclonal antibodies. T lymphocytes, both T helper/inducer (70% of T cells) and T suppressor/cytotoxic (30%) phenotypes, were found in every stage of atherosclerosis, constituting 30 to 40% of total cells in fatty streaks and transition zones of young subjects, and occasionally even in normal intima. Seventy percent of these T cells were HLA-DR positive, which indicated that most of them were activated. Macrophages were most frequent in fatty streaks and around the necrotic core of plaques. Smooth muscle cells, increasing from 5 to 30% with lesion progression, were HLA-DR positive where activated T helper cells occurred in the vicinity. The intracellular presence of the invariant gamma chain confirmed that HLA-DR was actually synthesized by these smooth muscle cells. Endothelial cells were HLA-DR positive above those regions of the lesions where HLA-DR-positive cells had accumulated, but not in normal intima, again suggesting induction of HLA-DR expression by T-cell-derived gamma-interferon. Furthermore, most HLA-DR-positive cells were also identified as HLA-DP and HLA-DQ positive. This aberrant major histocompatibility complex class II antigen expression in smooth muscle and endothelial cells may participate in the perpetuation of the atherogenetic autoimmune reaction.