Platelet secretory products increase low density lipoprotein oxidation, enhance its uptake by macrophages, and reduce its fluidity

Lipid Metabolism and Signaling in Platelet Function

Modern society has changed its diet composition, transitioning to a higher intake of saturated fat with a 50% increase of cardiovascular risk (CVD). Within the context of increased CVD, there is an induction of a prothrombotic phenotype mainly due to increased platelet reactivity as well as decreased platelet response to inhibitors. Platelets maintain haemostasis through both blood components and endothelial cells that secrete inhibitory or stimulatory molecules to regulate thrombus formation. There exist a correlation between platelets' polyunsaturated fatty acid (PUFA) and the increase in platelet reactivity. The aim of this chapter is to review the metabolism of the main PUFAs involved in platelet function associated with the role that their enzyme-derived oxidized metabolites exert in platelet function and fate. Finally, how lipid metabolism in the organism affect platelet aggregation and activation and the pharmacological modulation of these processes will also be discussed.

High fat diet induces adhesion of platelets to endothelium in two models of dyslipidemia

Cardiovascular diseases (CVD) represent about 30% of all global deaths. It is currently accepted that, in the atherogenic process, platelets play an important role, contributing to endothelial activation and modulation of the inflammatory phenomenon, promoting the beginning and formation of lesions and their subsequent thrombotic complications. The objective of the present work was to study using immunohistochemistry, the presence of platelets, monocytes/macrophages, and cell adhesion molecules (CD61, CD163, and CD54), in two stages of the atheromatous process. CF-1 mice fed a fat diet were used to obtain early stages of atheromatous process, denominated early stage of atherosclerosis, and ApoE(-/-) mice fed a fat diet were used to observe advanced stages of atherosclerosis. The CF-1 mice model presented immunostaining on endothelial surface for all three markers studied; the advanced atherosclerosis model in ApoE(-/-) mice also presented granular immunostaining on lesion thickness, for the same markers. These results suggest that platelets participate in atheromatous process from early stages to advance d stages. High fat diet induces adhesion of platelets to endothelial cells in vivo. These findings support studying the participation of platelets in the formation of atheromatous plate.

The effects of tirofiban on peripheral markers of oxidative stress and endothelial dysfunction in patients with acute coronary syndromes

BACKGROUND

Endothelial dysfunction and oxidative stress are believed to be central mechanisms in atherogenesis. We aimed to determine the effects of tirofiban on oxidative stress and neutrophil-endothelium interaction markers in patients with acute coronary syndromes (ACS).

MATERIALS AND METHODS

We measured malondialdehyde (MDA), interleukin-6 (IL-6) and soluble endothelial intercellular and vascular adhesion molecules (sICAM-1 and sVCAM-1) on admission, at 48 and 72 h and on 5th day of hospitalization in 34 patients (age 66.5+/-1.8 years) with ACS. Patients with recurrent angina, changes on the electrocardiogram and/or elevated troponin I received intravenously tirofiban for 48 h and the others received normal saline.

RESULTS

Baseline concentrations of all markers did not differ significantly and compared with placebo, tirofiban infusion markedly reduced MDA, IL-6, sICAM-1 and sVCAM-1 at 48 h (-31+/-6% vs. 84+/-49%, p=0.007, -12+/-14% vs. 23+/-10%, p=0.05, -20+/-6% vs. 36+/-25%, p=0.04 and -10+/-5% vs. 6+/-5%, p=0.02, respectively). Relative to baseline, significant reductions were observed for all 4 markers at 72 h and day 5 (p<0.05).

CONCLUSION

Tirofiban potentiates the decline in oxidative stress and may reverse abnormal endothelial activation in patients with ACS. This benefit seems to remain over the following 5 days.

LDL oxidized by hypochlorous acid causes irreversible platelet aggregation when combined with low levels of ADP, thrombin, epinephrine, or macrophage-derived chemokine (CCL22)

The in vitro oxidation of low-density lipoprotein (LDL) by hypochlorous acid produces a modified form (HOCl-LDL) capable of stimulating platelet function. We now report that HOCl-LDL is highly effective at inducing platelet function, causing stable aggregation and alpha-granule secretion. Such stimulation depended on the presence of low levels of primary agonists such as adenosine diphosphate (ADP) and thrombin, or others like epinephrine (EPI) and macrophage-derived chemokine (MDC, CCL22). Agonist levels, which by themselves induced little or reversible aggregation, caused strong stable aggregation when combined with low levels of HOCl-LDL. Platelet activation by HOCl-LDL and ADP (1 microM) caused P-selectin (CD62P) exposure, without serotonin or adenosine triphosphate (ATP) secretion. Intracellular calcium levels rose slowly (from 100 to 200 nM) in response to HOCl-LDL alone and rapidly when combined with ADP to about 300 nM. p38 mitogen-activated protein kinase (MAPK) became phosphorylated in response to HOCl-LDL alone. This phosphorylation was not blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide, which reduced the extent of aggregation and calcium increase. However, the p38 MAPK inhibitor SB203580 blocked platelet aggregation and phosphorylation of p38 MAPK. These findings suggest that HOCl-LDL exposed during atherosclerotic plaque rupture, coupled with low levels of primary agonists, can rapidly induce extensive and stable thrombus formation.

Potential health benefits from the flavonoids in grape products on vascular disease

In the dog, monkey, a nd human we have shown that 5 ml/kg of red wine or 5-10 ml/kg of purple grape juice but not orange or grapefruit juice inhibits platelet activity, and protects against epinephrine activation of platelets. Red wine and purple grape juice enhances platelet and endothelial production of nitric oxide (Fitzpatrick et al., 1993, Parker et al., 2000). This is thought to be one of the mechanisms whereby purple grape juice significantly improved endothelial function in 15 patients with coronary artery disease. The consumption of purple grape juice by the patients also offered increased protection against LDL cholesterol oxidation, even though all the patients were also taking another antioxidant vitamin E, 400 IU/day. The number of people and animals in these studies was small; however, each one acted as their own control as measurements were made in each before, and then after consumption of red wine or purple grape juice. Thus these studies are thought to be significant. We feel that the results of these studies are encouraging and justify further research on larger numbers of subjects. This suggests that the flavonoids in purple grape juice and red wine may inhibit the initiation of atherosclerosis by one or more of the mechanisms described above. It will take years to fully characterize the potential benefits of daily consumption of red wine or purple grape juice for maintaining a healthy heart. Based on the existing evidence of antiplatelet and antioxidant benefits and improved endothelial function from red wine and purple grape juice, it seems reasonable to suggest that moderate amounts of red wine or purple grape juice be included among the 5-7 daily servings of fruits and vegetables per day as recommended by the American Heart Association to help reduce the risk of developing cardiovascular disease.

Ramipril administration to atherosclerotic mice reduces oxidized low-density lipoprotein uptake by their macrophages and blocks the progression of atherosclerosis

Foam cell formation, the hallmark of early atherosclerosis, results from cholesterol accumulation in arterial macrophages. Angiotensin-II stimulates foam cell formation and angiotensin converting enzyme (ACE) inhibitors reduce atherosclerosis in animal models. The goal of the present study was to determine the effect of the ACE inhibitor Ramipril on the progression of atherosclerosis in apolipoprotein-E-deficient (E0) mice with already advanced atherosclerosis. Therefore, 4-month-old atherosclerotic E0 mice were treated with Ramipril for 2 and 4 months and compared to age-matched placebo-treated mice, as well as to control young (4-month-old) non-treated E0 mice, for their atherosclerosis. Histomorphometry showed that Ramipril treatment substantially inhibited atherogenesis as shown by 48 and 72% reduction in lesion size at 6 and 8 months of age, respectively, compared to the lesion size in age-matched placebo-treated mice. Moreover, the size of the atherosclerotic lesions in 6- and 8-month-old Ramipril-treated mice was almost identical to the size of atherosclerosis of the 4-month-old control mice. Moreover, Ramipril treatment of E0 mice, significantly reduced oxidized low-density lipoprotein (Ox-LDL) uptake by their peritoneal macrophages (MPM) by 32%, compared to Ox-LDL uptake by MPM from 6-month-old placebo mice, and even reduced it by 12% in comparison to Ox-LDL uptake by MPM from 4-month-old control mice. A significant decrease in the mRNA levels of the Ox-LDL receptor CD36 by 58% was observed in macrophages from 6-month-old Ramipril-treated mice compared to macrophages from the 6-month-old placebo-treated mice. There was even a significant reduction (by 32%) in CD36 mRNA levels in macrophages from the 6-month-old Ramipril-treated mice, compared to the CD36 mRNA levels in macrophages from the 4-month-old control mice. We thus conclude that administration of the ACE inhibitor Ramipril to E0 mice, which already exhibit significant atherosclerosis, blocked the progression of the atherosclerotic lesion build-up, a phenomenon that could be related to Ramipril-induced inhibition of Ox-LDL uptake by macrophages.

Pomegranate juice supplementation to atherosclerotic mice reduces macrophage lipid peroxidation, cellular cholesterol accumulation and development of atherosclerosis

Inhibition of lipid peroxidation contributes to the attenuation of macrophage cholesterol accumulation, foam-cell formation and atherosclerosis. Evidence suggests that nutritional antioxidants such as pomegranate juice (PJ) can contribute to the reduction of oxidative stress and atherogenesis. The goals of the present study were to determine whether such beneficial effects of PJ exist when supplemented to apolipoprotein E-deficient (E(0)) mice with advanced atherosclerosis and to analyze the antiatherosclerotic activity of a tannin-fraction isolated from PJ. Mice (4-mo-old) were supplemented with PJ in their drinking water for 2 mo and compared with age-matched placebo-treated mice, as well as to young (4-mo-old) control mice, for their mouse peritoneal macrophage (MPM) oxidative state, cholesterol flux and mice atherosclerotic lesion size. PJ supplementation reduced each of the proatherogenic variables determined in the present study compared with age-matched placebo-treated mice. It significantly induced serum paraoxonase activity and reduced MPM lipid peroxide content compared with placebo-treated mice and control mice. PJ administration to E(0) mice significantly reduced the oxidized (Ox)-LDL MPM uptake by 31% and MPM cholesterol esterification and increased macrophage cholesterol efflux by 39% compared with age-matched, placebo-treated mice. PJ consumption reduced macrophage Ox-LDL uptake and cholesterol esterification to levels lower than those in 4-mo-old, unsupplemented controls. PJ supplementation to E(0) mice with advanced atherosclerosis reduced the lesion size by 17% compared with placebo-treated mice. In a separate study, supplementation of young (2-mo-old) E(0) mice for 2 mo with a tannin fraction isolated from PJ reduced their atherosclerotic lesion size, paralleled by reduced plasma lipid peroxidation and decreased Ox-LDL MPM uptake. PJ supplementation to mice with advanced atherosclerosis reduced their macrophage oxidative stress, their macrophage cholesterol flux and even attenuated the development of atherosclerosis. Moreover, a tannin-fraction isolated from PJ had a significant antiatherosclerotic activity.

Retention of oxidized LDL by extracellular matrix proteoglycans leads to its uptake by macrophages: an alternative approach to study lipoproteins cellular uptake

Interaction between arterial macrophages and oxidized LDL (Ox-LDL) leads to foam cell formation, a critical step during early atherogenesis. Until now, cellular uptake of lipoproteins was studied through incubation of the media-soluble lipoprotein with cultured macrophages. However, as lipoproteins in the arterial wall are bound to subendothelial matrix, we questioned whether the retention (binding) of Ox-LDL to a macrophage-derived extracellular matrix (ECM) could lead to enhanced uptake by macrophages. The uptake of ECM-bound Ox-LDL by activated macrophages (by phorbol myristate acetate) was lipoprotein dose dependent, time dependent and higher (by 1.5-fold) than the uptake of ECM-bound native LDL. Preincubation of the ECM with lipoprotein lipase before the addition of Ox-LDL was essential for the uptake of ECM-bound Ox-LDL by the macrophages. After radiolabeling of the ECM glycosaminoglycans (GAGs), we found that ECM-bound Ox-LDL is taken up by the macrophages together with the ECM-GAG. Finally, these results were further confirmed through the use of ECM obtained from mouse peritoneal macrophages (MPMs), derived from atherosclerotic, apoE-deficient mice. In 24-week-old mice with developed atherosclerosis, the GAG content of their MPM-derived ECM increased by 52%, the ability of their MPM-derived ECM to bind Ox-LDL increased by 57%, and macrophage uptake of Ox-LDL that was retained by the MPM-derived ECM increased by 86%. In conclusion, the present study demonstrated that ECM-bound Ox-LDL is taken up by activated macrophages. This may represent a physiopathological phenomenon that leads to cholesterol and oxysterol accumulation in arterial macrophages, the hallmark of early atherosclerosis.

Modification of protein moiety of human low density lipoprotein by hypochlorite generates strong platelet agonist

Conflicting reports exist about the effects of mildly or extensively oxidized low density lipoproteins (LDLs) on the reactivity of human platelets. This platelet response is mainly caused by modification of the protein and lipid moiety, giving rise to very differently modified species with hardly predictable properties. The aim of this study was to prepare oxidized LDL with modifications essentially restricted to the protein moiety and to determine the eventual platelet responses. We treated LDL at 0 degrees C for 10 minutes with a 60- to 1000-fold molar excess of sodium hypochlorite in borate buffer in the presence of the radical scavenger butylated hydroxytoluene. Under these conditions, neither fragmentation of apolipoprotein B-100 nor formation of LDL aggregates was observed, and lipid oxidation products did not exceed the amount present in untreated LDLs. The degree of modification and the respective effects on platelet function were highly reproducible. Hypochlorite-modified LDLs act as strong platelet agonists, inducing morphological changes, dense granule release, and irreversible platelet aggregation. The evoked platelet effects are completely suppressed by inhibitors of the phosphoinositide cycle but not by EDTA or acetylsalicylic acid. Most likely, these effects are transmitted via high-affinity binding to a single class of sites, which does not recognize native or acetylated LDL. Obviously, modified lysines, and the secondary lipid modifications derived from them, are not essential for this interaction. We conclude that bioactive oxidized lipids are not directly involved in the stimulation of platelets by hypochlorite-modified LDLs.

Fluvastatin: effects beyond cholesterol lowering

Coronary heart disease (CHD) remains a major therapeutic challenge in the Western world, and strategies aimed at cholesterol lowering form the mainstay of treatment. Fluvastatin is an established 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor ("statin") for the treatment of hypercholesterolemia. Its efficacy and safety have been established in numerous clinical trials. Emerging evidence now indicates that treatment with fluvastatin slows the progression of atherosclerotic CHD and reduces the incidence of cardiovascular morbimortality in the secondary prevention setting. This effect of fluvastatin cannot be explained by cholesterol lowering alone; nonlipid-related mechanisms (so-called "pleiotropic effects") undoubtedly contribute to a certain extent, and are probably linked to modulation of the mevalonate pathway. This review discusses the experimental evidence regarding the antiatherosclerotic and antithrombotic effects of fluvastatin that may contribute to its beneficial action on disease progression and clinical events. Such effects include decreased expression of adhesion molecules in monocytes and leucocyte-endothelium adherence responses, immunomodulation, prevention of low-density lipoprotein oxidation, inhibition of cholesterol esterification and accumulation, along with effects on smooth muscle cell proliferation and migration. Pleiotropic actions aimed at plaque stabilization (eg, decreased secretion of matrix metalloproteinases by macrophages), together with effects on platelet activity, tissue factor expression, and endothelial function, may contribute to an antithrombotic effect of fluvastatin. Taken together, the results of these studies indicate that the effects of fluvastatin, at therapeutic doses, may extend beyond cholesterol lowering.

Effects of low-density lipoproteins on blood coagulation and fibrinolytic activity

In vitro experiments showed that copper-oxidized low-density lipoproteins activate factors of the prothrombin complex in the whole blood and inhibit fibrin generation in both blood and plasma. Moreover, oxidized low-density lipoproteins inhibit fibrinolysis and impair the structure of fibrin clot, which results in hypercoagulation.

Macrophage plasma membrane chondroitin sulfate proteoglycan binds oxidized low-density lipoprotein

Lipoprotein interactions with macrophage proteoglycans (PGs) is believed to play an important role in the cellular uptake of lipoproteins and in macrophage cholesterol accumulation. Recently, we have shown the participation of macrophage plasma membrane glycosaminoglycans (GAGs) in the cellular uptake of oxidized LDL (Ox-LDL). The aim of the present study was to identify the specific cell surface proteoglycans involved in this interaction. J-774 A.1 macrophage-like cell line plasma membrane proteoglycans were isolated by anion exchange chromatography from cells that were prelabeled with [35S]sodium sulfate. Using Sepharose 6B chromatography, cell surface major proteoglycans were identified as chondroitin sulfate (CS) proteoglycans (77%) and heparan sulfate (HS) proteoglycans (23%). Binding rates of these 35S-labeled proteoglycans to Ox-LDL and to native LDL were analyzed by their ability to bind lipoproteins coupled to a CnBr-activated Sepharose CL-4B chromatography. Of the total labeled cell surface proteoglycans added to the column, 57% were bound to the Sepharose-coupled Ox-LDL, whereas 73% of the cell surface proteoglycans were bound to the Sepharose-coupled native LDL. Binding of the plasma membrane macrophage 35S-labeled proteoglycans to Ox-LDL was inhibited by adding increasing concentrations of non-labeled chondroitin sulfate, or by pretreatment of the 35S-labeled proteoglycans fraction with chondroitinase ABC. In contrast, neither the addition of non-labeled heparan sulfate, nor pretreatment of the labeled proteoglycans fraction with heparinase III, had any significant effect on proteoglycan binding to Ox-LDL. These findings were further supported by using mutant cells characterized by specific glycosaminoglycan deficiencies. Ox-LDL binding and degradation by mutant 745 CHO cells which are characterized by a deficiency in both heparan sulfate and chondroitin sulfate, was decreased by 28 and 27% respectively, compared to the binding of Ox-LDL to the wild-type CHO cells. Ox-LDL binding and degradation by mutant 677 CHO cells, which lack heparan sulfate but have increased levels of chondroitin sulfate, however, was found to be increased by 29 and 19%, respectively, compared to Ox-LDL binding to the wild-type CHO cells. Finally, analysis of the cell surface proteoglycans in macrophages that were subjected to oxidative stress, by their preincubation with angiotensin II, exhibited a 51-59% increase in their cell surface proteoglycan content, with a major effect on chondroitin sulfate proteoglycans. The present study thus demonstrated that Ox-LDL can specifically bind to macrophage surface chondroitin sulfate proteoglycans, and the macrophage content of this proteoglycan is increased under oxidative stress. The interaction between macrophage chondroitin sulfate proteoglycans and Ox-LDL can contribute to enhanced uptake of Ox-LDL with the formation of cholesterol-loaded foam cells, and accelerated atherosclerosis.

Effects of fish oil supplementation on platelet survival and ex vivo platelet function in hypercholesterolemic patients

Little is known about the effects of dietary supplementation on platelet survival with low doses of n-3 and n-6 fatty acids in patients with hypercholesterolemia. The effects of a 6-week intervention with fish oil capsules (daily intake: 216 mg eicosapentaenoic acid, 140 mg docosahexaenoic acid, 390 mg gamma-linolenic acid, and 3480 mg linoleic acid) on in vivo platelet survival (111 In-oxine labeled platelets) and on ex vivo markers of platelet activation were investigated in a placebo-controlled, double-blind study with 26 hypercholesterolemic patients. In vivo platelet survival increased in the fish oil group (T) from a mean of 159+/-14 hours to a mean of 164+/-12 hours (p=0.025), whereas it remained unchanged in the placebo (P) group (T vs. P; p=0.055). Ex vivo, thromboxane B2 decreased from a mean of 225+/-16 to 212+/-21 ng/mL (p=0.003) in T but did not change in P (T vs. P: p=0.002). Malondialdehyde formation was lowered significantly by fish oil supplementation from a mean of 5.49+/-1.3 to 5.12+/-1.05 nM/10(9) platelets, p=0.005, as compared with P (T vs. P; p=0.018). The trendwise decrease in 11-DH-thromboxane B2 plasma levels was not significant nor was the increase in platelet sensitivity to prostaglandin I2 by fish oil. Baseline platelet survival in patients with hyperlipoproteinemia type IIa was not different from those with hyperlipoproteinemia IIb and response to treatment in terms of platelet activation markers was not either. The changes in platelet activation parameters in T were associated with significant reductions in cholesterol (-2.9%), low density lipoprotein cholesterol (-3.5%), and triglycerides (-12.4%). Both ex vivo and in vivo platelet activation parameters exhibited signs of decreased activation by a 6-week diet supplemented with n-3 and n-6 fatty acids, which might be beneficial in reducing atherothrombotic risk, in patients with hyperlipoproteinemia type IIa and IIb.

Association of age-related decrease in platelet membrane fluidity with platelet lipid peroxide

The influence of age on platelet lipid peroxide (LPO), platelet membrane fluidity and the composition of fatty acid was investigated in female Wistar rats widely ranging in age from 14 to 720 days old. LPO levels were significantly higher (p<0.05) in the platelets of upper age groups than in those of lower age groups, showing a significantly positive correlation with age (r=0.84, p<0.0001). Membrane fluidity, assessed by 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence polarization, was significantly reduced with age. The composition of fatty acid demonstrated an age-related elevation (p<0.05) in the unsaturation index. The rises in the LPO levels revealed a significantly positive correlation with DPH-polarization (r=0.73, p<0.0001). Thus our results suggested that the age-related deterioration of platelet membrane fluidity, despite a significant elevation in the unsaturation index, was due to the age-related higher basal levels of LPO in platelets.

The role of (E)-4-hydroxy-2-nonenal in platelet activation by low density lipoprotein and iron

(E)-4-Hydroxy-2-nonenal (HNE) is a highly reactive product of the oxidation of low density lipoprotein (LDL) which increases the platelet aggregation response to various agonists. HNE formation was increased during the enhanced platelet aggregation to thrombin, ADP. A23187 and epinephrine in the presence of LDL. The increase in platelet aggregation and HNE formation by LDL was inhibited by superoxide dismutase and catalase, suggesting superoxide and hydrogen peroxide produced by platelets during aggregation may be at least partly responsible. The responsiveness of platelets to LDL and the accompanying HNE formation was increased further in the presence of ferrous ion. The effect of ferrous ion on both platelet responses and HNE formation was decreased by superoxide dismutase, catalase and the antioxidants dipyridamole and probucol implicating platelet-derived free radicals. Ferrous ion caused an increase in the release of arachidonic acid from platelet membrane phospholipids in the presence of LDL which was probably caused by increased HNE production. The results suggest iron could increase platelet reactivity at sites of vascular injury by increasing HNE formation and promote the development of atherosclerotic lesions.

Role of macrophage glycosaminoglycans in the cellular catabolism of oxidized LDL by macrophages

Macrophage binding sites for oxidized LDL (Ox-LDL) include class A scavenger receptors (SR-As), the CD-36 molecule, and an additional but hitherto unidentified binding site. Because cell-surface glycosaminoglycans (GAGs) were previously shown to be involved in the cellular uptake of native LDL and lipoprotein(a), several strategies to assess the participation of heparan sulfate (HS) and chondroitin sulfate (CS) in macrophage catabolism of Ox-LDL were used. First, incubation of J-774 A.1 macrophage-like cells with either heparinase or chondroitinase, or with both enzymes together, reduced the binding, uptake, and degradation of 125I-Ox-LDL by 20% to 45%, in comparison with control nontreated cells, while catabolism of 125I-labeled acetylated LDL (Ac-LDL) and native LDL were unaffected. Second, the proteoglycan (PG) cellular content was increased by cell enrichment with exogenous GAGs or by using human monocyte-derived macrophages from two patients with Sanfilippo mucopolysaccharidosis, which are characterized by cellular HS accumulation. In these macrophages, cellular uptake of 125I-Ox-LDL increased, while catabolism of 125I-Ac-LDL and native LDL were unaffected. Experiments using conditioned media from control, heparinase-digested, or chondroitinase-digested macrophages indicated that neither secreted GAGs nor released digestion products played any role in Ox-LDL catabolism. To evaluate potential interactions between cell-surface GAGs and known receptors for Ox-LDL, we used excess unlabeled Ac-LDL to block SR-As or anti-CD-36 antibodies to block CD-36, and then examined the catabolism of 125I-Ox-LDL by GAG-enriched or -depleted macrophages. Both excess unlabeled Ac-LDL and anti-CD-36 antibodies reduced 125I-Ox-LDL catabolism, but only excess unlabeled Ac-LDL completely abolished the increase in 125I-Ox-LDL catabolism on GAG enrichment of the cells, indicating a cooperation between exogenous GAGs and cell-surface SR-As in the catabolism of OX-LDL. Moreover, the addition of GAGases to macrophages that were preincubated with anti-CD-36 antibodies and excess Ac-LDL further reduced macrophage degradation of Ox-LDL in comparison with cells that were pretreated only with anti-CD-36 antibodies and Ac-LDL, indicating a more complex role for endogenous GAGs. Overall, these studies demonstrate a substantial contribution of macrophage-associated GAGs in the catabolism of Ox-LDL, which is mediated in part by a cooperation between GAGs and cell-surface SR-As.

Interactions of platelets, macrophages, and lipoproteins in hypercholesterolemia: antiatherogenic effects of HMG-CoA reductase inhibitor therapy

To assess the effect of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors on plasma cholesterol concentrations and on platelet aggregation, lovastatin or fluvastatin, 40 mg daily, was given to hypercholesterolemic patients. After 24 weeks, plasma low-density lipoprotein (LDL) cholesterol concentrations were reduced by 37% after lovastatin therapy and 29% after fluvastatin therapy. The platelet cholesterol/phospholipid ratio was reduced by 33% and 26%, respectively. Platelet aggregation was significantly reduced by 12-15% (p < 0.01) after 4 weeks of therapy with either agent. Lovastatin or fluvastatin therapy reduced platelet aggregation through an in vivo hypocholesterolemic action on the platelet cholesterol content and also through a direct effect on platelet function, as a result of drug binding to the platelets. We also studied the effect of these HMG-CoA reductase inhibitors on LDL susceptibility to oxidation. LDL oxidation (induced by copper ions) was reduced by 31% after lovastatin therapy and by 37% after fluvastatin therapy. The inhibitory effect of HMG-CoA reductase inhibitors on LDL oxidation involved their stimulatory effect on the removal of LDL from the circulation and a direct binding effect of the drugs to the lipoprotein. Because HMG-CoA reductase inhibitors can inhibit platelet aggregation, macrophage foam cell formation, and LDL oxidation, major contributors to atherogenesis, the use of these drugs can significantly attenuate the atherosclerotic process.

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

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

Inhibition of macrophage scavenger receptor activity by tumor necrosis factor-alpha is transcriptionally and post-transcriptionally regulated

Regulation of expression of the scavenger receptor is thought to play a critical role in the accumulation of lipid by macrophages in atherosclerosis. Tumor necrosis factor-alpha (TNF-alpha) has been shown to suppress macrophage scavenger receptor function (van Lenten, B.J., and Fogelman, A.M. (1992) J. Immunol. 148, 112-6). However, the mechanism by which it does so is unknown. We evaluated the mechanism by which TNF-alpha inhibited macrophage scavenger receptor surface expression and binding of acetylated low density lipoprotein (aLDL). Binding of aLDL to phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 macrophages was suppressed by TNF-alpha in a dose-dependent manner. Inhibition of aLDL binding was paralleled by a reduction of macrophage scavenger receptor protein as detected by the Western blot. TNF-alpha partially decreased macrophage scavenger receptor mRNA steady state levels in PMA-differentiated THP-1 macrophages, a result that was confirmed by reverse transcription-polymerase chain reaction. PMA increased the luciferase activity driven by the macrophage scavenger receptor promoter in the transfected cells, whereas TNF-alpha partially reduced luciferase activity. However, macrophage scavenger receptor mRNA half-life was dramatically reduced in cells treated with TNF-alpha relative to untreated cells. Reduction in macrophage scavenger receptor message in response to TNF-alpha was dependent on new protein synthesis because it was blocked by cycloheximide. These results indicate that TNF-alpha regulates macrophage scavenger receptor expression in PMA-differentiated THP-1 macrophages by transcriptional and post-transcriptional mechanisms but principally by destabilization of macrophage scavenger receptor mRNA.

Angiotensin II-modified LDL is taken up by macrophages via the scavenger receptor, leading to cellular cholesterol accumulation

The incidence of myocardial infarction is significantly higher in hypertensive patients with increased plasma concentration of angiotensin (Ang) II. Ang II was shown to bind to LDL in vitro, and in the present study we showed its binding to LDL in vivo. Ang II (10(-7) mol/L) was incubated with LDL for 3 hours at 37 degrees C, followed by reseparation of the modified lipoprotein (Ang II-LDL) and its incubation with J-774 A.1 macrophages. Binding of Ang II to LDL significantly increased the lipoprotein protein degradation (by 25%) and its cell association (by 75%) compared with nontreated LDL. Unlike Ang II-LDL, both Ang I-LDL and Ang III-LDL were taken up by macrophages similar to native LDL. The lipid composition and size of Ang II-LDL were similar to those of native LDL, and it was not aggregated. Ang II-LDL was not oxidized, as the contents of malondialdehyde and peroxides were not different from those found in native LDL. On heparin-Sepharose column chromatography, Ang II-LDL was eluted in the void volume, like acetylated LDL (Ac-LDL) and unlike native LDL, which binds to heparin. The cellular degradation of Ang II-125I-labeled LDL by J-774 A.1 macrophages of Ang II-125I-labeled LDL by J-774 A.1 macrophages was studied in the presence of a 50-fold excess of nonlabeled native LDL, Ang II-LDL, Ac-LDL, or oxidized LDL (Ox-LDL). Whereas native LDL had no effect on the degradation of Ang II-125I-LDL by the macrophages, Ac-LDL, Ox-LDL, and Ang II-LDL reduced the cellular uptake of the lipoprotein by 77%, 82%, and 87%, respectively. Similarly, fucoidin but not free Ang II reduced macrophage degradation of the labeled Ang II-LDL. We conclude that Ang II can modify LDL to a form that is not oxidized or aggregated but is still taken up at an enhanced rate by macrophages via the scavenger receptor.

HDL apolipoprotein A-I attenuates oxidative modification of low density lipoprotein: studies in transgenic mice

Epidemiological evidence suggests that plasma high-density lipoprotein (HDL) is protective against coronary artery disease, whereas oxidatively modified low density lipoprotein is atherogenic. Human apolipoprotein A-I transgenic mice with overexpression of the human apolipoprotein A-I gene have increased plasma levels of apolipoprotein A-I and HDL-cholesterol. We analyzed LDL oxidation by determination of LDL associated malondialdehyde, peroxides and conjugated dienes. The present study demonstrates that HDL from both normal and human apolipoprotein A-I transgenic mice at similar concentrations inhibits LDL (protein concentration 500 mg/l) lipid peroxidation, but the effect of the human apolipoprotein A-I transgenic mice HDL was two-fold greater than that of HDL derived from normal mice. In addition, the electrophoretic mobility of oxidatively modified LDL was reduced about two-fold in the presence of human apolipoprotein A-I transgenic mice HDL than that obtained in the presence of normal mice HDL. We thus suggest that human apolipoprotein A-I possesses antioxidant properties which might neutralize LDL lipid peroxidation. This may underline the mechanism responsible for the lower prevalence of atherosclerosis in subjects with high plasma levels of HDL and apolipoprotein A-I.

Lipid peroxidation by activated platelets: a possible link between thrombosis and atherogenesis

Generation of reactive oxygen species (ROS) by platelets was detected by a cyano-tetrazolium dye (CTC) which forms fluorescent formazan on the cell surface upon reduction. Fluorescence was quantitated by a densitometric device. Resting platelets in plasma produced significant fluorescence (P < 0.0001), and addition of thrombin enhanced the fluorescence of the coagulated platelet mass even further (by 2 h, a 6- and 8-fold increase over fluorescence of platelet-free plasma was measured, respectively). Blood containing CTC was perfused through a glass capillary tubing and the action of shear forces resulted in the formation of an occlusive platelet thrombus. Such thrombi (formed either from whole blood or platelet-rich plasma) were intensely fluorescent, indicating formation of ROS in the platelet mass (a 10- and 8-fold increase in fluorescence over coagulated plasma, respectively). Lipid peroxide content of resting platelets in platelet-rich plasma was doubled over 24 h storage, while addition of thrombin caused a 7.4-fold increase (P < 0.0001) of lipid peroxides in the retracted platelet-rich plasma-clot. Transition metal chelator and antioxidant prevented lipid peroxidation by platelets in response to thrombin. Thrombin activation of (washed) platelets in plasma-free medium caused only 1.4-fold increase in oxidation of added low density lipoprotein (LDL). In contrast, thrombin activation of platelets suspended in de-lipidated autologous plasma resulted in a 5.25-fold increase (P < 0.0001) in LDL oxidation. Generation of ROS and lipid peroxides by platelets can be an important mechanism through which thrombotic events contribute to atherogenesis.

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.

Human macrophage-mediated oxidation of low-density lipoprotein is delayed and independent of superoxide production

There is growing evidence that oxidatively modified low-density lipoprotein (LDL) accumulates in the atherosclerotic intima of arteries. Cells present in the intima (including the monocyte/macrophage) are capable of oxidizing LDL in vitro, but the mechanisms by which this occurs are unknown. Several reports have claimed a crucial role for superoxide as a cell-derived radical species capable of enhancing the rate of LDL oxidation. We have used a sensitive h.p.l.c. system with chemiluminescence detection to measure LDL cholesteryl ester hydroperoxides at early stages of LDL oxidation. During the initial stages of LDL oxidation, there is at least a 2 h delay before human monocyte-derived macrophages enhance this process. Stimulation of these cells to produce large fluxes of superoxide does not increase the rate of LDL oxidation or decrease the delay of its onset. Prior exposure of LDL to a high flux of superoxide does not increase its susceptibility to oxidation by human monocyte-derived macrophages. We also show that the thiobarbituric acid-reactive substances (TBARS) assay does not always correlate with more direct methods of assessing LDL oxidation and confirm recent reports that superoxide dismutase only partially inhibits cell-mediated LDL oxidation. We conclude that superoxide does not play a major role in human monocyte-derived macrophage-mediated LDL oxidation under the conditions that we describe.

Low density lipoprotein isolated from patients with essential hypertension exhibits increased propensity for oxidation and enhanced uptake by macrophages: a possible role for angiotensin II

In patients with essential hypertension, the increased risk for atherosclerosis is related not only to the blood pressure levels per se, but also to other, unknown, factors. Recent observations have indicated that oxidation of low density lipoprotein (LDL) and macrophage uptake of oxidized LDL are implicated in human atherosclerosis. We tested both the susceptibility of LDL, derived from hypertensive patients, to lipid peroxidation as well as its uptake by macrophages, in comparison with control LDL obtained from healthy subjects. The LDL that was derived from 25 patients with essential hypertension demonstrated increased propensity for lipid peroxidation with a 63%, 91% and 69% elevation in the content of the lipoprotein malondialdehyde, peroxides and conjugated dienes, respectively, in comparison with control LDL. Minimally modified LDL (MM-LDL) (prepared by 6 months' storage of the LDL at 4 degrees C) derived from the hypertensive patients also demonstrated increased lipid peroxidation with a 94%, 130% and 96% elevation in lipoprotein malondialdehyde, peroxides and conjugated dienes, respectively, compared with the control LDL. The susceptibility of the patients' LDL to lipid peroxidation decreased by 32% and 44% (measured as malondialdehyde) after 3 weeks of therapy with the angiotensin converting enzyme inhibitors captopril and enalapril, respectively, with no parallel reduction in the patients' blood pressure. The patients' LDL was shown to contain increased content of lipid peroxides and unsaturated fatty acids, which may explain its increased susceptibility to lipid peroxidation. In vitro experiments revealed that LDL can bind angiotensin II, and that angiotensin II has a stimulatory effect on copper-mediated oxidation of LDL, as well as on LDL degradation by macrophages. These results were secondary to cell-mediated oxidation of the LDL and to its cellular uptake via the scavenger receptor. We conclude that LDL derived from patients with essential hypertension is more susceptible to lipid peroxidation than control LDL, and this may be secondary to angiotensin II stimulation of LDL lipid peroxidation in these patients. Furthermore, this LDL demonstrates enhanced cellular uptake by macrophages in comparison with normal LDL which can also be related to angiotensin II-mediated LDL oxidation. Both these phenomena have been shown to be associated with accelerated atherosclerosis, and thus suggest a new mechanism for increased atherogenecity in hypertensive patients.

Prevention of platelet dysfunction by vitamin E in diabetic atherosclerosis

Premature atherosclerosis and other vascular disorders are serious complications of diabetes mellitus. Contributing factors include (i) increased peroxidation of LDL leading to foam cell formation, fatty streaks and plaque formation in the arterial wall, and (ii) hyperreactivity of blood platelets leading to increased platelet adhesion and aggregation. Vitamin E may play a protective role as an antioxidant and/or membrane stabilizing agent in either mechanism. In platelets it appears to regulate arachidonic acid metabolism. Decreased vitamin E levels in platelets are associated with increased aggregation. This is reversible by correction of the vitamin E status. In diabetics, platelet vitamin E levels tend to be reduced with concomitant increase in platelet aggregation. Several studies in patients with insulin-dependent diabetes mellitus and, to some extent, in those with non-insulin-dependent diabetes mellitus have shown that supplementation with several hundred IU vitamin E significantly reduced platelet aggregation and lipid peroxidation. In healthy volunteers high-dose supplementation had no notable effect on platelet aggregation. However, doses as low as 200 IU vitamin E significantly reduced platelet adhesion and inhibited the formation of protruding pseudopods typically occurring in activated platelets. In diabetic patients a decrease in the nonenzymatic glycation of proteins by vitamin E supplementation has been observed. Controlled studies are needed to confirm the effect of vitamin E on platelet function in well-defined groups of diabetics, followed by large-scale trials investigating the prevention of diabetic vascular complications as clinical end point.

Accelerated coronary artery disease after heart transplantation: the role of enhanced platelet aggregation and thrombosis

The present study is a prospective examination of the relationship between platelet aggregation and the occurrence of graft failure in a single cohort of heart transplantation (HT) recipients. One-hundred-and-twenty-four patients underwent platelet function study and were then followed for 1 to 24 months (mean 6.7 months). There were nine re-transplantations and 13 deaths (11 related to ischaemic events, and two others). In 15 patients, pathologic examination confirmed or revealed that recent acute myocardial infarction was the obvious cause of the graft failure. In five patients, myocardial fibrosis related to severe and diffuse coronary disease was the only microscopic finding. In the last two patients, the cause of the heart failure was not clearly identified. In recent myocardial infarction there was a high incidence (14/15) of coronary thrombi. Thrombi were multiple, disseminated in the coronary tree end of different age. Their presence at autopsy or after explantation was associated with an enhanced ex vivo platelet aggregability as compared with patients without coronary thrombi (n = 8): 43.3 +/- 1.7% of maximal aggregation vs. 34.4 +/- 2.4 (P = 0.006) and 48.4 +/- 5.2 vs. 22.6 +/- 4.9 (P = 0.003) for the primary and secondary waves of ADP-induced aggregation. These results suggest that thrombosis and platelets may play a major role in the process of accelerated coronary artery disease after HT.

Preferential inhibition of LDL oxidation by the all-trans isomer of beta-carotene in comparison with 9-cis beta-carotene

The synthetic all-trans isomer of beta-carotene was recently shown to possess antioxidant properties towards the formation of oxidized low density lipoprotein. In the present study, the binding of the all-trans and the 9-cis isomers of beta-carotene to plasma lipoproteins was investigated, and the effect of these isomers on the susceptibility of plasma lipoprotein to lipid peroxidation and on macrophage uptake of oxidized LDL were studied. Both the synthetic all-trans isomer of beta-carotene and the natural beta-carotene from the algae Dunaliella Bardawil [which is composed of the all-trans (70%) and the 9-cis (30%) isomers], were found to bind similarly to all plasma lipoproteins, following the incubation of beta-carotene with purified lipoproteins or with whole plasma. Incubation of the beta-carotene isomers with whole plasma, followed by separation of the lipoproteins, revealed substantial carotene binding to very low density lipoprotein (VLDL) and to LDL and limited binding to high density lipoprotein (HDL). Lipid peroxidation of VLDL and LDL were significantly inhibited by beta-carotene. The synthetic beta-carotene, however, was twice as effective as the Dunaliella beta-carotene in inhibiting LDL lipid peroxidation (following LDL incubation with copper ions). Cellular degradation of oxidized lipoproteins (mediated via the scavenger receptor) was decreased by 40% and 18%, respectively, when they were prepared by incubation in the presence of synthetic or natural beta-carotene; the control oxidized LDL was prepared in the absence of beta-carotene.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional properties of apolipoprotein B in chemically modified low density lipoproteins

The structural and compositional changes occurring during in vitro chemical modification of apolipoprotein B-100 (apo B), the apolipoprotein component of low density lipoproteins (LDL), were investigated in this study. The functional properties of chemically modified apo B and especially its potential to induce accumulation of cholesterol esters in macrophages were related to the structural changes of apo B. Acetylation, maleylation or malondialdehyde conjugation did not significantly affect the lipid composition of LDL. However, the unsaturated cholesteryl esters content, especially that of cholesteryl arachidonate was significantly decreased through Cu-oxidation. The number of reactive lysine residues in apo B was decreased by Cu-catalyzed LDL oxidation, acetylation, maleylation and by malondialdehyde conjugation. The number of free cysteines decreased from six in native apo B-100 to three in Cu-oxidized LDL. The tryptophan fluorescence intensity decreased most in malondialdehyde-conjugated LDL and in Cu-oxidized LDL, compared with acetylated and maleylated LDL. The secondary structure of native and chemically modified LDL was measured by attenuated total reflection infrared spectroscopy and by circular dichroism. No significant changes were observed in the secondary structure of any of the modified LDL. These data suggest that neither acetylation, malondialdehyde treatment or even Cu-oxidation substantially altered the secondary structure of apo B, in spite of significant modifications in the primary structure. Incubation of chemically modified LDL with J774 macrophages induced an accumulation of cellular cholesteryl esters and foam cell formation. The highest cholesterol accumulation was induced after malondialdehyde treatment of LDL. These data suggest that the cellular uptake and accumulation of modified LDL is not modulated by changes in the apo B structure. Rather it seems dependent upon the net charge of the apo B protein and probably involves the modification of critical lysine residues.

Proteins derived from platelet alpha granules modulate the uptake of oxidized low density lipoprotein by macrophages

Activated platelets secrete from their alpha granules a protein-like factor which stimulates the uptake of oxidized low-density lipoprotein (Ox-LDL) by macrophages. The aim of the present study was to evaluate the effect of three purified proteins obtained from platelet alpha granules: platelet-derived growth factor (PDGF), platelet factor-4 (PF-4), and beta-thromboglobulin (B-TG), on the uptake of Ox-LDL by macrophages. Cellular degradation of Ox-LDL by the J-774 A.1 macrophage-like cell line, that was preincubated for 18 h at 37 degrees C, with increasing concentrations of partially purified PDGF, (designated PDGF-CMS-III) was increased by up to 36% in comparison to control cells preincubated without PDGF. This effect was due to PDGF-mediated increase in the number of macrophage receptors for Ox-LDL. The enhanced uptake of Ox-LDL by PDGF resulted in an increase in cellular cholesterol content. Preincubation of macrophages with two types of recombinant PDGF dimers (10 ng/ml), revealed that PDGF-BB stimulated Ox-LDL cellular degradation by 64%, whereas PDGF-AB demonstrated only 34% stimulation, in comparison to control cells that were not treated with PDGF. The stimulatory effect of PDGF-CMS-III and PDGF-AB were reduced by 20% and 28%, respectively, when incubated in the presence of H-7, a specific protein kinase C inhibitor. When macrophages were preincubated with B-TG, cellular uptake of Ox-LDL was reduced by up to 30% at 100 ng B-TG/ml. This effect, however, was obtained only when B-TG was present in the incubation medium. Cellular degradation of Ox-LDL was not affected by preincubation of the cells with PF-4. Pretreatment of PCM with anti-PDGF or anti-B-TG antibodies abolished the effects of PCM on Ox-LDL degradation by macrophages. PDGF, thus, may represent the protein-like factor present in PCM which stimulates Ox-LDL degradation by macrophages, whereas B-TG may have a role in the recognition of PCM particles by the macrophage scavenger receptor. Modulation of macrophage cholesterol content by proteins secreted from activated platelets may have an important role in foam cell formation and atherosclerosis.

Involvement of the macrophage low density lipoprotein receptor-binding domains in the uptake of oxidized low density lipoprotein

Macrophages, unlike most other cells, possess both low density lipoprotein (LDL) and scavenger receptors. The scavenger receptor has been shown to mediate the uptake of oxidized LDL (ox-LDL), which ultimately leads to cholesterol loading of the macrophages. The present study was undertaken to define epitopes on ox-LDL that are important for lipoprotein binding to macrophages and to ascertain whether ox-LDL can bind to the LDL receptor. Monoclonal antibodies (Mabs) directed against several epitopes along the apolipoprotein B-100 (apo B-100) molecule were used. LDL (300 micrograms/ml) was oxidized by incubation with 10 microM CuSO4 for 24 hours. Ox-LDL, as opposed to acetylated LDL (ac-LDL), reacted with Mabs directed against the LDL receptor-binding domains (Mabs B1B6 and B1B3). Similarly, uptake of ox-LDL but not ac-LDL by a murine J774 macrophage-like cell line was inhibited by as much as 40% after using Mab B1B6. The anti-LDL receptor antibody IgG-C7 also inhibited 125I-ox-LDL uptake by macrophages by 60%. Chromatography on heparin-Sepharose columns of LDL that was partially oxidized for only 3 hours resulted in two fractions: an unbound fraction with characteristics similar to those of ox-LDL and a bound fraction similar to native LDL. Macrophage degradation of the unbound fraction was inhibited by Mab IgG-C7 and Mab B1B6, which are directed toward the LDL receptor and the LDL receptor-binding domains on apo B-100, respectively. When incubated with three types of macrophages, J774 macrophage cells, mouse peritoneal macrophages, and human monocyte-derived macrophages, excess amounts of unlabeled ox-LDL, like native LDL but unlike ac-LDL, substantially suppressed the uptake and degradation of 125I-labeled LDL. Similar studies with fibroblasts, however, revealed that unlabeled LDL but not unlabeled ox-LDL or ac-LDL competed with 125I-LDL for cellular uptake and degradation. Mab directed against epitopes on the amino terminus domain of apo B-100 (C14) demonstrates a similar immunoreactivity with ox-LDL and native LDL but a much lower reactivity with ac-LDL. Mab C14 inhibited macrophage degradation of ox-LDL by 34% but had no inhibitory effect on the uptake of native LDL or ac-LDL. Thus, the ac-LDL and LDL receptor-binding domains as well as a unique epitope on the amino terminus of apo B-100 may be involved in macrophage binding of ox-LDL.(ABSTRACT TRUNCATED AT 400 WORDS)

Lovastatin inhibits low-density lipoprotein oxidation and alters its fluidity and uptake by macrophages: in vitro and in vivo studies

Under experimental conditions, oxidized low-density lipoprotein (Ox-LDL) may possess atherogenic properties, as is evidenced by its contribution to cholesterol accumulation in macrophages. LDL was oxidized in a cell-free system by predialysis of the lipoprotein against EDTA-free buffer and the addition of copper ions. Oxidation of LDL in the presence of lovastatin (10 to 1,000 mumol/L) resulted in a time- and dose-dependent reduction in thiobarbituric-acid-reactive substances (TBARS) concentration that was accompanied by increased LDL lysine-amino-group reactivity, in comparison with Ox-LDL produced in the absence of lovastatin. At 100 mumol/L, the drug reduced malondialdehyde concentration and increased amino-group reactivity by 24% and 42%, respectively. However, lovastatin's antioxidant effect was limited relative to other antioxidants, such as probucol and vitamin E. The fluidity of Ox-LDL was substantially reduced in comparison with native LDL. However, lovastatin inhibited this reduction in fluidity by 20%. Upon incubation of J-774 macrophage-like cell line with Ox-LDL, the lovastatin-treated Ox-LDL induced a reduction in the cellular cholesterol esterification rate in comparison with the effect of Ox-LDL that was produced in the absence of the drug. In four patients with hypercholesterolemia, the effect of lovastatin therapy (20 mg/d) on the sensitivity of their LDL to in vitro oxidation was studied. In all patients, Ox-LDL prepared from LDL obtained during lovastatin treatment demonstrated a reduced TBARS content, an increased trinitrobenzenesulfonic acid (TNBS) reactivity, an increased fluidity, and an impaired uptake by macrophages. These results were similar to those obtained by adding lovastatin in vitro.

Hypolipidemic drugs reduce lipoprotein susceptibility to undergo lipid peroxidation: in vitro and ex vivo studies

Oxidized LDL, which has been discovered in vivo in areas of proximity to the atherosclerotic lesion, has been shown to enhance macrophage cholesterol accumulation. We studied the anti-oxidant potential of pravastatin, bezafibrate and cholestyramine in 18 patients with hypercholesterolemia. In addition, we examined the electrophoretic mobility and the uptake of LDL by macrophages before and after drug therapy. Pravastatin lowered plasma levels of LDL cholesterol by 57%, cholestyramine by 27% and bezafibrate by 25%. Pravastatin and bezafibrate also altered the composition of LDL as evidenced by the reduction of its cholesterol/apo B100 ratio. Pravastatin and bezafibrate reduced plasma triglyceride levels by 45% and 25%, respectively, whereas cholestyramine raised plasma triglyceride concentrations by 28%. LDL propensity for in vitro oxidation was analyzed following lipoprotein incubation with 10 microM copper ions and determination of LDL malondialdehyde (MDA), peroxides (PD) and conjugated dienes (CD) content. All drugs inhibited the susceptibility to in vitro oxidation of LDL isolated after drug therapy in comparison to LDL isolated before commencing drug therapy. Pravastatin reduced MDA content by 22%, PD by 18% and CD by 20%. Cholestyramine reduced LDL content of MDA by 41%, PD by 25% and CD by 63%. Bezafibrate reduced MDA by 41%, PD by 38% and CD by 45%. LDL vitamin E content was reduced after treatment with bezafibrate, pravastatin and cholestyramine by 49%, 36% and 8%, respectively. The electrophoretic mobility of LDL after all drug therapies was reduced in comparison to LDL obtained before therapy. Macrophage uptake of LDL assessed by either the cellular cholesterol esterification rate or by lipoprotein degradation was not affected by drug therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidation of low density lipoprotein enhances its potential to increase intracellular free calcium concentration in vascular smooth muscle cells

There have been suggestions that oxidation of low density lipoproteins (LDL) might increase their atherogenic potential. Because changes in intracellular free calcium concentration [Ca2+]i have been linked to atherogenesis, we compared the influence of oxidized LDL (Ox-LDL) and native LDL (N-LDL) on [Ca2+]i in vascular smooth muscle cells cultured from rat aortas. For determination of [Ca2+]i, fura-2 fluorescence was used. LDL was isolated by ultracentrifugation from the sera of human donors (n = 17). In N-LDL, oxidation was prevented by addition of antioxidants, whereas Ox-LDL was obtained by auto-oxidation. The extent of oxidation was assessed by measurement of thiobarbituric acid-reactive substances. Addition of Ox-LDL (20 micrograms protein/ml) to the vascular smooth muscle cells induced a mean increase of 129 +/- 13% in [Ca2+]i compared with 81 +/- 7% with N-LDL (p less than 0.01). Dose-response curves from 1 to 20 micrograms/ml (six experiments) confirmed this difference within the entire dose range. These results indicate that a more pronounced increase in [Ca2+]i induced by Ox-LDL might be one of the cellular mechanisms responsible for the higher atherogenic potential of Ox-LDL compared with N-LDL, as [Ca2+]i is an important second-messenger system involved in many atherogenic processes such as hypertrophy, cell migration, and cell damage.

Dual effect of lovastatin and simvastatin on LDL-macrophage interaction

Lovastatin and simvastatin which are very potent cellular cholesterol biosynthesis inhibitors, significantly affect the plasma lipoprotein concentration. After incubation of plasma with 14C-labelled compounds, radioactivity was found in all lipoprotein fractions but mainly (40%) in high density lipoprotein (HDL), and in the lipoprotein-deficient plasma fraction (20-30%). Drug-treated lipoproteins showed reduced electrophoretic mobility on cellulose acetate in comparison with control lipoproteins. The lovastatin-treated low density lipoprotein (LDL) displayed 28% increased fluidity in comparison with control LDL. The immunoreactivity of drug-treated LDL with monoclonal antibody directed towards the LDL receptor binding domains (B1B6) was significantly less than that of control LDL, suggesting reduced binding to the LDL receptor. When drug-treated LDL was incubated with J-774 A.1 macrophage-like cell line, its binding (at 4 degrees C) was 28% less than that of control LDL, whereas a substantial increase in the cellular cholesterol esterification rate (by 83% with lovastatin and by 67% with simvastatin) was noted. Similarly, the degradation of lovastatin and simvastatin-treated LDL by macrophages was 87-89% greater than that of control LDL. The "apparent Vmax" for the macrophage degradation of lovastatin-treated LDL was 70% greater than that for control LDL. Thus, both drugs may have a dual effect on the macrophage uptake of LDL; they may increase the number of LDL receptors on the cell surface, but they may also reduce the affinity of LDL for its receptor, the former being the major effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Simple and specific test for measuring lipid peroxides in plasma

The specificity of an iodometric assay for measuring lipid peroxides in lipoproteins was tested, compared with the fluorimetric thiobarbituric acid assay, and adopted for detecting lipid peroxide in plasma samples. Oxidation of low density lipoproteins in vitro by Cu2+, lipoxidase, and phagocytosing polymorphonuclear leucocytes was sensitively detected by the iodometric assay. Unlike the thiobarbituric acid assay, neither non-lipid substances commonly present in plasma, nor platelet or polymorphonuclear leucocyte by-products interfered with the iodometric assay. The iodometric assay measured a normal mean (SD) plasma lipid peroxide concentration of 10.8 (2.1) microM; n = 63. Two weeks after the start of a high cholesterol diet in rabbits (n = 5), a sixfold increase in plasma lipid peroxide concentrations was measured by iodometric assay. The specificity of a simple and sensitive iodometric test of lipid peroxidation was superior to that of the thiobarbituric acid assay. This iodometric assay should therefore provide a much more accurate assessment of lipid peroxide in plasma samples.

Lipid-protein particles secreted from activated platelets reduce macrophage uptake of low density lipoprotein

Cellular uptake of low density lipoprotein (LDL) was reduced by 30-40% in macrophages that were preincubated with platelet conditioned medium (PCM) obtained from activated platelets. LDL mediated cholesterol accumulation and cholesterol esterification in macrophages were substantially inhibited by macrophages preincubation with PCM. This inhibitory effect was found to be dose dependent, and resulted from a reduction in the number of LDL receptors (decrement of 35% in "apparent Vmax"). The active component in PCM was present only in medium obtained from activated platelets and was found to be of a molecular weight higher than 25,000 dalton. It comprised of both protein and cholesterol but upon PCM delipidation only the lipid fraction demonstrated the inhibitory effect on macrophage uptake of LDL. Specific uptake of the PCM lipoprotein-like particle via the scavenger receptor on macrophages was found to be essential for the expression of LDL receptor reduced activity. Furthermore, LDL mediated cholesterol esterification was not inhibited by PCM in U937 macrophages, a cell line that lacks the scavenger receptors. It is concluded that activated platelets secrete a lipoprotein-like particle which is recognized by the macrophage scavenger receptor. Subsequent to PCM-macrophage interaction, cellular LDL uptake was reduced. This effect could be attributed to the PCM lipid constituents.

Enhanced in vitro oxidation of plasma lipoproteins derived from hypercholesterolemic patients

In vitro oxidation of plasma lipoproteins, derived from either normolipidemic or hypercholesterolemic subjects, was performed in the presence of copper ions. Following this procedure, hypercholesterolemic low-density lipoprotein (LDL), very-low-density lipoprotein (VLDL), and high-density lipoprotein (HDL) demonstrated greater propensity for oxidation than the corresponding normocholesterolemic lipoproteins. The oxidation was determined by the concentration of thiobarbituric acid-reactive substances (TBARS), which was 44%, 71%, and 54% greater in the patients' VLDL, LDL, and HDL in comparison to the normocholesterolemic lipoproteins, respectively. An associated reduction in trinitrobenzensulfonic acid (TNBS) reactivity in the patients' lipoproteins was noted. These changes were consistent whether expressed per lipoprotein protein or per concentration. Macrophage cholesterol esterification induced by oxidized LDL was substantially increased (up to 59%) when patients' lipoproteins were used, in comparison to control lipoproteins. A positive correlation was present between the LDL cholesterol to protein ratio, the extent of lipoprotein oxidation, and macrophage uptake of the oxidized lipoproteins. The lipoprotein content of pro-oxidant and antioxidant constituents was also analyzed. No measurable ferric or copper ions could be found in association with any of the lipoproteins. However, arachidonic acid content of the patients' LDL was 10.1% +/- 1.0% in comparison to 6.2% +/- 0.8% of total lipoprotein fatty acids in the control group (n = 5). Antioxidants such as vitamin E and carotenoids were significantly reduced in all patients' lipoproteins compared with those of controls. Thus, we suggest that increased cholesterol and arachidonic acid content and reduced concentration of antioxidants in lipoproteins of hypercholesterolemic patients may be responsible for the enhanced propensity for oxidation observed in these lipoproteins.

The pathogenesis of atherosclerosis: an overview

In this unifying hypothesis directed to the etiology and pathogenesis of atherosclerosis, the importance of focal arterial lesion-prone sites has been emphasized. Key initial participants in these sites include the focal intimal influx and accumulation of low-density lipoprotein (LDL) and a preferential recruitment of blood monocytes. Both are further enhanced in the presence of hyperlipidemia, when the quantity of intimal LDL and the oxidative potential of the intima exceed the capacity of macrophages to remove, via the non-down-regulating scavenger receptor, cytotoxic anionic (Ox-LDL) macromolecules. Foam cells, pathognomonic of the fatty streak, form during the receptor-mediated uptake of Ox-LDL by the macrophages. Interstitial free radicals and the excess of Ox-LDL particles injure and kill cells, including the foam cells, with the formation of the necrotic extracellular lipid core, a key transitional step in lesion progression. Monocyte-macrophage recruitment to the intima is likely to be regulated not only by a multiplicity of endothelial adhesive cytokines, integrins, and selectins, but also by the monocyte-specific chemoattractant, MCP-1, constitutively synthesized and secreted by intimal smooth muscle and endothelial cells. Its synthesis and secretion is augmented by mildly oxidized LDL. Free radicals, pivotal in the oxidation of LDL, and derived from activated macrophages, and also endothelial and smooth muscle cells. Smooth muscle cells migrate from the media through the intimal endothelial layer (IEL) and proliferate under the regulation of a number of mitogens, including platelet-derived growth factor (PDGF). Collagen synthesis by smooth muscle cells is substantial. Lymphocytes, as a source of interferons, invade the plaque and are present in the adventitia in substantial numbers, likely representing an autoimmune response in the later stages of plaque development. Platelets and mural thrombosis directly contribute to subsequent plaque growth, particularly after plaque rupture or fissure and disruption of the thromboresistant endothelial cells (EC). Plaque regression in all likelihood involves the conversion of the inert pool of extracellular lipid to a metabolically active intracellular pool and subsequent clearance by the high-density lipoprotein mediated reverse cholesterol transport system. The atherogenic cascades so described conceptually represent arterial inflammatory and healing processes occurring in a hyperlipidemic environment. Many components of pathogenesis are the targets for modulation by genetic, hemodynamic and selected risk factors. The prevention and treatment of the disease should logically target reduction in plasma LDL levels, the inhibition of the oxidative modification of lipoproteins, including LDL, by free radical scavengers, and augmentation of the reverse cholesterol transport system.

Malondialdehyde affects the physico-chemical and biological characteristics of oxidized low density lipoprotein

Oxidized LDL (Ox-LDL) was found in the atherosclerotic lesion and was associated with the formation of foam cells. Ox-LDL (30 micrograms protein/ml) reduced collagen induced platelet aggregation by 18% but following dialysis, this lipoprotein enhanced platelet aggregation by 21% in platelet-rich plasma. The addition of malondialdehyde to the dialyzed Ox-LDL prevented its stimulatory activity. Enhanced macrophage uptake of Ox-LDL however, was not affected by dialysis. Thus it is important to use similar preparations of Ox-LDL when comparing results with these lipoproteins.