Beta-carotene inhibits atherosclerosis in hypercholesterolemic rabbits

Clinical trials of vitamin E in coronary artery disease: is it time to reconsider the low-density lipoprotein oxidation hypothesis?

A wide range of structurally unrelated antioxidants inhibit atherosclerosis in animal models of hypercholesterolemia, implicating oxidation of low-density lipoprotein (LDL) in the pathogenesis of atherosclerosis. However, most prospective, randomized trials of one proposed antioxidant, vitamin E, have failed to demonstrate any reduction in cardiovascular events in humans with established coronary artery disease. Recent clinical studies suggest that vitamin E is also ineffectual in the primary prevention of atherosclerosis. These observations have led many to question the relevance of LDL oxidation to the pathogenesis of human cardiovascular disease. However, vitamin E's ineffectiveness in clinical trials might result from its failure to act as a physiologically relevant antioxidant. Indeed, vitamin E does not consistently inhibit atherosclerosis in hypercholesterolemic animals, and there is remarkably little evidence that clinically relevant doses of vitamin E result in inhibition of lipid peroxidation in vivo. Collectively, these observations indicate that there is little rationale for using vitamin E to prevent coronary artery disease in humans. They also strongly suggest that it will be critically important to establish that compounds with antioxidant activity in vitro actually prevent oxidative reactions in vivo before embarking on any new clinical trials.

The use of antioxidant supplements in coronary heart disease

There is clear evidence of lipoprotein oxidation in atherosclerotic lesions. Animal studies and observational prospective human cohort studies have been interpreted as supporting a role for antioxidants in the prevention of coronary heart disease (CHD). However, firm recommendations to take antioxidant supplements to treat or prevent CHD require evidence derived from randomised controlled studies. In primary prevention studies, low dose alpha-tocopherol does not reduce the incidence of coronary events (ATBC study), and beta-carotene either has no effect or increases the incidence of coronary events and cancer death (ATBC, CARET, Physician's Health studies). Secondary preventions, those with smaller populations and shorter duration of follow up have shown some benefit from alpha-tocopherol (CHAOS, SPACE), but larger randomised studies indicate no benefit from treatment with alpha-tocopherol (HOPE, GISSI, PPP). Recent studies with antioxidant combinations also show no benefit (HATS, MPS). On the basis of these data, supplements of alpha-tocopherol and beta-carotene cannot be recommended for the treatment or prevention of CHD. Fundamental and applied research may yet find a role for antioxidant supplements in the treatment of coronary disease. However, this will require positive results from combined antioxidant studies currently in progress, and the targeting of oxidative processes that operate in the artery wall and cause or contribute to disease.

Oxidant injury and antioxidant prevention: role of dietary antioxidants, minerals, and drugs in the management of coronary heart disease (Part II)

Antioxidants, trace minerals, and certain amino acids enhance antioxidant defense of the body by improving intracellular redox status, vascular endothelial function, and nitric oxide secretion. Antioxidant supplementation has been suggested for primary and secondary prevention of coronary heart disease.

all-trans-Retinoic acid increases nitric oxide synthesis by endothelial cells: a role for the induction of dimethylarginine dimethylaminohydrolase

all-trans-Retinoic acid (atRA) has important effects on the developing and mature cardiovascular system. Nitric oxide (NO) production has been associated with the atRA-induced differentiation of neuronal cells, and we hypothesized that NO may also mediate certain actions of atRA in the cardiovascular system. We studied the effects of atRA on NO production by endothelial cells and determined whether regulation of enzymes responsible for metabolism of asymmetric dimethylarginine (ADMA) contributed to the effects seen. Murine endothelioma (sEnd.1) cells were incubated with or without atRA. Nitrite production was determined using the Griess reaction. The expression of NO synthase (NOS) and dimethylarginine dimethylaminohydrolase (DDAH) genes was determined by Northern blotting. A reporter gene assay was also used to study the effect of atRA on the DDAH II promoter. atRA significantly increased nitrite production by sEnd.1 cells despite no increase in eNOS expression. atRA also increased DDAH II gene expression and promoter activity and reduced the ratio of ADMA to symmetric dimethylarginine (SDMA) in culture medium. The DDAH inhibitor 4124W significantly reduced the induction of NO synthesis by atRA. The present study demonstrates that atRA increases NO synthesis in endothelial cells without increasing eNOS expression. atRA also increases the expression of DDAH II, the predominant DDAH isoform in endothelial cells. Our data suggests that the induction of NO synthesis by atRA may be facilitated by DDAH II. This pathway may help to explain some of the effects of atRA on the cardiovascular system.

Hibiscus protocatechuic acid or esculetin can inhibit oxidative LDL induced by either copper ion or nitric oxide donor

Oxidation of low-density lipoprotein (LDL) could increase the incidence of atherosclerosis. Previous studies have shown that copper and sodium nitroprusside (SNP) possess the ability to oxidize LDL in a dose-dependent condition. They increase the existing negative charge in LDL and increase the electrophoretic mobility. In this study, we used protocatechuic acid (PCA) and/or esculetin (ECT) to define the antioxidative activity in oxidative LDL by relative electrophoretic mobility (REM) and thiobarbituric acid-relative substances (TBARS). The data showed that ECT and PCA possessed stronger antioxidative activity than vitamin E in oxidative LDL. A previous study showed that the level of oxidative LDL can be determined by the cholesterol degradation and fragmentation of Apo B. Our results showed that Cu(2+)-mediated oxidative LDL can induce 31% cholesterol degradation and significant fragmentation of Apo B. Both PCA and ECT exhibited remarkable ability to rescue the cholesterol degradation and Apo B fragmentation. Taken together, both PCA and ECT showed strong potency to inhibit oxidative LDL induced by copper or an NO donor. Additionally, their nontoxic characteristics elevated the possibility for their use in the daily diet; and should further prevent atherosclerosis effectively.

On the anti-atherogenic effect of the antioxidant BHT in cholesterol-fed rabbits: inverse relation between serum triglycerides and atheromatous lesions

We have shown that inclusion of the antioxidant butylated hydroxytoluene (BHT) in the diet protects against development of atherosclerotic lesions in cholesterol-fed rabbits. In parallel, BHT treatment results in increased plasma triglyceride levels. The present study explores the relationship between the triglyceride-inducing and protective effects of BHT in two different studies. The combined material contains 22 rabbits fed cholesterol and 18 rabbits fed cholesterol in combination with 1% BHT. In the BHT group there was an inverse relationship between triglyceride exposure/cholesterol exposure and extent of lesions with r=0.74 (P=0.0005). Our results show that increased triglyceride exposure parallels the anti-atherogenic effect of BHT. There was no significant correlation between atheromatosis and serum BHT levels. beta-very low density lipoprotein (beta-VLDL) from cholesterol and BHT animals was triglyceride-enriched and smaller compared to beta-VLDL from cholesterol-fed animals, but there was no significant association between the anti-atherogenic effect of BHT and particle size or apolipoprotein pattern of LDL or beta-VLDL. LDL isolated from rabbits treated with cholesterol and BHT was less sensitive to oxidative modification than LDL isolated from rabbits treated with cholesterol only. In conclusion, our results demonstrate that the degree of triglyceride exposure may be an important modulator of the anti-atherogenic effect of an antioxidant.

The smooth muscle cell membrane during atherogenesis: a potential target for amlodipine in atheroprotection

BACKGROUND

Atherosclerotic disease has been present in the human population apparently from the beginning of time. However, it has only been in the 20th century that improvements in the control of infectious diseases have allowed the average life span to increase to the point where atherosclerosis has been able to affect the general population. By the middle of the 20th century, atherosclerosis had reached epidemic levels, and it is currently pandemic and increasing worldwide. Despite its growing significance to health care, we still know relatively little about the cellular basis for plaque genesis in the vessel wall. Current thinking holds that atherosclerosis is caused by an unchecked chronic inflammatory process involving the cells of the arterial wall and their interaction with LDL and various inflammatory cells. Considerable evidence suggests that the principal insults underlying atherogenesis are serum dyslipidemias and oxidative stress mediated primarily by oxidized LDL. However, just how these insults alter the cell biology of vascular cells and lead to the atherosclerotic phenotype is still under intense investigation. Moreover, recent clinical trials have provided evidence that certain classes of drugs, including newer calcium channel blockers (CCBs), can remodel the arterial smooth muscle cell (SMC) membrane and inhibit the progression of atherosclerotic disease.

METHODS

This review summarizes our current thinking on atherogenesis in the arterial SMC and considers recent developments regarding alterations in the SMC membrane during the very early period of atherogenesis. We also discuss how certain CCBs might operate to produce atheroprotection.

RESULTS

The SMC membrane becomes enriched in unesterified cholesterol soon after the development of serum hypercholesterolemia. With excess membrane cholesterol, the membrane becomes thicker and develops distinct cholesterol domains. These alterations in the membrane increase the permeability of SMC to calcium and induce a variety of alterations in SMC function that contribute to cellular atherogenic processes during plaque genesis. Amlodipine, a third-generation CCB, markedly inhibits the progression of lesions. The explanation of this novel action may lie in the effects of this drug on various potential cellular targets.

CONCLUSIONS

Evidence is accumulating that excess membrane cholesterol may contribute to the cellular defects responsible for the transformation of the SMC to the atherosclerotic phenotype. Amlodipine, which has membrane-remodeling properties, is emerging as an important atheroprotective drug.

Effects of high-fat, low-cholesterol diets on hepatic lipid peroxidation and antioxidants in apolipoprotein E-deficient mice

The present study describes the effects of several high-fat low-cholesterol antiatherogenic diets on the hepatic lipid peroxidation and hepatic antioxidant systems in apolipoprotein E-deficient mice. Eighty mice were distributed into five groups and fed with regular mouse chow or chow supplemented with coconut, palm, olive and sunflower seed oils. After ten weeks, they were sacrificed and the livers were removed so that lipid peroxidation and alpha-tocopherol concentrations, and superoxide dismutase, glutathione peroxidase and glutathione reductase activities could be measured. The size of the atherosclerotic lesions in the aortas was also measured. Results showed that the diets supplemented with olive oil, palm oil or sunflower seed oil significantly decreased the size of the lesion. However, there was an association between those mice that were on diets supplemented with palm or coconut oils and a significant increase in hepatic lipid peroxidation. This association was not found in animals fed with olive or sunflower seed oils, the diets with the highest content of vitamin E. The dietary content of vitamin E was significantly correlated (r = 0.98; p < 0.05) with the hepatic concentration of this compound. Our study suggests that the high content of vitamin E in olive oil or sunflower seed oil may protect from the undesirable hepatotoxic effects of high-fat diets in apo E-deficient mice and that this should be taken into account when these diets are used to prevent atherosclerosis.

High plasma levels of alpha- and beta-carotene are associated with a lower risk of atherosclerosis: results from the Bruneck study

BACKGROUND AND PURPOSE

A large number of studies have contributed to the hypothesis that carotenoids, vitamins A and E are protective against atherosclerosis by acting as antioxidants. The aim of this study was to assess the relationship between plasma levels of carotenoids (alpha- and beta- carotene, lutein, lycopene, zeaxanthin, beta-cryptoxanthin), vitamins A and E, and atherosclerosis in the carotid and femoral arteries.

METHODS

This prospective and cross sectional study involved a randomly selected population sample of 392 men and women aged 45-65 years. Carotid and femoral artery atherosclerosis was assessed by high-resolution duplex ultrasound.

RESULTS

alpha- and beta- carotene plasma levels were inversely associated with the prevalence of atherosclerosis in the carotid and femoral arteries (P=0.004) and with the 5-year incidence of atherosclerotic lesions in the carotid arteries (P=0.04). These findings were obtained after adjustment for other cardiovascular risk factors (sex, age, LDL (low density lipoproteins), ferritin, systolic blood pressure, smoking, categories of alcohol consumption, social status, C-reactive protein). Atherosclerosis risk gradually decreased with increasing plasma alpha- and beta-carotene concentrations (P=0.004). No associations were found between vitamin A and E plasma levels and atherosclerosis.

CONCLUSIONS

This study provides further epidemiological evidence of a protective role of high alpha- and beta- carotene in early atherogenesis.

Palm tocotrienols protect ApoE +/- mice from diet-induced atheroma formation

We evaluated the effects of vitamin E and beta-carotene on apolipoprotein (apo)E +/- female mice, which develop atherosclerosis only when fed diets high in triglyceride and cholesterol. Mice were fed a nonpurified control diet (5.3 g/100 g triglyceride, 0.2 g/100 g cholesterol), an atherogenic diet alone (15.8 g/100 g triglyceride, 1.25 g/100 g cholesterol, 0.5 g/100 g Na cholate) or the atherogenic diet supplemented with either 0.5 g/100 g (+)-alpha-tocopherol (mixed isomers); 0.5 g/100 g palm tocopherols (palm-E; 33% alpha-tocopherol, 16.1% alpha-tocotrienol, 2.3% beta-tocotrienol, 32.2% gamma-tocotrienol, 16.1% delta-tocotrienol); 1.5 g/100 g palm-E; or 0.01 g/100 g palm-carotenoids (58% beta-carotene, 33% alpha-carotene, 9% other carotenoids). Compared with mice fed the control diet, plasma cholesterol was fourfold greater in mice fed the atherogenic diet. Mice fed the 1.5 g/100 g palm-E supplement had 60% lower plasma cholesterol than groups fed the other atherogenic diets. Mice fed the atherogenic diet had markedly higher VLDL, intermediate density lipoprotein (IDL) and LDL cholesterol and markedly lower HDL cholesterol than the controls. Lipoprotein patterns in mice supplemented with alpha-tocopherol or palm carotenoids were similar to those of the mice fed the atherogenic diet alone, but the pattern in mice supplemented with 1. 5 g/100 g palm-E was similar to that of mice fed the control diet. In mice fed the atherogenic diet, the hepatic cholesterol plus cholesterol ester concentration was 4.4-fold greater than in mice fed the control diet. Supplementing with 1.5 g/100 g palm-E lowered hepatic cholesterol plus cholesterol ester concentration 66% compared with the atherogenic diet alone. Mice fed the atherogenic diet had large atherosclerotic lesions at the level of the aortic valve. With supplements of 0.5 g/100 g palm-E or 1.5 g/100 g palm-E, the size of the lesions was 92 or 98% smaller, respectively. The 0.5 g/100 g alpha-tocopherol and palm carotenoid supplements had no effect. Supplements did not alter mRNA abundance for apolipoproteins A1, E, and C3. The beneficial effect of tocotrienols on atherogenesis, the plasma lipoprotein profile and accumulation of hepatic cholesterol esters cannot be attributed to their antioxidant properties.

The oxidative modification hypothesis of atherogenesis: an overview

The literature relating lipid and lipoprotein oxidation to atherosclerosis has expanded enormously in recent years. Papers on the "oxidative modification hypothesis" of atherogenesis have ranged from the most basic studies of the chemistry and enzymology of LDL oxidation, through studies of the biological effects of oxidized LDL on cultured cells, and on to in vivo studies of the effects of antioxidants on atherosclerosis in animals and humans. The data in support of this theory are mounting but many key questions remain unanswered. For example, while it is generally agreed that LDL undergoes oxidation and that oxidized LDL is present in arterial lesions, it is still not known how and where LDL gets oxidized in vivo nor which of its many biological effects demonstrable in vitro are relevant to atherogenesis in vivo. This brief review is not intended to be comprehensive but rather to offer a perspective and a context for this Forum. We discuss the strengths and weaknesses of each line of evidence, try to identify areas in which further research is needed, assess the relevance of the hypothesis to the human disease, and point to some of the potential targets for therapy.

Competitive carotenoid and cholesterol incorporation into liposomes: effects on membrane phase transition, fluidity, polarity and anisotropy

Pure 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) or mixed DPPC:1,2-dipalmitoyl phosphatidyletanolamine (DPPE):1,2-dipalmitoyl diphosphatidylserine (DPPS) (17:5:3) liposomes were incorporated with 5 mol% dietary carotenoids (beta-carotene, lutein and zeaxanthin) or with cholesterol (16 and 48 mol%) in the absence or presence of 15 mol% carotenoids, respectively. The carotenoid incorporation yields ranged from 0.42 in pure to 0.72 in mixed phospholipid liposomes. They decreased significantly, from 3 to 14%, in the corresponding cholesterol-doped liposomes, respectively. Highest incorporation yields were achieved by zeaxanthin and lutein in phospholipid liposomes while in cholesterol-containing liposomes, lutein was highest incorporated. The effects on membrane structure and dynamics were determined by differential scanning calorimetry, steady-state fluorescence and anisotropy measurements. Polar carotenoids and cholesterol cause similar, dose-dependent effects: ordering and rigidification revealed by broadening of the transition peak, and increase of anisotropy. Membrane hydrophobicity is determined by cholesterol content and carotenoid polarity. In cholesterol-doped liposomes, beta-carotene is less incorporated than in cholesterol-free liposomes. Our observations suggest effects of carotenoids, even at much lower effective concentrations than cholesterol (8 to 80-fold), on membrane structure and dynamics. Although they are minor constituents of animal membranes, carotenoids may act as modulators of membrane phase transition, fluidity, polarity and permeability, and therefore, can influence the membrane physiology and pathology.

The effect of carotenoids on the expression of cell surface adhesion molecules and binding of monocytes to human aortic endothelial cells

Several large epidemiological studies have shown a correlation between elevated plasma carotenoid levels and decreased risk of cardiovascular disease (CVD). One proposed mechanism for the beneficial effect of carotenoids is through functional modulation of potentially atherogenic processes associated with the vascular endothelium. To test this, we incubated confluent human aortic endothelial cell (HAEC) cultures (passages 4-8) for 24 h with each of the five most prevalent carotenoids in human plasma, which are alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, and lycopene, at an approximate concentration of 1 micromol/l. Carotenoids were solubilized in 0.7% (v/v) tetrahydrofuran and incorporated into FBS before adding to cell culture medium. Due to disparate solubilities in aqueous medium, final concentrations of alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, and lycopene were 1.7, 1.1, 0.7, 0.9, and 0.3 micromol/l and monolayers accumulated 647, 158, 7, 113, and 9 pmol/mg protein, respectively. Monolayers were then stimulated with IL-1beta (5 ng/ml) for 6 h with subsequent determination of cell surface expression of adhesion molecules as measured by an enzyme-linked immunosorbent assay (ELISA). To assess endothelial cell adhesion to monocytes, IL-1beta-stimulated monolayers were incubated for 10 min with 51Cr-labeled U937 monocytic cells and adhesion determined by isotope counting. Pre-incubation of HAEC with beta-carotene, lutein and lycopene significantly reduced VCAM-1 expression by 29, 28, and 13%, respectively. Pre-incubation with beta-carotene and lutein significantly reduced E-selectin expression by 38 and 34%, respectively. Pre-treatment with beta-carotene, lutein and lycopene significantly reduced the expression of ICAM-1 by 11, 14, and 18%, respectively. While other carotenoids were ineffective, lycopene attenuated both IL-1beta-stimulated and spontaneous HAEC adhesion to U937 monocytic cells by 20 and 25%, respectively. Thus, among the carotenoids, lycopene appears to be most effective in reducing both HAEC adhesion to monocytes and expression of adhesion molecules on the cell surface.

Hyperbaric oxygen reduces the progression and accelerates the regression of atherosclerosis in rabbits

We studied the effect of hyperbaric oxygen (HBO) treatment on the extent of diet-induced accumulation of lipid oxidation products in rabbit plasma and tissues, on plasma paraoxonase activity, and on the extent of progression and regression of atherosclerotic lesions in the rabbit aorta. HBO treatment of cholesterol-fed rabbits dramatically reduces the development of arterial lesions despite having little or no effect on plasma or individual lipoprotein cholesterol concentrations. Compared with no treatment in cholesterol-fed animals, HBO treatment also substantially reduces the accumulation of lipid oxidation products (conjugated dienes, trienes, and thiobarbituric acid-reactive substances) in plasma, in the low density lipoprotein and high density lipoprotein fractions of plasma, in the liver, and in the aortic tissues. In addition, HBO treatment prevents the decrease in plasma paraoxonase activity observed in rabbits fed cholesterol-rich diets. Similarly, in regression studies, HBO treatment has no effect on the rate of plasma (or lipoprotein) cholesterol decline but significantly accelerates aortic lesion regression compared with no treatment. Direct measures of aortic cholesterol content support these morphological observations. On the basis of these results, we conclude that repeated, but relatively short, exposure to HBO induces an antioxidant defense mechanism(s) that is responsible for retarding the development or accelerating the regression of atherosclerotic lesions.

High density lipoprotein oxidation: in vitro susceptibility and potential in vivo consequences

Elevated levels of plasma high density lipoprotein (HDL) are strongly predictive of protection against atherosclerotic vascular disease. HDL particles likely have several beneficial actions in vivo, including the initiation of reverse cholesterol transport. The apparent importance of oxidative modification of low density lipoprotein in atherogenesis raises the question of how oxidative modification of HDL might affect its cardioprotective actions. HDL is readily oxidized using numerous models of lipoprotein oxidation. In vitro evidence suggests oxidation might impair some protective actions, but actually enhance other mechanisms induced by HDL that prevent the accumulation of cholesterol in the artery wall. This article reviews the current literature concerning the relative oxidizability of HDL, the structural changes induced in HDL by oxidation in vitro, and the potential consequences of oxidative modification on the protective actions of HDL in vivo.

Inhibitory effect of Chinese green tea on endothelial cell-induced LDL oxidation

Green tea has been shown to inhibit Cu(2+)-induced LDL oxidation and suppress lipoxygenase activity. Since LDL oxidation is a characteristic feature of atherogenesis and lipoxygenase is involved in the disease process, the effect of Lung Chen Tea, a non-fermented Chinese green tea, on LDL oxidation induced by human umbilical cord vascular endothelial cell was investigated in the present study. Lung Chen Tea was extracted with methanol and the dried powder was redissolved in water before extraction with chloroform and then ethyl acetate. Lung Chen Tea, chloroform and ethyl acetate fractions dose-dependently reduced LDL oxidation and decreased its relative electrophoretic mobility (P<0.001) when compared to the oxidized LDL. The lipid peroxidation products, thiobarbituric acid reactive substances, and cellular cholesterol were also significantly lowered by 5 and 10 microg/ml Lung Chen Tea (P<0.001) in a dose-dependent manner. The remaining aqueous layer, which was devoid of catechins after chloroform and ethyl acetate extractions, did not prevent LDL oxidation. The results of this study demonstrated that Lung Chen Tea and catechin-rich fractions significantly prevented endothelial cell induced LDL oxidation. The consumption of Lung Chen Tea may therefore lower the risk of coronary heart diseases.

Redox control of vascular nitric oxide bioavailability

NO is an important component of vascular homeostasis and abnormal NO bioactivity has been implicated in number of disease states with important public health implications. One clear mechanism of impaired NO bioactivity and vascular disease is excess vascular oxidative stress. There is now a wealth of developing data that manipulation of vascular antioxidant stress is the considerable influence of the biologic activity of endothelium-derived NO. It remains to be seen if this influence can be exploited in a manner that truly alters the course of human disease.

Dietary and pharmacological antioxidants in atherosclerosis

Antioxidants that inhibit LDL oxidation are thought to be potential anti-atherogenic compounds. The results of major human randomized trials with antioxidants have, however, been disappointing, except for probucol, which consistently inhibits restenosis. Similarly, animal intervention studies show that antioxidants do not generally inhibit atherosclerosis, although some compounds provide protection. Direct evidence for the oxidation of LDL causing atherosclerosis is needed. This article summarizes results from antioxidant intervention studies, and highlights some of the key issues that need to be addressed to link biochemical changes in the arterial wall more directly to the oxidation theory of atherosclerosis.

Beta-carotene and other carotenoids as antioxidants

Carotenoids are natural pigments which are synthesized by plants and are responsible for the bright colors of various fruits and vegetables. There are several dozen carotenoids in the foods that we eat, and most of these carotenoids have antioxidant activity. Beta-carotene has been best studied since, in most countries it is the most common carotenoid in fruits and vegetables. However, in the U.S., lycopene from tomatoes now is consumed in approximately the same amount as beta-carotene. Antioxidants (including carotenoids) have been studied for their ability to prevent chronic disease. Beta-carotene and others carotenoids have antioxidant properties in vitro and in animal models. Mixtures of carotenoids or associations with others antioxidants (e.g. vitamin E) can increase their activity against free radicals. The use of animals models for studying carotenoids is limited since most of the animals do not absorb or metabolize carotenoids similarly to humans. Epidemiologic studies have shown an inverse relationship between presence of various cancers and dietary carotenoids or blood carotenoid levels. However, three out of four intervention trials using high dose beta-carotene supplements did not show protective effects against cancer or cardiovascular disease. Rather, the high risk population (smokers and asbestos workers) in these intervention trials showed an increase in cancer and angina cases. It appears that carotenoids (including beta-carotene) can promote health when taken at dietary levels, but may have adverse effects when taken in high dose by subjects who smoke or who have been exposed to asbestos. It will be the task of ongoing and future studies to define the populations that can benefit from carotenoids and to define the proper doses, lengths of treatment, and whether mixtures, rather than single carotenoids (e.g. beta-carotene) are more advantageous.

Intracellular generation of MDA-LYS epitope in foam cells

Oxidative stress plays a central role in atherogenesis. Antioxidants, such as probucol, inhibit oxidation of LDL, retard secretion of interleukin-1, growth factors and chemoattractants, and thus inhibit progression of atherosclerosis. Other antioxidants with an ability to inhibit LDL oxidation, however, could not prevent progression of atherosclerosis. The inconsistency between antioxidant potencies indicated oxidative events might have occurred at locations other than LDL. MDA-lysine epitope (MDA-lys) is closely associated with atherogenesis and was recognized as marker for oxidation. We traced formation of MDA-lys during oxidation of LDL and formation of foam cells. The results indicated that thiobarbituric acid reactive substance (TBARS) was primarily present in lipid fraction of ox-LDL not associated with protein fraction after Cu2+ oxidation in vitro. Oxidized LDL did not increase significant immunoreactivity of MDA-lys epitope under our experimental conditions. Foam cells, however, showed the presence of MDA-lys epitope suggesting that intracellular oxidation events occurred to internalized lipids. The uptake of non-oxidatively modified LDL (acetylated LDL) was sufficient to generate MDA-lys epitope in foam cells, consistent with the hypothesis that atherosclerosis is associated with oxidative events in addition to LDL oxidation. We hypothesized that MDA-lys may be generated through intracellular lipid metabolism during the formation of foam cells.

Dissociation of atherogenesis from aortic accumulation of lipid hydro(pero)xides in Watanabe heritable hyperlipidemic rabbits

Antioxidants can inhibit atherosclerosis, but it is unclear how inhibition of intimal lipid oxidation relates to atherogenesis. Here we tested the effect of probucol and its metabolite bisphenol on aortic lipid (per)oxidation and atherogenesis in Watanabe heritable hyperlipidemic (WHHL) rabbits. LDL and aortas from rabbits fed probucol contained bisphenol at concentrations comparable to those in bisphenol-treated animals. Bisphenol treatment increased plasma cholesterol slightly, and plasma and aortic alpha-tocopherol more substantially; these parameters were unaffected by probucol. Bisphenol and probucol treatment both enhanced the resistance of circulating LDL to peroxyl radical-induced lipid peroxidation; this was due to bisphenol, not probucol. Only probucol enhanced LDL's resistance to Cu(2+)-induced oxidation. Both bisphenol and probucol treatment strongly inhibited aortic accumulation of hydroperoxides and hydroxides of cholesteryl esters and triglycerides [LO(O)H]. Despite this, however, probucol had a modestly significant effect on the extent of lesion formation; bisphenol had no inhibitory effect. In addition, the extent of atherosclerosis did not correlate with amounts of aortic LO(O)H present, but, as expected, it did correlate with aortic alpha-tocopherol and cholesterol. Together, these results suggest that aortic accumulation of LO(O)H is not required for, nor is alpha-tocopherol depleted during, the initiation and progression of atherogenesis in WHHL rabbits.

Antiatherosclerotic activity of drugs in relation to nitric oxide function

Many studies have shown that loss of endothelium-derived nitric oxide is a major factor of ischemic episodes in patients with coronary artery disease and there is increasing evidence to suggest that nitric oxide might exert antiatherosclerotic actions. Based on these concepts, the results of animal studies on the effects of lipid lowering drugs, antioxidants, angiotensin converting enzyme inhibitors, Ca2+ channel blockers, estrogens and agents which modulate nitric oxide bioavailability are presented and compared to the results of patient studies and clinical trials. In spite of encouraging results obtained with antioxidants in animals, clinical trials could only show a clear positive effect of vitamin E treatment on the outcome of cardiovascular disease. Angiotensin converting enzyme inhibitors can ameliorate endothelial dysfunction in coronary heart disease, but their impact on disease progression remains unclear. There is evidence that estrogen replacement therapy in post-menopausal women may increase the bioavailability of nitric oxide. Finally, improved endothelial function and plaque stability clearly contribute to the clinical benefits of lipid lowering interventions, statins in particular. Taken together, these studies lend support to the concept that improving endothelial function and nitric oxide release might serve as valuable elements in the prevention or therapy of cardiovascular disease.

Dietary beta-carotene and alpha-tocopherol combination does not inhibit atherogenesis in an ApoE-deficient mouse model

Although lipid oxidation plays a major role in atherogenesis, the role of antioxidants in the prevention and treatment of the process is not clear. Apolipoprotein (apo) E-deficient mice develop spontaneous atherosclerotic lesions in major arteries. The presence of oxidized lipoprotein epitopes in the lesion suggests that oxidation reactions are involved in atherogenesis in this mouse model, but the inhibitory effect of antioxidants on atherogenesis in the model is controversial. To test the effect of dietary antioxidants on atherogenesis, male apoE-deficient mice (n=15) were fed a standard chow diet supplemented with 0.05% alpha-tocopherol and 0.05% all-trans beta-carotene. A control group (n=15) received no antioxidant supplement. At the end of the trial, mice consuming vitamins had 5x more plasma vitamin E but undetectable beta-carotene levels. However, liver levels of the beta-carotene metabolite, retinyl palmitate, were higher in antioxidant-treated mice compared with control mice. The antioxidants had no effect on lipoprotein or on plasma anti-oxidatively modified low density lipoproteins (anti-oxLDL) antibody levels. The vitamins had a small but insignificant effect on lipoprotein resistance to ex vivo oxidation, determined by a longer lag period of conjugated diene formation. Atherosclerosis, determined by the lesion size at the aortic sinus, was insignificantly suppressed in antioxidant-treated mice (mean area+/-SE, 20 000+/-7129 versus 13 281+/-5861 micrometer(2); P=0.40). The aortic atherosclerotic lesion area was similar in both experimental groups (2.55+/-0.65% and 2.08+/-0.5% of total aortic area in the control and antioxidant group, respectively; P=0.58). The results of the current study suggest that moderate levels of synthetic antioxidant vitamins have no effect on atherogenesis in apoE-deficient mice.

Antioxidants, but not B-group vitamins increase the resistance of low-density lipoprotein to oxidation: a randomized, factorial design, placebo-controlled trial

We have conducted an intervention trial to assess the effects of antioxidants and B-group vitamins on the susceptibility of low-density lipoprotein (LDL) to oxidation. A total of 509 men aged 30-49 from a local workforce were screened for total plasma homocysteine. The 132 selected (homocysteine concentration > or = 8.34 mumol/l) men were randomly assigned, using a factorial design, to one of four groups receiving supplementation with B group vitamins alone (1 mg folic acid, 7.2 mg pyridoxine, 0.02 mg cyanocobalamin), antioxidant vitamins (150 mg ascorbic acid, 67 mg alpha-tocopherol, 9 mg beta-carotene), B vitamins with antioxidant vitamins, or placebo. Intervention was double-blind. A total of 101 men completed the 8-week study. The lag time of LDL isolated ex vivo to oxidation (induced by 2 mumol/l cupric chloride) was increased in the two groups receiving antioxidants whether with (6.88 +/- 1.65 min) or without (8.51 +/- 1.77 min) B-vitamins, compared with placebo (-2.03 +/- 1.50) or B-vitamins alone (-3.34 +/- 1.08) (Mean +/- S.E., P < 0.001). Antibodies to malondialdehyde (MDA) modified LDL were also measured, but there were no significant changes in titers of these antibodies in any group of subjects whether receiving antioxidants or not. Contrast analysis showed that there was no interaction between antioxidants and B-group vitamins. This study indicates that while B-group vitamins lower plasma homocysteine they do not have an antioxidant effect. Thus B-group vitamins and antioxidants appear to have separate, independent effects in reducing cardiovascular risk.

Probucol enhances selective uptake of HDL-associated cholesteryl esters in vitro by a scavenger receptor B-I-dependent mechanism

Recently, the class B, type I scavenger receptor (SR-BI) has been shown to mediate the selective uptake of high density lipoprotein (HDL) cholesteryl esters (CEs), ie, lipid uptake independent of HDL holoparticle uptake. In vivo, this selective uptake delivers CEs to the liver for excretion and to steroidogenic tissues for hormone synthesis. Probucol, a hydrophobic antioxidant drug, lowers plasma cholesterol in humans and rodents and may inhibit progression of atherosclerosis and postangioplasty restenosis. In this study, the effect of probucol on HDL selective CE uptake was investigated in mice and in cells expressing SR-BI. Probucol feeding lowered plasma HDL cholesterol and markedly increased selective CE uptake from HDL in the liver and adrenal glands. However, probucol did not alter SR-BI protein levels in membranes from these organs. When incubated with control Chinese hamster ovary (CHO) cells, HDL isolated from probucol-treated mice (P-HDL) and HDL from control mice (C-HDL) showed similar low selective uptake of CEs. However, when incubated with SR-BI-transfected CHO cells, P-HDL showed a 2-fold increase in selective uptake compared with C-HDL. In an adrenal cell line (Y1-BS1), which expresses SR-BI in an adrenocorticotropic hormone-inducible manner, P-HDL showed significantly greater selective CE uptake than did C-HDL, and the differential response was amplified by adrenocorticotropic hormone treatment. In contrast to P-HDL, incorporation of this compound into HDL in vitro did not result in stimulation of selective CE uptake by SR-BI-transfected CHO cells, even though a significant mass of probucol could be detected in the HDL preparation. The specific interaction of P-HDL with SR-BI in cell culture could be observed after only 24 hours of probucol feeding, when there were minimal changes in HDL size and composition. Thus, probucol or one of its metabolites increases selective CE uptake in vivo by modifying HDL in a way that causes enhanced interaction with SR-BI. The increased interaction of P-HDL with SR-BI in the liver and arterial wall may be partly responsible for the effects of probucol on atherosclerosis and restenosis.

Inhibition by a coantioxidant of aortic lipoprotein lipid peroxidation and atherosclerosis in apolipoprotein E and low density lipoprotein receptor gene double knockout mice

Antioxidants can inhibit atherosclerosis in animals, though it is not clear whether this is due to the inhibition of aortic lipoprotein lipid (per)oxidation. Coantioxidants inhibit radical-induced, tocopherol-mediated peroxidation of lipids in lipoproteins through elimination of tocopheroxyl radical. Here we tested the effect of the bisphenolic probucol metabolite and coantioxidant H 212/43 on atherogenesis in apolipoprotein E and low density lipoprotein (LDL) receptor gene double knockout (apoE-/-;LDLr-/-) mice, and how this related to aortic lipid (per)oxidation measured by specific HPLC analyses. Dietary supplementation with H 212/43 resulted in circulating drug levels of approximately 200 microM, increased plasma total cholesterol slightly and decreased plasma and aortic alpha-tocopherol significantly relative to age-matched control mice. Treatment with H 212/43 increased the antioxidant capacity of plasma, as indicated by prolonged inhibition of peroxyl radical-induced, ex vivo lipid peroxidation. Aortic tissue from control apoE-/-;LDLr-/- mice contained lipid hydro(pero)xides and substantial atherosclerotic lesions, both of which were decreased strongly by supplementation of the animals with H 212/43. The results show that a coantioxidant effectively inhibits in vivo lipid peroxidation and atherosclerosis in apoE-/-;LDLr-/- mice, consistent with though not proving a causal relationship between aortic lipoprotein lipid oxidation and atherosclerosis in this model of the disease.

Oxidized amino acids in the urine of aging rats: potential markers for assessing oxidative stress in vivo

Oxidative damage of proteins has been implicated in disease and aging. In vitro studies demonstrate that two unnatural amino acids, o,o'-dityrosine and o-tyrosine, are stable markers of protein oxidation. We have investigated the possibility that assaying these compounds in urine could provide a noninvasive way to determine levels of protein oxidation in vivo. Isotope dilution gas chromatography-mass spectrometry was used to quantify levels of o,o'-dityrosine and o-tyrosine in skeletal muscle and urine of aging rats subjected to two interventions: 1) dietary antioxidant supplementation and 2) exercise training. In both sedentary rats and exercise-trained rats, antioxidant therapy reduced levels of protein-bound o,o'-dityrosine in skeletal muscle. In contrast, antioxidant therapy or exercise training minimally affected o-tyrosine levels in this tissue. Levels of the oxidized amino acids in urine samples mirrored those of skeletal muscle proteins. Quantification of the levels of oxidized amino acids in urine may thus serve as a noninvasive measure of oxidative stress in vivo because they change in parallel with levels of protein-bound oxidized amino acids in skeletal muscle.

Oxidants and antioxidants in the pathogenesis of atherosclerosis: implications for the oxidized low density lipoprotein hypothesis

The oxidation hypothesis proposes that low density lipoprotein must be oxidatively modified to trigger the pathological events of atherosclerosis. In this article, we evaluate recent studies addressing the pathways that promote low density lipoprotein oxidation in vivo and the impact of antioxidants on atherogenesis in animals, paying particular attention to the clinical implications of these studies for the oxidation hypothesis.

Varying dietary fat type of reduced-fat diets has little effect on the susceptibility of LDL to oxidative modification in moderately hypercholesterolemic subjects

The effect of the fatty acid composition of reduced-fat diets on the in vitro oxidation of LDL was examined in 14 moderately hypercholesterolemic [low density lipoprotein (LDL) > 3.36 mmol/L] postmenopausal female and male subjects (age 44-78 y). Each subject consumed each of five reduced-fat diets [30 energy percent (E%) fat, 17 E% protein and 53 E% carbohydrate] enriched in beef tallow, canola oil, corn oil, olive oil or rice bran oil (20 E%) for 32-d periods. In vitro oxidation of LDL was assessed by incubating LDL with hemin and hydrogen peroxide, and measuring the time required for the reaction to reach maximum velocity (lag time). LDL lag times were 93.2 +/- 25.8, 95.9 +/- 26.4, 104.2 +/- 32.7, 108.0 +/- 26.6 and 113.1 +/- 24.0 min for corn oil, beef tallow, rice bran oil, canola oil and olive oil periods, respectively. When the data from all dietary phases were pooled, LDL alpha-tocopherol level (r = 0.30, P = 0.01) and plasma 18:1/18:2 ratio (r = 0.22, P = 0.08) were positively related to resistance of LDL to oxidation. Differences induced by the dietary perturbations in LDL content of beta-cryptoxanthin, lutein/zeaxanthin, lycopene, alpha-carotene or beta-carotene, and LDL particle size were not related to resistance of LDL to oxidation. In conclusion, in middle-aged and elderly moderately hypercholesterolemic subjects, the consumption of reduced-fat diets enriched in animal fat or vegetable oils with a relatively wide range of fatty acid profiles did not alter the in vitro susceptibility of LDL to oxidation. The advantages of reducing the saturated fat content of the diet were reflected in lower total and LDL cholesterol levels.

all-Trans-retinoic acid reduces neointimal formation and promotes favorable geometric remodeling of the rat carotid artery after balloon withdrawal injury

BACKGROUND

The multifactorial and unpredictable nature of human restenosis will probably necessitate interventional strategies that target multiple processes involved in acute vascular narrowing. Retinoids (eg, all-trans-retinoic acid, atRA) represent a growing class of pleiotropic biological response modifiers with demonstrable efficacy in managing several pathological conditions. In this report, we have initiated studies to examine the hypothesis that atRA limits neointimal formation after experimental vascular injury.

METHODS AND RESULTS

Rats were predosed with atRA (30 mg . kg-1 . d-1 PO) or corn oil 4 days before balloon withdrawal injury (BWI) of the left common carotid artery and continued on this drug regimen for an additional 14 days. High-performance liquid chromatographic analysis documented therapeutic levels of atRA in serum and vascular tissue. atRA depressed peak DNA synthesis in the tunica media of BWI vessels (P<0.05). Histomorphometry revealed atRA-mediated reductions in neointimal area, neointimal cell number, and intimal/medial area ratio as well as significant increases in vessel wall perimeter (P<0. 05). Such changes in vascular architecture contributed to a 35% to 37% increase in the luminal area of BWI vessels exposed to atRA (P<0. 005 compared with controls).

CONCLUSIONS

atRA reduces neointimal mass and elicits favorable geometric remodeling of the injured rat carotid artery.

Dietary antioxidants inhibit development of fatty streak lesions in the LDL receptor-deficient mouse

Oxidized low density lipoprotein (LDL) promotes atherogenesis. Although pharmacological antioxidants such as probucol inhibit both LDL oxidation and atherosclerosis in hyperlipidemic animals, the effects of natural antioxidants such as vitamin E are inconclusive. To further determine the effects of supplemental dietary antioxidants in vivo, we evaluated whether combined dietary antioxidants (0.1% vitamin E, 0.5% beta-carotene, and 0.05% vitamin C) inhibit LDL oxidation and fatty streak lesion development in homozygous LDL receptor-null (LDLR-/-) mice fed a high-fat, high-cholesterol diet. An additional group of mice were fed black tea, which has been shown to inhibit LDL oxidation in vitro. After receiving a high-fat, high-cholesterol diet for 8 weeks, the combined antioxidant-supplemented (antioxidant) group (n=18), tea group (n=19), and control group (n=17) had equivalent plasma cholesterol levels. LDL oxidation, as measured by the lag phase of conjugated diene formation, was markedly inhibited in the antioxidant group compared with the tea or control groups [mean lag phases=143+/-7 (antioxidant), 100+/-5 (tea), and 84+/-4 (control) minutes; P<0.0001 antioxidant versus tea or control]. The cross-sectional surface area of fatty streak lesions in the aortic sinus was reduced by 60% in the antioxidant group compared with both the tea and control groups (P<0.0001 antioxidant versus tea or control). There was no difference in lesion area between tea and control groups. Although both LDL oxidation and atherosclerosis were significantly inhibited in the antioxidant group, no correlation between lag phase values and lesion size was observed among individual animals. Furthermore, black tea did not inhibit fatty streak development in LDLR-/- mice. These data suggest that combined natural dietary antioxidants inhibit both LDL oxidation and atherogenesis in animals with elevated LDL but that inhibition of LDL oxidation alone may not prevent the development of atherosclerosis.

Vitamin E combined with selenium inhibits atherosclerosis in hypercholesterolemic rabbits independently of effects on plasma cholesterol concentrations

Several antioxidants inhibit atherosclerosis. This study investigated the hypothesis that combining vitamin E, a lipophilic antioxidant, with vitamin C, a hydrophilic antioxidant, and/or selenium, a cofactor of peroxidases that detoxify lipid peroxides, would inhibit atherosclerosis more effectively than vitamin E alone. We also considered whether regional variation in inhibition of atherosclerosis by antioxidants would be associated with regional variation in aortic lipophilic antioxidants. Rabbits were fed an atherogenic diet (control) or an atherogenic diet supplemented with vitamin E, vitamins E and C, vitamin E+selenium, vitamins E and C+selenium, or probucol (positive control). Supplements were as follows: vitamin E, 146 IU/d; vitamin C, 791 mg/d; selenium, 22 microg/d; or probucol, 406 mg/d. Vitamin C did not influence atherosclerosis. After 22 weeks of treatment, rank order of aortic atherosclerosis was control>vitamin E (with or without vitamin C)>vitamin E+selenium (with or without vitamin C)>probucol. Antioxidant treatment reduced aortic cholesterol concentrations 21% to 56%, 29% to 86%, and 19% to 75% for the aortic arch, descending thoracic aorta, and abdominal aorta, respectively (P<0.025 to P<0.0003 by ANOVA), with slightly greatly reductions for areas of atherosclerotic lesions. Some treatments reduced plasma cholesterol concentrations, but none altered the distribution of cholesterol among lipoproteins. Corrected for differences in plasma cholesterol concentrations, aortic cholesterol concentrations were reduced up to 72% (P<0.02) by the antioxidant treatments, with equal reductions by vitamin E+selenium and by probucol. Aortic alpha-tocopherol standardized by aortic cholesterol as a measure of aortic lipids was lower in the abdominal aorta than in the aortic arch of rabbits not given alpha-tocopherol and increased relatively more in the abdominal aorta than in the aortic arch with alpha-tocopherol supplementation. The results of this study suggest that vitamin E+ selenium inhibited atherosclerosis as effectively as an equally hypocholesterolemic dose of probucol by a mechanism(s) that is in part independent of effects on plasma and lipoprotein cholesterol concentrations. The tendency for greater efficacy of antioxidant treatments in the abdominal aorta than aortic arch may relate to the lower concentrations of alpha-tocopherol in the abdominal aorta of unsupplemented rabbits.

Markers of protein oxidation by hydroxyl radical and reactive nitrogen species in tissues of aging rats

Many lines of evidence implicate oxidative damage in aging. Possible pathways include reactions that modify aromatic amino acid residues on proteins. o-Tyrosine is a stable marker for oxidation of protein-bound phenylalanine by hydroxyl radical, whereas 3-nitrotyrosine is a marker for oxidation of protein-bound tyrosine by reactive nitrogen species. To test the hypothesis that proteins damaged by hydroxyl radical and reactive nitrogen accumulate with aging, we used isotope dilution gas chromatography-mass spectrometry to measure levels of o-tyrosine and 3-nitrotyrosine in heart, skeletal muscle, and liver from young adult (9 mo) and old (24 mo) female Long-Evans/Wistar hybrid rats. We also measured these markers in young adult and old rats that received antioxidant supplements (alpha-tocopherol, beta-carotene, butylated hydroxytoluene, and ascorbic acid) from the age of 5 mo. We found that aging did not significantly increase levels of protein-bound o-tyrosine or 3-nitrotyrosine in any of the tissues. Antioxidant supplementation had no effect on the levels of protein-bound o-tyrosine and 3-nitrotyrosine in either young or old animals. These observations indicate that the o-tyrosine and 3-nitrotyrosine do not increase significantly in heart, skeletal muscle, and liver in old rats, suggesting that proteins damaged by hydroxyl radical and reactive nitrogen species do not accumulate in these tissues with advancing age.

Effects of green tea, black tea and dietary lipophilic antioxidants on LDL oxidizability and atherosclerosis in hypercholesterolaemic rabbits

The hypothesis that tea or dietary lipid-soluble antioxidants reduce atherogenesis by lowering the oxidizability of low-density lipoprotein (LDL) was investigated. Five groups of 20 female New Zealand white rabbits were fed a restricted amount of a high-fat (30 en%) semipurified diet supplemented with cholesterol (0.15%, w/w) for 21 weeks. The vitamin E content of the control diet was 40 mg/kg diet. The animals received either green tea or black tea in their drinking water or vitamin E (200 mg/kg diet) or beta-carotene (20 mg/kg). The serum cholesterol concentrations (in the order of 18-23 mmol/l) were not significantly different between the groups. Vitamin E was substantially increased as compared to controls in vitamin E supplemented animals (3-fold within 8 weeks in plasma and LDL; P < 0.01) and weakly (1.2-fold) by green and black tea (P < 0.05). Green tea consumption tended to reduce aortic lesion formation by 31% (24 +/- 3.2% versus 35 +/- 5.7% for control animals P = 0.11), while black tea, vitamin E and beta-carotene had no effect. This was in contrast to the resistance of isolated LDL to oxidation induced at high copper concentration. Green and black tea induced a 13% and 15% (P < 0.05) prolongation of the lag phase, respectively, with a correspondingly lower oxidation rate, while vitamin E increased the lag phase by 63% (P < 0.01) with a concomitant diminution of the oxidation rate and beta-carotene had no effect. Regression analysis showed that there was no relationship between the extent of atherosclerosis and LDL oxidizability or plasma malondialdehyde as marker of in vivo lipid peroxidation. The results of the present study raise the question whether LDL oxidizability (at least when tested at high induction rate ex vivo) is a primary causal mechanism in atherosclerosis in the cholesterol-fed rabbit. The suitability of the cholesterol-fed rabbit with extreme hypercholesterolaemia as a model to study antiatherosclerotic properties of dietary antioxidants, such as the tested polyphenols, is discussed.

Long-term probucol treatment reverses the severity of myocardial injury in watanabe heritable hyperlipidemic rabbits

We previously reported that administration of NO donors ameliorates the severity of myocardial injury in cholesterol-fed rabbits. We now evaluated the effects of probucol, a lipid-lowering antioxidant that can preserve endothelium-dependent relaxation (EDR), in the aortas of cholesterol-fed rabbits. We examined the effects of short-term (7 days) or long-term (24 weeks) administration of 1% probucol on the size of infarcts resulting from 30 minutes of coronary occlusion followed by reperfusion (for 48 hours) in Watanabe heritable hyperlipidemic (WHHL) rabbits. Infarcts in untreated WHHL rabbits were significantly larger than those in the rabbits receiving the long-term but not the short-term treatment with probucol (72.2 +/- 5.4%, 37.6 +/- 6.4%, and 66.7 +/- 3.5%, respectively). Long-term probucol treatment also significantly reduced myeloperoxidase activity in both ischemic and nonischemic myocardium and suppressed P-selectin expression in the coronary vasculature. No significant differences were observed in hemodynamic parameters during myocardial ischemia/reperfusion. Long-term probucol treatment significantly reduced the surface area of atherosclerotic plaque lesions in the aorta (24.4 +/- 3.8% vs 46.3 +/- 6.3, P < .05). Moreover, long-term probucol treatment restored acetylcholine-induced EDR in aortic rings but did not affect sodium nitroprusside-induced relaxation. Finally, long-term probucol treatment resulted in significantly elevated cGMP levels in the aorta. These results indicate that long-term probucol treatment significantly ameliorates myocardial injury in heritable atherosclerotic rabbits, perhaps by reducing the accumulation of leukocytes in the myocardium and atherosclerotic vascular lesions. Thus, long-term administration appears to suppress the progression of atherosclerotic vascular disease in this animal model.

Lipoprotein oxidation, antioxidants and cardiovascular risk: epidemiologic evidence

This review summarizes the scientific evidence for a possible role of antioxidants in the prevention of coronary heart disease (CHD). Dietary antioxidants include vitamin E, vitamin C and beta-carotene, whereas selenium is an integral part of the antioxidant enzyme glutathione peroxidase. Experimental studies suggest that the oxidation of low-density lipoproteins (LDL) in the vessel wall plays an important role in the development of atherosclerotic lesions. The resistance of LDL to oxidation is increased by antioxidant supplementation, at least in vitro. Epidemiological studies have not demonstrated unequivocally that a high intake of antioxidants leads to a decreased risk of CHD. Studies on dietary intake and serum levels of antioxidants do point in the direction of a preventive effect of antioxidants, whereas the results of intervention studies are less conclusive. Beta-carotene supplementation is not associated with any decrease in CHD; high doses of vitamin E may be beneficial, but results from large trials are to be awaited. General preventive measures based on antioxidant supplementation are not yet justifiable.