Extent of antioxidant protection of plasma LDL is not a predictor of the antiatherogenic effect of antioxidants

Syzygium cumini leaf extract inhibits LDL oxidation, but does not protect the liproprotein from glycation

ETNOPHARMACOLOGICAL RELEVANCE

Syzygium cumini (L.) Skeels is a plant widely used in folk medicine to treat diabetes mellitus (DM). The tea from its leaves is frequently used by diabetics for lowering hyperglycemia. There is a close relationship between DM and atherosclerosis, a chronic immuno-inflammatory disease, were the early stages encompass oxidative and glycative modifications in the structure of low density lipoprotein (LDL).

AIM OF THIS STUDY

To investigate the potential protective effects of aqueous-leaf extract from Syzygium cumini (S.cExt) against CuSO₄-induced oxidation and methylglyoxal (MG)-induced glycation of human LDL in vitro.

MATERIALS AND METHODS

LDL oxidative changes were evaluated by measuring conjugated dienes (CD) formation, thiobarbituric acid reactive substances (TBARS) levels, quenching of tryptophan (Trp) fluorescence and structural modifications in LDL particle. In LDL glycated by MG (glyLDL), we determined the levels of fluorescent advanced glycation end products (AGEs) and mobility by agarose gel electrophoresis.

RESULTS

S.cExt blocked oxidative events induced by CuSO₄ in human LDL, plasma and serum. Fourier transform infrared spectroscopy (FT-IR) revealed that specific regions of apoB100 were oxidized by CuSO₄ in human LDL and that S.cExt reduced these oxidations. Unlike, the increased AGEs levels and eletrophoretic mobility observed in LDL MG-glycated were not modified by S.cExt.

CONCLUSION

The findings herein indicate that S.cExt could be tested in atherogenesis models as potential protective agent against LDL oxidation.

Effects of Tanshinone IIA on the modulation of miR‑33a and the SREBP‑2/Pcsk9 signaling pathway in hyperlipidemic rats

Tanshinone IIA is the active compound isolated from Salvia miltiorrhiza bunge, which is a traditional Chinese medicine known as Danshen. The aim of the present study was to assess the effect of Tanshinone IIA on the regulation of lipid metabolism in the livers of hyperlipidemic rats and the underlying molecular events. An in vivo model of hyperlipidemia was established in rats, with the animals receiving a daily dose of Tanshinone IIA. The serum lipid profiles were analyzed using an automatic biochemical analyzer, and the histopathological alterations and lipid deposition in liver tissue were assessed using hematoxylin and eosin staining, and oil red O staining, respectively. The mRNA expression levels of microRNA (miR)‑33a, ATP‑binding cassette transporter (ABC)A1, ABCG1, sterol regulatory element‑binding protein 2 (SREBP‑2), proprotein convertase subtilisin/kexin type 9 (Pcsk9) and low‑density lipoprotein receptor (LDL‑R) in liver tissues were measured using reverse transcription‑quantitative polymerase chain reaction, and the protein expression levels of ABCA1, ABCG1, SREBP‑2, Pcsk9, and LDL‑R were analyzed using western blotting. Tanshinone IIA reduced lipid deposition and improved histopathology in the rat liver tissue, however, did not alter the lipid profile in rat serum. In addition, Tanshinone IIA treatment suppressed the expression of miR‑33a, whereas the protein expression levels of ABCA1, SREBP‑2, Pcsk9 in addition to LDL‑R mRNA and protein were upregulated. In conclusion, the present study indicated that Tanshinone IIA attenuated lipid deposition in the livers of hyperlipidemic rats and modulated the expression of miR‑33a and SREBP‑2/Pcsk9 signaling pathway proteins.

Oxidative stress and lectin-like ox-LDL-receptor LOX-1 in atherogenesis and tumorigenesis

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) has been identified as a major receptor for oxidized low-density lipoprotein (ox-LDL) in endothelial cells, monocytes, platelets, cardiomyocytes, and vascular smooth muscle cells. Its expression is minimal under physiological conditions but can be induced under pathological conditions. The upregulation of LOX-1 by ox-LDL appears to be important for physiologic processes, such as endothelial cell proliferation, apoptosis, and endothelium remodeling. Pathophysiologic effects of ox-LDL in atherogenesis have also been firmly established, including endothelial cell dysfunction, smooth muscle cell growth and migration, monocyte transformation into macrophages, and finally platelet aggregation-seen in atherogenesis. Recent studies show a positive correlation between increased serum ox-LDL levels and an increased risk of colon, breast, and ovarian cancer. As in atherosclerosis, ox-LDL and its receptor LOX-1 activate the inflammatory pathway through nuclear factor-kappa B, leading to cell transformation. LOX-1 is important for maintaining the transformed state in developmentally diverse cancer cell lines and for tumor growth, suggesting a molecular connection between atherogenesis and tumorigenesis.

Design, synthesis, and action of antiatherogenic antioxidants

Ample evidence supports the critical role of oxidized low-density lipoprotein (ox-LDL) in initiation and progression of atherosclerosis. Oxidation of LDL is a complex process involving several steps (processes) of reactions such as initiation and propagation. Both proteins and lipids in LDL undergo free radical-mediated oxidations leading to the formation of ox-LDL that plays a pivotal role in atherosclerosis. Antioxidants of various types (both aqueous and lipophilic) either arrest or retard the oxidation of LDL at various steps of the oxidation process (e.g., initiation or propagation). Certain lipophilic antioxidants act as the chain-terminating antioxidants leading to the inhibition of LDL oxidation. The current chapter describes the designing and efficacy of two novel lipophilic antioxidants (benzofuranol, BO-653 and aniline, BO-313) in inhibiting the LDL oxidation and atherogenesis in experimental animal model. Furthermore, the characteristics of an effective antioxidant to inhibit LDL oxidation and atherogenesis which dictates the designing of the antioxidant drug and its mechanism(s) of antiatherogenic action are discussed.

Protective effect of vitamin E supplements on experimental atherosclerosis is modest and depends on preexisting vitamin E deficiency

Vitamin E has failed to protect humans from cardiovascular disease outcome, yet its role in experimental atherosclerosis remains less clear. A previous study (Proc. Natl. Acad. Sci. USA 97:13830-13834; 2000) showed that vitamin E deficiency caused by disruption of the alpha-tocopherol transfer protein gene (Ttpa) is associated with a modest increase in atherosclerosis in apolipoprotein E gene deficient (Apoe(-/-)) mice. Here we confirm this finding and report that in Apoe(-/-)Ttpa(-/-) mice dietary alpha-tocopherol (alphaT) supplements restored circulating and aortic levels of alphaT, and decreased atherosclerosis in the aortic root to a level comparable to that seen in Apoe(-/-) mice. However, such dietary supplements did not decrease disease in Apoe(-/-) mice, whereas dietary supplements with a synthetic vitamin E analog (BO-653), either alone or in combination with alphaT, decreased atherosclerosis in Apoe(-/-) and in Apoe(-/-)Ttpa(-/-) mice. Differences in atherosclerosis were not associated with changes in the arterial concentrations of F(2)-isoprostanes and cholesterylester hydro(pero)xides, nor were they reflected in the resistance of plasma lipids to ex vivo oxidation. These results show that vitamin E at best has a modest effect on experimental atherosclerosis in hyperlipidemic mice, and only in situations of severe vitamin E deficiency and independent of lipid oxidation in the vessel wall.

Copper- and magnesium protoporphyrin complexes inhibit oxidative modification of LDL induced by hemin, transition metal ions and tyrosyl radicals

The oxidative modification of LDL may play an important role in the early events of atherogenesis. Thus the identification of antioxidative compounds may be of therapeutic and prophylactic importance regarding cardiovascular disease. Copper-chlorophyllin (Cu-CHL), a Cu(2+)-protoporphyrin IX complex, has been reported to inhibit lipid oxidation in biological membranes and liposomes. Hemin (Fe(3+)-protoporphyrin IX) has been shown to bind to LDL thereby inducing lipid peroxidation. As Cu-CHL has a similar structure as hemin, one may assume that Cu-CHL may compete with the hemin action on LDL. Therefore, in the present study Cu-CHL and the related compound magnesium-chlorophyllin (Mg-CHL) were examined in their ability to inhibit LDL oxidation initiated by hemin and other LDL oxidizing systems. LDL oxidation by hemin in presence of H(2)O(2) was strongly inhibited by both CHLs. Both chlorophyllins were also capable of effectively inhibiting LDL oxidation initiated by transition metal ions (Cu(2+)), human umbilical vein endothelial cells (HUVEC) and tyrosyl radicals generated by myeloperoxidase (MPO) in presence of H(2)O(2) and tyrosine. Cu- and Mg-CHL showed radical scavenging ability as demonstrated by the diphenylpicrylhydracylradical (DPPH)-radical assay and estimation of phenoxyl radical generated diphenyl (dityrosine) formation. As assessed by ultracentrifugation the chlorophyllins were found to bind to LDL (and HDL) in serum. The present study shows that copper chlorophyllin (Cu-CHL) and its magnesium analog could act as potent antagonists of atherogenic LDL modification induced by various oxidative stimuli. As inhibitory effects of the CHLs were found at concentrations as low as 1 mumol/l, which can be achieved in humans, the results may be physiologically/therapeutically relevant.

Oxidative Susceptibility of Unfractionated Serum or Plasma: Response to Antioxidants in Vitro and to Antioxidant Supplementation

Background: The susceptibility of plasma lipids to oxidation is thought to be a factor contributing to atherogenic risk. Various groups have studied the in vitro oxidizability of isolated LDL and examined the effects of conventional antioxidants. The drawbacks associated with the isolation of LDL for evaluation of in vitro oxidizability, however, have limited the application of this measurement in large-scale studies.

Methods: We developed and evaluated an assay that can be used to directly assess the oxidative susceptibility of unfractionated serum or plasma lipids, obviating the need for isolation of lipoprotein fractions. Oxidative conditions were initiated in vitro with cuprous chloride and 2,2′-azobis(2-amidinopropane) hydrochloride. The effects of antioxidants added in vitro, and as an oral supplement, were monitored by conjugated diene formation.

Results: The addition of ascorbic acid (0–50 μmol/L) in vitro elicited a dose-dependent protective effect, increasing the lag time to oxidation (P <0.001). In contrast, α-tocopherol demonstrated prooxidant behavior at increasing concentrations (0–50 μmol/L), although we observed a decrease in the maximum rate of oxidation. Our findings are supported by the results from plasma samples of participants in a randomized antioxidant (vitamins C and E) intervention study after acute ischemic stroke. The group receiving vitamins C and E for 14 days showed an increased lag time to plasma lipid oxidation in vitro compared with the nonsupplemented group (P <0.05).

Conclusion: The susceptibility of unfractionated plasma or serum lipids to oxidation in vitro offers an alternative to LDL for evaluating the efficacy of antioxidant regimens.

Role of oxidatively modified low density lipoproteins and anti-oxidants in atherothrombosis

Retrospective studies have demonstrated an association between coronary artery disease (CAD) and increased plasma levels of oxidised low density lipoproteins (LDL). A very recent prospective study in heart transplant patients has demonstrated that oxidised LDL is an independent risk factor for transplant CAD, thus further supporting the hypothesis that oxidised LDL is actively involved in the development of CAD. The increase of circulating oxidised LDL is most probably caused by back-diffusion from the atherosclerotic arterial wall in the blood, independent of plaque rupture. Indeed, plasma levels of oxidised LDL were very similar in patients with stable CAD and in patients with acute coronary syndromes. These were, however, associated with increased release of malondialdehyde (MDA)-modified LDL. Oxidised LDL may be generated by radical-mediated or by lipoxygenase or phospholipase catalysed lipid oxidation, and by myeloperoxidase catalysed protein and lipid oxidation. Prostaglandin synthesis by endothelial cells under oxidative stress and platelet activation are associated with the release of aldehydes; these induce the oxidative modification of the apolipoprotein B-100 moiety of LDL in the absence of lipid peroxidation, and thus generate MDA-modified LDL. Efficient prevention of in vivo oxidation may involve efficient cholesterol lowering, improving the anti-oxidative status of LDL by increasing the anti-oxidant content and increasing the oleate content of LDL, and by shifting the LDL away from phenotype B (characterised by small dense LDL particles). Anti-oxidative and anti-inflammatory enzymes associated with HDL may inhibit the oxidation of LDL or reverse the atherothrombotic effects of LDL.

Low plasma levels of oxidized low density lipoprotein in preeclampsia

BACKGROUND

Markers of lipid peroxidation are commonly used to assess oxidative stress in preeclampsia. The aim of this study was to assess the concentration of oxidized low density lipoprotein (oxLDL), a novel marker for lipid peroxidation, and that of the thiobarbituric acid reactive substances (TBARS) in the pathogenesis of severe preeclampsia and to investigate the influence of gestational age on these parameters.

METHOD

Plasma levels of oxLDL and TBARS were assayed in women with severe preeclampsia (n = 40), normotensive pregnant controls matched for gestational age (n = 24) and normotensive pregnant controls at full term (n = 16).

RESULTS

Women with preeclampsia showed lower oxLDL levels (mean +/- SE) than matched controls (181 +/- 12 vs. 219 +/- 14; p = 0.027), whereas no differences were found for the TBARS concentration (3.8 +/- 0.6 vs. 3.7 +/- 0.4). When women with preeclampsia were compared to control women at full term, TBARS were elevated (3.8 +/- 0.6 vs. 1.5 +/- 0.2; p = 0.01). However, in women with normotensive pregnancy TBARS were also lower in full-term control pregnancy compared to early third-trimester values (p < 0.0001).

CONCLUSION

Plasma TBARS decreased during the third trimester of pregnancy, underlining the importance of matching for gestational age when studying markers of lipid peroxidation in pregnant women. Women with preeclampsia had lower plasma levels of oxLDL compared to gestational age-matched controls, indicating that oxLDL could be a marker for preeclampsia.

Statins in atherosclerosis: lipid-lowering agents with antioxidant capabilities

Low-density lipoprotein (LDL) cholesterol is an established risk factor for coronary heart disease (CHD). In the presence of oxidative stress LDL particles can become oxidized to form a lipoprotein species that is particularly atherogenic. Indeed, oxidized LDL (oxLDL) is pro-inflammatory, it can cause endothelial dysfunction and it readily accumulates within the arterial wall. Several factors may influence the susceptibility of LDL to oxidation, including its size and composition, and the presence of endogenous antioxidant compounds, such as alpha-tocopherol. Individuals with type 2 diabetes or the metabolic syndrome have high levels of oxidative stress and consequently are at an increased risk for cardiovascular events. Reducing oxidative stress has been proposed as a potential approach to prevent CHD and antioxidant vitamins have been employed with encouraging results in experimental models of atherosclerosis. However, clinical trials have not demonstrated consistent beneficial effects of antioxidants on cardiovascular outcomes. Statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) are the first-line choice for lowering total and LDL cholesterol levels and they have been proven to reduce the risk of CHD. Recent data suggest that these compounds, in addition to their lipid-lowering ability, can also reduce the production of reactive oxygen species and increase the resistance of LDL to oxidation. It may be that the ability of statins to limit the oxidation of LDL contributes to their effectiveness at preventing atherosclerotic disease.

Dealcoholized red wine decreases atherosclerosis in apolipoprotein E gene-deficient mice independently of inhibition of lipid peroxidation in the artery wall

BACKGROUND

Oxidation of LDL is thought to be important in the development of atherosclerosis. Effective protection against lipoprotein oxidation is achieved by the use of alpha-tocopherol plus coantioxidants, ie. compounds that prevent the prooxidant activity of the vitamin. Wines contain a large number of polyphenols, micronutrients that may act as coantioxidants and may enhance the in vivo antioxidant activity of vitamin E.

OBJECTIVE

We examined whether wines and wine-derived fractions are able to act synergistically with vitamin E in vitro and whether dealcoholized red wine (DRW) retards the development of atherosclerosis.

DESIGN

Synergy with vitamin E was assessed in vitro by the ability of red and white wines to both attenuate alpha-tocopheroxyl radicals and inhibit in vitro oxidation of LDL in the presence of vitamin E. Female, 6-8-wk-old apolipoprotein E gene-deficient mice were fed a normal nonpurified stock diet for 24 wk to assess the effect on atherosclerosis of DRW at a dose equivalent to 200 mL x 80 kg body wt(-1) x d(-1).

RESULTS

DRW synergized with vitamin E as effectively as did red and white wine, and phenolic acids accounted for most of this activity. Administration of DRW increased plasma and aortic antioxidants concentrations and the resistance of plasma lipoproteins to ex vivo oxidation. Whereas lipoprotein oxidation in the artery wall was not affected, DRW significantly decreased atherosclerosis in the aortic arch, but not in the root, as assessed by morphometry.

CONCLUSIONS

DRW contains polyphenolic compounds capable of synergizing with vitamin E, and long-term moderate consumption of DRW can decrease atherosclerosis in apolipoprotein E gene-deficient mice.

The antiatherogenic effect of DiNAC: experimental findings supporting immunomodulation as a new treatment for atherosclerosis related diseases

Inflammatory processes in the arterial wall are important in atherogenesis. The present review discusses the development of DiNAC as a potential new treatment modality for atherosclerosis related diseases. DiNAC, N,N'-diacetyl-L-cystine, is the disulphide dimer of N-acetyl cysteine, NAC. It was selected as an immunomodulating drug candidate due to its ability to modify contact sensitivity/delayed type hypersensitivity (CS/DTH) reactions in vivo. Initial structure-activity relationship (SAR) studies indicated that an intact disulfide bridge was essential for this effect. Antioxidants, like probucol and some close analogs with two sulphurs in close proximity (but not disulphides), were also found to have similar effects on CS/DTH reactions. These antioxidants have antiatherosclerotic effects, while structurally related compounds without sulphurs do not. Therefore, it was hypothesized that DiNAC might also possess antiatherosclerotic effects. This was investigated in WHHL rabbits and mice. In both species, DiNAC had antiatherosclerotic activity similar to that of probucol. The effect of DiNAC was not due to an alteration of lipid metabolism. Impaired endothelium mediated relaxation is known to be associated with atherosclerosis. DiNAC was shown to reverse this process in WHHL rabbits with advanced atherosclerosis, probably due to an action on the vessel wall itself that is not related to the extent of atherosclerosis or to plasma lipid levels. Preliminary data from a clinical investigation in hypercholesterolemic subjects suggest that DiNAC is likely to have similar effects also in patients. Taken together, these findings suggest immunomodulation to be a potential new therapy for atherosclerosis related diseases. DiNAC may represent a new treatment modality for such diseases.

The role of vitamin E in atherosclerosis

Epidemiological and biochemical studies infer that oxidative processes, including the oxidation of low-density lipoprotein (LDL), are involved in atherosclerosis. Vitamin E has been the focus of several large supplemental studies of cardiovascular disease, yet its potential to attenuate or even prevent atherosclerosis has not been realised. The scientific rationale for vitamin E supplements protecting against atherosclerosis is based primarily on the oxidation theory of atherosclerosis, the assumption that vitamin E becomes depleted as disease progresses, and the expectation that vitamin E prevents the oxidation of LDL in vivo and atherogenic events linked to such oxidation. However, it is increasingly clear that the balance between vitamin E and other antioxidants may be crucial for in vivo antioxidant protection, that vitamin E is only minimally oxidised and not deficient in atherosclerotic lesions, and that vitamin E is not effective against two-electron oxidants that are increasingly implicated in both early and later stages of the disease. It also remains unclear as to whether oxidation plays a bystander or a casual role in atherosclerosis. This lack of knowledge may explain the ambivalence of vitamin E and other antioxidant supplementation in atherosclerosis.

Inhibition of low-density lipoprotein oxidation by the pure antiestrogens ICI 182780 and EM-652 (SCH 57068)

OBJECTIVE

The use of modulators of estrogen receptors activation is receiving increasing interest in menopausal medicine. Pure antiestrogens define a group of compounds exhibiting universal antagonistic properties. Nevertheless, we cannot disregard the possibility that they may still have some agonistic actions. Because estradiol (E2) has antioxidant properties, we tested the antioxidant capacity of two pure antiestrogens, ICI 182780 and EM-652 (SCH 57068).

DESIGN

The ability of both compounds to protect isolated low-density lipoproteins against copper-induced oxidation in vitro was used as an index of their antioxidant properties.

RESULTS

When compared with control (vehicle alone), ICI 182780 and EM-652 significantly increased the lag time of low-density lipoprotein oxidation when present at concentrations of 10 and 0.5 microM or greater, respectively. When compared with estrogens, ICI 182780 was as effective as E2 at the lower concentrations tested (0.1 and 1 microM) but less effective at higher concentrations. EM-652 had similar antioxidant potency as E2 but exerted maximal effects at lower concentration. Combined exposure to E2 and each of the antiestrogens resulted in a higher antioxidant effect than that of antiestrogen alone. Another estrogen (estrone), and selective estrogen-receptor modulators, such as tamoxifen and hydroxytamoxifen, displayed an antioxidant potency that was close to ICI 182780 but lower than EM-652. Progestogens, such as progesterone or medroxyprogesterone acetate, had no antioxidant effect.

CONCLUSIONS

Both ICI 182780 and EM-652 exhibited potent antioxidant activity, which could have important biological implications.

Comparison of the effects of alpha-tocopherol, ubiquinone-10 and probucol at therapeutic doses on atherosclerosis in WHHL rabbits

Oxidative modification of lipoproteins may trigger and maintain atherogenesis. We compared the effects of different antioxidants (alpha-tocopherol, probucol, ubiquinone-10) at doses similar to those used in humans in Watanabe Heritable Hyperlipidemic (WHHL) rabbits for 12 months. Aortic lesions were analyzed for their extent and cellular composition of lesions, mean thickness of fibrous caps and density of smooth muscle cells therein, content of antioxidants, non-oxidized and oxidized lipids. Compared to controls, probucol significantly lowered the extent and macrophage content of lesions and increased the existence and smooth muscle cell density of fibrous caps. alpha-Tocopherol supplementation increased the aortic content of vitamin E, but had no decreasing effect on either the accumulation of macrophage-specific antigen in the aorta or lesion size. Nevertheless, both probucol and alpha-tocopherol significantly decreased in vitro LDL oxidizability, measured under typically strong oxidative conditions. Ubiquinone-10 supplement increased lesion size and the fraction of lesions containing fibrous caps; however, LDL oxidizability remained unaffected by ubiquinone-10 treatment. None of the antioxidants tested lowered oxidized lipids within aortic tissue; however, long-term treatment with probucol provided the most effective anti-atherosclerotic effect, while alpha-tocopherol may be pro-atherogenic and ubiquinone-10 exerts ambivalent effects. Our data suggest that (i) widely used oxidation measures, such as ex-vivo LDL oxidizability, do not reflect the degree of atherosclerosis; and (ii) long-term beneficial effects of relatively low doses of antioxidants may be outweighed by high levels of plasma cholesterol in WHHL rabbits.

Treatment of hypertriglyceridemia with fenofibrate, fatty acid composition of plasma and LDL, and their relations to parameters of lipoperoxidation of LDL

The purpose of this study was to determine oxidation and oxidability of VLDL and LDL in connection with changes in their composition and content of FA in LDL after treatment with fenofibrate in patients with HTG.

Transdermal estradiol reduces plasma myeloperoxidase levels without affecting the LDL resistance to oxidation or the LDL particle size

OBJECTIVE

This study was designed to investigate the effects of different therapeutic range doses of transdermal estradiol (E(2)), alone or in combination with progesterone (P) or medroxyprogesterone acetate (MPA), on plasma lipoprotein levels and on three parameters related with LDL oxidizability, the resistance of LDL to oxidation by copper, the LDL particle size, and the myeloperoxidase levels.

DESIGN

Thirty-five healthy postmenopausal women who had been amenorrheic for at least 1 year received two consecutive, 2-month doses of transdermal estrogen (25-microg and 50-microg E(2) patch). Thereafter, they were randomly assigned to receive a 2-month treatment of either a 100-microg E(2) patch or a 50-microg E(2) patch combined with P (300 mg/day) or MPA (5 mg/day) during the last 14 days.

RESULTS

Neither transdermal E(2) alone nor transdermal E(2) plus progestogen modified the lipoprotein profile, the LDL resistance to oxidation, or the LDL particle size. However, all treatments similarly reduced the myeloperoxidase protein levels.

CONCLUSIONS

Different dosages of transdermal E(2) within the therapeutic range were equally effective in reducing myeloperoxidase protein levels. The effect remained after addition of P or MPA in a sequential regime.

Quantity versus quality of LDL cholesterol in patients with familial hypercholesterolemia--which is more important?

BACKGROUND

The aim of this study was to determine the effects of low-density lipoprotein (LDL) particle size and composition on the susceptibility of LDL to oxidation in subjects with Familial Hypercholesterolemia (FH).

METHODS

LDL was isolated from 20 FH homozygotes, 20 FH heterozygotes and 20 normal controls. Susceptibility of LDL to ex vivo copper-mediated oxidation was assessed by measuring conjugated diene production at 234 nm. Other factors known to influence LDL oxidation, namely particle size, vitamin E levels, and fatty acid composition of the LDL particles were also measured.

RESULTS

The mean duration of the lag phase was 1.42-fold longer in the FH homozygotes, and 1.21-fold longer in the FH heterozygotes than in the normal controls. LDL particle size was significantly larger in the FH homozygotes (26.45+/-0.37 nm) and FH heterozygotes (26.01+/-0.40 nm) compared to the normal control group (25.17+/-0.39 nm). LDL vitamin E concentrations, when expressed relative to LDL cholesterol concentrations, were similar in all the groups. In addition, no significant differences were observed in the total saturated, monounsaturated or polyunsaturated fatty acid content of LDL in the three groups of subjects.

CONCLUSION

These results suggest that it is the great excess in LDL quantity, rather than LDL 'quality', that is responsible for the severe and premature atherosclerosis in patients with FH.

Alpha-tocopherol and probucol reduce autoantibody titer to MDA-LDL in hypercholesterolemic rabbits

We considered the hypothesis that antioxidant supplementation that increases aortic antioxidant concentrations would reduce autoantibody titer to MDA-LDL, a measure that may indicate in vivo oxidation. We assessed autoantibody titer to MDA-LDL in rabbits before and after 5 months of treatment with a nutritionally adequate hypercholesterolemic diet alone (control) or supplemented with synthetic alpha-tocopherol or probucol. Aortic cholesterol and antioxidants were assessed at the end of treatment. alpha-Tocopherol supplementation increased the ratio of aortic alpha-tocopherol to cholesterol by 20-30-fold, while probucol supplementation increased the ratio of aortic probucol to cholesterol to 4-13 micromol/mol. Before treatment, MDA-LDL autoantibody titer averaged 5.09 +/- 0.24 with no difference among groups (p =.53 by ANOVA). However, after treatment, autoantibody titers differed among groups (p <.03 by ANOVA). Autoantibody titers were similar in rabbits supplemented with alpha-tocopherol and probucol (3.69 +/- 0.21 and 3.73 +/- 0.48, respectively, p = 0.81), and 26% (p <.009) lower in antioxidant supplemented rabbits than unsupplemented hypercholesterolemic rabbits (5.03 +/- 0.47). There was an inverse J relationship between autoantibody titer after treatment and aortic alpha-tocopherol/cholesterol and probucol/cholesterol, with minimum values for autoantibody titers above 8-10 micromol antioxidant/mmol cholesterol. The results of this study are consistent with inhibition of in vivo intra-aortic oxidation when aortic alpha-tocopherol or probucol exceed 8-10 micro;mol/mmol cholesterol.

Conjugated dienes: a critical trait of lipoprotein oxidizability in renal fibrosis

BACKGROUND

We assessed whether a differential oxidizability of apolipoprotein B (apo B)-containing lipoproteins (LDL and VLDL) may explain the oxidative stress that we had observed at the onset of renal fibrosis in Zucker obese (ZO) rats (Nephrol Dial Transplant 2000, 15: 467--476).

METHODS

Ex vivo copper-induced oxidation of lipoproteins was performed in 1-, 3-, and 9-month-old ZO and age-matched lean (ZL) rats. LDL/VLDL oxidizability was determined by spectrophotometry at 234 nm by monitoring the formation of conjugated diene hydroperoxides.

RESULTS

A significant increase in lag time (reflecting the resistance to oxidation) was observed in ZO rats at 3 months while the maximal diene production (reflecting the amount of hydroperoxides formed during oxidation) was higher in ZO than in ZL rats as early as 1 month. Lipoproteins were larger in ZO than in ZL rats, as shown by their core to surface component ratio. Furthermore, ZO lipoproteins had increased vitamin E and polyunsaturated fatty acid (PUFA) content, with no change in vitamin E/PUFA ratio.

CONCLUSIONS

Rather than oxidizability of apo B-containing lipoproteins, the ability of these molecules to produce high levels of conjugated dienes, which can act as toxic tissue messengers, appears to be a critical trait in the development of renal fibrosis in this rat model of obesity and renal fibrosis.

Oxidants and antioxidants in atherosclerosis

The oxidative theory suggests that LDL oxidation contributes to atherogenesis, implying that attenuation of this process by antioxidants should decrease atherosclerosis. However, a causative link between LDL oxidation and atherogenesis is not firmly established. It requires the identification of the oxidants that are responsible for the initiation of LDL oxidation, and an understanding of the modified moieties that are responsible for the proatherogenic activities of oxidized LDL. The present review summarizes recent data on potential biological oxidants for LDL in the vessel wall, and discusses the antiatherogenic role(s) of selected antioxidants.

The role of vitamin E in atherogenesis: linking the chemical, biological and clinical aspects of the disease

Atherosclerosis is a disease involving both oxidative modifications and disbalance of the immune system. Vitamin E, an endogenous redox-active component of circulating lipoproteins and (sub)cellular membranes whose levels can be manipulated by supplementation, has been shown to play a role in the initiation and progression of the disease. Recent data reveal that the activities of vitamin E go beyond its redox function. Moreover, it has been shown that vitamin E can exacerbate certain processes associated with atherogenesis. In this essay we review the role of biology of atherosclerosis, and suggest that these two facets decide the clinical manifestation and outcome of the disease.

The total peroxyl radical trapping potential in serum - an assay to define the stage of atherosclerosis

Lipid peroxides were identified among the factors that contribute to the atherosclerotic plaque formation in the arterial wall. We hypothesised that a correlation may exist between the content of antioxidant constituents in the serum and the gravity of atherosclerosis. To this purpose, we have determined the serum total peroxyl radical- trapping potential (TRAP), which is the combined capacity of all antioxidants to neutralize free radicals in serum and followed its variation in hyperlipemic animals in correlation with the stage of atherosclerosis. In addition, we compared TRAP values in the sera of coronary heart disease (CHD) patients, with or without type II diabetes mellitus. Results showed that after 18 weeks of hyperlipemic diet, the mean TRAP values measured in sera isolated from hyperlipemic hamsters exhibited an about 44% decrease, in good agreement with the increase of serum cholesterol and triglycerides. In the 3 groups of CHD patients, TRAP values decreased with about 10% in sera of stable angina patients, 20% in unstable patients, as compared with normal subjects. The lowest TRAP values were detected in the sera of patients with acute myocardial infarction. The results obtained for different experimental animals and for CHD patients sera indicate that the TRAP method, as adapted in our laboratory, is a reliable and reproducible assay, fit to be used in clinical studies as an ex vivo measurable parameter that correlates with the stage of the atherosclerosis.

Antioxidative and antiatherosclerotic effects of human apolipoprotein A-IV in apolipoprotein E-deficient mice

Mice expressing human apolipoprotein A-IV (apoA-IV) mainly in the intestine were obtained in an apolipoprotein E-deficient (apoE(0)) background (apoA-IV/E(0) mice). Quantification of aortic lesions and plasma lipid determination showed that compared with their control apoE(0) counterparts, the apoA-IV/E(0) mice are protected against atherosclerosis without an increase in HDL cholesterol. Because oxidized lipoproteins play an important role in atherogenesis, we tested whether the protection observed in these animals is accompanied by an in vivo reduction of the oxidation parameters. The lag time in the formation of conjugated dienes during copper-mediated oxidation, the aggregation state of LDL, and the presence of anti-oxidized LDL antibodies were measured. The presence of oxidized proteins in tissues and the presence of oxidation-specific epitopes in heart sections of atherosclerotic lesions were also analyzed. Except for lag time, the results showed that the oxidation parameters were reduced in the apoA-IV/E(0) mice compared with the apoE(0) mice. This suggests that human apoA-IV acts in vivo as an antioxidant. In addition, human apoA-IV accumulation was detected in the atherosclerotic lesions of apoA-IV/E(0) mice, suggesting that apoA-IV may inhibit oxidative damage to local tissues, thus decreasing the progression of atherosclerosis.

Effects of alpha-tocopherol on superoxide production and plasma intercellular adhesion molecule-1 and antibodies to oxidized LDL in chronic smokers

Antioxidants have been postulated to exert beneficial effects in atherosclerosis. Atherosclerosis is associated with raised plasma levels of soluble intercellular adhesion molecule-1 (sICAM-1) and autoantibodies against oxidized low-density lipoprotein (oxLDL). It is not known whether antioxidants affect these plasma factors in chronic smokers. In a randomized double-blind placebo-controlled study involving 128 male normolipidemic chronic smokers the effect of a 2-year alpha-tocopherol treatment (400 IU dL-alpha-tocopherol daily) on plasma levels of sICAM-1 and autoantibodies against oxLDL was evaluated. In addition, we monitored production of superoxide by leukocytes ex vivo. It was found that compared to nonsmokers (n = 33) plasma levels of IgG but not IgM autoantibodies against oxLDL and concentrations of sICAM-1 in smokers were significantly elevated (30 and 42%, respectively). After supplementation with alpha-tocopherol concentration of TBARS in plasma and in vitro oxidizability of LDL had decreased, but autoantibodies and sICAM-1 had not changed. Production of superoxide was not different between alpha-tocopherol- and placebo-treated smokers. It is concluded that in chronic smokers, long-term treatment with alpha-tocopherol does not normalize the raised levels of sICAM-1 and autoantibodies against oxLDL, both risk factors for initiation or progression of cardiovascular disease, despite a decrease in in vitro oxidizability of LDL.

The effect of oral hormone replacement therapy on lipoprotein profile, resistance of LDL to oxidation and LDL particle size

OBJECTIVES

To disclose if oral estradiol (E(2)), alone or in combination with natural progesterone (P) or medroxyprogesterone acetate (MPA), may modify the oxidizability of low density lipoprotein (LDL), and if the effect is achieved at physiological dosages. LDL oxidizability was assessed by the resistance to oxidation by copper and by the particle size profile, since small particles have increased oxidation susceptibility.

METHODS

Thirty-three women received two consecutive, two-month length doses of 1 and 2 mg/day of oral E(2). They were then randomly assigned to a fourteen-day treatment of 2 mg/day E(2) plus either 300 mg/day P or 5 mg/day MPA. A parallel group of experiments was performed on a pool of baseline plasma, where hormones were added at the desired concentration. Lipoprotein levels, resistance of LDL to oxidation, and LDL particle diameter, were measured at baseline and after each treatment.

RESULTS

Estradiol reduced LDL levels and increased high density lipoprotein (HDL) and triglycerides. P abolished these changes, whereas MPA only reversed the increase of HDL. Estradiol protected LDL from oxidation in a dose-dependent manner, although only at pharmacological concentrations (1 microM or higher). Both P and MPA were inert at either physiological or pharmacological concentrations. The size of the LDL particles remained unaffected except under MPA, in which it was reduced.

CONCLUSIONS

Estradiol has a protective effect against LDL oxidation, although only at pharmacological dosages. P and MPA did not limit the E(2) action. The size of the LDL particles remained unaltered after each E(2) dose, but MPA, and not P, was associated with a diminution.

The effect of pharmacological doses of different antioxidants on oxidation parameters and atherogenesis in hyperlipidaemic rabbits

The oxidation hypothesis of atherosclerosis implies that antioxidants are able to inhibit lipoprotein oxidation in the arterial wall and thereby retard atherogenesis. Since most of the animal studies performed have used very high doses of antioxidants, it is to date unknown whether antioxidants are effective antiatherosclerotic agents when given in pharmacological doses. Here we addressed this question using homozygous Watanabe heritable hyperlipidaemic (WHHL) rabbits as an animal model of atherosclerosis. The rabbits were divided into four groups, each consisting of ten animals. They received either a standard diet or a diet containing 4.3 mg ubiquinone-10, or 4.3 mg vitamin E or 15 mg probucol/kg body weight daily. After 12 months, the extent of aortic atherosclerosis was assessed as the intima thickness, media thickness and intima-to-media ratio in 14 cross sections equally distributed over the whole aorta. To evaluate the antioxidant effects of the diet, lipophilic and hydrophilic antioxidants, lipids, fatty acids and plasma oxidizability were measured after 0, 3 and 6 months of feeding. We found that supplementation with probucol significantly decreased aortic intima-to-media ratio compared to controls. The antiatherosclerotic action of probucol was accompanied by its beneficial action on plasma oxidizability and some plasma antioxidants. No decrease in aortic atherosclerosis was measured in ubiquinone-10- and vitamin E-supplemented rabbits, despite the fact that both antioxidants decreased plasma oxidizability and ubiquinone-10 increased the plasma levels of antioxidants. Taken together, these data suggest that pharmacological doses of probucol retard atherogenesis in WHHL rabbits by an antioxidant mechanism, while ubiquinone-10 and vitamin E at these dosages are ineffective in this highly hyperlipidaemic model. The measurement of some oxidation-related parameters in plasma, such as lipophilic antioxidants, polyunsaturated fatty acids and lipoprotein oxidizability, may be useful in assessing the risk of atherogenesis in humans.

Glibenclamide acts as an inhibitor of acyl-CoA:cholesterol acyltransferase enzyme

Sulfonylureas are used in the treatment of non-insulin-dependent diabetes mellitus. Little is known, however, about their effects on cholesterol metabolism. We tested in the present study the effects of glibenclamide (GB) on cholesterol esterification (CE) in macrophage-derived cells. GB inhibited intracellular accumulation of CE induced by acetylated LDL or oxidized LDL in J774 cells, but no such effect on total cholesterol, suggesting that the target of GB was acyl-CoA:cholesterol acyltransferase (ACAT). In the cell-free reconstitution ACAT assay, GB inhibited the ACAT activity with an IC(50) value of 20 microM. Furthermore, GB effectively inhibited the ACAT activity of PMA-stimulated THP-1 cells to the undifferentiated level of THP-1. In the whole-cell ACAT assay using CHO cells overexpressed with ACAT-1 or ACAT-2, GB inhibited the activity of both isozymes with similar potency. Our in vitro data suggest that sulfonylurea could be a potential seed for a new generation of ACAT inhibitors.

Lowering dietary saturated fat and total fat reduces the oxidative susceptibility of LDL in healthy men and women

The present study examined the effects of reducing dietary total fat and saturated fat (SFA) on LDL oxidative susceptibility in 27 healthy men and women (age 24-65 y). Each subject consumed each of three diets for 8 wk: an average American diet (AAD, 34% energy from fat, 15% from SFA), a Step-1 diet (29% fat, 9% SFA) and a very low SFA diet (Low-Sat, 25% fat, 6% SFA). In vitro LDL oxidation was assessed by copper-mediated oxidation, as measured by the kinetics of conjugated diene formation and lipid peroxide formation. Compared with the AAD, plasma LDL-cholesterol (LDL-C) and HDL cholesterol levels were 8% lower (P: = 0.16 and P: = 0.11, respectively), in subjects when they consumed the Step-1 diet and 11% (P: < 0.03) and 14% (P: < 0.057) lower, respectively, when they consumed the Low-Sat diet. Conjugated diene production and oxidation rate were 7% (P: < 0. 05) and 9% (P: < 0.05) lower, respectively. The reduction of lipid peroxide formation was 9% (P: < 0.05) in subjects when they consumed the Low-Sat diet vs. the AAD. In addition, lipid peroxide and conjugated diene formation were positively correlated with plasma total and LDL-C and apolipoprotein B (apo B) levels (r = 0.5-0.6, P: < 0.001), suggesting that quantity of LDL is an important determinant of oxidative modification. Furthermore, at the same level of apo B or LDL-C, LDL from subjects when they consumed either Step-1 or Low-Sat diets was less susceptible (P: < 0.05) to oxidation than those when they consumed the AAD, suggesting that qualitative changes also affect LDL oxidative susceptibility. Therefore, the benefits of lowering dietary SFA may extend beyond decreasing LDL-C levels and include favorable qualitative changes in LDL that further decrease risk of coronary heart disease.

Isoflavone phytoestrogens consumed in soy decrease F(2)-isoprostane concentrations and increase resistance of low-density lipoprotein to oxidation in humans

BACKGROUND

Oxidative damage to lipids may be involved in the etiology of atherosclerosis, cardiovascular disease in general, and cancer. The soy isoflavone phytoestrogens, genistein and daidzein, and equol (a daidzein metabolite produced by intestinal microflora) are antioxidants in vitro; equol is a particularly good inhibitor of LDL oxidation and membrane lipid peroxidation.

OBJECTIVE

We sought to investigate the effects of a diet enriched with soy containing isoflavones on in vivo biomarkers of lipid peroxidation and resistance of LDL to oxidation, compared with a diet enriched with soy from which the isoflavones had been extracted.

DESIGN

: A randomized, crossover design was used to compare diets enriched with soy that was low or high in isoflavones in 24 subjects. Plasma concentrations of an F(2)-isoprostane, 8-epi-prostaglandin F(2)(alpha) (8-epi-PGF(2)(alpha)), a biomarker of in vivo lipid peroxidation, and resistance of LDL to copper-ion-induced oxidation were determined.

RESULTS

Plasma concentrations of 8-epi-PGF(2)(alpha) were significantly lower after the high-isoflavone dietary treatment than after the low-isoflavone dietary treatment (326 +/- 32 and 405 +/- 50 ng/L, respectively; P = 0.028) and the lag time for copper-ion-induced LDL oxidation was longer (48 +/- 2.4 and 44 +/- 1.9 min, respectively; P = 0.017). Lag time for oxidation of unfractionated plasma and plasma concentrations of malondialdehyde, LDL alpha-tocopherol, polyunsaturated fatty acids, and isoflavonoids did not differ significantly between dietary treatments.

CONCLUSIONS

Consumption of soy containing naturally occurring amounts of isoflavone phytoestrogens reduced lipid peroxidation in vivo and increased the resistance of LDL to oxidation. This antioxidant action may be significant with regard to risk of atherosclerosis, cardiovascular disease in general, and cancer.

Current, new and future treatments in dyslipidaemia and atherosclerosis

The new therapeutic options available to clinicians treating dyslipidaemia in the last decade have enabled effective treatment for many patients. The development of the HMG-CoA reductase inhibitors (statins) have been a major advance in that they possess multiple pharmacological effects (pleiotropic effects) resulting in potent reductions of low density lipoproteins (LDL) and prevention of the atherosclerotic process. More recently, the newer fibric acid derivatives have also reduced LDL to levels comparable to those achieved with statins, have reduced triglycerides, and gemfibrozil has been shown to increase high density lipoprotein (HDL) levels. Nicotinic acid has been made tolerable with sustained-release formulations, and is still considered an excellent choice in elevating HDL cholesterol and is potentially effective in reducing lipoprotein(a) [Lp(a)] levels, an emerging risk factor for coronary heart disease (CHD). Furthermore, recent studies have reported positive lipid-lowering effects from estrogen and/or progestogen in postmenopausal women but there are still conflicting reports on the use of these agents in dyslipidaemia and in females at risk for CHD. In addition to lowering lipid levels, these antihyperlipidaemic agents may have directly or indirectly targeted thrombogenic, fibrinolytic and atherosclerotic processes which may have been unaccounted for in their overall success in clinical trials. Although LDL cholesterol is still the major target for therapy, it is likely that over the next several years other lipid/lipoprotein and nonlipid parameters will become more generally accepted targets for specific therapeutic interventions. Some important emerging lipid/lipoprotein parameters that have been associated with CHD include elevated triglyceride, oxidised LDL cholesterol and Lp(a) levels, and low HDL levels. The nonlipid parameters include elevated homocysteine and fibrinogen, and decreased endothelial-derived nitric oxide production. Among the new investigational agents are inhibitors of squalene synthetase, acylCoA: cholesterol acyltransferase, cholesteryl ester transfer protein, monocyte-macrophages and LDL cholesterol oxidation. Future applications may include thyromimetic therapy, cholesterol vaccination, somatic gene therapy, and recombinant proteins, in particular, apolipoproteins A-I and E. Non-LDL-related targets such as peroxisome proliferator-activating receptors, matrix metalloproteinases and scavenger receptor class B type I may also have clinical significance in the treatment of atherosclerosis in the near future. Before lipid-lowering therapy, dietary and lifestyle modification is and should be the first therapeutic intervention in the management of dyslipidaemia. Although current recommendations from the US and Europe are slightly different, adherence to these recommendations is essential to lower the risk of atherosclerotic vascular disease, more specifically CHD. New guidelines that are expected in the near future will encompass global opinions from the expert scientific community addressing the issue of target LDL goal (aggressive versus moderate lowering) and the application of therapy for newer emerging CHD risk factors.

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.

The effect of vitamin E, probucol, and lovastatin on oxidative status and aortic fatty lesions in hyperlipidemic-diabetic hamsters

Diabetes mellitus is associated with an increased risk of premature atherosclerosis, which may be due in part to an increased rate of low density lipoprotein (LDL) oxidation. Previous studies have shown that vitamin E, probucol, and lovastatin can reduce the oxidative susceptibility of LDL in normoglycemic animal models; however, few studies have investigated this in conjunction with aortic fatty streak lesion formation in diabetic hyperlipidemic models. Forty-eight Syrian hamsters were made diabetic by intraperitoneal injection of low dose streptozotocin. Diabetic animals (12 animals/groups) received a high saturated fat and cholesterol diet for 12.5 weeks. At 2.5 week of dietary treatments, the diet was supplemented with either: (1) 500 IU/day vitamin E (D+E); (2) 1% probucol w/w of the diet (D+P); (3) 25 mg/kg lovastatin (D+L); or (4) diabetic control (D). An age-matched group of hamsters (n=6) receiving the same diet but not made diabetic (ND) was used as control. At the end of the study, aortic arch foam cell-rich fatty streak lesion, plasma glucose, total cholesterol (TC), high density lipoprotein cholesterol (HDL-C), non-HDL-C, triglycerides (TG), phospholipids, alpha-tocopherol, plasma lipid peroxide and the susceptibility of LDL to copper-catalyzed oxidation were determined. Diabetes increased plasma glucose, and when combined with an atherogenic diet resulted in a further increase of plasma lipids. Vitamin E, probucol, and lovastatin significantly reduced plasma TG in the diabetic hamsters fed the atherogenic diet. Vitamin E treatment increased TC, probucol reduced HDL-C without affecting TC; whereas lovastatin reduced TC and selectively decreased non-HDL-C, and significantly reduced fatty streak lesion formation in the aortic arch. While vitamin E and probucol were effective in reducing several indices of oxidative stress including plasma lipid peroxides, cholesterol oxidation products and in vitro LDL oxidation, they had no effect on fatty streak lesion formation. Our results indicate that the LDL in diabetic animals is more susceptible to oxidation than in non-diabetic hamsters and that not only vitamin E and probucol but also lovastatin provide antioxidant protection. It appears that in this combined model of diabetes and hypercholesterolemia, progression of fatty streak lesion formation is mainly associated with changes in TC and non-HDL-C as affected by lovastatin, and is less dependent on the extent of LDL oxidation, changes in plasma TG level and oxidative stress status.

Tobacco-related diseases. Is there a role for antioxidant micronutrient supplementation?

It is clear that smoking causes an increase in free radicals, reactive nitrogen and oxygen species (RNS and ROS, respectively), and that cigarette smoking is associated with increases in the incidence and severity of several diseases including atherosclerosis, cancer, and chronic obstructive lung disease. Although there is still no unequivocal evidence that oxidative stress is a contributor to these diseases or that an increased intake of antioxidant nutrients is beneficial, the observation that smokers have lower circulating levels of some of these nutrients, raises concern. This article discusses the possible links between the observed oxidant-induced damage related to tobacco smoking, effects on cellular mechanisms, and their potential involvement in the causation and enhancement of disease processes.

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.

Novel physiological function of sphingomyelin in plasma. Inhibition of lipid peroxidation in low density lipoproteins

Although sphingomyelin (SPH) is a major constituent of all lipoproteins, its physiological function in plasma is not known. In this study, we tested the hypothesis that SPH inhibits lipid peroxidation in low density lipoproteins (LDL) because of its effects on surface fluidity and packing density and that the relative resistance of the buoyant LDL to oxidation, compared with the dense LDL, is partly due to their higher SPH content. Depletion of SPH by treatment with SPHase resulted in shortened lag times and increased rates of oxidation in both LDL subfractions, as measured by the conjugated diene formation in the presence of Cu(2+). Oxidation of LDL by soybean lipoxygenase was similarly stimulated by the degradation of SPH. Oxidation-induced fluorescence decay of diphenylhexatriene-labeled phosphatidylcholine (PC), equilibrated with LDL-PC, was accelerated significantly by the enzymatic depletion of SPH from the lipoprotein. Oxidation of 16:0-18:2 PC in the proteoliposomes was inhibited progressively by the incorporation of increasing amounts of egg SPH into the liposomes. Treatment of SPH-containing proteoliposomes with SPHase reversed the effect of SPH, showing that the presence of intact SPH is necessary for the inhibition of oxidation. Although the incorporation of SPH into the same liposome as the PC (intrinsic SPH) protected the PC against oxidation, the addition of SPH liposomes to PC liposomes (extrinsic SPH) was not effective. Oxidation of 16:0-18:2 PC in liposomes was also inhibited by the incorporation of dipalmitoyl-PC, but not by free cholesterol. These results suggest that SPH acts as a physiological inhibitor of lipoprotein oxidation, possibly by modifying the fluidity of the phospholipid monolayer and thereby inhibiting the lateral propagation of the lipid peroxy radicals.

Endothelial dysfunction in Type 1 diabetes mellitus: relationship with LDL oxidation and the effects of vitamin E

AIMS

To examine the hypothesis that increased susceptibility of low density lipoproteins (LDL) to oxidation predisposes to endothelial dysfunction in patients with Type 1 diabetes mellitus.

METHODS

A cross-sectional study of 46 non-nephropathic diabetic and 39 control subjects and in the diabetic patients, a 3-month duration, randomized, placebo-controlled double-blind trial of vitamin E 500 U/day. Flow-mediated vasodilatation (FMD) was measured in the forearm by high resolution ultrasound. LDL oxidation by Cu2+ was measured in vitro.

RESULTS

Diabetic patients had greater basal and reactive forearm blood flow (geometric mean (SD%) flow (ml/min) 110.15 (19.19%) vs. 74.99 (23.17%); P=0.045, and 344.35 (20.84%) vs. 205.17 (21.48%); P=0.007), compared with controls, but there was no difference in FMD (median (interquartile range) 0.00 (-0.01-0.02) vs. 0.02 (-0.01-0.02) cm2; P=0.78). Diabetic LDL oxidation lag time correlated with postdilatation brachial artery area (r= 0.32; P=0.05) but not with FMD. Lag-times and total LDL oxidation by Cu2+, lipoprotein and vitamin E concentrations were similar in diabetic and control groups. Antibody titres to oxidized LDL (oxLDL) were higher in non-diabetic than diabetic subjects, and were unrelated to FMD. In diabetic patients, vitamin E increased mean (SD) plasma vitamin E levels (24.0 (6.5) to 47.5 (7.5) gmol/l; P=0.0006) and resulted in increased FMD (delta 0.00 (-0.02-0.01) vs. 0.01 (0.01-0.02)) cm2; P=0.0036), but no changes in LDL Cu2+ oxidation profiles were observed.

CONCLUSIONS

FMD is no different in Type 1 diabetic and non-diabetic subjects and nor are indices of lipid peroxidation and in vitro LDL oxidation although levels of antibody to oxLDL are lower in diabetes. Vitamin E supplementation increases plasma vitamin E levels and may enhance FMD in diabetes but, in the absence of changes in LDL oxidation, this may not be mediated by reduced oxidation of LDL.

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.

Vitamin E supplementation in hyperlipidaemic patients: effect of increasing doses on in vitro and in vivo low-density lipoprotein oxidation

BACKGROUND

Vitamin E supplementation is associated with a reduced risk of developing atherosclerotic events; probably because it inhibits low-density lipoprotein (LDL) oxidation, an initial step in atherosclerosis. Metal ion-dependent LDL oxidation is a commonly used method to estimate oxidizability of LDL, but the effect of antioxidant supplementation on the levels of autoantibodies to oxidised LDL (ox-LDL), an in vivo indicator of LDL oxidation, is unknown.

DESIGN

This double-blind, placebo-controlled study investigated the susceptibility of LDL to copper induced oxidation and malondialdehyde (MDA) derivatized-LDL (MDA-LDL) in hyperlipidaemic patients on supplements of vitamin E. The vitamin E group (n = 20) took vitamin E 100 IU daily and the dose was doubled at six-weekly intervals to 1600 IU daily. The control group (n = 17) received placebo in the same fashion. Blood samples were obtained at baseline and each subsequent visit to measure vitamin E status and oxidation of LDL.

RESULTS

A significant increase in both alpha-tocopherol levels and the lengths of lag phase was seen in the vitamin E group after first week of supplementation (100 IU day-1). This continued to rise in a dose-dependent fashion with a doubling of the lag phase on 1600 IU daily. However, the titre of antibodies to MDA-LDL was not altered.

CONCLUSIONS

The results suggest that although regarded as an in vivo marker of LDL oxidation, antibodies to MDA-LDL may not be a suitable measure to evaluate the effect of short-term antioxidant supplementation. The failure of autoantibody titres to fall despite reduced oxidizability of LDL may possibly be attributable to a long half-life of the antibody or, once initiated, a continuous immunological response to ox-LDL contained in atherosclerotic lesions of the arterial wall.

F(2)-isoprostanes: sensitive and specific non-invasive indices of lipid peroxidation in vivo

Isoprostanes are members of a complex family of lipids, isomers of the conventional enzymatically derived prostaglandins (PG), which are produced in vivo primarily, if not exclusively, by a free radical-catalyzed peroxidation of polyunsaturated fatty acids. Most of the work has been focused upon a group of isomers of the enzyme-derived PGF(2alpha), called F(2)-isoprostanes (F(2)-iPs). Because of their mechanism of formation, chemical stability and the rapid development of sensitive methods for their measurement, they have the attraction as non-invasive indices of oxidant stress in vivo. Altered generation of F(2)-iPs has been reported in a variety of clinical settings putatively associated with oxidant stress. These include atherosclerosis, chronic obstructive pulmonary disease and Alzheimer's disease. Furthermore, the measurement of specific F(2)-iPs may provide a sensitive biochemical basis for rational dose-selection of natural and synthetic inhibitor of lipid peroxidation. Although F(2)-iPs possess biological activities in vitro and in vivo, much remains to be learned about their role and as mediators of the cellular effects of lipid peroxidation and oxidant stress.

Nonoxidative modifications of lipoproteins in atherogenesis

The key initiating event in atherosclerosis is the retention of plasma lipoproteins in the subendothelial matrix. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. There is considerable evidence that many of these biological responses, notably macrophage cholesteryl ester loading (foam cell formation), require subendothelial modification of the retained lipoproteins. Oxidation of lipoproteins is one such modification that likely occurs in vivo and promotes certain atherogenic events, but oxidation cannot explain all aspects of atherogenesis, including certain elements of macrophage foam cell formation. For this reason, there has been renewed interest in other modifications of lipoproteins that may be important in atherogenesis. This review addresses five such lipoprotein modifications, namely aggregation, glycation, immune complex formation, proteoglycan complex formation, and conversion to cholesterol-rich liposomes. The focus is on the evidence that these modifications occur in atherosclerotic lesions and on the potential role of these modified lipoproteins in atherogenesis, with an emphasis on macrophage foam cell formation.

Lipoprotein modification and atherosclerosis in aging

The development of extensive atherosclerosis of major arteries of the heart, brain, and lower extremities is a particularly frequent problem in elderly individuals and is responsible for the majority of the cardiovascular morbidity and mortality in this population. Although the frequency and severity of this problem is well recognized, there has been relatively little investigation of the effects of aging on the development of atherosclerosis. Work by a number of investigators over the last 10-15 years has demonstrated that modifications of lipoproteins, resulting from oxidative stress, glycoxidation, formation of AGE, or other processes may play an important role in atherosclerosis. As described in this review, the aging process may enhance lipoprotein modification and atherosclerosis in several ways. Conditions such as hypertension, diabetes, and menopause all increase in frequency with advancing age and may contribute both directly and indirectly to lipoprotein modification and vascular injury.Additionally, in some studies of older animals and humans, there seems to be evidence for greater in vivo oxidative stress. Whether this is a specific consequence of aging and associated medical conditions, or related to differences in dietary fatty acid or antioxidant content or other lifestyle differences is currently unknown. One important consequence of this may be enhanced susceptibility of lipoproteins to oxidation. Additional study of lipoprotein modifications associated with aging is clearly needed, and may provide new insight and solutions to the common problem of atherosclerosis in the elderly.

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.