Vitamin E fails to alter the aggregation and the oxygenated metabolism of arachidonic acid in normal human platelets

Platelet Oxidative Stress and its Relationship with Cardiovascular Diseases in Type 2 Diabetes Mellitus Patients

Enhanced platelet activation and thrombosis are linked to various cardiovascular diseases (CVD). Among other mechanisms, oxidative stress seems to play a pivotal role in platelet hyperactivity. Indeed, upon stimulation by physiological agonists, human platelets generate and release several types of reactive oxygen species (ROS) such as O2 -, H2O2 or OH-, further amplifying the platelet activation response via various signalling pathways, including, formation of isoprostanes, Ca2+ mobilization and NO inactivation. Furthermore, excessive platelet ROS generation, incorporation of free radicals from environment and/or depletion of antioxidants induce pro-oxidant, pro-inflammatory and platelet hyperaggregability effects, leading to the incidence of cardiovascular events. Here, we review the current knowledge regarding the effect of oxidative stress on platelet signaling pathways and its implication in CVD such as type 2 diabetes mellitus. We also summarize the role of natural antioxidants included in vegetables, fruits and medicinal herbs in reducing platelet function via an oxidative stress-mediated mechanism.

Mixed tocopherols inhibit platelet aggregation in humans: potential mechanisms

BACKGROUND

Epidemiologic studies have shown an inverse correlation between acute coronary events and high intake of dietary vitamin E. Recent clinical studies, however, failed to show any beneficial effects of alpha-tocopherol on cardiovascular events. Absence of tocopherols other than alpha-tocopherol in the clinical studies may account for the conflicting results.

OBJECTIVE

This study compared the effect of a mixed tocopherol preparation rich in gamma-tocopherol with that of alpha-tocopherol on platelet aggregation in humans and addressed the potential mechanisms of the effect.

DESIGN

Forty-six subjects were randomly divided into 3 groups: alpha-tocopherol, mixed tocopherols, and control. ADP and phorbol 12-myristate 13-acetate-induced platelet aggregation, nitric oxide (NO) release, activation of endothelial constitutive nitric-oxide synthase (ecNOS; EC 1.14.13.39) and of protein kinase C (PKC), and ecNOS, superoxide dismutase (SOD; EC 1.15.1.1), and PKC protein content in platelets were measured before and after 8 wk of administration of tocopherols.

RESULTS

ADP-induced platelet aggregation decreased significantly in the mixed tocopherol group but not in the alpha-tocopherol and control groups. NO release, ecNOS activation, and SOD protein content in platelets increased in the tocopherol-treated groups. PKC activation in platelets was markedly decreased in the tocopherol-treated groups. Mixed tocopherols were more potent than alpha-tocopherol alone in modulating NO release and ecNOS activation but not SOD protein content or PKC activation.

CONCLUSIONS

Mixed tocopherols were more potent in preventing platelet aggregation than was alpha-tocopherol alone. Effects of mixed tocopherols were associated with increased NO release, ecNOS activation, and SOD protein content in platelets, which may contribute to the effect on platelet aggregation.

Moderate supplementation with natural alpha-tocopherol decreases platelet aggregation and low-density lipoprotein oxidation

Previous studies have shown that oral administration of 300 mg alpha-tocopherol/day to healthy volunteers decreases platelet function and enhances their sensitivity to the platelet inhibitor, prostaglandin E(1), when full dose-response curves to a range of agonist concentrations are made. In this study, the effects of oral doses of natural alpha-tocopherol (75, 200 and 400 IU/day) were studied in order to determine whether the same effects might be achieved with lower intakes of vitamin E and whether inhibition is related to the platelet levels of the antioxidant in platelet membranes. Twenty two subjects undertook the supplementation regime, divided into three units of 2 weeks, each cycling through each of the dosages. The results show that uptake of vitamin E by the platelets was optimal at 75 IU/day, correlating with the maximal influence on platelet aggregation and platelet responsiveness to inhibition by PGE1, increased supplemental levels exerting no greater effects.

Vitamin E and vascular homeostasis: implications for atherosclerosis

Considerable epidemiologic data suggest that dietary consumption of vitamin E reduces the incidence of cardiovascular disease. The precise mechanisms are not clear, but emerging data indicate that vitamin E has numerous activities that may, in part, explain its effect on vascular disease. In particular, vitamin E enhances the bioactivity of nitric oxide, inhibits smooth muscle proliferation, and limits platelet aggregation. One common mechanism to account for these effects of vitamin E is the inhibition of protein kinase C stimulation. In the setting of atherosclerosis, inhibition of protein kinase C by vitamin E would be expected to maintain normal vascular homeostasis and thus reduce the clinical incidence of cardiovascular disease.

The influence of antioxidant nutrients on platelet function in healthy volunteers

There is mounting evidence that antioxidants may help to prevent coronary heart disease and modulate some thrombotic events such a platelet adhesion. However, the effects of antioxidant supplementation on platelet function in vivo are controversial. A double-blind, randomised, placebo-controlled study was performed on 40 healthy volunteers (20-50 years) supplemented daily with vitamin E (300 mg), vitamin C (250 mg) or beta-carotene (15 mg) for 8 weeks. Platelet function was assessed by platelet aggregation induced by ADP, arachidonic acid or collagen, platelet responsiveness to the inhibitor PGE1, beta-thromboglobulin release and ATP secretion. Supplementation with vitamin E resulted in a significant increase in platelet alpha-tocopherol level (+68%) reflecting closely the increase in plasma alpha-tocopherol level (+69%). Platelet function was significantly decreased by vitamin E as revealed by the decreased platelet aggregation in response to ADP and arachidonic acid, the increased sensitivity to inhibition by PGE1, the decreased plasma beta-thromboglobulin concentration and the decreased ATP secretion. Supplementation with vitamin C did not affect platelet function significantly although a trend towards a decreased platelet aggregability and an increased sensitivity to the inhibitor PGE1 were observed. No significant changes in platelet function occurred after supplementation with beta-carotene. In conclusion, supplementation of healthy volunteers with vitamin E decreased platelet function whereas supplementation with vitamin C or beta-carotene had no significant effects.

Oxygen free radicals and platelet activation

This article reviews our current understanding of the role of oxygen free radicals in platelet activation. Several studies have indicated that platelets, in analogy to other circulating blood cells, are able to produce oxygen free radicals, which are likely to play an important role in the mechanism of platelet activation and aggregation. Platelet activation has been obtained with very low, physiologically relevant concentrations of radicals generated chemically, by leukocytes, and by hemoglobin derived from membrane leakage of erythrocytes. Knowledge of the role of reactive species in platelet physiology is relevant because platelets are brought into close contact with other cells capable of producing free radicals, such as neutrophils, macrophages, and endothelial cells, during the formation of thrombus. The physiopatological importance of these findings is high because it is now emerging that free radicals may have a role in the mechanism of atherosclerosis and its thrombotic complications, where the causative role of platelets is well documented. This background suggests therapeutic interventions with antioxidants as antiplatelet agents to improve the pharmacological effect of classical antiplatelet drug such as aspirin.

alpha-tocopherol inhibits aggregation of human platelets by a protein kinase C-dependent mechanism

BACKGROUND

Epidemiological studies indicate that vitamin E (alpha-tocopherol) exerts a beneficial effect on cardiovascular disease. The effect of vitamin E has generally been attributed to its antioxidant activity and the antioxidant protection of LDL. Distinct from its effect on LDL, vitamin E is also known to inhibit platelet aggregation and adhesion in vitro, but the mechanism(s) responsible for these observations are not known.

METHODS AND RESULTS

Using gel-filtered platelets derived from platelet-rich plasma treated with alpha-tocopherol (500 mumol/L) or vehicle (0.5% ethanol), we found that inhibition of platelet aggregation by alpha-tocopherol was closely linked to its incorporation into platelets (r = -.78; P < .02). Platelet incorporation of alpha-tocopherol was associated with a significant reduction in platelet sensitivity to aggregation by adenosine 5'-diphosphate, arachidonic acid, and phorbol ester (PMA) by approximately, 0.15-, 2-, and 100-fold, respectively. In contrast, platelets treated similarly with butylated hydroxytoluene, another potent lipid-soluble antioxidant, did not demonstrate any change in sensitivity to these agents. Platelet incorporation of alpha-tocopherol inhibited PMA-induced stimulation of platelet protein kinase C (PKC) as determined by phosphorylation of the 47-kD PKC substrate. In 15 normal subjects, oral supplementation with alpha-tocopherol (400 to 1200 IU/d) resulted in an increase in platelet alpha-tocopherol content that correlated with marked inhibition of PMA-mediated platelet aggregation (r = .67; P < .01). Platelets derived from these subjects after supplementation also demonstrated apparent complete inhibition of PKC stimulation by PMA.

CONCLUSIONS

These data indicate that platelet incorporation of alpha-tocopherol at levels attained with oral supplementation is associated with inhibition of platelet aggregation through a PKC-dependent mechanism. These observations may represent one potential mechanism for the observed beneficial effect of alpha-tocopherol in preventing the development of coronary artery disease.

Levels of antioxidant nutrients in plasma and low density lipoproteins: a human volunteer supplementation study

A human supplementation study was undertaken in order to investigate the correlation between the intake of individual daily dosages of vitamin E (300 mg), vitamin C (250 mg), or beta-carotene (15 mg) of eight week duration and their uptake in vivo in plasma and LDL. The effects of a combined supplement of vitamin E, vitamin C and beta-carotene (Redoxon protector-75 mg, 150 mg, 15 mg respectively) were also investigated. The results show that on supplementation with the individual antioxidants the increases in plasma alpha-tocopherol:cholesterol levels lie in the 1.5-2 fold range and the beta-carotene:cholesterol ratios give a mean 3.5 fold enhancement. The combined supplement containing the same level of beta-carotene as the single dosage achieved comparative levels of uptake in plasma. The level of plasma vitamin C appears to be maximal at about 100 microM regardless of the pre-supplementation level.

Inhibition of platelet aggregation and eicosanoid production by phenolic components of olive oil

This study was designed to investigate the in vitro effects of phenolic compounds extracted from olive oil and from olive derived fractions. More specifically, we investigated the effects on platelets of 2-(3,4-di-hydroxyphenyl)-ethanol (DHPE), a phenol component of extra-virgin olive oil with potent antioxidant properties. The following variables were studied: aggregation of platelet rich plasma (PRP) induced by ADP or collagen, and thromboxane B2 production by collagen or thrombin-stimulated PRP. In addition, thromboxane B2 and 12-hydroxyeicosatetraenoic acid (12-HETE) produced during blood clotting were measured in serum. Preincubation of PRP with DHPE for at least 10 min resulted in maximal inhibition of the various measured variables. The IC50s (concentration resulting in 50% inhibition) of DHPE for ADP or collagen-induced PRP aggregations were 23 and 67 microM, respectively. At 400 microM DHPE, a concentration which completely inhibited collagen-induced PRP aggregation, TxB2 production by collagen- or thrombin-stimulated PRP was inhibited by over 80 percent. At the same DHPE concentration, the accumulation of TxB2 and 12-HETE in serum was reduced by over 90 and 50 percent, respectively. We also tested the effects of PRP aggregation of oleuropein, another typical olive oil phenol, and of selected flavnoids (luteolin, apigenin, quercetin) and found them to be much less active. On the other hand a partially characterized phenol-enriched extract obtained from aqueous waste from olive oil showed rather potent activities. Our results are the first evidence that components of the phenolic fraction of olive oil can inhibit platelet function and eicosanoid formation in vitro, and that other, partially characterized, olive derivatives share these biological activities.

Prevention of platelet dysfunction by vitamin E in diabetic atherosclerosis

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

Influence of vitamin E on platelet function in humans

alpha-tocopherol, a natural antioxidant, has been found to inhibit platelet aggregation and release when tested in an in vitro system. This effect of vitamin E was thought to be due to a slight reduction of platelet cyclooxygenase activity and inhibition of lipid peroxide formation. Aggregation of platelets derived from individuals on a dietary supplementation of alpha-tocopherol ranging from 400 to 1200 IU/day showed no significant reduction. The discrepancy between the effectiveness of alpha-tocopherol in vitro and ex vivo is probably related to the levels of alpha-tocopherol attainable in platelets and plasma. Investigation of the effect of alpha-tocopherol on platelet adhesion showed a major inhibitory activity at doses of vitamin E as low as 200 IU/day. Measurements were performed in a laminar flow chamber at both high and low shear rates. Reduced platelet adherence to collagen, fibrinogen, and fibronectin could be documented. alpha-tocopherol-enriched platelets that adhered to adhesive surfaces failed to show the usual long thin pseudopodia but exhibited short, rounded, blunt projections. The reason for this shape change is still unclear, but we speculate that it may be causing the vitamin E-induced reduction of platelet adhesiveness. We believe that dietary supplementation with this vitamin could play a role in the treatment of thromboembolic disease, especially when given in conjunction with an inhibitor of platelet aggregation.

Decrease in platelet reduced glutathione increases lipoxygenase activity and decreases vitamin E

Unstimulated normal human blood platelets were treated with azodicarboxylic acid bis(dimethylamide) (diamide), a thiol-oxidizing agent. Oxygenated arachidonic acid (AA) metabolites, malondialdehyde (MDA), and tocopherols were then quantified by high-performance liquid chromatography (HPLC). Diamide treatment partially decreased the amount of reduced glutathione (GSH) content and induced a subsequent decrease in peroxidase activity. However, formation of 12-hydroxy-eicosatetraenoic acid (12-HETE), the end-product of lipoxygenation of AA, increased. Formation of MDA, a marker of overall lipid peroxidation, was also enhanced. Furthermore, platelet alpha-tocopherol, but not gamma-tocopherol, significantly decreased. These results indicate that enhanced "basal" lipoxygenase activity, as a marker of specific AA oxygenation, may be linked to decreased platelet antioxidant status.