Prevention of platelet dysfunction by vitamin E in diabetic atherosclerosis

The platelet in diabetes: focus on prevention of ischemic events

Accelerated atherosclerosis and the increased risk of thrombotic vascular events in diabetes may result from dyslipidemia, endothelial dysfunction, platelet hyperreactivity, an impaired fibrinolytic balance, and abnormal blood flow. There is also a correlation between hyperglycemia and cardiovascular (CV) events. The importance of platelets in the atherothrombotic process has led to investigation of using antiplatelet agents to reduce CV risk. A meta-analysis conducted by the Antiplatelet Trialists' Collaboration demonstrated that aspirin reduced the risk of ischemic vascular events as a secondary prevention strategy in numerous high-risk groups, including patients with diabetes. Based on results from placebo-controlled randomized trials, the American Diabetes Association recommends low-dose enteric-coated aspirin as a primary prevention strategy for people with diabetes at high risk for CV events. Clopidogrel is recommended if aspirin allergy is present. There is occasionally a need for an alternative to aspirin or for additive antiplatelet therapy. Aspirin in low doses inhibits thromboxane production by platelets but has little to no effect on other sites of platelet reactivity. Agents such as ticlopidine and clopidogrel inhibit ADP-induced platelet activation, whereas the platelet glycoprotein (Gp) IIb/IIIa complex receptor antagonists block activity at the fibrinogen binding site on the platelet. These agents appear to be useful in acute coronary syndromes (ACSs) in diabetic and nondiabetic patients. A combination of clopidogrel plus aspirin was more effective than placebo plus standard therapy (including aspirin) in reducing a composite CV outcome in patients with unstable angina and non-ST segment elevation myocardial infarction. In a meta-analysis of six trials in diabetic patients with ACSs, intravenous GpIIb-IIIa inhibitors reduced 30-day mortality when compared with control subjects. Results from controlled prospective clinical trials justify the use of enteric-coated low-dose aspirin (81-325 mg) as a primary or secondary prevention strategy in adult diabetic individuals (aged >30 years) at high risk for CV events. Recent studies support the use of clopidogrel in addition to standard therapy, as well as the use of GpIIb-IIIa inhibitors in ACS patients.

Protective effect of three diallyl sulphides against glucose-induced erythrocyte and platelet oxidation, and ADP-induced platelet aggregation

Isolated human erythrocyte membrane and platelet were used to study the antioxidative and anti-aggregative effects of three diallyl sulphides (diallyl sulphide, DAS; diallyl disulphide, DADS; diallyl trisulphide, DAT) against glucose-induced oxidation and adenosine 5'-diphosphate (ADP)-induced platelet aggregation. Three sulphide agents showed dose-dependent antioxidative protection against glucose-induced erythrocyte membrane oxidation (p<.05), and these agents at 10 microM significantly increased the retention of alpha-tocopherol in erythrocyte membrane (p<.05), in which DAT was the most effective agent (p<.05). Three sulphide agents significantly inhibited 30 and 50 mM of glucose-induced platelet oxidation (p<.05). The anti-aggregative activity of each sulphide agent was dose dependent (p<.05), in which DAT showed the greatest inhibitory effect on platelet aggregation than DADS, followed by DAS (p<.05). After ADP stimulation, the malondialdehyde (MDA) formation in platelets treated with sulphide agents was significantly less (p<.05), in which DADS and DAT showed similar antioxidative activities (p>.05). These results suggested that DADS and DAT could be considered as strong antioxidative and antithrombotic agents to prevent or control oxidative damage and platelet hyperactivity.

Effects of long-term supplementation with moderate pharmacologic doses of vitamin E are saturable and reversible in patients with type 1 diabetes

BACKGROUND

Vitamin E supplementation has been proposed as adjunctive therapy to counteract the increased LDL oxidation in diabetes and thus prevent or delay cardiovascular complications.

OBJECTIVE

The objective of this study was to investigate the effect of a moderate pharmacologic dose of vitamin E for </=1 y in patients with type 1 diabetes.

DESIGN

The study was double blind and the subjects were randomly assigned to 2 groups: the supplemented group (group S; n = 22) received 250 IU (168 mg) RRR-alpha-tocopherol 3 times/d for 1 y and the placebo group (group P; n = 22) received a placebo for 6 mo followed by 250 IU (168 mg) RRR-alpha-tocopherol 3 times/d for an additional 6 mo.

RESULTS

Serum vitamin E doubled after 3 mo of supplementation, from a mean (+/-SD) of 36.9 +/- 10.9 to 66.4 +/- 18.3 micromol/L (P: < 0.0005). Although lipid profiles, glycated hemoglobin, and blood biochemistry values did not change significantly, copper-induced in vitro peroxidizability of LDL and VLDL decreased after 3 mo of supplementation: the production of thiobarbituric acid-reactive substances decreased by 30-60% (P: < 0. 005) and the lag time for the appearance of fluorescent products increased from 107 +/- 25 to 123 +/- 30 min in group S (P: = 0.002 compared with group P). Vitamin E supplementation for an additional 3-9 mo resulted in no further changes in serum vitamin E and lipoprotein peroxidizability. Values returned to baseline after supplementation ended.

CONCLUSIONS

Because the improvement in lipoprotein peroxidizability is saturable and reversible, life-long supplementation with vitamin E should be considered in patients with type 1 diabetes.

Alpha-tocopherol downregulates the expression of GPIIb promoter in HEL cells

Alpha-tocopherol is known to inhibit platelet aggregation but the mechanism responsible for this has not been elucidated. Glycoprotein IIb (GPIIb) is the alpha-subunit of the platelet membrane protein GPIIb/IIIa, which functions as a specific receptor for platelet aggregation. Human erythroleukemia (HEL) cells are megakaryocytelike and express the megakaryocyte-specific GPIIb gene. To understand the molecular mechanism of alpha-tocopherol on antiaggregation, we analyzed the effect of physiologically relevant concentrations of alpha-tocopherol on the expression of human GPIIb promoter in HEL cells. The enhancement of tetradecanoylphoerbol-12,13-acetate (TPA)-mediated transient and optimal expression of plasmids was achieved by adding 10(-7)-M TPA in the medium 24 h before lipofection. Transient expression of GPIIb promoter was determined in transfected cells pretreated with various concentrations of alpha-tocopherol. Our data shows that the GPIIb promoter activity was downregulated to 55, 23, 27, 20, and 15% in the presence of 10, 20, 40, 80, and 120 microg/ml of alpha-tocopherol, respectively, as compared with that in the absence of alpha-tocopherol. The downregulation of alpha-tocopherol on the TPA-mediated GPIIb promoter activity may result in a reduction of GPIIb protein expression and thus contribute to antiplatelet aggregation.

Effects of vitamin E on human platelet and mononuclear cell responses in vitro

Recent epidemiological studies have provided evidence supporting the potential benefits of antioxidants in coronary prevention. We have investigated the effects of vitamin E on platelets, monocytes and endothelial cells in vitro. Pre-incubation of platelets with vitamin E inhibited subsequent thrombin- (P < 0.05, n = 5), collagen- (P < 0. 0001, n = 5) and ADP-(P < 0.05, n = 4) induced platelet aggregation measured using a microtitre plate method, or conventional aggregometry. The adhesion of thrombin-activated platelets to collagen was also inhibited by vitamin E (P < 0.05, n = 8), but not by vitamin C (P > 0.05, n = 8); nor was the adhesion of unstimulated platelets significantly affected (P > 0.05, n = 8). Pre-incubation of monocytes with vitamin E inhibited their subsequent adhesion to plastic (P < 0.05, n = 9), and was also associated with an 18% reduction in adhesion to EA.hy 926 endothelial cells (n = 8), although this failed to reach statistical significance. Pre-incubation of the endothelial cells with vitamin E also significantly reduced subsequent mononuclear cell adhesion by 56% (P < 0.05, n = 3).

Contrast media effects on hemostatic and thrombotic parameters. Possible consequences for practical techniques and prophylactic measures

This paper reviews the basic mechanisms of the thrombohemorrhagic balance and ways in which contrast media (CM) influence these processes. Coagulation and platelet functions are strongly inhibited by ionic CM, but weakly so by nonionic CM, whereas the former are more detrimental to endothelium, and thus thrombogenic in this sense. Some observations indicate a lower rate of thromboembolic events with the ionic CM in percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass grafting (CABG), but this purported difference does not effect mortality or frequency of re-PTCA and emergency CABG. Thus, to challenge these events, strong acting antithrombotics, which also, unlike heparin, inactivate fibrin-bound thrombin, are necessary. Aggressive anti-atherogenic prophylaxis may hamper both thrombosis and reocclusion. The ideal antithrombotic in this setting is yet to be found.