α-Tocopherol Inhibits Aggregation of Human Platelets by a Protein Kinase C–Dependent Mechanism

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Abstract
Background Epidemiological studies indicate that vitamin E (α-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 α-tocopherol (500 μmol/L) or vehicle (0.5% ethanol), we found that inhibition of platelet aggregation by α-tocopherol was closely linked to its incorporation into platelets ( r =−.78; P <.02). Platelet incorporation of α-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 α-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 α-tocopherol (400 to 1200 IU/d) resulted in an increase in platelet α-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 α-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 α-tocopherol in preventing the development of coronary artery disease.

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