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.