α-Tocopherol Amplifies Phosphorylation of Endothelial Nitric Oxide Synthase at Serine 1177 and its Short-Chain Derivative Trolox Stabilizes Tetrahydrobiopterin

Abstract α-Tocopherol has been shown to increase nitric oxide (NO)-dependent relaxation but the underlying mechanisms have not been fully characterized. The present study investigates the effect of α-tocopherol and its derivative trolox on the synthesis of NO in human umbilical vein endothelial cells. NO was assayed as citrulline (co-product of NO) and cGMP (product of the NO-activated soluble guanylate cyclase) on ionomycin stimulation of cells. Ionomycin induced citrulline and cGMP formation partially through phosphorylation of endothelial NO synthase (eNOS) at its serine residue 1177, which was mediated mainly by calmodulin-dependent kinase II. Preincubation of cells with α-tocopherol or trolox increased eNOS activity in a concentration-dependent manner without changing eNOS expression. The effect of the water-soluble trolox was due to chemical stabilization of the eNOS cofactor tetrahydrobiopterin. On the contrary, α-tocopherol, located mainly in cellular membranes, did not affect tetrahydrobiopterin but increased ionomycin-induced eNOS phosphorylation at serine 1177. The effects of α-tocopherol on citrulline and cGMP formation and eNOS phosphorylation were amplified by co-incubation with ascorbate, which is suggested to regenerate oxidized α-tocopherol and to act synergistically with α-tocopherol. Our data describe a new vasoprotective function of α-tocopherol that may contribute to the prevention of endothelial dysfunction in vivo.