α-Tocopherol and Endothelial Nitric Oxide Synthesis

• Published in: Annals of the New York Academy of Sciences Vol. 1031; no. 1; pp. 74 - 85
• Main Authors: HELLER, REGINE, WERNER-FELMAYER, GABRIELE, WERNER, ERNST R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.12.2004
• Subjects: alpha-Tocopherol - pharmacology; ascorbic acid; Biopterins - analogs & derivatives; Biopterins - metabolism; Cells, Cultured; Citrulline - biosynthesis; Cyclic GMP - biosynthesis; endothelium; Endothelium, Vascular - drug effects; Endothelium, Vascular - metabolism; Humans; Nitric Oxide - biosynthesis; nitric oxide synthase; Nitric Oxide Synthase - metabolism; Nitric Oxide Synthase Type III; Phosphorylation; Serine - metabolism; Umbilical Veins; α-tocopherol
• ISSN: 0077-8923; 1749-6632
• DOI: 10.1196/annals.1331.007

: Nitric oxide (NO), a central regulator of vascular tone and homeostasis, is generated upon activation of endothelial NO synthase (eNOS), which is mediated by an increase of intracellular calcium and/or by eNOS phosphorylation. A reduction of NO bioavailability leads to endothelial dysfunction that has been shown to be improved by α‐tocopherol in certain conditions. The underlying mechanisms, however, are not completely clarified. The present study was performed to investigate whether α‐tocopherol is able to affect endothelial NO synthesis. The formation of NO was measured in human umbilical vein endothelial cells using citrulline (coproduct) and cGMP (product of the NO‐activated soluble guanylate cyclase) as indicator molecules. α‐Tocopherol (10‐200 μM, 24 hr) increased ionomycin‐induced citrulline and cGMP formation in intact cells in a concentration‐dependent manner. In parallel, ionomycin‐stimulated phosphorylation of eNOS at serine 1177, known to support enzyme activation, was increased by α‐tocopherol, suggesting that this was the mechanism responsible for enhanced NO formation. The effect of α‐tocopherol was dependent on its hydrophobic structure because it was mimicked by γ‐tocopherol but not by trolox, a hydrophilic derivative of α‐tocopherol. Coincubation with ascorbic acid (100 μM, 24 hr) amplified the effects of α‐tocopherol on eNOS phosphorylation and NO formation, which is possibly related to the regeneration of oxidized α‐tocopherol by ascorbate. Our data suggest that vasoprotective effects of α‐tocopherol in vivo may be related to an increase of NO formation. The effect of α‐tocopherol seems to be dependent on tissue saturation with ascorbic acid, and both vitamins may act synergistically to provide optimal conditions for endothelial NO formation.