R, R, R- α-tocopherol potentiates prostacyclin release in human endothelial cells. Evidence for structural specificity of the tocopherol molecule

Abstract

Human umbilical vein endothelial cells (HUVEC) in culture synthesize prostacyclin (PGI 2) as the predominant metabolite of arachidonic acid which is derived from the deacylation of phospholipids. Under basal-unstimulated condition, PGI 2 release from HUVEC is extremely low; however, when endothelial monolayers were preincubated with the natural vitamin E ( R, R, R- α-tocopherol), we found a dose-dependent potentiation of basal PGI 2 release. When HUVEC were stimulated with arachidonate or ionophore A23187, there was a dose-dependent increase of PGI 2 release in response to tocopherol enrichment. When HUVEC were labelled with [ Me- 3H]choline followed by A23187 stimulation, a significantly higher lysophosphatidylcholine was found in the tocopherol-enriched cells, suggesting a change in enzymes involved in phosphatidylcholine metabolism. Analysis of these enzymes revealed that phospholipase A 2 activity was enhanced by tocopherol enrichment, whereas lysophospholipase and acyl-CoA acyltransferase were unaffected. To determine the specificity of the tocopherol molecule, different analogues were tested for their PGI 2 potentiating activity. Results showed that the free hydroxyl group on the chromanol ring as well as the phytyl side-chain are absolutely required to stimulate PGI 2 release, whereas, different methyl locations and substituents on the chromanol ring had no effect. These studies demonstrated that tocopherol potentiates basal PGI 2 release in HUVEC and in contrast to its reported inhibitory role in rat platelets, myocadium and neutrophils, tocopherol stimulates phospholipase activity in HUVEC.