Up-regulation of thrombomodulin by activation of histamine H1-receptors in human umbilical-vein endothelial cells in vitro

Previous reports demonstrated that the expression of thrombomodulin (TM) in endothelial cells was modulated by various agents. Although TM was down-regulated by endotoxin or cytokines, up-regulation of TM was accomplished when endothelial cells were stimulated with unphysiologically high concentrations of cyclic AMP derivatives or tumour-promoting phorbol esters. We investigated the expression of TM in human umbilical-vein endothelial cells (HUVECs) by physiological substances that can be released into the bloodstream. Histamine (0.1-10 microM, 1-48 h) increased TM activity, TM antigen in cell lysates and TM mRNA levels, but 5-hydroxytryptamine and bradykinin had no effect. Enhancement of TM activity by histamine was completely blocked by the H1-selective antagonist pyrilamine, whereas the H2-antagonist cimetidine had no effect, showing that histamine up-regulates TM activity via H1-receptors on HUVECs. Enhanced TM activity by histamine and the resultant increase in protein C activation might play a role in a feedback regulation for prevention of vascular thrombosis.