Summary
Thrombin interacts with its receptor and thrombomodulin on endothelial cells. We evaluated
the respective roles of these two proteins on human umbilical vein endothelial cell
(HUVEC) growth by comparing thrombin, S195A (a mutant thrombin in which the serine
of the charge stabilizing system had been replaced by alanine), and the receptor activating
peptide (TRAP). Thrombin and TRAP induced DNA synthesis (half maximal cell proliferation
with 5 nM and 25 μM, respectively), whereas S195A thrombin was inactive, inferring
that growth is mediated through the thrombin receptor. Surprisingly, cells stimulated
by TRAP exhibited a maximal proliferation twice greater than that obtained with thrombin.
Combination of thrombin and TRAP resulted in a mitogenic response higher than by thrombin
alone, but lower than by TRAP alone. The role of thrombomodulin was evaluated by adding
an anti-thrombomodulin antibody, which prevents formation of the thrombin-thrombomodulin
complex. Antibody did not interfere with cell proliferation induced by TRAP, but enhanced
that induced by thrombin. We conclude that formation of the thrombin-thrombomodulin
complex restrains HUVEC proliferation mediated through the thrombin receptor.