Recombinant human thrombomodulin attenuates human endothelial cell activation by human thrombin

Two glycoforms of recombinant human thrombomodulin (TM; TMD1-105 and TMD1-75), an endothelial cell membrane protein, were tested for their ability to alter thrombin-induced activation of cultured human umbilical vein endothelial cells (HUVECs). After stimulation with 10 nmol/L thrombin, HUVEC generation of inositol-1,4,5-trisphosphate (IP₃), a potent Ca²⁺-mobilizing second messenger, was dose-dependently blocked by TMD1-105. Both TMD1-105 (IC₅₀ = 10 nmoI/L) and TMD1-75 (IC₅₀=100 nmol/L) blocked the enhanced prostacyclin synthesis by HUVEC monolayers treated with 10 nmol/L thrombin. HUVEC monolayer permeability to Evans blue dye-labeled albumin increased from 0.125±0.06 μL/min in control experiments to 0380±0.09 μL/min after treatment with 100 nmol/L thrombin (P<.05). Incubation of HUVECs with TMD1-105 alone (600 nmol/L) had no effect (0.114±0.04 μL/min) on basal permeability. In contrast, incubation of 100 nmol/L thrombin with 600 nmol/L TMD1-105 reduced this increase in HUVEC permeability to almost control levels (0.142±0.06 μL/min). These results demonstrate that recombinant human TM, a potent in vitro anticoagulant, also functions as an antagonist of thrombin receptor-mediated HUVEC activation. In addition to its anticoagulant functions, the high-affinity endothelial cell receptor TM may play a role in modulating endothelial cell activation by thrombin.