Angiopoietins bind thrombomodulin and inhibit its function as a thrombin cofactor

Angiopoietin-1 (Ang1) and Angiopoietin-2 (Ang2) are ligands for Tie2, an endothelial-specific receptor tyrosine kinase that is an essential regulator of angiogenesis. Here we report the identification, via expression cloning, of thrombomodulin (TM) as another receptor for Ang1 and Ang2. Thrombomodulin is an endothelial cell surface molecule that plays an essential role as a coagulation inhibitor via its function as a cofactor in the thrombin-mediated activation of protein C, an anticoagulant protein, as well as thrombin-activatable fibrinolysis inhibitor (TAFI). Ang1 and Ang2 inhibited the thrombin/TM-mediated generation of activated protein C and TAFI in cultured endothelial cells, and inhibited the binding of thrombin to TM in vitro. Ang2 appears to bind TM with higher affinity than Ang1 and is a more potent inhibitor of TM function. Consistent with a potential role for angiopoietins in coagulation, administration of thrombin to mice rapidly increased plasma Ang1 levels, presumably reflecting release from activated platelets (previously shown to contain high levels of Ang1). In addition, Ang1 levels were significantly elevated in plasma prepared from wound blood, suggesting that Ang1 is released from activated platelets at sites of vessel injury. Our results imply a previously undescribed role for angiopoietins in the regulation of hemostasis.