Structure and function of thrombomodulin: a natural anticoagulant.

NDOTHELIAL CELLS lining blood vessels possess E anticoagulant functions, although they can rapidly become procoagulant when injured. The anticoagulant actions include production of prostacyclin,’ secretion of plasminogen activator,* cell surface heparin containing proteoglycans that stimulate certain coagulation factor inhibitor^,^ and thrombomodulin, an endothelial cell surface glycoprotein that promotes protein C ac t i~a t ion . ~~’ When thrombin binds to thrombomodulin, the procoagulant activity of thrombin is altered. Thrombin activates the anticoagulant protein C much more efficiently (greater than 1,000-fold), but no longer clots fibrinogen or activates platelets as effectively as unbound thrombin. An anticoagulant activity in thrombin treated plasma was initially described in 1960 by Mammen et that later proved to be protein C,798 isolated by Stenflo in 1976.9 Protein C’0-’4 circulates in plasma a t 4 to 5 pg/mL as an inactive zymogen, primarily as a two-chain protein held together by a single disulfide bond. I t is activated by cleavage of a 12 amino acid peptide from the amino terminus of the heavy chain. Activated protein C, in the presence of a phospholipid surface, CA+’, and a second vitamin K-dependent protein cofactor, protein S, inhibits coagulation by proteolytically degrading factors Va and VIIIa. ”-” Protein C plays an important role in normal physiology as indicated by the occurrence of neonatal purpura fulminans in infants with severe or homozygous protein C deficiency. ” While some heterozygous protein C deficient subjects suffer thrombosis,” most do not. ” There are decreased levels of protein C in patients with disseminated intravascular coagulation. *’ Efforts to find an efficient protein activator of protein C were unsuccessful until 1981 when Esmon and Owen” performed a clever experiment that led to the discovery of thrombomodulin. They reasoned that if an activator of protein C was not in blood, perhaps it was on the endothelial lining of blood vessels. They perfused an isolated rabbit heart with thrombin and protein C simultaneously and found an increase in activation of protein C compared with the effect of thrombin without the endothelium. The activity was confirmed to be on endothelial cells using cultured cells23 and was isolated as a detergent solubilized protein from rabbit lungs.24 Thrombomodulin has subsequently been isolated from human p l a ~ e n t a , ~ ~ ~ ~ ~ bovine lung,27 mouse lung,” and rat lung.29 Thrombin forms a 1: 1 complex with thrombomodulin with a Kd of 0.5 nmol/L thrombin (Fig 1). The complex activates protein C with a K, of 0.5 pmol/L protein C on cell surfaces or in phospholipid vesicles. When bound to thrombomodulin, thrombin is less efficient in catalyzing procoagulant reactions. Clotting of f i b r i n ~ g e n ~ ~ . ~ ’ and activation of platelet^,^' factor V and factor XIII,33 and inactivation of protein S34 by thrombin are all inhibited by binding of thrombin to thrombomodulin. Thrombomodulin also decreases the activation of prothrombin when complexed with factor Xa.3’ Thus thrombomodulin provides a mechanism whereby thrombin found outside blood vessels, remote from the endothelium, promotes clot formation, while thrombin in proximity to endothelium generates the anticoagulant activated protein C to maintain blood fluidity.