ENDOTHELIUM AND PROTEIN S: SYNTHESIS, RELEASE AND REGULATION OF ANTICOAGULANT ACTIVITY

Abstract

The protein C-protein S pathway is closely linked to the vessel wall. In terms of protein C, endothelium has been shown to provide the receptor thrombomodulin, which promotes thrombin-mediated formation of activated protein C. Optimal anticoagulant function of activated protein C requires protein S and a cellular surface. Recent studies have indicated that endothelium can facilitate assembly of the activated protein C-protein S complex and that bovine endothelium expresses specific binding site(s) for protein S which promote its anticoagulant function. Expression of protein S binding sites is subject to down-regulation by Tumor Necrosis Factor (TNF) . Exposure of cultured bovine endothelium to TNF results in decreased 125I-protein s binding and attenuated rates of Factor Va inactivation after 2 hrs followed by negligible 125I-protein S binding and Factor Va inactivation by 10 hrs. These changes persist for over 48 hrs, in contrast to the more transient rise in endothelial cell tissue factor induced by TNF which returns to baseline by 24 hrs.In addition to providing binding sites for protein S, endothelium constitutively synthesizes and releases this vitamin K-dependent anticoagulant cofactor. Release of protein S is blocked by addition of warfarin, indicating that y-carboxylation facilitates the release of intracellular protein S. Morphologic studies, at the level of electron microscope, have shown protein S antigen to be present in cisternae of rough endoplasmic reticulum, the trans face of the golgi and a population of intracellular vesicles which appear to be distributed at the cellular periphery. By immunofluorescence, the distribution of protein S is distinct from that of von Willebrand Factor. The intracellular vesicles containing protein S constitute a storage pool potentially available for rapid release. Treatment of endothelium with norepinephrine results in release of protein S over the next 20 min. Release is half-maximal at a norepinephrine concentration of about 0.1 uM and is not observed with the biologically inactive entantiomer (+) norepinephrine. Norepinephrine-induced release of intracellular protein S can be blocked by prazosine (10-7 7 M), but not by propranolol (10-6 M) or yohimbine (10-5 M). These data are consistent with release of protein S being a receptor-mediated process dependent on an endothelial cell alpha 1 adrenergic receptor. Blockade of norepinephrine-induced release of protein S by pertussis toxin treatment of endothelium further defines the intracellular pathway of protein S and implicates regulatory G proteins in the stimulus-response coupling. Electron microscopic studies have shown that following exposure of endothelium to norepinephrine the intracellular vesicles containing protein S undergo exocytosis at the plasma membrane. These data define a new relationship between the autonomic nervous system and the coagulation mechanism.Protein S is clearly an endothelial cell-associated anticoagulant protein. A specific binding site on the endothelial cell surface can regulate its anticoagulant function on the vessel wall. Endothelial cell synthesis and release of protein S defines a new level of participation of endothelium in the protein C-protein S pathway.