Protein S : new insights into its functions

Protein S is a vitamin K dependent plasma protein that is mostly known for its anticoagulant properties. The pathophysiological importance of protein S became clear clinically by identification of infants with complete deficiency, who suffer purpura fulminans despite treatment with anticoagulants. Moreover, individuals with inherited heterozygous or acquired protein S deficiency are at enhanced risk of developing thrombotic diseases. Recently, the importance of protein S was also demonstrated in murine knockouts, which failed to survive development, and in heterozygous mice which have a pro-thrombotic phenotype. In addition to its role in coagulation, protein S is involved in the classical complement pathway and apoptotic clearance. It has also been reported to stimulate vascular injury repair by acting as a mitogen towards smooth muscle cells and has been found to have a neuroprotective effect during ischaemic brain injury in mice. Protein S is a 75 kDa protein, constituted by an N-terminal Gla domain, a thrombin sensitive region (TSR), four epidermal growth factor (EGF) domains and a C-terminal sex hormone-binding globulin (SHBG) domain. Protein S is mainly expressed by hepatocytes, but also by endothelial cells and megakarocytes and circulates in plasma at a concentration of ~333 nM. The Gla domain of protein S, containing 11 γ-carboxylated glutamic acid residues, mediates binding to phospholipid surfaces, which is essential for protein S cofactor function. Approximately 60% of protein S exists in a high affinity complex with C4b-binding protein (C4BP) to which it binds through its SHBG domain in a 1:1 stoichiometry, leaving approximately 133 nM of free protein S in circulation. The anticoagulant properties of protein S have mostly been investigated with respect to its cofactor activity towards activated protein C (APC) in the inactivation of coagulation factor Va (FVa) and FVIIIa. It has, however, been known since 1988 that protein S also has an APC-independent anticoagulant activity. Part of the APC-independent activity that was initially observed was later shown to be caused by protein S multimers, likely formed during in vitro manipulation and purification procedures. The protein S multimers were able to compete with the tenase and prothrombinase for binding to limiting phospholipid surfaces. However, an APC-independent activity of protein S was still observed in plasma at saturating concentrations of phospholipids. In 2006, the molecular mechanism behind this latter APC-independent anticoagulant activity of protein S was identified when it was shown that protein S can act as a cofactor for tissue factor pathway inhibitor (TFPI). Additional studies have suggested that there still are protein S direct effects, independent of both APC and TFPI, and that these are dependent on protein S binding to Zn. However, the physiological importance of this protein S direct effect in comparison to APC and TFPI cofactor activity of protein S still needs to be elucidated. Despite the pathophysiological importance of protein S in the inhibition of coagulation, the molecular mechanisms behind the cofactor functions for APC and TFPI are not completely understood. In the following sections, recent progress will be summarised.