Antithrombin: a new look at the actions of a serine protease inhibitor.

Antithrombin (AT) is a plasma-derived, single-chain glycoprotein with a molecular weight of 58 kDa. It is a serine protease inhibitor (serpin), sharing about 30% homology in amino acid sequence with other serpins. AT is a complex molecule with multiple biologically important properties. It is a potent anticoagulant that has been demonstrated to provide benefit in animal models and small cohorts of patients with coagulation disorders. AT also has remarkable anti-inflammatory properties, several of which result from its actions in the coagulation cascade. Activated coagulation proteases like activated factor X and thrombin contribute to inflammation; for instance, by the release of pro-inflammatory mediators. Inhibition of these proteases by AT prevents their specific interaction with cells and subsequent reactions. Anti-inflammatory properties of AT independent of coagulation involve direct interactions with cells leading to the release of, for instance, prostacyclin. Binding of AT to a recently identified cellular receptor, syndecan-4, leads to the interference with the intracellular signal induced by mediators like lipopolysaccharides and, thereby, to a down-modulation of the inflammatory response. AT has been shown to be effective in prospective and well-controlled small-scale studies of patients with inflammatory conditions, including sepsis. Although AT did not decrease overall patient mortality in a double-blind, placebo-controlled, phase III trial of patients with sepsis, it is important to note that AT improved the survival of individuals in this study not receiving heparin as a prophylactic regimen, which can be explained by the impaired interaction of AT with its cellular receptor in the presence of heparin, resulting in the reduction of the anti-inflammatory properties. Accordingly, the supplementation of AT without concomitant heparin may be beneficial in disorders with inflammatory characteristics, which has to be demonstrated in further clinical studies. Finally, recent results suggest that latent AT can induce apoptosis of endothelial cells by disrupting cell-matrix interactions. Further investigations will have to demonstrate whether latent and/or cleaved AT are physiological means to control angiogenesis. A potential prophylactic or therapeutic use as an anti-angiogenic and antitumor agent merits further exploration, including whether the growth of vessels in tumor tissues or close to tumors can be controlled by latent AT without affecting the formation of blood vessels during wound healing processes.

The frequent Marburg I polymorphism impairs the pro-urokinase activating potency of the factor VII activating protease (FSAP).

The recently reported plasmatic, Factor Seven Activating Protease (FSAP), has also been found to be a potent activator of pro-urokinase [single-chain plasminogen activator, urinary type (scuPA)]. An initial epidemiological study surprisingly showed that plasmas of 5-10% of healthy blood donors had an impaired potential to activate scuPA. Analysis of the respective genomic DNAs revealed one particular single nucleotide polymorphism of FSAP resulting in an identical amino acid exchange (G511E), which correlates with the reduced activities. The corresponding mutation was named FSAP Marburg I. Thrombelastographies of wild-type and mutant plasmas were performed, facilitating the auto-activation of the intrinsic FSAP pro-enzymes by addition of dextran sulfate (DXS) and accelerated clot lysis by addition of scuPA. On these conditions, tissue-factor-induced coagulation revealed that clot lysis was significantly delayed in the Marburg I mutant plasmas as compared with wild-type plasmas. Furthermore, in the presence of DXS and scuPA, a FSAP-deficient plasma revealed significantly prolonged plasma clot lysis times, whereas the addition of purified FSAP pro-enzyme plus scuPA reversed this effect. These results support the hypothesis that FSAP contributes to the scuPA-dependent plasma fibrinolytic potential, which can be impaired in plasmas containing the FSAP Marburg I polymorphism, for instance.