Esmon C T, Fukudome K, Mather T, Bode W, Regan L M, Stearns-Kurosawa D J, Kurosawa S
Oklahoma Medical Research Foundation Cardiovascular Biology Research, 825 N.E. 13th Street, Oklahoma City, Oklahoma 73104, USA.
Haematologica. 1999 Mar;84(3):254-9.
The molecular links between inflammation and coagulation are unquestioned. Inflammation promotes coagulation by leading to intravascular tissue factor expression, eliciting the expression of leukocyte adhesion molecules on the intravascular cell surfaces, and down regulating the fibrinolytic and protein C anticoagulant pathways. Thrombin, in turn, can promote inflammatory responses. This creates a cycle that logically progresses to vascular injury as occurs in septic shock. Most complex systems are regulated by product inhibition. This inflammation-coagulation cycle seems to follow this same principle with the protein C pathway serving as the regulatory mechanism. The molecular basis by which the protein C pathway functions as an anticoagulant is relatively well established compared to the mechanisms involved in regulating inflammation. As one approach to identifying the mechanisms involved in regulating inflammation, we set out to identify novel receptors that could modulate the specificity of APC in a manner analogous to the mechanisms by which thrombomodulin modulates thrombin specificity. This approach led to the identification of an endothelial cell protein C receptor (EPCR). To understand the mechanism, we obtained a crystal structure of APC (lacking the Gla domain). The crystal structure reveals a deep groove in a location analogous to anion binding exosite 1 of thrombin, the location of interaction for thrombomodulin, platelet thrombin receptor and fibrinogen. Thrombomodulin blocks the activation of platelets and fibrinogen without blocking reactivity with chromogenic substrates or inhibitors. Similarly, in solution, EPCR blocks factor Va inactivation without modulating reactivity with protease inhibitors. Thus, these endothelial cell receptors for the protein C system share many properties in common including the ability to be modulated by inflammatory cytokines. Current studies seek to identify the substrate for the APC-EPCR complex as the next step in elucidating the mechanisms by which the protein C pathway modulates the response to injury and inflammation.
炎症与凝血之间的分子联系是毋庸置疑的。炎症通过导致血管内组织因子表达、引发血管内细胞表面白细胞黏附分子的表达以及下调纤维蛋白溶解和蛋白C抗凝途径来促进凝血。反过来，凝血酶可促进炎症反应。这就形成了一个循环，在脓毒性休克中，这种循环会顺理成章地发展为血管损伤。大多数复杂系统是通过产物抑制来调节的。这种炎症 - 凝血循环似乎遵循相同的原理，蛋白C途径作为调节机制。与调节炎症的机制相比，蛋白C途径作为抗凝剂发挥作用的分子基础相对较为明确。作为确定调节炎症机制的一种方法，我们着手寻找新型受体，这些受体能够以类似于血栓调节蛋白调节凝血酶特异性的机制来调节活化蛋白C（APC）的特异性。这种方法导致了内皮细胞蛋白C受体（EPCR）的发现。为了解其机制，我们获得了APC（缺乏Gla结构域）的晶体结构。晶体结构显示在一个类似于凝血酶阴离子结合外位点1的位置有一个深沟，这是血栓调节蛋白、血小板凝血酶受体和纤维蛋白原相互作用的位置。血栓调节蛋白可阻断血小板和纤维蛋白原的活化，但不阻断与显色底物或抑制剂的反应性。同样，在溶液中，EPCR可阻断因子Va的失活，但不调节与蛋白酶抑制剂的反应性。因此，这些蛋白C系统的内皮细胞受体具有许多共同特性，包括能够被炎性细胞因子调节。目前的研究试图确定APC - EPCR复合物的底物，这是阐明蛋白C途径调节对损伤和炎症反应机制的下一步。
