Lysis of Platelet‐rich Thrombi: The Role of PAI–1a

Abstract

bDepamnent 0fMohurCW and cell B W b UnmerSity ofAbemkn, Mkiicbal GI@ Abrrdcnz AB9 1AS Scotkurd CLlepmnent of Meduine and Thnapeutics U n M t y 0fAbemkn Abeidw AB9 220 Scotland INTRODUCTIONThe process of thrombus formation depends on the concerted actions of a number of hemostatic mechanisms. These include generation of thrombin, resulting in fibrin formation, and platelet plug fbrmation, d i g fiom platelet adhesion and appegation. Platelets affect thrombus development, since they contribute procoagulant activity. In addition, platelet-rich thrombi are less susceptible to fibrinolysis than platelet-poor thrombi.lJ A number of factors contribute to this effect, including the dense structure of platelet-rich thrombi, due in part to the more dramatic retraction of such thrombi.3 The question addressed here is whether platelet constituents themselves make a contribution to this resistance to thrombus lysis, in particular by specific inhibition of the fibrinolytic process.Activation of plasminogen to the active protease, plasmin, is central to the fibrinolytic mechanism. This is accomplished either by tissue plasminogen activator (tPA) or by urokinase (uPA), the former having the dominant role in vascular fibrinolysis.* The process can be inhibited on two levels, by specific inhibitors of either plasmin or plasminogen activators. The principal inhibitor of plasmin is a2-antiplasmin (ar2hP) and its role in the regulation of fibrinolysis is well documented.' The role in hemostasis of plasminogen activator inhibitor 1 (PAI-1), the major inhibitor of plasminogen activation, is less clear. The plasma concentration of PAI-1 is I o w ,~~~ and even when elevated in disea~,~.* is relatively easily overcome by h e concentrations of tPA, such as those used therape~tically.~ Platelets also serve as a reservoir of PAI-llOJ1 and account for 93% of circulating PAI-l.5 This pool of PAI-1 is independent of the plasma pool, as evidenced by the lack of correlation between the PAI-1 concentmaons in the two pools and also by the normal plasma concentration of PAI-1 in a patient with Grey Platelet Syndrome, a We thank the British Heart Foundation and Grampian Health Board, who supported this Author to whom correspondence should be addrrssed; Tel.: 44-224273118146; FAX: project. 44-224-273144. 70

Characterization of a Small Molecule Inhibitor of Plasminogen Activator Inhibitor Type 1 That Accelerates the Transition into the Latent Conformation

“…This PAI-1 is not in complex with VN (28 -30) and is released upon platelet activation (28,59). Blood clots contain large amounts of PAI-1 that may originate from the ␣-granules of activated platelets and arterial clots contain 2-to 3-fold more PAI-1 than venous clots and are more resistant to thrombolysis (60,61). Thus, a chronic administration of a small molecule with the same mode of action as AZ3976 could potentially inactivate PAI-1 stored in platelets before it becomes excreted.…”

Type 1 Plasminogen Activator Inhibitor Binds to Fibrin via Vitronectin

“…The major inhibitor of t-PA, type 1 plasminogen activator inhibitor (PAI-1), circulates in plasma and is released from platelet ␣-granules during blood clotting (5,6). PAI-1 accumulates in thrombi, rendering them resistant to t-PA-mediated fibrinolysis (7)(8)(9)(10)(11)(12)(13)(14). In purified systems, PAI-1 has been shown to bind directly to fibrin, with a K d of 3.7 M (15-18).…”

Type 1 Plasminogen Activator Inhibitor Induces Multimerization of Plasma Vitronectin

“…The scenario is quite different in platelets. Both PAI-1 and Vn are contained within platelet ␣-granules (16), and platelets contain 26 nM Vn and 12 nM PAI-1/10 9 platelets (37,38). Interestingly, platelet Vn is present in high molecular weight multimers and expresses epitopes for conformationally sensitive antibodies (16,35), raising the possibility that the multimerization of platelet Vn is mediated by PAI-1.…”