Institute of Medical Sciences, University of Aberdeen, Aberdeen, UK.
J Thromb Haemost. 2010 Jun;8(6):1313-22. doi: 10.1111/j.1538-7836.2010.03813.x. Epub 2010 Feb 17.
Platelets are essential for hemostasis, and they cause resistance to fibrinolysis by tissue-type plasminogen activator. In contrast, platelets enhance fibrinolysis mediated by single-chain urokinase-type plasminogen activator (scu-PA). This study investigated the mechanism behind this profibrinolytic role of platelets.
Platelets enhanced scu-PA activity, but not urokinase-type plasminogen activator (u-PA) activity, in plasma clot lysis and chromogenic assays. We established, using the non-cleavable scu-PA mutant (Lys158-->Glu) and protease inhibitors, that platelets increased activation to u-PA by a serine protease. Activation of scu-PA was platelet-dependent, even in plasma. It occurred in platelet-rich but not in platelet-poor plasma, as assessed by sodium dodecylsulfate polyacrylamide gel electrophoresis and zymography after addition of plasminogen activator inhibitor-1. Candidate proteases that are known to activate scu-PA and are present in platelet preparations were investigated. Factor VII activating protease was detected in platelet preparations by western blotting, but its inhibition by antibodies did not inhibit activation of scu-PA by platelets. Plasmin and plasma kallikrein both mimicked the platelet effect, but were distinguished by their responses to a range of inhibitors. Analysis of platelet-associated protease activity and the time course of scu-PA activation pointed towards plasminogen, and the data were consistent with a mechanism of reciprocal activation. The essential role of plasminogen was revealed using platelets from plasminogen-deficient mice, which could not activate scu-PA. Local plasminogen on platelet membranes was markedly more effective than solution-phase plasminogen in activation of scu-PA.
Platelets enhance fibrinolysis by scu-PA through reciprocal activation of scu-PA and platelet-associated plasminogen, a system that is potentially important in the lysis of platelet-rich thrombi.
血小板对于止血至关重要，它们通过组织型纤溶酶原激活物（tissue-type plasminogen activator，t-PA）抵抗纤维蛋白溶解。相反，血小板增强了由单链尿激酶型纤溶酶原激活物（single-chain urokinase-type plasminogen activator，scu-PA）介导的纤维蛋白溶解。本研究旨在探讨血小板发挥这种促纤维蛋白溶解作用的机制。
血小板增强了血浆凝块溶解和显色分析中的 scu-PA 活性，但不增强 u-PA 活性。我们通过使用不可切割的 scu-PA 突变体（Lys158-->Glu）和蛋白酶抑制剂证实，血小板通过丝氨酸蛋白酶增加 u-PA 的激活。即使在血浆中，血小板依赖性的 scu-PA 激活也会发生。在加入纤溶酶原激活物抑制剂-1 后，通过十二烷基硫酸钠聚丙烯酰胺凝胶电泳和酶谱分析，在富含血小板的血浆中而不是在血小板缺乏的血浆中可以观察到这种情况。研究了已知能够激活 scu-PA 并存在于血小板制剂中的候选蛋白酶。通过 Western blot 在血小板制剂中检测到因子 VII 激活蛋白酶，但用抗体抑制其不能抑制血小板对 scu-PA 的激活。纤溶酶和血浆激肽均模拟了血小板的作用，但它们对一系列抑制剂的反应不同。对血小板相关蛋白酶活性的分析以及 scu-PA 激活的时程指向了纤溶酶原，并且数据与一种相互激活的机制一致。使用缺乏纤溶酶原的血小板证实了纤溶酶原的重要作用，缺乏纤溶酶原的血小板不能激活 scu-PA。血小板膜上的局部纤溶酶原比溶液相中的纤溶酶原在激活 scu-PA 方面更有效。
血小板通过 scu-PA 和血小板相关纤溶酶原的相互激活增强 scu-PA 介导的纤维蛋白溶解，这一系统在富含血小板的血栓溶解中可能具有重要作用。
