Institute of Medical Sciences, University of Aberdeen, Aberdeen, UK.
Haematologica. 2020 Dec 1;105(12):2824-2833. doi: 10.3324/haematol.2019.230367.
Platelets harbor the primary reservoir of circulating plasminogen activator inhibitor 1 (PAI-1), but the reportedly low functional activity of this pool of inhibitor has led to debate over its contribution to thrombus stability. Here we analyze the fate of PAI-1 secreted from activated platelets and examine its role in maintaining thrombus integrity. Activation of platelets results in translocation of PAI-1 to the outer leaflet of the membrane, with maximal exposure in response to strong dual agonist stimulation. PAI-1 is found to co-localize in the cap of PS-exposing platelets with its cofactor, vitronectin, and fibrinogen. Inclusion of tirofiban or Gly-Pro-Arg-Pro significantly attenuated exposure of PAI-1, indicating a crucial role for integrin αIIbβ3 and fibrin in delivery of PAI-1 to the activated membrane. Separation of platelets post-stimulation into soluble and cellular components revealed the presence of PAI-1 antigen and activity in both fractions, with approximately 40% of total platelet-derived PAI-1 remaining associated with the cellular fraction. Using a variety of fibrinolytic models we found that platelets produce a strong stabilizing effect against tPA-mediated clot lysis. Platelet lysate, as well as soluble and cellular fractions stabilize thrombi against premature degradation in a PAI-1 dependent manner. Our data show for the first time that a functional pool of PAI-1 is anchored to the membrane of stimulated platelets and regulates local fibrinolysis. We reveal a key role for integrin αIIbβ3 and fibrin in delivery of PAI-1 from platelet α-granules to the activated membrane. These data suggest that targeting platelet-associated PAI-1 may represent a viable target for novel profibrinolytic agents.
血小板是循环型纤溶酶原激活物抑制剂 1（PAI-1）的主要储存库，但据报道，该抑制剂池的功能活性较低，这导致了其对血栓稳定性的贡献存在争议。在这里，我们分析了从激活的血小板中分泌的 PAI-1 的命运，并研究了其在维持血栓完整性中的作用。血小板的激活导致 PAI-1 易位到膜的外叶，在强双重激动剂刺激下最大程度地暴露。发现 PAI-1 与它的辅因子，纤连蛋白和纤维蛋白原在暴露 PS 的血小板的帽中共同定位。包含替罗非班或 Gly-Pro-Arg-Pro 显著减弱了 PAI-1 的暴露，表明整合素 αIIbβ3 和纤维蛋白在将 PAI-1 递送到激活的膜中起着关键作用。刺激后将血小板分离成可溶性和细胞成分，揭示了两种成分中均存在 PAI-1 抗原和活性，大约 40%的总血小板衍生的 PAI-1 仍然与细胞成分相关。使用各种纤维蛋白溶解模型，我们发现血小板对 tPA 介导的血凝块溶解产生强烈的稳定作用。血小板裂解物以及可溶性和细胞成分以 PAI-1 依赖性方式稳定血栓，防止过早降解。我们的数据首次表明，功能型 PAI-1 池被锚定在受刺激的血小板的膜上，并调节局部纤维蛋白溶解。我们揭示了整合素 αIIbβ3 和纤维蛋白在将 PAI-1 从血小板 α-颗粒递送到激活的膜中的关键作用。这些数据表明，针对血小板相关的 PAI-1 可能是新型抗纤维蛋白溶解剂的可行靶点。
