Basic Science Advances for Clinicians

Alterations in thrombosis and fibrinolysis comprise important parts of stroke pathophysiology. A key step in the fibrinolytic process includes the tissue-type plasminogen activator (tPA)-mediated conversion of the proenzyme plasminogen into the active protease plasmin, which in turn degrades the fibrin structure of intravascular thrombi. There are a number of review articles well summarizing molecular mechanisms of the fibrinolytic system.1,2 Inhibition of the fibrinolytic system may occur at the level of plasminogen activation, mainly by a direct inhibition of tPA by plasminogen activator inhibitor 1 (PAI-1) or indirectly by thrombinactivatable fibrinolysis inhibitor and, at the level of plasmin, by 2-antiplasmin. The roles of these 3 inhibitors are complementary in thrombolysis.3 PAI-1 has become recognized as a central molecule linking pathogenesis and progression of thrombotic vascular events including stroke. As a main endogenous inhibitor of tPA, PAI-1 might be related to reperfusion efficacy and hemorrhagic risk of tPA thrombolytic therapy. Moreover, a clear association has been observed between elevated PAI-1 plasma levels and prothrombotic disease conditions such as hypertension, obesity, insulin resistance, and diabetes.4–7 Clinical and experimental studies show that PAI-1 deficiencies cause accelerated fibrinolysis and bleeding, whereas elevated PAI-1 plasma levels are associated with vascular thrombosis.8 Concentrations of active PAI-1 in newly formed thrombi can be several thousandfold greater than active PAI-1 concentrations in normal plasma9 and thus high enough to inhibit doses of tPA used for clinical thrombolysis. Furthermore, beyond its effects in modulating the activity of tPA and the functionally related urokinase-type plasminogen activator, which predominantly catalyzes plasmin formation in the extravascular space, PAI-1 also plays diverse roles in metabolic and vascular disease and may participate in the evolution of brain damage and recovery after stroke.4,10,11 In this translational review, we briefly survey the molecular mechanisms of PAI-1 and propose that it may serve as a critical crosstalk molecule, potential biomarker, and target that links risk factors to stroke mechanisms and response to thrombolytic and neuroprotective strategies.