Regulation of the conversion of plasminogen to plasmin by tissue plasminogen activator (tPA) is critical in the control of fibrin deposition. While several plasminogen activators have been described, soluble plasma cofactors that stimulate fibrinolysis have not been characterized. The purpose of this study was to investigate the effects of β2-glycoprotein I (β2GPI), an abundant plasma glycoprotein, on tPA-mediated plasminogen activation.
The effect of β2GPI on tPA-mediated activation of plasminogen was assessed using amidolytic assays, a fibrin gel, and plasma clots. Binding of β2GPI to tPA and plasminogen was determined in parallel. The effects of IgG fractions and anti-β2GPI antibodies from patients with antiphospholipid syndrome (APS) on tPA-mediated plasminogen activation were also measured.
Beta2-glycoprotein I stimulated tPA-dependent plasminogen activation in the fluid phase and within a fibrin gel. The β2GPI region responsible for stimulating tPA activity was shown to be at least partly contained within β2GPI domain V. In addition, β2GPI bound tPA with high affinity (Kd ∼20 nM), stimulated tPA amidolytic activity, and caused an overall 20-fold increase in the catalytic efficiency (Kcat/Km) of tPA-mediated conversion of Glu-plasminogen to plasmin. Moreover, depletion of β2GPI from plasma led to diminished rates of clot lysis, with restoration of normal lysis rates following β2GPI repletion. Stimulation of tPA-mediated plasminogen activity by β2GPI was inhibited by monoclonal anti-β2GPI antibodies as well as by anti-β2GPI antibodies from patients with APS.
These findings suggest that β2GPI may be an endogenous regulator of fibrinolysis. Impairment of β2GPI-stimulated fibrinolysis by anti-β2GPI antibodies may contribute to the development of thrombosis in patients with APS.