Abstract
Objective
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 β~2~‐glycoprotein I (β~2~GPI), an abundant plasma glycoprotein, on tPA‐mediated plasminogen activation.
Methods
The effect of β~2~GPI on tPA‐mediated activation of plasminogen was assessed using amidolytic assays, a fibrin gel, and plasma clots. Binding of β~2~GPI to tPA and plasminogen was determined in parallel. The effects of IgG fractions and anti‐β~2~GPI antibodies from patients with antiphospholipid syndrome (APS) on tPA‐mediated plasminogen activation were also measured.
Results
Beta~2~‐glycoprotein I stimulated tPA‐dependent plasminogen activation in the fluid phase and within a fibrin gel. The β~2~GPI region responsible for stimulating tPA activity was shown to be at least partly contained within β~2~GPI domain V. In addition, β~2~GPI bound tPA with high affinity (K~d~ ∼20 n__M__), stimulated tPA amidolytic activity, and caused an overall 20‐fold increase in the catalytic efficiency (K~cat~/K~m~) of tPA‐mediated conversion of Glu‐plasminogen to plasmin. Moreover, depletion of β~2~GPI from plasma led to diminished rates of clot lysis, with restoration of normal lysis rates following β~2~GPI repletion. Stimulation of tPA‐mediated plasminogen activity by β~2~GPI was inhibited by monoclonal anti‐β~2~GPI antibodies as well as by anti‐β~2~GPI antibodies from patients with APS.
Conclusion
These findings suggest that β~2~GPI may be an endogenous regulator of fibrinolysis. Impairment of β~2~GPI‐stimulated fibrinolysis by anti‐β~2~GPI antibodies may contribute to the development of thrombosis in patients with APS.