β2‐glycoprotein i is a cofactor for tissue plasminogen activator–mediated plasminogen activation

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 β₂‐glycoprotein I (β₂GPI), an abundant plasma glycoprotein, on tPA‐mediated plasminogen activation.

Methods

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.

Results

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 (K_d ∼20 nM), 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 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.

Conclusion

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.