The Role of Histidine-rich Glycoprotein in Coagulation & Fibrinolysis

Abstract

<p> The fibrinolytic system has an important role in maintaining vascular patency by restricting fibrin clot formation to prevent occlusion of the blood vessel. Plasminogen activation is the central event in fibrinolysis and is tightly regulated by activators and inhibitors. Histidine-rich glycoprotein (HRG) is an abundant plasma protein that has been proposed to have a regulatory role in many biological processes, including fibrinolysis. Approximately 50% ofplasminogen in the blood circulates in complex with HRG. Conflicting reports dispute the role of HRG in fibrinolysis, specifically whether it promotes or inhibits plasminogen activation. To elucidate the role of HRG in fibrinolysis, we isolated HRG from human plasma and analyzed its effect on plasminogen activation by tissue-type plasminogen activator in a kinetic assay. HRG had no significant effect on plasminogen activation by tissue-type plasminogen activator once contaminating plasminogen was eliminated from our HRG preparations. Based on these results, the focus of our research was redirected to analyzing the effect of HRG on additional plasminogen activators, namely urinary-type plasminogen activator and factor (F) Xlla. HRG inhibited plasminogen activation by both activators. HRG had the greatest inhibitory effect on FXIIa activity. This novel finding led us to explore the relationship between HRG and FX.IIa by measuring the affinity of HRG for FXIIa by surface plasmon resonance, and by analyzing the effect of HRG on FXIIa activity in various contact pathway reactions. ZnCh was also included in these reactions because it plays an important role in enhancing both HRG-and FXII-mediated interactions and is released by activated platelets. In the presence of 12.5 μM ZnCl2, FXIIa bound to the histidine-rich region of HRG with very high affinity (Kd = 56 ± 8.9 pM). Interestingly, HRG does not bind to FXII. Functional analysis of HRG revealed that it significantly inhibits a number of contact pathway reactions, including FXII autoactivation, kallikreinmediated FXII activation, and FXIIa-mediated FXI activation. Conversely, HRG enhanced FXIIa-mediated prekallikrein activation. Based on these findings, we hypothesize that HRG binds to an exosite on FXIIa, which is not expressed by the zymogen FXII, and alters FXIIa activity. The mechanism of HRG-mediated FXIIa inhibition is not fully understood and needs to be further analyzed by both binding and functional assays. These observations raise the possibility that the main function of HRG is to modulate FXIIa activity, rather than plasminogen activation. Because of its abundance, HRG may function as a modulator of haemostasis through its effect on coagulation and fibrinolysis. </p>