Protein C inhibitor directly and potently inhibits activated hepatocyte growth factor activator

Background: Hepatocyte growth factor (HGF) plays an important role in tissue repair and regeneration. HGF activator (HGFA), a factor XIIa-like serine protease, activates HGF precursor to HGF. The precursor of HGFA, proHGFA, is activated by thrombin generated at sites of tissue injury. It is known that protein C inhibitor (PCI), an inhibitor of activated protein C (APC), also inhibits thrombin-thrombomodulin (TM) complex. Objectives: In the present study we evaluated the effect of PCI on thrombin-catalyzed proHGFA activation in the presence of TM, and on HGFA activity. Results: PCI did not inhibit thrombin-TM-mediated proHGFA activation, but it directly inhibited activated HGFA by forming an enzyme inhibitor complex. The second-order rate constants (M-1 min(-1)) of the reaction between HGFA and PCI in the presence or absence of heparin (10 U mL(-1)) were 4.3 x 10(6) and 4.0 x 10(6), respectively. The inhibition of HGFA by PCI resulted in a significant decrease of HGFA-catalyzed activation of HGF precursor. Exogenous HGFA added to normal human plasma formed a complex with plasma PCI, and this complex formation was competitively inhibited by APC in the presence of heparin, but very weakly in the absence of heparin. We also demonstrated using recombinant R362A-PCI that Arg362 residue of PCI is important for HGFA inhibition by PCI as judged from the three-dimensional structures constructed using docking models of PCI and HGFA or APC. Conclusion: These observations indicate that PCI is a potent inhibitor of activated HGFA, suggesting a novel function for PCI in the regulation of tissue repair and regeneration.