Structural and functional characterization of a novel serine protease responsible for activation of hepatocyte growth factor

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Hepatocyte growth factor (HGF) is a potent mitogen for heptocytes and may play an important role in regeneration following hepatic injury. HGF is first synthesized as an inactive single chain precursor which is then converted to an active heterodimeric from by endoproteolytic processing. Recently, a novel serine protease responsible for this processing (HGF activator) has been purified from serum. In this study, we analyzed the structure and function of this protease and obtained the following results.1. Molecular cloning sequence analysis of a cDNA for HGF activator revealed that HGF activator purified from serum is derived from the C-terminal region of a precursor of 655 amino acids by proteolytic cleavage and that the precursor consists of multiple domains homologous to those observed in blood coagulation factor XII.2. HGF activator was present in plasma as an inactive zymogen. The zymogen was activated by the cleavage of the bond between Arg^<407> and Ile^<408> by thrombin in the presence of negatively charged substances. Thus, the HGF activator zymogen may be activated during blood coagulation pathway in response to tissue injury.3. HGF remained as an inactive single chain from in the liver, kidney, lung and spleen of normal rat. The production of HGF markedly increased in the liver after hepatotoxin treatment. A significant portion of the increased HGF was converted to the active form, whereas the conversion did not occur in other tissues. Thus, the proteolytic activation system functions in vivo as a mechanism for localizing HGF activities to injured tissues.4. An enzymatic activity that activates single chain HGF was detected in the injured liver but not in the normal liver. This activity was inhibited by an anti-HGF activator antibody. Thus, HGF activator is likely to be the key enzyme regulating the activities of HGF in the injured tissues.