Multiple sites of proteolytic cleavage to release soluble forms of hepatocyte growth factor activator inhibitor type I from a transmembrane form

Hepatocyte growth factor activator inhibitor type 1 (HAI-1) is a Kunitz-typ e serine protease inhibitor, which was identified as a potent inhibitor of hepatocyte growth factor (HGF) activator from the conditioned medium of a h uman carcinoma cell line. HGF activator is a blood coagulation factor XII-l ike serine protease that is responsible for proteolytic activation of the i nactive single chain precursor of HGF in injured tissues. The predicted seq uence of the primary translation product of HAI-1, which has a hydrophobic sequence in its COOH-terminal region, suggested that HAI-1 is first produce d in a membrane-associated form. In this study, we identified a transmembra ne form of HAI-1 integrated in the plasma membrane of cultured cells using a monoclonal antibody against HAI-1. We also identified several soluble for ms of HAI-1 in the conditioned medium of the cells, indicating that multipl e sites are present in the transmembrane form of HAI-1 at which proteolytic cleavage releases the extracellular domain, At least two proteases, one of which is a metalloprotease, appear to be responsible for the release. Furt her, the soluble forms of HAI-1 have different inhibitory activity against HGF activator. These findings suggest that proteolytic processing plays imp ortant roles in regulation of the inhibitory activity of HAI-1.