Hepatocyte growth factor activator inhibitor type 1 is a specific cell surface binding protein of hepatocyte growth factor activator (HGFA) and regulates HGFA activity in the pericellular microenvironment

Hepatocyte growth factor activator (HGFA) is responsible for proteolytic ac tivation of the precursor form of hepatocyte growth factor in injured tissu es. To date, two specific inhibitors of HGFA have been identified, namely H GFA inhibitor type 1 (HAI-1) and type 2 (HAI-2)/placental bikunin (PB). Bot h inhibitors are first synthesized as integral membrane proteins having two Kunitz domains and a transmembrane domain, and are subsequently released f rom cell surface by shedding. Here we show that an active form of HGFA is s pecifically complexed with membrane-form HAI-1, but not with HAI-2/PB, on t he surface of epithelial cells expressing both inhibitors. This binding req uired the enzyme activity of HGFA. The selective binding of HGFA to the cel l surface HAI-1 was further confirmed in an engineered system using Chinese hamster ovary cells, in which only the cells expressing HAI-1 retained exo genous HGFA. The binding of HGFA to HAI-1 was reversible, and no irreversib le modifications affecting the enzyme activity occurred during the binding. Importantly, HAI-1 and the HGFA HAI-1 complex were quickly released from t he cell surface by treatment with phorbol 12-myristate 13-acetate or interl eukin 1 beta accompanying the generation of 58-kDa fragments of HAI-1, whic h are less potent against HGFA, as well as significant recovery of HGFA act ivity in the culture supernatant. This regulated shedding was completely in hibited by BB3103, a synthetic zinc-metalloproteinase inhibitor. We conclud e that HAI-1 is not only an inhibitor but also a specific acceptor of activ e HGFA, acting as a reservoir of this enzyme on the cell surface. The latte r property appears to ensure the concentrated pericellular HGFA activity in certain cellular conditions, such as tissue injury and inflammation, via t he up-regulated shedding of HGFA.HAI-1 complex. These findings shed light o n a novel function of the integral membrane Kunitz-type inhibitor in the re gulation of pericellular proteinase activity.