Regulation of HAM function by proteolytic processing

HGF激活因子抑制剂（HAI-1）是HGF激活因子（HGFA）的蛋白性抑制剂，其蛋白水解激活肝细胞生长因子（HGF）。HGF是一种参与损伤后组织再生和修复的多功能细胞因子。HAI-1是一种I型膜蛋白，通过蛋白水解作用分泌为40 kD或58 kD的HAI-1。40kD HAI-1含有1个Kunitz结构域（Kunitz 1），而58 kD HAI-1含有2个Kunitz结构域（Kunitz 1和Kunitz 2）。我们生成了每个Kunitz结构域的缺失突变体，发现Kunitz 1主要参与抑制HGFA。虽然58 kD hai -1含有两个Kunitz结构域，但其对HGFA的抑制活性很弱。我们发现两个Kunitz结构域相互掩盖，从而降低了整个蛋白的抑制活性。当三分之二的大鼠肝脏被切除时，残余肝脏中的促生长因子显著增加，但活性生长因子仅少量存在。然后，我们在部分肝切除术后24小时通过门静脉给予活性HGFA，并检查对肝脏再生的影响。给药HGFA导致残肝中原HGF的激活，并诱导c-Met （HGF受体）的酪氨酸磷酸化。此外，肝细胞标记指数升高，促进肝块的恢复。然而，我们需要大量的HGFA才能获得肝脏再生的显著增强，这表明HAI-1等组织源性抑制剂在体内调节HGFA活性方面起着重要作用。

HGF activator inhibitor (HAI-1) is a proteinaceous inhibitor of HGF activator (HGFA), which proteolytically activates hepatocyte growth factor (HGF). HGF is a multifunctional cytokine that is involved in regeneration and tissue repair after injury. HAI-1 is synthesized as a type I membrane protein, and secreted as a 40 kD HAI-1 or 58 kD HAI-1 by proteolytic processing. 40kD HAI-1 contains one Kunitz domain (Kunitz 1), whereas 58 kD HAI-1 contains two Kunitz domains (Kunitz 1 and 2). We generated deletion mutants of each Kunitz domains, and found that Kunitz 1is mainly involved in inhibition of HGFA. Although 58 kD HAI-1contains two Kunitz domains, its inhibitory activity for HGFA is very weak. We found that two Kunitz domains masked each other, thus lowering the inhibitory activity of the whole protein.When two-thirds of rat liver is resected, pro-HGF in the residual liver increases dramatically, but the active HGF is present in only a small amount. We then administered active HGFA via portal vein 24 h after partial hepatectomy, and examined the effect on liver regeneration. Administration of HGFA caused activation of pro-HGF in the residual liver and induced tyrosine phosphorylation of c-Met (HGF receptor). In addition, labeling index of hepatocytes was increased, and recovery of liver mass was promoted. We needed, however, a large amount of HGFA to obtain significant enhancement of liver regeneration, indicating that tissue-derived inhibitors such as HAI-1 play important roles in regulating activity of HGFA in vivo.