Mechanism of retarded liver regeneration in plasminogen activator-deficient mice: Impaired activation of hepatocyte growth factor after Fas-mediated massive hepatic apoptosis

Abstract

Urokinase‐type plasminogen activator (uPA) is implicated in the regulation of hepatic regeneration by activating hepatocyte growth factor (HGF). Here, we investigated its role in the hepatic regeneration after Fas‐mediated massive hepatocyte death employing mice deficient in either uPA or its inhibitor, plasminogen activator inhibitor‐1 (PAI‐1). We measured kinetics of hepatic levels of proliferating cell nuclear antigen (PCNA)‐labeling index, plasmin activity, mature HGF, and its phosphorylated receptor, c‐Met. In the genetically targeted and wild‐type mice, hepatocytes fell into the same extent of apoptosis 6 to 12 hours after an intraperitoneal injection with anti‐Fas antibody, as judged from histologic analysis and a histon‐DNA enzyme‐linked immunosorbent assay (ELISA). In the wild‐type mice, mature HGF emerged in the liver 6 hours following anti‐Fas injection, and hepatic PCNA‐labeling index started to increase following 24 hours and peaked at 48 hours. In the uPA^−/− mice, emergence of mature HGF was delayed 12 hours and hepatic regeneration peaked at 96 hours. Supplementation with the uPA gene to the uPA^−/− mice by in vivo lipofection restored hepatic plasmin levels, and improved a delay in the expression of both mature HGF and phosphorylated c‐Met, accompanying a normal rate of liver regeneration. In contrast, PAI‐1^−/− mice showed accelerated liver regeneration; mature HGF emerged as early as 3 hours, and PCNA‐labeling index increased at 24 hours. This accelerated regeneration was abolished by administration with anti‐HGF antibody. These results strongly suggest a physiologic role of uPA in the proteolytic maturation of HGF, and thereby in hepatic regeneration after Fas‐mediated massive hepatocyte death.