Plasmin-mediated Proteolysis Is Required for Hepatocyte Growth Factor Activation during Liver Repair

The physiological relevance of the activation of hepatocyte growth factor (Hgf) by the plasminogen (Plg) system of proteases and its contribution to tissue repair are largely undefined. Here, we investigated whether the defective liver repair in mice lacking Plg is due to impaired activation of Hgf. Loss of Plg in vivo suppressed Hgf activation and signaling through its Met tyrosine kinase receptor. Without Plg, hepatocytes were unresponsive to Hgf-induced proliferation and migration, with a more pronounced impairment in hepatocyte movement within the hepatic environment. Most notably, circumventing the defect in proteolytic activation of Hgf by the downstream expression of an activated Met receptor corrected the functional deficits and improved liver repair in Plg-deficient mice. These findings support a fibrinolysis-unrelated role for Plg in modulating cell proliferation and migration by activation of Hgf.Tissue repair requires a prompt proliferative response in concert with the timely reorganization of the extracellular matrix. Each one of these processes can be disrupted by the loss of individual growth factors or proteases, but the precise regulatory relationship between these molecules in supporting tissue repair is not fully understood. Multiple in vitro studies have inferred that proteases in the plasminogen (Plg)² activation system may be important in the proteolytic activation of the hepatocyte growth factor (Hgf) (1–4), the ligand for the Met tyrosine kinase receptor that exerts potent mitogenic and motogenic properties to mesenchymal and epithelial cells. This concept is made even more attractive by the fact that Hgf is structurally related to Plg, with multiple kringle domains and a catalytically inactive serine protease-like domain. However, the physiological relevance of Plg to Hgf activation and Hgf-related reparative processes are controversial and effectively unexplored in vivo.We previously reported that a genetically imposed loss of circulating Plg severely impairs clearance of necrotic cells and the repopulation of injured zones by newly formed cells but without compromising the general hepatic proliferative response (5). Despite the indisputable role of Plg in fibrin clearance (6), complementary studies in mice with no capacity for fibrin deposition have shown that the loss of fibrinolytic function alone in Plg-deficient mice cannot account for the impediment in tissue repair (5). Multiple nonfibrin targets of plasmin-mediated proteolysis are known (e.g. serine and metalloprotease zymogens, and extracellular matrix glycoproteins, latent growth factors), and it is feasible that they may contribute to the focal clearance of necrotic tissue. However, based on recent findings pointing to a strikingly similar defect in hepatic repair in mice lacking Plg or a conditional loss of Met (7), an attractive hypothesis emerged that the Plg activation system supports physiological liver repair by activation of the Met ligand, Hgf. Testing this hypothesis, we found that the loss of Plg impairs Hgf activation, suppresses Met phosphorylation and signaling, and prevents Hgf-induced migration of hepatocytes. Most notably, consistent with a physiologically relevant contribution of Plg to Hgf-Met signaling, the expression of an autophosphorylated Met largely corrected the defective repair in Plg-deficient livers.