Extracellular matrix remodeling at the early stages of liver regeneration in the rat.

Previous studies have shown that activity of urokinase-type plasminogen activator (u-PA) increases very rapidly (within 1 minute) after partial hepatectomy. In view of the well-recognized roles of u-PA as one of the major initiators of the matrix proteolysis cascade and as an activator of plasminogen and hepatocyte growth factor (HGF), we studied matrix degradation in liver shortly after partial hepatectomy. The activation of plasminogen to plasmin following partial hepatectomy was examined by Western blot analysis, and a small increase in plasmin at approximately 15 minutes followed by a large elevation at approximately 3 to 6 hours after partial hepatectomy was detected. In addition, we found that fibrinogen, the major substrate for plasmin, begins to be degraded at approximately 15 to 30 minutes following partial hepatectomy. Using immunohistochemical staining, we detected that the distribution of fibrinogen in normal liver is localized to the perisinusoidal space surrounding the periportal region. A decreased distribution of fibrinogen in the periportal region was found by 15 minutes and continued through 24 hours following partial hepatectomy. In addition, the distribution of fibronectin in normal liver was localized to the perisinusoidal space surrounding the periportal and the pericentral regions. A strikingly decreased distribution of fibronectin in the periportal region was found at 5 minutes after partial hepatectomy. Furthermore, we observed that the protein levels of laminin, entactin, and fibronectin in an extracellular matrix (ECM)-enriched preparation decreased shortly after partial hepatectomy, and were restored later. No changes were observed with either vitronectin or the integrin chain alpha(v). In contrast to the protein levels of the ECM components, the messenger RNA (mRNA) levels of fibronectin, integrin chain beta1, and integrin chain alpha(v) gradually increased over 18 hours and then decreased thereafter. Taken together, these results suggest that rapid reorganization of selected ECM components are important for hepatocyte proliferation at the early stages of liver regeneration.