Differential regulation of activin A for hepatocyte growth and fibronectin synthesis in rat liver injury

Background/Aims: Both hepatocyte growth and production of extracellular matrix such as fibronectin are essential for liver regeneration. Although activin A is reported to inhibit DNA replication in rat hepatocytes, the role of activin A for liver regeneration after acute injury has not been fully assessed. This study investigated the mechanism by which hepatocyte growth is regulated by activin A during liver regeneration and the effects of activin A on extracellular matrix production. Methods: The mRNA for PA subunit of activin A and activin receptors in hepatocytes and hepatic stellate cells after CCl4 administration were studied by Northern blotting, Binding of I-125-activin A was tested in these cells, Effects of activin A were examined by DNA, collagen and fibronectin synthesis. Results: PA mRNA was expressed in quiescent hepatocytes, and this expression peaked 12 h after CCl4 administration. Activin receptor mRNAs and cross-linked ligand/receptor complexes were expressed in hepatocytes and hepatic stellate cells. However, these levels decreased specifically in hepatocytes at 24 h and had normalized by 72 h. The down-regulation of activin receptor was also observed after partial hepatectomy. Antiproliferative response to activin A decreased in hepatocytes at 24 h, Activin A stimulated production of fibronectin by hepatic stellate cells, but the synthesis of collagen was only slightly elevated in hepatic stellate cells following activin stimulation. Conclusions: The down-regulation of activin receptors in hepatocytes may be partly responsible for these cells becoming responsive to mitogenic stimuli, The increase of activin A at the early stage of liver injury has the potential to contribute to the regulation of fibronectin production in hepatic stellate cells.