To assess the usefulness of oncostatin M (osm) gene therapy in liver regeneration, we examined whether the introduction of OSM cDNA enhances the regeneration of livers damaged by dimethylnitrosamine (DMN) in rats. Repeated injection of OSM cDNA enclosed in hemagglutinating virus of Japan envelope into the spleen resulted in the exclusive expression of OSM protein in Kupffer cells of the liver, which was accompanied by increases in body weight, liver weight, and serum albumin levels and the reduction of serum liver injury parameters (bilirubin, aspartate aminotransferase, and alanine aminotransferase) and a serum fibrosis parameter (hyaluronic acid). Histological examination showed that osm gene therapy reduced centrilobular necrosis and inflammatory cell infiltration and augmented hepatocyte proliferation. The apoptosis of hepatocytes and fibrosis were suppressed by osm gene therapy. Time-course studies on osm gene therapy before or after DMN treatment showed that this therapy was effective not only in enhancing regeneration of hepatocytes damaged by DMN but in preventing hepatic cytotoxicity caused by subsequent treatment with DMN. These results indicate that OSM is a key mediator for proliferation and anti-apoptosis of hepatocytes and suggest that osm gene therapy is useful , as preventive and curative means , for the treatment of patients with liver damage. The liver has a remarkable ability to respond to injuries inflicted by various causes, such as partial hepatectomy, toxic exposure, and virus infection.1,2 Hepatocytes, which are liver parenchymal cells and normally in the quiescent G 0 phase, re-enter the cell cycle after injury to restore its mass, architecture, and function. In this process, a number of growth factors and cytokines have been reported to be involved. [3][4][5][6][7][8][9] For example, hepatocyte growth factor (HGF) functions as a potent mitogen for hepatocytes, 10 and the administration of HGF has been shown to ameliorate hepatic injury in animal models of fulminant hepatic failure. [11][12][13] It has also been shown that the introduction of hgf gene into rat cirrhotic livers using liposome with the hemagglutinating virus of Japan (HVJ) inhibits fibrogenesis and hepatocyte apoptosis, leading to the complete resolution of fibrosis and improvement of survival rate.14 Oncostatin M (OSM) is a member of the interleukin (IL)-6 cytokine family that includes IL-6, IL-11, leukemia inhibitory factor, ciliary neurotrophic factor, cardiotrophin-1, and novel neutrophin-1/B-cell-stimulating factor-3. [15][16][17][18] Mouse OSM receptor is composed of the gp130 subunit, common to all of the IL-6 family cytokines, and an OSM-specific subunit (hereafter called OSM-specific receptor; OSM-R).19 Binding of OSM to its receptor complex activates Janus tyrosine kinases (Jak1, Jak2, and Tyk2), which in turn activates downstream signaling pathways, including SHP-2 tyrosine phosphatase and signal transducer and activator of transcription protein 3 (STAT3). Recently, OSM has been shown to induce maturation of mou...