Targeted disruption of Smad3 confers resistance to the development of dimethylnitrosamine-induced hepatic fibrosis in mice

Background: Hepatic fibrosis is characterized by a progressive accumulation of fibrillar extracellular matrix (ECM) proteins including collagen, which occurs in most types of chronic liver diseases. Transforming growth factor-b (TGF-b)/Smad3 signalling plays a central role in tissue fibrogenesis, acting as a potent stimulus of ECM accumulation. Aim: To evaluate the potential protective role of Smad3 deficiency in the pathogenesis of liver fibrosis induced by dimethylnitrosamine (DMN) in Smad3 null mice. Methods: Chronic hepatitis-associated fibrosis was induced in 13 Smad3 null and 13 wild-type (WT) mice by intraperitoneal DMNadministration (10 mg/g body weight/ day) for three consecutive days per week for 6 weeks. The liver was excised for macroscopic examination and histological, morphometric and immunohistochemical (IHC) analyses. For IHC, a-smooth muscle actin (a-SMA), collagen types I–III, TGFb1, connective tissue growth factor (CTGF), Smad3, Smad7 and CD3 antibodies were used. Results: At macroscopic examination, the liver of DMN-treated Smad3 WT appeared harder with a dark brown colouring and necrotic areas compared with that from null mice.Histological and morphometric evaluation revealed a significantly higher degree of hepatic fibrosis and accumulation of connective tissue in the Smad3 WT compared with null mice. IHC evaluation showed a marked increase in a-SMA, CTGF, collagen I-III, TGF-b and Smad3 staining in the liver of Smad3 WT compared with that in null mice, whereas Smad7 was increased only in null mice. Conclusions: The results indicate that Smad3 loss confers resistance to the development of DMN-induced hepatic fibrosis. The reduced fibrotic response appears to be due to a reduction of fibrogenic myofibroblast activation and ECM production and accumulation. Smad3 could be a novel target for potential treatment of fibrosis complicating chronic hepatitis.