Mechanism of the transforming growth factor-beta induction of fibronectin expression in hepatic stem-like cells

Transforming growth factor-beta 1 (TGF-beta 1) plays an important role in the fibrogenic process in the liver. The aim of the present study was to explore the action of TGF-beta 1 on fibronectin expression in rat hepatic stem-like cells and the underlying mechanisms. The level of fibronectin expression was determined in hepatic stem-like cells (WB cells) before and after TGF-beta 1 stimulation by RT-PCR and Western blot methods. Using immunogold transmission electron microscopy and the Western blot method, we observed the result of the expression and the distribution of cAMP, phosphorylated Smad3 and Smad7 before and after TGF-beta 1 treatment. The levels of fibronectin expression in both mRNA and protein increased 4- to 5-fold after TGF-beta 1 stimulation, reaching an optimum level after 8 h and then gradually falling back. Similarly, TGF-beta 1 stimulation resulted in an increase of cAMP in WB cells, peaking at 8 h. After treatment with TGF-beta 1 for 24 h, the expression of cAMP gradually decreased. In addition, we found that TGF-beta 1 treatment also contributed to the increased expression and to changes in cellular distribution of phosphorylated Smad3 (translocation from the cytoplasm to the nucleus) and Smad7 (translocation from the nucleus to the cytoplasm) in WB cells. The present study demonstrates that TGF-beta is involved in the fibrogenic process in hepatic stem cells through up-regulation of fibronectin expression, and the mechanisms underlying this process may be associated with the activation of cAMP and Smad pathways.