Background/aims: During development of liver fibrosis, an important source of myofibroblasts is hepatocytes, which differentiate into myofibroblasts by epithelial to mesenchymal transition (EMT). In epithelial tumours and kidney fibrosis, hypoxia, through activation of hypoxia-inducible factors (HIFs), is an important stimulus of EMT. Our recent studies demonstrated that HIF-1alpha is important for the development of liver fibrosis. Accordingly, the hypothesis was tested that hypoxia stimulates hepatocyte EMT by a HIF-dependent mechanism.
Methods: Primary mouse hepatocytes were exposed to room air or 1% oxygen and EMT evaluated. In addition, bile duct ligations (BDLs) were performed in control and HIF-1alpha-deficient mice and EMT quantified.
Results: Exposure of hepatocytes to 1% oxygen increased expression of alpha-smooth muscle actin, vimentin, Snail and fibroblast-specific protein-1 (FSP-1). Levels of E-cadherin and zona occludens-1 were decreased. Upregulation of FSP-1 and Snail by hypoxia was completely prevented in HIF-1beta-deficient hepatocytes and by pretreatment with SB431542, a transforming growth factor-beta (TGF-beta) receptor inhibitor. HIFs promoted TGF-beta-dependent EMT by stimulating activation of latent TGF-beta1. To determine whether HIF-1alpha contributes to EMT in the liver during the development of fibrosis, control and HIF-1alpha-deficient mice were subjected to BDL. FSP-1 was increased to a greater extent in the livers of control mice when compared with HIF-1alpha-deficient mice.
Conclusions: Results from these studies demonstrate that hypoxia stimulates hepatocyte EMT by a HIF and TGF-beta-dependent mechanism. Furthermore, these studies suggest that HIF-1alpha is important for EMT in the liver during the development of fibrosis.