Hypoxia stimulates hepatocyte epithelial to mesenchymal transition by hypoxia-inducible factor and transforming growth factor-beta-dependent mechanisms.
OBJECTIVE
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.
Copple
Bryan L
BL
Department of Pharmacology, Toxicology, and Experimental Therapeutics, University of Kansas Medical Center, Kansas City, KS, USA. bcopple@kumc.edu
D000199:Actins / Q000737:chemistry; D000818:Animals; D015820:Cadherins / Q000032:analysis; D002135:Calcium-Binding Proteins / Q000032:analysis; D015687:Cell Hypoxia; D004847:Epithelial Cells / Q000473:pathology*; D022781:Hepatocytes / Q000473:pathology*; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000502:physiology*; D053539:Keratin-19 / Q000032:analysis; D008565:Membrane Proteins / Q000032:analysis; D008648:Mesoderm / Q000473:pathology*; D051379:Mice; D008810:Mice, Inbred C57BL; D010750:Phosphoproteins / Q000032:analysis; D000071999:S100 Calcium-Binding Protein A4; D009418:S100 Proteins; D053773:Transforming Growth Factor beta1 / Q000502:physiology*; D062826:Zonula Occludens-1 Protein
D000199:Actins; D015820:Cadherins; D002135:Calcium-Binding Proteins; C497443:Hif1a protein, mouse; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D053539:Keratin-19; D008565:Membrane Proteins; D010750:Phosphoproteins; D000071999:S100 Calcium-Binding Protein A4; D009418:S100 Proteins; C470944:S100a4 protein, mouse; C569043:Tjp1 protein, mouse; D053773:Transforming Growth Factor beta1; D062826:Zonula Occludens-1 Protein