Description
The liver is one of the organs in which hypoxia helps to regulate gene expression under normal physiological conditions and in diseases such as cirrhosis and cancer. We postulated that the expression/activity of some of the 'liver-enriched' transcription factors, which control liver-specific genes, was sensitive to hypoxia. We tested hepatocyte nuclear factor-1 (HNF-1), HNF-3 and HNF-4, which play key roles in differentiation, development and hepatic gene expression, using HepG2 human hepatoma cells cultured under hypoxic conditions. Severe hypoxia/anoxia downregulated HNF-4 DNA-binding activity while DNA-binding activity of HNF-1 and HNF-3 remained unaffected. These hypoxic conditions also strongly and specifically decreased cell contents of HNF-4 protein, indicating that the decrease in HNF-4 DNA-binding activity was due to the lower amount of protein and not to decreased DNA-binding affinity. Northern analysis indicated that the expression of the hnf-4 gene was also downregulated in HepG2 cells cultured under hypoxic conditions. These results provide evidence that hypoxic stress triggers a cascade of events that inhibits the transactivation potential of HNF-4 in HepG2 cells. This step may be crucial in modulating the expression of a subset of liver genes that are targets for this nuclear receptor. This relationship provides a new route for the investigation of the effects of hypoxia on the liver cell.