The role of hypoxia response element in TGF beta-induced carbonic anhydrase IX expression in Hep3B human hepatoma cells

Carbonic anhydrase IX (CAIX) is a hypoxia-regulated gene. It is overexpressed in a variety of cancers, including hepatocellular cancer. Transforming growth factor beta(TGF beta) is considered to have an impact on cancer biology due to its important roles in cell proliferation and differentiation. The effect of the TGF beta on CAIX expression under hypoxia and the mechanism underlying the role of the hypoxia response element (HRE) on this expression are unknown. In this study, we demonstrate that TGF beta upregulates CAIX expression under hypoxic conditions in the Hep3B hepatoma cell line, indicating that the mitogen-activated protein kinase (MAPK)-and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)-signaling pathways might be responsible for this response. Site-directed mutagenesis of the HRE region in CAIX promoter reduced the TGF beta-induced CAIX promoter activity, pointing to the significance of HRE for this response. Upregulation of TGF beta-stimulated CAIX expression was consistent with the upregulation of promoter activity of five different truncated constructs of the CAIX promoter under hypoxia. Our findings show that the HRE region is critical for TGF beta-induced CAIX expression, which is mainly controlled by MAPK and PI3K pathways.