Hypoxic induction of Ctgf is directly mediated by Hif-1

CTGF plays a significant role in the development of renal fibrosis by mediating the fibrotic effects of transforming growth factor (TGF)-beta(1) and has been shown to be hypoxia inducible in human breast cancer cells. It has been suggested that hypoxia is an important underlying cause for the development of renal fibrosis through the modulation of profibrotic genes. One of the key mediators of the cell's response to lowered oxygen environments is hypoxia-inducible-factor-1 (HIF-1), a basic helix-loop-helix transcription factor, which enables cells to adapt to hypoxia by regulating the expression of genes involved in increasing oxygen availability ( VEGF, erythropoietin) and enhancing glucose uptake and metabolism (Glut-1, PGK). In this paper, we have used primary tubular epithelial cell cultures from a tetracycline-inducible-Hif-1alpha knockout murine model to further elucidate the role of Hif-1 in the hypoxic-induction of Ctgf expression. We show that hypoxia response elements present upstream of Ctgf enable direct interaction of Hif-1 transcription factor with the Ctgf promoter, resulting in increased transcription of Ctgf mRNA. Cells deficient in Hif-1alpha were incapable of inducing Ctgf mRNA in response to hypoxia, suggesting an absolute requirement of Hif-1. Furthermore, the observed Hif-1-mediated hypoxic stimulation of Ctgf expression was found to occur independently of TGF-beta(1) signaling. Our findings have important implications for a number of fibrotic disorders in which hypoxia, CTGF, and TGF-beta(1) are involved, including renal, dermal, hepatic, and pulmonary fibrosis.