STAT1 represses hypoxia-inducible factor-1-mediated transcription

Tumor hypoxia is associated with tumor promotion, malignant progression, and resistance to cancer therapy. The hypoxia-induced phenotypic changes in tumors result, at least partially, from the induction of hypoxia-responsive genes, such as chemokine receptor CXCR4. Hypoxia-inducible factor 1 (HIF-1) is a critical transcription factor involved in the transcriptional regulation of these genes. Although interferon-gamma (IFNγ) exerts anti-tumor responses against various tumors, the effect of IFNγ on HIF-1-dependent transcription remains to be determined. In this study, we examined the inhibitory effect of IFNγ on hypoxia-induced CXCR4 gene expression in human glioblastoma cell lines and explored the mechanism of inhibition. Hypoxia (1% O2) and the iron chelator deferoxamine (DFX), a hypoxia mimetic, increased the levels of CXCR4 mRNA in A172 and T98G cells, and treatment with IFNγ inhibited the expression of CXCR4 mRNA. Although hypoxia and DFX induced the expression of HIF-1α protein and its hypoxia response element (HRE) DNA-binding activity, IFNγ failed to inhibit its expression or DNA-binding activity. The results of a luciferase reporter assay using a heterologous promoter construct containing the HRE sequence revealed that IFNγ suppressed the hypoxia- and DFX-induced reporter activities. Lentivirus-mediated RNAi of signal transducer and activator of transcription 1 (STAT1) expression abolished the inhibitory effect of IFNγ on hypoxia-induced reporter gene activity. Furthermore, this activity was not detected in a stable cell line expressing dominant-negative STAT1. These results indicate that IFNγ-activated STAT1 functions as a negative regulator of HIF-1-dependent transcription in tumor cells.