Abstract
Insulin-like growth factor binding protein (IGFBP)-3 regulates cell proliferation and apoptosis in esophageal squamous cell carcinoma (ESCC) cells. We have investigated how the hypoxic tumor microenvironment in ESCC fosters the induction of IGFBP3. RNA interference experiments revealed that hypoxia-inducible factor (HIF)-1α, but not HIF-2α, regulates IGFBP3 mRNA induction. By chromatin immunoprecipitation and transfection assays, HIF-1α was found to transactivate IGFBP3 through a novel hypoxia responsive element (HRE) located at 57 kb upstream from the transcription start site. Metabolic labeling experiments demonstrated hypoxia-mediated inhibition of global protein synthesis. 7-Methyl GTP-cap binding assays suggested that hypoxia suppresses cap-dependent translation. Experiments using pharmacological inhibitors for mammalian target of rapamycin (mTOR) suggested that a relatively weak mTOR activity may be sufficient for cap-dependent translation of IGFBP3 under hypoxic conditions. Bicistronic RNA reporter transfection assays did not validate the possibility of an internal ribosome entry site as a potential mechanism for cap-independent translation for IGFBP3 mRNA. Finally, IGFBP3 mRNA was found enriched to the polysomes. In aggregate, our study establishes IGFBP3 as a direct HIF-1α target gene and that polysome enrichment of IGFBP3 mRNA may permit continuous translation under hypoxic conditions.