| Abstract
| - Hypoxia-inducible factor 1 (HIF-1), a master heterodimeric transcriptional regulator consisting of HIF-1α and HIF-1β subunits for cellular response to hypoxia, plays an important role in carcinogenesis, while CCAAT/enhancer-binding protein α (C/EBPα) is proposed to act as a tumor suppressor in C/EBPα-expressing tissues. Previously, we reported that ectopically expressed HIF-1α protein interacts with and enhances transcriptional activity of C/EBPα, which favors leukemic cell differentiation. Here we further showed that such an interaction also occurred in their endogenously expressing state of leukemic U937 cells. Glutathione S-transferase pull-down assay proposed that the protein-protein interaction was direct, and transactivation domains of C/EBPα and the basic helix-loop-helix domain of HIF-1α were essential for such an interaction. More intriguingly, we provided the first demonstration that C/EBPα competed with HIF-1β for direct binding to HIF-1α protein. Correspondingly, C/EBPα overexpression significantly inhibited the DNA-binding ability of HIF-1 and expressions of hypoxia-responsive element-driven luciferase and HIF-1-targeted genes vascular endothelial growth factor, glucose transporter-1 and phosphoglycerate kinase 1. In parallel, suppression of C/EBPα expression by specific small hairpin RNA increased DNA-binding ability of HIF-1 and expression of these HIF-1-targeted genes in leukemic U937 cells. These results would provide new insights for antitumor potential of C/EBPα protein.
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