FIG. 4.

Hif-1α^−/− ES cells exhibit no HIF-2α activity under severe hypoxia or by activating HRE reporters. (A) Severe hypoxia increases the levels of HIF-1α and HIF-2α protein in WT ES cells (left panel) and HIF-2α protein levels in the Hif-1α^−/− ES cells (right panel). Nonspecific proteins in HIF-1α or HIF-2α Western blots labeled by asterisks serve as loading controls. (B) Severe hypoxia increases HIF-α common target gene expression in WT ES cells (left panel) but not in Hif-1α^−/− ES cells (right panel). (C) Hypoxic treatment of Hif-1α^−/− ES cells fails to enhance the transfected WT HRE reporter gene, while WT ES cells exhibit hypoxic induction of transfected WT-HRE reporter construct. Results are plotted as changes relative to a mutant (mt-HRE) reporter under normoxia.