HIF-1α, but not HIF-2α, mediates hypoxic induction of cord formation in HUVECs. HUVECs were seeded in a 10 mm dish and transfected with a negative control siRNA (NC) or siRNA specific for HIF-1α or HIF-2α. Cells were then treated in the absence or presence of DFO (100 μM) for 6 hours. HIF-1α and HIF-2α mRNA expression (A) was analyzed by real-time PCR, and HIF-1α and HIF-2α protein accumulation (B) was measured by immunoblotting.(C) HUVECs transfected with the indicated siRNA were subjected to hypoxic conditions for 16 hours in GF^– medium, and cord formation was examined. Results are representative of 4 independent experiments. (D) Length of cords from the experiment shown in panel C was quantified using the Bioquant Image Analysis System. Results are presented as mean of 3 different areas examined per well.