Topotecan inhibits HIF-1α but not HIF-2α in endothelial cells. (A) HUVECs cultured in GF^– medium were incubated under normoxic or hypoxic conditions in the presence or absence of topotecan at the indicated concentrations for 16 hours. HIF-1α and HIF-2α protein accumulation was performed by immunoblotting; β-actin is shown as loading control. (B) HUVECs were seeded on Matrigel in GF⁺ medium under normoxic condition or in GF^– medium under hypoxic conditions for 16 hours in the absence or presence of topotecan (1000 nM to 1 nM) or TNP-470 (100 nM), and cord formation was examined. Results are representative of 3 independent experiments.(C) Length of cords from the experiment shown in panel B was quantified using the Bioquant Image Analysis System.