TPO activates a human VEGF promoter fragment containing a HIF-1α–binding site. The pHRE luciferase plasmid was introduced into UT-7/TPO cells with internal control vector pRL-TKLuc. Twenty-four hours after transfection, the cells were starved for 24 hours and then stimulated with 100 ng/mL TPO (▨) or serum (▪). The cells were then harvested for luciferase assay. Luciferase activity was normalized to internal control and an average ± SD of 3 experiments is shown.