Endothelial PAS domain protein 1 (EPAS1) is a basic-helix-loop-helix/PAS domain transcription factor that is expressed preferentially in vascular endothelial cells. EPAS1 shares high homology with hypoxia-inducible factor-1α (HIF-1α) and is reported to transactivate vascular endothelial growth factor (VEGF), fetal liver kinase-1 (Flk-1), and Tie2 promoters. In this study, we analyzed the role of EPAS1 in the process of angiogenesis. Using microarray technology, we looked for target genes regulated by EPAS1 in vascular endothelial cells. A total of 130 genes were upregulated by EPAS1, including fms-like tyrosine kinase-1 (Flt-1). Reporter analysis using human Flt-1 promoter and gel mobility shift assays showed that the heterodimer of EPAS1 and aryl hydrocarbon receptor nuclear translocator binds directly to HIF-1-binding site upstream of Flt-1 promoter and transactivates it. Small interfering RNA targeted to EPAS1 but not HIF-1α attenuated desferrioxamine-induced Flt-1 mRNA expression, thus EPAS1 is though...