Endothelium-intrinsic requirement for Hif-2α during vascular development

Background-The development of the vascular system is a complex process that involves communications among multiple cell types. As such, it is important to understand whether a specific gene regulates vascular development directly from within the vascular system or indirectly from nonvascular cells. Hypoxia-inducible factor-2 alpha (Hif-2 alpha, or endothelial PAS protein-1 [EPAS-1]) is required for vascular development in mice, but it is not clear whether its requirement resides directly in endothelial cells. Methods and Results-To address this issue, we expressed Hif-2 alpha cDNA in the vascular endothelium of Hif-2 alpha(-/-) embryos by an embryonic stem (ES) cell-mediated transgenic approach and assessed whether endothelium-specific reexpression of Hif-2 alpha could rescue vascular development. Here we report that although ES cell-derived Hif-2 alpha(-/-) embryos developed severe vascular defects by embryonic day (E) 11.5 and died in utero before E12.5, endothelium-specific expression of Hif-2 alpha cDNA restored normal vascular development at all stages examined (up to E14.5) and allowed Hif-2 alpha(-/-) embryos to survive at a frequency comparable to that of Hif-2 alpha(-/-) embryos. Furthermore, we found that Tie-2 expression was significantly reduced in Hif-2 alpha(-/-) mutants but was restored by Hif-2 alpha cDNA expression. Conclusions-These data demonstrate an intrinsic requirement for Hif-2 alpha by endothelial cells and imply that hypoxia may control endothelial functions directly via Hif-2 alpha-regulated Tie-2 expression.