Hypoxia Inducible Factor (HIF), consisting of HIF1α and ARNT(HIF1β) subunits, activates multiple genes in response to oxygen(O2) deprivation. Arnt–/– mice exhibit substantial defects in blood cell and vessel development. We demonstrate that hypoxia accelerates the expression of Brachyury (a mesoderm-specific transcription factor), BMP4 (a mesoderm-promoting growth factor) and FLK1 (a marker of hemangioblasts, the bipotential progenitor of endothelial and hematopoietic cells) in differentiating ES cell cultures. Significantly, proliferation of embryonic hemangioblasts (BL-CFCs) is regulated by hypoxia, as Arnt+/+ ES cells generate increased numbers of FLK1+ cells, and BL-CFCs with accelerated kinetics in response to low O2. This response is HIF-dependent as Arnt–/– ES cells produce fewer FLK1+ cells and BL-CFCs, under both normoxic and hypoxic conditions. Interestingly, this defect is rescued when Arnt–/– ES cells are co-cultured with Arnt+/+ ES cells. Vegf+/–or Vegf–/– ES cells generate proper numbers of FLK1+ cells but fewer BL-CFCs, suggesting that additional factors regulated by HIF (other than VEGF) are involved in these early events. Thus,hypoxic responses are important for the establishment of various progenitor cells, including early mesoderm and its differentiation into hemangioblasts. Together these data suggest that ineffective responses to hypoxia in Arnt–/– embryos abrogate proper cardiovascular development during early embryogenesis, including the pathways controlling hemangioblast differentiation.

Decreased oxygen (O 2 ) levels activate hypoxia-inducible factor (HIF-1) to induce genes involved in glycolysis, glucose transport, erythropoiesis, and angiogenesis. Mutations in various HIF-1 subunits have contributed to our understanding of the role hypoxia plays during early embryonic development in general and the cardiovascular system in particular. We propose that HIF-1 is important for the generation, proliferation, maintenance, and differentiation of the early cardiovascular system. Understanding aberrations in these hypoxic responses is important since they contribute to serious human disease such as ischemia and tumorigenesis. In this review we will focus on the critical role of O 2 in regulating cardiovascular events during early embryonic development.