Endothelial HIF-2α regulates murine pathological angiogenesis and revascularization processes

Localized tissue hypoxia is a consequence of vascular compromise or rapid cellular proliferation and is a potent inducer of compensatory angiogenesis. The oxygen-responsive transcriptional regulator hypoxia-inducible factor 2 alpha (HIF-2 alpha) is highly expressed in vascular ECs and, along with HIF-1 alpha, activates expression of target genes whose products modulate vascular functions and angiogenesis. However, the mechanisms by which HIF-2 alpha regulates EC function and tissue perfusion under physiological and pathological conditions are poorly understood. Using mice in which Hif2a was specifically deleted in ECs, we demonstrate here that HIF-2 alpha expression is required for angiogenic responses during hindlimb ischemia and for the growth of autochthonous skin tumors. EC-specific Hif2a deletion resulted in increased vessel formation in both models; however, these vessels failed to undergo proper arteriogenesis, resulting in poor perfusion. Analysis of cultured HIF-2 alpha-deficient ECs revealed cell-autonomous increases in migration, invasion, and morphogenetic activity, which correlated with HIF-2 alpha-dependent expression of specific angiogenic factors, including delta-like ligand 4 (Dll4), a Notch ligand, and angiopoietin 2. By stimulating Dll4 signaling in cultured ECs or restoring Dll4 expression in ischemic muscle tissue, we rescued most of the HIF-2 alpha-dependent EC phenotypes in vitro and in vivo, emphasizing the critical role of Dll4/Notch signaling as a downstream target of HIF-2 alpha in ECs. These results indicate that HIP-1 alpha and HIF-2 alpha fulfill complementary, but largely nonoverlapping, essential functions in pathophysiological angiogenesis.