Endothelial HIF-2α Suppresses Retinal Angiogenesis in Neonatal Mice by Upregulating NOTCH Signaling.

Prolyl hydroxylase domain (PHD) proteins are oxygen sensors that use intracellular oxygen as a substrate to hydroxylate hypoxia inducible factor (HIF)-α proteins, routing them for polyubiquitination and proteasomal degradation. Typically, HIF-α accumulation in hypoxic or PHD-deficient tissues leads to upregulated angiogenesis. Here we report unexpected retinal phenotypes associated with endothelial cell (EC)-specific gene targeting of Phd2 and Hif-2α. EC-specific Phd2 disruption suppressed retinal angiogenesis, despite HIF-α accumulation and VEGF-A upregulation. Suppressed retinal angiogenesis was observed both in development and in the oxygen-induced retinopathy (OIR) model. On the other hand, EC-specific deletion of Hif-1α, Hif-2α, or both did not affect retinal vascular morphogenesis. Strikingly, retinal angiogenesis appeared normal in mice double-deficient for endothelial PHD2 and HIF-2α. In PHD2-deficient retinal vasculature, Delta-like 4 (DLL4, a NOTCH ligand) and HEY2 (a NOTCH target) were upregulated by HIF-2α-dependent mechanisms. Inhibition of NOTCH signaling by a chemical inhibitor or DLL4 antibody partially rescued retinal angiogenesis. Taken together, our data demonstrate that HIF-2α accumulation in retinal ECs inhibits rather than stimulates retinal angiogenesis, in part by upregulating DLL4 expression and NOTCH signaling.