Endothelial HIF2α 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 hypoxiainducible factor (HIF) alpha proteins, routing them for polyubiquitylation and proteasomal degradation. Typically, HIF alpha accumulation in hypoxic or PHD-deficient tissues leads to upregulated angiogenesis. Here, we report unexpected retinal phenotypes associated with endothelial cell EC-specific Phd2 disruption suppressed retinal angiogenesis, despite HIF alpha accumulation and VEGFA upregulation. Suppressed retinal angiogenesis was observed both in development and in the oxygeninduced retinopathy (OIR) model. On the other hand, EC-specific deletion of Hif1alpha (Hif1a), Hif2alpha, or both did not affect retinal vascular morphogenesis. Strikingly, retinal angiogenesis appeared normal in mice double-deficient for endothelial PHD2 and HIF2 alpha. In PHD2-deficient retinal vasculature, delta-like 4 (DLL4, a NOTCH ligand) and HEY2 (a NOTCH target) were upregulated by HIF2 alpha dependent mechanisms. Inhibition of NOTCH signaling by a chemical inhibitor or DLL4 antibody partially rescued retinal angiogenesis. Taken together, our data demonstrate that HIF2 alpha accumulation in retinal ECs inhibits rather than stimulates retinal angiogenesis, in part by upregulating DLL4 expression and NOTCH signaling.