Hypoxia reduces endothelial Ang1-induced Tie2 activity in a Tie1-dependent manner.

Despite the altered expression of Tie receptors and angiopoietin ligands during hypoxic conditions, the effect of hypoxia on Tie-mediated endothelial responses has not been elucidated. In this study, we found that hypoxia increased Tie receptor expression but attenuated angiopoietin-1 (Ang1)-induced Tie2 activity, including Tie2 phosphorylation, Tie2 downstream signaling activation, and endothelial cell tube formation. However, Ang1 binding to endothelial cells was increased during hypoxic conditions. We demonstrated that Tie1 suppression restored the Tie2 activity and that Tie1-mediated Tie2 suppression was independent of tyrosine phosphatase activity. These results suggest that under hypoxic conditions, Tie1 is critical for reducing Ang1-induced Tie2 activity and angiogenesis.