Angiopoietin-2 functions as an autocrine protective factor in stressed endothelial cells

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Abstract
Angiopoietin (Ang)-2, a context-dependent agonist/antagonist for the vascular-specific Tie2 receptor, is highly expressed by endothelial cells at sites of normal and pathologic angiogenesis. One prevailing model suggests that in these settings, Ang-2 acts as an autocrine Tie2 blocker, inhibiting the stabilizing influence of the Tie2 activator Ang-1, thereby promoting vascular remodeling. However, the effects of endogenous Ang-2 on cells that are actively producing it have not been studied in detail. Here, we demonstrate that Ang-2 expression is rapidly induced in endothelial cells by the transcription factor FOXO1 after inhibition of the phosphatidylinositol 3-kinase/Akt pathway. We employ RNAi and blocking antibodies to show that in this setting, Ang-2 unexpectedly functions as a Tie2 agonist, bolstering Akt activity so as to provide negative feedback on FOXO1-regulated transcription and apoptosis. In addition, we show that Ang-2, like Ang-1, activates Tie2/Akt signaling in vivo, thereby inhibiting the expression of FOXO1 target genes. Consistent with a role for Ang-2 as a Tie2 activator, we demonstrate that Ang-2 inhibits vascular leak. Our data suggests a model in which Ang-2 expression is induced in stressed endothelial cells, where it acts as an autocrine Tie2 agonist and protective factor.