Angiopoietin-2 Is Critical for Cytokine-Induced Vascular Leakage

Abstract

Genetic experiments (loss-of-function and gain-of-function) have established the role of Angiopoietin/Tie ligand/receptor tyrosine kinase system as a regulator of vessel maturation and quiescence. Angiopoietin-2 (Ang-2) acts on Tie2-expressing resting endothelial cells as an antagonistic ligand to negatively interfere with the vessel stabilizing effects of constitutive Ang-1/Tie-2 signaling. Ang-2 thereby controls the vascular response to inflammation-inducing as well as angiogenesis-inducing cytokines. This study was aimed at assessing the role of Ang-2 as an autocrine (i.e. endothelial-derived) regulator of rapid vascular responses (within minutes) caused by permeability-inducing agents. Employing two independent in vivo assays to quantitatively assess vascular leakage (tracheal microsphere assay, 1–5 min and Miles assay, 20 min), the immediate vascular response to histamine, bradykinin and VEGF was analyzed in Ang-2-deficient (Ang-2−/−) mice. In comparison to the wild type control mice, the Ang2−/− mice demonstrated a significantly attenuated response. The Ang-2−/− phenotype was rescued by systemic administration (paracrine) of an adenovirus encoding Ang-2. Furthermore, cytokine-induced intracellular calcium influx was impaired in Ang-2−/− endothelioma cells, consistent with reduced phospholipase activation in vivo. Additionally, recombinant human Ang-2 (rhAng-2) alone was unable to induce vascular leakage. In summary, we report here in a definite genetic setting that Ang-2 is critical for multiple vascular permeability-inducing cytokines.

Opposing actions of angiopoietin-2 on Tie2 signaling and FOXO1 activation

“…Capillary and Blood vessels that underwent remodeling under opposite states of Tie2 phosphorylation could be distinguished by differences in endothelial barrier function. Antileak effects of Tie2 activation and proleak effects of Tie2 suppression fit with many reports of the stabilizing actions of ANG1 and destabilizing actions of ANG2 (16,18,19,32,37,54).…”

“…Also, the findings demonstrate that both ANG1 and ANG2 act as Tie2 agonists and promote vascular enlargement and stability under otherwise normal conditions, consistent with previous reports (16,20,32,37), but also that, in inflammation, ANG2 acts as a Tie2 antagonist and promotes vascular destabilization and leakage.…”

“…When vascular enlargement is driven by Tie2 overactivation, vessels are resistant to leakage, but when it is driven by Tie2 inhibition, they are typically unstable and leaky (16,18,19,32,37,54). We exploited this background to distinguish vascular remodeling under opposite states of Tie2 activity in our model, using leakage of 50-nm fluorescent microspheres as a readout (55).…”

“…ANG2 competitively inhibits the action of ANG1 on Tie2 (7,15) and promotes vascular remodeling by suppression of Tie2 signaling (4,(16)(17)(18)(19). ANG2 overexpression causes embryonic lethality similar to deletion of Ang1 or Tie2 (15).…”

“…ANG1 dominates in quiescent blood vessels, but ANG2 plays important roles in inflammation, sepsis, and other pathological conditions (18,(24)(25)(26) and is rapidly released from endothelial cells by inflammatory stimuli (27,28). The effects of ANG2 are greatly amplified by TNF-α and other inflammatory cytokines (4,16,17,27). ANG2 inhibitors can reduce vascular remodeling and inflamAngiopoietin-2 (ANG2) regulates blood vessel remodeling in many pathological conditions through differential effects on Tie2 signaling.…”