Tie1 controls angiopoietin function in vascular remodeling and inflammation
Emilia A. Korhonen, … , Kari Alitalo, Pipsa Saharinen
Emilia A. Korhonen, … , Kari Alitalo, Pipsa Saharinen
Published September 1, 2016; First published August 22, 2016
Citation Information: J Clin Invest. 2016;126(9):3495-3510. https://doi.org/10.1172/JCI84923.
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Research Article Angiogenesis Vascular biology

Tie1 controls angiopoietin function in vascular remodeling and inflammation

Abstract

The angiopoietin/Tie (ANG/Tie) receptor system controls developmental and tumor angiogenesis, inflammatory vascular remodeling, and vessel leakage. ANG1 is a Tie2 agonist that promotes vascular stabilization in inflammation and sepsis, whereas ANG2 is a context-dependent Tie2 agonist or antagonist. A limited understanding of ANG signaling mechanisms and the orphan receptor Tie1 has hindered development of ANG/Tie-targeted therapeutics. Here, we determined that both ANG1 and ANG2 binding to Tie2 increases Tie1-Tie2 interactions in a β1 integrin–dependent manner and that Tie1 regulates ANG-induced Tie2 trafficking in endothelial cells. Endothelial Tie1 was essential for the agonist activity of ANG1 and autocrine ANG2. Deletion of endothelial Tie1 in mice reduced Tie2 phosphorylation and downstream Akt activation, increased FOXO1 nuclear localization and transcriptional activation, and prevented ANG1- and ANG2-induced capillary-to-venous remodeling. However, in acute endotoxemia, the Tie1 ectodomain that is responsible for interaction with Tie2 was rapidly cleaved, ANG1 agonist activity was decreased, and autocrine ANG2 agonist activity was lost, which led to suppression of Tie2 signaling. Tie1 cleavage also occurred in patients with hantavirus infection. These results support a model in which Tie1 directly interacts with Tie2 to promote ANG-induced vascular responses under noninflammatory conditions, whereas in inflammation, Tie1 cleavage contributes to loss of ANG2 agonist activity and vascular stability.

Authors

Emilia A. Korhonen, Anita Lampinen, Hemant Giri, Andrey Anisimov, Minah Kim, Breanna Allen, Shentong Fang, Gabriela D’Amico, Tuomas J. Sipilä, Marja Lohela, Tomas Strandin, Antti Vaheri, Seppo Ylä-Herttuala, Gou Young Koh, Donald M. McDonald, Kari Alitalo, Pipsa Saharinen

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Figure 1

Fluorescence energy transfer between Tie1 and Tie2 after angiopoietin stimulation of endothelial cells.

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Fluorescence energy transfer between Tie1 and Tie2 after angiopoietin st...
(A) HUVECs were transduced with retroviral vectors encoding FL Tie2-GFP and Tie1-mCherry, stimulated with CAng1, ANG2, or left untreated as a control. Nuclear DAPI stain is shown in blue. (B) Schematic view of the V5- and GFP-tagged Tie1 and Tie2 proteins used for FRET and FLIM. (C) HUVECs were transduced with retroviral vectors encoding the proteins shown in B. Representative confocal images of cells treated with CAng1, ANG1, and ANG2 (upper panel). FRET efficiency percentage was calculated from the ROI after acceptor photobleaching (lower panel). (D) Representative FLIM images (spatial distribution of lifetime value) of HUVECs transduced as in C. Decreased GFP lifetime is indicated by a color change from red to blue. (E) Quantification of FRET efficiency percentage of HUVECs in C from a representative experiment (n = 3). n(ROI) = 5 for unstimulated control,  n(ROI) = 14 for CAng1-stimulated, n(ROI) = 15 for ANG1-stimulated, and n(ROI) = 14 for ANG2-stimulated cells. (F) Quantification of GFP lifetimes (ns) of HUVECs in D from a representative experiment (n = 2). n(ROI) = 19 for unstimulated control, n(ROI) = 42 for CAng1-stimulated, n(ROI) = 26 for ANG1-stimulated, and n(ROI) = 33 for ANG2-stimulated cells. **P < 0.01; ***P < 0.001, Welch’s unequal variances t test, followed by Bonferroni’s post hoc test. Error bars indicate SD. Scale bars: 20 μm. Arrows indicate cell-cell junctions.