A closer look at

vascular instability

Ang-1–Tie2 signalling promotes vascular stability, maintaining homeostasis, whereas Ang-2–Tie2 signalling drives vascular instability, characterised by vascular leakage, neovascularisation, and inflammation.1
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Vascular instability

Under conditions of cellular stress, such as hyperglycaemia, ischemia, or hypoxia (i.e. in diseases such as DME, nAMD, and RVO), an angiogenic switch leads to elevated levels of Ang-2 and VEGF, potentiating angiogenic and inflammatory signals.1

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Use the switch to view different conditions

Vascular leakage & neovascularisation

Inflammation

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Vascular leakage and neovascularisation 1-4

Elevated Ang-2 levels inhibit Tie2:

  • Primes blood vessels for angiogenesis (pericyte apoptosis)
  • Reduces integrity of endothelial cell junctions, which contributes to leakage
  • Amplifies the effects of VEGF on blood vessels

Elevated VEGF levels:

  • Cause vascular leakage
  • Promote neovascularisation
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Inflammation 1-4

Inhibition of Tie2 by Ang-2 also modulates immune signalling, leading to:

  • Release of cytokines
  • Infiltration of leukocytes and macrophages into the surrounding tissues
  • Sensitisation of blood vessels to the effects of cytokines and amplification of those effects
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Angiopoietins:
Key regulators of vascular stability

Download a detailed illustration showing how Ang-1 and Ang-2
promote vascular stability or instability.

Downstream effects of Ang-Tie signalling

Both Ang-1 and Ang-2 bind to Tie2, with different effects on the downstream signalling pathways.1
Hear from the experts:
It’s all in the balance:
Ang-1 vs Ang-2

Anat Loewenstein, MD

Tel Aviv Medical Center

Ang-1–Tie2 signalling

Select a downstream pathway to learn more

Ang-2–Tie2 signalling

Select a downstream pathway to learn more

Context-dependent effects
of Ang-2 signalling

The effects of Ang-2 signalling on endothelial cells are context-dependent and mediated by
several factors, including expression of receptors and modulators and levels of ligands.1,7-12
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Ang, angiopoietin; DME, diabetic macular edema; eNOS, endothelial nitric oxide synthase; FAK, focal adhesion kinase; FOXO1, forkhead box O1; ICAM1, intercellular adhesion molecule 1; nAMD, neovascular age-related macular degeneration; NF-κB, nuclear factor κB; RVO, retinal vein occlusion; Tie, tyrosine kinase with immunoglobulin-like domains; TNF-α, tumour necrosis factor alpha; VCAM1, vascular cell adhesion molecule 1; VEGF, vascular endothelial growth factor; VEGFR, vascular endothelial growth factor receptor; VE-PTP, vascular endothelial protein tyrosine phosphate.

Discover the preclinical evidence that demonstrates how Ang-2 and VEGF work together to drive vascular instability…

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References

  1. Saharinen P, et al. Nat Rev Drug Discov. 2017;16:635–61
  2. Klaassen I, et al. Prog Retin Eye Res. 2013;34:19–48
  3. Bolinger MT, et al. Int J Mol Sci. 2016;17:1498
  4. Fiedler U, Augustin HG. Trends Immunol. 2006;27:552–8
  5. Augustin HG, et al. Nat Rev Mol Cell Biol. 2009;10:165–77
  6. Akwii RG, et al. Cells. 2019;8:E471
  7. Felcht M, et al. J Clin Invest. 2012;122:1991–2005
  8. Thomas M, et al. J Biol Chem. 2010;285:23842–9
  9. Lobov IB, et al. Proc Natl Acad Sci U S A. 2002;99:11205–10
  10. Korhonen EA, et al. J Clin Invest. 2016;126:3495–510 
  11. Kim M, et al. J Clin Invest. 2016;126:3511–25
  12. Souma T, et al. PNAS. 2018;115:1298–303
Vascular stability
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Healthy vasculature1,5,6

In healthy vasculature, activation of the Ang-1–Tie2 pathway actively maintains vascular homeostasis and serves as an important molecular brake against vascular instability.

Activated Tie2 promotes endothelial cells survival and integrity of cell junctions, thereby ensuring vascular stability.

Inhibiting inflammation
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Inflammation1,5

Tie2‑mediated activation of ABIN2 protects endothelial cells by inactivating the transcription factor NF‑κB, which is a key transcriptional regulator of inflammatory responses in endothelial cells.

Endothelial cell survival
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Endothelial cell survival1,5

Ang-1–induced Tie2 phosphorylation results in the activation of AKT. AKT signalling activates survival promoting pathways such as eNOS.

Inhibiting neovascularisation
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Neovascularisation, instability, and fibrosis1,5,6

FOXO1 phosphorylation prevents its translocation to the nucleus, reducing expression of its target genes:

  • Ang-2
  • Genes involved in:
    • Vascular remodelling and instability
    • Endothelial cell apoptosis
    • Fibrosis
Vascular instability
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Vascular instability1,5,6

In pathologic conditions, Ang-2–Tie2 binding inhibits Tie2, allowing activation of the FOXO1 pathway. This results in inhibition of survival signalling and promotion of vascular instability.

Ang-2 also cross-talks with the VEGF signalling pathway to promote vascular instability.

Inflammation
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Inflammation1,5

NF-κB drives expression of pro-inflammatory cytokines and adhesion molecules (i.e. ICAM1 and VCAM1) and primes the endothelium for TNF-α–induced leukocyte adhesion.

Neovascularisation & leakage
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Neovascularisation, vascular leakage, and fibrosis1,5,6

Tie2 inhibition dephosphorylates and activates FOXO1, which translocates to the nucleus and increases expression of its target genes:

  • Ang-2
  • Genes involved in:
    • Vascular remodelling and instability
    • Endothelial cell apoptosis
    • Fibrosis

FOXO1 nuclear translocation and upregulation of Ang-2 expression leads to a positive feedback loop of Ang-2 production and Tie2 inhibition.

Pathway cross-talk
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Pathway cross-talk1,5,6

Inhibition of Tie2 by Ang-2 amplifies the effects of VEGF.

Ang-2 and VEGF act in synergy to promote vascular instability.

Integrin signalling

Ang-2 signalling via integrins in endothelial cells7,8

The expression levels of Tie2 versus integrin receptors in endothelial cells impact the effects of Ang-2 signalling via the phosphorylation status of FAK, an integrin adaptor protein associated with barrier function.

Absence of Tie2
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Vascular sprouting
In the absence of Tie2, Ang-2 directly binds integrins. This stimulates endothelial cell migration and vascular sprouting.

Presence of Tie2

Vascular instability
In the presence of Tie2, Ang-2 binds Tie2 directly, stimulating integrin internalisation.

Internalised integrins are processed in intracellular compartments and targeted for either recycling or degradation. This results in vascular instability.

Role of VEGF

Role of VEGF concentration in Ang-2–Tie2 signalling9

The effects of Ang-2–Tie2 signalling differ depending on the presence or absence of VEGF, resulting in the promotion of endothelial cell survival or angiogenesis and vascular instability.

Low levels of VEGF
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Endothelial cell survival
VEGF is essential for endothelial cell survival. When VEGF is suppressed under experimental conditions, Ang-2–Tie2 signalling promotes endothelial cell death and vessel regression.

High levels of VEGF
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Vascular instability
Under pathologic conditions in which Ang-2 and VEGF are upregulated, high levels of VEGF modulate the effects of Ang-2. Therefore, Ang-2 and VEGF act synergistically to promote vascular instability and angiogenesis.

Role of Tie1

Role of Tie1 in Ang-2–Tie2 signalling10,11

Tie1 acts as a modulator of Tie2, and its expression impacts the role of Ang-2 in Ang-2–Tie2 signalling.

Homeostatic conditions

Ang-2 acts as a Tie2 agonist

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Normal conditions
Under normal conditions, Tie1 and Tie2 form a complex, promoting the Tie2 agonist function of Ang-2. This activates Tie2 and leads to the same downstream signalling effects as Ang-1–Tie2 signalling.

Inflammatory conditions

Ang-2 acts as a Tie2 antagonist

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Inflammatory conditions
Under inflammatory conditions, Tie1 is inactivated owing to ectodomain cleavage, preventing the formation of a Tie1–Tie2 complex. Inactivation of Tie1 promotes the Tie2 antagonist function of Ang-2.

Role of VE-PTP

Role of VE-PTP in Ang-2–Tie2 signalling12

VE-PTP acts as a modulator of Tie2, and its expression impacts the role of Ang-2 in Ang-2–Tie2 signalling.

Absence of VE-PTP

Ang-2 acts as a Tie2 agonist

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Tie2 responsiveness
VE-PTP sets the threshold for Tie2 responsiveness to Ang-1 and Ang-2. In the absence of VE-PTP (such as in lymphatic endothelial cells, where it is not expressed), Ang-2 acts as a Tie2 agonist, leading to Tie2 activation.

Presence of VE-PTP

Ang-2 acts as a Tie2 antagonist

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Tie2 affinity
In the presence of VE-PTP (such as in vascular endothelial cells, where it is highly expressed), Ang-2 has low affinity for Tie2 and acts as a Tie2 antagonist, leading to inhibition of Tie2.