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, inflammation, and neovascularisation.1
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The role of Ang-2 and VEGF in DME
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Vascular leakage and neovascularisation1-5
Elevated Ang-2 levels inhibit Tie2, which:
- Primes blood vessels for angiogenesis (pericyte dropout and apoptosis)
- Reduces integrity of endothelial cell junctions, which contributes to vascular leakage
- Amplifies the effects of VEGF on blood vessels
Elevated VEGF levels::
- Promote neovascularisation
- Lead to further vascular leakage
Inflammation1-5
Inhibition of Tie2 by Ang-2 also modulates immune signalling, leading to:
- Leukocyte adhesion to the vessel walls and transmigration into the surrounding tissues
- Release of pro-inflammatory and pro-angiogenic cytokines
- Sensitisation of blood vessels to the effects of cytokines and amplification of those effects
See how Ang-2 drives vascular instability in retinal and choroidal vascular diseases in this animated video.
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Ang-1–Tie2 signalling
Select a downstream pathway to learn more
Healthy vasculature1,2,6,7
In healthy vasculature, Ang-1–Tie2 signalling actively maintains vascular homeostasis and serves as an important molecular brake against vascular instability.
Activated Tie2 promotes endothelial cell integrity, thereby ensuring vascular stability.
Inhibiting inflammation1,2,6
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 and transmigration into the surrounding tissues.
Endothelial cell integrity1,2,6
Ang-1–induced Tie2 phosphorylation results in the activation of AKT. AKT signalling activates survival promoting pathways such as eNOS.
Inhibiting neovascularisation, vascular leakage and fibrosis1,2,6,7
Ang-1–induced Tie2 phosphorylation triggers AKT activation/phosphorylation; activated AKT phosphorylates and inactivates FOXO1, preventing its translocation to the nucleus and reducing expression of its target genes:
- Ang-2
- Genes involved in:
- Vascular remodelling and instability
- Endothelial cell apoptosis
- Fibrosis
Ang-2–Tie2 signalling
Select an effect to learn more
Pathway cross-talk1,2,6,7
Inhibition of Tie2 by Ang-2 amplifies the vascular leakage and neovascularisation effects of VEGF, and has other VEGF-independent effects.
Together, Ang-2 and VEGF act in synergy to promote vascular instability.
Vascular instability1,2,6,7
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, amplifying vascular leakage and neovascularisation to further promote vascular instability.
Inflammation1,2,6,7
Ang-2 prevents the Tie2-mediated activation of ABIN2, which in turn does not inactivate NF-kB. NF-kB drives expression of pro-inflammatory cytokines and adhesion molecules (i.e. ICAM1 and VCAM1) and primes the endothelium for TNF-α–induced leukocyte adhesion and transmigration into the surrounding tissues.
Promoting neovascularisation, vascular leakage, and fibrosis1,2,6,7
Tie2 inhibition by Ang-2 inactivates AKT; inactivated AKT 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.
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. 1,2,8-13
Role of VEGF concentration in Ang-2–Tie2 signalling8
The effects of Ang-2–Tie2 signalling differ depending on the presence or absence of VEGF, resulting in the promotion of endothelial cell integrity or angiogenesis and vascular instability.
Low levels of VEGF
Endothelial cell integrity
VEGF is necessary for the integrity of endothelial cells. When VEGF is suppressed under experimental conditions, Ang-2–Tie2 signalling promotes endothelial cell death and vessel regression.
High levels of VEGF
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.
Ang-2 signalling via integrins in endothelial cells2,9,10
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
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.
The role of Tie1 in Ang-2–Tie2 signalling2,11,12
Tie1 acts as a modulator for Tie2, and its expression affects the role of Ang-2 in Ang-2–Tie2 signaling.
Homeostatic conditions
Ang-2 acts as a Tie2 agonist
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
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 in Ang-2–Tie2 signalling2,13
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
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
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.
ABIN2, A20-binding inhibitor of NF-κB activation-2; 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.
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