DME, nAMD, and RVO are multifactorial diseases1,2–4 driven by vascular instability, characterised by vascular leakage, inflammation, and neovascularisation.5
Multiple signalling pathways, including the Ang–Tie pathway, work together to contribute to this vascular instability, and are stimulated by conditions of cellular stress.5,6
Angiopoietins (Ang) are a family of angiogenic growth factors, with Ang-1 and Ang-2 being best characterised for their key roles in vascular development and vascular stability.6
Ang-1 and Ang-2 bind to the Tie2 receptor and are important regulators of vascular stability.1
In addition to Ang-1 and Ang-2, other key players of the Ang–Tie signalling pathway include the Tie2 receptor and the Tie2 modulators Tie1 and VE-PTP.
The Ang–Tie pathway also cross talks with integrins (through direct Ang-2–integrin signalling) and VEGFR, via the signalling of downstream kinases.
Ang-1–Tie2 signalling promotes vascular stability, maintaining homeostasis, whereas Ang-2–Tie2 signalling drives vascular instability, characterised by vascular leakage, inflammation, and neovascularisation.1,2
DME and nAMD are multifactorial diseases involving numerous signalling pathways that contribute to vascular instability.2 Watch the videos below to discover how Ang-2 and VEGF work together to drive these retinal and choroidal vascular diseases.
Under conditions of cellular stress (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.6
Vascular leakage and neovascularisation1-5
Elevated Ang-2 levels inhibit Tie2, which:
Elevated VEGF levels:
Inhibition of Tie2 by Ang-2 also modulates immune signalling, leading to:
Both Ang-1 and Ang-2 bind to Tie2, with different effects on the downstream signalling pathways.1,2
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.
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.
Ang-1–induced Tie2 phosphorylation results in the activation of AKT. AKT signalling activates survival promoting pathways such as eNOS.
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:
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.
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.
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.
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:
FOXO1 nuclear translocation and upregulation of Ang-2 expression leads to a positive feedback loop of Ang-2 production and Tie2 inhibition.
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
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.
Endothelial cell integrity
VEGF is essential for endothelial cell integrity. When VEGF is suppressed under experimental conditions, Ang-2–Tie2 signalling promotes endothelial cell death and vessel regression.
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.
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.
In the absence of Tie2, Ang-2 directly binds integrins. This stimulates endothelial cell migration and vascular sprouting.
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.
Tie1 acts as a modulator of Tie2, and its expression impacts the role of Ang-2 in Ang-2–Tie2 signalling.
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.
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.
VE-PTP acts as a modulator of Tie2, and its expression impacts the role of Ang-2 in Ang-2–Tie2 signalling.
Ang-2 acts as a Tie2 agonist
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.
Ang-2 acts as a Tie2 antagonist
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.
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