ANGPT2

Angiopoietin 2 (Ang2), encoded by the gene ANGPT2, is a key regulator of vascular remodeling, angiogenesis and inflammation. The angiopoietin-Tie2 axis plays a central role in vascular biology. Ang2, a secreted glycoprotein, competes with its counterpart angiopoietin 1 (Ang1, encoded by the gene ANGPT1) for binding to the Tie2 receptor. In contrast to Ang1, which stabilizes the vasculature, Ang2 promotes vascular destabilization, angiogenesis and inflammation. Dysregulated Ang2 expression is associated with pathological angiogenesis and endothelial dysfunction, making it a critical target for therapeutic intervention.

NCBI Gene ID: 285

UniProtKB ID: O15123

Structure of Ang2

Angiopoietin 2 protein is composed of an N-terminal supercluster domain, a central coiled-coil domain, a linker region, and a C-terminal fibrinogen-like domain that is primarily responsible for binding to its receptor, Tie2; it shares an essentially similar structure with other angiopoietins, but has distinct functional properties due to its unique sequence variations.

Key points about Ang2 structure:

• Fibrinogen-like domain: The most important functional part of Ang2, located at the C-terminus, responsible for binding to the Tie2 receptor.
• Coiled-coil domain: A central region of the protein with repeating amino acid patterns that allows for protein-protein interactions.
• N-terminal supercluster domain: Plays a role in protein oligomerization and may contribute to the regulation of Ang2 activity.

Figure 1. Structure of Ang2 fibrinogen-like receptor binding domain (RBD). The refined model of Ang2-RBD with each of the three domains shown in a different color. The black sphere represents the bound calcium ion. (Barton et al., 2015)

Expression and Regulation of Ang2

Ang2 is primarily stored in Weibel-Palade bodies in endothelial cells and is rapidly released in response to various stimuli, including hypoxia, inflammation, and mechanical stress. Its expression is tightly regulated at both transcriptional and post-transcriptional levels, with hypoxia-inducible factor-1α (HIF-1α) being a key transcriptional regulator.

Biological Activity of Ang2 in Endothelial Cells

Blood Endothelial Cells (BECs)

The role of Ang2 in blood endothelial cells (BECs) remains complex and context-dependent. Under physiological conditions, Ang2 functions primarily as an antagonist of angiopoietin 1 by inhibiting Tie2 phosphorylation. Ang1 is known to rapidly auto phosphorylate the Tie2 receptor in endothelial cells, thereby stabilizing the vasculature and preventing excessive angiogenesis. However, Ang2 counteracts this effect by binding to Tie2 without inducing full phosphorylation, leading to endothelial destabilization and increased vascular permeability.

However, recent research suggests that Ang2 may also act as a Tie2 agonist under certain conditions. The presence of Tie1, a co-receptor, plays a critical role in determining whether Ang2 functions as an agonist or antagonist. In inflamed endothelium, Tie1 cleavage shifts Ang2 from an agonistic to an antagonistic role. This phenomenon is supported by studies in Tie1-deficient mice in which Ang2 loses its agonistic activity.

Lymphatic Endothelial Cells (LECs)

In lymphatic endothelial cells (LECs), Ang2 acts predominantly as an agonist rather than an antagonist. High concentrations of Ang2 induce Tie2 phosphorylation, which promotes LEC proliferation and survival. The observed angiogenic and anti-apoptotic effects of Ang2 in LECs are attributed to the reduced expression of Tie1, which otherwise inhibits Ang2-induced Tie2 activation. Compared to BECs, LECs have lower levels of Tie1, allowing a more agonistic role for Ang2.

In addition, the absence of vascular endothelial protein tyrosine phosphatase (VE-PTP) in LECs further modulates Ang2 function. VE-PTP has been identified as a key regulator of the agonistic and antagonistic roles of Ang2 in blood endothelial cells. Studies have shown that inhibition of VE-PTP converts Ang2 into a potent Tie2 activator, highlighting the context-dependent nature of Ang2 signaling.

Pathophysiological Role of Ang2

Dysregulated Ang2 contributes to a wide range of pathological conditions:

• Cancer: In cancer, Ang2 is frequently upregulated and contributes to tumor angiogenesis, a critical process for tumor growth and metastasis. High levels of Ang2 are associated with poor prognosis in several cancers, including breast, ovarian and colon cancer. Ang2 promotes vascular leakage, which facilitates tumor cell intravasation and extravasation, contributing to metastatic spread.
• Inflammatory Diseases: Ang2 has been implicated in several inflammatory conditions including sepsis, acute lung injury and rheumatoid arthritis. It exacerbates inflammation by increasing vascular permeability and promoting leukocyte extravasation. In sepsis, elevated Ang2 levels correlate with disease severity and mortality.
• Vascular Leakage Syndromes: Elevated Ang2 levels contribute to conditions such as sepsis and acute respiratory distress syndrome (ARDS) by increasing vascular permeability and exacerbating tissue damage.
• Ocular Disorders: In diabetic retinopathy and age-related macular degeneration (AMD), Ang2 contributes to pathological angiogenesis and vascular leakage. It destabilizes retinal vessels, leading to visual impairment and blindness.

Figure 2. Schematic representation of the Ang2 effect on the vascular bed in normal conditions, inflammation, and cancer. Under normal physiological conditions, Ang2 levels are low, but are upregulated during inflammation or cancer. Ang2 acts on endothelial cells, increasing endothelial permeability and also on the pericytes, causing pericyte detachment from the basement membrane, further inducing vascular leakiness, immune or/and cancer cell trans-endothelial migration, and deterioration of the condition. Ang2 has been proposed as a marker for inflammatory conditions and cancer. (Akwii et al., 2019)

Therapeutic Strategies Targeting Ang2

Given its pathological role, Ang2 is an attractive target for therapeutic intervention. Several approaches have been developed to inhibit Ang2 activity and restore vascular stability.

• Monoclonal Antibodies: Therapeutic monoclonal antibodies targeting Ang2 are designed to neutralize its activity by preventing its interaction with Tie2. One example is trebananib (AMG 386). Although primarily targeting both Ang1 and Ang2, this peptide-Fc fusion protein inhibits the angiopoietin-Tie2 interaction, thereby suppressing tumor angiogenesis and improving vascular normalization in cancer.
• Ang2-Tie2 Modulators: Compounds that enhance Tie2 activation while suppressing Ang2 effects are under development. These drugs aim to restore the balance between Ang1 and Ang2, thereby promoting vascular stability in diseases such as sepsis and diabetic complications.
• Gene Silencing Strategies: RNA-based therapies, such as siRNAs or antisense oligonucleotides, are being explored to downregulate Ang2 expression. These approaches offer specificity and the potential for long-term suppression of Ang2 in chronic disease.
• Peptide Inhibitors: Peptide-based inhibitors, such as L1-10, have been designed to block Ang2 binding to Tie2. These peptides are showing potential in preclinical models of cancer and inflammatory disease.

Clinical Applications and Challenges

Therapeutic targeting of Ang2 has broad applications in oncology, inflammatory diseases, and vascular leakage syndromes. However, challenges remain:

• Complex Role in Vascular Biology: Ang2 plays both pathological and physiological roles, necessitating precise modulation to avoid adverse effects.
• Tumor Resistance: Cancer cells may compensate for Ang2 inhibition by upregulating alternative angiogenic pathways, requiring combination therapies for effective treatment.
• Delivery Challenges: The development of targeted delivery systems is essential to maximize efficacy and minimize off-target effects, particularly in systemic diseases.

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References

1. Akwii RG, Sajib MS, Zahra FT, Mikelis CM. Role of angiopoietin-2 in vascular physiology and pathophysiology. Cells. 2019;8(5):471. doi:10.3390/cells8050471
2. Barton WA, Tzvetkova D, Nikolov DB. Structure of the angiopoietin-2 receptor binding domain and identification of surfaces involved in tie2 recognition. Structure. 2005;13(5):825-832. doi:10.1016/j.str.2005.03.009
3. Yang H, Zhang M, Mao XY, Chang H, Perez-Losada J, Mao JH. Distinct clinical impact and biological function of angiopoietin and angiopoietin-like proteins in human breast cancer. Cells. 2021;10(10):2590. doi:10.3390/cells10102590