ANGPT2 weakens TIE2-mediated vascular stabilization signals, making tumor vessels more responsive to VEGF and contributing to abnormal angiogenesis, immunosuppression, and tolerance to anti-angiogenic therapy.

Keywords: ANGPT2; TIE2; ANGPT1; angiogenesis; vascular normalization; VEGF; VEGFR2; tumor microenvironment; endothelial cells; anti-angiogenic therapy

1 Molecular Composition and Regulatory Features of the ANGPT2/TIE2 Signaling Axis

1.1 Molecular Features of ANGPT2

Angiopoietin-2 (ANGPT2) is a member of the angiopoietin family and is mainly produced by vascular endothelial cells. ANGPT2 contains an N-terminal region involved in multimerization, a linker region, and a C-terminal fibrinogen-like domain responsible for receptor binding. Its multimerization state can affect TIE2 binding and receptor clustering. In quiescent endothelial cells, ANGPT2 can be stored in Weibel-Palade bodies and rapidly released under hypoxia, inflammation, VEGF stimulation, or altered blood flow. Tumor vascular endothelial cells can also continuously upregulate ANGPT2, keeping local vessels in a persistently unstable and highly plastic state.

1.2 TIE2 Receptor and Auxiliary Regulatory Molecules

TIE2, also known as TEK, is a receptor tyrosine kinase mainly expressed in vascular endothelial cells and can also be found in some TIE2-positive monocytes and macrophages. After angiopoietin binding, TIE2 clustering and phosphorylation status can change, thereby regulating PI3K-AKT, ERK, FOXO1, and cell junction-related signaling. TIE1 can alter the response of TIE2 to ANGPT1 and ANGPT2. Vascular endothelial protein tyrosine phosphatase (VE-PTP) can dephosphorylate TIE2 and limit its activation. Therefore, TIE2 signaling is also influenced by cell junctions, shear stress, inflammatory state, and receptor complex composition.

1.3 Context-Dependent Effects of ANGPT2

ANGPT2 is often regarded as a functional antagonist of ANGPT1/TIE2 signaling, but its effect is not fixed. When ANGPT1 is sufficient and endothelial junctions are intact, ANGPT2 can compete for TIE2 binding and weaken vascular stabilization signals. When ANGPT1 is insufficient, ANGPT2 concentration is high, or receptor clustering patterns change, ANGPT2 may also partially activate TIE2. In cells with low TIE2 expression, ANGPT2 can also interact with certain integrins to regulate cell adhesion, migration, and inflammatory responses. Therefore, ANGPT2-related phenotypes cannot all be attributed to TIE2.

2 Balance of TIE2 Activity Mediated by ANGPT1 and ANGPT2

2.1 ANGPT1 Maintains Vascular Stability

ANGPT1 is mainly produced by pericytes, vascular smooth muscle cells, and some stromal cells. It can promote TIE2 clustering and phosphorylation, activate PI3K-AKT signaling, and inhibit FOXO1 activity, thereby enhancing VE-cadherin-mediated endothelial junctions, maintaining pericyte coverage, reducing vascular permeability, and decreasing endothelial cell sensitivity to inflammatory factors and persistent pro-angiogenic stimulation.

2.2 ANGPT2 Weakens the Vascular Quiescent State

Under tumor or inflammatory conditions, ANGPT2 released by endothelial cells can weaken ANGPT1-maintained TIE2 stabilization signaling, loosen endothelial junctions, reduce pericyte attachment, and increase vascular sensitivity to VEGF and inflammatory mediators. ANGPT2 mainly places existing vessels into a plastic state that is prone to sprouting, regression, or remodeling, rather than independently determining new vessel formation. When local endothelial survival signals such as VEGF are sufficient, vessels are more likely to sprout and expand; when survival signals are insufficient, regression may occur.

2.3 Comparison of ANGPT1 and ANGPT2 Functional Features

3 Cooperative Regulation of ANGPT2/TIE2 and VEGF/VEGFR2 Signaling

3.1 ANGPT2 Provides Structural Conditions for Vascular Sprouting

Stable vessels are jointly restricted by endothelial junctions, basement membrane, and pericyte coverage, making them less likely to directly enter a rapid sprouting state. After ANGPT2 weakens TIE2 stabilization signaling, endothelial junction loosening, pericyte detachment, and basement membrane remodeling can occur, providing structural conditions for VEGF-driven endothelial cell migration and proliferation.

3.2 VEGF Drives Endothelial Cell Proliferation and Migration

After VEGF-A binds VEGFR2, it can activate PLCγ-PKC, RAS-RAF-MEK-ERK, PI3K-AKT, and other pathways, promoting endothelial cell survival, proliferation, migration, and increased vascular permeability. When ANGPT2 and VEGF are simultaneously elevated, existing vascular stability decreases, and new vessel sprouting and abnormal branching increase. When ANGPT2 is elevated but survival signals such as VEGF are insufficient, vessels that have lost stabilization support may regress. Therefore, the vascular effects of ANGPT2 need to be interpreted together with local VEGF levels.

Figure 1. Schematic of ANGPT1/ANGPT2–TIE2 and VEGF/VEGFR2 Regulation of Vessel Stabilization and Angiogenesis

3.3 ANGPT2 Participates in Vascular Adaptation After Anti-VEGF Therapy

Anti-VEGF therapy can inhibit endothelial cell proliferation and reduce vascular permeability, but some tumors can maintain vascular remodeling by upregulating ANGPT2, promoting pericyte detachment, and altering myeloid cell recruitment. Elevated ANGPT2 in residual vessels may also aggravate hypoperfusion, hypoxia, and inflammation, induce alternative pro-angiogenic signals, and promote therapeutic tolerance.

4 ANGPT2-Mediated Tumor Vascular Remodeling and Tumor Progression

4.1 Abnormal Tumor Vessel Formation

Persistently elevated ANGPT2 and VEGF can cause tumor vessels to show disordered branching, irregular lumens, defective endothelial junctions, and insufficient pericyte coverage. Although these vessels may show increased density, perfusion efficiency is low and is accompanied by prominent leakage, uneven blood flow, and local hypoxia.

4.2 Hypoperfusion, Hypoxia, and Increased Interstitial Pressure

Vascular leakage allows plasma components to enter the tumor interstitium, increasing interstitial fluid pressure and compressing small vessels. Ineffective perfusion further causes hypoxia and an acidic metabolic environment. Hypoxia can activate HIF-related signaling and promote the expression of VEGF, ANGPT2, and invasion-related genes. Increased interstitial pressure also limits drug entry from vessels into tumor tissue, reducing effective exposure to chemotherapy and other systemic therapies.

4.3 Tumor Cell Intravasation and Distant Metastasis

Loosened endothelial junctions, incomplete basement membrane, and reduced pericyte coverage can lower the structural barrier for tumor cells to enter blood vessels. ANGPT2 can also alter endothelial adhesion molecules, inflammatory cell recruitment, and perivascular matrix, making it easier for tumor cells to approach vessels and enter the circulation. In distant organs, ANGPT2-related endothelial activation and increased permeability may also promote tumor cell extravasation and metastatic niche establishment, but its effects are influenced by tumor type, target organ, and local immune status.

5 ANGPT2/TIE2 Signaling and Tumor Microenvironment Regulation

5.1 TIE2-Positive Monocytes and Macrophages

Some tumor-associated monocytes and macrophages express TIE2 and can be influenced by ANGPT2, accumulating around vessels. These cells can produce VEGF, matrix-degrading enzymes, and immunoregulatory factors, participating in vascular sprouting, tumor cell intravasation, and local immunosuppression. TIE2-positive myeloid cells do not represent all tumor-associated macrophages. Their proportion and function need to be analyzed by combining TIE2 with markers such as CD14, CD68, and CD163.

5.2 Immune Barrier Formed by Vascular Endothelium

Abnormal tumor vessels can reduce the efficiency of effector T cell adhesion and entry into tumor tissue. ANGPT2-related endothelial activation can also alter the expression of ICAM-1, VCAM-1, and other adhesion molecules, causing myeloid cells and lymphocytes to show different recruitment patterns. After ANGPT2 blockade improves vascular structure and perfusion, the opportunity for effector immune cells to enter tumor tissue may increase. However, the final immune effect still depends on antigen presentation, immune checkpoint activity, and myeloid suppressive status.

5.3 Mutual Reinforcement Between Hypoxia and Immunosuppression

Hypoxia caused by inefficient perfusion can promote the expression of VEGF, ANGPT2, and multiple chemokines and drive the accumulation of immunosuppressive macrophages, myeloid-derived suppressor cells, and regulatory T cells. These cells can further release pro-angiogenic factors, matrix-degrading enzymes, and immunoregulatory molecules, forming a mutually reinforcing cycle among abnormal vessels, hypoxia, and immune escape.

6 ANGPT2-Targeted Inhibition and Combination Strategies with Anti-VEGF Therapy

6.1 Direct ANGPT2 Blockade

ANGPT2-neutralizing antibodies and other binding inhibitors can reduce interactions between ANGPT2 and TIE2 or integrins, restore part of the ANGPT1/TIE2 stabilization signal, and reduce vascular leakage, abnormal sprouting, and TIE2-positive myeloid cell recruitment. ANGPT2 blockade alone may be insufficient to continuously inhibit angiogenesis in tumors highly dependent on VEGF. Therefore, it is more suitable to evaluate ANGPT2 blockade from the perspectives of vascular stabilization, microenvironment remodeling, and sensitization to combination therapy.

6.2 Dual Blockade of ANGPT2 and VEGF

Combined inhibition of ANGPT2 and VEGF can simultaneously limit vascular instability and endothelial cell proliferation, reducing ANGPT2-mediated alternative remodeling after single-agent anti-VEGF therapy. Bispecific antibodies or combinations of two types of blocking agents can both be used to validate this strategy. However, overly strong or prolonged dual blockade may cause excessive vascular reduction, tissue hypoxia, and reduced drug delivery. Therefore, effective vascular normalization should be distinguished from nonselective vascular elimination.

6.3 Vascular Normalization and Immunotherapy Combination

Moderate inhibition of ANGPT2 and VEGF can improve pericyte coverage, endothelial junctions, and tumor perfusion, reduce hypoxia, and promote effector T cell infiltration, thereby providing a more favorable vascular condition for immune checkpoint blockade. Combination therapy should determine dosing sequence and duration according to the vascular normalization window. Chemotherapy, radiotherapy, or immunotherapy implemented after perfusion and immune cell entry improve is usually more consistent with the logic of combination therapy than simply pursuing maximal vessel reduction.

6.4 Efficacy Evaluation and Result Interpretation

ANGPT2/TIE2-targeted therapy should be evaluated from mutually complementary levels, including pathway status, vascular structure and function, and the immune microenvironment.

(1) Circulating and tissue pathway indicators

Circulating ANGPT2 can reflect endothelial activation and vascular injury, but it can also be affected by inflammation, infection, and cardiovascular status. Tumor tissue ANGPT2, ANGPT1, TIE2 phosphorylation, and the ANGPT1/ANGPT2 ratio are more suitable for evaluating the local signaling axis status.

(2) Vascular structure and perfusion function

CD31-positive vascular area can be used to observe changes in vascular density, but it cannot alone represent therapeutic efficacy. Evaluation of vascular normalization also requires perfusion, permeability, endothelial junction proteins such as Claudin-5, and pericyte markers such as NG2.

(3) Tumor hypoxia status

Vessel reduction does not necessarily improve the tumor environment. Increased HIF-1α and other hypoxia markers may indicate excessive vessel elimination, reduced perfusion, or enhanced alternative pro-angiogenic responses.

(4) Changes in the immune microenvironment

Reduction of TIE2-positive myeloid cells should be analyzed together with effector T cell infiltration, immunosuppressive factors, and tumor growth changes to determine whether therapy improves both the vascular barrier and antitumor immunity.

7 Products Related to ANGPT2/TIE2 and Tumor Vascular Research

ANGPT2 weakens ANGPT1/TIE2-mediated vascular stability, making tumor vessels more responsive to VEGF and driving vascular leakage, inefficient perfusion, and formation of an immunosuppressive microenvironment. Related research should evaluate ANGPT2 or TIE2 intervention together with VEGF/VEGFR2 signaling, endothelial junctions, pericyte coverage, tissue perfusion, and tumor hypoxia to distinguish vascular normalization, ineffective vessel elimination, and adaptive remodeling after therapy.

For more related articles, please see below:

[1] FGF2-Induced Angiogenesis Model: Standard Operating Procedure (Protocol)

[2] Tumor cell-induced angiogenesis model experiment

[3] Angiogenic Factors: Molecular Classes, Regulatory Mechanisms, and Experimental as Well as Translational Applications

[4] Key Nodes and Representative Mechanisms in Angiogenic Regulatory Networks: A Structured Review