Angiogenesis is a dynamic process by which new blood vessels form from pre-existing vascular networks. Through endothelial cell proliferation, migration, lumen formation, and maturation, it supports physiological functions such as embryonic development and tissue repair. This process is tightly regulated by a balance between pro-angiogenic factors (e.g., the VEGF family) and anti-angiogenic factors. Imbalances can lead to diseases such as tumors and ocular fundus disorders.
Within the VEGF family, VEGF165, a major splice variant of VEGF-A, is a core regulator of both physiological and pathological angiogenesis. Since the isolation of VEGF protein in 1989, studies have confirmed its role as a "central driver" in activating vascular endothelial cells, initiating new blood vessel sprouting, and maturing vascular networks. Notably, its expression level strongly correlates with angiogenesis efficiency during the "angiogenic switch" in tumor development.
VEGF165 arises from alternative splicing of the VEGF-A gene (located at 6p21.3). This gene contains 8 exons, and through combinatorial splicing, generates multiple isoforms. VEGF165 is the most functionally critical subtype due to its retention of both a complete heparin-binding domain and a receptor-binding domain.
Composed of 165 amino acids with a molecular weight of approximately 45 kDa, VEGF165 forms an antiparallel homodimer. Its N-terminal receptor-binding domain ensures specific interactions with VEGFR-1 and VEGFR-2, while its C-terminal heparin-binding domain endows it with dual properties: solubility in bodily fluids for diffusion and anchoring to extracellular matrix via heparan sulfate. This allows VEGF165 to form stable concentration gradients at angiogenesis sites, precisely regulating endothelial cell behavior—distinguishing it from VEGF121 (fully soluble) and VEGF189 (primarily matrix-anchored).
VEGF165 is constitutively expressed at basal levels in normal tissues, with higher concentrations in metabolically active organs such as the lungs, kidneys, and myocardium, where it maintains vascular permeability and endothelial homeostasis. During embryonic development, it is highly expressed in angiogenesis hotspots like limb buds and neural tubes, driving the formation of primitive vascular networks. In wound repair, cells surrounding the injury rapidly upregulate VEGF165 to induce angiogenesis in granulation tissue.
Unlike other isoforms, VEGF165 exhibits "demand-driven regulation": it remains low in normal states but is significantly upregulated in pathological conditions (e.g., tumors, diabetic retinopathy), making it a key link between physiological and pathological angiogenesis.
VEGF165 exerts its functions by binding to the VEGFR family (VEGFR-1, VEGFR-2, VEGFR-3), with VEGFR-2 as its primary target. All VEGFRs are tyrosine kinase receptors, consisting of an extracellular domain with 7 immunoglobulin-like structures (for ligand binding), a transmembrane region, and an intracellular tyrosine kinase domain (for signal transduction).
The dimeric structure of VEGF165 recognizes the D2 and D3 domains of VEGFR-2’s extracellular region, triggering receptor dimerization and intracellular tyrosine phosphorylation—initiating endothelial cell proliferation and migration. While VEGF165 binds VEGFR-1 with higher affinity, it induces weaker signaling, primarily acting as a competitive modulator to fine-tune VEGFR-2 activation and prevent excessive angiogenesis.
Binding of VEGF165 to VEGFR-2 activates multiple pathways:
PI3K/AKT pathway: Promotes endothelial cell survival, reducing cell death in nascent vessels.
ERK1/2 pathway: Drives endothelial proliferation and migration, serving as the core driver for "tip cell" extension during vascular sprouting.
p38 MAPK pathway: Increases vascular permeability to facilitate nutrient and immune cell transport.
Src family kinase pathway: Regulates cytoskeletal rearrangement in endothelial cells, supporting migration.
These pathways synergize to initiate endothelial cell proliferation and differentiation while coordinating intercellular interactions, ultimately forming intact new blood vessels.
Rapidly proliferating tumor cells depend on new blood vessels for nutrients. They secrete large amounts of VEGF165, disrupting the balance between pro- and anti-angiogenic factors and activating the "angiogenic switch." VEGF165 promotes tumor vascular proliferation through two mechanisms: at low concentrations, "sprouting angiogenesis" dominates, guiding tip cells to migrate from existing vessel walls toward the tumor; at high concentrations, "intussusception" is triggered, where existing vessels fold inward to form new lumens, rapidly expanding the vascular network. Concentration-dependent switching between these mechanisms accelerates abnormal vascular growth in tumors.
VEGF165-induced tumor vessels exhibit disorganized structures (thin walls, chaotic branching), high permeability (loose endothelial junctions), and low maturity (lack of pericyte coverage). While these features supply nutrients to tumors, they cause microenvironmental hypoxia and acidosis due to poor blood flow, further stimulating excessive VEGF165 secretion and creating a vicious cycle.
Clinical studies show that VEGF165 expression in tumors positively correlates with microvessel density, invasiveness, and metastatic potential. Patients with high VEGF165 expression have shorter disease-free survival, making it a potential prognostic marker for tumors.
VEGF165’s central role makes it a key therapeutic target, with several drug classes developed:
Monoclonal antibodies: e.g., bevacizumab, which binds VEGF165’s receptor-binding domain to block VEGFR-2 interaction, widely used in colorectal cancer and lung cancer.
Tyrosine kinase inhibitors: e.g., sunitinib, which inhibits VEGFR-2’s intracellular kinase activity to block signaling, used in renal and liver cancers.
Fusion proteins: e.g., aflibercept, which mimics VEGFR extracellular domains to competitively bind VEGF165, improving vision in patients with neovascular eye diseases.
Bispecific antibodies: e.g., PD-L1/VEGF165 bispecific antibodies, which simultaneously block angiogenesis and immune suppression, showing synergistic anti-tumor effects.
While effective, these drugs face challenges such as drug resistance (e.g., VEGFR mutations) and adverse reactions (hypertension, delayed wound healing). Future efforts will focus on developing specific inhibitors and targeted delivery technologies to enhance efficacy and reduce toxicity.
VEGF165, with its unique molecular structure and functional properties, occupies a central position in angiogenesis regulation. Its interaction with VEGFR-2 and downstream signaling networks drive endothelial cell behavior and vascular network formation. In tumors, its overexpression is a core mechanism underlying abnormal angiogenesis.
Advances in research have shifted VEGF165-targeted therapy from non-specific inhibition to precise regulation. Combining novel delivery systems and combination therapies holds promise for expanding its applications in angiogenesis-related diseases, offering more effective treatment strategies.
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