VEGF Pathway

• Background
• VEGF Pathway
  • Ligands
  • Receptors
• VEGF – Signal Transduction
• Factors Regulating VEGF and VEGFR Expression
• VEGFR in Human Diseases
• Conclusion

Background

Formation of new blood vessels, a process known as angiogenesis, is a fundamental biological process required for embryonic development, tissue repair, ovulation and menstruation. Angiogenesis involves sprouting of new blood vessels from the existing blood vessels through the proliferation, migration and invasion of endothelial cells. This process is tightly regulated by pro- and anti-angiogenic factors, dysregulation of which can lead to abnormal vasculature. Several signaling mechanisms function to form new blood vessels, including vascular endothelial growth factors (VEGFs)/VEGFRs, angiopoietin/Tie receptors¹, Platelet-derived growth factors (PDGFs)/PDGFRs² and EphirinB2/EphirinB4³ . Among these, VEGF signaling has been often recognized as the key regulator of angiogenesis.

VEGF and VEGF Receptors

Vascular permeability factor (VPF) was initially known to show hyperpermeability of tumor blood vessels and involved in formation of tumor-associated ascites⁴. Later, studies revealed vascular endothelial growth factors (VEGF) as an angiogenic growth factor displaying high specificity for endothelial cells⁵. Subsequently, it was discovered that both VEGF and VPF are encoded by a single VEGF gene and are products of alternate splicing. Analysis of VEGF/VPF cDNA clones also revealed significant homology to platelet-derived growth factor (PDGF).

Ligands

There are five structurally related VEGF ligands: VEGFA, VEGFB, VEGFC, VEGFD and Placenta Growth Factor (PIGF). All the VEGFs are homodimers with disulphide bonds. The native VEGF is heparin-binding homodimeric glycoprotein closely corresponds to the VEGF165 variant. VEGFA was shown to have five isoforms with 121, 145, 165, 189 and 206 amino acids as a result of alternate splicing⁶. VEGF ligands are secreted by various cell types and act in an autocrine and paracrine manner. Each VEGF ligand binds differently to the receptors and induces distinct biological responses⁷.

Receptors

VEGF ligands bind to three receptor tyrosine kinases, VEGFR1 (Flt-1), VEGFR2 (Flk-1, KDR) and VEGFR3 and to co-receptors like Neuropilin 1 (NRP1). While VEGF receptors have homology in kinase domains their signaling cascade differs significantly. Ligand binding induces receptor dimerization (homo/hetero dimers) and activation of the kinase domain.

Phylogenetic analyses revealed that D-VEGFR/PVR, found in Drosophila melanogaster is the common ancestor of the vertebrate VEGF receptors. Both VEGFR and PDGFR in vertebrates were most likely obtained from the duplication/triplication of single D-VEGFR/PVR gene^8,9.

VEGF – Signal Transduction

VEGF signaling involves the cascade of signals triggered when the ligand binds the VEGF receptors. The downstream signaling includes activation of phospholipase Cγ1, MAPK pathway via Ras/Raf1 activation and PI3K/Akt pathway. Phospholipase Cγ1 regulates the concentration of intracellular Ca⁺² ions and formation of endothelial nitric oxide synthase. The effect of all the cascades provides a balance of pro- and anti-angiogenic signals that maintain the vasculature and/or result in sprouting of new blood vessels, cell proliferation and cell migration (Figure 1). Table 1 summarizes signaling transduction through VEGF receptors.

Factors Regulating VEGF and VEGFR Expression

There are several factors regulating the expression of VEGF and VEGFR. They are broadly classified into external factors, transcription factors and oncogenes. External factors such as hypoxia in tissue microenvironment regulate VEGF expression through Hypoxia Inducible Factor-1 alpha (HIF-1α)¹². Under hypoxic conditions HIF-1α dimerizes with HIF-1β, binds to the VEGF promoter inducing VEGF transcription. Growth factors and cytokines secreted in tissue microenvironment such as epidermal growth factor receptors (ErbB1 and ErbB2), insulin like growth factor-I receptor (IGF-IR)¹³, hepatocyte growth factors¹⁴, platelet-derived growth factors (PDGF)¹⁵ and cyclooxygenases (COX)¹⁶ induce angiogenesis. Oncogenes and tumor suppressors such as like c-SRC (proto-oncogene)¹⁷, BCR-ABL¹⁸, Ras¹⁹, p53²⁰ and PTEN²¹ also regulate the expression of VEGF.

VEGFR in Human Diseases

Diseases that are associated with vascular abnormalities are related to anomalies in VEGF/VEGFR signaling axis. Anti-VEGF treatment is being employed to treat certain type of cancers, and condition like preeclampsia, macular degeneration and amyotrophic lateral sclerosis.^22-28 The efficacy of antiangiogenic therapy for tumors is often met with challenges such as drug resistance and factors such as increased expression of HIF-1alpha which leads to increase in the number of cancer stem cells in the tumor niche.

Conclusion

The role of VEGF and VEGFRs is well recognized in angiogenesis and has been implicated in chronic disease conditions. Despite the development of small molecules and antibodies that suppress angiogenesis, several tumors showed resistance to anti-VEGF therapy due to parallel angiogenesis signals from other growth factors. Factors that contribute to tumor resistance and angiogenesis are yet to be extensively defined.