Molecular Bases of VEGFR-2-Mediated Physiological Function and Pathological Role

Abstract

The vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) play crucial roles in vasculogenesis and angiogenesis. Angiogenesis is an important mechanism in many physiological and pathological processes, and is involved in endothelial cell proliferation, migration, and survival, then leads to further tubulogenesis, and finally promotes formation of vessels. This series of signaling cascade pathways are precisely mediated by VEGF/VEGFR-2 system. The VEGF binding to the IgD2 and IgD3 of VEGFR-2 induces the dimerization of the receptor, subsequently the activation and trans-autophosphorylation of the tyrosine kinase, and then the initiation of the intracellular signaling cascades. Finally the VEGF-activated VEGFR-2 stimulates and mediates variety of signaling transduction, biological responses, and pathological processes in angiogenesis. Several crucial phosphorylated sites Tyr801, Try951, Try1175, and Try1214 in the VEGFR-2 intracellular domains mediate several key signaling processes including PLCγ-PKC, TSAd-Src-PI3K-Akt, SHB-FAK-paxillin, SHB-PI3K-Akt, and NCK-p38-MAPKAPK2/3 pathways. Based on the molecular structure and signaling pathways of VEGFR-2, the strategy of the VEGFR-2-targeted therapy should be considered to employ in the treatment of the VEGF/VEGFR-2-associated diseases by blocking the VEGF/VEGFR-2 signaling pathway, inhibiting VEGF and VEGFR-2 gene expression, blocking the binding of VEGF and VEGFR-2, and preventing the proliferation, migration, and survival of vascular endothelial cells expressing VEGFR-2.

Keywords: VEGF; VEGFR-2; angiogenesis; function and role; structure; vasculogenesis.

FIGURE 1

The molecular structure of VEGF/VEGFR-2. (A) Diagram of the VEGFR-2 structure. VEGFR-2 is composed of a signal peptide, a extracellular domain (ECD) including seven Ig-like subdomains (IgD1∼7), a TMD, a JMD, a catalytic tyrosine kinas domain (TKD) including ATP binding domain (TKD1), kinase insert domain (KID) and phosphotransferase domain (TKD2), and a flexible C-terminal domain (CTD), and many functional sites. (B) VEGF-activated VEGFR-2 homodimer. After VEGFs binding to VEGFR-2, the crucial tyrosine residues on the TKD have been phosphorylated and are involved in mediating downstream signaling pathways. (C) Molecular structure of VEGF-A binding to IgD2 and IgD3 of VEGFR-2 [PDB ID: 3V2A (Brozzo et al., 2012)]. (D) Molecular structure of TKD of VEGFR-2 including TKD1 (N-lobe), KID, and TKD2 (C-lobe) [PDB ID: 4ASD (McTigue et al., 2012)]. There are three important motifs: glycine-rich loop (blue, 841–846 aa), catalytic loop (red, 1026–1033 aa), and activation loop (green, 1045–1075 aa), and three crucial phosphorylation sites (spheres) on the TKD: Tyr951 on the KID, and Tyr1054 and Try1059 on the TKD2.

FIGURE 2

VEGF/VEGFR-2 mediated signaling pathways during angiogenesis. (A) Diagram of VEGF-activated VEGFR-2 homodimer. Several crucial tyrosine residues have been phosphorylated after VEGFs binding to VEGFR-2. The Try801 on JMD is involved in cell permeability and proliferation by mediating the PLCγ-PKC, then eNOS-NO or MEK-ERK, respectively. The Try951 on KID mediates the cell survival and permeability via the TSAd-Src-PI3K-Akt pathway. The Try1054 and Try1059 on TKD2 can increase VEGFR-2 kinase activity. The Try1175 is involved in cell permeability, proliferation, and migration by regulating PLCγ-PKC, SHB-FAK-paxillin, and SHB-PI3K-Rac pathways. The Try1214 mediates cell migration through NCK-p38-MAPKAPK2/3 pathway. (B) Structure-based sequence alignment of PDGF-derived regions of the VEGF family members, includes human VEGF-A (NP_003367), human VEGF-B (NP_003368), human VEGF-C (NP_005420), human VEGF-D (NP_004460), human PGF (placental growth factor) (NP_002623), Orf virus orfVEGF (VEGF-E) (ABA00650), and Snake venom svVEGF (BAD38844), which are composed of two α-helix and five β-sheets. There are functional sites, including receptor binding site 1 (blue arrow) and site 2 (green arrows), cysteine knot motifs (red closed circles) that form three disulfide bridges including Cys263-Cys308, Cys267-Cys310, and Cys232-Cys274 in VEGF-A, and dimerization interface sites (orange stars).