The role of VEGF 165b in pathophysiology

Abstract

Anti-angiogenic vascular endothelial growth factor A (VEGF) 165b and pro-angiogenic VEGF 165 are generated from the same transcript, and their relative amounts are dependent on alternative splicing. The role of VEGF 165b has not been investigated in as much detail as VEGF 165, although it appears to be highly expressed in non-angiogenic tissues and, in contrast with VEGF 165, is downregulated in tumors and other pathologies associated with abnormal neovascularization such as diabetic retinopathy or Denys Drash syndrome. VEGF 165b inhibits VEGFR2 signaling by inducing differential phosphorylation, and it can be used to block angiogenesis in in vivo models of tumorigenesis and angiogenesis-related eye disease. Recent reports have identified three serine/arginine-rich proteins, SRSF1, SRSF2 and SRSF6, and studied their role in regulating terminal splice-site selection. Since the balance of VEGF isoforms is lost in cancer and angiogenesis-related conditions, control of VEGF splicing could also be used as a basis for therapy in these diseases.

Figure 1.vegf-a. (A) Gene structure. TSS is the transcriptional start site. (B) mRNA species. Alternative splicing of the vegf-a gene in the terminal exon results in two families of isoforms: the pro-angiogenic VEGF_xxx and the anti-angiogenic VEGF_xxxb isoforms. AUG, the start site for translation; UTR, the untranslated region; pA, the polyadenylation site. (C) Protein structure of the two major isoforms of each family, VEGF₁₆₅ and VEGF₁₆₅b. C-terminal splicing leads to an alternative last six amino acids (CDKPRR or SLTRKD). The isoforms are termed according to the amino acid number of the resulting protein (_xxx). HSPG, heparin sulfate proteoglycan; R1, VEGF Receptor 1; R2, VEGF Receptor 2 (from ref. 29).

Figure 2. VEGF₁₆₅b and VEGF₁₆₅ interaction with VEGF receptor 2 (VEGFR2). (A) The VEGFR2 binding site of VEGF₁₆₅ interacts with the VEGFR2 extracellular domain. VEGF₁₆₅ functions as a dimer and promotes the formation of VEGFR2 dimers resulting in activation of the kinase domains via the phosphorylation of its tyrosine residues. Robust tyrosine phosphorylation results in the activation of angiogenic signaling pathways. (B) VEGF₁₆₅b binds the VEGFR2 binding site with equal affinity to VEGF₁₆₅ but does not bind neuropilin 1 (NRP1) co-receptor. The C-terminus of VEGF₁₆₅b is neutral and there is insufficient torsional rotation for complete tyrosine phosphorylation, although weak phosphorylation can occur (adapted from ref. 11)

Figure 3. Regulation of VEGF alternative splicing in the terminal exon.