The anti-angiogenic isoforms of VEGF in health and disease

Abstract

Anti-angiogenic VEGF (vascular endothelial growth factor) isoforms, generated from differential splicing of exon 8, are widely expressed in normal human tissues but down-regulated in cancers and other pathologies associated with abnormal angiogenesis (cancer, diabetic retinopathy, retinal vein occlusion, the Denys-Drash syndrome and pre-eclampsia). Administration of recombinant VEGF(165)b inhibits ocular angiogenesis in mouse models of retinopathy and age-related macular degeneration, and colorectal carcinoma and metastatic melanoma. Splicing factors and their regulatory molecules alter splice site selection, such that cells can switch from the anti-angiogenic VEGF(xxx)b isoforms to the pro-angiogenic VEGF(xxx) isoforms, including SRp55 (serine/arginine protein 55), ASF/SF2 (alternative splicing factor/splicing factor 2) and SRPK (serine arginine domain protein kinase), and inhibitors of these molecules can inhibit angiogenesis in the eye, and splice site selection in cancer cells, opening up the possibility of using splicing factor inhibitors as novel anti-angiogenic therapeutics. Endogenous anti-angiogenic VEGF(xxx)b isoforms are cytoprotective for endothelial, epithelial and neuronal cells in vitro and in vivo, suggesting both an improved safety profile and an explanation for unpredicted anti-VEGF side effects. In summary, C-terminal distal splicing is a key component of VEGF biology, overlooked by the vast majority of publications in the field, and these findings require a radical revision of our understanding of VEGF biology in normal human physiology.

Figure 1. Structure of the VEGF isoforms

(A) mRNAs generated by alternative splicing of the VEGF-A gene. Exon 1 encodes the 5′-UTR and translational start (AUG), and exon 8 the stop codon and 3′-UTR. When pro-angiogenic isoforms are generated the stop codon is 19 nt from the splice site, and the remaining 47 nt of exon 8a are 3′-UTR as is the whole of exon 8b. In the anti-angiogenic isoforms, exon 8a is spliced out and the 3′-UTR is all but the first 19 nt of exon 8b. (B) Sequences of the mRNA and amino acids for the two families of isoforms.

Figure 2. Control for splicing of the VEGF isoforms

PSS, proximal splice site; DSS, dorsal splice site.