The growth and progression of tumours, in line with that of all expanding cellular structures such as the placenta and the developing embryo, depends on a proliferating vasculature ensuring adequate supply of nutrients and efficient removal of waste products. The advent of anti-angiogenic therapies such as sorafenib1
stems from a huge leap in our mechanistic understanding of the initiation, development, refinement and maintenance of new vessels and microvessels. This in turn originates from the discovery in the 1980s by Ferrara5
of the principal player in angiogenesis, vascular endothelial growth factor A (VEGF-A
, also referred to as VEGF). VEGF-A exists in multiple isoforms of variable exon content and strikingly contrasting properties and expression patterns. This range of products from the 8-exon VEGF-A
gene on chromosome 6 renders VEGF-A biology complex (), and alterations in isoform expression in cancer may be instructive for other genes involved in malignant change in general8
and in the pro-angiogenic cascade in particular. Indeed, the products of VEGF-A,
rather than just being targets for inhibition, may hold the key to impeding tumour growth and act as a model for controlling the qualitative expression of other malignancy-associated genes.
Protein and mRNA products of human vascular endothelial growth factor A (VEGF-A)
In tumours, and most other angiogenic situations, new vessel development is primarily dependent on this 46 kDa glycoprotein acting on its endothelial cell receptors VEGF receptor 1 (VEGFR1
and the co-receptor neuropilin 1
. This view is supported by the finding that even heterozygous Vegfa
knockouts are embryonically lethal9
. The first VEGF-A isoform described, VEGF-A165
), has been extensively investigated for its function, signalling, expression and roles in cancer10
. Other isoforms including VEGF-A121
, identified between 1989 and 2003, are generated by alternative splicing of exons 6 and 7, which code for motifs that bind to the highly negatively charged glycosaminoglycan carbohydrate heparin and similar molecules. In 2002, an additional isoform was identified11
b, which is generated by exon 8 distal splice site (DSS) selection. This DSS choice can also occur in conjunction with exon 6 and 7 inclusion or exclusion. It therefore became apparent that VEGF-A
mRNA splicing generates two families of proteins that differ by their C’ terminal six amino acids (), and these are termed VEGF-Axxx
(pro-angiogenic) and VEGF-Axxx
, xxx denoting the amino acid number of the mature protein.
Details of the molecular control of C’ terminal splice site choice (and the pro-angiogenic-anti-angiogenic balance) are emerging13
(). Upstream factors governing VEGF-A expression include hypoxia, cytokines, sex hormones, chemokines and growth factors (reviewed in REFS 10
), although most studies have assessed VEGF-A expression using agents that would not distinguish between the two VEGF-A families. Subsequent downstream VEGF-A signalling of the conventional pro-angiogenic VEGF-Axxx
isoforms has been identified (reviewed in REFS 15
) (). Alterations in these pathways have not been identified in as much detail for the VEGF-Axxx
b family ().
Vascular endothelial growth factor A (VEGF-A) C’ terminal splicing regulation
Signalling pathways downstream of vascular endothelial growth factor (VEGF-A)xxx and VEGF-Axxxb
In this article we consider the significant functional differences between the isoform families and the progress made in determining the mechanistic differences between them.