Cell release of bioactive fibroblast growth factor 2 by exon 6-encoded sequence of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor which is synthesized and secreted by many differentiated cells in response to various stimuli including hypoxia and growth factor exposure. Alternative splicing of vascular endothelial growth factor mRNA results in three distinct molecular forms: V189 and V165 or V121 which lack the exons 6 or 6 and 7, respectively. To clarify the functions of the 24-amino acid insertion, the biological activity of V165 was compared with that exerted by purified recombinant V189 and a synthetic peptide designed on the sequence encoded by exon 6 (Ex6P). V189 and Ex6P, but not V165, induced cell proliferation on corneal endothelial cells cultured in vitro. These effects were due to the release of fibroblast growth factor 2 (FGF2) stored in the extracellular matrix but not to direct interactions with FGF receptors since V189 was inefficient on heparan sulfate-deficient cells expressing constitutively FGF-R1. Moreover corneas incubated ex vivo with Ex6P solubilized 10-fold more FGF2 than a isocationic peptide containing a scrambled sequence. Ex6P elicited an angiogenic response in a corneal pocket assay which was totally inhibited by addition of anti-FGF2 IgG. Moreover the angiogenic response to V189, but not to V165, was inhibited by FGF2 immunoneutralization. These findings demonstrate that the presence of the exon 6-encoded sequence confers VEGF with the ability to exert its biological effects through FGF2 signaling pathways.