The most prevalent VEGF protein isoforms consist of 121, 165 and 189 amino acids in man and 120, 164 and 188 in the mouse. These isoforms differ in heparin binding affinity but all interact with the VEGF receptors, Flt1 (VEGFR-1) and Flk1/KDR (VEGFR-2). There is mounting evidence that the different VEGF isoforms can differentially signal, relating to their diverse abilities to bind co-receptors of VEGFR-2 such as the neuropilins, integrins and heparan sulphate proteoglycans.

Murine fibrosarcoma cell lines that express single VEGF isoforms (VEGF120 VEGF164 and VEGF188) and a wild-type control (VEGFWT) were developed and grown sub-cutaneously and in dorsal skin-fold 'window' chambers. Vascular development and response to treatment were investigated using immunohistochemistry and intravital microscopy. The most extreme differences were observed between VEGF120 and VEGF188 tumors, with VEGF188 expression uniquely associated with dense vascular networks and maturity of the vascular wall including pericyte recruitment. VEGF188-expressing tumors were relatively sensitive to treatment with ionizing radiation but resistant to the effects of a tubulin-binding vascular disrupting agent, combretastatin A4 phosphate (CA4P). Treatment with the VEGF receptor tyrosine kinase receptor inhibitor, SU5416, had only a moderate anti-angiogenic effect in both tumour types, but induced significant vascular normalization in VEGF120 tumours resulting in induced resistance to subsequent treatment with CA4P. Results from specific inhibition of VEGFR-1 and R-2 suggested a role for VEGFR-1 in the maturation phase of tumour angiogenesis. Results show that differential tumour expression of individual VEGF isoforms influences factors in the tumour microenvironment that relate to treatment response.

(This work was supported by Cancer Research UK)