Impact of VEGF-dependent tumour micro-environment on EDB fibronectin expression by subcutaneous human tumour xenografts in nude mice

Fibronectin (FN) is an extracellular matrix cell-adhesive glycoprotein. The alternative spliced isoform EDB-FN (extra domain B containing FN) is highly expressed in tumour blood vessels and stroma and represents a candidate for tumour targeting. To investigate the impact of different angiogenic micro-environments on EDB-FN expression, we used a tumour model in which human endometrial adenocarcinoma Tet-FGF2 cells overexpressing fibroblast growth factor-2 (FGF2) driven by the tetracycline-responsive promoter were further transfected with a VEGF antisense cDNA, generating AS-VEGF/Tet-FGF2 cells. In this model, the expression of FGF2 plus VEGF results in fast-growing, highly vascularized Tet-FGF2 tumours. Down-regulation of FGF2 production by tetracycline administration and/or of VEGF production by AS-VEGF transduction inhibited tumour growth and vascularization, with profound changes in tumour micro-environment. Quantitative RT-PCR analysis using human EDB-FN primers shows that subcutaneous grafting in immunodeficient mice is per se sufficient to cause a dramatic up-regulation of EDB-FN expression by these cells, as well as by human oesophageal cancer KYSE 30 cells and renal carcinoma Caki-1 cells. However, in vivo down-regulation of VEGF expression, as occurs in AS-VEGF/Tet-FGF2 tumours, and to a lesser extent of FGF2 expression, as occurs in tetracycline-treated Tet-FGF2 tumourbearing animals, causes significant inhibition of EDB-FN production in tumour grafts, as shown by immunohistochemistry and quantitative RT-PCR analysis. Accordingly, treatment of Tet-FGF2 tumour-bearing animals with the neutralizing anti-murine VEGF receptor- 2 antibody DC101, or of Caki-1 tumour-bearing animals with the anti-VEGF antibody bevacizumab, inhibited EDB-FN expression in tumour grafts. EDB-FN down-regulation was paralleled by a decrease in vascularity, thus confirming EDB-FN as a marker of tumour angiogenesis. These data demonstrate that the angiogenic micro-environment, and in particular the VEGF/VEGFR-2 system, plays a key role in modulating EDB-FN expression by tumour cells in vivo. This may have implications for the design of therapeutic strategies targeting EDB-FN in combination with anti-angiogenic and/or cytotoxic drugs.