We analysed the release of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) from squamous cell carcinoma (SCC) after irradiation and their potential contribution to radiation resistance. Three SCC cell lines were irradiated, and VEGF- and bFGF-release was quantified by Elisa-assay. Conditioned media (CM) were used in clonogenic assays for analysis of tumor cell survival. To evaluate the effect of tumor released VEGF and bFGF on survival, experiments with neutralizing monoclonal anti-VEGF and anti-bFGF antibodies were conducted in parallel. Cell cultures were irradiated with 2 Gy to analyse the effects of CM on tumor cell escape from radiation-induced death. We observed a marked increase in VEGF- and bFGF-release after irradiation by the surviving cells. Using these conditioned media, subsequently we observed an up to 10-fold increase in colony formation. The addition of anti-VEGF- and anti-bFGF-antibodies reduced colony formation, indicating that irradiation stimulates the release of growth promoting substances including VEGF and bFGF by the surviving cells. Additionally, irradiation of cells cultured with CM decreased colony formation about 50%, however, it was still increased 5-fold compared to the cultures without CM. The addition of VEGF- and/or bFGF-antibodies led to an additional 20% reduction of this radioprotective effect of the CM. This means, bFGF and VEGF contribute to a significant proportion of the survival stimulating activity. We thus show that irradiation might result in autologous protection of tumor cells from irradiation-induced cell death by the release of growth factors. These observations suggest that radiation might lead to unsuspected and undesired effects in tumorous tissue with possible clinical impact.

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