Impact of vascular endothelial growth factor release on radiation resistance

There is increasing evidence of an angiogenic response of irradiated tumors resulting in decreased radiation sensitivity. However, little is known about the contribution of tumor vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF)-release induced by irradiation to the individual level of resistance. In this in vitro study, we analysed the VEGF- and bFGF-release of six epithelial tumor cell lines before and after irradiation and correlated these data to the corresponding irradiation resistance. Two head and neck squamous cell carcinoma (HNSCC), two renal cell carcinoma (RCC), and two ovarian cancer (OC) cell lines were each exposed to 2 or 6 Gy single dose using a Cs-137-source. Non-irradiated controls were processed in parallel. Survival rates were assessed by colony assays as a measure of resistance. The released VEGF and bFGF was quantified by ELISA assays. Additionally, the expression of VEGF and its respective receptors (FLK, FLT, and NRP1) was visualized by immunohistochemistry. VEGF-release was significantly increased (p < 0.05) in all cell lines after irradiation. Release was most prominent in the RCC cell lines, less in the HNSCC cell lines and lowest in the OC cell lines. Radiation resistance correlated to the absolute level of released VEGF after irradiation as well as to its relative increase (r > 0.9, p < 0.01). bFGF levels were not correlated to resistance. VEGF and all three VEGF-receptors were detected in all cell lines analyzed supporting the concept of an autocrine protective mechanism. We suggest that tumor cell survival after irradiation may be enhanced by released VEGF and that the level of released VEGF directly corresponds to the resistance of the tumor to irradiation.