Negligible radiation protection of endothelial cells by vascular endothelial growth factor

Glioblastoma multiforme (GBM) is a radioresistant tumor. Tumor neoangiogenesis is an important mechanism for tumor sustenance. Angiogenesis is primarily mediated by vascular endothelial growth factor (VEGF), and earlier studies have suggested that VEGF protects human umbilical vein endothelial cells (HUVECs) against high doses of radiation. We tried to extend these findings to other endothelial cell lines and clinically relevant irradiation doses. Therefore, four different endothelial cell lines (HUVEC-C, primary HUVEC-P, an immortalized HUVEC cell line: EC-RF24, and bovine retina endothelial cells: BREC) were cultured without or with recombinant human VEGF165 (rhVEGF165). Cells were irradiated with gamma-rays from a 137Cs-source. Radiosensitivity was determined by proliferation or clonogenic assay. Apoptosis was assayed by flow cytometric determination of the sub-G1 population or by counting nuclear fragmentation. We found that the biologically active rhVEGF165 was able to improve clonogenic survival of HUVEC-C after 2 and 5 Gy. However, rhVEGF165 could not significantly alter the radiosensitivity of all cell lines studied in proliferation assays. rhVEGF165 only slightly reduced apoptosis in HUVEC-C after 3 Gy. In conclusion, the radioprotective effect from rhVEGF165 was found on different endothelial cell lines after clinically relevant radiation doses was negligible. We therefore hypothesize that the high VEGF-levels found in GBM in vivo do not reduce the radiosensitivity of endothelial cells, which is thought to contribute to the strong radioresistance of the tumor vasculature.