Basal HIF-1α expression levels are not predictive for radiosensitivity of human cancer cell lines

High levels of hypoxia inducible factor (HIF)-1 alpha in tumors are reported to be associated with tumor progression and resistance to therapy. To examine the impact of HIF-1 alpha on radioresistance under normoxia, the sensitivity towards irradiation was measured in human tumor cell lines that differ significantly in their basal HIF-1 alpha levels. HIF-1 alpha levels were quantified in lysates of H1339, EPLC-272H, A549, SAS, XF354, FaDu, BHY, and CX- tumor cell lines by ELISA. Protein levels of HIF-1 alpha, HIF-2 alpha, carbonic anhydrase IX (CA IX), and GAPDH were assessed by Western blot analysis. Knock-down experiments were performed using HIF-1 alpha siRNA. Clonogenic survival after irradiation was determined by the colony forming assay. According to their basal HIF-1 alpha status, the tumor cell lines were divided into low (SAS, XF354, FaDu, A549, CX-), intermediate (EPLC-272H, BHY), and high (H1339) HIF-1 alpha expressors. The functionality of the high basal HIF-1 alpha expression in H1339 cells was proven by reduced CA IX expression after knocking-down HIF-1 alpha. Linear regression analysis revealed no correlation between basal HIF-1 alpha levels and the survival fraction at either 2 or 4 Gy in all tumor cell lines investigated. Our data suggest that basal HIF-1 alpha levels in human tumor cell lines do not predict their radiosensitivity under normoxia.