Radiation Resistance in Glioma Cells Determined by DNA Damage Repair Activity of Ape1/Ref-1

Since radiation therapy remains a primary treatment modality for gliomas, the radioresistance of elioma cells and targets to modify their radiation tolerance are of significant interest. Human apurinic endonuclease I (Apel, Ref-1, APEX, HAP1, AP endo) is a multifunctional protein involved in base excision repair of DNA and a redox-dependent transcriptional co-activator. This study investigated whether there is a direct relationship between Ape I and radioresistance in glioma cells, employing the human U87 and U251 cell lines. U87 is intrinsically more radioresistant than U251, which is partly attributable to more cycling U25I cells found in G2/M, the most radiosensitive cell stage, while more U87 cells are found in S and GI, the more radioresistant cell stages. But observed radioresistance is also related to Apel activity. U87 has higher levels of Apel than does U251, as assessed by Western blot and enzyme activity assays (-1.5-2 fold higher in cycling cells, and -10 fold higher at G2/M). A direct relationship was seen in cells transfected with CMV-Apel constructs; there was a dose-dependent relationship between increasing Ape I overexpression and increasing radioresistance. Conversely, knock down by siRNA or by pharmacological down regulation of Apel resulted in decreased radioresistance. The inhibitors lucanthone and CRT004876 were employed, the former a thioxanthene previously under clinical evaluation as a radiosensitizer for brain tumors and the latter a more specific Apel inhibitor. These data suggest that Ape I may be a useful target for modifying radiation tolerance.