The human glioblastoma cell line U251 was obtained from Dr. Mark Israel (Norris Cotton Cancer Center, Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA) and cultured in DMEM supplemented with 10 % fetal bovine serum and 100 IU/ml penicillin-streptomycin. The cells were subcultivated at 80–90 % of confluence.