MRE11 participates in ATM activation following oxidative stress. (A) MRC5SV (normal), MRE11-defective HMfibroSV (HM) and ATLD2SV (ATLD) cells were treated with H₂O₂ (100 μM) for the indicated times and analysed using the indicated antibodies. (B) U2OS cells were transfected with MRE11 siRNA. After 2 days, these cells were treated with H₂O₂ (100 μM) for the indicated times and analyzsd using the indicated antibodies. (C) Normal (C2ABR) or ATLD2 (ATLD2ABR) lymphoblastoid cells were γ-ray-irradiated (10 Gy) or treated with 100 μM H₂O₂. After 1 h, cells were harvested and a kinase activity assay was measured using the immunocomplexes with an anti-ATM antibody. Phosphorylation was detected by western blot analysis using an anti-phospho-p53 antibody. The amount of substrate in the reaction was confirmed using an anti-GST antibody.