A549 cells were treated with 0, 5, 10 and 15 µg/ml of Nano-Co or Nano-TiO2 for 6 h (A) or with 15 µg/ml of Nano-Co for 0, 1, 3, 6 and 12 h (B). Cells without treatment were used as control. Nuclear protein was extracted for Western blot. Total ATM was used for verification of equality of lane loading. * Significant difference as compared with the control, p < 0.05; # Significant difference as compared with the same dose of Nano-TiO2-treated group, p < 0.05.