Temporally distinct response of irradiated normal human fibroblasts and their bystander cells to energetic heavy ions

Hamada, Nobuyuki*; Ni, M.*; Funayama, Tomoo; Sakashita, Tetsuya; Kobayashi, Yasuhiko

To characterize the energetic heavy ion-induced bystander response, we employed precise microbeams of C and Ne ions for targeting only a very small fraction of cells in confluent fibroblast cultures. Conventional broad field irradiation was conducted in parallel to see the effects in irradiated cells. Exposure of 0.00026% of cells led to nearly 10% reductions in the clonogenic survival and twofold rises in the apoptotic incidence regardless of ion species. Whilst apoptotic frequency increased with time up to 72 h post irradiation in irradiated cells, its frequency escalated up to 24 h post irradiation but declined at 48 h post irradiation in bystander cells, indicating that bystander cells exhibit transient commitment to apoptosis. C and Ne-ion microbeam irradiation similarly caused almost twofold increments in the levels of serine 15-phosphorylated p53 proteins, irrespective of whether 0.00026, 0.0013 or 0.0066% of cells were targeted. Whereas the levels of phosphorylated p53 were elevated and remained unchanged at 2 and 6 h post irradiation in irradiated cells, its levels rose at 6 h post irradiation but not at 2 h post irradiation in bystander cells, suggesting that bystander cells manifest delayed p53 phosphorylation.