Exposure to X-rays enhances autophagy in human fibroblast cells

Noguchi, Miho; Yokoya, Akinari; Suzuki, Keiji*; Fujii, Kentaro

The exposure to ionizing radiation causes damage to not only DNA but also proteins or organelles in cytoplasm of cells. Recent studies report that, for most of normal human fibroblasts and some of solid carcinoma cells, cells exposed to ionizing radiation sustain irreversible growth arrest, but still maintain their viability with showing senescence character even though they might have severe DNA damage. These suggests that, in irradiated fibroblasts, intracellular degradation system "autophagy" eliminates accumulated damaged proteins or dysfunctional organelles to avoid induction of apoptosis. In this study, to clarify the role of autophagy on cell survival, we investigated change of autophagic activity in irradiated fibroblast cells in terms of escape from apoptosis. Induction of autophagy in normal human fibroblast BJ1-hTERT cells irradiated with 20 Gy X-rays were measured using the fluorescent probe, Cyto-ID Green, which stains specifically autophagic vacuoles. Irradiated cells were treated with Cyto-ID Green 15 min before observation. The cells observed by a fluorescent microscope were quantified as mean values of the fluorescent intensity per cell. Irradiated cells showed about 3 times higher induction of autophagic activity than non-irradiated cells at 24 h after irradiation. The activation of autophagy persisted for at least 5 days (120 h) after irradiation. These results strongly suggest that highly induced autophagy is involved in the elimination of damaged proteins and organelles to keep their physiological functions, and avoid apoptosis. Frequency of apoptosis is expected to increase if the autophagic pathway is blocked. Results will also be presented for the treatments with pharmacological agent inhibiting the autophagic process.