PARENT SESSION

Risks and Modeling at Low Doses

(PP035) Mechanisms underlying ionizing radiation induced adaptive responses in normal human cells grown in three-dimensional architecture.

de Toledo, Sonia^*,1, Asaad, Nesrin¹, Venkatachalam, Perumal¹, Li, Ling², Howell, Roger¹, Spitz, Douglas², Azzam, Edouard¹, ¹ Department of Radiology/Division of Radiation Research, Newark, NJ, USA² Department of Radiation Oncology/Holden Comprehensive Cancer Center, Iowa City, IA, USA

ABSTRACT- To characterize low-dose effects and their potential impact on risk from high dose ionizing radiation, we investigated cellular, molecular and biochemical effects in -irradiated cells adapted to grow in three-dimensional architecture that mimics cell growth in vivo. Normal human diploid fibroblasts (AG1522) were irradiated with a single dose of 10 cGy (from a ¹³⁷Cs source) delivered acutely or protracted over various periods up to 48 h. Chromosomal damage and changes in the expression of stress-related proteins were measured following the exposure. Compared to sham-manipulated control cells, a significant increase in micronucleus formation (a surrogate form of DNA damage) was observed in the acutely irradiated cells. Protraction of the dose over 24 h resulted in significantly reduced effects; its protraction over 48 h reduced the micronucleus frequency to a level similar or lower than occurs spontaneously. This pattern of chromosomal damage correlated with that of changes in the phosphorylation of serine15 in the p53 protein, a marker we found to be highly sensitive to ionizing radiation, being increased in cells exposed to an acute dose as low as 1 cGy. Ectopic overexpression of either of the antioxidant enzymes CuZn-SOD, Mn-SOD, catalase or glutathione peroxidase, or the inhibition of oxidant generation by flavin-containing oxidases, significantly attenuated stressful effects in cells exposed to 10 cGy acutely or at moderate dose rates. Pre-exposure to 1, 5, or 10 cGy delivered at very low dose-rate (0.2 cGy/h) significantly protected against damage from a subsequent dose of 1 Gy (0.8 Gy/min). The induced mitigating effects were transient and disappeared by 48 h. Oxidative metabolism, DNA repair as well as cell cycle checkpoints are involved in mediating the induced adaptive responses. These studies show evidence for departure from the linear no threshold hypothesis, and suggest that distinct molecular events underlie high and low dose induced radiobiological effects. Cell culture in three-dimension offers an ideal system to investigate/reveal radiation-induced adaptive responses. Supported by Research Grant FG02-02ER63447 from the U.S. Department of Energy

Key words: Adaptive response, Oxidative metabolism, DNA repair, Cell cycle checkpoints