Radiation-Induced Bystander Effects in HT-29 Cell Culture

Abstract

Recent advances in radiation biology have caused a shift in focus from the classical view of direct DNA damage and repair to one that encompasses intricate cellular signaling pathways present at both the cell and tissue level. In this study, HT-29 human colon adenocarcinoma cells were irradiated with Cs-137 gamma rays to investigate the effects of varying dose and dose rate exposure scenarios of irradiated cell cultures on the bystander response elicited in unirradiated responder cell populations via transfer of irradiated cell conditioned medium (ICCM). Specifically, two endpoints were used to assess the bystander response. A microtetrazolium (MTT) colorimetric assay was used to evaluate metabolic reaction of responder cells, while a fluorescence assay was used to measure the level of caspase-3/7 activation—which has recently been shown to be a key regulator of growth-promoting signals generated from dying tumor cells. The results of this study demonstrate an active cellular response in cell cultures exposed to ionizing radiation, resulting from the secretion of potent signaling factors into the immediate cellular environment capable of inducing stimulatory responses in unirradiated responder cells. Stimulatory bystander responses were consistently observed over a wide array of exposure scenarios with all experimental points showing increased metabolic activity and, most interestingly, corresponding increases in caspase-3/7 activation. Clearly, traditional dose-response models are inadequate for predicting radiobiological effects at the cellular level and must be augmented by inclusion of cell-to-cell communication processes. This may have considerable implications for radiation therapy and radiation protection practices.