zing Radiation Activates the Nrf2 Antioxidant Response

nloaded transcription factor NF-E2-related factor 2 (Nrf2) binds the antioxidant DNA response element (ARE) to te important cellular cytoprotective defense systems. Recently several types of cancers have been shown rexpress Nrf2, but its role in the cellular response to radiation therapy has yet to be fully determined. In udy, we report that single doses of ionizing radiation from 2 to 8 Gy activate ARE-dependent transcripbreast cancer cells in a dose-dependent manner, but only after a delay of five days. Clinically relevant ose fractions of radiation also increased ARE-dependent transcription, but again only after five days. stream activation of Nrf2-ARE-dependent gene and protein markers, such as heme oxygenase-1, oc, whereas Nrf2-deficient fibroblasts were incapable of these responses. Compared with wild-type fibro, Nrf2-deficient fibroblasts had relatively high basal levels of reactive oxygen species that increased five days after radiation exposure. Further, in vitro clonogenic survival assays and in vivo sublethal body irradiation tests showed that Nrf2 deletion increased radiation sensitivity, whereas Nrf2-inducing did not increase radioresistance. Our results indicate that the Nrf2-ARE pathway is important to mainsistance to irradiation, but that it operates as a second-tier antioxidant adaptive response system actain re tivated by radiation only under specific circumstances, including those that may be highly relevant to tumor response during standard clinical dose-fractionated radiation therapy. Cancer Res; 70(21); 8886–95. ©2010 AACR.