Abstract 422: Early growth response 1 is required for radiation-induced apoptosis in normal tissues

Although radiation therapy is a mainstay of cancer treatments, it can have deleterious effects on normal tissues, leading to poor quality of life for cancer survivors. Radiation can cause depletion of stem cells in normal tissues that can result in late sequelae, including poor learning, cognitive disorders, intestinal malabsorption, infertility, and skin injuries. Our laboratory has shown that low doses of radiation can induce apoptosis selectively in undifferentiated normal stem cells compared to adjacent differentiated cells in multiple niches including the brain and intestine. To elucidate the molecular basis underlying stem cell sensitivity to radiation, we performed discovery research to detect genomic differences between stem cells and differentiated cells. Among the candidates, the Early growth response 1 (Egr1) gene was expressed at high levels in stem cells, but not in isogenic differentiated cells. Egr1 is a zinc-finger transcription factor that is rapidly induced in response to radiation and activates the transcription of pro-apoptotic genes. Since cancer survivors can suffer from cognitive difficulties and intestinal malabsorption, we investigated the role of Egr1 in neuronal and intestinal cell death following irradiation. Knockdown of Egr1 using shRNA increased clonogenic survival in irradiated HT-22 hippocampal neuronal cells and IEC-6 intestinal epithelial cells. This increased radioprotection was largely due to reduced apoptosis in Egr1 knockdown cells. Additionally, suppression of Egr1 promoter binding by mithramycin A and chromomycin A3 also conferred radioprotection to HT-22 and IEC-6 cells. Mouse models further supported Egr1’s role in radiation-induced apoptosis. Egr1 null mice had significantly fewer apoptotic cells in the hippocampus and small intestinal crypts following irradiation, as compared to their wild-type littermates. Finally, we examined mechanisms by which Egr1 regulates radiation-induced apoptosis. Irradiation of HT-22 and IEC-6 cells led to a G2/M cell cycle arrest, which was abrogated by the knockdown of Egr1. Also, knockdown of Egr1 by shRNA and inhibition of Egr1 promoter binding both attenuated radiation induction of the pro-apoptotic factors p53, Bim, and Bax, and upregulated the anti-apoptotic protein Bcl-2. Thus, this study establishes a crucial role for Egr1 in the radiation-induced apoptosis of hippocampal and intestinal tissues. Egr1 could be a potential molecular target to minimize the normal tissue complica