Although glycogen synthase kinase-3 beta (GSK-3β) was originally named for its ability to phosphorylate glycogen synthase and regulate glucose metabolism, this multifunctional kinase is presently known to be a key regulator of a wide range of cellular functions. GSK-3β is involved in modulating a variety of functions including cell signaling, growth metabolism, and various transcription factors that determine the survival or death of the organism. Secondary to the role of GSK-3β in various diseases including Alzheimer's disease, inflammation, diabetes, and cancer, small molecule inhibitors of GSK-3β are gaining significant attention. This paper is primarily focused on addressing the bifunctional or conflicting roles of GSK-3β in both the promotion of cell survival and of apoptosis. GSK-3β has emerged as an important molecular target for drug development.

Normal stem cells from tissues often exhibiting radiation injury are highly radiosensitive and exhibit a muted DNA damage response, in contrast to differentiated progeny. These radioresponses can be attributed to unique epigenetic regulation in stem cells, identifying potential therapeutic targets for radioprotection.

Normal tissue toxicity reduces the therapeutic index of radiotherapy and decreases the quality of life for cancer survivors. Apoptosis is a key element of the radiation response in normal tissues like the hippocampus and small intestine, resulting in neurocognitive disorders and intestinal malabsorption. The Early Growth Response 1 (Egr1) transcription factor mediates radiation-induced apoptosis by activating the transcription of pro-apoptosis genes in response to ionizing radiation (IR). Therefore, we hypothesized that the genetic abrogation of Egr1 and the pharmacological inhibition of its transcriptional activity could attenuate radiation-induced apoptosis in normal tissues. We demonstrated that Egr1 null mice had less apoptosis in the hippocampus and intestine following irradiation as compared to their wild-type littermates. A similar result was achieved using Mithramycin A (MMA) to prevent binding of Egr1 to target promoters in the mouse intestine. Egr1 expression using shRNA dampened apoptosis and enhanced the clonogenic survival of irradiated HT22 hippocampal neuronal cells and IEC6 intestinal epithelial cells. Mechanistically, these events involved an abrogation of p53 induction by IR and an increase in the ratio of Bcl-2/Bax expression. In contrast, targeted silencing of Egr1 in two cancer cell lines (GL261 glioma cells, HCT116 colorectal cancer cells) was not radioprotective, since it reduced their growth while also sensitizing them to radiation-induced death. Further, Egr1 depletion delayed the growth of heterotopically implanted GL261 and HCT116 tumors. These results support the potential of silencing Egr1 in order to minimize the normal tissue complications associated with radiotherapy while enhancing tumor control.

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. Stem cells in normal tissues are particularly sensitive to radiation. It is believed that they are depleted after radiation, resulting in late sequelae such as low IQ, cognitive disorders, intestinal malabsorption, infertility, and skin injuries. Our laboratory has found that normal tissue stem cells express high levels of Early Growth Response 1 (Egr1) protein and mRNA. Egr1 is a zinc-finger transcription factor that initiates early signaling events in response to ionizing radiation. Radiation enhances Egr1 expression and the Egr1 protein subsequently activates the transcription of genes involved in cell death. Therefore, we hypothesized that Egr1 could contribute to the sensitivity of stem cells to ionizing radiation. Using mouse models, we investigated the effect of Egr1 status on cell death in two clinically important stem cell niches-the dentate gyrus of the hippocampus and crypts of the intestine. Egr1 null mice had significantly less cell death in the dentate gyrus and small intestinal crypts following irradiation, as compared to their wild-type littermates. To elucidate the molecular basis underlying the resistance of Egr1 null hippocampi to radiation-induced cell death, we performed immunoblotting on ex vivo protein lysates made from Egr1 wild-type and knockout hippocampi. Protein lysates from Egr1 wild-type hippocampi showed that radiation treatment induced Egr1, as well as its known targets–p53 and regulators of the mitochondrial pathway of apoptosis, Bim and Bax. In contrast, the induction of these pro-apoptotic proteins by radiation was attenuated in Egr1 null hippocampal lysates. These results suggest that Egr1 is involved in the apoptosis mode of cell death in the irradiated hippocampus. The mechanism may involve Egr1 induction by ionizing radiation, followed by direct binding of Egr1 to the promoters of p53, Bim, and Bax to allow for transcription. We further conducted studies using mouse embryonic stem cell models and showed that knockdown of Egr1 using siRNA decreased cell death after radiation treatment. This finding supports our in vivo data that the hippocampal and gastrointestinal stem cell niches are radioprotected in Egr1 null mice. Thus, this study establishes an important role for Egr1 in radiation-induced apoptosis of hippocampal and intestinal tissues. Egr1 could be a potential molecular target to minimize the normal tissue complications associated with radiation therapy. Citation Format: Diana Y. Zhao, Keith M. Jacobs, Rowan M. Karvas, Jarrett L. Joubert, Dennis E. Hallahan, Dinesh Thotala. The Egr1 transcription factor contributes to radiation-induced apoptosis in the mouse hippocampus and intestinal crypts. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3941. doi:10.1158/1538-7445.AM2014-3941

<p>Radioprotection in Egr1 knockout mice was also observed at an earlier time point.</p>