Abstract 2510: eIF4E is a potential tumor specific target for radiosensitization
Authors: Hayman Thomas J. , Williams Eli S. , Camphausen Kevin , Tofilon Philip J. 
Published: June 2012

Abstract
The translational control of gene expression has been implicated as a component of the cellular response to ionizing radiation (IR). However, the mechanisms mediating this process and its specific role in radiosensitivity have not been defined. eIF4E, an mRNA 5’ cap-binding protein, is a critical protein involved in regulating gene translation in response to a variety of environmental signals. In the study described here, we have determined the effects of IR on eIF4E activity. Moreover, using 3 tumor cell lines (MDA-MB-231, DU145 and A549) and 2 normal cell lines (MRC9 and HMEC), the potential role of eIF4E as a molecular determinant of radiosensitivity was investigated. eIF4E activity is regulated by eIF4E binding protein 1 (4E-BP1); 4E-BP1 phosphorylation releases eIF4E to enhance eIF4F complex formation and cap-dependent translation. Exposure of MDA-MB-231 or MRC9 cells to IR (2 Gy, 1h) resulted in an increase in 4E-BP1 phosphorylation. Consistent with this effect, m7-GTP batch chromatography showed that IR induced an increase in cap-bound eIF4G and a decrease in cap-bound 4E-BP1 proteins. Thus, these results indicate that IR, via 4E-BP1 phosphorylation, enhances eIF4E activity and cap-dependent translation. Whereas phosphorylation of eIF4E has also been shown to influence its translational activity, IR had no effect on eIF4E phosphorylation in any of the cell lines evaluated. To determine the significance of eIF4E as a regulator of radiosensitivity, tumor and normal cell lines were treated with siRNA to eIF4E and radiation survival curves were generated based on the colony formation assay. Knockdown of eIF4E in each of the 3 tumor cell lines resulted in an increase in radiosensitivity with DEFs (dose enhancement factors at 0.1 surviving fraction) ranging from 1.24 to 1.44. In contrast, eIF4E knockdown had no effect on the radiosensitivity of normal fibroblasts (MRC9) or normal mammary epithelial cells (HMEC). To investigate the mechanism through which eIF4E influences tumor cell radiosensitivity, we initially focused on MDA-MB-231 cells. Knockdown of eIF4E had no significant effect on MDA-MB-231 cell cycle distribution indicating that a loss of S-phase cells does not account for the observed radiosensitization. In addition, eIF4E knockdown had no effect on apoptotic cell death after IR (6 Gy, 24 and 48h). However, knockdown of eIF4E significantly enhanced the level of mitotic catastrophe induced by IR (2 Gy). These initial results suggest that eIF4E may influence the ability of tumor cells to respond to genotoxic injury. Whereas the specific mechanisms remain to be determined, these data suggest that eIF4E selectively regulates the radiosensitivity of tumor cells over normal cells.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2510. doi:10.1158/1538-7445.AM2011-2510