The Translational Machinery As A Target For Radiosensitization

Current approaches aimed at improving the efficacy of radiation as a cancer treatment modality involve the development and application of molecularly targeted radiosensitizers, a strategy that requires a thorough understanding of the fundamental processes comprising the cellular radioresponse. Recent data indicating that radiation modifies gene expression primarily through translational control rather than transcriptional events suggests that mRNA translation contributes to cell survival after irradiation. The overall goal of this project is to determine whether the regulatory/rate-limiting components of the translational machinery provide targets for tumor cell radiosensitization. The majority of translation in mammalian cells occurs in a cap-dependent manner and is highly dependent on eIF4E. As such, we investigated a regulatory role for eIF4E in cellular radiosensitivity. eIF4E knockdown enhanced the radiosensitivity of tumor but not normal cells. eIF4E knockdown inhibited the dispersal of radiation-induced [gamma]H2AX foci. Furthermore, radiation was found to increase the binding of >1000 unique mRNAs to eIF4E, many involved in DNA replication, recombination, and repair. S6 kinase 1 (S6K1), also an important regulatory component of the translational machinery, enhances the translation of specific mRNA subpopulations, independent from eIF4E, and mediates ribosome biogenesis. The role of S6K1 in determing cell survival after radiation was determined in several tumor cell lines and one normal cell line. S6K1 knockdown enhanced the radiosensitivity of all 3 tumor lines. In contrast S6K1 knockdown had no effect on the cellular radiosensitivity of the one normal line tested. The mechanistic target of rapamycin (mTOR) is a critical kinase in the regulation of gene translation and has been suggested as a potential target for radiosensitization. Importantly, it plays a major role in regulating eIF4E availability as well as S6K1 activity.