Although ionizing radiation has been shown to influence gene transcription, there is little known about the effects of radiation on gene translational efficiency. To obtain a genome-wide perspective of the effects of radiation on gene translation, microarray analysis was performed on polysome-bound RNA isolated from irradiated human glioma cell line U87. To allow for a comparison with the effects of radiation on transcription, microarray analysis was also performed using total RNA. The number of genes whose translational activity was modified by radiation was approximately 10-fold greater than those whose transcription was affected. The radiation-induced change in a gene’s translational activity was shown to involve the recruitment of existing mRNAs to and away from polysomes, as observed by northern blot analysis of RNA collected from sucrose density gradients. Moreover, immunoblots, showed that the change in a gene’s translational activity after irradiation correlated with changes in the level of its corresponding protein. These data suggest that radiation modifies gene expression primarily at the level of translation. Genes identified as being altered include those coding for proteins related to cell cycle control, transcription, and DNA/RNA metabolism. In search for possible mechanisms by which certain transcripts are preferentially recruited to or away from polysomes, we focused on the microarray outliers coding for RNA binding proteins, which included FMRP, ataxin 2, staufen, TAP, PABP, G3BP, and IMP3. From these transcripts, the corresponding protein expression levels have been validated for FMRP, ataxin and PABP. FMRP, a well-documented mRNA binding protein, binds approximately 4% of neuronal transcripts, including itself, in a sequence-specific manner and forms large ribonucleoprotein complexes with proteins such as staufen and PABP, with which it was found to co-immunoprecipitate. In addition, the increase in FMRP expression upon irradiation was prevented by cycloheximide, but not by actinomycin D, consistent with the upregulation of FMRP being dependent on translation. These data suggest that altering the translation of mRNA-binding proteins may provide a mechanism through which radiation regulates the translation and thus the expression of subsets of genes. Moreover, these results suggest that translational control to form complexes may serve as a fundamental component of cellular radioresponse.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]