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Abstract.We focused on the transcriptional responses induced by low and very low doses of ionizing radiation with time effect. Regardless of their importance only a few limited studies have been done. Here we applied a large-scale gene transcript profile to elucidate the genes and biological pathways. Immortalized human mesenchymal stem cells were irradiated with 0.01, 0.05, 0.2 and 1 Gy of gamma radiation and total RNA was extracted from each cell line at 1, 4, 12 and 48 h after exposure. The essential transcriptional responses were identified according to dose and time. A total of 6,016 genes showed altered expression patterns at more than one time point or dose level among the investigated 10,800 genes. Genes that showed dose-dependent expression responses were involved in signal transduction, regulation of transcription, proteolysis, peptidolysis and metabolism. Those that showed time-dependent responses were divided into two distinct groups: the up-and-down group was associated with 'cellular defense mechanisms' such as apoptosis, cell adhesion, stress response and immune response and the down-and-up group with 'fundamental cellular processes' such as DNA replication, mitosis, RNA splicing, DNA repair and translation initiation. Genes showing both dose-and time-dependent responses exhibited a mixture of both features. A highly non-linear relationship between the IR dose and the transcriptional relative response was obtained from the dose-dependent group. The timedependent group also exhibited a non-linear relationship as the complex effect group did. Some of the early-reactive-phase (1-4 h) genes showed a differential expression response to 0.01, 0.05 and 0.2 Gy but were unresponsive to 1 Gy. Some of the late-recovery-phase (12-48 h) genes showed a differential expression to 1 Gy but were relatively unresponsive to other doses. We further characterized the gene expression patterns that could be implicated in the molecular mechanism of the cellular responses to low and very low-dose irradiation.

AKT phosphorylates components of the intrinsic cell survival machinery and promotes survival to various stimuli. In the present study, we identified CDC-like kinase 2 (CLK2) as a new substrate of AKT activation and elucidated its role in cell survival to ionizing radiation. AKT directly binds to and phosphorylates CLK2 on serine 34 and threonine 127, in vitro and in vivo. CLK2 phosphorylation was detected in HeLa cells overexpressing active AKT. In addition, we demonstrated that ionizing radiation induces CLK2 phosphorylation via AKT activation. In contrast, the suppression of endogenous AKT expression by siRNA inhibited CLK2 phosphorylation in response to 2 gray of ␥-ray or insulin. Furthermore, we examined the effect of CLK2 on the survival of irradiated CCD-18Lu cells overexpressing Myc-CLK2. CLK2 overexpression significantly increased cell growth and inhibited cell death induced by 2 gray. The role of CLK2 in cell survival to ionizing radiation was dependent on the phosphorylation of serine 34 and threonine 127. Our results suggest that AKT activation controls cell survival to ionizing radiation by phosphorylating CLK2, revealing an important regulatory mechanism required for promoting cell survival.

This study shows the human cellular responses and the mechanism of low-dose ionizing radiation in CCD 18 Lu cells, which are derived from normal human lung fibroblasts. Cell proliferation and viability assay were measured for the cells following gamma-irradiation using trypan blue, BrdU incorporation, and Wst-1 assay. We also examined genotoxicity using a micronuclei formation assay. The activation of the MAPKs pathway was determined by Western blot analysis, and the siRNA system was used to inhibit the expression of ERK1/2 and p38. We found that 0.05 Gy of ionizing radiation stimulated cell proliferation and did not change Micronuclei frequencies. In addition, 0.05 Gy of ionizing radiation activated ERK1/2 and p38, but did not activate JNK1/2 in cells. A specific ERK1/2 inhibitor, U0126, decreased the phosphorylation of ERK1/2 proteins induced by 0.05 Gy of ionizing radiation, and a similar suppressive effect was observed with a p38 inhibitor, PD169316. Suppression of ERK1/2 and p38 phosphorylation with these inhibitors decreased cell proliferation, which was stimulated by 0.05 Gy of ionizing radiation. Furthermore, downregulation of ERK1/2 and p38 expression using siRNA blocked the cell proliferation that had been increased by 0.05 Gy of ionizing radiation. These results suggest that 0.05 Gy of ionizing radiation enhances cell proliferation through the activation of ERK1/2 and p38 in normal human lung fibroblasts.

Ionizing radiation generates oxidative stress, which is thought to be a major cause of aging. Although living organisms are constantly exposed to low levels of radiation, most studies examining the effect of radiation have focused on accelerated aging and diminished life span that result from high-dose radiation. On the other hand, several studies have suggested that low-dose radiation enhances the longevity of Drosophila melanogaster. Therefore, investigation of the biological effects of low-dose radiation could contribute to a more comprehensive understanding of the aging process. In this study, microarray and quantitative real time-PCR were used to measure genome-wide changes in transcript levels in low-dose irradiated fruit flies that showed enhanced longevity. In response to radiation, approximately 13% of the genome exhibited changes in gene expression, and a number of aging-related genes were significantly regulated. These data were compared with quantitative trait loci affecting life-span to identify candidate genes involved in enhanced longevity induced by low-dose radiation. This genome-wide survey revealed novel information about changes in transcript levels in low-dose irradiated flies and identified 39 new candidate genes for molecular markers of extended longevity induced by ionizing radiation. In addition, this study also suggests a mechanism by which low-dose radiation extends longevity.