Human glioblastomas often develop resistance to radiation therapy. The molecular 
details of this phenomenon are not completely understood. Recent studies have 
suggested that deficiency in DNA repair pathways may alter the resistance to 
ionizing radiation in gliobastomas. The human glioma cell line M059J is 
deficient in DNA-dependent protein kinase (DNA-PK), whereas cell line M059K, 
isolated from the same malignant tumor, has normal DNA-PK activity. DNA-PK plays 
a central role in the repair of ionizing-radiation-induced double-strand break 
repair, and its deficiency has been correlated with ionizing radiation 
sensitivity in these glioblastoma cells. We argued that other cellular pathways 
could also play a role in the resistance to radiation therapy in gliomas. We 
hypothesized that micro-RNAs (miRNAs) are differentially modulated in M059J and 
M059K cells exposed to ionizing radiation and that the miRNA modulation 
contributes to the resistance to ionizing radiation. miRNAs are small nonprotein 
coding single-stranded RNA molecules, which are crucial posttranscriptional 
regulators of gene expression. Numerous studies have documented the 
participation of miRNAs in a wide range of biological processes. The 
contribution of miRNAs in mediating resistance of glioblastoma cell to ionizing 
radiation treatment has not been elucidated. To test this hypothesis, we 
examined the expression patterns of a number of miRNAs involved in 
carcinogenesis in irradiated M059J and M059K cells. The relative expression 
level as determined by real-time quantitative PCR for miRNAs belonging to the 
let-7 family indicated an upregulation in irradiated M059K cells. On the 
contrary, the analysis of irradiated M059J cells for the modulation of let-7 
family of miRNAs revealed an overall downregulation. The miR-17-3p, miR-17-5p, 
miR-19a, miR-19b, miR-142-3p, and miR-142-5p were upregulated in both M059K and 
M059J cells. The miR-15a, miR-16, miR-143, miR-155, and miR-21 were upregulated 
in M059K, and the modulation of these miRNAs fluctuated in M059J cells in a 
time-dependent manner. These results indicate the involvement of miRNAs in the 
differential response of glioblastoma cells to ionizing radiation treatment.