Radiation-induced highly-malignant cancer cells: the mechanism and how to deal with it

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Novel function of transcription factor ATF5: blockade of p53-dependent apoptosis induced by irradiation.Purpose: p53-dependent cell death is considered as a predominant mechanism of tumor cell apoptosis induced by ionizing irradiation, and a large number of studies have shown that mutant p53-harboring tumor cells with a p53 gene mutation exhibit radioresistance. However, even tumor cells that express wild-type p53 display various degrees of radiosensitivity to ionizing irradiation. This indicates that there are additional pathways that affect p53-dependent cell death mechanisms. Here we describe a novel molecule that represses radiation-induced apoptosis by inhibiting trans-activation activity of p53.Materials and Methods: We irradiated QRsP cells, a mouse transplantable malignant cell line, at 10Gy and from surviving colonies 24 sub-clones were established. These clonal cells were re-irradiated and the most readio-sensitive clone, QRsPIR-5, was used in the current experiment. All sub- … More cloned cells had the same morphological appearances as the parental QRsP cells and there were no p53 mutations of among any of the clones. Colony assay indicated that the survival fraction of QRsPIR-5 cells at a dose of 10 Gy was less than 20% of the survival fraction of the parental QRsP cells. Flow cytometer analysis also indicated a higher apoptotic index after infection with recombinant adenovirus containing wild-type p53 (Ad-p53) in QRsPIR-5 cells compared with the parent cell (26.5% vs. 7.1%). Interestingly, the parental and QRsPIR-5 cells had the same degree of tumorigenicity in a transplant experiment.Results: Comprehensive cDNA array analyses demonstrated differential gene expressions between the parental and QRsPIR-5 cells, both in vitro and in vivo. Among these, 23 genes were expressed differently both in vitro and in vivo between the parent and QRsP-5 cells with a high stringent threshold (less than 0.5 or more than 2.0). One such gene, bZIP transcription factor ATF5, which might explain difference in radio-sensitivity. Exogenous expression of ATF5 gave QRsPIR-5 cells a radioresistance level similar to that of the parental cells (colony assay). Moreover, QRsPIR-5 cells gained resistance to Ad-p53-induced apoptosis. A luciferase reporter assay demonstrated that over-expressed ATF5 repressed the transcriptional activity of wild-type p53 drastically. Interestingly, time lapse analysis indicated accelerated motility in ATF5-transfected QRsPIR-5 cells.Conclusion: It is likely that ATF5 is a potent repressor of p53. Elevated expression of ATF5 in a tumor may relate to enhanced malignant phenotypes, such as radio-resistance or greater cell motility. Less