Ionizing radiation-induced senescence of breast tumor cells is dependent on p53 and associated with telomere dysfunction. Free

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p53 wild-type MCF-7 breast tumor cells arrested by exposure to 10 Gy of ionizing radiation strongly express β-galactosidase, a marker of cellular senescence. Radiation fails to suppress expression of either the catalytic (hTERT) or RNA (hTR) subunits of telomerase, or to alter telomerase activity. Senescence arrest after irradiation is also observed in MCF-7/hTERT cells, which have elongated telomeres due to the ectopic expression of hTERT. No telomere shortening is detected in either MCF-7 or MCF-7/hTERT cells by the terminal restriction fragment assay, consistent with a telomere length independent mechanism of senescence. However, both of these isogenic cell lines demonstrate a significantly increased frequency of chromosomal abnormalities after exposure to ionizing radiation, with end fusions being the most prevalent cytogenetic aberration. We propose that these chromosome end-associated abnormalities induced by irradiation are critical elements leading to the onset of accelerated senescence. Fluorescence in-situ hybridization using a telomere-specific probe was performed with irradiated MCF-7/hTERT cells to evaluate the involvement of the telomere in the observed chromosomal aberrations. Telomeric signals were detected at the junction site of chromosomes involved in end fusions; however, the majority (84%) of the end fusions lacked a telomeric signal. These data suggest that deprotection of the telomere may allow for chromosome fusions; however, in most cases, the end fusions are likely due to telomeric or subtelomeric breaks, allowing for the fusion of chromosome ends. In contrast to parental and hTERT infected MCF-7 cells, p53 mutant MDA-MB-231 breast tumor cells and MCF-7/E6 cells in which p53 function has been abrogated by the viral E6 protein, undergo a delayed wave of apoptotic cell death in response to irradiation. Collectively, these data indicate that the senescence response induced by ionizing radiation in breast tumor cells is independent of alterations in telomere length or telomerase activity but is mediated by p53 and associated with telomere dysfunction. Supported in part by the DOD Breast Cancer Research Program DAMD17-03-1-0414.