Host: The Japan Radiation Research Society
Ionizing radiation induces DNA double strand breaks, which result in activation of ATM-dependent DNA damage checkpoint pathway. We have reported that a major mode of cell death induced by radiation is not apoptosis but senecsence-like growth arrest (SLGA). SLGA is an irreversible cell cycle arrest which resembles to the process induced in senescent cells. Because SLGA is dependent upon p53 function, it has been hypothesized that most human cancer cells, which abolish p53 function, may not be able to induce SLGA. As a result, human cancer cells proceed their cell cycle even with DNA double strand breaks, which lead to mitotic catastrophe. In the present study, we determine the process of cell death via mitotic catastrophe. Six human cancer cell lines were used. Every 24 hours, cells receiving 6 Gy of X-rays were fixed with 4% formaldehyde and stained with anti-phosphorylated ATM antibody together with DAPI. We found that mitotic catastrophe was observed in approximately 50% of cells, but all of them were phosphorylated ATM negative. Since 100% of cells were positive for phosphorylated ATM if they were exposed to 40J/m2 UVC, it was clear that apoptosis was not executed in cells induced mitotic catastrophe. Moreover, the cells were still phosphorylated ATM negative 48 hours after irradiation, however, we also found that parts of micronuclei were apoptosis positive, indicating that a mechanism degrading fragmented DNA is exist in cells. From these results it is indicated that mitotic catastrophe is a process of cell death which is independent of apoptosis. Although the cells progress cell cycle without successful cell division, they subsequently died by inducing irreversible SLGA.
