Loss of p53 function has been shown to cause increased resistance to ionizing radiation in normal murine cells; however, the role of p53 in radioresistance of human tumor cells is less clear. Since wild-type p53 function is required for radiation-induced G1 arrest, we measured G1 arrest in 12 human tumor cell lines that have a wide range of radiosensitivities (surviving fraction at 2 Gy, 0.11–0.8). We observed a significant correlation between the level of ionizing radiation-induced G1 arrest and radiosensitivity. Cell lines having G1 arrest are more radiosensitive. There is no correlation between maximal G2 arrest and radiosensitivity. Expression of a dominant-negative mutant of p53 (codon 143, Val to Ala) in transfectants of the radiosensitive human ovarian cell line A2780 abrogates the radiation-induced G1 arrest. Such mutant p53 transfectants are more resistant to ionizing radiation than the parental line and vector-alone transfectants, as measured by clonogenic assays. These results support the concept that wild-type p53 function is required for sensitivity of tumor cells to DNA-damaging agents, such as ionizing radiation, and that the loss of p53 function in certain human tumor cells can lead to resistance to ionizing radiation.

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This work was supported by the Cancer Research Campaign (United Kingdom) and the Scottish Hospitals Endowment Research Trust.

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