Induction of Accelerated Senescence by γ Radiation in Human Solid Tumor-Derived Cell Lines Expressing Wild-Type TP53

Abstract

Recent studies have demonstrated that p21WAF1 (now known as CDKN1A)-dependent and -independent accelerated senescence responses are a major determinant of the sensitivity of cancer cells to chemotherapeutic agents. The objective of the present study was to determine whether human solid tumor-derived cell lines that express wild-type TP53 can exhibit levels of CDKN1A induction after exposure to ionizing radiation that are sufficient to activate the accelerated senescence program. Exposure to 60Co gamma radiation (< or =8 Gy) triggered accelerated senescence in all five TP53 wild-type tumor cell lines examined, albeit to differing degrees. Three of the TP53 wild-type tumor cell lines, HCT116, A172 and SKNSH, activated the TP53 signaling pathway similarly to normal human fibroblasts, as judged by the nuclear accumulation of TP53, magnitude and duration of induction of CDKN1A mRNA and CDKN1A protein, and propensity to undergo accelerated senescence after radiation exposure. In the clonogenic survival assay, the degree of radiosensitivity of these three tumor cell lines was also in the range displayed by normal human fibroblasts. On the other hand, two other TP53 wild-type tumor cell lines, A498 and A375, did not maintain high levels of CDKN1A mRNA and CDKN1A protein at late times postirradiation and exhibited only low levels of accelerated senescence after radiation exposure. Studies with a CDKN1A knockout cell line (HCT116CDKN1A-/-) confirmed that the radiation-triggered accelerated senescence is dependent on CDKN1A function. We conclude that (1) clinically achievable doses of ionizing radiation can trigger CDKN1A-dependent accelerated senescence in some human tumor cell lines that express wild-type TP53; and (2) as previously documented for normal human fibroblasts, some TP53 wild-type tumor cell lines (e.g. HCT116, A172 and SKNSH) may lose their clonogenic potential in response to radiation-inflicted injury primarily through undergoing accelerated senescence.