Differential Radiation Sensitivity in p53 Wild-Type and p53-Deficient Tumor Cells Associated with Senescence but not Apoptosis or (Nonprotective) Autophagy.

Studies of radiation interaction with tumor cells often focus on apoptosis as an end point; however, clinically relevant doses of radiation also promote autophagy and senescence. Moreover, functional p53 has frequently been implicated in contributing to radiation sensitivity through the facilitation of apoptosis. To address the involvement of apoptosis, autophagy, senescence and p53 status in the response to radiation, the current studies utilized isogenic H460 non-small cell lung cancer cells that were either p53-wild type (H460wt) or null (H460crp53). As anticipated, radiosensitivity was higher in the H460wt cells than in the H460crp53 cell line; however, this differential radiation sensitivity did not appear to be a consequence of apoptosis. Furthermore, radiosensitivity did not appear to be reduced in association with the promotion of autophagy, as autophagy was markedly higher in the H460wt cells. Despite radiosensitization by chloroquine in the H460wt cells, the radiation-induced autophagy proved to be essentially nonprotective, as inhibition of autophagy via 3-methyl adenine (3-MA), bafilomycin A1 or ATG5 silencing failed to alter radiation sensitivity or promote apoptosis in either the H460wt or H460crp53 cells. Radiosensitivity appeared to be most closely associated with senescence, which occurred earlier and to a greater extent in the H460wt cells. This finding is consistent with the in-depth proteomics analysis on the secretomes from the H460wt and H460crp53 cells (with or without radiation exposure) that showed no significant association with radioresistance-related proteins, whereas several senescence-associated secretory phenotype (SASP) factors were upregulated in H460wt cells relative to H460crp53 cells. Taken together, these findings indicate that senescence, rather than apoptosis, plays a central role in determination of radiosensitivity; furthermore, autophagy is likely to have minimal influence on radiosensitivity under conditions where autophagy takes the nonprotective form.