Purpose: The aim of this work was to characterize the radioadaptive response at the molecular level.
Methods and materials: We used wild-type (wt) p53 and mutated (m) p53-containing cells derived from the human lung cancer H1299 cell line, which is p53-null. Cellular radiation sensitivities were determined with a colony-forming assay. The accumulations of p53, the human homolog of endogenous murine double minute 2 (Hdm2), and inducible nitric oxide synthase were analyzed with Western blotting. Quantification of chromosomal aberrations was estimated by scoring dicentrics per cell.
Results: In wtp53 cells, it was demonstrated that the lack of p53 accumulation was coupled with the activation of Hdm2 after low-dose irradiation (0.02 Gy). Although NO radicals were only minimally induced in wtp53 cells irradiated with a challenging irradiation (6 Gy) alone, NO radicals were seen to increase about two- to fourfold after challenging irradiation subsequent to a priming irradiation (0.02 Gy). Under similar irradiation conditions with a priming and challenging irradiation in wtp53 cells, induction of radioresistance and a depression of chromosomal aberrations were observed only in the absence of 5, 5'-(2, 5-Furanidiyl)bis-2-thiophenemethanol (RITA) or Nutlin-3 (p53-Hdm2 interaction inhibitors), aminoguanidine (an inducible nitric oxide synthase inhibitor), and c-PTIO (an NO radical scavenger). On the other hand, in p53 dysfunctional cells, a radioadaptive response was not observed in the presence or absence of those inhibitors. Moreover radioresistance developed when wtp53 cells were treated with isosorbide dinitrate (an NO-generating agent) alone.
Conclusions: These findings suggest that NO radicals are initiators of the radioadaptive response, acting through the activation of Hdm2 and the depression of p53 accumulations.