The role of p53 in gemcitabine-mediated cytotoxicity and radiosensitization

Purpose: We compared the cytotoxic and radiosensitizing effects of gemcitabine (2',2'-diffuoro-2'-deoxycytidine, dFdCyd), a clinically valuable radiosensitizer, in colon cancer RKO cells which differed in their p53 status. The parental RKO cells, RKO-P, contain wild-type p53 protein. In RKO-E6 cells, the p53 function has been disrupted by transfection of the cells with the human papillomavirus type-16 E6 gene. Results: We found that the RKO-P cells were significantly more sensitive to dFdCyd-mediated cytotoxicity and apoptosis than RKO-E6 cells (IC10 39.3 +/- 5.3 nM and 62.0 +/- 6.9 nM, respectively). The cytotoxic effect of dFdCyd in RKO-P cells was accompanied by induction of the proapoptotic protein Bax at the time when p53 was induced. In contrast, similar treatment of RKO-E6 cells with dFdCyd resulted in only limited expression of Bax, suggesting that the cytotoxic effect of dFdCyd was mediated, in part, by a p53-dependent apoptosis pathway. We also studied the effect of dFdCyd on radiation sensitivity. We found that at minimally cytotoxic concentrations dFdCyd failed to radiosensitize either RKO-P or RKO-E6 cells, whereas at cytotoxic concentrations equal sensitization was produced. Finally, we assessed the influence of dFdCyd on cell cycle distribution. We found that dFdCyd synchronized RKO-P cells, whereas synchrony was not produced in p53-disrupted RKO-E6 cells. Conclusion: These results suggest that p53 status may influence dFdCyd-mediated apoptosis, cytotoxicity, and cell cycle progression but do not support an important role for p53 in radiosensitization.