Variation in sensitizing effect of caffeine in human tumour cell lines after gamma-irradiation

Background and purpose: We have investigated whether the protective role of the G2 checkpoint has increasing importance when the p53-dependent G1 chec kpoint is inactivated. Materials and methods: We have studied the differential effect of caffeine by clonogenic assays and flow cytometry in three human tumour cell lines wi th different functionality of p53 protein. Results: The radiosensitizing effect of caffeine (2 mM) expressed itself as a significant decrease in surviving fraction at 2 Gy and a significant inc rease in alpha-values in RT112 and TE671, both with non-functional p53. How ever, no radiosensitizing effect was seen in cells with a normal p53 functi on (MCF-7 BUS). Two millimoles of caffeine also caused important changes in the cell cycle progression after irradiation. MCF-7 BUS showed a G1 arrest after irradiation and an early G2 arrest but those cells that reached the second G2 did not arrest significantly. In contrast, TE671 exhibited radios ensitization by caffeine, no G1 arrest, a G2 arrest in those cells irradiat ed in G2, no significant accumulation in the second G2 but an overall delay in release from the first eel cycle, which could be abrogated by caffeine. RT112 was similar to TE671 except that the emphasis in a G2 arrest was shi fted from the block in cells irradiated in G2 to those irradiated at other cell cycle phases. Conclusion: The data presented confirm that p53 status can be a significant determinant of the efficacy of caffeine as radiosensitizer in these tumour cell lines, and document the importance of the G2 checkpoint in this effec t. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.