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

Background and purpose: We have investigated whether the protective role of the G2 checkpoint has increasing importance when the p53-dependent G1 checkpoint 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 with 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 increase in alpha-values in RT112 and TE671, both with non-functional p53. However, no radiosensitizing effect was seen in cells with a normal p53 function (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 radiosensitization by caffeine, no G1 arrest, a G2 arrest in those cells irradiated 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 shifted 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 effect. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.