Caffeine and the G2/M block override: A concept resulting from a misleading cell kinetic delay, independent of functional p53

In the literature the sensitization of DNA to radiation-induced damage by c affeine has been attributed to an override of the G2/M block. This process was supposed to involve the tumor suppressor gene p53 as it was described t hat p53 negative cells were more sensitive to checkpoint inhibition by caff eine than the wildtype phenotype. We have recently shown that caffeine does not cause an override of the G2/M block induced by radiation in normal hum an fibroblasts. We demonstrate here that this also applies to a human trans formed cell line, the thyroid carcinoma K1, when submitted to gamma- rays i rradiation. Within 9 hr after irradiation over 70% of the cells accumulated in the G2/M phase. This block persisted at 16 hr. In caffeine containing c ultures the percentage of cells attaining the G2/M phase was reduced by ove r 30% at 16 hr. This was reflected in an accumulation of the cells in G1 ph ase and an inhibition of the S phase traverse. Cell cycle analyses from fur ther time points combined with cell proliferation measurements confirmed th ese data. These results were independent of p53 status as experiments perfo rmed with variant K1 cell lines having defective p53 functions, led to simi lar conclusions. In addition, caffeine restored a G1 delay after irradiatio n in the cell lines with abrogated p53 functions. The effects of caffeine u ndeniably cumulate with damages induced by irradiation but probably by inhi biting DNA repair mechanisms or by intervening with purine and pyrimidine m etabolisms and not by causing a G2/M block override. (C) 2001 Wiley-Liss, I nc.