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In the literature the sensitization of DNA to radiation-induced damage by caffeine 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 that p53 negative cells were more sensitive to checkpoint inhibition by caffeine 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 human fibroblasts. We demonstrate here that this also applies to a human transformed cell line, the thyroid carcinoma K1, when submitted to gamma- rays irradiation. 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 cultures the percentage of cells attaining the G2/M phase was reduced by over 30% at 16 hr. This was reflected in an accumulation of the cells in G1 phase and an inhibition of the S phase traverse. Cell cycle analyses from further time points combined with cell proliferation measurements confirmed these data. These results were independent of p53 status as experiments performed with variant K1 cell lines having defective p53 functions, led to similar conclusions. In addition, caffeine restored a G1 delay after irradiation in the cell lines with abrogated p53 functions. The effects of caffeine undeniably cumulate with damages induced by irradiation but probably by inhibiting DNA repair mechanisms or by intervening with purine and pyrimidine metabolisms and not by causing a G2/M block override., (Copyright 2001 Wiley-Liss, Inc.)

p53 mutations are found in about 70% of human cancers. In order to evaluate the role of these mutations in response to chemotherapeutic agents, it is important to distinguish between p53 response to DNA-damaging agents in normal and in tumour cells. Here, using normal human fibroblasts (NHFs), we show that cisplatin and UV radiation induce G2/M arrest which is temporally linked to p53-protein induction. To study the contribution of p53 to this G2/M arrest, we inhibited p53 induction in NHFs using p53 anti-sense oligonucleotides. Following exposure of NHFs to UV radiation, the inhibition of p53-protein induction leads to a greater accumulation of cells in the G2/M phase, but also to a decreased fraction of cells in the G1 phase. We propose that p53 does not induce G2/M arrest directly, and that the extent of this arrest may depend on the fraction of cells that do not stop at the G1 phase following exposure to DNA-damaging agents. Furthermore, inhibition of p53-protein induction leads to increased sensitivity of NHFs to UV radiation. These results suggest that inhibition of p53 protein enhances sensitivity to DNA-damaging agents in normal human cells.