Loss of G2/M arrest correlates with radiosensitization in two human sarcoma cell lines with mutant p53

We have examined the modulation of radiosensitivity by using caffeine in tw o human sarcoma cell lines both with a p53 mutation (US8-93 and LMS6-93). I n both cell lines a strong irradiation-induced G2/M arrest was coupled with a low rate of apoptosis. Incubation with caffeine resulted in a low percen tage of S and G2/M cells, associated with an accumulation in G1. With a hig her caffeine concentration, we detected a lower clonogenic survival with IC 50 at 2 mM. In both cell lines incubation with caffeine completely prevents the irradiation-induced G2/M arrest. This was connected to radiosensitizat ion, but without direct correlation to an induction of apoptosis. The effec t of radiosensitization rose with higher irradiation doses. However, in com parison with LMS6-93, it was stronger in cell line US8-93. A higher radiose nsitization in US8-93 correlated with the prevention of strong irradiation- induced G2/M response and higher initial DNA damage. Results of Western hyb ridization reveal a p53-independent mechanism of radiosensitization caused by caffeine. Our findings suggest that modulation in G2/M regulation may af fect a common checkpoint for tumor cells with defective p53 function. Furth ermore, our results show that the enhancer effect of caffeine is dependent on a strong reduction in the number of G2/M arrested cells and on an inhibi tion of DNA damage repair after irradiation. (C) 2001 Wiley-Liss, Inc.