Anomalous dose-response characteristics induced by caffeine in ultraviolet-irradiated V79--79 Chinese hamster cells

Survival and mutation of Chinese hamster cells after ultraviolet irradiation and caffeine treatment

UV-irradiated Chinese hamster cells on post-irradiation treatment with caffeine in growth medium for 24 h gave rise to biphasic UV-survival curves. At caffeine concentrations between 0.001 and 0.1 mM, control and caffeine-grown cells had similar survival curves initially from 0 to 30 J/m2. At fluences greater than 30 J/m2, there was effectively only little further killing of caffeine-grown cells beyond that observed at 30 J/m2. At concentrations of caffeine greater than 0.5 mM, there was a gradual sensitization in the early part of the survival curve with increasing caffeine concentrations; but at fluences greater than 30 J/m2, the slopes in the survival curves decreased very much. It has been proposed that the initial sensitization observed at low UV fluences is due to the suppression of post-replication repair by caffeine. After high fluences of UV exposures in these excision-deficient cells, in the presence of caffeine, the possibility of an induced 'SOS'-like repair process has been suggested. This suggestion was supported by the observation that caffeine increased the yield of the UV-induced 8-azaguanine-resistant mutants only for the cell population exposed to UV fluences greater than 30 J/m2.

The effect of caffeine on the cytotoxicity of misonidazole and some other nitroheterocyclic compounds

Caffeine was found to potentiate the cytotoxic effect of misonidazole (1-(2-nitroimidazol-1-yl)-3-methoxy-2-propanol) towards mammalian cells in vitro. This enhancement of toxicity is shown to occur under both aerobic and hypoxic conditions. Split dose experiments indicate that the general shape of the hypoxic survival curve can be restored by exposure of cells of O2 between doses. Exposure of cells to 2mM caffeine during the split dose experiment has no potentiating effect. Further experiments showed that caffeine affects the expression of misonidazole-induced potentially lethal damage. Other electron-affinic nitro compounds which show greater toxicity to hypoxic compared to aerobic cells, viz. metronidazole and nitrofurantoin, also have their toxicities enhanced by subsequent exposure to caffeine.

Complex interactions of caffeine and its structural analogs with ultraviolet light in cell killing

We measured the clonogenic survival response of cultured mouse 10T1/2 cells exposed to UV light and caffeine post-treatment. When 0.5 and 1 mM caffeine were present for 24 h immediately following UV, the D0 values of the biphasic survival curves suggest that one subpopulation was sensitized and one subpopulation was protected from killing by UV light. A cloned survivor from the radioprotected subpopulation responded to UV plus caffeine in identical manner as the parent cells. When the caffeine exposure was prolonged to 48 h, only the radiosensitizing effect was observed. Two demethylated analogs of caffeine were also tested. The response of 10T1/2 cells to 1 mM theophylline present for 24 h after UV irradiation was approximately the same as that for the same treatment with 1 mM caffeine. However, prolonging the theophylline exposure to 48 h failed to produce the same kind of potentiation of cell killing as that observed for caffeine. Xanthine by itself was as toxic to 10T1/2 cells as caffeine, but had no synergistic effect as caffeine when given to UV-irradiated cells for 24 or 48 h. It is therefore unlikely that all the effects of caffeine on UV-irradiated cells are mediated by its demethylated metabolites.