The structure of methylated xanthines in relation to their effects on DNA synthesis and cell lethality in nitrogen mustard-treated cells

Caffeine and human DNA metabolism: the magic and the mystery

The ability of caffeine to reverse cell cycle checkpoint function and enhance genotoxicity after DNA damage was examined in telomerase-expressing human fibroblasts. Caffeine reversed the ATM-dependent S and G2 checkpoint responses to DNA damage induced by ionizing radiation (IR), as well as the ATR- and Chk1-dependent S checkpoint response to ultraviolet radiation (UVC). Remarkably, under conditions in which IR-induced G2 delay was reversed by caffeine, IR-induced G1 arrest was not. Incubation in caffeine did not increase the percentage of cells entering the S phase 6-8h after irradiation; ATM-dependent phosphorylation of p53 and transactivation of p21(Cip1/Waf1) post-IR were resistant to caffeine. Caffeine alone induced a concentration- and time-dependent inhibition of DNA synthesis. It inhibited the entry of human fibroblasts into S phase by 70-80% regardless of the presence or absence of wildtype ATM or p53. Caffeine also enhanced the inhibition of cell proliferation induced by UVC in XP variant fibroblasts. This effect was reversed by expression of DNA polymerase eta, indicating that translesion synthesis of UVC-induced pyrimidine dimers by DNA pol eta protects human fibroblasts against UVC genotoxic effects even when other DNA repair functions are compromised by caffeine.

Cell cycle regulation in response to DNA damage in mammalian cells: a historical perspective

Cell cycle delay has long been known to occur in mammalian cells after exposure to DNA-damaging agents. It has been hypothesized that the function of this delay is to provide additional time for repair of DNA before the cell enters critical periods of the cell cycle, such as DNA synthesis in S phase or chromosome condensation in G2 phase. Recent evidence that p53 protein is involved in the delay in G1 in response to ionizing radiation has heightened interest in the importance of cell cycle delay, because mutations in p53 are commonly found in human cancer cells. Because mammalian cells defective in p53 protein show increased genomic instability, it is tempting to speculate that the instability is due to increased chromosome damage resulting from the lack of a G1 delay. Although this appears at first glance to be a highly plausible explanation, a review of the research performed on cell cycle regulation and DNA damage in mammalian cells provides little evidence to support this hypothesis. Studies involving cells treated with caffeine, cells from humans with the genetic disease ataxia telangiectasia, and cells that are deficient in p53 show no correlation between G1 delay and increased cell killing or chromosome damage in response to ionizing radiation. Instead, G1 delay appears to be only one aspect of a complex cellular response to DNA damage that also includes delays in S phase and G2 phase, apoptosis and chromosome repair. The exact mechanism of the genomic instability associated with p53, and its relationship to the failure to repair DNA before progression through the cell cycle, remains to be determined.

Abrogation of the effects of aphidicolin on NIH3T3 and V79 cells by caffeine

In this communication I show that caffeine (1,3,7-trimethylxanthine) stimulates [3H]thymidine incorporation in aphidicolin-treated V79 and NIH3T3 cells. Flow microfluorometric analysis showed that caffeine, partially or fully, abrogates the cell cycle progression block produced by aphidicolin. Increased cell growth is also observed in cultures treated with both aphidicolin and caffeine compared to cultures treated with aphidicolin only. Microscopic examination of V79 cultures treated with aphidicolin for 8 h showed a marked reduction in the frequency of round mitotic cells, as is expected from a drug which inhibits progression through the cell cycle by inhibiting DNA replication; this effect of aphidicolin was also reduced by caffeine. Biochemical analysis showed that caffeine did not directly interfere with the inhibition of DNA polymerase-alpha by aphidicolin. Analysis of dNTP pools indicated that caffeine increased the level of dCTP in V79 cells. In aphidicolin-treated V79 cells, the increase in the dCTP level due to exogenous cytidine was almost completely blocked; caffeine also substantially overcame this effect of aphidicolin. These results indicate that caffeine produces its effects on aphidicolin-treated cells by altering the dCTP metabolism.

Ca2+, Mg2+ and (adenosine diphosphate ribose)n in cellular response to irradiation

A model of cellular response to irradiation involving adenosine diphosphate ribosylation (ADP-ribosylation) is proposed. Its main assumptions are (a) control of accessibility of sites for ADP-ribosylation in chromatin by free Ca2+/Mg2+ ratio; and (b) regulation of the Ca2+/Mg2+ ratio by factors affecting intracellular free Ca2+ concentration; the regulation would be mediated by mitochondria. The model seeks to explain the mechanism of action of radiomodifiers such as caffeine, local anaesthetics, polyamines and 2,4-dinitrophenol.