Qi, W., Qiao, D. H. and Martinez, J. D. Caffeine Induces TP53-Independent G1-Phase Arrest and Apoptosis in Human Lung Tumor Cells in a Dose-Dependent Manner. Radiat. Res. 157, 166–174 (2002).
Caffeine is a model radiosensitizing agent that is thought to work by abrogating the radiation-induced G2-phase checkpoint. In this study, we examined the effect that various concentrations of caffeine had on cell cycle checkpoints and apoptosis in cells of a human lung carcinoma cell line and found that a concentration of 0.5 mM caffeine could abrogate the G2-phase arrest normally seen after exposure to ionizing radiation. Surprisingly, at a concentration of 5 mM, caffeine not only induced apoptosis by itself and acted synergistically to enhance radiation-induced apoptosis, but also induced a TP53-independent G1-phase arrest. Examination of the molecular mechanisms by which caffeine produced these effects revealed that caffeine had opposing effects on different cyclin-dependent kinases. CDK2 activity was suppressed by caffeine, whereas activity of CDC2 was enhanced by suppressing phosphorylation on Tyr15 and by interfering with 14-3-3 binding to CDC25C. These data indicate that the effect of caffeine on cell cycle checkpoints and apoptosis is dependent on dose and that caffeine acts through differential regulation of cyclin-dependent kinase activity.
