Comparative effects of caffeine on radiation- and heat-induced alterations in cell cycle progression

The effects of 3 mM caffeine on cell cycle progression of HeLa S3 cells exponentially and asynchronously growing in suspension culture were studied following exposure to 6.8 Gy gamma irradiation or 30 min at 45 degrees C hyperthermia. The stathmokinetic method, in which cells are grown in the presence of colcemid for the duration of experiment, in combination with two flow cytometric techniques, propidium iodide staining of DNA and acridine orange staining following acid denaturation of chromatin, were used to determine the fraction of cells in four cell cycle compartments, G1, S, G2, and M. Radiation and caffeine acted in a complementary manner, in which radiation reduced the caffeine-induced delays in cell cycle progression and caffeine prevented completely the radiation-induced accumulation of cells in G2 and mitotic delay. Heat and caffeine had additive effects on alterations in cell cycle progression. Cells containing spontaneous prematurely condensed chromatin were observed transiently immediately following heat exposure. These cells appeared to be in G2 and late S phase.