Effects of Actinomycin D on Cell Cycle Kinetics and the DMA of Chinese Hamster and Mouse Mammary Tumor Cells Cultivated in Vitro1

Actinomycin D produces changes in the cell cycle kinetics of Chinese hamster (HA1) and mouse mammary tumor cells (EMT6) cultivated in vitro. There was a reduced rate of progression of the cells through S phase and a G;, arrest, the duration and degree of which were drug dose depend ent. The lethal effects of the drug on the two cell lines were comparable. At the molecular level. DMA single-strand breaks (true breaks and/or alkali-labile bonds) appear initially. After drug removal, the extent of disappearance of these DNA strand breaks was drug dose dependent. Similar results were obtained for the two cell lines. However, at the lower drug concentration (0.1 ^g/ml), although the DNA strand breaks appeared to disappear after drug free incubation, about 60% of this cell population was not viable.