Effects of low concentrations of actinomycin D on the initiation of DNA synthesis in rapidly proliferating and stimulated cell cultures

Abstract

It has previously been shown that actinomycin D at a concentration of 0.08 μg/ml preferentially inhibits nucleolar (ribosomal) RNA synthesis in monolayer cultures of Chinese hamster cells. Treatment with 0.08 μg/ml actinomycin D of Chinese hamster cells, synchronized by mitotic selection, at the beginning of the G1 period causes a delay in the onset of DNA synthesis while a similar treatment in the late G1 period does not affect the process. The same effect has been produced by 9 μg/ml lucanthone (miracil D), a drug selectively inhibiting ribosomal RNA synthesis. The addition of 0.08 μg/ml actinomycin D to the stationary cultures stimulated to proliferate by medium changes completely prevents cells from entering the S period, when the drug is added to the medium in the first half of the pre-replicative period. However, in the second half of the pre-replicative period cells regain the ability to synthesize DNA in the presence of actinomycin D approximately to the same extent as do cells in the G1 period directly following mitosis. Thus, the demand for a de novo ribosomal RNA (rRNA) synthesis may be considered as a specific feature of cells induced to proliferate after a period of quiescence. The data obtained are consistent with the assumption that the first part of the pre-replicative period in stimulated quiescent cells is characteristic of G0 cells while later on the G0 cells enter the path of G1 cells typical for the rapidly proliferating cell populations.