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Interference of Daunomycin with Metabolic Events of the Cell Cycle in Synchronized Cultures of Rat Fibroblasts¹

Istituto Nazionale per lo Studio e la Cura dei Tumori, Sezione di Chemioterapia Sperimentale, Milan, Italy

The influence of daunomycin on nucleic acid synthesis during different stages of the cell cycle has been studied in mammalian cells. Explants from leg muscle of newborn rats were synchronized by two 24-hr treatments with excess thymidine, 16 hr apart. The metabolic activity of DNA and RNA was determined by autoradiographic methods by measuring the incorporation of thymidine-³H, deoxycytidine-³H, and uridine-³H into nuclear structures. In control cultures, DNA synthesis shows two distinct waves located in the early and late S phase, the latter being the most sensitive to the inhibiting effect of daunomycin. RNA synthesis takes place in the cells during the entire cell cycle from the end of mitosis to the late prophase, but two well-defined peaks can be recognized: the first in the G₂ phase just 1 or 2 hr before the mitotic peak and the second in the middle stages of G₁. These RNA syntheses involving both nucleolar and extranucleolar structures are strongly inhibited by daunomycin treatment. The significance of these RNA syntheses in control cells and the relationship between their inhibition and the antimitotic effect of daunomycin are discussed.

¹ These investigations were supported in part by the "Consiglio Nazionale delle Ricerche," Italia.

² Consiglio Nazionale delle Ricerche.

Received 7/ 8/69. Accepted 9/22/69.