On the regulation of DNA synthesis in a line of adrenocortical tumor cells: Effect of serum, adrenocorticotropin and pituitary factors

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Abstract
Y‐1 adrenal cells responded to serum step down by a several fold decrease in DNA synthesis. Serum starved cells resumed DNA synthesis upon serum step up. ACTH and cAMP inhibited DNA synthesis both at low and high serum concentrations, a fact previously known. Pituitary, brain and liver crude extracts stimulated DNA synthesis in serum starved cells. Purified pituitary factors preparations contained two activities: one specific for Y‐1 cells and another active with both fibroblasts and Y‐1 cells. The kinetics of restimulation of DNA synthesis by serum and pituitary factors was studied. DNA synthesis restimulation occurred after a lag of 11 hours. This lag did not vary irrespective of the type of stimulator or its concentration. Cells entered S phase continuously at a rate which increased with increasing concentrations of the stimulator. Cells became refractory to the inhibitory action of ACTH five hours before entering S phase. The implications of these data to the understanding of cell growth control are considered.