Cell cycle progression of C3H 10T½ and 3T3 cells in the absence of an increase in c-myc RNA levels

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Abstract
Normally, when confluent cells are stimulated to divide, they show a transient increase in c-myc RNA levels (1-5). This has led many investigators to believe that the rise in c-myc RNA is causally related to the control and regulation of cell proliferation. However, we report here that the rise in c-myc RNA levels is not observed in stimulated cycling C3H 10T1/2 and 3T3 cells that have been grown to confluence in the presence of the protease inhibitor antipain. Cells continue to progress from the G0/G1 phase to S phase of the cell cycle in the absence of an increase in c-myc RNA; the kinetics of [3H]thymidine incorporation after serum stimulation are comparable to those observed in cells in which c-myc RNA levels rise after serum stimulation. Our observations are of significance because they suggest a dissociation between the transient rise of c-myc RNA which occurs before DNA synthesis and the events that initiate DNA synthesis in quiescent fibroblasts; c-myc may not play as central a role in stimulating noncycling, quiescent cells to progress through the cell cycle, as has been generally assumed.