Regulation of ATP pools, rRNA and DNA synthesis in 3T3 cells in response to serum or hypoxanthine.

The serum-induced transition of 3T3 fibroblasts from resting to growing state is characterized by a marked increase in cellular ATP content, the maximal level of which is reached at the onset of DNA replication. This increase in cellular ATP during the G 1 period of the cell cycle is correlated with about 3-fold stimulation of transcription of rRNA measured in permeabilized cell in vitro. Addition of hypoxanthine to serum-depleted quiescent 3T3 cells gives rise to an increase in both the ATP pool and the rate of rRNA synthesis. The expansion of cellular ATP pools after growth induction by serum seems to be a prerequisite for initiation of DNA synthesis since inhibition of purine de novo biosynthesis by azaserine inhibits both ATP pool expansion and DNA replication. This effect of azaserine can be abolished by addition of hypoxanthine to the culture medium. It is concluded that (a) the increase of the rate of rRNA synthesis in 3T3 cells in response to growth factors or serum is controlled by the cellular purine nucleoside triphosphate concentration and (b) an increased ATP level is necessary for initiation of DNA synthesis but is not sufficient to trigger the events that lead to DNA replication.