Regulation of 40S ribosomal protein S6 phosphorylation in Swiss mouse 3T3 cells

Addition of serum to resting cultures of Swiss mouse 3T3 cells causes an immediate multiple phosphorylation of 40S ribosomal protein S6. After 60 min of stimulation, changing to medium containing no serum led to the net dephosphorylation of S6. During this same period, a second protein, as yet unidentified, became increasingly phosphorylated. Incubation of cells with cycloheximide prior to the addition of serum almost completely blocked the activation of protein synthesis. There was no effect on the serum-induced phosphorylation of S6. If cells were stimulated in the presence of cAMP phosphodiesterase inhibitors theophylline or SQ 20006, both S6 phosphorylation and the activation of protein synthesis were inhibited. Stimulation of cells with serum also led to an immediate drop in total intracellular cAMP levels. This was blocked by prostaglandin E1 (PGE1), which caused a 10 fold increase in total intracellular cyclic AMP. However, PGE1 had no effect on protein synthesis or S6 phosphorylation.