The effect of serum, EGF, PGF2 alpha and insulin on S6 phosphorylation and the initiation of protein and DNA synthesis.

To test the connection between S6 phosphorylation and the activation of protein and DNA synthesis, we compared the effects of serum, epidermal growth factor (EGF), prostaglandin F2 alpha (PGF2 alpha) and insulin (which is not mitogenic in these cells). Increasing concentrations of serum or EGF produced roughly parallel effects on all three processes, though the maximum response elicited by EGF (10(-9) M) was only a portion of that caused by saturating levels of serum (7.5% to 10%). PGF2 alpha (8.5 x 10(-7) M) alone acted similarly to EGF (10(-9) M) and with EGF produced a synergistic effect on all three processes. Insulin (10(-9) M) alone stimulated both S6 phosphorylation and protein synthesis to approximately the same level as EGF or PGF2 alpha, but had no effect on initiation of DNA synthesis. Thus neither stimulation of S6 phosphorylation nor activation of protein synthesis is sufficient for initiation of DNA synthesis. The requirement for S6 phosphorylation could not be dissociated from the activation of protein synthesis. Ribosomes containing the most highly phosphorylated forms of S6 appear to have a selective advantage in entering polysomes.