EGF, PGF2 alpha and insulin induce the phosphorylation of identical S6 peptides in swiss mouse 3T3 cells: effect of cAMP on early sites of phosphorylation.

Epidermal growth factor (10(-9)M), prostaglandin (8.5 X 10(-7)M), F2 alpha, and insulin (10(-9)M), each of which only leads to a partial phosphorylation of 40S ribosomal protein S6, generate the same first eight phosphopeptides induced by 10% serum, suggesting all three activate a common regulatory pathway for the phosphorylation of S6. Added together, they induce almost maximal S6 phosphorylation and a phosphopeptide pattern nearly equivalent to that of serum. Unlike the agents above, 8-Br-cAMP or PGE1 has no significant effect on protein synthesis, but does induce a small increase in S6 phosphorylation. Surprisingly, the three peptides that become phosphorylated are identical with insulin-induced phosphopeptides 10b, 11, and 9, based on either comigration, limited acid hydrolysis, or V8 protease digestion. Incubation of 40S subunits with cAMP-dependent protein kinase induces the phosphorylation of these same three phosphopeptides. The in vitro and in vivo studies described here raise the possibility that cAMP could, in part, be responsible for mediating the phosphorylation of S6 during the mitogenic response.