CYCLIC-AMP STIMULATES PROTEIN-SYNTHESIS IN L6 MYOBLASTS AND ITS EFFECTS ARE ADDITIVE TO THOSE OF INSULIN, VASOPRESSIN AND 12-0-TETRADECANOYLPHORBOL-13-ACETATE - POSSIBLE INVOLVEMENT OF MITOGEN-ACTIVATED PROTEIN-KINASE

The role of cyclic AMP as a second messenger in the stimulation of pro tein synthesis and the potential involvement of mitogen activated prot ein (MAP) kinase in this response was studied in L6 myoblasts. Dibutyr yl-cAMP (dbt-cAMP) increased protein synthesis at 90 min and 6 h in a concentration-dependent manner. The responses at 90 min were probably mediated by increased translation as they were not blocked by actinomy cin D; effects at 6 h were accompanied by increases in RNA content imp lying a transcriptional component. 100 nM 12-O-tetradecanoylphorbol-13 -acetate (TPA), 1 nM Insulin (90 min incubations) and 100 nM vasopress in (6 h incubation) also increased protein synthesis and these respons es were additive with those of 500 mu M dbt-cAMP. Responses to forskol in were similar to dbt-cAMP whilst 1,9-dideoxyforskolin had no effect. Cell extracts immunoblotted with MAP kinase antibody showed bands cor responding to approx. 42, 44, 54 and 83 kDa. 500 mu M dbt-cAMP elicite d an increase in activity of both the 42 and 44 kDa bands when assayed by the 'in gel' method and a similar response was also observed with forskolin. TPA and vasopressin also stimulated the activity of these t wo isoforms, but had no significant additive or inhibitory effects whe n added in combination with 500 mu M dbt-cAMP, In contrast, although 1 nM insulin alone had no effect, a synergistic response in terms of MA P kinase activation was observed in the presence of dbt-cAMP. The data demonstrate that cAMP stimulates protein synthesis in L6 cells and su ggest a role for MAP kinase in this event.