Stimulation of protein synthesis in pituitary cells by phorbol esters and cyclic AMP. Evidence for rapid induction of a component of translational initiation.

Phorbol esters such as phorbol myristate acetate (PMA) were employed to examine the involvement of protein kinase C in the regulation of protein synthesis in intact GH3 pituitary tumor cells. Amino acid incorporation increased as a function of time of pretreatment with these agents; 4-8- and 2-3-fold stimulations were observed for Ca2+-depleted and -restored preparations, respectively, following 2 h of exposure. PMA enhanced incorporation of amino acid into all detectable polypeptide species. Lysates of PMA-treated cells incorporated amino acid more efficiently than did lysates of untreated controls. Cells slowed at initiation by Ca2+ depletion responded to treatment with PMA with the production of low molecular weight polysomes and a concomitant decrease in 80 S monomers. In Ca2+-restored preparations, which form large polysomes, PMA treatment resulted in a decrease in 80 S monomers and a shift in average polysomal size from smaller to larger molecular weight. Ribosomal transit times, however, were not altered. PMA-stimulated amino acid incorporation and polysome formation were either eliminated or reduced significantly by actinomycin D and could not be ascribed to increased amino acid uptake or methionylation of tRNA. Substances which elevate cAMP in GH3 cells mimicked phorbol ester in its actions on protein synthesis. It is proposed that GH3 cells, in response to various stimuli, rapidly synthesize an mRNA that subsequently increases the synthesis of a rate-limiting component of translational initiation. Evidence that this pathway for translational control may function in alternative cell types is also presented.