Induction of specific protein synthesis by phorbol esters in mouse epidermal cell culture.

The induction of specific protein synthesis by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), was studied in cultured mouse epidermal cells by two-dimensional gel electrophoresis after pulse-labeling with [35S]methionine. While overall protein synthesis was moderately inhibited by TPA, the synthesis of five specific proteins was increased. Three of these proteins (M.W. 25,000, M.W. 55,000, and M.W. 70,000) were either not synthesized or synthesized at low rates in untreated cells, while two proteins (M.W. 35,000 and M.W. 50,000) were also synthesized in controls but to a lesser extent than in TPA-treated cells. Increased synthesis of the M.W. 50,000 protein could be seen as early as 1 hr after TPA exposure, while maximum induction of all proteins was observed at 6 hr. By 24 hr, synthesis rates of these proteins had returned to near basal levels even if TPA exposure was continued. The amino acid analog canavanine (at levels which inhibited protein synthesis to the same extent as does TPA) or nonpromoting analogs of TPA did not induce these proteins. Pulse-chase studies indicated that these proteins were not degradation products which may have resulted from TPA exposure and that the M.W. 25,000 and M.W. 35,000 protein appear to be rapidly turned over. 32PO4 labeling indicated that only the M.W. 55,000 protein was significantly phosphorylated and that TPA did not induce a qualitative change in the pattern of phosphorylation of epidermal proteins. Definitive identification of these proteins has not been made, but the specific stimulation of their synthesis supports a model of tumor promoter action in which promoters induce a specific program of changes in macromolecular synthesis in the epidermis.