Cholera toxin potentiates TPA-induced mitogenesis and c-fos expression in BALB/c-3T3-derived proadipocytes

Treatment of quiescent density-arrested A31T6 proadipocytes with medium supplemented with either 12-O-tetradecanoylphorbol-13-acetate (TPA), insulin, or cholera toxin alone did not stimulate G0/G1 traverse and initiation of DNA synthesis. Combinations of either TPA and cholera toxin or insulin and cholera toxin caused a small stimulation of proliferation. Addition of medium supplemented with TPA and insulin caused a marked stimulation of cell cycle traverse which was significantly potentiated by the coaddition of cholera toxin. The actions of cholera toxin were mimicked by forskolin. Expression of c-fos was regulated in a manner that reflected the results of the mitogenic experiments. TPA caused a marked induction of expression, while only a small increase in transcript levels was seen after treatment with cholera toxin. Addition of a combination of cholera toxin and TPA caused a synergistic induction of c-fos expression. The model system described in this paper allows a detailed analysis of the regulation, by independent second messenger systems, of the transcription of a gene in a mitogenically relevant manner.