Combinatorial action of cAMP and phorbol ester on synergistic expression of the human activin A gene

A number of synergistic effects of cAMP and 12-O-tetradecanoylphorbol 13-acetate (TPA) that mimic diacylglycerol function on transcription have been studied, implying the existence of "cross-talk" regulation. However, there is still less information available on the combinatorial action of such signal transduction convergence on cellular gene expression. In the present study, we have first examined the synergistic effects of cAMP and TPA on the gene expression of activin A, an important regulator of cell growth and differentiation, and next we revealed their ordered action in HT1080 cells. A combined treatment of these cells with cAMP and TPA synergistically increased accumulation of the activin A mRNA. Interestingly, this synergistic response also occurred when cells were treated with cAMP following short preexposure to TPA. In contrast, the increase in activin A mRNA levels in those cells preexposed to cAMP with subsequent treatment by TPA was limited to that caused by TPA alone, suggesting an importance of ordered stimulation. Addition of cycloheximide (CHX), an inhibitor of protein synthesis, to cultures led to superinduction of activin mRNA transcripts regardless of TPA exposure, but significantly attenuated the cAMP-induced mRNA levels. Moreover, the observed synergism between TPA and cAMP was not inhibited by CHX, showing that de novo protein synthesis was required for the mRNA induction by cAMP but not for those induced by TPA or those induced by both. These results demonstrated that transient stimulation of HT1080 cells by TPA with subsequent exposure to cAMP is sufficient for the synergistic induction of the activin A gene expression and suggested that the combinatorial action of TPA followed by cAMP stimulation plays an important role in regulating activin synthesis in these cells.