Cyclic AMP and phorbol esters interact synergistically to regulate expression of the chorionic gonadotropin genes

Previous studies have shown that activators of the protein kinase A pathway increase transcription of the genes encoding the alpha- and beta-subunits of human chorionic gonadotropin (hCG) in choriocarcinoma cell lines. Here, we show that treatment of choriocarcinoma cells with activators of protein kinase C, such as phorbol myristate acetate (PMA) and dioctanoylglycerol, increases accumulation of the mRNAs for both subunits of hCG by 3-4-fold. In contrast, a phorbol ester which fails to activate protein kinase C, phorbol 12 beta,13 alpha-didecanoate, has no effect on hCG mRNA levels. To test the possibility that these two major intracellular signaling pathways interact, we treated choriocarcinoma cells with PMA, forskolin, or PMA and forskolin together. Treatment with either agent led to a 2-3-fold increase in hCG mRNA levels, whereas treatment with both agents resulted in a 9-fold increase. This synergistic response also occurred when choriocarcinoma cells were treated with PMA and 8-Br-cAMP. Furthermore, PMA did not increase intracellular cAMP levels, suggesting that these two pathways interact subsequent to cAMP generation. PMA also increased transcription of the hCG alpha- and beta-genes by 2-3-fold. Whereas transcription of the alpha subunit gene increases synergistically after treatment with both PMA and forskolin, transcription of the hCG beta-gene was limited to the increase caused by either agent alone. This latter result suggests that regulation of hCG beta mRNA accumulation is more complex than that of alpha-subunit mRNA and probably involves both transcriptional and post-transcriptional components.