Multiple synergistic signal transduction pathways regulate c-fos expression in Swiss 3T3 cells: the role of cyclic AMP.

The promoter region of the c-fos gene contains several upstream enhancer elements which dictate the transcriptional response to specific intracellular signals. Among these is the cyclic AMP (cAMP)-responsive element, which is required for c-fos expression by cAMP in vitro. However, we have previously shown that cAMP-elevating agents cause only a slight increase in c-fos mRNA levels in intact Swiss 3T3 cells. Here, we show that cAMP can potentiate the induction of c-fos by other second messengers. A combination of forskolin and either phorbol-12,13-dibutyrate or epidermal growth factor leads to an increase in c-fos mRNA levels comparable to those induced by bombesin or serum. Furthermore, cAMP-elevating agents can act in synergy with calcium ionophores (which alone do not induce c-fos) to increase c-fos expression by up to 60% of the bombesin response, although, in parallel cultures, this combination does not stimulate the phosphorylation of 80K, an Mr 80,000 protein, which is a major substrate for protein kinase C. These results suggest that, in intact cells, cAMP acts synergistically with distinct intracellular signals to stimulate c-fos transcription through protein kinase C-dependent and -independent pathways.