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Phorbol esters activate protein kinase C and induce expression of the c-fos and c-myc protooncogenes in density-arrested BALB/c 3T3 (A31) cells; in contrast, epidermal growth factor (EGF) does not activate protein kinase C and is a poor inducer of c-fos and c-myc in these confluent cells. We show that, when A31 cells were subconfluent and made quiescent by serum deprivation, the phorbol ester phorbol 12-myristate 13-acetate induced c-fos and c-myc mRNA poorly, whereas EGF was a better inducer. Another platelet-derived growth factor-inducible gene, JE, did not show this differential regulation by phorbol 12-myristate 13-acetate and EGF. The ability of EGF to induce protooncogene mRNA was associated with elevated levels of intracellular cAMP. First, serum-deprived cells maintained cAMP at about 2-fold higher level than density-arrested cells. Second, induction was greatly enhanced by cholera toxin and 3-isobutyl-1-methylxanthine, which increased intracellular cAMP 3- to 10-fold. The calcium ionophore A23187 mimicked EGF in that it elevated c-fos and c-myc mRNA when administered with cholera toxin and isobutylmethylxanthine. Neither cholera toxin and isobutyl-methylxanthine nor A23187 appreciably induced these mRNAs when used alone. Our results suggest that c-fos and c-myc expression can be regulated by an EGF-directed pathway that utilizes calcium and cAMP as cooperating cytoplasmic messengers.

Induction of c-fos and c-myc mRNA by epidermal growth factor or calcium ionophore is cAMP dependent.