Inhibition of proto-oncogene c-fos transcription by inhibitors of protein kinase C and ion transport.

Both 4 beta-phorbol 12,13-didecanoate (PDD) and calcium ionophore A23187 induced c-fos mRNA accumulation with similar kinetics in human monocyte-like cells (U937). Their effects were additive. Cells pretreated with PDD were not induced to accumulate c-fos mRNA by PDD but remained responsive to the induction by A23187. Similarly cells pretreated with A23187 responded to PDD but not to A23187. Nuclear run-off transcription assay indicated that increase in the c-fos mRNA level after the second inducer treatment corresponded to transcriptional activation of the c-fos gene. The accumulation of c-fos mRNA and the transcriptional activation of c-fos induced by PDD or A23187 were inhibited by the protein kinase inhibitor H-7 and by quinidine and amiloride. The induction by A23187, but not that by PDD, was also inhibited by 4-aminopyridine, or tetraethylammonium. From these results, it is proposed that activation of protein kinase C and Na+ or K+ transport are required for c-fos induction caused by PDD and A23187.