Incubation with 1 nM triiodothyronine (T3) decreased cycloheximide-induced c-fos mRNA levels and the mRNA response to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 100 nM) or to forskolin (15 microM). T3 also reduced the abundance of nuclear proteins that bind to an AP-1 binding site and the levels of c-Fos protein, as determined by Western blot. In transient transfection assays with a c-fos promoter reporter construct T3 decreased basal promoter activity by more than 50-60% and strongly inhibited the TPA and forskolin-induced activity. T3 further suppressed basal promoter activity and led to a drastic decrease of the response to both stimulatory agents after co-transfection of the promoter with an expression vector for the receptor. A truncated receptor that lacks the DNA binding domain abolished the inhibitory effect of the endogenous and transfected T3 receptor. Retinoic acid (RA) had an effect similar to T3 reducing basal CAT activity and the response to TPA and forskolin by 40-50% in cells co-transfected with the c-fos promoter and an expression vector for the RA receptor. The inhibitory effect of the nuclear receptors on c-Fos is reciprocal, since overexpression of c-Fos repressed induction of the activity of the growth hormone promoter by T3 and RA. Co-transfection with an antisense c-fos vector relieved this inhibition and restored the response to both ligands. These results show the antagonism between the nuclear receptors and the membrane signal transduction pathways that converge on the c-fos oncogene.