This work describes the molecular mechanism of hormonal modulation of fatty-acid peroxisomal beta oxidation in liver. Morris 7800C1 hepatoma cells and isolated hepatocytes were cultured in the presence of myristic acid (1 mM) and tetradecylthioacetic acid, a 3-thia fatty acid (50 microM), separately or in combination with dexamethasone (0.25 microM) or insulin (0.4 microM). Myristic acid stimulated acyl-CoA oxidase and a synergistic action was observed with dexamethasone. Parallel changes were recognized in enzyme protein and mRNA levels as quantified from immunoblots and Northern analyses. Myristic acid and tetradecylthioacetic acid had similar effects on this enzyme, while insulin inhibited the basal activity and blocked all inductions by the fatty acids and dexamethasone. Parallel mRNA and immunoblot analyses of the subsequent enzymes in the peroxisomal beta-oxidation pathway, enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase/delta 3,delta 2-enoyl-CoA isomerase and 3-oxoacyl-CoA thiolase, showed an even stronger induction by tetradecylthioacetic acid and dexamethasone, while the counteraction by insulin was maintained in both 7800C1 hepatoma cells and hepatocytes. In hepatoma cells, the thiolase always showed the most pronounced induction (about 40-fold) after 14 days, with parallel changes in protein and mRNA levels. The results suggest that the changes in peroxisomal beta-oxidation enzymes in 7800C1 hepatoma cells are due to a major effect on steady-state mRNA levels giving rise to corresponding alterations in enzyme protein. These results may be explained by regulation at the level of transcription of corresponding genes, but mRNA stability changes and/or translational effects may also be of importance.