Long-chain fatty acids suppress the induction of urea cycle enzyme genes by glucocorticoid action

In order to test the possibility that free fatty acids are the mediator of the abnormal expression of urea cycle enzyme genes in carnitine-deficient juvenile visceral steatosis (JVS) mice, the effects of fatty acids on urea cycle enzyme, carbamoylphosphate synthetase (CPS) and argininosuccinate synthetase (ASS), mRNA levels were examined in rat primary cultured hepatocytes. Addition of a synthetic glucocorticoid hormone, dexamethasone, caused increases in CPS and ASS mRNAs. Further addition of oleic acid suppressed the induction of CPS and ASS mRNAs by dexamethasone. In contrast, the phosphoenolpyruvate carboxykinase (PEPCK) mRNA level induced by dexamethasone was enhanced in the presence of oleic acid. The effects were reversed on further addition of carnitine. The mRNA levels of these enzymes induced by dibutyryl cAMP were not affected by the addition of oleic acid. A study of the specificity of fatty acids revealed that long-chain fatty acids of more than 16 carbons chain length had a suppressive effect on the CPS mRNA level induced by dexamethasone and that the presence of double bonds enhanced the effect. The changes in gene expression of CPS, ASS and PEPCK caused by the fatty acids in the cultured hepatocytes were very similar to those observed in the liver of JVS mice. The AP-1 DNA binding activity in the presence of dexamethasone was slightly enhanced by the addition of oleic acid. These results suggest that the long-chain fatty acids not metabolized in JVS mice are mediators of the abnormal gene expression in the liver which results in hyperammonemia.