NUTRITIONAL AND HORMONAL-REGULATION OF MESSENGER-RNA LEVELS OF LIPOGENIC ENZYMES IN PRIMARY CULTURES OF RAT HEPATOCYTES

The effects of nutrients and hormones on the mRNA levels of acetyl-CoA carboxylase, fatty acid synthase, malic enzyme, and glucose 6-phosphate dehydrogenase were examined in primary cultures of rat hepatocytes during the process of induction. The addition of both glucose and insulin to the culture medium markedly enhanced the lipogenic enzyme mRNA induction due to either of them, in 16 h. Fructose or glycerol proved to be an effective substitute for glucose, suggesting that glycolytic metabolites were involved in the mRNA induction. It is remarkable that mRNA induction of acetyl-CoA carboxylase was the most sensitive to glucose and also to insulin among the lipogenic enzymes. Polyunsaturated fatty acids markedly reduced the mRNA induction of lipogenic enzymes. Dexamethasone enhanced all the lipogenic enzyme mRNA induction by insulin. On the other hand, triiodothyronine addition greatly increased the mRNA concentrations of lipogenic enzymes, but dexamethasone decreased rather than increased the mRNA induction by triiodothyronine. The effects of insulin on the induction of the lipogenic enzyme mRNAs were similar, but those of triiodothyronine were not. Triiodothyronine markedly enhanced malic enzyme mRNA induction by insulin with dexamethasone, and tended to enhance the induction of the acetyl-CoA carboxylase and fatty acid synthase mRNAs, but not that of glucose 6-phosphate dehydrogenase mRNA. It appeared that insulin and triiodothyronine synergistically enhanced lipogenic enzyme mRNA induction by glucose, but the mechanisms were different.