Abstract
In primary cultured monolayer hepatocytes of adult rats, insulin (1×10-8M) induced glucose-6-phosphate dehydrogenase [EC 1. 1. 1. 49, G 6 PDH] several fold in 48 h. It also induced lipogenesis, measured as [1-14C] acetate incorporation in 2 h, in these cells. Of the various lipids, triglycerides and phospholipids were induced markedly, while cholesterol and its esters were not induced. The increase of G 6 PDH and lipogenesis were parallel. Glucagon, dibutyryl cyclic AMP, triiodothyronine, dexamethasone, epinephrine, isoproterenol, and dibutyryl cyclic GMP were also tested under similar conditions, but none of them caused significant induction of G 6 PDH or lipogenesis.
Use of anti-G 6 PDH serum showed that induction of G 6 PDH by insulin was due to increase in the amount of enzyme protein. Insulin was found to increase the rate of synthesis of G 6 PDH about 3-fold. SDS-polyacrylamide gel electrophoresis of the immunoprecipitable protein revealed that besides G 6 PDH another radioactive fraction (Mr 37, 000) was increased by insulin. This suggests that complete synthesis of G 6 PDH protein is slowed down in primary cultured hepatocytes and that an apparent nascent peptide of the enzyme accumulates.
Although on long-term treatment (48 h), glucagon and dibutyryl cyclic AMP had no effect on lipogenesis, when added with [14C] acetate for 2 h they strongly inhibited lipogenesis. Significant inhibition of lipogenesis by short-term treatment with glucagon was seen even in cells with a high capacity for lipogenesis induced by long-term treatment with insulin. Insulin again stimulated lipogenesis in short-term treatment, but its effect was slight.
It is concluded from these results that insulin exerts long-term stimulation of lipogenesis by inducing enzymes related to lipogenesis including G 6 PDH as well as causing slight stimulation by enhancing supply of substrate for lipogenesis. Glucagon seems to play a minor role in long-term control, but it causes short-term inhibition of lipogenesis.
