Induction of Lipogenic Enzymes in Primary Cultures of Rat Hepatocytes

Abstract

Using primary cultures of adult rat hepatocytes, the regulation of the following lipogenic enzymes was studied: glucose-6-phosphate dehydrogenase, malic enzyme, ATP-citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase, and stearoyl-CoA desaturase. The addition to the culture medium of either insulin or triiodothyronine produced a 2-3-fold increase in each of the individual enzyme activities whereas glucagon slightly decreased enzyme activities. The addition to the medium of 8-bromoguanosine 3,'5'-monophosphate had no effect on any of the enzyme activities unless glucose was also added to the culture medium. Glucose addition alone to the culture medium was without any effect; however, glucose enhanced the stimulation of enzyme activity due to insulin. The addition of fructose or glycerol, even in the absence of insulin, increased the activities of each of the enzymes studied 2-3-fold. The increases in enzyme activity brought about by insulin or fructose were apparently the result of de n o w enzyme synthesis, as indicated by the observation that the increases were not noted in the presence of cordycepin or cycloheximide. Immunoprecipitation of ATP-citrate lyase from hepatocytes pulselabeled with [3H]leucine indicated that the induction of this enzyme in response to the addition of fructose or glycerol to the culture medium was the result of an increase in the rate of synthesis of the enzyme. These results indicate that the activity and synthesis of individual enzymes involved in lipogenesis are increased in response to the metabolism of carbohydrate independently in part from hormonal effects.Hepatic fatty acid synthesis is known to be regulated in response to the hormonal and nutritional state of the animal (see reviews, [1,2]). Dietary fructose has been shown to be as effective as glucose in increasing the hepatic production of fatty acids despite its weak action on insulin secretion [3]. Furthermore, Baker et al. [4] reported that dietary fructose, but not glucose, stimulated lipogenesis in diabetic rats. A number of enzymes involved in lipogenesis including ATPcitrate lyase [5,6], acetyl-CoA carboxylase [5,7], fatty acid synthetase [7,8] and stearoyl-CoA desaturase [9] have been induced in livers of diabetic rats by feeding fructose or glycerol.Based upon these and related observations, it has been suggested that the increase in fatty acid synthesis observed following carbohydrate feeding is the result of a buildup of carbohydrate metabolites [3,.Recently, we have presented evidence indicating that the synthesis of glucokinase is regulated in response to the breakdown of carbohydrates and that this regulation may be mediated via cGMP [lo]. It is intriguing to speculate that a common system of regulation may be responsible for the control of metabolic pathways which respond similarly to physiological stimuli. In this report, the regulation of the following lipogenic enzymes was studied using primary cultures of adult rat hepatocytes: glucose-6-phosphate de-

Enigmatic effect of cellular ATP on fatty acid biosynthesis

“…Cycloheximide‐induced inhibition of fatty acid synthesis, i.e . the opposite of that reported here, is also documented in the literature [25]. However, this is a long‐term effect resulting from down regulation of lipogenic enzymes as a consequence of inhibition of protein synthesis by cycloheximide.…”