Effect of insulin on ketogenesis and fatty acid synthesis in rat hepatocytes incubated with dichloroacetate

In parenchymal liver cells isolated from fed rats, insulin increased the formation of 14CO2 from [1-14C]pyruvate (and presumably the flux through pyruvate dehydrogenase) by 14%. Dichloroacetate, an activator of the pyruvate dehydrogenase complex, stimulated this process by 133%. As judged from the conversion of [2-14C]pyruvate to 14CO2, the tricarboxylic acid cycle activity was not affected by insulin, but it was depressed by dichloroacetate. In hepatocytes from fed rats, incubated with glucose as the only carbon source, dichloroacetate caused a stimulation (31%) of fatty acid synthesis, measured as 3H incorporation from 3H2O into fatty acid, and an increased (134%) accumulation of ketone bodies (acetoacetate + D-3-hydroxybutyrate). Dichloroacetate did not affect ketone body formation from [14C]palmitate, suggesting that the increased accumulation of ketone bodies resulted from acetyl-CoA derived from pyruvate. Insulin stimulated fatty acid synthesis in hepatocytes from fed rats. In the combined presence of insulin plus dichloroacetate, fatty acid synthesis was more rapid than in the presence of either insulin or dichloroacetate, whereas the accumulation of ketone bodies was smaller than in the presence of dichloroacetate alone. Although pyruvate dehydrogenase activity, which is rate-limiting for fatty acid synthesis in hepatocytes from fed rats, is stimulated both by insulin and by dichloroacetate, the reciprocal changes in fatty acid synthesis and ketone body accumulation brought about by insulin in the presence of dichloroacetate suggest that insulin is also involved in the regulation of fatty acid synthesis at a mitochondrial site after pyruvate dehydrogenase, possibly at the partitioning of acetyl-CoA between citrate and ketone body formation.     

Acute effects of insulin on fatty acid metabolism in isolated rat hepatocytes

Isolated rat hepatocytes, previously shown to display enhanced rates of fatty acid biosynthesis upon a brief exposure to insulin, were used to study acute effects of this hormone on other aspects of hepatic fatty acid metabolism. Insulin activates the incorporation of exogenously added fatty acids into glycerolipids and depresses their utilization in the formation of ketone bodies. Insulin increases both the activity of acetyl-CoA carboxylase and the cellular content of malonyl-CoA. Evidence is presented that malonyl-CoA plays an important role in the insulin-mediated control of both ketogenesis and de novo fatty acid synthesis. All metabolic parameters studied are affected by glucagon in a manner opposite to that of insulin.     

Persistent changes in the initial rate of pyruvate transport by isolated rat liver mitochondria after preincubation with adenine nucleotides and calcium ions

1. Preincubation of isolated rat-liver mitochondria in the presence of adenine nucleotides or Ca2+ results in definite and persistent changes in the initial rate of pyruvate transport. 2. These changes in the rate of pyruvate transport are accompanied by equally persistent changes in the opposite direction of the activity of pyruvate dehydrogenase (EC 1.2.4.1). 3. Changes of the transmembrane pH gradient and of the membrane potential, brought about by the pretreatments of the mitochondria, cannot account for the observed changes in the rate of pyruvate transport. 4. It is proposed that the pretreatment of the mitochondria directly modulates the activity of the mitochondrial pyruvate carrier. The possible regulatory role of such a modulation system is discussed.     

Stimulation by a tumor-promoting phorbol ester of acetyl-CoA carboxylase activity in isolated rat hepatocytes

Acetyl-CoA carboxylase (EC 6.4.1.2) in hepatocytes from meal-fed rats was activated by phorbol myristate acetate (PMA) in a time- and concentration-dependent fashion. This activation can account for the PMA-induced stimulation of de novo fatty acid synthesis. Purified rat-liver acetyl-CoA carboxylase was found to be phosphorylated and activated by protein kinase C, thus providing a possible mechanism for the metabolic action of PMA in intact hepatocytes.     

Time course of hormonal effects on acetyl-CoA carboxylase as measured in digitonin-permeabilized rat hepatocytes

Acetyl-CoA carboxylase activity was measured in digitonin-permeabilized rat hepatocytes by coupling the carboxylase reaction to the fatty acid synthase reaction. Using this assay the activity of acetyl-CoA carboxylase was covariant with the rate of fatty acid synthesis. Insulin and the tumor promotor phorbol myristate acetate were found to stimulate, and glucagon and noradrenaline to inhibit both cellular parameters. The stimulation of acetyl-CoA carboxylase by insulin developed slowly (15 to 30 min) whereas the phorbol myristate acetate effect developed faster (within 15 min). The inhibition of the enzyme caused by glucagon was already apparent within 1 min after hormone addition. Inhibition by noradrenaline, in the presence of propranolol, was also quite rapid and occurred within 2 min after addition of the agonist.