Regulation of Fatty Acid Synthesis in Isolated Hepatocytes

In isolated hepatocytes prepared from unfed neonatal chicks, stimulation of fatty acid synthesis by fructose or dihydroxyacetone required acetate or octanoate in the medium, suggesting that the stimulatory effect of these compounds required the enhanced production of intramitochondrial acetyl coenzyme A. Stimulation of fatty acid synthesis by lactate or pyruvate did not require a supplementary substrate. Distribution of acetyl-CoA synthetase was 80% intramitochondrial and 20% cytosolic. However, sufficient acetate could be activated in the cytosol to accommodate the most rapid rates of fatty acid synthesis observed in isolated cells. This was shown experimentally by an undiminished rate of incorporation of [1-14C]acetate plus [12C]lactate or [12C]fructose into fatty acids when ATP-citrate lyase was inhibited by (-)-hydroxycitrate. The citrate content of the isolated cells was positively correlated with fatty acid synthesis under all incubation conditions. The ATP content of the cells did not correlate with fatty acid synthesis. Fatty acyl-CoA and α-glycerophosphate levels only partially correlated with fatty acid synthesis. Hence, citrate appears to be an important determinant in the regulation of fatty acid synthesis in intact liver cells, probably via its ability to activate acetyl-CoA carboxylase. Inhibition of fatty acid synthesis by free fatty acids was reversible, did not increase with time of incubation, and was not relieved by simultaneous incubation with the inhibitor of fatty acid oxidation, (+)-decanoylcarnitine. Thus, the active inhibitor was not a stable product of the esterification of fatty acids nor a product of the oxidation of fatty acids. Inhibition of fatty acid synthesis by medium free fatty acids was accompanied by an increase in the fatty acyl-CoA level. Fatty acyl-CoA may inhibit fatty acid synthesis by directly inhibiting acetyl-CoA carboxylase or by inhibiting the mitochondrial citrate carrier and, thereby, reducing the activation of acetyl-CoA carboxylase caused by citrate.