Dietary cholesterol stimulates hepatic biosynthesis of triglyceride and reduces oxidation of fatty acids in the rat

Experiments were conducted in the intact rat and in the isolated, perfused rat liver to investigate the possibility that the increase in the concentration of hepatic triglyceride and increase in the secretion of the very low density lipoprotein (VLDL)-triglyceride (TG) resulting from addition of cholesterol to the diet are due to stimulation of synthesis of triglyceride, reduced fatty acid oxidation, or both. Male rats were fed for 7 days with either a cholesterol-free diet to which 5% (w/w) corn oil was added, or with the same diet supplemented with 0.5% cholesterol. Fed animals received [1-14C]oleic acid via the tail vein, as a complex with rat serum, and were killed 2 h later. Feeding cholesterol for 7 days increased hepatic triglyceride and cholesteryl ester (CE) concentrations, moderately elevated free cholesterol, but did not affect phospholipid (PL) levels, as we had previously observed after a feeding period of 3 weeks. Incorporation of [1-14C]oleic acid into hepatic and plasma triglyceride increased significantly (60 and 48%, respectively) with cholesterol feeding. Incorporation of [1-14C]oleic acid into hepatic and plasma cholesteryl esters increased by 63 and 79%, respectively, while incorporation into phospholipid was unaffected. Increasing the fat (corn oil) content of the diet to 20% (w/w) did not change these effects of dietary cholesterol. Studies using isolated, perfused rat livers were carried out in vitro after rats were fed the 5% corn oil diet for 3 weeks. [Perfusions lasted 4 h The perfusion medium contained 3% bovine serum albumin and 30% washed bovine erythrocytes in Krebs-Henseleit-HCO3 buffer. [1-14C]oleic acid was infused at a constant rate (140 micromoles/hr; 0.85 microcuries/hr) after an initial pulse (60 micromoles, 0.375 microcuries).] Hepatic uptake of free fatty acid was not altered by dietary cholesterol. Output of all VLDL lipids was stimulated, as was incorporation of 14C into hepatic and VLDL TG and CE (but not PL), by dietary cholesterol. Conversely, formation of 14CO2 and perfusate 14C-labeled perchloric acid-soluble compounds (ketones) was reduced in livers of animals fed cholesterol. Furthermore, the activity of mitochondrial carnitine palmitoyltransferase, the rate-limiting enzyme of fatty acid oxidation, was reduced in the liver of intact rats fed cholesterol. The increased level of triglyceride in liver and plasma may be secondary to reduced rates of fatty acid oxidation, to direct stimulation of the incorporation of hepatic acyl-CoA into TG, or to both of these factors. The additional TG synthesized may facilitate transport of cholesteryl ester in the VLDL.