Stimulation of fatty acid biosynthesis by dietary cholesterol and of cholesterol synthesis by dietary fatty acid.

We reported previously that dietary cholesterol produces hypertriglyceridemia in the rat, accompanied by reduced oxidation and increased incorporation of exogenous fatty acid into hepatic triglyceride and increased secretion of very low density lipoprotein. We now report that dietary cholesterol also increases net hepatic fatty acid synthesis and the incorporation of newly synthesized fatty acid into hepatic triglyceride in vivo. Male rats were fed a cholesterol-free, semisynthetic diet (5% [w/w] corn oil) for 7 days, or the same diet supplemented with 0.5% cholesterol. On the day of the experiments, fed animals received 5 mCi 3H2O intraperitoneally (i.p.) either at 1200 h (6 h into the light cycle) or at 2400 h (6 h into the dark cycle). Animals were killed 1 h after receiving the radioisotope. Feeding cholesterol increased hepatic triglyceride and cholesteryl ester concentrations, moderately elevated the content of free cholesterol, but did not affect phospholipid levels. Increased net synthesis of fatty acids by livers of animals receiving cholesterol was observed during the dark period; a similar increase during the light period was also observed for incorporation of newly synthesized fatty acid into hepatic phospholipid and cholesteryl ester, although incorporation into triglyceride was of borderline significance (P < 0.06). In other experiments male rats were fed similar diets for 3, 7, or 21 days. Fed animals received 10 mCi 3H2O, i.p. (900-1000 h), and were killed 24 h later. Duration of feeding did not influence rates of net fatty acid synthesis or the stimulation by cholesterol of incorporation of newly synthesized fatty acid into hepatic triglyceride and cholesteryl ester.(ABSTRACT TRUNCATED AT 250 WORDS)

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of hepatic secretion of very low density lipoprotein by dietary cholesterol.

Male rats were fed a cholesterol-free diet or the same diet supplemented with either 0.05, 0.1, 0.25, 0.5, 1, or 2% C for 21 days to investigate the effects of cholesterol on secretion of very low density lipoprotein (VLDL). Cholesterol feeding increased plasma and hepatic concentrations of triglyceride (TG) and cholesteryl esters (CE) in a dose-dependent manner. Plasma VLDL and low density lipoprotein (LDL) lipids were elevated by cholesterol feeding, while the high density lipoprotein (HDL) lipids were reduced. The secretion of the VLDL by perfused livers from these cholesterol-fed rats was examined to establish the relationship between the accumulation of lipids in the liver and the concurrent hyperlipemia. Liver perfusions were carried out for 4 h with a medium containing bovine serum albumin (3% w/v), glucose (0.1% w/v), bovine erythrocytes (30% v/v), and a 10-mCi 3H2O initial pulse. Oleic acid was infused to maintain a concentration of 0.6 mM. Hepatic secretion of VLDL-TG, PL (phospholipid), free cholesterol (FC), and CE increased in proportion to dietary cholesterol and was maximal at 0.5% cholesterol in these experiments in which TG synthesis was stimulated by oleic acid. Secretion of VLDL protein and apoB by the perfused liver was also increased. The molar ratios of surface (sum of PL and cholesterol) to core (sum of TG and CE) lipid components of the secreted VLDL, regardless of cholesterol feeding, were the same, as were the mean diameters of the secreted particles. The molar ratios of surface to core lipid of VLDL isolated from the plasma also were not affected by cholesterol feeding. During perfusion with oleic acid of livers from the rats fed the higher levels of cholesterol, the hepatic concentration of CE decreased, while the level of TG was not changed. We conclude that the hypercholesterolemia and hypertriglyceridemia that occur in vivo from cholesterol feeding, concurrent with accumulation of CE and TG in the liver, must result, in part, from increased hepatic secretion of all VLDL lipids and apoB. The VLDL particles produced by the liver of the cholesterol-fed rat are assembled without modification of the surface lipid ratios (PL/FC), but contain a greater proportion of cholesteryl esters compared to triglyceride in the core, because of the stimulated transport of CE from the expanded pool in the liver.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of selenium in the secretion of very-low-density lipoprotein in the isolated perfused rat liver.

A recirculating liver perfusion system was used to study the effects of dietary selenium (Se) on the hepatic secretion of very-low-density lipoprotein (VLDL). The perfusate from livers of rats fed on a Se-deficient diet incorporated about 50% more [1-14C]oleic acid into triacylglycerol (TG) and cholesteryl esters (ChoEs) than did the perfusate from livers of rats fed on a Se-supplemented diet. Similarly, livers from rats fed the Se-deficient diet secreted more VLDL and incorporated about 60% more [1-14C]oleic acid into VLDL TG and ChoEs than did livers from rats fed the Se-supplemented diet. The liver perfusate from rats in the Se-deficient group also showed significantly decreased fatty acid oxidation. We conclude that Se is a potent modulator of lipoprotein metabolism. A primary action of Se deficiency appears to be a decrease in fatty acid oxidation and a stimulation of fatty acid esterification, leading to increased VLDL TG and ChoEs formation and secretion.

Regulation of hepatic cholesterol biosynthesis by fatty acids: effect of feeding olive oil on cytoplasmic acetoacetyl-coenzyme A thiolase, beta-hydroxy-beta-methylglutaryl-CoA synthase, and acetoacetyl-coenzyme A ligase.

We reported previously that, in the perfused rat liver, oleic acid increased the specific activity of cytosolic enzymes of cholesterol biosynthesis. In this study, we examined the effects of oral administration of olive oil on the activities of HMG-CoA synthase, AcAc-CoA thiolase, AcAc-CoA ligase and HMG-CoA reductase. Olive oil feeding increased the specific activity of hepatic HMG-CoA synthase by 50%, AcAc-CoA thiolase by 2-fold, and AcAc-CoA ligase by 3-fold. Olive oil had no effect on HMG-CoA reductase activity. These data suggest that the enzymes that supply the HMG-CoA required for hepatic cholesterogenesis are regulated in parallel by a physiological substrate, fatty acid, independent of HMG-CoA reductase under these conditions.

Effects of dietary triglyceride on the properties and lipid composition of plasma lipoproteins: acute experiments in rats fed safflower oil.

Male rats were administered 1.5 ml safflower oil by gastric intubation 0, 4, and 8 hr after a 16 hr fast. Plasma, liver, and adipose tissue were collected 16 hr after the last fatty meal. Rats fasted for 16 hr served as controls. Following fat feeding, the fatty acid composition of the very low density lipoprotein, triglyceride, and hepatic triglyceride were similar, as were the percentages of 18:2 in the very low density lipoprotein and hepatic cholesteryl esters. The phospholipids of liver and plasma lipoproteins were similar in the control groups, except that more 16:0 was present in the plasma lipoproteins. After fat feeding, the plasma lipoproein phospholipids were enriched with 18:2 more than were the hepatic phospholipids. Furthermore, the percentage of 18:2 in phospholipid was much less than in triglyceride or cholesteryl esters. Clearly, esterified lipids of liver and plasma lipoproteins (very low density lipoprotein, low density lipoprotein, and high density lipoprotein), and to a lesser extent, adipose tissue, were enriched with 18:2 derived from dietary triglyceride fatty acid even 16 hr after the terminal meal. A major proportion of the very low density lipoprotein isolated by ultracentrifugation in zonal rotors from plasma of fat fed animals had a faster rate-zonal mobility than did the very low density lipoprotein isolated from plasma of control animals. The very low density lipoprotein isolated from plasma of fat fed rats contained fewer moles of phospholipids, cholesterol, and cholesteryl esters, relative to triglyceride than did the very low density lipoprotein from plasma of animals not receiving safflower oil. The molar ratio triglyceride:phospholipid:cholesterol:cholesterol esters in the very low denity lipoprotein was 100:42.0:22.1:44.5 in the control group and 100:35.4:17.8:19.5 in the fat fed animals. It is postulated that an important biochemical mechanism by which dietary triglyceride fatty acids consumed by the animal over a long period of time alter plasma concentrations of triglyceride, phospholipids, and cholesterol esters is the directive influence of plasma free fatty acid, derived from dietary triglyceride, on the secretion of very low density lipoprotein lipids by the liver.

Temporal relationships between dietary, plasma, hepatic, and adipose tissue lipids after short-term feeding of safflower oil to rats.

The objectives of this research were to study effects of dietary neutral fat on the concentration and the fatty acid composition of triglyceride (TG) and other lipid classes in plasma, liver, and adipose tissue and, particularly, to study the temporal and precursor-product relationship among those various lipid pools. Accordingly, male rats were administered 1.5 ml. of safflower oil by gastric intubation at 0, 4, and 8 hours after a 16-hour fast. Samples of plasma, liver, and adipose tissue were collected from groups of rats before fat feeding (0 time, controls), during the period of feeding (4 and 8 hours), and at several times thereafter up to 36 hours (28 hours after the last meal). The esterified lipids in all lipid fractions of liver and plasma became enriched with linoleic acid (18:2) during the experiment. The enrichment of plasma and hepatic TG, and of plasma FFA with 18:2 was substantial, reaching a maximum 8 hours after the third meal, whereas only a modest enrichment of adipose tissue TG was observed. The enrichment with 18:2 of various lipid fractions proceeded in the following sequence: total plasma TG became enriched first with 18:2 of chylomicron TG fatty acids during the period of active absorption of fat. Second, plasma FFA were enriched with 18:2 derived presumably from metabolism of chylomicron TG. Subsequently, enrichment of hepatic TG with 18:2 was observed. After maximum enrichment was attained, the percentage of 18:2 in these various lipid pools decreased in the following order. The percentage of 18:2 in plasma FFA fell rapidly, followed by a slower decrease of the 18:2 content of plasma TG and, finally, by an even slower decline of the 18:2 content of hepatic TG. All lipid fractions except plasma FFA contained an increased content of 18:2 even 28 hours following the last fattly meal. It can be deduced from these temporal relationships that plasma TG is derived indirectly from dietary TG long after active absorption of dietary fat has ceased. The composition of hepatic TG is altered by dietary neutral fat, through the intermediary uptake of plasma FFA derived from metabolism of chylomicron TG; this hepatic TG, which reflects dietary fat, is a significant precursor pool of the plasma TG in the postabsorptive state.

Enrichment of rat tissue lipids with fatty acids that are prostaglandin precursors.

The effects of supplementation of a complete diet with ethyl arachidonate and with ethyl dihomo-gamma-linolenate (20 : 3Omega6) on the fatty acid composition of plasma and tissue lipid classes were studied in normal rats. 2. These prostaglandin precursors were incorporated in varying degrees into all lipid classes of the tissues that were investigated. The largest elevations were seen in plasma and tissue triacylglycerols. Significant increases were also observed in phospholipids, cholesteryl esters and the free fatty acid fraction. 3. Following the feeding of the ester of 20 : 3Omega6, arachiodonate levels also rose in the lipids of some tissues. In others, such as the renal medulla and platelets, and increase in 20 : 3Omega6 content occurred without a rise in 20 : 4. 4. Platelet aggregation is known to be stimulated by 20 : 4 (via active metabolites), but not by 20 : 3Omega6. The ability to modify 20 : 3Omega6 levels selectively in certain tissues is of interest in light of such pharmacologic differences from 20 : 4.