Effects of saturated and unsaturated fats given with and without dietary cholesterol on hepatic cholesterol synthesis and hepatic lipid metabolism

Potential of Lipoprotein-Based Nanoparticulate Formulations for the Treatment of Eye Diseases

Lipoproteins are naturally occurring nanoparticles and their main physiological function is the promotion of lipid metabolism. They can be prepared in vitro for use as drug carriers, and these reconstituted lipoproteins show similar biological activity to their natural counterparts. Some lipoproteins can cross the blood-retinal barrier and are involved in intraocular lipid metabolism. Drug-loaded lipoproteins can be delivered to the retina for the treatment of posterior eye diseases. In this review, we have discussed the therapeutic applications of lipoproteins for eye diseases and introduced the emerging animal models used for the evaluation of their therapeutic effects.

Dietary fish oil reduces cholesterol and arachidonic acid levels in chick plasma and very low density lipoprotein

The mechanisms involved in the hypolipidemic effects of fish oil have not been clearly established. This study shows that supplementation of 10% menhaden oil to the chick diet for 7 days produced a significant hypocholesterolemia and hypotriglyceridemia. Fatty acid composition of chick plasma drastically changed by the same dietary manipulation. Percentages of 20:5 and 22:6 n-3 fatty acids strongly increased, while percentages of 20:4 n-6, 18:2 n-6, and 18:1 n-9 significantly decreased. Changes observed in the relative percentages were parallel to those obtained in the amount of each fatty acid. Ratio of n-3/n-6 clearly decreased in plasma by fish oil feeding. Total cholesterol and triacylglycerol contents decreased in high density lipoprotein (HDL) but did not change in low density lipoprotein (LDL). All chemical constituents of very low density lipoprotein (VLDL) significantly decreased after the first week of menhaden oil supplementation to the diet. Similar modifications in fatty acid composition of the three lipoprotein fractions were also found. Our results suggest that the hypocholesterolemic effects of fish oil may be mediated by the depletion in VLDL synthesis and secretion into the chick plasma. On the other hand, the strong decrease found in the arachidonic acid (AA) content of chick plasma and lipoproteins may contribute to the beneficial effects of fish oil consumption by lowering the production of its derived eicosanoids.

Differential effects of dietary fat on chick plasma and liver composition and HMG-CoA reductase activity

The comparative effects of diet supplementation with 10% saturated fat rich in 12:0 and 14:0 fatty acids (coconut oil), without and with 1% added cholesterol, and with 10% unsaturated fat rich in n-3 polyunsaturated fatty acids (menhaden oil) on cholesterol metabolism in neonatal chicks were examined to clarify the different mechanisms of their hyper- and hypolipidemic action. Supplementation of coconut oil produced a significant hypercholesterolemia after 7 days of treatment, with a similar increase in the amount of both free and esterified cholesterol. Supplementation of coconut oil plus cholesterol produced a higher increase of plasma cholesterol levels (approximately two to three times higher than those found with standard diet). However, supplementation of menhaden oil induced a significant decrease in total cholesterol after only 2 weeks of treatment. Levels of plasma triglycerides did not change by coconut oil addition to the diet, but a significant increase was observed after coconut oil plus cholesterol feeding. Menhaden oil produced a transient decrease in plasma triglycerides. Hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity did not change with coconut oil treatment. However, both coconut oil plus cholesterol and menhaden oil supplemented diets drastically decreased reductase activity after 1 week of dietary manipulation. These results show that different nutrients with the same inhibitory effect on reductase activity produced opposite effects on plasma cholesterol content, suggesting the existence of important differences in the regulatory mechanisms implied in cholesterol biosynthesis and its accumulation in plasma.

Comparison of the uptake and processing of cholesterol from chylomicrons of different fatty acid composition in rats fed high-fat and low-fat diets

The fate of [3H]cholesterol carried in chylomicrons prepared from rats given a meal of palm oil (rich in long-chain saturated fatty acids), olive oil (rich in monounsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids) was investigated in vivo in rats fed a low-fat diet or a diet supplemented with the corresponding oil (to provide 40% of the calories) for 21 days. In the low-fat-fed groups, radioactivity was removed from the blood and secreted into bile over 180 min more rapidly when the chylomicrons were derived from corn oil as compared to palm or olive oil. After feeding the corresponding high-fat diets, however, both parameters were decreased in rats fed palm and corn oil, but not olive oil. As a result of these changes, the rates of removal of radioactivity from the blood and secretion into bile were similar in animals given the olive oil and corn oil diets, and higher than those in rats fed the palm oil diet. All the high-fat diets tended to increase the proportion of the radioactivity in the plasma found in the 1.006-1.050-g/ml fraction (low-density lipoprotein) and decrease that in the 1.050-1.25-g/ml (high-density lipoprotein) fraction in comparison to the respective low-fat diet groups, but the transfer of radioactivity to the plasma high-density lipoprotein fraction was particularly slow in palm-oil-fed rats. These findings indicate that diets high in saturated or n-6 polyunsaturated fat retard the metabolism of chylomicron cholesterol in comparison to diets low in fat, while those high in monounsaturated fat do not have this effect. As a consequence of this, the rate of removal of cholesterol of dietary origin from the body is slower in animals fed saturated as compared to monounsaturated or n-6 polyunsaturated fat. Thus, differential metabolism of chylomicron cholesterol clearly plays an important role in the hyper- and hypo-cholesterolaemic effects of these dietary fats.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Comparison of the hepatic uptake and processing of cholesterol from chylomicrons of different fatty acid composition in the rat in vivo

The effect of the fatty acid composition of chylomicrons on the uptake and processing of the cholesterol they carry was investigated in the rat in vivo. Rats kept on a standard low fat pellet diet and tube fed a single dose of palm, olive, corn or fish oil (rich in saturated, n-9 monounsaturated, n-6 polyunsaturated and n-3 polyunsaturated fatty acids, respectively) were used to prepare [3H]cholesterol-labelled chylomicrons of different fatty acid composition. These were then injected intravenously into rats (kept on the standard diet), and the clearance of radioactivity from the blood, distribution in the plasma lipoprotein density fractions, uptake by the liver and appearance in the bile were studied. [3H]Cholesterol from fish and corn oil chylomicrons was cleared from the blood more rapidly than that from palm and olive oil chylomicrons. After 180 min the proportion of the radioactivity present in the plasma in high density lipoprotein (HDL) was less when the chylomicrons were derived from palm oil as compared to any of the other oils. Approx. 40% of the administered label was recovered in the liver after 180 min in all experiments. The percentage of the injected radioactivity secreted into bile during 180 min was significantly higher with corn and fish oil chylomicrons than with palm oil chylomicrons, with chylomicrons from olive oil in an intermediate position, and these differences were most pronounced between 60 and 120 min after administration of the label. These studies clearly demonstrate that the fatty acid composition of chylomicrons has important effects on the hepatic uptake and processing of the cholesterol they carry, with enrichment with polyunsaturated fatty acids leading to an increased rate of uptake and more rapid removal from the body via the bile as compared to enrichment with saturated or monounsaturated fatty acids.

Variations in composition of dietary fats affect hepatic uptake and metabolism of chylomicron remnants

The hepatic metabolism of [1-14C]oleate- and [1,2-3H]cholesterol-dual-labelled chylomicron remnants derived from olive, corn, palm and fish oil and butter fat was compared by adding each lipoprotein separately to the perfusate of isolated livers from rats fed on a normal diet. Labelled remnants from butter fat and fish oil were removed more rapidly from the perfusate than remnants derived from olive, corn and palm oil. The oxidation of labelled remnant fatty acid from olive oil, fish oil or butter fat was four to seven times greater than that from corn and palm oil. Labelled fatty acid in fish oil remnants was incorporated into phospholipid significantly more efficiently than the labelled fatty acid in olive, corn or palm oil remnants, with butter fat giving an intermediate value. For all the remnants, there was a significant amount of hydrolysis of labelled esterified cholesterol by the liver which was dependent on the magnitude of hepatic uptake of each type of remnant. The recovery of remnant [3H]cholesterol label in the bile was 50% less with palm oil remnants than with all the other remnants studied. The results indicate that the fatty acid composition of chylomicron remnants has a major impact on their uptake and metabolism by the liver.

Effect of dietary hyperlipidemic components and fish oil on concentration of lipids in liver and liver fatty acid profile of rats

This investigation was attempted to clarify the effects of dietary hyperlipidemic components and fish oil on concentration of lipids in liver and liver fatty acid profile of rats. In a first experiment male Sprague-Dawley rats were maintained on a semipurified low-fat diet with 6.5% coconut oil and 1.5% safflower oil, but without added cholesterol, or a high-fat hyperlipidemic diet supplemented with 7.3% coconut oil, 7.3% beef tallow, 0.4% safflower oil, and 1.5% cholesterol for 28 days. Rats fed the hyperlipidemic diet were then switched to fish oil diets with 1.4%, 2.8%, and 5.6% fish oil in exchange for coconut oil and beef tallow for, respectively, 10 and 20 days. In a second experiment male Sprague-Dawley rats were fed low-fat or high-fat diets without or with 1.5% added cholesterol for 28 days. Half of each group was then changed to a fish oil diet (for 20 days) which contained 5.6% fish oil in exchange for coconut oil and beef tallow. In experiment 1, rats fed the hyperlipidemic diet had enlarged fatty livers within 28 days. Experiment 2 showed that cholesterol in the diet was responsible for the accumulating liver lipids. Feeding diets with added cholesterol, the proportions of saturated fatty acids (SFA), especially 18:0, were markedly reduced in liver, whereas levels of monounsaturated fatty acids (MUFA) were greatly increased compared to diets without added cholesterol. This increase was most pronounced with respect to 16:1 and 18:1. In contrast, fish oil diets lowered MUFA level in liver in spite of its high MUFA content. Rats fed the highly saturated hyperlipidemic diet low in 18:2 n-6 had reduced 18:2 n-6 levels in liver compared to rats fed the low-fat diet without added cholesterol. Also, 20:4 n-6 level in liver was markedly reduced after the administration of the hyperlipidemic diet or the fish oil diets. Results of experiment 2 elucidate that cholesterol as well as fish oil sharply lowered 20:4 n-6 level in liver, which might be due to a reduced desaturation. In both experiments feeding fish oil increased all long-chain n-3 polyunsaturated fatty acids (PUFA) in liver. This occurred in a dose-dependent fashion and reached a maximum level with 5.6% fish oil in the diet. When additional cholesterol was applied 22:6 n-3 level in liver fell. This indicates also an impaired desaturation of PUFA due to dietary cholesterol.

Mechanism of the lipid-lowering effect of ethyl all-cis-5,8,11,14,17-icosapentaenoate

The effect of highly purified ethyl all-cis-5,8,11,14,17-icosapentaenoate (EPA-E) on cholesterol metabolism in rats was examined to clarify the mechanism of its hypolipidemic action. Pretreatment with EPA-E reduced the increase in plasma radioactivity after oral administration of [14C]cholesterol. The conversion of [14C]3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) to [14C]mevalonic acid was significantly inhibited in liver microsomes obtained from rats treated with EPA-E. There was an increase in free cholesterol and a marked rise in the eicosapentaenoic acid (EPA) content of phospholipids in these microsomes. EPA-E restored the suppression of biliary secretion induced by feeding a casein-rich diet to bile duct-cannulated rats. Furthermore, when serum lipoprotein (d < 1.210) from rats given EPA-E was i.v. injected into normal rats, a more rapid elimination of cholesterol was observed as compared to that in rats injected with lipoprotein from EPA-E-untreated rats. This rapid clearance was found in the lipoprotein fractions of d < 1.006 and 1.006 < d < 1.063. These findings suggest that EPA-E has an inhibitory effect on intestinal cholesterol absorption and hepatic cholesterol biosynthesis, and an enhancing effect on hepatic biliary secretion. EPA-E would also seem to cause modification of serum lipoproteins, whereby their clearance from the serum is increased.

Dietary fatty acids, lipoproteins, and cardiovascular disease

Dietary fat quality and quantity significantly affect the metabolism of all the plasma lipoproteins and probably constitute the most significant dietary determinants of plasma lipoprotein levels. Since the major role of the plasma lipoproteins is the transport of exogenous and endogenous fat, this would be expected of a highly regulated, metabolically homeostatic system. The data clearly show that dietary fat saturation affects all aspects of lipoprotein metabolism, from synthesis to intravascular remodeling and exchanges to receptor-mediated and nonspecific catabolism. The experimental data regarding dietary fatty acid effects on lipoprotein metabolism are complicated and at times contradictory due to the large degree of metabolic heterogeneity in the population, which, when coupled with the known abnormalities of lipoprotein metabolism associated with certain types of hyperlipoproteinemia, can present responses from A to Z. It is clear that the same dietary pattern has different effects in different individuals and that complicating factors of individuality raise some concerns regarding generalized dietary recommendations. As new knowledge of the role of dietary factors and CVD risk develops, and our abilities to characterize the individual patient's response to dietary interventions become more refined, it may be possible to specify dietary fat intervention from a patient-oriented concept rather than a single all-purpose diet approach. Thus it would be possible to design dietary interventions to match patient needs and gain both efficacy and compliance. With the spectrum of approaches possible--low fat, moderate fat with MUFA, n-3 PUFA, etc.--we should be able to approach dietary interventions to reduce CVD risk at both a population-based level and a patient-specific level. There remains much to learn regarding the effects of dietary fatty acids on the synthesis, intravascular modifications, and eventual catabolism of the plasma lipoproteins. The area of lipoprotein metabolism in health and disease, of its modifications by diets and drugs, and of the contributions of genetic heterogeneity to these processes is one of notable advances over the past two decades and continues to be an area of intense investigation.

Effect of fish oil and coconut oil diet on the LDL receptor activity of rat liver plasma membranes

The influence of 4 weeks treatment with fish oil and coconut oil enriched diets on the chemical composition of rat liver plasma membranes and LDL and on the binding of LDL to liver membranes was investigated. Rats fed fish oil diet showed a total, LDL and HDL plasma cholesterol concentration lower than the values observed in rats fed coconut oil and to a lesser extent lower than those of rats fed standard laboratory diet. LDL of rats on fish oil diet had a relative percentage of cholesterol and phospholipid lower, while that of triacylglycerol was greater. Furthermore, fish oil feeding was associated with a greater concentration of n - 3 fatty acids and a lower arachidonic and linoleic acid content in LDL. Liver plasma membranes isolated from fish oil rats showed a higher percentage of n - 3 fatty acids, while only a trace amount of these fatty acids was found in control and coconut oil fed animals. In binding experiments performed with LDL and liver membranes from fish oil fed rats and control rats, binding affinity (Kd = 3.47 +/- 0.93 and 4.56 +/- 1.27, respectively) was significantly higher (P less than 0.05) as compared to that found using membranes and lipoprotein from coconut oil fed rats (Kd = 6.82 +/- 2.69). In cross-binding experiments performed with fish oil LDL and coconut oil liver plasma membranes or coconut oil LDL and fish oil liver plasma membranes, the LDL binding affinity was comparable and similar to that found in fish oil fed animals. No difference was found in the Bmax among all the groups of binding experiments. Our data seem to indicate that during fish oil diet the higher binding affinity of LDL to liver plasma membranes might be partly responsible of the hypocholesterolemic action of marine oil rich diet as compared to saturated diet. Furthermore, the modifications of binding affinity induced by changes of LDL and membrane source, suggest that lipoprotein and liver plasma membrane composition may be an important variable in binding studies.

Whole body and hepatic cholesterol synthesis rates in the guinea-pig: effect of dietary fat quality

The effects of polyunsaturated, monounsaturated and saturated dietary fat on total and hepatic cholesterol synthesis were studied in the guinea-pig. Male Hartley guinea-pigs were fed semi-synthetic diets containing 7.5% (w/w) of either corn oil (CO), olive oil (OL) or lard for a period of 5 weeks and rates of endogenous cholesterol synthesis were determined from the incorporation of [3H]water into digitonin-precipitable sterols (DPS) and by measurement of sterol balance. In addition, total and expressed 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activities were determined in hepatic microsomes. Rates of whole body cholesterol synthesis determined by incorporation of [3H]water into DPS were significantly lower for guinea-pigs on the CO diet with values of 18.7 +/- 1.8 mumol/h (n = 4) vs. 26.7 +/- 4.8 and 24.6 +/- 1.8 mumol/h for animals on the OL (n = 4) and lard (n = 3) diets (P less than 0.001), respectively. Hepatic cholesterol synthesis rates were significantly decreased in animals on the OL diet, whether determined from incorporation of [3H]water into DPS or by analysis of HMG-CoA reductase activity. Hepatic total and free cholesterol levels were not different for animals on the three dietary fats; however, cholesteryl ester levels were 35% lower in guinea-pigs fed the lard diet (P less than 0.02). Sterol balance measurements indicated that whole body cholesterol synthesis rates were not affected by dietary fat quality (51.9 +/- 12.2, 42.8 +/- 7.6 and 51.2 +/- 20.2 mg/kg per day for animals on the CO, OL and lard diets, respectively). This is in striking contrast to the observed reduction in cholesterol synthesis rates for animals on the polyunsaturated CO diet as determined by incorporation of [3H]water into DPS. One possible explanation for the discrepancy between the sterol balance and [3H]water incorporation data is a polyunsaturated fat-mediated effect on energy utilization, which affects the equilibration of NADPH with the body water pool such that the [3H]NADPH has a lower specific activity than body [3H]water.

Effect of dietary cholesterol on phosphatidylcholines and phosphatidylethanolamines in bile and gallbladder mucosa in the prairie dog

Humans with cholesterol gallstones have been reported to have alterations in the molecular species of phospholipids in bile. Both decreases in phospholipids with linoleic acid and increases in those with arachidonic acid have been found. The purpose of this study was to investigate the effect of a lithogenic diet (0.34% cholesterol) on the relative abundance of individual molecular species of phospholipids in the biliary tract of the prairie dog. In hepatic bile, cholesterol feeding resulted in increases in phospholipid species containing arachidonate and decreases in the major species containing its precursor, linoleate. In gallbladder bile of both control and cholesterol-fed animals, phospholipid species containing linoleate were significantly less abundant than in hepatic bile, suggesting that linoleoyl species were selectively absorbed by the gallbladder epithelium. This apparent uptake was significantly increased by cholesterol feeding. Phosphatidylcholines and phosphatidylethanolamines containing arachidonate were also significantly increased in the gallbladder mucosa of the cholesterol-fed animals. These increases in arachidonate-containing phospholipids in the gallbladder mucosa may contribute to the increase in gallbladder prostaglandin synthesis that precedes gallstone formation in this animal model.

Cholesterol homeostasis in guinea pigs fed saturated and polyunsaturated fat diets

Whole body sterol balance, hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity, hepatic low-density lipoprotein (LDL) receptor levels and net tissue cholesterol concentrations were determined in guinea pigs fed either a corn oil- or lard-based purified diet for 6-7 weeks. In comparison to the saturated lard diet, the polyunsaturated corn oil diet resulted in a 34% reduction in plasma total cholesterol levels (P less than 0.02) and a 40% lower triacylglycerol level (P less than 0.02). Feeding the corn oil diet altered very-low-density lipoprotein (VLDL) and LDL composition; the percent cholesterol ester in both particles was decreased and the relative percentages of VLDL triacylglycerol and LDL phospholipid increased. The ratio of surface to core components of LDL from corn oil-fed guinea pigs was significantly higher compared to LDL from animals fed lard. Dietary fat quality had no effect on fecal neutral or acidic steroid excretion, net tissue accumulation of cholesterol, whole body cholesterol synthesis or gallbladder bile composition. Consistent with these results was the finding that fat quality did not alter either expressed (non-phosphorylated) or total hepatic HMG-CoA reductase activities. The hepatic concentrations of free and esterified cholesterol were significantly increased in corn oil-fed animals, as were cholesterol concentrations in intestine, adipose tissue, muscle and total carcass. Analysis of receptor-mediated LDL binding to isolated hepatic membranes demonstrated that the polyunsaturated corn-oil based diet caused a 1.9-fold increase in receptor levels (P less than 0.02). The data indicate that the hypocholesterolemic effects of dietary polyunsaturated fat in the guinea pig are not attributable to changes in endogenous cholesterol synthesis or catabolism but rather may result from a redistribution of plasma cholesterol to body tissue due to an increase in tissue LDL receptors.

The role of linoleic acid and its metabolites in the lowering of plasma cholesterol and the prevention of cardiovascular disease

An increase in linoleic acid intake lowers plasma cholesterol and is one of the safest methods for achieving this end. However, the amounts that must be consumed are large. Linoleic acid is metabolized via several routes and it is probable that a metabolite, rather than linoleic acid itself, is responsible for the cholesterol-lowering effect. If that metabolite could be identified, safe, drug-free, cholesterol-lowering might be achieved with much lower doses. Evidence is reviewed which suggests that a long-chain polyunsaturated fatty acid and/or a prostaglandin metabolite may be responsible for the cholesterol-controlling action of linoleic acid. Such metabolites may be effective also in controlling other risk factors for cardiovascular disease, such as elevated blood pressure and enhanced platelet aggregation. Epidemiological studies suggest that low levels of those metabolites, especially dihomogammalinolenic acid and arachidonic acid, are powerful independent risk factors for development of ischaemic heart disease. Further research in this area is urgently needed now that it is broadly accepted that cholesterol-lowering does indeed reduce the risk of cardiovascular disease.

Regulation of indices of cholesterol synthesis in human mononuclear leukocytes by dietary cholesterol and fat saturation

The responses of 2 indices of cholesterol synthesis, 3-hydroxy-3-methylglutaryl-CoA reductase activity and incorporation of [14C]acetate into sterols, in mononuclear leukocytes freshly isolated from peripheral blood to variation in the ratio of saturated to unsaturated fat (S:U) and the amount of cholesterol absorbed from the diet were examined in 24 free-living men. Increasing S:U was associated with increasing plasma cholesterol level (r = 0.27, P = 0.03) and increasing reductase activity in leukocytes (r = 0.60, p less than 0.001). This finding is consistent with the hypothesis that saturated fat decreases the flux of cholesterol from plasma into cells thereby releasing reductase from product feedback inhibition. Reductase activity, after controlling for the effect of S:U, was negatively correlated with absorbed cholesterol from sources other than eggs (r = 0.42, P = 0.02). Surprisingly, change in reductase activity was positively correlated with change in absorbed cholesterol upon eating eggs (r = 0.49, P = 0.008). Sterol labeling was negatively correlated with absorbed cholesterol from all sources including eggs (r = -0.64, P less than 0.001) and was uncorrelated with S:U. Reductase activity and sterol labeling responded in parallel to cholesterol in foods other than eggs but not to egg feeding nor to S:U, thus it is unclear which test best reflects endogenous sterol synthesis in these cells.

Changes of plasma lipids and long-chain n-3 and n-6 fatty acids in plasma, liver, heart and kidney phospholipids of rats fed variable levels of fish oil with or without cholesterol supplementation

Diets supplemented with 10% by weight of oil, either wholly safflower oil or proportinally (25, 50, 75 or 100%) replaced by fish oil, were given to 60 rats which had previously been deprived of dietary fat for 6 weeks. Half the animals on each dietary regimen were also given 1% of cholesterol. After 4 weeks of feeding, the plasma lipid contents and the phospholipid fatty acid compositions of plasma, liver, heart and kidney were determined. In general, the concentrations of plasma lipids were significantly reduced in animals fed a diet containing 5% or more of fish oil in comparison with those fed only safflower oil. Cholesterol feeding increased the levels of plasma cholesterol, whereas it lowered those of plasma triacylglycerols and phospholipids. The levels of 20:4(n - 6) in all four tissues were sharply reduced, whereas those of 18:2(n - 6) increased when 25% of dietary safflower oil was replaced by fish oil. Both 18:2(n - 6) and 20:4(n - 6) were decreased as the contents of dietary fish oil were further increased. The levels of long-chain n-3 fatty acids, e.g., 20:5(n - 3), 22:5(n - 3) and 22:6(n - 3) were increased as the intake of fish oil increased. The incorporation of 22:6(n - 3) was greater in plasma, liver and heart phospholipids, whereas that of 20:5(n - 3) was greater in kidney phospholipids. Cholesterol feeding also increased the levels of 18:2(n - 6) and 20:5(n - 3), whereas it decreased the levels of 20:4(n - 6) and 22:6(n - 3) in plasma and liver. However, these changes were not observed in heart and kidney.

Effect of dietary cholesterol on erythrocyte peroxidant stress in vitro and in vivo

The effect of dietary variation of plasma cholesterol concentrations on the susceptibility of erythrocytes to in vitro and in vivo peroxidant stress was studied in rats. Malonyldialdehyde, produced in vivo (endogenous malonyldialdehyde) or following in vitro exposure of cells to 10 mM H2O2 (H2O2 malonyldialdehyde), was used as a measure of peroxidant stress. After 5 weeks, the plasma cholesterol concentrations in rats receiving 1.2% cholesterol + 0.6% cholic acid in their diet rose to 6-times that of control rats receiving a diet without added cholesterol; at the same time, erythrocyte H2O2 malonyldialdehyde in the cholesterol-fed rats decreased significantly relative to the control rats. During subsequent exposure of both groups to in vivo peroxidant stress with phenylhydrazine in two separate dose trials, erythrocyte peroxidant stress remained significantly lower in the cholesterol-fed rats: at a dose of 100 mumol/100 g body weight, H2O2 malonyldialdehyde was lower; at a dose of 25 mumol/100 g body weight, both endogenous and H2O2 malonyldialdehyde were lower. Erythrocyte membrane cholesterol concentrations were 12% higher in the cholesterol-fed rats than in controls. The effects of in vivo peroxidant stress on plasma cholesterol were also studied. In vivo peroxidant stress at the higher dose of phenylhydrazine produced a decrease in plasma cholesterol concentrations of control rats. The lower dose had no effect on this group and the plasma cholesterol concentrations were unchanged in the cholesterol-fed rats during both treatments. The data suggest that elevated plasma cholesterol concentrations are protective against erythrocyte peroxidant stress. The mechanism of cholesterol's protective effect is probably mediated through elevated membrane cholesterol concentrations.

Influence of dietary cholesterol on the relative synthesis of hepatic glycerides and molecular classes of 1,2-diglycerides and phospholipids in the gerbil in vivo

The influence of dietary cholesterol on the relative rates of synthesis of hepatic lipids in the male Mongolian gerbil, Meriones unguiculatus, was studied. The semi-purified starch-based diet used lard as the dietary fat and was fed with or without a 0.5% (by wt.) cholesterol supplement. Each animal received 300 microCi [2-3H]-glycerol i.p. after 3 or 7 days on the dietary regimens. Relative rates of [2-3H]-glycerol incorporation into the major hepatic glycerides in vivo was not affected significantly by dietary cholesterol (0.5% level), suggesting that alteration in the relative biosynthesis of these lipids could not readily account for the higher triglyceride (TG) to phospholipid (PL) mass ratio in liver with cholesterol feeding. However, there was evidence for an increased formation of 1,2-diglyceride (1,2-DG). The complement of molecular species of hepatic 1,2-DG, phosphatidylcholine (PC), and phosphatidylethanolamine (PE) formed de novo, as measured using isotopic glycerol, was not influenced greatly by dietary cholesterol, although lower mean rates of synthesis of tetraenoic relative to dienoic species of phospholipids were indicated in cholesterol-fed gerbils.

The effects of dietary cholesterol on blood and liver polyunsaturated fatty acids and on plasma cholesterol in rats fed various types of fatty acid diet

Male rats were fed on a fat-free diet for 8 weeks and then switched to diets containing 10% hydrogenated coconut oil (HCO), safflower oil (SFO) or evening primrose oil (EPO). Half of each group was also given 1% of cholesterol in the diet. After 5 further weeks, plasma, red cell and liver fatty acids were measured in the various lipid fractions. Plasma and liver cholesterol also were estimated. In almost all fractions and on all three diets, feeding cholesterol led to accumulation of the substrates of desaturation reactions and to deficits of the products of these reactions. The results were consistent with inhibition of delta-6, delta-5 and delta-4 desaturation of n-6 essential fatty acids. Since the diets were deficient in n-3 fatty acids, levels were very low but were also consistent with inhibition of desaturation. In contrast, cholesterol had relatively less consistent effects on 20:3n-9, suggesting that desaturation of n-9 fatty acids was less inhibited. Plasma cholesterol levels rose sharply in the HCO and SFO groups but not at all in the EPO group. EPO contains the product of delta-6-desaturation, 18:3n-6, suggesting that conversion of linoleic acid to 18:3n-6 and possibly to further metabolites may be important for the cholesterol-lowering effect of polyunsaturates.

Effect of surfactant poloxalene 2930 on food intake, lipid absorption, and serum cholesterol in rats

Effect of hydrophobic surfactant, poloxalene 2930, on lipid absorption was studied in rats. Under acute conditions with surfactant infused intraduodenally with a lipid meal absorbed lipid accumulated abnormally in the enterocytes. This effect was quickly reversed after terminating treatment. Long-term administration of poloxalene given in semipurified diets resulted in changes in food intake, weight gain, fecal fat output, and serum cholesterol concentrations. The composition of the diet used as the vehicle for administration had a considerable effect on these results. When semipurified diets were used, food intake and weight gain were greatest when the dietary fat content was at the highest level. When the surfactant was given in ground chow, food intake was not affected and weight gain was only slightly, but significantly, less than the controls as a result of mild fat malabsorption. It is concluded that poloxalene 2930 affects lipid absorption, food intake, and serum cholesterol concentration but that results of this treatment are considerably affected by dietary factors.

Regulation of cholesterol synthesis in jejunal absorptive cells of the rat

Cholesterol metabolism was studied in jejunal mucosa of the rat with special emphasis on cholesterol synthesis of villous cells, the site of intestinal lipid absorption. The type of diet and nutritional state clearly affected the cholesterologenesis of villous cells. Consequently, the incorporation of 14C-acetate into nonsaponifiable lipids (NSL; includes squalene and sterols) decreased in the following order of magnitude: 1) fat-free diet supplemented with safflower oil (FFD-SO), 2) FFD alone, 3) standard rat chow, 4) 1% cholesterol in FFD-S(, 5) total fast. Both the cholesterol feeding and the fast increased the total cholesterol concentrations in the villous cells, but the concentrations were unaffected by the other diets. In rats fed FFD-SO diet the cholesterol synthesis was significantly higher in the villous than in the crypt cells, although the cellular cholesterol concentrations were similar. The hepatic cholesterol synthesis from 14C-acetate was low in rats fed FFD-SO as compared with chow diet, whereas the incorporation of 14C-acetate into hepatic fatty acids and the incorporation of 3H-mevalonate into NSL were not affected by the diet. 5% cholestyramine had no significant effect on the cholesterol synthesis or cholesterol concentrations of the villous cells during high (FFD-SO diet) or low cholesterol synthesis (chow or cholesterol feeding). FFD-SO increased serum total cholesterol compared with chow diet and total fast, whereas cholestyramine and cholesterol feeding had no effect. Intraperitoneal administration of the hypocholesterolemic agent, 4-aminopyrazolopyrimidine, to fasting rats decreased markedly crypt cell cholesterol and increased cholesterol synthesis in the crypt cells and in the villous cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dietary fat on bile salt synthesis in rat liver

The effect of dietary fat on conjugated cholic, chenodeoxycholic and tauro-beta-muricholic acid synthesis was studied using hepatocytes isolated from rats given a low-fat diet, or a low-fat diet mixed with 10% olive oil or 10% corn oil. The rats were totally biliary drained for 48 h prior to preparation of the cells in order to raise bile salt synthesis to a level which was measurable by radioimmunoassay. Synthesis of both conjugated cholic and chenodeoxycholic acid was raised in hepatocytes from rats given a fat supplement (either corn oil or olive oil) in the diet as compared to that in cells from low-fat-fed animals. Tauro-beta-muricholic acid synthesis, however, was unaffected by corn oil feeding. Production of conjugated cholic acid was increased to a greater extent when rats were given olive oil as opposed to corn oil, but these differences were not statistically significant. The conjugated cholic, chenodeoxycholic, and tauro-beta-muricholic acid and cholesterol content of bile collected at 2-h intervals during the biliary drainage of the same groups of rats was also determined. The pool size of both conjugated cholic and chenodeoxycholic acid in the enterohepatic circulation was found to be significantly decreased in rats given olive oil as compared to those given corn oil or the low-fat diet only. The pool size of tauro-beta-muricholic acid was also decreased in the olive oil-fed rats compared to the other two groups, but this difference was not statistically significant. After the pool had been drained out, animals which had received fat in the diet secreted more conjugated cholic and chenodeoxycholic acid into the bile than rats which had received the low-fat diet only. This effect was more marked when the fat given was olive oil rather than corn oil. Secretion of tauro-beta-muricholic acid into bile at this stage of biliary drainage was not changed by dietary fat supplements. Biliary cholesterol excretion was also increased in rats on diets containing 10% fat, with olive oil again having a greater effect than corn oil. The results show that supplementing the diet with fat leads to increased synthesis of conjugated cholic and chenodeoxycholic acids and biliary cholesterol secretion in the rat. The relatively more saturated fat, olive oil (85% oleate), gave a consistently larger increase than the more unsaturated, corn oil (50% linoleate), but the type of fat appeared less important than the presence of fat in the diet.

Cholesterol homeotasis in rats fed a purified diet

The rate of whole bodyb cholesterol synthesis was measured in male Sprague-Dawley rats fed either a standard chow, cereal-based diet or a semi-synthetic purified diet consisting of casein, sucrose and lard. The purified diet significantly decreased daily fecal excretion of neutral and acidic sterols, the specific acitvity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase, the bile acid pool size, and total daily cholesterol synthesis in the rat, while increasing plasma cholesterol concentrations and the total body content of cholesterol. The increased body content of cholesterol occurred primarily in muscle and connective tissue and not in the liver. The data demonstrate the importance of quantitating the net tissue accumulatin of cholesterol for accurate measurement of daily sterol synthesis in growing animals when sterol balance measurements are used. Tissue accumulation accounted for 7% of total daily cholesterol synthesis in rats fed the cereal diet, and 20% of daily synthesis in animals fed the purified diet.

Effect of cholesterol supplementation of diets of thyroidectomized dogs on the fatty acid composition of erythrocyte phospholipids and plasma phosphatidylcholine

Thyroidectomized foxhounds were fed: (a) a control diet, (b) the control diet supplemented with 15% beef tallow or cottonseed oil, with or without added cholesterol, or (c) the control diet supplemented with beef tallow (15%), safflower oil (1.5%) and cholesterol. After 23 weeks on the diet, the content of the individual phospholipids of the erythrocytes was not altered appreciably. However, supplementing either the saturated (beef tallow) or polyunsaturated (cottonseed oil) diets with cholesterol produced similar changes in the fatty acid compositions of the phospholipid classes. Most consistently, there were increases in the percentages on 18:2 omega 6 and 20:3 omega 6 and decreases in the percentages of 20:4 omega 6 and 22:4 omega 6. Cholesterol supplementation of the diets similarly affected the fatty acid composition of phosphatidylcholine isolated from the platelet-free plasma of these animals.

Dietary regulation of 3-hydroxy-3-methylglutaryl-CoA reductase from rat intestine

The effect of dietary cholesterol on rat intestinal 3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.34) varied depending upon whether animals received the dietary cholesterol with polyunsaturated or saturated fats. When cholesterol was fed with polyunsaturates, the enzyme activity in both the jejunum and ileum was significantly suppressed, whereas only the enzyme in the jejunum was significantly suppressed when cholesterol was given with saturated fats. It is concluded that dietary cholesterol has a negative feedback effect on intestinal cholesterol synthesis.

Effect of hydrophobic detergent on lipid absorption in the rat and on lipid and sterol balance in the swine

The effects of hydrophobic detergent on fat absorption in the rat were determined under two conditions. In the first, a high dose of detergent was given in a test lipid meal to rats not previously exposed to this agent. A marked delay in digestion of triglyceride in association with malabsorption was observed. In the second, a relatively small dose of detergent was given to rats pretreated with dietary supplement of detergent. No delay of digestion or uptake was observed but absorbed, reesterified lipid was noted to accumulate in the mucosa. Morphologic studies showed abnormal collections of fat droplets in the enterocytes. Sterol and fat balance studies were done on swine on chronic dietary detergent supplement. Mild steatorrhea with excess fecal excretion of neutral sterols was observed. It is concluded that hydrophobic detergents can have an inhibitory effect on both intraluminal and intracellular events of fat absorption.