Cholesterol dynamics in rats fed cis- and trans-octadecenoate in the form of triglyceride

Unavailability of liver triacylglycerol increases serum cholesterol concentration induced by dietary cholesterol in exogenously hypercholesterolemic (ExHC) rats

BACKGROUND

Exogenously hypercholesterolemic (ExHC) rats develop hypercholesterolemia and low hepatic triacylglycerol (TAG) levels when dietary cholesterol is loaded. The responsible gene Smek2 was identified via linkage analysis using the original strain Sprague-Dawley (SD) rats. In this study, we compared SD and ExHC rats to investigate a relationship between hypercholesterolemia and the low hepatic TAG levels observed in ExHC rats.

METHODS

Male 4-weeks-old ExHC and SD rats were fed a 1% cholesterol diet for 1 week. Serum and liver parameters were analyzed. Gene expression and enzyme activities related to TAG metabolism were also assessed.

RESULTS

We reproducibly observed higher serum cholesterol and lower hepatic TAG levels in ExHC rats than in SD rats. Golgi apparatus in the livers of ExHC rats secreted β-very-low-density lipoprotein (β-VLDL) that had higher cholesterol ester (CE) and lower TAG content than those in the β-VLDL secreted by SD rats. Gene expression related to fatty acid and TAG synthesis in ExHC rats was lower than that in SD rats. Enzymatic activities for fatty acid synthesis were also relatively lower in ExHC rats. Moreover, the fatty acid composition of hepatic and serum CE in ExHC rats showed that these CEs were not modified after secretion from the liver despite the similar activities of serum lecithin-cholesterol acyltransferase (LCAT) in ExHC rats to those in SD rats.

CONCLUSIONS

Low production of liver TAG and secretion of CE-rich, TAG-poor β-VLDL without modification by LCAT in the circulation contributed to hypercholesterolemia induced by dietary cholesterol in ExHC rats.

Trans fatty acids affect lipoprotein metabolism in rats

This study was designed to investigate the effects of oleic (CIS), palmitic (SAT) and trans fatty acids (TRANS) on cholesterol metabolism. Rats fed the TRANS diet had lower plasma total cholesterol (P < 0.005) and non-HDL-cholesterol (non HDL-C) concentrations (P < 0.005) compared with their CIS-fed counterparts. Plasma HDL-C was highest in rats fed the SAT diet (P = 0.01). An in vivo assay of reverse cholesterol transport (RCT) was performed whereby radiolabeled cholesterol was delivered to the liver as acetylated LDL and the reappearance of label into plasma and HDL was determined. Plasma radioactivity in TRANS-fed rats was lower than in their SAT-fed counterparts (P = 0.01), and consistent with the cholesterol distribution in plasma, the difference was due to lower [(3)H]-cholesterol in lower density lipoproteins. Despite diet-induced differences in the cholesterol and phospholipid concentrations and fatty acid composition of HDL, the amount of label in HDL did not differ among groups, suggesting that consumption of these diets resulted in HDL populations with similar capacity to participate in RCT. The present findings suggest that dietary trans fatty acids regulate the metabolism of apolipoprotein B-containing lipoproteins in rats and that the effect may be masked in species possessing high plasma cholesteryl ester transfer protein (CETP) activity. These results reinforce the important role of CETP activity in determining the distribution of plasma cholesterol in response to dietary trans fatty acids.

Dietary gamma-linolenic acid dose-dependently modifies fatty acid composition and immune parameters in rats

gamma-linolenic acid (GLA) has been reported to improve several inflammatory disorders through regulation of eicosanoid production. However, since GLA is a precursor of arachidonic acid, it may bring about increasing tissue arachidonic acid levels with subsequent pro-inflammatory events. To explore this possibility, we examined the effect of high-dose GLA acid on the fatty acid profile of immune cells, leukotriene B4 production by peritoneal exudate cells and immunoglobulin productivity of mesenteric lymph node lymphocytes of Sprague-Dawley rats. Male rats were fed 10% fat diets containing graded levels, 0, 20, 40 and 60% of GLA for 3 weeks. The results showed the distinction in activity of metabolizing GLA between immune cells and liver. Thus, in immune cells such as mesenteric lymph node and spleen lymphocytes and peritoneal exudate cells, more dihomo-gamma-linolenic acid was found than in the liver. Leukotriene B4 production by peritoneal exudate cells was significantly suppressed when fed the highest level of GLA suggesting a lower risk of allergic reaction. Moreover, immunoglobulin productivity in mesenteric lymph node lymphocytes was promoted by dietary GLA. The present study indicates that a high dose of GLA may exert anti-inflammatory effects through suppression of leukotriene B4 release and strengthening of gut immune system, thus ameliorating allergic reaction.

A buckwheat protein product suppresses gallstone formation and plasma cholesterol more strongly than soy protein isolate in hamsters

This study was conducted to investigate the effects of a buckwheat protein product (BWP) on plasma cholesterol, gallbladder bile composition and fecal steroid excretion in hamsters fed diets with 5 g/kg cholesterol. Diets also contained 200 g/kg of casein, soy protein isolate (SPI) or BWP as protein sources. After 2 wk, plasma and liver concentrations of cholesterol in the hamsters fed BWP were significantly lower than those in the hamsters fed casein and SPI. The molar proportion of cholesterol in gallbladder bile was significantly lower in the BWP group than in the other groups, whereas that of bile acids was slightly higher in the BWP group (P Collapse

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MESH Headings

• Animals
• Bile/chemistry
• Cholelithiasis/pathology
• Cholesterol/blood
• Cholesterol, Dietary/pharmacology
• Cricetinae
• Energy Intake/drug effects
• Fagopyrum
• Feces/chemistry
• Lipid Metabolism
• Lipids/blood
• Liver/metabolism
• Male
• Mesocricetus
• Plant Proteins/pharmacology
• Glycine max
• Weight Gain/drug effects

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Affiliation(s)

H Tomotake Development, Health Care, Kissei Pharmaceutical Company, Ltd., Matsumoto 399-8710, Japan
I Shimaoka
J Kayashita
F Yokoyama
M Nakajoh
N Kato

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Gemfibrozil reduces non-high-density lipoprotein cholesterol in exogenously hypercholesterolemic (ExHC) rats fed a high-cholesterol diet

Gemfibrozil is a widely used drug prescribed to elevate serum high-density lipoprotein (HDL) cholesterol levels and lower triacylglycerols. The present study was done to determine if the drug would alleviate hypercholesterolemia in exogenously hypercholesterolemic (ExHC) rats. In the drug-treated ExHC rats, the serum non-HDL cholesterol levels were lowered and the ratio of the non HDL cholesterol to serum triacylglycerols was decreased to the extent seen in the drug-treated SD rats. Liver cholesterol and triacylglycerols were lowered in the drug-treated rats. There was also an increase in fecal excretion of neutral sterols and bile acids, particularly chenodeoxycholic and beta-muricholic acids. The drug elevated cholesterol 7 alpha-hydroxylase activity and mRNA abundance and acyl-CoA cholesterol acyltransferase activity in the liver, but did not influence low-density lipoprotein receptor mRNA level in the liver. Thus, gemfibrozil is effective in alleviating hypercholesterolemia in exogenously hypercholesterolemic rats, by partitioning hepatic cholesterol into biliary excretion.

Linoleic and alpha-linolenic acids differently modify the effects of elaidic acid on polyunsaturated fatty acid metabolism and some immune indices in rats

To explore whether the metabolic responses to trans, compared with cis, fatty acids depend on the source of dietary polyunsaturated fatty acids (PUFA), male Sprague-Dawley rats, 5 weeks old, were fed on diets containing 30 g oleic (cis) or elaidic (trans) acids/kg in combination with either 70 g perilla oil (alpha-linolenic acid) or safflowerseed oil (linoleic acid)/kg for 3 weeks in separate experiments. The dietary fats were adjusted to have the same level of total PUFA. The dietary manipulation did not influence the growth indices, but spleen weight was greater when the dietary PUFA source was perilla oil. The incorporation of trans fatty acid into liver phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine and adipose tissue lipids, particularly phospholipids, was significantly higher when rats were fed on safflowerseed oil compared with perilla oil. However, only limited differences were observed in the effects of cis and trans fatty acids on the proportions of PUFA in liver phospholipids. Splenic production of prostaglandin E2 was reduced by trans fatty acid when safflowerseed oil was the PUFA source, but no trans effect was observed on leukotriene C4 production. Dietary PUFA significantly influenced the concentration of plasma immunoglobulins (Ig) but the effect of geometry was only seen in IgG which was increased by trans acid. Dietary trans fatty acid increased the CD4+:CD8+ T-lymphocyte ratio in the spleen, reflecting a decreasing trend of the proportion of CD8+, when combined with perilla oil. These observations indicate that the type of PUFA simultaneously ingested specifically influences the effect that trans acid exerts on PUFA metabolism, eicosanoid production and some immune indices.

Effects of randomization of partially hydrogenated corn oil on fatty acid and cholesterol absorption, and tissue lipid levels in rats

Randomization of partially hydrogenated corn oil containing approximately 45% of trans octadecenoic acid only slightly, but not significantly, increased the lymphatic fatty acid absorption in rats. No effect of randomization was observed on cholesterol absorption. When rats were fed these fats at the 8.8% level (with 1.2% safflower oil) for three weeks, the concentrations of serum cholesterol, and serum and liver phospholipid were significantly higher in randomized fat than in control fat, which was composed of 9% high-oleic safflower oil and 1% palm oil. Liver cholesterol tended to be higher in randomized fat. In contrast, nonrandomized fat was not hyperlipidemic compared to control fat. Although the fatty acid composition of liver phospholipids suggested a possible interference of trans fatty acid with the metabolism of linoleic acid to arachidonic acid, there was no effect of randomization. In the two hydrogenated fat groups, trans octadecenoic acid was incorporated and distributed similarly in adipose tissue triacylglycerol. These observations indicated that randomization of partially hydrogenated fat is not beneficial to various lipid parameters in rats.

Effects of oleic and elaidic acids on in vitro intestinal uptake of cholesterol in the rat

The effects of oleic and elaidic acid upon the in vitro intestinal uptake of cholesterol were studied in 9 weeks-old male and female rats. Elaidic acid increases the uptake of cholesterol when compared to oleic acid in both males and females. It is suggested that elaidic acid may enhance the esterification of cholesterol within the enterocyte through being more available for esterification and/or through being preferentially incorporated into cholesterol esters.

Effect of trans fatty acids on plasma lipids, platelet function and systolic blood pressure in stroke-prone spontaneously hypertensive rats

To investigate the effect of trans fatty acids on plasma lipid levels and systolic blood pressure, hydrogenated corn oil was fed to SHRSP (stroke-prone spontaneously hypertensive rats) and WKY (Wistar-Kyoto) rats for 30 days. Significantly lower systolic blood pressure and plasma total cholesterol were observed in SHRSP rats fed trans fatty acids when compared with rats fed cis fatty acids from olive oil. In addition, higher HDL cholesterol and lower VLDL plus chylomicron cholesterol levels were found in SHRSP rats fed trans fatty acids. Although no significant changes of systolic blood pressure and plasma total cholesterol levels were observed in WKY rats after trans fatty acids treatment, WKY rats fed trans fatty acids had lower plasma LDL cholesterol and higher HDL cholesterol levels. In addition, platelet aggregation induced by collagen was decreased in WKY rats fed trans fatty acids. It is interesting that trans fatty acids increased the activity of plasma lecithin:cholesterol acyltransferase (LCAT) in both SHRSP and WKY rats. The observed influence of trans fatty acids on plasma lipid levels, systolic blood pressure and platelet aggregation suggests that trans fatty acids might prevent thrombotic disorders in SHRSP rats.

Cholesterol-lowering activity of various undigested fractions of soybean protein in rats

The undigested high-molecular-weight fraction (HMF) of soybean protein prepared after exhaustive digestion by microbial proteases significantly decreased serum cholesterol levels to approximately 45% (p less than 0.05) of those observed with the parent protein in rats fed diets containing cholesterol (0.5%) and sodium cholate (0.125%). HMF bound conjugated bile salts in vitro and significantly increased fecal excretion of both neutral and acidic steroids by 65-95% and 80-170% more, respectively (p less than 0.05), than did the intact protein. Extraction of HMF with methanol slightly decreased the activity, but the methanol-soluble fraction was not regarded as a principal determinant. Soybean saponin at the dietary level equivalent to that contained in HMF did not effectively lower serum cholesterol. The activity was not necessarily duplicated even when methanol-treated fractions were recombined. Further degradation of the methanol-extracted HMF by various proteases resulted in loss of activity. HMF obtained after pepsin digestion exerted a potential similar to that of HMF prepared after digestion by microbial proteases.

Influence of dietary cis and trans fats on DMH-induced colon tumors, steroid excretion, and eicosanoid production in rats prone to colon cancer

The effect of geometrical isomerism of dietary fats on colon tumorigenesis was studied in male and female rats of a strain prone to colon cancer (Wistar-Furth-Osaka). The rats were fed purified diets containing either partially hydrogenated corn oil (trans fat) or high-oleic safflower (cis fat) at the 5% level for one week and received a single oral dose of 1,2-dimethylhydrazine. The difference in the fatty acid composition of dietary fats was confined solely to the geometry of octadecenoate. An appropriate level of linoleic acid (2% of total energy) was supplied. After about 60 weeks, neither fat-type nor sex-dependent differences in the incidence of colon and small intestinal tumors was observed. The fecal excretion of neutral but not acidic steroids was higher in male rats fed the trans fat than in those fed the cis fat, but the composition remained almost unchanged. Aortic production of prostacyclin and the plasma concentration of thromboxane B2 were not influenced by dietary fats, although these were significantly higher in females, irregardless of the fat source. Thus, trans fat behaved much like the cis fat in various parameters, except for steroid excretion.

Age-related changes in lipid metabolism in rats: the consequence of moderate food restriction

Male Sprague-Dawley rats, 3 weeks of age, received one of the four following dietary manipulations until 9 months of age: group AD, fed ad libitum; group R, restricted to 75% of food intake of group AD; group RAD, restricted until 5 months of age and then fed ad libitum; group ADR, fed ad libitum until 5 months of age and then restricted. The concentration of serum total and HDL cholesterol tended to be higher in group AD than in all groups that had experienced food restriction. Liver cholesterol was higher in groups AD and RAD than in groups R and ADR. Activity of hepatic microsomal hydroxymethylglutaryl-CoA reductase was comparable, whereas sterol synthesis from mevalonate was significantly higher in groups R and ADR than in groups AD and RAD. Cholesterol 7 alpha-hydroxylase activity also tended to be higher in groups R and ADR. It seems likely that hepatic cholesterol homeostasis functions effectively even when moderate food restriction was started after the growing period. In addition, food restriction reduced the ratio of arachidonate to linoleate, suggesting inhibition of the desaturation system.

The hypocholesterolemic action of the undigested fraction of soybean protein in rats

Soybean protein was exhaustively digested with endo- and exo-type microbial proteases and the effect of the digestible low molecular fraction (LMF) and the undigested high molecular fraction (HMF) on the serum cholesterol level was compared to that of the intact protein in rats given a cholesterol-enriched diet. The HMF, peptides relatively abundant in hydrophobic amino acids, was found to be substantially hypocholesterolemic when fed at the nitrogen level equivalent to that of the 20% soybean protein diet, and not only serum but also liver cholesterol levels were similar to those usually encountered in rats given diets free of cholesterol. There was a dose-dependent reduction of serum and liver cholesterol when casein was replaced stepwise with HMF. The cholesterol-lowering action could be attributable to an increased fecal steroid excretion.

Age-related changes in the regulation of cholesterol metabolism in rats

Activities of the hepatic cholesterol synthetic system including initial steps of the pathway and cholesterol 7 alpha-hydroxylase were all lower in adult (8 to 9-month-old) rats than in young (5 week-old) rats. The extent of diurnal fluctuation of 3-hydroxy-3-methylglutaryl coenzyme A reductase was, however, apparently greater in adult animals. When the cholesterol-enriched diet was fed to rats for 1 day, the extent of the depression of the cholesterogenic enzymes was dependent on age of animals. The enzyme activities rapidly increased on refeeding a cholesterol-free diet after the cholesterol challenge. In young rats the activity of cholesterol 7 alpha-hydroxylase exhibited a pattern inverse to that of HMG-CoA reductase whereas in adult rats it increased continuously during the entire experimental period. Cholesterol and triglyceride accumulated in the liver of adult animals, and their response to dietary cholesterol also depended on the age of the animals. The results indicate a specific modification of the cholesterol homeostatic mechanism with age.

Activities of liver mitochondrial and peroxisomal fatty acid oxidation enzymes in rats fed trans fat

The effect of trans fat on the activities of liver mitochondrial and peroxisomal fatty acid oxidation enzymes was examined in various strains of rats. When Wistar and Sprague-Dawley rats were fed for 30 days diets containing either olive oil or partially hydrogenated corn oil as a source of cis- or trans-octadecenoate, respectively, the activities of various enzymes of mitochondrial and peroxisomal beta-oxidation measured with cis- and trans-9-octadecenoic acid as substrates showed little dietary fat-dependent change. In Fischer 344 rats, feeding trans fat for 15 mo increased only moderately various enzymes of beta-oxidation except for carnitine acyltransferase. The rate of mitochondrial ketogenesis and the activity of carnitine acyltransferase measured with trans-9-octadecenoic acid as a substrate were about half those with the cis-counterpart. Peroxisomes oxidized trans-9-octadecenoyl-CoA at a rate comparable to the cis-counterpart. It was concluded from this study and previous ones that the difference in the geometry of dietary fatty acid had only a marginal effect in modulating the hepatic fatty acid oxidation system, in spite of marked differences in the metabolic behavior of cis- and trans fatty acid in cell-free preparations and perfused liver.

Activities of liver mixed function oxidase system in rats fed trans fat

Rats were fed diets containing either camellia oil or partially hydrogenated corn oil as a source of cis or trans octadecenoate, respectively, in the presence of adequate linoleic acid. After 35 days of feeding the diets, activities of several hepatic drug metabolizing enzymes as well as the content of hepatic microsomal cytochrome P-450 were determined. Geometrical difference in the dietary fat did not affect the amount of microsomal protein nor the content of cytochrome P-450. Also, activities of NADPH cytochrome C reductase, aminopyrine N-demethylase and biphenyl hydroxylase were approximately the same between two groups of rats. Aniline hydroxylase was slightly elevated in the rats fed trans fat. It was concluded that the difference in the geometry of dietary fatty acids had little effect in modulating the hepatic mixed function oxidase system.