Effect of n-3 fatty acids on the key enzymes involved in cholesterol and triglyceride turnover in rat liver

Regulation of cholesterol metabolism: New players for an old physiological process

Identified more than two centuries ago, cholesterol plays a pivotal role in human physiology. Since cholesterol metabolism is a physiologically significant process, it is not surprising that its alterations are associated with several pathologies. The discovery of new molecular targets or compounds able to modulate this sophisticated metabolism has been capturing the attention of research groups worldwide since many years. Endogenous and exogenous compounds are known to regulate cellular cholesterol synthesis and uptake, or reduce cholesterol absorption at the intestinal level, thereby regulating cholesterol homeostasis. However, there is a great need of new modulators and diverse new pathways have been uncovered. Here, after illustrating cholesterol metabolism and its well-known regulators, some new players of this important physiological process are also described.

A Mini-Review on the Effect of Docosahexaenoic Acid (DHA) on Cerulein-Induced and Hypertriglyceridemic Acute Pancreatitis

Acute pancreatitis refers to the sudden inflammation of the pancreas. It is associated with premature activation and release of digestive enzymes into the pancreatic interstitium and systemic circulation, resulting in pancreatic tissue autodigestion and multiple organ dysfunction, as well as with increased cytokine production, ultimately leading to deleterious local and systemic effects. Although mechanisms involved in pathogenesis of acute pancreatitis have not been completely elucidated, oxidative stress is regarded as a major risk factor. In human acute pancreatitis, lipid peroxide levels in pancreatic tissues increase. Docosahexaenoic acid (DHA), an omega-3 polyunsaturated fatty acid (C22:6n-3), exerts anti-inflammatory and antioxidant effects on various cells. Previous studies have shown that DHA activates peroxisome proliferator-activated receptor-γ and induces catalase, which inhibits oxidative stress-mediated inflammatory signaling required for cytokine expression in experimental acute pancreatitis using cerulein. Cerulein, a cholecystokinin analog, induces intra-acinar activation of trypsinogen in the pancreas, which results in human acute pancreatitis-like symptoms. Therefore, DHA supplementation may be beneficial for preventing or inhibiting acute pancreatitis development. Since DHA reduces serum triglyceride levels, addition of DHA to lipid-lowering drugs like statins has been investigated to reduce hypertriglyceridemic acute pancreatitis. However, high DHA concentrations increase cytosolic Ca²⁺, which activates protein kinase C and may induce hyperlipidemic acute pancreatitis. In this review, effect of DHA on cerulein-induced and hypertriglyceridemic acute pancreatitis has been discussed. The relation of high concentration of DHA to hyperlipidemic acute pancreatitis has been included.

Dietary DHA: time course of tissue uptake and effects on cytokine secretion in mice

Consumption of DHA has numerous beneficial effects, but little is known about these effects during the first few days of the DHA dietary intake. The main objectives of the present study were to determine the time course of DHA incorporation into phospholipids in different mouse tissues and the effects of DHA supplementation on adiponectin and leptin secretion. Mice were fed either a control diet or a DHA-rich diet, and some were killed on days 0, 4, 8, 16 and 32. Some mice were fed the DHA-rich diet for 16 d, and were then maintained on the control diet for sixteen more days (washout period). DHA supplementation increased plasma adiponectin secretion by 2·4-fold as early as 4 d after the initiation of the DHA-rich diet feeding. The adiponectin concentration remained 1·6-fold higher after the 16 d washout period. Plasma leptin levels were significantly lower after 4 d of feeding with DHA. These effects were associated with a significant increase in DHA incorporation in phosphatidylethanolamine and phosphatidylcholine of all analysed tissues (liver, heart and white adipose tissues). DHA mainly got incorporated at the expense of n-6 arachidonic acid. The present data show that DHA rapidly improved the profile of secreted adipokines, and that these protective effects were long lasting.

Effects of three different highly purified n-3 series highly unsaturated fatty acids on lipid metabolism in C57BL/KsJ-db/db mice

Triglycerides (TG) consisting of highly purified (>97%) n-3 series highly unsaturated fatty acids, eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA), were administered to C57BL/KsJ-db/db mice for 4 weeks by pair-feeding to compare their effects on lipid metabolism and to evaluate the effects of DPA on lipid metabolism. The hepatic TG level and total amount was decreased by treatment with DHA and DPA compared to the control. The efficacy of DPA was greater than that of EPA, but less than that of DHA. In contrast, EPA had the greatest serum TG reducing effect. The hepatic cytosol fraction of the DHA-treated group contained the lowest fatty acid synthase (FAS) and malic enzyme (ME) activity levels. Furthermore, the DHA-treated group contained the highest serum adiponectin concentrations. These findings indicate that the strong hepatic TG-lowering effect of DHA is due to the suppression of TG synthesis. The same tendencies were observed in DPA-treated mice, and the effect was stronger than that observed in EPA-treated mice, but equivalent to that observed in DHA-treated mice. Based on these results, DPA possesses lipid metabolism-improving effects. The beneficial effects of DPA for lipid metabolism were not superior to those of EPA and DHA, and the effect was always intermediate between those of EPA and DHA.

Bio-availability and metabolism of n-3 fatty acid rich garden cress (Lepidium sativum) seed oil in albino rats

The ratio of fatty acids namely linoleic acid (LA, 18:2, n-6) and alpha linolenic acid (ALA, 18:3, n-3) in the diet plays an important role in enrichment of ALA in tissues and further conversion to long-chain polyunsaturated fatty acids (LC-PUFA) like eicosapentaenoic acid (EPA, 20:5, n-3) and docosahexaenoic acid (DHA, 22:6, n-3). Garden cress seed oil (GCO) is one of the richest sources of omega-3 fatty acid and contains 29-34.5% of ALA. In this study, dietary supplementation of GCO on bio-availability and metabolism of alpha-linolenic acid was investigated in growing rats. Male wistar rats were fed with semi-purified diets supplemented with 10.0% sunflower oil (SFO 10%); 2.5% GCO and 7.5% SFO (GCO 2.5%); 5% GCO and 5% SFO (GCO 5.0%); 10% GCO (GCO 10%) for a period of 8 weeks. There was no significant difference with regard to the food intake, body weight gain and organ weights of rats in different dietary groups. Rats fed with GCO showed significant increase in ALA levels in serum and tissues compared to SFO fed rats. Feeding rats with 10% GCO lowered hepatic cholesterol by 12.3% and serum triglycerides by 40.4% compared to SFO fed group. Very low density lipoprotein cholesterol (VLDL-C) and low density lipoprotein cholesterol (LDL-C) levels decreased by 9.45% in serum of 10% GCO fed rats, while HDL remained unchanged among GCO fed rats. Adipose tissue showed incorporation of 3.3-17.4% of ALA and correlated with incremental intake of ALA. Except in adipose tissue, the EPA, DHA levels increased significantly in serum, liver, heart and brain tissues in GCO fed rats. A maximum level of DHA was registered in brain (11.6%) and to lesser extent in serum and liver tissues. A significant decrease in LA and its metabolite arachidonic acid (AA) was observed in serum and liver tissue of rats fed on GCO. Significant improvement in n-6/n-3 fatty acid ratio was observed in GCO based diets compared to diet containing SFO. This is the first study to demonstrate that supplementation of GCO increases serum and liver ALA, EPA, DHA and decreases LA and AA in rats. Therefore, the GCO can be considered as a potential, alternate dietary source of ALA.

Why do omega-3 fatty acids lower serum triglycerides?

PURPOSE OF REVIEW

Fish oils rich in n-3 fatty acids reduce serum triglyceride levels. This well known effect has been shown to be caused by decreased very low-density lipoprotein triglyceride secretion rates in kinetic studies in humans. Animal studies have explored the biochemical mechanisms underlying this effect. Triglyceride synthesis could be reduced by n-3 fatty acids in three general ways: reduced substrate (i.e. fatty acids) availability, which could be secondary to increase in beta-oxidation, decreased free fatty acids delivery to the liver, decreased hepatic fatty acids synthesis; increased phospholipid synthesis; or decreased activity of triglyceride-synthesizing enzymes (diacylgylcerol acyltranferase or phosphatidic acid phosphohydrolase).

RECENT FINDINGS

Rarely were experimental conditions used in rat studies physiologically relevant to the human situation in which 1.2% energy as n-3 fatty acids lowers serum triglyceride levels. Nevertheless, the most consistent effect of n-3 fatty acids feeding in rats is to decrease lipogenesis. Increased beta-oxidation was frequently, but not consistently, reported with similar numbers of studies reporting increased mitochondrial compared with peroxisomal oxidation. Inhibition of triglyceride-synthesizing enzymes was only occasionally noted.

SUMMARY

As the vast majority of studies fed unphysiologically high doses of n-3 fatty acids, these findings in rats must be considered tentative, and the mechanism by which n-3 fatty acids reduce triglyceride levels in humans remains speculative.

Dietary factors and the regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase: Implications for breast cancer development

A role for mevalonate in cancer development has long been suggested by findings that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity is elevated in malignant cells. Increased synthesis mevalonate and mevalonate-derived nonsterol isoprenoids supports increased cell proliferation through the activation of growth-regulatory proteins and oncoproteins, and by promoting DNA synthesis. We have recently shown that mevalonate promotes the growth of human breast cancer cells both in culture and as tumors grown in nude mice. Inhibition mevalonate synthesis, therefore, may be an effective strategy to impair the growth of malignant breast cells. Several dietary compounds with known anti-cancer effects are also reported to inhibit HMG-CoA reductase activity. Here, we review evidence suggesting that inhibition of mevalonate synthesis may mediate the protective effects of cholesterol, plant isoprenoids, genistein, and long-chain n-3 polyunsaturated fatty acids (PUFAs) on experimental breast cancer.

The role of acyl-CoA:diacylglycerol acyltransferase (DGAT) in energy metabolism

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC2.3.1.20), a key enzyme in triglyceride (TG) biosynthesis, not only participates in lipid metabolism but also influences metabolic pathways of other fuel molecules. Changes in the expression and/or activity levels of DGAT may lead to changes in systemic insulin sensitivity and energy homeostasis. The synthetic role of DGAT in adipose tissue, the liver, and the intestine, sites where endogenous levels of DGAT activity and TG synthesis are high, is relatively clear. Less clear is whether DGAT plays a mediating or preventive role in the development of ectopic lipotoxicity in tissues such as muscle and the pancreas, when their supply of free fatty acids (FFAs) exceeds their needs. Future studies with tissue-specific overexpression and/or knockout in these animal models would be expected to shed additional light on these issues.

The effects of dietary n-3 polyunsaturated fatty acids delivered in chylomicron remnants on the transcription of genes regulating synthesis and secretion of very-low-density lipoprotein by the liver: modulation by cellular oxidative state

The influence of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) (derived from fish or corn oil, respectively) on the expression of mRNA for four genes involved in the regulation of the synthesis, assembly, and secretion of very-low-density lipoprotein (VLDL) in the liver was investigated in normal rat hepatocytes and after manipulation of the cellular oxidative state by incubation with N-acetyl cysteine (NAC) or CuSO(4). The four genes investigated were those encoding apolipoprotein B (apoB), the microsomal triacylglycerol transfer protein (MTP), and the enzymes acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2), which play a role in the regulation of triacylglycerol and cholesteryl ester synthesis, respectively. mRNA levels for apoB, MTP, and DGAT were unaffected by either fish or corn oil chylomicron remnants, but the amount of ACAT2 mRNA was significantly reduced after incubation of the hepatocytes with fish oil remnants as compared with corn oil remnants or without remnants. These findings indicate that the delivery of dietary n-3 PUFA to hepatocytes in chylomicron remnants downregulates the expression of mRNA for ACAT2, and this may play a role in their inhibition of VLDL secretion. However, when the cells were shifted into a pro-oxidizing or pro-reducing state by pretreatment with CuSO(4) (1 mM) or NAC (5 mM) for 24 hr, levels of mRNA for MTP were increased by about 2- or 4-fold, respectively, by fish oil remnants, whereas corn oil remnants had no significant effect. Fish oil remnants also caused a smaller increase in apoB mRNA in comparison with corn oil remnants in NAC-treated cells (+38%). These changes would be expected to lead to increased VLDL secretion rather than the decrease associated with dietary n-3 PUFA in normal conditions. These findings suggest that relatively minor changes in cellular redox levels can have a major influence on important liver functions such as VLDL synthesis and secretion.

Increase in hepatic expression of SREBP-2 by gemfibrozil administration to rats

It is well known that gemfibrozil increases the biliary output of cholesterol and phospholipids, but we have little knowledge about the impact these changes have on liver cholesterol and phospholipid biosynthetic pathways. In the present study, no changes were detected in liver lipids and CTP:phosphocholine cytidylyltransferase after gemfibrozil administration to rats. On the contrary, 3-hydroxy-3-methylglutaryl-CoA reductase mRNA (9.9-fold) and Rd activity (16.7-fold) and phosphatidate phosphohydrolase activity (1.7-fold) increased, while plasma apo B-cholesterol (40%) and triglyceride (43%) levels decreased. As a part of a compensatory homeostatic response, we report for the first time that gemfibrozil administration to rats increased the hepatic sterol regulatory element binding protein-2 (SREBP-2) mRNA (2.9-fold) and mature protein (2.2-fold) levels. An early increase in the transcriptional activity of SREBP-2 elicited by gemfibrozil administration might be responsible for the observed changes in HMG-CoA reductase, phosphatidate phosphohydrolase, and SREBP-2 expression.

Response of plasma lipids to dietary cholesterol and wine polyphenols in rats fed polyunsaturated fat diets

This experiment was designed to evaluate the effects of dietary red wine phenolic compounds (WP) and cholesterol on lipid oxidation and transport in rats. For 5 wk, weanling rats were fed polyunsaturated fat diets (n-6/n-3 = 6.4) supplemented or not supplemented with either 3 g/kg diet of cholesterol, 5 g/kg diet of WP, or both. The concentrations of triacylglycerols (TAG, P < 0.01) and cholesterol (P < 0.0002) were reduced in fasting plasma of rats fed cholesterol despite the cholesterol enrichment of very low density lipoprotein + low density lipoprotein (VLDL + LDL). The response was due to the much lower plasma concentration of high density lipoprotein (HDL) (-35%, P < 0.0001). In contrast, TAG and cholesteryl ester (CE) accumulated in liver (+120 and +450%, respectively, P < 0.0001). However, the cholesterol content of liver microsomes was not affected. Dietary cholesterol altered the distribution of fatty acids mainly by reducing the ratio of arachidonic acid to linoleic acid (P < 0.0001) in plasma VLDL + LDL (-35%) and HDL (-42%) and in liver TAG (-42%), CE (-78%), and phospholipids (-28%). Dietary WP had little or no effect on these variables. On the other hand, dietary cholesterol lowered the alpha-tocopherol concentration in VLDL + LDL ( -40%, P < 0.003) and in microsomes (-60%, P < 0.0001). In contrast, dietary WP increased the concentration in microsomes (+21%, P < 0.0001), but had no effect on the concentration in VLDL + LDL. Cholesterol feeding decreased (P < 0.006) whereas WP feeding increased (P < 0.0001) the resistance of VLDL + LDL to copper-induced oxidation. The production of conjugated dienes after 25 h of oxidation ranged between 650 (WP without cholesterol) and 2,560 (cholesterol without WP) micromol/g VLDL + LDL protein. These findings show that dietary WP were absorbed at sufficient levels to contribute to the protection of polyunsaturated fatty acids in plasma and membranes. They could also reduce the consumption of alpha-tocopherol and endogenous antioxidants. The responses suggest that, in humans, these substances may be beneficial by reducing the deleterious effects of a dietary overload of cholesterol.

Eicosapentaenoic and docosahexaenoic acid affect mitochondrial and peroxisomal fatty acid oxidation in relation to substrate preference

Decreased triacylglycerol synthesis within hepatocytes due to decreased diacylglycerol acyltransferase (DGAT) activity has been suggested to be an important mechanism by which diets rich in fish oil lower plasma triacylglycerol levels. New findings suggest that eicosapentaenoic acid (EPA), and not docosahexaenoic acid (DHA), lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation and decreased availability of fatty acids for triacylglycerol synthesis. To contribute to the understanding of the triacylglycerol-lowering mechanism of fish oil, the different metabolic properties of EPA and DHA were studied in rat liver parenchymal cells and isolated rat liver organelles. EPA-CoA was a poorer substrate than DHA-CoA for DGAT in isolated rat liver microsomes, and in the presence of EPA, a markedly lower value for the triacyl[3H]glycerol/diacyl[3H]glycerol ratio was observed. The distribution of [1-14C]palmitic acid was shifted from incorporation into secreted glycerolipids toward oxidation in the presence of EPA (but not DHA) in rat liver parenchymal cells. [1-14C]EPA was oxidized to a much greater extent than [1-14C]DHA in rat liver parenchymal cells, isolated peroxisomes, and especially in purified mitochondria. As the oxidation of EPA was more effective and sensitive to the CPT-I inhibitor, etomoxir, when measured in a combination of both mitochondria and peroxisomes, we hypothesized that both are involved in EPA oxidation, whereas DHA mainly is oxidized in peroxisomes. In rats, EPA treatment lowered plasma triacylglycerol and increased hepatic mitochondrial fatty acid oxidation and carnitine palmitoyltransferase (CPT)-I activity in both the presence and absence of malonyl-CoA. Whereas only EPA treatment increased the mRNA levels of CPT-I, DHA treatment increased the mRNA levels of peroxisomal fatty acyl-CoA oxidase and fatty acid binding protein more effectively than EPA treatment. In conclusion, EPA and DHA affect cellular organelles in relation to their substrate preference. The present study strongly supports the hypothesis that EPA, and not DHA, lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation.

Selectivity of fatty acids on lipid metabolism and gene expression

Triacylglycerols represent the main form of storage for a wide spectrum of fatty acids. Their utilization first involves mobilization from adipose tissue through lipolysis. The release of individual fatty acids from adipose tissue is selective in vitro and in vivo in animal studies and also in human subjects. Generally, fatty acids are more readily mobilized from fat cells when they are short-chain and unsaturated. This selectivity could affect the storage of individual fatty acids in adipose tissue, and their subsequent supply to tissues. The nature of the dietary fats could affect lipid homeostasis and body fat deposition. Dietary fish oil influences adipose tissue development in a site-specific manner as a function of diet and feeding period. A diet high in n-3 polyunsaturated fatty acids (PUFA) results in a preferential partitioning of ingested energy towards oxidation at the expense of storage. Fatty acids are important mediators of gene expression in the liver. Indeed, genes encoding both glycolytic and lipogenic enzymes and key metabolic enzymes involved in fatty acid oxidation are regulated by dietary PUFA. White adipose tissue could also be a target for PUFA control of gene expression. The treatment of pre-adipose cells by fatty acids induces the expression of numerous genes that encode proteins involved in fatty acid metabolism. The mechanisms of PUFA-mediated repression of gene expression in adipocytes seem to be different, at least partly, from those described in liver. Tissue-specific and site-specific factors are possibly involved in the specific effect of PUFA on gene expression, although other mechanisms cannot be excluded.

Highly purified soybean protein is not hypocholesterolemic in rats but stimulates cholesterol synthesis and excretion and reduces polyunsaturated fatty acid biosynthesis

The specific effects of soybean protein on lipid metabolism were determined with highly purified soybean protein. At 5 wk of age, growing rats were fed diets containing 20% highly purified soybean protein or casein supplemented or not with 0.1% cholesterol for 2 mo. Plasma and liver lipid composition, fecal steroid excretion and several hepatic enzyme activities were measured. There were no significant dietary protein-related differences in plasma and liver cholesterol concentrations. When diets were cholesterol free, highly purified soybean protein stimulated fecal neutral and acidic steroid excretion associated with concomitantly higher hydroxy methylglutaryl CoA (HMG-CoA) reductase activity, but lower cholesterol 7alpha-hydroxylase activity. Soybean protein lowered the linoleate desaturation index [20:4(n-6)/18:2(n-6)] in liver microsomal lipids and phospholipids. This may have been due to the reduced microsomal Delta6(n-6) desaturase activity in rats fed soybean protein, whereas Delta5(n-6) desaturase activity did not differ between groups fed the two proteins. Cholesterol supplementation (0.1%) did not affect plasma cholesterol but increased liver cholesterol and triacylglycerol concentrations and reduced HMG-CoA reductase activity; this latter effect was greatest in rats fed soybean protein. Cholesterol 7alpha-hydroxylase activity, however, was diminished only in rats fed casein. Desaturase activities, and particularly Delta5(n-6) activity, were lowered by cholesterol supplementation in rats fed both protein diets, including a significantly lower 20:4(n-6)/18:2(n-6) ratio in liver microsomal lipids and liver phospholipids. Thus although dietary proteins have no effect on serum cholesterol in rats, they affect enzyme activities involved in cholesterol metabolism and fatty acid desaturation.

Comparative hypocholesterolemic effects of six dietary oils in cholesterol-fed rats after long-term feeding

Rats (8 wk of age) fed a conventional diet were shifted to diets containing 10% Oenothera biennis Linn oil (OBLO, linoleic acid + gamma-linolenic acid) from a wild plant, evening primrose oil (EPO, linoleic acid + gamma-linolenic acid) from a cultivated plant, bio-gamma-linolenic acid oil from mold (BIO, palmitic acid + oleic acid + linoleic acid + gamma-linolenic acid), safflower oil (linoleic acid), palm oil (PLO, palmitic acid + oleic acid + linoleic acid), or soybean oil (linoleic acid + alpha-linolenic acid) with 0.5% cholesterol for 13 wk. Though there were no significant differences in the food intake among the groups, the body weight gain of the OBLO group was significantly lower than that of the other groups except for the BIO and PLO groups, and that of the EPO and SBO groups were the highest among the groups. The liver weight of the OBLO group was significantly lower than that of other groups, and that of the PLO group was the highest among the groups. The serum total cholesterol and very low density lipoprotein (VLDL) + intermediate density lipoprotein (IDL) + low density lipoprotein (LDL) cholesterol concentrations of the OBLO and EPO groups were consistently lower than those in the other groups. However, those of the BIO group were higher than those in the OBLO and EPO groups. The liver cholesterol concentration of the PLO group was the highest among all groups except for the EPO group. The fecal neutral sterol and bile acid extraction of the BIO group tended to increase compared to the other groups. The results of this study demonstrate that OBLO and EPO inhibit the increasing of serum total cholesterol and VLDL + IDL + LDL-cholesterol concentrations in the presence of excess cholesterol in the diet compared with the other dietary oils.

On the interrelationship between hepatic carnitine, fatty acid oxidation, and triglyceride biosynthesis in nephrosis

The nephrotic syndrome is associated with disturbances in plasma lipid pattern and metabolism. However, the reason for these perturbations is poorly understood. In the present study, we have investigated hepatic triglyceride metabolism in puromycin aminonucleoside-induced nephrotic syndrome in rats. Nephrotic rats displayed a 70% increase in hepatic triglyceride levels compared to controls (16.9 +/- 1.6 vs. 9.8 +/- 0.6 mumol/g liver; means +/- SEM, P < 0.01). The capacity for hepatic mitochondrial beta-oxidation of fatty acids was substantially elevated (80%). This was associated with a rise in the liver content of the fatty acid carrier carnitine (1.24 +/- 0.06 vs. 0.85 +/- 0.07 mumol/g dry weight, P < 0.05). A positive correlation between the levels of acetylcarnitine and acetyl-CoA was found in normal as well as in nephrotic rats, implying that carnitine plays an important role as an acetyl group acceptor in the liver under normo- and hyperlipidemic conditions. Changes in carnitine levels seem to be tightly coupled to the rate of fatty acid oxidation. There was a significant elevation in the activity of phosphatidate phosphohydrolase (E.C. 3.1.3.4) in liver microsomes from nephrotic rats (1.07 +/- 0.09 vs. 0.81 +/- 0.04 nmol/min.mg protein, P < 0.02). Hepatic very low density lipoprotein (VLDL)-triglyceride secretion rate was 18% higher in nephrotic rats than in controls. The results demonstrate a deranged hepatic triglyceride metabolism in nephrosis, with an increased hepatic triglyceride biosynthesis, a sizable accumulation of hepatic triglycerides, and only a modest increase in VLDL triglyceride secretion. In addition, mitochondrial beta-oxidation of fatty acids was enhanced, associated with an increased availability of carnitine.

Level of polyunsaturated fatty acids and the n-6 to n-3 polyunsaturated fatty acid ratio in the rat diet alter serum lipid levels and lymphocyte functions

In order to further examine the effects of dietary polyunsaturated fatty acids (PUFA) upon blood lipid levels and lymphocyte functions, weanling rats were fed for 6 weeks on high fat (178 g/kg) diets which differed in the ratio of n-6:n-3 PUFA (100, 20, 10, 5, 1) and in the absolute level of PUFA (17.5 or 35 g/100 g fatty acids). The n-6:n-3 PUFA ratio of the diets was decreased by replacing linoleic acid with alpha-linolenic acid while the PUFA content of the diets was decreased by replacing PUFA with palmitic acid. Serum cholesterol concentrations decreased as the n-6:n-3 PUFA ratio of the low PUFA diet decreased. The ex vivo proliferation of spleen lymphocytes from rats fed the low PUFA diets decreased as the n-6:n-3 PUFA ratio of the diet decreased; the proliferation of spleen lymphocytes from high PUFA-fed rats was less affected by the n-6:n-3 PUFA ratio of the diet. Natural killer cell activity was lower for spleen lymphocytes from rats fed high PUFA diets with n-6:n-3 PUFA ratios of 100 or 20 than for those from rats fed low PUFA diets with these ratios. The natural killer cell activity of spleen lymphocytes decreased as the n-6:n-3 PUFA ratio of the low PUFA diet decreased. These findings indicate that dietary alpha-linolenic acid has significant blood lipid-lowering and immunomodulatory effects in rats, but that the effect is dependent upon the total PUFA content of the diet. The ratios of linoleic and alpha-linolenic acids to other fatty acids (e.g. palmitic, oleic) are important in determining the precise effect of manipulations of the fatty acid composition of the diet.

Dietary fish oil. Influence on lesion regression in the porcine model of atherosclerosis

We examined the influence of dietary fish oil on lesion regression in a porcine model of atherogenesis. Thirty-two female Yucatan miniature pigs were fed an atherogenic diet for 8 months. A no-regression group (n = 8) was killed to determine the extent of atherosclerosis at 8 months. Three regression groups were switched to normal minipig chow supplemented with either MaxEPA fish oil (FO group, n = 8), a control oil with the ratio of polyunsaturated to monounsaturated to saturated fatty acid matched to that of the fish oil (CO group, n = 8), or no oil supplement (NO group, n = 8) for a further 4 months. Plasma cholesterol levels reached between 15 and 20 mmol/L during the atherogenic phase and returned to normal (2 mmol/L) within 2 months of the beginning of the regression diet. Compared with the NO group, fish oil supplementation during the regression phase caused a decrease in VLDL and HDL cholesterol and an increase in LDL cholesterol. Similarly, the control oil also caused a decrease in VLDL cholesterol; however, in contrast to the FO group, HDL cholesterol increased and LDL cholesterol was unchanged. FO LDL, which had decreased levels of 20:4 (n-6 fatty acid) and increased levels of 18:3, 20:5, and 22:6 (n-3 fatty acids), was shown to be twice as susceptible to copper-mediated oxidation as CO LDL particles. Morphological examination of the major blood vessels revealed a significant reduction in lesion area in the ascending and thoracic aorta as well as the carotid artery after the regression diet; however, there was no significant difference between the fish oil and control oil groups in any of the vessels measured. Therefore, despite increased LDL, decreased HDL, and an increased susceptibility to in vitro oxidation of LDL, fish oil supplementation of a regression diet did not influence lesion regression.

Comparative hypocholesterolemic effects of capybara (Hydrochoerus hydrochaeris dabbenei) oil, horse oil, and sardine oil in cholesterol-fed rats

The hypocholesterolemic efficacy of various polyunsaturated fatty acids was compared in rats given cholesterol-enriched diets. Capybara oil (CO, linoleic + alpha-linolenic acids), horse oil (HO, alpha-linolenic acid), and sardine oil (SO, eicosapentaenoic + docosahexaenoic acids) were added to diets at 50 g/kg. The weight gain, food intake, and liver weight in the CO-fed group were significantly higher than those in other groups during the 6-wk experimental period. The serum total and very low density lipoprotein (VLDL) + intermediate density lipoprotein (IDL) + low density lipoprotein (LDL) cholesterol concentrations of the CO-fed and SO-fed groups were significantly lower than in the HO-fed group after 6 wk. The serum high density lipoprotein cholesterol concentration in the SO-fed group was significantly higher than that in the CO-fed and HO-fed groups. The fecal neutral sterol concentration in the CO-fed group was reduced significantly compared with the other groups, and the fecal bile acid concentration in the HO-fed group was significantly higher than that in the SO-fed group. The results of this study demonstrate that CO lowers the serum total cholesterol and VLDL + IDL + LDL-cholesterol concentrations in the presence of excess cholesterol in the diet as well as SO.

Dietary sunflower oil reduces plasma and liver triacylglycerols in fasting rats and is associated with decreased liver microsomal phosphatidate phosphohydrolase activity

Plasma and liver lipids were studied in male weanling rats fed diets containing moderate levels of fat (6% by weight) as sunflower oil (SF diet, rich in linoleic acid), salmon oil (SM diet, rich in long-chain n-3 fatty acids), or a blend of peanut and rapeseed oil (PR diet, rich in oleic acid). After nine weeks of feeding, the fasting plasma cholesterol concentrations were 49 and 24% lower in groups SM and SF, respectively, as compared to group PR. Both dietary salmon oil and sunflower oil lowered the triacylglycerol concentration of plasma and liver but, unexpectedly, the response was higher with sunflower oil. Indeed, in group SM the values were 15 and 30% lower in plasma and liver, whereas in group SF, they were 24 and 53% lower, respectively. As compared to group PR, liver triacylglycerols and microsomes contained 2.5- and 2.3-fold less oleic acid, respectively, in group SF, and they were 9.2- and 3.2-fold enriched in n-3 fatty acids, respectively, in group SM. The liver triacylglycerol concentrations were correlated with changes in the microsomal Mg(2+)-dependent phosphatidate phosphohydrolase activity (r = 0.47, P < 0.01). As oleic acid, unlike long-chain n-3 fatty acids, is considered to promote the triacylglycerol synthesis and secretion, our findings suggest that changes in the membrane fatty acid composition could affect the triacylglycerol content of liver and plasma. Moreover, the availability within the liver, of oleic acid, predominantly incorporated into triacylglycerols, might limit the triacylglycerol production in SF-fed rats.

The ratio of n-6 to n-3 polyunsaturated fatty acids in the rat diet alters serum lipid levels and lymphocyte functions

Previous studies have reported that feeding rats diets rich in fish oils, which contain high proportions of the n-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic and docosahexaenoic acids, results in lowering of blood lipid levels and suppression of lymphocyte functions tested ex vivo and in vivo. The effects of other n-3 PUFA, such as alpha-linolenic acid, which is found in high proportions in linseed oil, are not as well documented. Therefore, in the present study, weanling male rats were fed for six weeks on one of five high-fat (20% by weight) diets made by mixing together sunflower and linseed oils; the resulting blends had n-6/n-3 PUFA ratios of 112.5:1 (pure sunflower oil), 14.8:1, 6.5:1, 0.81:1, and 0.33:1 (pure linseed oil); the levels of all other components in the diet were identical. The final body weight and total dissectable fat were lowest in rats fed the pure linseed oil diet. Serum cholesterol, triacylglycerol and nonesterified fatty acid concentrations decreased as the n-6/n-3 PUFA ratio of the diet decreased. The fatty acid composition of the serum and of spleen lymphocytes was influenced by the diet fed-there was a progressive decrease in the proportions of linoleic and arachidonic acids and a progressive increase in the proportion of alpha-linolenic acid as the n-6/n-3 PUFA ratio of the diet decreased. Eicosapentaenoic and docosahexaenoic acids were detected in the serum but not in spleen lymphocytes. Inclusion of alpha-linolenic acid in the diet resulted in significant suppression of spleen lymphocyte proliferation in response to the T-cell mitogen concanavalin A and in spleen lymphocyte natural killer cell activity, both measured ex vivo. The localized graft vs. host response, a measure of cell-mediated immunity in vivo, progressively decreased as the n-6/n-3 PUFA ratio of the diet decreased. Thus, this study shows that dietary alpha-linolenic acid results in lowered blood lipid levels and suppressed lymphocyte functions ex vivo and in vivo. With respect to these effects, alpha-linolenic acid is as potent as dietary fish oil.

Eicosapentaenoic acid, but not docosahexaenoic acid, increases mitochondrial fatty acid oxidation and upregulates 2,4-dienoyl-CoA reductase gene expression in rats

The aim of the present study was to investigate whether eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) was responsible for the triglyceride-lowering effect of fish oil. In rats fed a single dose of EPA as ethyl ester (EPA-EE), the plasma concentration of triglycerides was decreased at 8 h after acute administration. This was accompanied by an increased hepatic fatty acid oxidation and mitochondrial 2,4-dienoyl-CoA reductase activity. The steady-state level of 2,4-dienoyl-CoA reductase mRNA increased in parallel with the enzyme activity. An increased hepatic long-chain acyl-CoA content, but a reduced amount of hepatic malonyl-CoA, was obtained at 8 h after acute EPA-EE treatment. On EPA-EE supplementation, both EPA (20:5n-3) and docosapentaenoic acid (DPA, 22:5n-3) increased in the liver, whereas the hepatic DHA (22:6n-3) concentration was unchanged. On DHA-EE supplementation retroconversion to EPA occurred. No statistically significant differences were found, however, for mitochondrial enzyme activities, malonyl-CoA, long-chain acyl-CoA, plasma lipid levels, and the amount of cellular fatty acids between DHA-EE treated rats and their controls at any time point studied. In cultured rat hepatocytes, the oxidation of [1-14C]palmitic acid was reduced by DHA, whereas it was stimulated by EPA. In the in vivo studies, the activities of phosphatidate phosphohydrolase and acetyl-CoA carboxylase were unaffected after acute EPA-EE and DHA-EE administration, but the fatty acyl-CoA oxidase, the rate-limiting enzyme in peroxisomal fatty acid oxidation, was increased after feeding these n-3 fatty acids. The hypocholesterolemic properties of EPA-EE may be due to decreased 3-hydroxy-3-methylglutaryl-CoA reductase activity. Furthermore, replacement of the ordinary fatty acids, i.e., the monoenes (16:1n-7, 18:1n-7, and 18:1n-9) with EPA and some conversion to DPA concomitant with increased fatty acid oxidation is probably the mechanism leading to changed fatty acid composition. In contrast, DHA does not stimulate fatty acid oxidation and, consequently, no such displacement mechanism operates. In conclusion, we have obtained evidence that EPA, and not DHA, is the fatty acid primarily responsible for the triglyceride-lowering effect of fish oil in rats.

Chronic administration of eicosapentaenoic acid and docosahexaenoic acid as ethyl esters reduced plasma cholesterol and changed the fatty acid composition in rat blood and organs

Fish oils rich in n-3 fatty acids have been shown to decrease plasma lipid levels, but the underlying mechanism has not yet been elucidated. This investigation was performed in order to further clarify the effects of purified ethyl esters of eicosapentaenoic acid (EPA-EE) and docosahexaenoic acid (DHA-EE) on lipid metabolism in rats. The animals were fed EPA-EE, DHA-EE, palmitic acid, or corn oil (1 g/kg/d) by orogastric intubation along with a chow background diet for three months. At the end the animals were sacrificed. Plasma and liver lipids were measured, as well as lipid-related enzyme activities and mRNA levels. The fatty acid composition of plasma and different tissues was also determined. This study shows that, compared to the corn oil control, EPA-EE and DHA-EE lowered plasma cholesterol level, whereas only EPA-EE lowered the amount of plasma triacylglycerol. In liver peroxisomes, both EE preparations increased fatty acyl-CoA oxidase FAO activities, and neither altered 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activities. In liver microsomes, EPA-EE raised HMG-CoA reductase and acyl-CoAicholesterol acyltransferase activities, whereas DHA-EE lowered the former and did not affect the latter. Neither product altered mRNA levels for HMG-CoA reductase, low density lipoprotein-receptor, or low density lipoprotein-receptor related protein. EPA-EE lowered plasma triacylglycerol, reflecting lowered very low density lipoprotein secretion, thus the cholesterol lowering effect in EPA-EE-treated rats may be secondary to the hypotriacylglycerolemic effect. An inhibition of HMG-CoA reductase activity in DHA-EE treated rats may contribute to the hypocholesterolemic effect. The present study reports that 20:5n-3, and not 22:6n-3, is the fatty acid primarily responsible for the triacylglycerol lowering effect of fish oil. Finally, 20:5n-3 was not converted to 22:6n-3, whereas retroconversion of 22:6n-3 to 20:5n-3 was observed.

Effects of a fish oil-lard diet on rat plasma lipoproteins, liver FAS, and lipolytic enzymes

The effects of a fish oil concentrate on blood lipids and lipoproteins were examined in relation to their effects on liver fatty acid synthase (FAS), 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, adipose tissue lipoprotein lipase (LPL), and hepatic triglyceride lipase (H-TGL). For 15 days, 2-mo-old rats were fed a control diet (10% of calories from fat, 4% fat by weight) or diets with 50% of calories (25% wt/wt) provided by lard, lard and fish oil calories (35%/15%), or lard and corn oil (35%/15%). The high-lard diet increased plasma chylomicron and liver triglycerides. The high-lard diet greatly decreased FAS, HMG-CoA reductase, and LPL activities; it also reduced H-TGL activity. Compared with the lard diet, the lard-fish oil diet decreased plasma TG by drastically lowering chylomicron (4-fold, P < 0.001) and very-low-density lipoprotein levels (P < 0.001). It also reduced high-density lipoprotein levels. The lard-fish oil diet prevented hepatic triglyceride accumulation and decreased FAS activity and mass by 3.5-fold (P < 0.001) but did not further decrease HMG-CoA reductase activity. Adipose tissue LPL activity was 2.5-fold (P < 0.001) higher with the lard-fish oil diet than with the lard diet, and H-TGL activity decreased significantly (-32%, P < 0.01), despite unaltered levels of H-TGL mRNA. These effects were significant with only 10% fish oil concentrate in the lard diet. They were not observed with the lard-corn oil diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

Effect of 3-thiadicarboxylic acid on lipid metabolism in experimental nephrosis

The effect of the sulfur-substituted fatty acid analogue 1,10 bis(carboxymethylthio)decane, also known as 3-thiadicarboxylic acid, on puromycin aminonucleoside-induced nephrotic hyperlipidemia was studied in rats. Treatment with 3-thiadicarboxylic acid (250 mg/kg) for 5 days reduced plasma levels of triglycerides from 5.8 to 2.7 mmol/L and cholesterol from 11.0 to 7.7 mmol/L. This was accounted for by decreases in very-low-density lipoprotein triglycerides, very-low-density lipoprotein cholesterol, and low-density lipoprotein cholesterol, without any major changes in the composition of plasma lipoproteins. The activities of two enzymes involved in fatty acid synthesis (ATP:citrate lyase and fatty acid synthetase) were inhibited by 3-thiadicarboxylic acid treatment, whereas acetyl-coenzyme A carboxylase activity was unchanged. In contrast, treatment with the sulfur-substituted fatty acid analogue induced the peroxisomal beta-oxidation of fatty acids ninefold and the mitochondrial beta-oxidation by 54% to 73%, depending on the substrate used. This was accompanied by a 26% reduction in hepatic triglyceride secretion rate. The hepatic phosphatidate phosphohydrolase activity was unchanged. 3-Thiadicarboxylic acid treatment suppressed the activity of the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, by 58%, whereas hepatic LDL receptor expression was unaltered. The activities of lipoprotein lipase and hepatic lipase were unchanged by treatment. These results demonstrated that treatment with 3-thiadicarboxylic acid ameliorates hyperlipidemia in experimental nephrosis primarily by decreasing the overproduction of very-low-density lipoprotein present. The data also indicate that hepatic very-low-density lipoprotein synthesis and secretion is strongly influenced by the availability of the fatty acid substrate under the same hyperlipidemic conditions.

The effects of n-3 fatty acids on plasma lipids and lipoproteins and other cardiovascular risk factors in patients with hyperlipidemia

This review discusses the effects of dietary n-3 fatty acids on the plasma concentrations and metabolism of lipoproteins with a particular focus on work in human subjects. The influence of dietary n-3 fatty acids on the concentrations of plasma lipoproteins are affected by the amount of n-3 fatty acids as well as by the lipoprotein phenotype in the patients under investigation. On the basis of the observed changes in lipoproteins, dietary n-3 fatty acids exert the greatest effects on the concentrations of triglyceride-rich lipoproteins; their therapeutic potential is greatest in patients with hypertriglyceridemia. In addition to their effects on plasma lipoproteins, dietary n-3 fatty acids have been reported to exert potentially favorable effects on blood pressure, platelet function and viscosity. These effects may justify the use of supplements of dietary n-3 fatty acids in selected patients with hypertriglyceridemia to reduce the risk of atherosclerosis; however, the benefits of such therapy remain to be demonstrated.

Studies on triglyceride metabolism: phosphatidate phosphohydrolase from guinea pig harderian gland

The guinea pig Harderian gland, located in the orbit, is characterized by a high production of lipids. However, little is known about the regulation of the metabolic pathways involved. In the present paper the properties of guinea pig Harderian gland phosphatidate phosphohydrolase, a key enzyme in triglyceride biosynthesis, was investigated. The enzyme was present both in the cytosolic and the microsomal fraction from the gland. Cytosolic phosphatidate phosphohydrolase was purified by hydroxylapatite chromatography. The enzyme was dependent on magnesium ions and inhibited in the presence of fluoride. The dependence on the substrate phosphatidate was investigated and the apparent Km for phosphatidate was about 0.6 mM. Phosphatidate phosphohydrolase activity was influenced by different phospholipids. As is the case for the rat liver enzyme, phosphatidylethanolamine was found to stimulate the enzyme activity. The results indicate that Harderian gland phosphatidate phosphohydrolase has similar properties as the corresponding enzyme from rat liver, suggesting that it may be of regulatory importance.

The hypotriglyceridemic effect of eicosapentaenoic acid in rats is reflected in increased mitochondrial fatty acid oxidation followed by diminished lipogenesis

The effect of eicosapentaenoic acid (EPA) on fatty acid oxidation and on key enzymes of triglyceride metabolism and lipogenesis was investigated in the liver of rats. Repeated administration of EPA to normolipidemic rats resulted in a time-dependent decrease in plasma triglycerides, phospholipids and cholesterol. The triglyceride-lowering effect was observed after one day of feeding whereas lowering of plasma cholesterol and phospholipids was observed after five days of treatment. The triglyceride content of liver was reduced after two-day treatment. At that time, increased mitochondrial fatty acid oxidation occurred whereas mitochondrial and microsomal glycerophosphate acyltransferase was inhibited. The phosphatidate phosphohydrolase activity was unchanged. Adenosine triphosphate:citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase and glucose-6-phosphate dehydrogenase were inhibited during the 15 d of EPA treatment whereas peroxisomal beta-oxidation was increased. At one day of feeding, however, when the hypotriglyceridemic effect was established, the lipogenic enzyme activities were reduced to the same extent in palmitic acid-treated animals as in EPA-treated rats. In cultured rat hepatocytes, the oxidation of [14C]palmitic acid to carbon dioxide and acid-soluble products was stimulated in the presence of EPA. These results suggest that the instant hypolipidemia in rats given EPA could be explained at least in part by a sudden increase in mitochondrial fatty acid oxidation, thereby reducing the availability of fatty acids for lipid synthesis in the liver for export, e.g., in the form of very low density lipoproteins, even before EPA induced peroxisomal fatty acid oxidation, reduced triglyceride biosynthesis and diminished lipogenesis.

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.