The lipolysis of chylomicrons derived from different dietary fats by lipoprotein lipase in vitro

Effects of Three-Month Administration of High-Saturated Fat Diet and High-Polyunsaturated Fat Diets with Different Linoleic Acid (LA, C18:2n-6) to α-Linolenic Acid (ALA, C18:3n-3) Ratio on the Mouse Liver Proteome

The aim of the study was to evaluate the effect of different types of high-fat diets (HFDs) on the proteomic profile of mouse liver. The analysis included four dietary groups of mice fed a standard diet (STD group), a high-fat diet rich in SFAs (SFA group), and high-fat diets dominated by PUFAs with linoleic acid (LA, C18:2n-6) to α-linolenic acid (ALA, C18:3n-3) ratios of 14:1 (14:1 group) and 5:1 (5:1 group). After three months of diets, liver proteins were resolved by two-dimensional gel electrophoresis (2DE) using 17 cm non-linear 3-10 pH gradient strips. Protein spots with different expression were identified by MALDI-TOF/TOF. The expression of 13 liver proteins was changed in the SFA group compared to the STD group (↓: ALB, APOA1, IVD, MAT1A, OAT and PHB; ↑: ALDH1L1, UniProtKB-Q91V76, GALK1, GPD1, HMGCS2, KHK and TKFC). Eleven proteins with altered expression were recorded in the 14:1 group compared to the SFA group (↓: ARG1, FTL1, GPD1, HGD, HMGCS2 and MAT1A; ↑: APOA1, CA3, GLO1, HDHD3 and IVD). The expression of 11 proteins was altered in the 5:1 group compared to the SFA group (↓: ATP5F1B, FTL1, GALK1, HGD, HSPA9, HSPD1, PC and TKFC; ↑: ACAT2, CA3 and GSTP1). High-PUFA diets significantly affected the expression of proteins involved in, e.g., carbohydrate metabolism, and had varying effects on plasma total cholesterol and glucose levels. The outcomes of this study revealed crucial liver proteins affected by different high-fat diets.

Effects of dietary fat quality on metabolic endotoxaemia: a systematic review

In this systematic review, we critically evaluated human clinical trials that assessed the effects of dietary fat quality on metabolic endotoxaemia. The studies were selected from three databases (PubMed, Scopus and Cochrane Library), and the keywords were defined according to the Medical Subject Headings indexing terminology. Two authors searched independently, according to the pre-defined selection criteria. Quality and risk assessment of bias for each selected study were also evaluated. The results of the included studies demonstrated associations between higher SFA intake and increased postprandial lipopolysaccharide (LPS) concentrations. On the other hand, after the consumption of PUFA, bloodstream LPS concentrations were lower. However, in none of the long-term studies, the consumption of dietary fats did not seem to exert effects on LPS concentration. Hence, SFA seem to act as a risk factor for transient increase in endotoxaemia, while PUFA demonstrated exerting a protective effect. Taken together, the evidence suggests that the dietary fatty acid profile may influence bloodstream endotoxin concentrations through modulation of factors such LPS clearance, alkaline phosphatase activity, bile acid metabolism, intestinal permeability and intestinal microbiota composition.

Delayed postprandial TAG peak after intake of SFA compared with PUFA in subjects with and without familial hypercholesterolaemia: a randomised controlled trial

Postprandial hypertriacylglycerolaemia is associated with an increased risk of developing CVD. How fat quality influences postprandial lipid response is scarcely explored in subjects with familial hypercholesterolaemia (FH). The aim of this study was to investigate the postprandial response of TAG and lipid sub-classes after consumption of high-fat meals with different fat quality in subjects with FH compared with normolipidaemic controls. A randomised controlled double-blind cross-over study with two meals and two groups was performed. A total of thirteen hypercholesterolaemic subjects with FH who discontinued lipid-lowering treatment 4 weeks before and during the study, and fourteen normolipidaemic controls, were included. Subjects were aged 18-30 years and had a BMI of 18·5-30·0 kg/m2. Each meal consisted of a muffin containing 60 g (70 E%) of fat, either mainly SFA (40 E%) or PUFA (40 E%), eaten in a random order with a wash-out period of 3-5 weeks between the meals. Blood samples were collected at baseline (fasting) and 2, 4 and 6 h after intake of the meals. In both FH and control subjects, the level of TAG and the largest VLDL sub-classes peaked at 2 h after intake of PUFA and at 4 h after intake of SFA. No significant differences were found in TAG levels between meals or between groups (0·25≤P≤0·72). The distinct TAG peaks may reflect differences in the postprandial lipid metabolism after intake of fatty acids with different chain lengths and degrees of saturation. The clinical impact of these findings remains to be determined.

Membrane lipid alterations in the metabolic syndrome and the role of dietary oils

The metabolic syndrome is a cluster of pathological conditions, including hypertension, hyperglycemia, hypertriglyceridemia, obesity and low HDL levels that is of great concern worldwide, as individuals with metabolic syndrome have an increased risk of type-2 diabetes and cardiovascular disease. Insulin resistance, the key feature of the metabolic syndrome, might be at the same time cause and consequence of impaired lipid composition in plasma membranes of insulin-sensitive tissues like liver, muscle and adipose tissue. Diet intervention has been proposed as a powerful tool to prevent the development of the metabolic syndrome, since healthy diets have been shown to have a protective role against the components of the metabolic syndrome. Particularly, dietary fatty acids are capable of modulating the deleterious effects of these conditions, among other mechanisms, by modifications of the lipid composition of the membranes in insulin-sensitive tissues. However, there is still scarce data based of high-level evidence on the effects of dietary oils on the effects of the metabolic syndrome and its components. This review summarizes the current knowledge on the effects of dietary oils on improving alterations of the components of the metabolic syndrome. It also examines their influence in the modulation of plasma membrane lipid composition and in the functionality of membrane proteins involved in insulin activity, like the insulin receptor, GLUT-4, CD36/FAT and ABCA-1, and their effect in the metabolism of glucose, fatty acids and cholesterol, and, in turn, the key features of the metabolic syndrome. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Comparison of hydrophobic properties of thoracic duct lymph chylomicrons from rats given different fats or oils by gavage

Lipoprotein aggregation is generated by hydrophobic nature of lipoproteins that is known to be one of the causes of atherosclerosis. Low density lipoproteins (LDL) has been extensively studied in this respect but not chylomicrons. There is strong evidence that post-prandial triacylglycerol-rich lipoproteins are atherogenic. Because biophysical properties of lipoproteins are largely determined by their lipid compositions, hydrophobic nature of thoracic lymph duct chylomicrons obtained from rats given different fats or oils by gavage was investigated by vortexing-induced aggregation and hydrophobic interaction chromatography. Contrary to LDL, vortexing did not cause aggregation in chylomicrons. Vortexing of fish oil and butter chylomicrons resulted in more prominent reduction in absorbances compared with chylomicrons from other sources that might indicate less micelle stability. Hydrophobic interaction chromatography of fish oil, palm oil and olive oil chylomicrons yielded three fractions, whereas that of sunflower, margarine and butter chylomicrons gave rise to two fractions. These results suggest that surface hydrophobicity of chylomicrons might be heterogenous. Our results also demonstrate that fish oil chylomicrons have less hydrophobicity and lower stability against vortexing compared with chylomicrons from other sources. Considering beneficial effects of fish oil in cardiovascular health, less hydrophobicity together with lower stability might provide an additional atherogeneicity index for lipoproteins.

Short-term, high-fat diet increases the expression of key intestinal genes involved in lipoprotein metabolism in healthy men

BACKGROUND

The modulation of cholesterol and fatty acid homeostasis by dietary fatty acids is thought to be mediated by changes in the expression of key intestinal genes involved in lipoprotein metabolism. However, the short-term effect of dietary fat intake on the expression of these genes has not been fully investigated in humans.

OBJECTIVE

To test whether short-term changes in dietary fatty acid intake affect the expression of key intestinal genes involved in lipoprotein metabolism, we conducted a randomized, double-blind, crossover study in 12 nonobese, healthy men with normal plasma lipid profiles.

DESIGN

Participants were subjected to the following 2 intensive 3-d dietary interventions under isocaloric conditions: 1) a high-fat diet (37% of energy from fat and 50% of energy from carbohydrates) and 2) a low-fat diet (25% of energy from fat and 62% of energy from carbohydrates). Expressions of key genes involved in lipoprotein metabolism were compared by using real-time polymerase chain reaction quantification on duodenal biopsy specimens obtained in a fasting state after each diet.

RESULTS

After the 3-d high-fat diet, plasma cholesterol, LDL cholesterol, and HDL cholesterol concentrations were significantly higher than concentrations observed after the low-fat diet was consumed. The high-fat diet also resulted in significant increases in the intestinal messenger RNA expression of several key genes involved in lipoprotein metabolism. Plasma triglycerides and apolipoprotein B-48 concentrations were significantly lower after the high-fat diet than after the low-fat diet.

CONCLUSION

These findings suggest that short-term exposure to a high-fat diet upregulates the expression of key genes involved in lipid and lipoprotein metabolism at the enterocyte level. This trial was registered at clinicaltrials.gov as NCT01806441.

Short-term adaptation of postprandial lipoprotein secretion and intestinal gene expression to a high-fat diet

Western diet is characterized by a hypercaloric and hyperlipidic intake, enriched in saturated fats, that is associated with the increased occurrence of metabolic diseases. To cope with this overload of dietary lipids, the intestine, which delivers dietary lipids to the body, has to adapt its capacity in lipid absorption and lipoprotein synthesis. We have studied the early effects of a high-fat diet (HFD) on intestinal lipid metabolism in mice. After 7 days of HFD, mice displayed normal fasting triglyceridemia but postprandial hypertriglyceridemia. HFD induced a decreased number of secreted chylomicrons with increased associated triglycerides. Secretion of larger chylomicrons was correlated with increased intestinal microsomal triglyceride transfer protein (MTP) content and activity. Seven days of HFD induced a repression of genes involved in fatty acid synthesis (FAS, ACC) and an increased expression of genes involved in lipoprotein assembly (apoB, MTP, and apoA-IV), suggesting a coordinated control of intestinal lipid metabolism to manage a high-fat loading. Of note, the mature form of the transcription factor SREBP-1c was increased and translocated to the nucleus, suggesting that it could be involved in the coordinated control of gene transcription. Activation of SREBP-1c was partly independent of LXR. Moreover, HFD induced hepatic insulin resistance whereas intestine remained insulin sensitive. Altogether, these results demonstrate that a short-term HFD is sufficient to impact intestinal lipid metabolism, which might participate in the development of dyslipidemia and metabolic diseases.

Contemporary trends in dyslipidemia in the Framingham Heart Study

BACKGROUND

Recent cross-sectional population studies in the United States have shown an increase in obesity, a decrease in cholesterol values, but no changes in levels of high-density lipoprotein cholesterol (HDL-C) or triglycerides (TG).

METHODS

Plasma total cholesterol, HDL-C, and TG levels, measured by the same methods at the 3 most recently completed examinations of Framingham Offspring Study participants (1991-2001), were compared in 1666 participants without prevalent cardiovascular disease, lipid therapy, or hormone replacement therapy (56% were men; mean ages of participants at the first and last examinations, 53 and 60 years, respectively). Changes in age- and multivariate-adjusted mean lipid levels were related to changes in body mass index (BMI).

RESULTS

Over the 3 examinations, comparing the findings of the earliest examination with those of the most recent examination, the mean HDL-C level was significantly increased (multivariate-adjusted means, 44.4 and 46.6 mg/dL in men; 56.9 and 60.1 mg/dL in women; P value for trend, P <.001 in both sexes), whereas levels of TG were decreased (144.5 and 134.1 mg/dL in men; 122.3 and 112.3 mg/dL in women; P value for trend, P = .004 in men and <.001 in women). Over the same time interval, BMI (calculated as weight in kilograms divided by height in meters squared) increased (27.8 and 28.5 in men; 27.0 and 27.6 in women; P value for trend, P < .001 in men and P = .001 in women). There was an inverse relationship between changes in BMI and magnitude of dyslipidemia (ie, individuals with the least increase in BMI had the most favorable changes in levels of HDL-C and TG).

CONCLUSION

During a 10-year period of recent examinations in the Framingham Heart Study there was a decrease in dyslipidemia with an increase in HDL-C levels and a decrease in levels of TG despite an overall increase in BMI.

Influence of minor components of olive oils on the composition and size of TRLs and on macrophage receptors involved in foam cell formation

Metabolic and epidemiologic studies support the idea that the type of dietary fat is more important than the total amount of fat with respect to the development of atherosclerosis and the risk of cardiovascular heart disease. Dietary fat is carried in CMs (chylomicrons), which can be taken up by macrophages without need of further oxidation, leading to the formation of foam cells and initiating or aggravating the atherogenic process. Evidence from different studies has shown that dietary fat can influence the composition and size of TRLs (triacylglycerol-rich lipoproteins), which might modulate their atherogenicity to a certain extent. In particular, experiments in vitro have shown the anti-atherogenic effects of minor components from olive oil when forming part of TRL, as these particles give minor lipid components the opportunity to interact with the cells implicated in endothelial dysfunction and atherogenesis. However, the exact mechanisms mediating CM uptake by macrophages still remain unclear. Thus further studies are needed to understand how the modifications of TRL composition caused by dietary fats could modulate the expression of macrophage receptors and foam cell formation, or even improve the atherogenic risk of these particles.

The differential hepatic uptake of chylomicron remnants of different fatty acid composition is not mediated by hepatic lipase

The hypothesis that hepatic lipase mediates the differential hepatic uptake of chylomicron remnants of different fatty acid composition, demonstrated in previous work from our laboratory, was tested by investigating the effect of antibodies to the enzyme on the uptake of remnants enriched with saturated orn-3 polyunsaturated fatty acids by the perfused rat liver. After perfusion of rat livers with polyclonal antibodies to rat hepatic lipase raised in rabbits or with rabbit non-immune serum for 15 min, [3H]oleate-labelled chylomicron remnants, derived from chylomicrons of rats given a bolus of either palm (rich in saturated fatty acids) oil or fish (rich inn-3 polyunsaturated fatty acids) oil, were added. The disappearance of radioactivity from the perfusate during 120 min and its recovery in the liver at the end of the experiments were then measured. Although the rabbit anti-rat hepatic lipase antiserum was shown to inhibit hepatic lipase activity by up to 90 %, and to bind extensively to hepatic sinusoidal surfaces when added to the perfusate, radioactivity from remnants of chylomicrons from rats given a bolus of fish oil as compared with palm oil disappeared from the perfusate and appeared in the liver more rapidly in the presence both the antiserum and the non-immune serum, and the differences between the uptake of the two types of remnants were similar. We conclude, therefore, that differential interaction with hepatic lipase is not responsible for the differences in the rate of removal of chylomicron remnants of different fatty acid composition from the blood.

Influence of different dietary fats on triacylglycerol deposition in rat adipose tissue

It has been demonstrated that triacylglycerol (TAG) mobilization from adipose tissue is selective and depends on fatty acid (FA) chain length, unsaturation and positional isomerism. The present study was performed to determine the influence of dietary fat on the composition of TAG stored in rat perirenal and subcutaneous adipose tissues. These results may provide information on the susceptibility of stored TAG to hydrolysis and further mobilization, and may help to establish an interrelationship between dietary composition and the FA efflux from adipose tissue. TAG molecular species and FA composition were determined by HPLC and GLC respectively. No significant differences were found in either FA or TAG composition between perirenal and subcutaneous adipose depots. The major FA in the dietary fats were present in the adipose tissues of the animals; in most cases, in similar proportions. However, differences were found between dietary and adipose tissue content of minor FA, which suggests that dietary FA composition is altered between ingestion and deposition in adipose tissue. The TAG molecular species of rat adipose tissue were enriched with the FA characteristic of each dietary fat. Dietary sunflower oil was responsible for enrichment with the most polar TAG. This finding may suggest easier mobilization of stored TAG. In conclusion, the process of fatty acid and TAG deposition in rat adipose tissue is selective, and depends on the composition of the diet.

Uptake of triacylglycerol-rich lipoproteins of differing triacylglycerol molecular species and unsaponifiable content by liver cells

The fatty acid composition of dietary oils can modulate the incorporation of triacylglycerol-rich lipoproteins (TRL) into hepatocytes, thus affecting the atherogenicity of these particles. However, nothing is known about the effect of the unsaponifiable fraction of the oils. In the present study, we evaluated the influence of these components on the uptake of TRL by rat primary hepatocytes. TRL were isolated from human serum after the intake of meals enriched in high-oleic sunflower oil (HOSO), virgin olive oil (VOO) or VOO enriched in its own unsaponifiable fraction (EVO). HOSO and HOSO-TRL differed from VOO and EVO and their corresponding TRL in the composition of triacylglycerol molecular species and of the unsaponifiable fraction. Furthermore, the increase in the unsaponifiable fraction of VOO led to changes in the triacylglycerol molecular species in the EVO-TRL. On incubation with hepatocytes, HOSO-TRL were taken up at a faster rate than VOO-TRL or EVO-TRL. In addition, in comparison to VOO-TRL, HOSO-TRL increased the expression of mRNA for the LDL receptor-related protein receptor, which plays an important role in the internalisation of remnant lipoproteins. EVO-TRL also increased LDL receptor-related protein mRNA expression in comparison with VOO-TRL, but this change was not accompanied by a rise in the uptake rate, suggesting that the unsaponifiable fraction of VOO may inhibit LDL receptor-related protein expression or activity post-transcriptionally. In conclusion, TRL from dietary oils with differing triacylglycerol molecular species and unsaponifiable fraction content are taken up by liver cells at different rates, and this may be important in the atherogenicity of these particles.

Postprandial serum triacylglycerols and oxidative stress in mice after consumption of fish oil, soy oil or olive oil: possible role for paraoxonase-1 triacylglycerol lipase-like activity

OBJECTIVE

Postprandial triacylglycerols and oxidative stress responses are influenced by the type of fat consumed. We investigated the effect of individual unsaturated fatty acids or oils (fish, soy, or olive) on postprandial triglyceridemia response in association with serum resistance to oxidation and paraoxonase-1 (PON1) activity.

METHODS

Balb/C mice were supplemented with phosphate buffered saline (control), docosahexaenoic acid (omega-3), linoleic acid (omega-6), or oleic acid (omega-9; 500 microg/300 microL of phosphate buffered saline) and with fish, soy, or olive oil (300 microL); blood samples were collected 2 h after feeding.

RESULTS

Serum triacylglycerol and oxidative stress responses increased after intake of all unsaturated fatty acids and oil supplements. However, ingestion of fish oil or its major fatty acid, docosahexaenoic acid, induced the most remarkable increase in postprandial serum triacylglycerols and in the susceptibility of serum to in vitro oxidation. Serum PON1 activity was decreased by 24% after fish oil ingestion. The increase in postprandial serum susceptibility to oxidation was lower after soy oil supplementation to PON1-transgenic mice in comparison with Balb/C mice, showing that PON1 attenuates the postprandial serum oxidative response. In parallel, in PON1-transgenic mice, a decreased postprandial triacylglycerol response was noted, suggesting PON1 involvement in triacylglycerol metabolism. PON1 exhibited a triacylglycerol lipase-like activity on chylomicrons.

CONCLUSION

PON1 attenuates the postprandial oxidative stress response, and this could have resulted from PON1 lipase-like activity on chylomicron triacylglycerols.

Differential uptake of subfractions of triglyceride-rich lipoproteins by THP-1 macrophages

It is well known that raised plasma triglycerides (TG) are positively linked to the development of coronary heart disease. However, triglycerides circulate in a range of distinct lipoprotein subfractions and the relative atherogenicity of these subfractions is not clear. In this study, three fractions of triglyceride rich lipoprotein (TRL) were isolated from normolipidaemic males according to their differing Svedberg flotation (S(f)) rates: chylomicron (CM, S(f)>400), very low-density lipoprotein (VLDL)-1 (S(f) 60-400) and VLDL-2 (S(f) 20-60). These fractions were incubated with THP-1 monocyte-derived macrophages for determination of cholesterol and TG accumulation, in the presence and absence of the lipoprotein lipase (LPL) inhibitor orlistat. Expression of LDL receptor related protein (LRP) and apolipoprotein B48 receptor (apoB48R) was also examined in both differentiating monocytes, and monocyte-derived macrophages, incubated with TRL. VLDL-1 caused a significantly greater accumulation of TG within macrophages compared to VLDL-2. Binding studies also tended to show a greater preference for VLDL-1. No change in expression of LRP or apoB48R was observed in fully differentiated macrophages incubated with VLDL-1, VLDL-2 or CM, although a greater expression of LRP mRNA was observed in differentiating monocytes exposed to VLDL-1, compared to those incubated with CM or VLDL-2. TG loading in response to all three TRL fractions was blocked by orlistat, suggesting that it is likely that the major pathway for uptake of TG was hydrolysis by LPL. Calculations suggested that direct uptake of particles accounts for between 12 and 25% of total TAG uptake. In conclusion, THP monocyte-derived macrophages demonstrate a preference for VLDL-1, both through the LPL pathway and by direct uptake of whole particles.

Greater enrichment of triacylglycerol-rich lipoproteins with apolipoproteins E and C-III after meals rich in saturated fatty acids than after meals rich in unsaturated fatty acids

BACKGROUND

Although there is considerable interest in the postprandial events involved in the absorption of dietary fats and the subsequent metabolism of diet-derived triacylglycerol-rich lipoproteins, little is known about the effects of meal fatty acids on the composition of these particles.

OBJECTIVE

We examined the effect of meal fatty acids on the lipid and apolipoprotein contents of triacylglycerol-rich lipoproteins.

DESIGN

Ten normolipidemic men received in random order a mixed meal containing 50 g of a mixture of palm oil and cocoa butter [rich in saturated fatty acids (SFAs)], safflower oil [n-6 polyunsaturated fatty acids (PUFAs)], or olive oil [monounsaturated fatty acids (MUFAs)] on 3 occasions. Fasting and postprandial apolipoproteins B-48, B-100, E, C-II, and C-III and lipids (triacylglycerol and cholesterol) were measured in plasma fractions with Svedberg flotation rates (S(f)) >400, S(f) 60-400, and S(f) 20-60.

RESULTS

Calculation of the composition of the triacylglycerol-rich lipoproteins (expressed per mole of apolipoprotein B) showed notable differences in the lipid and apolipoprotein contents of the SFA-enriched particles in the S(f) > 400 and S(f) 60-400 fractions. After the SFA meal, triacylglycerol-rich lipoproteins in these fractions showed significantly greater amounts of triacylglycerol and of apolipoproteins C-II (S(f) 60-400 fraction only), C-III, and E than were found after the MUFA meal (P < 0.02) and more cholesterol, apolipoprotein C-III (S(f) > 400 fraction only), and apolipoprotein E than after the PUFA meal (P < 0.02).

CONCLUSIONS

Differences in the composition of S(f) > 400 and S(f) 60-400 triacylglycerol-rich lipoproteins formed after saturated compared with unsaturated fatty acid-rich meals may explain differences in the metabolic handling of dietary fats.

The fatty acid composition of chylomicron remnants influences their propensity to oxidate

BACKGROUND AND AIM

Although the replacement of saturated with unsaturated dietary fat has been advocated as a means of reducing the risk of cardiovascular disease, diets high in polyunsaturated fatty acids (PUFAs) may increase lipid peroxidation, thus contributing to the pathogenesis of atherosclerosis. As the susceptibility of individual fatty acids to oxidation directly depends on their degree of unsaturation, and the oxidative modification of lipoproteins may be an important determinant of atherogenesis, the aim of this study was to evaluate the susceptibility to auto-oxidation and copper-mediated oxidation of chylomicron remnants (CMRs) enriched in n-3 or n-6 PUFA.

METHODS AND RESULTS

The remnants were prepared in vitro from chylomicrons obtained from rats given an oral dose of fish or corn oil, using rat plasma containing lipoprotein lipase. Their propensity to oxidate and the extent of the oxidation were estimated by measuring the formation of conjugated dienes and the detrimental products of lipid peroxidation. The results showed that: 1) the corn oil CMRs contained a relatively high proportion of n-6 PUFA (mainly linoleic acid), whereas the fish oil CMRs contained more n-3 PUFA, mainly eicosapentanoic and docosahexaenoic acids; 2) n-3-rich CMRs have a significantly lower propensity to oxidate than n-6-rich CMRs despite their 50% lower alpha-tocopherol content and 40% higher unsaturation index.

CONCLUSION

Our data indicate that the precise allocation of n-3 PUFA within the lipid core of CMRs may play a pivotal role in lowering the susceptibility to oxidation of fish CMRs by overcoming the effects of unfavourable alpha-tocopherol concentration. Eating n-3 rather than n-6 PUFAs seems to make CMRs more resistant against free radical attack, which may contribute to attenuating their potential atherogenic properties.

Dietary virgin olive oil triacylglycerols as an independent determinant of very low-density lipoprotein composition

OBJECTIVE

We examined the effects of virgin olive oil (VOO) triacylglycerols (TGs) on the lipid composition of human very low-density lipoprotein (VLDL).

METHODS

Twenty-one normocholesterolemic, normotensive, non-diabetic elderly subjects were recruited for the study. Two VOOs (VOO1 and VOO2) of the same variety, with an equivalent composition in minor components and differing only in the oleic and linoleic acid concentrations, were administered for 4 wk each to assess the effect of their TG molecular species compositions. Blood was collected after an overnight fast, VLDLs were isolated by ultracentrifugation, and lipid classes, TG molecular species, and TG fatty acid composition were determined.

RESULTS

Dietary VOOs significantly differed in TG molecular species composition. VOO1 represented larger amounts of triolein (P < 0.01), whereas VOO2 was significantly enriched with dilinoleoyl-oleoyl-glycerol, linoleoyl-dioleoyl-glycerol, and linoleoyl-oleoyl-palmitoyl-glycerol (P < 0.01). For VLDL, intake of VOO1 caused an increase of total TG (P < 0.01) due mainly to increases in triolein and linoleoyl-dioleoyl-glycerol. Conversely, VOO2 increased VLDL cholesteryl esters (P < 0.01) and TG rich in arachidonic acid (P < 0.01).

CONCLUSIONS

The different TG molecular species compositions of dietary oils may be an independent determinant of the lipid composition of VLDL in elderly people and therefore may play a role in regulating lipoprotein metabolism in these subjects.

Influence of the fatty acid composition of lipids in chylomicron remnants derived from fish or corn oil on the lipid profile of cultured rat hepatocytes

The aim of this work was to characterise the lipid and fatty acid composition of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) and to investigate their influence on the fatty acid profiles of the lipids of rat hepatocytes cultured in monolayers. Chylomicrons were prepared from the lymph collected from the thoracic duct of rats given an oral dose of fish or corn oil (high in n-3 and n-6 PUFA, respectively), and remnants were prepared in vitro from such chylomicrons using rat plasma containing lipoprotein lipase. The fatty acids predominating in the oils abounded also in their respective chylomicrons and remnants, especially in triacylglycerols. Chylomicrons as well as remnants contained small amounts of phospholipids and long-chain PUFA that were minor in, or absent from, the dietary oils, evidently provided by the intestinal epithelium. The incubation of hepatocytes for 6 h, with either n-3 or n-6 PUFA-rich remnants (0.25-0.75 mM triacylglycerol) resulted in a dose-dependent increase in the amount of triacylglycerols and phospholipids in the cells, which was not affected further by increasing the incubation time to 19 h. Whereas hepatocyte triacylglycerols mostly incorporated the PUFA predominating in each remnant type, the fatty acid profile of cell phospholipids was virtually unchanged. In addition, irrespective of whether they were enriched in n-3 or n-6 PUFA, remnants promoted a relative decrease in the amount of cholesteryl esters, a minor hepatocyte lipid class poor in PUFA. The results demonstrate that the hepatocyte fatty acid profile is modulated in a lipid-class specific way by the amount and type of dietary PUFA delivered to cells in chylomicron remnants.

Novel experimental protocol to increase specific plasma nonesterified fatty acids in humans

This study reports a novel protocol to increase plasma monounsaturated, polyunsaturated, and saturated nonesterified fatty acids (NEFA) in eight healthy volunteers (age 29-54 yr, body mass index 23-26 kg/m(2)). This was achieved by feeding small boluses of fat at different time points (35 g at 0 min and 8 g at 30, 60, 90, 120, 150, 180, and 210 min) in combination with a continuous low-dose heparin infusion. Olive oil, safflower oil, or palm stearin were used to increase monounsaturated, polyunsaturated, or saturated NEFAs, respectively. Plasma NEFA concentrations were increased for 2 h, when fat and heparin were given (olive oil: 745 +/- 35 micromol/l; safflower oil: 609 +/- 37 micromol/l, and palm stearin: 773 +/- 38 micromol/l) compared with the control test (no fat and no heparin: 445 +/- 41 micromol/l). During the heparin infusion, 18:1 n-9 was the most abundant fatty acid for the olive oil test compared with 18:2 n-6 for the safflower oil test and 16:0 for the palm stearin test (P < 0.01). The method described here successfully increases several types of plasma NEFA concentrations and could be used to investigate differential effects of elevated individual NEFAs on metabolic processes.

Butter differs from olive oil and sunflower oil in its effects on postprandial lipemia and triacylglycerol-rich lipoproteins after single mixed meals in healthy young men

Accumulation of postprandial triacylglycerol-rich lipoproteins is generated by assimilation of ingested dietary fat and has been increasingly related to atherogenic risk. Nevertheless, the influence of different kinds of dietary fatty acids on postprandial lipid metabolism is not well established, except for (n-3) polyunsaturated long-chain fatty acids. Our goal was to evaluate the effects of test meals containing a common edible fat source of saturated (butter), monounsaturated (olive oil) or (n-6) polyunsaturated (sunflower oil) fatty acids on postprandial lipid and triacylglycerol-rich lipoprotein responses. After a 12-h fast, 10 healthy young men ingested mixed meals containing 0 g (control) or 40 g fat, provided as butter, olive oil or sunflower oil in a random order. Fasting and postmeal blood samples were collected for 7 h. The no-fat test meal did not elicit any change over baseline except for plasma phospholipids, insulin and nonesterified fatty acids. Conversely, the three fat-containing meals elicited bell-shaped postprandial changes (P < 0.05) in serum triacylglycerols, free and esterified cholesterol, and nonesterified fatty acids. The butter meal induced a lower postprandial rise of triacylglycerols in serum and chylomicrons (incremental AUC, mmol.h/L: 0.72) than the two unsaturated oils (olive oil: 1.6, sunflower oil: 1.8), which did not differ. Circulating chylomicrons were smaller after the butter meal than after the two vegetable oil meals. The in vitro susceptibility of circulating chylomicrons to hydrolysis by postheparin plasma was higher after sunflower oil than after butter or olive oil. We conclude that butter results in lower postprandial lipemia and chylomicron accumulation in the circulation of young men than olive or sunflower oils after consumption of a single mixed meal.

The influence of chylomicron remnants on cholesteryl ester metabolism in cultured rat hepatocytes: comparison of the effects of particles enriched in n-3 or n-6 polyunsaturated fatty acids

The effect of chylomicron remnants derived from fish oil (rich in n-3 polyunsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids) on the formation and hydrolysis of cholesteryl esters in cultured rat hepatocytes was investigated. Hepatocytes were incubated with or without fish or corn oil chylomicron remnants (0.25-0.75 mM triacylglycerol), and the activity of acyl-CoA:cholesterol acyltranferase (ACAT) and cholesteryl ester hydrolases in the cytosol (cCEH) and endoplasmic reticulum (erCEH), and the expression of mRNA for ACAT1, ACAT2 and cCEH, and of enzyme protein for erCEH was determined. Addition of either type of remnants to hepatocyte cultures resulted in a decreased activity of erCEH, cCEH (after 6 and 19 h incubation), and of ACAT (after 6 h only). Hepatocyte levels of mRNA encoding ACAT1 and ACAT2 were not affected by either type of chylomicron remnants after 6 h of incubation, while ACAT2 mRNA levels were down-regulated by fish oil remnants as compared with corn oil remnants, and also with control cells in the long term (19 h). In contrast, cCEH mRNA levels were down-regulated by chylomicron remnants derived from corn oil but not fish oil. The expression of erCEH protein was induced in response to the inhibitory effect of both types of remnants on the activity of the enzyme, with corn oil remnants having a significantly greater effect. These findings demonstrate that dietary polyunsaturated fatty acids when delivered to hepatocytes in chylomicron remnants regulate the activity of the enzymes governing the intracellular cholesteryl ester balance, and suggest that dietary n-3 and n-6 polyunsaturated fatty acids or a metabolite thereof have differential effects on the expression of their genes at the mRNA and post-transcriptional levels.

Lipid and fatty acid composition of canine lipoproteins

Lipid classes and their fatty acids were studied in the major lipoprotein fractions from canine, in comparison with human, plasma. In dogs, high-density-lipoprotein (HDL), the main carrier of plasma phospholipid (PL), cholesterol ester (CE) and free cholesterol, was the most abundant lipoprotein, followed by low and very-low density lipoproteins (LDL and VLDL). Notably, LDL and VLDL contributed similarly to the total dog plasma triacylglycerol (TG). The PL composition was similar in all three lipoproteins, dominated by phosphatidylcholine (PC). Even though the content and composition of lipids within and among lipoproteins differed markedly between dog and man, the total amount of circulating lipid was similar. All canine lipoproteins were relatively richer than those from humans in long-chain (C20-C22) n-6 and n-3 polyunsaturated fatty acids (PUFA) but had comparable proportions of total saturated and monoenoic fatty acids, with 18:2n-6 being the main PUFA in both mammals. The fatty acid profile of canine and human lipoproteins differed because they had distinct proportions of their major lipids. There were more n-3 and n-6 long-chain PUFA in canine than in human plasma, because dogs had more HDL, their HDL had more PC and CE, and both these lipids were richer in such PUFA.

Normal oxidative stress and enhanced lipoprotein resistance to in vitro oxidation in hypertriglyceridemia: effects of bezafibrate therapy

Although there is evidence that hyperlipidemia and predominance of small dense low density lipoproteins (LDLs) are associated with increased oxidative stress, the oxidation status in patients with hypertriglyceridemia (HTG) has not been studied in detail. Therefore, we studied urinary levels of F(2)-isoprostanes (8-isoprostaglandin F(2alpha) and 2,3-dinor-5,6-dihydro-8-isoprostaglandin F(2alpha)) and susceptibility of very low density lipoproteins (VLDLs) and LDLs to oxidation ex vivo in 18 patients with endogenous HTG and 20 matched control subjects. In addition, the effects of 6 weeks of bezafibrate therapy were assessed in a double-blind, placebo-controlled, crossover trial. Urinary levels of F(2)-isoprostanes were similar in the HTG and normolipidemic group. Bezafibrate caused an increase in 8-isoprostaglandin F(2alpha) (762+/-313 versus 552+/-245 ng/24 h for bezafibrate and placebo therapy, respectively; P=0.03), whereas 2,3-dinor-5, 6-dihydro-8-isoprostaglandin F(2alpha) levels tended to be increased (1714+/-761 versus 1475+/-606 ng/24 h for bezafibrate and placebo therapy, respectively; P=0.11). VLDLs and LDLs were more resistant to copper-induced oxidation in patients with HTG than in control subjects. Bezafibrate reversed the oxidation resistance to the normal range. In conclusion, these results indicate the following: (1) HTG is associated with normal in vivo oxidative stress and enhanced ex vivo resistance of lipoproteins to oxidation. (2) Bezafibrate reduces the resistance of lipoproteins to copper-induced oxidation and enhances oxidative stress in HTG patients.

Lymphatic transport in rats of several dietary fats differing in fatty acid profile and triacylglycerol structure

We examined in rats the intestinal absorption of nine very different dietary fats (two rapeseed oils, corn, olive, palm and menhaden oil, butter, cocoa butter and lard) to investigate the influence of fatty acid profile and triacylglycerol structure on absorption. Absorption was followed for 24 h after administration of similar amounts of fats, and the accumulated lymphatic transport and amount of triacylglycerols found in lymph in response to the administered fats were calculated, revealing major differences. The transport of olive and low alpha-linolenic rapeseed oil was significantly higher than that of the other fats (P < 0.05), except corn oil. The lymphatic transport of the other fats followed a slower course, with cocoa butter and menhaden oil having the lowest amounts transported. The amount of triacylglycerols found in lymph in response to the administered fats at 8 h ranged from 27.5% of the administered dose for cocoa butter to 72.1% for olive oil. The value for cocoa butter was significantly lower than that for most other fats. At 24 h, the values ranged from 66.5% for cocoa butter to 121.2% for olive oil. The high value for olive oil suggested transport of endogenous as well as exogenous fatty acids. The low value observed after cocoa butter resulted from decreased lipolysis and possibly also low absorption of triacylglycerols with high levels of long-chain saturated fatty acids in the sn-1/3 position. Furthermore, a low value was observed after menhaden oil administration, suggesting decreased absorption of fats containing (n-3) long-chain polyunsaturated fatty acids. Overall, these results demonstrate the influence of the fatty acid composition and triacylglycerol structure on the lymphatic absorption of dietary fat.

Postprandial triacylglycerols from dietary virgin olive oil are selectively cleared in humans

The aims of the present study were to evaluate the effect of a meal rich in virgin olive oil on triacylglycerol composition of human postprandial triacylglycerol-rich lipoproteins (fraction Sf > 400), and to assess the role of the triacylglycerol molecular species concentration and polarity on lipoprotein clearance. Fasting (0 h) and postprandial blood samples were collected hourly for 7 h from eight healthy normolipidemic subjects after the ingestion of the meal. Plasma and lipoprotein triacylglycerol concentrations increased quickly over fasting values and peaked twice at 2 and 6 h during the 7-h postprandial period. The triacylglycerols in the lipoprotein fraction at 2 h generally reflected the composition of the olive oil, however, the proportions of the individualmolecular species were altered by the processes leading to their formation. Among the major triacylglycerols, the proportion of triolein (OOO; 43.6%) decreased (P < 0.05), palmitoyl-dioleoyl-glycerol (POO; 31. 1%) and stearoyl-dioleoyl-glycerol (SOO; 2.1%) were maintained and linoleoyl-dioleoyl-glycerol (LOO; 11.4%) and palmitoyl-oleoyl-linoleoyl-glycerol (POL; 4.6%) significantly increased (P < 0.05) compared with the composition of the triacylglycerols in the olive oil. Smaller amounts of endogenous triacylglycerol (0.8%), mainly constituted of the saturated myristic (14:0)and palmitic (16:0) fatty acids, were also identified. Analysis of total fatty acids suggested the presence of molecular species composed of long-chain polyunsaturated fatty acids of the (n-3) family, docosapentaenoic acid, [22:5(n-3)] and docosahexaenoic acid (DHA), [22:6(n-3)] and of the (n-6) family [arachidonic acid, [20:4(n-6)]. The fastest conversion of lipoproteins to remnants occurred from 2 to 4 h and was directly related to the concentration of the triacylglycerols in the lipoprotein particle (r = 0.9969, P < 0.05) and not with its polarity (r = 0.1769, P > 0.05). The rates of clearance were significantly different among the major triacylglycerols (OOO, POO, OOL and POL) (P < 0.05) and among the latter ones and PLL (palmitoyl-dilinoleoyl-glycerol, POS (palmitoyl-oleoyl-stearoyl-glycerol) and OLL (oleoyl-dilinoleoyl-glycerol) (P < 0.01). OOO was removed faster and was followed by POO, OOL, POL, PPO (dipalmitoyl-oleoyl-glycerol), SOO, PLL, POS and OLL.

Pinus pinaster oil affects lipoprotein metabolism in apolipoprotein E-deficient mice

The aim of the present study was to assess the antiatherogenic properties of Pinus pinaster (maritime pine) seed oil. To this end, the effects of P. pinaster oil supplementation on lipoprotein levels and atherosclerotic lesions were compared to those of lard or sunflower oil in apolipoprotein E-deficient mice. Plasma total cholesterol (P < 0.0001) and VLDL + intermediary density lipoprotein (IDL)-cholesterol (P < 0.0001) levels were lower in mice fed P. pinaster and sunflower oil than in those fed the lard diet. In contrast, triglycerides (P < 0.0001) and VLDL + IDL-triglycerides (P < 0.0001) levels were higher in mice fed P. pinaster oil than sunflower oil or lard. The VLDL + IDL lipid composition of apolipoprotein E-deficient mice fed P. pinaster oil was intermediate between that of lard-fed transgenic mice and that of wild-type mice fed nonpurified diet. Using the Triton WR1339 method, the fractional catabolic rate of plasma triglycerides was found to be lower in mice fed P. pinaster oil (P < 0.0001) than sunflower oil or lard diet, suggesting a defective clearance of triglycerides in the P. pinaster group. Finally, the susceptibility of triglyceride-rich lipoproteins to in vitro lipoprotein lipase-mediated lipolysis was lower in the P. pinaster oil-fed group than in the lard-fed group. Despite the differences in VLDL + IDL level and lipid composition, the surface areas of aortic atherosclerotic lesions were not significantly different among mice fed P. pinaster, sunflower or lard diets. In conclusion, the results of the present study indicated that feeding P. pinaster oil had no better preventive effect on aortic atherosclerotic lesion extension in apolipoprotein E-deficient mice than other saturated or polyunsaturated fats.

Hydrolysis of long-chain, n-3 fatty acid enriched chylomicrons by cardiac lipoprotein lipase

The hydrolysis of chylomicrons enriched in long-chain n-3 fatty acids by cardiac lipoprotein lipase was studied. In 60 min, 24.8% of the triacylglycerol fatty acids were released as free fatty acids. The fatty acids were hydrolyzed at different rates. DHA (docosahexaenoic acid, 22:6n-3) and EPA (eicosapentaenoic acid, 20:5n-3) were released at rates significantly less than average. Stearic acid (18:0), 20:1n-9, and alpha-linolenic acid (18:3n-3) were released significantly faster than average. There was no relationship between the rate of release of a fatty acid and the number of carbons or the number of double bonds. Lipoprotein lipase selectively hydrolyzes the fatty acids of chylomicron triacylglycerols. This selectively will result in remnants that are relatively depleted in 18:0, 20:1, and 18:3 and relatively enriched in 20:5 and 22:6.Key words: lipoprotein lipase, chylomicrons, fish oil, eicosapentaenoic acid, docosahexaenoic acid.

Effect of the fat composition of a single meal on the composition and cytotoxic potencies of lipolytically-releasable free fatty acids in postprandial plasma

Ingestion of a meal increases plasma levels of triglyceride (TG)-rich lipoproteins through the secretion of intestine-derived chylomcirons and liver-derived very low density lipoproteins (VLDL). We have determined the effects of the fat composition of a single meal on the composition of TG in TG-rich lipoproteins (VLDL + chylomicrons) and circulating and lipolytically-releasable free fatty acids (FFA) in postprandial (PP) plasma and on the cytotoxic potencies of the lipolytically-released FFA to cultured arterial wall cells. PP lipemia was induced by feeding fasted normolipidemic human subjects with a meal rich in saturated fat (SF) and another meal rich in polyunsaturated fat (PUF), or vice versa; each meal provided 65% of energy as fat, and polyunsaturated to saturated fatty acid ratios (P/S) of the SF and PUF in the meals were 0.40 and 2.49, respectively. The mean P/S of TG in TG-rich lipoproteins (1.43) and circulating FFA (1.46) in 4 h PP plasma of PUF were significantly higher than those in PP plasma of SF (0.44 and 0.59, respectively) in fasting plasma (0.52 and 0.53, respectively). In vitro lipolysis of fasting and PP serum by purified bovine milk lipoprotein lipase (LpL) resulted in a marked (8.8-12.3-fold) increase in the serum FFA level. The P/S of serum FFA in postlipolysis fasting and PP serum were consistently higher than that of FFA or that of TG associated with TG-rich lipoproteins in prelipolysis fasting and PP serum, indicating that polyunsaturated TG in VLDL and/or chylomicrons is more susceptible than saturated TG to lipolysis. When postlipolysis serum was interacted with cultured endothelial cells and mouse peritoneal macrophages (MPM), the lipolytically-released FFA in PP serum of SF and PUF disrupted the barrier function of endothelial cells and were cytotoxic to cultured MPM; FFA in postlipolysis fasting serum was not cytotoxic. FFA in postlipolysis PP serum of PUF were consistently more potent than that in postlipolysis PP serum of SF. Further study showed that all long-chain monounsaturated FFA and polyunsaturated FFA, but not saturated FFA, incorporated into lipoproteins (LDL) were cytotoxic to cultured MPM. In conclusion, despite the generally well-accepted belief that SF is more atherogenic than PUF, the present study provides in vitro evidence that the lipolytic remnant products of TG-rich lipoproteins produced after a meal rich in PUF are more injurious to arterial wall cells than those produced after a meal rich in SF.