Unsaturated fatty acids enhance low density lipoprotein uptake and degradation by peripheral blood mononuclear cells

Gene and protein expression profiling of the fat-1 mouse brain

Polyunsaturated fatty acids (PUFAs) are essential structural components of all cell membranes and, more so, of the central nervous system. Several studies revealed that n-3 PUFAs possess anti-inflammatory actions and are useful in the treatment of dyslipidemia. These actions explain the beneficial actions of n-3 PUFAs in the management of cardiovascular diseases, inflammatory conditions, neuronal dysfunction, and cancer. But, the exact molecular targets of these beneficial actions of n-3 PUFAs are not known. Mice engineered to carry a fat-1 gene from Caenorhabditis elegans add a double bond into an unsaturated fatty acid hydrocarbon chain and convert n-6 to n-3 fatty acids. This results in an abundance of n-3 eicosapentaenoic acid and docosapentaenoic acid specifically in the brain and a reduction in n-6 fatty acids of these mice that can be used to evaluate the actions of n-3 PUFAs. Gene expression profile, RT-PCR and protein microarray studies in the hippocampus and whole brain of wild-type and fat-1 transgenic mice revealed that genes and proteins concerned with inflammation, apoptosis, neurotransmission, and neuronal growth and synapse formation are specifically modulated in fat-1 mice. These results may explain as to why n-3 PUFAs are of benefit in the prevention and treatment of diseases such as Alzheimer's disease, schizophrenia and other diseases associated with neuronal dysfunction, low-grade systemic inflammatory conditions, and bronchial asthma. Based on these data, it is evident that n-3 PUFAs act to modulate specific genes and formation of their protein products and thus, bring about their various beneficial actions.

Oleic, linoleic and linolenic acids enhance receptor-mediated uptake of low density lipoproteins in Hep-G2 cells

In the present study, the binding, internalization and degradation of low-density lipoprotein (LDL) was investigated in Hep-G2 cells treated with 18:0, 18:1, 18:2 and 18:3. In non-treated control cells, the surface binding (heparin-releasable) of 125I-LDL progressed in a saturable manner reaching equilibrium within 2 h, amounting 24.0 +/- 1.1, 29.5 +/- 1.3 and 31.4 +/- 2.8 (ng/mg cell protein) at 1, 2 and 4 h, respectively. The cells rapidly internalized 125I-LDL reaching a plateau at 2 h (72.4 +/- 6.3/1 h, 96.7 +/- 4.3/2 h and 100.8 +/- 4.6 ng/mg protein/4 h, respectively). The degradation of internalized LDL progressed slowly during the first hour of incubation reflecting the time required to an uptake and delivery of LDL to the cellular lysosomes. The levels of degraded LDL discharged into the medium then increased rapidly in a linear manner after the initial lag period, amounting 16.8 +/- 1.2, 51.8 +/- 7.0 and 118.2 +/- 5.7 ng/mg protein at 1, 2 and 4 h, respectively. The treatment of cells with of 1.0 mM of fatty acids for 4 h resulted in a significant increase in the surface binding of 125I-LDL compared to the control (34.9 +/- 3.0), but it was significantly lower in cells exposed to 18:0 (48.2 +/- 2.0) than to 18:1 (56.8 +/- 5.1), 18:2 (56.0 +/- 3.5) and 18:3 (57.8 +/- 6.0 ng/mg protein/4 h) (P < 0.05). The levels of degraded LDL in cells remained nearly the same regardless of fatty acid treatments, but degraded LDL levels in the medium were much higher in cells exposed to 18:1 (167.6 +/- 10.1), 18:2 (159.8 +/- 7.7) and 18:3 (165.1 +/- 14.7) than to 18:0 (142.1 +/- 8.4) and the control (121.2 +/- 3.4 ng/mg protein/4 h) (P < 0.05). The present finding that 18:1 is equally effective in enhancing the receptor-mediated LDL uptake and its degradation as those of 18:2 and 18:3 suggests that the major action of 18:1 in lowering LDL-cholesterol levels also involves an increased clearance of LDL via hepatic LDL-receptors.

Free fatty acids modulate LDL receptor activity in BHK-21 cells

It has been shown that dietary fatty acids affect serum low density lipoprotein (LDL) levels, but the mechanism responsible for this effect is still under debate. Here we investigate the effect of different free fatty acids on LDL receptor activity in BHK-21 cells. These cells possess a classical LDL receptor strongly regulated by substances like 25-OH-cholesterol or lovastatin. Preincubation of cells for 24 h with both oleic (cis 18:1) and its trans counterpart, elaidic acid, enhanced 125I-LDL binding, internalization and degradation, being oleic acid more effective than elaidic acid. Among polyunsaturated fatty acids (PUFA) of the n-6 series arachidonic acid (20:4) enhanced LDL receptor activity more than linoleic acid (18:2), and among PUFA of the n-3 series docosahexaenoic (22:6) and eicosapentaenoic acids (20:5) were more effective compared to alpha-linolenic acid (18:3). Conversely, preincubation of cells with saturated fatty acids, palmitic (16:0) and stearic (18:0) acids, decreased binding, internalization and degradation of 125I-LDL. Scatchard analysis of binding data obtained with palmitic and oleic acids showed that these two fatty acids affect LDL receptor number without altering receptor affinity. The regulatory effect of free fatty acids on LDL receptor activity in BHK-21 cells is consistent with the hypothesis that the ability of fatty acids to modulate LDL-cholesterol levels in vivo is mediated, at least in part, by an action on receptor-dependent uptake of LDL.

In vitro regulation of low-density lipoprotein receptor interaction by fatty acids

Low-density lipoprotein (LDL) receptor binding is the initial step in receptor-mediated clearance. Dietary fat composition is known to affect LDL clearance, but the mechanism of the effect is unknown. We have examined the effects of altered membrane fatty acid composition, as might occur when specific dietary fats are consumed, on LDL binding using a Chinese hamster ovary (CHO) line that constitutively expresses the human LDL receptor. Binding of pooled human LDL to its receptor was compared in cells enriched with various fatty acids. Binding affinity was greater (lower Kd) for cells grown in 16:0-, 18:0-, or 18:1-enriched media than for those grown in 18:2 (P < .0001). The apparent receptor number (Bmax) was lower for cells enriched in saturated fatty acids and 18:1. Fluidity was assessed by measuring diphenylhexatriene (DPH) fluorescence anisotropy (rs). Cells enriched in 18:1 or 18:2 were the most fluid (P < .003). The correlation between binding and fluidity (r = .24, P = .27) was weak and did not appear to explain the effects of fatty acid modification on LDL receptor binding. Thus, it appears that cellular enrichment in 16:0, 18:0, and 18:1 increases binding affinity by affecting properties other than membrane fluidity. Changes in Bmax may also contribute to the observed differences in LDL binding.

Oleate and other long chain fatty acids stimulate low density lipoprotein receptor activity by enhancing acyl coenzyme A:cholesterol acyltransferase activity and altering intracellular regulatory cholesterol pools in cultured cells

Modification of dietary fatty acid composition results in changes in plasma cholesterol levels in man. We examined the effect of in vitro fatty acid supplementation on low density lipoprotein (LDL) receptor activity in cultured cells and questioned whether changes were related to fatty acid-induced alterations in acyl-CoA: cholesterol acyltransferase (ACAT) activity. Preincubation of cultured cells (i.e. human skin fibroblasts, J774 macrophages, and HepG2 cells) with oleic acid (oleic acid:bovine serum albumin molar ratio 2:1) at 37 degrees C for longer than 2 h resulted in a 1.2- to 1.5-fold increase in LDL cell binding at 4 degrees C and LDL cell degradation at 37 degrees C. Scatchard analysis showed that oleic acid increased LDL receptor number but not LDL affinity (Kd). Fatty acid supplementation of J774 macrophages increased both LDL receptor activity and cholesteryl ester accumulation. The ACAT inhibitor, 58-035, eliminated both effects, and increased ACAT activity preceded stimulation of LDL receptor activity by 1-2 h. Supplementation of macrophages with triolein emulsion particles also increased LDL cell binding and degradation, and addition of cholesterol to the emulsions abolished this effect. Among fatty acids tested, oleate (18:1), arachidonate (20:4), and eicosapentanoate (20:5) demonstrated the greatest effects. We hypothesize that certain fatty acids delivered to cells either in free form, or as triglyceride, first increase cellular ACAT activity, which then causes a decrease in an intracellular free cholesterol pool, signaling a need for increased LDL receptor activity. This mechanism may play a role in the effect of certain dietary fatty acids on LDL metabolism in vivo.

Plasma clearance and hepatic utilization of stearic, myristic and linoleic acids introduced via chylomicrons in rats

The primary objective of the present study was to compare the rates of plasma clearance and hepatic utilization of stearic (18:0), myristic (14:0) and linoleic (18:2) acids, as introduced via chylomicrons. Lymph chylomicrons were specifically labeled in vivo with [14C]stearic and (SA), [14C]myristic acid (MA), or [14C]linoleic acid (LA) by infusing donor rats intraduodenally with the labeled fatty acids in a lipid emulsion. Following intravenous injection of recipient rats with the labeled chylomicrons, the rates of plasma clearance and incorporation of the label in triglycerides (TG), phospholipids (PL) and other lipids in the liver were compared at 5, 15 and 30 min. [14C]SA was cleared at a slightly faster rate (t1/2 = 7.0 min) than [14C]MA (t1/2 = 8.1 min) and [14C]LA (t1/2 = 8.0 min) (P < 0.05). [14C]SA was accumulated in the liver at a significantly faster rate than [14C]MA and [14C]LA. At the peak (15 min) of hepatic uptake, 30.3% of [14C]SA, 26.2% of [14C]LA and 21.9% of [14C]MA were recovered in the liver. At 30 min, 33.5% of [14C]SA was taken up by the liver, whereas 27.8% of [14]LA and only 15.2% of [14C]MA were removed. In the liver, the percentage of [14C]SA incorporated into PL steadily increased with time, whereas the percent-age incorporated into TG decreased. [14C]SA was preferentially incorporated into PL at all time intervals, as compared with [14C]MA and [14C]LA. At 30 min, 38.6% of [14C]SA was found in PL, and only 5.2% of [14C]MA and 12.0% of [14C]LA were present in PL.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic effects of dietary stearic acid in mice: changes in the fatty acid composition of triglycerides and phospholipids in various tissues

The fatty acid patterns of triglycerides and phospholipids extracted from adipose tissue, liver, heart, kidney, spleen, and lung of 3 groups of C57BL/6 mice were determined after feeding diets rich in palmitic acid (16:0) (high palmitic: 16:0 = 45.1% of total fatty acids), stearic acid (18:0) (high stearic: 18:0 = 42.9% of total fatty acids) and oleic acid (18:1) (high oleic: 18:1 = 79.7% of total fatty acids) for 9 months. Triglyceride content of adipose, liver, heart, kidney, lung and spleen tissues was significantly enriched in palmitic acid in mice fed the high palmitic diet (range among all tissues: 19.9% +/- 0.2% to 29.0% +/- 1.9% of total fatty acids) and in oleic acid in mice fed the high oleic diet (range 56.0% +/- 1.9% to 71.6% +/- 1.2%). The stearic acid content of organ triglycerides in mice fed the high stearic diet ranged from 3.7% +/- 0.3% to 10.8% +/- 1.2%; however, the content of oleic acid on this diet (range: 57.0% +/- 1.8% to 71.4% +/- 1.7%) was similar to the one observed in mice fed the high oleic diet. In all organs, phospholipids had a significantly higher percentage of stearic acid (range: 23.5% +/- 0.9% to 51.5% +/- 6.6%) than triglycerides, regardless of diet. To evaluate the production of oleate from stearate and palmitate, 2 groups of mice were fed the high palmitic and the high stearic diets for 1 week and then injected intravenously with [1-14C]palmitate and [1-14C]stearate and the amount of labelled oleate in liver triglycerides was measured.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Lipoprotein interactions with T cells: an update

The role of plasma lipoproteins in atherogenesis is well recognized but the physiological relevance of their immunoregulatory properties is still questioned. Here Karine Traill and colleagues outline the recent advances that have been made towards unravelling the mechanisms of immunoregulation by lipoproteins in vitro and consider whether any of these mechanisms are operative in vivo. In particular they address the possible detrimental effects of high serum lipoprotein levels on immune function and the question of whether hyperlipidemia (or hypercholesterolemia) should be considered a risk factor for diminished immunity, for example in old age.

Effect of dietary fat saturation and cholesterol on LDL composition and metabolism. In vivo studies of receptor and nonreceptor-mediated catabolism of LDL in cebus monkeys

The mechanism(s) by which polyunsaturated fats reduce low density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B were investigated in 20 cebus monkeys (Cebus albifrons) fed diets containing corn oil or coconut oil as fat (31% of calories) with or without dietary cholesterol (0.1% by weight) for 3 to 10 years. Coconut-oil feeding compared to corn-oil feeding resulted in significant increases in levels of plasma total cholesterol (176%), very low density lipoprotein (VLDL)-LDL cholesterol (236%), high density lipoprotein (HDL) cholesterol (148%), apo B (78%), and apo A-I (112%). The addition of dietary cholesterol to corn oil compared to corn oil alone resulted in smaller, but significant, increases in levels of total cholesterol (44%), HDL cholesterol (40%), and apo A-I (33%). Although the increases in VLDL-LDL cholesterol were of similar magnitude (52%), they barely failed to reach statistical significance (p less than 0.08), while the changes in apo B levels were negligible. The addition of dietary cholesterol to coconut oil, compared to coconut oil alone, resulted in no significant changes in lipoprotein cholesterol or apoproteins, although levels of VLDL-LDL cholesterol and apo B values increased 22% and 16%, respectively. Although hepatic free cholesterol content was not altered by diet, coconut-oil compared to corn-oil feeding resulted in significant increases in hepatic cholesteryl esters (236%) and triglycerides (325%), the latter increasing still further when dietary cholesterol was added to coconut oil (563%). To further assess the effects of these dietary changes on LDL metabolism, radioiodinated normal and glucosylated LDL kinetics were performed. The production rate of LDL apo B was not altered by diet. With corn-oil feeding, 63% of LDL catabolism was via the receptor-mediated pathway. Coconut-oil compared to corn-oil feeding resulted in a 50% decrease in receptor-mediated LDL apo B fractional catabolic rate (FCR) and a 27% reduction in nonreceptor-mediated LDL apo B FCR. The addition of dietary cholesterol to corn oil, compared to corn oil alone, resulted in no significant effect on LDL apo B catabolism. The addition of dietary cholesterol to coconut oil, compared to coconut oil alone, was associated with no significant change in nonreceptor catabolism of LDL apo B but with a 58% decrease in receptor-mediated catabolism of LDL (p less than 0.059). The diet-induced alterations of LDL catabolism were significantly correlated with hepatic lipids, which were enriched in saturated fatty acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasma membrane enrichment with cis-unsaturated fatty acids enhances LDL metabolism in U937 monocytes

The mechanism by which dietary cis-unsaturated fatty acids lower low density lipoprotein (LDL) cholesterol is unknown. Because cis-unsaturated fatty acids incorporated into cell membranes increase membrane fluidity and, as a result, can alter membrane-dependent cell functions, we examined LDL binding, uptake, and degradation in upregulated U937 monocytes enriched in membrane oleate, a monounsaturated fatty acid, and membrane linoleate, a polyunsaturated fatty acid. The same parameters were also examined in upregulated U937 monocytes enriched in membrane stearate, a saturated fatty acid, and in upregulated, unmodified U937 monocytes. Monocytes enriched in cis-unsaturated fatty acids exhibited augmented LDL binding, internalization, and degradation compared with both stearate-enriched monocytes and unmodified monocytes. The molar potency of linoleate in augmenting LDL metabolism was 50% greater than that of oleate. Enrichment with oleate and linoleate resulted in a decrease in the fatty acyl mole-weighted melting point of the plasma membrane and an increase in plasma membrane fluidity, as indicated by a reduction in the steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene incorporated into the membrane. Stearate-enriched monocytes exhibited a slight increase in the plasma membrane fatty acyl mole-weighted melting point and essentially no change in plasma membrane fluidity. Thus, one mechanism by which cis-unsaturated fatty acids lower LDL cholesterol may involve alteration in membrane lipid composition and physical properties, thereby leading to an increase in cellular clearance of this atherogenic lipoprotein.

Effect of dietary fat saturation and cholesterol on low density lipoprotein degradation by mononuclear cells of Cebus monkeys

The mechanism by which dietary unsaturated fatty acids lower low density lipoprotein (LDL) cholesterol is unknown. Unsaturated fatty acids incorporated into the cell membrane can increase membrane fluidity and, as a result, dramatically alter membrane-dependent cell functions. Therefore, we examined the effect of long-term dietary consumption of corn oil and coconut oil with and without cholesterol in amounts equivalent to those of a typical Western diet on the degradation of human LDL by peripheral blood mononuclear cells in Cebus albifrons monkeys. Cellular LDL degradation was dramatically enhanced in the mononuclear cells isolated from animals fed corn oil in comparison with those from animals fed coconut oil. The addition of cholesterol to the diets resulted in a slight attenuation of LDL degradation in the corn oil group while no effect was noted in the coconut oil group. Crossover LDL binding and degradation experiments with LDL isolated from animals fed corn oil diets and coconut oil diets demonstrated increased binding and degradation of LDL in mononuclear cells from animals fed corn oil diets. Enhanced mononuclear cell LDL degradation was accompanied by increased cellular cis-unsaturated fatty acyl content, increased membrane fluidity, and decreased plasma cholesterol. Increased cellular cis-unsaturated fatty acyl content with its concomitant increase in membrane fluidity mirrored the dietary lipid profile of the host animal. A linear relationship was observed between cellular LDL degradation and both cellular cis-unsaturated fatty acyl content and membrane fluidity. These observations parallel results noted in whole-animal LDL catabolic studies with these same animals described elsewhere. These data suggest a novel mechanism by which dietary unsaturated fatty acids exert their LDL-lowering effect.

Food safety and health effects of canola oil

Canola oil is a newly marketed vegetable oil for use in salads and for cooking that contains 55% of the monounsaturated fatty acid; oleic acid, 25% linoleic acid and 10% alpha-linolenate [polyunsaturated fatty acid (PUFA)], and only 4% of the saturated fatty acids (SFAs) that have been implicated as factors in hypercholesterolemia. It is expressed from a cultivar of rapeseed that was selectively bred from old varieties in Canada to be very low in erucic acid--a fatty acid suspected to have pathogenic potential in diets high in the original rapeseed oil in experimental animals. Canola oil is free of those problems. It is the most widely consumed food oil in Canada, and has been approved for Generally Recognized as Safe (GRAS) status by the Food and Drug Administration (FDA) of the United States Department of Health and Human Services. The fatty acid composition of canola oil is consistent with its use as a substitute for SFAs, in meeting the dietary goals recommended by many health associations: an average diet containing about 30% of calories as fat made up of less than 10% SFAs, 8-10% PUFAs in a ratio of linoleic to linolenic acids between 4:1 and 10:1, the remainder being monounsaturated fatty acids. No single oil meets these current recommendations for ratios of PUFA/monounsaturated/polyunsaturated fatty acid ratios as the sole source of cooking and salad oil.

Dose-response effects of dietary marine oil on carbohydrate and lipid metabolism in normal subjects and patients with hypertriglyceridemia

Recent studies indicate that marine (omega-3) fatty acids decrease hypertriglyceridemia but worsen hyperglycemia in diabetes. We studied dose-response relationships between omega-3 intake and indices of carbohydrate and lipid metabolism in 21 hypertriglyceridemic patients with (n = 6) or without (n = 15) diabetes, and 6 normal volunteers. All subjects consumed isocaloric diets of 65% carbohydrate, 20% fat, and 15% protein. The basal diet contained 15% of total calories as vegetable oil (omega-6), and the test diets included 15%, 7.5%, or 3.75% calories as fish oil (MaxEPA). After three months of the basal diet, patients were randomized to receive two 3-month omega-3 diets in the following sequences: 15%/7.5%, 7.5%/15%, 7.5%/3.75%, or 3.75%/7.5%. Both 15% and 7.5% diets, regardless of sequence, significantly decreased serum triglycerides but increased low-density lipoprotein (LDL)-cholesterol levels as much as 98% and LDL/high-density lipoprotein (HDL)-cholesterol ratio as much as 1.6-fold. Daily insulin requirements of three diabetic patients increased progressively while they received an omega-3-enriched diet for up to 2 years. In healthy controls, favourable changes induced by an omega-3 fatty acid diet in serum lipids and lipoproteins were associated with a tendency toward an inhibition of C-peptide secretion following a meal challenge. We conclude that substitution of commercially available omega-3 for omega-6 fatty acids improves hypertriglyceridemia but may worsen other lipoproteins indices and may increase insulin requirements in diabetic hypertriglyceridemic subjects.