Chylomicrons or their remnants penetrate rabbit thoracic aorta as efficiently as do smaller macromolecules, including low-density lipoprotein, high-density lipoprotein, and albumin

Predictive utility of remnant cholesterol in atherosclerotic cardiovascular disease

PURPOSE OF REVIEW

Remnant cholesterol (RC) is the cholesterol carried in lipoproteins derived from the catabolism of chylomicrons and very low-density lipoproteins. Evidence supporting the causal relationship of RC with atherosclerotic cardiovascular disease (ASVD) is accumulating rapidly. The number of impactful contributions to this field are increasing and provide a pathophysiological insight into the current residual cardiovascular risk beyond low-density cholesterol (LDL)-cholesterol (LDL-C). They also raise the question of whether RC should be used in prediction models and become the target of new therapeutic interventions. The intent of this review is to highlight the recent advances on the role of RC in atherogenesis and the validation of RC as a predictor of ASVD.

RECENT FINDINGS

Numerous prospective and retrospective cohorts helped validate a significant causal relationship of RC with various forms of ASVD, independent of LDL-C. A recent large Mendelian randomization study reinforced the existence of this relationship and showed that the risk of atherosclerotic events was driven nearly entirely by a direct effect of RC.

SUMMARY

Both available and accumulating evidence suggest that a lifelong reduction in RC could translate into a substantial reduction in ASVD risk. The data support a revision of current guidelines to incorporate RC as an independent risk factor for ASVD. We propose that early screening of RC should be implemented and that RC lowering should become the target of future drug developments.

Postprandial dyslipidemia in insulin resistant states in adolescent populations

Obesity and the metabolic syndrome are becoming increasingly prevalent not only in adults, but also in adolescents. The metabolic syndrome, a complex cluster of metabolic abnormalities, increases one's risk of developing type 2 diabetes and cardiovascular disease (CVD). Dyslipidemia, a key component of the metabolic syndrome, is highly associated with insulin resistance and contributes to increased CVD risk. Dyslipidemia has traditionally been assessed using a fasting lipid profile [i.e. fasting triglycerides, total cholesterol, low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C)]. However, the postprandial state predominates over the course of a day and non-fasting triglycerides independently predict CVD risk. In insulin resistant states, the intestine overproduces triglyceride-rich lipoprotein (TRL) particles, termed chylomicrons (CMs), following ingestion of a fat-containing meal, as well as in the fasting state. Along with elevated hepatic TRLs (i.e. very-low density lipoproteins), CMs contribute to remnant lipoprotein accumulation, small dense LDL particles, and reduced HDL-C, which collectively increase CVD risk. Given the early genesis of atherosclerosis and physiological metabolic changes during adolescence, studying postprandial dyslipidemia in the adolescent population is an important area of study. Postprandial dyslipidemia in the pediatric population poses a significant public health concern, warranting a better understanding of its pathogenesis and association with insulin resistance and CVD. This review discusses the metabolic syndrome, focusing on the link between insulin resistance, postprandial dyslipidemia, and CVD risk. Furthermore, the clinical significance and functional assessment of postprandial dyslipidemia, specifically in the adolescent population, is discussed in more detail.

Differential Effects of Low-Density Lipoprotein and Chylomicron Remnants on Lipid Accumulation in Human Macrophages

The effects of low-density lipoprotein (LDL) and chylomicron remnants on lipid accumulation in human monocyte–derived macrophages (HMDMs) and in macrophages derived from the human monocyte cell line THP-1 were compared. The HMDMs or THP-1 macrophages were incubated with LDL, oxidized LDL (oxLDL), chylomicron remnant–like particles (CMR-LPs), or oxidized CMR-LPs (oxCMR-LPs), and the amount and type of lipid accumulated were determined. As expected, the lipid content of both cell types was increased markedly by oxLDL but not LDL, and this was due to a rise in cholesterol, cholesteryl ester (CE), and triacylglycerol (TG) levels. In contrast, both CMR-LPs and oxCMR-LPs caused a considerable increase in cellular lipid in HMDMs and THP-1 macrophages, but in this case there was a greater rise in the TG than in the cholesterol or CE content. Lipid accumulation in response to oxLDL, CMR-LPs, and oxCMR-LPs was prevented by the ACAT inhibitor CI976 in HMDMs but not in THP-1 macrophages, where TG levels remained markedly elevated. The rate of incorporation of [3H]oleate into CE and TG in THP-1 macrophages was increased by oxLDL, CMR-LPs, and oxCMR-LPs, but incorporation into TG was increased to a greater extent with CMR-LPs and oxCMR-LPs compared with oxLDL. These results demonstrate that both CMR-LPs and oxCMR-LPs cause lipid accumulation in human macrophages comparable to that seen with oxLDL and that oxidation of the remnant particles does not enhance this effect. They also demonstrate that a greater proportion of the lipid accumulated in response to CMR-LPs compared with oxLDL is TG rather than cholesterol or CE and that this is associated with a higher rate of TG synthesis. This study, therefore, provides further evidence to suggest that chylomicron remnants have a role in foam cell formation that is distinct from that of oxLDL.

Plasma triglyceride and high density lipoprotein cholesterol are poor surrogate markers of pro-atherogenic chylomicron remnant homeostasis in subjects with the metabolic syndrome

BACKGROUND

Subjects with metabolic syndrome (MetS) exhibit impaired lipoprotein metabolism and have an increased risk of cardiovascular disease. Although the risk is attributed primarily to the risk associated with individual components, it is also likely affected by other associated metabolic defects. Remnants of postprandial lipoproteins show potent atherogenicity in cell and animal models of insulin resistance and in pre-diabetic subjects with postprandial dyslipidemia. However, few studies have considered regulation of chylomicron remnant homeostasis in MetS per se. This study measured the plasma concentration in Caucasian men and women of small dense chylomicrons following fasting and explored associations with metabolic and anthropometric measures.

METHODS

A total of 215 Australian Caucasian participants (median age 62 years) were investigated. Of them, 40 participants were classified as having MetS. Apolipoprotein (apo) B-48, an exclusive marker of chylomicrons, metabolic markers and anthropometric measures were determined following an overnight fast.

RESULTS

The fasting apo B-48 concentration was 40 % higher in subjects with MetS than those without MetS. In all subjects, triglyceride (r = 0.445, P < 0.0005), non-HDL cholesterol (r = 0.28, P < 0.0005) and HDL cholesterol concentration (r = -0.272, P < 0.0005) were weakly associated with apo B-48 concentration. In subjects with MetS, the association of apo B-48 with triglyceride and non-HDL cholesterol was enhanced, but neither were robust markers of elevated apo B-48 in MetS (r = 0.618 and r = 0.595 respectively). There was no association between apo B-48 and HDL cholesterol in subjects with MetS.

CONCLUSION

This study demonstrates a substantial accumulation of pro-atherogenic remnants in subjects with MetS. We have shown that in a Caucasian cohort, the fasting plasma concentration of triglyceride or HDL/non-HDL cholesterol serves as poor surrogate markers of atherogenic chylomicron remnants. These findings suggest that subjects with MetS exhibit a chronic defect in chylomicron metabolism that is likely to contribute to their increased CV risk.

Brief Communication: Discordant ability of the triglyceride to apolipoprotein B ratio to predict triglyceride-rich lipoprotein particle size in normal-weight and obese men

The atherogenicity of triglyceride-rich lipoproteins (TRLs) is dependent of their particle size as it determines their metabolic fate. Since TRL possess a single apolipoprotein B (Apo B) molecule per particle, the triglyceride (TG)/Apo B ratio has been used as a convenient method to estimate TRL size. The aim of this study was to validate this approach by correlating the serum TG/Apo B ratio, and the TRL particle size measured by dynamic light scattering (DLS). Twenty-four male volunteers (12 normal-weight and 12 obese individuals) received a high-fat meal. Preprandial (0 h) and postprandial (2 and 4 h) serum samples were collected after meal ingestion, and TRLs were isolated. Serum TG and Apo B levels were quantified, and the TG/Apo B ratio was plotted against TRL particle size measured by DLS for correlation. A strong association between TRL particle size and serum TG/Apo B ratio for normal-weight subjects (P ≤ 0.001) was observed but not for obese subjects (P = 0.6116). TG/Apo B ratio correlates with particle size in healthy normal-weight males but not in obese individuals. Whether this ratio is useful to estimate TRL size in females and in other dyslipidemic patients should be subject of future investigations.

Palmitic acid increases medial calcification by inducing oxidative stress

BACKGROUND

Aortic medial calcification is a cellular-regulated process leading to arterial stiffness. Although epidemiological studies have suggested an association between the saturation of fatty acids (FA) and arterial stiffness, there is no evidence that saturated FA can induce arterial calcification. This study investigated the capacity of palmitic acid (PA) to induce medial calcification and the signaling pathway(s) implicated in this process.

METHODS

Rat aortic segments and vascular smooth muscle cells (VSMC) were exposed to calcification medium supplemented with PA. In vivo, rats were treated with warfarin to induce calcification and fed a PA-enriched diet.

RESULTS

In vitro and ex vivo, palmitate increases calcification and ROS production. Palmitate increases extracellular-signal-regulated kinase (ERK1/2) phosphorylation and osteogenic gene expression. Inhibition of NADPH oxidase with apocynin or an siRNA prevents these effects. ERK1/2 inhibition attenuates the amplification of osteogenic gene expression and calcification induced by palmitate. In vivo, a PA-enriched diet amplified medial calcification and pulse wave velocity (PWV). These effects are mediated by ROS production as indicated by the inhibition of calcification and PWV normalization in rats concomitantly treated with apocynin.

CONCLUSION

ROS induction by palmitate leads to ERK1/2 phosphorylation and subsequently induces the osteogenic differentiation of VSMC. © 2013 S. Karger AG, Basel.

Apolipoprotein A-V modulates multiple atherogenic mechanisms in a mouse model of disturbed clearance of triglyceride-rich lipoproteins

OBJECTIVE

Apolipoprotein A-V plays an important role in reducing plasma triglyceride levels. We hypothesized that expression of apoA-V would inhibit atherogenesis in apoE(-/-) mice fed chow diet which is a known model of hypercholesterolemia. Our aim was to study this protective effect and to explore possible mechanisms.

METHODS AND RESULTS

ApoA-V(+/+)ApoE(-/-) mice expressing human apolipoprotein A-V (hapoA-V) were generated and compared to apoE(-/-) mice. Atherosclerotic aortic sinus lesion area was 70% smaller in hapoA-V(+/+)apoE(-/-). This was accompanied by a 58% reduction in lesion macrophage content. Furthermore, advanced atherosclerotic lesions in hapoA-V(+/+)apoE(-/-) mice showed features of a more stable plaque, manifested by 59% and 37% higher collagen and α-actin content, respectively. Plasma triglyceride and cholesterol levels in hapoA-V(+/+)apoE(-/-) mice were 47% and 33% lower, respectively. These were associated with a 33% reduction in very low density lipoprotein triglyceride production and 2-fold acceleration in triglyceride-rich lipoprotein clearance in hapoA-V(+/+)apoE(-/-) mice. In addition, hapoA-V(+/+)apoE(-/-) mice showed enhanced insulin sensitivity (25% and 15% improvement in glucose tolerance and insulin responsiveness, respectively). Finally, hapoA-V(+/+)apoE(-/-) displayed a milder systemic inflammatory response compared to apoE(-/-) mice, manifested by 22%, 65% and 15% lower plasma levels of TNFα, IL-1β and IL-6, respectively.

CONCLUSIONS

We showed that human apolipoprotein A-V is a potent modulator of atherosclerosis in mice through multiple modes of action. These findings may identify apoA-V as a potential therapeutic target for treatment of atherosclerosis.

Consumption of low doses of fat prevents the postprandial rise in chylomicron particle concentration and remnant accumulation in healthy normolipidaemic males

Chylomicron particles are continually synthesised and secreted from the intestine even in the absence of ingested fat. It is possible that following consumption of low doses of fat the basal level of chylomicron secretion and subsequent metabolism are sufficient to metabolise this fat without an increase in postprandial chylomicron concentrations. To test this hypothesis, healthy male subjects were randomised to receive, on three separate occasions, meals containing a range of doses of fat (average 8·1–19 g) and effects on postprandial lipaemia and chylomicron concentration were determined. Furthermore, to delineate the effect on lipid-rich v. lipid-poor (remnant) forms lipid levels were also determined in a density <1·006 g/ml fraction. Following consumption of the very low dose of fat the postprandial concentration of chylomicrons was unaltered, whereas following the medium dose postprandial chylomicron concentrations were significantly increased. Interestingly, this increase was only detected in the lipid-rich chylomicron fraction, with postprandial levels of chylomicron remnants remaining unchanged. In conclusion, it appears that consumption of what would be considered low to medium doses of fat are not associated with transient postprandial increases in chylomicron remnants in healthy male subjects.

The oxidative state of chylomicron remnants influences their modulation of human monocyte activation

Chylomicron remnants (CMRs) contribute directly to human monocyte activation in vitro, by increasing reactive oxygen species (ROS) production and cell migration. In this study, the effects of the oxidative state of CMR on the degree of monocyte activation was investigated. CMR-like particles (CRLPs) were prepared in three different oxidative states, normal (CRLPs), protected from oxidation by incorporation of the antioxidant, probucol (pCRLPs), or oxidised with CuSO(4) (oxCRLPs). Lipid accumulation and ROS production were significantly increased in primary human monocytes incubated with CRLPs, whilst secretion on monocyte chemoattractant protein-1 was reduced, but oxCRLPs had no additional effect. In contrast, pCRLPs were taken up by monocytes to a lesser extent and had no significant effect on ROS or MCP-1 secretion. These studies suggest that the oxidative state of CMRs modulates their stimulation of the activation of peripheral blood human monocytes and that dietary antioxidants may provide some protection against these atherogenic effects.

Understanding postprandial inflammation and its relationship to lifestyle behaviour and metabolic diseases

Postprandial hyperlipidemia with accumulation of remnant lipoproteins is a common metabolic disturbance associated with atherosclerosis and vascular dysfunction, particularly during chronic disease states such as obesity, the metabolic syndrome and, diabetes. Remnant lipoproteins become attached to the vascular wall, where they can penetrate intact endothelium causing foam cell formation. Postprandial remnant lipoproteins can activate circulating leukocytes, upregulate the expression of endothelial adhesion molecules, facilitate adhesion and migration of inflammatory cells into the subendothelial space, and activate the complement system. Since humans are postprandial most of the day, the continuous generation of remnants after each meal may be one of the triggers for the development of atherosclerosis. Modulation of postprandial lipemia by lifestyle changes and pharmacological interventions could result in a further decrease of cardiovascular mortality and morbidity. This paper will provide an update on current concepts concerning the relationship between postprandial lipemia, inflammation, vascular function, and therapeutic options.

New insights into how the intestine can regulate lipid homeostasis and impact vascular disease: frontiers for new pharmaceutical therapies to lower cardiovascular disease risk

In recent years, evidence has emerged that the intestine is a significant regulator of systemic cholesterol homeostasis and can contribute to raised plasma cholesterol concentration. In this review we provide a context for the role the intestine may have in cardiovascular disease during conditions of chronic disease (insulin resistance, obesity). In particular, we highlight the physiological role of the intestine in lipid absorption, identify novel elements in enterocyte molecular biology, review the concept that chylomicrons and their remnants contribute to atherogenesis during chronic disease, and address new principles of chylomicron overproduction during conditions of insulin resistance including the associated hormonal control of the intestine during these conditions. Finally, we raise the issue of a growing need for novel lipid-lowering pharmaceutical therapies that target intestinal lipid metabolism.

Interaction between remnant-like lipoprotein particles and adipocytes

Inflammation plays a pivotal role in the pathophysiology of atherosclerosis. Remnant-like lipoprotein particles (RLPs) have been implicated as potentially atherogenic lipoproteins. RLPs can induce a pronounced inflammatory response especially during the postprandial phase. Increasing evidence demonstrates that adipose tissue may be a significant contributor to the increased systemic inflammation through secretion of various proinflammatory adipocytokines. In addition, adipocytes may potentially regulate RLPs metabolism. Therefore, we hypothesized that there might be an interaction between RLPs and adipocytes.

Review: Hypercholesterolemia-associated endothelial progenitor cell dysfunction

Hypercholesterolemia has been associated with increased cardiovascular risk by contributing to mechanical endothelial injury and dysfunction. There is evidence that chronic exposure to increased plasma cholesterol levels might also impair the repair of lipoprotein-mediated endothelial injury, possibly by reducing the availability and function of circulating endothelial progenitors. This review summarizes current knowledge about the mechanisms of lipoprotein-mediated endothelial injury and endothelial progenitor cell assisted vascular repair; the influence of hypercholesterolemia on endothelial progenitor cell dysfunction will be also addressed.

Chylomicron and apoB48 metabolism in the JCR:LA corpulent rat, a model for the metabolic syndrome

Postprandial (PP) lipaemia is a significant contributor to the development of dyslipidaemia and cardiovascular disease (CVD). It is also evident that PP lipaemia is prevalent during conditions of obesity and insulin resistance (IR) and may contribute to increased progression of CVD. Our group has assessed the potential of the obese JCR:LA-cp rat as a model of PP lipaemia in order to explore CM (chylomicron) metabolism during the onset and development of IR in the metabolic syndrome. Studies confirm that both fasting plasma and PP apoB48 (apolipoprotein B48) area under the curve are significantly elevated in the obese JCR:LA-cp phenotype as compared with lean controls. Mechanistic studies have also shown that the concentration of lymphatic CM apoB48 and CM size are significantly increased in this model. Furthermore, PP dyslipidaemia in the obese rat can be improved acutely with supplementation of n-3 polyunsaturated fatty acids. Using a different approach, we have subsequently hypothesized that the vascular remodelling that accompanies IR may explain accelerated entrapment of apoB48-containing particles. Small leucine-rich proteoglycans (including biglycan and decorin) have been observed to co-localize with apoB in human tissue. However, the potential impact of IR on vascular remodelling, particularly in the presence of obesity, remains unclear. Preliminary observations from the JCR:LA-cp model indicate that biglycan protein core content increases with age and is exacerbated by IR, suggestive of pro-atherogenic remodelling. The focus of this review is to contribute to the perspective of PP lipaemia in CVD risk associated with the metabolic syndrome through the use of animal models.

Remnant-like lipoprotein particles impair endothelial function: direct and indirect effects on nitric oxide synthase

Remnant-like lipoprotein particles (RLPs) have been implicated as potentially atherogenic lipoproteins. Endothelial dysfunction is known to be an early event in atherosclerosis and an important contributor to the pathogenesis of coronary artery disease. Moreover, there is considerable evidence linking increased RLP cholesterol levels with endothelial dysfunction, reflected by impaired endothelial vasodilatation and abnormal endothelial secretion. The underlying mechanisms by which RLPs may contribute to endothelial dysfunction are complex and have not been completely elucidated. Because the expression and activation of endothelial nitric oxide synthase (eNOS) are vital to endothelial function, and recent data have implied an association between RLPs and eNOS, this manuscript proposes the hypothesis that RLPs could impair endothelial function via direct and indirect effects on eNOS: RLPs may affect the autophosphorylation of focal adhesion kinase and its downstream phosphatidylinositol kinase/Akt (protein kinase B) signaling pathway, resulting in eNOS inactivation through induction of intracellular oxidative stress in endothelial cells; and RLPs could affect the expression or activation of eNOS indirectly by stimulating secretion of various inflammatory factors from multiple origins. The practical applications of this manuscript provide new insights for the future investigation of RLPs.

Differential modulation of hepatic very low-density lipoprotein secretion by triacylglycerol-rich lipoproteins derived from different oleic-acid rich dietary oils

Minor components from dietary oils can modulate the atherogenic response of the TAG-rich lipoproteins (TRL) in which they are transported. In the present study we investigated the influence of TRL isolated from man after the intake of oleic acid-rich oils with different minor component compositions on VLDL secretion by rat primary hepatocytes. TRL were isolated from nine men after the intake of meals enriched with high-oleic sunflower oil (HOSO) or virgin olive oil (VOO) or VOO enriched with minor components (EVO). TRL were incubated with rat primary hepatocytes and the lipid accumulation was analysed in the cells and the secreted VLDL. The expression of genes for proteins related to hepatic lipid metabolism and VLDL production was also measured. Incubation of hepatocytes with TRL derived from HOSO as compared to VOO led to lower intracellular lipid accumulation and VLDL production despite higher mRNA expression for diacylglycerol-acyltransferase, microsomal TAG transfer protein, apoB and PPARalpha. When TRL derived from EVO were used there were no changes in VLDL secretion. These results suggest that incorporation of minor components from dietary high-oleic oils into TRL modulates the effect of these atherogenic particles on VLDL secretion.

Oxidation of chylomicron remnant-like particles inhibits their uptake by THP-1 macrophages by apolipoprotein E-dependent processes

The influence of the oxidative state of chylomicron remnants (CMR) on the mechanisms of their uptake and induction of lipid accumulation by macrophages derived from the human monocyte cell line, THP-1, during foam cell formation was investigated using chylomicron-remnant-like particles (CRLPs) at 3 different levels of oxidation. The oxidative state of CRLPs was varied by exposure to CuSO₄ (oxCRLPs) or incorporation of the antioxidant, probucol (pCRLPs) into the particles. oxCRLPs caused significantly less accumulation of triacylglycerol in the macrophages than CRLPs, and their rate of uptake was lower, while pCRLPs caused more lipid accumulation and were taken up faster. Uptake of all 3 types of particles was inhibited to a similar extent when entry via the low density lipoprotein (LDL) receptor related protein (80–90%), LDL receptor (− 30–40%), CD36 (− 40%) and phagocytosis (− 35–40%) was blocked using lactoferrin, excess LDL, anti-CD36 and cytochalasin D, respectively, but blocking scavenger receptors-A or -B1 using poly inosinic acid or excess HDL had no effect. These findings show that oxidation of CRLPs lowers their rate of uptake and induction of lipid accumulation in macrophages. However, oxidation does not change the main pathways of internalisation of CRLPs into THP-1 macrophages, which occur mainly via the LRP with some contribution from the LDLr, while CD36 and phagocytosis have only a minor role, regardless of the oxidative state of the particles. Thus, the effects of CMR oxidation on foam cell formation contrast sharply with those of LDL oxidation and this may be important in the role of dietary oxidized lipids and antioxidants in modulating atherosclerosis.

Chronically gorgingv. nibbling fat and cholesterol increases postprandial lipaemia and atheroma deposition in the New Zealand White rabbit

In the present study, we compared the effects of nibbling and gorging on postprandial lipaemia and lipoproteins, hepatic lipid uptake and atheroma deposition. New Zealand White rabbits were fed on a low-fat (LF) control diet or a peanut oil- (10 g/d) and cholesterol- (0·5 g/d) enriched (HF) diet with the fat and cholesterol components given either by nibbling (HF-N) or gorging (HF-G). After 4 and 8 weeks, rabbits were given a test meal, which was either nibbled or taken as a bolus. The LF diet did not noticeably alter postprantial lipid variables. Triacylglycerol levels, 0–35 h lipid responses and plasma accumulation of dietary lipids were significantly higher in the HF-G group than in the HF-N group, despite higher post-heparin plasma lipase activities. Furthermore, as studied on cultured isolated hepatocytes, the higher the rate of supply of triacylglycerol- and cholesterol-rich lipoproteins (TCRL), the lower the rate of lipid uptake and bile salt secretion. Atheroma deposition was significantly increased by gorging the HF diet and was correlated with levels of most postprandial lipid variables. We conclude that gorgingv. nibbling a fat and cholesterol-enriched diet exacerbates postprandial lipaemia by reducing the rate of TCRL clearance and favours atheroma deposition.

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.

Modifications in Postprandial Triglyceride-Rich Lipoprotein Composition and Size after the Intake of Pomace Olive Oil

OBJECTIVE

This study was designed to determine the composition of postprandial triglyceride-rich lipoproteins (TRL) after the intake of pomace olive oil (POO), which is a subproduct of the extraction of virgin olive oil (VOO) and presents a high concentration of minor components with biological activity.

METHODS

Meals enriched in POO and refined olive oil (ROO) were administrated to 9 healthy young men and blood was extracted every hour during a postprandial period of 7 hours. Serum and TRL lipid composition were measured by enzymatic and chromatographic methods and apolipoprotein B composition by SDS-PAGE.

RESULTS

POO and ROO showed a very similar fatty acid composition but differed in their unsaponifiable fraction. The content of phytosterols, tocopherols, terpenic acids and alcohols and fatty alcohols was much higher in POO than in ROO. Serum lipids were not affected by the administration of the oils but the triglyceride concentration in TRL and the size of the particles (p < 0.05) after POO was higher at time point 2 h and lower at time point 4 h compared with ROO. In contrast, the number of TRL particles was lower after POO, although the rate of clearance was similar.

CONCLUSION

We suggest that the unsaponifiable fraction between the two olive oils affect the size and composition of postprandial TRL, which might have a relevant impact on their atherogenicity.

Fatty acid composition of chylomicron remnant-like particles influences their uptake and induction of lipid accumulation in macrophages

The influence of the fatty acid composition of chylomicron remnant-like particles (CRLPs) on their uptake and induction of lipid accumulation in macrophages was studied. CRLPs containing triacylglycerol enriched in saturated, monounsaturated, n-6 or n-3 polyunsaturated fatty acids derived from palm, olive, corn or fish oil, respectively, and macrophages derived from the human monocyte cell line THP-1 were used. Lipid accumulation (triacylglycerol and cholesterol) in the cells was measured after incubation with CRLPs for 5, 24 and 48 h, and uptake over 24 h was determined using CRLPs radiolabelled with [3H]triolein. Total lipid accumulation in the macrophages was significantly greater with palm CRLPs than with the other three types of particle. This was mainly due to increased triacylglycerol concentrations, whereas changes in cholesterol concentrations did not reach significance. There were no significant differences in lipid accumulation after incubation with olive, corn or fish CRLPs. Palm and olive CRLPs were taken up by the cells at a similar rate, which was considerably faster than that observed with corn and fish CRLPs. These findings demonstrate that CRLPs enriched in saturated or monounsaturated fatty acids are taken up more rapidly by macrophages than those enriched in n-6 or n-3 polyunsaturated fatty acids, and that the faster uptake rate results in greater lipid accumulation in the case of saturated fatty acid-rich particles, but not monounsaturated fatty acid-rich particles. Thus, dietary saturated fatty acids carried in chylomicron remnants may enhance their propensity to induce macrophage foam cell formation.

An investigation by electron microscopy of chylomicron remnant uptake by human monocyte-derived macrophages

Human monocyte-derived macrophages (HMM) internalise proatherogenic chylomicron remnants via several high affinity receptor pathways. However, the endocytic ultrastructures responsible for the uptake of chylomicron remnants by macrophages have not previously been described. In this study, we have utilised transmission electron microscopy together with colloidal gold-labelling of chylomicron remnants to investigate the pathways involved in macrophage uptake of chylomicron remnants. We found that macrophages internalise chylomicron remnants via surface-connected compartments of up to 600 nm as well as non-clathrin coated pits. Chylomicron remnants were found to be distributed internally in a number of endocytic vesicles including early cysternal endosomes, spherical late endosomes and tubular vesicular compartments. Uptake of chylomicron remnants by HMM via phagocytosis or macropinocytosis was excluded based on the observations that lipoproteins were not found in phagolysosomes nor modified by inhibitors of these two processes, respectively. The latter observation contrasts with previous reports of chylomicron remnant internalisation by macrophages of other species.

Diet and lipoprotein oxidation: analysis of oxidized triacylglycerols in pig lipoproteins

Oxidized lipoproteins have a recognized role in atherogenesis, but molecular-level research on oxidized lipids in lipoproteins and the effect of diet on these molecules have been limited. In the present study, the effects of three sunflower seed oil diets differing in oxidation levels (PV in oils 1, 84, and 223 mequiv O2/kg) on lipoprotein lipid oxidation in growing pigs were investigated. The emphasis was on the investigation of oxidized TAG molecules found in chylomicrons and VLDL. A method based on RP-HPLC and electrospray ionization-MS was used for the analysis of oxidized TAG molecules. The baseline diene conjugation method was used for the estimation of in vivo levels of lipoprotein lipid oxidation. Several oxidized TAG structures were found in the samples. These products consisted of TAG molecules with a hydroxy, an epoxy, or a keto group attached to a FA, and of TAG molecules containing an aldehyde structure derived from a FA. The lipoprotein lipids and TAG were more oxidized in the pigs fed on the most oxidized oil compared with those fed on nonoxidized oil. Oxidation of dietary fat was reflected in the lipoprotein oxidation. New, detailed information on oxidized TAG molecules of chylomicrons and VLDL was obtained.

Effects of hydrochlorothiazide and propranolol treatment on chylomicron metabolism in hypertensive subjects

Modifications in chylomicron metabolism caused by antihypertensive drugs were investigated in hypertensive subjects because previous studies had indicated that diuretics and beta-blockers modify the plasma lipid concentrations through mechanisms that were not fully understood. A triglyceride-rich emulsion resembling lymph chylomicrons, labeled with (3H) triolein and (14C) cholesteryl oleate, was infused intravenously into mildly hypertensive patients after 8 weeks on placebo and subsequently on hydrochlorothiazide (n = 10) or propranolol (n = 8). The residence time of both radioactivities in plasma was utilized for the simultaneous calculation of the particle remnant removal rate and of the lipoprotein lipase activity expressed as a delipidation index = 1 – [(3H) triolein residence time/(14C) cholesteryl oleate residence time]. Treatment with hydrochlorothiazide diminished the delipidation rate value whereas propranolol mildly increased the removal rate of the remnant particle. These alterations of the chylomicron kinetics were not accompanied by changes in plasma triglycerides, glucose, and insulin concentration as measured in the fasting state. The impairment of the lipoprotein lipase activity by thiazides and the faster removal rate of the whole particle by propranolol could explain the reason why in previous clinical studies the simultaneous use of these drugs does not aggravate the hyperlipidemia known to be induced by thiazides alone. Key words: hydrochlorothiazide, propranolol, hypertension, plasma lipoproteins, chylomicron metabolism.

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.

Triacylglycerol oxidation in pig lipoproteins after a diet rich in oxidized sunflower seed oil

The effects of two sunflower seed oil diets differing in oxidation levels (PV in oils 1 and 190 mequiv O2/kg) on lipoprotein TAG and total lipid oxidation were investigated in growing pigs. For 2 wk, two groups of 10 pigs were fed either of the diets, after which blood samples were collected. A method based on RP-HPLC and electrospray ionization-MS was used for the analysis of oxidized TAG molecules in chylomicrons and VLDL. The baseline diene conjugation method was used for the estimation of in vivo levels of lipoprotein lipid oxidation. TAG molecules with a hydroxy, an epoxy, or a keto group attached to a FA, as well as TAG core aldehydes were detected in the samples. Typically, lipoprotein TAG and total lipids were more oxidized in the pigs fed on the oxidized oil compared with those fed on nonoxidized oil. Oxidation of dietary fat was thus reflected in the lipoprotein oxidation, which confirmed our earlier findings.

Human triglyceride-rich lipoproteins impair glucose metabolism and insulin signalling in L6 skeletal muscle cells independently of non-esterified fatty acid levels

AIMS/HYPOTHESIS

Elevated fasting and postprandial plasma levels of triglyceride-rich lipoproteins (TGRLs), i.e. VLDL/remnants and chylomicrons/remnants, are a characteristic feature of insulin resistance and are considered a consequence of this state. The aim of this study was to investigate whether intact TGRL particles are capable of inducing insulin resistance.

METHODS

We studied the effect of highly purified TGRLs on glycogen synthesis, glycogen synthase activity, glucose uptake, insulin signalling and intramyocellular lipid (IMCL) content using fully differentiated L6 skeletal muscle cells.

RESULTS

Incubation with TGRLs diminished insulin-stimulated glycogen synthesis, glycogen synthase activity, glucose uptake and insulin-stimulated phosphorylation of Akt and glycogen synthase kinase 3. Insulin-stimulated tyrosine phosphorylation of IRS-1, and IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase (PI3K) activity were not impaired by TGRLs, suggesting that these steps were not involved in the lipoprotein-induced effects on glucose metabolism. The overall observed effects were time- and dose-dependent and paralleled IMCL accumulation. NEFA concentration in the incubation media did not increase in the presence of TGRLs indicating that the effects observed were solely due to intact lipoprotein particles. Moreover, co-incubation of TGRLs with orlistat, a potent active-site inhibitor of various lipases, did not alter TGRL-induced effects, whereas co-incubation with receptor-associated protein (RAP), which inhibits interaction of TGRL particles with members of the LDL receptor family, reversed the TGRL-induced effects on glycogen synthesis and insulin signalling.

CONCLUSIONS/INTERPRETATION

Our data suggest that the accumulation of TGRLs in the blood stream of insulin-resistant patients may not only be a consequence of insulin resistance but could also be a cause for it.

The unsaponifiable fraction of virgin olive oil in chylomicrons from men improves the balance between vasoprotective and prothrombotic factors released by endothelial cells

Minor components of virgin olive oil (VOO) may play a key role in the beneficial effects of VOO on atherosclerosis. In the present study we evaluated the influence of the unsaponifiable fraction of VOO on the production of eicosanoids and nitric oxide (NO) by endothelial cells (HUVECs). Triglyceride-rich lipoprotein (TRLs) were isolated from human serum after the intake of meals enriched in 3 high-oleic acid oils, i.e., high-oleic sunflower (HOSO), VOO, or enriched-virgin olive (EVO) oils, the last-mentioned containing 2.4% of unsaponifiable matter. HOSO induced a greater accumulation of triglycerides (TGs) in the postprandial serum than VOO or EVO, as measured by calculating the area under the curve. The incubation with TRLs increased NO release by endothelial cells compared with untreated control cells, but the effects of the various TRLs did not differ. EVO-derived TRLs reduced the production of prostaglandin E(2) (PGE(2)) and thromboxane B(2) (TxB(2)) (the stable metabolite of TxA(2)) compared with VOO- or HOSO-derived TRLs. The release of PGI(2) (as 6-keto PGF(1alpha)) was similarly diminished by all TRLs compared with the control. In conclusion, the unsaponifiable fraction of VOO does not affect postprandial triglyceridemia, but it has favorable effects on endothelial function, mainly by reducing proinflammatory and vasoconstrictor eicosanoid synthesis (PGE(2) and TxB(2)).

Delayed intravascular catabolism of chylomicron-like emulsions is an independent predictor of coronary artery disease

The atherogenic role of a delayed intravascular catabolism of chylomicrons has been suggested by univariate analysis of case-control studies. However, it is not established whether this association is caused by a direct atherogenic effect of these lipoproteins or results from the presence of concurrent and metabolically-related coronary artery disease (CAD) risk factors. In this study, the plasma kinetics of a chylomicron-like emulsion doubly labeled with 14C-cholesteryl oleate (CE) and 3H-triolein (TG) was determined in 93 subjects with or without angiographically-defined CAD. As compared with controls and even after adjustment for body mass index (BMI), LDL- and HDL-cholesterol, and the presence of traditional risk factors, CAD patients had 45% smaller fractional clearance rate (FCR) of TG, 41% smaller FCR-CE and 19% smaller dilapidation index (DI; P < 0.05). Among CAD patients, those with highest angiographic score had 66% smaller FCR-TG (P = 0.007), 50% smaller FCR-CE (P = 0.01) and 27% smaller DI (P = 0.004). In a multivariate logistic regression analysis, FCR-CE (P < 0.0001) and DI (P = 0.001) were the only independent predictors for the presence of CAD. In conclusion, we presently show that the rate of lipolysis and removal from the circulation of chylomicron-like emulsions constitutes an independent predictor of CAD and a marker of CAD severity.

Arterial permeability and efflux of apolipoprotein B-containing lipoproteins assessed by in situ perfusion and three-dimensional quantitative confocal microscopy

OBJECTIVES

There is accumulating evidence that an increased risk of cardiovascular disease (CVD) is not simply caused by the degree of arterial exposure to plasma lipoproteins but, in addition, is determined by the affinity of the vasculature for different lipoprotein phenotypes. In this study we compare the delivery and efflux of 2 atherogenic lipoproteins to further understand the factors that regulate cholesterol accumulation in early atherogenesis.

METHODS AND RESULTS

Lipoproteins containing apolipoprotein (apo) B100 (a low-density lipoprotein [LDL]) and apoB48 (chylomicron remnants) were isolated and differentially conjugated with fluorophores and simultaneously perfused at equivalent concentrations in situ through rabbit carotid vessels. Perfusion systems were established to quantify and differentiate between lipoprotein arterial delivery and efflux. The total average rate of delivery for LDL particles (23 nm) compared with chylomicron remnants (50 nm) was 4427 particles/min(-1) per microm3 and 452 particles/min(-1) per microm3, respectively. In contrast, the average rate of efflux was 3195 particles/min(-1) per microm3 and 163 particles/min(-1) per microm3 for LDL and chylomicron remnants, respectively.

CONCLUSIONS

Results indicate that although LDL particles have a higher rate of delivery, they efflux more readily from arterial tissue compared with the larger chylomicron remnants. Collectively, our findings highlight that lipoproteins permeate through arterial tissue differently and may be dependent on the phenotype and potential interactions with extracellular matrix components.

Protection of chylomicron remnants from oxidation by incorporation of probucol into the particles enhances their uptake by human macrophages and increases lipid accumulation in the cells

The effects of protection of chylomicron remnants from oxidation on their uptake and induction of lipid accumulation in macrophages were investigated using chylomicron remnant-like particles (CRLPs) containing the lipophilic antioxidant drug, probucol, and macrophages derived from the human monocyte cell line, THP-1. The total lipid content of THP-1 macrophages was markedly higher (x2.2) after 48 h of incubation of THP-1 macrophages with CRLPs containing probucol (pCRLPs) when compared to CRLPs without probucol, and this was because of increases in triacylglycerol (x2.3) and cholesterol (x1.8) levels, while cholesteryl ester concentrations were not significantly changed. Determination of the uptake of CRLPs and pCRLPs by the cells using particles labelled with the fluorescent probe 1,1'-dioctadecyl-3,3,3'3'-tetramethylindo-carbocyanine perchlorate showed that pCRLPs are taken up at a faster rate than CRLPs. The synthesis of triacylglycerol, as measured by the incorporation of [(3)H]oleate and [(3)H]glycerol, was also increased in macrophages incubated with pCRLPs as compared to CRLPs without probucol, but phospholipid and cholesteryl ester formation from [(3)H]oleate was unaffected. In addition, no differences between the effects of CRLPs and pCRLPs on the expression of mRNA for a range of genes believed to be involved in lipoprotein uptake, intracellular lipid metabolism and the efflux of cholesterol from macrophages was detected. These results suggest that antioxidants carried in chylomicron remnants enhance lipid accumulation in macrophages by increasing the rate of uptake of the particles and raising the intracellular synthesis of triacylglycerol, but not cholesteryl ester, and that these effects are brought about by changes at the post-transcriptional level. Antioxidants carried in chylomicron remnants therefore may promote the development of atherosclerosis, and this is likely to be particularly important in conditions where clearance of remnants from the circulation is delayed.

Triglyceride-rich lipoproteins inhibit cholesterol efflux to apolipoprotein (apo) A1 from human macrophage foam cells

High circulating levels of triglyceride-rich lipoproteins (TGRL) represent an independent risk factor for coronary artery disease. Here, we show that TGRL inhibit the efflux of cholesterol from 'foam cell' macrophages to lipid-poor apolipoprotein (apo) A1, and may thereby inhibit arterial reverse cholesterol transport and promote the formation of atherosclerotic lesions. Human (THP-1) monocyte-derived macrophages were pre-incubated (48 h) with acetylated low-density lipoprotein (AcLDL) to provide a foam cell model of cholesterol efflux to apoA1. Pre-incubation of macrophage 'foam cells' with TGRL (0-200 microg/ml, 0-24 h) inhibited the efflux of exogenously radiolabelled ([3H]), endogenously synthesised ([14C]) and cellular cholesterol mass to lipid-poor apoA1, but not control medium, during a (subsequent) efflux period. This inhibition is dependent upon the length of prior exposure to, and concentration of, TGRL employed, but is independent of changes in intracellular triglyceride accumulation or turnover of the cholesteryl ester pool. Despite the negative impact of TGRL on cholesterol efflux, major proteins involved in this process--namely apoE, ABCA1, SR-B1 and caveolin-1--were unaffected by TGRL pre-incubation, suggesting that exposure to these lipoproteins inhibits an alternate, and possibly novel, anti-atherogenic pathway.

Clearance of a 3H-labeled chylomicron-like emulsion following the acute phase of myocardial infarction

BACKGROUND

Plasma lipids may be altered during acute myocardial infarction and may not reflect patient baseline lipid profile. The metabolism of chylomicrons, the lipoproteins that carry the dietary lipids in the bloodstream has not yet been studied in acute myocardial infarction patients.

METHODS

In this study, a lipidic emulsion that mimics the intravascular behavior of chylomicrons labeled with cholesteryl oleate ((3)H-CO) was injected intravenously in 17 normolipidemic patients on the seventh and on the 45th day post-non complicated acute myocardial infarction after a 12-h fast. The plasma decay curve of the emulsion label was determined from blood samples collected during 60 min. Data were also compared with a group of 10 patients with chronic coronary artery disease.

RESULTS

In the acute myocardial infarction group, the plasma fractional catabolic rates of the emulsion (3)H-CO, expressed as median and confidence intervals, did not change from the seventh to the 45th day after the acute event [0.0773 (0.061, 0.1025) min(-1) vs. 0.0672 (0.00507, 0.1009) min(-1) P=0.61] and was similar to that determined in chronic coronary artery disease patients. High-density lipoprotein cholesterol and apolipoprotein AI were lower on the seventh day when compared to the 45th day post acute myocardial infarction (P=0.01 and P=0.004, respectively). No changes were found in LDL and total cholesterol as well as in plasma triglycerides in myocardial infarction group.

CONCLUSIONS

No changes were found in chylomicron metabolism is in the acute phase of myocardial infarction.

Postprandial lipemia and cardiovascular disease

Postprandial lipemia, characterized by a rise in triglyceride-rich lipoproteins after eating, is a dynamic, nonsteady-state condition in which humans spend the majority of time. There are several lines of evidence suggesting that postprandial lipemia increases risk of atherogenesis. Clinical data show a correlation between postprandial lipoproteins and the presence/progression of coronary artery disease and carotid intimal thickness. Mechanistic studies demonstrate that triglyceride-rich lipoprotein remnants may have adverse effects on endothelium and can penetrate into the subendothelial space. Exchange of core lipids between postprandial lipoproteins and low-density lipoprotein (LDL)/high-density lipoprotein (HDL) is increased during prolonged lipemia, resulting in small, dense LDL particles and reduced HDL cholesterol levels. Hemostatic variables, including clotting factors, platelet reactivity, and monocyte cytokine expression, may be increased during postprandial lipemia. Collectively, these data suggest that assessment and treatment of atherosclerosis should include parameters related to postprandial lipemia.

Intimal retention of cholesterol derived from apolipoprotein B100- and apolipoprotein B48-containing lipoproteins in carotid arteries of Watanabe heritable hyperlipidemic rabbits

OBJECTIVE

The arterial retention of apolipoprotein (apo) B100- and apoB48-containing lipoproteins was simultaneously determined in a rabbit model of human hypercholesterolemia using 3D confocal microscopy.

METHODS AND RESULTS

Lipoproteins containing apoB100 (LDL) and apoB48 (chylomicron remnants) were differentially conjugated with fluorophores and simultaneously perfused at equivalent concentrations under physiological conditions in situ through carotid vessels of Watanabe heritable hyperlipidemic rabbits and compared with controls. Retention of lipoproteins was defined as the amount remaining after an extensive washout phase. LDL and chylomicron remnants were both retained, primarily within the subendothelial space. Without a concomitant increase in exposure to lipoproteins, we found a marked increase in the retention of cholesterol within the intima of Watanabe heritable hyperlipidemic rabbits compared with controls, specifically because of increased entrapment of apoB48-containing lipoproteins.

CONCLUSIONS

Collectively, our data suggest that hypercholesterolemia induced as a consequence of LDL receptor deficiency differentially influences retention of LDL and of chylomicron remnants.

Chylomicron remnant induction of lipid accumulation in J774 macrophages is associated with up-regulation of triacylglycerol synthesis which is not dependent on oxidation of the particles

The influence of chylomicron remnants on lipid accumulation and synthesis and the activity and/or expression of mRNA for some of the key enzymes involved was investigated in the murine macrophage cell line J774. The effects of varying the polyunsaturated fatty acid (PUFA) composition and oxidation state of the remnants were also examined. Chylomicron remnants derived from corn oil (rich in n-6 PUFA) or fish oil (rich in n-3 PUFA) were prepared in vivo and oxidised by incubation with CuSO(4). The native and oxidised remnants caused a marked rise in intracellular triacylglycerol levels, but the rise induced by corn oil remnants (four- to sixfold) was greater than that observed with fish oil remnants (<2-fold). Triacylglycerol synthesis, as measured by the incorporation of [3H]oleate and [3H]glycerol into cellular triacylglycerol, was increased by all four remnant types tested, and corn oil remnants had a significantly greater effect than fish oil remnants. Oxidation of the remnants did not affect the results obtained. Although the incorporation of [3H]oleate into cholesteryl ester by the cells was not significantly changed by any of the four types of remnants tested, the activity and expression of mRNA for acyl Co-enzyme A: cholesterol acyltransferase (ACAT) was increased by corn oil, but not by fish or oxidised corn, remnants. Neutral cholesteryl ester hydrolase (nCEH) activity, however, was also raised by corn oil remnants. These studies indicate that chylomicron remnants induce the accumulation of triacylglycerol in J774 macrophages, and that increased synthesis of triacylglycerol plays a major role in this process. Furthermore, they demonstrate that these effects are enhanced when the remnants are enriched in n-6 PUFA as compared with n-3 PUFA, but not after oxidation of the particles, suggesting that the fatty acid composition of chylomicron remnants may be more important than their oxidation state in their ability to induce foam cell formation.

PPARdelta is a very low-density lipoprotein sensor in macrophages

Although triglyceride-rich particles, such as very low-density lipoprotein (VLDL), contribute significantly to human atherogenesis, the molecular basis for lipoprotein-driven pathogenicity is poorly understood. We demonstrate that in macrophages, VLDL functions as a transcriptional regulator via the activation of the nuclear receptor peroxisome proliferator-activated receptor delta. The signaling components of native VLDL are its triglycerides, whose activity is enhanced by lipoprotein lipase. Generation of peroxisome proliferator-activated receptor delta null macrophages verifies the absolute requirement of this transcription factor in mediating the VLDL response. Thus, our data reveal a pathway through which dietary triglycerides and VLDL can directly regulate gene expression in atherosclerotic lesions.

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.

Induction of Atherosclerosis by Human Chylomicron Remnants: A Hypothesis

Epidemiologic studies have provided support for the association between delayed remnant removal and premature atherosclerosis. Triglyceride-rich particles such as chylomicrons and chylomicron remnants that carry dietary derived fats, may play a role in the early stages of developing arteriosclerosis. Currently research focuses on these lipoprotein classes seeking distinguishing factors that causes some lipoproteins to be atherogenic while others are not. Such lipoproteins could be involved in atherogenesis directly or indirectly. Direct involvement occurs by interaction of triglyceride-rich particles with the arterial wall, possibly affecting the artery wall by oxidative stress, direct endothelial toxicity by constituents such as lysophosphatidylcholine or oxysterols, induction of prothrombotic changes, stimulation of endothelial expression of cell adhesion molecules and direct interaction with circulating blood cells. Indirect involvement refers to the influence of triglyceride-rich lipoproteins on other lipoproteins on the composition of low density lipoprotein (LDL) and high density lipoprotein (HDL) particles. We propose that in individuals with delayed removal of chylomicron remnants, the prolonged exposure of areas of endothelium that have been partially activated by turbulent flow, to specific components of the remnants, results in the endothelial cells becoming further activated and able to bind monocytes. During or shortly after the transcytosis to the intima and transformation of monocytes to macrophages, the macrophages become engorged with remnant derived lipids and form the nidus of a fatty streak.

Fibrinogen-coated chylomicrons in gastrointestinal lymph: a new rationale regarding the arterial deposition of postprandial lipids

The recent discovery that fibrinogen binds to chylomicrons in gastrointestinal lymph has prompted a new rationale regarding the arterial deposition of postprandial lipids, i.e., dietary fat. According to this new rationale, fibrinogen bound to chylomicrons in the gastrointestinal lymph renders those lipid particles and/or their remnants an adhesive potential, even before the particles reach the arterial system. It is proposed that such an adhesive potential, if realized in the vicinity of the arterial wall, can contribute to the nucleation and growth of atherosclerotic plaques, especially during and immediately following a fat-rich meal. Arguments in support of this proposal are made based on the proximity of the lymph outflow tract to the arteries most susceptible to atherosclerosis, and on the tissue distributions and activities of heparin, diamine oxidase, and lipoprotein lipase. This new rationale reconciles existing theories on atherosclerosis, and it also suggests novel means by which to prevent/treat the disease.

Arterial retention of apolipoprotein B(48)- and B(100)-containing lipoproteins in atherogenesis

PURPOSE OF REVIEW

The "response to retention" hypothesis of atherosclerosis suggests that the arterial deposition of cholesterol is directly proportional to the concentration of circulating plasma lipoproteins. However, there is increasing evidence to support the concept that specific lipoproteins may be preferentially retained within the arterial wall, possibly as a result of greater affinity for cell surface and extracellular matrices.

RECENT FINDINGS

Recently, key studies have provided insight into mechanisms involved in the interaction of apolipoprotein B (apoB)-containing lipoproteins with extracellular matrices. In addition, novel methods and innovative experimental design has enabled us to differentiate between the delivery, retention and efflux of apoB(48)- and apoB(100)-containing lipoproteins. Other studies have demonstrated a relationship between extracellular matrix proteoglycan expression and the development of atherosclerosis. Discussion in the present review also extends to the mechanisms that are involved in the relative intimal retention of apoB(48)- and apoB(100)-containing lipoproteins in order to explain the atherogenicity of these macromolecules.

SUMMARY

The perspective of this review is to highlight recent advances in the area of arterial lipoprotein retention and the physiological significance these processes may have in the aetiology of cardiovascular disease. Importantly, an understanding of the mechanisms responsible for the retention of apoB(48)/B(100)-containing lipoproteins will enable new strategies to be developed for the future management of cardiovascular disease.

The effects of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids on the transcription of genes regulating their uptake and metabolism by the liver: influence of cellular oxidative state

The influence of chylomicron remnants enriched in n-6 or n-3 polyunsaturated fatty acids (PUFA) on the expression of mRNA for the low density lipoprotein receptor (LDLr), LDLr-related protein (LRP), and peroxisome proliferator activated receptor alpha (PPAR(alpha)) was investigated in normal hepatocytes and after manipulation of the cellular oxidative state by incubation with N-acetyl cysteine (NAC) or CuSO(4). In normal cells, mRNA levels for the LDLr were unaffected by incubation with chylomicron remnants, but those for the LRP and PPAR(alpha) were downregulated by remnants enriched in n-3 as compared to n-6 PUFA, suggesting that the transcription of these genes are influenced directly by the type of fatty acid delivered to the liver from the diet. Treatment with NAC or CuSO(4) was found to shift the hepatocytes into a pro-reducing or pro-oxidizing state, respectively. The abundance of mRNA for the LDLr, LRP, and PPAR(alpha) was increased after incubation with remnants enriched in n-3, but not n-6, PUFA in pro-reducing as compared to pro-oxidizing cells, and PPAR(alpha) mRNA levels were also decreased by remnants high in n-6 PUFA in the more reduced cells. These results indicate that the effects of fatty acids from the diet delivered to the liver in chylomicron remnants on the expression of hepatic genes regulating their uptake and metabolism are modulated by the redox state of the cells, and that the type of fatty acid carried by the particles also plays a part in determining the response observed.

Triacylglycerol-rich lipoproteins alter the secretion, and the cholesterol-effluxing function, of apolipoprotein E-containing lipoprotein particles from human (THP-1) macrophages

Elevated plasma levels of triacylglycerol-rich lipoproteins (TGRLP) are associated with increased risk of atherogenesis and abnormal reverse cholesterol transport, as illustrated in Type II diabetes. Here we examine the effect of plasma triacylglycerol-rich or cholesteryl ester-rich lipoproteins on the secretion of nascent apolipoprotein E (apoE)-containing lipoprotein E (LpE) particles by human (THP-1) macrophages. As expected, preincubation with low-density lipoprotein (LDL) yielded small but significant increases in total cellular cholesterol content and also the secretion of apoE by macrophages. By contrast, preincubation with TGRLP resulted in higher, dose-dependent, increases in apoE secretion that reflected, but were not dependent on, cellular triacylglycerol accumulation. Secreted apoE was incorporated into a pre-beta migrating LpE fraction that differed in lipid composition and flotation density depending on preincubation conditions. Specifically, the LpE-containing lipoprotein fraction produced by macrophages preincubated with TGRLP was cholesterol-poor, markedly heterogeneous and of higher peak flotation density (d 1.14-1.18) when compared with particles produced after preincubation with LDL. Both the conditioned medium and the isolated (d<1.21) LpE-containing fraction, yielded by macrophages preincubated with TGRLP, seemed poorer at inducing cholesterol efflux than the equivalent fractions from cells preincubated with LDL, as judged by [(3)H]cholesterol efflux from untreated 'naïve' macrophages. Thus, although the interaction of TGRLP with macrophages can enhance apoE output from these cells, the LpE particles produced seem to be relatively inefficient mediators of cholesterol efflux. These factors might contribute to the increased risk of atherosclerosis in individuals with Type II diabetes.