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.

Apolipoprotein E enrichment of immuno-separated chylomicron and chylomicron remnants following saturated fatty acids

AIM

We examined the effect of meal fatty acids on lipid and apolipoprotein concentrations of very low density lipoprotein (VLDL) and chylomicron/chylomicron remnants in lipid fractions with a Svedberg flotation rate (Sf) 60-400 and Sf 20-60.

METHODS AND RESULTS

Six healthy middle-aged men received in random order mixed meals enriched with saturated (SFA), polyunsaturated (PUFA) or monounsaturated (MUFA) fatty acids on 3 occasions. VLDL and chylomicron/chylomicron remnants in the lipid fractions were separated by immunoaffinity chromatography against apo B-100. In the Sf 60-400 chylomicron/chylomicron remnants, triacylglycerol and cholesterol concentrations were significantly lower following PUFA compared with SFA and MUFA (P < or = 0.05). Apolipoprotein (apo) E responses were significantly higher after SFA in chylomicron/chylomicron remnants and VLDL compared with PUFA and MUFA (P < 0.007). However, apo B responses (particle number) were higher following MUFA than SFA (P = 0.039 for chylomicron/chylomicron remnants). Composition of the chylomicron/chylomicron remnants (expressed per particle) revealed differences in their triacylglycerol and apo E contents; in the Sf 60-400 fraction, SFA-rich chylomicron/chylomicron remnants contained significantly more triacylglycerol than MUFA (P = 0.028), more apo E than PUFA- and MUFA-rich particles (P < 0.05) and in the Sf 20-60 fraction, more apo E than MUFA (P = 0.009).

CONCLUSION

There are specific differences in the composition of chylomicron/chylomicron remnants formed after saturated compared with unsaturated fatty acid-rich meals which could determine their metabolic fate in the circulation and subsequent atherogenicity.

Evidence of Dual Pathways for Lipid Uptake during Chylomicron Remnant-Like Particle Processing by Human Macrophages

BACKGROUND

Though it is now clear that chylomicron remnants are pro-atherogenic lipoproteins, events leading to their incorporation by macrophages are poorly understood.

METHODS

This study investigates, in human macrophages, the fate of either [(3)H]cholesteryl oleate or [(3)H]triacylglycerol carried by human apolipoprotein-E-containing chylomicron remnant-like particles (CRLP) and the influence of CRLP containing trilinolein, (18:2)CRLP, or triolein, (18:1)CRLP, on lipid accumulation, newly synthesized cholesteryl ester (CE) and triacylglycerol (TG).

RESULTS

Labelled fatty acids from TG were markedly incorporated into TG and phospholipid and, to a lesser extent, into free fatty acids and were scarcely recovered in cholesteryl esters. [(3)H]CE from CRLP accumulated in cells in a dose-dependent manner with a significant difference between concentrations of 10 and 40 microg cholesterol/ml with (18:2)CRLP. In the same concentration range, TG synthesis was enhanced by about 46 and 30% by (18:2)CRLP and (18:1)CRLP cholesterol, respectively, whereas the esterification of cholesterol, evaluated by [(3)H]oleate incorporation, was decreased by about 30% with both types of CRLP. Endocytosis inhibition did not prevent cell cholesterol and TG accumulation, whereas lipoprotein lipase inhibition reduced the TG content.

CONCLUSIONS

The results are consistent with the hypotheses that in macrophages dietary remnants may support TG and CE internalization via different mechanisms. Extracellular lipolysis seems particularly important for internalization of dietary fatty acids, whereas the entrance of CE seems attributable to a concomitant selective CE uptake mediated by scavenger receptor class B type I, since the scavenger receptor class B type I antibody induces significant inhibition (38%) of [(3)H]CE transported by CRLP, but does not affect internalization of [(3)H]TG carried by the same particles.

Chylomicron remnants stimulate release of interleukin-1beta by THP-1 cells

Recent findings suggest that the oxidative modification of low-density lipoproteins (LDL) and an increase in triglyceride-rich lipoprotein particles including chylomicron remnants contribute to the progression of atherosclerosis, as does the inflammatory response. We therefore examined whether and how these lipoproteins affected interleukin (IL)-1beta release and mRNA expression for IL-1beta and IL-18 in THP-1 cells, a human monocyte cell line. Chylomicron remnants increased IL-1beta release into the conditioned medium by THP-1 in a dose- and time-dependent manner. At concentrations up to 1 microg/ml, chylomicron remnants increased IL-1beta release by 4-fold compared with the control. Neither native LDL nor oxidized LDL (OxLDL) significantly increased IL-1beta release. Chylomicron remnants increased IL-1beta mRNA expression by 3 times. Native LDL or OxLDL did not increase IL-1beta mRNA, while neither these lipoproteins nor chylomicron remnants increased IL-18 mRNA. Chylomicron remnants also increased the activities of caspase-1 and nuclear factor (NF)-kappaB significantly, while native LDL or OxLDL did not. In conclusion, chylomicron remnants stimulated IL-1beta mRNA expression and IL-1beta protein production probably via caspase-1 and NF-kappaB activation in THP-1 cells.

Does pravastatin increase chylomicron remnant catabolism in postmenopausal women with type 2 diabetes mellitus?

OBJECTIVE

We investigated the effects of pravastatin on chylomicron remnant catabolism measured with a 13C stable isotope breath test and plasma apolipoprotein (apo) B-48 and remnant-like particle (RLP)-cholesterol in postmenopausal women with type 2 diabetes mellitus.

PATIENTS AND MEASUREMENTS

Nineteen postmenopausal women with type 2 diabetes were randomized to receive 40 mg/day pravastatin or no treatment for 6 weeks followed by a 2-week washout period, and crossed over for a further 6 weeks. Fractional catabolic rate (FCR) of a chylomicron remnant-like emulsion was determined from 13CO2 enrichment in the breath and plasma using isotope-ratio mass spectrometry and multicompartmental modelling. Plasma apo B-48 and RLP-cholesterol concentrations were also measured as static markers of chylomicron remnant metabolism.

RESULTS

Pravastatin significantly reduced plasma concentrations of cholesterol (5.9 +/- 0.3 vs. 4.8 +/- 0.2 mmol/l; P < 0.001), low density lipoprotein (LDL)-cholesterol (3.5 +/- 0.2 vs. 2.6 +/- 0.2 mmol/l; P < 0.001), triglyceride (2.1 +/- 0.3 vs. 1.7 +/- 0.2 mmol/l; P = 0.017), non-high density lipoprotein (HDL)-cholesterol (4.4 +/- 0.3 vs. 3.3 +/- 0.2 mmol/l; P < 0.001), lathosterol/total cholesterol ratio (2.6 +/- 0.2 vs. 2.0 +/- 0.3, P = 0.035), apo B-100 (1.1 +/- 0.1 vs. 0.8 +/- 0.1 g/l; P = 0.001), apo B-48 (4.8 +/- 0.9 vs. 3.3 +/- 0.6 mg/l; P = 0.016), and RLP-cholesterol (31.4 +/- 8.2 vs. 18.6 +/- 4.6 mg/dl; P = 0.024). Pravastatin was also associated with an increase in sitosterol/total cholesterol ratio (2.8 +/- 0.3 vs. 3.1 +/- 0.3, P = 0.029). Chylomicron remnant-like emulsion catabolism was not, however, significantly altered by pravastatin estimated by either breath or plasma clearance measurements.

CONCLUSIONS

In postmenopausal women, pravastatin decreases plasma concentrations of remnant lipoproteins by a mechanism that may relate chiefly to inhibition of remnant production, but this requires further evaluation.

Effects of cholesterol in chylomicron remnant models of lipid emulsions on apoE-mediated uptake and cytotoxicity of macrophages

Chylomicron remnants have been suggested to be involved in the development of atherosclerosis. To investigate the mechanisms of chylomicron remnant-induced atherosclerosis, we prepared cholesterol (Chol)-containing emulsion particles as models for chylomicron remnants. Chol markedly increased the apolipoprotein E (apoE) binding maximum of emulsions without changing the binding affinity and thereby promoted emulsion uptake by J774 macrophages. Fluorescence measurements showed that Chol increased acyl chain order and head group hydration of the surface phospholipid (PL) layer of emulsions. The binding maximum of apoE was closely correlated with the hydration and the increase in the PL head group separation at the emulsion surface. From experiments using inhibitors for lipoprotein receptors, heparan sulfate proteoglycans and low density lipoprotein receptor-related protein were found to be the major contributors to the uptake of Chol-containing emulsions. Trypan blue dye exclusion revealed that the uptake of Chol-containing emulsions induced cytotoxicity to J774 macrophages. This study proposes a mechanism of atherosclerosis induced by chylomicron remnants.

Lipid metabolism and TNF-alpha secretion in response to dietary sterols in human monocyte derived macrophages

BACKGROUND

The postprandial phase is characterized by the circulation of atherogenic dietary-triacylglycerol rich lipoproteins. Little is known about the modulation of lipid and immune functions in macrophages by these particles or of the role of the oxysterols found in food such as 7beta-hydroxycholesterol and 7-ketocholesterol.

MATERIALS AND METHODS

Human macrophages were tested with different concentrations of chylomicron remnant-like particles (CRLP) with or without incorporated oxysterols to study their uptake by the cells, and their effects on cholesteryl ester and triacylglycerol synthesis and the secretion of inflammatory mediators, including tumour necrosis factor-alpha (TNF-alpha), interleukin 6 (IL-6) and interleukin 10 (IL-10).

RESULTS

Independently of the presence of oxysterols, CRLP caused cholesterol accumulation. However, the dose-dependent increase in [3H]cholesterol internalization by macrophages after incubation with [3H]cholesteryl ester-labelled CRLP was abolished by the presence of oxysterols in the particles. TNF-alpha secretion was decreased and that of IL-10 unaffected by CRLP independently of the presence of oxysterol. Exposure to CRLP containing 7beta-hydroxysterol, but not to CRLP or 7-ketosterol-containing CRLP, reduced IL-6 secretion with respect to cells not exposed to any particles. Because TNF-alpha levels have been related to scavenger receptor expression, we tested the uptake of modified LDL in macrophages exposed to human postprandial triacylglycerol-rich lipoproteins and found it to be markedly increased.

CONCLUSIONS

Cholesterol loading as a result of dietary lipids depresses the inflammatory response of macrophages and the presence of 7beta-hydroxysterol may exacerbate this effect. In addition, exposure to dietary lipids enhances scavenger receptor activity in macrophages. These results suggest that changes induced by dietary lipids in human macrophage function are related to an increased propensity of the cells to accumulate lipids during the postprandial phase.

Niemann-Pick C1 expression is not regulated by the amount of cholesterol flowing through cells in the mouse

The Niemann-Pick C1 (NPC1) protein functions to regulate the transport of cholesterol from late endosomes/lysosomes to other cellular compartments after lipoprotein uptake through the coated-pit pathway. The present study examines the relative expression of NPC1 mRNA and NPC1 protein in different tissues of the mouse in relation to the uptake of total cholesterol carried in chylomicron remnants (CMr-TC), low density lipoproteins (LDL-TC), cholesteryl ester carried in high density lipoproteins (HDL-CE), and cholesterol synthesis. Results from this study demonstrate that the highest relative expression of NPC1 is in the liver, which is also the tissue with the highest uptake of CMr-TC, LDL-TC, HDL-CE, and cholesterol synthesis. However, there was no similar relation in the remaining tissues. To examine the relative expression of NPC1 in relation to the amount of cholesterol that flowed through the coated-pit pathway, mice were fed a diet supplemented with increasing amounts of cholesterol or cholestyramine. The results from this study demonstrated that there was no relation between the relative expression of NPC1 and the amount of cholesterol that flowed through the coated-pit pathway. We conclude that the relative expression of NPC1 is not regulated by the flow of cholesterol through cells in the mouse and is therefore constitutive.

An Apolipoprotein(a) Peptide Delays Chylomicron Remnant Clearance and Increases Plasma Remnant Lipoproteins and Atherosclerosis In Vivo

Objective— Humans with high expression of apolipoprotein(a) [apo(a)] and high plasma levels of lipoprotein(a) [Lp(a)] are at increased risk for atherosclerosis, but the mechanism is not known. We have previously shown that the KIV 5–8 domain of apo(a) has unique cell-surface binding properties, and naturally occurring fragments of apo(a) encompassing this domain are thought to be atherogenic in humans. To investigate the effect of KIV 5–8 on lipoprotein metabolism and atherosclerosis in vivo, we created several independent lines of liver-targeted KIV 5–8 transgenic mice.

Methods and Results— The transgenic mice have plasma apo(a) peptide concentrations that are similar to Lp(a) concentrations in humans at risk for coronary artery disease. Remarkably, the transgenic mice had a 2- to 4-fold increase in cholesterol-rich remnant lipoproteins (RLPs) when fed a cholesterol-rich diet, and a 5- to 20-fold increase in atherosclerosis lesion area in the aortic root. Using an in vivo clearance study, we found only slight differences in the triglyceride and apolipoprotein B secretion rates between the 2 groups of mice, suggesting an RLP clearance defect. Using an isolated perfused mouse liver system, we showed that transgenic livers had a slower rate of RLP removal, which was retarded further when KIV 5–8 , full-length apo(a), or Lp(a) were added to the perfusate. An apo(a) peptide that does not interact with cells, K(IV 2 ) 3 , did not retard RLP removal, and low-density lipoprotein (LDL) had a much smaller effect than Lp(a).

Conclusions— We propose that high levels of apo(a)/Lp(a), perhaps acting via a specific cell-surface binding domain, inhibit hepatic clearance of remnants, leading to high plasma levels of RLPs and markedly enhanced atherosclerosis. We speculate that the KIV 5–8 region of apo(a) competes with one or more receptors for remnant clearance in the liver and that this process may represent one mechanism accounting for increased atherosclerosis in humans with high secretion levels of apo(a).

Biphasic insulin aspart compared to biphasic human insulin reduces postprandial hyperlipidemia in patients with Type 2 diabetes

BACKGROUND

Premixed insulin analogues reduce postprandial hyperglycemia in patients with Type 2 diabetes in comparison to premixed regular insulin. Insulin also plays an important role in the regulation of postprandial lipid metabolism. It is known that increased levels of postprandial insulin reduce postprandial hyperlipemia but, on the other hand, no information exists with regard to the possible effect of insulin analogues in comparison to human insulin.

MATERIALS AND METHODS

12 subjects (3 men; age 59 +/- 5 years; BMI 30.5 +/- 5.9 kg/m2, duration of diabetes 9 +/- 1 years, HbA1c 8.33 +/- 1.1 %) already on therapy with premixed insulin were treated either with biphasic human insulin (BHI30) or with biphasic insulin aspart (BIAsp30) (1.3 IU fast acting insulin/12 g KH) in the setting of a standardized test meal. Serum levels of glucose, insulin, C-peptide and triglycerides as well as retinylpalmitate in plasma and chylomicron remnants were determined before and up to 8 hours after the meal.

RESULTS

As was to be expected, therapy with BIAsp30 reduced the maximum increase of postprandial glucose from 7.10 +/- 2.00 mmol/l to 5.27 +/- 1.83 mmo/l (p = 0.007) compared to BHI30 insulin. In the same way, the maximum increase of triglycerides (from 2.33 +/- 1.03 to 1.65 +/- 0.69 mmol/l, p = 0.014) was reduced. The AUC 0 - 8 for triglycerides was not significantly influenced (34.20 +/- 19.86 vs. 31.46 +/- 16.21 mmol x 8 h/l) but the incremental area over baseline (AOB 0 - 8) was significantly reduced from 8.02 +/- 4.35 to 6.12 +/- 3.94 mmol x 8 h/l (p = 0.024).

CONCLUSIONS

Compared to conventional human premixed insulin the prandial therapy with biphasic insulin aspart results not only in an improvement of glucose tolerance but also in a significant reduction of postprandial hyperlipemia.

Efflux of lipid from macrophages after induction of lipid accumulation by chylomicron remnants

The fate of cholesterol and triacylglycerol taken up and accumulated by macrophages after exposure to chylomicron remnants was investigated using macrophages derived from the human monocyte cell line THP-1 and chylomicron remnant-like particles containing human apolipoprotein (apo) E (CRLPs) as the experimental model. In THP-1 macrophages lipid loaded with CRLPs and incubated with various cholesterol acceptors for 24 h, the mass of cholesterol and cholesteryl ester found in the cells was not changed by HDL, HDL3 or lipid-free ApoA-I, although it was decreased by 38% by ApoA-I-phosphatidylcholine vesicles (ApoA-I-PC). After loading of the macrophages with [3H]cholesterol-labelled CRLPs, only about 5% of the label was effluxed in 24 h in the absence of cholesterol acceptors, and this increased to about 10% with ApoA-I or PC only, and to about 30% with apoA-I-PC. In similar experiments with [3H]triolein, only about 4% of the labelled triacylglycerol taken up by the cells was released into the medium in 24 h, and a large (>60%) and consistent proportion of the intracellular radioactivity remained associated with the triacylglycerol throughout this period. These results suggest that cholesterol and triacylglycerol derived from chylomicron remnants are not readily cleared from macrophages, and this is likely to contribute to the atherogenicity of the remnant lipoproteins.

Chylomicron remnants regulate early growth response factor-1 in vascular smooth muscle cells

Early growth response factor-1 (Egr-1) is a zinc-finger transcription factor that induces genes that promote atherosclerosis. The goal of the present study was to determine whether Egr-1 expression is modulated by atherogenic, triglyceride rich lipoproteins known as chylomicron remnants. Chylomicron remnants induced Egr-1 mRNA and protein expression in rat cultured vascular smooth muscle cells (VSMCs) and activated extracellular signal-regulated kinase (ERK) 1/2 in VSMCs. Further, chylomicron remnant-induced Egr-1 expression was inhibited by PD98059, a selective inhibitor of MAPK kinase (MEK), suggesting that the action of chylomicron remnants on Egr-1 was dependent on the ERK/MEK pathway. Chylomicron remnants also induced mRNA expression of the pro-inflammatory cytokines, IL-2 and IFN-gamma in VSMCs. We conclude that chylomicron remnants act as atherogenic lipoproteins via induction of Egr-1 expression and via cytokine-mediated inflammation.

Effects of increasing doses of simvastatin on fasting lipoprotein subfractions, and the effect of high-dose simvastatin on postprandial chylomicron remnant clearance in normotriglyceridemic patients with premature coronary sclerosis

Postprandial hyperlipidemia has been linked to premature coronary artery disease (CAD) in fasting normotriglyceridemic patients. We investigated the effects of increasing doses of simvastatin up to 80 mg/day on fasting and postprandial lipoprotein metabolism in 18 normotriglyceridemic patients with premature CAD. Fasting lipoprotein subfractions and cholesteryl ester transfer protein (CETP) activity were determined after each 5-week dose titration (0, 20, 40 and 80 mg/day). At baseline and after treatment with simvastatin 80 mg/day, standardised Vitamin A oral fat loading tests (50 g/m2; 10 h) were carried out. Ten normolipidemic healthy control subjects matched for gender, age and BMI underwent tests without medication. Treatment with simvastatin resulted in dose-dependent reductions of fasting LDL-cholesterol, without changing cholesterol levels in the VLDL-1, VLDL-2 and IDL fractions. In addition, simvastatin decreased CETP activity dose-dependently, although HDL-cholesterol remained unchanged. Simvastatin 80 mg/day decreased fasting plasma triglycerides (TG) by 26% (P < 0.05), but did not decrease significantly TG levels in any of the subfractions. The TG/cholesterol ratio increased in all subfractions. The plasma TG response to the oral fat loading test, estimated as area under the curve (TG-AUC), improved by 30% (from 21.5 +/- 2.5 to 15.1 +/- 1.9 mmol h/L; P < 0.01). Treatment with simvastatin 80 mg/day improved chylomicron remnant clearance (RE-AUC) by 36% from 30.0 +/- 2.6 to 19.2 +/- 3.3 mg h/L (P < 0.01). After therapy, remnant clearance in patients was similar to controls (19.2 +/- 3.3 and 20.3 +/- 2.7 mg h/L, respectively), suggesting a normalization of this potentially atherogenic process. In conclusion, high-dose simvastatin has beneficial effects in normotriglyceridemic patients with premature CAD, due to improved chylomicron remnant clearance, besides effective lowering of LDL-cholesterol. In addition, the lipoprotein subfractions became more cholesterol-poor, as reflected by the increased TG/cholesterol ratio, which potentially makes them less atherogenic.

Removal of chylomicron remnants in transgenic mice overexpressing normal and membrane-anchored hepatic lipase

The LDL receptor and the LDL receptor-related protein (LRP) mediate the removal of chylomicron remnants. The LRP pathway involves sequestration of particles in the space of Disse. It has been proposed that either alone or in combination with other factors, such as apolipoprotein E and proteoglycans, hepatic lipase (HL) may contribute to the sequestration of chylomicron remnants. To test this hypothesis, we generated two lines of transgenic mice producing rat HL as a native or as a membrane-anchored form. These animals express HL at levels similar to normal rat. Chylomicron remnants were perfused in a single nonrecirculating pass into the livers of the rat HL transgenic, HL-deficient, and wild-type (WT) mice for 20 min, and the rate of chylomicron remnant removal was measured. Chylomicron remnants were removed at a rate of approximately 50% per pass in WT mice. It was slightly increased in both transgenic mice and reduced in HL-deficient mice compared with the WT mice. Confocal microscopy of liver sections showed that a modest amount of HL colocalized with chylomicron remnant clusters in the transgenic mice, suggesting that HL is a component of the LRP-proteoglycan clusters. These data suggest that HL helps to direct cholesterol to the tissues in which it is localized by a nonenzymatic mechanism.

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

BACKGROUND AND AIM

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

METHODS AND RESULTS

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

CONCLUSION

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

Chylomicron-remnant-like particles modify production of vasoactive mediators by endothelial cells

Endothelial-cell dysfunction is a critical initiating event in the pathogenesis of atherosclerosis. Although there is evidence to suggest that chylomicron remnants (CMRs), lipoproteins derived from the diet, influence endothelial-cell function to generate a pro-atherogenic phenotype, the mechanisms involved remain undefined. We have examined the effects of CMR-like particles (CMR-LPs) on human endothelial-cell function, focusing on the cyclo-oxygenase (COX) and nitric oxide synthase (NOS) pathways. CMR-LPs strongly enhanced the expression of the inducible cyclo-oxygenase COX-2 and increased prostacyclin synthesis in a biphasic manner. Studies with the COX-2-selective inhibitor NS-398 confirmed the COX-2 dependency of the later increase in prostanoid production. Pre-incubation with CMR-LPs reduced basal and thrombin-stimulated cGMP generation, whereas expression of endothelial NOS was not modified by remnant treatment.

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.

Chylomicron remnants and oxidised low density lipoprotein have differential effects on the expression of mRNA for genes involved in human macrophage foam cell formation

The effects of chylomicron remnants (non-oxidised or oxidised) and oxidised low density lipoprotein (oxLDL) on the expression of mRNA for a wide range of genes believed to play a role in macrophage foam cell formation were compared using macrophages derived from the human monocyte cell line THP-1. Chylomicron remnant-like particles (CMR-LPs), oxidised CMR-LPs (oxCMR-LPs) and oxLDL were incubated with THP-1 macrophages, and the relative abundance of mRNA transcripts for genes involved in lipoprotein uptake, intracellular lipid metabolism, transport and storage and cholesterol efflux from macrophages was determined. The results show that CMR-LPs and oxLDL differ markedly in their effects on the expression of mRNA for a number of the genes tested. OxLDL increased mRNA levels for the scavenger receptors CD36 (x3.2) and lectin-like oxLDL receptor 1 (x2.1), and peroxisome proliferator-activated receptor gamma while CMR-LPs did not. In contrast, the expression of mRNA for the LDL receptor-like protein was raised by CMR-LPs (x1.8) but not oxLDL. Furthermore, down-regulation of mRNA levels for the ATP-binding cassette transporter (ABC) A1 was observed with CMR-LPs (x0.6), compared to the up-regulation found with oxLDL (x4.4). In addition, a number of significant differences were found between the effects of CMR-LPs and oxCMR-LPs, with the oxidised particles causing a striking rise in mRNA expression for the multi-drug resistance 1 gene (x13.7), but otherwise showing pattern more similar to that seen with oxLDL. These findings provide evidence to indicate that chylomicron remnants cause lipid accumulation in macrophages by influencing the expression of genes which regulate lipid metabolism at the transcriptional level, and that the mechanisms involved differ in important respects from those triggered by oxLDL.

Comparison of ultracentrifugation and nuclear magnetic resonance spectroscopy in the quantification of triglyceride-rich lipoproteins after an oral fat load

BACKGROUND

The measurement of triglyceride (TG)-rich particles after an oral fat challenge has been used to provide a measure of risk for coronary artery disease independent of the fasting plasma triglyceride concentration. The analytical "gold standard" for measuring TG-rich lipoproteins uses density gradient ultracentrifugation; however, this technique is labor-intensive. Because of our need to perform numerous postprandial analyses of TG-rich lipoproteins for a large interventional study (Genetics of Lipid Lowering Drugs and Diet Network), we evaluated the use of nuclear magnetic resonance (NMR) spectroscopy for measuring TG-rich particles.

METHODS

EDTA-blood samples were obtained 0, 3.5, 6, and 8 h after ingestion of an oral fat meal (89% of calories from fat) in 20 apparently healthy individuals. The plasma TG concentrations of chylomicron and chylomicron remnant/VLDL fractions were analyzed by ultracentrifugation and NMR spectroscopy.

RESULTS

Comparison of all values (n = 78) by ultracentrifugation (x) and NMR (y) produced a linear regression equation of y = 0.979x - 0.035 mmol/L (R(2) = 0.90) for chylomicrons and y = 1.398x + 0.067 mmol/L (R(2) = 0.96) for the fraction containing chylomicron remnants and VLDL. Postprandial response of chylomicrons and chylomicron remnant/VLDL was similar, with maximum response occurring between 3.5 to 6 h regardless of method of measurement.

CONCLUSION

Chylomicron and chylomicron remnant/VLDL fraction measurements obtained by NMR have a high degree of correlation with results produced by ultracentrifugation. NMR may therefore be suitable as an alternative method for the measurement of postprandial TG-rich lipoproteins in individuals consuming a high-fat meal.

Remnant Lipoprotein Particles Induce Apoptosis in Endothelial Cells by NAD(P)H Oxidase–Mediated Production of Superoxide and Cytokines via Lectin-Like Oxidized Low-Density Lipoprotein Receptor-1 Activation

Background— Remnant lipoprotein particles (RLPs), products of lipolytic degradation of triglyceride-rich lipoprotein derived from VLDL, exert atherogenesis. In this study, we observed how RLPs induced cytotoxicity in human umbilical vein endothelial cells (HUVECs) and cilostazol prevented cell death.

Methods and Results— RLPs were isolated from the plasma of hyperlipidemic patients by use of an immunoaffinity gel mixture of anti–apolipoprotein A-1 and anti–apolipoprotein B-100 monoclonal antibodies. RLPs (50 μg/mL) significantly increased superoxide formation in HUVECs associated with elevated gp91phox mRNA and protein expression and Rac1 translocation, accompanied by increased production of tumor necrosis factor (TNF)-α and interleukin-1β, DNA fragmentation, and cell death. Cilostazol (1 to 100 μmol/L) significantly suppressed not only NAD(P)H oxidase–dependent superoxide production but also TNF-α and interleukin-1β release and restored viability. RLPs activated a lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), which was not inhibited by cilostazol. Treatment of HUVECs with monoclonal antibody for LOX-1 attenuated RLP-mediated production of superoxide, TNF-α, and interleukin-1β and DNA fragmentation.

Conclusions— RLPs stimulated NAD(P)H oxidase–dependent superoxide formation and induction of cytokines in HUVECs via activation of LOX-1, consequently leading to reduction in cell viability with DNA fragmentation, and cilostazol exerts a cell-protective effect by suppressing these variables.

Scavenger receptor BI plays a role in facilitating chylomicron metabolism

The function of scavenger receptor class B type I (SR-BI) in mediating the selective uptake of high density lipoprotein (HDL) cholesterol esters is well established. However, the potential role of SR-BI in chylomicron and chylomicron remnant metabolism is largely unknown. In the present investigation, we report that the cell association of 160 nm-sized triglyceride-rich chylomicron-like emulsion particles to freshly isolated hepatocytes from SR-BI-deficient mice is greatly reduced (>70%), as compared with wild-type littermate mice. Competition experiments show that the association of emulsion particles with isolated hepatocytes is efficiently competed for (>70%) by the well established SR-BI ligands, HDL and oxidized low density lipoprotein (LDL), whereas LDL is ineffective. Upon injection into SR-BI-deficient mice the hepatic association of emulsion particles is markedly decreased ( approximately 80%) as compared with wild-type mice. The relevance of these findings for in vivo chylomicron (remnant) metabolism was further evaluated by studying the effect of SR-BI deficiency on the intragastric fat load-induced postprandial triglyceride response. The postprandial triglyceride response is 2-fold higher in SR-BI-deficient mice as compared with wild-type littermates (area-under-the-curve 39.6 +/- 1.2 versus 21.1 +/- 3.6; p < 0.005), with a 4-fold increased accumulation of chylomicron (remnant)-associated triglycerides in plasma at 6 h after intragastric fat load. We conclude that SR-BI is important in facilitating chylomicron (remnant) metabolism and might function as an initial recognition site for chylomicron remnants whereby the subsequent internalization can be exerted by additional receptor systems like the LDL receptor and LDL receptor-related protein.

Lack of evidence for reduced plasma apo B48 catabolism in patients with heterozygous familial hypercholesterolemia carrying the same null LDL receptor gene mutation

Increasing evidence suggests that remnants of chylomicrons and very low density lipoprotein (VLDL) also known as triglyceride-rich lipoproteins (TRL) are directly related to the pathogenesis of atherosclerosis. While studies in animals suggest that low density lipoprotein (LDL) receptor deficiency delays clearance of chylomicron remnants, human data supporting this hypothesis are conflicting. The objective of this study was to compare the fractional catabolic rate (FCR) and production rate (PR) of TRL apolipoprotein B48, the principal structural protein of intestinally derived chylomicron remnants, between familial hypercholesterolemic (FH) heterozygotes and non-FH controls. This was achieved by examining the kinetics of TRL apo B48 labelled with a stable isotope (L-(5,5,5-D3)leucine) in five normolipidemic males (age: 24.7 +/- 1.3 years; body mass index (BMI): 23.9 +/- 1.4 kg/m2) and six genetically defined FH heterozygous males (age: 29.7 +/- 9.9 years; (BMI): 22.0 +/- 4.3 kg/m2) carrying the same null LDL receptor gene mutation. All participants were apo E3 homozygotes. During the kinetic study, the subjects consumed 1/30 of their daily food intake every 30 min over a 15 h period. No significant difference was observed between FH heterozygotes and controls for FCR of TRL apo B48 (7.9 +/- 2.1 versus 7.9 +/- 2.6 pools per day, P = 0.99) while the TRL apo B48 pool size (10.5 +/- 5.4 versus 5.7 +/- 2.4 mg, P = 0.03) and PR (1.1 +/- 0.3 versus 0.6 +/- 0.3 mg kg(-1) per day, P = 0.02) were significantly higher among FH than in controls. In conclusion, this study shows no evidence for reduced plasma apo B48 catabolism in patients with heterozygous FH carrying the same null LDL receptor gene mutation and suggests that the plasma levels of intestinally derived TRL are elevated in FH due to an increased production rate.

Incorporation of lycopene into chylomicron remnant-like particles enhances their induction of lipid accumulation in macrophages

Lipid accumulation in macrophages exposed to chylomicron remnant-like particles containing the dietary antioxidant lycopene was investigated. After incubation with THP-1 macrophages (48h), chylomicron remnant-like particles containing lycopene (lycCRLPs) as compared to those without (CRLPs) caused significantly more lipid accumulation in the cells, and this was due to increases in both the triacylglycerol (+100%) and cholesterol (+62%) content. In addition, expression of mRNA for diacylglycerol acyltransferase (DGAT), a key enzyme in triacylglycerol synthesis, was significantly decreased by lycCRLPs, but not CRLPs. These findings suggest that lycopene from the diet may promote, rather than retard, lipid accumulation in macrophages during its transport in the blood in chylomicron remnants.

Chylomicron remnants of various sizes are lowered more effectively by fenofibrate than by atorvastatin in patients with combined hyperlipidemia

Both atorvastatin and fenofibrate are known to lower postprandial chylomicrons and chylomicron remnants. However, until now it has not been investigated which of the two drugs is more effective in one and the same patient and, secondly, whether these drugs exert different effects on chylomicron remnants of different sizes. To this end 12 patients with mixed hyperlipidemia were treated in a crossover study with 40 mg atorvastatin or with 200 mg micronized fenofibrate once daily for 6 weeks. Oral fat loading was given before and after each treatment. Chylomicron remnants of various sizes were determined by fluorometric determinations of retinyl palmitate after lipoprotein separation by size-exclusion chromatography. As expected, atorvastatin was more effective than fenofibrate on total and LDL-cholesterol (P < 0.05). Fenofibrate, in contrast, was more effective on all triglyceride-rich lipoproteins in both the fasting and the postprandial state. The stronger effect of fenofibrate affected not only chylomicrons and VLDL but also chylomicron remnants. It reduced large chylomicron remnants by 66% at 6h and by 74% at 8 h. The action of atorvastatin was less pronounced, with corresponding reductions of 42 and 65% (P < 0.05 only after 8 h). Fenofibrate was even more effective on small chylomicron remnants, yielding reductions of 47, 74, and 66% at 4, 6, and 8 h. Atorvastatin, in contrast, gave reductions of 30 and 26% after 6 and 8 h, the effect reaching statistical significance only after 6h. Fenofibrate is therefore more effective than atorvastatin in lowering all triglyceride-rich lipoproteins, including large and small chylomicron remnants.

Chylomicron remnants induce monocyte chemoattractant protein-1 expression via p38 MAPK activation in vascular smooth muscle cells

Chylomicron remnants, major lipoproteins at postprandial hyperlipidemia, have been considered to be proatherogenic lipoproteins. However, the mechanisms by which chylomicron remnants enhance atherosclerosis have not been fully understood. Monocyte chemoattractant protein-1 (MCP-1) is a chemokine which stimulates migration of monocytes and plays a critical role in the development of atherosclerosis. In this study, we investigated the effect of chylomicron remnants on MCP-1 expression in cultured vascular smooth muscle cells (VSMCs). We prepared chylomicrons from the lymph of gastrostomized rats fed with egg solution and obtained chylomicron remnants from the plasma of hepatectomized rats which were injected with chylomicrons. Treatment of VSMC with chylomicron remnants resulted in a significant increase of the expression of MCP-1 mRNA and protein in a time-and a dose-dependent manner. Further, chylomicron remnants activated p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK1/2). Pretreatment of VSMCs with p38 MAPK inhibitors, SB203580 and SB202190, resulted in a dose-dependent inhibition of chylomicron remnants-induced MCP-1 mRNA and protein expression, whereas a MAPK kinase inhibitor, PD98059, had no effect on these responses. MCP-1 secretion by chylomicron remnants was much more pronounced than those by chylomicrons, oxidized low-density lipoproteins, or lysophosphatidylcholine. These results indicated that chylomicron remnants stimulated MCP-1 expression in VSMCs, and suggested that chylomicron remnants might contribute to the formation of atherosclerosis through this proinflammatory effect.

Chylomicron-remnant-like particles inhibit the basal nitric oxide pathway in porcine coronary artery and aortic endothelial cells

The effects of chylomicron remnants on the activity of basally produced nitric oxide (NO) from porcine coronary artery rings and porcine aortic endothelial cells were studied by investigating the effects of chylomicron-remnant-like particles (CMR-LPs) containing porcine apolipoprotein E on the vessel tone of porcine coronary arteries and on cGMP release by aortic endothelial cells. CMR-LPs were oxidized by incubation with CuSO(4) (10 microM) for 18 h at 37 degrees C. N (omega)-nitro-L-arginine (L-NOARG) and oxidized CMR-LPs (oxCMR-LPs), but not native CMR-LPs, increased the vessel tone of static porcine coronary artery rings (increase in tone as a percentage of the tone induced by depolarizing Krebs-Henseleit solution: L-NOARG, 14.24 +/- 2.09; oxCMR-LPs, 4.98 +/- 0.88; and native CMR-LPs, 0.47 +/- 0.21). L-NOARG, endothelium removal and oxCMR-LPs also all significantly increased the maximum relaxation of the vessels to S -nitroso- N -acetyl-DL-penicillamine. In addition, oxCMR-LPs reduced the amounts of cGMP released by porcine aortic endothelial cells into the culture medium from 116 +/- 12.0 to 84.2 +/- 11.6 fmol/microg of cellular protein, mimicking the effects of L-NOARG. These results indicate that oxCMR-LPs, but not native CMR-LPs, inhibit the activity, production or release of NO from unstimulated porcine coronary and aortic endothelial cells. oxCMR-LPs mimicked the addition of L-NOARG and endothelium removal in these experimental systems, suggesting that the lipoproteins were interfering with the L-arginine/NO pathway. This study provides further evidence to support a role of chylomicron remnants in the development of atherosclerosis.

Serum concentrations of remnant-like particles in hypothyroid patients before and after thyroxine replacement

OBJECTIVES

Remnant-like particles (RLPs) reflect chylomicron remnants and very-low-density lipoprotein remnants, which are most likely to be atherogenic particles. To investigate the effect of thyroxine replacement on the metabolism of RLPs in hypothyroidism, we measured serum concentrations of RLPs during an oral fat-loading test in patients with hypothyroidism before and after thyroxine replacement.

PATIENTS AND METHODS

Thirteen patients with hypothyroidism, having serum-free thyroxine (FT4) of 4.25 +/- 2.23 pmol/l (mean +/- SD) and TSH of 72.5 +/- 27.7 mU/l, participated in the study. Two-hundred grams of cream containing 32.9% of fat were given to each patient followed by blood draws every 2 h for 8 h. The patients became euthyroid after 3 months of T4 replacement, and the fat-loading tests were then repeated.

RESULTS

Fasting levels of serum total cholesterol and low-density lipoprotein cholesterol were remarkably decreased after T4 therapy (P < 0.0005). Serum high-density lipoprotein cholesterol and triglyceride were also decreased by T4 therapy, not so remarkably but significantly (P < 0.05). Activities of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) increased 52% and 85%, respectively, from the pretreatment values. Serum concentrations of remnant-like particle cholesterol (RLP-C) and remnant-like particle triglyceride (RLP-TG), measured by immunoseparation assays, significantly decreased from 0.14 +/- 0.03 to 0.09 +/- 0.03 mmol/l (P < 0.0005) and from 0.19 +/- 0.11-0.11 +/- 0.07 mmol/l (P < 0.01), respectively. In the fat-loading test, serum low-density lipoprotein cholesterol concentrations were not changed, while serum RLPs concentrations were increased and remained high throughout the test, with the peak value at 6 h in a hypothyroid condition. In an euthyroid condition after T4 therapy, the peak values of RLPs were obtained at 4 h, and the concentrations were decreased rapidly. As the result, areas under the curve of serum RLPs were decreased remarkably after T4 therapy.

CONCLUSIONS

Hypothyroidism seems to be associated with a decrease in metabolism of serum RLPs. Such altered metabolism of RLPs may be related to the decreased activities of LPL and HTGL and can be corrected by T4 replacement therapy.

Chylomicron remnant concentrations in patients with coronary artery disease

Persisting chylomicron remnants have been linked to premature atherosclerosis. The analysis of chylomicron remnant concentrations by an oral triglyceride tolerance test, however, is time-consuming for the study subjects and requires large resources in the laboratory. Therefore, only small numbers of subjects have been studied in the past. The aim of this study was to elucidate the prevalence of elevated chylomicron remnants, to identify effectors of chylomicron remnant clearance and to compare chylomicron remnants in the prediction of coronary artery disease with other risk factors. We applied a novel oral triglyceride tolerance test to 423 patients (368 males, 55 females) with a confirmed diagnosis of coronary artery disease (CAD) and to 390 control subjects (295 males, 95 females) in a case-control setting. This study revealed that elevated chylomicron remnant concentrations (retinyl esters > 1.5 micromol/l) are present in 20% of all subjects. Male gender, the apolipoprotein E2 isoform, and higher body mass index were associated with increased chylomicron remnant concentrations. However, chylomicron remnants were lower and plasma triglycerides higher in patients with CAD. We conclude that screening for a delayed clearance of chylomicron remnants is of little clinical value in CAD.

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.

Oxidized cholesterol in the diet is a source of oxidized lipoproteins in human serum

The aim of this study was to determine in humans whether oxidized cholesterol in the diet is absorbed and contributes to the pool of oxidized lipids in circulating lipoproteins. When a meal containing 400 mg cholestan-5alpha,6alpha-epoxy-3beta-ol (alpha-epoxy cholesterol) was fed to six controls and three subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol in serum was found in chylomicron/chylomicron remnants (CM/RM) and endogenous (VLDL, LDL, and HDL) lipoproteins. In controls, alpha-epoxy cholesterol in CM/RM was decreased by 10 h, whereas in endogenous lipoproteins it remained in the circulation for 72 h. In subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol was mainly in CM/RM. In vitro incubation of the CM/RM fraction containing alpha-epoxy cholesterol with human LDL and HDL that did not contain alpha-epoxy cholesterol resulted in a rapid transfer of oxidized cholesterol from CM/RM to both LDL and HDL. In contrast, no transfer was observed when human serum was substituted with rat serum, suggesting that cholesteryl ester transfer protein is mediating the transfer. Thus, alpha-epoxy cholesterol in the diet is incorporated into the CM/RM fraction and then transferred to LDL and HDL, contributing to lipoprotein oxidation. Moreover, LDL containing alpha-epoxy cholesterol displayed increased susceptibility to further copper oxidation in vitro. It is possible that oxidized cholesterol in the diet accelerates atherosclerosis by increasing oxidized cholesterol levels in circulating LDL and chylomicron remnants.

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.

Chylomicron-remnant-like particles inhibit receptor-mediated endothelium-dependent vasorelaxation in pig coronary arteries

The influence of native and oxidized chylomicron-remnant-like particles (CMR-LPs) on endothelium-dependent relaxation in pig coronary arteries was studied. Artificial lipid particles of a size and lipid composition resembling chylomicron remnants and containing pig apolipoprotein E were used to investigate the effects of chylomicron remnants on the relaxation of isolated segments of pig coronary arteries in response to three endothelium dilators: 5-hydroxytryptamine (5-HT), bradykinin and the calcium ionophore A23187. CMR-LPs caused significant inhibition of the maximum relaxation response of the vessels to 5-HT, but not that to bradykinin or A23187 ( P <0.05). In contrast, CMR-LPs that had been oxidized by incubation with 10 microM CuSO(4) (oxidized CMR-LPs) were found to significantly reduce maximal relaxation to bradykinin by 13% ( P <0.05) and to reduce the sensitivity of the tissue to A23187 by 1.7-fold ( P <0.05). In experiments in which either the L-arginine/nitric oxide (NO) pathway or the endothelium-derived hyperpolarizing factor (EDHF) pathway was selectively inhibited, leaving the other intact, the inhibitory effect of oxidized CMR-LPs was observed only in vessels in which the -arginine/NO-mediated pathway was operative. Furthermore, the oxidized particles had no inhibitory effect on the relaxation of the vessel segments to the non-endothelium-dependent agonists S -nitro- N -acetylpenicillamine, 5'-( N -ethylcarboxamido)adenosine or pinacidil. These results demonstrate that CMR-LPs inhibit vascular relaxation in pig coronary arteries by an endothelium-dependent mechanism involving the L-arginine/NO pathway, but not the EDHF pathway, and provide evidence in support of a role for chylomicron remnants in the endothelial dysfunction associated with hypercholesterolaemia and atherogenesis.

Effect of atorvastatin on chylomicron remnant metabolism in visceral obesity: a study employing a new stable isotope breath test

Elevated plasma concentration of chylomicron remnants may be causally related to atherosclerosis in obesity. We examined the effect of atorvastatin on chylomicron remnant metabolism in 25 obese men with dyslipidaemia. A remnant-like emulsion labeled with cholesteryl [(13)C]oleate was injected intravenously into patients; the fractional catabolic rate (FCR) of the remnant-like emulsion was determined by measurement of (13)CO(2) in the breath and analyzed using compartmental modelling. Compared with placebo, atorvastatin significantly decreased the plasma concentrations of total cholesterol, triglycerides, LDL cholesterol, apolipoprotein B (apoB), and lathosterol (P < 0.001). ApoB-48 and remnant-like particle-cholesterol (RLP-C) both decreased significantly by 23% (P = 0.002) and 33% (P = 0.045), respectively. The FCR of the remnant-like emulsion increased significantly from 0.054 +/- 0.008 to 0.090 +/- 0.010 pools/h (P = 0.002). The decrease in RLP-C was associated with the decrease in plasma triglycerides (r = 0.750, P = 0.003). Furthermore, the change in FCR of remnant-like emulsions was inversely associated with the change in LDL-C (r = -0.575, P = 0.040), suggesting removal of LDL and chylomicron remnants by similar hepatic receptor pathways. We conclude that in obese subjects, inhibition of cholesterol synthesis with atorvastatin decreases the plasma concentrations of both LDL-C and triglyceride-rich remnants and that this may be partially due to an enhancement in hepatic clearance of these lipoproteins.

Effects of atorvastatin on postprandial plasma lipoproteins in postinfarction patients with combined hyperlipidaemia

Enhanced and prolonged postprandial lipaemia is implicated in coronary and carotid artery disease. This study assessed the effects of atorvastatin, a 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor, on postprandial plasma concentrations of triglyceride-rich lipoproteins (TRLs). Sixteen middle-aged men with combined hyperlipidaemia (baseline low density lipoprotein (LDL) cholesterol and plasma triglyceride concentrations (median (interquartile range) of 4.54 (4.17-5.26)) and 2.66 (2.04-3.20) mmol/l, respectively) and previous myocardial infarction were randomised to atorvastatin 40 mg or placebo once daily for 8 weeks in a double-blind, cross-over design. The apolipoprotein (apo) B-48 and B-100 contents were determined in subfractions of TRLs as a measure of chylomicron remnant and very low density lipoprotein (VLDL) particle concentrations (expressed as mg apo B-48 or apo B-100 per litre of plasma), in the fasting state and after intake of a mixed meal. Atorvastatin treatment reduced significantly the fasting plasma concentrations of VLDL cholesterol, LDL cholesterol and VLDL triglycerides (median% change) by 29, 44 and 27%, respectively, and increased high density lipoprotein (HDL) cholesterol by 19%, compared with baseline. The postprandial plasma concentrations of large (Svedberg flotation rate (Sf) 60-400) and small (Sf 20-60) VLDLs and chylomicron remnants were almost halved compared with baseline (mean 0-6 h plasma concentrations were reduced by 48% for Sf 60-400 apo B-100, by 46% for Sf 60-400 apo B-48, by 46% for Sf 20-60 apo B-100 and by 27% for Sf 20-60 apo B-48), and the postprandial triglyceridaemia was reduced by 23% during active treatment. In conclusion, atorvastatin 40 mg once daily causes profound reductions of postprandial plasma concentrations of all TRLs in combined hyperlipidaemic patients with premature coronary artery disease.

Food restriction normalizes chylomicron remnant metabolism in murine models of obesity as assessed by a novel stable isotope breath test

Evidence is increasing that defective metabolism of postprandial remnants of triglyceride-rich lipoproteins contributes to atherogenesis. In obesity, postprandial lipemia is increased by mechanisms that are not currently established. In the present study, a recently developed (13)CO(2) breath test was used to assess the metabolism of chylomicron remnants (CR) in obese mice. Six murine obese models ob/ob, fat/fat, New Zealand Obese (NZO), db/db, gold thioglucose (GTG)-treated and agouti (A(y)) were studied. All obese mice were hyperphagic and their breath test metabolism was markedly impaired (P < 0.01) compared with control, nonobese mice. The breath test was also impaired (P < 0.01) in all obese mice except A(y) mice after 24-h food deprivation. However, after restriction to the food intake of paired control mice for 6 wk, the breath test in all obese mice improved to values of control, nonobese mice. The obese NZO, fat/fat and ob/ob mice had significant (P < 0.02) weight loss when food restricted, whereas A(y), GTG, and db/db mice did not. In all obese mice, plasma cholesterol levels decreased (P < 0.02) after the 6-wk period of food restriction. Plasma triglyceride levels significantly decreased (P < 0.02) in NZO, GTG and db/db mice, but not in other obese mice. Plasma glucose levels were significantly decreased (P < 0.02) after the 6-wk period in the obese mice except for the A(y) and NZO mice; levels were greater in food-restricted db/db mice. Although some of the obese models such as db/db were diabetic, our data suggest that the defective breath test was independent of diabetes because all obese and diabetic models responded similarly to food restriction. Impaired hepatic catabolism of CR was excluded as a cause of the abnormal breath tests. In summary, the impairment (P < 0.05) in remnant metabolism as assessed by the breath test in obese mice was corrected by food restriction, associated with improvements in plasma glucose, triglyceride and cholesterol levels.

Preliminary experience with a new stable isotope breath test for chylomicron remnant metabolism: a study in central obesity

We aimed to investigate the metabolism of chylomicron remnants in the postabsorptive state employing a new stable isotope breath test in centrally obese men without overt hyperlipidaemia. Groups of 12 centrally obese and 12 non-obese men of similar age and with similar plasma cholesterol and triacylglycerol (triglyceride) levels were studied. The catabolism of chylomicron remnants was measured using an intravenous injection of a remnant-like emulsion containing cholesteryl [(13)C]oleate. Isotopic enrichment of (13)CO(2) in breath was determined using isotope-ratio mass spectrometry, and a multi-compartmental model (SAAM II program) was used to estimate the fractional catabolic rate (FCR) of the chylomicron remnant-like particles. The plasma concentrations of low-density lipoprotein (LDL)-cholesterol, non-high-density lipoprotein (HDL)-cholesterol and insulin were significantly higher (P<0.05) in the obese than the control subjects. The obese subjects had significantly lower HDL-cholesterol (P<0.05) and, in particular, a decreased FCR of the remnant-like particles compared with lean subjects (0.061+/-0.014 and 0.201+/-0.048 pools/h respectively; P=0.016). In the obese group, the FCR of remnant-like particles was inversely associated with the waist/hip ratio, and with plasma triacylglycerol, cholesterol, LDL-cholesterol and non-HDL-cholesterol levels. In multiple regression analysis, the waist/hip ratio was the best predictor of the FCR of the emulsion. In conclusion, this new test suggests that postabsorptive chylomicron remnant catabolism is impaired in centrally obese subjects without overt hyperlipidaemia. This defect may be due to the degree of adiposity.

Lipid synthesis in macrophages derived from the human cell line THP-1: modulation of the effects of native and oxidized chylomicron-remnant-like particles by oestrogen

The effects of native and oxidized chylomicron remnants on the synthesis of cholesteryl ester and triacylglycerol in macrophages, and the way that this is influenced by exposure of the cells to oestrogen, was investigated using the human monocyte cell line THP-1 and chylomicron-remnant-like particles containing human apolipoprotein E (CRLPs). Synthesis of the lipids was measured by the incorporation of [(3)H]oleate into cholesteryl ester and triacylglycerol. CRLPs (5-40 microgram of cholesterol/ml) containing either trilinolein or triolein as the triacylglycerol component caused a dose-dependent decrease in cholesteryl ester formation, while triacylglycerol production was unchanged. After oxidation of the CRLPs, the level of thiobarbituric acid-reactive substances was increased by 6.3-fold and 2.2-fold in particles containing trilinolein and triolein respectively. Furthermore, CRLPs containing oxidized trilinolein lost their ability to down-regulate cholesterol esterification, while CRLPs containing oxidized triolein did not. Both types of oxidized CRLPs decreased triacylglycerol synthesis. Treatment of the macrophages with 17beta-oestradiol caused increases of approx. 94% and 34% in the synthesis of cholesteryl ester and triacylglycerol respectively in the absence of CRLPs. The differences between control and oestrogen-treated cells were abolished, however, when CRLPs (40 microgram of cholesterol/ml) were added to the incubations. In addition, in contrast with their lack of effect in control cells, CRLPs containing oxidized trilinolein decreased cholesterol esterification in oestrogen-treated cells by approx. 48%. These findings with CRLPs suggest that chylomicron remnants have significant effects on cholesteryl ester and triacylglycerol synthesis in macrophages, which may be modulated both by the oxidation state of the particles and by oestrogen.

Chylomicron remnant metabolism in familial hypercholesterolaemia studied with a stable isotope breath test

Chylomicron remnant metabolism was studied using a stable isotope breath test in 25 patients with familial hypercholesterolaemia (FH) (10 homozygotes, 15 heterozygotes), and in 15 normolipidaemic controls. A lipid emulsion mimicking the composition of chylomicron remnants and labelled with cholesteryl (13)C-oleate was injected intravenously; (13)CO(2) was measured subsequently in breath using isotope-ratio mass spectrometry. The fractional catabolic rate (pools/h) of the emulsion, derived from a compartmental model, did not differ significantly among the groups: homozygous FH mean 0.20 (S.E.M. 0.05), heterozygous FH 0.12 (0.02), controls 0.16 (0.03). We suggest that the catabolism of chylomicron remnants from plasma is not impaired in FH and that the hepatic uptake of these particles is not dependent on functional LDL receptors.

An improved method for the rapid assessment of persisting chylomicron remnant concentrations

Persisting chylomicron remnant concentrations have been linked to premature atherosclerosis. The analysis of persisting chylomicron remnant concentrations by an oral triglyceride tolerance test, however, is time-consuming for the study subjects and requires large resources in the laboratory. Therefore, only small numbers of subjects have been studied in the past. We describe major improvements of the testing procedure in regard of composition of the fatty meal, of patient testing, and measurement of postprandial remnants. Shifting the time of the (ready-to-use) fatty drink from the morning hours to bedtime was well accepted by the study subjects and allowed the analysis of blood samples drawn at the morning with minimal impact on the participants' time and with minimal interferences by confounding factors (e.g. smoking, additional food intake, physical activity). Chylomicron remnants were measured by fluorometry of the supernatant after ultracentrifugation. This procedure was sensitive, was specific for chylomicron remnants, and was easy to perform. The biological validity of the improved procedure was evaluated by studying type III hyperlipoproteinemia patients and normolipemic apolipoprotein (Apo) E2 homozygotes. In conclusion, this improved test permits the rapid testing for persisting chylomicron remnants in the clinical routine and in large epidemiological studies.

LDL receptor-related protein mediates cell-surface clustering and hepatic sequestration of chylomicron remnants in LDLR-deficient mice

It has been proposed that in the liver, chylomicron remnants (lipoproteins carrying dietary lipid) may be sequestered before being internalized by hepatocytes. To study this, chylomicron remnants labeled with a fluorescent dye were perfused into isolated livers of LDL receptor-deficient (LDLR-deficient) mice (Ldlr(-/-)) and examined by confocal microscopy. In contrast to livers from normal mice, there was clustering of the chylomicron remnants on the cell surface in the space of DISSE: These remnant clusters colocalized with clusters of LDLR-related protein (LRP) and could be eliminated by low concentrations of receptor-associated protein, an inhibitor of LRP. When competed with ligands of heparan sulfate proteoglycans (HSPGs), the remnant clusters still appeared but were fewer in number, although syndecans (membrane HSPGs) colocalized with the remnant clusters. This suggests that the clustering of remnants is not dependent on syndecans but that the syndecans may modify the binding of remnants. These results establish that sequestration is a novel process, the clustering of remnants in the space of DISSE: The clustering involves remnants binding to the LRP, and this may be stabilized by binding with syndecans, eventually followed by endocytosis.

Chylomicron remnant metabolism in familial dyslipidemias studied with a remnant-like emulsion breath test

We have developed a stable isotope breath test for the assessment of chylomicron remnant metabolism and report the results from the breath test in human subjects selected for disorders of chylomicron or remnant metabolism. In type I hyperlipemia, the phenotype is extreme hypertriglyceridemia due to a lack of lipoprotein lipase activity, which causes the failure of remnant formation. The type III dyslipidemia phenotype is caused by the inefficient removal of chylomicron remnants from plasma, generally because of homozygosity for apolipoprotein E2 alleles. The breath test was predicted to be abnormal in type III hyperlipemia, whereas a priori in type I hyperlipemia defective remnant clearance was not anticipated. Subjects were injected with lipid emulsions prepared with a composition similar to normal chylomicron remnants. The emulsions contained cholesteryl ester incorporating the stable nonradioactive isotope (13)C in the fatty acid moiety. End exhalation breath was collected at intervals after intravenous injection of the remnant-like emulsions and analyzed for (13)C enrichment by isotope-ratio mass spectrometry. Compared with the group of normolipemic men, the fractional catabolic rate of remnants measured by the breath test was significantly decreased (P = 0.006) in subjects with type III dyslipidemia. In the group with type I hyperlipemia, the fractional catabolic rate was not different (P = 0.233) from the control group. Therefore, the underlying capacity for remnant catabolism was normal in this group of markedly hypertriglyceridemic subjects. By short-circuiting the step of lipolysis, the remnant-like emulsion breath test provides direct information about remnant clearance and metabolism, which should assist in investigations of postprandial lipid metabolism.

Oxidation affects the regulation of hepatic lipid synthesis by chylomicron remnants

The effects of native and oxidized chylomicron remnants on lipid synthesis in normal and oxidatively stressed liver cells were investigated using MET murine hepatocytes (MMH cells), a nontransformed mouse hepatocyte cell line that maintains a highly differentiated hepatic phenotype in culture. Lipid synthesis was determined by measuring the incorporation of [(3)H]oleate into cholesteryl ester, triacylglycerol, and phospholipid by the cells. The formation of cholesteryl ester and phospholipid was decreased by chylomicron remnants in a dose-dependent manner, while triacylglycerol synthesis was increased. Exposure of MMH cells to mild oxidative stress by incubation with CuSO(4) (2.5 microM) for 24 h led to significantly increased incorporation of [(3)H]oleate into triacylglycerol and phospholipid, but not cholesteryl ester, in the absence of chylomicron remnants. In the presence of the lipoproteins, however, similar effects to those found in untreated cells were observed. Oxidatively modified chylomicron remnants prepared by incubation with CuSO(4) (10 microM, 18 h, 37 degrees C) did not influence cholesteryl ester or phospholipid synthesis in MMH cells, but had a similar effect to that found with native remnants on triacylglycerol synthesis. These findings show that hepatic lipid metabolism is altered by exposure to mild oxidative stress and by lipids from the diet delivered to the liver in chylomicron remnants, and these effects may play a role in the development of atherosclerosis.

Transcriptional regulation of apolipoprotein C-III gene expression by the orphan nuclear receptor RORalpha

Triglyceride-rich remnant lipoproteins are considered as major risk factors contributing to the pathogenesis of atherosclerosis. Because apolipoprotein (apo) C-III is a major determinant of plasma triglyceride and remnant lipoprotein metabolism, it is important to understand how the expression of this gene is regulated. In the present study, we identified the orphan nuclear receptor RORalpha1 as a regulator of human and mouse apo C-III gene expression. Plasma triglyceride and apo C-III protein concentrations in staggerer (sg/sg) mice, homozygous for a deletion in the RORalpha gene, were significantly lower than in wild type littermates. The lowered plasma apo C-III levels were associated with reduced apo C-III mRNA levels in liver and intestine of sg/sg mice. Transient transfection experiments in human hepatoma HepG2, human colonic CaCO2, and rabbit kidney RK13 cells demonstrated that overexpression of the human RORalpha1 isoform specifically increases human apo C-III promoter activity, indicating that RORalpha1 enhances human apo C-III gene transcription. RORalpha1 response elements were mapped by promoter deletion analysis and gel shift experiments to two AGGTCA half-sites located at positions -83/-78 (within the C3P site) and -23/-18 (downstream of the TATA box) in the human apo C-III promoter, with the -23/-18 site exhibiting the highest binding affinity. Transfection of site-directed mutated constructs in HepG2 cells indicated that the RORalpha1 effect is predominantly mediated by the -23/-18 site. This site is conserved in the mouse apo C-III gene promoter. Moreover, RORalpha binds to the equivalent mouse site and activates constructs containing three copies of the mouse site cloned in front of an heterologous promoter. Taken together, our data identify RORalpha as a transcriptional regulator of apo C-III gene expression, providing a novel, physiological role for RORalpha1 in the regulation of genes controlling triglyceride metabolism.

Remnant lipoproteins as therapeutic targets

Increasing evidence suggests that subsets of triglyceride-rich lipoproteins are particularly atherogenic. These include particles with some, but not necessarily all the properties classically attributed to remnants. Cholesteryl ester-enrichment seems to be a common feature of these particles, some of which can be taken up by macrophages by a novel receptor that recognizes species of apolipoprotein B but not apolipoprotein E. These characteristics seem to be common to postprandial and hypertriglyceridemic very low density lipoproteins as well as chylomicron remnants. Remnant-like triglyceride-rich lipoproteins that exhibit several potentially atherogenic properties can be quantified by a simple test that shows promise for identifying individuals at high risk for lesion formation and clinical events. Available hygienic and pharmaceutical measures that effectively lower the concentration of atherogenic triglyceride-rich lipoproteins deserve wider use.

Detection of LDL receptor by ligand blotting with chylomicron remnants labelled with colloidal gold

The LDL receptor plays a pivotal role in the clearance of pro-atherogenic lipoproteins, and LDL receptor deficiency may be the underlying cause of several primary and secondary dyslipidaemic conditions. Intervention strategies are often targeted to increase hepatic LDL receptor expression. It is difficult to quantitate hepatic LDL receptor activity and to monitor changes post-therapy. In order to avoid liver biopsy, human skin fibroblasts or circulating mononuclear cells have often been used as surrogate markers for the hepatic receptor. Fibroblasts, and particularly mononuclear cells, are relatively easy to isolate and can be stored for extensive lengths of time without significant loss of LDL receptor expression. Leucocytes or fibroblasts are normally probed with isotopically or gold-labelled LDL. However, the specific activity of the LDL conjugate is usually too low to enable accurate quantitation of differences, or changes, in LDL receptor expression. In this study, we describe an enhanced colloidal gold-labelling procedure for the detection of LDL receptor binding activity. The binding of colloidal gold-labelled chylomicron remnants to human hepatocytes (HepG2 cells) was compared with that of gold-conjugated LDL. Labelled remnants bound specifically to a cell surface protein with a molecular weight of approximately 130 kDa. Binding was blocked in the presence of unlabelled remnants, LDL, or antiserum specific to the LDL receptor. The binding of gold-labelled remnants was substantially greater than that of gold-labelled LDL. Compared with gold-labelled LDL, we found a much clearer demarcation of remnant binding with hepatocytes incubated in the presence or absence of sterols. Our observations suggest that, because of the greater affinity of the LDL receptor for lipoproteins containing apolipoprotein E, changes in LDL receptor expression might be more readily identified using gold-labelled remnants. We conclude that gold-conjugated chylomicron remnants might provide a useful means of detecting subtle changes in LDL receptor expression.

Plasma levels of remnant particles are determined in part by variation in the APOC3 gene insulin response element and the APOCI-APOE cluster

Remnant particles of triglyceride-rich lipoproteins (RLP) are known to be a strong predictor of atherogenicity. The serum concentrations of remnant-like particle triglyceride (RLPTG) and remnant-like particle cholesterol (RLPC) have been determined in a representative sample of the Czech MONICA study (n = 285). The relationship was investigated between remnant particle triglyceride/cholesterol concentrations and polymorphisms in the genes APOC3 (-482C-->T/3238C-->G), APOE (epsilon2/epsilon3/epsilon4), APOCI (-317-321ins), APOB (signal peptide), hepatic lipase (LIPE, -480C-->T), and lipoprotein lipase (LPL, S447X). Univariate analysis showed significant effects on RLPTG associated only with the APOE genotype (P = 0.009), the APOC3 -482C-->T genotype (P = 0.018), and the APOCI -317-321ins (P = 0.014) genotype and significant effects on RLPC with APOE (P = 0.01) and APOCI -317-321ins (P = 0.021). The raising effect of the APOE genotype for both remnant cholesterol and triglyceride was confined to the epsilon2/4 (n = 6) and varepsilon4/4 (n = 3) groups, and thus when the epsilon2/4 group was omitted in order to analyze by allele (epsilon2+/epsilon3+/epsilon4+), significance was lost (P = 0.6). There was strong linkage disequilibrium between the APOE and APOCI alleles (chi(2), P < 0.001) and a multivariate ANOVA of RLPTG with all three significantly associated variants as factors demonstrated that while the APOC3 -482C-->T effect was independent of the others (P = 0.003), the APOCI -317-321ins and APOE effects were not. This was also true for the APOCI -317-321ins and APOE effects on RLPC. To assess whether APOE-CI effects on RLPC were independent of their effects on total cholesterol and triglyceride levels, multiple linear regression was used. Using multiple linear regression, it appeared that the APOE-CI effects on RLPC were independent of their effects on plasma cholesterol, but the effects of APOC3 and APOE-CI on RLPTG could not be separated from their effects on plasma Tg levels. Further characterization of this remnant particle phenotype and its genetic determinants may lead to a better understanding of its metabolism and contribution to atherosclerosis.