Chylomicron/chylomicron remnant turnover in humans: evidence for margination of chylomicrons and poor conversion of larger to smaller chylomicron remnants

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 [3H]oleate into cholesteryl ester and triacylglycerol. CRLPs (5-40μg 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 17β-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 (40μg 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.

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

Clinical Science ◽

10.1042/cs1010683 ◽

2001 ◽

Vol 101 (6) ◽

pp. 683-690 ◽

Cited By ~ 11

Author(s):

G.F. WATTS ◽

D.C.F. CHAN ◽

P.H.R. BARRETT ◽

I.J. MARTINS ◽

T.G. REDGRAVE

Keyword(s):

Stable Isotope ◽

Breath Test ◽

Ldl Cholesterol ◽

Plasma Concentrations ◽

Density Lipoprotein ◽

Hdl Cholesterol ◽

Chylomicron Remnant ◽

Waist Hip Ratio ◽

Obese Subjects ◽

Chylomicron Remnants

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 [13C]oleate. Isotopic enrichment of 13CO2 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±0.048pools/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.

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Chylomicron-remnant uptake by freshly isolated hepatocytes. Effect of heparin and of hepatic triacylglycerol lipase

Biochemical Journal ◽

10.1042/bj2580587 ◽

1989 ◽

Vol 258 (2) ◽

pp. 587-594 ◽

Cited By ~ 25

Author(s):

F Sultan ◽

D Lagrange ◽

X Le Liepvre ◽

S Griglio

Keyword(s):

Cholesteryl Ester ◽

Hepatic Lipase ◽

Isolated Hepatocytes ◽

Chylomicron Remnant ◽

Triacylglycerol Lipase ◽

Cell Pellet ◽

Chylomicron Remnants ◽

Dose Response Study ◽

Cholesteryl Ester Accumulation

Chylomicron remnants labelled biologically with [3H]cholesterol were efficiently taken up by freshly isolated hepatocytes during a 3 h incubation in Krebs bicarbonate medium. Their [3H]cholesteryl ester was hydrolysed (74% net hydrolysis), and 0.1 mM-chloroquine could partially inhibit this hydrolysis, provided that hepatocytes were first preincubated for 2 h 30 min at 37 degrees C. This hydrolysis was also measured in preincubated cells with remnants double-labelled (3H and 14C) on their free cholesterol moiety; [3H]cholesterol arising from [3H]cholesteryl ester hydrolysis was recovered in the free [3H]cholesterol pool. A dose-response study showed saturation of remnant uptake at 180 micrograms of remnant protein/10(7) cells. Heparin (10 units/ml) increased remnant uptake by 63% (P less than 0.01), [3H]cholesteryl ester accumulation in the cell pellet by 110% (P less than 0.025) and hepatic lipase activity secreted in the medium by 2.4-fold (P less than 0.01) and by 3.3-fold (P less than 0.01) at the end of the preincubation and incubation periods respectively. Addition of 100 munits of semi-purified hepatic lipase preparation/flask stimulated remnant uptake by 44-69%, and [3H]cholesteryl ester accumulation in the presence of chloroquine by 2.1-fold (P less than 0.025). When hepatic lipase was incubated solely with the remnants, it decreased their triacylglycerol and phospholipid contents by 24% and 26% respectively. Thus freshly isolated hepatocytes may be used to study chylomicron-remnant uptake. Hepatic lipase, which seems to underly the stimulating effect of heparin, facilitates remnant uptake in vitro, and this could be mediated by at least one (or both) of its hydrolytic properties.

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Phospholipase A2 Mediates Apolipoprotein-Independent Uptake of Chylomicron Remnant-Like Particles by Human Macrophages

International Journal of Vascular Medicine ◽

10.1155/2012/501954 ◽

2012 ◽

Vol 2012 ◽

pp. 1-11 ◽

Cited By ~ 3

Author(s):

Mariarosaria Napolitano ◽

Howard S. Kruth ◽

Elena Bravo

Keyword(s):

Phospholipase A2 ◽

Cholesteryl Ester ◽

Apolipoprotein E ◽

Lipoprotein Lipase ◽

Human Monocyte ◽

Cholesteryl Ester Hydrolase ◽

Chylomicron Remnant ◽

Ester Hydrolase ◽

Chylomicron Remnants ◽

Cytosolic Pla2

Apolipoprotein E-receptor-mediated pathways are the main routes by which macrophages take up chylomicron remnants, but uptake may also be mediated by receptor-independent routes. To investigate these mechanisms, triacylglycerol (TG) accumulation induced by apolipoprotein-free chylomicron remnant-like particles (CRLPw/o) in human monocyte-derived macrophages was evaluated. Macrophage TG content increased about 5-fold after incubation with CRLPw/o, and this effect was not reduced by the inhibition of phagocytosis, macropinocytosis, apolipoprotein E function, or proteoglycan bridging. The role of lipases, including lipoprotein lipase, cholesteryl ester hydrolase, and secretory (sPLA2) and cytosolic phospholipase A2, was studied using [3H]TG-labelled CRLPw/o. Total cell radioactivity after incubation with [3H]TG CRLPw/o was reduced by 15–30% by inhibitors of lipoprotein lipase and cholesteryl ester hydrolase and by about 45% by inhibitors of sPLA2 and cytosolic PLA2. These results suggest that macrophage lipolytic enzymes mediate the internalization of postprandial TG-rich lipoproteins and that sPLA2and cytosolic PLA2, play a more important role than extracellular lipoprotein lipase-mediated TG hydrolysis.

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

Clinical Science ◽

10.1042/cs20030039 ◽

2003 ◽

Vol 105 (3) ◽

pp. 363-371 ◽

Cited By ~ 10

Author(s):

Andrew B. GOULTER ◽

Michael AVELLA ◽

Kathleen M. BOTHAM ◽

Jonathan ELLIOTT

Keyword(s):

Nitric Oxide ◽

Endothelial Cells ◽

Coronary Artery ◽

Cellular Protein ◽

Chylomicron Remnant ◽

Aortic Endothelial Cells ◽

Porcine Coronary Artery ◽

Chylomicron Remnants ◽

Porcine Aortic Endothelial Cells ◽

Aortic Endothelial

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 CuSO4 (10 μM) for 18 h at 37 °C. Nω-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/μg 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.

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Chylomicron-remnant-induced foam cell formation and cytotoxicity: a possible mechanism of cell death in atherosclerosis

Clinical Science ◽

10.1042/cs0980183 ◽

2000 ◽

Vol 98 (2) ◽

pp. 183-192 ◽

Cited By ~ 33

Author(s):

Kenneth C.-W. YU ◽

John C. L. MAMO

Keyword(s):

Cell Death ◽

Smooth Muscle ◽

Free Radical ◽

Smooth Muscle Cells ◽

Muscle Cells ◽

Chylomicron Remnant ◽

Free Radical Production ◽

Radical Production ◽

Superoxide Free Radical ◽

Chylomicron Remnants

The effects of chylomicron remnants on cytoplasmic lipid loading and cell viability were assessed in cultures of human monocyte-derived macrophages and rabbit arterial smooth muscle cells. At a cholesterol concentration of 150 μg/ml, chylomicron remnants induced substantial cytoplasmic lipid loading of macrophages, but not of smooth muscle cells, within 6 h of exposure. Chylomicron remnants were found to be cytotoxic to macrophages and smooth muscle cells, although the latter were generally more resistant. Chylomicron remnants contained no detectable oxysterols (> 1 ng) and contained less non-esterified (‘free’) fatty acids than non-lipolysed nascent chylomicrons. Chylomicron-remnant-induced cytotoxicity appeared to be time- and dose-dependent. Macrophage and smooth muscle cell viability were inversely related to the production of superoxide free radicals and were significantly improved in the combined presence of superoxide dismutase and catalase. Collectively, our data suggest that, in macrophages, cell viability is compromised as a consequence of superoxide free radical production following uptake of chylomicron remnants. We would suggest that, in arterial smooth muscle cells, chylomicron-remnant-induced cell death also occurs as a consequence of superoxide free radical production. Our observations in the present study suggest that macrophage foam cells in atherosclerotic plaques might be derived from the cellular uptake of chylomicron remnants. Furthermore, arterial accumulation of chylomicron remnants might contribute to plaque destabilization as a consequence of cell death following superoxide free radical production by macrophages and smooth muscle cells.

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Binding, interiorization and degradation of cholesterol ester-labelled chylomicron-remnant particles by rat hepatocyte monolayers

Biochemical Journal ◽

10.1042/bj1680483 ◽

1977 ◽

Vol 168 (3) ◽

pp. 483-494 ◽

Cited By ~ 86

Author(s):

Claes-Henrik Florén ◽

Åke Nilsson

Keyword(s):

Cell Surface ◽

Cholesteryl Ester ◽

Time Course ◽

Inhibitory Effect ◽

Metabolic Inhibitors ◽

High Density Lipoproteins ◽

Chylomicron Remnant ◽

Very Low Density Lipoproteins ◽

Chylomicron Remnants ◽

Hydrolysis Of

1. The cholesteryl ester of isolated chylomicron-remnant particles was efficiently degraded by hepatocyte monolayers. The degradation was sensitive to metabolic inhibitors. 2. With increasing amounts of remnant cholesteryl ester the rate of uptake approached saturation and conformed to a linear double-reciprocal plot. The Vmax. was determined as 80ng of cholesteryl ester/h per mg of protein and the apparent Km as 1.4μg of cholesteryl ester per mg of protein. The time course for the uptake and hydrolysis suggested that binding of particles to the cell surface preceded the degradation. 3. Cholesteryl esters of native chylomicrons were degraded to a much smaller extent and their presence had only a small inhibitory effect on the degradation of chylomicron remnants. Intestinal very-low-density lipoproteins were degraded somewhat faster than chylomicrons, and caused more inhibition of remnant degradation. Rat high-density lipoproteins inhibited the hydrolysis of remnant cholesteryl ester by up to 50%, but had less influence on the amount of cholesteryl ester that was bound to the cells. Serum decreased both the uptake and hydrolysis, whereas d=1.21 infranatant had no effect. 4. The cholesteryl ester hydrolysis after the uptake by the cells was inhibited by chloroquine and by colchicine. Only 28–36% of the unhydrolysed cholesteryl ester could be released from these cells by trypsin treatment, indicating that the major portion was truly intracellular. The particles that could be released from the cell surface by trypsin and those remaining in the medium had the same triacylglycerol/cholesteryl ester ratio as the added remnant particles. Significant amounts of denser particles were thus not formed during contact with the cell surface. 5. The presence of heparin, as well as preincubation of the cells with heparin, increased the uptake of chylomicron remnants. This effect was most marked in the presence of serum. A much smaller proportion of the other serum lipoproteins was taken up, and this proportion was not increased by heparin.

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Uptake and degradation of iodine-labelled chylomicron remnant particles by monolayers of rat hepatocytes

Biochemical Journal ◽

10.1042/bj1740827 ◽

1978 ◽

Vol 174 (3) ◽

pp. 827-838 ◽

Cited By ~ 33

Author(s):

C H Florén ◽

A Nilsson

Keyword(s):

Protein A ◽

Sodium Dodecyl ◽

Rat Hepatocytes ◽

Relative Increase ◽

Cell Protein ◽

Chylomicron Remnant ◽

Protein Mass ◽

Apolipoprotein A ◽

Chylomicron Remnants ◽

Protein Moiety

1. Rat chylomicrons were labelled with 125I with 69–72% of the iodine in the protein moiety. Less than 1 nmol of iodine was incorporated per nmol of protein. Of the peptide radioactivity 44–56% was in apolipoprotein A-1, 30–40% in the C peptides and 11–15% in apolipoprotine B. The arginine-rich peptide, which accounted for about 14% of the chylomicron protein mass as determined by scanning of sodium dodecyl sulphate-polyacrylamide gels, contained very little radioactivity. 2. Chylomicron remnants generated with postheparin plasma from iodine-labelled chylomicrons showed a relative increase in the percentage of the arginine-rich peptide (76–90% of the apolipoprotein mass according to gel scanning). The major portion of the peptide iodine label was present in apolipoprotein A-1 (43–57%), B (22–32%) and C peptides (17–35%). 3. When iodine-labelled chylomicron remnants were added to rat hepatocytes in primary culture, labelled peptides were taken up and degraded by the hepatocytes by a saturable process. The Vmax. for the uptake was calculated to the 300ng of protein/h per mg of cell protein and the apparent Km as 7.7 microgram of protein/mg of cell protein. A larger proportion of the 125I-labelled lipids of the remnants (mainly polar lipids) was taken up. This suggest that these may also enter the cells by a mechanism that does not involve particulate uptake, such as phospholipid exchange. 4. The degradation of labelled peptides was inhibited by colchicine, concanavalin A, chloroquine and NH4Cl, which also inhibit degradation of the cholesteryl ester portion. All these drugs exerted their inhibition mainly after the uptake of labelled peptide. No degradation occurred at 4 degrees C, and also the uptake was markedly decreased. 5. The uptake of labelled chylomicron remnant peptide was 77 times as effective as that of labelled sucrose, which is likely to be taken up randomly by pinocytosis.

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Chylomicron-remnant clearance in homozygote and heterozygote Watanabe-heritable-hyperlipidaemic rabbits is defective. Lack of evidence for an independent chylomicron-remnant receptor

Biochemical Journal ◽

10.1042/bj2760381 ◽

1991 ◽

Vol 276 (2) ◽

pp. 381-386 ◽

Cited By ~ 54

Author(s):

A Bowler ◽

T G Redgrave ◽

J C L Mamo

Keyword(s):

Cholesteryl Ester ◽

Plasma Cholesterol ◽

Low Density Lipoproteins ◽

Low Density ◽

Lipid Emulsions ◽

Chylomicron Remnant ◽

Unesterified Cholesterol ◽

Apolipoprotein B100 ◽

Plasma Triacylglycerol ◽

Chylomicron Remnants

Lymph chylomicrons radiolabelled in triacylglycerol and cholesteryl ester were injected into control and Watanabe heritable-hyperlipidaemic (WHHL) rabbits. Clearance of chylomicrons was slower in heterozygote and homozygote WHHL rabbits. Slower remnant clearance in WHHL rabbits was confirmed by monitoring the clearance from plasma of preformed chylomicron remnants. Use of chylomicron-like lipid emulsions injected into control and WHHL rabbits also confirmed the defect in remnant clearance in heterozygote WHHL and homozygote WHHL groups. Clearance from plasma of emulsion triolein was delayed in both WHHL groups compared with controls, owing to slower remnant clearance. The clearance from plasma of radioiodinated rabbit low-density lipoproteins (LDL) in heterozygote WHHL rabbits was the same as control rabbits. Defective chylomicron-remnant removal but normal LDL clearance in the heterozygote WHHL corresponded to elevated concentrations of plasma triacylglycerol and normal concentrations of plasma cholesterol. Receptor versus non-receptor clearances of chylomicron remnants were studied by comparing the clearance of emulsions with and without unesterified cholesterol respectively. Unlike control rabbits, there were no significant differences in the clearances of the two emulsion types in either the homozygote or heterozygote WHHL rabbits, indicating that the apolipoprotein-B100/E receptor is the primary route for clearance of chylomicron remnants from plasma.

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Mechanisms involved in chylomicron remnant lipid uptake by macrophages

Biochemical Society Transactions ◽

10.1042/bst0350459 ◽

2007 ◽

Vol 35 (3) ◽

pp. 459-463 ◽

Cited By ~ 11

Author(s):

E. Bravo ◽

M. Napolitano

Keyword(s):

Oxidation Products ◽

Cholesterol Oxidation ◽

Oxidized Lipids ◽

Chylomicron Remnant ◽

Lipid Uptake ◽

Lipase Hydrolysis ◽

Atherosclerotic Lesions ◽

Apoe Receptor ◽

Quantitative Changes ◽

Chylomicron Remnants

Although it is clear that chylomicron remnants are atherogenic, events leading to their internalization by macrophages are still debated. The lack of apoE (apolipoprotein E) in CRLPs (chylomicron remnant-like particles) reduces macrophage TAG (triacylglycerol) content by approx. 50%, suggesting that, as well as apoE-mediated endocytic uptake, apoE receptor-independent mechanisms are involved in the induction of foam cells by chylomicron remnants. Evaluation of the radioactivity associated with macrophages after incubation with CRLPs containing radiolabelled lipids suggests that the TAG and cholesterol carried by the particles have different kinetics of internalization. In addition, inhibition-based experiments indicate that cholesteryl ester-selective uptake and the extracellular lipoprotein lipase hydrolysis of TAG contribute to cholesterol and TAG accumulation respectively. Thus plasma TAG and cholesterol carried by remnant particles have to be considered two independent and non-interchangeable risk factors for athero-related diseases. In addition, the interaction between CRLPs and macrophages is modulated by dietary oxidized lipids and other lipophilic components. The presence of oxidized lipids, such as 7β-hydroxycholesterol and 7-oxocholesterol, the major cholesterol oxidation products found in atherosclerotic lesions, in CRLPs interferes with the mechanisms of their internalization, but does not cause quantitative changes of accumulated lipids, while the presence of the plant carotenoid, lycopene, or the antioxidant drug, probucol, enhances lipid accumulation in macrophages by increasing the rate of uptake of the particles and raising the intracellular synthesis of TAG. In conclusion, several mechanisms contribute to the macrophage uptake of postprandial lipoproteins, however, little is known of the balance and modulation between the different pathways.

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