Apoprotein E mediates the interaction of beta-VLDL with macrophages

Interaction between VLDL and phosphatidylcholine liposomes generates new γ-LpE-like particles

One of the subfractions of HDL involved in reverse cholesterol transport is γ-LpE. It has been assumed that, like preβ-LpAI, it can be generated during the interaction between phosphatidylcholine liposomes and lipoproteins and can contribute to more efficient cholesterol efflux after the introduction of liposomes to plasma. However, there has been no evidence concerning what the sources of these particles in plasma might be. Here, we determined whether the interaction of phosphatidylcholine liposomes with VLDL and the subsequent conversions of particles could be a source of new γ-LpE particles. We found that the interaction between liposomes and VLDL affected its lipid and protein composition. The content of phospholipids increased (~96 %) while the content of free cholesterol and apolipoprotein E decreased in VLDL during the reaction with liposomes (~100 and ~24 %, respectively). New particles which did not contain apolipoprotein B were generated. Heterogeneous HDL-sized populations of particles were generated, containing phospholipids and apolipoprotein E as the sole apolipoprotein, with densities from 1.063 to 1.21 g/ml, either with γ-mobility on agarose gel and Stokes diameters from 8.58 to 22.07 nm or with preβ-mobility and Stokes diameters from 9.9 to 21.08 nm. The obtained results contribute to the understanding of changes in lipoproteins under the influence of phosphatidylcholine liposomes, showing the formation of new (γ-LpE)-like and (preβ-LpE)-like particles, similar in mobility and size to plasma HDL-LpE. These newly generated particles can claim a share of the antiatherogenic effects of liposomes, observed in studies both in vitro and in vivo.

Apoprotein c-III and E-containing lipoparticles are markedly increased in HIV-infected patients treated with protease inhibitors: association with the development of lipodystrophy

Long-term therapy with protease inhibitors (PIs) can induce hypertriglyceridemia and development of a lipodystrophy. To better understand these metabolic alterations, the apoprotein and lipoparticle profile was investigated in male HIV patients under antiretroviral therapy: 49 received PIs, and 14 were given only two reverse transcriptase inhibitors. As controls, 63 male subjects were selected from a population study carried out in the Toulouse, France, area. Fasting glucose, insulin, and C-peptide were also determined. All patients under PIs displayed low levels of plasma glucose and increased insulin. PI administration was associated with moderate hypertriglyceridemia, low high-density cholesterol and apolipoprotein (apo) A-I levels. The most striking changes were a 2- to 3-fold increase in apo E and apo C-III, essentially recovered as associated to apo B-containing lipoparticles. Levels of those lipoparticles were two to eight times above control values. About 50% of PI-treated patients had developed a patent lipodystrophy. Multivariate analysis revealed that, among the investigated parameters, apo C-III was the only one found strongly associated with the occurrence of lipodystrophy (odds ratio, 5.5; P: < 0.015). Finally, 13 PI-receiving subjects with patent hypertriglyceridemia were given fenofibrate and were reevaluated 2 months later. Triglycerides, apo E, apo C-III, and the corresponding lipoparticles had returned to nearly normal levels. These results document the accumulation of potentially atherogenic lipoparticles under PIs. Apo C-III may play a pivotal role in the development of hypertriglyceridemia and lipodystrophy.

The very low- and intermediate-density lipoprotein fraction isolated from apolipoprotein E-knockout mice transforms macrophages to foam cells through an apolipoprotein E-independent pathway

Apolipoprotein E (apoE)-knockout mice develop severe atherosclerosis associated with high levels of very low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) in plasma. To investigate the atherogenic role of VLDL and IDL, the lipoprotein fraction containing both VLDL and IDL (apoEko-VLDL/IDL) was isolated from plasma of apoE-knockout mice by ultracentrifugation, and its interaction with macrophages was studied. When peritoneal macrophages obtained from apoE-knockout mice were incubated with apoEko-VLDL/IDL, the level of cellular cholesteryl esters (CE) increased with the concentration of apoEko-VLDL/IDL. The level of cellular cholesteryl [3H]oleate formed reached 15.1 nmol/mg of cell protein upon incubation with 50 microg/mL apoEko-VLDL/IDL for 18 h, which was an 8.4-fold increase over the corresponding level induced by low-density lipoprotein (LDL). The cellular CE mass was also significantly increased by apoEko-VLDL/IDL. Morphologically, after exposure to apoEko-VLDL/IDL, macrophages became strongly stained with Sudan black B. The total binding of [125I]apoEko-VLDL/IDL to macrophages was effectively replaced by more than 80% by an excess of the unlabeled ligand. Specific binding, calculated by subtracting the nonspecific binding from the total binding, exhibited a saturation pattern. Similar results were obtained with cell association and degradation experiments. In addition, the endocytic degradation of [125I]apoEko-VLDL/IDL was partially inhibited by LDL, whereas acetyl-LDL did not show any effect. These results indicated that apoEko-VLDL/IDL in its unmodified form produced significant CE accumulation in macrophages through a specific and apoE-independent pathway. This pathway may explain, in part, the mechanisms of foam cell formation in arterial walls and the subsequent development of atherosclerosis in apoE-knockout mice.

The influence of apolipoprotein E on the interactions between normal human very low density lipoproteins and U937 human macrophages: heterogeneity among persons

Apolipoprotein E (apo E) can mediate the cell binding of normal human very low density lipoproteins (VLDL). However, the extent to which apo E is involved in the cell binding and uptake of VLDL from different normolipidemic persons is not well defined. The VLDL (d < 1.006 g/l) of eight subjects were fractionated into VLDL with apo E and without apo E using a monoclonal antibody that binds to the LDL receptor recognition region of apo E. VLDL particles that expressed the 1D7 binding region of apo E comprised an average of 34% (range 7-51%) of the VLDL particles. Anti-apo E blocked an average of 43% (range 8-63%) of the binding of unfractionated VLDL to U937 cells. Anti-apo E blocked a similar proportion of binding to U937 cells of three VLDL subfractions of different density ranges (Sf20-60, Sf60-100, Sf100-400). The proportion of the VLDL particles that contained apo E correlated with the extent of uptake of the total VLDL by U937 cells, but not with stimulation by total VLDL of cholesterol ester formation. The binding to cells of VLDL without apo E varied by six-fold among persons, and caused most of the binding of the total VLDL of some subjects. Therefore, normolipidemic VLDL contains particles across its density range that use apo E to bind to U937 macrophages. In some VLDL samples, apo E provides most of the cell binding activity, whereas in others the binding activity occurs by other means.

Uptake of type III hypertriglyceridemic VLDL by macrophages is enhanced by oxidation, especially after remnant formation

We previously showed that hypertriglyceridemic VLDL (HTG-VLDL, Sf 60 to 400) from subjects with type III (E2/E2) hyperlipoproteinemia do not induce appreciable cholesteryl ester (CE) accumulation in cultured macrophages (J774A.1). In the present study, we examined whether oxidation of type III HTG-VLDL would enhance their uptake by J774A.1 cells. Type III HTG-VLDL were oxidized as measured by both conjugated-diene formation and increased electrophoretic mobility on agarose gels. Both LDL and type III HTG-VLDL undergo oxidation, albeit under different kinetic parameters. From the conjugated-diene curve, type III HTG-VLDL, compared with LDL, were found to have a 6-fold longer lag time, to take 6-fold longer to reach maximal diene production, and to produce a 2-fold greater amount of dienes but at half the rate (all P < .005). Incubation of macrophages with either native type III HTG-VLDL or LDL (50 micrograms lipoprotein cholesterol/mL media for 16 hours) caused small increases (4-fold and 2.7-fold, respectively) in cellular CE levels relative to control cells (both P = .0001). After 24 hours of CuSO4 exposure, we found that oxidized type III HTG-VLDL and LDL caused a 9.4-fold and 10.5-fold increase, respectively, in cellular CE levels (P = .0001). We next examined whether extending the exposure period for type III HTG-VLDL to CuSO4 beyond 24 hours would further enhance its ability to induce macrophage CE accumulation. After 48 hours of CuSO4 exposure, type III HTG-VLDL and LDL caused 21.3-fold and 11.6-fold increases, respectively, in cellular CE levels (P = .0001). The cellular CE loading achieved with 48 hour-oxidized type III HTG-VLDL was significantly higher than either 24 hour-oxidized type III HTG-VLDL (2.3-fold, P = .003) or 48 hour-oxidized LDL (1.8-fold, P = .012). There was no significant difference between the CE loading achieved by incubation of cells with either 24 hour-oxidized type III HTG-VLDL, 24 hour-oxidized LDL, or 48 hour-oxidized LDL (P > or = .518). In this study, we also examined whether partial lipolysis (19% to 50% triglyceride hydrolysis) of type III HTG-VLDL to produce remnants would increase the susceptibility of the lipoprotein to oxidative modification and subsequent cellular CE loading. Forty-eight hour-oxidized type III VLDL-remnants stimulated CE accumulation 30.4-fold over baseline (P = .0001). In contrast, nonoxidized type III VLDL-remnants caused the same very low level of CE loading as did native type III HTG-VLDL (P = .680). The increase in cellular CE levels achieved with 48 hour-oxidized type III VLDL-remnants was significantly higher than that achieved with 48 hour-oxidized type III HTG-VLDL (P = .047). In conclusion, we have shown that oxidized type III HTG-VLDL will induce macrophage CE accumulation well above levels achieved with oxidized LDL. In addition, we also showed that by forming a VLDL-remnant before oxidative modification, we can further enhance macrophage CE accumulation. These results provide a potential mechanism for the atherogenicity of type III HTG-VLDL and their remnants.

Influence of purified apoprotein E on platelet activation induced by serotonin

Serotonin induces platelet activation. Purified apoprotein E of 300 micrograms/ml prevented morphological alterations of blood platelets stimulated with serotonin (5 microM). Lower concentrated apoprotein E showed no such clear effects. These findings suggest that apoprotein E may liter atherosclerosis by suppressing agonist-induced platelet activation.

Slow beta-migrating lipoprotein: an atherogenic subclass of low density lipoproteins

OBJECTIVE

To clarify the possibility that midband Lp in LDL fractions might act as an atherogenic lipoprotein in their interaction with macrophages.

DESIGN AND METHODS

Low density lipoproteins (LDL) isolated by zonal ultracentrifugation from midband lipoprotein-positive serum in type lib hyperlipidemics were subjected to polyacrylamide gel disc electrophoresis.

RESULTS

A part of midband lipoprotein was observed between pre beta-and beta-band, in addition to the main beta-band. We named this midband lipoprotein "slow beta-migrating Lp (slow beta-Lp)." The larger LDL subfraction from midband-lipoprotein positive serum on Sepharose 2B column chromatography contained much slow beta-Lp, named slow beta-Lp-rich LDL. The smaller LDL subfraction contained a little slow beta-Lp, named slow beta-Lp-poor LDL. Slow beta-Lp-rich LDL had similar composition to the control LDL except for apolipoprotein E. The uptake of [3H]cholesteryl linoleate-labeled slow beta-Lp-rich LDL by J774 macrophages was higher than that of control LDL. The cholesterol ester content of J774 macrophages incubated with slow beta-Lp-rich LDL increased significantly compared with slow beta-Lp-poor LDL, beta-VLDL, and control LDL.

CONCLUSION

These results suggest that slow beta-Lp in type llb might generate foam cells from macrophages in atherosclerotic lesions.

Effects of efonidipine hydrochloride on cholesterol esterification mediated by beta-very low density lipoprotein in J774 macrophages

The effects of efonidipine hydrochloride (efonidipine), a dihydropyridine calcium antagonist, on the cholesterol ester metabolism induced by beta-migrating very low density lipoprotein (beta-VLDL) in J774 macrophages were studied. The cholesteryl ester content in the macrophages was increased by incubation with beta-VLDL, and the increase was inhibited by efonidipine. Oleic acid incorporation into cellular cholesteryl ester was increased by beta-VLDL in J774 macrophages. The incorporation at an early phase of beta-VLDL induction (0-3 hr) was inhibited by efonidipine. This inhibitory effect of efonidipine was greater at an early phase of beta-VLDL induction (0-3 hr) than at a late phase of the induction (8-11 hr). Pretreatment of the cells with efonidipine enhanced the inhibitory effect. Efonidipine also inhibited beta-VLDL degradation but not the binding and association in macrophages without pretreatment. beta-VLDL binding and association to macrophages were decreased by pretreatment of the cells with efonidipine. beta-VLDL metabolism was also decreased by dibutyryl cyclic AMP pretreatment. The decrease of beta-VLDL metabolism by efonidipine was prevented by co-treatment with efonidipine and HA1004, a protein kinase A inhibitor. Furthermore, efonidipine increased the intracellular cyclic AMP content in J774 macrophages. These findings suggest that efonidipine suppresses cholesterol ester deposition in atherosclerotic foam cells by inhibiting the modified lipoprotein metabolism and cholesterol esterification mainly through elevation of the cellular cyclic AMP level.

Effect of fluvastatin on intermediate density lipoprotein (remnants) and other lipoprotein levels in hypercholesterolemia

The accelerated atherosclerosis in diseases associated with elevated remnant lipoprotein levels has directed interest toward the response of this lipoprotein species to lipid-lowering treatment. The effect of fluvastatin--a synthetic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor--was compared with that of placebo on parameters of remnant metabolism in 57 patients with moderate hypercholesterolemia, but not heterozygous familial hypercholesterolemia, type III hyperlipidemia, or endogenous hypertriglyceridemia. Fluvastatin therapy resulted in decreases versus baseline in plasma total cholesterol, low density lipoprotein cholesterol (LDL-C) and LDL apolipoprotein (apo) B levels of 18%, 20%, and 18%, respectively (p < 0.01). Plasma parameters related to remnant metabolism were also significantly decreased: intermediate density lipoprotein by 43% and apo E by 22% (p < 0.01). The percent decrease in plasma intermediate density lipoprotein cholesterol level was twice that of LDL-C and 50% greater than the decrease seen in very low density lipoprotein cholesterol (VLDL-C), which was decreased by 28%. Total triglycerides were reduced by 11% and VLDL apo B by 24%, whereas high density lipoprotein cholesterol (HDL-C) rose significantly by 8%, HDL2-C by 24%, and HDL3-C by 3%. There were no increases in apo A-I levels compared with placebo nor any significant change in plasma lipoprotein(a) levels. The composition of LDL and VLDL particles did not appear to be altered by therapy, as assessed by the LDL-C:LDL-B, VLDL-C:VLDL-B, or triglyceride:VLDL-B ratios.(ABSTRACT TRUNCATED AT 250 WORDS)

Clustering of cardiovascular risk factors: targeting high-risk individuals

Cardiovascular risk factors have traditionally been divided into 2 categories: modifiable risk factors (smoking, hypertension, elevated cholesterol, reduced high density lipoprotein cholesterol, and diabetes), and nonmodifiable risk factors (age, gender, and hereditary factors). However, more recent data indicate clustering of several metabolic and familial factors that are often related to each other. A pattern of lipoprotein abnormalities characterized by increased hepatic production of apolipoprotein B-containing lipoprotein particles, high blood pressure, visceral obesity, and peripheral insulin resistance are identified with increasing frequency in subjects with premature coronary artery disease (CAD). The metabolic substrates for many such disorders are being uncovered, and genetic analysis of affected kindred have, often with conflicting results, suggested associations with candidate genes. In the context of a multifactorial approach, aggressive treatment of lipoprotein disorders in high-risk individuals, or in the secondary prevention of cardiovascular diseases, has resulted in a decreased rate of progression of CAD and a marked reduction in clinical events. Further work in the field of hemostatic factors has shown that fibrinogen, activated coagulation factor VII, spontaneous platelet aggregation, and elevated levels of plasminogen activator inhibitor-1 (PAI-1), are all associated with CAD. There is a strong association between lipids (especially triglyceride-rich lipoproteins) and fibrinogen, PAI-1, and activation of factor VII. In addition, vascular function, especially endothelial cell physiology, has been shown to be compromised in the presence of multiple risk factors and to be improved with intensive therapy aimed at reducing risk factors, especially plasma lipoprotein levels. The implications for clinical practice are important. In the primary prevention of cardiovascular disease, proper risk stratification must be carried out with specific attention given to lifestyle changes. Cessation of smoking and changes in diet (both qualitative and quantitative), exercise, and serenity are often required. In the prevention of cardiovascular disease in subjects at high risk, or in the secondary prevention of CAD, a clear justification exists for aggressive lifestyle changes, often coupled with lipid-lowering therapy and adequate blood pressure control. Basic research is providing us with a better understanding of the molecular interactions between lipoproteins and hemostatic factors. It is becoming increasingly necessary to develop novel pharmaceutical agents with the combined ability to reduce atherogenic lipoprotein levels while also reducing susceptibility to thrombosis.

Proteinuria, lipoproteins and renal apolipoprotein deposits in uninephrectomized female analbuminemic rats

To elucidate the pathogenetic role of hyperlipidemia per se in the development of glomerulosclerosis, severely hyperlipidemic female analbuminemic rats (NAR) and mildly hyperlipidemic male NAR were studied for a period of 37 weeks after uninephrectomy (UNX). Plasma cholesterol increased from 6.3 +/- 0.4 (week 4) to 11.9 +/- 0.6 mmol/liter (week 37) in the female NAR, and from 4.3 +/- 0.1 to 6.4 +/- 0.5 mmol/liter in the male NAR in the same period. Plasma protein concentration was also consistently higher in female NAR (60 +/- 1 g/liter) as compared to male NAR (52 +/- 1 g/liter). Plasma viscosity was higher in female NAR than in male NAR, but there were no differences in blood viscosity. Proteinuria increased progressively in the UNX female NAR from 25 weeks after surgery, reaching a final value of 141 +/- 37 mg/day. No proteinuria occurred in the UNX male NAR (final value 15 +/- 2 mg/day). Glomerular capillary pressure, measured prior to the onset of proteinuria, was not significantly different in UNX female NAR and UNX male NAR. At the end of the study glomerulosclerosis and lipid deposition was only found in the UNX female NAR. Throughout the study hyperfiltration and hyperperfusion, relative to the one-kidney clearances of the sham-operated (2K) animals, were not different in UNX male and female NAR. No differences were observed in blood pressure. Hypertrophy, evaluated by glomerular diameters, was less pronounced in UNX female NAR (174 +/- 3 microns) than in UNX male NAR (190 +/- 7 microns). Glomerular diameters in 2K female and male NAR were similar (respectively 158 +/- 2 and 157 +/- 4 microns). Plasma apo B levels were similar (2K female NAR: 204 +/- 8 U; 2K male NAR 204 +/- 13 U), but cholesterol and triglyceride content of apo B-containing lipoproteins, namely VLDL, IDL and LDL, was increased twofold in the female NAR as compared to the male NAR, implying a larger particle size in the female NAR. Deposition of apo B and apo E was observed in the glomerular mesangium of UNX female NAR, particularly in sclerotic lesions. Glomerular apo A-I deposits were localized primarily in visceral epithelial cells and were not associated with sclerotic lesions. The development of proteinuria and glomerulosclerosis after UNX in female NAR but not in male NAR may depend upon differences in plasma lipoprotein composition, but is apparently not related to differences in whole kidney hyperfiltration and hyperperfusion, glomerular capillary pressure, or blood viscosity.

The significance of apolipoprotein E structure to the metabolism of plasma triglyceride-rich lipoproteins

In this paper we analyse the structural organization of human apolipoprotein E (apoE) at the surface of triglyceride (TG)-rich lipoproteins, in relation to the metabolic pathway of these particles. ApoE acts as a receptor-binding ligand at the surface of chylomicrons and VLDL (very low density lipoproteins). The degree of exposure of apoE at the surface of lipoproteins and its affinity for the receptor both determine the uptake and catabolism of these lipoproteins. ApoE and/or apoB100, the major apolipoprotein constituent of LDL, contribute to the interaction of lipoproteins with five different cellular receptors: 1) the low density lipoprotein (LDL) receptor; 2) the LDL receptor-related protein (LRP); 3) the macrophage receptor for hypertriglyceridemic VLDL; 4) the scavenger receptor; 5) the VLDL receptor. The degree of exposure of apoE at the surface of normo- and hyperlipidemic VLDL can modulate their uptake by the LDL receptor. Normolipidemic VLDL are poorly recognized by the LDL receptor whereas hypertriglyceridemic VLDL are cleared more efficiently through this pathway. On the other hand, the extent of apoE self-association, which is dependent upon the degree of hydrolysis of the TG-rich particles, can control their interaction with the LDL-receptor related protein. The lateral organization of apoE at the surface of TG-rich particles, its interaction with other apoproteins and its extent of self-association might therefore be important factors in the clearance of these lipoproteins. Finally, structural defects of apoE might result in an impaired interaction of apoE-containing lipoproteins with these receptors and lead to the development of atherogenic dyslipidemias.

Internalization of poly(D,L-lactic acid) nanoparticles by isolated human leukocytes and analysis of plasma proteins adsorbed onto the particles

The objective of this work was to investigate the interactions of poly(D,L-lactic acid) nanoparticles prepared by a recently developed salting-out process, with lymphocytes and monocytes isolated from healthy human donors. Nanoparticles were labeled with a hydrophobic fluorescent dye and incubated with lymphocytes and monocytes, and their uptake was followed by flow cytometry in the presence and absence of plasma. Plasma protein adsorption increased nanoparticle uptake by monocytes, whereas a decrease of cellular binding of the nanoparticles to lymphocytes was noted. The cellular uptake for both cell types consisted in a passive adsorption and in an energy-requiring process, because the cells became 2-3 times more fluorescent when the incubation temperature was increased from 4 to 37 degrees C. When nanoparticles were coated with polyethylene glycol 20,000, uptake by monocytes decreased by 43 and 78% in phosphate-buffered saline and plasma, respectively; a similar decrease in nanoparticle uptake was observed for lymphocytes. Two-dimensional gel electrophoresis was performed to identify the plasma opsonins adsorbed onto the nanoparticle surface. Protein mappings for uncoated and polyethylene glycol-coated nanoparticles differed for two spot series. These spots, not yet clearly identified, may represent specific apolipoproteins involved in the metabolism of human lipoproteins, indicating the possible involvement of specific receptors in the uptake of the nanoparticles.

Inheritability of atherosclerosis and the role of lipoproteins as risk factors in the development of atherosclerosis in WHHL rabbits: risk factors related to coronary atherosclerosis are different from those related to aortic atherosclerosis

Inheritability of atherosclerosis and the influences of serum lipids on atherosclerosis were examined by following its progression in selectively bred WHHL rabbits. Our studies indicate (1) coronary atherosclerosis is clearly inherited from parents by offspring whereas inheritability of aortic atherosclerosis is uncertain; (2) coronary stenosis is positively correlated to serum cholesterol level, although the correlation coefficient is markedly low: in contrast, no relationship between serum lipid levels and aortic atherosclerosis was observed; (3) cholesterol-rich VLDL showed atherogenicity in aorta, but not in coronary arteries; (4) an unknown lipoprotein detected by 3.6% polyacrylamide gel electrophoresis was related to coronary atherosclerosis, although no relationship between the unknown lipoprotein and aortic atherosclerosis was observed. These findings suggest that there are two types of genetic factors involved in atherosclerosis, one of which is unique to coronary atherosclerosis whereas the other is related to only aortic atherosclerosis.

Cholesterol accumulation in J774 macrophages induced by triglyceride-rich lipoproteins. Comparison of very low density lipoprotein from subjects with type III, IV, and V hyperlipoproteinemias

The capacity of human triglyceride-rich lipoproteins to induce cholesterol accumulation in the murine J774 macrophage cell line was investigated with large very low density lipoprotein (VLDL, Sf 60-400) obtained from subjects with type III, IV, and V hyperlipoproteinemias. After incubation for 24 hours, VLDLs from type IV and type V subjects were similar in their ability to raise cellular cholesterol deposition threefold to fourfold and cellular triglyceride 16-fold. The increase in cholesterol was entirely due to the dramatic increase in cholesterol ester, from less than 1 to greater than 50 micrograms/mg cell protein. Total cholesterol accumulation was fourfold to fivefold greater than the cholesterol accumulation observed for VLDL or low density lipoprotein (LDL) from normal subjects. Cholesterol esterification (acyl coenzyme A: cholesterol acyltransferase [ACAT] activity) paralleled the rate of cholesterol accumulation in these cells. Treating the macrophages with the ACAT inhibitor 58035, which is known to downregulate the LDL receptor in these cells, diminished cholesterol accumulation by 40% for type IV VLDL and by 23% for normal LDL. Since hypertriglyceridemic VLDL carries excess apoprotein (apo) E molecules, we investigated the role of normal and abnormal apo E. An anti-apo E monoclonal antibody, known to block the binding of apo E to the LDL receptor, blocked type IV VLDL-induced cholesterol ester accumulation by approximately 70%. In contrast to type IV subjects, VLDL from type III subjects (homozygous for apo E2) when incubated with J774 macrophages (which do not secrete apo E) caused only a modest 1.5-2-fold increase in cellular cholesterol. Pre-beta- and beta-migrating VLDL subfractions from type III subjects were equally ineffective in causing cholesterol accumulation. By contrast, beta-VLDL from cholesterol-fed rabbits caused a sevenfold to eightfold increase in cellular cholesterol content. These results indicate that triglyceride-rich lipoproteins from type IV and type V subjects can cause substantial cholesterol ester accumulation and enhanced cholesterol esterification in J774 cells. The lower cholesterol accumulation with type IV VLDL in the presence of apo E antibodies and VLDL from type III subjects demonstrates the importance of functional apo E in this process.

The enhanced cellular uptake of very-low-density lipoprotein enriched in apolipoprotein E

We have recently reported an increased clearance of plasma very-low-density lipoprotein (VLDL) after intravenous injection of apolipoprotein (apo) E in Watanabe heritable hyperlipidemic (WHHL) rabbits. In the present study, we have investigated the cellular uptake of VLDL enriched in apo E (VLDL-E) which had been incubated with purified rabbit apo E. VLDL-E was taken up approx. 2-fold more than VLDL in human skin fibroblast, human monocyte-derived macrophage and Hep G2 cell and its degradation was least in macrophage. To characterize the binding of VLDL-E, we performed a binding assay using hepatic endosome isolated from estradiol-treated rats and we observed both increased EDTA-sensitive and -resistant binding of VLDL-E on endosome. Ligand blotting of hepatic endosome demonstrated two major bands of LDL receptor (130 and 260 kDa protein) and a minor band of LDL receptor-related protein (580 kDa protein) with a ligand of VLDL-E. These results suggested that VLDL-E was endocytosed in liver through a similar pathway among three cell types, and enrichment of apo E in VLDL enhanced the uptake of VLDL not only via an EDTA-sensitive binding site (classical LDL receptor) but also via other binding sites including an EDTA-resistant binding site and an LDL receptor-related protein.

Comparison of fatty acid and triacylglycerol metabolism of macrophages and smooth muscle cells

The response of macrophages and smooth muscle cells to culture in free fatty acid has been compared. Because oleate and linoleate promoted triacylglycerol enrichment of smooth muscle cells, whereas palmitate had little effect, oleate was used for these studies. The kinetics of the accumulation of triacylglycerol produced by oleate was comparable between smooth muscle cells and macrophages. When grown in increasing concentrations of oleic acid at various fatty acid to albumin molar ratios, the extent of triacylglycerol accumulation in both cell types was dependent on the concentration of oleate, the concentration of albumin, and the oleate to albumin molar ratio. However, macrophages contained 2.6-fold more triacylglycerol than smooth muscle cells in the presence of oleate at 0.36 mM or greater and at levels of albumin higher than 0.15 mM. The cellular triacylglycerol content of macrophages was linearly related to the oleate to albumin molar ratio at both a constant albumin concentration and a constant oleate concentration, whereas the accumulation of triacylglycerol in smooth muscle cells showed a curvilinear relationship. When cells were preloaded with triacylglycerol, smooth muscle cells showed a greater loss of lipid when exposed to albumin than macrophages did. Over a two-hr time period, macrophages incorporated twice as much labeled fatty acid as smooth muscle cells. Thus, while smooth muscle cells and macrophages showed similar responses to exogenous fatty acid and albumin, there were also significant quantitative distinctions.