Effect of particle size and temperature on the conformation and physiological behavior of apolipoprotein E bound to model lipoprotein particles

Analysis of the structural and dynamic properties of human N-terminal domain of apolipoprotein E by molecular dynamics simulations

Whereas the lipid-free N-terminal domain of apolipoprotein E (apoE-NT) adopts a four-helix bundle, the lipid-bound form is believed to undergo a large conformational change likely to be characterized by the opening of the bundle. ApoE-NT in a water/alcohol mixture was also shown to experience conformational changes exhibiting similarities with those induced upon lipid binding. The structure and dynamics of apoE-NT have been here investigated by analyzing 40 ns and 60 ns molecular dynamics simulations performed in water and in a water/propanol mixture, respectively. The overall structural properties show alterations of the tertiary structure of apoE-NT in the water/alcohol system in agreement with those observed experimentally. In contrast, in the water simulation, the sampled conformations remain closer to the crystal structure that served as a starting point for both simulations. Interestingly, several propanol molecules are seen to penetrate two hydrophobic regions of the bundle interior. One of these regions is enclosed in part by the short helix (H1') connecting helices 1 and 2 of the bundle which has been experimentally shown to be important for modulating lipid binding activity of apoE-NT. Principal component analysis of the water/propanol trajectory confirms that the region including H1' is the locus of the largest motion. Another region involves the loop connecting helix 2 and helix 3 which has been hypothesized to play the role of a hinge in the opening of the bundle.

Differential effects of apolipoprotein E isoforms on lipolysis of very low-density lipoprotein triglycerides

Apolipoprotein (apo) E plays a key role in lipoprotein metabolism and has been proposed to modulate triglyceride (TG) lipolysis. However, no systematic investigation on lipolysis using all 3 isoforms of apoE has been performed. To clarify the role of common human apoE isoforms in the lipolysis of very low-density lipoprotein (VLDL) TGs, we overexpressed human apoE isoforms in apoE and low-density lipoprotein receptor-deficient mice using adenoviral-mediated gene transfer and used VLDL particles obtained from these mice for in vitro lipolysis assay. Overexpression of apoE, regardless of its isoforms, increased the TG content of VLDL in mice in vivo. In vitro analysis of the effect of apoE on lipolysis revealed that irrespective of its isoforms, apoE did inhibit TG lipolysis at every concentration of apoE examined, and this inhibitory effect became more pronounced as the apoE content of VLDL increased. No difference was observed in TG lipolysis activity among isoforms at low apoE/TG ratio; however, intermediate ratios of apoE/TG, which reflect physiologic VLDL apoE/TG ratios, demonstrated a significantly greater level of lipolysis inhibition in apoE2, but less so in apoE4 compared with other isoforms. This differential effect by apoE isoforms on lipolysis was attenuated at higher apoE/TG ratios; nevertheless, apoE2 still inhibited lipolysis significantly more than did apoE4. Enrichment of VLDL with apoE decreased both the apoC contents and apoC-II/C-III ratios of VLDL, contributing, at least in part, to the inhibitory function of apoE on lipolysis. The present study clarifies the differential lipolysis-modulating effect of apoE isoforms, which would help explain the difference in pre- and postprandial TG levels among humans carrying different apoE isoforms.

Evaluation of the carrier potential for the lipid dispersion system with lipophilic compound

KW-3902 (a newly synthesized adenosine A(1)-receptor antagonist) has potent diuretic and renal protective activities and was formulated in lipid dispersion systems, i.e., lipid emulsions and liposomes. The objective of the present study was to evaluate the carrier potential of these lipid dispersion systems, which is explained here as the ability of the formulation to retain the drug in its dispersed phase. The relative affinity of the drug to the formulation, K(f/b), was defined as a parameter in order to assess the performance of the formulations and was obtained from the in vitro blood component binding study. The results indicated that KW-3902 showed higher relative affinity to the liposome formulation than to the lipid emulsion. Moreover, the total amount of drug retained in the dispersion system depended on both K(f/b) and the dosing volume. The usefulness of the parameter, K(f/b), was discussed as an indicator for a carrier potential to understand the properties of the formulations.

Effects of plasma apolipoproteins on lipoprotein lipase-mediated lipolysis of small and large lipid emulsions

Large (ca. 120 nm) and small (ca. 35 nm) emulsions consisting of triolein (TO) and phosphatidylcholine (PC) were prepared as the primary protein-free models of chylomicrons and their remnants, respectively. Lipoprotein lipase (LPL)-mediated lipolysis of emulsion TO was retarded in chylomicron-free human plasma compared with the hydrolysis activated by isolated apolipoprotein C-II (apoC-II). In 30% plasma, free fatty acid (FFA) release rate was higher for large emulsions than for small ones, while both emulsions were hydrolyzed at similar rates in the presence of isolated apoC-II. Isolated apolipoprotein C-III (apoC-III) or apolipoprotein E (apoE) worked as LPL-inhibitor of the lipolysis activated by apoC-II. It was also observed that apolipoprotein A-I (apoA-I) showed distinct inhibitory effects on the lipolysis of large and small emulsions: more effective inhibition for small emulsions. Kinetic analyses showed that K(m)(app) and V(max)(app) for the lipolysis of emulsions were lower in the presence of 30% plasma than isolated apoC-II. ApoA-I also markedly decreased K(m)(app) and V(max)(app) for LPL-catalyzed hydrolysis of both emulsions. In chylomicron-free serum, the density of bound apoA-I at small emulsion surfaces was about three fold greater than large emulsion surfaces, but the binding densities of apoC-II, apoC-III and apoE were less for small emulsion surfaces than for large ones, suggesting that apoA-I preferentially binds to small particles and displaces other exchangeable apolipoproteins from particle surfaces. These results indicate that, in addition to the well known inhibitory effects of apoC-III and apoE, apoA-I in plasma regulates the lipolysis of triglyceride (TG)-rich emulsions and lipoproteins in a size-dependent manner.

Influence of apoE domain structure and polymorphism on the kinetics of phospholipid vesicle solubilization

We examined the effects of apolipoprotein E (apoE) domain structure and polymorphism on the kinetics of solubilization (clearance) of dimyristoyl-phosphatidylcholine multilamellar vesicles. This second order reaction consisted of two simultaneous kinetic phases; it also exhibited saturable kinetics when the apolipoprotein concentration was increased at a constant lipid concentration. Rigid connections between alpha-helices in the 4-helix bundle formed by the 22 kDa N-terminal domain of apoE reduced the reaction rate. In contrast, the more flexible interhelical connections in apoA-I and the 10 kDa C-terminal domain of apoE promoted rapid solubilization of dimyristoyl-phosphatidylcholine (DMPC) multilamellar vesicles (mLV). Full-length apoE-3 reacted at about half the rate of the C-terminal domain alone. This decrease occurred because the hinge region probably decreased the interhelical flexibility of the 10 kDa domain and because both domains are conformationally restricted when covalently linked. Furthermore, the mLV surface affinities and reaction rates of the N-terminal domain fragments of the three common apoE isoforms tended to vary inversely with the stabilities of these fragments. These results confirm the importance of apoE's structure on the kinetics of lipid interaction. They suggest that flexibility in an apolipoprotein molecule increases the time-averaged exposure of hydrophobic surface area, thereby increasing the rate of phospholipid solubilization.

Effect of dietary cholesterol oxidation products on the plasma clearance of chylomicrons in the rat

Oxidized cholesterols in the diet have been shown to exacerbate arterial cholesterol deposition and the development of atherosclerosis in animal models. Dietary oxidized cholesterols are absorbed through the intestine and incorporated into lymph chylomicrons. The aim of this study was to investigate the effect of oxidized cholesterols on the metabolism of nascent chylomicrons in vivo. It was shown that oxidized cholesterols markedly delay the clearance of chylomicrons from plasma compared to rats given TG alone. However, there was no difference in the clearance of chylomicrons containing oxidized cholesterols vs. purified cholesterol, although the presence of oxysterols did appear to exacerbate the removal of these particles from circulation. The impaired clearance of chylomicrons containing oxidized cholesterols was not due to impaired lipolysis and slower conversion to the remnant form. Moreover, the incorporation of oxidized cholesterols did not alter the hepatic or splenic uptake of chylomicrons compared to chylomicrons isolated from rats given purified cholesterol or TG alone. Collectively, the results of this study suggest that the exacerbated delay in clearance of chylomicron remnants enriched with oxysterols may be due to impaired uptake by tissues other than the liver and spleen. Apolipoprotein (apo) analysis showed that oxysterol incorporation reduced the apoE content and altered the apoC phenotype of chylomicrons, which may have an impact on the removal of chylomicron remnants from plasma. In conclusion, dietary oxysterols appear to have the potential to adversely affect chylomicron metabolism. Therefore, further investigations in humans are required to determine whether dietary oxidized cholesterols found in cholesterol-rich processed foods delay the clearance of postprandial remnants, which may contribute to and exacerbate the development of atherosclerosis.

Role of the lipid emulsion on an injectable formulation of lipophilic KW-3902, a newly synthesized adenosine A1-receptor antagonist

KW-3902 (a newly synthesized adenosine A1-receptor antagonist) has potent diuretic and renal protective activities. We investigated the influence of the emulsion formulation on the pharmacokinetics of KW-3902 and its metabolite (M1) in rats using three different formulations, i.e., a lipid emulsion about 130 nm in diameter composed of egg yolk lecithin: soybean oil: oleic acid=1:1:0.048, a liposome about 100 nm in diameter composed of egg yolk lecithin, and a saline solution containing 1% (v/v) each of dimethyl sulfoxide and 1 N NaOH. There was no significant difference in the pharmacokinetic parameters of KW-3902 (elimination half-life (T1/2), area under the plasma concentration-time curves (AUC0-infinity), total body clearance (CL), mean residence time (MRT) and volume of distribution at steady-state (Vdss) and M1 (Cmax, T1/2, AUC0-infinity and MRT) after injection of these three dosage forms. Moreover, we investigated in vitro the binding of KW-3902 to blood components using these three formulations. KW-3902 was completely partitioned into the blood components regardless of its dosage form. These findings suggested that KW-3902 dissociated rapidly from the lipid emulsion or liposome in blood after injection and showed intrinsic pharmacokinetics of KW-3902 at doses of 0.1 and 1 mg/kg. Thus, the lipid emulsion formulation of KW-3902 was defined as a solvent, which was a vehicle for dissolving the drugs to prepare the injection, at its expected effective doses.

Modulation of apolipoprotein E-mediated plasma clearance and cell uptake of emulsion particles by cholesteryl ester

Cholesteryl ester, along with triglyceride (TG), is the major core component of plasma lipoproteins. We investigated the effect of core composition on the physical state and metabolic behavior of lipid emulsions, as model particles of lipoproteins. Fluorescence studies using 1,6-diphenylhexatriene analogs showed that although cholesteryl oleate (CO) significantly decreased core mobility, the surface rigidity of phosphatidylcholine (PC) monolayers was independent of core composition. When intravenously injected into rats, the increased amount of core CO tended to retard TG emulsion removal from plasma, and the initial clearance rate was correlated with the amount of apolipoprotein E (apoE) bound from plasma. In addition, PC liposomes with a similar emulsion particle size showed negligible binding of apoE and were cleared at a slower rate compared to all emulsions. Furthermore, the effect of CO on the binding behavior of apoE to the emulsion surface and the emulsion uptake by hepatocytes was assessed in vitro. Replacing core TG with CO was found to decrease the apoE binding capacity to emulsions markedly without changing the binding affinity and thereby to reduce the cell uptake of emulsion particles by HepG2 cells. These results indicate that the physical state of core lipids, which can be modulated by CO content, plays a role in emulsion metabolism through the alteration in apoE binding.

Conformational reorganization of the four-helix bundle of human apolipoprotein E in binding to phospholipid

Conformational reorganization of the amino-terminal four-helix bundle (22-kDa fragment) of apolipoprotein E (apoE) in binding to the phospholipid dimyristoylphosphatidylcholine (DMPC) to form discoidal particles was investigated by introducing single, double, and triple interhelical disulfide bonds to restrict the opening of the bundle. Interaction of apoE with DMPC was assessed by vesicle disruption, turbidimetric clearing, and gel filtration assays. The results indicate that the formation of apoE.DMPC discoidal particles occurs in a series of steps. A triple disulfide mutant, in which all four helices were tethered, did not form complexes but could release encapsulated 5-(6)-carboxylfluorescein from DMPC vesicles, indicating that the initial interaction does not involve major reorganization of the helical bundle. Initial interaction is followed by the opening of the four-helix bundle to expose the hydrophobic faces of the amphipathic helices. In this step, helices 1 and 2 and helices 3 and 4 preferentially remain paired, since these disulfide-linked mutants bound to DMPC in a manner similar to that of the 22-kDa fragment of apoE4. In contrast, mutants in which helices 2 and 3 and/or helices 1 and 4 paired bound poorly to DMPC. However, all single and double helical pairings resulted in the formation of larger discs than were formed by the 22-kDa fragment, indicating that further reorganization of the helices occurs following the initial opening of the four-helix bundle in which the protein assumes its final lipid-bound conformation. In support of this rearrangement, reducing the disulfide bonds converted the large disulfide mutant discs to normal size.

Mimicking lipid-binding-induced conformational changes in the human apolipoprotein E N-terminal receptor binding domain effects of low pH and propanol

We studied the effects of n-propanol and pH on the structure of the apolipoprotein E3 N-terminal receptor binding domain, apo E3(1-191), to determine whether conditions similar to those occurring near lipid surfaces (decreased dielectric constant and pH) can mimic lipid-induced conformational changes in apo E3. The addition of 30% n-propanol, at pH 7, induces a conformational change in apo E3(1-191) as shown by changes in the intrinsic tryptophan fluorescence and by an increase in the Stokes radius of the majority of the protein from 3.0 to 4.1 nm, although the protein remains monomeric as shown by chemical cross-linking. These changes are accompanied by increased resistance to limited proteolysis with trypsin, chymotrypsin, subtilisin and endoproteinase glu-C, as is the case for apo E3(1-191) reconstituted into phospholipid/cholesterol lipid bicelles. Far and near UV circular dichroism showed that n-propanol increases the amount of calculated alpha-helical structure (42-65%) and alters the tertiary structure of the protein although not as much as when apo E3(1-191) is incorporated into lipid bicelles. In the absence of n-propanol, lowering the pH to 4.5 decreases the Stokes radius of the majority of the protein somewhat, with little effect upon the secondary and the tertiary structures. The addition of 30% n-propanol at pH 4.5 increases the Stokes radius of apo E3(1-191) from 2.2 to 5.0 nm, even more than at pH 7 (3.0-4.1 nm) although the protein still remains predominantly monomeric. There is increased resistance to limited proteolysis with endoproteinase glu-C. As assessed by far and near UV circular dichroism, the addition of 30% n-propanol at pH 4.5, in contrast to pH 7, markedly increases the alpha-helical structure and changes the tertiary structure of the protein similarly to that resulting from the incorporation of apo E3(1-191) into lipid bicelles. The results suggest that a combination of n-propanol and low pH in aqueous solutions may be useful as a simple model system for studying conformational changes in apo E3 similar to those, which occur upon interaction of the protein with lipids.

Effects of apolipoprotein B-100 on the metabolism of a lipid microemulsion model in rats

In previous studies, it was shown that lipid microemulsions resembling LDL (LDE) but not containing protein, acquire apolipoprotein E when injected into the bloodstream and bind to LDL receptors (LDLR) using this protein as ligand. Aiming to evaluate the effects of apolipoprotein (apo) B-100 on the catabolism of these microemulsions, LDE with incorporated apo B-100 (LDE-apoB) and native LDL, all labeled with radioactive lipids were studied after intraarterial injection into Wistar rats. Plasma decay curves of the labels were determined in samples collected over 10 h and tissue uptake was assayed from organs excised from the animals sacrificed 24 h after injection. LDE-apo B had a fractional clearance rate (FCR) similar to native LDL (0.40 and 0.33, respectively) but both had FCR pronouncedly smaller than LDE (0.56, P<0.01). Liver was the main uptake site for LDE, LDE-apoB, and native LDL, but LDE-apoB and native LDL had lower hepatic uptake rates than LDE. Pre-treatment of the rats with 17alpha-ethinylestradiol, known to upregulate LDLR, accelerated the removal from plasma of both LDE and LDE-apoB, but the effect was greater upon LDE than LDE-apoB. These differences in metabolic behavior documented in vivo can be interpreted by the lower affinity of LDLR for apo B-100 than for apo E, demonstrated in in vitro studies. Therefore, our study shows in vivo that, in comparison with apo E, apo B is a less efficient ligand to remove lipid particles such as microemulsions or lipoproteins from the intravascular compartment.

Receptor-mediated mechanisms of lipoprotein remnant catabolism

Chylomicron and VLDL are triglyceride-rich lipoprotein particles assembled by the intestine and liver respectively. These particles are not metabolized by the liver in their native form. However, upon entry into the plasma, their triglyceride component is rapidly hydrolyzed by lipoprotein lipase and they are converted to cholesterol-rich remnant particles. The remnant particles are recognized by the liver and rapidly cleared from the plasma. This process is believed to occur in two steps. (i) An initial sequestration of remnant particles on hepatic cell surface proteoglycans, and (ii) receptor-mediated endocytosis of remnants by hepatic parenchymal cells. The initial binding to proteoglycans may be facilitated by lipoprotein lipase and hepatic lipase which possess both lipid- and heparin-binding domains. The subsequent endocytic process may be mediated by LDL receptors and/or LRP. Both receptors have a high affinity for apoE, a major apolipoprotein component of remnant particles. The lipases may also serve as ligands for these receptors. An impairment of any component of this complex process may result in an accumulation of remnant particles in the plasma leading to atherosclerosis and coronary heart disease.

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.

Secretory processing of amyloid precursor protein is inhibited by increase in cellular cholesterol content

Plasma-membrane composition plays a crucial role in most of the cellular functions that depend on membrane processes. In virtually all cell types the proteolytic processing of Alzheimer amyloid precursor protein (APP) to generate soluble APP (sAPP) is believed to occur at the plasma membrane or in its immediate proximity. Alteration of this metabolic pathway has been linked to the pathogenesis of Alzheimer's disease. We analysed the effect of membrane cholesterol enrichment on APP metabolism. Incubation of COS cells with increasing concentrations of non-esterified cholesterol carried by rabbit beta-very low-density lipoprotein caused a dose-dependent inhibition of sAPP release: 70% inhibition with 10 microg/ml non-esterified cholesterol. A less pronounced inhibitory effect was observed on treatment with human low-density lipoprotein. Inhibition of sAPP release was independent of receptor-mediated lipoprotein metabolism since simultaneous treatment with chloroquine did not modify the effect of lipoprotein treatment. In addition, treatment with cholesterol dissolved in either ethanol or methyl-beta-cyclodextrin elicited the same effect. Excess non-esterified cholesterol did not cause cell toxicity. Cell cholesterol mass inversely correlated with sAPP release. Progesterone, which inhibits shuttling of non-esterified cholesterol between the plasma membrane and intracellular pools, had no effect on the inhibition of sAPP release from cholesterol-loaded cells, providing indirect evidence that cholesterol may act at the plasma membrane.

Physical states of surface and core lipids in lipid emulsions and apolipoprotein binding to the emulsion surface

Plasma triglyceride-rich lipoproteins vary in lipid composition during their metabolism. We investigated the effects of the lipid composition of emulsion particles, specifically those of cholesterol enrichment and core replacement (replacing core triglyceride with cholesteryl oleate), on the physical states of surface and core lipids. Steady-state and time-resolved fluorescence anisotropies were measured in lipid emulsions using 1,6-diphenylhexatriene to probe the core and 1,6-diphenylhexatriene analogues for the outer and inner hydrophobic portions of surface phospholipids. In the absence of cholesterol, core replacement had little effect on the surface rigidity, despite the large difference in core mobility. However, core replacement caused a marked increase in surface rigidity in the presence of cholesterol. Quenching experiments using the fluorescent cholesterol analogue, dehydroergosterol, indicated that core replacement allowed surface dehydroergosterol to redistribute from the inner to the outer regions in the emulsion surface. These results indicated that core replacement modulates the surface properties of the emulsion particles through the redistribution of cholesterol in the surface layers. Furthermore, core replacement significantly decreased the binding of apolipoprotein E to the emulsion surface, whereas the binding of apolipoprotein CII responded to the cholesterol enrichment. This binding behavior of exchangeable apolipoproteins may closely correlate with the location of surface cholesterol and the mobility of core lipids.

FH Afrikaner-3 LDL receptor mutation results in defective LDL receptors and causes a mild form of familial hypercholesterolemia

Three founder-related gene mutations (FH Afrikaner-1, -2, and -3) that affect the LDL receptor are responsible for 90% of the familial hypercholesterolemia (FH) in South African Afrikaners. Patients heterozygous for the FH Afrikaner-1 (FH1) mutation, which results in receptors having approximately 20% of normal receptor activity, have significantly lower plasma cholesterol levels and milder clinical symptoms than heterozygotes with the FH Afrikaner-2 mutation, which completely abolishes LDL receptor activity. In this study we re-created the FH3 mutation (Asp154-->Asn) in exon 4 by site-directed mutagenesis and analyzed the expression of the mutant receptors in Chinese hamster ovary cells. The mutation resulted in the formation of LDL receptors that are markedly defective in their ability to bind LDL, whereas binding of apoE-containing beta-VLDL is less affected. The mutant receptors are poorly expressed on the cell surface as a result of significant degradation of receptor precursors. The plasma cholesterol levels of 31 FH3 heterozygotes were similar to FH1 heterozygotes but significantly lower than FH2 heterozygotes. The FH1 and FH3 heterozygotes also tended to be less severely affected clinically (by coronary heart disease and xanthomata) than FH2 patients. This study demonstrates that mutational heterogeneity in the LDL receptor gene influences the phenotypic expression of heterozygous FH and that severity of expression correlates with the activity of the LDL receptor measured in vitro. The results further indicate that knowledge of the specific mutation underlying FH in heterozygotes is valuable in determining the potential risk of premature atherosclerosis and should influence the clinical management of FH patients.

In vivo gene therapy for hyperlipidemia: phenotypic correction in Watanabe rabbits by hepatic delivery of the rabbit LDL receptor gene

Elevations of plasma total or LDL cholesterol are major risk factors for cardiovascular disease. Efforts directed at preventing and treating cardiovascular disease have often focused on reducing the levels of these substances in the blood. The Watanabe Heritable Hyperlipidemic Rabbit, which has exceedingly high plasma cholesterol levels resulting from an LDL receptor deficiency, provides an excellent animal model for testing new treatments. A recombinant adenoviral vector containing the rabbit LDL receptor cDNA was administered to Watanabe rabbits. Plasma total cholesterol levels in the treated animals were reduced from 825.5 +/- 69.8 (mean +/- SD) to 247.3 +/- 61.5 mg/dl 6 d after infusion. These animals also demonstrated a 300-400% increase in plasma levels of HDL cholesterol and apo AI 10 d after treatment. As a result, the LDL:HDL ratio exhibited a dramatic decrease. Because only the rabbit LDL receptor gene was used for treatment, the results strongly suggest that the elevations of plasma HDL cholesterol and apo AI were secondary to a reduction in plasma total cholesterol in the treated animals. These results suggest an inverse relationship between plasma LDL and HDL cholesterol levels and imply that reduction of LDL cholesterol levels may have a beneficial effect on plasma HDL cholesterol.

Effects of particle size on cell uptake of model triglyceride-rich particles with and without apoprotein E

The effect of apoprotein E on cellular uptake of "VLDL-size" and "IDL-size" triacylglycerol-phospholipid emulsion particles was studied in J-774 macrophages and fibroblasts. In the absence of apoprotein E (apo E), uptake of the smaller IDL-size particles was up to 2-fold higher by mass and 100-fold higher as calculated by particle number. Apo E enhanced the uptake of both VLDL-size and IDL-size emulsion particles, but the effect was greater on the uptake of larger particles (4-5-fold) as compared to up to a 2-fold increase in the uptake of IDL-size particles. In fibroblasts, particle uptake was less than in macrophages (30-50%), but preferential uptake of smaller particles was similarly observed. Particle internalization was demonstrated by 125I-apo E degradation and resistance to particle release by heparin-suramin. In the absence of apo E, cholesteryl ester of emulsion particles (prepared with trace amounts of [3H]cholesteryl ester) was hydrolyzed to free cholesterol, proving internalization and intracellular metabolism. Double-label experiments using DiI-labeled emulsion particles, in the absence and presence of apo E, showed that emulsion particles are rapidly targeted to perinuclear lysosomes. Thus, at physiological concentrations of triglyceride-rich particles, non-receptor-mediated uptake is a mechanism for the uptake of VLDL-size and IDL-size particles into cells.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.