Low Density Lipoprotein Complex Research Articles

Features of the immune system and levels of blood transport components in residents of the arctic of the Russian Federation.

The peripheral venous blood of 191 residents of the Arctic of the Russian Federation (Murmansk Oblast) aged 21-55 analyzed comprehensively. Blood was sampled from the ulnar vein on an empty stomach in the morning. The hemogram, phagocytic activity of neutrophils, lymphocyte content with CD3, CD4, CD8, CD10, CD19, CD16, CD71, CD25, HLA-DR and CD95 phenotypes, concentrations of cytokines: TNF-α, IFN-γ, IL-6, IL-10, extracellular receptor pool: sCD71, sCD62L, sApo-1/Fas, sFasL, circulating immune complexes (CIC) and blood transport components: haptoglobin (Hp), transferrin, IgM, IgG, IgA, IgE (immunoglobulins M, G, A and E) low-density lipoproteins (LDL), and high density lipoprotein (HDL) were evaluated. The results were analyzed using descriptive statistics and comparative, factorial, and regression analyses. Residents of the Murmansk Oblast exhibit a high prevalence of erythrocytosis (62.3%), thrombocytosis (25.0%), leukocytosis (20.1%), and increased hemoglobin (Hb) concentrations (42.5%), coupled with a significantly decreased level of active phagocytes (47.6%). A significantly decreased level of lymphocytes with phenotypes CD3 (92.4%), CD4 (40.4%), CD71 (62.3%) was revealed, coupled with the activation of lymphocytes with phenotypes CD16 (50.1%), CD8 (37.5%), CD19 (15.9%) and increased concentrations of pro-inflammatory IFN-γ (47.9%), IL-6 (33.3%), and TNF-α (20.1%). Elevated levels of sCD71 (56.6%), sCD62L (32.1%), sApo-1/Fas (22.1%), sFasL (10.2%), and autoantibodies to double-stranded DNA (57.1%), RNA (10.4%), and oLDL (oxidized-modified LDL) (16.3%) were recorded in residents of the Murmansk Oblast. Arctic residents showed elevated concentrations of Hp (63.3%), IgM (63.9%), IgA (42.4%), IgE (18.8%), LDL (24.9%), and a decreased level of HDL (21.1%) owing to an ApoA-I ligand deficiency (58.1%). Elevated levels of Hp in the blood are associated with increased erythrocyte aggregation frequency, T-lymphocyte activation, and increased concentration of free receptors of the extracellular pool. Increased IgM and IgA levels in the blood are attributed to the need to bind components of the extracellular receptor pool sCD71, sCD62L, sApo-1/Fas, sFasL and are associated with decreased levels lymphocyte with CD8 and CD16 phenotypes. The humoral immune response in Arctic residents is triggered when the cellular component of immunity is under strain which compounded by ineffective clearance of metabolic byproducts owing to imbalanced LDL and HDL lipid transport complexes. The immune system of Arctic residents is characterized by excessive cytotoxic activity of lymphocytes and increased concentrations of pro-inflammatory cytokines, free forms of receptors of the extracellular pool, and autoantibodies. The change in the content of transport components of the blood system is directed at maintaining homeostasis by exhibiting antioxidant, anti-inflammatory, and immunoregulatory properties.

Hypericin can cross barriers in the chicken's chorioallantoic membrane model when delivered in low-density lipoproteins.

Low-density lipoproteins (LDL) were used as a natural drug delivery system for the transport of hypericin (Hyp) in the bloodstream of the chicken's chorioallantoic membrane model (CAM). Hyp was chosen as a model for hydrophobic drug used in photo-diagnosis and photo-treatments (PDT). The extravasation of the Hyp:LDL complexes for different concentration ratios and the redistribution of Hyp between different serum components were investigated with an innovative statistical treatment. Hyp biodistribution was monitored in CAM by intravital fluorescence microscopy. The innovative statistical treatment of experimental data presented here enabled us to obtain highly detailed information from the weak Hyp fluorescence distribution in CAM blood vessels. Hyp redistribution between the serum components was studied by fluorescence spectroscopy in lipids/protein composed solutions. Extravasation of Hyp was dependent on Hyp:LDL concentration ratio. While Hyp:LDL = 50:1 resulted in a significant Hyp extravasation, the Hyp extravasation from Hyp:LDL = 100:1 was weak. The redistribution of Hyp was found to be faster for lipidic particles than for proteins. We have demonstrated that lipids composition has a significant control over Hyp delivery in CAM.

Effects of different polysaccharides on the formation of egg yolk LDL complex nanogels for nutrient delivery

Five polysaccharides, pectin, carboxymethyl cellulose (CMC), gum arabic, carrageenan and alginate, were studied to form complex nanogels with egg yolk low density lipoprotein (LDL). All nanogels were smaller than 85nm with high negative zeta potential, while LDL/carrageenan and LDL/alginate nanogels exhibited more heterogeneous size distribution. Fourier transform infrared spectrum suggested that hydrogen bonds, hydrophobic and electrostatic interactions were involved to form nanogels. Overall, significant expansion of nanogels was observed after encapsulation of curcumin, being studied as a model lipophilic nutrient. Fluorescence spectra evidenced that LDL provided non-polar microenvironment for curcumin and polysaccharides played an important role in the encapsulation process. All nanogels showed sustained release of curcumin under simulated gastrointestinal conditions. Furthermore, nanoscale, smooth and spherical ultrafine dry powders of nanogels were obtained by innovative nano spray drying technology. Our study indicated that LDL/polysaccharides may serve as potential oral delivery systems for lipophilic nutrients.

α-Defensins Induce a Post-translational Modification of Low Density Lipoprotein (LDL) That Promotes Atherosclerosis at Normal Levels of Plasma Cholesterol

Approximately one-half of the patients who develop clinical atherosclerosis have normal or only modest elevations in plasma lipids, indicating that additional mechanisms contribute to pathogenesis. In view of increasing evidence that inflammation contributes to atherogenesis, we studied the effect of human neutrophil α-defensins on low density lipoprotein (LDL) trafficking, metabolism, vascular deposition, and atherogenesis using transgenic mice expressing human α-defensins in their polymorphonuclear leukocytes (Def(+/+)). Accelerated Def(+/+) mice developed α-defensin·LDL complexes that accelerate the clearance of LDL from the circulation accompanied by enhanced vascular deposition and retention of LDL, induction of endothelial cathepsins, increased endothelial permeability to LDL, and the development of lipid streaks in the aortic roots when fed a regular diet and at normal plasma levels of LDL. Transplantation of bone marrow from Def(+/+) to WT mice increased LDL clearance, increased vascular permeability, and increased vascular deposition of LDL, whereas transplantation of WT bone marrow to Def(+/+) mice prevented these outcomes. The same outcome was obtained by treating Def(+/+) mice with colchicine to inhibit the release of α-defensins. These studies identify a potential new link between inflammation and the development of atherosclerosis.

Circulating Lp(a):LDL complexes contain LDL molecules proportionate to Lp(a) size and bind to galectin-1: a possible route for LDL entry into cells.

The molecular mechanism of vascular pathology mediated by circulating lipoprotein(a) [Lp(a)] remains unknown. We examined the role of two distinguishing features of Lp(a) viz non-covalent complex formation with a low density lipoprotein (LDL) and heavy glycosylation as determinants of binding of this lipoprotein and its LDL complex to cell-surface receptors. LDL isolated from the Lp(a):LDL complex, free LDL and oxidized LDL were equally efficient in forming a reconstituted complex with pure Lp(a). Complexed LDL in healthy individuals was equal in oxidation status to free LDL. The number of LDL molecules associated with each Lp(a) molecule (LDL index) in plasma samples increased steadily with Lp(a) size (correlation coefficient r = 0.834). Complex reconstituted from purified plasma Lp(a) and LDL maintained the same LDL index as plasma in accordance with Lp(a) size. Consequently, the percentage of complex-free Lp(a) in the plasma decreased sharply with Lp(a) size (r = -0.887). Although O-glycosylation measured in terms of lectin binding increased with Lp(a) size, the LDL index increased significantly faster than O-glycosylation among Lp(a) phenotypes of different plasma samples. Complexes with varying stoichiometry existed in the same plasma. Extra LDL complex molecules were not recognized by LDL receptors on human macrophages or rat cardiac fibroblasts indicating attachment to Lp(a) involved LDL receptor-binding sites. However, unlike free LDL complex LDL could attach through Lp(a) to immobilized form of galectin-1, a lectin ubiquitous on mammalian cells. Results suggest that phenotype-dependence of the physiological and pathological functions of Lp(a) may operate through differential LDL-carrier activity.

Enhancement on the Europium emission band of Europium chlortetracycline complex in the presence of LDL

Low-density lipoprotein (LDL) particles are the major cholesterol-carrying lipoprotein in the human circulation from the liver to peripheral tissues. High levels of LDL-Cholesterol (LDL-C) are known risk factor for the development of coronary artery disease (CAD). The most common approach to determine the LDL-C in the clinical laboratory involves the Friedewald formula. However, in certain situations, this approach is inadequate. In this paper we report on the enhancement on the Europium emission band of Europium chlortetracycline complex (CTEu) in the presence of LDL. The emission intensity at 615 nm of the CTEu increases with increasing amounts of LDL. This phenomenon allowed us to propose a method to determine the LDL concentration in a sample composed by an aqueous solution of LDL. With this result we obtained LDL calibration curve, LOD (limit of detection) of 0.49 mg/mL and SD (standard deviation) of 0.003. We observed that CTEu complex provides a wider dynamic concentration-range for LDL determination than that from Eu–tetracycline previously. The averaged emission lifetimes of the CTEu and CTEu with LDL (1.5 mg/mL) complexes were measured as 15 and 46 μs, respectively. Study with some metallic interferents is presented.

The association between adiponectin, HDL-cholesterol and α1-antitrypsin-LDL in female subjects without metabolic syndrome

BackgroundOxidized low-density lipoprotein (LDL) may act as an atheroprotective (anti-atherosclerotic) agent under some conditions. While the α1-antitrypsin (AT)-LDL complex is considered a type of oxidized LDL, its clinical relevance remains unknown. The aim of the present study was to investigate the association between AT-LDL and anti-atherosclerotic variables such as HDL-cholesterol and adiponectin in subjects with and without metabolic syndrome (MetS).MethodsIn asymptomatic females (n = 194; mean age, 54 years) who were divided into non-MetS (n = 108) and MetS groups (n = 86), the fasting levels of serum AT-LDL, adiponectin and glucose/lipid panels were measured, in addition to body mass index (BMI) and blood pressure.ResultsThe MetS group showed significantly higher BMI, blood pressure, glucose and triglyceride levels as well as significantly lower levels of HDL-cholesterol and adiponectin than the non-MetS group. A multivariate-adjusted analysis revealed that in the non-MetS group, AT-LDL was significantly, independently and positively correlated with adiponectin (β = 0.297, P < 0.05), along with HDL-cholesterol (β = 0.217, P < 0.05). In the MetS group, AT-LDL was significantly, independently and positively correlated with LDL-cholesterol only (β = 0.342, P < 0.05).ConclusionsThese data suggest that AT-LDL may exert anti-atherosclerotic effects in female subjects without MetS. More studies are required to clarify the clinical roles of AT-LDL in relation to the pathophysiology of MetS.

Loading anticancer drugs into HDL as well as LDL has little affect on properties of complexes and enhances cytotoxicity to human carcinoma cells

Low density lipoprotein (LDL) has been found to represent a suitable carrier for cytotoxic drugs that may target them to cancer. This study investigated whether very low density lipoprotein (VLDL), LDL and high density lipoprotein (HDL) can be used to effectively incorporate four cytotoxic drugs, 5-fluorouracil (5-FU), 5-iododeoxyuridine (IUdR), doxorubicin (Dox) and vindesine; characterized the complexes; and examined the effect of incorporation on drug cytotoxicity against HeLa cervical and MCF-7 breast carcinoma cells. Significant drug loading was achieved into all three classes of lipoproteins, consistent with the sizes and hydrophobicity of the drugs. The relative loading efficiency was found to be vindesine>IUdR>Dox>5-FU for all three classes of lipoproteins. As shown by electron microscopy (EM), drug incorporation did not affect the size or morphology of the lipoproteins. Differential scanning calorimetry (DSC) showed that drug loading did not significantly change the thermal transition temperature of core lipids in the lipoproteins. The transition enthalpy was changed only for LDL–Dox and LDL–vindesine. The drugs remained stable in the lipoproteins as determined by high performance liquid chromatography (HPLC). EM, DSC and HPLC data suggest that drugs were incorporated into lipoproteins without disrupting their integrity and drugs remained in their stable forms inside lipoproteins. Compared with free drugs in cytotoxicity assays, the IC50 values of LDL– and HDL–drug complexes were significantly lower (2.4- to 8.6-fold for LDL complexes and 2.5- to 23-fold for HDL complexes). All free or lipoprotein-bound drug formulations were comparably more cytotoxic against MCF-7 than HeLa cells. Upregulating the lipoprotein receptors enhanced, and downregulating them inhibited, the cytotoxicity, indicating the mechanistic involvement of lipoprotein receptor pathways. Complexes of all four drugs with VLDL, in contrast to LDL and HDL, had the same cytotoxicity as the four corresponding free drugs. Our results suggest that further studies are required of the potential of HDL to be a cancer targeting drug carrier.

A quantitative assay for the non-covalent association between apolipoprotein[a] and apolipoprotein B: an alternative measure of Lp[a] assembly

Increasing evidence suggests that the assembly of lipoprotein[a] (Lp[a]) proceeds in two steps. In the first step, non-covalent interactions between apolipoprotein[a] (apo[a]) and apolipoprotein B (apoB) of low density lipoprotein (LDL) form a dissociable apo[a]:LDL complex. In the second step, a covalent disulfide linkage forms the stable Lp[a] particle. Several methods are currently used to study the assembly of Lp[a], however, these methods are laborious, time-consuming, and not suitable for a high throughput screening. We report here the development of a rapid and simple assay based on the binding of labeled LDL to a Lp[a]/apo[a] substrate which is immobilized on the surface of a microtiter plate. Quantification of bound LDL provides a measure of the extent of complex formation. Labeled LDL bound to both Lp[a] and apo[a] substrates with similar affinity. Plasma lipoproteins containing apoB as well as free apo[a] were capable of competing with LDL binding. The binding of LDL to Lp[a]/apo[a] was inhibited by L-proline and lysine analogs, which are known to inhibit the non-covalent association between apo[a] and apoB. Using this method we have found that nicotinic acid and captopril are able to inhibit the association of apo[a] with apoB. This method is compatible with automation and can be applied to a high throughput screening of inhibitors of Lp[a] formation.

Plasma stability and cytotoxicity of lipophilic daunorubicin derivatives incorporated into low density lipoproteins

The selective targeting of antineoplastic drugs to tumours by incorporation in low density lipoproteins (LDL) is an attractive possibility if the drug–LDL complex remains stable in the circulation and is taken up by the tumour. In previous studies we have shown that vincristine– and N-trifluoroacetyladriamycin-14-valerate–LDL complexes were unstable in vivo. We synthesized five N-substituted lipophilic derivatives of daunorubicin and studied their incorporation into LDL. Three out of five daunorubicin derivatives incorporated successfully into LDL. In vitro these complexes were more cytotoxic towards LDL receptor positive Chinese hamster ovary cells than LDL receptor negative cells. Non-specific cytotoxicity was explained by slow dissociation of the drug–LDL complex in plasma. Our results underline the importance of careful studies of plasma stability when investigating lipoproteins and other carriers in drug targeting.

Inhibitory effect of dexamethasone palmitate-low density lipoprotein complex on low density lipoprotein-induced macrophage foam cell formation.

We used low density lipoprotein (LDL) as a carrier of site-specific delivery of drugs to atherosclerotic lesions, prepared a dexamethasone palmitate (DP)-LDL complex, and examined the effect of the DP-LDL complex on foam cell formation of macrophages in vitro. LDL was isolated from human plasma and the DP-LDL complex was prepared by incubation in the presence of Celite 545. The complex contained about 50 mol of DP in 1 mol of LDL. When macrophages were incubated with LDL for 48 h, cholesterol ester was accumulated in the macrophages, indicating foam cell formation. This accumulation of cholesterol ester was significantly inhibited by incubation with the DP-LDL complex. The potency of the DP-LDL complex was similar to that of dexamethasone alone. The DP-LDL complex also significantly attenuated the accumulation of cholesterol ester induced by incubation with LDL prior to the incubation with the DP-LDL complex. These findings indicated that the DP-LDL complex showed similar characteristics to LDL, and the DP-LDL complex inhibited the foam cell formation of macrophages in in vitro experiments. This DP-LDL complex has a possibility as a drug-carrier complex for use in atherosclerosis.

Low Density Lipoprotein as a Carrier of Cytostatics in Cancer Chemotherapy: Study of Stability of Drug-carrier Complexes in Blood

Several solid tumour and leukemia cell types have a higher low density lipoprotein (LDL) uptake than the corresponding normal cells. We are investigating the possibilities to use LDL as a drug carrier to increase the selectivity of antineoplastic drugs in cancer chemotherapy. We have developed a method to incorporate lipophilic cytotoxic agents without interfering with the in vitro and in vivo properties of LDL. In this study, we examined the stability of some drug-LDL complexes in blood and plasma as this is an important prerequisite to achieve a selective therapy.The in vitro dialysis of N-trifluoroacetyl-adriamycin-14-valerat-LDL (AD-32-LDL) against plasma revealed a slow dissociation of the complex. The same method showed a fast and total leakage of paclitaxel from paclitaxel—LDL into the plasma chamber. The dissociation of paclitaxel was confirmed by an autoradiographic study of the distribution of paclitaxel—LDL in tumour-bearing mice. In patients with leukemia the rapid plasma dissociation of AD-32 from LDL illustrated a much higher in vivo instability of this complex. With this method, cholesteryl-linoleate only could be incorporated into LDL in a stable manner as shown by dialysis and autoradiography results. The incorporation of cytotoxic drug derivatives, containing lipophilic anchors, is now under study in order to obtain LDL complexes with better plasma stability.

Kinetics of hemin distribution in plasma reveals its role in lipoprotein oxidation

Hemin is a powerful in vitro inducer of low-density lipoprotein (LDL) oxidation, implicated in development of atherosclerosis. To support the proposed role of hemin in atherogenesis, the question of whether hemin has any chance of getting together with LDL in vivo, must be addressed. A stopped-flow technique was employed in order to investigate the fast kinetics of hemin binding to LDL and to other plasma hemin-binding proteins: high-density lipoprotein (HDL), albumin and hemopexin. Based on the measured rate constants of hemin association with and dissociation from each of these proteins, time-dependent hemin distribution in plasma was analyzed. The analysis shows that as much as 80% of total hemin binds initially to LDL and HDL, the plasma components which are most susceptible to oxidation. Only then hemin partially transfers to the antioxidants albumin and hemopexin. The half time of the hemin–LDL complex in plasma, initially comprising 27% of total hemin, was more than 20 s. Not only transient, but also oxidatively active steady-state hemin–lipoprotein complexes in plasma were both predicted from the kinetic analysis and found in experiment. Our data suggest that the hemin–LDL complex may exist in vivo and that its oxidative potential should be considered pro-atherogenic.

Targeting of an antitumor agent, RS-1541 (palmitoyl-rhizoxin), via low-density lipoprotein receptor

RS-1541 is a 13- O-palmitoyl derivative of rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective uptake by tumors in mice. We demonstrated that: (1) 99% of RS-1541 bound to human lipoproteins after incubation with human serum, among which the low-density lipoprotein (LDL) fraction attained 10%; (2) RS-1541/LDL complex was obtained by simply incubating human native LDL with a detergent-aided RS-1541 solution; (3) uptake of the [ 14C]RS-1541/LDL complex in cultured human skin fibroblasts was dependent on the LDL receptor activities of the cells; (4) excess amounts of native LDL or 10 μM monensin, a proton ionophore, significantly inhibited the uptake; and (5) the detergent-aided solution of [ 14C]RS-1541 showed very low cellular uptake. These results suggest the possibility of LDL as an endogenous targeting carrier of RS-1541 into tumor cells, which have higher LDL receptor activities.

Oxidation of low-density lipoprotein by hypochlorite causes aggregation that is mediated by modification of lysine residues rather than lipid oxidation.

Peroxidation of low-density lipoprotein (LDL) lipid is generally thought to represent the initial step in a series of modification reactions that ultimately transform the protein moiety of the lipoprotein into a form recognized by receptors different from those that bind native LDL. Uptake of LDL via these alternative receptors can lead to the formation of lipid-laden cells, which are typical for the early stages of atherogenesis. We have studied the oxidative modification of LDL by hypochlorite (-OCl), a powerful oxidant produced from H2O2 and chloride via the action of myeloperoxidase which is released from activated neutrophils and monocytes. Exposure of LDL to reagent or enzymically generated -OCl at 4 or 37 degrees C resulted in immediate and preferential oxidation of amino acid residues of apolipoprotein B-100, the single protein associated with LDL. Lysine residues quantitatively represented the major target and, like tryptophan, were oxidized to approximately the same extent with reagent or enzymically generated -OCl. In contrast, LDL lipid oxidation was less favoured than protein oxidation, as judged by the amounts of lipid hydroperoxides, chlorohydrins, cholesterol or fatty acid oxidation products formed. Treatment with -OCl caused aggregation of LDL, as shown by an increased turbidity of the oxidized LDL solution and elution from a size-exclusion h.p.l.c. column of high-molecular-mass LDL complexes. Chemical modification of lysine residues before oxidation with -OCl prevented aggregation, while it enhanced the extent of lipid peroxidation. Treatment of LDL with -OCl also caused the formation of carbonyl groups and release of ammonia; both these modifications were inhibited by lysine-residue modification before oxidation. These results demonstrate that aggregation reactions are dependent on initial lysine oxidation by -OCl, followed by deamination and carbonyl formation, but do not involve lipid (per)oxidation. We propose that the observed -OCl-mediated aggregation of LDL is caused, at least in part, by cross-linking of apoproteins by Schiff-base formation independently of lipid peroxidation.

Immunologic detection and measurement of glycated apolipoprotein B with site specific monoclonal antibodies

Nonenzymatic glycation of apolopoprotein B (apo B) is a post-secretory modification of low density lipoprotein (LDL) that affects its atherogenic potential and is implicated in the accelerated atherosclerosis associated with diabetes. To facilitate assessment of apo B glycation, we produced hybridomas secreting monoclonal antibodies specific for glycated apo B. SP 2/0 myeloma cells were fused with spleen cells from BALB/c mice immunized with purified apo B glycated non-reductively in vitro. Specificity of monoclonal antibodies secreted by the cloned cell line designated ES12 was demonstrated by immunoblotting and by direct ELISA, wherein the antibodies reacted with glycated epitopes residing in LDL but not in other plasma proteins, and did not react with nonglycated apo B or nonglycated LDL. Immunoblotting of human plasma with ES12 monoclonal antibody yielded an approx. 180,000 molecular weight component showing co-identity with apo B, indicating site specificity for glycated epitopes residing in apo B of the LDL complex and absence of reactivity with other nonezymatically glycated plasma proteins. This reactivity of ES12 with the physiologic form of glycated apo B that occurs in vivo differs from properties of other antibodies raised against glycated lipoproteins, which recognize glycated residues only after reductive conversion to glucitol-lysine and which do not discriminate between different glycated proteins. In a competitive ELISA, mean concentration of glycated LDL, measured as apo B equivalents, in eight separate plasma samples was 19.7 ± 1.9 μg/ml, representing 3.5 ± 0.3% of total apo B. The ES12 monoclonal antibody allows specific determination of plasma glycated LDL concentrations, which may have diagnostic and pathogenic importance.

Altered susceptibility to in vitro oxidation of LDL in LDL complexes and LDL aggregates.

Low density lipoprotein (LDL) is known to form complexes with polysulfated compounds, like heparin, dextran sulfate (DS), and chondroitin sulfate. In particular, chondroitin 6-sulfate (C6S)-rich proteoglycans of the arterial intima can associate with LDL, resulting in accumulation of LDL in atherosclerotic lesions. Besides LDL complex formation, LDL self-aggregation has been recently suggested to play a role in atherogenesis. Oxidative modification of LDL has also been implicated as a factor in the generation of the atherosclerotic plaque. Assuming that LDL self-aggregation may alter the molecule's susceptibility to oxidative modification, we have studied the sensitivity of LDL in LDL a aggregates as well as in insoluble and soluble LDL-C6S, LDL-heparin, and LDL-DS complexes to in vitro oxidation by cooper ions. Complexing the LDL with C6S and heparin resulted in an increased susceptibility of LDL to in vitro oxidation, whereas the oxidation of LDL complexed with DS was unaffected. In great contrast to the oxidation of LDL in LDL complexes, the in vitro oxidation of LDL in LDL aggregates (self-aggregation by denaturation) was strongly reduced. The results suggest that complex or aggregate formation may alter the susceptibility of the lipoprotein to oxidative modification and finally its metabolic fate or biological activity.

Comparative study of the precipitation of low density lipoproteins by aortic proteodermatan sulphate and heparin

The aortic proteoglycans and heparin were shown to form insoluble complexes with human low density lipoproteins (LDL). The effect of temperature, polyethylene glycol and ionic strength on the formation of complexes between porcine aortic proteodermatan sulphate (PDS) and LDL has been studied by laser nephelometry and comparisons made with heparin LDL complexes. Turbidity was a nonlinear function of the quantity of LDL precipitated by PDS. The turbidity of aggregates was constant at temperatures between 2°C and 30°C but increased with temperature above 30°C up to 50°C. The formation of insoluble complexes decreased rapidly with increasing NaCl concentration. Polyethylene glycol enchanced the turbidity at 20°C but not at 37°C. It also increased the resistance of complexes to dissociation by increasing ionic strength. The turbidity of heparin-LDL complexes was linearly correlated with the quantity of precipitated LDL. The heparin-LDL aggregates were less sensitive to modification of temperature and ionic strength than the PDS-LDL aggregates. These results suggest that ionic interactions are weaker in PDS-LDL complexes than in the heparin-LDL complexes. Non-coulombic interactions and/or temperature dependent conformational changes may be involved in the stabilization of supramolecular PDS-LDL aggregates. No such interactions or changes appear to be involved in complex formation between heparin and LDL.

Macrophage lipoprotein receptors

Macrophages possess a number of surface receptors that are capable of mediating the internalization of lipoproteins. The low-density lipoprotein (LDL) receptor of human monocyte macrophages recognizes apolipoprotein B-100 and apolipoprotein E and is rapidly regulated in response to changes in intracellular cholesterol levels. In contrast, in J774 macrophages LDL receptor regulation is defective and LDL can cause massive cholesterol accumulation. The beta migrating very low density lipoprotein (beta-VLDL) receptor is poorly regulated by cellular cholesterol concentrations, readily recognizes apolipoprotein E, poorly recognizes apolipoprotein B-100, and is immunologically related to the LDL receptor. The scavenger receptor (acetyl-LDL receptor) appears to have a molecular weight of 250,000 and is not regulated by cellular cholesterol levels. This receptor recognizes LDL that has been chemically or biologically altered. LDL complexes can also enter macrophages and cause cholesterol accumulation. Examples of such complexes are LDL-dextran sulphate complexes, LDL-proteoglycan aggregates, LDL-mast cell granule complexes, LDL-heparin-fibronectin-denatured collagen complexes, and LDL-antibody complexes. The entry of lipoprotein into macrophages by a pathway that is poorly regulated or is not regulated by cellular cholesterol concentrations appears to be a prerequisite for the formation of arterial foam cells.

The effect of proteoglycans, collagen and lysyl oxidase on the metabolism of low density lipoprotein by macrophages

To study the interactions of lipoproteins, connective tissue components and cells, mouse peritoneal macrophages were incubated in the presence of human low density lipoproteins (LDL) that had been complexed with pig aortic proteoglycans (PG) or incubated in the presence of soluble collagen and/or lysyl oxidase, which catalyses the formation of cross-linkages in collagen and elastin by oxidising ϵ-amino groups of lysine residues to aldehydes. Soluble and insoluble PG-LDL complexes increased the incorporation of [ 3H]oleate into cellular cholesteryl esters (CE) 1.6- and 2.8-fold, respectively, while LDL incubated with collagen and lysyl oxidase had no effect compared to control LDL. As judged on the basis of incubations with fucoidin, spermine and 125I-labelled lipoproteins, the mechanism of internalisation of the PG-LDL complexes is different from that of acetylated LDL or dextran sulphate-LDL complexes. The formation of PG-LDL complexes in the arterial intima may lead to an increased uptake of lipoproteins by intimal macrophages during the early phase of atherogenesis.