Interaction of high molecular weight chondroitin sulfate from human aorta with plasma low density lipoproteins

A novel assay to measure low-density lipoproteins binding to proteoglycans

BACKGROUND

The binding of low-density lipoprotein (LDL) to proteoglycans (PGs) in the extracellular matrix (ECM) of the arterial intima is a key initial step in the development of atherosclerosis. Although many techniques have been developed to assess this binding, most of the methods are labor-intensive and technically challenging to standardize across research laboratories. Thus, sensitive, and reproducible assay to detect LDL binding to PGs is needed to screen clinical populations for atherosclerosis risk.

OBJECTIVES

The aim of this study was to develop a quantitative, and reproducible assay to evaluate the affinity of LDL towards PGs and to replicate previously published results on LDL-PG binding.

METHODS

Immunofluorescence microscopy was performed to visualize the binding of LDL to PGs using mouse vascular smooth muscle (MOVAS) cells. An in-cell ELISA (ICE) was also developed and optimized to quantitatively measure LDL-PG binding using fixed MOVAS cells cultured in a 96-well format.

RESULTS

We used the ICE assay to show that, despite equal APOB concentrations, LDL isolated from adults with cardiovascular disease bound to PG to a greater extent than LDL isolated from adults without cardiovascular disease (p<0.05).

CONCLUSION

We have developed an LDL-PG binding assay that is capable of detecting differences in PG binding affinities despite equal APOB concentrations. Future work will focus on candidate apolipoproteins that enhance or diminish this interaction.

Endothelial glycocalyx in retina, hyperglycemia, and diabetic retinopathy

The endothelial glycocalyx (EG) is a meshlike network present on the apical surface of the endothelium. Membrane-bound proteoglycans, the major backbone molecules of the EG, consist of glycosaminoglycans attached to core proteins. In addition to maintaining the integrity of the endothelial barrier, the EG regulates inflammation and perfusion and acts as a mechanosensor. The loss of the EG can cause endothelial dysfunction and drive the progression of vascular diseases including diabetic retinopathy. Therefore, the EG presents a novel therapeutic target for treatment of vascular complications. In this review article, we provide an overview of the structure and function of the EG in the retina. Our particular focus is on hyperglycemia-induced perturbations in the glycocalyx structure in the retina, potential underlying mechanisms, and clinical trials studying protective treatments against degradation of the EG.

Differences in the distribution of versican, decorin, and biglycan in atherosclerotic human coronary arteries

The distributions of versican, biglycan, and decorin have been examined in segments of normal and atherosclerotic human coronary arteries using antibodies directed against the core proteins of these macromolecules. Versican immunostaining was prominent throughout the extracellular matrix (ECM) in regions of the vessels that contained abundant smooth-muscle cells, such as in diffuse intimal thickenings, fibrous caps, and in zones of loose, myxoid connective tissue. Versican also was present in smooth-muscle-rich thrombi and at borders of the lipid-rich cores of advanced atherosclerotic lesions. Biglycan immunostaining was observed in diffuse intimal thickenings, fibrous caps, and myxoid areas, but, unlike versican, it was abundant in the lipid-rich core of advanced plaques. However, biglycan immunostaining was absent in smooth-muscle cell-enriched thrombi. Decorin immunostaining paralleled biglycan immunostaining except that it was conspicuously absent in the myxoid areas of the plaque and markedly reduced in diffuse intimal thickenings. Both biglycan and decorin immunostaining were consistently associated with some of the microvessels in the thrombi and in advanced atherosclerotic plaques. Taken together, these results indicate that specific proteoglycans distribute to topographically defined regions of normal and atherosclerotic human coronary arteries and that these different distributions may indicate a diversity of functions in normal and pathologic processes of the arterial wall.

Anionic biopolyelectrolytes of the syndecan/perlecan superfamily: physicochemical properties and medical significance

In the review article presented here, we demonstrate that the connective tissue is more than just a matrix for cells and a passive scaffold to provide physical support. The extracellular matrix can be subdivided into proteins (collagen, elastin), glycoconjugates (structural glycoproteins, proteoglycans) and glycosaminoglycans (hyaluronan). Our main focus rests on the anionic biopolyelectrolytes of the perlecan/syndecan superfamily which belongs to extracellular matrix and cell membrane integral proteoglycans. Though the extracellular domain of the syndecans may well be performing a structural role within the extracellular matrix, a key function of this class of membrane intercalated proteoglycans may be to act as signal transducers across the plasma membrane and thus be more appropriately included in the group of cell surface receptors. Nevertheless, there is a continuum in functions of syndecans and perlecans, especially with respect to their structural role and biomedical significance. HS/CS proteoglycans are receptor sites for lipoprotein binding thus intervening directly in lipid metabolism. We could show that among all lipoproteins, HDL has the highest affinity to these proteoglycans and thus instals a feedforward forechecking loop against atherogenic apoB100 lipoprotein deposition on surface membranes and in subendothelial spaces. Therefore, HDL is not only responsible for VLDL/IDL/LDL cholesterol exit but also controls thoroughly the entry. This way, it inhibits arteriosclerotic nanoplaque formation. The ternary complex 'lipoprotein receptor (HS/CS-PG) - lipoprotein (LDL, oxLDL, Lp(a)) - calcium' may be interpreted as arteriosclerotic nanoplaque build-up on the molecular level before any cellular reactivity, possibly representing the arteriosclerotic primary lesion combined with endothelial dysfunction. With laser-based ellipsometry we could demonstrate that nanoplaque formation is a Ca(2+)-driven process. In an in vitro biosensor application of HS-PG coated silica surfaces we tested nanoplaque formation and size in clinical trials with cardiovascular high-risk patients who underwent treatment with ginkgo or fluvastatin. While ginkgo reduced nanoplaque formation (size) by 14.3% (23.4%) in the isolated apoB100 lipid fraction at a normal blood Ca(2+) concentration, the effect of the statin with a reduction of 44.1% (25.4%) was more pronounced. In addition, ginkgo showed beneficial effects on several biomarkers of oxidative stress and inflammation. Besides acting as peripheral lipoprotein binding receptor, HS/CS-PG is crucially implicated in blood flow sensing. A sensor molecule has to fulfil certain mechanochemical and mechanoelectrical requirements. It should possess viscoelastic and cation binding properties capable of undergoing conformational changes caused both mechanically and electrostatically. Moreover, the latter should be ion-specific. Under no-flow conditions, the viscoelastic polyelectrolyte at the endothelium - blood interface assumes a random coil form. Blood flow causes a conformational change from the random coil state to the directed filament structure state. This conformational transition effects a protein unfurling and molecular elongation of the GAG side chains like in a 'stretched' spring. This configuration is therefore combined with an increase in binding sites for Na(+) ions. Counterion migration of Na(+) along the polysaccharide chain is followed by transmembrane Na(+) influx into the endothelial cell and by endothelial cell membrane depolarization. The simultaneous Ca(2+) influx releases NO and PGI2, vasodilatation is the consequence. Decrease in flow reverses the process. Binding of Ca(2+) and/or apoB100 lipoproteins (nanoplaque formation) impairs the flow sensor function. The physicochemical and functional properties of proteoglycans are due to their amphiphilicity and anionic polyelectrolyte character. Thus, they potently interact with cations, albeit in a rather complex manner. Utilizing (23)Na(+) and (39)K(+) NMR techniques, we could show that, both in HS-PG solutions and in native vascular connective tissue, the mode of interaction for monovalent cations is competition. Mg(2+) and Ca(2+) ions, however, induced a conformational change leading to an increased allosteric, cooperative K(+) and Na(+) binding, respectively. Since extracellular matrices and basement membranes form a tight-fitting sheath around the cell membrane of muscle and Schwann cells, in particular around sinus node cells of the heart, and underlie all epithelial and endothelial cell sheets and tubes, a release of cations from or an adsorption to these polyanionic macromolecules can transiently lead to fast and drastic activity changes in these tiny extracellular tissue compartments. The ionic currents underlying pacemaker and action potential of sinus node cells are fundamentally modulated. Therefore, these polyelectrolytic ion binding characteristics directly contribute to and intervene into heart rhythm.

Characterization of human placental glycosaminoglycans and regional binding to VAR2CSA in malaria infected erythrocytes

Placental malaria is a serious problem in sub-Saharan Africa. Young women are particular susceptible to contracting this form of malaria during their first or second pregnancy despite previously acquired immunity from past infections. Placental malaria is caused by Plasmodium falciparum parasites expressing VAR2CSA on the erythrocyte surface. This protein adheres to a low-sulfated chondroitin sulfate-A found in placental tissue causing great harm to both mother and developing fetus. In rare cases, the localization of infected erythrocytes to the placenta can even result in the vertical transmission of malaria. In an effort to better understand this infection, chondroitin sulfate was isolated from the cotyledon part of the placenta, which should be accessible for parasite adhesion, as well as two non-accessible parts of the placenta to serve as controls. The placental chondroitin sulfate structures and their VAR2CSA binding were characterized. All portions of human placenta contained sufficient amounts of the appropriate low-sulfated chondroitin sulfate-A to display high-affinity binding to a recombinant truncated VAR2CSA construct, as determined using surface plasmon resonance. The cotyledon is the only placental tissue accessible to parasites in the bloodstream, suggesting it is the primary receptor for parasite infected red blood cells.

In patients with pseudoxanthoma elasticum a thicker and more elastic carotid artery is associated with elastin fragmentation and proteoglycans accumulation

Skin biopsies in patients with pseudoxanthoma elasticum (PXE) show elastic fiber fragmentation and calcium and proteoglycans accumulation. Assuming such changes to be present in the artery wall as well, we studied the influence of such alterations on function and structure of the human common carotid artery (CCA). Indeed, elastin fragmentation and increased calcium and proteoglycans content were present in the arteries of the two PXE patients examined. Internal diameter, distension and intima-media thickness (IMT) in the CCA of PXE patients (n = 19) and controls (n = 39) were determined by ultrasound (US). Pulse pressure was assessed in the brachial artery. The distensibility and compliance coefficients as well as the Young's modulus were calculated. Diameter and pulse pressure were not significantly different in PXE patients and controls. The distensibility and compliance coefficients were significantly greater in older PXE patients than in older controls. The distensibility coefficient decreased with age in both PXE patients and in controls. Unlike in controls, the compliance coefficient did not decrease and the Young's modulus barely increased with age in PXE patients. IMT was significantly greater at both younger and older ages and the Young's modulus was significantly smaller at older ages in PXE patients than in controls. The carotid artery is thicker and more elastic in PXE patients than in control subjects; differences are most pronounced at older ages. These alterations might be explained by the elastin fragmentation and proteoglycans accumulation as observed in these patients.

Human venous and arterial glycosaminoglycans have similar affinity for plasma low-density lipoproteins

We compared the glycosaminoglycan content of human venous and arterial walls. The most abundant glycosaminoglycan in human veins is dermatan sulfate whereas chondroitin 4/6-sulfate is preponderant in arteries. The concentrations of chondroitin 4/6-sulfate and heparan sulfate are approximately 4.8- and approximately 2.5-fold higher in arteries than in veins whereas dermatan sulfate contents are similar in the two types of blood vessels. Normal and varicose saphenous veins do not differ in their glycosaminoglycan contents. It is known that certain glycosaminoglycan species from the arterial wall, mainly high-molecular-weight fractions of dermatan sulfate+chondroitin 4/6-sulfate have greater affinity for plasma LDL. These types of glycosaminoglycans can be identified on a LDL-affinity column. We now demonstrated that a similar population of glycosaminoglycan also occurs in veins, although with a lower concentration than in the arteries due to less chondroitin 4/6-sulfate with affinity for LDL. The concentrations of dermatan sulfate species, which interact with LDL, are similar in arteries and veins. The presence of these glycosaminoglycans with affinity to plasma LDL in veins raises interesting questions concerning the role of these molecules in the pathogenesis of atherosclerosis. Possibly, the presence of these glycosaminoglycans in the vessel wall are not sufficient to cause retention of LDL and consequently endothelial dysfunction, but may require additional intrinsic factors and/or the hydrodynamic of the blood under the arterial pressure.

Glycosaminoglycan-lipoprotein interaction

Glycosaminoglycans (GAGs) bound to various proteoglycans (PGs) present in the cardiovascular system have been proposed to perform a wide range of functions. These include conferring viscoelastic properties; interacting with and modulating growth factors and enzymes; and as receptors and co-receptors in lipoprotein metabolism. Binding of apoB-100 lipoproteins, particularly low density lipoproteins (LDL), to GAGs of extracellular matrix PGs in arteries has been proposed to be an initiating event in development of atherosclerosis. This study was initiated with the aim of getting an overview of the binding patterns of different lipoprotein subclasses with individual GAG categories. We thus evaluated the interaction of lipoproteins with GAGs commonly found in the cardiovascular system using a gel mobility-shift assay developed for this purpose. The same procedure was used to measure lipoproteins binding to metabolically [(35)S]-labeled whole PGs prepared from three cell types, arterial smooth muscle cells, THP-1 macrophages and from HepG2 cells. The effect of GAG composition on PGs on lipoprotein binding was evaluated by enzymatic degradation of the carbohydrate chains. Heparan sulfate was found to bind beta very low density lipoproteins (beta-VLDL) and a chylomicron remnant model (beta-VLDL+apoE), but not LDL. Dermatan sulfate was found to bind LDL, but not beta-VLDL or the chylomicron remnant model. Chondroitin sulfate and heparin were found to bind all lipoproteins tested (LDL, beta-VLDL and beta-VLDL+apoE) although with different affinities. We can conclude that each lipoprotein subclass tested binds a specific assortment of the GAGs tested. The observations made contribute to the understanding of new and complex mechanisms by which carbohydrate and lipid metabolism may be linked.

Changes in matrix proteoglycans induced by insulin and fatty acids in hepatic cells may contribute to dyslipidemia of insulin resistance

Insulin resistance and type 2 diabetes are associated with elevated circulating levels of insulin, nonesterified fatty acids (NEFAs), and lipoprotein remnants. Extracellular matrix proteoglycan (PG) alterations are also common in macro- and microvascular complications of type 2 diabetes. In liver, extracellular heparan sulfate (HS) PGs contribute to the uptake of triglyceride-rich lipoprotein remnants. We found that HepG2 cells cultured with 10 or 50 nmol/l insulin or 300 micromol/l albumin-bound linoleic acid changed their PG secretion. The glycosaminoglycans (GAGs) of the secreted PGs from insulin-treated HepG2 cells were enriched in chondroitin sulfate (CS) PGs. In contrast, cells exposed to linoleic acid secreted PGs with decreased content of CS. Insulin caused a moderate increase in mRNA for versican (secreted CS PG), whereas linoleic acid markedly decreased mRNA for versican in HepG2 cells, as did the peroxisomal proliferator-activated receptor-alpha agonist bezafibrate. The effects of insulin or linoleic acid on syndecan 1, a cell surface HS PG, were similar to those on versican, but less pronounced. The livers of obese Zucker fa/fa rats, which are insulin-resistant and have high levels of insulin, NEFAs, and triglyceride-rich remnants, showed increased expression of CS PGs when compared with lean littermates. These changes in PG composition decreased the affinity of remnant beta-VLDL particles to PGs isolated from insulin-treated HepG2 cells and obese rat livers. The results indicated that insulin and NEFAs modulate the expression of PGs in hepatic cells. We speculate that in vivo this exchange of CS for HS may reduce the clearance of remnant beta-VLDLs and contribute to the dyslipidemia of insulin resistance.

Human monocyte-derived macrophages secrete two forms of proteoglycan-macrophage colony-stimulating factor that differ in their ability to bind low density lipoproteins

This study evaluated whether human monocyte-derived macrophages synthesize specific types of proteoglycans with lipoprotein-binding capability that could contribute to lipid retention in the arterial wall. After labeling with either [35S]SO4 or [35S]methionine, macrophages secreted a high molecular mass proteoglycan, with glycosaminoglycan chains of approximately 18 kDa and core protein bands of approximately 100 and 55 kDa. Both core protein bands were recognized by an antibody to PG-100, an antibody that recognizes the proteoglycan form of macrophage colony-stimulating factor (PG-100/PG-MCSF). The interaction between PG-100/PG-MCSF and low density lipoproteins (LDL) was examined by gel mobility shift. In this system, PG-100/PG-MCSF was resolved further into two forms. The two forms had the same core proteins but differed in their overall size and glycosaminoglycan content. The larger form contained glycosaminoglycan chains that were entirely chondroitin ABC lyase-sensitive, whereas the smaller form contained chains that were sensitive to both chondroitin ABC lyase and heparinase. Both forms bound native LDL with high affinity, but the larger form bound LDL with higher affinity than the smaller form. The glycosaminoglycan chains of PG-100/PG-MCSF, but not the core proteins, were responsible for binding to native LDL. Mildly oxidized LDL and methyl-LDL, which have an electrophoretic charge similar to that of native LDL, also bound PG-100/PG-MCSF. In contrast, extensively oxidized LDL and acetyl-LDL, which are more electronegative than native LDL, did not bind to either form of PG-100/PG-MCSF. The demonstration of two forms of human monocyte-derived macrophage PG-100/PG-MCSF which bind LDL may represent an additional role for macrophages in the extracellular trapping of lipoproteins in atherosclerosis.

Age-related changes in populations of aortic glycosaminoglycans: species with low affinity for plasma low-density lipoproteins, and not species with high affinity, are preferentially affected

Glycosaminoglycans were extracted from the intima and media layers of normal human thoracic aortas from donors of different ages. The arterial segments were devoid of macroscopically visible lesions obtained from patients who had no clinically evident cardiovascular disease. Total glycosaminoglycan content increases during the first 40 years of life. Changes in the content of hyaluronic acid and heparan sulfate are less noticeable. The content of chondroitin sulfate (mainly the 6-isomer) increases, whereas dermatan sulfate remains constant. Plasma LDL-affinity chromatography of dermatan sulfate+chondroitin 4/6-sulfate fractions allowed the separation of LDL high- and low-affinity glycosaminoglycan species. Remarkably, only glycosaminoglycan species with low affinity for plasma LDL increase with age in the disease-free areas of human thoracic aortas studied. These results suggest that age-related changes in glycosaminoglycan composition of the arterial wall do not contribute to increased deposition of plasma LDL. However, the alternative explanation that individuals with arterial glycosaminoglycans that avidly bind LDL would develop early and severe cardiovascular disease and would thus be excluded from our analysis cannot be ruled out.

High affinity of a fucosylated chondroitin sulfate for plasma low density lipoprotein

Factors that influence the binding of sulfated polysaccharides to plasma low density lipoprotein (LDL) were investigated. Among the naturally occurring polysaccharides tested, a fucosylated chondroitin sulfate from an echinoderm exhibited the strongest interaction with LDL. Defucosylation and desulfation totally abolished the interaction with LDL while reduction of carboxyl groups had little effect. These data indicate that the sulfated fucose branches are essential for binding of fucosylated chondroitin sulfate to LDL. In addition, there was a positive correlation between the binding to LDL and increasing length of the sulfated polysaccharide chains. The possibility of a practical use of this fucosylated chondroitin sulfate for the binding of LDL is discussed.

Influence of plasma lipid and LDL-subfraction profile on the interaction between low density lipoprotein with human arterial wall proteoglycans

Low density lipoprotein (LDL) is known to bind to arterial wall proteoglycans (APG), an interaction which may initiate cholesterol deposition in the arterial wall. The objective of this study was to determine whether a predominance of small, dense LDL (LDL-III, d = 1.044-1.063 g/ml) in the circulation in association with an atherogenic lipoprotein phenotype (ALP) (i.e. LDL-III > 100 mg/dl, an elevated plasma triglyceride and a low high density lipoprotein cholesterol) alters LDL reactivity towards APG. Total LDL (d = 1.019-1.063 g/ml) was isolated from 59 patients undergoing coronary angiography (39 males and 20 females) and the LDL subfraction profile determined by non-equilibrium density gradient centrifugation. A binding assay was developed in which total LDL (0.1 mg/ml apo LDL) was mixed with a standard preparation of APG containing 2.5 micrograms/ml chondroitin sulphate and the extent of APG-LDL complex formation followed by absorbance measurement and the amount of precipitated LDL cholesterol. APG-LDL complex formation was positively associated with (a) the percentage of LDL-III within total LDL (r = 0.48, P < 0.0001); (b) the plasma triglyceride level (r = 0.27, P < 0.04); and negatively associated with (a) the percentage of the buoyant LDL-I (d = 1.019-1.033 g/ml)(r = -0.47, P < 0.0001); and (b) the HDL cholesterol concentration (r = -0.37, P < 0.004). There was no association with the percentage of the major LDL species LDL-II. When the patients were divided according to the presence or absence of an ALP i.e. LDL-III greater or less than 100 mg/dl respectively, proteoglycan-LDL complex formation was significantly higher in the former compared to the latter group of patients (P < 0.0001). This study therefore provides evidence that the extent of the interaction of LDL with APG varies considerably between individuals and is enhanced in the presence of ALP. It is postulated that the increased atherogenicity associated with ALP may in part be due to prolonged and enhanced retention of LDL by the arterial wall.

Interferon gamma binds to extracellular matrix chondroitin-sulfate proteoglycans, thus enhancing its cellular response

The amino acid sequence of interferon gamma (IFN-gamma) has basic amino acid clusters similar to the heparin-binding consensus sequences found in other proteins that bind to proteoglycans (PGs). We investigated whether recombinant human IFN-gamma could bind to extracellular matrix (ECM) PGs secreted by human arterial smooth muscle cells (HASMCs) in vitro and whether the interaction affected the cellular response to IFN-gamma. As an in vitro model of ECM we used the basement membrane from HASMCs in culture. The binding of 125I-IFN-gamma to ECM was reduced significantly by pretreatment of ECM with chondroitinase ABC, an enzyme that degrades chondroitin-sulfate glycosaminoglycans. IFN-gamma binding to ECM was reduced by increasing concentrations of chondroitin-6-sulfate. 125I-IFN-gamma (0.05 to 2 ng/mL) binding data indicated an apparent Kd of 2 x 10(-11) mol/L and a maximum binding of 1.6 x 10(6) IFN-gamma molecules bound per square millimeter of ECM. Experiments with synthetic peptides suggested that residues 127 through 135 (AKTGKRKRS) are involved in the binding. The binding to chondroitin-sulfate PGs was confirmed by affinity chromatography of isolated [35S]chondroitin-sulfate PGs from ECM and cell-culture medium on immobilized IFN-gamma. The binding was abolished by treatment with chondroitinase ABC. ECM-bound IFN-gamma was more effective in inducing the expression of class II major histocompatibility antigens such as HLA-DR in HASMCs and human arterial endothelial cells than soluble IFN-gamma. These results suggest a role for chondroitin-sulfate PGs in immobilizing IFN-gamma in the ECM compartment and enhancing the cellular response to IFN-gamma.

Glycosaminoglycan fractions from human arteries presenting diverse susceptibilities to atherosclerosis have different binding affinities to plasma LDL

The topographic distribution of atherosclerotic lesions is influenced by biochemical factors intrinsic to the arterial wall. In the present work we have investigated whether the composition/chemical structure of glycosaminoglycans constitutes one of these factors. Normal human arteries were obtained at necropsy, and in order of decreasing susceptibility to atherosclerosis, consisted of the abdominal and thoracic aortas and the iliac and pulmonary arteries. The results showed similar concentrations of total glycosaminoglycan and collagen. Of the glycosaminoglycans known to interact with low-density lipoprotein (LDL), dermatan sulfate was present in all arteries in comparable concentrations, but the aortas had a 30% higher content of chondroitin 4/6-sulfate, which in turn was slightly enriched in 6-sulfated disaccharide units. LDL-affinity chromatography with dermatan sulfate+chondroitin 4/6-sulfate fractions demonstrated that increasing affinity to LDL matched an increasing susceptibility to atherosclerosis. Analysis of glycosaminoglycans in the eluates indicated a positive correlation between affinity to LDL and increasing molecular weight and the existence of a fraction of glycosaminoglycans of high affinity to LDL in the aortas only. These results suggest that arterial glycosaminoglycans participate in the multifactorial mechanisms that modulate the differential localization of atherosclerotic lesions.

Effect of arterial proteoglycans and glycosaminoglycans on low density lipoprotein oxidation and its uptake by human macrophages and arterial smooth muscle cells

The reversible interaction of low density lipoprotein (LDL) with arterial chondroitin sulfate proteoglycans (CSPGs) or glycosaminoglycans (GAGs) selects LDL particles with a high affinity for sulfated GAGs and also induces modifications in apolipoprotein B (apo B) and the lipid organization of the lipoprotein. In the present work we studied the effect that the reversible interaction with sulfated polysaccharides has on the susceptibility of LDL to in vitro oxidation. For this purpose soluble, nonaggregated CSPG- or GAG-treated LDL was subjected to oxidation in the presence of 5 microM CuSO4 for as long as 48 hours. The rate of formation of thiobarbituric acid-reactive substances, the decrease in isoelectric point, the increase in relative electrophoretic mobility of LDL, the higher degradation rate by human macrophages, and the lower degradation rate by human arterial smooth muscle cells showed that LDLs exposed to CSPGs and GAGs were significantly more susceptible to oxidation than native LDL. Results from competition experiments indicate that C6S-treated LDL after 4 hours of oxidation is taken up via the acetylated LDL receptor in human macrophages. Coincubation of lipoproteins with human macrophages or human arterial smooth muscle cells for 24 hours also indicated that C6S-treated LDL was more susceptible to cell-induced modifications than native LDL. The occurrence in vivo of similar processes may contribute to focal retention, increased rate oxidation of LDL in the arterial intima, and foam cell formation during atherogenesis.

Molecular parameters that control the association of low density lipoprotein apo B-100 with chondroitin sulphate

The association of low density lipoprotein (LDL) with proteoglycans of the arterial intima, in particular chondroitin 6-sulphate proteoglycans, may contribute to LDL accumulation during atherogenesis. We studied the interactions of apolipoprotein B-100 (apo B-100) peptide segments and model peptides with chondroitin 6-sulphate. The ability of these peptides to inhibit complex formation between LDL and chondroitin 6-sulphate was used as a measurement of the interaction. Results from earlier studies suggest that surface located segments of apo B-100 are responsible for the interaction of LDL with heparin and chondroitin sulphate-rich arterial proteoglycans. Therefore 16 hydrophilic apo B-100 peptides were selected for studies and synthesized with a peptide synthesizer. These synthetic peptides were 7 to 26 amino acids long. Four of the peptides inhibited the association of LDL with chondroitin 6-sulphate, namely apo B segments 4230-4254, 3359-3377, 3145-3157 and 2106-2121. The 3359-3377 segment was the most efficient. A common feature between the interacting peptides was an excess of positively charged side chains and based on these results we synthesized nine model peptides that shared sequence characteristics with the interacting apo B-100 peptides. Five of these: RSGRKRSGK, RSSRKRSGK, RGGRKRGGK, RSRSRSRSR and RGRGRGRGR were shown to block the LDL-chondroitin-6-sulphate association, RSRSRSRSR being the most effective. The results suggest that the optimal association of the peptides with chondroitin 6-sulphate is obtained with a minimal chain length of nine amino acids and a minimum of five positive charges and that flexibility in the binding region is important.