Human monocyte-derived macrophages bind low-density-lipoprotein-proteoglycan complexes by a receptor different from the low-density-lipoprotein receptor

Hyaluronan forms complexes with low density lipoprotein while also inducing foam cell infiltration in the dermis

BACKGROUND

Xanthoma is a foam cell infiltrating lesion similar to atherosclerosis. Glycosaminoglycans and proteoglycans have long been considered to play a role in atherogenesis.

OBJECTIVE

The purpose of this study is to investigate the role of hyaluronan, the main dermal glycosaminoglycan, in xanthoma formation.

METHODS

The complex formation of low density lipoprotein (LDL) with hyaluronan was investigated by assaying the cholesterol level of precipitates that were formed by incubating LDL, hyaluronan and cetylpyridinium chloride in the presence of Ca2+. The uptake of LDL by mouse peritoneal macrophages was studied by assaying the cellular cholesterol esterification activity. The responsible receptor for the LDL internalization was examined by saturating hyaluronan receptor and blocking class A macrophage scavenger receptor (CD204). Hyaluronan was injected into the dorsal skin of diet-induced hypercholesterolemic rabbits to reveal the xanthoma inducing activity of hyaluronan.

RESULTS

Cetylpyridinium chloride precipitated hyaluronan, which had formed complexes with LDL. The macrophages incorporated hyaluronan-LDL complexes and oxidized LDL via CD204. Foam cell infiltration and cholesterol accumulation were induced by intradermal injections of hyaluronan in diet-induced hypercholesterolemic rabbits.

CONCLUSION

Hyaluronan, like other sulfated glycosaminoglycans, retains LDL by forming a complex. Via macrophage scavenger receptors, macrophages incorporate not only LDL-hyaluronan complexes, but also oxidized LDL, which has been oxidized during the retention time.

Human coronary smooth muscle cells internalize versican-modified LDL through LDL receptor-related protein and LDL receptors

Versican-like proteoglycans are the main component of the intimal extracellular matrix interacting with low density lipoprotein (LDL). The aim of this study has been to investigate the receptors involved in versican-modified LDL uptake by human vascular smooth muscle cells (VSMCs). We have found that versican-LDL interaction leads to the following: (1) monomeric LDL particles that are similar in size and electrophoretic mobility to native LDL but that have a higher capacity to induce intracellular cholesteryl ester (CE) accumulation and (2) fused LDL particles similar in size to those obtained by vortexing. The precipitable fraction of versican-LDL, composed of 50% monomeric and 50% fused LDL particles, induced a dose-response increase in the CE content of VSMCs. Anti-LDL receptor antibody decreased the CE accumulation derived from monomeric LDL particles by 88 +/- 3% and that derived from the total precipitable fraction by 45 +/- 3%. Inhibition of LDL receptor-related protein expression by antisense oligodeoxynucleotides reduced the CE accumulation derived from the precipitable fraction by 65 +/- 2.8%, whereas it did not produce any effect on the CE accumulation derived from monomeric LDL. These results suggest that versican-LDL induces CE accumulation in human VSMCs by the LDL receptor (monomeric particles) and LDL receptor-related protein (fused LDL).

Binding of [99mTc]chondroitin sulfate to scavenger receptors on human chondrocytes as compared to binding of oxidized [125I]LDL on human macrophages

Chondroitin sulfate (CS) used for treatment of osteoarthritis exerts distinct effects on human articular chondrocytes in vitro. We performed a binding analysis with 99mTc-labeled CS (Condrosulf, a commercial CS preparation containing calcium stearate) and cultured human chondrocytes in order to evaluate the presence of specific receptors. Saturation binding at 37 degrees C for 2 h revealed the presence of high-affinity binding sites for CS with a Kd of 2.3 x 10(-9) mol/L and a Bmax of 5.0 x 10(8). Extensive dialysis of Chondrosulf led to a decrease of the binding affinity by 52.5 +/- 19.5% and of the number of CS binding sites/cell by 62.0 +/- 14.0%, demonstrating that the additive present in the Condrosulf preparation enhances CS binding. The nature of the binding site is not yet known but evidence exists in the literature that the scavenger receptor CD36, thoroughly investigated on macrophages, is also found on chondrocytes and might be involved in CS binding. Therefore, we undertook a comparative binding study with human monocytes and labelled LDL and oxidized LDL, the latter being a postulated atherogenic agent in atherosclerosis. For [125I]-LDL binding we found a Kd of 0.45 x 10(-8) mol/L and a Bmax of 0.14 x 10(6) on quiescent monocytes and for [125I]-(ox)LDL binding a Kd of 1.8 x 10(-8) mol/L and a Bmax of 1.3 x 10(6) using LPS-activated monocytes. These data are comparable to the binding affinity found for lipoprotein-proteoglycan-complexes and hence are an indication but not a proof that CD36 is involved in CS binding to human chondrocytes.

Macrophage-released proteoglycans enhance LDL aggregation: studies in aorta from apolipoprotein E-deficient mice

Aggregated low-density lipoprotein (LDL) was shown to be present in the atherosclerotic lesion, but the mechanism responsible for its formation in vivo is not known yet. To find out whether LDL aggregation occurs in the arterial wall during atherogenesis, LDLs were extracted from the aortas of apolipoprotein E-deficient (E(0)) mice during their aging (and the development of atherosclerosis), and were analyzed for their aggregation states, in comparison to LDLs isolated from aortas of control mice. LDL isolated from aortas of E(0) mice was already aggregated at 1 month of age and its aggregation state substantially increased with age, with 3-fold elevation at 6 months of age compared to younger, 1-month-old, mice. Only minimal aggregation could be detected in LDL derived from control mice. Electron microscopy examination revealed that LDL particles from aortas of the E(0) mice were heterogeneous in their size, ranging between 20 and 300 nm. The mouse aortic LDL contained proteoglycans (PGs) and their content increased with the age of the mice, with about 2-fold higher levels than those found in LDLs derived from aortas of control mice. Macrophage-released PGs were previously demonstrated to enhance LDL aggregation in vitro. However, their involvement in LDL aggregation in vivo has not been studied yet. Thus, we next studied the effect of arterial macrophage-released PGs on the susceptibility of plasma LDL to aggregation by Bacillus cereus sphingomyelinase (SMase). Foam cell macrophages were isolated from aortas of the atherosclerotic E(0) mice at 6 months of age and were found to be loaded with cholesterol and to contain oxidized lipids. To analyze the effect of macrophage-released PGs on LDL aggregation, PGs were prelabeled by cell incubation with [35S]sulfate, followed by incubation of macrophage-released PGs with E(0) mouse plasma LDL (200 microg protein/ml) for 1 h at 37 degrees C. [35S]Sulfated PGs were found to be LDL-associated and the susceptibility of PG-associated LDL to aggregation by SMase was increased by up to 45% in comparison to control LDL. Similar results demonstrating the involvement of PGs in LDL aggregation were obtained upon incubation of LDL with increasing concentrations of PGs that were isolated from the entire aorta of E(o) mice (rather than the isolated macrophages). The stimulatory effect of macrophage-released PGs on LDL aggregation was markedly reduced when the PGs were pretreated with the glycosaminoglycan-hydrolyzing enzymes, chondroitinase ABC or chondroitinase AC, and to a much lesser extent with heparinase. We thus conclude that macrophage-released chondroitin sulfate PG can contribute to the formation of atherogenic aggregated LDL in the arterial wall.

Expression and localization of the proteoglycan decorin during the progression of cholesterol induced atherosclerosis in Japanese quail: implications for interaction with collagen type I and lipoproteins

The temporal and spatial distribution, and relative levels of the proteoglycan decorin and collagen type I were examined during the progression of atherosclerosis in the dorsal aortas of Japanese quail selected for cholesterol induced atherosclerosis. The quail were placed on either a control or 0.5% added cholesterol diet at approximately 16 weeks of age. Dorsal aortas were collected at 1- or 2-week intervals over a 15-week period after initiating cholesterol feeding. Biochemical analysis for decorin and collagen type I showed that both increased in the cholesterol-fed birds compared to control-fed birds beginning at 9 weeks and continued through the duration of the study. Through immunohistochemical staining for decorin and collagen type I, the spatial localization of decorin and collagen type I in control and less severe plaques in cholesterol-fed birds was most prominent in the arterial adventitia. However, in severe atherosclerotic plaques, decorin was localized in foam cell regions and collagen type I was found surrounding the foam cell regions where decorin accumulated. These results demonstrated a localization of decorin in the core of the atherosclerotic plaque foam cell region with collagen type I being located on the plaque surface.

Cholesteryl esterase-treated LDL augments oxidized LDL-mediated cholesteryl ester deposition in mouse peritoneal macrophages

Arterial unesterified cholesterol, phospholipid particles have been isolated from atherosclerotic lesions and characterized. However, the role of these 'liposomes' in macrophage foam cell formation is unclear. Recently, LDL, after trypsin and cholesteryl esterase treatment (T/CE LDL), was shown to have physical properties similar to the unesterified cholesterol, phospholipid particles isolated from atherosclerotic lesions. Yet, when mouse peritoneal macrophages were incubated with these model particles in culture medium (DMEM and 5% LPDS), only an insignificant accumulation of cellular cholesteryl esters was observed. Previously, we demonstrated that complex formation between unesterified cholesterol, phosphatidylcholine liposomes and cupric sulfate-oxidized LDL dramatically enhances the ability of the liposomes to augment cellular cholesterol accretion (Greenspan P, Yu H, Mao F, Gutman RL. J Lipid Res 1997;38:101-109). When T/CE LDL, another cholesterol-rich phospholipid particle, was substituted for unesterified cholesterol phosphatidylcholine liposomes in our complex, mouse peritoneal macrophages accumulated a significant amount of both cellular unesterifed cholesterol (61 microg/mg cell protein) and cholesteryl esters (76 microg/mg cell protein) after 48 h of incubation. These results demonstrate again that the interaction of two cholesterol-bearing particles (T/CE LDL and oxidized LDL), which individually can not promote significant cholesterol accumulation in cells, will, when combined, produce macrophage foam cells.

Aggregation of dimyristoylphosphatidylglycerol liposomes by human plasma low density lipoprotein

Turbidity (absorbance at 470 nm) measurements revealed human serum low density lipoprotein (LDL) to cause, within a few minutes and at physiological pH and [NaCl], the aggregation of liquid crystalline large unilamellar liposomes (LUVs) of dimyristoylphosphatidylglycerol (DMPG). No evidence for concomitant lipid or aqueous contents mixing was obtained with fluorescent assays for these processes, in keeping with the lack of fusion of LUVs. Involvement of apoB is implicated by the finding that tryptic digestion of LDL abrogates its ability to cause aggregation. Aggregation is not caused by VLDL, HDL2, or HDL3. Interestingly, also oxidised LDL failed to aggregate DMPG vesicles. Aggregation of DMPG LUVs by LDL did depend on the ionic strength of the medium as well as on the phase state of the lipid. More specifically, below the main transition temperature Tm maximal aggregation was seen in the presence of 25-100 mM NaCl, whereas slightly higher (up to 150 mM) [NaCl] were required when T>Tm. Aggregation due to LDL was also observed for dimyristoylphosphatidylserine as well as for dipalmitoylphosphatidylglycerol LUVs, whereas liposomes composed of either unsaturated acidic phospholipids or different phosphatidylcholines were not aggregated. Involvement of electrostatic attraction between the acidic phosphate of DMPG and cationic residues in apoB is suggested by the finding that increasing the content of dimyristoylphosphatidylcholine (DMPC) in DMPG liposomes reduced their aggregation and at XDMPC=0.50 no response was evident. Notably, increasing the mole fraction of 1-palmitoyl-2-oleyl-PG (POPG) in DMPG LUVs progressively reduced their aggregation by LDL and at XPOPG=0.50 there was complete inhibition. The latter effect of POPG is likely to be due to augmented hydration of the unsaturated lipid constituting a barrier for the contact between apoB and the vesicle surface. In keeping with this view, the presence of the strongly hygroscopic polymer, poly(ethylene glycol) at 1% (by weight) enhanced the aggregation and could partly reverse the inhibition by POPG.

Proteoglycans in macrophages: characterization and possible role in the cellular uptake of lipoproteins

The murine macrophage cell line J774 was incubated with [35S]sulphate. The cell-associated 35S-labelled macromolecules were shown to be proteoglycans and glycosaminoglycans in similar amounts. The possible presence of cell-surface proteoglycans was investigated by incubating [35S]sulphate-labelled cells with trypsin for 15 min. The released material contained approx. 70% free glycosaminoglycan chains and 30% proteoglycans. The latter component was demonstrated by HNO2 treatment to contain heparan sulphate. In the total cell fraction not treated with trypsin a small but significant portion was shown to be chondroitin sulphate proteoglycan. The cell-associated glycosaminoglycans contained both chondroitin sulphate and heparan sulphate. To investigate possible biological functions of cell-surface proteoglycans in macrophages, cells were incubated with NaClO3 to inhibit sulphation of proteoglycans and beta-d-xyloside to abrogate proteoglycan expression. The uptake of oxidized 125I-tyraminylcellobiose-labelled low-density lipoprotein (125I-TC-LDL) was typically two to three times higher than that of native 125I-TC-LDL in untreated J774 cells. The cellular uptake at 37 degreesC of native 125I-TC-LDL was decreased 25% after both NaClO3 and xyloside treatment, whereas the uptake of oxidized 125I-TC-LDL was decreased 35% after both types of treatment. The mRNA levels for the scavenger receptor A-II and the LDL receptor were not affected by NaClO3 or xyloside treatment. Furthermore, fluid-phase endocytosis, measured as uptake of horseradish peroxidase, and receptor-mediated endocytosis, measured as uptake of 125I-TC-ovalbumin, were not affected by NaClO3 treatment of J774 cells. Removal of cell-surface chondroitin sulphate with chondroitinase ABC decreased only the binding of native 125I-TC-LDL, whereas removal of heparan sulphate with heparitinase decreased the binding of both oxidized and native 125I-TC-LDL. Addition of lipoprotein lipase increased the uptake of oxidized 125I-TC-LDL 1.7 times and the uptake of native 125I-TC-LDL 2.1 times. The binding of the former was more sensitive to NaClO3 treatment than the latter. The results presented support the notion that some of the uptake pathways for lipoproteins in the foam-cell-forming macrophages depend on the presence of cell-surface heparan sulphate and chondroitin sulphate.

Scavenger receptors are present on rabbit aortic endothelial cells in vivo

Endothelial cells metabolize modified LDL, but attempts to detect scavenger receptors in this cell type in vitro have been unsuccessful. To determine whether scavenger receptors are present on endothelial cells in vivo, species-specific reagents were developed to detect rabbit scavenger receptor protein. Antiserum against the rabbit scavenger receptor was generated with the use of synthetic peptides of two distinct regions: residues 3 to 21 in the cytoplasmic tail and residues 282 to 304 in the collagen-like region. Reactivity of antiserum against the synthetic peptides was confirmed with an enzyme-linked immunosorbent assay. Positive reactivity was also observed against fragments of scavenger receptor protein expressed in bacteria. Antiserum to both regions reacted with liver membrane proteins of sizes consistent with the scavenger receptor, as confirmed by Western blotting under reduced and nonreduced conditions. Immunocytochemical examination of rabbit aortic tissue by use of antiserum to both regions of scavenger receptor protein produced striking and identical patterns of staining of aortic endothelium. Immunostaining was abolished for both antisera by preadsorption with the specific peptide region used as immunogen. In contrast, incubation of scavenger receptor antiserum with a peptide of a region of the rabbit LDL receptor failed to influence immunoreactivity against endothelium. These data demonstrate the presence of scavenger receptors in rabbit endothelium in vivo, which may have fundamental implications for lipoprotein metabolism by the arterial wall.

Isolation and characterization of a proteoglycan variant from human aorta exhibiting a marked affinity for low density lipoprotein and demonstration of its enhanced expression in atherosclerotic plaques

Proteoglycans (PG) are implicated in the pathophysiology of atherosclerosis due to their ability to complex with plasma low density lipoproteins (LDL). Studies were conducted to determine whether human aorta contains PG subclasses that exhibit enhanced LDL binding ability. PG were isolated from normal and atherosclerotic aortas by a combination of dissociative extraction and ion-exchange chromatography. The PG were further subfractionated on an LDL affinity column based on their binding affinity to LDL. Two PG fractions exhibiting high-affinity binding to LDL, as evidenced by their elution at 1.0 and 1.5 M NaCl, respectively, were isolated from both normal and atherosclerotic tissue. Compared with normal tissue, atherosclerotic tissue showed a twofold increase in the high-affinity PG that eluted at 1.5 M NaCl. Gel filtration of the high-affinity PG from normal tissue yielded two peaks (nPG2 and nPG3), while the high-affinity PG from plaque tissue was resolved into three peaks (pPG1, pPG2, and pPG3). pPG1 eluted at the void volume of the column, indicating that it was of very large molecular size. The hydrodynamic size of pPG2 was larger than that of the corresponding nPG2 (Kav = 0.44 versus 0.51), while pPG3 had the same hydrodynamic size as nPG3 (Kav = 0.86). The high-affinity PG subfractions from normal aorta contained varying proportions of chondroitin sulfates, dermatan sulfates, and heparan sulfate. In contrast, the PG subfractions from plaque tissue contained predominantly chondroitin sulfates and heparan sulfate. In vitro complexes of LDL and the high-affinity PG fractions from normal aorta and plaque tissue stimulated cholesteryl ester synthesis in human monocyte-derived macrophages. However, the LDL-plaque PG complex was significantly more potent than the LDL-normal aorta PG complex in this respect. These results indicate that PG subclasses with enhanced binding affinity to LDL occur in the normal human aorta and that their concentration increases significantly in atherosclerotic lesions. In addition, the high-affinity PG in plaque tissue have altered characteristics and increased ability to stimulate LDL-mediated cholesterol ester synthesis in macrophages. This could lead to increased lipid deposition during atherogenesis.

Cholesterol-dependent changes of glycosaminoglycan pattern in human aorta

Glycosaminoglycans are regular constituents of the arterial wall and essential for its structure and function. The arteriosclerosis-dependent changes of glycosaminoglycans were investigated, the degree of arteriosclerosis was monitored by the cholesterol content of the tissue. Histological characterization was achieved by electron microscopy. Total glycosaminoglycans were isolated from 33 delipidated segments of human aorta thoracica after exhaustive proteolytic digestion, and fractionated into the individual glycosaminoglycans by a multistep purification procedure. Chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), and hyaluronate (HA) were identified and quantified by chemical and enzymatic analysis. The concentration of total and individual glycosaminoglycans, expressed as mg/g delipidated dry weight of tissue, decreased significantly with increasing cholesterol content of tissue (p = 0.0005-0.005). The extent of decrease differed between the individual glycosaminoglycans as indicated by a shift in the CS/DS:HA:HS ratio from 47:32:21 in low cholesterol aortic segments to 59:29:12 in cholesterol-rich specimens. Determination of the relative molecular masses (Mr) revealed 58 kDa for CS/DS and 92 kDa for HS with a (statistically not significant) increase of the molecular mass of CS/DS and a decrease of HS with increasing cholesterol content. The copolymeric CS/DS glycosaminoglycans were disintegrated enzymatically into CS and DS containing fragments. A significantly higher relative DS content (p = 0.01) was found in cholesterol-rich arterial tissue (32.5%) as compared with low cholesterol tissue samples (28.8%). Cell culture experiments revealed that human arterial HS is able to inhibit the proliferation of cultured human arterial smooth muscle cells. The HS concentration required for a 30% inhibition of smooth muscle cell proliferation was in the same order as the tissue concentration of HS. This confirms the function of HS as an endogenous inhibitor of cell division and its impact for the development of atherosclerosis.

Lipoprotein lipase stimulates the binding and uptake of moderately oxidized low-density lipoprotein by J774 macrophages

Lipoprotein lipase (LPL) stimulates the uptake of low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL) in different cell types, including macrophages, through bridging of LPL between lipoproteins and extracellular heparan sulphate proteoglycans (HSPG). Because macrophages produce LPL and because modified lipoproteins are present in the arterial wall in vivo, we wondered whether LPL also enhances the uptake of oxidized LDL by J774 macrophages. LDL samples with different degrees of oxidation, as evaluated by relative electrophoretic mobility (REM) as compared with native LDL are used as well as native and acetylated LDL. Addition of 5 microg/ml LPL to the J774 cell culture medium stimulated the binding of both native LDL and moderately oxidized LDL (REM < 3.5) 50-100-fold, and their uptake was stimulated approx. 20-fold. The LPL-mediated binding of native LDL and moderately oxidized LDL was dose-dependent. Preincubation of the cells with heparinase (2.4 units/ml) inhibited the stimulatory effect of LPL, indicating that this LPL-mediated stimulation was due to bridging between the lipoproteins and HSPG. The binding to J774 macrophages of severely oxidized LDL (REM=4.3) was stimulated less than 3-fold by LPL, whereas its uptake was not stimulated significantly. The binding and uptake of acetylated LDL (AcLDL) were not stimulated by LPL, although the LPL-molecule itself does bind to AcLDL. Measurements of the cellular lipid content showed that addition of LPL also stimulated the accumulation in the cells of cholesteryl ester derived from both native LDL and moderately oxidized LDL in a dose-dependent manner. We conclude that our results present experimental evidence for the hypothesis that LPL serves as an atherogenic component in the vessel wall.

Association of negatively-charged phospholipids with low-density lipoprotein (LDL) increases its uptake and the deposition of cholesteryl esters by macrophages

LDL, the major carrier of cholesterol in blood, is poorly metabolized by macrophages. In contrast, macrophages can recognize and endocytose anionic phospholipids such as phosphatidylserine, phosphatidylglycerol and cardiolipin. Since macrophages can take up large amounts of these phospholipids, experiments were performed to ascertain whether pre-incubation of native LDL with negatively-charged phospholipids would enhance the metabolism of LDL by macrophages. When 125I-LDL was incubated with cardiolipin liposomes for 18 h at 37 degrees C before addition to macrophages, an approx. 40-fold increase of LDL metabolism by these cells was observed. Similar results were found when LDL was pre-incubated with phosphatidylserine or phosphatidylglycerol; however, pre-incubation of LDL with phosphatidylcholine liposomes did not lead to an increase of LDL metabolism. The macrophage uptake of LDL pre-incubated with cardiolipin was reduced to approx. 40% of control values in the presence of dextran sulfate and fucoidin, inhibitors of anionic phospholipid uptake. Cytochalasin D, an inhibitor of phagocytosis, reduced the lysosomal degradation of LDL pre-incubated with cardiolipin to approx. 10% of control values. When the LDL-cardiolipin mixture was chromatographed on agarose gel, two peaks containing LDL were observed in the elution profile: the first peak appeared at the void volume and the second peak was detected just ahead of native LDL. The LDL in both peaks was much more extensively metabolized by macrophages than was native LDL; the LDL in the first peak was metabolized at a rate that was 8 times the second peak. The results demonstrate that negatively-charged phospholipids can form a complex with LDL which facilitates its phagocytosis by macrophages.

The injection of heparin prolongs the plasma clearance of oxidized low density lipoprotein in the rat

There is evidence that oxidized-LDL plays an important role in atherogenesis. We now report on the in vivo interaction between unfractionated heparin and oxidized LDL in rats. The recovery rates of the native LDL particles ranged between 75% and 85% of the injected dose. Heparin did not interfere with the clearance rates of native LDL. After administration of radioactive labeled oxidized-LDL particles, 26% of the material was measured in circulation after 5 minutes, 8% after 20 minutes, and 3% after 60 minutes. After injection of heparin 2 minutes prior to oxidized-LDL tracer particles, 44% of the tracer was found in blood after 5 minutes, 23% after 20 minutes, and 9% after 60 minutes. Oxidized-LDL tracer particles disappeared from blood with an alpha half-life of 5 minutes and a beta half-life of 7.5 minutes. After receptor blocking with unfractionated heparin the alpha half-life of the oxidized-LDL tracer was prolonged to 17.5 minutes and the beta half-life to 27.5 minutes. These results indicate that heparin molecules of a comparatively small molecular weight competed the scavenger receptor mediated uptake of oxidized-LDL particles in vivo. Oxidized-LDL particles are known to mediate their pro-atherosclerotic activity in part by stimulating smooth muscle cell proliferation by a scavenger receptor-mediated pathway. It can be speculated, if heparins interfere with the uptake of oxidized-LDL, heparins might thus in part exert their known antiatherosclerotic properties.

Proteoglycan form of macrophage colony-stimulating factor binds low density lipoprotein

We recently isolated a proteoglycan form of macrophage colony-stimulating factor (PG-M-CSF) that carries a chondroitin sulfate glycosaminoglycan chain. Here, we examined the interaction of PG-M-CSF with low density lipoprotein (LDL). When LDL preincubated with PG-M-CSF was fractionated by molecular size sieving chromatography, it was eluted earlier than untreated LDL. When LDL was preincubated with chondroitin sulfate-free 85-kD M-CSF instead of PG-M-CSF, the elution profile of LDL remained unchanged, indicating specific interaction between PG-M-CSF and LDL. The level of PG-M-CSF binding in the wells of a plastic microtitration plate precoated with LDL was significant, this binding being completely abolished by pretreatment of PG-M-CSF with chondroitinase AC, which degrades chondroitin sulfate. The addition of exogenous chondroitin sulfate or apolipoprotein B inhibited the binding of PG-M-CSF to LDL in a dose-dependent manner, indicating that the interaction between PG-M-CSF and LDL was mediated by the binding of the chondroitin sulfate chain of PG-M-CSF to LDL apolipoprotein B. PG-M-CSF was also demonstrated in the arterial wall, and there were increased amounts of PG-M-CSF in atherosclerotic lesions. The in vitro interaction between PG-M-CSF and LDL thus appears to have physiological significance.

Regulation of the metabolism of lipoprotein-proteoglycan complexes in human monocyte-derived macrophages

Studies were performed to evaluate the effect of several factors on the metabolism of lipoprotein-proteoglycan complexes in human monocyte-derived macrophages. In vivo apoB-lipoprotein-proteoglycan complex was isolated from human aorta fibrous-plaque lesions and low-density lipoprotein (LDL)-proteoglycan complex was formed in vitro. Degradation of LDL-proteoglycan complex and cholesteryl ester synthesis mediated by the in vivo and in vitro complexes were lowest in freshly isolated monocytes. With the maturation of monocytes into macrophages, there was a dramatic rise in both. The degradation of the complex and the resultant stimulation of cholesterol esterification increased significantly with increasing cell density. Preincubation of macrophages in medium containing lipoprotein cholesterol did not down-regulate the subsequent degradation of LDL-proteoglycan complex. Macrophage-conditioned medium had a profound stimulatory effect on the degradation of LDL-proteoglycan complex and cholesterol esterification by mature macrophages and freshly isolated monocytes. The conditioned medium lost its stimulatory activity after boiling, dialysis and trypsin digestion. Macrophage activation with phorbol ester and bacterial lipopolysaccharide resulted in a marked suppression of the binding and degradation of the complex, as well as the complex-mediated cholesteryl ester synthesis. These results demonstrate that several factors regulate the metabolism of lipoprotein-proteoglycan complexes in human monocyte-derived macrophages.