Interaction of a high-affinity heparin subfraction with low-density lipoprotein stimulates cholesteryl ester accumulation in mouse macrophages

Are novel scavenger-like receptors involved in the hepatic uptake of heparin?

Heparin is an anionic macromolecular drug. It has been widely used as an anticoagulant, and numerous efforts to clarify the mechanism of its disposition in the body have been made to help expand its clinical applications, using its newly found biological activities, as well as to further improve its use in anticoagulant therapy. It has now been shown that heparin is taken up extensively not only by Kupffer cells but also by parenchymal cells in the liver, the major distribution organ, and a receptor-mediated endocytotic mechanism, which is shared by heparin analogs and various anionic macromolecules, is responsible for heparin uptake in both types of cells. Although the characteristics of the receptors for heparin in both cells have lots of similarities to those of scavenger receptors, the receptors in parenchymal cells do not accept acetylated low density lipoprotein (Ac-LDL) as a ligand, which is the only striking difference between them and major scavenger receptors. Although the receptors in Kupffer cells, which accept Ac-LDL as a ligand, may belong to class A scavenger receptors, this remains to be established. We therefore conclude at present that it is likely that novel scavenger-like receptors for heparin (heparin receptors) or unidentified scavenger receptors are responsible for heparin uptake in the liver.

Surface Plasmon Resonance Measurement of the Binding of Low-Density Lipoprotein at a Heparin Surface

Low-density lipoprotein (LDL) adsorption to heparin-like surface is of wide clinical interest since such a mechanism might be responsible for cholesterol accumulation in the arterial wall. By modifying the surface plasmon resonance sensor surface with heparin and albumin (BSA) LDL adsorption to this surface was investigated and characterized. Heparin was seen to be a potentially useful ligand for LDL detection and analysis in a clinical context. It was found that N-acetylheparin had a lower affinity for LDL than heparin and that the binding strength of LDL to N-acetylheparin was reduced. Assuming a random distribution of heparin on the surface, it was calculated from the data obtained that a maximum of 4.0 x 10(9) heparin or 4.8 x 10(9) N-acetylheparin "rods" can be found on a millimeter square and that one LDL molecule (380 nm(2)) covers on average only 1.5 heparin molecules or 1.8 N-acetylheparin molecules, yet maximum LDL binding cannot be increased beyond a surface coverage of 7.5 or 5.8% for heparin and N-acetyl heparin, respectively. This could lead to the suggestion that the glycosaminoglycan-LDL atheroclerosis mechanism would involve only 1-2 heparin molecules in "binding" each LDL, but 20-30 molecules are required to attract it to the surface in the first place. Copyright 1999 Academic Press.

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.

Circulating plasma xanthine oxidase contributes to vascular dysfunction in hypercholesterolemic rabbits

Reactive oxygen species play a central role in vascular inflammation and atherogenesis, with enhanced superoxide (O2.-) production contributing significantly to impairment of nitric oxide (.NO)-dependent relaxation of vessels from cholesterol-fed rabbits. We investigated potential sources of O2.- production, which contribute to this loss of endothelium-dependent vascular responses. The vasorelaxation elicited by acetylcholine (ACh) in phenylephrine-contracted, aortic ring segments was impaired by cholesterol feeding. Pretreatment of aortic vessels with either heparin, which competes with xanthine oxidase (XO) for binding to sulfated glycosaminoglycans, or the XO inhibitor allopurinol resulted in a partial restoration (36-40% at 1 muM ACh) of ACh-dependent relaxation. Furthermore, O2.(-)-dependent lucigenin chemiluminescence, measured in intact ring segments from hypercholesterolemic rabbits, was decreased by addition of heparin, allopurinol or a chimeric, heparin-binding superoxide dismutase. XO activity was elevated more than two-fold in plasma of hypercholesterolemic rabbits. Incubation of vascular rings from rabbits on a normal diet with purified XO (10 milliunits/ml) also impaired .NO-dependent relaxation but only in the presence of purine substrate. As with vessels from hypercholesterolemic rabbits, this effect was prevented by heparin and allopurinol treatment. We hypothesize that increases in plasma cholesterol induce the release of XO into the circulation, where it binds to endothelial cell glycosaminoglycans. Only in hypercholesterolemic vessels is sufficient substrate available to sustain the production of O2.- and impair NO-dependent vasorelaxation. Chronically, the continued production of peroxynitrite, (ONOO-) which the simultaneous generation of NO and O2.- implies, may irreversibly impair vessel function.

Marked alteration of proteoglycan metabolism in cholesterol-enriched human arterial smooth muscle cells

To elucidate the correlation between vascular cholesterol metabolism and proteoglycan (PrGl) biosynthesis, we investigated PrGl synthesis in human aortic smooth muscle cells (SMCs) after cholesterol enrichment with cationized low-density lipoproteins (LDL). Compared with normal SMCs, total PrGl synthesis by cholesterol-enriched cells decreased 2.4-fold (11874 +/- 530 d.p.m. per 10(5) cells compared with 4890 +/- 385 d.p.m. per 10(5) cells). This was the net result of a 6.9-fold reduction in medium PrGl (11000 +/- 490 d.p.m. per 10(5) cells compared with 1580 +/- 246 d.p.m. per 10(5) cells) and a 3.8-fold increase in cellular PrGl over controls (874 +/- 27 d.p.m. per 10(5) cells compared with 3310 +/- 193 d.p.m. per 10(5) cells). Prior incubation of SMCs with native LDL had no effect on PrGl synthesis by these cells. The decrease in PrGl synthesis in cholesterol-enriched cells correlated with a 90% and 20% reduction in the steady-state level of mRNA for biglycan and decorin respectively, and a virtual elimination of the steady-state level of mRNA for versican over controls. Despite the down-regulation of PrGl synthesis, cholesterol-loaded cells produced a 2-fold increase in a PrGl subfraction with high affinity for LDL. Compared with the corresponding PrGl subfraction from normal cells, that from the cholesterol-enriched cells exhibited increased charge density and a higher molecular mass and contained relatively larger proportions of chondroitin 6-sulphate and dermatan sulphate. These results show that PrGl metabolism is dramatically altered in cholesterol-enriched human SMCs.

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.

Enhanced synthesis and accumulation of proteoglycans in cholesterol-enriched arterial smooth muscle cells

To determine the effects of lipid accumulation on proteoglycan synthesis, we studied proteoglycan biosynthesis in rabbit aortic smooth muscle cells in culture. Cholesterol-enrichment was accomplished by incubating confluent smooth muscle cells with cationized low-density lipoprotein. Control and cholesterol-enriched cells were incubated with [35S]sulphate, [3H]glucosamine, or [3H]serine. Metabolically labelled proteoglycans in the cell layer and medium were quantified. During a 20 h incubation period, proteoglycan synthesis in cholesterol-enriched cells increased by 40-50% above that in control cells. A similar increase in precursor incorporation into proteoglycans was also noted following a short 15 min pulse. The cholesterol-enriched cells also showed a 45-50% increase over control rates in the intralysosomal accumulation of a large chondroitin sulphate proteoglycan and a small dermatan sulphate proteoglycan. The enhanced synthesis of proteoglycans in cholesterol-enriched cultures was inhibited by cycloheximide and actinomycin D, which are inhibitors of protein synthesis and transcription respectively. Proteoglycan turnover was investigated by pulse-chase analysis. Following a 2-h pulse, intracellular proteoglycans in cholesterol-enriched cells disappeared, having a half-life of 26.5 h compared with 2.8 h for those in the control cells. The amount of trypsin-releasable proteoglycan was significantly reduced in cholesterol-enriched cells. In addition, the degradation of proteoglycans was severely retarded in cholesterol-enriched cultures. The activities of three acid hydrolases, N-acetyl-beta-hexosaminidase, beta-glucuronidase and cathepsin C, were significantly reduced in cholesterol-enriched cells compared with activities in control cells. The results indicate that proteoglycan metabolism is altered in cholesterol-enriched smooth muscle cells.

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

We have shown recently that lipoprotein-proteoglycan complexes isolated from human atherosclerotic lesions stimulated cholesteryl ester synthesis in human monocyte-derived macrophages [Vijayagopal, Srinivasan, Radhakrishnamurthy and Berenson (1992) Arterioscler. Thromb. 12, 237-249]. The present study was conducted to determine the mechanism of cellular uptake of the complexes. A chondroitin sulphate-dermatan sulphate proteoglycan was isolated from normal human aorta and complexed to 125I-labelled human low-density lipoprotein (LDL). The binding and degradation of 125I-LDL-proteoglycan complex were then studied in human monocyte-derived macrophages. The specific binding and degradation of the complex showed saturability and concentration-dependency. The Kd for binding was 1.5 x 10(-8) M, which was greater than that reported for LDL in monocyte-derived macrophages. Binding of the complex was not subject to down-regulation. Chloroquine inhibited degradation of the complex and the resultant stimulation of cholesteryl ester synthesis. Limited treatment of macrophages with proteolytic enzymes abolished binding and degradation of the complex significantly. Macrophages bound 125I-methyl-LDL-proteoglycan complex to the same extent as 125I-LDL-proteoglycan complex. Excess LDL and proteoglycan did not compete against the binding of the complex; however, excess acetyl-LDL competed for 61% of the binding. Likewise, excess LDL-proteoglycan complex inhibited the binding of 125I-acetyl-LDL by 64%. Polyinosinic acid and cytochalasin D inhibited the binding of 125I-LDL-proteoglycan complex by 60% and 36% respectively. Compared with that of acetyl-LDL, the degradation of LDL-proteoglycan complex was retarded in human macrophages. The results indicate that the uptake of LDL-proteoglycan complex in human monocyte-derived macrophages is not mediated through binding to the LDL receptor; but occurs predominantly via the scavenger receptor, with phagocytosis playing a minor role in the process.

Hepatic uptake of a modified low molecular weight heparin in rats

Fractionated and unfractionated heparins are widely used as antithrombotic agents. Because of their heterogeneous composition, it is difficult to study the pharmacokinetics of these drugs. We now report on a new method for labeling low molecular weight heparins with 131I by binding tyramine to the anhydromannose end of the molecules. We examined the pharmacokinetics of the compound by intravenous injection of 131I-tyramine-heparin into Wistar rats. About 18% of the activity was found in the liver, whereas 33% was detected in urine. Biological activity in terms of Factor Xa inhibition was measurable. Since evidence from cell culture experiments implies that reticuloendothelial cell system receptors might be involved in heparin metabolism, maleylated BSA, a substance known to block scavenger receptors, was injected before the radiolabeled heparin compound. The liver uptake was reduced from 17.4 to 4.8%. Injection of unfractionated heparin before tracer application caused a considerable increase in urine excretion of the tracer substance. To our knowledge, this is the first report that liver uptake of heparins is linked to scavenger receptor mediated mechanisms in vivo. This interaction of heparins with scavenger receptors might play an important role in the biology of the vessel wall.

Lipoprotein-proteoglycan complexes from atherosclerotic lesions promote cholesteryl ester accumulation in human monocytes/macrophages

Lipoprotein-proteoglycan complexes from human atherosclerotic lesions were studied to determine their ability to stimulate cholesteryl ester accumulation in human monocytes/macrophages. Complexes containing apolipoprotein (apo) B lipoproteins and proteoglycans were extracted from fatty streaks and fibrous plaque lesions of human aortas by extraction with 0.15 M NaCl. Fractionation of the complex with Bio-Gel A-50m yielded a single fraction from fatty streaks and two fractions from fibrous plaques. The complexes were further purified by anti-apo B affinity chromatography and analyzed for apolipoproteins, lipids, and glycosaminoglycans Apo B was the only apolipoprotein present in the complexes. Although the complexes from fatty streaks and fibrous plaques contained varying proportions of hyaluronic acid, chondroitin 6-sulfate, and dermatan sulfate, heparin was present in only the fibrous plaque complexes. All three lipoprotein-proteoglycan complexes increased the rate of incorporation of [14C]oleate into cholesteryl [14C]oleate and stimulated cholesteryl ester accumulation in monocytes/macrophages. However, the complexes from fibrous plaques were more potent than those from fatty streaks in this regard. Cholesteryl ester synthesis that is mediated by the uptake of the complexes was dose dependent and showed apparent saturation, suggesting that cell surface binding may be required. Chloroquine, a lysosomotropic agent, inhibited cholesteryl ester synthesis that is induced by the complexes, indicating that lysosomal hydrolysis was essential. Cholesteryl ester synthesis that is mediated by the complexes was inhibited 70-79% by polyinosinic acid. Furthermore, excess unlabeled fibrous plaque complexes significantly inhibited the binding and internalization of in vitro 125I-low density lipoprotein (LDL)-proteoglycan complexes and 125I-acetylated-LDL and not 125I-LDL. These results suggest the involvement of the scavenger receptor in the uptake of the complexes. Phagocytosis played a minor role in the metabolism of these ligands because cytochalasin D inhibited cholesteryl ester synthesis, which is mediated by fibrous plaque complexes, by 7.5-25%. Cholesteryl ester synthesis increased linearly over 32 hours in macrophages incubated with the complexes, indicating an apparent lack of downregulation of binding sites. This resulted in the appearance of intracellular oil red O-positive lipid droplets. These studies show for the first time that apo B lipoprotein-proteoglycan complexes isolated from human atherosclerotic lesions can induce cholesteryl ester accumulation in monocytes/macrophages.

Studies on the mechanism of uptake of low density lipoprotein-proteoglycan complex in macrophages

Earlier, we (Vijayagopal, P. et al. (1988) Biochim. Biophys. Acta 960, 210) showed that mouse peritoneal macrophages metabolize low density lipoprotein (LDL)-proteoglycan complex by a receptor pathway distinct from the acetyl-LDL receptor. Further studies were conducted to probe further into the mechanism of LDL-proteoglycan complex uptake by macrophages. Both 125I-methyl-LDL-proteoglycan complex and 125I-LDL-proteoglycan complex were taken up and degraded by the cells to the same extent. Similarly, the ability of these ligands to stimulate cholesteryl ester synthesis was also indistinguishable. These results rule out the possibility of apoB,E receptor involvement in the uptake of LDL-proteoglycan complex in macrophages. Sodium fluoride, cytochalasin D and aggregated LDL inhibited degradation of the complex by 24%, 26% and 28%, respectively, indicating that phagocytosis is only a minor pathway for the uptake. Both binding and degradation of the complex were not inhibited by excess hyaluronic acid suggesting that ligand recognition was not through hyaluronic acid binding sites. As compared to acetyl-LDL, the cellular degradation of LDL-proteoglycan complex was retarded. Macrophages exhibited a rapid stimulation of [3H]inositol trisphosphate (IP3) release and diacylglycerol production when incubated with LDL-proteoglycan complex. Furthermore, pertussis toxin produced a 62% inhibition of LDL-proteoglycan complex mediated IP3 release, suggesting that LDL-proteoglycan complex metabolism in macrophages is dependent upon the G-protein coupled signal transduction mechanism. These results show that receptor mediated endocytosis plays a major role in the metabolism of LDL-proteoglycan complex in macrophages.