A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection

Innate recognition systems in insect immunity and development: new approaches in Drosophila

Phagocytosis is important in immune defense and development in vertebrates and invertebrates. It leads to swift ingestion of microorganisms and dying cells by phagocytes. How particles are recognized is not well understood. Studies in insects, and Drosophila in particular, argue that these are powerful model systems to dissect this process.

Macrophage scavenger receptor class A: A multifunctional receptor in atherosclerosis

In atherogenesis, elevated plasma levels of low density lipoprotein (LDL) lead to the chronic presence of LDL in the arterial wall. There, LDL is modified (eg, oxidized), and these modified lipoproteins activate endothelial cells, which attract circulating monocytes. These monocytes enter the vessel wall, differentiate into macrophages, and subject the modified lipoproteins to endocytosis through scavenger receptor pathways. This unrestricted uptake, which is not limited by intracellular cholesterol levels, eventually leads to the formation of lipid-filled foam cells, the initial step in atherosclerosis. Macrophage scavenger receptor class A (SRA) is thought to be one of the main receptors involved in foam cell formation, mediating the influx of lipids into the macrophages. In addition to this role in modified lipoprotein uptake by macrophages, the SRA has been shown to be important in the inflammatory response in host defense, cellular activation, adhesion, and cell-cell interaction. Given the importance of these processes in atherogenesis, these latter functions may prove to make the SRA a multifunctional player in the atherosclerotic process.

Protection from lethal gram-positive infection by macrophage scavenger receptor-dependent phagocytosis

Infections with gram-positive bacteria are a major cause of morbidity and mortality in humans. Opsonin-dependent phagocytosis plays a major role in protection against and recovery from gram-positive infections. Inborn and acquired defects in opsonin generation and/or recognition by phagocytes are associated with an increased susceptibility to bacterial infections. In contrast, the physiological significance of opsonin-independent phagocytosis is unknown. Type I and II class A scavenger receptors (SR-AI/II) recognize a variety of polyanions including bacterial cell wall products such as lipopolysaccharide (LPS) and lipoteichoic acid (LTA), suggesting a role for SR-AI/II in innate immunity to bacterial infections. Here, we show that SR-AI/II-deficient mice (MSR-A(-/-)) are more susceptible to intraperitoneal infection with a prototypic gram-positive pathogen, Staphylococcus aureus, than MSR-A(+/+) control mice. MSR-A(-/-) mice display an impaired ability to clear bacteria from the site of infection despite normal killing of S. aureus by neutrophils and die as a result of disseminated infection. Opsonin-independent phagocytosis of gram-positive bacteria by MSR-A(-/-) macrophages is significantly decreased although their phagocytic machinery is intact. Peritoneal macrophages from control mice phagocytose a variety of gram-positive bacteria in an SR-AI/II-dependent manner. Our findings demonstrate that SR-AI/II mediate opsonin-independent phagocytosis of gram-positive bacteria, and provide the first evidence that opsonin-independent phagocytosis plays a critical role in host defense against bacterial infections in vivo.

Retrovirus-mediated, stable scavenger-receptor gene transfer leads to functional endocytotic receptor expression, foam cell formation, and increased susceptibility to apoptosis in rabbit aortic smooth muscle cells

The type II, class A macrophage scavenger receptor (SR-A) plays an important role in the pathogenesis of atherosclerosis and foam cell formation. However, its role in nonmacrophage cell lines remains unknown. To test the hypothesis that SR-A activity leads to proatherogenic changes in nonmacrophage cell lines, we generated Moloney murine leukemia virus- and vesicular stomatitis virus G protein-pseudotyped retroviruses containing SR-A type II cDNA, which were used for stable transfection of SR-A activity into mouse fibroblasts and rabbit aortic smooth muscle cells (SMCs). beta-Galactosidase-transfected cell lines were used as controls. Transfected cell lines expressed functional SR-A mRNA and protein. Expression of SR-A activity was stable for at least 9 months. By electron microscopy, transfected receptors were located in coated pits and in intracellular structures resembling endocytotic vesicles. Expression of SR-A on the cell surface was verified by flow cytometry and by uptake and degradation of (125)I-labeled acetylated low density lipoprotein (LDL). Increases of 5- to 25-fold and of 6- to 8-fold in the rate of acetylated LDL degradation were observed in transfected fibroblasts and SMCs, respectively, compared with beta-galactosidase-transfected control cell lines. Incubation of the transfected SMCs and fibroblasts with acetylated or oxidized LDL led to foam cell formation. Incubation with oxidized LDL also led to increased apoptosis and cell death. An altered morphology with increased cell size and granularity was observed in the most active SR-A SMC clones. It is concluded that stable overexpression of SR-A leads to foam cell formation and other proatherogenic changes in nonmacrophage cell lines. Stable SMC and fibroblast cell lines can be used as models for foam cell formation. The results also suggest that increased SR activity may play an important role in SMC-related pathology in atherosclerotic arteries.

Inverse relationship between circulating oxidized low density lipoprotein (oxLDL) and anti-oxLDL antibody levels in healthy subjects

Oxidized low density lipoprotein (oxLDL) has been implicated in the pathogenesis of atherosclerosis. Recent studies have shown that immunization of animals with oxLDL results in suppression of atherogenesis. Antibody against oxLDL (oxLDL Ab) is detectable in human sera, although its biological significance is not well established. We examined the relationship between oxLDL Ab titer and circulating oxLDL level in 130 healthy Japanese subjects. OxLDL was measured as apolipoprotein (apo) B-containing lipoproteins carrying oxidized phosphatidylcholines by a sensitive ELISA. IgG class oxLDL Ab titer was measured by ELISA. Plasma oxLDL concentration was very low and it corresponded on average to one to two out of 1000 apoB-containing lipoproteins in plasma. Plasma oxLDL correlated positively with LDL cholesterol and inversely with oxLDL Ab titer. These associations remained significant and independent in multiple regression analysis including age, gender, smoking, and high-density lipoprotein cholesterol. These data indicate that healthy subjects have a very low concentration of oxLDL in the circulation, and that oxLDL Ab titer is in an inverse relationship with plasma oxLDL concentration in this population. Although these results suggest that oxLDL Ab may play a role in maintaining the low level of plasma oxLDL, its role in atherogenesis awaits further studies.

Stimulation of phospholipase D activity by oxidized LDL in mouse peritoneal macrophages

Oxidation of LDL is an important factor in the development of atherosclerosis. However, the mechanisms by which oxidized LDL exerts its atherogenic actions are poorly understood. In the present work, we show that oxidized LDL stimulates phospholipase D (PLD) activity in mouse peritoneal macrophages and that this effect increases with the degree of LDL oxidation. Oxidative modification of LDL results in the production of lipid peroxides and the conversion of phosphatidylcholine to lysophosphatidylcholine. Although we found that lysophosphatidylcholine alone activates PLD, the stimulation of this enzyme activity by oxidized LDL is independent of lysophosphatidylcholine formation. Also, 7-ketocholesterol, the major oxysterol in oxidized LDL, failed to stimulate PLD activity. To determine the mechanism(s) whereby oxidized LDL activates PLD, the possible involvements of protein kinase C and tyrosine phosphorylation were investigated. Pretreatment of macrophages with the protein kinase C inhibitor Ro-32-0432 or downregulation of protein kinase C activity by prolonged incubation with 100 nmol/L 4beta-phorbol 12-myristate 13-acetate did not alter the stimulatory effect of oxidized LDL on PLD activation. However, oxidized LDL stimulated tyrosine phosphorylation of several macrophage proteins, and preincubation of the macrophages with genistein, a tyrosine kinase inhibitor, blocked the activation of PLD by oxidized LDL. In addition, pretreatment with orthovanadate, which inhibits tyrosine phosphatases, enhanced basal and oxidized LDL-stimulated PLD activity. Pretreatment of macrophages with pertussis toxin decreased the stimulatory effect of oxidized LDL, indicating that GTP-binding proteins may also be involved in the activation of PLD by oxidized LDL. We also found that the platelet-activating factor receptor antagonists WEB 2086 and L-659,989 inhibit the oxidized LDL stimulation of PLD, suggesting a role for platelet-activating factor receptor in this process. The stimulation of the PLD pathway by oxidized LDL may be of importance in atherogenesis, because PLD activation leads to generation of important second messengers such as phosphatidate, lysophosphatidate, and diacylglycerol, which are known to regulate many cellular functions.

The processing of ligands by the class A scavenger receptor is dependent on signal information located in the cytoplasmic domain

The mechanisms that regulate the transport of the macrophage class A scavenger receptor during ligand uptake were investigated. Kinetic analysis of the changes in receptor phosphorylation demonstrated that serine phosphorylation increased during the internalization of acetyl-low density lipoproteins (LDL) by macrophages. The increase was maximal at about 2.5 min after the initiation of ligand uptake. Oxidized LDL also stimulated serine phosphorylation, but the relative increase was smaller and the time to maximum was shorter. Receptor mutants expressed in Chinese hamster ovary and COS cells showed that elimination of the potential phosphorylation site at Ser(21) increased acetyl-LDL metabolism, whereas inactivation of the site at Ser(49) reduced acetyl-LDL uptake. The increase in uptake by the Ser(21) mutant was due to an increase in surface receptor expression. In contrast, elimination of the site at Ser(49) did not affect receptor expression but slowed receptor internalization. To identify potential internalization signal sequences, beta-turn structure in the cytosolic domain was targeted for mutagenesis. Disruption of one region near Asp(25) inhibited receptor activity. The studies support a model whereby receptor internalization requires the presence of an internalization signal motif but that the rate of receptor internalization is governed by the pattern of receptor phosphorylation induced by the ligand.

Estrogen and progesterone reduce lipid accumulation in human monocyte-derived macrophages: a sex-specific effect

BACKGROUND

Males have an earlier onset and greater prevalence of clinical atherosclerosis than age-matched females, which is consistent with an atheroprotective effect of the female sex steroids, estrogen and progesterone. We therefore examined the effects of estrogen and progesterone on human foam cell formation, a key early event in atherogenesis.

METHODS AND RESULTS

Monocytes from healthy female and male donors were obtained from white cell concentrates and allowed to differentiate into macrophages over 10 days. These human monocyte-derived macrophages (MDMs) were exposed to either control (0.1% vol/vol ethanol) or estrogen or progesterone treatment on days 3 through 10. Lipid loading was achieved on days 8 through 10 by incubation with acetylated LDL. Lipid from the MDMs was then extracted for analysis of cholesteryl ester (CE) content. 17beta-Estradiol at both physiological (2 nmol/L) and supraphysiological (20 and 200 nmol/L) concentrations produced a significant reduction in macrophage CE content (88+/-3%, 88+/-2%, and 85+/-4%, respectively; P<0.02 compared with control). Physiological and supraphysiological levels of progesterone (2, 10, and 200 nmol/L) produced an even more dramatic reduction in CE content (74+/-9%, 56+/-10%, and 65+/-8%, respectively; P<0.002 compared with control). This effect could be abrogated by coincubation with the progesterone receptor antagonist RU486. Neither estrogen nor progesterone produced a reduction in lipid loading in male-donor-derived MDMs. Detailed lipid trafficking studies demonstrated that both estrogen and progesterone altered macrophage uptake and/or processing of modified LDL.

CONCLUSIONS

Physiological levels of estrogen and progesterone are associated with a female-sex-specific reduction in human macrophage lipid loading, which is consistent with an atheroprotective effect.

Macrophage specific overexpression of the human macrophage scavenger receptor in transgenic mice, using a 180-kb yeast artificial chromosome, leads to enhanced foam cell formation of isolated peritoneal macrophages

Macrophage scavenger receptors class A (MSR) are thought to play an important role in atherogenesis by mediating the unrestricted uptake of modified lipoproteins by macrophages in the vessel wall leading to foam cell formation. To investigate the in vivo role of the MSR in this process, a transgenic mouse model expressing both isoforms of the human MSR was generated. A 180-kb yeast artificial chromosome (YAC) containing the human MSR gene (MSR1) with 60- and 40-kb flanking sequence at the 5' and 3' end, respectively, was obtained by reducing the size of a 1050-kb YAC by homologous recombination. This 180-kb YAC was microinjected into mouse oocytes. In the resulting transgenic mice, high levels of mRNA for both type I and type II human MSR1 were detected in peritoneal macrophages and trace levels in other organs, known to contain macrophage-derived cells. Using an antibody against the human MSR, the Kupffer cells in the liver were shown to contain the MSR protein. In vivo clearance of acetyl-LDL was not changed in the MSR1-transgenic mice. However, in vitro studies using peritoneal macrophages from the transgenic mice showed a two-fold increased degradation of acetyl-LDL and cholesterolester accumulation concomitant with a four-fold increase in foam cell formation, as compared to wild-type macrophages. Thus, macrophage specific overexpression of the MSR may lead to increased foam cell formation, which is one of the initial and crucial steps in atherogenesis.

Structure and function of type I and II macrophage scavenger receptors

Type I and II macrophage scavenger receptors are implicated in the pathologic deposition of cholesterol during the atherogenesis. There is a charged collagen structure of type I and II receptors identified as a ligand binding domain, which can recognize a wide range of negatively charged macromolecules including oxidized LDL as well as damaged or apoptotic cells and pathogenic micro-organisms. After binding these ligands can be either internalized by endocytosis, phagocytosis, or remain at cell surface and mediate the adhesion. Under physiological condition, scavenger receptors serve to scavenge or clean up cellular debris and other related materials, as well as playing a role in the hosts defence. In pathological condition, they mediate the recruitment, activation and transformation of macrophages and other cells, which may be related to the development of atherosclerosis and to disorders caused by the accumulation of denatured materials, such as Alzheimer's disease.

Expression of scavenger receptor class A and CD14 in lipopolysaccharide-induced lung injury

CD14 and macrophage scavenger receptor class A type I and II (MSR-A) are receptors for lipopolysaccharide (LPS). In this study, the expressions of both receptors in the lung after administration of LPS in aerosol to mice with a nebulizer were observed. Bronchiolar epithelial cells and alveolar macrophages immediately incorporated LPS and expressed CD14. CD14-positive neutrophils then appeared in the alveolar space followed by the appearance of MSR-A-expressing cells in the vascular lumen, pulmonary interstitium, and alveolar space. Numbers of apoptotic cells increased after 1 day, and MSR-A-expressing macrophages actively incorporated apoptotic bodies. Daily administration of macrophage colony stimulating factor (M-CSF) to the mice resulted in increased levels of MSR-A expression and reduced levels of CD14 as well as several cytokine expressions, leading to shortening of the inflammatory process. The numbers of apoptotic cells were reduced in M-CSF injected mice. These findings imply that CD14 acts as an immediate expressing receptor for LPS and MSR-A exerts a protective function by scavenging LPS and apoptotic cells in LPS-induced lung injury.

Elimination of the class A scavenger receptor does not affect amyloid plaque formation or neurodegeneration in transgenic mice expressing human amyloid protein precursors

The class A scavenger receptor (SR) is expressed on reactive microglia surrounding cerebral amyloid plaques in Alzheimer's disease (AD). Interactions between the SR and amyloid beta peptides (Abeta) in microglial cultures elicit phagocytosis of Abeta aggregates and release of neurotoxins. To assess the role of the SR in amyloid clearance and Abeta-associated neurodegeneration in vivo, we used the platelet-derived growth factor promoter to express human amyloid protein precursors (hAPPs) in neurons of transgenic mice. With increasing age, hAPP mice develop AD-like amyloid plaques. We bred heterozygous hAPP (hAPP(+/-)) mice that were wild type for SR (SR(+/+)) with SR knockout (SR(-/-)) mice. Crosses among the resulting hAPP(+/-)SR(+/-) offspring yielded hAPP(+/-) and hAPP(-/-) littermates that were SR(+/+) or SR(-/-). These second-generation mice were analyzed at 6 and 12 months of age for extent of cerebral amyloid deposition and loss of synaptophysin-immunoreactive presynaptic terminals. hAPP(-/-)SR(-/-) mice showed no lack of SR expression, plaque formation, or synaptic degeneration, indicating that lack of SR expression does not result in significant accumulation of endogenous amyloidogenic or neurotoxic factors. In hAPP(+/-) mice, ablation of SR expression did not alter number, extent, distribution, or age-dependent accumulation of plaques; nor did it affect synaptic degeneration. Our results do not support a critical pathogenic role for microglial SR expression in neurodegenerative alterations associated with cerebral beta amyloidosis.

Inflammation and immune response in atherosclerosis

Atherosclerosis is an inflammatory disease with a significant autoimmune component. Studies using transgenic murine models have clarified that recruitment of mononuclear leukocytes through vascular leukocyte-adhesion molecules and chemokines, differentiation of monocytes to macrophages, and endocytosis through scavenger receptors all are of decisive importance for atherosclerosis in hypercholesterolemic mice. T and B cells modulate disease progression and lesion development is reduced in mice lacking adaptive immunity. In particular, local immune responses eliciting Th1 effector mechanisms appear to be proatherogenic, whereas protective immune responses can be induced by immunization with oxidized low-density lipoprotein. Thus, innate immunity is necessary for atherosclerosis, whereas adaptive immunity is an important modulator of disease development.

Nonoxidative modifications of lipoproteins in atherogenesis

The key initiating event in atherosclerosis is the retention of plasma lipoproteins in the subendothelial matrix. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. There is considerable evidence that many of these biological responses, notably macrophage cholesteryl ester loading (foam cell formation), require subendothelial modification of the retained lipoproteins. Oxidation of lipoproteins is one such modification that likely occurs in vivo and promotes certain atherogenic events, but oxidation cannot explain all aspects of atherogenesis, including certain elements of macrophage foam cell formation. For this reason, there has been renewed interest in other modifications of lipoproteins that may be important in atherogenesis. This review addresses five such lipoprotein modifications, namely aggregation, glycation, immune complex formation, proteoglycan complex formation, and conversion to cholesterol-rich liposomes. The focus is on the evidence that these modifications occur in atherosclerotic lesions and on the potential role of these modified lipoproteins in atherogenesis, with an emphasis on macrophage foam cell formation.

Influence of the high density lipoprotein receptor SR-BI on reproductive and cardiovascular pathophysiology

The high density lipoprotein (HDL) receptor SR-BI (scavenger receptor class B type I) mediates the selective uptake of plasma HDL cholesterol by the liver and steroidogenic tissues. As a consequence, SR-BI can influence plasma HDL cholesterol levels, HDL structure, biliary cholesterol concentrations, and the uptake, storage, and utilization of cholesterol by steroid hormone-producing cells. Here we used homozygous null SR-BI knockout mice to show that SR-BI is required for maintaining normal biliary cholesterol levels, oocyte development, and female fertility. We also used SR-BI/apolipoprotein E double homozygous knockout mice to show that SR-BI can protect against early-onset atherosclerosis. Although the mechanisms underlying the effects of SR-BI loss on reproduction and atherosclerosis have not been established, potential causes include changes in (i) plasma lipoprotein levels and/or structure, (ii) cholesterol flux into or out of peripheral tissues (ovary, aortic wall), and (iii) reverse cholesterol transport, as indicated by the significant reduction of gallbladder bile cholesterol levels in SR-BI and SR-BI/apolipoprotein E double knockout mice relative to controls. If SR-BI has similar activities in humans, it may become an attractive target for therapeutic intervention in a variety of diseases.

Genetic background determines the extent of atherosclerosis in ApoE-deficient mice

Two strains of ApoE-deficient mice were found to have markedly different plasma lipoprotein profiles and susceptibility to atherosclerosis when fed either a low-fat chow or a high-fat Western-type diet. FVB/NJ ApoE-deficient (FVB E0) mice had higher total cholesterol, HDL cholesterol, ApoA1, and ApoA2 levels when compared with C57BL/6J ApoE-deficient (C57 E0) mice. At 16 weeks of age, mean aortic root atherosclerotic lesion area was 7- to 9-fold higher in chow diet-fed C57 E0 mice and 3.5-fold higher in Western diet-fed C57 E0 mice compared with FVB E0 mice fed similar diets. Lesion area in chow diet-fed first-generation mice from a strain intercross was intermediate in size compared with parental values. The distribution of the lesion area in 150 chow diet-fed second-generation progeny spanned the range of the lesion area in both parental strains. There were no correlations between total cholesterol, non-HDL cholesterol, HDL cholesterol, ApoA1, ApoA2, ApoJ, or anti-cardiolipin antibodies and lesion area in the second-generation progeny. Thus, a genomic approach may succeed in identifying the genes responsible for the variation in atherosclerosis susceptibility in these 2 strains of ApoE-deficient mice, which could not be explained by measured plasma parameters.

Uptake of oxidized LDL by macrophages differs from that of acetyl LDL and leads to expansion of an acidic endolysosomal compartment

Accumulation of cholesterol by macrophage foam cells in atherosclerotic lesions is thought to involve the uptake of modified low density lipoproteins (LDLs). Previous studies have shown that there is impaired degradation of oxidized LDL in macrophages. The present study was done to determine whether the differences in intracellular metabolism of oxidized LDL and acetyl LDL were associated with delivery to different intracellular compartments. Mouse peritoneal macrophages were incubated with 1,1'-dioctadecyl-3, 3,3',3'-tetramethylindocarbocyanine perchlo- rate-labeled oxidized LDL or 3,3'-dioctadecyloxacarbocyanine perchlorate-labeled acetyl LDL and examined by fluorescence microscopy. Deconvolution image analysis showed <10% colocalization of the 2 lipoproteins at incubation times ranging from 30 minutes to 6 hours. Subcellular fractionation of macrophages after incubation with (99m)Tc-labeled oxidized LDL revealed accumulation of the tracer in a compartment with a d=1.042 g/mL, consistent with endosomes. Surprisingly, there was a concurrent dramatic shift of the density of lysosomal marker enzymes from d=1.1 g/mL to the same fractions that contained (99m)Tc, indicating that this compartment was formed after fusion with primary lysosomes. Parallel experiments in J774 cells, a murine macrophage-like cell line, did not show a similar density shift, perhaps because of the slower rate of accumulation of oxidized LDL by these cells. Fluorescence microscopy of macrophages labeled with a lysosomotropic dye revealed a marked expansion of the acidic compartment after exposure of cells to oxidized LDL. We conclude that oxidized LDL and acetyl LDL are internalized by morphologically distinct pathways. Furthermore, because of its impaired lysosomal degradation, oxidized LDL causes expansion of and a decrease in the density of the lysosomal compartment in macrophages.

Scavenger receptors and oxidized low density lipoproteins

Oxidized LDL has been shown to exhibit a number of potentially proatherogenic actions and properties, including receptor-mediated uptake and lipid accumulation within macrophages. It has been postulated that rapid, unregulated uptake of oxidatively modified LDL could account for the transformation of monocyte-derived macrophages to foam cells in atherosclerotic lesions. In support of this hypothesis, oxidized LDL and lipid peroxidation products have been shown to exist in atheromas in vivo. Furthermore, a number of cell membrane proteins that can bind oxidized LDL with high affinity have been identified on the surface of macrophages, endothelial cells and smooth muscle cells. One characteristic that almost all of these 'scavenger receptors' share is the ability to bind with high affinity to a broad spectrum of structurally unrelated ligands. Of all of the different classes of scavenger receptors that have been identified, the scavenger receptor class A type I/II (SR-AI/II) has received the most attention. Studies with macrophages from mice deficient in the gene for SR-AI/II provide direct evidence that a receptor other than the SR-AI/II is responsible for most of the uptake of oxidized LDL in murine macrophages. This article provides an overview of the characterization and functions of the scavenger receptors that have been shown to interact with oxidized LDL, including SR-AI/II, CD36, SR-BI, macrosialin/CD68, LOX-1, and SREC. Isolation and characterization of these and other scavenger receptors has increased our understanding of their role in the uptake of oxidized LDL and the pathogenesis of atherosclerosis.

Angiogenesis and the atherosclerotic carotid plaque: an association between symptomatology and plaque morphology

PURPOSE

Symptomatic carotid disease resulting from generation of thromboemboli has been associated with plaque instability and intraplaque hemorrhage. These features of the lesion could be influenced by the fragility and position of neovessels within the plaque. The purpose of this study was to determine whether any association exists between neovessel density, position, morphology, and thromboembolic sequelae.

METHODS

Carotid endarterectomy samples were collected from 15 asymptomatic patients with greater than 80% stenoses and from 13 highly symptomatic patients who had suffered ipsilateral carotid stenotic events within 1 month of surgery. Both groups were matched for gender, age, risk factors, degree of carotid artery stenosis, and plaque size. Samples were stained with hematoxylin/eosin and van Geison. Immunohistochemistry was performed by using an endothelial specific antibody to CD31. Plaques were assessed for histologic characteristics, and neovessels were counted and characterized by size, site, and shape.

RESULTS

There were significantly more neovessels in plaques (P <.00001) and fibrous caps (P <.0001) in symptomatic compared with asymptomatic plaques. Neovessels in symptomatic plaques were larger (P <.004) and more irregular. There was a significant increase in plaque necrosis and rupture in symptomatic plaques. Plaque hemorrhage and rupture were associated with more neovessels within the plaque (P <.017, P <.001) and within the fibrous cap (P <.046, P <.004). Patients with preoperative and intraoperative embolization had significantly more plaque and fibrous cap neovessels (P <.025, P <.001).

CONCLUSION

Symptomatic carotid disease is associated with increased neovascularization within the atherosclerotic plaque and fibrous cap. These vessels are larger and more irregular and may contribute to plaque instability and the onset of thromboembolic sequelae.

Apolipoprotein E and atherosclerosis: insight from animal and human studies

Major advances have been made in our understanding of the role of apolipoprotein E (apoE) in the onset and development of atherosclerosis. Increasing evidence from both animal and human studies suggests that apoE is able to protect against atherosclerosis by: a) promoting efficient uptake of triglyceride-rich lipoproteins from the circulation; b) maintaining normal macrophage lipid homeostasis; c) playing a role in cellular cholesterol efflux and reverse cholesterol transport; d) acting as an antioxidant; e) inhibiting platelet aggregation; and f) modulating immune function. In humans, apoE is polymorphic, and this genetic variation has a strong effect on its antiatherogenic characteristics. Thus, compared to the epsilon3 allele, the epsilon4 allele promotes atherosclerosis, whereas the epsilon2 allele is either pro- or anti-atherogenic, depending on the influence of both environmental and genetic factors. ApoE and its gene are prime targets for therapeutic intervention aimed at preventing or treating atherosclerotic vascular disease.

Transcriptional inhibition by interleukin-6 of the class A macrophage scavenger receptor in macrophages derived from human peripheral monocytes and the THP-1 monocytic cell line

Expression of the class A macrophage scavenger receptor (MSR) contributes to the uptake of modified low density lipoproteins (LDL) by macrophages and transformation of these cells into lipid-laden foam cells, which characterize atherosclerosis. Many environmental factors, in particular, proinflammatory cytokines and growth factors, can exert regulatory effects on MSR expression, whereas intracellular accumulation of cholesterol itself does not influence MSR levels to any considerable extent. In the present study, by using an in vitro model, we examined whether stimulation with interleukin-6 (IL-6), an immunoregulatory, multipotential cytokine, modulates the expression and activities of the MSR in macrophages. When treated with IL-6, macrophages derived from peripheral monocytes and phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 monocytic cells showed significantly reduced uptake and/or binding of the MSR ligand, acetylated LDL. This effect was paralleled by a reduction in the expression of MSR protein and mRNA. Analysis of MSR promoter activity in THP-1 cells transfected with an MSR promoter-reporter gene construct demonstrated decreased activity of the MSR promoter in IL-6-treated THP-1 macrophages. Electrophoretic mobility gel shift assay also showed a reduction in the binding of a transcription factor to the MSR promoter AP-1/ets elements in IL-6-treated cells. Thus, exposure to IL-6 may inhibit expression of the class A MSR in differentiated macrophages at transcriptional levels. This result suggests that this cytokine may modulate foam cell formation during atherogenesis.

The role of scavenger receptor class A in the adhesion of cells is dependent on cell type and cellular activation state

Scavenger receptor class A (SR-A) facilitates the development of atherosclerosis, which might be due to its role in the uptake of modified low-density lipoproteins. However, the receptor is also suggested to be important for cell adhesion, thereby potentially influencing the residence time of cells in vivo. Using SR-A-deficient mice, we investigated the role of SR-A in the adhesion of peritoneal macrophages (PM) and tissue macrophages (Kupffer cells). In resident PM no effect of the absence or presence of SR-A on cell adhesion was observed, either in the presence or in the absence of serum. However, in thioglycollate-induced PM, SR-A is important for adhesion both in the presence and in the absence of serum and more than 85% of the divalent-cation-independent adhesion in the presence of serum is mediated by SR-A. In unactivated Kupffer cells, like in resident PM, adhesion is not influenced by the absence or presence of SR-A. In vivo administration of phorbol 12-myristate 13-acetate leads to the activation of Kupffer cells, and it appears that under these conditions SR-A does contribute to adhesion, since both in the absence and in the presence of serum SR-A is responsible for about 35% of cell adhesion. It is concluded that SR-A is important for the divalent-cation-independent adhesion of activated PM and Kupffer cells, suggesting that SR-A may influence the residence time of cells at sites of cellular activation, e.g., in atherosclerotic plaques and during liver infection.

A null mutation in murine CD36 reveals an important role in fatty acid and lipoprotein metabolism

A null mutation in the scavenger receptor gene CD36 was created in mice by targeted homologous recombination. These mice produced no detectable CD36 protein, were viable, and bred normally. A significant decrease in binding and uptake of oxidized low density lipoprotein was observed in peritoneal macrophages of null mice as compared with those from control mice. CD36 null animals had a significant increase in fasting levels of cholesterol, nonesterified free fatty acids, and triacylglycerol. The increase in cholesterol was mainly within the high density lipoprotein fraction, while the increase in triacylglycerol was within the very low density lipoprotein fraction. Null animals had lower fasting serum glucose levels when compared with wild type controls. Uptake of 3H-labeled oleate was significantly reduced in adipocytes from null mice. However, the decrease was limited to the low ratios of fatty acid:bovine serum albumin, suggesting that CD36 was necessary for the high affinity component of the uptake process. The data provide evidence for a functional role for CD36 in lipoprotein/fatty acid metabolism that was previously underappreciated.

Differentiation of the Mononuclear Phagocyte System During Mouse Embryogenesis: The Role of Transcription Factor PU.1

During mouse embryogenesis, macrophage-like cells arise first in the yolk sac and are produced subsequently in the liver. The onset of liver hematopoiesis is associated with the transition from primitive to definitive erythrocyte production. This report addresses the hypothesis that a similar transition in phenotype occurs in myelopoiesis. We have used whole mount in situ hybridization to detect macrophage-specific genes expressed during mouse development. The mouse c-fms mRNA, encoding the receptor for macrophage colony-stimulating factor (CSF-1), was expressed on phagocytic cells in the yolk sac and throughout the embryo before the onset of liver hematopoiesis. Similar cells were detected using the mannose receptor, the complement receptor (CR3), or the Microphthalmia transcription factor (MITF) as mRNA markers. By contrast, other markers including the F4/80 antigen, the macrophage scavenger receptor, the S-100 proteins, S100A8 and S100A9, and the secretory product lysozyme appeared later in development and appeared restricted to only a subset of c-fms–positive cells. Two-color immunolabeling on disaggregated cells confirmed that CR3 and c-fmsproteins are expressed on the same cells. Among the genes appearing later in development was the macrophage-restricted transcription factor, PU.1, which has been shown to be required for normal adult myelopoiesis. Mice with null mutations in PU.1 had normal numbers of c-fms–positive phagocytes at 11.5dpc. PU.1(−/−) embryonic stem cells were able to give rise to macrophage-like cells after cultivation in vitro. The results support previous evidence that yolk sac–derived fetal phagocytes are functionally distinct from those arising in the liver and develop via a different pathway.

Requirement for croquemort in phagocytosis of apoptotic cells in Drosophila

Macrophages in the Drosophila embryo are responsible for the phagocytosis of apoptotic cells and are competent to engulf bacteria. Croquemort (CRQ) is a CD36-related receptor expressed exclusively on these macrophages. Genetic evidence showed that crq was essential for efficient phagocytosis of apoptotic corpses but was not required for the engulfment of bacteria. The expression of CRQ was regulated by the amount of apoptosis. These data define distinct pathways for the phagocytosis of corpses and bacteria in Drosophila.

The ambivalence of vitamin E in atherogenesis

The early events in atherogenesis might be due to the oxidation of low- density lipoprotein. The antioxidant vitamin E, therefore, has received much attention as a potential anti-atherogenic agent. Recent mechanistic studies of the early stage of lipoprotein-lipid oxidation show that the role of vitamin E in this process is not simply that of a classical antioxidant. Unless additional compounds are present, vitamin E can have antioxidant, neutral or pro-oxidant activity. This more complex function is reflected in the results of vitamin-E-intervention studies of atherosclerosis in animals and of controlled prospective trials on the incidence of cardiovascular disease in humans, which, overall, are inconclusive.

Lipopolysaccharide recognition, CD14, and lipopolysaccharide receptors

The ability of a host to sense invasion by a pathogenic organism, and to respond appropriately to control infection, is paramount to survival. To that end, an array of receptors and binding proteins has evolved as part of the innate immune system to detect Gram-negative bacteria. This article reviews the role of CD14, other LPS binding proteins, and the Toll family of receptors in the innate recognition of bacterial lipopolysaccharide.

Scavenger receptor deficiency leads to more complex atherosclerotic lesions in APOE3Leiden transgenic mice

Apolipoprotein (apo) E3Leiden is a dysfunctional apo E variant associated with familial dysbetalipoproteinemia in humans. Transgenic mice carrying the APOE3Leiden gene develop hyperlipidemia and are highly susceptible to diet-induced atherosclerosis. An early step in atherosclerosis is foam cell formation, which is thought to result from the unrestricted uptake of modified lipoproteins by macrophages. To investigate the role of the macrophage scavenger receptor type I and II (MSR-A) in this process, APOE3Leiden transgenic mice were crossed onto a MSR-A deficient background and the development of atherosclerosis was examined. In view of recent results with apo E deficient mice (Suzuki H et al., A role for the macrophage scavenger receptors in atherosclerosis. Nature 1997; 386(6622):292-296), absence of the MSR-A in APOE3Leiden mice was expected to lead to a reduction of atherosclerosis. In our study we compared APOE3Leiden/MSR-A deficient mice (E3L MSR-A -/-) to APOE3Leiden/MSR-A wild-type mice (E3L MSR-A +/+). These animals were fed an atherogenic diet for 10 weeks. Quantification of the lesion area showed no significant difference between E3L MSR-A -/- and E3L MSR-A +/+ mice although there was a trend towards the development of larger lesions in the E3L MSR-A -/- mice. All lesions were typed according to their cellular composition. In both male and female E3L MSR-A -/- mice, significantly more severe lesions developed as compared to E3L MSR-A +/+ mice. These results indicate that the effect of MSR-A deficiency on atherogenesis may depend on the presence or absence of apo E.

Vitamin E and vascular homeostasis: implications for atherosclerosis

Considerable epidemiologic data suggest that dietary consumption of vitamin E reduces the incidence of cardiovascular disease. The precise mechanisms are not clear, but emerging data indicate that vitamin E has numerous activities that may, in part, explain its effect on vascular disease. In particular, vitamin E enhances the bioactivity of nitric oxide, inhibits smooth muscle proliferation, and limits platelet aggregation. One common mechanism to account for these effects of vitamin E is the inhibition of protein kinase C stimulation. In the setting of atherosclerosis, inhibition of protein kinase C by vitamin E would be expected to maintain normal vascular homeostasis and thus reduce the clinical incidence of cardiovascular disease.

Scavenger receptor class B type I in the rat ovary: possible role in high density lipoprotein cholesterol uptake and in the recognition of apoptotic granulosa cells

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of high density lipoprotein cholesterol. SR-BI is expressed at high levels in the ovary, indicating that it plays a role in the delivery of cholesterol as substrate for steroid hormone production. However, SR-BI also binds anionic phospholipids with high affinity and could therefore be involved in the recognition of apoptotic cells. In this study we have characterized the expression of SR-BI in rat ovarian follicles undergoing atresia. Atretic follicles with cells undergoing apoptosis were identified by in situ DNA end labeling, and SR-BI expression was determined by in situ hybridization and immunohistochemistry. SR-BI was expressed in thecal cells at all stages of follicular development, including atretic follicles, and in corpus luteum. Isolated apoptotic granulosa cells (but not viable granulosa cells) bound annexin V, indicating that they display anionic phospholipids on the cell surface. Transfection of COS-7 cells with an expression vector carrying the rat SR-BI complementary DNA resulted in increased binding to apoptotic granulosa cells (46 +/- 2% of the SR-BI-expressing cells bound at least one granulosa cell compared with 24 +/- 3% for the mock-transfected cells; P < 0.0001), whereas the binding to viable granulosa cells was unchanged. Apoptotic granulosa cells also bound to isolated thecal shells. We conclude that thecal cells of both nonatretic and atretic follicles express SR-BI. The location of SR-BI expression in the ovary supports a role of this receptor in the uptake of high density lipoprotein cholesterol. In addition, our data suggest that SR-BI mediates the recognition of apoptotic granulosa cells by the surrounding thecal cells and that it therefore may play a role in the remodeling of atretic follicles to secondary interstitial cells.

Receptors for oxidized low-density lipoprotein on elicited mouse peritoneal macrophages can recognize both the modified lipid moieties and the modified protein moieties: implications with respect to macrophage recognition of apoptotic cells

It has been shown previously that the binding of oxidized low-density lipoprotein (OxLDL) to resident mouse peritoneal macrophages can be inhibited (up to 70%) by the apoprotein B (apoB) isolated from OxLDL, suggesting that macrophage recognition of OxLDL is primarily dependent on its modified protein moiety. However, recent experiments have demonstrated that the lipids isolated from OxLDL and reconstituted into a microemulsion can also strongly inhibit uptake of OxLDL (up to 80%). The present studies show that lipid microemulsions prepared from OxLDL bind to thioglycollate-elicited macrophages at 4 degrees C in a saturable fashion and inhibit the binding of intact OxLDL and also of the apoB from OxLDL. Reciprocally, the binding of the OxLDL-lipid microemulsions was strongly inhibited by intact OxLDL. A conjugate of synthetic 1-palmitoyl 2(5-oxovaleroyl) phosphatidylcholine (an oxidation product of 1-palmitoyl 2-arachidonoyl phosphatidylcholine) with serum albumin, shown previously to inhibit macrophage binding of intact OxLDL, also inhibited the binding of both the apoprotein and the lipid microemulsions prepared from OxLDL. Finally, a monoclonal antibody against oxidized phospholipids, one that inhibits binding of intact OxLDL to macrophages, also inhibited the binding of both the resolubilized apoB and the lipid microemulsions prepared from OxLDL. These studies support the conclusions that: (i) at least some of the macrophage receptors for oxidized LDL can recognize both the lipid and the protein moieties; and (ii) oxidized phospholipids, in the lipid phase of the lipoprotein and/or covalently linked to the apoB of OxLDL, likely play a role in that recognition.

Structure and chromosomal localization of the human and murine genes for the macrophage MARCO receptor

The structures of the human and mouse genes for the macrophage receptor with collagenous structure were determined. Both genes have 17 exons, of which exons 4-15 encode the collagenous domain. The transcription initiation sites in the mouse gene were identified using primer extension, SI nuclease mapping, and 5' capturing rapid amplification of cDNA ends assays. All three methods revealed two major initiation sites, one starting 27 bp downstream of a TATA box and another at positions -63 and -66 downstream of an AT-rich region. Several potential binding sites for transcription factors were identified in the promoter region, neither gene has a CAAT box or GC boxes. The human and mouse genes were localized to syntenic regions on chromosomes 2 and 1, respectively, using fluorescence in situ hybridization.

Role of the scavenger receptor MARCO in alveolar macrophage binding of unopsonized environmental particles

Alveolar macrophages (AMs) avidly bind and ingest unopsonized environmental particles and bacteria through scavenger-type receptors (SRs). AMs from mice with a genetic deletion of the major macrophage SR (types AI and AII; SR-/-) showed no decrease in particle binding compared with SR+/+ mice, suggesting that other SRs are involved. To identify these receptors, we generated a monoclonal antibody (mAb), PAL-1, that inhibits hamster AM binding of unopsonized particles (TiO2, Fe2O3, and latex beads; 66 +/- 5, 77 +/- 2, and 85 +/- 2% inhibition, respectively, measured by flow cytometry). This antibody identifies a protein of approximately 70 kD on the AM surface (immunoprecipitation) that is expressed by AMs and other macrophages in situ. A cDNA clone encoding the mAb PAL-1-reactive protein isolated by means of COS cell expression was found to be 84 and 77% homologous to mouse and human scavenger receptor MARCO mRNA, respectively. Transfection of COS cells with MARCO cDNA conferred mAb-inhibitable TiO2 binding. Hamster MARCO also mediates AM binding of unopsonized bacteria (67 +/- 5 and 47 +/- 4% inhibition of Escherichia coli and Staphylococcus aureus binding by mAb PAL-1). A polyclonal antibody to human MARCO identified the expected approximately 70-kD band on Western blots of lysates of normal bronchoalveolar lavage (BAL) cells (>90% AMs) and showed strong immunolabeling of human AMs in BAL cytocentrifuge preparations and within lung tissue specimens. In normal mouse AMs, the anti-MARCO mAb ED31 also showed immunoreactivity and inhibited binding of unopsonized particles (e.g., TiO2 approximately 40%) and bacteria. The novel function of binding unopsonized environmental dusts and pathogens suggests an important role for MARCO in the lungs' response to inhaled particles.

Lovastatin decreases the receptor-mediated degradation of acetylated and oxidized LDLs in human blood monocytes during the early stage of differentiation into macrophages

3-Hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors are used therapeutically to upregulate the LDL receptor-mediated removal of plasma cholesterol by the liver. Several lines of evidence indicate that these drugs also exert direct effects on the metabolism of native and modified LDL in extrahepatic cells. We studied the effects of lovastatin (LOV) on the degradation of native, acetylated, and oxidized LDL, and on levels of mRNA encoding for the LDL, types I and II class A macrophage scavenger, and CD36 receptors in human blood monocytes at different stages of their maturation into adherent macrophages. LOV (10 micromol/L) reduced the degradation of acetylated LDL when added to freshly isolated cells cultured for 2 (81+/-4% of control, P<0.05) and 5 (76+/-6%, of control, P<0.05) days. The degradation of oxidized LDL was also reduced in cells treated with LOV for 2 days after seeding (51+/-3% of control, P<0. 001) but not in 5-day-old cells. LOV had no significant effect on the degradation of either acetylated or oxidized LDL when added to fully matured macrophages allowed to differentiate under control conditions for 7 days before incubations with 10 micromol/L LOV for an additional 2 days. In contrast, LOV increased the degradation of native LDL in these cells at all 3 stages of cell differentiation. LOV also reduced class A types I and II macrophage scavenger receptor and CD36 mRNA levels in 2- and 5-day-old cells but not in the more mature macrophages. These data suggest that 3-hydroxy-3-methylglutaryl-coenzyme A inhibitors may reduce the expression and function of the class A types I and II macrophage scavenger receptor and CD36 in monocytes, during the early stages of their differentiation into adherent macrophages. These effects, if operative in vivo, may slow down the development of the atherosclerotic plaque and thus contribute to the beneficial effects of these drugs.

Scavenger receptor activity is increased in macrophages from rabbits with low atherosclerotic response: studies in normocholesterolemic high and low atherosclerotic response rabbits

We have previously described 2 strains of New Zealand White rabbits with a high (HAR) or low (LAR) atherosclerotic response to hypercholesterolemia. In the present study, we focused on class A scavenger receptor (SR-A) activity and ApoE expression in macrophages from both rabbit strains. These parameters play a crucial role in maintaining cholesterol homeostasis in the arterial wall and may be involved in the development of atherosclerosis. SR activity, as measured by uptake of DiI-labeled acetylated LDL, was significantly higher in macrophages from LAR rabbits (2177+/-253 ng/mg cell protein) than in macrophages from HAR rabbits (1153+/-200 ng/mg cell protein). The higher SR activity was caused by a greater number of SRs (apparent Vmax, 4100 ng/mg in LAR and 1980 ng/mg in HAR rabbits). The high SR activity in macrophages from LAR rabbits was associated with a significantly higher expression of SR-A mRNA compared with macrophages from HAR rabbits. However, the latter finding could not be explained by differences in the activity of transcription factor-activating protein 1 (AP-1), which was comparable in macrophages from both strains of rabbits. Because under certain circumstances SR-A mRNA expression is regulated in parallel with ApoE expression, we also evaluated this parameter. Although ApoE mRNA was 74% higher in macrophages from LAR rabbits, the difference did not reach statistical significance. In conclusion, the increased expression of SR-A in macrophages in the presence of adequate amounts of ApoE may play a role in attenuating atherosclerosis in LAR rabbits.

Modification of type III VLDL, their remnants, and VLDL from ApoE-knockout mice by p-hydroxyphenylacetaldehyde, a product of myeloperoxidase activity, causes marked cholesteryl ester accumulation in macrophages

Very low density lipoproteins (VLDLs) from apolipoprotein (apo) E2/E2 subjects with type III hyperlipoproteinemia, VLDL remnants, and VLDL from apoE-knockout (EKO) mice are taken up poorly by macrophages. The present study examined whether VLDL modification by the reactive aldehyde p-hydroxyphenylacetaldehyde (pHA) enhances cholesteryl ester (CE) accumulation by J774A.1 macrophages. pHA is the major product derived from the oxidation of L-tyrosine by myeloperoxidase and is a component of human atherosclerotic lesions. Incubation of J774A.1 cells with native type III VLDL, their remnants, and EKO-VLDL increased cellular CE by only 3-, 5-, and 5-fold, respectively, compared with controls. In striking contrast, cells exposed to VLDL modified by purified pHA (pHA-VLDL) exhibited marked increases in cellular CE of 38-, 47-, and 35-fold, respectively (P</=0.0001). Addition of the lipoprotein lipase inhibitor tetrahydrolipstatin decreased cellular CE accumulation induced by the 3 pHA-modified VLDL preparations by 73%, 59%, and 73%, respectively. Addition of the acyl coenzyme A:cholesterol acyltransferase inhibitor DuP 128 to cells incubated with the pHA-modified lipoproteins decreased cellular CE by 100%, 82%, and 95%, respectively, but had no effect on cellular triglycerides. To examine whether the type A scavenger receptors (SR-As) mediated the uptake of pHA-VLDL, incubations were performed in the presence of polyinosine (poly I), a polynucleotide known to block binding to SR-As (types I and II), or in cells preincubated with interferon-gamma (IFN-gamma), a cytokine known to decrease expression of SR-A type I. Coincubation of pHA-VLDL with poly I reduced cellular CE by only 38%, 44%, and 49%, respectively, whereas coincubation with IFN-gamma reduced CE by only 18%, 27%, and 65%, respectively. In marked contrast to pHA-VLDL, both poly I and IFN-gamma inhibited, by>95%, CE accumulation induced by copper-oxidized VLDL. These results demonstrate a novel mechanism for the conversion of type III VLDLs, their remnants, and EKO-VLDL into atherogenic particles and suggest that macrophage uptake of pHA-VLDL (1) requires catalytically active lipoprotein lipase, (2) involves acyl coenzyme A:cholesterol acyltransferase-mediated cholesterol esterification, and (3) involves pathways distinct from the SR-A.

Age-related decrease in proteoglycan synthesis of human articular chondrocytes: the role of nonenzymatic glycation

OBJECTIVE

To examine the effect of nonenzymatic glycation of cartilage extracellular matrix on the synthetic activity of chondrocytes.

METHODS

The proteoglycan-synthesis rate (35SO4(2-) incorporation) and levels of advanced nonenzymatic glycation (determined by high-performance liquid chromatography measurement of pentosidine) were evaluated in human articular cartilage from 129 donors, varying in age from 25 to 88 years, and in cartilage with enhanced levels of advanced glycation end-products (AGEs) resulting from incubation with ribose.

RESULTS

Cartilage showed a strong age-related increase in pentosidine levels (r = 0.97, P < 0.0005) and, concomitantly, a decrease in proteoglycan synthesis (r = -0.98, P < 0.0002). This decrease in proteoglycan synthesis correlated with the increase in pentosidine (r = -0.95, P < 0.02). Moreover, the elevation of pentosidine levels in the in vitro-ribosylated cartilage was proportional with the decrease in proteoglycan synthesis (r = -0.95, P < 0.005).

CONCLUSION

In both aged and in vitro AGE-enriched cartilage, the rate of proteoglycan synthesis was negatively correlated with the degree of glycation. This suggests that the age-related increase in cartilage AGE levels may be responsible, at least in part, for the age-related decline in the synthetic capacity of cartilage.

MCP-1 and IL-8 trigger firm adhesion of monocytes to vascular endothelium under flow conditions

Monocytes contribute to the development of atherosclerotic lesions in mouse models. The chemoattractant proteins (chemokines), monocyte chemoattractant protein-1 (MCP-1) and interleukin-8 (IL-8), are found in human atheroma, and mice lacking receptors for these chemokines are less susceptible to atherosclerosis and have fewer monocytes in vascular lesions. Although MCP-1 has a powerful effect on monocytes, IL-8 is thought to act predominantly on neutrophils and it is unclear how it could recruit monocytes. Here we investigate the ability of chemokines to control the interaction of monocytes under flow conditions with vascular endothelium that has been transduced to express specific leukocyte-adherence receptors. We find that MCP-1 and IL-8 can each rapidly cause rolling monocytes to adhere firmly onto monolayers expressing E-selectin, whereas related chemokines do not. These effects do not correlate with either the induction of a calcium transient or chemotaxis. We conclude that chemokines are important modulators of monocyte-endothelial interactions under flow conditions. Moreover, our finding that IL-8 is a powerful trigger for firm adhesion of monocytes to vascular endothelium reveals an unexpected role for this chemokine in monocyte recruitment.

A modification of apolipoprotein B accounts for most of the induction of macrophage growth by oxidized low density lipoprotein

It has recently been shown that macrophage proliferation occurs during the progression of atherosclerotic lesions and that oxidized low density lipoprotein (LDL) stimulates macrophage growth. Possible mechanisms for this include the interaction of oxidized LDL with integral plasma membrane proteins coupled to signaling pathways, the release of growth factors and autocrine activation of growth factor receptors, or the potentiation of mitogenic signal transduction by a component of oxidized LDL after internalization. The present study was undertaken to further elucidate the mechanisms involved in the growth-stimulating effect of oxidized LDL in macrophages. Only extensively oxidized LDL caused significant growth stimulation, whereas mildly oxidized LDL, native LDL, and acetyl LDL were ineffective. LDL that had been methylated before oxidation (to block lysine derivatization by oxidation products and thereby prevent the formation of a scavenger receptor ligand) did not promote growth, even though extensive lipid peroxidation had occurred. The growth stimulation could not be attributed to lysophosphatidylcholine (lyso-PC) because incubation of oxidized LDL with fatty acid-free bovine serum albumin resulted in a 97% decrease in lyso-PC content but only a 20% decrease in mitogenic activity. Similarly, treatment of acetyl LDL with phospholipase A2 converted more than 90% of the initial content of phosphatidylcholine (PC) to lyso-PC, but the phospholipase A2-treated acetyl LDL was nearly 10-fold less potent than oxidized LDL at stimulating growth. Platelet-activating factor receptor antagonists partly inhibited growth stimulation by oxidized LDL, but platelet-activating factor itself did not induce growth. Digestion of oxidized LDL with phospholipase A2 resulted in the hydrolysis of PC and oxidized PC but did not attenuate growth induction. Native LDL, treated with autoxidized arachidonic acid under conditions that caused extensive modification of lysine residues by lipid peroxidation products but did not result in oxidation of LDL lipids, was equal to oxidized LDL in potency at stimulating macrophage growth. Albumin modified by arachidonic acid peroxidation products also stimulated growth, demonstrating that LDL lipids are not essential for this effect. These findings suggest that oxidatively modified apolipoprotein B is the main growth-stimulating component of oxidized LDL, but that oxidized phospholipids may play a secondary role.

Expression of macrophage MARCO receptor induces formation of dendritic plasma membrane processes

MARCO is a novel macrophage-specific receptor structurally related to macrophage class A scavenger receptors. It is constitutively expressed in macrophages of the marginal zone of the spleen and in lymph nodes and is up-regulated in other tissues during systemic bacterial infections. In this study, we show that ectopic expression of MARCO in cell lines such as Chinese hamster ovary, HeLa, NIH3T3, and 293 induces dramatic cell shape changes. Typically these changes include formation of large lamellipodia-like structures and of long dendritic processes. The morphological changes are accompanied by disassembly of actin stress fibers and often also by complete loss of focal adhesions. The MARCO-induced changes are dependent on cell adhesion and are inhibited, but not completely abolished, when the cells are plated on fibronectin-coated surfaces. Similarly, a dominant-negative mutant of the Rho family GTPase Rac1 partially inhibited the morphogenic effects of MARCO in Chinese hamster ovary cells, whereas a dominant-negative form of a related protein, Cdc42, did not. Expression studies with a variety of truncated MARCO forms indicated that the proximal segment of the cysteine-rich domain V is important for the morphoregulatory activity. The results indicate that expression of MARCO has a direct effect in generating the phenotype of activated macrophages necessary for the trapping and removal of pathogens and other foreign substances.

Intraperitoneal administration of anti-c-fms monoclonal antibody prevents initial events of atherogenesis but does not reduce the size of advanced lesions in apolipoprotein E-deficient mice

BACKGROUND

Atherosclerosis results from complex inflammatory-fibroproliferative responses. To elucidate the central role of macrophage and macrophage-colony stimulating factor (M-CSF) during atherogenesis, we used a new strategy to administer to adult apolipoprotein E (apoE)-deficient mice a monoclonal antibody (AFS98) raised against c-fms, the receptor of M-CSF.

METHODS AND RESULTS

When 6-week-old apoE-deficient mice were fed a high-fat diet and injected with 2 mg of AFS98 intraperitoneally on alternate days for 6 weeks, accumulation of macrophage-derived foam cells in the aortic root was suppressed by 70% compared with that in controls. This preventive effect was associated with neither remarkable decrease of the number of circulating monocytes nor systemic growth retardation. In contrast, when apoE-deficient mice that had been fed a high-fat diet from 6 weeks of age were given AFS98 from 12 to 18 weeks of age, a minimal protective effect on lesion size was observed.

CONCLUSIONS

These results suggest that (1) macrophage and M-CSF/c-fms play an essential role in the arterial wall during development of the fatty streak lesion and (2) blockade of the M-CSF/c-fms pathway could act as protection from at least early atherogenesis but could have a less preventive effect on maintenance of the advanced lesions.

Atherogenecity of lipoprotein(a) and oxidized low density lipoprotein: insight from in vivo studies of arterial wall influx, degradation and efflux

The accumulation of atherogenic lipoproteins in the arterial intima is pathognomonic of atherosclerosis. Modification of LDL by covalent linkage of apo(a) (resulting in the formation of Lp(a)) or oxidation probably enhances its atherogenecity. Although the metabolism of LDL in arterial intima has been rather extensively characterized, little has been known about the interaction of Lp(a) and oxidized LDL (ox-LDL) with the arterial wall. The present paper reviews a series of recent in vivo studies of the interaction of Lp(a) and ox-LDL with the arterial wall. The results have identified several factors that affect the accumulation of Lp(a) and ox-LDL in the arterial intima and have provided fresh insight into unique metabolic characteristics of Lp(a) and ox-LDL that may explain the large atherogenic potential of these modified LDL species.

Characterization of plasmid DNA binding and uptake by peritoneal macrophages from class A scavenger receptor knockout mice

PURPOSE

Plasmid DNA (pDNA) has become an important class of macromolecular agent suitable for non-viral gene therapy as well as DNA vaccination. Our recent study has suggested that pDNA is taken up by mouse peritoneal macrophages via a specific mechanism mediated by a receptor similar to the scavenger receptor (SR). This study was designed to further characterize the pDNA uptake by macrophages in order to elucidate the mechanism.

METHODS

The binding and uptake of pDNA labeled with 32P or a fluorescent marker were studied in vitro using cultured Chinese hamster ovary (CHO) cells expressing the class A scavenger receptor (SRA) and peritoneal macrophages from SRA-knockout mice.

RESULTS

pDNA binding and uptake by CHO(SRA) cells were minimal and almost identical to that by wild-type CHO cells. Macrophages from the knockout mice showed pronounced pDNA binding and uptake as did the control macrophages. In both types of macrophage, pDNA binding was significantly inhibited by cold pDNA, polyinosinic acid and dextran sulfate but not by polycytidylic acid or Ac-LDL. These results provide direct evidence that SRA is not responsible for the significant binding and subsequent uptake of pDNA by mouse peritoneal macrophages. Further binding experiments revealed that, in addition to polyinosinic acid and dextran sulfate, heparin was a potent inhibitor among a variety of polyanionic compounds such as polynucleotides, anionic polysaccharides and modified proteins including Ox-LDL.

CONCLUSIONS

The present study suggest that pDNA binding and uptake by mouse peritoneal macrophages are mediated by a specific mechanism to some defined polyanions not by scavenger receptors. The finding would be an important basis for further studies to elucidate the mechanism(s) of pDNA uptake by macrophages.

Activation of receptor for advanced glycation end products: a mechanism for chronic vascular dysfunction in diabetic vasculopathy and atherosclerosis

Receptor for advanced glycation end products (RAGE) is a member of the immunoglobulin superfamily of cell surface molecules and engages diverse ligands relevant to distinct pathological processes. One class of RAGE ligands includes glycoxidation products, termed advanced glycation end products, which occur in diabetes, at sites of oxidant stress in tissues, and in renal failure and amyloidoses. RAGE also functions as a signal transduction receptor for amyloid beta peptide, known to accumulate in Alzheimer disease in both affected brain parenchyma and cerebral vasculature. Interaction of RAGE with these ligands enhances receptor expression and initiates a positive feedback loop whereby receptor occupancy triggers increased RAGE expression, thereby perpetuating another wave of cellular activation. Sustained expression of RAGE by critical target cells, including endothelium, smooth muscle cells, mononuclear phagocytes, and neurons, in proximity to these ligands, sets the stage for chronic cellular activation and tissue damage. In a model of accelerated atherosclerosis associated with diabetes in genetically manipulated mice, blockade of cell surface RAGE by infusion of a soluble, truncated form of the receptor completely suppressed enhanced formation of vascular lesions. Amelioration of atherosclerosis in these diabetic/atherosclerotic animals by soluble RAGE occurred in the absence of changes in plasma lipids or glycemia, emphasizing the contribution of a lipid- and glycemia-independent mechanism(s) to atherogenesis, which we postulate to be interaction of RAGE with its ligands. Future studies using mice in which RAGE expression has been genetically manipulated and with selective low molecular weight RAGE inhibitors will be required to definitively assign a critical role for RAGE activation in diabetic vasculopathy. However, sustained receptor expression in a microenvironment with a plethora of ligand makes possible prolonged receptor stimulation, suggesting that interaction of cellular RAGE with its ligands could be a factor contributing to a range of important chronic disorders.

Receptors for proteins modified by advanced glycation endproducts (AGE)--their functional role in atherosclerosis

Long-term incubation of proteins with glucose leads, through the formation of early stage products such as Schiff base and Amadori rearrangement products, to the formation of advanced glycation end products (AGE). Recent studies of AGE-structures as well as the receptor for AGE-proteins (AGE-receptors) have emphasized the involvement of protein modification by AGE in aging and age-enhanced disease processes. Immunohistochemical analyses of human atherosclerotic lesions using a monoclonal anti-AGE antibody have demonstrated diffuse extracellular AGE-deposition as well as dense intracellular AGE-deposition in macrophage- and vascular smooth muscle cell (SMC)-derived foam cells. In vitro experiments using both CHO cells overexpressing macrophage scavenger receptor-A (MSR-A) and peritoneal macrophages from MSR-A-knockout mice have shown that the MSR-A plays a major role in endocytic uptake of AGE-proteins by macrophages. Furthermore, in vitro experiments with rabbit arterial SMCs demonstrated a novel AGE-receptor mediating endocytosis of AGE-proteins. These in vivo and in vitro experiments suggest that AGE-proteins formed extracellularly in atherosclerotic lesions are endocytosed by macrophages through MSR-A in the early stage, and by SMCs through the novel AGE-receptor in the advanced stage, implicating functional contribution of the AGE-receptor-mediated interaction of AGE-proteins with these cells to atherosclerotic processes in arterial walls.

Progression and regression of atherosclerosis in APOE3-Leiden transgenic mice: an immunohistochemical study

Apolipoprotein E3-Leiden (APOE3-Leiden) transgenic mice develop hyperlipidemia and are highly susceptible to diet-induced atherosclerosis. We have studied the progression and regression of atherosclerosis using immunohistochemistry. Female transgenic mice were fed a moderate fat diet to study atherosclerosis over a longer time period. Fatty streaks arose in the intima and consisted of lipid filled macrophages which differed in origin. All macrophages expressed the macrophage scavenger receptor while two thirds expressed sialoadhesin and were positive for an antibody recognizing marginal zone macrophages (MOMA-1). All macrophages were negative for the scavenger receptor MARCO and 50% were positive for CD4. Small fatty streaks contained CD-3 positive T-lymphocytes which were for more than 70% CD4-positive. ICAM-1 was positive both in atherosclerotic and control mice. In early plaques, fibrosis was observed on the luminal and medial site of the foam cells while smooth muscle cells were only observed in the fibrous cap. To study regression, we used a high fat, high cholesterol diet to rapidly induce atherosclerosis (14 weeks). The animals were then fed normal chow. Subsequently, atherosclerosis was assayed over time (4, 8, 16 weeks). Cholesterol levels dropped in 4 weeks to control levels. The animals did not show a significantly decrease in plaque size over time. but the percentage macrophages was significantly smaller in the animals after 4 weeks. In conclusion, the APOE3-Leiden mouse is a useful model to study the progression and regression of atherosclerosis.

Roles of a macrophage receptor with collagenous structure (MARCO) in host defense and heterogeneity of splenic marginal zone macrophages

Class A type I and type II macrophage scavenger receptors (MSR-A) and a macrophage receptor with collagenous structure (MARCO) are trimeric membrane glycoproteins mediating the uptake of chemically modified low density lipoproteins. MSR-A is expressed constitutively in several tissue macrophages and in liver sinusoidal endothelial cells, whereas MARCO is expressed constitutively in splenic marginal zone macrophages and in macrophages and endothelial cells in the lymphatic medullary sinuses of lymph nodes. The administration of LPS, zymosan, BCG, or L. monocytogenes to mice resulted in marked and transient MARCO expression and in the upregulation of MSR-A expression in the liver and spleen. In osteopetrotic (op) mutant mice defective in the production on M-CSF, ER-TR9-positive marginal zone macrophages and MOMA-1-positive marginal metallophilic macrophages were absent, whereas MARCO-expressing marginal zone macrophages were present, indicating the heterogeneity of marginal zone macrophages. Intravenous administration of BCG resulted in marked accumulation of BCG bacilli in the both marginal zone macrophages and marginal metallophilic macrophages in littermate control mice. In contrast, BCG bacilli were incorporated almost exclusively by MARCO-expressing marginal zone macrophages in op/op mice. These results indicate that MARCO is not only expressed constitutively in specific macrophage subpopulations but is also induced by various bacterial antigens and plays a role in host defense against bacteria.