Phagocytosis of aggregated lipoprotein by macrophages: low density lipoprotein receptor-dependent foam-cell formation

Intracellular tPA-PAI-1 interaction determines VLDL assembly in hepatocytes

Apolipoprotein B (apoB)-lipoproteins initiate and promote atherosclerotic cardiovascular disease. Plasma tissue plasminogen activator (tPA) activity is negatively associated with atherogenic apoB-lipoprotein cholesterol levels in humans, but the mechanisms are unknown. We found that tPA, partially through the lysine-binding site on its Kringle 2 domain, binds to the N terminus of apoB, blocking the interaction between apoB and microsomal triglyceride transfer protein (MTP) in hepatocytes, thereby reducing very-low-density lipoprotein (VLDL) assembly and plasma apoB-lipoprotein cholesterol levels. Plasminogen activator inhibitor 1 (PAI-1) sequesters tPA away from apoB and increases VLDL assembly. Humans with PAI-1 deficiency have smaller VLDL particles and lower plasma levels of apoB-lipoprotein cholesterol. These results suggest a mechanism that fine-tunes VLDL assembly by intracellular interactions among tPA, PAI-1, and apoB in hepatocytes.

Macrophage Anti-inflammatory Behaviour in a Multiphase Model of Atherosclerotic Plaque Development

Atherosclerosis is an inflammatory disease characterised by the formation of plaques, which are deposits of lipids and cholesterol-laden macrophages that form in the artery wall. The inflammation is often non-resolving, due in large part to changes in normal macrophage anti-inflammatory behaviour that are induced by the toxic plaque microenvironment. These changes include higher death rates, defective efferocytic uptake of dead cells, and reduced rates of emigration. We develop a free boundary multiphase model for early atherosclerotic plaques, and we use it to investigate the effects of impaired macrophage anti-inflammatory behaviour on plaque structure and growth. We find that high rates of cell death relative to efferocytic uptake results in a plaque populated mostly by dead cells. We also find that emigration can potentially slow or halt plaque growth by allowing material to exit the plaque, but this is contingent on the availability of live macrophage foam cells in the deep plaque. Finally, we introduce an additional bead species to model macrophage tagging via microspheres, and we use the extended model to explore how high rates of cell death and low rates of efferocytosis and emigration prevent the clearance of macrophages from the plaque.

Cell-Based Drug Delivery Systems with Innate Homing Capability as a Novel Nanocarrier Platform

Nanoparticle-based drug delivery systems have been designed to treat various diseases. However, many problems remain, such as inadequate tumor targeting and poor therapeutic outcomes. To overcome these obstacles, cell-based drug delivery systems have been developed. Candidates for cell-mediated drug delivery include blood cells, immune cells, and stem cells with innate tumor tropism and low immunogenicity; they act as a disguise to deliver the therapeutic payload. In drug delivery systems, therapeutic agents are encapsulated intracellularly or attached to the surface of the plasma membrane and transported to the desired site. Here, we review the pros and cons of cell-based therapies and discuss their homing mechanisms in the tumor microenvironment. In addition, different strategies to load therapeutic agents inside or on the surface of circulating cells and the current applications for a wide range of disease treatments are summarized.

Receptor-independent fluid-phase macropinocytosis promotes arterial foam cell formation and atherosclerosis

Accumulation of lipid-laden foam cells in the arterial wall plays a central role in atherosclerotic lesion development, plaque progression, and late-stage complications of atherosclerosis. However, there are still fundamental gaps in our knowledge of the underlying mechanisms leading to foam cell formation in atherosclerotic arteries. Here, we investigated the role of receptor-independent macropinocytosis in arterial lipid accumulation and pathogenesis of atherosclerosis. Genetic inhibition of fluid-phase macropinocytosis in myeloid cells (LysMCre+ Nhe1fl/fl) and repurposing of a Food and Drug Administration (FDA)-approved drug that inhibits macrophage macropinocytosis substantially decreased atherosclerotic lesion development in low-density lipoprotein (LDL) receptor-deficient and Apoe-/- mice. Stimulation of macropinocytosis using genetic (H-RASG12V) and physiologically relevant approaches promoted internalization of unmodified native (nLDL) and modified [e.g., acetylated (ac) and oxidized (ox) LDL] lipoproteins in both wild-type and scavenger receptor (SR) knockout (Cd36-/-/Sra-/-) macrophages. Pharmacological inhibition of macropinocytosis in hypercholesterolemic wild-type and Cd36-/-/Sra-/- mice identified an important role of macropinocytosis in LDL uptake by lesional macrophages and development of atherosclerosis. Furthermore, serial section high-resolution imaging, LDL immunolabeling, and three-dimensional (3D) reconstruction of subendothelial foam cells provide visual evidence of lipid macropinocytosis in both human and murine atherosclerotic arteries. Our findings complement the SR paradigm of atherosclerosis and identify a therapeutic strategy to counter the development of atherosclerosis and cardiovascular disease.

Lipid accumulation and novel insight into vascular smooth muscle cells in atherosclerosis

Atherosclerosis is a chronic and progressive process. It is the most important pathological basis of cardiovascular disease and stroke. Vascular smooth muscle cells (VSMCs) are an essential cell type in atherosclerosis. Previous studies have revealed that VSMCs undergo phenotypic transformation in atherosclerosis to participate in the retention of atherogenic lipoproteins as well as the formation of the fibrous cap and the underlying necrotic core in plaques. The emergence of lineage-tracing studies indicates that the function and number of VSMCs in plaques have been greatly underestimated. In addition, recent studies have revealed that VSMCs make up at least 50% of the foam cell population in human and mouse atherosclerotic lesions. Therefore, understanding the formation of lipid-loaded VSMCs and their regulatory mechanisms is critical to elucidate the pathogenesis of atherosclerosis and to explore potential therapeutic targets. Moreover, combination of many complementary technologies such as lineage tracing, single-cell RNA sequencing (scRNA-seq), flow cytometry, and mass cytometry (CyTOF) with immunostaining has been performed to further understand the complex VSMC function. Correct identification of detrimental and beneficial processes may reveal successful therapeutic treatments targeting VSMCs and their derivatives during atherosclerosis. The purpose of this review is to summarize the process of lipid-loaded VSMC formation in atherosclerosis and to describe novel insight into VSMCs gained by using multiple advanced methods.

Molybdenum derived from nanomaterials incorporates into molybdenum enzymes and affects their activities in vivo

Many nanoscale biomaterials fail to reach the clinical trial stage due to a poor understanding of the fundamental principles of their in vivo behaviour. Here we describe the transport, transformation and bioavailability of MoS2 nanomaterials through a combination of in vivo experiments and molecular dynamics simulations. We show that after intravenous injection molybdenum is significantly enriched in liver sinusoid and splenic red pulp. This biodistribution is mediated by protein coronas that spontaneously form in the blood, principally with apolipoprotein E. The biotransformation of MoS2 leads to incorporation of molybdenum into molybdenum enzymes, which increases their specific activities in the liver, affecting its metabolism. Our findings reveal that nanomaterials undergo a protein corona-bridged transport-transformation-bioavailability chain in vivo, and suggest that nanomaterials consisting of essential trace elements may be converted into active biological molecules that organisms can exploit. Our results also indicate that the long-term biotransformation of nanomaterials may have an impact on liver metabolism.

Anti-Atherogenic Effect of Stem Cell Nanovesicles Targeting Disturbed Flow Sites

Atherosclerosis development leads to irreversible cascades, highlighting the unmet need for improved methods of early diagnosis and prevention. Disturbed flow formation is one of the earliest atherogenic events, resulting in increased endothelial permeability and subsequent monocyte recruitment. Here, a mesenchymal stem cell (MSC)-derived nanovesicle (NV) that can target disturbed flow sites with the peptide GSPREYTSYMPH (PREY) (PMSC-NVs) is presented which is selected through phage display screening of a hundred million peptides. The PMSC-NVs are effectively produced from human MSCs (hMSCs) using plasmid DNA designed to functionalize the cell membrane with PREY. The potent anti-inflammatory and pro-endothelial recovery effects are confirmed, similar to those of hMSCs, employing mouse and porcine partial carotid artery ligation models as well as a microfluidic disturbed flow model with human carotid artery-derived endothelial cells. This nanoscale platform is expected to contribute to the development of new theragnostic strategies for preventing the progression of atherosclerosis.

Macrophages, Endothelial Cells, and Lipoprotein Oxidation in the Pathogenesis of Atherosclerosis*

One of the earliest phenomena in the atherogenic process in cholesterol-fed rabbits appears to be the trapping of low density lipoproteins (LDL) at lesion-prone sites in the aorta. The resulting increase in residence time may facilitate oxidation of the lipoproteins, which, in turn, may be a chemotactic signal for monocytes to enter the intima. Oxidized lipoproteins may also be the major source of the cholesterol that the cells accumulate during their transformation into macrophage-derived foam cells (MFC). Adherent monocytes appear to cluster over small groups of subendothelial foam cells, perhaps in response to the enhanced expression of specific adhesion molecules on the surface of endothelial cells and/or monocytes following activation by oxidized lipoproteins. Lipoproteins oxidized by MFC may also injure endothelial cells causing them to retract or rupture. The resulting exposure of the MFC facilitates the formation of mural thrombi. MFC contain oxidation-specific lipid-protein adducts and specifically express the mRNA for 15-lipoxygenase, an enzyme potentially involved in lipoprotein oxidation. MFC isolated from atherosclerotic lesions and containing up to 600 μg cholesterol/mg protein are still capable of binding and degrading modified lipoproteins and affecting the oxidation of LDL.

Detection of a High Ratio of Soluble to Membrane-Bound LOX-1 in Aspirated Coronary Thrombi From Patients With ST-Segment-Elevation Myocardial Infarction

Background

The circulating level of soluble lectin‐like oxidized low‐density lipoprotein receptor‐1 (sLOX‐1) is a valuable biomarker of acute myocardial infarction (AMI). The most electronegative low‐density lipoprotein, L5, signals through LOX‐1 to trigger atherogenesis. We examined the characteristics of LOX‐1 and the role of L5 in aspirated coronary thrombi of AMI patients.

Methods and Results

Intracoronary thrombi were aspirated by performing interventional thrombosuction in patients with ST‐segment–elevation myocardial infarction (STEMI; n=32) or non–ST‐segment–elevation myocardial infarction (n=12). LOX‐1 level and the ratio of sLOX‐1 to membrane‐bound LOX‐1 were higher in thrombi of STEMI patients than in those of non–ST‐segment–elevation myocardial infarction patients. In all aspirated thrombi, LOX‐1 colocalized with apoB100. When we explored the role of L5 in AMI, deconvolution microscopy showed that particles of L5 but not L1 (the least electronegative low‐density lipoprotein) quickly formed aggregates prone to retention in thrombi. Treating human monocytic THP‐1 cells with L5 or L1 showed that L5 induced cellular adhesion and promoted the differentiation of monocytes into macrophages in a dose‐dependent manner. In a second cohort of AMI patients, the L5 percentage and plasma concentration of sLOX‐1 were higher in STEMI patients (n=33) than in non–ST‐segment–elevation myocardial infarction patients (n=25), and sLOX‐1 level positively correlated with L5 level in AMI patients.

Conclusions

The level of LOX‐1 and the ratio of sLOX‐1 to membrane‐bound LOX‐1 in aspirated thrombi, as well as the circulating level of sLOX‐1 were higher in STEMI patients than in non–ST‐segment–elevation myocardial infarction patients. L5 may play a role in releasing a high level of sLOX‐1 into the circulation of STEMI patients.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

The effect of intakes of fish and Camelina sativa oil on atherogenic and anti-atherogenic functions of LDL and HDL particles: A randomized controlled trial

BACKGROUND AND AIMS

Omega-3 fatty acids are known to have several cardioprotective effects. Our aim was to investigate the effects of intakes of fish and Camelina sativa oil (CSO), rich in alpha-linolenic acid, on the atherogenic and anti-atherogenic functions of LDL and HDL particles.

METHODS

Altogether, 88 volunteers with impaired glucose metabolism were randomly assigned to CSO (10 g of alpha-linolenic acid/day), fatty fish (4 fish meals/week), lean fish (4 fish meals/week) or control group for 12 weeks. 79 subjects completed the study. The binding of lipoproteins to aortic proteoglycans, LDL aggregation and activation of endothelial cells by LDL and cholesterol efflux capacity of HDL were determined in vitro.

RESULTS

Intake of CSO decreased the binding of lipoproteins to aortic proteoglycans in a non-normalized model (p = 0.006). After normalizing with serum concentrations of non-HDL cholesterol, apolipoprotein B (apoB) or LDL cholesterol, which decreased in the CSO group, the change was no longer statistically significant. In the fish groups, there were no changes in the binding of lipoproteins to proteoglycans. Regarding other lipoprotein functions, there were no changes in any of the groups.

CONCLUSIONS

Intake of CSO decreases the binding of lipoproteins to aortic proteoglycans by decreasing serum LDL cholesterol concentration, which suggests that the level of apoB-containing lipoproteins in the circulation is the main driver of lipoprotein retention within the arterial wall. Intake of fish or CSO has no effects on other lipoprotein functions.

Circulating low density lipoprotein (LDL)

Low-density lipoprotein (LDL) particles are known as atherogenic agents in coronary artery diseases. They modify to other electronegative forms and may be the subject for improvement of inflammatory events in vessel subendothelial spaces. The circulating LDL value is associated with the plasma PCSK-9 level. They internalize into macrophages using the lysosomal receptor-mediated pathways. LDL uptake is related to the membrane scavenger receptors, modifications of lipid and protein components of LDL particles, vesicular maturation and lipid stores of cells. Furthermore, LDL vesicular trafficking is involved with the function of some proteins such as Rab and Lamp families. These proteins also help in the transportation of free cholesterol from lysosome into the cytosol. The aggregation of lipids in the cytosol is a starting point for the formation of foam cells so that they may participate in the primary core of atherosclerosis plaques. The effects of macrophage subclasses are different in the formation and remodeling of plaques. This review is focused on the cellular and molecular events involved in cholesterol homeostasis.

Exploiting scavenger receptors in cancer immunotherapy: Lessons from CD5 and SR-B1

Scavenger receptors (SRs) are structurally heterogeneous cell surface receptors characterized by their capacity to remove extraneous or modified self-macromolecules from circulation, thus avoiding the accumulation of noxious agents in the extracellular space. This scavenging activity makes SRs important molecules for host defense and homeostasis. In turn, SRs keep the activation of the steady-state immune response in check, and participate as co-receptors in the priming of the effector immune responses when the macromolecules are associated with a threat that might compromise host homeostasis. Therefore, SRs built up sophisticated sensor mechanisms controlling the immune system, which may be exploited to develop novel drugs for cancer immunotherapy. In this review, we focus on the regulation of the anti-tumor immune response by two paradigmatic SRs: the lymphocyte receptor CD5 and the more broadly distributed scavenger receptor class B type 1 (SR-B1). Cancer immunity can be boosted by blockade of SRs working as immune checkpoint inhibitors (CD5) and/or by proper engagement of SRs working as innate danger receptor (SR-B1). Thus, these receptors illustrate both the complexity of targeting SRs in cancer immunotherapy and also the opportunities offered by such an approach.

Inflammatory Mediators in Xanthelasma Palpebrarum: Histopathologic and Immunohistochemical Study

PURPOSE

To evaluate the expression of inflammatory mediators in xanthelasma palpebrarum.

METHODS

In this retrospective histopathologic case-control study, xanthelasma specimens obtained from the private practice and pathology archives of 1 author (R.Z.S.) were analyzed and compared with the blepharoplasty tissues from age- and sex-matched controls. Paraffin-embedded tissue sections were stained with hematoxylin-eosin and CD3, CD20, CD163, cyclooxygenase-1, inducible nitric oxide synthase, matrix metallopeptidase-9, and myeloperoxidase antibodies. Immunostaining was quantified by light microscopy and with a computerized image analysis system of scanned images.

RESULTS

Hematoxylin-eosin-stained preparations of xanthelasma specimens demonstrated significantly more intense chronic lymphocytic infiltrate when compared with the control blepharoplasty tissues (p < 0.001). Immunohistochemical studies revealed more intense CD3+ T cell and CD163+ histiocytic infiltrate (11% vs. 5%; p = 0.02 and 28% vs. 5%; p = 0.003, respectively) and increased expression of cyclooxygenase-1 (44% vs. 20% expressing cells; p < 0.001 and 21% vs. 9% strongly expressing cells; p = 0.008) and inducible nitric oxide synthase (43% vs. 26% expressing cells; p = 0.03 and 42% vs. 25% strongly expressing cells; p = 0.02) in xanthelasma specimens compared with control tissues.

CONCLUSIONS

The inflammatory milieu in xanthelasma appears to be analogous to descriptions of the early stages of cardiac atherosclerotic plaque formation. These findings may contribute to the understanding of xanthelasma pathogenesis and to the development of potential targeted therapies.

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

Low-density lipoproteins (LDLs, also known as 'bad cholesterol') are the major carriers of circulating cholesterol and the main causative risk factor of atherosclerosis. Plasma LDLs are 20- to 25-nm nanoparticles containing a core of cholesterol esters surrounded by a phospholipid monolayer and a single copy of apolipoprotein B (550 kDa). An early sign of atherosclerosis is the accumulation of LDL-derived lipid droplets in the arterial wall. According to the widely accepted 'response-to-retention hypothesis', LDL binding to the extracellular matrix proteoglycans in the arterial intima induces hydrolytic and oxidative modifications that promote LDL aggregation and fusion. This enhances LDL uptake by the arterial macrophages and triggers a cascade of pathogenic responses that culminate in the development of atherosclerotic lesions. Hence, LDL aggregation, fusion, and lipid droplet formation are important early steps in atherogenesis. In vitro, a variety of enzymatic and nonenzymatic modifications of LDL can induce these reactions and thereby provide useful models for their detailed analysis. Here, we summarize current knowledge of the in vivo and in vitro modifications of LDLs leading to their aggregation, fusion, and lipid droplet formation; outline the techniques used to study these reactions; and propose a molecular mechanism that underlies these pro-atherogenic processes. Such knowledge is essential in identifying endogenous and exogenous factors that can promote or prevent LDL aggregation and fusion in vivo and to help establish new potential therapeutic targets to decelerate or even block these pathogenic reactions.

Lysosomal acid lipase: at the crossroads of normal and atherogenic cholesterol metabolism

Unregulated cellular uptake of apolipoprotein B-containing lipoproteins in the arterial intima leads to the formation of foam cells in atherosclerosis. Lysosomal acid lipase (LAL) plays a crucial role in both lipoprotein lipid catabolism and excess lipid accumulation as it is the primary enzyme that hydrolyzes cholesteryl esters derived from both low density lipoprotein (LDL) and modified forms of LDL. Evidence suggests that as atherosclerosis progresses, accumulation of excess free cholesterol in lysosomes leads to impairment of LAL activity, resulting in accumulation of cholesteryl esters in the lysosome as well as the cytosol in foam cells. Impaired metabolism and release of cholesterol from lysosomes can lead to downstream defects in ATP-binding cassette transporter A1 regulation, needed to offload excess cholesterol from plaque foam cells. This review focuses on the role LAL plays in normal cholesterol metabolism and how the associated changes in its enzymatic activity may ultimately contribute to atherosclerosis progression.

Contribution of monocyte-derived macrophages and smooth muscle cells to arterial foam cell formation

Smooth muscle cells (SMCs) are the main cell type in intimal thickenings and some stages of human atherosclerosis. Like monocyte-derived macrophages, SMCs accumulate excess lipids and contribute to the total intimal foam cell population. In contrast, apolipoprotein (Apo)E-deficient and LDL receptor-deficient mice develop atherosclerotic lesions that are macrophage- as opposed to SMC-rich. The lesser contribution of SMCs to lesion development in these mouse models has distracted attention away from the importance of SMC cholesterol homeostasis in the artery wall. Intimal SMCs accumulate excess amounts of cholesteryl esters when compared with medial layer SMCs, possibly explained by reduced ATP-binding cassette transporter A1 expression and ApoA-I binding to intimal-type SMCs. The aim of this review is to compare the relative contribution of monocyte-derived macrophages and SMCs to human vs. mouse atherosclerosis, and describe what is known about lipid uptake and removal mechanisms contributing to arterial macrophage and SMC foam cell formation. An increased understanding of the contribution of these cell types to lesion development will help to delineate their relative importance in atherogenesis and as potential therapeutic targets.

Size-selective uptake of colloidal low density lipoprotein aggregates by cultured white blood cells

This paper illustrates how principles of colloid science are useful in studying atherosclerosis. Accumulation of foam cells in the arterial intima is a key step in atherogenesis. The extent of foam cell formation is enhanced by low density lipoprotein (LDL) aggregates, and we have previously shown that the size of sphingomyelinase (Smase)-hydrolysis-induced aggregates depends directly on the concentration of ceramide generated in the LDL phospholipid monolayer, mediated by the hydrophobic effect. Here, we focus on the effect of LDL aggregate particle sizes on their subsequent uptake by macrophages. Our data show the first direct measurement of uptake as a function of aggregate size and the first direct comparison of uptake after Smase-catalyzed and vortex-mixing-mediated aggregation. Vortex-mixed aggregates with radii 20-77 nm showed maximal uptake approximately 118 microg sterol/mg protein at a 53 nm intermediate size, consistent with a mathematical model describing competition between aggregate surface area and volume. Smase-treated aggregates with radii 25-211 nm also showed maximal uptake at an intermediate size, approximately 58 microg sterol/mg protein for 132 nm particles, and fit a modified model that incorporated ceramide concentration expressed as aggregate size. This study shows that particle size is significant and composition may also be a factor in LDL uptake.

Apolipoprotein A-V N-terminal domain lipid interaction properties in vitro explain the hypertriglyceridemic phenotype associated with natural truncation mutants

The N-terminal 146 residues of apolipoprotein (apo) A-V adopt a helix bundle conformation in the absence of lipid. Because similarly sized truncation mutants in human subjects correlate with severe hypertriglyceridemia, the lipid binding properties of apoA-V(1-146) were studied. Upon incubation with phospholipid in vitro, apoA-V(1-146) forms reconstituted high density lipoproteins 15-17 nm in diameter. Far UV circular dichroism spectroscopy analyses of lipid-bound apoA-V(1-146) yielded an alpha-helix secondary structure content of 60%. Fourier transformed infrared spectroscopy analysis revealed that apoA-V(1-146) alpha-helix segments align perpendicular with respect to particle phospholipid fatty acyl chains. Fluorescence spectroscopy of single Trp variant apoA-V(1-146) indicates that lipid interaction is accompanied by a conformational change. The data are consistent with a model wherein apoA-V(1-146) alpha-helices circumscribe the perimeter of a disk-shaped bilayer. The ability of apoA-V(1-146) to solubilize dimyristoylphosphatidylcholine vesicles at a rate faster than full-length apoA-V suggests that N- and C-terminal interactions in the full-length protein modulate its lipid binding properties. Preferential association of apoA-V(1-146) with murine plasma HDL, but not with VLDL, suggests that particle size is a determinant of its lipoprotein binding specificity. It may be concluded that defective lipoprotein binding of truncated apoA-V contributes to the hypertriglyceridemia phenotype associated with truncation mutations in human subjects.

Macrophages create an acidic extracellular hydrolytic compartment to digest aggregated lipoproteins

A critical event in atherogenesis is the interaction of macrophages with subendothelial lipoproteins. Although most studies model this interaction by incubating macrophages with monomeric lipoproteins, macrophages in vivo encounter lipoproteins that are aggregated. The physical features of the lipoproteins require distinctive mechanisms for their uptake. We show that macrophages create an extracellular, acidic, hydrolytic compartment to carry out digestion of aggregated low-density lipoproteins. We demonstrate delivery of lysosomal contents to these specialized compartments and their acidification by vacuolar ATPase, enabling aggregate catabolism by lysosomal acid hydrolases. We observe transient sealing of portions of the compartments, allowing formation of an "extracellular" proton gradient. An increase in free cholesterol is observed in aggregates contained in these compartments. Thus, cholesteryl ester hydrolysis can occur extracellularly in a specialized compartment, a lysosomal synapse, during the interaction of macrophages with aggregated low-density lipoprotein. A detailed understanding of these processes is essential for developing strategies to prevent atherosclerosis.

Low density lipoprotein undergoes oxidation within lysosomes in cells

The oxidized low density lipoprotein (LDL) hypothesis of atherosclerosis proposes that LDL undergoes oxidation in the interstitial fluid of the arterial wall. We have shown that aggregated (vortexed) nonoxidized LDL was taken up by J774 mouse macrophages and human monocyte-derived macrophages and oxidized intracellularly, as assessed by the microscopic detection of ceroid, an advanced lipid oxidation product. Confocal microscopy showed that the ceroid was located in the lysosomes. To confirm these findings, J774 macrophages were incubated with acetylated LDL, which is internalized rapidly to lysosomes, and then incubated (chase incubation) in the absence of any LDL. The intracellular levels of oxysterols, measured by HPLC, increased during the chase incubation period, showing that LDL must have been oxidized inside the cells. Furthermore, we found that this oxidative modification was inhibited by lipid-soluble antioxidants, an iron chelator taken up by fluid-phase pinocytosis and the lysosomotropic drug chloroquine, which increases the pH of lysosomes. The results indicate that LDL oxidation can occur intracellularly, most probably within lysosomes.

Contribution of xanthoma tissue-derived LDL density substances in the transformation of macrophages to foam cells

BACKGROUND

The source of accumulated lipids in the foam cells of xanthoma is primarily the lipoproteins existing in the lesional dermis.

OBJECTIVE

This study was designated to clarify the contribution of low density lipoprotein (LDL) density substances existing in xanthoma tissue to foam cell formation.

METHODS

An LDL density fraction was obtained from homogenized rabbit experimental xanthoma tissue. Biochemical and functional characteristics of xanthoma-extracted LDL density substance were examined. The in vivo foam cell-inducing ability of xanthoma-extracted LDL density substance was examined microscopically at the intradermal injection site.

RESULTS

Xanthoma-extracted LDL density substance showed more negatively charged mobility on agarose gel electrophoresis than plasma native LDL. A small amount of aggregated material remained at the origin on agarose gel electrophoresis. Xanthoma-extracted LDL density substance contained much higher level of lipid peroxides than native LDL. Mouse peritoneal macrophages internalized xanthoma-extracted LDL density substance extensively and transformed into foam cells by incubation with xanthoma-extracted LDL density substance. Intradermal injection of the xanthoma-extracted LDL density substance induced foam cell infiltration in the skin of a normolipemic rabbit.

CONCLUSION

LDL density substances prepared ex vivo from experimental xanthoma tissue contained lipid-protein complexes that have physiochemical properties of oxidized LDL. The lipid-protein complexes were incorporated into foam cells. The substances were considered to contribute to foam cell recruitment during the persistence of xanthoma lesions.

Sphingomyelinase-to-LDL molar ratio determines low density lipoprotein aggregation size: biological significance

The subendothelial retention of low density lipoproteins (LDL) is believed to be the central pathogenic event in atherosclerosis, as stated by the response-to-retention hypothesis. Sphingomyelinase, an enzyme present in the arteries, has been proven to promote LDL aggregation. This study investigates the hypothesis that the extent of LDL aggregation is determined by the molar ratio of sphingomyelinase (SMase)-to-LDL, rather than the absolute concentrations. A mass action model is used to describe the aggregation process, and binding and dissociation rate constants are determined by fitting of dynamic light scattering data. The model predicts aggregate sizes that agree well with experimental observations. This study also tests the hypothesis that monocyte uptake of LDL correlates with aggregate size. LDL aggregates of three specific sizes (75, 100, and 150 nm) were incubated with J774A.1 cells and the net accumulation of LDL was monitored by measuring changes in the cellular cholesterol and protein content. Relative to a control sample, cholesterol accumulation was enhanced for aggregate sizes of 75 and 150 nm. The intermediate size aggregates, 100 nm, led to a very striking result demonstrating that cholesterol accumulation was markedly greater than the other samples, and was sufficient to cause cell death. These results underscore an important role of colloidal aggregation, and the influence of LDL aggregate size, in atherosclerosis.

Apolipoprotein E polymorphism and lipoprotein compositions in normolipidaemic xanthelasma patients

BACKGROUND

Apolipoprotein E (apoE) phenotypes and lipoprotein compositions in xanthelasma patients have been reported in different series.

OBJECTIVE

To investigate the apoE polymorphism and lipoprotein compositions in xanthelasma patients by using rapid polymerase chain reaction, and searched for an association between apoE polymorphism and the lipoprotein levels in xanthelasma patients.

DESIGN

ApoE polymorphism and the different types of serum lipoproteins were studied in 25 patients with xanthelasma and compared with 27 normal subjects.

RESULTS

All of patients were found to be normolipidaemic. The patients had significantly higher concentrations of total cholesterol and apolipoprotein B, and lower concentrations of apolipoprotein A. There was no difference in serum triglyceride, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol concentrations. The distribution of apoE genotypes and alleles was the same in both groups.

CONCLUSIONS

The apoB, apoA and cholesterol levels did show statistically significant differences in the direction of an increased risk of atherosclerosis. Patients with xanthelasma demonstrated slight differentiations in the apoE polymorphism and metabolism of lipoproteins that require further clarifications.

Further evaluation of plasma sphingomyelin levels as a risk factor for coronary artery disease

BACKGROUND

Sphingomyelin (SM) is the major phospholipid in cell membranes and in lipoproteins. In human plasma, SM is mainly found in atherogenic lipoproteins; thus, high levels of SM may promote atherogenesis.

METHODS

We investigated in a median follow up of 6.0 years the association of SM with the incidence of a combined endpoint (myocardial infarction and cardiovascular death) in stable and unstable patients, and its relation to other marker of atherosclerosis in 1,102 patients with angiographically documented CAD and 444 healthy controls.

RESULTS AND DISCUSSION

Logistic regression analysis showed that SM categorized by median was associated with an elevated risk for CAD (HR 3.2, 95%CI 2.5-4.0, p < 0.05). SM levels were correlated with apoB (r = 0.34) and triglyceride levels (r = 0.31). In patients with stable angina (n = 614), SM categorized by median was not related to incidence of a combined endpoint (cardiovascular death and myocardial infarction) (p = 0.844 by Log-rank test). However, in patients with acute coronary syndrome (n = 488), elevated SM was related to the combined endpoint (p < 0.05 by Log-rank test), also in a multivariate Cox regression analysis including potential confounders (HR 1.8, 95%CI 1.0-3.3, p < 0.05).

CONCLUSION

The results of our study reveal that 1) human plasma SM levels are a risk factor for CAD; 2) the pro-atherogenic property of plasma SM might be related to metabolism of apoB-containing or triglyceride-rich lipoproteins; and 3) plasma SM levels are a predictor for outcome of patients with acute coronary syndrome.

Aggregated LDL and lipid dispersions induce lysosomal cholesteryl ester accumulation in macrophage foam cells

Macrophage foam cells in atherosclerotic lesions accumulate substantial cholesterol stores within large, swollen lysosomes. Previous studies with mildly oxidized low density lipoprotein (OxLDL)-treated THP-1 macrophages suggest an initial buildup of free cholesterol (FC), followed by an inhibition of lysosomal cholesteryl ester (CE) hydrolysis and a subsequent lysosomal accumulation of unhydrolyzed lipoprotein CE. We examined whether other potential sources of cholesterol found within atherosclerotic lesions could also induce similar lysosomal accumulation. Biochemical analysis combined with microscopic analysis showed that treatment of THP-1 macrophages with aggregated low density lipoprotein (AggLDL) or CE-rich lipid dispersions (DISP) produced a similar lysosomal accumulation of both FC and CE. Co-treatment with an ACAT inhibitor, CP113,818, confirmed that the CE accumulation was primarily the result of the inhibition of lysosomal CE hydrolysis. The rate of unhydrolyzed CE buildup was more rapid with DISP than with AggLDL. However, with both treatments, FC appeared to accumulate in lysosomes before the inhibition in hydrolysis and CE accumulation, a sequence shared with mildly OxLDL. Thus, lysosomal accumulation of FC and CE can be attributable to more general mechanisms than just the inhibition of hydrolysis by oxidized lipids.

Group V Secretory Phospholipase A2-modified Low Density Lipoprotein Promotes Foam Cell Formation by a SR-A- and CD36-independent Process That Involves Cellular Proteoglycans

Accumulating evidence indicates that secretory phospholipase A2 (sPLA2) enzymes promote atherogenic processes. We have previously showed the presence of Group V sPLA2 (GV sPLA2) in human and mouse atherosclerotic lesions, its hydrolysis of low density lipoprotein (LDL) particles, and the ability of GV sPLA2-modified LDL (GV-LDL) to induce macrophage foam cell formation in vitro. The goal of this study was to investigate the mechanisms involved in macrophage uptake of GV-LDL. Peritoneal macrophages from C57BL/6 mice (wild type (WT)), C57BL/6 mice deficient in LDL receptor (LDLR-/-), or SR-A and CD36 (DKO) were treated with control LDL, GV-LDL, oxidized LDL (ox-LDL) or LDL aggregated by vortexing (vx-LDL). As expected, ox-LDL induced significantly more cholesterol ester accumulation in WT and LDLR-/- compared with DKO macrophages. In contrast, there was no difference in the accumulation of GV-LDL or vx-LDL in the three cell types. 125I-ox-LDL exhibited high affinity, saturable binding to WT cells that was significantly reduced in DKO cells. Vx-LDL and GV-LDL showed low affinity, non-saturable binding that was similar for both cell types, and significantly higher compared with control LDL. GV-LDL degradation in WT and DKO cells was similar. Analyses by confocal microscopy indicated a distinct intracellular distribution of Alexa-568-labeled GV-LDL and Alexa-488-labeled ox-LDL. Uptake of GV-LDL (but not ox-LDL or vx-LDL) was significantly reduced in cells preincubated with heparin or NaClO3, suggesting a role for proteoglycans in GV-LDL uptake. Our data point to a physiological modification of LDL that has the potential to promote macrophage foam cell formation independent of scavenger receptors.

Thematic review series: The Immune System and Atherogenesis. Lipoprotein-associated inflammatory proteins: markers or mediators of cardiovascular disease?

In humans, a chronically increased circulating level of C-reactive protein (CRP), a positive acute-phase reactant, is an independent risk factor for cardiovascular disease. This observation has led to considerable interest in the role of inflammatory proteins in atherosclerosis. In this review, after discussing CRP, we focus on the potential role in the pathogenesis of human vascular disease of inflammation-induced proteins that are carried by lipoproteins. Serum amyloid A (SAA) is transported predominantly on HDL, and levels of this protein increase markedly during acute and chronic inflammation in both animals and humans. Increased SAA levels predict the risk of cardiovascular disease in humans. Recent animal studies support the proposal that SAA plays a role in atherogenesis. Evidence is accruing that secretory phospholipase A(2), an HDL-associated protein, and platelet-activating factor acetylhydrolase, a protein associated predominantly with LDL in humans and HDL in mice, might also play roles both as markers and mediators of human atherosclerosis. In contrast to positive acute-phase proteins, which increase in abundance during inflammation, negative acute-phase proteins have received less attention. Apolipoprotein A-I (apoA-I), the major apolipoprotein of HDL, decreases during inflammation. Recent studies also indicate that HDL is oxidized by myeloperoxidase in patients with established atherosclerosis. These alterations may limit the ability of apoA-I to participate in reverse cholesterol transport. Paraoxonase-1 (PON1), another HDL-associated protein, also decreases during inflammation. PON1 is atheroprotective in animal models of hypercholesterolemia. Controversy over its utility as a marker of human atherosclerosis may reflect the fact that enzyme activity rather than blood level (or genotype) is the major determinant of cardiovascular risk. Thus, multiple lipoprotein-associated proteins that change in concentration during acute and chronic inflammation may serve as markers of cardiovascular disease. In future studies, it will be important to determine whether these proteins play a causal role in atherogenesis.

Aggregation kinetics of low density lipoproteins upon exposure to sphingomyelinase

The response-to-retention hypothesis in atherosclerosis states that subendothelial retention of cholesterol-rich, atherogenic lipoproteins is the central pathogenic event that is both necessary and sufficient to provoke lesion initiation in an otherwise normal artery. Sphingomyelinase-induced aggregation of low density lipoproteins (LDL) is known to facilitate LDL retention, and the only available measurements of LDL aggregates suggest LDL aggregate size is approximately 100 nm. This study investigates the hypothesis that LDL aggregate size is determined by the relative rates of sphingomyelinase hydrolysis and LDL collisions. Using a combination of dynamic light scattering and UV-vis absorbance spectroscopy to measure aggregation kinetics and particle sizes, a mass action model was developed to describe the aggregation process. It is found that LDL aggregation is sensitive to the relative amounts of sphingomyelinase and LDL and to pH. Model rate parameters were fit to experimental data in vitro and used to predict LDL aggregate sizes in vivo. The value of 100 nm in vivo does not appear to be fixed; rather, it is the value expected for the prevailing enzyme-to-LDL molar ratio.

Group V sPLA2 hydrolysis of low-density lipoprotein results in spontaneous particle aggregation and promotes macrophage foam cell formation

OBJECTIVE

Secretory phospholipase A2 (sPLA2) enzymes hydrolyze the sn-2 fatty acyl ester bond of phospholipids to produce a free fatty acid and a lysophospholid. Group V sPLA2 is expressed in cultured macrophage cells and has high affinity for phosphatidyl choline-containing substrates. The present study assesses the presence of group V sPLA2 in human and mouse atherosclerotic lesions and its activity toward low-density lipoprotein (LDL) particles.

METHODS AND RESULTS

Group V sPLA2 was detected in human and mouse atherosclerotic lesions by immunohistochemical staining. Electron microscopic analysis showed that mouse group V sPLA2-modified LDL is significantly smaller (mean diameter+/-SEM=25.3+/-0.25 nm) than native LDL (mean diameter+/-SEM=27.7+/-0.29 nm). Hydrolysis by group V sPLA2 induced spontaneous particle aggregation; the extent of aggregation was directly proportional to the degree of LDL hydrolysis. Group V sPLA2 modification of LDL led to enhanced lipid accumulation in cultured mouse peritoneal macrophage cells.

CONCLUSIONS

Group V sPLA2 may play an important role in promoting atherosclerotic lesion development by modifying LDL particles in the arterial wall, thereby enhancing particle aggregation, retention, and macrophage uptake.

Decreased Macrophage Paraoxonase 2 Expression in Patients With Hypercholesterolemia Is the Result of Their Increased Cellular Cholesterol Content: Effect of Atorvastatin Therapy

OBJECTIVE

To analyze paraoxonase2 (PON2) expression in human monocyte-derived macrophages (HMDM) from patients with hypercholesterolemia in relation to cellular cholesterol and oxidative stress.

METHODS AND RESULTS

Ten healthy subjects (controls) and 10 patients with hypercholesterolema who received 20-mg/d atorvastatin participated in the study. The patients' versus controls' HMDM demonstrated increased cholesterol content (270%) and oxidative stress (30% to 45%). Atorvastatin therapy reduced these parameters (59% and 25%, respectively). The patients' versus controls' macrophage-PON2 mRNA expression and PON2 activity were lower (100% and 40%, respectively), and atorvastatin therapy increased these parameters (76% and 200%, respectively). Untreated patient HMDM incubation with atorvastatin (0 to 10 micromol/L) resulted in a dose-dependent reduction in cellular cholesterol content and in cell-mediated low-density lipoprotein (LDL) oxidation up to 79% and 66%, respectively. In parallel, PON2 mRNA expression and PON2 activity increased dose-dependently up to 3.6- and 2.1-fold, respectively. On incubation of control HMDM with acetylated-LDL or aggregated-LDL, cellular cholesterol content increased (77% and 100%), and macrophage-PON2 activity decreased (49% and 22%), respectively. In contrast, oxidized LDL increased both cellular oxidative stress and PON2 expression.

CONCLUSIONS

HMDM-PON2 expression is reduced in patients with hypercholesterolemia as a result of their increased cellular cholesterol content. Atorvastatin therapy reduced both macrophage oxidative stress and cholesterol content, and upregulated PON2 expression, thus contributing to attenuation of foam cells formation.

Reduction of Atherosclerotic Plaques by Lysosomal Acid Lipase Supplementation

Objective— Proof of principle is presented for targeted enzyme supplementation by using lysosomal acid lipase to decrease aortic and coronary wall lipid accumulation in a mouse model of atherosclerosis.

Methods and Results— Mice with LDL receptor deficiency were placed on an atherogenic diet and developed predictable aortic and coronary atheroma. α-Mannosyl-terminated human lysosomal acid lipase (phLAL) was produced in Pichia pastoris , purified, and administered intravenously to such mice with either early or late lesions. phLAL injections reduced plasma, hepatic, and splenic cholesteryl esters and triglycerides in affected mice. phLAL was detected in hepatic Kupffer cells and in atheromatous foam cells. Repeated enzyme injections were well tolerated, with no obvious adverse effects. In addition, the coronary and aortic atheromatous lesions were (1) eliminated in their early stages and (2) quantitatively and qualitatively reduced in their advanced stages.

Conclusion— These results support the potential utility of lysosomal acid lipase supplementation for the treatment of atherosclerosis, a leading cause of mortality and morbidity in Westernized nations.

The cytoplasmic domain of the low density lipoprotein (LDL) receptor-related protein, but not that of the LDL receptor, triggers phagocytosis

The macrophage LDL receptor and LDL receptor-related protein (LRP, CD91) mediate the phagocytic-like uptake of atherogenic lipoproteins and apoptotic cells, yet the structural basis of their phagocytic functions is not known. To address this issue, we transfected macrophages with chimeric proteins containing the cytoplasmic tails and transmembrane regions of the LDL receptor or LRP and the ectodomain of CD2, which can bind non-opsonized sheep red blood cells (SRBCs). Macrophages expressing receptors containing the LDL receptor domains were able to bind but not internalize SRBCs. In contrast, macrophages expressing receptors containing the cytoplasmic tail of LRP were able to bind and internalize SRBCs. Chimeras in which the LRP cytoplasmic tail was mutated in two di-leucine motifs and a tyrosine in an NPXYXXL motif were able to endocytose anti-CD2 antibody and bind SRBCs, but SRBC phagocytosis was decreased by 70%. Thus, the phagocytic-like functions of LRP, but not those of the LDL receptor, can be explained by the ability of the LRP cytoplasmic tail to trigger phagocytosis. These findings have important implications for atherogenesis and apoptotic cell clearance and for a fundamental cell biological understanding of how the LDL receptor and LRP function in internalization processes.

Structural changes of low density lipoproteins with Cu2+and glucose induced oxidation

The compositional and structural changes of lipids and apolipoproteins during in vitro oxidation of low density lipoprotein (LDL) are investigated in this study by IR spectroscopy. For comparison, LDL samples containing either copper or glucose at physiological or pathological concentrations are considered in order to know the separate affects of these chemical factors on LDL oxidation. The results show that the initial steps of lipid oxidation proceed through hydrogen atom loss from methylene groups, as well as loss of cholesteryl ester molecules, and later a recovering of carbonyl compounds resulting from aldehyde formation that generally occurs in autooxidation processes. Lipid oxidation is induced by copper ions, and glucose enhances metal ion induced LDL oxidation as determined by conjugated diene formation. As to the protein conformational changes, IR spectroscopy reveals for the first time that LDL oxidation involves formation of beta-sheet structures, these being more abundant in LDL samples with pathological concentrations of glucose or copper. Consequently, the LDL structural changes may contribute to the recognition of oxidized LDL particles by scavenger receptors.

Lysosomal enzymes are released from cultured human macrophages, hydrolyze LDL in vitro, and are present extracellularly in human atherosclerotic lesions

OBJECTIVE

Human atherosclerotic lesions have been shown to contain lipid droplets and vesicles resembling those of in vitro enzymatically modified LDL. However, little is known about the hydrolytic enzymes in the arterial intima that induce fusion of LDL particles and so produce lipid droplets or that induce foam cell formation.

METHODS AND RESULTS

Human coronary atherosclerotic lesions obtained at surgery and at autopsy were stained for lysosomal acid lipase and cathepsin D. The extracellular areas of macrophage-rich intimal regions of the atherosclerotic lesions stained positively for both cathepsin D and lysosomal acid lipase, whereas normal arteries were negative. When monocyte-derived macrophages were incubated with opsonized zymosan to stimulate the release of lysosomal enzymes from the cells and LDL was incubated with the macrophage-conditioned media, the apolipoprotein B-100, cholesteryl esters, and triacylglycerols of LDL were hydrolyzed. These hydrolytic modifications rendered the LDL particles unstable and induced their fusion. Cultured macrophages and smooth muscle cells took up the hydrolase-modified LDL particles avidly and were transformed into foam cells.

CONCLUSIONS

Our in vivo and in vitro results suggest that lysosomal enzymes released from macrophages may induce hydrolytic modification of LDL and foam cell formation in the human arterial intima.

Inhibition of oxidized LDL aggregation with the calcium channel blocker amlodipine: role of electrostatic interactions

Atherogenic low-density lipoproteins (LDL) are characterized by elevations in cholesterol content and increased electronegativity, factors that contribute to aggregation and foam cell formation. This study was designed to test the effect of the positively charged calcium channel blocker (CCB) amlodipine on the aggregation properties of oxidized LDL lipids. Large unilamellar vesicles (LUVs) (100 nm diameter) labeled with a non-exchangeable marker [3H]cholesteryl hexadecyl ether were prepared with lipids extracted from human LDL following oxidation. The LUVs were shown to bind, in a reversible fashion, to charged diethylaminoethyl Sephadex columns. The addition of amlodipine inhibited binding of the oxidized LDL lipids in a dose-dependent fashion with an IC(50) in the nanomolar range as a result of its high lipophilicity and positively charged amino group (pK(a) of 9.02). The activity of amlodipine was reproduced in model membranes that contained fixed amounts of charged phospholipid (glycerophospholipid) in a concentration-dependent manner. By contrast, drugs lacking a formal positive charge, including CCBs (felodipine, nifedipine, diltiazem, verapamil) and an angiotensin-converting enzyme-inhibitor (ramiprilate) had no effect on the column binding of the modified, electronegative lipids. These effects of amlodipine on LDL lipid aggregation and electrostatic properties may represent a novel antiatherosclerotic mechanism of action.

Effect of fluid mechanical stresses and plasma constituents on aggregation of LDL

LDL aggregates when exposed to even moderate fluid mechanical stresses in the laboratory, yet its half-life in the circulation is 2-3 days, implying that little aggregation occurs. LDL may be protected from aggregation in vivo by components of plasma, or by a qualitative difference in flows. Previous studies have shown that HDL and albumin inhibit the aggregation induced by vortexing. Using a more reproducible method of inducing aggregation and assessing aggregation both spectrophotometrically and by sedimentation techniques, we showed that at physiological concentrations, albumin is the more effective inhibitor, and that aggregation is substantially but not completely inhibited in plasma. Heat denatured and fatty-acid-stripped albumin were more effective inhibitors than normal albumin, supporting the idea that hydrophobic interactions are involved. Aggregation of LDL in a model reproducing several aspects of flow in the circulation was 200-fold slower, but was still inhibited by HDL and albumin, suggesting similar mechanisms are involved. Within the sensitivity of our technique, LDL aggregation did not occur in plasma exposed to these flows. Thus, as a result of the characteristics of blood flow and the inhibitory effects of plasma components, particularly albumin, LDL aggregation is unlikely to occur within the circulation.

Diverse fibrillar peptides directly bind the Alzheimer's amyloid precursor protein and amyloid precursor-like protein 2 resulting in cellular accumulation

The Alzheimer's disease Abeta peptide can increase the levels of cell-associated amyloid precursor protein (APP) in vitro. To determine the specificity of this response for Abeta and whether it is related to cytotoxicity, we tested a diverse range of fibrillar peptides including amyloid-beta (Abeta), the fibrillar prion peptides PrP106-126 and PrP178-193 and human islet-cell amylin. All these peptides increased the levels of APP and amyloid precursor-like protein 2 (APLP2) in primary cultures of astrocytes and neurons. Specificity was shown by a lack of change to amyloid precursor-like protein 1, tau-1 and cellular prion protein (PrP(c)) levels. APP and APLP2 levels were elevated only in cultures exposed to fibrillar peptides as assessed by electron microscopy and not in cultures treated with non-fibrillogenic peptide variants or aggregated lipoprotein. We found that PrP106-126 and the non-toxic but fibril-forming PrP178-193 increased APP levels in cultures derived from both wild-type and PrP(c)-deficient mice indicating that fibrillar peptides up-regulate APP through a non-cytotoxic mechanism and irrespective of parental protein expression. Fibrillar PrP106-126 and Abeta peptides bound recombinant APP and APLP2 suggesting the accumulation of these proteins was mediated by direct binding to the fibrillated peptide. This was supported by decreased APP accumulation following extensive washing of the cultures to remove fibrillar aggregates. Pre-incubation of fibrillar peptide with recombinant APP18-146, the putative fibril binding site, also abrogated the accumulation of APP. These findings show that diverse fibrillogenic peptides can induce accumulation of APP and APLP2 and this mechanism could contribute to pathogenesis in neurodegenerative disorders.

Macrophage-expressed group IIA secretory phospholipase A2 increases atherosclerotic lesion formation in LDL receptor-deficient mice

OBJECTIVE

Transgenic mice expressing human group IIA secretory phospholipase A(2) (group IIA sPLA(2)) spontaneously develop atherosclerotic lesions. The mechanism for this proatherogenic effect is likely multifactorial, because HDL-cholesterol is significantly lower and LDL/VLDL cholesterol is slightly higher in transgenic mice compared with nontransgenic littermates. In the present study, we show for the first time that elicited peritoneal macrophages from transgenic mice express human group IIA sPLA(2). This study tested whether macrophage-expressed sPLA(2) contributes to atherogenesis.

METHODS AND RESULTS

Bone marrow cells from either sPLA(2) transgenic mice or control C57BL/6 mice were transplanted into LDL receptor-deficient mice. After hematopoietic engraftment, animals were fed a diet enriched with saturated fat and cholesterol for 12 weeks. Despite a lack of effect on serum lipoprotein concentrations, the presence of bone marrow-derived cells expressing human group IIA sPLA(2) resulted in a significant increase in the extent of atherosclerosis in the aortic arch (12.8+/-1.4% versus 7.4+/-0.9%; P<0.005) and aortic sinus (0.3+/-0.03 mm(2) versus 0.2+/-0.04 mm(2); P<0.05).

CONCLUSIONS

Group IIA sPLA(2) can contribute to atherosclerotic lesion development through a mechanism that is independent of systemic lipoprotein metabolism.

Sequestration of aggregated low-density lipoproteins by macrophages

PURPOSE OF REVIEW

Evidence suggests that much of the LDL in atherosclerotic plaques is aggregated. Aggregation of LDL could be an important factor that determines how this lipoprotein is metabolized by plaque macrophages and the fate of aggregated LDL cholesterol within plaques. This review discusses a novel endocytic pathway by which macrophages process aggregated LDL.

RECENT FINDINGS

Recently, it has been shown that aggregated LDL can be sequestered in macrophage surface-connected compartments and plasma membrane invaginations by a process termed patocytosis. In contrast to rapid degradation of LDL and aggregated LDL taken up by macrophages through pinocytosis and phagocytosis, respectively, aggregated LDL sequestered in macrophages undergoes only limited degradation. Macrophages can disaggregate and release sequestered aggregated LDL by activating plasminogen to plasmin. Plasmin degrades LDL apolipoprotein B sufficiently to disaggregate the aggregated LDL, releasing it from the macrophage surface-connected compartments. In contrast, activating macrophages with phorbol-myristate-acetate stimulates degradation of aggregated LDL and inhibits plasminogen-mediated release of the aggregated lipoprotein from macrophage surface-connected compartments.

SUMMARY

Macrophage sequestration of aggregated LDL is a unique endocytic pathway relevant not only to the processing of aggregated LDL in atherosclerotic plaques but also for the processing of other materials, such as hydrophobic particles that trigger this endocytic pathway. Macrophage sequestration of aggregated LDL can result in different fates for the aggregated LDL, depending on the state of macrophage activation and the functioning of the plasminogen-based fibrinolytic system. Patocytosis of aggregated LDL should be considered in addition to phagocytosis as a possible uptake pathway in studies of macrophage processing of aggregated LDL.

Biological effects of secretory phospholipase A(2) group IIA on lipoproteins and in atherogenesis

Secretory phospholipase A(2) group IIA(sPLA(2) IIA) can be produced and secreted by various cell types either constitutionally or as an acute-phase reactant upon stimulation by proinflammatory cytokines. The enzyme prefers phosphatidylethanolamine and phosphatidylserine as substrates. One important biological function may be the hydrolytic destruction of bacterial membranes. It has been demonstrated, however, that sPLA(2) can also hydrolyse the phospholipid monolayers of high density lipoprotein (HDL) and low density lipoprotein (LDL) in vitro. Secretory phospholipase A(2)-modified LDL show increased affinity to glycosaminoglycans and proteoglycans, a tendency to aggregate, and an enhanced ability to deliver cholesterol to cells. Incubation of cultured macrophages with PLA(2)-treated LDL and HDL is associated with increased intracellular lipid accumulation, resulting in the formation of foam cells. Elevated sPLA(2)(IIA) activity in blood serum leads to an increased clearance of serum cholesterol. Secretory phospholipase A(2)(IIA) can also be detected in the intima, adventitia and media of the atherosclerotic wall not only in developed lesions but also in very early stages of atherosclerosis. The presence of DNA of Chlamydia pneumoniae, herpes simplex virus, and cytomegalovirus was found to be associated with sPLA(2)(IIA) expression and other signs of local inflammation. Thus, sPLA(2)(IIA) appears to be one important link between the lipid and the inflammation hypothesis of atherosclerosis.

Platelet factor 4 binds to low-density lipoprotein receptors and disrupts the endocytic machinery, resulting in retention of low-density lipoprotein on the cell surface

The influence of platelets on the cellular metabolism of atherogenic lipoproteins has not been characterized in detail. Therefore, we investigated the effect of platelet factor 4 (PF4), a cationic protein released in high concentration by activated platelets, on the uptake and degradation of low-density lipoprotein (LDL) via the LDL receptor (LDL-R). LDL-R-dependent binding, internalization, and degradation of LDL by cultured cells were inhibited 50%, 80%, and 80%, respectively, on addition of PF4. PF4 bound specifically to the ligand-binding domain of recombinant soluble LDL-R (half-maximal binding 0.5 microg/mL PF4) and partially (approximately 50%) inhibited the binding of LDL. Inhibition of internalization and degradation by PF4 required the presence of cell-associated proteoglycans, primarily those rich in chondroitin sulfate. PF4 variants with impaired heparin binding lacked the capacity to inhibit LDL. PF4, soluble LDL-R, and LDL formed ternary complexes with cell-surface proteoglycans. PF4 induced the retention of LDL/LDL-R complexes on the surface of human fibroblasts in multimolecular clusters unassociated with coated pits, as assessed by immuno-electron microscopy. These studies demonstrate that PF4 inhibits the catabolism of LDL in vitro in part by competing for binding to LDL-R, by promoting interactions with cell-associated chondroitin sulfate proteoglycans, and by disrupting the normal endocytic trafficking of LDL/LDL-R complexes. Retention of LDL on cell surfaces may facilitate proatherogenic modifications and support an expanded role for platelets in the pathogenesis of atherosclerosis.

Cytomegalovirus infection accelerates inflammation in vascular tissue overexpressing monocyte chemoattractant protein-1

Cardiovascular disease is the leading cause of mortality in the United States. Atherosclerosis is responsible for most of this pathology and is an inflammatory disease with multiple cytokines and adhesion molecules expressed during atherogenesis. Cytomegalovirus (CMV), monocytes, and monocyte chemoattractant protein-1 (MCP-1) have all been implicated in human atherogenesis. A transgenic mouse overexpressing MCP-1 in the myocardium and pulmonary arteries develops myocarditis and pulmonary vascular inflammation. We infected MCP-1 transgenic mice with a sublethal dose of murine cytomegalovirus (MCMV) to look for evidence of accelerated inflammation in vascular tissues overexpressing MCP-1 to determine if MCMV could interact with monocytes and MCP-1 in a manner similar to what may occur in atherogenesis. MCMV infection of MCP-1 transgenic mice caused ascites, myocarditis, and pulmonary artery inflammation, which was not present in mock-infected MCP-1 or MCMV-infected wild-type mice. Inflammatory infiltrates in these tissues consisted of macrophages and T lymphocytes similar to the infiltrates seen in atherosclerosis. Virus presence in inflamed tissues was demonstrated by infecting transgenic mice with MCMV recombinant virus containing the gene sequence for the enhanced green fluorescent protein (EGFP). Human CMV could be involved in atherogenesis in a similar manner by interacting with monocytes and MCP-1 specifically expressed in vascular walls.

The uptake and degradation of matrix-bound lipoproteins by macrophages require an intact actin Cytoskeleton, Rho family GTPases, and myosin ATPase activity

A key cellular event in atherogenesis is the interaction of macrophages with lipoproteins in the subendothelium. In vivo, these lipoproteins are bound to matrix and often aggregated, yet most cell-culture experiments explore these events using soluble monomeric lipoproteins. We hypothesized that the internalization and degradation of matrix-retained and aggregated low density lipoprotein (LDL) by macrophages may involve the actin-myosin cytoskeleton in a manner that distinguishes this process from the endocytosis of soluble LDL. To explore these ideas, we plated macrophages on sphingomyelinase-aggregated LDL bound to smooth muscle cell-derived matrix in the presence of lipoprotein lipase. The macrophages internalized and degraded the LDL, which was mediated partially by the LDL receptor-related protein. Cytochalasin D and latrunculin A, which block actin polymerization, markedly inhibited the uptake and degradation of matrix-retained LDL but not soluble LDL. Inhibition of Rho family GTPases by Clostridium difficile toxin B blocked the degradation of matrix-retained and aggregated LDL by >90% without any inhibition of soluble LDL degradation. However, specific inhibition of Rho had no effect, suggesting the importance of Rac1 and Cdc42. Degradation of matrix-retained, but not soluble, LDL was also blocked by inhibitors of tyrosine kinase, phosphatidylinositol 3-kinase, and myosin ATPase. These findings define fundamental cytoskeletal pathways that may be involved in macrophage foam cell formation in vivo but have been missed by the use of previous cell culture models.

Enzymatically degraded low density lipoproteins are more potent inducers of egr-1 mRNA than oxidized or native low density lipoproteins

OBJECTIVES

The transcription factor early growth response gene-1 (Egr-1) may contribute to atherosclerosis by inducing genes that mediate inflammation and thrombosis. Egr-1 mRNA is highly expressed in human atherosclerotic lesions. Enzymatic modification transforms LDL into atherogenic molecules (E-LDL) which are also present in atherosclerotic lesions. We have investigated whether E-LDL induces egr-1 mRNA in human monocytes.

DESIGN AND METHODS

Mono-Mac-6 cells were incubated with E-LDL, oxidized (Ox-LDL) and native LDL (N-LDL). Egr-1 mRNA expression was followed by quantitative RT-PCR.

RESULTS

E-LDL (25 microg cholesterol/mL) induced egr-1 mRNA maximally within 1 h and were 2.3 and 3.6 fold (p < 0.05) more effective than Ox-LDL or N-LDL. At a concentration of 10 microg/mL cholesterol, E-LDL were twofold less effective.

CONCLUSIONS

These results show that E-LDL are potent inducers of egr-1 mRNA and may therefore represent a link between lipoproteins trapped in the subendothelium and enhanced expression of egr-1 in human atherosclerotic lesions.

Lipolysis of LDL by human secretory phospholipase A(2) induces particle fusion and enhances the retention of LDL to human aortic proteoglycans

The first morphological sign of atherogenesis is the accumulation of extracellular lipid droplets in the proteoglycan-rich subendothelial layer of the arterial intima. Secretory nonpancreatic phospholipase A(2) (snpPLA(2)), an enzyme capable of lipolyzing LDL particles, is found in the arterial extracellular matrix and in contact with the extracellular lipid droplets. We have recently shown that in the presence of heparin, lipolysis of LDL with bee venom PLA(2) induces aggregation and fusion of the particles. Here, we studied the effect of human snpPLA(2) on the integrity of LDL particles and on their interaction with human aortic proteoglycans. In addition, the capacity of the proteoglycans to retain PLA(2)-lipolyzed LDL particles was tested in a microtiter well assay. We found that lipolysis of LDL induced fusion of proteoglycan-bound LDL particles, which increased their binding strength to the proteoglycans. Moreover, lipolysis of LDL with snpPLA(2) under physiological salt and albumin concentrations induced a 3-fold increase in the amount of LDL bound to proteoglycans. The results imply a role for PLA(2) in the retention and accumulation of LDL to the proteoglycan matrix in atherosclerosis.