Unique cellular events occurring during the initial interaction of macrophages with matrix-retained or methylated aggregated low density lipoprotein (LDL). Prolonged cell-surface contact during which ldl-cholesteryl ester hydrolysis exceeds ldl protein degradation

Signaling pathways regulating the extracellular digestion of lipoprotein aggregates by macrophages

The interaction between aggregated low-density lipoprotein (agLDL) and macrophages in arteries plays a major role in atherosclerosis. Macrophages digest agLDL and generate free cholesterol in an extracellular, acidic, hydrolytic compartment known as the lysosomal synapse. Macrophages form a tight seal around agLDL through actin polymerization and deliver lysosomal contents into this space in a process termed digestive exophagy. Our laboratory has identified TLR4 activation of MyD88/Syk as critical for digestive exophagy. Here we use pharmacological agents and siRNA knockdown to characterize signaling pathways downstream of Syk that are involved in digestive exophagy. Syk activates Bruton's tyrosine kinase (BTK) and phospholipase Cγ2 (PLCγ2). We show that PLCγ2 and to a lesser extent BTK regulate digestive exophagy. PLCγ2 cleaves PI(4,5)P2 into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). Soluble IP3 activates release of Ca2+ from the endoplasmic reticulum (ER). We demonstrate that Ca2+ release from the ER is upregulated by agLDL and plays a key role in digestive exophagy. Both DAG and Ca2+ activate protein kinase Cα (PKCα). We find that PKCα is an important regulator of digestive exophagy. These results expand our understanding of the mechanisms of digestive exophagy, which could be useful in developing therapeutic interventions to slow development of atherosclerosis.

The formation and consequences of cholesterol-rich deposits in atherosclerotic lesions

Cardiovascular diseases remain the leading cause of death throughout the world. Accumulation of lipoprotein-associated lipids and their interaction with macrophages are early steps in the development of atherosclerotic lesions. For decades, it has been known that aggregates of lipoproteins in the subendothelial space are found in early plaques, and these aggregates are tightly associated with extracellular matrix fibers. Additionally, most of the cholesterol in these subendothelial aggregates is unesterified, in contrast to the core of low-density lipoproteins (LDL), in which cholesteryl esters predominate. This suggests that the hydrolysis of cholesteryl esters occurs extracellularly. At the cellular level, macrophages in early plaques engage with the LDL and ingest large amounts of cholesterol, which is esterified and stored in lipid droplets. When excessive lipid droplets have accumulated, endoplasmic reticulum stress responses are activated, leading to cell death. The cholesterol-laden dead cells must be cleared by other macrophages. For many years, it was unclear how unesterified (free) cholesterol could be formed extracellularly in early lesions. Papers in the past decade have shown that macrophages form tightly sealed extracellular attachments to aggregates of LDL. These sealed regions become acidified, and lysosomal contents are secreted into these compartments. Lysosomal acid lipase hydrolyzes the cholesteryl esters, and the free cholesterol is transported into the macrophages. High concentrations of cholesterol can also lead to formation of crystals of cholesterol hydrate, and these crystals have been observed in atherosclerotic blood vessels. Characterization of this process may lead to novel therapies for the prevention and treatment of atherosclerosis.

Autophagy Is Differentially Regulated in Leukocyte and Nonleukocyte Foam Cells During Atherosclerosis

RATIONALE

Atherosclerosis is characterized by an accumulation of foam cells within the arterial wall, resulting from excess cholesterol uptake and buildup of cytosolic lipid droplets (LDs). Autophagy promotes LD clearance by freeing stored cholesterol for efflux, a process that has been shown to be atheroprotective. While the role of autophagy in LD catabolism has been studied in macrophage-derived foam cells, this has remained unexplored in vascular smooth muscle cell (VSMC)-derived foam cells that constitute a large fraction of foam cells within atherosclerotic lesions.

OBJECTIVE

We performed a comparative analysis of autophagy flux in lipid-rich aortic intimal populations to determine whether VSMC-derived foam cells metabolize LDs similarly to their macrophage counterparts.

METHODS AND RESULTS

Atherosclerosis was induced in GFP-LC3 transgenic mice by PCSK9 (proprotein convertase subtilisin/kexin type 9)-adeno-associated viral injection and Western diet feeding. Using flow cytometry of aortic digests, we observed a significant increase in dysfunctional autophagy of VSMC-derived foam cells during atherogenesis relative to macrophage-derived foam cells. Using cell culture models of lipid-loaded VSMC and macrophage, we show that autophagy-mediated cholesterol efflux from VSMC foam cells was poor relative to macrophage foam cells, and largely occurs when HDL (high-density lipoprotein) is used as a cholesterol acceptor, as opposed to apoA-1 (apolipoproteinA-1). This was associated with the predominant expression of ABCG1 in VSMC foam cells. Using metformin, an autophagy activator, cholesterol efflux to HDL was significantly increased in VSMC, but not in macrophage, foam cells.

CONCLUSIONS

These data demonstrate that VSMC and macrophage foam cells perform cholesterol efflux by distinct mechanisms, and that autophagy flux is highly impaired in VSMC foam cells, but can be induced by pharmacological means. Further investigation is warranted into targeting autophagy specifically in VSMC foam cells, the predominant foam cell subtype of advanced atherosclerotic plaques, to promote reverse cholesterol transport and resolution of the atherosclerotic plaque.

Stable reduction of STARD4 alters cholesterol regulation and lipid homeostasis

STARD4, a member of the evolutionarily conserved START gene family, is a soluble sterol transport protein implicated in cholesterol sensing and maintenance of cellular homeostasis. STARD4 is widely expressed and has been shown to transfer sterol between liposomes as well as organelles in cells. However, STARD4 knockout mice lack an obvious phenotype, so the overall role of STARD4 is unclear. To model long term depletion of STARD4 in cells, we use short hairpin RNA technology to stably decrease STARD4 expression in human U2OS osteosarcoma cells (STARD4-KD). We show that STARD4-KD cells display increased total cholesterol, slower cholesterol trafficking between the plasma membrane and the endocytic recycling compartment, and increased plasma membrane fluidity. These effects can all be rescued by transient expression of a short hairpin RNA-resistant STARD4 construct. Some of the cholesterol increase was due to excess storage in late endosomes or lysosomes. To understand the effects of reduced STARD4, we carried out transcriptional and lipidomic profiling of control and STARD4-KD cells. Reduction of STARD4 activates compensatory mechanisms that alter membrane composition and lipid homeostasis. Based on these observations, we propose that STARD4 functions as a critical sterol transport protein involved in sterol sensing and maintaining lipid homeostasis.

Dynamic Actin Reorganization and Vav/Cdc42-Dependent Actin Polymerization Promote Macrophage Aggregated LDL (Low-Density Lipoprotein) Uptake and Catabolism

Objective- During atherosclerosis, LDLs (low-density lipoproteins) accumulate in the arteries, where they become modified, aggregated, and retained. Such deposits of aggregated LDL (agLDL) can be recognized by macrophages, which attempt to digest and clear them. AgLDL catabolism promotes internalization of cholesterol and foam cell formation, which leads to the progression of atherosclerosis. Therapeutic blockade of this process may delay disease progression. When macrophages interact with agLDL in vitro, they form a novel extracellular, hydrolytic compartment-the lysosomal synapse (LS)-aided by local actin polymerization to digest agLDL. Here, we investigated the specific regulators involved in actin polymerization during the formation of the LS. Approach and Results- We demonstrate in vivo that atherosclerotic plaque macrophages contacting agLDL deposits polymerize actin and form a compartment strikingly similar to those made in vitro. Live cell imaging revealed that macrophage cortical F-actin depolymerization is required for actin polymerization to support the formation of the LS. This depolymerization is cofilin-1 dependent. Using siRNA-mediated silencing, pharmacological inhibition, genetic knockout, and stable overexpression, we elucidate key roles for Cdc42 Rho GTPase and GEF (guanine nucleotide exchange factor) Vav in promoting actin polymerization during the formation of the LS and exclude a role for Rac1. Conclusions- These results highlight critical roles for dynamic macrophage F-actin rearrangement and polymerization via cofilin-1, Vav, and Cdc42 in LS formation, catabolism of agLDL, and foam cell formation. These proteins might represent therapeutic targets to treat atherosclerotic disease.

Digestive exophagy: Phagocyte digestion of objects too large for phagocytosis

Mammalian phagocytes carry out several essential functions, including killing and digesting infectious organisms, clearing denatured proteins, removing dead cells and removing several types of debris from the extracellular space. Many of these functions involve phagocytosis, the engulfment of a target in a specialized endocytic process and then fusion of the new phagosome with lysosomes. Phagocytes such as macrophages can phagocytose targets that are several micrometers in diameter (eg, dead cells), but in some cases they encounter much larger objects. We have studied two such examples in some detail: large deposits of lipoproteins such as those in the wall of blood vessels associated with atherosclerosis, and dead adipocytes, which are dozens of micrometers in diameter. We describe a process, which we call digestive exophagy, in which macrophages create a tight seal in contact with the target, acidify the sealed zone and secrete lysosomal contents into the contact zone. By this process, hydrolysis by lysosomal enzymes occurs in a compartment that is outside the cell. We compare this process to the well characterized digestion of bone by osteoclasts, and we point out key similarities and differences.

High-density lipoprotein or cyclodextrin extraction of cholesterol from aggregated LDL reduces foam cell formation

Low-density lipoprotein (LDL) deposition, aggregation and retention in the endothelial sub-intima are critical initiating events during atherosclerosis. Macrophages digest aggregated LDL (agLDL) through a process called exophagy. High-density lipoprotein (HDL) plays an atheroprotective role, but studies attempting to exploit it therapeutically have been unsuccessful, highlighting gaps in our current understanding of HDL function. Here, we characterized the role of HDL during exophagy of agLDL. We find that atherosclerotic plaque macrophages contact agLDL and form an extracellular digestive compartment similar to that observed in vitro During macrophage catabolism of agLDL in vitro, levels of free cholesterol in the agLDL are increased. HDL can extract free cholesterol directly from this agLDL and inhibit macrophage foam cell formation. Cholesterol-balanced hydroxypropyl-β-cyclodextrin similarly reduced macrophage cholesterol uptake and foam cell formation. Finally, we show that HDL can directly extract free cholesterol, but not cholesterol esters, from agLDL in the absence of cells. Together, these results suggest that the actions of HDL can directly extract free cholesterol from agLDL during catabolism, and provide a new context in which to view the complex relationship between HDL and atherosclerosis.

TLR4 (Toll-Like Receptor 4)-Dependent Signaling Drives Extracellular Catabolism of LDL (Low-Density Lipoprotein) Aggregates

OBJECTIVE

Aggregation and modification of LDLs (low-density lipoproteins) promote their retention and accumulation in the arteries. This is a critical initiating factor during atherosclerosis. Macrophage catabolism of agLDL (aggregated LDL) occurs using a specialized extracellular, hydrolytic compartment, the lysosomal synapse. Compartment formation by local actin polymerization and delivery of lysosomal contents by exocytosis promotes acidification of the compartment and degradation of agLDL. Internalization of metabolites, such as cholesterol, promotes foam cell formation, a process that drives atherogenesis. Furthermore, there is accumulating evidence for the involvement of TLR4 (Toll-like receptor 4) and its adaptor protein MyD88 (myeloid differentiation primary response 88) in atherosclerosis. Here, we investigated the role of TLR4 in catabolism of agLDL using the lysosomal synapse and foam cell formation. Approach and Results: Using bone marrow-derived macrophages from knockout mice, we find that TLR4 and MyD88 regulate compartment formation, lysosome exocytosis, acidification of the compartment, and foam cell formation. Using siRNA (small interfering RNA), pharmacological inhibition and knockout bone marrow-derived macrophages, we implicate SYK (spleen tyrosine kinase), PI3K (phosphoinositide 3-kinase), and Akt in agLDL catabolism using the lysosomal synapse. Using bone marrow transplantation of LDL receptor knockout mice with TLR4 knockout bone marrow, we show that deficiency of TLR4 protects macrophages from lipid accumulation during atherosclerosis. Finally, we demonstrate that macrophages in vivo form an extracellular compartment and exocytose lysosome contents similar to that observed in vitro for degradation of agLDL.

CONCLUSIONS

We present a mechanism in which interaction of macrophages with agLDL initiates a TLR4 signaling pathway, resulting in formation of the lysosomal synapse, catabolism of agLDL, and lipid accumulation in vitro and in vivo.

Progranulin in the hematopoietic compartment protects mice from atherosclerosis

BACKGROUND AND AIMS

Progranulin is a circulating protein that modulates inflammation and is found in atherosclerotic lesions. Here we determined whether inflammatory cell-derived progranulin impacts atherosclerosis development.

METHODS

Ldlr^-/- mice were transplanted with bone marrow from wild-type (WT) or Grn^-/- (progranulin KO) mice (referred to as Tx-WT and Tx-KO, respectively).

RESULTS

After 10 weeks of high-fat diet feeding, both groups displayed similarly elevated plasma levels of cholesterol and triglycerides. Despite abundant circulating levels of progranulin, the size of atherosclerotic lesions in Tx-KO mice was increased by 47% in aortic roots and by 62% in whole aortas. Aortic root lesions in Tx-KO mice had increased macrophage content and larger necrotic cores, consistent with more advanced lesions. Progranulin staining was markedly reduced in the lesions of Tx-KO mice, indicating little or no uptake of circulating progranulin. Mechanistically, cultured progranulin-deficient macrophages exhibited increased lysosome-mediated exophagy of aggregated low-density lipoproteins resulting in increased cholesterol uptake and foam cell formation.

CONCLUSIONS

We conclude that hematopoietic progranulin deficiency promotes diet-induced atherosclerosis in Ldlr^-/- mice, possibly due to increased exophagy-mediated cholesterol uptake. Circulating progranulin was unable to prevent the increased lesion development, consistent with the importance of progranulin acting via cell-autonomous or local effects.

Ceramide activation of RhoA/Rho kinase impairs actin polymerization during aggregated LDL catabolism

Macrophages use an extracellular, hydrolytic compartment formed by local actin polymerization to digest aggregated LDL (agLDL). Catabolism of agLDL promotes foam cell formation and creates an environment rich in LDL catabolites, including cholesterol and ceramide. Increased ceramide levels are present in lesional LDL, but the effect of ceramide on macrophage proatherogenic processes remains unknown. Here, we show that macrophages accumulate ceramide in atherosclerotic lesions. Using macrophages from sphingosine kinase 2 KO (SK2KO) mice to mimic ceramide-rich conditions of atherosclerotic lesions, we show that SK2KO macrophages display impaired actin polymerization and foam cell formation in response to contact with agLDL. C16-ceramide treatment impaired wild-type but not SK2KO macrophage actin polymerization, confirming that this effect is due to increased ceramide levels. We demonstrate that knockdown of RhoA or inhibition of Rho kinase restores agLDL-induced actin polymerization in SK2KO macrophages. Activation of RhoA in macrophages was sufficient to impair actin polymerization and foam cell formation in response to agLDL. Finally, we establish that during catabolism, macrophages take up ceramide from agLDL, and inhibition of ceramide generation modulates actin polymerization. These findings highlight a critical regulatory pathway by which ceramide impairs actin polymerization through increased RhoA/Rho kinase signaling and regulates foam cell formation.

Macrophages of genetically characterized familial hypercholesterolaemia patients show up-regulation of LDL-receptor-related proteins

Familial hypercholesterolaemia (FH) is a major risk for premature coronary heart disease due to severe long‐life exposure to high LDL levels. Accumulation of LDL in the vascular wall triggers atherosclerosis with activation of the innate immunity system. Here, we have investigated (i) gene expression of LDLR and LRPs in peripheral blood cells (PBLs) and in differentiated macrophages of young FH‐patients; and (ii) whether macrophage from FH patients have a differential response when exposed to high levels of atherogenic LDL. PBLs in young heterozygous genetically characterized FH patients have higher expression of LRP5 and LRP6 than age‐matched healthy controls or patients with secondary hypercholesterolaemia. LRP1 levels were similar among groups. In monocyte‐derived macrophages (MACs), LRP5 and LRP1 transcript levels did not differ between FHs and controls in resting conditions, but when exposed to agLDL, FH‐MAC showed a highly significant up‐regulation of LRP5, while LRP1 was unaffected. PBL and MAC cells from FH patients had significantly lower LDLR expression than control cells, independently of the lipid‐lowering therapy. Furthermore, exposure of FH‐MAC to agLDL resulted in a reduced expression of CD163, scavenger receptor with anti‐inflammatory and atheroprotective properties. In summary, our results show for first time that LRPs, active lipid‐internalizing receptors, are up‐regulated in innate immunity cells of young FH patients that have functional LDLR mutations. Additionally, their reduced CD163 expression indicates less atheroprotection. Both mechanisms may play a synergic effect on the onset of premature atherosclerosis in FH patients.

Degradation of aggregated LDL occurs in complex extracellular sub-compartments of the lysosomal synapse

Monocyte-derived cells use an extracellular, acidic, lytic compartment (a lysosomal synapse) for initial degradation of large objects or species bound to the extracellular matrix. Akin to osteoclast degradation of bone, extracellular catabolism is used by macrophages to degrade aggregates of low density lipoprotein (LDL) similar to those encountered during atherogenesis. However, unlike osteoclast catabolism, the lysosomal synapse is a highly dynamic and intricate structure. In this study, we use high resolution three dimensional imaging to visualize compartments formed by macrophages to catabolize aggregated LDL. We show that these compartments are topologically complex, have a convoluted structure and contain sub-regions that are acidified. These sub-regions are characterized by a close apposition of the macrophage plasma membrane and aggregates of LDL that are still connected to the extracellular space. Compartment formation is dependent on local actin polymerization. However, once formed, compartments are able to maintain a pH gradient when actin is depolymerized. These observations explain how compartments are able to maintain a proton gradient while remaining outside the boundaries of the plasma membrane.

Monocyte-Derived Dendritic Cells Upregulate Extracellular Catabolism of Aggregated Low-Density Lipoprotein on Maturation, Leading to Foam Cell Formation

OBJECTIVE

Although dendritic cells are known to play a role in atherosclerosis, few studies have examined the contribution of the wide variety of dendritic cell subsets. Accordingly, their roles in atherogenesis remain largely unknown. We investigated the ability of different dendritic cell subsets to become foam cells after contact with aggregated low-density lipoprotein (LDL; the predominant form of LDL found in atherosclerotic plaques).

APPROACH AND RESULTS

We demonstrate that both murine and human monocyte-derived dendritic cells use exophagy to degrade aggregated LDL, leading to foam cell formation, whereas monocyte-independent dendritic cells are unable to clear LDL aggregates by this mechanism. Exophagy is a catabolic process in which objects that cannot be internalized by phagocytosis (because of their size or association with extracellular structures) are initially digested in an extracellular acidic lytic compartment. Surprisingly, we found that monocyte-derived dendritic cells upregulate exophagy on maturation. This contrasts various forms of endocytic internalization in dendritic cells, which decrease on maturation. Finally, we show that our in vitro results are consistent with dendritic cell lipid accumulation in plaques of an ApoE(-/-) mouse model of atherosclerosis.

CONCLUSIONS

Our results show that monocyte-derived dendritic cells use exophagy to degrade aggregated LDL and become foam cells, whereas monocyte-independent dendritic cells are unable to clear LDL deposits. Furthermore, we find that exophagy is upregulated on dendritic cell maturation. Thus, exophagy-mediated foam cell formation in monocyte-derived dendritic cells could play a significant role in atherogenesis.

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.

Plasmin promotes foam cell formation by increasing macrophage catabolism of aggregated low-density lipoprotein

OBJECTIVE

The plasmin/plasminogen system is involved in atherosclerosis. However, the mechanisms by which it stimulates disease are not fully defined. A key event in atherogenesis is the deposition of low-density lipoprotein (LDL) on arterial walls where it is modified, aggregated, and retained. Macrophages are recruited to clear the lipoproteins, and they become foam cells. The goal of this study was to assess the role of plasmin in macrophage uptake of aggregated LDL and foam cell formation.

APPROACH AND RESULTS

Plasminogen treatment of macrophages catabolizing aggregated LDL significantly accelerated foam cell formation. Macrophage interaction with aggregated LDL increased the surface expression of urokinase-type plasminogen activator receptor and plasminogen activator activity, resulting in increased ability to generate plasmin at the cell surface. The high local level of plasmin cleaves cell-associated aggregated LDL, allowing a portion of the aggregate to become sequestered in a nearly sealed, yet extracellular, acidic compartment. The low pH in the plasmin-induced compartment allows lysosomal enzymes, delivered via lysosome exocytosis, greater activity, resulting in more efficient cholesteryl ester hydrolysis and delivery of a large cholesterol load to the macrophage, thereby promoting foam cell formation.

CONCLUSIONS

These findings highlight a critical role for plasmin in the catabolism of aggregated LDL by macrophages and provide a new context for considering the atherogenic role of plasmin.

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

Lipoproteins, platelets and atherothrombosis

Atherosclerosis and thrombosis associated with the rupture of vulnerable plaque are the main causes of cardiovascular events, including acute coronary syndrome. Low-density lipoprotein (LDL) plays a key role in the pathogenesis of atherothrombotic processes. LDLs modify the antithrombotic properties of the vascular endothelium and change vessel contractility by reducing the availability of endothelial nitric oxide and activating proinflammatory signaling pathways. In addition, LDLs also influence the functions and interactions of cells present in atherosclerotic lesions, whether they come from the circulation or are resident in vessel walls. In fact, LDLs entering affected vessels undergo modifications (e.g. oxidation, aggregation and glycosylation) that potentiate their atherogenic properties. Once modified, these intravascular LDLs promote the formation of foam cells derived from smooth muscle cells and macrophages, thereby increasing the vulnerability of atherosclerotic plaque. Moreover, they also increase the thrombogenicity of both plaque and blood, in which circulating tissue factor levels are raised and platelet reactivity is enhanced. This review focuses on the importance of native and modified LDL for the pathogenesis of atherothrombosis. It also discusses current studies on LDL and its effects on the actions of vascular cells and blood cells, particularly platelets, and considers novel potential therapeutic targets.

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.

Aggregated LDL in contact with macrophages induces local increases in free cholesterol levels that regulate local actin polymerization

OBJECTIVE

Interaction of macrophages with aggregated matrix-anchored lipoprotein deposits is an important initial step in atherogenesis. Aggregated lipoproteins require different cellular uptake processes than those used for endocytosis of monomeric lipoproteins. In this study, we tested the hypothesis that engagement of aggregated LDL (agLDL) by macrophages could lead to local increases in free cholesterol levels and that these increases in free cholesterol regulate signals that control cellular actin.

METHODS AND RESULTS

AgLDL resides for prolonged periods in surface-connected compartments. Although agLDL is still extracellular, we demonstrate that an increase in free cholesterol occurs at sites of contact between agLDL and cells because of hydrolysis of agLDL-derived cholesteryl ester. This increase in free cholesterol causes enhanced actin polymerization around the agLDL. Inhibition of cholesteryl ester hydrolysis results in decreased actin polymerization.

CONCLUSIONS

We describe a novel process that occurs during agLDL-macrophage interactions in which local release of free cholesterol causes local actin polymerization, promoting a pathological positive feedback loop for increased catabolism of agLDL and eventual foam cell formation.

Evolution of foamy macrophages in the pulmonary granulomas of experimental tuberculosis models

The chronic phase of Mycobacterium tuberculosis infection in mouse experimental models is characterized by the accumulation of foamy macrophages (FM)--which shape the outer ring of the granuloma - in the alveolar spaces, as detected in paraffin-embedded tissues stained with hematoxylin-eosin. In this study, the use of semi- and ultra-thin sections offers more detailed information about the origin of FM both in mouse and guinea-pig experimental models. Lipid bodies (LB) are present in macrophages from the beginning of infection and accumulate in the chronic phase. LB progress from an early (ELB) to a late (LLB) stage, defined according to their progressive capacity to generate cholesterol crystals, resembling atherosclerotic lesions. FM arise from massive accumulation of LLB. Electronic microscopy reveals intracellular lipophilic inclusions (ILIs) in those M. tuberculosis bacilli inside FM. It is our hypothesis that the accumulation of lipids in M. tuberculosis concomitant to the establishment of the non-replicating state prepares the bacilli for future reactivation and for facing future stressful environments.

Foamy macrophages from tuberculous patients' granulomas constitute a nutrient-rich reservoir for M. tuberculosis persistence

Tuberculosis (TB) is characterized by a tight interplay between Mycobacterium tuberculosis and host cells within granulomas. These cellular aggregates restrict bacterial spreading, but do not kill all the bacilli, which can persist for years. In-depth investigation of M. tuberculosis interactions with granuloma-specific cell populations are needed to gain insight into mycobacterial persistence, and to better understand the physiopathology of the disease. We have analyzed the formation of foamy macrophages (FMs), a granuloma-specific cell population characterized by its high lipid content, and studied their interaction with the tubercle bacillus. Within our in vitro human granuloma model, M. tuberculosis long chain fatty acids, namely oxygenated mycolic acids (MA), triggered the differentiation of human monocyte-derived macrophages into FMs. In these cells, mycobacteria no longer replicated and switched to a dormant non-replicative state. Electron microscopy observation of M. tuberculosis–infected FMs showed that the mycobacteria-containing phagosomes migrate towards host cell lipid bodies (LB), a process which culminates with the engulfment of the bacillus into the lipid droplets and with the accumulation of lipids within the microbe. Altogether, our results suggest that oxygenated mycolic acids from M. tuberculosis play a crucial role in the differentiation of macrophages into FMs. These cells might constitute a reservoir used by the tubercle bacillus for long-term persistence within its human host, and could provide a relevant model for the screening of new antimicrobials against non-replicating persistent mycobacteria.

Mycobacterium tuberculosis, the causative agent of tuberculosis, is responsible for dramatic health problems globally. It is estimated that this pathogen infects one-third of the human population and causes three million deaths annually. Most individuals remain asymptomatic for several years before developing an active disease. In such individuals, the bacilli are not cleared but rather persist in a dormant state. Major goals of TB research are to (i) understand how the bacilli remain alive for years within infected individuals, and (ii) find how to prevent their reactivation and hence clinical disease. During dormancy, most of the bacilli are confined to granulomas that consist of well-defined aggregates of different host immune cells. Granulomas prevent spreading of bacilli. In this study, we analyzed the role of a particular cell population found within granulomas, the “foamy macrophages”. These cells are filled with droplets of lipids, a well-known nutrient for persistent bacilli. We found that within these cells, the bacilli do not replicate, but remain alive and seem to internalize host lipids. The foamy macrophages might thus constitute a reservoir for persisting bacilli within their human host, and could provide a relevant model for screening of new antimicrobials against non-replicating persistent mycobacteria.

Elevated cholesterol levels in the plasma membranes of macrophages inhibit migration by disrupting RhoA regulation

OBJECTIVE

Atherogenesis begins as small subendothelial accumulations of foam cells that develop through unregulated uptake of modified and aggregated low-density lipoprotein (LDL). The reason why foam cells remain in the atherosclerotic plaque rather than migrating out of the area is unclear. We tested the hypothesis that elevated membrane cholesterol levels, which may result from interactions with aggregated LDL, affect macrophage migration.

METHODS AND RESULTS

Cholesterol loading by incubation with cholesterol-chelated methyl-beta-cyclodextrin decreased migration of J774A.1 macrophages toward complement 5a (C5a) in transwell migration assays, even though cholesterol-loaded macrophages responded to a bath application of C5a. In a micropipette polarization assay, cholesterol-loaded cells polarized toward a C5a gradient. In a transwell migration assay, cholesterol-loaded cells extended lamellae through the filter pores but were unable to translocate their cell bodies. Cholesterol loading decreased both the cellular levels of GTP-bound active RhoA and the phosphorylation of myosin light chain. Expression of constitutively active RhoA largely prevented the inhibition of cell migration by cholesterol loading.

CONCLUSIONS

These results suggest that increases in plasma membrane cholesterol content alter RhoA activation, resulting in inhibition of cell migration. These findings provide one possible explanation for the retention of foam cells in atherosclerotic lesions.

Fluorescent sterols as tools in membrane biophysics and cell biology

Cholesterol is an important constituent of cellular membranes playing a fundamental role in many biological processes. This sterol affects membrane permeability, lateral lipid organization, signal transduction and membrane trafficking. Intracellular sterol transport modes and pathways as well as the regulation of sterol metabolism and disposition in various tissues are areas of intense research. Progress is intimately linked to development and use of appropriate analogs, which closely mimic the properties of cholesterol while allowing to be detected by spectroscopic or microscopic methods. This review provides an overview of various fluorescent sterols used in membrane biophysics and cell biology including analogs of cholesterol and cholesteryl esters. Attention is paid to the natural fluorescent sterol dehydroergosterol (DHE). A survey of the many applications of DHE in biological research is presented. Special emphasis is on recent developments in fluorescence microscopy instrumentation to visualize DHE as an intrinsically fluorescent analog of cholesterol in living cells.

Plasma membrane sterol distribution resembles the surface topography of living cells

Cholesterol is an important constituent of cellular membranes. It has been suggested that cholesterol segregates into sterol-rich and -poor domains in the plasma membrane, although clear evidence for this is lacking. By fluorescence imaging of the natural sterol dehydroergosterol (DHE), the lateral sterol distribution has been visualized in living cells. The spatial labeling pattern of DHE coincided with surface structures such as ruffles, microvilli, and filopodia with correlation lengths in the range of 0.8-2.5 microm. DHE staining of branched tubules and of nanotubes connecting two cells was detected. Dynamics of DHE in folded and plane membrane regions was comparable as determined by fluorescence recovery after photobleaching. DHE colocalized with fluid membrane-preferring phospholipids in surface structures and at sites of cell attachment as well as in the cleavage furrow of dividing cells, but it was not particularly enriched in those regions. Fluorescent sterol showed homogeneous staining in membrane blebs induced by F-actin disruption. Cross-linking the ganglioside GM1--a putative raft marker--did not affect the cell surface distribution of DHE. The results suggest that spatial heterogeneities of plasma membrane staining of DHE resolvable by light microscopy reflect the cell surface topography but not phase-separated sterol domains in the bilayer plane.

Advanced atherosclerotic foam cell formation has features of an acquired lysosomal storage disorder

Atherosclerosis is a disease of large- and medium-sized arteries. Complications from atherosclerosis remain a serious cause of morbidity and mortality in industrialized countries. The disease begins very early in life and effects most people in the West. However, because the progression of the disease is slow, symptoms usually do not occur until after the fifth decade of life. Because atherosclerosis is a ubiquitous occurrence throughout the world, as life expectancy is prolonged most populations will see increasing numbers of deaths from complications of atherosclerosis unless there are dramatic advances in treatment. Because it begins so early in life, current treatment is aimed at slowing or reversing the progression of the disease rather than eliminating the initiating steps. Changes in diet and exercise, cholesterol-lowering drugs, and improvements in surgical treatments have made significant inroads into prolonging life, but much work is still required. To proceed further, a better understanding is needed of the underlying causes of disease progression. In this regard, evidence is mounting that the foam cells of the lesion (a critical cell in atherosclerosis progression) exhibit characteristics of an acquired lysosomal storage disorder. In this review the evidence for this conclusion is reviewed and the ramifications of this conclusion are explored with regard to the understanding of disease progression mechanisms, possible improvements in treatment, and their role in increasing life expectancy.

Sterol regulatory element-binding protein-2 negatively regulates low density lipoprotein receptor-related protein transcription

Low density lipoprotein receptor-related protein (LRP1) binds aggregated LDL (agLDL) leading to a high intracellular cholesteryl ester (CE) accumulation. AgLDL up-regulates LRP1 expression concomitantly with an LDL receptor (LDLR) and sterol regulatory element binding protein (SREBP-2) down-regulation. The objectives were to investigate whether SREBP-2 regulates LRP1 transcription and determine the molecular mechanisms involved in the process. Down-regulation of active SREBP-2 by nLDL and agLDL led to LDLR down-regulation and LRP1 up-regulation. Enforced expression of an active form of SREBP-2 (SREBP-2-NT, amino acid residues 1-468) decreased LRP1 expression and LRP1 promoter (WT-LRP1) luciferase activity in a dose-dependent manner. LDL did not exert any significant effect on LRP1 promoter activity when a putative sterol regulatory element (SRE) (5-GTGGGGTGA-3'; +225 to +233) was mutated (SRE-MT-LRP1). SREBP-2 overexpression exerted stronger down-regulatory effects on WT-LRP1 than on SRE-MT-LRP1 promoter activity both in control, nLDL- and agLDL-exposed HeLa cells. Gel mobility shift assays showed that recombinant SREBP-2-NT protein (1-468) binds to a double-stranded LRP1 DNA fragment (215 to 245) containing a wild-type (wt) SRE sequence but not to a mutated SRE (mt) sequence (5-GAATTCGA-3'). Our results demonstrate that LDL stimulates LRP1 transcription and decreases SREBP-2 active form which negatively regulates LRP1 transcription. SRE sequence (+225 to +233) plays a pivotal role for the down-regulatory effect of SREBP-2 on LRP1 promoter activity.

Role of cholesterol and lipid organization in disease

Membrane lipids are essential for biological functions ranging from membrane trafficking to signal transduction. The composition of lipid membranes influences their organization and properties, so it is not surprising that disorders in lipid metabolism and transport have a role in human disease. Significant recent progress has enhanced our understanding of the molecular and cellular basis of lipid-associated disorders such as Tangier disease, Niemann-Pick disease type C and atherosclerosis. These insights have also led to improved understanding of normal physiology.

Elevated plasma membrane cholesterol content alters macrophage signaling and function

OBJECTIVE

During atherogenesis, macrophages migrate into the subendothelial space where they ingest deposited lipoproteins, accumulate lipids, and transform into foam cells. It is unclear why these macrophages do not remove their lipid loads from the region. This study was aimed at testing the hypothesis that macrophage behavior is altered when membrane cholesterol levels are elevated, as might be the case for cells in contact with lipoproteins within atherosclerotic lesions.

METHODS AND RESULTS

We examined the effects of elevating membrane cholesterol on macrophage behavior. J774 macrophages were treated with either acetylated low-density lipoprotein (ac-LDL) and ACAT inhibitor or cholesterol-chelated methyl-beta-cyclodextrin (chol-MbetaCD) to increase membrane cholesterol levels. Our results show that elevating the membrane cholesterol of J774 macrophages induced dramatic ruffling, stimulated cell spreading, and affected F-actin organization. Cellular adhesion was required for these effects, and Rac-mediated signaling pathways were involved. Additionally, 3-dimensional transwell chemotaxis assays showed that migration of J774 macrophages was significantly inhibited when membrane cholesterol levels were raised.

CONCLUSIONS

These findings indicate that increased membrane cholesterol causes dramatic effects on macrophage cellular functions related to the actin cytoskeleton. They should provide new insights into the early steps of atherogenesis.

Lysosomal acid lipase and atherosclerosis

PURPOSE OF REVIEW

Atherosclerosis remains the leading cause of death in the developed countries. In addition to lipid-lowering drugs - statins, dietary control, and exercise, new approaches are needed for the treatment and prevention of atherosclerosis. This review will focus on the role(s) of lysosomal acid lipase and its use as an enzyme therapy to reduce atherosclerotic lesions in a mouse model and to examine the molecular basis supporting this novel strategy and its mechanism of effect.

RECENT FINDINGS

Administration of human lysosomal acid lipase via tail vein into mice with atherosclerosis eliminates early aortic and coronary ostial lesions and reduces lesional size in advanced disease. The reduction of advanced lesional area is related to decreases in foamy macrophages, collagen positive areas, and necrotic areas. Compared with sham-treated mice, the human lysosomal acid lipase-treated mice also have reduced levels of plasma cholesteryl esters, and reduced levels of hepatic cholesterol and triglycerides.

SUMMARY

These studies indicate that administrated lysosomal acid lipase affects the atherogenesis by at least two mechanisms: (1) direct targeting of lesional macrophages with resultant decreases in cholesteryl esters and triglyceride in the lysosomes of macrophages in the lesions; (2) systemic effects that mediate the liver to reduce the hepatic cholesteryl ester and triglyceride release, possibly leading to reduced production of VLDL and LDL.

Cellular mechanisms of vitamin E uptake: relevance in α-tocopherol metabolism and potential implications for disease

alpha-Tocopherol is an essential micronutrient involved in various oxidative stress-related processes. Because of its hydrophobic nature, alpha-tocopherol is transported in plasma lipoproteins, and the pathways involved in its cellular uptake are closely related to the lipoprotein metabolism. alpha-Tocopherol transfer from plasma to cells can occur by different mechanisms such as uptake facilitated by lipid transfer proteins and lipases, receptor-mediated lipoprotein endocytosis, and selective lipid uptake. Here we discuss recent progress in understanding the physiological and pathophysiological relevance of these different pathways for cellular uptake of vitamin E in vivo. This review is mainly focused on the role of the scavenger receptor class B type I (SR-BI) on alpha-tocopherol metabolism and its potential implications for disease conditions.

Reduced atherosclerosis in hormone-sensitive lipase transgenic mice overexpressing cholesterol acceptors

Macrophage-specific overexpression of cholesteryl ester hydrolysis in hormone-sensitive lipase transgenic (HSL Tg) female mice paradoxically increases cholesterol esterification and cholesteryl ester accumulation in macrophages, and thus susceptibility to diet-induced atherosclerosis compared to nontransgenic C57BL/6 mice. The current studies suggest that whereas increased cholesterol uptake could contribute to transgenic foam cell formation, there are no differences in cholesterol synthesis and the expression of cholesterol efflux mediators (ABCA1, ABCG1, apoE, PPARgamma, and LXRalpha) compared to wild-type macrophages. HSL Tg macrophages exhibit twofold greater efflux of cholesterol to apoA-I in vitro, suggesting the potential rate-limiting role of cholesteryl ester hydrolysis in efflux. However, macrophage cholesteryl ester levels appear to depend on the relative efficacy of alternate pathways for free cholesterol in either efflux or re-esterification. Thus, increased atherosclerosis in HSL Tg mice appears to be due to the coupling of the efficient re-esterification of excess free cholesterol to its limited removal mediated by the cholesterol acceptors in these mice. The overexpression of cholesterol acceptors in HSL-apoA-IV double-transgenic mice increases plasma HDL levels and decreases diet-induced atherosclerosis compared to HSL Tg mice, with aortic lesions reduced to sizes in nontransgenic littermates. The results in vivo are consistent with the effective efflux from HSL Tg macrophages supplemented with HDL and apoA-I in vitro, and highlight the importance of cholesterol acceptors in inhibiting atherosclerosis caused by imbalances in the cholesteryl ester cycle.

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.

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.

Distinct endosomal compartments in early trafficking of low density lipoprotein-derived cholesterol

We previously studied the early trafficking of low density lipoprotein (LDL)-derived cholesterol in mutant Chinese hamster ovary cells defective in Niemann-Pick type C1 (NPC1) using cyclodextrin (CD) to monitor the arrival of cholesterol from the cell interior to the plasma membrane (PM) (Cruz, J. C., Sugii, S., Yu, C., and Chang, T.-Y. (2000) J. Biol. Chem. 275, 4013-4021). We found that newly hydrolyzed cholesterol derived from LDL first appears in certain CD-accessible pool(s), which we assumed to be the PM, before accumulating in the late endosome/lysosome, where NPC1 resides. To determine the identity of the early CD-accessible pool(s), in this study, we performed additional experiments, including the use of revised CD incubation protocols. We found that prolonged incubation with CD (>30 min) caused cholesterol in internal membrane compartment(s) to redistribute to the PM, where it became accessible to CD. In contrast, a short incubation with CD (5-10 min) did not cause such an effect. We also show that one of the early compartments contains acid lipase (AL), the enzyme required for liberating cholesterol from cholesteryl ester in LDL. Biochemical and microscopic evidence indicates that most of the AL is present in endocytic compartment(s) distinct from the late endosome/lysosome. Our results suggest that cholesterol is liberated from LDL cholesteryl ester in the hydrolytic compartment containing AL and then moves to the NPC1-containing late endosome/lysosome before reaching the PM or the endoplasmic reticulum.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

Hyperhomocysteinemia accelerates atherosclerosis in cystathionine beta-synthase and apolipoprotein E double knock-out mice with and without dietary perturbation

Although hyperhomocysteinemia is an independent risk factor for cardiovascular disease, a direct role for homocysteine (Hcy) in this disease remains to be shown. Whereas diet-induced hyperhomocysteinemia promotes atherosclerosis in animal models, the effects of Hcy on atherogenesis in the absence of dietary perturbations is not known. We have generated double knock-out mice with targeted deletions of the genes for apolipoprotein E (apoE) and cystathionine beta-synthase (CBS), which converts Hcy to cystathionine. ApoE(-/-)/CBS(-/-) mice developed aortic lesions even in the absence of dietary manipulation; lesion area and lesion cholesteryl ester (CE) and triglyceride (TG) contents increased with animal age and plasma Hcy levels. Plasma total cholesterol was significantly increased, whereas high density lipoprotein (HDL) cholesterol and TG concentrations of apoE(-/-)/CBS(-/-) mice were decreased. Cholesterol esterification and activities of enzymes catalyzing CE or TG formation in the vessel wall and in peritoneal macrophages were not changed by hyperhomocysteinemia. However, uptake of human acetyl-LDL, but not native low density lipoprotein (LDL), by mouse peritoneal macrophages was higher in the presence of hyperhomocysteinemia. These results suggest that isolated hyperhomocysteinemia is atherogenic and alters hepatic and macrophage lipoprotein metabolism, in part, by enhancing uptake of modified LDL.

Differential role of heparan sulfate proteoglycans on aggregated LDL uptake in human vascular smooth muscle cells and mouse embryonic fibroblasts

OBJECTIVE

Low density lipoprotein (LDL) receptor-related protein (LRP) binds and internalizes aggregated LDL (agLDL) in human vascular smooth muscle cells (VSMCs). To analyze the contribution of proteoglycans (PGs) to agLDL uptake in human VSMCs, in wild-type mouse embryonic fibroblasts (MEF line), and in LRP-deficient mouse embryonic fibroblasts (PEA13 line).

METHODS AND RESULTS

PGs in the medium and cellular and extracellular matrix have been isolated by metabolic radiolabeling with [35S]Na2SO4 and characterized by selective digestion with heparinase I and III (4 U/mL each) and chondroitinase ABC (2 U/mL). To examine the contribution of PGs and LRPs to agLDL internalization, nonexpressing and LRP-expressing cells, treated or not with polysaccharidase, were incubated with agLDL (25, 50, and 100 micro g/mL) for 18 hours. In human VSMCs, agLDL was unable to induce cholesteryl ester (CE) accumulation in antisense LRP-oligodeoxynucleotide-treated cells, and heparan sulfate (HS)-PG depletion leads to a reduction of the CE accumulation. In mouse fibroblasts, PEA13 compared with MEF showed lower, but still considerable, CE accumulation, and HS-PG depletion almost completely inhibited CE accumulation.

CONCLUSIONS

In MEF, HS-PGs can function alone as receptors that bind and internalize agLDL in the absence of LRP, but in human VSMCs, although HS-PGs facilitate agLDL binding to the cells, LRP is essential for agLDL internalization.

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.

Trypsin-sensitive and lipid-containing sites of the macrophage extracellular matrix bind apolipoprotein A-I and participate in ABCA1-dependent cholesterol efflux

A unique property of the extracellular matrix of J774 and THP-1 cells has been identified, which contributes to the ability of these cells to promote cholesterol efflux. We demonstrate high level apolipoprotein (apo) A-I binding to macrophage cells (THP-1 and J774) and to their extracellular matrix (ECM). However, high level apoA-I binding is not observed on fibroblasts, HepG2 cells, or U937 cells (a macrophage cell line that does not efflux cholesterol to apoA-I or bind apoA-I on their respective ECM). Binding to the ECM of THP-1 or J774 macrophages depends on the presence of apoA-I C-terminal helices and is markedly reduced with a mutant lacking residues 187-243 (apoA-I Delta(187-243)), suggesting that the hydrophobic C terminus forms a hydrophobic interaction with the ECM. ApoA-I binding is lost upon trypsin treatment or with Triton X-100, a preparation method that de-lipidates the ECM. However, binding is recovered with re-lipidation, and is preserved with ECM prepared using cytochalasin B, which conserves the endogenous phospholipid levels of the ECM. We also demonstrate that specific cholesterol efflux to apoA-I is much reduced in cells released from their native ECM, but fully restored when ECM-depleted cells are added back to ECM in the presence of apoA-I. The apoA-I-mediated efflux is deficient in plated or suspension U937 macrophages, but is restored to high levels when the suspension U937 cells are reconstituted with the ECM of J774 cells. The ECM-dependent activity was much reduced in the presence of glyburide, indicating participation of ABCA1 (ATP-binding cassette transporter 1) in the efflux mechanism. These studies establish a novel binding site for apoA-I on the macrophage ECM that may function together with ABCA1 in promoting cholesterol efflux.

Macrophage uptake of low-density lipoprotein bound to aggregated C-reactive protein: possible mechanism of foam-cell formation in atherosclerotic lesions

Foam cells found in atherosclerotic lesions are believed to derive from macrophages that take up aggregated low-density lipoprotein (LDL) particles bound to the extracellular matrix of arterial walls. C-reactive protein (CRP) is an acute-phase protein found in atherosclerotic lesions, which when immobilized on a solid phase, can bind and cluster LDL particles in a calcium-dependent manner. In the present study, we examined whether CRP-bound aggregated LDL could be taken up by macrophages in culture. CRP molecules were aggregated in the presence of calcium and immobilized on the surface of polystyrene microtitre wells. Human LDL added to the wells bound to and aggregated on the immobilized CRP, also in a calcium-dependent manner. On incubation with macrophages, the immobilized CRP-bound LDL aggregates were readily taken up by the cells, as demonstrated by immunofluorescence microscopy, by the cellular accumulation of cholesterol and by the overexpression of adipophilin. Immunofluorescence microscopy and flow-cytometry analysis established that the uptake of the LDL-CRP complex was not mediated by the CRP receptor CD32. These observations with immobilized CRP and LDL, approximating the conditions that exist in the extracellular matrix of the arterial wall, thus suggest that CRP may contribute to the formation of foam cells in atherosclerotic lesions by causing the aggregation of LDL molecules that are then taken up by macrophages through a CD32-independent pathway.

Murine sclerodermatous graft-versus-host disease, a model for human scleroderma: cutaneous cytokines, chemokines, and immune cell activation

Murine sclerodermatous graft-vs-host disease (Scl GVHD) models human scleroderma, with prominent skin thickening, lung fibrosis, and up-regulation of cutaneous collagen mRNA. Fibrosis in Scl GVHD may be driven by infiltrating TGF-beta1-producing mononuclear cells. Here we characterize the origin and types of those cutaneous effector cells, the cytokine and chemokine environments, and the effects of anti-TGF-beta Ab on skin fibrosis, immune cell activation markers, and collagen and cytokine synthesis. Donor cells infiltrating skin in Scl GVHD increase significantly at early time points post-transplantation and are detectable by PCR analysis of Y-chromosome sequences when female mice are transplanted with male cells. Cutaneous monocyte/macrophages and T cells are the most numerous cells in Scl GVHD compared with syngeneic controls. These immune cells up-regulate activation markers (MHC class II I-A(d) molecules and class A scavenger receptors), suggesting Ag presentation by cutaneous macrophages in early fibrosing disease. Early elevated cutaneous mRNA expression of TGF-beta1, but not TGF-beta2 or TGF-beta3, and elevated C-C chemokines macrophage chemoattractant protein-1, macrophage inflammatory protein-1alpha, and RANTES precede subsequent skin and lung fibrosis. Therefore, TGF-beta1-producing donor mononuclear cells may be critical effector cells, and C-C chemokines may play important roles in the initiation of Scl GVHD. Abs to TGF-beta prevent Scl GVHD by effectively blocking the influx of monocyte/macrophages and T cells into skin and by abrogating up-regulation of TGF-beta1, thereby preventing new collagen synthesis. The Scl GVHD model is valuable for testing new interventions in early fibrosing diseases, and chemokines may be new potential targets in scleroderma.

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.

Arterial wall chondroitin sulfate proteoglycans: diverse molecules with distinct roles in lipoprotein retention and atherogenesis

Chondroitin sulfate proteoglycans (CSPGs) of the arterial wall are generally considered to be atherogenic because of their ability to trap cholesterol-rich lipoproteins in vitro. Nevertheless, CSPGs are a diverse group of molecules with a long evolutionary history and distinct biologic functions. The three principal CSPGs in the arterial wall are versican, which is part of the hyalectan gene family; and decorin and biglycan, which are members of a separate gene family, the small leucine-rich proteoglycans. Importantly, there is now substantial evidence that the different molecular species of CSPGs participate unequally in lipoprotein retention, and that they exert unequal regulatory effects that are related to atherogenesis. Recently available murine models with genetic manipulations that affect CSPGs now allow causal studies of the roles of these molecules to be conducted in vivo, with occasionally surprising results. Moreover, tools are being developed to examine human genetic variations that are relevant to CSPGs, which may provide additional important insights into the human disease. The era in which proteoglycans are regarded as a nondescript backdrop, playing purely nonspecific structural roles, is over. Studies in manipulated animals and in human populations will continue to reveal precise, dynamic roles for these fascinating and ancient molecules.

Retention of oxidized LDL by extracellular matrix proteoglycans leads to its uptake by macrophages: an alternative approach to study lipoproteins cellular uptake

Interaction between arterial macrophages and oxidized LDL (Ox-LDL) leads to foam cell formation, a critical step during early atherogenesis. Until now, cellular uptake of lipoproteins was studied through incubation of the media-soluble lipoprotein with cultured macrophages. However, as lipoproteins in the arterial wall are bound to subendothelial matrix, we questioned whether the retention (binding) of Ox-LDL to a macrophage-derived extracellular matrix (ECM) could lead to enhanced uptake by macrophages. The uptake of ECM-bound Ox-LDL by activated macrophages (by phorbol myristate acetate) was lipoprotein dose dependent, time dependent and higher (by 1.5-fold) than the uptake of ECM-bound native LDL. Preincubation of the ECM with lipoprotein lipase before the addition of Ox-LDL was essential for the uptake of ECM-bound Ox-LDL by the macrophages. After radiolabeling of the ECM glycosaminoglycans (GAGs), we found that ECM-bound Ox-LDL is taken up by the macrophages together with the ECM-GAG. Finally, these results were further confirmed through the use of ECM obtained from mouse peritoneal macrophages (MPMs), derived from atherosclerotic, apoE-deficient mice. In 24-week-old mice with developed atherosclerosis, the GAG content of their MPM-derived ECM increased by 52%, the ability of their MPM-derived ECM to bind Ox-LDL increased by 57%, and macrophage uptake of Ox-LDL that was retained by the MPM-derived ECM increased by 86%. In conclusion, the present study demonstrated that ECM-bound Ox-LDL is taken up by activated macrophages. This may represent a physiopathological phenomenon that leads to cholesterol and oxysterol accumulation in arterial macrophages, the hallmark of early atherosclerosis.

Cholesterol and phospholipid metabolism in macrophages

Cholesterol-loaded macrophages are present at all stages of atherogenesis, and recent in vivo data indicate that these cells play important roles in both early lesion development and late lesion complications. To understand how these cells promote atherogenesis, it is critical that we understand how lesional macrophages interact with subendothelial lipoproteins, the consequences of this interaction, and the impact of subsequent intracellular metabolic events. In the arterial wall, macrophages likely interact with both soluble and matrix-retained lipoproteins, and a new challenge is to understand how certain consequences of these two processes might differ. Initially, the major intracellular metabolic route of the lipoprotein-derived cholesterol is esterification to fatty acids, but macrophages in advanced atherosclerotic lesions progressively accumulate large amounts of unesterified, or free, cholesterol (FC). In cultured macrophages, excess FC accumulation stimulates phospholipid biosynthesis, which is an adaptive response to protect the macrophage from FC-induced cytotoxicity. This phospholipid response eventually decreases with continued FC loading, leading to a series of cellular death reactions involving both death receptor-induced signaling and mitochondrial dysfunction. Because macrophage death in advanced lesions is thought to promote plaque instability, these intracellular processes involving cholesterol, phospholipid, and death pathways may play a critical role in the acute clinical manifestations of advanced atherosclerotic lesions.

Sphingomyelinase converts lipoproteins from apolipoprotein E knockout mice into potent inducers of macrophage foam cell formation

The apoE knockout (E0) mouse is one of the most widely used animal models of atherosclerosis, and there may be similarities to chylomicron remnant-induced atherosclerosis in humans. Although the lesions of these mice contain large numbers of cholesteryl ester (CE)-laden macrophages (foam cells), E0 plasma lipoproteins are relatively weak inducers of cholesterol esterification in macrophages. Previous in vivo work has suggested that arterial wall sphingomyelinase (SMase) may promote atherogenesis in the E0 mouse, perhaps by inducing subendothelial lipoprotein aggregation and subsequent foam cell formation. The goal of the present study was to test the hypothesis that the modification of E0 lipoproteins by SMase converts these lipoproteins into potent inducers of macrophage foam cell formation. When d<1.063 E0 lipoproteins were pretreated with SMase and then incubated with E0 macrophages, cellular CE mass and stimulation of the cholesterol esterification pathway were increased approximately 5-fold compared with untreated lipoproteins. SMase-treated E0 lipoproteins were more potent stimulators of cholesterol esterification than either E0 lipoproteins in the presence of lipoprotein lipases or oxidized E0 lipoproteins. The uptake and degradation of SMase-treated E0 lipoproteins by macrophages were saturable and specific and substantially inhibited by partial proteolysis of cell-surface proteins. Uptake and degradation were diminished by an anti-apoB antibody and by competition with human S(f) 100-400 hypertriglyceridemic VLDL, raising the possibility that a receptor that recognizes apoB-48 might be involved. In conclusion, SMase-modification of E0 lipoproteins, a process previously shown to occur in lesions, may be an important mechanism for foam cell formation in this widely studied model of atherosclerosis. Moreover, the findings in this report may provide important clues regarding the atherogenicity of chylomicron remnants in humans.