Targeting of neutral cholesterol ester hydrolase to the endoplasmic reticulum via its N-terminal sequence

Unraveling the significance of innate inflammation in vascular disease

Atheroma formation is initiated by the activation of endothelial and smooth muscle cells, as well as immune cells, including neutrophils, lymphocytes, monocytes, macrophages, and dendritic cells. Monocytes, macrophages, and neutrophils are the innate immune cells that provide a rapid initial line of defence against vascular disease. These cells have a short lifespan and cannot retain memories, making them potential therapeutic targets for the inflammatory process associated with atherosclerosis. In addition, macrophages comprise the majority of vessel wall infiltrates and are, therefore, implicated in all stages of atherosclerosis progression. Neutrophils are the most common type of leukocyte found in circulation, and their high levels of matrix-degrading protease explain their significance in fibrous cap destabilization. However, the activation of immune cells becomes more complex by various microenvironmental stimuli and cytokines, which ultimately transform immune cells into their pro-inflammatory state. Different types of macrophage subsets with distinct functions in inflammation, such as M1 macrophages, cause an increase in pro-inflammatory cytokines and produce reactive oxygen species and nitric oxide, further worsening the disease. This review aims to shed light on immune-mediated inflammation in cardiovascular disease by focusing on the role of macrophage subsets in vascular inflammation and plaque stability, as well as the interaction between neutrophils and monocyte-macrophages.

Validation and Functional Analysis of Reference and Tissue-Specific Genes in Adipose Tissue of Freshwater Drum, Aplodinotus grunniens, under Starvation and Hypothermia Stress

Adipose tissue is critical to the growth, development, and physiological health of animals. Reference genes play an essential role in normalizing the expression of mRNAs. Tissue-specific genes are preferred for their function and expression in specific tissues or cell types. Identification of these genes contributes to understanding the tissue-gene relationship and the etiology and discovery of new tissue-specific targets. Therefore, reference genes and tissue-specific genes in the adipose tissue of Aplodinotus grunniens were identified to explore their function under exogenous starvation (1 d, 2 w, 6 w) and hypothermic stress (18 °C and 10 °C for 2 d and 8 d) in this study. Results suggest that 60SRP was the most stable reference gene in adipose tissue. Meanwhile, eight genes were validated as tissue-specific candidates from the high-throughput sequencing database, while seven of them (ADM2, β₂GP1, CAMK1G, CIDE3, FAM213A, HSL, KRT222, and NCEH1) were confirmed in adipose tissue. Additionally, these seven tissue-specific genes were active in response to starvation and hypothermic stress in a time- or temperature-dependent manner. These results demonstrate that adipose-specific genes can be identified using stable internal reference genes, thereby identifying specific important functions under starvation and hypothermic stress, which provides tissue-specific targets for adipose regulation in A. grunniens.

ISG15 Is a Novel Regulator of Lipid Metabolism during Vaccinia Virus Infection

Interferon-stimulated gene 15 (ISG15) is a 15-kDa ubiquitin-like modifier that binds to target proteins in a process termed ISGylation. ISG15, first described as an antiviral molecule against many viruses, participates in numerous cellular processes, from immune modulation to the regulation of genome stability. Interestingly, the role of ISG15 as a regulator of cell metabolism has recently gained strength. We previously described ISG15 as a regulator of mitochondrial functions in bone marrow-derived macrophages (BMDMs) in the context of Vaccinia virus (VACV) infection. Here, we demonstrate that ISG15 regulates lipid metabolism in BMDMs and that ISG15 is necessary to modulate the impact of VACV infection on lipid metabolism. We show that Isg15-/- BMDMs demonstrate alterations in the levels of several key proteins of lipid metabolism that result in differences in the lipid profile compared with Isg15+/+ (wild-type [WT]) BMDMs. Specifically, Isg15-/- BMDMs present reduced levels of neutral lipids, reflected by decreased lipid droplet number. These alterations are linked to increased levels of lipases and are independent of enhanced fatty acid oxidation (FAO). Moreover, we demonstrate that VACV causes a dysregulation in the proteomes of BMDMs and alterations in the lipid content of these cells, which appear exacerbated in Isg15-/- BMDMs. Such metabolic changes are likely caused by increased expression of the metabolic regulators peroxisome proliferator-activated receptor-γ (PPARγ) and PPARγ coactivator-1α (PGC-1α). In summary, our results highlight that ISG15 controls BMDM lipid metabolism during viral infections, suggesting that ISG15 is an important host factor to restrain VACV impact on cell metabolism. IMPORTANCE The functions of ISG15 are continuously expanding, and growing evidence supports its role as a relevant modulator of cell metabolism. In this work, we highlight how the absence of ISG15 impacts macrophage lipid metabolism in the context of viral infections and how poxviruses modulate metabolism to ensure successful replication. Our results open the door to new advances in the comprehension of macrophage immunometabolism and the interaction between VACV and the host.

KIAA1363-A Multifunctional Enzyme in Xenobiotic Detoxification and Lipid Ester Hydrolysis

KIAA1363, annotated as neutral cholesterol ester hydrolase 1 (NCEH1), is a member of the arylacetamide deacetylase (AADAC) protein family. The name-giving enzyme, AADAC, is known to hydrolyze amide and ester bonds of a number of xenobiotic substances, as well as clinical drugs and of endogenous lipid substrates such as diglycerides, respectively. Similarly, KIAA1363, annotated as the first AADAC-like protein, exhibits enzymatic activities for a diverse substrate range including the xenobiotic insecticide chlorpyrifos oxon and endogenous substrates, acetyl monoalkylglycerol ether, cholesterol ester, and retinyl ester. Two independent knockout mouse models have been generated and characterized. However, apart from reduced acetyl monoalkylglycerol ether and cholesterol ester hydrolase activity in specific tissues and cell types, no gross-phenotype has been reported. This raises the question of its physiological role and whether it functions as drug detoxifying enzyme and/or as hydrolase/lipase of endogenous substrates. This review delineates the current knowledge about the structure, function and of the physiological role of KIAA1363, as evident from the phenotypical changes inflicted by pharmacological inhibition or by silencing as well as knockout of KIAA1363 gene expression in cells, as well as mouse models, respectively.

KIAA1363 affects retinyl ester turnover in cultured murine and human hepatic stellate cells

Large quantities of vitamin A are stored as retinyl esters (REs) in specialized liver cells, the hepatic stellate cells (HSCs). To date, the enzymes controlling RE degradation in HSCs are poorly understood. In this study, we identified KIAA1363 (also annotated as arylacetamide deacetylase 1 or neutral cholesterol ester hydrolase 1) as a novel RE hydrolase. We show that KIAA1363 is expressed in the liver, mainly in HSCs, and exhibits RE hydrolase activity at neutral pH. Accordingly, addition of the KIAA1363-specific inhibitor JW480 largely reduced RE hydrolase activity in lysates of cultured murine and human HSCs. Furthermore, cell fractionation experiments and confocal microscopy studies showed that KIAA1363 localizes to the endoplasmic reticulum. We demonstrate that overexpression of KIAA1363 in cells led to lower cellular RE content after a retinol loading period. Conversely, pharmacological inhibition or shRNA-mediated silencing of KIAA1363 expression in cultured murine and human HSCs attenuated RE degradation. Together, our data suggest that KIAA1363 affects vitamin A metabolism of HSCs by hydrolyzing REs at the endoplasmic reticulum, thereby counteracting retinol esterification and RE storage in lipid droplets.

The Diverse Cellular Functions of Inner Nuclear Membrane Proteins

The nuclear compartment is delimited by a specialized expanded sheet of the endoplasmic reticulum (ER) known as the nuclear envelope (NE). Compared to the outer nuclear membrane and the contiguous peripheral ER, the inner nuclear membrane (INM) houses a unique set of transmembrane proteins that serve a staggering range of functions. Many of these functions reflect the exceptional position of INM proteins at the membrane-chromatin interface. Recent research revealed that numerous INM proteins perform crucial roles in chromatin organization, regulation of gene expression, genome stability, and mediation of signaling pathways into the nucleus. Other INM proteins establish mechanical links between chromatin and the cytoskeleton, help NE remodeling, or contribute to the surveillance of NE integrity and homeostasis. As INM proteins continue to gain prominence, we review these advancements and give an overview on the functional versatility of the INM proteome.

2-Hydroxypropyl-β-cyclodextrin reduces retinal cholesterol in wild-type and Cyp27a1-/- Cyp46a1-/- mice with deficiency in the oxysterol production

BACKGROUND AND PURPOSE

2-Hydroxypropyl-β-cyclodextrin (HPCD) is an FDA approved vehicle for drug delivery and an efficient cholesterol-lowering agent. HPCD was proposed to lower tissue cholesterol via multiple mechanisms including those mediated by oxysterols. CYP27A1 and CYP46A1 are the major oxysterol-producing enzymes in the retina that convert cholesterol to 27- and 24-hydroxycholesterol, respectively. We investigated whether HPCD treatments affected the retina of wild-type and Cyp27a1^-/- Cyp46a1^-/- mice that do not produce the major retinal oxysterols.

EXPERIMENTAL APPROACH

HPCD administration was either by i.p., p.o. or s.c. Delivery to the retina was confirmed by angiography using the fluorescently labelled HPCD. Effects on the levels of retinal sterols, mRNA and proteins were evaluated by GC-MS, qRT-PCR and label-free approach, respectively.

KEY RESULTS

In both wild-type and Cyp27a1^-/- Cyp46a1^-/- mice, HPCD crossed the blood-retinal barrier when delivered i.p. and lowered the retinal cholesterol content when administered p.o. and s.c. In both genotypes, oral HPCD treatment affected the expression of cholesterol-related genes as well as the proteins involved in endocytosis, lysosomal function and lipid homeostasis. Mechanistically, liver X receptors and the altered expression of Lipe (hormone-sensitive lipase), Nceh1 (neutral cholesterol ester hydrolase 1) and NLTP (non-specific lipid-transfer protein) could mediate some of the HPCD effects.

CONCLUSIONS AND IMPLICATIONS

HPCD treatment altered retinal cholesterol homeostasis and is a potential therapeutic approach for the reduction of drusen and subretinal drusenoid deposits, cholesterol-rich lesions and hallmarks of age-related macular degeneration.

LINKED ARTICLES

This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Autoinflammation in addition to combined immunodeficiency: SLC29A3 gene defect

INTRODUCTION

H Syndrome is an autosomal recessive (AR) disease caused by defects in SLCA29A3 gene. This gene encodes the equilibrative nucleoside transporter, the protein which is highly expressed in spleen, lymph node and bone marrow. Autoinflammation and autoimmunity accompanies H Syndrome (HS).

AIM

The aim was to further elucidate the mechanisms of disease by molecular studies in a patient with SLC29A3 gene defect.

PATIENT AND METHODS

Mitochondrial dysfunction, lysosomal integrity, cytokine response in response to stimulation with different pattern recognition receptor ligands, and circulating cell-free mitochondrial-DNA(ccf-mtDNA) level in plasma were analyzed compared to controls to understand the cellular triggers of autoinflammation. RNA sequencing (RS) analyses were also performed in monocytes before/after culture with lipopolysaccharide.

RESULTS

Patient had progressive destructive arthropathy in addition to clinical findings due to combined immunodeficiency. Pure red cell aplasia (PRCA), vitiligo, diabetes, multiple autoantibody positivity, lymphopenia, increased acute phase reactants were present. Recent thymic emigrants (RTE), naïve T cells were decreased, effector memory CD4 + T cells, nonclassical inflammatory monocytes were increased. Patient's peripheral blood mononuclear cells secreted more IL-1β and IL-6, showed lysosomal disruption and significant mitochondrial dysfunction compared to healthy controls. Plasma ccf-mtDNA level was significantly elevated compared to age-matched controls (p < 0.05). RNA sequencing studies revealed decreased expression of NLR Family Caspase Recrument-Domain Containing 4(NLRC4), 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4(PFKFB4), serine dehydratase(SDS), heparan sulfate(Glucosamine) 3-O-sulfotransferase 1(HS3ST1), neutral cholesterol ester hydrolase 1 (NCEH1), and interleukin-8 (IL-8) in patient's monocytes compared to controls. Longstanding PRCA, which is possibly autoimmune, resolved after initiating monthly intravenous immunoglobulins (IVIG) and low dose steroids to the patient.

CONCLUSION

Although autoinflammation and autoimmunity are reported in HS, by functional analyses we here show in the present patient that over-active inflammasome pathway in HS might be related with mitochondrial and lysosomal dysfunction. Increased plasma ccf-mtDNA may be used as a biomarker of inflammasomopathy in HS. HS should be included in the classification of primary immunodeficiency diseases.

Immunobiology of Atherosclerosis: A Complex Net of Interactions

Cardiovascular disease is the leading cause of mortality worldwide, and atherosclerosis the principal factor underlying cardiovascular events. Atherosclerosis is a chronic inflammatory disease characterized by endothelial dysfunction, intimal lipid deposition, smooth muscle cell proliferation, cell apoptosis and necrosis, and local and systemic inflammation, involving key contributions to from innate and adaptive immunity. The balance between proatherogenic inflammatory and atheroprotective anti-inflammatory responses is modulated by a complex network of interactions among vascular components and immune cells, including monocytes, macrophages, dendritic cells, and T, B, and foam cells; these interactions modulate the further progression and stability of the atherosclerotic lesion. In this review, we take a global perspective on existing knowledge about the pathogenesis of immune responses in the atherosclerotic microenvironment and the interplay between the major innate and adaptive immune factors in atherosclerosis. Studies such as this are the basis for the development of new therapies against atherosclerosis.

Loss of ACAT1 Attenuates Atherosclerosis Aggravated by Loss of NCEH1 in Bone Marrow-Derived Cells

AIM

Acyl-CoA cholesterol acyltransferase 1 (ACAT1) esterifies free cholesterol to cholesteryl esters (CE), which are subsequently hydrolyzed by neutral cholesterol ester hydrolase 1 (NCEH1). The elimination of ACAT1 in vitro reduces the amounts of CE accumulated in Nceh1-deficient macrophages. The present study aimed at examining whether the loss of ACAT1 attenuates atherosclerosis which is aggravated by the loss of NCEH1 in vivo.

METHODS

Low density lipoprotein receptor (Ldlr)-deficient mice were transplanted with bone marrow from wild-type mice and mice lacking ACAT1, NCEH1, or both. The four types of mice were fed a high-cholesterol diet and, then, were examined for atherosclerosis.

RESULTS

The cross-sectional lesion size of the recipients of Nceh1-deficient bone marrow was 1.6-fold larger than that of the wild-type bone marrow. The lesions of the recipients of Nceh1-deficient bone marrow were enriched with MOMA2-positive macrophages compared with the lesions of the recipients of the wild-type bone marrow. The size and the macrophage content of the lesions of the recipients of bone marrow lacking both ACAT1 and NCEH1 were significantly smaller than the recipients of the Nceh1-deficient bone marrow, indicating that the loss of ACAT1 decreases the excess CE in the Nceh1-deficient lesions. The collagen-rich and/or mucin-rich areas and en face lesion size were enlarged in the recipients of the Acat1^－/－ bone marrow compared with those of the recipients of the WT bone marrow.

CONCLUSION

The loss of ACAT1 in bone marrow-derived cells attenuates atherosclerosis, which is aggravated by the loss of NCEH1, corroborating the in vitro functions of ACAT1 (formation of CE) and NCEH1 (hydrolysis of CE).

Foam cell formation: A new target for fighting atherosclerosis and cardiovascular disease

During atherosclerosis, the gradual accumulation of lipids into the subendothelial space of damaged arteries results in several lipid modification processes followed by macrophage uptake in the arterial wall. The way in which these modified lipoproteins are dealt with determines the likelihood of cholesterol accumulation within the monocyte-derived macrophage and thus its transformation into the foam cell that makes up the characteristic fatty streak observed in the early stages of atherosclerosis. The unique expression of chemokine receptors and cellular adhesion molecules expressed on the cell surface of monocytes points to a particular extravasation route that they can take to gain entry into atherosclerotic site, in order to undergo differentiation into the phagocytic macrophage. Indeed several GWAS and animal studies have identified key genes and proteins required for monocyte recruitment as well cholesterol handling involving lipid uptake, cholesterol esterification and cholesterol efflux. A re-examination of the previously accepted paradigm of macrophage foam cell origin has been called into question by recent studies demonstrating shared expression of scavenger receptors, cholesterol transporters and pro-inflammatory cytokine release by alternative cell types present in the neointima, namely; endothelial cells, vascular smooth muscle cells and stem/progenitor cells. Thus, therapeutic targets aimed at a more heterogeneous foam cell population with shared functions, such as enhanced protease activity, and signalling pathways, mediated by non-coding RNA molecules, may provide greater therapeutic outcome in patients. Finally, studies targeting each aspect of foam cell formation and death using both genetic knock down and pharmacological inhibition have provided researchers with a clearer understanding of the cellular processes at play, as well as helped researchers to identify key molecular targets, which may hold significant therapeutic potential in the future.

NCEH-1 modulates cholesterol metabolism and protects against α-synuclein toxicity in a C. elegans model of Parkinson's disease

Parkinson's disease (PD) is an aging-associated neurodegenerative disease affecting millions worldwide. Misfolding, oligomerization and accumulation of the human α-synuclein protein is a key pathological hallmark of PD and is associated with the progressive loss of dopaminergic neurons over the course of aging. Lifespan extension via the suppression of IGF-1/insulin-like signaling (IIS) offers a possibility to retard disease onset through induction of metabolic changes that provide neuroprotection. The nceh-1 gene of Caenorhabditis elegans encodes an ortholog of neutral cholesterol ester hydrolase 1 (NCEH-1), an IIS downstream protein that was identified in a screen as a modulator of α-synuclein accumulation in vivo. The mechanism whereby cholesterol metabolism functionally impacts neurodegeneration induced by α-synuclein is undefined. Here we report that NCEH-1 protects dopaminergic neurons from α-synuclein-dependent neurotoxicity in C. elegans via a mechanism that is independent of lifespan extension. We discovered that the presence of cholesterol, LDLR-mediated cholesterol endocytosis, and cholesterol efflux are all essential to NCEH-1-mediated neuroprotection. In protecting from α-synuclein neurotoxicity, NCEH-1 also stimulates cholesterol-derived neurosteroid formation and lowers cellular reactive oxygen species in mitochondria. Collectively, this study augments our understanding of how cholesterol metabolism can modulate a neuroprotective mechanism that attenuates α-synuclein neurotoxicity, thereby pointing toward regulation of neuronal cholesterol turnover as a potential therapeutic avenue for PD.

Selective inhibition of sterolO-acyltransferase 1 isozyme by beauveriolide III in intact cells

Beauveriolide III (BeauIII) inhibited sterol O-acyltransferases 1 and 2 (SOAT1 and SOAT2), which are endoplasmic reticulum (ER) membrane proteins, in an enzyme-based assay, and selectively inhibited SOAT1 in a cell-based assay using SOAT1-/SOAT2-CHO cells. This discrepancy in SOAT inhibition by BeauIII was investigated. In the enzyme-based assay, BeauIII inhibited SOAT1 and SOAT2 to a similar extent using microsomes prepared from cells disrupted under the strongest sonication condition. In semi-intact SOAT1-/SOAT2-CHO cells prepared by a treatment with digitonin (plasma membrane permeabilized), BeauIII selectively inhibited SOAT1 (IC₅₀; 5.0 µM (SOAT1) vs >90 µM (SOAT2)), while in those treated with saponin (plasma membrane and ER membrane permeabilized), BeauIII inhibited SOAT1 (IC₅₀, 1.8 µM) and SOAT2 (5.9 µM). SOAT1-selective inhibition by BeauIII was reproduced in intact ER fractions prepared from SOAT1/SOAT2-CHO cells. A Western blotting analysis revealed that biotin-labeled beauveriolide bound to the SOAT1 protein prepared from SOAT1-CHO cells. We concluded that BeauIII binds to a putative active site responsible for SOAT1 that is located on the cytosolic side of the ER, while BeauIII is not accessible to the corresponding active site for SOAT2 located on the luminal side.

Lighting-up breast cancer cells by a near-infrared fluorescent probe based on KIAA1363 enzyme-targeting

NB-AX, the first NIR KIAA1363-targeting fluorescent probe, could be used for the ultrafast lighting up of breast cancer cells and detection of tissues and tumors in vivo.

Macrophage-mediated cholesterol handling in atherosclerosis

Formation of foam cells is a hallmark at the initial stages of atherosclerosis. Monocytes attracted by pro-inflammatory stimuli attach to the inflamed vascular endothelium and penetrate to the arterial intima where they differentiate to macrophages. Intimal macrophages phagocytize oxidized low-density lipoproteins (oxLDL). Several scavenger receptors (SR), including CD36, SR-A1 and lectin-like oxLDL receptor-1 (LOX-1), mediate oxLDL uptake. In late endosomes/lysosomes of macrophages, oxLDL are catabolysed. Lysosomal acid lipase (LAL) hydrolyses cholesterol esters that are enriched in LDL to free cholesterol and free fatty acids. In the endoplasmic reticulum (ER), acyl coenzyme A: cholesterol acyltransferase-1 (ACAT1) in turn catalyses esterification of cholesterol to store cholesterol esters as lipid droplets in the ER of macrophages. Neutral cholesteryl ester hydrolases nCEH and NCEH1 are involved in a secondary hydrolysis of cholesterol esters to liberate free cholesterol that could be then out-flowed from macrophages by cholesterol ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 and SR-BI. In atherosclerosis, disruption of lipid homoeostasis in macrophages leads to cholesterol accumulation and formation of foam cells.

Absence of Nceh1 augments 25-hydroxycholesterol-induced ER stress and apoptosis in macrophages

An excess of cholesterol and/or oxysterols induces apoptosis in macrophages, contributing to the development of advanced atherosclerotic lesions. In foam cells, these sterols are stored in esterified forms, which are hydrolyzed by two enzymes: neutral cholesterol ester hydrolase 1 (Nceh1) and hormone-sensitive lipase (Lipe). A deficiency in either enzyme leads to accelerated growth of atherosclerotic lesions in mice. However, it is poorly understood how the esterification and hydrolysis of sterols are linked to apoptosis. Remarkably, Nceh1-deficient thioglycollate-elicited peritoneal macrophages (TGEMs), but not Lipe-deficient TGEMs, were more susceptible to apoptosis induced by oxysterols, particularly 25-hydroxycholesterol (25-HC), and incubation with 25-HC caused massive accumulation of 25-HC ester in the endoplasmic reticulum (ER) due to its defective hydrolysis, thereby activating ER stress signaling such as induction of CCAAT/enhancer-binding protein-homologous protein (CHOP). These changes were nearly reversed by inhibition of ACAT1. In conclusion, deficiency of Nceh1 augments 25-HC-induced ER stress and subsequent apoptosis in TGEMs. In addition to reducing the cholesteryl ester content of foam cells, Nceh1 may protect against the pro-apoptotic effect of oxysterols and modulate the development of atherosclerosis.

Novel lipid droplet-associated serine hydrolase regulates macrophage cholesterol mobilization

OBJECTIVE

Lipid-laden macrophages or foam cells are characterized by massive cytosolic lipid droplet (LD) deposition containing mostly cholesterol ester (CE) derived from the lipoproteins cleared from the arterial wall. Cholesterol efflux from foam cells is considered to be atheroprotective. Because cholesterol is effluxed as free cholesterol, CE accumulation in LDs may limit free cholesterol efflux. Our objective was to identify proteins that regulate cholesterol trafficking through LDs.

APPROACH AND RESULTS

In a proteomic analysis of the LD fraction of RAW 264.7 macrophages, we identified an evolutionarily conserved protein with a canonical GXSXG lipase catalytic motif and a predicted α/β-hydrolase fold, the RIKEN cDNA 1110057K04 gene, which we named LD-associated hydrolase (LDAH). LDAH association with LDs was confirmed by immunoblotting and immunocytochemistry. LDAH was labeled with a probe specific for active serine hydrolases. LDAH showed relatively weak in vitro CE hydrolase activity. However, cholesterol measurements in intact cells supported a significant role of LDAH in CE homeostasis because LDAH upregulation and downregulation decreased and increased, respectively, intracellular cholesterol and CE in human embryonic kidney-293 cells and RAW 264.7 macrophages. Mutation of the putative nucleophilic serine impaired active hydrolase probe binding, in vitro CE hydrolase activity, and cholesterol-lowering effect in cells, whereas this mutant still localized to the LD. LDAH upregulation increased CE hydrolysis and cholesterol efflux from macrophages, and, interestingly, LDAH is highly expressed in macrophage-rich areas within mouse and human atherosclerotic lesions.

CONCLUSIONS

The data identify a candidate target to promote reverse cholesterol transport from atherosclerotic lesions.

ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice

Leukocytosis is associated with increased cardiovascular disease risk in humans and develops in hypercholesterolemic atherosclerotic animal models. Leukocytosis is associated with the proliferation of hematopoietic stem and multipotential progenitor cells (HSPCs) in mice with deficiencies of the cholesterol efflux-promoting ABC transporters ABCA1 and ABCG1 in BM cells. Here, we have determined the role of endogenous apolipoprotein-mediated cholesterol efflux pathways in these processes. In Apoe⁻/⁻ mice fed a chow or Western- type diet, monocytosis and neutrophilia developed in association with the proliferation and expansion of HSPCs in the BM. In contrast, Apoa1⁻/⁻ mice showed no monocytosis compared with controls. ApoE was found on the surface of HSPCs, in a proteoglycan-bound pool, where it acted in an ABCA1- and ABCG1-dependent fashion to decrease cell proliferation. Accordingly, competitive BM transplantation experiments showed that ApoE acted cell autonomously to control HSPC proliferation, monocytosis, neutrophilia, and monocyte accumulation in atherosclerotic lesions. Infusion of reconstituted HDL and LXR activator treatment each reduced HSPC proliferation and monocytosis in Apoe⁻/⁻ mice. These studies suggest a specific role for proteoglycanbound ApoE at the surface of HSPCs to promote cholesterol efflux via ABCA1/ABCG1 and decrease cell proliferation, monocytosis, and atherosclerosis. Although endogenous apoA-I was ineffective, pharmacologic approaches to increasing cholesterol efflux suppressed stem cell proliferative responses.

Cytoskeleton disruption in J774 macrophages: consequences for lipid droplet formation and cholesterol flux

Macrophages store excess unesterified cholesterol (free, FC) in the form of cholesteryl ester (CE) in cytoplasmic lipid droplets. The hydrolysis of droplet-CE in peripheral foam cells is critical to HDL-promoted reverse cholesterol transport because it represents the first step in cellular cholesterol clearance, as only FC is effluxed from cells to HDL. Cytoplasmic lipid droplets move within the cell utilizing the cytoskeletal network, but, little is known about the influence of the cytoskeleton on lipid droplet formation. To understand this role we employed cytochalasin D (cyt.D) to promote actin depolymerization in J774 macrophages. Incubating J774 with acetylated LDL creates foam cells having a 4-fold increase in cellular cholesterol content (30-40% cholesterol present as cholesteryl ester (CE)) in cytoplasmic droplets. Lipid droplets formed in the presence of cyt.D are smaller in diameter. CE-deposition and -hydrolysis are decreased when cells are cholesterol-enriched in the presence of cyt.D or latrunculin A, another cytoskeleton disrupting agent. However, when lipid droplets formed in the presence of cyt.D are isolated and incubated with an exogenous CE hydrolase, the CE is more rapidly metabolized compared to droplets from control cells. This is apparently due to the smaller size and altered lipid composition of the droplets formed in the presence of cyt.D. Cytoskeletal proteins found on CE droplets influence droplet lipid composition and maturation in model foam cells. In J774 macrophages, cytoskeletal proteins are apparently involved in facilitating the interaction of lipid droplets and a cytosolic neutral CE hydrolase and may play a role in foam cell formation. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Cholesteryl ester hydrolase activity is abolished in HSL-/- macrophages but unchanged in macrophages lacking KIAA1363

Cholesteryl ester (CE) accumulation in macrophages represents a crucial event during foam cell formation, a hallmark of atherogenesis. Here we investigated the role of two previously described CE hydrolases, hormone-sensitive lipase (HSL) and KIAA1363, in macrophage CE hydrolysis. HSL and KIAA1363 exhibited marked differences in their abilities to hydrolyze CE, triacylglycerol (TG), diacylglycerol (DG), and 2-acetyl monoalkylglycerol ether (AcMAGE), a precursor for biosynthesis of platelet-activating factor (PAF). HSL efficiently cleaved all four substrates, whereas KIAA1363 hydrolyzed only AcMAGE. This contradicts previous studies suggesting that KIAA1363 is a neutral CE hydrolase. Macrophages of KIAA1363(-/-) and wild-type mice exhibited identical neutral CE hydrolase activity, which was almost abolished in tissues and macrophages of HSL(-/-) mice. Conversely, AcMAGE hydrolase activity was diminished in macrophages and some tissues of KIAA1363(-/-) but unchanged in HSL(-/-) mice. CE turnover was unaffected in macrophages lacking KIAA1363 and HSL, whereas cAMP-dependent cholesterol efflux was influenced by HSL but not by KIAA1363. Despite decreased CE hydrolase activities, HSL(-/-) macrophages exhibited CE accumulation similar to wild-type (WT) macrophages. We conclude that additional enzymes must exist that cooperate with HSL to regulate CE levels in macrophages. KIAA1363 affects AcMAGE hydrolase activity but is of minor importance as a direct CE hydrolase in macrophages.

Macrophage cholesteryl ester mobilization and atherosclerosis

Accumulation of cholesteryl esters (CE) stored as cytoplasmic lipid droplets is the main characteristic of macrophage foam cells that are central to the development of atherosclerotic plaques. Since only unesterified or free cholesterol (FC) can be effluxed from the cells to extracellular cholesterol acceptors, hydrolysis of CE is the obligatory first step in CE mobilization from macrophages. This reaction, catalyzed by neutral cholesteryl ester hydrolase (CEH), is increasingly being recognized as the rate-limiting step in FC efflux. CEH, therefore, regulates the process of reverse cholesterol transport and ultimate elimination of cholesterol from the body. In this review, we summarize the earlier controversies surrounding the identity of CEH in macrophages, discuss the characteristics of the various candidates recognized to date and examine their role in mobilizing cellular CE and thus regulating atherogenesis. In addition, physiological requirements to hydrolyze lipid droplet-associated substrate and complexities of interfacial catalysis are also discussed to emphasize the importance of evaluating the biochemical characteristics of candidate enzymes that may be targeted in the future to attenuate atherosclerosis.