Low-dose expression of a human apolipoprotein E transgene in macrophages restores cholesterol efflux capacity of apolipoprotein E-deficient mouse plasma

Interactions Between HDL and CD4+ T Cells: A Novel Understanding of HDL Anti-Inflammatory Properties

Several studies in animal models and human cohorts have recently suggested that HDLs (high-density lipoproteins) not only modulate innate immune responses but also adaptative immune responses, particularly CD4+ T cells. CD4+ T cells are central effectors and regulators of the adaptive immune system, and any alterations in their homeostasis contribute to the pathogenesis of cardiovascular diseases, autoimmunity, and inflammatory diseases. In this review, we focus on how HDLs and their components affect CD4+ T-cell homeostasis by modulating cholesterol efflux, immune synapsis, proliferation, differentiation, oxidative stress, and apoptosis. While the effects of apoB-containing lipoproteins on T cells have been relatively well established, this review focuses specifically on new connections between HDL and CD4+ T cells. We present a model where HDL may modulate T cells through both direct and indirect mechanisms.

The Impact of the Apolipoprotein E Genotype on Cardiovascular Disease and Cognitive Disorders

Apolipoprotein E (ApoE) plays a critical role in cholesterol transport and protection against the development of atherosclerotic cardiovascular disease (ASCVD). Humans have 3 prevalent isoforms of ApoE: apolipoprotein E2 (ApoE2), apolipoprotein E3 (ApoE3), and apolipoprotein E4 (ApoE4). The E4 allele has been associated with higher ASCVD risk. While E4 patients do have higher cholesterol levels, they do not have enough to account for the substantially elevated ASCVD risk relative to E2 and E3 patients. ASCVD risk calculators would underestimate the true effect of E4 if the difference was caused entirely by a difference in cholesterol level. This article reviews the function of ApoE in atherosclerosis, and how each isoform functions differently. We review what is known about the molecular mechanisms through which ApoE prevents endothelial dysfunction and damage, how ApoE stimulates macrophage efflux of cholesterol from atherogenic lesions, and the ways in which ApoE decreases inflammation throughout atherosclerosis. The impact of ApoE on Alzheimer's disease and a discussion of why it is possibly unrelated to ASCVD prevention are included. Clinical applications to hyperlipidemia management and ASCVD prevention in specific patient populations are discussed.

Correlations between the NMR Lipoprotein Profile, APOE Genotype, and Cholesterol Efflux Capacity of Fasting Plasma from Cognitively Healthy Elderly Adults

Cholesterol efflux capacity (CEC) is of interest given its potential relationship with several important clinical conditions including Alzheimer's disease. The inactivation of the APOE locus in mouse models supports the idea that it is involved in determining the CEC. With that in mind, we examine the impact of the plasma metabolome profile and the APOE genotype on the CEC in cognitively healthy elderly subjects. The study subjects were 144 unrelated healthy individuals. The plasma CEC was determined by exposing cultured mouse macrophages treated with BODIPY-cholesterol to human plasma. The metabolome profile was determined using NMR techniques. Multiple regression was performed to identify the most important predictors of CEC, as well as the NMR features most strongly associated with the APOE genotype. Plasma 3-hydroxybutyrate was the variable most strongly correlated with the CEC (r = 0.365; p = 7.3 × 10^-6). Male sex was associated with a stronger CEC (r = -0.326, p = 6.8 × 10^-5). Most of the NMR particles associated with the CEC did not correlate with the APOE genotype. The NMR metabolomics results confirmed the APOE genotype to have a huge effect on the concentration of plasma lipoprotein particles as well as those of other molecules including omega-3 fatty acids. In conclusion, the CEC of human plasma was associated with ketone body concentration, sex, and (to a lesser extent) the other features of the plasma lipoprotein profile. The APOE genotype exerted only a weak effect on the CEC via the modulation of the lipoprotein profile. The APOE locus was associated with omega-3 fatty acid levels independent of the plasma cholesterol level.

Apolipoprotein E in Cardiometabolic and Neurological Health and Diseases

A preponderance of evidence obtained from genetically modified mice and human population studies reveals the association of apolipoprotein E (apoE) deficiency and polymorphisms with pathogenesis of numerous chronic diseases, including atherosclerosis, obesity/diabetes, and Alzheimer’s disease. The human APOE gene is polymorphic with three major alleles, ε2, ε3 and ε4, encoding apoE2, apoE3, and apoE4, respectively. The APOE gene is expressed in many cell types, including hepatocytes, adipocytes, immune cells of the myeloid lineage, vascular smooth muscle cells, and in the brain. ApoE is present in subclasses of plasma lipoproteins, and it mediates the clearance of atherogenic lipoproteins from plasma circulation via its interaction with LDL receptor family proteins and heparan sulfate proteoglycans. Extracellular apoE also interacts with cell surface receptors and confers signaling events for cell regulation, while apoE expressed endogenously in various cell types regulates cell functions via autocrine and paracrine mechanisms. This review article focuses on lipoprotein transport-dependent and -independent mechanisms by which apoE deficiency or polymorphisms contribute to cardiovascular disease, metabolic disease, and neurological disorders.

ApoE Lipidation as a Therapeutic Target in Alzheimer's Disease

Apolipoprotein E (APOE) is the major cholesterol carrier in the brain, affecting various normal cellular processes including neuronal growth, repair and remodeling of membranes, synaptogenesis, clearance and degradation of amyloid β (Aβ) and neuroinflammation. In humans, the APOE gene has three common allelic variants, termed E2, E3, and E4. APOE4 is considered the strongest genetic risk factor for Alzheimer’s disease (AD), whereas APOE2 is neuroprotective. To perform its normal functions, apoE must be secreted and properly lipidated, a process influenced by the structural differences associated with apoE isoforms. Here we highlight the importance of lipidated apoE as well as the APOE-lipidation targeted therapeutic approaches that have the potential to correct or prevent neurodegeneration. Many of these approaches have been validated using diverse cellular and animal models. Overall, there is great potential to improve the lipidated state of apoE with the goal of ameliorating APOE-associated central nervous system impairments.

The future of apolipoprotein E mimetic peptides in the prevention of cardiovascular disease

PURPOSE OF REVIEW

This review aims to discuss the recent developments in the area of apolipoprotein E (apoE) mimetics and their therapeutic potential for treating cardiovascular disease, the leading cause of mortality worldwide.

RECENT FINDINGS

Ongoing research efforts target the development of novel therapies that would not only reduce circulating levels of atherogenic lipoproteins, but could also increase high density lipoprotein cholesterol (HDL-C) levels and/or improve HDL function. Among them, synthetic peptides that mimic the structure of natural human apoE, a component of triglyceride-rich lipoproteins and HDL, have been designed and proven to be functionally similar to apoE. In specific, apoE mimetic peptides mediate hepatic clearance of circulating atherogenic lipoproteins, dramatically reduce plasma cholesterol, and lead to attenuation of atherosclerosis development in vivo. These peptides also exhibit pleiotropic antiatherogenic properties, such as macrophage cholesterol efflux capacity, as well as anti-inflammatory and antioxidative functions.

SUMMARY

ApoE mimetics are undergoing preclinical and clinical evaluation with promising results to date that render them attractive candidates in cardiovascular disease prevention and treatment.

Current and Emerging Reconstituted HDL-apoA-I and HDL-apoE Approaches to Treat Atherosclerosis

Atherosclerosis affects millions of people worldwide. However, the wide variety of limitations in the current therapeutic options leaves much to be desired in future lipid-lowering therapies. For example, although statins, which are the first-line treatment for coronary heart disease (CHD), reduce the risk of cardiovascular events in a large percentage of patients, they lead to optimal levels of low density lipoprotein-cholesterol (LDL-C) in only about one-third of patients. A new promising research direction against atherosclerosis aims to improve lipoprotein metabolism. Novel therapeutic approaches are being developed to increase the levels of functional high density lipoprotein (HDL) particles. This review aims to highlight the atheroprotective potential of the in vitro synthesized reconstituted HDL particles containing apolipoprotein E (apoE) as their sole apolipoprotein component (rHDL-apoE). For this purpose, we provide: (1) a summary of the atheroprotective properties of native plasma HDL and its apolipoprotein components, apolipoprotein A-I (apoA-I) and apoE; (2) an overview of the anti-atherogenic functions of rHDL-apoA-I and apoA-I-containing HDL, i.e., natural HDL isolated from transgenic Apoa1^−/− × Apoe^−/− mice overexpressing human apoA-I (HDL-apoA-I); and (3) the latest developments and therapeutic potential of HDL-apoE and rHDL-apoE. Novel rHDL formulations containing apoE could possibly present enhanced biological functions, leading to improved therapeutic efficacy against atherosclerosis.

The phenomenon of atherosclerosis reversal and regression: Lessons from animal models

Studies in non-rodent and murine models showed that atherosclerosis can be reversed. Atherosclerosis progression induced by high-fat or cholesterol-rich diet can be reduced and reversed to plaque regression after switching to a normal diet or through administration of lipid-lowering agents. The similar process should exist in humans after implementation of lipid-lowering therapy and as a result of targeting of small rupture-prone plaques that are major contributors for acute atherosclerotic complications. Lowering of low density lipoprotein (LDL) cholesterol and the activation of reverse cholesterol transport lead to a decline in foam cell content, to the depletion of plaque lipid reservoirs, a decrease in lesional macrophage numbers through the activation of macrophage emigration and, probably, apoptosis, dampening plaque inflammation, and the induction of anti-inflammatory macrophages involved in clearance of the necrotic core and plaque healing. By contrast, plaque regression is characterized by opposite events, leading to the retention of atherogenic LDL and oxidized LDL particles in the plaque, an increased flux of monocytes, the immobilization of macrophages in the intimal vascular tissues, and the propagation of intraplaque inflammation. Transfer of various apolipoprotein (apo) genes to spontaneously hypercholesterolemic mice deficient for either apoE or LDL receptor and, especially, the implementation of the transplantation murine model allowed studying molecular mechanisms of atherosclerotic regression, associated with the depletion of atherogenic lipids in the plaque, egress of macrophages and phenotypic switch of macrophages from the proinflammatory M1 to the anti-inflammatory M2.

Modulation of autoimmune arthritis severity in mice by apolipoprotein E (ApoE) and cholesterol

Apolipoprotein E (ApoE) deficiency promoted an exacerbation of autoimmune arthritis in mice by inducing proinflammatory immune responses. In this study we analysed the contribution of hypercholesterolaemia and/or the absence of ApoE anti-inflammatory properties, unrelated to its function in the control of cholesterol metabolism, towards the acceleration of arthritis in these mutant animals. The induction and severity of collagen type II-induced arthritis (CIA) were compared for B10.RIII wild-type (WT), B10.RIII.ApoE^+/- , B10.RIII.ApoE^-/- and B10.RIII.low-density lipoprotein receptor (LDLR^-/- ) mice with different concentrations of circulating ApoE and cholesterol. A 50-70% reduction in serum levels of ApoE was observed in heterozygous B10.RIII.ApoE^+/- mice in comparison to B10.RIII.WT, although both strains of mice exhibited similar circulating lipid profiles. This ApoE reduction was associated with an increased CIA severity that remained lower than in homozygous B10.RIII.ApoE^-/- mice. An important rise in circulating ApoE concentration was observed in hypercholesterolaemic B10.RIII.LDLR^-/- mice fed with a normal chow diet, and both parameters increased further with an atherogenic hypercholesterolaemic diet. However, the severity of CIA in B10.RIII.LDLR^-/- mice was similar to that of B10.RIII.WT controls. In conclusion, by comparing the evolution of CIA between several strains of mutant mice with different levels of serum ApoE and cholesterol, our results demonstrate that both hypercholesterolaemia and ApoE regulate the intensity of in-vivo systemic autoimmune responses.

ApoE knockout and knockin mice: the history of their contribution to the understanding of atherogenesis

ApoE is a multifunctional protein that is expressed by many cell types that influences many aspects of cardiovascular physiology. In humans, there are three major allelic variants that differentially influence lipoprotein metabolism and risk for the development of atherosclerosis. Apoe-deficient mice and human apoE isoform knockin mice, as well as hypomorphic Apoe mice, have significantly contributed to our understanding of the role of apoE in lipoprotein metabolism, monocyte/macrophage biology, and atherosclerosis. This brief history of these mouse models will highlight their contribution to the understanding of the role of apoE in these processes. These Apoe(-/-) mice have also been extensively utilized as an atherosensitive platform upon which to assess the impact of modulator genes on the development and regression of atherosclerosis.

Anti-inflammatory and cholesterol-reducing properties of apolipoprotein mimetics: a review

Reduced levels of HDL cholesterol (HDL-C) are a strong independent predictor of coronary artery disease (CAD) risk. The major anti-atherogenic function of HDL is to mediate reverse cholesterol transport. This response is highly dependent on apoA-I and apoE, protein components of HDL. Randomized clinical trials have assessed effects of several classes of drugs on plasma cholesterol levels in CAD patients. Agents including cholestyramine, fibrates, niacin, and statins significantly lower LDL cholesterol (LDL-C) and induce modest increases in HDL-C, but tolerance issues and undesirable side effects are common. Additionally, residual risk may be present in patients with persistently low HDL-C and other complications despite a reduction in LDL-C. These observations have fueled interest in the development of new pharmacotherapies that positively impact circulating lipoproteins. The goal of this review is to discuss the therapeutic potential of synthetic apolipoprotein mimetic peptides. These include apoA-I mimetic peptides that have undergone initial clinical assessment. We also discuss newer apoE mimetics that mediate the clearance of atherogenic lipids from the circulation and possess anti-inflammatory properties. One of these (AEM-28) has recently been given orphan drug status and is undergoing clinical trials.

ApoE production in human monocytes and its regulation by inflammatory cytokines

The apoE production by tissue macrophages is crucial for the prevention of atherosclerosis and the aim of this study was to further elucidate how this apolipoprotein is regulated by cytokines present during inflammation. Here we studied apoE production in peripheral blood mononuclear cells (PBMC) and analysis was made with a newly developed apoE ELISpot assay. In PBMC, apoE secretion was restricted to monocytes with classical (CD14⁺⁺CD16⁻) and intermediate (CD14⁺CD16⁺) monocytes being the main producers. As earlier described for macrophages, production was strongly upregulated by TGF-β and downregulated by bacterial lipopolysaccharide (LPS) and the inflammatory cytokines IFN-γ, TNF-α and IL-1β. We could here show that a similar down-regulatory effect was also observed with the type I interferon, IFN-α, while IL-6, often regarded as one of the more prominent inflammatory cytokines, did not affect TGF-β-induced apoE production. The TNF-α inhibitor Enbrel could partly block the down-regulatory effect of IFN-γ, IFN-α and IL-1β, indicating that inhibition of apoE by these cytokines may be dependent on or synergize with TNF-α. Other cytokines tested, IL-2, IL-4, IL-12, IL-13, IL-17A and IL-23, had no inhibitory effect on apoE production. In contrast to the effect on monocytes, apoE production by primary hepatocytes and the hepatoma cell line HepG2 was more or less unaffected by treatment with cytokines or LPS.

Type VIII collagen mediates vessel wall remodeling after arterial injury and fibrous cap formation in atherosclerosis

Collagens in the atherosclerotic plaque signal regulation of cell behavior and provide tensile strength to the fibrous cap. Type VIII collagen, a short-chain collagen, is up-regulated in atherosclerosis; however, little is known about its functions in vivo. We studied the response to arterial injury and the development of atherosclerosis in type VIII collagen knockout mice (Col8(-/-) mice). After wire injury of the femoral artery, Col8(-/-) mice had decreased vessel wall thickening and outward remodeling when compared with Col8(+/+) mice. We discovered that apolipoprotein E (ApoE) is an endogenous repressor of the Col8a1 chain, and, therefore, in ApoE knockout mice, type VIII collagen was up-regulated. Deficiency of type VIII collagen in ApoE(-/-) mice (Col8(-/-);ApoE(-/-)) resulted in development of plaques with thin fibrous caps because of decreased smooth muscle cell migration and proliferation and reduced accumulation of fibrillar type I collagen. In contrast, macrophage accumulation was not affected, and the plaques had large lipid-rich necrotic cores. We conclude that in atherosclerosis, type VIII collagen is up-regulated in the absence of ApoE and functions to increase smooth muscle cell proliferation and migration. This is an important mechanism for formation of a thick fibrous cap to protect the atherosclerotic plaque from rupture.

Apolipoprotein E regulation of myeloid cell plasticity in atherosclerosis

PURPOSE OF REVIEW

Apolipoprotein (apo) E is a multifunctional protein that has long been recognized for its ability to safeguard against atherosclerosis. Among its pleiotropic roles known to suppress atherosclerosis, mechanisms by which apoE regulates cells of the immune system have remained elusive. Because atherosclerosis is a chronic inflammatory disease that remains on the rise, understanding in more detail how apoE controls immune cell activation and function is of much interest.

RECENT FINDINGS

Literature reported in the past year introduces apoE as a regulator of monocyte and macrophage plasticity. Through signals delivered by its interaction with cell surface receptors, apoE has been shown to influence the polarity and inflammatory phenotypes of the macrophage. By promoting cellular cholesterol efflux in a cell autonomous manner and through its ability to enhance HDL function in hyperlipidemic plasma, apoE is now known to suppress atherosclerosis by controlling myeloid cell proliferation, monocyte activation and their capacity to infiltrate the vascular wall. Lastly, the structural basis for apoE isoform-specific effects in macrophage dysfunction and atherosclerosis susceptibility is beginning to emerge.

SUMMARY

Collectively, these findings introduce a new dimension to our understanding of how apoE links lipoprotein biology to monocyte and macrophage function in atherosclerosis susceptibility.

Macrophage-specific apoE gene repair reduces diet-induced hyperlipidemia and atherosclerosis in hypomorphic Apoe mice

Background

Apolipoprotein (apo) E is best known for its ability to lower plasma cholesterol and protect against atherosclerosis. Although the liver is the major source of plasma apoE, extra-hepatic sources of apoE, including from macrophages, account for up to 10% of plasma apoE levels. This study examined the contribution of macrophage-derived apoE expression levels in diet-induced hyperlipidemia and atherosclerosis.

Methodology/Principal Findings

Hypomorphic apoE (Apoe^h/h) mice expressing wildtype mouse apoE at ∼2–5% of physiological levels in all tissues were derived by gene targeting in embryonic stem cells. Cre-mediated gene repair of the Apoe^h/h allele in Apoe^h/hLysM-Cre mice raised apoE expression levels by 26 fold in freshly isolated peritoneal macrophages, restoring it to 37% of levels seen in wildtype mice. Chow-fed Apoe^h/hLysM-Cre and Apoe^h/h mice displayed similar plasma apoE and cholesterol levels (55.53±2.90 mg/dl versus 62.70±2.77 mg/dl, n = 12). When fed a high-cholesterol diet (HCD) for 16 weeks, Apoe^h/hLysM-Cre mice displayed a 3-fold increase in plasma apoE and a concomitant 32% decrease in plasma cholesterol when compared to Apoe^h/h mice (602.20±22.30 mg/dl versus 888.80±24.99 mg/dl, n = 7). On HCD, Apoe^h/hLysM-Cre mice showed increased apoE immunoreactivity in lesional macrophages and liver-associated Kupffer cells but not hepatocytes. In addition, Apoe^h/hLysM-Cre mice developed 35% less atherosclerotic lesions in the aortic root than Apoe^h/h mice (167×10³±16×10³ µm² versus 259×10³±56×10³ µm², n = 7). This difference in atherosclerosis lesions size was proportional to the observed reduction in plasma cholesterol.

Conclusions/Significance

Macrophage-derived apoE raises plasma apoE levels in response to diet-induced hyperlipidemia and by such reduces atherosclerosis proportionally to the extent to which it lowers plasma cholesterol levels.

Activation of the TLR4 signaling pathway and abnormal cholesterol efflux lead to emphysema in ApoE-deficient mice

Smokers with airflow obstruction have an increased risk of atherosclerosis, but the relationship between the pathogenesis of these diseases is not well understood. To determine whether hypercholesterolemia alters lung inflammation and emphysema formation, we examined the lung phenotype of two hypercholesterolemic murine models of atherosclerosis at baseline and on a high-fat diet. Airspace enlargement developed in the lungs of apolipoprotein E-deficient (Apoe(-/-)) mice exposed to a Western-type diet for 10 wk. An elevated number of macrophages and lymphocytes accompanied by an increase in matrix metalloproteinase-9 (MMP-9) activity and MMP-12 expression was observed in the lungs of Apoe(-/-) mice on a Western-type diet. In contrast, low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice did not exhibit lung destruction or inflammatory changes. Most importantly, we revealed augmented expression of the downstream targets of the Toll-like receptor (TLR) pathway, interleukin-1 receptor-associated kinase 1, and granulocyte colony-stimulating factor, in the lungs of Apoe(-/-) mice fed with a Western-type diet. In addition, we demonstrated overexpression of MMP-9 in Apoe(-/-) macrophages treated with TLR4 ligand, augmented with the addition of oxidized LDL, suggesting that emphysema in these mice results from the activation of the TLR pathway secondary to known abnormal cholesterol efflux. Our findings indicate that, in Apoe(-/-) mice fed with an atherogenic diet, abnormal cholesterol efflux leads to increased systemic inflammation with subsequent lung damage and emphysema formation.

Effect of rosiglitazone on HDL metabolism in subjects with metabolic syndrome and low HDL

Treatment with the peroxisome proliferator-activated receptor γ agonist rosiglitazone has been reported to increase HDL-cholesterol (HDL-C) levels, although the mechanism responsible for this is unknown. We sought to determine the effect of rosiglitazone on HDL apolipoprotein A-I (apoA-I) and apoA-II metabolism in subjects with metabolic syndrome and low HDL-C. Subjects were treated with placebo followed by rosiglitazone (8 mg) once daily. At the end of each 8 week treatment, subjects (n = 15) underwent a kinetic study to measure apoA-I and apoA-II production rate (PR) and fractional catabolic rate. Rosiglitazone significantly reduced fasting insulin and high-sensitivity C-reactive protein (hsCRP) and increased apoA-II levels. Mean apoA-I and HDL-C levels were unchanged following rosiglitazone treatment, although there was considerable individual variability in the HDL-C response. Rosiglitazone had no effect on apoA-I metabolism, whereas the apoA-II PR was increased by 23%. The change in HDL-C in response to rosiglitazone was significantly correlated with the change in apoA-II concentration but not to changes in apoA-I, measures of glucose homeostasis, or hsCRP. Treatment with rosiglitazone significantly increased apoA-II production in subjects with metabolic syndrome and low HDL-C but had no effect on apoA-I metabolism. The change in HDL-C in response to rosiglitazone treatment was unrelated to effects on apoA-I, instead being related to the change in the metabolism of apoA-II.

Leptin modulates ACAT1 expression and cholesterol efflux from human macrophages

Leptin is an adipose tissue-derived hormone implicated in atherosclerosis and macrophage foam cell formation. The current study was conducted to examine the effect of leptin on cholesteryl ester accumulation in human monocytes/macrophages. Exogenously added leptin at 5 nM during differentiation of monocytes into macrophages for 7 days accelerated acetylated LDL (acetyl-LDL)-induced cholesteryl ester accumulation by 30-50%. Leptin did not affect endocytic uptake of acetyl-LDL; however, it increased ACAT activity 1.8-fold and ACAT-1 protein expression 1.9-fold. Among the four ACAT-1 mRNA transcripts, two shorter transcripts (2.8 and 3.6 kb) were upregulated approximately 1.7-fold upon leptin treatment. The enhanced expression of ACAT-1 protein by leptin was suppressed by inhibitors of Janus-activated kinase2 (JAK2) and phosphatidylinositol 3-kinase (PI3K). HDL-mediated cholesterol efflux was suppressed by leptin, which was canceled by K-604, an ACAT-1 inhibitor. Expression of long form of leptin receptor was upregulated during monocytic differentiation into macrophages and sustained after differentiation. Thus, the results suggest that leptin accelerates cholesteryl ester accumulation in human monocyte-derived macrophages by increasing ACAT-1 expression via JAK2 and PI3K, thereby suppressing cholesterol efflux.

Role of apolipoprotein E genotype in coronary artery disease

Recent gene-targeting technology has provided good animal models that provide insight into the pathology of complex diseases such as atherosclerosis. The apolipoprotein E gene polymorphism is one of the most extensively studied in cardiovascular medicine. The scope of the present review is to briefly outline the biochemical characteristics and the genetic variation of apolipoprotein E. Apolipoprotein E is best known for its role in modulating lipoprotein metabolism as a ligand for cellular receptors. Other functions unrelated to lipid transport are becoming known, including reverse cholesterol transport, immunoregulation and modulation of cell growth. This review will examine recent work that addresses how apolipoprotein E participates in atherosclerosis. Genotypic variation of apolipoprotein E has been associated with certain phenotypes regarding vascular disease, such as the presence of atherosclerosis and coronary heart disease outcomes. This article will also review evidence regarding the association between apolipoprotein E gene polymorphisms and coronary artery disease based upon experimental and clinical studies.

Impaired secretion of apolipoprotein E2 from macrophages

Human apoE is a multifunctional and polymorphic protein synthesized and secreted by liver, brain, and tissue macrophages. Here we show that apoE isoforms and mutants expressed through lentiviral transduction display cell-specific differences in secretion efficiency. Whereas apoE3, apoE4, and a natural mutant of apoE4 (apoE-Cys(142)) were efficiently secreted from macrophages, apoE2 and a non-natural apoE mutant (apoE-Cys(112)/Cys(142)) were retained in the perinuclear region and only minimally secreted. The secretory block for apoE2 in macrophages was not affected by the ablation of LDLR (low density lipoprotein receptor), ABCA-1, or SR-BI (scavenger receptor class B type I) but was released in the absence of low density lipoprotein receptor related protein (LRP). In co-immunoprecipitation experiments, an anti-apoE antibody pulled down two times more LRP in apoE2-transduced macrophages than in apoE3-expressing macrophages. Non-reducing SDS-PAGE/Western blot analyses showed that macrophage apoE2 is mostly dimeric and multimeric, whereas apoE3 is predominantly monomeric. ApoE2 retention and multimer formation also occurred in human macrophages derived from the monocyte cell line THP-1. These results were specific for macrophages, as in transduced mouse primary hepatocytes: 1) ApoE2 was secreted as efficiently as apoE3 and apoE4; 2) all isoforms were exclusively in monomeric form; 3) there was no co-immunoprecipitation of apoE and LRP. A microsomal triglyceride transfer protein (MTP) inhibitor nearly deleted apoB100 secretion from hepatocytes without affecting apoE secretion. These data show that macrophages retain apoE2, a highly expressed protein carried by about 8% of the human population. Given the role of locally produced apoE in regulating cholesterol efflux, modulating inflammation, and controlling oxidative stress, this unique property of apoE2 may have important impacts on atherogenesis.

A pathway-dependent on apoE, ApoAI, and ABCA1 determines formation of buoyant high-density lipoprotein by macrophage foam cells

OBJECTIVE

ABCA1-dependent and ABCA1-independent pathways may operate in high-density lipoprotein formation by macrophages secreting apolipoprotein (apo) E. We examined the impact of ABCA1 on apoE-mediated efflux from cholesterol-enriched macrophages.

METHODS AND RESULTS

Without acceptors, wild-type, ABCA1-/-, and apoE-/- macrophages released 5.7%+/-0.3%, 1.8%+/-0.1%, and 2.3%+/-0.2% of their cholesterol, and the LXR agonist, TO-901317, enhanced efflux by 137%, 10%, and 20%. Although similar amounts of apoE were secreted from ABCA1-/- and wild-type cells, apoE from ABCA1-/- cells was only partially phospholipidated and floated at density > 1.21 g/mL, whereas apoE from wild-type cells floated at density of 1.09 to 1.17 g/mL and paralleled the density of cholesterol. With apoAI, LXR stimulation increased efflux by 139% and 86% from wild-type and apoE-/- cells, resulting in a large difference in efflux (29.5%+/-0.2% versus 17.0%+/-0.5%). The density of apoE and cholesterol from wild-type cells did not change with apoAI, and most apoAI floated at density > or = 1.17 g/mL. In apoE-/- cells, apoAI and cholesterol floated at similar density, but the peak fraction only contained 4 microg cholesterol/mg protein versus 18 in WT cells.

CONCLUSIONS

Macrophage apoE requires ABCA1 for formation of high-density lipoprotein. ApoAI facilitates association of apoE with more buoyant high-density lipoprotein, suggesting that apoE, plasma apoAI, and ABCA1 operate together to optimize mobilization of macrophage cholesterol, a process critical to limiting plaque development.

Macrophage apolipoprotein E reduces atherosclerosis and prevents premature death in apolipoprotein E and scavenger receptor-class BI double-knockout mice

OBJECTIVE

Mice null for both apolipoprotein (apo)E and scavenger receptor (SR)-BI (DKO) develop severe hypercholesterolemia, occlusive coronary atherosclerosis, myocardial infarction, and premature death. The current study examines the ability of macrophage apoE to improve the dyslipidemia, reduce atherosclerosis, and rescue the lethal phenotype of DKO mice.

METHODS AND RESULTS

Initially, bone marrow transplantation (BMT) was unsuccessful, because the DKO mice died from a rapidly fatal anemia 3 to 5 days after lethal irradiation. Therefore, probucol was used to rescue the DKO mice during BMT and was discontinued 2-weeks after BMT, allowing successful reconstitution with donor marrow. Twelve male apoE(-/-)SR-BI(-/-) mice fed 0.5% probucol in a chow diet were lethally irradiated and transplanted with either wild-type (WT) or DKO bone marrow. Two-weeks after BMT, apoE was detected in serum in WT-->DKO mice, and mean serum cholesterol levels were reduced by 70% versus DKO-->DKO mice. Lipoprotein profiles and HDL subpopulations in WT-->DKO mice were similar to apoE(+/+)SR-BI(-/-)-->DKO mice and resembled those of SR-BI(-/-) mice. In WT-->DKO mice, aortic atherosclerosis was reduced by 88% to 90% versus DKO-->DKO mice. Furthermore, the DKO-->DKO mice died &8 weeks after BMT, whereas WT-->DKO mice exhibited a life span >40 weeks after BMT.

CONCLUSIONS

Macrophage apoE is able to rescue the lethal phenotype of apoE(-/-)SR-BI(-/-) mice by improving the dyslipidemia and dramatically reducing atherosclerotic lesion development.

Understanding hyperlipidemia and atherosclerosis: lessons from genetically modified apoe and ldlr mice

Hyperlipidemia is the most important risk factor for atherosclerosis, which is the major cause of cardiovascular disease. The etiology of hyperlipidemia and atherosclerosis is complex and governed by multiple interacting genes. However, mutations in two genes have been shown to be directly involved, i.e., the low-density lipoprotein receptor (LDLR) and apolipoprotein E (ApoE). Genetically modified mouse models have been instrumental in elucidating the underlying molecular mechanisms in lipid metabolism. In this review, we focus on the use of two of the most widely used mouse models, ApoE- and LDLR-deficient mice. After almost a decade of applications, it is clear that each model has unique strengths and drawbacks when carrying out studies of the role of additional genes and environmental factors such as nutrition and lipid-lowering drugs. Importantly, we elaborate on mice expressing mutant forms of APOE, including the

Testing the role of apoA-I, HDL, and cholesterol efflux in the atheroprotective action of low-level apoE expression

Low levels of transgenic mouse apolipoprotein E (apoE) suppress atherosclerosis in apoE knockout (apoE-/-) mice without normalizing plasma cholesterol. To test whether this is due to facilitation of cholesterol efflux from the vessel wall, we produced apoA-I-/-/apoE-/- mice with or without the transgene. Even without apoA-I and HDL, apoA-I-/-/apoE-/- mice had the same amount of aorta cholesteryl ester as apoE-/- mice. Low apoE in the apoA-I-/-/apoE-/- transgenic mice reduced aortic lesions by 70% versus their apoA-I-/-/apoE-/- siblings. To define the free cholesterol (FC) efflux capacity of lipoproteins from the various genotypes, sera were assayed on macrophages expressing ATP-binding cassette transporter A1 (ABCA1). Surprisingly, ABCA1 FC efflux was twice as high to sera from the apoA-I-/-/apoE-/- or apoE-/- mice compared with wild-type mice, and this activity correlated with serum apoA-IV. Immunodepletion of apoA-IV from apoA-I-/-/apoE-/- serum abolished ABCA1 FC efflux, indicating that apoAI-V serves as a potent acceptor for FC efflux via ABCA1. With increasing apoE expression, apoA-IV and FC acceptor capacity decreased, indicating a reciprocal relationship between plasma apoE and apoA-IV. Low plasma apoE (1-3 x 10(-8) M) suppresses atherosclerosis by as yet undefined mechanisms, not dependent on the presence of apoA-I or HDL or an increased capacity of serum acceptors for FC efflux.

Structural and functional properties of human plasma high density-sized lipoprotein containing only apoE particles

To investigate the metabolism of HDL-apolipoprotein E (apoE) particles in human plasma, we isolated a fraction of plasma HDL-apoEs that lack apoA-I (HDL-LpE) from subjects with apoE3/3 phenotype by immunoaffinity. Plasma HDL-LpE had a particle size ranging from 9 nm to 18.5 nm in diameter and was characterized by two-dimensional nondenaturing gradient gel electrophoresis as having either gamma-, prebeta1-, prebeta2-, or alpha-electrophoretic mobility. HDL-LpE was also present in the medium of cultured human hepatoma cell lines and monocyte-derived macrophages. The majority of apoE3 was found as a monomeric form in HDL-LpE and floated at density d > 1.21 g/ml. Plasma levels of HDL-LpE in normolipidemic, CETP-deficient, and ABCA1-deficient subjects were 0.72 +/- 0.15 mg/dl (n = 12), 1.77 +/- 0.75 mg/dl (n = 3), and 0.55 +/- 0.11 mg/dl (n = 3), respectively. The ratio of HDL-apoE containing apoA-I to HDL-LpE was significantly higher 4 h after a fat load, representing a 35 +/- 9% increase (n = 3). Isolated plasma HDL-LpE3 was as effective as apoE3, reconstituted HDL particles, or apoA-I in promoting cellular cholesterol efflux. These results demonstrate that 1) plasma HDL-LpE may have hepatogenous and macrophagic origins; 2) HDL-LpE was preserved even with large reductions in apoA-I-containing lipoproteins; 3) HDL-LpE was active in the transfer of apoE to triglyceride-rich lipoproteins, and 4) HDL-LpEs efficiently take up cell-derived cholesterol.

Apolipoprotein E and lipoprotein lipase increase triglyceride-rich particle binding but decrease particle penetration in arterial wall

OBJECTIVE

Liver-derived apolipoprotein E (apoE) decreases atherosclerosis without altering the circulating concentrations of plasma lipoproteins. We evaluated the effects of apoE and lipoprotein lipase (LpL) on the interactions of triglyceride-rich particles (TGRPs) in the arterial wall.

METHODS AND RESULTS

Quantitative fluorescence microscopy was used to study the interactions of TGRPs (25- to 35-nm diameter) in the arterial wall. Carotid arteries were harvested from rats, placed in a perfusion chamber, and perfused with fluorescently labeled TGRPs. In the absence of apoE or LpL, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine-TGRP (100 microg neutral lipid/mL) was poorly retained in the arterial wall. The addition of either apoE (10 microg/mL) or LpL (10 microg/mL) increased TGRP accumulation 220% and 100%, respectively. This effect was attenuated by heparin (10.0 IU/mL). Histological analyses of cross sections from these vessels demonstrate that in the absence of apoE or LpL, there is deep penetration of lipid into the arterial wall. With the addition of either apoE or LpL, arterial wall penetration of TGRP is blocked.

CONCLUSIONS

These results demonstrate that although apoE and LpL increase arterial wall accumulation of TGRPs, these proteins also reduce the penetration of TGRPs into the arterial wall. We postulate that this may represent a novel antiatherogenic property of apoE and LpL.

Apolipoprotein E kinetics: influence of insulin resistance and type 2 diabetes

BACKGROUNDS AND AIMS

Insulin resistance related to obesity and diabetes is characterized by an increase in plasma TG-rich lipoprotein concentrations. Apolipoprotein (apo) E plays a crucial role in the metabolism of these lipoproteins and particularly in the hepatic clearance of their remnants. The aim of this study was to explore apoE kinetics of obese subjects and to determine what parameters could influence its metabolism.

METHODS

Using stable-isotope labelling technique ([(2)H(3)]-leucine-primed constant infusion) and monocompartmental model (SAAM II computer software), we have studied the plasma kinetics of very-low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) apoE in 12 obese subjects (body mass index (BMI) 27.4-36.6 kg/m(2)): Seven were type 2 diabetics (age 47-65 y; HbA1c 7.1-10.2%) and five were non-diabetics (age 40-51 y, HbA1c: 4.9-5.3%). Six of the diabetic subjects were insulin resistant as assessed by insulin sensitivity index (HOMA 2.6-10.0), while non-diabetic subjects were all insulin sensitive (HOMA 1.2-2.1).

RESULTS

Plasma VLDL and HDL apoE concentrations were significantly higher in diabetic than in non-diabetic subjects (5.74+/-1.60 vs 1.46+/-1.74 mg/l, P<0.01 and 17.81+/-6.67 vs 9.97+/-3.32 mg/l, P<0.05). These increased levels were associated with significantly higher absolute production rate (APR) of VLDL and HDL apoE (0.714+/-0.343 vs 0.130+/-0.200 mg/kg/day, P<0.01, and 0.197+/-0.087 vs 0.080+/-0.060 mg/kg/day, P<0.05, respectively) while no significant difference was found for fractional catabolic rate (FCR) of VLDL and HDL apoE (3.44+/-1.64 vs 1.97+/-0.84/day and 0.30+/-0.12 vs 0.19+/-0.09/day, respectively). In the whole population, BMI was not correlated with any of apoE kinetic data. HOMA was positively correlated with FCR of VLDL apoE (r=0.64, P<0.05) and tended to be correlated with APR of VLDL apoE (r=0.58, P=0.06). HbA1c was positively correlated with APR and FCR of both VLDL apoE (r=0.91 and 0.78, P<0.01, respectively) and HDL apoE (r=0.66 and 0.69, P<0.05, respectively).

CONCLUSION

Obese diabetics are characterized by elevated VLDL and HDL apoE levels associated with enhancement of VLDL and HDL apoE production rates. Whereas obesity did not influence apoE kinetic parameters in itself, insulin resistance may lead to an increase in VLDL apoE production and fractional catabolic rates. Diabetes and the glycemic control may also specifically influence the kinetics of both VLDL and HDL apoE. All together, these disorders should explain at least part of the increase in VLDL and HDL apoE observed in diabetes.

Reconstituted discoidal ApoE-phospholipid particles are ligands for the scavenger receptor BI. The amino-terminal 1-165 domain of ApoE suffices for receptor binding

The high density lipoprotein receptor, scavenger receptor class B type I (SR-BI), recognizes lipid-bound apolipoprotein A-I (apoA-I) and other apolipoproteins. Here, we have used large scale cultures of apoE-expressing cells to purify apoE and prepare apoE containing reconstituted discoidal 1-palmitoyl-2-oleoyl-l-phosphatidylcholine (POPC)-apoE particles. These particles have been used to examine their binding to wild-type and mutant forms of SR-BI expressed in transfected ldlA-7 cells. Specific binding to SR-BI was determined by subtracting from the total binding, nonspecific values measured using either control untransfected ldlA-7 cells or by inhibiting SR-BI-mediated binding with a high titer antireceptor-blocking antibody. POPC-apoE particles generated using apoE2, apoE3, apoE4, or the carboxyl-terminally truncated forms apoE165, apoE202, apoE229, and apoE259 all bound tightly to wild-type SR-BI with similar affinities (K(d) = 35-45 microg/ml). Binding was nearly abolished in a cell line expressing the ldlA (Q402R/Q418R) double mutant form of SR-BI that is unable to bind native high density lipoprotein but binds low density lipoprotein normally. The findings establish that apoE is a ligand for SR-BI and that the receptor binding domain is located in the amino-terminal 1-165-region of the protein. SR-BI-apoE interactions may contribute to cholesterol homeostasis in tissues and cells expressing SR-BI that are accessible to apoE-containing lipoproteins.

Negative and independent influence of apolipoprotein E on C-reactive protein (CRP) concentration in obese adults. Potential anti-inflammatory role of apoE in vivo

BACKGROUND

Obesity is frequently associated with an increase in the early inflammation marker C-reactive protein (CRP), insulin resistance and changes in lipoprotein metabolism. Increased CRP is known as an independent cardiovascular risk factor. Since the apolipoproteins (apo) E and CIII components of HDL are associated with reduced cardiovascular risk and since apoE has in vitro anti-inflammatory effect, we have investigated the relationships between apoE, apoCIII (in apoB and non apoB containing lipoproteins) and CRP in obese adults.

METHODS

The following parameters from 34 healthy obese fasting women (age 22-64 y, body mass index (BMI) 28-68 kg/m2) were measured: (1) ApoE and apoCIII, in total plasma, in apoB- (E LpB, CIII LpB) and non-apoB-containing lipoproteins (E LpnonB, CIII LpnonB); (2) CRP and cytokine secreted by adipose tissue (TNF-alpha and its soluble receptor TNFR2); (3) triglyceride, HDL-cholesterol, systolic blood pressure, diastolic blood pressure, waist and hip circumferences, insulin, glucose. HOMA, a marker of insulin sensitivity, and the ratio E/CIII in LpB and LpnonB were calculated.

RESULTS

CRP was positively correlated with BMI (P<0.05), waist circumference (WC, P<0.05), triglyceride (P<0.05) and negatively correlated with apoE (P<0.01) and E LpnonB (P<0.05). Two multiple regression models including parameters related to CRP with a P<0.25 were run stepwise to assess their independent contribution to CRP concentration. In the first model (including BMI, WC, HOMA, insulin, triglyceride, apoE, E LpnonB), apoE was the best predictor of CRP (P=0.01) together with triglyceride (P=0.02) and BMI (P=0.08). The second model took into account E/CIII LpnonB ratio with the parameters included in the first model. In this second model, E/CIII LpnonB was the best predictor of CRP (P=0.007), explaining 39% of CRP variance.

CONCLUSION

ApoE is strongly correlated with CRP and could have an anti-inflammatory effect in vivo in obese subjects. This correlation could be limited to LpnonB lipoproteins, depending on their apoE and CIII relative content.

The peroxisome proliferator-activated receptor delta promotes lipid accumulation in human macrophages

The peroxisome proliferator-activated receptors (PPARs) are a family of fatty acid-activated transcription factors which control lipid homeostasis and cellular differentiation. PPARalpha (NR1C1) controls lipid oxidation and clearance in hepatocytes and PPARgamma (NR1C3) promotes preadipocyte differentiation and lipogenesis. Drugs that activate PPARalpha are effective in lowering plasma levels of lipids and have been used in the management of hyperlipidemia. PPARgamma agonists increase insulin sensitivity and are used in the management of type 2 diabetes. In contrast, there are no marketed drugs that selectively target PPARdelta (NR1C2) and the physiological roles of PPARdelta are unclear. In this report we demonstrate that the expression of PPARdelta is increased during the differentiation of human macrophages in vitro. In addition, a highly selective agonist of PPARdelta (compound F) promotes lipid accumulation in primary human macrophages and in macrophages derived from the human monocytic cell line, THP-1. Compound F increases the expression of genes involved in lipid uptake and storage such as the class A and B scavenger receptors (SRA, CD36) and adipophilin. PPARdelta activation also represses key genes involved in lipid metabolism and efflux, i.e. cholesterol 27-hydroxylase and apolipoprotein E. We have generated THP-1 sublines that overexpress PPARdelta and have confirmed that PPARdelta is a powerful promoter of macrophage lipid accumulation. These data suggest that PPARdelta may play a role in the pathology of diseases associated with lipid-filled macrophages, such as atherosclerosis, arthritis, and neurodegeneration.

Retrovirus-mediated expression of apolipoprotein A-I in the macrophage protects against atherosclerosis in vivo

We have previously reported that the lack of apolipoprotein (apo) E expression by macrophages promotes foam cell formation in vivo. Because transgenic mice overexpressing human apoA-I from the liver (h-apoA-I TgN) are protected from the atherogenesis induced by apoE deficiency, we hypothesized that the presence of apoA-I in the vessel wall could reduce the negative effect of apoE deficiency on lesion growth. To address this issue, we used both retroviral transduction and transgenic approaches to produce in vivo systems where apoA-I is expressed from macrophages. In the retroviral transduction study, apoA-I-deficient (apoA-I(-/-)) mice reconstituted with apoE-deficient (apoE(-/-)) bone marrow cells that were infected with a retroviral vector expressing human apoA-I (MFG-HAI) had 95% lower atherosclerotic lesion area than that of recipients of apoE(-/-) bone marrow cells infected with the parental virus (MFG). To determine whether the protective effect of locally produced apoA-I was due to the lack of systemic apoA-I, we conducted a different experiment using h-apoA-I TgN mice as recipients of apoE(-/-) bone marrow with or without human apoA-I (driven by a macrophage-specific transgene defined as mphi-AI). Aortic lesion area in apoE(-/-)/mphi-AI --> h-apoA-I TgN mice was decreased by 85% compared with apoE(-/-) --> h-apoA-I TgN mice. These data demonstrate that expression of apoA-I from macrophages protects against atherogenesis without affecting plasma apoA-I and high density lipoprotein cholesterol levels.

Triacylglycerol-rich lipoproteins alter the secretion, and the cholesterol-effluxing function, of apolipoprotein E-containing lipoprotein particles from human (THP-1) macrophages

Elevated plasma levels of triacylglycerol-rich lipoproteins (TGRLP) are associated with increased risk of atherogenesis and abnormal reverse cholesterol transport, as illustrated in Type II diabetes. Here we examine the effect of plasma triacylglycerol-rich or cholesteryl ester-rich lipoproteins on the secretion of nascent apolipoprotein E (apoE)-containing lipoprotein E (LpE) particles by human (THP-1) macrophages. As expected, preincubation with low-density lipoprotein (LDL) yielded small but significant increases in total cellular cholesterol content and also the secretion of apoE by macrophages. By contrast, preincubation with TGRLP resulted in higher, dose-dependent, increases in apoE secretion that reflected, but were not dependent on, cellular triacylglycerol accumulation. Secreted apoE was incorporated into a pre-beta migrating LpE fraction that differed in lipid composition and flotation density depending on preincubation conditions. Specifically, the LpE-containing lipoprotein fraction produced by macrophages preincubated with TGRLP was cholesterol-poor, markedly heterogeneous and of higher peak flotation density (d 1.14-1.18) when compared with particles produced after preincubation with LDL. Both the conditioned medium and the isolated (d<1.21) LpE-containing fraction, yielded by macrophages preincubated with TGRLP, seemed poorer at inducing cholesterol efflux than the equivalent fractions from cells preincubated with LDL, as judged by [(3)H]cholesterol efflux from untreated 'naïve' macrophages. Thus, although the interaction of TGRLP with macrophages can enhance apoE output from these cells, the LpE particles produced seem to be relatively inefficient mediators of cholesterol efflux. These factors might contribute to the increased risk of atherosclerosis in individuals with Type II diabetes.

Domains of apolipoprotein E contributing to triglyceride and cholesterol homeostasis in vivo. Carboxyl-terminal region 203-299 promotes hepatic very low density lipoprotein-triglyceride secretion

Apolipoprotein (apo) E has been implicated in cholesterol and triglyceride homeostasis in humans. At physiological concentration apoE promotes efficient clearance of apoE-containing lipoprotein remnants. However, high apoE plasma levels correlate with high plasma triglyceride levels. We have used adenovirus-mediated gene transfer in apoE-deficient mice (E(-)/-) to define the domains of apoE required for cholesterol and triglyceride homeostasis in vivo. A dose of 2 x 10(9) plaque-forming units of apoE4-expressing adenovirus reduced slightly the cholesterol levels of E(-)/- mice and resulted in severe hypertriglyceridemia, due to accumulation of cholesterol and triglyceride-rich very low density lipoprotein particles in plasma. In contrast, the truncated form apoE4-202 resulted in a 90% reduction in the plasma cholesterol levels but did not alter plasma triglyceride levels in the E(-)/- mice. ApoE secretion by cell cultures, as well as the steady-state hepatic mRNA levels in individual mice expressing apoE4 or apoE4-202, were similar. In contrast, very low density lipoprotein-triglyceride secretion in mice expressing apoE4, but not apoE4-202, was increased 10-fold, as compared with mice infected with a control adenovirus. The findings suggest that the amino-terminal 1-202 region of apoE4 contains the domains required for the in vivo clearance of lipoprotein remnants. Furthermore, the carboxyl-terminal 203-299 residues of apoE promote hepatic very low density lipoprotein-triglyceride secretion and contribute to apoE-induced hypertriglyceridemia.

Effect of atherogenic diet on reverse cholesterol transport in vivo in atherosclerosis susceptible (C57BL/6) and resistant (C3H) mice

Mice susceptible (C57BL/6) or resistant (C3H) to atherosclerosis induced by a high cholesterol-cholate containing diet (A-diet) were used to study reverse cholesterol transport (RCT) in vivo as measured by loss of cholesterol from a depot created by injection of cationized LDL into the rectus femoris muscle. Plasma total and HDL-cholesterol (HDL-C), total and HDL phospholipid (HDL-PL) levels in chow fed C3H male and female mice were higher than in C57BL/6 mice. After one month on A-diet, plasma cholesterol more than doubled in both strains and genders. The decrease in HDL-C and HDL-PL was twice as great in C57BL/6 as in C3H female mice, while in male C3H mice there was no decrease. The loss of exogenous cholesterol mass (ECM) after injection of cationized LDL was more rapid in C3H than in C57BL/6 mice. In chow fed mice, ECM retained in muscle on day 12 was 37% in C57BL/6 and 20% in C3H females; in males it was 39% and 18% in C57BL/6 and C3H, respectively. On A-diet, 76% were retained in C57BL/6 and 28% in C3H females; these values were 59% and 28% in C57BL/6 and C3H males. Thus, the slow clearance of ECM (which represents RCT) in C57BL/6 mice on A-diet, that could be related to a marked decrease of HDL-PL, might contribute towards their susceptibility to atherosclerosis.

Apolipoprotein E polymorphisms and age at first coronary artery bypass graft

Apolipoprotein E (apoE) polymorphisms are heritable determinants of total and low-density lipoprotein cholesterol. The impact of apoE4 genotypes on the severity of atherosclerosis has been debated; however, recent studies have identified a correlation between apoE4 genotype and atherosclerosis. We assessed the impact of apoE4 genotype on age at first coronary artery bypass graft (CABG), hypothesizing that patients with the apoE4 allele are predisposed to coronary artery disease and present earlier for coronary revascularization. We assessed individual apoE genotypes and age in 560 patients undergoing primary CABG, by using analysis of variance (ANOVA) and controlling for gender. Because of the small number of patients in individual genotype groups, we compared patients with one or more copies of the apoE4 allele with those having no copies of the allele, again controlling for gender. A comparison of patients with one or more copies of the apoE4 allele with patients without the allele showed an earlier age at first CABG for those with the allele (P: = 0.032). Gene-dose analysis was also significant (P: = 0.012); patients with two copies of the allele presented at 54.2 +/- 6.9 yr. We report that the apoE4 allele is linked to age at first CABG. Identifying at-risk individuals may help prevent atherosclerosis. Further study is needed to define the mechanism of this association, and to define which coronary intervention is appropriate, based on long-term outcome.

IMPLICATIONS

A correlation exists between apolipoprotein E (apoE) genotypes and the severity of atherosclerosis. We hypothesized that patients with the apoE4 allele are predisposed to coronary artery disease and present earlier for coronary artery bypass graft (CABG). Individuals with the apoE4 allele presented earlier for CABG, and the apoE4 allele is linked to age at first CABG.

Macrophage cholesterol metabolism, apolipoprotein E, and scavenger receptor AI/II mRNA in atherosclerosis-susceptible and -resistant mice

Female mice known to be susceptible (C57BL) and resistant (C3H and BALB/c) to diet-induced atherosclerosis were studied. Feeding of a cholate-containing atherogenic diet for 1 month resulted in an increase in plasma total cholesterol, little or no change in total phospholipids and high density lipoprotein (HDL) cholesterol, and a fall in HDL phospholipid, which was most pronounced in the C57BL strain. In elicited macrophages, cholesterol esterification was lower with acetylated low density lipoprotein (acLDL) and higher with beta-very low density lipoprotein (beta-VLDL) in C57BL than in C3H or BALB/C strains. In resident macrophages, acLDL enhanced cholesterol esterification more than did rabbit beta-VLDL. With acLDL, more apolipoprotein E (apoE) was recovered in all macrophage cultures. In macrophages from chow-fed mice, most apoE was in the medium, whereas in mice fed an atherogenic diet, half of the apoE was in the cells. ApoE protein was highest in macrophages from BALB/c mice fed an atherogenic diet; an increase in apoE mRNA occurred in BALB/c and C3H macrophages. Scavenger receptor AI/II mRNA was significantly higher in macrophages from atherosclerosis-resistant mice. Thus, higher HDL phospholipid and plasma apoE levels (reported by others), together with high macrophage scavenger receptor AI/II mRNA, could inhibit accretion of cholesterol in the vessel wall in the 2 resistant strains.

Acceleration of reverse cholesterol transport

A low level of high-density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease. Levels of HDL cholesterol and composition of HDL subclasses in plasma are regulated by many factors, including apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. Reverse transport of cholesterol from cells of the arterial wall to the liver is an important mechanism by which HDL exerts its anti-atherogenic properties. Enhancement of reverse cholesterol transport is considered as a potential target for anti-atherosclerotic drug therapy. It is suggested, however, that the serum level of HDL cholesterol does not necessarily reflect the efficacy of reverse cholesterol transport.

Low levels of extrahepatic nonmacrophage ApoE inhibit atherosclerosis without correcting hypercholesterolemia in ApoE-deficient mice

The prevention of atherosclerosis by apolipoprotein E (apoE) is generally attributed to the removal of plasma lipoprotein remnant particles. We developed transgenic apoE-knockout mice expressing apoE specifically in the adrenal gland and found that only 3% of the wild-type plasma level of apoE was sufficient to normalize plasma cholesterol levels in the apoE-deficient mouse. As expected, mice expressing apoE at levels that correct hypercholesterolemia had almost no cholesteryl ester deposition in their aortas. In contrast, their nontransgenic siblings had significant atherosclerosis. Unexpectedly, we found that atherosclerosis was also reduced in 2 transgenic lines expressing too little apoE (<1% to 2% of wild type) to correct their hypercholesterolemia. Gel exclusion chromatographic profiles of plasma lipoproteins and the size distributions of lipoproteins with density <1.063 (low density and very low density lipoproteins), as determined by dynamic laser light scattering, were the same in mice expressing <2 microg/mL plasma apoE and their nontransgenic littermates. We conclude that the antiatherogenic action of low levels of plasma apoE is not due to the clearance of remnant lipoproteins. Thus, low levels of apoE provided systemically, but not made in the liver or in macrophages, can block atherogenesis in the vascular wall independently of normalizing the plasma concentration of atherogenic remnant lipoprotein particles.

Apolipoprotein E and atherosclerosis

Apolipoprotein E plays a key protective role in atherosclerosis. Its capacity to safeguard against this disease can be attributed to at least three distinct functions. First, plasma apolipoprotein E maintains overall plasma cholesterol homeostasis by facilitating efficient hepatic uptake of lipoprotein remnants. Second, lesion apolipoprotein E in concert with apolipoprotein A-I facilitates cellular cholesterol efflux from macrophage foam cells within the intima of the lesion. Third, lesion apolipoprotein E directly modifies both macrophage- and T lymphocyte-mediated immune responses that contribute to this chronic inflammatory disease.

Apolipoprotein-mediated cellular cholesterol/phospholipid efflux and plasma high density lipoprotein level in mice

Helical apolipoprotein(apo)s generate pre-beta-high density lipoprotein (HDL) by removing cellular cholesterol and phospholipid upon the interaction with cells. To investigate its physiological relevance, we studied the effect of an in vitro inhibitor of this reaction, probucol, in mice on the cell-apo interaction and plasma HDL levels. Plasma HDL severely dropped in a few days with probucol-containing chow while low density protein decreased more mildly over a few weeks. The peritoneal macrophages were assayed for apoA-I binding, apoA-I-mediated release of cellular cholesterol and phospholipid and the reduction by apoA-I of the ACAT-available intracellular cholesterol pool. All of these parameters were strongly suppressed in the probucol-fed mice. In contrast, the mRNA levels of the potential regulatory proteins of the HDL level such as apoA-I, apoE, LCAT, PLTP, SRB1 and ABC1 did not change with probucol. The fractional clearance rate of plasma HDL-cholesteryl ester was uninfluenced by probucol, but that of the HDL-apoprotein was slightly increased. No measurable CETP activity was detected either in the control or probucol-fed mice plasma. The change in these functional parameters is consistent with that observed in the Tangier disease patients. We thus concluded that generation of HDL by apo-cell interaction is a major source of plasma HDL in mice.

Accelerated atherosclerosis in C57Bl/6 mice transplanted with ApoE-deficient bone marrow

Apolipoprotein E (apoE), a high affinity ligand for lipoprotein receptors, is synthesized by the liver and extrahepatic tissues, including cells of the monocyte/macrophage cell lineage. The role of monocyte/macrophage-derived apoE in atherogenesis was assessed by transplantation of apoE-deficient (apoE-/-) bone marrow into normolipidemic C57Bl/6 mice. No significant effect could be demonstrated on serum apoE levels in C57Bl/6 mice, transplanted with apoE-deficient bone marrow compared with control transplanted mice. Furthermore, no consistent effect on serum cholesteryl esters and triglyceride concentrations could be demonstrated on either a standard chow diet or a high cholesterol diet. Quantitative analysis of atherosclerosis in mice transplanted with apoE-deficient bone marrow, after two months on a high cholesterol diet, revealed a 4-fold increase in the atherosclerotic lesion area as compared to animals transplanted with apoE+/+ bone marrow. Analysis of the ability of apoE-deficient macrophages to release cholesterol after loading with acetylated LDL revealed that the release of cholesterol from apoE-deficient macrophages was impaired as compared to wild-type macrophages in the absence and the presence of specific cholesterol acceptors. In conclusion, apoE production by macrophages retards the formation of atherosclerotic plaques, possibly by mediating cholesterol efflux. We anticipate that pharmacological approaches to increase apoE synthesis and/or secretion by macrophages might be beneficial for the treatment of atherosclerosis.

The value of apolipoprotein E knockout mice for studying the effects of dietary fat and cholesterol on atherogenesis

The ability of the apolipoprotein E-deficient mouse to develop spontaneous atherosclerosis, which resembles the human process, is an excellent model in which to assess the impact of dietary factors. This review discusses the role of several nutrients in the development of atherosclerosis and the mechanisms through which they act.

Hepatic expression of apolipoprotein E inhibits progression of atherosclerosis without reducing cholesterol levels in LDL receptor-deficient mice

Apolipoprotein E (apoE) is a multifunctional protein synthesized by the liver and by tissue macrophages. Plasma apoE (derived primarily from the liver) regulates plasma lipoprotein metabolism, but macrophage-derived apoE was shown to slow the progression of atherosclerosis independent of plasma lipid levels. We utilized liver-directed gene transfer to test the hypothesis that hepatic expression of human apoE would inhibit atherogenesis even in a model in which apoE expression has little effect on plasma lipoproteins. LDL receptor-deficient mice fed a western-type diet for 5 weeks were injected with a second-generation recombinant adenovirus encoding human apoE3 or control virus. Plasma cholesterol levels were not significantly different in the two groups of mice after virus injection. Four weeks after injection, atherosclerosis was examined using three independent assays. Expression of apoE was associated with significantly reduced atherosclerosis compared with control mice in both the aortic arch (decreased by 43%) and the aortic root (decreased by 59%). In summary, hepatic overexpression of apoE inhibited progression of atherosclerosis in LDL receptor-deficient mice without reducing plasma cholesterol levels. This finding indicates that liver-derived plasma apoE can influence early atherogenesis through mechanisms other than modulation of lipoprotein metabolism and that liver-directed gene transfer and overexpression of apoE may be a therapeutic approach to atherosclerosis.

Effect of macrophage-derived mouse ApoE, human ApoE3-Leiden, and human ApoE2 (Arg158-->Cys) on cholesterol levels and atherosclerosis in ApoE-deficient mice

The effect of monocyte/macrophage-derived wild-type mouse apolipoprotein E (apoE), human apoE3-Leiden, and human apoE2 on serum cholesterol levels and the development of atherosclerosis in apoE-deficient (apoe-/-) mice was investigated by using bone marrow transplantation (BMT). At 4 weeks after BMT, murine apoe+/+ bone marrow reduced serum cholesterol levels by 87% in apoe-/- mice, whereas macrophage-derived human apoE3-Leiden and human apoE2 induced a maximal, transient reduction of 35% and 48%, respectively. At 4 months after BMT, atherosclerosis was 23-fold (P<0.001) reduced in apoe+/+-->apoe-/- mice, whereas no significant reduction in apoE3-Leiden.apoe-/--->apoe-/- and apoE2.apoe-/--->apoe-/- mice could be demonstrated. A highly significant decrease in serum cholesterol levels (78% reduction) and atherosclerosis (21-fold, P<0. 001) was found in apoE3-Leiden.apoe-/- animals expressing high levels of apoE in multiple tissues, whereas apoE2 was ineffective even at high concentrations. Furthermore, in contrast to apoE-deficient macrophages, cholesterol efflux from apoE2 or apoE3-Leiden macrophages was not impaired. In conclusion, apoE3-Leiden as well as apoE2 are less effective in reducing cholesterol levels and atherosclerosis in apoe-/- animals, compared with apoe+/+, with apoE2<apoE3-Leiden<apoe+/+, irrespective of the observed adequate efflux of cholesterol from macrophages expressing apoE2 and apoE3-Leiden, indicating that normalization of cholesterol efflux by macrophages is not accompanied by measurable effects on lesion growth.

Rapid regression of atherosclerosis induced by liver-directed gene transfer of ApoE in ApoE-deficient mice

Apolipoprotein E (apoE) is a multifunctional protein synthesized by the liver and tissue macrophages. ApoE-deficient mice have severe hyperlipidemia and develop accelerated atherosclerosis on a chow diet. Both liver-derived and macrophage-derived apoEs have been shown to reduce plasma lipoprotein levels and slow the progression of atherosclerosis in apoE-deficient mice, but regression of atherosclerosis has not been demonstrated in this model. We utilized second-generation adenoviruses to achieve hepatic expression of human apoE in chow-fed, apoE-deficient mice with established atherosclerotic lesions of different stages. As expected, hepatic expression of human apoE3 significantly reduced plasma cholesterol levels. Liver-derived apoE also accumulated substantially within preexisting atherosclerotic lesions, indicating that plasma apoE gained access to the arterial intima. Hepatic expression of human apoE3 for 6 weeks resulted in significant quantitative regression of both early fatty streak lesions as well as advanced, complex lesions in both the aortic root and the aortic arch. In addition, hepatic expression of apoE induced substantial morphological changes in lesions, including decreased foam cells and increased smooth muscle cells and extracellular matrix content. In parallel, human apoE4 and apoE2 were also expressed in the liver by using recombinant adenoviruses. ApoE4 reduced cholesterol levels to the same extent as did apoE3 and also prevented progression but did not induce significant regression of preexisting lesions. ApoE2 reduced cholesterol levels to a lesser degree than did apoE3 and apoE4 and lesion progression was reduced, but regression was not induced. In summary, (1) regression of preexisting atherosclerotic lesions in apoE-deficient mice can be rapidly induced by hepatic expression of apoE, despite the absence of macrophage-derived apoE; (2) the morphological changes seen in this model of regression resemble those in other animal models, induced over longer periods of time; (3) liver-derived apoE gained access to and was retained by intimal atherosclerotic lesions; and (4) apoE4 was less effective in inducing regression, despite its effects on plasma lipoproteins that were similar to those of apoE3. The rapid regression of preexisiting atherosclerotic lesions induced by apoE gene transfer in apoE-deficient mice could provide a convenient murine model for investigation of the molecular events associated with atherosclerosis regression.

Dissection of the complex role of apolipoprotein E in lipoprotein metabolism and atherosclerosis using mouse models

Transgenic and knockout mice have been instrumental in delineating the role of apolipoprotein (apo) E in lipoprotein metabolism and atherosclerosis. The severe hypercholesterolemia and premature atherosclerosis of the apoE knockout mouse have been the starting point from which various physiologic processes have been identified in which apoE plays a critical role. These processes include 1) very low density lipoprotein (VLDL) triglyceride production; 2) lipoprotein lipase mediated triglyceride lipolysis; 3) VLDL remnant clearance and intracellular processing; and 4) the efflux of cellular cholesterol. In this review we will discuss the recent insight in the role of apoE in these processes, which has been obtained using a variety of in vivo and in vitro approaches to modify apoE expression and function.

Apolipoprotein E and atherosclerosis: insight from animal and human studies

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