Macrophage free cholesterol content regulates apolipoprotein E synthesis

An Overview of Atherosclerosis: A Look to the Future*

This overview briefly summarizes the cellular pathobiology of experimental atherosclerosis and is then followed by a consideration of how 3 major risk factors interact with the hypothesized pathogenetic process. First, since hemodynamics and blood flow influence the localization of atherosclerotic plaques, possible mechanisms and directions of research are considered. Secondly, the recent hypothesis relating the oxidation of LDL to several of the early processes of atherogenesis is briefly discussed in view of the fact that hyperlipidemia is a major risk factor. The possibility that subsets of LDL and lipoproteins other than LDL might be involved is also discussed. Family history is the last of the 3 contributors to atherosclerosis reviewed and some prototypes of gene abnormalities are considered. Finally, the needs and prospects of future research are summarized.

Comparing Data-Independent Acquisition and Parallel Reaction Monitoring in Their Abilities To Differentiate High-Density Lipoprotein Subclasses

High-density lipoprotein (HDL) is a diverse group of particles with multiple cardioprotective functions. HDL proteome follows HDL particle complexity. Many proteins were described in HDL, but consistent quantification of HDL protein cargo is still a challenge. To address this issue, the aim of this work was to compare data-independent acquisition (DIA) and parallel reaction monitoring (PRM) methodologies in their abilities to differentiate HDL subclasses through their proteomes. To this end, we first evaluated the analytical performances of DIA and PRM using labeled peptides in pooled digested HDL as a biological matrix. Next, we compared the quantification capabilities of the two methodologies for 24 proteins found in HDL₂ and HDL₃ from 19 apparently healthy subjects. DIA and PRM exhibited comparable linearity, accuracy, and precision. Moreover, both methodologies worked equally well, differentiating HDL subclasses' proteomes with high precision. Our findings may help to understand HDL functional diversity.

Apoproteins E, A-I, and SAA in Macrophage Pathobiology Related to Atherogenesis

Macrophages are core cellular elements of both early and advanced atherosclerosis. They take up modified lipoproteins and become lipid-loaded foam cells and secrete factors that influence other cell types in the artery wall involved in atherogenesis. Apoproteins E, AI, and SAA are all found on HDL which can enter the artery wall. In addition, apoE is synthesized by macrophages. These three apoproteins can promote cholesterol efflux from lipid-loaded macrophages and have other functions that modulate macrophage biology. Mimetic peptides based on the sequence or structure of these apoproteins replicate some of these properties and are potential therapeutic agents for the treatment of atherosclerosis to reduce cardiovascular diseases.

Apoprotein E and Reverse Cholesterol Transport

Apoprotein E (apoE) is a multifunctional protein. Its best-characterized function is as a ligand for low-density lipoprotein (LDL) receptor family members to mediate the clearance of apoB-containing atherogenic lipoproteins. Among its other functions, apoE is involved in cholesterol efflux, especially from cholesterol-loaded macrophage foam cells and other atherosclerosis-relevant cells, and in reverse cholesterol transport. Reverse cholesterol transport is a mechanism by which excess cellular cholesterol is transported via lipoproteins in the plasma to the liver where it can be excreted from the body in the feces. This process is thought to have a role in the attenuation of atherosclerosis. This review summarizes studies on the role of apoE in cellular cholesterol efflux and reverse cholesterol transport and discusses the identification of apoE mimetic peptides that may promote these pathways.

Protein kinase C controls vesicular transport and secretion of apolipoprotein E from primary human macrophages

Macrophage-specific apolipoprotein E (apoE) secretion plays an important protective role in atherosclerosis. However, the precise signaling mechanisms regulating apoE secretion from primary human monocyte-derived macrophages (HMDMs) remain unclear. Here we investigate the role of protein kinase C (PKC) in regulating basal and stimulated apoE secretion from HMDMs. Treatment of HMDMs with structurally distinct pan-PKC inhibitors (calphostin C, Ro-31-8220, Go6976) and a PKC inhibitory peptide all significantly decreased apoE secretion without significantly affecting apoE mRNA or apoE protein levels. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated apoE secretion, and both PMA-induced and apoAI-induced apoE secretion were inhibited by PKC inhibitors. PKC regulation of apoE secretion was found to be independent of the ATP binding cassette transporter ABCA1. Live cell imaging demonstrated that PKC inhibitors inhibited vesicular transport of apoE to the plasma membrane. Pharmacological or peptide inhibitor and knockdown studies indicate that classical isoforms PKCα/β and not PKCδ, -ε, -θ, or -ι/ζ isoforms regulate apoE secretion from HMDMs. The activity of myristoylated alanine-rich protein kinase C substrate (MARCKS) correlated with modulation of PKC activity in these cells, and direct peptide inhibition of MARCKS inhibited apoE secretion, implicating MARCKS as a downstream effector of PKC in apoE secretion. Comparison with other secreted proteins indicated that PKC similarly regulated secretion of matrix metalloproteinase 9 and chitinase-3-like-1 protein but differentially affected the secretion of other proteins. In conclusion, PKC regulates the secretion of apoE from primary human macrophages.

Apolipoprotein E enhances endothelial-NO production by modulating caveolin 1 interaction with endothelial NO synthase

Apolipoprotein E (apoE) is widely expressed in mammalian tissues, and one of the important tissue-specific effects is the atheroprotection ascribed to macrophage-derived apoE in the arterial wall. However, underlying mechanisms are not well understood. In this study, using subcellular fractionation, confocal microscopy, and coimmunoprecipitation, we demonstrated that macrophage-derived apoE is internalized by endothelial cells and impacts the subcellular distribution/interaction of caveolin 1 (cav-1) and endothelial NO synthase (eNOS). The addition of apoE disrupts the heteromeric complex formed between cav-1 and eNOS, and increases NO production. Sterol and oxysterol enhance endothelial cav-1/eNOS interaction and suppress NO production, but these effects are reversed by apoE. Silencing endothelial cav-1 attenuates apoE-induced NO production, establishing the importance of the cav-1-eNOS interaction for the increment in endothelial NO produced by apoE. Consistent with these observations, macrophage-derived apoE significantly improves vasodilation to acetylcholine in resistance arteries isolated from adipose tissue of obese humans. We conclude that macrophage-derived apoE enhances endothelial NO production by disrupting the inhibitory interaction of eNOS with cav-1. These results establish a novel mechanism by which apoE modulates endothelial cell function.

Hyperglycemia and advanced glycosylation end products suppress adipocyte apoE expression: implications for adipocyte triglyceride metabolism

Endogenous adipocyte apolipoprotein E (apoE) plays an important role in adipocyte lipoprotein metabolism and lipid flux. A potential role for hyperglycemia in regulating adipocyte apoE expression and triglyceride metabolism was examined. Exposure of adipocytes to high glucose or advanced glycosylation end product-BSA significantly suppressed apoE mRNA and protein levels. This suppression was significantly attenuated by antioxidants or inhibitors of the NF-κB transcription pathway. Hyperglycemia in vivo led to adipose tissue oxidant stress and significant reduction in adipose tissue and adipocyte apoE mRNA level. Incubation with antioxidant in organ culture completely reversed this suppression. Hyperglycemia also reduced adipocyte triglyceride synthesis, and this could be completely reversed by adenoviral-mediated increases in apoE. To more specifically evaluate an in vivo role for adipocyte apoE expression on organismal triglyceride distribution in vivo, WT or apoE knockout (EKO) adipose tissue was transplanted in EKO recipient mice. After 12 wk, WT adipocytes transplanted in EKO mice accumulated more triglyceride compared with transplanted EKO adipocytes. In addition, EKO recipients of WT adipose tissue had reduced hepatic triglyceride content compared with EKO recipients transplanted with EKO adipose tissue. Our results demonstrate that hyperglycemia and advanced glycosylation end products suppress the expression of adipocyte apoE in vitro and in vivo and thereby reduce adipocyte triglyceride synthesis. In vivo results using adipose tissue transplantation suggest that reduction of adipocyte apoE, and subsequent reduction of adipocyte triglyceride accumulation, could influence lipid accumulation in nonadipose tissue.

Decreased lipid efflux and increased susceptibility to cholesterol-induced apoptosis in macrophages lacking phosphatidylcholine transfer protein

Macrophages are the predominant cellular component of atherosclerotic lesions, where they scavenge oxidatively modified lipoproteins while defending themselves against cholesterol-induced cytotoxicity by adaptive mechanisms that depend in part on the synthesis, distribution and efflux of phosphatidylcholines. PC-TP (phosphatidylcholine transfer protein) is a START (steroidogenic acute regulatory protein-related lipid transfer) domain protein that catalyses the intermembrane transfer of phosphatidylcholines and promotes apolipoprotein AI-mediated lipid efflux when overexpressed in the cytosol of Chinese-hamster ovary cells. To explore a role for PC-TP in the adaptive responses of macrophages to cholesterol loading, we utilized peritoneal macrophages from mice with homozygous disruption of the gene encoding PC-TP (Pctp(-/-)) and wild-type littermate controls. PC-TP was abundantly expressed in macrophages from wild-type but not Pctp(-/-) mice. In cholesteryl ester-loaded macrophages from Pctp(-/-) mice, the apolipoprotein AI-mediated efflux of phospholipids and cholesterol was decreased. This could be attributed to proportional decreases in the expression levels of ATP-binding cassette A1. Also, in response to free cholesterol loading, the absence of PC-TP from macrophages was associated with marked increases in apoptotic cell death. These findings suggest that PC-TP in macrophages may serve an atheroprotective role by defending against cholesterol-induced cytotoxicity.

Apoprotein E as a lipid transport and signaling protein in the blood, liver, and artery wall

Apoprotein E (apoE) is synthesized by a number of tissues including the liver, brain, adipose tissue, and artery wall. The majority of apoE is found in the plasma associated with specific lipoprotein subclasses and is derived primarily from the liver. However the fact that apoE expression is sustained in nonhepatic tissues suggests that the local production must have some unique functional attribute. ApoE is involved in many steps in lipid and lipoprotein homeostasis, for the triglyceride-rich lipoproteins and for HDL. ApoE is also important for lipid homeostasis in the brain, artery wall, and adipose tissue through its synthesis by glial cells, adipocytes, and macrophages. In addition, nonlipid related functions have also been attributed to apoE, including effects on immune response and inflammation, oxidation, and smooth muscle proliferation and migration. Some of these effects have been shown to be dependent upon different domains of the protein, different concentrations, and lipidation state. Thus, this multifunctional protein impacts normal and pathophysiology at multiple levels.

Reinioside C, a triterpene saponin of Polygala aureocauda Dunn, exerts hypolipidemic effect on hyperlipidemic mice

Reinioside C is a triterpene saponin from the the root of Polygala aureocauda Dunn (PAD). This study examined the effects of reinioside C on hyperlipidemic mice in vivo and endothelium cells, macrophages and smooth muscle cells in vitro. Mice were given a hyperlipidemic diet for 30 days, then administered reinioside C (4, 8, 16 mg/kg/day, p.o.) for 30 days. Then the serum lipid, superoxide dismutase (SOD), malonaldehyde (MDA), the total cholesterol (TC) and triglyceride (TG) in the liver extract were measured. Human umbilical vein endothelial (HUVECs), peritoneal macrophages and smooth muscle cells (SMCs) pre-treated with reinioside C were treated with oxidized low-density lipoprotein (OxLDL). The results showed that reinioside C decreased serum and liver tissue lipid profiles in hyperlipidemic mice. Moreover, reinioside C protected the HUVECs against the Ox-LDL induced LDH leakage and exerted a protective effect on oxidative lesions induced by OxLDL, inhibited cholesteryl ester accumulation in macrophages, and decreased [Ca2+](i) and SMC proliferation in vitro. Based on these results, it is suggested that reinioside C is a promising hypolipidemic candidate.

Regulation of endogenous apolipoprotein E secretion by macrophages

Apolipoprotein E has critical roles in the protection against atherosclerosis and is understood to follow the classical constitutive secretion pathway. Recent studies have indicated that the secretion of apoE from macrophages is a regulated process of unexpected complexity. Cholesterol acceptors such as apolipoprotein A-I, high density lipoprotein, and phospholipid vesicles can stimulate apoE secretion. The ATP binding cassette transporter ABCA1 is involved in basal apoE secretion and in lipidating apoE-containing particles secreted by macrophages. However, the stimulation of apoE secretion by apoA-I is ABCA1-independent, indicating the existence of both ABCA1-dependent and -independent pathways of apoE secretion. The release of apoE under basal conditions is also regulated, requiring intact protein kinase A activity, intracellular calcium, and an intact microtubular network. Mathematical modeling of apoE turnover indicates that whereas some pools of apoE are committed to either secretion or degradation, other pools can be diverted from degradation toward secretion. Targeted inhibition or stimulation of specific apoE trafficking pathways will provide unique opportunities to regulate the biology of this important molecule.

Secretion of apolipoprotein E from macrophages occurs via a protein kinase A and calcium-dependent pathway along the microtubule network

Macrophage-specific expression of apolipoprotein (apo)E protects against atherosclerosis; however, the signaling and trafficking pathways regulating secretion of apoE are unknown. We investigated the roles of the actin skeleton, microtubules, protein kinase A (PKA) and calcium (Ca2+) in regulating apoE secretion from macrophages. Disrupting microtubules with vinblastine or colchicine inhibited basal secretion of apoE substantially, whereas disruption of the actin skeleton had no effect. Structurally distinct inhibitors of PKA (H89, KT5720, inhibitory peptide PKI(14-22)) all decreased basal secretion of apoE by between 50% to 80% (P<0.01). Pulse-chase experiments demonstrated that inhibition of PKA reduced the rate of apoE secretion without affecting its degradation. Confocal microscopy and live cell imaging of apoE-green fluorescent protein-transfected RAW macrophages identified apoE-green fluorescent protein in vesicles colocalized with the microtubular network, and inhibition of PKA markedly inhibited vesicular movement. Chelation of intracellular calcium ([Ca2+]i) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate-acetoxymethyl ester (BAPTA-AM) inhibited apoE secretion by 77.2% (P<0.01). Injection of c57Bl6 apoE+/+ bone marrow-derived macrophages into the peritoneum of apoE-/- C57Bl6 mice resulted in time-dependent secretion of apoE into plasma, which was significantly inhibited by transient exposure of macrophages to BAPTA-AM and colchicine and less effectively inhibited by H89. We conclude that macrophage secretion of apoE occurs via a PKA- and calcium-dependent pathway along the microtubule network.

Endogenous ApoE expression modulates adipocyte triglyceride content and turnover

Apolipoprotein E (apoE) is highly expressed in adipose tissue and adipocytes in which its expression is regulated by peroxisome proliferator-activated receptor (PPAR)-gamma agonists and tumor necrosis factor-alpha. There is, however, no information regarding a role for endogenous apoE in differentiated adipocyte function. In this report, we define a novel role for apoE in modulating adipocyte lipid metabolism. ApoE(-/-) mice have less body fat and smaller adipocytes compared with wild-type controls. Freshly isolated adipose tissue from apoE(-/-) mice contains lower levels of triglyceride and free fatty acid, and these differences are maintained in cultured adipocytes derived from preadipocytes. Adenoviral expression of apoE in apoE(-/-)-cultured adipocytes increases triglyceride and fatty acid content. During incubation with apoE-containing triglyceride-rich lipoproteins, apoE(-/-) adipose tissue accumulates less triglyceride than wild type. The absence of apoE expression in primary cultured adipocytes also leads to changes in the expression of genes involved in the metabolism/turnover of fatty acids and the triglyceride droplet. Markers of adipocyte differentiation were lower in freshly isolated and cultured apoE(-/-) adipocytes. Importantly, PPAR-gamma-mediated changes in lipid content and gene expression are markedly altered in cultured apoE(-/-) adipocytes. These results establish a novel role for endogenous apoE in adipocyte lipid metabolism and have implications for constructing an integrated model of adipocyte physiology in health and disease.

Evaluation of apolipoprotein E secretion by macrophages in type 2 diabetic patients: role of HDL and apolipoprotein A-I

It has been shown that apolipoprotein A-I (ApoA-I) stimulates the secretion of apolipoprotein E (ApoE) from human macrophages. ApoA-I is a major protein constituent of HDL which because of its role in reverse cholesterol transport, has been implicated in the prevention of atherosclerosis. We herein investigated the ability of monocyte-derived macrophages (MDMs) in 42 patients with type 2 diabetes to secrete ApoE; these patients commonly have low plasma HDL and ApoA-I levels. Our data showed that ApoE secretion from these cells was reduced in patients with low plasma HDL and ApoA-I levels; there were positive correlation between ApoE secretion from MDMs and plasma HDL (r2=0.33, p=0.03) and ApoA-I (r2=0.31, p=0.03). Furthermore, we found that ApoE secretion increased concomitantly with an increase in HDL or ApoA-I in treated diabetics (n=24) from 1.99+/-1.86 to 3.40+/-1.77 ng/mg cell protein. These findings suggest another possible link between HDL and ApoA-I metabolism and atherosclerosis in patients with type 2 diabetes.

The key role of apolipoprotein E in atherosclerosis

Apolipoprotein E is a multifunctional protein that is synthesized by the liver and several peripheral tissues and cell types, including macrophages. The protein is involved in the efficient hepatic uptake of lipoprotein particles, stimulation of cholesterol efflux from macrophage foam cells in the atherosclerotic lesion, and the regulation of immune and inflammatory responses. Apolipoprotein E deficiency in mice leads to the development of atherosclerosis and re-expression of the protein reduces the extent of the disease. This review presents evidence for the potent anti-atherogenic action of apolipoprotein E and describes our current understanding of its multiple functions and regulation by factors implicated in the pathogenesis of cardiovascular disease.

ApoE-mediated cholesterol efflux from macrophages: separation of autocrine and paracrine effects

Macrophages in the vessel wall secrete high levels of apolipoprotein E (apoE). Cholesterol efflux from macrophages to apoE has been shown to decrease foam cell formation and prevent atherosclerosis. An apoE molecule can mediate cholesterol efflux from the macrophage that originally secreted it (autocrine effect) or from surrounding macrophages (paracrine effect). Traditional methodologies have not been able to separate these serial effects. The novel methodology presented here was developed to separate autocrine and paracrine effects by using a simple mathematical model to interpret the effects of dilution on apoE-mediated cholesterol efflux. Our results show that, at very dilute concentrations, the paracrine effect of apoE is not evident and the autocrine effect becomes the dominant mediator of efflux. However, at saturating concentrations, paracrine apoE causes 80-90% of the apoE-mediated cholesterol efflux, whereas autocrine apoE is responsible for the remaining 10-20%. These results suggest that the relative importance of autocrine and paracrine apoE depends on the size of the local distribution volume, a factor not considered in previous in vitro studies of apoE function. Furthermore, autocrine effects of apoE could be critical in the prevention of foam cell formation in vivo. This novel methodology may be applicable to other types of mixed autocrine/paracrine systems, such as signal transduction systems.

Divergent effects of peroxisome proliferator-activated receptor gamma agonists and tumor necrosis factor alpha on adipocyte ApoE expression

ApoE is expressed in multiple mammalian cell types in which it supports cellular differentiated function. In this report we demonstrate that apoE expression in adipocytes is regulated by factors involved in modulating systemic insulin sensitivity. Systemic treatment with pioglitazone increased systemic insulin sensitivity and increased apoE mRNA levels in adipose tissue by 2-3-fold. Treatment of cultured 3T3-L1 adipocytes with ciglitazone increased apoE mRNA levels by 2-4-fold in a dose-dependent manner and increased apoE secretion from cells. Conversely, treatment of adipocytes with tumor necrosis factor (TNF) alpha reduced apoE mRNA levels and apoE secretion by 60%. Neither insulin nor a peroxisome proliferator-activated receptor (PPAR) alpha agonist regulated adipocyte apoE gene expression. In addition, treatment of human monocyte-derived macrophages with ciglitazone did not regulate expression of apoE. Additional analyses using reporter genes indicated that the effect of TNFalpha and PPARgamma agonists on the apoE gene was mediated via distinct gene control elements. The TNFalpha effect was mediated by elements within the proximal promoter, whereas the PPARgamma effect was mediated by elements within a downstream enhancer. However, the addition of TNFalpha substantially reduced the absolute levels of apoE reporter gene response even in the presence of ciglitazone. These results indicate for the first time that adipose tissue expression of apoE is modulated by physiologic regulators of insulin sensitivity.

Apolipoprotein A-I-stimulated apolipoprotein E secretion from human macrophages is independent of cholesterol efflux

Apolipoprotein A-I (apoA-I)-mediated cholesterol efflux involves the binding of apoA-I to the plasma membrane via its C terminus and requires cellular ATP-binding cassette transporter (ABCA1) activity. ApoA-I also stimulates secretion of apolipoprotein E (apoE) from macrophage foam cells, although the mechanism of this process is not understood. In this study, we demonstrate that apoA-I stimulates secretion of apoE independently of both ABCA1-mediated cholesterol efflux and of lipid binding by its C terminus. Pulse-chase experiments using (35)S-labeled cellular apoE demonstrate that macrophage apoE exists in both relatively mobile (E(m)) and stable (E(s)) pools, that apoA-I diverts apoE from degradation to secretion, and that only a small proportion of apoA-I-mobilized apoE is derived from the cell surface. The structural requirements for induction of apoE secretion and cholesterol efflux are clearly dissociated, as C-terminal deletions in recombinant apoA-I reduce cholesterol efflux but increase apoE secretion, and deletion of central helices 5 and 6 decreases apoE secretion without perturbing cholesterol efflux. Moreover, a range of 11- and 22-mer alpha-helical peptides representing amphipathic alpha-helical segments of apoA-I stimulate apoE secretion whereas only the C-terminal alpha-helix (domains 220-241) stimulates cholesterol efflux. Other alpha-helix-containing apolipoproteins (apoA-II, apoA-IV, apoE2, apoE3, apoE4) also stimulate apoE secretion, implying a positive feedback autocrine loop for apoE secretion, although apoE4 is less effective. Finally, apoA-I stimulates apoE secretion normally from macrophages of two unrelated subjects with genetically confirmed Tangier Disease (mutations C733R and c.5220-5222delTCT; and mutations A1046D and c.4629-4630insA), despite severely inhibited cholesterol efflux. We conclude that apoA-I stimulates secretion of apoE independently of cholesterol efflux, and that this represents a novel, ABCA-1-independent, positive feedback pathway for stimulation of potentially anti-atherogenic apoE secretion by alpha-helix-containing molecules including apoA-I and apoE.

Adipophilin Enhances Lipid Accumulation and Prevents Lipid Efflux From THP-1 Macrophages

Objective— Uptake of modified low-density lipoprotein (LDL) by macrophages through scavenger receptors results in lipid droplets accumulation and foam cell formation. Excess lipid deposition in macrophages has been reported to modulate expression of several genes including adipophilin. In this study, we investigated the function of adipophilin in lipid accumulation and cholesterol efflux in THP-1 macrophages.

Methods and Results— Adipophilin mRNA expression was 3.5-fold higher in human atherosclerotic plaques compared with healthy areas of the same arteries. Moreover, in the presence of acetylated LDL (AcLDL), triglycerides and cholesteryl esters were increased in macrophages overexpressing adipophilin by 40% and 67%, respectively, whereas their accumulation was reduced when endogenous cellular adipophilin was depleted using siRNA approach. In addition, neither overexpression nor downregulation of adipophilin altered expression of genes involved in lipid efflux. However, the affinity and the number of AcLDL receptors were not affected. After 24-hour incubation of lipid-loaded macrophages with apolipoprotein A-I, cholesterol efflux was reduced by 47% in adipophilin transfected cells versus control cells.

Conclusion— Our results showed that stimulation of adipophilin expression in macrophages by modified LDL promotes triglycerides and cholesterol storage and reduces cholesterol efflux. Therefore, adipophilin might contribute, in vivo, to lipid accumulation in the intima of the arterial wall.

Effect of atorvastatin on ApoE and ApoC-I synthesis and secretion by THP-1 macrophages

Apolipoprotein (apo) E and C-I are plasma apolipoproteins that have been implicated in the etiology of atherosclerosis and obesity, respectively. Both proteins are synthesized and secreted by macrophages, though pharmacological regulation of their production is poorly understood. The authors compared the effect of 2 HMG-CoA reductase inhibitors, atorvastatin and cerivastatin, on the synthesis and secretion of apoE and apoC-I by THP-1 macrophages. Atorvastatin reduced medium apoE and cellular apoE mRNA of PMA-activated THP-1 cells in a dose-dependent manner (-24% and -22%, respectively, at 1-micromol/L, P < 0.01). ApoC-I in the medium was also reduced by atorvastatin in a dose-dependent manner, though to a lesser extent (-15% at 1-micromol/L, P < 0.05). Cerivastatin similarly reduced medium apoE (-20% at 1-micromol/L, P < 0.05) and cellular apoE mRNA (-31% at 1-micromol/L, P < 0.05), and significantly lowered cellular apoC-I mRNA (-15%, P < 0.05), but not apoC-I in the medium. In experiments with THP-1 macrophages loaded with cholesterol (ie, 24-hour incubation with acetyl-LDL), atorvastatin and cerivastatin (1-micromol/L) significantly (P < 0.05) reduced both medium apoE (-30% and -25%, respectively) and cellular apoE mRNA (-25% and -17%, respectively). A lower and less consistent effect was observed on medium apoC-I (-6% and -18%, respectively) and cellular apoC-I mRNA (-13% and -19%, respectively). These data demonstrate that statins have the capacity to reduce the synthesis and secretion of both apoE and apoC-I in THP-1 macrophages loaded or unloaded with cholesterol.

Regulation of macrophage ApoE expression and processing by extracellular matrix

Macrophage-derived apoE in the vessel wall has important effects on atherogenesis in vivo, making it important to understand factors that regulate its expression. Vessel wall macrophages are embedded in an extracellular matrix produced largely by arterial smooth muscle cells and endothelial cells. In this series of studies, we evaluated the influence of extracellular matrix on macrophage apoE expression. Subendothelial matrix, fibronectin, or collagen I stimulated macrophage apoE gene expression and apoE synthesis. Adhesion of macrophages to a polylysine substrate had no effect. An increase in apoE synthesis after plating on fibronectin could be observed by 2 h and was inhibited by blocking antibodies to the alpha(5)beta(1) integrin receptor for fibronectin. Fibronectin also regulated the post-translational processing of newly synthesized macrophage apoE by inhibiting its degradation. The increment in apoE resulting from suppressed degradation was retained in the cell-fibronectin monolayer in a pool that was resistant to release by exogenous high density lipoprotein subfraction 3. These observations establish a new pathway for the regulation of macrophage apoE expression in the vessel wall. The composition of the extracellular matrix changes after vessel wall injury and in response to locally produced cytokines and growth factors. The evolving composition of this matrix will, therefore, be important for regulating apoE expression and processing by vessel wall macrophages.

Sterol efflux mediated by endogenous macrophage ApoE expression is independent of ABCA1

Sterol efflux importantly contributes to preservation of cellular cholesterol homeostasis, and multiple pathways may be involved for mediating such efflux. Recently, an important role has been ascribed to ABCA1 in facilitating lipid efflux from cells, including macrophages, to extracellular lipid-free apolipoproteins. Macrophages are relatively unique among cells because they express apoprotein E (apoE) as a major protein product, and this endogenous expression of apoE increases sterol and phospholipid efflux from macrophages. The studies in this article were designed to test whether the sterol efflux mediated by the endogenous expression of apoE in macrophages was dependent on ABCA1 expression. These studies were facilitated by comparing apoE-expressing J774 cells (J774E(+)) with nonexpressing parental cells (J774E(-)). Sterol efflux was higher from J774E(+) cells compared with J774E(-) cells, but the increment in efflux between these cell types was not increased by induction of ABCA1 expression with cAMP. Induction of ABCA1 with cAMP, however, did increase sterol efflux to exogenously added apoA1 from both cell types. Inhibitors of ABCA1 activity significantly reduced (by 40% to 50%) sterol efflux from both J774E(+) and J774E(-) cells treated with cAMP and apoA1. This inhibitor did not, however, reduce the increment in sterol efflux due to the expression of endogenous apoE. The results of these studies indicate that the increment in sterol efflux mediated by the endogenous expression of apoE in macrophages does not depend on ABCA1 expression or activity.

Apolipoprotein E is a putative autocrine regulator of the rat ovarian theca cell compartment

Apolipoprotein (apo) E inhibits androgen production by ovarian theca cells. We found that apo E, as a synthetic peptide mimicked the full-size protein, induced theca and interstitial cell (TIC) apoptosis indicated by pyknotic cell morphology, increased DNA end-labeling (TUNEL), and DNA ladders. None of the low-density lipoprotein (LDL) receptor superfamily members were involved because the universal antagonist of these receptors, receptor-associated protein (RAP), did not block apo E-induced apoptosis. Furthermore, several apo E synthetic peptides that do not bind the LDL receptor did induce TIC apoptosis. Similar to apo E, apoptogenic agents such as ceramide and LY 294002, a phosphatidylinositol (PI) 3-kinase inhibitor, induced apoptosis and suppressed androstenedione production. However, apoptosis alone was not responsible for apo E suppression of androstenedione production because both insulin and IGF-I prevented apo E-induced apoptosis, but neither restored androstenedione production. Theca cells of atretic follicles express the greatest apo E mRNA, and here we show that cultured TIC produce apo E. When considered with the observation of TUNEL-positive theca cells in atretic follicles these results support our hypothesis that intraovarian apo E controls theca cell production of androgen as well as limiting the size of the theca cell compartment.

Control of apolipoprotein E secretion in the human hepatoma cell line KYN-2

Even though it is known that apolipoprotein E (apoE) is deeply involved in major age-related disorders such as atherosclerosis or Alzheimer's disease (AD), the control of cell-specific apoE expression is still poorly understood. We compared the apoE secretion as previously described in astrocytic cell17 to hepatic cell apoE secretion. We used the human hepatoma cell line KYN-2 to better delineate the characteristics of apoE secretion and to validate it with respect to the classical human hepatoma cell line HepG2. Interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) significantly inhibited, while IL-2, IL-6 and tumour necrosis factor-alpha (TNF-alpha) were inactive on apoE secretion by KYN-2 as well as HepG2 cells. Cholesterol and 25-OH cholesterol had no effect, while forskolin exerted a significant inhibitory effect, on apoE secretion in KYN-2 cells. Our results suggest that the KYN-2 cell line represents an appropriate cell model, and in any case an alternative model to the HepG2 cell line, to study the control of apoE secretion. The response of KYN-2 cells to both cytokines and cholesterol differs from that found in astrocytoma cells, suggesting that blood variations of apoE concentrations in AD may not reflect the dysregulations taking place in the brain.

Effects of high-density lipoprotein(2) on cholesterol transport and acyl-coenzyme A:cholesterol acyltransferase activity in P388D1 macrophages

High-density lipoproteins are the putative vehicles for cholesterol removal from monocyte-derived macrophages, which are an important cell type in all stages of atherosclerosis. The role of HDL(2), an HDL subclass that accounts for most variation in plasma HDL-cholesterol concentration, in cholesterol metabolism in monocyte-derived macrophages is not known. In this study, the dose-dependent effects of HDL(2) on cellular cholesterol mass, efflux, and esterification, and on cellular cholesteryl ester (CE) hydrolysis using the mouse macrophage P388D1 cell line was investigated. HDL(2) at low concentrations (40 microg protein/ml) decreased CE content without affecting cellular free cholesterol content (FC), CE hydrolysis, or cholesterol biosynthesis. In addition, HDL(2) at low concentrations reduced cellular acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity and increased FC efflux from macrophages. Thus, HDL(2) has two potential roles in reverse cholesterol transport. In one, HDL(2) is an acceptor of macrophage FC. In the other, more novel role, HDL(2) increases the availability of macrophage FC through the inhibition of ACAT. Elucidation of the mechanism by which HDL(2) inhibits ACAT could identify new therapeutic targets that enhance the transfer of cholesterol from macrophages to the liver.

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.

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.

Sterols and inhibitors of sterol transport modulate the degradation and secretion of macrophage ApoE: requirement for the C-terminal domain

Macrophage-derived apoE, produced in the vessel wall, may have important effects during atherogenesis. The production of apoE by macrophages can be regulated at a transcriptional level by cellular differentiation state, cytokines and sterol loading. In addition, there are post-transcriptional and post-translational loci for regulation. We have recently identified an intermediate density cell membrane fraction in which the degradation of apoE can be modulated by sterols. Suppressing degradation of apoE in this fraction by pre-incubating cells in sterols led to enhanced apoE secretion. In this report we demonstrate that the suppressive effect of sterols on the degradation of newly synthesized apoE in this fraction depends on the presence on its C-terminal domain, by studying a macrophage cell line transfected to express a mutant form of apoE in which amino acids beyond amino acid 202 were deleted. In addition, two modulators of cellular sterol transport, progesterone and U1866A, inhibited the degradation of full-length apoE. In contrast, incubation of cells in the acyl-CoA:cholesterol acyltransferase inhibitor S58035 did not influence apoE degradation. As would be predicted based on the results of degradation assays, U1866A, but not S58035, increased the secretion of apoE from a cell line transfected to constitutively express full-length apoE cDNA. The effect of U1866A on apoE degradation, like the effect of sterol, required the presence of the apoE C-terminal domain. Our results indicate that alteration of intracellular sterol homeostasis by pre-incubation in sterols or by drugs that modify the subcellular transport of sterol, modulates the susceptibility of apoE to degradation and that this modulation requires the presence of C-terminal lipid binding domains.

Apolipoprotein A-I stimulates secretion of apolipoprotein E by foam cell macrophages

Apolipoprotein A-I (apoA-I) overexpression inhibits atherogenesis in mice, and apolipoprotein E (apoE) secreted by foam cell macrophages may exert antiatherogenic effects within the arterial wall. We hypothesized that interaction between apoA-I and apoE contributed to the antiatherogenic properties of apoA-I, and therefore investigated whether apoA-I stimulated secretion of apoE by foam cell macrophages. Cholesterol enrichment of primary murine and human macrophages increased spontaneous apoE secretion 2-fold, as quantified by Western blot and chemiluminescence detection. Human apoA-I caused a further marked increase of apoE secretion from both murine (3.8-fold, p < 0.01) and human (3.2-fold, p = 0.01) foam cells in a time- and concentration- dependent manner, and this increase was confirmed by immunoprecipitation of [(35)S]methionine-labeled macrophage apoE. The protein synthesis inhibitor cycloheximide, but not the transcription inhibitor actinomycin D, markedly inhibited apoE secretion to apoA-I (73.1 +/- 9.8% inhibition at 4 h) and completely suppressed apoE secretion beyond 4 h. Pretreatment of macrophages with Pronase inhibited initial apoA-I-mediated apoE secretion by 70.5 +/- 6.5% at 2 h, but by 8 h apoA-I-induced apoE secretion was the same in Pronase-pretreated and non-pretreated cells. Non-apolipoprotein-mediated cholesterol efflux induced by trimethyl-beta cyclodextrin did not enhance apoE secretion, whereas phospholipid vesicles inducing the same degree of cholesterol efflux substantially enhanced apoE secretion, and apoA-I and phospholipid vesicles in combination demonstrated additive induction of apoE secretion. We conclude that apoA-I concurrently stimulates apoE secretion and cholesterol efflux from foam cell macrophages and that lipoprotein-derived apoA-I may enhance local secretion and accumulation of apoE in atherosclerotic lesions.

Binding of beta-VLDL to heparan sulfate proteoglycans requires lipoprotein lipase, whereas ApoE only modulates binding affinity

The binding of beta-VLDL to heparan sulfate proteoglycans (HSPG) has been reported to be stimulated by both apoE and lipoprotein lipase (LPL). In the present study we investigated the effect of the isoform and the amount of apoE per particle, as well as the role of LPL on the binding of beta-VLDL to HSPG. Therefore, we isolated beta-VLDL from transgenic mice, expressing either APOE*2(Arg158-->Cys) or APOE*3-Leiden (E2-VLDL and E3Leiden-VLDL, respectively), as well as from apoE-deficient mice containing no apoE at all (Enull-VLDL). In the absence of LPL, the binding affinity and maximal binding capacity of all beta-VLDL samples for HSPG-coated microtiter plates was very low. Addition of LPL to this cell-free system resulted in a 12- to 55-fold increase in the binding affinity and a 7- to 15-fold increase in the maximal binding capacity (Bmax). In the presence of LPL, the association constant (Ka) tended to increase in the order Enull-VLDL<E2-VLDL<E3Leiden-VLDL, whereas Bmax increased in the reverse order: E3Leiden-VLDL approximately E2-VLDL<Enull-VLDL. Addition of LPL resulted in a marked stimulation of both Ka and Bmax for binding of beta-VLDL samples to J774 cells similar to that found for the binding to HSPG-LPL complexes. Our results indicate that both Ka and Bmax for binding of beta-VLDL to HSPG are increased more than 1 order of magnitude on addition of LPL. In addition, for the binding of beta-VLDL to HSPG-LPL complexes, the presence of apoE is not a prerequisite, but results in an increased binding affinity, depending on the apoE isoform used.

Beta-glucocerebrosidase gene locus as a link for Gaucher's disease and familial hypo-alpha-lipoproteinaemia

BACKGROUND

Gaucher's disease is the most common lysosomal storage disorder, caused by deficiency of glucocerebrosidase resulting from homozygosity for any of several mutations of the glucocerebrosidase gene locus. Affected people have decreased concentrations of LDL cholesterol (LDL-C) and HDL cholesterol (HDL-C). We assessed the association between mutations in the glucocerebrosidase locus and hypo-alpha-lipoproteinaemia.

METHODS

We studied 258 people from 43 unrelated Spanish families. 57 participants were affected, 137 were non-affected carriers, and 64 were non-carriers. We determined glucocerebrosidase genotypes and measured plasmid lipids, apolipoproteins A-I, B, and E, and leucocyte glucocerebrosidase activity.

FINDINGS

The most common glucocerebrosidase mutations were N370S (45%), L444P (23%), and G377S (5%). Deletions and recombinants accounted for another 5%, and point mutations in exons 5, 6, 9, and 10 were present in 12%. Affected participants had lower LDL-C and HDL-C concentrations than non-affected carriers (p<0.001) and non-carriers (p<0.001). HDL-C values were also significantly different between the non-affected carriers and non-carriers. Mutations at this locus may account for as much as 19.5% of the genetic variability in HDL-C in the population studied.

INTERPRETATION

Heterozygosity for these mutations at the glucocerebrosidase locus does not result in clinical expression of Gaucher's disease but can decrease HDL-C concentrations. Given the high frequency of these mutations, the glucocerebrosidase locus might lead to familial low alpha-lipoproteinaemia in up to 2% of the general population and be one of the most common known genetic causes of HDL-C.

Degradation of macrophage ApoE in a nonlysosomal compartment. Regulation by sterols

Macrophage-derived apoE has been shown to play an important role in the susceptibility of the vessel wall to atherosclerosis. Previous studies have shown that macrophage sterol content modulates apoE synthesis and secretion, associated with a large transcriptional response of the apoE gene. The current studies were undertaken to evaluate the existence of additional post-transcriptional regulatory loci for the effect of sterols on apoE synthesis and secretion. Using a macrophage cell line transfected to constitutively express an apoE cDNA to facilitate detection of a post-transcriptional regulatory locus, we demonstrated that preincubations in 25-hydroxycholesterol and cholesterol lead to increased apoE secretion in pulse/chase experiments. Examination of cell lysates in these experiments showed that apoE not secreted by control cells was degraded and not detectable, suggesting that the preincubation in sterols increased secretion by decreasing degradation of newly synthesized apoE. The measurement of total protein and apoE degradation in cell fractions revealed an intermediate density fraction that degraded significant amounts of newly synthesized total protein and newly synthesized apoE. In this fraction, degradation of total protein and apoE was unaffected by chloroquine but was substantially reduced by N-acetyl-Leu-Leu-norleucinal plus N-acetyl-Leu-Leu-methioninal or by lactacystin, suggesting the involvement of proteasomes. Preincubation in sterol/oxysterol or acetylated low density lipoprotein did not modify total protein degradation by this fraction but inhibited apoE degradation. Similar results were obtained using intermediate density fractions isolated from human monocyte-derived macrophages. The results of our studies indicate that newly synthesized apoE in the macrophage can be degraded in an intermediate density nonlysosomal cellular compartment, which is sensitive to proteasomal inhibitors. Alteration of cellular lipid homeostasis by preincubation in sterol/oxysterol or acetylated low density lipoprotein inhibits apoE, but not total protein, degradation in this fraction. Inhibition of the degradation of apoE in this fraction likely contributes to the increased apoE secretion observed in sterol-enriched cells.

Bone marrow transplantation in apolipoprotein E-deficient mice. Effect of ApoE gene dosage on serum lipid concentrations, (beta)VLDL catabolism, and atherosclerosis

Apolipoprotein E (apoE), a high-affinity ligand for lipoprotein receptors, is synthesized by the liver and extrahepatic tissues, including cells of the monocyte/macrophage lineage. Inactivation of the apoE gene in mice leads to a prominent increase in serum cholesterol and triglyceride levels and the development of premature atherosclerosis. In this study, the role of monocyte/macrophage-derived apoE in lipoprotein remnant metabolism and atherogenesis was assessed. The influence of apoE gene dosage on serum lipid concentrations was determined by transplantation of homozygous apoE-deficient (apoE-/-), heterozygous apoE-deficient (apoE+/-), and wild-type (apoE+/+) bone marrow in homozygous apoE-deficient mice. The concentration of apoE detected in serum was found to be gene dosage dependent, being 3.52 +/- 0.30%, 1.87 +/- 0.17%, and 0% of normal in transplanted mice receiving either apoE+/+, apoE+/-, or apoE-/- bone marrow, respectively. These low concentrations of apoE nevertheless dramatically reduced serum cholesterol levels owing to a reduction of VLDL and, to a lesser extent, LDL, while HDL levels were slightly raised. After 4 months on a "Western-type" diet, atherosclerosis was evidently reduced in mice transplanted with apoE+/+ bone marrow, compared with control transplanted mice. To study the mechanism of the lipoprotein changes on bone marrow transplantation, the in vivo turnover of autologous serum (beta)VLDL was studied. The serum half-life of (beta)VLDL in transplanted mice, compared with control apoE-deficient mice, was shortened mainly as a consequence of an increased recognition and uptake by the liver. Analysis of the relative contribution of the liver parenchymal cells, endothelial cells, and Kupffer cells (liver tissue macrophages) indicated an increased uptake by parenchymal cells, while the relative contribution to Kupffer cells was decreased. In conclusion, macrophage-derived apoE can dose-dependently reduce hypercholesterolemia in apoE-deficient mice owing to increased recognition and uptake of (beta)VLDL by parenchymal liver cells, leading to a decreased susceptibility to atherosclerosis.

Cholesterol-mediated changes of neutral cholesterol esterase activity in macrophages. Mechanism for mobilization of cholesteryl esters in lipid droplets by HDL

Cholesteryl esters (CE) in lipid droplets undergo a continual cycle of hydrolysis and reesterification by neutral cholesterol esterase (N-CEase) and acyl CoA:cholesterol acyltransferase (ACAT), respectively. The mechanism by which HDL mobilizes CE from lipid droplets in J774 A.1 cells was investigated, focusing on N-CEase activity. We asked whether HDL enhances the activity and, if so, what signals induce the change of the activity. An incubation of cells with HDL enhanced the decline of cholesteryl-[l-14C]-oleate in foam cells and increased N-CEase activity in the supernatant of cell homogenate in a concentration-dependent manner, whereas incubation with LDL decreased the activity. In addition, N-CEase activity was fivefold higher when cells were cultured in 10% lipoprotein-deficient serum (LPDS) medium (2 micrograms cholesterol/mL) than when cultured in 10% fetal calf serum medium (31 micrograms cholesterol/mL), suggesting that changes in N-CEase activity are mediated by cholesterol. An addition of cholesterol (0 to 30 micrograms/mL) in LPDS medium markedly inhibited N-CEase activity with a concomitant increase in cellular cholesterol concentration. This inhibitory effect of cholesterol was also observed in mouse peritoneal macrophages. In vitro addition of cholesterol did not affect N-CEase activity. Treatment of cells with HMG-CoA reductase inhibitors enhanced N-CEase activity, whereas ACAT inhibitor decreased the activity. Northern blot analysis of N-CEase mRNA showed that the expression was not altered by the presence of cholesterol in LPDS medium. These results suggest that cholesterol downregulates N-CEase activity, probably through cholesterol-dependent appearance of some factors.

High-density lipoprotein-binding protein (HBP)/vigilin is expressed in human atherosclerotic lesions and colocalizes with apolipoprotein E

Accumulation of cholesteryl esters within cells of the arterial intima is a hallmark of atherosclerosis. A small number of proteins have been shown in vitro to be upregulated by cellular cholesterol loading, including apolipoprotein E (apoE) and the recently cloned HDL-binding protein (HBP), but only apoE has been shown to be upregulated in cholesterol-loaded cells in atherosclerosis. To determine whether HBP (also called vigilin) might be expressed in human atherosclerosis, immunohistochemistry and in situ hybridization were performed on coronary arteries of 18 patients. HBP/vigilin was detected on all endothelial cells. HBP/vigilin mRNA and protein also were detected on a subset of macrophages and occasionally on smooth muscle cells (SMC) in atherosclerotic plaques but were not detected on these cell types in nondiseased coronary intima. The majority of HBP/vigilin-expressing macrophages were foam cells, but HBP/vigilin expression also was detected rarely in nonfoam cell macrophages. Foam cell macrophage HBP/vigilin expression was present in 100% of atherosclerotic quadrants, and nonfoam cell macrophage HBP/vigilin expression was present in 6% of atherosclerotic quadrants. HBP/vigilin-expressing human plaque cells also expressed apoE. However, HBP/vigilin was detected in cardiac myocyte foam cells of an apoE-deficient mouse, demonstrating that HBP/vigilin expression can occur independently of apoE. These results suggest that in vivo HBP/vigilin expression is upregulated by intracellular cholesterol loading but also that other factors present in atherosclerotic plaques may upregulate HBP/vigilin. Although the exact function of HBP/vigilin is unknown, its expression in plaque macrophages suggests a role for this molecule in atherogenesis.

Design, synthesis and structure-activity relationship studies of novel 4,4-bis(trifluoromethyl)imidazolines as acyl-CoA: cholesterol acyltransferase (ACAT) inhibitors and antihypercholesterolemic agents

Novel 4,4-bis(trifluoromethyl)imidazolines have been found to be the potent acyl-CoA cholesterol acyltransferase (ACAT) inhibitors. ACAT is responsible for cholesterol esterification in the intestine, liver, and the arterial wall. These novel imidazolines also inhibit cholesterol ester formation in the macrophage. Several compounds have shown potent serum cholesterol-lowering activity in several animal models. Para-substitution of the 2-phenyl is critical for in vitro and in vivo activity. The 4,4-bis(trifluoromethyl)imidazolines with a para-cyano group on 2-phenyl and a 4-alkylcyclohexyl amide as the side-chain at the 5-position possess the most potent inhibitory activity in this series. Based on biochemical studies, this series acts as a competitive inhibitor with respect to cholesterol binding at the enzyme, which distinguishes it from most of the ACAT inhibitors discovered to date. Preliminary biological studies supported by X-ray crystal structures, molecular modeling, and structure-activity relationship (SAR) studies suggest that this series may be a cholesterol mimic.

Regulation of ApoE gene expression in mouse peritoneal macrophages by VLDL

Mouse peritoneal macrophages (MPM) were incubated with ApoE-poor VLDL or ApoE-rich VLDL at same concentrations for 24 h. The ApoE mRNA content increased in both groups than that in control and the highest ApoE mRNA content was seen in MPM incubated with ApoE-poor VLDL. The results suggest that VLDL could stimulate ApoE gene expression in MPM and the ApoE-poor VLDL has more pronounced effect. We think that the ApoE secreted by MPM may be incorporated into VLDL, especially the ApoE-poor VLDL, and thereby enhance the uptake of those lipoproteins by MPM or other local cells via ApoE-mediated receptor pathways.

Cyclic AMP induces apolipoprotein E binding activity and promotes cholesterol efflux from a macrophage cell line to apolipoprotein acceptors

RAW 264 mouse macrophage cells were stably transfected with human apolipoprotein E (apoE) expression vectors. Clonal derivatives were characterized for expression of the human apoE2, apoE3, and apoE4 isoforms. An apoE4-expressing clonal cell line and a non-expressing clonal control cell line were loaded overnight with either [3H]cholesterol or [3H]choline. The cells were washed and incubated for 24 h in serum-free medium with or without the addition of 8-bromo-cyclic AMP (8-Br-cAMP). Only the apoE-secreting cells and only in the presence of 8-Br-cAMP released large amounts of labeled cholesterol or phosphatidylcholine into the medium. Mass analyses of cellular free and esterified cholesterol confirmed the results of the labeling studies; a decrease in cellular cholesterol content was observed in the 8-Br-cAMP-treated apoE-secreting cells, concurrent with an increase in cholesterol found in the medium. FPLC analysis of the medium demonstrated that 8-Br-cAMP treatment of the apoE-secreting cells led to an increased size fraction and amount of a peak of secreted cholesterol which comigrated with apoE. The 8-Br-cAMP-mediated increase in cholesterol efflux was also observed in non-apoE-secreting cells incubated with exogenous apoE or apoAI, and the effect of apoE was saturable. The apoE2, apoE3, and apoE4 isoforms were equally efficient in promoting 8-Br-cAMP-dependent cholesterol efflux. Reductive methylation of apoE abolished its ability to promote 8-Br-cAMP-dependent cholesterol efflux. Brefeldin A and monensin, inhibitors of protein processing through the Golgi, both blocked the 8-Br-cAMP stimulation of cholesterol efflux to exogenous apoE. 8-Br-cAMP induced specific apoE and apoAI binding, but not apoE degradation, by the RAW cells. We present a model wherein cAMP induces a membrane apolipoprotein receptor that does not lead to endocytosis and degradation, but instead promotes the transfer of lipids to apolipoproteins, which can then be released from the cell.

Apolipoprotein E produced by human monocyte-derived macrophages mediates cholesterol efflux that occurs in the absence of added cholesterol acceptors

Human monocyte-derived macrophages can efflux accumulated cholesterol without exogenously added cholesterol acceptors (Kruth, H. S., Skarlatos, S. I., Gaynor, P. M., and Gamble, W. (1994) J. Biol. Chem. 269, 24511-24518). Most of the effluxed cholesterol accumulates in the medium as apolipoprotein E-discoidal lipid particles. In the current study, we determined whether and to what degree cholesterol efflux from human monocyte-macrophages depended on apolipoprotein E secretion. Unexpectedly, 2-week-old differentiated monocyte-macrophages secreted similar amounts of apolipoprotein E without or with cholesterol enrichment. Apolipoprotein E mRNA levels in these macrophages were not increased by cholesterol enrichment and were comparable with levels in HepG2 cells. Without cholesterol enrichment, monocyte-macrophages secreted lipid-poor apolipoprotein E with a density >1.21 g/ml. By contrast, cholesterol enrichment of monocyte-macrophages induced the association of apoE with phospholipid and cholesterol to form discoidal particles that floated at densities of 1.08-1.10 g/ml. An anti-apolipoprotein E monoclonal antibody added to the culture medium significantly inhibited cholesterol and phospholipid efflux from the monocyte-macrophages. This showed that apolipoprotein E was required for most of the cholesterol efflux, and that apolipoprotein E did not leave macrophages with lipid but rather associated with lipid after it was secreted. Thus, 1) apolipoprotein E was constitutively secreted by differentiated human monocyte-macrophages, 2) apolipoprotein E only formed discoidal particles following macrophage cholesterol enrichment, 3) apolipoprotein E was necessary for cholesterol efflux to occur in the absence of added cholesterol acceptors and, in addition 4) the level of macrophage unesterified cholesterol was not rate-limiting for this cholesterol efflux, and 5) net phospholipid synthesis occurred in macrophages secondary to apoE-mediated loss of macrophage phospholipid. In conclusion, apolipoprotein E functions in an autocrine pathway that mediates cholesterol efflux from human monocyte-derived macrophages.

Lymphocyte populations in atherosclerotic lesions of apoE -/- and LDL receptor -/- mice. Decreasing density with disease progression

Lymphocytes are prominent components of human atherosclerotic lesions, but their presence in murine models of disease has not been confirmed. Lymphocyte subpopulations have been identified in apoE -/- and LDL receptor -/- mice fed a cholesterol-enriched diet for up to 3 months. ApoE -/- mice had higher serum cholesterol concentrations than did LDL receptor -/- mice during most of the feeding period, primarily due to large increases in VLDL concentrations. Total area of atherosclerotic lesions was greater at all times in apoE -/- than LDL receptor -/- mice (lesion area after 3 months on cholesterol-enriched diet: apoE -/-, 993 +/- 193 and LDL receptor -/-, 560 +/- 131 microns2 x 10(3), mean +/- SEM, n = 6 in each group). Lesions in apoE -/- mice contained larger macrophage-rich necrotic cores and more calcification than did those in LDL receptor -/- mice. Immunocytochemical analyses of tissue sections of ascending aortas performed with monoclonal antibodies to T and B lymphocytes and macrophages revealed that T lymphocytes immunoreactive for Thy 1.2, CD5, CD4, and CD8 were observed in lesions from both strains, but no B lymphocytes were detected. The density of Thy 1.2+ T lymphocytes in lesions was greatest at 1 month (apoE -/-, 98 +/- 23 and LDL receptor -/-, 201 +/- 40 lymphocytes/mm2, n = 6 in each group), decreasing in apoE -/- mice to 12 +/- 3 and in LDL receptor -/- mice to 51 +/- 20 lymphocytes/mm2 at 3 months. The presence of T lymphocytes in murine atherosclerotic lesions makes these animals potentially useful for studying the involvement of the immune system in atherogenesis.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

The rat, a useful animal model for pharmacological studies on apolipoprotein E

Apolipoprotein E is a major protein component of lipoproteins and plays an important role in cholesterol transport. The structure of the gene and the polymorphism of apolipoprotein E have been studied in human and rat, which show similar structures of apolipoprotein E. The wide tissue distribution of this apolipoprotein suggests diverse functions like cholesterol distribution between cells, intracellular cholesterol trafficking and tissue reparation. Nevertheless, the presence of apolipoprotein E in atherosclerotic plaques and amyloid deposits in brains of Alzheimer's disease patients also indicate pathologic functions staying misunderstood. The aim of this paper is to review the present knowledge on the distribution of apolipoprotein E between the different organs with the related functions and to make an overview of the implications of this apolipoprotein is physiological events and pathological states in the rat. The rat is widely used for drug metabolism studies. Its serum levels are 5-10 times higher than in human and thus this animal provides an useful pharmacological model to elucidate the functions of apo E.

Characterization of an upstream regulatory sequence and its binding protein in the mouse apolipoprotein E gene

The mouse apolipoprotein (apo) E gene from strain C57BL/6 was isolated from a genomic DNA library and its complete nucleotide sequence, together with 1.3 kilobase of 5' flanking DNA and 300 base pairs of the 3' flanking DNA, was determined. Regulatory sequences in the proximal 5' flanking region of the gene were identified. Using a chloramphenicol acetyltransferase transient assay system, positive and negative cis-acting sequences were mapped within 380 base pairs of the 5' flanking region of the mouse apoE gene. Two nuclear protein binding sites were identified within this region by DNase I footprinting. We have characterized one of these regions, termed mouse apoE regulatory sequence (MARS-2), which spans nucleotides -151 to -133. Gel mobility shift assays using oligonucleotides of the MARS-2 sequence having specific deletions or substitutions as probes or competitors showed that the essential sequence of MARS-2 required for nuclear protein binding consists of 16 nucleotides encompassing -151 to -136. When nuclear extracts from different cells were examined, L cells and mouse liver nuclear protein contained the highest levels of binding protein for the MARS-2 probe. This protein, termed MARS-2 binding protein, was purified from mouse liver nuclear extracts to homogeneity using gel filtration and MARS-2 oligonucleotide-specific column chromatographic procedures. The Mr = 66,000 binding protein showed a gel mobility shift band that was identical to that of crude nuclear extracts.