Distinct Cellular Loci for the ABCA1-Dependent and ABCA1-Independent Lipid Efflux Mediated by Endogenous Apolipoprotein E Expression

APOE Alleles and Diet in Brain Aging and Alzheimer's Disease

The APOE gene alleles modify human aging and the response to the diet at many levels with diverse pleotropic effects from gut to brain. To understand the interactions of APOE isoforms and diet, we analyze how cellular trafficking of apoE proteins affects energy metabolism, the immune system, and reproduction. The age-accelerating APOE4 allele alters the endosomal trafficking of cell surface receptors that mediate lipid and glucose metabolism. The APOE4 allele is the ancestral human allele, joined by APOE3 and then APOE2 in the human species. Under conditions of high infection, uncertain food, and shorter life expectancy, APOE4 may be adaptive for reducing mortality. As humans transitioned into modern less-infectious environments and longer life spans, APOE4 increased risks of aging-related diseases, particularly impacting arteries and the brain. The association of APOE4 with glucose dysregulation and body weight promotes many aging-associated diseases. Additionally, the APOE gene locus interacts with adjacent genes on chromosome 19 in haplotypes that modify neurodegeneration and metabolism, for which we anticipate complex gene-environment interactions. We summarize how diet and Alzheimer's disease (AD) risk are altered by APOE genotype in both animal and human studies and identify gaps. Much remains obscure in how APOE alleles modify nutritional factors in human aging. Identifying risk variant haplotypes in the APOE gene complex will clarify homeostatic adaptive responses to environmental conditions.

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.

LAL (Lysosomal Acid Lipase) Promotes Reverse Cholesterol Transport In Vitro and In Vivo

OBJECTIVE

To explore the role of LAL (lysosomal acid lipase) in macrophage cholesterol efflux and whole-body reverse cholesterol transport.

APPROACH AND RESULTS

Immortalized peritoneal macrophages from lal^-/- mice showed reduced expression of ABCA1 (ATP-binding cassette transporter A1) and ABCG1 (ATP-binding cassette transporter G1), reduced production of the regulatory oxysterol 27-hydroxycholesterol, and impaired suppression of cholesterol synthesis on exposure to acetylated low-density lipoprotein when compared with lal^+/+ macrophages. LAL-deficient mice also showed reduced hepatic ABCG5 (ATP-binding cassette transporter G5) and ABCG8 (ATP-binding cassette transporter G8) expression compared with lal^+/+ mice. LAL-deficient macrophages loaded with [³H]-cholesteryl oleate-labeled acetylated low-density lipoprotein showed impaired efflux of released [³H]-cholesterol to apoA-I (apolipoprotein A-I), with normalization of [³H]-cholesteryl ester levels and partial correction of ABCA1 expression and cholesterol efflux to apoA-I when treated with exogenous rhLAL (recombinant human LAL protein). LAL-deficient mice injected intraperitoneally with lal^-/- macrophages cholesterol loaded and labeled in the same way exhibited only 1.55±0.35% total injected [³H]-cholesterol counts appearing in the feces for 48 h (n=30), compared with 5.38±0.92% in lal^+/+ mice injected with labeled lal^+/+ macrophages (n=27), P<0.001. To mimic the therapeutic condition of delivery of supplemental LAL in vivo, injection of labeled lal^-/- macrophages into lal^+/+ mice resulted in a significant increase in reverse cholesterol transport (2.60±0.46% of ³H-cholesterol counts in feces at 48 hours [n=19]; P<0.001 when compared with injection into lal^-/- mice).

CONCLUSIONS

These results indicate a critical role for LAL in promoting both macrophage and whole-body reverse cholesterol transport and the ability of supplemental LAL to be taken up and correct reverse cholesterol transport in vivo.

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.

Apolipoprotein E gene polymorphism influences aggressive behavior in prostate cancer cells by deregulating cholesterol homeostasis

High circulating cholesterol and its deregulated homeostasis may facilitate prostate cancer progression. Genetic polymorphism in Apolipoprotein (Apo) E, a key cholesterol regulatory protein may effect changes in systemic cholesterol levels. In this investigation, we determined whether variants of the Apo E gene can trigger defective intracellular cholesterol efflux, which could promote aggressive prostate cancer. ApoE genotypes of weakly (non-aggressive), moderate and highly tumorigenic (aggressive) prostate cancer cell lines were characterized, and we explored whether the ApoE variants were associated with tumor aggressiveness generated by intra cellular cholesterol imbalance, using the expression of caveolin-1 (cav-1), a pro-malignancy surrogate of cholesterol overload. Restriction isotyping of ApoE isoforms revealed that the non-aggressive cell lines carried ApoE ε3/ε3 or ε3/ε4 alleles, while the aggressive cell lines carried the Apoε2/ε4 alleles. Our data suggest a contrast between the non-aggressive and the aggressive prostate cancer cell lines in the pattern of cholesterol efflux and cav-1 expression. Our exploratory results suggest a relationship between prostate aggressiveness, ApoE isoforms and cholesterol imbalance. Further investigation of this relationship may elucidate the molecular basis for considering cholesterol as a risk factor of aggressive prostate tumors, and underscore the potential of the dysfunctional ApoE2/E4 isoform as a biomarker of aggressive disease.

Effect of repeated apoA-IMilano/POPC infusion on lipids, (apo)lipoproteins, and serum cholesterol efflux capacity in cynomolgus monkeys

MDCO-216, a complex of dimeric recombinant apoA-IMilano (apoA-IM) and palmitoyl-oleoyl-phosphatidylcholine (POPC), was administered to cynomolgus monkeys at 30, 100, and 300 mg/kg every other day for a total of 21 infusions, and effects on lipids, (apo)lipoproteins, and ex-vivo cholesterol efflux capacity were monitored. After 7 or 20 infusions, free cholesterol (FC) and phospholipids (PL) were strongly increased, and HDL-cholesterol (HDL-C), apoA-I, and apoA-II were strongly decreased. We then measured short-term effects on apoA-IM, lipids, and (apo)lipoproteins after the first or the last infusion. After the first infusion, PL and FC went up in the HDL region and also in the LDL and VLDL regions. ApoE shifted from HDL to LDL and VLDL regions, while ApoA-IM remained located in the HDL region. On day 41, ApoE levels were 8-fold higher than on day 1, and FC, PL, and apoE resided mostly in LDL and VLDL regions. Drug infusion quickly decreased the endogenous cholesterol esterification rate. ABCA1-mediated cholesterol efflux on day 41 was markedly increased, whereas scavenger receptor type B1 (SRB1) and ABCG1-mediated effluxes were only weakly increased. Strong increase of FC is due to sustained stimulation of ABCA1-mediated efflux, and drop in HDL and formation of large apoE-rich particles are due to lack of LCAT activation.

APOE genotype modifies the association between plasma omega-3 fatty acids and plasma lipids in the Multi-Ethnic Study of Atherosclerosis (MESA)

OBJECTIVE

The benefits of fish oil fatty acids eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) on plasma lipid profiles have been inconsistent but may partially depend on individual Apolipoprotein E (APOE) genotypes. We aimed to determine whether APOE genotype modifies the association of lipid profile characteristics with plasma EPA and DHA levels.

METHODS

APOE genotype was determined in this cross-sectional analysis of 2340 Multi-Ethnic Study of Atherosclerosis (MESA) participants. Relative plasma phospholipid EPA and DHA levels, plasma lipids, and lipoprotein subclass particle sizes and concentrations were measured.

RESULTS

Significant gene-EPA interactions were found with HDL-C, and particle concentrations of large and total HDL (p(interaction) = 0.0002, 0.006, and 0.007, respectively). The above lipid targets were positively associated with EPA in the E2 groups, whereas negative trends were observed among the E4 participants. Gene-DHA interactions were noted for small LDL particle concentrations alone (p(interaction) = 0.01), where a positive trend was found among E4 but not E2 or E3 participants.

CONCLUSIONS

These results indicate a significant contribution of the APOE genotype to the EPA-lipid profile relationship; however, the results do not explain the differences in previous findings regarding LDL-C, triglycerides or total cholesterol. Future investigators examining the effects of EPA on HDL-C or lipoprotein characteristics may consider including APOE genotype in their analyses.

Cholesterol accumulation inhibits ER to Golgi transport and protein secretion: studies of apolipoprotein E and VSVGt

Cholesterol excess is typical of various diseases including atherosclerosis. We have investigated whether cholesterol accumulation in the ER (endoplasmic reticulum) can inhibit exit of vesicular cargo and secretion of proteins by studying apoE (apolipoprotein E), a significant glycoprotein in human health and disease. CHO (Chinese hamster ovary) cells expressing human apoE under a cholesterol-independent promoter incubated with cholesterol-cyclodextrin complexes showed increased levels of cellular free and esterified cholesterol, inhibition of SREBP-2 (sterol-regulatory-element-binding protein 2) processing, and a mild induction of ER stress, indicating significant accumulation of cholesterol in the ER. Secretion of apoE was markedly inhibited by cholesterol accumulation, and similar effects were observed in cells enriched with lipoprotein-derived cholesterol and in primary human macrophages. Removal of excess cholesterol by a cyclodextrin vehicle restored apoE secretion, indicating that the transport defect was reversible. That cholesterol impaired protein trafficking was supported by the cellular accumulation of less sialylated apoE glycoforms, and by direct visualization of altered ER to Golgi transport of thermo-reversible VSVG (vesicular stomatitis virus glycoprotein) linked to GFP (green fluorescent protein). We conclude that intracellular accumulation of cholesterol in the ER reversibly inhibits protein transport and secretion. Strategies to correct ER cholesterol may restore homoeostatic processes and intracellular protein transport in conditions characterized by cholesterol excess.

ApoE derived from adipose tissue does not suppress atherosclerosis or correct hyperlipidemia in apoE knockout mice

The synthesis of apoE by adipocytes has profound effects on adipose tissue lipid flux and gene expression. Using adipose tissue transplantation from wild-type (WT) to apoE knockout (EKO) mice, we show that adipose tissue also contributes to circulating apoE. Different from circulating apoE produced by bone marrow transplantation (BMT), however, adipose tissue-derived apoE does not correct hyperlipidemia or suppress atherosclerosis. ApoE secreted by macrophages has a more acidic isoform distribution, and it increases binding of reconstituted VLDL particles to hepatocytes and fibroblasts more effectively than apoE secreted by adipocytes. The incremental binding can be entirely accounted for by binding to the LDL receptor. After BMT into EKO hosts, plasma cholesterol and macrophage-derived apoE are largely within IDL/LDL- and HDL-sized particles. After adipose tissue transplantation, most cholesterol and adipocyte apoE remain in VLDL. After BMT, circulating apoE no longer demonstrates predominance of acidic isoforms compared with that circulating after fat transplantation. In conclusion, fat transplantation provides circulating apoE levels similar to those provided by bone marrow transplantation, but it does not suppress hyperlipidemia or atherosclerosis. A potential mechanism contributing to this difference is differential binding to cell surface lipoprotein receptors.

Apolipoprotein E4 is deficient in inducing macrophage ABCA1 expression and stimulating the Sp1 signaling pathway

ATP binding cassette A1 (ABCA1) is a membrane protein that promotes cellular cholesterol efflux. Using RAW 264.7 macrophages, we studied the relative effects of apolipoprotein (apo) E3 and apoE4 on ABCA1 and on the signaling pathway that regulates its expression. Both lipid-associated and lipid-free apoE4 forms induced ∼30% lower levels of ABCA1 protein and mRNA than apoE3 forms. Phosphorylated levels of phosphoinositol 3-kinase (PI3K), protein kinase Cζ (PKCζ) and specificity protein 1 (Sp1) were also lower when treated with apoE4 compared to apoE3. The reduced ability of apoE4 to induce ABCA1 expression, PKCζ and Sp1 phosphorylation were confirmed in human THP-1 monocytes/macrophages. Sequential phosphorylation of PI3K, PKCζ and Sp1 has been suggested as a mechanism for upregulation of ABCA1 expression. Both apoE3 and apoE4 reduced total cholesterol and cholesterol esters in lipid-laden RAW 264.7 cells, and induced apoAI-mediated cholesterol efflux. However, the cholesterol esters and cholesterol efflux in apoE4-treated cells were ∼50% and ∼24% lower, respectively, compared to apoE3-treated cells. Accumulation of cholesterol esters in macrophages is a mechanism for foam cell formation. Thus the reduced ability of apoE4 to activate the PI3K-PKCζ-Sp1 signaling pathway and induce ABCA1 expression likely impairs cholesterol ester removal, and increases foam cell formation.

Regulation of reverse cholesterol transport - a comprehensive appraisal of available animal studies

Plasma levels of high density lipoprotein (HDL) cholesterol are strongly inversely correlated to the risk of atherosclerotic cardiovascular disease. A major recognized functional property of HDL particles is to elicit cholesterol efflux and consequently mediate reverse cholesterol transport (RCT). The recent introduction of a surrogate method aiming at determining specifically RCT from the macrophage compartment has facilitated research on the different components and pathways relevant for RCT. The current review provides a comprehensive overview of studies carried out on macrophage-specific RCT including a quick reference guide of available data. Knowledge and insights gained on the regulation of the RCT pathway are summarized. A discussion of methodological issues as well as of the respective relevance of specific pathways for RCT is also included.

Apolipoprotein E4 domain interaction accelerates diet-induced atherosclerosis in hypomorphic Arg-61 apoe mice

OBJECTIVE

Apolipoprotein (apo) E4 is an established risk factor for atherosclerosis, but the structural components underlying this association remain unclear. ApoE4 is characterized by 2 biophysical properties: domain interaction and molten globule state. Substituting Arg-61 for Thr-61 in mouse apoE introduces domain interaction without molten globule state, allowing us to delineate potential proatherogenic effects of domain interaction in vivo.

METHODS AND RESULTS

We studied atherosclerosis susceptibility of hypomorphic Apoe mice expressing either Thr-61 or Arg-61 apoE (ApoeT(h/h) or ApoeR(h/h)mice). On a chow diet, both mouse models were normolipidemic with similar levels of plasma apoE and lipoproteins. However, on a high-cholesterol diet, ApoeR(h/h) mice displayed increased levels of total plasma cholesterol and very-low-density lipoprotein as well as larger atherosclerotic plaques in the aortic root, arch, and descending aorta compared with ApoeT(h/h) mice. In addition, evidence of cellular dysfunction was identified in peritoneal ApoeR(h/h) macrophages which released lower amounts of apoE in culture medium and displayed increased expression of major histocompatibility complex class II molecules.

CONCLUSIONS

These data indicate that domain interaction mediates proatherogenic effects of apoE4 in part by modulating lipoprotein metabolism and macrophage biology. Pharmaceutical targeting of domain interaction could lead to new treatments for atherosclerosis in apoE4 individuals.

ABCA1 promotes the efflux of bacterial LPS from macrophages and accelerates recovery from LPS-induced tolerance

Macrophages play important roles in both lipid metabolism and innate immunity. We show here that macrophage ATP-binding cassette transporter A1 (ABCA1), a transporter known for its ability to promote apolipoprotein-dependent cholesterol efflux, also participates in the removal of an immunostimulatory bacterial lipid, lipopolysaccharide (LPS). Whereas monocytes require an exogenous lipoprotein acceptor to remove cell-associated LPS, macrophages released LPS in the absence of an exogenous acceptor by a mechanism that was driven, in part, by endogenous apolipoprotein E (apoE). Agents that increased ABCA1 expression increased LPS efflux from wild-type but not ABCA1-deficient macrophages. Preexposure of peritoneal macrophages to LPS for 24 h increased the expression of ABCA1 and increased LPS efflux with a requirement for exogenous apolipoproteins due to suppression of endogenous apoE production. In contrast, LPS preconditioning of ABCA1-deficient macrophages significantly decreased LPS efflux and led to prolonged retention of cell-surface LPS. Although the initial response to LPS was similar in wild-type and ABCA1-deficient macrophages, LPS-induced tolerance was greater and more prolonged in macrophages that lacked ABCA1. Our results define a new role for macrophage ABCA1 in removing cell-associated LPS and restoring normal macrophage responsiveness.

Apolipoprotein E inhibits toll-like receptor (TLR)-3- and TLR-4-mediated macrophage activation through distinct mechanisms

Previous studies have shown that apoE (apolipoprotein E) expression in macrophages suppresses inflammatory responses; however, whether endogenously synthesized apoE acts intracellularly or after its secretion in suppressing macrophage inflammation remains unclear. The present study used the murine monocyte macrophage cell line RAW 264.7 to examine the influence of exogenous apoE on macrophage inflammatory responses induced by TLR (Toll-like receptor)-4 and TLR-3 agonists LPS (lipopolysaccharide) and poly(I-C) respectively. Results showed that exogenously added apoE suppressed the LPS and poly(I-C) induction of IL (interleukin)-6, IL-1beta and TNF-alpha (tumour necrosis factor-alpha) secretion by RAW 264.7 cells. The mechanism was related to apoE suppression of TLR-agonist-induced phosphorylation of JNK (c-Jun N-terminal kinase) and c-Jun. A peptide containing the tandem repeat sequence of the receptor-binding domain of apoE, apoE-(141-155)2, was similarly effective in inhibiting LPS- and poly(I-C)-induced macrophage inflammatory responses. Reductive methylation of lysine residues in apoE, which abolished its receptor-binding capability without affecting its ability to interact with HSPGs (heparin sulfate proteoglycans), inhibited the ability of apoE to suppress macrophage responses to LPS, but had no effect on apoE suppression of poly(I-C)-induced macrophage activation. The ability of apoE to suppress poly(I-C)-induced pro-inflammatory cytokine production was abolished by heparinase treatment of RAW 264.7 cells to remove cell-surface HSPGs. Taken together, these results indicate that exogenous apoE inhibits macrophage inflammatory responses to TLR-4 and TLR-3 agonists through distinct mechanisms related to receptor and HSPG binding respectively, and that these inhibitory effects converged on suppression of JNK and c-Jun activation which are necessary for macrophage activation.

High density lipoprotein structure-function and role in reverse cholesterol transport

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

Anti-inflammatory and recycling properties of an apolipoprotein mimetic peptide, Ac-hE18A-NH(2)

Apolipoprotein E (apoE) exerts prominent anti-inflammatory effects and undergoes recycling by target cells. We previously reported that the peptide Ac-hE18A-NH(2), composed of the receptor binding domain (LRKLRKRLLR) of apoE covalently linked to the Class A amphipathic peptide 18A, dramatically lowers plasma cholesterol and lipid hydroperoxides and enhances paraoxonase activity in dyslipidemic animal models. The objective of this study was to determine whether this peptide, analogous to apoE, exerts anti-inflammatory effects and undergoes recycling under in vitro conditions. Pulse chase studies using [(125)I]-Ac-hE18A-NH(2) in THP-1 derived macrophages and HepG2 cells showed greater amounts of intact peptide in the cells at later time points indicating recycling of the peptide. Ac-hE18A-NH(2) induced a 2.5-fold increase in prebeta-HDL in the conditioned media of HepG2 cells. This effect persisted for 3 days after removal of the peptide from culture medium. Ac-hE18A-NH(2) also induced the secretion of cell surface apoE from THP-1 macrophages. In addition, the peptide increased cholesterol efflux from THP-1 cells by an ABCA1 independent mechanism. Moreover, Ac-hE18A-NH(2) inhibited LPS-induced vascular cell adhesion molecule-1 (VCAM-1) expression, and reduced monocyte adhesion in human umbilical vein endothelial cells (HUVECs). It also reduced the secretion of interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) from THP-1 macrophages even when administered post-LPS and abolished the 18-fold increase in LPS-induced mRNA levels for MCP-1 in THP-1 cells. Taken together, these results suggest that addition of the putative apoE receptor-domain to the Class A amphipathic peptide 18A results in a peptide that, similar to apoE, recycles, thus enabling the potentiation and prolongation of its anti-atherogenic and anti-inflammatory effects. Such a peptide has great potential as a therapeutic agent in the management of atherosclerosis and other inflammatory diseases.

Inhibition of ERK1/2 and activation of liver X receptor synergistically induce macrophage ABCA1 expression and cholesterol efflux

ATP-binding cassette transporter A1 (ABCA1), a molecule mediating free cholesterol efflux from peripheral tissues to apoAI and high density lipoprotein (HDL), inhibits the formation of lipid-laden macrophage/foam cells and the development of atherosclerosis. ERK1/2 are important signaling molecules regulating cellular growth and differentiation. The ERK1/2 signaling pathway is implicated in cardiac development and hypertrophy. However, the role of ERK1/2 in the development of atherosclerosis, particularly in macrophage cholesterol homeostasis, is unknown. In this study, we investigated the effects of ERK1/2 activity on macrophage ABCA1 expression and cholesterol efflux. Compared with a minor effect by inhibition of other kinases, inhibition of ERK1/2 significantly increased macrophage cholesterol efflux to apoAI and HDL. In contrast, activation of ERK1/2 reduced macrophage cholesterol efflux and ABCA1 expression. The increased cholesterol efflux by ERK1/2 inhibitors was associated with the increased ABCA1 levels and the binding of apoAI to cells. The increased ABCA1 by ERK1/2 inhibitors was due to increased ABCA1 mRNA and protein stability. The induction of ABCA1 expression and cholesterol efflux by ERK1/2 inhibitors was concentration-dependent. The mechanism study indicated that activation of liver X receptor (LXR) had little effect on ERK1/2 expression and activation. ERK1/2 inhibitors had no effect on macrophage LXRalpha/beta expression, whereas they did not influence the activation or the inhibition of the ABCA1 promoter by LXR or sterol regulatory element-binding protein (SREBP). However, inhibition of ERK1/2 and activation of LXR synergistically induced macrophage cholesterol efflux and ABCA1 expression. Our data suggest that ERK1/2 activity can play an important role in macrophage cholesterol trafficking.

Atherogenic lipids induce high-density lipoprotein uptake and cholesterol efflux in human macrophages by up-regulating transmembrane chemokine CXCL16 without engaging CXCL16-dependent cell adhesion

Atherosclerosis is a complex pathologic process in which chemokine-mediated leukocyte accumulation in arterial walls is thought to be an important mechanism of pathogenesis. An interesting exception to this paradigm is the chemokine CXCL16, also known as the scavenger receptor for phosphatidylserine and oxidized low density lipoprotein, which is highly expressed in mouse and human atherosclerotic lesions, yet appears to be atheroprotective. In this study, we address potential mechanisms responsible for this activity. Consistent with its presence in atherosclerotic plaque, we found that atherogenic lipids up-regulated CXCL16 in primary human monocyte-derived macrophages. However, the same lipids down-regulated the CXCL16-targeted protease ADAM10, resulting in preferential expression of CXCL16 as the transmembrane form, not the shed form. Although transmembrane CXCL16 is known to mediate cell-cell adhesion by binding its receptor CXCR6, and atherogenic lipids are known to stimulate macrophage adhesion to coronary artery smooth muscle cells, we found that heterotypic adhesion of these cell types occurred in a CXCL16-independent manner. Instead we found that in macrophages, CXCL16 promoted internalization of both oxidized low density lipoprotein and high density lipoprotein, as well as release of cholesterol. Moreover, CXCL16 deficiency in macrophages interfered with oxidized low density lipoprotein-induced up-regulation of atheroprotective genes: adenosine triphosphate-binding cassette transporter A1 and G1 as well as apolipoprotein E. Thus, our findings support the hypothesis that CXCL16 mediates atheroprotection through its scavenger role in macrophages and not by cell-cell adhesion.

Altered expression of genes functioning in lipid homeostasis is associated with lipid deposition in NOD mouse lacrimal gland

Functional atrophy and accompanying lymphocytic infiltration and destruction of the lacrimal gland (LG) are characteristics of Sjögren's Syndrome (SjS). The male NOD mouse is an experimental model for the autoimmune exocrinopathy that develops in the LG of SjS patients. Acinar cells in LG of male NOD mice aged 3-4 months were previously shown to accumulate lipid droplets. In the current study, analysis of lipid components revealed that the accumulated lipids were mostly cholesteryl esters (CE). Gene expression microarray analysis followed by real-time RT-PCR revealed alterations in the expression of several genes involved in lipid homeostasis in LG of 12-week-old male NOD mice relative to matched BALB/c controls. A series of upregulated genes including apolipoprotein E, apolipoprotein F, hepatic lipase, phosphomevalonate kinase, ATP-binding cassette D1 and ATP-binding cassette G1 were identified. Comparison of liver mRNAs to LG mRNAs in BALB/c and NOD mice revealed that the differential expressions were LG-specific. Gene expression profiles were also characterized in LGs of female mice, younger mice and immune-incompetent NOD SCID mice. Investigation of the cellular distribution of Apo-E and Apo-F proteins suggested that these proteins normally coordinate to mediate lipid efflux from the acinar cells but that dysfunction of these processes due to missorting of Apo-F may contribute to CE deposition. Finally, the initiation and extent of lipid deposition were correlated with lymphocytic infiltration in the LG of male NOD mice. We propose that impaired lipid efflux contributes to lipid deposition, an event that may contribute to the development and/or progression of dacryoadenitis in the male NOD mouse.

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.

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.

Combined deficiency of ABCA1 and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in mice

HDLs protect against the development of atherosclerosis, but the underlying mechanisms are poorly understood. HDL and its apolipoproteins can promote cholesterol efflux from macrophage foam cells via the ATP-binding cassette transporters ABCA1 and ABCG1. Experiments addressing the individual roles of ABCA1 and ABCG1 in the development of atherosclerosis have produced mixed results, perhaps because of compensatory upregulation in the individual KO models. To clarify the role of transporter-mediated sterol efflux in this disease process, we transplanted BM from Abca1(-/-)Abcg1(-/-) mice into LDL receptor-deficient mice and administered a high-cholesterol diet. Compared with control and single-KO BM recipients, Abca1(-/-)Abcg1(-/-) BM recipients showed accelerated atherosclerosis and extensive infiltration of the myocardium and spleen with macrophage foam cells. In experiments with isolated macrophages, combined ABCA1 and ABCG1 deficiency resulted in impaired cholesterol efflux to HDL or apoA-1, profoundly decreased apoE secretion, and increased secretion of inflammatory cytokines and chemokines. In addition, these cells showed increased apoptosis when challenged with free cholesterol or oxidized LDL loading. These results suggest that the combined effects of ABCA1 and ABCG1 in mediating macrophage sterol efflux are central to the antiatherogenic properties of HDL.

Nutritional regulation of adipose tissue apolipoprotein E expression

Apolipoprotein E (apoE) is a multifunctional protein that is highly expressed in human and murine adipose tissue. Endogenous adipocyte apoE expression influences adipocyte triglyceride turnover and modulates the expression of genes involved in lipid synthesis and oxidation. We now demonstrate the regulation of adipose tissue apoE expression by nutritional status in lean and obese mice. Obesity induced by high-fat diet, or by hyperphagia in ob/ob mice, produces significant reduction of adipose tissue apoE expression at the protein and messenger RNA level. Fasting in C57BL/6J mice for 24 h significantly increased apoE protein and messenger RNA levels. In ob/ob mice, transplantation of adipose tissue from lean littermate controls to restore circulating leptin levels produced significant weight loss over 12 wk and also produced an increase in adipose tissue apoE expression. The increase in adipose tissue apoE expression in this model, however, did not require leptin. Adipose tissue apoE was also significantly increased in ob/ob mice after a 48-h fast or after 7 days of caloric restriction. In summary, obesity suppresses adipose tissue apoE expression, whereas fasting or weight loss increases it. From our previous observations, these changes in adipose tissue apoE expression will have significant impact on adipose tissue lipid flux and lipoprotein metabolism. Furthermore, these results suggest adipose tissue apoE participates in defending adipose tissue and organismal energy homeostasis in response to nutritional perturbation.

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.

Decreased atherosclerosis in low-density lipoprotein receptor knockout mice transplanted with Abcg1-/- bone marrow

OBJECTIVE

Recent studies indicate that the ATP-binding cassette transporter ABCG1 can promote cholesterol efflux from macrophages to high-density lipoprotein. This study was designed to assess the in vivo role of macrophage ABCG1 in atherosclerosis.

METHODS AND RESULTS

Bone marrow from Abcg1-/- mice was transplanted into irradiated Ldlr-/- recipients, and atherosclerosis was evaluated by aortic root assay after 7 or 11 weeks of feeding on a Western diet. After 7 weeks, there was no difference in lesion area in mice receiving either wild-type or Abcg1-/- bone marrow, whereas after 11 weeks, lesion area was moderately but significantly reduced in Abcg1-/- recipients. ABCG1-deficient peritoneal macrophages showed induction of several liver X receptor target genes, such as Abca1 and Srebp1c, and a dramatic increase in apolipoprotein E (apoE) protein both in cell media and lysates, without parallel change in apoE mRNA. Abca1 knockdown prevented the increase in apoE secretion but had minimal effects on apoE accumulation in cell lysates of Abcg1-/- macrophages. Plasma apoE levels were markedly increased in recipients of Abcg1-/- bone marrow.

CONCLUSIONS

These studies reveal an inverse relationship between Abcg1 expression and apoE accumulation and secretion in macrophages. The reduced atherosclerosis in recipients of Abcg1-deficient bone marrow may be explained by induction of Abca1 and an associated increase in macrophage apoE secretion.

Lysophosphatidylcholine promotes cholesterol efflux from mouse macrophage foam cells via PPARγ-LXRα-ABCA1-dependent pathway associated with apoE

Formation of macrophage-derived foam cells is a hallmark in earlier stages of atherosclerosis (AS). Increased cholesterol efflux from macrophage foam cells promote atherosclerotic regression. In the present study, lysophosphatidylcholine (LPC) promoting cholesterol efflux from macrophage foam cells was observed, and the mechanism underlying the action was investigated. Macrophage foam cells from mice were incubated with different concentrations of LPC (10, 20, 40, 80 microM), and the free cholesterol in medium increased but total intracellular cholesterol decreased. At the same time, the expression of PPARgamma, LXRalpha, ABCA1 was enhanced in a dose-dependent manner. The treatment of macrophage foam cells with 40 microM LPC for 12, 24 and 48 h promoted cellular cholesterol efflux in a time-dependent manner, meanwhile expression of PPARgamma, LXRalpha, ABCA1 was also raised respectively. Addition of different specific inhibitors of PPARgamma (GW9662), LXRalpha (GGPP), ABCA1 (DIDS) to the foam cells significantly suppressed LPC-induced cholesterol efflux. Also treatment with specific inhibitors of PPARgamma or LXRalpha decreased ABCA1 mRNA and protein expressions. LPC (40 microM)-induced cholesterol efflux was significantly lower in macrophage foam cells from apoE deficient mice than from normal C57BL/6J mice. In contrast, 10 microg apoAI-induced cholesterol efflux from foam cells remained in apoE deficient mice. The present results indicate that LPC promotes cholesterol efflux from macrophage foam cells via a PPARgamma-LXRalpha-ABCA1-dependent pathway. Furthermore, apoE may be involved in this process.