Transport and processing of endogenously synthesized ApoE on the macrophage cell surface

Inhibition of Sirt2 Decreases ApoE Secretion in Astrocytes and Microglial Cells

Amyloid-β (Aβ) accumulation caused by an imbalance of the production and clearance of Aβ in the brain is associated with the development of Alzheimer's disease (ad). Apolipoprotein E (ApoE) (the strongest genetic risk factor) enhances Aβ clearance, preventing Aβ deposition. Sirtuin 2 (Sirt2) is an NAD+-dependent histone deacetylase and its inhibition has been reported to ameliorate memory impairment in ad-like model mice. However, the role of Sirt2 in ApoE secretion is unknown. Here, we found that inhibition of Sirt2 activity in primary cultured astrocytes and BV2 cells decreased ApoE secretion, resulting in the accumulation of intracellular ApoE and inhibiting extracellular Aβ degradation. However, the reduction of Sirt2 protein level by Sirt2 siRNA decreased ApoE protein level, which ultimately reduces ApoE secretion. In addition, the knockdown of Sirt2 in the HEK293-APP cells also decreased levels of intracellular ApoE leading to reduction of its secretion, which is accompanied by increased Aβ levels without altering APP and APP processing enzymes. Our findings provide a novel role of Sirt2 in ApoE secretion.

Impact of protein glycosylation on lipoprotein metabolism and atherosclerosis

Protein glycosylation is a post-translational modification consisting in the enzymatic attachment of carbohydrate chains to specific residues of the protein sequence. Several types of glycosylation have been described, with N-glycosylation and O-glycosylation being the most common types impacting on crucial biological processes, such as protein synthesis, trafficking, localization, and function. Genetic defects in genes involved in protein glycosylation may result in altered production and activity of several proteins, with a broad range of clinical manifestations, including dyslipidaemia and atherosclerosis. A large number of apolipoproteins, lipoprotein receptors, and other proteins involved in lipoprotein metabolism are glycosylated, and alterations in their glycosylation profile are associated with changes in their expression and/or function. Rare genetic diseases and population genetics have provided additional information linking protein glycosylation to the regulation of lipoprotein metabolism.

Cell-specific production, secretion, and function of apolipoprotein E

Apolipoprotein E (apoE) is a 34-kDa glycoprotein that is secreted from many cells throughout the body. ApoE is best known for its role in lipoprotein metabolism. Recent studies underline the association of circulating lipoprotein-associated apoE levels and the development for cardiovascular disease (CVD). Besides its well-established role in pathology of CVD, it is also implicated in neurodegenerative diseases and recent new data on adipose-produced apoE point to a novel metabolic role for apoE in obesity. The regulation of apoE production and secretion is remarkably cell and tissue specific. Here, we summarize recent insights into the differential regulation apoE production and secretion by hepatocytes, monocytes/macrophages, adipocytes, and the central nervous system and relevant variations in apoE biochemistry and function.

ApoA-I/HDL Generation and Intracellular Cholesterol Transport through Cytosolic Lipid-Protein Particles in Astrocytes

Exogenous apolipoprotein A-I (apoA-I) associates with ATP-binding cassette transporter A1 (ABCA1) on the cell surface of astrocytes like various peripheral cells and enhances the translocation of newly synthesized cholesterol from the endoplasmic reticulum/Golgi apparatus (ER/Golgi) to the cytosol. The cholesterol translocated to the cytosol is incorporated to cytosolic lipid-protein particles (CLPP) together with phospholipids and proteins such as sphingomyelin, phosphatidylcholine, caveolin-1, protein kinase Cα (PK-Cα), and cyclophilin A. The CLPP are high density lipoproteins- (HDL-)like cytosolic lipid-protein complex with densities of 1.09–1.16 g/mL and diameters of 17-18 nm. The association of exogenous apoA-I with cellular ABCA1 induces tyrosine phosphorylation, activation, and translocation to the CLPP of ABCA1-associated phospholipase Cγ (PL-Cγ) in rat astrocytes. Furthermore, PK-Cα is translocated and activated to/in the CLPP through theproduction of diacylglyceride in the CLPP. ApoA-I enhances both the association of CLPP with microtubules and the phosphorylation of α-tubulin as a component of microtubules. The CLPP are dissociated from microtubules after α-tubulin in microtubules is phosphorylated by the CLPP-associated PK-Cα. The association and dissociation between CLPP and microtubules may participate in the intracellular transport of cholesterol to the plasma membrane.

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.

Glycosylation and sialylation of macrophage-derived human apolipoprotein E analyzed by SDS-PAGE and mass spectrometry: evidence for a novel site of glycosylation on Ser290

Apolipoprotein E (apoE) is a 34-kDa glycoprotein secreted from various cells including hepatocytes and macrophages and plays an important role in remnant lipoprotein clearance, immune responses, Alzheimer disease, and atherosclerosis. Cellular apoE and plasma apoE exist as multiple glycosylated and sialylated glycoforms with plasma apoE being less glycosylated/sialylated than cell-derived apoE. Some of the glycan structures on plasma apoE are characterized; however, the more complicated structures on plasma and cellular/secreted apoE remain unidentified. We investigated glycosylation and sialylation of cellular and secreted apoE from primary human macrophages by one- and two-dimensional gel electrophoresis and mass spectrometry. Our results identify eight different glycoforms with (HexNAc)(2)-Hex(2)-(NeuAc)(2) being the most complex glycan detected on Thr(194) in both cellular and secreted apoE. Four additional glycans were identified on apoE(283-299), and using beta-elimination/alkylation by methylamine in vitro, we identified Ser(290) as a novel site of glycan attachment. Comparison of plasma and cellular/secreted apoE from the same donor confirmed that cell-derived apoE is more extensively sialylated than plasma apoE. Given the importance of the C terminus of apoE in regulating apoE solubility, stability, and lipid binding, these results may have important implications for our understanding of apoE biochemistry.

Mechanism for endogenously expressed ApoE modulation of adipocyte very low density lipoprotein metabolism: role in endocytic and lipase-mediated metabolic pathways

Triglyceride-rich lipoproteins distribute energy in the form of fatty acids to peripheral tissues. We have previously shown that the absence of endogenous adipocyte apoE expression impairs adipocyte triglyceride acquisition from apoE-containing triglyceride-rich lipoproteins in vitro and in vivo. Studies were performed to evaluate the mechanism(s) for this impairment. We excluded a role for secreted apoE in accounting for the difference in very low density lipoprotein (VLDL)-induced adipocyte triglyceride accumulation using cross-incubation studies to show that secreted apoE did not enhance triglyceride synthesis in apoE knockout (EKO) adipocytes incubated with apoE-containing VLDL. Subsequent experiments established that both endocytic and lipase-mediated pathways for lipid acquisition from VLDL were impaired in EKO adipocytes. Binding and internalization of VLDL to EKO adipocytes were significantly lower due to decreased expression or redistribution of low density lipoprotein receptor family proteins. An important role for the VLDL receptor for contributing to differences in VLDL binding between wild-type and EKO adipocytes was identified. Lipoprotein lipase-dependent adipocyte lipogenesis was also significantly decreased in EKO adipocytes even though they secreted as much or more lipolytic activity. This decrease was related to impaired fatty acid internalization in EKO cells. Evaluation of potential mechanisms revealed reduced caveolin-1 and plasma membrane raft expression in EKO adipocytes. Increasing caveolin expression in EKO adipocytes increased fatty acid internalization. Our results establish a role for endogenous adipocyte apoE in VLDL-induced adipocyte lipogenesis by impacting both endocytic and lipoprotein lipase-mediated metabolic pathways. Reduced adipocyte apoE expression, for example that accompanying obesity, will suppress adipocyte acquisition of lipid from apoE-containing VLDL.

Cyclosporin A decreases apolipoprotein E secretion from human macrophages via a protein phosphatase 2B-dependent and ATP-binding cassette transporter A1 (ABCA1)-independent pathway

Cyclosporin A (CsA) is an immunosuppressant that inhibits protein phosphatase 2B (PP2B/calcineurin) and is associated with hyperlipidemia, decreased cholesterol efflux via ATP-binding cassette transporter A1 (ABCA1), and increased risk of atherosclerosis. Apolipoprotein E (apoE) is an important regulator of lipid metabolism and atherosclerosis, the secretion of which from human macrophages is regulated by the serine/threonine protein kinase A (PKA) and intracellular calcium (Ca(2+)) (Kockx, M., Guo, D. L., Huby, T., Lesnik, P., Kay, J., Sabaretnam, T., Jary, E., Hill, M., Gaus, K., Chapman, J., Stow, J. L., Jessup, W., and Kritharides, L. (2007) Circ. Res. 101, 607-616). As PP2B is Ca(2+)-dependent and has been linked to PKA-dependent processes, we investigated whether CsA modulated apoE secretion. CsA dose- and time-dependently inhibited secretion of apoE from primary human macrophages and from Chinese hamster ovary cells stably transfected with human apoE and increased cellular apoE levels without affecting apoE mRNA. [(35)S]Met kinetic modeling studies showed that CsA inhibited both secretion and degradation of apoE, increasing the half-life of cellular apoE 2-fold. CsA also inhibited secretion from primary human Tangier disease macrophages and from mouse macrophages deficient in ABCA1, indicating that the effect is independent of the known inhibition of ABCA1 by CsA. The role of PP2B in mediating apoE secretion was confirmed using additional peptide and chemical inhibitors of PP2B. Importantly, kinetic modeling, live-cell imaging, and confocal microscopy all indicated that CsA inhibited apoE secretion by mechanisms quite distinct from those of PKA inhibition, most likely inducing accumulation of apoE in the endoplasmic reticulum compartment. Taken together, these results establish a novel mechanism for the pro-atherosclerotic effects of CsA, and establish for the first time a role for PP2B in regulating the intracellular transport and secretion of apoE.

Tumor necrosis factor-alpha-mediated suppression of adipocyte apolipoprotein E gene transcription: primary role for the nuclear factor (NF)-kappaB pathway and NFkappaB p50

The adipose tissue inflammation accompanying obesity has important consequences for adipocyte lipid metabolism, and increased adipose tissue TNFalpha plays an important role for mediating the effect of inflammation on adipocyte function. Recent studies have shown that apolipoprotein E (apoE) is highly expressed in adipose tissue where it plays an important role in modulating adipocyte triglyceride metabolism, triglyceride mass, and adipocyte size. We have previously reported that TNFalpha reduces adipocyte apoE, and the current studies were undertaken to evaluate the molecular mechanism for this regulation. TNFalpha repression of adipocyte apoE gene expression required an intact nuclear factor (NF)-kappaB binding site at -43 in the apoE promoter. Site-directed mutagenesis at this site completely eliminated TNFalpha regulation of an apoE gene reporter. TNFalpha treatment activated binding of NFkappaB p50, isolated from adipocyte nuclei, to the apoE promoter. Two structurally distinct inhibitors of NFkappaB complex activation or translocation abrogated the TNFalpha effect on the apoE gene. Using chromatin immunoprecipitation assays, we demonstrated that treatment of adipocytes with TNFalpha led to increased binding of NFkappaB p50, and decreased binding of p65 and Sp1, to this region of the apoE promoter in living cells. The key role played by increased p50 binding was confirmed by p50 knockdown experiments. Reduction of p50 expression using small interference RNA completely eliminated TNFalpha-mediated reduction of endogenous adipocyte apoE gene expression. These results establish the molecular link between adipose tissue inflammation and apoE gene expression in adipocytes. The suppression of adipocyte apoE by the proinflammatory adipose tissue milieu associated with obesity will have important downstream effects on adipocyte triglyceride turnover and content.

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.

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

OBJECTIVE

Macrophage expression of both apolipoprotein E (apoE) and ABCA1 have been shown to modulate lipid efflux from these cells and to play an important atheroprotective role in vivo. We evaluated the relationship between apoE and ABCA1 for regulating cellular sterol efflux.

METHODS AND RESULTS

ApoE-mediated, but ABCA1-independent, lipid efflux was demonstrated in 3 model systems. First, adenoviral-mediated expression of apoE in dermal fibroblasts isolated from ABCA1(-/-) mice significantly increased both sterol and phospholipid efflux. Second, expression of human apoE in a macrophage cell line increased sterol efflux, and this increment in efflux was not reduced by suppressing ABCA1 expression. Third, reduction of apoE expression using an apoE small interfering RNA significantly reduced sterol efflux from ABCA1(-/-) mouse peritoneal macrophages. ApoE-mediated, but ABCA1-independent, lipid efflux could be differentiated from lipid efflux that was dependent on the extracellular accumulation of secreted apoE, because exogenous cell-derived apoE stimulated efflux only from cells expressing ABCA1. Sterol efflux was usually highest in cells expressing both ABCA1 and apoE, likely representing a summation of the ABCA1-dependent and -independent pathways for apoE-mediated sterol efflux.

CONCLUSIONS

ABCA1 expression is required for apoE-mediated efflux when endogenously synthesized apoE accumulates extracellularly. Our results, however, establish the existence of an ABCA1-independent pathway for lipid efflux that requires the intracellular synthesis and/or transport of apoE.

The recycling of apolipoprotein E in macrophages

The ability of apolipoprotein E (apoE) to be spared degradation in lysosomes and to recycle to the cell surface has been demonstrated by our group and others, but its physiologic relevance is unknown. In this study, we characterized apoE recycling in primary murine macrophages and probed the effects of HDL and apoA-I on this process. In cells pulsed with (125)I.apoE bound to VLDL, intact apoE was found in the chase medium for up to 24 h after the pulse. Approximately 27 +/- 5% of the apoE internalized during the pulse was recycled after 4 h of chase. Addition of apoA-I and HDL increased apoE recycling to 45 +/- 3% and 46 +/- 3%, respectively, similar to the amount of apoE recycled after pulsing the cells with (125)I.apoE.HDL. In addition, apoA-I-producing macrophages from transgenic mice showed increased apoE recycling at 4 h (38 +/- 3%). Increased ABCA1 expression potentiated apoE recycling, suggesting that recycling occurs via ABCA1. Finally, in the presence of apoA-I, recycled apoE exited the cells on HDL-like particles. These results suggest that apoE recycling in macrophages may be part of a larger signaling loop activated by HDL and directed at maximizing cholesterol losses from the cell.

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.

Heterogeneous expression of apolipoprotein-E by human macrophages

Apolipoprotein-E (apoE) is expressed at high levels by macrophages. In addition to its role in lipid transport, macrophage-derived apoE plays an important role in immunoregulation. Previous studies have identified macrophage subpopulations that differ substantially in their ability to synthesize specific cytokines and enzymes, however, potential heterogeneous macrophage apoE expression has not been studied. Here we examined apoE expression in human THP-1 macrophages and monocyte-derived macrophages (MDM). Using immunocytochemistry and flow cytometry methods we reveal a striking heterogeneity in macrophage apoE expression in both cell types. In phorbol-ester-differentiated THP-1 macrophages, 5% of the cells over-expressed apoE at levels more than 50-fold higher than the rest of the population. ApoE over-expressing THP-1 macrophages contained condensed/fragmented nuclei and increased levels of activated caspase-3 indicating induction of apoptosis. In MDM, 3-5% of the cells also highly over-expressed apoE, up to 50-fold higher than the rest of the population; however, this was not associated with obvious nuclear alterations. The apoE over-expressing MDM were larger, more granular, and more autofluorescent than the majority of cells and they contained numerous vesicle-like structures that appeared to be coated by apoE. Flow cytometry experiments indicated that the apoE over-expressing subpopulation of MDM were positive for CD14, CD11b/Mac-1 and CD68. These observations suggest that specific macrophage subpopulations may be important for apoE-mediated immunoregulation and clearly indicate that subpopulation heterogeneity should be taken into account when investigating macrophage apoE expression.

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.

Oleic acid modulates the post-translational glycosylation of macrophage ApoE to increase its secretion

There has been increasing interest in a potential role for fatty acids in adversely affecting organismal substrate utilization and contributing to the cardiovascular complications in insulin resistance. Fatty acids have already been implicated in regulating the expression of a number of genes in resident cells of the vessel wall. In the current studies, we evaluated a potential role for fatty acids in the regulation of macrophage apoE expression. Incubation in oleic acid increased the synthesis and secretion of apoE by human monocyte-derived macrophages. Part of this stimulation was mediated at a post-translational locus. Oleic acid increased the secretion of apoE from macrophages that constitutively expressed a human apoE3 cDNA. Incubation in palmitic acid decreased apoE secretion from these cells. The effect of oleic acid on apoE secretion could not be accounted for by the known effect of fatty acid on cellular sterol, because incubation in oleic acid did not suppress the degradation of nascent apoE. Incubation in oleic acid for at least 6 h was required to observe an effect on apoE secretion. Oleic acid altered the glycosylation pattern of cellular and secreted apoE, with a loss of the most heavily sialylated isoform. Oleic acid had no effect on the glycosylation of interleukin 6 secreted from macrophages. Elimination of apoE glycosylation, by substitution of threonine 194 with alanine, eliminated oleic acid-mediated stimulation of apoE secretion. These results indicate that oleic acid increases apoE secretion from macrophages at a locus involving post-translational glycosylation.

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.

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.

Polarized cholesterol and phospholipid efflux in cultured gall-bladder epithelial cells: evidence for an ABCA1-mediated pathway

Gall-bladder epithelial cells (GBEC) are exposed to high concentrations of cholesterol in bile. Whereas cholesterol absorption by GBEC is established, the fate of this absorbed cholesterol is not known. The aim of this study was to determine whether ABCA1 (ATP-binding cassette transporter A1) mediates cholesterol efflux in GBEC. Polarized canine GBEC were cultured on porous membrane filters allowing separate access to apical (AP) and basolateral (BL) compartments. After AP loading of cells with model bile and [14C]cholesterol, cholesterol efflux was measured. Cholesterol loading together with 8-bromo-cAMP treatment, which increased ABCA1 expression, led to a significant increase in cholesterol efflux with apolipoprotein A-I (apoA-I) as the acceptor. Cholesterol efflux was observed predominantly into the BL compartment. Similar results were found for phospholipid efflux. Confocal immunofluorescence microscopy showed a predominantly BL ABCA1 localization. Interestingly, apoA-I added to either the AP or the BL compartments elicited BL lipid efflux with cAMP treatment. No paracellular or transcellular passage of 125I-apoA-I occurred. Ligands for the nuclear hormone receptors liver X receptor alpha (LXRalpha) and retinoid X receptor (RXR) elicited AP and BL cholesterol efflux, suggesting the involvement of both ABCA1- and non-ABCA1-mediated pathways. In summary, BL cholesterol/phospholipid efflux consistent with an ABCA1-mediated mechanism occurs in GBEC. This efflux pathway is stimulated by cAMP and by LXRalpha/RXR ligands, and in the case of the cAMP pathway appears to involve a role for biliary apoA-I.

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.

ATP binding cassette transporter ABCA1 modulates the secretion of apolipoprotein E from human monocyte-derived macrophages

Apolipoprotein E (apoE) produced by macrophages in the arterial wall protects against atherosclerosis, but the regulation of its secretion by these cells is poorly understood. Here we investigated the contribution of the adenosine triphosphate binding cassette transporters ABCA1 and ABC8 to the secretion of apoE from either primary human monocyte-derived macrophages (HMDM) or human THP1 macrophages. During incubations of up to 6 h, apoE secretion from both THP1 macrophages and HMDM was stimulated by 8-Br-cAMP, which activates ABCA1 expression. The putative ABCA1 inhibitor glyburide and antisense oligonucleotides directed against ABCA1 mRNA significantly reduced apoE secretion from THP1 macrophages and HMDM. Antisense oligonucleotides directed against ABC8 mRNA also inhibited apoE secretion, although this inhibition was less pronounced and consistent than in the case of ABCA1. ApoE secretion from HMDM of ABCA1-deficient patients with Tangier disease was also decreased. ApoE mRNA expression was not affected by inhibition of ABCA1 or ABC8 in normal HMDM or the lack of functional ABCA1 in HMDM from Tangier disease patients. Inhibition of ABCA1 in HMDM prevented the occurrence of anti-apoE-immunoreactive granular structures in the plasma membrane. We conclude that ABCA1 and, to a lesser extent, ABC8 both promote secretion of apoE from human macrophages.