Regulation of apolipoprotein E secretion by high density lipoprotein 3 in mouse macrophages

Isoform-dependent lysosomal degradation and internalization of apolipoprotein E requires autophagy proteins

The human apolipoprotein E4 isoform (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (AD), and lysosomal dysfunction has been implicated in AD pathogenesis. We found, by examining cells stably expressing each APOE isoform, that APOE4 increases lysosomal trafficking, accumulates in enlarged lysosomes and late endosomes, alters autophagic flux and the abundance of autophagy proteins and lipid droplets, and alters the proteomic contents of lysosomes following internalization. We investigated APOE-related lysosomal trafficking further in cell culture, and found that APOE from the post-Golgi compartment is degraded through autophagy. We found that this autophagic process requires the lysosomal membrane protein LAMP2 in immortalized neuron-like and hepatic cells, and in mouse brain tissue. Several macroautophagy-associated proteins were also required for autophagic degradation and internalization of APOE in hepatic cells. The dysregulated autophagic flux and lysosomal trafficking of APOE4 that we observed suggest a possible novel mechanism that might contribute to AD pathogenesis. This article has an associated First Person interview with the first author of the paper.

Cholesterol loading suppresses the atheroinflammatory gene polarization of human macrophages induced by colony stimulating factors

In atherosclerotic lesions, blood-derived monocytes differentiate into distinct macrophage subpopulations, and further into cholesterol-filled foam cells under a complex milieu of cytokines, which also contains macrophage-colony stimulating factor (M-CSF) and granulocyte-macrophage-colony stimulating factor (GM-CSF). Here we generated human macrophages in the presence of either M-CSF or GM-CSF to obtain M-MØ and GM-MØ, respectively. The macrophages were converted into cholesterol-loaded foam cells by incubating them with acetyl-LDL, and their atheroinflammatory gene expression profiles were then assessed. Compared with GM-MØ, the M-MØ expressed higher levels of CD36, SRA1, and ACAT1, and also exhibited a greater ability to take up acetyl-LDL, esterify cholesterol, and become converted to foam cells. M-MØ foam cells expressed higher levels of ABCA1 and ABCG1, and, correspondingly, exhibited higher rates of cholesterol efflux to apoA-I and HDL2. Cholesterol loading of M-MØ strongly suppressed the high baseline expression of CCL2, whereas in GM-MØ the low baseline expression CCL2 remained unchanged during cholesterol loading. The expression of TNFA, IL1B, and CXCL8 were reduced in LPS-activated macrophage foam cells of either subtype. In summary, cholesterol loading converged the CSF-dependent expression of key genes related to intracellular cholesterol balance and inflammation. These findings suggest that transformation of CSF-polarized macrophages into foam cells may reduce their atheroinflammatory potential in atherogenesis.

The Patatin-Like Phospholipase Domain Containing Protein 7 Facilitates VLDL Secretion by Modulating ApoE Stability

BACKGROUND AND AIMS

The regulation of hepatic very-low-density lipoprotein (VLDL) secretion is vital for lipid metabolism whose pathogenetic status is involved in fatty liver disease and dyslipidemia seen in hepatic steatosis. Accumulated evidence suggest that apolipoprotein E (ApoE) is closely related to hepatic VLDL secretion. Here, we report that the expression of patatin-like phospholipase domain containing protein 7 (PNPLA7) is strongly induced by hepatic steatosis and positively correlates with plasma triacylglycerol (TAG) levels in the human subjects, whereas the role of PNPLA7 in hepatic VLDL secretion is unknown.

APPROACH AND RESULTS

Herein, with genetic manipulation in the mice, the deficiency of hepatic PNPLA7 expression resulted in reduced VLDL secretion accompanied by enhanced hepatic lipid accumulation and decreased hepatic ApoE expression. Furthermore, knockdown of PNPLA7 in the livers of the db/db mice also resulted in significant reduction in plasma TAG level but aggravated hepatic steatosis. Importantly, we observed that PNPLA7 interacted with ApoE and presumably at the site of endoplasmic reticulum. Mechanistically, we have shown that PNPLA7 could modulate polyubiquitination and proteasomal-mediated degradation of ApoE. Overexpressed ApoE restored the impaired VLDL-TAG metabolism in PNPLA7-knockdown primary hepatocytes.

CONCLUSION

PNPLA7 plays a critical role in regulating hepatic VLDL secretion by modulating ApoE stability through its interaction with ApoE.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

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.

The multiscale backbone of the human phenotype network based on biological pathways

Background

Networks are commonly used to represent and analyze large and complex systems of interacting elements. In systems biology, human disease networks show interactions between disorders sharing common genetic background. We built pathway-based human phenotype network (PHPN) of over 800 physical attributes, diseases, and behavioral traits; based on about 2,300 genes and 1,200 biological pathways. Using GWAS phenotype-to-genes associations, and pathway data from Reactome, we connect human traits based on the common patterns of human biological pathways, detecting more pleiotropic effects, and expanding previous studies from a gene-centric approach to that of shared cell-processes.

Results

The resulting network has a heavily right-skewed degree distribution, placing it in the scale-free region of the network topologies spectrum. We extract the multi-scale information backbone of the PHPN based on the local densities of the network and discarding weak connection. Using a standard community detection algorithm, we construct phenotype modules of similar traits without applying expert biological knowledge. These modules can be assimilated to the disease classes. However, we are able to classify phenotypes according to shared biology, and not arbitrary disease classes. We present examples of expected clinical connections identified by PHPN as proof of principle.

Conclusions

We unveil a previously uncharacterized connection between phenotype modules and discuss potential mechanistic connections that are obvious only in retrospect. The PHPN shows tremendous potential to become a useful tool both in the unveiling of the diseases’ common biology, and in the elaboration of diagnosis and treatments.

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.

An ABCA1-independent pathway for recycling a poorly lipidated 8.1 nm apolipoprotein E particle from glia

Lipid transport in the brain is coordinated by glial-derived lipoproteins that contain apolipoprotein E (apoE) as their primary protein. Here we show that apoE is secreted from wild-type (WT) primary murine mixed glia as nascent lipoprotein subspecies ranging from 7.5 to 17 nm in diameter. Negative-staining electron microscropy (EM) revealed rouleaux, suggesting a discoidal structure. Potassium bromide (KBr) density gradient ultracentrifugation showed that all subspecies, except an 8.1 nm particle, were lipidated. Glia lacking the cholesterol transporter ABCA1 secreted only 8.1 nm particles, which were poorly lipidated and nondiscoidal but could accept lipids to form the full repertoire of WT apoE particles. Receptor-associated-protein (RAP)-mediated inhibition of apoE receptor function blocked appearance of the 8.1 nm species, suggesting that this particle may arise through apoE recycling. Selective deletion of the LDL receptor (LDLR) reduced the level of 8.1 nm particle production by approximately 90%, suggesting that apoE is preferentially recycled through the LDLR. Finally, apoA-I stimulated secretion of 8.1 nm particles in a dose-dependent manner. These results suggest that nascent glial apoE lipoproteins are secreted through multiple pathways and that a greater understanding of these mechanisms may be relevant to several neurological disorders.

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.

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.

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.

Cellular apoptosis is associated with increased caveolin-1 expression in macrophages

Macrophage apoptosis is an important factor in determining the efficiency of the immune response, atherosclerotic lesion stability, and clearance of aged cells by phagocytosis. The involvement of caveolin-1 in the regulation of apoptosis has been previously suggested in fibroblasts and epithelial cells. Here we show that treatment of thioglycollate-elicited mouse peritoneal macrophages with various unrelated apoptotic agents, including simvastatin, camptothecin, or glucose deprivation, is associated with a specific and large increase in caveolin-1 expression. In contrast, caveolin-2 levels remain unaffected. Induction of apoptosis was measured by changes in cell morphology, annexin V-labeling, and DNA fragmentation. We demonstrate that caveolin-1 in macrophages is present in lipid rafts and colocalizes with phosphatidylserine (PS) at the cell surface of apoptotic macrophages. Our data suggest that caveolin-1 increase is an early event, closely accompanied by PS externalization and independent of caspase activation and nuclear DNA fragmentation. The increase in caveolin-1 levels does not require new protein synthesis, as cycloheximide does not prevent the apoptosis-mediated increase in caveolin-1 levels. We propose that increased levels of caveolin-1 characterize the apoptotic phenotype of macrophages. Caveolin-1 may be involved in the efficient externalization of PS at the surface of the apoptotic cells.

Caveolins and macrophage lipid metabolism

The identification of caveolin-1 more than a decade ago initiated active research into its role in the formation of caveolae, membrane trafficking, signal transduction pathways, and lipid homeostasis. Although caveolins are ubiquitously expressed, the majority of the available information comes from differentiated cells rich in caveolins, such as fibroblasts, adipocytes, and endothelial cells. During the development of atherosclerosis, macrophages play a pivotal role in the formation of the fatty streak lesions. They take up large amounts of lipids and accumulate in the subendothelial space, forming foam cells that fill up the lesion area. Since caveolins have been implicated in the regulation of cellular cholesterol metabolism in several cell types, it is of interest to examine their potential function in macrophages. In this review, we attempt to summarize current knowledge and views on the role of caveolins in cholesterol metabolism with emphasis on macrophages.

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.

Apolipoprotein E inhibits serum-stimulated cell proliferation and enhances serum-independent cell proliferation

Independently of its role in lipid homeostasis, apolipoprotein E (apoE) inhibits cell proliferation. We compared the effects of apoE added to media (exogenous apoE) with the effects of stably expressed apoE (endogenous apoE) on cell proliferation. Exogenous and endogenous apoE increased population doubling times by 30-50% over a period of 14 days by prolonging the G(1) phase of the cell cycle. Exogenous and endogenous apoE also decreased serum-stimulated DNA synthesis by 30-50%. However, apoE did not cause cell cycle arrest; both apoE-treated and control cells achieved equivalent saturation densities at 14 days. Further analyses demonstrated that exogenous and endogenous apoE prevented activation of MAPK but not induction of c-fos expression in response to serum growth factors. Endogenous (but not exogenous) apoE altered serum concentration-dependent effects on proliferation. Whereas control (non-apoE-expressing) cell numbers increased with increasing serum concentrations (1.6-fold for every 2-fold increase in serum), apoE-expressing cell numbers did not differ as serum levels were raised from 2.5 to 10%. In addition, in low serum (0.1%), apoE-expressing cells had elevated DNA synthesis levels compared with control cells. We conclude that apoE does not simply inhibit cell proliferation; rather, the presence of apoE alters the response to and requirement for serum mitogens.

Regulation of apolipoprotein E secretion in rat primary hippocampal astrocyte cultures

Apolipoprotein E isoforms may have differential effects on a number of pathological processes underlying Alzheimer's disease. Recent studies suggest that the amount, rather than the type, of apolipoprotein E may also be an important determinant for Alzheimer's disease. Therefore, understanding the regulated synthesis of apolipoprotein E is important for determining its role in Alzheimer's disease. We show here that in rat primary hippocampal astrocyte cultures, dibutyryl-cAMP increased apolipoprotein E secretion with time in a dose-dependent manner (to 177% at 48 h) and that retinoic acid potentiated this effect (to 298% at 48 h). Dibutyryl-cAMP also gave a rapid, albeit transient, increase of apolipoprotein E mRNA expression (to 200% at 1 h). In contrast, the protein kinase C activator phorbol 12-myristate 13-acetate decreased both apolipoprotein E secretion (to 59% at 48 h) and mRNA expression (to 22% at 1 h). Phorbol 12-myristate 13-acetate also reversed the effects of dibutyryl-cAMP. Apolipoprotein E secretion was also modulated by receptor agonists for the adenylyl cyclase/cAMP pathway. Isoproterenol (50 nM, a beta-adrenoceptor agonist) enhanced, while clonidine (250 nM, an alpha2-adrenoceptor agonist) decreased, secreted apolipoprotein E. We also analysed the effects of agonists for the phospholipase C/protein kinase C pathway. Arterenol (1 microM, an alpha1-adrenoceptor agonist) and serotonin (2.5 microM) enhanced, whereas carbachol (10 microM, an acetylcholine muscarinic receptor agonist) decreased secreted apolipoprotein E. The effects of these non-selective receptor agonists were modest, probably due to effects on different signalling pathways. Arterenol also potentiated the isoproterenol-mediated increase. We also show that phorbol 12-myristate 13-acetate and dibutyryl-cAMP have opposite effects on nerve growth factor, as compared to apolipoprotein E, secretion, suggesting that the results obtained were unlikely to be due to a general effect on protein synthesis. We conclude that astrocyte apolipoprotein E production can be regulated by factors that affect cAMP intracellular concentration or activate protein kinase C. Alterations in these signalling pathways in Alzheimer's disease brain may have consequences for apolipoprotein E secretion in this disorder.

Caveolin-1 and caveolin-2 expression in mouse macrophages. High density lipoprotein 3-stimulated secretion and a lack of significant subcellular co-localization

Evidence for caveolin expression in macrophages is scarce and conflicting. We therefore examined caveolin-1 and caveolin-2 expression in resident and thioglycollate-elicited mouse peritoneal macrophages (tg-MPM) and in the J774 mouse macrophage cell line by RT-PCR, ribonuclease protection assay, immunoblotting, and immunofluorescence. We found that relative to 3T3 cells, resident MPM and tg-MPM express low amounts of caveolin-1 (45 and 15% of those in 3T3 fibroblasts, respectively), while J774.A1 cells do not express any. Caveolin-2, on the other hand, is expressed in all cells examined, with highest expression in tg-MPM and the lowest in J774 cells. The relative levels of caveolin expression in the various cells correspond well with their respective mRNA levels, as measured by ribonuclease protection assay. Caveolin-1, present primarily on the cell surface, does not co-localize significantly with caveolin-2, which is present primarily in the Golgi compartment in all macrophages studied. Loading of tg-MPM with cholesterol or variations in unesterified cholesterol content appear to have little effect on the level of caveolin-1 or -2 expression or their distribution. Stimulation of cholesterol efflux by HDL(3) leads to caveolin-1 and caveolin-2 secretion to the cell culture medium, a process not detected in the absence of HDL(3). The lack of significant co-localization of the two caveolin isoforms in primary macrophages and their secretion in the presence of HDL(3) provides an interesting and physiologically relevant model system to study additional aspects of caveolin function.

Effects of chronic alcohol treatment on the synthesis, sialylation, and disposition of nascent apolipoprotein E by peritoneal macrophages of rats

BACKGROUND

Plasma apolipoprotein (apo) E, a sialoprotein, plays an important role in reverse cholesterol transport. Previously, we showed that chronic alcohol consumption impairs glycosylation of apo E in rat liver. Peritoneal macrophages are another significant apo E synthesis site.

OBJECTIVE

The main purpose of this study was to determine the effects of chronic alcohol feeding of rats on the synthesis, sialylation, and sialic acid content of macrophage apo E and its ability to bind to the HDL(3) molecule in vitro.

DESIGN

Rats were fed an alcoholic diet or an isoenergetic control diet for 8 wk, after which peritoneal macrophages isolated from them were cultured and analyzed for apo E metabolism.

RESULTS

Macrophages from alcohol-fed rats accumulated 33.3% more (P < 0.05) cholesterol than did those from control rats when incubated with acetylated LDL. These macrophages showed a 51-57% lower relative sialylation rate of apo E (P < 0.001) but no significant difference in relative protein synthetic rate. The sialic acid content of the intracellular and secreted forms of apo E was reduced by 41.8% (P < 0.001) and 50.3% (P < 0.001), respectively, with chronic alcohol treatment. Secretion of newly synthesized apo E was impaired by 53.7% (P < 0.001) and 26. 1% (P < 0.001) in the absence and presence of HDL in the medium, respectively. Macrophages of alcohol-treated rats secreted apo E with 47.6-67.2% lower (P < 0.001) HDL(3) binding ability; binding ability was restored completely by resialylation of the desialylated apo E.

CONCLUSION

In rats, an alcohol-mediated decrease in sialylation rate resulting in loss of sialic acid residues in apo E impairs the ability of apo E to bind to HDL and consequently in defective reverse cholesterol transport.

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.

Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis

Apolipoprotein E (apoE) is an important determinant for the uptake of triglyceride-rich lipoproteins and emulsions by the liver, but the intracellular pathway of apoE following particle internalization is poorly defined. In the present study, we investigated whether retroendocytosis is a unique feature of apoE as compared with apoB by studying the intracellular fate of very low density lipoprotein-sized apoE-containing triglyceride-rich emulsion particles and LDL after LDLr-mediated uptake. Incubation of HepG2 cells with [(3)H]cholesteryl oleate-labeled particles at 37 degrees C led to a rapid release of [(3)H]cholesterol within 30 min for both LDL and emulsion particles. In contrast, emulsion-derived (125)I-apoE was more resistant to degradation (>/=120 min) than LDL-derived (125)I-apoB (30 min). Incubation at 18 degrees C, which allows endosomal uptake but prevents lysosomal degradation, with subsequent incubation at 37 degrees C resulted in a time-dependent release of intact apoE from the cells (up to 14% of the endocytosed apoE at 4 h). The release of apoE was accelerated by the presence of protein-free emulsion (20%) or high density lipoprotein (26%). Retroendocytosis of intact particles could be excluded since little intact [(3)H]cholesteryl oleate was released (<3%). In contrast, the degradation of LDL was complete with virtually no secretion of intact apoB into the medium. The intracellular stability of apoE was also demonstrated after hepatic uptake in C57Bl/6 mice. Intravenous injection of (125)I-apoE and [(3)H]cholesteryl oleate-labeled emulsions resulted in efficient LDLr-mediated uptake of both components by the liver (45-50% of the injected dose after 20 min). At 1 h after injection, only 15-20% of the hepatic (125)I-apoE was degraded, whereas 75% of the [(3)H]cholesteryl oleate was hydrolyzed. From these data we conclude that following LDLr-mediated internalization by liver cells, apoE can escape degradation and can be resecreted. This sequence of events may allow apoE to participate in its hypothesized intracellular functions such as mediator of the post-lysosomal trafficking of lipids and very low density lipoprotein assembly.

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.

Lipopolysaccharide inhibits the expression of the scavenger receptor Cla-1 in human monocytes and macrophages

Human Cla-1 is the likely homologue of the murine scavenger receptor class B type I (SR-BI). SR-BI mediates selective transfer of cholesterol to high-density lipoprotein (HDL) and the efflux of endogenously synthesized and plasma membrane sterols to HDL. HDL protects against atherosclerosis but also reduces endotoxic activity by complexation and neutralization of LPS. We found that Cla-1 is upregulated during phagocytic as well as dendritic differentiation of monocytes, indicating a function of this receptor for cholesterol homeostasis in phagocytes and antigen-presenting cells. Cla-1 expression is suppressed by the proinflammatory stimuli lipopolysaccharide, interferon-gamma, and tumor necrosis factor alpha in monocytes and macrophages. Downregulation of Cla-1 mRNA by LPS is likely due to a modification and subsequent destabilization of the mRNA. We propose that suppression of Cla-1 expression may help to stabilize the lipoprotein status in the blood compartment important for host defense.

Probucol selectively increases oxidation of atherogenic lipoproteins in cholesterol-fed mice and in Watanabe heritable hyperlipidemic rabbits

The anti-atherogenic and cholesterol-lowering drug probucol (0.5-1%) or quercetin (1%), a natural antioxidant, was given to cholesterol-fed (1.5%) mice for a period of 6 weeks and to Watanabe heritable hyperlipidemic (WHHL) rabbits for a period of 8 weeks to investigate the oxidative changes in plasma and lipoproteins. Oxidation was measured as the total amount of malondialdehyde (nmol MDA/g protein) by a very specific MDA-HPLC method. A large and significant increase in MDA was seen in LDL from probucol treated WHHL rabbits (1778.7+/-585.5 nmol/g vs. 394.4+/-144.5 nmol/g, P < 0.001) and cholesterol-fed mice (579.7 + 47.3 nmol/g vs. 408.1+/-85.8 nmol/g, P < 0.05) as compared to controls while LDL cholesterol was lowered (WHHL rabbits: P < 0.05; mice: P < 0.01). In WHHL rabbits VLDL oxidation was determined additionally, and also revealed a large increase in the probucol group (2102.7+/-1156.1 nmol/g vs. 455.0+/-207.8 nmol/g, P< 0.01). In contrast, the oxidation of plasma and HDL from probucol treated animals was not statistically significantly increased, implying that probucol mediates a selective oxidation of atherogenic cholesterol-transporting lipoproteins. Quercetin treated animals did not show increased oxidation of LDL (and VLDL in rabbits) and cholesterol levels were not decreased. Furthermore, no protective antioxidant effect of quercetin was seen. In conclusion, the results suggest that a prooxidant mechanism rather than antioxidative effects influences lipoprotein metabolism in these animals. It is hypothesized that the oxidation of lipoproteins might be a physiological mechanism performed by macrophages or other cells for uptake and degradation (by macrophages and liver) of excessive amounts of LDL or VLDL and that probucol oxidizes atherogenic lipoproteins and thereby leads to a decrease in cholesterol levels.

Increased apolipoprotein E mRNA in the hippocampus in Alzheimer disease and in rats after entorhinal cortex lesioning

The distribution of apolipoprotein E (ApoE) mRNA was characterized in the hippocampus of humans with Alzheimer disease (AD) and in rats with experimental lesions (unilateral ablation of the entorhinal cortex) that model selected features of AD. In both AD and the lesion model, we observed a shift in the location of astrocytes containing prevalent ApoE mRNA from the neuropil to regions with densely packed neurons. The increased abundance of ApoE mRNA in astrocytes close to neuron cell bodies could be indicative of lipid uptake in regions where neurons are degenerating or where synaptic remodeling is taking place.

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.

Oxidative-modified and acetylated low-density lipoproteins differ in their effects on cholesterol synthesis and stimulate synthesis of apolipoprotein E in rat peritoneal macrophages by different mechanisms

Apolipoprotein (apo) E plays an important role in the recognition of lipoproteins by cellular lipoprotein receptors. Unlike other apolipoproteins, apo E is expressed by many extrahepatic tissues including macrophages (M phi). Resident M phi express low levels of apo E. However, their synthesis of apo E is substantially increased after M phi have been incubated with acetylated low-density lipoprotein (LDL). But acetylation of LDL is not known to occur in vivo. On the other hand, modification of LDL by oxidation and by enzymatic action is believed to happen physiologically. In this report, we compared the effects of various modified LDLs on the synthesis of apo E by M phi. Freshly isolated human LDL was modified by (1) repeated addition of acetic anhydride (Ac-LDL); (2) incubation with 20 mumol/L CuSO4 at 37 degrees C for 24 hours (Ox-LDL); and (3) incubation with phospholipase C at 37 degrees C for 1 hour (PI-LDL). Resident peritoneal M phi were collected by lavage from rats and allowed to attach to plastic culture dishes. Although native LDL had no effect, treatment with Ac-, Ox-, and PI-LDL (50 micrograms/mL each) was found to increase medium apo E by (-fold) 4.19 +/- 0.26, 4.20 +/- 0.34, and 2.02 +/- 0.20 (mean +/- SEM, n = 5), respectively, as compared with untreated cells. Northern blot analysis revealed that cellular apo E mRNA was increased in parallel to apo E protein by Ac-LDL and PI-LDL. However, increases of apo E protein and mRNA by Ox-LDL were not equal.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of human apoE4 on the clearance of chylomicron-like lipid emulsions and atherogenesis in transgenic mice

Apolipoprotein (apo) E is a ligand for lipoprotein receptors and mediates the cellular uptake of several different lipoproteins. Human apoE occurs in three allelic forms designated E2, E3, and E4. The E2 isoform is associated with changes in lipoprotein metabolism, and the E4 isoform is associated with Alzheimer's disease and an increased risk of coronary heart disease. In this study transgenic mice were generated to assess the effect of a sustained increase in plasma apoE4 concentration. The transgenic animals had three- to sixfold increases in total plasma apoE, associated primarily with the non-high-density lipoprotein (HDL) fractions of plasma lipoproteins. In response to an atherogenic diet the transgenic mice developed hypercholesterolemia similar to that in nontransgenic mice but did not experience the decrease in HDL cholesterol normally observed in this strain of C57BL/6 mice. The rate of plasma clearance of a lipid emulsion mimicking lymph chylomicrons was measured in transgenic mice expressing the human apoE4 gene and compared with the clearance rate in nontransgenic control animals. In animals fed a low-fat diet the emulsion lipids were cleared significantly more rapidly from the plasma of transgenic than control mice. In animals adapted to a high-fat diet, the clearance of chylomicron remnants was slowed markedly in both transgenic and control mice and was not significantly accelerated in transgenic compared with control animals. We also investigated the effect of increasing the plasma concentration of apoE4 on the progression of atherosclerotic heart disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Exogenous glucocorticoids increase macrophage secretion of apo E by cholesterol-independent pathways

Macrophage apo E synthesis and secretion has been previously demonstrated to be regulated by intracellular free cholesterol levels and is decreased by cytokines and other inflammatory stimuli associated with macrophage activation. In a recent study, the opposing effects of TGF beta and GM-CSF were reported with the former increasing and the latter decreasing apo E secretion and apo E mRNA levels. In an attempt to further understand the mechanisms by which TGF beta increased apo E expression in mouse peritoneal macrophages, the present study was performed to determine whether pharmacological agents could up-regulate apo E secretion by a mechanism independent of intracellular free cholesterol levels. Agents which resulted in increased apo E secretion were subdivided based on their effects on cAMP elevation. In addition to TGF beta, dexamethasone resulted in significant increases in apo E secretion. The 2-4-fold enhancement in apo E secretion by both TGF beta and dexamethasone occurred without concomitant changes in intracellular cAMP or free cholesterol. Other agents which increased apo E secretion included cholera toxin and 8-bromo-cAMP. While these agents did not affect intracellular cholesterol levels, cholera toxin did increase macrophage cAMP. The changes in apo E secretion by dexamethasone and 8-bromo-cAMP were associated with elevations in apo E mRNA. Dexamethasone-treated macrophages had 6-fold increases in apo E mRNA by 48 h when compared with control macrophages. Macrophages stimulated with 8-bromo-cAMP for 48 h demonstrated a more modest but statistically significant (P < 0.001) 2.2-fold increase. Similar effects of dexamethasone, cholera toxin, TGF beta, and 8-bromo-cAMP on apo E secretion were also apparent in macrophage-derived foam cells. In addition to increasing apo E secretion in macrophages and foam cells, dexamethasone and 8-bromo-cAMP inhibited the down-regulation of apo E secretion mediated by LPS and GM-CSF. Finally, the increased apo E secretion by exogenous glucocorticoids or TGF beta was not species specific as similar effects were observed in rabbit peritoneal macrophages. Therefore, while macrophage activation results in decreased apo E synthesis, macrophages exposed to anti-inflammatory agents including dexamethasone, TGF beta, or following cAMP elevation demonstrate increased apo E secretion by a cholesterol-independent mechanism.

Cytokine regulation of macrophage apo E secretion: opposing effects of GM-CSF and TGF-beta

Biosynthesis of apolipoprotein (apo) E has been previously demonstrated to be regulated in macrophages by intracellular free cholesterol levels as well as by macrophage activating factors. In this report, the regulation of apo E secretion by cytokines detected within atherosclerotic lesions has been investigated. Granulocyte macrophage-colony stimulating factor (GM-CSF) stimulated macrophages had a 3-5-fold reduction in apo E secretion, comparable to that observed for gamma interferon (IFN gamma), while tumor necrosis factor alpha (TNF alpha) and interleukin 1 beta (IL-1 beta) resulted in a 2-fold decrease. In contrast to the reduction in apo E secretion by these cytokines, transforming growth factor beta (TGF-beta) stimulated macrophages secreted 3-fold greater amounts of apo E than controls. The reduced secretion of apo E by GM-CSF was reversible, heat labile, dose dependent, maximal 48 h after cytokine exposure and was coincident with an increase in fibronectin secretion. The opposing effects of GM-CSF and TGF-beta on apo E secretion were consistent with similar changes detected in apo E mRNA levels. Cytokine effects on apo E secretion in cholesterol loaded macrophages were also investigated and found to be similar to the non-loaded cells with GM-CSF decreasing and TGF-beta increasing apo E secretion. The observed differences in apo E secretion did not correlate with any significant changes in either cellular cholesterol distribution in the non-cholesterol loaded macrophages or in basal ACAT activity. In addition to changes in apo E secretion, cytokine treated macrophages pulsed with [14C]oleate and acetylated LDL for 2-6 h had a 2-fold increase (GM-CSF) or decrease (TGF-beta) in cholesterol esterification. Therefore, GM-CSF and TGF-beta mediated changes in apo E secretion may occur through a mechanism independent of changes in cellular free cholesterol levels. These results suggest that cytokines expressed within an atheroma may play an important role in the modulation of macrophage mediated reverse cholesterol transport.

High density lipoprotein mediates selective reduction in cholesteryl esters from macrophage foam cells

To elucidate an anti-atherogenic nature of high density lipoprotein (HDL) at cellular level, its in vitro effect on macrophage foam cells was examined. Rat peritoneal macrophages were converted to foam cells by incubation with [3H]cholesterol-labeled acetylated LDL. HDL addition to these foam cells resulted in a reduction in cellular radioactive cholesteryl esters (CE) as well as its CE mass. The radioactive free cholesterol (FC) was similarly reduced with time, whereas its FC mass level was unaltered. Other lipoproteins such as very low density lipoprotein and low density lipoprotein also reduced the radioactive FC. However, their CE-reducing capacity was negligibly weak. These results suggest that (i) CE reduction is selective to HDL, (ii) FC transfer from plasma membrane to lipoprotein (cholesterol efflux) expressed by reduction in radioactive FC is not selective to HDL but occurs to other lipoproteins, (iii) the CE-reducing capacity of HDL became weaker when cellular binding of HDL was reduced by chemical modification with tetranitromethane or a chemical cross-linker, dithiobis-succinimidylpropionate, suggesting an importance of the specific binding in the HDL-mediated CE reduction. These in vitro results gave an experimental support to a definite role of HDL as an anti-atherogenic lipoprotein in vivo.