Published online before print April 3, 2008, doi: 10.1161/ATVBAHA.108.164350
© 2008 American Heart Association, Inc.
Brief Reviews |
Regulation of Endogenous Apolipoprotein E Secretion by Macrophages
From the Macrophage Biology Group (M.K., W.J., L.K.), Centre for Vascular Research, University of New South Wales, Sydney; and the Department of Cardiology (L.K.), Concord Repatriation General Hospital, Sydney South Western Area Health Service, University of Sydney, Australia.
Correspondence to Leonard Kritharides, Macrophage Biology Group, Centre for Vascular Research, Room 405C Wallace Wurth Building, University of New South Wales, High Street, Kensington, Sydney, NSW 2050, Australia. E-mail l.kritharides{at}unsw.edu.au
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Abstract |
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 Top Abstract IntroductionBasic Biology of ApoE...Regulation of ApoE Secretion...Future DirectionsReferences |
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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.
Key Words: lipid and lipoprotein metabolism • atherosclerosis pathophysiology • mechanisms of atherosclerosis • cell biology/structural biology • cell signalling/signal transduction
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Introduction |
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TopAbstractIntroductionBasic Biology of ApoE...Regulation of ApoE Secretion...Future DirectionsReferences |
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Apolipoprotein E (apoE) is a 34-kDa protein that is produced and secreted by many cell types such as hepatocytes, smooth muscle cells, neuronal cells, and macrophages. As a constituent of plasma lipopoproteins, apoE directs movement of lipids from the periphery to the liver, where high affinity binding of apoE to the LDL receptor and other members of the LDL-receptor family facilitates uptake of the lipoprotein particles. A critical role for apoE in the protection against atherosclerosis was clearly established by the development of the apoE knockout mouse and subsequent studies which showed that macrophage-specific expression of apoE was antiatherogenic.1,2 The mechanisms by which macrophage apoE is antiatherogenic may include stimulating the removal of excess cholesterol from macrophage foam cells,3 as well as anti-inflammatory,4 antiproliferative,5 and immunomodulatory properties.6,7 The regulation of the secretion of apoE from macrophages is thus of great importance to our understanding of the atherosclerotic process.
The aim of this review is to describe the regulation of the secretion of apoE by macrophages, in light of recent observations indicating that its trafficking and secretion are under the control of signaling pathways. Although recycling of lipoprotein-derived apoE is referenced briefly for comparison, a detailed analysis of apoE recycling is beyond the scope of the present review and the interested reader is directed to recent excellent reviews on this topic.8–10
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Basic Biology of ApoE in Macrophages |
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TopAbstractIntroductionBasic Biology of ApoE...Regulation of ApoE Secretion...Future DirectionsReferences |
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Transcriptional Regulation of ApoE Synthesis
Transcription of macrophage apoE can be induced by differentiation, exposure to cytokines, hormones, and lipids such as cholesterol or oxysterols. In addition to proximal promoter elements, apoE transcription is modulated by 2 distal flanking sequences termed multienhancer 1 (ME.1) and multienhancer 2 (ME.2), which were shown to direct macrophage and adipose-specific expression in transgenic mice.11 Transcription factors involved in the direct transcriptional regulation of apoE in macrophages included: AP-1,12,13 NF-
B,12 LXR,14 and PPAR
.15 The LXR/RXR heterodimer regulates apoE transcription in response to lipids through interaction with conserved LXR response elements in the multienhancer regions. Studies in LXR-deficient mice indicate that LXRs mediate lipid-inducible rather than tissue-specific expression of apoE, and point to an important role of LXR in foam cell–related apoE expression in atherosclerosis.14 Regulation of apoE transcription by PPAR is mediated via a PPRE element present in the intergenic region between the apoE and apoC1 genes, which is also well outside the proximal promoter.15
Other modulators of macrophage apoE transcription have been described, including transforming growth factor (TGF)-beta which stimulates apoE expression by promoting binding of c-Jun to AP-1,13 and lipopolysaccharide (LPS), which represses apoE gene expression via both Tpl-2 and MEKK1 signaling pathways, leading to c-Jun and NF-B action on distinct regions in the proximal promoter.12
Posttranslational Modifications to ApoE
ApoE is understood to follow the classical secretory pathway, whereby synthesis in the ER is followed by its movement to the Golgi and Trans Golgi network, and incorporation into vesicular structures, before being transported to the plasma membrane and secreted (Figure 1, pathway 1). Secreted apoE is typically released as phospholipid discs16 (Figure 1, pathway 6), however secreted apoE can vary in its density and association with lipid.17–19 As well as being released into the extracellular medium, a proportion of secreted apoE can be found bound to the cell surface (Figure 1, pathway 7), particularly in association with heparan sulfate proteoglycans.20,21 Cell surface pools may be reinternalized and subsequently degraded (Figure 1, pathways 3 and 4), or transported to the Golgi network for further modification22 (Figure 1, pathways 3 and 5), or released into the extracellular medium (Figure 1, pathway 3, 2, and 6).
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ApoE is synthesized as a 38 500 Mr protein designated preapoE, containing a NH2-terminal 18-AA extension23 which mediates entry into the ER. After cleavage of the signal peptide in the ER, the protein is trafficked to the Golgi, where apoE is O-glycosylated on threonine 19424 and extensively sialylated.16 Glycosylation and sialylation can therefore act as useful markers of the intracellular trafficking of apoE. Glycosylation and sialylation are uniformly present in apoE freshly secreted from hepatocytes and macrophages.16,25 However, the majority of apoE found in the plasma is at least partially desialylated.16,23 These posttranslational modifications of apoE do not appear to affect receptor binding or lipoprotein clearance but may determine the preferential association with high density lipoprotein (HDL) particles.26 Studies using Chinese Hamster Ovary (CHO) cells defective in O-glycosylation demonstrated that glycosylation/sialylation is not obligatory for secretion of apoE.24,27 This is supported by other studies showing that oleic acid stimulates secretion of macrophage apoE while apparently reducing its sialylation.28 The possibility that glycosylation and sialylation may affect recycling of secreted apoE (see below) remains unexplored.
Cellular Localization in Macrophages
ApoE represents up to 8% to 10% of protein constitutively secreted from macrophages.29 Although a number of studies have investigated the localization of endogenous apoE in hepatocytes,30–32 detailed microscopic studies in macrophages are limited. Werb et al29 showed that apoE was distributed in the Golgi complex, ER, secretory vesicles, and coated vesicles of mouse peritoneal macrophages. Kruth et al33 demonstrated that cellular apoE can localize to invaginations of the plasma membrane containing cholesterol crystals or modified LDL, in so-called "surface-connected" compartments where it may facilitate cholesterol solubilization. In the studies of Werb, apoE was released from secretory storage granules in response to frustrated phagocytosis that occurred after contact with immobilized immune complexes.34 The same investigators also described reuptake and resecretion of secreted apoE.29 This observation of apparent "recycling" of macrophage-derived apoE has been confirmed and extended by several groups (see below). ApoE does not accumulate in lysosomes under normal circumstances, consistent with usually efficient lysosomal degradation, but may be observed when lysosomal activity is inhibited.35
The quantitative importance of cell surface–bound apoE, relative to internal vesicular apoE, as a source for secreted apoE varies between cell types. In hepatocytes (studies principally performed in HepG2 cells) up to 54% of total cellular apoE at steady-state is at the cell surface,36 existing in distinct heparinase- and lipid-releasable pools.20 In contrast, in primary macrophages,17,22,37 less than 10% of cell apoE is on the cell surface. This suggests that the binding of endogenous apoE to cell-surface proteoglycans varies between cell types and appears to be quantitatively more important in HepG2 cells than in macrophages. There is also variation between primary macrophages of different species, as primary murine macrophages have a larger cell surface pool of apoE (sensitive to pronase) than do primary human macrophages.38 In primary human macrophages heparinase has no effect on apoE secretion,37,38 suggesting that the cell-surface binding of apoE to proteoglycans may be a quantitatively minor regulator of the rate of apoE secretion under steady state conditions.
Recycling of Macrophage-Derived ApoE in Macrophages
Reuptake of endogenously synthesized and secreted apoE by macrophages was first observed by Werb et al.29 Recent studies have shown that apoE can undergo repeated cycles of internalisation and resecretion (Figure 1, pathways 3 and 2), with approximately 27% of internalized apoE recycled and resecreted within 4 hours.39 Endocytosis of exogenous lipoprotein-derived apoE was reported to follow a quite distinct intracellular route relative to endogenous apoE, the latter having a predominantly Golgi localization and the former having an endosomal/lysosomal localization.40 However, reendocytosed lipoprotein-derived apoE has also been identified in the Golgi network.41 Apparent inconsistencies in the literature may relate to the relative sizes of endogenous and recycled exogenous pools. The predominance of endogenous apoE in the Golgi and exogenous apoE in the endosomal compartment may still allow for some overlap of intracellular transport routes for exogenous and endogenous apoE.
Resecretion of reinternalized apoE is observed in macrophages, hepatocytes, fibroblasts, and apoE-transfected CHO cells. Recycling of 125I-apoE-VLDL delivered to CHO cells was enhanced by cyclodextrins but not by apoA-I,42 whereas apoA-I and HDL promoted recycling of exogenous lipoprotein-derived apoE in primary murine macrophages39 and hepatoma cells.43 Monensin44 and brefeldin A35 inhibit endogenous apoE secretion, but fail to affect recycling of 125I-apoE-VLDL delivered to CHO cells,42 supporting the existence of different secretory pathways for endogenous and recycled apoE. Recycling of lipoprotein-derived apoE has been associated with cholesterol removal from hepatoma cells,9 however in vivo hepatic recycling of macrophage-derived apoE is not associated with significant alterations to hepatocyte lipid metabolism.45 The quantitative importance of recycling to the stimulation of secretion of macrophage apoE by apoA-I, HDL, and other apolipoproteins (see below) requires clarification.
Intracellular Degradation
ApoE undergoes intracellular degradation in a post Golgi compartment in macrophages (Figure 1, pathway 4), HepG2 and CHO cells.35,46 Newly synthesized apoE has a half life of 22 minutes in macrophages47 and of 2.6 hours in HepG2 cells.46 Some studies have concluded degradation only occurs within lysosomes, whereas others have shown that degradation is sensitive to both lysosomal and proteasomal chemical inhibitors.35,46,48 Brefeldin A, which perturbs ER to Golgi transport, inhibits apoE degradation and causes intracellular accumulation of unglycosylated apoE with lower molecular weight.35 This suggests that entry of apoE into the Golgi network, and glycosylation of apoE, precede entry into the degradatory compartment.35 Degradation of apoE is inhibited by ALLN, an inhibitor of Ca2+ dependent proteases, in HepG2 cells, macrophages, and CHO cells expressing apoE.46,48 Furthermore, the degradation of apoE by HepG2 cells is inhibited by incubation in Ca2+-free medium. This has been interpreted as indicating that Ca2+-dependent proteases may contribute to intracellular degradation.46 However, interpretation is complicated by the effects of Ca2+ on intracellular signaling, and more recently, its direct role in apoE secretion.49
ApoE degradation has been inhibited by up to 60% to 70% by the use of lysosomal chemical inhibitors.35 Concentrations of the lysosomal inhibitor chloroquine, which effectively inhibited apoE degradation (25 to 100 µmol/L), did not stimulate apoE secretion.35 In other studies, a cysteine protease inhibitor (ALLN) and lysosomal inhibitors did not stimulate apoE secretion despite effectively inhibiting degradation.21 The lack of stimulation of apoE secretion when degradation is inhibited suggests that some cellular pools of apoE are committed either to degradation or secretion.21 In this model, committed entry into the lysosomal or other degradatory compartment precludes reentry into the secretory compartment. However, as a number of studies have shown that stimulation of apoE secretion by HDL and apoA-I decreases net degradation of apoE,17,47,50 it is likely that there are other "uncommitted" pools of apoE otherwise destined for degradation which can be redirected to secretion.
We have recently modeled the secretion and degradation of apoE in human macrophages measured by [35S]-methionine labeling pulse-chase turnover studies and fitting the data to a first order equation (Figure 2A).17 The data best fit a model in which [35S]-apoE could exist within mobile (Em) or stable (Es) pools, where Em, but not Es, was available for secretion or degradation. In response to stimulation of apoE secretion with apoA-I (see below), the rate of degradation decreased, whereas Es was unchanged. These results support that some Em pools are "uncommitted" and accessible to either secretory or degradatory pathways.
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Em/
Stimulated Secretion of ApoE
As distinct from factors that increase apoE secretion by stimulating its synthesis (eg, LXR/RXR ligands, cholesterol enrichment, cytokines,14,51,52 a number of agents have been shown to directly increase apoE secretion (independent of transcription). These include, LDL,48,53 HDL,47,50 apoA-I, A-II, A-IV, apoE,17–19,38 phospholipid vesicles (PLV),38,50 heparinase, oleic acid, and lactoferrin.20,21,28 Phospholipid discs containing apoA-I are more potent stimuli of apoE secretion than phospholipid vesicles or lipid-free apoA-I.17 Such stimulation presumably occurs at the level of the Golgi or post-Golgi compartments, as the secreted apoE is almost always to some extent glycosylated and sialylated. Most studies to date have not been designed to discriminate between stimulated de novo secretion of endogenous apoE and stimulated resecretion of reinternalized endogenous apoE.39
The observation that apoE can stimulate its own secretion from macrophages raises the potential of a positive feedback loop, which may be of particular importance in the microenvironment of the arterial wall. Physiological concentrations of apoE in the arterial wall have been demonstrated to be atheroprotective.54 The natural isoforms of apoE vary in their ability to exogenously stimulate secretion. Exogenous apoE4 is less effective at stimulating apoE3 secretion from macrophages than are apoE3 and apoE2,17 which may be relevant for the increased atherosclerotic risk associated with the apoE4 phenotype.55 This compares with studies on the secretion of endogenous apoE isoforms where apoE2 is secreted by macrophages less effectively than apoE3 or apoE456 and recycling studies of exogenous apoE4 which suggest that apoE4 recycles less efficiently than other isoforms.57
The amount of apoE bound to, and released from, the cell surface can be expected to affect the stimulation achieved by various agents. Increased secretion when proteoglycan synthesis is inhibited suggests that cell surface binding can sequester apoE and inhibit its secretion.21 Cell surface apoE can also be bound to lipoprotein receptors, such that antiserum to the LDL receptor (LDLr)58 and suramin (which blocks ligand binding to the LDL-receptor related protein LRP) can reduce sequestration on the macrophage surface and increase apoE secretion.59 Simple displacement of apoE from the macrophage surface is unlikely to fully explain apoA-I–stimulated apoE secretion.17
As cell surface pools are relatively small, rapidly internalized, and degraded,21,37 vesicular traffic and exocytosis of apoE probably continuously replenish cell surface pools of apoE, in which case cell surface and intracellular apoE transport mechanisms may need to be considered as closely connected. Recent studies from our own group indicate that inhibition of intracellular vesicular trafficking results in very rapid inhibition of apoE secretion,49 implying that residence time on the cell surface is either relatively brief in primary human macrophages or contributes relatively little to apoE secretion.
ATP-Binding Cassette Transporters and ApoE Secretion
ABCA1, Constitutive Macrophage ApoE Secretion and Cholesterol Efflux
The ATP binding cassette transporter ABCA1 has been identified as the key regulator of cholesterol efflux to apoA-I and the protein which is defective in Tangier disease.60–62 ABCA1 activity contributes to basal constitutive secretion of apoE, as macrophages from patients with Tangier disease17,63 and normal macrophages in which ABCA1 activity or expression is inhibited show reduced apoE secretion.63 Similar observations were made in the brain, where murine ABCA1 deficiency was associated with lower tissue apoE levels and a markedly reduced secretion of apoE from astrocytes and microglial cells.64,65 The mechanism by which ABCA1 regulates apoE secretion is unclear. Cells from patients with Tangier disease and mice with ABCA1 deficiency have a morphologically expanded Golgi complex,66 in parallel with impaired intracellular transport, likely attributable to perturbed vesicular budding between the Golgi and the plasma membrane,67 and these characteristics would be expected to disturb apoE trafficking. The ABCA1 transporter has also been reported to modulate late endocytic trafficking of cholesterol and the Niemann Pick Type 1 (NPC1) protein,68 raising the additional possibility that late endosomal dysfunction could perturb apoE secretion in Tangier disease.
Although the pathways stimulating apoE secretion and cholesterol efflux are distinct,17,44 apoE, whether added to cells or during its secretion from cells, can itself mediate cholesterol efflux. ABCA1 primarily, but not exclusively, mediates cholesterol efflux to delipidated or lipid-poor apolipoproteins. There is evidence to suggest that exogenous apoE removes cholesterol via an ABCA1-mediated pathway, whereas secreted apoE may also remove cholesterol in an ABCA1-independent manner.69 Increasing lipidation of cholesterol acceptors decreases the contribution of ABCA1 to cholesterol efflux relative to that of ATP-binding cassette transporter G1 (ABCG1).70,71 This raises the possibility that ABCG1 may contribute to cholesterol efflux mediated by phospholipid-containing secreted apoE particles.
ABCG1 and ApoE Secretion
ABCG1 is a major mediator of cholesterol efflux to phospholipid-containing acceptor particles such as HDL, contributes to reverse cholesterol transport of macrophage cholesterol (reviewed in71), and may have a role in apoE expression or secretion. Deletion of macrophage ABCG1 has recently been shown to be associated with increased expression of ABCA1, increased cellular apoE levels, and increased secretion of apoE.72 Transplantation of these ABCG1-deficient macrophages into LDLr–/– mice led to an increase in plasma apoE concentrations.72 However, in another study, total body ABCG1-deletion had no effect on plasma levels of apoE.73 The discrepancy between the 2 studies may arise from differences between macrophage-specific and total body ABCG1-deletion, differences between LDLr-deficient and LDLr-replete animal models, or from differences in the severity of hypercholesterolemia achieved in the 2 studies. Assuming that there are circumstances in which ABCG1-deletion does modulate apoE secretion, it remains to be established whether this is secondary to increased apoE synthesis, to indirect effects on ABCA1 activity or cellular cholesterol status, or a direct effect of ABCG1 on apoE trafficking.
ApoE in the Formation of Nascent HDL Particles
ApoE secreted from macrophages is lipid-poor.74 However, on cholesterol enrichment of macrophages, apoE is secreted as discoidal particles with a density of <1.21g/mL.16–18,74 In these cholesterol-enriched conditions, secretion of apoE can promote cholesterol efflux in the absence of added cholesterol acceptors.74 Secreted apoE can also associate with exogenous HDL particles.44 After exposure of foam cell macrophages to apoA-I, most apoA-I–cholesterol–phospholipid particles formed are free of apoE, and secreted apoE is found on both apoE-only and apoE/apoA-I–containing particles.19 Interestingly, although apoA-I–stimulated apoE secretion (see below) is not directly dependent on ABCA1, its lipidation does require expression of ABCA1, as apoE secreted from ABCA1-deficient macrophages is only partially lipidated and floats at a higher density (>1.21 g/mL) than from normal macrophages.18 This suggests that lipidation of apoE may be dependent on efflux of phospholipid or cholesterol mediated by ABCA1.
ApoA-I–Mediated ApoE Secretion Is ABCA1-Independent
Whereas apoA-I–mediated cholesterol efflux is critically dependent on the expression and activity of ABCA1, we and others have demonstrated that apoA-I–stimulated apoE secretion is ABCA1-independent.17,18 Studies using recombinant apoA-I with selected deletions and point mutations established that there are different structural requirements for apoA-I–induced apoE secretion and apoA-I–induced cholesterol efflux.17 For example, deletion of the C-terminal domain (AA190–243) reduced apoA-I–induced cholesterol efflux, but promoted apoA-I–induced apoE secretion, and deletion of the central region (AA123-166) did not significantly affect cholesterol efflux but did inhibit apoE secretion. Additionally, a range of synthetic 
-helical peptides which stimulated apoE secretion to an extent similar to intact apoA-I were incapable of stimulating cholesterol efflux.17 Stimulation of apoE secretion is a general property of -helix containing molecules including apoA-II, A-IV, and apoE.17 Most importantly, macrophages from patients with Tangier disease, while secreting far less apoE under basal conditions, secreted normal amounts of apoE when exposed to apoA-I.17 These results indicate that apoA-I induces apoE secretion via an ABCA1-independent pathway, and that this is distinct from basal apoE secretion, which is ABCA1-dependent.
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Regulation of ApoE Secretion From Macrophages |
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TopAbstractIntroductionBasic Biology of ApoE...Regulation of ApoE Secretion...Future DirectionsReferences |
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General Aspects of Regulated Protein Secretion From Macrophages
In the classical constitutive pathway implicated in apoE secretion, proteins travel from the ER to the Golgi, and thence through the Golgi apparatus to the plasma membrane in tubulo-vesicular carriers.75,76 In the secretion of other proteins, pathways regulating exocytic and endocytic vesicle traffic can interact at different levels,77,78 while using the same intermediate endo/lysosomal vesicles. Recent studies have shown that in macrophages the recycling endosome can act as a sorting station for interleukin (IL)-6 and tumor necrosis factor (TNF)
.77 The role of "sorting stations" for apoE secretion requires investigation. Regulated secretion, as observed in mast cells and platelets,79,80 is usually distinguished from classical constitutive secretion by the involvement of secretory granules containing accumulated secretory products that are released on cellular recognition of appropriate stimuli.75 A third, lysosomal secretory pathway, is used by cathepsins and involves a subset of specialized lysosomes with unique morphology and content that release their constituents in response to external stimuli.81 Finally, a nonclassical or ER/Golgi-independent secretory pathway is described.82 This is relevant to proteins which do not possess a signal sequence necessary for translocation to the ER, and this pathway is not inhibited by brefeldin A or monensin. IL-1 and high mobility group Box-1 (HMBG1) protein are secreted from macrophages via this route.82 Although constitutive secretion and regulated exocytosis of proteins have traditionally been considered quite distinct, apoE appears to combine constitutive secretion with acute regulation.
Macrophage ApoE Secretion Involves Protein Kinase A, Intracellular Calcium, and the Microtubular Network
Recent studies in other systems have demonstrated that constitutive protein secretion is regulated, and involves kinases, phosphatases, receptors, and typically, the microtubule network.76,83,84 In view of previous studies indicating that apoE secretion could be stimulated by apoA-I and by HDL,17,19,38,47 we hypothesized that even basal apoE secretion must be under some form of regulation. We showed that disruption of microtubules inhibited basal apoE secretion, whereas disruption of the actin skeleton had no effect. Microscopy studies supported this by showing alignment of apoE-containing vesicles along the microtubules but not the actin cytoskeleton in RAW macrophages.49
Protein kinase A (PKA) regulates traffic of several proteins at different steps in the constitutive secretory pathway,85 and a range of PKA inhibitors decreased secretion of apoE.49 Live cell imaging of apoE-GFP–transfected RAW macrophages indicated that PKA is required for the movement of apoE-containing vesicles. Pulse-chase apoE turnover studies showed that inhibition of PKA specifically reduces the rate of apoE secretion, expanding the size of the intracellular apoE pool, but does not affect its degradation.49 This suggests that apoE-containing secretory vesicles accumulate within the cell in response to PKA inhibition, at a site inaccessible to lysosomal degradation (Figure 2). Importantly, an extracellular PKA agonist did not stimulate apoE secretion, indicating constitutive PKA activity is sufficient.
Another important regulator of secretion is the second messenger calcium, which is also required for the regulated secretion of neurotransmitters and hormones from endocrine and neuronal cells.86–88 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, but chelation of extracellular calcium with EGTA had no effect. As inhibitors of phospholipase C (PLC) and the inositol triphosphate (IP3) receptor (IP3R) also decreased secretion of apoE, a role for PLC and IP3R in the mobilization of [Ca2+]i during secretion of apoE is most likely.49
Although PKA and [Ca2+]i are required for apoE secretion, neither stimulation of PKA activity nor increased [Ca2+]i stimulated basal apoE secretion from macrophages; conversely apoA-1-stimulated apoE secretion was not accompanied by an increase in cellular PKA activity or [Ca2+]i. Thus [Ca2+]i and PKA can be considered permissive but do not explain how apoE secretion is stimulated. The IC50 for inhibiting basal apoE secretion with the PKA inhibitor H89 is half that for its inhibition of apoA-I–stimulated apoE secretion.49 Therefore it is likely that ABCA1-mediated basal apoE secretion occurs predominantly via a PKA-sensitive pathway, and that apoA-I–stimulated secretion additionally involves a contribution from an as yet undefined PKA-insensitive pathway (Figure 2B). Whether PKA and calcium interact during apoE secretion as described elsewhere,86,89,90 or act at different steps in the secretory pathway, requires further investigation.
As constitutive release of lysozyme and other proteins from macrophages were also found to be PKA-, microtubule-, and calcium-dependent,49 the regulation of apoE secretion can be considered a prototype for understanding the processes regulating the secretion of other macrophage proteins.
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Future Directions |
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TopAbstractIntroductionBasic Biology of ApoE...Regulation of ApoE Secretion...Future DirectionsReferences |
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The regulation of apoE secretion from macrophages provides an important avenue for rapidly regulating the concentration of apoE in local cellular environments such as the arterial wall, and potentially in the plasma. However there are quite major limitations to our current understanding of how secretion of apoE is regulated. The intracellular routes for recycling of macrophage-secreted apoE require further careful mapping and comparison made with those required for recycling of lipoprotein-derived apoE. Once these routes are defined, the relative quantitative contribution of resecretion of macrophage-derived apoE and secretion of Golgi-network-derived de novo apoE under basal and apoA-I–induced conditions can be understood. The signaling and vesicular pathways regulating apoE secretion in macrophages clearly need deeper exploration and will require identifying the different pathways involved and how they interact, including the signaling partners and subcellular sites of action for PKA. The relevance of these pathways to cells other than macrophages cannot be automatically assumed and will require in vitro evaluation before targeted manipulation of apoE secretion in defined tissues can be undertaken. Targeted inhibition or stimulation of specific apoE trafficking pathways in vivo will provide unique opportunities to test directly the role of secreted macrophage-derived apoE in regulating atherosclerosis.
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Acknowledgments |
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The contributions of Kat Gaus to modelling studies and Jenny Stow and Jay Kay to trafficking studies performed in our laboratory are gratefully acknowledged.
Disclosures
Supported by National Health and Medical Research Council of Australia (grants 455252 and 482800).
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Footnotes |
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Original received February 4, 2008; final version accepted March 21, 2008.
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References |
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1. Linton MF, Atkinson JB, Fazio S. Prevention of atherosclerosis in apolipoprotein E-deficient mice by bone marrow transplantation. Science. 1995; 267: 1034–1037.
2. Mahley RW, Rall SC Jr. Apolipoprotein E. far more than a lipid transport protein. Annu Rev Genomics Hum Genet. 2000; 1: 507–537.[CrossRef][Medline] [Order article via Infotrieve]
3. Mazzone T, Reardon C. Expression of heterologous human apolipoprotein E by J774 macrophages enhances cholesterol efflux to HDL3. J Lipid Res. 1994; 35: 1345–1353.[Abstract]
4. Kelly ME, Clay MA, Mistry MJ, Hsieh-Li HM, Harmony JA. Apolipoprotein E inhibition of proliferation of mitogen-activated T lymphocytes: production of interleukin 2 with reduced biological activity. Cell Immunol. 1994; 159: 124–139.[CrossRef][Medline] [Order article via Infotrieve]
5. Kothapalli D, Fuki I, Ali K, Stewart SA, Zhao L, Yahil R, Kwiatkowski D, Hawthorne EA, FitzGerald GA, Phillips MC, Lund-Katz S, Pure E, Rader DJ, Assoian RK. Antimitogenic effects of HDL and APOE mediated by Cox-2-dependent IP activation. J Clin Invest. 2004; 113: 609–618.[CrossRef][Medline] [Order article via Infotrieve]
6. van den Elzen P, Garg S, Leon L, Brigl M, Leadbetter EA, Gumperz JE, Dascher CC, Cheng TY, Sacks FM, Illarionov PA, Besra GS, Kent SC, Moody DB, Brenner MB. Apolipoprotein-mediated pathways of lipid antigen presentation. Nature. 2005; 437: 906–910.[CrossRef][Medline] [Order article via Infotrieve]
7. Ali K, Middleton M, Pure E, Rader DJ. Apolipoprotein E suppresses the type I inflammatory response in vivo. Circ Res. 2005; 97: 922–927.
8. Fazio S, Linton MF, Swift LL. The cell biology and physiologic relevance of ApoE recycling. Trends Cardiovasc Med. 2000; 10: 23–30.[CrossRef][Medline] [Order article via Infotrieve]
9. Heeren J, Beisiegel U, Grewal T. Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis. Arterioscler Thromb Vasc Biol. 2006; 26: 442–448.
10. Swift LL, Hasty AH, LInton MF, Fazio S. Apolipoprotein E and reverse cholesterol transport: the role of recycling in plasma and intracellular lipid trafficking. In: Fielding CJ, ed. High-Density Lipoproteins: From Basic Biology to Clinical Aspects. John Wiley; 2007.
11. Shih SJ, Allan C, Grehan S, Tse E, Moran C, Taylor JM. Duplicated downstream enhancers control expression of the human apolipoprotein E gene in macrophages and adipose tissue. J Biol Chem. 2000; 275: 31567–31572.
12. Gafencu AV, Robciuc MR, Fuior E, Zannis VI, Kardassis D, Simionescu M. Inflammatory signaling pathways regulating ApoE gene expression in macrophages. J Biol Chem. 2007; 282: 21776–21785.
13. Singh NN, Ramji DP. Transforming growth factor-beta-induced expression of the apolipoprotein E gene requires c-Jun N-terminal kinase, p38 kinase, and casein kinase 2. Arterioscler Thromb Vasc Biol. 2006; 26: 1323–1329.
14. Laffitte BA, Repa JJ, Joseph SB, Wilpitz DC, Kast HR, Mangelsdorf DJ, Tontonoz P. LXRs control lipid-inducible expression of the apolipoprotein E gene in macrophages and adipocytes. Proc Natl Acad Sci U S A. 2001; 98: 507–512.
15. Galetto R, Albajar M, Polanco JI, Zakin MM, Rodriguez-Rey JC. Identification of a peroxisome-proliferator-activated-receptor response element in the apolipoprotein E gene control region. Biochem J. 2001; 357: 521–527.[CrossRef][Medline] [Order article via Infotrieve]
16. Basu SK, Ho YK, Brown MS, Bilheimer DW, Anderson RG, Goldstein JL. Biochemical and genetic studies of the apoprotein E secreted by mouse macrophages and human monocytes. J Biol Chem. 1982; 257: 9788–9795.
17. Kockx M, Rye KA, Gaus K, Quinn CM, Wright J, Sloane T, Sviridov D, Fu Y, Sullivan D, Burnett JR, Rust S, Assmann G, Anantharamaiah GM, Palgunachari MN, Katz SL, Phillips MC, Dean RT, Jessup W, Kritharides L. Apolipoprotein A-I-stimulated apolipoprotein E secretion from human macrophages is independent of cholesterol efflux. J Biol Chem. 2004; 279: 25966–25977.
18. Yancey PG, Yu H, Linton MF, Fazio S. A pathway-dependent on apoE, ApoAI, and ABCA1 determines formation of buoyant high-density lipoprotein by macrophage foam cells. Arterioscler Thromb Vasc Biol. 2007; 27: 1123–1131.
19. Bielicki JK, McCall MR, Forte TM. Apolipoprotein A-I promotes cholesterol release and apolipoprotein E recruitment from THP-1 macrophage-like foam cells. J Lipid Res. 1999; 40: 85–92.
20. Burgess JW, Gould DR, Marcel YL. The HepG2 extracellular matrix contains separate heparinase- and lipid-releasable pools of ApoE. Implications for hepatic lipoprotein metabolism. J Biol Chem. 1998; 273: 5645–5654.
21. Lucas M, Mazzone T. Cell surface proteoglycans modulate net synthesis and secretion of macrophage apolipoprotein E. J Biol Chem. 1996; 271: 13454–13460.
22. Zhao Y, Mazzone T. Transport and processing of endogenously synthesized ApoE on the macrophage cell surface. J Biol Chem. 2000; 275: 4759–4765.
23. Zannis VI, McPherson J, Goldberger G, Karathanasis SK, Breslow JL. Synthesis, intracellular processing, and signal peptide of human apolipoprotein E. J Biol Chem. 1984; 259: 5495–5499.
24. Wernette-Hammond ME, Lauer SJ, Corsini A, Walker D, Taylor JM, Rall SC Jr. Glycosylation of human apolipoprotein E. The carbohydrate attachment site is threonine 194. J Biol Chem. 1989; 264: 9094–9101.
25. Zannis VI, vanderSpek J, Silverman D. Intracellular modifications of human apolipoprotein E. J Biol Chem. 1986; 261: 13415–13421.
26. Marmillot P, Rao MN, Liu QH, Lakshman MR. Desialylation of human apolipoprotein E decreases its binding to human high-density lipoprotein and its ability to deliver esterified cholesterol to the liver. Metabolism. 1999; 48: 1184–1192.[CrossRef][Medline] [Order article via Infotrieve]
27. Zanni EE, Kouvatsi A, Hadzopoulou-Cladaras M, Krieger M, Zannis VI. Expression of ApoE gene in Chinese hamster cells with a reversible defect in O-glycosylation. Glycosylation is not required for apoE secretion. J Biol Chem. 1989; 264: 9137–9140.
28. Huang ZH, Gu D, Mazzone T. Oleic acid modulates the post-translational glycosylation of macrophage ApoE to increase its secretion. J Biol Chem. 2004; 279: 29195–29201.
29. Werb Z, Chin JR, Takemura R, Oropeza RL, Bainton DF, Stenberg P, Taylor JM, Reardon C. The cell and molecular biology of apolipoprotein E synthesis by macrophages. Ciba Found Symp. 1986; 118: 155–171.[Medline] [Order article via Infotrieve]
30. Dahan S, Ahluwalia JP, Wong L, Posner BI, Bergeron JJ. Concentration of intracellular hepatic apolipoprotein E in Golgi apparatus saccular distensions and endosomes. J Cell Biol. 1994; 127: 1859–1869.
31. Hamilton RL, Wong JS, Guo LS, Krisans S, Havel RJ. Apolipoprotein E localization in rat hepatocytes by immunogold labeling of cryothin sections. J Lipid Res. 1990; 31: 1589–1603.[Abstract]
32. Vance JE. Phospholipid synthesis in a membrane fraction associated with mitochondria. J Biol Chem. 1990; 265: 7248–7256.
33. Kruth HS, Skarlatos SI, Lilly K, Chang J, Ifrim I. Sequestration of acetylated LDL and cholesterol crystals by human monocyte-derived macrophages. J Cell Biol. 1995; 129: 133–145.
34. Werb Z, Takemura R, Stenberg PE, Bainton DF. Directed exocytosis of secretory granules containing apolipoprotein E to the adherent surface and basal vacuoles of macrophages spreading on immobile immune complexes. Am J Pathol. 1989; 134: 661–670.[Abstract]
35. Deng J, Rudick V, Dory L. Lysosomal degradation and sorting of apolipoprotein E in macrophages. J Lipid Res. 1995; 36: 2129–2140.[Abstract]
36. Schmitt M, Grand-Perret T. Regulated turnover of a cell surface-associated pool of newly synthesized apolipoprotein E in HepG2 cells. J Lipid Res. 1999; 40: 39–49.
37. Deng J, Rudick V, Rudick M, Dory L. Investigation of plasma membrane-associated apolipoprotein E in primary macrophages. J Lipid Res. 1997; 38: 217–227.[Abstract]
38. Rees D, Sloane T, Jessup W, Dean RT, Kritharides L. Apolipoprotein A-I stimulates secretion of apolipoprotein E by foam cell macrophages. J Biol Chem. 1999; 274: 27925–27933.
39. Hasty AH, Plummer MR, Weisgraber KH, Linton MF, Fazio S, Swift LL. The recycling of apolipoprotein E in macrophages: influence of HDL and apolipoprotein A-I. J Lipid Res. 2005; 46: 1433–1439.
40. Ho YY, Al-Haideri M, Mazzone T, Vogel T, Presley JF, Sturley SL, Deckelbaum RJ. Endogenously expressed apolipoprotein E has different effects on cell lipid metabolism as compared to exogenous apolipoprotein E carried on triglyceride-rich particles. Biochemistry. 2000; 39: 4746–4754.[CrossRef][Medline] [Order article via Infotrieve]
41. Fazio S, Linton MF, Hasty AH, Swift LL. Recycling of apolipoprotein E in mouse liver. J Biol Chem. 1999; 274: 8247–8253.
42. Braun NA, Mohler PJ, Weisgraber KH, Hasty AH, Linton MF, Yancey PG, Su YR, Fazio S, Swift LL. Intracellular trafficking of recycling apolipoprotein E in Chinese hamster ovary cells. J Lipid Res. 2006; 47: 1176–1186.
43. Heeren J, Grewal T, Laatsch A, Rottke D, Rinninger F, Enrich C, Beisiegel U. Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization. J Biol Chem. 2003; 278: 14370–14378.
44. Basu SK, Goldstein JL, Brown MS. Independent pathways for secretion of cholesterol and apolipoprotein E by macrophages. Science. 1983; 219: 871–873.
45. Zhu MY, Hasty AH, Harris C, Linton MF, Fazio S, Swift LL. Physiological relevance of apolipoprotein E recycling: studies in primary mouse hepatocytes. Metabolism. 2005; 54: 1309–1315.[CrossRef][Medline] [Order article via Infotrieve]
46. Ye SQ, Reardon CA, Getz GS. Inhibition of apolipoprotein E degradation in a post-Golgi compartment by a cysteine protease inhibitor. J Biol Chem. 1993; 268: 8497–8502.
47. Dory L. Regulation of apolipoprotein E secretion by high density lipoprotein 3 in mouse macrophages. J Lipid Res. 1991; 32: 783–792.[Abstract]
48. Duan H, Lin CY, Mazzone T. Degradation of macrophage ApoE in a nonlysosomal compartment. Regulation by sterols. J Biol Chem. 1997; 272: 31156–31162.
49. Kockx M, Guo DL, Huby T, Lesnik P, Kay J, Sabaretnam T, Jary E, Hill M, Gaus K, Chapman J, Stow JL, Jessup W, Kritharides L. Secretion of apolipoprotein E from macrophages occurs via a protein kinase A and calcium-dependent pathway along the microtubule network. Circ Res. 2007; 101: 607–616.
50. Mazzone T, Pustelnikas L, Reardon CA. Post-translational regulation of macrophage apoprotein E production. J Biol Chem. 1992; 267: 1081–1087.
51. Mazzone T, Gump H, Diller P, Getz GS. Macrophage free cholesterol content regulates apolipoprotein E synthesis. J Biol Chem. 1987; 262: 11657–11662.
52. Zuckerman SH, Evans GF, O’Neal L. Cytokine regulation of macrophage apo E secretion: opposing effects of GM-CSF and TGF-beta. Atherosclerosis. 1992; 96: 203–214.[CrossRef][Medline] [Order article via Infotrieve]
53. Ye SQ, Olson LM, Reardon CA, Getz GS. Human plasma lipoproteins regulate apolipoprotein E secretion from a post-Golgi compartment. J Biol Chem. 1992; 267: 21961–21966.
54. Fazio S, Babaev VR, Burleigh ME, Major AS, Hasty AH, Linton MF. Physiological expression of macrophage apoE in the artery wall reduces atherosclerosis in severely hyperlipidemic mice. J Lipid Res. 2002; 43: 1602–1609.
55. Ilveskoski E, Perola M, Lehtimaki T, Laippala P, Savolainen V, Pajarinen J, Penttila A, Lalu KH, Mannikko A, Liesto KK, Koivula T, Karhunen PJ. Age-dependent association of apolipoprotein E genotype with coronary and aortic atherosclerosis in middle-aged men: an autopsy study. Circulation. 1999; 100: 608–613.
56. Fan D, Qiu S, Overton CD, Yancey PG, Swift LL, Jerome WG, Linton MF, Fazio S. Impaired secretion of apolipoprotein E2 from macrophages. J Biol Chem. 2007; 282: 13746–13753.
57. Heeren J, Grewal T, Laatsch A, Becker N, Rinninger F, Rye KA, Beisiegel U. Impaired recycling of apolipoprotein E4 is associated with intracellular cholesterol accumulation. J Biol Chem. 2004; 279: 55483–55492.
58. Zhao Y, Mazzone T. LDL receptor binds newly synthesized apoE in macrophages. A precursor pool for apoe secretion. J Lipid Res. 1999; 40: 1029–1035.
59. Cullen P, Cignarella A, Brennhausen B, Mohr S, Assmann G, von Eckardstein A. Phenotype-dependent differences in apolipoprotein E metabolism and in cholesterol homeostasis in human monocyte-derived macrophages. J Clin Invest. 1998; 101: 1670–1677.[Medline] [Order article via Infotrieve]
60. Brooks-Wilson A, Marcil M, Clee SM, Zhang LH, Roomp K, van Dam M, Yu L, Brewer C, Collins JA, Molhuizen HO, Loubser O, Ouelette BF, Fichter K, Ashbourne-Excoffon KJ, Sensen CW, Scherer S, Mott S, Denis M, Martindale D, Frohlich J, Morgan K, Koop B, Pimstone S, Kastelein JJ, Genest J Jr, Hayden MR. Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency. Nat Genet. 1999; 22: 336–345.[CrossRef][Medline] [Order article via Infotrieve]
61. Rust S, Rosier M, Funke H, Real J, Amoura Z, Piette JC, Deleuze JF, Brewer HB, Duverger N, Denefle P, Assmann G. Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1. Nat Genet. 1999; 22: 352–355.[CrossRef][Medline] [Order article via Infotrieve]
62. Bodzioch M, Orso E, Klucken J, Langmann T, Bottcher A, Diederich W, Drobnik W, Barlage S, Buchler C, Porsch-Ozcurumez M, Kaminski WE, Hahmann HW, Oette K, Rothe G, Aslanidis C, Lackner KJ, Schmitz G. The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease. Nat Genet. 1999; 22: 347–351.[CrossRef][Medline] [Order article via Infotrieve]
63. Von Eckardstein A, Langer C, Engel T, Schaukal I, Cignarella A, Reinhardt J, Lorkowski S, Li Z, Zhou X, Cullen P, Assmann G. ATP binding cassette transporter ABCA1 modulates the secretion of apolipoprotein E from human monocyte-derived macrophages. Faseb J. 2001; 15: 1555–1561.
64. Hirsch-Reinshagen V, Zhou S, Burgess BL, Bernier L, McIsaac SA, Chan JY, Tansley GH, Cohn JS, Hayden MR, Wellington CL. Deficiency of ABCA1 impairs apolipoprotein E metabolism in brain. J Biol Chem. 2004; 279: 41197–41207.
65. Wahrle SE, Jiang H, Parsadanian M, Legleiter J, Han X, Fryer JD, Kowalewski T, Holtzman DM. ABCA1 is required for normal central nervous system ApoE levels and for lipidation of astrocyte-secreted apoE. J Biol Chem. 2004; 279: 40987–40993.
66. Robenek H, Schmitz G. Abnormal processing of Golgi elements and lysosomes in Tangier disease. Arterioscler Thromb. 1991; 11: 1007–1020.
67. Orso E, Broccardo C, Kaminski WE, Bottcher A, Liebisch G, Drobnik W, Gotz A, Chambenoit O, Diederich W, Langmann T, Spruss T, Luciani MF, Rothe G, Lackner KJ, Chimini G, Schmitz G. Transport of lipids from golgi to plasma membrane is defective in tangier disease patients and Abc1-deficient mice. Nat Genet. 2000; 24: 192–196.[CrossRef][Medline] [Order article via Infotrieve]
68. Neufeld EB, Stonik JA, Demosky SJ Jr, Knapper CL, Combs CA, Cooney A, Comly M, Dwyer N, Blanchette-Mackie J, Remaley AT, Santamarina-Fojo S, Brewer HB Jr. The ABCA1 transporter modulates late endocytic trafficking: insights from the correction of the genetic defect in Tangier disease. J Biol Chem. 2004; 279: 15571–15578.
69. Huang ZH, Fitzgerald ML, Mazzone T. Distinct cellular loci for the ABCA1-dependent and ABCA1-independent lipid efflux mediated by endogenous apolipoprotein E expression. Arterioscler Thromb Vasc Biol. 2006; 26: 157–162.
70. Gelissen IC, Harris M, Rye KA, Quinn C, Brown AJ, Kockx M, Cartland S, Packianathan M, Kritharides L, Jessup W. ABCA1 and ABCG1 synergize to mediate cholesterol export to apoA-I. Arterioscler Thromb Vasc Biol. 2006; 26: 534–540.
71. Jessup W, Gelissen IC, Gaus K, Kritharides L. Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages. Curr Opin Lipidol. 2006; 17: 247–257.[Medline] [Order article via Infotrieve]
72. Ranalletta M, Wang N, Han S, Yvan-Charvet L, Welch C, Tall AR. Decreased atherosclerosis in low-density lipoprotein receptor knockout mice transplanted with Abcg1-/- bone marrow. Arterioscler Thromb Vasc Biol. 2006; 26: 2308–2315.[CrossRef][Medline] [Order article via Infotrieve]
73. Out R, Hoekstra M, Meurs I, de Vos P, Kuiper J, Van Eck M, Van Berkel TJ. Total body ABCG1 expression protects against early atherosclerotic lesion development in mice. Arterioscler Thromb Vasc Biol. 2007; 27: 594–599.
74. Zhang WY, Gaynor PM, Kruth HS. Apolipoprotein E produced by human monocyte-derived macrophages mediates cholesterol efflux that occurs in the absence of added cholesterol acceptors. J Biol Chem. 1996; 271: 28641–28646.
75. Burgess TL, Kelly RB. Constitutive and regulated secretion of proteins. Annu Rev Cell Biol. 1987; 3: 243–293.[CrossRef][Medline] [Order article via Infotrieve]
76. Hirschberg K, Lippincott-Schwartz J. Secretory pathway kinetics and in vivo analysis of protein traffic from the Golgi complex to the cell surface. Faseb J. 1999; 13 Suppl 2: S251–256.
77. Manderson AP, Kay JG, Hammond LA, Brown DL, Stow JL. Subcompartments of the macrophage recycling endosome direct the differential secretion of IL-6 and TNFalpha. J Cell Biol. 2007; 178: 57–69.
78. Stow JL, Manderson AP, Murray RZ. SNAREing immunity: the role of SNAREs in the immune system. Nat Rev Immunol. 2006; 6: 919–929.[CrossRef][Medline] [Order article via Infotrieve]
79. Rendu F, Brohard-Bohn B. The platelet release reaction: granules’ constituents, secretion and functions. Platelets. 2001; 12: 261–273.[CrossRef][Medline] [Order article via Infotrieve]
80. Blank U, Rivera J. The ins and outs of IgE-dependent mast-cell exocytosis. Trends Immunol. 2004; 25: 266–273.[CrossRef][Medline] [Order article via Infotrieve]
81. Blott EJ, Griffiths GM. Secretory lysosomes. Nat Rev Mol Cell Biol. 2002; 3: 122–131.[CrossRef][Medline] [Order article via Infotrieve]
82. Nickel W. The mystery of nonclassical protein secretion. A current view on cargo proteins and potential export routes. Eur J Biochem. 2003; 270: 2109–2119.[Medline] [Order article via Infotrieve]
83. Webb RJ, Judah JD, Lo LC, Thomas GM. Constitutive secretion of serum albumin requires reversible protein tyrosine phosphorylation events in trans-Golgi. Am J Physiol Cell Physiol. 2005; 289: C748–C756.
84. Luini A, De Matteis MA. Receptor-mediated regulation of constitutive secretion. Trends Cell Biol. 1993; 3: 290–292.[CrossRef][Medline] [Order article via Infotrieve]
85. Muniz M, Alonso M, Hidalgo J, Velasco A. A regulatory role for cAMP-dependent protein kinase in protein traffic along the exocytic route. J Biol Chem. 1996; 271: 30935–30941.
86. Nakahari T, Ito S, Yoshida H, Furuya E, Imai Y. Accumulation of cAMP evoked by acetylcholine stimulation in rat submandibular acinar cells: observation of exocytosis, fluid secretion and [Ca2+]i. Exp Physiol. 2000; 85: 159–169.[Abstract]
87. Takei T, Takano K, Yasufuku-Takano J, Fujita T, Yamashita N. Enhancement of Ca2+ currents by GHRH and its relation to PKA and [Ca2+]i in human GH-secreting adenoma cells. Am J Physiol. 1996; 271: E801–E807.[Medline] [Order article via Infotrieve]
88. Neher E. Vesicle pools and Ca2+ microdomains: new tools for understanding their roles in neurotransmitter release. Neuron. 1998; 20: 389–399.[CrossRef][Medline] [Order article via Infotrieve]
89. Bugrim AE. Regulation of Ca2+ release by cAMP-dependent protein kinase. A mechanism for agonist-specific calcium signaling? Cell Calcium. 1999; 25: 219–226.[CrossRef][Medline] [Order article via Infotrieve]
90. Renstrom E, Eliasson L, Rorsman P. Protein kinase A-dependent and -independent stimulation of exocytosis by cAMP in mouse pancreatic B-cells. J Physiol. 1997; 502: 105–118.
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