Apolipoprotein E expression by human-monocyte-derived macrophages. Modulation by opsonised zymosan and cholesterol

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.

Adipophilin increases triglyceride storage in human macrophages by stimulation of biosynthesis and inhibition of beta-oxidation

Lipid accumulation alters macrophage biology and contributes to lipid retention within the vessel wall. In this study, we investigated the role of adipophilin on triglyceride accumulation and lipid-droplet formation in THP-1-derived macrophages (THP-1 macrophages). In the presence of acetylated low-density lipoprotein, macrophages infected with an adenovirus expressing human adipophilin showed a 31% increase in triglyceride content and a greater number of lipid droplets compared with control cells. Incubation of macrophages with very low-density lipoprotein (VLDL) dramatically increased cellular triglyceride content similarly in control and adipophilin-overexpressing cells. By itself, VLDL increased adipophilin expression, which explains the lack of effect of adipophilin overexpression on cellular triglyceride content in macrophages loaded with VLDL. The lipid-droplet content of macrophages was increased by overexpression of adipophilin and/or loading with VLDL. In contrast, inhibition of adipophilin expression using siRNA prevented lipid-droplet formation and significantly reduced intracellular triglyceride content. Using inhibitors of beta-oxidation and acyl-coenzyme A synthetase, results were obtained which suggest that adipophilin elevates cellular lipids by inhibition of beta-oxidation and stimulation of long-chain fatty acid incorporation into triglycerides. Adipophilin expression in THP-1 macrophages altered the cellular content of different lipids and enhanced the size of lipid droplets, consistent with a role for adipophilin in human foam cell formation.

Spatiotemporal changes of apolipoprotein E immunoreactivity and apolipoprotein E mRNA expression after transient middle cerebral artery occlusion in rat brain

Apolipoprotein E (ApoE) is a constituent of lipoprotein and plays an important role in the maintenance of neural networks. However, spatiotemporal differences in ApoE expression and its long-term role in neural process after brain ischemia have not been studied. We investigated changes of ApoE immunoreactivity and ApoE mRNA expression both in the core and in the periischemic area at 1, 7, 21, or 56 days after 90 min of transient middle cerebral artery occlusion. Double stainings for ApoE plus NeuN or plus ED1 were performed in order to identify cell type of ApoE-positive stainings. The maximal increase of ApoE expression was observed at 7 days in the core and at 7 and 21 days in the periischemic area. In the core, ApoE plus NeuN double-positive cells increased at 1 and 7 days, without ApoE mRNA expression, whereas they increased in the periischemic area, with a peak at 21 days, with ApoE mRNA expression in glial cells but not in neurons. On the other hand, ApoE plus ED1 double-positive cells increased only in the core, with a peak in number at 7 and 21 days and marked ApoE mRNA expression in macrophages. The present study suggests that ApoE plays various important roles in different type of cells, reflecting spatiotemporal dissociation between degenerative and regenerative processes after brain ischemia, and that ApoE is profoundly involved in pathological conditions, such as brain ischemia.

Delayed, but marked, expression of apolipoprotein E is involved in tissue clearance after cerebral infarction

Clearance of infarct tissue would be an important process for tissue repair after a stroke. Delayed clearance may hamper reconstitution of the blood-brain barrier and glial boundary formation. Recent growing evidence has indicated that apolipoprotein E (APOE), a major apoprotein, plays an important role in lipid transport and homeostasis in the brain. The tissue in the infarction contains abundant lipids must be removed for tissue clearance. In the current study, the authors investigated APOE expression after focal ischemia and the functional role of APOE in tissue clearance using APOE-knockout mice. Expression of APOE was delayed, but marked, in immunohistochemistry and immunoblotting 7 days after permanent focal ischemia. Macrophages were found to express APOE in the infarct center. Infarct size was similar after focal ischemia between wild-type and APOE-knockout mice, although there was no APOE protein expression in knockout mice. However, clearance of infarct tissue 2 weeks after ischemia was significantly delayed in APOE-knockout mice compared with wild-type mice. The current study supports current thinking that APOE is a key molecule for tissue remodeling in the brain. Clearance of damaged tissue may be one of the important functions of APOE in the brain.

Preferential reduction of very low density lipoprotein-1 particle number by fenofibrate in type IIB hyperlipidemia: consequences for lipid accumulation in human monocyte-derived macrophages

The combined (mixed) type IIB phenotype is typically associated with premature atherosclerosis and characterised by concomitant elevation of plasma levels of atherogenic triglyceride-rich lipoproteins, consisting of very low density lipoprotein (VLDL)-1 (Sf 60-400), VLDL-2 (Sf 20-60), and intermediate density lipoprotein (IDL) (Sf 12-20), as well as small dense LDL. After dietary stabilisation, type IIB patients received micronised fenofibrate (267 mg/day) for up to 12 months. At baseline (T0), patients (n=11) displayed fasting triglyceride, cholesterol and apoB levels of 308+/-13, 350+/-17 and 187+/-9 mg/dl, respectively. Micronised fenofibrate (M-fenofibrate) induced marked reductions in plasma triglyceride (TG) (-61%, P<0.0001), total cholesterol (-32%, P=0.0005) and apolipoprotein (apo) B (-33%, P<0.001) at 12 months (T12); similar effects were seen after 3 months (T3) of treatment. These changes resulted from significant reductions in VLDL-1 (-75%, P=0.00001), VLDL-2 (-46%, P=0.002) and LDL (-33%, P<0.0003); IDL concentrations were unchanged. At baseline, VLDL-1 constituted the major TG-rich lipoprotein (TRL) fraction (50% of total mass), but only 25% at T12. These drug effects were accompanied by marked increase in HDL-C (+20%, P=0.018). Quantitative changes in triglyceride-rich lipoproteins were accompanied by significant qualitative modifications in particle size and chemical composition (VLDL-1: TG, -10.7%, P<0.001; FC, +59%, P=0.0002; PL, +19%, P=0.033; VLDL-2: FC, +11%, P=0.027; IDL: FC, +14%, P=0.0004; PL, +12%, P=0.002). Reduction in the TG content of VLDL-1 was reflected in a shift of particle size distribution to smaller diameters (mean 45.4 and 42.3 nm, respectively, at T0 and T12). We evaluated the relative atherogenicity of TRL subfractions by determining their capacity, when normalised to equal particle numbers (as apoB 100 content), to induce lipid accumulation in human monocyte-derived macrophages. Among TRL subfractions, VLDL-1 (100 microg apoB/ml) possessed the highest capacity to induce macrophage lipid loading (up to sevenfold increase in TG content, P<0.001; free cholesterol, up to 1.7-fold; P<0.05). At 100 microg apoB/ml, cellular TG loading from VLDL-1 was twofold greater than that for VLDL-2 (P<0.01), and fivefold greater than for IDL (P<0.01). Despite drug-induced changes in the qualitative properties of TRL subfractions, the activity of VLDL-1, VLDL-2 and IDL as ligands which lead to induction of macrophage lipid accumulation, at equivalent particle numbers, was not detectably altered. By contrast, the fibrate-mediated reduction in the number of circulating VLDL-1 and VLDL-2 particles (four and twofold, respectively) resulted in marked decrease in cellular lipid loading. Considered together, these findings suggest that fenofibrate may act at systemic and arterial levels to reduce the cardiovascular risk associated with VLDL subfractions in patients with a combined hyperlipidemic (type IIB) phenotype. Indeed, we speculate that reductions in circulating levels of VLDL-1 and VLDL-2 may diminish intimal penetration of these particles and thus their propensity to enhance arterial macrophage lipid accumulation and foam cell formation. Finally, fenofibrate further attenuated the atherogenic lipid profile in these patients by inducing marked reduction in LDL and elevation in cardioprotective HDL.

Effects of pro-inflammatory cytokines on apolipoprotein E secretion by a human astrocytoma cell line (CCF-STTG1)

Apolipoprotein (apo) E has been implicated in Alzheimer's disease; however, little is known about the regulation of its secretion in astrocytes. To investigate the effects of pro-inflammatory cytokines such as interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) on apoE secretion by CCF-STTG1 cells, a sensitive and specific double sandwich Enzyme-Linked ImmunoSorbent Assay (ELISA) was developed. Using a monoclonal anti-human apoE antibody as the capture antibody, this assay was carried out with commercially available reagents. The assay had a sensitivity of 0.013 ng per well, within-run and between-run variation coefficients of 6.0 and 8.6 per cent respectively. There was no cross-reactions between antibodies used and apoAI, apoAII, apoB, apoCI, apoCII and apoCIII. Low apoE concentrations were assessed using a serum-free HepG2 culture medium as secondary calibrator, containing 59 microg l(-1) of apoE. In serum-free medium, CCF-STTG1 cells secreted apoE, the accumulation of which in the cell medium increased linearly with time (27 microg per 48 h). After 48 h of incubation, apoE secretion was inhibited by TNF-alpha but not affected by IL-1beta and IFN-gamma. However, the effect of regulatory factors may depend upon culture conditions since in the presence of 10 per cent fetal calf serum, IFN-gamma significantly inhibited apoE secretion. Thus, apoE secretion by CCF-STTG1 cells is inhibited by specific pro-inflammatory cytokines. This new apoE ELISA presents the great advantage of using commercially available reagents which permit inter-laboratory comparability of results, involves relatively low cost and is adaptable for the measurement of low levels of apoE.

Tissue factor pathway inhibitor is expressed by human monocyte-derived macrophages : relationship to tissue factor induction by cholesterol and oxidized LDL

Lipid-laden macrophages express tissue factor (TF), which may activate the extrinsic coagulation pathway on rupture of the atherosclerotic plaque. Tissue factor pathway inhibitor (TFPI) is a major regulator of TF-induced coagulation. We evaluated the possibility that monocyte-derived macrophages express this protein, thereby contributing to regulation of TF activity (TFact). Equally, we investigated the effect of cholesterol and of oxidized LDL (Ox-LDL) on the expression of TFPI and TF by human monocyte-derived macrophages (HMDMs). Northern blot analysis of TFPI mRNA from cultured HMDMs revealed a single band at 4.2 kb with weak intensity; this finding was confirmed by reverse transcription-polymerase chain reaction. Gel filtration of HMDM supernatants showed the presence of an active 100-kDa form of TFPI, which was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions; under reducing conditions, however, the immunoblot revealed a 40-kDa form of TFPI. The TFPI in HMDM supernatants possessed heparin-binding affinity, suggesting potential interaction of TFPI with heparan sulfate proteoglycans. Stimulation of foam cell formation by incubation of macrophages for 48 hours with exogenous free cholesterol indicated that neither the biological activity nor the de novo synthesis of TFPI protein was affected. In contrast, cholesterol loading with exogenous free cholesterol induced significant upregulation of total TFact (2.6-fold: 25.0 versus 9.4 mU/mg cell protein, cholesterol-treated versus control cells; P<0. 05); such induction was not correlated with an elevation in TF antigen (8.5 versus 7.8 ng/mg cell protein, cholesterol-treated versus control cells). Similarly, cholesterol-rich Ox-LDL induced an increase in TFact (1.9-fold: 18.9 versus 10.0 mU/mg cell protein, Ox-LDL-treated versus control cells; P<0.05); by contrast, the amount of TF antigen remained unchanged (7.1 versus 7.9 ng/mg cell protein, Ox-LDL-treated versus control cells). Our data indicate that enhancement of the procoagulant activity of TF in macrophage-derived foam cells is not counterbalanced by upregulation of TFPI activity, suggesting that lesion foam cells are in a procoagulant state; they may therefore contribute to thrombus generation on plaque rupture.

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.

Functional characterization of two different Kupffer cell populations of normal rat liver

BACKGROUND

Kupffer cells of the liver represent the largest population of tissue macrophages. Small and large Kupffer cells were distinguished in normal liver, leading to the suggestion that they have different functions. This study intends to further characterize small and large Kupffer cells of normal rat liver in vivo and in vitro.

METHODS

Sections of rat liver were investigated by double-staining immunofluorescence with the monoclonal antibodies ED1 and ED2. Isolated nonparenchymal liver cells were separated according to size to obtain small and large Kupffer cells. In culture, phagocytosis was studied by zymosan ingestion and cell proliferation by incorporation of 3H-thymidine. Synthesis of the proteins C1-inhibitor, apolipoprotein E and interleukin-1 was studied by endogenous labeling of newly synthesized proteins, immunoprecipitation and sodium dodecylsulfate-polyacrylamide gel electrophoresis.

RESULTS

ED1+ ED2+ Kupffer cells were located in the liver along the sinusoids. ED1+ ED2+ cells were found mainly located around the central vein and portal vessels. By counterflow elution, small ED1+ ED2- cells were separated from larger ED1+ ED2+ cells and cultured. The larger cells abundantly synthesized C1-inhibitor and apolipoprotein E, while the small cells synthesized only trace amounts of these proteins. Interferon-gamma increased C1-inhibitor synthesis in small (5-fold) and large cells (1.5-fold). 3H-thymidine incorporation was 11-fold higher in small than in large cells. However, lipopolysaccharide-induced pro-interleukin-1 alpha and pro-interleukin-1 beta synthesis and phagocytic activity were similar in both populations.

CONCLUSIONS

The data demonstrate two different populations of mononuclear phagocytes in normal rat liver well distinguished by immunocytochemical and functional markers.

Lecithin:cholesterol acyltransferase overexpression generates hyperalpha-lipoproteinemia and a nonatherogenic lipoprotein pattern in transgenic rabbits

Cholesterol esterification within plasma lipoprotein particles is catalyzed by lecithin:cholesterol acyltransferase (LCAT). The impact of the overexpression of this enzyme on plasma concentrations of the different plasma lipoproteins in an animal model expressing cholesteryl ester transfer protein was evaluated by generating rabbits expressing human LCAT. A 6.2-kilobase human genomic DNA construct was injected into the pronuclei of rabbit embryos. Of the 1002 embryos that were injected, 3 founder rabbits were characterized that expressed the human LCAT gene. As in mice and humans, the principal sites of mRNA expression in these rabbits is in the liver and brain, indicating that the regulatory elements required for tissue-specific expression among these species are similar. The alpha-LCAT activity correlated with the number of copies of LCAT that integrated into the rabbit DNA. Compared with controls, the high expressor LCAT-transgenic rabbits total and high density lipoprotein (HDL) cholesterol concentrations were increased 1.5-2.5-fold with a 3.1-fold increase in the plasma cholesterol esterification rate. Analysis of the plasma lipoproteins by fast protein liquid chromatography indicates that these changes reflected an increased concentration of apolipoprotein E-enriched, HDL1-sized particles, whereas atherogenic apolipoprotein B particles disappeared from the plasma. The concentrations of plasma HDL cholesterol were highly correlated with both human LCAT mass (r = 0.93; p = 0.001) and the log LCAT activity (r = 0.94; p < 0.001) in the transgenic rabbits. These results indicate that overexpression of LCAT in the presence of cholesteryl ester transfer protein leads to both hyperalpha-lipoproteinemia and reduced concentrations of atherogenic lipoproteins.

Phagocytic activation induces formation of platelet-activating factor in human monocyte-derived macrophages and in macrophage-derived foam cells. Relevance to the inflammatory reaction in atherogenesis

Monocyte-derived macrophages and macrophage-derived foam cells in arterial tissue may undergo phagocytic activation and thereby contribute to an inflammatory reaction. We have investigated the effect of phagocytic activation on the formation of platelet-activating factor (1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF-acether, PAF), a proinflammatory phospholipid, in human monocyte-derived macrophages (macrophages) and in cholesterol-loaded macrophage foam cells (foam cells). Adherent human monocyte-derived macrophages were transformed into foam cells upon incubation with acetylated low-density lipoproteins (Ac-LDL). Such foam cells characteristically displayed a markedly increased content of cholesteryl esters compared with macrophages (4.3 +/- 1.3 microgram/microgram DNA and 0.2 +/- 0.3 microgram/microgram DNA, n = 5, respectively). After phagocytic stimulation with serum-opsonized zymosan (OPZ), both macrophages and foam cells synthesized PAF transiently with maximal production (0.5-1.1 pmol PAF/microgram DNA, n = 5, corresponding to 4.0-8.8 pmol PAF/10(6) cells, as assessed by bioassay) occurring approximately 15 min after stimulation. A major fraction of the synthesized PAF remained cell-associated; such PAF was composed mainly of the hexadecyl (16:0 PAF, approximately 75%) and the octadecenyl (18:1 PAF) species and of trace amounts of octadecyl (18:0 PAF), as assessed by reverse-phase liquid chromatography. Addition of exogenous 16:0 lyso-PAF alone triggered PAF formation (0.9-1.7 pmol PAF/microgram DNA, after 15 min of cellular stimulation); simultaneous cellular stimulation with OPZ and 16:0 lyso-PAF increased PAF formation in an additive manner. Acetyltransferase, the enzyme which acetylates the precursor lyso-PAF and transforms it into PAF, displayed elevated activity both in macrophages and in foam cells, attaining 83-240 pmol PAF formed per min per mg DNA (n = 4); such elevated activity was not increased by OPZ-stimulation. The activity of acetylhydrolase, the PAF-degrading enzyme, was similar in macrophages and in foam cells, and varied between 120 pmol and 320 pmol PAF degraded per min per mg DNA (n = 5). Cell-associated acetylhydrolase activity was increased significantly by 40+/-15 % (P < 0.003, n = 5) after 15 - 30 min of activation with OPZ compared with non-stimulated cells and may account for the rapid decrease in cellular PAF content observed approximately 30 min after stimulation. These studies have established that metabolism of PAF in foam cells closely resembles that in macrophages, and thus PAF metabolism is largely independent of cellular cholesterol content. Moreover our data are consistent with the hypothesis that both macrophages and macrophage-derived foam cells upon phagocytic-activation constitute a significant transient source of PAF at inflammatory sites in the arterial intima where this phospholipidic mediator may exert potent proatherogenic and prothrombotic effects.

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.

Uptake of exogenous free cholesterol induces upregulation of tissue factor expression in human monocyte-derived macrophages

Lipid-laden macrophages present as foam cells may contribute to the hyperthrombotic state of human atherosclerotic lesions by the production of tissue factor (TF). We investigated the effect of exogenous nonlipoprotein cholesterol on the expression of TF by human monocyte-derived macrophages in culture. Nonlipoprotein cholesterol at 50 micrograms/ml increased TF activity 4-fold; TF induction was dose- and time-dependent. Expression of TF activity was positively correlated with the free cholesterol content of monocyte-derived macrophages, was increased upon inhibition of cholesterol esterification, and reflected de novo synthesis of TF protein. TF expression in cholesterol-loaded macrophages remained sensitive to stimulation (approximately 12-fold) by bacterial lipopolysaccharide, indicating that intracellular free cholesterol and lipopolysaccharide act by distinct mechanisms in inducing TF procoagulant activity. Our results suggest that loading human monocyte-derived macrophages with free cholesterol induces upregulation of TF expression, thereby contributing to thrombus formation at sites of plaque rupture.

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.

Dexamethasone modulates alpha 2-macroglobulin and apolipoprotein E gene expression in cultured rat liver fat-storing (Ito) cells

Fat-storing (Ito) cells are perisinusoidal liver cells thought to play a central role in vitamin A metabolism and fibrongenesis. Glucocorticoids have been shown to be beneficial in the treatment of certain types of liver diseases by delaying the development of cirrhosis. To study the regulatory effects of dexamethasone on Ito cell gene expression, Ito cells were isolated from normal rat liver and primary cultures were established. The effect of dexamethasone on the synthesis of alpha 2-macroglobulin, apolipoprotein E, fibronectin and actin was examined. Protein synthesis was studied both at the protein level and at the RNA level by means of biosynthetic labeling, immunoprecipitation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by Northern blot analysis of total RNA. After exposure to dexamethasone for 20 hr, alpha 2-macroglobulin protein synthesis was increased threefold, whereas apolipoprotein E expression was decreased 80%. Biosynthesis of fibronectin remained unaffected by hormone treatment. The dexamethasone effect became detectable 5 hr after beginning the exposure. Deinduction kinetic experiments showed that the glucocorticoid effect was detectable more than 12 hr after the replacement of the dexamethasone-containing culture medium by medium without the hormone. Corresponding to the data obtained at the protein level, dexamethasone increased the steady-state levels of alpha 2-macroglobulin-specific messenger RNA and reduced apolipoprotein E-specific transcripts, whereas fibronectin and actin messenger mRNA remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokine production by cholesterol-loaded human peripheral monocyte-macrophages: the effect on fibrinogen mRNA levels in a hepatoma cell-line (HepG2)

Conditioned medium from human monocyte-macrophages incubated under various conditions was tested for its ability to stimulate fibrinogen mRNA levels in the hepatoma cell line HepG2. Recombinant human interleukin-6 (IL-6) stimulated fibrinogen mRNA levels 4.4-fold over control levels; this response was blocked by an anti-IL-6 antibody. Conditioned medium from 3-day-cultured monocyte-macrophages produced a slight stimulation of fibrinogen synthesis in HepG2 cells which was enhanced when the monocyte-macrophages had been treated with lipopolysaccharide (LPS). This stimulation was blocked by the anti IL-6 antibody. The cytokines, interleukin-1 (IL-1) and tumour necrosis factor (TNF) were also detected in the conditioned medium from the 3-day-cultured monocyte-macrophages. Monocyte-macrophages were cultured for 17 days and then incubated with acetylated low density lipoprotein (AcLDL) for 48 h. Such cells were 'foamy' in appearance and showed a 4-fold increase in apoE mRNA and a 10 to 50-fold increase in apoE secretion. This increase in apoE production was suppressed by almost a third when cells were coincubated with AcLDL and LPS. Conditioned medium from these 17-day-cultured AcLDL-treated human monocyte-macrophages did not stimulate fibrinogen mRNA synthesis in HepG2 cells, nor did the conditioned medium contain detectable levels of cytokines. These results suggest that cytokine production from foam cells in the atherosclerotic lesion is unlikely to be a major contributing factor in determining the elevated fibrinogen levels seen in the plasma of patients with IHD.

Native low-density lipoproteins stimulate leukotriene B4 production by human monocyte-derived macrophages

We have evaluated the effect of native low-density lipoproteins (LDL) on the production of leukotriene B4 (LTB4), a potent inflammatory and chemotactic factor, by human monocyte-derived macrophages. The capacity of LDL (d, 1.024-1.050 g/ml) to increase LTB4 secretion was dose-dependent with an optimal response at 100 micrograms LDL protein/ml, representing an approx. 7.5-fold stimulation over basal levels at 10 days of culture; the half-maximal response occurred at 20 micrograms/ml. The effect of LDL on LTB4 production was rapid (within 15 min) and was maintained for at least 21 h. The generation of LTB4 in response to LDL was partially inhibited (approx. 70% inhibition) by EDTA (5 mM) and by a monoclonal antibody (IgG-C7; 160 micrograms/ml) directed against the binding site of the cellular LDL receptor. In addition, the effects of native LDL and acetylated LDL were additive. These findings suggest that the specific interaction of LDL with its high affinity receptor represents a major component in the stimulation of the production of LTB4 by human monocyte-derived macrophages.