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

Effects of autologous serum on TREM2 and APOE in a personalized monocyte-derived macrophage assay of late-onset Alzheimer's patients

BACKGROUND

Age-associated deterioration of the immune system contributes to a chronic low-grade inflammatory state known as "inflammaging" and is implicated in the pathogenesis of late-onset Alzheimer's disease (LOAD). Whether changes in the tissue environment caused by circulatory factors associated with aging may alter the innate immune response is unknown. Monocyte-derived macrophages (Mo-MФs) infiltrating the brain alongside microglia are postulated to play a modulatory role in LOAD and both express triggering receptor expressed on myeloid cells 2 (TREM2). Apolipoprotein E (APOE) acts as a ligand for TREM2, and their role in amyloid beta (Aβ) clearance highlights their importance in LOAD. However, the influence of the patient's own milieu (autologous serum) on the synthesis of TREM2 and APOE in infiltrating macrophages remains unknown.

OBJECTIVES

To functionally assess patient-specific TREM2 and APOE synthesis, we designed a personalized assay based on Mo-MФs using monocytes from LOAD patients and matched controls (CO). We assessed the influence of each participant's own milieu, by examining the effect of short- (1 day) and long- (10 days) term differentiation of the cells in the presence of the donor´s autologous serum (AS) into M1-, M2- or M0-macrophages. Additionally, sex differences and Aβ-uptake ability in short- and long-term differentiated Mo-MФs were assessed.

RESULTS

We showed a time-dependent increase in TREM2 and APOE protein levels in LOAD- and CO-derived cells. While AS did not differentially modulate TREM2 compared to standard fetal calf serum (FCS), AS decreased APOE levels in M2 macrophages but increased levels in M1 macrophages. Interestingly, higher levels of TREM2 and lower levels of APOE were detected in female- than in male- LOAD patients. Finally, we report decreased Aβ-uptake in long-term differentiated CO- and LOAD-derived cells, particularly in APOEε4(+) carriers.

CONCLUSIONS

We demonstrate for the first time the suitability of a personalized Mo-MФ cell culture-based assay for studying functional TREM2 and APOE synthesis in a patient's own aged milieu. Our strategy may thus provide a useful tool for future research on diagnostic and therapeutic aspects of personalized medicine.

The contribution of amyloid deposition in the aortic valve to calcification and aortic stenosis

Calcific aortic valve disease (CAVD) and stenosis have a complex pathogenesis, and no therapies are available that can halt or slow their progression. Several studies have shown the presence of apolipoprotein-related amyloid deposits in close proximity to calcified areas in diseased aortic valves. In this Perspective, we explore a possible relationship between amyloid deposits, calcification and the development of aortic valve stenosis. These amyloid deposits might contribute to the amplification of the inflammatory cycle in the aortic valve, including extracellular matrix remodelling and myofibroblast and osteoblast-like cell proliferation. Further investigation in this area is needed to characterize the amyloid deposits associated with CAVD, which could allow the use of antisense oligonucleotides and/or isotype gene therapies for the prevention and/or treatment of CAVD.

Lipid efflux mechanisms, relation to disease and potential therapeutic aspects

Lipids are hydrophobic and amphiphilic molecules involved in diverse functions such as membrane structure, energy metabolism, immunity, and signaling. However, altered intra-cellular lipid levels or composition can lead to metabolic and inflammatory dysfunction, as well as lipotoxicity. Thus, intra-cellular lipid homeostasis is tightly regulated by multiple mechanisms. Since most peripheral cells do not catabolize cholesterol, efflux (extra-cellular transport) of cholesterol is vital for lipid homeostasis. Defective efflux contributes to atherosclerotic plaque development, impaired β-cell insulin secretion, and neuropathology. Of these, defective lipid efflux in macrophages in the arterial walls leading to foam cell and atherosclerotic plaque formation has been the most well studied, likely because a leading global cause of death is cardiovascular disease. Circulating high density lipoprotein particles play critical roles as acceptors of effluxed cellular lipids, suggesting their importance in disease etiology. We review here mechanisms and pathways that modulate lipid efflux, the role of lipid efflux in disease etiology, and therapeutic options aimed at modulating this critical process.

Complement Factor H and Apolipoprotein E Participate in Regulation of Inflammation in THP-1 Macrophages

The alternative pathway (AP) of complement is constantly active in plasma and can easily be activated on self surfaces and trigger local inflammation. Host cells are protected from AP attack by Factor H (FH), the main AP regulator in plasma. Although complement is known to play a role in atherosclerosis, the mechanisms of its contribution are not fully understood. Since FH via its domains 5-7 binds apoliporotein E (apoE) and macrophages produce apoE we examined how FH could be involved in the antiatherogenic effects of apoE. We used blood peripheral monocytes and THP-1 monocyte/macrophage cells which were also loaded with acetylated low-density lipoprotein (LDL) to form foam cells. Binding of FH and apoE on these cells was analyzed by flow cytometry. High-density lipoprotein (HDL)-mediated cholesterol efflux of activated THP-1 cells was measured and transcriptomes of THP-1 cells using mRNA sequencing were determined. We found that binding of FH to human blood monocytes and cholesterol-loaded THP-1 macrophages increased apoE binding to these cells. Preincubation of fluorescent cholesterol labeled THP-1 macrophages in the presence of FH increased cholesterol efflux and cholesterol-loaded macrophages displayed reduced transcription of proinflammatory/proatherogenic factors and increased transcription of anti-inflammatory/anti-atherogenic factors. Further incubation of THP-1 cells with serum reduced C3b/iC3b deposition. Overall, our data indicate that apoE and FH interact with monocytic cells in a concerted action and this interaction reduces complement activation and inflammation in the atherosclerotic lesions. By this way FH may participate in mediating the beneficial effects of apoE in suppressing atherosclerotic lesion progression.

IRF2BP2 Reduces Macrophage Inflammation and Susceptibility to Atherosclerosis

RATIONALE

Inflammation impairs macrophage cholesterol clearance from vascular tissues and promotes atherosclerosis. Inflammatory macrophages suppress expression of the transcription cofactor interferon regulatory factor 2-binding protein 2 (IRF2BP2), and genetic variants near IRF2BP2 associate with ischemic heart disease progression in humans.

OBJECTIVES

To test whether IRF2BP2 in macrophages affects atherosclerosis in mice and humans.

METHODS AND RESULTS

We generated mice that delete IRF2BP2 in macrophages. IRF2BP2-deficient macrophages worsened atherosclerosis in irradiated low-density lipoprotein receptor null-recipient mice and in apolipoprotein E null mice. IRF2BP2-deficient macrophages were inflammatory and had impaired cholesterol efflux because of their inability to activate the cholesterol transporter ABCA1 in response to cholesterol loading. Their expression of the anti-inflammatory transcription factor Krüppel-like factor 2 was markedly reduced. Promoter studies revealed that IRF2BP2 is required for MEF2-dependent activation of Krüppel-like factor 2. Importantly, restoring Krüppel-like factor 2 in IRF2BP2-deficient macrophages attenuated M1 inflammatory and rescued M2 anti-inflammatory gene activation and improved the cholesterol efflux deficit by restoring ABCA1 activation in response to cholesterol loading. In a cohort of 1066 angiographic cases and 1011 controls, homozygous carriers of a deletion polymorphism (rs3045215) in the 3' untranslated region sequence of human IRF2BP2 mRNA had a higher risk of coronary artery disease (recessive model, odds ratio [95% confidence interval]=1.560 [1.179-2.065], P=1.73E-03) and had lower IRF2BP2 (and Krüppel-like factor 2) protein levels in peripheral blood mononuclear cells. The effect of this deletion polymorphism to suppress protein expression was confirmed in luciferase reporter studies.

CONCLUSION

Ablation of IRF2BP2 in macrophages worsens atherosclerosis in mice, and a deletion variant that lowers IRF2BP2 expression predisposes to coronary artery disease in humans.

ApoE production in human monocytes and its regulation by inflammatory cytokines

The apoE production by tissue macrophages is crucial for the prevention of atherosclerosis and the aim of this study was to further elucidate how this apolipoprotein is regulated by cytokines present during inflammation. Here we studied apoE production in peripheral blood mononuclear cells (PBMC) and analysis was made with a newly developed apoE ELISpot assay. In PBMC, apoE secretion was restricted to monocytes with classical (CD14⁺⁺CD16⁻) and intermediate (CD14⁺CD16⁺) monocytes being the main producers. As earlier described for macrophages, production was strongly upregulated by TGF-β and downregulated by bacterial lipopolysaccharide (LPS) and the inflammatory cytokines IFN-γ, TNF-α and IL-1β. We could here show that a similar down-regulatory effect was also observed with the type I interferon, IFN-α, while IL-6, often regarded as one of the more prominent inflammatory cytokines, did not affect TGF-β-induced apoE production. The TNF-α inhibitor Enbrel could partly block the down-regulatory effect of IFN-γ, IFN-α and IL-1β, indicating that inhibition of apoE by these cytokines may be dependent on or synergize with TNF-α. Other cytokines tested, IL-2, IL-4, IL-12, IL-13, IL-17A and IL-23, had no inhibitory effect on apoE production. In contrast to the effect on monocytes, apoE production by primary hepatocytes and the hepatoma cell line HepG2 was more or less unaffected by treatment with cytokines or LPS.

Regulation of ADAMTS-1, -4 and -5 expression in human macrophages: differential regulation by key cytokines implicated in atherosclerosis and novel synergism between TL1A and IL-17

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Atherosclerosis is an inflammatory disorder regulated by cytokines.

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ADAMTS proteases have been suggested to play an important role in this disease.

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The action of key cytokines on the expression of ADAMTS proteases in macrophages is poorly understood.

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The effect of IFN-γ, TGF-β, TL1A and IL-17A on the expression of ADAMTS-1, -4 and -5 was studied.

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Novel differential actions and synergistic interactions were identified.

Atherosclerosis is an inflammatory disease of the vasculature regulated by cytokines. Macrophages play a crucial role at all stages of this disease, including regulation of foam cell formation, the inflammatory response and stability of atherosclerotic plaques. For example, matrix metalloproteinases produced by macrophages play an important role in modulating plaque stability. More recently, the ADAMTS proteases, which are known to play a key role in the control of cartilage degradation during arthritis, have been found to be expressed in atherosclerotic lesions and suggested to have potentially important functions in the control of plaque stability. Unfortunately, the action of cytokines on the expression of ADAMTS family in macrophages is poorly understood. We have investigated the effect of classical cytokines (IFN-γ and TGF-β) and those that have been recently identified (TL1A and IL-17) on the expression of ADAMTS-1, -4 and -5 in human macrophages. The expression of all three ADAMTS members was induced during differentiation of monocytes into macrophages. TGF-β had a differential action with induction of ADAMTS-1 and -5 expression and attenuation in the levels of ADAMTS-4. In contrast, IFN-γ suppressed the expression of ADAMTS-1 without having an effect on ADAMTS-4 and -5. Although TL-1A or IL-17A alone had little effect on the expression of all the members, they induced their expression synergistically when present together. These studies provide new insight into the regulation of key ADAMTS family members in human macrophages by major cytokines in relation to atherosclerosis.

Lipid metabolism and neuroinflammation in Alzheimer's disease: a role for liver X receptors

Liver X receptors (LXR) are nuclear receptors that have emerged as key regulators of lipid metabolism. In addition to their functions as cholesterol sensors, LXR have also been found to regulate inflammatory responses in macrophages. Alzheimer's disease (AD) is a neurodegenerative disease characterized by a progressive cognitive decline associated with inflammation. Evidence indicates that the initiation and progression of AD is linked to aberrant cholesterol metabolism and inflammation. Activation of LXR can regulate neuroinflammation and decrease amyloid-β peptide accumulation. Here, we highlight the role of LXR in orchestrating lipid homeostasis and neuroinflammation in the brain. In addition, diabetes mellitus is also briefly discussed as a significant risk factor for AD because of the appearing beneficial effects of LXR on glucose homeostasis. The ability of LXR to attenuate AD pathology makes them potential therapeutic targets for this neurodegenerative disease.

Granulocyte Macrophage Colony Stimulating Factor Treatment is Associated with Improved Cognition in Cancer Patients

BACKGROUND: Endogenous Granulocyte Macrophage Colony Stimulating Factor (GMCSF) is released in rheumatoid arthritis patients, who are largely protected from Alzheimer's disease (AD). Introducing exogenous GMCSF into an AD mouse model reduced amyloid deposition by 55% and restored normal cognition. No published studies have examined exogenous GMCSF and cognitive functioning in humans. OBJECTIVES/DESIGN: The goal of the current study was to examine the association between receipt of GMCSF and cognitive functioning in patients receiving colony stimulating factors as part of routine supportive care for hematopoietic cell transplantation (HCT). SETTING AND PARTICIPANTS: Archived neuropsychological data were examined from a longitudinal study of cognitive functioning in 95 patients receiving HCT at the Moffitt Cancer Center. INTERVENTION: Receipt of GMCSF and/or Granulocyte Colony Stimulating Factor (GCSF) was ascertained through patient billing records. MEASUREMENTS: Patients were assessed with a battery of neuropsychological tests prior to transplant and 6 and 12 months post-transplant. RESULTS: Patients treated with GMCSF and GCSF (n=19) showed significantly greater improvement in total neuropsychological functioning (TNP) at 6 months than patients treated with GCSF only (n=76) (p=.04). There was no group difference in TNP at 12 months (p=.24). Improvement in TNP from baseline to 6 months post-HCT was significant in the GMCSF+GCSF group (p=.01) but not the GCSF only group (p=.33). Improvement in TNP from baseline to 12 months post-HCT was significant in both groups (ps<.01). CONCLUSION: Preliminary data from this study of humans receiving colony stimulating factors suggest that receipt of GMCSF+GCSF was associated with greater cognitive improvement than GCSF alone. Randomized controlled trials of the effects of GMCSF on cognitive functioning in humans are warranted and underway to confirm these findings.

Cross-talk between apolipoprotein E and cytokines

Apolipoprotein E (apoE) is a multifunctional glycosylated protein characterized by its wide tissue distribution. Despite its importance in lipid transport and atherosclerosis pathogenesis, apoE is associated with neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson disease, and autoimmune disorders such as multiple sclerosis and psoriasis. Among others, the role of apoE in modulating inflammation and oxidation is crucial in elucidating the risk factors of the above diseases since the function of apoE is closely linked with both proinflammatory and antiinflammatory cytokines. Moreover, apoE modulates inflammatory and immune responses in an isoform-dependent manner. Correspondingly, inflammatory cytokines can either upregulate or downregulate the production of apoE in various tissue types. However, studies on the interactions between apoE and cytokines occasionally yield conflicting results, highlighting the complex roles of apoE and cytokines in various disorders. The present paper summarizes the current knowledge about the cross-talk between apoE and cytokines, with emphasis on the effects of apoE on the Th1/Th2 balance.

The role of apolipoprotein E in Guillain-Barré syndrome and experimental autoimmune neuritis

Apolipoprotein E (apoE) is a 34.2 kDa glycosylated protein characterized by its wide tissue distribution and multiple functions. ApoE has been widely studied in lipid metabolism, cardiocerebrovascular diseases, and neurodegenerative diseases like Alzheimer's disease and mild cognitive impairment, and so forth. Recently, a growing body of evidence has pointed to nonlipid related properties of apoE, including suppression of T cell proliferation, regulation of macrophage function, facilitation of lipid antigen presentation by CD1 molecules to natural killer T (NKT) cells, and modulation of inflammation and oxidation. By these properties, apoE impacts physiology and pathophysiology at multiple levels. The present paper summarizes updated studies on the immunoregulatory function of apoE, with special focus on isoform-specific effects of apoE on Guillain-Barré syndrome (GBS) and its animal model experimental autoimmune neuritis (EAN).

Interleukin-1beta and tumour necrosis factor-alpha impede neutral lipid turnover in macrophage-derived foam cells

Background

Pro-inflammatory cytokines can affect intracellular lipid metabolism. A variety of effects have been described for different cell types; hepatocyte lipid turnover pathways are inhibited during inflammation, whereas interleukin-1β (IL-1β) reduces intracellular cholesterol levels in fibroblasts. Levels of the pro-inflammatory cytokines IL-1β and tumour necrosis factor-α (TNF-α) are up-regulated at sites of formation of atherosclerotic plaques. Plaque formation is though to begin with infiltration of monocytes to the intimal layer of the vascular wall, followed by differentiation to macrophages and macrophage uptake of modified lipoproteins, resulting in accumulation of intracellular lipids. The lipid-filled cells are referred to as macrophage foam cells, a key feature of atherosclerotic plaques. We have investigated the effects of IL-1β and TNF-α on macrophage foam cells in order to assess whether presence of the pro-inflammatory cytokines improves or aggravates macrophage foam cell formation by affecting lipid accumulation and lipid turn-over in the cells.

Results

Differentiated primary human macrophages or THP-1 cells were lipid loaded by uptake of aggregated low density lipoproteins (AgLDL) or very low density lipoproteins (VLDL), and then incubated with IL-1β (0 – 5000 pg/ml) in lipoprotein-free media for 24 h. Cells incubated in absence of cytokine utilized accumulated neutral lipids, in particular triglycerides. Addition of exogenous IL-1β resulted in a dose-dependent retention of intracellular cholesterol and triglycerides. Exchanging IL-1β with TNF-α gave a similar response. Analysis of fatty acid efflux and intracellular fatty acid activation revealed a pattern of decreased lipid utilization in cytokine-stimulated cells.

Conclusion

IL-1β and TNF-α enhance macrophage foam cell formation, in part by inhibition of macrophage intracellular lipid catabolism. If present in vivo, these mechanisms will further augment the pro-atherogenic properties of the two cytokines.

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.

GM-CSF Deficiency Reduces Macrophage PPAR-γ Expression and Aggravates Atherosclerosis in ApoE-Deficient Mice

Objective— Granulocyte-macrophage colony-stimulating factor (GM-CSF) is expressed in atherosclerotic lesions but its significance for lesion development is unknown. Consequently, we investigated the significance of GM-CSF expression for development of atherosclerotic lesions in apolipoprotein E-deficient (apoE −/− ) mice.

Methods and Results— We generated apoE −/− mice deficient in GM-CSF (apoE −/− .GM-CSF −/− mice), fed them a high-fat diet, and compared lesion development with apoE −/− mice. We measured lesion size, macrophage, smooth muscle cell, and collagen accumulation at the aortic sinus, and expression of genes that regulate cholesterol transport and inflammation. No differences in serum cholesterol were found between the 2 groups. Lesion size in hyperlipidemic apoE −/− .GM-CSF −/− increased by 30% ( P <0.05), macrophage accumulation doubled, and collagen content reduced by 15% ( P <0.05); smooth muscle cell accumulation and vascularity were unaffected. Analysis of PPAR-γ, ABCA1, and CD36 in lesions showed reduced expression (50%, 65%, and 55%, respectively), whereas SR-A doubled. In peritoneal macrophages, PPAR-γ and ABCA1 expression was also reduced by 50% and 70%, respectively, as was cholesterol efflux, by 50%. In lesions, pro-inflammatory MCP-1 and tumor necrosis factor (TNF)-α expression increased 2- and 3.5-fold, respectively, vascular cell adhesion molecule (VCAM)-1 expression enhanced and interleukin (IL)-1 receptor antagonist reduced by 50%.

Conclusions— GM-CSF deficiency increases atherosclerosis under hypercholesterolemic conditions, indicating antiatherogenic role for GM-CSF. We suggest this protective role is mediated by PPAR-γ and ABCA1, molecules that affect cholesterol homeostasis and inflammation.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Transforming growth factor-beta-induced expression of the apolipoprotein E gene requires c-Jun N-terminal kinase, p38 kinase, and casein kinase 2

OBJECTIVE

The cytokine transforming growth factor-beta (TGF-beta) and apolipoprotein E (apoE) play potent antiatherogenic roles. Despite such importance, the mechanisms underlying the regulation of apoE expression by TGF-beta have not been characterized and were therefore investigated.

METHODS AND RESULTS

Using THP-1 cell line as a model system, with key findings confirmed in primary cultures, we show that TGF-beta induces the expression of apoE, and this is prevented by pharmacological inhibitors of c-Jun N-terminal kinase (JNK), p38 kinase, and casein kinase 2 (CK2). In support for an important role for these pathways, TGF-beta activates JNK, p38 kinase, and CK2, and dominant-negative (DN) forms of these proteins inhibit the cytokine-induced apoE expression. TGF-beta also increases the phosphorylation and expression of c-Jun, a downstream target for JNK action and a component of activator protein-1 (AP-1), and DN c-Jun inhibits the induction of apoE expression in response to the cytokine. AP-1 DNA binding was also induced by TGF-beta, and the action of p38 kinase, JNK, and CK2 converged on the activation of c-Jun/AP-1.

CONCLUSIONS

These studies reveal a novel role for JNK, p38 kinase, CK2, and c-Jun/AP-1 in the TGF-beta-induced expression of apoE.

Cytokines in Atherosclerosis: Pathogenic and Regulatory Pathways

Atherosclerosis is a chronic disease of the arterial wall where both innate and adaptive immunoinflammatory mechanisms are involved. Inflammation is central at all stages of atherosclerosis. It is implicated in the formation of early fatty streaks, when the endothelium is activated and expresses chemokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into the subendothelium. It also acts at the onset of adverse clinical vascular events, when activated cells within the plaque secrete matrix proteases that degrade extracellular matrix proteins and weaken the fibrous cap, leading to rupture and thrombus formation. Cells involved in the atherosclerotic process secrete and are activated by soluble factors, known as cytokines. Important recent advances in the comprehension of the mechanisms of atherosclerosis provided evidence that the immunoinflammatory response in atherosclerosis is modulated by regulatory pathways, in which the two anti-inflammatory cytokines interleukin-10 and transforming growth factor-β play a critical role. The purpose of this review is to bring together the current information concerning the role of cytokines in the development, progression, and complications of atherosclerosis. Specific emphasis is placed on the contribution of pro- and anti-inflammatory cytokines to pathogenic (innate and adaptive) and regulatory immunity in the context of atherosclerosis. Based on our current knowledge of the role of cytokines in atherosclerosis, we propose some novel therapeutic strategies to combat this disease. In addition, we discuss the potential of circulating cytokine levels as biomarkers of coronary artery disease.

Reverse cholesterol transport and cholesterol efflux in atherosclerosis

Reverse cholesterol transport (RCT) is a pathway by which accumulated cholesterol is transported from the vessel wall to the liver for excretion, thus preventing atherosclerosis. Major constituents of RCT include acceptors such as high-density lipoprotein (HDL) and apolipoprotein A-I (apoA-I), and enzymes such as lecithin:cholesterol acyltransferase (LCAT), phospholipid transfer protein (PLTP), hepatic lipase (HL) and cholesterol ester transfer protein (CETP). A critical part of RCT is cholesterol efflux, in which accumulated cholesterol is removed from macrophages in the subintima of the vessel wall by ATP-binding membrane cassette transporter A1 (ABCA1) or by other mechanisms, including passive diffusion, scavenger receptor B1 (SR-B1), caveolins and sterol 27-hydroxylase, and collected by HDL and apoA-I. Esterified cholesterol in the HDL is then delivered to the liver for excretion. In patients with mutated ABCA1 genes, RCT and cholesterol efflux are impaired and atherosclerosis is increased. In studies with transgenic mice, disruption of ABCA1 genes can induce atherosclerosis. Levels of HDL are inversely correlated with incidences of cardiovascular disease. Supplementation with HDL or apoA-I can reverse atherosclerosis by accelerating RCT and cholesterol efflux. On the other hand, pro-inflammatory factors such as interferon-gamma (IFN-gamma), endotoxin, tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta), can be atherogenic by impairing RCT and cholesterol efflux, according to in vitro studies. RCT and cholesterol efflux play a major role in anti-atherogenesis, and modification of these processes may provide new therapeutic approaches to cardiovascular disease. Further research on new modifying factors for RCT and cholesterol efflux is warranted.

Transcriptional regulatory networks in lipid metabolism control ABCA1 expression

The ATP-binding cassette transporters, ABCA1 and ABCG1, are major players in mediating cellular efflux of phospholipids and cholesterol to apoA-I containing lipoproteins including prebeta-HDL and alphaHDL and thereby exert important antiatherogenic properties. Although the exact mechanisms how ABC transporters mediate lipid transport are not completely resolved, recent evidence from several laboratories including ours suggests that vesicular transport processes involving different interactive proteins like beta2-syntrophin, alpha1-syntrophin, Lin7, and cdc42 are critically involved in cellular lipid homeostasis controlled by ABCA1 and ABCG1. Besides sterols and fatty acids as known physiological modulators of the LXR/RXR and SREBP pathways, a growing list of natural and synthetic substances and metabolic regulators such as retinoids, PPAR-ligands, hormones, cytokines, and drugs are particularly effective in modulating ABCA1 and ABCG1 gene expression. Although ABCA1 protein amounts are regulated at the level of stability, the majority of potent activating and repressing mechanisms on ABCA1 function directly act on the ABCA1 gene promoter. Among the inducing factors, liver-X-receptors (LXR), retinoic acid receptors (RAR) and peroxisome proliferator-activated receptors (PPARs) along with their coactivators provide an amplification loop for ABCA1 and ABCG1 expression. The ABCA1 promoter is further stimulated by the ubiquitous factor Sp1 and the hypoxia-induced factor 1 (HIF1), which bind to GC-boxes and the E-box, respectively. Shutdown of ABCA1 expression in the absence of sterols or in certain tissues is mediated by corepressor complexes involving unliganded LXR, sterol-regulatory element binding protein 2 (SREBP2), Sp3, and the SCAN-domain protein ZNF202, which also impacts nuclear receptor signaling. Thus, a highly sophisticated transcriptional network controls the balanced expression of ABCA1.

Thematic review series: The Immune System and Atherogenesis. Cytokine regulation of macrophage functions in atherogenesis

This review will focus on the role of cytokines in the behavior of macrophages, a prominent cell type of atherosclerotic lesions. Once these macrophages have immigrated into the vessel wall, they propagate the development of atherosclerosis by modifying lipoproteins, accumulating intracellular lipids, remodeling the extracellular environment, and promoting local coagulation. The numerous cytokines that have been detected in atherosclerosis, combined with the expression of large numbers of cytokine receptors on macrophages, are consistent with this axis being an important contributor to lesion development. Given the vast literature on cytokine-macrophage interactions, this review will be selective, with an emphasis on the major cytokines that have been detected in atherosclerotic lesions and their effects on properties that are relevant to lesion formation and maturation. There will be an emphasis on the role of cytokines in regulating lipid metabolism by macrophages. We will provide an overview of the major findings in cell culture and then put these in the context of in vivo studies.

Dyslipidemia and inflammation: an evolutionary conserved mechanism

Inflammation leads to changes in lipid metabolism aimed at decreasing the toxicity of a variety of harmful agents and tissue repair by redistributing nutrients to cells involved in host defence. Acute phase response, mediated by cytokines, preserves the host from acute injury. When this inflammation becomes chronic, it might lead to chronic disorders as atherosclerosis and the metabolic syndrome. The activation of the inflammatory cascade will induce a decrease in HDL-cholesterol (HDL-C), with impairment in reverse cholesterol transport, and parallel changes in apolipoproteins, enzymes, anti-oxidant capacity and ATP binding cassette A1-dependent efflux. This decrease in HDL-C and phospholipids could stimulate compensatory changes, as synthesis and accumulation of phospholipid-rich VLDL which binds bacterial products and other toxic substances, resulting in hypertriglyceridemia. The final consequence is an increased accumulation of cholesterol in cells. When the compensatory response (inflammation) is not able to repair injury, it turns into a harmful reaction, and the lipid changes will become chronic, either by repeated or overwhelming stimulus, enhancing the formation of atherosclerotic lesions. Thus, the classical lipid changes associated with the metabolic syndrome (increased triglycerides and decreased HDL-C) may be envisioned as a highly conserved evolutionary response aimed at tissue repair. Under this assumption, the problem is not the response but the persistence of the stimulus.

Sex steroids, APOE genotype and the innate immune system

Microglia are a primary cellular component of the CNS innate immune system. Their response to conserved pathogen motifs is inherent and leads to the release of cytoactive factors that impact surrounding neurons and glia. The microglial response is modified by the local tissue environment and by "global" factors such as gender. Exposure to estrogen and testosterone, in general, down regulate microglia and peripheral macrophage function, promoting an anti-inflammatory phenotype. Other global factors, however, can "override" the gender-based effects demonstrated by estrogen or testosterone. Apolipoprotein E (APOE) genotype and the expression of specific isoforms of apolipoprotein E differentially regulate microglial and peripheral macrophage function. Our studies have shown that the presence of the APOE4 gene, a known risk factor for AD and other neurodegenerative diseases, promotes a pro-inflammatory macrophage phenotype in neonatal microglia. However, in adult mice, the APOE genotype-specific effect depends on gender. Peritoneal macrophages from female adult APOE3 and APOE4 targeted replacement mice do not demonstrate an APOE genotype-specific response, whereas adult male APOE4 targeted replacement mice show enhanced macrophage responsiveness compared to adult male APOE3 mice. At least part of the altered macrophage response in APOE4 male mice may be due to differences in androgen receptor sensitivity to testosterone. These data re-enforce the concept that classical activation in macrophages has multiple levels of regulation, dictated by competing or synergistic factors and genotype.

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.

Regulation of sterol 27-hydroxylase in human monocyte-derived macrophages: up-regulation by transforming growth factor β1

Regulatory mechanisms for human CYP27A1 enzyme have not yet been fully investigated. Our approach was to add different hormones and cytokines to cultured human monocyte-derived macrophages, and assess the effects on the CYP27A1 by measuring the production of 27-hydroxylated cholesterol in the media. Of the different hormones and cytokines tested, only transforming growth factor beta1 (TGF-beta1) had a clear effect on CYP27A1. Further experiments showed a significant increase in 27-hydroxylated cholesterol products (27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid). A concomitant increase in CYP27A1 mRNA levels was also seen and this positive effect was confirmed using a human CYP27A1 luciferase reporter gene expressed in HepG2 cells. Experiments with progressive deletion/luciferase reporter gene constructs indicated that a TGF-beta1 responsive sequence might be localized in a region about 400 bp upstream of the CYP27A1 translation start. The possibility is discussed that induction of CYP27A1 by TGF-beta1 may be responsible for some of the anti-atherogenic properties of this cytokine.

Heterogeneous expression of apolipoprotein-E by human macrophages

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

Transforming growth factor-beta down-regulates apolipoprotein M in HepG2 cells

Apolipoprotein M (apoM) is a novel apolipoprotein presented mostly in high-density lipoprotein (HDL) in human plasma, and is exclusively expressed in liver and in kidney. The pathophysiological function of apoM has not yet been elucidated. Apolipoprotein B (apoB), the characteristic apolipoprotein of low-density lipoprotein (LDL), is like apoM, a very hydrophobic protein, and thereafter they both must co-circulate with lipoprotein particles in plasma. The cytokine, transforming growth factor-beta (TGF-beta), has been shown to decreased apoB secretion in HepG2 cells, and we hypothesized that TGF-beta may have the same effects on apoM expression in HepG2 cells. In the present study, we used real-time RT-PCR to analyze apoM and apoB mRNA levels during administration of TGF-beta, as well as TGF-alpha, epidermal growth factor (EGF) and hepatic growth factor (HGF). TGF-beta significantly inhibited both apoM and apoB mRNA expression in HepG2 cells. The inhibitory effects of TGF-beta were dose-dependent, i.e. 1 ng/ml of TGF-beta decreased apoM mRNA levels by 30%, and 10 or 100 ng/ml of TGF-beta decreased apoM mRNA levels more than 65%. The effect of TGF-beta on apoB mRNA expression was slightly weaker than that of apoM, with a maximum effect at 10 or 100 ng/ml TGF-beta where apoB mRNA levels decreased about 55%. The inhibitory effects of TGF-beta on apoM and apoB mRNA levels also increased with increasing incubation time, where the maximum effect was obtained at 24 h. Moreover TGF-alpha, EGF and HGF all decreased both apoM and apoB mRNA levels, but to a less extent than TGF-beta. Further, all four cytokines had more pronounced effects on apoM mRNA expression than apoB mRNA expression. The present study suggested that apoM, like apoB, may be involved in the hepatic lipoprotein assembly in vivo.

Effect of atorvastatin on ApoE and ApoC-I synthesis and secretion by THP-1 macrophages

Apolipoprotein (apo) E and C-I are plasma apolipoproteins that have been implicated in the etiology of atherosclerosis and obesity, respectively. Both proteins are synthesized and secreted by macrophages, though pharmacological regulation of their production is poorly understood. The authors compared the effect of 2 HMG-CoA reductase inhibitors, atorvastatin and cerivastatin, on the synthesis and secretion of apoE and apoC-I by THP-1 macrophages. Atorvastatin reduced medium apoE and cellular apoE mRNA of PMA-activated THP-1 cells in a dose-dependent manner (-24% and -22%, respectively, at 1-micromol/L, P < 0.01). ApoC-I in the medium was also reduced by atorvastatin in a dose-dependent manner, though to a lesser extent (-15% at 1-micromol/L, P < 0.05). Cerivastatin similarly reduced medium apoE (-20% at 1-micromol/L, P < 0.05) and cellular apoE mRNA (-31% at 1-micromol/L, P < 0.05), and significantly lowered cellular apoC-I mRNA (-15%, P < 0.05), but not apoC-I in the medium. In experiments with THP-1 macrophages loaded with cholesterol (ie, 24-hour incubation with acetyl-LDL), atorvastatin and cerivastatin (1-micromol/L) significantly (P < 0.05) reduced both medium apoE (-30% and -25%, respectively) and cellular apoE mRNA (-25% and -17%, respectively). A lower and less consistent effect was observed on medium apoC-I (-6% and -18%, respectively) and cellular apoC-I mRNA (-13% and -19%, respectively). These data demonstrate that statins have the capacity to reduce the synthesis and secretion of both apoE and apoC-I in THP-1 macrophages loaded or unloaded with cholesterol.

Peroxisome proliferator-activated receptor alpha,gamma coagonist LY465608 inhibits macrophage activation and atherosclerosis in apolipoprotein E knockout mice

The apolipoprotein E (apoE) knockout mouse has provided an approach to the investigation of the effect of both cellular and humoral processes on atherosclerotic lesion progression. In the present study, pharmacologic modulation of both interferon gamma (IFNgamma)-inducible macrophage effector functions, and atherosclerotic lesions in the apoE knockout mouse were investigated using the peroxisome proliferator-activated receptor (PPAR) alpha,gamma coagonist LY465608. LY465608 inhibited, in a concentration-dependent manner, IFNgamma induction of both nitric oxide synthesis and the beta 2 integrin CD11a in elicited peritoneal macrophages from apoE knockout mice. Similar effects were observed ex vivo following 10 d of treating mice with 10 mg/kg of LY465608. Treatment of apoE knockout mice for 18 wk with LY465608 resulted in a statistically significant 2.5-fold reduction in atherosclerotic lesion area in en face aorta preparations. These effects were apparent in the absence of any reduction in total serum cholesterol or in lipoprotein distribution. Finally, treatment of apoF knockout mice with established atherosclerotic disease resulted in a modest but not statistically significant decrease in aortic lesional surface area. These results demonstrate the utility of PPAR coagonists in reducing the progression of the atherosclerotic lesion.

Interferon-gamma-mediated downregulation of cholesterol efflux and ABC1 expression is by the Stat1 pathway

The pathological role of interferon-gamma (IFN-gamma) in atherosclerosis is mediated through effects on macrophages, foam cells, and other vascular cells. Recently, we reported that ATP-binding cassette transporter 1(ABC1) message and protein levels were decreased 3- to 4-fold in foam cells by IFN-gamma. In the present study, the pathway by which IFN-gamma inhibited ABC1 expression was investigated with signal transducers and activators of transcription (Stat1) knockout mice. IFN-gamma-stimulated, wild-type, macrophage-derived foam cells, as previously reported, exhibited a decrease in cholesterol efflux and ABC1 expression as well as an increase in acyl coenzyme A:cholesterol-O-acyltransferase activity. However, IFN-gamma treatment of foam cells from Stat1 knockout mice failed to demonstrate reductions in efflux or ABC1 expression at the message or protein levels, nor were there any increases in acyl coenzyme A:cholesterol-O-acyltransferase activity. However, ABC1 mRNA expression in macrophages from Stat1 knockout mice could still be demonstrated to be increased by lipid loading with acetylated low density lipoprotein. Finally, Stat1-independent gene activation by IFN-gamma was intact in the Stat1 KO macrophages, inasmuch as IFN-gamma was shown to stimulate increases in interleukin-6 production in the Stat1 KO macrophages that were comparable to those observed in the wild-type macrophages. Therefore, Stat1 signaling is necessary and sufficient for the inhibitory effects of IFN-gamma on cholesterol efflux and ABC1 expression.

Interferon-beta-1b and interferon-gamma have similar inhibitory effects on apolipoprotein-E production in the monocyte/macrophage

Apolipoprotein-E (Apo-E) is the major lipid carrier in the brain, and is therefore important in the recyding of lipids and cholesterol to regenerating neurons during the remission phase of multiple sclerosis (MS). Interferon (IFN)-gamma has been shown to inhibit Apo-E production by a mainly post-transcriptional method in a macrophage cell line, and reduced Apo-E in cerebrospinal fluid is noted during the remission phase in patients. IFN-beta-1b is a recombinantly produced, anti-inflammatory cytokine, which has been shown to reduce the severity of MS relapses and reduce relapse rate. We have examined the effects of IFN-gamma and IFN-beta-1b on the production of Apo-E mRNA, cellular protein and secreted protein in primary monocytes derived from donor blood. IFN-beta-1b does not relieve the dose-dependent inhibition of Apo-E seen with IFN-gamma at up to 100 U/ml in these cells, and when used alone inhibits Apo-E production in a dose-dependent manner. This inhibition by IFN-beta-1b was seen to be at a transcriptional level, and dose dependent up to 100 U/ml. Apo-E genotype, which has also been linked to failure to recover from MS relapses, did not affect this inhibition. The mode of action of IFN-beta-1b in MS is therefore not thought to be through modification of Apo-E production.

Modulation of THP-1 macrophage and cholesterol-loaded foam cell apolipoprotein E levels by glycosphingolipids

Macrophages synthesize and secrete apolipoprotein E (apoE) constitutively. This process is upregulated under conditions of cholesterol loading. The response to cholesterol is antiatherogenic as it is believed to promote cholesterol efflux from the artery wall. The concentration of lactosyl ceramide (LacCer), a glycosphingolipid recently discovered to regulate cellular signaling, proliferation, and expression of adhesion molecules, is also increased in atherosclerotic tissues. Here we have investigated the effect of exogenous LacCer on macrophage apoE levels. We show that increasing macrophage LacCer levels sevenfold led to reductions in cellular and secreted apoE (15 and 30%, respectively, over a 24-h period) as determined by enzyme-linked immunosorbent assay. A similar effect was also induced by glucosyl ceramide (GlcCer) but not by ganglioside species. When macrophages were converted to cholesterol-loaded foam cells by incubation with acetylated LDL, the resulting increase in cellular apoE levels was inhibited by 26% when the cells were subsequently enriched with LacCer. After metabolic labeling of cellular glycosphingolipids with [14C]palmitate, we also discovered that high-density lipoprotein (HDL) stimulates the efflux of glycosphingolipids from foam cells. These data imply that LacCer and GlcCer may be proatherogenic due to the suppression of macrophage apoE production. Furthermore, the efflux of glycosphingolipids from macrophage foam cells to HDL could indicate a potential pathway for their removal from the artery wall and subsequent delivery to the liver.

Molecular link between cholesterol, cytokines and atherosclerosis

Current investigation on the origin of atherosclerosis has initiated an intense debate over whether atherosclerosis results from hypercholesterolemia or an inappropriate immune response to vascular injury. Although the role of the immune system has been questioned, the overwhelming body of evidence clearly indicates that atherogenesis is initiated by the interplay between cholesterol and cellular secretion of cytokines (especially IL-6) and apolipoprotein 'E' within the arterial wall. Recent studies have revealed that cells possess two cholesterol-sensors: (a) Receptor-Ck which senses the extracellular cholesterol and initiates signalling pathway responsible for the regulation of genes involved in the cell cycle, cell death, cellular cholesterol homeostasis and cytokines including IL-6; (b) LxR alpha which senses intracellular oxysterols and controls genes involved in cell death, cellular cholesterol homeostasis and cytokine IL-8. These cholesterol sensors define the molecular mechanism responsible for cholesterol-depended regulation of cellular synthesis and secretion of cytokines (IL-6, IL-8) within arterial wall. On the basis of this mechanism, presence of cholesterol and its oxy-derivative in the modified LDL will result in transient activation/deactivation of Receptor-Ck-dependent genes which will give rise to repeated cycles of growth coupled with apoptosis leading to a situation where apoptotic-deficient cells in the arterial wall, would be selected resulting in their accumulation and formation of oligoclonal atherosclerotic plaque.

TGF-beta reduced binding of high-density lipoproteins in murine macrophages and macrophage-derived foam cells

The expression of macrophage scavenger receptors is regulated by intracellular cholesterol levels, as well as by cytokines affecting macrophage effector functions. CD36, a member of the type B scavenger receptor family, will bind a variety of nonlipoprotein and lipoprotein ligands including high-density lipoprotein (HDL). Transforming growth factor-beta (TGF-beta) has been demonstrated to modulate macrophage effector functions and is present within atherosclerotic lesions. In the present study, the effect of TGF-beta on HDL binding by both macrophages and macrophage-derived foam cells was evaluated. TGF-beta, in a dose-dependent manner, reduced the binding of flurochrome-labeled HDL to both macrophages and foam cells. These effects were observed in macrophages derived from nonatherosclerotic (BALB/c) as well as from macrophages obtained from both apolipoprotein E and low-density lipoprotein receptor knockout mice. The decrease in HDL binding was consistent with a significant reduction in CD36 message levels. The effect of TGF-beta on type B scavenger receptor expression was not limited to CD36 as SR-BI message was also downregulated, although the effect was more modest. A similar reduction in HDL binding and CD36 message was also observed with the immunosuppressive glucocorticoid dexamethasone. These results suggest that within the microenvironment of an atherosclerotic lesion, TGF-beta and other agents that inhibit macrophage inflammatory responses may impact lesion progression through mechanisms that include the modulation of HDL-foam cell interactions.

The effect of gamma-interferon to inhibit macrophage-high density lipoprotein interactions is reversed by 15-deoxy-delta12,14-prostaglandin J2

Macrophage activation has been recognized as playing a central role in chronic inflammatory diseases in general and, more specifically, in the vascular wall during the progression of atherosclerotic lesions. Macrophage-activating factors present within the atherosclerotic lesion include the colony-stimulating factors and gamma interferon (IFNgamma). In the present study, the effects of IFNgamma on macrophage binding and uptake of fluorochrome-labeled high density lipoprotein (HDL) were investigated by flow cytometry and by measuring the amount of the type B scavenger receptors CD36 and scavenger receptor type B (SR-BI) by Northern blot analysis. IFNgamma-, but not granulocyte macrophage colony-stimulating factor (GM-CSF)-treated murine peritoneal macrophages displayed a two- to threefold decrease in Dil-labeled HDL uptake. This effect was observed in the absence of a comparable decrease in SR-BI message and protein or CD36 message. This decrease in both HDL binding and uptake was reversed by the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist, 15-deoxy-delta12,4-prostaglandin J2 (15d-PGJ2), which also inhibited the IFNgamma induction of the beta2 integrin CD11a. Furthermore, 15d-PGJ2 increased the expression of SR-BI and CD36 message and SR-BI protein which was reflected in an increase in HDL binding and uptake. These results suggest a role for PPARgamma agonists in modulating the IFNgamma-mediated macrophage effector functions relevant to atherosclerotic disease progression.

Enhanced upregulation of the Fc gamma receptor IIIa (CD16a) during in vitro differentiation of ApoE4/4 monocytes

We recently reported a positive correlation of the pool size of lipopolysaccharide receptor (CD14)dim and Fc gamma receptor IIIa (CD16a)+ monocytes in peripheral blood to the apolipoprotein E4 (apoE4) phenotype and a negative correlation to high density lipoprotein (HDL) cholesterol levels (Arterioscler Thromb Vasc Biol. 1996;16:1437-1447). In this study, the in vitro differentiation of mononuclear phagocytes derived from healthy blood donors homozygous for the E3/3 or the E4/4 phenotype was analyzed during 7 days of culture in serum-free medium supplemented with macrophage colony-stimulating factor (M-CSF). The CD16a expression, which indicates Fc receptor-dependent phagocytic activity, increased to a significantly higher level in apoE4/4 monocytes than in apoE3/3 cells. The costimulatory molecule CD40, which indicates antigen-presenting capacity, was upregulated more strongly in apoE3/3 monocytes compared with E4/4 cells, but the difference did not reach a significant level. The expression of differentiation-associated surface proteins (CD14, CD33, CD45) and adhesion molecules (CD11a, CD11b, CD11c, CD49d) was not significantly different between apoE3/3 and apoE4/4 monocytes. However, a significantly decreased intracellular apoE concentration and a reduced amount of secreted apoE were found in apoE4/4 monocytes during in vitro differentiation. No differences were found in the surface expression of the low density lipoprotein receptor-related protein (CD91) and the uptake of fluorescence labeled low density lipoprotein between apoE3/3 and apoE4/4 monocytes. These data indicate that the apoE4/4 phenotype significantly influences the M-CSF-dependent differentiation of monocytes toward a more CD16a-positive phagocytic phenotype.

Comparison of apolipoprotein and proteoglycan deposits in human coronary atherosclerotic plaques: colocalization of biglycan with apolipoproteins

BACKGROUND

Because the content of specific proteoglycans and apolipoproteins is increased in atherosclerotic plaques and in vitro studies have suggested a role for proteoglycans in mediating plaque apolipoprotein (apo) retention, immunohistochemistry was performed to systematically examine the relative locations of proteoglycans and apolipoproteins in human atherosclerosis.

METHODS AND RESULTS

The spatial relationships of versican, biglycan, and apoE were compared on 68 human coronary artery segments; apoA-I and apoB also were evaluated on an additional 20 segments. Nonatherosclerotic intima contained extensive deposits of versican, whereas deposits of apoE, apoB, and apoA-I were much less prevalent. In contrast, nearly all atherosclerotic segments contained substantial deposits of biglycan, apoE, apoA-I, and apoB. There was a high degree of colocalization of apoE and biglycan deposits. ApoA-I, the major apolipoprotein of HDL, and apoB also were detected in regions with apoE and biglycan deposition. Exceptions to the localization of biglycan with apolipoproteins were found in regions that lacked intact extracellular matrix because of necrosis or dense macrophage accumulation. In vitro studies demonstrated that biglycan binds apoE-containing but not apoE-free HDL and that biglycan also binds LDL.

CONCLUSIONS

These results suggest that biglycan may bind apoE and apoB in atherosclerotic intima. They also raise the possibility that apoE may act as a "bridging" molecule that traps apoA-I-containing HDL in atherosclerotic intima. Taken together, these findings are consistent with the hypothesis that biglycan may contribute to the pathogenesis of atherosclerosis by trapping lipoproteins in the artery wall.

Acquisition of secretion of transforming growth factor-beta 1 leads to autonomous suppression of scavenger receptor activity in a monocyte-macrophage cell line, THP-1

Macrophage cells derived from the human monocytic leukemia cell line, THP-1, accumulate esterified cholesterol when cultivated in the presence of acetylated low density lipoprotein (Ac-LDL) through scavenger receptors (ScR). In the present study, we isolated a subtype of THP-1 cells that failed to accumulate esterified cholesterol when cultivated in the presence of Ac-LDL. The cells had negligible amounts of cell association and degradation of Ac-LDL compared with the parent THP-1 cells. The subtype THP-1 cells did not express ScR mRNA as well as that of lipoprotein lipase. In contrast, the expression of apolipoprotein E mRNA was greater than that found in parent THP-1 cells. The culture medium of subtype THP-1 cells treated with 12-O-tetradecanoylphorbol-13-acetate inhibited the uptake of Ac-LDL and the expression of ScR in parent THP-1 cells. After a 48-h incubation in the culture medium containing 12-O-tetradecanoylphorbol-13-acetate, the culture medium of differentiated subtype THP-1 cells contained 6.9 ng/ml transforming growth factor (TGF)-beta 1, while that of parent THP-1 cells secreted below detection level, which was less than 3 ng/ml. This inhibitory effect of the conditioned medium on the expression of ScR in parent THP-1 cells was abolished by pretreatment of the culture medium with anti-TGF-beta 1 antibodies. Parent THP-1 cells expressed as much TGF-beta 1 mRNA as sTHP-1 cells after stimulation of differentiation. Although the precursor forms of TGF-beta 1 that were synthesized in both parent and subtype THP-1 cells were of similar size and were expressed at similar levels, latent TGF-beta 1-binding protein, which is necessary for the secretion of TGF-beta 1, could only be co-immunoprecipitated with anti-TGF-beta 1 antibody from subtype THP-1 cells. This suggests that subtype THP-1 cells secrete TGF-beta 1 into the medium by forming a functional complex with the latent TGF-beta 1-binding protein. We conclude that subtype THP-1 cells could not take up Ac-LDL because ScR was inhibited (leading to a loss of function) caused by the secreted TGF-beta 1.

High-density lipoprotein-binding protein (HBP)/vigilin is expressed in human atherosclerotic lesions and colocalizes with apolipoprotein E

Accumulation of cholesteryl esters within cells of the arterial intima is a hallmark of atherosclerosis. A small number of proteins have been shown in vitro to be upregulated by cellular cholesterol loading, including apolipoprotein E (apoE) and the recently cloned HDL-binding protein (HBP), but only apoE has been shown to be upregulated in cholesterol-loaded cells in atherosclerosis. To determine whether HBP (also called vigilin) might be expressed in human atherosclerosis, immunohistochemistry and in situ hybridization were performed on coronary arteries of 18 patients. HBP/vigilin was detected on all endothelial cells. HBP/vigilin mRNA and protein also were detected on a subset of macrophages and occasionally on smooth muscle cells (SMC) in atherosclerotic plaques but were not detected on these cell types in nondiseased coronary intima. The majority of HBP/vigilin-expressing macrophages were foam cells, but HBP/vigilin expression also was detected rarely in nonfoam cell macrophages. Foam cell macrophage HBP/vigilin expression was present in 100% of atherosclerotic quadrants, and nonfoam cell macrophage HBP/vigilin expression was present in 6% of atherosclerotic quadrants. HBP/vigilin-expressing human plaque cells also expressed apoE. However, HBP/vigilin was detected in cardiac myocyte foam cells of an apoE-deficient mouse, demonstrating that HBP/vigilin expression can occur independently of apoE. These results suggest that in vivo HBP/vigilin expression is upregulated by intracellular cholesterol loading but also that other factors present in atherosclerotic plaques may upregulate HBP/vigilin. Although the exact function of HBP/vigilin is unknown, its expression in plaque macrophages suggests a role for this molecule in atherogenesis.

Altered apolipoprotein E secretion in cytokine treated human astrocyte cultures

Apolipoprotein E (ApoE), postulated to be a major lipid carrier protein in brain, is synthesized and secreted primarily by astrocytes and is involved in brain development and repair. We have analyzed its secretion in primary cultures of older (high passage) slowly dividing and younger (lower passage) rapidly dividing fetal human astrocytes exposed to various inflammatory and anti-inflammatory cytokines, alone and in combination. ApoE secretion was reduced in high passage astrocytes when compared to lower passage astrocytes. A further reduction in ApoE secretion in high passage cells was consistently produced by the combination of cytokines interleukin 1 (IL-1) alpha and beta and interferon (IFN-gamma) cytokines or by the basic fibroblast growth factor (basic-FGF) alone. Epidermal growth factor (EGF) increased ApoE secretion. The combination of these cytokine effects in chronically degenerating brain regions of Alzheimer's disease and other neurodegenerative diseases could reduce the amount of ApoE available for neuronal regeneration. EGF, or agents inducing EGF, could ameliorate these ApoE deficiencies.

LPS and cytokines regulate extra hepatic mRNA levels of apolipoproteins during the acute phase response in Syrian hamsters

Altered hepatic expression of apolipoproteins occurs during the acute phase response. Here we examined whether the acute phase response alters extra hepatic expression of apolipoproteins. Syrian hamsters were injected with endotoxin (LPS), tumor necrosis factor (TNF), interleukin (IL)-1, or the combination of TNF + IL-1 and mRNAs for serum amyloid A (apoSAA), apolipoprotein (apo) J, apo E. apo A-I, and apo D, were analyzed. LPS increased mRNA levels for apoSAA in all tissues examined. LPS and TNF + IL-1 increased mRNA levels for apo J in kidney, heart, stomach, intestine, and muscle. Individually, TNF and IL-1 were less potent than the combination of the two cytokines. LPS decreased mRNA levels for apo E in all tissues, except for mid and distal intestine. TNF and IL-1 were less effective than LPS. LPS, TNF + IL-1 and TNF decreased mRNA levels for apo A-I in duodenum. mRNA for apo D decreased in heart, were unchanged in brain and increased in muscle, following LPS. The widespread extra hepatic regulation of the apolipoproteins during the acute phase response may be important for the alterations in lipid metabolism that occur during infection and inflammation as well as the immune response.