Oxidized low density lipoprotein and innate immune receptors

Individual variation of scavenger receptor expression in human macrophages with oxidized low-density lipoprotein is associated with a differential inflammatory response

Atherosclerosis is an inflammatory disease in which oxidized low-density lipoprotein (oxLDL) plays important roles. Scavenger receptors (SR) CD36, SR-A, and LOX-1 uptake over 90% of the oxLDL leading to foam cell formation and secretion of inflammatory cytokines. To investigate whether the interindividual differences in macrophage SR gene expression could determine the inflammatory variability in response to oxLDL, we quantified the gene and protein expression of SR and inflammatory molecules from macrophages isolated from 18 volunteer subjects and incubated with oxLDL for 1, 3, 6, and 18 h. The individual gene expression profile of the studied SR at 1 h of incubation was highly variable, showing a wide fold-change range: CD36: -3.57-4.22, SR-A: -5.0-4.43, and LOX-1: -1.56-75.32. We identified subjects as high and low responders depending on whether their SR gene expression was above or below the median, showing a different inflammation response pattern. CD36 and LOX-1 gene expression correlated positively with IL-1beta; SR-A correlated negatively with IL-8 and positively with PPARgamma and NF-kappaBIotaA. These results were confirmed in the same subjects 3 mo after the first sampling. Furthermore, a negative correlation existed between CD36 and SR-A at protein level after 18 h of oxLDL incubation (R = -0.926, p = 0.024). These data would suggest that the type of SR could determine the macrophage activation: more proinflammatory when associated to CD36 and LOX-1 than when associated with SR-A.

Inhibition of the Renin-Angiotensin System Abolishes the Proatherogenic Effect of Uremia in Apolipoprotein E-Deficient Mice

Objective— Uremia accelerates atherosclerosis in apolipoprotein E-deficient (apoE −/− ) mice. We examined whether this effect may be preventable by pharmacological blockade of the renin-angiotensin system (RAS).

Methods and Results— Uremia was induced in apoE −/− mice by 5/6 nephrectomy (NX). Treatment with the angiotensin converting enzyme inhibitor enalapril (2 or 12 mg/kg/d) from week 4 to 36 after NX reduced the aortic plaque area fraction from 0.23±0.02 (n=20) in untreated mice to 0.11±0.01 (n=21) and 0.08±0.01 (n=23), respectively ( P <0.0001); the aortic plaque area fraction was 0.09±0.01 (n=22) in sham-operated controls. Enalapril from week 20 to 44 after NX also retarded the progression of atherosclerosis. Plasma levels of soluble intercellular adhesion molecule-1 (sICAM-1) and vascular cell adhesion molecule-1 (sVCAM-1) and concentrations of IgM antibodies against oxidized low density lipoprotein (OxLDL) increased after NX ( P <0.01). Enalapril (12 mg/kg/d) attenuated these increases ( P <0.05) and reduced aortic expression of vascular cell adhesion molecule (VCAM)-1 mRNA ( P <0.05). Atherosclerosis in NX mice was also reduced by losartan (an angiotensin II receptor-blocker), but not when blood pressure was lowered with hydralazine (a non-RAS-dependent vasodilator).

Conclusion— The results suggest that inhibition of RAS abolishes the proatherogenic effect of uremia independent of its blood pressure-lowering effect, possibly because of antiinflammatory and antioxidative mechanisms.

Induction of dendritic cell-like phenotype in macrophages during foam cell formation

Foam cell formation from monocyte-derived macrophages is a hallmark of atherosclerotic lesions. Aspects of this process can be recapitulated in vitro by exposing M-CSF-induced or platelet factor 4 (CXCL4)-induced macrophages to oxidized (ox) or minimally modified (mm) low density lipoprotein (LDL). We measured gene expression in peripheral blood mononuclear cells, monocytes, and macrophages treated with CXCL1 (GRO-α) or CCL2 (MCP-1), as well as foam cells induced by native LDL, mmLDL, or oxLDL using 22 Affymetrix gene chips. Using an advanced Bayesian error-pooling approach and a heterogeneous error model with a false discovery rate <0.05, we found 5,303 of 22,215 probe sets to be significantly regulated in at least one of the conditions. Among a subset of 917 candidate genes that were preselected for their known biological functions in macrophage foam-cell differentiation, we found that 290 genes met the above statistical criteria for significant differential expression patterns. While many expected genes were found to be upregulated by LDL and oxLDL, very few were induced by mmLDL. We also found induction of unexpected genes, most strikingly MHC-II and other dendritic cell markers such as CD11c. The gene expression patterns in response to oxLDL were similar in M-CSF-induced and CXCL4-induced macrophages. Our findings suggest that LDL and oxLDL, but not mmLDL, induce a dendritic cell-like phenotype in macrophages, suggesting that these cells may be able to present antigens and support an immune response.

Proteins specifically hyperexpressed in a coeliac disease patient with aberrant T cells

An aberrant T cell population is the basis for diagnosis of refractory coeliac disease and determines the risk of enteropathy-associated T cell lymphoma. This disease is serious with a poor survival. Pathogenetic mechanisms sustaining aberrant T cell proliferation remain unknown. Recently, alemtuzumab has been proposed as a promising new approach to treat these patients. Only few single cases have been tested at present; nevertheless, in all the cases a clinical improvement was observed. However, whether intraepithelial lymphocytes have been targeted effectively by alemtuzumab is still debated. This study reports, using two-dimensional difference gel electrophoresis (2D DIGE), hyperexpressed proteins associated specifically with aberrant T cells found in a patient with coeliac disease by comparison of the protein expression of this sample with that of patients with coeliac disease and polyclonal T cells or with control subjects. The data demonstrated a significantly higher expression of IgM, apolipoprotein C-III and Charcot-Leyden crystal proteins in a duodenal biopsy specimen of the patient with clonal T cells compared with that of other patients. These preliminary results allow hypothesizing different clinical effects of alemtuzumab in patients with coeliac disease and aberrant T cell proliferation, because as well as the probable effect on T cells, alemtuzumab could exert its effect by acting on inflammatory associated CD52(+) IgM(+) B cells and eosinophil cells, known to produce IgM and Charcot-Leyden crystal proteins, that we demonstrated to be altered in this patient. The results also emphasize the possible association of apolipoprotein with aberrant T cell proliferation.

Innate immune receptor expression in normal brain aging

Brain aging often results in cognitive impairment and is considered to be a major risk factor for neurodegenerative diseases. Earlier studies reported inflammatory responses in aged brain that could contribute to age-related neurodegeneration. Recently, innate immune receptors such as toll-like receptors (TLRs), so far implicated in defense against microorganisms, have been linked to pathogenesis of Alzheimer's disease. Therefore, we asked whether the transcription of TLRs (1-9) and CD14, could also be altered in physiological brain aging. Using real-time polymerase chain reaction (PCR), we indeed observed that TLR1, TLR2, TLR4, TLR5, TLR7 and CD14 expression was up-regulated in mouse brain in correlation with age. In contrast, transcriptions of TLR3, TLR6 and TLR8 were unchanged and the one of TLR9 was down-regulated. In situ hybridization further confirmed these results and identified the cellular source of TLR2 and TLR7 as mononuclear phagocytes. Together, this first systematic analysis demonstrates altered regulation of those innate immune receptors even in normal brain aging, which might be of relevance for understanding susceptibility to neurodegenerative processes associated with aging.

Role of naturally occurring CD4+ CD25+ regulatory T cells in experimental atherosclerosis

OBJECTIVE

Naturally occurring CD4+ CD25+ regulatory T cells (Tregs) exert suppressive effects on effector CD4 cells and downregulate experimental autoimmune disorders. We investigated the importance and potential role of Tregs in murine atherogenesis.

METHODS AND RESULTS

Tregs were investigated comparatively between aged and young apolipoprotein E-knockout (ApoE-KO) mice and age-matched C57BL/6 littermates. The effect of oxidized LDL (oxLDL) was tested on the functional suppressive properties of Tregs from ApoE-KO and C57BL/6 mice. Tregs, CD4+ CD25- cells, and saline were infused into ApoE-KO mice to study their effects on atherogenesis. Treg numbers were reduced in atherosclerotic compared with nonatherosclerotic ApoE-KO mice. The functional suppressive properties of Tregs from ApoE-KO mice were compromised in comparison with those from their C57BL/6 littermates. Thus, oxLDL attenuated the suppressive properties of Tregs from C57BL/6 mice and more so in ApoE-KO mice. Transfer of Tregs from age-matched ApoE-KO mice resulted in significant attenuation of atherosclerosis compared with that after delivery of CD4+ CD25+/- T cells or phosphate-buffered saline.

CONCLUSIONS

CD4+ CD25+ Tregs may play a protective role in the progression of atherosclerosis and could be considered a therapeutic tool if results from human studies can solidify observations in murine models.

The human IgM antibody SAM-6 induces tumor-specific apoptosis with oxidized low-density lipoprotein

Lipids are essential for normal and malignant cells during growth and differentiation. The turnover is strictly regulated because an uncontrolled uptake and accumulation is cytotoxic and can lead to lipoapoptosis: lipoptosis. The human monoclonal antibody SAM-6 binds to a cell surface receptor on malignant cells and to oxidized low-density lipoprotein (LDL). SAM-6 induces an excess of intracellular lipids, by overfeeding malignant cells with oxidized LDL, via a receptor-mediated endocytosis. The treated cells overaccumulate depots of cholesteryl esters and triglycerides. This lipid overaccumulation is tumor specific; nonmalignant cells neither bind the antibody nor harvest lipids after incubation. Because for both forms of apoptosis, the death domain dependent ("extrinsic") and independent ("intrinsic"), the activation of proteases is crucial, we also investigated this pathway in more detail. It was found that shortly after internalization of antibody/oxidized LDL/receptor complex and formation of lipid depots, cytochrome c is released by mitochondria. Followed by this, initiator caspase-8 and caspase-9 and effector caspase-3 and caspase-6 are activated. The mechanism of mitochondrial trigger (e.g., by free fatty acids) is under investigation. However, the present data indicate that the SAM-6 antibody induces an intrinsic-like form of apoptosis by overfeeding malignant cells with lipoproteins.

Pathogen-sensing plasmacytoid dendritic cells stimulate cytotoxic T-cell function in the atherosclerotic plaque through interferon-alpha

BACKGROUND

Unstable atherosclerotic plaque is characterized by an infiltrate of inflammatory cells. Both macrophages and T cells have been implicated in mediating the tissue injury leading to plaque rupture; however, signals regulating their activation remain unidentified. Infectious episodes have been suspected to render plaques vulnerable to rupture. We therefore explored whether plasmacytoid dendritic cells (pDCs) that specialize in sensing bacterial and viral products can regulate effector functions of plaque-residing T cells and thus connect host infection and plaque instability.

METHODS AND RESULTS

pDCs were identified in 53% of carotid atheromas (n=30) in which they localized to the shoulder region and produced the potent immunoregulatory cytokine interferon (INF)-alpha. IFN-alpha transcript concentrations in atheroma tissues correlated strongly with plaque instability (P<0.0001). Plaque-residing pDCs responded to pathogen-derived motifs, CpG-containing oligodeoxynucleotides binding to toll-like receptor 9, with enhanced IFN-alpha transcription (P=0.03) and secretion (P=0.007). IFN-alpha emerged as a potent regulator of T-cell function, even in the absence of antigen recognition. Specifically, IFN-alpha induced a 10-fold increase of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) on the surface of CD4 T cells (P<0.0001) and enabled them to effectively kill vascular smooth muscle cells (P=0.0003).

CONCLUSIONS

pDCs in atherosclerotic plaque sense microbial motifs and amplify cytolytic T-cell functions, thus providing a link between host-infectious episodes and acute immune-mediated complications of atherosclerosis.

Multiple roles of Toll-like receptor signaling in atherosclerosis

PURPOSE OF REVIEW

Toll-like receptors are key regulators of both innate and adaptive immune responses. This review outlines the recently emerged multiple roles of Toll-like receptor signaling in atherosclerosis.

RECENT FINDINGS

Mice deficient in TLR4, TLR2 and MyD88 all have reduced atherosclerosis which establishes that Toll-like receptor-dependent pathways contribute to disease development. Although it is likely that total "infectious burden" contributes to atherosclerosis progression, endogenous ligands may also initiate and modulate Toll-like receptor signaling pathways. CD36, with established roles in recognition of endogenous ligands and atherosclerotic disease, facilitates TLR2 signaling and might therefore represent a bridge between endogenous lipid ligands and Toll-like receptor pathways. Furthermore, lipoprotein oxidation generates ligands that activate Toll-like receptor pathways. At the same time, Toll-like receptor activation may be inhibited by accumulating oxidized phospholipids, which could result in reduced dendritic cell maturation and impaired immunological priming.

SUMMARY

Activation of Toll-like receptor signaling can promote atherosclerosis by multiple mechanisms, while some beneficial Toll-like receptor pathways may be inhibited by lipid accumulation. Due to their central role in the disease process, Toll-like receptor signaling pathways represent a target of immunomodulatory therapy with the goal of tipping the balance from excessive chronic inflammation towards resolution of inflammation, while not compromising host defense or atheroprotective immune functions.

C-reactive protein inhibits in vitro oxidation of low-density lipoprotein

C-reactive protein (CRP) is elevated in cardiovascular disease and binds to oxidized phosphatidylcholine (oxPtC) in the low-density lipoprotein (LDL) surface. In the present study, we tested if CRP influences the susceptibility of LDL to oxidation. At physiological concentrations of 1-7mug/ml, CRP strongly inhibited copper-mediated oxidation of LDL and phospholipid liposomes in a concentration-dependent manner. Similar concentrations of different monoclonal antibodies or albumin did not influence LDL oxidation. Antioxidant activity of CRP was inhibited by phosphocholine (PC), indicating that the observed activity involves binding of CRP to oxPtC. These results suggest that CRP may limit atherogenic oxidation of LDL in vivo.

Toll-like receptor-2 is essential for the development of palmitate-induced insulin resistance in myotubes

Fatty acids can activate proinflammatory pathways leading to the development of insulin resistance, but the mechanism is undiscovered. Toll like receptor 2 (TLR2) recognizes lipids, activates proinflammatory pathways, and is genetically associated with inflammatory diseases. This study aimed to examine the role of TLR2 in palmitate-induced insulin resistance in C2C12 myotubes. Treatment with palmitate rapidly induced the association of myeloid differentiation factor 88 (MyD88) with the TLR2 receptor, activated the stress-linked kinases p38, JNK, and protein kinase C, induced degradation of IkappaBalpha, and increased NF-kappaB DNA binding. The activation of these pathways by palmitate was sensitive and temporally regulated and occurred within the upper physiologic range of saturated fatty acid concentrations in vivo, suggesting a receptor-mediated event and not simple lipotoxicity. When compared with an equimolar concentration of palmitate, fibroblast-stimulating lipopeptide-1, a known TLR2 ligand, was a slightly more potent activator of signal transduction and interleukin (IL)-6 production. Palmitate inhibited insulin signal transduction in C2C12 cells beginning 1-2 h after exposure and reached a maximum at 12-16 h. An antagonist TLR2 antibody, mAb 2.5, led to a 50-60% decrease in palmitate-induced IL-6 production and partially restored insulin signal transduction, whereas an isotype-matched control antibody had no effect. RNA interference-mediated inhibition of TLR2 and MyD88 expression in C2C12 muscle cells resulted in a near complete inhibition of palmitate-induced insulin resistance and IL-6 production. This study provides strong evidence that TLR2 mediates the initial events of fatty acid-induced insulin resistance in muscle.

Oxidative stress and atherosclerosis

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality in the Western world. Its incidence has been increasing lately in developing countries. Several lines of evidence support a role for oxidative stress in atherogenesis. Growing evidence indicates that chronic and acute overproduction of reactive oxygen species (ROS) under pathophysiologic conditions is integral in the development of cardiovascular diseases (CVD). ROS mediate various signaling pathways that underlie vascular inflammation in atherogenesis from the initiation of fatty streak development through lesion progression to ultimate plaque rupture. Various animal models of oxidative stress support the notion that ROS have a causal role in atherosclerosis and other cardiovascular diseases. Human investigations also support the oxidative stress hypothesis of atherosclerosis. Oxidative stress is the unifying mechanism for many CVD risk factors, which additionally supports its central role in CVD. A main source of ROS in vascular cells is the reduced nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase system. This is a membrane-associated enzyme, composed of five subunits, catalyzing the one-electron reduction of oxygen, using NADH or NADPH as the electron donor. This system is an important target for genetic investigations. Identification of groups of patients with genetically prone or resistant of oxidative stress is therefore an obvious target of investigation. A better understanding of the complexity of cellular redox reactions, development of a new class of antioxidants targeted to specific subcellular sites, and the phenotype-genotype linkage analysis for oxidative stress will likely be avenues for future research with regards to the broader use of pharmacological therapies in the treatment and prevention of CVD.

Myeloperoxidase Predicts Progression of Carotid Stenosis in States of Low High-Density Lipoprotein Cholesterol

OBJECTIVES

We investigated the effect of myeloperoxidase (MPO) on progression of carotid stenosis in states of high and low high-density lipoprotein-cholesterol (HDL-C) and low-density lipoprotein-cholesterol (LDL-C) levels.

BACKGROUND

Myeloperoxidase is pivotally involved in the pathogenesis of atherosclerosis. In vitro data suggest that MPO exerts deleterious effects via oxidative modulation of lipoproteins.

METHODS

We prospectively studied 1,019 of 1,268 consecutive patients who were asymptomatic with respect to carotid artery disease. Patients underwent serial carotid ultrasound investigations at baseline and after a follow-up interval of median 7.5 months (range 6 to 9 months), categorizing carotid arteries as 0% to 29%, 30% to 49%, 50% to 69%, 70% to 89%, or 90% to 99% stenosed or occluded. The MPO, HDL-C, and LDL-C levels were measured at baseline, grouped by medians, and correlated with progression of carotid atherosclerosis.

RESULTS

Progression of carotid atherosclerosis was found in 100 of 1,019 patients (9.8%). Myeloperoxidase (p = 0.014) but not HDL-C (p = 0.95) or LDL-C (p = 0.30) were associated with progressive disease. However, MPO > or =310 ng/ml was significantly associated with progressive disease (adjusted odds ratio [OR] 2.57, 95% confidence interval [CI] 1.39 to 4.75) only in patients with HDL-C levels <49 mg/dl. Otherwise, in patients with higher HDL-C levels (> or =49 mg/dl), MPO > or =310 ng/ml did not predict disease progression (adjusted OR 1.42, 95% CI 0.72 to 2.78). No interaction of MPO with LDL-C was observed.

CONCLUSIONS

Myeloperoxidase was associated with progression of carotid atherosclerosis in patients with HDL cholesterol levels below 49 mg/dl.

OxLDL–IgG Immune Complexes Induce Survival of Human Monocytes

OBJECTIVE

Immune complexes containing oxidatively modified low-density lipoprotein (oxLDL) particles are deposited in human atherosclerotic lesions during atherogenesis. Here we studied whether OxLDL-IgG immune complexes (OxLDL-IgG ICs) affect survival of human monocytes.

METHODS AND RESULTS

As demonstrated by light microscopy, and analysis of cell proliferation, caspase-3 activity, and DNA fragmentation, OxLDL-IgG ICs promoted survival of cultured human monocytes by decreasing their spontaneous apoptosis. OxLDL-IgG ICs induced a concentration-dependent production of the major monocyte growth factor, monocyte colony-stimulating factor (M-CSF), by the monocytes, but its inhibition was without effect on OxLDL-IgG IC-induced monocyte survival. Rather, OxLDL-IgG ICs induced rapid phosphorylation of Akt, suggesting a direct anti-apoptotic effect mediated by cross-linking of Fcgamma receptors. Experiments with receptor blocking antibodies revealed that the OxLDL-IgG IC-induced monocyte survival was mediated by Fcgamma receptor I.

CONCLUSIONS

The results show that OxLDL-IgG ICs promote survival of monocytes by cross-linking Fcgamma receptor I and activating Akt-dependent survival signaling. The results reveal a novel mechanism by which an immune reaction toward oxLDL can play a role in the accumulation of macrophages in human atherosclerotic lesions.

Oxidized phospatidylcholine but not native phosphatidylcholine inhibits inducible nitric oxide synthase in RAW 264.7 macrophages

This study was designed to compare the effects of oxidized 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphorylcholine (PAPC) and native PAPC on the inducible nitric oxide synthase (iNOS) in the macrophage cell line RAW 264.7. Macrophages stimulated by bacterial lipopolysaccharide (1 microg/ml) were incubated with increasing amounts of native or oxidized PAPC (oxPAPC, 10-20 microg/ml). Cells incubated with oxPAPC showed a dose-dependent inhibition of inducible nitric oxide synthesis, as well as reduced iNOS protein expression and mRNA levels. Additionally, chromatin immunoprecipitation assay revealed that oxPAPC reduced the interaction of the active NF-kappaB subunit p65 with the iNOS promoter region when compared to native PAPC.

Effect of the functional toll-like receptor 4 Asp299Gly polymorphism on susceptibility to late-onset Alzheimer's disease

Experimental data have shown an upregulated expression of toll-like receptors, particularly toll-like receptor 4 (TLR4), in neurodegeneration. The Asp299Gly polymorphism of the TLR4 gene has been associated with an attenuated receptor signalling and a blunted inflammatory response. In the present study, we sought to determine whether this common genetic variant could influence susceptibility to late-onset Alzheimer's disease (LOAD) in an Italian population sample. A cohort of 277 LOAD patients and 300 cognitively healthy controls were genotyped for the TLR4 Asp299Gly polymorphism using restriction isotyping. The frequency of the minor 299Gly allele was significantly higher in the controls than in the LOAD cases (7.2% versus 3.1%, respectively, P=0.003). Additionally, the frequency of the variant genotypes (Asp/Gly and Gly/Gly) was 13.0% in the controls and 5.4% in LOAD patients (P=0.002). After adjustment for age, gender, and the APOE varepsilon4 carrier status, the odds ratio for the development of LOAD associated with the Asp/Gly and Gly/Gly versus Asp/Asp genotype was 0.37 (95% CI: 0.20-0.69, P=0.002). Our data further support a role for innate immunity in neurodegeneration and give the first evidence that the TLR4 Asp299Gly variant may be protective toward the development of LOAD.

Lipoproteomics I: mapping of proteins in low-density lipoprotein using two-dimensional gel electrophoresis and mass spectrometry

The molecular mechanisms underlying the relationship between low-density lipoprotein (LDL) and the risk of atherosclerosis are not clear. Therefore, detailed information about the protein composition of LDL may contribute to reveal its role in atherogenesis and the mechanisms that lead to coronary disease in humans. Here, we sought to map the proteins in human LDL by a proteomic approach. LDL was isolated by two-step discontinuous density-gradient ultracentrifugation and the proteins were separated with two-dimensional gel electrophoresis and identified with peptide mass fingerprinting, using matrix assisted laser desorption/ionization-time of flight-mass spectrometry and with amino acid sequencing using electrospray ionization tandem mass spectrometry. These procedures identified apo B-100, apo C-II, apo C-III (three isoforms), apo E (four isoforms), apo A-I (two isoforms), apo A-IV, apo J and apo M (three isoforms not previously described). In addition, three proteins that have not previously been identified in LDL were found: serum amyloid A-IV (two isoforms), calgranulin A, and lysozyme C. The identities of apo M, calgranulin A, and lysozyme C were confirmed by sequence information obtained after collision-induced dissociation fragmentation of peptides characteristic for these proteins. Moreover, the presence of lysozyme C was further corroborated by demonstrating enriched hydrolytic activity in LDL against Micrococcus lysodeikticus. These results indicate that in addition to the dominating apo B-100, LDL contains a number of other apolipoproteins, many of which occur in different isoforms. The demonstration, for the first time, that LDL contains calgranulin A and lysozyme C raises the possibility that LDL proteins may play hitherto unknown role(s) in immune and inflammatory reactions of the arterial wall.

Expression of Human Myeloperoxidase by Macrophages Promotes Atherosclerosis in Mice

Background— Myeloperoxidase (MPO) colocalizes with macrophages in the human artery wall, and its characteristic oxidation products have been detected in atherosclerotic lesions. Thus, oxidants produced by the enzyme might promote atherosclerosis. However, macrophages in mouse atherosclerotic tissue do not express MPO. Therefore, mice are an inappropriate model for testing the role of MPO in vascular disease. To overcome this problem, we generated and studied transgenic (Tg) mice that contained the human MPO gene.

Methods and Results— We produced human MPO -Tg mice with use of a Visna virus promoter. To confine MPO expression to macrophages, we lethally irradiated LDL receptor–deficient mice and repopulated their bone marrow with cells from wild-type mice or MPO -Tg mice. Despite having similarly high levels of cholesterol after maintenance on a high-fat, high-cholesterol diet, the MPO -Tg animals developed a 2-fold greater atherosclerotic area in the aorta than did mice transplanted with wild-type bone marrow ( P =0.00003).

Conclusions— Our observations indicate that expression of human MPO in macrophages promotes atherosclerosis in hypercholesterolemic mice, raising the possibility that the enzyme might be a potential therapeutic target for preventing cardiovascular disease in humans.

A frequent toll-like receptor (TLR)-2 polymorphism is a risk factor for coronary restenosis

Restenosis is a major problem for patients undergoing percutaneous transluminal coronary angioplasty (PTCA). Inflammatory processes and genetic factors have been suggested to be involved in the pathogenesis of both atherosclerosis and restenosis. The recently discovered family of Toll-like receptors (TLRs) consists of molecules that initiate signaling after host-pathogen interactions. Recently it has been shown that the TLRs are involved in the development and progression of atherosclerosis by interfering with lipid metabolisms and by mediating inflammation. TLR-2 is a key innate immunity receptor for sensing both endogenous inflammatory mediators and ligands of several microbial pathogens postulated to be involved in atherosclerosis. A frequent single nucleotide polymorphism (SNP) for the TLR-2 gene, resulting in a non-functional receptor, has been described. By genotyping two independent groups of patients receiving PTCA, followed by stent implantation in one group, we found a significantly enhanced frequency of the TLR-2 Arg753Gln SNP in patients with restenosis as compared to those without restenosis (PTCA: 7.21 versus 2.45%, P = 0.014; PTCA/stent: 6.86 versus 1.53%, P = 0.013). In contrast, a common TLR-4 SNP was similarly distributed among the patient groups investigated. We furthermore compared the frequency of both SNPs in the patients with an age-matched group of individuals without atherosclerosis and found a trend towards a lower frequency of the TLR-4 SNP in the atherosclerotic group (PTCA: 5.58; PTCA/stent: 3.85 versus 7.14%). We conclude that in restenosis a functional TLR-2 is protective and potentially involved in a reaction pattern preventing restenosis. Screening for the TLR-2 Arg753Gln SNP may be of importance for stratifying a patient's risk and for preventive and therapeutic measures.

Lipoproteins, macrophage function, and atherosclerosis: beyond the foam cell?

Atherogenesis requires and is highly influenced by the interaction between lipoproteins and macrophages. Most of the focus to date has been on the ability of atherogenic lipoproteins (such as low-density lipoproteins, LDL) to promote and of anti-atherogenic lipoproteins (such as high-density lipoproteins, HDL) to prevent the development of the cholesteryl ester-enriched macrophage-derived foam cell. However, lipoprotein-macrophage interactions have the potential to modulate macrophage function in a variety of additional ways that may impact on atherosclerosis. These include modulating cellular cholesterol and oxysterol content, providing fatty acids as ligands for PPARs, and acting as ligands for macrophage scavenger and Toll-like receptors. We suggest that atherogenic lipoproteins promote and anti-atherogenic lipoproteins inhibit atherogenesis by modulating macrophage function in a variety of ways beyond cholesteryl ester accumulation and foam cell formation.

Toll-like receptor 4-dependent and -independent cytokine secretion induced by minimally oxidized low-density lipoprotein in macrophages

OBJECTIVE

Innate immune responses to oxidized low-density lipoprotein LDL (LDL) regulate the development of atherosclerosis. We demonstrated previously that an early form of oxidized LDL, minimally modified LDL (mmLDL), triggers cytoskeletal rearrangements in macrophages via CD14 and Toll-like receptor 4 (TLR4)/MD-2. Because lipopolysaccharide (LPS) activation of TLR4 leads to proinflammatory gene expression, in this study, we asked whether mmLDL also induced proinflammatory signaling.

METHODS AND RESULTS

We studied cytokine secretion and signaling in J774 and primary peritoneal macrophages stimulated with mmLDL, which was prepared by incubating LDL with cells expressing human 15-lipoxygenase. MmLDL stimulated robust phosphoinositide 3-kinase (PI3K) activation, and Akt and extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, which exceeded that induced by LPS. On the other hand, although mmLDL induced nuclear factor kappaB (NF-kappaB) p65 translocation to the nucleus, there was no detectable NF-kappaB activation. However, mmLDL induced early mRNA and protein expression of the cytokines MIP-2, MCP-1, tumor necrosis factor-alpha, and interleukin-6. Chemokine MIP-2 but not MCP-1 secretion depended on TLR4/MyD88, ERK1/2, and PI3K signaling. In turn, TLR4 regulated phosphorylation of ERK1/2 but not of Akt, suggesting that mmLDL-induced PI3K activation is TLR4 independent.

CONCLUSIONS

In macrophages, mmLDL activates TLR4-dependent and -independent signaling pathways, resulting in secretion of proinflammatory cytokines. These results provide new insights into the inflammatory origins of atherosclerosis.

What causes acute coronary syndromes? Applying Koch's postulates

The term "acute coronary syndromes" (ACS) is used to describe a heterogeneous spectrum of clinical conditions. This includes myocardial infarction, non-ST-elevation myocardial infarction, and unstable angina. These conditions are linked by a similar constellation of signs and symptoms but not necessarily by a common pathophysiology. They are syndromes. Several different hypotheses exist that have attempted to explain the pathological mechanisms that are involved in these conditions, however, it is not clear whether ACS are caused by variations of a single disease process or by several disease processes. The contribution of both vessel wall- and blood-related factors in the pathogenesis of acute coronary syndromes is herein discussed with the guidance of Koch's postulates.

Effects of silencing leukocyte-type 12/15-lipoxygenase using short interfering RNAs

The leukocyte-type 12/15-lipoxygenase (12/15-LO) has been implicated in the pathogenesis of atherosclerosis, hypertension, and diabetes. 12/15-LO and its products are associated with LDL oxidation, cellular growth, migration, adhesion, and inflammatory gene expression in monocytes/macrophages, endothelial cells, and vascular smooth muscle cells (VSMCs). Our objective, therefore, was to develop novel expression vectors for short interfering RNAs (siRNAs) targeting 12/15-LO to evaluate its functional relevance in macrophages and VSMCs. We used a PCR-based approach to rapidly identify effective siRNA target sites on mouse 12/15-LO and initially tested their efficacy on a fusion construct of 12/15-LO cDNA and enhanced green fluorescent protein. We then cloned these U6 promoter+siRNA PCR products into plasmid vectors [short hairpin siRNAs (shRNAs)] to knockdown endogenous 12/15-LO expression in mouse macrophages and also rat and mouse VSMCs. Furthermore, the functional effects of shRNA-mediated 12/15-LO knockdown were noted by the reduced oxidant stress and chemokine [monocyte chemoattractant protein-1 (MCP-1)] expression in a differentiated mouse monocytic cell line as well as by the reduced cellular adhesion and fibronectin expression in VMSCs. Knocking down 12/15-LO expression also reduced the expression of inflammatory genes, MCP-1, vascular cell adhesion molecule-1, and interleukin-6 in VSMCs. Our results illustrate the functional relevance of 12/15-LO activation in macrophages and VSMCs and its relationship to oxidant stress and inflammation.

Immunogenetic Susceptibility of Atherosclerotic Stroke

The understanding of the pathophysiology governing atherosclerosis supports a prominent role for inflammation pathways in plaque initiation and progression that result in stroke and myocardial infarction. Elevated levels of inflammatory markers in the blood, such as C-reactive protein and CD40 ligand/CD40, in concert with increased expression of adhesion molecules, chemokines, cytokines, matrix metalloproteinases (MMP), and inflammatory cells in the plaque, characterize the symptomatic atherothrombotic state. Advances in predictive capabilities of vascular events using a number of these biomarkers are beginning to remodel our clinical practice in the use of medications such as statins and angiotensin receptor blockers for stroke prevention. Although the general inflammatory features of atherosclerosis are becoming widely recognized, factors resulting in individual variability in plaque formation and instability remain poorly defined. Emerging literature points toward several acquired and innate susceptibility factors in the immune pathways that may provide insight into why many plaques rapidly evolve from a “stable” to an “unstable” or symptomatic state. First, exposure of plaque memory T-lymphocytes to infectious or endogenous antigens may result in rapid clonal expansion of T-cell variable β chain subtypes and stimulate macrophages to release MMPs, causing plaque destabilization. The effects of infectious agents can further be influenced by an individual’s major histocompatibility complex class II molecule profiles, which can affect susceptibility to specific organisms. Second, functional polymorphisms of genes that regulate the immune pathway can predispose patients to a more robust inflammatory expression after risk factor exposure. Identification of a susceptibility gene profile and immunologic mediators that promote T-cell activation provides a unique opportunity for early identification of stroke risk and targets for future therapy.

Functional role for toll-like receptors in atherosclerosis and arterial remodeling

PURPOSE OF REVIEW

Activation of inflammatory cascades is causally related to the development of atherosclerotic disease. Toll-like receptors are innate immune receptors that recognize pathogen-associated molecular patterns. In this review the pathways by which toll-like receptors might play a role in the development and progression of atherosclerosis will be discussed according to recent literature.

RECENT FINDINGS

Toll-like receptors are expressed in atherosclerotic tissue. Next to pathogens, endogenous toll-like receptor ligands have been linked with the development of arterial occlusive disease. In mouse models of hyperlipidemia, a potential role for the toll-like receptor pathway has been suggested in hypercholesterolemia-induced atherosclerosis. Recent in-vitro studies revealed a mechanism by which toll-like receptor ligation results in a strong inhibition of cholesterol efflux from macrophages. In addition, oxidized lipoproteins interact with toll-like receptors. Furthermore, activation of the apoptotic cascade, which is important during atherogenesis, enhances the toll-like receptor pathway resulting in upregulation of proinflammatory cytokines. Human epidemiologic studies have linked TLR4 polymorphism with atherosclerosis. However, data on the association between atherosclerosis progression and TLR4 polymorphisms are conflicting. Next to plaque growth, arterial remodeling is an important determinant of luminal narrowing in atherosclerosis. Recently, a possible role for TLR4 signaling in arterial remodeling has been revealed in mouse models.

SUMMARY

A clarification of the molecule [corrected] mechanisms by which the toll-like receptor signaling cascade influences atherosclerosis might [corrected] lead to novel strategies to intervene in the development of this life-threatening disease.

The oxidation hypothesis of atherogenesis: the role of oxidized phospholipids and HDL

For more than two decades, there has been continuing evidence of lipid oxidation playing a central role in atherogenesis. The oxidation hypothesis of atherogenesis has evolved to focus on specific proinflammatory oxidized phospholipids that result from the oxidation of LDL phospholipids containing arachidonic acid and that are recognized by the innate immune system in animals and humans. These oxidized phospholipids are largely generated by potent oxidants produced by the lipoxygenase and myeloperoxidase pathways. The failure of antioxidant vitamins to influence clinical outcomes may have many explanations, including the inability of vitamin E to prevent the formation of these oxidized phospholipids and other lipid oxidation products of the myeloperoxidase pathway. Preliminary data suggest that the oxidation hypothesis of atherogenesis and the reverse cholesterol transport hypothesis of atherogenesis may have a common biological basis. The levels of specific oxidized lipids in plasma and lipoproteins, the levels of antibodies to these lipids, and the inflammatory/anti-inflammatory properties of HDL may be useful markers of susceptibility to atherogenesis. Apolipoprotein A-I (apoA-I) and apoA-I mimetic peptides may both promote a reduction in oxidized lipids and enhance reverse cholesterol transport and therefore may have therapeutic potential.