CD36, a scavenger receptor implicated in atherosclerosis

Viral Inhibition of Bacterial Phagocytosis by Human Macrophages: Redundant Role of CD36

Macrophages are essential to maintaining lung homoeostasis and recent work has demonstrated that influenza-infected lung macrophages downregulate their expression of the scavenger receptor CD36. This receptor has also been shown to be involved in phagocytosis of Streptococcus pneumoniae, a primary agent associated with pneumonia secondary to viral infection. The aim of this study was to investigate the role of CD36 in the effects of viral infection on macrophage phagocytic function. Human monocyte-derived macrophages (MDM) were exposed to H3N2 X31 influenza virus, M37 respiratory syncytial virus (RSV) or UV-irradiated virus. No infection of MDM was seen upon exposure to UV-irradiated virus but incubation with live X31 or M37 resulted in significant levels of viral detection by flow cytometry or RT-PCR respectively. Infection resulted in significantly diminished uptake of S. pneumoniae by MDM and significantly decreased expression of CD36 at both the cell surface and mRNA level. Concurrently, there was a significant increase in IFNβ gene expression in response to infection and we observed a significant decrease in bacterial phagocytosis (p = 0.031) and CD36 gene expression (p = 0.031) by MDM cultured for 24 h in 50IU/ml IFNβ. Knockdown of CD36 by siRNA resulted in decreased phagocytosis, but this was mimicked by transfection reagent alone. When MDM were incubated with CD36 blocking antibodies no effect on phagocytic ability was observed. These data indicate that autologous IFNβ production by virally-infected cells can inhibit bacterial phagocytosis, but that decreased CD36 expression by these cells does not play a major role in this functional deficiency.

Lipid partitioning during cardiac stress

It is well documented that fatty acids serve as the primary fuel substrate for the contracting myocardium. However, extensive research has identified significant changes in the myocardial oxidation of fatty acids during acute or chronic cardiac stress. As a result, the redistribution or partitioning of fatty acids due to metabolic derangements could have biological implications. Fatty acids can be stored as triacylglycerols, serve as critical components for biosynthesis of phospholipid membranes, and form the potent signaling molecules, diacylglycerol and ceramides. Therefore, the contribution of lipid metabolism to health and disease is more intricate than a balance of uptake and oxidation. In this review, the available data regarding alterations that occur in endogenous cardiac lipid pathways during the pathological stressors of ischemia-reperfusion and pathological hypertrophy/heart failure are highlighted. In addition, changes in endogenous lipids observed in exercise training models are presented for comparison. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

CD36 deficiency impairs the small intestinal barrier and induces subclinical inflammation in mice

BACKGROUND & AIMS

CD36 has immuno-metabolic actions and is abundant in the small intestine on epithelial, endothelial and immune cells. We examined the role of CD36 in gut homeostasis using mice null for CD36 (CD36KO) and with CD36 deletion specific to enterocytes (Ent-CD36KO) or endothelial cells (EC-CD36KO).

METHODS

Intestinal morphology was evaluated using immunohistochemistry and electron microscopy (EM). Intestinal inflammation was determined from neutrophil infiltration and expression of cytokines, toll-like receptors and COX-2. Barrier integrity was assessed from circulating lipopolysaccharide (LPS) and dextran administered intragastrically. Epithelial permeability to luminal dextran was visualized using two photon microscopy.

RESULTS

The small intestines of CD36KO mice fed a chow diet showed several abnormalities including extracellular matrix (ECM) accumulation with increased expression of ECM proteins, evidence of neutrophil infiltration, inflammation and compromised barrier function. EM showed shortened desmosomes with decreased desmocollin 2 expression. Systemically, leukocytosis and neutrophilia were present together with 80% reduction of anti-inflammatory Ly6C^low monocytes. Bone marrow transplants supported the primary contribution of non-hematopoietic cells to the inflammatory phenotype. Specific deletion of endothelial but not of enterocyte CD36 reproduced many of the gut phenotypes of germline CD36KO mice including fibronectin deposition, increased interleukin 6, neutrophil infiltration, desmosome shortening and impaired epithelial barrier function.

CONCLUSIONS

CD36 loss results in chronic neutrophil infiltration of the gut, impairs barrier integrity and systemically causes subclinical inflammation. Endothelial cell CD36 deletion reproduces the major intestinal phenotypes. The findings suggest an important role of the endothelium in etiology of gut inflammation and loss of epithelial barrier integrity.

Characterisation and comparison of adipose tissue macrophages from human subcutaneous, visceral and perivascular adipose tissue

Background and aims

Macrophages play important roles in adipose tissue inflammation and its consequences. Unfortunately, a detailed description of the macrophage phenotypes in different human adipose tissues is not available.

Subjects and methods

Subcutaneous, visceral and perivascular adipose tissues were obtained from 52 living kidney donors during live donor nephrectomy. Stromal vascular fractions were isolated, and the macrophage phenotypes were analyzed by flow cytometry using surface markers (CD14, CD16, CD36, and CD163).

Results

In addition to CD16 positivity, pro-inflammatory macrophages also display high scavenger receptor CD36 expression. The great majority of CD16 negative macrophages express the anti-inflammatory CD163 marker. The presence of pro-inflammatory macrophages was almost twice as high in visceral (p < 0.0001) and perivascular (p < 0.0001) adipose tissues than in subcutaneous tissue. This difference was substantially more pronounced in the postmenopausal women subgroup, consequentlly, the total difference was driven by this subgroup.

Conclusion

We obtained detailed information about M1 and M2 macrophage phenotypes in human adipose tissue. The visceral and perivascular adipose tissues had substantially higher pro-inflammatory characteristics than the subcutaneous tissue. The higher proportion of pro-inflammatory macrophages in the visceral adipose tissue of postmenopausal women might be related to an increased cardiovascular risk.

A CD36 ectodomain mediates insect pheromone detection via a putative tunnelling mechanism

CD36 transmembrane proteins have diverse roles in lipid uptake, cell adhesion and pathogen sensing. Despite numerous in vitro studies, how they act in native cellular contexts is poorly understood. A Drosophila CD36 homologue, sensory neuron membrane protein 1 (SNMP1), was previously shown to facilitate detection of lipid-derived pheromones by their cognate receptors in olfactory cilia. Here we investigate how SNMP1 functions in vivo. Structure-activity dissection demonstrates that SNMP1's ectodomain is essential, but intracellular and transmembrane domains dispensable, for cilia localization and pheromone-evoked responses. SNMP1 can be substituted by mammalian CD36, whose ectodomain can interact with insect pheromones. Homology modelling, using the mammalian LIMP-2 structure as template, reveals a putative tunnel in the SNMP1 ectodomain that is sufficiently large to accommodate pheromone molecules. Amino-acid substitutions predicted to block this tunnel diminish pheromone sensitivity. We propose a model in which SNMP1 funnels hydrophobic pheromones from the extracellular fluid to integral membrane receptors.

ApoE deficiency exacerbates the development and sustainment of a semi-chronic K/BxN serum transfer-induced arthritis model

Background

The risk for developing cardiovascular disease is greater in patients with rheumatoid arthritis (RA) than in the general population. While patients with RA also have dyslipidemia, the impact of dyslipidemia on the severity of inflammatory arthritis and associated cardiovascular disease is unclear. Currently, there are conflicting results regarding arthritis incidence in apolipoprotein E (ApoE) deficient mice, which spontaneously exhibit both hyperlipidemia and atherosclerosis. Here, we utilize a distinct approach to investigate the contribution of a hyperlipidemic environment on the development of arthritis and atherosclerosis in mice lacking ApoE.

Methods

K/BxN serum transfer-induced arthritis (STIA) was assessed in C57BL/6 (control) and ApoE^−/− mice using clinical indices and immunohistochemical staining. Ankle synoviums were processed for flow cytometry. Aortic atherosclerosis was quantitated using Sudan IV staining. Serum cholesterol and cytokine levels were determined via enzymatic and luminex bead-based assays, respectively.

Results

ApoE^−/− mice developed a sustained and enhanced semi-chronic inflammatory arthritis as compared to control mice. ApoE^−/− mice had increased numbers of foamy macrophages, enhanced joint inflammation and amplified collagen deposition versus controls. The presence of arthritis did not exacerbate serum cholesterol levels or significantly augment the level of atherosclerosis in ApoE^−/− mice. However, arthritic ApoE^−/− mice exhibited a marked elevation of IL-6 as compared to non-arthritic ApoE^−/− mice and arthritic C57BL/6 mice.

Conclusions

Loss of ApoE potentiates a semi-chronic inflammatory arthritis. This heightened inflammatory response was associated with an increase in circulating IL-6 and in the number of foamy macrophages within the joint. Moreover, the foamy macrophages within the arthritic joint are reminiscent of those within unstable atherosclerotic lesions and suggest a pathologic role for foamy macrophages in propagating arthritis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0912-y) contains supplementary material, which is available to authorized users.

Inhibition of ABCA1 Protein Expression and Cholesterol Efflux by TNF α in MLO-Y4 Osteocytes

Hip fracture and myocardial infarction cause significant morbidity and mortality. In vivo studies raising serum cholesterol levels as well as pro-inflammatory cytokines such as TNF α manifest bone loss and atherosclerotic vascular disease, suggesting that abnormalities of cholesterol transport may contribute to osteoporosis. We used the mouse osteocyte cell line (MLO-Y4) to investigate the effects of TNF α on the expression of cholesterol acceptor proteins such as apolipoprotein A-I (apo A-I) and apolipoprotein E (apo E), as well as on the cholesterol transporters ATP-binding cassette-1 (ABCA1), scavenger receptor class B type 1 (SRB1), and cluster of differentiation 36 (CD36). MLO-Y4 cells do not express apo A-I or apo E; however, they do express all three cholesterol transporters (ABCA1, SRB1, and CD36). Treatment of MLO-Y4 cells with TNF α had no effect on SRB1, CD36, and osteocalcin levels; however, TNF α reduced ABCA1 protein levels in a dose-dependent manner and cholesterol efflux to apo A-I. Interestingly, TNF α treatment increased ABCA1 promoter activity and ABCA1 mRNA levels, and increased liver X receptor α protein expression, but had no effect on retinoid X receptor α and retinoic acid receptor α levels. Pharmacological inhibition of p38 mitogen-activated protein (MAP) kinase, but not c-jun-N-terminal kinase 1 or mitogen-activated protein kinase (MEK), restored ABCA1 protein levels in TNF α-treated cells. These results suggest that pro-inflammatory cytokines regulate cholesterol metabolism in osteocytes in part by suppressing ABCA1 levels post-translationally in a p38 MAP kinase-dependent manner.

Detection and treatment of atherosclerosis using nanoparticles

Atherosclerosis is the key pathogenesis of cardiovascular disease, which is a silent killer and a leading cause of death in the United States. Atherosclerosis starts with the adhesion of inflammatory monocytes on the activated endothelial cells in response to inflammatory stimuli. These monocytes can further migrate into the intimal layer of the blood vessel where they differentiate into macrophages, which take up oxidized low-density lipoproteins and release inflammatory factors to amplify the local inflammatory response. After accumulation of cholesterol, the lipid-laden macrophages are transformed into foam cells, the hallmark of the early stage of atherosclerosis. Foam cells can die from apoptosis or necrosis, and the intracellular lipid is deposed in the artery wall forming lesions. The angiogenesis for nurturing cells is enhanced during lesion development. Proteases released from macrophages, foam cells, and other cells degrade the fibrous cap of the lesion, resulting in rupture of the lesion and subsequent thrombus formation. Thrombi can block blood circulation, which represents a major cause of acute heart events and stroke. There are generally no symptoms in the early stages of atherosclerosis. Current detection techniques cannot easily, safely, and effectively detect the lesions in the early stages, nor can they characterize the lesion features such as the vulnerability. While the available therapeutic modalities cannot target specific molecules, cells, and processes in the lesions, nanoparticles appear to have a promising potential in improving atherosclerosis detection and treatment via targeting the intimal macrophages, foam cells, endothelial cells, angiogenesis, proteolysis, apoptosis, and thrombosis. Indeed, many nanoparticles have been developed in improving blood lipid profile and decreasing inflammatory response for enhancing therapeutic efficacy of drugs and decreasing their side effects. WIREs Nanomed Nanobiotechnol 2017, 9:e1412. doi: 10.1002/wnan.1412 For further resources related to this article, please visit the WIREs website.

The macrophage and its related cholesterol efflux as a HDL function index in atherosclerosis

The macrophage and its related cholesterol efflux are considered to be a key player in atherosclerotic formation in relation to the function of high-density lipoprotein (HDL). The HDL function can be evaluated by the reaction between lipid-loaded macrophages and lipid-acceptors in the HDL fraction from the plasma, apolipoprotein B-depleted serum, and/or whole serum/plasma. Recent studies have reported that an impaired cholesterol efflux of HDL is observed in patients with cardiometabolic diseases, such as dyslipidemia, diabetes mellitus, and chronic kidney disease. A population-based cohort study has reported an inverse association between the cholesterol efflux capacity of HDL and the incidence of atherosclerotic disease, regardless of the serum HDL-cholesterol level. Moreover, in this paper, when we summarized several clinical interventional studies of statin treatment that examined cholesterol efflux, a potential increase in the efflux in patients treated with statins was implied. However, the effect was not fully defined in the current situation because of the small sample sizes, lack of a unified protocol for measuring the efflux, and short-term intervention periods without cardiovascular outcomes in available studies. Further investigation is necessary to determine the effect of drugs on cholesterol efflux. With additional advanced studies, cholesterol efflux is a promising laboratory index to understand the HDL function.

Preventive Role of Tetraspanin CD9 in Systemic Inflammation of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is frequently associated with extrapulmonary complications, including cardiovascular disease, diabetes, and osteoporosis. Persistent, low-grade, systemic inflammation underlies these comorbid disorders. Tetraspanins, which have a characteristic structure spanning the membrane four times, facilitate lateral organization of molecular complexes and thereby form tetraspanin-enriched microdomains that are distinct from lipid rafts. Recent basic research has suggested a preventive role of tetraspanin CD9 in COPD. CD9-enriched microdomains negatively regulate LPS-induced receptor formation by preventing CD14 from accumulating into the rafts, and decreased CD9 in macrophages enhances inflammation in mice. Mice doubly deficient in CD9 and a related tetraspanin, CD81, show pulmonary emphysema, weight loss, and osteopenia, a phenotype akin to human COPD. A therapeutic approach to up-regulating CD9 in macrophages might improve the clinical course of patients with COPD with comorbidities.

Zebrafish scarb2a insertional mutant reveals a novel function for the Scarb2/Limp2 receptor in notochord development

BACKGROUND

Scarb2 or Limp2 belong to a subfamily of Scavenger receptors described as lysosomal transmembrane glycosylated receptors, that are mutated in the human syndrome AMRF (action myoclonus-renal failure). The zebrafish insertional mutant scarb2a(hi1463Tg) has notochord defects, the notochord is a defining feature of chordates running along the center of the longitudinal axis and it is essential for forming the spinal column in all vertebrates.

RESULTS

There are three paralogous scarb2 genes in zebrafish; scarb2a, scarb2b, and scarb2c. Both Scarb2a and Scarb2b proteins lack the classical di-leucine motif. We found that scarb2a(hi1463Tg) homozygous zebrafish embryos have a null mutation impairing vacuole formation in the notochord and simultaneously disrupting proper formation of the basement membrane resulting in its thickening at the ventral side of the notochord, which may be the cause for the anomalous upward bending observed in the trunk. Through whole-mount in situ hybridization, we detected scarb2a mRNA expression in the notochord and in the brain early in development. However, it is puzzling that scarb2a notochord mRNA expression is short-lived in the presumptive notochord and precedes the complete differentiation of the notochord.

CONCLUSIONS

This work describes a novel function for the Scarb2 receptor as an essential glycoprotein for notochord development.

CD36 actions in the heart: Lipids, calcium, inflammation, repair and more?

CD36 is a multifunctional immuno-metabolic receptor with many ligands. One of its physiological functions in the heart is the high-affinity uptake of long-chain fatty acids (FAs) from albumin and triglyceride rich lipoproteins. CD36 deletion markedly reduces myocardial FA uptake in rodents and humans. The protein is expressed on endothelial cells and cardiomyocytes and at both sites is likely to contribute to FA uptake by the myocardium. CD36 also transduces intracellular signaling events that influence how the FA is utilized and mediate metabolic effects of FA in the heart. CD36 transduced signaling regulates AMPK activation in a way that adjusts oxidation to FA uptake. It also impacts remodeling of myocardial phospholipids and eicosanoid production, effects exerted via influencing intracellular calcium (iCa(2+)) and the activation of phospholipases. Under excessive FA supply CD36 contributes to lipid accumulation, inflammation and dysfunction. However, it is also important for myocardial repair after injury via its contribution to immune cell clearance of apoptotic cells. This review describes recent progress regarding the multiple actions of CD36 in the heart and highlights those areas requiring future investigation. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Soluble cluster of differentiation 36 concentrations are not associated with cardiovascular risk factors in middle-aged subjects

Cluster of differentiation 36 (CD36) is involved in the development of atherosclerosis by enhancing macrophage endocytosis of oxidized low-density lipoproteins and foam cell formation. Soluble CD36 (sCD36) was found to be elevated in type 2 diabetic patients and possibly acted as a marker of insulin resistance and atherosclerosis. In young subjects, sCD36 was associated with cardiovascular risk factors including obesity and hypertriglyceridemia. The present study was conducted to further investigate the association between plasma sCD36 and cardiovascular risk factors among middle-aged patients with metabolic syndrome (MetS) and healthy controls. sCD36 concentrations were determined by enzyme-linked immunosorbent assays (ELISA) for 41 patients with MetS and 36 healthy controls. Data for other variables were obtained from patient medical records. sCD36 concentrations were relatively low compared to the majority of other studies and were not significantly different between the MetS group and controls (P=0.17). sCD36 was also not correlated with age, body mass index, glucose, lipid profile, serum electrolytes and blood counts. sCD36 was not significantly different between subjects with obesity, hyperglycemia, dyslipidemia, hypertension or cardiovascular disease, and those without these abnormalities (P>0.05). The inconsistency between results reported in the present study and other studies may be unique to the study population or be a result of the lack of a reliable standardized method for determining absolute sCD36 concentrations. However, further investigations are required to assess CD36 tissue expression in the study population and to assess the accuracy of various commercially available sCD36 ELISA kits. Thus, the availability of a standardized simple sCD36 ELISA that could be performed in any basic laboratory would be more favorable to the specialized flow cytometry methods that detect CD36+ microparticles if it was to be used as a biomarker.

CD36 is a co-receptor for hepatitis C virus E1 protein attachment

The cluster of differentiation 36 (CD36) is a membrane protein related to lipid metabolism. We show that HCV infection in vitro increased CD36 expression in either surface or soluble form. HCV attachment was facilitated through a direct interaction between CD36 and HCV E1 protein, causing enhanced entry and replication. The HCV co-receptor effect of CD36 was independent of that of SR-BI. CD36 monoclonal antibodies neutralized the effect of CD36 and reduced HCV replication. CD36 inhibitor sulfo-N-succinimidyl oleate (SSO), which directly bound CD36 but not SR-BI, significantly interrupted HCV entry, and therefore inhibited HCV replication. SSO’s antiviral effect was seen only in HCV but not in other viruses. SSO in combination with known anti-HCV drugs showed additional inhibition against HCV. SSO was considerably safe in mice. Conclusively, CD36 interacts with HCV E1 and might be a co-receptor specific for HCV entry; thus, CD36 could be a potential drug target against HCV.

Novel flow cytometric approach for the detection of adipocyte subpopulations during adipogenesis

The ability of mesenchymal stromal cells (MSCs) to differentiate into adipocytes provides a cellular model of human origin to study adipogenesis in vitro. One of the major challenges in studying adipogenesis is the lack of tools to identify and monitor the differentiation of various subpopulations within the heterogeneous pool of MSCs. Cluster of differentiation (CD)36 plays an important role in the formation of intracellular lipid droplets, a key characteristic of adipocyte differentiation/maturation. The objective of this study was to develop a reproducible quantitative method to study adipocyte differentiation by comparing two lipophilic dyes [Nile Red (NR) and Bodipy 493/503] in combination with CD36 surface marker staining. We identified a subpopulation of adipose-derived stromal cells that express CD36 at intermediate/high levels and show that combining CD36 cell surface staining with neutral lipid-specific staining allows us to monitor differentiation of adipose-derived stromal cells that express CD36^{intermediate/high} during adipocyte differentiation in vitro. The gradual increase of CD36^{intermediate/high/}NR^positive cells during the 21 day adipogenesis induction period correlated with upregulation of adipogenesis-associated gene expression.

The Impact of Lipoprotein-Associated Oxidative Stress on Cell-Specific Microvesicle Release in Patients with Familial Hypercholesterolemia

Objective. Microvesicles (MVs) are small cell-derived particles shed upon activation. Familial hypercholesterolemia (FH) particularly when associated with Achilles tendon xanthomas (ATX) predisposes to atherosclerosis, possibly through oxLDL-C interaction with the CD36 receptor. To investigate the hypothesis that MVs derived from cells involved in atherosclerosis are increased in FH and that CD36 expressing MVs (CD36+ MVs) may be markers of oxLDL-C-induced cell activation, cell-specific MVs were measured in FH patients with and without ATX and their association with atherogenic lipid profile was studied. Approach and Results. Thirty FH patients with and without ATX and twenty-three controls were included. Plasma concentrations of MVs and CD36+ MVs derived from platelets (PMVs), erythrocytes (ErytMVs), monocytes (MMVs), and endothelial cells (EMVs), as well as tissue factor-positive cells (TF+ MVs), were measured by flow cytometry. Total MVs, MMVs, EMVs, ErytMVs, and TF+ MVs were significantly increased in FH patients, compared to controls. CD36+ MVs derived from endothelial cells and monocytes were significantly higher in FH patients and oxLDL-C predicted all the investigated cell-specific CD36+ MVs in FH patients with ATX. Conclusions. MVs derived from cells involved in atherosclerosis were increased in FH and may contribute to elevated atherothrombosis risk. The increased cell-specific CD36+ MVs observed in FH may represent markers of oxLDL-C-induced cell activation.

The cytokine-serum amyloid A-chemokine network

Levels of serum amyloid A (SAA), a major acute phase protein in humans, are increased up to 1000-fold upon infection, trauma, cancer or other inflammatory events. However, the exact role of SAA in host defense is yet not fully understood. Several pro- and anti-inflammatory properties have been ascribed to SAA. Here, the regulated production of SAA by cytokines and glucocorticoids is discussed first. Secondly, the cytokine and chemokine inducing capacity of SAA and its receptor usage are reviewed. Thirdly, the direct (via FPR2) and indirect (via TLR2) chemotactic effects of SAA and its synergy with chemokines are unraveled. Altogether, a complex cytokine-SAA-chemokine network is established, in which SAA plays a key role in regulating the inflammatory response.

Genetic variations of CD36 and low platelet CD36 expression - a risk factor for lipemic plasma donation in Taiwanese apheresis donors

BACKGROUND

New CD36 mutations are constantly being identified, although no study has specifically targeted a Taiwanese population. CD36 deficiency can result in dyslipid state and slow clearance of chylomicron. This could be linked to more frequent lipemic donations.

STUDY DESIGN AND METHODS

We used flow cytometric methods to study the CD36 deficiency in 640 regular volunteer platelet apheresis donors from Taipei blood centre. The coding exons of CD36 gene were sequenced in CD36-deficient individuals, and the allele frequencies of CD36 variants were determined in the larger population by mutation-specific PCR and oligonucleotide hybridization. Visual inspection of lipemic plasma was routinely performed on samples taken before commencement of apheresis. Individuals found to have lipemic plasma are deferred until next donation. We investigated the link between positive lipemic deferral record and low platelet CD36 expression status.

RESULTS

We found four donors (0·6%) with type I CD36 deficiency (both platelets and monocytes CD36(null) ) and six (1·0%) with type II CD36 deficiency (PLT: CD36(null) , monocyte: CD36(low) ). Six CD36 genetic variants were identified, two of them were novel, all but one are found exclusively in CD36(null) and CD36(low) expressors. Subjects with CD36 genetic variants also displayed deficient or reduced CD36 on monocytes. Donors with null or low PLT CD36 expression were more likely to have a lipemic deferral record than control subjects with normal PLT CD36 expression (X(2) = 27·36, odds ratio = 2·6, 95% conference interval: 1·8-3·8, P < 0·0001).

CONCLUSION

Through this study, we established a donor registry to supply CD36-negative platelets for patients in need.

Curcumin retunes cholesterol transport homeostasis and inflammation response in M1 macrophage to prevent atherosclerosis

Lipoprotein cholesterol metabolism dysfunction in the arterial wall is a major contributor to atherosclerosis, and excessive lipid intake and failed cholesterol homeostasis may accelerate the atherogenic process. Curcumin exerts multiple effects by alleviating inflammation, hyperlipidemia, and atherosclerosis; however, its role in cholesterol transport homeostasis and its underlying impact on inflammatory M1 macrophages are poorly understood. This work aimed to investigate the effect of curcumin on cholesterol transport, the inflammatory response and cell apoptosis in M1 macrophages. RAW264.7 macrophages (M0) were induced with LPS plus IFN-γ for 12 h to develop a M1 subtype and were then incubated with curcumin at different concentrations (6.25 and 12.5 μmol/L) in the presence or absence of oxLDL. Then, cholesterol influx/efflux and foam cell formation as well as inflammation and apoptosis were evaluated. It was found that curcumin increased cholesterol uptake measured by the Dil-oxLDL binding assay, and simultaneously increased cholesterol efflux carried out by Apo-A1 and HDL in M1 cells. Curcumin further reinforced ox-LDL-induced cholesterol esterification and foam cell formation as determined by Oil Red O and BODIPY staining. Moreover, curcumin dramatically reduced ox-LDL-induced cytokine production such as IL-1β, IL-6 as well as TNF-α and M1 cell apoptosis. We also found that curcumin upregulated CD36 and ABCA1 in M1 macrophages. Curcumin increased PPARγ expression, which in turn promoted CD36 and ABCA1 expression. In conclusion, curcumin may increase the ability of M1 macrophages to handle harmful lipids, thus promoting lipid processing, disposal and removal, which may support cholesterol homeostasis and exert an anti-atherosclerotic effect.

Scavenger receptors and non-coding RNAs: relevance in atherogenesis

Scavenger receptors (SRs), which recognize modified low-density lipoprotein (LDL) by oxidation or acetylation, are a group of receptors on plasma membrane of macrophages and other cell types. These receptors by facilitating modified LDL uptake are a primary step in the intracellular accumulation of modified LDL and formation of fatty streak. Non-coding RNAs (ncRNAs) are a group of functional RNA nucleotides that are not translated into protein, and include microRNAs (miRs), snoRNAs, siRNAs, snRNAs, exRNAs, piRNAs, and the long ncRNAs (lncRNAs). Recently, ncRNAs have received much attention due to their effects in a variety of disease states such as atherosclerotic cardiovascular disease and cancers. A host of ncRNAs, such as miRs and lncRNAs, have been found to be involved in the regulation of SRs and the inflammatory cascade and subsequently atherosclerosis. Here, we review this important area to create interest in this growing field among researchers and clinicians alike.

Immune-mediated and lipid-mediated platelet function in atherosclerosis

PURPOSE OF REVIEW

Cardiovascular disease (CVD) is the leading cause of death and morbidity worldwide. Detailed knowledge of the mechanisms of atherosclerosis, the main underlying disease of CVD, will enable improved preventive and therapeutic options, thus potentially limiting the burden of vascular disease in aging societies. A large body of evidence illustrates the contribution of platelets to processes beyond their traditionally recognized role as mediators in thrombosis and hemostasis. Recent advances in molecular biology help to understand the complexity of atherosclerosis.

RECENT FINDINGS

This article outlines the role of platelets as modulators of immune responses in the context of atherosclerosis. It provides a short overview of interactions between platelets and endothelial cells or immune cells via direct cell contact or soluble factors during atherogenesis. By means of some well examined, exemplary pathways (e.g. CD40/CD40L dyad), this article will discuss recent discoveries in immune-related function of platelets. We also focus on the relationship between platelets and the lipid metabolism highlighting potential consequences to atherosclerosis and dyslipidemia.

SUMMARY

A better understanding of the molecular mechanisms of platelet-related immune activity allows their utilization as powerful diagnostic tools or targets of therapeutic intervention. Those findings might help to develop new classes of drugs which may supplement or replace classical anticoagulants and help clinicians to tackle CVD more efficiently.

Oxidized LDL induces phosphorylation of non-muscle myosin IIA heavy chain in macrophages

Oxidized LDL (oxLDL) performs critical roles in atherosclerosis by inducing macrophage foam cell formation and promoting inflammation. There have been reports showing that oxLDL modulates macrophage cytoskeletal functions for oxLDL uptake and trapping, however, the precise mechanism has not been clearly elucidated. Our study examined the effect of oxLDL on non-muscle myosin heavy chain IIA (MHC-IIA) in macrophages. We demonstrated that oxLDL induces phosphorylation of MHC-IIA (Ser1917) in peritoneal macrophages from wild-type mice and THP-1, a human monocytic cell line, but not in macrophages deficient for CD36, a scavenger receptor for oxLDL. Protein kinase C (PKC) inhibitor-treated macrophages did not undergo the oxLDL-induced MHC-IIA phosphorylation. Our immunoprecipitation revealed that oxLDL increased physical association between PKC and MHC-IIA, supporting the role of PKC in this process. We conclude that oxLDL via CD36 induces PKC-mediated MHC-IIA (Ser1917) phosphorylation and this may affect oxLDL-induced functions of macrophages involved in atherosclerosis.

Macrophage Activation by Ursolic and Oleanolic Acids during Mycobacterial Infection

Oleanolic (OA) and ursolic acids (UA) are triterpenes that are abundant in vegetables, fruits and medicinal plants. They have been described as active moieties in medicinal plants used for the treatment of tuberculosis. In this study, we analyzed the effects of these triterpenes on macrophages infected in vitro with Mycobacterium tuberculosis (MTB). We evaluated production of nitric oxide (NO), reactive oxygen species (ROS), and cytokines (TNF-α and TGF-β) as well as expression of cell membrane receptors (TGR5 and CD36) in MTB-infected macrophages following treatment with OA and UA. Triterpenes caused reduced MTB growth in macrophages, stimulated production of NO and ROS in the early phase, stimulated TNF-α, suppressed TGF-β and caused over-expression of CD36 and TGR5 receptors. Thus, our data suggest immunomodulatory properties of OA and UA on MTB infected macrophages. In conclusion, antimycobacterial effects induced by these triterpenes may be attributable to the conversion of macrophages from stage M2 (alternatively activated) to M1 (classically activated).

CD36 Is Essential for Regulation of the Host Innate Response to Staphylococcus aureus α-Toxin-Mediated Dermonecrosis

Staphylococcus aureus is the primary cause of skin and skin structure infections (SSSIs) in the United States. α-Hemolysin (Hla), a pore-forming toxin secreted by S. aureus and a major contributor to tissue necrosis, prompts recruitment of neutrophils critical for host defense against S. aureus infections. However, the failure to clear apoptotic neutrophils can result in damage to host tissues, suggesting that mechanisms of neutrophil clearance are essential to limiting Hla-mediated dermonecrosis. We hypothesized that CD36, a scavenger receptor which facilitates recognition of apoptosing cells, would play a significant role in regulating Hla-mediated inflammation and tissue injury during S. aureus SSSI. In this study, we show that CD36 on macrophages negatively regulates dermonecrosis caused by Hla-producing S. aureus. This regulation is independent of bacterial burden, as CD36 also limits dermonecrosis caused by intoxication with sterile bacterial supernatant or purified Hla. Dermonecrotic lesions of supernatant intoxicated CD36(-/-) mice are significantly larger, with increased neutrophil accumulation and IL-1β expression, compared with CD36(+/+) (wild-type) mice. Neutrophil depletion of CD36(-/-) mice prevents this phenotype, demonstrating the contribution of neutrophils to tissue injury in this model. Furthermore, administration of CD36(+/+) but not CD36(-/-) macrophages near the site of intoxication reduces dermonecrosis, IL-1β production and neutrophil accumulation to levels seen in wild-type mice. This therapeutic effect is reversed by inhibiting actin polymerization in the CD36(+/+) macrophages, supporting a mechanism of action whereby CD36-dependent macrophage phagocytosis of apoptotic neutrophils regulates Hla-mediated dermonecrosis. Taken together, these data demonstrate that CD36 is essential for controlling the host innate response to S. aureus skin infection.

Ethanol extract of propolis protects macrophages from oxidized low density lipoprotein-induced apoptosis by inhibiting CD36 expression and endoplasmic reticulum stress-C/EBP homologous protein pathway

Background

Ethanol extract of propolis (EEP), rich in flavones, has been known for various biological activities including antioxidant, antiinflammatory and antibiotic activities. Our previous studies have shown that EEP protects endothelial cells from oxidized low-density lipoprotein (ox-LDL)-induced apoptosis and inhibits atherosclerotic lesion development. In this present study, we explored the protective effect of EEP on ox-LDL-induced cytotoxicity in macrophages and specifically the endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP) pathway-mediated apoptosis.

Methods

EEP was prepared and the total flavonoids content of EEP was determined by the colorimetric method of Chinese Standard (GB/T 20574-2006). The effects of EEP on lipid accumulation, cytotoxicity and apoptosis in RAW264.7 cells induced by ox-LDL or tunicamycin (TM, an ER stress inducer) were assayed using oil red O staining, MTT assay, flow cytometric analysis and so on. Immunofluorescence, Western blot and real time-PCR analysis were then used to further investigate the molecular mechanisms by which EEP protects macrophages from ox-LDL-induced apoptosis. 4-phenylbutyric acid (PBA), an ER stress inhibitor, was used as a positive control.

Results

EEP (7.5, 15 and 30 mg/L) not only attenuated ox-LDL-induced lipid accumulation in RAW264.7 macrophages in a dose-dependent manner but also inhibited the decreased cell viability and the increased lactate dehydrogenase (LDH) leakage, caspase-3 activation and apoptosis induced by ox-LDL or tunicamycin (TM, a classical ER stress inducer), which were similar to 4-phenylbutyric acid (PBA, an inhibitor of ER stress) treatment. In addition, like PBA, EEP significantly suppressed the ox-LDL- or TM-induced activation of ER stress signaling pathway including the phosphorylation of double-stranded RNA-activated protein kinase-like ER kinase (PERK) and eukaryotic translation initiation factor 2α (eIF2α) as well as upregulation of glucose regulated protein 78 (GRP78) and the pro-apoptotic protein CHOP. Furthermore, EEP significantly suppressed ox-LDL intake by macrophages and the upregulation of CD36 induced by ox-LDL.

Conclusion

These data indicate that EEP may protect macrophages from ox-LDL-induced apoptosis and the mechanism at least partially involves its ability to suppress the CD36-mediated ox-LDL intake and subsequent activation of ER stress-CHOP signalling pathway.

The Role of CD36 in the Effect of Arginine in Atherosclerotic Rats

Background

The aim of this study was to investigate the effects of arginine in the development of atherosclerosis in rats fed a high-fat diet supplemented with arginine and to evaluate the role of CD36 in this process.

Material/Methods

A total of 40 Sprague-Dawley rats were randomly assigned to 4 groups: control group, fat diet group, simvastatin group, and arginine group. They were fed for 12 weeks and were then sacrificed. Immunohistochemical CD36 expression and pathology was investigated in the aorta; CD36 expression in mononuclear cells was detected by Western blot and RT-PCR.

Results

The thickness of the aortal intima, media, and I/M significantly decreased in the arginine group rats compared with those in the fat diet group (P<0.05). CD36 expression was up-regulated in rats in the fat diet group compared with the control group and was down-regulated in rats in the arginine group compared with rats in the fat diet group.

Conclusions

The addition of arginine has a significant effect on reducing rat atherosclerosis development, which may be attributed to both the down-regulation of CD36 expression in rat aortic endothelial and blood mononuclear cells and the NO pathway.

Regulation of EMMPRIN (CD147) on monocyte subsets in patients with symptomatic coronary artery disease

INTRODUCTION

The role of individual monocyte subsets in inflammatory cardiovascular diseases is insufficiently understood. Although the Extracellular Matrix Metalloproteinase Inducer (EMMPRIN) regulates important processes for inflammation such as MMP-release, its expression and regulation on monocyte subsets has not been characterized.

MATERIALS AND METHODS

In this clinical study, blood was obtained from 80 patients with stable coronary artery disease (CAD), 49 with acute myocardial infarction (AMI) and 34 healthy controls. Monocytes were divided into 3 subsets: CD14(++)CD16(-) (low), CD14(++)CD16(+) (intermediate), CD14(+)CD16(++) (high) according to phenotypic markers analyzed by flow cytometry. Surface expression of EMMPRIN was evaluated and compared with CD36 and CD47 expression.

RESULTS

In all patients, EMMPRIN expression was significantly different among monocyte subsets with the highest expression on "classical" CD14(++)CD16(-) monocytes. EMMPRIN was upregulated on all monocyte subsets in patients with AMI as compared to patients with stable CAD. Notably, neither CD47 nor CD36 revealed a significant difference in patients with AMI compared to patients with stable CAD.

CONCLUSION

EMMPRIN could serve as a marker for classical monocytes, which is upregulated in patients with acute myocardial infarction.

Proliferation of macrophages due to the inhibition of inducible nitric oxide synthesis by oxidized low-density lipoproteins

Oxidized low-density lipoprotein (ox-LDL) is assumed to be a major causal agent in hypercholesteraemia-induced atherosclerosis. Because the proliferation of lipid-loaden macrophages within atherosclerotic lesions has been described, we investigated the dependence of macrophage proliferation on the inhibition of inducible nitric oxide synthase (iNOS) by hypochlorite oxidized LDL. Ox-LDL induces a dose dependent inhibition of inducible nitric oxide synthesis in lipopolysaccharide-interferon stimulated mouse macrophages (J774.A1) with concomitant macrophage proliferation as assayed by cell counting, tritiated-thymidine incorporation and measurement of cell protein. Native LDL did not influence macrophage proliferation and inducible nitric oxide synthesis. iNOS protein and mRNA was reduced by HOCl-oxidized LDL (0-40 µg/ml) as revealed by immunoblotting and competitive semiquantitative PCR. Macrophage proliferation was increased by the addition of the iNOS inhibitor L-NAME. The addition of ox-LDL to L-NAME containing incubations induced no further statistically significant increase in cell number. Nitric oxide donors decreased ox-LDL induced macrophage proliferation and nitric oxide scavengers restored macrophage proliferation to the initial values achieved by ox-LDL. The decrease of cytosolic DNA fragments in stimulated macrophages incubated with ox-LDL demonstrates that the proliferative actions of ox-LDL are associated with a decrease of NO-induced apoptosis. Our data show that inhibition of iNOS dependent nitric oxide production caused by hypochlorite oxidized LDL enhances macrophage proliferation. This might be a key event in the pathogenesis of atherosclerotic lesions.

Diverse CD36 expression among Japanese population: defective CD36 mutations cause platelet and monocyte CD36 reductions in not only deficient but also normal phenotype subjects

INTRODUCTION

CD36 is a multifunctional glycoprotein expressed on various human cells, including platelets and monocytes. Five CD36 gene mutations (C268T, 949insA, 329-339del, 1228-1239del and 629-631del/insAAAAC) are mainly responsible for CD36-deficient phenotypes in Japan. It has also been reported that platelet CD36 expression varies widely among normal phenotype individuals. Here, in order to obtain further insight into CD36 expression, we investigated the association between platelet and monocyte CD36 expression levels and defective mutations in the Japanese population.

MATERIALS AND METHODS

Blood samples were collected from 135 healthy Japanese volunteers. CD36 expression levels on platelets and monocytes were quantitatively analyzed by flow cytometry. Real-time PCR, PCR-RFLP and allele-specific PCR were performed to detect mutant genotypes.

RESULTS

In this population, we found 2 (1.5%) and 9 (6.7%) CD36-deficient subjects as type I and type II, respectively. Among normal phenotype subjects, CD36 expression levels ranged from 1,259 to 11,002 (4,487±2,017) molecules/platelet and from 211 to 5,150 (1,628±986) molecules/monocyte. Genotyping assay showed that heterozygotes with the defective mutations were present in normal (12.9%) and type II-deficient (66.7%) subjects, and that these heterozygous mutations led to decreases in CD36 surface expression on platelets and monocytes.

CONCLUSIONS

Heterozygous CD36 mutations, previously known to lead to deficiency in this molecule, are one of the factors responsible for the diversity of CD36 surface expression levels on platelets and monocytes in normal phenotype subjects.

Trypanosoma cruzi infection is a potent risk factor for non-alcoholic steatohepatitis enhancing local and systemic inflammation associated with strong oxidative stress and metabolic disorders

BACKGROUND

The immune mechanisms underlying experimental non-alcoholic steatohepatitis (NASH), and more interestingly, the effect of T. cruzi chronic infection on the pathogenesis of this metabolic disorder are not completely understood.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated immunological parameters in male C57BL/6 wild type and TLR4 deficient mice fed with a standard, low fat diet, LFD (3% fat) as control group, or a medium fat diet, MFD (14% fat) in order to induce NASH, or mice infected intraperitoneally with 100 blood-derived trypomastigotes of Tulahuen strain and also fed with LFD (I+LFD) or MFD (I+MFD) for 24 weeks. We demonstrated that MFD by itself was able to induce NASH in WT mice and that parasitic infection induced marked metabolic changes with reduction of body weight and steatosis revealed by histological studies. The I+MFD group also improved insulin resistance, demonstrated by homeostasis model assessment of insulin resistance (HOMA-IR) analysis; although parasitic infection increased the triglycerides and cholesterol plasma levels. In addition, hepatic M1 inflammatory macrophages and cytotoxic T cells showed intracellular inflammatory cytokines which were associated with high levels of IL6, IFNγ and IL17 plasmatic cytokines and CCL2 chemokine. These findings correlated with an increase in hepatic parasite load in I+MFD group demonstrated by qPCR assays. The recruitment of hepatic B lymphocytes, NK and dendritic cells was enhanced by MFD, and it was intensified by parasitic infection. These results were TLR4 signaling dependent. Flow cytometry and confocal microscopy analysis demonstrated that the reactive oxygen species and peroxinitrites produced by liver inflammatory leukocytes of MFD group were also exacerbated by parasitic infection in our NASH model.

CONCLUSIONS

We highlight that a medium fat diet by itself is able to induce steatohepatitis. Our results also suggest a synergic effect between damage associated with molecular patterns generated during NASH and parasitic infection, revealing an intense cross-talk between metabolically active tissues, such as the liver, and the immune system. Thus, T. cruzi infection must be considered as an additional risk factor since exacerbates the inflammation and accelerates the development of hepatic injury.

Glycated Serum Albumin and AGE Receptors

In vivo modification of proteins by molecules with reactive carbonyl groups leads to intermediate and advanced glycation end products (AGE). Glucose is a significant glycation reagent due to its high physiological concentration and poorly controlled diabetics show increased albumin glycation. Increased levels of glycated and AGE-modified albumin have been linked to diabetic complications, neurodegeneration, and vascular disease. This review discusses glycated albumin formation, structural consequences of albumin glycation on drug binding, removal of circulating AGE by several scavenger receptors, as well as AGE-induced proinflammatory signaling through activation of the receptor for AGE. Analytical methods for quantitative detection of protein glycation and AGE formation are compared. Finally, the use of glycated albumin as a novel clinical marker to monitor glycemic control is discussed and compared to glycated hemoglobin (HbA1c) as long-term indicator of glycemic status.

Macrophage Heterogeneity and Plasticity: Impact of Macrophage Biomarkers on Atherosclerosis

Cardiovascular disease (CVD) is a global epidemic, currently representing the worldwide leading cause of morbidity and mortality. Atherosclerosis is the fundamental pathophysiologic component of CVD, where the immune system plays an essential role. Monocytes and macrophages are key mediators in this aspect: due to their heterogeneity and plasticity, these cells may act as either pro- or anti-inflammatory mediators. Indeed, monocytes may develop heterogeneous functional phenotypes depending on the predominating pro- or anti-inflammatory microenvironment within the lesion, resulting in classic, intermediate, and non-classic monocytes, each with strikingly differing features. Similarly, macrophages may also adopt heterogeneous profiles being mainly M1 and M2, the former showing a proinflammatory profile while the latter demonstrates anti-inflammatory traits; they are further subdivided in several subtypes with more specialized functions. Furthermore, macrophages may display plasticity by dynamically shifting between phenotypes in response to specific signals. Each of these distinct cell profiles is associated with diverse biomarkers which may be exploited for therapeutic intervention, including IL-10, IL-13, PPAR-γ, LXR, NLRP3 inflammasomes, and microRNAs. Direct modulation of the molecular pathways concerning these potential macrophage-related targets represents a promising field for new therapeutic alternatives in atherosclerosis and CVD.

Histone deacetylases and atherosclerosis

Atherosclerosis is the most common pathological process that leads to cardiovascular diseases, a disease of large- and medium-sized arteries that is characterized by a formation of atherosclerotic plaques consisting of necrotic cores, calcified regions, accumulated modified lipids, smooth muscle cells (SMCs), endothelial cells, leukocytes, and foam cells. Recently, the question about how to suppress the occurrence of atherosclerosis and alleviate the progress of cardiovascular disease becomes the hot topic. Accumulating evidence suggests that histone deacetylases(HDACs) play crucial roles in arteriosclerosis. This review summarizes the effect of HDACs and HDAC inhibitors(HDACi) on the progress of atherosclerosis.

CD36-dependent 7-ketocholesterol accumulation in macrophages mediates progression of atherosclerosis in response to chronic air pollution exposure

RATIONALE

Air pollution exposure has been shown to potentiate plaque progression in humans and animals. Our previous studies have suggested a role for oxidized lipids in mediating adverse vascular effect of air pollution. However, the types of oxidized lipids formed in response to air pollutants and how this occurs and their relevance to atherosclerosis are not fully understood.

OBJECTIVE

To investigate the mechanisms by which particulate matter <2.5 μm (PM2.5) induces progression of atherosclerosis.

METHODS AND RESULTS

Atherosclerosis-prone ApoE(-/-) or LDLR(-/-) mice were exposed to filtered air or concentrated ambient PM2.5 using a versatile aerosol concentrator enrichment system for 6 months. PM2.5 increased 7-ketocholesterol (7-KCh), an oxidatively modified form of cholesterol, in plasma intermediate density lipoprotein/low-density lipoprotein fraction and in aortic plaque concomitant with progression of atherosclerosis and increased CD36 expression in plaque macrophages from PM2.5-exposed mice. Macrophages isolated from PM2.5-exposed mice displayed increased uptake of oxidized lipids without alterations in their efflux capacity. Consistent with these finding, CD36-positive macrophages displayed a heightened capacity for oxidized lipid uptake. Deficiency of CD36 on hematopoietic cells diminished the effect of air pollution on 7-KCh accumulation, foam cell formation, and atherosclerosis.

CONCLUSIONS

Our results suggest a potential role for CD36-mediated abnormal accumulations of oxidized lipids, such as 7-KCh, in air pollution-induced atherosclerosis progression.

Commentary on Myers et al.: growing role of the innate immunity receptor CD36 in central nervous system diseases

Activation of innate immunity by sterile inflammation has emerged as a key event in selected CNS diseases, with a defining impact on all stages of the pathological process. Due to its multiple functions and assembly with other pattern recognition receptors, the innate immunity receptor CD36 has been implicated in a wide variety of brain pathologies, ranging from acute brain injury to neurodegeneration. However, the role of CD36 is complex involving both tissue destruction, related mainly to oxidative stress and inflammation, and beneficial reparative effects due to the involvement of CD36 in tissue repair and reorganization. A recent paper of Meyer at al. provided novel evidence for a role of CD36 also in spinal cord trauma, a condition in which the effect of CD36 was found to be univocally deleterious. This commentary will provide a brief overview of the pathobiology of CD36 and its expanding role in diseases of the brain and spinal cord.