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      Molecular Pathways Regulating Macrophage Polarization: Implications for Atherosclerosis

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          Abstract

          Recent years have seen a tremendous development of our insight into the biology of atherosclerosis and its acute thrombotic manifestations. Inflammation now takes center stage among traditional risk factors as a decisive factor in cardiovascular risk. Consequently, its assessment and modulation have become key to clinical care and fundamental research alike. Plaque macrophages orchestrate many of the inflammatory processes that occur throughout atherogenesis. These cells are characteristically heterogeneous and adopt diverse activation states in response to micro-environmental triggers. In this review, macrophage-mediated inflammation in atherosclerosis sets the scene for a discussion of the gene regulatory mechanisms that facilitate and shape polarized macrophage phenotypes. When applicable, we consider these factors within the context of atherosclerosis and reflect on opportunities for future application.

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          Most cited references59

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          Macrophage-specific PPARgamma controls alternative activation and improves insulin resistance.

          Obesity and insulin resistance, the cardinal features of metabolic syndrome, are closely associated with a state of low-grade inflammation. In adipose tissue chronic overnutrition leads to macrophage infiltration, resulting in local inflammation that potentiates insulin resistance. For instance, transgenic expression of Mcp1 (also known as chemokine ligand 2, Ccl2) in adipose tissue increases macrophage infiltration, inflammation and insulin resistance. Conversely, disruption of Mcp1 or its receptor Ccr2 impairs migration of macrophages into adipose tissue, thereby lowering adipose tissue inflammation and improving insulin sensitivity. These findings together suggest a correlation between macrophage content in adipose tissue and insulin resistance. However, resident macrophages in tissues display tremendous heterogeneity in their activities and functions, primarily reflecting their local metabolic and immune microenvironment. While Mcp1 directs recruitment of pro-inflammatory classically activated macrophages to sites of tissue damage, resident macrophages, such as those present in the adipose tissue of lean mice, display the alternatively activated phenotype. Despite their higher capacity to repair tissue, the precise role of alternatively activated macrophages in obesity-induced insulin resistance remains unknown. Using mice with macrophage-specific deletion of the peroxisome proliferator activated receptor-gamma (PPARgamma), we show here that PPARgamma is required for maturation of alternatively activated macrophages. Disruption of PPARgamma in myeloid cells impairs alternative macrophage activation, and predisposes these animals to development of diet-induced obesity, insulin resistance, and glucose intolerance. Furthermore, gene expression profiling revealed that downregulation of oxidative phosphorylation gene expression in skeletal muscle and liver leads to decreased insulin sensitivity in these tissues. Together, our findings suggest that resident alternatively activated macrophages have a beneficial role in regulating nutrient homeostasis and suggest that macrophage polarization towards the alternative state might be a useful strategy for treating type 2 diabetes.
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            IRF5 promotes inflammatory macrophage polarization and TH1-TH17 responses.

            Polymorphisms in the gene encoding the transcription factor IRF5 that lead to higher mRNA expression are associated with many autoimmune diseases. Here we show that IRF5 expression in macrophages was reversibly induced by inflammatory stimuli and contributed to the plasticity of macrophage polarization. High expression of IRF5 was characteristic of M1 macrophages, in which it directly activated transcription of the genes encoding interleukin 12 subunit p40 (IL-12p40), IL-12p35 and IL-23p19 and repressed the gene encoding IL-10. Consequently, those macrophages set up the environment for a potent T helper type 1 (T(H)1)-T(H)17 response. Global gene expression analysis demonstrated that exogenous IRF5 upregulated or downregulated expression of established phenotypic markers of M1 or M2 macrophages, respectively. Our data suggest a critical role for IRF5 in M1 macrophage polarization and define a previously unknown function for IRF5 as a transcriptional repressor.
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              Cytokines in atherosclerosis: pathogenic and regulatory pathways.

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

                Contributors
                +31-20-5664811 , m.a.hoeksema@amc.uva.nl
                +31-20-5664811 , l.stoger@amc.uva.nl
                +31-20-5666762 , +31-20-6915519 , m.dewinther@amc.uva.nl
                Journal
                Curr Atheroscler Rep
                Curr Atheroscler Rep
                Current Atherosclerosis Reports
                Current Science Inc. (New York )
                1523-3804
                1534-6242
                11 March 2012
                11 March 2012
                June 2012
                : 14
                : 3
                : 254-263
                Affiliations
                Department of Medical Biochemistry, Academic Medical Center, University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands
                Article
                240
                10.1007/s11883-012-0240-5
                3348484
                22407286
                810a0ed8-1a0a-464e-808f-f08575cc33cf
                © The Author(s) 2012
                History
                Categories
                Vascular Biology (RS Rosenson, Section Editor)
                Custom metadata
                © Springer Science+Business Media, LLC 2012

                Immunology
                macrophages,heterogeneity,atherosclerosis,human pathology,cardiovascular disease,gene regulation,transcription factors,microenvironment,cytokines,inflammation,coronary artery disease,polarization,mouse models,innate immunity,foam cells,epigenetics

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