PPARgamma activation primes human monocytes into alternative M2 macrophages with anti-inflammatory properties

Do PPAR-Gamma Agonists Have a Future in Parkinson's Disease Therapy?

Thiazolidinediones (TZDs) are peroxisome proliferator-activated receptor (PPAR)-γ agonists commonly used as insulin-sensitizing drugs for the treatment of type 2 diabetes. In the last decade, PPAR-γ agonists have received increasing attention for their neuroprotective properties displayed in a variety of neurodegenerative diseases, including Parkinson's disease (PD), likely related to the anti-infammatory activity of these compounds. Recent studies indicate that neuroinflammation, specifically reactive microglia, plays important roles in PD pathogenesis. Moreover, after the discovery of infiltrating activated Limphocytes in the substantia nigra (SN) of PD patients, most recent research supports a role of immune-mediated mechanisms in the pathological process leading to chronic neuroinflammation and dopaminergic degeneration. PPAR-γ are highly expressed in cells of both central and peripheral immune systems, playing a pivotal role in microglial activation as well as in monocytes and T cells differentiation, in which they act as key regulators of immune responses. Here, we review preclinical evidences of PPAR-γ-induced neuroprotection in experimental PD models and highlight relative anti-inflammatory mechanisms involving either central or peripheral immunomodulatory activity. Specific targeting of immune functions contributing to neuroinflammation either directly (central) or indirectly (peripheral) may represent a novel therapeutic approach for disease modifying therapies in PD.

Eosinophils sustain adipose alternatively activated macrophages associated with glucose homeostasis

Eosinophils are associated with helminth immunity and allergy, often in conjunction with alternatively activated macrophages (AAMs). Adipose tissue AAMs are necessary to maintain glucose homeostasis and are induced by the cytokine interleukin-4 (IL-4). Here, we show that eosinophils are the major IL-4-expressing cells in white adipose tissues of mice, and, in their absence, AAMs are greatly attenuated. Eosinophils migrate into adipose tissue by an integrin-dependent process and reconstitute AAMs through an IL-4- or IL-13-dependent process. Mice fed a high-fat diet develop increased body fat, impaired glucose tolerance, and insulin resistance in the absence of eosinophils, and helminth-induced adipose tissue eosinophilia enhances glucose tolerance. Our results suggest that eosinophils play an unexpected role in metabolic homeostasis through maintenance of adipose AAMs.

Blockade of class IB phosphoinositide-3 kinase ameliorates obesity-induced inflammation and insulin resistance

Obesity and insulin resistance, the key features of metabolic syndrome, are closely associated with a state of chronic, low-grade inflammation characterized by abnormal macrophage infiltration into adipose tissues. Although it has been reported that chemokines promote leukocyte migration by activating class IB phosphoinositide-3 kinase (PI3Kγ) in inflammatory states, little is known about the role of PI3Kγ in obesity-induced macrophage infiltration into tissues, systemic inflammation, and the development of insulin resistance. In the present study, we used murine models of both diet-induced and genetically induced obesity to examine the role of PI3Kγ in the accumulation of tissue macrophages and the development of obesity-induced insulin resistance. Mice lacking p110γ (Pik3cg(-/-)), the catalytic subunit of PI3Kγ, exhibited improved systemic insulin sensitivity with enhanced insulin signaling in the tissues of obese animals. In adipose tissues and livers of obese Pik3cg(-/-) mice, the numbers of infiltrated proinflammatory macrophages were markedly reduced, leading to suppression of inflammatory reactions in these tissues. Furthermore, bone marrow-specific deletion and pharmacological blockade of PI3Kγ also ameliorated obesity-induced macrophage infiltration and insulin resistance. These data suggest that PI3Kγ plays a crucial role in the development of both obesity-induced inflammation and systemic insulin resistance and that PI3Kγ can be a therapeutic target for type 2 diabetes.

Intraplaque haemorrhages as the trigger of plaque vulnerability

Atherothrombosis remains one of the main causes of morbidity and mortality in the western countries. Human atherothrombotic disease begins early in life in relation to circulating lipid retention in the inner vascular wall. Risk factors enhance the progression towards clinical expression: dyslipidaemia, diabetes, smoking, hypertension, ageing, etc. The evolution from the initial lipid retention in the arterial wall to clinical events is a continuum of increasingly complex biological processes. Current strategies to fight the consequences of atherothrombosis are orientated either towards the promotion of a healthy life style and preventive treatment of risk factors, or towards late interventional strategies. Despite this therapeutic arsenal, the incidence of clinical events remains dramatically high, dependent, at least in part, on the increasing frequency of type 2 diabetes and ageing. But some medical treatments, focusing only on prevention of the metabolic risk, have failed to reduce cardiovascular mortality, thus illustrating that our understanding of the pathophysiology of human atherothrombosis leading to clinical events remain incomplete. New paradigms are now emerging which may give rise to novel experimental strategies to improve therapeutic efficacy and prediction of disease progression. Recent studies strengthen the concept that the intraplaque neovascularization and bleeding (Figure 1, upper panel) are events that could play a major role in plaque progression and leucocyte infiltration, and may also serve as a measure of risk for the development of future events. The recent advances in our understanding of IntraPlaque Hemorrhage as a critical event in triggering acute clinical events have important implications for clinical research and possibly future clinical practice.

PPARs as therapeutic targets in cardiovascular disease

IMPORTANCE OF THE FIELD

The role of peroxisome proliferator-activated receptors PPARα, PPARδ and PPARγ in cardiovascular disease is receiving widespread attention. As ligand-activated nuclear receptors, they play a role in regulation of lipid and glucose metabolism. This feature of the PPARs has been successfully exploited to treat systemic metabolic diseases, like hyperlipidemia and type-2 diabetes. Indirectly, their lipid lowering effect also leads to a reduction of the risk for cardiovascular diseases, primarily atherosclerosis.

AREAS COVERED IN THIS REVIEW

The pleiotropic effects of each of the PPAR isotypes on vascular and cardiac disease are discussed, with special emphasis on the molecular mechanism of action and on preclinical observations. The mechanism underlying the beneficial effect of PPARs is not confined to whole body metabolism, but also includes modulation of other vital processes, such as inflammation and cell fate (proliferation, differentiation, apoptosis).

WHAT THE READER WILL GAIN

A large body of preclinical studies indicates that, in addition to their effect on atherogenesis, PPAR ligands also impact on ischemic heart disease and the development of cardiac failure. It remains to be established to what extent these intriguing observations can be translated into clinical practice.

TAKE HOME MESSAGE

The versatile mechanism of action extends the potential therapeutic profile of the PPARs enormously. Conversely, this versatility makes it harder to attain a specific therapeutic effect, without increasing the risk of undesirable side effects. The future challenge will be to design PPAR-based therapeutic strategies that minimize the detrimental side effects.

Reversal of hyperlipidemia with a genetic switch favorably affects the content and inflammatory state of macrophages in atherosclerotic plaques

BACKGROUND

We previously showed that the progression of atherosclerosis in the Reversa mouse (Ldlr(-/-Apob100/100Mttpfl/fl) Mx1Cre(+/+)) was arrested when the hyperlipidemia was normalized by inactivating the gene for microsomal triglyceride transfer protein. Here, we tested whether atherosclerosis would regress if the lipid levels were reduced after advanced plaques formed.

METHODS AND RESULTS

Reversa mice were fed an atherogenic diet for 16 weeks. Plasma lipid levels were then reduced. Within 2 weeks, this reduction led to decreased monocyte-derived (CD68(+)) cells in atherosclerotic plaques and was associated with emigration of these cells out of plaques. In addition, the fall in lipid levels was accompanied by lower plaque lipid content and by reduced expression in plaque CD68(+) cells of inflammatory genes and higher expression of genes for markers of antiinflammatory M2 macrophages. Plaque composition was affected more than plaque size, with the decreased content of lipid and CD68(+) cells balanced by a higher content of collagen. When the reduced lipid level was combined with the administration of pioglitazone to simulate the clinical aggressive lipid management and proliferator-activated receptor-γ agonist treatment, the rate of depletion of plaque CD68(+) cells was unaffected, but there was a further increase in their expression of antiinflammatory macrophage markers.

CONCLUSION

The Reversa mouse is a new model of atherosclerosis regression. After lipid lowering, favorable changes in plaque composition were independent of changes in size. In addition, plaque CD68(+) cells became less inflammatory, an effect enhanced by treatment with pioglitazone.

PPARs are a unique set of fatty acid regulated transcription factors controlling both lipid metabolism and inflammation

Cells are constantly exposed to a large variety of lipids. Traditionally, these molecules were thought to serve as simple energy storing molecules. More recently it has been realized that they can also initiate and regulate signaling events that will decisively influence development, cellular differentiation, metabolism and related functions through the regulation of gene expression. Multicellular organisms dedicate a large family of nuclear receptors to these tasks. These proteins combine the defining features of both transcription factors and receptor molecules, and therefore have the unique ability of being able to bind lipid signaling molecules and transduce the appropriate signals derived from lipid environment to the level of gene expression. Intriguingly, the members of a subfamily of the nuclear receptors, the peroxisome proliferator-activated receptors (PPARs) are able to sense and interpret fatty acid signals derived from dietary lipids, pathogenic lipoproteins or essential fatty acid metabolites. Not surprisingly, Peroxisome proliferator-activated receptors were found to be key regulators of lipid and carbohydrate metabolism. Unexpectedly, later studies revealed that Peroxisome proliferator-activated receptors are also able to modulate inflammatory responses. Here we summarize our understanding on how these transcription factors/receptors connect lipid metabolism to inflammation and some of the novel regulatory mechanisms by which they contribute to homeostasis and certain pathological conditions. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Differential lipid partitioning between adipocytes and tissue macrophages modulates macrophage lipotoxicity and M2/M1 polarization in obese mice

OBJECTIVE

Obesity-associated insulin resistance is characterized by a state of chronic, low-grade inflammation that is associated with the accumulation of M1 proinflammatory macrophages in adipose tissue. Although different evidence explains the mechanisms linking the expansion of adipose tissue and adipose tissue macrophage (ATM) polarization, in the current study we investigated the concept of lipid-induced toxicity as the pathogenic link that could explain the trigger of this response.

RESEARCH DESIGN AND METHODS

We addressed this question using isolated ATMs and adipocytes from genetic and diet-induced murine models of obesity. Through transcriptomic and lipidomic analysis, we created a model integrating transcript and lipid species networks simultaneously occurring in adipocytes and ATMs and their reversibility by thiazolidinedione treatment.

RESULTS

We show that polarization of ATMs is associated with lipid accumulation and the consequent formation of foam cell-like cells in adipose tissue. Our study reveals that early stages of adipose tissue expansion are characterized by M2-polarized ATMs and that progressive lipid accumulation within ATMs heralds the M1 polarization, a macrophage phenotype associated with severe obesity and insulin resistance. Furthermore, rosiglitazone treatment, which promotes redistribution of lipids toward adipocytes and extends the M2 ATM polarization state, prevents the lipid alterations associated with M1 ATM polarization.

CONCLUSIONS

Our data indicate that the M1 ATM polarization in obesity might be a macrophage-specific manifestation of a more general lipotoxic pathogenic mechanism. This indicates that strategies to optimize fat deposition and repartitioning toward adipocytes might improve insulin sensitivity by preventing ATM lipotoxicity and M1 polarization.

Apolipoprotein E induces antiinflammatory phenotype in macrophages

OBJECTIVE

Apolipoprotein E (apoE) exerts potent antiinflammatory effects. Here, we investigated the effect of apoE on the functional phenotype of macrophages.

METHODS AND RESULTS

Human apoE receptors very-low-density lipoprotein receptor (VLDL-R) and apoE receptor-2 (apoER2) were stably expressed in RAW264.7 mouse macrophages. In these cells, apoE downregulated markers of the proinflammatory M1 phenotype (inducible nitric oxide synthase, interleukin [IL]-12, macrophage inflammatory protein-1α) but upregulated markers of the antiinflammatory M2 phenotype (arginase I, SOCS3, IL-1 receptor antagonist [IL-1RA]). In addition, M1 macrophage responses (migration, generation of reactive oxygen species, antibody-dependent cell cytotoxicity, phagocytosis), as well as poly(I:C)- or interferon-γ-induced production of proinflammatory cytokines; cyclooxygenase-2 expression; and activation of nuclear factor-κB, IκB, and STAT1, were suppressed in VLDL-R- or apoER2-expressing cells. Conversely, the suppression of the M2 phenotype and the enhanced response to poly(I:C) were observed in apoE-producing bone marrow macrophages derived from VLDL-R-deficient mice but not wild-type or low-density lipoprotein receptor-deficient mice. The modulatory effects of apoE on macrophage polarization were inhibited in apoE receptor-expressing RAW264.7 cells exposed to SB220025, a p38 mitogen-activated protein kinase inhibitor, and PP1, a tyrosine kinase inhibitor. Accordingly, apoE induced tyrosine kinase-dependent activation of p38 mitogen-activated protein kinase in VLDL-R- or apoER2-expressing macrophages. Under in vivo conditions, apoE-/- mice transplanted with apoE-producing wild-type bone marrow showed increased plasma IL-1RA levels, and peritoneal macrophages of transplanted animals were shifted to the M2 phenotype (increased IL-1RA production and CD206 expression).

CONCLUSIONS

ApoE signaling via VLDL-R or apoER2 promotes macrophage conversion from the proinflammatory M1 to the antiinflammatory M2 phenotype. This effect may represent a novel antiinflammatory activity of apoE.

Human atherosclerotic plaque alternative macrophages display low cholesterol handling but high phagocytosis because of distinct activities of the PPARγ and LXRα pathways

RATIONALE

A crucial step in atherogenesis is the infiltration of the subendothelial space of large arteries by monocytes where they differentiate into macrophages and transform into lipid-loaded foam cells. Macrophages are heterogeneous cells that adapt their response to environmental cytokines. Th1 cytokines promote monocyte differentiation into M1 macrophages, whereas Th2 cytokines trigger an "alternative" M2 phenotype.

OBJECTIVE

We previously reported the presence of CD68(+) mannose receptor (MR)(+) M2 macrophages in human atherosclerotic plaques. However, the function of these plaque CD68(+)MR(+) macrophages is still unknown.

METHODS AND RESULTS

Histological analysis revealed that CD68(+)MR(+) macrophages locate far from the lipid core of the plaque and contain smaller lipid droplets compared to CD68(+)MR(-) macrophages. Interleukin (IL)-4-polarized CD68(+)MR(+) macrophages display a reduced capacity to handle and efflux cellular cholesterol because of low expression levels of the nuclear receptor liver x receptor (LXR)α and its target genes, ABCA1 and apolipoprotein E, attributable to the high 15-lipoxygenase activity in CD68(+)MR(+) macrophages. By contrast, CD68(+)MR(+) macrophages highly express opsonins and receptors involved in phagocytosis, resulting in high phagocytic activity. In M2 macrophages, peroxisome proliferator-activated receptor (PPAR)γ activation enhances the phagocytic but not the cholesterol trafficking pathways.

CONCLUSIONS

These data identify a distinct macrophage subpopulation with a low susceptibility to become foam cells but high phagocytic activity resulting from different regulatory activities of the PPARγ-LXRα pathways.

Different responses to oxidized low-density lipoproteins in human polarized macrophages

Background

Oxidized low-density lipoprotein (oxLDL) uptake by macrophages plays an important role in foam cell formation. It has been suggested the presence of heterogeneous subsets of macrophage, such as M1 and M2, in human atherosclerotic lesions. To evaluate which types of macrophages contribute to atherogenesis, we performed cDNA microarray analysis to determine oxLDL-induced transcriptional alterations of each subset of macrophages.

Results

Human monocyte-derived macrophages were polarized toward the M1 or M2 subset, followed by treatment with oxLDL. Then gene expression levels during oxLDL treatment in each subset of macrophages were evaluated by cDNA microarray analysis and quantitative real-time RT-PCR. In terms of high-ranking upregulated genes and functional ontologies, the alterations during oxLDL treatment in M2 macrophages were similar to those in nonpolarized macrophages (M0). Molecular network analysis showed that most of the molecules in the oxLDL-induced highest scoring molecular network of M1 macrophages were directly or indirectly related to transforming growth factor (TGF)-β1. Hierarchical cluster analysis revealed commonly upregulated genes in all subset of macrophages, some of which contained antioxidant response elements (ARE) in their promoter regions. A cluster of genes that were specifically upregulated in M1 macrophages included those encoding molecules related to nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-κB) signaling pathway. Quantitative real-time RT-PCR showed that the gene expression of interleukin (IL)-8 after oxLDL treatment in M2 macrophages was markedly lower than those in M0 and M1 cells. HMOX1 gene expression levels were almost the same in all 3 subsets of macrophages even after oxLDL treatment.

Conclusions

The present study demonstrated transcriptional alterations in polarized macrophages during oxLDL treatment. The data suggested that oxLDL uptake may affect TGF-β1- and NF-κB-mediated functions of M1 macrophages, but not those of M0 or M2 macrophages. It is likely that M1 macrophages characteristically respond to oxLDL.

Targeting type 2 diabetes

The evolving concept of how nutrient excess and inflammation modulate metabolism provides new opportunities for strategies to correct the detrimental health consequences of obesity. In this review, we focus on the complex interplay among lipid overload, immune response, proinflammatory pathways and organelle dysfunction through which excess adiposity might lead to type 2 diabetes. We then consider evidence linking dysregulated CNS circuits to insulin resistance and results on nutrient-sensing pathways emerging from studies with calorie restriction. Subsequently, recent recommendations for the management of type 2 diabetes are discussed with emphasis on prevailing current therapeutic classes of biguanides, thiazolidinediones and incretin-based approaches.

Liver X receptors as regulators of macrophage inflammatory and metabolic pathways

The liver X receptors (LXRα and LXRβ) are members of the nuclear receptor family of transcription factors that play essential roles in the transcriptional control of lipid metabolism. LXRs are endogenously activated by modified forms of cholesterol known as oxysterols and control the expression of genes important for cholesterol uptake, efflux, transport, and excretion in multiple tissues. In addition to their role as cholesterol sensors, a number of studies have implicated LXRs in the modulation of innate and adaptive immune responses. Both through activation and repression mechanisms, LXRs regulate diverse aspects of inflammatory gene expression in macrophages. The ability of LXRs to coordinate metabolic and immune responses constitutes an attractive therapeutic target for the treatment of chronic inflammatory disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Mechanisms of failed apoptotic cell clearance by phagocyte subsets in cardiovascular disease

Recent evidence in humans indicate that defective phagocytic clearance of dying cells is linked to progression of advanced atherosclerotic lesions, the precursor to atherothrombosis, ischemic heart disease, and leading cause of death in the industrialized world. During atherogenesis, apoptotic cell turnover in the vascular wall is counterbalanced by neighboring phagocytes with high clearance efficiency, thereby limiting cellularity and maintaining lesion integrity. However, as lesions mature, phagocytic removal of apoptotic cells (efferocytosis) becomes defective, leading to secondary necrosis, expansion of plaque necrotic cores, and susceptibility to rupture. Recent genetic causation studies in experimental rodents have implicated key molecular regulators of efferocytosis in atherosclerotic progression. These include MER tyrosine kinase (MERTK), milk fat globule-EGF factor 8 (MFGE8), and complement C1q. At the cellular level, atheromata are infiltrated by a heterogenous population of professional phagocytes, comprised of monocytes, differentiated macrophages, and CD11c(+) dendritic-like cells. Each cell type is characterized by disparate clearance efficiencies and varying activities of key phagocytic signaling molecules. It is in this context that we outline a working model whereby plaque necrosis and destabilization is jointly promoted by (1) direct inhibition of core phagocytic signaling pathways and (2) expansion of phagocyte subsets with poor clearance capacity. Towards identifying targets for promoting efficient apoptotic cell clearance and resolving inflammation in atherosclerosis and during ischemic heart disease and post myocardial infarction, this review will discuss potential in vivo suppressors of efferocytosis at each stage of clearance and how these putative interventional targets may differentially affect uptake at the level of vascular phagocyte subsets.

Antagonistic regulation of macrophage phenotype by M-CSF and GM-CSF: implication in atherosclerosis

OBJECTIVES

We characterized the transcriptional profiles of GM-CSF- (GM-MØ) and M-CSF-induced macrophages (M-MØ) and investigated in situ a subset of differentially expressed genes in human and mouse atherosclerotic lesions.

METHODS AND RESULTS

Using microarrays we identified a number of genes and biological processes differentially regulated in M-MØ vs GM-MØ. By varying in culture the M-CSF/GM-CSF ratio (0-10), a spectrum of macrophage phenotypes was explored by RT-QPCR. M-CSF (10 ng/ml) stimulated expression of several genes, including selenoprotein-1 (SEPP1), stabilin-1 (STAB1) and CD163 molecule-like-1 (CD163L1) which was inhibited by a low dose of GM-CSF (1 ng/ml); M-CSF inhibited the expression of pro-platelet basic protein (PPBP) induced by GM-CSF. Combining tissue microarrays/quantitative immunohistochemistry of human aortic lesions with RT-QPCR expression data either from human carotids vs mammary non-atherosclerotic arteries or from the apoE null mice normal and atherosclerotic aortas showed that, STAB1, SEPP1 and CD163L1 (M-CSF-sensitive genes) and PPBP (GM-CSF-sensitive gene) were expressed in both human arterial and apoE null mice atherosclerotic tissues.

CONCLUSION

A balance between M-CSF vs GM-CSF defines macrophage functional polarisation and may contribute to the progression of atherosclerosis.

Oxidized LDL enhances pro-inflammatory responses of alternatively activated M2 macrophages: a crucial role for Krüppel-like factor 2

OBJECTIVE

Macrophages are key players in atherogenesis because of their properties to form foam cells that produce a large variety of pro-inflammatory mediators. We addressed the potency of phenotypic different macrophages to accumulate oxidized LDL.

METHODS AND RESULTS

Surprisingly, anti-inflammatory M2 macrophages but not pro-inflammatory M1 macrophages rapidly accumulated oxidized LDL. Simultaneously, expression of Krüppel-like factor 2, a nuclear transcription factor known to suppress inflammation in endothelial cells and monocytes, decreased and the functional phenotype of M2 macrophages shifted towards a pro-inflammatory profile, characterized by higher production of IL-6, IL-8 and MCP-1 and lower expression of IL-10 upon stimulation with LPS. In contrast, Krüppel-like factor 2 expression and the phenotype of M1 macrophages remained largely unchanged upon oxidized LDL exposure. Downregulation of Krüppel-like factor 2 expression of M2 macrophages using siRNA technology led to a significant increase of LPS-induced MCP-1 secretion.

CONCLUSIONS

We show that (1) anti-inflammatory M2 macrophages are more susceptible to foam cell formation than pro-inflammatory M1 macrophages, (2) exposure to oxidized LDL renders M2 macrophages pro-inflammatory, and (3) Krüppel-like factor 2 is involved in the enhanced secretion of MCP-1 by M2 macrophages loaded with oxidized LDL. The phenotype switch of M2 macrophages from an anti- to a pro-inflammatory profile may play an important role in pathogenesis of atherosclerosis, and could represent a novel therapeutic target.

Palmitate and insulin synergistically induce IL-6 expression in human monocytes

BACKGROUND

Insulin resistance is associated with a proinflammatory state that promotes the development of complications such as type 2 diabetes mellitus (T2DM) and atherosclerosis. The metabolic stimuli that initiate and propagate proinflammatory cytokine production and the cellular origin of proinflammatory cytokines in insulin resistance have not been fully elucidated. Circulating proinflammatory monocytes show signs of enhanced inflammation in obese, insulin resistant subjects and are thus a potential source of proinflammatory cytokine production. The specific, circulating metabolic factors that might stimulate monocyte inflammation in insulin resistant subjects are poorly characterized. We have examined whether saturated nonesterified fatty acids (NEFA) and insulin, which increase in concentration with developing insulin resistance, can trigger the production of interleukin (IL)-6 and tumor necrosis factor (TNF)-α in human monocytes.

METHODS

Messenger RNA and protein levels of the proinflammatory cytokines IL-6 and TNF-α were measured by quantitative real-time PCR (qRT-PCR) and Luminex bioassays. Student's t-test was used with a significance level of p < 0.05 to determine significance between treatment groups.

RESULTS

Esterification of palmitate with coenzyme A (CoA) was necessary, while β-oxidation and ceramide biosynthesis were not required, for the induction of IL-6 and TNF-α in THP-1 monocytes. Monocytes incubated with insulin and palmitate together produced more IL-6 mRNA and protein, and more TNF-α protein, compared to monocytes incubated with palmitate alone. Incubation of monocytes with insulin alone did not affect the production of IL-6 or TNF-α. Both PI3K-Akt and MEK/ERK signalling pathways are important for cytokine induction by palmitate. MEK/ERK signalling is necessary for synergistic induction of IL-6 by palmitate and insulin.

CONCLUSIONS

High levels of saturated NEFA, such as palmitate, when combined with hyperinsulinemia, may activate human monocytes to produce proinflammatory cytokines and support the development and propagation of the subacute, chronic inflammatory state that is characteristic of insulin resistance. Results with inhibitors of β-oxidation and ceramide biosynthesis pathways suggest that increased fatty acid flux through the glycerolipid biosynthesis pathway may be involved in promoting proinflammatory cytokine production in monocytes.

Peroxisome proliferator-activated receptors, metabolic syndrome and cardiovascular disease

Metabolic syndrome (MetS) is a constellation of risk factors including insulin resistance, central obesity, dyslipidemia and hypertension that markedly increase the risk of Type 2 diabetes (T2DM) and cardiovascular disease (CVD). The peroxisome proliferators-activated receptor (PPAR) isotypes, PPARα, PPARδ/ß and PPARγ are ligand-activated nuclear transcription factors, which modulate the expression of an array of genes that play a central role in regulating glucose, lipid and cholesterol metabolism, where imbalance can lead to obesity, T2DM and CVD. They are also drug targets, and currently, PPARα (fibrates) and PPARγ (thiazolodinediones) agonists are in clinical use for treating dyslipidemia and T2DM, respectively. These metabolic characteristics of the PPARs, coupled with their involvement in metabolic diseases, mean extensive efforts are underway worldwide to develop new and efficacious PPAR-based therapies for the treatment of additional maladies associated with the MetS. This article presents an overview of the functional characteristics of three PPAR isotypes, discusses recent advances in our understanding of the diverse biological actions of PPARs, particularly in the vascular system, and summarizes the developmental status of new single, dual, pan (multiple) and partial PPAR agonists for the clinical management of key components of MetS, T2DM and CVD. It also summarizes the clinical outcomes from various clinical trials aimed at evaluating the atheroprotective actions of currently used fibrates and thiazolodinediones.

In aged mice, outgrowth of intraocular melanoma depends on proangiogenic M2-type macrophages

Macrophages are part of the tumor microenvironment and have been associated with poor prognosis in uveal melanoma. We determined the presence of macrophages and their differentiation status in a murine intraocular melanoma model. Inoculation of B16F10 cells into the anterior chamber of the eye resulted in rapid tumor outgrowth. Strikingly, in aged mice, tumor progression depended on the presence of macrophages, as local depletion of these cells prevented tumor outgrowth, indicating that macrophages in old mice had a strong tumor-promoting role. Immunohistochemistry and gene expression analysis revealed that macrophages carried M2-type characteristics, as shown by CD163 and peroxisome proliferator-activated receptor gamma expression, and that multiple angiogenic genes were heavily overrepresented in tumors of old mice. The M2-type macrophages were also shown to have immunosuppressive features. We conclude that tumor-associated macrophages are directly involved in tumor outgrowth of intraocular melanoma and that macrophages in aged mice have a predisposition for an M2-type profile.

Therapeutic Implications of PPARgamma in Cardiovascular Diseases

Peroxisome proliferator-activated receptor-γ (PPARγ) is the members of the nuclear receptor superfamily as a master transcriptional factor that promotes differentiation of preadipocytes by activating adipose-specific gene expression. Although PPARγ is expressed predominantly in adipose tissue and associated with adipocyte differentiation and glucose homeostasis, PPARγ is also present in a variety of cell types including vascular cells and cardiomyocytes. Activation of PPARγ suppresses production of inflammatory cytokines, and there is accumulating data that PPARγ ligands exert antihypertrophy of cardiomyocytes and anti-inflammatory, antioxidative, and antiproliferative effects on vascular wall cells and cardiomyocytes. In addition, activation of PPARγ is implicated in the regulation of endothelial function, proliferation and migration of vascular smooth muscle cells, and activation of macrophages. Many studies suggest that PPARγ ligands not only ameliorate insulin sensitivity, but also have pleiotropic effects on the pathophysiology of atherosclerosis, cardiac hypertrophy, ischemic heart, and myocarditis.

Myeloid mineralocorticoid receptor controls macrophage polarization and cardiovascular hypertrophy and remodeling in mice

Inappropriate excess of the steroid hormone aldosterone, which is a mineralocorticoid receptor (MR) agonist, is associated with increased inflammation and risk of cardiovascular disease. MR antagonists are cardioprotective and antiinflammatory in vivo, and evidence suggests that they mediate these effects in part by aldosterone-independent mechanisms. Here we have shown that MR on myeloid cells is necessary for efficient classical macrophage activation by proinflammatory cytokines. Macrophages from mice lacking MR in myeloid cells (referred to herein as MyMRKO mice) exhibited a transcription profile of alternative activation. In vitro, MR deficiency synergized with inducers of alternatively activated macrophages (for example, IL-4 and agonists of PPARgamma and the glucocorticoid receptor) to enhance alternative activation. In vivo, MR deficiency in macrophages mimicked the effects of MR antagonists and protected against cardiac hypertrophy, fibrosis, and vascular damage caused by L-NAME/Ang II. Increased blood pressure and heart rates and decreased circadian variation were observed during treatment of MyMRKO mice with L-NAME/Ang II. We conclude that myeloid MR is an important control point in macrophage polarization and that the function of MR on myeloid cells likely represents a conserved ancestral MR function that is integrated in a transcriptional network with PPARgamma and glucocorticoid receptor. Furthermore, myeloid MR is critical for blood pressure control and for hypertrophic and fibrotic responses in the mouse heart and aorta.

Identification of a novel macrophage phenotype that develops in response to atherogenic phospholipids via Nrf2

RATIONALE

Macrophages change their phenotype and biological functions depending on the microenvironment. In atherosclerosis, oxidative tissue damage accompanies chronic inflammation; however, macrophage phenotypic changes in response to oxidatively modified molecules are not known.

OBJECTIVE

To examine macrophage phenotypic changes in response to oxidized phospholipids that are present in atherosclerotic lesions.

METHODS AND RESULTS

We show that oxidized phospholipid-treated murine macrophages develop into a novel phenotype (Mox) that is strikingly different from the conventional M1 and M2 macrophage phenotypes. Compared to M1 and M2, Mox macrophages show a different gene expression pattern, as well as decreased phagocytotic and chemotactic capacity. Treatment with oxidized phospholipids induces both M1 and M2 macrophages to switch to the Mox phenotype. Whole-genome expression array analysis and subsequent gene ontology clustering revealed that the Mox phenotype was characterized by abundant overrepresentation of Nrf2-mediated expression of redox-regulatory genes. In macrophages isolated from Nrf2(-/-) mice, oxidized phospholipid-induced gene expression and regulation of redox status were compromised. Moreover, we found that Mox macrophages comprise 30% of all macrophages in advanced atherosclerotic lesions of low-density lipoprotein receptor knockout (LDLR(-/-)) mice.

CONCLUSIONS

Together, we identify Nrf2 as a key regulator in the formation of a novel macrophage phenotype (Mox) that develops in response to oxidative tissue damage. The unique biological properties of Mox macrophages suggest this phenotype may play an important role in atherosclerotic lesion development as well as in other settings of chronic inflammation.

Low-intensity exercise enhances expression of markers of alternative activation in circulating leukocytes: roles of PPARγ and Th2 cytokines

OBJECTIVE

Pharmacological activation of the nuclear receptor PPARγ is linked to numerous beneficial effects in the contexts of inflammation, lipid homeostasis, Type-2 Diabetes (T2D) and atherosclerosis. These beneficial effects include priming of circulating monocytes for differentiation towards an 'alternative' anti-inflammatory M2 macrophage phenotype. As we have recently shown that participation in low-intensity exercise increases PPARγ expression and activity in leukocytes from previously sedentary individuals, we aimed to elucidate whether low-intensity exercise elicited a pattern of gene expression similar to that reported for M2 monocyte-macrophage differentiation.

METHODS

17 sedentary individuals undertook an 8-week low-intensity exercise programme (walking 10,000steps/day, three times/week). Changes in expression of PPARs and the PPARγ co-activators PGC-1α and PGC-1β; Th2 (IL-4; IL-10) and Th1 (IL-6) cytokines; and markers for the M2 (AMAC1, CD14, MR, IL-4) and the 'classical' pro-inflammatory M1 (MCP-1, TNFα, IL-6) phenotypes, were determined using RT-PCR (to assess leukocyte mRNA expression) and ELISA (to assess plasma cytokine levels).

RESULTS

Exercise was associated with upregulation of M2 markers, PGC-1α and PGC-1β, and with downregulation of M1 markers. Moreover, plasma levels of Th2 cytokines increased after exercise, while those of Th1 cytokines decreased. However, other PPARs (PPARα; PPARβ/δ) did not undergo marked exercise-induced activation or upregulation. Thus, participation in low-intensity exercise may prime monocytes for differentiation towards an M2 macrophage phenotype via PPARγ/PGC-1α/β.

CONCLUSION

Given the similarities between these effects and pharmacologically induced M2 polarisation, we propose that exercise-induced PPARγ/PGC-1α/β-mediated M2 polarisation may constitute a novel anti-inflammatory benefit of low-intensity exercise.

Macrophages heterogeneity in atherosclerosis - implications for therapy

Atherosclerosis is a chronic inflammatory disease occurring within the artery wall and is an underlying cause of cardiovascular complications, including myocardial infarction, stroke and peripheral vascular disease. Its pathogenesis involves many immune cell types with a well accepted role for monocyte/macrophages. Cholesterol-loaded macrophages are a characteristic feature of plaques and are major players in all stages of plaque development. As well as modulating lipid metabolism, macrophages secrete inflammatory cytokines, chemokines and reactive oxygen and nitrogen species that drive pathogenesis. They also produce proteases and tissue factor that contribute to plaque rupture and thrombosis. Macrophages are however heterogeneous cells and when appropriately activated, they phagocytose cytotoxic lipoproteins, clear apoptotic bodies, secrete anti-inflammatory cytokines and synthesize matrix repair proteins that stabilize vulnerable plaques. Pharmacological modulation of macrophage activity therefore represents a potential therapeutic strategy for atherosclerosis. The aim of this review is to provide an overview of the current understanding of the different macrophage subsets and their monocyte precursors, and, the implications of these subsets for atherosclerosis. This will present a foundation for highlighting novel opportunities to exploit the heterogeneity of macrophages as important diagnostic and therapeutic targets for atherosclerosis and its associated diseases.

Adiponectin primes human monocytes into alternative anti-inflammatory M2 macrophages

Altered macrophage kinetics is a pivotal mechanism of visceral obesity-induced inflammation and cardiometabolic risk. Because monocytes can differentiate into either proatherogenic M1 macrophages or anti-inflammatory M2 macrophages, approaches that limit M1 while promoting M2 differentiation represent a unique therapeutic strategy. We hypothesized that adiponectin may prime human monocytes toward the M2 phenotype. Adiponectin promoted the alternative activation of human monocytes into anti-inflammatory M2 macrophages as opposed to the classically activated M1 phenotype. Adiponectin-treated cells displayed increased M2 markers, including the mannose receptor (MR) and alternative macrophage activation-associated CC chemokine-1. Incubation of M1 macrophages with adiponectin-treated M2-derived culture supernatant resulted in a pronounced inhibition of tumor necrosis factor-alpha and monocyte chemotactic protein-1 secretion. Activation of human monocytes into M2 macrophages by adiponectin was mediated, in addition to AMP-activated protein kinase and peroxisome proliferator-activated receptor (PPAR)-gamma, via PPAR-alpha. Furthermore, macrophages isolated from adiponectin knockout mice demonstrated diminished levels of M2 markers such as MR, which were restored with adiponectin treatment. We report a novel immunoregulatory mechanism through which adiponectin primes human monocyte differentiation into anti-inflammatory M2 macrophages. Conditions associated with low adiponectin levels, such as visceral obesity and insulin resistance, may promote atherosclerosis, in part through aberrant macrophage kinetics.

Control of macrophage activation and function by PPARs

Abstract: Macrophages, a key component of the innate defense against pathogens, participate in the initiation and resolution of inflammation, and in the maintenance of tissues. These diverse and at times antithetical functions of macrophages are executed via distinct activation states, ranging from classical to alternative to deactivation. Because the dysregulation of macrophage activation is pathogenically linked to various metabolic, inflammatory and immune disorders, regulatory proteins controlling macrophage activation have emerged as important new therapeutic targets. Here, the mechanisms by which peroxisome proliferator-activated receptors (PPARs) transcriptionally regulate macrophage activation in health and disease states, including obesity, insulin resistance and cardiovascular disease, are reviewed.

Visfatin is induced by peroxisome proliferator-activated receptor gamma in human macrophages

Obesity is a low-grade chronic inflammatory disease associated with an increased number of macrophages (adipose tissue macrophages) in adipose tissue. Within the adipose tissue, adipose tissue macrophages are the major source of visfatin/pre-B-cell colony-enhancing factor/nicotinamide phosphoribosyl transferase. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) exerts anti-inflammatory effects in macrophages by inhibiting cytokine production and enhancing alternative differentiation. In this study, we investigated whether PPARgamma modulates visfatin expression in murine (bone marrow-derived macrophage) and human (primary human resting macrophage, classical macrophage, alternative macrophage or adipose tissue macrophage) macrophage models and pre-adipocyte-derived adipocytes. We show that synthetic PPARgamma ligands increase visfatin gene expression in a PPARgamma-dependent manner in primary human resting macrophages and in adipose tissue macrophages, but not in adipocytes. The threefold increase of visfatin mRNA was paralleled by an increase of protein expression (30%) and secretion (30%). Electrophoretic mobility shift assay experiments and transient transfection assays indicated that PPARgamma induces visfatin promoter activity in human macrophages by binding to a DR1-PPARgamma response element. Finally, we show that PPARgamma ligands increase NAD(+) production in primary human macrophages and that this regulation is dampened in the presence of visfatin small interfering RNA or by the visfatin-specific inhibitor FK866. Taken together, our results suggest that PPARgamma regulates the expression of visfatin in macrophages, leading to increased levels of NAD(+).

Sex dimorphic actions of rosiglitazone in generalised peroxisome proliferator-activated receptor-gamma (PPAR-gamma)-deficient mice

AIMS/HYPOTHESIS

The aim of this study was to determine the dependency on peroxisome proliferator-activated receptor-gamma (PPAR-gamma) of insulin sensitisation and glucose homeostasis by thiazolidinediones using a global Ppar-gamma (also known as Pparg)-knockout mouse model.

METHODS

Global Mox2-Cre-Ppar-gamma-knockout (MORE-PGKO) mice were treated with rosiglitazone and analysed for insulin sensitivity and glucose metabolism. Metabolic and hormonal variables were determined. Adipose and other tissues were measured and analysed for gene expression.

RESULTS

Rosiglitazone induced regrowth of fat in female but not male MORE-PGKO mice, and only in specific depots. Insulin sensitivity increased but, surprisingly, was not associated with the typical changes in adipokines, plasma NEFA or tissue triacylglycerol. However, increases in alternatively activated macrophage markers, which have been previously associated with metabolic improvement, were observed in the regrown fat. Rosiglitazone improved glucose homeostasis but not insulin sensitivity in male MORE-PGKO mice, with further increase of insulin associated with an apparent expansion of pancreatic islets.

CONCLUSIONS/INTERPRETATION

Stimulating fat growth by rosiglitazone is sufficient to improve insulin sensitivity in female mice with 95% PPAR-gamma deficiency. This increase in insulin sensitivity is not likely to be due to changes typically seen in adipokines or lipids but may involve changes in macrophage polarisation that occur independent of PPAR-gamma. In contrast, rosiglitazone improves glucose homeostasis in male mice with similar PPAR-gamma deficiency by increasing insulin production independent of changes in adiposity. Further, the insulin-sensitising effect of rosiglitazone is dependent on PPAR-gamma in this male lipodystrophic model.

Dietary intervention-induced weight loss decreases macrophage content in adipose tissue of obese women

OBJECTIVE

Accumulation of adipose tissue macrophages (ATMs) is observed in obesity and may participate in the development of insulin resistance and obesity-related complications. The aim of our study was to investigate the effect of long-term dietary intervention on ATM content in human adipose tissue.

DESIGN

We performed a multi-phase longitudinal study.

SUBJECTS AND MEASUREMENTS

A total of 27 obese pre-menopausal women (age 39 ± 2 years, body mass index 33.7 ± 0.5 kg m(-2)) underwent a 6-month dietary intervention consisting of two periods: 4 weeks of very low-calorie diet (VLCD) followed by weight stabilization composed of 2 months of low-calorie diet and 3 to 4 months of weight maintenance diet. At baseline and at the end of each dietary period, samples of subcutaneous adipose tissue (SAT) were obtained by needle biopsy and blood samples were drawn. ATMs were determined by flow cytometry using combinations of cell surface markers. Selected cytokine and chemokine plasma levels were measured using enzyme-linked immunosorbent assay. In addition, in a subgroup of 16 subjects, gene expression profiling of macrophage markers in SAT was performed using real-time PCR.

RESULTS

Dietary intervention led to a significant decrease in body weight, plasma insulin and C-reactive protein levels. After VLCD, ATM content defined by CD45+/14+/206+ did not change, whereas it decreased at the end of the intervention. This decrease was associated with a downregulation of macrophage marker mRNA levels (CD14, CD163, CD68 and LYVE-1 (lymphatic vessel endothelial hyaluronan receptor-1)) and plasma levels of monocyte-chemoattractant protein-1 (MCP-1) and CXCL5 (chemokine (C-X-C motif) ligand 5). During the whole dietary intervention, the proportion of two ATM subpopulations distinguished by the CD16 marker was not changed.

CONCLUSION

A 6-month weight-reducing dietary intervention, but not VLCD, promotes a decrease in the number of the whole ATM population with no change in the relative distribution of ATM subsets.

Tumor-conditioned macrophages secrete migration-stimulating factor: a new marker for M2-polarization, influencing tumor cell motility

Tumor-associated macrophages (TAMs) are key orchestrators of the tumor microenvironment directly affecting neoplastic cell growth, neoangiogenesis, and extracellular matrix remodeling. In turn, the tumor milieu strongly influences maturation of TAMs and shapes several of their features. To address the early macrophage (M) differentiation phase in a malignant context, we mimicked a tumor microenvironment by in vitro coculturing human blood monocytes with conditioned media from different cancer cell lines. Only 2 out of 16 tumor cell lines induced M differentiation due to secreted M-CSF isoforms, including high molecular mass species. A global gene profiling of tumor-conditioned M was performed. Comparison with other datasets (polarized M1-M, M2-M, and TAMs isolated from human tumors) highlighted the upregulation of several genes also shared by TAM and M2-polarized M. The most expressed genes were selenoprotein 1, osteoactivin, osteopontin, and, interestingly, migration-stimulating factor (MSF), a poorly studied oncofoetal isoform of fibronectin. MSF (present in fetal/cancer epithelial and stromal cells but not in healthy tissues) was never identified in M. MSF production was confirmed by immunohistochemistry in human TAMs. MSF was induced by M-CSF, IL-4, and TGFbeta but not by proinflammatory stimuli. RNA and protein analysis clearly demonstrated that it is specifically associated with the M2 polarization of M. Tumor-conditioned M-derived MSFs strongly stimulated tumor cell migration, thus contributing to the motile phenotype of neoplastic cells. In conclusion, MSF is a new molecule associated with the M2 polarization of M and expressed by TAMs. Its biological function may contribute to M-mediated promotion of cancer cell invasion and metastasis.

Involvement of inducible 6-phosphofructo-2-kinase in the anti-diabetic effect of peroxisome proliferator-activated receptor gamma activation in mice

PFKFB3 is the gene that codes for the inducible isoform of 6-phosphofructo-2-kinase (iPFK2), a key regulatory enzyme of glycolysis. As one of the targets of peroxisome proliferator-activated receptor gamma (PPARgamma), PFKFB3/iPFK2 is up-regulated by thiazolidinediones. In the present study, using PFKFB3/iPFK2-disrupted mice, the role of PFKFB3/iPFK2 in the anti-diabetic effect of PPARgamma activation was determined. In wild-type littermate mice, PPARgamma activation (i.e. treatment with rosiglitazone) restored euglycemia and reversed high fat diet-induced insulin resistance and glucose intolerance. In contrast, PPARgamma activation did not reduce high fat diet-induced hyperglycemia and failed to reverse insulin resistance and glucose intolerance in PFKFB3(+/-) mice. The lack of anti-diabetic effect in PFKFB3(+/-) mice was associated with the inability of PPARgamma activation to suppress adipose tissue lipolysis and proinflammatory cytokine production, stimulate visceral fat accumulation, enhance adipose tissue insulin signaling, and appropriately regulate adipokine expression. Similarly, in cultured 3T3-L1 adipocytes, knockdown of PFKFB3/iPFK2 lessened the effect of PPARgamma activation on stimulating lipid accumulation. Furthermore, PPARgamma activation did not suppress inflammatory signaling in PFKFB3/iPFK2-knockdown adipocytes as it did in control adipocytes. Upon inhibition of excessive fatty acid oxidation in PFKFB3/iPFK2-knockdown adipocytes, PPARgamma activation was able to significantly reverse inflammatory signaling and proinflammatory cytokine expression and restore insulin signaling. Together, these data demonstrate that PFKFB3/iPFK2 is critically involved in the anti-diabetic effect of PPARgamma activation.

Inhibition of T cell response to native low-density lipoprotein reduces atherosclerosis

Immune responses to oxidized low-density lipoprotein (oxLDL) are proposed to be important in atherosclerosis. To identify the mechanisms of recognition that govern T cell responses to LDL particles, we generated T cell hybridomas from human ApoB100 transgenic (huB100(tg)) mice that were immunized with human oxLDL. Surprisingly, none of the hybridomas responded to oxidized LDL, only to native LDL and the purified LDL apolipoprotein ApoB100. However, sera from immunized mice contained IgG antibodies to oxLDL, suggesting that T cell responses to native ApoB100 help B cells making antibodies to oxLDL. ApoB100 responding CD4(+) T cell hybridomas were MHC class II-restricted and expressed a single T cell receptor (TCR) variable (V) beta chain, TRBV31, with different Valpha chains. Immunization of huB100(tg)xLdlr(-/-) mice with a TRBV31-derived peptide induced anti-TRBV31 antibodies that blocked T cell recognition of ApoB100. This treatment significantly reduced atherosclerosis by 65%, with a concomitant reduction of macrophage infiltration and MHC class II expression in lesions. In conclusion, CD4(+) T cells recognize epitopes on native ApoB100 protein, this response is associated with a limited set of clonotypic TCRs, and blocking TCR-dependent antigen recognition by these T cells protects against atherosclerosis.

Induction of fatty acid synthesis is a key requirement for phagocytic differentiation of human monocytes

Monocytes are precursors of macrophages. Here we demonstrate that macrophage colony-stimulating factor (M-CSF)-dependent differentiation of primary human monocytes from healthy volunteers induces transcription of SREBP-1c target genes required for fatty acid (FA) biosynthesis and impairs transcription of SREBP-2 target genes required for cholesterol synthesis. Detailed lipid metabolic profiling showed that this transcriptional regulation leads to a dramatically increased fatty acid synthesis as driving force for enhanced phospholipid synthesis. During cell differentiation the major lipid class switches from cholesterol in monocytes to phosphatidylcholine in macrophages. Ultrastructural analysis revealed that this transcriptional and metabolic regulation is essential for development of macrophage filopodia and cellular organelles including primary lysosomes, endoplasmic reticulum, and Golgi network. Additional functional studies showed that suppression of fatty acid synthesis prevents phagocytosis representing a central macrophage function. Therefore induction of fatty acid synthesis is a key requirement for phagocyte development and function.

Thromboxane synthase expression and thromboxane A2 production in the atherosclerotic lesion

Thromboxane A(2) (TXA(2)) is a potent prothrombotic and immune modulating lipid mediator, which is implicated in cardiovascular diseases, in particular, atherosclerotic lesion development and thrombogenicity. Here, we tested the hypothesis that thromboxane synthase (TXAS), the obligate enzyme required to synthesize TXA(2), is expressed within the human atherosclerotic lesion, thus potentially contributing to TXA(2) synthesis and disease development. In an animal study, different atherosclerosis-prone mouse strains were investigated and compared with control mice. In a patient study (n = 134), endarterectomies of carotid atherosclerotic lesions were compared with non-atherosclerotic arteries (n = 11). Expression of TXAS was evaluated by real-time quantitative reverse transcription PCR and immunohistochemistry. TXAS mRNA expression was increased within the vascular wall in mouse models of atherosclerosis with advanced lesions. In humans, TXAS was expressed in the atherosclerotic lesion, associated with increased inflammatory cells, in particular M2 polarized macrophages, and increased in atherosclerotic lesions of patients with recent symptoms of thrombotic events. Production of TXA(2) by plaque tissue, verified by gas chromatography-mass spectrometry, increased after addition of arachidonic acid or lipopolysaccharide, and was inhibited by the TXAS inhibitor furegrelate. The findings suggest that intraplaque TXA(2) generation may contribute to the development of atherosclerosis and its thrombotic complications in humans.

Stromal cell biology--a way to understand the evolution of cardiovascular diseases

Stromal cells, composed of fibroblasts, microvascular endothelial cells, immune cells and inflammatory cells, are critical determinants of the mechanical properties and function of the heart and vasculature, and the mechanisms whereby these types of cells are activated are important to understand the progression of cardiovascular diseases. Emerging studies have suggested that the activation of autocrine and paracrine signaling pathways by stromal cell-derived growth factors, cytokines and bioactive molecules contributes to disease progression. Disruption of the stromal network will result in alterations in the geometry and function in these organs. Interventions targeting the stromal cells (eg, myofibroblasts, microvascular endothelial cells, inflammatory cells) by pharmacological agents or direct gene delivery/small interfering RNA would be potential novel therapeutic strategies to prevent/attenuate the progression of cardiovascular disorders.

Hydroxyoctadecadienoic acids: novel regulators of macrophage differentiation and atherogenesis

Hydroxyoctadecadienoic acids (HODEs) are stable oxidation products of linoleic acid, the generation of which is increased where oxidative stress is increased, such as in diabetes. In early atherosclerosis, 13-HODE is generated in macrophages by 15-lipoxygenase-1. This enhances protective mechanisms through peroxisome proliferator-activated receptor (PPAR)-g activation leading to increased clearance of lipid and lipid-laden cells from the arterial wall. In later atherosclerosis, both 9-HODE and 13-HODE are generated nonenzymatically. At this stage, early protective mechanisms are overwhelmed and pro-inflammatory effects of 9-HODE, acting through the receptor GPR132, and increased apoptosis predominate leading to a fragile, acellular plaque. Increased HODE levels thus contribute to atherosclerosis progression and the risk of clinical events such as myocardial infarction or stroke. Better understanding of the role of HODEs may lead to new pharmacologic approaches to modulate their production or action, and therefore lessen the burden of atherosclerotic disease in high-risk patients.

Pioglitazone and mechanisms of CV protection

BACKGROUND AND AIM

Pioglitazone has diverse multiple effects on metabolic and inflammatory processes that have the potential to influence cardiovascular disease pathophysiology at various points in the disease process, including atherogenesis, plaque inflammation, plaque rupture, haemostatic disturbances and microangiopathy.

RESULTS

Linking the many direct and indirect effects on the vasculature to the reduction in key macrovascular outcomes reported with pioglitazone in patients with type 2 diabetes presents a considerable challenge. However, recent large-scale clinical cardiovascular imaging studies are beginning to provide some mechanistic insights, including a potentially important role for improvements in high-density lipoprotein cholesterol with pioglitazone. In addition to a role in prevention, animal studies also suggest that pioglitazone may minimize damage and improve recovery during and after ischaemic cardio- and cerebrovascular events.

DESIGN AND METHODS

In this review, we consider potential cardiovascular protective mechanisms of pioglitazone by linking preclinical data and clinical cardiovascular outcomes guided by insights from recent imaging studies.

CONCLUSION

Pioglitazone may influence CVD pathophysiology at multiple points in the disease process, including atherogenesis, plaque inflammation, plaque rupture and haemostatic disturbances (i.e. thrombus/embolism formation), as well as microangiopathy.

Adipose tissue macrophages: their role in adipose tissue remodeling

The adipose tissue secretes a large number of bioactive substances, adipocytokines, which may be involved in a variety of physiologic and pathologic processes. Unbalanced production of pro- and anti-inflammatory adipocytokines seen in visceral fat obesity contributes critically to the development of the metabolic syndrome. Evidence has accumulated indicating that obesity is associated with a state of chronic, low-grade inflammation, suggesting that inflammation may be a potential mechanism, whereby obesity leads to insulin resistance. Indeed, obese adipose tissue is characterized by adipocyte hypertrophy, followed by increased angiogenesis, immune cell infiltration, extracellular matrix overproduction, and thus, increased production of proinflammatory adipocytokines during the progression of chronic inflammation. The dynamic change found in the adipose tissue can be referred to as "adipose tissue remodeling," in which stromal cells change dramatically in number and cell type during the course of obesity. Among stromal cells, infiltration of macrophages in the adipose tissue precedes the development of insulin resistance in animal models, suggesting that they are crucial for obesity-related adipose tissue inflammation. We have demonstrated that a paracrine loop involving saturated fatty acids and TNF-alpha derived from adipocytes and macrophages, respectively, aggravates obesity-induced adipose tissue inflammation. Notably, saturated fatty acids, which are released from hypertrophied adipocytes via the macrophage-induced lipolysis, serve as a naturally occurring ligand for TLR4 complex, thereby activating macrophages. Understanding the molecular mechanism underlying adipose tissue remodeling may lead to the identification of novel, therapeutic strategies to prevent or treat obesity-induced adipose tissue inflammation.

CXC chemokine ligand 4 induces a unique transcriptome in monocyte-derived macrophages

In atherosclerotic arteries, blood monocytes differentiate to macrophages in the presence of growth factors, such as macrophage colony-stimulation factor (M-CSF), and chemokines, such as platelet factor 4 (CXCL4). To compare the gene expression signature of CXCL4-induced macrophages with M-CSF-induced macrophages or macrophages polarized with IFN-gamma/LPS (M1) or IL-4 (M2), we cultured primary human peripheral blood monocytes for 6 d. mRNA expression was measured by Affymetrix gene chips, and differences were analyzed by local pooled error test, profile of complex functionality, and gene set enrichment analysis. Three hundred seventy-five genes were differentially expressed between M-CSF- and CXCL4-induced macrophages; 206 of them overexpressed in CXCL4 macrophages coding for genes implicated in the inflammatory/immune response, Ag processing and presentation, and lipid metabolism. CXCL4-induced macrophages overexpressed some M1 and M2 genes and the corresponding cytokines at the protein level; however, their transcriptome clustered with neither M1 nor M2 transcriptomes. They almost completely lost the ability to phagocytose zymosan beads. Genes linked to atherosclerosis were not consistently upregulated or downregulated. Scavenger receptors showed lower and cholesterol efflux transporters showed higher expression in CXCL4- than M-CSF-induced macrophages, resulting in lower low-density lipoprotein content. We conclude that CXCL4 induces a unique macrophage transcriptome distinct from known macrophage types, defining a new macrophage differentiation that we propose to call M4.

Abscisic acid synergizes with rosiglitazone to improve glucose tolerance and down-modulate macrophage accumulation in adipose tissue: possible action of the cAMP/PKA/PPAR γ axis

BACKGROUND & AIMS

Abscisic acid (ABA) is effective in preventing insulin resistance and obesity-related inflammation through a PPAR γ-dependent mechanism. The objective of this study was to assess the efficacy ABA in improving glucose homeostasis and suppress inflammation when administered in combination with rosiglitazone (Ros) and to determine whether PPAR γ activation by ABA is initiated via cAMP/protein kinase A (PKA) signaling.

METHODS

Obese db/db mice were fed high-fat diets containing 0, 10, or 70 mg/kg Ros with and without racemic ABA (100 mg/kg) for 60 days. Glucose tolerance and fasting insulin levels were assessed at 6 and 8 weeks, respectively, and adipose tissue macrophage (ATM) infiltration was examined by flow cytometry. Gene expression was examined on white adipose tissue (WAT) and stromal vascular cells (SVCs) cultured with ABA, Ros, or an ABA/Ros combination.

RESULTS

Both Ros and ABA improved glucose tolerance, and ABA decreased plasma insulin levels while having no effect on Ros-induced weight gain. ABA in combination with low-dose Ros (10 mg/kg; Roslo) synergistically inhibited ATM infiltration. Treatment of SVCs with Ros, ABA or ABA/Ros suppressed expression of the M1 marker CCL17. ABA and Ros synergistically increased PPAR γ activity and pretreatment with a cAMP-inhibitor or a PKA-inhibitor abrogated ABA-induced PPAR γ activation.

CONCLUSIONS

ABA and Ros act synergistically to modulate PPAR γ activity and macrophage accumulation in WAT and ABA enhances PPAR γ activity through a membrane-initiated mechanism dependent on cAMP/PKA signaling.

Regular tart cherry intake alters abdominal adiposity, adipose gene transcription, and inflammation in obesity-prone rats fed a high fat diet

Obesity, systemic inflammation, and hyperlipidemia are among the components of metabolic syndrome, a spectrum of phenotypes that can precede the development of type 2 diabetes and cardiovascular disease. Animal studies show that intake of anthocyanin-rich extracts can affect these phenotypes. Anthocyanins can alter the activity of tissue peroxisome proliferator-activated receptors (PPARs), which affect energy substrate metabolism and inflammation. However, it is unknown if physiologically relevant, anthocyanin-containing whole foods confer similar effects to concentrated, anthocyanin extracts. The effect of anthocyanin-rich tart cherries was tested in the Zucker fatty rat model of obesity and metabolic syndrome. For 90 days, rats were pair-fed a higher fat diet supplemented with either 1% (wt/wt) freeze-dried, whole tart cherry powder or with a calorie- and macronutrient-matched control diet. Tart cherry intake was associated with reduced hyperlipidemia, percentage fat mass, abdominal fat (retroperitoneal) weight, retroperitoneal interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) expression, and plasma IL-6 and TNF-alpha. Tart cherry diet also increased retroperitoneal fat PPAR-alpha and PPAR-gamma mRNA (P = .12), decreased IL-6 and TNF-alpha mRNA, and decreased nuclear factor kappaB activity. In conclusion, in at-risk obese rats fed a high fat diet, physiologically relevant tart cherry consumption reduced several phenotypes of metabolic syndrome and reduced both systemic and local inflammation. Tart cherries may reduce the degree or trajectory of metabolic syndrome, thereby reducing risk for the development of type 2 diabetes and heart disease.

Cell-specific determinants of peroxisome proliferator-activated receptor gamma function in adipocytes and macrophages

The nuclear receptor peroxisome proliferator activator receptor gamma (PPARgamma) is the target of antidiabetic thiazolidinedione drugs, which improve insulin resistance but have side effects that limit widespread use. PPARgamma is required for adipocyte differentiation, but it is also expressed in other cell types, notably macrophages, where it influences atherosclerosis, insulin resistance, and inflammation. A central question is whether PPARgamma binding in macrophages occurs at genomic locations the same as or different from those in adipocytes. Here, utilizing chromatin immunoprecipitation and high-throughput sequencing (ChIP-seq), we demonstrate that PPARgamma cistromes in mouse adipocytes and macrophages are predominantly cell type specific. In thioglycolate-elicited macrophages, PPARgamma colocalizes with the hematopoietic transcription factor PU.1 in areas of open chromatin and histone acetylation, near a distinct set of immune genes in addition to a number of metabolic genes shared with adipocytes. In adipocytes, the macrophage-unique binding regions are marked with repressive histone modifications, typically associated with local chromatin compaction and gene silencing. PPARgamma, when introduced into preadipocytes, bound only to regions depleted of repressive histone modifications, where it increased DNA accessibility, enhanced histone acetylation, and induced gene expression. Thus, the cell specificity of PPARgamma function is regulated by cell-specific transcription factors, chromatin accessibility, and histone marks. Our data support the existence of an epigenomic hierarchy in which PPARgamma binding to cell-specific sites not marked by repressive marks opens chromatin and leads to local activation marks, including histone acetylation.

Immunomodulatory activity of andrographolide on macrophage activation and specific antibody response

AIM

To investigate the immunomodulatory effects of andrographolide on both innate and adaptive immune responses.

METHODS

Andrographolide (10 microg/mL in vitro or 1 mg/kg in vivo) was used to modulate LPS-induced classical activated (M1) or IL-4-induced alternative activated (M2) macrophages in vitro and humor immune response to HBsAg in vivo. Cytokine gene expression profile (M1 vs M2) was measured by real-time PCR, IL-12/IL-10 level was detected by ELISA, and surface antigen expression was evaluated by flow cytometry, whereas phosphorylation level of ERK 1/2 and AKT was determined by Western blot. The level of anti-HBs antibodies in HBsAg immunized mice was detected by ELISA, and the number of HBsAg specific IL-4-producing splenocyte was enumerated by ELISPOT.

RESULTS

Andrographolide treatment in vitro attenuated either LPS or IL-4 induced macrophage activation, inhibited both M1 and M2 cytokines expression and decreased IL-12/IL-10 ratio (the ratio of M1/M2 polarization). Andrographolide down-regulated the expression of mannose receptor (CD206) in IL-4 induced macrophages and major histocompability complex/costimulatory molecules (MHC I, CD40, CD80, CD86) in LPS-induced macrophages. Correspondingly, anti-HBs antibody production and the number of IL-4-producing splenocytes were reduced by in vivo administration of andrographolide. Reduced phosphorylation levels of ERK1/2 and AKT were observed in macrophages treated with andrographolide.

CONCLUSION

Andrographolide can modulate the innate and adaptive immune responses by regulating macrophage phenotypic polarization and Ag-specific antibody production. MAPK and PI3K signaling pathways may participate in the mechanisms of andrographolide regulating macrophage activation and polarization.

Macrophage plasticity in experimental atherosclerosis

As in human disease, macrophages (MØ) are central players in the development and progression of experimental atherosclerosis. In this study we have evaluated the phenotype of MØ associated with progression of atherosclerosis in the apolipoprotein E (ApoE) knockout (KO) mouse model.

We found that bone marrow-derived MØ submitted to M1 and M2 polarization specifically expressed arginase (Arg) II and Arg I, respectively. This distinct arginase expression was used to evaluate the frequency and distribution of M1 and M2 MØ in cross-sections of atherosclerotic plaques of ApoE KO mice. Early lesions were infiltrated by Arg I⁺ (M2) MØ. This type of MØ favored the proliferation of smooth muscle cells, in vitro. Arg II⁺ (M1) MØ appeared and prevailed in lesions of aged ApoE KO mice and lesion progression was correlated with the dominance of M1 over the M2 MØ phenotype. In order to address whether the M2->M1 switch could be due to a phenotypic switch of the infiltrated cells, we performed in vitro repolarization experiments. We found that fully polarized MØ retained their plasticity since they could revert their phenotype. The analysis of the distribution of Arg I- and Arg II-expressing MØ also argued against a recent recruitment of M1 MØ in the lesion. The combined data therefore suggest that the M2->M1 switch observed in vivo is due to a conversion of cells already present in the lesion. Our study suggests that interventional tools able to revert the MØ infiltrate towards the M2 phenotype may exert an atheroprotective action.

Effects of cold stress on the messenger ribonucleic acid levels of peroxisome proliferator-activated receptor-{gamma} in spleen, thymus, and bursa of Fabricius of chickens

This study was to investigate the expression trait of the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) gene and the effect of cold stress on the mRNA levels of PPAR-gamma in spleen, thymus, and bursa of Fabricius of chickens. Eighty-four 1-d-old male chickens were randomly allocated to 12 groups (7 chickens per group). There was 1 control group and 5 treatment groups for acute cold stress and 3 control groups and 3 treatment groups for chronic cold stress. Chickens were maintained in our animal facility, kept under a 16L:8D cycle and temperature (30 +/- 2 degrees C), and given free access to standard chow and water. The cold stress was initiated when the birds were 15 d of age, with the duration of the acute cold stress being 1, 3, 6, 12, and 24 h, and the chronic cold stress was 5, 10, and 20 d, respectively. Cold stress temperature was 12 +/- 1 degrees C. Spleen, thymus, and bursa of Fabricius were collected for the assessment of the mRNA levels by real-time PCR after stress termination. The results showed that the PPAR-gamma gene is expressed in spleen, thymus, and bursa of Fabricius, and its expression level is different in different tissues and at different ages. Acute cold stress significantly decreased (P < 0.05) the mRNA levels of the PPAR-gamma gene of spleen and thymus in all treatment groups and significantly increased (P < 0.05) the mRNA levels of the PPAR-gamma gene of bursa of Fabricius in all treatment groups. Compared with the corresponding control groups, chronic cold stress resulted in a significant increase (P < 0.05) of the mRNA levels of the PPAR-gamma gene in spleen and a significant decrease (P < 0.05) of the mRNA levels of the PPAR-gamma gene in thymus and bursa of Fabricius. The results indicate that the PPAR-gamma gene is expressed in all 3 immune organs and has different expression traits. The magnitude and direction of change in PPAR-gamma gene expression differs with the type of cold stress applied and also varies by tissue.

ANG II receptor blockade enhances anti-inflammatory macrophages in anti-glomerular basement membrane glomerulonephritis

Macrophages are heterogeneous immune cell populations that include classically activated and alternatively activated (M2) macrophages. We examined the anti-inflammatory effect of ANG II type 1 receptor (AT(1)R) blocker (ARB) on glomerular inflammation in a rat model of anti-glomerular basement membrane (GBM) glomerulonephritis (GN). The study focused on infiltrating CD8(+) and CD4(+) cells and macrophages, as well as the heterogeneity of intraglomerular macrophages. Wistar-Kyoto rats were treated with high-dose olmesartan (3 mg.kg(-1).day(-1)), low-dose olmesartan (0.3 mg.kg(-1).day(-1)), or vehicle (control) 7 days before induction of anti-GBM GN. Control rats showed mainly CD8(+) cells and ED1(+) macrophages, with a few CD4(+) cells infiltrating the glomeruli. Necrotizing and crescentic glomerular lesions developed by day 7 with the increase of proteinuria. AT(1)R was expressed on CD8(+) and CD4(+) cells and on ED1(+) macrophages. Low-dose ARB had no anti-inflammatory effects in anti-GBM GN. However, high-dose ARB reduced glomerular infiltration of CD8(+) cells and ED1(+) macrophages and suppressed necrotizing and crescentic lesions by days 5 to 7 (P < 0.05). In addition, high-dose ARB reduced the numbers of ED3(+)-activated macrophages, suppressed glomerular TNF-alpha and IFN-gamma production, and downregulated M1-related chemokine and cytokines (monocyte chemoattractant protein type 1, IL-6, and IL-12). High-dose ARB also enhanced ED2(+) M2 macrophages by day 7 with upregulation of glomerular IL-4 and IL-13 and augmented CCL17, IL-1 receptor antagonist, and IL-10. We concluded that high-dose ARB inhibits glomerular inflammation by increasing the numbers of M2 macrophages and upregulation of anti-inflammatory cytokines and by suppressing M1 macrophage development with downregulation of M1-related proinflammatory cytokines.