Alternative M2 activation of Kupffer cells by PPARdelta ameliorates obesity-induced insulin resistance

The C3a anaphylatoxin receptor is a key mediator of insulin resistance and functions by modulating adipose tissue macrophage infiltration and activation

OBJECTIVE

Significant new data suggest that metabolic disorders such as diabetes, obesity, and atherosclerosis all posses an important inflammatory component. Infiltrating macrophages contribute to both tissue-specific and systemic inflammation, which promotes insulin resistance. The complement cascade is involved in the inflammatory cascade initiated by the innate and adaptive immune response. A mouse genomic F2 cross biology was performed and identified several causal genes linked to type 2 diabetes, including the complement pathway.

RESEARCH DESIGN AND METHODS

We therefore sought to investigate the effect of a C3a receptor (C3aR) deletion on insulin resistance, obesity, and macrophage function utilizing both the normal-diet (ND) and a diet-induced obesity mouse model.

RESULTS

We demonstrate that high C3aR expression is found in white adipose tissue and increases upon high-fat diet (HFD) feeding. Both adipocytes and macrophages within the white adipose tissue express significant amounts of C3aR. C3aR(-/-) mice on HFD are transiently resistant to diet-induced obesity during an 8-week period. Metabolic profiling suggests that they are also protected from HFD-induced insulin resistance and liver steatosis. C3aR(-/-) mice had improved insulin sensitivity on both ND and HFD as seen by an insulin tolerance test and an oral glucose tolerance test. Adipose tissue analysis revealed a striking decrease in macrophage infiltration with a concomitant reduction in both tissue and plasma proinflammatory cytokine production. Furthermore, C3aR(-/-) macrophages polarized to the M1 phenotype showed a considerable decrease in proinflammatory mediators.

CONCLUSIONS

Overall, our results suggest that the C3aR in macrophages, and potentially adipocytes, plays an important role in adipose tissue homeostasis and insulin resistance.

Loss of Kupffer cells in diet-induced obesity is associated with increased hepatic steatosis, STAT3 signaling, and further decreases in insulin signaling

While adipose tissue-associated macrophages contribute to development of chronic inflammation and insulin resistance of obesity, little is known about the role of hepatic Kupffer cells in this environment. Here we address the impact of Kupffer cell ablation using clodronate-encapsulated liposome depletion in a diet-induced obese (DIO) and insulin resistant mouse model. Hepatic expression of macrophage markers measured by realtime RT-PCR remained unaltered in DIO mice despite characteristic expansion of adipose tissue-associated macrophages. DIO mouse livers displayed increased expression of alternative activation markers but unaltered proinflammatory cytokine expression when compared to lean mice. Kupffer cell ablation reduced hepatic anti-inflammatory cytokine IL-10 mRNA expression in lean and DIO mice by 95% and 84%, respectively. Despite decreased hepatic IL-6 gene expression after ablation in lean and DIO mice, hepatic STAT3 phosphorylation, Socs3 and acute phase protein mRNA expression increased. Kupffer cell ablation in DIO mice resulted in additional hepatic triglyceride accumulation and a 30-40% reduction in hepatic insulin receptor autophosphorylation and Akt activation. Implicating systemic loss of IL-10, high-fat-fed IL-10 knockout mice also displayed increased hepatic STAT3 signaling and hepatic triglyceride accumulation. Insulin signaling was not altered, however. In conclusion, Kupffer cells are a major source of hepatic IL-10 expression, the loss of which is associated with increased STAT3-dependent signaling and steatosis. One or more additional factors appear to be required, however, for the Kupffer cell-dependent protective effect on insulin receptor signaling in DIO mice.

Retinoid X receptor heterodimer variants and cardiovascular risk factors

Nuclear receptors are transcription factors that can be activated by specific ligands. Recent progress has shown that retinoid X receptor (RXR) and its heterodimerization partners, including peroxisome proliferator-activated receptors, regulate many important genes involved in energy homeostasis and atherosclerosis, and should be promising therapeutic targets of metabolic syndrome. RXR heterodimers regulate a number of complex cellular processes, and genetic studies of RXR heterodimers have provided important clinical information in addition to knowledge gained from basic research. Genetic variants of RXR heterodimers were screened and investigated, and some variants were shown to have a considerable impact on metabolic disorders, including phenotypic components of familial combined hyperlipidemia. The combined efforts of basic and clinical science regarding nuclear receptors have achieved significant progress in unraveling the inextricably linked control system of energy expenditure, lipid and glucose homeostasis, inflammation, and atherosclerosis.This review summarizes the current understanding regarding RXR heterodimers based on their human genetic variants, which will provide new clues to uncover the background of multifactorial disease, such as metabolic syndrome or familial combined hyperlipidemia.

PPARs: the vasculature, inflammation and hypertension

PURPOSE OF REVIEW

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear receptors activated by nutrient molecules and their derivatives. Their role has been increasingly recognized to be important in hypertension, metabolic disorders and cardiovascular disease, including atherosclerosis. Control of innate inflammatory processes mostly through alteration of monocyte/macrophage phenotype promises to be a unifying paradigm in understanding the pleiotropic effects of PPAR agonists.

RECENT FINDINGS

Although PPAR-gamma was the first to be described as an anti-inflammatory agent, both PPAR-alpha and PPAR-delta are now known to have similar effects as well. Inflammation is an important part of the damage caused by hypertensive diseases. PPARs have now been recognized as important determinants of macrophage polarization. Monocyte precursors of classical and alternatively activated macrophages are being defined as important changes in progression of cardiovascular disease associated with metabolic syndrome including hypertension, hyperlipidemia and obesity.

SUMMARY

A major unifying role for PPARs in hypertension and its complications through modification of the innate immune system and inflammation is increasingly likely. PPAR agonists may be beneficial, alone or in combination with other drugs that modify the inflammatory response, in treating hypertension, atherosclerosis and metabolic derangements associated with obesity.

Kupffer cells in non-alcoholic fatty liver disease: the emerging view

Non-alcoholic fatty liver disease (NAFLD) has become the most common liver disorder of our times. Simple steatosis, a seemingly innocent manifestation of NAFLD, may progress into steatohepatitis and cirrhosis, but this process is not well understood. Since NAFLD is associated with obesity and insulin resistance, mechanisms that link lipid metabolism to inflammation offer insights into the pathogenesis. An important parallel between obesity-related pathology of adipose tissue and liver pertains to the emerging role of macrophages and evidence is growing that Kupffer cells critically contribute to progression of NAFLD. Toll-like receptors, in particular TLR4, represent a major conduit for danger recognition linked to Kupffer cell activation and this process may be perturbed at multiple steps in NAFLD. Steatosis may interfere with sinusoid microcirculation and hepatocellular clearance of microbial and host-derived danger signals, enhancing responsiveness of Kupffer cells. Altered lipid homeostasis in NAFLD may unfavourably affect TLR4 receptor complex assembly and sorting, interfere with signalling flux redistribution, promote amplification loops, and impair negative regulation including alternative activation of Kupffer cells. These events are further promoted by altered adipokine secretion and reactive oxygen species production. Specific targeting of these interactions may provide more effective strategies in the treatment of NAFLD.

Unlike PPARgamma, PPARalpha or PPARbeta/delta activation does not promote human monocyte differentiation toward alternative macrophages

Macrophages adapt their response to micro-environmental signals. While Th1 cytokines promote pro-inflammatory M1 macrophages, Th2 cytokines promote an "alternative" anti-inflammatory M2 macrophage phenotype. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors expressed in macrophages where they control the inflammatory response. It has been shown that PPARgamma promotes the differentiation of monocytes into anti-inflammatory M2 macrophages in humans and mice, while a role for PPARbeta/delta in this process has been reported only in mice and no data are available for PPARalpha. Here, we show that in contrast to PPARgamma, expression of PPARalpha and PPARbeta/delta overall does not correlate with the expression of M2 markers in human atherosclerotic lesions, whereas a positive correlation with genes of lipid metabolism exists. Moreover, unlike PPARgamma, PPARalpha or PPARbeta/delta activation does not influence human monocyte differentiation into M2 macrophages in vitro. Thus, PPARalpha and PPARbeta/delta do not appear to modulate the alternative differentiation of human macrophages.

Muscle inflammatory response and insulin resistance: synergistic interaction between macrophages and fatty acids leads to impaired insulin action

Obesity is characterized by adipose tissue expansion as well as macrophage infiltration of adipose tissue. This results in an increase in circulating inflammatory cytokines and nonesterified fatty acids, factors that cause skeletal muscle insulin resistance. Whether obesity also results in skeletal muscle inflammation is not known. In this study, we quantified macrophages immunohistochemically in vastus lateralis biopsies from eight obese and eight lean subjects. Our study demonstrates that macrophages infiltrate skeletal muscle in obesity, and we developed an in vitro system to study this mechanistically. Myoblasts were isolated from vastus lateralis biopsies and differentiated in culture. Coculture of differentiated human myotubes with macrophages in the presence of palmitic acid, to mimic an obese environment, revealed that macrophages in the presence of palmitic acid synergistically augment cytokine and chemokine expression in myotubes, decrease IkappaB-alpha protein expression, increase phosphorylated JNK, decrease phosphorylated Akt, and increase markers of muscle atrophy. These results suggest that macrophages alter the inflammatory state of muscle cells in an obese milieu, inhibiting insulin signaling. Thus in obesity both adipose tissue and skeletal muscle inflammation may contribute to insulin resistance.

PPARs and the cardiovascular system

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone-receptor superfamily. Originally cloned in 1990, PPARs were found to be mediators of pharmacologic agents that induce hepatocyte peroxisome proliferation. PPARs also are expressed in cells of the cardiovascular system. PPAR gamma appears to be highly expressed during atherosclerotic lesion formation, suggesting that increased PPAR gamma expression may be a vascular compensatory response. Also, ligand-activated PPAR gamma decreases the inflammatory response in cardiovascular cells, particularly in endothelial cells. PPAR alpha, similar to PPAR gamma, also has pleiotropic effects in the cardiovascular system, including antiinflammatory and antiatherosclerotic properties. PPAR alpha activation inhibits vascular smooth muscle proinflammatory responses, attenuating the development of atherosclerosis. However, PPAR delta overexpression may lead to elevated macrophage inflammation and atherosclerosis. Conversely, PPAR delta ligands are shown to attenuate the pathogenesis of atherosclerosis by improving endothelial cell proliferation and survival while decreasing endothelial cell inflammation and vascular smooth muscle cell proliferation. Furthermore, the administration of PPAR ligands in the form of TZDs and fibrates has been disappointing in terms of markedly reducing cardiovascular events in the clinical setting. Therefore, a better understanding of PPAR-dependent and -independent signaling will provide the foundation for future research on the role of PPARs in human cardiovascular biology.

Significance of peroxisome proliferator-activated receptors in the cardiovascular system in health and disease

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear transcription factors that belong to the nuclear receptor superfamily. Three isoforms of PPAR have been identified, alpha, delta and gamma, which play distinct roles in the regulation of key metabolic processes, such as glucose and lipid redistribution. PPARalpha is expressed predominantly in the liver, kidney and heart, and is primarily involved in fatty acid oxidation. PPARgamma is mainly associated with adipose tissue, where it controls adipocyte differentiation and insulin sensitivity. PPARdelta is abundantly and ubiquitously expressed, but as yet its function has not been clearly defined. Activators of PPARalpha (fibrates) and gamma (thiazolidinediones) have been used clinically for a number of years in the treatment of hyperlipidaemia and to improve insulin sensitivity in diabetes. More recently, PPAR activation has been found to confer additional benefits on endothelial function, inflammation and thrombosis, suggesting that PPAR agonists may be good candidates for the treatment of cardiovascular disease. In this regard, it has been demonstrated that PPAR activators are capable of reducing blood pressure and attenuating the development of atherosclerosis and cardiac hypertrophy. This review will provide a detailed discussion of the current understanding of basic PPAR physiology, with particular reference to the cardiovascular system. It will also examine the evidence supporting the involvement of the different PPAR isoforms in cardiovascular disease and discuss the current and potential future clinical applications of PPAR activators.

Activating transcription factor 3 constitutes a negative feedback mechanism that attenuates saturated Fatty acid/toll-like receptor 4 signaling and macrophage activation in obese adipose tissue

Obese adipose tissue is markedly infiltrated by macrophages, suggesting that they may participate in the inflammatory pathways that are activated in obese adipose tissue. Evidence has suggested that saturated fatty acids released via adipocyte lipolysis serve as a naturally occurring ligand that stimulates Toll-like receptor (TLR)4 signaling, thereby inducing the inflammatory responses in macrophages in obese adipose tissue. Through a combination of cDNA microarray analyses of saturated fatty acid-stimulated macrophages in vitro and obese adipose tissue in vivo, here we identified activating transcription factor (ATF)3, a member of the ATF/cAMP response element-binding protein family of basic leucine zipper-type transcription factors, as a target gene of saturated fatty acids/TLR4 signaling in macrophages in obese adipose tissue. Importantly, ATF3, when induced by saturated fatty acids, can transcriptionally repress tumor necrosis factor-alpha production in macrophages in vitro. Chromatin immunoprecipitation assay revealed that ATF3 is recruited to the region containing the activator protein-1 site of the endogenous tumor necrosis factor-alpha promoter. Furthermore, transgenic overexpression of ATF3 specifically in macrophages results in the marked attenuation of proinflammatory M1 macrophage activation in the adipose tissue from genetically obese KKA(y) mice fed high-fat diet. This study provides evidence that ATF3, which is induced in obese adipose tissue, acts as a transcriptional repressor of saturated fatty acids/TLR4 signaling, thereby revealing the negative feedback mechanism that attenuates obesity-induced macrophage activation. Our data also suggest that activation of ATF3 in macrophages offers a novel therapeutic strategy to prevent or treat obesity-induced adipose tissue inflammation.

Alternative activation of macrophages: an immunologic functional perspective

Macrophages are innate immune cells with well-established roles in the primary response to pathogens, but also in tissue homeostasis, coordination of the adaptive immune response, inflammation, resolution, and repair. These cells recognize danger signals through receptors capable of inducing specialized activation programs. The classically known macrophage activation is induced by IFN-gamma, which triggers a harsh proinflammatory response that is required to kill intracellular pathogens. Macrophages also undergo alternative activation by IL-4 and IL-13, which trigger a different phenotype that is important for the immune response to parasites. Here we review the cellular sources of these cytokines, receptor signaling pathways, and induced markers and gene signatures. We draw attention to discrepancies found between mouse and human models of alternative activation. The evidence for in vivo alternative activation of macrophages is also analyzed, with nematode infection as prototypic disease. Finally, we revisit the concept of macrophage activation in the context of the immune response.

Critical role of Kupffer cells in the management of diet-induced diabetes and obesity

The aim of this study was to investigate the role of Kupffer cell in glucose metabolism and hepatic insulin sensitivity in mice. Both phagocytic activity and secretory capacity of Kupffer cells were blunted 24h after GdCl3 administration. Glucose tolerance--evaluated following an oral glucose tolerance test (OGTT)--was higher in GdCl3-treated mice whereas fasting insulinemia and HOMA-IR index decreased. The improvement of glucose tolerance and hepatic insulin signalling pathway after inhibition of Kupffer cells was supported by a lower hepatic gluconeogenic enzyme expression and a higher phosphorylation of Akt upon insulin challenge. Moreover, fasting hyperglycemia, insulin resistance and impaired glucose tolerance--induced by high fat (HF) diet--were improved through chronic administration of GdCl3. Interestingly, the inhibition of Kupffer cell exerted antiobesity effects in HF-fed mice, and lowered hepatic steatosis. Therefore, strategies targeting Kupffer cell functions could be a promising approach to counteract obesity and related metabolic disorders.

PPAR gamma is highly expressed in F4/80(hi) adipose tissue macrophages and dampens adipose-tissue inflammation

Macrophage infiltration into adipose tissue is a hallmark of obesity. We recently reported two phenotypically distinct subsets of adipose tissue macrophages (ATM) based on the surface expression of the glycoprotein F4/80 and responsiveness to treatment with a peroxisome proliferator-activated receptor (PPAR) gamma agonist. Hence, we hypothesized that F4/80(hi) and F4/80(lo) ATM differentially express PPAR gamma. This study phenotypically and functionally characterizes F4/80(hi) and F4/80(lo) ATM subsets during obesity. Changes in gene expression were also examined on sorted F4/80(lo) and F4/80(hi) ATM by quantitative real-time RT-PCR. We show that while F4/80(lo) macrophages predominate in adipose tissue of lean mice, obesity causes accumulation of both F4/80(lo) and F4/80(hi) ATM. Moreover, accumulation of F4/80(hi) ATM in adipose tissue is associated with impaired glucose tolerance. Phenotypically, F4/80(hi) ATM express greater amounts of CD11c, MHC II, CD49b, and CX3CR1 and produce more TNF-alpha, MCP-1, and IL-10 than F4/80(lo) ATM. Gene expression analyses of the sorted populations revealed that only the F4/80(lo) population produced IL-4, whereas the F4/80(hi) ATM expressed greater amounts of PPAR gamma, delta, CD36 and toll-like receptor-4. In addition, the deficiency of PPAR gamma in immune cells favors expression of M1 and impairs M2 macrophage marker expression in adipose tissue. Thus, PPAR gamma is differentially expressed in F4/80(hi) versus F4/80(low) ATM subsets and its deficiency favors a predominance of M1 markers in WAT.

Mechanisms and consequences of efferocytosis in advanced atherosclerosis

Throughout atherosclerotic lesion development, intimal macrophages undergo apoptosis, a form of death that usually prevents cellular necrosis. In advanced atherosclerotic lesions, however, these apoptotic macrophages become secondarily necrotic and coalesce over time into a key feature of vulnerable plaques, the necrotic core. This event is critically important, as necrotic core formation in these advanced atheromata is thought to promote plaque disruption and ultimately, acute atherothrombotic vascular disease. Increasing evidence suggests that the mechanism behind postapoptotic macrophage necrosis in advanced atherosclerosis is defective phagocytic clearance or "efferocytosis" of the apoptotic cells. Thus, understanding the cellular and molecular mechanisms of efferocytosis in atherosclerosis and why efferocytosis becomes defective in advanced lesions is an important goal. Molecular-genetic causation studies in mouse models of advanced atherosclerosis have provided evidence that several molecules known to be involved in efferocytosis, including TG2, MFG-E8, complement C1q, Mertk, lysoPC, and Fas, play important roles in the clearance of apoptotic cells in advanced plaques. These and future insights into the molecular mechanisms of defective efferocytosis in advanced atheromata may open the way for novel therapeutic strategies for atherothrombotic vascular disease, the leading cause of death in the industrialized world.

Preserved glucose tolerance in high-fat-fed C57BL/6 mice transplanted with PPARgamma-/-, PPARdelta-/-, PPARgammadelta-/-, or LXRalphabeta-/- bone marrow

Macrophage lipid metabolism and inflammatory responses are both regulated by the nuclear receptors PPAR and LXR. Emerging links between inflammation and metabolic disease progression suggest that PPAR and LXR signaling may alter macrophage function and thereby impact systemic metabolism. In this study, the function of macrophage PPAR and LXR in Th1-biased C57BL/6 mice was tested using a bone marrow transplantation approach with PPARgamma(-/-), PPARdelta(-/-), PPARgammadelta(-/-), and LXRalphabeta(-/-) cells. Despite their inhibitory effects on inflammatory gene expression, loss of PPARs or LXRs in macrophages did not exert major effects on obesity or glucose tolerance induced by a high-fat diet. Treatment with rosiglitazone effectively improved glucose tolerance in mice lacking macrophage PPARgamma, suggesting that cell types other than macrophages are the primary mediators of the anti-diabetic effects of PPARgamma agonists in our model system. C57BL/6 macrophages lacking PPARs or LXRs exhibited normal expression of most alternative activation gene markers, indicating that macrophage alternative activation is not absolutely dependent on these receptors in the C57BL/6 background under the conditions used here. These studies suggest that genetic background may be an important modifier of nuclear receptor effects in macrophages. Our results do not exclude a contribution of macrophage PPAR and LXR expression to systemic metabolism in certain contexts, but these factors do not appear to be dominant contributors to glucose tolerance in a high-fat-fed Th1-biased bone marrow transplant model.

Impact of increased adipose tissue mass on inflammation, insulin resistance, and dyslipidemia

Obesity is associated with increased prevalence of metabolic disorders, such as inflammation, insulin resistance, and dyslipidemia, which can predispose an individual to develop diabetes and cardiovascular disease. Adipose tissue (AT) is now recognized as a metabolically active organ that controls plasma free fatty acid levels and contributes to systemic metabolic homeostasis by secreting adipokines. In obesity, the recruitment of immune cells, such as T cells and macrophages, to AT causes inflammation, which is thought to contribute to local insulin resistance. This loss of insulin sensitivity within AT can lead to uncontrolled release of fatty acids, secretion of inflammatory cytokines, and alterations in the balance of adipokines, which ultimately impact lipoprotein metabolism and insulin sensitivity systemically. Thus, AT itself plays an important role in the increased risk of diabetes and cardiovascular disease that is associated with obesity.

Dietary interventions for metabolic syndrome: role of modifying dietary fats

In humans, insulin sensitivity is relatively impaired by diets that are low in oleic acid (OA), a cis monounsaturated fatty acid (MUFA), or rich in trans MUFA or palmitic acid (PA), a saturated fatty acid (FA). Emerging evidence exists that PA, in contrast to OA, causes insulin resistance via stimulation of inflammatory signaling and production of cytosolic lipid compounds (diacylglycerol and ceramide), leading one to presume that dietary or pharmacologic maneuvers that facilitate transport of FA into the mitochondria would be beneficial. However, in some models, insulin resistance is caused by excessive FA transport into the mitochondria, coupled with deficient electron transport and possibly increased reactive oxygen species formation; PA may impair electron transport via effects on gene expression. A research challenge is to determine whether feeding humans diets with markedly different contents of PA and OA would alter insulin sensitivity and/or critical biochemical mechanisms impacting muscle insulin signaling.

Alternative macrophage activation and the regulation of metabolism

Macrophages are white blood cells that have important roles in phagocytosis and immune responses. A series of recent papers reveals that nuclear receptors influence the precise pathway of macrophage phenotype polarization and that these effects protect against insulin resistance and metabolic syndrome, the most important group of diseases facing the industrialized world.

Ambient air pollution exaggerates adipose inflammation and insulin resistance in a mouse model of diet-induced obesity

BACKGROUND

There is a strong link between urbanization and type 2 diabetes mellitus. Although a multitude of mechanisms have been proposed, there are no studies evaluating the impact of ambient air pollutants and the propensity to develop type 2 diabetes mellitus. We hypothesized that exposure to ambient fine particulate matter (<2.5 mum; PM(2.5)) exaggerates diet-induced insulin resistance, adipose inflammation, and visceral adiposity.

METHODS AND RESULTS

Male C57BL/6 mice were fed high-fat chow for 10 weeks and randomly assigned to concentrated ambient PM(2.5) or filtered air (n=14 per group) for 24 weeks. PM(2.5)-exposed C57BL/6 mice exhibited marked whole-body insulin resistance, systemic inflammation, and an increase in visceral adiposity. PM(2.5) exposure induced signaling abnormalities characteristic of insulin resistance, including decreased Akt and endothelial nitric oxide synthase phosphorylation in the endothelium and increased protein kinase C expression. These abnormalilties were associated with abnormalities in vascular relaxation to insulin and acetylcholine. PM(2.5) increased adipose tissue macrophages (F4/80(+) cells) in visceral fat expressing higher levels of tumor necrosis factor-alpha/interleukin-6 and lower interleukin-10/N-acetyl-galactosamine specific lectin 1. To test the impact of PM(2.5) in eliciting direct monocyte infiltration into fat, we rendered FVBN mice expressing yellow fluorescent protein (YFP) under control of a monocyte-specific promoter (c-fms, c-fms(YFP)) diabetic over 10 weeks and then exposed these mice to PM(2.5) or saline intratracheally. PM(2.5) induced YFP cell accumulation in visceral fat and potentiated YFP cell adhesion in the microcirculation.

CONCLUSIONS

PM(2.5) exposure exaggerates insulin resistance and visceral inflammation/adiposity. These findings provide a new link between air pollution and type 2 diabetes mellitus.

PPARdelta Agonism for the Treatment of Obesity and Associated Disorders: Challenges and Opportunities

The prevalence of obesity in the USA and worldwide has reached epidemic proportions during the last two decades. Drugs currently available for the treatment of obesity provide no more than 5% placebo-adjusted weight loss and are associated with undesirable side effects. Peroxisome proliferator-activated receptor (PPAR) modulators offer potential benefits for the treatment of obesity and its associated complications but their development has been complicated by biological, technical, and regulatory challenges. Despite significant challenges, PPAR modulators are attractive targets for the treatment of obesity and could offer a viable alternative to the millions of patients who fail to lose weight following rigorous dieting and exercise protocols. In addition, PPAR modulators have the potential-added benefit of ameliorating the associated comorbidities.

Mechanisms of macrophage activation in obesity-induced insulin resistance

Chronic inflammation is now recognized as a key step in the pathogenesis of obesity-induced insulin resistance and type 2 diabetes mellitus. This low-grade inflammation is mediated by the inflammatory (classical) activation of recruited and resident macrophages that populate metabolic tissues, including adipose tissue and liver. These findings have led to the concept that infiltration by and activation of macrophages in adipose tissue are causally linked to obesity-induced insulin resistance. Studies have shown, however, that alternatively activated macrophages taking residence in adipose tissue and liver perform beneficial functions in obesity-induced metabolic disease. Alternatively activated macrophages reduce insulin resistance in obese mice by attenuating tissue inflammation and increasing oxidative metabolism in liver and skeletal muscle. The discovery that distinct subsets of macrophages are involved in the promotion or attenuation of insulin resistance suggests that pathways controlling macrophage activation can potentially be targeted to treat these comorbidities of obesity. Thus, this Review focuses on the stimuli and mechanisms that control classical and alternative activation of tissue macrophages, and how these macrophage activation programs modulate insulin action in peripheral tissues. The functional importance of macrophage activation is further discussed in the context of host defense to highlight the crosstalk between innate immunity and metabolism.

Coordination of inflammation and metabolism by PPAR and LXR nuclear receptors

Biological systems are integrated networks constantly responding to internal and external stimulators. Understanding the intrinsic response to an imbalanced system provides the opportunity to develop therapeutic approaches to reinstate the natural balanced state. Increasing evidence suggests that members of the nuclear receptor superfamily integrate both inflammatory and metabolic signals to maintain homeostasis in immune cells such as macrophages and lymphocytes. PPAR and LXR are nuclear receptors activated by fatty acid and cholesterol derivatives respectively that control the expression of an array of genes involved in lipid metabolism and inflammation. Recent studies have uncovered distinct mechanisms for transcriptional regulation of metabolic and inflammatory target genes by PPAR and LXR and have expanded the biology of these receptors to include roles in alternative macrophage activation and adaptive immunity.

Coordination of inflammation and metabolism by PPAR and LXR nuclear receptors

Biological systems are integrated networks constantly responding to internal and external stimulators. Understanding the intrinsic response to an imbalanced system provides the opportunity to develop therapeutic approaches to reinstate the natural balanced state. Increasing evidence suggests that members of the nuclear receptor superfamily integrate both inflammatory and metabolic signals to maintain homeostasis in immune cells such as macrophages and lymphocytes. PPAR and LXR are nuclear receptors activated by fatty acid and cholesterol derivatives respectively that control the expression of an array of genes involved in lipid metabolism and inflammation. Recent studies have uncovered distinct mechanisms for transcriptional regulation of metabolic and inflammatory target genes by PPAR and LXR and have expanded the biology of these receptors to include roles in alternative macrophage activation and adaptive immunity.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) attenuates carbon tetrachloride hepatotoxicity by downregulating proinflammatory gene expression

Peroxisome proliferator-activated receptor (PPAR) beta/delta-null mice exhibit exacerbated hepatotoxicity in response to administration of carbon tetrachloride (CCl(4)). To determine whether ligand activation of the receptor protects against chemical toxicity in the liver, wild-type and PPARbeta/delta-null mice were administered CCl(4) with or without coadministration of the highly specific PPARbeta/delta ligand GW0742. Biomarkers of liver toxicity, including serum alanine aminotransferase (ALT) and hepatic tumor necrosis factor (TNF) alpha mRNA, were significantly higher in CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. Hepatic expression of TNF-like weak inducer of apoptosis receptor (TWEAKr) and S100 calcium-binding protein A6 (S100A6/calcyclin), genes involved in nuclear factor kappa B signaling, was higher in the CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. GW0742 treatment resulted in reduced serum ALT concentration and lower expression of CCl(4)-induced TNF-alpha, S100A6, monocyte chemoattractant protein-1 (MCP1), and TWEAKr in wild-type mice, and these effects were not observed in PPARbeta/delta-null mice. Expression of TNF-alpha was higher in PPARbeta/delta-null primary hepatocytes in response to interleukin-1beta treatment compared to wild-type hepatocytes, but GW0742 did not significantly modulate TNF-alpha expression in hepatocytes from either genotype. While PPARbeta/delta-null hepatic stellate exhibited higher rates of proliferation compared to wild-type cells, GW0742 did not affect alpha-smooth muscle actin expression in these cells. Combined, these findings demonstrate that ligand activation of PPARbeta/delta protects against chemically induced hepatotoxicity by downregulating expression of proinflammatory genes. Hepatocytes and hepatic stellate cells do not appear to directly mediate the inhibitory effects of ligand activation of PPARbeta/delta in liver, suggesting the involvement of paracrine and autocrine events mediated by hepatic cells.

PPARdelta/beta: the lobbyist switching macrophage allegiance in favor of metabolism

Macrophages, which belong to the immune system, are increasingly being recognized for their contribution to metabolic regulation. In two studies by Kang et al. (2008) and Odegaard et al. (2008) in this issue of Cell Metabolism, we learn that alternative activation (M2a) of resident macrophages in liver and adipose tissue depends highly on PPARdelta/beta activity, leading to improved fatty acid metabolism and insulin sensitivity.