Loss of PPAR gamma in immune cells impairs the ability of abscisic acid to improve insulin sensitivity by suppressing monocyte chemoattractant protein-1 expression and macrophage infiltration into white adipose tissue

Abscisic acid (ABA) is a natural phytohormone and peroxisome proliferator-activated receptor gamma (PPARgamma) agonist that significantly improves insulin sensitivity in db/db mice. Although it has become clear that obesity is associated with macrophage infiltration into white adipose tissue (WAT), the phenotype of adipose tissue macrophages (ATMs) and the mechanisms by which insulin-sensitizing compounds modulate their infiltration remain unknown. We used a loss-of-function approach to investigate whether ABA ameliorates insulin resistance through a mechanism dependent on immune cell PPARgamma. We characterized two phenotypically distinct ATM subsets in db/db mice based on their surface expression of F4/80. F4/80(hi) ATMs were more abundant and expressed greater concentrations of chemokine receptor (CCR) 2 and CCR5 when compared to F4/80(lo) ATMs. ABA significantly decreased CCR2(+) F4/80(hi) infiltration into WAT and suppressed monocyte chemoattractant protein-1 (MCP-1) expression in WAT and plasma. Furthermore, the deficiency of PPARgamma in immune cells, including macrophages, impaired the ability of ABA to suppress the infiltration of F4/80(hi) ATMs into WAT, to repress WAT MCP-1 expression and to improve glucose tolerance. We provide molecular evidence in vivo demonstrating that ABA improves insulin sensitivity and obesity-related inflammation by inhibiting MCP-1 expression and F4/80(hi) ATM infiltration through a PPARgamma-dependent mechanism.     

Naringenin suppresses macrophage infiltration into adipose tissue in an early phase of high-fat diet-induced obesity

Obese adipose tissue is characterized by increased macrophage infiltration, which results in chronic inflammation in adipose tissue and leads to obesity-related diseases such as type 2 diabetes mellitus and atherosclerosis. The regulation of macrophage infiltration into adipose tissue is an important strategy for preventing and treating obesity-related diseases. In this study, we report that naringenin, a citrus flavonoid, suppressed macrophage infiltration into adipose tissue induced by short-term (14 days) feeding of a high-fat diet in mice; although naringenin did not show any differences in high-fat diet-induced changes of serum biochemical parameters in this short administration period. Naringenin suppressed monocyte chemoattractant protein-1 (MCP-1) in adipose tissue, and this effect was mediated in part through inhibition of c-Jun NH₂-terminal kinase pathway. Naringenin also inhibited MCP-1 expression in adipocytes, macrophages, and a co-culture of adipocytes and macrophages. Our results suggest a mechanism by which daily consumption of naringenin may exhibit preventive effects on obesity-related diseases.     

Recent advances in the relationship between obesity, inflammation, and insulin resistance

It now appears that, in most obese patients, obesity is associated with a low-grade inflammation of white adipose tissue (WAT) resulting from chronic activation of the innate immune system and which can subsequently lead to insulin resistance, impaired glucose tolerance and even diabetes. WAT is the physiological site of energy storage as lipids. In addition, it has been more recently recognized as an active participant in numerous physiological and pathophysiological processes. In obesity, WAT is characterized by an increased production and secretion of a wide range of inflammatory molecules including TNF-alpha and interleukin-6 (IL-6), which may have local effects on WAT physiology but also systemic effects on other organs. Recent data indicate that obese WAT is infiltrated by macrophages, which may be a major source of locally-produced pro-inflammatory cytokines. Interestingly, weight loss is associated with a reduction in the macrophage infiltration of WAT and an improvement of the inflammatory profile of gene expression. Several factors derived not only from adipocytes but also from infiltrated macrophages probably contribute to the pathogenesis of insulin resistance. Most of them are overproduced during obesity, including leptin, TNF-alpha, IL-6 and resistin. Conversely, expression and plasma levels of adiponectin, an insulin-sensitising effector, are down-regulated during obesity. Leptin could modulate TNF-alpha production and macrophage activation. TNF-alpha is overproduced in adipose tissue of several rodent models of obesity and has an important role in the pathogenesis of insulin resistance in these species. However, its actual involvement in glucose metabolism disorders in humans remains controversial. IL-6 production by human adipose tissue increases during obesity. It may induce hepatic CRP synthesis and may promote the onset of cardiovascular complications. Both TNF-alpha and IL-6 can alter insulin sensitivity by triggering different key steps in the insulin signalling pathway. In rodents, resistin can induce insulin resistance, while its implication in the control of insulin sensitivity is still a matter of debate in humans. Adiponectin is highly expressed in WAT, and circulating adiponectin levels are decreased in subjects with obesity-related insulin resistance, type 2 diabetes and coronary heart disease. Adiponectin inhibits liver neoglucogenesis and promotes fatty acid oxidation in skeletal muscle. In addition, adiponectin counteracts the pro-inflammatory effects of TNF-alpha on the arterial wall and probably protects against the development of arteriosclerosis. In obesity, the pro-inflammatory effects of cytokines through intracellular signalling pathways involve the NF-kappaB and JNK systems. Genetic or pharmacological manipulations of these effectors of the inflammatory response have been shown to modulate insulin sensitivity in different animal models. In humans, it has been suggested that the improved glucose tolerance observed in the presence of thiazolidinediones or statins is likely related to their anti-inflammatory properties. Thus, it can be considered that obesity corresponds to a sub-clinical inflammatory condition that promotes the production of pro-inflammatory factors involved in the pathogenesis of insulin resistance.     

Adipocyte death defines macrophage localization and function in adipose tissue of obese mice and humans

Macrophage infiltration of white adipose tissue (WAT) is implicated in the metabolic complications of obesity. The precipitating event(s) and function(s) of macrophage infiltration into WAT are unknown. We demonstrate that >90% of all macrophages in WAT of obese mice and humans are localized to dead adipocytes, where they fuse to form syncytia that sequester and scavenge the residual "free" adipocyte lipid droplet and ultimately form multinucleate giant cells, a hallmark of chronic inflammation. Adipocyte death increases in obese (db/db) mice (30-fold) and humans and exhibits ultrastructural features of necrosis (but not apoptosis). These observations identify necrotic-like adipocyte death as a pathologic hallmark of obesity and suggest that scavenging of adipocyte debris is an important function of WAT macrophages in obese individuals. The frequency of adipocyte death is positively correlated with increased adipocyte size in obese mice and humans and in hormone-sensitive lipase-deficient (HSL-/-) mice, a model of adipocyte hypertrophy without increased adipose mass. WAT of HSL-/- mice exhibited a 15-fold increase in necrotic-like adipocyte death and formation of macrophage syncytia, coincident with increased tumor necrosis factor-alpha gene expression. These results provide a novel framework for understanding macrophage recruitment, function, and persistence in WAT of obese individuals.     

Microparticles Release by Adipocytes Act as “Find-Me” Signals to Promote Macrophage Migration

Macrophage infiltration of adipose tissue during weight gain is a central event leading to the metabolic complications of obesity. However, what are the mechanisms attracting professional phagocytes to obese adipose tissue remains poorly understood. Here, we demonstrate that adipocyte-derived microparticles (MPs) are critical “find-me” signals for recruitment of monocytes and macrophages. Supernatants from stressed adipocytes stimulated the attraction of monocyte cells and primary macrophages. The activation of caspase 3 was required for release of these signals. Adipocytes exposed to saturated fatty acids showed marked release of MPs into the supernatant while common genetic mouse models of obesity demonstrate high levels of circulating adipocyte-derived MPs. The release of MPs was highly regulated and dependent on caspase 3 and Rho-associated kinase. Further analysis identified these MPs as a central chemoattractant in vitro and in vivo. In addition, intravenously transplanting circulating MPs from the ob/ob mice lead to activation of monocytes in circulation and adipose tissue of the wild type mice. These data identify adipocyte-derived MPs as novel “find me” signals that contributes to macrophage infiltration associated with obesity.     

Active spice-derived components can inhibit inflammatory responses of adipose tissue in obesity by suppressing inflammatory actions of macrophages and release of monocyte chemoattractant protein-1 from adipocytes

Inflammation plays a key role in obesity-related pathologies such as cardiovascular disease, type II diabetes, and several types of cancer. Obesity-induced inflammation entails the enhancement of the recruitment of macrophages into adipose tissue and the release of various proinflammatory proteins from fat tissue. Therefore, the modulation of inflammatory responses in obesity may be useful for preventing or ameliorating obesity-related pathologies. Some spice-derived components, which are naturally occurring phytochemicals, elicit antiobesity and antiinflammatory properties. In this study, we investigated whether active spice-derived components can be applied to the suppression of obesity-induced inflammatory responses. Mesenteric adipose tissue was isolated from obese mice fed a high-fat diet and cultured to prepare an adipose tissue-conditioned medium. Raw 264.7 macrophages were treated with the adipose tissue-conditioned medium with or without active spice-derived components (i.e., diallyl disulfide, allyl isothiocyanate, piperine, zingerone and curcumin). Chemotaxis assay was performed to measure the degree of macrophage migration. Macrophage activation was estimated by measuring tumor necrosis factor-alpha (TNF-alpha), nitric oxide, and monocyte chemoattractant protein-1 (MCP-1) concentrations. The active spice-derived components markedly suppressed the migration of macrophages induced by the mesenteric adipose tissue-conditioned medium in a dose-dependent manner. Among the active spice-derived components studied, allyl isothiocyanate, zingerone, and curcumin significantly inhibited the cellular production of proinflammatory mediators such as TNF-alpha and nitric oxide, and significantly inhibited the release of MCP-1 from 3T3-L1 adipocytes. Our findings suggest that the spice-derived components can suppress obesity-induced inflammatory responses by suppressing adipose tissue macrophage accumulation or activation and inhibiting MCP-1 release from adipocytes. These spice-derived components may have a potential to improve chronic inflammatory conditions in obesity.     

The Roles of Adipokines,Proinflammatory Cytokines,and Adipose Tissue Macrophages in Obesity-Associated Insulin Resistance in Modest Obesity and Early Metabolic Dysfunction

The roles of adipokines, proinflammatory cytokines, and adipose tissue macrophages in obesity-associated insulin resistance have been explored in both animal and human studies. However, our current understanding of obesity-associated insulin resistance relies on studies of artificial metabolic extremes. The purpose of this study was to explore the roles of adipokines, proinflammatory cytokines, and adipose tissue macrophages in human patients with modest obesity and early metabolic dysfunction. We obtained omental adipose tissue and fasting blood samples from 51 females undergoing gynecologic surgery. We investigated serum concentrations of proinflammatory cytokines and adipokines as well as the mRNA expression of proinflammatory and macrophage phenotype markers in visceral adipose tissue using ELISA and quantitative RT-PCR. We measured adipose tissue inflammation and macrophage infiltration using immunohistochemical analysis. Serum levels of adiponectin and leptin were significantly correlated with HOMA-IR and body mass index. The levels of expression of MCP-1 and TNF-α in visceral adipose tissue were also higher in the obese group (body mass index ≥ 25). The expression of mRNA MCP-1 in visceral adipose tissue was positively correlated with body mass index (r = 0.428, p = 0.037) but not with HOMA-IR, whereas TNF-α in visceral adipose tissue was correlated with HOMA-IR (r = 0.462, p = 0.035) but not with body mass index. There was no obvious change in macrophage phenotype or macrophage infiltration in patients with modest obesity or early metabolic dysfunction. Expression of mRNA CD163/CD68 was significantly related to mitochondrial-associated genes and serum inflammatory cytokine levels of resistin and leptin. These results suggest that changes in the production of inflammatory biomolecules precede increased immune cell infiltration and induction of a macrophage phenotype switch in visceral adipose tissue. Furthermore, serum resistin and leptin have specific roles in the regulation of adipose tissue macrophages in patients with modest obesity or early metabolic dysfunction.     

Myeloid heme oxygenase-1 haploinsufficiency reduces high fat diet-induced insulin resistance by affecting adipose macrophage infiltration in mice

Increased adipose tissue macrophages contribute to obesity-induced metabolic syndrome. Heme oxygenase-1 (HO-1) is a stress-inducible enzyme with potent anti-inflammatory and proangiogenic activities in macrophages. However, the role of macrophage HO-1 on obesity-induced adipose inflammation and metabolic syndrome remains unclear. Here we show that high-fat diet (HFD) feeding in C57BL/6J mice induced HO-1 expression in the visceral adipose tissue, particularly the stromal vascular fraction. When the irradiated C57BL/6J mice reconstituted with wild-type or HO-1(+/-) bone marrow were fed with HFD for over 24 weeks, the HO-1(+/-) chimeras were protected from HFD-induced insulin resistance and this was associated with reduced adipose macrophage infiltration and angiogenesis, suggesting that HO-1 affects myeloid cell migration toward adipose tissue during obesity. In vivo and in vitro migration assays revealed that HO-1(+/-) macrophages exhibited an impaired migration response. Chemoattractant-induced phosphorylation of p38 and focal adhesion kinase (FAK) declined faster in HO-1(+/-) macrophages. Further experiments demonstrated that carbon monoxide and bilirubin, the byproducts derived from heme degradation by HO-1, enhanced macrophage migration by increasing phosphorylation of p38 and FAK, respectively. These data disclose a novel role of hematopoietic cell HO-1 in promoting adipose macrophage infiltration and the development of insulin resistance during obesity.     

Effect of Angiotensin II Type 2 Receptor-Interacting Protein on Adipose Tissue Function via Modulation of Macrophage Polarization

We demonstrated that angiotensin II type 2 (AT₂) receptor-interacting protein (ATIP) 1 ameliorates inflammation-mediated vascular remodeling independent of the AT₂ receptor, leading us to explore the possibility of whether ATIP1 could exert anti-inflammatory effects and play a role in other pathophysiological conditions. We examined the possible anti-inflammatory effects of ATIP1 in adipose tissue associated with amelioration of insulin resistance. In mice fed a high-cholesterol diet, adipose tissue macrophage (ATM) infiltration and M1-to-M2 ratio were decreased in ATIP1 transgenic mice (ATIP1-Tg) compared with wild-type mice (WT), with decreased expression of inflammatory cytokines such as tumor necrosis factor-α and monocyte chemoattractant protein-1 in white adipose tissue (WAT), but an increase in interleukin-10, an anti-inflammatory cytokine. Moreover, 2-[³H]deoxy-d-glucose (2-[³H]DG) uptake was significantly increased in ATIP1-Tg compared with WT. Next, we examined the roles of ATIP1 in BM-derived hematopoietic cells, employing chimeric mice produced by BM transplantation into irradiated type 2 diabetic mice with obesity, KKAy, as recipients. ATM infiltration and M1-to-M2 ratio were decreased in ATIP1 chimera (ATIP1-tg as BM donor), with improvement of insulin-mediated 2-[³H]DG uptake and amelioration of inflammation in WAT. Moreover, serum adiponectin concentration in ATIP1 chimera was significantly higher than that in WT chimera (WT as BM donor) and KKAy chimera (KKAy as BM donor). These results indicate that ATIP1 could exert anti-inflammatory effects in adipose tissue via macrophage polarization associated with improvement of insulin resistance, and ATIP1 in hematopoietic cells may contribute to these beneficial effects on adipose tissue functions in type 2 diabetes.     

Macrophage Gene Expression in Adipose Tissue is Associated with Insulin Sensitivity and Serum Lipid Levels Independent of Obesity

Objective: Obesity is linked to both increased metabolic disturbances and increased adipose tissue macrophage infiltration. However, whether macrophage infiltration directly influences human metabolism is unclear. The aim of this study was to investigate if there are obesity‐independent links between adipose tissue macrophages and metabolic disturbances. Design and Methods: Expression of macrophage markers in adipose tissue was analyzed by DNA microarrays in the SOS Sib Pair study and in patients with type 2 diabetes and a BMI‐matched healthy control group. Results: The expression of macrophage markers in adipose tissue was increased in obesity and associated with several metabolic and anthropometric measurements. After adjustment for BMI, the expression remained associated with insulin sensitivity, serum levels of insulin, C‐peptide, high density lipoprotein cholesterol (HDL‐cholesterol) and triglycerides. In addition, the expression of most macrophage markers was significantly increased in patients with type 2 diabetes compared to the control group. Conclusion: Our study shows that infiltration of macrophages in human adipose tissue, estimated by the expression of macrophage markers, is increased in subjects with obesity and diabetes and associated with insulin sensitivity and serum lipid levels independent of BMI. This indicates that adipose tissue macrophages may contribute to the development of insulin resistance and dyslipidemia.     

Identification of Pigment Epithelium-Derived Factor as an Adipocyte-Derived Inflammatory Factor

Obesity is a major risk factor for insulin resistance, type 2 diabetes mellitus and cardiovascular disease. The pathophysiology of obesity is associated with chronic low-grade inflammation. Adipose tissue in obesity is significantly infiltrated by macrophages that secrete cytokines. The mechanisms of interaction between macrophages and adipocytes, leading to macrophage activation and increased cytokine release, remain to be elucidated. We reasoned that an adipocyte-derived factor might stimulate activation of macrophages. We have identified pigment epithelium-derived factor (PEDF) as a mediator of inflammation that is secreted by adipocytes and mediates macrophage activation. Recombinant PEDF activates macrophages to release tumor necrosis factor (TNF) and interleukin-1 (IL-1). The PEDF receptor adipose triglyceride lipase (ATGL) is required for PEDF-mediated macrophage activation. Selective inhibition of ATGL on macrophages attenuates PEDF-induced TNF production, and PEDF enhances the phosphorylation of p38 and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinases. PEDF administration to rats results in increased serum TNF levels, and insulin resistance. Together, these findings suggest that PEDF secreted by adipocytes contributes to the onset and maintenance of chronic inflammation in obesity, and may be a therapeutic target in ameliorating insulin resistance.     

Momordica charantia (Bitter Melon) Reduces Obesity-Associated Macrophage and Mast Cell Infiltration as well as Inflammatory Cytokine Expression in Adipose Tissues

Obesity is a world-wide epidemic disease that correlates closely with type 2 diabetes and cardiovascular diseases. Obesity-induced chronic adipose tissue inflammation is now considered as a critical contributor to the above complications. Momordica charantia (bitter melon, BM) is a traditional Chinese food and well known for its function of reducing body weight gain and insulin resistance. However, it is unclear whether BM could alleviate adipose tissue inflammation caused by obesity. In this study, C57BL/6 mice were fed high fat diet (HFD) with or without BM for 12 weeks. BM-contained diets ameliorated HFD-induced obesity and insulin resistance. Histological and real-time PCR analysis demonstrated BM not only reduced macrophage infiltration into epididymal adipose tissues (EAT) and brown adipose tissues (BAT). Flow cytometry show that BM could modify the M1/M2 phenotype ratio of macrophages in EAT. Further study showed that BM lowered mast cell recruitments in EAT, and depressed pro-inflammatory cytokine monocyte chemotactic protein-1 (MCP-1) expression in EAT and BAT as well as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) expression in EAT. Finally, ELISA analysis showed BM-contained diets also normalized serum levels of the cytokines. In summary, in concert with ameliorated insulin resistance and fat deposition, BM reduced adipose tissue inflammation in diet-induced obese (DIO) mice.     

Adiponectin Promotes Macrophage Polarization toward an Anti-inflammatory Phenotype

It is established that the adipocyte-derived cytokine adiponectin protects against cardiovascular and metabolic diseases, but the effect of this adipokine on macrophage polarization, an important mediator of disease progression, has never been assessed. We hypothesized that adiponectin modulates macrophage polarization from that resembling a classically activated M1 phenotype to that resembling alternatively-activated M2 cells. Peritoneal macrophages and the stromal vascular fraction (SVF) cells of adipose tissue isolated from adiponectin knock-out mice displayed increased M1 markers, including tumor necrosis factor-α, interleukin-6, and monocyte chemoattractant protein-1 and decreased M2 markers, including arginase-1, macrophage galactose N-acetyl-galactosamine specific lectin-1, and interleukin-10. The systemic delivery of adenovirus expressing adiponectin significantly augmented arginase-1 expression in peritoneal macrophages and SVF cells in both wild-type and adiponectin knock-out mice. In culture, the treatment of macrophages with recombinant adiponectin protein led to an increase in the levels of M2 markers and a reduction of reactive oxygen species and reactive oxygen species-related gene expression. Adiponectin also stimulated the expression of M2 markers and attenuated the expression of M1 markers in human monocyte-derived macrophages and SVF cells isolated from human adipose tissue. These data show that adiponectin functions as a regulator of macrophage polarization, and they indicate that conditions of high adiponectin expression may deter metabolic and cardiovascular disease progression by favoring an anti-inflammatory phenotype in macrophages.