Phenotype Of Adipose Tissue Macrophages Research Articles

Omega-3 Fatty Acids Prevent Peripheral Neuropathy and Pro Inflammatory Adipose Tissue Macrophage Phenotype in High-Fat-Fed Mice

Obesity and/or consumption of a high fat diet is associated with the development of peripheral neuropathy (PN) which is characterized by changes in epidermal innervation and increased sensitivity in the extremities. Evidence indicates that omega-3 fatty acids prevent the development of PN and attenuate adipose inflammation, but the interaction of PN and adipose tissue macrophage (ATMΦ) phenotype have not been investigated. We hypothesized that a diet high in unsaturated fatty acids may prevent PN and adipose inflammation, despite the high fat content. PURPOSE: Assess the effects of omega-3 fatty acids on signs of PN and ATMΦ phenotype in high-fat-fed mice. Methods: Male C57Bl/6 mice were randomized to (n = 12-14/group): 10% low fat fish oil (10% FO); 54% high fat fish oil (54% FO); 10% low fat lard (10% L); or 54% high fat lard (54% L) for 14 weeks. PN was characterized by the von Frey test for hindpaw sensitivity at baseline then biweekly through week 14. After 14 weeks, intraepidermal nerve fiber density (IENFD) was quantified by immunohistochemistry of the hindpaw footpad skin using anti-PGP 9.5. The relative percentage of total macrophage cells (F4/80+CD11b+) and M1 (CD206-CD11c+) and M2 (CD206+CD11c-) cell populations in epididymal adipose were identified using flow cytometry. Group differences were identified by 2 x 2 factorial ANOVA. Pearson correlations were used to assess the relationship between hindpaw withdrawal threshold and ATMΦ. Results: 54% L (35.3 ± 1.0 g) had greater body mass compared to all other groups: 10% FO (26.5 ± 0.7, p < 0.001); 10% L (27.8 ± 0.6, p = 0.001); and 54% L (35.3 ± 1.0, p < 0.001). Similarly, fasting insulin was greater in 54% L (3.1 ± 0.4 ng/mL) compared to all other groups: 10% FO (1.0 ± 1.0, p < 0.001); 10% L (1.6 ± 0.2, p < 0.001), and 54% FO (1.0 ± 0.1, p < 0.001). Fasting glucose was greater in 54% L (225.8 ± 7.5 mg/dl) compared to 10% FO (216.1 ± 7.7, p =0.04) and 54% FO (182.6 ± 6.6, p = 0.03), but not 10% L (180.7 ± 17.1, p = 0.483). Following the same pattern as body mass and insulin, homeostatic model assessment of insulin resistance (HOMA-IR) was greater in 54% L (8.1 ± 1.1) compared to all other groups: 10% FO (2.2 ± 0.4, p < 0.001); 10% L (4.1 ± 0.6, p < 0.001); and 54% FO (2.2 ± 0.3, p < 0.001). Hindpaw sensitivity was greater in 54% L (0.08 ± 0.02 g) compared to 54% FO (0.19 ± 0.03, p = 0.02). IENFD was greater in 54% L (52.5 ± 2.2 fibers per mm) compared to 10% FO (44.1 ± 1.5, p = 0.04), but was not different from 10% L (49.7 ± 2.8, p = 0.47) or 54% FO (48.1 ± 3.0, p = 0.24). M1 ATMΦ percentage was greater in 54% L (11.7 ± 2.5 %) compared to all other groups: 10% L (3.8 ± 0.8, p = 0.001), 10% FO (5.2 ± 0.7, p = 0.014), and 54% FO (5.1 ± 0.6 %, p = 0.016). M2 ATMΦ percentage was greater in 10% L (59.7 ± 4.5 %) compared to all other groups: 10% FO (42.1 ± 5.0, p = 0.036), 54% L (41.4 ± 3.9, p = 0.027), and 54% FO (31.4 ± 3.6 %, p < 0.001), but there were no significant differences in M2 ATMΦ percentage in 54% L vs. 54% FO. There were no significant correlations between hindpaw sensitivity and M1 ATMΦ percentage, M2 ATMΦ percentage, or M1:M2 ratio. CONCLUSION: The negative metabolic effects of a high fat lard diet on body mass, glucose, insulin, and insulin resistance were prevented when the dietary fat source was omega-3 fatty acids instead of lard. Notably, mice fed a high fat lard diet had a higher percentage of the pro inflammatory M1 ATMΦ phenotype and developed signs of painful PN including hindpaw hypersensitivity and increased IENFD but these effects were prevented by omega-3 fatty acids in the 54% FO group, despite the high dietary fat content. Supported by the Southern Illinois University Edwardsville Vaughnie Lindsay New Investigator Award. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The endoplasmic reticulum stress protein GRP94 modulates cathepsin L activity in M2 macrophages in conditions of obesity-associated inflammation and contributes to their pro-inflammatory profile.

Adipose tissue macrophages (ATM) are key actors in the pathophysiology of obesity-related diseases. They have a unique intermediate M2-M1 phenotype which has been linked to endoplasmic reticulum (ER) stress. We previously reported that human M2 macrophages treated with the ER stress inducer thapsigargin switched to a pro-inflammatory phenotype that depended on the stress protein GRP94. In these conditions, GRP94 promoted cathepsin L secretion and was co-secreted with complement C3. As cathepsin L and complement C3 have been reported to play a role in the pathophysiology of obesity, in this work we studied the involvement of GRP94 in the pro-inflammatory phenotype of ATM. GRP94, cathepsin L and C3 expression were analyzed in CD206 + ATM from mice, WT or obesity-resistant transgenic fat-1, fed a high-fat diet (HFD) or a standard diet. GRP94 colocalization with cathepsin L and C3 and its effects were analyzed in human primary macrophages using thapsigargin as a control to induce ER stress and palmitic acid (PA) as a driver of metabolic activation. In WT, but not in fat-1 mice, fed a HFD, we observed an increase in crown-like structures consisting of CD206 + pSTAT1+ macrophages showing high expression of GRP94 that colocalized with cathepsin L and C3. In vitro experiments showed that PA favored a M2-M1 switch depending on GRP94. This switch was prevented by omega-3 fatty acids. PA-induced GRP94-cathepsin L colocalization and a decrease in cathepsin L enzymatic activity within the cells (while the enzymatic activity in the extracellular medium was increased). These effects were prevented by the GRP94 inhibitor PU-WS13. GRP94 is overexpressed in macrophages both in in vivo and in vitro conditions of obesity-associated inflammation and is involved in changing their profile towards a more pro-inflammatory profile. It colocalizes with complement C3 and cathepsin L and modulates cathepsin L activity.

A lipid-associated macrophage lineage rewires the spatial landscape of adipose tissue in early obesity.

Adipose tissue macrophage (ATM) infiltration is associated with adipose tissue dysfunction and insulin resistance in mice and humans. Recent single-cell data highlight increased ATM heterogeneity in obesity but do not provide a spatial context for ATM phenotype dynamics. We integrated single-cell RNA-Seq, spatial transcriptomics, and imaging of murine adipose tissue in a time course study of diet-induced obesity. Overall, proinflammatory immune cells were predominant in early obesity, whereas nonresident antiinflammatory ATMs predominated in chronic obesity. A subset of these antiinflammatory ATMs were transcriptomically intermediate between monocytes and mature lipid-associated macrophages (LAMs) and were consistent with a LAM precursor (pre-LAM). Pre-LAMs were spatially associated with early obesity crown-like structures (CLSs), which indicate adipose tissue dysfunction. Spatial data showed colocalization of ligand-receptor transcripts related to lipid signaling among monocytes, pre-LAMs, and LAMs, including Apoe, Lrp1, Lpl, and App. Pre-LAM expression of these ligands in early obesity suggested signaling to LAMs in the CLS microenvironment. Our results refine understanding of ATM diversity and provide insight into the dynamics of the LAM lineage during development of metabolic disease.

Therapeutic targeting of adipose tissue macrophages ameliorates liver fibrosis in non-alcoholic fatty liver disease

Background & Aims: The accumulation of adipose tissue macrophages (ATMs) in obesity has been associated with hepatic injury. However, the contribution of ATMs to hepatic fibrosis in non-alcoholic fatty liver disease (NAFLD) remains to be elucidated. Herein, we investigate the relationship between ATMs and liver fibrosis in patients with patients with NAFLD and evaluate the impact of modulation of ATMs over hepatic fibrosis in an experimental non-alcoholic steatohepatitis (NASH) model. MethodsAdipose tissue and liver biopsies from 42 patients with NAFLD with different fibrosis stages were collected. ATMs were characterised by immunohistochemistry and flow cytometry and the correlation between ATMs and liver fibrosis stages was assessed. Selective modulation of the ATM phenotype was achieved by i.p. administration of dextran coupled with dexamethasone in diet-induced obesity and NASH murine models. Chronic administration effects were evaluated by histology and gene expression analysis in adipose tissue and liver samples. In vitro crosstalk between human ATMs and hepatic stellate cells (HSCs) and liver spheroids was performed. ResultsPatients with NAFLD presented an increased accumulation of pro-inflammatory ATMs that correlated with hepatic fibrosis. Long-term modulation of ATMs significantly reduced pro-inflammatory phenotype and ameliorated adipose tissue inflammation. Moreover, ATMs modulation was associated with an improvement in steatosis and hepatic inflammation and significantly reduced fibrosis progression in an experimental NASH model. In vitro, the reduction of the pro-inflammatory phenotype of human ATMs with dextran–dexamethasone treatment reduced the secretion of inflammatory chemokines and directly attenuated the pro-fibrogenic response in HSCs and liver spheroids. ConclusionsPro-inflammatory ATMs increase in parallel with fibrosis degree in patients with NAFLD and their modulation in an experimental NASH model improves liver fibrosis, uncovering the potential of ATMs as a therapeutic target to mitigate liver fibrosis in NAFLD. Impact and implicationsWe report that human adipose tissue pro-inflammatory macrophages correlate with hepatic fibrosis in non-alcoholic fatty liver disease (NAFLD). Furthermore, the modulation of adipose tissue macrophages (ATMs) by dextran-nanocarrier conjugated with dexamethasone shifts the pro-inflammatory phenotype of ATMs to an anti-inflammatory phenotype in an experimental murine model of non-alcoholic steatohepatitis. This shift ameliorates adipose tissue inflammation, hepatic inflammation, and fibrosis. Our results highlight the relevance of adipose tissue in NAFLD pathophysiology and unveil ATMs as a potential target for NAFLD.

Oral ω-3 PUFA supplementation modulates inflammation in adipose tissue depots in morbidly obese women: A randomized trial

ObjectivesObesity is characterized by local and systemic low-grade inflammatory responses. Adipose tissue macrophages (ATM) play decisive roles in inflammation, insulin signaling, and various metabolic dysfunctions. Diets enriched with ω-3 polyunsaturated fatty acids (PUFAs) have been shown to improve health and mitigate pathologic conditions. However, the effects of ω-3 PUFA on adipose tissue inflammation, ATM number, and phenotype are poorly defined in human obesity. The aim of this study was to examine differences in expression of metabolic-inflammatory markers in omental, mesenteric, and subcutaneous fat depots of obese women supplemented with ω-3 PUFAs for 4 wk compared with a low-calorie diet before bariatric surgery. MethodsIn a randomized controlled trial, inflammatory markers in the abdominal adipose tissue and the systemic response in obese women were studied. Patients were treated with a 2-wk low-calorie diet (LCD) or a 4-wk ω-3 PUFA-enriched diet (920 mg eicosapentaenoic acid, 760 mg docosahexaenoic acid daily) before laparoscopic bypass surgery. Omental, mesenteric, and subcutaneous adipose tissue biopsies were collected during surgery and analyzed for quantity and phenotype of ATMs, and profiled for adipokines, cytokines, and signal transduction molecules. ResultsThe chronic inflammatory state characterized by ATM markers was mostly improved by ω-3 PUFAs in visceral adipose tissue. We observed a decreased expression of CD45, CCL2, and CD68, indicating a lower inflammatory state. In patients with type 2 diabetes, ω-3 PUFAs lowered the expression of Netrin-1. ConclusionsCompared with an LCD, a diet enriched with ω-3 PUFAs influences the inflammatory state in different adipose tissue depots, by affecting markers of adipose tissue inflammation, macrophage phenotype, and retention. However, this was not reflected in clinical parameters such as insulin resistance and inflammatory cytokines. Subcutaneous adipose tissue and visceral adipose tissue have different responses to an LCD or a ω-3 PUFA-enriched diet. The presence of diabetes modifies the expression of inflammatory markers.

Omega-3 Fatty Acids Prevent Hindpaw Hypersensitivity And Proinflammatory Adipose Tissue Macrophage Phenotype In High-fat-fed Mice

Peripheral neuropathy (PN) is a debilitating complication of diabetes, characterized by damage to peripheral nerves and loss of sensation and/or pain in the extremities. Evidence indicates that omega-3 (n-3) fatty acids prevent the development of PN and attenuate adipose inflammation, but the interaction of PN and adipose tissue macrophage (ATMφ) phenotype have not been investigated. PURPOSE: Assess the effects of n-3 fatty acids on PN and ATMφ phenotype in high fat fed mice. METHODS: Male C57Bl/6 mice were randomized to (n = 12-14/group): 10% low fat fish oil (10% FO); 54% high fat fish oil (54% FO); 10% low fat lard (10% L); or 54% high fat lard (54% L). The von Frey test for hindpaw sensitivity was used to characterize PN at baseline then biweekly through week 14. After 14 weeks, the relative percentage of total macrophage cells (F4/80+CD11b+) and M1 (CD206-CD11c+) and M2 (CD206+CD11c-) cell populations were identified using flow cytometry. Group differences were identified by 2 x 2 factorial ANOVA. Pearson correlations were used to assess the relationship between hindpaw withdrawal threshold and ATMφ. RESULTS: Body weight, fasting insulin and glucose, and HOMA-IR were greater in 54% L compared to all other groups. 54% L had greater hindpaw sensitivity compared to 54% FO. M1 macrophage percentage was greater in 54% L compared to all other groups, but there were no significant differences in M2 percentage in 54% L vs. 54% FO. Paw withdrawal threshold was correlated with M2 ATMφ (r = 0.34, p = 0.049). CONCLUSION: n-3 fatty acids prevented excess body weight, insulin resistance, hindpaw hypersensitivity and increased proinflammatory macrophages in high fat fed mice. Less hindpaw sensitivity was positively associated with M2 anti-inflammatory ATMφ. Adipose inflammation may play a role in the development of PN and n-3 fatty acids may inhibit development of PN in part by increasing anti-inflammatory ATMφ cell populations. Supported by SIUE Vaughnie Lindsay New Investigator Award

Organokines and Exosomes: Integrators of Adipose Tissue Macrophage Polarization and Recruitment in Obesity.

The prevalence of obesity is escalating and has become a worldwide health challenge coinciding with the development of metabolic diseases. Emerging evidence has shown that obesity is accompanied by the infiltration of macrophages into adipose tissue, contributing to a state of low-grade chronic inflammation and dysregulated metabolism. Moreover, in the state of obesity, the phenotype of adipose tissue macrophages switches from the M2 polarized state to the M1 state, thereby contributing to chronic inflammation. Notably, multiple metabolic organs (adipose tissue, gut, skeletal muscle, and the liver) communicate with adipose tissue macrophages via secreting organokines or exosomes. In this review, we systematically summarize how the organokines (adipokines, gut microbiota and its metabolites, gut cytokines, myokines, and hepatokines) and exosomes (adipocyte-, skeletal muscle-, and hepatocyte-derived exosomes) act as important triggers for macrophage recruitment in adipose tissue and adipose tissue macrophage polarization, thus providing further insight into obesity treatment. In addition, we also highlight the complex interaction of organokines with organokines and organokines with exosomes, revealing new paths in understanding adipose tissue macrophage recruitment and polarization.

Adipocyte death triggers a pro-inflammatory response and induces metabolic activation of resident macrophages

A chronic low-grade inflammation within adipose tissue (AT) seems to be the link between obesity and some of its associated diseases. One hallmark of this AT inflammation is the accumulation of AT macrophages (ATMs) around dead or dying adipocytes, forming so-called crown-like structures (CLS). To investigate the dynamics of CLS and their direct impact on the activation state of ATMs, we established a laser injury model to deplete individual adipocytes in living AT from double reporter mice (GFP-labeled ATMs and tdTomato-labeled adipocytes). Hence, we were able to detect early ATM-adipocyte interactions by live imaging and to determine a precise timeline for CLS formation after adipocyte death. Further, our data indicate metabolic activation and increased lipid metabolism in ATMs upon forming CLS. Most importantly, adipocyte death, even in lean animals under homeostatic conditions, leads to a locally confined inflammation, which is in sharp contrast to other tissues. We identified cell size as cause for the described pro-inflammatory response, as the size of adipocytes is above a critical threshold size for efferocytosis, a process for anti-inflammatory removal of dead cells during tissue homeostasis. Finally, experiments on parabiotic mice verified that adipocyte death leads to a pro-inflammatory response of resident ATMs in vivo, without significant recruitment of blood monocytes. Our data indicate that adipocyte death triggers a unique degradation process and locally induces a metabolically activated ATM phenotype that is globally observed with obesity.

Macrophage beta2-adrenergic receptor is dispensable for the adipose tissue inflammation and function

ObjectiveNeuroimmune interactions between the sympathetic nervous system (SNS) and macrophages are required for the homeostasis of multiple tissues, including the adipose tissue. It has been proposed that the SNS maintains adipose tissue macrophages (ATMs) in an anti-inflammatory state via direct norepinephrine (NE) signaling to macrophages. This study aimed to investigate the physiological importance of this paradigm by utilizing a mouse model in which the adrenergic signaling from the SNS to macrophages, but not to other adipose tissue cells, was disrupted. MethodsWe generated a macrophage-specific B2AR knockout mouse (Adrb2ΔLyz2) by crossing Adrb2fl/fl and Lyz2Cre/+ mice. We have previously shown that macrophages isolated from Adrb2ΔLyz2 animals do not respond to NE stimulation in vitro. Herein we performed a metabolic phenotyping of Adrb2ΔLyz2 mice on either chow or high-fat diet (HFD). We also assessed the adipose tissue function of Adrb2ΔLyz2 animals during fasting and cold exposure. Finally, we transplanted Adrb2ΔLyz2 bone marrow to low-density lipoprotein receptor (LDLR) knockout mice and investigated the development of atherosclerosis during Western diet feeding. ResultsWe demonstrated that SNS-associated ATMs have a transcriptional profile indicative of activated beta-2 adrenergic receptor (B2AR), the main adrenergic receptor isoform in myeloid cells. However, Adrb2ΔLyz2 mice have unaltered energy balance on a chow or HFD. Furthermore, Adrb2ΔLyz2 mice show similar levels of adipose tissue inflammation and function during feeding, fasting, or cold exposure, and develop insulin resistance during HFD at the same rate as controls. Finally, macrophage-specific B2AR deletion does not affect the development of atherosclerosis on an LDL receptor-null genetic background. ConclusionsOverall, our data suggest that the SNS does not directly modulate the phenotype of adipose tissue macrophages in either lean mice or mouse models of cardiometabolic disease. Instead, sympathetic nerve activity exerts an indirect effect on adipose tissue macrophages through the modulation of adipocyte function.

Specific macrophage subsets accumulate in human subcutaneous and omental fat depots during obesity.

Obesity is a chronic inflammatory disease associated with adipose tissue macrophage (ATM) activation. ATMs from lean mice contribute to tissue homeostasis by their M2-oriented polarization, whereas obesity leads to an increase of M1 inflammatory ATMs that underlies obesity-related metabolic disorders. In humans, studies characterizing ATMs and their functional status are limited. Here we investigated ATM phenotype in visceral (VAT) and subcutaneous (SAT) adipose tissue from healthy lean and obese individuals using two molecules previously identified as markers of M1-like and M2-like/tissue-resident macrophages, the C-type lectin CLEC5A and the scavenger receptor CD163L1, respectively. CD163L1 was expressed by the majority of ATMs, and CD163L1+ ATM density was greater with respect to cells expressing the pan-macrophage markers CD68 or CD11b. ATM counts in SAT, but not in VAT, increased in obese compared to lean individuals, measured with the three markers. Accordingly, CD163L1, CD68 and ITGAM gene expression was significantly enhanced in obese with respect to control individuals only in SAT. CLEC5A+ ATMs had a proinflammatory profile and were abundant in the lean VAT, but their density diminished in obesity. The only ATM subset that increased its counts in the obese VAT had a mixed M1-like (CD11c+ CD163- CD209- ) and M2-like (CLEC5A- CD206+ ) phenotype. ATM expansion was dominated by a subset of M2-like macrophages (CD11c- CLEC5A- CD163+ CD206+ CD209+ ) in the obese SAT, with a minor contribution of a CD11c+ CLEC5A- ATM subpopulation. Thus, both SAT and VAT seems to limit inflammation during obesity by differentially altering their ATM subset composition.

SMRT Regulates Metabolic Homeostasis and Adipose Tissue Macrophage Phenotypes in Tandem.

The Silencing Mediator of Retinoid and Thyroid Hormone Receptors (SMRT) is a nuclear corepressor, regulating the transcriptional activity of many transcription factors critical for metabolic processes. While the importance of the role of SMRT in the adipocyte has been well-established, our comprehensive understanding of its in vivo function in the context of homeostatic maintenance is limited due to contradictory phenotypes yielded by prior generalized knockout mouse models. Multiple such models agree that SMRT deficiency leads to increased adiposity, although the effects of SMRT loss on glucose tolerance and insulin sensitivity have been variable. We therefore generated an adipocyte-specific SMRT knockout (adSMRT-/-) mouse to more clearly define the metabolic contributions of SMRT. In doing so, we found that SMRT deletion in the adipocyte does not cause obesity-even when mice are challenged with a high-fat diet. This suggests that adiposity phenotypes of previously described models were due to effects of SMRT loss beyond the adipocyte. However, an adipocyte-specific SMRT deficiency still led to dramatic effects on systemic glucose tolerance and adipocyte insulin sensitivity, impairing both. This metabolically deleterious outcome was coupled with a surprising immune phenotype, wherein most genes differentially expressed in the adipose tissue of adSMRT-/- mice were upregulated in pro-inflammatory pathways. Flow cytometry and conditioned media experiments demonstrated that secreted factors from knockout adipose tissue strongly informed resident macrophages to develop a pro-inflammatory, MMe (metabolically activated) phenotype. Together, these studies suggest a novel role for SMRT as an integrator of metabolic and inflammatory signals to maintain physiological homeostasis.

IEX-1 deficiency in hematopoietic cells mitigates high fat diet-induced obesity and glucose intolerance

Abstract We previously showed that IEX-1 paucity in mice inhibited high fat diet (HFD)-induced obesity and insulin resistance. IEX-1 deficiency induced browning of white adipose tissue (WAT) by promoting alternative activation of macrophages (AAMs), increasing energy expenditure. We now hypothesize that IEX-1 deficiency in macrophages promotes alternative activation of adipose tissue macrophages (ATM) and induces browning to inhibit obesity. We performed reciprocal bone marrow (BM) transplantation between IEX-1 knockout (KO) and wild-type (WT) mice to generate four types of chimeras. We fed these mice with HFD for 20 weeks and monitored their body weight, glucose disposal, and determined ATM phenotype using flow cytometry. While WT+WT-BM control mice (WT mice received WT bone marrow) gained ~80% body weight on HFD, WT+KO-BM mice (WT mice received KO bone marrow) increased their body weight only by 50% (p<0.01). Conversely, KO+WT-BM mice (KO mice received WT bone marrow) exhibited increased weight gain on HFD as compared to KO+KO-BM control mice that remained lean (p<0.05). Likewise, WT+KO-BM mice displayed greater glucose disposal as compared to WT control, and KO+WT-BM mice became glucose intolerant as compared to KO control as determined by glucose tolerance test. Analysis of ATM phenotype in mice fed with HFD for 5 weeks showed that HFD increased the percentage of classically activated macrophages (CAMs) by 3–4-folds (p<0.01), and decreased AAMs by ~30% (P<0.05) in epidydmal and inguinal WAT of WT+WT-BM mice. However, these effects were markedly suppressed in WT+KO-BM mice. Collectively, these data suggest that IEX-1 in hematopoietic cells contributes to HFD-induced obesity and glucose intolerance via inhibiting AAMs polarization.

Adipose tissue macrophages: Unique polarization and bioenergetics in obesity.

Macrophages comprise a majority of the resident immune cells in adipose tissue (AT) and regulate both tissue homeostasis in the lean state and metabolic dysregulation in obesity. Since the AT environment rapidly changes based upon systemic energy status, AT macrophages (ATMs) must adapt phenotypically and metabolically. There is a distinct dichotomy in the polarization and bioenergetics of in vitro models, with M2 macrophages utilizing oxidative phosphorylation (OX PHOS) and M1 macrophages utilizing glycolysis. Early studies suggested differential polarization of ATMs, with M2-like macrophages predominant in lean AT and M1-like macrophages in obese AT. However, recent studies show that the phenotypic plasticity of ATMs is far more complicated, which is also reflected in their bioenergetics. Multiple ATM populations exist along the M2 to M1 continuum and appear to utilize both glycolysis and OX PHOS in obesity. The significance of the dual fuel bioenergetics is unclear and may be related to an intermediate polarization, their buffering capacity, or the result of a mixed population of distinct polarized ATMs. Recent evidence also suggests that ATMs of lean mice serve as a substrate buffer or reservoir to modulate lipid, catecholamine, and iron availability. Furthermore, recent models of weight loss and weight cycling reveal additional roles for ATMs in systemic metabolism. Evaluating ATM phenotype and intracellular metabolism together may more accurately illuminate the consequences of ATM accumulation in obese AT, lending further insight into obesity-related comorbidities in humans.

Adipose Tissue Macrophage Phenotypes and Characteristics: The Key to Insulin Resistance in Obesity and Metabolic Disorders.

Obesity is one of the most serious global health problems, with an incidence that increases yearly and coincides with the development of a variety of associated comorbidities (e.g., type 2 diabetes, nonalcoholic fatty liver disease, some immune-related disorders). Although many studies have investigated the pathogenesis of overweight and obesity, multiple regulatory factors underlying the onset of obesity-related metabolic disorders remain elusive. Macrophages contribute to modulation of obesity-related inflammation and insulin resistance (IR); adipose tissue macrophages are particularly important in this context. Based on newly identified links between the chemokine system and obesity, macrophage polarization has become an essential target of new therapies for obesity-related IR. The findings of multiple studies imply that variations in gut microbiota and its metabolites might contribute to the regulation of obesity and related metabolic disorders. Recently, several novel antidiabetic drugs, applied as treatment for weight loss, were shown to be effective for obesity-induced IR and other comorbidities. The present review will discuss the properties and functions of macrophages in adipose tissue under conditions of obesity from three perspectives: the chemokine system, the gut microbiota, and antidiabetic drug application. It is proposed that macrophages might be a key therapeutic target for obesity-induced complications.

Peritoneal macrophages have an impaired immune response in obesity which can be reversed by subsequent weight loss

IntroductionObesity is recognized as a risk factor for various microbial infections. The immune system, which is affected by obesity, plays an important role in the pathophysiology of these infections and...

Alteration of adipose tissue immune cell milieu towards the suppression of inflammation in high fat diet fed mice by flaxseed oil supplementation

The present study evaluates the effect of flaxseed oil (FXO) supplementation on adipose tissue macrophages (ATM’s), E and D series resolvin (Rv) levels and adipose tissue inflammation. Male C57BL/6J mice were divided into five groups (n = 5): lean group (given standard chow diet), HFD group given high fat diet (approx. 18 weeks) till they developed insulin resistance and 4, 8 or 16 mg/kg group (HFD group later orally supplemented with 4, 8 or 16 mg/kg body weight flaxseed oil) for 4 weeks.The present study showed that FXO supplementation led to enhanced DHA, EPA, RvE1-E2, RvD2, RvD5- D6, IL-4, IL-10 and arginase 1 levels in ATMs together with altered immune cell infiltration and reduced NF-κB expression. The FXO supplementation suppresses immune cell infiltration into adipose tissue and alters adipose tissue macrophage phenotype towards the anti-inflammatory state via enhancement of E and D series resolvins, arginase 1 expression and anti-inflammatory cytokines level (IL-4 and IL-10.) leading to amelioration of insulin resistance in flaxseed oil supplemented HFD mice.

Hoxa5 alleviates obesity-induced chronic inflammation by reducing ER stress and promoting M2 macrophage polarization in mouse adipose tissue.

Obesity‐induced chronic inflammation is associated with endoplasmic reticulum stress (ERS) in adipocytes and changes in both the number and phenotype of adipose tissue macrophages (ATMs). In addition, ERS enhances macrophage activation. So far, the function of Hoxa5 in obesity‐induced chronic inflammation has been poorly understood. Herein, we demonstrate the importance of the transcription factor, Hoxa5, in determining adipose tissue macrophage (ATM) polarity and ERS. Hoxa5 decreased bodyweight, reduced inflammatory cytokine secretion and corresponded with an increased number of M2 macrophages in the adipose tissue of high‐fat diet (HFD) mice. Transcriptome sequencing data showed that overexpression of Hoxa5 in adipocytes changed expression of endoplasmic reticulum (ER) protein processing‐related genes. Based on transcriptome sequencing data and bioinformatics prediction, we have been suggested that Hoxa5 alleviated inflammatory responses by inhibiting ERS and by activating PPARγ pathway in mouse adipose tissue. Hoxa5 alleviated ERS and inflammatory responses by inhibiting the eIF2α/PERK signalling pathway in adipocytes. Hoxa5 also inhibited chronic inflammation of adipocytes by promoting M2 macrophage polarization. In addition, Hoxa5 transcriptionally activated the PPARγ pathway to promote polarization of M2 macrophages, which in turn alleviated chronic inflammation of adipocytes. Taken together, these results shed light on the mechanisms underlying Hoxa5‐dependent inhibition of obesity‐induced chronic inflammation by reducing ERS and promoting polarization of M2 macrophages. These results suggest that Hoxa5 may be a potential therapeutic target for obesity and other metabolic syndromes.

Netrin-1 Alters Adipose Tissue Macrophage Fate and Function in Obesity.

Macrophages accumulate prominently in the visceral adipose tissue (VAT) of obese humans and high fat diet (HFD) fed mice, and this is linked to insulin resistance and type II diabetes. While the mechanisms regulating macrophage recruitment in obesity have been delineated, the signals directing macrophage persistence in VAT are poorly understood. We previously showed that the neuroimmune guidance cue netrin-1 is expressed in the VAT of obese mice and humans, where it promotes macrophage accumulation. To better understand the source of netrin-1 and its effects on adipose tissue macrophage (ATM) fate and function in obesity, we generated mice with myeloid-specific deletion of netrin-1 (Ntn1fl/fl LysMCre+/–; Ntn1Δmac). Interestingly, Ntn1Δmac mice showed a modest decrease in HFD-induced adiposity and adipocyte size, in the absence of changes in food intake or leptin, that was accompanied by an increase in markers of adipocyte beiging (Prdm16, UCP-1). Using single cell RNA-seq, combined with conventional histological and flow cytometry techniques, we show that myeloid-specific deletion of netrin-1 caused a 50% attrition of ATMs in HFD-fed mice, particularly of the resident macrophage subset, and altered the phenotype of residual ATMs to enhance lipid handling. Pseudotime analysis of single cell transcriptomes showed that in the absence of netrin-1, macrophages in the obese VAT underwent a phenotypic switch with the majority of ATMs activating a program of genes specialized in lipid handling, including fatty acid uptake and intracellular transport, lipid droplet formation and lipolysis, and regulation of lipid localization. Furthermore, Ntn1Δmac macrophages had reduced expression of genes involved in arachidonic acid metabolism, and targeted LCMS/MS metabololipidomics analysis revealed decreases in proinflammatory eicosanoids (5-HETE, 6-trans LTB4, TXB2, PGD2) in the obese VAT. Collectively, our data show that targeted deletion of netrin-1 in macrophages reprograms the ATM phenotype in obesity, leading to reduced adipose inflammation, and improved lipid handling and metabolic function.

The Effect of Magnolia Bark on the Metabolic Inflammation and Insulin Resistance of ob/ob Mice

Objective This study was undertaken to investigate how magnolia bark extract affects ob/ob mouse in terms of metabolic inflammation and insulin resistance. Methods Leptin-deficient ob/ob mice were divided into 2 groups (n=5): a normal saline treatment (=control) and magnolia bark treatment. Wild type mice were the lean group (n=5). After 5 weeks, we measured fasting blood sugar (FBS) and conducted oral glucose tolerance tests (OGTTs) in each group. After 6 weeks, we measured body weight, epididymal fat pad weight, liver weight, serum glucose, serum insulin, and gene expression of tumor necrosis factor-α, interferon-γ, and interleukin-6. We characterized the phenotype of adipose tissue macrophages (ATMs) and analyzed fractions of the phenotype in each group by flow cytometry. Results In the magnolia bark group, fasting blood sugar, oral glucose tolerance levels, and insulin resistance (HOMA-IR) were significantly decreased. The population and proportion of ATMs among leukocytes in adipose tissue were significantly decreased in the magnolia bark group. The population and proportion of M1 type ATMs among ATMs were significantly decreased in the magnolia bark group. Gene expression of tumor necrosis factor-α was significantly decreased in the magnolia bark group. Conclusions These results support a positive effect of magnolia bark on metabolic diseases such as insulin resistance and metabolic inflammation in leptin-deficient ob/ob mice. Keywords: magnolia bark, ob/ob mouse, metabolic inflammation, insulin resistance, ATMs

Adipose tissue inflammation in breast cancer survivors: effects of a 16-week combined aerobic and resistance exercise training intervention.

Obesity is a leading modifiable contributor to breast cancer mortality due to its association with increased recurrence and decreased overall survival rate. Obesity stimulates cancer progression through chronic, low-grade inflammation in white adipose tissue, leading to accumulation of adipose tissue macrophages (ATMs), in particular, the pro-inflammatory M1 phenotype macrophage. Exercise has been shown to reduce M1 ATMs and increase the more anti-inflammatory M2 ATMs in obese adults. The purpose of this study was to determine whether a 16-week exercise intervention would positively alter ATM phenotype in obese postmenopausal breast cancer survivors. Twenty obese postmenopausal breast cancer survivors were randomized to a 16-week aerobic and resistance exercise (EX) intervention or delayed intervention control (CON). The EX group participated in 16weeks of supervised exercise sessions 3 times/week. Participants provided fasting blood, dual-energy X-ray absorptiometry (DXA), and superficial subcutaneous abdominal adipose tissue biopsies at baseline and following the 16-week study period. EX participants experienced significant improvements in body composition, cardiometabolic biomarkers, and systemic inflammation (all p<0.03 vs. CON). Adipose tissue from EX participants showed a significant decrease in ATM M1 (p<0.001), an increase in ATM M2 (p<0.001), increased adipose tissue secretion of anti-inflammatory cytokines such as adiponectin, and decreased secretion of the pro-inflammatory cytokines IL-6 and TNF- α (all p<0.055). A 16-week aerobic and resistance exercise intervention attenuates adipose tissue inflammation in obese postmenopausal breast cancer survivors. Future large randomized trials are warranted to investigate the impact of exercise-induced reductions in adipose tissue inflammation and breast cancer recurrence.