Short term high fat diet challenge promotes alternative macrophage polarization in adipose tissue via natural killer T cells and interleukin-4

Regulatory Roles of Invariant Natural Killer T Cells in Adipose Tissue Inflammation: Defenders Against Obesity-Induced Metabolic Complications

Adipose tissue is a metabolic organ that plays a central role in controlling systemic energy homeostasis. Compelling evidence indicates that immune system is closely linked to healthy physiologic functions and pathologic dysfunction of adipose tissue. In obesity, the accumulation of pro-inflammatory responses in adipose tissue subsequently leads to dysfunction of adipose tissue as well as whole body energy homeostasis. Simultaneously, adipose tissue also activates anti-inflammatory responses in an effort to reduce the unfavorable effects of pro-inflammation. Notably, the interplay between adipocytes and resident invariant natural killer T (iNKT) cells is a major component of defensive mechanisms of adipose tissue. iNKT cells are leukocytes that recognize lipids loaded on CD1d as antigens, whereas most other immune cells are activated by peptide antigens. In adipose tissue, adipocytes directly interact with iNKT cells by presenting lipid antigens and stimulate iNKT cell activation to alleviate pro-inflammation. In this review, we provide an overview of the molecular and cellular determinants of obesity-induced adipose tissue inflammation. Specifically, we focus on the roles of iNKT cell-adipocyte interaction in maintaining adipose tissue homeostasis as well as the consequent modulation in systemic energy metabolism. We also briefly discuss future research directions regarding the interplay between adipocytes and adipose iNKT cells in adipose tissue inflammation.

De-silencing Grb10 contributes to acute ER stress-induced steatosis in mouse liver

The growth factor receptor bound protein GRB10 is an imprinted gene product and a key negative regulator of the insulin, IGF1 and mTORC1 signaling pathways. GRB10 is highly expressed in mouse fetal liver but almost completely silenced in adult mice, suggesting a potential detrimental role of this protein in adult liver function. Here we show that the Grb10 gene could be reactivated in adult mouse liver by acute endoplasmic reticulum stress (ER stress) such as tunicamycin or a short-term high-fat diet (HFD) challenge, concurrently with increased unfolded protein response (UPR) and hepatosteatosis. Lipogenic gene expression and acute ER stress-induced hepatosteatosis were significantly suppressed in the liver of the liver-specific GRB10 knockout mice, uncovering a key role of Grb10 reactivation in acute ER stress-induced hepatic lipid dysregulation. Mechanically, acute ER stress induces Grb10 reactivation via an ATF4-mediated increase in Grb10 gene transcription. Our study demonstrates for the first time that the silenced Grb10 gene can be reactivated by acute ER stress and its reactivation plays an important role in the early development of hepatic steatosis.

Obesity associated disease risk: the role of inherent differences and location of adipose depots

Obesity and associated metabolic co-morbidities are a worldwide public health problem. Negative health outcomes associated with obesity, however, do not arise from excessive adiposity alone. Rather, deleterious outcomes of adipose tissue accumulation are a result of how adipocytes are distributed to individual regions in the body. Due to our increased understanding of the dynamic relationship that exists between specific adipose depots and disease risk, an accurate characterization of total body adiposity as well as location is required to properly evaluate a population's disease risk. Specifically, distinctive tissue depots within the body include the lower body, upper body and abdominal (deep and superficial) subcutaneous regions, as well as visceral (mesenteric and omental) regions. Upper body and visceral adipose tissues are highly associated with metabolic dysfunction and chronic disease development, whereas lower body gluteofemoral subcutaneous adipose tissue imparts protection against diet-induced metabolic derangement. Each adipose depot functions distinctly as an endocrine organ hence it has a different level of impact on health outcomes. Effluent from adipose tissue can modulate the functions of other tissues, whilst receiving differential communication from the rest of the body via central nervous system innervation, metabolites and other signaling molecules. More so, adipose depots contain a diverse reservoir of tissue-resident immune cells that play an integral part in both maintaining tissue homeostasis, as well as propagating metabolically-induced inflammation. Overall, the conceptualization of obesity and associated risks needs updating to reflect the complexities of obesity. We review adipose tissue characteristics that are linked to deleterious or beneficial adipose tissue distributions.

Adipocyte CD1d determines adipose inflammation and insulin resistance in obesity

Obesity-induced adipose tissue inflammation is regulated by various immune cells for innate and adaptive immunity. Among adipose tissue immune cells, it has been proposed that invariant Natural Killer T (iNKT) cells play crucial roles in anti-inflammatory responses in obesity. iNKT cells recognize 'lipid' antigens loaded on CD1d of antigen presenting cells and modulate immune responses by secreting Th1 or Th2 type cytokines depending on species of lipid antigens, antigen presenting cell types, and environmental cytokine milieu. However, the regulatory mechanisms of antigen presenting cells for adipose iNKT cell stimulation have not been clearly elucidated. Recently, we have reported that CD1d expressing adipocytes could act as an antigen presenting cell for adipose iNKT cells by characterization of adipocyte-specific CD1d knockout (CD1d^ADKO) mice. Upon high-fat diet (HFD) feeding, CD1d^ADKO mice aggravated adipose tissue inflammation and insulin resistance compared with CD1d^f/f mice. In this commentary, we provide the additional data of adipocyte CD1d-dependent regulation of adipose iNKT cell responses as well as systemic insulin sensitivity. In addition, we discuss how the interaction between adipocytes and iNKT cells would be regulated with the progression of obesity.

Frontline Science: Rapid adipose tissue expansion triggers unique proliferation and lipid accumulation profiles in adipose tissue macrophages

Obesity-related changes in adipose tissue leukocytes, in particular adipose tissue macrophages (ATMs) and dendritic cells (ATDCs), are implicated in metabolic inflammation, insulin resistance, and altered regulation of adipocyte function. We evaluated stromal cell and white adipose tissue (WAT) expansion dynamics with high fat diet (HFD) feeding for 3-56 days, quantifying ATMs, ATDCs, endothelial cells (ECs), and preadipocytes (PAs) in visceral epididymal WAT and subcutaneous inguinal WAT. To better understand mechanisms of the early response to obesity, we evaluated ATM proliferation and lipid accumulation. ATMs, ATDCs, and ECs increased with rapid WAT expansion, with ATMs derived primarily from a CCR2-independent resident population. WAT expansion stimulated proliferation in resident ATMs and ECs, but not CD11c⁺ ATMs or ATDCs. ATM proliferation was unperturbed in Csf2- and Rag1-deficient mice with WAT expansion. Additionally, ATM apoptosis decreased with WAT expansion, and proliferation and apoptosis reverted to baseline with weight loss. Adipocytes reached maximal hypertrophy at 28 days of HFD, coinciding with a plateau in resident ATM accumulation and the appearance of lipid-laden CD11c⁺ ATMs in visceral epididymal WAT. ATM increases were proportional to tissue expansion and adipocyte hypertrophy, supporting adipocyte-mediated regulation of resident ATMs. The appearance of lipid-laden CD11c⁺ ATMs at peak adipocyte size supports a role in responding to ectopic lipid accumulation within adipose tissue. In contrast, ATDCs increase independently of proliferation and may be derived from circulating precursors. These changes precede and establish the setting in which large-scale adipose tissue infiltration of CD11c⁺ ATMs, inflammation, and adipose tissue dysfunction contributes to insulin resistance.

Effects of hereditary moderate high fat diet on metabolic performance and physical endurance capacity in C57BL/6 offspring

Obesity in pregnant women presents a risk to fetal health, leading to numerous metabolic syndromes and chronic inflammation risks. Previously, physical exercise was considered to be one of the primary treatments for obesity. However, the effect of fat consumption throughout the life cycle on physical endurance capacity remains unknown. A total of two groups of female mice (age, 6 weeks; C57BL/6J) were fed with a normal chow diet and a moderate high fat diet (MHFD), during pregnancy and lactation (8 weeks), with the offspring receiving the same diet as the mother. When filial mice were 8, 16 and 24 weeks old, they were tested for endurance, blood pressure (BP) and glucose tolerance, as well as adipose tissue infiltration and macrophage subtype. Compared with the control group, filial mice in MHFD groups exhibited increased BP and glucose levels and larger adipose cells (~4‑fold). During adolescence, the obese filial mice demonstrated increased endurance compared with controls. Endurance declines in middle and old age; the endurance of aged obese mice was 29% that of lean ones. In addition, body coordination and movement memory did not notably change. The expression of cluster of differentiation 68, one of the most reliable markers of macrophages, increased by 2.48‑fold, demonstrating that macrophages were recruited and underwent infiltration. In addition, increased tumor necrosis factor‑α and decreased interleukin‑10 expression demonstrated that infiltrated macrophages are polarized to the M1 state, which weakens physical endurance and resists type M2 macrophages, which exhibit repairing functions. In conclusion, hereditary MHFD weakens physical endurance and alters the metabolic characteristics of C57BL/6 offspring.

Chronic Trypanosoma cruzi infection potentiates adipose tissue macrophage polarization toward an anti-inflammatory M2 phenotype and contributes to diabetes progression in a diet-induced obesity model

Chronic obesity and Chagas disease (caused by the protozoan Trypanosoma cruzi) represent serious public health concerns. The interrelation between parasite infection, adipose tissue, immune system and metabolism in an obesogenic context, has not been entirely explored. A novel diet-induced obesity model (DIO) was developed in C57BL/6 wild type mice to examine the effect of chronic infection (DIO+I) on metabolic parameters and on obesity-related disorders. Dyslipidemia, hyperleptinemia, and cardiac/hepatic steatosis were strongly developed in DIO mice. Strikingly, although these metabolic alterations were collectively improved by infection, plasmatic apoB100 levels remain significantly increased in DIO+I, suggesting the presence of pro-atherogenic small and dense LDL particles. Moreover, acute insulin resistance followed by chronic hyperglycemia with hypoinsulinemia was found, evidencing an infection-related-diabetes progression. These lipid and glucose metabolic changes seemed to be highly dependent on TLR4 expression since TLR4-/- mice were protected from obesity and its complications. Notably, chronic infection promoted a strong increase in MCP-1 producing macrophages with a M2 (F4/80+CD11c-CD206+) phenotype associated to oxidative stress in visceral adipose tissue of DIO+I mice. Importantly, infection reduced lipid content but intensified inflammatory infiltrates in target tissues. Thus, parasite persistence in an obesogenic environment and the resulting host immunometabolic dysregulation may contribute to diabetes/atherosclerosis progression.

Endogenous lipid antigens for invariant natural killer T cells hold the reins in adipose tissue homeostasis

The global obesity epidemic and its associated co-morbidities, including type 2 diabetes, cardiovascular disease and certain types of cancers, have drawn attention to the pivotal role of adipocytes in health and disease. Besides their 'classical' function in energy storage and release, adipocytes interact with adipose-tissue-resident immune cells, among which are lipid-responsive invariant natural killer T (iNKT) cells. The iNKT cells are activated by lipid antigens presented by antigen-presenting cells as CD1d/lipid complexes. Upon activation, iNKT cells can rapidly secrete soluble mediators that either promote or oppose inflammation. In lean adipose tissue, iNKT cells elicit a predominantly anti-inflammatory immune response, whereas obesity is associated with declining iNKT cell numbers. Recent work showed that adipocytes act as non-professional antigen-presenting cells for lipid antigens. Here, we discuss endogenous lipid antigen processing and presentation by adipocytes, and speculate on how these lipid antigens, together with 'environmental factors' such as tissue/organ environment and co-stimulatory signals, are able to influence the fate of adipose-tissue-resident iNKT cells, and thereby the role of these cells in obesity and its associated pathologies.

Increased hepatic fatty acid polyunsaturation precedes ectopic lipid deposition in the liver in adaptation to high-fat diets in mice

OBJECTIVE

We monitored hepatic lipid content (HLC) and fatty acid (FA) composition in the context of enhanced lipid handling induced by a metabolic high-fat diet (HFD) challenge and fasting.

MATERIALS AND METHODS

Mice received a control diet (10% of kilocalories from fat, N = 14) or an HFD (45% or 60% of kilocalories from fat, N = 10 and N = 16, respectively) for 26 weeks. A subset of five mice receiving an HFD (60% of kilocalories from fat) were switched to the control diet for the final 7 weeks. At nine time points, magnetic resonance spectroscopy was performed in vivo at 14.1 T, interleaved with glucose tolerance tests.

RESULTS

Glucose intolerance promptly developed with the HFD, followed by a progressive increase of fasting insulin level, simultaneously with that of HLC. These metabolic defects were normalized by dietary reversal. HFD feeding immediately increased polyunsaturation of hepatic FA, before lipid accumulation. Fasting-induced changes in hepatic lipids (increased HLC and FA polyunsaturation, decreased FA monounsaturation) in control-diet-fed mice were not completely reproduced in HFD-fed mice, not even after dietary reversal.

CONCLUSION

A similar adaptation of hepatic lipids to both fasting and an HFD suggests common mechanisms of lipid trafficking from adipose tissue to the liver. Altered hepatic lipid handling with fasting indicates imperfect metabolic recovery from HFD exposure.

Insulin action and resistance in obesity and type 2 diabetes

Nutritional excess is a major forerunner of type 2 diabetes. It enhances the secretion of insulin, but attenuates insulin's metabolic actions in the liver, skeletal muscle and adipose tissue. However, conflicting evidence indicates a lack of knowledge of the timing of these events during the development of obesity and diabetes, pointing to a key gap in our understanding of metabolic disease. This Perspective reviews alternate viewpoints and recent results on the temporal and mechanistic connections between hyperinsulinemia, obesity and insulin resistance. Although much attention has addressed early steps in the insulin signaling cascade, insulin resistance in obesity seems to be largely elicited downstream of these steps. New findings also connect insulin resistance to extensive metabolic cross-talk between the liver, adipose tissue, pancreas and skeletal muscle. These and other advances over the past 5 years offer exciting opportunities and daunting challenges for the development of new therapeutic strategies for the treatment of type 2 diabetes.

CD40 signaling and hepatic steatosis: Unanticipated links

Obesity predisposes to an increased risk of nonalcoholic fatty liver disease (NAFLD). Hepatic steatosis is the key pathological feature of NAFLD and has emerged as a metabolic disorder in which innate and adaptive arms of the immune response play a central role in disease pathogenesis. Recent studies have revealed unexpected relationships between CD40 signaling and hepatic steatosis in high fat diet rodent models. CD154, the ligand of CD40, is a mediator of inflammation and controls several critical events of innate and adaptive immune responses. In the light of these reports, we discuss potential links between CD40 signaling and hepatic steatosis in NAFLD.

Unusual Suspects in the Development of Obesity-Induced Inflammation and Insulin Resistance: NK cells, iNKT cells, and ILCs

The notion that obesity-induced inflammation mediates the development of insulin resistance in animal models and humans has been gaining strong support. It has also been shown that immune cells in local tissues, in particular in visceral adipose tissue, play a major role in the regulation of obesity-induced inflammation. Specifically, obesity increases the numbers and activation of proinflammatory immune cells, including M1 macrophages, neutrophils, Th1 CD4 T cells, and CD8 T cells, while simultaneously suppressing anti-inflammatory cells such as M2 macrophages, CD4 regulatory T cells, regulatory B cells, and eosinophils. Recently, however, new cell types have been shown to participate in the development of obesity-induced inflammation and insulin resistance. Some of these cell types also appear to regulate obesity. These cells are natural killer (NK) cells and innate lymphoid cells (ILCs), which are closely related, and invariant natural killer T (iNKT) cells. It should be noted that, although iNKT cells resemble NK cells in name, they are actually a completely different cell type in terms of their development and functions in immunity and metabolism. In this review, we will focus on the roles that these relatively new players in the metabolism field play in obesity-induced insulin resistance and the regulation of obesity.

M2-specific reduction of CD1d switches NKT cell-mediated immune responses and triggers metaflammation in adipose tissue

Metaflammation is responsible for several metabolic syndromes, such as type 2 diabetes. However, the mechanisms by which metabolic disorders trigger metaflammation remain unclear. We identified a cell type-specific downregulation of CD1d expression in M2 macrophages during the progression of obesity prior to the onset of inflammation in visceral adipose tissues. A reduction in CD1d expression influenced the ability of M2 macrophages to present antigens and caused a change in antigen-presenting cells from M2 macrophages to M1 macrophages. With CD1d conditional knockout (KO) mice, we further demonstrated that natural killer T (NKT) cell activation by M2 macrophages inhibited metaflammation and insulin resistance by promoting Th2 responses and M2 polarization in visceral adipose tissues of obese mice, whereas NKT cell activation by M1 macrophages exacerbated metaflammation and insulin resistance by promoting Th1 responses and inhibiting M2 polarization. Our results suggest that an M2-specific reduction of CD1d is an initiating event that switches NKT cell-mediated immune responses and disrupts the immune balance in visceral adipose tissues in obese mice.

Deletion of CD1d in Adipocytes Aggravates Adipose Tissue Inflammation and Insulin Resistance in Obesity

Adipose tissue inflammation is an important factor in obesity that promotes insulin resistance. Among various cell types in adipose tissue, immune cells actively regulate inflammatory responses and affect whole-body energy metabolism. In particular, invariant natural killer T (iNKT) cells contribute to mitigating dysregulation of systemic energy homeostasis by counteracting obesity-induced inflammation in adipose tissue. However, the molecular mechanisms by which adipose iNKT cells become activated and mediate anti-inflammatory roles in obese adipose tissue have not been thoroughly understood yet. In the current study, we demonstrate that adipocyte CD1d plays a key role in the stimulation of adipose iNKT cells, leading to anti-inflammatory responses in high-fat diet (HFD)-fed mice. Accordingly, adipocyte-specific CD1d-knockout (CD1d^ADKO) mice showed reduced numbers of iNKT cells in adipose tissues and decreased responses to α-galactosylceramide-induced iNKT cell activation. Additionally, HFD-fed CD1d^ADKO mice revealed reduced interleukin-4 expression in adipose iNKT cells and aggravated adipose tissue inflammation and insulin resistance. Collectively, these data suggest that adipocytes could selectively stimulate adipose iNKT cells to mediate anti-inflammatory responses and attenuate excess proinflammatory responses in obese adipose tissue.

Lower levels of IL-4 and IL-10 influence lipodystrophy in HIV/AIDS patients under antiretroviral therapy

BACKGROUND

The role of interleukins in the pathogenesis of lipodystrophy in HIV/AIDS-patients is still not understood. The aim of this study was to evaluate the relationship between serum levels of interleukins between HIV/AIDS-patients with or without lipodystrophy, as well as between different subgroups of lipodystrophy (lipoatrophy, lipohypertrophy, mixed-fat-redistribution) and patients without lipodystrophy.

METHODS

Cross-sectional study of 66 HIV/AIDS patients, all Caucasians. Serum levels of interleukins (IL-1α, IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10) were measured using Cytokine-Array-1 on Evidence Investigator, Biochip Array Technology. The associations between interleukins and anthropometric and metabolic variables were estimated by Spearman-correlation. Analysis of covariance with bootstrapping method (ACBM) was used to examine relationship between interleukins and lipodystrophy categories adjusted for confounding variables.

RESULTS

The lipodystrophy was observed in 29 (44%) patients, while 15 (52%) had lipoatrophy, 4 (14%) lipohypertrophy and 10 (34%) patients had mixed fat redistribution. There were 37 (56%) patients without lipodystrophy. Significantly lower levels of IL-4 and IL-10 were observed in lipodystrophy vs. non-lipodystrophy (p=0.008; p=0.027, respectively). No differences were found relating IL-1α, IL-1β, IL-2, IL-6 and IL-8 levels in lipodystrophy vs. non-lipodystrophy. In patient subgroup with lipoatrophy, significantly lower levels of IL-4 and IL-10 were found when compared to non-lipodystrophy (p=0.043; p=0.031, respectively). In lipohypertrophy subgroup significantly lower levels of IL-4 were found when compared to non-lipodystrophy (p=0.003). In order to estimate the correlation of IL-4 and IL-10 and the presence of lipodystrophy, ACBM showed that correlation of IL-4 levels in patients with lipodystrophy remains statistically significant (p=0.004) in all types of lipodystrophy: lipoatrophy, lipohypertrophy and mix-fat-redistribution (p=0.027; p=0.009; p=0.017, respectively) after adjustment for age, BMI.

CONCLUSIONS

IL-4 and IL-10 levels were significantly lower in lipodystrophy vs. non-lipodystrophy. According to our knowledge, we showed for the first time significant correlation between IL-4 levels and lipodystrophy development in HIV/AIDS patients.

The initiation of metabolic inflammation in childhood obesity

An understanding of the events that initiate metabolic inflammation (metainflammation) can support the identification of targets for preventing metabolic disease and its negative effects on health. There is ample evidence demonstrating that the initiating events in obesity-induced inflammation start early in childhood. This has significant implications on our understanding of how early life events in childhood influence adult disease. In this Review we frame the initiating events of metainflammation in the context of child development and discuss what this reveals about the mechanisms by which this unique form of chronic inflammation is initiated and sustained into adulthood.

The ominous triad of adipose tissue dysfunction: inflammation, fibrosis, and impaired angiogenesis

There are three dominant contributors to the pathogenesis of dysfunctional adipose tissue (AT) in obesity: unresolved inflammation, inappropriate extracellular matrix (ECM) remodeling and insufficient angiogenic potential. The interactions of these processes during AT expansion reflect both a linear progression as well as feed-forward mechanisms. For example, both inflammation and inadequate angiogenic remodeling can drive fibrosis, which can in turn promote migration of immune cells into adipose depots and impede further angiogenesis. Therefore, the relationship between the members of this triad is complex but important for our understanding of the pathogenesis of obesity. Here we untangle some of these intricacies to highlight the contributions of inflammation, angiogenesis, and the ECM to both "healthy" and "unhealthy" AT expansion.

Peripheral invariant natural killer T cell deficiency in metabolically unhealthy but normal weight versus metabolically healthy but obese individuals

Objective

To investigate the proportion of circulating invariant natural killer T (iNKT) cells in four body health types.

Methods

In this cross-sectional study, participants were classified into four body health types according to the body mass index and metabolic status: metabolically healthy and normal weight (MHNW), metabolically unhealthy but normal weight (MUNW), metabolically healthy but obese (MHO), or metabolically unhealthy and obese (MUO). Demographic and clinical characteristics were measured, and the homeostasis model assessment of insulin resistance (HOMA-IR) and visceral adiposity index (VAI) were calculated. The proportion of circulating iNKT cells was also evaluated by flow cytometry.

Results

The study enrolled 41 MHNW, 37 MUNW, 30 MHO, and 43 MUO participants. Compared with the MHNW group, the MUNW, MHO, and MUO groups had significantly lower iNKT cell proportions. The iNKT cell proportion was significantly higher in the MHO group than the MUNW and MUO groups. The iNKT cell proportion was inversely correlated with high-sensitivity C-reactive protein, HOMA-IR, and VAI values.

Conclusion

The proportion of iNKT cells was lower in people (lean or obese) with excessive visceral fat accumulation, suggesting that iNKT cell deficiency may be involved in the pathophysiology of obesity-related metabolic disorders.

Regulation of Nlrp3 inflammasome by dietary metabolites

The bidirectional communication between innate immune cells and energy metabolism is now widely appreciated to regulate homeostasis as well as chronic diseases that emerge from dysregulated inflammation. Macronutrients-derived from diet or endogenous pathways that generate and divert metabolites into energetic or biosynthetic pathways – regulate the initiation, duration and cessation of the inflammatory response. The NLRP3 inflammasome is an important innate sensor of structurally diverse metabolic damage-associated molecular patterns (DAMPs) that has been implicated in a wide range of inflammatory disorders associated with caloric excess, adiposity and aging. Understanding the regulators of immune-metabolic interactions and their contribution towards chronic disease mechanisms, therefore, has the potential to reduce disease pathology, improve quality of life in elderly and promote the extension of healthspan. Just as specialized subsets of immune cells dampen inflammation through the production of negative regulatory cytokines; specific immunoregulatory metabolites can deactivate inflammasome-mediated immune activation. Here, we highlight the role of energy substrates, alternative fuels and metabolic DAMPs in the regulation of the NLRP3 inflammasome and discuss potential dietary interventions that may impact sterile inflammatory disease.

iNKT Cells Induce FGF21 for Thermogenesis and Are Required for Maximal Weight Loss in GLP1 Therapy

Adipose-resident invariant natural killer T (iNKT) cells are key players in metabolic regulation. iNKT cells are innate lipid sensors, and their activation, using their prototypic ligand α-galactosylceramide (αGalCer), induces weight loss and restores glycemic control in obesity. Here, iNKT activation induced fibroblast growth factor 21 (FGF21) production and thermogenic browning of white fat. Complete metabolic analysis revealed that iNKT cell activation induced increased body temperature, V02, VC02, and fatty acid oxidation, without affecting food intake or activity. FGF21 induction played a major role in iNKT cell-induced weight loss, as FGF21 null mice lost significantly less weight after αGalCer treatment. The glucagon-like peptide 1 (GLP-1) receptor agonist, liraglutide, also activated iNKT cells in humans and mice. In iNKT-deficient mice, liraglutide promoted satiety but failed to induce FGF21, resulting in less weight loss. These findings reveal an iNKT cell-FGF21 axis that defines a new immune-mediated pathway that could be targeted for glycemic control and weight regulation.

The Sel1L-Hrd1 Endoplasmic Reticulum-Associated Degradation Complex Manages a Key Checkpoint in B Cell Development

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a principal mechanism that targets ER-associated proteins for cytosolic proteasomal degradation. Here, our data demonstrate a critical role for the Sel1L-Hrd1 complex, the most conserved branch of ERAD, in early B cell development. Loss of Sel1L-Hrd1 ERAD in B cell precursors leads to a severe developmental block at the transition from large to small pre-B cells. Mechanistically, we show that Sel1L-Hrd1 ERAD selectively recognizes and targets the pre-B cell receptor (pre-BCR) for proteasomal degradation in a BiP-dependent manner. The pre-BCR complex accumulates both intracellularly and at the cell surface in Sel1L-deficient pre-B cells, leading to persistent pre-BCR signaling and pre-B cell proliferation. This study thus implicates ERAD mediated by Sel1L-Hrd1 as a key regulator of B cell development and reveals the molecular mechanism underpinning the transient nature of pre-BCR signaling.

Regulation of metabolism by the innate immune system

Low-grade tissue inflammation induced by obesity can result in insulin resistance, which in turn is a key cause of type 2 diabetes mellitus. Cells of the innate immune system produce cytokines and other factors that impair insulin signalling, which contributes to the connection between obesity and the onset of type 2 diabetes mellitus. Here, we review the innate immune cells involved in secreting inflammatory factors in the obese state. In the adipose tissue, these cells include proinflammatory adipose tissue macrophages and natural killer cells. We also discuss the role of innate immune cells, such as anti-inflammatory adipose tissue macrophages, eosinophils, group 2 innate lymphoid cells and invariant natural killer T cells, in maintaining an anti-inflammatory and insulin-sensitive environment in the lean state. In the liver, both Kupffer cells and recruited hepatic macrophages can contribute to decreased hepatic insulin sensitivity. Proinflammatory macrophages might also adversely affect insulin sensitivity in the skeletal muscle and pancreatic β-cell function. Finally, this Review provides an overview of the mechanisms for regulating proinflammatory immune responses that could lead to future therapeutic opportunities to improve insulin sensitivity.

Adipose Tissue Remodeling: Its Role in Energy Metabolism and Metabolic Disorders

The adipose tissue is a central metabolic organ in the regulation of whole-body energy homeostasis. The white adipose tissue functions as a key energy reservoir for other organs, whereas the brown adipose tissue accumulates lipids for cold-induced adaptive thermogenesis. Adipose tissues secrete various hormones, cytokines, and metabolites (termed as adipokines) that control systemic energy balance by regulating appetitive signals from the central nerve system as well as metabolic activity in peripheral tissues. In response to changes in the nutritional status, the adipose tissue undergoes dynamic remodeling, including quantitative and qualitative alterations in adipose tissue-resident cells. A growing body of evidence indicates that adipose tissue remodeling in obesity is closely associated with adipose tissue function. Changes in the number and size of the adipocytes affect the microenvironment of expanded fat tissues, accompanied by alterations in adipokine secretion, adipocyte death, local hypoxia, and fatty acid fluxes. Concurrently, stromal vascular cells in the adipose tissue, including immune cells, are involved in numerous adaptive processes, such as dead adipocyte clearance, adipogenesis, and angiogenesis, all of which are dysregulated in obese adipose tissue remodeling. Chronic overnutrition triggers uncontrolled inflammatory responses, leading to systemic low-grade inflammation and metabolic disorders, such as insulin resistance. This review will discuss current mechanistic understandings of adipose tissue remodeling processes in adaptive energy homeostasis and pathological remodeling of adipose tissue in connection with immune response.

A short bout of HFD promotes long-lasting hepatic lipid accumulation

A short bout of high fat diet (HFD) impairs glucose tolerance and induces hepatic steatosis in mice. Here, we aimed to elaborate on long-lasting effects of short-term high fat feeding. As expected, one week of HFD significantly impaired glucose tolerance. Intriguingly, recovery feeding with a standard rodent diet for 8 weeks did not fully normalize glucose tolerance. In addition, mice exposed to a short bout of HFD revealed significantly increased liver fat accumulation paralleled by elevated portal free fatty acid levels after 8 weeks of recovery feeding compared to exclusively chow-fed littermates. In conclusion, a short bout of HFD has long-lasting effects on hepatic lipid accumulation and glucose tolerance.

From neutrophils to macrophages: differences in regional adipose tissue depots

Currently, we do not fully understand the underlying mechanisms of how regional adiposity promotes metabolic dysregulation. As adipose tissue expands, there is an increase in chronic systemic low-grade inflammation due to greater infiltration of immune cells and production of cytokines. This chronic inflammation is thought to play a major role in the development of metabolic complications and disease such as insulin resistance and diabetes. We know that different adipose tissue depots contribute differently to the risk of metabolic disease. People who have an upper body fat distribution around the abdomen are at greater risk of disease than those who tend to store fat in their lower body around the hips and thighs. Thus, it is conceivable that adipose tissue depots contribute differently to the inflammatory milieu as a result of varied infiltration of immune cell types. In this review, we describe the role and function of major resident immune cells in the development of adipose tissue inflammation and discuss their regional differences in the context of metabolic disease risk. We find that although initial studies have found regional differences, a more comprehensive understanding of how immune cells interrupt adipose tissue homeostasis is needed.

Short- or long-term high-fat diet feeding plus acute ethanol binge synergistically induce acute liver injury in mice: an important role for CXCL1

Obesity and alcohol consumption often coexist and work synergistically to promote steatohepatitis; however, the underlying mechanisms remain obscure. Here, we demonstrate that feeding mice a high-fat diet (HFD) for as little as 3 days markedly exacerbated acute ethanol binge-induced liver neutrophil infiltration and injury. Feeding mice with an HFD for 3 months plus a single binge of ethanol induced much more severe steatohepatitis. Moreover, 3-day or 3-month HFD-plus-ethanol binge (3d-HFD+ethanol or 3m-HFD+ethanol) treatment markedly up-regulated the hepatic expression of several chemokines, including chemokine (C-X-C motif) ligand 1 (Cxcl1), which showed the highest fold (approximately 20-fold and 35-fold, respectively) induction. Serum CXCL1 protein levels were also markedly elevated after the HFD+ethanol treatment. Blockade of CXCL1 with a CXCL1 neutralizing antibody or genetic deletion of the Cxcl1 gene reduced the HFD+ethanol-induced hepatic neutrophil infiltration and injury, whereas overexpression of Cxcl1 exacerbated steatohepatitis in HFD-fed mice. Furthermore, expression of Cxcl1 messenger RNA was up-regulated in hepatocytes, hepatic stellate cells, and endothelial cells isolated from HFD+ethanol-fed mice compared to mice that were only given the HFD, with the highest fold induction observed in hepatocytes. In vitro stimulation of hepatocytes with palmitic acid up-regulated the expression of Cxcl1 messenger RNA, and this up-regulation was attenuated after treatment with an inhibitor of extracellular signal-regulated kinase 1/2, c-Jun N-terminal kinase, or nuclear factor κB. In addition, hepatic or serum levels of free fatty acids were higher in HFD+ethanol-fed mice than in the control groups.

CONCLUSION

An HFD combined with acute ethanol consumption synergistically induces acute liver inflammation and injury through the elevation of hepatic or serum free fatty acids and subsequent up-regulation of hepatic CXCL1 expression and promotion of hepatic neutrophil infiltration.

Inflammation-induced formation of fat-associated lymphoid clusters

Fat-associated lymphoid clusters (FALCs) are a type of lymphoid tissue associated with visceral fat. Here we found that the distribution of FALCs was heterogeneous, with the pericardium containing large numbers of these clusters. FALCs contributed to the retention of B-1 cells in the peritoneal cavity through high expression of the chemokine CXCL13, and they supported B cell proliferation and germinal center differentiation during peritoneal immunological challenges. FALC formation was induced by inflammation, which triggered the recruitment of myeloid cells that expressed tumor-necrosis factor (TNF) necessary for signaling via the TNF receptors in stromal cells. Natural killer T cells (NKT cells) restricted by the antigen-presenting molecule CD1d were likewise required for the inducible formation of FALCs. Thus, FALCs supported and coordinated the activation of innate B cells and T cells during serosal immune responses.

Estradiol regulates insulin signaling and inflammation in adipose tissue

BACKGROUND

Obesity-associated low-grade inflammation at white adipose tissue (WAT) leads to metabolic defects. Sex steroid hormone estrogen may be protective against high-fat diet (HFD)-induced obesity and insulin resistance. This has been tested by many previous studies utilizing rodent models of ovariectomy (OVX) and/or treatment of estradiol (E2), the major biologically active form of estrogen. Body weight and adiposity are increased by OVX and reduced following E2 treatment, however. Thus, the protective roles of E2 may be secondary effects to the changes in body weight and adiposity. We hypothesize that E2 directly prevents inflammation and maintains insulin sensitivity in WAT independent of energy status using mice with similar body weights and adiposity.

MATERIALS AND METHODS

Four groups of female C57BL/6 mice were used, including sham-operated mice treated with vehicle for E2 and fed with either a low-fat diet (LFD; Sham-Veh-LFD) or a HFD (Sham-Veh-HFD), and HFD-fed OVX mice treated with either vehicle (OVX-Veh-HFD) or E2 (OVX-E2-HFD). Body weight and abdominal parametrial WAT mass, insulin signaling, and expression levels of genes related to low-grade inflammation in WAT were compared between these groups pair-fed with equal amounts of calories for a period of 4 days.

RESULTS

Body weights and WAT mass were similar in all four groups. OVX-Veh-HFD mice had impaired insulin signaling associated with rapid activation of inflammation, whereas OVX-E2-HFD group maintained insulin sensitivity without showing inflammation in WAT.

CONCLUSIONS

E2 directly contributed to the maintenance of insulin sensitivity during the early phase of development of metabolic dysfunction, possibly via preventing low-grade inflammation in WAT.

IL-4 and IL-6 levels and adipose tissue distribution in HIV-1 patients under antiretroviral therapy

PURPOSE

Combined antiretroviral therapy (cART) for the treatment of HIV-1 infection has been associated with complications, including lipodystrophy. Several interleukins have been implicated in the pathology and physiology of lipodystrophy. The present study aimed to compare the levels of IL-4 and IL-6 in HIV-1 patients under cART with and without, clinically and fat mass ratio defined, lipodystrophy and in four different groups of fat distribution: (1) no lipodystrophy; (2) isolated central fat accumulation; (3) isolated lipoatrophy and (4) mixed forms of lipodystrophy.

METHODS

In the present cross-sectional study we evaluated IL-4 and IL-6 levels, insulin resistance and insulin sensitivity indexes in 86 HIV-infected adults under cART.

RESULTS

No significant differences in IL-4 and IL-6 levels between the four groups of body composition were observed. Patients with HOMA-IR >4 presented higher levels of IL-6 and lower levels of IL-4, although without statistical significance. No correlation between IL-6, or IL-4, HOMA-IR and quantitative body fat mass distribution was found.

CONCLUSION

Although there was a tendency for patients with isolated lipoatrophy and isolated fat accumulation to present higher IL-6 levels, these differences were not statistically significant. No differences were found relating IL-4 levels.

The gut-adipose-liver axis in the metabolic syndrome

Obesity is associated with altered gut microbiota composition and impaired gut barrier function. These changes, together with interrelated mesenteric adipose tissue inflammation, result in increased release of pro-inflammatory cytokines, bacteria-derived factors, and lipids into the portal circulation, promoting the development of (hepatic) insulin resistance. Herein, the potential impact of obesity-related changes in gut and visceral adipose tissue biology on the development of insulin resistance and Type 2 diabetes is reviewed.

Understanding the Mysterious M2 Macrophage through Activation Markers and Effector Mechanisms

The alternatively activated or M2 macrophages are immune cells with high phenotypic heterogeneity and are governing functions at the interface of immunity, tissue homeostasis, metabolism, and endocrine signaling. Today the M2 macrophages are identified based on the expression pattern of a set of M2 markers. These markers are transmembrane glycoproteins, scavenger receptors, enzymes, growth factors, hormones, cytokines, and cytokine receptors with diverse and often yet unexplored functions. This review discusses whether these M2 markers can be reliably used to identify M2 macrophages and define their functional subdivisions. Also, it provides an update on the novel signals of the tissue environment and the neuroendocrine system which shape the M2 activation. The possible evolutionary roots of the M2 macrophage functions are also discussed.

In adipose tissue, increased mitochondrial emission of reactive oxygen species is important for short-term high-fat diet-induced insulin resistance in mice

AIMS/HYPOTHESIS

Consuming a high-fat diet (HFD) induces insulin resistance in white adipose tissue (WAT) within 1 week. However, little is known about the initiating events. One potential mechanism that has remained largely unexplored is excessive mitochondrial emission of reactive oxygen species (ROS).

METHODS

To determine the role of mitochondrial ROS emissions at the onset of insulin resistance, wild-type (WT) mice were placed on an HFD for 1 week. WAT insulin sensitivity and inflammation were assessed by western blot. In addition, we optimised/validated a method to determine ROS emissions in permeabilised WAT.

RESULTS

An HFD for 1 week resulted in impaired insulin signalling, increased c-Jun NH2-terminal kinase (JNK) phosphorylation and an increase in oxidative stress. These changes were associated with an increase in fatty-acid-mediated mitochondrial ROS emissions without any change in mitochondrial respiration/content. To determine that mitochondrial ROS causes insulin resistance, we used transgenic mice that express human catalase in mitochondria (MCAT) as a model of upregulated mitochondrial antioxidant enzyme capacity. MCAT mice displayed attenuated mitochondrial ROS emission, preserved insulin signalling and no inflammatory response following an HFD.

CONCLUSIONS/INTERPRETATION

Findings from this study suggest that elevated mitochondrial ROS emission contributes to HFD-induced WAT insulin resistance.

Male bovine GH transgenic mice have decreased adiposity with an adipose depot-specific increase in immune cell populations

White adipose tissue (WAT) is composed of mature adipocytes and a stromal vascular fraction (SVF), which contains a variety of cells, including immune cells that vary among the different WAT depots. Growth hormone (GH) impacts immune function and adiposity in an adipose depot-specific manner. However, its effects on WAT immune cell populations remain unstudied. Bovine GH transgenic (bGH) mice are commonly used to study the in vivo effects of GH. These giant mice have an excess of GH action, impaired glucose metabolism, decreased adiposity, increased lean mass, and a shortened lifespan. Therefore, the purpose of this study was to characterize the WAT depot-specific differences in immune cell populations in the presence of excess GH in vivo. Three WAT depots were assessed: inguinal (sc), epididymal (EPI), and mesenteric (MES). Subcutaneous and MES bGH WAT depots showed a significantly higher number of total SVF cells, yet only MES bGH WAT had higher leukocyte counts compared with control samples. By means of flow cytometry analysis of the SVF, we detected greater macrophage and regulatory T-cell infiltration in sc and MES bGH WAT depots compared with controls. However, no differences were observed in the EPI WAT depot. RNA-sequencing confirmed significant alterations in pathways related to T-cell infiltration and activation in the sc depot with fewer significant changes in the EPI bGH WAT depot. These findings collectively point to a previously unrecognized role for GH in influencing the distribution of WAT immune cell populations in a depot-specific manner.

Innate immunity drives the initiation of a murine model of primary biliary cirrhosis

Invariant natural killer T (iNKT) cells play complex roles in bridging innate and adaptive immunity by engaging with glycolipid antigens presented by CD1d. Our earlier work suggested that iNKT cells were involved in the initiation of the original loss of tolerance in primary biliary cirrhosis (PBC). To address this issue in more detail and, in particular, to focus on whether iNKT cells activated by a Th2-biasing agonist (2s,3s,4r)-1-O-(α-_D-galactopyranosyl)-N-tetracosanoyl-2-amino-1,3,4-nonanetriol (OCH), can influence the development of PBC in a xenobiotic-induced PBC murine model. Groups of mice were treated with either OCH or, as a control, α-galactosylceramide (α-GalCer) and thence serially followed for cytokine production, markers of T cell activation, liver histopathology and anti-mitochondrial antibody responses. Further, additional groups of CD1d deleted mice were similarly studied. Our data indicate that administration of OCH has a dramatic influence with exacerbation of portal inflammation and hepatic fibrosis similar to mice treated with α-GalCer. Further, iNKT cell deficient CD1d knockout mice have decreased inflammatory portal cell infiltrates and reduced anti-mitochondrial antibody responses. We submit that activation of iNKT cells can occur via overlapping and/or promiscuous pathways and highlight the critical role of innate immunity in the natural history of autoimmune cholangitis. These data have implications for humans with PBC and emphasize that therapeutic strategies must focus not only on suppressing adaptive responses, but also innate immunity.

Alzheimer-associated Aβ oligomers impact the central nervous system to induce peripheral metabolic deregulation

Alzheimer's disease (AD) is associated with peripheral metabolic disorders. Clinical/epidemiological data indicate increased risk of diabetes in AD patients. Here, we show that intracerebroventricular infusion of AD-associated Aβ oligomers (AβOs) in mice triggered peripheral glucose intolerance, a phenomenon further verified in two transgenic mouse models of AD. Systemically injected AβOs failed to induce glucose intolerance, suggesting AβOs target brain regions involved in peripheral metabolic control. Accordingly, we show that AβOs affected hypothalamic neurons in culture, inducing eukaryotic translation initiation factor 2α phosphorylation (eIF2α-P). AβOs further induced eIF2α-P and activated pro-inflammatory IKKβ/NF-κB signaling in the hypothalamus of mice and macaques. AβOs failed to trigger peripheral glucose intolerance in tumor necrosis factor-α (TNF-α) receptor 1 knockout mice. Pharmacological inhibition of brain inflammation and endoplasmic reticulum stress prevented glucose intolerance in mice, indicating that AβOs act via a central route to affect peripheral glucose homeostasis. While the hypothalamus has been largely ignored in the AD field, our findings indicate that AβOs affect this brain region and reveal novel shared molecular mechanisms between hypothalamic dysfunction in metabolic disorders and AD.

Stages versus subsets: Invariant Natural Killer T cell lineage differentiation

Invariant Natural Killer T (iNKT) cells represent a population of innate T lymphocytes which act as 'first-responders' to infection. While they have long been considered a versatile cell, capable of secretion of multiple cytokines upon activation, recent evidence now indicates that distinct lineages of iNKT cells with unique transcriptional and cytokine profiles exist in different peripheral tissue and as such represent 'fine-tuning' of these cells, which act as mediators between the innate and adaptive immune systems. Here we discuss the molecules regulating the differentiation of iNKT cell lineages, the transcription factors associated with their development, and the role of E protein transcription factors and their negative regulators the Id proteins, as these cells develop from immature progenitor cells to terminally differentiated cells in peripheral tissue.

Enalapril treatment increases T cell number and promotes polarization towards M1-like macrophages locally in diabetic nephropathy

Diabetic nephropathy (DN) is a serious complication of longstanding diabetes affecting up to 30% of all diabetes patients and is the main cause of end-stage kidney disease globally. Current standard treatment e.g. ACE-inhibitors like enalapril merely offers a delay in the progression leading to DN. Herein, we describe in two preclinical models evidence to local effects on the inflammatory signatures after intervention treatment with enalapril which provides enhanced understanding of the mechanism of ACE inhibitors. Enalapril transiently reduced albuminuria in both the db/db and the STZ-induced DN models with established disease, without modulating the HbA1c%. Albuminuria was strongly associated with loss of leukocytes, particularly B cells, but also of sub-populations of macrophages and CD4(+) T cells. The remaining kidney macrophages were polarized into a M2-like sub-population with reduced surface expression of the M1-like macrophage marker CD11c and enhanced expression of galectin-3. Enalapril treatment counteracted the reduction of leukocytes in the diabetic kidney towards levels noted in the non-diabetic kidney. Particularly, a subset of macrophages was increased and a clear expansion of CD4(+) and CD8(+) T cells was observed. However, enalapril failed to modulate the B cell compartment. Interestingly, enalapril treatment resulted in a re-polarization of the macrophages towards a M1-like phenotype characterized by elevated levels of CD11c with moderate down-regulation of the M2 marker galectin-3. The data demonstrate that ACE inhibition in pre-clinical models of DN shows a transient beneficial effect on albuminuria which is unexpectedly associated with restoration of T cells and M1-like macrophages in the kidney.

Regulatory iNKT cells lack expression of the transcription factor PLZF and control the homeostasis of T(reg) cells and macrophages in adipose tissue

Invariant natural killer T cells (iNKT cells) are lipid-sensing innate T cells that are restricted by the antigen-presenting molecule CD1d and express the transcription factor PLZF. iNKT cells accumulate in adipose tissue, where they are anti-inflammatory, but the factors that contribute to their anti-inflammatory nature, as well as their targets in adipose tissue, are unknown. Here we found that iNKT cells in adipose tissue had a unique transcriptional program and produced interleukin 2 (IL-2) and IL-10. Unlike other iNKT cells, they lacked PLZF but expressed the transcription factor E4BP4, which controlled their IL-10 production. The adipose iNKT cells were a tissue-resident population that induced an anti-inflammatory phenotype in macrophages and, through the production of IL-2, controlled the number, proliferation and suppressor function of regulatory T cells (Treg cells) in adipose tissue. Thus, iNKT cells in adipose tissue are unique regulators of immunological homeostasis in this tissue.

High-fat diet consumption disrupts memory and primes elevations in hippocampal IL-1β, an effect that can be prevented with dietary reversal or IL-1 receptor antagonism

High-fat diet (HFD)-induced obesity is reaching worldwide proportions. In addition to causing obesity, HFDs also induce a variety of health disorders, which includes cognitive decline. Hippocampal function may be particularly vulnerable to the negative consequences of HFD, and it is suspected that 'primed' neuroinflammatory processes may mediate this response. To examine the link between diet, hippocampal function and neuroinflammation, male Wistar rats were fed a medium or HFD. Hippocampal memory function was measured using contextual pre-exposure fear conditioning (CPE-FC). Rats fed a HFD demonstrated impaired memory, an effect that was augmented with longer duration of HFD consumption. HFD-induced memory impairments were linked to potentiated levels of interleukin-1 beta (IL-1β) protein in the hippocampus 2h after the foot-shock that occurs during CPE-FC. Central IL-1 receptor antagonism, with intracisterna magna (ICM) administration of hIL-1RA prior to the foot-shock prevented the diet-induced memory disruption, suggesting a critical role for IL-1β in this phenomenon. Additionally, obese animals whose diet regimen was reversed from HFD back to standard chow recovered memory function and did not demonstrate a foot-shock-induced hippocampal IL-1β increase. Interestingly, dietary reversal neutralized the negative impact of HFD on memory and IL-1β, yet animals maintained physiological evidence of obesity (increased body mass and serum leptin), indicating that dietary components, not body mass, may mediate the negative effects on memory.

Adipose invariant natural killer T cells

Adipose tissue is a dynamic organ that makes up a substantial proportion of the body; in severe obesity it can account for 50% of body mass. Details of the unique immune system resident in human and murine adipose tissue are only recently emerging, and so it has remained a largely unexplored and unappreciated immune site until now. Adipose tissue harbours a unique collection of immune cells, which often display unusual functions compared with their counterparts elsewhere in the body. These resident immune cells are key to maintaining tissue and immune homeostasis, yet in obesity their chronic aberrant stimulation can contribute to the inflammation and pathogenesis associated with obesity. Anti-inflammatory adipose-resident lymphocytes are often depleted in obesity, whereas pro-inflammatory immune cells accumulate, leading to an overall inflammatory state, which is a key step in the development of obesity-induced metabolic disease. A good example is invariant natural killer T (iNKT) cells, which make up a large proportion of lymphocytes in human and murine adipose tissue. Here, they are unusually poised to produce anti-inflammatory or regulatory cytokines, however in obesity, iNKT cells are greatly reduced. As iNKT cells are potent transactivaors of other immune cells, and can act as a bridge between innate and adaptive immunity, their loss in obesity represents the loss of a major regulatory population. Restoring iNKT cells, or activating them in obese mice leads to improved glucose handling, insulin sensitivity, and even weight loss, and hence represents an exciting therapeutic avenue to be explored for restoring homeostasis in obese adipose tissue.

Adipose-tissue and intestinal inflammation - visceral obesity and creeping fat

Obesity has become one of the main threats to health worldwide and therefore gained increasing clinical and economic significance as well as scientific attention. General adipose-tissue accumulation in obesity is associated with systemically increased pro-inflammatory mediators and humoral and cellular changes within this compartment. These adipose-tissue changes and their systemic consequences led to the concept of obesity as a chronic inflammatory state. A pathognomonic feature of Crohn’s disease (CD) is creeping fat (CF), a locally restricted hyperplasia of the mesenteric fat adjacent to the inflamed segments of the intestine. The precise role of this adipose-tissue and its mediators remains controversial, and ongoing work will have to define whether this compartment is protecting from or contributing to disease activity. This review aims to outline specific cellular changes within the adipose-tissue, occurring in either obesity or CF. Hence the potential impact of adipocytes and resident immune cells from the innate and adaptive immune system will be discussed for both diseases. The second part focuses on the impact of generalized adipose-tissue accumulation in obesity, respectively on the locally restricted form in CD, on intestinal inflammation and on the closely related integrity of the mucosal barrier.

CMHX008, a novel peroxisome proliferator-activated receptor γ partial agonist, enhances insulin sensitivity in vitro and in vivo

The peroxisome proliferator-activated receptor γ (PPARγ) plays an important role in adipocyte differentiation and insulin sensitivity. Its ligand rosiglitazone has anti-diabetic effect but is frequently accompanied with some severe unwanted effects. The aim of the current study was to compare the anti-diabetic effect of CMHX008, a novel thiazolidinedione-derivative, with rosiglitazone. A luciferase assay was used to evaluate in vitro PPARγ activation. 3T3-L1 cells were used to examine adipocyte differentiation. High fat diet (HFD) mice were used to examine in vivo insulin sensitivity. The mRNA levels were evaluated by real-time RT-PCR. Serum biochemical and hormonal variables were assessed using a clinical chemistry analyser. CMHX008 displayed a moderate PPARγ agonist activity, and promoted 3T3-L1 preadipocyte differentiation with lower activity than rosiglitazone. CMHX008 regulated the expression of PPARγ target genes in a different manner from rosiglitazone. CMHX008 increased the expression and secretion of adiponectin with the similar efficacy as rosiglitazone, but only 25% as potent as rosiglitazone for the induction of adipocyte fatty acid binding protein. Treatment of CMHX008 and rosiglitazone protected mice from high fat diet (HFD)-induced glucose intolerance, hyperinsulinemia and inflammation. CMHX008 reduced the mRNA expression of M1 macrophage markers, and significantly increased the expressions of M2 markers. In conclusion, CMHX008 shared the comparable insulin-sensitizing effects as rosiglitazone with lower adipogenic capacity and might potentially be developed into an effective agent for the treatment of diabetes and metabolic disorders.

CD1d-mediated presentation of endogenous lipid antigens by adipocytes requires microsomal triglyceride transfer protein

Obesity-induced adipose tissue (AT) dysfunction results in a chronic low-grade inflammation that predisposes to the development of insulin resistance and type 2 diabetes. During the development of obesity, the AT-resident immune cell profile alters to create a pro-inflammatory state. Very recently, CD1d-restricted invariant (i) natural killer T (NKT) cells, a unique subset of lymphocytes that are reactive to so called lipid antigens, were implicated in AT homeostasis. Interestingly, recent data also suggest that human and mouse adipocytes can present such lipid antigens to iNKT cells in a CD1d-dependent fashion, but little is known about the lipid antigen presentation machinery in adipocytes. Here we show that CD1d, as well as the lipid antigen loading machinery genes pro-saposin (Psap), Niemann Pick type C2 (Npc2), α-galactosidase (Gla), are up-regulated in early adipogenesis, and are transcriptionally controlled by CCAAT/enhancer-binding protein (C/EBP)-β and -δ. Moreover, adipocyte-induced Th1 and Th2 cytokine release by iNKT cells also occurred in the absence of exogenous ligands, suggesting the display of endogenous lipid antigen-D1d complexes by 3T3-L1 adipocytes. Furthermore, we identified microsomal triglyceride transfer protein, which we show is also under the transcriptional regulation of C/EBPβ and -δ, as a novel player in the presentation of endogenous lipid antigens by adipocytes. Overall, our findings indicate that adipocytes can function as non-professional lipid antigen presenting cells, which may present an important aspect of adipocyte-immune cell communication in the regulation of whole body energy metabolism and immune homeostasis.

Diet-induced alterations in gut microflora contribute to lethal pulmonary damage in TLR2/TLR4-deficient mice

Chronic intake of Western diet has driven an epidemic of obesity and metabolic syndrome, but how it induces mortality remains unclear. Here, we show that chronic intake of a high-fat diet (HFD), not a low-fat diet, leads to severe pulmonary damage and mortality in mice deficient in Toll-like receptors 2 and 4 (DKO). Diet-induced pulmonary lesions are blocked by antibiotic treatment and are transmissible to wild-type mice upon either cohousing or fecal transplantation, pointing to the existence of bacterial pathogens. Indeed, diet and innate deficiency exert significant impact on gut microbiota composition. Thus, chronic intake of HFD promotes severe pulmonary damage and mortality in DKO mice in part via gut dysbiosis, a finding that may be important for immunodeficient patients, particularly those on chemotherapy or radiotherapy, where gut-microbiota-caused conditions are often life threatening.

MitoNEET-mediated effects on browning of white adipose tissue

MitoNEET is an outer mitochondrial membrane protein that, upon overexpression in white adipose tissue (WAT), exerts a positive impact on tissue expansion and whole-body lipid and carbohydrate homeostasis by altering mitochondrial matrix iron metabolism. Here we determine the key transcriptional events in subcutaneous WAT of mice in response to mitoNEET overexpression and a high-fat diet (HFD). Microarray analyses at key points during weight gain upon body-weight divergence with wild-type mice demonstrate that mitoNEET-enriched sWAT early on upregulates a browning signature program that limits WAT expansion in transgenic mice for a period of up to 12-weeks of HFD. This compensatory browning phenotype is subsequently lost, resulting in rapid WAT expansion and body-weight gain. Exposure to thermoneutral temperatures during HFD prompts weight gain significantly earlier. Similar WAT expansion is achieved upon infection with an adeno-associated virus expressing mitoNEET. Collectively, the mitoNEET enriched fat-pads feature a more vascularized, anti-inflammatory and less fibrotic environment.

Crosstalk between adipocytes and immune cells in adipose tissue inflammation and metabolic dysregulation in obesity

Recent findings, notably on adipokines and adipose tissue inflammation, have revised the concept of adipose tissues being a mere storage depot for body energy. Instead, adipose tissues are emerging as endocrine and immunologically active organs with multiple effects on the regulation of systemic energy homeostasis. Notably, compared with other metabolic organs such as liver and muscle, various inflammatory responses are dynamically regulated in adipose tissues and most of the immune cells in adipose tissues are involved in obesity-mediated metabolic complications, including insulin resistance. Here, we summarize recent findings on the key roles of innate (neutrophils, macrophages, mast cells, eosinophils) and adaptive (regulatory T cells, type 1 helper T cells, CD8 T cells, B cells) immune cells in adipose tissue inflammation and metabolic dysregulation in obesity. In particular, the roles of natural killer T cells, one type of innate lymphocyte, in adipose tissue inflammation will be discussed. Finally, a new role of adipocytes as antigen presenting cells to modulate T cell activity and subsequent adipose tissue inflammation will be proposed.

Short-term HFD does not alter lipolytic function of adipocytes

A short bout of high fat diet (HFD) impairs glucose tolerance and hepatic insulin sensitivity. We recently identified adipose tissue inflammation and resulting dysfunctional adipose tissue-liver cross-talk as an early event in the development of HFD-induced hepatic insulin resistance. In particular, reducing white adipose tissue (WAT) inflammation by adipocyte-specific depletion of Fas/CD95 protected mice from developing hepatic insulin resistance but not hepatic steatosis. Herein, we expanded our previous work and determined the impact of four days of HFD on lipolytic activity of isolated adipocytes. Compared with chow-fed mice, the degree of basal and isoproterenol-stimulated free fatty acid (FFA) and glycerol release was similar in HFD-fed animals. Moreover, insulin's ability to suppress lipolysis remained intact, suggesting retained insulin sensitivity. Despite unaltered lipolysis, circulating FFA concentrations were greatly increased in non-fasted HFD-fed mice. In conclusion, a short-term HFD challenge does not affect lipolytic function of adipocytes. The observed increase of circulating FFA levels in randomly fed animals may rather be the result of increased dietary fat supply.

Adipokines and insulin action: A sensitive issue

Obesity is a major public health concern and a strong risk factor for insulin resistance, type 2 diabetes mellitus (T2DM), and cardiovascular disease. The last two decades have seen a reconsideration of the role of white adipose tissue (WAT) in whole body metabolism and insulin action. Adipose tissue-derived cytokines and hormones, or adipokines, are likely mediators of metabolic function and dysfunction. While several adipokines have been associated with obese and insulin-resistant phenotypes, a select group has been linked with insulin sensitivity, namely leptin, adiponectin, and more recently, adipolin. What is known about these insulin-sensitizing molecules and their effects in healthy and insulin resistant states is the subject of this review. There remains a significant amount of research to do to fully elucidate the mechanisms of action of these adipokines for development of therapeutics in metabolic disease.

Morphological and inflammatory changes in visceral adipose tissue during obesity

Obesity is a major health burden worldwide and is a major factor in the development of insulin resistance and metabolic complications such as type II diabetes. Chronic nutrient excess leads to visceral adipose tissue (VAT) expansion and dysfunction in an active process that involves the adipocytes, their supporting matrix, and immune cell infiltrates. These changes contribute to adipose tissue hypoxia, adipocyte cell stress, and ultimately cell death. Accumulation of lymphocytes, macrophages, and other immune cells around dying adipocytes forms the so-called "crown-like structure", a histological hallmark of VAT in obesity. Cross talk between immune cells in adipose tissue dictates the overall inflammatory response, ultimately leading to the production of pro-inflammatory mediators which directly induce insulin resistance in VAT. In this review, we summarize recent studies demonstrating the dramatic changes that occur in visceral adipose tissue during obesity leading to low-grade chronic inflammation and metabolic disease.