Intrinsic Properties of Brown and White Adipocytes Have Differential Effects on Macrophage Inflammatory Responses

Senescent adipocytes and type 2 diabetes - current knowledge and perspective concepts

Among civilization diseases, the number of individuals suffering from type 2 diabetes (T2DM) is expected to increase to more than a billion in less than 20 years, which is associated with, e.g., populational aging, poor diet, sedentary lifestyle, genetic predispositions, and immunological factors. T2DM affects many organs and is characterized by insulin resistance, high glucose levels, and adipocyte dysfunction, which are related to senescence. Although this type of cellular aging has beneficial biological functions, it can also act unfavorable since senescent adipocytes resist apoptosis, enhance cytokine secretion, downregulate cell identity genes, and acquire the senescence-associated secretory phenotype that renders a more oxidative environment. Opposing T2DM is possible via a wide variety of senotherapies, including senolytics and senomorphics; nevertheless, further research is advised to expand therapeutic possibilities and benefits. Consequences that ought to be deeply researched include secretory phenotype, chronic inflammation, increasing insulin resistance, as well as impairment of adipogenesis and functioning of adipocyte cells. Herein, despite reviewing T2DM and fat tissue senescence, we summarized the latest adipocyte-related anti-diabetes solutions and suggested further research directions.

Brown adipose tissue transplantation improves skin fibrosis in localized scleroderma

Adipose tissue transplantation shows great therapeutic potential in reversing localized scleroderma-associated skin fibrosis. Brown adipose tissue (BAT) can specifically secrete various cytokines against fibrosis, but its therapeutic potential in improving skin fibrosis has not yet been demonstrated. In this study, we have demonstrated the superior therapeutic efficacy of BAT transplantation for sclerotic skin by transplanting two distinct types of adipose tissue. In comparison to the white adipose tissue (WAT) group, mice treated with BAT transplantation exhibited a significant reduction in dermal thickness. BAT transplantation effectively reverses skin sclerosis through mechanisms involving inflammation reduction, promotion of angiogenesis, inhibition of myofibroblast accumulation, and collagen deposition. This therapeutic effect can be attributed to its unique paracrine effects. Furthermore, transcriptome sequencing (RNA-Seq) revealed upregulation of pathways associated with lipogenesis and fatty acid metabolism in BAT while downregulating pathways are related to transforming growth factor β(TGF-β), epithelial-mesenchymal transition (EMT), and inflammatory response. These findings suggest that BAT transplantation holds great promise as a novel approach for localized scleroderma treatment.

The HIF1α polymorphism rs2301104 is associated with obesity and obesity-related cytokines in Han Chinese population

AIMS

To evaluate the association between genetic polymorphisms of HIF1α and obesity and obesity-related cytokines in the Han Chinese population.

METHODS

The study consisted of 160 subjects with obesity and 166 age- and gender-matched healthy controls. We genotyped three tag single nucleotide polymorphisms (SNPs) of HIF1α by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS)-based genotyping technology. Plasma cytokine concentrations were determined on the Luminex platform. The genetic associations were analysed statistically.

RESULTS

Obese subjects had significant obesity-related metabolic abnormalities, including hyperglycaemia, insulin resistance, and abnormalities in blood lipids, liver enzymes, and uric acid levels. SNP analysis of HIF1α revealed that the allele and genotype frequencies of rs2301104 were significantly associated with obesity. Our results suggest that the minor allele C of rs2301104 might be a protective mutation of obesity, and CC/CG genotypes of rs2301104 could be protective genotype of obesity. We also found that subjects with CC/CG genotypes of rs2301104 had significantly lower levels of IL-6, TNF-α, IL-1β, IL-8, and IL-10 than subjects with GG genotypes.

CONCLUSIONS

This is the first study to report an association between HIF1α polymorphisms and obesity and obesity-related cytokines in a Han Chinese population. These results require replication in larger populations but suggest that HIF1α may play an important role in obesity development.

Food phenolics stimulate adipocyte browning via regulating gut microecology

Fat browning has piqued the interest of researchers as a potential target for treating obesity and related metabolic disorders. Recruitment of brown adipocytes leads to enhanced energy dissipation and reduced adiposity, thus facilitating the maintenance of metabolic homeostasis. Evidence is increasing to support the crucial roles of polyphenols and gut microecology in turning fat "brown". However, it is not clear whether the intestinal microecology is involved in polyphenol-mediated regulation of adipose browning, so this concept is worthy of exploration. In this review, we summarize the current knowledge, mostly from studies with murine models, supporting the concept that the effects of food phenolics on brown fat activation and white fat browning can be attributed to their regulatory actions on gut microecology, including microbial community profile, gut metabolites, and gut-derived hormones. Furthermore, the potential underlying pathways involved are also discussed. Basically, understanding gut microecology paves the way to determine the underlying roles and mechanisms of food phenolics in adipose browning.

Exercise tolls the bell for key mediators of low-grade inflammation in dysmetabolic conditions

Metabolic conditions share a common low-grade inflammatory milieu, which represents a key-factor for their ignition and maintenance. Exercise is instrumental for warranting systemic cardio-metabolic balance, owing to its regulatory effect on inflammation. This review explores the effect of physical activity in the modulation of sub-inflammatory framework characterizing dysmetabolic conditions. Regular exercise suppresses plasma levels of TNFα, IL-1β, FFAs and MCP-1, in dysmetabolic subjects. In addition, a single session of training increases the anti-inflammatory IL-10, IL-1 receptor antagonist (IL-1ra), and muscle-derived IL-6, mitigating low-grade inflammation. Resting IL-6 levels are decreased in trained-dysmetabolic subjects, compared to sedentary. On the other hand, the acute release of muscle-IL-6, after exercise, seems to exert a regulatory effect on the metabolic and inflammatory balance. In fact, muscle-released IL-6 is presumably implicated in fat loss and boosts plasma levels of IL-10 and IL-1ra. The improvement of adipose tissue functionality, following regular exercise, is also critical for the mitigation of sub-inflammation. This effect is likely mediated by muscle-released IL-15 and IL-6 and partly relies on the brown-shifting of white adipocytes, induced by exercise. In obese-dysmetabolic subjects, moderate training is shown to restore gut-microbiota health, and this mitigates the translocation of bacterial-LPS into bloodstream. Finally, regular exercise can lower plasma advanced glycated endproducts. The articulated physiology of circulating mediators and the modulating effect of the pathophysiological background, render the comprehension of the exercise-regulatory effect on sub-inflammation a key issue, in dysmetabolism.

Remodeling of Macrophages in White Adipose Tissue under the Conditions of Obesity as well as Lipolysis

Adipose tissue macrophages (ATM) are a major source of low-grade inflammation in obesity, and yet reasons driving ATM accumulation in white adipose tissue (WAT) are not fully understood. Emerging evidence suggested that ATM underwent extensive remodeling in obesity. In addition to abundance, ATM in obesity were lipid-laden and metabolically reprogrammed, which in turn was tightly related to their functional alterations and persistence in obesity. Herein, we aimed to discuss that activation of lipid sensing signaling associated with metabolic reprogramming in ATM was indispensible for their migration, retention, or proliferation in obesity. Likewise, lipolysis also induced similar but transient ATM remodeling. Therefore, we assumed that obesity might share overlapping mechanisms with lipolysis in remodeling ATM. Formation of crown-like structures (CLS) in WAT was presumably a common event initiating ATM remodeling, with a spectrum of lipid metabolites released from adipocytes being potential signaling molecules. Moreover, adipose interlerkin-6 (IL-6) exhibited homologous alterations by obesity and lipolysis. Thus, we postulated a positive feedback loop between ATM and adipocytes via IL-6 signaling backing ATM persistence by comparison of ATM remodeling under obesity and lipolysis. An elucidation of ATM persistence could help to provide novel therapeutic targets for obesity-associated inflammation.

Impact of thermogenesis induced by chronic β3-adrenergic receptor agonist treatment on inflammatory and infectious response during bacteremia in mice

White adipocytes store energy differently than brown and brite adipocytes which dissipate energy under the form of heat. Studies have shown that adipocytes are able to respond to bacteria thanks to the presence of Toll-like receptors at their surface. Despite this, little is known about the involvement of each class of adipocytes in the infectious response. We treated mice for one week with a β3-adrenergic receptor agonist to induce activation of brown adipose tissue and brite adipocytes within white adipose tissue. Mice were then injected intraperitoneally with E. coli to generate acute infection. The metabolic, infectious and inflammatory parameters of the mice were analysed during 48 hours after infection. Our results shown that in response to bacteria, thermogenic activity promoted a discrete and local anti-inflammatory environment in white adipose tissue characterized by the increase of the IL-1RA secretion. More generally, activation of brown and brite adipocytes did not modify the host response to infection including no additive effect with fever and an equivalent bacteria clearance and inflammatory response. In conclusion, these results suggest an IL-1RA-mediated immunomodulatory activity of thermogenic adipocytes in response to acute bacterial infection and open a way to characterize their effect along more chronic infection as septicaemia.

Effects of stem cells from inducible brown adipose tissue on diet-induced obesity in mice

Adipose-derived mesenchymal stem cells (ASCs) are a promising option for the treatment of obesity and its metabolic co-morbidities. Despite the recent identification of brown adipose tissue (BAT) as a potential target in the management of obesity, the use of ASCs isolated from BAT as a therapy for patients with obesity has not yet been explored. Metabolic activation of BAT has been shown to have not only thermogenic effects, but it also triggers the secretion of factors that confer protection against obesity. Herein, we isolated and characterized ASCs from the visceral adipose tissue surrounding a pheochromocytoma (IB-hASCs), a model of inducible BAT in humans. We then compared the anti-obesity properties of IB-hASCs and human ASCs isolated from visceral white adipose tissue (W-hASCs) in a murine model of diet-induced obesity. We found that both ASC therapies mitigated the metabolic abnormalities of obesity to a similar extent, including reducing weight gain and improving glucose tolerance. However, infusion of IB-hASCs was superior to W-hASCs in suppressing lipogenic and inflammatory markers, as well as preserving insulin secretion. Our findings provide evidence for the metabolic benefits of visceral ASC infusion and support further studies on IB-hASCs as a therapeutic option for obesity-related comorbidities.

Defining the lineage of thermogenic perivascular adipose tissue

Brown adipose tissue can expend large amounts of energy, and therefore increasing its size or activity is a promising therapeutic approach to combat metabolic disease. In humans, major deposits of brown fat cells are found intimately associated with large blood vessels, corresponding to perivascular adipose tissue (PVAT). However, the cellular origins of PVAT are poorly understood. Here, we determine the identity of perivascular adipocyte progenitors in mice and humans. In mice, thoracic PVAT develops from a fibroblastic lineage, consisting of progenitor cells (Pdgfra+, Ly6a+ and Pparg-) and preadipocytes (Pdgfra+, Ly6a+ and Pparg+), which share transcriptional similarity with analogous cell types in white adipose tissue. Interestingly, the aortic adventitia of adult animals contains a population of adipogenic smooth muscle cells (Myh11+, Pdgfra- and Pparg+) that contribute to perivascular adipocyte formation. Similarly, human PVAT contains presumptive fibroblastic and smooth muscle-like adipocyte progenitor cells, as revealed by single-nucleus RNA sequencing. Together, these studies define distinct populations of progenitor cells for thermogenic PVAT, providing a foundation for developing strategies to augment brown fat activity.

Brown Adipose Tissue and Its Role in Insulin and Glucose Homeostasis

The increased worldwide prevalence of obesity, insulin resistance, and their related metabolic complications have prompted the scientific world to search for new possibilities to combat obesity. Brown adipose tissue (BAT), due to its unique protein uncoupling protein 1 (UPC1) in the inner membrane of the mitochondria, has been acknowledged as a promising approach to increase energy expenditure. Activated brown adipocytes dissipate energy, resulting in heat production. In other words, BAT burns fat and increases the metabolic rate, promoting a negative energy balance. Moreover, BAT alleviates metabolic complications like dyslipidemia, impaired insulin secretion, and insulin resistance in type 2 diabetes. The aim of this review is to explore the role of BAT in total energy expenditure, as well as lipid and glucose homeostasis, and to discuss new possible activators of brown adipose tissue in humans to treat obesity and metabolic disorders.

The Role of Brown Adipose Tissue Dysfunction in the Development of Cardiovascular Disease

Brown adipose tissue (BAT), consisted of brown adipocytes and stromal vascular fraction, which includes endothelial cells, lymphocytes, fibroblasts and stem cells, plays a vital role in regulating cardiovascular health and diseases. As a thermogenic organ, BAT can influence body through strengthening energy expenditure by promoting glucose and lipid metabolism. In addition, BAT is also an endocrine organ which is able to secret adipokines in an autocrine and/or paracrine fashion. BAT plays a protective role in cardiovascular system through attenuating cardiac remodeling and suppressing inflammatory response. In this review, we summarize the advances from the discovery of BAT to the present and provide an overview on the role of BAT dysfunction in cardiovascular diseases.

Adipose Tissue Immunomodulation: A Novel Therapeutic Approach in Cardiovascular and Metabolic Diseases

Adipose tissue is a critical regulator of systemic metabolism and bodily homeostasis as it secretes a myriad of adipokines, including inflammatory and anti-inflammatory cytokines. As the main storage pool of lipids, subcutaneous and visceral adipose tissues undergo marked hypertrophy and hyperplasia in response to nutritional excess leading to hypoxia, adipokine dysregulation, and subsequent low-grade inflammation that is characterized by increased infiltration and activation of innate and adaptive immune cells. The specific localization, physiology, susceptibility to inflammation and the heterogeneity of the inflammatory cell population of each adipose depot are unique and thus dictate the possible complications of adipose tissue chronic inflammation. Several lines of evidence link visceral and particularly perivascular, pericardial, and perirenal adipose tissue inflammation to the development of metabolic syndrome, insulin resistance, type 2 diabetes and cardiovascular diseases. In addition to the implication of the immune system in the regulation of adipose tissue function, adipose tissue immune components are pivotal in detrimental or otherwise favorable adipose tissue remodeling and thermogenesis. Adipose tissue resident and infiltrating immune cells undergo metabolic and morphological adaptation based on the systemic energy status and thus a better comprehension of the metabolic regulation of immune cells in adipose tissues is pivotal to address complications of chronic adipose tissue inflammation. In this review, we discuss the role of adipose innate and adaptive immune cells across various physiological and pathophysiological states that pertain to the development or progression of cardiovascular diseases associated with metabolic disorders. Understanding such mechanisms allows for the exploitation of the adipose tissue-immune system crosstalk, exploring how the adipose immune system might be targeted as a strategy to treat cardiovascular derangements associated with metabolic dysfunctions.

Modulation of the inflammatory response to LPS by the recruitment and activation of brown and brite adipocytes in mice

Numerous studies have shown that the recruitment and activation of thermogenic adipocytes, which are brown and beige/brite, reduce the mass of adipose tissue and normalize abnormal glycemia and lipidemia. However, the impact of these adipocytes on the inflammatory state of adipose tissue is still not well understood, especially in response to endotoxemia, which is a major aspect of obesity and metabolic diseases. First, we analyzed the phenotype and metabolic function of white and brite primary adipocytes in response to lipopolysaccharide (LPS) treatment in vitro. Then, 8-wk-old male BALB/c mice were treated for 1 wk with a β3-adrenergic receptor agonist (CL316,243, 1 mg/kg/day) to induce recruitment and activation of brown and brite adipocytes and were subsequently injected with LPS (Escherichia coli lipopolysaccharide, 100 μg/mouse ip) to generate acute endotoxemia. The metabolic and inflammatory parameters of the mice were analyzed 6 h later. Our results showed that in response to LPS, thermogenic activity promoted a local anti-inflammatory environment with high secretion of IL-1 receptor antagonist (IL-1RA) without affecting other anti- or proinflammatory cytokines. Interestingly, activation of brite adipocytes reduced the LPS-induced secretion of leptin. However, thermogenic activity and adipocyte function were not altered by LPS treatment in vitro or by acute endotoxemia in vivo. In conclusion, these results suggest an IL-1RA-mediated immunomodulatory activity of thermogenic adipocytes specifically in response to endotoxemia. This encourages potential therapy involving brown and brite adipocytes for the treatment of obesity and associated metabolic diseases.NEW & NOTEWORTHY Recruitment and activation of brown and brite adipocytes in the adipose tissue of mice lead to a local low-grade anti-inflammatory phenotype in response to acute endotoxemia without alteration of adipocyte phenotype and function.

Adipocyte Reprogramming by the Transcriptional Coregulator GPS2 Impacts Beta Cell Insulin Secretion

Glucose homeostasis is maintained through organ crosstalk that regulates secretion of insulin to keep blood glucose levels within a physiological range. In type 2 diabetes, this coordinated response is altered, leading to a deregulation of beta cell function and inadequate insulin secretion. Reprogramming of white adipose tissue has a central role in this deregulation, but the critical regulatory components remain unclear. Here, we demonstrate that expression of the transcriptional coregulator GPS2 in white adipose tissue is correlated with insulin secretion rate in humans. The causality of this relationship is confirmed using adipocyte-specific GPS2 knockout mice, in which inappropriate secretion of insulin promotes glucose intolerance. This phenotype is driven by adipose-tissue-secreted factors, which cause increased pancreatic islet inflammation and impaired beta cell function. Thus, our study suggests that, in mice and in humans, GPS2 controls the reprogramming of white adipocytes to influence pancreatic islet function and insulin secretion.

Lipocalin-2 deficiency may predispose to the progression of spontaneous age-related adiposity in mice

Lipocalin-2 (Lcn2) is an innate immune protein elevated by several orders of magnitude in various inflammatory conditions including aging and obesity. Recent studies have shown that Lcn2 is secreted by adipocytes in response to inflammation and is categorized as a new adipokine cross-linking innate immunity and metabolic disorders including obesity. However, the involvement of Lcn2 and its function during the progression of obesity is largely unknown. Recently, browning of white adipose tissue (WAT) has gained attention as a therapeutic strategy to combat obesity. Herein, we have shown that treatment of mature 3T3-L1 adipocytes with recombinant Lcn2 (rec-Lcn2) resulted in the up-regulation of thermogenic and beige/brown markers (UCP1, PRDM16, ZIC-1 and TBX1) and increased mitochondrial activity. Additionally, global Lcn2 genetic knockout (Lcn2KO) mice exhibited accelerated weight gain and visceral fat deposition with age, when compared to wild type (WT) mice. Taken together, both in vitro and in vivo studies suggest that Lcn2 is a naturally occurring adipokine, and may serve as an anti-obesity agent by upregulating the thermogenic markers resulting in the browning of WAT. Therefore, Lcn2 and its downstream signaling pathways could be a potential therapeutic target for obesity.

Age-Dependent Changes of Adipokine and Cytokine Secretion From Rat Adipose Tissue by Endogenous and Exogenous Toll-Like Receptor Agonists

White adipose tissue but recently also brown adipose tissue have emerged as endocrine organs. Age-associated obesity is accompanied by prolonged and elevated lipopolysaccharide (LPS)-induced sickness symptoms and increased cytokine and adipokine levels in the circulation partially originating from adipose tissue. In the present study, ex vivo fat explants were used to investigate how the exogenous pathogen-associated molecular pattern (PAMP) LPS or the endogenous danger-associated molecular patterns (DAMPs) high mobility group box-1 protein (HMGB1) and biglycan modulate the release of cytokines and adipokines/batokines and, thus, could influence systemic and/or local inflammation. The response of adipose tissue (epididymal, retroperitoneal, subcutaneous, and brown) was compared between young lean and old obese rats (2 vs. 24 months old). LPS induced a strong interleukin (IL)-6 and tumor necrosis factor (TNF) alpha release into the supernatant of all adipose tissue types investigated. HMGB1 (subcutaneous) and biglycan (retroperitoneal) led to an increased release of IL-6 and TNFalpha (HMGB1) and decreased visfatin and adiponectin (biglycan) secretion from epididymal adipose tissue (young rats). Visfatin was also decreased by HMGB1 in retroperitoneal adipose tissue of old rats. We found significantly higher leptin (all fat pads) and adiponectin (subcutaneous) levels in supernatants of adipose tissue from old compared to young rats, whereas visfatin secretion showed the opposite. The expression of the biglycan receptor Toll-like receptor (TLR) 2 as well as the LPS and HMGB1 receptors TLR4 and receptor for advanced glycation end products (RAGE) were reduced with age (TLR4/RAGE) and by stimulation with their ligands (subcutaneous). Overall, we revealed that adipokines/adipose-tissue released cytokines show some modulation of their release caused by mediators of septic (batokines) and sterile inflammation with potential implication for acute and chronic disease. Moreover, aging may increase or decrease the release of fat-derived mediators. These data show that DAMPS and LPS locally modulate cytokine secretion while only DAMPS but not LPS can locally alter adipokine secretion during inflammation.

Inflammatory Signaling and Brown Fat Activity

Obesity is characterized by a state of chronic inflammation in adipose tissue mediated by the secretion of a range of inflammatory cytokines. In comparison to WAT, relatively little is known about the inflammatory status of brown adipose tissue (BAT) in physiology and pathophysiology. Because BAT and brown/beige adipocytes are specialized in energy expenditure they have protective roles against obesity and associated metabolic diseases. BAT appears to be is less susceptible to developing inflammation than WAT. However, there is increasing evidence that inflammation directly alters the thermogenic activity of brown fat by impairing its capacity for energy expenditure and glucose uptake. The inflammatory microenvironment can be affected by cytokines secreted by immune cells as well as by the brown adipocytes themselves. Therefore, pro-inflammatory signals represent an important component of the thermogenic potential of brown and beige adipocytes and may contribute their dysfunction in obesity.

Brown Adipocyte and Splenocyte Co-Culture Maintains Regulatory T Cell Subset in Intermittent Hypobaric Conditions

Background

Brown adipocytes have thermogenic characteristics in neonates and elicit anti-inflammatory responses. We postulated that thermogenic brown adipocytes produce distinctive intercellular effects in a hypobaric state. The purpose of this study is to analyze the correlation between brown adipocyte and regulatory T cell (T_reg) expression under intermittent hypobaric conditions.

Methods

Brown and white adipocytes were harvested from the interscapular and flank areas of C57BL6 mice, respectively. Adipocytes were cultured with syngeneic splenocytes after isolation and differentiation. Intermittent hypobaric conditions were generated using cyclic negative pressure application for 48 h in both groups of adipocytes. Expression levels of T_regs (CD4 + CD25 + Foxp3 + T cells), cytokines [tumor necrosis factor-α (TNF-α) and interleukin-10 (IL-10), and the programmed death-ligand 1 (PD-L1)] co-inhibitory ligand were examined.

Results

Splenocytes, cultured with brown and white adipocytes, exhibited comparable T_reg expression in a normobaric state. Under hypobaric conditions, brown adipocytes maintained a subset of T_regs. However, a decrease in T_regs was found in the white adipocyte group. TNF-α levels increased in both groups under hypobaric conditions. In the brown adipocyte group, anti-inflammatory IL-10 expression increased significantly; meanwhile, IL-10 expression decreased in the white adipocyte group. PD-L1 levels increased more significantly in brown adipocytes than in white adipocytes under hypobaric conditions.

Conclusion

Both brown and white adipocytes support T_reg expression when they are cultured with splenocytes. Of note, brown adipocytes maintained T_reg expression in intermittent hypobaric conditions. Anti-inflammatory cytokines and co-inhibitory ligands mediate the immunomodulatory effects of brown adipocytes under altered atmospheric conditions. Brown adipocytes showed the feasibility as a source of adjustment in physical stresses.

Inhibitory Effects of Toll-Like Receptor 4, NLRP3 Inflammasome, and Interleukin-1β on White Adipocyte Browning

Adipose tissue expansion is accompanied by infiltration and accumulation of pro-inflammatory macrophages, which links obesity to pathologic conditions such as type 2 diabetes. However, little is known regarding the role of pro-inflammatory adipose tissue remodeling in the thermogenic activation of brown/beige fat. Here, we investigated the effect of pattern recognition receptors (PRR) activation in macrophages, especially the toll-like receptor 4 (TLR4) and Nod-like receptor 3 (NLRP3), on white adipocyte browning. We report that TLR4 activation by lipopolysaccharide repressed white adipocyte browning in response to β3-adrenergic receptor activation and caused ROS production and mitochondrial dysfunction, while genetic deletion of TLR4 protected mitochondrial function and thermogenesis. In addition, activation of NLRP3 inflammasome in macrophages attenuated UCP1 induction and mitochondrial respiration in cultures of primary adipocytes, while the absence of NLRP3 protected UCP1 in adipocytes. The effect of NLRP3 inflammasome activation on browning was mediated by IL-1β signaling, as blocking IL-1 receptor in adipocytes protected thermogenesis. We also report that IL-1β interferes with thermogenesis via oxidative stress stimulation and mitochondrial dysfunction as we observed a statistically significant increase in ROS production, decrease in SOD enzyme activity, and increase in mitochondrial depolarization in adipocytes treated with IL-1β. Collectively, we demonstrated that inflammatory response to obesity, such as TLR4 and NLRP3 inflammasome activation as well as IL-1β secretion, attenuates β3-adrenoreceptor-induced beige adipocyte formation via oxidative stress and mitochondrial dysfunction. Our findings provide insights into targeting innate inflammatory system for enhancement of the adaptive thermogenesis against obesity.

Toward an Understanding of How Immune Cells Control Brown and Beige Adipobiology

Immune cells were recently found to have an unexpected involvement in controlling the thermogenic activity of brown and beige adipose tissue. Here, we review how macrophages, eosinophils, type 2 innate lymphoid cells, and T lymphocytes are linked to this process. In particular, the recruitment of alternatively activated macrophages and eosinophils is associated with brown fat activation and white fat browning. Conversely, pro-inflammatory immune cell recruitment represses the thermogenic activity of brown and beige adipose tissues via cytokines that inhibit noradrenergic signaling. Macrophages also influence the noradrenergic tone by degrading norepinephrine locally and by inhibiting sympathetic innervation over time.

Macrophage polarization and meta-inflammation

Chronic overnutrition and obesity induces low-grade inflammation throughout the body. Termed "meta-inflammation," this chronic state of inflammation is mediated by macrophages located within the colon, liver, muscle, and adipose tissue. A sentinel orchestrator of immune activity and homeostasis, macrophages adopt variable states of activation as a function of time and environmental cues. Meta-inflammation phenotypically skews these polarization states and has been linked to numerous metabolic disorders. The past decade has revealed several key regulators of macrophage polarization, including the signal transducer and activator of transcription family, the peroxisome proliferator-activated receptor gamma, the CCAAT-enhancer-binding proteins (C/EBP) family, and the interferon regulatory factors. Recent studies have also suggested that microRNAs and long noncoding RNA influence macrophage polarization. The pathogenic alteration of macrophage polarization in meta-inflammation is regulated by both extracellular and intracellular cues, resulting in distinct secretome profiles. Meta-inflammation-altered macrophage polarization has been linked to insulin insensitivity, atherosclerosis, inflammatory bowel disease, cancer, and autoimmunity. Thus, further mechanistic exploration into the skewing of macrophage polarization promises to have profound impacts on improving global health.

The Lives and Times of Brown Adipokines

Brown adipose tissue (BAT) is responsible for adaptive non-shivering thermogenesis. Moreover, brown fat secretes regulatory factors, so-called brown adipokines, that have autocrine, paracrine, and endocrine actions. Brown adipokines are either polypeptides or nonpeptidic molecules including lipid molecules and microRNAs. The secretory properties of brown fat are essential for tissue remodeling adaptations to thermogenic necessities. The endocrine properties of brown adipokines are thought to contribute to the association between BAT activity and a healthy metabolic profile in relation to glucose and lipid homeostasis. The identification and characterization of brown adipokines may allow the discovery of circulating biomarkers of BAT activity in humans, and will lead to the development of candidate tools for therapeutic interventions in metabolic diseases.

Development and characterization of cholangioids from normal and diseased human cholangiocytes as an in vitro model to study primary sclerosing cholangitis

Primary sclerosing cholangitis (PSC) is an incurable, fibroinflammatory biliary disease for which there is no effective pharmacotherapy. We recently reported cholangiocyte senescence as an important phenotype in PSC while others showed that portal macrophages accumulate in PSC. Unfortunately, our ability to explore cholangiocyte senescence and macrophage accumulation has been hampered by limited in vitro models. Thus, our aim was to develop and characterize a three-dimensional (3D) model of normal and diseased bile ducts (cholangioids) starting with normal human cholangiocytes (NHC), senescent NHC (NHC-sen), and cholangiocytes from PSC patients. In 3D culture, NHCs formed spheroids of ~5000 cells with a central lumen of ~150 μm. By confocal microscopy and western blot, cholangioids retained expression of cholangiocyte proteins (cytokeratin 7/19) and markers of epithelial polarity (secretin receptor and GM130). Cholangioids are functionally active, and upon secretin stimulation, luminal size increased by ~80%. Cholangioids exposed to hydrogen peroxide exhibited cellular senescence and the senescence-associated secretory phenotype (SASP; increased IL-6, p21, SA-β-Gal, yH2A.x and p16 expression). Furthermore, cholangioids derived from NHC-sen or PSC patients were smaller and had slower growth than the controls. When co-cultured with THP-1 macrophages, the number of macrophages associated with NHC-sen or PSC cholangioids was five- to seven-fold greater compared to co-culture with non-senescent NHC. We observed that NHC-sen and PSC cholangioids release greater number of extracellular vesicles (EVs) compared to controls. Moreover, conditioned media from NHC-sen cholangioids resulted in an ~2-fold increase in macrophage migration. In summary, we developed a method to generate normal and diseased cholangioids, characterized them morphologically and functionally, showed that they can be induced to senescence and SASP, and demonstrated both EV release and macrophage attraction. This novel model mimics several features of PSC, and thus will be useful for studying the pathogenesis of PSC and potentially identifying new therapeutic targets.

Obesity Alters B Cell and Macrophage Populations in Brown Adipose Tissue

OBJECTIVE

The prevalence of obesity continues to rise, and it is understood that regulation of white adipose tissue (WAT) function is important to systemic metabolic homeostasis. Immune cells play a central role in the maintenance of WAT, and their compositions change in number and inflammatory phenotype with the progression of obesity. Because of its energy-burning capabilities, brown adipose tissue (BAT) has become a focus of obesity research. Although novel studies have focused on the function of brown adipocytes in thermogenesis, the tissue as a whole has not been immunologically characterized.

METHODS

BAT immune cell populations were analyzed by flow cytometry and immunohistochemistry in mice with diet-induced obesity (3, 8, or 16 weeks of diet) and in aged mice (1, 6-7, and 10-15 months).

RESULTS

The data confirmed the presence of macrophages and eosinophils, as previously reported, and showed that 20% to 30% of the immune cells in BAT were B cells. The number of B cells and eosinophils increased with diet-induced obesity, whereas macrophages decreased. There was no change in number of any immune cell quantified with age.

CONCLUSIONS

These studies reveal a novel finding of B220 + B cells in BAT and show that BAT immune cell populations change in response to diet-induced obesity.