Between brown and white: novel aspects of adipocyte differentiation

Lesson on obesity and anatomy of adipose tissue: new models of study in the era of clinical and translational research

Obesity is a serious global illness that is frequently associated with metabolic syndrome. Adipocytes are the typical cells of adipose organ, which is composed of at least two different tissues, white and brown adipose tissue. They functionally cooperate, interconverting each other under physiological conditions, but differ in their anatomy, physiology, and endocrine functions. Different cellular models have been proposed to study adipose tissue in vitro. They are also useful for elucidating the mechanisms that are responsible for a pathological condition, such as obesity, and for testing therapeutic strategies. Each cell model has its own characteristics, culture conditions, advantages and disadvantages. The choice of one model rather than another depends on the specific study the researcher is conducting. In recent decades, three-dimensional cultures, such as adipose spheroids, have become very attractive because they more closely resemble the phenotype of freshly isolated cells. The use of such models has developed in parallel with the evolution of translational research, an interdisciplinary branch of the biomedical field, which aims to learn a scientific translational approach to improve human health and longevity. The focus of the present review is on the growing body of data linking the use of new cell models and the spread of translational research. Also, we discuss the possibility, for the future, to employ new three-dimensional adipose tissue cell models to promote the transition from benchside to bedsite and vice versa, allowing translational research to become routine, with the final goal of obtaining clinical benefits in the prevention and treatment of obesity and related disorders.

Kinin B1 receptor deficiency promotes enhanced adipose tissue thermogenic response to β3-adrenergic stimulation

OBJECTIVE AND DESIGN

Kinin B1 receptor (B1R) has a key role in adipocytes to protect against obesity and glycemic metabolism, thus becoming a potential target for regulation of energy metabolism and adipose tissue thermogenesis.

MATERIAL OR SUBJECTS

Kinin B1 knockout mice (B1KO) were subjected to acute induction with CL 316,243 and chronic cold exposure.

METHODS

Metabolic and histological analyses, gene and protein expression and RNA-seq were performed on interscapular brown adipose tissue (iBAT) and inguinal white adipose tissue (iWAT) of mice.

RESULTS

B1KO mice, under acute effect of CL 316,243, exhibited increased energy expenditure and upregulated thermogenic genes in iWAT. They were also protected from chronic cold, showing enhanced non-shivering thermogenesis with increased iBAT mass (~ 90%) and recruitment of beige adipocytes in iWAT (~ 50%). Positive modulation of thermogenic and electron transport chain genes, reaching a 14.5-fold increase for Ucp1 in iWAT. RNA-seq revealed activation of the insulin signaling pathways for iBAT and oxidative phosphorylation, tricarboxylic acid cycle, and browning pathways for iWAT.

CONCLUSION

B1R deficiency induced metabolic and gene expression alterations in adipose tissue, activating thermogenic pathways and increasing energy metabolism. B1R antagonists emerge as promising therapeutic targets for regulating obesity and associated metabolic disorders, such as inflammation and diabetes.

MAFB in macrophages regulates cold-induced neuronal density in brown adipose tissue

Transcription factor MAFB regulates various homeostatic functions of macrophages. This study explores the role of MAFB in brown adipose tissue (BAT) thermogenesis using macrophage-specific Mafb-deficient (Mafbf/f::LysM-Cre) mice. We find that Mafb deficiency in macrophages reduces thermogenesis, energy expenditure, and sympathetic neuron (SN) density in BAT under cold conditions. This phenotype features a proinflammatory environment that is characterized by macrophage/granulocyte accumulation, increases in interleukin-6 (IL-6) production, and IL-6 trans-signaling, which lead to decreases in nerve growth factor (NGF) expression and reduction in SN density in BAT. We confirm MAFB regulation of IL-6 expression using luciferase readout driven by IL-6 promoter in RAW-264.7 macrophage cell lines. Immunohistochemistry shows clustered organization of NGF-producing cells in BAT, which are primarily TRPV1+ vascular smooth muscle cells, as additionally shown using single-cell RNA sequencing and RT-qPCR of the stromal vascular fraction. Treating Mafbf/f::LysM-Cre mice with anti-IL-6 receptor antibody rescues SN density, body temperature, and energy expenditure.

Protective role of perivascular adipose tissue in the cardiovascular system

This review provides an overview of the key role played by perivascular adipose tissue (PVAT) in the protection of cardiovascular health. PVAT is a specific type of adipose tissue that wraps around blood vessels and has recently emerged as a critical factor for maintenance of vascular health. Through a profound exploration of existing research, this review sheds light on the intricate structural composition and cellular origins of PVAT, with a particular emphasis on combining its regulatory functions for vascular tone, inflammation, oxidative stress, and endothelial function. The review then delves into the intricate mechanisms by which PVAT exerts its protective effects, including the secretion of diverse adipokines and manipulation of the renin-angiotensin complex. The review further examines the alterations in PVAT function and phenotype observed in several cardiovascular diseases, including atherosclerosis, hypertension, and heart failure. Recognizing the complex interactions of PVAT with the cardiovascular system is critical for pursuing breakthrough therapeutic strategies that can target cardiovascular disease. Therefore, this review aims to augment present understanding of the protective role of PVAT in cardiovascular health, with a special emphasis on elucidating potential mechanisms and paving the way for future research directions in this evolving field.

Brown Fat and Nutrition: Implications for Nutritional Interventions

Brown and beige adipocytes are renowned for their unique ability to generate heat through a mechanism known as thermogenesis. This process can be induced by exposure to cold, hormonal signals, drugs, and dietary factors. The activation of these thermogenic adipocytes holds promise for improving glucose metabolism, reducing fat accumulation, and enhancing insulin sensitivity. However, the translation of preclinical findings into effective clinical therapies poses challenges, warranting further research to identify the molecular mechanisms underlying the differentiation and function of brown and beige adipocytes. Consequently, research has focused on the development of drugs, such as mirabegron, ephedrine, and thyroid hormone, that mimic the effects of cold exposure to activate brown fat activity. Additionally, nutritional interventions have been explored as an alternative approach to minimize potential side effects. Brown fat and beige fat have emerged as promising targets for addressing nutritional imbalances, with the potential to develop strategies for mitigating the impact of metabolic diseases. Understanding the influence of nutritional factors on brown fat activity can facilitate the development of strategies to promote its activation and mitigate metabolic disorders.

Myoglobin in Brown Adipose Tissue: A Multifaceted Player in Thermogenesis

Brown adipose tissue (BAT) plays an important role in energy homeostasis by generating heat from chemical energy via uncoupled oxidative phosphorylation. Besides its high mitochondrial content and its exclusive expression of the uncoupling protein 1, another key feature of BAT is the high expression of myoglobin (MB), a heme-containing protein that typically binds oxygen, thereby facilitating the diffusion of the gas from cell membranes to mitochondria of muscle cells. In addition, MB also modulates nitric oxide (NO•) pools and can bind C16 and C18 fatty acids, which indicates a role in lipid metabolism. Recent studies in humans and mice implicated MB present in BAT in the regulation of lipid droplet morphology and fatty acid shuttling and composition, as well as mitochondrial oxidative metabolism. These functions suggest that MB plays an essential role in BAT energy metabolism and thermogenesis. In this review, we will discuss in detail the possible physiological roles played by MB in BAT thermogenesis along with the potential underlying molecular mechanisms and focus on the question of how BAT-MB expression is regulated and, in turn, how this globin regulates mitochondrial, lipid, and NO• metabolism. Finally, we present potential MB-mediated approaches to augment energy metabolism, which ultimately could help tackle different metabolic disorders.

Adipose Tissue Hyperplasia and Hypertrophy in Common and Syndromic Obesity-The Case of BBS Obesity

Obesity is a metabolic state generated by the expansion of adipose tissue. Adipose tissue expansion depends on the interplay between hyperplasia and hypertrophy, and is mainly regulated by a complex interaction between genetics and excess energy intake. However, the genetic regulation of adipose tissue expansion is yet to be fully understood. Obesity can be divided into common multifactorial/polygenic obesity and monogenic obesity, non-syndromic and syndromic. Several genes related to obesity were found through studies of monogenic non-syndromic obesity models. However, syndromic obesity, characterized by additional features other than obesity, suggesting a more global role of the mutant genes related to the syndrome and, thus, an additional peripheral influence on the development of obesity, were hardly studied to date in this regard. This review summarizes present knowledge regarding the hyperplasia and hypertrophy of adipocytes in common obesity. Additionally, we highlight the scarce research on syndromic obesity as a model for studying adipocyte hyperplasia and hypertrophy, focusing on Bardet-Biedl syndrome (BBS). BBS obesity involves central and peripheral mechanisms, with molecular and mechanistic alternation in adipocyte hyperplasia and hypertrophy. Thus, we argue that using syndromic obesity models, such as BBS, can further advance our knowledge regarding peripheral adipocyte regulation in obesity.

PDGF receptor signal mediates the contribution of Nestin-positive cell lineage to subcutaneous fat development

The beiging of white adipose tissue (WAT) is expected to improve systemic metabolic conditions; however, the regulation and developmental origin of this process remain insufficiently understood. In the present study, the implication of platelet-derived growth factor receptor alpha (PDGFRα) was examined in the beiging of inguinal WAT (ingWAT) of neonatal mice. Using in vivo Nestin expressing cell (Nestin⁺) lineage tracing and deletion mouse models, we found that, in the mice with Pdgfra gene inactivation in Nestin⁺ lineage (N-PRα-KO mice), the growth of inguinal WAT (ingWAT) was suppressed during neonatal periods as compared with control wild-type mice. In the ingWAT of N-PRα-KO mice, the beige adipocytes appeared earlier that were accompanied by the increased expressions of both adipogenic and beiging markers compared to control wild-type mice. In the perivascular adipocyte progenitor cell (APC) niche of ingWAT, many PDGFRα⁺ cells of Nestin⁺ lineage were recruited in Pdgfra-preserving control mice, but were largely decreased in N-PRα-KO mice. This PDGFRα⁺ cell depletion was replenished by PDGFRα⁺ cells of non-Nestin⁺ lineage, unexpectedly resulting in an increase of total PDGFRα⁺ cell number in APC niche of N-PRα-KO mice over that of control mice. These represented a potent homeostatic control of PDGFRα⁺ cells between Nestin⁺ and non-Nestin⁺ lineages that was accompanied by the active adipogenesis and beiging as well as small WAT depot. This highly plastic nature of PDGFRα⁺ cells in APC niche may contribute to the WAT remodeling for the therapeutic purpose against metabolic diseases.

Nogo-B deficiency suppresses white adipogenesis by regulating β-catenin signaling

AIMS

Obesity is a global epidemic around the world. Reticulon-4B (Nogo-B) is an endoplasmic reticulum-resident protein. Our previous work demonstrated that Nogo-B deficiency inhibited obesity and decreased the size of white adipocytes. However, the underlying molecular mechanism of Nogo-B in white adipogenesis remains poorly understood. This study aims to explore the effect of Nogo-B in white adipogenesis, as well as its underlying molecular mechanisms.

MAIN METHODS AND FINDINGS

The study adopted mouse embryonic fibroblasts (MEFs) and 3T3-L1 preadipocytes to induce white adipogenesis and investigate the effect of Nogo-B on adipogenesis using qRT-PCR, Western blotting, immunofluorescence, lipid quantification, and Oil Red O staining. During white adipogenesis, Nogo-B expression was increased accompanied by upregulation of adipogenic markers. In contrast, Nogo-B deficiency inhibited white adipocyte markers expression and lipid accumulation. Furthermore, the mechanism study showed that Nogo-B deficiency decreased the destruction complex [AXIN1-APC-glycogen synthase kinase 3β (GSK3β)] levels through activating protein kinase B 2 (AKT2), resulting in β-catenin translocating into the nucleus and inhibiting the expression of adipogenic markers. Moreover, Nogo-B deficiency promoted the expression of brown/beige adipocytes markers while improving mitochondrial thermogenesis by activating β-catenin pathway. In addition, Nogo-B deficiency reduced the levels of inflammatory molecules during white adipogenic differentiation.

SIGNIFICANCE

This study revealed that Nogo-B deficiency inhibited white adipogenesis through AKT2/GSK3β/β-catenin pathway. Meanwhile, Nogo-B deficiency increased the expression of brown/beige adipocyte markers and promoted mitochondrial thermogenesis. In addition, Nogo-B deficiency reduced inflammatory cytokine levels caused by adipogenesis. Collectively, blocking Nogo-B expression may be a potential strategy to suppress white adipogenesis.

Insights by which TUDCA is a potential therapy against adiposity

Adipose tissue is an organ with metabolic and endocrine activity. White, brown and ectopic adipose tissues have different structure, location, and function. Adipose tissue regulates energy homeostasis, providing energy in nutrient-deficient conditions and storing it in high-supply conditions. To attend to the high demand for energy storage during obesity, the adipose tissue undergoes morphological, functional and molecular changes. Endoplasmic reticulum (ER) stress has been evidenced as a molecular hallmark of metabolic disorders. In this sense, the ER stress inhibitor tauroursodeoxycholic acid (TUDCA), a bile acid conjugated to taurine with chemical chaperone activity, has emerged as a therapeutic strategy to minimize adipose tissue dysfunction and metabolic alterations associated with obesity. In this review, we highlight the effects of TUDCA and receptors TGR5 and FXR on adipose tissue in the setting of obesity. TUDCA has been demonstrated to limit metabolic disturbs associated to obesity by inhibiting ER stress, inflammation, and apoptosis in adipocytes. The beneficial effect of TUDCA on perivascular adipose tissue (PVAT) function and adiponectin release may be related to cardiovascular protection in obesity, although more studies are needed to clarify the mechanisms. Therefore, TUDCA has emerged as a potential therapeutic strategy for obesity and comorbidities.

Ciliary control of adipocyte progenitor cell fate regulates energy storage

The primary cilium is a cellular sensory organelle found in most cells in our body. This includes adipocyte progenitor cells in our adipose tissue, a complex organ involved in energy storage, endocrine signaling, and thermogenesis. Numerous studies have shown that the primary cilium plays a critical role in directing the cell fate of adipocyte progenitor cells in multiple adipose tissue types. Accordingly, diseases with dysfunctional cilia called ciliopathies have a broad range of clinical manifestations, including obesity and diabetes. This review summarizes our current understanding of how the primary cilium regulates adipocyte progenitor cell fate in multiple contexts and illustrates the importance of the primary cilium in regulating energy storage and adipose tissue function.

Endothelial TrkA coordinates vascularization and innervation in thermogenic adipose tissue and can be targeted to control metabolism

OBJECTIVE

Brown adipogenesis and thermogenesis in brown and beige adipose tissue (AT) involve vascular remodeling and sympathetic neuronal guidance. Here, we investigated the molecular mechanism coordinating these processes.

METHODS

We used mouse models to identify the molecular target of a peptide CPATAERPC homing to the endothelium of brown and beige AT.

RESULTS

We demonstrate that CPATAERPC mimics nerve growth factor (NGF) and identify a low molecular weight isoform of NGF receptor, TrkA, as the CPATAERPC cell surface target. We show that the expression of truncated endothelial TrkA is selective for brown and subcutaneous AT. Analysis of mice with endothelium-specific TrkA knockout revealed the role of TrkA in neuro-vascular coordination supporting the thermogenic function of brown adipocytes. A hunter-killer peptide D-BAT, composed of CPATAERPC and a pro-apoptotic domain, induced cell death in the endothelium and adipocytes. This resulted in thermogenesis impairment, and predisposed mice to obesity and glucose intolerance. We also tested if this treatment can inhibit the tumor recruitment of lipids mobilized from adipocytes from adjacent AT. Indeed, in a mouse model of breast cancer D-BAT suppressed tumor-associated AT lipolysis, which resulted in reduced fatty acid utilization by cancer cells.

CONCLUSION

Our study demonstrates that TrkA signaling in the endothelium supports neuro-vascular coordination enabling beige adipogenesis.

Potential Therapeutic Effect of All-Trans Retinoic Acid on Atherosclerosis

Atherosclerosis is a major risk factor for myocardial infarction and ischemic stroke, which are the leading cause of death worldwide. All-trans retinoic acid (ATRA) is a natural derivative of essential vitamin A. Numerous studies have shown that ATRA plays an important role in cell proliferation, cell apoptosis, cell differentiation, and embryonic development. All-trans retinoic acid (ATRA) is a ligand of retinoic acid receptors that regulates various biological processes by activating retinoic acid signals. In this paper, the metabolic processes of ATRA were reviewed, with emphasis on the effects of ATRA on inflammatory cells involved in the process of atherosclerosis.

A closer look at adipose tissue lymphatics and their markers

PURPOSE OF REVIEW

Lymphatic vessels are found in most tissues, with the exception of the cornea and the central nervous system. Tissues that have high exposure to antigens, such as the skin and the intestine, have especially extensive lymphatic vascular networks. Despite being densely vascularized with blood vessels, adipose tissue is poorly permeated with lymphatic vasculature. Here, we focus on the recent advances in the research on adipose tissue lymphatics and present a lymphatic-focused analysis of published single-cell and single-nucleus RNA sequencing datasets of adipose tissues.

RECENT FINDINGS

Although lymphatic expansion in obesity may limit inflammation and promote glycerol efflux from adipose tissue, lymphatic endothelial cells (LECs) secrete factors that reduce brown adipocyte thermogenesis. Transcriptomic analyses of these cells show that they express common lymphatic markers such as Prox1, but datasets from different studies show great variation in gene expression values due to the low number of captured LECs, depot differences, and species-specific gene expression patterns.

SUMMARY

As the importance of LECs in the homeostasis of adipose tissue has become evident, investigators want to shed light on the specific interactions of lymphatics with other cell types in adipose tissues. Extracting LECs from readily available transcriptomics datasets provides a standpoint for investigators for future research. However, systematic studies are needed to reveal unique identities according to depot and species-specific LEC signatures.

Contributions of microRNAs to Peripheral Insulin Sensitivity

An extensive literature base combined with advances in sequencing technologies demonstrate microRNA levels correlate with various metabolic diseases. Mechanistic studies also establish microRNAs regulate central metabolic pathways and thus play vital roles in maintaining organismal energy balance and metabolic homeostasis. This review highlights research progress on the roles and regulation of microRNAs in the peripheral tissues that confer insulin sensitivity. We discuss sequencing technologies used to comprehensively define the target spectrum of microRNAs in metabolic disease that complement studies reporting physiologic roles for microRNAs in the regulation of glucose and lipid metabolism in animal models. We also discuss the emerging roles of exosomal microRNAs as endocrine signals to regulate lipid and carbohydrate metabolism.

Adiponectin: a pleiotropic hormone with multifaceted roles

Adipose tissue mostly composed of different types of fat is one of the largest endocrine organs in the body playing multiple intricate roles including but not limited to energy storage, metabolic homeostasis, generation of heat, participation in immune functions and secretion of a number of biologically active factors known as adipokines. The most abundant of them is adiponectin. This adipocite-derived hormone exerts pleiotropic actions and exhibits insulin-sensitizing, antidiabetic, anti-obesogenic, anti-inflammatory, antiatherogenic, cardio- and neuroprotective properties. Contrariwise to its protective effects against various pathological events in different cell types, adiponectin may have links to several systemic diseases and malignances. Reduction in adiponectin levels has an implication in COVID-19-associated respiratory failure, which is attributed mainly to a phenomenon called 'adiponectin paradox'. Ample evidence about multiple functions of adiponectin in the body was obtained from animal, mostly rodent studies. Our succinct review is entirely about multifaceted roles of adiponectin and mechanisms of its action in different physiological and pathological states.

The Associations Between Serum Concentrations of Irisin and Glucose-dependent Insulinotropic Polypeptide with Body Mass Index Among Women with and Without Polycystic Ovary Syndrome

BACKGROUND

The actions of adipocytokines may be a connective factor between obesity and polycystic ovary syndrome (PCOS). It is suggested that irisin, as recently described cytokine secreted by skeletal muscles and glucose-dependent insulinotropic polypeptide (GIP) as an incretin hormone that induces cytokine expression, may play key roles in favoring obesity in these women.

OBJECTIVES

We aimed to evaluate the association between body mass index (BMI) and serum concentrations of irisin/GIP in women with and without PCOS in the linear and non-linear models.

METHODS

This cross-sectional study was conducted among 159 PCOS and 82 healthy eumenorrheic non-hirsute women aged 20 - 50 years. The fractional-polynomial model was used to develop a model of continuous risk factors, which evaluates non-linear associations between irisin/GIP and BMI among women with and without PCOS.

RESULTS

Women with PCOS were significantly younger (28.2 ± 5.8 vs. 33.0 ± 7.8 years, P < 0.001) and had a greater BMI (26.6 ± 5.2 vs. 25.2 ± 4.8 kg/m², P = 0.04) than the healthy counterparts. There were no significant linear and non-linear associations between serum concentration of irisin/GIP and BMI in both groups. The analysis of pair-wise age and BMI matching of women with PCOS and controls confirmed these findings.

CONCLUSIONS

This study showed that irisin and GIP have no association with BMI in women with or without PCOS. This finding could help to better understand the underlying pathophysiological status of PCOS, insulin resistance, and obesity-related disorders. Further large cohort studies are needed to confirm these findings.

Effectiveness of Magnolol, a Lignan from Magnolia Bark, in Diabetes, Its Complications and Comorbidities-A Review

Diabetes mellitus is a chronic metabolic disease characterized by disturbances in carbohydrate, protein, and lipid metabolism, often accompanied by oxidative stress. Diabetes treatment is a complicated process in which, in addition to the standard pharmacological action, it is necessary to append a comprehensive approach. Introducing the aspect of non-pharmacological treatment of diabetes allows one to alleviate its many adverse complications. Therefore, it seems important to look for substances that, when included in the daily diet, can improve diabetic parameters. Magnolol, a polyphenolic compound found in magnolia bark, is known for its health-promoting activities and multidirectional beneficial effects on the body. Accordingly, the goal of this review is to systematize the available scientific literature on its beneficial effects on type 2 diabetes and its complications. Taking the above into consideration, the article collects data on the favorable effects of magnolol on parameters related to glycemia, lipid metabolism, or oxidative stress in the course of diabetes. After careful analysis of many scientific articles, it can be concluded that this lignan is a promising agent supporting the conventional therapies with antidiabetic drugs in order to manage diabetes and diabetes-related diseases.

Exercise induces favorable metabolic changes in white adipose tissue preventing high-fat diet obesity

Diet and/or exercise are cost effective interventions to treat obesity. However, it is unclear if the type of exercise undertaken can prevent the onset of obesity and if it can act through different effects on fat depots. In this study we did not allow obesity to develop so we commenced the high-fat diet (HFD) and exercise programs concurrently and investigated the effect of endurance exercise (END) and high-intensity interval training (HIIT) on changes in cellular adipogenesis, thermogenesis, fibrosis, and inflammatory markers in three different fat depots, on a HFD and a chow diet. This was to assess the effectiveness of exercise to prevent the onset of obesity-induced changes. Mice fed with chow or HFD (45% kcal fat) were trained and performed either END or HIIT for 10 weeks (3 x 40 min sessions/week). In HFD mice, both exercise programs significantly prevented the increase in body weight (END: 17%, HIIT: 20%), total body fat mass (END: 46%, HIIT: 50%), increased lean mass as a proportion of body weight (Lean mass/BW) by 14%, and improved insulin sensitivity by 22%. Further evidence of the preventative effect of exercise was seen significantly decreased markers for adipogenesis, inflammation, and extracellular matrix accumulation in both subcutaneous adipose tissue (SAT) and epididymal adipose tissue (EPI). In chow, no such marked effects were seen with both the exercise programs on all the three fat depots. This study establishes the beneficial effect of both HIIT and END exercise in preventing metabolic deterioration, collagen deposition, and inflammatory responses in fat depots, resulting in an improved whole body insulin resistance in HFD mice.

Cajanolactone A, a Stilbenoid From Cajanus canjan (L.) Millsp, Prevents High-Fat Diet-Induced Obesity via Suppressing Energy Intake

Cajanolactone A (CLA) is a stilbenoid isolated from Cajanus canjan (L.) Millsp with the potential to prevent postmenopausal obesity. In this study, the effect of CLA on high-fat diet (HFD)-induced obesity in female C57BL/6 mice was investigated. It was found that, treatment with CLA reduced the energy intake and effectively protected the mice from HFD-induced body weight gain, fat accumulation within the adipose tissues and liver, and impairment in energy metabolism. Further investigation revealed that CLA significantly down-regulated the expression of ORX, ORXR2, pMCH, and Gal in the hypothalamus and antagonized HFD-induced changes in the expression of UCP1, Pgc-1α, Tfam, and Mfn1 in the inguinal white adipose tissue (iWAT); Caveolin-1, MT and UCP3 in the perigonadal white adipose tissue (pWAT); and Pdhb, IRS2, Mttp, Hadhb, and Cpt1b in the liver. CLA also protected the pWAT and liver from HFD-induced mitochondrial damage. However, neither HFD nor CLA showed an effect on the mass of brown adipose tissue (BAT) or the expression of UCP1 in the BAT. In summary, our findings suggest that CLA is a potential drug candidate for preventing diet-induced obesity, at least in females. CLA works most likely by suppressing the hypothalamic expression of orexigenic genes, which leads to reduced energy intake, and subsequently, reduced fat accumulation, thereby protecting the adipose tissues and the liver from lipid-induced mitochondrial dysfunction.

Mitochondrial activity regulates the differentiation of skin-derived mesenchymal stem cells into brown adipocytes to contribute to hypertension

BACKGROUND

Brown adipocytes (BAs) are major components of brown adipose tissue (BAT), which is involved in blood pressure regulation. BAs are derived from multiple progenitors, including PDGFRα⁺ adipose-derived stem cells (ASCs). Skin-derived mesenchymal stem cells (S-MSCs) have the capacity to differentiate into adipocytes; however, their ability to differentiate into BAs remains unexplored. We aim to study the ability and regulatory mechanism of the differentiation of S-MSCs into BAs and the direct role of BAT in blood pressure regulation.

METHODS

Protein expression was measured by flow cytometry or Western blotting, and gene mRNA levels were quantified by real-time quantitative PCR (RT-PCR). To induce the differentiation of S-MSCs into BAs, S-MSCs were stimulated with a brown adipogenic cocktail comprising insulin, IBMX, dexamethasone, triiodothyronine (T3), and rosiglitazone for the indicated periods. The oxygen consumption rate (OCR) was measured with an XF24 Extracellular Flux Analyzer. Mitochondrial mass was determined by flow cytometry and fluorescence staining. Hypertension was induced in WT mice by infusion of angiotensin II (Ang II), and systolic blood pressure (SBP) was measured using a tail cuff. Interscapular brown adipose tissue (iBAT)-deficient mice were generated by surgical removal of the iBAT depot, after which the animals were allowed to recover for 6 days. Aortic, iBAT, and heart tissue sections were analyzed by hematoxylin and eosin (HE) staining.

RESULTS

We found that in vitro, S-MSCs isolated from the mouse dermis expressed the stem cell markers CD90/105 and PDGFRα and readily differentiated into BAs. Mitochondrial biogenesis and oxygen consumption were markedly increased during differentiation of S-MSCs into BAs. In vivo, iBAT was converted to white adipose tissue (WAT) in Ang II-induced hypertensive mice. We assessed the direct role of BAT in blood pressure (BP) regulation by using iBAT-deficient mice (generated by surgical removal of iBAT) and C57BL/6 (wild-type (WT)) mice and found that Ang II-induced BP elevation and vascular damage were markedly aggravated in iBAT-deficient mice compared with WT mice.

CONCLUSIONS

This study demonstrates that PDGFRα⁺ S-MSCs are able to differentiate into BAs and that this differentiation is regulated by mitochondrial activity. We also show that BAT plays a direct role in ameliorating Ang II-induced hypertension. The therapeutic potential of BAT for the prevention of hypertension-induced organ remodeling thus warrants further investigation.

All-trans-retinoic acid ameliorates atherosclerosis, promotes perivascular adipose tissue browning, and increases adiponectin production in Apo-E mice

All-trans-retinoic acid (atRA), an active metabolite of vitamin A, exerts a potential role in the prevention of cardiovascular diseases. It has been shown that atRA ameliorates atherosclerosis while the exact mechanism underlying this protection remains unknown. This study investigated the influence of atRA on insulin resistance (IR), atherosclerosis, and the process of perivascular adipose tissue (PVAT) browning. Moreover, syntheses of adiponectin, adipokine with anti-atherogenic effects, and tumor necrosis factor-alpha (TNF-α), a pro-inflammatory cytokine, were determined in PVAT. Apolipoprotein E-deficient mice (Apo-E) and control C57BL/6J wild-type mice were treated with atRA (5 mg/kg/day) or vehicle (corn oil) by plastic feeding tubes for 8 weeks. Long-term atRA treatment in Apo-E mice did not affect insulin resistance. AtRa administration ameliorated atherosclerosis, induced PVAT browning, and increased adiponectin production in PVAT in Apo-E mice. Furthermore, atRA increased nitric oxide (NO) level but did not affect adiponectin concentration in the aorta of Apo-E mice. These results indicate that atRA ameliorates atherosclerosis in Apo-E mice. We also observed the browning of PVAT. Besides, atRA increased the synthesis of adiponectin in PVAT and augmented NO level in the aorta in ApoE mice.

The relationship of irisin with metabolic syndrome components and insulin secretion and resistance in schoolchildren

Irisin, a novel myokine, is believed to be the crucial factor in converting white adipose tissue to beige adipose tissue. For this paper, we studied the relationship among irisin and components of metabolic syndrome (MetS), and insulin secretion and resistance in schoolchildren of Taiwan.Subjects receiving routine annual health examination at elementary school were enrolled. Demographic data, anthropometry, MetS components, irisin, and insulin secretion and resistance were collected. Subjects were divided into normal, overweight, and obese groups for evaluation of irisin in obesity. Finally, the relationship between irisin and MetS was analyzed.There were 376 children (179 boys and 197 girls), aged 10.3 ± 1.5 years, were enrolled. In boys, irisin levels were not associated with body mass index percentile, body fat, blood pressure, lipid profiles, insulin secretion or resistance. After adjusting for age, the irisin level in boys was negatively related to fasting plasma glucose (FPG) (r = -0.21, P = .006). In girls, after adjusting for age, the irisin levels were positively related only to FPG (r = 1.49, P = .038). In both genders, irisin levels were similar among normal, overweight, and obese groups, and between subjects with and without MetS.The irisin levels were not associated with MetS in either boys or girls. In girls, circulating irisin levels have a nonsignificant declining trend in overweight and obese girls. However, irisin levels were negatively related to FPG in boys and positively related to FPG in girls. The contrary relationship between irisin and FPG in boys and girls needs further exploration.

Adaptive Thermogenesis in a Mouse Model Lacking Selenoprotein Biosynthesis in Brown Adipocytes

Selenoproteins are a class of proteins with the selenium-containing amino acid selenocysteine (Sec) in their primary structure. Sec is incorporated into selenoproteins via recoding of the stop codon UGA, with specific cis and trans factors required during translation to avoid UGA recognition as a stop codon, including a Sec-specific tRNA, tRNA[Ser]Sec, encoded in mice by the gene Trsp. Whole-body deletion of Trsp in mouse is embryonically lethal, while targeted deletion of Trsp in mice has been used to understand the role of selenoproteins in the health and physiology of various tissues. We developed a mouse model with the targeted deletion of Trsp in brown adipocytes (Trspf/f-Ucp1-Cre+/-), a cell type predominant in brown adipose tissue (BAT) controlling energy expenditure via activation of adaptive thermogenesis, mostly using uncoupling protein 1 (Ucp1). At room temperature, Trspf/f-Ucp1-Cre+/- mice maintain oxygen consumption and Ucp1 expression, with male Trspf/f-Ucp1-Cre+/- mice accumulating more triglycerides in BAT than both female Trspf/f-Ucp1-Cre+/- mice or Trspf/f controls. Acute cold exposure neither reduced core body temperature nor changed the expression of selenoprotein iodothyronine deiodinase type II (Dio2), a marker of adaptive thermogenesis, in Trspf/f-Ucp1-Cre+/- mice. Microarray analysis of BAT from Trspf/f-Ucp1-Cre+/- mice revealed glutathione S-transferase alpha 3 (Gsta3) and ELMO domain containing 2 (Elmod2) as the transcripts most affected by the loss of Trsp. Male Trspf/f-Ucp1-Cre+/- mice showed mild hypothyroidism while downregulating thyroid hormone-responsive genes Thrsp and Tshr in their BATs. In summary, modest changes in the BAT of Trspf/f-Ucp1-Cre +/- mice implicate a mild thyroid hormone dysfunction in brown adipocytes.

Phosphodiesterase 2A2 regulates mitochondria clearance through Parkin-dependent mitophagy

Programmed degradation of mitochondria by mitophagy, an essential process to maintain mitochondrial homeostasis, is not completely understood. Here we uncover a regulatory process that controls mitophagy and involves the cAMP-degrading enzyme phosphodiesterase 2A2 (PDE2A2). We find that PDE2A2 is part of a mitochondrial signalosome at the mitochondrial inner membrane where it interacts with the mitochondrial contact site and organizing system (MICOS). As part of this compartmentalised signalling system PDE2A2 regulates PKA-mediated phosphorylation of the MICOS component MIC60, resulting in modulation of Parkin recruitment to the mitochondria and mitophagy. Inhibition of PDE2A2 is sufficient to regulate mitophagy in the absence of other triggers, highlighting the physiological relevance of PDE2A2 in this process. Pharmacological inhibition of PDE2 promotes a 'fat-burning' phenotype to retain thermogenic beige adipocytes, indicating that PDE2A2 may serve as a novel target with potential for developing therapies for metabolic disorders.

Adipose Stromal Cell Expansion and Exhaustion: Mechanisms and Consequences

Adipose tissue (AT) is comprised of a diverse number of cell types, including adipocytes, stromal cells, endothelial cells, and infiltrating leukocytes. Adipose stromal cells (ASCs) are a mixed population containing adipose progenitor cells (APCs) as well as fibro-inflammatory precursors and cells supporting the vasculature. There is growing evidence that the ability of ASCs to renew and undergo adipogenesis into new, healthy adipocytes is a hallmark of healthy fat, preventing disease-inducing adipocyte hypertrophy and the spillover of lipids into other organs, such as the liver and muscles. However, there is building evidence indicating that the ability for ASCs to self-renew is not infinite. With rates of ASC proliferation and adipogenesis tightly controlled by diet and the circadian clock, the capacity to maintain healthy AT via the generation of new, healthy adipocytes appears to be tightly regulated. Here, we review the contributions of ASCs to the maintenance of distinct adipocyte pools as well as pathogenic fibroblasts in cancer and fibrosis. We also discuss aging and diet-induced obesity as factors that might lead to ASC senescence, and the consequences for metabolic health.

HYPOTHesizing about central comBAT against obesity

The hypothalamus is a brain region in charge of many vital functions. Among them, BAT thermogenesis represents an essential physiological function to maintain body temperature. In the metabolic context, it has now been established that energy expenditure attributed to BAT function can contribute to the energy balance in a substantial extent. Thus, therapeutic interest in this regard has increased in the last years and some studies have shown that BAT function in humans can make a real contribution to improve diabetes and obesity-associated diseases. Nevertheless, how the hypothalamus controls BAT activity is still not fully understood. Despite the fact that much has been known about the mechanisms that regulate BAT activity in recent years, and that the central regulation of thermogenesis offers a very promising target, many questions remain still unsolved. Among them, the possible human application of knowledge obtained from rodent studies, and drug administration strategies able to specifically target the hypothalamus. Here, we review the current knowledge of homeostatic regulation of BAT, including the molecular insights of brown adipocytes, its central control, and its implication in the development of obesity.

Suppression of Brown Adipocyte Autophagy Improves Energy Metabolism by Regulating Mitochondrial Turnover

The high abundance of mitochondria and the expression of mitochondrial uncoupling protein 1 (UCP1) confer upon brown adipose tissue (BAT) the unique capacity to convert chemical energy into heat at the expense of ATP synthesis. It was long believed that BAT is present only in infants, and so, it was not considered as a potential therapeutic target for metabolic syndrome; however, the discovery of metabolically active BAT in adult humans has re-stimulated interest in the contributions of BAT metabolic regulation and dysfunction to health and disease. Here we demonstrate that brown adipocyte autophagy plays a critical role in the regulation BAT activity and systemic energy metabolism. Mice deficient in brown adipocyte autophagy due to BAT-specific deletion of Atg7-a gene essential for autophagosome generation-maintained higher mitochondrial content due to suppression of mitochondrial clearance and exhibited improved insulin sensitivity and energy metabolism. Autophagy was upregulated in BAT of older mice compared to younger mice, suggesting its involvement in the age-dependent decline of BAT activity and metabolic rate. These findings suggest that brown adipocyte autophagy plays a crucial role in metabolism and that targeting this pathway may be a potential therapeutic strategy for metabolic syndrome.

Neural Crest Cells Differentiate Into Brown Adipocytes and Contribute to Periaortic Arch Adipose Tissue Formation

OBJECTIVE

Periaortic arch adipose tissue (PAAT) plays critical roles in regulating vascular homeostasis; however, its anatomic features, developmental processes, and origins remain unclear. Approach and Results: Anatomic analysis and genetic lineage tracing of Wnt1 (wingless-type MMTV [mouse mammary tumor virus] integration site family member 1)-Cre⁺;Rosa26^RFP/+ mice, Myf5 (myogenic factor 5)-Cre⁺;Rosa26^RFP/+ mice, and SM22α-Cre⁺;Rosa26^RFP/+ mice are performed, and the results show that PAAT has unique anatomic features, and the developmental processes of PAAT are independent of the others periaortic adipose tissues. PAAT adipocytes are mainly derived from neural crest cells (NCCs) rather than from Myf5⁺ progenitors. Most PAAT adipocyte progenitors expressed SM22α⁺ (smooth muscle protein 22-alpha) during development. Using Wnt1-Cre⁺;PPARγ^flox/flox mice, we found that knockout of PPAR (peroxisome proliferator-activated receptor)-γ in NCCs results in PAAT developmental delay and dysplasia, further confirming that NCCs contribute to PAAT formation. And we further indicated PAAT dysplasia aggravates Ang II (angiotensin II)-induced inflammation and remodeling of the common carotid artery close to aorta arch. We also found that NCCs can be differentiated into both brown and white adipocytes in vivo and in vitro. RNA sequencing results suggested NCC-derived adipose tissue displays a distinct transcriptional profile compared with the non-NCC-derived adipose tissue in PAAT.

CONCLUSIONS

PAAT has distinctive anatomic features and developmental processes. Most PAAT adipocytes are originated from NCCs which derive from ectoderm. NCCs are progenitors not only of white adipocytes but also of brown adipocytes. This study indicates that the PAAT is derived from multiple cell lineages, the adipocytes derived from different origins have distinct transcriptional profiles, and PAAT plays a critical role in Ang II-induced common carotid artery inflammation and remodeling.Visual OvervieW: An online visual overview is available for this article.

The Importance of Peripheral Nerves in Adipose Tissue for the Regulation of Energy Balance

Brown and white adipose tissues are essential for maintenance of proper energy balance and metabolic health. In order to function efficiently, these tissues require both endocrine and neural communication with the brain. Brown adipose tissue (BAT), as well as the inducible brown adipocytes that appear in white adipose tissue (WAT) after simulation, are thermogenic and energy expending. This uncoupling protein 1 (UCP1)-mediated process requires input from sympathetic nerves releasing norepinephrine. In addition to sympathetic noradrenergic signaling, adipose tissue contains sensory nerves that may be important for relaying fuel status to the brain. Chemical and surgical denervation studies of both WAT and BAT have clearly demonstrated the role of peripheral nerves in browning, thermogenesis, lipolysis, and adipogenesis. However, much is still unknown about which subtypes of nerves are present in BAT versus WAT, what nerve products are released from adipose nerves and how they act to mediate metabolic homeostasis, as well as which cell types in adipose are receiving synaptic input. Recent advances in whole-depot imaging and quantification of adipose nerve fibers, as well as other new research findings, have reinvigorated this field of research. This review summarizes the history of research into adipose innervation and brain⁻adipose communication, and also covers landmark and recent research on this topic to outline what we currently know and do not know about adipose tissue nerve supply and communication with the brain.

Developmental and functional characteristics of the thoracic aorta perivascular adipocyte

Thoracic aorta perivascular adipose tissue (T-PVAT) has critical roles in regulating vascular homeostasis. However, the developmental characteristics and cellular lineage of adipocyte in the T-PVAT remain unclear. We show that T-PVAT contains three long strip-shaped fat depots, anterior T-PVAT (A-T-PVAT), left lateral T-PVAT (LL-T-PVAT), and right lateral T-PVAT (RL-T-PVAT). A-T-PVAT displays a distinct transcriptional profile and developmental origin compared to the two lateral T-PVATs (L-T-PVAT). Lineage tracing studies indicate that A-T-PVAT adipocytes are primarily derived from SM22α+ progenitors, whereas L-T-PVAT contains both SM22α+ and Myf5+ cells. We also show that L-T-PVAT contains more UCP1+ brown adipocytes than A-T-PVAT, and L-T-PVAT exerts a greater relaxing effect on aorta than A-T-PVAT. Angiotensin II-infused hypertensive mice display greater macrophage infiltration into A-T-PVAT than L-T-PVAT. These combined results indicate that L-T-PVAT has a distinct development from A-T-PVAT with different cellular lineage, and suggest that L-T-PVAT and A-T-PVAT have different physiological and pathological functions.

Environmental Pollutants Effect on Brown Adipose Tissue

Brown adipose tissue (BAT) with its thermogenic function due to the presence of the mitochondrial uncoupling protein 1 (UCP1), has been positively associated with improved resistance to obesity and metabolic diseases. During recent years, the potential influence of environmental pollutants on energetic homoeostasis and obesity development has drawn increased attention. The purpose of this review is to discuss how regulation of BAT function could be involved in the environmental pollutant effect on body energy metabolism. We mainly focused in reviewing studies on animal models, which provide a better insight into the cellular mechanisms involved in this effect on body energy metabolism. The current literature supports the hypothesis that some environmental pollutants, acting as endocrine disruptors (EDCs), such as dichlorodiphenyltrichoroethane (DDT) and its metabolite dichlorodiphenylethylene (DDE) as well as some, traffic pollutants, are associated with increased obesity risk, whereas some other chemicals, such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), had a reverse association with obesity. Noteworthy, the EDCs associated with obesity and metabolic disorders impaired BAT mass and function. Perinatal exposure to DDT impaired BAT thermogenesis and substrate utilization, increasing susceptibility to metabolic syndrome. Ambient particulate air pollutions induced insulin resistance associated with BAT mitochondrial dysfunction. On the other hand, the environmental pollutants (PFOS/PFOA) elicited a reduction in body weight and adipose mass associated with upregulation of UCP1 and increased oxidative capacity in brown-fat mitochondria. Further research is needed to better understand the physiological role of BAT in response to exposure to both obesogenic and anti-obesogenic pollutants and to confirm the same role in humans.

Ectopic BAT mUCP-1 overexpression in SKM by delivering a BMP7/PRDM16/PGC-1a gene cocktail or single PRMD16 using non-viral UTMD gene therapy

Here we present our progress in inducing an ectopic brown adipose tissue (BAT) phenotype in skeletal muscle (SKM) as a potential gene therapy for obesity and its comorbidities. We used ultrasound-targeted microbubble destruction (UTMD), a novel targeted, non-viral approach to gene therapy, to deliver genes in the BAT differentiation pathway into rodent SKM to engineer a thermogenic BAT phenotype with ectopic mUCP-1 overexpression. In parallel, we performed a second protocol using wild-type Ucp-1-null knockout mice to test whether the effects of the gene therapy are UCP-1 dependent. Our main findings were a robust cellular presence of mUCP-1 immunostaining (IHC), significantly higher expression levels of mUCP-1 measured by qRT-PCR, and highest temperature elevation measured by infrared thermography in the treated thigh, achieved in rats after delivering the UTMD-PRDM16/PGC-1a/BMP7/hyPB gene cocktail. Interestingly, the weight loss obtained in the treated rats with the triple gene delivery, never recovered the levels observed in the controls in spite of food intake recovery. Our results establish the feasibility of minimally invasive UTMD gene-based therapy administration in SKM, to induce overexpression of ectopic mUCP-1 after delivery of the thermogenic BAT gene program, and describe systemic effects of this intervention on food intake, weight loss, and thermogenesis.

Expression, purification and biological characterisation of recombinant human irisin (12.5 kDa)

Fibronectin type III domain containing 5 (FNDC5) is a transmembrane protein. Upon cleavage, it yields a peptide called irisin that is supposedly bind to an unknown receptor and facilitates browning of white adipose tissue (WAT). Increased levels of irisin are associated with increased levels of energy expenditure markers PGC-1α, UCP-1, besides abundance of beige adipocytes in WAT. Though varied sizes of irisin were reported in humans and rodents it is not yet clear about the actual size of the irisin produced physiologically. Hence, we cloned and expressed human irisin (32–143 aa of FNDC5) in Escherichia coli based on the proposed cleavage site that yields 12.5 kDa peptide to study its antigenicity and other biological functions in vitro. We purified recombinant human irisin (rh-irisin) to 95% homogeneity with simple purification method with a yield of 25 mg/g wet cell pellet. rh-irisin has been detected by commercially available antibodies from different sources with similar antigenicity. Biological activity of the rh-irisin was confirmed by using 3T3-L1 pre-adipocyte differentiation by Oil red O staining. Further, rh-irisin treatment on pre-adipocytes showed increased expression of markers associated with energy expenditure. As it is involved in energy expenditure process, it could be considered as potential therapeutic option for various metabolic diseases.

Proteomic profiling of Ganoderma tsugae ethanol extract-induced adipogenesis displaying browning features

Ganoderma is classified as a top grade traditional Chinese medicine for promoting human health by regulating 'vital energy'. Its potency towards metabolism and energy homeostasis, particularly, metabolic adaptations of adipocytes, needs to be re-evaluated through an evidence-based study. Here, the triterpenoid-rich Ganoderma tsugae ethanol extract (GTEE) was found to contribute towards adipogenesis accompanied with elevated intracellular lipid metabolic flux. Additionally, proteomic profiling revealed GTEE-upregulated mitochondrial remodeling and chemical energy redox modifications, which display UCP1-positive browning fat-selective features and a NADH-mediated adaptive mechanism. GTEE-treated mice with diet-induced obesity also resulted in the amelioration of white adipocyte hypertrophy and the appearance of UCP1-positive browning adipocytes. Our novel findings unravel that GTEE could promote intracellular metabolic flexibility and plasticity followed by the induction of adipocyte browning.

Glucocorticoids Suppress the Browning of Adipose Tissue via miR-19b in Male Mice

Physiological levels of glucocorticoids (GCs) are required for proper metabolic control, and excessive GC action has been linked to a variety of pandemic metabolic diseases. MicroRNA (miRNA)-19b plays a critical role in the pathogenesis of GC-induced metabolic diseases. This study explored the potential of miRNA-based therapeutics targeting adipose tissue. Our results showed that overexpressed miR-19b in stromal vascular fraction (SVF) cells derived from subcutaneous adipose tissue had the same effects as dexamethasone (DEX) treatment on the inhibition of adipose browning and oxygen consumption rate. The inhibition of miR-19b blocked DEX-mediated suppression of the expression of browning marker genes as well as the oxygen consumption rate in differentiated SVF cells derived from subcutaneous and brown adipose tissue. Overexpressed miR-19b in SVF cells derived from brown adipose tissue had the same effects as DEX treatment on the inhibition of brown adipose differentiation and energy expenditure. Glucocorticoids transcriptionally regulate the expression of miR-19b via a GC receptor-mediated direct DNA binding mechanism. This study confirmed that miR-19b is an essential target for GC-mediated control of adipose tissue browning. It is hoped that the plasticity of the adipose organ can be exploited in the next generation of therapeutic strategies to combat the increasing incidence of metabolic diseases, including obesity and diabetes.

The Role of Insulin Resistance on FGF-21 and Inflammatory Markers in Obese Adolescents Undergoing Multicomponent Long-Term Weight Loss Therapy

Objective: The purpose of this study was to investigate the effects of a long-term weight loss therapy in two groups (insulin resistance [IR] and non-insulin resistance [non-IR]) of obese adolescents based on metabolic profile, biomarkers of inflammation, and fibroblast growth factor-21 (FGF-21) concentrations. Methods: Obese adolescents (15–19 years) were randomised into two groups (IR=8 and non-IR=9) and monitored through clinical, exercise training, nutritional, and psychological counselling over 1 year. Measurements of inflammatory biomarkers and FGF-21 were performed. The effects of therapy were verified by two-way ANOVA and post hoc analyses were performed (α ≤5%). Results: A reduction in body mass, visceral fat, and an increase in adiponectin in both groups was found. Only the non-IR group demonstrated improved BMI, body fat mass, lean body mass, and waist circumference. Indeed, in the non-IR group, FGF-21 presence was positively correlated with high-density lipoprotein cholesterol and lean body mass and inversely correlated with plasminogen activator inhibitor-1 and triglycerides. In the IR group, there was a reduction in FGF-21 concentration, adiponectin/leptin ratio, insulin, total cholesterol, low-density lipoprotein cholesterol, and plasminogen activator inhibitor-1. FGF-21 was negatively correlated with delta-triglycerides, waist circumference, and low-density lipoprotein cholesterol. The IR prevalence reduced from 47% to 23.5% in the studied population. Conclusions: Although the multicomponent clinical approach improves, in both analysed groups and in both metabolic and inflammatory states, the presence of IR resulted in a reduction in both FGF-21 concentration and adiponectin/leptin ratio. Additionally, in the IR group, FGF-21 was negatively correlated with proinflammatory markers, and in the non-IR group it was positively associated with high-density lipoprotein, suggesting its role in the control of inflammation counteracting IR. In this way, we suggest that IR can impair the anti-inflammatory effects of FGF-21. It will be helpful if these results can be confirmed in a large cohort, underlying physiological mechanisms to explore how these results can help in setting up more prospective studies.

Comparisons between perivascular adipose tissue and the endothelium in their modulation of vascular tone

The endothelium is an established modulator of vascular tone; however, the recent discovery of the anti-contractile nature of perivascular adipose tissue (PVAT) suggests that the fat, which surrounds many blood vessels, can also modulate vascular tone. Both the endothelium and PVAT secrete vasoactive substances, which regulate vascular function. Many of these factors are common to both the endothelium and PVAT; therefore, this review will highlight the potential shared mechanisms in the modulation of vascular tone. Endothelial dysfunction is a hallmark of many vascular diseases, including hypertension and obesity. Moreover, PVAT dysfunction is now being reported in several cardio-metabolic disorders. Thus, this review will also discuss the mechanistic insights into endothelial and PVAT dysfunction in order to evaluate whether PVAT modulation of vascular contractility is similar to that of the endothelium in health and disease.

LINKED ARTICLES

This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.

Impact of Adrenal Steroids on Regulation of Adipose Tissue

Corticosteroids are secreted by the adrenal glands and control the functions of adipose tissue via the activation of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR). In turn, adipocytes release a large variety of adipokines into the bloodstream, regulating the function of several organs and tissues, including the adrenal glands, hereby controlling corticosteroid production. In adipose tissue, the activation of the MR by glucocorticoids (GC) and aldosterone affects important processes such as adipocyte differentiation, oxidative stress, autophagic flux, adipokine expression as well as local production of GC through upregulation of the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). Notably, the proinflammatory responses induced by the MR are counteracted by activation of the GR, whose activity inhibits the expression of inflammatory adipokines. Both GR and MR are deeply involved in adipogenesis and adipose expansion; hence pharmacological blockade of these two receptors has proven effective against adipose tissue dysfunction in experimental models of obesity and metabolic syndrome (MetS), suggesting a potential use for MR and GR antagonists in these clinical settings. Importantly, obesity and Cushing's syndrome (CS) share metabolic similarities and are characterized by high levels of circulating corticosteroids, which in turn are able to deeply affect adipose tissue. In addition, pharmacological approaches aimed at reducing aldosterone and GC levels, by means of the inhibition of CYP11B2 (aldosterone synthase) or 11β-HSD1, represent alternative strategies to counter the detrimental effects of excessive levels of corticosteroids, which are often observed in obesity and, more general, in MetS. © 2017 American Physiological Society. Compr Physiol 7:1425-1447, 2017.

Tbx18-dependent differentiation of brown adipose tissue-derived stem cells toward cardiac pacemaker cells

A cell-sourced biological pacemaker is a promising therapeutic approach for sick sinus syndrome (SSS) or severe atrial ventricular block (AVB). Adipose tissue-derived stem cells (ATSCs), which are optimal candidate cells for possible use in regenerative therapy for acute or chronic myocardial injury, have the potential to differentiate into spontaneous beating cardiomyocytes. However, the pacemaker characteristics of the beating cells need to be confirmed, and little is known about the underlying differential mechanism. In this study, we found that brown adipose tissue-derived stem cells (BATSCs) in mice could differentiate into spontaneous beating cells in 15% FBS Dulbecco's modified Eagle's medium (DMEM) without additional treatment. Subsequently, we provide additional evidence, including data regarding ultrastructure, protein expression, electrophysiology, and pharmacology, to support the differentiation of BATSCs into a cardiac pacemaker phenotype during the course of early cultivation. Furthermore, we found that silencing Tbx18, a key transcription factor in the development of pacemaker cells, terminated the differentiation of BATSCs into a pacemaker phenotype, suggesting that Tbx18 is required to direct BATSCs toward a cardiac pacemaker fate. The expression of Tbx3 and shox2, the other two important transcription factors in the development of pacemaker cells, was decreased by silencing Tbx18, which suggests that Tbx18 mediates the differentiation of BATSCs into a pacemaker phenotype via these two downstream transcription factors.

A pilot study on the correlation between fat fraction values and glucose uptake values in supraclavicular fat by simultaneous PET/MRI

PURPOSE

To assess the spatial correlation between MRI and 18F-fludeoxyglucose positron emission tomography (FDG-PET) maps of human brown adipose tissue (BAT) and to measure differences in fat fraction (FF) between glucose avid and non-avid regions of the supraclavicular fat depot using a hybrid FDG-PET/MR scanner.

METHODS

In 16 healthy volunteers, mean age of 30 and body mass index of 26, FF, R2*, and FDG uptake maps were acquired simultaneously using a hybrid PET/MR system while employing an individualized cooling protocol to maximally stimulate BAT.

RESULTS

Fourteen of the 16 volunteers reported BAT-positive FDG-PET scans. MR FF maps of BAT correlate well with combined FDG-PET/MR maps of BAT only in subjects with intense glucose uptake. The results indicate that the extent of the spatial correlation positively correlates with maximum FDG uptake in the supraclavicular fat depot. No consistent, significant differences were found in FF or R2* between FDG avid and non-avid supraclavicular fat regions. In a few FDG-positive subjects, a small but significant linear decrease in BAT FF was observed during BAT stimulation.

CONCLUSION

MR FF, when used in conjunction with FDG uptake maps, can be seen as a valuable, radiation-free alternative to CT and can be used to measure tissue hydration and lipid consumption in some subjects. Magn Reson Med 78:1922-1932, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Secretory function of adipose tissue

There are two kinds of adipose tissue in mammals: white adipose tissue - WAT and brown adipose tissue - BAT. The main function of WAT is accumulation of triacylglycerols whereas the function of BAT is heat generation. At present, WAT is also considered to be an endocrine gland that produces bioactive adipokines, which take part in glucose and lipid metabolism. Considering its endocrine function, the adipose tissue is not a homogeneous gland but a group of a few glands which act differently. Studies on the secretory function of WAT began in 1994 after discovery of leptin known as the satiation hormone, which regulates body energy homeostasis and maintainence of body mass. Apart from leptin, the following belong to adipokines: adiponectin, resistin, apelin, visfatin and cytokines: TNF and IL 6. Adiponectin is a polypeptide hormone of antidiabetic, anti-inflammatory and anti-atherogenic activity. It plays a key role in carbohydrate and fat metabolism. Resistin exerts a counter effect compared to adiponectin and its physiological role is to maintain fasting glycaemia. Visfatin stimulates insulin secretion and increases insulin sensitivity and glucose uptake by muscle cells and adipocytes. Apelin probably increases the insulin sensitivity of tissues. TNF evokes insulin resistance by blocking insulin receptors and inhibits insulin secretion. Approximately 30% of circulating IL 6 comes from adipose tissue. It causes insulin resistance by decreasing the expression of insulin receptors, decreases adipogenesis and adiponectin and visfatin secretion, and stimulates hepatic gluconeogenesis. In 2004, Bays introduced the notion of adiposopathy, defined as dysfunction of the adipose tissue, whose main feature is insulin and leptin resistance as well as the production of inflammatory cytokines: TNF and IL 6 and monocyte chemoattractant protein. This means that excess of adipose tissue, especially visceral adipose tissue, leads to the development of a chronic subclinical inflammatory condition, which favours the development of insulin resistance and Type 2 diabetes. Obesity is a systemic illness caused by energy transformation homeostasis disorder which results in an increase in the amount of body fat mass. It effects approximately 40% of dogs and 20% of cats. Illnesses which accompany obesity result, to a great extent, from the secretive role of adipose tissue, which is still little known, which should be included when planning treatment of an obese animal.

Endothelial Dysfunction in Obesity

Chronic inflammatory state in obesity causes dysregulation of the endocrine and paracrine actions of adipocyte-derived factors, which disrupt vascular homeostasis and contribute to endothelial vasodilator dysfunction and subsequent hypertension. While normal healthy perivascular adipose tissue (PVAT) ensures the dilation of blood vessels, obesity-associated PVAT leads to a change in profile of the released adipo-cytokines, resulting in a decreased vasorelaxing effect. Adipose tissue inflammation, nitric oxide (NO)-bioavailability, insulin resistance and oxidized low-density lipoprotein (oxLDL) are main participating factors in endothelial dysfunction of obesity. In this chapter, disruption of inter-endothelial junctions between endothelial cells, significant increase in the production of reactive oxygen species (ROS), inflammation mediators, which are originated from inflamed endothelial cells, the balance between NO synthesis and ROS , insulin signaling and NO production, and decrease in L-arginine/endogenous asymmetric dimethyl-L-arginine (ADMA) ratio are discussed in connection with endothelial dysfunction in obesity.

Paradoxical leanness in the imprinting-centre deletion mouse model for Prader-Willi syndrome

Prader-Willi syndrome (PWS), a neurodevelopmental disorder caused by loss of paternal gene expression from 15q11-q13, is characterised by growth retardation, hyperphagia and obesity. However, as single gene mutation mouse models for this condition display an incomplete spectrum of the PWS phenotype, we have characterised the metabolic impairment in a mouse model for 'full' PWS, in which deletion of the imprinting centre (IC) abolishes paternal gene expression from the entire PWS cluster. We show that PWS-IC^del mice displayed postnatal growth retardation, with reduced body weight, hyperghrelinaemia and marked abdominal leanness; proportionate retroperitoneal, epididymal/omental and inguinal white adipose tissue (WAT) weights being reduced by 82%, 84% and 67%, respectively. PWS-IC^del mice also displayed a 48% reduction in proportionate interscapular brown adipose tissue (isBAT) weight with significant 'beiging' of abdominal WAT, and a 2°C increase in interscapular surface body temperature. Maintenance of PWS-IC^del mice under thermoneutral conditions (30°C) suppressed the thermogenic activity in PWS-IC^del males, but failed to elevate the abdominal WAT weight, possibly due to a normalisation of caloric intake. Interestingly, PWS-IC^del mice also showed exaggerated food hoarding behaviour with standard and high-fat diets, but despite becoming hyperphagic when switched to a high-fat diet, PWS-IC^del mice failed to gain weight. This evidence indicates that, unlike humans with PWS, loss of paternal gene expression from the PWS cluster in mice results in abdominal leanness. Although reduced subcutaneous insulation may lead to exaggerated heat loss and thermogenesis, abdominal leanness is likely to arise from a reduced lipid storage capacity rather than increased energy utilisation in BAT.

Physiological functions of Vitamin D in adipose tissue

Adipose tissue has long been identified as the major site of vitamin D storage. Recent studies have demonstrated that VDR and vitamin D metabolizing enzymes are expressed in adipocytes. Furthermore, it has been shown that vitamin D regulates adipogenic gene expression as well as adipocyte apoptosis. Vitamin D is active in adipocytes at all levels. It interacts with membrane receptors, adaptor molecules, and nuclear coregulator proteins. Several functions of unliganded nVDR were discovered by studying human samples from patients having hereditary vitamin D resistant rickets, transgenic mice overexpressing the VDR and VDR knockout mice. Through its genomic action, vitamin D participates in the regulation of energy metabolism by controlling the expression of uncoupling proteins. In vitro, vitamin D stimulates lipogenesis and inhibits lipolysis by interacting with mVDR. mVDR is present in caveolae of the plasma membrane and is the same as the classic nVDR. In addition, vitamin D affects directly the expression of the appetite regulating hormone, leptin. Some researchers reported also that vitamin D regulates the expression of the insulin sensitizing hormone, adiponectin. Vitamin D reduced cytokine release and adipose tissue inflammation through the inhibition of NF-κB signaling. Scientific research investigating the role of adipose tissue resident immune cells in the pathogenesis of obesity-associated inflammation is scarce. Obesity is associated with vitamin D deficiency. However there is no scientific evidence to prove that vitamin D deficiency predispose to obesity. Vitamin D supplementation may prevent obesity but it does not lead to weight loss in obese subjects.

The role of adipose tissue in cancer-associated cachexia

Adipose tissue (fat) is a heterogeneous organ, both in function and histology, distributed throughout the body. White adipose tissue, responsible for energy storage and more recently found to have endocrine and inflammation-modulatory activities, was historically thought to be the only type of fat present in adult humans. The recent demonstration of functional brown adipose tissue in adults, which is highly metabolic, shifted this paradigm. Additionally, recent studies demonstrate the ability of white adipose tissue to be induced toward the brown adipose phenotype - "beige" or "brite" adipose tissue - in a process referred to as "browning." While these adipose tissue depots are under investigation in the context of obesity, new evidence suggests a maladaptive role in other metabolic disturbances including cancer-associated cachexia, which is the topic of this review. This syndrome is multifactorial in nature and is an independent factor associated with poor prognosis. Here, we review the contributions of all three adipose depots - white, brown, and beige - to the development and progression of cancer-associated cachexia. Specifically, we focus on the local and systemic processes involving these adipose tissues that lead to increased energy expenditure and sustained negative energy balance. We highlight key findings from both animal and human studies and discuss areas within the field that need further exploration. Impact statement Cancer-associated cachexia (CAC) is a complex, multifactorial syndrome that negatively impacts patient quality of live and prognosis. This work reviews a component of CAC that lacks prior discussion: adipose tissue contributions. Uniquely, it discusses all three types of adipose tissue, white, beige, and brown, their interactions, and their contributions to the development and progression of CAC. Summarizing key bench and clinical studies, it provides information that will be useful to both basic and clinical researchers in designing experiments, studies, and clinical trials.

Sympathetic Innervation of Cold-Activated Brown and White Fat in Lean Young Adults

Recent work in rodents has demonstrated that basal activity of the local sympathetic nervous system is critical for maintaining brown adipocyte phenotypes in classic brown adipose tissue (BAT) and white adipose tissue (WAT). Accordingly, we sought to assess the relationship between sympathetic innervation and cold-induced activation of BAT and WAT in lean young adults. Methods: Twenty adult lean normal subjects (10 women and 10 men; mean age ± SD, 23.3 ± 3.8 y; body mass index, 23.7 ± 2.5 kg/m²) underwent ¹¹C-meta-hydroxyephedrin (¹¹C-HED) and ¹⁵O-water PET imaging at rest and after exposure to mild cold (16°C) temperature. In addition, ¹⁸F-FDG images were obtained during the cold stress condition to assess cold-activated BAT mass. Subjects were divided into 2 groups (high BAT and low BAT) based on the presence of ¹⁸F-FDG tracer uptake. Blood flow and ¹¹C-HED retention index (RI, an indirect measure of sympathetic innervation) were calculated from dynamic PET scans at the location of BAT and WAT. Whole-body daily energy expenditure (DEE) during rest and cold stress was measured by indirect calorimetry. Tissue level oxygen consumption (MRO₂) was determined and used to calculate the contribution of cold-activated BAT and WAT to daily DEE. Results:¹⁸F-FDG uptake identified subjects with high and low levels of cold-activated BAT mass (high BAT, 96 ± 37 g; low-BAT, 16 ± 4 g). ¹¹C-HED RI under thermoneutral conditions significantly predicted ¹⁸F-FDG uptake during cold stress (R² = 0.68, P < 0.01). In contrast to the significant increase of ¹¹C-HED RI during cold in BAT (2.42 ± 0.85 vs. 3.43 ± 0.93, P = 0.02), cold exposure decreased the ¹¹C-HED RI in WAT (0.44 ± 0.22 vs. 0.41 ± 0.18) as a consequence of decreased perfusion (1.22 ± 0.20 vs. 1.12 ± 0.16 mL/100 g/min). The contribution of WAT to whole-body DEE was approximately 150 kcal/d at rest (149 ± 52 kcal/d), which decreased to approximately 100 kcal/d during cold (102 ± 47 kcal/d). Conclusion: The level of sympathetic innervation, as determined by ¹¹C-HED RI, can predict levels of functional BAT. Overall, blood flow is the best independent predictor of ¹¹C-HED RI and ¹⁸F-FDG uptake across thermoneutral and cold conditions. In contrast to BAT, cold stress reduces blood flow and ¹⁸F-FDG uptake in subcutaneous WAT, indicating that the physiologic response is to reduce heat loss rather than to generate heat.

Deletion of AMPKα1 attenuates the anticontractile effect of perivascular adipose tissue (PVAT) and reduces adiponectin release

BACKGROUND AND PURPOSE

Perivascular adipose tissue (PVAT) surrounds most blood vessels and secretes numerous active substances, including adiponectin, which produce a net anticontractile effect in healthy individuals. AMPK is a key mediator of cellular energy balance and may mediate the vascular effects of adiponectin. In this study, we investigated the role of AMPK within PVAT in mediating the anticontractile effect of PVAT.

EXPERIMENTAL APPROACH

Endothelium-denuded aortic rings from wild-type (WT; Sv129) and α₁ AMPK knockout (KO) mice were mounted on a wire myograph. Dose-response curves to the AMPK-independent vasodilator cromakalim were studied in vessels with and without PVAT, and effect of pre-incubation with conditioned media and adiponectin on relaxation was also studied. The effect of AMPKα1 KO on the secretory profile of PVAT was assessed by elisa.

KEY RESULTS

Thoracic aortic PVAT from KO mice was morphologically indistinct from that of WT and primarily composed of brown adipose tissue. PVAT augmented relaxation to cromakalim in WT but not KO aortic rings. Addition of WT PVAT augmented relaxation in KO aortic rings but KO PVAT had no effect in WT rings. PVAT from KO mice secreted significantly less adiponectin and addition of adiponectin to either KO or WT aortic rings without PVAT augmented relaxation to cromakalim. An adiponectin blocking peptide significantly attenuated relaxation in WT rings with PVAT but not in KO rings.

CONCLUSIONS AND IMPLICATIONS

AMPKα1 has a critical role in maintaining the anticontractile actions of PVAT; an effect independent of the endothelium but likely mediated through altered adiponectin secretion or sensitivity.

LINKED ARTICLES

This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.