Brown adipose tissue functions in humans

Increased Aquaporin-7 Expression Is Associated with Changes in Rat Brown Adipose Tissue Whitening in Obesity: Impact of Cold Exposure and Bariatric Surgery

Glycerol is a key metabolite for lipid accumulation in insulin-sensitive tissues. We examined the role of aquaporin-7 (AQP7), the main glycerol channel in adipocytes, in the improvement of brown adipose tissue (BAT) whitening, a process whereby brown adipocytes differentiate into white-like unilocular cells, after cold exposure or bariatric surgery in male Wistar rats with diet-induced obesity (DIO) (n = 229). DIO promoted BAT whitening, evidenced by increased BAT hypertrophy, steatosis and upregulation of the lipogenic factors Pparg2, Mogat2 and Dgat1. AQP7 was detected in BAT capillary endothelial cells and brown adipocytes, and its expression was upregulated by DIO. Interestingly, AQP7 gene and protein expressions were downregulated after cold exposure (4 °C) for 1 week or one month after sleeve gastrectomy in parallel to the improvement of BAT whitening. Moreover, Aqp7 mRNA expression was positively associated with transcripts of the lipogenic factors Pparg2, Mogat2 and Dgat1 and regulated by lipogenic (ghrelin) and lipolytic (isoproterenol and leptin) signals. Together, the upregulation of AQP7 in DIO might contribute to glycerol influx used for triacylglycerol synthesis in brown adipocytes, and hence, BAT whitening. This process is reversible by cold exposure and bariatric surgery, thereby suggesting the potential of targeting BAT AQP7 as an anti-obesity therapy.

Brown fat triglyceride content is associated with cardiovascular risk markers in adults from a tropical region

Brown adipose tissue (BAT) is regarded as an interesting potential target for the treatment of obesity, diabetes, and cardiovascular diseases, and the detailed characterization of its structural and functional phenotype could enable an advance in these fields. Most studies evaluating BAT structure and function were performed in temperate climate regions, and we are yet to know how these findings apply to the 40% of the world's population living in tropical areas. Here, we used 18F-fluorodeoxyglucose positron emission tomography - magnetic resonance imaging to evaluate BAT in 45 lean, overweight, and obese volunteers living in a tropical area in Southeast Brazil. We aimed at investigating the associations between BAT activity, volume, metabolic activity, and BAT content of triglycerides with adiposity and cardiovascular risk markers in a sample of adults living in a tropical area and we showed that BAT glucose uptake is not correlated with leanness; instead, BAT triglyceride content is correlated with visceral adiposity and markers of cardiovascular risk. This study expands knowledge regarding the structure and function of BAT in people living in tropical areas. In addition, we provide evidence that BAT triglyceride content could be an interesting marker of cardiovascular risk.

AMPKα1 regulates Idh2 transcription through H2B O-GlcNAcylation during brown adipogenesis

AMP-activated protein kinase (AMPK) is indispensable for the development and maintenance of brown adipose tissue (BAT), and its activity is inhibited due to obesity. The isocitrate dehydrogenase 2 (IDH2) is a mitochondrial enzyme responsible for the production of α-ketoglutarate, a key intermediate metabolite integrating multiple metabolic processes. We previously found that AMPKα1 ablation reduced cellular α-ketoglutarate concentration during brown adipocyte differentiation, but the effect of AMPKα1 on Idh2 expression remains undefined. In the present study, mouse C3H10T1/2 cells were transfected with Idh2-CRISPR/Cas9, and induced to brown adipogenesis. Our data suggested that brown adipogenesis was compromised due to IDH2 deficiency in vitro, which was accompanied by down-regulation of PR-domain containing 16. Importantly, the IDH2 content was reduced in brown stromal vascular cells (BSVs) separated from AMPKα1 knockout (KO) BAT, which was associated with lower contents of histone 2B (H2B) O-GlcNAcylation and monoubiquitination. Furthermore, both GlcNAcylated-H2B (S112) and ubiquityl-histone 2B (K120) contents in the Idh2 promoter were decreased in AMPKα1 KO BSVs. Meanwhile, ectopic O-linked N-acetylglucosamine transferase (OGT) expression was positively correlated with Idh2 expression, while OGT (T444A) mutation abolished the regulatory effect of AMPKα1 on Idh2. In vivo, reduced AMPKα1 activity and lower IDH2 abundance were observed in BAT of obese mice when compared with those in control mice. Taken together, our data demonstrated that IDH2 is necessary for brown adipogenesis and that AMPKα1 deficiency attenuates Idh2 expression, which might be by suppressing H2B O-GlcNAcylation modification.

Arsenite exposure suppresses adipogenesis, mitochondrial biogenesis and thermogenesis via autophagy inhibition in brown adipose tissue

Arsenite, a trivalent form of arsenic, is an element that occurs naturally in the environment. Humans are exposed to high dose of arsenite through consuming arsenite-contaminated drinking water and food, and the arsenite can accumulate in the human tissues. Arsenite induces oxidative stress, which is linked to metabolic disorders such as obesity and diabetes. Brown adipocytes dissipating energy as heat have emerging roles for obesity treatment and prevention. Therefore, understanding the pathophysiological role of brown adipocytes can provide effective strategies delineating the link between arsenite exposure and metabolic disorders. Our study revealed that arsenite significantly reduced differentiation of murine brown adipocytes and mitochondrial biogenesis and respiration, leading to attenuated thermogenesis via decreasing UCP1 expression. Oral administration of arsenite in mice resulted in heavy accumulation in brown adipose tissue and suppression of lipogenesis, mitochondrial biogenesis and thermogenesis. Mechanistically, arsenite exposure significantly inhibited autophagy necessary for homeostasis of brown adipose tissue through suppression of Sestrin2 and ULK1. These results clearly confirm the emerging mechanisms underlying the implications of arsenite exposure in metabolic disorders.

Adipose Tissue Composition in Obesity and After Bariatric Surgery

The adipose tissue is a complex organ that regulates food intake and energy expenditure as well as induces low-grade inflammation. This review deals with changes in the composition and activity of the adipose organ after bariatric surgery, focusing on epicardial and ectopic fat and on relationships between white and brown adipose tissues. Postoperative improvements of ectopic fat and epicardial fat size and composition account for the metabolic recovery and the decreased cardiovascular risk. Following Roux-en-Y gastric bypass or biliopancreatic diversion, a proportional increase in the size and activity of the metabolically active brown adipose tissue was observed, most likely related to the postoperative rearrangement of the entero-hormonal pattern with an increase of GLP-1 production: this aspect would promote the postoperative weight loss and maintenance of post-surgery benefits.

Sympathetic innervation of the interscapular brown adipose tissue in mouse

The recent discovery of significant brown fat depots in adult humans has revived discussion of exploiting brown fat thermogenesis in the control of energy balance and body weight. The sympathetic nervous system (SNS) has a key role in the activation of brown fat and functional mapping of its components will be crucial for the development of specific neuromodulation techniques. The mouse is an important species used for molecular genetic modulations, but its small size is not ideal for anatomical dissections, thus brown fat innervation studies are mostly available in larger rodents such as rats and hamsters. Here, we use pseudorabies virus retrograde tracing, whole tissue clearing, and confocal/light sheet microscopy to show the location of pre- and postganglionic neurons selectively innervating the interscapular brown adipose tissue (iBAT) in the mouse. Using iDISCO whole tissue clearing, we identified iBAT projecting postganglionic neurons in the caudal parts of the ipsilateral fused stellate/T1, as well as the T2-T5 sympathetic chain ganglia and preganglionic neurons between levels T2 and T6 of the ipsilateral spinal cord. The methodology enabled high-resolution imaging and 3D rendering of the specific SNS innervation of iBAT and will be helpful to discern peripheral nervous system innervation of other organs and tissues.

Emerging Functions of Regulatory T Cells in Tissue Homeostasis

CD4⁺Foxp3⁺ regulatory T-cells (Tregs) are a unique subset of helper T-cells, which regulate immune response and establish peripheral tolerance. Tregs not only maintain the tone and tenor of an immune response by dominant tolerance but, in recent years, have also been identified as key players in resolving tissue inflammation and as mediators of tissue healing. Apart from being diverse in their origin (thymic and peripheral) and location (lymphoid and tissue resident), Tregs are also phenotypically heterogeneous as per the orientation of ongoing immune response. In this review, we discuss the recent advances in the field of Treg biology in general, and non-lymphoid and tissue-resident Tregs in particular. We elaborate upon well-known visceral adipose tissue, colon, skin, and tumor-infiltrating Tregs and newly identified tissue Treg populations as in lungs, skeletal muscle, placenta, and other tissues. Our attempt is to differentiate Tregs based on distinctive properties of their location, origin, ligand specificity, chemotaxis, and specific suppressive mechanisms. Despite ever expanding roles in maintaining systemic homeostasis, Tregs are employed by large varieties of tumors to dampen antitumor immunity. Thus, a comprehensive understanding of Treg biology in the context of inflammation can be instrumental in effectively managing tissue transplantation, autoimmunity, and antitumor immune responses.

Cryptotanshinone promotes commitment to the brown adipocyte lineage and mitochondrial biogenesis in C3H10T1/2 mesenchymal stem cells via AMPK and p38-MAPK signaling

Although white adipose tissue (WAT) stores triglycerides and contributes to obesity, brown adipose tissue (BAT) dissipates energy as heat. Therefore, browning of WAT is regarded as an attractive way to counteract obesity. Our previous studies have revealed that treatment with cryptotanshinone (CT) during adipogenesis of 3T3-L1 cells inhibits their differentiation. Here, we found that pretreatment of C3H10T1/2 mesenchymal stem cells with CT before exposure to adipogenic hormonal stimuli promotes the commitment of these mesenchymal stem cells to the adipocyte lineage as confirmed by increased triglyceride accumulation. Furthermore, CT treatment induced the expression of early B-cell factor 2 (Ebf2) and bone morphogenetic protein 7 (Bmp7), which are known to drive differentiation of C3H10T1/2 mesenchymal stem cells toward preadipocytes and to the commitment to brown adipocytes. Consequently, CT treatment yielded brown-adipocyte-like features as evidenced by elevated expression of brown-fat signature genes including Ucp1, Prdm16, Pgc-1α, Cidea, Zic1, and beige-cell-specific genes such as CD137, Hspb7, Cox2, and Tmem26. Additionally, CT treatment induced mitochondrial biogenesis through upregulation of Sirt1, Tfam, Nrf1, and Cox7a and increased mitochondrial mass and DNA content. Our data also showed that cotreatment with CT and BMP4 was more effective at activating brown-adipocyte-specific genes. Mechanistic experiments revealed that treatment with CT activated AMPKα and p38-MAPK via their phosphorylation: the two major signaling pathways regulating energy metabolism. Thus, these findings suggest that CT is a candidate therapeutic agent against obesity working via activation of browning and mitochondrial biogenesis in C3H10T1/2 mesenchymal stem cells.

Factors involved in white-to-brown adipose tissue conversion and in thermogenesis: a review

Obesity is the result of energy intake chronically exceeding energy expenditure. Classical treatments against obesity do not provide a satisfactory long-term outcome for the majority of patients. After the demonstration of functional brown adipose tissue in human adults, great effort is being devoted to develop therapies based on the adipose tissue itself, through the conversion of fat-accumulating white adipose tissue into energy-dissipating brown adipose tissue. Anti-obesity treatments that exploit endogenous, pharmacological and nutritional factors to drive such conversion are especially in demand. In the present review, we summarize the current knowledge about the various molecules that can be applied in promoting white-to-brown adipose tissue conversion and energy expenditure and the cellular mechanisms involved.

Contributors to Metabolic Disease Risk Following Spinal Cord Injury

Spinal cord injury (SCI) induced changes in neurological function have significant impact on the metabolism and subsequent metabolic-related disease risk in injured individuals. This metabolic-related disease risk relationship is differential depending on the anatomic level and severity of the injury, with high level anatomic injuries contributing a greater risk of glucose and lipid dysregulation resulting in type 2 diabetes and cardiovascular disease risk elevation. Although alterations in body composition, particularly excess adiposity and its anatomical distribution in the visceral depot or ectopic location in non-adipose organs, is known to significantly contribute to metabolic disease risk, changes in fat mass and fat-free mass do not fully account for this elevated disease risk in subjects with SCI. There are other negative adaptations in body composition including reductions in skeletal muscle mass and alterations in muscle fiber type, in addition to significant reduction in physical activity, that contribute to a decline in metabolic rate and increased metabolic disease risk following SCI. Recent studies in adult humans suggest cold- and diet-induced thermogenesis through brown adipose tissue metabolism may be important for energy balance and substrate metabolism, and particularly sensitive to sympathetic nervous signaling. Considering the alterations that occur in the autonomic nervous system (SNS) (sympathetic and parasympathetic) following a SCI, significant dysfunction of brown adipose function is expected. This review will highlight metabolic alterations following SCI and integrate findings from brown adipose tissue studies as potential new areas of research to pursue.

Visfatin expression analysis in association with recruitment and activation of human and rodent brown and brite adipocytes

Human brown adipocytes are able to burn fat and glucose and are now considered as a potential strategy to treat obesity, type 2 diabetes and metabolic disorders. Besides their thermogenic function, brown adipocytes are able to secrete adipokines. One of these is visfatin, a nicotinamide phosphoribosyltransferase involved in nicotinamide dinucleotide synthesis, which is known to participate in the synthesis of insulin by pancreatic β cells. In a therapeutic context, it is of interest to establish whether a potential correlation exists between brown adipocyte activation and/or brite adipocyte recruitment, and adipokine expression. We analyzed visfatin expression, as a pre-requisite to its secretion, in rodent and human biopsies and cell models of brown/brite adipocytes. We found that visfatin was preferentially expressed in mature adipocytes and that this expression was higher in brown adipose tissue of rodents compared to other fat depots. However, using various rodent models we were unable to find any correlation between visfatin expression and brown or brite adipocyte activation or recruitment. Interestingly, the situation is different in humans where visfatin expression was found to be equivalent between white and brown or brite adipocytes in vivo and in vitro. In conclusion, visfatin can be considered only as a rodent brown adipocyte biomarker, independently of tissue activation.

Fibroblast growth factor-21, energy balance and obesity

Fibroblast growth factor (FGF)-21 is an endocrine member of the FGF family with healthy effects on glucose and lipid metabolism. FGF21 reduces glycemia and lipidemia in rodent models of obesity and type 2 diabetes. In addition to its effects improving insulin sensitivity, FGF21 causes weight loss by increasing energy expenditure. Activation of the thermogenic activity of brown adipose tissue and promotion of the appearance of the so-called beige/brite type of brown adipocytes in white fat are considered the main mechanisms underlying the leaning effects of FGF21. Paradoxically, however, obesity in rodents and humans is characterized by high levels of FGF21 in the blood. Some degree of resistance to the actions of FGF21 has been proposed as part of the endocrine alterations in obesity. The resistance in adipose tissue from obese rodents and patients is likely attributable to abnormally low levels of the FGF co-receptor β-Klotho, required for FGF21 cellular action. However, native FGF21 and FGF21 derivatives retain their healthy metabolic and weight-loss effects when used as pharmacological agents to treat obese rodents and humans. FGF21 derivatives or molecules mimicking FGF21 action appear to be interesting candidates for the development of novel anti-obesity drugs designed to increase energy expenditure.

Effective treatment of mitochondrial myopathy by nicotinamide riboside, a vitamin B3

Nutrient availability is the major regulator of life and reproduction, and a complex cellular signaling network has evolved to adapt organisms to fasting. These sensor pathways monitor cellular energy metabolism, especially mitochondrial ATP production and NAD⁺/NADH ratio, as major signals for nutritional state. We hypothesized that these signals would be modified by mitochondrial respiratory chain disease, because of inefficient NADH utilization and ATP production. Oral administration of nicotinamide riboside (NR), a vitamin B3 and NAD⁺ precursor, was previously shown to boost NAD⁺ levels in mice and to induce mitochondrial biogenesis. Here, we treated mitochondrial myopathy mice with NR. This vitamin effectively delayed early- and late-stage disease progression, by robustly inducing mitochondrial biogenesis in skeletal muscle and brown adipose tissue, preventing mitochondrial ultrastructure abnormalities and mtDNA deletion formation. NR further stimulated mitochondrial unfolded protein response, suggesting its protective role in mitochondrial disease. These results indicate that NR and strategies boosting NAD⁺ levels are a promising treatment strategy for mitochondrial myopathy.

Adipocyte lineages: tracing back the origins of fat

The obesity epidemic has intensified efforts to understand the mechanisms controlling adipose tissue development. Adipose tissue is generally classified as white adipose tissue (WAT), the major energy storing tissue, or brown adipose tissue (BAT), which mediates non-shivering thermogenesis. It is hypothesized that brite adipocytes (brown in white) may represent a third adipocyte class. The recent realization that brown fat exist in adult humans suggests increasing brown fat energy expenditure could be a therapeutic strategy to combat obesity. To understand adipose tissue development, several groups are tracing the origins of mature adipocytes back to their adult precursor and embryonic ancestors. From these studies emerged a model that brown adipocytes originate from a precursor shared with skeletal muscle that expresses Myf5-Cre, while all white adipocytes originate from a Myf5-negative precursors. While this provided a rational explanation to why BAT is more metabolically favorable than WAT, recent work indicates the situation is more complex because subsets of white adipocytes also arise from Myf5-Cre expressing precursors. Lineage tracing studies further suggest that the vasculature may provide a niche supporting both brown and white adipocyte progenitors; however, the identity of the adipocyte progenitor cell is under debate. Differences in origin between adipocytes could explain metabolic heterogeneity between depots and/or influence body fat patterning particularly in lipodystrophy disorders. Here, we discuss recent insights into adipose tissue origins highlighting lineage-tracing studies in mice, how variations in metabolism or signaling between lineages could affect body fat distribution, and the questions that remain unresolved. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

The origin and definition of brite versus white and classical brown adipocytes

White adipose tissue stores energy while brown adipose tissue contributes to body temperature maintenance through non-shivering thermogenesis. In addition, brite (brown-in-white) adipocytes resembling classical brown adipocytes within predominantly white adipose tissue can be found in response to cold adaptation or other stimuli. Even though our understanding of brite adipocyte formation has increased substantially in the last few years, it is still unclear how brite and classical brown adipocytes are formed in vivo. In this review, we outline and discuss the current understanding of brite adipocyte nomenclature, developmental origin and possible mechanisms of their recruitment. We reason that future work in the field will bridge in vivo tracing studies and primary cell characterization with molecular mechanistic data from in vitro approaches to devise new means to increase energy expenditure.

Transient receptor potential activated brown fat thermogenesis as a target of food ingredients for obesity management

PURPOSE OF REVIEW

Cold exposure activates brown adipose tissue (BAT), the major site of sympathetically activated nonshivering thermognenesis, via transient receptor potential (TRP) channels. Capsaicin and its nonpungent analogue (capsinoids) are agonists for a vanilloid subtype one of TRP, and have the potential to increase whole-body energy expenditure and reduce body fat. This article reviews the regulatory roles of BAT for energy expenditure and body fat in humans, particularly focusing on food ingredients activating the TRP-BAT axis.

RECENT FINDINGS

Acute cold exposure increased energy expenditure in humans with metabolically active BAT, but not those without it. Quite similar responses were found after a single oral ingestion of either capsinoids or an alcohol extract of Guinea pepper seeds, indicating that these food ingredients activate BAT and thereby increase energy expenditure. When individuals without active BAT were exposed to cold every day for 6 weeks, BAT was recruited in association with increased energy expenditure and decreased body fat. A 6-week daily ingestion of capsinoids mimicked the effects of repeated cold exposure. These findings indicate that human BAT can be reactivated/recruited, thereby increasing energy expenditure and decreasing body fat.

SUMMARY

Human BAT recruited by prolonged ingestion of a vanilloid subtype one of TRP agonists increases energy expenditure and decreases body fat. In addition to capsinoids, there are numerous food ingredients acting as TRP agonists, which are expected to activate BAT and so be useful for the prevention of obesity in daily life.

Diurnal and menstrual cycles in body temperature are regulated differently: a 28-day ambulatory study in healthy women with thermal discomfort of cold extremities and controls

Diurnal cycle variations in body-heat loss and heat production, and their resulting core body temperature (CBT), are relatively well investigated; however, little is known about their variations across the menstrual cycle under ambulatory conditions. The main purpose of this study was to determine whether menstrual cycle variations in distal and proximal skin temperatures exhibit similar patterns to those of diurnal variations, with lower internal heat conductance when CBT is high, i.e. during the luteal phase. Furthermore, we tested these relationships in two groups of women, with and without thermal discomfort of cold extremities (TDCE). In total, 19 healthy eumenorrheic women with regular menstrual cycles (28-32 days), 9 with habitual TDCE (ages 29 ± 1.5 year; BMI 20.1 ± 0.4) and 10 controls without these symptoms (CON: aged 27 ± 0.8 year; BMI 22.7 ± 0.6; p < 0.004 different to TDCE) took part in the study. Twenty-eight days continuous ambulatory skin temperature measurements of distal (mean of hands and feet) and proximal (mean of sternum and infraclavicular regions) skin regions, thighs, and calves were carried out under real-life, ambulatory conditions (i-Buttons® skin probes, sampling rate: 2.5 min). The distal minus proximal skin temperature gradient (DPG) provided a valuable measure for heat redistribution from the core to the shell, and, hence, for internal heat conduction. Additionally, basal body temperature was measured sublingually directly after waking up in bed. Mean diurnal amplitudes in skin temperatures increased from proximal to distal skin regions and the 24-h mean values were inversely related. TDCE compared to CON showed significantly lower hand skin temperatures and DPG during daytime. However, menstrual cycle phase did not modify these diurnal patterns, indicating that menstrual and diurnal cycle variations in skin temperatures reveal additive effects. Most striking was the finding that all measured skin temperatures, together with basal body temperature, revealed a similar menstrual cycle variation (independent of BMI), with highest and lowest values during the luteal and follicular phases, respectively. These findings lead to the conclusion that in contrast to diurnal cycle, variations in CBT variation across the menstrual cycle cannot be explained by changes in internal heat conduction under ambulatory conditions. Although no measurements of metabolic heat production were carried out increased metabolic heat generation during the luteal phase seems to be the most plausible explanation for similar body temperature increases.