Brown Adipose Tissue: an Update on Recent Findings

Adrenergic-Independent Signaling via CHRNA2 Regulates Beige Fat Activation

Maintaining energy homeostasis upon environmental challenges, such as cold or excess calorie intake, is essential to the fitness and survival of mammals. Drug discovery efforts targeting β-adrenergic signaling have not been fruitful after decades of intensive research. We recently identified a new beige fat regulatory pathway mediated via the nicotinic acetylcholine receptor subunit CHRNA2. Here, we generated fat-specific Chrna2 KO mice and observed thermogenic defects in cold and metabolic dysfunction upon dietary challenges caused by adipocyte-autonomous regulation in vivo. We found that CHRNA2 signaling is activated after acute high fat diet feeding and this effect is manifested through both UCP1- and creatine-mediated mechanisms. Furthermore, our data suggested that CHRNA2 signaling may activate glycolytic beige fat, a subpopulation of beige adipocytes mediated by GABPα emerging in the absence of β-adrenergic signaling. These findings reveal the biological significance of the CHRNA2 pathway in beige fat biogenesis and energy homeostasis.

Metabolic Factors Determining the Susceptibility to Weight Gain: Current Evidence

PURPOSE OF REVIEW

There is substantial inter-individual variability in body weight change, which is not fully accounted by differences in daily energy intake and physical activity levels. The metabolic responses to short-term perturbations in energy intake can explain part of this variability by quantifying the degree of metabolic "thriftiness" that confers more susceptibility to weight gain and more resistance to weight loss. It is unclear which metabolic factors and pathways determine this human "thrifty" phenotype. This review will investigate and summarize emerging research in the field of energy metabolism and highlight important metabolic mechanisms implicated in body weight regulation in humans.

RECENT FINDINGS

Dysfunctional adipose tissue lipolysis, reduced brown adipose tissue activity, blunted fibroblast growth factor 21 secretion in response to low-protein hypercaloric diets, and impaired sympathetic nervous system activity might constitute important metabolic factors characterizing "thriftiness" and favoring weight gain in humans. The individual propensity to weight gain in the current obesogenic environment could be ascertained by measuring specific metabolic factors which might open up new pathways to prevent and treat human obesity.

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.

The gut hormone secretin triggers a gut-brown fat-brain axis in the control of food intake

NEW FINDINGS

What is the topic of this review? Brown fat's role in meal-associated thermogenesis and the related consequences for energy balance regulation with a focus on the gut hormone secretin, which has been identified as the endocrine molecular mediator of meal-associated brown fat thermogenesis. What advances does it highlight? The finding of the secretin-induced gut-brown fat-brain axis creates new opportunities to manipulate brown fat and thereby energy balance in a natural way while living in a thermoneutral environment. The role of brown fat as a mere catabolic heater organ needs to be revised and more attention should be directed towards the regulatory role of brown fat beyond energy expenditure.

ABSTRACT

Brown fat research concentrates on the energy expenditure function of this heating organ, whereas previous evidence for a role of brown fat in regulating energy intake has been mostly neglected. Ingestion of a single mixed meal activates human brown fat thermogenesis to the same degree as cold. In mice, activation of brown fat thermogenesis with a β₃ -adrenergic receptor agonist inhibits food intake. Pharmacological β-blockade, however, inhibits neither meal-associated thermogenesis nor food intake. We recently identified the gut hormone secretin as a non-adrenergic activator of brown fat. In vivo, secretin treatment acutely increases energy expenditure and inhibits food intake in wild-type, but not in uncoupling protein 1 (UCP1)-knockout (KO) mice, which lack thermogenic brown fat function. Concurrently, secretin alters gene expression of melanocortinergic peptides of hypothalamic neurons in wild-type mice, but not UCP1-KO. Blocking endogenous secretin with a neutralizing antibody attenuates brown fat thermogenesis during refeeding, increases food intake of mice, and alters ad libitum feeding behaviour. Taken together, these findings demonstrate that secretin triggers an endocrine gut-brown adipose tissue-brain axis in the control of satiation. We hypothesize that meal-associated activation of brown adipose tissue thermogenesis induced by secretin results in a rise in brain temperature and increased melanocortinergic signalling. Taken together, brown fat is not a mere heating organ dissipating excess calories but also involved in gut-brain communication in the control of food intake.

Vigorous-Intensity Physical Activities Are Associated with High Brown Adipose Tissue Density in Humans

Brown adipose tissue (BAT) plays a role in adaptive thermogenesis in response to cold environments and dietary intake via sympathetic nervous system (SNS) activation. It is unclear whether physical activity increases BAT density (BAT-d). Two-hundred ninety-eight participants (age: 41.2 ± 12.1 (mean ± standard deviation), height: 163.6 ± 8.3 cm, weight: 60.2 ± 11.0 kg, body mass index (BMI): 22.4 ± 3.0 kg/m2, body fat percentage: 25.4 ± 7.5%) without smoking habits were categorized based on their physical activity levels (a group performing physical activities including walking and moderate physical activity (WM) and a group performing WM + vigorous-intensity physical activities (VWM)). We measured the total hemoglobin concentration ([Total-Hb]) in the supraclavicular region, an index of BAT-d, and anthropometric parameters. [Total-Hb] was significantly higher in VWM than WM for all participant groups presumably owing to SNS activation during vigorous-intensity physical activities, and unrelated to the amount of total physical activity levels. Furthermore, multiple regression analysis revealed that BAT-d was related to visceral fat area and VWM in men and related to body fat percentage in women. We conclude that vigorous-intensity physical activities are associated with high BAT-d in humans, especially in men.

The influence of hypertensive environment on adipose tissue remodeling measured by fluorescence lifetime imaging in spontaneously hypertensive rats

There is a lack of information correlating low adiposity with hypertension experienced by Spontaneous Hypertensive Rats (SHR) or overweight and normotension in Wistar-Kyoto (WKY). We aimed to investigate this lipodystrophy phenomenon by measuring fluorescence lifetime (FLIM), optical redox ratio (ORR), serum levels of hypothalamic-pituitary-adrenal (HPA) and/or hypothalamic-pituitary-thyroid (HPT) hormones axes between Wistar, WKY and SHR before and after establishment of hypertension. Under high blood pressure, we evaluated serum adipokines. Brown adipose tissue was characterized as lower ORR and shorter FLIM compared to white adipose tissue. HPT axis showed a crucial role in the SHR adipose tissue configuration by attenuating whitening. The increased adiposity in WKY may act as a preventive agent for hypertension, since SHR, with low adiposity, establishes the disease. The hypertensive environment can highlight key adipokines that may result in new therapeutic approaches to the treatment of adiposity dysfunctions and hypertension.

The effect of whole-body cryostimulation on body composition and leukocyte expression of HSPA1A, HSPB1, and CRP in obese men

In recent years, the prevalence of obesity has increased dramatically and has become a 21st century epidemic. Obesity is associated with the development of many diseases, and therefore treatments that can reduce body mass are actively sought. The aim of this study was to examine the effect of 20 cryostimulation sessions on body composition in obese/high body mass (HBM, n = 12) males and normal body mass (NBM, n = 9) controls. The HBM group had a mean age = 29.08 ± 4.19 years, body fat percentage = 32.08 ± 6.16%, body mass index = 36.23 ± 8.13 kg/m²) and NBM group had a mean age = 22.00 ± 2.45 years, body fat percentage = 12.14 ± 4.93%, body mass index = 23.58 ± 2.00 kg/m². Kilocalorie intake was similar for both groups. All participants received 20 sessions of systemic cryostimulation at -120°C for 2-3 min in a cryochamber. Blood samples were collected before the first session, 1 h after the 10th session, and 1 h after the 20th cryostimulation session. C-reactive protein (CRP) plasma concentrations, and expression of the heat shock protein genes (HSPA1A, HSPB1) and CRP mRNA in leukocytes were evaluated after 10 and 20 cryostimulation sessions. In both groups, 20 sessions were associated with a significant decrease in body mass, fat mass and the percentage of body fat. CRP concentrations were significantly higher in obese people before the first session and after 10 treatments, but not at the end of study. Expression of HSPA1A and HSPB1 mRNA gradually decreased with the number of cryostimulation sessions. A significant difference in HSPA1A expression was found after 20 sessions (NBM > HBM) and for HSPB1 at baseline and after 20 sessions (HBM > NBM). Our results show that cryostimulation influences body composition and that cryostimulation-induced HSP genes expression depends on the number of cryosessions and baseline body mass, and is differentially altered in HBM individuals. Further research on the interaction between body mass and cold adaptation is warranted.

Indirubin, a small molecular deriving from connectivity map (CMAP) screening, ameliorates obesity-induced metabolic dysfunction by enhancing brown adipose thermogenesis and white adipose browning

Background

Obesity occurs when the body’s energy intake is constantly greater than its energy consumption and the pharmacological enhancing the activity of brown adipose tissue (BAT) and (or) browning of white adipose tissue (WAT) has been considered promising strategies to treat obesity.

Methods

In this study, we took a multi-pronged approach to screen UCP1 activators, including in silico predictions, in vitro assays, as well as in vivo experiments.

Results

Base on Connectivity MAP (CMAP) screening, we obtained multiple drugs that possess a remarkably correlating gene expression pattern to that of enhancing activity in BAT and (or) sWAT signature. Particularly, we focused on a previously unreported drug-indirubin, a compound obtained from the Indigo plant, which is now mainly used for the treatment of chronic myelogenous leukemia (CML). In the current study, our results shown that indirubin could enhance the BAT activity, as evidenced by up-regulated Ucp1 expression and enhanced mitochondrial respiratory function in vitro cellular model. Furthermore, indirubin treatment restrained high-fat diet (HFD)-induced body weight gain, improved glucose homeostasis and ameliorated hepatic steatosis which were associated with the increase of energy expenditure in the mice model. Moreover, we revealed that indirubin treatment increased BAT activity by promoting thermogenesis and mitochondrial biogenesis in BAT and induced browning of subcutaneous inguinal white adipose tissue (sWAT) of mice under HFD. Besides, our results indicated that indirubin induced UCP1 expression in brown adipocytes, at least in part, via activation of PKA and p38MAPK signaling pathways.

Conclusions

Our results clearly show that as an effective BAT (as well as beige cells) activator, indirubin may have a protective effect on the prevention and treatment of obesity and its complications.

Humans in the cold: Regulating energy balance

For humans to maintain a stable core temperature in cold environments, an increase in energy expenditure (EE) is required. However, little is known about how cold stimulus impacts energy balance as a whole, as energy intake (EI) has been largely overlooked. This review focuses on the current state of knowledge regarding how cold exposure (CE) impacts both EE and EI, while highlighting key gaps and shortcomings in the literature. Animal models clearly reveal that CE produces large increases in EE, while decreasing environmental temperatures results in a significant negative dose-response effect in EI (r=-.787, P<.001), meaning animals eat more as temperature decreases. In humans, multiple methods are used to administer cold stimuli, which result in consistent yet quantitatively small increases in EE. However, only two studies have measured ad libitum food intake in combination with acute CE in humans. Chronic CE (i.e., cold acclimation) studies have been shown to produce minimal changes in body weight, with an average compensation of ~126%. Although more studies are required to investigate how cold impacts EI in humans, results presented in this review warrant caution before presenting or considering CE as a potential adjunct to weight loss strategies.

Weight loss induced by whole grain-rich diet is through a gut microbiota-independent mechanism

The prevalence of overweight and obesity has increased worldwide. Obesity is a well-known risk factor of type 2 diabetes mellitus and cardiovascular disease and raises public health concerns. Many dietary guidelines encourage the replacement of refined grains with whole grains (WGs) to enhance body weight management. Current evidence regarding interrelationships among WGs, body weight, and gut microbiota is limited and inconclusive. In this editorial, we comment on the article by Roager et al published in the recent issue of the Gut 2019; 68(1): 83-93. In the study, obese patients (25 < body mass index < 35 kg/m²) were randomly assigned to receive two 8-wk dietary controlling periods with WGs and refined grain-rich diet. The results showed significantly decreased body weight in the WG group. Either the composition of gut microbiota or short-chain fatty acids, the leading end product of fermentation of non-digestible carbohydrate by gut microbiota, did not differ between the two groups. The study highly indicated that a WG-rich diet reduced body weight independent of gut microbiota. We then raised some plausible mechanisms of how WGs might influence body weight and demonstrated more literature in line with WGs enhance body weight control through a microbiota-independent pathway. Possible mechanisms include: (1) The abundant dietary fiber contents of WGs increase satiety, satiation, energy excretion from stool, and energy expenditure simultaneously decreasing energy absorption and fat storage; (2) The plentiful amount of polyphenols of WGs improve energy expenditure by hampering adipocyte maturation and function; (3) The sufficient magnesium and zinc of WGs guarantee lean body mass growth and decrease fat mass; (4) The effect of WGs on brown adipose tissue is a key component of non-shivering thermogenesis; and (5) The increase of adiponectin by WGs enhances glucose utilization, lipid oxidation, and energy expenditure.

Comprehensive amelioration of high-fat diet-induced metabolic dysfunctions through activation of the PGC-1α pathway by probiotics treatment in mice

Although the beneficial effects of probiotics in the prevention or treatment of metabolic disorders have been extensively researched, the precise mechanisms by which probiotics improve metabolic homeostasis are still not clear. Given that probiotics usually exert a comprehensive effect on multiple metabolic disorders, defining a concurrent mechanism underlying the multiple effects is critical to understand the function of probiotics. In this study, we identified the SIRT1-dependent or independent PGC-1α pathways in multiple organs that mediate the protective effects of a strain of Lactobacillus plantarum against high-fat diet-induced adiposity, glucose intolerance, and dyslipidemia. L. plantarum treatment significantly enhanced the expression of SIRT1, PPARα, and PGC-1α in the liver and adipose tissues under HFD-fed condition. L. plantarum treated mice also exhibited significantly increased expressions of genes involved in bile acid synthesis and reverse cholesterol transport in the liver, browning and thermogenesis of adipose tissue, and fatty acid oxidation in the liver and adipose tissue. Additionally, L. plantarum treatment significantly upregulated the expressions of adiponectin in adipose tissue, irisin in skeletal muscle and subcutaneous adipose tissue (SAT), and FGF21 in SAT. These beneficial changes were associated with a significantly improved HFD-induced alteration of gut microbiota. Our findings suggest that the PGC-1α-mediated pathway could be regarded as a potential target in the development of probiotics-based therapies for the prevention and treatment of metabolic disorders.

Neofunctionalization of the UCP1 mediated the non-shivering thermogenesis in the evolution of small-sized placental mammals

The acquisition of UCP1-mediated non-shivering thermogenesis (NST) was an important event during the evolution of mammals. Here, we assessed the thermogenic neofunctionalization that occurred in the mammalian UCP1, by performing detailed comparative evolutionary genomics analyses (including phylogenetic and selection analyses) of the UCP family members across all major vertebrate classes. Heterogeneously distributed positive selection signatures were found in several UCPs, being preferably located in the mitochondrial matrix domains. Additionally, comparisons with non-mammalian orthologs showed increased evolutionary rates of the mammalian UCP1, not observable in the phylogenetically related UCP2 and UCP3 paralogs. Also, parallel signatures of episodic positive selection (ω > 1) were found in the ancestral branches of both Glires (rodents and lagomorphs) and Afroinsectivores (afrosoricids and macroscelids), underlining the importance of the UCP1 thermogenic activity in these mammalian groups. Finally, we hypothesize that the independent positive selection events that occurred in these two lineages resulted in two UCP1-mediated NST approaches, namely the cold acute response in the Glires and the reproduction success enhancement in the Afroinsectivores.

Ecklonia Cava Extract Attenuates Endothelial Cell Dysfunction by Modulation of Inflammation and Brown Adipocyte Function in Perivascular Fat Tissue

It is well known that perivascular fat tissue (PVAT) dysfunction can induce endothelial cell (EC) dysfunction, an event which is related with various cardiovascular diseases. In this study, we evaluated whether Ecklonia cava extract (ECE) and pyrogallol-phloroglucinol-6,6-bieckol (PPB), one component of ECE, could attenuate EC dysfunction by modulating diet-induced PVAT dysfunction mediated by inflammation and ER stress. A high fat diet (HFD) led to an increase in the number and size of white adipocytes in PVAT; PPB and ECE attenuated those increases. Additionally, ECE and PPB attenuated: (i) an increase in the number of M1 macrophages and the expression level of monocyte chemoattractant protein-1 (MCP-1), both of which are related to increases in macrophage infiltration and induction of inflammation in PVAT, and (ii) the expression of pro-inflammatory cytokines (e.g., tumor necrosis factor-α (TNF-α) and interleukin (IL)-6, chemerin) in PVAT which led to vasoconstriction. Furthermore, ECE and PPB: (i) enhanced the expression of adiponectin and IL-10 which had anti-inflammatory and vasodilator effects, (ii) decreased HFD-induced endoplasmic reticulum (ER) stress and (iii) attenuated the ER stress mediated reduction in sirtuin type 1 (Sirt1) and peroxisome proliferator-activated receptor γ (PPARγ) expression. Protective effects against decreased Sirt1 and PPARγ expression led to the restoration of uncoupling protein -1 (UCP-1) expression and the browning process in PVAT. PPB or ECE attenuated endothelial dysfunction by enhancing the pAMPK-PI3K-peNOS pathway and reducing the expression of endothelin-1 (ET-1). In conclusion, PPB and ECE attenuated PVAT dysfunction and subsequent endothelial dysfunction by: (i) decreasing inflammation and ER stress, and (ii) modulating brown adipocyte function.

Intentional Exposure to Extreme Cold Temperature to Influence Shape and/or Weight and Its Association to Eating Disorder Pathology

Very little is known about potentially dangerous forms of weight control or compensatory behaviors involving deliberately exposing oneself to cold temperature. We investigated frequency of intentional cold exposure behavior to influence shape/weight and its relation to eating disorder pathology. Participants (496; 94.0% females) were recruited via print advertisements and social media. Items were based on a review of scientific literature, popular media, pro-ED forums, and input from clinicians and persons with a lived experience. Lifetime cold exposure and frequency during the past 28 days ("never" to "very often") were assessed. Participants completed a new self-report questionnaire (Intentional Cold Exposure-Questionnaire; ICE-Q) and measures of eating pathology. Approximately 1/2 of individuals with a current self-reported ED had engaged in at least one type of cold exposure behavior during the past 28 days versus 17% of individuals without an ED, although average frequency was low. Common cold exposure behaviors included underdressing, turning down the heat, ingesting ice-cold beverages, and ice baths. Significant moderate correlations existed between the ICE-Q and measures of ED pathology. This is the first study to assess intentional exposure to cold temperature as a means of controlling shape and/or weight. Cold exposure does not appear to be socially normative as a weight control method, but a markedly pathological behavior associated with ED symptomology. Findings indicate a propensity toward experimentation, but sporadic uptake, of deliberate cold exposure by individuals with an ED. Future research is needed to assess replicability and to investigate the clinical, theoretical, and prognostic significance of deliberate cold exposure behavior.

Phytochemicals as potential candidates to combat obesity via adipose non-shivering thermogenesis

Obesity is a chronic metabolic disease caused by a long-term imbalance between energy intake and expenditure. The discovery of three different shades of adipose tissues has implications in terms of understanding the pathogenesis and potential interventions for obesity and its related complications. Fat browning, as well as activation of brown adipocytes and new beige adipocytes differentiated from adipogenic progenitor cells, are emerging as interesting and promising methods to curb obesity because of their unique capacity to upregulate non-shivering thermogenesis. This capacity is due to catabolism of stored energy to generate heat through the best characterized thermogenic effector uncoupling protein 1 (UCP1). A variety of phytochemicals have been shown in the literature to contribute to thermogenesis by acting as chemical uncouplers, UCP1 inducers or regulators of fat differentiation and browning. In this review, we summarize the mechanisms and strategies for targeting adipose-mediated thermogenesis and highlight the role of phytochemicals in targeting adipose thermogenesis to fight against obesity. We also discuss proposed targets for how these phytochemical molecules promote BAT activity, WAT browning and beige cell development, thereby offering novel insights into interventional strategies of how phytochemicals may help prevent and manage obesity via adipose thermogenesis.

Interscapular and Perivascular Brown Adipose Tissue Respond Differently to a Short-Term High-Fat Diet

Brown adipose tissue (BAT) function may depend on its anatomical location and developmental origin. Interscapular BAT (iBAT) regulates acute macronutrient metabolism, whilst perivascular BAT (PVAT) regulates vascular function. Although phenotypically similar, whether these depots respond differently to acute nutrient excess is unclear. Given their distinct anatomical locations and developmental origins and we hypothesised that iBAT and PVAT would respond differently to brief period of nutrient excess. Sprague-Dawley rats aged 12 weeks (n=12) were fed either a standard (10% fat, n=6) or high fat diet (HFD: 45% fat, n=6) for 72h and housed at thermoneutrality. Following an assessment of whole body physiology, fat was collected from both depots for analysis of gene expression and the proteome. HFD consumption for 72h induced rapid weight gain (c. 2.6%) and reduced serum non-esterified fatty acids (NEFA) with no change in either total adipose or depot mass. In iBAT, an upregulation of genes involved in insulin signalling and lipid metabolism was accompanied by enrichment of lipid-related processes and functions, plus glucagon and peroxisome proliferator-activated receptor (PPAR) signalling pathways. In PVAT, HFD induced a pronounced down-regulation of multiple metabolic pathways which was accompanied with increased abundance of proteins involved in apoptosis (e.g., Hdgf and Ywaq) and toll-like receptor signalling (Ube2n). There was also an enrichment of DNA-related processes and functions (e.g., nucleosome assembly and histone exchange) and RNA degradation and cell adhesion pathways. In conclusion, we show that iBAT and PVAT elicit divergent responses to short-term nutrient excess highlighting early adaptations in these depots before changes in fat mass.

Metabolic Determinants of Weight Gain in Humans

One of the fundamental challenges in obesity research is to identify subjects prone to weight gain so that obesity and its comorbidities can be promptly prevented or treated. The principles of thermodynamics as applied to human body energetics demonstrate that susceptibility to weight gain varies among individuals as a result of interindividual differences in energy expenditure and energy intake, two factors that counterbalance one another and determine daily energy balance and, ultimately, body weight change. This review focuses on the variability among individuals in human metabolism that determines weight change. Conflicting results have been reported about the role of interindividual differences in energy metabolism during energy balance in relation to future weight change. However, recent studies have shown that metabolic responses to acute, short-term dietary interventions that create energy imbalance, such as low-protein overfeeding or fasting for 24 hours, may reveal the underlying metabolic phenotype that determines the degree of resistance to diet-induced weight loss or the propensity to spontaneous weight gain over time. Metabolically "thrifty" individuals, characterized by a predilection for saving energy in settings of undernutrition and dietary protein restriction, display a minimal increase in plasma fibroblast growth factor 21 concentrations in response to a low-protein overfeeding diet and tend to gain more weight over time compared with metabolically "spendthrift" individuals. Similarly, interindividual variability in the causal relationship between energy expenditure and energy intake ("energy sensing") and in the metabolic response to cold exposure (e.g., brown adipose tissue activation) seems, to some extent, to be indicative of individual propensity to weight gain. Thus, an increased understanding and the clinical characterization of phenotypic differences in energy metabolism among individuals (metabolic profile) may lead to new strategies to prevent weight gain or improve weight-loss interventions by targeted therapies on the basis of metabolic phenotype and susceptibility to obesity in individual persons.

Cold and Exercise: Therapeutic Tools to Activate Brown Adipose Tissue and Combat Obesity

The rise in obesity over the last several decades has reached pandemic proportions. Brown adipose tissue (BAT) is a thermogenic organ that is involved in energy expenditure and represents an attractive target to combat both obesity and type 2 diabetes. Cold exposure and exercise training are two stimuli that have been investigated with respect to BAT activation, metabolism, and the contribution of BAT to metabolic health. These two stimuli are of great interest because they have both disparate and converging effects on BAT activation and metabolism. Cold exposure is an effective mechanism to stimulate BAT activity and increase glucose and lipid uptake through mitochondrial uncoupling, resulting in metabolic benefits including elevated energy expenditure and increased insulin sensitivity. Exercise is a therapeutic tool that has marked benefits on systemic metabolism and affects several tissues, including BAT. Compared to cold exposure, studies focused on BAT metabolism and exercise display conflicting results; the majority of studies in rodents and humans demonstrate a reduction in BAT activity and reduced glucose and lipid uptake and storage. In addition to investigations of energy uptake and utilization, recent studies have focused on the effects of cold exposure and exercise on the structural lipids in BAT and secreted factors released from BAT, termed batokines. Cold exposure and exercise induce opposite responses in terms of structural lipids, but an important overlap exists between the effects of cold and exercise on batokines. In this review, we will discuss the similarities and differences of cold exposure and exercise in relation to their effects on BAT activity and metabolism and its relevance for the prevention of obesity and the development of type 2 diabetes.

Adipocyte MTERF4 regulates non-shivering adaptive thermogenesis and sympathetic-dependent glucose homeostasis

In humans, low brown adipose tissue (BAT) mass and activity have been associated with increased adiposity and fasting glucose levels, suggesting that defective BAT-dependent thermogenesis could contribute to the development of obesity and/or type 2 diabetes. The thermogenic function of BAT relies on a vast network of mitochondria exclusively equipped with UCP1. Mitochondrial biogenesis is exquisitely regulated by a well-defined network of transcription factors that coordinate the expression of nuclear genes required for the formation of functional mitochondria. However, less is known about the mitochondrial factors that control the expression of the genes encoded by the mitochondrial genome. Here, we have studied the role of mitochondrial transcription termination factor-4 (MTERF4) in BAT by using a new mouse model devoid of MTERF4 specifically in adipocytes (MTERF4-FAT-KO mice). Lack of MTERF4 in BAT leads to reduced OxPhos mitochondrial protein levels and impaired assembly of OxPhos complexes I, III and IV due to deficient translation of mtDNA-encoded proteins. As a result, brown adipocytes lacking MTERF4 exhibit impaired respiratory capacity. MTERF4-FAT-KO mice show a blunted thermogenic response and are unable to maintain body temperature when exposed to cold. Despite impaired BAT function, MTERF4-FAT-KO mice do not develop obesity or insulin resistance. Still, MTERF4-FAT-KO mice became resistant to the insulin-sensitizing effects of β₃-specific adrenergic receptor agonists. Our results demonstrate that MTERF4 regulates mitochondrial protein translation and is essential for proper BAT thermogenic activity. Our study also supports the notion that pharmacological activation of BAT is a plausible therapeutic target for the treatment of insulin resistance.

Regulation of Adaptive Thermogenesis and Browning by Prebiotics and Postbiotics

Prebiotics are non-digestible food components able to modify host microbiota toward a healthy profile, concomitantly conferring general beneficial health effects. Numerous research works have provided wide evidence regarding the effects of prebiotics on the protection against different detrimental phenotypes related to cancer, immunity, and features of the metabolic syndrome, among others. Nonetheless, one topic less studied so far, but relevant, relates to the connection between prebiotics and energy metabolism regulation (and the prevention or treatment of obesity), especially by means of their impact on adaptive (non-shivering) thermogenesis in brown adipose tissue (BAT) and in the browning of white adipose tissue (WAT). In the present review, a key link between prebiotics and the regulation of adaptive thermogenesis and lipid metabolism (in both BAT and WAT) is proposed, thus connecting prebiotic consumption, microbiota selection (especially gut microbiota), production of microbiota metabolites, and the regulation of energy metabolism in adipose tissue, particularly regarding the effects on browning promotion, or on BAT recruitment. In this sense, various types of prebiotics, from complex carbohydrates to phenolic compounds, have been studied regarding their microbiota-modulating role and their effects on crucial tissues for energy metabolism, including adipose tissue. Other studies have analyzed the effects of the main metabolites produced by selected microbiota on the improvement of metabolism, such as short chain fatty acids and secondary bile acids. Here, we focus on state-of-the-art evidence to demonstrate that different prebiotics can have an impact on energy metabolism and the prevention or treatment of obesity (and its associated disorders) by inducing or regulating adaptive thermogenic capacity in WAT and/or BAT, through modulation of microbiota and their derived metabolites.

Association between brown adipose tissue and bone mineral density in humans

BACKGROUND

Brown adipose tissue (BAT) seems to play a role in bone morphogenesis. A negative association has been reported between BAT and bone mineral density (BMD) in women, but not in men. A panel of experts has recently published a set of recommendations for BAT assessment, and thus, to re-address previously reported associations is needed. This study aimed to investigate the association between cold-induced BAT ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) uptake and BMD in young healthy adults.

METHODS

Ninety-eight healthy adults (68 women; 22 ± 2.2 years old; 24.3 ± 4.5 kg/m²) cold-induced BAT was assessed by means of an ¹⁸F-FDG positron emission tomography-computed tomography (PET-CT) scan preceded by a personalized cold stimulation. The cold exposure consisted in 2 h in a mild cold room at 19.5-20 °C wearing a water perfused cooling vest set 4 °C above the individual shivering threshold. Total body and lumbar spine BMD were assessed by a whole-body DXA scan.

RESULTS

We found no association between BMD and cold-induced BAT volume, mean, and maximal activity (all P > 0.1) in neither young and healthy men nor women. These results remained unchanged when adjusting by height, by body composition, and by objectively assessed physical activity. Sensitivity analyses using the criteria to quantify cold-induced BAT-related parameters applied in previous studies did not change the results.

CONCLUSIONS

In summary, our study shows that there is no association between cold-induced BAT and BMD in young healthy adults. Moreover, our data support the notion that previously shown associations between BAT and BMD in healthy non-calorically restricted individuals, could be driven by methodological issues related to BAT assessment and/or sample size limitations.

Proton leak regulates mitochondrial reactive oxygen species generation in endothelial cell activation and inflammation - A novel concept

Mitochondria are capable of detecting cellular insults and orchestrating inflammatory responses. Mitochondrial reactive oxygen species (mtROS) are intermediates that trigger inflammatory signaling cascades in response to our newly proposed conditional damage associated molecular patterns (DAMP). We recently reported that increased proton leak regulates mtROS generation and thereby exert physiological and pathological activation of endothelial cells. Herein, we report the recent progress in determining the roles of proton leak in regulating mtROS, and highlight several important findings: 1) The majority of mtROS are generated in the complexes I and III of electron transport chain (ETC); 2) Inducible proton leak and mtROS production are mutually regulated; 3) ATP synthase-uncoupled ETC activity and mtROS regulate both physiological and pathological endothelial cell activation and inflammation initiation; 4) Mitochondrial Ca²⁺ uniporter and exchanger proteins have an impact on proton leak and mtROS generation; 5) MtROS connect signaling pathways between conditional DAMP-regulated immunometabolism and histone post-translational modifications (PTM) and gene expression. Continuous improvement of our understanding in this aspect of mitochondrial function would provide novel insights and generate novel therapeutic targets for the treatment of sterile inflammatory disorders such as metabolic diseases, cardiovascular diseases and cancers.

In silico identification of lipid-binding α helices of uncoupling protein 1

Uncoupling protein 1 (UCP1) located at the mitochondrial inner membrane serves an important role in adaptive non-shivering thermogenesis. Previous data has demonstrated that membrane lipids regulate the biological functions of membrane proteins. However, how mitochondrial lipids interact with UCP1 still remains elusive. In this study, the interactions between UCP1 and membrane lipids were investigated, using bioinformatic approaches due to the limitations associated with experimental techniques. A total of 8 UCP1 peptide regions with α-helices were identified and related to functional sites of UCP1. These were all novel peptide sequences compared with the known protein-lipid interactions. Among several types of UCP1-binding molecules, cardiolipin appeared to serve as a key interacting molecule of the 8 lipid-binding α-helix regions of UCP1. Two cardiolipin-binding lysines (K¹⁷⁵ and K²⁶⁹) of UCP1 may be crucial for this UCP1-cardiolipin recognition and UCP1 function. The present findings provide novel insight into the associations of UCP1 with lipids and the potential drug targets in UCP1-associated diseases.

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.

Norepinephrine and T4 Are Predictors of Fat Mass Gain in Humans With Cold-Induced Brown Adipose Tissue Activation

CONTEXT

In healthy adults with detectable cold-induced brown adipose tissue activation (CIBA), the relationships between sympathetic nervous system (SNS) or thyroid activity during energy balance (EBL) with CIBA and body composition change are undetermined.

OBJECTIVE

To investigate the relationships between CIBA and thermoneutral catecholamines and thyroid hormones measured during EBL and to determine if CIBA, catecholamines, or thyroid hormones predict body composition changes.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTIONS

Twelve healthy volunteers (seven male and five female) with positive CIBA [>2 standardized uptake value (g/mL)] had 24-hour energy expenditure (24hEE) assessed during EBL via whole-room indirect calorimetry while residing on a clinical research unit. Positron emission tomography/computed tomography scans were performed after exposure to 16°C for 2 hours to quantify CIBA.

MAIN OUTCOME MEASURES

CIBA, 24hEE during EBL, and thermoneutrality with concomitant measurement of urinary catecholamines and plasma free T3 and free T4. Body composition at baseline and 6 months by dual-energy X-ray absorptiometry.

RESULTS

Lower urinary norepinephrine and free T4 were associated with higher CIBA (r = -0.65, P = 0.03; and r = -0.75, P < 0.01, respectively), but CIBA was not associated with 24hEE at thermoneutrality (P = 0.77). Lower CIBA (β = -3.5 kg/standardized uptake value; P < 0.01) predicted fat mass gain, whereas higher urinary norepinephrine and free T4 predicted future fat mass gain at 6 months (β = 3.0 kg per twofold difference in norepinephrine, P = 0.03; and β = 1.2 kg per 0.1-ng/dL difference in free T4, P = 0.03, respectively).

CONCLUSION

Lower SNS and free thyroid measurements at baseline indicate a greater capacity for CIBA, which may be predictive against fat mass gain.

Neck adipose tissue - tying ties in metabolic disorders

Upper body adipose tissue accumulation has been associated with clustering of metabolic disorders and increased cardiovascular risk. Neck circumference (NC) indicated that subcutaneous adipose tissue (SAT) in that region is an independent pathogenic depot that might account for the additional risk missed by visceral adipose tissue (VAT). Neck adipose tissue (NAT) is not only one more ectopic depot but has several particular features that might modulate its metabolic role. Besides a controversial impact on obstructive apnea syndrome, neck fat encompasses carotid arteries as an important perivascular adipose tissue (PVAT) depot. With dysfunctional changes in obesity, physiologic vascular regulation is lost and inflammatory signals accelerate atherogenesis. Unexpected was the discovery of brown and beige adipocytes in the neck of human adults. When stimulated, brown adipose tissue (BAT) dissipates energy through thermogenesis and it is associated with other favorable metabolic effects. Moreover, the neck is the region where the browning mechanism was disclosed. With this unique plastic nature, NAT revealed multiple ties, challenging dynamics and potential new therapeutic targets that might have significant implications on metabolic outcomes and vascular risk.