Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans

Plasma Docosahexaenoic Acid and Eicosapentaenoic Acid Concentrations Are Positively Associated with Brown Adipose Tissue Activity in Humans

Brown adipose tissue (BAT) activation is a possible therapeutic strategy to increase energy expenditure and improve metabolic homeostasis in obesity. Recent studies have revealed novel interactions between BAT and circulating lipid species—in particular, the non-esterified fatty acid (NEFA) and oxylipin lipid classes. This study aimed to identify individual lipid species that may be associated with cold-stimulated BAT activity in humans. A panel of 44 NEFA and 41 oxylipin species were measured using mass-spectrometry-based lipidomics in the plasma of fourteen healthy male participants before and after 90 min of mild cold exposure. Lipid measures were correlated with BAT activity measured via ¹⁸F-fluorodeoxyglucose ([¹⁸F]FDG) positron emission tomography/computed tomography (PET/CT), along with norepinephrine (NE) concentration (a surrogate marker of sympathetic activity). The study identified a significant increase in total NEFA concentration following cold exposure that was positively associated with NE concentration change. Individually, 33 NEFA and 11 oxylipin species increased significantly in response to cold exposure. The concentration of the omega-3 NEFA, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) at baseline was significantly associated with BAT activity, and the cold-induced change in 18 NEFA species was significantly associated with BAT activity. No significant associations were identified between BAT activity and oxylipins.

Rhubarb Supplementation Prevents Diet-Induced Obesity and Diabetes in Association with Increased Akkermansia muciniphila in Mice

Obesity and obesity-related disorders, such as type 2 diabetes have been progressively increasing worldwide and treatments have failed to counteract their progression. Growing evidence have demonstrated that gut microbiota is associated with the incidence of these pathologies. Hence, the identification of new nutritional compounds, able to improve health through a modulation of gut microbiota, is gaining interest. In this context, the aim of this study was to investigate the gut-driving effects of rhubarb extract in a context of diet-induced obesity and diabetes. Eight weeks old C57BL6/J male mice were fed a control diet (CTRL), a high fat and high sucrose diet (HFHS) or a HFHS diet supplemented with 0.3% (g/g) of rhubarb extract for eight weeks. Rhubarb supplementation fully prevented HFHS-induced obesity, diabetes, visceral adiposity, adipose tissue inflammation and liver triglyceride accumulation, without any modification in food intake. By combining sequencing and qPCR methods, we found that all these effects were associated with a blooming of Akkermansia muciniphila, which is strongly correlated with increased expression of Reg3γ in the colon. Our data showed that rhubarb supplementation is sufficient to protect against metabolic disorders induced by a diet rich in lipid and carbohydrates in association with a reciprocal interaction between Akkermansia muciniphila and Reg3γ.

[The immune system of adipose tissue: obesity-associated inflammation]

Adipose tissue is an important endocrine organ. Via its secretion products, it cross-talks with other organs of the body and communicates the filling state of its triglyceride stores. Obesity is characterized by the excessive accumulation of body fat and leads to the infiltration and accumulation of immune cells in white adipose tissue. In this review article we introduce the various immune cell populations of adipose tissue and discuss their local and systemic influence.

Basal and cold-induced fatty acid uptake of human brown adipose tissue is impaired in obesity

Fatty acids (FA) are important substrates for brown adipose tissue (BAT) metabolism, however, it remains unclear whether there exists a difference in FA metabolism of BAT between lean and obese healthy humans. In this study we evaluated supraclavicular BAT fatty acid uptake (FAU) along with blood perfusion in lean and obese subjects during cold exposure and at room temperature using positron emission tomography (PET)/computed tomography (CT). Additionally, tissue samples were taken from supraclavicular region (typical BAT region) from a subset of subjects to evaluate histological presence of BAT. Non-shivering cold stress elevated FAU and perfusion of BAT in lean, but not in obese subjects. Lean subjects had greater FAU in BAT compared to obese subjects during cold exposure and interestingly also at room temperature. The higher BAT FAU was related to younger age and several indicators of superior systemic metabolic health. The subjects who manifested BAT histologically had several folds higher BAT FAU compared to subjects with no such histological manifestation. Together, obese subjects have less active tissue in supraclavicular region both in basal and cold-activated state and the FA metabolism of BAT is blunted in obesity.

Brown adipose tissue lipoprotein and glucose disposal is not determined by thermogenesis in uncoupling protein 1-deficient mice

Adaptive thermogenesis is highly dependent on uncoupling protein 1 (UCP1), a protein expressed by thermogenic adipocytes present in brown adipose tissue (BAT) and white adipose tissue (WAT). Thermogenic capacity of human and mouse BAT can be measured by positron emission tomography-computed tomography quantifying the uptake of ¹⁸F-fluodeoxyglucose or lipid tracers. BAT activation is typically studied in response to cold exposure or treatment with β-3-adrenergic receptor agonists such as CL316,243 (CL). Currently, it is unknown whether cold-stimulated uptake of glucose or lipid tracers is a good surrogate marker of UCP1-mediated thermogenesis. In metabolic studies using radiolabeled tracers, we found that glucose uptake is increased in mildly cold-activated BAT of Ucp1 ^-/- versus WT mice kept at subthermoneutral temperature. Conversely, lower glucose disposal was detected after full thermogenic activation achieved by sustained cold exposure or CL treatment. In contrast, uptake of lipoprotein-derived fatty acids into chronically activated thermogenic adipose tissues was substantially increased in UCP1-deficient mice. This effect is linked to higher sympathetic tone in adipose tissues of Ucp1 ^-/- mice, as indicated by elevated levels of thermogenic genes in BAT and WAT. Thus, glucose and lipoprotein handling does not necessarily reflect UCP1-dependent thermogenic activity, but especially lipid uptake rather mirrors sympathetic activation of adipose tissues.

Human Brown Adipocyte Thermogenesis Is Driven by β2-AR Stimulation

Stimulation of brown adipose tissue (BAT) thermogenesis in humans has emerged as an attractive target to improve metabolic health. Pharmacological stimulations targeting the β₃-adrenergic receptor (β₃-AR), the adrenergic receptor believed to mediate BAT thermogenesis, have historically performed poorly in human clinical trials. Here we report that, in contrast to rodents, human BAT thermogenesis is not mediated by the stimulation of β₃-AR. Oral administration of the β₃-AR agonist mirabegron only elicited increases in BAT thermogenesis when ingested at the maximal allowable dose. This led to off-target binding to β₁-AR and β₂-AR, thereby increasing cardiovascular responses and white adipose tissue lipolysis, respectively. ADRB2 was co-expressed with UCP1 in human brown adipocytes. Pharmacological stimulation and inhibition of the β₂-AR as well as knockdown of ADRB1, ADRB2, or ADRB3 in human brown adipocytes all confirmed that BAT lipolysis and thermogenesis occur through β₂-AR signaling in humans (ClinicalTrials.govNCT02811289).

Imaging Biotin Trafficking In Vivo with Positron Emission Tomography

The water-soluble vitamin biotin is essential for cellular growth, development, and well-being, but its absorption, distribution, metabolism, and excretion are poorly understood. This paper describes the radiolabeling of biotin with the positron emission tomography (PET) radionuclide carbon-11 ([¹¹C]biotin) to enable the quantitative study of biotin trafficking in vivo. We show that intravenously administered [¹¹C]biotin is quickly distributed to the liver, kidneys, retina, heart, and brain in rodents-consistent with the known expression of the biotin transporter-and there is a surprising accumulation in the brown adipose tissue (BAT). Orally administered [¹¹C]biotin was rapidly absorbed in the small intestine and swiftly distributed to the same organs. Preadministration of nonradioactive biotin inhibited organ uptake and increased excretion. [¹¹C]Biotin PET imaging therefore provides a dynamic in vivo map of transporter-mediated biotin trafficking in healthy rodents. This technique will enable the exploration of biotin trafficking in humans and its use as a research tool for diagnostic imaging of obesity/diabetes, bacterial infection, and cancer.

Biomarkers of Browning in Cold Exposed Siberian Adults

Cold-exposure promotes energy expenditure by inducing brown adipose tissue (BAT) thermogenesis, which over time, is also sustained by browning, the appearance, or increase, of brown-like cells into white fat depots. Identification of circulating markers reflecting BAT activity and browning is crucial to study this phenomenon and its triggers, also holding possible implications for the therapy of obesity and metabolic diseases. Using RT-qPCR, we evaluated the peripheral blood mononuclear cells (PBMC) expression profile of regulators of BAT activity (CIDEA, PRDM16), white adipocytes browning (HOXC9 and SLC27A1), and fatty acid β-oxidation (CPT1A) in 150 Siberian healthy miners living at extremely cold temperatures compared to 29 healthy subjects living in thermoneutral conditions. Anthropometric parameters, glucose, and lipid profiles were also assessed. The cold-exposed group showed significantly lower weight, BMI, hip circumference, and PBMC expression of CIDEA, but higher expression of HOXC9 and higher circulating glucose compared to controls. Within the cold-exposed group, BMI, total cholesterol, and the atherogenic coefficient were lower in individuals exposed to low temperatures for a longer time. In conclusion, human PBMC expresses the brown adipocytes marker CIDEA and the browning marker HOXC9, which, varying according to cold-exposure, possibly reflect changes in BAT activation and white fat browning.

A novel tracer for in vivo optical imaging of fatty acid metabolism in the heart and brown adipose tissue

Multiplexed imaging is essential for the evaluation of substrate utilization in metabolically active organs, such as the heart and brown adipose tissue (BAT), where substrate preference changes in pathophysiologic states. Optical imaging provides a useful platform because of its low cost, high throughput and intrinsic ability to perform composite readouts. However, the paucity of probes available for in vivo use has limited optical methods to image substrate metabolism. Here, we present a novel near-infrared (NIR) free fatty acid (FFA) tracer suitable for in vivo imaging of deep tissues such as the heart. Using click chemistry, Alexa Fluor 647 DIBO Alkyne was conjugated to palmitic acid. Mice injected with 0.05 nmol/g bodyweight of the conjugate (AlexaFFA) were subjected to conditions known to increase FFA uptake in the heart (fasting) and BAT [cold exposure and injection with the β₃ adrenergic agonist CL 316, 243(CL)]. Organs were subsequently imaged both ex vivo and in vivo to quantify AlexaFFA uptake. The blood kinetics of AlexaFFA followed a two-compartment model with an initial fast compartment half-life of 0.14 h and a subsequent slow compartment half-life of 5.2 h, consistent with reversible protein binding. Ex vivo fluorescence imaging after overnight cold exposure and fasting produced a significant increase in AlexaFFA uptake in the heart (58 ± 12%) and BAT (278 ± 19%) compared to warm/fed animals. In vivo imaging of the heart and BAT after exposure to CL and fasting showed a significant increase in AlexaFFA uptake in the heart (48 ± 20%) and BAT (40 ± 10%) compared to saline-injected/fed mice. We present a novel near-infrared FFA tracer, AlexaFFA, that is suitable for in vivo quantification of FFA metabolism and can be applied in the context of a low cost, high throughput, and multiplexed optical imaging platform.

The Presence of Active Brown Adipose Tissue Determines Cold-Induced Energy Expenditure and Oxylipin Profiles in Humans

BACKGROUND

Accumulating evidence links brown adipose tissue (BAT) to increased cold-induced energy expenditure (CIEE) and regulation of lipid metabolism in humans. BAT has also been proposed as a novel source for biologically active lipid mediators including polyunsaturated fatty acids (PUFAs) and oxylipins. However, little is known about cold-mediated differences in energy expenditure and various lipid species between individuals with detectable BAT positive (BATpos) and those without BAT negative (BATneg).

METHODS

Here we investigated a unique cohort of matched BATpos and BATneg individuals identified by 18F-fluorodeoxyglucose positron emission tomography combined with computed tomography ([18F]-FDG PET/CT). BAT function, CIEE, and circulating oxylipins, were analyzed before and after short-term cold exposure using [18F]-FDG PET/CT, indirect calorimetry, and high-resolution mass spectrometry, respectively.

RESULTS

We found that active BAT is the major determinant of CIEE since only BATpos individuals experienced significantly increased energy expenditure in response to cold. A single bout of moderate cold exposure resulted in the dissipation of an additional 20 kcal excess energy in BATpos but not in BATneg individuals. The presence of BAT was associated with a unique systemic PUFA and oxylipin profile characterized by increased levels of anti-inflammatory omega-3 fatty acids as well as cytochrome P450 products but decreased concentrations of some proinflammatory hydroxyeicosatetraenoic acids when compared with BATneg individuals. Notably, cold exposure raised circulating levels of various lipids, including the recently identified BAT-derived circulating factors (BATokines) DiHOME and 12-HEPE, only in BATpos individuals.

CONCLUSIONS

In summary, our data emphasize that BAT in humans is a major contributor toward cold-mediated energy dissipation and a critical organ in the regulation of the systemic lipid pool.

Fruit of Hovenia dulcis Thunb. Induces Nonshivering Thermogenesis through Mitochondrial Biogenesis and Activation by SIRT1 in High-Fat Diet-Fed Obese Mice and Primary Cultured Brown Adipocytes

Brown adipocytes, which contain abundant mitochondria, use stored energy as fuel during a process named nonshivering thermogenesis. Thus, the pharmacological activation of thermogenesis in brown adipose tissue (BAT) has become a promising target for treating obesity. We investigated the effect of fruit of Hovenial dulcis Thunb. (FHD), a frequently used herbal treatment for liver diseases, on thermogenesis and its mechanism using primary cultured brown adipocytes and BAT of high-fat-diet (HFD)-induced obese mice. Thermogenesis-related factors including UCP1 and PGC1α increased with FHD treatment. FHD also increased mitochondrial biogenesis and activation factors such as nuclear respiratory factor (NRF)1 and oxidative phosphorylation (OXPHOS) complex. Furthermore, FHD increased the intercellular nicotinamide adenine dinucleotide (NAD⁺) level and sirtuin 1 (SIRT1) activity, which may be responsible for the activation of the thermogenic reaction. Overall, our results suggest that FHD can be a novel option for obesity treatment due to its thermogenic action through mitochondrial biogenesis and activation.

Nutritional Requirements for Sustaining Health and Performance During Exposure to Extreme Environments

Dietary guidelines are formulated to meet minimum nutrient requirements, which prevent deficiencies and maintain health, growth, development, and function. These guidelines can be inadequate and contribute to disrupted homeostasis, lean body mass loss, and deteriorated performance in individuals who are working long, arduous hours with limited access to food in environmentally challenging locations. Environmental extremes can elicit physiological adjustments that alone alter nutrition requirements by upregulating energy expenditure, altering substrate metabolism, and accelerating body water and muscle protein loss. The mechanisms by which the environment, including high-altitude, heat, and cold exposure, alters nutrition requirements have been studied extensively. This contemporary review discusses physiological adjustments to environmental extremes, particularly when those adjustments alter dietary requirements.

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.

The scaffold protein p62 regulates adaptive thermogenesis through ATF2 nuclear target activation

During β-adrenergic stimulation of brown adipose tissue (BAT), p38 phosphorylates the activating transcription factor 2 (ATF2) which then translocates to the nucleus to activate the expression of Ucp1 and Pgc-1α. The mechanisms underlying ATF2 target activation are unknown. Here we demonstrate that p62 (Sqstm1) binds to ATF2 to orchestrate activation of the Ucp1 enhancer and Pgc-1α promoter. P62^Δ69-251 mice show reduced expression of Ucp1 and Pgc-1α with impaired ATF2 genomic binding. Modulation of Ucp1 and Pgc-1α expression through p62 regulation of ATF2 signaling is demonstrated in vitro and in vivo in p62^Δ69-251 mice, global p62^−/− and Ucp1-Cre p62^flx/flx mice. BAT dysfunction resulting from p62 deficiency is manifest after birth and obesity subsequently develops despite normal food intake, intestinal nutrient absorption and locomotor activity. In summary, our data identify p62 as a master regulator of BAT function in that it controls the Ucp1 pathway through regulation of ATF2 genomic binding.

Beta-adrenergic stimulation of brown adipose tissue leads to thermogenesis via the activating transcription factor 2 (ATF2) mediated expression of the thermogenic genes Ucp1 and Pgc-1α. Here, the authors show that the scaffold protein p62 regulates brown adipose tissue function through modifying ATF2 genomic binding and subsequent Ucp1 and Pgc-1α induction.

Cold-Activated Brown Adipose Tissue is Associated with Less Cardiometabolic Dysfunction in Young Adults with Obesity

OBJECTIVE

This study aimed to test the hypothesis that young adults with obesity and cold-activated brown adipose tissue (BAT) are less likely to have metabolic dysfunction (dyslipidemia, insulin resistance, and hypertension) than those without cold-activated BAT. Previous studies have noted a potentially protective effect of BAT and higher adiponectin/leptin ratios, but they have acknowledged that the clinical implications of these findings remain uncertain.

METHODS

Twenty-one females and twenty-three males with obesity (BMI ≥ 30 kg/m² ) underwent a 2-hour cooling protocol before ¹⁸ F-fluorodeoxyglucose (¹⁸ F-FDG)-positron emission tomography/x-ray computed tomography scan to determine the prevalence, volume, and ¹⁸ F-FDG uptake of cold-activated BAT.

RESULTS

Cold-activated BAT was identified in 43% of participants (11 female, 8 male); females had greater ¹⁸ F-FDG uptake. Those with cold-activated BAT had a lesser degree of metabolic dysfunction. Cold-activated BAT volume correlated with triglycerides (inversely) and adiponectin (concordantly). Body-mass-adjusted cold-activated BAT activity correlated with high-density lipoprotein cholesterol (concordantly). Males with cold-activated BAT had lower leptin and higher adiponectin/leptin ratio.

CONCLUSIONS

A high prevalence of cold-activated BAT was found in the study participants. BAT could be important in decreasing metabolic dysfunction among young adults with obesity, making it a potential target for treating metabolically unhealthy obesity.

21st Century Advances in Multimodality Imaging of Obesity for Care of the Cardiovascular Patient

Although obesity is typically defined by body mass index criteria, this does not differentiate true body fatness, as this includes both body fat and muscle. Therefore, other fat depots may better define cardiometabolic and cardiovascular disease (CVD) risk imposed by obesity. Data from translational, epidemiological, and clinical studies over the past 3 decades have clearly demonstrated that accumulation of adiposity in the abdominal viscera and within tissue depots lacking physiological adipose tissue storage capacity (termed "ectopic fat") is strongly associated with the development of a clinical syndrome characterized by atherogenic dyslipidemia, hyperinsulinemia/glucose intolerance/type 2 diabetes mellitus, hypertension, atherosclerosis, and abnormal cardiac remodeling and heart failure. This state-of-the-art paper discusses the impact of various body fat depots on cardiometabolic parameters and CVD risk. Specifically, it reviews novel and emerging imaging techniques to evaluate adiposity and the risk of cardiometabolic diseases and CVD.

Liraglutide decreases energy expenditure and does not affect the fat fraction of supraclavicular brown adipose tissue in patients with type 2 diabetes

BACKGROUND AND AIMS

Several studies have shown that glucagon-like peptide-1 (GLP-1) analogues can affect resting energy expenditure, and preclinical studies suggest that they may activate brown adipose tissue (BAT). The aim of the present study was to investigate the effect of treatment with liraglutide on energy metabolism and BAT fat fraction in patients with type 2 diabetes.

METHODS AND RESULTS

In a 26-week double-blind, placebo-controlled trial, 50 patients with type 2 diabetes were randomized to treatment with liraglutide (1.8 mg/day) or placebo added to standard care. At baseline and after treatment for 4, 12 and 26 weeks, we assessed resting energy expenditure (REE) by indirect calorimetry. Furthermore, at baseline and after 26 weeks, we determined the fat fraction in the supraclavicular BAT depot using chemical-shift water-fat MRI at 3T. Liraglutide reduced REE after 4 weeks, which persisted after 12 weeks and tended to be present after 26 weeks (week 26 vs baseline: liraglutide -52 ± 128 kcal/day; P = 0.071, placebo +44 ± 144 kcal/day; P = 0.153, between group P = 0.057). Treatment with liraglutide for 26 weeks did not decrease the fat fraction in supraclavicular BAT (-0.4 ± 1.7%; P = 0.447) compared to placebo (-0.4 ± 1.4%; P = 0.420; between group P = 0.911).

CONCLUSION

Treatment with liraglutide decreases REE in the first 12 weeks and tends to decrease this after 26 weeks without affecting the fat fraction in the supraclavicular BAT depot. These findings suggest reduction in energy intake rather than an increase in REE to contribute to the liraglutide-induced weight loss.

TRIAL REGISTRY NUMBER

NCT01761318.

Pheochromocytoma and Paraganglioma Patients With Poor Survival Often Show Brown Adipose Tissue Activation

CONTEXT

Pheochromocytomas/paragangliomas (PPGLs) are neuroendocrine tumors that can secrete norepinephrine (NE). Brown adipose tissue (BAT) activation is mediated through the action of NE on β-adrenoceptors (β-ARs). In some malignancies, BAT activation is associated with higher cancer activity.

OBJECTIVE

To study the relationship between BAT activation and PPGL clinical outcomes.

DESIGN

A retrospective case-control study that included 342 patients with PPGLs who underwent 18F-fluoro-2-deoxy-D-glucose positron emission tomography-computed tomography (18F-FDG PET/CT) imaging at the National Institutes of Health (NIH). We excluded all patients with parasympathetic tumors and those who underwent 18F-FDG PET/CT after PPGL resection. Scans of 205 patients were reviewed by 2 blinded nuclear medicine physicians; 16 patients had BAT activation on 18F-FDG PET/CT [7.80%; age 27.50 (15.00-45.50) years; 10 female/6 male; body mass index [BMI] 24.90 [19.60-25.35] kg/m2). From the remaining 189 patients, we selected 36 matched controls (age 34.4 [25.4-45.5] years; 21 female/15 male; BMI 25.0 [22.0-26.0] kg/m2).

PRIMARY OUTCOME MEASURE

Overall survival.

RESULTS

The presence of active BAT on 18F-FDG PET/CT was associated with decreased overall survival when compared with the control group (HRz 5.80; 95% CI, 1.05-32.05; P = 0.02). This association remained significant after adjusting for the SDHB mutation. Median plasma NE in the BAT group was higher than the control group [4.65 vs 0.55 times above the upper limit of normal; P < 0.01]. There was a significant association between higher plasma NE levels and mortality in PPGLs in both groups.

CONCLUSIONS

Our findings suggest that the detection of BAT activity in PPGL patients is associated with higher mortality. We suggest that BAT activation could either be reflecting or contributing to a state of increased host stress that may predict poor outcome in metastatic PPGL.

The generation of immune-induced fever and emotional stress-induced hyperthermia in mice does not involve brown adipose tissue thermogenesis

We examined the role of brown adipose tissue (BAT) for fever and emotional stress-induced hyperthermia. Wild-type and uncoupling protein-1 (UCP-1) knockout mice were injected with lipopolysaccharide intraperitoneally or intravenously, or subjected to cage exchange, and body temperature monitored by telemetry. Both genotypes showed similar febrile responses to immune challenge and both displayed hyperthermia to emotional stress. Neither procedure resulted in the activation of BAT, such as the induction of UCP-1 or peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) mRNA, or reduced BAT weight and triglyceride content. In contrast, in mice injected with a β₃ agonist, UCP-1 and PGC-1α were strongly induced, and BAT weight and triglyceride content reduced. Both lipopolysaccharide and the β₃ agonist, and emotional stress, induced UCP-3 mRNA in skeletal muscle. A β₃ antagonist did not attenuate lipopolysaccharide-induced fever, but augmented body temperature decrease and inhibited BAT activation when mice were exposed to cold. An α₁ /α_2b antagonist or a 5HT_1A agonist, which inhibit vasoconstriction, abolished lipopolysaccharide-induced fever, but had no effect on emotional stress-induced hyperthermia. These findings demonstrate that in mice, UCP-1-mediated BAT thermogenesis does not take part in inflammation-induced fever, which is dependent on peripheral vasoconstriction, nor in stress-induced hyperthermia. However, both phenomena may involve UCP-3-mediated muscle thermogenesis.

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.

Adipose Tissue Distribution, Inflammation and Its Metabolic Consequences, Including Diabetes and Cardiovascular Disease

Adipose tissue plays essential roles in maintaining lipid and glucose homeostasis. To date several types of adipose tissue have been identified, namely white, brown, and beige, that reside in various specific anatomical locations throughout the body. The cellular composition, secretome, and location of these adipose depots define their function in health and metabolic disease. In obesity, adipose tissue becomes dysfunctional, promoting a pro-inflammatory, hyperlipidemic and insulin resistant environment that contributes to type 2 diabetes mellitus (T2DM). Concurrently, similar features that result from adipose tissue dysfunction also promote cardiovascular disease (CVD) by mechanisms that can be augmented by T2DM. The mechanisms by which dysfunctional adipose tissue simultaneously promote T2DM and CVD, focusing on adipose tissue depot-specific adipokines, inflammatory profiles, and metabolism, will be the focus of this review. The impact that various T2DM and CVD treatment strategies have on adipose tissue function and body weight also will be discussed.

Incendiary Leptin

Leptin is a hormone released by adipose tissue that plays a key role in the control of energy homeostasis through its binding to leptin receptors (LepR), mainly expressed in the hypothalamus. Most scientific evidence points to leptin’s satiating effect being due to its dual capacity to promote the expression of anorexigenic neuropeptides and to reduce orexigenic expression in the hypothalamus. However, it has also been demonstrated that leptin can stimulate (i) thermogenesis in brown adipose tissue (BAT) and (ii) the browning of white adipose tissue (WAT). Since the demonstration of the importance of BAT in humans 10 years ago, its study has aroused great interest, mainly in the improvement of obesity-associated metabolic disorders through the induction of thermogenesis. Consequently, several strategies targeting BAT activation (mainly in rodent models) have demonstrated great potential to improve hyperlipidemias, hepatic steatosis, insulin resistance and weight gain, leading to an overall healthier metabolic profile. Here, we review the potential therapeutic ability of leptin to correct obesity and other metabolic disorders, not only through its satiating effect, but by also utilizing its thermogenic properties.

Through fat and thin – a journey with the adipose tissues

The paper is based on the lecture that I gave on receiving the Nutrition Society's inaugural Gowland Hopkins Award for contributions to Cellular and Molecular Nutrition. It reviews studies on the adipose tissues, brown and white, conducted by the groups that I have led since entering nutrition research in 1975. The initial focus was on exploring metabolic factors that underpin the development of obesity using animal models. This resulted in an interest in non-shivering thermogenesis with brown adipose tissue being identified as the key effector of facultative heat production. Brown fat is less thermogenically active in various obese rodents, and major changes in activity are exhibited under physiological conditions such as lactation and fasting consistent with a general role for the tissue in nutritional energetics. My interests moved to white adipose tissue following the cloning of the Ob gene. Our initial contributions in this area included demonstrating nutritional regulation of Ob gene expression and circulating leptin levels, as well as a regulatory role for the sympathetic nervous system operating through β3-adrenoceptors. My interests subsequently evolved to a wider concern with the endocrine/signalling role of adipose tissue. Inflammation is a characteristic of white fat in obesity with the release of inflammation-related adipokines, and we proposed that hypoxia underlies this inflammatory state. O2-deprivation was shown to have substantial effects on gene expression and cellular function in white adipocytes. The hypoxia studies led to the proposition that O2 should be considered as a critical macronutrient.

Brown adipose tissue volume and 18F-fluorodeoxyglucose uptake are not associated with energy intake in young human adults

BACKGROUND

Several studies have explored the role of human brown adipose tissue (BAT) in energy expenditure. However, the link between BAT and appetite regulation needs to be more rigorously examined.

OBJECTIVES

We aimed to investigate the associations of BAT volume and 18F-fluordeoxyglucose (18F-FDG) uptake after a personalized cold exposure with energy intake and appetite-related sensations in young healthy humans.

METHODS

A total of 102 young adults (65 women; age: 22.08 ± 2.17 y; BMI: 25.05 ± 4.93 kg/m 2) took part in this cross-sectional study. BAT volume, BAT 18F-FDG uptake, and skeletal muscle 18F-FDG uptake were assessed by means of static 18F-FDG positron-emission tomography and computed tomography scans after a 2-h personalized exposure to cold. Energy intake was estimated via an objectively measured ad libitum meal and three nonconsecutive 24-h dietary recalls. Appetite-related sensations (i.e., hunger and fullness) were recorded by visual analog scales before and after a standardized breakfast (energy content = 50% of basal metabolic rate) and the ad libitum meal. Body composition was assessed by a whole-body DXA scan.

RESULTS

BAT volume and 18F-FDG uptake were not associated with quantified ad libitum energy intake (all P > 0.088), nor with habitual energy intake estimated from the 24-h dietary recalls (all P  > 0.683). Lean mass was positively associated with both the energy intake from the ad libitum meal (β: 17.612, R2 = 0.213; P < 0.001) and the habitual energy intake (β: 16.052, R2 = 0.123; P = 0.001). Neither the interaction BAT volume × time elapsed after meal consumption nor that of BAT 18F-FDG uptake × time elapsed after meal consumption had any significant influence on appetite-related sensations after breakfast or after meal consumption (all P > 0.3).

CONCLUSIONS

Neither BAT volume, nor BAT 18F-FDG uptake after cold stimulation, are related to appetite regulation in young adults. These results suggest BAT plays no important role in the regulation of energy intake in humans.This trial was registered at clinicaltrials.gov as NCT02365129.

Long non-coding RNA 2310069B03Rik functions as a suppressor of Ucp1 expression under prolonged cold exposure in murine beige adipocytes

Specific conditions, such as exposure to cold, can induce the production of brown-like adipocytes in white adipose tissue. These adipocytes express high levels of uncoupling protein 1 (UCP1) and energy expended by generating heat. Thus, these are a potential target for the prevention or treatment of obesity. The present study involved a comprehensive analysis of the adipose tissue to understand the relationship between long non-coding RNA (lncRNA) 2310069B03Rik and UCP1. Cold exposure increased both lncRNA 2310069B03Rik and Ucp1 expression in inguinal white adipose tissue (iWAT). However, overexpression of lncRNA 2310069B03Rik suppressed the Ucp1 mRNA expression and the promoter activity of UCP1 in the iWAT primary adipocytes. In addition, compared to the early induction of Ucp1 expression by cold stimulation, the induction of lncRNA 2310069B03Rik expression was later. These results suggest that lncRNA 2310069B03Rik functions as a suppression factor of Ucp1 expression.

Isocitrate dehydrogenase 2 protects mice from high-fat diet-induced metabolic stress by limiting oxidative damage to the mitochondria from brown adipose tissue

Isocitrate dehydrogenase 2 (IDH2) is an NADP+-dependent enzyme that catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate in the mitochondrial matrix, and is critical for the production of NADPH to limit the accumulation of mitochondrial reactive oxygen species (ROS). Here, we showed that high-fat diet (HFD) feeding resulted in accelerated weight gain in the IDH2KO mice due to a reduction in whole-body energy expenditure. Moreover, the levels of NADP+, NADPH, NAD+, and NADH were significantly decreased in the brown adipose tissue (BAT) of the HFD-fed IDH2KO animals, accompanied by decreased mitochondrial function and reduced expression of key genes involved in mitochondrial biogenesis, energy expenditure, and ROS resolution. Interestingly, these changes were partially reversed when the antioxidant butylated hydroxyanisole was added to the HFD. These observations reveal a crucial role for IDH2 in limiting ROS-dependent mitochondrial damage when BAT metabolism is normally enhanced to limit weight gain in response to dietary caloric overload.

Hydrocarboxylic acid receptor 1 in BAT regulates glucose uptake in mice fed a high-fat diet

Interscapular brown adipose tissue (BAT) has the capability to take up glucose from the circulation. Despite the important role of BAT in the control of glucose homeostasis, the metabolic fate and function of glucose in BAT remain elusive as there is clear dissociation between glucose uptake and BAT thermogenesis. Interestingly, intracellular glycolysis and lactate production appear to be required for glucose uptake by BAT. Here, we specifically examine whether activation of lactate receptors in BAT plays a key role in regulating glucose homeostasis in mice fed a high-fat diet (HFD). When C57BL/6J mice are given HFD for 5 weeks at 28°C, male, but not female, mice gain body weight and develop hyperglycemia. Importantly, high-fat feeding upregulates expression of the lactate receptor hydroxycarboxylic acid receptor 1 (HCAR1) in female C57BL/6J mice, whereas male C57BL/6J mice show reduced HCAR1 expression in BAT. Treatment with the HCAR1 agonist lowers systemic glucose levels in male DIO mice. This reduction is associated with increased glucose uptake in BAT. Therefore, our results suggest that HCAR1 in BAT may contribute to the development of hyperglycemia in male C57BL/6J DIO mice.

ZW290 Increases Cold Tolerance by Inducing Thermogenesis via the Upregulation of Uncoupling Protein 1 in Brown Adipose Tissue In Vitro and In Vivo

To provide molecular evidence on the thermogenic mechanism of primary brown adipocytes, western blot analysis was used to detect brown adipose tissue (BAT)-specific gene expressions. BAT protects the mammals from hypothermia injury with a large amount of mitochondria and high expression of uncoupling Protein 1 (UCP1), which is the vital protein to determine the heat production in BAT. In our previous study, the compound ZW290 (the structure shown in Fig. 1) was obtained by molecular docking with a UCP1 inducer. In the present study, ZW290 not only significantly upregulated the expression of UCP1 protein (p < 0.01) and its related signaling pathway in the primary brown adipocytes, but also remarkably decreased the mitochondrial membrane potential and the concentration of adenosine triphosphate (ATP) (p < 0.01). Kunming (KM) mice were kept under acute cold exposure (-20°C) to evaluate the preventive and protective effects of ZW290 on cold injury, and revealed its regulating mechanism in vitro. The rectal and body temperatures of ZW290-treated mice were significantly higher than those of the control (or model) group both at room temperature and at -20°C (p < 0.001). Hematoxylin-eosin (HE) staining and immunohistochemistry indicated that ZW290 notably decreased the size of lipid droplets in BAT and increased the content of mitochondria and the expression of UCP1 in BAT and white adipose tissue (WAT). Furthermore, the survival rate showed that ZW290 could prolong the overall survival of mice. Therefore, we obtained the conclusion that ZW290 might transform energy into heat by inhibiting ATP synthesis and increasing the expression of UCP1. Additionally, ZW290 may enhance cold tolerance by increasing heat production through increasing the content of mitochondria and the expression of UCP1 in BAT and WAT.

Low- and high-thermogenic brown adipocyte subpopulations coexist in murine adipose tissue

Brown adipose tissue (BAT), as the main site of adaptive thermogenesis, exerts beneficial metabolic effects on obesity and insulin resistance. BAT has been previously assumed to contain a homogeneous population of brown adipocytes. Utilizing multiple mouse models capable of genetically labeling different cellular populations, as well as single-cell RNA sequencing and 3D tissue profiling, we discovered a brown adipocyte subpopulation with low thermogenic activity coexisting with the classical high-thermogenic brown adipocytes within the BAT. Compared with the high-thermogenic brown adipocytes, these low-thermogenic brown adipocytes had substantially lower Ucp1 and Adipoq expression, larger lipid droplets, and lower mitochondrial content. Functional analyses showed that, unlike the high-thermogenic brown adipocytes, the low-thermogenic brown adipocytes have markedly lower basal mitochondrial respiration, and they are specialized in fatty acid uptake. Upon changes in environmental temperature, the 2 brown adipocyte subpopulations underwent dynamic interconversions. Cold exposure converted low-thermogenic brown adipocytes into high-thermogenic cells. A thermoneutral environment had the opposite effect. The recruitment of high-thermogenic brown adipocytes by cold stimulation is not affected by high-fat diet feeding, but it does substantially decline with age. Our results revealed a high degree of functional heterogeneity of brown adipocytes.

Magnetic Resonance Imaging Techniques for Brown Adipose Tissue Detection

Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) methods can non-invasively assess brown adipose tissue (BAT) structure and function. Recently, MRI and MRS have been proposed as a means to differentiate BAT from white adipose tissue (WAT) and to extract morphological and functional information on BAT inaccessible by other means. Specifically, proton MR (¹H) techniques, such as proton density fat fraction mapping, diffusion imaging, and intermolecular multiple quantum coherence imaging, have been employed to access BAT microstructure; MR thermometry, relaxometry, and MRI and MRS with ³¹P, ²H, ¹³C, and ¹²⁹Xe have shown to provide complementary information on BAT function. The purpose of the present review is to provide a comprehensive overview of MR imaging and spectroscopy techniques used to detect BAT in rodents and in humans. The present work discusses common challenges of current methods and provides an outlook on possible future directions of using MRI and MRS in BAT studies.

Brown Adipose Tissue, Diet-Induced Thermogenesis, and Thermogenic Food Ingredients: From Mice to Men

Since the recent rediscovery of brown adipose tissue (BAT) in adult humans, this thermogenic tissue has been attracting increasing interest. The inverse relationship between BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. Cold exposure activates and recruits BAT, resulting in increased energy expenditure and decreased body fatness. The stimulatory effects of cold exposure are mediated through transient receptor potential (TRP) channels and the sympathetic nervous system (SNS). Most TRP members also function as chemesthetic receptors for various food ingredients, and indeed, agonists of TRP vanilloid 1 such as capsaicin and its analog capsinoids mimic the effects of cold exposure to decrease body fatness through the activation and recruitment of BAT. The antiobesity effect of other food ingredients including tea catechins may be attributable, at least in part, to the activation of the TRP-SNS-BAT axis. BAT is also involved in the facultative thermogenesis induced by meal intake, referred to as diet-induced thermogenesis (DIT), which is a significant component of the total energy expenditure in our daily lives. Emerging evidence suggests a crucial role for the SNS in BAT-associated DIT, particularly during the early phase, but several gut-derived humoral factors may also participate in meal-induced BAT activation. One intriguing factor is bile acids, which activate BAT directly through Takeda G-protein receptor 5 (TGR5) in brown adipocytes. Given the apparent beneficial effects of some TRP agonists and bile acids on whole-body substrate and energy metabolism, the TRP/TGR5-BAT axis represents a promising target for combating obesity and related metabolic disorders in humans.

Electrical Neurostimulation Promotes Brown Adipose Tissue Thermogenesis

BACKGROUND

Brown adipose tissue (BAT) is present in humans and rodents, and contributes to energy expenditure by converting energy stored in lipids and glucose into heat. Beta adrenergic receptor (β-AR) agonists have been proposed as pharmacological tools to activate BAT, but they lack selectivity for this tissue. This study aimed to investigate the possibility to apply electrical neurostimulation as a novel approach to activate BAT by promoting the sympathetic outflow towards BAT.

METHODS

Male C57BL/6J mice were treated with either unilateral electrical neurostimulation of interscapular BAT or with the β3-AR agonist CL316,243. Thermogenesis, nutrient uptake by BAT and downstream signaling of adrenergic receptors in BAT were examined.

RESULTS

Electrical neurostimulation and β3-AR agonism acutely increased heat production by BAT, as evidenced by an increase in local temperature in BAT, without influencing the core body temperature. Both treatments acutely increased tyrosine hydroxylase content in the nerve terminals thereby confirming enhanced sympathetic activity. In addition, we identified increased phosphorylation of hormone-sensitive lipase coinciding with reduced intracellular lipids in BAT, without affecting acute nutrient uptake from plasma. The increased BAT temperature as induced by electrical neurostimulation was reversed by β3-AR antagonism.

CONCLUSION

Electrical neurostimulation acutely promotes thermogenesis in BAT as dependent on β3-AR signaling. We anticipate that electrical neurostimulation may be further developed as a novel strategy to activate BAT and thereby combat (cardio)metabolic diseases.

Brown Adipose Crosstalk in Tissue Plasticity and Human Metabolism

Infants rely on brown adipose tissue (BAT) as a primary source of thermogenesis. In some adult humans, residuals of brown adipose tissue are adjacent to the central nervous system and acute activation increases metabolic rate. Brown adipose tissue (BAT) recruitment occurs during cold acclimation and includes secretion of factors, known as batokines, which target several different cell types within BAT, and promote adipogenesis, angiogenesis, immune cell interactions, and neurite outgrowth. All these processes seem to act in concert to promote an adapted BAT. Recent studies have also provided exciting data on whole body metabolic regulation with a broad spectrum of mechanisms involving BAT crosstalk with liver, skeletal muscle, and gut as well as the central nervous system. These widespread interactions might reflect the property of BAT of switching between an active thermogenic state where energy is highly consumed and drained from the circulation, and the passive thermoneutral state, where energy consumption is turned off. (Endocrine Reviews 41: XXX - XXX, 2020).

The Association of Obesity with Autoimmune Thyroiditis and Thyroid Function-Possible Mechanisms of Bilateral Interaction

A growing number of patients suffer from autoimmune diseases, including autoimmune thyroid disease. There has simultaneously been a significant increase in the prevalence of obesity worldwide. It is still an open question whether adiposity can directly influence activation of inflammatory processes affecting the thyroid in genetically predisposed individuals. Adipokines, biologically active substances derived from the adipocytes, belong to a heterogenic group of compounds involved in numerous physiological functions, including the maintenance of metabolism, hormonal balance, and immune response. Notably, the presence of obesity worsens the course of selected autoimmune diseases and impairs response to treatment. Moreover, the excess of body fat may result in the progression of autoimmune diseases. Nutritional status, body weight, and energy expenditure may influence thyroid hormone secretion. Interestingly, thyroid hormones might influence the activity of adipose tissue as metabolic alterations related to fat tissue are observed under pathological conditions in which there are deficits or overproduction of thyroid hormones. Functioning TSH receptors are expressed on adipocytes. Thermogenesis may presumably be stimulated by TSH binding to its receptor on brown adipocytes. There could be a bilateral interaction between the thyroid and adipose. Obesity may influence the onset and course of autoimmune disease.

Investigation of the Relationship between MR-Based Supraclavicular Fat Fraction and Thyroid Hormones

PURPOSE

Brown adipose tissue (BAT) plays a potential role in energy and glucose metabolism in humans. Thyroid hormones (TH) are main regulators of BAT development and function. However, it remains unknown how the magnetic resonance (MR)-based proton density fat fraction (PDFF) of supraclavicular adipose tissue used as a surrogate marker for BAT presence relates to TH. Therefore, the purpose of this analysis was to investigate the relationship between supraclavicular PDFF and serum levels of TH.

METHODS

In total, 96 adult volunteers from a large cross-sectional study who underwent additional MR examination of the neck and pelvis were included in this analysis. Segmented PDFF maps of the supraclavicular and gluteal subcutaneous adipose tissue were generated. Delta PDFF was calculated as the difference between gluteal and supraclavicular PDFF and grouped as high (≥12%) or low (<12%) based on the median and the clinical rationale of a high versus low probability of BAT being present. Thyroid-stimulating hormone (mIU/L), free triiodothyronine (FT3, pg/mL) and free thyroxine (FT4, ng/dL) levels were determined in blood samples. Body mass index (BMI) was calculated as weight (kg)/height (m)2. Statistical analyses included the use of paired samples ttest, simple linear regression analysis and a multivariable linear regression analysis.

RESULTS

The median age of the subjects (77% female) was 33 years, BMI ranged from 17.2 to 43.1 kg/m2. Supraclavicular and gluteal PDFF differed significantly (76.5 ± 4.8 vs. 89.4 ± 3.5 %, p < 0.01). Supraclavicular PDFF was associated with FT3 in subjects with high delta PDFF (R2 = 0.17, p < 0.01), with higher FT3 being associated with lower supraclavicular PDFF (y = 85.2 + -3.6 x). In a multivariable linear regression analysis considering further potential prognostic factors, the interaction between the delta PDFF group and FT3 remained a predictor for supraclavicular PDFF (B = -4.65, p < 0.01).

DISCUSSION/CONCLUSIONS

Supraclavicular PDFF corresponds to the presence of BAT. In the present analysis, supraclavicular PDFF is correlated with FT3 in subjects with high delta PDFF. Therefore, the present findings suggest that biologically active T3 may be involved in the development of supraclavicular BAT.

Metabolic Improvement via Enhancing Thermogenic Fat-Mediated Non-shivering Thermogenesis: From Rodents to Humans

Brown and beige adipose tissues play a large role in non-shivering thermogenesis (NST) in mammals, and subsequently have been studied for decades as potential therapeutic targets to treat obesity and its related metabolic diseases. However, the mechanistic regulation of brown/beige adipose tissue induction and maintenance in humans is very limited due to the ethical reasons. In fact, metabolic signaling has primarily been investigated using rodent models. A better understanding of non-shivering thermogenesis in humans is thus vital and urgent in order to treat obesity by targeting human brown adipose tissue (BAT). In this review, we summarize the anatomical and physiological differences between rodent and human BAT, current useful and mostly non-invasive methods in studying human BAT, as well as recent advancements targeting thermogenic adipocytes as a means to combat metabolic diseases in humans. Furthermore, we also discuss several novel relevant strategies of therapeutic interventions, which has been attempted in rodent experiments, and possible future investigations in humans in this field.

Substrate Utilization by Brown Adipose Tissue: What's Hot and What's Not?

Our understanding of brown adipose tissue (BAT) function in humans has increased rapidly over the past 10 years. This is predominantly due to the development of powerful non-invasive imaging techniques such as positron emission tomography that can quantify BAT mass and function using metabolic tracers. Activation of BAT during cold-induced thermogenesis is an effective way to dissipate energy to generate heat and requires utilization of multiple energy substrates for optimal function. This has led to interest in the activation of BAT as a potential therapeutic target for type 2 diabetes, dyslipidaemia, and obesity. Here, we provide an overview of the current understanding of BAT substrate utilization in humans and highlight additional mechanisms found in rodents, where BAT more prominently contributes to energy expenditure. During thermogenesis, BAT demonstrates substantially increased glucose uptake which appears to be critical for BAT function. However, glucose is not fully oxidized, with a large proportion converted to lactate. The primary energy substrate for thermogenesis is fatty acids, released from brown adipocyte triglyceride stores. Active BAT also sequesters circulating lipids to sustain optimal thermogenesis. Recent evidence reveals that metabolic intermediates from the tricarboxylic acid cycle and glycolytic pathways also play a critical role in BAT function. Understanding the role of these metabolites in regulating thermogenesis and whole body substrate utilization may elucidate novel strategies for therapeutic BAT activation.

Opportunities and challenges in the therapeutic activation of human energy expenditure and thermogenesis to manage obesity

The current obesity pandemic results from a physiological imbalance in which energy intake chronically exceeds energy expenditure (EE), and prevention and treatment strategies remain generally ineffective. Approaches designed to increase EE have been informed by decades of experiments in rodent models designed to stimulate adaptive thermogenesis, a long-term increase in metabolism, primarily induced by chronic cold exposure. At the cellular level, thermogenesis is achieved through increased rates of futile cycling, which are observed in several systems, most notably the regulated uncoupling of oxidative phosphorylation from ATP generation by uncoupling protein 1, a tissue-specific protein present in mitochondria of brown adipose tissue (BAT). Physiological activation of BAT and other organ thermogenesis occurs through β-adrenergic receptors (AR), and considerable effort over the past 5 decades has been directed toward developing AR agonists capable of safely achieving a net negative energy balance while avoiding unwanted cardiovascular side effects. Recent discoveries of other BAT futile cycles based on creatine and succinate have provided additional targets. Complicating the current and developing pharmacological-, cold-, and exercise-based methods to increase EE is the emerging evidence for strong physiological drives toward restoring lost weight over the long term. Future studies will need to address technical challenges such as how to accurately measure individual tissue thermogenesis in humans; how to safely activate BAT and other organ thermogenesis; and how to sustain a negative energy balance over many years of treatment.

Interaction between adipocytes and high-density lipoprotein:new insights into the mechanism of obesity-induced dyslipidemia and atherosclerosis

Obesity is the most common nutritional disorder worldwide and is associated with dyslipidemia and atherosclerotic cardiovascular disease. The hallmark of dyslipidemia in obesity is low high density lipoprotein (HDL) cholesterol (HDL-C) levels. Moreover, the quality of HDL is also changed in the obese setting. However, there are still some disputes on the explanations for this phenomenon. There is increasing evidence that adipose tissue, as an energy storage tissue, participates in several metabolism activities, such as hormone secretion and cholesterol efflux. It can influence overall reverse cholesterol transport and plasma HDL-C level. In obesity individuals, the changes in morphology and function of adipose tissue affect plasma HDL-C levels and HDL function, thus, adipose tissue should be the main target for the treatment of HDL metabolism in obesity. In this review, we will summarize the cross-talk between adipocytes and HDL related to cardiovascular disease and focus on the new insights of the potential mechanism underlying obesity and HDL dysfunction.

MRI Reveals Human Brown Adipose Tissue Is Rapidly Activated in Response to Cold

CONTEXT

In rodents, cold exposure induces the activation of brown adipose tissue (BAT) and the induction of intracellular triacylglycerol (TAG) lipolysis. However, in humans, the kinetics of supraclavicular (SCV) BAT activation and the potential importance of TAG stores remain poorly defined.

OBJECTIVE

To determine the time course of BAT activation and changes in intracellular TAG using MRI assessment of the SCV (i.e., BAT depot) and fat in the posterior neck region (i.e., non-BAT).

DESIGN

Cross-sectional.

SETTING

Clinical research center.

PATIENTS OR OTHER PARTICIPANTS

Twelve healthy male volunteers aged 18 to 29 years [body mass index = 24.7 ± 2.8 kg/m2 and body fat percentage = 25.0% ± 7.4% (both, mean ± SD)].

INTERVENTIONS

Standardized whole-body cold exposure (180 minutes at 18°C) and immediate rewarming (30 minutes at 32°C).

MAIN OUTCOME MEASURES

Proton density fat fraction (PDFF) and T2* of the SCV and posterior neck fat pads. Acquisitions occurred at 5- to 15-minute intervals during cooling and subsequent warming.

RESULTS

SCV PDFF declined significantly after only 10 minutes of cold exposure [-1.6% (SE: 0.44%; P = 0.007)] and continued to decline until 35 minutes, after which time it remained stable until 180 minutes. A similar time course was also observed for SCV T2*. In the posterior neck fat (non-BAT), there were no cold-induced changes in PDFF or T2*. Rewarming did not result in a change in SCV PDFF or T2*.

CONCLUSIONS

The rapid cold-induced decline in SCV PDFF suggests that in humans BAT is activated quickly in response to cold and that TAG is a primary substrate.

The gut microbiota modulates both browning of white adipose tissue and the activity of brown adipose tissue

Given the increasing worldwide prevalence of obesity and associated metabolic disturbances, novel therapeutic strategies are imperatively required. A plausible manner to increase energy expenditure is the enhancement of thermogenic pathways in white (WAT) and brown adipose tissue (BAT). In the last 15 years, the identification of novel endogenous mechanisms to promote BAT activity or browning of WAT has pointed at gut microbiota as an important modulator of host metabolic homeostasis and energy balance. In this review, we focused on the relationship between gut microbiota composition and adipose tissue thermogenic program (including BAT activity and browning of WAT) in both physiological and stress conditions. Specifically, we reviewed the effects of fasting, caloric restriction, cold stress and metabolic endotoxemia on both browning and gut microbiota shifts. Mechanistically speaking, processes related to bile acid metabolism and the endocannabinoid system seem to play an important role. In summary, the gut microbiota seems to impact WAT and BAT physiology at multiple levels.

Metabolically Active Brown Adipose Tissue Is Found in Adult Subjects with Type 1 Diabetes

Type 1 diabetes (T1D) is characterized by the loss of insulin-producing cells and hence insulin secretion and metabolic control. In addition to insulin, there are a number of hormones and cytokines that influence metabolism, and many of these can be secreted from brown adipose tissue (BAT). However, the presence and activity of BAT in T1D have not been studied, despite the fact that preclinical studies have shown that transplantation of BAT in mouse models of T1D can restore metabolic control. The metabolic activity of BAT, white adipose tissue (WAT), and skeletal muscle was investigated in patients with T1D (n = 11) by 2-deoxy-2-(¹⁸F)fluoro-D-glucose PET/CT after cold stimulation. Functional BAT was detected in 4 out of 11 individuals with T1D with a prevalence of 36%. The glucose utilization rate in the supraclavicular BAT regions ranged from 0.75–38.7 µmol × min⁻¹ × 100 g⁻¹. The glucose utilization per gram tissue was higher in BAT when compared with both WAT (p = 0.049) and skeletal muscle (p = 0.039). However, no correlation between BAT activity and metabolic control or insulin requirements was found. In conclusion, for the first time, cold-induced BAT was detected in patients with T1D with a wide range in metabolic activity. Contrary to findings in animal models, the metabolic activity of BAT had negligible impact on insulin requirements or metabolic control in T1D under normal physiological conditions.

Specific knockout of p85α in brown adipose tissue induces resistance to high-fat diet-induced obesity and its metabolic complications in male mice

Objective

An increase in mass and/or brown adipose tissue (BAT) functionality leads to an increase in energy expenditure, which may be beneficial for the prevention and treatment of obesity. Moreover, distinct class I PI3K isoforms can participate in metabolic control as well as in systemic dysfunctions associated with obesity. In this regard, we analyzed in vivo whether the lack of p85α in BAT (BATp85αKO) could modulate the activity and insulin signaling of this tissue, thereby improving diet-induced obesity and its associated metabolic complications.

Methods

We generated BATp85αKO mice using Cre-LoxP technology, specifically deleting p85α in a conditional manner. To characterize this new mouse model, we used mice of 6 and 12 months of age. In addition, BATp85αKO mice were submitted to a high-fat diet (HFD) to challenge BAT functionality.

Results

Our results suggest that the loss of p85α in BAT improves its thermogenic functionality, high-fat diet–induced adiposity and body weight, insulin resistance, and liver steatosis. The potential mechanisms involved in the improvement of obesity include (1) increased insulin signaling and lower activation of JNK in BAT, (2) enhanced insulin receptor isoform B (IRB) expression and association with IRS-1 in BAT, (3) lower production of proinflammatory cytokines by the adipose organ, (4) increased iWAT browning, and (5) improved liver steatosis.

Conclusions

Our results provide new mechanisms involved in the resistance to obesity development, supporting the hypothesis that the gain of BAT activity induced by the lack of p85α has a direct impact on the prevention of diet-induced obesity and its associated metabolic complications.

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The lack of p85α in brown adipose tissue confers obesity resistance.

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BATp85αKO mice show improved thermogenic function, fatty liver and insulin resistance.

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High IRB levels in BAT and iWAT browning might explain the improvement of obesity.

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Increase in BAT functionality has a direct impact on the prevention of obesity.

Whole-body and adipose tissue-specific mechanisms underlying the metabolic effects of fibroblast growth factor 21 in the Siberian hamster

Objective

Fibroblast growth factor 21 (FGF21) has been shown to rapidly lower body weight in the Siberian hamster, a preclinical model of adiposity. This induced negative energy balance mediated by FGF21 is associated with both lowered caloric intake and increased energy expenditure. Previous research demonstrated that adipose tissue (AT) is one of the primary sites of FGF21 action and may be responsible for its ability to increase the whole-body metabolic rate. The present study sought to determine the relative importance of white (subcutaneous AT [sWAT] and visceral AT [vWAT]), and brown (interscapular brown AT [iBAT]) in governing FGF21-mediated metabolic improvements using the tissue-specific uptake of glucose and lipids as a proxy for metabolic activity.

Methods

We used positron emission tomography-computed tomography (PET-CT) imaging in combination with both glucose (¹⁸F-fluorodeoxyglucose) and lipid (¹⁸F-4-thiapalmitate) tracers to assess the effect of FGF21 on the tissue-specific uptake of these metabolites and compared responses to a control group pair-fed to match the food intake of the FGF21-treated group. In vivo imaging was combined with ex vivo tissue-specific functional, biochemical, and molecular analyses of the nutrient uptake and signaling pathways.

Results

Consistent with previous findings, FGF21 reduced body weight via reduced caloric intake and increased energy expenditure in the Siberian hamster. PET-CT studies demonstrated that FGF21 increased the uptake of glucose in BAT and WAT independently of reduced food intake and body weight as demonstrated by imaging of the pair-fed group. Furthermore, FGF21 increased glucose uptake in the primary adipocytes, confirming that these in vivo effects may be due to a direct action of FGF21 at the level of the adipocytes. Mechanistically, the effects of FGF21 are associated with activation of the ERK signaling pathway and upregulation of GLUT4 protein content in all fat depots. In response to treatment with FGF21, we observed an increase in the markers of lipolysis and lipogenesis in both the subcutaneous and visceral WAT depots. In contrast, FGF21 was only able to directly increase the uptake of lipid into BAT.

Conclusions

These data identify brown and white fat depots as primary peripheral sites of action of FGF21 in promoting glucose uptake and also indicate that FGF21 selectively stimulates lipid uptake in brown fat, which may fuel thermogenesis.

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FGF21 increases glucose and lipid uptake in adipose tissue.

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The selective FGF21-induced increase in lipid uptake in BAT may fuel thermogenesis.

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Unlike BAT, glucose uptake in WAT may be used for lipogenesis.

Imaging Metabolically Active Fat: A Literature Review and Mechanistic Insights

Currently, obesity is one of the leading causes death in the world. Shortly before 2000, researchers began describing metabolically active adipose tissue on cancer-surveillance ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in adult humans. This tissue generates heat through mitochondrial uncoupling and functions similar to classical brown and beige adipose tissue in mice. Despite extensive research, human brown/beige fat's role in resistance to obesity in humans has not yet been fully delineated. FDG uptake is the de facto gold standard imaging technique when studying brown adipose tissue, although it has not been rigorously compared to other techniques. We, therefore, present a concise review of established and emerging methods to image brown adipose tissue activity in humans. Reviewed modalities include anatomic imaging with CT and magnetic resonance imaging (MRI); molecular imaging with FDG, fatty acids, and acetate; and emerging techniques. FDG-PET/CT is the most commonly used modality because of its widespread use in cancer imaging, but there are mechanistic reasons to believe other radiotracers may be more sensitive and accurate at detecting brown adipose tissue activity. Radiation-free modalities may help the longitudinal study of brown adipose tissue activity in the future.

Brown Adipose Tissue Response to Cold Stimulation Is Reduced in Girls With Autoimmune Hypothyroidism

Objective

The interaction between thyroid status and brown adipose tissue (BAT) activation is complex. We assessed the effect of autoimmune hypothyroidism (ATD) in female children on BAT activation, measured using infrared thermography.

Design

Twenty-six female participants (14 with ATD and 12 healthy controls) between 5 and 17 years of age attended a single study session. Thermal images were taken of the supraclavicular region before, and after, the introduction of a cool stimulus.

Results

Participants with ATD had lower resting (hypothyroid, 34.9 ± 0.7°C; control, 35.4 ± 0.5°C; P = 0.03) and stimulated (hypothyroid, 35.0 ± 0.6°C; control, 35.5 ± 0.5°C; P = 0.04) supraclavicular temperatures compared with controls, but there was no difference between groups in the temperature increase with stimulation. BAT activation, calculated as the relative temperature change comparing the supraclavicular temperature to a sternal reference region, was reduced in participants with ATD (hypothyroid, 0.1 ± 0.1°C; control, 0.2 ± 0.2°C; P = 0.04). Children with ATD were frequently biochemically euthyroid due to replacement therapy, but, despite this, increased relative supraclavicular temperature was closely associated with increased TSH (r = 0.7, P = 0.01) concentrations.

Conclusions

Girls with ATD had an attenuated thermogenic response to cold stimulation compared with healthy controls, but, contrary to expectation, those with suboptimal biochemical control (with higher TSH) showed increased BAT activation. This suggests that the underlying disease process may have a negative effect on BAT response, but high levels of TSH can mitigate, and even stimulate, BAT activity. In summary, thyroid status is a complex determinant of BAT activity in girls with ATD.

Quantification of the Capacity for Cold-Induced Thermogenesis in Young Men With and Without Obesity

OBJECTIVE

Cold exposure increases energy expenditure (EE) and could have a role in combating obesity. To understand this potential, we determined the capacity for cold-induced thermogenesis (CIT), the EE increase above the basal metabolic rate at the individualized coldest tolerable temperature before overt shivering.

DESIGN

During a 13-day inpatient protocol, we quantitated the EE of 12 lean men and 9 men with obesity at various randomly ordered ambient temperatures in a room calorimeter. Subjects underwent brown fat imaging after exposure to their coldest tolerable temperature.

RESULTS

CIT capacity was 300 ± 218 kcal/d (mean ± SD) or 17 ± 11% in lean men and 125 ± 146 kcal/d or 6 ± 7% in men with obesity (P = 0.01). The temperature below which EE increased, lower critical temperature (Tlc), was warmer in lean men than men with obesity (22.9 ± 1.2 vs 21.1 ± 1.7°C, P = 0.03), but both had similar skin temperature (Tskin) changes and coldest tolerable temperatures. Whereas lean subjects had higher brown fat activity, skeletal muscle activity increased synchronously with CIT beginning at the Tlc in both groups, indicating that muscle is recruited for CIT in parallel with brown fat, not sequentially after nonshivering thermogenesis is maximal.

CONCLUSIONS

Despite greater insulation from fat, men with obesity had a narrower range of tolerable cool temperatures available for increasing EE and less capacity for CIT than lean men, likely as a result of greater basal heat production and similar perception to Tskin cooling. Further study of the reduced CIT capacity in men with obesity may inform treatment opportunities for obesity.

Lipokines and Thermogenesis

Adaptive thermogenesis is a catabolic process that consumes energy-storing molecules and expends that energy as heat in response to environmental changes. This process occurs primarily in brown and beige adipose tissue. Thermogenesis is regulated by many factors, including lipid derived paracrine and endocrine hormones called lipokines. Recently, technologic advances for identifying new lipid biomarkers of thermogenic activity have shed light on a diverse set of lipokines that act through different pathways to regulate energy expenditure. In this review, we highlight a few examples of lipokines that regulate thermogenesis. The biosynthesis, regulation, and effects of the thermogenic lipokines in several families are reviewed, including oloeylethanolamine, endocannabinoids, prostaglandin E2, and 12,13-diHOME. These thermogenic lipokines present potential therapeutic targets to combat states of excess energy storage, such as obesity and related metabolic disorders.

Cold exposure induces dynamic, heterogeneous alterations in human brown adipose tissue lipid content

Brown adipose tissue undergoes a dynamic, heterogeneous response to cold exposure that can include the simultaneous synthesis, uptake, and oxidation of fatty acids. The purpose of this work was to quantify these changes in brown adipose tissue lipid content (fat-signal fraction (FSF)) using fat-water magnetic resonance imaging during individualized cooling to 3 °C above a participant's shiver threshold. Eight healthy men completed familiarization, perception-based cooling, and MRI-cooling visits. FSF maps of the supraclavicular region were acquired in thermoneutrality and during cooling (59.5 ± 6.5 min). Brown adipose tissue regions of interest were defined, and voxels were grouped into FSF decades (0-10%, 10-20%…90-100%) according to their initial value. Brown adipose tissue contained a heterogeneous morphology of lipid content. Voxels with initial FSF values of 60-100% (P < 0.05) exhibited a significant decrease in FSF while a simultaneous increase in FSF occurred in voxels with initial FSF values of 0-30% (P < 0.05). These data suggest that in healthy young men, cold exposure elicits a dynamic and heterogeneous response in brown adipose tissue, with areas initially rich with lipid undergoing net lipid loss and areas of low initial lipid undergoing a net lipid accumulation.