Exosomal microRNA miR-92a concentration in serum reflects human brown fat activity.

Brown adipose tissue (BAT) dissipates energy and its activity correlates with leanness in human adults. (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography coupled with computer tomography (PET/CT) is still the standard for measuring BAT activity, but exposes subjects to ionizing radiation. To study BAT function in large human cohorts, novel diagnostic tools are needed. Here we show that brown adipocytes release exosomes and that BAT activation increases exosome release. Profiling miRNAs in exosomes released from brown adipocytes, and in exosomes isolated from mouse serum, we show that levels of miRNAs change after BAT activation in vitro and in vivo. One of these exosomal miRNAs, miR-92a, is also present in human serum exosomes. Importantly, serum concentrations of exosomal miR-92a inversely correlate with human BAT activity measured by (18)F-FDG PET/CT in two unique and independent cohorts comprising 41 healthy individuals. Thus, exosomal miR-92a represents a potential serum biomarker for BAT activity in mice and humans.

Short-term Cold Acclimation Recruits Brown Adipose Tissue in Obese Humans.

Recruitment of brown adipose tissue (BAT) has emerged as a potential tool to combat obesity and associated metabolic complications. Short-term cold acclimation has been shown not only to enhance the presence and activity of BAT in lean humans but also to improve the metabolic profile of skeletal muscle to benefit glucose uptake in patients with type 2 diabetes. Here we examined whether short-term cold acclimation also induced such adaptations in 10 metabolically healthy obese male subjects. A 10-day cold acclimation period resulted in increased cold-induced glucose uptake in BAT, as assessed by [(18)F]fluorodeoxyglucose positron emission tomography/computed tomography. BAT activity was negatively related to age, with a similar trend for body fat percentage. In addition, cold-induced glucose uptake in BAT was positively related to glucose uptake in visceral white adipose tissue, although glucose uptake in visceral and subcutaneous white adipose tissue depots was unchanged upon cold acclimation. Cold-induced skeletal muscle glucose uptake tended to increase upon cold acclimation, which was paralleled by increased basal GLUT4 localization in the sarcolemma, as assessed through muscle biopsies. Proximal skin temperature was increased and subjective responses to cold were slightly improved at the end of the acclimation period. These metabolic adaptations to prolonged exposure to mild cold may lead to improved glucose metabolism or prevent the development of obesity-associated insulin resistance and hyperglycemia.

Supraclavicular skin temperature and BAT activity in lean healthy adults.

The 'gold standard' for measuring brown adipose tissue (BAT) in humans is [(18)F]FDG-PET/CT-imaging. With this technique subjects are exposed to ionizing radiation and are therefore limited in the number of scans that can be performed. We investigated the relation between supraclavicular skin temperatures and BAT activity values using a strictly temperature-controlled air-cooling protocol. Data of 36 male subjects was analyzed. BAT activity was evaluated by [(18)F]FDG-PET/CT-imaging and skin temperature was measured by means of wireless temperature sensors. Supraclavicular skin temperature dropped less compared to skin temperatures at other sites (all P values <0.01). A significant positive correlation was found between the change in supraclavicular skin temperature with BAT activity (R (2) 0.23), and the change in supraclavicular skin temperature and non-shivering thermogenesis (R (2) 0.18, both P values <0.01). The correlations indicate that supraclavicular skin temperature (changes) can potentially be used as a qualitative measure of BAT activity and BAT thermogenesis.

Short-term cold acclimation improves insulin sensitivity in patients with type 2 diabetes mellitus.

Cold exposure may be a potential therapy for diabetes by increasing brown adipose tissue (BAT) mass and activity. Here we report that 10 d of cold acclimation (14-15 °C) increased peripheral insulin sensitivity by ∼43% in eight type 2 diabetes subjects. Basal skeletal muscle GLUT4 translocation markedly increased, without effects on insulin signaling or AMP-activated protein kinase (AMPK) activation and only a minor increase in BAT glucose uptake.

Cold acclimation affects immune composition in skeletal muscle of healthy lean subjects.

Low environmental temperatures have a profound effect on biological processes in the body, including the immune system. Cold exposure coincides with hormonal changes, which may directly or indirectly alter the immune system, even in the skeletal muscle. The aim of the present study was to investigate the effect of cold acclimation on immune composition in skeletal muscle. Skeletal muscle biopsies were obtained from 17 healthy lean subjects before and after 10 days of mild cold exposure (15 °: C, 6 h/day). Nonshivering thermogenesis was calculated by indirect calorimetry. We found that cold acclimation increased nonshivering thermogenesis from 10.8 ± 7.5 before to 17.8 ± 11.1% after cold acclimation (P < 0.01), but did not affect plasma catecholamine nor cytokine levels. In contrast, cold acclimation affected mRNA expression of several immune cell markers in skeletal muscle. It downregulated expression of the Th17 markers RORC (-28%, P < 0.01) and NEDD4L (-15%, P < 0.05), as well as the regulatory T-cell marker FOXP3 (-13%, P < 0.05). Furthermore, cold acclimation downregulated expression of the M2 macrophage markers CCL22 (-50%, P < 0.05), CXCL13 (-17%, P < 0.05) and CD209 (-15%, P < 0.05), while the M1 macrophage marker IL12B was upregulated (+141%, P < 0.05). Cold acclimation also enhanced several markers related to interferon (IFN) signaling, including TAP1 (+12%, P < 0.01), IFITM1/3 (+11%, P < 0.05), CD274 (+36%, P < 0.05) and STAT 2 (+10%, P < 0.05). In conclusion, 10 days of intermittent cold exposure induces marked changes in the expression of immune cell markers in skeletal muscle of healthy lean subjects. The physiological consequences and therapeutic relevance of these changes remain to be determined.

Serum FGF21 levels are associated with brown adipose tissue activity in humans.

The obesity pandemic has spurred a need for novel therapies to prevent and treat metabolic complications. The recent rediscovery of brown adipose tissue (BAT) in humans made this tissue a possible therapeutic target, due to its potentially substantial contributions to energy homeostasis. Fibroblast growth factor 21 (FGF21) has been identified as a facilitator of cold-induced thermogenesis in humans. Furthermore, pre-clinical studies revealed that FGF21 administration leads to improvement in the metabolic consequences of obesity, such as dyslipidemia and type 2 diabetes. Here we studied plasma FGF21 levels in two cohorts of human subjects, in whom BAT activity was determined using an individualized cooling protocol by [(18)F]FDG-PET/CT scan. Importantly, we found that circulating FGF21 levels correlated with BAT activity during acute cold exposure in male subjects. In addition, FGF21 levels were related to the change in core temperature upon acute cold exposure, indicating a role for FGF21 in maintaining normothermia, possibly via activation of BAT. Furthermore, cold acclimation increased BAT activity in parallel with increased FGF21 levels. In conclusion, our results demonstrate that FGF21 levels in humans are related to BAT activity, suggesting that FGF21 may represent a novel mechanism via which BAT activity in humans may be enhanced.

Cold-activated brown adipose tissue in human adults: methodological issues.

The relevance of functional brown adipose tissue (BAT) depots in human adults was undisputedly proven approximately seven years ago. Here we give an overview of all dedicated studies that were published on cold-induced BAT activity in adult humans that appeared since then. Different cooling protocols and imaging techniques to determine BAT activity are reviewed. BAT activation can be achieved by means of air- or water-cooling protocols. The most promising approach is individualized cooling, during which subjects are studied at the lowest temperature for nonshivering condition, probably revealing maximal nonshivering thermogenesis. The highest BAT prevalence (i.e., close to 100%) is observed using the individualized cooling protocol. Currently, the most widely used technique to study the metabolic activity of BAT is deoxy-2-[18F]fluoro-d-glucose ([18F]FDG)-positron emission tomography/computed tomography (PET/CT) imaging. Dynamic imaging provides quantitative information about glucose uptake rates, whereas static imaging reflects overall BAT glucose uptake, localization, and distribution. In general, standardized uptake values (SUV) are used to quantify BAT activity. An accurate determination of total BAT volume is hampered by the limited spatial resolution of the PET image, leading to spillover. Different research groups use different SUV threshold values, which make it difficult to directly compare BAT activity levels between studies. Another issue is the comparison of [18F]FDG uptake in BAT with respect to other tissues or upon with baseline values. This comparison can be performed by using the "fixed volume" methodology. Finally, the potential use of other relatively noninvasive methods to quantify BAT, like magnetic resonance imaging or thermography, is discussed.

Cold acclimation recruits human brown fat and increases nonshivering thermogenesis.

In recent years, it has been shown that humans have active brown adipose tissue (BAT) depots, raising the question of whether activation and recruitment of BAT can be a target to counterbalance the current obesity pandemic. Here, we show that a 10-day cold acclimation protocol in humans increases BAT activity in parallel with an increase in nonshivering thermogenesis (NST). No sex differences in BAT presence and activity were found either before or after cold acclimation. Respiration measurements in permeabilized fibers and isolated mitochondria revealed no significant contribution of skeletal muscle mitochondrial uncoupling to the increased NST. Based on cell-specific markers and on uncoupling protein-1 (characteristic of both BAT and beige/brite cells), this study did not show "browning" of abdominal subcutaneous white adipose tissue upon cold acclimation. The observed physiological acclimation is in line with the subjective changes in temperature sensation; upon cold acclimation, the subjects judged the environment warmer, felt more comfortable in the cold, and reported less shivering. The combined results suggest that a variable indoor environment with frequent cold exposures might be an acceptable and economic manner to increase energy expenditure and may contribute to counteracting the current obesity epidemic.

Brown adipose tissue activity after a high-calorie meal in humans.

BACKGROUND: Studies in rodents have shown that brown adipose tissue (BAT) is activated on food intake, thereby reducing metabolic efficiency. OBJECTIVE: The current study investigated whether a single high-calorie, carbohydrate-rich meal activates BAT in lean human adults. DESIGN: BAT activity was studied in 11 lean adult men [age: 23.6 ± 2.1 y; body mass index (BMI; in kg/m(2)): 22.4 ± 2.1] after consumption of a high-calorie, carbohydrate-rich meal (1622 ± 222 kcal; 78% carbohydrate, 12% protein, 10% fat). BAT activity during 2 h of mild cold exposure served as a positive control experiment. BAT activity was assessed by [(18)F]fluorodeoxyglucose (FDG)-positron emission tomography-computed tomography. Energy expenditure was measured by indirect calorimetry. RESULTS: Postprandial [(18)F]FDG uptake was significantly higher in BAT [1.65 ± 0.99 mean standard uptake value (SUVmean)] than in subcutaneous (0.35 ± 0.15 SUVmean; P < 0.05) and visceral (0.49 ± 0.24 SUVmean; P < 0.05) white adipose tissue and liver (0.95 ± 0.28 SUVmean; P < 0.05). Postprandial BAT activity was lower than cold-induced BAT activity (7.19 ± 2.09 SUVmean). However, postprandial BAT activity may have been underestimated because of high postprandial [(18)F]FDG uptake in skeletal muscle compared with cold (1.36 ± 0.31 compared with 0.59 ± 0.07 SUVmean, P < 0.05), which reduces [(18)F]FDG bioavailability for BAT and other tissues. No direct relation was found between BAT and diet-induced thermogenesis (DIT). CONCLUSIONS: Glucose uptake in BAT increases after a meal in humans, which indicates a role for BAT in reducing metabolic efficiency. However, the quantitative contribution of BAT to DIT relative to other tissues, such as skeletal muscle, remains to be investigated. This trial was registered at www.controlled-trials.com as ISRCTN21413505.

Imaging cold-activated brown adipose tissue using dynamic T2*-weighted magnetic resonance imaging and 2-deoxy-2-[18F]fluoro-D-glucose positron emission tomography.

OBJECTIVES: The objective of this study was to explore the use of magnetic resonance imaging (MRI) to identify and quantify active brown adipose tissue (BAT) in adult humans. 2-Deoxy-2-[F]fluoro-D-glucose (FDG) positron emission tomography (PET) combined with computed tomography was used as a reference method to identify active BAT depots and to guide the MRI data analysis. MATERIALS AND METHODS: The ethics committee of the institute approved the protocol, and all participants provided written informed consent before participation. Both PET combined with computed tomography and MRI of BAT were performed in 11 healthy volunteers. Brown adipose tissue was activated by cooling the participants using a dedicated water-perfused suit. For the MRI examination of BAT, water-fat imaging and dynamic T2* imaging were performed at an effective temporal resolution of 2 minutes per volume. Water-fat images were derived from a multiecho MRI sequence using the Dixon technique. RESULTS: 2-Deoxy-2-[F]fluoro-D-glucose-PET identified active BAT in 8 of the 11 participants. Water-fat MRI showed that BAT depots had a fat fraction of 65.2% (7.0%) compared with 81.5% (5.4%) for the subcutaneous white adipose tissue (paired difference of 16.3% [4.9%]; P < 0.05). Dynamic T2* imaging during cold stimulation revealed signal fluctuations that were sensitive to BAT activation. The presence of these components correlated with BAT activation quantified from FDG-PET (r = 0.63; P < 0.05). CONCLUSIONS: Although FDG-PET has superior contrast for identifying active BAT, the MRI temporal resolution provides insight in activation dynamics. In addition, the flexibility of MRI allows for simultaneous mapping of tissue fat content and functional responses. The results indicate that MRI is a promising addition to PET for the identification of BAT and its activity responses to stimulation. An MRI-based methodology to quantify BAT activity is a highly desirable step in addressing the role of BAT in obesity disorders.

Systemic ß-adrenergic stimulation of thermogenesis is not accompanied by brown adipose tissue activity in humans.

Brown adipose tissue (BAT) is currently considered as a target to combat obesity and diabetes in humans. BAT is densely innervated by the sympathetic nervous system (SNS) and can be stimulated by ß-adrenergic agonists, at least in animals. However, the exact role of the ß-adrenergic part of the SNS in BAT activation in humans is not known yet. In this study, we measured BAT activity by 2-deoxy-2-[(18)F]fluoro-d-glucose ([(18)F]FDG) positron emission tomography/computed tomography imaging in 10 lean men during systemic infusion of the nonselective ß-agonist isoprenaline (ISO) and compared this with cold-activated BAT activity. ISO successfully mimicked sympathetic stimulation as shown by increased cardiovascular and metabolic activity. Energy expenditure increased to similar levels as during cold exposure. Surprisingly, BAT was not activated during ß-adrenergic stimulation. We next examined whether the high plasma free fatty acid (FFA) levels induced by ISO competed with glucose ([(18)F]FDG) uptake in BAT locations by blocking lipolysis with acipimox (ACI). ACI successfully lowered plasma FFA, but did not increase [(18)F]FDG-uptake in BAT. We therefore conclude that systemic nonselective ß-adrenergic stimulation by ISO at concentrations that increase energy expenditure to the same extent as cold exposure does not activate BAT in humans, indicating that other tissues are responsible for the increased ß-adrenergic thermogenesis.