Increased expression of glucose transporter GLUT-4 in brown adipose tissue of fasted rats after cold exposure

Ozone modified hypothalamic signaling enhancing thermogenesis in the TDP-43A315T transgenic model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS), a devastating progressive neurodegenerative disease, has no effective treatment. Recent evidence supports a strong metabolic component in ALS pathogenesis. Indeed, metabolic abnormalities in ALS correlate to disease susceptibility and progression, raising additional therapeutic targets against ALS. Ozone (O₃), a natural bioactive molecule, has been shown to elicit beneficial effects to reduce metabolic disturbances and improved motor behavior in TDP-43^A315T mice. However, it is fundamental to determine the mechanism through which O₃ acts in ALS. To characterize the association between O₃ exposure and disease-associated weight loss in ALS, we assessed the mRNA and protein expression profile of molecular pathways with a main role in the regulation of the metabolic homeostasis on the hypothalamus and the brown adipose tissue (BAT) at the disease end-stage, in TDP-43^A315T mice compared to age-matched WT littermates. In addition, the impact of O₃ exposure on the faecal bacterial community diversity, by Illumina sequencing, and on the neuromuscular junctions (NMJs), by confocal imaging, were analysed. Our findings suggest the effectiveness of O₃ exposure to induce metabolic effects in the hypothalamus and BAT of TDP-43^A315T mice and could be a new complementary non-pharmacological approach for ALS therapy.

Effect of Acute Cold Exposure on Energy Metabolism and Activity of Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis

Background: The benefit of cold exposure for humans against obesity has brought the energy metabolism and activity of brown adipose tissue (BAT) induced by cold into focus. But the results are inconsistent. This review is aimed to systematically explore the effect of cold exposure on the activity of BAT and energy metabolism in humans.

Methods: We searched relevant papers that were published from 1990 to 2021 and were cited in PubMed Central, Web of science, Embase and Cochrane Library databases to conduct this systematic review and meta-analysis. Energy metabolism, BAT volume, BAT activity and non-esterified fatty acids (NEFA) data reported in eligible researches were extracted. Meta-analysis was applied to combine the mean difference or standard mean difference with their 95% confidence intervals (95%CI). RevMan 5.3 software was used for meta-analysis and evaluating the risk of bias. Stata 16.0 was used for evaluating the publication bias.

Results: Ten randomized controlled trials were included in meta-analysis. Compared with human exposed in room temperature at 24°C, the energy expenditure (EE) was increased after acute cold exposure at 16∼19°C (Z = 7.58, p < 0.05, mean different = 188.43kal/d, 95% CI = 139.73–237.13); BAT volume (Z = 2.62, p < 0.05; standard mean different = 0.41, 95% CI = 0.10–0.73); BAT activity (Z = 2.05, p = 0.04, standard mean difference = 1.61, 95% CI = 0.07–3.14) and the intake of BAT NEFA (Z = 2.85, p < 0.05; standard mean different = 0.53, 95% CI = 0.17–0.90) also increased.

Conclusion: Acute cold exposure could improve the energy expenditure and BAT activity in adults, which is beneficial for human against obesity.

Brown Fat as a Regulator of Systemic Metabolism beyond Thermogenesis

Brown adipose tissue (BAT) is a specialized tissue for nonshivering thermogenesis to dissipate energy as heat. Although BAT research has long been limited mostly in small rodents, the rediscovery of metabolically active BAT in adult humans has dramatically promoted the translational studies on BAT in health and diseases. Moreover, several remarkable advancements have been made in brown fat biology over the past decade: The molecular and functional analyses of inducible thermogenic adipocytes (socalled beige adipocytes) arising from a developmentally different lineage from classical brown adipocytes have been accelerated. In addition to a well-established thermogenic activity of uncoupling protein 1 (UCP1), several alternative thermogenic mechanisms have been discovered, particularly in beige adipocytes. It has become clear that BAT influences other peripheral tissues and controls their functions and systemic homeostasis of energy and metabolic substrates, suggesting BAT as a metabolic regulator, other than for thermogenesis. This notion is supported by discovering that various paracrine and endocrine factors are secreted from BAT. We review the current understanding of BAT pathophysiology, particularly focusing on its role as a metabolic regulator in small rodents and also in humans.

Examining the benefits of cold exposure as a therapeutic strategy for obesity and type 2 diabetes

The pathogenesis of metabolic diseases such as obesity and type 2 diabetes are characterized by a progressive dysregulation in energy partitioning, often leading to end-organ complications. One emerging approach proposed to target this metabolic dysregulation is the application of mild cold exposure. In healthy individuals, cold exposure can increase energy expenditure and whole body glucose and fatty acid utilization. Repeated exposures can lower fasting glucose and insulin levels and improve dietary fatty acid handling, even in healthy individuals. Despite its apparent therapeutic potential, little is known regarding the effects of cold exposure in populations for which this stimulation could benefit the most. The few studies available have shown that both acute and repeated exposures to the cold can improve insulin sensitivity and reduce fasting glycemia in individuals with type 2 diabetes. However, critical gaps remain in understanding the prolonged effects of repeated cold exposures on glucose regulation and whole body insulin sensitivity in individuals with metabolic syndrome. Much of the metabolic benefits appear to be attributable to the recruitment of shivering skeletal muscles. However, further work is required to determine whether the broader recruitment of skeletal muscles observed during cold exposure can confer metabolic benefits that surpass what has been historically observed from endurance exercise. In addition, although cold exposure offers unique cardiovascular responses for a physiological stimulus that increases energy expenditure, further work is required to determine how acute and repeated cold exposure can impact cardiovascular responses and myocardial function across a broader scope of individuals.

Brown Adipose Tissue and Its Role in Insulin and Glucose Homeostasis

The increased worldwide prevalence of obesity, insulin resistance, and their related metabolic complications have prompted the scientific world to search for new possibilities to combat obesity. Brown adipose tissue (BAT), due to its unique protein uncoupling protein 1 (UPC1) in the inner membrane of the mitochondria, has been acknowledged as a promising approach to increase energy expenditure. Activated brown adipocytes dissipate energy, resulting in heat production. In other words, BAT burns fat and increases the metabolic rate, promoting a negative energy balance. Moreover, BAT alleviates metabolic complications like dyslipidemia, impaired insulin secretion, and insulin resistance in type 2 diabetes. The aim of this review is to explore the role of BAT in total energy expenditure, as well as lipid and glucose homeostasis, and to discuss new possible activators of brown adipose tissue in humans to treat obesity and metabolic disorders.

Thermogenesis in Adipose Tissue Activated by Thyroid Hormone

Thermogenesis is the production of heat that occurs in all warm-blooded animals. During cold exposure, there is obligatory thermogenesis derived from body metabolism as well as adaptive thermogenesis through shivering and non-shivering mechanisms. The latter mainly occurs in brown adipose tissue (BAT) and muscle; however, white adipose tissue (WAT) also can undergo browning via adrenergic stimulation to acquire thermogenic potential. Thyroid hormone (TH) also exerts profound effects on thermoregulation, as decreased body temperature and increased body temperature occur during hypothyroidism and hyperthyroidism, respectively. We have termed the TH-mediated thermogenesis under thermoneutral conditions “activated” thermogenesis. TH acts on the brown and/or white adipose tissues to induce uncoupled respiration through the induction of the uncoupling protein (Ucp1) to generate heat. TH acts centrally to activate the BAT and browning through the sympathetic nervous system. However, recent studies also show that TH acts peripherally on the BAT to directly stimulate Ucp1 expression and thermogenesis through an autophagy-dependent mechanism. Additionally, THs can exert Ucp1-independent effects on thermogenesis, most likely through activation of exothermic metabolic pathways. This review summarizes thermogenic effects of THs on adipose tissues.

Uncoupling protein 1 and the capacity for nonshivering thermogenesis are components of the glucose homeostatic system

Uncoupling protein 1 (Ucp1) provides nonshivering thermogenesis (NST) fueled by the dissipation of energy from macronutrients in brown and brite adipocytes. The availability of thermogenic fuels is facilitated by the uptake of extracellular glucose. This conjunction renders thermogenic adipocytes in brown and white adipose tissue (WAT) a potential target against obesity and glucose intolerance. We employed wild-type (WT) and Ucp1-ablated mice to elucidate this relationship. In three experiments of similar setup, Ucp1-ablated mice fed a high-fat diet (HFD) had either reduced or similar body mass gain, food intake, and metabolic efficiency compared with WT mice, challenging the hypothesized role of this protein in the development of diet-induced obesity. Despite the absence of increased body mass, oral glucose tolerance was robustly impaired in Ucp1-ablated mice in response to HFD. Postprandial glucose uptake was attenuated in brown adipose tissue but enhanced in subcutaneous WAT of Ucp1-ablated mice. These differences were explainable by expression of the insulin-responsive member 4 of the facilitated glucose transporter family and fully in line with the capacity for NST in these very tissues. Thus, the postprandial glucose uptake of adipose tissues serves as a surrogate measure for Ucp1-dependent and independent capacity for NST. Collectively, our findings corroborate Ucp1 as a modulator of adipose tissue glucose uptake and systemic glucose homeostasis but challenge its hypothesized causal effect on the development of obesity.

Hypothermia induces changes in the alternative splicing pattern of cold-inducible RNA-binding protein transcripts in a non-hibernator, the mouse

Cold-inducible RNA-binding protein (CIRBP) plays important roles in protection against harmful effects of cold temperature. We previously found that several splicing variants of CIRBP mRNA are constitutively expressed in the heart of non-hibernating euthermic hamsters and that one of the variants is predominantly expressed with remarkable reduction in the expression of other variants in hibernating hypothermic hamsters. The aim of this study was to determine whether the regulation of alternative splicing is a common function in a non-hibernator, the mouse. The expression of CIRBP mRNA was assessed by RT-PCR. In euthermic control mice, several splicing variants of CIRBP mRNA were detected in various organs. When hypothermia was induced in mice by using isoflurane anesthesia, the short form variant, which encodes full-length functional CIRBP, was predominantly detected. Keeping body temperature of anesthetized mice at 37°C prevented changes in the splicing pattern. Exposure of mice to a low temperature did not change the splicing pattern, suggesting that endogenous neuronal and/or humoral pathways activated in response to cold stimuli applied to the body surface play minor roles. In agreement with this, the shift in alternative splicing was reproduced in isolated leukocytes in vitro when they were incubated at 28°C. Since application of a TRPM8 or TRPA1 agonist at 37°C failed to promote the shift in the splicing pattern, it seems likely that cold-sensitive channels are not involved in the splicing regulation. Therefore, it is probable that a substantial reduction of temperature is a major cause of the regulation of alternative splicing of CIRBP transcripts. The regulatory system of CIRBP expression at the level of alternative splicing, which was originally discovered in the hibernating hamster, commonly exists in non-hibernators such as mice.

During Adipocyte Remodeling, Lipid Droplet Configurations Regulate Insulin Sensitivity through F-Actin and G-Actin Reorganization

Adipocytes have unique morphological traits in insulin sensitivity control. However, how the appearance of adipocytes can determine insulin sensitivity has not been understood. Here, we demonstrate that actin cytoskeleton reorganization upon lipid droplet (LD) configurations in adipocytes plays important roles in insulin-dependent glucose uptake by regulating GLUT4 trafficking. Compared to white adipocytes, brown/beige adipocytes with multilocular LDs exhibited well-developed filamentous actin (F-actin) structure and potentiated GLUT4 translocation to the plasma membrane in the presence of insulin. In contrast, LD enlargement and unilocularization in adipocytes downregulated cortical F-actin formation, eventually leading to decreased F-actin-to-globular actin (G-actin) ratio and suppression of insulin-dependent GLUT4 trafficking. Pharmacological inhibition of actin polymerization accompanied with impaired F/G-actin dynamics reduced glucose uptake in adipose tissue and conferred systemic insulin resistance in mice. Thus, our study reveals that adipocyte remodeling with different LD configurations could be an important factor to determine insulin sensitivity by modulating F/G-actin dynamics.

Decreased Blood Asprosin in Hyperglycemic Menopausal Women as a Result of Whole-Body Cryotherapy Regardless of Metabolic Syndrome

Endocrine dysfunction often occurs in metabolic syndrome (MetS), resulting in hyperglycemia and atherogenic blood lipid profile disorders. Asprosin is a newly discovered glucose-regulating hormone. The study aim was to determine whether the application of whole-body cryotherapy (WBC) affects asprosin and selected adipocytokines as well as insulin resistance in menopausal women with metabolic disorders. A total of 37 menopausal women were exposed to 20 WBC (-130 °C, 3 min). Blood glucose, asprosin, irisin, leptin, adiponectin, and insulin were measured before and after 20 WBC treatments, after which a homeostasis model assessment of insulin resistance (HOMA-IR) and atherogenic index of plasma (AIP) were calculated. The results were analyzed in the MetS group compared to the controls (CON) without MetS, and in the hyperglycemic (HG) group compared to the normoglycemic group (NG). After 20 WBC, a significant reduction (p < 0.05) in asprosin concentration was found in the MetS, HG, and CON groups, and a significant decrease (p < 0.05) in glucose concentration was noted in the HG group. Changes in asprosin concentration positively correlated with changes in glucose concentration. Asprosin concentration before WBC correlated positively with metabolic disorder risk factor levels, and the change in asprosin concentration after 20 WBC correlated negatively with metabolic disorder risk factor levels: fasting glucose, AIP, and the leptin/adiponectin index. Research indicates the possibility of using WBC in supporting metabolic disorders, type 2 diabetes (T2DM), and insulin resistance.

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

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

Imaging of Brown Adipose Tissue: State of the Art

The rates of diabetes, obesity, and metabolic disease have reached epidemic proportions worldwide. In recent years there has been renewed interest in combating these diseases not only by modifying energy intake and lifestyle factors, but also by inducing endogenous energy expenditure. This approach has largely been stimulated by the recent recognition that brown adipose tissue (BAT)-long known to promote heat production and energy expenditure in infants and hibernating mammals-also exists in adult humans. This landmark finding relied on the use of clinical fluorine 18 fluorodeoxyglucose positron emission tomography/computed tomography, and imaging techniques continue to play a crucial and increasingly central role in understanding BAT physiology and function. Herein, the authors review the origins of BAT imaging, discuss current preclinical and clinical strategies for imaging BAT, and discuss imaging methods that will provide crucial insight into metabolic disease and how it may be treated by modulating BAT activity. (©) RSNA, 2016.

Mice fed fish oil diet and upregulation of brown adipose tissue thermogenic markers

PURPOSE

Fish oil (FO) elicits diverse beneficial effects. Reduction in or prevention of body mass (BM) gain in animal models may be associated with modulation of brown adipose tissue (BAT). We aimed to evaluate the effects of different high-fat diets with FO on BAT metabolism and thermogenic markers.

METHODS

C57BL/6 male mice (3-month-old) were fed different diets during 8 weeks: standard-chow diet (SC 10% fat), high-fat lard diet (HF-L 50% fat), high-fat lard plus FO diet (HF-L+FO 50% fat), and high-fat FO diet (HF-FO 50% fat). We evaluated BM and performed an oral glucose tolerance test. At euthanasia, plasma was collected for leptin, and triacylglycerol measurement and interscapular BAT was dissected and stored for molecular analyses.

RESULTS

HF-L group showed elevated BM; glucose intolerance associated with diminished TC10 and GLUT4 expressions; hypertriglyceridemia associated with increased CD36 and diminished CPT1 expression; elevated expression of pro-inflammatory cytokines; and reduced PPAR expression. Furthermore, these animals showed hyperleptinemia with increased expression of thermogenic markers (beta3-AR, PGC1alpha, and UCP1). Conversely, HF-L+FO and HF-FO groups showed reduced BM gain with regularization of glucose tolerance and triglyceridemia, GLUT4, TC10, CD36, CPT1, and cytokines expressions. Both groups exhibited elevated PPAR and thermogenic markers expression in a dose-dependent way.

CONCLUSIONS

FO improves metabolic profile and upregulates thermogenic markers, suggesting an elevated thermogenesis that leads to reduced BM gain.

Neural innervation of white adipose tissue and the control of lipolysis

White adipose tissue (WAT) is innervated by the sympathetic nervous system (SNS) and its activation is necessary for lipolysis. WAT parasympathetic innervation is not supported. Fully-executed SNS-norepinephrine (NE)-mediated WAT lipolysis is dependent on β-adrenoceptor stimulation ultimately hinging on hormone sensitive lipase and perilipin A phosphorylation. WAT sympathetic drive is appropriately measured electrophysiologically and neurochemically (NE turnover) in non-human animals and this drive is fat pad-specific preventing generalizations among WAT depots and non-WAT organs. Leptin-triggered SNS-mediated lipolysis is weakly supported, whereas insulin or adenosine inhibition of SNS/NE-mediated lipolysis is strongly supported. In addition to lipolysis control, increases or decreases in WAT SNS drive/NE inhibit and stimulate white adipocyte proliferation, respectively. WAT sensory nerves are of spinal-origin and sensitive to local leptin and increases in sympathetic drive, the latter implicating lipolysis. Transsynaptic viral tract tracers revealed WAT central sympathetic and sensory circuits including SNS-sensory feedback loops that may control lipolysis.

Brown adipose tissue: endocrine determinants of function and therapeutic manipulation as a novel treatment strategy for obesity

Introduction

Recent observation of brown adipose tissue (BAT) being functional in adult humans provides a rationale for its stimulation to increase energy expenditure through ‘adaptive thermogenesis’ for an anti-obesity strategy. Many endocrine dysfunctions are associated with changes in metabolic rate that over time may result in changes in body weight. It is likely that human BAT plays a role in such processes.

Review

In this brief review article, we explore the endocrine determinants of BAT activity, and discuss how these insights may provide a basis for future developments of novel therapeutic strategies for obesity management.

A review of electronic and print data comprising original and review articles retrieved from PubMed search up to December 2013 was conducted (Search terms: brown adipose tissue, brown fat, obesity, hormone). In addition, relevant references from the articles were screened for papers containing original data.

Conclusion

There is promising data to suggest that targeting endocrine hormones for BAT modulation can yield a cellular bioenergetics answer for successful prevention and management of human obesity. Further understanding of the physiological link between various endocrine hormones and BAT is necessary for the development of new therapeutic options.

Electronic supplementary material

The online version of this article (doi:10.1186/s40608-014-0013-5) contains supplementary material, which is available to authorized users.

Responses of the insulin signaling pathways in the brown adipose tissue of rats following cold exposure

The insulin signaling pathway is critical for the control of blood glucose levels. Brown adipose tissue (BAT) has also been implicated as important in glucose homeostasis. The effect of short-term cold exposure on this pathway in BAT has not been explored. We evaluated the effect of 4 hours of cold exposure on the insulin pathway in the BAT of rats. Whole genomic microarray chips were used to examine the transcripts of the pathway in BAT of rats exposed to 4°C and 22°C for 4 hours. The 4 most significantly altered pathways following 4 hours of cold exposure were the insulin signaling pathway, protein kinase A, PI3K/AKT and ERK/MAPK signaling. The insulin signaling pathway was the most affected. In the documented 142 genes of the insulin pathway, 42 transcripts (29.6%) responded significantly to this cold exposure with the least false discovery rate (Benjamini-Hochberg Multiple Testing: −log10 (p-value)  = 7.18). Twenty-seven genes (64%) were up-regulated, including the insulin receptor (Insr), insulin substrates 1 and 2 (Irs1 and Irs2). Fifteen transcripts (36%) were down-regulated. Multiple transcripts of the primary target and secondary effector targets for the insulin signaling were also up-regulated, including those for carbohydrate metabolism. Using western blotting, we demonstrated that the cold induced higher Irs2, Irs1, and Akt-p protein levels in the BAT than in the BAT of controls maintained at room temperature, and higher Akt-p protein level in the muscle. Conclusion: this study demonstrated that 4 hours of cold exposure stimulated the insulin signaling pathway in the BAT and muscle of overnight fasted rats. This raises the possibility that acute cold stimulation may have potential to improve glucose clearance and insulin sensitivity.

Brown adipose tissue as a regulator of energy expenditure and body fat in humans

Brown adipose tissue (BAT) is recognized as the major site of sympathetically activated nonshivering thermogenesis during cold exposure and after spontaneous hyperphagia, thereby controling whole-body energy expenditure and body fat. In adult humans, BAT has long been believed to be absent or negligible, but recent studies using fluorodeoxyglucose-positron emission tomography, in combination with computed tomography, demonstrated the existence of metabolically active BAT in healthy adult humans. Human BAT is activated by acute cold exposure, being positively correlated to cold-induced increases in energy expenditure. The metabolic activity of BAT differs among individuals, being lower in older and obese individuals. Thus, BAT is recognized as a regulator of whole-body energy expenditure and body fat in humans as in small rodents, and a hopeful target combating obesity and related disorders. In fact, there are some food ingredients such as capsaicin and capsinoids, which have potential to activate and recruit BAT via activity on the specific receptor, transient receptor potential channels, thereby increasing energy expenditure and decreasing body fat modestly and consistently.

Metabolic adaptation of mice in a cool environment

Homeothermic animals, including humans, live by adapting to changes in ambient temperature. Numerous studies have demonstrated cold exposure (at approximately 5 degrees C) improves glucose tolerance despite reducing insulin secretion and increasing energy expenditure. To determine the effects of a small reduction in ambient temperature on energy metabolism, we compared two groups of mice; one exposed to a cool environment (20 degrees C) and the other maintained in a near-thermoneutral environment (25 degrees C) for 10 days. Both glucose-induced insulin secretion and glucose response were significantly impaired in mice exposed to a cool environment. In the cool temperature-exposed mice, skin temperatures were reduced, and plasma norepinephrine levels were increased, suggesting that impairment of insulin secretion was facilitated by induction of sympathetic nervous activity due to skin cooling. In addition, expression of GLUT4 mRNA was increased significantly in inguinal subcutaneous adipose tissue (IWAT) but not in epididymal or brown adipose tissue or skeletal muscle in these mice. Moreover, expression of Dok1, a molecule linked to activation of insulin receptors in adipocyte hypertrophy, and Cd36, a molecule related to NEFA uptake, were also increased at mRNA and/or protein levels only in IWAT of the cool temperature-exposed mice. Fatty acid synthesis was also facilitated, and fat weights were increased only in IWAT from mice kept at 20 degrees C. These results suggest that a small reduction in ambient temperature can affect glucose homeostasis through regulation of insulin secretion and preferentially enhances fat storage in IWAT. These adaptations can be interpreted as preparation for a further reduction in ambient temperature.

Histochemical demonstration of monocarboxylate transporters in mouse brown adipose tissue

Proton-coupled monocarboxylate transporters (MCTs) are essential for the transport of lactate, ketone bodies, and other monocarboxylates through the plasma membrane. The present immunohistochemical study aimed to examine the expression of MCTs in the brown adipose tissue (BAT) of mice. An intense immunoreactivity for MCT1 was found in the plasma membrane of brown adipose cells at light and electron microscopic levels but not in white adipose cells. The expression of MCT1 in BAT was confirmed by Western blot and in situ hybridization analyses. In fetuses (E17.5) and neonates, the MCT1 mRNA expression of BAT was abundant and appeared more intense than that in adult animals. These results, together with the intense expression of CD147 (a functional partner of MCTs) and acetyl-CoA carboxylase-2 (a component of fatty acid oxidation) in perinatal periods, suggest the involvement of MCT1 in the uptake of monocarboxylates from the circulation for thermogenesis rather than lipogenesis.

High incidence of metabolically active brown adipose tissue in healthy adult humans: effects of cold exposure and adiposity

OBJECTIVE

The significant roles of brown adipose tissue (BAT) in the regulation of energy expenditure and adiposity are established in small rodents but have been controversial in humans. The objective is to examine the prevalence of metabolically active BAT in healthy adult humans and to clarify the effects of cold exposure and adiposity.

RESEARCH DESIGN AND METHODS

In vivo 2-[(18)F]fluoro-2-deoxyglucose (FDG) uptake into adipose tissue was measured in 56 healthy volunteers (31 male and 25 female subjects) aged 23-65 years by positron emission tomography (PET) combined with X-ray computed tomography (CT).

RESULTS

When exposed to cold (19 degrees C) for 2 h, 17 of 32 younger subjects (aged 23-35 years) and 2 of 24 elderly subjects (aged 38-65 years) showed a substantial FDG uptake into adipose tissue of the supraclavicular and paraspinal regions, whereas they showed no detectable uptake when kept warm (27 degrees C). Histological examinations confirmed the presence of brown adipocytes in these regions. The cold-activated FDG uptake was increased in winter compared with summer (P < 0.001) and was inversely related to BMI (P < 0.001) and total (P < 0.01) and visceral (P < 0.001) fat areas estimated from CT image at the umbilical level.

CONCLUSIONS

Our findings, being against the conventional view, indicate the high incidence of metabolically active BAT in adult humans and suggest a role in the control of body temperature and adiposity.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Cold exposure induces tissue-specific modulation of the insulin-signalling pathway in Rattus norvegicus

Cold exposure provides a reproducible model of improved glucose turnover accompanied by reduced steady state and glucose-induced insulin levels. In the present report we performed immunoprecipitation and immunoblot studies to evaluate the initial and intermediate steps of the insulin-signalling pathway in white and brown adipose tissues, liver and skeletal muscle of rats exposed to cold. Basal and glucose-induced insulin secretion were significantly impaired, while glucose clearance rates during a glucose tolerance test and the constant for glucose decay during a 15 min insulin tolerance test were increased, indicating a significantly improved glucose turnover and insulin sensitivity in rats exposed to cold. Evaluation of protein levels and insulin-induced tyrosine (insulin receptor, insulin receptor substrates (IRS)-1 and -2, ERK (extracellular signal-related kinase)) or serine (Akt; protein kinase B) phosphorylation of proteins of the insulin signalling cascade revealed a tissue-specific pattern of regulation of the molecular events triggered by insulin such that in white adipose tissue and skeletal muscle an impaired molecular response to insulin was detected, while in brown adipose tissue an enhanced response to insulin was evident. In muscle and white and brown adipose tissues, increased 2-deoxy-D-glucose (2-DG) uptake was detected. Thus, during cold exposure there is a tissue-specific regulation of the insulin-signalling pathway, which seems to favour heat-producing brown adipose tissue. Nevertheless, muscle and white adipose tissue are able to take up large amounts of glucose, even in the face of an apparent molecular resistance to insulin.

Effects of rearing temperature on sympathoadrenal activity in young adult rats

Animals reared at 18 degrees C exhibit enhanced innervation of brown adipose tissue (BAT) and greater cold tolerance as adults, yet gain more weight when fed an enriched diet compared with rats reared at 30 degrees C. To explore this paradox, sympathoadrenal activity was examined using techniques of [(3)H]norepinephrine ([(3)H]NE) turnover and urinary catecholamine excretion in male and female rats reared until 2 mo of age at 18 or 30 degrees C. Gene expression in BAT was also analyzed for several sympathetically related proteins. Although [(3)H]NE turnover in heart did not differ between groups, [(3)H]NE turnover in BAT was consistently elevated in the 18 degrees C-reared animals, even 2 mo after removal from the cool environment. Gene expression for uncoupling proteins 1 and 3, GLUT-4, leptin, and the alpha(1A)-adrenergic receptor was more abundant in BAT and the increase in epinephrine excretion with fasting suppressed in 18 degrees C-reared animals. These studies demonstrate that obesity consequent to exposure to 18 degrees C in early life occurs despite tonic elevation of sympathetic input to BAT. Diminished adrenal epinephrine responsiveness to fasting may play a contributory role.

Effects of noradrenaline on the cell-surface glucose transporters in cultured brown adipocytes: novel mechanism for selective activation of GLUT1 glucose transporters

Glucose transport into rat brown adipocytes has been shown to be stimulated directly by the sympathetic neurotransmitter, noradrenaline, without a significant increase in the protein content of either GLUT1 or GLUT4 glucose transporter in the plasma membrane [Shimizu, Kielar, Minokoshi and Shimazu (1996) Biochem. J. 314, 485-490]. In the present study, we labelled the exofacial glucose-binding sites of GLUT1 and GLUT4 with a membrane-impermeant photoaffinity reagent, 2-N-[4-(1-azitrifluoroethyl)benzoyl]-[2-3H]1,3-bis- (D-mannos-4-yloxy)-2-propylamine (ATB-[3H]BMPA), to determine which isoform is responsible for the noradrenaline-induced increase in glucose transport into intact brown adipocytes in culture. Insulin stimulated the rate of hexose transport by increasing ATB-[3H]BMPA-labelled cell-surface GLUT4. In contrast, the noradrenaline-induced increase in glucose transport was not accompanied by an increased ATB-[3H]BMPA labelling of GLUT4, nor with an increased amount of GLUT4 in the plasma membrane fraction as assessed by Western blotting, indicating that noradrenaline does not promote the translocation of GLUT4. However, noradrenaline induced an increase in photoaffinity labelling of cell-surface GLUT1 without an apparent increase in the immunoreactive GLUT1 protein in the plasma membrane. This is suggestive of an increased affinity of GLUT1 for the ligand. In fact, the Ki value of non-radioactive ATB-BMPA for 2-deoxy-D-glucose uptake was significantly decreased after treatment of the cells with noradrenaline. The increased photoaffinity labelling of GLUT1 and increased glucose transport caused by noradrenaline were inhibited by a cAMP antagonist, cAMP-S Rp-isomer. These results demonstrate that noradrenaline stimulates glucose transport in brown adipocytes by enhancing the functional activity of GLUT1 through a cAMP-dependent mechanism.

Adrenergic activation of vascular endothelial growth factor mRNA expression in rat brown adipose tissue: implication in cold-induced angiogenesis

Cold exposure produces adaptive hyperplasia and growth of brown adipose tissue (BAT), the major site of non-shivering thermogenesis in rodents, associated with increased angiogenesis in this tissue. Vascular endothelial growth factor (VEGF), one of the most potent angiogenic factors, was found to be expressed abundantly in BAT of the rat. When rats were exposed to cold at 4 degrees C, the VEGF mRNA level in BAT was increased by 2-3-fold in 1-4 h, but returned to the basal level within 24 h. VEGF expression in other tissues such as heart, kidney and lung did not change after cold exposure. The cold-induced increase in VEGF mRNA was abolished by surgical sympathetic denervation, but mimicked by administration of noradrenaline or a beta3-adrenoceptor agonist CL316,243, indicating the critical role of the beta-adrenergic pathway in VEGF expression in BAT. Among three isoforms of VEGF, the mRNA of a short form (VEGF120) lacking heparin-binding activity was preferentially increased after cold exposure and treatment with the adrenergic agonists. These results suggest that cold exposure activates the sympathetic nerves and leads to a rapid increase in synthesis of VEGF in BAT, which in turn stimulates the proliferation of surrounding vascular endothelial cells.

Expression of uncoupling protein in skeletal muscle and white fat of obese mice treated with thermogenic beta 3-adrenergic agonist

The mitochondrial uncoupling protein (UCP) is usually expressed only in brown adipose tissue (BAT) and a key molecule for metabolic thermogenesis. The effects of a highly selective beta 3-adrenergic agonist, CL316,243 (CL), on UCP expression in skeletal muscle and adipose tissues were examined in mice. Daily injection of CL (0.1 mg/kg, sc) to obese yellow KK mice for two weeks caused a significant reduction of body weight, associated with a marked decrease of white fat pad weight and hypertrophy of the interscapular BAT with a sixfold increase in UCP content. Clear signals of UCP protein and mRNA were detected by Western and Northern blot analyses in inguinal, mesenteric and retroperitoneal white fat pads, and also in gastrocnemius and quadriceps muscles, whereas no signal in saline-treated mice. The presence of UCP mRNA in muscle tissues was also confirmed by reverse transcription-PCR analysis. Weaker UCP signals were also detected in control C57BL mice treated with CL, but only in inguinal and retroperitoneal fat pads. Immunohistochemical examinations revealed that UCP stains in the white fat pads were localized on multilocular cells quite similar to typical brown adipocyte, and those in the muscle tissues on myocytes. The mitochondrial localization of UCP in myocytes was confirmed by immunoelectron microscopy. In addition to UCP protein, UCP mRNA was also detected in myocytes by in situ hybridization analysis. Thus, chronic stimulation of the beta 3-adrenergic receptor induces ectopic expression of UCP in adipose tissues conventionally considered as white fat and even in skeletal muscle, which probably contributes to the potent anti-obesity effect of the beta 3-adrenergic agonist.

Hormonal regulation of glucose transport in a brown adipose cell preparation isolated from rats that shows a large response to insulin

Isolated brown adipose cells from rats are prepared whose viability is indicated by the expected stimulation of oxygen consumption by noradrenaline and counter-regulation of this oxygen consumption response by insulin. Insulin stimulates 3-O-methyl-D-glucose transport by approx. 15-fold in the absence of adenosine, and adenosine augments this response at least 2-fold. The insulin-stimulated translocation of the glucose transporter GLUT4 from an intracellular compartment to the plasma membrane is readily detected by subcellular fractionation and Western blotting, and the appearance of GLUT4 on the cell surface in response to insulin is demonstrated by bis-mannose photolabelling. Isoprenaline also stimulates glucose transport activity but only by approx. 3-fold; this effect is not altered by adenosine. Isoprenaline increases insulin-stimulated glucose transport activity in the absence of adenosine but decreases it in the presence of adenosine. These results demonstrate that although the regulation of glucose transport by insulin in brown adipose cells is qualitatively similar to that in white adipose cells, counter-regulation by adenosine and isoprenaline is at least quantitatively and may be qualitatively different. Isolated brown adipose cells from rats thus represent an excellent model for further examination of the mechanism by which multiple hormone signalling pathways interact to control glucose transport and GLUT4 subcellular trafficking.

Noradrenaline increases glucose transport into brown adipocytes in culture by a mechanism different from that of insulin

Glucose uptake into brown adipose tissue has been shown to be enhanced directly by noradrenaline (norepinephrine) released from sympathetic nerves. In this study we characterized the glucose transport system in cultured brown adipocytes, which responds to noradrenaline as well as insulin, and analysed the mechanism underlying the noradrenaline-induced increase in glucose transport. Insulin increased 2-deoxyglucose (dGlc) uptake progressively at concentrations from 10(-11) to 10(-6) M, with maximal stimulation at 10(-7) M. Noradrenaline concentrations ranging from 10(-8) to 10(-6) M also enhanced dGlc uptake, even in the absence of insulin. The effects of noradrenaline and insulin on dGlc uptake were additive. The stimulatory effect of noradrenaline was mimicked by the beta3-adrenergic agonist, BRL37344, at concentrations two orders lower than noradrenaline. Dibutyryl cyclic AMP also mimicked the stimulatory effect of noradrenaline, and the antagonist of cyclic AMP, cyclic AMP-S Rp-isomer, blocked the enhancement of glucose uptake due to noradrenaline. Furthermore Western blot analysis with an anti-phosphotyrosine antibody revealed that, in contrast with insulin, noradrenaline apparently does not stimulate intracellular phosphorylation of tyrosine, suggesting that the noradrenaline-induced increase in dGlc uptake depends on elevation of the intracellular cyclic AMP level and not on the signal chain common to insulin. When cells were incubated with insulin, the content of the muscle/adipocyte type of glucose transporter (GLUT4) in the plasma membrane increased, with a corresponding decrease in the amount in the microsomal membrane. In contrast, noradrenaline did not affect the subcellular distribution of GLUT4 or that of the HepG2/erythrocyte type of glucose transporter. Although insulin increased Vmax. and decreased the Km value for glucose uptake, the effect of noradrenaline was restricted to a pronounced decrease in Km. These results suggest that the mechanism by which noradrenaline stimulates glucose transport into brown adipocytes is not due to translocation of GLUT but is probably due to an increase in the intrinsic activity of GLUT, which is mediated by a cyclic AMP-dependent pathway.