Neuregulin 4: A “Hotline” Between Brown Fat and Liver

The discovery that functional brown adipose tissue (BAT) in adult humans is inversely related to body fat mass and may reflect metabolic health has stimulated adipose tissue research to explore activation of BAT as a potential target for antiobesity treatments. In addition to the capacity of BAT to increase energy expenditure and glucose and lipid uptake, BAT secretes factors that may contribute to the regulation of whole‐body metabolism. Among signals released from BAT, neuregulin 4 (NRG4) has been recently identified as an endocrine factor that may link the activation of BAT to protection against diet‐induced obesity, insulin resistance, and hepatic steatosis. NRG4 was shown to directly reduce lipogenesis in hepatocytes, and it could indirectly activate BAT via sympathetic neurons or via inducing brown adipocyte–like signatures in white adipocytes in a paracrine manner. However, the potential relevance of NRG4 as a diagnostic tool or target for the treatment of obesity‐related diseases remains to be explored.     

Biology and pathological implications of brown adipose tissue: promises and caveats for the control of obesity and its associated complications

The discovery of metabolically active brown adipose tissue (BAT) in adult humans has fuelled the research of diverse aspects of this previously neglected tissue. BAT is solely present in mammals and its clearest physiological role is non‐shivering thermogenesis, owing to the capacity of brown adipocytes to dissipate metabolic energy as heat. Recently, a number of other possible functions have been proposed, including direct regulation of glucose and lipid homeostasis and the secretion of a number of factors with diverse regulatory actions. Herein, we review recent advances in general biological knowledge of BAT and discuss the possible implications of this tissue in human metabolic health. In particular, we confront the claimed thermogenic potential of BAT for human energy balance and body mass regulation, mostly based on animal studies, with the most recent quantifications of human BAT.     

Autophagy in Myf5+ progenitors regulates energy and glucose homeostasis through control of brown fat and skeletal muscle development

Macroautophagy (MA) regulates cellular quality control and energy balance. For example, loss of MA in aP2‐positive adipocytes converts white adipose tissue (WAT) into brown adipose tissue (BAT)‐like, enhancing BAT function and thereby insulin sensitivity. However, whether MA regulates early BAT development is unknown. We report that deleting Atg7 in myogenic Myf5+ progenitors inhibits MA in Myf5‐cell‐derived BAT and muscle. Knock out (KO) mice have defective BAT differentiation and function. Surprisingly, their body temperature is higher due to WAT lipolysis‐driven increases in fatty acid oxidation in ‘Beige’ cells in inguinal WAT, BAT and muscle. KO mice also present impaired muscle differentiation, reduced muscle mass and glucose intolerance. Our studies show that ATG7 in Myf5+ progenitors is required to maintain energy and glucose homeostasis through effects on BAT and muscle development. Decreased MA in myogenic progenitors with age and/or overnutrition might contribute to the metabolic defects and sarcopenia observed in these conditions.     

Let's burn whatever you have: mitofusin 2 metabolically re‐wires brown adipose tissue

Brown adipose tissue (BAT) has received enormous scientific and lay attention in the recent past as its thermogenic, energy‐consuming capacities represent prime candidates for therapeutic interventions toward obesity, glucose intolerance, and diabetes even in humans. The overall positive effects of BAT activation and recruitment on systemic energy homeostasis have been largely attributed to the inherent ability of brown adipocytes to combust fatty acid and glucose energy substrates through mitochondrial uncoupling, driven by the unique expression of uncoupling protein 1 (UCP1). Two recent reports by Boutant et al and Mahdaviani et al now identify the GTPase mitofusin (Mfn) 2 as a key determinant of BAT thermogenic function that is largely independent of its previously described role in mitochondrial fusion [1,2].     

Inducible beige adipocytes improve impaired glucose metabolism in interscapular BAT-removal mice

Inducible beige adipocytes are emerging as an interesting issue in obesity and metabolism research. There is a neglected possibility that brown adipocytes are equally activated when external stimuli induce the formation of beige adipocytes. Thus, the question is whether beige adipocytes have the same functions as brown adipocytes when brown adipose tissue (BAT) is lacking. This question has not been well studied. Therefore we determine the beneficial effects of beige adipocytes upon cold challenge or CL316243 treatments in animal models of interscapular BAT (iBAT) ablation by surgical denervation. We found that denervated iBAT were activated by cold exposure and CL316243 treatments. The data show that beige adipocytes partly contribute to the improvement of impaired glucose metabolism resulting from denervated iBAT. Thus, we further used iBAT-removal animal models to abolish iBAT functions completely. We found that beige adipocytes upon cold exposure or CL316243 treatments improved impaired glucose metabolism and enhanced glucose uptake in iBAT-removal mice. The insulin signaling was activated in iBAT-removal mice upon cold exposure. Both the activation of insulin signaling and up-regulation of glucose transporter expression were observed in iBAT-removal mice with CL316243 treatments. The data show that inducible beige adipocytes may have different mechanisms to improve impaired glucose metabolism. Inducible beige adipocytes can also enhance energy expenditure and lipolytic activity of white adipose tissues when iBAT is lacking. We provide direct evidences for the beneficial effect of inducible beige adipocytes in glucose metabolism and energy expenditure in the absence of iBAT in vivo.     

Lack of TRPV2 impairs thermogenesis in mouse brown adipose tissue

Brown adipose tissue (BAT), a major site for mammalian non‐shivering thermogenesis, could be a target for prevention and treatment of human obesity. Transient receptor potential vanilloid 2 (TRPV2), a Ca²⁺‐permeable non‐selective cation channel, plays vital roles in the regulation of various cellular functions. Here, we show that TRPV2 is expressed in brown adipocytes and that mRNA levels of thermogenic genes are reduced in both cultured brown adipocytes and BAT from TRPV2 knockout (TRPV2KO) mice. The induction of thermogenic genes in response to β‐adrenergic receptor stimulation is also decreased in TRPV2KO brown adipocytes and suppressed by reduced intracellular Ca²⁺ concentrations in wild‐type brown adipocytes. In addition, TRPV2KO mice have more white adipose tissue and larger brown adipocytes and show cold intolerance, and lower BAT temperature increases in response to β‐adrenergic receptor stimulation. Furthermore, TRPV2KO mice have increased body weight and fat upon high‐fat‐diet treatment. Based on these findings, we conclude that TRPV2 has a role in BAT thermogenesis and could be a target for human obesity therapy.     

Human brown adipose tissue: Underestimated target in metabolic disease?

Active brown adipose tissue (BAT) has, since it rediscovery in adult humans in 2009, received much attention for its ability to increase energy expenditure when activated. By means of mitochondrial uncoupling activity BAT's main function is to produce heat instead of storing energy such as in white adipose tissue (WAT). Therefore, BAT is considered a new potential target to treat obesity and the metabolic syndrome. However, the contribution of this thermogenic tissue is still a matter of debate among researchers.The aim of this review is to give an overview of the differences between classical brown adipocytes and inducible beige adipocytes in humans, and the potential activators of BAT in humans. Furthermore newly described genetic markers for identification of these two types of brown adipocytes are examined. Finally, the potential of the current measurement techniques, and the contribution of BAT activity to whole body energy expenditure are discussed.     

Changes in the properties of cytosolic acetyl-CoA carboxylase studied in cold-clamped liver samples from fed, starved and starved-refed rats.

The mechanism responsible for the insulin resistance described in vivo in brown adipose tissue (BAT) of lactating rats was investigated. The effect of insulin on glucose metabolism was studied on isolated brown adipocytes of non-lactating and lactating rats. Insulin stimulation of total glucose metabolism is 50% less in brown adipocytes from lactating than from non-lactating rats. This reflects a decreased effect of insulin on glucose oxidation and lipogenesis. However, the effect of noradrenaline (8 microM) on glucose metabolism was preserved in brown adipocytes from lactating rats as compared with non-lactating rats. The number of insulin receptors is similar in BAT of lactating and non-lactating rats. The insulin-receptor tyrosine kinase activity is not altered during lactation, for receptor autophosphorylation as well as tyrosine kinase activity towards the synthetic peptide poly(Glu4-Tyr1). The defect in the action of insulin is thus localized at a post-receptor level. The insulin stimulation of pyruvate dehydrogenase activity during euglycaemic/hyperinsulinaemic clamps is 2-fold lower in BAT from lactating than from non-lactating rats. However, the percentage of active form of pyruvate dehydrogenase is similar in non-lactating and lactating rats (8.6% versus 8.9% in the basal state, and 37.0% versus 32.3% during the clamp). A decrease in the amount of pyruvate dehydrogenase is likely to be involved in the insulin resistance described in BAT during lactation.     

The orphan nuclear receptor SHP regulates PGC-1alpha expression and energy production in brown adipocytes

Brown adipocytes increase energy production in response to induction of PGC-1alpha, a dominant regulator of energy metabolism. We have found that the orphan nuclear receptor SHP (NR0B2) is a negative regulator of PGC-1alpha expression in brown adipocytes. Mice lacking SHP show increased basal expression of PGC-1alpha, increased energy expenditure, and resistance to diet-induced obesity. Increased PGC-1alpha expression in SHP null brown adipose tissue is not due to beta-adrenergic activation, since it is also observed in primary cultures of SHP(-/-) brown adipocytes that are not exposed to such stimuli. In addition, acute inhibition of SHP expression in cultured wild-type brown adipocytes increases basal PGC-1alpha expression, and SHP overexpression in SHP null brown adipocytes decreases it. The orphan nuclear receptor ERRgamma is expressed in BAT and its transactivation of the PGC-1alpha promoter is potently inhibited by SHP. We conclude that SHP functions as a negative regulator of energy production in BAT.     

Blockade of the activin receptor IIb activates functional brown adipogenesis and thermogenesis by inducing mitochondrial oxidative metabolism

Brown adipose tissue (BAT) is a key tissue for energy expenditure via fat and glucose oxidation for thermogenesis. In this study, we demonstrate that the myostatin/activin receptor IIB (ActRIIB) pathway, which serves as an important negative regulator of muscle growth, is also a negative regulator of brown adipocyte differentiation. In parallel to the anticipated hypertrophy of skeletal muscle, the pharmacological inhibition of ActRIIB in mice, using a neutralizing antibody, increases the amount of BAT without directly affecting white adipose tissue. Mechanistically, inhibition of ActRIIB inhibits Smad3 signaling and activates the expression of myoglobin and PGC-1 coregulators in brown adipocytes. Consequently, ActRIIB blockade in brown adipose tissue enhances mitochondrial function and uncoupled respiration, translating into beneficial functional consequences, including enhanced cold tolerance and increased energy expenditure. Importantly, ActRIIB inhibition enhanced energy expenditure only at ambient temperature or in the cold and not at thermoneutrality, where nonshivering thermogenesis is minimal, strongly suggesting that brown fat activation plays a prominent role in the metabolic actions of ActRIIB inhibition.     

Shc depletion stimulates brown fat activity in vivo and in vitro

Adipose tissue is an important metabolic organ that integrates a wide array of homeostatic processes and is crucial for whole‐body insulin sensitivity and energy metabolism. Brown adipose tissue (BAT) is a key thermogenic tissue with a well‐established role in energy expenditure. BAT dissipates energy and protects against both hypothermia and obesity. Thus, BAT stimulation therapy is a rational strategy for the looming pandemic of obesity, whose consequences and comorbidities have a huge impact on the aged. Shc‐deficient mice (ShcKO) were previously shown to be lean, insulin sensitive, and resistant to high‐fat diet and obesity. We investigated the contribution of BAT to this phenotype. Insulin‐dependent BAT glucose uptake was higher in ShcKO mice. Primary ShcKO BAT cells exhibited increased mitochondrial respiration; increased expression of several mitochondrial and lipid‐oxidative enzymes was observed in ShcKO BAT. Levels of brown fat‐specific markers of differentiation, UCP1, PRDM16, ELOVL3, and Cox8b, were higher in ShcKO BAT. In vitro, Shc knockdown in BAT cell line increased insulin sensitivity and metabolic activity. In vivo, pharmacological stimulation of ShcKO BAT resulted in higher energy expenditure. Conversely, pharmacological inhibition of BAT abolished the improved metabolic parameters, that is the increased insulin sensitivity and glucose tolerance of ShcKO mice. Similarly, in vitro Shc knockdown in BAT cell lines increased their expression of UCP1 and metabolic activity. These data suggest increased BAT activity significantly contributes to the improved metabolic phenotype of ShcKO mice.     

Distinction of white,beige and brown adipocytes derived from mesenchymal stem cells简

Adipose tissue is a major metabolic organ, and it has been traditionally classified as either white adipose tissue (WAT) or brown adipose tissue (BAT). WAT and BAT are characterized by different anatomical locations, morphological structures, functions, and regulations. WAT and BAT are both involved in energy balance. WAT is mainly involved in the storage and mobilization of energy in the form of triglycerides, whereas BAT specializes in dissipating energy as heat during cold- or diet-induced thermogenesis. Recently, brown-like adipocytes were discovered in WAT. These brown-like adipocytes that appear in WAT are called beige or brite adipocytes. Interestingly, these beige/brite cells resemble white fat cells in the basal state, but they respond to thermogenic stimuli with increased levels of thermogenic genes and increased respiration rates. In addition, beige/brite cells have a gene expression pattern distinct from that of either white or brown fat cells. The current epidemic of obesity has increased the interest in studying adipocyte formation (adipogenesis), especially in beige/brite cells. This review summarizes the developmental process of adipose tissues that originate from the mesenchymal stem cells and the features of these three different types of adipocytes.     

Distinction of white,beige and brown adipocytes derived from mesenchymal stem cells

Adipose tissue is a major metabolic organ, and it has been traditionally classified as either white adipose tissue(WAT) or brown adipose tissue(BAT). WAT and BAT are characterized by different anatomical locations, morphological structures, functions, and regulations. WAT and BAT are both involved in energy balance. WAT is mainly involved in the storage and mobilization of energy in the form of triglycerides, whereas BAT specializes in dissipating energy as heat during cold- or diet-induced thermogenesis. Recently, brownlike adipocytes were discovered in WAT. These brownlike adipocytes that appear in WAT are called beige or brite adipocytes. Interestingly, these beige/brite cells resemble white fat cells in the basal state, but they respond to thermogenic stimuli with increased levels of thermogenic genes and increased respiration rates. In addition, beige/brite cells have a gene expressionpattern distinct from that of either white or brown fat cells. The current epidemic of obesity has increased the interest in studying adipocyte formation(adipogenesis), especially in beige/brite cells. This review summarizes the developmental process of adipose tissues that originate from the mesenchymal stem cells and the features of these three different types of adipocytes.     

Knockdown of NPY expression in the dorsomedial hypothalamus promotes development of brown adipocytes and prevents diet-induced obesity

Hypothalamic neuropeptide Y (NPY) has been implicated in control of energy balance, but the physiological importance of NPY in the dorsomedial hypothalamus (DMH) remains unclear. Here we report that knockdown of NPY expression in the DMH by adeno-associated virus-mediated RNAi reduced fat depots in rats fed regular chow and ameliorated high-fat diet-induced hyperphagia and obesity. DMH NPY knockdown resulted in development of brown adipocytes in inguinal white adipose tissue through the sympathetic nervous system. This knockdown increased uncoupling protein 1 expression in both inguinal fat and interscapular brown adipose tissue (BAT). Consistent with the activation of BAT, DMH NPY knockdown increased energy expenditure and enhanced the thermogenic response to a cold environment. This knockdown also increased locomotor activity, improved glucose homeostasis, and enhanced insulin sensitivity. Together, these results demonstrate critical roles of DMH NPY in body weight regulation through affecting food intake, body adiposity, thermogenesis, energy expenditure, and physical activity.     

Metallothionein is expressed in adipocytes of brown fat and is induced by catecholamines and zinc

Metallothionein (MT) is thought to have an antioxidant function and is strongly expressed during activation of thermogenesis and increased oxidative stress in brown adipose tissue (BAT). Localization and regulation of MT expression in BAT was therefore investigated in rats and mice. Immunohistochemical analysis of BAT from rats exposed to 4 degrees C for 24 h showed that MT and uncoupling protein 1 (UCP1) were coexpressed in differentiated adipocytes, and both cytoplasmic and nuclear localization of MT was observed. Cold induction of MT-1 expression in BAT was also observed in mice. Administration of norepinephrine to rats and isoproterenol to mice stimulated MT and UCP1 expression in BAT, implying a sympathetically mediated pathway for MT induction. In mice, zinc, and particularly dexamethasone, induced MT-2 expression in BAT and liver. Surprisingly, zinc also induced UCP1 in BAT, suggesting that elevated zinc may induce thermogenesis. We conclude that expression of MT in mature brown adipocytes upon beta-adrenoceptor activation is consistent with a role in protecting against physiological oxidative stress or in facilitating the mobilization or utilization of energy reserves.     

Argonaute-2 is associated to brown adipose tissue activation

MicroRNAs (miRNAs) are important modulators of thermogenic brown adipose tissue (BAT). They have been involved in its differentiation and hence its functioning. While different regulators of the miRNA machinery have been shown to be essential for BAT differentiation, little is known about their implication in BAT activation. The aim of this work was to evaluate the role of AGO2, the chief miRNA mediator, in BAT activation.We took advantage of two non-genetic models of BAT activation to analyze the miRNA machinery and miRNA expression in BAT. We used principal component analysis (PCA) to obtain an overview of miRNA expression according to the BAT activation state. In vitro, we examined AGO2 expression during brown adipocyte differentiation and activation. Finally, we downregulated AGO2 to reveal its potential role in the thermogenic function of brown adipocytes.PCA analysis allowed to cluster animals on their miRNA signature in active BAT. Moreover, hierarchical clustering showed a positive correlation between global upregulation of miRNA expression and active BAT. Consistently, the miRNA machinery, particularly AGO2, was upregulated in vivo in active BAT and in vitro in mature brown adipocytes. Finally, the partial loss-of-function of AGO2 in mature brown adipocytes is sufficient to lead to a diminished expression of UCP1 associated to a decreased uncoupled respiration.Therefore, our study shows the potential contribution of AGO2 in BAT activation. Since BAT is a calorie-burning tissue these data have a translational potential in terms of therapeutic target in the field of altered fuel homeostasis associated to obesity and diabetes.     

Regulatory networks of non-coding RNAs in brown/beige adipogenesis

BAT (brown adipose tissue) is specialized to burn fatty acids for heat generation and energy expenditure to defend against cold and obesity. Accumulating studies have demonstrated that manipulation of BAT activity through various strategies can regulate metabolic homoeostasis and lead to a healthy phenotype. Two classes of ncRNA (non-coding RNA), miRNA and lncRNA (long non-coding RNA), play crucial roles in gene regulation during tissue development and remodelling. In the present review, we summarize recent findings on regulatory role of distinct ncRNAs in brown/beige adipocytes, and discuss how these ncRNA regulatory networks contribute to brown/beige fat development, differentiation and function. We suggest that targeting ncRNAs could be an attractive approach to enhance BAT activity for protecting the body against obesity and its pathological consequences.     

Activated brown adipose tissue releases exosomes containing mitochondrial methylene tetrahydrofolate dehydrogenase (NADP dependent) 1-like protein (MTHFD1L)

Brown adipose tissue (BAT) is a promising weapon to combat obesity and metabolic disease. BAT is thermogenic and consumes substantial amounts of glucose and fatty acids as fuel for thermogenesis and energy expenditure. To study BAT function in large human longitudinal cohorts, safe and precise detection methodologies are needed. Although regarded a gold standard, the foray of PET-CT into BAT research and clinical applications is limited by its high ionizing radiation doses. Here, we show that brown adipocytes release exosomes in blood plasma that can be utilized to assess BAT activity. In the present study, we investigated circulating protein biomarkers that can accurately and reliably reflect BAT activation triggered by cold exposure, capsinoids ingestion and thyroid hormone excess in humans. We discovered an exosomal protein, methylene tetrahydrofolate dehydrogenase (NADP⁺ dependent) 1-like (MTHFD1L), to be overexpressed and detectable in plasma for all three modes of BAT activation in human subjects. This mitochondrial protein is packaged as a cargo within multivesicular bodies of the endosomal compartment and secreted as exosomes via exocytosis from activated brown adipocytes into the circulation. To support MTHFD1L as a conserved BAT activation response in other vertebrates, we examined a rodent model and also proved its presence in blood of rats following BAT activation by cold exposure. Plasma concentration of exosomal MTHFD1L correlated with human BAT activity as confirmed by PET-MR in humans and supported by data from rats. Thus, we deduce that MTHFD1L appears to be overexpressed in activated BAT compared to BAT in the basal nonstimulated state.