UCP1 in brite/beige adipose tissue mitochondria is functionally thermogenic

Dibenzazepine-Loaded Nanoparticles Induce Local Browning of White Adipose Tissue to Counteract Obesity

Inhibition of Notch signaling via systemic drug administration triggers conversion of white adipocytes into beige adipocytes (browning) and reduces adiposity. However, translation of this discovery into clinical practice is challenged by potential off-target side effects and lack of control over the location and temporal extent of beige adipocyte biogenesis. Here, we demonstrate an alternative approach to stimulate browning using nanoparticles (NPs) composed of FDA-approved poly(lactide-co-glycolide) that enable sustained local release of a Notch inhibitor (dibenzazepine, DBZ). These DBZ-loaded NPs support rapid cellular internalization and inhibit Notch signaling in adipocytes. Importantly, focal injection of these NPs into the inguinal white adipose tissue depots of diet-induced obese mice results in localized NP retention and browning of adipocytes, consequently improving the glucose homeostasis and attenuating body-weight gain of the treated mice. These findings offer new avenues to develop a potential therapeutic strategy for clinical treatment of obesity and its associated metabolic syndrome.

A self-sustained loop of inflammation-driven inhibition of beige adipogenesis in obesity

In obesity, inflammation of white adipose tissue (AT) is associated with diminished generation of beige adipocytes ('beige adipogenesis'), a thermogenic and energy-dissipating function mediated by beige adipocytes that express the uncoupling protein UCP1. Here we delineated an inflammation-driven inhibitory mechanism of beige adipogenesis in obesity that required direct adhesive interactions between macrophages and adipocytes mediated by the integrin α4 and its counter-receptor VCAM-1, respectively; expression of the latter was upregulated in obesity. This adhesive interaction reciprocally and concomitantly modulated inflammatory activation of macrophages and downregulation of UCP1 expression dependent on the kinase Erk in adipocytes. Genetic or pharmacological inactivation of the integrin α4 in mice resulted in elevated expression of UCP1 and beige adipogenesis of subcutaneous AT in obesity. Our findings, established in both mouse systems and human systems, reveal a self-sustained cycle of inflammation-driven impairment of beige adipogenesis in obesity.

The β3-adrenergic receptor is dispensable for browning of adipose tissues

Brown and brite/beige adipocytes are attractive therapeutic targets to treat metabolic diseases. To maximally utilize their functional potential, further understanding is required about their identities and their functional differences. Recent studies with β₃-adrenergic receptor knockout mice reported that brite/beige adipocytes, but not classical brown adipocytes, require the β₃-adrenergic receptor for cold-induced transcriptional activation of thermogenic genes. We aimed to further characterize this requirement of the β₃-adrenergic receptor as a functional distinction between classical brown and brite/beige adipocytes. However, when comparing wild-type and β₃-adrenergic receptor knockout mice, we observed no differences in cold-induced thermogenic gene expression (Ucp1, Pgc1a, Dio2, and Cidea) in brown or white (brite/beige) adipose tissues. Irrespective of the duration of the cold exposure or the sex of the mice, we observed no effect of the absence of the β₃-adrenergic receptor. Experiments with the β₃-adrenergic receptor agonist CL-316,243 verified the functional absence of β₃-adrenergic signaling in these knockout mice. The β₃-adrenergic receptor knockout model in the present study was maintained on a FVB/N background, whereas earlier reports used C57BL/6 and 129Sv mice. Thus our data imply background-dependent differences in adrenergic signaling mechanisms in response to cold exposure. Nonetheless, the present data indicate that the β₃-adrenergic receptor is dispensable for cold-induced transcriptional activation in both classical brown and, as opposed to earlier studies, brite/beige cells.

Blocking FSH induces thermogenic adipose tissue and reduces body fat

Menopause is associated with bone loss and enhanced visceral adiposity. A polyclonal antibody that targets the β-subunit of the pituitary hormone follicle-stimulating hormone (Fsh) increases bone mass in mice. Here, we report that this antibody sharply reduces adipose tissue in wild-type mice, phenocopying genetic haploinsufficiency for the Fsh receptor gene Fshr. The antibody also causes profound beiging, increases cellular mitochondrial density, activates brown adipose tissue and enhances thermogenesis. These actions result from the specific binding of the antibody to the β-subunit of Fsh to block its action. Our studies uncover opportunities for simultaneously treating obesity and osteoporosis.

A homologous stem cell therapy for obesity and its related metabolic disorders

Obesity and overweight have become a leading health problem in the world. But we have not yet had any optimal therapy to prevent this health issue. Accumulating evidence suggests that there is existence of functional brown/brite adipocytes in both infants and adult humans, and the activated brown/brite can burn energy by generating heat. These adipocytes can be differentiated from stem cells and transplantation of pre-activated human thermogenic adipocytes in vitro benefits to glucose homeostasis, insulin sensitivity and reduces body fat in normal or obese immunodeficient mice. These investigations lead us to hypothesize a homologous stem cell therapy to treat obesity and its related disorders. The therapy bases on functional brown/brite adipocytes which are differentiated from homologous stem cells isolated from fat depot of an obese person, and those adipocytes are activated to have a full thermogenic program before being transplanted back to that patient to improve glucose homeostasis and reduce fat content.

Mitochondrial Patch Clamp of Beige Adipocytes Reveals UCP1-Positive and UCP1-Negative Cells Both Exhibiting Futile Creatine Cycling

Cold and other environmental factors induce "browning" of white fat depots-development of beige adipocytes with morphological and functional resemblance to brown fat. Similar to brown fat, beige adipocytes are assumed to express mitochondrial uncoupling protein 1 (UCP1) and are thermogenic due to the UCP1-mediated H⁺ leak across the inner mitochondrial membrane. However, this assumption has never been tested directly. Herein we patch clamped the inner mitochondrial membrane of beige and brown fat to provide a direct comparison of their thermogenic H⁺ leak (I_H). All inguinal beige adipocytes had robust UCP1-dependent I_H comparable to brown fat, but it was about three times less sensitive to purine nucleotide inhibition. Strikingly, only ∼15% of epididymal beige adipocytes had I_H, while in the rest UCP1-dependent I_H was undetectable. Despite the absence of UCP1 in the majority of epididymal beige adipocytes, these cells employ prominent creatine cycling as a UCP1-independent thermogenic mechanism.

Quantification of UCP1 function in human brown adipose tissue

Brown adipose tissue (BAT) mitochondria are distinct from their counterparts in other tissues in that ATP production is not their primary physiologic role. BAT mitochondria are equipped with a specialized protein known as uncoupling protein 1 (UCP1). UCP1 short-circuits the electron transport chain, allowing mitochondrial membrane potential to be transduced to heat, making BAT a tissue capable of altering energy expenditure and fuel metabolism in mammals without increasing physical activity. The recent discovery that adult humans have metabolically active BAT has rekindled an interest in this intriguing tissue, with the overarching aim of manipulating BAT function to augment energy expenditure as a countermeasure for obesity and the metabolic abnormalities it incurs. Subsequently, there has been heightened interest in quantifying BAT function and more specifically, determining UCP1-mediated thermogenesis in BAT specimens - including in those obtained from humans. In this article, BAT mitochondrial bioenergetics will be described and compared with more conventional mitochondria in other tissues. The biochemical methods typically used to quantify BAT mitochondrial function will also be discussed in terms of their specificity for assaying UCP1 mediated thermogenesis. Finally, recent data concerning BAT UCP1 function in humans will be described and discussed.

Keeping fat on time: Circadian control of adipose tissue

Circadian clocks harmonize processes ranging from intracellular biochemistry to whole-body physiology in accordance with the Earth's 24h rotation. These intrinsic oscillators are based on an interlocked transcriptional-translational feedback loop comprised from a set of core clock factors. In addition to maintaining rhythmicity in nearly every cell of the body, these clock factors also mediate tissue specific metabolic functions. In this review, we will explore how the molecular clock shapes the unique features of different adipose depots.

Near-Infrared Photoluminescent Carbon Nanotubes for Imaging of Brown Fat

Near-infrared photoluminescent single-walled carbon nanotubes (CNTs) are expected to provide effectual bio-imaging tools, although, as yet, only limited applications have been reported. Here, we report that CNTs coated with an amphiphilic and biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate; PMB), generate high-quality images of brown fat. Brown fat is a heat-productive adipose tissue, which is attracting increasing attention as a new therapeutic target for obesity-associated metabolic disorders. Its brown colour is mainly attributed to densely packed capillaries, which facilitate its high heat-exchanging efficiency. Currently, positron emission tomography-computed tomography is the only practical technique to identify brown fat distribution in the living body; however, it is expensive to use. By virtue of their high affinity to apolipoproteins and exemption from macrophage phagocytosis, PMB-CNTs selectively accumulate on capillary endothelial cells but not larger vessels in adipose tissue. Therefore, the image brightness of adipose tissue can directly reflect the capillary density, and indirectly the thermogenic capability and brownness. PMB-CNTs provide clearer images than conventional organic dyes, as the high level of transmitted light passes through the body with less light scattering. Thus, PMB-CNT-based imaging methods could open a new phase in thermogenic adipose tissue research.

Effects of physical activity on the link between PGC-1a and FNDC5 in muscle, circulating Ιrisin and UCP1 of white adipocytes in humans: A systematic review

Background: Exercise may activate a brown adipose-like phenotype in white adipose tissue. The aim of this systematic review was to identify the effects of physical activity on the link between peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1a) and fibronectin type III domain-containing protein 5 (FNDC5) in muscle, circulating Irisin and uncoupling protein one (UCP1) of white adipocytes in humans. Methods: Two databases (PubMed 1966 to 08/2016 and EMBASE 1974 to 08/2016) were searched using an appropriate algorithm. We included articles that examined physical activity and/or exercise in humans that met the following criteria: a) PGC-1a in conjunction with FNDC5 measurements, and b) FNDC5 and/or circulating Irisin and/or UCP1 levels in white adipocytes. Results: We included 51 studies (12 randomised controlled trials) with 2474 participants. Out of the 51 studies, 16 examined PGC-1a and FNDC5 in response to exercise, and only four found increases in both PGC-1a and FNDC5 mRNA and one showed increased FNDC5 mRNA. In total, 22 out of 45 studies that examined circulating Irisin in response to exercise showed increased concentrations when ELISA techniques were used; two studies also revealed increased Irisin levels measured via mass spectrometry. Three studies showed a positive association of circulating Irisin with physical activity levels. One study found no exercise effects on UCP1 mRNA in white adipocytes. Conclusions: The effects of physical activity on the link between PGC-1a, FNDC5 mRNA in muscle and UCP1 in white human adipocytes has attracted little scientific attention. Current methods for Irisin identification lack precision and, therefore, the existing evidence does not allow for conclusions to be made regarding Irisin responses to physical activity. We found a contrast between standardised review methods and accuracy of the measurements used. This should be considered in future systematic reviews.

Effects of physical activity on the link between PGC-1a and FNDC5 in muscle, circulating Ιrisin and UCP1 of white adipocytes in humans: A systematic review

Background: Exercise may activate a brown adipose-like phenotype in white adipose tissue. The aim of this systematic review was to identify the effects of physical activity on the link between peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1a) and fibronectin type III domain-containing protein 5 (FNDC5) in muscle, circulating Irisin and uncoupling protein one (UCP1) of white adipocytes in humans. Methods: Two databases (PubMed 1966 to 08/2016 and EMBASE 1974 to 08/2016) were searched using an appropriate algorithm. We included articles that examined physical activity and/or exercise in humans that met the following criteria: a) PGC-1a in conjunction with FNDC5 measurements, and b) FNDC5 and/or circulating Irisin and/or UCP1 levels in white adipocytes. Results: We included 51 studies (12 randomised controlled trials) with 2474 participants. Out of the 51 studies, 16 examined PGC-1a and FNDC5 in response to exercise, and only four found increases in both PGC-1a and FNDC5 mRNA and one showed increased FNDC5 mRNA. In total, 22 out of 45 studies that examined circulating Irisin in response to exercise showed increased concentrations when ELISA techniques were used; two studies also revealed increased Irisin levels measured via mass spectrometry. Three studies showed a positive association of circulating Irisin with physical activity levels. One study found no exercise effects on UCP1 mRNA in white adipocytes. Conclusions: The effects of physical activity on the link between PGC-1a, FNDC5 mRNA in muscle and UCP1 in white human adipocytes has attracted little scientific attention. Current methods for Irisin identification lack precision and, therefore, the existing evidence does not allow for conclusions to be made regarding Irisin responses to physical activity. We found a contrast between standardised review methods and accuracy of the measurements used. This should be considered in future systematic reviews.

Circadian Rhythms in Adipose Tissue Physiology

The different types of adipose tissues fulfill a wide range of biological functions-from energy storage to hormone secretion and thermogenesis-many of which show pronounced variations over the course of the day. Such 24-h rhythms in physiology and behavior are coordinated by endogenous circadian clocks found in all tissues and cells, including adipocytes. At the molecular level, these clocks are based on interlocked transcriptional-translational feedback loops comprised of a set of clock genes/proteins. Tissue-specific clock-controlled transcriptional programs translate time-of-day information into physiologically relevant signals. In adipose tissues, clock gene control has been documented for adipocyte proliferation and differentiation, lipid metabolism as well as endocrine function and other adipose oscillations are under control of systemic signals tied to endocrine, neuronal, or behavioral rhythms. Circadian rhythm disruption, for example, by night shift work or through genetic alterations, is associated with changes in adipocyte metabolism and hormone secretion. At the same time, adipose metabolic state feeds back to central and peripheral clocks, adjusting behavioral and physiological rhythms. In this overview article, we summarize our current knowledge about the crosstalk between circadian clocks and energy metabolism with a focus on adipose physiology. © 2017 American Physiological Society. Compr Physiol 7:383-427, 2017.

AT1 receptor antagonist induces thermogenic beige adipocytes in the inguinal white adipose tissue of obese mice

PURPOSE

To evaluate whether losartan is able to induce beige adipocytes formation, focusing on the thermogenic gene expression and adipocyte remodeling in the subcutaneous white adipose tissue of diet-induced obese mice.

METHODS

Male C57BL/6 mice received a control diet (10% energy as lipids) or a high-fat diet (50% energy as lipids) for 10 weeks, followed by a 5-week treatment with losartan: control group, control-losartan group (10 mg/Kg/day), high-fat group and high-fat-losartan group (10 mg/Kg/day). Biochemical, morphometrical, stereological and molecular approaches were used to evaluate the outcomes.

RESULTS

The high-fat diet elicited overweight, insulin resistance and adipocyte hypertrophy in the high-fat group, all of which losartan rescued in the high-fat-losartan group. These effects comply with the induction of beige adipocytes within the inguinal fat pads in high-fat-losartan group as they exhibited the greatest energy expenditure among the groups along with the presence uncoupling protein 1 positive multilocular adipocytes with enhanced peroxisome proliferator-activated receptor gamma coactivator 1-alpha and PR domain containing 16 mRNA levels, indicating a significant potential for mitochondrial biogenesis and adaptive thermogenesis.

CONCLUSIONS

Our results show compelling evidence that losartan countered diet-induced obesity in mice by enhancing energy expenditure through beige adipocytes induction. Reduced body mass, increased insulin sensitivity, decreased adipocyte size and marked expression of uncoupling protein 1 by ectopic multilocular adipocytes support these findings. The use of losartan as a coadjutant medicine to tackle obesity and its related disorders merits further investigation.

Mitochondrial homeostasis in adipose tissue remodeling

Mitochondrial homeostasis is regulated by a balance between mitochondrial biogenesis and degradation. Emerging evidence suggests that mitophagy, a selective form of autophagy that degrades mitochondria, plays a key role in the physiology and pathophysiology of mitochondria-enriched cells, such as brown and beige adipocytes. This review discusses findings regarding the roles of autophagy and mitophagy in cellular development, maintenance, and functions of metabolic organs, including adipose tissue, liver, and pancreas. A better understanding of the molecular links between mitophagy and energy metabolism will help to identify promising targets for the treatment of obesity and obesity-associated disorders.

Targeting PPARγ in the epigenome rescues genetic metabolic defects in mice

Obesity causes insulin resistance, and PPARγ ligands such as rosiglitazone are insulin sensitizing, yet the mechanisms remain unclear. In C57BL/6 (B6) mice, obesity induced by a high-fat diet (HFD) has major effects on visceral epididymal adipose tissue (eWAT). Here, we report that HFD-induced obesity in B6 mice also altered the activity of gene regulatory elements and genome-wide occupancy of PPARγ. Rosiglitazone treatment restored insulin sensitivity in obese B6 mice, yet, surprisingly, had little effect on gene expression in eWAT. However, in subcutaneous inguinal fat (iWAT), rosiglitazone markedly induced molecular signatures of brown fat, including the key thermogenic gene Ucp1. Obesity-resistant 129S1/SvImJ mice (129 mice) displayed iWAT browning, even in the absence of rosiglitazone. The 129 Ucp1 locus had increased PPARγ binding and gene expression that were preserved in the iWAT of B6x129 F1-intercrossed mice, with an imbalance favoring the 129-derived alleles, demonstrating a cis-acting genetic difference. Thus, B6 mice have genetically defective Ucp1 expression in iWAT. However, when Ucp1 was activated by rosiglitazone, or by iWAT browning in cold-exposed or young mice, expression of the B6 version of Ucp1 was no longer defective relative to the 129 version, indicating epigenomic rescue. These results provide a framework for understanding how environmental influences like drugs can affect the epigenome and potentially rescue genetically determined disease phenotypes.

Impaired adrenergic agonist-dependent beige adipocyte induction in aged mice

OBJECTIVE

There are two types of thermogenic adipocytes expressing uncoupling protein (UCP)-1: the brown adipocyte activated by adrenergic stimulation and the beige adipocyte that appears within the white adipose tissue (WAT) in response to chronic adrenergic stimulation. This study examined age-related changes in responses of both types of adipocytes to adrenergic stimulation in mice.

METHODS

Aged (age 20 months) and young (4 months) mice were injected daily with either saline or β3-adrenergic receptor agonist CL316,243 (CL; 0.1 mg/kg, once a day) for 1 week.

RESULTS

The body and WAT weight tended to be higher in aged mice. CL treatment increased UCP-1 protein amounts in both brown adipose tissue and inguinal WAT, suggesting activation of brown and beige adipocytes. However, induction of beige adipocytes was impaired in aged mice, whereas brown adipocyte activation was comparable to young mice. The number of platelet-derived growth factor receptor α-expressing progenitor cells, which were reported to differentiate into beige adipocytes, significantly decreased in inguinal WAT of aged mice compared with that of young mice.

CONCLUSIONS

Inductive ability of beige adipocytes in WAT declines with aging in mice. It may be partly because of a decreased number of progenitor cells associated with aging.

Thyroid hormones induce browning of white fat

The canonical view about the effect of thyroid hormones (THs) on thermogenesis assumes that the hypothalamus acts merely as a modulator of the sympathetic outflow on brown adipose tissue (BAT). Recent data have challenged that vision by demonstrating that THs act on the ventromedial nucleus of the hypothalamus (VMH) to inhibit AMP-activated protein kinase (AMPK), which regulates the thermogenic program in BAT, leading to increased thermogenesis and weight loss. Current data have shown that in addition to activation of brown fat, the browning of white adipose tissue (WAT) might also be an important thermogenic mechanism. However, the possible central effects of THs on the browning of white fat remain unclear. Here, we show that 3,3',5,5' tetraiodothyroxyne (T₄)-induced hyperthyroidism promotes a marked browning of WAT. Of note, central or VMH-specific administration of 3,3',5-triiodothyronine (T₃) recapitulates that effect. The specific genetic activation of hypothalamic AMPK in the VMH reversed the central effect of T₃ on browning. Finally, we also showed that the expression of browning genes in human WAT correlates with serum T₄ Overall, these data indicate that THs induce browning of WAT and that this mechanism is mediated via the central effects of THs on energy balance.

Beyond obesity – thermogenic adipocytes and cardiometabolic health

The global prevalence of obesity and related cardiometabolic disease continues to increase through the 21st century. Whilst multi-factorial, obesity is ultimately caused by chronic caloric excess. However, despite numerous interventions focussing on reducing caloric intake these either fail or only elicit short-term changes in body mass. There is now a focus on increasing energy expenditure instead which has stemmed from the recent ‘re-discovery’ of cold-activated brown adipose tissue (BAT) in adult humans and inducible ‘beige’ adipocytes. Through the unique mitochondrial uncoupling protein 1 (UCP1), these thermogenic adipocytes are capable of combusting large amounts of chemical energy as heat and in animal models can prevent obesity and cardiometabolic disease. At present, human data does not point to a role for thermogenic adipocytes in regulating body weight or fat mass but points to a pivotal role in regulating metabolic health by improving insulin resistance as well as glucose and lipid homeostasis. This review will therefore focus on the metabolic benefits of BAT activation and the mechanisms and signalling pathways by which these could occur including improvements in insulin signalling in peripheral tissues, systemic lipid and cholesterol metabolism and cardiac and vascular function.

Browning of white adipose tissue: lessons from experimental models

Beige or brite (brown-in-white) adipocytes are present in white adipose tissue (WAT) and have a white fat-like phenotype that when stimulated acquires a brown fat-like phenotype, leading to increased thermogenesis. This phenomenon is known as browning and is more likely to occur in subcutaneous fat depots. Browning involves the expression of many transcription factors, such as PR domain containing 16 (PRDM16) and peroxisome proliferator-activated receptor (PPAR)-γ, and of uncoupling protein (UCP)-1, which is the hallmark of thermogenesis. Recent papers pointed that browning can occur in the WAT of humans, with beneficial metabolic effects. This fact indicates that these cells can be targeted to treat a range of diseases, with both pharmacological and nutritional activators. Pharmacological approaches to induce browning include the use of PPAR-α agonist, adrenergic receptor stimulation, thyroid hormone administration, irisin and FGF21 induction. Most of them act through the induction of PPAR-γ coactivator (PGC) 1-α and the consequent mitochondrial biogenesis and UCP1 induction. About the nutritional inducers, several compounds have been described with multiple mechanisms of action. Some of these activators include specific amino acids restriction, capsaicin, bile acids, Resveratrol, and retinoic acid. Besides that, some classes of lipids, as well as many plant extracts, have also been implicated in the browning of WAT. In conclusion, the discovery of browning in human WAT opens the possibility to target the adipose tissue to fight a range of diseases. Studies have arisen showing promising results and bringing new opportunities in thermogenesis and obesity control.

Tissue Immunometabolism: Development, Physiology, and Pathobiology

Evolution of metazoans resulted in the specialization of cellular and tissue function. This was accomplished by division of labor, which allowed tissue parenchymal cells to prioritize their core functions while ancillary functions were delegated to tissue accessory cells, such as immune, stromal, and endothelial cells. In metabolic organs, the accessory cells communicate with their clients, the tissue parenchymal cells, to optimize cellular processes, allowing organisms to adapt to changes in their environment. Here, we discuss tissue immunometabolism from this vantage point and use examples from adipose tissues (white, beige, and brown) and liver to outline the general principles by which accessory cells support metabolic homeostasis in parenchymal cells. A corollary of this model is that disruption of communication between client and accessory cells might predispose metabolic organs to the development of disease.

Plasticity of adipose tissue in response to fasting and refeeding in male mice

Background

Fasting is the most widely prescribed and self-imposed strategy for treating excessive weight gain and obesity, and has been shown to exert a number of beneficial effects. The aim of the present study was to determine the exact role of fasting and subsequent refeeding on fat distribution in mice.

Methods

C57/BL6 mice fasted for 24 to 72 h and were then subjected to refeeding for 72 h. At 24, 48 and 72 h of fasting, and 12, 24, 48 and 72 h of refeeding, the mice were sacrificed, and serum and various adipose tissues were collected. Serum biochemical parameters, adipose tissue masses and histomorphological analysis of different depots were detected. MRNA was isolated from various adipose tissues, and the expressions of thermogenesis, visceral signature and lipid metabolism-related genes were examined. The phenotypes of adipose tissues between juvenile and adult mice subjected to fasting and refeeding were also compared.

Results

Fasting preferentially consumed mesenteric fat mass and decreased the cell size of mesenteric depots; however, refeeding recovered the mass and morphology of inguinal adipose tissues preferentially compared with visceral depots. Thermogenesis-related gene expression in the inguinal WAT and interscapular BAT were suppressed. Mitochondrial biogenesis was affected by fasting in a depot-specific manner. Furthermore, a short period of fasting led to an increase in visceral signature genes (Wt1, Tcf21) in subcutaneous adipose tissue, while the expression of these genes decreased sharply as the fasting time increased. Additionally, lipogenesis-related markers were enhanced to a greater extent greater in subcutaneous depots compared with those in visceral adipose tissues by refeeding. Although similar phenotypic changes in adipose tissue were observed between juvenile mice and adult mice subjected to fasting and refeeding, the alterations appeared earlier and more sensitively in juvenile mice.

Conclusions

Fasting preferentially consumes lipids in visceral adipose tissues, whereas refeeding recovers lipids predominantly in subcutaneous adipose tissues, which indicated the significance of plasticity of adipose organs for fat distribution when subject to food deprivation or refeeding.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-016-0159-x) contains supplementary material, which is available to authorized users.

Metabolic heterogeneity of activated beige/brite adipocytes in inguinal adipose tissue

Sustained β3 adrenergic receptor (ADRB3) activation simultaneously upregulates fatty acid synthesis and oxidation in mouse brown, beige, and white adipose tissues; however, the cellular basis of this dual regulation is not known. Treatment of mice with the ADRB3 agonist CL316,243 (CL) increased expression of fatty acid synthase (FASN) and medium chain acyl-CoA dehydrogenase (MCAD) protein within the same cells in classic brown and white adipose tissues. Surprisingly, in inguinal adipose tissue, CL-upregulated FASN and MCAD in distinct cell populations: high MCAD expression occurred in multilocular adipocytes that co-expressed UCP1+, whereas high FASN expression occurred in paucilocular adipocytes lacking detectable UCP1. Genetic tracing with UCP1-cre, however, indicated nearly half of adipocytes with a history of UCP1 expression expressed high levels of FASN without current expression of UCP1. Global transcriptomic analysis of FACS-isolated adipocytes confirmed the presence of distinct anabolic and catabolic phenotypes, and identified differential expression of transcriptional pathways known to regulate lipid synthesis and oxidation. Surprisingly, paternally-expressed genes of the non-classical gene imprinted network were strikingly enriched in anabolic phenotypes, suggesting possible involvement in maintaining the balance of metabolic phenotypes. The results indicate that metabolic heterogeneity is a distinct property of activated beige/brite adipocytes that might be under epigenetic control.

Reduction of Hypothalamic Endoplasmic Reticulum Stress Activates Browning of White Fat and Ameliorates Obesity

The chaperone GRP78/BiP (glucose-regulated protein 78 kDa/binding immunoglobulin protein) modulates protein folding in reply to cellular insults that lead to endoplasmic reticulum (ER) stress. This study investigated the role of hypothalamic GRP78 on energy balance, with particular interest in thermogenesis and browning of white adipose tissue (WAT). For this purpose, we used diet-induced obese rats and rats administered thapsigargin, and by combining metabolic, histologic, physiologic, pharmacologic, thermographic, and molecular techniques, we studied the effect of genetic manipulation of hypothalamic GRP78. Our data showed that rats fed a high-fat diet or that were centrally administered thapsigargin displayed hypothalamic ER stress, whereas genetic overexpression of GRP78 specifically in the ventromedial nucleus of the hypothalamus was sufficient to alleviate ER stress and to revert the obese and metabolic phenotype. Those effects were independent of feeding and leptin but were related to increased thermogenic activation of brown adipose tissue and induction of browning in WAT and could be reversed by antagonism of β3 adrenergic receptors. This evidence indicates that modulation of hypothalamic GRP78 activity may be a potential strategy against obesity and associated comorbidities.

Flow Cytometric Isolation and Differentiation of Adipogenic Progenitor Cells into Brown and Brite/Beige Adipocytes

Aside from mature adipocytes, adipose tissue harbors several distinct cell populations including immune cells, endothelial cells, and adipogenic progenitor cells (AdPCs). AdPCs represent the reservoir of regenerative cells that replenishes adipocytes during normal cellular turnover and during times of increased demand for triglyceride-storage capacity. The worldwide increase in pathologies associated with the metabolic syndrome, such as obesity and type-2 diabetes, has heightened public and scientific interest in adipose tissues and the cell biological processes of adipose tissue formation and function. Two distinct types of fat cells are known: White and brown adipocytes. Especially brown adipose tissue (BAT) has received considerable attention due to its unique capacity for thermogenic energy expenditure and potential role in the treatment of adiposity. Accordingly, the cold-induced conversion of white into brown-like adipocytes has become a feasible approach in humans and a study-subject in rodents to better understand the underlying molecular processes. Fluorescence-activated cell sorting (FACS) provides a method to isolate AdPCs and other cell populations from adipose tissue by using antibodies detecting unique surface markers. We here describe an approach to isolate cells committed to the adipogenic lineage and summarize established protocols to differentiate FACS-purified primary AdPCs into UCP1-expressing brown adipocytes under in vitro conditions.

Brown adipose tissue as a secretory organ

Brown adipose tissue (BAT) is the main site of adaptive thermogenesis and experimental studies have associated BAT activity with protection against obesity and metabolic diseases, such as type 2 diabetes mellitus and dyslipidaemia. Active BAT is present in adult humans and its activity is impaired in patients with obesity. The ability of BAT to protect against chronic metabolic disease has traditionally been attributed to its capacity to utilize glucose and lipids for thermogenesis. However, BAT might also have a secretory role, which could contribute to the systemic consequences of BAT activity. Several BAT-derived molecules that act in a paracrine or autocrine manner have been identified. Most of these factors promote hypertrophy and hyperplasia of BAT, vascularization, innervation and blood flow, processes that are all associated with BAT recruitment when thermogenic activity is enhanced. Additionally, BAT can release regulatory molecules that act on other tissues and organs. This secretory capacity of BAT is thought to be involved in the beneficial effects of BAT transplantation in rodents. Fibroblast growth factor 21, IL-6 and neuregulin 4 are among the first BAT-derived endocrine factors to be identified. In this Review, we discuss the current understanding of the regulatory molecules (the so-called brown adipokines or batokines) that are released by BAT that influence systemic metabolism and convey the beneficial metabolic effects of BAT activation. The identification of such adipokines might also direct drug discovery approaches for managing obesity and its associated chronic metabolic diseases.

UCP1 inhibition in Cidea-overexpressing mice is physiologically counteracted by brown adipose tissue hyperrecruitment

Cidea is a gene highly expressed in thermogenesis-competent (UCP1-containing) adipose cells, both brown and brite/beige. Here, we initially demonstrate a remarkable adipose-depot specific regulation of Cidea expression. In classical brown fat, Cidea mRNA is expressed continuously and invariably, irrespective of tissue recruitment. However, Cidea protein levels are regulated posttranscriptionally, being conspicuously induced in the thermogenically recruited state. In contrast, in brite fat, Cidea protein levels are regulated at the transcriptional level, and Cidea mRNA and protein levels are proportional to tissue "briteness." Although routinely followed as a thermogenic molecular marker, Cidea function is not clarified. Here, we employed a gain-of-function approach to examine a possible role of Cidea in the regulation of thermogenesis. We utilized transgenic aP2-hCidea mice that overexpress human Cidea in all adipose tissues. We demonstrate that UCP1 activity is markedly suppressed in brown-fat mitochondria isolated from aP2-hCidea mice. However, mitochondrial UCP1 protein levels were identical in wild-type and transgenic mice. This implies a regulatory effect of Cidea on UCP1 activity, but as we demonstrate that Cidea itself is not localized to mitochondria, we propose an indirect inhibitory effect. The Cidea-induced inhibition of UCP1 activity (observed in isolated mitochondria) is physiologically relevant since the mice, through an appropriate homeostatic compensatory mechanism, increased the total amount of UCP1 in the tissue to exactly match the diminished thermogenic capacity of the UCP1 protein and retain unaltered nonshivering thermogenic capacity. Thus, we verified Cidea as being a marker of thermogenesis-competent adipose tissues, but we conclude that Cidea, unexpectedly, functions molecularly as an indirect inhibitor of thermogenesis.

Hypothalamus and thermogenesis: Heating the BAT, browning the WAT

Brown adipose tissue (BAT) has been also considered as the main thermogenic organ responsible of maintenance body temperature through heat production. However, a new type of thermogenic fat has been characterized during the last years, the beige or brite fat, that is developed from white adipose tissue (WAT) in response to different stimuli by a process known as browning. The activities of brown and beige adipocytes ameliorate metabolic disease, including obesity in mice and correlate with leanness in humans. Many genes and pathways that regulate brown and beige adipocyte biology have now been identified, providing a variety of promising therapeutic targets for metabolic disease. The hypothalamus is the main central place orchestrating the outflow signals that drive the sympathetic nerve activity to BAT and WAT, controlling heat production and energy homeostasis. Recent data have revealed new hypothalamic molecular mechanisms, such as hypothalamic AMP-activated protein kinase (AMPK), that control both thermogenesis and browning. This review provides an overview of the factors influencing BAT and WAT thermogenesis, with special focus on the integration of peripheral information on hypothalamic circuits controlling thermoregulation.

Activation of β3-adrenoceptors increases in vivo free fatty acid uptake and utilization in brown but not white fat depots in high-fat-fed rats

Activation of brown adipose tissue (BAT) and browning of white adipose tissue (WAT) present potential new therapies for obesity and type 2 diabetes. Here, we examined the effects of β₃-adrenergic stimulation on tissue-specific uptake and storage of free fatty acids (FFA) and its implications for whole body FFA metabolism in diet-induced obese rats using a multi-radiotracer technique. Male Wistar rats were high fat-fed for 12 wk and administered β3-agonist CL316,243 (CL, 1 mg·kg^-1·day^-1) or saline via osmotic minipumps during the last 3 wk. The rats were then fasted and acutely infused with a tracer mixture ([¹⁴C]palmitate and the partially metabolized R-[³H]bromopalmitate) under anesthesia. CL infusion decreased body weight gain and fasting plasma glucose levels. While core body temperature was unaffected, infrared thermography showed an increase in tail heat dissipation following CL infusion. Interestingly, CL markedly increased both FFA storage and utilization in interscapular and perirenal BAT, whereas the flux of FFA to skeletal muscle was decreased. In this rat model of obesity, only sporadic populations of beige adipocytes were detected in the epididymal WAT depot of CL-infused rats, and there was no change in FFA uptake or utilization in WAT following CL infusion. In summary, β₃-agonism robustly increased FFA flux to BAT coupled with enhanced utilization. Increased BAT activation most likely drove the increased tail heat dissipation to maintain thermostasis. Our results emphasize the quantitative role of brown fat as the functional target of β₃-agonism in obesity.

Separate and shared sympathetic outflow to white and brown fat coordinately regulates thermoregulation and beige adipocyte recruitment

White adipose tissue (WAT) and brown adipose tissue (BAT) are innervated and regulated by the sympathetic nervous system (SNS). It is not clear, however, whether there are shared or separate central SNS outflows to WAT and BAT that regulate their function. We injected two isogenic strains of pseudorabies virus, a retrograde transneuronal viral tract tracer, with unique fluorescent reporters into interscapular BAT (IBAT) and inguinal WAT (IWAT) of the same Siberian hamsters to define SNS pathways to both. To test the functional importance of SNS coordinated control of BAT and WAT, we exposed hamsters with denervated SNS nerves to IBAT to 4°C for 16-24 h and measured core and fat temperatures and norepinephrine turnover (NETO) and uncoupling protein 1 (UCP1) expression in fat tissues. Overall, there were more SNS neurons innervating IBAT than IWAT across the neuroaxis. However, there was a greater percentage of singly labeled IWAT neurons in midbrain reticular nuclei than singly labeled IBAT neurons. The hindbrain had ~30-40% of doubly labeled neurons while the forebrain had ~25% suggesting shared SNS circuitry to BAT and WAT across the brain. The raphe nucleus, a key region in thermoregulation, had ~40% doubly labeled neurons. Hamsters with IBAT SNS denervation maintained core body temperature during acute cold challenge and had increased beige adipocyte formation in IWAT. They also had increased IWAT NETO, temperature, and UCP1 expression compared with intact hamsters. These data provide strong neuroanatomical and functional evidence of WAT and BAT SNS cross talk for thermoregulation and beige adipocyte formation.

The lipid sensor GPR120 promotes brown fat activation and FGF21 release from adipocytes

The thermogenic activity of brown adipose tissue (BAT) and browning of white adipose tissue are important components of energy expenditure. Here we show that GPR120, a receptor for polyunsaturated fatty acids, promotes brown fat activation. Using RNA-seq to analyse mouse BAT transcriptome, we find that the gene encoding GPR120 is induced by thermogenic activation. We further show that GPR120 activation induces BAT activity and promotes the browning of white fat in mice, whereas GRP120-null mice show impaired cold-induced browning. Omega-3 polyunsaturated fatty acids induce brown and beige adipocyte differentiation and thermogenic activation, and these effects require GPR120. GPR120 activation induces the release of fibroblast growth factor-21 (FGF21) by brown and beige adipocytes, and increases blood FGF21 levels. The effects of GPR120 activation on BAT activation and browning are impaired in FGF21-null mice and cells. Thus, the lipid sensor GPR120 activates brown fat via a mechanism that involves induction of FGF21.

GPR120 is a G-protein-coupled receptor that binds polyunsaturated fatty acids. Here, the authors show that GPR120 is upregulated in brown fat in cold-exposed mice, and mediates thermogenic activation of brown fat via a mechanism that, at least in part, depends on the release of the adipokine FGF21.

'Browning' the cardiac and peri-vascular adipose tissues to modulate cardiovascular risk

Excess visceral adiposity, in particular that located adjacent to the heart and coronary arteries is associated with increased cardiovascular risk. In the pathophysiological state, dysfunctional adipose tissue secretes an array of factors modulating vascular function and driving atherogenesis. Conversely, brown and beige adipose tissues utilise glucose and lipids to generate heat and are associated with improved cardiometabolic health. The cardiac and thoracic perivascular adipose tissues are now understood to be composed of brown adipose tissue in the healthy state and undergo a brown-to-white transition i.e. during obesity which may be a driving factor of cardiovascular disease. In this review we discuss the risks of excess cardiac and vascular adiposity and potential mechanisms by which restoring the brown phenotype i.e. “re-browning” could potentially be achieved in clinically relevant populations.

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Epicardial, paracardial and thoracic perivascular adipose tissues resemble BAT at birth.

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Despite ‘whitening’ in early life these depots remain metabolically active and potentially thermogenic into adulthood.

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Obesity induces further ‘whitening’ and inflammation in these depots likely driving the atherogenesis.

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Maintaining or inducing the brown phenotype in these depots could prevent atherosclerotic disease.

Salsalate (Salicylate) Uncouples Mitochondria, Improves Glucose Homeostasis, and Reduces Liver Lipids Independent of AMPK-β1

Salsalate is a prodrug of salicylate that lowers blood glucose in patients with type 2 diabetes (T2D) and reduces nonalcoholic fatty liver disease (NAFLD) in animal models; however, the mechanism mediating these effects is unclear. Salicylate directly activates AMPK via the β1 subunit, but whether salsalate requires AMPK-β1 to improve T2D and NAFLD has not been examined. Therefore, wild-type (WT) and AMPK-β1-knockout (AMPK-β1KO) mice were treated with a salsalate dose resulting in clinically relevant serum salicylate concentrations (∼1 mmol/L). Salsalate treatment increased VO₂, lowered fasting glucose, improved glucose tolerance, and led to an ∼55% reduction in liver lipid content. These effects were observed in both WT and AMPK-β1KO mice. To explain these AMPK-independent effects, we found that salicylate increases oligomycin-insensitive respiration (state 4o) and directly increases mitochondrial proton conductance at clinical concentrations. This uncoupling effect is tightly correlated with the suppression of de novo lipogenesis. Salicylate is also able to stimulate brown adipose tissue respiration independent of uncoupling protein 1. These data indicate that the primary mechanism by which salsalate improves glucose homeostasis and NAFLD is via salicylate-driven mitochondrial uncoupling.

Rutin ameliorates obesity through brown fat activation

Increasing energy expenditure through activation of brown adipose tissue (BAT) is a critical approach to treating obesity and diabetes. In this study, rutin, a natural compound extracted from mulberry and a drug used as a capillary stabilizer clinically for many years without any side effects, regulated whole-body energy metabolism by enhancing BAT activity. Rutin treatment significantly reduced adiposity, increased energy expenditure, and improved glucose homeostasis in both genetically obese (Db/Db) and diet-induced obesity (DIO) mice. Rutin also induced brown-like adipocyte (beige) formation in subcutaneous adipose tissue in both obesity mouse models. Mechanistically, we found that rutin directly bound to and stabilized SIRT1, leading to hypoacetylation of peroxisome proliferator-activated receptor γ coactivator-1α protein, which stimulated Tfam transactivation and eventually augmented the number of mitochondria and UCP1 activity in BAT. These findings reveal that rutin is a novel small molecule that activates BAT and may provide a novel therapeutic approach to the treatment of metabolic disorders.-Yuan, X., Wei, G., You, Y., Huang, Y., Lee, H. J., Dong, M., Lin, J., Hu, T., Zhang, H., Zhang, C., Zhou, H., Ye, R., Qi, X., Zhai, B., Huang, W., Liu, S., Xie, W., Liu, Q., Liu, X., Cui, C., Li, D., Zhan, J., Cheng, J., Yuan, Z., Jin, W. Rutin ameliorates obesity through brown fat activation.

β-Lapachone Prevents Diet-Induced Obesity by Increasing Energy Expenditure and Stimulating the Browning of White Adipose Tissue via Downregulation of miR-382 Expression

There has been great interest in the browning of fat for the treatment of obesity. Although β-lapachone (BLC) has potential therapeutic effects on obesity, the fat-browning effect and thermogenic capacity of BLC on obesity have never been demonstrated. Here, we showed that BLC stimulated the browning of white adipose tissue (WAT), increased the expression of brown adipocyte-specific genes (e.g., uncoupling protein 1 [UCP1]), decreased body weight gain, and ameliorated metabolic parameters in mice fed a high-fat diet. Consistently, BLC-treated mice showed significantly higher energy expenditure compared with control mice. In vitro, BLC increased the expression of brown adipocyte-specific genes in stromal vascular fraction-differentiated adipocytes. BLC also controlled the expression of miR-382, which led to the upregulation of its direct target, Dio2. Upregulation of miR-382 markedly inhibited the differentiation of adipocytes into beige adipocytes, whereas BLC recovered beige adipocyte differentiation and increased the expression of Dio2 and UCP1. Our findings suggest that the BLC-mediated increase in the browning of WAT and the thermogenic capacity of BAT significantly results in increases in energy expenditure. Browning of WAT by BLC was partially controlled via the regulation of miR-382 targeting Dio2 and may lead to the prevention of diet-induced obesity.

Mitochondria-targeted dodecyltriphenylphosphonium (C12TPP) combats high-fat-diet-induced obesity in mice

Background:

A membrane-penetrating cation, dodecyltriphenylphosphonium (C₁₂TPP), facilitates the recycling of fatty acids in the artificial lipid membrane and mitochondria. C₁₂TPP can dissipate mitochondrial membrane potential and may affect total energy expenditure and body weight in animals and humans.

Methods:

We investigated the metabolic effects of C₁₂TPP in isolated brown-fat mitochondria, brown adipocyte cultures and mice in vivo. Experimental approaches included the measurement of oxygen consumption, carbon dioxide production, western blotting, magnetic resonance imaging and bomb calorimetry.

Results:

In mice, C₁₂TPP (50 μmol per (day•kg body weight)) in the drinking water significantly reduced body weight (12%, P<0.001) and body fat mass (24%, P<0.001) during the first 7 days of treatment. C₁₂TPP did not affect water palatability and intake or the energy and lipid content in feces. The addition of C₁₂TPP to isolated brown-fat mitochondria resulted in increased oxygen consumption. Three hours of pretreatment with C₁₂TPP also increased oligomycin-insensitive oxygen consumption in brown adipocyte cultures (P<0.01). The effects of C₁₂TPP on mitochondria, cells and mice were independent of uncoupling protein 1 (UCP1). However, C₁₂TPP treatment increased the mitochondrial protein levels in the brown adipose tissue of both wild-type and UCP1-knockout mice. Pair-feeding revealed that one-third of the body weight loss in C₁₂TPP-treated mice was due to reduced food intake. C₁₂TPP treatment elevated the resting metabolic rate (RMR) by up to 18% (P<0.05) compared with pair-fed animals. C₁₂TPP reduced the respiratory exchange ratio, indicating enhanced fatty acid oxidation in mice.

Conclusions:

C₁₂TPP combats diet-induced obesity by reducing food intake, increasing the RMR and enhancing fatty acid oxidation.

Hormonal and nutritional signalling in the control of brown and beige adipose tissue activation and recruitment

Recent research has revealed that the activity of adipose tissue (BAT) in adult humans is higher than previously thought, and that obese patients show abnormally low levels of brown fat activity. Studies in experimental animals have shown that BAT is a site of energy expenditure, and that BAT activity protects against obesity and associated metabolic diseases. The action of the sympathetic nervous activity on BAT depots is considered the main regulator of BAT activity in rodent models and possibly also in humans. However, recent research has revealed the existence of additional hormonal factors, produced by distinct peripheral tissues or present in the diet, that influence the amount and activity of BAT. These hormonal factors may act on BAT directly, but also indirectly by targeting the brain and determining the intensity of sympathetic action upon BAT. Identification and characterization of novel factors that control BAT may provide clues for the development of new strategies to treat obesity and metabolic diseases.

Suppression of Resting Metabolism by the Angiotensin AT2 Receptor

Activation of the brain renin-angiotensin system (RAS) stimulates energy expenditure through increasing of the resting metabolic rate (RMR), and this effect requires simultaneous suppression of the circulating and/or adipose RAS. To identify the mechanism by which the peripheral RAS opposes RMR control by the brain RAS, we examined mice with transgenic activation of the brain RAS (sRA mice). sRA mice exhibit increased RMR through increased energy flux in the inguinal adipose tissue, and this effect is attenuated by angiotensin II type 2 receptor (AT2) activation. AT2 activation in inguinal adipocytes opposes norepinephrine-induced uncoupling protein-1 (UCP1) production and aspects of cellular respiration, but not lipolysis. AT2 activation also opposes inguinal adipocyte function and differentiation responses to epidermal growth factor (EGF). These results highlight a major, multifaceted role for AT2 within inguinal adipocytes in the control of RMR. The AT2 receptor may therefore contribute to body fat distribution and adipose depot-specific effects upon cardio-metabolic health.

Metabolic activity of brown, "beige," and white adipose tissues in response to chronic adrenergic stimulation in male mice

Classical brown adipocytes such as those found in interscapular brown adipose tissue (iBAT) represent energy-burning cells, which have been postulated to play a pivotal role in energy metabolism. Brown adipocytes can also be found in white adipose tissue (WAT) depots [e.g., inguinal WAT (iWAT)] following adrenergic stimulation, and they have been referred to as "beige" adipocytes. Whether the presence of these adipocytes, which gives iWAT a beige appearance, can confer a white depot with some thermogenic activity remains to be seen. In consequence, we designed the present study to investigate the metabolic activity of iBAT, iWAT, and epididymal white depots in mice. Mice were either 1) kept at thermoneutrality (30°C), 2) kept at 30°C and treated daily for 14 days with an adrenergic agonist [CL-316,243 (CL)], or 3) housed at 10°C for 14 days. Metabolic activity was assessed using positron emission tomography imaging with fluoro-[(18)F]deoxyglucose (glucose uptake), fluoro-[(18)F]thiaheptadecanoic acid (fatty acid uptake), and [(11)C]acetate (oxidative activity). In each group, substrate uptakes and oxidative activity were measured in anesthetized mice in response to acute CL. Our results revealed iBAT as a major site of metabolic activity, which exhibited enhanced glucose and nonesterified fatty acid uptakes and oxidative activity in response to chronic cold and CL. On the other hand, beige adipose tissue failed to exhibit appreciable increase in oxidative activity in response to chronic cold and CL. Altogether, our results suggest that the contribution of beige fat to acute-CL-induced metabolic activity is low compared with that of iBAT, even after sustained adrenergic stimulation.

A novel thermoregulatory role for PDE10A in mouse and human adipocytes

Phosphodiesterase type 10A (PDE10A) is highly enriched in striatum and is under evaluation as a drug target for several psychiatric/neurodegenerative diseases. Preclinical studies implicate PDE10A in the regulation of energy homeostasis, but the mechanisms remain unclear. By utilizing small-animal PET/MRI and the novel radioligand [(18)F]-AQ28A, we found marked levels of PDE10A in interscapular brown adipose tissue (BAT) of mice. Pharmacological inactivation of PDE10A with the highly selective inhibitor MP-10 recruited BAT and potentiated thermogenesis in vivo In diet-induced obese mice, chronic administration of MP-10 caused weight loss associated with increased energy expenditure, browning of white adipose tissue, and improved insulin sensitivity. Analysis of human PET data further revealed marked levels of PDE10A in the supraclavicular region where brown/beige adipocytes are clustered in adults. Finally, the inhibition of PDE10A with MP-10 stimulated thermogenic gene expression in human brown adipocytes and induced browning of human white adipocytes. Collectively, our findings highlight a novel thermoregulatory role for PDE10A in mouse and human adipocytes and promote PDE10A inhibitors as promising candidates for the treatment of obesity and diabetes.

No insulating effect of obesity

The development of obesity may be aggravated if obesity itself insulates against heat loss and thus diminishes the amount of food burnt for body temperature control. This would be particularly important under normal laboratory conditions where mice experience a chronic cold stress (at ≈20°C). We used Scholander plots (energy expenditure plotted against ambient temperature) to examine the insulation (thermal conductance) of mice, defined as the inverse of the slope of the Scholander curve at subthermoneutral temperatures. We verified the method by demonstrating that shaved mice possessed only half the insulation of nonshaved mice. We examined a series of obesity models [mice fed high-fat diets and kept at different temperatures, classical diet-induced obese mice, ob/ob mice, and obesity-prone (C57BL/6) vs. obesity-resistant (129S) mice]. We found that neither acclimation temperature nor any kind or degree of obesity affected the thermal insulation of the mice when analyzed at the whole mouse level or as energy expenditure per lean weight. Calculation per body weight erroneously implied increased insulation in obese mice. We conclude that, in contrast to what would be expected, obesity of any kind does not increase thermal insulation in mice, and therefore, it does not in itself aggravate the development of obesity. It may be discussed as to what degree of effect excess adipose tissue has on insulation in humans and especially whether significant metabolic effects are associated with insulation in humans.

Capsaicin induces browning of white adipose tissue and counters obesity by activating TRPV1 channel-dependent mechanisms

BACKGROUND AND PURPOSE

The growing epidemic of obesity and metabolic diseases necessitates the development of novel strategies to prevent and treat such diseases. Current research suggests that browning of white adipose tissue (WAT) promotes energy expenditure to counter obesity. Recent research suggests that activation of the TRPV1 channels counters obesity. However, the mechanism by which activation of TRPV1 channels counters obesity still remains unclear.

EXPERIMENTAL APPROACH

We evaluated the effect of dietary capsaicin to induce a browning program in WAT by activating TRPV1 channels to prevent diet-induced obesity using wild-type and TRPV1(-/-) mouse models. We performed experiments using preadipocytes and fat pads from these mice.

KEY RESULTS

Capsaicin stimulated the expression of brown fat-specific thermogenic uncoupling protein-1 and bone morphogenetic protein-8b in WAT. Capsaicin triggered browning of WAT by promoting sirtuin-1 expression and activity via TRPV1 channel-dependent elevation of intracellular Ca(2) (+) and phosphorylation of Ca(2) (+) /calmodulin-activated protein kinase II and AMP-activated kinase. Capsaicin increased the expression of PPARγ 1 coactivator α and enhanced metabolic and ambulatory activity. Further, capsaicin stimulated sirtuin-1-dependent deacetylation of PPARγ and the transcription factor PRDM-16 and facilitated PPARγ-PRDM-16 interaction to induce browning of WAT. Dietary capsaicin did not protect TRPV1(-/-) mice from obesity.

CONCLUSIONS AND INTERPRETATIONS

Our results show for the first time that activation of TRPV1 channels by dietary capsaicin triggers browning of WAT to counteract obesity. Our results suggest that activation of TRPV1 channels is a promising strategy to counter obesity.

Severe Burn Injury Induces Thermogenically Functional Mitochondria in Murine White Adipose Tissue

Chronic cold exposure induces functionally thermogenic mitochondria in the inguinal white adipose tissue (iWAT) of mice. Whether this response occurs in pathophysiological states remains unclear. The purpose of this study was to determine the impact of severe burn trauma on iWAT mitochondrial function in mice. Male BALB/c mice (10-12 weeks) received full-thickness scald burns to ∼30% of the body surface area. Inguinal white adipose tissue was harvested from mice at 1, 4, 10, 20, and 40 days postinjury. Total and uncoupling protein 1 (UCP1)-dependent mitochondrial thermogenesis were determined in iWAT. Citrate synthase activity was determined as a proxy of mitochondrial abundance. Immunohistochemistry was performed to assess iWAT morphology and UCP1 expression. Uncoupling protein 1-dependent respiration was significantly greater at 4 and 10 days after burn compared with sham, peaking at 20 days after burn (P < 0.001). Citrate synthase activity was threefold greater at 4, 10, 20, and 40 days after burn versus sham (P < 0.05). Per mitochondrion, UCP1 function increased after burn trauma (P < 0.05). After burn trauma, iWAT exhibited numerous multilocular lipid droplets that stained positive for UCP1. The current findings demonstrate the induction of thermogenically competent mitochondria within rodent iWAT in a model of severe burn trauma. These data identify a specific pathology that induces the browning of white adipose tissue in vivo and may offer a mechanistic explanation for the chronic hypermetabolism observed in burn victims.

Emerging Complexities in Adipocyte Origins and Identity

The global incidence of obesity and its comorbidities continues to rise along with a demand for novel therapeutic interventions. Brown adipose tissue (BAT) is attracting attention as a therapeutic target because of its presence in adult humans and high capacity to dissipate energy as heat, and thus burn excess calories, when stimulated. Another potential avenue for therapeutic intervention is to induce, within white adipose tissue (WAT), the formation of brown-like adipocytes called brite (brown-like-in-white) or beige adipocytes. However, understanding how to harness the potential of these thermogenic cells requires a deep understanding of their developmental origins and regulation. Recent cell-labeling and lineage-tracing experiments are beginning to shed light on this emerging area of adipocyte biology. We review here adipocyte development, giving particular attention to thermogenic adipocytes.

A Synergistic Antiobesity Effect by a Combination of Capsinoids and Cold Temperature Through Promoting Beige Adipocyte Biogenesis

Beige adipocytes emerge postnatally within the white adipose tissue in response to certain environmental cues, such as chronic cold exposure. Because of its highly recruitable nature and relevance to adult humans, beige adipocytes have gained much attention as an attractive cellular target for antiobesity therapy. However, molecular circuits that preferentially promote beige adipocyte biogenesis remain poorly understood. We report that a combination of mild cold exposure at 17°C and capsinoids, a nonpungent analog of capsaicin, synergistically and preferentially promotes beige adipocyte biogenesis and ameliorates diet-induced obesity. Gain- and loss-of-function studies show that the combination of capsinoids and cold exposure synergistically promotes beige adipocyte development through the β2-adrenoceptor signaling pathway. This synergistic effect on beige adipocyte biogenesis occurs through an increased half-life of PRDM16, a dominant transcriptional regulator of brown/beige adipocyte development. We document a previously unappreciated molecular circuit that controls beige adipocyte biogenesis and suggest a plausible approach to increase whole-body energy expenditure by combining dietary components and environmental cues.

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.