UCP1 in brite/beige adipose tissue mitochondria is functionally thermogenic

QKI regulates adipose tissue metabolism by acting as a brake on thermogenesis and promoting obesity

Adipose tissue controls numerous physiological processes, and its dysfunction has a causative role in the development of systemic metabolic disorders. The role of posttranscriptional regulation in adipose metabolism has yet to be fully understood. Here, we show that the RNA-binding protein quaking (QKI) plays an important role in controlling metabolic homeostasis of the adipose tissue. QKI-deficient mice are resistant to high-fat-diet (HFD)-induced obesity. Additionally, QKI depletion increased brown fat energy dissipation and browning of subcutaneous white fat. Adipose tissue-specific depletion of QKI in mice enhances cold-induced thermogenesis, thereby preventing hypothermia in response to cold stimulus. Further mechanistic analysis reveals that QKI is transcriptionally induced by the cAMP-cAMP response element-binding protein (CREB) axis and restricts adipose tissue energy consumption by decreasing stability, nuclear export, and translation of mRNAs encoding UCP1 and PGC1α. These findings extend our knowledge of the significance of posttranscriptional regulation in adipose metabolic homeostasis and provide a potential therapeutic target to defend against obesity and its related metabolic diseases.

ESRRG and PERM1 Govern Mitochondrial Conversion in Brite/Beige Adipocyte Formation

When exposed to cold temperatures, mice increase their thermogenic capacity by an expansion of brown adipose tissue mass and the formation of brite/beige adipocytes in white adipose tissue depots. However, the process of the transcriptional changes underlying the conversion of a phenotypic white to brite/beige adipocytes is only poorly understood. By analyzing transcriptome profiles of inguinal adipocytes during cold exposure and in mouse models with a different propensity to form brite/beige adipocytes, we identified ESRRG and PERM1 as modulators of this process. The production of heat by mitochondrial uncoupled respiration is a key feature of brite/beige compared to white adipocytes and we show here that both candidates are involved in PGC1α transcriptional network to positively regulate mitochondrial capacity. Moreover, we show that an increased expression of ESRRG or PERM1 supports the formation of brown or brite/beige adipocytes in vitro and in vivo. These results reveal that ESRRG and PERM1 are early induced in and important regulators of brite/beige adipocyte formation.

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

PURPOSE

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

METHODS

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

RESULTS

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

DISCUSSION/CONCLUSIONS

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

Obesity: a neuroimmunometabolic perspective

Neuroimmunology and immunometabolism are burgeoning topics of study, but the intersection of these two fields is scarcely considered. This interplay is particularly prevalent within adipose tissue, where immune cells and the sympathetic nervous system (SNS) have an important role in metabolic homeostasis and pathology, namely in obesity. In the present Review, we first outline the established reciprocal adipose-SNS relationship comprising the neuroendocrine loop facilitated primarily by adipose tissue-derived leptin and SNS-derived noradrenaline. Next, we review the extensive crosstalk between adipocytes and resident innate immune cells as well as the changes that occur in these secretory and signalling pathways in obesity. Finally, we discuss the effect of SNS adrenergic signalling in immune cells and conclude with exciting new research demonstrating an immutable role for SNS-resident macrophages in modulating SNS-adipose crosstalk. We posit that the latter point constitutes the existence of a new field - neuroimmunometabolism.

HYPOTHesizing about central comBAT against obesity

The hypothalamus is a brain region in charge of many vital functions. Among them, BAT thermogenesis represents an essential physiological function to maintain body temperature. In the metabolic context, it has now been established that energy expenditure attributed to BAT function can contribute to the energy balance in a substantial extent. Thus, therapeutic interest in this regard has increased in the last years and some studies have shown that BAT function in humans can make a real contribution to improve diabetes and obesity-associated diseases. Nevertheless, how the hypothalamus controls BAT activity is still not fully understood. Despite the fact that much has been known about the mechanisms that regulate BAT activity in recent years, and that the central regulation of thermogenesis offers a very promising target, many questions remain still unsolved. Among them, the possible human application of knowledge obtained from rodent studies, and drug administration strategies able to specifically target the hypothalamus. Here, we review the current knowledge of homeostatic regulation of BAT, including the molecular insights of brown adipocytes, its central control, and its implication in the development of obesity.

Towards a Better Understanding of Beige Adipocyte Plasticity

Beige adipocytes are defined as Ucp1⁺, multilocular adipocytes within white adipose tissue (WAT) that are capable of thermogenesis, the process of heat generation. In both mouse models and humans, the increase of beige adipocyte population, also called WAT browning, is associated with certain metabolic benefits, such as reduced obesity and increased insulin sensitivity. In this review, we summarize the current knowledge regarding WAT browning, with a special focus on the beige adipocyte plasticity, collectively referring to a bidirectional transition between thermogenic active and latent states in response to environmental changes. We further exploit the utility of a unique beige adipocyte ablation system to interrogate anti-obesity effect of beige adipocytes in vivo.

Amelioration of perivascular adipose inflammation reverses vascular dysfunction in a model of nonobese prediabetic metabolic challenge: potential role of antidiabetic drugs

The onset of vascular impairment precedes that of diagnostic hyperglycemia in diabetic patients suggesting a vascular insult early in the course of metabolic dysfunction without a well-defined mechanism. Mounting evidence implicates adipose inflammation in the pathogenesis of insulin resistance and diabetes. It is not certain whether amelioration of adipose inflammation is sufficient to preclude vascular dysfunction in early stages of metabolic disease. Recent findings suggest that antidiabetic drugs, metformin, and pioglitazone, improve vascular function in prediabetic patients, without an indication if this protective effect is mediated by reduction of adipose inflammation. Here, we used a prediabetic rat model with delayed development of hyperglycemia to study the effect of metformin or pioglitazone on adipose inflammation and vascular function. At the end of the metabolic challenge, these rats were neither obese, hypertensive, nor hyperglycemic. However, they showed increased pressor responses to phenylephrine and augmented aortic and mesenteric contraction. Vascular tissues from prediabetic rats showed increased Rho-associated kinase activity causing enhanced calcium sensitization. An elevated level of reactive oxygen species was seen in aortic tissues together with increased Transforming growth factor β1 and Interleukin-1β expression. Although, no signs of systemic inflammation were detected, perivascular adipose inflammation was observed. Adipocyte hypertrophy, increased macrophage infiltration, and elevated Transforming growth factor β1 and Interleukin-1β mRNA levels were seen. Two-week treatment with metformin or pioglitazone or switching to normal chow ameliorated adipose inflammation and vascular dysfunction. Localized perivascular adipose inflammation is sufficient to trigger vascular dysfunction early in the course of diabetes. Interfering with this inflammatory process reverses this early abnormality.

Catechins and Caffeine Promote Lipid Metabolism and Heat Production Through the Transformation of Differentiated 3T3-L1 Adipocytes from White to Beige Adipocytes

To elucidate effects of catechins and caffeine on lipid metabolism in adipocytes and identify the mechanism of action, differentiated 3T3-L1 adipocytes were incubated in culture media containing catechins at 1, 2.5, 5, and 10 µg/mL and caffeine at 50 and 100 µg/mL, singly or in combination, for 8 days. Intracellular lipid accumulation and glycerol-3-phosphate dehydrogenase activity were strongly suppressed by catechins and caffeine combination treatment. The mRNA expression of PPARɤ, GLUT4, HSL, UCP-1, and TMEM26 were significantly increased in the combined groups. These findings suggest that the combined treatment inhibited lipid synthesis and improved lipid metabolism in adipocytes. Moreover, it was indicated that the differentiated 3T3-L1 adipocytes could be transformed from white adipocytes to beige-like adipocytes by catechins and caffeine, and accordingly that this transformation could promote calorigenic action. PRACTICAL APPLICATION: In this study, we revealed that the combined treatment of catechins and caffeine inhibited lipid synthesis and improved lipid metabolism in adipocytes. Moreover, the treatment may contribute to the transforming from white adipocytes to beige-like adipocytes, which could strongly promote calorigenic action.

Milagro F, Sánchez J, de la Garza AL, Castro H. miRNAs and Novel Food Compounds Related to the Browning Process

Obesity prevalence is rapidly increasing worldwide. With the discovery of brown adipose tissue (BAT) in adult humans, BAT activation has emerged as a potential strategy for increasing energy expenditure. Recently, the presence of a third type of fat, referred to as beige or brite (brown in white), has been recognized to be present in certain kinds of white adipose tissue (WAT) depots. It has been suggested that WAT can undergo the process of browning in response to stimuli that induce and enhance the expression of thermogenesis: a metabolic feature typically associated with BAT. MicroRNAs (miRNAs) are small transcriptional regulators that control gene expression in a variety of tissues, including WAT and BAT. Likewise, it was shown that several food compounds could influence miRNAs associated with browning, thus, potentially contributing to the management of excessive adipose tissue accumulation (obesity) through specific nutritional and dietetic approaches. Therefore, this has created significant excitement towards the development of a promising dietary strategy to promote browning/beiging in WAT to potentially contribute to combat the growing epidemic of obesity. For this reason, we summarize the current knowledge about miRNAs and food compounds that could be applied in promoting adipose browning, as well as the cellular mechanisms involved.

Flow Cytometry of Mouse and Human Adipocytes for the Analysis of Browning and Cellular Heterogeneity

Adipocytes, once considered simple lipid-storing cells, are rapidly emerging as complex cells with many biologically diverse functions. A powerful high-throughput method for analyzing single cells is flow cytometry. Several groups have attempted to analyze and sort freshly isolated adipocytes; however, using an adipocyte-specific reporter mouse, we demonstrate that these studies fail to detect the majority of white adipocytes. We define critical settings required for adipocyte flow cytometry and provide a rigid strategy for analyzing and sorting white and brown adipocyte populations. The applicability of our protocol is shown by sorting mouse adipocytes based on size or UCP1 expression and demonstrating that a subset of human adipocytes lacks the β₂-adrenergic receptor, particularly in the insulin-resistant state. In conclusion, the present study confers key technological insights for analyzing and sorting mature adipocytes, opening up numerous downstream research applications.

Non-invasive in vivo imaging of UCP1 expression in live mice via near-infrared fluorescent protein iRFP720

Uncoupling protein 1 (UCP1) is a mitochondrial protein that is expressed in both brown and beige adipocytes. UCP1 uncouples the mitochondrial electron transport chain from ATP synthesis to produce heat via non-shivering thermogenesis. Due to their ability to dissipate energy as heat and ameliorate metabolic disorders, UCP1-expressing adipocytes are considered as a potential target for anti-obesity treatment. To monitor the expression of UCP1 in live mice in a non-invasive manner, we generated the Ucp1-iRFP720 knock-in (Ucp1-iRFP720 KI) mice, in which the gene encoding a near-infrared fluorescent protein iRFP720 is inserted into the Ucp1 gene locus. Using the heterozygous Ucp1-iRFP720 KI mice, we observed robust iRFP fluorescence in the interscapular region where brown adipose tissue is located. Moreover, the iRFP fluorescence was clearly observable in inguinal white adipose tissues in live mice administered with β3-adrenergic receptor agonist CL316,243. We also found that the homozygous Ucp1-iRFP720 KI mice, which are deficient in UCP1, displayed prominent iRFP fluorescence in the inguinal regions at the standard housing temperature. Consistent with this, the mice exhibited expanded populations of beige-like adipocytes in inguinal white adipose tissue, in which the Ucp1 promoter was dramatically activated. Thus, the Ucp1-iRFP720 KI mice provide a convenient model for non-invasive in vivo imaging of UCP1 expression in both brown and beige adipocytes in live mice.

TFEB drives PGC-1α expression in adipocytes to protect against diet-induced metabolic dysfunction

TFEB is a basic helix-loop-helix transcription factor that confers protection against metabolic diseases such as atherosclerosis by targeting a network of genes involved in autophagy-lysosomal biogenesis and lipid catabolism. In this study, we sought to characterize the role of TFEB in adipocyte and adipose tissue physiology and evaluate the therapeutic potential of adipocyte-specific TFEB overexpression in obesity. We demonstrated that mice with adipocyte-specific TFEB overexpression (Adipo-TFEB) were protected from diet-induced obesity, insulin resistance, and metabolic sequelae. Adipo-TFEB mice were lean primarily through increased metabolic rate, suggesting a role for adipose tissue browning and enhanced nonshivering thermogenesis in fat. Transcriptional characterization revealed that TFEB targeted genes involved in adipose tissue browning rather than those involved in autophagy. One such gene encoded PGC-1α, an established target of TFEB that promotes adipocyte browning. To dissect the role of PGC-1α in mediating the downstream effects of TFEB overexpression, we generated mice with adipocyte-specific PGC-1α deficiency and TFEB overexpression. Without PGC-1α, the ability of TFEB overexpression to brown adipose tissue and to elicit beneficial metabolic effects was blunted. Overall, these data implicate TFEB as a PGC-1α-dependent regulator of adipocyte browning and suggest its therapeutic potential in treating metabolic disease.

Prediction of Adipose Browning Capacity by Systematic Integration of Transcriptional Profiles

Activation and recruitment of thermogenic cells in human white adipose tissues ("browning") can counteract obesity and associated metabolic disorders. However, quantifying the effects of therapeutic interventions on browning remains enigmatic. Here, we devise a computational tool, named ProFAT (profiling of fat tissue types), for quantifying the thermogenic potential of heterogeneous fat biopsies based on prediction of white and brown adipocyte content from raw gene expression datasets. ProFAT systematically integrates 103 mouse-fat-derived transcriptomes to identify unbiased and robust gene signatures of brown and white adipocytes. We validate ProFAT on 80 mouse and 97 human transcriptional profiles from 14 independent studies and correctly predict browning capacity upon various physiological and pharmacological stimuli. Our study represents the most exhaustive comparative analysis of public data on adipose biology toward quantification of browning after personalized medical intervention. ProFAT is freely available and should become increasingly powerful with the growing wealth of transcriptomics data.

The Role of Adipose Tissue Mitochondria: Regulation of Mitochondrial Function for the Treatment of Metabolic Diseases

: Mitochondria play a key role in maintaining energy homeostasis in metabolic tissues, including adipose tissues. The two main types of adipose tissues are the white adipose tissue (WAT) and the brown adipose tissue (BAT). WAT primarily stores excess energy, whereas BAT is predominantly responsible for energy expenditure by non-shivering thermogenesis through the mitochondria. WAT in response to appropriate stimuli such as cold exposure and β-adrenergic agonist undergoes browning wherein it acts as BAT, which is characterized by the presence of a higher number of mitochondria. Mitochondrial dysfunction in adipocytes has been reported to have strong correlation with metabolic diseases, including obesity and type 2 diabetes. Dysfunction of mitochondria results in detrimental effects on adipocyte differentiation, lipid metabolism, insulin sensitivity, oxidative capacity, and thermogenesis, which consequently lead to metabolic diseases. Recent studies have shown that mitochondrial function can be improved by using thiazolidinedione, mitochondria-targeted antioxidants, and dietary natural compounds; by performing exercise; and by controlling caloric restriction, thereby maintaining the metabolic homeostasis by inducing adaptive thermogenesis of BAT and browning of WAT. In this review, we focus on and summarize the molecular regulation involved in the improvement of mitochondrial function in adipose tissues so that strategies can be developed to treat metabolic diseases.

Lipid Regulators of Thermogenic Fat Activation

The global prevalence of obesity continues to increase, suggesting a need for alternative treatment approaches. Targeting brown fat function to promote energy expenditure represents one such approach. Brown adipocytes and the related beige adipocytes oxidize fatty acids and glucose to generate heat and are activated by cold exposure or consumption of high-calorie diets. Alternative, more practical means to activate thermogenic fat are needed. Here, we review emerging data suggesting new roles for lipids in activating thermogenesis that extend beyond their serving as a fuel source for heat generation. Lipids have also been implicated in mediating interorgan communication, crosstalk between organelles, and cellular signaling regulating thermogenesis. Understanding how lipids regulate thermogenesis could identify innovative therapeutic interventions for obesity.

Epigallocatechin Exerts Anti-Obesity Effect in Brown Adipose Tissue

Catechins in green tea are well-known to be effective in reducing the risk of obesity. The purpose of this study was to elucidate the effects of catechins present in green tea on adipocyte differentiation and mature adipocyte metabolism. Treatment of 3T3-L1 mouse adipocyte during differentiation adipocytes with (-)-epigallocatechin (EGC) and gallic acid (GA) resulted in dose-dependent inhibition of adipogenesis. Specifically, EGC increased adiponectin and uncoupling protein 1 (UCP1) transcription in mature adipocytes. Transcription levels of adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) were not significantly impacted by either of the compounds. These results suggest that the EGC is the most effective catechin having anti-obesity activity. Finally, EGC is an attractive candidate component for remodeling obesity.

Cold-induced lipid dynamics and transcriptional programs in white adipose tissue

Background

In mammals, cold exposure induces browning of white adipose tissue (WAT) and alters WAT gene expression and lipid metabolism to boost adaptive thermogenesis and maintain body temperature. Understanding the lipidomic and transcriptomic profiles of WAT upon cold exposure provides insights into the adaptive changes associated with this process.

Results

Here, we applied mass spectrometry and RNA sequencing (RNA-seq) to provide a comprehensive resource for describing the lipidomic or transcriptome profiles in cold-induced inguinal WAT (iWAT). We showed that short-term (3-day) cold exposure induces browning of iWAT, increases energy expenditure, and results in loss of body weight and fat mass. Lipidomic analysis shows that short-term cold exposure leads to dramatic changes of the overall composition of lipid classes WAT. Notably, cold exposure induces significant changes in the acyl-chain composition of triacylglycerols (TAGs), as well as the levels of glycerophospholipids and sphingolipids in iWAT. RNA-seq and qPCR analysis suggests that short-term cold exposure alters the expression of genes and pathways involved in fatty acid elongation, and the synthesis of TAGs, sphingolipids, and glycerophospholipids. Furthermore, the cold-induced lipid dynamics and gene expression pathways in iWAT are contrary to those previously observed in metabolic syndrome, neurodegenerative disorders, and aging, suggesting beneficial effects of cold-induced WAT browning on health and lifespan.

Conclusion

We described the significant alterations in the composition of glyphospholipids, glycerolipids, and sphingolipids and expression of genes involved in thermogenesis, fatty acid elongation, and fatty acid metabolism during the response of iWAT to short-term cold exposure. We also found that some changes in the levels of specific lipid species happening after cold treatment of iWAT are negatively correlated to metabolic diseases, including obesity and T2D.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0693-x) contains supplementary material, which is available to authorized users.

Black Raspberry (Rubus coreanus Miquel) Promotes Browning of Preadipocytes and Inguinal White Adipose Tissue in Cold-Induced Mice

The alteration of white adipose tissue (WAT) "browning", a change of white into beige fat, has been considered as a new therapeutic strategy to treat obesity. In this study, we investigated the browning effect of black raspberry (Rubus coreanus Miquel) using in vitro and in vivo models. Black raspberry water extract (BRWE) treatment inhibited lipid accumulation in human mesenchymal stem cells (hMSCs) and zebrafish. To evaluate the thermogenic activity, BRWE was orally administered for 2 weeks, and then, the mice were placed in a 4 °C environment. As a result, BRWE treatment increased rectal temperature and inguinal WAT (iWAT) thermogenesis by inducing the expression of beige fat specific markers such as PR domain zinc-finger protein 16 (PRDM16), uncoupling protein 1 (UCP1), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α), and t-box protein 1 (TBX1) in cold-exposed mice. Furthermore, ellagic acid (EA), a constituent of BRWE, markedly promoted beige specific markers: UCP1, PGC1α, TBX1, and nuclear respiratory factor 1 in beige differentiation media (DM)-induced 3T3-L1 adipocytes. Our findings indicate that BRWE can promote beige differentiation/activation, and EA is the active compound responsible for such effect. Thus, we suggest the nature-derived agents BRWE and EA as potential agents for obesity treatment.

Central nicotine induces browning through hypothalamic κ opioid receptor

Increased body weight is a major factor that interferes with smoking cessation. Nicotine, the main bioactive compound in tobacco, has been demonstrated to have an impact on energy balance, since it affects both feeding and energy expenditure at the central level. Among the central actions of nicotine on body weight, much attention has been focused on its effect on brown adipose tissue (BAT) thermogenesis, though its effect on browning of white adipose tissue (WAT) is unclear. Here, we show that nicotine induces the browning of WAT through a central mechanism and that this effect is dependent on the κ opioid receptor (KOR), specifically in the lateral hypothalamic area (LHA). Consistent with these findings, smokers show higher levels of uncoupling protein 1 (UCP1) expression in WAT, which correlates with smoking status. These data demonstrate that central nicotine-induced modulation of WAT browning may be a target against human obesity.

Nicotine reduces food intake and increases energy expenditure in brown adipose tissue. Here the authors show that nicotine also induces white adipose tissue browning via central kappa opioid receptor action.

Circadian lipid synthesis in brown fat maintains murine body temperature during chronic cold

Ambient temperature influences the molecular clock and lipid metabolism, but the impact of chronic cold exposure on circadian lipid metabolism in thermogenic brown adipose tissue (BAT) has not been studied. Here we show that during chronic cold exposure (1 wk at 4 °C), genes controlling de novo lipogenesis (DNL) including Srebp1, the master transcriptional regulator of DNL, acquired high-amplitude circadian rhythms in thermogenic BAT. These conditions activated mechanistic target of rapamycin 1 (mTORC1), an inducer of Srebp1 expression, and engaged circadian transcriptional repressors REV-ERBα and β as rhythmic regulators of Srebp1 in BAT. SREBP was required in BAT for the thermogenic response to norepinephrine, and depletion of SREBP prevented maintenance of body temperature both during circadian cycles as well as during fasting of chronically cold mice. By contrast, deletion of REV-ERBα and β in BAT allowed mice to maintain their body temperature in chronic cold. Thus, the environmental challenge of prolonged noncircadian exposure to cold temperature induces circadian induction of SREBP1 that drives fuel synthesis in BAT and is necessary to maintain circadian body temperature during chronic cold exposure. The requirement for BAT fatty acid synthesis has broad implications for adaptation to cold.

Zeaxanthin promotes mitochondrial biogenesis and adipocyte browning via AMPKα1 activation

Zeaxanthin (ZEA), a type of oxygenated carotenoid with strong antioxidant activity, has previously been found to exhibit an anti-lipogenesis effect. In the present study, we investigated the effect of ZEA on brown-like adipocyte formation and mitochondrial biogenesis in 3T3-L1 adipocytes. Brown adipocyte-specific markers, mitochondrial biogenesis and oxidative stress, and the involvement of AMP-activated protein kinase (AMPK) α1 were assessed. ZEA treated adipocytes demonstrated a brown-like pattern, with upregulated expression of uncoupling protein 1 (UCP1) and other brown adipocyte markers. In addition, ZEA intervention induced a dramatic increase in mitochondrial DNA (mtDNA) content and in the mRNA levels of genes associated with mitochondrial biogenesis. Furthermore, ZEA attenuated mitochondrial oxidative damage caused by lipid peroxidation in adipocytes, significantly improved the mitochondrial membrane potential (MMP), and scavenged intracellular reactive oxygen species (ROS) and mitochondrial superoxide. Finally, we concluded that AMPKα1 mediated the ZEA-caused inhibition of lipid accumulation and promotion of brown and beige adipocyte-biomarker expression, as the positive effects of ZEA were diminished by Prkaa1 (AMPKα1) knockdown. These findings demonstrated that ZEA promoted the expression of brown and beige adipogenesis markers and mitochondrial biogenesis, which involved AMPKα1 activation, thus contributing to the anti-obesity effects of ZEA.

Human brown adipose tissue is phenocopied by classical brown adipose tissue in physiologically humanized mice

Human and rodent brown adipose tissues (BAT) appear morphologically and molecularly different. Here we compare human BAT with both classical brown and brite/beige adipose tissues of 'physiologically humanized' mice: middle-aged mice living under conditions approaching human thermal and nutritional conditions, that is, prolonged exposure to thermoneutral temperature (approximately 30 °C) and to an energy-rich (high-fat, high-sugar) diet. We find that the morphological, cellular and molecular characteristics (both marker and adipose-selective gene expression) of classical brown fat, but not of brite/beige fat, of these physiologically humanized mice are notably similar to human BAT. We also demonstrate, both in silico and experimentally, that in physiologically humanized mice only classical BAT possesses a high thermogenic potential. These observations suggest that classical rodent BAT is the tissue of choice for translational studies aimed at recruiting human BAT to counteract the development of obesity and its comorbidities.

Beige Fat, Adaptive Thermogenesis, and Its Regulation by Exercise and Thyroid Hormone

While it is now understood that the proper expansion of adipose tissue is critically important for metabolic homeostasis, it is also appreciated that adipose tissues perform far more functions than simply maintaining energy balance. Adipose tissue performs endocrine functions, secreting hormones or adipokines that affect the regulation of extra-adipose tissues, and, under certain conditions, can also be major contributors to energy expenditure and the systemic metabolic rate via the activation of thermogenesis. Adipose thermogenesis takes place in brown and beige adipocytes. While brown adipocytes have been relatively well studied, the study of beige adipocytes has only recently become an area of considerable exploration. Numerous suggestions have been made that beige adipocytes can elicit beneficial metabolic effects on body weight, insulin sensitivity, and lipid levels. However, the potential impact of beige adipocyte thermogenesis on systemic metabolism is not yet clear and an understanding of beige adipocyte development and regulation is also limited. This review will highlight our current understanding of beige adipocytes and select factors that have been reported to elicit the development and activation of thermogenesis in beige cells, with a focus on factors that may represent a link between exercise and 'beiging', as well as the role that thyroid hormone signaling plays in beige adipocyte regulation.

Brown Adipose Tissue Development and Metabolism

Brown adipose tissue is well known to be a thermoregulatory organ particularly important in small rodents and human infants, but it was only recently that its existence and significance to metabolic fitness in adult humans have been widely realized. The ability of active brown fat to expend high amounts of energy has raised interest in stimulating thermogenesis therapeutically to treat metabolic diseases related to obesity and type 2 diabetes. In parallel, there has been a surge of research aimed at understanding the biology of rodent and human brown fat development, its remarkable metabolic properties, and the phenomenon of white fat browning, in which white adipocytes can be converted into brown like adipocytes with similar thermogenic properties. Here, we review the current understanding of the developmental and metabolic pathways involved in forming thermogenic adipocytes, and highlight some of the many unknown functions of brown fat that make its study a rich and exciting area for future research.

The Mechanism FA-Dependent H+ Transport by UCP1

Uncoupling protein 1 (UCP1) is an integral protein of the inner mitochondrial membrane (IMM) that is expressed specifically in brown and beige fat depots. UCP1 is responsible for the production of heat to control core body temperature, the regulation of fat metabolism, and the energy balance. As an uncoupling protein, UCP1 transports H⁺ across the IMM in presence of long-chain fatty acids (FA), which makes brown fat mitochondria produce heat at the expense of ATP. However, the exact mechanism of UCP1 action has remained difficult to elucidate, because direct methods for studying currents generated by UCP1 were unavailable. Recently, the patch-clamp technique was successfully applied to brown and beige fat mitochondria to directly study H⁺ currents across the IMM and characterize UCP1 function. A new model of the UCP1 mechanism was proposed based on the patch-clamp analysis. In this model, both FA anions (FA^-) and H⁺ are transport substrates of UCP1, and UCP1 operates as a non-canonical FA^-/H⁺ symporter. Here, we summarize recent findings obtained with the patch-clamp technique that describe how UCP1 can transport not only H⁺ but also FA^-.

The role of brown and beige adipose tissue in glycaemic control

For the past decade, brown adipose tissue (BAT) has been extensively studied as a potential therapy for obesity and metabolic diseases due to its thermogenic and glucose-consuming properties. It is now clear that the function of BAT goes beyond heat production, as it also plays an important endocrine role by secreting the so-called batokines to communicate with other metabolic tissues and regulate systemic energy homeostasis. However, despite numerous studies showing the benefits of BAT in rodents, it is still not clear whether recruitment of BAT can be utilized to treat human patients. Here, we review the advances on understanding the role of BAT in metabolism and its benefits on glucose and lipid homeostasis in both humans and rodents. Moreover, we discuss the latest methodological approaches to assess the contribution of BAT to human metabolism as well as the possibility to target BAT, pharmacologically or by lifestyle adaptations, to treat metabolic disorders.

SIRT3 controls brown fat thermogenesis by deacetylation regulation of pathways upstream of UCP1

OBJECTIVE

Brown adipose tissue (BAT) is important for thermoregulation in many mammals. Uncoupling protein 1 (UCP1) is the critical regulator of thermogenesis in BAT. Here we aimed to investigate the deacetylation control of BAT and to investigate a possible functional connection between UCP1 and sirtuin 3 (SIRT3), the master mitochondrial lysine deacetylase.

METHODS

We carried out physiological, molecular, and proteomic analyses of BAT from wild-type and Sirt3KO mice when BAT is activated. Mice were either cold exposed for 2 days or were injected with the β3-adrenergic agonist, CL316,243 (1 mg/kg; i.p.). Mutagenesis studies were conducted in a cellular model to assess the impact of acetylation lysine sites on UCP1 function. Cardiac punctures were collected for proteomic analysis of blood acylcarnitines. Isolated mitochondria were used for functional analysis of OXPHOS proteins.

RESULTS

Our findings showed that SIRT3 absence in mice resulted in impaired BAT lipid use, whole body thermoregulation, and respiration in BAT mitochondria, without affecting UCP1 expression. Acetylome profiling of BAT mitochondria revealed that SIRT3 regulates acetylation status of many BAT mitochondrial proteins including UCP1 and crucial upstream proteins. Mutagenesis work in cells suggested that UCP1 activity was independent of direct SIRT3-regulated lysine acetylation. However, SIRT3 impacted BAT mitochondrial proteins activities of acylcarnitine metabolism and specific electron transport chain complexes, CI and CII.

CONCLUSIONS

Our data highlight that SIRT3 likely controls BAT thermogenesis indirectly by targeting pathways upstream of UCP1.

The role of DNA methylation in thermogenic adipose biology

The two types of thermogenic fat cells, beige and brown adipocytes, play a significant role in regulating energy homeostasis. Their development and thermogenesis are tightly regulated by dynamic epigenetic mechanisms, which could potentially be targeted to treat metabolic disorders such as obesity. However, we are just beginning to catalog and understand these dynamic changes. In this review, we will discuss the current understanding of the role of DNA (de)methylation events in beige and brown adipose biology in order to highlight the holes in our knowledge and to point the way forward for future studies.

Neuroendocrine Regulation of Energy Metabolism Involving Different Types of Adipose Tissues

Despite tremendous research efforts to identify regulatory factors that control energy metabolism, the prevalence of obesity has been continuously rising, with nearly 40% of US adults being obese. Interactions between secretory factors from adipose tissues and the nervous system innervating adipose tissues play key roles in maintaining energy metabolism and promoting survival in response to metabolic challenges. It is currently accepted that there are three types of adipose tissues, white (WAT), brown (BAT), and beige (BeAT), all of which play essential roles in maintaining energy homeostasis. WAT mainly stores energy under positive energy balance, while it releases fuels under negative energy balance. Thermogenic BAT and BeAT dissipate energy as heat under cold exposure to maintain body temperature. Adipose tissues require neural and endocrine communication with the brain. A number of WAT adipokines and BAT batokines interact with the neural circuits extending from the brain to cooperatively regulate whole-body lipid metabolism and energy homeostasis. We review neuroanatomical, histological, genetic, and pharmacological studies in neuroendocrine regulation of adipose function, including lipid storage and mobilization of WAT, non-shivering thermogenesis of BAT, and browning of BeAT. Recent whole-tissue imaging and transcriptome analysis of differential gene expression in WAT and BAT yield promising findings to better understand the interaction between secretory factors and neural circuits, which represents a novel opportunity to tackle obesity.

Adipose Lipocalin 2 overexpression protects against age-related decline in thermogenic function of adipose tissue and metabolic deterioration

OBJECTIVES

Aging increases the risk for development of adipose tissue dysfunction, insulin resistance, dyslipidemia, and liver steatosis. Lipocalin 2 (Lcn2) deficient mice are more prone to diet-induced obesity and metabolic dysfunction, indicating a protective role for Lcn2 in younger mice. In this study, we determined whether overexpressing Lcn2 in adipose tissue can protect against age-related metabolic deterioration.

METHODS

We developed ap2-promoter-driven Lcn2 transgenic (Tg) mice and aged Lcn2 Tg mice for the metabolic assessments.

RESULTS

We found decreased adipocyte size in inguinal white adipose tissue (iWAT) from 10-month-old Lcn2 Tg mice relative to WT. This was accompanied by increased markers of adipogenesis in iWAT and attenuation of the age-related decline in AMP-activated protein kinase (AMPK) phosphorylation in adipose tissue depots. In addition to improvements in adipose tissue function, whole-body metabolic homeostasis was maintained in aged Lcn2 Tg mice. This included improved glucose tolerance and reduced serum triglycerides in older Lcn2 Tg mice relative to WT mice. Further, liver morphology and liver lipid levels were improved in older Lcn2 Tg mice, alongside a decrease in markers of liver inflammation and fibrosis.

CONCLUSIONS

We demonstrate that overexpression of Lcn2 in adipose tissue not only preserves adipose tissue function during aging but also promotes maintenance of glucose tolerance, decreases dyslipidemia, and prevents liver lipid accumulation and steatosis.

Browning is activated in the subcutaneous white adipose tissue of mice metabolically challenged with a high-fructose diet submitted to high-intensity interval training

Fructose may induce an endocrine dysfunction in adipose tissue in rodents. Browning is identified by deposits of beige adipocytes in subcutaneous white adipose tissue (sWAT). We study the effects of the high-intensity interval training (HIIT) on the formation of beige adipocytes in the sWAT of mice fed a high-fructose diet. Sixty male mice (3 months old; C57BL/6) were fed two diets for 18 weeks (n=30 each): control diet (C) or high-fructose diet (F). At the 10th week, for an additional 8-week period, the groups were (n=15 each) nontrained (NT) or trained (HIIT): C-NT, C-HIIT, F-NT and F-HIIT. We evaluated body mass, energy expenditure and molecular analyses for browning and thermogenic markers in sWAT. The HIIT groups showed significantly lower body mass and increased energy expenditure. The consumption of fructose was linked with an increased sWAT mass. However, HIIT caused a reduction of sWAT mass compared to the NT groups. Energy intake was parallel in the groups, regardless of the diet type and HIIT. Fructose was related to higher glucose and insulin levels and hypertrophied sWAT adipocytes, but HIIT decreased both glucose and insulin levels and led to the appearance of brown fat-like adipocytes dispersed in sWAT with higher expression of browning markers. Also, fructose reduced the sWAT markers of mitochondrial biogenesis and beta-oxidation, which were enhanced by HIIT. In conclusion, HIIT might stimulate the sWAT browning in mice fed a high-fructose diet associated with beneficial changes in mitochondrial biogenesis and beta-oxidation markers, contributing to a whole-body metabolic improvement.

The Impact of Different Animal-Derived Protein Sources on Adiposity and Glucose Homeostasis during Ad Libitum Feeding and Energy Restriction in Already Obese Mice

Low-fat diets and energy restriction are recommended to prevent obesity and to induce weight loss, but high-protein diets are popular alternatives. However, the importance of the protein source in obesity prevention and weight loss is unclear. The aim of this study was to investigate the ability of different animal protein sources to prevent or reverse obesity by using lean or obese C57BL/6J mice fed high-fat/high-protein or low-fat diets with casein, cod or pork as protein sources. Only the high-fat/high-protein casein-based diet completely prevented obesity development when fed to lean mice. In obese mice, ad libitum intake of a casein-based high-fat/high-protein diet modestly reduced body mass, whereas a pork-based high-fat/high-protein diet aggravated the obese state and reduced lean body mass. Caloric restriction of obese mice fed high-fat/high-protein diets reduced body weight and fat mass and improved glucose tolerance and insulin sensitivity, irrespective of the protein source. Finally, in obese mice, ad libitum intake of a low-fat diet stabilized body weight, reduced fat mass and increased lean body mass, with the highest loss of fat mass found in mice fed the casein-based diet. Combined with caloric restriction, the casein-based low-fat diet resulted in the highest loss of fat mass. Overall, the dietary protein source has greater impact in obesity prevention than obesity reversal.

Mitochondrial electron transport chain, ROS generation and uncoupling (Review)

The mammalian mitochondrial electron transport chain (ETC) includes complexes I-IV, as well as the electron transporters ubiquinone and cytochrome c. There are two electron transport pathways in the ETC: Complex I/III/IV, with NADH as the substrate and complex II/III/IV, with succinic acid as the substrate. The electron flow is coupled with the generation of a proton gradient across the inner membrane and the energy accumulated in the proton gradient is used by complex V (ATP synthase) to produce ATP. The first part of this review briefly introduces the structure and function of complexes I-IV and ATP synthase, including the specific electron transfer process in each complex. Some electrons are directly transferred to O₂ to generate reactive oxygen species (ROS) in the ETC. The second part of this review discusses the sites of ROS generation in each ETC complex, including sites I_F and I_Q in complex I, site II_F in complex II and site III_Qo in complex III, and the physiological and pathological regulation of ROS. As signaling molecules, ROS play an important role in cell proliferation, hypoxia adaptation and cell fate determination, but excessive ROS can cause irreversible cell damage and even cell death. The occurrence and development of a number of diseases are closely related to ROS overproduction. Finally, proton leak and uncoupling proteins (UCP_S) are discussed. Proton leak consists of basal proton leak and induced proton leak. Induced proton leak is precisely regulated and induced by UCPs. A total of five UCPs (UCP1-5) have been identified in mammalian cells. UCP1 mainly plays a role in the maintenance of body temperature in a cold environment through non-shivering thermogenesis. The core role of UCP2-5 is to reduce oxidative stress under certain conditions, therefore exerting cytoprotective effects. All diseases involving oxidative stress are associated with UCPs.

Compensation for cold-induced thermogenesis during weight loss maintenance and regain

Prevalence of obesity is exacerbated by low rates of successful long-term weight loss maintenance (WLM). In part, relapse from WLM to obesity is due to a reduction in energy expenditure (EE) that persists throughout WLM and relapse. Thus, interventions that increase EE might facilitate WLM. In obese mice that were calorically restricted to reduce body weight by ~20%, we manipulated EE throughout WLM and early relapse using intermittent cold exposure (ICE; 4°C, 90 min/day, 5 days/wk, within the last 3 h of the light cycle). EE, energy intake, and spontaneous physical activity were measured during the obese, WLM, and relapse phases. During WLM and relapse, the ICE group expended more energy during the light cycle because of cold exposure but expended less energy in the dark cycle, which led to no overall difference in total daily EE. The compensation in EE appeared to be mediated by activity, whereby the ICE group was more active during the light cycle because of cold exposure but less active during the dark cycle, which led to no overall effect on total daily activity during WLM and relapse. In brown adipose tissue of relapsing mice, the ICE group had greater mRNA expression of Dio2 and protein expression of UCP1 but lower mRNA expression of Prdm16. In summary, these findings indicate that despite robust increases in EE during cold exposures, ICE is unable to alter total daily EE during WLM or early relapse, likely due to compensatory behaviors in activity.

Inhibitory Effect of Olive Leaf Extract on Obesity in High-fat Diet-induced Mice

BACKGROUND/AIM

The rapid increase in the number of people who are overweight or obese, which increases the risk of diseases and health problems, is becoming an important issue. Herein, we investigated whether olive leaf extract (OLE) has potent anti-obesity effects in high-fat induced mouse models.

MATERIALS AND METHODS

C57BL/6 mice were randomized into normal control, high-fat diet (HFD), HFD with OLE, and HFD with garcinia groups and administered experimental diets for 12 weeks. Body weight and food intake were measured once per week and obesity-related biomarkers were evaluated in the serum and adipose tissue.

RESULTS

OLE significantly suppressed weight gain, food efficiency ratio, visceral fat accumulation, and serum lipid composition in HFD-induced mice. Furthermore, the expression of adipogenesis- and thermogenesis-related molecules was decreased in the OLE-treated group.

CONCLUSION

OLE prevents obesity development by regulating the expression of molecules involved in adipogenesis and thermogenesis.

Important Trends in UCP3 Investigation

Membrane uncoupling protein 3 (UCP3), a member of the mitochondrial uncoupling protein family, was discovered in 1997. UCP3's properties, such as its high homology to other mitochondrial carriers, especially to UCP2, its short lifetime and low specificity of UCP3 antibodies, have hindered progress in understanding its biological function and transport mechanism over decades. The abundance of UCP3 is highest in murine brown adipose tissue (BAT, 15.0 pmol/mg protein), compared to heart (2.7 pmol/mg protein) and the gastrocnemius muscle (1.7 pmol/mg protein), but it is still 400-fold lower than the abundance of UCP1, a biomarker for BAT. Investigation of UCP3 reconstituted in planar bilayer membranes revealed that it transports protons only when activated by fatty acids (FA). Although purine nucleotides (PN) inhibit UCP3-mediated transport, the molecular mechanism differs from that of UCP1. It remains a conundrum that two homologous proton-transporting proteins exist within the same tissue. Recently, we proposed that UCP3 abundance directly correlates with the degree of FA β-oxidation in cell metabolism. Further development in this field implies that UCP3 may have dual function in transporting substrates, which have yet to be identified, alongside protons. Evaluation of the literature with respect to UCP3 is a complex task because (i) UCP3 features are often extrapolated from its "twin" UCP2 without additional proof, and (ii) the specificity of antibodies against UCP3 used in studies is rarely evaluated. In this review, we primarily focus on recent findings obtained for UCP3 in biological and biomimetic systems.

Redox control of non-shivering thermogenesis

Background

Thermogenic adipocytes reorganize their metabolism during cold exposure. Metabolic reprogramming requires readily available bioenergetics substrates, such as glucose and fatty acids, to increase mitochondrial respiration and produce heat via the uncoupling protein 1 (UCP1). This condition generates a finely-tuned production of mitochondrial reactive oxygen species (ROS) that support non-shivering thermogenesis.

Scope of review

Herein, the findings underlining the mechanisms that regulate ROS production and control of the adaptive responses tuning thermogenesis in adipocytes are described. Furthermore, this review describes the metabolic responses to substrate availability and the consequence of mitochondrial failure to switch fuel oxidation in response to changes in nutrient availability. A framework to control mitochondrial ROS threshold to maximize non-shivering thermogenesis in adipocytes is provided.

Major conclusions

Thermogenesis synchronizes fuel oxidation with an acute and transient increase of mitochondrial ROS that promotes the activation of redox-sensitive thermogenic signaling cascade and UCP1. However, an overload of substrate flux to mitochondria causes a massive and damaging mitochondrial ROS production that affects mitochondrial flexibility. Finding novel thermogenic redox targets and manipulating ROS concentration in adipocytes appears to be a promising avenue of research for improving thermogenesis and counteracting metabolic diseases.

•

Mitochondrial ROS support non-shivering thermogenesis.

•

Thermogenic ROS are tightly related to mitochondrial metabolic reorganization.

•

Uncontrolled mitochondrial ROS production is causative of metabolic inflexibility.

Adrenoceptors in white, brown, and brite adipocytes

Adrenoceptors play an important role in adipose tissue biology and physiology that includes regulating the synthesis and storage of triglycerides (lipogenesis), the breakdown of stored triglycerides (lipolysis), thermogenesis (heat production), glucose metabolism, and the secretion of adipocyte-derived hormones that can control whole-body energy homeostasis. These processes are regulated by the sympathetic nervous system through actions at different adrenoceptor subtypes expressed in adipose tissue depots. In this review, we have highlighted the role of adrenoceptor subtypes in white, brown, and brite adipocytes in both rodents and humans and have included detailed analysis of adrenoceptor expression in human adipose tissue and clonally derived adipocytes. We discuss important considerations when investigating adrenoceptor function in adipose tissue or adipocytes. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Rosiglitazone remodels the lipid droplet and britens human visceral and subcutaneous adipocytes ex vivo

Treatment with PPARγ agonists in vivo improves human adipocyte metabolism, but the cellular mechanisms and possible depot differences in responsiveness to their effects are poorly understood. To examine the ex vivo metabolic effects of rosiglitazone (Rosi), we cultured explants of human visceral (omental) and abdominal subcutaneous adipose tissues for 7 days. Rosi increased mRNA levels of transcriptional regulators of brite/beige adipocytes (PGC1α, PRDM16), triglyceride synthesis (GPAT3, DGAT1), and lipolysis (ATGL) similarly in adipose tissues from both depots. In parallel, Rosi increased key modulators of FA oxidation (UCP1, FABP3, PLIN5 protein), rates of FA oxidation, and protein levels of electron transport complexes, suggesting an enhanced respiratory capacity as confirmed in newly differentiated adipocytes. Rosi led to the formation of small lipid droplets (SLDs) around the adipocyte central lipid droplet; each SLD was decorated with redistributed mitochondria that colocalized with PLIN5. SLD maintenance required lipolysis and FA reesterification. Rosi thus coordinated a structural and metabolic remodeling in adipocytes from both visceral and subcutaneous depots that enhanced oxidative capacity. Selective targeting of these cellular mechanisms to improve adipocyte FA handling may provide a new approach to treat metabolic complications of obesity and diabetes.

Rosiglitazone Enhances Browning Adipocytes in Association with MAPK and PI3-K Pathways During the Differentiation of Telomerase-Transformed Mesenchymal Stromal Cells into Adipocytes

Obesity is a major risk for diabetes. Brown adipose tissue (BAT) mediates production of heat while white adipose tissue (WAT) function in the storage of fat. Roles of BAT in the treatment of obesity and related disorders warrants more investigation. Peroxisome proliferator activator receptor gamma (PPAR-γ) is the master regulator of both BAT and WAT adipogenesis and has roles in glucose and fatty acid metabolism. Adipose tissue is the major expression site for PPAR-γ. In this study, the effects of rosiglitazone on the brown adipogenesis and the association of MAPK and PI3K pathways was investigated during the in vitro adipogenic differentiation of telomerase transformed mesenchymal stromal cells (iMSCs). Our data indicate that 2 µM rosiglitazone enhanced adipogenesis by over-expression of PPAR-γ and C/EBP-α. More specifically, brown adipogenesis was enhanced by the upregulation of EBF2 and UCP-1 and evidenced by multilocular fatty droplets morphology of the differentiated adipocytes. We also found that rosiglitazone significantly activated MAPK and PI3K pathways at the maturation stage of differentiation. Overall, the results indicate that rosiglitazone induced overexpression of PPAR-γ that in turn enhanced adipogenesis, particularly browning adipogenesis. This study reports the browning effects of rosiglitazone during the differentiation of iMSCs into adipocytes in association with the activation of MAPK and PI3K signaling pathways.

Parkin controls brown adipose tissue plasticity in response to adaptive thermogenesis

Parkin is an ubiquitin-E3 ligase that acts as a key component of the cellular machinery for mitophagy. We show here that Parkin expression is reciprocally regulated in brown adipose tissue in relation to thermogenic activity. Thermogenic stimuli repress Parkin gene expression via transcriptional mechanisms that are elicited by noradrenergic and PPARα-mediated pathways that involve intracellular lipolysis in brown adipocytes. Parkin-KO mice show over-activated brown adipose tissue thermogenic activity and exhibit improved metabolic parameters, especially when fed a high-fat diet. Deacclimation, which is the return of a cold-adapted mouse to a thermoneutral temperature, dramatically induces mitophagy in brown adipocytes, with a concomitant induction of Parkin levels. We further reveal that Parkin-KO mice exhibit defects in the degradative processing of mitochondrial proteins in brown adipose tissue in response to deacclimation. These results suggest that the transcriptional control of Parkin in brown adipose tissue may contribute to modulating the mitochondrial mass and activity for adaptation to thermogenic requirements.

Adult Hippocampal Neurogenesis Can Be Enhanced by Cold Challenge Independently From Beigeing Effects

In this study, we investigated the effects of cold challenge on adult hippocampal neurogenesis (AHN) and hippocampal gene expression and whether these are mediated by beigeing of peripheral fat tissues. Cold challenge (6 ± 2°C) for 1 and 4 weeks was found to induce beigeing effects in inguinal white adipose tissue based on hematoxylin and eosin staining as well as uncoupled protein-1 immunohistochemical staining. In the hippocampus, cold challenge for 1 or 4 weeks increased dentate gyrus neurogenesis and expression of genes related to AHN, including notch signaling, G protein-coupled receptor signaling, and adrenergic beta receptor-1. However, this enhancement of neurogenesis and gene expression by cold challenge was not shown by administration of CL 316,243, which induces peripheral beigeing similar to cold challenge but does not cross the blood-brain barrier. These results suggest that cold challenge promotes AHN and central expression of AHN-related, signaling, and β1-adrenergic receptors genes, and that peripheral beigeing by itself is not sufficient to mediate these effects. Considering the increase in AHN and gene expression changes, cold challenge may offer a novel approach to hippocampal modulation.

Intact innervation is essential for diet-induced recruitment of brown adipose tissue

The possibility that recruitment and activation of brown adipose tissue (BAT) thermogenesis could be beneficial for curtailing obesity development in humans prompts a need for a better understanding of the control of these processes [that are often referred to collectively as diet-induced thermogenesis (DIT)]. Dietary conditions are associated with large changes in blood-borne factors that could be responsible for BAT recruitment, but BAT is also innervated by the sympathetic nervous system. To examine the significance of the innervation for DIT recruitment, we surgically denervated the largest BAT depot, i.e., the interscapular BAT depot in mice and exposed the mice at thermoneutrality to a high-fat diet versus a chow diet. Denervation led to an alteration in feeding pattern but did not lead to enhanced obesity, but obesity was achieved with a lower food intake, as denervation increased metabolic efficiency. Conclusively, denervation totally abolished the diet-induced increase in total UCP1 protein levels observed in the intact mice, whereas basal UCP1 expression was not dependent on innervation. The denervation of interscapular BAT did not discernably hyper-recruit other BAT depots, and no UCP1 protein could be detected in the principally browning-competent inguinal white adipose tissue depot under any of the examined conditions. We conclude that intact innervation is essential for diet-induced thermogenesis and that circulating factors cannot by themselves initiate recruitment of brown adipose tissue under obesogenic conditions. Therefore, the processes that link food intake and energy storage to activation of the nervous system are those of significance for the further understanding of diet-induced thermogenesis.

Sympathetic nerve innervation is required for beigeing in white fat

It is increasingly recognized that activation of beige adipocyte thermogenesis by pharmacological or genetic approaches increases energy expenditure and alleviates obesity. Sympathetic nervous system (SNS) directly innervating brown adipose tissue (BAT) and white adipose tissue (WAT) plays a key role in promoting nonshivering thermogenesis. However, direct evidence that supports the importance of SNS innervation for beige adipocyte formation is still lacking, and the significance of beige adipocyte thermogenesis in protection of body temperature during cold challenge is not clear. Here we tested the necessity of SNS innervation into WAT for beige adipocyte formation in mice with defective brown fat thermogenesis via interscapular BAT (iBAT) SNS denervation. SNS denervation was achieved by microinjection of 6-hydroxydopamine (6-OHDA), a selective neurotoxin to SNS nerves, into iBAT, inguinal WAT (iWAT), or both. The partial chemical denervation of iBAT SNS down-regulated UCP-1 protein expression in iBAT demonstrated by immunoblotting and immunohistochemical measurements. This was associated with an up-regulation of UCP1 protein expression and enhanced formation of beige cells in iWAT of mice with iBAT SNS denervation. In contrast, the chemical denervation of iWAT SNS completely abolished the upregulated UCP-1 protein and beige cell formation in iWAT of mice with iBAT SNS denervation. Our data demonstrate that SNS innervation in WAT is required for beige cell formation during cold-induced thermogenesis. We conclude that there exists a coordinated thermoregulation for BAT and WAT thermogenesis via a functional cross talk between BAT and WAT SNS.

Germinated soy germ extract ameliorates obesity through beige fat activation

Obesity is a worldwide public health concern requiring safe and effective strategies. Recent studies suggest that bioactive compounds from soybeans have beneficial effects on weight loss and reducing fat accumulation. However, despite the biochemical and nutritional changes during germination, the biological effects of germinated soy germ have not been fully investigated. In this article, germinated soy germ extract (GSGE) was evaluated as a potential treatment option for obesity using 3T3-L1 pre-adipocyte and high-fat diet (HFD)-induced obese mice. In vitro studies demonstrated that GSGE suppressed the differentiation of 3T3-L1 cells into mature adipocytes, along with reductions in lipid accumulation and lipid droplet formation. In vivo studies also showed that a daily dose of 1 mg kg-1 of GSGE reduced weight gain, adipocyte area, serum triglyceride, and LDL-cholesterol in HFD-fed mice. The GSGE treatment promoted browning, which was associated with increased UCP1 expression in vitro and in vivo. In addition, GSGE treatment induced beige fat activation by upregulation of lipolysis and beta-oxidation. Furthermore, gene and protein expression levels of endocannabinoid system-related factors such as NAPE-PLD, FAAH, DAGL-α, and CB2 were altered along with browning and beige fat activation by GSGE. The present study indicates that GSGE effectively inhibits lipid accumulation and promotes beige fat transition and activation. Therefore, we suggest that GSGE treatment could be a promising strategy for the prevention of obesity by promoting weight loss, reducing fat accumulation, and improving obesity-related metabolic disorders.

Mechanisms underlying UCP1 dependent and independent adipocyte thermogenesis

The growing focus on brown adipocytes has spurred an interest in their potential benefits for metabolic diseases. Brown and beige (or brite) adipocytes express high levels of uncoupling protein 1 (Ucp1) to dissipate heat instead of generating ATP. Ucp1 induction by stimuli including cold, exercise, and diet increases nonshivering thermogenesis, leading to increased energy expenditure and prevention of obesity. Recently, studies in adipocytes have indicated the existence of functional Ucp1-independent thermogenic regulators. Furthermore, substrate cycling involving creatine metabolites, cold-induced N-acyl amino acids, and oxidized lipids in white adipocytes can increase energy expenditure in the absence of Ucp1. These studies emphasize the need for a better understanding of the mechanisms governing energy expenditure in adipocytes and their potential applications in the prevention of human obesity and metabolic diseases.