Mitophagy suppression by miquelianin-rich lotus leaf extract induces 'beiging' of white fat via AMPK/DRP1-PINK1/PARKIN signaling axis

BACKGROUND

Lotus (Nelumbo nucifera) leaf has been described to have anti-obesity activity, but the role of white fat 'browning' or 'beiging' in its beneficial metabolic actions remains unclear. Here, 3T3-L1 cells and high-fat-diet (HFD)-fed mice were used to evaluate the effects of miquelianin-rich lotus leaf extract (LLE) on white-to-beige fat conversion and its regulatory mechanisms.

RESULTS

Treatment with LLE increased mitochondrial abundance, mitochondrial membrane potential and NAD+ /NADH ratio in 3T3-L1 cells, suggesting its potential in promoting mitochondrial activity. qPCR and/or western blotting analysis confirmed that LLE induced the expression of beige fat-enriched gene signatures (e.g. Sirt1, Cidea, Dio2, Prdm16, Ucp1, Cd40, Cd137, Cited1) and mitochondrial biogenesis-related markers (e.g. Nrf1, Cox2, Cox7a, Tfam) in 3T3-L1 cells and inguinal white adipose tissue of HFD-fed mice. Furthermore, we found that LLE treatment inhibited mitochondrial fission protein DRP1 and blocked mitophagy markers such as PINK1, PARKIN, BECLIN1 and LC-3B. Chemical inhibition experiments revealed that AMPK/DRP1 signaling was required for LLE-induced beige fat formation via suppressing PINK1/PARKIN/mitophagy.

CONCLUSION

Our data reveal a novel mechanism underlying the anti-obesity effect of LLE, namely the induction of white fat beiging via AMPK/DRP1/mitophagy signaling. © 2023 Society of Chemical Industry.

Browning of white adipose tissue may be an appropriate adaptive response to critical illness

Both the baseline amount of brown adipose tissue (BAT) and the capacity to stimulate browning of white adipose tissue (WAT) may provide a protective effect to the patient in a critical care setting. Critical illness is associated with reduced mitochondrial volume and function resulting in the increased production of reactive oxygen species, greater demand for adenosine triphosphate, a switch to uncoupled fat metabolism, and hibernation of the organelle, which all contribute to multiple organ failure. Increasing insulin resistance, decreasing fatty acid oxidation, and dependence on carbohydrate metabolism result. Browning of WAT may oppose many of these adverse effects. The presence of BAT and the changes associated with browning may help dissipate oxidative stress, increase consumption and utilization of metabolites, and reduce pro-inflammatory actions. The number of mitochondria increases, and there is greater infiltration of macrophages into adipose tissue. A shift occurs in macrophage expression from the M1 to M2 phenotype, an effect which further dampens inflammation, increases insulin sensitivity, and improves tissue healing and remodeling. Any benefit from these responses may be lost in the disease states of chronic hypermetabolism (such as burns or cancer cachexia) in which the persistence of these physiologic effects may become detrimental, contributing to excessive weight loss, adipose wasting, and loss of lean body mass. This paper discusses the plasticity of adipose tissue and whether shifts in its physiology provide clinical advantages in the intensive care unit.

Rna M6 a Methylation Regulates Glycolysis of Beige Fat and Contributes to Systemic Metabolic Homeostasis

N6-methyladenosine (m6 A) modification has been implicated in the progression of obesity and metabolic diseases. However, its impact on beige fat biology is not well understood. Here, via m6 A-sequencing and RNA-sequencing, this work reports that upon beige adipocytes activation, glycolytic genes undergo major events of m6 A modification and transcriptional activation. Genetic ablation of m6 A writer Mettl3 in fat tissues reveals that Mettl3 deficiency in mature beige adipocytes leads to suppressed glycolytic capability and thermogenesis, as well as reduced preadipocytes proliferation via glycolytic product lactate. In addition, specific modulation of Mettl3 in beige fat via AAV delivery demonstrates consistently Mettl3's role in glucose metabolism, thermogenesis, and beige fat hyperplasia. Mechanistically, Mettl3 and m6 A reader Igf2bp2 control mRNA stability of key glycolytic genes in beige adipocytes. Overall, these findings highlight the significance of m6 A on fat biology and systemic energy homeostasis.

SOX4 promotes beige adipocyte-mediated adaptive thermogenesis by facilitating PRDM16-PPARγ complex

Brown and beige fat protect against cold environments and obesity by catabolizing stored energy to generate heat. This process is achieved by controlling thermogenesis-related gene expression and the development of brown/beige fat through the induction of transcription factors, most notably PPARγ. However, the cofactors that induce the expression of thermogenic genes with PPARγ are still not well understood. In this study, we explored the role of SOX4 in adaptive thermogenesis and its relationship with PPARγ. Methods: Whole transcriptome deep sequencing (RNA-seq) analysis of inguinal subcutaneous white adipose tissue (iWAT) after cold stimulation was performed to identify genes with differential expression in mice. Indirect calorimetry detected oxygen consumption rate and heat generation. mRNA levels were analyzed by qPCR assays. Proteins were detected by immunoblotting and immunofluorescence. Interaction of proteins was detected by endogenous and exogenous Co-IP. ChIP-qPCR, FAIRE assay and luciferase reporter assays were used to investigate transcriptional regulation. Results: SOX4 was identified as the main transcriptional effector of thermogenesis. Mice with either adipocyte-specific or UCP1⁺ cells deletion of SOX4 exhibited significant cold intolerance, decreased energy expenditure, and beige adipocyte formation, which was attributed to decreased thermogenic gene expression. In addition, these mice developed obesity on a high-fat diet, with severe hepatic steatosis, insulin resistance, and inflammation. At the cell level, loss of SOX4 from preadipocytes inhibited the development of beige adipocytes, and loss of SOX4 from mature beige adipocytes reduced the expression of thermogenesis-related genes and energy metabolism. Mechanistically, SOX4 stimulated the transcriptional activity of Ucp1 by binding to PPARγ and activating its transcriptional function. These actions of SOX4 were, at least partly, mediated by recruiting PRDM16 to PPARγ, thus forming a transcriptional complex to elevate the expression of thermogenic genes. Conclusion: SOX4, as a coactivator of PPARγ, drives the thermogenic gene expression program and thermogenesis of beige fat, promoting energy expenditure. It has important physiological significance in resisting cold and obesity.

Dietary lipids inhibit mitochondria transfer to macrophages to divert adipocyte-derived mitochondria into the blood

Adipocytes transfer mitochondria to macrophages in white and brown adipose tissues to maintain metabolic homeostasis. In obesity, adipocyte-to-macrophage mitochondria transfer is impaired, and instead, adipocytes release mitochondria into the blood to induce a protective antioxidant response in the heart. We found that adipocyte-to-macrophage mitochondria transfer in white adipose tissue is inhibited in murine obesity elicited by a lard-based high-fat diet, but not a hydrogenated-coconut-oil-based high-fat diet, aging, or a corn-starch diet. The long-chain fatty acids enriched in lard suppress mitochondria capture by macrophages, diverting adipocyte-derived mitochondria into the blood for delivery to other organs, such as the heart. The depletion of macrophages rapidly increased the number of adipocyte-derived mitochondria in the blood. These findings suggest that dietary lipids regulate mitochondria uptake by macrophages locally in white adipose tissue to determine whether adipocyte-derived mitochondria are released into systemic circulation to support the metabolic adaptation of distant organs in response to nutrient stress.

LncRNA-Mediated Adipogenesis in Different Adipocytes

Long-chain noncoding RNAs (lncRNAs) are RNAs that do not code for proteins, widely present in eukaryotes. They regulate gene expression at multiple levels through different mechanisms at epigenetic, transcription, translation, and the maturation of mRNA transcripts or regulation of the chromatin structure, and compete with microRNAs for binding to endogenous RNA. Adipose tissue is a large and endocrine-rich functional tissue in mammals. Excessive accumulation of white adipose tissue in mammals can cause metabolic diseases. However, unlike white fat, brown and beige fats release energy as heat. In recent years, many lncRNAs associated with adipogenesis have been reported. The molecular mechanisms of how lncRNAs regulate adipogenesis are continually investigated. In this review, we discuss the classification of lncRNAs according to their transcriptional location. lncRNAs that participate in the adipogenesis of white or brown fats are also discussed. The function of lncRNAs as decoy molecules and RNA double-stranded complexes, among other functions, is also discussed.

Muscle-Derived Beige Adipose Precursors Secrete Promyogenic Exosomes That Treat Rotator Cuff Muscle Degeneration in Mice and Are Identified in Humans by Single-Cell RNA Sequencing

BACKGROUND

Muscle atrophy, fibrosis, and fatty infiltration are common to a variety of sports-related and degenerative conditions and are thought to be irreversible. Fibroadipogenic progenitors (FAPs) are multipotent resident muscle stem cells with the capacity to differentiate into fibrogenic as well as white and beige adipose tissue (BAT). FAPs that have assumed a BAT differentiation state (FAP-BAT) have proven efficacious in treating muscle degeneration in numerous injury models.

PURPOSE

To characterize the subpopulation of murine FAPs with FAP-BAT activity, determine whether their promyogenic effect is mediated via exosomes, and analyze human FAPs for an analogous promyogenic exosome-rich subpopulation.

STUDY DESIGN

Controlled laboratory study.

METHODS

FAPs from UCP1 reporter mice were isolated via fluorescence-activated cell sorting and sorted according to the differential intensity of the UCP1 signal observed: negative for UCP1 (UCP1-), intermediate intensity (UCP1+), and high intensity (UCP1++). Bulk RNA sequencing was performed on UCP1-, UCP1+, and UCP1++ FAPs to evaluate distinct characteristics of each population. Exosomes were harvested from UCP1++ FAP-BAT exosomes (Exo-FB) as well as UCP1- non-FAP-BAT exosomes (Exo-nFB) cells using cushioned-density gradient ultracentrifugation and used to treat C2C12 cells and mouse embryonic fibroblasts in vitro, and the myotube fusion index was assessed. Exo-FB and Exo-nFB were then used to treat wild type C57B/L6J mice that had undergone a massive rotator cuff tear. At 6 weeks mice were sacrificed, and supraspinatus muscles were harvested and analyzed for muscle atrophy, fibrosis, fatty infiltration, and UCP1 expression. Single-cell RNA sequencing was then performed on FAPs isolated from human muscle that were treated with the beta-agonist formoterol or standard media to assess for the presence of a parallel promyogenic subpopulation of FAP-BAT cells in humans.

RESULTS

Flow cytometry analysis of sorted UCP1 reporter mouse FAPs revealed a trimodal distribution of UCP1 signal intensity, which correlated with 3 distinct transcriptomic profiles characterized with bulk RNA sequencing. UCP1++ cells were marked by high mitochondrial gene expression, BAT markers, and exosome surface makers; UCP1- cells were marked by fibrogenic markers; and UCP1+ cells were characterized differential enrichment of white adipose tissue markers. Exo-FB treatment of C2C12 cells resulted in robust myotube fusion, while treatment of mouse embryonic fibroblasts resulted in differentiation into myotubes. Treatment of cells with Exo-nFB resulted in poor myotube formation. Mice that were treated with Exo-FB at the time of rotator cuff injury demonstrated markedly reduced muscle atrophy and fatty infiltration as compared with treatment with Exo-nFB or phosphate-buffered saline. Single-cell RNA sequencing of human FAPs from the rotator cuff revealed 6 distinct subpopulations of human FAPs, with one subpopulation demonstrating the presence of UCP1+ beige adipocytes with a distinct profile of BAT, mitochondrial, and extracellular vesicle-associated markers.

CONCLUSION

FAP-BAT cells form a subpopulation of FAPs with upregulated beige gene expression and exosome production that mediate promyogenic effects in vitro and in vivo, and they are present as a transcriptomically similar subpopulation of FAPs in humans.

CLINICAL RELEVANCE

FAP-BAT cells and their exosomes represent a potential therapeutic avenue for treating rotator cuff muscle degeneration.

Local hyperthermia therapy induces browning of white fat and treats obesity

Beige fat plays key roles in the regulation of systemic energy homeostasis; however, detailed mechanisms and safe strategy for its activation remain elusive. In this study, we discovered that local hyperthermia therapy (LHT) targeting beige fat promoted its activation in humans and mice. LHT achieved using a hydrogel-based photothermal therapy activated beige fat, preventing and treating obesity in mice without adverse effects. HSF1 is required for the effects since HSF1 deficiency blunted the metabolic benefits of LHT. HSF1 regulates Hnrnpa2b1 (A2b1) transcription, leading to increased mRNA stability of key metabolic genes. Importantly, analysis of human association studies followed by functional analysis revealed that the HSF1 gain-of-function variant p.P365T is associated with improved metabolic performance in humans and increased A2b1 transcription in mice and cells. Overall, we demonstrate that LHT offers a promising strategy against obesity by inducing beige fat activation via HSF1-A2B1 transcriptional axis.

Exercise-induced changes on exerkines that might influence brown adipose tissue metabolism in young sedentary adults

In rodents, exercise alters the plasma concentration of exerkines that regulate white adipose tissue (WAT) browning or brown adipose tissue (BAT) metabolism. This study aims to analyse the acute and chronic effect of exercise on the circulating concentrations of 16 of these exerkines in humans. Ten young sedentary adults (6 female) performed a maximum walking effort test and a resistance exercise session. The plasma concentration of 16 exerkines was assessed before, and 3, 30, 60, and 120 minutes after exercise. Those exerkines modified by exercise were additionally measured in another 28 subjects (22 women). We also measured the plasma concentrations of the exerkines before and after a 24-week exercise program (endurance + resistance; 3-groups: control, moderate-intensity and vigorous-intensity) in 110 subjects (75 women). Endurance exercise acutely increased the plasma concentration of lactate, norepinephrine, brain-derived neurotrophic factor, interleukin 6, and follistatin-like protein 1 (3 minutes after exercise), and musclin and fibroblast growth factor 21 (30 and 60 minutes after exercise), decreasing the plasma concentration of leptin (30 minutes after exercise). Adiponectin, atrial natriuretic peptide (ANP), β-aminoisobutyric acid, meteorin-like, follistatin, pro-ANP, irisin and myostatin were not modified or not detectable. The resistance exercise session increased the plasma concentration of lactate 3 minutes after exercise. Chronic exercise did not alter the plasma concentration of these exerkines. In sedentary young adults, acute endurance exercise releases to the bloodstream exerkines that regulate BAT metabolism and WAT browning. In contrast, neither a low-volume resistance exercise session nor a 24-week training program modified plasma levels of these molecules.Trial registration: ClinicalTrials.gov identifier: NCT02365129..

Adipose-tissue plasticity in health and disease

Adipose tissue, colloquially known as "fat," is an extraordinarily flexible and heterogeneous organ. While historically viewed as a passive site for energy storage, we now appreciate that adipose tissue regulates many aspects of whole-body physiology, including food intake, maintenance of energy levels, insulin sensitivity, body temperature, and immune responses. A crucial property of adipose tissue is its high degree of plasticity. Physiologic stimuli induce dramatic alterations in adipose-tissue metabolism, structure, and phenotype to meet the needs of the organism. Limitations to this plasticity cause diminished or aberrant responses to physiologic cues and drive the progression of cardiometabolic disease along with other pathological consequences of obesity.

Obesity-resistance of UCP1-deficient mice associates with sustained FGF21 sensitivity in inguinal adipose tissue

Metabolic diseases represent the major health burden of our modern society. With the need of novel therapeutic approaches, fibroblast growth factor 21 (FGF21) is a promising target, based on metabolic improvements upon FGF21 administration in mice and humans. Endogenous FGF21 serum levels, however, are increased during obesity-related diseases, suggesting the development of FGF21 resistance during obesity and thereby lowering FGF21 efficacy. In uncoupling protein 1 knockout (UCP1 KO) mice, however, elevated endogenous FGF21 levels mediate resistance against diet-induced obesity. Here, we show that after long-term high fat diet feeding (HFD), circulating FGF21 levels become similarly high in obese wildtype and obesity-resistant UCP1 KO mice, suggesting improved FGF21 sensitivity in UCP1 KO mice. To test this hypothesis, we injected FGF21 after long-term HFD and assessed the metabolic and molecular effects. The UCP1 KO mice lost weight directly upon FGF21 administration, whereas body weights of WT mice resisted weight loss in the initial phase of the treatment. The FGF21 treatment induced expression of liver Pck1, a typical FGF21-responsive gene, in both genotypes. In iWAT, FGF21-responsive genes were selectively induced in UCP1 KO mice, strongly associating FGF21-sensitivity in iWAT with healthy body weights. Thus, these data support the concept that FGF21-sensitivity in adipose tissue is key for metabolic improvements during obesogenic diets.

Role of irisin in physiology and pathology

Irisin, out-membrane part of fibronectin type III domain-containing 5 protein (FNDC5), was activated by Peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) during physical exercise in skeletal muscle tissues. Most studies have reported that the concentration of irisin is highly associated with health status. For instance, the level of irisin is significantly lower in patients with obesity, osteoporosis/fractures, muscle atrophy, Alzheimer's disease, and cardiovascular diseases (CVDs) but higher in patients with cancer. Irisin can bind to its receptor integrin αV/β5 to induce browning of white fat, maintain glucose stability, keep bone homeostasis, and alleviate cardiac injury. However, it is unclear whether it works by directly binding to its receptors to regulate muscle regeneration, promote neurogenesis, keep liver glucose homeostasis, and inhibit cancer development. Supplementation of recombinant irisin or exercise-activated irisin might be a successful strategy to fight obesity, osteoporosis, muscle atrophy, liver injury, and CVDs in one go. Here, we summarize the publications of FNDC5/irisin from PubMed/Medline, Scopus, and Web of Science until March 2022, and we review the role of FNDC5/irisin in physiology and pathology.

CHRNA2: a new paradigm in beige thermoregulation and metabolism

The contribution of thermogenic adipocytes to maintain systemic metabolic homeostasis has been increasingly appreciated in recent years. It is now recognized that different types (e.g., brown, beige) and subtypes of thermogenic adipocytes may arise from various developmental origins. In addition to the adrenergic pathway, other signals can activate thermogenesis, including paracrine communication between immune cells within the adipose tissue niche and thermogenic adipocytes. In this opinion article we highlight the recently discovered beige-selective signaling between acetylcholine from immune cells and cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) in activated beige adipocytes. We present our current knowledge of how this previously unrecognized adipose non-neuronal cholinergic signaling pathway mediates beige thermoregulation, and discuss its impact on whole-body fitness and its therapeutic potential as a novel target for combating metabolic disease.

MLR-1023 Treatment in Mice and Humans Induces a Thermogenic Program, and Menthol Potentiates the Effect

A glucose-lowering medication that acts by a different mechanism than metformin, or other approved diabetes medications, can supplement monotherapies when patients fail to meet blood glucose goals. We examined the actions underlying the effects of an insulin sensitizer, tolimidone (MLR-1023) and investigated its effects on body weight. Diet-induced obesity (CD1/ICR) and type 2 diabetes (db/db) mouse models were used to study the effect of MLR-1023 on metabolic outcomes and to explore its synergy with menthol. We also examined the efficacy of MLR-1023 alone in a clinical trial (NCT02317796), as well as in combination with menthol in human adipocytes. MLR-1023 produced weight loss in humans in four weeks, and in mice fed a high-fat diet it reduced weight gain and fat mass without affecting food intake. In human adipocytes from obese donors, the upregulation of Uncoupling Protein 1, Glucose (UCP)1, adiponectin, Glucose Transporter Type 4 (GLUT4), Adipose Triglyceride Lipase (ATGL), Carnitine palmitoyltransferase 1 beta (CPT1β), and Transient Receptor Potential Melastin (TRPM8) mRNA expression suggested the induction of thermogenesis. The TRPM8 agonist, menthol, potentiated the effect of MLR-1023 on the upregulation of genes for energy expenditure and insulin sensitivity in human adipocytes, and reduced fasting blood glucose in mice. The amplification of the thermogenic program by MLR-1023 and menthol in the absence of adrenergic activation will likely be well-tolerated, and bears investigation in a clinical trial.

The Forkhead Box Transcription Factor FoxP4 Regulates Thermogenic Programs in Adipocytes

Forkhead box transcription factors have been shown to be involved in various developmental and differentiation processes. In particular, members of the FoxP family have been previously characterized in depth for their participation in the regulation of lung and neuronal cell differentiation and T-cell development and function; however, their role in adipocyte functionality has not yet been investigated. Here, we report for the first time that Forkhead box P4 (FoxP4) is expressed at high levels in subcutaneous fat depots and mature thermogenic adipocytes. Through molecular and gene expression analyses, we revealed that FoxP4 is induced in response to thermogenic stimuli, both in vivo and in isolated cells, and is regulated directly by the heat shock factor protein 1 through a heat shock response element identified in the proximal promoter region of FoxP4. Further detailed analysis involving chromatin immunoprecipitation and luciferase assays demonstrated that FoxP4 directly controls the levels of uncoupling protein 1, a key regulator of thermogenesis that uncouples fatty acid oxidation from ATP production. In addition, through our gain-of-function and loss-of-function studies, we showed that FoxP4 regulates the expression of a number of classic brown and beige fat genes and affects oxygen consumption in isolated adipocytes. Overall, our data demonstrate for the first time the novel role of FoxP4 in the regulation of thermogenic adipocyte functionality.

Oil does more than light the lamp: The multifaceted role of lipids in thermogenic fat

Brown and beige adipocytes, or thermogenic fat, were initially thought to be merely a thermogenic organ. However, emerging evidence suggests its multifaceted roles in the regulation of systemic glucose and lipid homeostasis that go beyond enhancing thermogenesis. One of the important functions of thermogenic fat is as a "metabolic sink" for glucose, fatty acids, and amino acids, which profoundly impacts metabolite clearance and oxidation. Importantly, lipids are not only the predominant fuel source used for thermogenesis but are also essential molecules for development, cellular signaling, and structural components. Here, we review the multifaceted role of lipids in thermogenic adipocytes.

Inhibition of the sodium-glucose co-transporter SGLT2 by canagliflozin ameliorates diet-induced obesity by increasing intra-adipose sympathetic innervation

BACKGROUND AND PURPOSE

Inhibition of the sodium-glucose cotransporter 2 (SGLT2) induces hypoglycaemia by increasing urinary glucose excretion and increasing the use of fat. However, the underlying mechanism is poorly understood. This study was aimed to determine the effects of canagliflozin, a selective SGLT2 inhibitor, on diet-induced obesity and the underlying mechanism(s).

EXPERIMENTAL APPROACH

Adult C57BL/6J male mice were fed with a standard chow diet or high-fat diet supplemented with vehicle or canagliflozin. Whole body energy expenditure was monitored by metabolic cages, noradrenaline levels were measured by HPLC, glucose uptake was measured by PET/CT, and mRNA and protein expression were measured by RT-PCR and western blotting analysis.

KEY RESULTS

Mice treated with canagliflozin were resistant to high-fat diet-induced obesity and its metabolic consequences. Canagliflozin treatment decreased fat mass and increased energy expenditure via increasing thermogenesis and lipolysis in adipose tissue. Mechanistically, SGLT2 inhibition by canagliflozin elevated adipose sympathetic innervation and fat mobilization via a β₃ -adrenoceptor-cAMP-PKA signalling pathway. Finally, we showed that canagliflozin improved insulin resistance and hepatic steatosis in mice fed with a high-fat diet.

CONCLUSIONS AND IMPLICATIONS

Chronic inhibition of SGLT2 increased energy consumption by increasing intra-adipose sympathetic innervation to counteract diet-induced obesity. The present study reveals a new therapeutic function for SGLT2 inhibitors in regulating energy homeostasis.

β3-Adrenergic receptor agonist treats rotator cuff fatty infiltration by activating beige fat in mice

BACKGROUND

Rotator cuff (RC) muscle atrophy and fatty infiltration (FI) are independent factors correlated with failure of attempted tendon repair in larger RC tears. However, there is no effective treatment for RC muscle atrophy and FI at this time. The recent discovery of beige adipose tissue (BAT) in adults shed light on a new avenue in treating obesity and excessive fat deposition by promoting BAT activity. The goal of this study was to define the role of intramuscular BAT in RC muscle FI and the effect of β₃-adrenergic receptor agonists in treating RC muscle FI by promoting BAT activity.

MATERIALS AND METHODS

Three-month-old wild-type C57BL/6J, platelet derived growth factor receptor-alpha (PDGFRα) green fluorescent protein (GFP) reporter and uncoupling protein 1 (UCP-1) knockout mice underwent a unilateral RC injury procedure, which included supraspinatus (SS) and infraspinatus tendon resection and suprascapular nerve transection. To stimulate BAT activity, amibegron, a selective β₃-adrenergic receptor agonist, was administered to C57BL/6J mice either on the same day as surgery or 6 weeks after surgery through daily intraperitoneal injections. Gait analysis was conducted to measure forelimb function at 6 weeks or 12 weeks (in groups receiving delayed amibegron treatment) after surgery. Animals were killed humanely at 6 weeks (or 12 weeks for delayed amibegron groups) after surgery. SS muscles were harvested and analyzed histologically and biochemically.

RESULTS

Histologic analysis of SS muscles from PDGFRα-GFP reporter mice showed that PDGFRα-positive fibroadipogenic progenitors in RC muscle expressed UCP-1, a hallmark of BAT during the development of FI after RC tears. Impairing BAT activity by knocking out UCP-1 resulted in more severe muscle atrophy and FI with inferior forelimb function in UCP-1 knockout mice compared with wild-type mice. Promoting BAT activity with amibegron significantly reduced muscle atrophy and FI after RC tears and improved forelimb function. Delayed treatment with amibegron reversed muscle atrophy and FI in muscle.

CONCLUSIONS

Fat accumulated in muscle after RC tears possesses BAT characteristics. Impairing BAT activity results in worse RC muscle atrophy and FI. Amibegron reduces and reverses RC atrophy and FI by promoting BAT activity.

CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling

Adipose tissues dynamically remodel their cellular composition in response to external cues by stimulating beige adipocyte biogenesis; however, the developmental origin and pathways regulating this process remain insufficiently understood owing to adipose tissue heterogeneity. Here, we employed single-cell RNA-seq and identified a unique subset of adipocyte progenitor cells (APCs) that possessed the cell-intrinsic plasticity to give rise to beige fat. This beige APC population is proliferative and marked by cell-surface proteins, including PDGFRα, Sca1, and CD81. Notably, CD81 is not only a beige APC marker but also required for de novo beige fat biogenesis following cold exposure. CD81 forms a complex with αV/β1 and αV/β5 integrins and mediates the activation of integrin-FAK signaling in response to irisin. Importantly, CD81 loss causes diet-induced obesity, insulin resistance, and adipose tissue inflammation. These results suggest that CD81 functions as a key sensor of external inputs and controls beige APC proliferation and whole-body energy homeostasis.

Adrenergic-Independent Signaling via CHRNA2 Regulates Beige Fat Activation

Maintaining energy homeostasis upon environmental challenges, such as cold or excess calorie intake, is essential to the fitness and survival of mammals. Drug discovery efforts targeting β-adrenergic signaling have not been fruitful after decades of intensive research. We recently identified a new beige fat regulatory pathway mediated via the nicotinic acetylcholine receptor subunit CHRNA2. Here, we generated fat-specific Chrna2 KO mice and observed thermogenic defects in cold and metabolic dysfunction upon dietary challenges caused by adipocyte-autonomous regulation in vivo. We found that CHRNA2 signaling is activated after acute high fat diet feeding and this effect is manifested through both UCP1- and creatine-mediated mechanisms. Furthermore, our data suggested that CHRNA2 signaling may activate glycolytic beige fat, a subpopulation of beige adipocytes mediated by GABPα emerging in the absence of β-adrenergic signaling. These findings reveal the biological significance of the CHRNA2 pathway in beige fat biogenesis and energy homeostasis.

Human brown adipose tissue: Classical brown rather than brite/beige?

NEW FINDINGS

What is the topic of this review? It has been suggested that human brown adipose tissue (BAT) is more similar to the brite/beige adipose tissue of mice than to classical BAT of mice. The basis of this is discussed in relationship to the physiological conditions of standard experimental mice. What advances does it highlight? We highlight that, provided mouse adipose tissues are examined under physiological conditions closer to those prevalent for most humans, the gene expression profile of mouse classical BAT is more similar to that of human BAT than is the profile of mouse brite/beige adipose tissue. Human BAT is therefore not different in nature from classical mouse BAT.

ABSTRACT

Since the presence of brown adipose tissue (BAT) was established in adult humans some 13 years ago, its physiological significance and molecular characteristics have been discussed. In particular, it has been proposed that the mouse adipose tissue depot most closely resembling and molecularly parallel to human BAT is not classical mouse BAT. Instead, so-called brite or beige adipose tissue, which is characteristically observed in the inguinal 'white' adipose tissue depot of mice, has been proposed to be the closest mouse equivalent of human BAT. We summarize here the published evidence examining this question. We emphasize the differences in tissue appearance and tissue transcriptomes from 'standard' mice [young, chow fed and, in effect semi-cold exposed (20°C)] versus 'physiologically humanized' mice [middle-aged, high-fat diet-fed mice living at thermoneutrality (30°C)]. We find that in the physiologically humanized mice, classical BAT displays molecular and cellular characteristics that are more akin to human BAT than are those of brite/beige adipose tissues from either standard or physiologically humanized mice. We suggest, therefore, that mouse BAT is the more relevant tissue for translational studies. This is an invited summary of a presentation given at Physiology 2019 (Aberdeen).

Bilirubin remodels murine white adipose tissue by reshaping mitochondrial activity and the coregulator profile of peroxisome proliferator-activated receptor α

Activation of lipid-burning pathways in the fat-storing white adipose tissue (WAT) is a promising strategy to improve metabolic health and reduce obesity, insulin resistance, and type II diabetes. For unknown reasons, bilirubin levels are negatively associated with obesity and diabetes. Here, using mice and an array of approaches, including MRI to assess body composition, biochemical assays to measure bilirubin and fatty acids, MitoTracker-based mitochondrial analysis, immunofluorescence, and high-throughput coregulator analysis, we show that bilirubin functions as a molecular switch for the nuclear receptor transcription factor peroxisome proliferator-activated receptor α (PPARα). Bilirubin exerted its effects by recruiting and dissociating specific coregulators in WAT, driving the expression of PPARα target genes such as uncoupling protein 1 (Ucp1) and adrenoreceptor β 3 (Adrb3). We also found that bilirubin is a selective ligand for PPARα and does not affect the activities of the related proteins PPARγ and PPARδ. We further found that diet-induced obese mice with mild hyperbilirubinemia have reduced WAT size and an increased number of mitochondria, associated with a restructuring of PPARα-binding coregulators. We conclude that bilirubin strongly affects organismal body weight by reshaping the PPARα coregulator profile, remodeling WAT to improve metabolic function, and reducing fat accumulation.

Exercise Reverses the Alterations in Gut Microbiota Upon Cold Exposure and Promotes Cold-Induced Weight Loss

Gut microbiota has been reported to contribute to reduced diet-induced obesity upon cold exposure. Furthermore, gut microbiome fermentation determines the efficacy of exercise for diabetes prevention and enhances exercise performance. However, there have been no systematic examinations of changes in gut microbiome composition in relation to exercise performed under low-temperature conditions. In this study, we investigated the effects of exercise performed under different conditions (room temperature, acute cold, intermittent cold, and sustained cold) in obese rats maintained on a high-fat diet at four time points during experimental trials (days 0, 1, 3, and 35), including observations on white fat browning, weight loss, cardiovascular effects, and changes in gut microbiota among treatment groups. We found that exercise under sustained cold conditions produced a remarkable shift in microbiota composition. Unexpectedly, exercise was found to reverse the alterations in gut microbiota alpha-diversity and the abundance of certain bacterial phyla observed in response to cold exposure (e.g., Proteobacteria decreased upon cold exposure but increased in response to exercise under cold conditions). Moreover, exercise under cold conditions (hereafter referred to “cold exercise”) promoted a considerably higher level of white fat browning and greater weight loss and protected against the negative cardiovascular effects of cold exposure. Correlation analysis revealed that cold exercise-related changes in gut microbial communities were significantly correlated with white fat browning and cardiovascular phenotypes. These results could reveal novel mechanisms whereby additional health benefits attributable to both cold and exercise are mediated via altered gut microbes differently compared with either of them alone.

Beige FAPs Transplantation Improves Muscle Quality and Shoulder Function After Massive Rotator Cuff Tears

Rotator cuff (RC) tears are a common cause of upper extremity disability. Any tear size can result in subsequent muscle atrophy and fatty infiltration (FI). Preoperative muscle degeneration can predict repair and postoperative functional outcomes. Muscle residential fibro-adipogenic progenitors (FAPs) are found to be capable of differentiating into beige adipocytes that release factors to promote muscle growth. This study evaluated the regenerative potential of local cell transplantation of beige FAPs to mitigate muscle degeneration in a murine massive RC tear model. Beige FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1⁺ /Sca1⁺ /PDGFR⁺ /CD31^- /CD45^- /integrin α7^- . C57/BL6J mice undergoing supraspinatus tendon tear with suprascapular nerve transection (TT + DN) received either no additional treatment, phosphate-buffered saline injection, or beige FAP injection 2 weeks after the initial injury. Forelimb gait analysis was used to assess shoulder function with DigiGait. Mice were sacrificed 6 weeks after cell transplantation. FI, fibrosis, fiber size, vascularity were analyzed and quantified via ImageJ. Our results showed that beige FAP transplantation significantly decreased fibrosis, FI, and atrophy, enhanced vascularization compared with saline injection and non-treatment groups. Beige FAP transplantation also significantly improved shoulder function as measured by gait analysis. This study suggests that beige-differentiated FAPs may serve as a treatment option for RC muscle atrophy and FI, thus improving shoulder function in patients with massive RC tendon tears. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1159-1166, 2020.

Patch-Clamp Analysis of the Mitochondrial H+ Leak in Brown and Beige Fat

Mitochondria convert the chemical energy of metabolic substrates into adenosine triphosphate (ATP) and heat. Although ATP production has become a focal point of research in bioenergetics, mitochondrial thermogenesis is also crucial for energy metabolism. Mitochondria generate heat due to H+ leak across the inner mitochondrial membrane (IMM) which is mediated by mitochondrial uncoupling proteins. The mitochondrial H+ leak was first identified, and studied for many decades, using mitochondrial respiration technique. Unfortunately, this method measures H+ leak indirectly, and its precision is insufficient for the rigorous insight into the mitochondrial function at the molecular level. Direct patch-clamp recording of H+ leak would have a significantly higher amplitude and time resolution, but application of the patch-clamp technique to a small subcellular organelle such as mitochondria has been challenging. We developed a method that facilitates patch-clamp recording from the whole IMM, enabling the direct measurement of small H+ leak currents via uncoupling proteins and thus, providing a rigorous understanding of the molecular mechanisms involved. In this paper we cover the methodology of measuring the H+ leak in mitochondria of specialized thermogenic tissues brown and beige fat.

Beige fibro-adipogenic progenitor transplantation reduces muscle degeneration and improves function in a mouse model of delayed repair of rotator cuff tears

BACKGROUND

Muscle atrophy and fatty infiltration (FI) are common occurrences following rotator cuff (RC) tears. Tears of all sizes are subject to muscle degeneration. The degree of muscle degeneration following RC tears is highly correlated with repair success and functional outcomes. We have recently discovered that muscle fibro-adipogenic progenitors (FAPs) can differentiate into uncoupling protein 1 (UCP-1)-expressing beige adipocytes and induce muscle regeneration. This study evaluated the potential of local cell transplantation of beige adipose FAPs (BAT-FAPs) to treat RC muscle degeneration in a murine model of RC repair.

METHODS

BAT-FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1⁺/Sca1⁺/PDGFR⁺/CD31^-/CD45^-/integrin α7^-. C57/BL6J mice underwent supraspinatus tendon tear with suprascapular nerve transection followed by repair 2 or 6 weeks after the initial injury. At the time of repair, mice received either no additional treatment, phosphate-buffered saline injection, or BAT-FAP injection. Functional outcomes were assessed by gait analysis. Mice were humanely killed at 6 weeks after cell transplantation. Supraspinatus muscle FI, fibrosis, muscle fiber size, and vascularity were analyzed and quantified via ImageJ. Analysis of variance with post hoc Tukey test and P <.05 was used to determine statistical significance.

RESULTS

Cell transplantation diminished fibrosis, FI, and atrophy and enhanced vascularization in both delayed repair models. Cell transplantation resulted in improved shoulder function as assessed with gait analysis in both the delayed repair models.

CONCLUSIONS

BAT-FAPs significantly reduced muscle degeneration and improved shoulder function after RC repair. BAT-FAPs hold significant promise as a therapeutic adjunct to repair for patients with advanced RC pathology.

EoTHINophils: Eosinophils as key players in adipose tissue homeostasis

Eosinophils are granular cells of the innate immune system that are found in almost all vertebrates and some invertebrates. Knowledge of their wide-ranging roles in health and disease has largely been attained through studies in mice and humans. Although eosinophils are typically associated with helminth infections and allergic diseases such as asthma, there is building evidence that beneficial homeostatic eosinophils residing in specific niches are important for tissue development, remodelling and metabolic control. In recent years, the importance of immune cells in the regulation of adipose tissue homeostasis has been a focal point of research efforts. There is an abundance of anti-inflammatory innate immune cells in lean white adipose tissue, including macrophages, eosinophils and group 2 innate lymphoid cells, which promote energy homeostasis and stimulate the development of thermogenic beige adipocytes. This review will evaluate evidence for the role of adipose-resident eosinophils in local tissue homeostasis, beiging and systemic metabolism, highlighting where more research is needed to establish the specific effector functions that adipose eosinophils perform in response to different internal and external cues.

Biliverdin Reductase A (BVRA) Knockout in Adipocytes Induces Hypertrophy and Reduces Mitochondria in White Fat of Obese Mice

Biliverdin reductase (BVR) is an enzymatic and signaling protein that has multifaceted roles in physiological systems. Despite the wealth of knowledge about BVR, no data exist regarding its actions in adipocytes. Here, we generated an adipose-specific deletion of biliverdin reductase-A (BVRA) (Blvra^FatKO) in mice to determine the function of BVRA in adipocytes and how it may impact adipose tissue expansion. The Blvra^FatKO and littermate control (Blvra^Flox) mice were placed on a high-fat diet (HFD) for 12 weeks. Body weights were measured weekly and body composition, fasting blood glucose and insulin levels were quantitated at the end of the 12 weeks. The data showed that the percent body fat and body weights did not differ between the groups; however, Blvra^FatKO mice had significantly higher visceral fat as compared to the Blvra^Flox. The loss of adipocyte BVRA decreased the mitochondrial number in white adipose tissue (WAT), and increased inflammation and adipocyte size, but this was not observed in brown adipose tissue (BAT). There were genes significantly reduced in WAT that induce the browning effect such as Ppara and Adrb3, indicating that BVRA improves mitochondria function and beige-type white adipocytes. The Blvra^FatKO mice also had significantly higher fasting blood glucose levels and no changes in plasma insulin levels, which is indicative of decreased insulin signaling in WAT, as evidenced by reduced levels of phosphorylated AKT (pAKT) and Glut4 mRNA. These results demonstrate the essential role of BVRA in WAT in insulin signaling and adipocyte hypertrophy.

In vivo study of metabolic dynamics and heterogeneity in brown and beige fat by label-free multiphoton redox and fluorescence lifetime microscopy

In this work, the metabolic characteristics of adipose tissues in live mouse model were investigated using a multiphoton redox ratio and fluorescence lifetime imaging technology. By analyzing the intrinsic fluorescence of metabolic coenzymes, we measured the optical redox ratios of adipocytes in vivo and studied their responses to thermogenesis. The fluorescence lifetime imaging further revealed changes in protein bindings of metabolic coenzymes in the adipocytes during thermogenesis. Our study uncovered significant heterogeneity in the cellular structures and metabolic characteristics of thermogenic adipocytes in brown and beige fat. Subgroups of brown and beige adipocytes were identified based on the distinct lipid size distributions, redox ratios, fluorescence lifetimes and thermogenic capacities. The results of our study show that this label-free imaging technique can shed new light on in vivo study of metabolic dynamics and heterogeneity of adipose tissues in live organisms.

Combating Obesity With Thermogenic Fat: Current Challenges and Advancements

Brown fat and beige fat are known as thermogenic fat due to their contribution to non-shivering thermogenesis in mammals following cold stimulation. Beige fat is unique due to its origin and its development in white fat. Subsequently, both brown fat and beige fat have become viable targets to combat obesity. Over the last few decades, most therapeutic strategies have been focused on the canonical pathway of thermogenic fat activation via the β3-adrenergic receptor (AR). Notwithstanding, administering β3-AR agonists often leads to side effects including hypertension and particularly cardiovascular disease. It is thus imperative to search for alternative therapeutic approaches to combat obesity. In this review, we discuss the current challenges in the field with respect to stimulating brown/beige fat thermogenesis. Additionally, we include a summary of other newly discovered pathways, including non-AR signaling- and non-UCP1-dependent mechanisms, which could be potential targets for the treatment of obesity and its related metabolic diseases.

Beige Fat Maintenance; Toward a Sustained Metabolic Health

Understanding the mammalian energy balance can pave the way for future therapeutics that enhance energy expenditure as an anti-obesity and anti-diabetic strategy. Several studies showed that brown adipose tissue activity increases daily energy expenditure. However, the size and activity of brown adipose tissue is reduced in individuals with obesity and type two diabetes. Humans have an abundance of functionally similar beige adipocytes that have the potential to contribute to increased energy expenditure. This makes beige adipocytes a promising target for metabolic disease therapies. While brown adipocytes tend to be stable, beige adipocytes have a high level of plasticity that allows for the rapid and dynamic induction of thermogenesis by external stimuli such as low environmental temperatures. This means that after browning stimuli have been withdrawn beige adipocytes quickly transition back to their white adipose state. The detailed molecular mechanisms regulating beige adipocytes development, function, and reversibility are not fully understood. The goal of this review is to give a comprehensive overview of beige fat development and origins, along with the transcriptional and epigenetic programs that lead to beige fat formation, and subsequent thermogenesis in humans. An improved understanding of the molecular pathways of beige adipocyte plasticity will enable us to selectively manipulate beige cells to induce and maintain their thermogenic output thus improving the whole-body energy homeostasis.

Dynamic contrast-enhanced MRI of brown and beige adipose tissues

PURPOSE

The vascular blood flow in brown adipose tissue (BAT) is important for handling triglyceride clearance, increased blood flow and oxygenation. We used dynamic contrast-enhanced (DCE)-MRI and fat fraction (FF) imaging for investigating vascular perfusion kinetics in brown and beige adipose tissues with cold exposure or treatment with β3-adrenergic agonist.

METHODS

FF imaging and DCE-MRI using gadolinium-diethylenetriaminepentaacetic acid were performed in interscapular BAT (iBAT) and beige tissues using male Wister rats (n = 38). Imaging was performed at thermoneutral condition and with either cold exposure, treatment with pharmacological agent CL-316,243, or saline. DCE-MRI and FF data were co-registered to enhance the understanding of metabolic activity.

RESULTS

Uptake of contrast agent in activated iBAT and beige tissues were significantly (P < .05) higher than nonactivated iBAT. The K^trans and k_ep increased significantly in iBAT and beige tissues after treatment with either cold exposure or β3-adrenergic agonist. The FF decreased in activated iBAT and beige tissues. The K^trans and FF from iBAT and beige tissues were inversely correlated (r = 0.97; r = 0.94). Significant increase in vascular endothelial growth factor expression and K^trans in activated iBAT and beige tissues were in agreement with the increased vasculature and vascular perfusion kinetics. The iBAT and beige tissues were validated by measuring molecular markers.

CONCLUSION

Increased K^trans and decreased FF in iBAT and beige tissues were in agreement with the vascular perfusion kinetics facilitating the clearance of free fatty acids. The methodology can be extended for the screening of browning agents.

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.

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.

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.

A PRDM16-Driven Metabolic Signal from Adipocytes Regulates Precursor Cell Fate

The precursor cells for metabolically beneficial beige adipocytes can alternatively become fibrogenic and contribute to adipose fibrosis. We found that cold exposure or β3-adrenergic agonist treatment of mice decreased the fibrogenic profile of precursor cells and stimulated beige adipocyte differentiation. This fibrogenic-to-adipogenic transition was impaired in aged animals, correlating with reduced adipocyte expression of the transcription factor PRDM16. Genetic loss of Prdm16 mimicked the effect of aging in promoting fibrosis, whereas increasing PRDM16 in aged mice decreased fibrosis and restored beige adipose development. PRDM16-expressing adipose cells secreted the metabolite β-hydroxybutyrate (BHB), which blocked precursor fibrogenesis and facilitated beige adipogenesis. BHB catabolism in precursor cells, mediated by BDH1, was required for beige fat differentiation in vivo. Finally, dietary BHB supplementation in aged animals reduced adipose fibrosis and promoted beige fat formation. Together, our results demonstrate that adipocytes secrete a metabolite signal that controls beige fat remodeling.

Identification of a natural beige adipose depot in mice

Beige fat is a potential therapeutic target for obesity and other metabolic diseases due to its inducible brown fat-like functions. Inguinal white adipose tissue (iWAT) can undergo robust brown remodeling with appropriate stimuli and is therefore widely considered as a representative beige fat depot. However, adipose tissues residing in different anatomic depots exhibit a broad range of plasticity, raising the possibility that better beige fat depots with greater plasticity may exist. Here we identified and characterized a novel, naturally-existing beige fat depot, thigh adipose tissue (tAT). Unlike classic WATs, tAT maintains beige fat morphology at room temperature, whereas high-fat diet (HFD) feeding or aging promotes the development of typical WAT features, namely unilocular adipocytes. The brown adipocyte gene expression in tAT is consistently higher than in iWAT under cold exposure, HFD feeding, and rosiglitazone treatment conditions. Our molecular profiling by RNA-Seq revealed up-regulation of energy expenditure pathways and repressed inflammation in tAT relative to eWAT and iWAT. Furthermore, we demonstrated that the master fatty acid oxidation regulator peroxisome proliferator-activated receptor α is dispensable for maintaining and activating the beige character of tAT. Therefore, we have identified tAT as a natural beige adipose depot in mice with a unique molecular profile that does not require peroxisome proliferator-activated receptor α.

New Advances in Adaptive Thermogenesis: UCP1 and Beyond

Brown and beige adipocytes can catabolize stored energy to generate heat, and this distinct capacity for thermogenesis could be leveraged as a therapy for metabolic diseases like obesity and type 2 diabetes. Thermogenic adipocytes drive heat production through close coordination of substrate supply with the mitochondrial oxidative machinery and effectors that control the rate of substrate oxidation. Together, this apparatus affords these adipocytes with tremendous capacity to drive thermogenesis. The best characterized thermogenic effector is uncoupling protein 1 (UCP1). Importantly, additional mechanisms for activating thermogenesis beyond UCP1 have been identified and characterized to varying extents. Acute regulation of these thermogenic pathways has been an active area of study, and numerous regulatory factors have been uncovered in recent years. Here we will review the evidence for regulators of heat production in thermogenic adipocytes in the context of the thermodynamic and kinetic principles that govern their therapeutic utility.

Transcriptional Control of Brown and Beige Fat Development and Function

Adipose tissue, once viewed as an inert organ of energy storage, is now appreciated to be a central node for the dynamic regulation of systemic metabolism. There are three general types of adipose tissue: white, brown, and brown-in-white or "beige" fat. All three types of adipose tissue communicate extensively with other organs in the body, including skin, liver, pancreas, muscle, and brain, to maintain energy homeostasis. When energy intake chronically exceeds energy expenditure, obesity and its comorbidities can develop. Thus, understanding the molecular mechanisms by which different types of adipose tissues develop and function could uncover new therapies for combating disorders of energy imbalance. In this review, the recent findings on the transcriptional and chromatin-mediated regulation of brown and beige adipose tissue activity are highlighted.

Functional Gut Microbiota Remodeling Contributes to the Caloric Restriction-Induced Metabolic Improvements

Caloric restriction (CR) stimulates development of functional beige fat and extends healthy lifespan. Here we show that compositional and functional changes in the gut microbiota contribute to a number of CR-induced metabolic improvements and promote fat browning. Mechanistically, these effects are linked to a lower expression of the key bacterial enzymes necessary for the lipid A biosynthesis, a critical lipopolysaccharide (LPS) building component. The decreased LPS dictates the tone of the innate immune response during CR, leading to increased eosinophil infiltration and anti-inflammatory macrophage polarization in fat of the CR animals. Genetic and pharmacological suppression of the LPS-TLR4 pathway or transplantation with Tlr4^-/- bone-marrow-derived hematopoietic cells increases beige fat development and ameliorates diet-induced fatty liver, while Tlr4^-/- or microbiota-depleted mice are resistant to further CR-stimulated metabolic alterations. These data reveal signals critical for our understanding of the microbiota-fat signaling axis during CR and provide potential new anti-obesity therapeutics.

PexRAP Inhibits PRDM16-Mediated Thermogenic Gene Expression

How the nuclear receptor PPARγ regulates the development of two functionally distinct types of adipose tissue, brown and white fat, as well as the browning of white fat, remains unclear. Our previous studies suggest that PexRAP, a peroxisomal lipid synthetic enzyme, regulates PPARγ signaling and white adipogenesis. Here, we show that PexRAP is an inhibitor of brown adipocyte gene expression. PexRAP inactivation promoted adipocyte browning, increased energy expenditure, and decreased adiposity. Identification of PexRAP-interacting proteins suggests that PexRAP function extends beyond its role as a lipid synthetic enzyme. Notably, PexRAP interacts with importin-β1, a nuclear import factor, and knockdown of PexRAP in adipocytes reduced the levels of nuclear phospholipids. PexRAP also interacts with PPARγ, as well as PRDM16, a critical transcriptional regulator of thermogenesis, and disrupts the PRDM16-PPARγ complex, providing a potential mechanism for PexRAP-mediated inhibition of adipocyte browning. These results identify PexRAP as an important regulator of adipose tissue remodeling.

Multimodal imaging approach to monitor browning of adipose tissue in vivo

The discovery that white adipocytes can undergo a browning process to become metabolically active beige cells has attracted significant interest in the fight against obesity. However, the study of adipose browning has been impeded by a lack of imaging tools that allow longitudinal and noninvasive monitoring of this process in vivo. Here, we report a preclinical imaging approach to detect development of beige adipocytes during adrenergic stimulation. In this approach, we expressed near-infrared fluorescent protein, iRFP720, driven under an uncoupling protein-1 (Ucp1) promoter in mice by viral transduction, and used multispectral optoacoustic imaging technology with ultrasound tomography (MSOT-US) to assess adipose beiging during adrenergic stimulation. We observed increased photoacoustic signal at 720 nm, coupled with attenuated lipid signals in stimulated animals. As a proof of concept, we validated our approach against hybrid positron emission tomography combined with magnetic resonance (PET/MR) imaging modality, and quantified the extent of adipose browning by MRI-guided segmentation of 2-deoxy-2-18F-fluoro-d-glucose uptake signals. The browning extent detected by MSOT-US and PET/MR are well correlated with Ucp1 induction. Taken together, these systems offer great opportunities for preclinical screening aimed at identifying compounds that promote adipose browning and translation of these discoveries into clinical studies of humans.

Chronic Rapamycin Treatment Improved Metabolic Phenotype but Inhibited Adipose Tissue Browning in High-Fat Diet-Fed C57BL/6J Mice

Rapamycin (Rap) has been demonstrated to affect lipid metabolism through stimulating lipolysis, inhibiting de novo lipogenesis and reducing adiposity. In the present study, we investigated rapamycin exposure's influence on adipose tissue browning in high-fat diet-induced fatty mice. Four-week old C57BL/6J mice were fed normal chow or high-fat diet for a period of 6 weeks and then divided into three groups: (1) Nor group: mice fed with normal chow; (2) high fat diet (HFD) group: fatty mice fed with high-fat diet; (3) Rap group: high-fat diet-fed fatty mice treated intragastrically with rapamycin at a dose of 2.5 mg/kg per day for 5 weeks. Body weights and food intakes of the mice were recorded weekly. At the end of the study, blood samples were collected for glucose, lipid and insulin evaluations. Adipose tissues were weighed and lipid contents were monitored. Moreover, real-time PCR and Western blotting were applied to detect the expression levels of beige and brown fat marker genes in white adipose tissue (WAT) and brown adipose tissue (BAT). Our data demonstrated that Rap exposure significantly ameliorated metabolic defects including hyperglycaemia, dyslipidaemia and insulin resistance in the fatty mice. Furthermore, Rap treatment led to decreased tissue weights and lipid contents both in WAT and BAT. Remarkably, expression levels of BAT marker genes including uncoupling protein-1 (UCP-1), cell death-inducing DNA fragmentation factor-alpha-like effector A (CIDEA), PR-domain containing protein-16 (PRDM16) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) were significantly down-regulated in Rap-treated fatty mice. This report demonstrates Rap exposure is capable of inhibiting adipose tissue browning in high-fat diet-induced fatty mice, and provides evidence for deeper understanding of Rap's influence on lipid homeostasis.

What Can 'Brown-ing' Do For You?

Human stem cell-based models of thermogenic adipocytes provide an opportunity for the establishment of new therapeutics, modeling of disease mechanisms, and understanding of development. Pluripotent stem cells, adipose-derived stem cells/preadipocytes, and programming-reprogramming-based approaches have been used to develop cell-based platforms for drug screening and transplantable therapeutics in the metabolic disease arena. Here we provide a detailed overview of these approaches, the latest advances in this field, and the opportunities and shortcomings they present. Moreover, we comment on how stem-cell-based platforms can be best utilized in the future for the treatment and understanding of metabolic diseases, including type 2 diabetes and associated medical issues such as obesity.

Kynurenic Acid and Gpr35 Regulate Adipose Tissue Energy Homeostasis and Inflammation

The role of tryptophan-kynurenine metabolism in psychiatric disease is well established, but remains less explored in peripheral tissues. Exercise training activates kynurenine biotransformation in skeletal muscle, which protects from neuroinflammation and leads to peripheral kynurenic acid accumulation. Here we show that kynurenic acid increases energy utilization by activating G protein-coupled receptor Gpr35, which stimulates lipid metabolism, thermogenic, and anti-inflammatory gene expression in adipose tissue. This suppresses weight gain in animals fed a high-fat diet and improves glucose tolerance. Kynurenic acid and Gpr35 enhance Pgc-1α1 expression and cellular respiration, and increase the levels of Rgs14 in adipocytes, which leads to enhanced beta-adrenergic receptor signaling. Conversely, genetic deletion of Gpr35 causes progressive weight gain and glucose intolerance, and sensitizes to the effects of high-fat diets. Finally, exercise-induced adipose tissue browning is compromised in Gpr35 knockout animals. This work uncovers kynurenine metabolism as a pathway with therapeutic potential to control energy homeostasis.

Brown and beige adipose tissues: phenotype and metabolic potential in mice and men

With the recent rediscovery of brown fat in adult humans, our outlook on adipose tissue biology has undergone a paradigm shift. While we attempt to identify, recruit, and activate classic brown fat stores in humans, identification of beige fat has also raised the possibility of browning our white fat stores. Whether such transformation of human white fat depots can be achieved to enhance the whole body oxidative potential remains to be seen. Evidence to date, however, largely points toward a major oxidative role only for classic brown fat depots, at least in rodents. White fat stores seem to provide the main fuel for sustaining thermogenesis via lipolysis. Interestingly, molecular markers consistent with both classic brown and beige fat identity can be observed in human supraclavicular depot, thereby complicating the discussion on beige fat in humans. Here, we review the recent advances made in our understanding of brown and beige fat in humans and mice. We further provide an overview of their plausible physiological relevance to whole body energy metabolism.

Zbtb7b engages the long noncoding RNA Blnc1 to drive brown and beige fat development and thermogenesis

Brown and beige adipocytes convert chemical energy into heat through uncoupled respiration to defend against cold stress. Beyond thermogenesis, brown and beige fats engage other metabolic tissues via secreted factors to influence systemic energy metabolism. How the protein and long noncoding RNA (lncRNA) regulatory networks act in concert to regulate key aspects of thermogenic adipocyte biology remains largely unknown. Here we developed a genome-wide functional screen to interrogate the transcription factors and cofactors in thermogenic gene activation and identified zinc finger and BTB domain-containing 7b (Zbtb7b) as a potent driver of brown fat development and thermogenesis and cold-induced beige fat formation. Zbtb7b is required for activation of the thermogenic gene program in brown and beige adipocytes. Genetic ablation of Zbtb7b impaired cold-induced transcriptional remodeling in brown fat, rendering mice sensitive to cold temperature, and diminished browning of inguinal white fat. Proteomic analysis revealed a mechanistic link between Zbtb7b and the lncRNA regulatory pathway through which Zbtb7b recruits the brown fat lncRNA 1 (Blnc1)/heterogeneous nuclear ribonucleoprotein U (hnRNPU) ribonucleoprotein complex to activate thermogenic gene expression in adipocytes. These findings illustrate the emerging concept of a protein-lncRNA regulatory network in the control of adipose tissue biology and energy metabolism.

The significance of beige and brown fat in humans

Promotion of brown adipose tissue (BAT) activity or browning of white adipose tissue has shown great potential as anti-obesity strategy in numerous preclinical models. The discovery of active BAT in humans and the recent advances in the understanding of human BAT biology and function have significantly propelled this field of research. Pharmacological stimulation of energy expenditure to counteract obesity has always been an intriguing therapeutic concept; with the identification of the specific molecular pathways of brown fat function, this idea has now become as realistic as ever. Two distinct strategies are currently being pursued; one is the activation of bone fide BAT, the other is the induction of BAT-like cells or beige adipocytes within white fat depots, a process called browning. Recent evidence suggests that both phenomena can occur in humans. Cold-induced promotion of BAT activity is strongly associated with enhanced thermogenesis and energy expenditure in humans and has beneficial effects on fat mass and glucose metabolism. Despite these encouraging results, a number of issues deserve additional attention including the distinct characteristics of human vs rodent BAT, the heterogeneity of human BAT depots or the identification of the adipocyte precursors that can give rise to thermogenic cells in human adipose tissue. In addition, many pharmaceutical compounds are being tested for their ability to promote a thermogenic program in human adipocytes. This review summarizes the current knowledge about the various cellular and molecular aspects of human BAT as well as the relevance for energy metabolism including its therapeutic potential for obesity.

Lsd1 prevents age-programed loss of beige adipocytes

Aging is accompanied by major changes in adipose tissue distribution and function. In particular, with time, thermogenic-competent beige adipocytes progressively gain a white adipocyte morphology. However, the mechanisms controlling the age-related transition of beige adipocytes to white adipocytes remain unclear. Lysine-specific demethylase 1 (Lsd1) is an epigenetic eraser enzyme positively regulating differentiation and function of adipocytes. Here we show that Lsd1 levels decrease in aging inguinal white adipose tissue concomitantly with beige fat cell decline. Accordingly, adipocyte-specific increase of Lsd1 expression is sufficient to rescue the age-related transition of beige adipocytes to white adipocytes in vivo, whereas loss of Lsd1 precipitates it. Lsd1 maintains beige adipocytes by controlling the expression of peroxisome proliferator-activated receptor α (Ppara), and treatment with a Ppara agonist is sufficient to rescue the loss of beige adipocytes caused by Lsd1 ablation. In summary, our data provide insights into the mechanism controlling the age-related beige-to-white adipocyte transition and identify Lsd1 as a regulator of beige fat cell maintenance.