Brown adipose tissue improves whole-body glucose homeostasis and insulin sensitivity in humans

Short-term cold exposure induces persistent epigenomic memory in brown fat

Deficiency of the epigenome modulator histone deacetylase 3 (HDAC3) in brown adipose tissue (BAT) impairs the ability of mice to survive in near-freezing temperatures. Here, we report that short-term exposure to mild cold temperature (STEMCT: 15°C for 24 h) averted lethal hypothermia of mice lacking HDAC3 in BAT (HDAC3 BAT KO) exposed to 4°C. STEMCT restored the induction of the thermogenic coactivator PGC-1α along with UCP1 at 22°C, which is greatly impaired in HDAC3-deficient BAT, and deletion of either UCP1 or PGC-1α prevented the protective effect of STEMCT. Remarkably, this protection lasted for up to 7 days. Transcriptional activator C/EBPβ was induced by short-term cold exposure in mouse and human BAT and, uniquely, remained high for 7 days following STEMCT. Adeno-associated virus-mediated knockdown of BAT C/EBPβ in HDAC3 BAT KO mice erased the persistent memory of STEMCT, revealing the existence of a C/EBPβ-dependent and HDAC3-independent cold-adaptive epigenomic memory.

Thermoneutrality Inhibits Thermogenic Markers and Exacerbates Nonalcoholic Fatty Liver Disease in Mice

Nonalcoholic fatty liver disease (NAFLD) affects over a third of the US population and 25% globally, with current treatments proving ineffective. This study investigates whether manipulating brown adipose tissue (BAT) and beige fat activity by housing C57BL/6J mice at thermoneutral (27 °C) or standard temperatures (22 °C) impacts NAFLD development. Male mice were fed either a chow diet (CHD) or a "fast food" diet (FFD) for 10 weeks. Mice at 27 °C had reduced food intake but increased body weight and plasma leptin levels. FFD-fed mice at 27 °C had greater liver weight (2.6 vs. 1.8 g), triglyceride content (7.6 vs. 3.9 mg/g), and hepatic steatosis compared to those at 22 °C. Gene expression of fatty acid synthase, sterol regulatory element-binding protein 1, and fatty acid translocase CD36 was elevated in FFD-fed mice at 27 °C, but not in CHD-fed mice. Thermoneutral housing also reduced expression of thermogenic markers in BAT and inguinal white adipose tissue (WAT) and caused BAT whitening. In conclusion, thermoneutrality inhibits thermogenic markers and exacerbates NAFLD. Activating BAT or promoting WAT browning via cold exposure or other stimuli may offer a strategy for managing NAFLD.

Mitochondrial lipidomes are tissue specific - low cholesterol contents relate to UCP1 activity

Lipid composition is conserved within sub-cellular compartments to maintain cell function. Lipidomic analyses of liver, muscle, white and brown adipose tissue (BAT) mitochondria revealed substantial differences in their glycerophospholipid (GPL) and free cholesterol (FC) contents. The GPL to FC ratio was 50-fold higher in brown than white adipose tissue mitochondria. Their purity was verified by comparison of proteomes with ER and mitochondria-associated membranes. A lipid signature containing PC and FC, calculated from the lipidomic profiles, allowed differentiation of mitochondria from BAT of mice housed at different temperatures. Elevating FC in BAT mitochondria prevented uncoupling protein (UCP) 1 function, whereas increasing GPL boosted it. Similarly, STARD3 overexpression facilitating mitochondrial FC import inhibited UCP1 function in primary brown adipocytes, whereas a knockdown promoted it. We conclude that the mitochondrial GPL/FC ratio is key for BAT function and propose that targeting it might be a promising strategy to promote UCP1 activity.

Single-nucleus transcriptomics identifies separate classes of UCP1 and futile cycle adipocytes

Adipose tissue can recruit catabolic adipocytes that utilize chemical energy to dissipate heat. This process occurs either by uncoupled respiration through uncoupling protein 1 (UCP1) or by utilizing ATP-dependent futile cycles (FCs). However, it remains unclear how these pathways coexist since both processes rely on the mitochondrial membrane potential. Utilizing single-nucleus RNA sequencing to deconvolute the heterogeneity of subcutaneous adipose tissue in mice and humans, we identify at least 2 distinct subpopulations of beige adipocytes: FC-adipocytes and UCP1-beige adipocytes. Importantly, we demonstrate that the FC-adipocyte subpopulation is highly metabolically active and utilizes FCs to dissipate energy, thus contributing to thermogenesis independent of Ucp1. Furthermore, FC-adipocytes are important drivers of systemic energy homeostasis and linked to glucose metabolism and obesity resistance in humans. Taken together, our findings identify a noncanonical thermogenic adipocyte subpopulation, which could be an important regulator of energy homeostasis in mammals.

Lipid-associated macrophages reshape BAT cell identity in obesity

Obesity and type 2 diabetes cause a loss in brown adipose tissue (BAT) activity, but the molecular mechanisms that drive BAT cell remodeling remain largely unexplored. Using a multilayered approach, we comprehensively mapped a reorganization in BAT cells. We uncovered a subset of macrophages as lipid-associated macrophages (LAMs), which were massively increased in genetic and dietary model of BAT expansion. LAMs participate in this scenario by capturing extracellular vesicles carrying damaged lipids and mitochondria released from metabolically stressed brown adipocytes. CD36 scavenger receptor drove LAM phenotype, and CD36-deficient LAMs were able to increase brown fat genes in adipocytes. LAMs released transforming growth factor β1 (TGF-β1), which promoted the loss of brown adipocyte identity through aldehyde dehydrogenase 1 family member A1 (Aldh1a1) induction. These findings unfold cell dynamic changes in BAT during obesity and identify LAMs as key responders to tissue metabolic stress and drivers of loss of brown adipocyte identity.

Weighing in on the role of brown adipose tissue for treatment of obesity

Brown adipose tissue (BAT) activation is an emerging target for obesity treatments due to its thermogenic properties stemming from its ability to shuttle energy through uncoupling protein 1 (Ucp1). Recent rodent studies show how BAT and white adipose tissue (WAT) activity can be modulated to increase the expression of thermogenic proteins. Consequently, these alterations enable organisms to endure cold-temperatures and elevate energy expenditure, thereby promoting weight loss. In humans, BAT is less abundant in obese subjects and impacts of thermogenesis are less pronounced, bringing into question whether energy expending properties of BAT seen in rodents can be translated to human models. Our review will discuss pharmacological, hormonal, bioactive, sex-specific and environmental activators and inhibitors of BAT to determine the potential for BAT to act as a therapeutic strategy. We aim to address the feasibility of utilizing BAT modulators for weight reduction in obese individuals, as recent studies suggest that BAT's contributions to energy expenditure along with Ucp1-dependent and -independent pathways may or may not rectify energy imbalance characteristic of obesity.

Thermogenic Fat as a New Obesity Management Tool: From Pharmaceutical Reagents to Cell Therapies

Obesity is a complex medical condition caused by a positive imbalance between calorie intake and calorie consumption. Brown adipose tissue (BAT), along with the newly discovered "brown-like" adipocytes (called beige cells), functions as a promising therapeutic tool to ameliorate obesity and metabolic disorders by burning out extra nutrients in the form of heat. Many studies in animal models and humans have proved the feasibility of this concept. In this review, we aim to summarize the endeavors over the last decade to achieve a higher number/activity of these heat-generating adipocytes. In particular, pharmacological compounds, especially agonists to the β3 adrenergic receptor (β3-AR), are reviewed in terms of their feasibility and efficacy in elevating BAT function and improving metabolic parameters in human subjects. Alternatively, allograft transplantation of BAT and the transplantation of functional brown or beige adipocytes from mesenchymal stromal cells or human induced pluripotent stem cells (hiPSCs) make it possible to increase the number of these beneficial adipocytes in patients. However, practical and ethical issues still need to be considered before the therapy can eventually be applied in the clinical setting. This review provides insights and guidance on brown- and beige-cell-based strategies for the management of obesity and its associated metabolic comorbidities.

UCP1 expression in human brown adipose tissue is inversely associated with cardiometabolic risk factors

OBJECTIVE

Brown adipose tissue (BAT) is a therapeutic target for obesity. 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) is commonly used to quantify human BAT mass and activity. Detectable 18F-FDG uptake by BAT is associated with reduced prevalence of cardiometabolic disease. However, 18F-FDG uptake may not always be a reliable marker of BAT thermogenesis, for example, insulin resistance may reduce glucose uptake. Uncoupling protein 1 (UCP1) is the key thermogenic protein in BAT. Therefore, we hypothesised that UCP1 expression may be altered in individuals with cardiometabolic risk factors.

METHODS

We quantified UCP1 expression as an alternative marker of thermogenic capacity in BAT and white adipose tissue (WAT) samples (n = 53) and in differentiated brown and white pre-adipocytes (n = 85).

RESULTS

UCP1 expression in BAT, but not in WAT or brown/white differentiated pre-adipocytes, was reduced with increasing age, obesity, and adverse cardiometabolic risk factors such as fasting glucose, insulin, and blood pressure. However, UCP1 expression in BAT was preserved in obese subjects of <40 years of age. To determine if BAT activity was also preserved in vivo, we undertook a case-control study, performing 18F-FDG scanning during mild cold exposure in young (mean age ∼22 years) normal weight and obese volunteers. 18F-FDG uptake by BAT and BAT volume were similar between groups, despite increased insulin resistance.

CONCLUSION

18F-FDG uptake by BAT and UCP1 expression are preserved in young obese adults. Older subjects retain precursor cells with the capacity to form new thermogenic adipocytes. These data highlight the therapeutic potential of BAT mass expansion and activation in obesity.

Intranasal insulin treatment ameliorates spatial memory, muscular strength, and frailty deficits in 5xFAD mice

The 5xFAD mouse model shows age-related weight loss as well as cognitive and motor deficits. Metabolic dysregulation, especially impaired insulin signaling, is also present in AD. This study examined whether intranasal delivery of insulin (INI) at low (0.875 U) or high (1.750 U) doses would ameliorate these deficits compared to saline in 10-month-old female 5xFAD and B6SJL wildtype (WT) mice. INI increased forelimb grip strength in the wire hang test in 5xFAD mice in a dose-dependent manner but did not improve the performance of 5xFAD mice on the balance beam. High INI doses reduced frailty scores in 5xFAD mice and improved spatial memory in both acquisition and reversal probe trials in the Morris water maze. INI increased swim speed in 5xFAD mice but had no effect on object recognition memory or working memory in the spontaneous alternation task, nor did it improve memory in the contextual or cued fear memory tasks. High doses of insulin increased the liver, spleen, and kidney weights and reduced brown adipose tissue weights. P-Akt signaling in the hippocampus was increased by insulin in a dose-dependent manner. Altogether, INI increased strength, reduced frailty scores, and improved visual spatial memory. Hypoglycemia was not present after INI, however alterations in tissue and organ weights were present. These results are novel and important as they indicate that intra-nasal insulin can reverse cognitive, motor and frailty deficits found in this mouse model of AD.

A novel fatty acid mimetic with pan-PPAR partial agonist activity inhibits diet-induced obesity and metabolic dysfunction-associated steatotic liver disease

OBJECTIVE

The prevalence of metabolic diseases is increasing globally at an alarming rate; thus, it is essential that effective, accessible, low-cost therapeutics are developed. Peroxisome proliferator-activated receptors (PPARs) are transcription factors that tightly regulate glucose homeostasis and lipid metabolism and are important drug targets for the treatment of type 2 diabetes and dyslipidemia. We previously identified LDT409, a fatty acid-like compound derived from cashew nut shell liquid, as a novel pan-active PPARα/γ/δ compound. Herein, we aimed to assess the efficacy of LDT409 in vivo and investigate the molecular mechanisms governing the actions of the fatty acid mimetic LDT409 in diet-induced obese mice.

METHODS

C57Bl/6 mice (6-11-month-old) were fed a chow or high fat diet (HFD) for 4 weeks; mice thereafter received once daily intraperitoneal injections of vehicle, 10 mg/kg Rosiglitazone, 40 mg/kg WY14643, or 40 mg/kg LDT409 for 18 days while continuing the HFD. During treatments, body weight, food intake, glucose and insulin tolerance, energy expenditure, and intestinal lipid absorption were measured. On day 18 of treatment, tissues and plasma were collected for histological, molecular, and biochemical analysis.

RESULTS

We found that treatment with LDT409 was effective at reversing HFD-induced obesity and associated metabolic abnormalities in mice. LDT409 lowered food intake and hyperlipidemia, while improving insulin tolerance. Despite being a substrate of both PPARα and PPARγ, LDT409 was crucial for promoting hepatic fatty acid oxidation and reducing hepatic steatosis in HFD-fed mice. We also highlighted a role for LDT409 in white and brown adipocytes in vitro and in vivo where it decreased fat accumulation, increased lipolysis, induced browning of WAT, and upregulated thermogenic gene Ucp1. Remarkably, LDT409 reversed HFD-induced weight gain back to chow-fed control levels. We determined that the LDT409-induced weight-loss was associated with a combination of increased energy expenditure (detectable before weight loss was apparent), decreased food intake, increased systemic fat utilization, and increased fecal lipid excretion in HFD-fed mice.

CONCLUSIONS

Collectively, LDT409 represents a fatty acid mimetic that generates a uniquely favorable metabolic response for the treatment of multiple abnormalities including obesity, dyslipidemia, metabolic dysfunction-associated steatotic liver disease, and diabetes. LDT409 is derived from a highly abundant natural product-based starting material and its development could be pursued as a therapeutic solution to the global metabolic health crisis.

Global burden of type 1 diabetes in adults aged 65 years and older, 1990-2019: population based study

OBJECTIVES

To estimate the burden, trends, and inequalities of type 1 diabetes mellitus (T1DM) among older adults at global, regional, and national level from 1990 to 2019.

DESIGN

Population based study.

POPULATION

Adults aged ≥65 years from 21 regions and 204 countries and territories (Global Burden of Disease and Risk Factors Study 2019)from 1990 to 2019.

MAIN OUTCOME MEASURES

Primary outcomes were T1DM related age standardised prevalence, mortality, disability adjusted life years (DALYs), and average annual percentage change.

RESULTS

The global age standardised prevalence of T1DM among adults aged ≥65 years increased from 400 (95% uncertainty interval (UI) 332 to 476) per 100 000 population in 1990 to 514 (417 to 624) per 100 000 population in 2019, with an average annual trend of 0.86% (95% confidence interval (CI) 0.79% to 0.93%); while mortality decreased from 4.74 (95% UI 3.44 to 5.9) per 100 000 population to 3.54 (2.91 to 4.59) per 100 000 population, with an average annual trend of -1.00% (95% CI -1.09% to -0.91%), and age standardised DALYs decreased from 113 (95% UI 89 to 137) per 100 000 population to 103 (85 to 127) per 100 000 population, with an average annual trend of -0.33% (95% CI -0.41% to -0.25%). The most significant decrease in DALYs was observed among those aged <79 years: 65-69 (-0.44% per year (95% CI -0.53% to -0.34%)), 70-74 (-0.34% per year (-0.41% to -0.27%)), and 75-79 years (-0.42% per year (-0.58% to -0.26%)). Mortality fell 13 times faster in countries with a high sociodemographic index versus countries with a low-middle sociodemographic index (-2.17% per year (95% CI -2.31% to -2.02%) v -0.16% per year (-0.45% to 0.12%)). While the highest prevalence remained in high income North America, Australasia, and western Europe, the highest DALY rates were found in southern sub-Saharan Africa, Oceania, and the Caribbean. A high fasting plasma glucose level remained the highest risk factor for DALYs among older adults during 1990-2019.

CONCLUSIONS

The life expectancy of older people with T1DM has increased since the 1990s along with a considerable decrease in associated mortality and DALYs. T1DM related mortality and DALYs were lower in women aged ≥65 years, those living in regions with a high sociodemographic index, and those aged <79 years. Management of high fasting plasma glucose remains a major challenge for older people with T1DM, and targeted clinical guidelines are needed.

Futile cycles: Emerging utility from apparent futility

Futile cycles are biological phenomena where two opposing biochemical reactions run simultaneously, resulting in a net energy loss without appreciable productivity. Such a state was presumed to be a biological aberration and thus deemed an energy-wasting "futile" cycle. However, multiple pieces of evidence suggest that biological utilities emerge from futile cycles. A few established functions of futile cycles are to control metabolic sensitivity, modulate energy homeostasis, and drive adaptive thermogenesis. Yet, the physiological regulation, implication, and pathological relevance of most futile cycles remain poorly studied. In this review, we highlight the abundance and versatility of futile cycles and propose a classification scheme. We further discuss the energetic implications of various futile cycles and their impact on basal metabolic rate, their bona fide and tentative pathophysiological implications, and putative drug interactions.

Structural mechanisms of mitochondrial uncoupling protein 1 regulation in thermogenesis

In mitochondria, the oxidation of nutrients is coupled to ATP synthesis by the generation of a protonmotive force across the mitochondrial inner membrane. In mammalian brown adipose tissue (BAT), uncoupling protein 1 (UCP1, SLC25A7), a member of the SLC25 mitochondrial carrier family, dissipates the protonmotive force by facilitating the return of protons to the mitochondrial matrix. This process short-circuits the mitochondrion, generating heat for non-shivering thermogenesis. Recent cryo-electron microscopy (cryo-EM) structures of human UCP1 have provided new molecular insights into the inhibition and activation of thermogenesis. Here, we discuss these structures, describing how purine nucleotides lock UCP1 in a proton-impermeable conformation and rationalizing potential conformational changes of this carrier in response to fatty acid activators that enable proton leak for thermogenesis.

Estrogen counteracts age-related decline in beige adipogenesis through the NAMPT-regulated ER stress response

Thermogenic beige adipocytes are recognized as potential therapeutic targets for combating metabolic diseases. However, the metabolic advantages that they offer are compromised with aging. Here we show that treating mice with estrogen (E2), a hormone that decreases with age, can counteract the age-related decline in beige adipogenesis when exposed to cold temperature while concurrently enhancing energy expenditure and improving glucose tolerance in mice. Mechanistically, we found that nicotinamide phosphoribosyl transferase (NAMPT) plays a pivotal role in facilitating the formation of E2-induced beige adipocytes, which subsequently suppresses the onset of age-related endoplasmic reticulum (ER) stress. Furthermore, we found that targeting NAMPT signaling, either genetically or pharmacologically, can restore the formation of beige adipocytes by increasing the number of perivascular adipocyte progenitor cells. Conversely, the absence of NAMPT signaling prevents this process. Together, our findings shed light on the mechanisms regulating the age-dependent impairment of beige adipocyte formation and underscore the E2-NAMPT-controlled ER stress pathway as a key regulator of this process.

The Notch-PDGFRβ axis suppresses brown adipocyte progenitor differentiation in early post-natal mice

De novo brown adipogenesis holds potential in combating the epidemics of obesity and diabetes. However, the identity of brown adipocyte progenitor cells (APCs) and their regulation have not been extensively explored. Here, through in vivo lineage tracing and mouse modeling, we observed that platelet-derived growth factor receptor beta (PDGFRβ)+ pericytes give rise to developmental brown adipocytes but not to those in adult homeostasis. By contrast, T-box 18 (TBX18)+ pericytes contribute to brown adipogenesis throughout both developmental and adult stages, though in a depot-specific manner. Mechanistically, Notch inhibition in PDGFRβ+ pericytes promotes brown adipogenesis by downregulating PDGFRβ. Furthermore, inhibition of Notch signaling in PDGFRβ+ pericytes mitigates high-fat, high-sucrose (HFHS)-induced glucose and metabolic impairment in mice during their development and juvenile phases. Collectively, these findings show that the Notch/PDGFRβ axis negatively regulates developmental brown adipogenesis, and its repression promotes brown adipose tissue expansion and improves metabolic health.

The dairy-derived peptide Miltin exerts anti-obesity effects by increasing adipocyte thermogenesis

Growing research has highlighted that the consumption of dairy products improves the metabolic health in obese individuals by functioning as regulatory modulators. However, the molecular basis of this effect remains largely unknown. Herein, we report a dairy-derived peptide, which we named Miltin, that activates the thermogenesis of brown adipocytes and increases white adipocyte browning. Previously, Miltin was merely identified for its antioxidant capacity, although it is commonly present in different dairy products. In this study, we revealed the effect of Miltin in modulating adipose thermogenesis and further explored its potential in treating obesity through in vivo and in vitro strategies. The administration of Miltin in mice fed with a high-fat diet resulted in enhanced thermogenesis, improved glucose homeostasis, and reduced body mass and lipid accumulation, indicating the anti-obesity effect of Miltin. Genomic analysis revealed that Miltin modulates thermogenesis by inducing the activation of the MAPK signaling pathway by preferentially interacting with GADD45γ to promote its stability. Together, our findings indicate that Miltin's role in initiating the thermogenesis of adipocytes makes it a potential anti-obesity therapy for future development.

Liver ACOX1 regulates levels of circulating lipids that promote metabolic health through adipose remodeling

The liver gene expression of the peroxisomal β-oxidation enzyme acyl-coenzyme A oxidase 1 (ACOX1), which catabolizes very long chain fatty acids (VLCFA), increases in the context of obesity, but how this pathway impacts systemic energy metabolism remains unknown. Here, we show that hepatic ACOX1-mediated β-oxidation regulates inter-organ communication involved in metabolic homeostasis. Liver-specific knockout of Acox1 (Acox1-LKO) protects mice from diet-induced obesity, adipose tissue inflammation, and systemic insulin resistance. Serum from Acox1-LKO mice promotes browning in cultured white adipocytes. Global serum lipidomics show increased circulating levels of several species of ω-3 VLCFAs (C24-C28) with previously uncharacterized physiological role that promote browning, mitochondrial biogenesis and Glut4 translocation through activation of the lipid sensor GPR120 in adipocytes. This work identifies hepatic peroxisomal β-oxidation as an important regulator of metabolic homeostasis and suggests that manipulation of ACOX1 or its substrates may treat obesity-associated metabolic disorders.

Developmentally programmed obesity: Is there a role for anti-inflammatory nutritional strategies?

Pregnancy represents a period of immense maternal physiological adaptation, with progressive increases in lipid storage potential and insulin resistance to support fetal/placental growth. This requires significant change in the adipose tissue. Women living with obesity/overweight are more susceptible to these changes causing complications such as gestational diabetes. This is particularly worrying as up to 60% of European women are living with overweight/obesity at the onset of pregnancy. Furthermore, less than 1% meet all nutrition guidelines. There is now evidence that these deep metabolic changes can result in a predisposition to metabolic disease in both the mother and child in later life. Health and nutrition status during this period therefore represents a window to future health. This period offers a valuable opportunity for intervention to prevent the negative consequences of poor in utero environments and increases the long-term quality of life for mother and offspring. This review will examine a range of in utero factors which determine adipose tissue development, the impact of these factors on later-life obesity and metabolic health and the therapeutic value of dietary anti-inflammatory nutritional interventions during pregnancy and early life. When it comes to early life nutrition, a 'one size fits all' approach is not always appropriate. Understanding the mechanisms of adipose tissue development in response to differing nutritional strategies may be important in the context of complicated or adverse in utero environments and represents a substantial step towards a more personalised nutritional approach for the prevention of obesity, metabolic syndrome and related non-communicable diseases in future generations.

Histone proteoform analysis reveals epigenetic changes in adult mouse brown adipose tissue in response to cold stress

BACKGROUND

Regulation of the thermogenic response by brown adipose tissue (BAT) is an important component of energy homeostasis with implications for the treatment of obesity and diabetes. Our preliminary analyses of RNA-Seq data uncovered many nodes representing epigenetic modifiers that are altered in BAT in response to chronic thermogenic activation. Thus, we hypothesized that chronic thermogenic activation broadly alters epigenetic modifications of DNA and histones in BAT.

RESULTS

Motivated to understand how BAT function is regulated epigenetically, we developed a novel method for the first-ever unbiased top-down proteomic quantitation of histone modifications in BAT and validated our results with a multi-omic approach. To test our hypothesis, wildtype male C57BL/6J mice were housed under chronic conditions of thermoneutral temperature (TN, 28°C), mild cold/room temperature (RT, 22°C), or severe cold (SC, 8°C) and BAT was analyzed for DNA methylation and histone modifications. Methylation of promoters and intragenic regions in genomic DNA decrease in response to chronic cold exposure. Integration of DNA methylation and RNA expression datasets suggest a role for epigenetic modification of DNA in regulation of gene expression in response to cold. In response to cold housing, we observe increased bulk acetylation of histones H3.2 and H4, increased histone H3.2 proteoforms with di- and trimethylation of lysine 9 (K9me2 and K9me3), and increased histone H4 proteoforms with acetylation of lysine 16 (K16ac) in BAT.

CONCLUSIONS

Our results reveal global epigenetically-regulated transcriptional "on" and "off" signals in murine BAT in response to varying degrees of chronic cold stimuli and establish a novel methodology to quantitatively study histones in BAT, allowing for direct comparisons to decipher mechanistic changes during the thermogenic response. Additionally, we make histone PTM and proteoform quantitation, RNA splicing, RRBS, and transcriptional footprint datasets available as a resource for future research.

Sex-specific regulation of miR-22 and ERα in white adipose tissue of obese dam's female offspring impairs the early postnatal development of functional beige adipocytes in mice

During inguinal adipose tissue (iWAT) ontogenesis, beige adipocytes spontaneously appear between postnatal 10 (P10) and P20 and their ablation impairs iWAT browning capacity in adulthood. Since maternal obesity has deleterious effects on offspring iWAT function, we aimed to investigate its effect in spontaneous iWAT browning in offspring. Female C57BL/6 J mice were fed a control or obesogenic diet six weeks before mating. Male and female offspring were euthanized at P10 and P20 or weaned at P21 and fed chow diet until P60. At P50, mice were treated with saline or CL316,243, a β3-adrenoceptor agonist, for ten days. Maternal obesity induced insulin resistance at P60, and CL316,243 treatment effectively restored insulin sensitivity in male but not female offspring. This discrepancy occurred due to female offspring severe browning impairment. During development, the spontaneous iWAT browning and sympathetic nerve branching at P20 were severely impaired in female obese dam's offspring but occurred normally in males. Additionally, maternal obesity increased miR-22 expression in the iWAT of male and female offspring during development. ERα, a target and regulator of miR-22, was concomitantly upregulated in the male's iWAT. Next, we evaluated miR-22 knockout (KO) offspring at P10 and P20. The miR-22 deficiency does not affect spontaneous iWAT browning in females and, surprisingly, anticipates iWAT browning in males. In conclusion, maternal obesity impairs functional iWAT development in the offspring in a sex-specific way that seems to be driven by miR-22 levels and ERα signaling. This impacts adult browning capacity and glucose homeostasis, especially in female offspring.

The pathophysiology of visceral adipose tissues in cardiometabolic diseases

Central pattern of fat distribution, especially fat accumulation within the intraabdominal cavity increases risks for cardiometabolic diseases. Portal hypothesis combined with a pathological remodeling in visceral fat is considered the major etiological factor explaining the independent contribution of visceral obesity to cardiometabolic diseases. Excessive remodeling in visceral fat during development of obesity leads to dysfunctions in the depot, characterized by hypertrophy and death of adipocytes, hypoxia, inflammation, and fibrosis. Dysfunctional visceral fat secretes elevated levels of fatty acids, glycerol, and proinflammatory and profibrotic cytokines into the portal vein directly impacting the liver, the central regulator of systemic metabolism. These metabolic and endocrine products induce ectopic fat accumulation, insulin resistance, inflammation, and fibrosis in the liver, which in turn causes or exacerbates systemic metabolic derangements. Elucidation of underlying mechanisms that lead to the pathological remodeling and higher degree of dysfunctions in visceral adipose tissue is therefore, critical for the development of therapeutics to prevent deleterious sequelae in obesity. We review depot differences in metabolic and endocrine properties and expendabilities as well as underlying mechanisms that contribute to the pathophysiological aspects of visceral adiposity in cardiometabolic diseases. We also discuss impacts of different weight loss interventions on visceral adiposity and cardiometabolic diseases.

Ianus Bifrons: The Two Faces of Metformin

The ancient Roman god Ianus was a mysterious divinity with two opposite faces, one looking at the past and the other looking to the future. Likewise, metformin is an "old" drug, with one side looking at the metabolic role and the other looking at the anti-proliferative mechanism; therefore, it represents a typical and ideal bridge between diabetes and cancer. Metformin (1,1-dimethylbiguanidine hydrochloride) is a drug that has long been in use for the treatment of type 2 diabetes mellitus, but recently evidence is growing about its potential use in other metabolic conditions and in proliferative-associated diseases. The aim of this paper is to retrace, from a historical perspective, the knowledge of this molecule, shedding light on the subcellular mechanisms of action involved in metabolism as well as cellular and tissue growth. The intra-tumoral pharmacodynamic effects of metformin and its possible role in the management of different neoplasms are evaluated and debated. The etymology of the name Ianus is probably from the Latin term ianua, which means door. How many new doors will this old drug be able to open?

Indication of mixed glucose and fatty acid use by inferred brown adipose tissue activity in Samoans

OBJECTIVES

Despite the growing rates of global obesity and the known positive associations between brown adipose tissue (BAT) and cardiovascular health, little is known about the metabolic effects of BAT activity in Samoans, a population at high risk of obesity and type II diabetes. Here we assessed the potential effects of inferred BAT activity on metabolic health markers in Samoan adults exposed to mild cold.

METHODS

Using point-of-care finger prick technology we measured fasting glucose, total cholesterol, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) levels before and after 30 min of cold exposure among 61 individuals (38 females, 23 males, ages 31-54) from 'Upolu Island, Samoa. Respiratory quotient was measured by indirect calorimetry to determine substrate metabolism at room temperature and cold exposure.

RESULTS

Fasting glucose levels decreased significantly (p < .001) after cold exposure while neither total cholesterol (p = .88), HDL (p = .312), nor LDL (p = .089) changed. Respiratory quotient decreased significantly (p = .009) between exposures, suggesting an increased preference for lipid metabolism as a response to cold.

CONCLUSIONS

The observed effects of inferred BAT activity on biomarkers suggest BAT activity utilizes both glucose and lipid-derived fatty acids as fuel for thermogenesis. Our work provides evidence for the beneficial metabolic effects of BAT and emphasizes the need for the population-specific development of metabolic treatments involving BAT to ensure the successful and equitable minimization of extreme consequences of obesity and metabolic health.

Spermidine activates adipose tissue thermogenesis through autophagy and fibroblast growth factor 21

Spermidine exerts protective roles in obesity, while the mechanism of spermidine in adipose tissue thermogenesis remains unclear. The present study first investigated the effect of spermidine on cold-stimulation and β3-adrenoceptor agonist-induced thermogenesis in lean and high-fat diet-induced obese mice. Next, the role of spermidine on glucose and lipid metabolism in different types of adipose tissue was determined. Here, we found that spermidine supplementation did not affect cold-stimulated thermogenesis in lean mice, while significantly promoting the activation of adipose tissue thermogenesis under cold stimulation and β3-adrenergic receptor agonist treatment in obese mice. Spermidine treatment markedly enhanced glucose and lipid metabolism in adipose tissues, and these results were associated with the activated autophagy pathway. Moreover, spermidine up-regulated fibroblast growth factor 21 (FGF21) signaling and its downstream pathway, including PI3K/AKT and AMPK pathways in vivo and in vitro. Knockdown of Fgf21 or inhibition of PI3K/AKT and AMPK pathways in brown adipocytes abolished the thermogenesis-promoting effect of spermidine, suggesting that the effect of spermidine on adipose tissue thermogenesis might be regulated by FGF21 signaling via the PI3K/AKT and AMPK pathways. The present study provides new insight into the mechanism of spermidine on obesity and its metabolic complications, thereby laying a theoretical basis for the clinical application of spermidine.

Multimodality Imaging in Metabolic Syndrome: State-of-the-Art Review

Metabolic syndrome comprises a set of risk factors that include abdominal obesity, impaired glucose tolerance, hypertriglyceridemia, low high-density lipoprotein levels, and high blood pressure, at least three of which must be fulfilled for diagnosis. Metabolic syndrome has been linked to an increased risk of cardiovascular disease and type 2 diabetes mellitus. Multimodality imaging plays an important role in metabolic syndrome, including diagnosis, risk stratification, and assessment of complications. CT and MRI are the primary tools for quantification of excess fat, including subcutaneous and visceral adipose tissue, as well as fat around organs, which are associated with increased cardiovascular risk. PET has been shown to detect signs of insulin resistance and may detect ectopic sites of brown fat. Cardiovascular disease is an important complication of metabolic syndrome, resulting in subclinical or symptomatic coronary artery disease, alterations in cardiac structure and function with potential progression to heart failure, and systemic vascular disease. CT angiography provides comprehensive evaluation of the coronary and systemic arteries, while cardiac MRI assesses cardiac structure, function, myocardial ischemia, and infarction. Liver damage results from a spectrum of nonalcoholic fatty liver disease ranging from steatosis to fibrosis and possible cirrhosis. US, CT, and MRI are useful in assessing steatosis and can be performed to detect and grade hepatic fibrosis, particularly using elastography techniques. Metabolic syndrome also has deleterious effects on the pancreas, kidney, gastrointestinal tract, and ovaries, including increased risk for several malignancies. Metabolic syndrome is associated with cerebral infarcts, best evaluated with MRI, and has been linked with cognitive decline. ©RSNA, 2024 Test Your Knowledge questions for this article are available in the supplemental material. See the invited commentary by Pickhardt in this issue.

Dexamethasone Inhibits White Adipose Tissue Browning

White adipose tissue (WAT) regulates energy balance through energy storage, adipokines secretion and the thermogenesis process. Beige adipocytes are responsible for WAT thermogenesis. They are generated by adipogenesis or transdifferentiation during cold or β3-adrenergic agonist stimulus through a process called browning. Browning has gained significant interest for to its preventive effect on obesity. Glucocorticoids (GCs) have several functions in WAT biology; however, their role in beige adipocyte generation and WAT browning is not fully understood. The aim of our study was to determine the effect of dexamethasone (DXM) on WAT thermogenesis. For this purpose, rats were treated with DXM at room temperature (RT) or cold conditions to determine different thermogenic markers. Furthermore, the effects of DXM on the adipogenic potential of beige precursors and on mature beige adipocytes were evaluated in vitro. Our results showed that DXM decreased UCP-1 mRNA and protein levels, mainly after cold exposure. In vitro studies showed that DXM decreased the expression of a beige precursor marker (Ebf2), affecting their ability to differentiate into beige adipocytes, and inhibited the thermogenic response of mature beige adipocytes (Ucp-1, Dio2 and Pgc1α gene expressions and mitochondrial respiration). Overall, our data strongly suggest that DXM can inhibit the thermogenic program of both retroperitoneal and inguinal WAT depots, an effect that could be exerted, at least partially, by inhibiting de novo cell generation and the thermogenic response in beige adipocytes.

Whitening of brown adipose tissue inhibits osteogenic differentiation via secretion of S100A8/A9

The mechanism by which brown adipose tissue (BAT) regulates bone metabolism is unclear. Here, we reveal that BAT secretes S100A8/A9, a previously unidentified BAT adipokine (batokine), to impair bone formation. Brown adipocytes-specific knockout of Rheb (RhebBAD KO), the upstream activator of mTOR, causes BAT malfunction to inhibit osteogenesis. Rheb depletion induces NF-κB dependent S100A8/A9 secretion from brown adipocytes, but not from macrophages. In wild-type mice, age-related Rheb downregulation in BAT is associated with enhanced S100A8/A9 secretion. Either batokines from RhebBAD KO mice, or recombinant S100A8/A9, inhibits osteoblast differentiation of mesenchymal stem cells in vitro by targeting toll-like receptor 4 on their surfaces. Conversely, S100A8/A9 neutralization not only rescues the osteogenesis repressed in the RhebBAD KO mice, but also alleviates age-related osteoporosis in wild-type mice. Collectively, our data revealed an unexpected BAT-bone crosstalk driven by Rheb-S100A8/A9, uncovering S100A8/A9 as a promising target for the treatment, and potentially, prevention of osteoporosis.

Aging adipose tissue, insulin resistance, and type 2 diabetes

With the increase of population aging, the prevalence of type 2 diabetes (T2D) is also rising. Aging affects the tissues and organs of the whole body, which is the result of various physiological and pathological processes. Adipose tissue has a high degree of plasticity and changes with aging. Aging changes the distribution of adipose tissue, affects adipogenesis, browning characteristics, inflammatory status and adipokine secretion, and increases lipotoxicity. These age-dependent changes in adipose tissue are an important cause of insulin resistance and T2D. Understanding adipose tissue changes can help promote healthy aging process. This review summarizes changes in adipose tissue ascribable to aging, with a focus on the role of aging adipose tissue in insulin resistance and T2D.

When the cold gets under your skin: Evidence for brown adipose tissue activity in Samoan adults

OBJECTIVES

Brown adipose tissue (BAT) is a heat-producing organ aiding nonshivering thermogenesis (NST) during cold stress. Due to its potential cold-adaptive role BAT has been predominantly studied in cold and temperate climate populations, but not among warm-climate adults. This work explores if BAT activity can be inferred in Samoans.

MATERIALS AND METHODS

We inferred BAT activity by comparing metabolic rate and surface heat dissipation using indirect calorimetry and thermal imaging between room temperature and cold exposure among Samoans (N = 61, females: n = 38) from 'Upolu Island, Samoa. BAT activity was inferred using ANOVA linear regression models with the variables measured at cold exposure as outcomes. T-tests were used to compare changes in surface temperature between room temperature and cold exposure.

RESULTS

Metabolic rate significantly increased after cooling. In both the supraclavicular area, a known BAT location, and the sternum, a non-BAT location, temperatures decreased significantly upon cold exposure. Differences in supraclavicular temperatures between room temperature and cold were significantly smaller than differences in sternum temperatures between exposures. These results suggest that BAT thermogenesis occurred in known BAT-locations and thus contributed to NST during cooling.

CONCLUSIONS

This study adds to our understanding of BAT activity across different populations and climates. Further study may illuminate whether the cold-adaptive properties of BAT may have played a role in the successful expansion of populations across the globe, including warm-climate groups.

Pinuseldarone, a Clerodane-Type Diterpene from Pinus eldarica Needles and Phytochemicals as Novel Agents for Regulating Brown Adipogenesis and Thermogenesis

Phytochemical investigation of the MeOH extract of Pinus eldarica needles led to the isolation and identification of a new clerodane-type diterpene, pinuseldarone (1), along with a known flavonoid, 5,4'-dihydroxy-3,7,8-trimethoxy-6-C-methylflavone (2), through HPLC purification. The structure of the new compound 1 was elucidated using spectroscopic methods, including 1D and 2D NMR, as well as HRESIMS. Its absolute configuration was established through NOESY analysis and computational methods, including electronic circular dichroism (ECD) calculations and gauge-including atomic orbital NMR chemical shift calculations, followed by DP4+ probability analysis. The metabolic implications of the isolated compounds were assessed using a cultured brown adipocyte model derived from murine brown adipose tissue. It was observed that treatment with dihydroxy-3,7,8-trimethoxy-6-C-methylflavone (2) downregulates the adipogenic marker C/EBPδ and fatty acid transporter CD36, resulting in a significant reduction in lipid accumulation during brown adipocyte differentiation. However, pinuseldarone (1) treatment did not affect brown adipocyte differentiation. Interestingly, pretreatment with pinuseldarone (1) potentiated the pharmacological stimulation of brown adipocytes, seemingly achieved by sensitizing their response to β3-adrenoreceptor signaling. Therefore, our findings indicate that phytochemicals derived from P. eldarica needles could potentially serve as valuable compounds for adjusting the metabolic activity of brown adipose tissue, a vital component in maintaining whole-body metabolic homeostasis.

Histone proteoform analysis reveals epigenetic changes in adult mouse brown adipose tissue in response to cold stress

Regulation of the thermogenic response by brown adipose tissue (BAT) is an important component of energy homeostasis with implications for the treatment of obesity and diabetes. Our preliminary analyses uncovered many nodes representing epigenetic modifiers that are altered in BAT in response to chronic thermogenic activation. Thus, we hypothesized that chronic thermogenic activation broadly alters epigenetic modifications of DNA and histones in BAT. Motivated to understand how BAT function is regulated epigenetically, we developed a novel method for the first-ever unbiased top-down proteomic quantitation of histone modifications in BAT and validated our results with a multi-omic approach. To test our hypothesis, wildtype male C57BL/6J mice were housed under chronic conditions of thermoneutral temperature (TN, 28.8°C), mild cold/room temperature (RT, 22°C), or severe cold (SC, 8°C) and BAT was analyzed for DNA methylation and histone modifications. Methylation of promoters and intragenic regions in genomic DNA decrease in response to chronic cold exposure. Integration of DNA methylation and RNA expression data suggest a role for epigenetic modification of DNA in gene regulation in response to cold. In response to cold housing, we observe increased bulk acetylation of histones H3.2 and H4, increased histone H3.2 proteoforms with di- and trimethylation of lysine 9 (K9me2 and K9me3), and increased histone H4 proteoforms with acetylation of lysine 16 (K16ac) in BAT. Taken together, our results reveal global epigenetically-regulated transcriptional "on" and "off" signals in murine BAT in response to varying degrees of chronic cold stimuli and establish a novel methodology to quantitatively study histones in BAT, allowing for direct comparisons to decipher mechanistic changes during the thermogenic response. Additionally, we make histone PTM and proteoform quantitation, RNA splicing, RRBS, and transcriptional footprint datasets available as a resource for future research.

Very-low-density lipoprotein triglyceride and free fatty acid plasma kinetics in women with high or low brown adipose tissue volume and overweight/obesity

Although a high amount of brown adipose tissue (BAT) is associated with low plasma triglyceride concentration, the mechanism responsible for this relationship in people is not clear. Here, we evaluate the interrelationships among BAT, very-low-density lipoprotein triglyceride (VLDL-TG), and free fatty acid (FFA) plasma kinetics during thermoneutrality in women with overweight/obesity who had a low (<20 mL) or high (≥20 mL) volume of cold-activated BAT (assessed by using positron emission tomography in conjunction with 2-deoxy-2-[18F]-fluoro-glucose). We find that plasma TG and FFA concentrations are lower and VLDL-TG and FFA plasma clearance rates are faster in women with high BAT than low BAT volume, whereas VLDL-TG and FFA appearance rates in plasma are not different between the two groups. These findings demonstrate that women with high BAT volume have lower plasma TG and FFA concentrations than women with low BAT volumes because of increased VLDL-TG and FFA clearance rates. This study was registered at ClinicalTrials.gov (NCT02786251).

Electroacupuncture stimulation improves cognitive ability and regulates metabolic disorders in Alzheimer's disease model mice: new insights from brown adipose tissue thermogenesis

Background

Metabolic defects play a crucial role in Alzheimer's disease (AD) development. Brown adipose tissue (BAT) has been identified as a novel potential therapeutic target for AD due to its unique role in energy metabolism. Electroacupuncture (EA) shows promise in improving cognitive ability and brain glucose metabolism in AD, but its effects on peripheral and central metabolism are unclear.

Methods

In this study, SAMP8 mice (AD model) received EA stimulation at specific acupoints. Cognitive abilities were evaluated using the Morris water maze test, while neuronal morphology and tau pathology were assessed through Nissl staining and immunofluorescence staining, respectively. Metabolic variations and BAT thermogenesis were measured using ELISA, HE staining, Western blotting, and infrared thermal imaging.

Results

Compared to SAMR1 mice, SAMP8 mice showed impaired cognitive ability, neuronal damage, disrupted thermoregulation, and metabolic disorders with low BAT activity. Both the EA and DD groups improved cognitive ability and decreased tau phosphorylation (p<0.01 or p<0.05). However, only the EA group had a significant effect on metabolic disorders and BAT thermogenesis (p<0.01 or p<0.05), while the DD group did not.

Conclusion

These findings indicate that EA not only improves the cognitive ability of SAMP8 mice, but also effectively regulates peripheral and central metabolic disorders, with this effect being significantly related to the activation of BAT thermogenesis.

CD73-dependent generation of extracellular adenosine by vascular endothelial cells modulates de novo lipogenesis in adipose tissue

Next to white and brown adipocytes present in white and brown adipose tissue (WAT, BAT), vascular endothelial cells, tissue-resident macrophages and other immune cells have important roles in maintaining adipose tissue homeostasis but also contribute to the etiology of obesity-associated chronic inflammatory metabolic diseases. In addition to hormonal signals such as insulin and norepinephrine, extracellular adenine nucleotides modulate lipid storage, fatty acid release and thermogenic responses in adipose tissues. The complex regulation of extracellular adenine nucleotides involves a network of ectoenzymes that convert ATP via ADP and AMP to adenosine. However, in WAT and BAT the processing of extracellular adenine nucleotides and its relevance for intercellular communications are still largely unknown. Based on our observations that in adipose tissues the adenosine-generating enzyme CD73 is mainly expressed by vascular endothelial cells, we studied glucose and lipid handling, energy expenditure and adaptive thermogenesis in mice lacking endothelial CD73 housed at different ambient temperatures. Under conditions of thermogenic activation, CD73 expressed by endothelial cells is dispensable for the expression of thermogenic genes as well as energy expenditure. Notably, thermoneutral housing leading to a state of low energy expenditure and lipid accumulation in adipose tissues resulted in enhanced glucose uptake into WAT of endothelial CD73-deficient mice. This effect was associated with elevated expression levels of de novo lipogenesis genes. Mechanistic studies provide evidence that extracellular adenosine is imported into adipocytes and converted to AMP by adenosine kinase. Subsequently, activation of the AMP kinase lowers the expression of de novo lipogenesis genes, most likely via inactivation of the transcription factor carbohydrate response element binding protein (ChREBP). In conclusion, this study demonstrates that endothelial-derived extracellular adenosine generated via the ectoenzyme CD73 is a paracrine factor shaping lipid metabolism in WAT.

Fructose-induced FGF21 secretion does not activate brown adipose tissue in Japanese young men: randomized cross-over and randomized controlled trials

BACKGROUND

Human brown adipose tissue (BAT) activity is associated with lower body fatness and favorable glucose metabolism. Previous studies reported that oral fructose loading induces postprandial fibroblast growth factor 21 (FGF21) secretion. FGF21 is a known inducer of adipose tissue thermogenesis; however, the effects of diet-induced FGF21 secretion on BAT thermogenesis remain to be elucidated.

METHODS

The effects of both single load and daily consumption of fructose on BAT activity were examined using a randomized cross-over trial and a 2-week randomized controlled trial (RCT), respectively. In the cross-over trial, 15 young men consumed a single dose of fructose solution or water and then consumed the other on a subsequent day. The RCT enrolled 22 young men, and the participants were allocated to a group that consumed fructose and a group that consumed water daily for 2 weeks. BAT activity was analyzed using thermography with cold exposure. Plasma FGF21 level was determined by enzyme-linked immunosorbent assay.

RESULTS

In the cross-over single-load trial, plasma FGF21 levels were significantly increased at 2 h after oral fructose load (p < 0.01); however, there was no significant difference in BAT activity between the fructose load and drinking water. The 2-week RCT revealed that both plasma FGF21 levels and BAT activity were not significantly increased by daily fructose consumption compared to water. Correlation analyses revealed that BAT activity at the baseline and the final measurements were strongly and positively associated with the RCT (r = 0.869, p < 0.001). Changes in BAT activity were significantly and negatively correlated with changes in plasma glucose levels during the 2-week intervention (r = - 0.497, p = 0.022).

CONCLUSIONS

Oral fructose load induces a temporary increase in circulating FGF21 levels; however, this does not activate BAT thermogenesis in healthy young men. Further studies are needed to elucidate the effect of endogenous FGF21 on physiological function.

TRIAL REGISTRATION

This study is registered with the University Hospital Medical Information Network in Japan (number 000051761, registered 1 August 2023, retrospectively registered, https://center6.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000052680 ).

Emerging role of N6-methyladenosine in the homeostasis of glucose metabolism

N6-methyladenosine (m6A) is the most prevalent post-transcriptional internal RNA modification, which is involved in the regulation of diverse physiological processes. Dynamic and reversible m6A modification has been shown to regulate glucose metabolism, and dysregulation of m6A modification contributes to glucose metabolic disorders in multiple organs and tissues including the pancreas, liver, adipose tissue, skeletal muscle, kidney, blood vessels, and so forth. In this review, the role and molecular mechanism of m6A modification in the regulation of glucose metabolism were summarized, the potential therapeutic strategies that improve glucose metabolism by targeting m6A modifiers were outlined, and feasible directions of future research in this field were discussed as well, providing clues for translational research on combating metabolic diseases based on m6A modification in the future.

Impact of physical fitness and exercise training on subcutaneous adipose tissue beiging markers in humans with and without diabetes and a high-fat diet-fed mouse model

AIMS

Exercise training induces white adipose tissue (WAT) beiging and improves glucose homeostasis and mitochondrial function in rodents. This could be relevant for type 2 diabetes in humans, but the effect of physical fitness on beiging of subcutaneous WAT (scWAT) remains unclear. This translational study investigates if beiging of scWAT associates with physical fitness in healthy humans and recent-onset type 2 diabetes and if a voluntary running wheel intervention is sufficient to induce beiging in mice.

MATERIALS AND METHODS

Gene expression levels of established beiging markers were measured in scWAT biopsies of humans with (n = 28) or without type 2 diabetes (n = 28), stratified by spiroergometry into low (L-FIT; n = 14 each) and high physical fitness (H-FIT; n = 14 each). High-fat diet-fed FVB/N mice underwent voluntary wheel running, treadmill training or no training (n = 8 each group). Following the training intervention, mitochondrial respiration and content of scWAT were assessed by high-resolution respirometry and citrate synthase activity, respectively.

RESULTS

Secreted CD137 antigen (Tnfrsf9/Cd137) expression was three-fold higher in glucose-tolerant H-FIT than in L-FIT, but not different between H-FIT and L-FIT with type 2 diabetes. In mice, both training modalities increased Cd137 expression and enhanced mitochondrial content without changing respiration in scWAT. Treadmill but not voluntary wheel running led to improved whole-body insulin sensitivity.

CONCLUSIONS

Higher physical fitness and different exercise interventions associated with higher gene expression levels of the beiging marker CD137 in healthy humans and mice on a high-fat diet. Humans with recent-onset type 2 diabetes show an impaired adipose tissue-specific response to physical activity.

Rat brown adipose tissue thermogenic markers are modulated by estrous cycle phases and short-term fasting

Brown adipose tissue (BAT) converts chemical energy into heat to maintain body temperature. Although fatty acids (FAs) represent a primary substrate for uncoupling protein 1 (UCP1)-dependent thermogenesis, BAT also utilizes glucose for the same purpose. Considering that estrous cycle effects on BAT are not greatly explored, we examined those of 6-h fasting on interscapular BAT (iBAT) thermogenic markers in proestrus and diestrus. We found that the percentage of multilocular adipocytes was lower in proestrus than in diestrus, although it was increased after fasting in both analyzed estrous cycle stages. Furthermore, the percentage of paucilocular adipocytes was increased by fasting, unlike the percentage of unilocular cells, which decreased in both analyzed stages of the estrous cycle. The UCP1 amount was lower in proestrus irrespectively of the examined dietary regimens. Regarding FA transporters, it was shown that iBAT CD36 content was increased in fasted rats in diestrus. In contrast to GLUT1, the level of GLUT4 was interactively modulated by selected estrous cycle phases and fasting. There was no change in insulin receptor and ERK1/2 activation, while AKT activation was interactively modulated by fasting and estrous cycle stages. Our study showed that iBAT exhibits morphological and functional changes in proestrus and diestrus. Moreover, iBAT undergoes additional dynamic functional and morphological changes during short-term fasting to modulate nutrient utilization and adjust energy expenditure.

GPR84-mediated signal transduction affects metabolic function by promoting brown adipocyte activity

The G protein-coupled receptor 84 (GPR84), a medium-chain fatty acid receptor, has garnered attention because of its potential involvement in a range of metabolic conditions. However, the precise mechanisms underlying this effect remain elusive. Our study has shed light on the pivotal role of GPR84, revealing its robust expression and functional significance within brown adipose tissue (BAT). Mice lacking GPR84 exhibited increased lipid accumulation in BAT, rendering them more susceptible to cold exposure and displaying reduced BAT activity compared with their WT counterparts. Our in vitro experiments with primary brown adipocytes from GPR84-KO mice revealed diminished expression of thermogenic genes and reduced O2 consumption. Furthermore, the application of the GPR84 agonist 6-n-octylaminouracil (6-OAU) counteracted these effects, effectively reinstating the brown adipocyte activity. These compelling in vivo and in vitro findings converge to highlight mitochondrial dysfunction as the primary cause of BAT anomalies in GPR84-KO mice. The activation of GPR84 induced an increase in intracellular Ca2+ levels, which intricately influenced mitochondrial respiration. By modulating mitochondrial Ca2+ levels and respiration, GPR84 acts as a potent molecule involved in BAT activity. These findings suggest that GPR84 is a potential therapeutic target for invigorating BAT and ameliorating metabolic disorders.

Intermittent fasting favorably modulates adipokines and potentially attenuates atherosclerosis

Adipose tissue is now recognized as an endocrine organ that secretes bioactive molecules called adipokines. These biomolecules regulate key physiological functions, including insulin sensitivity, energy metabolism, appetite regulation, endothelial function and immunity. Dysregulated secretion of adipokines is intimately associated with obesity, and translates into increased risk of obesity-related cardiovasculo-metabolic diseases. In particular, emerging evidence suggests that adipokine imbalance contributes to the pathogenesis of atherosclerosis. One of the promising diet regimens that is beneficial in the fight against obesity and cardiometabolic disorders is intermittent fasting (IF). Indeed, IF robustly suppresses inflammation, meditates weight loss and mitigates many aspects of the cardiometabolic syndrome. In this paper, we review the main adipokines and their role in atherosclerosis, which remains a major contributor to cardiovascular-associated morbidity and mortality. We further discuss how IF can be employed as an effective management modality for obesity-associated atherosclerosis. By exploring a plethora of the beneficial effects of IF, particularly on inflammatory markers, we present IF as a possible intervention to help prevent atherosclerosis.

Is the β3-Adrenoceptor a Valid Target for the Treatment of Obesity and/or Type 2 Diabetes?

β3-Adrenoceptors mediate several functions in rodents that could be beneficial for the treatment of obesity and type 2 diabetes. This includes promotion of insulin release from the pancreas, cellular glucose uptake, lipolysis, and thermogenesis in brown adipose tissue. In combination, they lead to a reduction of body weight in several rodent models including ob/ob mice and Zucker diabetic fatty rats. These findings stimulated drug development programs in various pharmaceutical companies, and at least nine β3-adrenoceptor agonists have been tested in clinical trials. However, all of these projects were discontinued due to the lack of clinically relevant changes in body weight. Following a concise historical account of discoveries leading to such drug development programs we discuss species differences that explain why β3-adrenoceptors are not a meaningful drug target for the treatment of obesity and type 2 diabetes in humans.

Pharmacological modulation of adaptive thermogenesis: new clues for obesity management?

BACKGROUND

Adaptive thermogenesis represents the main mechanism through which the body generates heat in response to external stimuli, a phenomenon that includes shivering and non-shivering thermogenesis. The non-shivering thermogenesis is mainly exploited by adipose tissue characterized by a brown aspect, which specializes in energy dissipation. A decreased amount of brown adipose tissue has been observed in ageing and chronic illnesses such as obesity, a worldwide health problem characterized by dysfunctional adipose tissue expansion and associated cardiometabolic complications. In the last decades, the discovery of a trans-differentiation mechanism ("browning") within white adipose tissue depots, leading to the generation of brown-like cells, allowed to explore new natural and synthetic compounds able to favour this process and thus enhance thermogenesis with the aim of counteracting obesity. Based on recent findings, brown adipose tissue-activating agents could represent another option in addition to appetite inhibitors and inhibitors of nutrient absorption for obesity treatment.

PURPOSE

This review investigates the main molecules involved in the physiological (e.g. incretin hormones) and pharmacological (e.g. β3-adrenergic receptors agonists, thyroid receptor agonists, farnesoid X receptor agonists, glucagon-like peptide-1, and glucagon receptor agonists) modulation of adaptive thermogenesis and the signalling mechanisms involved.

Wnt10b knockdown promotes UCP1 expression in brown adipose tissue in mice

The effect of Wnt10b overexpression on adipose tissue development has been reported. However, the impact of Wnt10b knockdown on the function of brown adipose tissue (BAT) is yet largely unknown. Here, we used the CRISPR/Cas9 technique to generate Wnt10b-knockdown (Wnt10b+/- ) mice. We compared the development and thermogenic gene expression of interscapular BAT (iBAT) between Wnt10b+/- and Wnt10b+/+ mice under a chow diet, high-fat diet (HFD), and cold exposure conditions. Moreover, the effect of Wnt10b knockdown on brown adipocyte function was tested via in vitro experiments. Results indicated that Wnt10b knockdown decreased the iBAT mass and the brown adipocyte size and enhanced thermogenic gene expression, including UCP1, under chow diet conditions. In addition, Wnt10b+/- mice appeared to be able to maintain their body temperature better than the control in a cold environment, accompanied by higher UCP1 protein expression. Intriguingly, even under HFD conditions, Wnt10b+/- mice still showed higher UCP1 expression, which was associated with an alleviated obesity phenotype. In vitro studies further evidenced the Wnt10b knockdown stimulation of UCP1 expression and suppression of the adipogenic program. This study indicates that Wnt10b knockdown enhances UCP1 expression and inhibits the adipogenic differentiation of brown adipocytes, providing a novel option for therapeutic interventions in adiposity.

Inhibition of adipose tissue angiogenesis prevents rebound weight gain after caloric restriction in mice fed a high-fat diet

AIMS

We investigated whether modulation of white adipose tissue (WAT) vasculature regulates rebound weight gain (RWG) after caloric restriction (CR) in mice fed a high-fat diet (HFD).

MAIN METHODS

We compared changes in energy balance, hypothalamic neuropeptide gene expression, and characteristics of WAT by RT-qPCR, ELISA, immunohistochemistry, and adipose-derived stromal vascular fraction spheroid sprouting assay in obese mice fed a HFD ad libitum (HFD-AL), mice under 40 % CR for 3 or 4 weeks, mice fed HFD-AL for 3 days after CR (CRAL), and CRAL mice treated with TNP-470, an angiogenic inhibitor.

KEY FINDINGS

WAT angiogenic genes were expressed at low levels, but WAT vascular density was maintained in the CR group compared to that in the HFD-AL group. The CRAL group showed RWG, fat regain, and hyperphagia with higher expression of angiogenic genes and reduced pericyte coverage of the endothelium in WAT on day 3 after CR compared to the CR group, indicating rapidly increased angiogenic activity after CR. Administration of TNP-470 suppressed RWG, fat regain, and hyperphagia only after CR compared to the CRAL group. Changes in circulating leptin levels and hypothalamic neuropeptide gene expression were correlated with changes in weight and fat mass, suggesting that TNP-470 suppressed hyperphagia independently of the hypothalamic melanocortin system. Additionally, TNP-470 increased gene expression related to thermogenesis, fuel utilization, and browning in brown adipose tissue (BAT) and WAT, indicating TNP-470-induced increase in thermogenesis.

SIGNIFICANCE

Modulation of the WAT vasculature attenuates RWG after CR by suppressing hyperphagia and increasing BAT thermogenesis and WAT browning.

Glutamine Production by Glul Promotes Thermogenic Adipocyte Differentiation Through Prdm9-Mediated H3K4me3 and Transcriptional Reprogramming

Thermogenic adipocytes have been extensively investigated because of their energy-dissipating property and therapeutic potential for obesity and diabetes. Besides serving as fuel sources, accumulating evidence suggests that intermediate metabolites play critical roles in multiple biological processes. However, their role in adipocyte differentiation and thermogenesis remains unexplored. Here, we report that human and mouse obesity is associated with marked downregulation of glutamine synthetase (Glul) expression and activity in thermogenic adipose tissues. Glul is robustly upregulated during brown adipocyte (BAC) differentiation and in brown adipose tissue (BAT) upon cold exposure and Cl316,243 stimulation. Further genetic, pharmacologic, or metabolic manipulations of Glul and glutamine levels reveal that glutamine cells autonomously stimulate BAC differentiation and function and BAT remodeling and improve systemic energy homeostasis in mice. Mechanistically, glutamine promotes transcriptional induction of adipogenic and thermogenic gene programs through histone modification-mediated chromatin remodeling. Among all the glutamine-regulated writer and eraser genes responsible for histone methylation and acetylation, only Prdm9, a histone lysine methyltransferase, is robustly induced during BAC differentiation. Importantly, Prdm9 inactivation by shRNA knockdown or a selective inhibitor attenuates glutamine-triggered adipogenic and thermogenic induction. Furthermore, Prdm9 gene transcription is regulated by glutamine through the recruitment of C/EBPb to its enhancer region. This work reveals glutamine as a novel activator of thermogenic adipocyte differentiation and uncovers an unexpected role of C/EBPb-Prdm9-mediated H3K4me3 and transcriptional reprogramming in adipocyte differentiation and thermogenesis.

ARTICLE HIGHLIGHTS

Angiotensin II type 2 receptor as a novel activator of brown adipose tissue in obesity

The angiotensin II type 2 receptor (AT2R) exerts vasorelaxant, anti-inflammatory, and antioxidant properties. In obesity, its activation counterbalances the adverse cardiovascular effects of angiotensin II mediated by the AT1R. Preliminary results indicate that it also promotes brown adipocyte differentiation in vitro. Our hypothesis is that AT2R activation could increase BAT mass and activity in obesity. Five-week-old male C57BL/6J mice were fed a standard or a high-fat (HF) diet for 6 weeks. Half of the animals were treated with compound 21 (C21), a selective AT2R agonist, (1 mg/kg/day) in the drinking water. Electron transport chain (ETC), oxidative phosphorylation, and UCP1 proteins were measured in the interscapular BAT (iBAT) and thoracic perivascular adipose tissue (tPVAT) as well as inflammatory and oxidative parameters. Differentiation and oxygen consumption rate (OCR) in the presence of C21 was tested in brown preadipocytes. In vitro, C21-differentiated brown adipocytes showed an AT2R-dependent increase of differentiation markers (Ucp1, Cidea, Pparg) and increased basal and H+ leak-linked OCR. In vivo, HF-C21 mice showed increased iBAT mass compared to HF animals. Both their iBAT and tPVAT showed higher protein levels of the ETC protein complexes and UCP1, together with a reduction of inflammatory and oxidative markers. The activation of the AT2R increases BAT mass, mitochondrial activity, and reduces markers of tissue inflammation and oxidative stress in obesity. Therefore, insulin reduction and better vascular responses are achieved. Thus, the activation of the protective arm of the renin-angiotensin system arises as a promising tool in the treatment of obesity.

Lipocalin 2 regulates mitochondrial phospholipidome remodeling, dynamics, and function in brown adipose tissue in male mice

Mitochondrial function is vital for energy metabolism in thermogenic adipocytes. Impaired mitochondrial bioenergetics in brown adipocytes are linked to disrupted thermogenesis and energy balance in obesity and aging. Phospholipid cardiolipin (CL) and phosphatidic acid (PA) jointly regulate mitochondrial membrane architecture and dynamics, with mitochondria-associated endoplasmic reticulum membranes (MAMs) serving as the platform for phospholipid biosynthesis and metabolism. However, little is known about the regulators of MAM phospholipid metabolism and their connection to mitochondrial function. We discover that LCN2 is a PA binding protein recruited to the MAM during inflammation and metabolic stimulation. Lcn2 deficiency disrupts mitochondrial fusion-fission balance and alters the acyl-chain composition of mitochondrial phospholipids in brown adipose tissue (BAT) of male mice. Lcn2 KO male mice exhibit an increase in the levels of CLs containing long-chain polyunsaturated fatty acids (LC-PUFA), a decrease in CLs containing monounsaturated fatty acids, resulting in mitochondrial dysfunction. This dysfunction triggers compensatory activation of peroxisomal function and the biosynthesis of LC-PUFA-containing plasmalogens in BAT. Additionally, Lcn2 deficiency alters PA production, correlating with changes in PA-regulated phospholipid-metabolizing enzymes and the mTOR signaling pathway. In conclusion, LCN2 plays a critical role in the acyl-chain remodeling of phospholipids and mitochondrial bioenergetics by regulating PA production and its function in activating signaling pathways.

Absolute thermometry of human brown adipose tissue by magnetic resonance with laser polarized 129Xe

BACKGROUND

Absolute temperature measurements of tissues inside the human body are difficult to perform non-invasively. Yet, for brown adipose tissue (BAT), these measurements would enable direct monitoring of its thermogenic activity and its association with metabolic health.

METHODS

Here, we report direct measurement of absolute BAT temperature in humans during cold exposure by magnetic resonance (MR) with laser polarized xenon gas. This methodology, which leverages on the sensitivity of the chemical shift of the 129Xe isotope to temperature-induced changes in fat density, is first calibrated in vitro and then tested in vivo in rodents. Finally, it is used in humans along with positron emission tomography (PET) scans with fluorine-18-fluorodeoxyglucose to detect BAT thermogenic activity during cold exposure.

RESULTS

Absolute temperature measurements, obtained in rodents with an experimental error of 0.5 °C, show only a median deviation of 0.12 °C against temperature measurements made using a pre-calibrated optical temperature probe. In humans, enhanced uptake of 129Xe in BAT during cold exposure leads to background-free detection of this tissue by MR. Global measurements of supraclavicular BAT temperature, made over the course of four seconds and with an experimental error ranging from a minimum of 0.4 °C to more than 2 °C, in case of poor shimming, reveal an average BAT temperature of 38.8° ± 0.8 °C, significantly higher (p < 0.02 two-sided t test) than 37.7 °C. Hot BAT is also detected in participants with a PET scan negative for BAT.

CONCLUSIONS

Non-invasive, radiation-free measurements of BAT temperature by MRI with hyperpolarized 129Xe may enable longitudinal monitoring of human BAT activity under various stimulatory conditions.

The obesity-linked human lncRNA AATBC stimulates mitochondrial function in adipocytes

Adipocytes are critical regulators of metabolism and energy balance. While white adipocyte dysfunction is a hallmark of obesity-associated disorders, thermogenic adipocytes are linked to cardiometabolic health. As adipocytes dynamically adapt to environmental cues by functionally switching between white and thermogenic phenotypes, a molecular understanding of this plasticity could help improving metabolism. Here, we show that the lncRNA Apoptosis associated transcript in bladder cancer (AATBC) is a human-specific regulator of adipocyte plasticity. Comparing transcriptional profiles of human adipose tissues and cultured adipocytes we discovered that AATBC was enriched in thermogenic conditions. Using primary and immortalized human adipocytes we found that AATBC enhanced the thermogenic phenotype, which was linked to increased respiration and a more fragmented mitochondrial network. Expression of AATBC in adipose tissue of mice led to lower plasma leptin levels. Interestingly, this association was also present in human subjects, as AATBC in adipose tissue was inversely correlated with plasma leptin levels, BMI, and other measures of metabolic health. In conclusion, AATBC is a novel obesity-linked regulator of adipocyte plasticity and mitochondrial function in humans.

TMEM135 links peroxisomes to the regulation of brown fat mitochondrial fission and energy homeostasis

Mitochondrial morphology, which is controlled by mitochondrial fission and fusion, is an important regulator of the thermogenic capacity of brown adipocytes. Adipose-specific peroxisome deficiency impairs thermogenesis by inhibiting cold-induced mitochondrial fission due to decreased mitochondrial membrane content of the peroxisome-derived lipids called plasmalogens. Here, we identify TMEM135 as a critical mediator of the peroxisomal regulation of mitochondrial fission and thermogenesis. Adipose-specific TMEM135 knockout in mice blocks mitochondrial fission, impairs thermogenesis, and increases diet-induced obesity and insulin resistance. Conversely, TMEM135 overexpression promotes mitochondrial division, counteracts obesity and insulin resistance, and rescues thermogenesis in peroxisome-deficient mice. Mechanistically, thermogenic stimuli promote association between peroxisomes and mitochondria and plasmalogen-dependent localization of TMEM135 in mitochondria, where it mediates PKA-dependent phosphorylation and mitochondrial retention of the fission factor Drp1. Together, these results reveal a previously unrecognized inter-organelle communication regulating mitochondrial fission and energy homeostasis and identify TMEM135 as a potential target for therapeutic activation of BAT.