Selective Impairment of Glucose but Not Fatty Acid or Oxidative Metabolism in Brown Adipose Tissue of Subjects With Type 2 Diabetes

The effects of the β1-adrenergic receptor antagonist bisoprolol administration on mirabegron-stimulated human brown adipose tissue thermogenesis

AIM

Pharmacological stimulation of human brown adipose tissue (BAT) has been hindered by ineffective activation or undesirable off-target effects. Oral administration of the maximal allowable dose of mirabegron (200 mg), a β3-adrenergic receptor (β3-AR) agonist, has been effective in stimulating BAT thermogenesis and whole-body energy expenditure. However, this has been accompanied by undesirable cardiovascular effects. Therefore, we hypothesized that combining mirabegron with a β1-AR antagonist could suppress these unwanted effects and increase the stimulation of the β3-AR and β2-AR in BAT.

METHODS

We performed a randomized crossover trial (NCT04823442) in 8 lean men. Mirabegron (200 mg) was administered orally with or without the β1-AR antagonist bisoprolol (10 mg). Dynamic [11C]-acetate and 2-deoxy-2-[18F]fluoro-d-glucose PET/CT scans were performed sequentially after oral administration of mirabegron ± bisoprolol.

RESULTS

Compared to room temperature, mirabegron alone increased BAT oxidative metabolism (0.84 ± 0.46 vs. 1.79 ± 0.91 min-1, p = 0.0433), but not when combined with bisoprolol. The metabolic rate of glucose in BAT, measured using [18F]FDG PET, was significantly higher with mirabegron than mirabegron with bisoprolol (24 ± 10 vs. 16 ± 8 nmol/g/min, p = 0.0284). Bisoprolol inhibited the mirabegron-induced increase in systolic blood pressure and heart rate.

CONCLUSION

The administration of bisoprolol decreases the adverse cardiovascular effects of mirabegron. However, the provided dose also blunted the mirabegron-stimulated increase in BAT lipolysis, thermogenesis, and glucose uptake. The attenuation in BAT blood flow induced by the large dose of bisoprolol may have limited BAT thermogenesis.

Brown adipose tissue metabolism in women is dependent on ovarian status

In rodents, loss of estradiol (E2) reduces brown adipose tissue (BAT) metabolic activity. Whether E2 impacts BAT activity in women is not known. BAT oxidative metabolism was measured in premenopausal (n = 27; 35 ± 9 yr; body mass index = 26.0 ± 5.3 kg/m2) and postmenopausal (n = 25; 51 ± 8 yr; body mass index = 28.0 ± 5.0 kg/m2) women at room temperature and during acute cold exposure using [11C]acetate with positron emission tomography coupled with computed tomograph. BAT glucose uptake was also measured during acute cold exposure using 2-deoxy-2-[18F]fluoro-d-glucose. To isolate the effects of ovarian hormones from biological aging, measurements were repeated in a subset of premenopausal women (n = 8; 40 ± 4 yr; BMI = 28.0 ± 7.2 kg/m2) after 6 mo of gonadotropin-releasing hormone agonist therapy to suppress ovarian hormones. At room temperature, there was no difference in BAT oxidative metabolism between premenopausal (0.56 ± 0.31 min-1) and postmenopausal women (0.63 ± 0.28 min-1). During cold exposure, BAT oxidative metabolism (1.28 ± 0.85 vs. 0.91 ± 0.63 min-1, P = 0.03) and net BAT glucose uptake (84.4 ± 82.5 vs. 29.7 ± 31.4 nmol·g-1·min-1, P < 0.01) were higher in premenopausal than postmenopausal women. In premenopausal women who underwent gonadotropin-releasing hormone agonist, cold-stimulated BAT oxidative metabolism was reduced to a similar level (from 1.36 ± 0.66 min-1 to 0.91 ± 0.41 min-1) to that observed in postmenopausal women (0.91 ± 0.63 min-1). These results provide the first evidence in humans that reproductive hormones are associated with BAT oxidative metabolism and suggest that BAT may be a target to attenuate age-related reduction in energy expenditure and maintain metabolic health in postmenopausal women.NEW & NOTEWORTHY In rodents, loss of estrogen reduces brown adipose tissue (BAT) activity. Whether this is true in humans is not known. We found that BAT oxidative metabolism and glucose uptake were lower in postmenopausal compared to premenopausal women. In premenopausal women who underwent ovarian suppression to reduce circulating estrogen, BAT oxidative metabolism was reduced to postmenopausal levels. Thus the loss of ovarian function in women leads to a reduction in BAT metabolic activity independent of age.

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.

Plasticity of Adipose Tissues: Interconversion among White, Brown, and Beige Fat and Its Role in Energy Homeostasis

Obesity, characterized by the excessive accumulation of adipose tissue, has emerged as a major public health concern worldwide. To develop effective strategies for treating obesity, it is essential to comprehend the biological properties of different adipose tissue types and their respective roles in maintaining energy balance. Adipose tissue serves as a crucial organ for energy storage and metabolism in the human body, with functions extending beyond simple fat storage to encompass the regulation of energy homeostasis and the secretion of endocrine factors. This review provides an overview of the key characteristics, functional differences, and interconversion processes among white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue. Moreover, it delves into the molecular mechanisms and recent research advancements concerning the browning of WAT, activation of BAT, and whitening of BAT. Although targeting adipose tissue metabolism holds promise as a potential approach for obesity treatment, further investigations are necessary to unravel the intricate biological features of various adipose tissue types and elucidate the molecular pathways governing their interconversion. Such research endeavors will pave the way for the development of more efficient and targeted therapeutic interventions in the fight against obesity.

Healthy Aging in Times of Extreme Temperatures: Biomedical Approaches

Climate extremes and rising energy prices present interconnected global health risks. Technical solutions can be supplemented with biomedical approaches to promote healthy longevity in hot and cold conditions. In summer, reducing basal metabolic rate through mild caloric restriction or CR mimetics, such as resveratrol, can potentially be used to lower body temperature. In winter, activating brown adipose tissue (BAT) for non-shivering thermogenesis and improved metabolic health can help adaptation to colder environments. Catechins found in green tea and in other food could be alternatives to drugs for these purposes. This review examines and discusses the biomedical evidence supporting the use of CR mimetics and BAT activators for health benefits amid increasingly extreme temperatures.

An Update on the Role and Potential Molecules in Relation to Ruminococcus gnavus in Inflammatory Bowel Disease, Obesity and Diabetes Mellitus

Ruminococcus gnavus (R. gnavus) is a gram-positive anaerobe commonly resides in the human gut microbiota. The advent of metagenomics has linked R. gnavus with various diseases, including inflammatory bowel disease (IBD), obesity, and diabetes mellitus (DM), which has become a growing area of investigation. The initial focus of research primarily centered on assessing the abundance of R. gnavus and its potential association with disease presentation, taking into account variations in sample size, sequencing and analysis methods. However, recent investigations have shifted towards elucidating the underlying mechanistic pathways through which R. gnavus may contribute to disease manifestation. In this comprehensive review, we aim to provide an updated synthesis of the current literature on R. gnavus in the context of IBD, obesity, and DM. We critically analyze relevant studies and summarize the potential molecular mediators implicated in the association between R. gnavus and these diseases. Across numerous studies, various molecules such as methylation-controlled J (MCJ), glucopolysaccharides, ursodeoxycholic acid (UDCA), interleukin(IL)-10, IL-17, and capric acid have been proposed as potential contributors to the link between R. gnavus and IBD. Similarly, in the realm of obesity, molecules such as hydrogen peroxide, butyrate, and UDCA have been suggested as potential mediators, while glycine ursodeoxycholic acid (GUDCA) has been implicated in the connection between R. gnavus and DM. Furthermore, it is imperative to emphasize the necessity for additional studies to evaluate the potential efficacy of targeting pathways associated with R. gnavus as a viable strategy for managing these diseases. These findings have significantly expanded our understanding of the functional role of R. gnavus in the context of IBD, obesity, and DM. This review aims to offer updated insights into the role and potential mechanisms of R. gnavus, as well as potential strategies for the treatment of these diseases.

Tracers and Imaging of Fatty Acid and Energy Metabolism of Human Adipose Tissues

White adipose tissue and brown adipose tissue (WAT and BAT) regulate fatty acid metabolism and control lipid fluxes to other organs. Dysfunction of these key metabolic processes contributes to organ insulin resistance and inflammation leading to chronic diseases such as type 2 diabetes, metabolic dysfunction-associated steatohepatitis, and cardiovascular diseases. Metabolic tracers combined with molecular imaging methods are powerful tools for the investigation of these pathogenic mechanisms. Herein, I review some of the positron emission tomography and magnetic resonance imaging methods combined with stable isotopic metabolic tracers to investigate fatty acid and energy metabolism, focusing on human WAT and BAT metabolism. I will discuss the complementary strengths offered by these methods for human investigations and current gaps in the field.

Neural circuits of long-term thermoregulatory adaptations to cold temperatures and metabolic demands

The mammalian brain controls heat generation and heat loss mechanisms that regulate body temperature and energy metabolism. Thermoeffectors include brown adipose tissue, cutaneous blood flow and skeletal muscle, and metabolic energy sources include white adipose tissue. Neural and metabolic pathways modulating the activity and functional plasticity of these mechanisms contribute not only to the optimization of function during acute challenges, such as ambient temperature changes, infection and stress, but also to longitudinal adaptations to environmental and internal changes. Exposure of humans to repeated and seasonal cold ambient conditions leads to adaptations in thermoeffectors such as habituation of cutaneous vasoconstriction and shivering. In animals that undergo hibernation and torpor, neurally regulated metabolic and thermoregulatory adaptations enable survival during periods of significant reduction in metabolic rate. In addition, changes in diet can activate accessory neural pathways that alter thermoeffector activity. This knowledge may be harnessed for therapeutic purposes, including treatments for obesity and improved means of therapeutic hypothermia.

Cold exposure modulates potential brown adipokines in humans, but only FGF21 is associated with brown adipose tissue volume

OBJECTIVE

The study objective was to investigate the effect of cold exposure on the plasma levels of five potential human brown adipokines (chemokine ligand 14 [CXCL14], growth differentiation factor 15 [GDF15], fibroblast growth factor 21 [FGF21], interleukin 6 [IL6], and bone morphogenic protein 8b [BMP8b]) and to study whether such cold-induced effects are related to brown adipose tissue (BAT) volume, activity, or radiodensity in young humans.

METHODS

Plasma levels of brown adipokines were measured before and 1 h and 2 h after starting an individualized cold exposure in 30 young adults (60% women, 21.9 ± 2.3 y; 24.9 ± 5.1 kg/m2 ). BAT volume, 18 F-fluorodeoxyglucose uptake, and radiodensity were assessed by a static positron emission tomography-computerized tomography scan after cold exposure.

RESULTS

Cold exposure increased the concentration of CXCL14 (Δ2h = 0.58 ± 0.98 ng/mL; p = 0.007), GDF15 (Δ2h = 19.63 ± 46.2 pg/mL; p = 0.013), FGF21 (Δ2h = 33.72 ± 55.13 pg/mL; p = 0.003), and IL6 (Δ1h = 1.98 ± 3.56 pg/mL; p = 0.048) and reduced BMP8b (Δ2h = -37.12 ± 83.53 pg/mL; p = 0.022). The cold-induced increase in plasma FGF21 was positively associated with BAT volume (Δ2h: β = 0.456; R2  = 0.307; p = 0.001), but not with 18 F-fluorodeoxyglucose uptake or radiodensity. None of the changes in the other studied brown adipokines was related to BAT volume, activity, or radiodensity.

CONCLUSIONS

Cold exposure modulates plasma levels of several potential brown adipokines in humans, whereas only cold-induced changes in FGF21 levels are associated with BAT volume. These findings suggest that human BAT might contribute to the circulatory pool of FGF21.

Adults with metabolically healthy overweight or obesity present more brown adipose tissue and higher thermogenesis than their metabolically unhealthy counterparts

BACKGROUND

There is a subset of individuals with overweight/obesity characterized by a lower risk of cardiometabolic complications, the so-called metabolically healthy overweight/obesity (MHOO) phenotype. Despite the relatively higher levels of subcutaneous adipose tissue and lower visceral adipose tissue observed in individuals with MHOO than individuals with metabolically unhealthy overweight/obesity (MUOO), little is known about the differences in brown adipose tissue (BAT).

METHODS

This study included 53 young adults (28 women) with a body mass index (BMI) ≥25 kg/m2 which were classified as MHOO (n = 34) or MUOO (n = 19). BAT was assessed through a static 18F-FDG positron emission tomography/computed tomography scan after a 2-h personalized cooling protocol. Energy expenditure, skin temperature, and thermal perception were assessed during a standardized mixed meal test (3.5 h) and a 1-h personalized cold exposure. Body composition was assessed by dual-energy x-ray absorptiometry, energy intake was determined during an ad libitum meal test and dietary recalls, and physical activity levels were determined by a wrist-worn accelerometer.

FINDINGS

Participants with MHOO presented higher BAT volume (+124%, P = 0.008), SUVmean (+63%, P = 0.001), and SUVpeak (+133%, P = 0.003) than MUOO, despite having similar BAT mean radiodensity (P = 0.354). In addition, individuals with MHOO exhibited marginally higher meal-induced thermogenesis (P = 0.096) and cold-induced thermogenesis (+158%, P = 0.050). Moreover, MHOO participants showed higher supraclavicular skin temperature than MUOO during the first hour of the postprandial period and during the cold exposure, while no statistically significant differences were observed in other skin temperature parameters. We observed no statistically significant differences between MHOO and MUOO in thermal perception, body composition, outdoor ambient temperature exposure, resting metabolic rate, energy intake, or physical activity levels.

INTERPRETATION

Adults with MHOO present higher BAT volume and activity than MUOO. The higher meal- and cold-induced thermogenesis and cold-induced supraclavicular skin temperature are compatible with a higher BAT activity. Overall, these results suggest that BAT presence and activity might be linked to a healthier phenotype in young adults with overweight or obesity.

FUNDING

See acknowledgments section.

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.

Standardized In Vitro Models of Human Adipose Tissue Reveal Metabolic Flexibility in Brown Adipocyte Thermogenesis

Functional human brown and white adipose tissue (BAT and WAT) are vital for thermoregulation and nutritional homeostasis, while obesity and other stressors lead, respectively, to cold intolerance and metabolic disease. Understanding BAT and WAT physiology and dysfunction necessitates clinical trials complemented by mechanistic experiments at the cellular level. These require standardized in vitro models, currently lacking, that establish references for gene expression and function. We generated and characterized a pair of immortalized, clonal human brown (hBA) and white (hWA) preadipocytes derived from the perirenal and subcutaneous depots, respectively, of a 40-year-old male individual. Cells were immortalized with hTERT and confirmed to be of a mesenchymal, nonhematopoietic lineage based on fluorescence-activated cell sorting and DNA barcoding. Functional assessments showed that the hWA and hBA phenocopied primary adipocytes in terms of adrenergic signaling, lipolysis, and thermogenesis. Compared to hWA, hBA were metabolically distinct, with higher rates of glucose uptake and lactate metabolism, and greater basal, maximal, and nonmitochondrial respiration, providing a mechanistic explanation for the association between obesity and BAT dysfunction. The hBA also responded to the stress of maximal respiration by using both endogenous and exogenous fatty acids. In contrast to certain mouse models, hBA adrenergic thermogenesis was mediated by several mechanisms, not principally via uncoupling protein 1 (UCP1). Transcriptomics via RNA-seq were consistent with the functional studies and established a molecular signature for each cell type before and after differentiation. These standardized cells are anticipated to become a common resource for future physiological, pharmacological, and genetic studies of human adipocytes.

Precision nutrition for targeting pathophysiology of cardiometabolic phenotypes

Obesity is a heterogenous disease accompanied by a broad spectrum of cardiometabolic risk profiles. Traditional paradigms for dietary weight management do not address biological heterogeneity between individuals and have catastrophically failed to combat the global pandemic of obesity-related diseases. Nutritional strategies that extend beyond basic weight management to instead target patient-specific pathophysiology are warranted. In this narrative review, we provide an overview of the tissue-level pathophysiological processes that drive patient heterogeneity to shape distinct cardiometabolic phenotypes in obesity. Specifically, we discuss how divergent physiology and postprandial phenotypes can reveal key metabolic defects within adipose, liver, or skeletal muscle, as well as the integrative involvement of the gut microbiome and the innate immune system. Finally, we highlight potential precision nutritional approaches to target these pathways and discuss recent translational evidence concerning the efficacy of such tailored dietary interventions for different obesity phenotypes, to optimise cardiometabolic benefits.

Assessment of cold exposure-induced metabolic changes in mice using untargeted metabolomics

Background: Cold exposure (CE) can effectively modulate adipose tissue metabolism and improve metabolic health. Although previous metabolomics studies have primarily focused on analyzing one or two samples from serum, brown adipose tissue (BAT), white adipose tissue (WAT), and liver samples, there is a significant lack of simultaneous analysis of multiple tissues regarding the metabolic changes induced by CE in mice. Therefore, our study aims to investigate the metabolic profiles of the major tissues involved. Methods: A total of 14 male C57BL/6J mice were randomly assigned to two groups: the control group (n = 7) and the CE group (n = 7). Metabolite determination was carried out using gas chromatography-mass spectrometry (GC-MS), and multivariate analysis was employed to identify metabolites exhibiting differential expression between the two groups. Results: In our study, we identified 32 discriminant metabolites in BAT, 17 in WAT, 21 in serum, 7 in the liver, 16 in the spleen, and 26 in the kidney, respectively. Among these metabolites, amino acids, fatty acids, and nucleotides emerged as the most significantly altered compounds. These metabolites were found to be associated with 12 differential metabolic pathways closely related to amino acids, fatty acids, and energy metabolism. Conclusion: Our study may provide valuable insights into the metabolic effects induced by CE, and they have the potential to inspire novel approaches for treating metabolic diseases.

The serotonin transporter sustains human brown adipose tissue thermogenesis

Activation of brown adipose tissue (BAT) in humans is a strategy to treat obesity and metabolic disease. Here we show that the serotonin transporter (SERT), encoded by SLC6A4, prevents serotonin-mediated suppression of human BAT function. RNA sequencing of human primary brown and white adipocytes shows that SLC6A4 is highly expressed in human, but not murine, brown adipocytes and BAT. Serotonin decreases uncoupled respiration and reduces uncoupling protein 1 via the 5-HT2B receptor. SERT inhibition by the selective serotonin reuptake inhibitor (SSRI) sertraline prevents uptake of extracellular serotonin, thereby potentiating serotonin's suppressive effect on brown adipocytes. Furthermore, we see that sertraline reduces BAT activation in healthy volunteers, and SSRI-treated patients demonstrate no 18F-fluorodeoxyglucose uptake by BAT at room temperature, unlike matched controls. Inhibition of BAT thermogenesis may contribute to SSRI-induced weight gain and metabolic dysfunction, and reducing peripheral serotonin action may be an approach to treat obesity and metabolic disease.

Human thermogenic adipose tissue

Human thermogenic adipose tissue has long been touted as a promising therapeutic target for obesity and its associated metabolic diseases. Here, we provide a brief overview of the current knowledge of in vivo human thermogenic adipose tissue metabolism. We explore the evidence provided by retrospective and prospective studies describing the association of brown adipose tissue (BAT) [¹⁸F]fluorodeoxyglucose accumulation and various cardiometabolic risk factors. Although these studies have been invaluable in generating hypothesis, it has also raised some questions about the reliability of this method as an indicator of BAT thermogenic capacity. We discuss the evidence in support of human BAT functioning as a local thermogenic organ and energy sink, as an endocrine organ, and as a biomarker of adipose tissue health.

Effects of the pesticide deltamethrin on high fat diet-induced obesity and insulin resistance in male mice

Worldwide, rates of metabolic diseases are rapidly increasing and environmental exposure to pesticides, pollutants and/or other chemicals may play a role. Reductions in Brown Adipose Tissue (BAT) thermogenesis, mediated in part by uncoupling protein 1 (Ucp1), are associated with metabolic diseases. In the current study, we investigated whether the pesticide deltamethrin (0.01-1 mg/kg bw/day) incorporated into a high-fat diet and fed to mice housed at either room temperature (21 °C) or thermoneutrality (29 °C) would suppress BAT activity and accelerate the development of metabolic disease. Importantly, thermoneutrality allows for more accurate modeling of human metabolic disease. We found that, 0.01mg/kg bw/day of deltamethrin induced weight loss, improved insulin sensitivity and increased energy expenditure, effects that were associated with increases in physical activity. In contrast, exposure to 0.1 and 1 mg/kg bw/day deltamethrin had no effect on any of the parameters examined. Deltamethrin treatment in mice did not alter molecular markers of BAT thermogenesis, despite observing suppression of UCP1 expression in cultured brown adipocytes. These data indicate that while deltamethrin inhibits UCP1 expression in vitro, 16wks exposure does not alter BAT thermogenesis markers nor exacerbates the development of obesity and insulin resistance in mice.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

Stimulation of the beta-2-adrenergic receptor with salbutamol activates human brown adipose tissue

While brown adipose tissue (BAT) is activated by the beta-3-adrenergic receptor (ADRB3) in rodents, in human brown adipocytes, the ADRB2 is dominantly present and responsible for noradrenergic activation. Therefore, we performed a randomized double-blinded crossover trial in young lean men to compare the effects of single intravenous bolus of the ADRB2 agonist salbutamol without and with the ADRB1/2 antagonist propranolol on glucose uptake by BAT, assessed by dynamic 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography-computed tomography scan (i.e., primary outcome). Salbutamol, compared with salbutamol with propranolol, increases glucose uptake by BAT, without affecting the glucose uptake by skeletal muscle and white adipose tissue. The salbutamol-induced glucose uptake by BAT positively associates with the increase in energy expenditure. Notably, participants with high salbutamol-induced glucose uptake by BAT have lower body fat mass, waist-hip ratio, and serum LDL-cholesterol concentration. In conclusion, specific ADRB2 agonism activates human BAT, which warrants investigation of ADRB2 activation in long-term studies (EudraCT: 2020-004059-34).

Physiological and molecular mechanisms of cold-induced improvements in glucose homeostasis in humans beyond brown adipose tissue

Exposure to low ambient temperatures has previously been demonstrated to markedly improve glucose homeostasis in both rodents and humans. Although the brown adipose tissue is key in mediating these beneficial effects in rodents, its contribution appears more limited in humans. Hence, the exact tissues and underlying mechanisms that mediate cold-induced improvements in glucose homeostasis in humans remain to be fully established. In this review, we evaluated the response of the main organs involved in glucose metabolism (i.e. pancreas, liver, (white) adipose tissue, and skeletal muscle) to cold exposure and discuss their potential contribution to cold-induced improvements in glucose homeostasis in humans. We here show that cold exposure has widespread effects on metabolic organs involved in glucose regulation. Nevertheless, cold-induced improvements in glucose homeostasis appear primarily mediated via adaptations within the skeletal muscle and (presumably) white adipose tissue. Since the underlying mechanisms remain elusive, future studies should be aimed at pinpointing the exact physiological and molecular mechanisms involved in humans. Nonetheless, cold exposure holds great promise as a novel, additive lifestyle approach to improve glucose homeostasis in insulin resistant individuals. Parts of this graphical abstract were created using (modified) images from Servier Medical Art, licensed under the Creative Commons Attribution 3.0 Unported License. TG = thermogenesis, TAG = triacylglycerol, FFA = free fatty acid, SLN = sarcolipin, UCP3 = uncoupling protein 3, β2-AR = beta-2 adrenergic receptor, SNS = sympathetic nervous system.

Brown adipose tissue is not associated with cachexia or increased mortality in a retrospective study of patients with cancer

Although brown fat is strongly associated with a constellation of cardiometabolic benefits in animal models and humans, it has also been tied to cancer cachexia. In humans, cancer-associated cachexia increases mortality, raising the possibility that brown fat in this context may be associated with increased cancer death. However, the effect of brown fat on cancer-associated cachexia and survival in humans remains unclear. Here, we retrospectively identify patients with and without brown fat on fluorodeoxyglucose (18F-FDG) positron-emission tomography (PET) scans obtained as part of routine cancer care and assemble a cohort to address these questions. We did not find an association between brown fat status and cachexia. Furthermore, we did not observe an association between brown fat and increased mortality in patients with cachexia. Our analyses controlled for confounding factors including age at cancer diagnosis, sex, body mass index, cancer site, cancer stage, outdoor temperature, comorbid conditions (heart failure, type 2 diabetes mellitus, coronary artery disease, hypertension, dyslipidemia, cerebrovascular disease), and β-blocker use. Taken together, our results suggest that brown fat is not linked to cancer-associated cachexia and does not worsen overall survival in patients with cachexia.NEW & NOTEWORTHY This study finds that brown fat is not linked to cancer-associated cachexia. Moreover, this work shows that brown fat does not worsen overall survival in patients with cachexia.

Aqp5-/- mice exhibit reduced maximal body O2 consumption under cold exposure, normal pulmonary gas exchange, and impaired formation of brown adipose tissue

The fundamental body functions that determine maximal O₂ uptake (V̇o_2max) have not been studied in Aqp5^-/- mice (aquaporin 5, AQP5). We measured V̇o_2max to globally assess these functions and then investigated why it was found altered in Aqp5^-/- mice. V̇o_2max was measured by the Helox technique, which elicits maximal metabolic rate by intense cold exposure of the animals. We found V̇o_2max reduced in Aqp5^-/- mice by 20%-30% compared with wild-type (WT) mice. As AQP5 has been implicated to act as a membrane channel for respiratory gases, we studied whether this is caused by the known lack of AQP5 in the alveolar epithelial membranes of Aqp5^-/- mice. Lung function parameters as well as arterial O₂ saturation were normal and identical between Aqp5^-/- and WT mice, indicating that AQP5 does not contribute to pulmonary O₂ exchange. The cause for the decreased V̇o_2max thus might be found in decreased O₂ consumption of an intensely O₂-consuming peripheral organ such as activated brown adipose tissue (BAT). We found indeed that absence of AQP5 greatly reduces the amount of interscapular BAT formed in response to 4 wk of cold exposure, from 63% in WT to 25% in Aqp5^-/- animals. We conclude that lack of AQP5 does not affect pulmonary O₂ exchange, but greatly inhibits transformation of white to brown adipose tissue. As under cold exposure, BAT is a major source of the animals' heat production, reduction of BAT likely causes the decrease in V̇o_2max under this condition.

Cooling down with Entresto. Can sacubitril/valsartan combination enhance browning more than coldness?

BACKGROUND

It is a growing importance to induce a new treatment approach to encourage weight loss but also to improve maintenance of lost weight. It has been shown that promotion of brown adipose tissue (BAT) function or acquisition of BAT characteristics in white adipose tissue (terms referred as "browning") can be protective against obesity.

MAIN TEXT

Amongst numerous established environmental influences on BAT activity, cold exposure is the best interested technique due to its not only effects on of BAT depots in proliferation process but also de novo differentiation of precursor cells via β-adrenergic receptor activation. A novel combination drug, sacubitril/valsartan, has been shown to be more efficient in reducing cardiovascular events and heart failure readmission compared to conventional therapy. Also, this combination of drugs increases the postprandial lipid oxidation contributing to energy expenditure, promotes lipolysis in adipocytes and reduces body weight. To date, there is no research examining potential of combined sacubitril/valsartan use to promote browning or mechanisms in the basis of this thermogenic process.

CONCLUSION

Due to the pronounced effects of cold and sacubitril/valsartan treatment on function and metabolism of BAT, the primary goal of further research should focused on investigation of the synergistic effects of the sacubitril/valsartan treatment at low temperature environmental conditions.

Cold exposure induces dynamic changes in circulating triacylglycerol species, which is dependent on intracellular lipolysis: A randomized cross-over trial

BACKGROUND

The application of cold exposure has emerged as an approach to enhance whole-body lipid catabolism. The global effect of cold exposure on the lipidome in humans has been reported with mixed results depending on intensity and duration of cold.

METHODS

This secondary study was based on data from a previous randomized cross-over trial (ClinicalTrials.gov ID: NCT03012113). We performed sequential lipidomic profiling in serum during 120 min cold exposure of human volunteers. Next, the intracellular lipolysis was blocked in mice (eighteen 10-week-old male wild-type mice C57BL/6J) using a small-molecule inhibitor of adipose triglyceride lipase (ATGL; Atglistatin), and mice were exposed to cold for a similar duration. The quantitative lipidomic profiling was assessed in-depth using the Lipidyzer platform.

FINDINGS

In humans, cold exposure gradually increased circulating free fatty acids reaching a maximum at 60 min, and transiently decreased total triacylglycerols (TAGs) only at 30 min. A broad range of TAG species was initially decreased, in particular unsaturated and polyunsaturated TAG species with ≤5 double bonds, while after 120 min a significant increase was observed for polyunsaturated TAG species with ≥6 double bonds in humans. The mechanistic study in mice revealed that the cold-induced increase in polyunsaturated TAGs was largely prevented by blocking adipose triglyceride lipase.

INTERPRETATION

We interpret these findings as that cold exposure feeds thermogenic tissues with TAG-derived fatty acids for combustion, resulting in a decrease of circulating TAG species, followed by increased hepatic production of polyunsaturated TAG species induced by liberation of free fatty acids stemming from adipose tissue.

FUNDING

This work was supported by the Netherlands CardioVascular Research Initiative: 'the Dutch Heart Foundation, Dutch Federation of University Medical Centers, the Netherlands Organisation for Health Research and Development and the Royal Netherlands Academy of Sciences' [CVON2017-20 GENIUS-II] to Patrick C.N. Rensen. Borja Martinez-Tellez is supported by individual postdoctoral grant from the Fundación Alfonso Martin Escudero and by a Maria Zambrano fellowship by the Ministerio de Universidades y la Unión Europea - NextGenerationEU (RR_C_2021_04). Lucas Jurado-Fasoli was supported by an individual pre-doctoral grant from the Spanish Ministry of Education (FPU19/01609) and with an Albert Renold Travel Fellowship from the European Foundation for the Study of Diabetes (EFSD). Martin Giera was partially supported by NWO XOmics project #184.034.019.

Synthesis and Preliminary Evaluation of an 18F-labeled Oleate Analog to Image Fatty Acid Beta-Oxidation in the Absence of Metabolic Defluorination

PURPOSE

Fatty acid oxidation (FAO) is a key parameter for evaluating cardiovascular, oncologic, neurologic, and other metabolic diseases. Several single-photon emission computed tomography and positron emission tomography (PET) tracers have been developed to measure FAO. Among these, 18-[¹⁸F]fluoro-4-thia-oleate ([¹⁸F]FTO), first developed by DeGrado et al., is well characterized. Here, we synthesized several analogs of [¹⁸F]FTO to improve the metabolic stability of the C-¹⁸F bond, and preliminarily evaluated their performance in monkey PET studies.

PROCEDURES

Several secondary ¹⁸F-fluorinated analogs, 17-[¹⁸F]fluoro-4-thia-oleate (17-[¹⁸F]FTO), 15-[¹⁸F]fluoro-4-thia-oleate (15-[¹⁸F]FTO), 12-[¹⁸F]fluoro-4-thia-oleate (12-[¹⁸F]FTO), 7-[¹⁸F]fluoro-4-thia-oleate, (7-[¹⁸F]FTO, [¹⁸F]AS3504073-00), and 6-[¹⁸F]fluoro-4-thia-oleate (6-[¹⁸F]FTO), were synthesized from tosylate or bromide precursors using similar procedures. Nucleophilic ¹⁸F fluorination on each precursor was performed using [¹⁸F]tetrabutylammonium fluoride/tetrabutylammonium hydrocarbonate, followed by hydrolysis of methylester. All synthesized ¹⁸F-labeled compounds were administered to cynomolgus monkeys, and PET measurements were performed. From the monkey PET studies, 7-[¹⁸F]FTO was selected as the best tracer and used to perform preliminary evaluations in mice.

RESULTS

All five compounds had sufficient quality and stability for animal experiments. In monkey PET studies, 12-, 7-, and 6-[¹⁸F]FTO showed greater accumulation in the heart than [¹⁸F]FTO, but not 17- and 15-[¹⁸F]FTO. Only 7-[¹⁸F]FTO did not show significant accumulation in the bone. The standardized uptake values (SUVs) for 12-[¹⁸F]FTO, 7-[¹⁸F]FTO, and 6-[¹⁸F]FTO were 9.77, 9.26, and 7.25 in the heart, and 3.17, n.d., and 1.96 in the bone 1 h after administration, respectively. In mouse distribution studies, SUVs 1 h after administration of 7-[¹⁸F]FTO and [¹⁸F]FTO were 10.4 and 10.0 in the heart, and 0.37 and 3.48 in the femur, respectively. Administration of etomoxir, a carnitine palmitoyltransferase inhibitor, reduced SUVs of 7-[¹⁸F]FTO and [¹⁸F]FTO in the heart by 91% and 87%, respectively.

CONCLUSIONS

We developed a novel PET tracer 7-[¹⁸F]FTO/[¹⁸F]AS3504073-00 for FAO imaging. 7-[¹⁸F]FTO had an excellent PET tracer profile, suggesting it may be a useful tracer for FAO imaging. Further evaluations of the tracer are ongoing.

High-fructose feeding suppresses cold-stimulated brown adipose tissue glucose uptake independently of changes in thermogenesis and the gut microbiome

Diets rich in added sugars are associated with metabolic diseases, and studies have shown a link between these pathologies and changes in the microbiome. Given the reported associations in animal models between the microbiome and brown adipose tissue (BAT) function, and the alterations in the microbiome induced by high-glucose or high-fructose diets, we investigated the potential causal link between high-glucose or -fructose diets and BAT dysfunction in humans. Primary outcomes are changes in BAT cold-induced thermogenesis and the fecal microbiome (clinicaltrials.gov, NCT03188835). We show that BAT glucose uptake, but not thermogenesis, is impaired by a high-fructose but not high-glucose diet, in the absence of changes in the gastrointestinal microbiome. We conclude that decreased BAT glucose metabolism occurs earlier than other pathophysiological abnormalities during fructose overconsumption in humans. This is a potential confounding factor for studies relying on ¹⁸F-FDG to assess BAT thermogenesis.

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Fructose overfeeding decreases brown adipose tissue glucose metabolism

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These changes occur independently of oxidative metabolism

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No change is observed with glucose overfeeding

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The gut microbiome is not affected by fructose/glucose overfeeding

A larger brown fat volume and lower radiodensity are related to a greater cardiometabolic risk, especially in young men

OBJECTIVES

Brown adipose tissue (BAT) is important in the maintenance of cardiometabolic health in rodents. Recent reports appear to suggest the same in humans, although if this is true remains elusive partly because of the methodological bias that affected previous research. This cross-sectional work reports the relationships of cold-induced BAT volume, activity (peak standardized uptake, SUVpeak), and mean radiodensity (an inverse proxy of the triacylglycerols content) with the cardiometabolic and inflammatory profile of 131 young adults, and how these relationships are influenced by sex and body weight.

DESIGN

This is a cross-sectional study.

METHODS

Subjects underwent personalized cold exposure for 2 h to activate BAT, followed by static 18F-fluorodeoxyglucose PET-CT scanning to determine BAT variables. Information on cardiometabolic risk (CMR) and inflammatory markers was gathered, and a CMR score and fatty liver index (FLI) were calculated.

RESULTS

In men, BAT volume was positively related to homocysteine and liver damage markers concentrations (independently of BMI and seasonality) and the FLI (all P ≤ 0.05). In men, BAT mean radiodensity was negatively related to the glucose and insulin concentrations, alanine aminotransferase activity, insulin resistance, total cholesterol/HDL-C, LDL-C/HDL-C, the CMR score, and the FLI (all P ≤ 0.02). In women, it was only negatively related to the FLI (P < 0.001). These associations were driven by the results for the overweight and obese subjects. No relationship was seen between BAT and inflammatory markers (P > 0.05).

CONCLUSIONS

A larger BAT volume and a lower BAT mean radiodensity are related to a higher CMR, especially in young men, which may support that BAT acts as a compensatory organ in states of metabolic disruption.

Regulatory networks determining substrate utilization in brown adipocytes

Brown adipose tissue (BAT) is often considered as a sink for nutrients to generate heat. However, when the complex hormonal and nervous inputs and intracellular signaling networks regulating substrate utilization are considered, BAT appears much more as a tightly controlled rheostat, regulating body temperature and balancing circulating nutrient levels. Here we provide an overview of key regulatory circuits, including the diurnal rhythm, determining glucose, fatty acid, and amino acid utilization and the interdependency of these nutrients in thermogenesis. Moreover, we discuss additional factors mediating sympathetic BAT activation beyond β-adrenergic signaling and the limitations of glucose-based BAT activity measurements to foster a better understanding and interpretation of BAT activity data.

Dietary adenosine 5’-monophosphate supplementation increases food intake and remodels energy expenditure in mice

Background

Methods

Results

Conclusions

Effect of Acute Cold Exposure on Energy Metabolism and Activity of Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis

Background: The benefit of cold exposure for humans against obesity has brought the energy metabolism and activity of brown adipose tissue (BAT) induced by cold into focus. But the results are inconsistent. This review is aimed to systematically explore the effect of cold exposure on the activity of BAT and energy metabolism in humans.

Methods: We searched relevant papers that were published from 1990 to 2021 and were cited in PubMed Central, Web of science, Embase and Cochrane Library databases to conduct this systematic review and meta-analysis. Energy metabolism, BAT volume, BAT activity and non-esterified fatty acids (NEFA) data reported in eligible researches were extracted. Meta-analysis was applied to combine the mean difference or standard mean difference with their 95% confidence intervals (95%CI). RevMan 5.3 software was used for meta-analysis and evaluating the risk of bias. Stata 16.0 was used for evaluating the publication bias.

Results: Ten randomized controlled trials were included in meta-analysis. Compared with human exposed in room temperature at 24°C, the energy expenditure (EE) was increased after acute cold exposure at 16∼19°C (Z = 7.58, p < 0.05, mean different = 188.43kal/d, 95% CI = 139.73–237.13); BAT volume (Z = 2.62, p < 0.05; standard mean different = 0.41, 95% CI = 0.10–0.73); BAT activity (Z = 2.05, p = 0.04, standard mean difference = 1.61, 95% CI = 0.07–3.14) and the intake of BAT NEFA (Z = 2.85, p < 0.05; standard mean different = 0.53, 95% CI = 0.17–0.90) also increased.

Conclusion: Acute cold exposure could improve the energy expenditure and BAT activity in adults, which is beneficial for human against obesity.

Impaired Cold-Stimulated Supraclavicular Brown Adipose Tissue Activity in Young Boys With Obesity

Childhood obesity is a growing worldwide problem. In adults, lower cold-induced brown adipose tissue (BAT) activity is linked to obesity and metabolic dysfunction; this relationship remains uncertain in children. In this cross-sectional study, we compared cold-induced supraclavicular (SCV) BAT activity (percent change in proton density fat fraction [PDFF]) within the SCV region after 1 h of whole-body cold exposure (18°C), using MRI in 26 boys aged 8-10 years: 13 with normal BMI and 13 with overweight/obesity. Anthropometry, body composition, hepatic fat, visceral adipose tissue (VAT), and pre- and postcold PDFF of the subcutaneous adipose tissue (SAT) in the posterior neck region and the abdomen were measured. Boys with overweight/obesity had lower cold-induced percent decline in SCV PDFF compared with those with normal BMI (1.6 ± 0.8 vs. 4.7 ± 1.2%, P = 0.044). SCV PDFF declined significantly in boys with normal BMI (2.7 ± 0.7%, P = 0.003) but not in boys with overweight/obesity (1.1 ± 0.5%, P = 0.053). No cold-induced changes in the PDFF of either neck SAT (-0.89 ± 0.7%, P = 0.250, vs. 0.37 ± 0.3%, P = 0.230) or abdominal SAT (-0.39 ± 0.5%, P = 0.409, and 0.25 ± 0.2%, P = 0.139, for normal BMI and overweight/obesity groups, respectively) were seen. The cold-induced percent decline in SCV PDFF was inversely related to BMI (r = -0.39, P = 0.047), waist circumference (r = -0.48, P = 0.014), and VAT (r = -0.47, P = 0.014). Thus, in young boys, as in adults, BAT activity is lower in those with overweight/obesity, suggesting that restoring activity may be important for improving metabolic health.

Thermogenesis is supported by high rates of circulatory fatty acid and triglyceride delivery in highland deer mice

Highland native deer mice (Peromyscus maniculatus) have greater rates of lipid oxidation during maximal cold challenge in hypoxia (hypoxic cold-induced V˙O2max) compared to their lowland conspecifics. Lipid oxidation is also increased in deer mice acclimated to simulated high altitude (cold hypoxia), regardless of altitude ancestry. The underlying lipid metabolic pathway traits responsible for sustaining maximal thermogenic demand in deer mice is currently unknown. The objective of this study was to characterize key steps in the lipid oxidation pathway in highland and lowland deer mice acclimated to control (23oC, 21kPa O2) or cold hypoxic (5oC, 12kPa O2) conditions. We hypothesized that capacities for lipid delivery and tissue uptake will be greater in highlanders and further increase with cold hypoxia acclimation. With the transition from rest to hypoxic cold-induced V˙O2max, both highland and lowland deer mice showed increased plasma glycerol concentrations and fatty acid availability. Interestingly, cold hypoxia acclimation led to increased plasma triglyceride concentrations at cold-induced V˙O2max, but only in highlanders. Highlanders also had significantly greater delivery rates of circulatory free fatty acids and triglycerides due to higher plasma flow rates at cold-induced V˙O2max. We found no population or acclimation differences in fatty acid translocase (FAT/CD36) abundance in the gastrocnemius or brown adipose tissue, suggesting fatty acid uptake across membranes is not limiting during thermogenesis. Our data indicate that circulatory lipid delivery plays a major role in supporting the high thermogenic rates observed in highland versus lowland deer mice.

Many Ways to Rome: Exercise, Cold Exposure and Diet-Do They All Affect BAT Activation and WAT Browning in the Same Manner?

The discovery of functional brown adipose tissue (BAT) in adult humans and the possibility to recruit beige cells with high thermogenic potential within white adipose tissue (WAT) depots opened the field for new strategies to combat obesity and its associated comorbidities. Exercise training as well as cold exposure and dietary components are associated with the enhanced accumulation of metabolically-active beige adipocytes and BAT activation. Both activated beige and brown adipocytes increase their metabolic rate by utilizing lipids to generate heat via non-shivering thermogenesis, which is dependent on uncoupling protein 1 (UCP1) in the inner mitochondrial membrane. Non-shivering thermogenesis elevates energy expenditure and promotes a negative energy balance, which may ameliorate metabolic complications of obesity and Type 2 Diabetes Mellitus (T2DM) such as insulin resistance (IR) in skeletal muscle and adipose tissue. Despite the recent advances in pharmacological approaches to reduce obesity and IR by inducing non-shivering thermogenesis in BAT and WAT, the administered pharmacological compounds are often associated with unwanted side effects. Therefore, lifestyle interventions such as exercise, cold exposure, and/or specified dietary regimens present promising anchor points for future disease prevention and treatment of obesity and T2DM. The exact mechanisms where exercise, cold exposure, dietary interventions, and pharmacological treatments converge or rather diverge in their specific impact on BAT activation or WAT browning are difficult to determine. In the past, many reviews have demonstrated the mechanistic principles of exercise- and/or cold-induced BAT activation and WAT browning. In this review, we aim to summarize not only the current state of knowledge on the various mechanistic principles of diverse external stimuli on BAT activation and WAT browning, but also present their translational potential in future clinical applications.

Thermogenic responses to different clamped skin temperatures in cold-exposed men and women

Despite many decades of research examining thermoregulatory responses under varying cold stresses in humans, very little is known about the variability in metabolic heat production and shivering activity. Here, we used a novel closed-loop mean skin temperature clamping technique with a liquid-conditioned suit to isolate the effects of mean skin temperature on the subjective evaluation of thermal sensation, heat production, shivering responses, and oxidative fuel selection in young, lean and healthy men (n = 12) and women (n = 12). Our results showed a skin temperature-dependent increase in metabolic heat production (5.2±1.0 kJ/min, 5.9±1.0 kJ/min and 7.0±1.0 kJ/min with skin temperature maintained at 31°C, 29°C and 27°C, respectively; P< 0.0001) and shivering intensity in both men and women (0.6±0.1 %MVC, 1.1±0.4 %MVC and 2.5±0.7 %MVC, respectively; P<0.0001), including sex-dependent differences in heat production at all three temperatures (P < 0.005). Even when controlling for lean body mass and fat mass, sex differences persisted (P = 0.048 and P = 0.004, respectively), whereas controlling for differences in body surface area eliminated these differences. Interestingly, there were no sex differences in the cold-induced change in thermogenesis. Despite clamping skin temperature, there was tremendous variability in the rate of heat production and shivering intensity. Collectively this data suggests that many of the inter-individual differences in thermogenesis and shivering may be explained by differences in morphology and body composition.

Angiopoietin-like 4 governs diurnal lipoprotein lipase activity in brown adipose tissue

Objective

Brown adipose tissue (BAT) burns fatty acids (FAs) to produce heat, and shows diurnal oscillation in glucose and triglyceride (TG)-derived FA-uptake, peaking around wakening. Here we aimed to gain insight in the diurnal regulation of metabolic BAT activity.

Methods

RNA-sequencing, chromatin immunoprecipitation (ChIP)-sequencing, and lipidomics analyses were performed on BAT samples of wild type C57BL/6J mice collected at 3-hour intervals throughout the day. Knockout and overexpression models were used to study causal relationships in diurnal lipid handling by BAT.

Results

We identified pronounced enrichment of oscillating genes involved in extracellular lipolysis in BAT, accompanied by oscillations of FA and monoacylglycerol content. This coincided with peak lipoprotein lipase (Lpl) expression, and was predicted to be driven by peroxisome proliferator-activated receptor gamma (PPARγ) activity. ChIP-sequencing for PPARγ confirmed oscillation in binding of PPARγ to Lpl. Of the known LPL-modulators, angiopoietin-like 4 (Angptl4) showed the largest diurnal amplitude opposite to Lpl, and both Angptl4 knockout and overexpression attenuated oscillations of LPL activity and TG-derived FA-uptake by BAT.

Conclusions

Our findings highlight involvement of PPARγ and a crucial role of ANGPTL4 in mediating the diurnal oscillation of TG-derived FA-uptake by BAT, and imply that time of day is essential when targeting LPL activity in BAT to improve metabolic health.

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The transcriptome and lipidome of brown fat show clusters with distinct circadian phases.

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The peak in metabolic brown fat activity is defined by activation of lipolytic processes.

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PPARγ shows oscillating binding to lipolytic genes and may drive diurnal brown fat activity.

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Genetic modulation of the lipoprotein lipase inhibitor Angptl4 flattens rhythmic activity in brown fat.

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Time of day should be considered when studying the metabolic benefits of targeting brown fat.

Combined training increases thermogenic fat activity in patients with overweight and type 2 diabetes

BACKGROUND

Exercise is an important strategy in the management of diabetes. Experimental studies have shown that exercise acts, at least in part, by inducing the production of myokines that improve metabolic control and activate brown/beige adipose tissue depots. Combined training (CT) is recommended by the major diabetes guidelines due to its metabolic and cardiovascular benefits, however, its impact on brown/beige adipose tissue activities has never been tested in humans with overweight and type 2 diabetes (T2D). Here, we evaluated the effects of 16-week combined training (CT) program on brown adipose tissue activity; browning and autophagy markers, and serum pro-thermogenic/inflammatory inducers in patients with overweight and T2D.

METHODS

Thirty-four patients with overweight and T2D were assigned to either a control group (CG) or a combined training group (CTG) in a randomized and controlled study. Functional/fitness parameters, anthropometry/body composition parameters, blood hormone/biochemical parameters, thermogenic/autophagic gene expression in subcutaneous adipose tissue were evaluated before and at the end of the intervention. In addition, cold-induced 18-Fluoroxyglucose Positron Emission Computed Tomography (18^F-FDG PET/CT) was performed in the training group before and after the end of the intervention.

RESULTS

CT increased cervical/supraclavicular brown adipose tissue (BAT) thermogenic activity (p = 0.03) as well as in perirenal adipose tissue (p = 0.02). In addition, CT increased the expression of genes related to thermogenic profile (TMEM26: + 95%, p = 0.04; and EPSTI1: + 26%, p = 0.03) and decreased autophagic genes (ULK1: -15%, p = 0.04; LC3: -5%, p = 0.02; and ATG4: -22%, p < 0.001) in subcutaneous adipose tissue. There were positive correlations between Δ% BAT activity with Δ% of post training energy expenditure cold exposure, HDL-c, IL4, adiponectin, irisin, meteorin-like, and TMEM26 and ZIC1 genes, besides negative correlations with LDL-c, total cholesterol and C-reactive protein.

CONCLUSION

This is the first evidence of the beneficial actions of CT on adipose tissue thermogenic activity in humans, and it adds important support for the recommendation of CT as a strategy in the management of diabetes.

n-3 polyunsaturated fatty acids in the regulation of adipose tissue browning and thermogenesis in obesity: Potential relationship with gut microbiota

BACKGROUND

Obesity is a worldwide public health problem characterized by fat tissue accumulation, favouring adipose tissue and metabolic alterations. Increasing energy expenditure (EE) through brown adipose tissue activation and white adipose tissue (WAT) browning has gained relevance as a therapeutic approach. Different bioactive compounds, such as n-3 polyunsaturated fatty acids (PUFA), have been shown to induce those thermogenic effects. This process is regulated by the gut microbiota as well. Nevertheless, obesity is characterized by gut microbiota dysbiosis, which can be restored by weight loss and n-3 PUFA intake, among other factors. Knowledge gap: However, the role of the gut microbiota on the n-3 PUFA effect in inducing thermogenesis in obesity has not been fully elucidated.

OBJECTIVE

This review aims to elucidate the potential implications of this interrelation on WAT browning adiposw sittue (BAT), BAT activity, and EE regulation in obesity models.

Brown Adipose Tissue Prevalence Is Lower in Obesity but Its Metabolic Activity Is Intact

Due to its high metabolic activity, brown adipose tissue (BAT) has become a promising target for the development of novel treatment concepts for metabolic disease. Despite several reports of a negative association between the presence of active BAT and obesity, very little is known about the quantitative and qualitative differences of BAT in lean and obese individuals. Systematic studies directly comparing cold-induced BAT activity in leanness and obesity are currently lacking. Here we studied BAT mass and function in 31 lean and 64 obese men and women. After a standardized cooling protocol using a water-perfused vest, ¹⁸F-FDG-positron emission tomography/computed tomography scans were performed, and BAT was delineated using lean body-mass adjusted standardized uptake value (SUV) thresholds in anatomic regions with fat radiodensity. Cold-induced thermogenesis (CIT), a functional readout of BAT activity, was quantified by indirect calorimetry. Active BAT was present in a significantly higher proportion of lean than obese individuals (58% vs. 33%, p=0.019). In these participants with active BAT, however, BAT volume and activity did not differ between leanness and obesity. Accordingly, CIT was similar in both weight groups. BAT metrics were not related to adiposity or total fat mass per se. However, in obese participants a strong negative correlation existed between visceral adipose tissue and BAT volume, ¹⁸F-FDG uptake and CIT. In summary, despite a significantly lower prevalence of BAT, the metabolic activity and thermogenic capacity of BAT appears to be still intact in obesity and is inversely associated with visceral fat mass.

Brown fat triglyceride content is associated with cardiovascular risk markers in adults from a tropical region

Brown adipose tissue (BAT) is regarded as an interesting potential target for the treatment of obesity, diabetes, and cardiovascular diseases, and the detailed characterization of its structural and functional phenotype could enable an advance in these fields. Most studies evaluating BAT structure and function were performed in temperate climate regions, and we are yet to know how these findings apply to the 40% of the world's population living in tropical areas. Here, we used ¹⁸F-fluorodeoxyglucose positron emission tomography - magnetic resonance imaging to evaluate BAT in 45 lean, overweight, and obese volunteers living in a tropical area in Southeast Brazil. We aimed at investigating the associations between BAT activity, volume, metabolic activity, and BAT content of triglycerides with adiposity and cardiovascular risk markers in a sample of adults living in a tropical area and we showed that BAT glucose uptake is not correlated with leanness; instead, BAT triglyceride content is correlated with visceral adiposity and markers of cardiovascular risk. This study expands knowledge regarding the structure and function of BAT in people living in tropical areas. In addition, we provide evidence that BAT triglyceride content could be an interesting marker of cardiovascular risk.

ADH5-mediated NO bioactivity maintains metabolic homeostasis in brown adipose tissue

Brown adipose tissue (BAT) thermogenic activity is tightly regulated by cellular redox status, but the underlying molecular mechanisms are incompletely understood. Protein S-nitrosylation, the nitric-oxide-mediated cysteine thiol protein modification, plays important roles in cellular redox regulation. Here we show that diet-induced obesity (DIO) and acute cold exposure elevate BAT protein S-nitrosylation, including UCP1. This thermogenic-induced nitric oxide bioactivity is regulated by S-nitrosoglutathione reductase (GSNOR; alcohol dehydrogenase 5 [ADH5]), a denitrosylase that balances the intracellular nitroso-redox status. Loss of ADH5 in BAT impairs cold-induced UCP1-dependent thermogenesis and worsens obesity-associated metabolic dysfunction. Mechanistically, we demonstrate that Adh5 expression is induced by the transcription factor heat shock factor 1 (HSF1), and administration of an HSF1 activator to BAT of DIO mice increases Adh5 expression and significantly improves UCP1-mediated respiration. Together, these data indicate that ADH5 controls BAT nitroso-redox homeostasis to regulate adipose thermogenesis, which may be therapeutically targeted to improve metabolic health.

Human Brown Adipose Tissue and Metabolic Health: Potential for Therapeutic Avenues

Obesity-associated metabolic abnormalities comprise a cluster of conditions including dyslipidemia, insulin resistance, diabetes and cardiovascular diseases that has affected more than 650 million people all over the globe. Obesity results from the accumulation of white adipose tissues mainly due to the chronic imbalance of energy intake and energy expenditure. A variety of approaches to treat or prevent obesity, including lifestyle interventions, surgical weight loss procedures and pharmacological approaches to reduce energy intake and increase energy expenditure have failed to substantially decrease the prevalence of obesity. Brown adipose tissue (BAT), the primary source of thermogenesis in infants and small mammals may represent a promising therapeutic target to treat obesity by promoting energy expenditure through non-shivering thermogenesis mediated by mitochondrial uncoupling protein 1 (UCP1). Since the confirmation of functional BAT in adult humans by several groups, approximately a decade ago, and its association with a favorable metabolic phenotype, intense interest on the significance of BAT in adult human physiology and metabolic health has emerged within the scientific community to explore its therapeutic potential for the treatment of obesity and metabolic diseases. A substantially decreased BAT activity in individuals with obesity indicates a role for BAT in the setting of human obesity. On the other hand, BAT mass and its prevalence correlate with lower body mass index (BMI), decreased age and lower glucose levels, leading to a lower incidence of cardio-metabolic diseases. The increased cold exposure in adult humans with undetectable BAT was associated with decreased body fat mass and increased insulin sensitivity. A deeper understanding of the role of BAT in human metabolic health and its interrelationship with body fat distribution and deciphering proper strategies to increase energy expenditure, by either increasing functional BAT mass or inducing white adipose browning, holds the promise for possible therapeutic avenues for the treatment of obesity and associated metabolic disorders.

Dietary Iron Deficiency Modulates Adipocyte Iron Homeostasis, Adaptive Thermogenesis, and Obesity in C57BL/6 Mice

BACKGROUND

Adaptive thermogenesis is an iron-demanding pathway, significantly contributing to whole-body energy expenditure. However, the effects of iron-deficient diets on adaptive thermogenesis and obesity remain unknown.

OBJECTIVES

We aimed to determine the impact of dietary iron deficiency on iron homeostasis in adipocytes, adaptive thermogenic capacity, and metabolic consequences in obesity.

METHODS

C57BL/6 male mice were assigned to either the iron-adequate (IA, 35 ppm) or the iron-deficient group (ID, 3 ppm) at weaning. Upon 8 wk of age, both IA and ID groups received an isocaloric high-fat diet (45% kcal from fat) for 10 wk, maintaining the same iron content. Mice (n = 8) were used to determine the iron status at the systemic and tissue levels and lipid metabolism and inflammatory signaling in adipose tissue. The same mice were used to evaluate cold tolerance (4°C) for 3 h. For assessing adaptive thermogenesis, mice (n = 5) received an intraperitoneal injection of β3-adrenoceptor agonist CL316243 (CL) for 5 d.

RESULTS

Compared with the IA group, the ID group had nonanemic iron deficiency, lower serum ferritin (42.8%, P < 0.01), and greater weight gain (8.67%, P < 0.05) and insulin resistance (159%, P < 0.01), partly due to reduced AMP-activated protein kinase activation (61.0%, P < 0.05). Upon cold exposure, the ID group maintained a core body temperature 2°C lower than the IA group. The ID group had lower iron content (47.0%, P < 0.01) in the inguinal adipose tissue (iWAT) than the IA group, which was associated with impaired adaptive thermogenesis. In response to CL, ID mice showed decreased heat production (P < 0.01) and defective upregulation of beige adipocyte-specific markers, including uncoupling protein 1 (41.1%, P < 0.001), transferrin receptor 1 (47.5%, P < 0.001), and mitochondrial respiratory chain complexes (P < 0.05) compared with IA mice.

CONCLUSIONS

Dietary iron deficiency deregulates iron balance in the iWAT and impairs adaptive thermogenesis, thereby escalating the diet-induced weight gain in C57BL/6 mice.

Lower brown adipose tissue activity is associated with non-alcoholic fatty liver disease but not changes in the gut microbiota

In rodents, lower brown adipose tissue (BAT) activity is associated with greater liver steatosis and changes in the gut microbiome. However, little is known about these relationships in humans. In adults (n = 60), we assessed hepatic fat and cold-stimulated BAT activity using magnetic resonance imaging and the gut microbiota with 16S sequencing. We transplanted gnotobiotic mice with feces from humans to assess the transferability of BAT activity through the microbiota. Individuals with NAFLD (n = 29) have lower BAT activity than those without, and BAT activity is inversely related to hepatic fat content. BAT activity is not related to the characteristics of the fecal microbiota and is not transmissible through fecal transplantation to mice. Thus, low BAT activity is associated with higher hepatic fat accumulation in human adults, but this does not appear to have been mediated through the gut microbiota.

Molecular Imaging of Brown Adipose Tissue Mass

Brown adipose tissue (BAT), a uniquely thermogenic tissue that plays an important role in metabolism and energy expenditure, has recently become a revived target in the fight against metabolic diseases, such as obesity, diabetes, and non-alcoholic fatty liver disease (NAFLD). Different from white adipose tissue (WAT), the brown adipocytes have distinctive features including multilocular lipid droplets, a large number of mitochondria, and a high expression of uncoupling protein-1 (UCP-1), as well as abundant capillarity. These histologic characteristics provide an opportunity to differentiate BAT from WAT using imaging modalities, such as PET/CT, SPECT/CT, MRI, NIRF and Ultrasound. However, most of the reported imaging methods were BAT activation dependent, and the imaging signals could be affected by many factors, including environmental temperatures and the states of the sympathetic nervous system. Accurate BAT mass detection methods that are independent of temperature and hormone levels have the capacity to track the development and changes of BAT throughout the lifetime of mammals, and such methods could be very useful for the investigation of potential BAT-related therapies. In this review, we focus on molecular imaging modalities that can detect and quantify BAT mass. In addition, their detection mechanism and limitations will be discussed as well.

Metabolic effects of brown fat in transitioning from hyperthyroidism to euthyroidism

OBJECTIVE

Brown adipose tissue (BAT) controls metabolic rate through thermogenesis. As its regulatory factors during the transition from hyperthyroidism to euthyroidism are not well established, our study investigated the relationships between supraclavicular brown adipose tissue (sBAT) activity and physiological/metabolic changes with changes in thyroid status.

DESIGN

Participants with newly diagnosed Graves' disease were recruited. A thionamide antithyroid drug (ATD) such as carbimazole (CMZ) or thiamazole (TMZ) was prescribed in every case. All underwent energy expenditure (EE) measurement and supraclavicular infrared thermography (IRT) within a chamber calorimeter, as well as 18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography/magnetic resonance (PET/MR) imaging scanning, with clinical and biochemical parameters measured during hyperthyroidism and repeated in early euthyroidism. PET sBAT mean/maximum standardized uptake value (SUV mean/max), MR supraclavicular fat fraction (sFF) and mean temperature (Tscv) quantified sBAT activity.

RESULTS

Twenty-one (16 female/5 male) participants aged 39.5 ± 2.5 years completed the study. The average duration to attain euthyroidism was 28.6 ± 2.3 weeks. Eight participants were BAT-positive while 13 were BAT-negative. sFF increased with euthyroidism (72.3 ± 1.4% to 76.8 ± 1.4%; P < 0.01), but no changes were observed in PET SUV mean and Tscv. Significant changes in serum-free triiodothyronine (FT3) levels were related to BAT status (interaction P value = 0.04). FT3 concentration at hyperthyroid state was positively associated with sBAT PET SUV mean (r = 0.58, P = 0.01) and resting metabolic rate (RMR) (P < 0.01).

CONCLUSION

Hyperthyroidism does not consistently lead to a detectable increase in BAT activity. FT3 reduction during the transition to euthyroidism correlated with BAT activity.

Defective insulin-stimulated GLUT4 translocation in brown adipocytes induces systemic glucose homeostasis dysregulation independent of thermogenesis in female mice

OBJECTIVE

Recent studies indicate that brown adipose tissue, in addition to its role in thermogenesis, has a role in the regulation of whole-body metabolism. Here we characterize the metabolic effects of deleting Rab10, a protein key for insulin stimulation of glucose uptake into white adipocytes, solely from brown adipocytes.

METHODS

We used a murine brown adipocyte cell line and stromal vascular fraction-derived in vitro differentiated brown adipocytes to study the role of Rab10 in insulin-stimulated GLUT4 translocation to the plasma membrane and insulin-stimulated glucose uptake. We generated a brown adipocyte-specific Rab10 knockout for in vivo studies of metabolism and thermoregulation.

RESULTS

We demonstrate that deletion of Rab10 from brown adipocytes results in a two-fold reduction of insulin-stimulated glucose transport by reducing translocation of the GLUT4 glucose transporter to the plasma membrane, an effect linked to whole-body glucose intolerance and insulin resistance in female mice. This effect on metabolism is independent of the thermogenic function of brown adipocytes, thereby revealing a metabolism-specific role for brown adipocytes in female mice. The reduced glucose uptake induced by Rab10 deletion disrupts ChREBP regulation of de novo lipogenesis (DNL) genes, providing a potential link between DNL in brown adipocytes and whole-body metabolic regulation in female mice. However, deletion of Rab10 from male mice does not induce systemic insulin resistance, although ChREBP regulation is disrupted.

CONCLUSIONS

Our studies of Rab10 reveal the role of insulin-regulated glucose transport into brown adipocytes in whole-body metabolic homeostasis of female mice. Importantly, the contribution of brown adipocytes to whole-body metabolic regulation is independent of its role in thermogenesis. It is unclear whether the whole-body metabolic sexual dimorphism is because female mice are permissive to the effects of Rab10 deletion from brown adipocytes or because male mice are resistant to the effect.

Obeticholic acid ameliorates obesity and hepatic steatosis by activating brown fat

Obeticholic acid (OCA) is exemplified as a potent drug for treating primary biliary cirrhosis and nonalcoholic fatty liver disease by inhibiting bile acid synthesis. However, it remains unclear whether the effect of OCA is mediated by the function of brown adipose tissue (BAT). In the present study, brown adipogenesis differentiation in vitro and db/db mouse model treated with OCA were used to assess the anti-obesity function by body weight tracking, O₂ consumption, food intake, physical activity, glucose tolerance tests. In addition, uncoupling protein 1 (Ucp1) protein expression in brown adipose tissue was measured by western blotting, morphometry of brown adipose tissue was analyzed by hematoxylin and eosin staining. Hepatic steatosis was detected by Oil-Red O staining and serological analysis was performed to assess the effect of OCA on hyperlipidemia. OCA treatment enhanced brown adipocyte cell differentiation and upregulated the expression of the BAT-specific gene Ucp1) in C3H10T1/2 cells in vitro. Consistent with these findings, OCA increased whole-body energy metabolism and glucose homeostasis by enhancing BAT activity in vivo, and ultimately decreased body weight gain in db/db mice. In addition, the results demonstrated that spontaneous hepatic steatosis in db/db mice was ameliorated following OCA treatment. In summary, OCA functioned as a BAT activator to help ameliorate obesity and maintain glucose homeostasis in db/db mice. The present results may provide a novel potential therapeutic approach to activate brown fat in patients with obesity and other metabolic disorders.

Brown Adipose Tissue Volume and Fat Content Are Positively Associated With Whole-Body Adiposity in Young Men-Not in Women

Human brown adipose tissue (BAT) volume has consistently been claimed to be inversely associated with whole-body adiposity. However, recent advances in the assessment of human BAT suggest that previously reported associations may have been biased. The present cross-sectional study investigates the association of BAT volume, mean radiodensity, and 18F-fluorodeoxyglucose (18F-FDG) uptake (assessed via a static positron emission tomography [PET]-computed tomography [CT] scan after a 2-h personalized cold exposure) with whole-body adiposity (measured by DXA) in 126 young adults (42 men and 84 women; mean ± SD BMI 24.9 ± 4.7 kg/m2). BAT volume, but not 18F-FDG uptake, was positively associated with BMI, fat mass, and visceral adipose tissue (VAT) mass in men but not in women. These associations were independent of the date when the PET-CT was performed, insulin sensitivity, and body surface area. BAT mean radiodensity, an inverse proxy of BAT fat content, was negatively associated with BMI, fat mass, and VAT mass in men and in women. These results refute the widely held belief that human BAT volume is reduced in obese persons, at least in young adults, and suggest that it might even be the opposite in young men.