Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans

GPCR-mediated regulation of beige adipocyte formation: Implications for obesity and metabolic health

Obesity and its associated complications pose a significant burden on health. The non-shivering thermogenesis (NST) and metabolic capacity properties of brown adipose tissue (BAT), which are distinct from those of white adipose tissue (WAT), in combating obesity and its related metabolic diseases has been well documented. However, beige adipose tissue, the third and relatively novel type of adipose tissue, which emerges in extensive presence of WAT and shares similar favorable metabolic properties with BAT, has garnered considerable attention in recent years. In this review, we focused on the role of G protein-coupled receptors (GPCRs), the largest receptor family and the most successful class of drug targets in humans, in the induction of beige adipocytes. More importantly, we highlight researchers' clinical treatment attempts to ameliorate obesity and other related metabolic diseases through the formation and activation of beige adipose tissue. In summary, this review provides valuable insights into the formation of beige adipose tissue and the involvement of GPCRs, based on the latest advancements in scientific research.

Cold exposure affects glucose metabolism, lipid droplet deposition and mitophagy in skeletal muscle of newborn goats

Cold exposure is a common stressor for newborn goats. Skeletal muscle plays an important role in maintaining whole-body homeostasis of glucose and lipid metabolism. However, the molecular mechanisms underlying regulation of skeletal muscle of newborn goats by cold exposure remains unclear. In this study, we found a significant increase (P < 0.01) in serum glucagon levels after 24 h of cold exposure (COLD, 6°C), while glucose and insulin concentrations were significantly decreased (P < 0.01) compared to room temperature (RT, 25°C). Additionally, we found that cold exposure reduced glycogen content (P < 0.01) in skeletal muscle. Pathway enrichment analysis revealed that cold exposure activated skeletal muscle glucose metabolism pathways (including insulin resistance and the insulin signaling pathway) and mitophagy-related pathways. Cold exposure up-regulated the expression of genes involved in fatty acid and triglyceride synthesis, promoting skeletal muscle lipid deposition. Notably, cold exposure induced mitophagy in skeletal muscle.

Is stimulation of browning of human adipose tissue a relevant therapeutic target?

Brown adipose tissue (BAT) and beige adipose tissues are important contributors to cold-induced whole body thermogenesis in rodents. The documentation in humans of cold- and ß-adrenergic receptor agonist-stimulated BAT glucose uptake using positron emission tomography (PET) and of a decrease of this response in individuals with cardiometabolic disorders led to the suggestion that BAT/beige adipose tissues could be relevant targets for prevention and treatment of these conditions. In this brief review, we will critically assess this question by first describing the basic rationale for this affirmation, second by examining the evidence in human studies, and third by discussing the possible means to activate the thermogenic response of these tissues in humans.

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.

New Mediators in the Crosstalk between Different Adipose Tissues

Adipose tissue is a multifunctional organ that regulates many physiological processes such as energy homeostasis, nutrition, the regulation of insulin sensitivity, body temperature, and immune response. In this review, we highlight the relevance of the different mediators that control adipose tissue activity through a systematic review of the main players present in white and brown adipose tissues. Among them, inflammatory mediators secreted by the adipose tissue, such as classical adipokines and more recent ones, elements of the immune system infiltrated into the adipose tissue (certain cell types and interleukins), as well as the role of intestinal microbiota and derived metabolites, have been reviewed. Furthermore, anti-obesity mediators that promote the activation of beige adipose tissue, e.g., myokines, thyroid hormones, amino acids, and both long and micro RNAs, are exhaustively examined. Finally, we also analyze therapeutic strategies based on those mediators that have been described to date. In conclusion, novel regulators of obesity, such as microRNAs or microbiota, are being characterized and are promising tools to treat obesity in the future.

The Different Shades of Thermogenic Adipose Tissue

PURPOSE OF REVIEW

By providing a concise overview of adipose tissue types, elucidating the regulation of adipose thermogenic capacity in both physiological contexts and chronic wasting diseases (a protracted hypermetabolic state that precipitates sustained catabolism and consequent progressive corporeal atrophy), and most importantly, delving into the ongoing discourse regarding the role of adipose tissue thermogenic activation in chronic wasting diseases, this review aims to provide researchers with a comprehensive understanding of the field.

RECENT FINDINGS

Adipose tissue, traditionally classified as white, brown, and beige (brite) based on its thermogenic activity and potential, is intricately regulated by complex mechanisms in response to exercise or cold exposure. This regulation is adipose depot-specific and dependent on the duration of exposure. Excessive thermogenic activation of adipose tissue has been observed in chronic wasting diseases and has been considered a pathological factor that accelerates disease progression. However, this conclusion may be confounded by the detrimental effects of excessive lipolysis. Recent research also suggests that such activation may play a beneficial role in the early stages of chronic wasting disease and provide potential therapeutic effects. A more comprehensive understanding of the changes in adipose tissue thermogenesis under physiological and pathological conditions, as well as the underlying regulatory mechanisms, is essential for the development of novel interventions to improve health and prevent disease.

Cold-induced FOXO1 nuclear transport aids cold survival and tissue storage

Cold-induced injuries severely limit opportunities and outcomes of hypothermic therapies and organ preservation, calling for better understanding of cold adaptation. Here, by surveying cold-altered chromatin accessibility and integrated CUT&Tag/RNA-seq analyses in human stem cells, we reveal forkhead box O1 (FOXO1) as a key transcription factor for autonomous cold adaptation. Accordingly, we find a nonconventional, temperature-sensitive FOXO1 transport mechanism involving the nuclear pore complex protein RANBP2, SUMO-modification of transporter proteins Importin-7 and Exportin-1, and a SUMO-interacting motif on FOXO1. Our conclusions are supported by cold survival experiments with human cell models and zebrafish larvae. Promoting FOXO1 nuclear entry by the Exportin-1 inhibitor KPT-330 enhances cold tolerance in pre-diabetic obese mice, and greatly prolongs the shelf-life of human and mouse pancreatic tissues and islets. Transplantation of mouse islets cold-stored for 14 days reestablishes normoglycemia in diabetic mice. Our findings uncover a regulatory network and potential therapeutic targets to boost spontaneous cold adaptation.

ACSS2 controls PPARγ activity homeostasis to potentiate adipose-tissue plasticity

The appropriate transcriptional activity of PPARγ is indispensable for controlling inflammation, tumor and obesity. Therefore, the identification of key switch that couples PPARγ activation with degradation to sustain its activity homeostasis is extremely important. Unexpectedly, we here show that acetyl-CoA synthetase short-chain family member 2 (ACSS2) critically controls PPARγ activity homeostasis via SIRT1 to enhance adipose plasticity via promoting white adipose tissues beiging and brown adipose tissues thermogenesis. Mechanistically, ACSS2 binds directly acetylated PPARγ in the presence of ligand and recruits SIRT1 and PRDM16 to activate UCP1 expression. In turn, SIRT1 triggers ACSS2 translocation from deacetylated PPARγ to P300 and thereafter induces PPARγ polyubiquitination and degradation. Interestingly, D-mannose rapidly activates ACSS2-PPARγ-UCP1 axis to resist high fat diet induced obesity in mice. We thus reveal a novel ACSS2 function in coupling PPARγ activation with degradation via SIRT1 and suggest D-mannose as a novel adipose plasticity regulator via ACSS2 to prevent 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.

Bone Morphogenetic Protein 2 Enhances Porcine Beige Adipogenesis via AKT/mTOR and MAPK Signaling Pathways

Bone morphogenetic protein 2 (BMP2) has been reported to regulate adipogenesis, but its role in porcine beige adipocyte formation remains unclear. Our data reveal that BMP2 is significantly induced at the early stages of porcine beige adipocyte differentiation. Additionally, supplementing rhBMP2 during the early stages, but not the late stages of differentiation, significantly enhances porcine SVF adipogenesis, thermogenesis, and proliferation. Furthermore, compared to the empty plasmid-transfected-SVFs, BMP2-overexpressed SVFs had the enhanced lipid accumulation and thermogenesis, while knockdown of BMP2 in SVFs exhibited the opposite effect. The RNA-seq of the above three types of cells revealed the enrichment of the annotation of thermogenesis, brown cell differentiation, etc. In addition, the analysis also highlights the significant enrichment of cell adhesion, the MAPK cascade, and PPARγ signaling. Mechanistically, BMP2 positively regulates the adipogenic and thermogenic capacities of porcine beige adipocytes by activating PPARγ expression through AKT/mTOR and MAPK signaling pathways.

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.

Human cold adaptation: An unfinished agenda v2.0

BACKGROUND

Research on human extreme cold climate adaptations has benefitted from a recent resurgence since Ted Steegmann laid out his Human Cold Adaptation Agenda in 2007. Human biologists have drastically expanded our knowledge in this area during the last 15 years, but we still have a great deal more work to do to fulfill the cold climate adaptation agenda.

METHODS

Here, I follow Steegmann's example by providing a review of cold climate adaptations and setting forth a new, expanded agenda.

RESULTS

I review the foundational work on cold climate adaptations including classic Bergmann, Allen, and Thomson rules as well as early work assessing metabolic differences among Indigenous cold climate populations. From there, I discuss some of the groundbreaking work currently taking place on cold climate adaptations such as brown adipose tissue (a heat generating organ), physical activity levels, metabolic rates, and behavioral/cultural mechanisms. Finally, I present a path forward for future research with a focus on some of the basic extreme cold adaptations as well as how human biologists should approach the effects of climate change on human health and well-being, particularly within a cold climate context.

CONCLUSION

The Arctic has felt the dramatic effects of climate change sooner and more acutely than other parts of the world, making it an ideal location for studying both cold climate adaptations and climate change resilience. Human biologists have a great deal to contribute to the conversation on not only adaptations to extreme cold, but also the ways in which climate change is being embodied by cold climate populations.

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.

Obesity causes mitochondrial fragmentation and dysfunction in white adipocytes due to RalA activation

Mitochondrial dysfunction is a characteristic trait of human and rodent obesity, insulin resistance and fatty liver disease. Here we show that high-fat diet (HFD) feeding causes mitochondrial fragmentation in inguinal white adipocytes from male mice, leading to reduced oxidative capacity by a process dependent on the small GTPase RalA. RalA expression and activity are increased in white adipocytes after HFD. Targeted deletion of RalA in white adipocytes prevents fragmentation of mitochondria and diminishes HFD-induced weight gain by increasing fatty acid oxidation. Mechanistically, RalA increases fission in adipocytes by reversing the inhibitory Ser637 phosphorylation of the fission protein Drp1, leading to more mitochondrial fragmentation. Adipose tissue expression of the human homolog of Drp1, DNM1L, is positively correlated with obesity and insulin resistance. Thus, chronic activation of RalA plays a key role in repressing energy expenditure in obese adipose tissue by shifting the balance of mitochondrial dynamics toward excessive fission, contributing to weight gain and metabolic dysfunction.

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.

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.

Additive impact of soy protein dietary intake and exercise on visceral fat mass reduction and mitochondrial complex I activation in skeletal muscle

Soy protein has shown remarkable effectiveness in reducing fat mass compared with other protein sources, and exercise has the potential to further enhance this fat loss effect. Previous studies have demonstrated that soy protein intake leads to decreased fatty acid synthesis, which contributes to its fat-loss properties. However, the exact mechanism by which these lipids are consumed remains unclear. To investigate this, we conducted a comprehensive study using C57/BL6 male mice, comparing the effects of soy and casein proteins with and without exercise (Casein-Sed, Casein-Ex, Soy-Sed, and Soy-Ex groups) under high- and low-protein conditions (14% or 40% protein). Our findings revealed that combining soy protein intake with exercise significantly reduced epididymal white adipose tissue (eWAT) weight, particularly in the high-protein diet group. Further analysis revealed that exercise increased the expression of lipid oxidation-regulatory proteins, including mitochondrial oxidative phosphorylation protein (OXPHOS) complexes, in the plantaris muscle regardless of the protein source. Although soy protein intake did not directly affect muscle mitochondrial protein expression, the activity of OXPHOS complex I was additively enhanced by exercise and soy protein under the 40% protein condition. Notably, complex I activity inversely correlated with eWAT weight in the soy protein diet group. These results highlight the potential link between improved complex I activity induced by soy protein and fat mass reduction, which emphasizes the promising benefits of combining soy protein with exercise in promoting fat loss.NEW & NOTEWORTHY The findings revealed that soy protein intake combined with exercise resulted in reduced adipose tissue weight compared with that obtained with casein protein intake. Furthermore, the joint impact of exercise and soy protein consumption resulted in enhanced activity of oxidative phosphorylation protein (OXPHOS) complex I in fast-twitch muscles, which appears to be associated with fat mass reduction. These findings elucidate the potential additive effects of soy protein and exercise on body weight management.

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).

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.

Adipose tissue browning and thermogenesis under physiologically energetic challenges: a remodelled thermogenic system

Metabolic diseases such as obesity and diabetes are often thought to be caused by reduced energy expenditure, which poses a serious threat to human health. Cold exposure, exercise and caloric restriction have been shown to promote adipose tissue browning and thermogenesis. These physiological interventions increase energy expenditure and thus have emerged as promising strategies for mitigating metabolic disorders. However, that increased adipose tissue browning and thermogenesis elevate thermogenic consumption is not a reasonable explanation when humans and animals confront energetic challenges imposed by these interventions. In this review, we collected numerous results on adipose tissue browning and whitening and evaluated this bi-directional conversion of adipocytes from the perspective of energy homeostasis. Here, we propose a new interpretation of the role of adipose tissue browning under energetic challenges: increased adipose tissue browning and thermogenesis under energy challenge is not to enhance energy expenditure, but to reestablish a more economical thermogenic pattern to maintain the core body temperature. This can be achieved by enhancing the contribution of non-shivering thermogenesis (adipose tissue browning and thermogenesis) and lowering shivering thermogenesis and high intensity shivering. Consequently, the proportion of heat production in fat increases and that in skeletal muscle decreases, enabling skeletal muscle to devote more energy reserves to overcoming environmental stress.

Candidate biomarkers in brown adipose tissue for post-mortem diagnosis of fatal hypothermia

Post-mortem diagnosis of fatal hypothermia (FHT) is challenging in forensic practice because traditional morphological and biochemical methods lack specificity. Recent studies have reported that brown adipose tissue (BAT) is activated during cold-induced non-shivering thermogenesis in mammals, but BAT has not been used to diagnose FHT. The aim of this study was to identify novel biomarkers in BAT for FHT based on morphological changes and differential protein expression. Two FHT animal models were created by exposing mice to 4 or -20 °C at 50% humidity. Morphologically, the unilocular lipid droplet content was significantly increased in BAT of FHT model mice compared with that of control mice. Proteomics analysis revealed a total of 283 and 266 differentially expressed proteins (DEPs) between the 4 or -20 °C FHT subgroups and control group, respectively. In addition, 140 proteins were shared between the FHT subgroups. GO and KEGG analyses revealed that the shared DEPs were mainly enriched in pathways associated with metabolism, oxidative phosphorylation, and thermogenesis. Further screening (|log2FC| > 1.6, q-value (FDR) < 0.05) identified GMFB, KDM1A, DDX6, RAB1B, SHMT-1, CLPTM1, and LMF1 as candidate biomarkers of FHT. Subsequent validation experiments were performed in FHT model mice using classic immunohistochemistry and western blotting. RAB1B and GMFB expression was further verified in BAT specimens from human cases of FHT. The results demonstrate that BAT can be used as a target organ for FHT diagnosis employing RAB1B and GMFB as biological markers, thus providing a new strategy for the post-mortem diagnosis of FHT in forensic practice.

Brown adipose tissue-derived metabolites and their role in regulating metabolism

The discovery and rejuvenation of metabolically active brown adipose tissue (BAT) in adult humans have offered a new approach to treat obesity and metabolic diseases. Beyond its accomplished role in adaptive thermogenesis, BAT secretes signaling molecules known as "batokines", which are instrumental in regulating whole-body metabolism via autocrine, paracrine, and endocrine action. In addition to the intrinsic BAT metabolite-oxidizing activity, the endocrine functions of these molecules may help to explain the association between BAT activity and a healthy systemic metabolic profile. Herein, we review the evidence that underscores the significance of BAT-derived metabolites, especially highlighting their role in controlling physiological and metabolic processes involving thermogenesis, substrate metabolism, and other essential biological processes. The conversation extends to their capacity to enhance energy expenditure and mitigate features of obesity and its related metabolic complications. Thus, metabolites derived from BAT may provide new avenues for the discovery of metabolic health-promoting drugs with far-reaching impacts. This review aims to dissect the complexities of the secretory role of BAT in modulating local and systemic metabolism in metabolic health and disease.

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.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

A new era of understanding in vivo metabolic flux in thermogenic adipocytes

Nonshivering thermogenesis by brown adipose tissue (BAT) is an adaptive mechanism for maintaining body temperature in cold environments. BAT is critical in rodents and human infants and has substantial influence on adult human metabolism. Stimulating BAT therapeutically is also being investigated as a strategy against metabolic diseases because of its ability to function as a catabolic sink. Thus, understanding how brown adipocytes and the related brite/beige adipocytes use nutrients to fuel their demanding metabolism has both basic and translational implications. Recent advances in mass spectrometry and isotope tracing are improving the ability to study metabolic flux in vivo. Here, we review how such strategies are advancing our understanding of adipocyte thermogenesis and conclude with key future questions.

Update on Adipose Tissue and Cancer

Adipose tissue is the largest endocrine organ and an accepted contributor to overall energy homeostasis. There is strong evidence linking increased adiposity to the development of 13 types of cancer. With increased adiposity comes metabolic dysfunction and insulin resistance, and increased systemic insulin and glucose support the growth of many cancers, including those of the colon and endometrium. There is also an important direct crosstalk between adipose tissue and various organs. For instance, the healthy development and function of the mammary gland, as well as the development, growth, and progression of breast cancer, are heavily impacted by the breast adipose tissue in which breast epithelial cells are embedded. Cells of the adipose tissue are responsive to external stimuli, including overfeeding, leading to remodeling and important changes in the secretion of factors known to drive the development and growth of cancers. Loss of factors like adiponectin and increased production of leptin, endotrophin, steroid hormones, and inflammatory mediators have been determined to be important mediators of the obesity-cancer link. Obesity is also associated with a structural remodeling of the adipose tissue, including increased localized fibrosis and disrupted angiogenesis that contribute to the development and progression of cancers. Furthermore, tumor cells feed off the adipose tissue, where increased lipolysis within adipocytes leads to the release of fatty acids and stromal cell aerobic glycolysis leading to the increased production of lactate. Both have been hypothesized to support the higher energetic demands of cancer cells. Here, we aim to provide an update on the state of the literature revolving around the role of the adipose tissue in cancer initiation and progression.

Comprehensive quantification of metabolic flux during acute cold stress in mice

Cold-induced thermogenesis (CIT) is widely studied as a potential avenue to treat obesity, but a thorough understanding of the metabolic changes driving CIT is lacking. Here, we present a comprehensive and quantitative analysis of the metabolic response to acute cold exposure, leveraging metabolomic profiling and minimally perturbative isotope tracing studies in unanesthetized mice. During cold exposure, brown adipose tissue (BAT) primarily fueled the tricarboxylic acid (TCA) cycle with fat in fasted mice and glucose in fed mice, underscoring BAT's metabolic flexibility. BAT minimally used branched-chain amino acids or ketones, which were instead avidly consumed by muscle during cold exposure. Surprisingly, isotopic labeling analyses revealed that BAT uses glucose largely for TCA anaplerosis via pyruvate carboxylation. Finally, we find that cold-induced hepatic gluconeogenesis is critical for CIT during fasting, demonstrating a key functional role for glucose metabolism. Together, these findings provide a detailed map of the metabolic rewiring driving acute CIT.

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.

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.

Lower Prevalence of Body Fat-Defined Obesity at Higher Altitudes in Peruvian Adults

Woolcott, Orison O., Till Seuring, and Oscar A. Castillo. Lower prevalence of body fat-defined obesity at higher altitudes in Peruvian adults. High Alt Med Biol. 24:214-222, 2023. Background: Previous studies have reported a lower prevalence of obesity (defined as a body mass index [BMI] ≥30 kg/m2) in populations from higher altitudes. Since BMI does not distinguish fat mass and fat-free mass, it is unclear whether there is an inverse association between altitude and body fat-defined obesity. Methods: We performed an analysis of cross-sectional data to examine the association between altitude and body fat-defined obesity (as opposed to BMI-defined obesity) using individual-level data from a nationally representative sample of the Peruvian adult population living between 0 and 5,400 m altitude. Body fat-defined obesity was diagnosed using the relative fat mass (RFM), an anthropometric index validated to estimate whole-body fat percentage. RFM cutoffs for obesity diagnosis were ≥40% for women and ≥30% for men. We utilized Poisson regression to estimate the prevalence ratio and confidence intervals (CIs) as the measure of the association, adjusting for age, cigarette use, and diabetes. Results: Analysis comprised 36,727 individuals (median age, 39 years; 50.1% women). In rural areas, for a one-km increase in altitude, the prevalence of body fat-defined obesity decreased by 12% among women (adjusted prevalence ratio: 0.88; 95% CI, 0.86 - 0.90; p < 0.001) and 19% among men (adjusted prevalence ratio: 0.81; 95% CI, 0.77 - 0.86; p < 0.001), on average, when all the other variables were held constant. The inverse association between altitude and obesity was less strong in urban areas than in rural areas but remained significant among women (p = 0.001) and men (p < 0.001). However, the relationship between altitude and obesity in women who live in urban areas appears to be nonlinear. Conclusions: In Peruvian adults, the prevalence of body fat-defined obesity was inversely associated with altitude. Whether this inverse association is explained by altitude per se or confounded by socioeconomic or other environmental factors, or differences in race/ethnicity or lifestyle, warrants further investigation.

Amino acids contribute to adaptive thermogenesis. New insights into the mechanisms of action of recent drugs for metabolic disorders are emerging

Adaptive thermogenesis is the heat production by muscle contractions (shivering thermogenesis) or brown adipose tissue (BAT) and beige fat (non-shivering thermogenesis) in response to external stimuli, including cold exposure. BAT and beige fat communicate with peripheral organs and the brain through a variegate secretory and absorption processes - controlling adipokines, microRNAs, extracellular vesicles, and metabolites - and have received much attention as potential therapeutic targets for managing obesity-related disorders. The sympathetic nervous system and norepinephrine-releasing adipose tissue macrophages (ATM) activate uncoupling protein 1 (UCP1), expressed explicitly in brown and beige adipocytes, dissolving the electrochemical gradient and uncoupling tricarboxylic acid cycle and the electron transport chain from ATP production. Mounting evidence has attracted attention to the multiple effects of dietary and endogenously synthesised amino acids in BAT thermogenesis and metabolic phenotype in animals and humans. However, the mechanisms implicated in these processes have yet to be conclusively characterized. In the present review article, we aim to define the principal investigation areas in this context, including intestinal microbiota constitution, adipose autophagy modulation, and secretome and metabolic fluxes control, which lead to increased brown/beige thermogenesis. Finally, also based on our recent epicardial adipose tissue results, we summarise the evidence supporting the notion that the new dual and triple agonists of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon (GCG) receptor - with never before seen weight loss and insulin-sensitizing efficacy - promote thermogenic-like amino acid profiles in BAT with robust heat production and likely trigger sympathetic activation and adaptive thermogenesis by controlling amino acid metabolism and ATM expansion in BAT and beige fat.

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.

Latest advances in the regulatory genes of adipocyte thermogenesis

An energy imbalance cause obesity: more energy intake or less energy expenditure, or both. Obesity could be the origin of many metabolic disorders, such as type 2 diabetes and cardiovascular disease. UCP1 (uncoupling protein1), which is highly and exclusively expressed in the thermogenic adipocytes, including beige and brown adipocytes, can dissipate proton motive force into heat without producing ATP to increase energy expenditure. It is an attractive strategy to combat obesity and its related metabolic disorders by increasing non-shivering adipocyte thermogenesis. Adipocyte thermogenesis has recently been reported to be regulated by several new genes. This work provided novel and potential targets to activate adipocyte thermogenesis and resist obesity, such as secreted proteins ADISSP and EMC10, enzyme SSU72, etc. In this review, we have summarized the latest research on adipocyte thermogenesis regulation to shed more light on this topic.

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.

Estimated energetic demands of thermoregulation during ancient canoe passages from Tahiti to Hawaii and New Zealand, a simulation analysis

Prehistoric colonization of East Polynesia represents the last and most extensive of human migrations into regions previously uninhabited. Although much of East Polynesia is tropical, the southern third, dominated by New Zealand-by far the largest Polynesian landmass-ranges from a warm- to cool-temperate climate with some islands extending into the Subantarctic. The substantial latitudinal variation implies questions about biocultural adaptations of tropical people to conditions in which most of their familiar resources were absent and their agriculture marginal. Perhaps the most basic question, but one which has never been explored, is the extent to which sailing out of the tropics on long-distance colonizing voyages imposed physiological stress on canoe crews and passengers. In this paper we use trajectories of simulated voyages from Tahiti to New Zealand and Tahiti to Hawaii to obtain along-trip environmental parameters which are then used to model the energy expenditure of these long overseas journeys. Results show that travelers to New Zealand are exposed to much harsher environmental conditions, leading to significantly greater in-trip thermoregulatory demands. For both destinations, travelers with larger body sizes exhibit lower modeled heat loss and hence obtain an energetic advantage, with greater gains for females. Such physiological features, notably of Samoans who probably formed the founding population in East Polynesia, may help explain successful voyaging to temperate latitudes.

Increasing adipocyte number and reducing adipocyte size: the role of retinoids in adipose tissue development and metabolism

The rising prevalence of obesity is a grave public health threat. In response to excessive energy intake, adipocyte hypertrophy impairs cellular function and leads to metabolic dysfunctions while de novo adipogenesis leads to healthy adipose tissue expansion. Through burning fatty acids and glucose, the thermogenic activity of brown/beige adipocytes can effectively reduce the size of adipocytes. Recent studies show that retinoids, especially retinoic acid (RA), promote adipose vascular development which in turn increases the number of adipose progenitors surrounding the vascular vessels. RA also promotes preadipocyte commitment. In addition, RA promotes white adipocyte browning and stimulates the thermogenic activity of brown/beige adipocytes. Thus, vitamin A is a promising anti-obesity micronutrient.

Quantitative analysis of metabolic fluxes in brown fat and skeletal muscle during thermogenesis

Adaptive thermogenesis by brown adipose tissue (BAT) dissipates calories as heat, making it an attractive anti-obesity target. Yet how BAT contributes to circulating metabolite exchange remains unclear. Here, we quantified metabolite exchange in BAT and skeletal muscle by arteriovenous metabolomics during cold exposure in fed male mice. This identified unexpected metabolites consumed, released and shared between organs. Quantitative analysis of tissue fluxes showed that glucose and lactate provide ~85% of carbon for adaptive thermogenesis and that cold and CL316,243 trigger markedly divergent fuel utilization profiles. In cold adaptation, BAT also dramatically increases nitrogen uptake by net consuming amino acids, except glutamine. Isotope tracing and functional studies suggest glutamine catabolism concurrent with synthesis via glutamine synthetase, which avoids ammonia buildup and boosts fuel oxidation. These data underscore the ability of BAT to function as a glucose and amino acid sink and provide a quantitative and comprehensive landscape of BAT fuel utilization to guide translational studies.

High Fat Diet Suppresses Energy Expenditure Via Neurons in the Brainstem

Oxidation of fat by brown adipose tissue (BAT) contributes to energy balance and heat production. During cold exposure, BAT thermogenesis produces heat to warm the body. Obese subjects and rodents, however, show impaired BAT thermogenesis to the cold. Our previous studies suggest that vagal afferents synapsing in the nucleus tractus solitarius (NTS), tonically inhibit BAT thermogenesis to the cold in obese rats. NTS neurons send projections to the dorsal aspect of the lateral parabrachial nucleus (LPBd), which is a major integrative center that receives warm afferent inputs from the periphery and promotes inhibition of BAT thermogenesis. This study investigated the contribution of LPBd neurons in the impairment of BAT thermogenesis in rats fed a high-fat diet (HFD). By using a targeted dual viral vector approach, we found that chemogenetic activation of an NTS-LPB pathway inhibited BAT thermogenesis to the cold. We also found that the number of Fos-labelled neurons in the LPBd was higher in rats fed a HFD than in chow diet-fed rats after exposure to a cold ambient temperature. Nanoinjections of a GABAA receptor agonist into the LPBd area rescued BAT thermogenesis to the cold in HFD rats. These data reveal the LPBd as a critical brain area that tonically suppresses energy expenditure in obesity during skin cooling. These findings reveal novel effects of high-fat diets in the brain and in the control of metabolism and can contribute to the development of therapeutic approaches to regulate fat metabolism.

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.

m 6 A mRNA Methylation in Brown Adipose Tissue Regulates Systemic Insulin Sensitivity via an Inter-Organ Prostaglandin Signaling Axis

Brown adipose tissue (BAT) has the capacity to regulate systemic metabolism through the secretion of signaling lipids. N6-methyladenosine (m 6 A) is the most prevalent and abundant post-transcriptional mRNA modification and has been reported to regulate BAT adipogenesis and energy expenditure. In this study, we demonstrate that the absence of m 6 A methyltransferase-like 14 (METTL14), modifies the BAT secretome to initiate inter-organ communication to improve systemic insulin sensitivity. Importantly, these phenotypes are independent of UCP1-mediated energy expenditure and thermogenesis. Using lipidomics, we identified prostaglandin E2 (PGE2) and prostaglandin F2a (PGF2a) as M14 KO -BAT-secreted insulin sensitizers. Notably, circulatory PGE2 and PGF2a levels are inversely correlated with insulin sensitivity in humans. Furthermore, in vivo administration of PGE2 and PGF2a in high-fat diet-induced insulin-resistant obese mice recapitulates the phenotypes of METTL14 deficient animals. PGE2 or PGF2a improves insulin signaling by suppressing the expression of specific AKT phosphatases. Mechanistically, METTL14-mediated m 6 A installation promotes decay of transcripts encoding prostaglandin synthases and their regulators in human and mouse brown adipocytes in a YTHDF2/3-dependent manner. Taken together, these findings reveal a novel biological mechanism through which m 6 A-dependent regulation of BAT secretome regulates systemic insulin sensitivity in mice and humans.

Highlights

Mettl14 KO -BAT improves systemic insulin sensitivity via inter-organ communication; PGE2 and PGF2a are BAT-secreted insulin sensitizers and browning inducers;PGE2 and PGF2a sensitize insulin responses through PGE2-EP-pAKT and PGF2a-FP-AKT axis; METTL14-mediated m 6 A installation selectively destabilizes prostaglandin synthases and their regulator transcripts; Targeting METTL14 in BAT has therapeutic potential to enhance systemic insulin sensitivity.

Abstract Figure

Physiological and pathological roles of lipogenesis

Lipids are essential metabolites, which function as energy sources, structural components and signalling mediators. Most cells are able to convert carbohydrates into fatty acids, which are often converted into neutral lipids for storage in the form of lipid droplets. Accumulating evidence suggests that lipogenesis plays a crucial role not only in metabolic tissues for systemic energy homoeostasis but also in immune and nervous systems for their proliferation, differentiation and even pathophysiological roles. Thus, excessive or insufficient lipogenesis is closely associated with aberrations in lipid homoeostasis, potentially leading to pathological consequences, such as dyslipidaemia, diabetes, fatty liver, autoimmune diseases, neurodegenerative diseases and cancers. For systemic energy homoeostasis, multiple enzymes involved in lipogenesis are tightly controlled by transcriptional and post-translational modifications. In this Review, we discuss recent findings regarding the regulatory mechanisms, physiological roles and pathological importance of lipogenesis in multiple tissues such as adipose tissue and the liver, as well as the immune and nervous systems. Furthermore, we briefly introduce the therapeutic implications of lipogenesis modulation.

Rats lacking Ucp1 present a novel translational tool for the investigation of thermogenic adaptation during cold challenge

AIM

Valuable studies have tested the role of UCP1 on body temperature maintenance in mice, and we sought to knockout Ucp1 in rats (Ucp1-/- ) to provide insight into thermogenic mechanisms in larger mammals.

METHODS

We used CRISPR/Cas9 technology to create Ucp1-/- rats. Body weight and adiposity were measured, and rats were subjected to indirect calorimetry. Rats were maintained at room temperature or exposed to 4°C for either 24 h or 14 days. Analyses of brown and white adipose tissue and skeletal muscle were conducted via histology, western blot comparison of oxidative phosphorylation proteins, and qPCR to compare mitochondrial DNA levels and mRNA expression profiles. RNA-seq was performed in skeletal muscle.

RESULTS

Ucp1-/- rats withstood 4°C for 14 days, but core temperature steadily declined. All rats lost body weight after 14 days at 4°C, but controls increased food intake more robustly than Ucp1-/- rats. Brown adipose tissue showed signs of decreased activity in Ucp1-/- rats, while mitochondrial lipid metabolism markers in white adipose tissue and skeletal muscle were increased. Ucp1-/- rats displayed more visible shivering and energy expenditure than controls at 4°C. Skeletal muscle transcriptomics showed more differences between genotypes at 23°C than at 4°C.

CONCLUSION

Room temperature presented sufficient cold stress to rats lacking UCP1 to activate compensatory thermogenic mechanisms in skeletal muscle, which were only activated in control rats following exposure to 4°C. These results provide novel insight into thermogenic responses to UCP1 deficiency; and highlight Ucp1-/- rats as an attractive translational model for the study of thermogenesis.

CD36 regulates substrates utilisation in brown adipose tissue of spontaneously hypertensive rats: In vitro study

Thermogenesis in brown adipose tissue (BAT) uses intracellular triglycerides, circulating free fatty acids and glucose as the main substrates. The objective of the current study was to analyse the role of CD36 fatty acid translocase in regulation of glucose and fatty acid utilisation in BAT. BAT isolated from spontaneously hypertensive rat (SHR) with mutant Cd36 gene and SHR-Cd36 transgenic rats with wild type variant was incubated in media containing labeled glucose and palmitate to measure substrate incorporation and oxidation. SHR-Cd36 versus SHR rats showed significantly increased glucose incorporation into intracellular lipids associated with reduced glycogen synthase kinase 3β (GSK-3β) protein expression and phosphorylation and increased oxidation of exogenous palmitate. It can be concluded that CD36 enhances glucose transport for lipogenesis in BAT by suppressing GSK-3β and promotes direct palmitate oxidation.

Thermogenic adipose tissue in energy regulation and metabolic health

The ability to generate thermogenic fat could be a targeted therapy to thwart obesity and improve metabolic health. Brown and beige adipocytes are two types of thermogenic fat cells that regulate energy balance. Both adipocytes share common morphological, biochemical, and thermogenic properties. Yet, recent evidence suggests unique features exist between brown and beige adipocytes, such as their cellular origin and thermogenic regulatory processes. Beige adipocytes also appear highly plastic, responding to environmental stimuli and interconverting between beige and white adipocyte states. Additionally, beige adipocytes appear to be metabolically heterogenic and have substrate specificity. Nevertheless, obese and aged individuals cannot develop beige adipocytes in response to thermogenic fat-inducers, creating a key clinical hurdle to their therapeutic promise. Thus, elucidating the underlying developmental, molecular, and functional mechanisms that govern thermogenic fat cells will improve our understanding of systemic energy regulation and strive for new targeted therapies to generate thermogenic fat. This review will examine the recent advances in thermogenic fat biogenesis, molecular regulation, and the potential mechanisms for their failure.

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.

The energetics of childhood: Current knowledge and insights into human variation, evolution, and health

How organisms capture and ultimately use metabolic energy-a limiting resource of life-has profound implications for understanding evolutionary legacies and current patterns of phenotypic variation, adaptation, and health. Energetics research among humans has a rich history in biological anthropology and beyond. The energetics of childhood, however, remains relatively underexplored. This shortcoming is notable given the accepted importance of childhood in the evolution of the unique human life history pattern as well as the known sensitivity of childhood development to local environments and lived experiences. In this review, I have three objectives: (1) To overview current knowledge regarding how children acquire and use energy, highlighting work among diverse human populations and pointing to recent advances and remaining areas of uncertainty; (2) To discuss key applications of this knowledge for understanding human variation, evolution, and health; (3) To recommend future avenues for research. A growing body of evidence supports a model of trade-offs and constraint in childhood energy expenditure. This model, combined with advancements on topics such as the energetics of immune activity, the brain, and the gut, provides insights into the evolution of extended human subadulthood and the nature of variation in childhood development, lifetime phenotype, and health.

Neuroendocrine control of brown adipocyte function by prolactin and growth hormone

Growth hormone (GH) is fundamental for growth and glucose homeostasis, and prolactin for optimal pregnancy and lactation outcome, but additionally, both hormones have multiple functions that include a strong impact on energetic metabolism. In this respect, prolactin and GH receptors have been found in brown, and white adipocytes, as well as in hypothalamic centers regulating thermogenesis. This review describes the neuroendocrine control of the function and plasticity of brown and beige adipocytes, with a special focus on prolactin and GH actions. Most evidence points to a negative association between high prolactin levels and the thermogenic capacity of BAT, except in early development. During lactation and pregnancy, prolactin may be a contributing factor that limits unneeded thermogenesis, downregulating BAT UCP1. Furthermore, animal models of high serum prolactin have low BAT UCP1 levels and whitening of the tissue, while lack of Prlr induces beiging in WAT depots. These actions may involve hypothalamic nuclei, particularly the DMN, POA and ARN, brain centers that participate in thermogenesis. Studies on GH regulation of BAT function present some controversies. Most mouse models with GH excess or deficiency point to an inhibitory role of GH on BAT function. Even so, a stimulatory role of GH on WAT beiging has also been described, in accordance with whole-genome microarrays that demonstrate divergent response signatures of BAT and WAT genes to the loss of GH signaling. Understanding the physiology of BAT and WAT beiging may contribute to the ongoing efforts to curtail obesity.

Ssu72 phosphatase is essential for thermogenic adaptation by regulating cytosolic translation

Brown adipose tissue (BAT) plays a pivotal role in maintaining body temperature and energy homeostasis. BAT dysfunction is associated with impaired metabolic health. Here, we show that Ssu72 phosphatase is essential for mRNA translation of genes required for thermogenesis in BAT. Ssu72 is found to be highly expressed in BAT among adipose tissue depots, and the expression level of Ssu72 is increased upon acute cold exposure. Mice lacking adipocyte Ssu72 exhibit cold intolerance during acute cold exposure. Mechanistically, Ssu72 deficiency alters cytosolic mRNA translation program through hyperphosphorylation of eIF2α and reduces translation of mitochondrial oxidative phosphorylation (OXPHOS) subunits, resulting in mitochondrial dysfunction and defective thermogenesis in BAT. In addition, metabolic dysfunction in Ssu72-deficient BAT returns to almost normal after restoring Ssu72 expression. In summary, our findings demonstrate that cold-responsive Ssu72 phosphatase is involved in cytosolic translation of key thermogenic effectors via dephosphorylation of eIF2α in brown adipocytes, providing insights into metabolic benefits of Ssu72.