Assessment of brown adipose tissue function

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.

Differential responses to UCP1 ablation in classical brown versus beige fat, despite a parallel increase in sympathetic innervation

In the cold, the absence of the mitochondrial uncoupling protein 1 (UCP1) results in hyper-recruitment of beige fat, but classical brown fat becomes atrophied. Here we examine possible mechanisms underlying this phenomenon. We confirm that in brown fat from UCP1-knockout (UCP1-KO) mice acclimated to the cold, the levels of mitochondrial respiratory chain proteins were diminished; however, in beige fat, the mitochondria seemed to be unaffected. The macrophages that accumulated massively not only in brown fat but also in beige fat of the UCP1-KO mice acclimated to cold did not express tyrosine hydroxylase, the norepinephrine transporter (NET) and monoamine oxidase-A (MAO-A). Consequently, they could not influence the tissues through the synthesis or degradation of norepinephrine. Unexpectedly, in the cold, both brown and beige adipocytes from UCP1-KO mice acquired an ability to express MAO-A. Adipose tissue norepinephrine was exclusively of sympathetic origin, and sympathetic innervation significantly increased in both tissues of UCP1-KO mice. Importantly, the magnitude of sympathetic innervation and the expression levels of genes induced by adrenergic stimulation were much higher in brown fat. Therefore, we conclude that no qualitative differences in innervation or macrophage character could explain the contrasting reactions of brown versus beige adipose tissues to UCP1-ablation. Instead, these contrasting responses may be explained by quantitative differences in sympathetic innervation: the beige adipose depot from the UCP1-KO mice responded to cold acclimation in a canonical manner and displayed enhanced recruitment, while the atrophy of brown fat lacking UCP1 may be seen as a consequence of supraphysiological adrenergic stimulation in this tissue.

Time-restricted feeding restores metabolic flexibility in adult mice with excess adiposity

Introduction

Obesity is prevalent with the adult population in the United States. Energy-dense diets and erratic eating behavior contribute to obesity. Time-restricted eating is a dietary strategy in humans that has been advanced to reduce the propensity for obesity. We hypothesized that time-restricted feeding (TRF) would improve metabolic flexibility and normalize metabolic function in adult mice with established excess adiposity.

Methods

Male C57BL/6NHsd mice were initially fed a high-fat diet (HFD) for 12 weeks to establish excess body adiposity, while control mice were fed a normal diet. Then, the HFD-fed mice were assigned to two groups, either ad libitum HFD or TRF of the HFD in the dark phase (12 h) for another 12 weeks.

Results and discussion

Energy intake and body fat mass were similar in TRF and HFD-fed mice. TRF restored rhythmic oscillations of respiratory exchange ratio (RER), which had been flattened by the HFD, with greater RER amplitude in the dark phase. Insulin sensitivity was improved and plasma cholesterol and hepatic triacylglycerol were decreased by TRF. When compared to HFD, TRF decreased transcription of circadian genes Per1 and Per2 and genes encoding lipid metabolism (Acaca, Fads1, Fads2, Fasn, Scd1, and Srebf1) in liver. Metabolomic analysis showed that TRF created a profile that was distinct from those of mice fed the control diet or HFD, particularly in altered amino acid profiles. These included aminoacyl-tRNA-biosynthesis, glutathione metabolism, and phenylalanine, tyrosine, and tryptophan biosynthesis pathways. In conclusion, TRF improved metabolic function in adult mice with excess adiposity. This improvement was not through a reduction in body fat mass but through the restoration of metabolic flexibility.

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.

Highly recruited brown adipose tissue does not in itself protect against obesity

OBJECTIVE

The possibility to counteract the development of obesity in humans by recruiting brown or brite/beige adipose tissue (and thus UCP1) has attracted much attention. Here we examine if a diet that can activate diet-induced thermogenesis can exploit pre-enhanced amounts of UCP1 to counteract the development of diet-induced obesity.

METHODS

To investigate the anti-obesity significance of highly augmented amounts of UCP1 for control of body energy reserves, we physiologically increased total UCP1 amounts by recruitment of brown and brite/beige tissues in mice. We then examined the influence of the augmented UCP1 levels on metabolic parameters when the mice were exposed to a high-fat/high-sucrose diet under thermoneutral conditions.

RESULTS

The total UCP1 levels achieved were about 50-fold higher in recruited than in non-recruited mice. Contrary to underlying expectations, in the mice with highly recruited UCP1 and exposed to a high-fat/high-sucrose diet the thermogenic capacity of this UCP1 was completely inactivate. The mice even transiently (in an adipostat-like manner) demonstrated a higher metabolic efficiency and fat gain than did non-recruited mice. This was accomplished without altering energy expenditure or food absorption efficiency. The metabolic efficiency here was indistinguishable from that of mice totally devoid of UCP1.

CONCLUSIONS

Although UCP1 protein may be available, it is not inevitably utilized for diet-induced thermogenesis. Thus, although attempts to recruit UCP1 in humans may become successful as such, it is only if constant activation of the UCP1 is also achieved that amelioration of obesity development could be attained.

Kappa opioid receptor activation increases thermogenic energy expenditure which drives increased feeding

Opioid receptors, including the kappa opioid receptor (KOR), exert control over thermoregulation and feeding behavior. Notably, activation of KOR stimulates food intake, leading to postulation that KOR signaling plays a central role in managing energy intake. KOR has also been proposed as a target for treating obesity. Herein, we report studies examining how roles for KOR signaling in regulating thermogenesis, feeding, and energy balance may be interrelated using pharmacological interventions, genetic tools, quantitative thermal imaging, and metabolic profiling. Our findings demonstrate that activation of KOR in the central nervous system causes increased energy expenditure via brown adipose tissue activation. Importantly, pharmacologic, or genetic inhibition of brown adipose tissue thermogenesis prevented the elevated food intake triggered by KOR activation. Furthermore, our data reveal that KOR-mediated thermogenesis elevation is reversibly disrupted by chronic high-fat diet, implicating KOR signaling as a potential mediator in high-fat diet-induced weight gain.

Defective extracellular matrix remodeling in brown adipose tissue is associated with fibro-inflammation and reduced diet-induced thermogenesis

The relevance of extracellular matrix (ECM) remodeling is reported in white adipose tissue (AT) and obesity-related dysfunctions, but little is known about the importance of ECM remodeling in brown AT (BAT) function. Here, we show that a time course of high-fat diet (HFD) feeding progressively impairs diet-induced thermogenesis concomitantly with the development of fibro-inflammation in BAT. Higher markers of fibro-inflammation are associated with lower cold-induced BAT activity in humans. Similarly, when mice are housed at thermoneutrality, inactivated BAT features fibro-inflammation. We validate the pathophysiological relevance of BAT ECM remodeling in response to temperature challenges and HFD using a model of a primary defect in the collagen turnover mediated by partial ablation of the Pepd prolidase. Pepd-heterozygous mice display exacerbated dysfunction and BAT fibro-inflammation at thermoneutrality and in HFD. Our findings show the relevance of ECM remodeling in BAT activation and provide a mechanism for BAT dysfunction in obesity.

Glucagon and energy expenditure; Revisiting amino acid metabolism and implications for weight loss therapy

Glucagon receptor (GCGR)-targeted multi-agonists are being developed for the treatment of obesity and metabolic disease. GCGR activity is utilised for its favourable weight loss and metabolic properties, including increased energy expenditure (EE) and hepatic lipid metabolism. GLP1R and GIPR activities are increasingly present in a multi-agonist strategy. Due to the compound effect of increased satiety, reduced food intake and increased energy expenditure, the striking weight loss effects of these multi-agonists has been demonstrated in pre-clinical models of obesity. The precise contribution and mechanism of GCGR activity to enhanced energy expenditure and weight loss in both rodents and humans is not fully understood. In this review, our understanding of glucagon-mediated EE is explored, and an amino acid-centric paradigm contributing to this phenomenon is presented. The current progress of GCGR-targeted multi-agonists in development is also highlighted with a focus on the implications of glucagon-stimulated hypoaminoacidemia.

Astrocytic circadian clock control of energy expenditure by transcriptional stress responses in the ventromedial hypothalamus

Hypothalamic circuits compute systemic information to control metabolism. Astrocytes residing within the hypothalamus directly sense nutrients and hormones, integrating metabolic information, and modulating neuronal responses. Nevertheless, the role of the astrocytic circadian clock on the control of energy balance remains unclear. We used mice with a targeted ablation of the core-clock gene Bmal1 within Gfap-expressing astrocytes to gain insight on the role played by this transcription factor in astrocytes. While this mutation does not substantially affect the phenotype in mice fed normo-caloric diet, under high-fat diet we unmasked a thermogenic phenotype consisting of increased energy expenditure, and catabolism in brown adipose and overall metabolic improvement consisting of better glycemia control, and body composition. Transcriptomic analysis in the ventromedial hypothalamus revealed an enhanced response to moderate cellular stress, including ER-stress response, unfolded protein response and autophagy. We identified Xbp1 and Atf1 as two key transcription factors enhancing cellular stress responses. Therefore, we unveiled a previously unknown role of the astrocytic circadian clock modulating energy balance through the regulation of cellular stress responses within the VMH.

Effects of Poria cocos extract and protein powder mixture on glucolipid metabolism and rhythm changes in obese mice

Herein, we explored the effects of Poria cocos extract, protein powder mixture, and their combined intervention on weight loss in high-fat diet (HFD)-induced obese mice. Male C57BL/6J mice were selected and fed a HFD for 8 weeks; obese mice that were successfully modeled were divided into modeling and five intervention groups, and given the corresponding treatment for 10 weeks. Body weight, fat, and muscle tissue, blood glucose, lipids, inflammatory factors, and other glucose and lipid metabolism-related indicators were measured to evaluate the effect of P. cocos and protein powder intervention on weight loss in obese mice. The body weight of the intervention group was reduced compared with the HFD group. Fat content of mice in F3PM group decreased significantly (p < .05). Levels of blood glucose, lipids, adiponectin, leptin, and inflammatory factors, including interleukin-1 β and tumor necrosis factor- α showed improvement. Lipoprotein lipase (lower about 2.97 pg/ml, vs. HFD mice 10.65 mmoL/ml) and sterol regulatory element-binding transcription factor (lower about 1413.63 pg/ml, vs. HFD mice 3915.33 pg/ml) levels in liver tissue were decreased. The respiratory exchange rate (RER) of mice in the HFD and subject intervention groups had no circadian rhythm and was maintained at approximately 0.80. The protein powder mixture (PM) group had the lowest RER (p < .05), the P. cocos extract (FL) and F1PM groups had similar RER to the HFD group (p < .05), and the F2PM group had a higher RER than the HFD group (p < .05). And food intake and energy metabolism returned to circadian rhythm, with an increase in the dose of P. cocos extract, the feeding rhythms of F1PM, F2PM, and F3PM were closer to that of the normal diet (ND) group. Feeding intervention with P. cocos and protein powder improved fat distribution, glucolipid metabolism, and energy metabolism, with the combination of F3PM showing more diverse benefits.

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.

Repeated short excursions from thermoneutrality suffice to restructure brown adipose tissue

Given the presence of brown adipose tissue in adult humans, an important issue is whether human brown adipose tissue is recruitable. Cold exposure is the canonical recruitment treatment; however, in experimental animals (mice), recruitment of brown adipose tissue is normally induced by placing the mice in constant cold, a procedure not feasible in humans. For possible translational applications, we have therefore investigated whether shorter daily excursions from thermoneutrality would suffice to qualitatively and quantitatively induce recruitment in mice. Mice, housed at thermoneutrality (30 °C) to mimic human conditions, were transferred every day for 4 weeks to cool conditions (18 °C), for 0, 15, 30, 120 and 420 min (or placed constantly in 18 °C). On the examination day, the mice were not exposed to cold. Very short daily exposures (≤30 min) were sufficient to induce structural changes in the form of higher protein density in brown adipose tissue, changes that may affect the identification of the tissue in e.g. computer tomography and other scan studies. To estimate thermogenic capacity, UCP1 protein levels were followed. No UCP1 protein was detectable in inguinal white adipose tissue. In the interscapular brown adipose tissue, a remarkable two-phase reaction was seen. Very short daily exposures (≤30 min) were sufficient to induce a significant increase in total UCP1 levels. For attainment of full cold acclimation, the mice had, however, to remain exposed to the cold. The studies indicate that marked alterations in brown adipose tissue composition can be induced in mammals through relatively modest stimulation events.

miR-130b/301b Is a Negative Regulator of Beige Adipogenesis and Energy Metabolism In Vitro and In Vivo

Thermogenic brown or beige adipocytes dissipate energy in the form of heat and thereby counteract obesity and related metabolic complications. The miRNA cluster miR-130b/301b is highly expressed in adipose tissues and has been implicated in metabolic diseases as a posttranscriptional regulator of mitochondrial biogenesis and lipid metabolism. We investigated the roles of miR-130b/301b in regulating beige adipogenesis in vivo and in vitro. miR-130b/301b declined in adipose progenitor cells during beige adipogenesis, while forced overexpression of miR-130b-3p or miR-301b-3p suppressed uncoupling protein 1 (UCP1) and mitochondrial respiration, suggesting that a decline in miR-130b-3p or miR-301b-3p is required for adipocyte precursors to develop the beige phenotype. Mechanistically, miR-130b/301b directly targeted AMP-activated protein kinase (AMPKα1) and suppressed peroxisome proliferator-activated receptor γ coactivator-1α (Pgc-1α), key regulators of brown adipogenesis and mitochondrial biogenesis. Mice lacking the miR-130b/301b miRNA cluster showed reduced visceral adiposity and less weight gain. miR-130b/301b null mice exhibited improved glucose tolerance, increased UCP1 and AMPK activation in subcutaneous fat (inguinal white adipose tissue [iWAT]), and increased response to cold-induced energy expenditure. Together, these data identify the miR-130b/301b cluster as a new regulator that suppresses beige adipogenesis involving PGC-1α and AMPK signaling in iWAT and is therefore a potential therapeutic target against obesity and related metabolic disorders.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

Sex and Species Differences in the Development of Diet-Induced Obesity and Metabolic Disturbances in Rodents

Prevalence and health consequences of obesity differ between men and women. Yet, most preclinical studies investigating the etiology of obesity have, to date, been conducted in male rodents. Notably, diet is a major determinant of obesity, but sex differences in rodent models of diet-induced obesity, and the mechanisms that underlie such differences, are still understudied. Here, we aim to determine whether time course and characteristics of diet-induced obesity differ between sexes in rats and mice, and to investigate the potential causes of the observed divergence. To achieve this, we offered the most commonly tested rodents of both sexes, SD rats and C57BL/6 mice, a free choice of 60 % high-fat diet (HFD) and regular chow; body weight, food intake, fat mass, brown adipose responses, locomotor activity and glucose tolerance were assessed in a similar manner in both species. Our results indicate that overall diet-induced hyperphagia is greater in males but that females display a higher preference for the HFD, irrespective of species. Female rats, compared to males, showed a delay in diet-induced weight gain and less metabolic complications. Although male rats increased brown adipose tissue thermogenesis in response to the HFD challenge, this was not sufficient to counteract increased adiposity. In contrast to rats, female and male mice presented with a dramatic adiposity and impaired glucose tolerance, and a decreased energy expenditure. Female mice showed a 5-fold increase in visceral fat, compared to 2-fold increase seen in male mice. Overall, we found that male and female rodents responded very differently to HFD challenge, and engaged different compensatory energy expenditure mechanisms. In addition, these sex differences are divergent in rats and mice. We conclude that SD rats have a better face validity for the lower prevalence of overweight in women, while C57BL/6 mice may better model the increased prevalence of morbid obesity in women.

Imprinted lncRNA Dio3os preprograms intergenerational brown fat development and obesity resistance

Maternal obesity (MO) predisposes offspring to obesity and metabolic disorders but little is known about the contribution of offspring brown adipose tissue (BAT). We find that MO impairs fetal BAT development, which persistently suppresses BAT thermogenesis and primes female offspring to metabolic dysfunction. In fetal BAT, MO enhances expression of Dio3, which encodes deiodinase 3 (D3) to catabolize triiodothyronine (T3), while a maternally imprinted long noncoding RNA, Dio3 antisense RNA (Dio3os), is inhibited, leading to intracellular T3 deficiency and suppression of BAT development. Gain and loss of function shows Dio3os reduces D3 content and enhances BAT thermogenesis, rendering female offspring resistant to high fat diet-induced obesity. Attributing to Dio3os inactivation, its promoter has higher DNA methylation in obese dam oocytes which persists in fetal and adult BAT, uncovering an oocyte origin of intergenerational obesity. Overall, our data uncover key features of Dio3os activation in BAT to prevent intergenerational obesity and metabolic dysfunctions.

Maternal obesity predisposes offspring to obesity and metabolic disorders through incompletely understood mechanisms. Here the authors report that Dio3os is an imprinted long-coding RNA that modulates brown adipose tissue development and obesity resistance in the offspring.

m6A methylation promotes white-to-beige fat transition by facilitating Hif1a translation

Obesity mainly results from a chronic energy imbalance. Promoting browning of white adipocytes is a promising strategy to enhance energy expenditure and combat obesity. N6-methyladenosine (m6A), the most abundant mRNA modification in eukaryotes, plays an important role in regulating adipogenesis. However, whether m6A regulates white adipocyte browning was unknown. Here, we report that adipose tissue-specific deletion of Fto, an m6A demethylase, predisposes mice to prevent high-fat diet (HFD)-induced obesity by enhancing energy expenditure. Additionally, deletion of FTO in vitro promotes thermogenesis and white-to-beige adipocyte transition. Mechanistically, FTO deficiency increases the m6A level of Hif1a mRNA, which is recognized by m6A-binding protein YTHDC2, facilitating mRNA translation and increasing HIF1A protein abundance. HIF1A activates the transcription of thermogenic genes, including Ppaggc1a, Prdm16, and Pparg, thereby promoting Ucp1 expression and the browning process. Collectively, these results unveil an epigenetic mechanism by which m6A-facilitated HIF1A expression controls browning of white adipocytes and thermogenesis, providing a potential target to counteract obesity and metabolic disease.

Increased expression level of ANGPTL8 in white adipose tissue under acute and chronic cold treatment

Background

Angiopoietin-like proteins (ANGPTL), primarily 3, 4, and 8, play a major role in maintaining energy homeostasis by regulating triglyceride metabolism. This study evaluated the level of ANGPTL3, 4, and 8 in the liver, brown adipose tissue (BAT), and subcutaneous white adipose tissue (SAT) of mice maintained under acute and chronic cold conditions.

Methods

C57BL/6J mice were exposed to cold temperature (4 °C) for 10 days with food provided ad libitum. Animal tissues were harvested at Day 0 (Control group, n = 5) and Days 1, 3, 5, and 10 (cold treatment groups, n = 10 per group). The expression levels of various genes were measured in the liver, SAT, and BAT. ANGPTL3, 4, and 8 expressions were measured in the liver. ANGPTL4, 8, and genes involved in browning and lipid metabolism [uncoupling protein 1 (UCP1), lipoprotein lipase (LPL), and adipose triglyceride lipase (ATGL)] were measured in SAT and BAT. Western blotting (WB) analysis and immunohistochemistry (IHC) were performed to confirm ANGPTL8 expression in these tissues.

Results

The expressions of ANGPTL3 and 8 mRNA were significantly reduced in mouse liver tissues after cold treatment (P < 0.05); however, the expression of ANGPTL4 was not significantly altered. In BAT, ANGPTL8 expression was unchanged after cold treatment, whereas ANGPTL4 expression was significantly reduced (P < 0.05). ANGPTL4 levels were also significantly reduced in SAT, whereas ANGPTL8 gene expression exhibited over a 5-fold increase. Similarly, UCP1 gene expression was also significantly increased in SAT. The mRNA levels of LPL and ATGL showed an initial increase followed by a gradual decrease with an increase in the days of cold exposure. ANGPTL8 protein overexpression was further confirmed by WB and IHC.

Conclusions

This study shows that exposure to acute and chronic cold treatment results in the differential expression of ANGPTL proteins in the liver and adipose tissues (SAT and BAT). The results show a significant reduction in ANGPTL4 in BAT, which is linked to improved thermogenesis in response to acute cold exposure. ANGPTL8 was activated under acute and chronic cold conditions in SAT, suggesting that it is involved in regulating lipolysis and enhancing SAT browning.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01547-0.

Crosstalk between Drp1 phosphorylation sites during mitochondrial remodeling and their impact on metabolic adaptation

Mitochondria constantly undergo fusion and fission events, referred as mitochondrial dynamics, which determine mitochondrial architecture and bioenergetics. Cultured cell studies demonstrate that mitochondrial dynamics are acutely regulated by phosphorylation of the mitochondrial fission orchestrator dynamin-related protein 1 (Drp1) at S579 or S600. However, the physiological impact and crosstalk of these phosphorylation sites is poorly understood. Here, we describe the functional interrelation between S579 and S600 phosphorylation sites in vivo and their role on mitochondrial remodeling. Mice carrying a homozygous Drp1 S600A knockin (Drp1 KI) mutation display larger mitochondria and enhanced lipid oxidation and respiratory capacities, granting improved glucose tolerance and thermogenic response upon high-fat feeding. Housing mice at thermoneutrality blunts these differences, suggesting a role for the brown adipose tissue in the protection of Drp1 KI mice against metabolic damage. Overall, we demonstrate crosstalk between Drp1 phosphorylation sites and provide evidence that their modulation could be used in the treatment and prevention of metabolic diseases.

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Drp1 phosphorylation at S600 promotes the phosphorylation at the S579 site

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Both Drp1 P-S600 and P-S579 are required for maximal mitochondrial fragmentation

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Drp1 S600A knockin mice are protected against diet-induced metabolic damage

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Drp1 phosphorylation controls brown adipose tissue thermogenic capacity in mice

Impaired brown adipose tissue is differentially modulated in insulin-resistant obese wistar and type 2 diabetic Goto-Kakizaki rats

Brown adipose tissue (BAT) is a potential target to treat obesity and diabetes, dissipating energy as heat. Type 2 diabetes (T2D) has been associated with obesogenic diets; however, T2D was also reported in lean individuals to be associated with genetic factors. We aimed to investigate the differences between obese and lean models of insulin resistance (IR) and elucidate the mechanism associated with BAT metabolism and dysfunction in different IR animal models: a genetic model (lean GK rats) and obese models (diet-induced obese Wistar rats) at 8 weeks of age fed a high-carbohydrate (HC), high-fat (HF) diet, or high-fat and high-sugar (HFHS) diet for 8 weeks. At 15 weeks of age, BAT glucose uptake was evaluated by 18F-FDG PET under basal (saline administration) or stimulated condition (CL316,243, a selective β3-AR agonist). After CL316, 243 administrations, GK animals showed decreased glucose uptake compared to HC animals. At 16 weeks of age, the animals were euthanized, and the interscapular BAT was dissected for analysis. Histological analyses showed lower cell density in GK rats and higher adipocyte area compared to all groups, followed by HFHS and HF compared to HC. HFHS showed a decreased batokine FGF21 protein level compared to all groups. However, GK animals showed increased expression of genes involved in fatty acid oxidation (CPT1 and CPT2), BAT metabolism (Sirt1 and Pgc1-α), and obesogenic genes (leptin and PAI-1) but decreased gene expression of glucose transporter 1 (GLUT-1) compared to other groups. Our data suggest impaired BAT function in obese Wistar and GK rats, with evidence of a whitening process in these animals.

TFEB deficiency attenuates mitochondrial degradation upon brown adipose tissue whitening at thermoneutrality

OBJECTIVE

Brown adipose tissue (BAT) thermogenesis offers the potential to improve metabolic health in mice and humans. However, humans predominantly live under thermoneutral conditions, leading to BAT whitening, a reduction in BAT mitochondrial content and metabolic activity. Recent studies have established mitophagy as a major driver of mitochondrial degradation in the whitening of thermogenic brite/beige adipocytes, yet the pathways mediating mitochondrial breakdown in whitening of classical BAT remain largely elusive. The transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy belonging to the MiT family of transcription factors, is the only member of this family that is upregulated during whitening, pointing toward a role of TFEB in whitening-associated mitochondrial breakdown.

METHODS

We generated brown adipocyte-specific TFEB knockout mice, and induced BAT whitening by thermoneutral housing. We characterized gene and protein expression patterns, BAT metabolic activity, systemic metabolism, and mitochondrial localization using in vivo and in vitro approaches.

RESULTS

Under low thermogenic activation conditions, deletion of TFEB preserves mitochondrial mass independently of mitochondriogenesis in BAT and primary brown adipocytes. However, this does not translate into elevated thermogenic capacity or protection from diet-induced obesity. Autophagosomal/lysosomal marker levels are altered in TFEB-deficient BAT and primary adipocytes, and lysosomal markers co-localize and co-purify with mitochondria in TFEB-deficient BAT, indicating trapping of mitochondria in late stages of mitophagy.

CONCLUSION

We identify TFEB as a driver of BAT whitening, mediating mitochondrial degradation via the autophagosomal and lysosomal machinery. This study provides proof of concept that interfering with the mitochondrial degradation machinery can increase mitochondrial mass in classical BAT under human-relevant conditions. However, it must be considered that interfering with autophagy may result in accumulation of non-functional mitochondria. Future studies targeting earlier steps of mitophagy or target recognition are therefore warranted.

Brown and beige adipose tissue regulate systemic metabolism through a metabolite interorgan signaling axis

Brown and beige adipose tissue are emerging as distinct endocrine organs. These tissues are functionally associated with skeletal muscle, adipose tissue metabolism and systemic energy expenditure, suggesting an interorgan signaling network. Using metabolomics, we identify 3-methyl-2-oxovaleric acid, 5-oxoproline, and β-hydroxyisobutyric acid as small molecule metabokines synthesized in browning adipocytes and secreted via monocarboxylate transporters. 3-methyl-2-oxovaleric acid, 5-oxoproline and β-hydroxyisobutyric acid induce a brown adipocyte-specific phenotype in white adipocytes and mitochondrial oxidative energy metabolism in skeletal myocytes both in vitro and in vivo. 3-methyl-2-oxovaleric acid and 5-oxoproline signal through cAMP-PKA-p38 MAPK and β-hydroxyisobutyric acid via mTOR. In humans, plasma and adipose tissue 3-methyl-2-oxovaleric acid, 5-oxoproline and β-hydroxyisobutyric acid concentrations correlate with markers of adipose browning and inversely associate with body mass index. These metabolites reduce adiposity, increase energy expenditure and improve glucose and insulin homeostasis in mouse models of obesity and diabetes. Our findings identify beige adipose-brown adipose-muscle physiological metabokine crosstalk.

Melanotan II, a melanocortin agonist, partially rescues the impaired thermogenic capacity of pituitary adenylate cyclase-activating polypeptide deficient mice

NEW FINDINGS

What is the central question of this study? Can chronic treatment of pituitary adenylate cyclase-activating polypeptide (PACAP) deficient mice with the melanocortin agonist melanotan II during cold acclimation rescue the impaired thermogenic capacity previously observed in PACAP deficient mice? What is the main finding and its importance? Using a genetic model of PACAP deficiency, this study provides evidence that PACAP acts upstream of the melanocortin system in regulating sympathetic nerve activity to brown adipose tissue in mice.

ABSTRACT

Impaired adipose tissue function in obesity, including reduced thermogenic potential, has detrimental consequences for metabolic health. Hormonal regulation of adaptive thermogenesis is being explored as a potential therapeutic target for human obesity. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide expressed in nuclei of the hypothalamus known to regulate energy expenditure, and functional studies reveal a role for PACAP in the central regulation of thermogenesis, although mechanisms are not well understood. We hypothesized that PACAP acts upstream of the melanocortin system to regulate sympathetic nerve activity to stimulate thermogenesis. To assess this, female PACAP^-/- and PACAP^+/+ mice were given daily peripheral injections of a melanocortin receptor agonist, melanotan II (MTII), for 3 weeks during cold acclimation, and the effect of MTII on thermogenic capacity and adipose tissue remodelling was examined by physiological and histological analyses. MTII partially rescued the impaired thermogenic capacity in PACAP^-/- mice as compared to PACAP^+/+ mice as determined by measuring noradrenaline-induced metabolic rate. In addition, MTII treatment during cold acclimation corrected the previously identified deficit in lipid utilization in response to adrenergic stimulation in PACAP^-/- null mice, suggesting impaired lipid mobilization may contribute to the impaired thermogenic capacity of PACAP^-/- mice. Results presented here provide physiological evidence to suggest that PACAP acts upstream of melanocortin receptors to facilitate sympathetically induced mechanisms of adaptive thermogenesis in response to cold acclimation.

Adipose ABHD6 regulates tolerance to cold and thermogenic programs

Enhanced energy expenditure in brown (BAT) and white adipose tissues (WAT) can be therapeutic against metabolic diseases. We examined the thermogenic role of adipose α/β-hydrolase domain 6 (ABHD6), which hydrolyzes monoacylglycerol (MAG), by employing adipose-specific ABHD6-KO mice. Control and KO mice showed similar phenotypes at room temperature and thermoneutral conditions. However, KO mice were resistant to hypothermia, which can be accounted for by the simultaneously increased lipolysis and lipogenesis of the thermogenic glycerolipid/free fatty acid (GL/FFA) cycle in visceral fat, despite unaltered uncoupling protein 1 expression. Upon cold stress, nuclear 2-MAG levels increased in visceral WAT of the KO mice. Evidence is provided that 2-MAG causes activation of PPARα in white adipocytes, leading to elevated expression and activity of GL/FFA cycle enzymes. In the ABHD6-ablated BAT, glucose and oxidative metabolism were elevated upon cold induction, without changes in GL/FFA cycle and lipid turnover. Moreover, response to in vivo β₃-adrenergic stimulation was comparable between KO and control mice. Our data reveal a MAG/PPARα/GL/FFA cycling metabolic signaling network in visceral adipose tissue, which contributes to cold tolerance, and that adipose ABHD6 is a negative modulator of adaptive thermogenesis.

Visceral adipose adipose α/β-hydrolase domain 6 regulates cold adaptation and acts as a brake for heat production via the regulation of thermogenic glycerolipid/free fatty acid cycling.

Differences in [18F]FDG uptake in BAT of UCP1 -/- and UCP1 +/+ during adrenergic stimulation of non-shivering thermogenesis

BACKGROUND

Brown adipose tissue (BAT) is a fat tissue found in most mammals that helps regulate energy balance and core body temperature through a sympathetic process known as non-shivering thermogenesis. BAT activity is commonly detected and quantified in [¹⁸F]FDG positron emission tomography/computed tomography (PET/CT) scans, and radiotracer uptake in BAT during adrenergic stimulation is often used as a surrogate measure for identifying thermogenic activity in the tissue. BAT thermogenesis is believed to be contingent upon the expression of the protein UCP1, but conflicting results have been reported in the literature concerning [¹⁸F]FDG uptake within BAT of mice with and without UCP1. Differences in animal handling techniques such as feeding status, type of anesthetic, type of BAT stimulation, and estrogen levels were identified as possible confounding variables for [¹⁸F]FDG uptake. In this study, we aimed to assess differences in BAT [¹⁸F]FDG uptake between wild-type and UCP1-knockout mice using a protocol that minimizes possible variations in BAT stimulation caused by different stress responses to mouse handling.

RESULTS

[¹⁸F]FDG PET/CT scans were run on mice that were anesthetized with pentobarbital after stimulation of non-shivering thermogenesis by norepinephrine. While in wild-type mice [¹⁸F]FDG uptake in BAT increased significantly with norepinephrine stimulation of BAT, there was no consistent change in [¹⁸F]FDG uptake in BAT of mice lacking UCP1.

CONCLUSIONS

[¹⁸F]FDG uptake within adrenergically stimulated BAT of wild-type and UCP1-knockout mice can significantly vary such that an [¹⁸F]FDG uptake threshold cannot be used to differentiate wild-type from UCP1-knockout mice. However, while an increase in BAT [¹⁸F]FDG uptake during adrenergic stimulation is consistently observed in wild-type mice, in UCP1-knockout mice [¹⁸F]FDG uptake in BAT seems to be independent of β₃-adrenergic stimulation of non-shivering thermogenesis.

PGC-1β-expressing POMC neurons mediate the effect of leptin on thermoregulation in the mouse

The arcuate nucleus (ARC) of the hypothalamus is a key regulator of food intake, brown adipose tissue (BAT) thermogenesis, and locomotor activity. Whole-body deficiency of the transcriptional coactivator peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1β (PGC-1β) disrupts mouse circadian locomotor activity and BAT-regulated thermogenesis, in association with altered gene expression at the central level. We examined whether PGC-1β expression in the ARC is required for proper energy balance and locomotor behavior by generating mice lacking the PGC-1β gene specifically in pro-opiomelanocortin (POMC) neurons. POMC neuron-specific deletion of PGC-1β did not impact locomotor behavior, food intake, body composition, energy fuel utilization and metabolic rate in fed, 24-h fasted and 24-h refed conditions. In contrast, in the fed state, deletion of PGC-1β in POMC cells elevated core body temperature during the nighttime period. Importantly, this higher body temperature is not associated with changes in BAT function and gene expression. Conversely, we provide evidence that mice lacking PGC-1β in POMC neurons are more sensitive to the effect of leptin on heat dissipation. Our data indicate that PGC-1β-expressing POMC neurons are part of a circuit controlling body temperature homeostasis and that PGC-1β function in these neurons is involved in the thermoregulatory effect of leptin.

Creatine metabolism: energy homeostasis, immunity and cancer biology

Perturbations in metabolic processes are associated with diseases such as obesity, type 2 diabetes mellitus, certain infections and some cancers. A resurgence of interest in creatine biology is developing, with new insights into a diverse set of regulatory functions for creatine. This resurgence is primarily driven by technological advances in genetic engineering and metabolism as well as by the realization that this metabolite has key roles in cells beyond the muscle and brain. Herein, we highlight the latest advances in creatine biology in tissues and cell types that have historically received little attention in the field. In adipose tissue, creatine controls thermogenic respiration and loss of this metabolite impairs whole-body energy expenditure, leading to obesity. We also cover the various roles that creatine metabolism has in cancer cell survival and the function of the immune system. Renewed interest in this area has begun to showcase the therapeutic potential that lies in understanding how changes in creatine metabolism lead to metabolic disease.

Chronic arsenic exposure impairs adaptive thermogenesis in male C57BL/6J mice

The global prevalence of type 2 diabetes (T2D) has doubled since 1980. Human epidemiological studies support arsenic exposure as a risk factor for T2D, although the precise mechanism is unclear. We hypothesized that chronic arsenic ingestion alters glucose homeostasis by impairing adaptive thermogenesis, i.e., body heat production in cold environments. Arsenic is a pervasive environmental contaminant, with more than 200 million people worldwide currently exposed to arsenic-contaminated drinking water. Male C57BL/6J mice exposed to sodium arsenite in drinking water at 300 μg/L for 9 wk experienced significantly decreased metabolic heat production when acclimated to chronic cold tolerance testing, as evidenced by indirect calorimetry, despite no change in physical activity. Arsenic exposure increased total fat mass and subcutaneous inguinal white adipose tissue (iWAT) mass. RNA sequencing analysis of iWAT indicated that arsenic dysregulated mitochondrial processes, including fatty acid metabolism. Western blotting in WAT confirmed that arsenic significantly decreased TOMM20, a correlate of mitochondrial abundance; PGC1A, a master regulator of mitochondrial biogenesis; and, CPT1B, the rate-limiting step of fatty acid oxidation (FAO). Our findings show that chronic arsenic exposure impacts the mitochondrial proteins of thermogenic tissues involved in energy expenditure and substrate regulation, providing novel mechanistic evidence for arsenic's role in T2D development.

Apolipoprotein A-IV Enhances Fatty Acid Uptake by Adipose Tissues of Male Mice via Sympathetic Activation

Apolipoprotein A-IV (ApoA-IV) synthesized by the gut regulates lipid metabolism. Sympathetic innervation of adipose tissues also controls lipid metabolism. We hypothesized that ApoA-IV required sympathetic innervation to increase fatty acid (FA) uptake by adipose tissues and brown adipose tissue (BAT) thermogenesis. After 3 weeks feeding of either a standard chow diet or a high-fat diet (HFD), mice with unilateral denervation of adipose tissues received intraperitoneal administration of recombinant ApoA-IV protein and intravenous infusion of lipid mixture with radioactive triolein. In chow-fed mice, ApoA-IV administration increased FA uptake by intact BAT but not the contralateral denervated BAT or intact white adipose tissue (WAT). Immunoblots showed that, in chow-fed mice, ApoA-IV increased expression of lipoprotein lipase and tyrosine hydroxylase in both intact BAT and inguinal WAT (IWAT), while ApoA-IV enhanced protein levels of β3 adrenergic receptor, adipose triglyceride lipase, and uncoupling protein 1 in the intact BAT only. In HFD-fed mice, ApoA-IV elevated FA uptake by intact epididymal WAT (EWAT) but not intact BAT or IWAT. ApoA-IV increased sympathetic activity assessed by norepinephrine turnover (NETO) rate in BAT and EWAT of chow-fed mice, whereas it elevated NETO only in EWAT of HFD-fed mice. These observations suggest that, in chow-fed mice, ApoA-IV activates sympathetic activity of BAT and increases FA uptake by BAT via innervation, while in HFD-fed mice, ApoA-IV stimulates sympathetic activity of EWAT to shunt FAs into the EWAT.

Leptin: Is It Thermogenic?

Animals that lack the hormone leptin become grossly obese, purportedly for 2 reasons: increased food intake and decreased energy expenditure (thermogenesis). This review examines the experimental evidence for the thermogenesis component. Analysis of the data available led us to conclude that the reports indicating hypometabolism in the leptin-deficient ob/ob mice (as well as in the leptin-receptor-deficient db/db mice and fa/fa rats) derive from a misleading calculation artefact resulting from expression of energy expenditure per gram of body weight and not per intact organism. Correspondingly, the body weight-reducing effects of leptin are not augmented by enhanced thermogenesis. Congruent with this, there is no evidence that the ob/ob mouse demonstrates atrophied brown adipose tissue or diminished levels of total UCP1 mRNA or protein when the ob mutation is studied on the inbred C57BL/6 mouse background, but a reduced sympathetic nerve activity is observed. On the outbred "Aston" mouse background, brown adipose tissue atrophy is seen, but whether this is of quantitative significance for the development of obesity has not been demonstrated. We conclude that leptin is not a thermogenic hormone. Rather, leptin has effects on body temperature regulation, by opposing torpor bouts and by shifting thermoregulatory thresholds. The central pathways behind these effects are largely unexplored.

Mechanistic Links Between Obesity, Diabetes, and Blood Pressure: Role of Perivascular Adipose Tissue

Obesity is increasingly prevalent and is associated with substantial cardiovascular risk. Adipose tissue distribution and morphology play a key role in determining the degree of adverse effects, and a key factor in the disease process appears to be the inflammatory cell population in adipose tissue. Healthy adipose tissue secretes a number of vasoactive adipokines and anti-inflammatory cytokines, and changes to this secretory profile will contribute to pathogenesis in obesity. In this review, we discuss the links between adipokine dysregulation and the development of hypertension and diabetes and explore the potential for manipulating adipose tissue morphology and its immune cell population to improve cardiovascular health in obesity.

Knockout of the X-linked Fgf13 in the hypothalamic paraventricular nucleus impairs sympathetic output to brown fat and causes obesity

Fibroblast growth factor (FGF)13, a nonsecreted, X-linked, FGF homologous factor, is differentially expressed in adipocytes in response to diet, yet Fgf13's role in metabolism has not been explored. Heterozygous Fgf13 knockouts fed normal chow and housed at 22°C showed hyperactivity accompanying reduced core temperature and obesity when housed at 30°C. Those heterozygous knockouts showed defects in thermogenesis even at 30°C and an inability to protect core temperature. Surprisingly, we detected trivial FGF13 in adipose of wild-type mice fed normal chow and no obesity in adipose-specific heterozygous knockouts housed at 30°C, and we detected an intact brown fat response through exogenous β3 agonist stimulation, suggesting a defect in sympathetic drive to brown adipose tissue. In contrast, hypothalamic-specific ablation of Fgf13 recapitulated weight gain at 30°C. Norepinephrine turnover in brown fat was reduced at both housing temperatures. Thus, our data suggest that impaired CNS regulation of sympathetic activation of brown fat underlies obesity and thermogenesis in Fgf13 heterozygous knockouts fed normal chow.-Sinden, D. S., Holman, C. D., Bare, C. J., Sun, X., Gade, A. R., Cohen, D. E., Pitt, G. S. Knockout of the X-linked Fgf13 in the hypothalamic paraventricular nucleus impairs sympathetic output to brown fat and causes obesity.

Mitochondrial 2,4-dienoyl-CoA reductase (Decr) deficiency and impairment of thermogenesis in mouse brown adipose tissue

A large number of studies have demonstrated significance of polyunsaturated fatty acids (PUFAs) for human health. However, many aspects on signals translating PUFA-sensing into body homeostasis have remained enigmatic. To shed light on PUFA physiology, we have generated a mouse line defective in mitochondrial dienoyl-CoA reductase (Decr), which is a key enzyme required for β-oxidation of PUFAs. Previously, we have shown that these mice, whose oxidation of saturated fatty acid is intact but break-down of unsaturated fatty acids is blunted, develop severe hypoglycemia during metabolic stresses and fatal hypothermia upon acute cold challenge. In the current work, indirect calorimetry and thermography suggested that cold intolerance of Decr^−/− mice is due to failure in maintaining appropriate heat production at least partly due to failure of brown adipose tissue (BAT) thermogenesis. Magnetic resonance imaging, electron microscopy, mass spectrometry and biochemical analysis showed attenuation in activation of lipolysis despite of functional NE-signaling and inappropriate expression of genes contributing to thermogenesis in iBAT when the Decr^−/− mice were exposed to cold. We hypothesize that the failure in turning on BAT thermogenesis occurs due to accumulation of unsaturated long-chain fatty acids or their metabolites in Decr^−/− mice BAT suppressing down-stream propagation of NE-signaling.

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

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

Glucocorticoids and Brown Adipose Tissue: Do glucocorticoids really inhibit thermogenesis?

A reduction in the thermogenic activity of brown adipose tissue (BAT) is presently discussed as a possible determinant for the development of obesity in humans. One group of endogenous factors that could potentially affect BAT activity is the glucocorticoids (e.g. cortisol). We analyse here studies examining the effects of alterations in glucocorticoid signaling on BAT recruitment and thermogenic capacity. We find that irrespective of which manipulation of glucocorticoid signaling is examined, a seemingly homogeneous picture of lowered thermogenic capacity due to glucocorticoid stimulation is apparently obtained: e.g. lowered uncoupling protein 1 (UCP1) protein levels per mg protein, and an increased lipid accumulation in BAT. However, further analyses generally indicate that these effects result from a dilution effect rather than a true decrease in total capacity; the tissue may thus be said to be in a state of pseudo-atrophy. However, under conditions of very low physiological stimulation of BAT, glucocorticoids may truly inhibit Ucp1 gene expression and consequently lower total UCP1 protein levels, but the metabolic effects of this reduction are probably minor. It is thus unlikely that glucocorticoids affect organismal metabolism and induce the development of obesity through alterations of BAT activity.

SUCNR1 controls an anti-inflammatory program in macrophages to regulate the metabolic response to obesity

Succinate is a signaling metabolite sensed extracellularly by succinate receptor 1 (SUNCR1). The accumulation of succinate in macrophages is known to activate a pro-inflammatory program; however, the contribution of SUCNR1 to macrophage phenotype and function has remained unclear. Here we found that activation of SUCNR1 had a critical role in the anti-inflammatory responses in macrophages. Myeloid-specific deficiency in SUCNR1 promoted a local pro-inflammatory phenotype, disrupted glucose homeostasis in mice fed a normal chow diet, exacerbated the metabolic consequences of diet-induced obesity and impaired adipose-tissue browning in response to cold exposure. Activation of SUCNR1 promoted an anti-inflammatory phenotype in macrophages and boosted the response of these cells to type 2 cytokines, including interleukin-4. Succinate decreased the expression of inflammatory markers in adipose tissue from lean human subjects but not that from obese subjects, who had lower expression of SUCNR1 in adipose-tissue-resident macrophages. Our findings highlight the importance of succinate-SUCNR1 signaling in determining macrophage polarization and assign a role to succinate in limiting inflammation.

Androgen Receptor in Neurons Slows Age-Related Cortical Thinning in Male Mice

Androgens via the androgen receptor (AR) are required for optimal male bone health. The target cell(s) for the effects of androgens on cortical bone remain(s) incompletely understood. In females, estrogen receptor alpha in neurons is a negative regulator of cortical and trabecular bone. Whether neuronal AR regulates bone mass in males remains unexplored. Here, we inactivated AR in neurons using a tamoxifen-inducible CreERT2 under the control of the neuronal promoter Thy1. Tamoxifen induced a 70% to 80% reduction of AR mRNA levels in Thy1-CreERT2-positive brain regions cerebral cortex and brainstem as well as in the peripheral nervous tissue of male neuronal AR knockout (N-ARKO) mice. Hypothalamic AR mRNA levels were only marginally reduced and the hypothalamic-pituitary-gonadal axis remained unaffected, as determined by normal levels of serum testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). In contrast to orchidectomy, deletion of neuronal AR did not alter body weight, body composition, hindlimb muscle mass, grip strength, or wheel running. MicroCT analysis of the femur revealed no changes in bone accrual during growth in N-ARKO mice. However, 36- and 46-week-old N-ARKO mice displayed an accelerated age-related cortical involution, namely a more pronounced loss of cortical thickness and strength, which occurred in the setting of androgen sufficiency. Neuronal AR inactivation decreased the cancellous bone volume fraction in L₅ vertebra but not in the appendicular skeleton of aging mice. MicroCT findings were corroborated in the tibia and after normalization of hormonal levels. Serum markers of bone turnover and histomorphometry parameters were comparable between genotypes, except for a 30% increase in osteoclast surface in the trabecular compartment of 36-week-old N-ARKO mice. Cortical bone loss in N-ARKO mice was associated with an upregulation of Ucp1 expression in brown adipose tissue, a widely used readout for sympathetic tone. We conclude that androgens preserve cortical integrity in aging male mice via AR in neurons. © 2018 American Society for Bone and Mineral Research.

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

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

Interleukin-18 in Health and Disease

Interleukin (IL)-18 was originally discovered as a factor that enhanced IFN-γ production from anti-CD3-stimulated Th1 cells, especially in the presence of IL-12. Upon stimulation with Ag plus IL-12, naïve T cells develop into IL-18 receptor (IL-18R) expressing Th1 cells, which increase IFN-γ production in response to IL-18 stimulation. Therefore, IL-12 is a commitment factor that induces the development of Th1 cells. In contrast, IL-18 is a proinflammatory cytokine that facilitates type 1 responses. However, IL-18 without IL-12 but with IL-2, stimulates NK cells, CD4⁺ NKT cells, and established Th1 cells, to produce IL-3, IL-9, and IL-13. Furthermore, together with IL-3, IL-18 stimulates mast cells and basophils to produce IL-4, IL-13, and chemical mediators such as histamine. Therefore, IL-18 is a cytokine that stimulates various cell types and has pleiotropic functions. IL-18 is a member of the IL-1 family of cytokines. IL-18 demonstrates a unique function by binding to a specific receptor expressed on various types of cells. In this review article, we will focus on the unique features of IL-18 in health and disease in experimental animals and humans.

Organic cation transporter 3 (Oct3) is a distinct catecholamines clearance route in adipocytes mediating the beiging of white adipose tissue

Beiging of white adipose tissue (WAT) is a particularly appealing target for therapeutics in the treatment of metabolic diseases through norepinephrine (NE)-mediated signaling pathways. Although previous studies report NE clearance mechanisms via SLC6A2 on sympathetic neurons or proinflammatory macrophages in adipose tissues (ATs), the low catecholamine clearance capacity of SLC6A2 may limit the cleaning efficiency. Here, we report that mouse organic cation transporter 3 (Oct3; Slc22a3) is highly expressed in WAT and displays the greatest uptake rate of NE as a selective non-neural route of NE clearance in white adipocytes, which differs from other known routes such as adjacent neurons or macrophages. We further show that adipocytes express high levels of NE degradation enzymes Maoa, Maob, and Comt, providing the molecular basis on NE clearance by adipocytes together with its reuptake transporter Oct3. Under NE administration, ablation of Oct3 induces higher body temperature, thermogenesis, and lipolysis compared with littermate controls. After prolonged cold challenge, inguinal WAT (ingWAT) in adipose-specific Oct3-deficient mice shows much stronger browning characteristics and significantly elevated expression of thermogenic and mitochondrial biogenesis genes than in littermate controls, and this response involves enhanced β-adrenergic receptor (β-AR)/protein kinase A (PKA)/cyclic adenosine monophosphate (cAMP)-responsive element binding protein (Creb) pathway activation. Glycolytic genes are reprogrammed to significantly higher levels to compensate for the loss of ATP production in adipose-specific Oct3 knockout (KO) mice, indicating the fundamental role of glucose metabolism during beiging. Inhibition of β-AR largely abolishes the higher lipolytic and thermogenic activities in Oct3-deficient ingWAT, indicating the NE overload in the vicinity of adipocytes in Oct3 KO adipocytes. Of note, reduced functional alleles in human OCT3 are also identified to be associated with increased basal metabolic rate (BMR). Collectively, our results demonstrate that Oct3 governs β-AR activity as a NE recycling transporter in white adipocytes, offering potential therapeutic applications for metabolic disorders.

Arterial blood sampling in male CD-1 and C57BL/6J mice with 1% isoflurane is similar to awake mice

Isoflurane (ISO) is a commonly used anesthetic that offers rapid recovery for laboratory animal research. Initial studies indicated no difference in arterial Pco₂ (PaCO2) or pH between conscious (NO ISO) and 1% ISO-exposed CD-1 mice. Our laboratory investigated whether arterial blood sampling with 1% ISO is a suitable alternative to NO ISO sampling for monitoring ventilation in a commonly studied mouse strain. We hypothesized similar blood chemistry, breathing patterns, and cardiovascular responses with NO ISO and 1% ISO. C57BL/6J mice underwent unrestrained barometric plethysmography to quantify the pattern of breathing. Mice exposed to hypoxic and hypercapnic gas under 1% ISO displayed blunted responses; with air, there were no breathing differences. Blood pressure and heart rate were not different between NO ISO and 1% ISO-exposed mice breathing air. Oxygen saturation was not different between groups receiving 2% ISO, 1% ISO, or air. Breathing frequency stabilized at ~11 min of 1% ISO following 2% ISO exposure, suggesting that 11 min is the optimal time for a sample in C57BL/6J mice. Blood samples at 1% ISO and NO ISO revealed no differences in blood pH and PaCO2 in C57BL/6J mice. Overall, this method reveals similar arterial blood sampling values in awake and 1% ISO CD-1 and C57BL/6J mice exposed to air. Although this protocol may be appropriate in other mouse strains when a conscious sample is not feasible, caution is warranted first to identify breathing frequency responses at 1% ISO to tailor the protocol.

NEW & NOTEWORTHY Conscious arterial blood sampling is influenced by extraneous factors and is a challenging method due to the small size of mice. Through a series of experiments, we show that arterial blood sampling with 1% isoflurane (ISO) is an alternative to awake sampling in C57BL/6J and CD-1 male mice breathing air. Monitoring breathing frequency during 1% ISO is important to the protocol and should be closely followed to confirm adequate recovery after the catheter implantation.

Prior exercise training improves cold tolerance independent of indices associated with non-shivering thermogenesis

KEY POINTS

Mammals defend against cold-induced reductions in body temperature through both shivering and non-shivering thermogenesis. The activation of non-shivering thermogenesis is primarily driven by uncoupling protein-1 in brown adipose tissue and to a lesser degree by the browning of white adipose tissue. Endurance exercise has also been shown to increase markers of white adipose tissue browning. This study aimed to determine whether prior exercise training would alter the response to a cold challenge and if this would be associated with differences in indices of non-shivering thermogenesis. It is shown that exercise training protects against cold-induced weight loss by increasing food intake. Exercise-trained mice were better able to maintain their core temperature, independent of differences in markers of non-shivering thermogenesis.

ABSTRACT

Shivering is one of the first defences against cold, and as skeletal muscle fatigues there is an increased reliance on non-shivering thermogenesis. Brown and beige adipose tissues are the primary thermogenic tissues regulating this process. Exercise has also been shown to increase the thermogenic capacity of subcutaneous white adipose tissue. Whether exercise has an effect on the adaptations to cold stress within adipose tissue and skeletal muscle remains to be shown. Male C57BL/6 mice were either subjected to voluntary wheel running or remained sedentary for 12 days. Exercise led to decreased body weight and increased glucose tolerance. Mice were then divided into groups kept at 25°C room temperature or a cold challenge of 4°C for 48 h. Exercised mice were protected against cold-induced reductions in weight and in parallel with increased food intake. Providing exercised mice with the same amount of food as sedentary mice eliminated the protection against cold-induced weight loss. Cold exposure led to greater reductions in rectal temperature in sedentary compared to exercised mice. This protective effect was not explained by differences in the browning of white adipose tissue or brown adipose tissue mass. Similarly, the ability of the β₃ -adrenergic agonist CL 316,243 to increase energy expenditure was attenuated in previously exercised mice, suggesting that the activation of uncoupling protein-1 in brown and/or beige adipocytes is not the source of protective effects. We speculate that the protection against cold-induced reductions in rectal temperature could potentially be linked to exercise-induced alterations in skeletal muscle.

Norepinephrine and T4 Are Predictors of Fat Mass Gain in Humans With Cold-Induced Brown Adipose Tissue Activation

CONTEXT

In healthy adults with detectable cold-induced brown adipose tissue activation (CIBA), the relationships between sympathetic nervous system (SNS) or thyroid activity during energy balance (EBL) with CIBA and body composition change are undetermined.

OBJECTIVE

To investigate the relationships between CIBA and thermoneutral catecholamines and thyroid hormones measured during EBL and to determine if CIBA, catecholamines, or thyroid hormones predict body composition changes.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTIONS

Twelve healthy volunteers (seven male and five female) with positive CIBA [>2 standardized uptake value (g/mL)] had 24-hour energy expenditure (24hEE) assessed during EBL via whole-room indirect calorimetry while residing on a clinical research unit. Positron emission tomography/computed tomography scans were performed after exposure to 16°C for 2 hours to quantify CIBA.

MAIN OUTCOME MEASURES

CIBA, 24hEE during EBL, and thermoneutrality with concomitant measurement of urinary catecholamines and plasma free T3 and free T4. Body composition at baseline and 6 months by dual-energy X-ray absorptiometry.

RESULTS

Lower urinary norepinephrine and free T4 were associated with higher CIBA (r = -0.65, P = 0.03; and r = -0.75, P < 0.01, respectively), but CIBA was not associated with 24hEE at thermoneutrality (P = 0.77). Lower CIBA (β = -3.5 kg/standardized uptake value; P < 0.01) predicted fat mass gain, whereas higher urinary norepinephrine and free T4 predicted future fat mass gain at 6 months (β = 3.0 kg per twofold difference in norepinephrine, P = 0.03; and β = 1.2 kg per 0.1-ng/dL difference in free T4, P = 0.03, respectively).

CONCLUSION

Lower SNS and free thyroid measurements at baseline indicate a greater capacity for CIBA, which may be predictive against fat mass gain.

Cold-Induced Thermogenesis Depends on ATGL-Mediated Lipolysis in Cardiac Muscle, but Not Brown Adipose Tissue

Fatty acids (FAs) activate and fuel UCP1-mediated non-shivering thermogenesis (NST) in brown adipose tissue (BAT). Release of FAs from intracellular fat stores by adipose triglyceride lipase (ATGL) is considered a key step in NST. Accordingly, the severe cold intolerance of global ATGL knockout (AKO) mice has been attributed to defective BAT lipolysis. Here we show that this conclusion is incorrect. We demonstrate that although the BAT-specific loss of ATGL impairs BAT lipolysis and alters BAT morphology, it does not compromise the β₃-adrenergic thermogenic response or cold-induced NST. Instead, NST depends on nutrient supply or lipolysis in white adipose tissue during fasting, suggesting that circulating energy substrates are sufficient to fuel NST. Cold intolerance in AKO mice is not caused by BAT dysfunction as previously suspected but by severe cardiomyopathy. We conclude that functional NST requires adequate substrate supply and cardiac function, but does not depend on ATGL-mediated lipolysis in BAT.

A brain-sparing diphtheria toxin for chemical genetic ablation of peripheral cell lineages

Conditional expression of diphtheria toxin receptor (DTR) is widely used for tissue-specific ablation of cells. However, diphtheria toxin (DT) crosses the blood-brain barrier, which limits its utility for ablating peripheral cells using Cre drivers that are also expressed in the central nervous system (CNS). Here we report the development of a brain-sparing DT, termed BRAINSPAReDT, for tissue-specific genetic ablation of cells outside the CNS. We prevent blood-brain barrier passage of DT through PEGylation, which polarizes the molecule and increases its size. We validate BRAINSPAReDT with regional genetic sympathectomy: BRAINSPAReDT ablates peripheral but not central catecholaminergic neurons, thus avoiding the Parkinson-like phenotype associated with full dopaminergic depletion. Regional sympathectomy compromises adipose tissue thermogenesis, and renders mice susceptible to obesity. We provide a proof of principle that BRAINSPAReDT can be used for Cre/DTR tissue-specific ablation outside the brain using CNS drivers, while consolidating the link between adiposity and the sympathetic nervous system.

Parkin-mediated mitophagy is downregulated in browning of white adipose tissue

OBJECTIVE

Browning of white adipose tissue (WAT) promotes increased energy expenditure through the action of uncoupling protein 1 (UCP1) and is an attractive target to promote weight loss in obesity. Lowering of mitochondrial membrane potential by UCP1 is uniquely beneficial in this context; in other tissues, reduced membrane potential promotes mitochondrial clearance via mitophagy. It is unknown how parkin-mediated mitophagy is regulated in beige adipocytes.

METHODS

The relationship between parkin expression and WAT browning was investigated in 3T3-L1 adipocytes and parkin-deficient male C57BL/6 mice in response to pharmacological browning stimuli.

RESULTS

Rosiglitazone treatment in 3T3-L1 adipocytes promoted mitochondrial biogenesis, UCP1 expression, and mitochondrial uncoupling. Parkin expression was decreased and reduced mitochondrial-associated parkin, and p62 indicated a reduction in mitophagy activity. Parkin overexpression prevented mitochondrial remodeling in response to rosiglitazone. In CL 316,243-treated wild-type mice, decreased parkin expression was observed in subcutaneous inguinal WAT, where UCP1 was strongly induced. CL 316,243 treatment weakly induced UCP1 expression in the gonadal depot, where parkin expression was unchanged. In contrast, parkin-deficient mice exhibited robust UCP1 expression in gonadal WAT following CL 316,243 treatment.

CONCLUSIONS

WAT browning was associated with a decrease in parkin-mediated mitophagy, and parkin expression antagonized browning of WAT.

Role of leptin in energy expenditure: the hypothalamic perspective

The adipocyte-derived hormone leptin is a peripheral signal that informs the brain about the metabolic status of an organism. Although traditionally viewed as an appetite-suppressing hormone, studies in the past decade have highlighted the role of leptin in energy expenditure. Leptin has been shown to increase energy expenditure in particular through its effects on the cardiovascular system and brown adipose tissue (BAT) thermogenesis via the hypothalamus. The current review summarizes the role of leptin signaling in various hypothalamic nuclei and its effects on the sympathetic nervous system to influence blood pressure, heart rate, and BAT thermogenesis. Specifically, the role of leptin signaling on three different hypothalamic nuclei, the dorsomedial hypothalamus, the ventromedial hypothalamus, and the arcuate nucleus, is reviewed. It is known that all of these brain regions influence the sympathetic nervous system activity and thereby regulate BAT thermogenesis and the cardiovascular system. Thus the current work focuses on how leptin signaling in specific neuronal populations within these hypothalamic nuclei influences certain aspects of energy expenditure.

Mfn2 is critical for brown adipose tissue thermogenic function

Mitochondrial fusion and fission events, collectively known as mitochondrial dynamics, act as quality control mechanisms to ensure mitochondrial function and fine‐tune cellular bioenergetics. Defective mitofusin 2 (Mfn2) expression and enhanced mitochondrial fission in skeletal muscle are hallmarks of insulin‐resistant states. Interestingly, Mfn2 is highly expressed in brown adipose tissue (BAT), yet its role remains unexplored. Using adipose‐specific Mfn2 knockout (Mfn2‐adKO) mice, we demonstrate that Mfn2, but not Mfn1, deficiency in BAT leads to a profound BAT dysfunction, associated with impaired respiratory capacity and a blunted response to adrenergic stimuli. Importantly, Mfn2 directly interacts with perilipin 1, facilitating the interaction between the mitochondria and the lipid droplet in response to adrenergic stimulation. Surprisingly, Mfn2‐adKO mice were protected from high‐fat diet‐induced insulin resistance and hepatic steatosis. Altogether, these results demonstrate that Mfn2 is a mediator of mitochondria to lipid droplet interactions, influencing lipolytic processes and whole‐body energy homeostasis.

UCP1 inhibition in Cidea-overexpressing mice is physiologically counteracted by brown adipose tissue hyperrecruitment

Cidea is a gene highly expressed in thermogenesis-competent (UCP1-containing) adipose cells, both brown and brite/beige. Here, we initially demonstrate a remarkable adipose-depot specific regulation of Cidea expression. In classical brown fat, Cidea mRNA is expressed continuously and invariably, irrespective of tissue recruitment. However, Cidea protein levels are regulated posttranscriptionally, being conspicuously induced in the thermogenically recruited state. In contrast, in brite fat, Cidea protein levels are regulated at the transcriptional level, and Cidea mRNA and protein levels are proportional to tissue "briteness." Although routinely followed as a thermogenic molecular marker, Cidea function is not clarified. Here, we employed a gain-of-function approach to examine a possible role of Cidea in the regulation of thermogenesis. We utilized transgenic aP2-hCidea mice that overexpress human Cidea in all adipose tissues. We demonstrate that UCP1 activity is markedly suppressed in brown-fat mitochondria isolated from aP2-hCidea mice. However, mitochondrial UCP1 protein levels were identical in wild-type and transgenic mice. This implies a regulatory effect of Cidea on UCP1 activity, but as we demonstrate that Cidea itself is not localized to mitochondria, we propose an indirect inhibitory effect. The Cidea-induced inhibition of UCP1 activity (observed in isolated mitochondria) is physiologically relevant since the mice, through an appropriate homeostatic compensatory mechanism, increased the total amount of UCP1 in the tissue to exactly match the diminished thermogenic capacity of the UCP1 protein and retain unaltered nonshivering thermogenic capacity. Thus, we verified Cidea as being a marker of thermogenesis-competent adipose tissues, but we conclude that Cidea, unexpectedly, functions molecularly as an indirect inhibitor of thermogenesis.

Seipin deficiency alters brown adipose tissue thermogenesis and insulin sensitivity in a non-cell autonomous mode

Loss-of-function mutations in BSCL2 are responsible for Berardinelli-Seip congenital lipodystrophy, a rare disorder characterized by near absence of adipose tissue associated with insulin resistance. Seipin-deficient (Bscl2^−/−) mice display an almost total loss of white adipose tissue (WAT) with residual brown adipose tissue (BAT). Previous cellular studies have shown that seipin deficiency alters white adipocyte differentiation. In this study, we aimed to decipher the consequences of seipin deficiency in BAT. Using a brown adipocyte cell-line, we show that seipin knockdown had very little effect on adipocyte differentiation without affecting insulin sensitivity and oxygen consumption. However, when submitted to cold acclimation or chronic β3 agonist treatment, Bscl2^−/− mice displayed altered thermogenic capacity, despite several signs of BAT remodeling. Under cold activation, Bscl2^−/− mice were able to maintain their body temperature when fed ad libitum, but not under short fasting. At control temperature (i.e. 21 °C), fasting worsened Bscl2^−/− BAT properties. Finally, Bscl2^−/− BAT displayed obvious signs of insulin resistance. Our results in these lipodystrophic mice strongly suggest that BAT activity relies on WAT as an energetic substrate provider and adipokine-producing organ. Therefore, the WAT/BAT dialogue is a key component of BAT integrity in guaranteeing its response to insulin and cold-activated adrenergic signals.