Brown Adipose Tissue Thermogenic Activity Research Articles

Acute Administration of Calafate (Berberis microphylla) Extract Induces the Expression of Thermogenic Markers and Modulates Gut Microbiota in Mice Fed a High-Fat Chow Diet

Introduction: Obesity, characterized by excess adipose tissue, is a major public health problem worldwide. Brown adipose tissue (BAT) and beige adipose tissue participate in thermogenesis through uncoupling protein 1 (UCP1). Polyphenols including those from Calafate (a native polyphenol-rich Patagonian berry), are considered as potential anti-obesity compounds due to their pro-thermogenic characteristics. However, polyphenols are mainly metabolized by the gut microbiota (GM) that may influence their bioactivity and bioavailability. The aim of this study was to determine the impact of dietary administration with a Calafate polyphenol-rich extract on thermogenic activity of BAT and beige adipose tissue and GM composition. Methods: Eight-week-old C57BL6 mice (n = 30) were divided into 4 groups to receive for 24 weeks a control diet (C), a high-fat diet alone (HF), or high-fat diet supplemented with Calafate extract (HFC) or the same high-fat diet supplemented with Calafate extract but treated with antibiotics (HFCAB) from week 19–20. Administration with Calafate extract (50 mg/kg per day) was carried out for 3 weeks from week 21–23 in the HFC and HFCAB groups. After euthanasia, gene expression of thermogenic markers was analyzed in BAT and inguinal white adipose tissue (iWAT). Transmission electron microscopy was performed to assess mitochondrial morphology and cristae density in BAT. GM diversity and composition were characterized by deep sequencing with the MiSeq Illumina platform. Results: Calafate extract administration had no effect on weight gain in mice fed a high-fat diet. However, it prevented alterations in mitochondrial cristae induced by HFD and increased Dio2 expression in BAT and iWAT. The intervention also influenced the GM composition, preventing changes in specific bacterial taxa induced by the high-fat diet. However, the antibiotic treatment prevented in part these effects, suggesting the implications of GM. Conclusion: These results suggest that the acute administration of a Calafate extract modulates the expression of thermogenic markers, prevents alterations in mitochondrial cristae and intestinal microbiota in preclinical models. The study highlights the complex interaction between polyphenols, thermogenesis, and the GM, providing valuable insights into their potential roles in the treatment of obesity-related metabolic diseases.

UCP1-dependent brown adipose activation accelerates cardiac metabolic remodeling and reduces initial hypertrophic and fibrotic responses to pathological stress.

Brown adipose tissue (BAT) is correlated to cardiovascular health in rodents and humans, but the physiological role of BAT in the initial cardiac remodeling at the onset of stress is unknown. Activation of BAT via 48 h cold (16°C) in mice following transverse aortic constriction (TAC) reduced cardiac gene expression for LCFA uptake and oxidation in male mice and accelerated the onset of cardiac metabolic remodeling, with an early isoform shift of carnitine palmitoyltransferase 1 (CPT1) toward increased CPT1a, reduced entry of long chain fatty acid (LCFA) into oxidative metabolism (0.59 ± 0.02 vs. 0.72 ± 0.02 in RT TAC hearts, p < .05) and increased carbohydrate oxidation with altered glucose transporter content. BAT activation with TAC reduced early hypertrophic expression of β-MHC by 61% versus RT-TAC and reduced pro-fibrotic TGF-β1 and COL3α1 expression. While cardiac natriuretic peptide expression was yet to increase at only 3 days TAC, Nppa and Nppb expression were elevated in Cold TAC versus RT TAC hearts 2.7- and 2.4-fold, respectively. Eliminating BAT thermogenic activation with UCP1 KO mice eliminated differences between Cold TAC and RT TAC hearts, confirming effects of BAT activation rather than autonomous cardiac responses to cold. Female responses to BAT activation were blunted, with limited UCP1 changes with cold, partly due to already activated BAT in females at RT compared to thermoneutrality. These data reveal a previously unknown physiological mechanism of UCP1-dependent BAT activation in attenuating early cardiac hypertrophic and profibrotic signaling and accelerating remodeled metabolic activity in the heart at the onset of cardiac stress.

Cold-induced expression of a truncated adenylyl cyclase 3 acts as rheostat to brown fat function.

Promoting brown adipose tissue (BAT) activity innovatively targets obesity and metabolic disease. While thermogenic activation of BAT is well understood, the rheostatic regulation of BAT to avoid excessive energy dissipation remains ill-defined. Here, we demonstrate that adenylyl cyclase 3 (AC3) is key for BAT function. We identified a cold-inducible promoter that generates a 5' truncated AC3 mRNA isoform (Adcy3-at), whose expression is driven by a cold-induced, truncated isoform of PPARGC1A (PPARGC1A-AT). Male mice lacking Adcy3-at display increased energy expenditure and are resistant to obesity and ensuing metabolic imbalances. Mouse and human AC3-AT are retained in the endoplasmic reticulum, unable to translocate to the plasma membrane and lack enzymatic activity. AC3-AT interacts with AC3 and sequesters it in the endoplasmic reticulum, reducing the pool of adenylyl cyclases available for G-protein-mediated cAMP synthesis. Thus, AC3-AT acts as a cold-induced rheostat in BAT, limiting adverse consequences of cAMP activity during chronic BAT activation.

Adaptive thermogenesis of the brown adipose tissue in Apodemus chevrieri during cold acclimation

Abstract To investigate the effect of low temperature on body mass, thermogenic activity, and Uncoupling protein-1 (UCP1) content of brown adipose tissue (BAT) in Chevrier’s field mouse (Apodemus chevrieri), 50 healthy adult mice with similar body mass were used in our experiment. They were divided into five groups of ten individuals as follows: a control group (0 d), where animals were maintained under 25 ± 1°C and a 12L:12D (light:dark, lights on 08:00) photoperiod; the other groups were maintained under 5 ± 1°C and a 12L:12D photoperiod for 7 d, 14 d, 21 d and 28 d, respectively. At the end of the experiment, the changes in body mass, resting metabolic rate (RMR), nonshivering thermogenesis (NST), BAT mass, mitochondrial protein (MP) content, cytochrome c oxidase (COX) activity, and UCP1 content were measured. The results showed that compared with the control group (0 d), the body mass of the cold acclimation groups decreased significantly. In contrast, RMR, NST, BAT mass, MP content, COX activity, and UCP1 content of the cold acclimation group increased significantly. After cold acclimation for 28 days, RMR increased by 89.4%, NST increased by 50.4%, BAT mass increased by 44.6%, and UCP1 content increased by 36.0%. The ratio of (NST − RMR)/RMR was 1.03 after seven days and then dropped to 0.59 on day 21, and remained at a steady level thereafter. Evidently, NST was significantly positively correlated with BAT mass and UCP1 content. The results indicated that under continuous cold exposure, A. chevrieri could take appropriate measures to reduce body mass, increase RMR, induce BAT tissue proliferation, and unregulated UCP1 expression, thereby enhancing BAT thermogenic activity to cope with a low-temperature environment. In the early stage of cold acclimation, NST was dominant in thermogenesis, but as time continued, it reduced. This may represent a unique energy adaptation strategy of small rodents originating from the north and spreading southward toward the Hengduan Mountain regions.

Ubiquitin ligase RNF20 coordinates sequential adipose thermogenesis with brown and beige fat-specific substrates

In mammals, brown adipose tissue (BAT) and inguinal white adipose tissue (iWAT) execute sequential thermogenesis to maintain body temperature during cold stimuli. BAT rapidly generates heat through brown adipocyte activation, and further iWAT gradually stimulates beige fat cell differentiation upon prolonged cold challenges. However, fat depot-specific regulatory mechanisms for thermogenic activation of two fat depots are poorly understood. Here, we demonstrate that E3 ubiquitin ligase RNF20 orchestrates adipose thermogenesis with BAT- and iWAT-specific substrates. Upon cold stimuli, BAT RNF20 is rapidly downregulated, resulting in GABPα protein elevation by controlling protein stability, which stimulates thermogenic gene expression. Accordingly, BAT-specific Rnf20 suppression potentiates BAT thermogenic activity via GABPα upregulation. Moreover, upon prolonged cold stimuli, iWAT RNF20 is gradually upregulated to promote de novo beige adipogenesis. Mechanistically, iWAT RNF20 mediates NCoR1 protein degradation, rather than GABPα, to activate PPARγ. Together, current findings propose fat depot-specific regulatory mechanisms for temporal activation of adipose thermogenesis.

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

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

Acute deep brain stimulation of the paraventricular nucleus of the hypothalamus increases brown adipose tissue thermogenesis in rats

Brown adipose tissue (BAT) activity is controlled by the sympathetic nervous system. Activation of BAT has shown significant promise in preclinical studies to elicit weight loss. Since the hypothalamic paraventricular nucleus (PVN) contributes to the regulation of BAT thermogenic activity, we sought to determine the effects of electrical stimulation of the PVN as a model of deep brain stimulation (DBS) for increasing BAT sympathetic nerve activity (SNA). The rostral raphe pallidus area (rRPa) was also chosen as a target for DBS since it contains the sympathetic premotor neurons for BAT. Electrical stimulation (100 µA, 100 µs, 100 Hz, for 5 min at a 50 % duty cycle) of the PVN increased BAT SNA and BAT thermogenesis. These effects were prevented by a local nanoinjection of bicuculline, a GABAA receptor antagonist. We suggest that electrical stimulation of the PVN elicited local release of GABA, which inhibited BAT sympathoinhibitory neurons in PVN, thereby releasing a restraint on BAT SNA. Electrical stimulation of the rRPa inhibited BAT thermogenesis and this was prevented by a local nanoinjection of bicuculline, suggesting that local release of GABA suppressed BAT SNA. Electrical stimulation of the PVN activates BAT metabolism via a mechanism that may include activation of local GABAA receptors. These findings contribute to our understanding of the mechanisms underlying the effects of DBS in the regulation of fat metabolism and provide a foundation for further DBS studies targeting hypothalamic circuits regulating BAT thermogenesis as a therapy for obesity.

Anti-Obesity Effects of Metformin: A Scoping Review Evaluating the Feasibility of Brown Adipose Tissue as a Therapeutic Target.

Brown adipose tissue (BAT) is increasingly recognized as the major therapeutic target to promote energy expenditure and ameliorate diverse metabolic complications. There is a general interest in understanding the pleiotropic effects of metformin against metabolic complications. Major electronic databases and search engines such as PubMed/MEDLINE, Google Scholar, and the Cochrane library were used to retrieve and critically discuss evidence reporting on the impact of metformin on regulating BAT thermogenic activity to ameliorate complications linked with obesity. The summarized evidence suggests that metformin can reduce body weight, enhance insulin sensitivity, and improve glucose metabolism by promoting BAT thermogenic activity in preclinical models of obesity. Notably, this anti-diabetic agent can affect the expression of major thermogenic transcriptional factors such as uncoupling protein 1 (UCP1), nuclear respiratory factor 1 (NRF1), and peroxisome-proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α) to improve BAT mitochondrial function and promote energy expenditure. Interestingly, vital molecular markers involved in glucose metabolism and energy regulation such as AMP-activated protein kinase (AMPK) and fibroblast growth factor 21 (FGF21) are similarly upregulated by metformin treatment in preclinical models of obesity. The current review also discusses the clinical relevance of BAT and thermogenesis as therapeutic targets. This review explored critical components including effective dosage and appropriate intervention period, consistent with the beneficial effects of metformin against obesity-associated complications.

DEPTOR loss impairs brown adipocyte development in vitro but has limited impacts in mice

ObjectivesThe mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that regulates growth and metabolism. In mice, activation of mTOR controls cold adaptation by promoting the recruitment and the activation of brown adipose tissue (BAT). DEP-domain containing mTOR-interacting protein (DEPTOR) interacts with mTOR to modulate its activity. Whether DEPTOR levels are modulated by cold in BAT and whether this protein regulates brown adipocyte development and thermogenic activation has never been tested. MethodsDEPTOR levels were measured in mouse tissues upon cold exposure and in brown preadipocytes following the induction of adipogenesis. Lentiviruses expressing short-hairpin RNA were used to repress DEPTOR expression in brown preadipocytes in vitro. Conditional deletion of DEPTOR in brown preadipocytes and in mature brown fat cells was achieved by crossing DEPTOR floxed mice with either Myf5-Cre or Ucp1-CreERT2 mice. These animals were exposed to cold and extensively phenotyped. ResultsDEPTOR is highly expressed in BAT and its levels are induced by chronic cold exposure, a condition that triggers BAT expansion and activation. Supporting a role for DEPTOR in brown fat cell recruitment, we found that DEPTOR is induced during brown adipocyte development and that its depletion impairs adipogenesis in vitro. This adipogenic lesion was associated with defects in both Akt activation and the expression of key adipogenic regulators. Conditional deletion of DEPTOR in brown preadipocytes or mature brown fat cells did not impact BAT recruitment and thermogenesis in mice but slightly reduced the expression of adipogenic and lipogenic genes. ConclusionsDEPTOR is highly expressed in BAT and its levels are dynamically regulated during brown fat cell development and upon cold exposure. Although DEPTOR depletion severely represses brown fat adipogenesis in vitro, its deletion is dispensable for BAT development, recruitment, and thermogenic activation in mice.

Nutraceuticals in Brown Adipose Tissue Activation.

Obesity and its associated comorbidities have become pandemic, and challenge the global healthcare system. Lifestyle changes, nutritional interventions and phamaceuticals should be differently combined in a personalized strategy to tackle such a public health burden. Altered brown adipose tissue (BAT) function contributes to the pathophysiology of obesity and glucose metabolism dysfunctions. BAT thermogenic activity burns glucose and fatty acids to produce heat through uncoupled respiration, and can dissipate the excessive calorie intake, reduce glycemia and circulate fatty acids released from white adipose tissue. Thus, BAT activity is expected to contribute to whole body energy homeostasis and protect against obesity, diabetes and alterations in lipid profile. To date, pharmacological therapies aimed at activating brown fat have failed in clinical trials, due to cardiovascular side effects or scarce efficacy. On the other hand, several studies have identified plant-derived chemical compounds capable of stimulating BAT thermogenesis in animal models, suggesting the translational applications of dietary supplements to fight adipose tissue dysfunctions. This review describes several nutraceuticals with thermogenic properties and provides indications, at a molecular level, of the regulation of the adipocyte thermogenesis by the mentioned phytochemicals.

Mass Spectrometry-Based Lipidomics of Brown Adipose Tissue and Plasma of New-Born Lambs Subjected to Short-Term Cold Exposure.

Simple SummaryThis study evaluated the impact of short-term cold exposure (2-days) on new-born lamb brown adipose tissue (BAT) and plasma lipid composition, and identified potential biomarkers to predict BAT activity. Results from the present study propose that short-term cold exposure provokes profound changes in BAT and the plasma lipidome composition of new-born lambs. A significant increase in key lipid classes was observed, where they seem to cooperate as one in order to enhance lipid metabolism via BAT thermogenic activation and adipocyte survival during cold adaptation.During cold exposure, brown adipose tissue (BAT) holds the key mechanism in the generation of heat, thus inducing thermogenic adaptation in response to cooler environmental changes. This process can lead to a major lipidome remodelling in BAT, where the increase in abundance of many lipid classes plays a significant role in the thermogenic mechanisms for heat production. This study aimed to identify different types of lipids, through liquid chromatography–mass spectrometry (LC-MS), in BAT and plasma during a short-term cold challenge (2-days), or not, in new-born lambs. Fifteen new-born Romney lambs were selected randomly and divided into three groups: Group 1 (n = 3) with BAT and plasma obtained within 24 h after birth, as a control; Group 2 (n = 6) kept indoors for two days at an ambient temperature (20–22 °C) and Group 3 (n = 6) kept indoors for two days at a cold temperature (4 °C). Significant differences in lipid composition of many lipid categories (such as glycerolipids, glycerophospholipids, sphingolipids and sterol lipids) were observed in BAT and plasma under cold conditions, compared with ambient conditions. Data obtained from the present study suggest that short-term cold exposure induces profound changes in BAT and plasma lipidome composition of new-born lambs, which may enhance lipid metabolism via BAT thermogenic activation and adipocyte survival during cold adaptation. Further analysis on the roles of these lipid changes, validation of potential biomarkers for BAT activity, such as LPC 18:1 and PC 35:6, should contribute to the improvement of new-born lamb survival. Collectively, these observations help broaden the knowledge on the variations of lipid composition during cold exposure.

Dose-Dependent Effect of Melatonin on BAT Thermogenesis in Zücker Diabetic Fatty Rat: Future Clinical Implications for Obesity.

Experimental data have revealed that melatonin at high doses reduced obesity and improved metabolic outcomes in experimental models of obesity, mainly by enhancing brown adipose tissue (BAT) thermogenesis. A potential dose-response relationship has yet to be performed to translate these promising findings into potential clinical therapy. This study aimed to assess the effects of different doses of melatonin on interscapular BAT (iBAT) thermogenic capacity in Zücker diabetic fatty (ZDF) rats. At 6 wk of age, male ZDF rats were divided into four groups (n = 4 per group): control and those treated with different doses of melatonin (0.1, 1, and 10 mg/kg of body weight) in their drinking water for 6 wk. Body weight (BW) was significantly decreased at doses of 1 and 10 mg/kg of melatonin, but not at 0.1 mg/kg compared with the control, with a similar rate of BW decrease being reached at the dose of 1 mg/kg (by ~11%) and 10 mg/kg (by ~12%). This effect was associated with a dose-dependent increase in the thermal response to the baseline condition or acute cold challenge in the interscapular area measurable by infrared thermography, with the highest thermal response being recorded at the 10 mg/kg dose. Upon histology, melatonin treatment markedly restored the typical brownish appearance of the tissue and promoted a shift in size distribution toward smaller adipocytes in a dose-dependent fashion, with the most pronounced brownish phenotype being observed at 10 mg/kg of melatonin. As a hallmark of thermogenesis, the protein level of uncoupled protein 1 (UCP1) from immunofluorescence and Western blot analysis increased significantly and dose-dependently at all three doses of melatonin, reaching the highest level at the dose of 10 mg/kg. Likewise, all three doses of melatonin modulated iBAT mitochondrial dynamics by increasing protein expression of the optic atrophy protein type 1 (OPA1) fusion marker and decreasing that of the dynamin-related protein1 (DRP1) fission marker, again dose-dependently, with the highest and lowest expression levels, respectively, being reached at the 10 mg/kg dose. These findings highlight for the first time the relevance of the dose-dependency of melatonin toward BW control and BAT thermogenic activation, which may have potential therapeutic implications for the treatment of obesity. To clinically apply the potential therapeutic of melatonin for obesity, we consider that the effective animal doses that should be extrapolated to obese individuals may be within the dose range of 1 to 10 mg/kg.

Coffee intake mitigated high fat diet‐induced whitening of brown adipose tissue in obese mice

The aim of this study was to evaluate the preventive and therapeutic effect of coffee consumption on metabolic and molecular aspects of adipose tissue (AT) remodeling in a high‐fat diet‐induced obesity mice. Our hypothesis is that coffee consumption mitigates brown adipose tissue (BAT) whitening and prevent obesity related comorbidities. 50 male mice of 90 days of the C57BL6 lineage divided into 3 groups: Control (C); High‐fat (HF) and prevention 1% w/w coffee (HF‐CP). After 10 weeks, the HF group gave rise to the treatment 1% w/w coffee group (HF‐CT), totaling 4 different groups for another 4 weeks, totaling 14 weeks of experiment. The HF‐CP group has lower body mass than the high fat (HF) group (‐8%, p &lt; 0.05), better AT distribution, with smaller adipocytes and smaller mean sectional area (p&lt;0.05). Both groups that received coffee associated with a high‐fat diet (HF‐CP and HF‐CT) improved glucose metabolism when compared to the HF group, the glucose tolerance test in the HF‐CP group was similar to the C. Coffee consumption prevented hyperinsulinemia and hyperleptinemia in the HF‐CP group, this group did not develop insulin resistance and in the group that consumed coffee after induction of obesity (HF‐CT) the hyperinsulinemia was attenuated. The plasma concentration of IL‐6 was reduced (p&lt;0.05) in HF‐CP and HF‐CT when compared to the HF group. The immunofluorescence expression of UCP‐1was more intense in BAT of HF‐CP compared to others groups. Thermogenesis and mitochondrial biogenesis in BAT were higher with increased gene expression of UCP‐1, PPARγ, PRDM16, PCG1α, Rβ3A, OPA‐1in HF‐CP. Coffee has better effects when consumed preventively with better AT remodeling, with maintenance of the brown phenotype, thermogenic activity in BAT. Furthermore, coffee consumption was able to attenuate body mass gain, improve glucose and leptin homeostasis. When consumed after obesity induction, as a treatment, it was not as effective as prevention, however, there was also better BAT remodeling with attenuation of lightening, greater expression of thermogenic genes and reversal of hyperinsulinemia.

199-LB: ATF4 Deletion in Brown Adipose Tissue Impairs Adaptive Thermogenesis but Attenuates Diet-Induced Insulin Resistance

Short-term cold exposure induces the integrated stress response (ISR) and its main effector, the activating transcription factor 4 (ATF4), in brown adipose tissue (BAT). Furthermore, induction of ATF4 in BAT in response to mitochondrial stress is required for systemic metabolic adaptations mediated by BATokine secretion. To test the hypothesis that ATF4 in BAT is required for cold-induced thermogenesis and to promote BATokine secretion, we selectively deleted ATF4 in BAT by crossing Atf4 floxed mice with mice harboring the Cre recombinase under the control of the Ucp1 promoter (ATF4 BAT KO). KO mice experienced a significant reduction in core body temperature after 3 days of cold exposure relative to wild type (WT) mice. Impaired thermogenesis correlated with reduced BAT mass, and decreased UCP1 and tyrosine hydroxylase protein levels. In addition, KO mice had impaired cold-induced browning of white adipose tissue (WAT) and reduced expression of fibroblast growth factor 21 (Fgf21) and growth and differentiation factor 15 (Gdf15), BATokines shown to be induced in response to BAT thermogenic activation. Surprisingly, although KO mice had equivalent weight gain and fat mass relative to WT mice after 12 weeks of high-fat feeding, glucose and insulin intolerance were ameliorated, and fasting insulin levels were significantly reduced, indicating increased insulin sensitivity. Improved glucose clearance correlated with increased levels of glucose transporters 1 and 4 (GLUT1 and GLUT4) in BAT, but not in WAT or skeletal muscle. Taken together, our data indicate that Atf4 expression is required for full thermogenic activation of BAT and to regulate FGF21 and GDF15 release as BATokines, likely coordinating BAT thermogenesis with systemic adaptations. Finally, our data suggest that in diet-induced obesity, ATF4 downregulation may increase glucose utilization in BAT, contributing to improved glucose homeostasis and insulin sensitivity. Disclosure S. Bjorkman: None. A. A. Marti: None. E. T. Weatherford: None. L. M. García-peña: None. M. J. Potthoff: None. E. D. Abel: Board Member; Self; MyoKardia, Other Relationship; Self; AstraZeneca. R. Pereira: None. Funding National Institutes of Health (R01DK125405)

Lrg1 is a Driver of Brown Adipose Tissue Dysfunction in Obesity

Background Energy is stored as lipid within white adipose tissue (WAT) however, over-nutrition can lead to excess WAT, obesity and metabolic disease. Brown adipose tissue (BAT) has a primary role in energy expenditure through futile cycling of the electron transport chain (ETC) to generate heat. BATs heat producing capacity may be a novel approach to treating obesity. Adult humans possess metabolically active BAT, but the onset of obesity can reduce BAT thermogenic activity and mass. Obesity can bring about morphological changes to BAT, causing BAT to resemble WAT in a process termed “whitening”. WAT is a well-known endocrine organ. Specific endocrine signals from WAT could be responsible for reduced BAT activity in obesity. Leucine-rich-α2-glycoprotein 1 (LRG1) is an adipose-associated secretory protein and potential adipokine, with plasma levels positively correlated to body mass index in humans. Hypothesis It is hypothesized that LRG1 is an adipokine which negatively regulates thermogenesis in BAT and contributes to obesity related loss of BAT. Methods Male Lrg1 global knockout (KO) mice or wild type (WT) controls were fed standard chow (STD) or high fat diet (HFD) for 10 weeks. At 18 weeks of age systemic and BAT metabolic characteristics were determined by whole body indirect calorimetry, high-resolution tissue respirometry, quantitative PCR and immunoblots. Results After 10 weeks of HFD feeding Lrg1 KO mice fed STD and HFD had significantly higher percentage of body mass comprised of BAT than WT controls (Figure 1A). Lrg1 KO mice fed HFD had a lower respiratory exchange ratio (RER), indicating a greater use of fat oxidation compared to WT controls (Figure 1B). Mitochondrial respiration was used to measure changes in cellular energy. In response to HFD WT mice exhibit a decrease in mitochondrial respiration through complex 1 of the electron transport chain (ETC) (Figure 2A). The mitochondrial function of Lrg1 KO mice BAT is protected from HFD-induced dysfunction at Complex 1 of the ETC (Figure 2A). Uncoupling protein 1 (UCP1), a key protein involved in thermogenesis, Ucp1 mRNA is significantly increased in the BAT of Lrg1 KO STD and HFD fed mice compared to WT (Figure 2B). Ucp1 mRNA expression increase in Lrg1 KO mice was confirmed by measuring UCP1 protein with an immunoblot (Figure 2C). Conclusion Loss of Lrg1 protects BAT from dysfunction in diet-induced obesity.

GHS-R in brown fat potentiates differential thermogenic responses under metabolic and thermal stresses.

In response to cold or diet, fatty acids are dissipated into heat through uncoupling protein 1 (UCP1) in brown adipose tissue (BAT). This process is termed non-shivering thermogenesis, which is important for body temperature maintenance and contributes to obesity pathogenesis. Thermogenic enhancement has been considered a promising anti-obesity strategy. Ghrelin and its receptor Growth Hormone Secretagogue Receptor (GHS-R) have critical roles in energy intake, nutrient sensing, and lipid metabolism. We previously reported that global Ghsr-knockout mice have increased energy expenditure due to enhanced thermogenesis. To determine the site of action for GHS-R mediated thermogenesis, we generated brown adipocyte-specific Ghsr knockout mice (UCP1-CreER/Ghsrf/f) and assessed thermogenic responses under regular diet (RD) fed homeostatic metabolic state or high-fat diet (HFD) fed metabolically-impaired obese state, under normal or cold housing environment. Under a RD-feeding, UCP1-CreER/Ghsrf/f mice showed increased body fat and a slightly elevated core body temperature under cold but not under normal temperature. Consistently, the expression of thermogenic genes in BAT of RD-fed UCP1-CreER/Ghsrf/f mice was increased in reposes to cold. Under HFD feeding, HFD-fed UCP1-CreER/Ghsrf/f mice showed no difference in body fat or body temperature under either normal or cold exposure. Interestingly, the expression of thermogenic genes in BAT of HFD-fed UCP1-CreER/Ghsrf/f mice was upregulated under normal temperature but downregulated under cold exposure. Overall, our data show that GHS-R has cell-autonomous effect in brown adipocytes, and GHS-R regulates BAT thermogenic activity in a temperature- and metabolic state-dependent manner. The thermogenic effect of GHS-R in BAT is more pronounced in cold environment and differentially variable based on metabolic state; under cold exposure, GHS-R inhibition in BAT activates thermogenesis under homeostatic state but suppresses thermogenesis under obese state. Our finding collectively suggests that GHS-R in BAT, acting as a “metabolic thermostat”, differentially regulates thermogenesis in response to different metabolic and thermal stimuli.

The endocrine role of brown adipose tissue: An update on actors and actions.

In recent years, brown adipose tissue (BAT) has been recognized not only as a main site of non-shivering thermogenesis in mammals, but also as an endocrine organ. BAT secretes a myriad of regulatory factors. These so-called batokines exert local autocrine and paracrine effects, as well as endocrine actions targeting tissues and organs at a distance. The endocrine batokines include peptide factors, such as fibroblast growth factor-21 (FGF21), neuregulin-4 (NRG4), phospholipid transfer protein (PLTP), interleukin-6, adiponectin and myostatin, and also lipids (lipokines; e.g., 12,13-dihydroxy-9Z-octadecenoic acid [12,13-diHOME]) and miRNAs (e.g., miR-99b). The liver, heart, and skeletal muscle are the most commonly reported targets of batokines. In response to BAT thermogenic activation, batokines such as NRG4 and PLTP are released and act to reduce hepatic steatosis and improve insulin sensitivity. Stress-induced interleukin-6-mediated signaling from BAT to liver favors hepatic glucose production through enhanced gluconeogenesis. Batokines may act on liver to induce the secretion of regulatory hepatokines (e.g. FGF21 and bile acids in response to miR-99b and PLTP, respectively), thereby resulting in a systemic expansion of BAT-originating signals. Batokines also target extrahepatic tissues: FGF21 and 12,13-diHOME are cardioprotective, whereas BAT-secreted myostatin and 12,13-diHOME influence skeletal muscle development and performance. Further research is needed to ascertain in humans the role of batokines, which have been identified mostly in experimental models. The endocrine role of BAT may explain the association between active BAT and a healthy metabolism in the human system, which is characterized by small amounts of BAT and a likely moderate BAT-mediated energy expenditure.

Embryonic exposure to hyper glucocorticoids suppresses brown fat development and thermogenesis via REDD1

Maternal stress during pregnancy is prevailing worldwide, which exposes fetuses to intrauterine hyper glucocorticoids (GC), programming offspring to obesity and metabolic diseases. Despite the importance of brown adipose tissue (BAT) in maintaining long-term metabolic health, impacts of prenatal hyper GC on postnatal BAT thermogenesis and underlying regulations remain poorly defined. Pregnant mice were administrated with synthetic GC dexamethasone (DEX) at levels comparable to fetal GC exposure of stressed mothers. Prenatal GC exposure dose-dependently reduced BAT thermogenic activity, contributing to lower body temperature and higher mortality of neonates; such difference was abolished under thermoneutrality, underscoring BAT deficiency was the major contributor to adverse changes in postnatal thermogenesis due to excessive GC. Prenatal GC exposure highly activated Redd1 expression and reduced Ppargc1a transcription from the alternative promoter (Ppargc1a-AP) in neonatal BAT. During brown adipocyte differentiation, ectopic Redd1 expression reduced Ppargc1a-AP expression and mitochondrial biogenesis; and the inhibitory effects of GC on mitochondrial biogenesis and Ppargc1a-AP expression were blocked by Redd1 ablation. Redd1 reduced protein kinase A phosphorylation and suppressed cyclic adenosine monophosphate (cAMP) -responsive element-binding protein (CREB) binding to the cAMP regulatory element (CRE) in Ppargc1a-AP promoter, leading to Ppargc1a-AP inactivation. In summary, excessive maternal GC exposure during pregnancy dysregulates Redd1-Ppargc1a-AP axis, which impairs fetal BAT development, hampering postnatal thermogenic adaptation and metabolic health of offspring.

The kallikrein\u2013kinin pathway as a mechanism for auto-control of brown adipose tissue activity

Brown adipose tissue (BAT) is known to secrete regulatory factors in response to thermogenic stimuli. Components of the BAT secretome may exert local effects that contribute to BAT recruitment and activation. Here, we found that a thermogenic stimulus leads to enhanced secretion of kininogen (Kng) by BAT, owing to induction of kininogen 2 (Kng2) gene expression. Noradrenergic, cAMP-mediated signals induce KNG2 expression and release in brown adipocytes. Conversely, the expression of kinin receptors, that are activated by the Kng products bradykinin and [Des-Arg9]-bradykinin, are repressed by thermogenic activation of BAT in vivo and of brown adipocytes in vitro. Loss-of-function models for Kng (the circulating-Kng-deficient BN/Ka rat) and bradykinin (pharmacological inhibition of kinin receptors, kinin receptor-null mice) signaling were coincident in showing abnormal overactivation of BAT. Studies in vitro indicated that Kng and bradykinin exert repressive effects on brown adipocyte thermogenic activity by interfering the PKA/p38 MAPK pathway of control of Ucp1 gene transcription, whereas impaired kinin receptor expression enhances it. Our findings identify the kallikrein–kinin system as a relevant component of BAT thermogenic regulation that provides auto-regulatory inhibitory signaling to BAT.

The role of microRNA-23b-5p in regulating brown adipogenesis and thermogenic program.

Enhanced brown adipose tissue (BAT) mass and activity have been demonstrated to promote the expenditure of excess stored energy and reduce prevalence of obesity. Cold is known as a potent stimulator of BAT and activates BAT primarily through the β3-adrenergic-cAMP signaling. Here, we performed RNA-sequencing to identify differential miRNAs in mouse BAT upon cold exposure and a total of 20 miRNAs were validated. With the treatment of CL-316,243 (CL) and forskolin (Fsk) in mouse and human differentiated brown adipocyte cells in vitro, miR-23b-5p, miR-133a-3p, miR-135-5p, miR-491-5p, and miR-150-3p expression decreased and miR-455-5p expression increased. Among these deferentially expressed miRNAs, miR-23b-5p expression was differentially regulated in activated and aging mouse BAT and negatively correlated with Ucp1 expression. Overexpression of miR-23b-5p in the precursor cells from BAT revealed no significant effects on lipid accumulation, but diminished mitochondrial function and decreased expression of BAT specific markers. Though luciferase reporter assays did not confirm the positive association of miR-23b-5p with the 3′UTRs of the predicted target Ern1, miR-23b-5p overexpression may affect brown adipocyte thermogenic capacity mainly through regulating genes expression involving in lipolysis and fatty acid β-oxidation pathways. Our results suggest that miRNAs are involved in cold-mediated BAT thermogenic activation and further acknowledged miR-23b-5p as a negative regulator in controlling thermogenic programs, further providing potential molecular therapeutic targets to increase surplus energy and treat obesity.