Cold Exposure Induces Proliferation of Mature Brown Adipocyte in a ß3-Adrenergic Receptor-Mediated Pathway

Neuronostatin regulates proliferation and differentiation of rat brown primary preadipocytes

Neuronostatin suppresses the differentiation of white preadipocytes. However, the role of neuronostatin in brown adipose tissue remains elusive. Therefore, we investigated the impact of neuronostatin on the proliferation and differentiation of isolated rat brown preadipocytes. We report that neuronostatin and its receptor (GPR107) are synthesized in brown preadipocytes and brown adipose tissue. Furthermore, neuronostatin promotes the replication of brown preadipocytes via the AKT pathway. Notably, neuronostatin suppresses the expression of markers associated with brown adipogenesis (PGC-1α, PPARγ, PRDM16, and UCP1) and reduces cellular mitochondria content. Moreover, neuronostatin impedes the differentiation of preadipocytes by activating the JNK signaling pathway. These effects were not mimicked by somatostatin. Our results suggest that neuronostatin is involved in regulating brown adipogenesis.

Immune response to cold exposure: Role of γδ T cells and TLR2-mediated inflammation

The mammalian body possesses remarkable adaptability to cold exposure, involving intricate adjustments in cellular metabolism, ultimately leading to thermogenesis. However, cold-induced stress can impact immune response, primarily through noradrenaline-mediated pathways. In our study, we utilized a rat model subjected to short-term or long-term mild cold exposure to investigate systemic immune response during the cold acclimation. To provide human relevance, we included a group of regular cold swimmers in our study. Our research revealed complex relationship between cold exposure, neural signaling, immune response, and thermogenic regulation. One-day cold exposure triggered stress response, including cytokine production in white adipose tissue, subsequently activating brown adipose tissue, and inducing thermogenesis. We further studied systemic immune response, including the proportion of leukocytes and cytokines production. Interestingly, γδ T cells emerged as possible regulators in the broader systemic response, suggesting their possible contribution in the dynamic process of cold adaptation. We employed RNA-seq to gain further insights into the mechanisms by which γδ T cells participate in the response to cold. Additionally, we challenged rats exposed to cold with the Toll-like receptor 2 agonist, showing significant modulation of immune response. These findings significantly contribute to understanding of the physiological acclimation that occur in response to cold exposure.

[18F]F-AraG imaging reveals association between neuroinflammation and brown- and bone marrow adipose tissue

Brown and brown-like adipose tissues have attracted significant attention for their role in metabolism and therapeutic potential in diabetes and obesity. Despite compelling evidence of an interplay between adipocytes and lymphocytes, the involvement of these tissues in immune responses remains largely unexplored. This study explicates a newfound connection between neuroinflammation and brown- and bone marrow adipose tissue. Leveraging the use of [18F]F-AraG, a mitochondrial metabolic tracer capable of tracking activated lymphocytes and adipocytes simultaneously, we demonstrate, in models of glioblastoma and multiple sclerosis, the correlation between intracerebral immune infiltration and changes in brown- and bone marrow adipose tissue. Significantly, we show initial evidence that a neuroinflammation-adipose tissue link may also exist in humans. This study proposes the concept of an intricate immuno-neuro-adipose circuit, and highlights brown- and bone marrow adipose tissue as an intermediary in the communication between the immune and nervous systems. Understanding the interconnectedness within this circuitry may lead to advancements in the treatment and management of various conditions, including cancer, neurodegenerative diseases and metabolic disorders.

Adiponectin stimulates Sca1+CD34--adipocyte precursor cells associated with hyperplastic expansion and beiging of brown and white adipose tissue

BACKGROUND

The adipocyte hormone adiponectin improves insulin sensitivity and there is an inverse correlation between adiponectin levels and type-2 diabetes risk. Previous research shows that adiponectin remodels the adipose tissue into a more efficient metabolic sink. For instance, mice that overexpress adiponectin show increased capacity for hyperplastic adipose tissue expansion as evident from smaller and metabolically more active white adipocytes. In contrast, the brown adipose tissue (BAT) of these mice looks "whiter" possibly indicating reduced metabolic activity. Here, we aimed to further establish the effect of adiponectin on adipose tissue expansion and adipocyte mitochondrial function as well as to unravel mechanistic aspects in this area.

METHODS

Brown and white adipose tissues from adiponectin overexpressing (APN tg) mice and littermate wildtype controls, housed at room and cold temperature, were studied by histological, gene/protein expression and flow cytometry analyses. Metabolic and mitochondrial functions were studied by radiotracers and Seahorse-based technology. In addition, mitochondrial function was assessed in cultured adiponectin deficient adipocytes from APN knockout and heterozygote mice.

RESULTS

APN tg BAT displayed increased proliferation prenatally leading to enlarged BAT. Postnatally, APN tg BAT turned whiter than control BAT, confirming previous reports. Furthermore, elevated adiponectin augmented the sympathetic innervation/activation within adipose tissue. APN tg BAT displayed reduced metabolic activity and reduced mitochondrial oxygen consumption rate (OCR). In contrast, APN tg inguinal white adipose tissue (IWAT) displayed enhanced metabolic activity. These metabolic differences between genotypes were apparent also in cultured adipocytes differentiated from BAT and IWAT stroma vascular fraction, and the OCR was reduced in both brown and white APN heterozygote adipocytes. In both APN tg BAT and IWAT, the mesenchymal stem cell-related genes were upregulated along with an increased abundance of Lineage-Sca1+CD34- "beige-like" adipocyte precursor cells. In vitro, the adiponectin receptor agonist Adiporon increased the expression of the proliferation marker Pcna and decreased the expression of Cd34 in Sca1+ mesenchymal stem cells.

CONCLUSIONS

We propose that the seemingly opposite effect of adiponectin on BAT and IWAT is mediated by a common mechanism; while reduced adiponectin levels are linked to lower adipocyte OCR, elevated adiponectin levels stimulate expansion of adipocyte precursor cells that produce adipocytes with intrinsically higher metabolic rate than classical white but lower metabolic rate than classical brown adipocytes. Moreover, adiponectin can modify the adipocytes' metabolic activity directly and by enhancing the sympathetic innervation within a fat depot.

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

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

Brown adipose tissue and aging: A potential role for exercise

Aging is one of the primary risk factors for the development of type 2 diabetes and cardiovascular disease, and regular physical activity can help to delay, prevent, or manage the onset and development of many chronic diseases present in older adults. Brown adipose tissue (BAT) is thermogenic tissue that protects against age-related disease, but BAT activity decreases with age. In this review, we discuss how aging contributes to impaired BAT function by inducing a 'whitening' of the BAT and altering beta 3 adrenergic receptor (β3AR) signaling, uncoupling protein 1 (UCP1) gene expression, and mitochondria respiration, and potential mechanisms for exercise to counteract the effects of aging on BAT.

Mevalonate biosynthesis pathway regulates the development and survival of brown adipocytes

The high thermogenic activity of brown adipose tissue (BAT) has received considerable attention. Here, we demonstrated the role of the mevalonate (MVA) biosynthesis pathway in the regulation of brown adipocyte development and survival. The inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), the rate-limiting enzyme in the MVA pathway and the molecular target of statins, suppressed brown adipocyte differentiation by suppressing protein geranylgeranylation-mediated mitotic clonal expansion. The development of BAT in neonatal mice exposed to statins during the fetal period was severely impaired. Moreover, statin-induced geranylgeranyl pyrophosphate (GGPP) deficiency led to the apoptosis of mature brown adipocytes. Brown adipocyte-specific Hmgcr knockout induced BAT atrophy and disrupted thermogenesis. Importantly, both genetic and pharmacological inhibition of HMGCR in adult mice induced morphological changes in BAT accompanied by an increase in apoptosis, and statin-treated diabetic mice showed worsened hyperglycemia. These findings revealed that MVA pathway-generated GGPP is indispensable for BAT development and survival.

p27Kip1 Deficiency Impairs Brown Adipose Tissue Function Favouring Fat Accumulation in Mice

The aim of this work was to investigate the effect of the whole-body deletion of p27 on the activity of brown adipose tissue and the susceptibility to develop obesity and glucose homeostasis disturbances in mice, especially when subjected to a high fat diet. p27 knockout (p27^-/-) and wild type (WT) mice were fed a normal chow diet or a high fat diet (HFD) for 10-weeks. Body weight and composition were assessed. Insulin and glucose tolerance tests and indirect calorimetry assays were performed. Histological analysis of interscapular BAT (iBAT) was carried out, and expression of key genes/proteins involved in BAT function were characterized by qPCR and Western blot. iBAT activity was estimated by ¹⁸F-fluorodeoxyglucose (¹⁸FDG) uptake with microPET. p27^-/- mice were more prone to develop obesity and insulin resistance, exhibiting increased size of all fat depots. p27^-/- mice displayed a higher respiratory exchange ratio. iBAT presented larger adipocytes in p27^-/- HFD mice, accompanied by downregulation of both Glut1 and uncoupling protein 1 (UCP1) in parallel with defective insulin signalling. Moreover, p27^-/- HFD mice exhibited impaired response to cold exposure, characterized by a reduced iBAT ¹⁸FDG uptake and difficulty to maintain body temperature when exposed to cold compared to WT HFD mice, suggesting reduced thermogenic capacity. These data suggest that p27 could play a role in BAT activation and in the susceptibility to develop obesity and insulin resistance.

The regulation of adipocyte growth in white adipose tissue

Adipocytes can increase in volume up to a thousand-fold, storing excess calories as triacylglycerol in large lipid droplets. The dramatic morphological changes required of adipocytes demands extensive cytoskeletal remodeling, including lipid droplet and plasma membrane expansion. Cell growth-related signalling pathways are activated, stimulating the production of sufficient amino acids, functional lipids and nucleotides to meet the increasing cellular needs of lipid storage, metabolic activity and adipokine secretion. Continued expansion gives rise to enlarged (hypertrophic) adipocytes. This can result in a failure to maintain growth-related homeostasis and an inability to cope with excess nutrition or respond to stimuli efficiently, ultimately leading to metabolic dysfunction. We summarize recent studies which investigate the functional and cellular structure remodeling of hypertrophic adipocytes. How adipocytes adapt to an enlarged cell size and how this relates to cellular dysfunction are discussed. Understanding the healthy and pathological processes involved in adipocyte hypertrophy may shed light on new strategies for promoting healthy adipose tissue expansion.

Brown Fat as a Regulator of Systemic Metabolism beyond Thermogenesis

Brown adipose tissue (BAT) is a specialized tissue for nonshivering thermogenesis to dissipate energy as heat. Although BAT research has long been limited mostly in small rodents, the rediscovery of metabolically active BAT in adult humans has dramatically promoted the translational studies on BAT in health and diseases. Moreover, several remarkable advancements have been made in brown fat biology over the past decade: The molecular and functional analyses of inducible thermogenic adipocytes (socalled beige adipocytes) arising from a developmentally different lineage from classical brown adipocytes have been accelerated. In addition to a well-established thermogenic activity of uncoupling protein 1 (UCP1), several alternative thermogenic mechanisms have been discovered, particularly in beige adipocytes. It has become clear that BAT influences other peripheral tissues and controls their functions and systemic homeostasis of energy and metabolic substrates, suggesting BAT as a metabolic regulator, other than for thermogenesis. This notion is supported by discovering that various paracrine and endocrine factors are secreted from BAT. We review the current understanding of BAT pathophysiology, particularly focusing on its role as a metabolic regulator in small rodents and also in humans.

Chronic cold exposure induces autophagy to promote fatty acid oxidation, mitochondrial turnover, and thermogenesis in brown adipose tissue

Autophagy plays an important role in lipid breakdown, mitochondrial turnover, and mitochondrial function during brown adipose tissue (BAT) activation by thyroid hormone, but its role in BAT during adaptive thermogenesis remains controversial. Here, we examined BAT from mice exposed to 72 h of cold challenge as well as primary brown adipocytes treated with norepinephrine and found increased autophagy as well as increased β-oxidation, mitophagy, mitochondrial turnover, and mitochondrial activity. To further understand the role of autophagy of BAT in vivo, we generated BAT-specific Atg5 knockout (Atg5cKO) mice and exposed them to cold for 72 h. Interestingly, BAT-specific Atg5cKO mice were unable to maintain body temperature after chronic cold exposure and displayed deranged mitochondrial morphology and reactive oxygen species damage in their BAT. Our findings demonstrate the critical role of autophagy in adaptive thermogenesis, fatty acid metabolism, and mitochondrial function in BAT during chronic cold exposure.

Progenitor-like characteristics in a subgroup of UCP1+ cells within white adipose tissue

Thermogenic beige fat found in white adipose tissue is a potential therapeutic target to curb the global obesity and diabetes epidemic. However, these inducible thermogenic beige adipocytes have been thought to be short-lived and to rapidly convert to "white-like" adipocytes after discontinuing stimuli. In this study, using effective labeling techniques and genetic mouse tools, we demonstrate that a subset of UCP1+ cells that exist within white adipose tissue are able to self-divide and contribute to new beige adipocyte recruitment in response to β3 stimuli. When these cells are depleted or their adipogenic capability is blocked, β3-induced beige adipocyte formation is impaired. We also identify a cell-cycle machinery of p21 and CDKN2A as a molecular basis of beige adipocyte regulation. Collectively, our findings provide new insights into the cellular and molecular mechanisms of beige adipocyte regulation and potential therapeutic opportunities to induce the beige phenotype and treat metabolic disease.

Effects of Caffeine on Brown Adipose Tissue Thermogenesis and Metabolic Homeostasis: A Review

The impact of brown adipose tissue (BAT) metabolism on understanding energy balance in humans is a relatively new and exciting field of research. The pathogenesis of obesity can be largely explained by an imbalance between caloric intake and energy expenditure, but the underlying mechanisms are far more complex. Traditional non-selective sympathetic activators have been used to artificially elevate energy utilization, or suppress appetite, however undesirable side effects are apparent with the use of these pharmacological interventions. Understanding the role of BAT, in relation to human energy homeostasis has the potential to dramatically offset the energy imbalance associated with obesity. This review discusses paradoxical effects of caffeine on peripheral adenosine receptors and the possible role of adenosine in increasing metabolism is highlighted, with consideration to the potential of central rather than peripheral mechanisms for caffeine mediated BAT thermogenesis and energy expenditure. Research on the complex physiology of adipose tissue, the embryonic lineage and function of the different types of adipocytes is summarized. In addition, the effect of BAT on overall human metabolism and the extent of the associated increase in energy expenditure are discussed. The controversy surrounding the primary β-adrenoceptor involved in human BAT activation is examined, and suggestions as to the lack of translational findings from animal to human physiology and human in vitro to in vivo models are provided. This review compares and distinguishes human and rodent BAT effects, thus developing an understanding of human BAT thermogenesis to aid lifestyle interventions targeting obesity and metabolic syndrome. The focus of this review is on the effect of BAT thermogenesis on overall metabolism, and the potential therapeutic effects of caffeine in increasing metabolism via its effects on BAT.

Lactate fluxes mediated by the monocarboxylate transporter-1 are key determinants of the metabolic activity of beige adipocytes

Activation of energy-dissipating brown/beige adipocytes represents an attractive therapeutic strategy against metabolic disorders. While lactate is known to induce beiging through the regulation of Ucp1 gene expression, the role of lactate transporters on beige adipocytes' ongoing metabolic activity remains poorly understood. To explore the function of the lactate-transporting monocarboxylate transporters (MCTs), we used a combination of primary cell culture studies, 13C isotopic tracing, laser microdissection experiments, and in situ immunofluorescence of murine adipose fat pads. Dissecting white adipose tissue heterogeneity revealed that the MCT1 is expressed in inducible beige adipocytes as the emergence of uncoupling protein 1 after cold exposure was restricted to a subpopulation of MCT1-expressing adipocytes suggesting MCT1 as a marker of inducible beige adipocytes. We also observed that MCT1 mediates bidirectional and simultaneous inward and outward lactate fluxes, which were required for efficient utilization of glucose by beige adipocytes activated by the canonical β3-adrenergic signaling pathway. Finally, we demonstrated that significant lactate import through MCT1 occurs even when glucose is not limiting, which feeds the oxidative metabolism of beige adipocytes. These data highlight the key role of lactate fluxes in finely tuning the metabolic activity of beige adipocytes according to extracellular metabolic conditions and reinforce the emerging role of lactate metabolism in the control of energy homeostasis.

UCP1-dependent and UCP1-independent metabolic changes induced by acute cold exposure in brown adipose tissue of mice

BACKGROUND

Brown adipose tissue (BAT) is a site of metabolic thermogenesis mediated by mitochondrial uncoupling protein 1 (UCP1) and represents a target for a therapeutic intervention in obesity. Cold exposure activates UCP1-mediated thermogenesis in BAT and causes drastic changes in glucose, lipid, and amino acid metabolism; however, the relationship between these metabolic changes and UCP1-mediated thermogenesis is not fully understood.

METHODS

We conducted metabolomic and GeneChip array analyses of BAT after 4-h exposure to cold temperature (10 °C) in wild-type (WT) and UCP1-KO mice.

RESULTS

Cold exposure largely increased metabolites of the glycolysis pathway and lactic acid levels in WT, but not in UCP1-KO, mice, indicating that aerobic glycolysis is enhanced as a consequence of UCP1-mediated thermogenesis. GeneChip array analysis of BAT revealed that there were 2865 genes upregulated by cold exposure in WT mice, and 838 of these were upregulated and 74 were downregulated in UCP1-KO mice. Pathway analysis revealed the enrichment of genes involved in fatty acid (FA) β oxidation and triglyceride (TG) synthesis in both WT and UCP1-KO mice, suggesting that these metabolic pathways were enhanced by cold exposure independently of UCP1-mediated thermogenesis. FA and cholesterol biosynthesis pathways were enhanced only in UCP1-KO mice. Cold exposure also significantly increased the BAT content of proline, tryptophan, and phenylalanine amino acids in both WT and UCP1-KO mice. In WT mice, cold exposure significantly increased glutamine content and enhanced the expression of genes related to glutamine metabolism. Surprisingly, aspartate was almost completely depleted after cold exposure in UCP1-KO mice. Gene expression analysis suggested that aspartate was actively utilized after cold exposure both in WT and UCP1-KO mice, but it was replenished from intracellular N-acetyl-aspartate in WT mice.

CONCLUSIONS

These results revealed that cold exposure induces UCP1-mediated thermogenesis-dependent glucose utilization and UCP1-independent active lipid metabolism in BAT. In addition, cold exposure largely affects amino acid metabolism in BAT, especially UCP1-dependently enhances glutamine utilization. These results contribute a comprehensive understanding of UCP1-mediated thermogenesis-dependent and thermogenesis-independent metabolism in BAT.

Both proliferation and lipogenesis of brown adipocytes contribute to postnatal brown adipose tissue growth in mice

Brown adipose tissue (BAT) is the primary non-shivering thermogenesis organ in mammals, which plays essential roles in maintaining the body temperature of infants. Although the development of BAT during embryogenesis has been well addressed in rodents, how BAT grows after birth remains unknown. Using mouse interscapular BAT (iBAT) as an example, we studied the cellular and molecular mechanisms that regulate postnatal BAT growth. By analyzing the developmental dynamics of brown adipocytes (BAs), we found that BAs size enlargement partially accounts for iBAT growth. By investigating the BAs cell cycle activities, we confirmed the presence of proliferative BAs in the neonatal mice. Two weeks after birth, most of the BAs exit cell cycle, and the further expansion of the BAT was mainly due to lipogenesis-mediated BAs volume increase. Microscopy and fluorescence-activated cell sorting analyses suggest that most BAs are mononuclear and diploid. Based on the developmental dynamics of brown adipocytes, we propose that the murine iBAT has two different growth phases between birth and weaning: increase of BAs size and number in the first two weeks, and BAs size enlargement thereafter. In summary, our data demonstrate that both lipogenesis and proliferation of BAs contribute to postnatal iBAT growth in mice.

Adropin stimulates proliferation but suppresses differentiation in rat primary brown preadipocytes

Adropin is a peptide hormone encoded by Energy Homeostasis Associated (Enho) gene. Adropin modulates glucose and lipid metabolism, and adiposity. Recently, we found that adropin suppresses differentiation of rodent white preadipocytes into mature fat cells. By contrast, the role of adropin in controlling brown adipogenesis is largely unknown. Therefore, in the present study we evaluated the effects of adropin on proliferation and differentiation of adipocyte precursor cells in rats. Brown adipocyte precursor cells were isolated from male Wistar rats. Cell replication was measured by BrdU incorporation. Gene expression was studied using real time PCR. Protein phosphorylation and production was assessed by Western blot. Lipid accumulation was evaluated by Oil Red O staining. Colorimetric kits were used to evaluate glycerol and free fatty acids release. We report here that adropin stimulates proliferation of brown preadipocytes. Moreover, in brown preadipocytes, adropin suppresses mRNA expression of adipogenic genes (C/ebpα, C/ebpβ, Pgc1α, Pparγ and Prdm16) during differentiation process. In addition, adropin suppresses UCP1 protein production in brown adipocytes. Finally, adropin reduces intracellular lipid content in brown adipocytes. These results indicate that adropin stimulates proliferation of brown preadipocytes and suppresses their differentiation into mature adipocytes.

Glucose metabolism in brown adipose tissue determined by deuterium metabolic imaging in rats

BACKGROUND/OBJECTIVES

Brown adipose tissue (BAT) has gained growing interest as a potential target for treatment of obesity. Currently, the most widely used technique/method for in vivo measurements of BAT activity in humans is ¹⁸FDG PET/CT. To supplement these investigations novel radiation-free methods are warranted. Deuterium metabolic imaging (DMI) is a novel modality that combines magnetic resonance spectroscopic (MRS) imaging with deuterium-labelled glucose (²H-glucose). This allows for spatio-temporal and metabolic imaging beyond glucose uptake. We aimed to evaluate if DMI could discriminate glucose metabolism in BAT of cold-acclimatised and thermoneutral rats.

SUBJECTS/METHODS

Male Sprague-Dawley rats were housed in a cold environment (9 °C, n = 10) or at thermoneutrality (30 °C, n = 11) for 1 week. For imaging rats were anaesthetized, received a ²H-glucose (1 M, 1.95 g/kg) bolus and DMI was acquired at baseline followed by 20 min time intervals up to 2 h. Furthermore, Dixon MRI was performed for anatomical determination of the interscapular BAT (iBAT) depot along with dynamic contrast enhanced (DCE) MRI to evaluate perfusion.

RESULTS

²H-glucose signal was higher in cold-acclimatised rats compared with thermoneutral rats (p ≤ 0.001) indicating an overall increase in glucose uptake and metabolism. This was in line with a lower fat/water threshold, higher perfusion and increased UCP1 mRNA expression in iBAT (ninefold increment) of cold-acclimatised rats compared with thermoneutral rats.

CONCLUSIONS

We find that DMI can discriminate cold-acclimatised and thermoneutral BAT in rats. This is the first study to evaluate BAT activity by DMI, which may open up for the use of the non-radioactive DMI method for BAT measurements in humans.

Activation of the sympathetic nervous system suppresses mouse white adipose tissue hyperplasia through the β1 adrenergic receptor

Adipose tissue (AT) expands via both hypertrophy and hyperplasia during the development of obesity. While AT hypertrophy involves the increase in size of existing adipocytes, hyperplasia is the process of creating new adipocytes from the pool of adipocyte precursor cells (APCs), which includes adipocyte progenitor cells and preadipocytes. Prior studies have implicated a role of the sympathetic nervous system (SNS) in regulation of hyperplasia in white adipose tissue (WAT). Here, we aimed to determine the mechanisms underlying SNS regulation of APC proliferation in mouse WAT. Using flow cytometry with antibodies against various cell surface markers, along with an intracellular marker of proliferation (Ki67), we quantitated the percentages and proliferative status of adipocyte progenitor cells and preadipocytes in the stromal vascular fraction (SVF) of WAT. In vivo SNS activation through cold exposure, as well as in vitro adrenergic stimulation via exposure to the canonical SNS neurotransmitter norepinephrine (NE), inhibited preadipocyte proliferation. Pretreatment with propranolol, a β1‐ and β2‐adrenergic receptor (AR) antagonist, trended toward rescuing the inhibitory effects of NE in primary cell culture. The selective β1‐AR agonist dobutamine diminished preadipocyte proliferation both in vivo and in vitro, whereas the selective β2‐AR agonist, salbutamol, promoted proliferation in vitro, suggesting that the β1‐AR may mediate the inhibitory effect of NE on preadipocyte proliferation. Taken together, we conclude that SNS activation suppresses preadipocyte proliferation via activation of the β1 AR in WAT.

Mechanisms of Impaired Brown Adipose Tissue Recruitment in Obesity

Brown adipose tissue (BAT) dissipates energy to produce heat. Thus, it has the potential to regulate body temperature by thermogenesis. For the last decade, BAT has been in the spotlight due to its rediscovery in adult humans. This is evidenced by over a hundred clinical trials that are currently registered to target BAT as a therapeutic tool in the treatment of metabolic diseases, such as obesity or diabetes. The goal of most of these trials is to activate the BAT thermogenic program via several approaches such as adrenergic stimulation, natriuretic peptides, retinoids, capsinoids, thyroid hormones, or glucocorticoids. However, the impact of BAT activation on total body energy consumption and the potential effect on weight loss is still limited. Other studies have focused on increasing the mass of thermogenic BAT. This can be relevant in obesity, where the activity and abundance of BAT have been shown to be drastically reduced. The aim of this review is to describe pathological processes associated with obesity that may influence the correct differentiation of BAT, such as catecholamine resistance, inflammation, oxidative stress, and endoplasmic reticulum stress. This will shed light on the thermogenic potential of BAT as a therapeutic approach to target obesity-induced metabolic diseases.

Metformin enhances mitochondrial biogenesis and thermogenesis in brown adipocytes of mice

AIMS

We studied the effect of metformin on the brown adipose tissue (BAT) in a fructose-rich-fed model, focusing on BAT proliferation, differentiation, and thermogenic markers.

MAIN METHODS

C57Bl/6 mice received isoenergetic diets for ten weeks: control (C) or high-fructose (F). For additional eight weeks, animals received metformin hydrochloride (M, 250 mg/kg/day) or saline. After sacrifice, BAT and white fat pads were prepared for light microscopy and molecular analyses.

KEY FINDINGS

Body mass gain, white fat pads, and adiposity index were not different among the groups. There was a reduction in energy intake in the F group and energy expenditure in the F and FM groups. Metformin led to a more massive BAT in both groups CM and FM, associated with a higher adipocyte proliferation (β1-adrenergic receptor, proliferating cell nuclear antigen, and vascular endothelial growth factor), and differentiation (PR domain containing 16, bone morphogenetic protein 7), in part by activating 5' adenosine monophosphate-activated protein kinase. Metformin also enhanced thermogenic markers in the BAT (uncoupling protein type 1, peroxisome proliferator-activated receptor gamma coactivator-1 alpha) through adrenergic stimuli and fibroblast growth factor 21. Metformin might improve mitochondrial biogenesis in the BAT (nuclear respiratory factor 1, mitochondrial transcription factor A), lipolysis (perilipin, adipose triglyceride lipase, hormone-sensitive lipase), and fatty acid uptake (lipoprotein lipase, cluster of differentiation 36, adipocyte protein 2).

SIGNIFICANCE

Metformin effects are not linked to body mass changes, but affect BAT thermogenesis, mitochondrial biogenesis, and fatty acid uptake. Therefore, BAT may be a metformin adjuvant target for the treatment of metabolic disorders.

Differentiation of bone marrow-derived cells toward thermogenic adipocytes in white adipose tissue induced by the β3 adrenergic stimulation

Bone marrow provides progenitors of several types of cells, including muscle and white adipocytes, ensuring peripheral tissue homeostasis. However, the role of bone marrow-derived cells (BMCs) in induction of thermogenic adipocytes is unresolved. The purpose of this study is to examine whether BMCs are involved in the emergence of thermogenic adipocytes through adrenergic activation. Irradiation of mice with 8 Gy of X-ray-depleted BMCs and peripheral blood mononucleated cells (PBMCs), which in turn impaired induction of uncoupling protein 1 (UCP1) through administration of β3 adrenergic receptor agonist, CL 316,243 (CL), in inguinal white adipose tissue (iWAT). In contrast, CL-induced UCP1 induction in brown adipose tissue was unaffected by BMC depletion. Transplantation of normal BMCs into mice depleted of BMCs recovered PBMC levels and rescued the ability of iWAT browning by CL. Furthermore, analyses of mice transplanted with green fluorescent protein (GFP)-labeled BMCs revealed that the number of GFP-positive BMCs and PBMCs were significantly decreased by CL and that GFP-positive stromal cells and GFP-positive UCP1-expressing multilocular adipocytes appeared in iWAT after CL administration, demonstrating differentiation of BMC-derived preadipocytes into UCP1-expressing thermogenic adipocytes. These results unveiled a crucial role of the BMC as a nonresident origin for a subset of thermogenic adipocytes, contributing to browning of white adipose tissue.-Yoneshiro, T., Shin, W., Machida, K., Fukano, K., Tsubota, A., Chen, Y., Yasui, H., Inanami, O., Okamatsu-Ogura, Y., Kimura, K. Differentiation of bone marrow-derived cells toward thermogenic adipocytes in white adipose tissue induced by the β3 adrenergic stimulation.

Effect of ambient temperature on the proliferation of brown adipocyte progenitors and endothelial cells during postnatal BAT development in Syrian hamsters

In Syrian hamsters, brown adipose tissue (BAT) develops postnatally through the proliferation and differentiation of brown adipocyte progenitors. In the study reported here, we investigated how ambient temperature influenced BAT formation in neonatal hamsters. In both hamsters raised at 23 or 30 °C, the interscapular fat changed from white to brown coloration in an age-dependent manner and acquired the typical morphological features of BAT by day 16. However, the expression of uncoupling protein 1, a brown adipocyte marker, and of vascular endothelial growth factor α were lower in the group raised at 30 °C than in that raised at 23 °C. Immunofluorescent staining revealed that the proportion of Ki67-expressing progenitors and endothelial cells was lower in the 30 °C group than in the 23 °C group. These results indicate that warm ambient temperature suppresses the proliferation of brown adipocyte progenitors and endothelial cells and negatively affects the postnatal development of BAT in Syrian hamsters.

Evaluation of Active Brown Adipose Tissue by the Use of Hyperpolarized [1-13C]Pyruvate MRI in Mice

The capacity to increase energy expenditure makes brown adipose tissue (BAT) a putative target for treatment of metabolic diseases such as obesity. Presently, investigation of BAT in vivo is mainly performed by fluoro-d-glucose positron emission tomography (FDG PET)/CT. However, non-radioactive methods that add information on, for example, substrate metabolism are warranted. Thus, the aim of this study was to evaluate the potential of hyperpolarized [1-¹³C]pyruvate Magnetic Resonance Imaging (HP-MRI) to determine BAT activity in mice following chronic cold exposure. Cold (6 °C) and thermo-neutral (30 °C) acclimated mice were scanned with HP-MRI for assessment of the interscapular BAT (iBAT) activity. Comparable mice were scanned with the conventional method FDG PET/MRI. Finally, iBAT was evaluated for gene expression and protein levels of the specific thermogenic marker, uncoupling protein 1 (UCP1). Cold exposure increased the thermogenic capacity 3–4 fold (p < 0.05) as measured by UCP1 gene and protein analysis. Furthermore, cold exposure as compared with thermo-neutrality increased iBAT pyruvate metabolism by 5.5-fold determined by HP-MRI which is in good agreement with the 5-fold increment in FDG uptake (p < 0.05) measured by FDG PET/MRI. iBAT activity is detectable in mice using HP-MRI in which potential changes in intracellular metabolism may add useful information to the conventional FDG PET studies. HP-MRI may also be a promising radiation-free tool for repetitive BAT studies in humans.

Mirabegron, a β3-adrenoceptor agonist reduced platelet aggregation through cyclic adenosine monophosphate accumulation

Mirabegron is a β₃-adrenoceptor agonist and released on the marked for the treatment of overactive bladder. Because mirabegron is the only β₃-adrenoceptor agonist available and substances that increase the levels of cyclic adenosine monophosphate (cAMP) inhibit platelet activity, we tested the hypothesis that mirabegron could have antiplatelet activity. Collagen- and thrombin induced platelet aggregation, thromboxane B2 (TXB₂) and cyclic nucleotides quantification and calcium (Ca²⁺) mobilization were determined in the absence and presence of mirabegron in human washed platelets. Our results revealed that mirabegron (10-300 µM) produced significant inhibitions on platelet aggregation induced by collagen- or thrombin, accompanied by greater intracellular levels of cAMP. The β₃-adrenoceptor antagonist L 748,337 (1 µM) and the adenylate cyclase inhibitor, SQ 22,536 (100 µM) reversed the inhibition induced by mirabegron in thrombin-stimulated platelets. The selective antagonists for β₁-and β₂-adrenoceptors, atenolol and ICI 117,551 (3 µM), respectively did not interfere on the inhibition induced by mirabegron. In Fluo-4 loaded platelets, mirabegron reduced the total and intracellular Ca²⁺ levels. Pre-incubation with mirabegron almost abolished the levels of TXB2. Mirabegron did not augment the intracellular levels of cyclic guanosine monophosphate. In conclusion, mirabegron inhibited human platelet aggregation through cAMP accumulation, thus suggesting that substances that activate β₃-adrenoceptor could be beneficial as adjuvant antiplatelet therapy.

Cell-cycle arrest in mature adipocytes impairs BAT development but not WAT browning, and reduces adaptive thermogenesis in mice

We previously reported brown adipocytes can proliferate even after differentiation. To test the involvement of mature adipocyte proliferation in cell number control in fat tissue, we generated transgenic (Tg) mice over-expressing cell-cycle inhibitory protein p27 specifically in adipocytes, using the aP2 promoter. While there was no apparent difference in white adipose tissue (WAT) between wild-type (WT) and Tg mice, the amount of brown adipose tissue (BAT) was much smaller in Tg mice. Although BAT showed a normal cellular morphology, Tg mice had lower content of uncoupling protein 1 (UCP1) as a whole, and attenuated cold exposure- or β3-adrenergic receptor (AR) agonist-induced thermogenesis, with a decrease in the number of mature brown adipocytes expressing proliferation markers. An agonist for the β3-AR failed to increase the number of proliferating brown adipocytes, UCP1 content in BAT, and oxygen consumption in Tg mice, although the induction and the function of beige adipocytes in inguinal WAT from Tg mice were similar to WT mice. These results show that brown adipocyte proliferation significantly contributes to BAT development and adaptive thermogenesis in mice, but not to induction of beige adipocytes.