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

A Closer Look into White Adipose Tissue Biology and the Molecular Regulation of Stem Cell Commitment and Differentiation

White adipose tissue (WAT) makes up about 20-25% of total body mass in healthy individuals and is crucial for regulating various metabolic processes, including energy metabolism, endocrine function, immunity, and reproduction. In adipose tissue research, "adipogenesis" is commonly used to refer to the process of adipocyte formation, spanning from stem cell commitment to the development of mature, functional adipocytes. Although, this term should encompass a wide range of processes beyond commitment and differentiation, to also include other stages of adipose tissue development such as hypertrophy, hyperplasia, angiogenesis, macrophage infiltration, polarization, etc.… collectively, referred to herein as the adipogenic cycle. The term "differentiation", conversely, should only be used to refer to the process by which committed stem cells progress through distinct phases of subsequent differentiation. Recognizing this distinction is essential for accurately interpreting research findings on the mechanisms and stages of adipose tissue development and function. In this review, we focus on the molecular regulation of white adipose tissue development, from commitment to terminal differentiation, and examine key functional aspects of WAT that are crucial for normal physiology and systemic metabolic homeostasis.

Tetrahydroberberrubine exhibits preventive effect on obesity by activating PGC1α-mediated thermogenesis in white and brown adipose tissue

The escalating prevalence of obesity presents formidable challenges, necessitating the development of effective therapeutic strategies. In this study, we aimed to elucidate the preventive effects on obesity of tetrahydroberberrubine (THBru), a derivative of berberine (BBR) and to unravel its underlying mechanism. Using an obese mouse model induced by a high-fat diet (HFD), THBru was found to markedly ameliorate obesity, as evidenced by reduced body weight, decreased Lee's index, diminished fat mass in epididymal white adipose tissue (WAT) and brown adipose tissue (BAT), alongside improved dyslipidemia. Notably, at the same dose, THBru exhibited superior efficacy compared to BBR. RNA-sequencing and gene set enrichment analysis indicated THBru activated thermogenesis, which was further confirmed in WAT, BAT, and 3T3-L1 cells. Bioinformatics analysis of RNA-sequencing data revealed the candidate gene Pgc1α, a key regulator involved in thermogenesis. Moreover, THBru was demonstrated to elevate the expression of PGC1α by stabilizing its mRNA in WAT, BAT and 3T3-L1 cells. Furthermore, PGC1α knockdown blocked the pro-thermogenic and anti-obesity action of THBru both in vivo and in vitro. This study unravels the preventive effects of THBru on obesity through the activation of PGC1α-mediated thermogenesis, thereby delineating its potential therapeutic implications for obesity and associated disorders.

Lycopene and Garcinia cambogia Induce White-to-Brown Adipose Differentiation: An Innovative Strategy to Curb Obesity

Obesity is considered one of the main risk factors for cardiovascular diseases. The browning process has been recently recognized as a promising anti-obesity therapy. Lycopene (LYC) and Garcinia cambogia fruit extract (GE) might be important resources for anti-obesity drugs; therefore, the aim of this study was to investigate the anti-obesity effects of LYC and GE on 3T3-L1 adipocytes and Zucker rats. Mouse 3T3-L1 pre-adipocytes were differentiated in mature adipocytes and then treated with LYC (0.5 μM), GE (30 mg/mL) or LYC + GE for 24 h. Moreover, male Zucker Crl:ZUC-Leprfa rats were randomly assigned to 5 groups of 10 animals to orally receive Vehicle (Ctrl), Orlistat (20 mg/kg), LYC (5 mg/kg), GE (1000 mg/kg) or LYC + GE for 28 days. LYC, GC extracts and even more LYC + GE stimulated the mRNA and protein expression of thermogenic genes UCP1, CIDEA and DIO2, significantly reduced lipid droplet size and increased lipid droplet number in adipocytes. UCP1 mRNA and protein expression was also increased in the visceral adipose tissue of the rats that received the dietary intake of LYC, GE and even more LYC + GE. Moreover, LYC + GE induced the reorganization of visceral fat depots that showed a great number of small adipocytes and a significant reduction in weight gain and food intake compared to the control group. The obtained results demonstrated that LYC + GE might be used as new approaches for obesity management in order to induce the browning process and achieve a metabolically active tissue instead of a tissue characterized by lipid depot accumulation.

Exercise training and cold exposure trigger distinct molecular adaptations to inguinal white adipose tissue

Exercise training and cold exposure both improve systemic metabolism, but the mechanisms are not well established. Here, we tested the hypothesis that inguinal white adipose tissue (iWAT) adaptations are critical for these beneficial effects and determined the impact of exercise-trained and cold-exposed iWAT on systemic glucose metabolism and the iWAT proteome and secretome. Transplanting trained iWAT into sedentary mice improves glucose tolerance, while cold-exposed iWAT transplantation shows no such benefit. Compared to training, cold leads to more pronounced alterations in the iWAT proteome and secretome, downregulating >2,000 proteins but also boosting the thermogenic capacity of iWAT. In contrast, only training increases extracellular space and vesicle transport proteins, and only training upregulates proteins that correlate with favorable fasting glucose, suggesting fundamental changes in trained iWAT that mediate tissue-to-tissue communication. This study defines the unique exercise training- and cold exposure-induced iWAT proteomes, revealing distinct mechanisms for the beneficial effects of these interventions on metabolic health.

Depletion of TBC1D4 Improves the Metabolic Exercise Response by Overcoming Genetically Induced Peripheral Insulin Resistance

The Rab-GTPase-activating protein (RabGAP) TBC1D4 (AS160) represents a key component in the regulation of glucose transport into skeletal muscle and white adipose tissue (WAT) and is therefore crucial during the development of insulin resistance and type 2 diabetes. Increased daily activity has been shown to be associated with improved postprandial hyperglycemia in allele carriers of a loss-of-function variant in the human TBC1D4 gene. Using conventional Tbc1d4-deficient mice (D4KO) fed a high-fat diet, we show that moderate endurance exercise training leads to substantially improved glucose and insulin tolerance and enhanced expression levels of markers for mitochondrial activity and browning in WAT from D4KO animals. Importantly, in vivo and ex vivo analyses of glucose uptake revealed increased glucose clearance in interscapular brown adipose tissue and WAT from trained D4KO mice. Thus, chronic exercise is able to overcome the genetically induced insulin resistance caused by Tbc1d4 depletion. Gene variants in TBC1D4 may be relevant in future precision medicine as determinants of exercise response.

ARTICLE HIGHLIGHTS

Sexual dimorphism in thermogenic regulators and metrnl expression in adipose tissue of offspring mice exposed to maternal and postnatal overnutrition

Current study investigated the impact of maternal and postnatal overnutrition on phenotype of adipose, in relation to offspring thermogenesis and sex. Female C57BL/6 J mice were fed with CHOW or high fat diet (HFD) for 2 weeks before mating, throughout gestation and lactation. At weaning, pups were fed to 9 weeks old with CHOW or HFD, which resulted in four groups for each gender--male or female: CHOW-CHOW (CC), CHOW-HFD (CH), HFD-CHOW (HC), HFD-HFD (HH). Maternal and post-weaning HFD enhanced thermogenic factors such as Acox1, Dio2 and Cox8b in iBAT of male and female offspring, but increased SIRT1, PGC-1α and UCP1 only in female. However, Acox1, Dio2 and Cox8b mRNA expression and SIRT1, PGC-1α and UCP1 protein expression were only enhanced upon maternal and post-weaning HFD in sWAT and pWAT of female offspring. Increased metrnl expression in adipose were observed in sex- and depot-specific manner, while enhanced circulating metrnl level was only observed in male offspring undergoing maternal HFD. Palmitic acid changed metrnl expression during preadipocytes differentiation and siRNA-mediated knockdown of metrnl inhibited preadipocyte differentiation. Female offspring were more prone to resist adverse outcomes induced by maternal and post-weaning overnutrition, which probably related to metrnl expression and thermogenesis.

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

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

Trim21 Regulates the Postnatal Development and Thermogenesis of Brown Adipose Tissue

Brown adipose tissue undergoes rapid postnatal development to mature and plays a crucial role in thermoregulation and energy expenditure, which protects against cold and obesity. Herein, it is shown that the expression of Trim21 mRNA level of interscapular brown adipose tissue elevates after birth, and peaks at P14 (postnatal day 14). Trim21 depletion severely impairs the maturation of interscapular brown adipose tissue, decreases the expression of a series of thermogenic genes, and reduces energy expenditure. Consistently, the loss of Trim21 also leads to a suppression of white adipose tissue "browning", in response to cold exposure and a β-adrenergic agonist, CL316,243. In addition, Trim21-/- mice are more prone to high-fat diet-induced obesity compared with the control littermates. Taken together, the study for the first time reveals a critical role of Trim21 in regulating iBAT postnatal development and thermogenesis.

Brown adipose Vanin-1 is required for the maintenance of mitochondrial homeostasis and prevents diet-induced metabolic dysfunction

BACKGROUND

Energy-dissipating brown adipocytes have significant potential for improving systemic metabolism. Vanin-1, a membrane-bound pantetheinase, is involved in various biological processes in mice. However, its role in BAT mitochondrial function is still unclear. In this study, we aimed to elucidate the impact of Vanin-1 on BAT function and contribution during overnutrition-induced obesity.

METHODS

Vanin-1 expression was analyzed in different adipose depots in mice. The cellular localization of Vanin-1 was analyzed by confocal microscopy and western blots. Mice lacking Vanin-1 (Vanin-1-/-) were continuously fed either a chow diet or a high-fat diet (HFD) to establish an obesity model. RNA-seq analysis was performed to identify the molecular changes associated with Vanin-1 deficiency during obesity. BAT-specific Vanin-1 overexpression mice were established to determine the effects of Vanin-1 in vivo. Cysteamine treatment was used to examine the effect of enzymatic reaction products of Vanin-1 on BAT mitochondria function in Vanin-1-/- mice.

RESULTS

The results indicate that the expression of Vanin-1 is reduced in BAT from both diet-induced and leptin-deficient obese mice. Study on the subcellular location of Vanin-1 shows that it has a mitochondrial localization. Vanin-1 deficiency results in increased adiposity, BAT dysfunction, aberrant mitochondrial structure, and promotes HFD induced-BAT whitening. This is attributed to the impairment of the electron transport chain (ETC) in mitochondria due to Vanin-1 deficiency, resulting in reduced mitochondrial respiration. Overexpression of Vanin-1 significantly enhances energy expenditure and thermogenesis in BAT, renders mice resistant to diet-induced obesity. Furthermore, treatment with cysteamine rescue the mitochondrial dysfunction in Vanin-1-/- mice.

CONCLUSIONS

Collectively, these findings suggest that Vanin-1 plays a crucial role in promoting mitochondrial respiration to counteract diet-induced obesity, making it a potential therapeutic target for obesity.

Are MTV and TLG Accurate for Quantifying the Intensity of Brown Adipose Tissue Activation?

Recent research has suggested that one novel mechanism of action for anti-obesity medications is to stimulate the activation of brown adipose tissue (BAT). 18FDG PET/CT remains the gold standard for defining and quantifying BAT. SUVmax is the most often used quantification tool in clinical practice. However, this parameter does not reflect the entire BAT volume. As a potential method for precisely evaluating BAT, we have utilised metabolic tumour volume (MTV) and total lesion glycolysis (TLG) to answer the question: Are MTV and TLG accurate in quantifying the intensity of BAT activation? After analysing the total number of oncological 18F-FDG PET/CT scans between 2021-2023, we selected patients with active BAT. Based on the BAT SUVmax, the patients were divided into BAT-moderate activation (MA) vs. BAT-high activation (HA). Furthermore, we statistically analysed the accuracy of TLG and MTV in assessing BAT activation intensity. The results showed that both parameters increased their predictive value regarding BAT activation, and presented a significantly high sensitivity and specificity for the correct classification of BAT activation intensity. To conclude, these parameters could be important indicators with increased accuracy for classifying BAT expression, and could bring additional information about the volume of BAT to complement the limitations of the SUVmax.

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

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

Impact of physical fitness and exercise training on subcutaneous adipose tissue beiging markers in humans with and without diabetes and a high-fat diet-fed mouse model

AIMS

Exercise training induces white adipose tissue (WAT) beiging and improves glucose homeostasis and mitochondrial function in rodents. This could be relevant for type 2 diabetes in humans, but the effect of physical fitness on beiging of subcutaneous WAT (scWAT) remains unclear. This translational study investigates if beiging of scWAT associates with physical fitness in healthy humans and recent-onset type 2 diabetes and if a voluntary running wheel intervention is sufficient to induce beiging in mice.

MATERIALS AND METHODS

Gene expression levels of established beiging markers were measured in scWAT biopsies of humans with (n = 28) or without type 2 diabetes (n = 28), stratified by spiroergometry into low (L-FIT; n = 14 each) and high physical fitness (H-FIT; n = 14 each). High-fat diet-fed FVB/N mice underwent voluntary wheel running, treadmill training or no training (n = 8 each group). Following the training intervention, mitochondrial respiration and content of scWAT were assessed by high-resolution respirometry and citrate synthase activity, respectively.

RESULTS

Secreted CD137 antigen (Tnfrsf9/Cd137) expression was three-fold higher in glucose-tolerant H-FIT than in L-FIT, but not different between H-FIT and L-FIT with type 2 diabetes. In mice, both training modalities increased Cd137 expression and enhanced mitochondrial content without changing respiration in scWAT. Treadmill but not voluntary wheel running led to improved whole-body insulin sensitivity.

CONCLUSIONS

Higher physical fitness and different exercise interventions associated with higher gene expression levels of the beiging marker CD137 in healthy humans and mice on a high-fat diet. Humans with recent-onset type 2 diabetes show an impaired adipose tissue-specific response to physical activity.

Unravelling the Influence of Endocrine-Disrupting Chemicals on Obesity Pathophysiology Pathways

Obesity represents a global health concern, affecting individuals of all age groups across the world. The prevalence of excess weight and obesity has escalated to pandemic proportions, leading to a substantial increase in the incidence of various comorbidities, such as cardiovascular diseases, type 2 diabetes, and cancer. This chapter seeks to provide a comprehensive exploration of the pathways through which endocrine-disrupting chemicals can influence the pathophysiology of obesity. These mechanisms encompass aspects such as the regulation of food intake and appetite, intestinal fat absorption, lipid metabolism, and the modulation of inflammation. This knowledge may help to elucidate the role of exogenous molecules in both the aetiology and progression of obesity.

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

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

Physical exercise for a healthy pregnancy: the role of placentokines and exerkines

Complications such as diabetes and preeclampsia can occur during pregnancy. Moderate-intensity exercise can prevent such complications by releasing placentokines and exerkines, such as apelin, adiponectin, leptin, irisin, and chemerin. Exercise and apelin increase thermogenesis and glucose uptake in pregnancy by activating AMPK, PI3K, PGC-1α, AKT1, UCP3, and sarcolipin. Exercise increases apelin levels to reduce preeclampsia symptoms by increasing eNOS, NO, placental growth factor (PlGF), and VEGF and decreasing levels of fms-like tyrosine kinase 1 (sFlt-1), soluble endoglin (sEng), and oxidative stress. A negative relationship has been reported between plasma leptin and VO2peak/kg and VO2peak in women with gestational diabetes. In active women, decreases in leptin levels reduce the risk of preeclampsia by ~ 40%. Higher adiponectin levels are associated with greater physical activity and lead to increased insulin sensitivity. Increased adiponectin levels in preeclampsia and exercise counteract inflammatory and atherogenic activities while also having vascular protective effects. Exercise increases irisin levels that correlate negatively with fasting glucose, insulin concentration, and glycosylated hemoglobin levels. Irisin augments mRNA expression levels of UCP1 and cell death-inducing DNA fragmentation factor-like effector A (cidea) to cause browning of adipose tissue, increased thermogenesis, and increased energy consumption. Irisin concentrations in mothers with preeclampsia in the third trimester negatively correlate with systolic and diastolic blood pressure. Expression levels of chemerin, IL-6, and TNF-α are increased in gestational diabetes, and the increases in chemerin in late pregnancy positively correlate with the ratio of sFlt-1 to PlGF as a marker of preeclampsia. The effects of physical exercise on placentokines and exerkines in women at various stages of pregnancy remain poorly understood.

Exercise Training and Cold Exposure Trigger Distinct Molecular Adaptations to Inguinal White Adipose Tissue

Exercise training and cold exposure both improve systemic metabolism, but the mechanisms are not well-established. We tested the hypothesis that adaptations to inguinal white adipose tissue (iWAT) are critical for these beneficial effects by determining the impact of exercise-trained and cold-exposed iWAT on systemic glucose metabolism and the iWAT proteome and secretome. Transplanting trained iWAT into sedentary mice improved glucose tolerance, while cold-exposed iWAT transplantation showed no such benefit. Compared to training, cold led to more pronounced alterations in the iWAT proteome and secretome, downregulating >2,000 proteins but also boosting iWAT's thermogenic capacity. In contrast, only training increased extracellular space and vesicle transport proteins, and only training upregulated proteins that correlate with favorable fasting glucose, suggesting fundamental changes in trained iWAT that mediate tissue-to-tissue communication. This study defines the unique exercise training- and cold exposure-induced iWAT proteomes, revealing distinct mechanisms for the beneficial effects of these interventions on metabolic health.

Skeletal muscles and gut microbiota-derived metabolites: novel modulators of adipocyte thermogenesis

Obesity occurs when overall energy intake surpasses energy expenditure. White adipose tissue is an energy storage site, whereas brown and beige adipose tissues catabolize stored energy to generate heat, which protects against obesity and obesity-associated metabolic disorders. Metabolites are substrates in metabolic reactions that act as signaling molecules, mediating communication between metabolic sites (i.e., adipose tissue, skeletal muscle, and gut microbiota). Although the effects of metabolites from peripheral organs on adipose tissue have been extensively studied, their role in regulating adipocyte thermogenesis requires further investigation. Skeletal muscles and intestinal microorganisms are important metabolic sites in the body, and their metabolites play an important role in obesity. In this review, we consolidated the latest research on skeletal muscles and gut microbiota-derived metabolites that potentially promote adipocyte thermogenesis. Skeletal muscles can release lactate, kynurenic acid, inosine, and β-aminoisobutyric acid, whereas the gut secretes bile acids, butyrate, succinate, cinnabarinic acid, urolithin A, and asparagine. These metabolites function as signaling molecules by interacting with membrane receptors or controlling intracellular enzyme activity. The mechanisms underlying the reciprocal exchange of metabolites between the adipose tissue and other metabolic organs will be a focal point in future studies on obesity. Furthermore, understanding how metabolites regulate adipocyte thermogenesis will provide a basis for establishing new therapeutic targets for obesity.

Cytokines and exosomal miRNAs in skeletal muscle-adipose crosstalk

Skeletal muscle and adipose tissues (ATs) are secretory organs that release secretory factors including cytokines and exosomes. These factors mediate muscle-adipose crosstalk to regulate systemic metabolism via paracrine and endocrine pathways. Myokines and adipokines are cytokines secreted by skeletal muscle and ATs, respectively. Exosomes loaded with nucleic acids, proteins, lipid droplets, and organelles can fuse with the cytoplasm of target cells to perform regulatory functions. A major regulatory component of exosomes is miRNA. In addition, numerous novel myokines and adipokines have been identified through technological innovations. These discoveries have identified new biomarkers and sparked new insights into the molecular regulation of skeletal muscle growth and adipose deposition. The knowledge may contribute to potential diagnostic and therapeutic targets in metabolic disease.

The Emerging Importance of Mitochondria in White Adipocytes: Neither Last nor Least

The growing recognition of mitochondria's crucial role in the regulation of white adipose tissue remodeling and energy balance underscores its significance. The marked metabolic diversity of mitochondria provides the molecular and cellular foundation for enabling adipose tissue plasticity in response to various metabolic cues. Effective control of mitochondrial function at the cellular level, not only in thermogenic brown and beige adipocytes but also in energy-storing white adipocytes, exerts a profound influence on adipose homeostasis. Furthermore, mitochondria play a pivotal role in intercellular communication within adipose tissue via production of metabolites with signaling properties. A more comprehensive understanding of mitochondrial regulation within white adipocytes will empower the development of targeted and efficacious strategies to enhance adipose function, leading to advancements in overall metabolic health.

Progress and obstacles in transplantation of brown adipose tissue or engineered cells with thermogenic potential for metabolic benefits

Transplantation of brown adipose tissue (BAT), engineered thermogenic progenitor cells, and adipocytes have received much attention for the improvement of obesity and metabolic disorders. However, even though the thermogenic and metabolic potential exists early after transplantation, the whitening of the brown fat graft occurs with metabolic function significantly impaired. In this review, specific experiment designs, graft outcomes, and metabolic benefits for the transplantation of BAT or engineered cells will be discussed. The current advancements will offer guidance to further investigation, and the obstacles appearing in previous studies will require innovation of BAT transplantation methods.

Brown Adipose Tissue: A New Potential Target for Glucagon-like Peptide 1 Receptor Agonists in the Treatment of Obesity

Adipose tissue can be divided into white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue, according to the differences in morphology. WAT acts as a buffer for increased energy intake and decreased energy expenditure during the development of obesity, resulting in visceral and ectopic WAT accumulation. These WAT depots are strongly associated with chronic systemic inflammation, insulin resistance, and cardiometabolic risk related to obesity. They represent a primary weight loss target in anti-obesity management. Second-generation anti-obesity medications glucagon-like peptide-1 receptor agonists (GLP-1RAs) cause weight loss and improve body composition by reducing visceral and ectopic fat depots of WAT, resulting in improved cardiometabolic health. Recently, the understanding of the physiological significance of BAT beyond its primary function in generating heat through non-shivering thermogenesis has been expanded. This has raised scientific and pharmaceutical interest in the manipulation of BAT to further enhance weight reduction and body weight maintenance. This narrative review focuses on the potential impact of GLP-1 receptor agonism on BAT, particularly in human clinical studies. It provides an overview of the role of BAT in weight management and highlights the need for further research to elucidate the mechanisms by which GLP-1RAs affect energy metabolism and weight loss. Despite encouraging preclinical data, limited clinical evidence supports the notion that GLP-1RAs contribute to BAT activation.

Physical exercise alleviates oxidative stress in brown adipose tissue and causes changes in body composition and nutritional behavior in rats with polycystic ovary syndrome

AIM

Polycystic Ovary Syndrome (PCOS) is a very common endocrine disorder in women. We investigate the effect of physical exercise on body composition, nutritional parameters, and oxidative stress in rats with PCOS.

METHODS

Female rats were into three groups: Control, PCOS, and PCOS + Exercise. PCOS was induced by letrozole (1 mg/kg via p.o.) for 21 days consecutively. Physical exercise was swimming, for 21 consecutive days, 1 h/day with 5 % load. In all groups, we assessed the nutritional and murinometric parameters, body composition, thermography, and oxidative stress in brown adipose tissue (BAT) and peri-ovarian adipose tissue (POAT).

KEY FINDINGS

In PCOS we observed an increase (P < 0.05) in body weight vs. the Control group. But, the PCOS + Exercise group prevent this weight gain (P < 0.05). The temperature in BAT, decrease (P < 0.05) in the PCOS group vs. Control group. PCOS + Exercise prevented this reduction (P < 0.05) in BAT temperature vs. PCOS groups. We observed decreases (P < 0.05) in Lee Index and BMI in POS + Exercise vs. PCOS group. In PCOS rats, we observed an increase (P < 0.05) in murinometric (SRWG, EI, and FE) and body composition parameters (TWB, ECF, ICF, and FFM) vs. the Control group. The PCOS + Exercise prevents (P < 0.05) these changes in all groups, compared with PCOS. Regarding the BAT, we observe an increase (P < 0.05) in MPO and MDA levels in the PCOS vs. Control group. PCOS + Exercise prevents (P < 0.05) these increases vs. the PCOS group.

SIGNIFICANCE

PCOS modifies body composition, and nutritional parameters, and induces changes in oxidative stress in BAT. Physical exercise prevented these alterations.

Central angiotensin 1-7 triggers brown fat thermogenesis

We tested the hypothesis that third ventricular (3V) injections of angiotensin 1-7 (Ang 1-7) increases thermogenesis in brown adipose tissue (BAT), and whether the Mas receptor mediates this response. First, in male Siberian hamsters (n = 18), we evaluated the effect of Ang 1-7 in the interscapular BAT (IBAT) temperature and, using selective Mas receptor antagonist A-779, the role of Mas receptor in this response. Each animal received 3V injections (200 nL), with 48 h intervals: saline; Ang 1-7 (0.03, 0.3, 3, and 30 nmol); A-779 (3 nmol); and Ang 1-7 (0.3 nmol) + A-779 (3 nmol). IBAT temperature increased after 0.3 nmol Ang 1-7 compared with Ang 1-7 + A-779 at 20, 30, and 60 min. Also, 0.3 nmol Ang 1-7 increased IBAT temperature at 10 and 20 min, and decreased at 60 min compared with pretreatment. IBAT temperature decreased after A-779 at 60 min and after Ang 1-7 + A-779 at 30 and 60 min compared with the respective pretreatment. A-779 and Ang 1-7 + A-779 decreased core temperature at 60 min compared with 10 min. Then, we evaluated blood and tissue Ang 1-7 levels, and the expression of hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) in IBAT. Male Siberian hamsters (n = 36) were killed 10 min after one of the injections. No changes were observed in blood glucose, serum and IBAT Ang 1-7 levels, and ATGL. Ang 1-7 (0.3 nmol) increased p-HSL expression compared with A-779 and increased p-HSL/HSL ration compared with other injections. Ang 1-7 and Mas receptor immunoreactive cells were found in brain regions that coincide with the sympathetic nerves outflow to BAT. In conclusion, 3V injection of Ang 1-7 induced thermogenesis in IBAT in a Mas receptor-dependent manner.

Cardiac ischemia modulates white adipose tissue in a depot-specific manner

The incidence of heart failure after myocardial infarction (MI) remains high and the underlying causes are incompletely understood. The crosstalk between heart and adipose tissue and stimulated lipolysis has been identified as potential driver of heart failure. Lipolysis is also activated acutely in response to MI. However, the role in the post-ischemic remodeling process and the contribution of different depots of adipose tissue is unclear. Here, we employ a mouse model of 60 min cardiac ischemia and reperfusion (I/R) to monitor morphology, cellular infiltrates and gene expression of visceral and subcutaneous white adipose tissue depots (VAT and SAT) for up to 28 days post ischemia. We found that in SAT but not VAT, adipocyte size gradually decreased over the course of reperfusion and that these changes were associated with upregulation of UCP1 protein, indicating white adipocyte conversion to the so-called ‘brown-in-white’ phenotype. While this phenomenon is generally associated with beneficial metabolic consequences, its role in the context of MI is unknown. We further measured decreased lipogenesis in SAT together with enhanced infiltration of MAC-2+ macrophages. Finally, quantitative PCR analysis revealed transient downregulation of the adipokines adiponectin, leptin and resistin in SAT. While adiponectin and leptin have been shown to be cardioprotective, the role of resistin after MI needs further investigation. Importantly, all significant changes were identified in SAT, while VAT was largely unaffected by MI. We conclude that targeted interference with lipolysis in SAT may be a promising approach to promote cardiac healing after ischemia.

The Role of Crosstalk between Adipose Cells and Myocytes in the Pathogenesis of Sarcopenic Obesity in the Elderly

As a result of aging, body composition changes, with a decline in muscle mass and an increase in adipose tissue (AT), which reallocates from subcutaneous to visceral depots and stores ectopically in the liver, heart and muscles. Furthermore, with aging, muscle and AT, both of which have recognized endocrine activity, become dysfunctional and contribute, in the case of positive energy balance, to the development of sarcopenic obesity (SO). SO is defined as the co-existence of excess adiposity and low muscle mass and function, and its prevalence increases with age. SO is strongly associated with greater morbidity and mortality. The pathogenesis of SO is complex and multifactorial. This review focuses mainly on the role of crosstalk between age-related dysfunctional adipose and muscle cells as one of the mechanisms leading to SO. A better understanding of this mechanisms may be useful for development of prevention strategies and treatments aimed at reducing the occurrence of SO.

Possible roles of exercise and apelin against pregnancy complications

The prevalence of maternal obesity during pregnancy is associated with the risk of gestational diabetes, preeclampsia, and cardiomyopathy. Environmental factors such as active lifestyles and apelin may lead to beneficial changes. In rats, apelin and exercise (45 to 65% VO_2max for 6 to 9 weeks) during pregnancy increase brown adipose tissue (BAT) proteins such as Cidea, Elovl3, UCP1, PRDM16, and PGC-1α in males and females fetuses, while white adipose tissue (WAT) is reduced. In humans and animals, apelin and exercise stimulate the expression of the glucose transporters (GLUT1/2/4) in the muscle and adipose tissue through the PI3K/Akt and AMPK pathways. Hence, exercise and apelin may are known as regulators of energy metabolism and be anti-obesity and anti-diabetic properties. In mice, exercise also creates a short-term hypoxic environment in the pregnant mother, activating HIF-1, VEGF, and VEGFR, and increasing angiogenesis. Exercise and apelin also increase vasodilation, angiogenesis, and suppression of inflammation through the L-arginine/eNOS/NO pathway in humans. Exercise can stimulate the ACE2-Ang-(1-7)-Mas axis in parallel with inhibiting the ACE-Ang II-AT1 pathway. Exercise and apelin seem to prevent preeclampsia through these processes. In rats, moderate-intensity exercise (60 to 70% VO_2max for 8 weeks) and apelin/APJ also may prevent pathological hypertrophy in pregnancy by activating the PI3K/Akt/mTOR/p70S6K pathway, PI3k-Akt-ERK1/2-p70S6K pathway, and the anti-inflammatory cytokine IL-10. Since pre-clinical studies have been more on animal models, future research with scientific guidelines should pay more attention to human specimens. In future research, time factors such as the first, second, and third trimesters of pregnancy and the intensity and duration of exercise are important variables that should be considered to determine the optimal intensity and duration of exercise.