Brown adipose tissue as a therapeutic target for human obesity

White adipose tissue undergoes browning during preweaning period in association with microbiota formation in mice

Beige adipocytes are transiently induced during early postnatal period in mice. Previous studies have suggested that, unlike in adults, the induction is independent of the sympathetic nerve activity; however, the mechanism is yet unknown. Here, we showed that beige adipocytes are induced during the preweaning period in association with the formation of microbiota in mice. Alteration of gut microbiota composition in preweaning mice by maternal treatment with antibiotics or high-fat diet feeding substantially suppressed WAT browning. The suppression was also found in pups transplanted cecal microbiota from pups of high-fat diet-fed dams. These treatments reduced the hepatic expression of genes involved in bile acid synthesis and the serum bile acids level. The abundance of Porphyromonadaceae and Ruminococcaceae in microbiota showed a positive and negative correlation with the induction of beige adipocytes, respectively. This finding may provide comprehensive understanding of the association between gut microbiota and adipose tissue development in the neonatal period.

Selenoprotein P-mediated reductive stress impairs cold-induced thermogenesis in brown fat

Reactive oxygen species (ROS) activate uncoupler protein 1 (UCP1) in brown adipose tissue (BAT) under physiological cold exposure and noradrenaline (NA) stimulation to increase thermogenesis. However, the endogenous regulator of ROS in activated BAT and its role in pathological conditions remain unclear. We show that serum levels of selenoprotein P (SeP; encoded by SELENOP) negatively correlate with BAT activity in humans. Physiological cold exposure downregulates Selenop in BAT. Selenop knockout mice show higher rectal temperatures and UCP1 sulfenylation during cold exposure. SeP treatment to brown adipocytes eliminated the NA-induced mitochondrial ROS by upregulating glutathione peroxidase 4 and impaired cellular thermogenesis. A high-fat/high-sucrose diet elevates serum SeP levels and diminishes the elevated NA-induced thermogenesis in BAT-Selenop KO mice. Therefore, SeP is the intrinsic factor inducing reductive stress that impairs thermogenesis in BAT and may be a potential therapeutic target for obesity and diabetes.

Targeting PLD2 in adipocytes augments adaptive thermogenesis by improving mitochondrial quality and quantity in mice

Phospholipase D (PLD)2 via its enzymatic activity regulates cell proliferation and migration and thus is implicated in cancer. However, the role of PLD2 in obesity and type 2 diabetes has not previously been investigated. Here, we show that during diet-induced thermogenesis and obesity, levels of PLD2 but not PLD1 in adipose tissue are inversely related with uncoupling protein 1, a key thermogenic protein. We demonstrate that the thermogenic program in adipose tissue is significantly augmented in mice with adipocyte-specific Pld2 deletion or treated with a PLD2-specific inhibitor and that these mice are resistant to high fat diet–induced obesity, glucose intolerance, and insulin resistance. Mechanistically, we show that Pld2 deletion in adipose tissue or PLD2 pharmacoinhibition acts via p62 to improve mitochondrial quality and quantity in adipocytes. Thus, PLD2 inhibition is an attractive therapeutic approach for obesity and type 2 diabetes by resolving defects in diet-induced thermogenesis.

Delivery of miRNAs to the adipose organ for metabolic health

Despite the increasing prevalence of obesity and diabetes, there is no efficient treatment to combat these epidemics. The adipose organ is the main site for energy storage and plays a pivotal role in whole body lipid metabolism and energy homeostasis, including remodeling and dysfunction of adipocytes and adipose tissues in obesity and diabetes. Thus, restoring and balancing metabolic functions in the adipose organ is in demand. MiRNAs represent a novel class of drugs and drug targets, as they are heavily involved in the regulation of many cellular and metabolic processes and diseases, likewise in adipocytes. In this review, we summarize key regulatory activities of miRNAs in the adipose organ, discuss various miRNA replacement and inhibition strategies, promising delivery systems for miRNAs and reflect the future of novel miRNA-based therapeutics to target adipose tissues with the ultimate goal to combat metabolic disorders.

Adipose browning response to burn trauma is impaired with aging

BACKGROUND

The incidence of burn injuries in older patients is dramatically increasing as the population of older people grows. Despite the increased demand for elderly burn care, the mechanisms that mediate increased morbidity and mortality in older trauma patients are unknown. We recently showed that a burn injury invokes white adipose tissue browning that leads to a substantially increased hypermetabolic response associated with poor outcomes. Therefore, the aim of this study was to determine the effect of age on the metabolic adipose response of browning after a burn injury.

METHOD

One hundred and seventy patients with burn injury admitted to the Ross Tilley Burn Centre were prospectively enrolled and grouped by age as older (≥50 years) and young (≤35 years). Adipose tissue and sera were collected and analyzed for browning markers and metabolic state via histology, gene expression, and resting energy expenditure assays.

RESULTS

We found that older patients with burn injury lacked the adipose browning response, as they showed significant reductions in uncoupling protein 1 (UCP1) expression. This failure of the browning response was associated with reduced whole-body metabolism and decreased survival in older patients with burn injury. Mechanistically, we found that the adipose of both aged patients after burn trauma and aged mice after a burn showed impairments in macrophage infiltration and IL-6, key immunological regulators of the browning process after a severe trauma.

CONCLUSION

Targeting pathways that activate the browning response represents a potential therapeutic approach to improve outcomes after burn trauma for elderly patients.

FUNDING

NIH (R01-GM087285-01), Canadian Institutes of Health Research (grant no. 123336), and Canada Foundation for Innovation Leaders Opportunity Fund (no. 25407).

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.

Modeling Adipogenesis: Current and Future Perspective

Adipose tissue is contemplated as a dynamic organ that plays key roles in the human body. Adipogenesis is the process by which adipocytes develop from adipose-derived stem cells to form the adipose tissue. Adipose-derived stem cells’ differentiation serves well beyond the simple goal of producing new adipocytes. Indeed, with the current immense biotechnological advances, the most critical role of adipose-derived stem cells remains their tremendous potential in the field of regenerative medicine. This review focuses on examining the physiological importance of adipogenesis, the current approaches that are employed to model this tightly controlled phenomenon, and the crucial role of adipogenesis in elucidating the pathophysiology and potential treatment modalities of human diseases. The future of adipogenesis is centered around its crucial role in regenerative and personalized medicine.

Multiorgan contribution to non-shivering and shivering thermogenesis and vascular responses during gradual cold exposure in humans

PURPOSE

Human brown adipose tissue (BAT) is known to be a significant thermoeffector in non-shivering thermogenesis (NST), albeit with individual variations in the BAT activity. We hypothesized that humans with less BAT would have more contribution from the skeletal muscle (SM) to NST or earlier shivering onset and greater vasoconstriction to compensate for less BAT-mediated thermogenesis.

METHODS

Eighteen males participated in this study. Their BAT activity and detectable volume were investigated. A gradual cold exposure was conducted for inducing NST at 18.6 °C and initiating shivering at 11.6 °C. The energy expenditure, electromyograph of the pectoralis major, skin blood flow, and rectal (T_re) and skin temperatures were evaluated.

RESULTS

BAT volume significantly correlated with the change in metabolic heat production during mild cold phase relative to baseline (NST; r = 0.562, P < 0.05), but not with shivering initiation phase (NST+ ST). SM mass correlated with baseline metabolic heat production (M_base; r = 0.839, P < 0.01) but not with NST or NST + ST. A positive correlation was noted between BAT volume and T_re at the end of the 18.6 °C exposure period (r = 0.586, P < 0.05), which positively correlated with shivering onset time (r = 0.553, P < 0.05). The skin blood flow, mean skin temperature, and forearm and finger skin temperature difference at the end of the 18.6 °C exposure period did not correlate with NST or BAT volume.

CONCLUSION

BAT volume positively correlated with NST. Notably, lower T_re in individuals with less BAT volume induced earlier shivering onset for offsetting the less NST. Whereas, no correlation between metabolic and vasomotor responses was observed.

Phenylbutyrate Ameliorates High-Fat Diet-Induced Obesity via Brown Adipose Tissue Activation

Obesity, which is characterized by an excessive accumulation of body fat, is one of the critical factors causing metabolic syndrome. Many studies have been performed to identify appropriate agents to control obesity, but toxicity remains a problem. Herein, we identified that phenylbutyrate (PBA), which has been used to treat urea cycle disorder with very low toxicity for a long time, efficiently inhibited high fat-induced body weight gain in a diet-induced obesity mouse model (DIO model). PBA treatment decreased body fat mass and increased lean composition. Moreover, PBA increased brown adipose tissue (BAT) activity by increasing glucose uptake, thereby improving glucose tolerance and insulin tolerance. Interestingly, PBA could induce the expression of liver type phosphofructokinase (PFKL), a key enzyme in the glycolytic pathway, and knocking down PFKL dramatically repressed the expression level of Ucp1 as well as those of Prdm16, Cidea, Pgc1α, and Pparγ, which are marker genes for BAT activation. These results strongly suggested that PBA could increase energy expenditure by increasing BAT activity via the induction of PFKL. Taken together, PBA could be used as a therapeutic agent for people with obesity to prevent the development of metabolic syndrome.

Recombinant Lactococcus lactis NZ3900 expressing bioactive human FGF21 reduced body weight of Db/Db mice through the activity of brown adipose tissue

Fibroblast growth factor 21 (FGF21), a metabolism regulator, has an important effect on metabolic diseases, such as obesity and diabetes. It is also expressed in mice, and the murine source has high homology with human FGF21. Recently, it has been extensively studied and has become a potential drug target for the treatment of metabolic diseases. As it is a protein-based hormone, FGF21 cannot be easily and quickly absorbed into the blood through oral administration. Moreover, it has a 0-2 h half-life in vivo, as shown in a previous study, thus its efficacy lasts for a short period of time when used to treat metabolic diseases, limiting its clinical applications. To avoid these limitations, we used Lactococcus lactis, a food-grade bacterium, as the host to express FGF21. It could be used successfully for the expression and long-term effect of FGF21 in vivo. Instead of antibiotic resistance genes, the LacF gene was used as a selection marker in the NZ3900/PNZ8149 expression system, which is safe and could reduce the antibiotic resistance crisis. In this study, we a constructed human FGF21 expressing L. lactis strain and administered it to Db/Db mice by gavage. Compared with the control group, the body weight of mice in the experimental group was significantly reduced, and the overall homeostasis was improved in mice treated with human FGF21. Moreover, the activity of brown adipose tissue was enhanced. These results revealed that oral administration of FGF21 through heterologous expression in L. lactis appears to be an effective approach for its clinical application.

Brown adipocytes from induced pluripotent stem cells-how far have we come?

A global increase in the number of individuals who are either overweight or obese is leading to a higher incidence of type 2 diabetes (T2D). Behavioral interventions for the treatment of obesity have yet to deliver desired outcomes, thus introducing a pressing need for molecular- and cellular-based therapies. Excess energy from food is stored in the form of triglycerides in white adipose tissue, which expands during weight gain and can lead to insulin resistance and T2D. By contrast, brown adipose tissue (BAT) releases energy from metabolic substrates in the form of heat and secretes factors that can reverse metabolic disease by acting systemically. Therefore, the ability to increase BAT activity is a promising approach to improve energy balance and metabolic homeostasis. Methods are now being developed to generate brown adipocytes from human induced pluripotent stem cells (hiPSCs), which would (1) provide an unlimited source of cellular material to study human brown adipogenesis, and (2) could be used to develop drug- and cell-based therapies for the treatment of metabolic complications associated with obesity. This article reviews basic BAT biology and details the current progress toward developing brown adipocytes from hiPSCs.

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

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

Systems genetic analysis of brown adipose tissue function

Brown adipose tissue (BAT) has been suggested to play an important role in lipid and glucose metabolism in rodents and possibly also in humans. In the current study, we used genetic and correlation analyses in the BXH/HXB recombinant inbred (RI) strains, derived from Brown Norway (BN) and spontaneously hypertensive rats (SHR), to identify genetic determinants of BAT function. Linkage analyses revealed a quantitative trait locus (QTL) associated with interscapular BAT mass on chromosome 4 and two closely linked QTLs associated with glucose oxidation and glucose incorporation into BAT lipids on chromosome 2. Using weighted gene coexpression network analysis (WGCNA) we identified 1,147 gene coexpression modules in the BAT from BXH/HXB rats and mapped their module eigengene QTLs. Through an unsupervised analysis, we identified modules related to BAT relative mass and function. The Coral4.1 coexpression module is associated with BAT relative mass (includes Cd36 highly connected gene), and the Darkseagreen coexpression module is associated with glucose incorporation into BAT lipids (includes Hiat1, Fmo5, and Sort1 highly connected transcripts). Because multiple statistical criteria were used to identify candidate modules, significance thresholds for individual tests were not adjusted for multiple comparisons across modules. In summary, a systems genetic analysis using genomic and quantitative transcriptomic and physiological information has produced confirmation of several known genetic factors and significant insight into novel genetic components functioning in BAT and possibly contributing to traits characteristic of the metabolic syndrome.

Impaired adrenergic agonist-dependent beige adipocyte induction in aged mice

OBJECTIVE

There are two types of thermogenic adipocytes expressing uncoupling protein (UCP)-1: the brown adipocyte activated by adrenergic stimulation and the beige adipocyte that appears within the white adipose tissue (WAT) in response to chronic adrenergic stimulation. This study examined age-related changes in responses of both types of adipocytes to adrenergic stimulation in mice.

METHODS

Aged (age 20 months) and young (4 months) mice were injected daily with either saline or β3-adrenergic receptor agonist CL316,243 (CL; 0.1 mg/kg, once a day) for 1 week.

RESULTS

The body and WAT weight tended to be higher in aged mice. CL treatment increased UCP-1 protein amounts in both brown adipose tissue and inguinal WAT, suggesting activation of brown and beige adipocytes. However, induction of beige adipocytes was impaired in aged mice, whereas brown adipocyte activation was comparable to young mice. The number of platelet-derived growth factor receptor α-expressing progenitor cells, which were reported to differentiate into beige adipocytes, significantly decreased in inguinal WAT of aged mice compared with that of young mice.

CONCLUSIONS

Inductive ability of beige adipocytes in WAT declines with aging in mice. It may be partly because of a decreased number of progenitor cells associated with aging.

Chili pepper as a body weight-loss food

Chili has culinary as well as medical importance. Studies in humans, using a wide range of doses of chili intake (varying from a single meal to a continuous uptake for up to 12 weeks), concluded that it facilitates weight loss. In regard to this, the main targets of chili are fat metabolism, energy expenditure, and thermogenesis. To induce weight loss, the active substance of chili, capsaicin, activates Transient Receptor Potential Cation Channel sub-family V member 1 (TRPV1) channels) receptors causing an increase in intracellular calcium levels and triggering the sympathetic nervous system. Apart from TRPV1, chili directly reduces energy expenditure by activating Brown Adipose Tissue. Weight loss by chili is also the result of an improved control of insulin, which supports weight management and has positive effects for treatment for diseases like obesity, diabetes and cardiovascular disorders. This review summarizes the major pathways by which chili contributes to ameliorating parameters that help weight management and how the consumption of chili can help in accelerating weight loss through dietary modifications.

Mitochondrial uncoupling proteins and energy metabolism

Understanding the metabolic factors that contribute to energy metabolism (EM) is critical for the development of new treatments for obesity and related diseases. Mitochondrial oxidative phosphorylation is not perfectly coupled to ATP synthesis, and the process of proton-leak plays a crucial role. Proton-leak accounts for a significant part of the resting metabolic rate (RMR) and therefore enhancement of this process represents a potential target for obesity treatment. Since their discovery, uncoupling proteins have stimulated great interest due to their involvement in mitochondrial-inducible proton-leak. Despite the widely accepted uncoupling/thermogenic effect of uncoupling protein one (UCP₁), which was the first in this family to be discovered, the reactions catalyzed by its homolog UCP₃ and the physiological role remain under debate. This review provides an overview of the role played by UCP₁ and UCP₃ in mitochondrial uncoupling/functionality as well as EM and suggests that they are a potential therapeutic target for treating obesity and its related diseases such as type II diabetes mellitus.

Fighting obesity: When muscle meets fat

The prevalence of obesity has risen to an unprecedented level. According to World Health Organization, over 500 million adults, equivalent to 10%-14% of the world population, were obese with a body mass index (BMI) of 30 kg/m(2) or greater in 2008.(1) This rising prevalence and earlier onset of obesity is believed to be resulted from an interplay of genetic factors, over-nutrition and physical inactivity in modern lifestyles. Obesity also increases the susceptibility to metabolic syndromes, hypertension, cardiovascular diseases, Type 2 diabetes mellitus (T2DM) and cancer.(2-4) The global obesity epidemic has sparked substantial interests in the biology of adipose tissue (fat). In addition, the skeletal muscle and its secretive factors (myokines) have also been shown to play a critical role in controlling body energy balance, adipose homeostasis and inflammation status.(5) Interestingly, skeletal muscle cells share a common developmental origin with brown adipocytes,(6,7) which breaks down lipids to generate heat - thus reducing obesity. Here, we provide a brief overview of the basics and recent progress in muscle-fat crosstalk in the context of body energy metabolism, obesity, and diabetes. We summarize the different types of adipocytes, their developmental origins and implications in body composition. We highlight the role of several novel myokines in regulating fat mass and systemic energy balance, and evaluate the potential of skeletal muscles as a therapeutic target to treat obesity.