Adipogenesis: new insights into brown adipose tissue differentiation

Transcriptome analysis of adipose tissue in grazing cattle: Identifying key regulators of fat metabolism

The taste and tenderness of meat are the main determinants of carcass quality in many countries. This study aimed to discuss the mechanisms of intramuscular fat deposition in grazing and house-breeding cattle. We performed transcriptome analysis to characterize messenger RNA and microRNA (miRNA) expression profiles. A total of 456 and 66 differentially expressed genes (DEGs) and differentially expressed (DE) miRNAs were identified in the adipose tissue of grazing and house-breeding cattle. Kyoto Encyclopedia of Genes and Genomes pathway analysis identified the association of DEGs with fatty acid metabolism, fatty acid degradation, peroxisome proliferator-activated receptors signaling pathway, adenosine monophosphate-activated protein kinase signaling pathway, adipocytokine signaling pathway, and the association of DE miRNAs with mitogen-activated protein kinase signaling pathway. Apolipoprotein L domain containing 1, pyruvate dehydrogenase kinase 4, and sphingosine-1-phosphate lyase 1 genes may be the key regulators of fat metabolism in grazing cattle. Finally, we found that miR-211 and miR-331-5p were negatively correlated with the elongation of very long-chain fatty acids protein 6 (ELOVL6), and miR-331-5p might be the new regulator involved in fat metabolism. The results indicated that ELOVL6 participated in various functions and pathways related to fat metabolism. Meanwhile, miR-331-5p, as a new regulator, might play an essential role in this process. Our findings laid a more in-depth and systematic research foundation for the formation mechanism and characteristics of adipose tissue in grazing cattle.

Adipose Tissue Dysfunction Determines Lipotoxicity and Triggers the Metabolic Syndrome: Current Challenges and Clinical Perspectives

The adipose tissue organ is organised as distinct anatomical depots located all along the body axis, and it is constituted of three different types of adipocytes: white, beige and brown, which are integrated with vascular, immune, neural, and extracellular stroma cells. These distinct adipocytes serve different specialised functions. The main function of white adipocytes is to ensure healthy storage of excess nutrients/energy and its rapid mobilisation to supply the demand of energy imposed by physiological cues in other organs, whereas brown and beige adipocytes are designed for heat production through uncoupling lipid oxidation from energy production. The concerted action of the three types of adipocytes/tissues ensures an optimal metabolic status. However, when one or several of these adipose depots become dysfunctional because of sustained lipid/nutrient overload, then insulin resistance and associated metabolic complications ensue. These metabolic alterations close a vicious cycle that negatively affects the adipose tissue functionality and compromises global metabolic homeostasis. Optimising white adipose tissue expandability and ensuring its functional metabolic flexibility and/or promoting brown/beige mediated thermogenic activity are complementary strategies that counteract obesity and its associated lipotoxic metabolic effects. However, the development of these therapeutic approaches requires a deep understanding of adipose tissue in all broad aspects. In this chapter, we will discuss the characteristics of the different adipose tissue depots with respect to origins and precursors recruitment, plasticity, cellular composition, and expandability capacity potential as well as molecular and metabolic characteristic signatures in both physiological and pathophysiological conditions. Current antilipotoxic strategies for future clinical application are also discussed in this chapter.

Regenerative Medicine for Polycystic Ovary Syndrome: Stem Cell-Based Therapies and Brown Adipose Tissue Activation

Polycystic ovary syndrome (PCOS) is a pathological condition prevalent among women of reproductive age: it is associated with varied etiological factors (lifestyle, genetic, environmental…) and characterized by an increased polycystic morphology of the ovaries leading to disturbances in the menstrual cycle and its correlated infertility. Interconnections between PCOS, obesity, and insulin resistance have been recently investigated thoroughly in the scientific community; these findings directed PCOS therapies into unraveling possibilities to target insulin resistance and central adiposity as efficient treatment. On the other hand, brown adipose tissue is known to possess a thermogenic activity that increases lipolysis and directly attenuates fat deposition. Therefore, brown adipose tissue activation lands itself as a potential target for reducing obesity and its induced insulin resistance, subsequently rescuing PCOS phenotypes. In addition, regenerative medicine has proven efficacy in resolving PCOS-associated infertility and its metabolic symptoms. In particular, many stem/progenitor cells have been verified to possess the differentiation capacity into functional brown adipocytes. Thus, throughout this review, we will discuss the different brown adipose tissue activation strategies and stem-cell-based therapies applied to PCOS models and the possible combination of both therapeutic approaches to synergistically act on the activation of brown adipose tissue and attenuate PCOS-correlated infertility and retract the consequences of the metabolic syndrome on the physiological state of patients.

Deficiency of lipopolysaccharide binding protein facilitates adipose browning, glucose uptake and oxygen consumption in mouse embryonic fibroblasts via activating PI3K/Akt/mTOR pathway and inhibiting autophagy

This study aimed to explore the role of lipopolysaccharide-binding protein (LBP) in adipose browning. Mouse embryonic fibroblasts (MEFs) were treated with differentiation induction reagents and Perifosine (Akt inhibitor), with the transfection of Atg5, short hairpin RNA targeting LBP (shLBP), and Atg5 (shAtg5). The expression levels of LBP, inflammatory markers , brown fat markers, lipid metabolism marker, autophagy markers, insulin signaling-related molecules , p-mTOR, mTOR, p-Akt, Akt, p-PI3K, and PI3K were quantified or determined by Western blot, qRT-PCR, and immunofluorescence assay. The formation of lipid was examined through Oil red O staining assay. The consumption of oxygen was assessed using a Seahorse XF96 analyzer, and the uptake of glucose was evaluated by [³H]-2-deoxy-D-glucose uptake assay. Deficiency of LBP promoted adipose browning, oxygen consumption, glucose uptake, and insulin sensitivity in differentiated MEFs, where it inhibited inflammation and autophagy. All of the effects above were reversed by Atg5 overexpression. Meanwhile, the knockdown of Atg5 strengthened the activation of PI3K/Akt/mTOR pathway induced by the depletion of LBP, while Perifosine partly reversed the activation of differentiated MEFs. The knockdown of LBP facilitated adipose browning, glucose uptake, and oxygen consumption in MEFs via the activation of PI3K/Akt/mTOR pathway and the inhibition of autophagy.

An insight into brown/beige adipose tissue whitening, a metabolic complication of obesity with the multifactorial origin

Brown adipose tissue (BAT), a thermoregulatory organ known to promote energy expenditure, has been extensively studied as a potential avenue to combat obesity. Although BAT is the opposite of white adipose tissue (WAT) which is responsible for energy storage, BAT shares thermogenic capacity with beige adipose tissue that emerges from WAT depots. This is unsurprising as both BAT and beige adipose tissue display a huge difference from WAT in terms of their secretory profile and physiological role. In obesity, the content of BAT and beige adipose tissue declines as these tissues acquire the WAT characteristics via the process called "whitening". This process has been rarely explored for its implication in obesity, whether it contributes to or exacerbates obesity. Emerging research has demonstrated that BAT/beige adipose tissue whitening is a sophisticated metabolic complication of obesity that is linked to multiple factors. The current review provides clarification on the influence of various factors such as diet, age, genetics, thermoneutrality, and chemical exposure on BAT/beige adipose tissue whitening. Moreover, the defects and mechanisms that underpin the whitening are described. Notably, the BAT/beige adipose tissue whitening can be marked by the accumulation of large unilocular lipid droplets, mitochondrial degeneration, and collapsed thermogenic capacity, by the virtue of mitochondrial dysfunction, devascularization, autophagy, and inflammation.

Mitochondrial RNA stimulates beige adipocyte development in young mice

Childhood obesity is a serious public health crisis and a critical factor that determines future obesity prevalence. Signals affecting adipocyte development in early postnatal life have a strong potential to trigger childhood obesity; however, these signals are still poorly understood. We show here that mitochondrial (mt)RNA efflux stimulates transcription of nuclear-encoded genes for mitobiogenesis and thermogenesis in adipocytes of young mice and human infants. While cytosolic mtRNA is a potential trigger of the interferon (IFN) response, young adipocytes lack such a response to cytosolic mtRNA due to the suppression of IFN regulatory factor (IRF)7 expression by vitamin D receptor signalling. Adult and obese adipocytes, however, strongly express IRF7 and mount an IFN response to cytosolic mtRNA. In turn, suppressing IRF7 expression in adult adipocytes restores mtRNA-induced mitobiogenesis and thermogenesis and eventually mitigates obesity. Retrograde mitochondrion-to-nucleus signalling by mtRNA is thus a mechanism to evoke thermogenic potential during early adipocyte development and to protect against obesity.

Co-Evolution of Breast Milk Lipid Signaling and Thermogenic Adipose Tissue

Breastfeeding is a unique and defining behavior of mammals and has a fundamental role in nourishing offspring by supplying a lipid-rich product that is utilized to generate heat and metabolic fuel. Heat generation from lipids is a feature of newborn mammals and is mediated by the uncoupling of mitochondrial respiration in specific fat depots. Breastfeeding and thermogenic adipose tissue have a shared evolutionary history: both have evolved in the course of homeothermy evolution; breastfeeding mammals are termed "thermolipials", meaning "animals with warm fat". Beyond its heat-producing capacity, thermogenic adipose tissue is also necessary for proper lipid metabolism and determines adiposity in offspring. Recent advances have demonstrated that lipid metabolism in infants is orchestrated by breast milk lipid signals, which establish mother-to-child signaling and control metabolic development in the infant. Breastfeeding rates are declining worldwide, and are paralleled by an alarming increase in childhood obesity, which at least in part may have its roots in the impaired metabolic control by breast milk lipid signals.

Transcriptional Landscaping Identifies a Beige Adipocyte Depot in the Newborn Mouse

The present study sought to identify gene networks that are hallmarks of the developing inguinal subcutaneous adipose tissue (iWAT) and the interscapular brown adipose tissue (BAT) in the mouse. RNA profiling revealed that the iWAT of postnatal (P) day 6 mice expressed thermogenic and lipid catabolism transcripts, along with the abundance of transcripts associated with the beige adipogenesis program. This was an unexpected finding, as thermogenic BAT was believed to be the only site of nonshivering thermogenesis in the young mouse. However, the transcriptional landscape of BAT in P6 mice suggests that it is still undergoing differentiation and maturation, and that the iWAT temporally adopts thermogenic and lipolytic potential. Moreover, P6 iWAT and adult (P56) BAT were similar in their expression of immune gene networks, but P6 iWAT was unique in the abundant expression of antimicrobial proteins and virus entry factors, including a possible receptor for SARS-CoV-2. In summary, postnatal iWAT development is associated with a metabolic shift from thermogenesis and lipolysis towards fat storage. However, transcripts of beige-inducing signal pathways including β-adrenergic receptors and interleukin-4 signaling were underrepresented in young iWAT, suggesting that the signals for thermogenic fat differentiation may be different in early postnatal life and in adulthood.

Transcriptional Factors of Thermogenic Adipocyte Development and Generation of Brown and Beige Adipocytes From Stem Cells

Brown and beige adipocytes have been widely known for their potential to dissipate excessive energy into heat form, resulting in an alleviation of obesity and other overweight-related conditions. This review highlights the origins, characteristics, and functions of the various kinds of adipocytes, as well as their anatomic distribution inside the human body. This review mainly focuses on various essential transcriptional factors such as PRDM16, FGF21, PPARα, PPARγ and PGC-1α, which exert their effects on the development and activation of thermogenic adipocytes via important pathways such as JAK-STAT, cAMP-PKA and PI3K-AKT signaling pathways. Additionally, this review will underline promising strategies to generate an unexhausted source of thermogenic adipocytes differentiated from human stem cells. These exogenous thermogenic adipocytes offer therapeutic potential for improvement of metabolic disorders via application as single cell or whole tissue transplantation. Graphical abstract Caption is required. Please provide.

Unraveling the Developmental Roadmap toward Human Brown Adipose Tissue

Increasing brown adipose tissue (BAT) mass and activation is a therapeutic strategy to treat obesity and complications. Obese and diabetic patients possess low amounts of BAT, so an efficient way to expand their mass is necessary. There is limited knowledge about how human BAT develops, differentiates, and is optimally activated. Accessing human BAT is challenging, given its low volume and anatomical dispersion. These constraints make detailed BAT-related developmental and functional mechanistic studies in humans virtually impossible. We have developed and characterized functionally and molecularly a new chemically defined protocol for the differentiation of human pluripotent stem cells (hPSCs) into brown adipocytes (BAs) that overcomes current limitations. This protocol recapitulates step by step the physiological developmental path of human BAT. The BAs obtained express BA and thermogenic markers, are insulin sensitive, and responsive to β-adrenergic stimuli. This new protocol is scalable, enabling the study of human BAs at early stages of development.

NOTCH Receptors and DLK Proteins Enhance Brown Adipogenesis in Mesenchymal C3H10T1/2 Cells

The NOTCH family of receptors and ligands is involved in numerous cell differentiation processes, including adipogenesis. We recently showed that overexpression of each of the four NOTCH receptors in 3T3-L1 preadipocytes enhances adipogenesis and modulates the acquisition of the mature adipocyte phenotype. We also revealed that DLK proteins modulate the adipogenesis of 3T3-L1 preadipocytes and mesenchymal C3H10T1/2 cells in an opposite way, despite their function as non-canonical inhibitory ligands of NOTCH receptors. In this work, we used multipotent C3H10T1/2 cells as an adipogenic model. We used standard adipogenic procedures and analyzed different parameters by using quantitative-polymerase chain reaction (qPCR), quantitative reverse transcription-polymerase chain reaction (qRT-PCR), luciferase, Western blot, and metabolic assays. We revealed that C3H10T1/2 multipotent cells show higher levels of NOTCH receptors expression and activity and lower Dlk gene expression levels than 3T3-L1 preadipocytes. We found that the overexpression of NOTCH receptors enhanced C3H10T1/2 adipogenesis levels, and the overexpression of NOTCH receptors and DLK (DELTA-like homolog) proteins modulated the conversion of cells towards a brown-like adipocyte phenotype. These and our prior results with 3T3-L1 preadipocytes strengthen the idea that, depending on the cellular context, a precise and highly regulated level of global NOTCH signaling is necessary to allow adipogenesis and determine the mature adipocyte phenotype.

Brown Adipocyte and Splenocyte Co-Culture Maintains Regulatory T Cell Subset in Intermittent Hypobaric Conditions

Background

Brown adipocytes have thermogenic characteristics in neonates and elicit anti-inflammatory responses. We postulated that thermogenic brown adipocytes produce distinctive intercellular effects in a hypobaric state. The purpose of this study is to analyze the correlation between brown adipocyte and regulatory T cell (T_reg) expression under intermittent hypobaric conditions.

Methods

Brown and white adipocytes were harvested from the interscapular and flank areas of C57BL6 mice, respectively. Adipocytes were cultured with syngeneic splenocytes after isolation and differentiation. Intermittent hypobaric conditions were generated using cyclic negative pressure application for 48 h in both groups of adipocytes. Expression levels of T_regs (CD4 + CD25 + Foxp3 + T cells), cytokines [tumor necrosis factor-α (TNF-α) and interleukin-10 (IL-10), and the programmed death-ligand 1 (PD-L1)] co-inhibitory ligand were examined.

Results

Splenocytes, cultured with brown and white adipocytes, exhibited comparable T_reg expression in a normobaric state. Under hypobaric conditions, brown adipocytes maintained a subset of T_regs. However, a decrease in T_regs was found in the white adipocyte group. TNF-α levels increased in both groups under hypobaric conditions. In the brown adipocyte group, anti-inflammatory IL-10 expression increased significantly; meanwhile, IL-10 expression decreased in the white adipocyte group. PD-L1 levels increased more significantly in brown adipocytes than in white adipocytes under hypobaric conditions.

Conclusion

Both brown and white adipocytes support T_reg expression when they are cultured with splenocytes. Of note, brown adipocytes maintained T_reg expression in intermittent hypobaric conditions. Anti-inflammatory cytokines and co-inhibitory ligands mediate the immunomodulatory effects of brown adipocytes under altered atmospheric conditions. Brown adipocytes showed the feasibility as a source of adjustment in physical stresses.

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.

Epigenetic Regulation of Adipogenic Differentiation by Histone Lysine Demethylation

Obesity is a rising public health problem that contributes to the development of several metabolic diseases and cancer. Adipocyte precursors outside of adipose depots that expand due to overweight and obesity may have a negative impact on human health. Determining how progenitor cells acquire a preadipocyte commitment and become mature adipocytes remains a significant challenge. Over the past several years, we have learned that the establishment of cellular identity is widely influenced by changes in histone marks, which in turn modulate chromatin structure. In this regard, histone lysine demethylases (KDMs) are now emerging as key players that shape chromatin through their ability to demethylate almost all major histone methylation sites. Recent research has shown that KDMs orchestrate the chromatin landscape, which mediates the activation of adipocyte-specific genes. In addition, KDMs have functions in addition to their enzymatic activity, which are beginning to be revealed, and their dysregulation seems to be related to the development of metabolic disorders. In this review, we highlight the biological functions of KDMs that contribute to the establishment of a permissive or repressive chromatin environment during the mesenchymal stem cell transition into adipocytes. Understanding how KDMs regulate adipogenesis might prompt the development of new strategies for fighting obesity-related diseases.

Role of brown adipose tissue in modulating adipose tissue inflammation and insulin resistance in high-fat diet fed mice

Obesity results in the chronic activation of innate immune system and subsequently sets in diabetes. Aim of the study was to investigate the immunometabolic role of brown adipose tissue (BAT) in the obesity. We performed both BAT transplantation as well as extirpation experiments in the mouse model of high-fat diet (HFD)-induced obesity. We carried out immune cell profiling in the stromal vascular fraction (SVF) isolated from epididymal white adipose tissue (eWAT). BAT transplantation reversed HFD-induced increase in body weight gain and insulin resistance without altering diet intake. Importantly, BAT transplantation attenuated the obesity-associated adipose tissue inflammation in terms of decreased pro-inflammatory M1-macrophages, cytotoxic CD8a T-cells and restored anti-inflammatory regulatory T-cells (Tregs) in the eWAT. BAT transplantation also improved endogenous BAT activity by elevating protein expression of browning markers (UCP-1, PRDM16 and PGC1α) in it. In addition, BAT transplantation promoted the eWAT expression of various genes involved in fatty acid oxidation (such as Elvol3 and Tfam,). In contrast, extirpation of the interscapular BAT exacerbated HFD-induced obesity, insulin resistance and adipose tissue inflammation (by increasing M1 macrophages, CD8a T-cell and decreasing Tregs in eWAT). Taken together, our results suggested an important role of BAT in combating obesity-associated metabolic complications. These results open a novel therapeutic option to target obesity and related metabolic disorders like type 2 diabetes.

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.

Evaluation and optimization of differentiation conditions for human primary brown adipocytes

As an effective way to improve energy expenditure, increasing the mass and activity of brown adipose tissue (BAT) has become a promising treatment for obesity and its associated disorders. Many efforts have been made to promote brown adipogenesis and increase the thermogenic capacity of brown adipose cells (BACs). The present culture schemes for human BAC differentiation are mostly derived from white adipocyte differentiation schemes. To solve this issue, we compared the adipogenic and thermogenic effects of various components on human BAC differentiation and optimized their concentrations as well as the culture time for BAC differentiation. In this study, we found that the induction factors did not show a dose-dependent promotion of brown adipogenesis or thermogenic capacity. The higher differentiation levels did not inevitably result in higher BAT-specific gene expression levels or increased β₃-receptor agonist sensitivity. As an important element of culture medium, triiodothyronine was found to be essential for differentiation and metabolic property maintenance. Furthermore, compared with other reported methods, this protocol induced a specific intrinsic differentiation program. Our study provides not only an optimized method for human BAC differentiation but also a cell model with good differentiation and thermogenic capacity for brown adipose research.

Adipose Tissue Function and Expandability as Determinants of Lipotoxicity and the Metabolic Syndrome

The adipose tissue organ is organised as distinct anatomical depots located all along the body axis and it is constituted of three different types of adipocytes : white, beige and brown which are integrated with vascular, immune, neural and extracellular stroma cells. These distinct adipocytes serve different specialised functions. The main function of white adipocytes is to ensure healthy storage of excess nutrients/energy and its rapid mobilisation to supply the demand of energy imposed by physiological cues in other organs, whereas brown and beige adipocytes are designed for heat production through uncoupling lipid oxidation from energy production. The concert action of the three type of adipocytes/tissues has been reported to ensure an optimal metabolic status in rodents. However, when one or multiple of these adipose depots become dysfunctional as a consequence of sustained lipid/nutrient overload, then insulin resistance and associated metabolic complications ensue. These metabolic alterations negatively affects the adipose tissue functionality and compromises global metabolic homeostasis. Optimising white adipose tissue expandability and its functional metabolic flexibility and/or promoting brown/beige mediated thermogenic activity counteracts obesity and its associated lipotoxic metabolic effects. The development of these therapeutic approaches requires a deep understanding of adipose tissue in all broad aspects. In this chapter we will discuss the characteristics of the different adipose tissue depots with respect to origins and precursors recruitment, plasticity, cellular composition and expandability capacity as well as molecular and metabolic signatures in both physiological and pathophysiological conditions.

Concomitant beige adipocyte differentiation upon induction of mesenchymal stem cells into brown adipocytes

The accumulation of fat, which results in obesity, is related to many metabolic disorders. Besides white and brown adipose tissue, beige adipose tissue has recently been recognized as a new type of accumulated fat. Mesenchymal stem cells (MSCs) have been shown to differentiate into brown adipocytes. Through analyzing levels of mRNA and protein markers associated with beige adipocyte, we found concomitant beige adipocyte differentiation upon induction of MSCs into brown adipocytes in a defined medium containing triiodothyronine, insulin, dexamethasone, and indomethacin. Moreover, we found that protein kinase A (PKA) modulators regulated MSC differentiation into brown or beige adipocytes. Activation of PKA by isobutylmethylxanthine or forskolin increased brown adipocyte differentiation and reduced beige adipocyte differentiation, while inactivation of PKA by KT-5720 or SC-3010 or the knockdown of PKA downstream cAMP response element-binding protein (CREB) decreased brown adipocyte differentiation and increased beige adipocyte differentiation. We also showed that increased brown adipocyte differentiation was accompanied by an increase in mitochondrial mass. In conclusion, we propose a model of beige/brown co-differentiation in MSCs and develop a method for controlling this differentiation via PKA modulation.

The Engrailed-1 Gene Stimulates Brown Adipogenesis

As a thermogenic organ, brown adipose tissue (BAT) has received a great attention in treating obesity and related diseases. It has been reported that brown adipocyte was derived from engrailed-1 (EN1) positive central dermomyotome. However, functions of EN1 in brown adipogenesis are largely unknown. Here we demonstrated that EN1 overexpression increased while EN1 knockdown decreased lipid accumulation and the expressions of key adipogenic genes including PPARγ2 and C/EBPα and mitochondrial OXPHOS as well as BAT specific marker UCP1. Taken together, our findings clearly indicate that EN1 is a positive regulator of brown adipogenesis.

Skeletal integration of energy homeostasis: Translational implications

New evidence has recently emerged defining a close relationship between fat and bone metabolism. Adipose tissue is one of the largest organs in the body but its functions vary by location and origin. Adipocytes can act in an autocrine manner to regulate energy balance by sequestering triglycerides and then, depending on demand, releasing fatty acids through lipolysis for energy utilization, and in some cases through uncoupling protein 1 for generating heat. Adipose tissue can also act in an endocrine or paracrine manner by releasing adipokines that modulate the function of other organs. Bone is one of those target tissues, although recent evidence has emerged that the skeleton reciprocates by releasing its own factors that modulate adipose tissue and beta cells in the pancreas. Therefore, it is not surprising that these energy-modulating tissues are controlled by a central regulatory mechanism, primarily the sympathetic nervous system. Disruption in this complex regulatory circuit and its downstream tissues is manifested in a wide range of metabolic disorders, for which the most prevalent is type 2 diabetes mellitus. The aim of this review is to summarize our knowledge of common determinants in the bone and adipose function and the translational implications of recent work in this emerging field.

Ambient Temperature and Prevalence of Obesity: A Nationwide Population-Based Study in Korea

Background

Recent studies have suggested a possible association between outdoor or indoor temperature and obesity. We aimed to examine whether ambient temperature is associated with the prevalence of obesity or abdominal obesity in the Korean population.

Methods

Data on anthropometric, socio-demographic, laboratory and lifestyle factors were retrieved from National Health Insurance System data obtained in 2009–2010. Thirty years (1981 to 2010) of meteorological parameters for 71 observation areas were acquired from the Korea Meteorological Administration. Included in this analysis were 124,354 individuals. A body mass index (BMI) ≥ 25 kg/m² and a waist circumference (WC) ≥ 90 cm (men) or 85 cm (women) were considered to represent obesity and abdominal obesity, respectively.

Results

The mean annual temperature (MAT) ranged from 6.6°C to 16.6°C, and BMI was positively correlated with MAT (r = 0.0078, P = 0.0065). WC was positively correlated with MAT (r = 0.0165, P < 0.0001) and negatively correlated with the number of days with mean temperature < 0°C (DMT0; r = –0.0129, P = 0.0002). After adjusting for age, sex, smoking status, alcohol consumption, exercise, income, residential area and altitude, the odds ratios (95% CI) for obesity and abdominal obesity in the highest quintile MAT group were 1.045 (1.010, 1.081) and 1.082 (1.042, 1.124), respectively, compared with the lower four quintiles of the MAT group. Similarly, subjects in the area of the lowest quintile of DMT0 had significantly higher odds of abdominal obesity compared with the higher four quintile groups of DMT0.

Conclusion

This study finds an association between ambient temperature and prevalence of obesity in the Korean population when controlling for several confounding factors. Adaptive thermogenesis might be a possible explanation for this phenomenon.

The brain and brown fat

Brown adipose tissue (BAT) is a specialized organ responsible for thermogenesis, a process required for maintaining body temperature. BAT is regulated by the sympathetic nervous system (SNS), which activates lipolysis and mitochondrial uncoupling in brown adipocytes. For many years, BAT was considered to be important only in small mammals and newborn humans, but recent data have shown that BAT is also functional in adult humans. On the basis of this evidence, extensive research has been focused on BAT function, where new molecules, such as irisin and bone morphogenetic proteins, particularly BMP7 and BMP8B, as well as novel central factors and new regulatory mechanisms, such as orexins and the canonical ventomedial nucleus of the hypothalamus (VMH) AMP- activated protein kinase (AMPK)-SNS-BAT axis, have been discovered and emerged as potential drug targets to combat obesity. In this review we provide an overview of the complex central regulation of BAT and how different neuronal cell populations co-ordinately work to maintain energy homeostasis.

Insulin resistance as a physiological defense against metabolic stress: implications for the management of subsets of type 2 diabetes

Stratifying the management of type 2 diabetes (T2D) has to take into account marked variability in patient phenotype due to heterogeneity in its pathophysiology, different stages of the disease process, and multiple other patient factors including comorbidities. The focus here is on the very challenging subgroup of patients with T2D who are overweight or obese with insulin resistance (IR) and the most refractory hyperglycemia due to an inability to change lifestyle to reverse positive energy balance. For this subgroup of patients with T2D, we question the dogma that IR is primarily harmful to the body and should be counteracted at any cost. Instead we propose that IR, particularly in this high-risk subgroup, is a defense mechanism that protects critical tissues of the cardiovascular system from nutrient-induced injury. Overriding IR in an effort to lower plasma glucose levels, particularly with intensive insulin therapy, could therefore be harmful. Treatments that nutrient off-load to lower glucose are more likely to be beneficial. The concepts of "IR as an adaptive defense mechanism" and "insulin-induced metabolic stress" may provide explanation for some of the unexpected outcomes of recent major clinical trials in T2D. Potential molecular mechanisms underlying these concepts; their clinical implications for stratification of T2D management, particularly in overweight and obese patients with difficult glycemic control; and future research requirements are discussed.

Adipose Tissues Characteristics of Normal, Obesity, and Type 2 Diabetes in Uygurs Population

Our results showed that, at the same BMI level, Uygurs have greater WHR values, abdominal visceral fat content, and diabetes risks than Kazaks. In addition, values of HDL-C in Uygur subjects were lower than those in Kazak subjects, and values of creatinine, uric acid, diastolic blood pressure, blood glucose, and fructosamine in Uygur male subjects were lower than those in Kazak male subjects. In contrast, systolic blood pressure values in Uygur subjects were greater than those in Kazak subjects, and blood glucose values were greater in Uygur female subjects than in Kazak female subjects. Additionally, in Uygurs, visceral adipose tissue expression levels of TBX1 and TCF21 were greater in obesity group than in normal and T2DM groups and lower in T2DM group than in normal group (P < 0.01). The visceral adipose tissue expression levels of APN in normal group was greater than those in obesity and T2DM groups, and visceral adipose tissue expression levels of TNF-α and MCP-1 in normal group were lower than those in obesity and T2DM groups (P < 0.01). In conclusion, T2DM in Uygurs was mainly associated with not only distribution of adipose tissue in body, but also change in metabolic activity and adipocytokines secretion of adipose tissue.

Ambient temperature and prevalence of obesity in the Spanish population: The Di@bet.es study

OBJECTIVE

The aim of this study was to examine possible associations between ambient temperature and obesity in the Spanish population using an ecological focus.

METHODS

The Di@bet.es study is a national, cross-sectional, population-based survey of cardiometabolic risk factors and their association with lifestyle.

SAMPLE

5,061 subjects in 100 clusters.

VARIABLES

Clinical, demographic and lifestyle survey, physical examination, and blood sampling. The mean annual temperature (°C) for each study site was collected from the Spanish National Meteorology Agency (1971-2000).

RESULTS

The prevalence rates of obesity in the different geographical areas divided according to mean annual temperature quartiles were 26.9% in quartile 1 (10.4-14.5°C), 30.5% in quartile 2 (14.5-15.5°C), 32% in quartile 3 (15.5-17.8°C), and 33.6% in quartile 4 (17.8-21.3°C) (P = 0.003). Logistic regression analyses including multiple socio-demographic (age, gender, educational level, marital status) and lifestyle (physical activity, Mediterranean diet score, smoking) variables showed that, as compared with quartile 1, the odd ratios for obesity were 1.20 (1.01-1.42), 1.35 (1.12-1.61), and 1.38 (1.14-1.67) in quartiles 2, 3, and 4, respectively (P = 0.001 for difference, P < 0.001 for trend).

CONCLUSIONS

Our study reports an association between ambient temperature and obesity in the Spanish population controlled for known confounders.

Unraveling biochemical pathways affected by mitochondrial dysfunctions using metabolomic approaches

Mitochondrial dysfunction(s) (MDs) can be defined as alterations in the mitochondria, including mitochondrial uncoupling, mitochondrial depolarization, inhibition of the mitochondrial respiratory chain, mitochondrial network fragmentation, mitochondrial or nuclear DNA mutations and the mitochondrial accumulation of protein aggregates. All these MDs are known to alter the capacity of ATP production and are observed in several pathological states/diseases, including cancer, obesity, muscle and neurological disorders. The induction of MDs can also alter the secretion of several metabolites, reactive oxygen species production and modify several cell-signalling pathways to resolve the mitochondrial dysfunction or ultimately trigger cell death. Many metabolites, such as fatty acids and derived compounds, could be secreted into the blood stream by cells suffering from mitochondrial alterations. In this review, we summarize how a mitochondrial uncoupling can modify metabolites, the signalling pathways and transcription factors involved in this process. We describe how to identify the causes or consequences of mitochondrial dysfunction using metabolomics (liquid and gas chromatography associated with mass spectrometry analysis, NMR spectroscopy) in the obesity and insulin resistance thematic.

Syndecan-1 is required to maintain intradermal fat and prevent cold stress

Homeostatic temperature regulation is fundamental to mammalian physiology and is controlled by acute and chronic responses of local, endocrine and nervous regulators. Here, we report that loss of the heparan sulfate proteoglycan, syndecan-1, causes a profoundly depleted intradermal fat layer, which provides crucial thermogenic insulation for mammals. Mice without syndecan-1 enter torpor upon fasting and show multiple indicators of cold stress, including activation of the stress checkpoint p38α in brown adipose tissue, liver and lung. The metabolic phenotype in mutant mice, including reduced liver glycogen, is rescued by housing at thermoneutrality, suggesting that reduced insulation in cool temperatures underlies the observed phenotypes. We find that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for adipocyte differentiation in vitro. Intradermal fat shows highly dynamic differentiation, continuously expanding and involuting in response to hair cycle and ambient temperature. This physiology probably confers a unique role for Sdc1 in this adipocyte sub-type. The PPARγ agonist rosiglitazone rescues Sdc1−/− intradermal adipose tissue, placing PPARγ downstream of Sdc1 in triggering adipocyte differentiation. Our study indicates that disruption of intradermal adipose tissue development results in cold stress and complex metabolic pathology.

All mammals strive to maintain a fixed body temperature, and do so using a remarkable array of different strategies, which vary depending upon the degree of cold challenge. Physiologists many decades ago observed that a fat layer right underneath the epidermis (and above the dermal muscle layer) thickens in response to colder ambient temperatures. This “intradermal fat” provided insulation within days of climate changes. We have found that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for intradermal fat expansion in response to cold exposure. This is a highly specific phenotype not shared by other adipocytes. When intradermal fat is absent, mice do not adapt normally to cold stress, and show altered systemic physiologies, including increased brown adipose tissue thermogenesis and hyper-activation of a stress checkpoint (p38α), designed to protect the body against mutagenic and oxidative stressors. The phenotypes associated with loss of Sdc1 function are reversed when mice are housed in warm temperatures, where defense of body temperature is not required. This study is the first to show that intradermal fat can be genetically regulated, with systemic effects on physiology.

Inhibition of the central melanocortin system decreases brown adipose tissue activity

The melanocortin system is an important regulator of energy balance, and melanocortin 4 receptor (MC4R) deficiency is the most common monogenic cause of obesity. We investigated whether the relationship between melanocortin system activity and energy expenditure (EE) is mediated by brown adipose tissue (BAT) activity. Therefore, female APOE*3-Leiden.CETP transgenic mice were fed a Western-type diet for 4 weeks and infused intracerebroventricularly with the melanocortin 3/4 receptor (MC3/4R) antagonist SHU9119 or vehicle for 2 weeks. SHU9119 increased food intake (+30%) and body fat (+50%) and decreased EE by reduction in fat oxidation (-42%). In addition, SHU9119 impaired the uptake of VLDL-TG by BAT. In line with this, SHU9119 decreased uncoupling protein-1 levels in BAT (-60%) and induced large intracellular lipid droplets, indicative of severely disturbed BAT activity. Finally, SHU9119-treated mice pair-fed to the vehicle-treated group still exhibited these effects, indicating that MC4R inhibition impairs BAT activity independent of food intake. These effects were not specific to the APOE*3-Leiden.CETP background as SHU9119 also inhibited BAT activity in wild-type mice. We conclude that inhibition of central MC3/4R signaling impairs BAT function, which is accompanied by reduced EE, thereby promoting adiposity. We anticipate that activation of MC4R is a promising strategy to combat obesity by increasing BAT activity.

The different shades of fat

Our understanding of adipose tissue biology has progressed rapidly since the turn of the century. White adipose tissue has emerged as a key determinant of healthy metabolism and metabolic dysfunction. This realization is paralleled only by the confirmation that adult humans have heat-dissipating brown adipose tissue, an important contributor to energy balance and a possible therapeutic target for the treatment of metabolic disease. We propose that the development of successful strategies to target brown and white adipose tissues will depend on investigations that elucidate their developmental origins and cell-type-specific functional regulators.

On the evolutionary origins of obesity: a new hypothesis

Obesity is an escalating threat of pandemic proportions, currently affecting billions of people worldwide and exerting a devastating socioeconomic influence in industrialized countries. Despite intensive efforts to curtail obesity, results have proved disappointing. Although it is well recognized that obesity is a result of gene-environment interactions and that predisposition to obesity lies predominantly in our evolutionary past, there is much debate as to the precise nature of how our evolutionary past contributed to obesity. The "thrifty genotype" hypothesis suggests that obesity in industrialized countries is a throwback to our ancestors having undergone positive selection for genes that favored energy storage as a consequence of the cyclical episodes of famine and surplus after the advent of farming 10 000 years ago. Conversely, the "drifty genotype" hypothesis contends that the prevalence of thrifty genes is not a result of positive selection for energy-storage genes but attributable to genetic drift resulting from the removal of predative selection pressures. Both theories, however, assume that selection pressures the ancestors of modern humans living in western societies faced were the same. Moreover, neither theory adequately explains the impact of globalization and changing population demographics on the genetic basis for obesity in developed countries, despite clear evidence for ethnic variation in obesity susceptibility and related metabolic disorders. In this article, we propose that the modern obesity pandemic in industrialized countries is a result of the differential exposure of the ancestors of modern humans to environmental factors that began when modern humans left Africa around 70 000 years ago and migrated through the globe, reaching the Americas around 20 000 years ago. This article serves to elucidate how an understanding of ethnic differences in genetic susceptibility to obesity and the metabolic syndrome, in the context of historic human population redistribution, could be used in the treatment of obesity in industrialized countries.