Secretory Proteome of Brown Adipocytes in Response to cAMP-Mediated Thermogenic Activation

The browning and mobilization of subcutaneous white adipose tissue supports efficient skin repair

Adipocytes in dermis are considered to be important participants in skin repair and regeneration, but the role of subcutaneous white adipose tissue (sWAT) in skin repair is poorly understood. Here, we revealed the dynamic changes of sWAT during wound healing process. Lineage-tracing mouse studies revealed that sWAT would enter into the large wound bed and participate in the formation of granulation tissue. Moreover, sWAT undergoes beiging after skin injury. Inhibition of sWAT beiging by genetically silencing PRDM16, a key regulator to beiging, hindered wound healing process. The transcriptomics results suggested that beige adipocytes in sWAT abundantly express neuregulin 4 (NRG4), which regulated macrophage polarization and the function of myofibroblasts. In diabetic wounds, the beiging of sWAT was significantly suppressed. Thus, adipocytes from sWAT regulate multiple aspects of repair and may be therapeutic for inflammatory diseases and defective wound healing associated with aging and diabetes.

Defective extracellular matrix remodeling in brown adipose tissue is associated with fibro-inflammation and reduced diet-induced thermogenesis

The relevance of extracellular matrix (ECM) remodeling is reported in white adipose tissue (AT) and obesity-related dysfunctions, but little is known about the importance of ECM remodeling in brown AT (BAT) function. Here, we show that a time course of high-fat diet (HFD) feeding progressively impairs diet-induced thermogenesis concomitantly with the development of fibro-inflammation in BAT. Higher markers of fibro-inflammation are associated with lower cold-induced BAT activity in humans. Similarly, when mice are housed at thermoneutrality, inactivated BAT features fibro-inflammation. We validate the pathophysiological relevance of BAT ECM remodeling in response to temperature challenges and HFD using a model of a primary defect in the collagen turnover mediated by partial ablation of the Pepd prolidase. Pepd-heterozygous mice display exacerbated dysfunction and BAT fibro-inflammation at thermoneutrality and in HFD. Our findings show the relevance of ECM remodeling in BAT activation and provide a mechanism for BAT dysfunction in obesity.

The evolving view of thermogenic fat and its implications in cancer and metabolic diseases

The incidence of metabolism-related diseases like obesity and type 2 diabetes mellitus has reached pandemic levels worldwide and increased gradually. Most of them are listed on the table of high-risk factors for malignancy, and metabolic disorders systematically or locally contribute to cancer progression and poor prognosis of patients. Importantly, adipose tissue is fundamental to the occurrence and development of these metabolic disorders. White adipose tissue stores excessive energy, while thermogenic fat including brown and beige adipose tissue dissipates energy to generate heat. In addition to thermogenesis, beige and brown adipocytes also function as dynamic secretory cells and a metabolic sink of nutrients, like glucose, fatty acids, and amino acids. Accordingly, strategies that activate and expand thermogenic adipose tissue offer therapeutic promise to combat overweight, diabetes, and other metabolic disorders through increasing energy expenditure and enhancing glucose tolerance. With a better understanding of its origins and biological functions and the advances in imaging techniques detecting thermogenesis, the roles of thermogenic adipose tissue in tumors have been revealed gradually. On the one hand, enhanced browning of subcutaneous fatty tissue results in weight loss and cancer-associated cachexia. On the other hand, locally activated thermogenic adipocytes in the tumor microenvironment accelerate cancer progression by offering fuel sources and is likely to develop resistance to chemotherapy. Here, we enumerate current knowledge about the significant advances made in the origin and physiological functions of thermogenic fat. In addition, we discuss the multiple roles of thermogenic adipocytes in different tumors. Ultimately, we summarize imaging technologies for identifying thermogenic adipose tissue and pharmacologic agents via modulating thermogenesis in preclinical experiments and clinical trials.

The Gut Microbiota Metabolite Succinate Promotes Adipose Tissue Browning in Crohn's Disease

BACKGROUND AND AIMS

Crohn's disease [CD] is associated with complex microbe-host interactions, involving changes in microbial communities, and gut barrier defects, leading to the translocation of microorganisms to surrounding adipose tissue [AT]. We evaluated the presence of beige AT depots in CD and questioned whether succinate and/or bacterial translocation promotes white-to-beige transition in adipocytes.

METHODS

Visceral [VAT] and subcutaneous [SAT] AT biopsies, serum and plasma were obtained from patients with active [n = 21] or inactive [n = 12] CD, and from healthy controls [n = 15]. Adipose-derived stem cells [ASCs] and AT macrophages [ATMs] were isolated from VAT biopsies.

RESULTS

Plasma succinate levels were significantly higher in patients with active CD than in controls and were intermediate in those with inactive disease. Plasma succinate correlated with the inflammatory marker high-sensitivity C-reactive protein. Expression of the succinate receptor SUCNR1 was higher in VAT, ASCs and ATMs from the active CD group than from the inactive or control groups. Succinate treatment of ASCs elevated the expression of several beige AT markers from controls and from patients with inactive disease, including uncoupling protein-1 [UCP1]. Notably, beige AT markers were prominent in ASCs from patients with active CD. Secretome profiling revealed that ASCs from patients with active disease secrete beige AT-related proteins, and co-culture assays showed that bacteria also trigger the white-to-beige switch of ASCs from patients with CD. Finally, AT depots from patients with CD exhibited a conversion from white to beige AT together with high UCP1 expression, which was corroborated by in situ thermal imaging analysis.

CONCLUSIONS

Succinate and bacteria trigger white-to-beige AT transition in CD. Understanding the role of beige AT in CD might aid in the development of therapeutic or diagnostic interventions.

Secretome of Adipose Tissue as the Key to Understanding the Endocrine Function of Adipose Tissue

The prevalence of obesity has reached pandemic levels and is becoming a serious health problem in developed and developing countries. Obesity is associated with an increased prevalence of comorbidities that include type II diabetes, cardiovascular diseases and some cancers. The recognition of adipose tissue as an endocrine organ capable of secreting adipokines that influence whole-body energy homeostasis was a breakthrough leading to a better molecular understanding of obesity. Of the adipokines known to be involved in the regulation of energy metabolism, very few are considered central regulators of insulin sensitivity, metabolism and energy homeostasis, and the discovery and characterization of new adipocyte-derived factors are still ongoing. Proteomics techniques, such as liquid chromatography-mass spectrometry or gas chromatography-mass spectrometry, have proven to be useful tools for analyzing the secretory function of adipose tissue (the secretome), providing insights into molecular events that influence body weight. Apart from the identification of novel proteins, the considerable advantage of this approach is the ability to detect post-translational modifications that cannot be predicted in genomic studies. In this review, we summarize recent efforts to identify novel bioactive secretory factors through proteomics.

Immune Cells in Thermogenic Adipose Depots: The Essential but Complex Relationship

Brown adipose tissue (BAT) is a unique organ in mammals capable of dissipating energy in form of heat. Additionally, white adipose tissue (WAT) can undergo browning and perform thermogenesis. In recent years, the research community has aimed to harness thermogenic depot functions for new therapeutic strategies against obesity and the metabolic syndrome; hence a comprehensive understanding of the thermogenic fat microenvironment is essential. Akin to WAT, immune cells also infiltrate and reside within the thermogenic adipose tissues and perform vital functions. As highly plastic organs, adipose depots rely on crucial interplay with these tissue resident cells to conserve their healthy state. Evidence has accumulated to show that different immune cell populations contribute to thermogenic adipose tissue homeostasis and activation through complex communicative networks. Furthermore, new studies have identified -but still not fully characterized further- numerous immune cell populations present in these depots. Here, we review the current knowledge of this emerging field by describing the immune cells that sway the thermogenic adipose depots, and the complex array of communications that influence tissue performance.

Mutations of NRG4 Contribute to the Pathogenesis of Nonalcoholic Fatty Liver Disease and Related Metabolic Disorders

Neuregulin 4 (Nrg4), an adipose tissue-enriched endocrine factor, participates in adipocyte-to-hepatocyte communication, eliciting beneficial metabolic effects in nonalcoholic fatty liver disease (NAFLD). We evaluate the physiological roles of NRG4 in humans and unravel the role of NRG4 variants in the pathogenesis of NAFLD and related metabolic disorders. We identified two rare missense mutations-p.R44H and p.E47Q-in the NRG4 EGF-like domain by whole-exome sequencing in 224 severely obese subjects and exome genotyping in 2,388 subjects from the Shanghai Obesity Study. The overexpression animal models showed that wild-type (WT) Nrg4 could attenuate high-fat diet-induced hepatic lipogenesis and improve energy metabolism. Nrg4 E47Q enhanced the protective effect, whereas Nrg4 R44H lost this function. Unlike Nrg4 R44H, Nrg4 E47Q activated the phosphorylation of ErbB4 and negatively regulated de novo lipogenesis through the ErbB4-STAT5-SREBP-1C pathway. The surface plasmon resonance experiments revealed a higher affinity of E47Q Nrg4 than WT to bind ErbB4, while R44H showed no binding. In conclusion, the study suggests that genetic variations in NRG4 could produce mutant proteins with aberrant functions and that impaired or enhanced Nrg4 function could be either a risk factor or a protective factor for NAFLD and associated metabolic disorders.

Physical Activity Attenuates the Obesity-Induced Dysregulated Expression of Brown Adipokines in Murine Interscapular Brown Adipose Tissue

In recent years, brown adipose tissue (BAT), which has a high heat-producing capacity, has been confirmed to exist even in adults, and it has become a focal point for the prevention and the improvement of obesity and lifestyle-related diseases. However, the influences of obesity and physical activity (PA) on the fluid factors secreted from BAT (brown adipokines) are not well understood. In this study, therefore, we focused on brown adipokines and investigated the effects of obesity and PA. The abnormal expressions of gene fluid factors such as galectin-3 (Lgals3) and Lgals3 binding protein (Lgals3bp), whose proteins are secreted from HB2 brown adipocytes, were observed in the interscapular BAT of obese mice fed a high-fat diet for 4 months. PA attenuated the abnormalities in the expressions of these genes. Furthermore, although the gene expressions of factors related to brown adipocyte differentiation such as peroxisome proliferator-activated receptor gamma coactivator 1-α were also down-regulated in the BAT of the obese mice, PA suppressed the down-regulation of these factors. On the other hand, lipogenesis was increased more in HB2 cells overexpressing Lgals3 compared with that in control cells, and the overexpression of Lgals3bp decreased the mitochondrial mass. These results indicate that PA attenuates the obesity-induced dysregulated expression of brown adipokines and suggests that Lgals3 and Lgals3bp are involved in brown adipocyte differentiation.

Effects of stem cells from inducible brown adipose tissue on diet-induced obesity in mice

Adipose-derived mesenchymal stem cells (ASCs) are a promising option for the treatment of obesity and its metabolic co-morbidities. Despite the recent identification of brown adipose tissue (BAT) as a potential target in the management of obesity, the use of ASCs isolated from BAT as a therapy for patients with obesity has not yet been explored. Metabolic activation of BAT has been shown to have not only thermogenic effects, but it also triggers the secretion of factors that confer protection against obesity. Herein, we isolated and characterized ASCs from the visceral adipose tissue surrounding a pheochromocytoma (IB-hASCs), a model of inducible BAT in humans. We then compared the anti-obesity properties of IB-hASCs and human ASCs isolated from visceral white adipose tissue (W-hASCs) in a murine model of diet-induced obesity. We found that both ASC therapies mitigated the metabolic abnormalities of obesity to a similar extent, including reducing weight gain and improving glucose tolerance. However, infusion of IB-hASCs was superior to W-hASCs in suppressing lipogenic and inflammatory markers, as well as preserving insulin secretion. Our findings provide evidence for the metabolic benefits of visceral ASC infusion and support further studies on IB-hASCs as a therapeutic option for obesity-related comorbidities.

The cellular and functional complexity of thermogenic fat

Brown and beige adipocytes are mitochondria-enriched cells capable of dissipating energy in the form of heat. These thermogenic fat cells were originally considered to function solely in heat generation through the action of the mitochondrial protein uncoupling protein 1 (UCP1). In recent years, significant advances have been made in our understanding of the ontogeny, bioenergetics and physiological functions of thermogenic fat. Distinct subtypes of thermogenic adipocytes have been identified with unique developmental origins, which have been increasingly dissected in cellular and molecular detail. Moreover, several UCP1-independent thermogenic mechanisms have been described, expanding the role of these cells in energy homeostasis. Recent studies have also delineated roles for these cells beyond the regulation of thermogenesis, including as dynamic secretory cells and as a metabolic sink. This Review presents our current understanding of thermogenic adipocytes with an emphasis on their development, biological functions and roles in systemic physiology.

Thermogenic Fat: Development, Physiological Function, and Therapeutic Potential

The concerning worldwide increase of obesity and chronic metabolic diseases, such as T2D, dyslipidemia, and cardiovascular disease, motivates further investigations into preventive and alternative therapeutic approaches. Over the past decade, there has been growing evidence that the formation and activation of thermogenic adipocytes (brown and beige) may serve as therapy to treat obesity and its associated diseases owing to its capacity to increase energy expenditure and to modulate circulating lipids and glucose levels. Thus, understanding the molecular mechanism of brown and beige adipocytes formation and activation will facilitate the development of strategies to combat metabolic disorders. Here, we provide a comprehensive overview of pathways and players involved in the development of brown and beige fat, as well as the role of thermogenic adipocytes in energy homeostasis and metabolism. Furthermore, we discuss the alterations in brown and beige adipose tissue function during obesity and explore the therapeutic potential of thermogenic activation to treat metabolic syndrome.

The Effect of Omega-3 Fatty Acid Supplementation on Serum Adipocytokines, Lipid Profile and Biochemical Markers of Inflammation in Recreational Runners

BACKGROUND

The study aimed to evaluate the effects of a 3-week ω-3 PUFA supplementation on serum adipocytokines (i.e., adiponectin, leptin), neuregulin-4 (NRG4) and erythrocyte omega-3 (ω-3) fatty acid content, as well as the blood antioxidant defense capacity in non-elite endurance runners.

METHODS

Twenty-four runners were randomized into two groups: the supplemented group, who received omega free fatty acids extract containing 142 mg of EPA, 267 mg of DHA, 12 mg of vitamin E and 5 µg of vitamin D, each administrated at a dose of six capsules twice a day for three weeks, or the placebo group. Venous blood samples were withdrawn at the start and at the end of the study protocols to estimate serum biochemical variables.

RESULTS

A significantly higher ω-3 index and lower AA/EPA ratio was observed after ω-3 PUFA compared to pre-supplementation levels (p < 0.001 and p < 0.001, respectively). An increase in baseline adiponectin and NRG4 levels, as well as a decrease of leptin concentration and lipid profile improvement, were observed in subjects after a ω-3 PUFA diet. The increased ω-3 index had a significant effect on TNFα levels and a serum marker of antioxidant defense.

CONCLUSIONS

The ω-3 PUFA extract with added vitamin E and D supplementation may have a positive effect on the function of the adipocyte tissue, as well as the ability to prevent cardiovascular complications in athletes.

Genetic background and diet affect brown adipose gene coexpression networks associated with metabolic phenotypes

Adipose is a dynamic endocrine organ that is critical for regulating metabolism and is highly responsive to nutritional environment. Brown adipose tissue is an exciting potential therapeutic target; however, there are no systematic studies of gene-by-environment interactions affecting function of this organ. We leveraged a weighted gene coexpression network analysis to identify transcriptional networks in brown adipose tissue from LG/J and SM/J inbred mice fed high- or low-fat diets and correlate these networks with metabolic phenotypes. We identified eight primary gene network modules associated with variation in obesity and diabetes-related traits. Four modules were enriched for metabolically relevant processes such as immune and cytokine response, cell division, peroxisome functions, and organic molecule metabolic processes. The relative expression of genes in these modules is highly dependent on both genetic background and dietary environment. Genes in the immune/cytokine response and cell division modules are particularly highly expressed in high fat-fed SM/J mice, which show unique brown adipose-dependent remission of diabetes. The interconnectivity of genes in these modules is also heavily dependent on diet and strain, with most genes showing both higher expression and coexpression under the same context. We highlight several genes of interest, Col28a1, Cyp26b1, Bmp8b, and Ngef, that have distinct expression patterns among strain-by-diet contexts and fall under metabolic quantitative trait loci previously mapped in an F16 generation of an advanced intercross between LG/J and SM/J. Each of these genes have some connection to obesity and diabetes-related traits, but have not been studied in brown adipose tissue. Our results provide important insights into the relationship between brown adipose and systemic metabolism by being the first gene-by-environment study of brown adipose transcriptional networks.

Neuregulin 4: A "Hotline" Between Brown Fat and Liver

The discovery that functional brown adipose tissue (BAT) in adult humans is inversely related to body fat mass and may reflect metabolic health has stimulated adipose tissue research to explore activation of BAT as a potential target for antiobesity treatments. In addition to the capacity of BAT to increase energy expenditure and glucose and lipid uptake, BAT secretes factors that may contribute to the regulation of whole-body metabolism. Among signals released from BAT, neuregulin 4 (NRG4) has been recently identified as an endocrine factor that may link the activation of BAT to protection against diet-induced obesity, insulin resistance, and hepatic steatosis. NRG4 was shown to directly reduce lipogenesis in hepatocytes, and it could indirectly activate BAT via sympathetic neurons or via inducing brown adipocyte-like signatures in white adipocytes in a paracrine manner. However, the potential relevance of NRG4 as a diagnostic tool or target for the treatment of obesity-related diseases remains to be explored.