A direct tissue-grafting approach to increasing endogenous brown fat

Lipid Droplet-Mitochondria Contacts in Health and Disease

The orchestration of cellular metabolism and redox balance is a complex, multifaceted process crucial for maintaining cellular homeostasis. Lipid droplets (LDs), once considered inert storage depots for neutral lipids, are now recognized as dynamic organelles critical in lipid metabolism and energy regulation. Mitochondria, the powerhouses of the cell, play a central role in energy production, metabolic pathways, and redox signaling. The physical and functional contacts between LDs and mitochondria facilitate a direct transfer of lipids, primarily fatty acids, which are crucial for mitochondrial β-oxidation, thus influencing energy homeostasis and cellular health. This review highlights recent advances in understanding the mechanisms governing LD-mitochondria interactions and their regulation, drawing attention to proteins and pathways that mediate these contacts. We discuss the physiological relevance of these interactions, emphasizing their role in maintaining energy and redox balance within cells, and how these processes are critical in response to metabolic demands and stress conditions. Furthermore, we explore the pathological implications of dysregulated LD-mitochondria interactions, particularly in the context of metabolic diseases such as obesity, diabetes, and non-alcoholic fatty liver disease, and their potential links to cardiovascular and neurodegenerative diseases. Conclusively, this review provides a comprehensive overview of the current understanding of LD-mitochondria interactions, underscoring their significance in cellular metabolism and suggesting future research directions that could unveil novel therapeutic targets for metabolic and degenerative diseases.

Transplantation of Cold-Stimulated Subcutaneous Adipose Tissue Improves Fat Retention and Recipient Metabolism

BACKGROUND

Induction of beige fat for grafting is an emerging transplantation strategy. However, safety concerns associated with pharmaceutical interventions limit its wider application. Moreover, because beige fat is a special type of fat with strong metabolic functions, its effect on the metabolism of recipients after grafting has not been explored in the plastic surgery domain.

OBJECTIVES

The aim of this study was to explore whether cold-induced inguinal white adipose tissue (iWAT) transplantation has a higher retention rate and beneficial effects on recipient metabolism.

METHODS

C57/BL6 mice were subjected to cold stimulation for 48 hours to induce the browning of iWAT and harvested immediately. Subsequently, each mouse received a transplant of 0.2 mL cold-induced iWAT or normal iWAT. Fat grafts and recipients' iWAT, epididymal adipose tissue, and brown adipose tissue were harvested at 8 weeks after operation. Immunofluorescence staining, real-time polymerase chain reaction, and western blot were used for histological and molecular analysis.

RESULTS

Cold-induced iWAT grafting had a higher mean [standard error of the mean] retention rate (67.33% [1.74%] vs 55.83% [2.94%], P < .01) and more satisfactory structural integrity than normal iWAT. Histological changes identified improved adipose tissue homeostasis after cold challenge, including abundant smaller adipocytes, higher levels of adipogenesis, angiogenesis, and proliferation, but lower levels of fibrosis. More importantly, cold-induced iWAT grafting suppressed the inflammation of epididymal adipose tissue caused by conventional fat grafting, and activated the glucose metabolism and thermogenic activity of recipients' adipose tissues.

CONCLUSIONS

Cold-induced iWAT grafting is an effective nonpharmacological intervention strategy to improve the retention rate and homeostasis of grafts. Furthermore, it improves the adverse effects caused by traditional fat grafting, while also conferring metabolic benefits.

Unleashing the potential of adipose organoids: A revolutionary approach to combat obesity-related metabolic diseases

Obesity-related metabolic diseases, including obesity, diabetes, hyperlipidemia, and non-alcoholic fatty liver diseases pose a significant threat to health. However, comprehensive pathogenesis exploration and effective therapy development are impeded by the limited availability of human models. Notably, advances in organoid technology enable the generation of adipose organoids that recapitulate structures and functions of native human adipose tissues to investigate mechanisms and develop corresponding treatments for obesity-related metabolic diseases. Here, we review the general principles, sources, and three-dimensional techniques for engineering adipose organoids, along with strategies to promote maturation. We also outline the application of white adipose organoids, primarily for disease modeling and drug screening, and highlight the therapeutic potential of thermogenic beige and brown adipose organoids in promoting weight loss and glucose and lipid metabolic homeostasis. We also discuss the challenges and prospects in the establishment and bench-to-bedside of adipose organoids, as well as their potential applications.

Castration promotes the browning of the prostate tumor microenvironment

BACKGROUND

Adipose tissue has gained attention due to its potential paracrine role. Periprostatic adipose tissue surrounds the prostate and the prostatic urethra, and it is an essential player in prostate cancer progression. Since obesity is directly related to human tumor progression, and adipose tissue depots are one of the significant components of the tumor microenvironment, the molecular mediators of the communication between adipocytes and epithelial cells are in the spotlight. Although periprostatic white adipose tissue contributes to prostate cancer progression, brown adipose tissue (BAT), which has beneficial effects in metabolic pathologies, has been scarcely investigated concerning cancer progression. Given that adipose tissue is a target of androgen signaling, the actual role of androgen removal on the periprostatic adipose tissue was the aim of this work.

METHODS

Surgical castration of the transgenic adenocarcinoma of the mouse prostate (TRAMP) was employed. By histology examination and software analysis, WAT and BAT tissue was quantified. 3T3-like adipocytes were used to study the role of Casodex® in modifying adipocyte differentiation and to investigate the function of the secretome of adipocytes on the proliferation of androgen-dependent and independent prostate cancer cells. Finally, the role of cell communication was assayed by TRAMP-C1 xenograft implanted in the presence of 3T3-like adipocytes.

RESULTS

Androgen removal increases brown/beige adipose tissue in the fat immediately surrounding the prostate glands of TRAMP mice, concomitant with an adjustment of the metabolism. Castration increases body temperature, respiratory exchange rate, and energy expenditure. Also, in vitro, it is described that blocking androgen signaling by Casodex® increases the uncoupling protein 1 (UCP1) marker in 3T3-like adipocytes. Finally, the effect of brown/beige adipocyte secretome was studied on the proliferation of prostate cancer cells in vivo and in vitro. The secretome of brown/beige adipocytes reduces the proliferation of prostate cancer cells mediated partly by the secretion of extracellular vesicles.

CONCLUSIONS

Consequently, we concluded that hampering androgen signaling plays a crucial role in the browning of the periprostatic adipose tissue. Also, the presence of brown adipocytes exhibits the opposite effect to that of white adipocytes in vitro regulating processes that govern the mechanisms of cell proliferation of prostate cancer cells. And finally, promoting the browning of adipose tissue in the periprostatic adipose tissue might be a way to handle prostate cancer cell progression. Video Abstract.

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

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

Imaging Brown Adipose Tissue with TSPO PET Tracers in Preclinical Animal Studies

Brown adipose tissue (BAT) is closely associated with thermogenesis and related to numerous diseases, including type 2 diabetes, nonalcoholic fatty liver disease (NAFLD), and obesity. Using molecular imaging technologies to monitor BAT could facilitate etiology elucidation, disease diagnosis, and therapeutics development. Translocator protein (TSPO), an 18 kDa protein that mainly locates on the outer mitochondrial membrane, has been proven as a promising biomarker for monitoring BAT mass. Here, we lay out the steps for imaging BAT with TSPO PET tracer [¹⁸F]-DPA in mouse studies.

A Review of Structural Features, Biological Functions and Biotransformation Studies in Adipose Tissues and an Assessment of Progress and Implications

Roles for adipose tissues in energy metabolism, health maintenance and disease onset have been established. Evidence indicates that white, brown and beige fats are quite different in terms of their cellular origin and biological characteristics. These differences are significant in targeting adipocytes to study the pathogenesis and prevention strategies of related diseases. The biotransformations of white, brown and beige fat cells constitute an intriguing topic worthy of further study, and the molecular mechanisms underlying the biotransformations of white, brown and beige fat cells remain to be elucidated. Hence, we herein collected evidence from studies on adipose tissue or adipocytes, and we extracted the structural features, biologic functions, and biotransformations of adipose tissue/adipocytes. The present review aimed to summarize the latest research progress and propose novel research directions with respect to adipose tissue and adipocytes. We posit that this work will provide new insights and opportunities in the effective treatment strategies for obesity, diabetes and other lipid-related diseases. It will also contribute to our knowledge of the basic biologic underpinnings of adipocyte biology.

Regulation of Adipose Progenitor Cell Expansion in a Novel Micro-Physiological Model of Human Adipose Tissue Mimicking Fibrotic and Pro-Inflammatory Microenvironments

The expansion of adipose progenitor cells (APCs) plays an important role in the regeneration of the adipose tissue in physiological and pathological situations. The major role of CD26-expressing APCs in the generation of adipocytes has recently been highlighted, revealing that the CD26 APC subtype displays features of multipotent stem cells, giving rise to CD54- and CD142-expressing preadipocytes. However, a relevant human in vitro model to explore the regulation of the APC subpopulation expansion in lean and obese adipose tissue microenvironments is still lacking. In this work, we describe a novel adipose tissue model, named ExAdEx, that can be obtained from cosmetic surgery wastes. ExAdEx products are adipose tissue units maintaining the characteristics and organization of adipose tissue as it presents in vivo. The model was viable and metabolically active for up to two months and could adopt a pathological-like phenotype. The results revealed that inflammatory and fibrotic microenvironments differentially regulated the expansion of the CD26 APC subpopulation and its CD54 and CD142 APC progenies. The approach used significantly improves the method of generating adipose tissue models, and ExAdEx constitutes a relevant model that could be used to identify pathways promoting the expansion of APCs in physiological and pathological microenvironments.

Browning of the white adipose tissue regulation: new insights into nutritional and metabolic relevance in health and diseases

Adipose tissues are dynamic tissues that play crucial physiological roles in maintaining health and homeostasis. Although white adipose tissue and brown adipose tissue are currently considered key endocrine organs, they differ functionally and morphologically. The existence of the beige or brite adipocytes, cells displaying intermediary characteristics between white and brown adipocytes, illustrates the plastic nature of the adipose tissue. These cells are generated through white adipose tissue browning, a process associated with augmented non-shivering thermogenesis and metabolic capacity. This process involves the upregulation of the uncoupling protein 1, a molecule that uncouples the respiratory chain from Adenosine triphosphate synthesis, producing heat. β-3 adrenergic receptor system is one important mediator of white adipose tissue browning, during cold exposure. Surprisingly, hyperthermia may also induce beige activation and white adipose tissue beiging. Physical exercising copes with increased levels of specific molecules, including Beta-Aminoisobutyric acid, irisin, and Fibroblast growth factor 21 (FGF21), which induce adipose tissue browning. FGF21 is a stress-responsive hormone that interacts with beta-klotho. The central roles played by hormones in the browning process highlight the relevance of the individual lifestyle, including circadian rhythm and diet. Circadian rhythm involves the sleep-wake cycle and is regulated by melatonin, a hormone associated with UCP1 level upregulation. In contrast to the pro-inflammatory and adipose tissue disrupting effects of the western diet, specific food items, including capsaicin and n-3 polyunsaturated fatty acids, and dietary interventions such as calorie restriction and intermittent fasting, favor white adipose tissue browning and metabolic efficiency. The intestinal microbiome has also been pictured as a key factor in regulating white tissue browning, as it modulates bile acid levels, important molecules for the thermogenic program activation. During embryogenesis, in which adipose tissue formation is affected by Bone morphogenetic proteins that regulate gene expression, the stimuli herein discussed influence an orchestra of gene expression regulators, including a plethora of transcription factors, and chromatin remodeling enzymes, and non-coding RNAs. Considering the detrimental effects of adipose tissue browning and the disparities between adipose tissue characteristics in mice and humans, further efforts will benefit a better understanding of adipose tissue plasticity biology and its applicability to managing the overwhelming burden of several chronic diseases.

Engineering Human Beige Adipose Tissue

In this study, we described a method for generating functional, beige (thermogenic) adipose microtissues from human microvascular fragments (MVFs). The MVFs were isolated from adipose tissue acquired from adults over 50 years of age. The tissues express thermogenic gene markers and reproduce functions essential for the potential therapeutic impact of beige adipose tissues such as enhanced lipid metabolism and increased mitochondrial respiration. MVFs serve as a potential single, autologous source of cells that can be isolated from adult patients, induced to recreate functional aspects of beige adipose tissue and enable rapid vascularization post-transplantation. This approach has the potential to be used as an autologous therapy for metabolic diseases or as a model for the development of a personalized approach to high-throughput drug development/screening for adipose tissue.

Transplantation of fat tissues and iPSC-derived energy expenditure adipocytes to counteract obesity-driven metabolic disorders: Current strategies and future perspectives

Several therapeutic options have been developed to address the obesity epidemic and treat associated metabolic diseases. Despite the beneficial effects of surgery and drugs, effective therapeutic solutions have been held back by the poor long-term efficiency and detrimental side effects. The development of alternative approaches is thus urgently required. Fat transplantation is common practice in many surgical procedures, including aesthetic and reconstructive surgery, and is a budding future direction for treating obesity-related metabolic defects. This review focuses on adipose tissue transplantation and the recent development of cell-based therapies to boost the mass of energy-expenditure cells. Brown adipocyte transplantation is a promising novel therapy to manage obesity and associated metabolic disorders, but the need to have an abundant and relevant source of brown fat tissue or brown adipocytes for transplantation is a major hurdle to overcome. Current studies have focused on the rodent model to obtain a proof of concept of a tissue-transplantation strategy able to achieve effective long-term effects to reverse metabolic defects in obese patients. Future perspectives and opportunities to develop innovative human fat tissue models and 3D engineered hiPSC-adipocytes are discussed.

Brown and beige adipose tissue: a novel therapeutic strategy for obesity and type 2 diabetes mellitus

Mammalian adipose tissue can be divided into two major types, namely, white adipose tissue (WAT) and brown adipose tissue (BAT). According to classical view, the main function of WAT is to store excess energy in the form of triglycerides, while BAT is a thermogenic tissue that acts a pivotal part in maintaining the core body temperature. White adipocytes display high plasticity and can transdifferentiate into beige adipocytes which have many similar morphological and functional properties with brown adipocytes under the stimulations of exercise, cold exposure and other factors. This phenomenon is also known as 'browning of WAT'. In addition to transdifferentiation, beige adipocytes can also come from de novo differentiation from tissue-resident progenitors. Activating BAT and inducing browning of WAT can accelerate the intake of glycolipids and reduce the insulin secretion requirement, which may be a new strategy to improve glycolipids metabolism and insulin resistance of obese and type 2 diabetes mellitus (T2DM) patients. This review mainly discusses the significance of brown and beige adipose tissues in the treatment of obesity and T2DM, and focuses on the effect of the browning agent on obesity and T2DM, which provides a brand-new theoretical reference for the prevention and treatment of obesity and T2DM.

CRISPR-enhanced human adipocyte browning as cell therapy for metabolic disease

Obesity and type 2 diabetes are associated with disturbances in insulin-regulated glucose and lipid fluxes and severe comorbidities including cardiovascular disease and steatohepatitis. Whole body metabolism is regulated by lipid-storing white adipocytes as well as "brown" and "brite/beige" adipocytes that express thermogenic uncoupling protein 1 (UCP1) and secrete factors favorable to metabolic health. Implantation of brown fat into obese mice improves glucose tolerance, but translation to humans has been stymied by low abundance of primary human beige adipocytes. Here we apply methods to greatly expand human adipocyte progenitors from small samples of human subcutaneous adipose tissue and then disrupt the thermogenic suppressor gene NRIP1 by CRISPR. Ribonucleoprotein consisting of Cas9 and sgRNA delivered ex vivo are fully degraded by the human cells following high efficiency NRIP1 depletion without detectable off-target editing. Implantation of such CRISPR-enhanced human or mouse brown-like adipocytes into high fat diet fed mice decreases adiposity and liver triglycerides while enhancing glucose tolerance compared to implantation with unmodified adipocytes. These findings advance a therapeutic strategy to improve metabolic homeostasis through CRISPR-based genetic enhancement of human adipocytes without exposing the recipient to immunogenic Cas9 or delivery vectors.

Intercellular and inter-organ crosstalk in browning of white adipose tissue: molecular mechanism and therapeutic complications

Adipose tissue (AT) is highly plastic and heterogeneous in response to environmental and nutritional changes. The development of heat-dissipating beige adipocytes in white AT (WAT) through a process known as browning (or beiging) has garnered much attention as a promising therapeutic strategy for obesity and its related metabolic complications. This is due to its inducibility in response to thermogenic stimulation and its association with improved metabolic health. WAT consists of adipocytes, nerves, vascular endothelial cells, various types of immune cells, adipocyte progenitor cells, and fibroblasts. These cells contribute to the formation of beige adipocytes through the release of protein factors that significantly influence browning capacity. In addition, inter-organ crosstalk is also important for beige adipocyte biogenesis. Here, we summarize recent findings on fat depot-specific differences, secretory factors participating in intercellular and inter-organ communications that regulate the recruitment of thermogenic beige adipocytes, as well as challenges in targeting beige adipocytes as a potential anti-obese therapy.

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.

Mitochondrial activity regulates the differentiation of skin-derived mesenchymal stem cells into brown adipocytes to contribute to hypertension

BACKGROUND

Brown adipocytes (BAs) are major components of brown adipose tissue (BAT), which is involved in blood pressure regulation. BAs are derived from multiple progenitors, including PDGFRα⁺ adipose-derived stem cells (ASCs). Skin-derived mesenchymal stem cells (S-MSCs) have the capacity to differentiate into adipocytes; however, their ability to differentiate into BAs remains unexplored. We aim to study the ability and regulatory mechanism of the differentiation of S-MSCs into BAs and the direct role of BAT in blood pressure regulation.

METHODS

Protein expression was measured by flow cytometry or Western blotting, and gene mRNA levels were quantified by real-time quantitative PCR (RT-PCR). To induce the differentiation of S-MSCs into BAs, S-MSCs were stimulated with a brown adipogenic cocktail comprising insulin, IBMX, dexamethasone, triiodothyronine (T3), and rosiglitazone for the indicated periods. The oxygen consumption rate (OCR) was measured with an XF24 Extracellular Flux Analyzer. Mitochondrial mass was determined by flow cytometry and fluorescence staining. Hypertension was induced in WT mice by infusion of angiotensin II (Ang II), and systolic blood pressure (SBP) was measured using a tail cuff. Interscapular brown adipose tissue (iBAT)-deficient mice were generated by surgical removal of the iBAT depot, after which the animals were allowed to recover for 6 days. Aortic, iBAT, and heart tissue sections were analyzed by hematoxylin and eosin (HE) staining.

RESULTS

We found that in vitro, S-MSCs isolated from the mouse dermis expressed the stem cell markers CD90/105 and PDGFRα and readily differentiated into BAs. Mitochondrial biogenesis and oxygen consumption were markedly increased during differentiation of S-MSCs into BAs. In vivo, iBAT was converted to white adipose tissue (WAT) in Ang II-induced hypertensive mice. We assessed the direct role of BAT in blood pressure (BP) regulation by using iBAT-deficient mice (generated by surgical removal of iBAT) and C57BL/6 (wild-type (WT)) mice and found that Ang II-induced BP elevation and vascular damage were markedly aggravated in iBAT-deficient mice compared with WT mice.

CONCLUSIONS

This study demonstrates that PDGFRα⁺ S-MSCs are able to differentiate into BAs and that this differentiation is regulated by mitochondrial activity. We also show that BAT plays a direct role in ameliorating Ang II-induced hypertension. The therapeutic potential of BAT for the prevention of hypertension-induced organ remodeling thus warrants further investigation.

Integrins and extracellular matrix proteins modulate adipocyte thermogenic capacity

Obesity and the metabolic disease epidemic has led to an increase in morbidity and mortality. A rise in adipose thermogenic capacity via activation of brown or beige fat is a potential treatment for metabolic diseases. However, an understanding of how local factors control adipocyte fate is limited. Mice with a null mutation in the laminin α4 (LAMA4) gene (KO) exhibit resistance to obesity and enhanced expression of thermogenic fat markers in white adipose tissue (WAT). In this study, changes in WAT extracellular matrix composition in the absence of LAMA4 were evaluated using liquid chromatography/tandem mass spectrometry. KO-mice showed lower levels of collagen 1A1 and 3A1, and integrins α7 (ITA7) and β1 (ITB1). ITA7-ITB1 and collagen 1A1-3A1 protein levels were lower in brown adipose tissue compared to WAT in wild-type mice. Immunohistochemical staining confirmed lower levels and different spatial distribution of ITA7 in KO-WAT. In culture studies, ITA7 and LAMA4 levels decreased following a 12-day differentiation of adipose-derived stem cells into beige fat, and knock-down of ITA7 during differentiation increased beiging. These results demonstrate that extracellular matrix interactions regulate adipocyte thermogenic capacity and that ITA7 plays a role in beige adipose formation. A better understanding of the mechanisms underlying these interactions can be used to improve systemic energy metabolism and glucose homeostasis.

Induction of Adipose Tissue Browning as a Strategy to Combat Obesity

The ongoing obesity pandemic generates a constant need to develop new therapeutic strategies to restore the energy balance. Therefore, the concept of activating brown adipose tissue (BAT) in order to increase energy expenditure has been revived. In mammals, two developmentally distinct types of brown adipocytes exist; the classical or constitutive BAT that arises during embryogenesis, and the beige adipose tissue that is recruited postnatally within white adipose tissue (WAT) in the process called browning. Research of recent years has significantly increased our understanding of the mechanisms involved in BAT activation and WAT browning. They also allowed for the identification of critical molecules and critical steps of both processes and, therefore, many new therapeutic targets. Several non-pharmacological approaches, as well as chemical compounds aiming at the induction of WAT browning and BAT activation, have been tested in vitro as well as in animal models of genetically determined and/or diet-induced obesity. The therapeutic potential of some of these strategies has also been tested in humans. In this review, we summarize present concepts regarding potential therapeutic targets in the process of BAT activation and WAT browning and available strategies aiming at them.

The application of stem cell therapy and brown adipose tissue transplantation in metabolic disorders

The discovery of brown fat in adult humans has led to increased research of the thermogenic function of this tissue in various metabolic diseases. In addition, high levels of brown fat have been correlated with lower body mass index values. Therefore, increasing brown fat mass and/or activity through methods such as the browning of white fat is considered a promising strategy to prevent and treat obesity-associated diseases. Cell-based approaches using mesenchymal stromal cells and brown adipose tissue (BAT) have been utilized to directly increase BAT mass/activity through cell and tissue implantation into animals. In addition, recent studies evaluating the transplantation of human embryonic stem cells and induced pluripotent stem (iPS) cells have shown promising results in terms of positive metabolic function. In this comprehensive review, we provide a summary of the research over the past 10 years with regard to stem cell therapy and brown fat tissue transplantation for the effective treatment of metabolic syndrome. Recent advancements in stem cell methods have allowed for the production of brown adipocytes from human iPS cells, which represent an unlimited source of cellular material with which to study adipocyte development. In addition, this process is expected to be used to further explore drug- and cell-based therapies to treat obesity-related metabolic complications.

Retinol-binding protein 2 (RBP2) binds monoacylglycerols and modulates gut endocrine signaling and body weight

Expressed in the small intestine, retinol-binding protein 2 (RBP2) facilitates dietary retinoid absorption. Rbp2-deficient (Rbp2-/- ) mice fed a chow diet exhibit by 6-7 months-of-age higher body weights, impaired glucose metabolism, and greater hepatic triglyceride levels compared to controls. These phenotypes are also observed when young Rbp2-/- mice are fed a high fat diet. Retinoids do not account for the phenotypes. Rather, RBP2 is a previously unidentified monoacylglycerol (MAG)-binding protein, interacting with the endocannabinoid 2-arachidonoylglycerol (2-AG) and other MAGs with affinities comparable to retinol. X-ray crystallographic studies show that MAGs bind in the retinol binding pocket. When challenged with an oil gavage, Rbp2-/- mice show elevated mucosal levels of 2-MAGs. This is accompanied by significantly elevated blood levels of the gut hormone GIP (glucose-dependent insulinotropic polypeptide). Thus, RBP2, in addition to facilitating dietary retinoid absorption, modulates MAG metabolism and likely signaling, playing a heretofore unknown role in systemic energy balance.

Adipose tissue browning in mice and humans

In the midst of an obesity epidemic, the promotion of brown adipose tissue (BAT) function and the browning of white adipose tissue (WAT) have emerged as promising therapeutic targets to increase energy expenditure and counteract weight gain. Despite the fact that the thermogenic potential of bone fide BAT in rodents is several orders of magnitudes higher than white fat containing brite/beige adipocytes, WAT browning represents a particularly intriguing concept in humans given the extreme amount of excess WAT in obese individuals. In addition, the clear distinction between classic brown and beige fat that has been proposed in mice does not exist in humans. In fact, studies of human BAT biopsies found controversial results suggesting both classic brown and beige characteristics. Irrespective of the true ‘color’, accumulating evidence suggests the induction of thermogenic adipocytes in human WAT depots in response to specific stimuli, highlighting that WAT browning may occur in both, mice and humans. These observations also emphasize the great plasticity of human fat depots and raise important questions about the metabolic properties of thermogenically active adipose tissue in humans and the potential therapeutic implications. We will first review the cellular and molecular aspects of selected adipose tissue browning concepts that have been identified in mouse models with emphasis on neuronal factors, the microbiome, immune cells and several hormones. We will also summarize the evidence for adipose tissue browning in humans including some experimental pharmacologic approaches.