Regulation of brown and beige fat by microRNAs

An update on the secretory functions of brown, white, and beige adipose tissue: Towards therapeutic applications

Adipose tissue, including white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue, is vital in modulating whole-body energy metabolism. While WAT primarily stores energy, BAT dissipates energy as heat for thermoregulation. Beige adipose tissue is a hybrid form of adipose tissue that shares characteristics with WAT and BAT. Dysregulation of adipose tissue metabolism is linked to various disorders, including obesity, type 2 diabetes, cardiovascular diseases, cancer, and infertility. Both brown and beige adipocytes secrete multiple molecules, such as batokines, packaged in extracellular vesicles or as soluble signaling molecules that play autocrine, paracrine, and endocrine roles. A greater understanding of the adipocyte secretome is essential for identifying novel molecular targets in treating metabolic disorders. Additionally, microRNAs show crucial roles in regulating adipose tissue differentiation and function, highlighting their potential as biomarkers for metabolic disorders. The browning of WAT has emerged as a promising therapeutic approach in treating obesity and associated metabolic disorders. Many browning agents have been identified, and nanotechnology-based drug delivery systems have been developed to enhance their efficacy. This review scrutinizes the characteristics of and differences between white, brown, and beige adipose tissues, the molecular mechanisms involved in the development of the adipocytes, the significant roles of batokines, and regulatory microRNAs active in different adipose tissues. Finally, the potential of WAT browning in treating obesity and atherosclerosis, the relationship of BAT with cancer and fertility disorders, and the crosstalk between adipose tissue with circadian system and circadian disorders are also investigated.

MicroRNAs as Epigenetic Regulators of Obesity

In obesity, the process of adipogenesis largely determines the number of adipocytes in body fat depots. Adipogenesis is regulated by several adipocyte-selective micro-ribonucleic acids (miRNAs) and transcription factors that modulate adipocyte proliferation and differentiation. However, some miRNAs block the expression of master regulators of adipogenesis. Since the specific miRNAs display different expressions during adipogenesis, in mature adipocytes and permanent obesity, their use as biomarkers or therapeutic targets is feasible. Upregulated miRNAs in persistent obesity are downregulated during adipogenesis. Moreover, some of the downregulated miRNAs in obese individuals are upregulated in mature adipocytes. Induction of adipocyte stress and hypertrophy leads to the release of adipocyte-derived exosomes (AdEXs) that contain the cargo molecules, miRNAs. miRNAs are important messengers for intercellular communication involved in metabolic responses and have very specific signatures that direct the metabolic activity of target cells. While each miRNA targets multiple messenger RNAs (mRNAs), which may coordinate or antagonize each other's functions, several miRNAs are dysregulated in other tissues during obesity-related comorbidities. Deletion of the miRNA-processing enzyme DICER in pro-opiomelanocortin-expressing cells results in obesity, which is characterized by hyperphagia, increased adiposity, hyperleptinemia, defective glucose metabolism, and alterations in the pituitary-adrenal axis. In recent years, RNA-based therapeutical approaches have entered clinical trials as novel therapies against overweight and its complications. Development of lipid droplets, macrophage accumulation, macrophage polarization, tumor necrosis factor receptor-associated factor 6 activity, lipolysis, lipotoxicity, and insulin resistance are effectively controlled by miRNAs. Thereby, miRNAs as epigenetic regulators are used to determine the new gene transcripts and therapeutic targets.

Transcriptome analysis reveals FABP5 as a key player in the development of chicken abdominal fat, regulated by miR-122-5p targeting

BACKGROUND

The development of abdominal fat and meat quality are closely related and can impact economic efficiency. In this study, we conducted transcriptome sequencing of the abdominal fat tissue of Gushi chickens at 6, 14, 22, and 30 weeks, and selected key miRNA-mRNA regulatory networks related to abdominal fat development through correlation analysis.

RESULTS

A total of 1893 differentially expressed genes were identified. Time series analysis indicated that at around 6 weeks, the development of chicken abdominal fat was extensively regulated by the TGF-β signaling pathway, Wnt signaling pathway, and PPAR signaling pathway. However, at 30 weeks of age, the apoptosis signaling pathway was the most significant, and correlation analysis revealed several genes highly correlated with abdominal fat development, including Fatty Acid Binding Protein 5 (FABP5). Based on miRNA transcriptome data, it was discovered that miR-122-5p is a potential target miRNA for FABP5. Cell experiments showed that miR-122-5p can directly target FABP5 to promote the differentiation of preadipocytes.

CONCLUSION

The present study confirms that the key gene FABP5 and its target gene miR-122-5p are critical regulatory factors in the development of chicken abdominal fat. These results provide new insights into the molecular regulatory mechanisms associated with the development of abdomen-al fat in chickens.

H19X-encoded microRNAs induced by IL-4 in adipocyte precursors regulate proliferation to facilitate differentiation

Adipose tissue, an organ critical for systemic energy homeostasis, is influenced by type 2 immunity in its development and function. The type 2 cytokine interleukin (IL)-4 induces the proliferation of bipotential adipocyte precursors (APs) in white fat tissue and primes these cells for differentiation into beige adipocytes, which are specialized for thermogenesis. However, the underlying mechanisms have not yet been comprehensively examined. Here, we identified six microRNA (miRNA) genes upregulated upon IL-4 stimulation in APs, miR-322, miR-503, miR-351, miR-542, miR-450a, and miR-450b; these are encoded in the H19X locus of the genome. Their expression is positively regulated by the transcription factor Klf4, whose expression also increases upon IL-4 stimulation. These miRNAs shared a large set of target genes, of which 381 genes were downregulated in mRNA expression upon IL-4 stimulation and enriched in Wnt signaling pathways. Two genes with downregulated expression, Ccnd1 and Fzd6, were repressed by H19X-encoded miRNAs. Additionally, the Wnt signaling activator LiCl downregulated the expression of this group of miRNAs in APs, indicating that Wnt signaling-related genes and these miRNAs form a double-negative feedback regulatory loop. This miRNA/Wnt feedback regulation modulated the elevated proliferation of APs induced by IL-4 stimulation and contributed to priming them for beige adipocyte differentiation. Moreover, the aberrant expression of these miRNAs attenuates the differentiation of APs into beige adipocytes. Collectively, our results suggest that H19X-encoded miRNAs facilitate the transition of APs from proliferation to differentiation in the IL-4-mediated regulation.

Analysis of differential metabolites and metabolic pathways in adipose tissue of tree shrews (Tupaia belangeri) under gradient cooling acclimation

In order to investigate the influence of gradient cooling acclimation on body mass regulation in tree shrews (Tupaia belangeri), white adipose tissue (WAT) and brown adipose tissue (BAT) in T. belangeri between the control group and gradient cooling acclimation group on day 56 were collected, body mass, food intake, thermogenic capacity, differential metabolites, and related metabolic pathways in WAT and BAT were measured, the changes of differential metabolites were analyzed by non-targeted metabolomics method based on liquid chromatography-mass spectrometry. The results shown that gradient cooling acclimation significantly increased body mass, food intake, resting metabolic rate (RMR), non-shivering thermogenesis (NST), and masses of WAT and BAT. 23 significant differential metabolites in WAT between the gradient cooling acclimation group and the control group, of which the relative contents of 13 differential metabolites were up-regulated and 10 differential metabolites were down-regulated. 27 significant differential metabolites in BAT, of which 18 differential metabolites decreased and 9 differential metabolites increased. 15 differential metabolic pathways in WAT, 8 differential metabolic pathways in BAT, and 4 differential metabolic pathways involved in both WAT and BAT, including Purine metabolism, Pyrimidine metabolism, Glycerol phosphate metabolism, Arginine and proline metabolism, respectively. All of the above results suggested that T. belangeri could use different metabolites of adipose tissue to withstand low temperature environments and enhance their survival.

Adipose Rheb deficiency promotes miR-182-5p expression via the cAMP/PPARγ signaling pathway

Dysregulation of microRNAs (miRNAs) in adipocytes plays a critical role in the pathogenesis of obesity. However, the signaling mechanisms regulating miRNAs production in adipose tissue remain largely unclear. Here, we show that adipose tissue-specific knockout of Ras homolog enriched in brain (Rheb), a direct upstream activator of mTOR, increases miR-182-5p level in mouse subcutaneous white adipose tissues. Interestingly, the inhibition of mTOR signaling by rapamycin has no effect on miR-182-5p level in primary subcutaneous white adipocytes, suggesting the presence of a mTOR-independent mechanism regulating Rheb-mediated miR-182-5p expression. Consistent with this view, Rheb-ablation activates the cAMP/PPARγ signaling pathway. In addition, treatment of white adipocytes with pioglitazone, a PPARγ agonist, dramatically upregulates miR-182-5p levels. Our study reveals a unique mechanism by which Rheb regulates miR-182-5p in adipocytes. Given that increasing miR-182-5p in adipose tissue promotes beige fat development, our study also suggests a unique mechanism by which Rheb promotes thermogenesis and energy expenditure.

miR-1275 Inhibits Human Omental Adipose-Derived Stem Cells Differentiation Toward the Beige Phenotype via PRDM16

Beige adipocytes have recently attracted attention for their potential as new therapeutic targets in the management of obesity and related metabolic disorders. MicroRNAs (miRNAs) have been reported as transcriptional regulators or biomarkers of brown and beige adipogenesis. Nevertheless, the effects of miRNAs involved in beige differentiation of human visceral adipocytes remain to be investigated. In this study, microarray screening showed that miR-1275 was significantly decreased during the differentiation of beige adipocytes induced by human omental adipose-derived stem cells (hASCs). Overexpression of miR-1275 suppressed the "brown-like" differentiation of hASCs by inhibiting the key transcriptional factor PR domain containing 16 (PRDM16) without affecting the proliferation. Adipogenesis and mitochondrial biogenesis of beige adipocytes derived from hASCs were impaired by miR-1275 overexpression. The regulatory effect of miR-1275 was determined by direct binding to the 3'-untranslated region of PRDM16, which was demonstrated by a dual-luciferase assay. Taken together, this study identified miR-1275 as a negative regulator of beige cell development in hASCs by inhibiting PRDM16. Thus, miR-1275 might be a potential target in the management of visceral obesity and related metabolic diseases.

Integrated analysis of microRNAs, circular RNAs, long non-coding RNAs, and mRNAs revealed competing endogenous RNA networks involved in brown adipose tissue whitening in rabbits

BACKGROUND

The brown adipose tissue (BAT) is a target for treating obesity. BAT losses thermogenic capacity and gains a "white adipose tissue-like" phenotype ("BAT whitening") under thermoneutral environments, which is a potential factor causing a low curative effect in BAT-related obesity treatments. Circular RNAs (circRNAs) and long non-coding RNAs (lncRNAs) can act as competing endogenous RNAs (ceRNA) to mRNAs and function in various processes by sponging shared microRNAs (miRNAs). However, the roles of circRNA- and lncRNA-related ceRNA networks in regulating BAT whitening remain litter known.

RESULTS

In this study, BATs were collected from rabbits at day0 (D0), D15, D85, and 2 years (Y2). MiRNA-seq was performed to investigate miRNA changes during BAT whitening. Then, a combined analysis of circRNA-seq and whole-transcriptome sequencing was used for circRNA assembly and quantification during BAT whitening. Our data showed that 1187 miRNAs and 6204 circRNAs were expressed in the samples, and many of which were identified as significantly changed during BAT whitening. Target prediction showed that D0-selective miRNAs were significantly enriched in the Ras, MAPK, and PI3K-Akt signaling pathways, and Y2-selective miRNAs were predicted to be involved in cell proliferation. The cyclization of several circRNAs could form novel response elements of key thermogenesis miRNAs at the back-splicing junction (BSJ) sites, and in combination with a dual-luciferase reporter assay confirmed the binding between the BSJ site of novel_circ_0013792 and ocu-miR-378-5p. CircRNAs and lncRNAs have high cooperativity in sponging miRNAs during BAT whitening. Both circRNA-miRNA-mRNA and lncRNA-miRNA-mRNA triple networks were significantly involved in immune response-associated biological processes. The D15-selective circRNA might promote BAT whitening by increasing the expression of IDH2. The Y2-selective circRNA-related ceRNA network and lncRNA-related ceRNA network might regulate the formation of the WAT-like phenotype of BAT via MAPK and Ras signaling pathways, respectively.

CONCLUSIONS

Our work systematically revealed ceRNA networks during BAT whitening in rabbits and might provide new insight into BAT-based obesity treatments.

LPS-Induced Inhibition of miR-143 Expression in Brown Adipocytes Promotes Thermogenesis and Fever

Fever is an important part of inflammatory response to infection. Although brown adipose tissue (BAT) thermogenesis is known to be potently influenced by systemic inflammation, the role of BAT during infection-induced fever remains largely unknown. Here, we injected mice with a low dose of LPS and found that low-dose LPS can directly induce thermogenesis of brown adipocytes. It is known that miR-143 is highly expressed in the BAT, and miR-143 knockout mice exhibited stronger thermogenesis under cold exposure. Interestingly, miR-143 was negatively correlated with an LPS-induced increase of TNFα and IL-6 mRNA levels, and the IL-6 pathway may mediate the inhibition of miR-143 expression. Moreover, miR-143 is down-regulated by LPS, and overexpression of miR-143 in brown adipocytes by lentivirus could rescue the enhancement of UCP1 protein expression caused by LPS, hinting miR-143 may be an important regulator of the thermogenesis in brown adipocytes. More importantly, the knockout of miR-143 further enhanced the LPS-induced increase of body temperature and BAT thermogenesis, and this result was further confirmed by in vitro experiments by using primary brown adipocytes. Mechanistically, adenylate cyclase 9 (AC9) is a new target gene of miR-143 and LPS increases BAT thermogenesis by a way of inhibiting miR-143 expression, a negative regulator for AC9. Our study considerably improves our collective understanding of the important function of miR-143 in inflammatory BAT thermogenesis.

Integrated Analysis of Transcriptome, microRNAs, and Chromatin Accessibility Revealed Potential Early B-Cell Factor1-Regulated Transcriptional Networks during the Early Development of Fetal Brown Adipose Tissues in Rabbits

In domestic mammals, cold stress decreases the survival rate of newborns and increases the cost of management. Brown adipose tissue (BAT) is the main thermogenic organ for cubs, and well-developed fetal BAT (FBAT) is beneficial for newborns to maintain core temperatures during the first several days of life. However, our knowledge of the epigenetic mechanisms during the early development of FBAT remains largely unknown. Rabbits (Oryctolagus cuniculus) are economically important domestic animals. In this study, a histological analysis shows that the tissue content, thermogenic capacity, and lipid content of FBAT dramatically increases from gestational day 21 (G21) to gestational day 24 (G24) in rabbits. RNA-seq, microRNA-seq (miRNA-seq), and the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) show that many genes, miRNAs, and chromatin-accessible regions (referred to as peaks) were identified as significantly changed from G21 to G24, respectively. The upregulated genes from G21 to G24 were significantly enriched in the mitochondrial metabolism and thermogenesis-related signal pathways. The integrated analysis of transcriptome and chromatin accessibility reveals that the peaks in the promoters have a more regulatory effect than peaks in other genomic elements on the expression of their nearby genes in FBATs. The upregulated genes that are associated with increased chromatin accessibility in the promoter regions are involved in the energy metabolism-related signaling pathways. The genes that have a greater tendency to be regulated by miRNAs than the chromatin accessibility in gene promoters are involved in the apelin, insulin, and endocytosis signaling pathways. Furthermore, genome-wide transcription factor (TF) footprinting analysis identifies early B-cell factor1 (EBF1) as playing a key role during early FBAT development. The carbon metabolism, citrate cycle, and PPAR signaling pathways are significantly enriched by the predicted EBF1-regulated cascade TF-network. In conclusion, our work provides a framework for understanding epigenetics regulatory mechanisms underlying the early development of FBAT and identifies potential TF involved in the early development of FBAT in rabbits.

A natural anti-obesity reagent derived from sea buckthorn polysaccharides: Structure characterization and anti-obesity evaluation in vivo

Sea buckthorn polysaccharide (SBP) has received increasing attention for its various bioactive functions. In this study, a novel polysaccharide SBP-1 was initially separated from crude SBP and further purified to obtain its main fraction SBP-1-A with a Mw of 9944 Da, consisting of Rha, Ara, Gal, Glc, and GalA. The structure of SBP-1-A was characterized based on FT-IR, GC-MS, and 1D/2D NMR, and its backbone was composed of a repeated unit of → 3,4)-β-l-Rhap-(1 → 4)-α-d-GalAp-(1 → 4)-α-d-GalAp-(1 → with branches at C-4 position comprised of α-l-Araf, β-d-Galp, β-d-Glcp, α-d-Glcp. Besides, the anti-obesity effects of SBP-1 on high-fat diet mice were evaluated, indicating it could restrain the body weight gain and lipids accumulation by promoting the expression of PGC1α, UCP-1, and PRDM16 to activate the brown adipocyte and improve the thermogenesis. In summary, the results offered new supports for the structural information of SBP and its feasibility to be used as a natural anti-obesity reagent.

Inhibition of Mitochondrial Fission by Drp-1 Blockade by Short-Term Leptin and Mdivi-1 Treatment Improves White Adipose Tissue Abnormalities in Obesity and Diabetes

BACKGROUND

Obesity and type 2 diabetes are chronic diseases characterized by insulin resistance, mitochondrial dysfunction and morphological abnormalities.

OBJECTIVE

We have investigated if dysregulation of mitochondrial dynamics and biogenesis is involved in an animal model of obesity and diabetes.

METHODS

The effect of short-term leptin and mdivi-1 -a selective inhibitor of Drp-1 fission-protein- treatment on mitochondrial dynamics and biogenesis was evaluated in epididymal white adipose tissue (WAT) from male ob/ob mice.

RESULTS

An increase in Drp-1 protein levels and a decrease in Mfn2 and OPA-1 protein expression were observed with enhanced and sustained mitochondrial fragmentation in ob/ob mice compared to wt C57BL/6 animals (p<0.05). The content of mitochondrial DNA and PGC-1α mRNA expression -both parameters of mitochondrial biogenesis- were reduced in ob/ob mice (p<0.05). Treatment with leptin and mdivi-1 significantly increased mitochondrial biogenesis, improved fusion-to-fission balance and attenuated mitochondrial dysfunction, thus inducing white-to-beige adipocyte transdifferentiation. Measurements of glucose and lipid oxidation in adipocytes revealed that both leptin and mdivi-1 increase substrates oxidation while in vivo determination of blood glucose concentration showed decreased levels by 50% in ob/ob mice, almost to the wt level.

CONCLUSIONS

Pharmacological targeting of Drp-1 fission protein may be a potential novel therapeutic tool for obesity and type 2 diabetes.

Astragalus polysaccharide regulates brown adipocytes differentiation by miR-6911 targeting Prdm16

Brown adipose tissue (BAT) is a specialized tissue in mammals related to thermogenesis. The Astragalus polysaccharide (APS) is the major natural active component of Astragalus membranaceus, which has been recognized as one of the most popular herbal medicines worldwide. The role and possible mechanisms of APS on brown adipocytes differentiation is not well defined. Here, we explored the effect of APS on the differentiation of brown adipocytes in C3H10T 1/2 cells. The results showed that APS promoted the differentiation of brown adipocytes and improved insulin sensitivity along with significant increases in the expression of brown adipogenic marker proteins (C/EBPα, C/EBPβ, and PPARγ), thermogenesis marker proteins (UCP1, PRDM16, and PGC-1α), and insulin sensitivity marker protein (GLUT4). Meanwhile, the results showed that the amount of the phosphorylation of insulin receptor substrate 1 (p-IRS1) and phospho-AKT (p-AKT) which are critical factors in the insulin signaling pathway was increased without changing the total amount of IRS and AKT. Furthermore, the results of RNA-seq showed that APS altered the expression profiles of various miRNAs, and among which the expression of miR-6911 as a universal regulatory factor was significantly decreased. Importantly, we found that miR-6911 regulated the differentiation of brown adipocytes by targeting PR domain-containing 16 (Prdm16). In addition, after transfection of miR-6911 mimics, compared with the control and inhibitor group, PRDM16 protein expression significantly decreased, which was accompanied by the decrease of PPARγ, UCP1, and PGC-1α. Collectively, our results indicated that APS regulated brown adipocytes differentiation in C3H10T 1/2 cells via miRNA-6911 targeting Prdm16.

Human Brown Adipose Tissue and Metabolic Health: Potential for Therapeutic Avenues

Obesity-associated metabolic abnormalities comprise a cluster of conditions including dyslipidemia, insulin resistance, diabetes and cardiovascular diseases that has affected more than 650 million people all over the globe. Obesity results from the accumulation of white adipose tissues mainly due to the chronic imbalance of energy intake and energy expenditure. A variety of approaches to treat or prevent obesity, including lifestyle interventions, surgical weight loss procedures and pharmacological approaches to reduce energy intake and increase energy expenditure have failed to substantially decrease the prevalence of obesity. Brown adipose tissue (BAT), the primary source of thermogenesis in infants and small mammals may represent a promising therapeutic target to treat obesity by promoting energy expenditure through non-shivering thermogenesis mediated by mitochondrial uncoupling protein 1 (UCP1). Since the confirmation of functional BAT in adult humans by several groups, approximately a decade ago, and its association with a favorable metabolic phenotype, intense interest on the significance of BAT in adult human physiology and metabolic health has emerged within the scientific community to explore its therapeutic potential for the treatment of obesity and metabolic diseases. A substantially decreased BAT activity in individuals with obesity indicates a role for BAT in the setting of human obesity. On the other hand, BAT mass and its prevalence correlate with lower body mass index (BMI), decreased age and lower glucose levels, leading to a lower incidence of cardio-metabolic diseases. The increased cold exposure in adult humans with undetectable BAT was associated with decreased body fat mass and increased insulin sensitivity. A deeper understanding of the role of BAT in human metabolic health and its interrelationship with body fat distribution and deciphering proper strategies to increase energy expenditure, by either increasing functional BAT mass or inducing white adipose browning, holds the promise for possible therapeutic avenues for the treatment of obesity and associated metabolic disorders.

Geniposide suppresses thermogenesis via regulating PKA catalytic subunit in adipocytes

Geniposide has been widely found to ameliorate many metabolic diseases. The recruitment and activation of brown/beige adipocytes are effective and promising methods for counteracting obesity and related diseases. However, the effect of geniposide on thermogenesis of adipocytes and its underlying mechanism have not yet been investigated. Here, we demonstrate that geniposide (25 mg/kg) reduces body temperature and cold tolerance of mice via suppressing thermogenic genes in interscapular brown adipose tissue (iBAT) and inguinal white adipose tissue (iWAT). Consistently, geniposide (20 mg/mL) suppresses thermogenic capacity of adipocytes (brown adipocytes and 3T3L1 preadipocyte cells) in vitro. Mechanistically, geniposide reduces the level of protein kinase A (PKA) catalytic subunit and further suppresses transcription activity and protein stability of uncoupling protein 1 (UCP1), leading to reduction of thermogenic capacity in adipocytes. Moreover, pharmacological PKA activation reverses geniposide-induced UCP1 inhibition, which indicated that geniposide suppresses thermogenesis of adipocytes via regulating PKA signaling. Together, our findings suggest that geniposide is an inhibitor of fat thermogenesis, establishing a novel function characteristic of geniposide.

Biochemical and immunological changes in obesity

Obesity is a syndemia that promotes high expenditures for public health, and is defined by the excess of adipose tissue that is classified according to its function and anatomical distribution. In obese people, this tissue generates oxidative stress associated with a chronic inflammatory response, in which there is an imbalance in relation to the release of hormones and adipokines that cause loss of body homeostasis and predisposition to the development of some comorbidities. The purpose of this review is to summarize the main events that occur during the onset and progression of obesity with a special focus on biochemical and immunological changes. Hypertrophied and hyperplasia adipocytes have biomarkers and release adipokines capable of regulating pathways and expressing genes that culminate in the development of metabolic changes, such as changes in energy balance and intestinal microbiota, and the development of some comorbidities, diabetes mellitus, dyslipidemias, arterial hypertension, liver disease, cancer, allergies, osteoporosis, sarcopenia and obstructive sleep apnea. Thus, it is necessary to treat and/or prevent pathology, using traditional methods based on healthy eating, and regular physical and leisure activities.

Challenges in tackling energy expenditure as obesity therapy: From preclinical models to clinical application

BACKGROUND

A chronic imbalance of energy intake and energy expenditure results in excess fat storage. The obesity often caused by this overweight is detrimental to the health of millions of people. Understanding both sides of the energy balance equation and their counter-regulatory mechanisms is critical to the development of effective therapies to treat this epidemic.

SCOPE OF REVIEW

Behaviors surrounding ingestion have been reviewed extensively. This review focuses more specifically on energy expenditure regarding bodyweight control, with a particular emphasis on the organs and attractive metabolic processes known to reduce bodyweight. Moreover, previous and current attempts at anti-obesity strategies focusing on energy expenditure are highlighted. Precise measurements of energy expenditure, which consist of cellular, animal, and human models, as well as measurements of their translatability, are required to provide the most effective therapies.

MAJOR CONCLUSIONS

A precise understanding of the components surrounding energy expenditure, including tailored approaches based on genetic, biomarker, or physical characteristics, must be integrated into future anti-obesity treatments. Further comprehensive investigations are required to define suitable treatments, especially because the complex nature of the human perspective remains poorly understood.

Regulatory microRNAs in Brown, Brite and White Adipose Tissue

MicroRNAs (miRNAs) constitute a class of short noncoding RNAs which regulate gene expression by targeting messenger RNA, inducing translational repression and messenger RNA degradation. This regulation of gene expression by miRNAs in adipose tissue (AT) can impact on the regulation of metabolism and energy homeostasis, particularly considering the different types of adipocytes which exist in mammals, i.e., white adipocytes (white AT; WAT), brown adipocytes (brown AT; BAT), and inducible brown adipocytes in WAT (beige or brite or brown-in-white adipocytes). Indeed, an increasing number of miRNAs has been identified to regulate key signaling pathways of adipogenesis in BAT, brite AT, and WAT by acting on transcription factors that promote or inhibit adipocyte differentiation. For example, MiR-328, MiR-378, MiR-30b/c, MiR-455, MiR-32, and MiR-193b-365 activate brown adipogenesis, whereas MiR-34a, MiR-133, MiR-155, and MiR-27b are brown adipogenesis inhibitors. Given that WAT mainly stores energy as lipids, whilst BAT mainly dissipates energy as heat, clarifying the effects of miRNAs in different types of AT has recently attracted significant research interest, aiming to also develop novel miRNA-based therapies against obesity, diabetes, and other obesity-related diseases. Therefore, this review presents an up-to-date comprehensive overview of the role of key regulatory miRNAs in BAT, brite AT, and WAT.

Research and prospect of peptides for use in obesity treatment (Review)

Obesity and its related diseases, such as type 2 diabetes, hypertension and cardiovascular disease, are steadily increasing worldwide. Over the past few decades, numerous studies have focused on the differentiation and function of brown and beige fat, providing evidence for their therapeutic potential in treating obesity. However, no specific novel drug has been developed to treat obesity in this way. Peptides are a class of chemically active substances, which are linked together by amino acids using peptide bonds. They have specific physiological activities, including browning of white fat. As signal molecules regulated by the neuroendocrine system, the role of polypeptides, such as neuropeptide Y, brain-gut peptide and glucagon-like peptide in obesity and its related complications has been revealed. Notably, with the rapid development of peptidomics, peptide drugs have been widely used in the prevention and treatment of metabolic diseases, due to their short half-life, small apparent distribution volume, low toxicity and low side effects. The present review summarizes the progress and the new trend of peptide research, which may provide novel targets for the prevention and treatment of obesity.

Canine colostrum exosomes: characterization and influence on the canine mesenchymal stem cell secretory profile and fibroblast anti-oxidative capacity

Background

Canine colostrum milk (CCM) is a specific secretion of the mammary gland that is fundamental for the survival of the newborn. CCM has many described components (immunoglobulins, proteins or fat), but its small vesicles, named exosomes, are largely unknown.

Results

A characterization of CCM exosomes was performed. Exosomes were abundant in CCM and appeared with the characteristic cup-shaped morphology and well-defined round vesicles. The size distribution of exosomes was between 37 and 140 nm, and western blot analysis showed positive expression of specific exosomal markers. Proteomic analysis revealed a total of 826 proteins in exosome cargo. We also found that exosomes modified the proliferation and secretory profiles in canine mesenchymal stem cells derived from bone marrow (cBM-MSCs) and adipose tissue (cAd-MSCs). Additionally, CCM exosomes demonstrated a potent antioxidant effect on canine fibroblasts in culture.

Conclusions

Our findings highlight, for the first time, the abundant presence of exosomes in CCM and their ability to interact with mesenchymal stem cells (MSCs). The addition of exosomes to two types of MSCs in culture resulted in specific secretory profiles with functions related to angiogenesis, migration and chemotaxis of immune cells. In particular, the cAd-MSCs secretory profile showed higher potential in adipose tissue development and neurogenesis, while cBM-MSC production was associated with immunity, cell mobilization and haematopoiesis. Finally, exosomes also presented antioxidant capacity on fibroblasts against reactive oxygen species activity within the cell, demonstrating their fundamental role in the development and maturation of dogs in the early stages of their life.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s12917-020-02623-w.

Glycogen Dynamics Drives Lipid Droplet Biogenesis during Brown Adipocyte Differentiation

Browning induction or transplantation of brown adipose tissue (BAT) or brown/beige adipocytes derived from progenitor or induced pluripotent stem cells (iPSCs) can represent a powerful strategy to treat metabolic diseases. However, our poor understanding of the mechanisms that govern the differentiation and activation of brown adipocytes limits the development of such therapy. Various genetic factors controlling the differentiation of brown adipocytes have been identified, although most studies have been performed using in vitro cultured pre-adipocytes. We investigate here the differentiation of brown adipocytes from adipose progenitors in the mouse embryo. We demonstrate that the formation of multiple lipid droplets (LDs) is initiated within clusters of glycogen, which is degraded through glycophagy to provide the metabolic substrates essential for de novo lipogenesis and LD formation. Therefore, this study uncovers the role of glycogen in the generation of LDs.

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Brown adipocytes are functionally differentiated at E17.5 in the mouse embryo

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Lipid droplets are formed within glycogen clusters

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Glycogen production is crucial for lipid droplet biogenesis during BAT differentiation

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Glycophagy-mediated glycogen degradation drives lipid droplet formation

Elucidating the Regulatory Role of Melatonin in Brown, White, and Beige Adipocytes

The high prevalence of obesity and its associated metabolic diseases has heightened the importance of understanding control of adipose tissue development and energy metabolism. In mammals, 3 types of adipocytes with different characteristics and origins have been identified: white, brown, and beige. Beige and brown adipocytes contain numerous mitochondria and have the capability to burn energy and counteract obesity, while white adipocytes store energy and are closely associated with metabolic disorders and obesity. Thus, regulation of the development and function of different adipocytes is important for controlling energy balance and combating obesity and related metabolic disorders. Melatonin is a neurohormone, which plays multiple roles in regulating inflammation, blood pressure, insulin actions, and energy metabolism. This article summarizes and discusses the role of melatonin in white, beige, and brown adipocytes, especially in affecting adipogenesis, inducing beige formation or white adipose tissue browning, enhancing brown adipose tissue mass and activities, improving anti-inflammatory and antioxidative effects, regulating adipokine secretion, and preventing body weight gain. Based on the current findings, melatonin is a potential therapeutic agent to control energy metabolism, adipogenesis, fat deposition, adiposity, and related metabolic diseases.

PVAT: an important guardian of the cardiovascular system

Perivascular adipose tissue (PVAT) had long been considered to serve only structural, vessel-supporting purposes, but today PVAT is recognized to be an endocrine organ with important physiological and pathological effects. The expansion of PVAT in vascular homeostasis and vascular disease has attracted much interest. PVAT has been shown to release a wide spectrum of molecules, such as PVAT-derived relaxing factors (PVATRFs) and PVAT-derived contracting factors (PVATCFs). PVAT dysfunction may lead to obesity, atherosclerosis, and other cardiovascular diseases. This review describes recent advances in our understanding of PVAT's important effects on the cardiovascular system.

Comprehensive Analysis of the Characteristics and Differences in Adult and Newborn Brown Adipose Tissue (BAT): Newborn BAT Is a More Active/Dynamic BAT

Brown adipose tissue (BAT) plays an essential role in maintaining body temperature and in treating obesity and diabetes. The adult BAT (aBAT) and neonatal BAT (neBAT) vary greatly in capacity, but the characteristics and differences between them on the molecular level, as well as the related features of BAT as it develops post-delivery, have not yet been fully determined. In this study, we examined the morphological features of aBAT and neBAT of mice by using hematoxylin-eosin (H&E) staining, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). We found that neBAT contains a smaller number and size of lipid droplets, as well as more abundant mitochondria, compared with aBAT. The dynamic morphological changes revealed that the number and size of lipid droplets increase, but the number of mitochondria gradually decrease during the post-delivery development, which consisted of some differences in RNA or protein expression levels, such as gradually decreased uncoupling protein 1 (UCP1) expression levels and mitochondrial genes, such as mitochondrial transcription factor A (Tfam). The adipocyte differentiation-related genes, such as transcription factor CCAAT enhancer-binding protein β (CEBPβ), were also continuously upregulated. Additionally, the different features of aBAT and neBAT were analyzed from the global transcription (RNA-Seq) level, which included messenger RNA (mRNA), microRNA, long non-coding RNA (lncRNA), circRNA, and DNA methylation, as well as proteins (proteomics). Differentially methylated region (DMR) analysis identified 383 hyper- and 503 hypo-methylated genes, as well as 1221 new circRNA in ne-BAT and 1991 new circRNA in a-BAT, with significantly higher expression of circRNA in aBAT compared with neBAT. Gene ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that mitochondrial activity, protein synthesis, and cell life activity levels were higher in neBAT, and pathways related to ribosomes, spliceosomes, and metabolism were significantly activated in neBAT compared to aBAT. Collectively, this study describes the dynamic changes occurring throughout post-delivery development from the morphological, molecular and omics perspectives. Our study provides information that may be utilized in improving BAT functional activity through gene regulation and/or epigenetic regulation.

Beige Fat Maintenance; Toward a Sustained Metabolic Health

Understanding the mammalian energy balance can pave the way for future therapeutics that enhance energy expenditure as an anti-obesity and anti-diabetic strategy. Several studies showed that brown adipose tissue activity increases daily energy expenditure. However, the size and activity of brown adipose tissue is reduced in individuals with obesity and type two diabetes. Humans have an abundance of functionally similar beige adipocytes that have the potential to contribute to increased energy expenditure. This makes beige adipocytes a promising target for metabolic disease therapies. While brown adipocytes tend to be stable, beige adipocytes have a high level of plasticity that allows for the rapid and dynamic induction of thermogenesis by external stimuli such as low environmental temperatures. This means that after browning stimuli have been withdrawn beige adipocytes quickly transition back to their white adipose state. The detailed molecular mechanisms regulating beige adipocytes development, function, and reversibility are not fully understood. The goal of this review is to give a comprehensive overview of beige fat development and origins, along with the transcriptional and epigenetic programs that lead to beige fat formation, and subsequent thermogenesis in humans. An improved understanding of the molecular pathways of beige adipocyte plasticity will enable us to selectively manipulate beige cells to induce and maintain their thermogenic output thus improving the whole-body energy homeostasis.

Transcriptome analysis of miRNA and mRNA in the livers of pigs with highly diverged backfat thickness

Fat deposition is very important in pig production, and its mechanism is not clearly understood. MicroRNAs (miRNAs) play critical roles in fat deposition and energy metabolism. In the current study, we investigated the mRNA and miRNA transcriptome in the livers of Landrace pigs with extreme backfat thickness to explore miRNA-mRNA regulatory networks related to lipid deposition and metabolism. A comparative analysis of liver mRNA and miRNA transcriptomes from pigs (four pigs per group) with extreme backfat thickness was performed. We identified differentially expressed genes from RNA-seq data using a Cufflinks pipeline. Seventy-one differentially expressed genes (DEGs), including twenty-eight well annotated on the porcine reference genome genes, were found. The upregulation genes in pigs with higher backfat thickness were mainly involved in fatty acid synthesis, and included fatty acid synthase (FASN), glucokinase (GCK), phosphoglycerate dehydrogenase (PHGDH), and apolipoprotein A4 (APOA4). Cytochrome P450, family 2, subfamily J, polypeptide 34 (CYP2J34) was lower expressed in pigs with high backfat thickness, and is involved in the oxidation of arachidonic acid. Moreover, 13 differentially expressed miRNAs were identified. Seven miRNAs were associated with fatty acid synthesis, lipid metabolism, and adipogenic differentiation. Based on comprehensive analysis of the transcriptome of both mRNAs and miRNAs, an important regulatory network, in which six DEGs could be regulated by differentially expressed miRNAs, was established for fat deposition. The negative correlate in the regulatory network including, miR-545-5p and GRAMD3, miR-338 and FASN, and miR-127, miR-146b, miR-34c, miR-144 and THBS1 indicate that direct suppressive regulation may be involved in lipid deposition and energy metabolism. Based on liver mRNA and miRNA transcriptomes from pigs with extreme backfat thickness, we identified 28 differentially expressed genes and 13 differentially expressed miRNAs, and established an important miRNA-mRNA regulatory network. This study provides new insights into the molecular mechanisms that determine fat deposition in pigs.

cAMP-MicroRNA-203-IFNγ network regulates subcutaneous white fat browning and glucose tolerance

OBJECTIVE

Brown and beige adipocytes in humans and rodents are specialized to burn lipids for heat generation as a natural defense against cold and obesity, which is advantageous to metabolic homeostasis. MicroRNAs as another regulatory layer to regulate metabolic homeostasis attracted a lot of attentions. Our previous work revealed microRNA (miR)-203 as a brown adipocyte-enriched microRNA involved in brown adipocytes development. However, the potential role of miR-203 in adipose tissue metabolic homeostasis has not been determined in vivo. In this study, we investigate the potential role of miR-203 in subcutaneous white adipose tissue (sub-WAT) browning and metabolic homeostasis.

METHODS

We investigated the relationship between miR-203 and energy homeostasis in adipose tissue from cold exposed, high fat diet (HFD) fed, ob/ob and db/db mice. The functions of miR-203 on sub-WAT browning were validated through miR-203 knockdown or overexpression. The miR-203 targeted signal pathway was screened by RNAseq analysis. Luciferase report assay, western blot, and qPCR were performed to establish the miR-203 related upstream and downstream signal pathway in vivo and in vitro. The functions of miR-203 on obesity and metabolic homeostasis were validated through GTT/ITT and western blot on high fat diet-induced obesity in C57 mice. ELISA was used to determine the concentration of IFN-γ. Flow cytometry analysis was performed to determine the infiltration of macrophages in adipose tissue.

RESULTS

MiR-203 expression positively correlates with energy expenditure, and overexpression of miR-203 could enhance sub-WAT browning in normal diet (ND) condition. Mechanistically, the expression of miR-203 is activated by cAMP-dependent C/EBPβ up-regulation. Subsequently, miR-203 inhibits IFN-γ signal pathway activation by directly targeting Lyn, which is an activator of Jak1-Stat1. Moreover, the forced expression of miR-203 could improve insulin sensitivity and resist high fat diet-induced obesity by inhibiting IFN-γ.

CONCLUSIONS

MicroRNA-203 (miR-203) promotes white adipose tissue browning in cold exposed mice and improves glucose tolerance in HFD fed mice by repressing IFN-γ. Since miR-203 is activated by cAMP-dependent C/EBPβ up-regulation and directly represses IFN-γ signal pathway, we declare that miR-203 acts as a messenger between cAMP signal pathway and IFN-γ signal pathway.

Associations between body condition score at parturition and microRNA profile in colostrum of dairy cows as evaluated by paired mapping programs

MicroRNA (miRNA) are abundant in milk, and likely have regulatory activity involving lactation and immunity. The objective of this study was to determine the miRNA profile in colostrum of overconditioned cows compared with cows of more moderate body condition score (BCS) at calving. Multiparous cows with either high (≥4.0 on a scale of 1 to 5; n = 7) or moderate BCS (2.75 to 3.50; n = 9) in the week before parturition were selected from a commercial dairy herd. Blood and colostrum were sampled within 24 h after calving. Blood serum was analyzed for free fatty acid (FFA) concentration. MicroRNA was isolated from colostrum samples after removing milk fat and cells. MicroRNA were sequenced, and reads were mapped to the bovine genome and to the existing database of miRNA at miRBase.org. Two programs, Oasis 2.0 and miRDeep2, were employed in parallel for read alignment, and analysis of miRNA count data was performed using DESeq2. Identification of differentially expressed miRNA from DESeq2 was not affected by the differences in miRNA detected by the 2 mapping programs. Most abundant miRNA included miR-30a, miR-148a, miR-181a, let-7f, miR-26a, miR-21, miR-22, and miR-92a. Large-scale shifts in miRNA profile were not observed; however, colostrum of cows with high BCS contained less miR-486, which has been linked with altered glucose metabolism. Colostrum from cows with elevated serum FFA contained less miR-885, which may be connected to hepatic function during the transition period. Potential functions of abundant miRNA suggest involvement in development and maintenance of cellular function in the mammary gland, with the additional possibility of influencing neonatal tissue and immune system development.

Administration of eicosapentaenoic and docosahexaenoic acids may improve the remodeling and browning in subcutaneous white adipose tissue and thermogenic markers in brown adipose tissue in mice

The role of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in browning and thermogenesis has not been fully elucidated. Thus, we meant to evaluate the effect of EPA and DHA, administered alone or combined, with the activation of browning markers in subcutaneous white adipose tissue (sWAT), and thermogenic markers in brown adipose tissue (BAT). C57BL/6 adult male mice received a control diet or a high-fructose diet (HFru) for eight weeks, but after the first three weeks, HFru was divided into new groups: HFru, HFru + EPA, HFru + DHA, and HFru-EPA + DHA. EPA and DHA diminished adipocyte hypertrophy, recovered markers of browning in sWAT and thermogenic factors in the BAT, and improved gene expressions linked with mitochondrial biogenesis and lipid metabolism. Importantly, EPA and DHA administrated alone showed stronger results than the combination of EPA + DHA. The results suggest that EPA and DHA might be useful as adjuvant strategies to treat metabolic-associated disorders.

Maternal n-3 PUFA supplementation promotes fetal brown adipose tissue development through epigenetic modifications in C57BL/6 mice

Brown adipose tissue (BAT) is a crucial regulator of energy expenditure. Emerging evidence suggests that n-3 PUFA potentiate brown adipogenesis in vitro. Since the pregnancy and lactation is a critical time for brown fat formation, we hypothesized that maternal supplementation of n-3 PUFA promotes BAT development in offspring. Female C57BL/6 mice were fed a diet containing n-3 PUFA (3%) derived from fish oil (FO), or an isocaloric diet devoid of n-3 PUFA (Cont) during pregnancy and lactation. Maternal n-3 PUFA intake was delivered to the BAT of neonates significantly reducing the n-6/n-3 ratio. The maternal n-3 PUFA exposure was linked with upregulated brown-specific gene and protein profiles and the functional cluster of brown-specific miRNAs. In addition, maternal n-3 PUFA induced histone modifications in the BAT evidenced by 1) increased epigenetic signature of brown adipogenesis, i.e., H3K27Ac and H3K9me2, 2) modified chromatin-remodeling enzymes, and 3) enriched the H3K27Ac in the promoter region of Ucp1. The offspring received maternal n-3 PUFA nutrition exhibited a significant increase in whole-body energy expenditure and better maintenance of core body temperature against acute cold treatment. Collectively, our results suggest that maternal n-3 PUFA supplementation potentiates fetal BAT development via the synergistic action of miRNA production and histone modifications, which may confer long-lasting metabolic benefits to offspring.

New treatment modalities for obesity

The treatment of childhood obesity represents a greater challenge for pediatricians. To date, it is multidisciplinary, including behavioral, dietary, pharmacological, and surgical options. Given the limited efficacy of available treatments, scientific research on finding new solutions is very active. Several drugs comprising Metformin, Glucagon-like peptide- 1 receptor agonists, Naltrexone-bupropion, Phentermine-Topiramate, and Lorcaserin have been studied as pediatric antiobesity agents. Findings from clinical trials showed a modest but significant effect of these drugs on weight loss, but long-term studies are needed to better define their exact role. Bariatric surgery is also promising for extremely obese adolescents. Moreover, a novel approach to treat obesity might be represented by compounds inducing browning of white adipose tissue, a complex process involved in body energy homeostasis, but at present evidence in humans is lacking. We aimed to review the current knowledge regarding the available new options for pediatric obesity treatment.

Fed-EXosome: extracellular vesicles and cell-cell communication in metabolic regulation

Extracellular vesicles (EVs) have emerged as a novel messaging system of the organism, mediating cell-cell and interorgan communication. Through their content of proteins and nucleic acids, as well as membrane proteins and lipid species, EVs can interact with and modulate the function of their target cells. The regulation of whole-body metabolism requires cross-talk between key metabolic tissues including adipose tissue (AT), the liver and skeletal muscle. Furthermore, the regulation of nutrient/energy allocation during pregnancy requires co-ordinated communication between the foetus and metabolic organs of the mother. A growing body of evidence is suggesting that EVs play a role in communication between and within key metabolic organs, both physiologically during metabolic homoeostasis but also contributing to pathophysiology during metabolic dysregulation observed in metabolic diseases such as obesity and diabetes. As obesity and its associated metabolic complications are reaching epidemic proportions, characterization of EV-mediated communication between key metabolic tissues may offer important insights into the regulation of metabolic functions during disease and offer global therapeutic opportunities. Here, we focus on the role of EVs in metabolic regulation and, in particular, EV-mediated cross-talk between cells of the AT.

PVAT and Its Relation to Brown, Beige, and White Adipose Tissue in Development and Function

Adipose tissue is commonly categorized into three types with distinct functions, phenotypes, and anatomical localizations. White adipose tissue (WAT) is the major energy store; the largest depots of WAT are found in subcutaneous or intravisceral sites. Brown adipose tissue (BAT) is responsible for energy dissipation during cold-exposure (i.e., non-shivering thermogenesis) and is primarily located in the interscapular region. Beige or brite (brown-in-white) adipose tissue can be found interspersed in WAT and can attain a brown-like phenotype. These three types of tissues also have endocrine functions and play major roles in whole body metabolism especially in obesity and its co-morbidities, such as cardiovascular disease. Over the last years, perivascular adipose tissue (PVAT) has emerged as an adipose organ with endocrine and paracrine functions. Pro and anti-inflammatory agents released by PVAT affect vascular health, and are implicated in the inflammatory aspects of atherosclerosis. PVAT shares several of the defining characteristics of brown adipose tissue, including its cellular morphology and expression of thermogenic genes characteristic for brown adipocytes. However, PVATs from different vessels are phenotypically different, and significant developmental differences exist between PVAT and other adipose tissues. Whether PVAT represents classical BAT, beige adipose tissue, or WAT with changing characteristics, is unclear. In this review, we summarize the current knowledge on how PVAT relates to other types of adipose tissue, both in terms of functionality, developmental origins, and its role in obesity-related cardiovascular disease and inflammation.

The clinical potential of adipogenesis and obesity-related microRNAs

Obesity is a growing health problem commonly associated with numerous metabolic disorders including type 2 diabetes, hypertension, cardiovascular disease, and some forms of cancer. The burden of obesity and associated cardiometabolic diseases are believed to arise through complex interplay between genetics and epigenetics predisposition, nutrition, environment, and lifestyle. However, the molecular basis and the repertoire of obesity-affecting factors are still unknown. Emerging evidence is connecting microRNAs (miRNAs) dysregulation with adipogenesis and obesity. Alteration in miRNAs expression could result in changes in the pattern of genes controlling a range of biological processes including inflammation, lipid metabolism, insulin resistance and adipogenesis. Hence, understanding exact roles of miRNAs as well as the degree of their contribution to the regulation of adipogenesis and fat cell development in obesity would provide new therapeutic targets for the development of novel and effective anti-obesity drugs. The objective of the current review is to: (i) discuss some of the latest development on relevant miRNAs dysregulation mainly in human adipogenesis and obesity, (ii) emphasize the role of circulating miRNAs as new promising therapeutics and attractive potential biomarkers for treating obesity and associated risk factor diseases, (iii) describe how dietary factors may influence obesity through modulation of miRNAs expression, (iv) highlight some of the actual limitations to the promise of miRNAs as novel therapeutics as well as to their translation for the benefit of patients, and finally (v) provide recommendations for future research on miRNA-based therapeutics that could lead to a breakthrough in the treatment of obesity and its associated pathologies.

'Basic and Applied Thermogenesis Research' Bridging the Gap

Obesity is a major health problem without satisfactory pharmacological treatment. A promising strategy is to promote energy dissipation by activating brown/beige adipose tissue. However, for this strategy to succeed it requires improving the transferability amongst cellular, murine, and human systems and bridging the gap between basic and clinical research.

No evidence of white adipocyte browning after endurance exercise training in obese men

BACKGROUND/OBJECTIVES

The phenomenon of adipocyte 'beiging' involves the conversion of non-classic brown adipocytes to brown-like adipose tissue with thermogenic, fat-burning properties, and this phenomenon has been shown in rodents to slow the progression of obesity-associated metabolic diseases. Rodent studies consistently report adipocyte beiging after endurance exercise training, indicating that increased thermogenic capacity in these adipocytes may underpin the improved health benefits of exercise training. The aim of this study was to determine whether prolonged endurance exercise training induces beige adipogenesis in subcutaneous adipose tissues of obese men.

SUBJECTS/METHODS

Molecular markers of beiging were examined in adipocytes obtained from abdominal subcutaneous (AbSC) and gluteofemoral (GF) subcutaneous adipose tissues before and after 6 weeks of endurance exercise training in obese men (n=6, 37.3±2.3 years, 30.1±2.3 kg m^-2).

RESULTS

The mRNAs encoding the brown or beige adipocyte-selective proteins were very lowly expressed in AbSC and GF adipose tissues and exercise training did not alter the mRNA expression of UCP1, CD137, CITED, TBX1, LHX8 and TCF21. Using immunohistochemistry, neither multilocular adipocytes, nor UCP1 or CD137-positive adipocytes were detected in any sample. MicroRNAs known to regulate brown and/or beige adipose development were highly expressed in white adipocytes but endurance exercise training did not impact their expression.

CONCLUSIONS

The present study reaffirms emerging data in humans demonstrating no evidence of white adipose tissue beiging in response to exercise training, and supports a growing body of work demonstrating divergence of brown/beige adipose location, molecular characterization and physiological function between rodents and humans.

Small non coding RNAs in adipocyte biology and obesity

Obesity has reached epidemic proportions world-wide and constitutes a substantial risk factor for hypertension, type 2 diabetes, cardiovascular diseases and certain cancers. So far, regulation of energy intake by dietary and pharmacological treatments has met limited success. The main interest of current research is focused on understanding the role of different pathways involved in adipose tissue function and modulation of its mass. Whole-genome sequencing studies revealed that the majority of the human genome is transcribed, with thousands of non-protein-coding RNAs (ncRNA), which comprise small and long ncRNAs. ncRNAs regulate gene expression at the transcriptional and post-transcriptional level. Numerous studies described the involvement of ncRNAs in the pathogenesis of many diseases including obesity and associated metabolic disorders. ncRNAs represent potential diagnostic biomarkers and promising therapeutic targets. In this review, we focused on small ncRNAs involved in the formation and function of adipocytes and obesity.

MiR-185 inhibits 3T3-L1 cell differentiation by targeting SREBP-1

Adipogenesis involves a highly orchestrated series of complex events in which microRNAs (miRNAs) may play an essential role. In this study, we found that the miR-185 expression increased gradually during 3T3-L1 cells differentiation. To explore the role of miR-185 in adipogenesis, miRNA agomirs and antagomirs were used to perform miR-185 overexpression and knockdown, respectively. Overexpression of miR-185 dramatically reduced the mRNA expression of the adipogenic markers, PPARγ, FABP4, FAS, and LPL, and the protein level of PPARγ and FAS. MiR-185 overexpression also led to a notable reduction in lipid accumulation. In contrast, miR-185 inhibition promoted differentiation of 3T3-L1 cells. By target gene prediction and luciferase reporter assay, we demonstrated that sterol regulatory element binding protein 1 (SREBP-1) may be the target of miR-185. These results indicate that miR-185 negatively regulates the differentiation of 3T3-L1 cells by targeting SREBP-1, further highlighting the importance of miRNAs in adipogenesis.

Physiological regulation and metabolic role of browning in white adipose tissue

Great progress has been made in our understanding of the browning process in white adipose tissue (WAT) in rodents. The recognition that i) adult humans have physiologically inducible brown adipose tissue (BAT) that may facilitate resistance to obesity and ii) that adult human BAT molecularly and functionally resembles beige adipose tissue in rodents, reignited optimism that obesity and obesity-related diabetes type 2 can be battled by controlling the browning of WAT. In this review the main cellular mechanisms and molecular mediators of browning of WAT in different physiological states are summarized. The relevance of browning of WAT in metabolic health is considered primarily through a modulation of biological role of fat tissue in overall metabolic homeostasis.

Genetics-based manipulation of adipose tissue sympathetic innervation

There is renewed interest in leveraging the thermogenic capacity of brown adipose tissue (BAT) and browning of white adipose tissue (WAT) to improve energy balance and prevent obesity. In addition to these effects on energy expenditure, both BAT and WAT secrete large numbers of hormones and cytokines that play important roles in maintaining metabolic health. Both BAT and WAT are densely innervated by the sympathetic nervous system (SNS) and this innervation is crucial for BAT thermogenesis and WAT browning, making it a potentially interesting target for manipulating energy balance and treatment of obesity and metabolic disease. Peripheral neuromodulation in the form of electrical manipulation of the SNS and parasympathetic nervous system (PSNS) has been used for the management of pain and many other conditions, but progress is hampered by lack of detailed knowledge of function-specific neurons and nerves innervating particular organs and tissues. Therefore, the goal of the National Institutes of Health (NIH) Common Fund project "Stimulating Peripheral Activity to Relieve Conditions (SPARC)" is to comprehensively map both anatomical and neurochemical aspects of the peripheral nervous system in animal model systems to ultimately guide optimal neuromodulation strategies in humans. Compared to electrical manipulation, neuron-specific opto- and chemogenetic manipulation, now being extensively used to decode the function of brain circuits, will further increase the functional specificity of peripheral neuromodulation.

Muscle Conditional Medium Reduces Intramuscular Adipocyte Differentiation and Lipid Accumulation through Regulating Insulin Signaling

Due to the paracrine effects of skeletal muscle, the lipid metabolism of porcine intramuscular (i.m.) preadipocytes was different from that of subcutaneous (s.c.) preadipocytes. To investigate the development of i.m. preadipocytes in vivo, the s.c. preadipocytes were cultured with muscle conditional cultured medium (MCM) for approximating extracellular micro-environment of the i.m. preadipocytes. Insulin signaling plays a fundamental role in porcine adipocyte differentiation. The expression levels of insulin receptor (INSR) and insulin-like growth factor 1 receptor (IGF-1R) in i.m. Preadipocytes were higher than that in s.c. preadipocytes. The effects of MCM on adipocyte differentiation, lipid metabolism and insulin signaling transdution were verified. MCM induced the apoptosis of s.c. preadipocytes but not of s.c. adipocytes. Moreover, MCM inhibited adipocyte differentiation at pre-differentiation and early stages of differentiation, while the expression levels of INSR and IGF-1R were increased. Furthermore, MCM treatment increased adipocyte lipolysis and fatty acid oxidation through induction of genes involved in lipolysis, thermogenesis, and fatty acid oxidation in mitochondria. Consistent with the above, treatment of s.c. adipocytes with MCM upregulated mitochondrial biogenesis. Taken together, MCM can approximate the muscle micro-environment and reduce intramuscular adipocyte differentiation and lipid accumulation via regulating insulin signaling.

Brown Fat-Derived Exosomes: Small Vesicles with Big Impact

Adipose tissue (AT) not only stores energy, but also secretes hormones and releases small vesicles known as exosomes. Thomou et al. (2017) now show that exosomes secreted by brown fat carry miRNAs that regulate the liver. Thus, AT exosomes might have therapeutic and diagnostic relevance for metabolic disorders.