β-ADRENERGICRECEPTORS ANDREGULATION OFENERGYEXPENDITURE: A Family Affair

Inosine: novel activator of brown adipose tissue and energy homeostasis

Extracellular purinergic molecules act as signaling molecules that bind to cellular receptors and regulate signaling pathways. Growing evidence suggests that purines regulate adipocyte function and whole-body metabolism. Here, we focus on one specific purine: inosine. Brown adipocytes, which are important regulators of whole-body energy expenditure (EE), release inosine when they are stressed or become apoptotic. Unexpectedly, inosine activates EE in neighboring brown adipocytes and enhances differentiation of brown preadipocytes. Increasing extracellular inosine, either directly by increasing inosine intake or indirectly via pharmacological inhibition of cellular inosine transporters, increases whole-body EE and counteracts obesity. Thus, inosine and other closely related purines might be a novel approach to tackle obesity and associated metabolic disorders by enhancing EE.

"Humanizing" mouse environments: Humidity, diurnal cycles and thermoneutrality

Thermoneutral housing has been shown to promote more accurate and robust development of several pathologies in mice. Raising animal housing temperatures a few degrees may create a relatively straightforward opportunity to improve translatability of mouse models. In this commentary, we discuss the changes of physiology induced in mice housed at thermoneutrality, and review techniques for measuring systemic thermogenesis, specifically those affecting storage and mobilization of lipids in adipose depots. Environmental cues are a component of the information integrated by the brain to calculate food consumption and calorie deposition. We show that relative humidity is one of those cues, inducing a rapid sensory response that is converted to a more chronic susceptibility to obesity. Given high inter-institutional variability in the regulation of relative humidity, study reproducibility may be improved by consideration of this factor. We evaluate a "humanized" environmental cycling protocol, where mice sleep in warm temperature housing, and are cool during the wake cycle. We show that this protocol suppresses adaptation to cool exposure, with consequence for adipose-associated lipid storage. To evaluate systemic cues in mice housed at thermoneutral temperatures, we characterized the circulating lipidome, and show that sera are highly depleted in some HDL-associated phospholipids, specifically phospholipids containing the essential fatty acid, 18:2 linoleic acid, and its derivative, arachidonic acid (20:4) and related ether-phospholipids. Given the role of these fatty acids in inflammatory responses, we propose they may underlie the differences in disease progression observed at thermoneutrality.

Effects of aging-induced obesity on the transcriptional expression of adipogenesis and thermogenic activity in the gonadal white adipose, brown adipose, and skeletal muscle tissues

PURPOSE

Aging is closely associated with chronic metabolic diseases, such as obesity, which lead to increased adiposity, skeletal muscle wasting, and imbalanced cellular energy metabolism. However, transcriptional profiles representing energy imbalances in aging-induced obesity are not fully understood. Thus, this study aimed to investigate the candidate genes predominantly regulated in aging-related obesity in spontaneously aged mice.

METHODS

Male C57BL/6J mice were divided into three age groups according to age: 2- (young), 12- (middle-aged), and 24- (old) months. Body weight and body composition parameters were measured in all mice. Gonadal white adipose tissue (gWAT), brown adipose tissue (BAT), and skeletal muscle (SM) were dissected and weighed. The target tissues were assessed using biochemical and histological assays.

RESULTS

Aging-induced obesity increased adipose mass and decreased SM weight through processes of adipocyte hypertrophy; however, recruitment of modulating adipogenesis-inducing transcription factors did not occur. Among adipokines, leptin level was greatly increased in the gWAT during aging. Interestingly, the β2-adrenergic receptor had a higher affinity than the β3-adrenergic receptor in aging-induced obesity. For the thermogenic regulation through β-adrenergic receptors (β-ARs), a declined uncoupling protein-1 (UCP-1) in the BAT was relevant to aging-induced obesity.

CONCLUSION

Aging-induced obesity increases leptin levels in adipocytes and decreases UCP-1 in BAT through β-ARs, according to transcriptional gene profiling. WAT browning increases energy expenditure due to exercise training adaptations. Further research is needed to discover more effective methods, such as exercise, against aging-induced obesity.

Swimming in cold water upregulates genes involved in thermogenesis and the browning of white adipose tissues

The purpose of this study was to investigate whether there is an interacting effect of six weeks of swimming in cold water on the gene expression of browning markers in adipose tissue in rodents. Twenty male Wistar rats were randomly divided into four groups: Control (C, 25 °C), Cold Exposure (CE, 4 °C), Swimming in tepid Water (STW, 30 °C), and Swimming in Cold Water (SCW, 15 °C). The swimming included 2-3 min intervals, 1 min rest, until exhaustion, three days a week for six weeks, with 3 to 6% of bodyweight overload. Rats from CE were exposed to cold for 2 h per day, five days per week. After the experimental protocol, interscapular brown (BAT) and inguinal subcutaneous white (WAT) fat tissues were excised, weighed, and processed for beiging and mitochondrial biogenesis markers gene expression. The experimental protocols resulted in an apparent increase in the number of brown adipocytes (per mm²) in the adipose deposits compared to the C group; substantial changes were observed in the SCW group. Compared to other groups, cold exposure alone increased significantly serum norepinephrine, and also β₂-adrenergic receptor expression was upregulated in the adipocytes compared to the C group. The STW group increased the expression of peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator-1 alpha (PGC-1α), β₂-adrenergic receptor, and CCAAT/enhancer-binding proteins-α(c/EBP-α) in WAT in comparison with the C group(p < 0.05). In both adipocytes, the SCW intervention significantly upregulated the expression of PGC-1α, PPAR-γ, and c/EBP-α genes in comparison with the C and CE groups. In addition, the expression of TFAM and UCP1 was upregulated substantially in the SCW group compared to other groups. Our data demonstrate that swim training and cold exposure present additive effects in the expression of genes involved in the beiging process and mitochondrial biogenesis markers in BAT and WAT. In addition, it seems that the upregulation of these genes is related to the activation of β₂-adrenergic receptors.

Adrenoceptors and SCD1 in adipocytes/adipose tissues: The expression and variation in health and obesity

Obesity, considered a metabolic disorder, is one of the most significant health issues that the community has to cope with today. A rising number of studies have been conducted to find out promising genetic targets for obese treatment. The sympathetic nervous system was proven to possess remarkable roles in energy metabolism, including the stimulation of lipolysis as well as thermogenesis, via distinct adrenoceptors appearing on the membrane of adipocyte. A decrease of β-adrenoceptor expression has been observed in obese individuals, which is related to reducing energy expenditure and developing obesity. While that the deficiency of stearoyl-CoA desaturase-1 (SCD1), which is a promising target for treatments of metabolic diseases, decreases oxidation and promotes the synthesis of fatty acids. Here, we emphasized several differences between distinct adrenoceptor subtypes, including their mRNA expression level and function in white adipose tissue and brown adipose tissue. We also highlighted SCD1's roles related to the progression of adipocytes and its changing expression under the obese condition in both rodents and humans, and furthermore, tried to figure out the interaction between adrenoceptors and SCD1 in adipose tissue.

Energy Trade-Off and Four Extreme Human Body Types

BACKGROUND

Resource trade-off theory suggests that increased performance on a given trait comes at the cost of decreased performance on other traits.

METHODS

Growth data from 1889 subjects (996 girls) were used from the GrowUp1974 Gothenburg study. Energy Trade-Off (ETO) between height and weight for individuals with extreme body types was characterized using a novel ETO-Score (ETOS). Four extreme body types were defined based on height and ETOI at early adulthood: tall-slender, short-stout, short-slender, and tall-stout; their growth trajectories assessed from ages 0.5-17.5 years.A GWAS using UK BioBank data was conducted to identify gene variants associated with height, BMI, and for the first time with ETOS.

RESULTS

Height and ETOS trajectories show a two-hit pattern with profound changes during early infancy and at puberty for tall-slender and short-stout body types.Several loci (including FTO, ADCY3, GDF5, ) and pathways were identified by GWAS as being highly associated with ETOS. The most strongly associated pathways were related to 'extracellular matrix', 'signal transduction', 'chromatin organization', and 'energy metabolism'.

CONCLUSIONS

ETOS represents a novel anthropometric trait with utility in describing body types. We discovered the multiple genomic loci and pathways probably involved in energy trade-off.

Role of thermogenic adipose tissue in lipid metabolism and atherosclerotic cardiovascular disease: lessons from studies in mice and humans

Brown adipocytes within brown adipose tissue (BAT) and beige adipocytes within white adipose tissue dissipate nutritional energy as heat. Studies in mice have shown that activation of thermogenesis in brown and beige adipocytes enhances the lipolytic processing of triglyceride-rich lipoproteins (TRLs) in plasma to supply these adipocytes with fatty acids for oxidation. This process results in formation of TRL remnants that are removed from the circulation through binding of apolipoprotein E (ApoE) on their surface to the low-density lipoprotein receptor (LDLR) on hepatocytes, followed by internalization. Concomitantly, lipolytic processing of circulating TRLs leads to generation of excess surface phospholipids that are transferred to nascent high-density lipoproteins (HDL), increasing their capacity for reverse cholesterol transport. Activation of thermogenic adipocytes thus lowers circulating triglycerides and non-HDL-cholesterol, while it increases HDL-cholesterol. The combined effect is protection from atherosclerosis development, which becomes evident in humanized mouse models with an intact ApoE-LDLR clearance pathway only, and is additive to the effects of classical lipid-lowering drugs including statins and proprotein convertase subtilisin/kexin type 9 inhibitors. A large recent study revealed that the presence of metabolically active BAT in humans is associated with lower triglycerides, higher HDL-cholesterol and lower risk of cardiovascular diseases. This narrative review aims to provide leads for further exploration of thermogenic adipose tissue as a therapeutic target. To this end, we describe the latest knowledge on the role of BAT in lipoprotein metabolism and address, for example, the discovery of the β2-adrenergic receptor as the dominant adrenergic receptor in human thermogenic adipocytes.

Adtrp regulates thermogenic activity of adipose tissue via mediating the secretion of S100b

Brown and beige adipose tissues dissipate chemical energy in the form of heat to maintain your body temperature in cold conditions. The impaired function of these tissues results in various metabolic diseases in humans and mice. By bioinformatical analyses, we identified a functional thermogenic regulator of adipose tissue, Androgen-dependent tissue factor pathway inhibitor [TFPI]-regulating protein (Adtrp), which was significantly overexpressed in and functionally activated the mature brown/beige adipocytes. Hereby, we knocked out Adtrp in mice which led to multiple abnormalities in thermogenesis, metabolism, and maturation of brown/beige adipocytes causing excess lipid accumulation in brown adipose tissue (BAT) and cold intolerance. The capability of thermogenesis in brown/beige adipose tissues could be recovered in Adtrp KO mice upon direct β3-adrenergic receptor (β3-AR) stimulation by CL316,243 treatment. Our mechanistic studies revealed that Adtrp by binding to S100 calcium-binding protein b (S100b) indirectly mediated the secretion of S100b, which in turn promoted the β3-AR mediated thermogenesis via sympathetic innervation. These results may provide a novel insight into Adtrp in metabolism via regulating the differentiation and thermogenesis of adipose tissues in mice.

Loss of adipose TET proteins enhances β-adrenergic responses and protects against obesity by epigenetic regulation of β3-AR expression

β-adrenergic receptor (β-AR) signaling plays predominant roles in modulating energy expenditure by triggering lipolysis and thermogenesis in adipose tissue, thereby conferring obesity resistance. Obesity is associated with diminished β3-adrenergic receptor (β3-AR) expression and decreased β-adrenergic responses, but the molecular mechanism coupling nutrient overload to catecholamine resistance remains poorly defined. Ten-eleven translocation (TET) proteins are dioxygenases that alter the methylation status of DNA by oxidizing 5-methylcytosine to 5-hydroxymethylcytosine and further oxidized derivatives. Here, we show that TET proteins are pivotal epigenetic suppressors of β3-AR expression in adipocytes, thereby attenuating the responsiveness to β-adrenergic stimulation. Deletion of all three Tet genes in adipocytes led to increased β3-AR expression and thereby enhanced the downstream β-adrenergic responses, including lipolysis, thermogenic gene induction, oxidative metabolism, and fat browning in vitro and in vivo. In mouse adipose tissues, Tet expression was elevated after mice ate a high-fat diet. Mice with adipose-specific ablation of all TET proteins maintained higher levels of β3-AR in both white and brown adipose tissues and remained sensitive to β-AR stimuli under high-fat diet challenge, leading to augmented energy expenditure and decreased fat accumulation. Consequently, they exhibited improved cold tolerance and were substantially protected from diet-induced obesity, inflammation, and metabolic complications, including insulin resistance and hyperlipidemia. Mechanistically, TET proteins directly repressed β3-AR transcription, mainly in an enzymatic activity-independent manner, and involved the recruitment of histone deacetylases to increase deacetylation of its promoter. Thus, the TET-histone deacetylase-β3-AR axis could be targeted to treat obesity and related metabolic diseases.

Adipocyte Gq signaling is a regulator of glucose and lipid homeostasis in mice

Obesity is the major driver of the global epidemic in type 2 diabetes (T2D). In individuals with obesity, impaired insulin action leads to increased lipolysis in adipocytes, resulting in elevated plasma free fatty acid (FFA) levels that promote peripheral insulin resistance, a hallmark of T2D. Here we show, by using a combined genetic/biochemical/pharmacologic approach, that increased adipocyte lipolysis can be prevented by selective activation of adipocyte Gq signaling in vitro and in vivo (in mice). Activation of this pathway by a Gq-coupled designer receptor or by an agonist acting on an endogenous adipocyte Gq-coupled receptor (CysLT2 receptor) greatly improved glucose and lipid homeostasis in obese mice or in mice with adipocyte insulin receptor deficiency. Our findings identify adipocyte Gq signaling as an essential regulator of whole-body glucose and lipid homeostasis and should inform the development of novel classes of GPCR-based antidiabetic drugs.

Adipose tissue: a neglected organ in the response to severe trauma?

Despite the manifold recent efforts to improve patient outcomes, trauma still is a clinical and socioeconomical issue of major relevance especially in younger people. The systemic immune reaction after severe injury is characterized by a strong pro- and anti-inflammatory response. Besides its functions as energy storage depot and organ-protective cushion, adipose tissue regulates vital processes via its secretion products. However, there is little awareness of the important role of adipose tissue in regulating the posttraumatic inflammatory response. In this review, we delineate the local and systemic role of adipose tissue in trauma and outline different aspects of adipose tissue as an immunologically active modifier of inflammation and as an immune target of injured remote organs after severe trauma.

PaOctβ2R: Identification and Functional Characterization of an Octopamine Receptor Activating Adenylyl Cyclase Activity in the American Cockroach Periplaneta americana

Biogenic amines constitute an important group of neuroactive substances that control and modulate various neural circuits. These small organic compounds engage members of the guanine nucleotide-binding protein coupled receptor (GPCR) superfamily to evoke specific cellular responses. In addition to dopamine- and 5-hydroxytryptamine (serotonin) receptors, arthropods express receptors that are activated exclusively by tyramine and octopamine. These phenolamines functionally substitute the noradrenergic system of vertebrates Octopamine receptors that are the focus of this study are classified as either α- or β-adrenergic-like. Knowledge on these receptors is scarce for the American cockroach (Periplaneta americana). So far, only an α–adrenergic-like octopamine receptor that primarily causes Ca²⁺ release from intracellular stores has been studied from the cockroach (PaOctα1R). Here we succeeded in cloning a gene from cockroach brain tissue that encodes a β-adrenergic-like receptor and leads to cAMP production upon activation. Notably, the receptor is 100-fold more selective for octopamine than for tyramine. A series of synthetic antagonists selectively block receptor activity with epinastine being the most potent. Bioinformatics allowed us to identify a total of 19 receptor sequences that build the framework of the biogenic amine receptor clade in the American cockroach. Phylogenetic analyses using these sequences and receptor sequences from model organisms showed that the newly cloned gene is an β2-adrenergic-like octopamine receptor. The functional characterization of PaOctβ2R and the bioinformatics data uncovered that the monoaminergic receptor family in the hemimetabolic P. americana is similarly complex as in holometabolic model insects like Drosophila melanogaster and the honeybee, Apis mellifera. Thus, investigating these receptors in detail may contribute to a better understanding of monoaminergic signaling in insect behavior and physiology.

Borage oil intake by overweight young adults: no effect on metabolic rate; beneficial effects on plasma triglyceride and HDL cholesterol readings

Some research has raised the possibility that gamma linolenic acid (GLA) can increase resting metabolic rate (RMR), which can help with weight control. However, in overweight young adults with a family history of obesity, no effect on RMR was seen after a 6 weeks treatment with borage oil (880 mg GLA per day) or evening primrose oil (540 mg GLA per day). On the other hand, borage oil did lower plasma triglyceride readings and raise HDL cholesterol readings (mean starting values in normal range for triglycerides, borderline low for HDL). No effect was seen for body mass index, plasma total cholesterol, LDL cholesterol, or glucose. Thus, in the type of subjects studied here, borage oil, a source of GLA, did not show promise as a weight control aid, but could help prevent undesirable readings for two blood lipid measures.

Inhibition of a Novel CLK1-THRAP3-PPARγ Axis Improves Insulin Sensitivity

Increasing energy expenditure by promoting "browning" in adipose tissues is a promising strategy to prevent obesity and associated diabetes. To uncover potential targets of cold exposure, which induces energy expenditure, we performed phosphoproteomics profiling in brown adipose tissue of mice housed in mild cold environment at 16°C. We identified CDC2-like kinase 1 (CLK1) as one of the kinases that were significantly downregulated by mild cold exposure. In addition, genetic knockout of CLK1 or chemical inhibition in mice ameliorated diet-induced obesity and insulin resistance at 22°C. Through proteomics, we uncovered thyroid hormone receptor-associated protein 3 (THRAP3) as an interacting partner of CLK1, further confirmed by co-immunoprecipitation assays. We further demonstrated that CLK1 phosphorylates THRAP3 at Ser243, which is required for its regulatory interaction with phosphorylated peroxisome proliferator-activated receptor gamma (PPARγ), resulting in impaired adipose tissue browning and insulin sensitivity. These data suggest that CLK1 plays a critical role in controlling energy expenditure through the CLK1-THRAP3-PPARγ axis.

Functional Characterization of the Obesity-Linked Variant of the β3-Adrenergic Receptor

Adrenergic receptor β₃ (ADRβ₃) is a member of the rhodopsin-like G protein-coupled receptor family. The binding of the ligand to ADRβ₃ activates adenylate cyclase and increases cAMP in the cells. ADRβ₃ is highly expressed in white and brown adipocytes and controls key regulatory pathways of lipid metabolism. Trp64Arg (W64R) polymorphism in the ADRβ₃ is associated with the early development of type 2 diabetes mellitus, lower resting metabolic rate, abdominal obesity, and insulin resistance. It is unclear how the substitution of W64R affects the functioning of ADRβ₃. This study was initiated to functionally characterize this obesity-linked variant of ADRβ₃. We evaluated in detail the expression, subcellular distribution, and post-activation behavior of the WT and W64R ADRβ₃ using single cell quantitative fluorescence microscopy. When expressed in HEK 293 cells, ADRβ₃ shows a typical distribution displayed by other GPCRs with a predominant localization at the cell surface. Unlike adrenergic receptor β₂ (ADRβ₂), agonist-induced desensitization of ADRβ₃ does not involve loss of cell surface expression. WT and W64R variant of ADRβ₃ displayed comparable biochemical properties, and there was no significant impact of the substitution of tryptophan with arginine on the expression, cellular distribution, signaling, and post-activation behavior of ADRβ₃. The obesity-linked W64R variant of ADRβ₃ is indistinguishable from the WT ADRβ₃ in terms of expression, cellular distribution, signaling, and post-activation behavior.

NT3/TrkC Pathway Modulates the Expression of UCP-1 and Adipocyte Size in Human and Rodent Adipose Tissue

Neurotrophin-3 (NT3), through activation of its tropomyosin-related kinase receptor C (TrkC), modulates neuronal survival and neural stem cell differentiation. It is widely distributed in peripheral tissues (especially vessels and pancreas) and this ubiquitous pattern suggests a role for NT3, outside the nervous system and related to metabolic functions. The presence of the NT3/TrkC pathway in the adipose tissue (AT) has never been investigated. Present work studies in human and murine adipose tissue (AT) the presence of elements of the NT3/TrkC pathway and its role on lipolysis and adipocyte differentiation. qRT-PCR and immunoblot indicate that NT3 (encoded by NTF3) was present in human retroperitoneal AT and decreases with age. NT3 was also present in rat isolated adipocytes and retroperitoneal, interscapular, perivascular, and perirenal AT. Histological analysis evidences that NT3 was mainly present in vessels irrigating AT close associated to sympathetic fibers. Similar mRNA levels of TrkC (encoded by NTRK3) and β-adrenoceptors were found in all ATs assayed and in isolated adipocytes. NT3, through TrkC activation, exert a mild effect in lipolysis. Addition of NT3 during the differentiation process of human pre-adipocytes resulted in smaller adipocytes and increased uncoupling protein-1 (UCP-1) without changes in β-adrenoceptors. Similarly, transgenic mice with reduced expression of NT3 (Ntf3 knock-in lacZ reporter mice) or lacking endothelial NT3 expression (Ntf3flox1/flox2;Tie2-Cre+/0) displayed enlarged white and brown adipocytes and lower UCP-1 expression.

Conclusions

NT3, mainly released by blood vessels, activates TrkC and regulates adipocyte differentiation and browning. Disruption of NT3/TrkC signaling conducts to hypertrophied white and brown adipocytes with reduced expression of the thermogenesis marker UCP-1.

Chemogenetic approaches to identify metabolically important GPCR signaling pathways: Therapeutic implications

DREADDs (Designer Receptors Exclusively Activated by a Designer Drug) are designer G protein-coupled receptors (GPCRs) that are widely used in the neuroscience field to modulate neuronal activity. In this review, we will focus on DREADD studies carried out with genetically engineered mice aimed at elucidating signaling pathways important for maintaining proper glucose and energy homeostasis. The availability of muscarinic receptor-based DREADDs endowed with selectivity for one of the four major classes of heterotrimeric G proteins (G_s , G_i , G_q , and G₁₂ ) has been instrumental in dissecting the physiological and pathophysiological roles of distinct G protein signaling pathways in metabolically important cell types. The novel insights gained from this work should inform the development of novel classes of drugs useful for the treatment of several metabolic disorders including type 2 diabetes and obesity.

Lipodistrophy: a paradigm for understanding the consequences of "overloading" adipose tissue

Lipodystrophies have been recognized since at least the nineteenth century and, despite their rarity, tended to attract considerable medical attention because of the severity and somewhat paradoxical nature of the associated metabolic disease that so closely mimics that of obesity. Within the last 20 yr most of the monogenic subtypes have been characterized, facilitating family genetic screening and earlier disease detection as well as providing important insights into adipocyte biology and the systemic consequences of impaired adipocyte function. Even more recently, compelling genetic studies have suggested that subtle partial lipodystrophy is likely to be a major factor in prevalent insulin-resistant type 2 diabetes mellitus (T2DM), justifying the longstanding interest in these disorders. This progress has also underpinned novel approaches to treatment that, in at least some patients, can be of considerable therapeutic benefit.

The ADRB3 rs4994 polymorphism increases risk of childhood and adolescent overweight/obesity for East Asia's population: an evidence-based meta-analysis

Whether the Adrenoceptor Beta 3 (ADRB3) gene rs4994 polymorphism could affect the individual risk of childhood and adolescent overweight/obesity remains controversial. This meta-analysis was performed to estimate the prevalence of this polymorphism in overweight/obesity, and test the potential association by summarizing existing evidence. Comprehensive literature search in PubMed, Web of Science, Cochrane Library, Wanfang, and CNKI databases was performed to identify eligible data sets. Finally, 16 studies involving 5,147 overweight/obese cases and 7,350 non-obese controls were included for further synthetic analyses. Odds ratio (OR) and its corresponding 95% confidence intervals (CIs) were statistically calculated. Totally, 69.9% of the included subjects came from East Asia. In the meta-analysis for overall population, statistically significant associations with increased risk of childhood and adolescent overweight/obesity were identified in allele model (OR 1.23, 95% CI 1.10–1.38), heterozygote model (OR 1.39, 95% CI 1.16–1.68), and dominant model (OR 1.31, 95% CI 1.12–1.54). Further stratified analysis according to geographical regions revealed that the statistical significance could only be detected in the East Asia subgroup in allele model, homozygote model, heterozygote model, and dominant model. In summary, our meta-analysis indicated that the ADRB3 rs4994 polymorphism could significantly increase the risk of childhood and adolescent overweight/obesity, especially for the East Asia’s population.

Adipose Tissue Metabolic Function and Dysfunction: Impact of Burn Injury

For decades, adipose tissue had been considered as merely a storage depot and cushion to protect organs against trauma and injury. However, in recent years, a number of impactful studies have pinpointed the adipose tissue as an endocrine organ mediating systemic dysfunction in not only metabolic disorders such as obesity, but also in the stages following traumatic events such as severe burns. For instance, thermal injury induces a chronic β-adrenergic response associated with drastic increases in adipose lipolysis, macrophage infiltration and IL-6 mediated browning of white adipose tissue (WAT). The downstream consequences of these physiological changes to adipose, such as hepatomegaly and muscle wasting, are only now coming to light and suggest that WAT is both a culprit in and initiator of metabolic disorders after burn injury. To that effect, the aim of this review is to chronicle and critically analyze the scientific advances made in the study of adipose tissue with regards to its role in orchestrating the hypermetabolic response and detrimental effects of burn injury. The topics covered include the magnitude of the lipolytic response following thermal trauma and how WAT browning and inflammation perpetuate this cycle as well as how WAT physiology impacts insulin resistance and hyperglycemia post-burn. To conclude, we discuss how these findings can be translated from bench to bedside in the form of therapeutic interventions which target physiological changes to WAT to restore systemic homeostasis following a severe burn.

Use of DREADD Technology to Identify Novel Targets for Antidiabetic Drugs

G protein-coupled receptors (GPCRs) form a superfamily of plasma membrane receptors that couple to four major families of heterotrimeric G proteins, G_s, G_i, G_q, and G₁₂. GPCRs represent excellent targets for drug therapy. Since the individual GPCRs are expressed by many different cell types, the in vivo metabolic roles of a specific GPCR expressed by a distinct cell type are not well understood. The development of designer GPCRs known as DREADDs (designer receptors exclusively activated by a designer drug) that selectively couple to distinct classes of heterotrimeric G proteins has greatly facilitated studies in this area. This review focuses on the use of DREADD technology to explore the physiological and pathophysiological roles of distinct GPCR/G protein cascades in several metabolically important cell types. The novel insights gained from these studies should stimulate the development of GPCR-based treatments for major metabolic diseases such as type 2 diabetes and obesity.

The S100B Protein and Partners in Adipocyte Response to Cold Stress and Adaptive Thermogenesis: Facts, Hypotheses, and Perspectives

In mammals, adipose tissue is an active secretory tissue that responds to mild hypothermia and as such is a genuine model to study molecular and cellular adaptive responses to cold-stress. A recent study identified a mammal-specific protein of the endoplasmic reticulum that is strongly induced in the inguinal subcutaneous white adipocyte upon exposure to cold, calsyntenin 3β (CLSTN3β). CLSTN3β regulates sympathetic innervation of thermogenic adipocytes and contributes to adaptive non-shivering thermogenesis. The calcium- and zinc-binding S100B is a downstream effector in the CLSTN3β pathways. We review, here, the literature on the transcriptional regulation of the S100b gene in adipocyte cells. We also rationalize the interactions of the S100B protein with its recognized or hypothesized intracellular (p53, ATAD3A, CYP2E1, AHNAK) and extracellular (Receptor for Advanced Glycation End products (RAGE), RPTPσ) target proteins in the context of adipocyte differentiation and adaptive thermogenesis. We highlight a chaperon-associated function for the intracellular S100B and point to functional synergies between the different intracellular S100B target proteins. A model of non-classical S100B secretion involving AHNAK/S100A10/annexin2-dependent exocytosis by the mean of exosomes is also proposed. Implications for related areas of research are noted and suggestions for future research are offered.

Urolithin A exerts antiobesity effects through enhancing adipose tissue thermogenesis in mice

Obesity leads to multiple health problems, including diabetes, fatty liver, and even cancer. Here, we report that urolithin A (UA), a gut-microflora-derived metabolite of pomegranate ellagitannins (ETs), prevents diet-induced obesity and metabolic dysfunctions in mice without causing adverse effects. UA treatment increases energy expenditure (EE) by enhancing thermogenesis in brown adipose tissue (BAT) and inducing browning of white adipose tissue (WAT). Mechanistically, UA-mediated increased thermogenesis is caused by an elevation of triiodothyronine (T3) levels in BAT and inguinal fat depots. This is also confirmed in UA-treated white and brown adipocytes. Consistent with this mechanism, UA loses its beneficial effects on activation of BAT, browning of white fat, body weight control, and glucose homeostasis when thyroid hormone (TH) production is blocked by its inhibitor, propylthiouracil (PTU). Conversely, administration of exogenous tetraiodothyronine (T4) to PTU-treated mice restores UA-induced activation of BAT and browning of white fat and its preventive role on high-fat diet (HFD)-induced weight gain. Together, these results suggest that UA is a potent antiobesity agent with potential for human clinical applications.

Foxp1 controls brown/beige adipocyte differentiation and thermogenesis through regulating β3-AR desensitization

β-Adrenergic receptor (β-AR) signaling is a pathway controlling adaptive thermogenesis in brown or beige adipocytes. Here we investigate the biological roles of the transcription factor Foxp1 in brown/beige adipocyte differentiation and thermogenesis. Adipose-specific deletion of Foxp1 leads to an increase of brown adipose activity and browning program of white adipose tissues. The Foxp1-deficient mice show an augmented energy expenditure and are protected from diet-induced obesity and insulin resistance. Consistently, overexpression of Foxp1 in adipocytes impairs adaptive thermogenesis and promotes diet-induced obesity. A robust change in abundance of the β3-adrenergic receptor (β3-AR) is observed in brown/beige adipocytes from both lines of mice. Molecularly, Foxp1 directly represses β3-AR transcription and regulates its desensitization behavior. Taken together, our findings reveal Foxp1 as a master transcriptional repressor of brown/beige adipocyte differentiation and thermogenesis, and provide an important clue for its targeting and treatment of obesity.

Beta3-adrenergic receptor (b3-AR) signaling in response to cold activates adipose tissue thermogenesis. Here the authors identify the transcription factor FoxP1 as a direct negative regulator of b3-AR expression and show that loss of FoxP1 leads to enhanced development of thermogenic adipose tissue.

Transcriptome Analysis Reveals Differential Expression of Genes Regulating Hepatic Triglyceride Metabolism in Pekin Ducks During Dietary Threonine Deficiency

Dietary threonine (Thr) deficiency increases hepatic triglyceride accumulation in Pekin ducks, which results in fatty liver disease and impairs hepatic function. However, the underlying molecular mechanisms altered by dietary Thr deficiency are still unknown. To identify the underlying molecular changes, 180 one-day-old ducklings were divided into three groups, including Thr deficiency group (Thr-D), Thr sufficiency group (Thr-S), and pair-fed group (Pair-F) that was fed with a Thr-sufficient diet but with reduced daily feed intake. The results showed that feed intake was similar between Thr-D and Pair-F groups, but weight gain rate and final body weight in the Thr-D group were lower than those in the Pair-F group. Feed intake, weight gain, and body weight in Thr-D and Pair-F groups were lower than those in the Thr-S group. The Thr-D diet reduced abdominal fat percentage but increased hepatic triglyceride content when compared with that of the Thr-S and Pair-F groups. The Pair-F reduced hepatic levels of C15:0, C17:0, C18:0, C20:0, C20:4n6, and C22:0 and also reduced total fatty acid, saturated fatty acid, and unsaturated fatty acid content when compared with those of the Thr-D and Thr-S groups. The Thr-D diet increased hepatic content of C6:0, C17:1, C18:3n6, C20:0, C20:1n9, and C22:2, as well as reduced the content of C18:2n6t and C23:0 when compared with those of the Thr-S group. Transcriptome analysis in the liver indicated that the Thr-D diet upregulated genes related to fatty acid and triglyceride synthesis and downregulated genes related to fatty acid oxidation and triglyceride transport. Gene ontology analysis showed that more genes related to lipid metabolism processes and molecular function were differentially expressed in the Thr-D group relative to Thr-S and Pair-F groups than in the Pair-F group relative to the Thr-S group. KEGG pathway analysis showed that differentially expressed genes were enriched in signal transduction, immune, hormone, lipid, and amino acid metabolism pathways. Our findings indicated that the Thr-D diet increased hepatic triglyceride and fatty acid accumulation via increasing fatty acid and triglyceride synthesis and reducing fatty acid oxidation and triglyceride transport. These findings provide novel insights into our understanding of the molecular mechanisms underlying fat accumulation in the liver caused by dietary threonine deficiency.

Translational Pharmacology and Physiology of Brown Adipose Tissue in Human Disease and Treatment

Human brown adipose tissue (BAT) is experimentally modeled to better understand the biology of this important metabolic tissue, and also to enable the potential discovery and development of novel therapeutics for obesity and sequelae resulting from the persistent positive energy balance. This chapter focuses on translation into humans of findings and hypotheses generated in nonhuman models of BAT pharmacology. Given the demonstrated challenges of sustainably reducing caloric intake in modern humans, potential solutions to obesity likely lie in increasing energy expenditure. The energy-transforming activities of a single cell in any given tissue can be conceptualized as a flow of chemical energy from energy-rich substrate molecules into energy-expending, endergonic biological work processes through oxidative degradation of organic molecules ingested as nutrients. Despite the relatively tight coupling between metabolic reactions and products, some expended energy is incidentally lost as heat, and in this manner a significant fraction of the energy originally captured from the environment nonproductively transforms into heat rather than into biological work. In human and other mammalian cells, some processes are even completely uncoupled, and therefore purely energy consuming. These molecular and cellular actions sum up at the physiological level to adaptive thermogenesis, the endogenous physiology in which energy is nonproductively released as heat through uncoupling of mitochondria in brown fat and potentially skeletal muscle. Adaptive thermogenesis in mammals occurs in three forms, mostly in skeletal muscle and brown fat: shivering thermogenesis in skeletal muscle, non-shivering thermogenesis in brown fat, and diet-induced thermogenesis in brown fat. At the cellular level, the greatest energy transformations in humans and other eukaryotes occur in the mitochondria, where creating energetic inefficiency by uncoupling the conversion of energy-rich substrate molecules into ATP usable by all three major forms of biological work occurs by two primary means. Basal uncoupling occurs as a passive, general, nonspecific leak down the proton concentration gradient across the membrane in all mitochondria in the human body, a gradient driving a key step in ATP synthesis. Inducible uncoupling, which is the active conduction of protons across gradients through processes catalyzed by proteins, occurs only in select cell types including BAT. Experiments in rodents revealed UCP1 as the primary mammalian molecule accounting for the regulated, inducible uncoupling of BAT, and responsive to both cold and pharmacological stimulation. Cold stimulation of BAT has convincingly translated into humans, and older clinical observations with nonselective 2,4-DNP validate that human BAT's participation in pharmacologically mediated, though nonselective, mitochondrial membrane decoupling can provide increased energy expenditure and corresponding body weight loss. In recent times, however, neither beta-adrenergic antagonism nor unselective sympathomimetic agonism by ephedrine and sibutramine provide convincing evidence that more BAT-selective mechanisms can impact energy balance and subsequently body weight. Although BAT activity correlates with leanness, hypothesis-driven selective β3-adrenergic agonism to activate BAT in humans has only provided robust proof of pharmacologic activation of β-adrenergic receptor signaling, limited proof of the mechanism of increased adaptive thermogenesis, and no convincing evidence that body weight loss through negative energy balance upon BAT activation can be accomplished outside of rodents. None of the five demonstrably β3 selective molecules with sufficient clinical experience to merit review provided significant weight loss in clinical trials (BRL 26830A, TAK 677, L-796568, CL 316,243, and BRL 35135). Broader conclusions regarding the human BAT therapeutic hypothesis are limited by the absence of data from most studies demonstrating specific activation of BAT thermogenesis in most studies. Additionally, more limited data sets with older or less selective β3 agonists also did not provide strong evidence of body weight effects. Encouragingly, β3-adrenergic agonists, catechins, capsinoids, and nutritional extracts, even without robust negative energy balance outcomes, all demonstrated increased total energy expenditure that in some cases could be associated with concomitant activation of BAT, though the absence of body weight loss indicates that in no cases did the magnitude of negative energy balance reach sufficient levels. Glucocorticoid receptor agonists, PPARg agonists, and thyroid hormone receptor agonists all possess defined molecular and cellular pharmacology that preclinical models predicted to be efficacious for negative energy balance and body weight loss, yet their effects on human BAT thermogenesis upon translation were inconsistent with predictions and disappointing. A few new mechanisms are nearing the stage of clinical trials and may yet provide a more quantitatively robust translation from preclinical to human experience with BAT. In conclusion, translation into humans has been demonstrated with BAT molecular pharmacology and cell biology, as well as with physiological response to cold. However, despite pharmacologically mediated, statistically significant elevation in total energy expenditure, translation into biologically meaningful negative energy balance was not achieved, as indicated by the absence of measurable loss of body weight over the duration of a clinical study.

Intermittent Fasting Promotes White Adipose Browning and Decreases Obesity by Shaping the Gut Microbiota

While activation of beige thermogenesis is a promising approach for treatment of obesity-associated diseases, there are currently no known pharmacological means of inducing beiging in humans. Intermittent fasting is an effective and natural strategy for weight control, but the mechanism for its efficacy is poorly understood. Here, we show that an every-other-day fasting (EODF) regimen selectively stimulates beige fat development within white adipose tissue and dramatically ameliorates obesity, insulin resistance, and hepatic steatosis. EODF treatment results in a shift in the gut microbiota composition leading to elevation of the fermentation products acetate and lactate and to the selective upregulation of monocarboxylate transporter 1 expression in beige cells. Microbiota-depleted mice are resistance to EODF-induced beiging, while transplantation of the microbiota from EODF-treated mice to microbiota-depleted mice activates beiging and improves metabolic homeostasis. These findings provide a new gut-microbiota-driven mechanism for activating adipose tissue browning and treating metabolic diseases.

Red blood cell β-adrenergic receptors contribute to diet-induced energy expenditure by increasing O2 supply

Diet-induced obesity (DIO) represents the major cause for the current obesity epidemic, but the mechanism underlying DIO is unclear. β-Adrenergic receptors (β-ARs) play a major role in sympathetic nervous system-mediated (SNS-mediated) diet-induced energy expenditure (EE). Rbc express abundant β-ARs; however, a potential role for rbc in DIO remains untested. Here, we demonstrated that high-fat, high-caloric diet (HFD) feeding increased both EE and blood O2 content, and the HFD-induced increases in blood O2 level and in body weight gain were negatively correlated. Deficiency of β-ARs in rbc reduced glycolysis and ATP levels, diminished HFD-induced increases in both blood O2 content and EE, and resulted in DIO. Importantly, specific activation of cAMP signaling in rbc promoted HFD-induced EE and reduced HFD-induced tissue hypoxia independent of obesity. Both HFD and pharmacological activation cAMP signaling in rbc led to increased glycolysis and ATP levels. These results identify a previously unknown role for rbc β-ARs in mediating the SNS action on HFD-induced EE by increasing O2 supply, and they demonstrate that HFD-induced EE is limited by blood O2 availability and can be augenmented by increased O2 supply.

Purple corn silk: A potential anti-obesity agent with inhibition on adipogenesis and induction on lipolysis and apoptosis in adipocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Corn silk or the stigma of Zea mays L. has traditionally been used in weight loss stimulation and treatment of cystitis, urinary infections and obesity. Purple corn silk, rich of polyphenolic substances, was reported on anti-diabetic and anti-obesity effect in animal studies. However, scientific evidence on mechanisms and targets of action of purple corn silk related to adipocyte life cycle has been limited.

AIM OF STUDY

To determine phytochemical compositions and investigate anti-obesity potential of the purple corn silk focusing on interruption of adipocyte life cycle; effect on pre-adipocyte proliferation, adipogenesis, adipocyte lipolysis, and apoptosis.

MATERIALS AND METHODS

The ethanolic purple corn silk extract (PCS) was prepared and investigated for phytochemical compositions by LC/MS/MS technique and anti-obesity potential using murine 3T3-L1 cell line. Using methyl thiazole tetrazolium (MTT) assay, the effects on pre-adipocytes and adipocyte viability and on pre-adipocytes proliferation at 24-, 48-, and 72-h incubation period were evaluated. In addition, anti-adipogenesis via inhibition on adipocyte differentiation and reduction of total lipid accumulation was evaluated using Oil Red O staining and spectrophotometric methods, respectively. The lipolysis effect was determined by measurement of glycerol released content using glycerol test kit after 48-h treatment of PCS to adipocytes. Apoptosis inductive effect was done by using 2-(4-Amidinophenyl)-6-indolecarbamidine dihydrochloride (DAPI) staining method.

RESULTS

The polyphenols including anthocyanins, quercetin and phenolic acids and derivatives were found as the major chemical compositions of the PCS. With multiple-stages interruption on the adipocyte life cycle, anti-obesity effect of PCS was interestingly demonstrated. When compared to the control, the PCS at concentration range between 250-1000 μg/mL showed anti-adipogenesis effect as expressing of significant inhibition on pre-adipocyte proliferation at all incubation period (43.52±5.28 - 75.51±9.09%) and significant decreasing of total lipid accumulation at concentration of 500μg/mL (80.22±6.58%) and 1000μg/mL (69.62±5.42%). Moreover, the PCS exhibited lipolysis and apoptosis inductive effect with dose dependent manner and significance at concentration of 1000μg/mL by increase of released glycerol content (173.88±6.13% of the control) and of nuclei condensing and apoptotic bodies (with relative apoptosis induction as 131.74±1.64% of the control).

CONCLUSION

Our data has evidenced the anti-obesity potential of PCS related interruption at multiple stages of adipocyte life cycle. Its potency was attributed to inhibition on adipocyte proliferation and adipogenesis as well as induction on lipolysis and apoptosis at high concentration. However, further in vivo investigation should be considered to insist the possibility in applications of PCS in prevention and treatment of obesity.

Characterization of cold-induced remodelling reveals depot-specific differences across and within brown and white adipose tissues in mice

AIM

Brown and beige adipose tissues dissipate energy in the form of heat via mitochondrial uncoupling protein 1, defending against hypothermia and potentially obesity. The latter has prompted renewed interest in understanding the processes involved in browning to realize the potential therapeutic benefits. To characterize the temporal profile of cold-induced changes and browning of brown and white adipose tissues in mice.

METHODS

Male C57BL/6J mice were singly housed in conventional cages under cold exposure (4 °C) for 1, 2, 3, 4, 5 and 7 days. Food intake and body weight were measured daily. Interscapular brown adipose tissue (iBAT), inguinal subcutaneous (sWAT) and epididymal white adipose tissue (eWAT) were harvested for histological, immunohistochemical, gene and protein expression analysis.

RESULTS

Upon cold exposure, food intake increased, whilst body weight and adipocyte size were found to be transiently reduced. iBAT mass was found to be increased, whilst sWAT and eWAT were found to be transiently decreased. A combination of morphological, genetic (Ucp-1, Pgc-1α and Elov13) and biochemical (UCP-1, PPARγ and aP2) analyses demonstrated the depot-specific remodelling in response to cold exposure.

CONCLUSION

Our results demonstrate the differential responses to cold-induced changes across discrete BAT and WAT depots and support the notion that the effects of short-term cold exposure are achieved by expansion, activation and increasing thermogenic capacity of iBAT, as well as browning of sWAT and, to a lesser extent, eWAT.

cGMP and Brown Adipose Tissue

The second messenger cyclic guanosine monophosphate (cGMP) is a key mediator in physiological processes such as vascular tone, and its essential involvement in pathways regulating metabolism has been recognized in recent years. Here, we focus on the fundamental role of cGMP in brown adipose tissue (BAT) differentiation and function. In contrast to white adipose tissue (WAT), which stores energy in the form of lipids, BAT consumes energy stored in lipids to generate heat. This so-called non-shivering thermogenesis takes place in BAT mitochondria, which express the specific uncoupling protein 1 (UCP1). The energy combusting properties of BAT render it a promising target in antiobesity strategies in which BAT could burn the surplus energy that has accumulated in obese and overweight individuals. cGMP is generated by guanylyl cyclases upon activation by nitric oxide or natriuretic peptides. It affects several downstream molecules including cGMP-receptor proteins such as cGMP-dependent protein kinase and is degraded by phosphodiesterases. The cGMP pathway contains several signaling molecules that can increase cGMP signaling, resulting in activation and recruitment of brown adipocytes, and hence can enhance the energy combusting features of BAT. In this review we highlight recent results showing the physiological significance of cGMP signaling in BAT, as well as pharmacological options targeting cGMP signaling that bear a high potential to become BAT-centered therapies for the treatment of obesity.

Brown adipose tissue: The heat is on the heart

The study of brown adipose tissue (BAT) has gained significant scientific interest since the discovery of functional BAT in adult humans. The thermogenic properties of BAT are well recognized; however, data generated in the last decade in both rodents and humans reveal therapeutic potential for BAT against metabolic disorders and obesity. Here we review the current literature in light of a potential role for BAT in beneficially mediating cardiovascular health. We focus mainly on BAT's actions in obesity, vascular tone, and glucose and lipid metabolism. Furthermore, we discuss the recently discovered endocrine factors that have a potential beneficial role in cardiovascular health. These BAT-secreted factors may have a favorable effect against cardiovascular risk either through their metabolic role or by directly affecting the heart.

Molecular clock integration of brown adipose tissue formation and function

The circadian clock is an essential time-keeping mechanism that entrains internal physiology to environmental cues. Despite the well-established link between the molecular clock and metabolic homeostasis, an intimate interplay between the clock machinery and the metabolically active brown adipose tissue (BAT) is only emerging. Recently, we came to appreciate that the formation and metabolic functions of BAT, a key organ for body temperature maintenance, are under an orchestrated circadian clock regulation. Two complementary studies from our group uncover that the cell-intrinsic clock machinery exerts concerted control of brown adipogenesis with consequent impacts on adaptive thermogenesis, which adds a previously unappreciated temporal dimension to the regulatory mechanisms governing BAT development and function. The essential clock transcriptional activator, Bmal1, suppresses adipocyte lineage commitment and differentiation, whereas the clock repressor, Rev-erbα, promotes these processes. This newly discovered temporal mechanism in fine-tuning BAT thermogenic capacity may enable energy utilization and body temperature regulation in accordance with external timing signals during development and functional recruitment. Given the important role of BAT in whole-body metabolic homeostasis, pharmacological interventions targeting the BAT-modulatory activities of the clock circuit may offer new avenues for the prevention and treatment of metabolic disorders, particularly those associated with circadian dysregulation.

Perceived stress and anhedonia predict short-and long-term weight change, respectively, in healthy adults

OBJECTIVE

Perceived stress; emotional eating; anhedonia; depression and dietary restraint, hunger, and disinhibition have been studied as risk factors for obesity. However, the majority of studies have been cross-sectional and the directionality of these relationships remains unclear. In this longitudinal study, we assess their impact on future weight change.

METHODS

Psychological predictors of weight change in short- (6month) and long-term (>1year) periods were studied in 65 lean and obese individuals in two cohorts. Subjects participated in studies of food intake and metabolism that did not include any type of medication or weight loss interventions. They completed psychological questionnaires at baseline and weight change was monitored at follow-up visits.

RESULTS

At six months, perceived stress predicted weight gain (r(2)=0.23, P=0.02). There was a significant interaction (r(2)=.38, P=0.009) between perceived stress and positive emotional eating, such that higher scores in both predicted greater weight gain, while those with low stress but high emotional eating scores lost weight. For long-term, higher anhedonia scores predicted weight gain (r(2)=0.24, P=0.04). Depression moderated these effects such that higher scores in both predicted weight gain but higher depression and lower anhedonia scores predicted weight loss.

CONCLUSION

There are different behavioral determinants for short- and long-term weight change. Targeting perceived stress may help with short-term weight loss while depression and anhedonia may be better targets for long-term weight regulation.

AHNAK deficiency promotes browning and lipolysis in mice via increased responsiveness to β-adrenergic signalling

In adipose tissue, agonists of the β3-adrenergic receptor (ADRB3) regulate lipolysis, lipid oxidation, and thermogenesis. The deficiency in the thermogenesis induced by neuroblast differentiation-associated protein AHNAK in white adipose tissue (WAT) of mice fed a high-fat diet suggests that AHNAK may stimulate energy expenditure via development of beige fat. Here, we report that AHNAK deficiency promoted browning and thermogenic gene expression in WAT but not in brown adipose tissue of mice stimulated with the ADRB3 agonist CL-316243. Consistent with the increased thermogenesis, Ahnak(-/-) mice exhibited an increase in energy expenditure, accompanied by elevated mitochondrial biogenesis in WAT depots in response to CL-316243. Additionally, AHNAK-deficient WAT contained more eosinophils and higher levels of type 2 cytokines (IL-4/IL-13) to promote browning of WAT in response to CL-316243. This was associated with enhanced sympathetic tone in the WAT via upregulation of adrb3 and tyrosine hydroxylase (TH) in response to β-adrenergic activation. CL-316243 activated PKA signalling and enhanced lipolysis, as evidenced by increased phosphorylation of hormone-sensitive lipase and release of free glycerol in Ahnak(-/-) mice compared to wild-type mice. Overall, these findings suggest an important role of AHNAK in the regulation of thermogenesis and lipolysis in WAT via β-adrenergic signalling.

ECM microenvironment unlocks brown adipogenic potential of adult human bone marrow-derived MSCs

Key to realizing the diagnostic and therapeutic potential of human brown/brite adipocytes is the identification of a renewable, easily accessible and safe tissue source of progenitor cells, and an efficacious in vitro differentiation protocol. We show that macromolecular crowding (MMC) facilitates brown adipocyte differentiation in adult human bone marrow mesenchymal stem cells (bmMSCs), as evidenced by substantially upregulating uncoupling protein 1 (UCP1) and uncoupled respiration. Moreover, MMC also induced ‘browning’ in bmMSC-derived white adipocytes. Mechanistically, MMC creates a 3D extracellular matrix architecture enshrouding maturing adipocytes in a collagen IV cocoon that is engaged by paxillin-positive focal adhesions also at the apical side of cells, without contact to the stiff support structure. This leads to an enhanced matrix-cell signaling, reflected by increased phosphorylation of ATF2, a key transcription factor in UCP1 regulation. Thus, tuning the dimensionality of the microenvironment in vitro can unlock a strong brown potential dormant in bone marrow.

Gsα deficiency in adipose tissue improves glucose metabolism and insulin sensitivity without an effect on body weight

Gsα, the G protein that transduces receptor-stimulated cAMP generation, mediates sympathetic nervous system stimulation of brown adipose tissue (BAT) thermogenesis and browning of white adipose tissue (WAT), which are both potential targets for treating obesity, as well as lipolysis. We generated a mouse line with Gsα deficiency in mature BAT and WAT adipocytes (Ad-GsKO). Ad-GsKO mice had impaired BAT function, absent browning of WAT, and reduced lipolysis, and were therefore cold-intolerant. Despite the presence of these abnormalities, Ad-GsKO mice maintained normal energy balance on both standard and high-fat diets, associated with decreases in both lipolysis and lipid synthesis. In addition, Ad-GsKO mice maintained at thermoneutrality on a standard diet also had normal energy balance. Ad-GsKO mice had improved insulin sensitivity and glucose metabolism, possibly secondary to the effects of reduced lipolysis and lower circulating fatty acid binding protein 4 levels. Gsα signaling in adipose tissues may therefore affect whole-body glucose metabolism in the absence of an effect on body weight.

What We Talk About When We Talk About Evolution

Currently, the biologic sciences are a Tower of Babel, having become so highly specialized that one discipline cannot effectively communicate with another. A mechanism for evolution that integrates development and physiologic homeostasis phylogenetically has been identified—cell-cell interactions. By reducing this process to ligand-receptor interactions and their intermediate down-stream signaling partners, it is possible, for example, to envision the functional homologies between such seemingly disparate structures and functions as the lung alveolus and kidney glomerulus, the skin and brain, or the skin and lung. For example, by showing the continuum of the lung phenotype for gas exchange at the cell-molecular level, being selected for increased surface area by augmenting lung surfactant production and function in lowering surface tension, we have determined an unprecedented structural-functional continuum from proximate to ultimate causation in evolution. It is maintained that tracing the changes in structure and function that have occurred over both the short-term history of the organism (as ontogeny), and the long-term history of the organism (as phylogeny), and how the mechanisms shared in common can account for both biologic stability and novelty, will provide the key to understanding the mechanisms of evolution. We need to better understand evolution from its unicellular origins as the Big Bang of biology.

Mitochondrial oxidative metabolism and uncoupling proteins in the failing heart

Despite significant progress in cardiovascular medicine, myocardial ischemia and infarction, progressing eventually to the final end point heart failure (HF), remain the leading cause of morbidity and mortality in the USA. HF is a complex syndrome that results from any structural or functional impairment in ventricular filling or blood ejection. Ultimately, the heart's inability to supply the body's tissues with enough blood may lead to death. Mechanistically, the hallmarks of the failing heart include abnormal energy metabolism, increased production of reactive oxygen species (ROS) and defects in excitation-contraction coupling. HF is a highly dynamic pathological process, and observed alterations in cardiac metabolism and function depend on the disease progression. In the early stages, cardiac remodeling characterized by normal or slightly increased fatty acid (FA) oxidation plays a compensatory, cardioprotective role. However, upon progression of HF, FA oxidation and mitochondrial oxidative activity are decreased, resulting in a significant drop in cardiac ATP levels. In HF, as a compensatory response to decreased oxidative metabolism, glucose uptake and glycolysis are upregulated, but this upregulation is not sufficient to compensate for a drop in ATP production. Elevated mitochondrial ROS generation and ROS-mediated damage, when they overwhelm the cellular antioxidant defense system, induce heart injury and contribute to the progression of HF. Mitochondrial uncoupling proteins (UCPs), which promote proton leak across the inner mitochondrial membrane, have emerged as essential regulators of mitochondrial membrane potential, respiratory activity and ROS generation. Although the physiological role of UCP2 and UCP3, expressed in the heart, has not been clearly established, increasing evidence suggests that these proteins by promoting mild uncoupling could reduce mitochondrial ROS generation and cardiomyocyte apoptosis and ameliorate thereby myocardial function. Further investigation on the alterations in cardiac UCP activity and regulation will advance our understanding of their physiological roles in the healthy and diseased heart and also may facilitate the development of novel and more efficient therapies.

The brown fat secretome: metabolic functions beyond thermogenesis

Brown fat is highly active in fuel oxidation and dissipates chemical energy through uncoupling protein (UCP)1-mediated heat production. Activation of brown fat leads to increased energy expenditure, reduced adiposity, and lower plasma glucose and lipid levels, thus contributing to better homeostasis. Uncoupled respiration and thermogenesis have been considered to be responsible for the metabolic benefits of brown adipose tissue. Recent studies have demonstrated that brown adipocytes also secrete factors that act locally and systemically to influence fuel and energy metabolism. This review discusses the evidence supporting a thermogenesis-independent role of brown fat, particularly through its release of secreted factors, and their implications in physiology and therapeutic development.

Acute effects of acylated ghrelin on salbutamol-induced metabolic actions in humans

The aim of this study is to describe a potential modulatory effect of acute acylated ghrelin (AG) administration on the glucose, insulin, and free fatty acids (FFA) responses to salbutamol (SALBU). Six healthy young male volunteers underwent the following four testing sessions in random order at least 7 days apart: (a) acute AG administration (1.0 μg/kg i.v. as bolus at 0'); (b) SALBU infusion (0.06 μg/kg/min i.v. from -15' to +45'); (c) SALBU infusion+AG; and (d) isotonic saline infusion. Blood samples for glucose, insulin, and FFA levels were collected every 15 min. As expected, with respect to saline, SALBU infusion induced a remarkable increase in glucose (10.8±5.6 mmol/l×min; P<0.05), insulin (2436.8±556.9 pmol/l×min; P<0.05), and FFA (18.9±4.5 mmol/l×min; P<0.01) levels. A significant increase in glucose (7.4±3.9 mmol/l×min; P<0.05) and FFA levels (10.0±2.8 mmol/l×min; P<0.01) without significant variations in insulin levels were recorded after AG administration. Interestingly, the hyperglycemic effect of AG appeared to be significantly potentiated during SALBU infusion (26.7±4.8 mmol/l×min; P<0.05). On the other hand, the stimulatory effect of SALBU on insulin and FFA was not significantly modified by AG administration. The results of this study show that acute AG administration has a synergic effect with β2-adrenergic receptor activation by SALBU on blood glucose increase, suggesting that their pharmacological hyperglycemic action takes place via different mechanisms. On the other hand, AG has a negligible influence on the other pharmacological metabolic effects of SALBU infusion.

Cannabinoid type 1 (CB1) receptors on Sim1-expressing neurons regulate energy expenditure in male mice

The paraventricular nucleus of the hypothalamus (PVN) regulates energy balance by modulating not only food intake, but also energy expenditure (EE) and brown adipose tissue thermogenesis. To test the hypothesis that cannabinoid type 1 (CB1) receptor in PVN neurons might control these processes, we used the Cre/loxP system to delete CB1 from single-minded 1 (Sim1) neurons, which account for the majority of PVN neurons. On standard chow, mice lacking CB1 receptor in Sim1 neurons (Sim1-CB1-knockout [KO]) had food intake, body weight, adiposity, glucose metabolism, and EE comparable with wild-type (WT) (Sim1-CB1-WT) littermates. However, maintenance on a high-fat diet revealed a gene-by-diet interaction whereby Sim1-CB1-KO mice had decreased adiposity, improved insulin sensitivity, and increased EE, whereas feeding behavior was similar to Sim1-CB1-WT mice. Additionally, high-fat diet-fed Sim1-CB1-KO mice had increased mRNA expression of the β3-adrenergic receptor, as well as of uncoupling protein-1, cytochrome-c oxidase subunit IV and mitochondrial transcription factor A in the brown adipose tissue, all molecular changes suggestive of increased thermogenesis. Pharmacological studies using β-blockers suggested that modulation of β-adrenergic transmission play an important role in determining EE changes observed in Sim1-CB1-KO. Finally, chemical sympathectomy abolished the obesity-resistant phenotype of Sim1-CB1-KO mice. Altogether, these findings reveal a diet-dependent dissociation in the CB1 receptor control of food intake and EE, likely mediated by the PVN, where CB1 receptors on Sim1-positive neurons do not impact food intake but hinder EE during dietary environmental challenges that promote body weight gain.

Brown, beige, and white: the new color code of fat and its pharmacological implications

Brown adipose tissue (BAT) was previously regarded as a special type of fat relevant only for defending hibernating animals and newborns against a cold environment. Recently, BAT has received considerable attention following its (re)discovery in humans. Using glucose tracers, multiple laboratories independently found metabolically active BAT in adults. The enormous metabolic powers of BAT in animal models could make it an attractive target for antiobesity therapies in humans. Here, we review the present knowledge on the role of BAT in energy homeostasis and metabolism, focusing on signaling pathways and potential targets for novel therapeutics. We also shine light on ongoing debates, including those about the true color of brown fat in adults, as well as on the requirements for translation of basic research on BAT into clinical medicine.

Dissecting adipose tissue lipolysis: molecular regulation and implications for metabolic disease

Lipolysis is the process by which triglycerides (TGs) are hydrolyzed to free fatty acids (FFAs) and glycerol. In adipocytes, this is achieved by sequential action of adipose TG lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase. The activity in the lipolytic pathway is tightly regulated by hormonal and nutritional factors. Under conditions of negative energy balance such as fasting and exercise, stimulation of lipolysis results in a profound increase in FFA release from adipose tissue (AT). This response is crucial in order to provide the organism with a sufficient supply of substrate for oxidative metabolism. However, failure to efficiently suppress lipolysis when FFA demands are low can have serious metabolic consequences and is believed to be a key mechanism in the development of type 2 diabetes in obesity. As the discovery of ATGL in 2004, substantial progress has been made in the delineation of the remarkable complexity of the regulatory network controlling adipocyte lipolysis. Notably, regulatory mechanisms have been identified on multiple levels of the lipolytic pathway, including gene transcription and translation, post-translational modifications, intracellular localization, protein-protein interactions, and protein stability/degradation. Here, we provide an overview of the recent advances in the field of AT lipolysis with particular focus on the molecular regulation of the two main lipases, ATGL and HSL, and the intracellular and extracellular signals affecting their activity.

Vascular rarefaction mediates whitening of brown fat in obesity

Brown adipose tissue (BAT) is a highly vascularized organ with abundant mitochondria that produce heat through uncoupled respiration. Obesity is associated with a reduction of BAT function; however, it is unknown how obesity promotes dysfunctional BAT. Here, using a murine model of diet-induced obesity, we determined that obesity causes capillary rarefaction and functional hypoxia in BAT, leading to a BAT "whitening" phenotype that is characterized by mitochondrial dysfunction, lipid droplet accumulation, and decreased expression of Vegfa. Targeted deletion of Vegfa in adipose tissue of nonobese mice resulted in BAT whitening, supporting a role for decreased vascularity in obesity-associated BAT. Conversely, introduction of VEGF-A specifically into BAT of obese mice restored vascularity, ameliorated brown adipocyte dysfunction, and improved insulin sensitivity. The capillary rarefaction in BAT that was brought about by obesity or Vegfa ablation diminished β-adrenergic signaling, increased mitochondrial ROS production, and promoted mitophagy. These data indicate that overnutrition leads to the development of a hypoxic state in BAT, causing it to whiten through mitochondrial dysfunction and loss. Furthermore, these results link obesity-associated BAT whitening to impaired systemic glucose metabolism.

Glial cell line-derived neurotrophic factor protects against high-fat diet-induced obesity

Obesity is a growing epidemic with limited effective treatments. The neurotrophic factor glial cell line-derived neurotrophic factor (GDNF) was recently shown to enhance β-cell mass and improve glucose control in rodents. Its role in obesity is, however, not well characterized. In this study, we investigated the ability of GDNF to protect against high-fat diet (HFD)-induced obesity. GDNF transgenic (Tg) mice that overexpress GDNF under the control of the glial fibrillary acidic protein promoter and wild-type (WT) littermates were maintained on a HFD or regular rodent diet for 11 wk, and weight gain, energy expenditure, and insulin sensitivity were monitored. Differentiated mouse brown adipocytes and 3T3-L1 white adipocytes were used to study the effects of GDNF in vitro. Tg mice resisted the HFD-induced weight gain, insulin resistance, dyslipidemia, hyperleptinemia, and hepatic steatosis seen in WT mice despite similar food intake and activity levels. They exhibited significantly (P<0.001) higher energy expenditure than WT mice and increased expression in skeletal muscle and brown adipose tissue of peroxisome proliferator activated receptor-α and β1- and β3-adrenergic receptor genes, which are associated with increased lipolysis and enhanced lipid β-oxidation. In vitro, GDNF enhanced β-adrenergic-mediated cAMP release in brown adipocytes and suppressed lipid accumulation in differentiated 3T3L-1 cells through a p38MAPK signaling pathway. Our studies demonstrate a novel role for GDNF in the regulation of high-fat diet-induced obesity through increased energy expenditure. They show that GDNF and its receptor agonists may be potential targets for the treatment or prevention of obesity.