Novel aspects of brown adipose tissue biology

Temperature modulates PVN pre-sympathetic neurones via transient receptor potential ion channels

The paraventricular nucleus (PVN) of the hypothalamus plays a vital role in maintaining homeostasis and modulates cardiovascular function via autonomic pre-sympathetic neurones. We have previously shown that coupling between transient receptor potential cation channel subfamily V Member 4 (Trpv4) and small-conductance calcium-activated potassium channels (SK) in the PVN facilitate osmosensing, but since TRP channels are also thermosensitive, in this report we investigated the temperature sensitivity of these neurones. Methods: TRP channel mRNA was quantified from mouse PVN with RT-PCR and thermosensitivity of Trpv4-like PVN neuronal ion channels characterised with cell-attached patch-clamp electrophysiology. Following recovery of temperature-sensitive single-channel kinetic schema, we constructed a predictive stochastic mathematical model of these neurones and validated this with electrophysiological recordings of action current frequency. Results: 7 thermosensitive TRP channel genes were found in PVN punches. Trpv4 was the most abundant of these and was identified at the single channel level on PVN neurones. We investigated the thermosensitivity of these Trpv4-like channels; open probability (Po) markedly decreased when temperature was decreased, mediated by a decrease in mean open dwell times. Our neuronal model predicted that PVN spontaneous action current frequency (ACf) would increase as temperature is decreased and in our electrophysiological experiments, we found that ACf from PVN neurones was significantly higher at lower temperatures. The broad-spectrum channel blocker gadolinium (100 µM), was used to block the warm-activated, Ca2+-permeable Trpv4 channels. In the presence of gadolinium (100 µM), the temperature effect was largely retained. Using econazole (10 µM), a blocker of Trpm2, we found there were significant increases in overall ACf and the temperature effect was inhibited. Conclusion: Trpv4, the abundantly transcribed thermosensitive TRP channel gene in the PVN appears to contribute to intrinsic thermosensitive properties of PVN neurones. At physiological temperatures (37°C), we observed relatively low ACf primarily due to the activity of Trpm2 channels, whereas at room temperature, where most of the previous characterisation of PVN neuronal activity has been performed, ACf is much higher, and appears to be predominately due to reduced Trpv4 activity. This work gives insight into the fundamental mechanisms by which the body decodes temperature signals and maintains homeostasis.

A cardiac amino-terminal GRK2 peptide inhibits insulin resistance yet enhances maladaptive cardiovascular and brown adipose tissue remodeling in females during diet-induced obesity

Obesity and metabolic disorders are increasing in epidemic proportions, leading to poor outcomes including heart failure. With a growing recognition of the effect of adipose tissue dysfunction on heart disease, it is less well understood how the heart can influence systemic metabolic homeostasis. Even less well understood is sex differences in cardiometabolic responses. Previously, our lab investigated the role of the amino-terminus of GRK2 in cardiometabolic remodeling using transgenic mice with cardiac restricted expression of a short peptide, βARKnt. Male mice preserved insulin sensitivity, enhanced metabolic flexibility and adipose tissue health, elicited cardioprotection, and improved cardiac metabolic signaling. To examine the effect of cardiac βARKnt expression on cardiac and metabolic function in females in response to diet-induced obesity, we subjected female mice to high fat diet (HFD) to trigger cardiac and metabolic adaptive changes. Despite equivalent weight gain, βARKnt mice exhibited improved glucose tolerance and insulin sensitivity. However, βARKnt mice displayed a progressive reduction in energy expenditure during cold challenge after acute and chronic HFD stress. They also demonstrated reduced cardiac function and increased markers of maladaptive remodeling and tissue injury, and decreased or aberrant metabolic signaling. βARKnt mice exhibited reduced lipid deposition in the brown adipose tissue (BAT), but delayed or decreased markers of BAT activation and function suggested multiple mechanisms contributed to the decreased thermogenic capacity. These data suggest a non-canonical cardiac regulation of BAT lipolysis and function that highlights the need for studies elucidating the mechanisms of sex-specific responses to metabolic dysfunction.

The Role of NAD+ in Metabolic Regulation of Adipose Tissue: Implications for Obesity-Induced Insulin Resistance

Obesity-induced insulin resistance is among the key factors in the development of type 2 diabetes, atherogenic dyslipidemia and cardiovascular disease. Adipose tissue plays a key role in the regulation of whole-body metabolism and insulin sensitivity. In obesity, adipose tissue becomes inflamed and dysfunctional, exhibiting a modified biochemical signature and adipokine secretion pattern that promotes insulin resistance in peripheral tissues. An important hallmark of dysfunctional obese adipose tissue is impaired NAD+/sirtuin signaling. In this chapter, we summarize the evidence for impairment of the NAD+/sirtuin pathway in obesity, not only in white adipose tissue but also in brown adipose tissue and during the process of beiging, together with correlative evidence from human studies. We also describe the role of PARPs and CD38 as important NAD+ consumers and discuss findings from experimental studies that investigated potential NAD+ boosting strategies and their efficacy in restoring impaired NAD+ metabolism in dysfunctional obese adipose tissue. In sum, these studies suggest a critical role of NAD+ metabolism in adipose biology and provide a basis for the potential development of strategies to restore metabolic health in obesity.

A Cardiac Amino-Terminal GRK2 Peptide Inhibits Maladaptive Adipocyte Hypertrophy and Insulin Resistance During Diet-Induced Obesity

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Heart disease remains the leading cause of death, in part due to increasing diabetes and obesity, though the exact mechanisms linking these disorders are not fully understood.

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In a diet-induced obesity model, we found that cardiac expression of an amino-terminal peptide of GRK2, βARKnt, preserves systemic glucose tolerance and insulin sensitivity despite normal weight gain.

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βARKnt enhanced metabolic flexibility, increased energy expenditure, protected against maladaptive visceral adipocyte hypertrophy, and induced visceral fat browning.

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βARKnt further elicited cardioprotection and increased insulin-mediated AS160 signaling during metabolic stress.

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These data point to a noncanonical cardiac regulation of systemic metabolic homeostasis that may lead to new treatment modalities for metabolic syndrome.

Heart disease remains the leading cause of death, and mortality rates positively correlate with the presence of obesity and diabetes. Despite the correlation between cardiac and metabolic dysregulation, the mechanistic pathway(s) of interorgan crosstalk still remain undefined. This study reveals that cardiac-restricted expression of an amino-terminal peptide of GRK2 (βARKnt) preserves systemic and cardiac insulin responsiveness, and protects against adipocyte maladaptive hypertrophy in a diet-induced obesity model. These data suggest a cardiac-driven mechanism to ameliorate maladaptive cardiac remodeling and improve systemic metabolic homeostasis that may lead to new treatment modalities for cardioprotection in obesity and obesity-related metabolic syndromes.

Obesity I: Overview and molecular and biochemical mechanisms

Obesity is a chronic, relapsing condition characterized by excess body fat. Its prevalence has increased globally since the 1970s, and the number of obese and overweight people is now greater than those underweight. Obesity is a multifactorial condition, and as such, many components contribute to its development and pathogenesis. This is the first of three companion reviews that consider obesity. This review focuses on the genetics, viruses, insulin resistance, inflammation, gut microbiome, and circadian rhythms that promote obesity, along with hormones, growth factors, and organs and tissues that control its development. It shows that the regulation of energy balance (intake vs. expenditure) relies on the interplay of a variety of hormones from adipose tissue, gastrointestinal tract, pancreas, liver, and brain. It details how integrating central neurotransmitters and peripheral metabolic signals (e.g., leptin, insulin, ghrelin, peptide YY_3-36) is essential for controlling energy homeostasis and feeding behavior. It describes the distinct types of adipocytes and how fat cell development is controlled by hormones and growth factors acting via a variety of receptors, including peroxisome proliferator-activated receptor-gamma, retinoid X, insulin, estrogen, androgen, glucocorticoid, thyroid hormone, liver X, constitutive androstane, pregnane X, farnesoid, and aryl hydrocarbon receptors. Finally, it demonstrates that obesity likely has origins in utero. Understanding these biochemical drivers of adiposity and metabolic dysfunction throughout the life cycle lends plausibility and credence to the "obesogen hypothesis" (i.e., the importance of environmental chemicals that disrupt these receptors to promote adiposity or alter metabolism), elucidated more fully in the two companion reviews.

The Sick Adipose Tissue: New Insights Into Defective Signaling and Crosstalk With the Myocardium

Adipose tissue (AT) biology is linked to cardiovascular health since obesity is associated with cardiovascular disease (CVD) and positively correlated with excessive visceral fat accumulation. AT signaling to myocardial cells through soluble factors known as adipokines, cardiokines, branched-chain amino acids and small molecules like microRNAs, undoubtedly influence myocardial cells and AT function via the endocrine-paracrine mechanisms of action. Unfortunately, abnormal total and visceral adiposity can alter this harmonious signaling network, resulting in tissue hypoxia and monocyte/macrophage adipose infiltration occurring alongside expanded intra-abdominal and epicardial fat depots seen in the human obese phenotype. These processes promote an abnormal adipocyte proteomic reprogramming, whereby these cells become a source of abnormal signals, affecting vascular and myocardial tissues, leading to meta-inflammation, atrial fibrillation, coronary artery disease, heart hypertrophy, heart failure and myocardial infarction. This review first discusses the pathophysiology and consequences of adipose tissue expansion, particularly their association with meta-inflammation and microbiota dysbiosis. We also explore the precise mechanisms involved in metabolic reprogramming in AT that represent plausible causative factors for CVD. Finally, we clarify how lifestyle changes could promote improvement in myocardiocyte function in the context of changes in AT proteomics and a better gut microbiome profile to develop effective, non-pharmacologic approaches to CVD.

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.

Tissue-specific functional interaction between apolipoproteins A1 and E in cold-induced adipose organ mitochondrial energy metabolism

White (WAT) and brown (BAT) adipose tissue, the two main types of adipose organ, are responsible for lipid storage and non-shivering thermogenesis, respectively. Thermogenesis is a process mediated by mitochondrial uncoupling protein 1 (UCP1) which uncouples oxidative phosphorylation from ATP production, leading to the conversion of free fatty acids to heat. This process can be triggered by exposure to low ambient temperatures, caloric excess, and the immune system. Recently mitochondrial thermogenesis has also been associated with plasma lipoprotein transport system. Specifically, apolipoprotein (APO) E3 is shown to have a bimodal effect on WAT thermogenesis that is highly dependent on its site of expression. Similarly, APOE2 and APOE4 differentially affect BAT and WAT mitochondrial metabolic activity in processes highly modulated by APOA1. Furthermore, the absence of classical APOA1 containing HDL (APOA1-HDL), is associated with no measurable non-shivering thermogenesis in WAT of mice fed high fat diet. Based on these previous observations which indicate important regulatory roles for both APOA1 and APOE in adipose tissue mitochondrial metabolic activity, here we sought to investigate the potential roles of these apolipoproteins in BAT and WAT metabolic activation in mice, following stimulation by cold exposure (7 °C). Our data indicate that APOA1-HDL promotes metabolic activation of BAT only in the presence of very low levels (virtually undetectable) of APOE3-containing HDL (APOE3-HDL), which acts as an inhibitor in this process. In contrast, induction of WAT thermogenesis is subjected to a more complicated regulation which requires the combined presence of both APOA1-HDL and APOE3-HDL.

Insulin sensing by astrocytes is critical for normal thermogenesis and body temperature regulation

The important role of astrocytes in the central control of energy balance and glucose homeostasis has recently been recognized. Changes in thermoregulation can lead to metabolic dysregulation, but the role of astrocytes in this process is not yet clear. Therefore, we generated mice congenitally lacking insulin receptors (Ir) in astrocytes (IrKOGFAP mice) to investigate the involvement of astrocyte insulin signaling. IrKOGFAP mice displayed significantly lower energy expenditure and a strikingly lower basal and fasting body temperature. When exposed to cold, however, they were able to mount a thermogenic response. IrKOGFAP mice displayed sex differences in metabolic function and thermogenesis that may contribute to the development of obesity and type II diabetes as early as 2 months of age. While brown adipose tissue exhibited higher adipocyte size in both sexes, more apoptosis was seen in IrKOGFAP males. Less innervation and lower BAR3 expression levels were also observed in IrKOGFAP brown adipose tissue. These effects have not been reported in models of astrocyte Ir deletion in adulthood. In contrast, body weight and glucose regulatory defects phenocopied such models. These findings identify a novel role for astrocyte insulin signaling in the development of normal body temperature control and sympathetic activation of BAT. Targeting insulin signaling in astrocytes has the potential to serve as a novel target for increasing energy expenditure.

Dietary Silk Peptide Prevents High-Fat Diet-Induced Obesity and Promotes Adipose Browning by Activating AMP-Activated Protein Kinase in Mice

Obesity is associated with metabolic syndrome and other chronic diseases, and is caused when the energy intake is greater than the energy expenditure. We aimed to determine the mechanism whereby acid-hydrolyzed silk peptide (SP) prevents high-fat diet-induced obesity, and whether it induces browning and fatty acid oxidation (FAO) in white adipose tissue (WAT), using in vivo and ex vivo approaches. We determined the effects of dietary SP in high-fat diet-fed obese mice. The expression of adipose tissue-specific genes was quantified by western blotting, qRT-PCR, and immunofluorescence analysis. We also investigated whether SP directly induces browning in primarily subcutaneous WAT-derived adipocytes. Our findings demonstrate that SP has a browning effect in WAT by upregulating AMP-activated Protein Kinase (AMPK) phosphorylation and uncoupling protein 1 (UCP1) expression. SP also suppresses adipogenesis and promotes FAO, implying that it may have potential as an anti-obesity drug.

Inhibition of lipolysis: A novel explanation for the hypothermic actions of acetaminophen in non-febrile rodents

Acetaminophen is both widely used to treat children with fever and is also responsible for thousands being hospitalised annually. Historically the antipyretic actions of acetaminophen were attributed to the inhibition of cyclooxygenase (COX-1/2) enzymes and more recently a novel COX-1 variant (COX-3) located in the brain. However, the evidence for acetaminophen-mediated COX inhibition remains contentious. This study assesses the impact of acetaminophen and other putative COX-3 inhibitors on the release of fatty acids during lipolysis as an alternative mechanism by which antipyretics can reduce body temperature during fever. 3T3-L1 adipocytes, primary brown adipocytes and isolated mitochondria were exposed to COX-3 inhibitors and lipolysis and mitochondrial electron transport chain function assessed. Acetaminophen, aminopyrine and antipyrine at 1-10 mM caused a significant decrease (up to 70%; P < 0.01, from control) in lipolysis within 1, 3 and 24 h without affecting cell viability. The inhibition was observed regardless of where along its signalling pathway lipolysis was stimulated. All three compounds were found to significantly attenuate mitochondrial function by up to 30% for complex I and 40% for complex II (P < 0.01, from control). These novel observations combined with the known limited inhibition of the COX enzymes by acetaminophen suggest both the antipyretic and hypothermia induced by acetaminophen and related compounds could be attributed to the direct inhibition of lipolysis and mitochondrial function, rather than cyclooxygenase inhibition centrally. Further these observations could provide new drug targets for reducing fever with the added bonus of fewer individuals being hospitalized by accidental acetaminophen overdose.

Isoform and tissue dependent impact of apolipoprotein E on adipose tissue metabolic activation: The role of apolipoprotein A1

Adipose organ is made of white (WAT) and brown (BAT) adipose tissue which are primarily responsible for lipid storage and energy production (heat and ATP) respectively. Metabolic activation of WAT may ascribe to this tissue characteristics of BAT, namely non-shivering thermogenesis and ATP production. Recent data indicate that apolipoproteins E (APOE) and A1 (APOA1) regulate WAT mitochondrial metabolic activation. Here, we investigated the functional cross-talk between natural human APOE2 and APOE4 isoforms with APOA1 in this process, using Apoe2^knock-in and Apoe4^knock-in mice. At baseline when Apoe2^knock-in and Apoe4^knock-in mice express both APOE and Apoa1, the Apoe2^knock-in strain appears to have higher mitochondrial oxidative phosphorylation levels and non-shivering thermogenesis in WAT compared to Apoe4^knock-in mice. When mice were switched to a high-fat diet for 18 weeks, circulating levels of endogenous Apoa1 in Apoe2^knock-in mice became barely detectable though significant levels of APOE2 were still present. This change was accompanied by a significant reduction in WAT mitochondrial Ucp1 expression while BAT Ucp1 was unaffected. Ectopic APOA1 expression in Apoe2^knock-in animals potently stimulated WAT but not BAT mitochondrial Ucp1 expression providing further evidence that APOA1 potently stimulates WAT non-shivering thermogenesis in the presence of APOE2. Ectopic expression of APOA1 in Apoe4^knock-in mice stimulated BAT but no WAT mitochondrial Ucp1 levels, suggesting that in the presence of APOE4, APOA1 is a trigger of BAT non-shivering thermogenesis. Overall, our data identified a tissue-specific role of the natural human APOE2 and APOE4 isoforms in WAT- and BAT-metabolic activation respectively, that appears dependent on circulating APOA1 levels.

Rats that are predisposed to excessive obesity show reduced (leptin-induced) thermoregulation even in the preobese state

Both feeding behavior and thermogenesis are regulated by leptin. The sensitivity to leptin's anorexigenic effects on chow diet was previously shown to predict the development of diet-induced obesity. In this study, we determined whether the sensitivity to leptin's anorexigenic effects correlates with leptin's thermogenic response, and if this response is exerted at the level of the dorsomedial hypothalamus (DMH), a brain area that plays an important role in thermoregulation. Based on the feeding response to injected leptin on a chow diet, rats were divided into leptin-sensitive (LS) and leptin-resistant (LR) groups. The effects of leptin on core body, brown adipose tissue (BAT) and tail temperature were compared after intravenous versus intra-DMH leptin administration. After intravenous leptin injection, LS rats increased their BAT thermogenesis and reduced heat loss via the tail, resulting in a modest increase in core body temperature. The induction of these thermoregulatory mechanisms with intra-DMH leptin was smaller, but in the same direction as with intravenous leptin administration. In contrast, LR rats did not show any thermogenic response to either intravenous or intra-DMH leptin. These differences in the thermogenic response to leptin were associated with a 1°C lower BAT temperature and reduced UCP1 expression in LR rats under ad libitum feeding. The preexisting sensitivity to the anorexigenic effects of leptin, a predictor for obesity, correlates with the sensitivity to the thermoregulatory effects of leptin, which appears to be exerted, at least in part, at the level of the DMH.

How the association between obesity and inflammation may lead to insulin resistance and cancer

BACKGROUND AND OBJECTIVES

Obesity is associated with metabolic dysfunction and over nutrition. Increased body mass index and obesity are strongly amalgamated with changes in the physiological function of adipose tissue, leading to altered secretion of adipocytokines, inflammatory mediators release as well as chronic inflammation and insulin resistance. The purposes of this study were to review the evidence of how obesity and inflammation may lead to insulin resistance and cancer.

SUMMARY

Recent findings suggested that increased level of inflammatory mediators in obesity, plays an introductory and cabalistic role in the development of different types of inflammatory disorders including type 2 diabetes mellitus. Link between elevated body mass index and type 2 diabetes mellitus (T2DM). Several of the factors-such as increased levels of leptin, plasminogen activator inhibitor-1, decreased levels of adiponectin, insulin resistance, chronic inflammation etc. consequently result in carcinogenesis and carcinogenic progression too.

CONCLUSION

This review summarizes how cytokine production in adipose tissue of obese subject creates a chronic inflammatory environment that favors tumor cell motility and invasion to enhance the metastatic potential of tumor cells. High levels of cytokine in the circulation of affected individuals have been associated with a significantly worse outcome. This article also reconnoiters the mechanisms that link obesity to numerous disorders such as inflammation, diabetes, cancers and most specifically combine these processes in a single image. Understanding these mechanisms may assist to understand the consequences of obesity.

Association between circulating irisin levels and epicardial fat in patients with treatment-naïve overt hyperthyroidism

BACKGROUND

Hyperthyroidism is associated with increased metabolic activity and thermogenesis. Irisin is a key molecule in thermogenesis and energy expenditure via adipose tissue browning. Epicardial fat was previously defined as brown-like fat. Thus, here we aimed to evaluate the association between serum irisin level and epicardial fat thickness (EFT) in patients with hyperthyroidism.

METHODS

A total of 25 hyperthyroid patients and 24 age-, sex- and BMI-matched healthy controls were enrolled. Serum irisin levels, thyroid hormone levels, and body compositions were compared. EFT was measured via transthoracic echocardiography.

RESULTS

Serum irisin level and EFT were significantly higher in the hyperthyroid group (p < 0.001 and p = 0.001, respectively). The distributions of fat-free mass, muscle mass and fat mass were similar between the study groups. Serum irisin level was negatively correlated with TSH (p < 0.001) and positively correlated with fT3 (p < 0.001), fT4 (p < 0.001) and TSH receptor antibody (p = 0.002) levels and EFT (p = 0.001). In multivariate linear regression analysis, TSH (β = -0.475, p < 0.001) and EFT (β = 0.290, p = 0.023) levels were significantly associated with serum irisin levels.

CONCLUSIONS

An increased serum irisin level associated with EFT might contribute to metabolic derangement in hyperthyroidism. Further studies are needed to elucidate whether irisin levels and EFT are affected by hyperthyroidism or vice versa.

Anatomical projections of the dorsomedial hypothalamus to the periaqueductal grey and their role in thermoregulation: a cautionary note

The DMH is known to regulate brown adipose tissue (BAT) thermogenesis via projections to sympathetic premotor neurons in the raphe pallidus, but there is evidence that the periaqueductal gray (PAG) is also an important relay in the descending pathways regulating thermogenesis. The anatomical projections from the DMH to the PAG subdivisions and their function are largely elusive, and may differ per anterior-posterior level from bregma. We here aimed to investigate the anatomical projections from the DMH to the PAG along the entire anterior-posterior axis of the PAG, and to study the role of these projections in thermogenesis in Wistar rats. Anterograde channel rhodopsin viral tracing showed that the DMH projects especially to the dorsal and lateral PAG. Retrograde rabies viral tracing confirmed this, but also indicated that the PAG receives a diffuse input from the DMH and adjacent hypothalamic subregions. We aimed to study the role of the identified DMH to PAG projections in thermogenesis in conscious rats by specifically activating them using a combination of canine adenovirus-2 (CAV2Cre) and Cre-dependent designer receptor exclusively activated by designer drugs (DREADD) technology. Chemogenetic activation of DMH to PAG projections increased BAT temperature and core body temperature, but we cannot exclude the possibility that at least some thermogenic effects were mediated by adjacent hypothalamic subregions due to difficulties in specifically targeting the DMH and distinct subdivisions of the PAG because of diffuse virus expression. To conclude, our study shows the complexity of the anatomical and functional connection between the hypothalamus and the PAG, and some technical challenges in studying their connection.

Association between active brown adipose tissue and coronary artery calcification in healthy men

Aim: We compared various clinical factors between persons with active brown adipose tissue (ABAT) and matched controls, and investigated the relationship between the presence of ABAT and coronary artery calcification (CAC) with respect to arterial inflammation.Methods: We retrospectively reviewed fluorine-18-labeled fluoro- 2-deoxy-D-glucose (F-18 FDG) positron emission tomography/computed tomography (PET/CT) data from men who underwent general health check-ups. Sixty-seven men with ABAT were identified and were matched with controls at a 1:1 ratio. Peripheral blood samples were obtained and the levels of various laboratory parameters were measured just before FDG PET/CT studies. Arterial inflammation was measured in the ascending aorta, venous mean standardized uptake value (SUV) was collected from the superior vena cava as FDG uptake on PET, and background-corrected SUV was calculated as the target-to-background ratio (TBR) and blood- subtracted SUVmax (bsSUVmax). CAC was as-sessed using CT images acquired from a PET/ CT scanner.Results: The prevalence of fatty liver (p = 0.048) and CAC (p = 0.026) was lower in men with ABAT compared to matched controls. Arterial SUVmax (1.72 ± 0.23 vs. 1.88 ± 0.23, p < 0.001), TBR (1.18 ± 0.14 vs. 1.29 ± 0.13, p < 0.001), and bsSUVmax (0.25 ± 0.18 vs. 0.41 ± 0.16, p < 0.001) were significantly lower in men with ABAT. ABAT (odds ratio [OR] = 0.19, p=0.024) and high- density lipoprotein cholesterol (OR = 0.95, p = 0.037) were independent factors associated with CAC according to multiple logistic regression analysis.Conclusion: ABAT is associated with down-regulated arterial inflammation and may exert a protective effect against the development of atherosclerosis.

Karavia E, Constantinou C, Hatziri A, Kalogeropoulou C, Xepapadaki E, Zvintzou E. Apolipoprotein E in diet-induced obesity: a paradigm shift from conventional perception

Apolipoprotein E (APOE) is a major protein component of peripheral and brain lipoprotein transport systems. APOE in peripheral circulation does not cross blood brain barrier or blood cerebrospinal fluid barrier. As a result, peripheral APOE expression does not affect brain APOE levels and vice versa. Numerous epidemiological studies suggest a key role of peripherally expressed APOE in the development and progression of coronary heart disease while brain APOE has been associated with dementia and Alzheimer's disease. More recent studies, mainly in experimental mice, suggested a link between Apoe and morbid obesity. According to the latest findings, expression of human apolipoprotein E3 (APOE3) isoform in the brain of mice is associated with a potent inhibition of visceral white adipose tissue (WAT) mitochondrial oxidative phosphorylation leading to significantly reduced substrate oxidation, increased fat accumulation and obesity. In contrast, hepatically expressed APOE3 is associated with a notable shift of substrate oxidation towards non-shivering thermogenesis in visceral WAT mitochondria, leading to resistance to obesity. These novel findings constitute a major paradigm shift from the widely accepted perception that APOE promotes obesity via receptor-mediated postprandial lipid delivery to WAT. Here, we provide a critical review of the latest facts on the role of APOE in morbid obesity.

The color of fat and its central role in the development and progression of metabolic diseases

Excess caloric intake does not always translate to an expansion of the subcutaneous adipose tissue (SAT) and increase in fat mass. It is now recognized that adipocyte type (white, WAT, or brown, BAT), size (large vs. small) and metabolism are important factors for the development of cardiometabolic diseases. When the subcutaneous adipose tissue is not able to expand in response to increased energy intake the excess substrate is stored as visceral adipose tissue or as ectopic fat in tissues as muscle, liver and pancreas. Moreover, adipocytes become dysfunctional (adiposopathy, or sick fat), adipokines secretion is increased, fat accumulates in ectopic sites like muscle and liver and alters insulin signaling, increasing the demand for insulin secretion. Thus, there are some subjects that despite having normal weight have the metabolic characteristics of the obese (NWMO), while some obese expand their SAT and remain metabolically healthy (MHO). In this paper we have reviewed the recent findings that relate the metabolism of adipose tissue and its composition to metabolic diseases. In particular, we have discussed the possible role of dysfunctional adipocytes and adipose tissue resistance to the antilipolytic effect of insulin on the development of impaired glucose metabolism. Finally we have reviewed the possible role of BAT vs. WAT in the alteration of lipid and glucose metabolism and the recent studies that have tried to stimulate browning in human adipose tissue.

Ventromedial hypothalamic melanocortin receptor activation: regulation of activity energy expenditure and skeletal muscle thermogenesis

KEY POINTS

The ventromedial hypothalamus (VMH) and the central melanocortin system both play vital roles in regulating energy balance by modulating energy intake and utilization. Recent evidence suggests that activation of the VMH alters skeletal muscle metabolism. We show that intra-VMH melanocortin receptor activation increases energy expenditure and physical activity, switches fuel utilization to fats, and lowers work efficiency such that excess calories are dissipated by skeletal muscle as heat. We also show that intra-VMH melanocortin receptor activation increases sympathetic nervous system outflow to skeletal muscle. Intra-VMH melanocortin receptor activation also induced significant changes in the expression of mediators of energy expenditure in muscle. These results support the role of melanocortin receptors in the VMH in the modulation of skeletal muscle metabolism.

ABSTRACT

The ventromedial hypothalamus (VMH) and the brain melanocortin system both play vital roles in increasing energy expenditure (EE) and physical activity, decreasing appetite and modulating sympathetic nervous system (SNS) outflow. Because of recent evidence showing that VMH activation modulates skeletal muscle metabolism, we propose the existence of an axis between the VMH and skeletal muscle, modulated by brain melanocortins, modelled on the brain control of brown adipose tissue. Activation of melanocortin receptors in the VMH of rats using a non-specific agonist melanotan II (MTII), compared to vehicle, increased oxygen consumption and EE and decreased the respiratory exchange ratio. Intra-VMH MTII enhanced activity-related EE even when activity levels were held constant. MTII treatment increased gastrocnemius muscle heat dissipation during controlled activity, as well as in the home cage. Compared to vehicle-treated rats, rats with intra-VMH melanocortin receptor activation had higher skeletal muscle norepinephrine turnover, indicating an increased SNS drive to muscle. Lastly, intra-VMH MTII induced mRNA expression of muscle energetic mediators, whereas short-term changes at the protein level were primarily limited to phosphorylation events. These results support the hypothesis that melanocortin peptides act in the VMH to increase EE by lowering the economy of activity via the enhanced expression of mediators of EE in the periphery including skeletal muscle. The data are consistent with the role of melanocortins in the VMH in the modulation of skeletal muscle metabolism.

Metabolic interplay between white, beige, brown adipocytes and the liver

In mammalian evolution, three types of adipocytes have developed, white, brown and beige adipocytes. White adipocytes are the major constituents of white adipose tissue (WAT), the predominant store for energy-dense triglycerides in the body that are released as fatty acids during catabolic conditions. The less abundant brown adipocytes, the defining parenchymal cells of brown adipose tissue (BAT), internalize triglycerides that are stored intracellularly in multilocular lipid droplets. Beige adipocytes (also known as brite or inducible brown adipocytes) are functionally very similar to brown adipocytes and emerge in specific WAT depots in response to various stimuli including sustained cold exposure. The activation of brown and beige adipocytes (together referred to as thermogenic adipocytes) causes both the hydrolysis of stored triglycerides as well as the uptake of lipids and glucose from the circulation. Together, these fuels are combusted for heat production to maintain body temperature in mammals including adult humans. Given that heating by brown and beige adipocytes is a very-well controlled and energy-demanding process which entails pronounced shifts in energy fluxes, it is not surprising that an intensive interplay exists between the various adipocyte types and parenchymal liver cells, and that this influences systemic metabolic fluxes and endocrine networks. In this review we will emphasize the role of hepatic factors that regulate the metabolic activity of white and thermogenic adipocytes. In addition, we will discuss the relevance of lipids and hormones that are secreted by white, brown and beige adipocytes regulating liver metabolism in order to maintain systemic energy metabolism in health and disease.

Voluntary exercise adapts the hypothalamus-pituitary-thyroid axis in male rats

The hypothalamic-pituitary thyroid (HPT) axis modulates energy homeostasis. Its activity decreases in conditions of negative energy balance but the effects of chronic exercise on the axis are controversial and unknown at hypothalamic level. Wistar male rats were exposed for up to 14 days to voluntary wheel running (WR), or pair-feeding (PF; 18% food restriction), or to repeated restraint (RR), a mild stressor. WR and RR diminished food intake; body weight gain decreased in the 3 experimental groups, but WAT mass and serum leptin more intensely in the WR group. WR, but not RR, produced a delayed inhibition of central markers of HPT axis activity. At day 14, in WR rats paraventricular nucleus-pro-TRH mRNA and serum TSH levels decreased, anterior pituitary TRH-receptor 1 mRNA levels increased, but serum thyroid hormone levels were unaltered, which is consistent with decreased secretion of TRH and clearance of thyroid hormones. A similar pattern was observed if WR animals were euthanized during their activity phase. In contrast, in PF animals the profound drop of HPT axis activity included decreased serum T3 levels and hepatic deiodinase 1 activity; these changes were correlated with an intense increase in serum corticosterone levels. WR effects on HPT axis were not associated with changes in the activity of the hypothalamic-pituitary adrenal axis, but correlated positively with serum leptin levels. These data demonstrate that voluntary WR adapts the status of the HPT axis, through pathways that are distinct from those observed during food restriction or repeated stress.

NAG-1/GDF-15 prevents obesity by increasing thermogenesis, lipolysis and oxidative metabolism

Objective

Obesity is a major health problem associated with high morbidity and mortality. NSAID activated gene, (NAG-1) is a TGF-β superfamily member reported to alter adipose tissue levels in mice. We investigated whether hNAG-1 acts as a regulator of adiposity and energy metabolism.

Design/Subjects

hNAG-1 mice, ubiquitously expressing hNAG-1, were placed on a control or high fat diet (HFD) for 12 weeks. hNAG-1 expressing B16/F10 melanoma cells were used in a xenograft model to deliver hNAG-1 to obese C57BL/6 mice.

Results

As compared to wild-type littermates, transgenic hNAG-1 mice have less white fat and brown fat despite equivalent food intake, improved glucose tolerance, lower insulin levels and are resistant to dietary- and genetic-induced obesity. hNAG-1 mice are more metabolically active with higher energy expenditure. Obese C57BL/6 mice treated with hNAG-1 expressing xenografts show decreases in adipose tissue and serum insulin levels. hNAG-1 mice and obese mice treated with hNAG-1 expressing xenografts show increased thermogenic gene expression (UCP1, PGC1α, ECH1, Cox8b, Dio2, Cyc1, PGC1β, PPARα, Elvol3) in brown adipose tissue (BAT) and increased expression of lipolytic genes (Adrb3, ATGL, HSL) in both white adipose tissue (WAT) and BAT, consistent with higher energy metabolism

Conclusion

hNAG-1 modulates metabolic activity by increasing the expression of key thermogenic and lipolytic genes in BAT and WAT. hNAG-1 appears to be a novel therapeutic target in preventing and treating obesity and insulin resistance.

Adipose tissue browning and metabolic health

Accumulation of excess white adipose tissue (WAT) has deleterious consequences for metabolic health. The activation of brown adipose tissue (BAT), the primary organ for heat production, confers beneficial effects on adiposity, insulin resistance and hyperlipidaemia, at least in mice. As the amount of metabolically active BAT seems to be particularly low in patients with obesity or diabetes mellitus who require immediate therapy, new avenues are needed to increase the capacity for adaptive thermogenesis. In this light, we review the findings that BAT in human adults might consist of not only classic brown adipocytes but also inducible brown adipocytes (also called beige, brown-in-white, or brite adipocytes), which are phenotypically distinct from both white and brown adipocytes. Stimulating the development of beige adipocytes in WAT (so called 'browning') might reduce adverse effects of WAT and could help to improve metabolic health. This article focuses on the development and regulatory control of beige adipocytes at the transcriptional and hormonal levels. Emerging insights into the metabolic role of beige adipocytes are also discussed, along with the developments that can be expected from these promising targets for therapy of metabolic disease in the future.