Brown adipose tissue thermogenesis in the resistance to and reversal of obesity: A potential new mechanism contributing to the metabolic benefits of proglucagon-derived peptides

Spotlight on the Mechanism of Action of Semaglutide

Initially intended to control blood glucose levels in patients with type 2 diabetes, semaglutide, a potent glucagon-like peptide 1 analogue, has been established as an effective weight loss treatment by controlling appetite. Integrating the latest clinical trials, semaglutide in patients with or without diabetes presents significant therapeutic efficacy in ameliorating cardiometabolic risk factors and physical functioning, independent of body weight reduction. Semaglutide may modulate adipose tissue browning, which enhances human metabolism and exhibits possible benefits in skeletal muscle degeneration, accelerated by obesity and ageing. This may be attributed to anti-inflammatory, mitochondrial biogenesis, antioxidant and autophagy-regulating effects. However, most of the supporting evidence on the mechanistic actions of semaglutide is preclinical, demonstrated in rodents and not actually confirmed in humans, therefore warranting caution in the interpretation. This article aims to explore potential innovative molecular mechanisms of semaglutide action in restoring the balance of several interlinking aspects of metabolism, pointing to distinct functions in inflammation and oxidative stress in insulin-sensitive musculoskeletal and adipose tissues. Moreover, possible applications in protection from infections and anti-aging properties are discussed. Semaglutide enhancement of the core molecular mechanisms involved in the progress of obesity and diabetes, although mostly preclinical, may provide a framework for future research applications in human diseases overall.

Thermoregulatory responses, heart rate, and the susceptibility to anxiety in obese animals subjected to stress

Obesity and stress are related to cardiovascular diseases. Rats fed a high-fat diet (HFD) show increased cardiovascular reactivity to emotional stress and altered defensive behavioral responses. Indeed, changes in thermoregulatory responses in an aversive environment are observed in these animals. However, studies aimed at clarifying the physiological mechanisms linking obesity, stress hyperreactivity and behavioral changes are needed. The aim of this study was to evaluate the changes in thermoregulatory responses, heart rate, and the susceptibility to anxiety in obese animals subjected to stress. Nine-week high-fat diet protocol was effective in inducing obesity by increasing weight gain, fat mass, adiposity index, white epididymal, retroperitoneal, inguinal and brown adipose tissue. Animals induced to obesity and subjected to stress (HFDS group) by the intruder animal method showed increases in heart rate (HR), core body temperature and tail temperature. HFDS showed an increase in the first exposure to the closed arm (anxiety-like behavior) in elevated T-Maze (ETM). The groups did not differ with respect to panic behavior assessed in the ETM and locomotor activity in the open field test. Our study shows that HFDS animals presented increased reactivity to stress with higher stress hyperthermia and anxious behavior. Thus, our results present relevant information regarding stress responsiveness and behavioral changes in obese animals.

Genetic advancements in obesity management and CRISPR-Cas9-based gene editing system

Human genome research has reached new heights in the recent decade thanks to a major advance in genome editing. Genome editing enables scientists to understand better the functions of a single gene and its impact on a wide range of diseases. In brief, genome editing is a technique for introducing alterations into specific DNA sequences, such as insertions, deletions, or base substitutions. Several methods are adopted to perform genome editing and clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9) systems. Unfortunately, despite substantial progress in understanding the molecular pathways behind obesity, anti-obesity medications are now ineffective. If you are obese, a 10% weight decrease would be preferable to healthy body weight for most people. CRISPR-Cas9, on the other hand, has been shown to reduce body weight by an astonishing 20%. Hence, this updated review elaborates on the molecular basis of obesity, risk factors, types of gene therapy, possible mechanisms, and advantages of the CRISPR-Cas9 system over other methods.

Association of high-fat diet with neuroinflammation, anxiety-like defensive behavioral responses, and altered thermoregulatory responses in male rats

Overweight and obesity are a worldwide pandemic affecting billions of people. These conditions have been associated with a chronic low-grade inflammatory state that is recognized as a risk factor for a range of somatic diseases as well as neurodevelopmental disorders, anxiety disorders, trauma- and stressor-related disorders, and affective disorders. We previously reported that the ingestion of a high-fat diet (HFD; 45% fat kcal/g) for nine weeks was capable of inducing obesity in rats in association with increased reactivity to stress and increased anxiety-related defensive behavior. In this study, we conducted a nine-week diet protocol to induce obesity in rats, followed by investigation of anxiety-related defensive behavioral responses using the elevated T-maze (ETM), numbers of FOS-immunoreactive cells after exposure of rats to the avoidance or escape task of the ETM, and neuroinflammatory cytokine expression in hypothalamic and amygdaloid nuclei. In addition, we investigated stress-induced cutaneous thermoregulatory responses during exposure to an open-field (OF). Here we demonstrated that nine weeks of HFD intake induced obesity, in association with increased abdominal fat pad weight, increased anxiety-related defensive behavioral responses, and increased proinflammatory cytokines in hypothalamic and amygdaloid nuclei. In addition, HFD exposure altered avoidance- or escape task-induced FOS-immunoreactivity within brain structures involved in control of neuroendocrine, autonomic, and behavioral responses to aversive stimuli, including the basolateral amygdala (BLA) and dorsomedial (DMH), paraventricular (PVN) and ventromedial (VMH) hypothalamic nuclei. Furthermore, rats exposed to HFD, relative to control diet-fed rats, responded with increased tail skin temperature at baseline and throughout exposure to an open-field apparatus. These data are consistent with the hypothesis that HFD induces neuroinflammation, alters excitability of brain nuclei controlling neuroendocrine, autonomic, and behavioral responses to stressful stimuli, and enhances stress reactivity and anxiety-like defensive behavioral responses.

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.

Brown adipose tissue: what have we learned since its recent identification in human adults

Brown adipose tissue, an essential organ for thermoregulation in small and hibernating mammals due to its mitochondrial uncoupling capacity, was until recently considered to be present in humans only in newborns. The identification of brown adipose tissue in adult humans since the development and use of positron emission tomography marked with 18-fluorodeoxyglucose (PET-FDG) has raised a series of doubts and questions about its real importance in our metabolism. In this review, we will discuss what we have learnt since its identification in humans as well as both new and old concepts, some of which have been marginalized for decades, such as diet-induced thermogenesis.

Mice Deficient in Proglucagon-Derived Peptides Exhibit Glucose Intolerance on a High-Fat Diet but Are Resistant to Obesity

Homozygous glucagon-GFP knock-in mice (Gcg^gfp/gfp) lack proglucagon derived-peptides including glucagon and GLP-1, and are normoglycemic. We have previously shown that Gcg^gfp/gfp show improved glucose tolerance with enhanced insulin secretion. Here, we studied glucose and energy metabolism in Gcg^gfp/gfp mice fed a high-fat diet (HFD). Male Gcg^gfp/gfp and Gcg^gfp/+ mice were fed either a normal chow diet (NCD) or an HFD for 15–20 weeks. Regardless of the genotype, mice on an HFD showed glucose intolerance, and Gcg^gfp/gfp mice on HFD exhibited impaired insulin secretion whereas Gcg^gfp/+ mice on HFD exhibited increased insulin secretion. A compensatory increase in β-cell mass was observed in Gcg^gfp/+mice on HFD, but not in Gcg^gfp/gfp mice on the same diet. Weight gain was significantly lower in Gcg^gfp/gfp mice than in Gcg^gfp/+mice. Oxygen consumption was enhanced in Gcg^gfp/gfp mice compared to Gcg^gfp/+ mice on an HFD. HFD feeding significantly increased uncoupling protein 1 mRNA expression in brown adipose and inguinal white adipose tissues of Gcg^gfp/gfp mice, but not of Gcg^gfp/+mice. Treatment with the glucagon-like peptide-1 receptor agonist liraglutide (200 mg/kg) improved glucose tolerance in Gcg^gfp/gfp mice and insulin content in Gcg^gfp/gfp and Gcg^gfp/+ mice was similar after liraglutide treatment. Our findings demonstrate that Gcg^gfp/gfp mice develop diabetes upon HFD-feeding in the absence of proglucagon-derived peptides, although they are resistant to diet-induced obesity.

Contribution of brown adipose tissue activity to the control of energy balance by GLP-1 receptor signalling in mice

AIMS/HYPOTHESIS

We assessed the contribution of glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) signalling to thermogenesis induced by high-fat diet (HFD) consumption. Furthermore, we determined whether brown adipose tissue (BAT) activity contributes to weight loss induced by chronic subcutaneous treatment with the GLP-1R agonist, liraglutide, in a model of diet-induced obesity.

METHODS

Metabolic phenotyping was performed using indirect calorimetry in wild-type (WT) and Glp1r-knockout (KO) mice during chow and HFD feeding at room temperature and at thermoneutrality. In a separate study, we investigated the contribution of BAT thermogenic capacity to the weight lowering effect induced by GLP-1 mimetics by administering liraglutide (10 or 30 nmol kg(-1) day(-1) s.c.) to diet-induced obese (DIO) mice for 6 or 4 weeks, respectively. In both studies, animals were subjected to a noradrenaline (norepinephrine)-stimulated oxygen consumption [Formula: see text] test.

RESULTS

At thermoneutrality, HFD-fed Glp1r-KO mice had similar energy expenditure (EE) compared with HFD-fed WT controls. However, HFD-fed Glp1r-KO mice exhibited relatively less EE when housed at a cooler standard room temperature, and had relatively lower [Formula: see text] in response to a noradrenaline challenge, which is consistent with impaired BAT thermogenic capacity. In contrast to the loss of function model, chronic peripheral liraglutide treatment did not increase BAT activity as determined by noradrenaline-stimulated [Formula: see text] and BAT gene expression.

CONCLUSIONS/INTERPRETATION

These data suggest that although endogenous GLP-1R signalling contributes to increased BAT thermogenesis, this mechanism does not play a significant role in the food intake-independent body weight lowering effect of the GLP-1 mimetic liraglutide in DIO mice.

GLP-1 based therapeutics: simultaneously combating T2DM and obesity

Glucagon-like peptide-1 (GLP-1) enhances meal-related insulin secretion, which lowers blood glucose excursions. In addition to its incretin action, GLP-1 acts on the GLP-1 receptor (GLP-1R) in the brain to suppress feeding. These combined actions of GLP-1R signaling cause improvements in glycemic control as well as weight loss in type II diabetes (T2DM) patients treated with GLP-1R agonists. This is a superior advantage of GLP-1R pharmaceuticals as many other drugs used to treat T2DM are weight neutral or actual cause weight gain. This review summarizes GLP-1R action on energy and glucose metabolism, the effectiveness of current GLP-1R agonists on weight loss in T2DM patients, as well as GLP-1R combination therapies.

The brain and brown fat

Brown adipose tissue (BAT) is a specialized organ responsible for thermogenesis, a process required for maintaining body temperature. BAT is regulated by the sympathetic nervous system (SNS), which activates lipolysis and mitochondrial uncoupling in brown adipocytes. For many years, BAT was considered to be important only in small mammals and newborn humans, but recent data have shown that BAT is also functional in adult humans. On the basis of this evidence, extensive research has been focused on BAT function, where new molecules, such as irisin and bone morphogenetic proteins, particularly BMP7 and BMP8B, as well as novel central factors and new regulatory mechanisms, such as orexins and the canonical ventomedial nucleus of the hypothalamus (VMH) AMP- activated protein kinase (AMPK)-SNS-BAT axis, have been discovered and emerged as potential drug targets to combat obesity. In this review we provide an overview of the complex central regulation of BAT and how different neuronal cell populations co-ordinately work to maintain energy homeostasis.

Metabolic characteristics and therapeutic potential of brown and ?beige? adipose tissues

According to the International Diabetes Federation, 10.9 million people have diabetes mellitus (DM) in Russia; however, only up to 4 million are registered. In addition, 11.9 million people have impaired glucose tolerance and impaired fasting glucose levels [1]. One of the significant risk factors for type 2 DM (T2DM) is obesity, which increases insulin resistance (IR). IR is the major pathogenetic link to T2DM. According to current concepts, there are three types of adipose tissue: white adipose tissue (WAT), brown adipose tissue (BAT) and ?beige?, of which the last two types have a thermogenic function. Some research results have revealed the main stages in the development of adipocytes; however, there is no general consensus regarding the development of ?beige? adipocytes. Furthermore, the biology of BAT and ?beige? adipose tissue is currently being intensively investigated, and some key transcription factors, signalling pathways and hormones that promote the development and activation of these tissues have been identified. The most discussed hormones are irisin and fibroblast growth factor 21, which have established positive effects on BAT and ?beige? adipose tissue with regard to carbohydrate, lipid and energy metabolism. The primary imaging techniques used to investigate BAT are PET-CT with 18F-fluorodeoxyglucose and magnetic resonance spectroscopy. With respect to the current obesity epidemic and associated diseases, including T2DM, there is a growing interest in investigating adipogenesis and the possibility of altering this process. BAT and ?beige? adipose tissue may be targets for developing drugs directed against obesity and T2DM.