Gastric GDF15 levels are regulated by age, sex, and nutritional status in rodents and humans

AIM

Growth differentiation factor 15 (GDF15) is a stress response cytokine that has been proposed as a relevant metabolic hormone. Descriptive studies have shown that plasma GDF15 levels are regulated by short term changes in nutritional status, such as fasting, or in obesity. However, few data exist regarding how GDF15 levels are regulated in peripheral tissues. The aim of the present work was to study the variations on gastric levels of GDF15 and its precursor under different physiological conditions, such as short-term changes in nutritional status or overfeeding achieved by HFD. Moreover, we also address the sex- and age-dependent alterations in GDF15 physiology.

METHODS

The levels of gastric and plasma GDF15 and its precursor were measured in lean and obese mice, rats and humans by western blot, RT-PCR, ELISA, immunohistochemistry and by an in vitro organ culture system.

RESULTS

Our results show a robust regulation of gastric GDF15 production by fasting in rodents. In obesity an increase in GDF15 secretion from the stomach is reflected with an increase in circulating levels of GDF15 in rats and humans. Moreover, gastric GDF15 levels increase with age in both rats and humans. Finally, gastric GDF15 levels display sexual dimorphism, which could explain the difference in circulating GFD15 levels between males and females, observed in both humans and rodents.

CONCLUSIONS

Our results provide clear evidence that gastric GDF15 is a critical contributor of circulating GDF15 levels and can explain some of the metabolic effects induced by GDF15.

Hippocampal Pyk2 regulates specific social skills: Implications for schizophrenia

Pyk2 has been shown previously to be involved in several psychological and cognitive alterations related to stress, Huntington's disease, and Alzheimer's disease. All these disorders are accompanied by different types of impairments in sociability, which has recently been linked to improper mitochondrial function. We hypothesize that Pyk2, which regulates mitochondria, could be associated with the regulation of mitochondrial dynamics and social skills. In the present manuscript, we report that a reduction of Pyk2 levels in mouse pyramidal neurons of the hippocampus decreased social dominance and aggressivity. Furthermore, social interactions induced robust Pyk2-dependent hippocampal changes in several oxidative phosphorylation complexes. We also observed that Pyk2 levels were increased in the CA1 pyramidal neurons of schizophrenic subjects, occurring alongside changes in different direct and indirect regulators of mitochondrial function including DISC1 and Grp75. Accordingly, overexpressing Pyk2 in hippocampal CA1 pyramidal cells mimicked some specific schizophrenia-like social behaviors in mice. In summary, our results indicate that Pyk2 might play a role in regulating specific social skills likely via mitochondrial dynamics and that there might be a link between Pyk2 levels in hippocampal neurons and social disturbances in schizophrenia.

Novel mechanisms involved in leptin sensitization in obesity

Leptin is a hormone that is secreted by adipocytes in proportion to adipose tissue size, and that informs the brain about the energy status of the body. Leptin acts through its receptor LepRb, expressed mainly in the hypothalamus, and induces a negative energy balance by potent inhibition of feeding and activation of energy expenditure. These actions have led to huge expectations for the development of therapeutic targets for metabolic complications based on leptin-derived compounds. However, the majority of patients with obesity presents elevated leptin production, suggesting that in this setting leptin is ineffective in the regulation of energy balance. This resistance to the action of leptin in obesity has led to the development of "leptin sensitizers," which have been tested in preclinical studies. Much research has focused on generating combined treatments that act on multiple levels of the gastrointestinal-brain axis. The gastrointestinal-brain axis secretes a variety of different anorexigenic signals, such as uroguanylin, glucagon-like peptide-1, amylin, or cholecystokinin, which can alleviate the resistance to leptin action. Moreover, alternative mechanism such as pharmacokinetics, proteostasis, the role of specific kinases, chaperones, ER stress and neonatal feeding modifications are also implicated in leptin resistance. This review will cover the current knowledge regarding the interaction of leptin with different endocrine factors from the gastrointestinal-brain axis and other novel mechanisms that improve leptin sensitivity in obesity.

Liver Brain Interactions: Focus on FGF21 a Systematic Review

Fibroblast growth factor 21 is a pleiotropic hormone secreted mainly by the liver in response to metabolic and nutritional challenges. Physiologically, fibroblast growth factor 21 plays a key role in mediating the metabolic responses to fasting or starvation and acts as an important regulator of energy homeostasis, glucose and lipid metabolism, and insulin sensitivity, in part by its direct action on the central nervous system. Accordingly, pharmacological recombinant fibroblast growth factor 21 therapies have been shown to counteract obesity and its related metabolic disorders in both rodents and nonhuman primates. In this systematic review, we discuss how fibroblast growth factor 21 regulates metabolism and its interactions with the central nervous system. In addition, we also state our vision for possible therapeutic uses of this hepatic-brain axis.

Metabolic actions of the growth hormone-insulin growth factor-1 axis and its interaction with the central nervous system

The growth hormone/insulin growth factor-1 axis is a key endocrine system that exerts profound effects on metabolism by its actions on different peripheral tissues but also in the brain. Growth hormone together with insulin growth factor-1 perform metabolic adjustments, including regulation of food intake, energy expenditure, and glycemia. The dysregulation of this hepatic axis leads to different metabolic disorders including obesity, type 2 diabetes or liver disease. In this review, we discuss how the growth hormone/insulin growth factor-1 axis regulates metabolism and its interactions with the central nervous system. Finally, we state our vision for possible therapeutic uses of compounds based in the components of this hepatic axis.

Prolonged breastfeeding protects from obesity by hypothalamic action of hepatic FGF21

Early-life determinants are thought to be a major factor in the rapid increase of obesity. However, while maternal nutrition has been extensively studied, the effects of breastfeeding by the infant on the reprogramming of energy balance in childhood and throughout adulthood remain largely unknown. Here we show that delayed weaning in rat pups protects them against diet-induced obesity in adulthood, through enhanced brown adipose tissue thermogenesis and energy expenditure. In-depth metabolic phenotyping in this rat model as well as in transgenic mice reveals that the effects of prolonged suckling are mediated by increased hepatic fibroblast growth factor 21 (FGF21) production and tanycyte-controlled access to the hypothalamus in adulthood. Specifically, FGF21 activates GABA-containing neurons expressing dopamine receptor 2 in the lateral hypothalamic area and zona incerta. Prolonged breastfeeding thus constitutes a protective mechanism against obesity by affecting long-lasting physiological changes in liver-to-hypothalamus communication and hypothalamic metabolic regulation.

Multifaceted actions of melanin-concentrating hormone on mammalian energy homeostasis

Melanin-concentrating hormone (MCH) is a small cyclic peptide expressed in all mammals, mainly in the hypothalamus. MCH acts as a robust integrator of several physiological functions and has crucial roles in the regulation of sleep-wake rhythms, feeding behaviour and metabolism. MCH signalling has a very broad endocrine context and is involved in physiological functions and emotional states associated with metabolism, such as reproduction, anxiety, depression, sleep and circadian rhythms. MCH mediates its functions through two receptors (MCHR1 and MCHR2), of which only MCHR1 is common to all mammals. Owing to the wide variety of MCH downstream signalling pathways, MCHR1 agonists and antagonists have great potential as tools for the directed management of energy balance disorders and associated metabolic complications, and translational strategies using these compounds hold promise for the development of novel treatments for obesity. This Review provides an overview of the numerous roles of MCH in energy and glucose homeostasis, as well as in regulation of the mesolimbic dopaminergic circuits that encode the hedonic component of food intake.

Sirt3 in POMC neurons controls energy balance in a sex- and diet-dependent manner

Sirtuin 3 (SIRT3) is one of the seven mammalian sirtuin homologs of the yeast Sir2 gene that has emerged as an important player in the regulation of energy metabolism in peripheral tissues. However, its role in the hypothalamus has not been explored. Herein, we show that the genetic inhibition of SIRT3 in the hypothalamic arcuate nucleus (ARC) induced a negative energy balance and improvement of several metabolic parameters. These effects are specific for POMC neurons, because ablation of SIRT3 in POMC, but not in AgRP neurons, decreased body weight and adiposity, increased energy expenditure and brown adipose tissue (BAT) activity, and induced browning in white adipose tissue (WAT). Notably, the depletion of SIRT3 in POMC neurons caused these effects in male mice fed a chow diet but failed to affect energy balance in males fed a high fat diet and females under both type of diets. Overall, we provide the first evidence pointing for a key role of SIRT3 in POMC neurons in the regulation of energy balance.

Ghrelin and liver disease

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are two of the most common liver diseases associated with obesity, type 2 diabetes and metabolic syndrome. The prevalence of these conditions are increasingly rising and presently there is not a pharmacological option available in the market. Elucidation of the mechanism of action and the molecular underpinnings behind liver disease could help to better understand the pathophysiology of these illnesses. In this sense, in the last years modulation of the ghrelin system in preclinical animal models emerge as a promising therapeutic tool. In this review, we compile the latest knowledge of the modulation of ghrelin system and its intracellular pathways that regulates lipid metabolism, hepatic inflammation and liver fibrosis. We also describe novel processes implicated in the regulation of liver disease by ghrelin, such as autophagy or dysregulated circadian rhythms. In conclusion, the information displayed in this review support that the ghrelin system could be an appealing strategy for the treatment of liver disease.

Oral Pharmacological Activation of Hypothalamic Guanylate Cyclase 2C Receptor Stimulates Brown Fat Thermogenesis to Reduce Body Weight

Linaclotide is a synthetic peptide approved by the FDA for the treatment of constipation-predominant irritable bowel syndrome and chronic constipation. Linaclotide binds and activates the transmembrane receptor guanylate cyclase 2C (Gucy2c). Uroguanylin (UGN) is a 16 amino acid peptide that is mainly secreted by enterochromaffin cells in the duodenum and proximal small intestine. UGN is the endogenous ligand of Gucy2c and decreases body weight in diet-induced obese (DIO) mice via the activation of the thermogenic program in brown adipose tissue. Therefore, we wanted to evaluate whether oral linaclotide could also improve DIO mice metabolic phenotype. In this study, we have demonstrated that DIO mice orally treated with linaclotide exhibited a significant reduction of body weight without modifying food intake. Linaclotide exerts its actions through the central nervous system, and more specifically, via Gucy2c receptors located in the mediobasal hypothalamus, leading to the activation of the sympathetic nervous system to trigger the thermogenic activity of brown fat stimulating energy expenditure. These findings indicate for first time that, in addition to its effects at intestinal level to treat irritable bowel syndrome with constipation and chronic constipation, linaclotide also exerts a beneficial effect in whole body metabolism.

MCH Regulates SIRT1/FoxO1 and Reduces POMC Neuronal Activity to Induce Hyperphagia, Adiposity, and Glucose Intolerance

Melanin-concentrating hormone (MCH) is an important regulator of food intake, glucose metabolism, and adiposity. However, the mechanisms mediating these actions remain largely unknown. We used pharmacological and genetic approaches to show that the sirtuin 1 (SIRT1)/FoxO1 signaling pathway in the hypothalamic arcuate nucleus (ARC) mediates MCH-induced feeding, adiposity, and glucose intolerance. MCH reduces proopiomelanocortin (POMC) neuronal activity, and the SIRT1/FoxO1 pathway regulates the inhibitory effect of MCH on POMC expression. Remarkably, the metabolic actions of MCH are compromised in mice lacking SIRT1 specifically in POMC neurons. Of note, the actions of MCH are independent of agouti-related peptide (AgRP) neurons because inhibition of γ-aminobutyric acid receptor in the ARC did not prevent the orexigenic action of MCH, and the hypophagic effect of MCH silencing was maintained after chemogenetic stimulation of AgRP neurons. Central SIRT1 is required for MCH-induced weight gain through its actions on the sympathetic nervous system. The central MCH knockdown causes hypophagia and weight loss in diet-induced obese wild-type mice; however, these effects were abolished in mice overexpressing SIRT1 fed a high-fat diet. These data reveal the neuronal basis for the effects of MCH on food intake, body weight, and glucose metabolism and highlight the relevance of SIRT1/FoxO1 pathway in obesity.

Exciting advances in GPCR-based drugs discovery for treating metabolic disease and future perspectives

INTRODUCTION

Current pharmacological therapies that target single receptors have limited efficacy for the treatment of diabetes and obesity. Novel approaches with hybrid peptides that activate more than one receptor at once to generate beneficial effects through synergistic effects have shown promising results. Several unimolecular dual and tri-agonists, mainly associated with GPCR like GLP-1/GCG/GIP receptors, have shown exceptional efficacy in preclinical models, and are currently being evaluated in clinical trials to investigate their safety and beneficial effects in humans. Areas covered: Herein, the authors review the development of drugs used in the treatment of metabolic disease, from single agonists to the new generation of tri-agonist peptides and compile the latest knowledge available on GPCR-based drug discovery. The authors also provide the reader with their expert perspectives on this exciting area of drug development. Expert opinion: The co-agonists that have been clinically tested so far have been well tolerated and reduce body weight as well as fasting glucose levels in patients with Type 2 Diabetes Mellitus to a higher degree than single agonists alone. The promising data collected so far now warrant large scale randomized clinical trials to assess whether a unimolecular polypharmacy-based approach could translate into safe and efficacious treatments for obesity and its comorbidities.

p53 and energy balance: meeting hypothalamic AgRP neurons

Cancer cells feature strong metabolic changes to cope with the high energy demand for cell growth and division. Given the importance of metabolic reprogramming in tumor development, it seems logical that tumor suppressors and oncogenes are also regulating the molecular pathways controlling these processes. The p53 tumor suppressor gene has been extensively studied for its role in responding to DNA damage, hypoxia, and oncogenic activation. During the last years, we have learnt that p53 has also the capacity to modulate metabolic changes in cells by regulating a large variety of pathways such as glycolysis, oxidative phosphorylation or fatty acid metabolism. Our group has recently found that the lack of p53 in AgRP neurons, but not POMC neurons, causes that mice are more prone to develop diet-induced obesity (Nat Commun. 9(1):3432). The reason for this is that these mice showed a late increase in food intake and especially because they had a reduced thermogenic activity in BAT. The mechanism modulating these actions involves the upregulation of MKK7 that activates c-Jun N-terminal kinase.

Pharmacological inhibition of cannabinoid receptor 1 stimulates gastric release of nesfatin-1 via the mTOR pathway

AIM

To determine whether Nucb2/nesfatin1 production is regulated by the cannabinoid system through the intracellular mTOR pathway in the stomach.

METHODS

Sprague Dawley rats were treated with vehicle, rimonabant, rapamycin or rapamycin+rimonabant. Gastric tissue obtained from the animals was used for biochemical assays: Nucb2 mRNA measurement by real time PCR, gastric Nucb2/nesfatin protein content by western blot, and gastric explants to obtain gastric secretomes. Nucb2/nesfatin levels were measured in gastric secretomes and plasma using enzyme-linked immunosorbent assay.

RESULTS

The inhibition of cannabinoid receptor 1 (CB1) by the peripheral injection of an inverse agonist, namely rimonabant, decreases food intake and increases the gastric secretion and circulating levels of Nucb2/nesfatin-1. In addition, rimonabant treatment activates mTOR pathway in the stomach as showed by the increase in pmTOR/mTOR expression in gastric tissue obtained from rimonabant treated animals. These effects were confirmed by the use of a CB1 antagonist, AM281. When the intracellular pathway mTOR/S6k was inactivated by chronic treatment with rapamycin, rimonabant treatment was no longer able to stimulate the gastric secretion of Nucb2/nesfatin-1.

CONCLUSION

The peripheral cannabinoid system regulates food intake through a mechanism that implies gastric production and release of Nucb2/Nesfatin-1, which is mediated by the mTOR/S6k pathway.

Pharmacological and Genetic Manipulation of p53 in Brown Fat at Adult But Not Embryonic Stages Regulates Thermogenesis and Body Weight in Male Mice

p53 is a well-known tumor suppressor that plays multiple biological roles, including the capacity to modulate metabolism at different levels. However, its metabolic role in brown adipose tissue (BAT) remains largely unknown. Herein we sought to investigate the physiological role of endogenous p53 in BAT and its implication on BAT thermogenic activity and energy balance. To this end, we generated and characterized global p53-null mice and mice lacking p53 specifically in BAT. Additionally we performed gain-and-loss-of-function experiments in the BAT of adult mice using virogenetic and pharmacological approaches. BAT was collected and analyzed by immunohistochemistry, thermography, real-time PCR, and Western blot. p53-deficient mice were resistant to diet-induced obesity due to increased energy expenditure and BAT activity. However, the deletion of p53 in BAT using a Myf5-Cre driven p53 knockout did not show any changes in body weight or the expression of thermogenic markers. The acute inhibition of p53 in the BAT of adult mice slightly increased body weight and inhibited BAT thermogenesis, whereas its overexpression in the BAT of diet-induced obese mice reduced body weight and increased thermogenesis. On the other hand, pharmacological activation of p53 improves body weight gain due to increased BAT thermogenesis by sympathetic nervous system in obese adult wild-type mice but not in p53(-/-) animals. These results reveal that p53 regulates BAT metabolism by coordinating body weight and thermogenesis, but these metabolic actions are tissue specific and also dependent on the developmental stage.

FNDC5 is produced in the stomach and associated to body composition

The fibronectin type III domain-containing protein 5 (FNDC5) discovered in 2002 has recently gained attention due to its potential role in protecting against obesity. In rat, no data exist regarding FNDC5 production and regulation in the stomach. The aim of the present work was to determine the expression of FNDC5 in the rat stomach and its potential regulation by body composition. The present data shows FNDC5 gene expression in the gastric mucosa. Immunohistochemical studies found FNDC5 immunopositivity in chief cells of gastric tissue. By the use of three different antibodies FNDC5 was found expressed in gastric mucosa and secreted by the stomach. The rate of gastric FNDC5 secretion parallels the circulating levels of FNDC5. The body fat mass increase after intervention with high fat diet coincided with a decrease in the secretion of FNDC5 from the stomach and a diminution in the FNDC5 circulating levels. In summary, the present data shows, for the first time, the expression of FNDC5 in the stomach of rats and its regulation by body composition, suggesting a potential role of gastric FNDC5 in energy homeostasis.

Uroguanylin Action in the Brain Reduces Weight Gain in Obese Mice via Different Efferent Autonomic Pathways

The gut-brain axis is of great importance in the control of energy homeostasis. The identification of uroguanylin (UGN), a peptide released in the intestines that is regulated by nutritional status and anorectic actions, as the endogenous ligand for the guanylyl cyclase 2C receptor has revealed a new system in the regulation of energy balance. We show that chronic central infusion of UGN reduces weight gain and adiposity in diet-induced obese mice. These effects were independent of food intake and involved specific efferent autonomic pathways. On one hand, brain UGN induces brown adipose tissue thermogenesis, as well as browning and lipid mobilization in white adipose tissue through stimulation of the sympathetic nervous system. On the other hand, brain UGN augments fecal output through the vagus nerve. These findings are of relevance as they suggest that the beneficial metabolic actions of UGN through the sympathetic nervous system do not involve nondesirable gastrointestinal adverse effects, such as diarrhea. The present work provides mechanistic insights into how UGN influences energy homeostasis and suggests that UGN action in the brain represents a feasible pharmacological target in the treatment of obesity.

Hypothalamic CaMKKβ mediates glucagon anorectic effect and its diet-induced resistance

Objective

Glucagon receptor antagonists and humanized glucagon antibodies are currently studied as promising therapies for obesity and type II diabetes. Among its variety of actions, glucagon reduces food intake, but the molecular mechanisms mediating this effect as well as glucagon resistance are totally unknown.

Methods

Glucagon and adenoviral vectors were administered in specific hypothalamic nuclei of lean and diet-induced obese rats. The expression of neuropeptides controlling food intake was performed by in situ hybridization. The regulation of factors of the glucagon signaling pathway was assessed by western blot.

Results

The central injection of glucagon decreased feeding through a hypothalamic pathway involving protein kinase A (PKA)/Ca²⁺-calmodulin-dependent protein kinase kinase β (CaMKKβ)/AMP-activated protein kinase (AMPK)-dependent mechanism. More specifically, the central injection of glucagon increases PKA activity and reduces protein levels of CaMKKβ and its downstream target phosphorylated AMPK in the hypothalamic arcuate nucleus (ARC). Consistently, central glucagon significantly decreased AgRP expression. Inhibition of PKA and genetic activation of AMPK in the ARC blocked glucagon-induced anorexia in lean rats. Genetic down-regulation of glucagon receptors in the ARC stimulates fasting-induced hyperphagia. Although glucagon was unable to decrease food intake in DIO rats, glucagon sensitivity was restored after inactivation of CaMKKβ, specifically in the ARC. Thus, glucagon decreases food intake acutely via PKA/CaMKKβ/AMPK dependent pathways in the ARC, and CaMKKβ mediates its obesity-induced hypothalamic resistance.

Conclusions

This work reveals the molecular underpinnings by which glucagon controls feeding that may lead to a better understanding of disease states linked to anorexia and cachexia.

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Glucagon stimulates PKA and inhibits CaMKKβ and AMPK in the arcuate nucleus (ARC).

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Down-regulation of glucagon receptor in the ARC increases fasting-induced hyperphagia.

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Glucagon is unable to decrease food intake in diet-induced obese (DIO) rats.

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In DIO rats, glucagon fails to alter CaMKKβ and its downstream targets AMPK and pACC.

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Down-regulation of CaMKKβ in the ARC restores glucagon sensitivity in obese rodents.

Regulation of NUCB2/nesfatin-1 production in rat's stomach and adipose tissue is dependent on age, testosterone levels and lactating status

Nesfatin-1, which is derived from the NEFA/nucleobindin 2 (NUCB2) precursor, was recently identified as an anorexigenic peptide that is produced in several tissues including the hypothalamus. Currently, no data exist regarding the regulation of NUCB2/nesfatin-1 production in peripheral tissues, such as gastric mucosa and adipose tissue, through different periods of development. The aim of the present work was to study the variations on circulating levels, mRNA expression and tissue content in gastric mucosa and adipose tissue of NUCB2/nesfatin-1 with age and specially in two clue periods of maturation, weaning and puberty. The weaning period affected NUCB2/nesfatin-1 production in gastric tissue. The testosterone changes associated with the initiation of puberty regulated NUCB2/nesfatin-1 production via adipose tissue and gastric NUCB2/nesfatin-1 production. In conclusion, the production of NUCB2/nesfatin-1 by the stomach and adipose tissue fluctuates with age to regulate energy homeostasis during different states of development.

Cross-talk between SIRT1 and endocrine factors: effects on energy homeostasis

The mammalian sirtuins (SIRT1-7) are a family of highly conserved nicotine adenine dinucleotide (NAD(+))-dependent deacetylases that act as cellular sensors to detect energy availability. SIRT1 is a multifaceted protein that is involved in a wide variety of cellular processes. SIRT1 is activated in response to caloric restriction, acting on multiple targets in a wide range of tissues. SIRT1 regulates the role of multiple hormones implicated in energy balance, including glucose and lipid metabolism. Here, we review the relevant role of SIRT1 as a mediator of endocrine function of several hormones to modulate energy balance. In addition, we analyze the potential of targeting SIRT1 for the treatment of obesity and type 2 diabetes mellitus.

Duodenal nutrient exclusion improves metabolic syndrome and stimulates villus hyperplasia

OBJECTIVE

Surgical interventions that prevent nutrient exposure to the duodenum are among the most successful treatments for obesity and diabetes. However, these interventions are highly invasive, irreversible and often carry significant risk. The duodenal-endoluminal sleeve (DES) is a flexible tube that acts as a barrier to nutrient-tissue interaction along the duodenum. We implanted this device in Zucker Diabetic Fatty (ZDF) rats to gain greater understanding of duodenal nutrient exclusion on glucose homeostasis.

DESIGN

ZDF rats were randomised to four groups: Naive, sham ad libitum, sham pair-fed, and DES implanted. Food intake, body weight (BW) and body composition were measured for 28 days postoperatively. Glucose, lipid and bile acid metabolism were evaluated, as well as histological assessment of the upper intestine.

RESULTS

DES implantation induced a sustained decrease in BW throughout the study that was matched by pair-fed sham animals. Decreased BW resulted from loss of fat, but not lean mass. DES rats were also found to be more glucose tolerant than either ad libitum-fed or pair-fed sham controls, suggesting fat mass independent metabolic benefits. DES also reduced circulating triglyceride and glycerol levels while increasing circulating bile acids. Interestingly, DES stimulated a considerable increase in villus length throughout the upper intestine, which may contribute to metabolic improvements.

CONCLUSIONS

Our preclinical results validate DES as a promising therapeutic approach to diabetes and obesity, which offers reversibility, low risk, low invasiveness and triple benefits including fat mass loss, glucose and lipid metabolism improvement which mechanistically may involve increased villus growth in the upper gut.

High-density lipoprotein maintains skeletal muscle function by modulating cellular respiration in mice

BACKGROUND

Abnormal glucose metabolism is a central feature of disorders with increased rates of cardiovascular disease. Low levels of high-density lipoprotein (HDL) are a key predictor for cardiovascular disease. We used genetic mouse models with increased HDL levels (apolipoprotein A-I transgenic [apoA-I tg]) and reduced HDL levels (apoA-I-deficient [apoA-I ko]) to investigate whether HDL modulates mitochondrial bioenergetics in skeletal muscle.

METHODS AND RESULTS

ApoA-I ko mice exhibited fasting hyperglycemia and impaired glucose tolerance test compared with wild-type mice. Mitochondria isolated from gastrocnemius muscle of apoA-I ko mice displayed markedly blunted ATP synthesis. Endurance capacity during exercise exhaustion test was impaired in apoA-I ko mice. HDL directly enhanced glucose oxidation by increasing glycolysis and mitochondrial respiration rate in C2C12 muscle cells. ApoA-I tg mice exhibited lower fasting glucose levels, improved glucose tolerance test, increased lactate levels, reduced fat mass, associated with protection against age-induced decline of endurance capacity compared with wild-type mice. Circulating levels of fibroblast growth factor 21, a novel biomarker for mitochondrial respiratory chain deficiencies and inhibitor of white adipose lipolysis, were significantly reduced in apoA-I tg mice. Consistent with an increase in glucose utilization of skeletal muscle, genetically increased HDL and apoA-I levels in mice prevented high-fat diet-induced impairment of glucose homeostasis.

CONCLUSIONS

In view of impaired mitochondrial function and decreased HDL levels in type 2 diabetes mellitus, our findings indicate that HDL-raising therapies may preserve muscle mitochondrial function and address key aspects of type 2 diabetes mellitus beyond cardiovascular disease.

Chronic sympathoexcitation through loss of Vav3, a Rac1 activator, results in divergent effects on metabolic syndrome and obesity depending on diet

The role of the sympathetic nervous system, stress, and hypertension in metabolic syndrome and obesity remains unclear. To clarify this issue, we utilized genetically engineered mice showing chronic sympathoexcitation and hypertension due to lack of Vav3, a Rac1 activator. Here, we report that these animals develop metabolic syndrome under chow diet. However, they show protection from metabolic syndrome and obesity under fatty diets. These effects are elicited by α1-adrenergic- and diet-dependent metabolic changes in liver and the α1/β3 adrenergic-mediated stimulation of brown adipocyte thermogenesis. These responses seem to be engaged by the local action of noradrenaline in target tissues rather than by long-range effects of adrenaline. By contrast, they are not triggered by low parasympathetic drive or the hypertensive state present in Vav3-deficient mice. These results indicate that the sympathetic system plays divergent roles in the etiology of metabolic diseases depending on food regimen, sympathoexcitation source, and disease stage.

Sirt1 inhibits the transcription factor CREB to regulate pituitary growth hormone synthesis

Growth hormone (GH) is a major anabolic hormone and the primary regulator of organism growth. Its transcription is triggered by GH-releasing hormone (GHRH) through the transcription factor cAMP response element-binding protein (CREB) and by caloric intake. In contrast, the deacetylase Sirt1 is activated by caloric restriction. Therefore, the present study investigates how Sirt1 affects CREB function and GH synthesis. Sirt1 pharmacological activation with resveratrol (IC₅₀=87 μM) suppressed GHRH-induced GH secretion from rat anterior pituitary cells in vivo and in vitro, while vehicle controls showed no effect. Resveratrol's effects were abolished after knocking down Sirt1 with RNA interference, but not in control scrambled siRNA-transfected rat somatotrophs, confirming the Sirt1 specificity. Sirt1 activation and overexpression suppressed forskolin-induced CREB-Ser(133) phosphorylation, but no effect was seen with vehicle and empty plasmid controls. The deacetylase-dead mutant Sirt1 retained CREB-Ser(133) phosphorylation by keeping protein phosphatase protein phosphatase 1 activity low. Sirt1 activation suppressed glycogen synthase kinase 3 β acetylation, and a mutation on the GSK3β-Lys(205) residue mimicking a hypoacetylated form revealed increased activity. In summary, this is a novel mechanism through which Sirt1 intercepts the cAMP pathway by suppressing CREB transcriptional activation, resulting in decreased GH synthesis.

The obestatin/GPR39 system is up-regulated by muscle injury and functions as an autocrine regenerative system

The maintenance and repair of skeletal muscle are attributable to an elaborate interaction between extrinsic and intrinsic regulatory signals that regulate the myogenic process. In the present work, we showed that obestatin, a 23-amino acid peptide encoded by the ghrelin gene, and the GPR39 receptor are expressed in rat skeletal muscle and are up-regulated upon experimental injury. To define their roles in muscle regeneration, L6E9 cells were used to perform in vitro assays. For the in vivo assays, skeletal muscle tissue was obtained from male rats and maintained under continuous subcutaneous infusion of obestatin. In differentiating L6E9 cells, preproghrelin expression and correspondingly obestatin increased during myogenesis being sustained throughout terminal differentiation. Autocrine action was demonstrated by neutralization of the endogenous obestatin secreted by differentiating L6E9 cells using a specific anti-obestatin antibody. Knockdown experiments by preproghrelin siRNA confirmed the contribution of obestatin to the myogenic program. Furthermore, GPR39 siRNA reduced obestatin action and myogenic differentiation. Exogenous obestatin stimulation was also shown to regulate myoblast migration and proliferation. Furthermore, the addition of obestatin to the differentiation medium increased myogenic differentiation of L6E9 cells. The relevance of the actions of obestatin was confirmed in vivo by the up-regulation of Pax-7, MyoD, Myf5, Myf6, myogenin, and myosin heavy chain (MHC) in obestatin-infused rats when compared with saline-infused rats. These data elucidate a novel mechanism whereby the obestatin/GPR39 system is coordinately regulated as part of the myogenic program and operates as an autocrine signal regulating skeletal myogenesis.

Obestatin as a regulator of adipocyte metabolism and adipogenesis

The role of obestatin, a 23-amino-acid peptide encoded by the ghrelin gene, on the control of the metabolism of pre-adipocyte and adipocytes as well as on adipogenesis was determined. For in vitro assays, pre-adipocyte and adipocyte 3T3-L1 cells were used to assess the obestatin effect on cell metabolism and adipogenesis based on the regulation of the key enzymatic nodes, Akt and AMPK and their downstream targets. For in vivo assays, white adipose tissue (WAT) was obtained from male rats under continuous subcutaneous infusion of obestatin. Obestatin activated Akt and its downstream targets, GSK3α/β, mTOR and S6K1, in 3T3-L1 adipocyte cells. Simultaneously, obestatin inactivated AMPK in this cell model. In keeping with this, ACC phosphorylation was also decreased. This fact was confirmed in vivo in white adipose tissue (omental, subcutaneous and gonadal) obtained from male rats under continuous sc infusion of obestatin (24 and 72 hrs). The relevance of obestatin as regulator of adipocyte metabolism was supported by AS160 phosphorylation, GLUT4 translocation and augment of glucose uptake in 3T3-L1 adipocyte cells. In contrast, obestatin failed to modify translocation of fatty acid transporters, FATP1, FATP4 and FAT/CD36, to plasma membrane. Obestatin treatment in combination with IBMX and DEX showed to regulate the expression of C/EBPα, C/EBPβ, C/EBPδ and PPARγ promoting adipogenesis. Remarkable, preproghrelin expression, and thus obestatin expression, increased during adipogenesis being sustained throughout terminal differentiation. Neutralization of endogenous obestatin secreted by 3T3-L1 cells by anti-obestatin antibody decreased adipocyte differentiation. Furthermore, knockdown experiments by preproghrelin siRNA supported that obestatin contributes to adipogenesis. In summary, obestatin promotes adipogenesis in an autocrine/paracrine manner, being a regulator of adipocyte metabolism. These data point to a putative role in the pathogenesis of metabolic syndrome.

Preproghrelin expression is a key target for insulin action on adipogenesis

This study aimed to investigate the role of preproghrelin-derived peptides in adipogenesis. Immunocytochemical analysis of 3T3-L1 adipocyte cells showed stronger preproghrelin expression compared with that observed in 3T3-L1 preadipocyte cells. Insulin promoted this expression throughout adipogenesis identifying mTORC1 as a critical downstream substrate for this profile. The role of preproghrelin-derived peptides on the differentiation process was supported by preproghrelin knockdown experiments, which revealed its contribution to adipogenesis. Neutralization of endogenous O-acyl ghrelin (acylated ghrelin), unacylated ghrelin, and obestatin by specific antibodies supported their adipogenic potential. Furthermore, a parallel increase in the expression of ghrelin-associated enzymatic machinery, prohormone convertase 1/3 (PC1/3) and membrane-bound O-acyltransferase 4 (MBOAT4), was dependent on the expression of preproghrelin in the course of insulin-induced adipogenesis. The coexpression of preproghrelin system and their receptors, GHSR1a and GPR39, during adipogenesis supports an autocrine/paracrine role for these peptides. Preproghrelin, PC1/3, and MBOAT4 exhibited dissimilar expression depending on the white fat depot, revealing their regulation in a positive energy balance situation in mice. The results underscore a key role for preproghrelin-derived peptides on adipogenesis through an autocrine/paracrine mechanism.

The vagus nerve as a regulator of growth hormone secretion

The growth hormone (GH) axis is mainly regulated by the growth hormone releasing hormone (GHRH) and somatostatin (SS) hypothalamic peptides. Nevertheless, since ghrelin peptide was discovered as the natural ligand for growth hormone secretagogue receptor (GHS-R), the mechanism of GH regulation has acquired a new dimension. It was described that ghrelin possesses a relevant effect inducing GH secretion when it is administered peripherally. A role of the vagus nerve mediating ghrelin action has been described although this effect is not understood. Under this context the main objective of this work was to determine the possible involvement of the vagus in the somatotroph axis regulation. The results in this manuscript show that animals with a disruption of the vagus connection presented lower basal IGF-1 and GH levels, a decreased GH response to peripheral GHRH administration and a marked diminution in the GH response to peripheral and central ghrelin treatments. In addition, vagotomized animals showed a down-regulation of GHRH mRNA in the arcuate nucleus of the hypothalamus and a down-regulation in both GHRH and GHS receptors' mRNA at the pituitary level. In conclusion, the present work reveals that the vagus nerve is crucial in growth hormone regulation and essential for the full GH-releasing effect of ghrelin.

Peripheral endocannabinoid system-mediated actions of rimonabant on growth hormone secretion are ghrelin-dependent

The somatotroph axis is a crucial pathway regulating metabolism. Despite the fact that the endocannabinoid system has been also revealed as a potent modulator of energy homeostasis, little information is available concerning a putative interaction between these two systems. The aim of the present study was to determine the in vivo effects of the blockade of the cannabinoid receptor type 1 (CB1) over growth hormone (GH) secretion using the CB1 antagonist rimonabant. The results obtained show that the blockade of the CB1 peripheral receptor by i.p. injection of rimonabant significantly inhibited pulsatile GH secretion. Similarly, it was found that this injection significantly decreased ghrelin-induced GH secretion without any effect on growth hormone-releasing hormone (GHRH)-induced GH discharge. In situ hybridisation showed that the peripheral blockade of CB1 did not affect hypothalamic somatostatin mRNA levels; however, GHRH mRNA expression was significantly decreased. The blockade of the vagus nerve signal by surgical vagotomy eliminated the inhibitory action of rimonabant on GHRH mRNA and consequently on GH. On the other hand, the central CB1 blockade by i.c.v. rimonabant treatment was unable to reproduce the effect of peripheral blockade on GHRH mRNA, nor the GH response to ghrelin. In conclusion, the data reported in the present study establish, from a physiological point of view, the existence of a novel mechanism of GH regulation implicating the action of the cannabinoid receptor on the somatotroph axis.

Peripheral leptin and ghrelin receptors are regulated in a tissue-specific manner in activity-based anorexia

The aim of this research was to investigate the effect of long-term exposure to low leptin and high ghrelin levels, inherent to activity-based anorexia (ABA), on peripheral metabolism-implicated tissues such as muscle and fat depots. For this purpose, rats under ABA were submitted to a global study which included the characterization of body weight and composition change, the evaluation of leptin and ghrelin levels as well as their receptors expression at peripheral level. Our results confirm that feeding restriction to 1 h per day, and particularly the combination of this fasting regime with exercise (ABA), significantly reduces fat mass, decreases leptin circulating levels, increases ghrelin levels strikingly and enhances insulin sensitivity. By direct in vitro assays, we show that visceral and gonadal fat participate more than subcutaneous fat in the hypoleptinemia of these animals. The study of ghrelin (GHS-R1a) and leptin (LEPR) receptors at peripheral level exhibits a tissue-specific expression pattern. Concretely, oxidative-soleus type of muscle appears to be more susceptible to ghrelin and leptin circulating levels than glycolytic-gastrocnemius type under exercise and food restriction situations. In relation to adipose tissue, chronic hyperghrelinemia induces GHS-R1a expression on visceral and subcutaneous fat which might suggest the prevention of lipid loss. On the other hand, only subcutaneous fat express the active long form of LEPR compared to visceral and gonadal fat under low leptin levels in ABA animals. All together, these findings indicate tissue-specific mechanisms for the control of energy homeostasis in response to nutrient and energy availability.

Age, sex, and lactating status regulate ghrelin secretion and GOAT mRNA levels from isolated rat stomach

Ghrelin is a stomach derivate peptide involved in energy homeostasis regulation, and ghrelin O-acyltransferase (GOAT) is the enzyme responsible for ghrelin acylation. Puberty is a period characterized by profound changes in the metabolic requirements and notable variations of sexual hormone levels. On the other hand, the weaning process is a fundamental modification of the diet, which implicates several adaptations of the gastrointestinal tract physiology. Until now the direct secretion of ghrelin by the stomach in these conditions, without interferences from other organs, has never been studied. The main objective of this article was to investigate how the stomach modulates ghrelin production and secretion as well as GOAT expression on these periods of life. Gastric ghrelin secretion is regulated through postnatal life in an independent way of gastric expression and circulating levels of this hormone. The present work shows a strong regulation of gastric ghrelin secretion by estrogens. The weaning strongly regulates gastric ghrelin secretion. Animals subjected to delayed weaning present a lower body weight than the corresponding controls. For the first time, it is shown that a noticeable decrease in circulating levels of testosterone and estrogens is associated with delay of weaning. GOAT mRNA levels in the stomach are strongly regulated by age, breastfeeding, and testosterone. In conclusion, the stomach itself regulates ghrelin and GOAT production to adapt the organism to the metabolic requirements demanded through each stage of life.

Role of obestatin on growth hormone secretion: An in vitro approach

Obestatin, the ghrelin-associated peptide, showed to activate MAPK signaling with no effect on Akt nor cell proliferating activity in rat tumor somatotroph cells (growth cells, GC). A sequential analysis of the obestatin transmembrane signaling pathway indicated a route involving the consecutive activation of G(i), PI3k, novel PKCepsilon, and Src for ERK1/2 activation. Furthermore, obestatin treatment triggers growth hormone (GH) release in the first 30min, being more acute at 15min. At 1h, obestatin treated cells showed the same levels in GH secretion than controls. Added to this functionality, obestatin was secreted by GC cells. Based on the capacity to stimulate GH release from somatotroph cells, obestatin may act directly in the pituitary through an autocrine/paracrine mechanism.

Growth hormone and somatostatin directly inhibit gastric ghrelin secretion. An in vitro organ culture system

Ghrelin is a 28-amino-acid hormone produced mainly by the stomach which strongly promotes food intake. It is the only known peripheral orexigenic hormone that induces the release of GH. Ghrelin has been proposed as a link between the enteric system and central regulation of energy balance and growth. Although it has recently been the focus of extensive study, the secretion mechanism is not yet well characterized. The aim of this study was to test the direct effect of hormones from the somatotropic axis on ghrelin release directly from the stomach. To this end, an organ culture model of gastric tissue explants from rat donors was used. These stomach explants were incubated in 6 well plates for 1, 2, and 3 h after treatment with either GH, GHRH, SS or IGF-I, all them at 10(-6) M. After incubation, the medium was collected and the amount of ghrelin secreted by the gastric tissue was measured by radioimmunoassay. It was observed that GH and SS significantly decreased gastric ghrelin secretion, while GHRH and IGF-1 had no effect on the present model. These results would confirm the capacity of GH and SS to act directly upon gastric level, inhibiting ghrelin secretion in vitro.

Sensory stimuli directly acting at the central nervous system regulate gastric ghrelin secretion. an ex vivo organ culture study

Ghrelin, a novel gastrointestinal hormone involved in GH regulation, has been postulated as a relevant orexigenic peptide released by splanchnic tissues. Descriptive studies have shown that plasma ghrelin levels increase in states of negative energy balance or fasting, while decreasing in obesity and after feeding. In the present study, a novel organ-culture model of gastric tissue explants obtained from rat donors has been validated for ex vivo experiments. Fasting induced gastric ghrelin release as well as ghrelin mRNA expression that were reflected in plasma. Interestingly, those changes were fully reverted by 15 min of refeeding before stomach extraction. Unexpectedly, when animals were allowed 15 min before explant extraction to see or smell, but not eat, the food (tease feeding), ghrelin secretion was suppressed just like in gastric explants from refed animals. This effect was blocked when the animals were subjected to surgical vagotomy or treated with atropine sulphate. In conclusion, gastric explants were a suitable model for testing ghrelin mechanism of secretion in vitro, and they were found to maintain memory of the previously received signals. Similar to feeding, tease feeding resulted in suppression of ghrelin discharge by explants.

Central obestatin administration does not modify either spontaneous or ghrelin-induced food intake in rats

The isolation of ghrelin unveiled a new system implicated in food intake regulation. The recently isolated hormone obestatin derives from the same precursor of ghrelin and seems to perform opposite actions. It could be part of a dual system connecting gut and brain to regulate energy homeostasis. The ability of intracerebroventricular administration of obestatin to modify food intake was evaluated. Obestatin had no effect on spontaneous food intake in both ad libitum and food restricted rats. The obestatin injection was not able to antagonize the ghrelin-stimulated increase in food intake either. In conclusion, the present work does not support a role for obestatin on the regulation of food intake in any model studied.

Ghrelin: from a GH-secretagogue to the regulation of food intake, sleep and anxiety

Grhelin is an endogenous ligand for the growth hormone secretagogue receptor from the stomach. It is a 28-aminoacid peptide of which the serine 3 residue is n-octanoylated. Ghrelin strongly stimulates GH secretion in vivo as well as in vitro. This endogenous ligand promotes the production of orexigenic neuropeptides (NPY and AgRP) in the hypothalamic arcuate nuclei and activates the neurons that produce these orexigenic peptides, resulting in an increase in feeding and body weight. Ghrelin has other significant actions, including control of acid secretion, influences on sleep and on the regulation of anxiety.

[Role of ghrelin in the pathophysiology of eating behaviour]

Ghrelin, the endogenous ligand for GHS-R, was isolated from rat stomach, although other tissues exist expressing ghrelin, such as pituitary, hypothalamus, placent, ovary, testes, etc. It was showed that ghrelin is implicated in GH secretion, in vivo and in vitro. There are direct evidences that proof that ghrelin administration induces GH secretion. There are in vivo data, showing ghrelin as a most potent GH secretor than GHRH. Evidences exist of ghrelin actions in the regulation of food intake and energy homeostasis. Ghrelin has a clear role in the differents pathologies such as obesity, anorexia and bulimia.