Mediobasal hypothalamic p70 S6 kinase 1 modulates the control of energy homeostasis

Role of the hypothalamus in mediating protective effects of dietary restriction during aging

Dietary restriction (DR) can extend lifespan and reduce disease burden across a wide range of animals and yeast but the mechanisms mediating these remarkably protective effects remain to be elucidated despite extensive efforts. Although it has generally been assumed that protective effects of DR are cell-autonomous, there is considerable evidence that many whole-body responses to nutritional state, including DR, are regulated by nutrient-sensing neurons. In this review, we explore the hypothesis that nutrient sensing neurons in the ventromedial hypothalamus hierarchically regulate the protective responses of dietary restriction. We describe multiple peripheral responses that are hierarchically regulated by the hypothalamus and we present evidence for non-cell autonomous signaling of dietary restriction gathered from a diverse range of models including invertebrates, mammalian cell culture, and rodent studies.

Central leucine sensing in the control of energy homeostasis

This article reviews recent studies identifying two key brain regions as two critical nodes in the neural network where central leucine sensing contributes to whole body energy homeostasis: the mediobasal hypothalamus and the dorsal vagal complex of the caudal brainstem. Activation of these leucine sensing sites engages multiple determinants of energy balance, including glucose homeostasis, food intake, and adiposity.

CNS control of glucose metabolism: response to environmental challenges

Over the last 15 years, considerable work has accumulated to support the role of the CNS in regulating postprandial glucose levels. As discussed in the first section of this review, the CNS receives and integrates information from afferent neurons, circulating hormones, and postprandially generated nutrients to subsequently direct changes in glucose output by the liver and glucose uptake by peripheral tissues. The second major component of this review focuses on the effects of external pressures, including high fat diet and changes to the light:dark cycle on CNS-regulating glucose homeostasis. We also discuss the interaction between these different pressures and how they contribute to the multifaceted mechanisms that we hypothesize contribute to the dysregulation of glucose in type 2 diabetes mellitus (T2DM). We argue that while current peripheral therapies serve to delay the progression of T2DM, generating combined obesity and T2DM therapies targeted at the CNS, the primary site of dysfunction for both diseases, would lead to a more profound impact on the progression of both diseases.

Wired on sugar: the role of the CNS in the regulation of glucose homeostasis

Obesity and type 2 diabetes mellitus (T2DM)--disorders of energy homeostasis and glucose homeostasis, respectively--are tightly linked and the incidences of both conditions are increasing in parallel. The CNS integrates information regarding peripheral nutrient and hormonal changes and processes this information to regulate energy homeostasis. Recent findings indicate that some of the neural circuits and mechanisms underlying energy balance are also essential for the regulation of glucose homeostasis. We propose that disruption of these overlapping pathways links the metabolic disturbances associated with obesity and T2DM. A better understanding of these converging mechanisms may lead to therapeutic strategies that target both T2DM and obesity.

Regulatory T cells, mTOR kinase, and metabolic activity

The field that links immunity and metabolism is rapidly expanding. Apparently, non-immunological disorders such as obesity and type 2 diabetes have been linked to immune dysregulation, suggesting that metabolic alterations can be induced by or be a consequence of an altered self-immune tolerance. In this context, a key role is played by signaling systems acting as metabolic "sensors" linking energy/nutritional status to regulatory T (Treg) cell functions. We propose that a dynamic/oscillatory activity of intracellular metabolism, through mTOR modulation, might represent a shift in understanding the molecular mechanisms governing Treg cell tolerance. In particular, the decision between Treg cell proliferation and hyporesponsiveness arises from their ability to probe the extracellular milieu and, modulating the metabolic intracellular signaling, to determine different qualitative and quantitative functional outcomes.

Leptin and insulin pathways in POMC and AgRP neurons that modulate energy balance and glucose homeostasis

With the steady rise in the prevalence of obesity and its associated diseases, research aimed at understanding the mechanisms that regulate and control whole body energy homeostasis has gained new interest. Leptin and insulin, two anorectic hormones, have key roles in the regulation of body weight and energy homeostasis, as highlighted by the fact that several obese patients develop resistance to these hormones. Within the brain, the hypothalamic proopiomelanocortin and agouti-related protein neurons have been identified as major targets of leptin and insulin action. Many studies have attempted to discern the individual contributions of various components of the principal pathways that mediate the central effects of leptin and insulin. The aim of this review is to discuss the latest findings that might shed light on, and lead to a better understanding of, energy balance and glucose homeostasis. In addition, recently discovered targets and mechanisms that mediate hormonal action in the brain are highlighted.

Brainstem nutrient sensing in the nucleus of the solitary tract inhibits feeding

Direct detection of circulating nutrients by the central nervous system has been implicated in the regulation of energy balance, and the mediobasal hypothalamus is considered as the primary sensing site mediating these effects. Neurons sensitive to energyrelated signals have also been identified outside the hypothalamus, particularly within the caudomedial nucleus of the solitary tract (cmNTS) in brainstem, but the consequences of direct cmNTS nutrient detection on energy balance remain poorly characterized. Here we determined the behavioral and metabolic consequences of direct L-leucine detection by the cmNTS and investigated the intracellular signaling and neurochemical pathways implicated in cmNTS L-leucine sensing in rats. Our results support the distributed nature of central nutrient detection, evidence a role for the cmNTS S6K1 pathway in the regulation of meal size and body weight, and suggest that the cmNTS integrates direct cmNTS nutrient detection with gut-derived, descending forebrain, and adiposity signals of energy availability to regulate food intake.

TXNIP in Agrp neurons regulates adiposity, energy expenditure, and central leptin sensitivity

Thioredoxin interacting protein (TXNIP) has recently been described as a key regulator of energy metabolism through pleiotropic actions that include nutrient sensing in the mediobasal hypothalamus (MBH). However, the role of TXNIP in neurochemically specific hypothalamic subpopulations and the circuits downstream from MBH TXNIP engaged to regulate energy homeostasis remain unexplored. To evaluate the metabolic role of TXNIP activity specifically within arcuate Agrp neurons, we generated Agrp-specific TXNIP gain-of-function and loss-of-function mouse models using Agrp-Ires-cre mice, TXNIP (flox/flox) mice, and a lentivector expressing the human TXNIP isoform conditionally in the presence of Cre recombinase. Overexpression of TXNIP in Agrp neurons predisposed to diet-induced obesity and adipose tissue storage by decreasing energy expenditure and spontaneous locomotion, without affecting food intake. Conversely, Agrp neuronal TXNIP deletion protected against diet-induced obesity and adipose tissue storage by increasing energy expenditure and spontaneous locomotion, also without affecting food intake. TXNIP overexpression in Agrp neurons did not primarily affect glycemic control, whereas deletion of TXNIP in Agrp neurons improved fasting glucose levels and glucose tolerance independently of its effects on body weight and adiposity. Bidirectional manipulation of TXNIP expression induced reciprocal changes in central leptin sensitivity and the neural regulation of lipolysis. Together, these results identify a critical role for TXNIP in Agrp neurons in mediating diet-induced obesity through the regulation of energy expenditure and adipose tissue metabolism, independently of food intake. They also reveal a previously unidentified role for Agrp neurons in the brain-adipose axis.

Rapamycin ameliorates age-dependent obesity associated with increased mTOR signaling in hypothalamic POMC neurons

VIDEO ABSTRACT

The prevalence of obesity in older people is the leading cause of metabolic syndromes. Central neurons serving as homeostatic sensors for body-weight control include hypothalamic neurons that express pro-opiomelanocortin (POMC) or neuropeptide-Y (NPY) and agouti-related protein (AgRP). Here, we report an age-dependent increase of mammalian target of rapamycin (mTOR) signaling in POMC neurons that elevates the ATP-sensitive potassium (K(ATP)) channel activity cell-autonomously to silence POMC neurons. Systemic or intracerebral administration of the mTOR inhibitor rapamycin causes weight loss in old mice. Intracerebral rapamycin infusion into old mice enhances the excitability and neurite projection of POMC neurons, thereby causing a reduction of food intake and body weight. Conversely, young mice lacking the mTOR-negative regulator TSC1 in POMC neurons, but not those lacking TSC1 in NPY/AgRP neurons, were obese. Our study reveals that an increase in mTOR signaling in hypothalamic POMC neurons contributes to age-dependent obesity.

Hypothalamic mTOR signaling mediates the orexigenic action of ghrelin

Current evidence suggests that ghrelin, a stomach derived peptide, exerts its orexigenic action through specific modulation of Sirtuin1 (SIRT1)/p53 and AMP-activated protein kinase (AMPK) pathways, which ultimately increase the expression of agouti-related protein (AgRP) and neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus (ARC). However, there is a paucity of data about the possible action of ghrelin on alternative metabolic pathways at this level. Here, we demonstrate that ghrelin elicits a marked upregulation of the hypothalamic mammalian target of rapamycin (mTOR) signaling pathway. Of note, central inhibition of mTOR signaling with rapamycin decreased ghrelin’s orexigenic action and normalized the mRNA expression of AgRP and NPY, as well as their key downstream transcription factors, namely cAMP response-element binding protein (pCREB) and forkhead box O1 (FoxO1, total and phosphorylated). Taken together, these data indicate that, in addition to previous reported mechanisms, ghrelin also promotes feeding through modulation of hypothalamic mTOR pathway.

S6K1 in the central nervous system regulates energy expenditure via MC4R/CRH pathways in response to deprivation of an essential amino acid

It is well established that the central nervous system (CNS), especially the hypothalamus, plays an important role in regulating energy homeostasis and lipid metabolism. We have previously shown that hypothalamic corticotropin-releasing hormone (CRH) is critical for stimulating fat loss in response to dietary leucine deprivation. The molecular mechanisms underlying the CNS regulation of leucine deprivation-stimulated fat loss are, however, still largely unknown. Here, we used intracerebroventricular injection of adenoviral vectors to identify a novel role for hypothalamic p70 S6 kinase 1 (S6K1), a major downstream effector of the kinase mammalian target of rapamycin, in leucine deprivation stimulation of energy expenditure. Furthermore, we show that the effect of hypothalamic S6K1 is mediated by modulation of Crh expression in a melanocortin-4 receptor-dependent manner. Taken together, our studies provide a new perspective for understanding the regulation of energy expenditure by the CNS and the importance of cross-talk between nutritional control and regulation of endocrine signals.

Leptin-induced mTOR activation defines a specific molecular and transcriptional signature controlling CD4+ effector T cell responses

The sensing by T cells of metabolic and energetic changes in the microenvironment can determine the differentiation, maturation, and activation of these cells. Although it is known that mammalian target of rapamycin (mTOR) gauges nutritonal and energetic signals in the extracellular milieu, it is not known how mTOR and metabolism influence CD4+CD25-FOXP3- effector T cell (Teff) responses. In this article, we show that leptin-induced activation of mTOR, which, in turn, controls leptin production and signaling, causes a defined cellular, biochemical, and transcriptional signature that determine the outcome of Teff responses, both in vitro and in vivo. The blockade of leptin/leptin receptor signaling, induced by genetic means or by starvation, leads to impaired mTOR activity that inhibits the proliferation of Teffs in vivo. Notably, the transcriptional signature of Teffs in the presence of leptin blockade appears similar to that observed in rapamycin-treated Teffs. These results identify a novel link between nutritional status and Teff responses through the leptin-mTOR axis and define a potential target for Teff modulation in normal and pathologic conditions.

p70S6 kinase phosphorylates AMPK on serine 491 to mediate leptin's effect on food intake

The PI3K-AKT, mTOR-p70S6 kinase and AMPK pathways play distinct and critical roles in metabolic regulation. Each pathway is necessary for leptin's anorexigenic effects in the hypothalamus. Here we show that these pathways converge in an integrated phosphorylation cascade to mediate leptin action in the hypothalamus. We identify serine(491) on α2AMPK as the site of convergence and show that p70S6 kinase forms a complex with α2AMPK, resulting in phosphorylation on serine(491). Blocking α2AMPK-serine(491) phosphorylation increases hypothalamic AMPK activity, food intake, and body weight. Serine(491) phosphorylation is necessary for leptin's effects on hypothalamic α2AMPK activity, neuropeptide expression, food intake, and body weight. These results identify an inhibitory AMPK kinase, p70S6 kinase, and demonstrate that AMPK is a substrate for mTOR-p70S6 kinase. This discovery has broad biologic implications since mTOR-p70S6 kinase and AMPK have multiple, fundamental and generally opposing cellular effects that regulate metabolism, cell growth, and development.

mTOR signaling in growth control and disease

The mechanistic target of rapamycin (mTOR) signaling pathway senses and integrates a variety of environmental cues to regulate organismal growth and homeostasis. The pathway regulates many major cellular processes and is implicated in an increasing number of pathological conditions, including cancer, obesity, type 2 diabetes, and neurodegeneration. Here, we review recent advances in our understanding of the mTOR pathway and its role in health, disease, and aging. We further discuss pharmacological approaches to treat human pathologies linked to mTOR deregulation.

Anorexigenic and orexigenic hormone modulation of mammalian target of rapamycin complex 1 activity and the regulation of hypothalamic agouti-related protein mRNA expression

Activation of mammalian target of rapamycin 1 (mTORC1) by nutrients, insulin and leptin leads to appetite suppression (anorexia). Contrastingly, increased AMP-activated protein kinase (AMPK) activity by ghrelin promotes appetite (orexia). However, the interplay between these mechanisms remains poorly defined. The relationship between the anorexigenic hormones, insulin and leptin, and the orexigenic hormone, ghrelin, on mTORC1 signalling was examined using S6 kinase phosphorylation as a marker for changes in mTORC1 activity in mouse hypothalamic GT1-7 cells. Additionally, the contribution of AMPK and mTORC1 signalling in relation to insulin-, leptin- and ghrelin-driven alterations to mouse hypothalamic agouti-related protein (AgRP) mRNA levels was examined. Insulin and leptin increase mTORC1 activity in a phosphoinositide-3-kinase (PI3K)- and protein kinase B (PKB)-dependent manner, compared to vehicle controls, whereas increasing AMPK activity inhibits mTORC1 activity and blocks the actions of the anorexigenic hormones. Ghrelin mediates an AMPK-dependent decrease in mTORC1 activity and increases hypothalamic AgRP mRNA levels, the latter effect being prevented by insulin in an mTORC1-dependent manner. In conclusion, mTORC1 acts as an integration node in hypothalamic neurons for hormone-derived PI3K and AMPK signalling and mediates at least part of the assimilated output of anorexigenic and orexigenic hormone actions in the hypothalamus.

Metabolic control by S6 kinases depends on dietary lipids

Targeted deletion of S6 kinase (S6K) 1 in mice leads to higher energy expenditure and improved glucose metabolism. However, the molecular mechanisms controlling these effects remain to be fully elucidated. Here, we analyze the potential role of dietary lipids in regulating the mTORC1/S6K system. Analysis of S6K phosphorylation in vivo and in vitro showed that dietary lipids activate S6K, and this effect is not dependent upon amino acids. Comparison of male mice lacking S6K1 and 2 (S6K-dko) with wt controls showed that S6K-dko mice are protected against obesity and glucose intolerance induced by a high-fat diet. S6K-dko mice fed a high-fat diet had increased energy expenditure, improved glucose tolerance, lower fat mass gain, and changes in markers of lipid metabolism. Importantly, however, these metabolic phenotypes were dependent upon dietary lipids, with no such effects observed in S6K-dko mice fed a fat-free diet. These changes appear to be mediated via modulation of cellular metabolism in skeletal muscle, as shown by the expression of genes involved in energy metabolism. Taken together, our results suggest that the metabolic functions of S6K in vivo play a key role as a molecular interface connecting dietary lipids to the endogenous control of energy metabolism.

Hypothalamic mTOR pathway mediates thyroid hormone-induced hyperphagia in hyperthyroidism

Hyperthyroidism is characterized in rats by increased energy expenditure and marked hyperphagia. Alterations of thermogenesis linked to hyperthyroidism are associated with dysregulation of hypothalamic AMPK and fatty acid metabolism; however, the central mechanisms mediating hyperthyroidism-induced hyperphagia remain largely unclear. Here, we demonstrate that hyperthyroid rats exhibit marked up-regulation of the hypothalamic mammalian target of rapamycin (mTOR) signalling pathway associated with increased mRNA levels of agouti-related protein (AgRP) and neuropeptide Y (NPY), and decreased mRNA levels of pro-opiomelanocortin (POMC) in the arcuate nucleus of the hypothalamus (ARC), an area where mTOR co-localizes with thyroid hormone receptor-α (TRα). Central administration of thyroid hormone (T3) or genetic activation of thyroid hormone signalling in the ARC recapitulated hyperthyroidism effects on feeding and the mTOR pathway. In turn, central inhibition of mTOR signalling with rapamycin in hyperthyroid rats reversed hyperphagia and normalized the expression of ARC-derived neuropeptides, resulting in substantial body weight loss. The data indicate that in the hyperthyroid state, increased feeding is associated with thyroid hormone-induced up-regulation of mTOR signalling. Furthermore, our findings that different neuronal modulations influence food intake and energy expenditure in hyperthyroidism pave the way for a more rational design of specific and selective therapeutic compounds aimed at reversing the metabolic consequences of this disease.

Selfish cellular networks and the evolution of complex organisms

Human gametogenesis takes years and involves many cellular divisions, particularly in males. Consequently, gametogenesis provides the opportunity to acquire multiple de novo mutations. A significant portion of these is likely to impact the cellular networks linking genes, proteins, RNA and metabolites, which constitute the functional units of cells. A wealth of literature shows that these individual cellular networks are complex, robust and evolvable. To some extent, they are able to monitor their own performance, and display sufficient autonomy to be termed "selfish". Their robustness is linked to quality control mechanisms which are embedded in and act upon the individual networks, thereby providing a basis for selection during gametogenesis. These selective processes are equally likely to affect cellular functions that are not gamete-specific, and the evolution of the most complex organisms, including man, is therefore likely to occur via two pathways: essential housekeeping functions would be regulated and evolve during gametogenesis within the parents before being transmitted to their progeny, while classical selection would operate on other traits of the organisms that shape their fitness with respect to the environment.

Regulation and function of ribosomal protein S6 kinase (S6K) within mTOR signalling networks

The ribosomal protein S6K (S6 kinase) represents an extensively studied effector of the TORC1 [TOR (target of rapamycin) complex 1], which possesses important yet incompletely defined roles in cellular and organismal physiology. TORC1 functions as an environmental sensor by integrating signals derived from diverse environmental cues to promote anabolic and inhibit catabolic cellular functions. mTORC1 (mammalian TORC1) phosphorylates and activates S6K1 and S6K2, whose first identified substrate was rpS6 (ribosomal protein S6), a component of the 40S ribosome. Studies over the past decade have uncovered a number of additional S6K1 substrates, revealing multiple levels at which the mTORC1-S6K1 axis regulates cell physiology. The results thus far indicate that the mTORC1-S6K1 axis controls fundamental cellular processes, including transcription, translation, protein and lipid synthesis, cell growth/size and cell metabolism. In the present review we summarize the regulation of S6Ks, their cellular substrates and functions, and their integration within rapidly expanding mTOR (mammalian TOR) signalling networks. Although our understanding of the role of mTORC1-S6K1 signalling in physiology remains in its infancy, evidence indicates that this signalling axis controls, at least in part, glucose homoeostasis, insulin sensitivity, adipocyte metabolism, body mass and energy balance, tissue and organ size, learning, memory and aging. As dysregulation of this signalling axis contributes to diverse disease states, improved understanding of S6K regulation and function within mTOR signalling networks may enable the development of novel therapeutics.

Bone morphogenetic protein 7 (BMP7) reverses obesity and regulates appetite through a central mTOR pathway

Body weight is regulated by coordinating energy intake and energy expenditure. Transforming growth factor β (TGFβ)/bone morphogenetic protein (BMP) signaling has been shown to regulate energy balance in lower organisms, but whether a similar pathway exists in mammals is unknown. We have previously demonstrated that BMP7 can regulate brown adipogenesis and energy expenditure. In the current study, we have uncovered a novel role for BMP7 in appetite regulation. Systemic treatment of diet-induced obese mice with BMP7 resulted in increased energy expenditure and decreased food intake, leading to a significant reduction in body weight and improvement of metabolic syndrome. Similar degrees of weight loss with reduced appetite were also observed in BMP7-treated ob/ob mice, suggesting a leptin-independent mechanism utilized by BMP7. Intracerebroventricular administration of BMP7 to mice led to an acute decrease in food intake, which was mediated, at least in part, by a central rapamycin-sensitive mTOR-p70S6 kinase pathway. Together, these results underscore the importance of BMP7 in regulating both food intake and energy expenditure, and suggest new therapeutic approaches for obesity and its comorbidities.

Homeostatic regulation of protein intake: in search of a mechanism

Free-living organisms must procure adequate nutrition by negotiating an environment in which both the quality and quantity of food vary markedly. Recent decades have seen marked progress in our understanding of neural regulation of feeding behavior. However, this progress has occurred largely in the context of energy intake, despite the fact that food intake is influenced by more than just the energy content of the diet. A large number of behavioral studies indicate that both the quantity and quality of dietary protein can markedly influence food intake. High-protein diets tend to reduce intake, low-protein diets tend to increase intake, and rodent models seem to self-select between diets in order to meet protein requirements and avoid diets that are imbalanced in amino acids. Recent work suggests that the amino acid leucine regulates food intake by altering mTOR and AMPK signaling in the hypothalamus, while activation of GCN2 within the anterior piriform cortex contributes to the detection and avoidance of amino acid-imbalanced diets. This review focuses on the role that these and other signaling systems may play in mediating the homeostatic regulation of protein balance, and in doing so, highlights our lack of knowledge regarding the physiological and neurobiological mechanisms that might underpin such a regulatory phenomenon.

SIRT1 and CLOCK 3111T> C combined genotype is associated with evening preference and weight loss resistance in a behavioral therapy treatment for obesity

BACKGROUND

A new negative feedback loop has been proposed, which suggests connections between the circadian clock and SIRTUIN1 (SIRT1)-dependent functions associated with cell survival, development and metabolism.

OBJECTIVE

To develop a SIRT1 and circadian locomotor output cycles kaput (CLOCK) combined genotype and to assess its associations with the chronotype of subjects and their potential resistance to weight loss in a behavioral treatment for obesity based on a Mediterranean diet.

DESIGN

Overweight /obese subjects (n=1465), aged 20-65 years, who attended outpatient obesity clinics, were genotyped for SIRT1 (rs1467568) and CLOCK (3111T>C, rs1801260). Anthropometric, biochemical and dietary-intake variables were analyzed. Effectiveness of the program and weight loss progression during 30 weeks of treatment was assessed.

RESULTS

We found highly consistent associations between the morning/evening questionnaires across the different genotype categories. Subjects carrying minor alleles at SIRT1 and CLOCK loci (R group) displayed a higher resistance to weight loss and a lower weekly weight loss rate as compared with homozygotes for both major alleles (P group). Significant differences were found across genotypes in weight loss progression during the 30 weeks of treatment (P=0.039). Dietary habits indicated that R carriers had a lower intake of total carbohydrates and monounsaturated fats, and a higher intake of saturated fats than those carrying the intermediate (M) and the P genotype (P=0.02). Plasma ghrelin concentrations were also significantly higher in subjects carrying the R genotype.

CONCLUSION

Variants of both SIRT1 and CLOCK have an additive effect on resistance to weight loss that could be related to the chronotype of the subject, higher plasma levels of ghrelin and less adherence to Mediterranean diet patterns.

Intracerebroventricular administration of metformin inhibits ghrelin-induced Hypothalamic AMP-kinase signalling and food intake

BACKGROUND/AIMS

The antihyperglycaemic drug metformin reduces food consumption through mechanisms that are not fully elucidated. The present study investigated the effects of intracerebroventricular administration of metformin on food intake and hypothalamic appetite-regulating signalling pathways induced by the orexigenic peptide ghrelin.

METHODS

Rats were injected intracerebroventricularly with ghrelin (5 µg), metformin (50, 100 or 200 µg), 5-amino-imidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR, 25 µg) and L-leucine (1 µg) in different combinations. Food intake was monitored during the next 4 h. Hypothalamic activation of AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), regulatory-associated protein of mTOR (Raptor), mammalian target of rapamycin (mTOR) and p70 S6 kinase 1 (S6K) after 1 h of treatment was analysed by immunoblotting.

RESULTS

Metformin suppressed the increase in food consumption induced by intracerebroventricular ghrelin in a dose-dependent manner. Ghrelin increased phosphorylation of hypothalamic AMPK and its targets ACC and Raptor, which was associated with the reduced phosphorylation of mTOR. The mTOR substrate, S6K, was activated by intracerebroventricular ghrelin despite the inhibition of mTOR. Metformin treatment blocked ghrelin-induced activation of hypothalamic AMPK/ACC/Raptor and restored mTOR activity without affecting S6K phosphorylation. Metformin also reduced food consumption induced by the AMPK activator AICAR while the ghrelin-triggered food intake was inhibited by the mTOR activator L-leucine.

CONCLUSION

Metformin could reduce food intake by preventing ghrelin-induced AMPK signalling and mTOR inhibition in the hypotalamus.

Gastric mammalian target of rapamycin signaling, hormone production and energy metabolism

The obesity epidemic imposes a significant health burden on human beings. Current understanding of the mechanisms underlying the development of obesity is incomplete and contemporary treatment is often ineffective. Gastrointestinal hormones are important regulators of food intake and energy metabolism. Previous studies indicate that the mammalian target of rapamycin signaling pathway in the gastric mucosa is crucially involved in fuel sensing in the gastrointestinal tract and plays a critical role in the coordination of nutrient availability and ingestive behavior via the production of gastric hormones. As an important component of the brain-gut axis regulating food intake and energy homeostasis, energy sensing in the gastrointestinal tract may provide a novel insight into our understanding of the precise coordination between the organism and cellular energy state.

Central nervous control of energy and glucose balance: focus on the central melanocortin system

Studies have suggested that manipulations of the central melanocortin circuitry by pharmacological agents produce robust effects on the regulation of body weight and glucose homeostasis. In this review, we discuss recent findings from genetic mouse models that have further established the physiological relevance of this circuitry in the context of glucose and energy balance. In addition, we will discuss distinct neuronal populations that respond to central melanocortins to regulate food intake, energy expenditure, insulin sensitivity, and insulin secretion, respectively. Finally, multiple hormonal and neural cues (e.g., leptin, estrogen, and serotonin) that use the melanocortin systems to regulate energy and glucose homeostasis will be reviewed. These findings suggest that targeting the specific branches of melanocortin circuits may be potential avenues to combat the current obesity and diabetes epidemics.

Rapamycin passes the torch: a new generation of mTOR inhibitors

Mammalian target of rapamycin (mTOR) is an atypical protein kinase that controls growth and metabolism in response to nutrients, growth factors and cellular energy levels, and it is frequently dysregulated in cancer and metabolic disorders. Rapamycin is an allosteric inhibitor of mTOR, and was approved as an immuno-suppressant in 1999. In recent years, interest has focused on its potential as an anticancer drug. However, the performance of rapamycin and its analogues (rapalogues) has been undistinguished despite isolated successes in subsets of cancer, suggesting that the full therapeutic potential of targeting mTOR has yet to be exploited. A new generation of ATP-competitive inhibitors that directly target the mTOR catalytic site display potent and comprehensive mTOR inhibition and are in early clinical trials.

Hypoxia-inducible factor directs POMC gene to mediate hypothalamic glucose sensing and energy balance regulation

Hypoxia-inducible factor (HIF) is a nuclear transcription factor that responds to environmental and pathological hypoxia to induce metabolic adaptation, vascular growth, and cell survival. Here we found that HIF subunits and HIF2α in particular were normally expressed in the mediobasal hypothalamus of mice. Hypothalamic HIF was up-regulated by glucose to mediate the feeding control of hypothalamic glucose sensing. Two underlying molecular pathways were identified, including suppression of PHDs by glucose metabolites to prevent HIF2α degradation and the recruitment of AMPK and mTOR/S6K to regulate HIF2α protein synthesis. HIF activation was found to directly control the transcription of POMC gene. Genetic approach was then employed to develop conditional knockout mice with HIF inhibition in POMC neurons, revealing that HIF loss-of-function in POMC neurons impaired hypothalamic glucose sensing and caused energy imbalance to promote obesity development. The metabolic effects of HIF in hypothalamic POMC neurons were independent of leptin signaling or pituitary ACTH pathway. Hypothalamic gene delivery of HIF counteracted overeating and obesity under conditions of nutritional excess. In conclusion, HIF controls hypothalamic POMC gene to direct the central nutrient sensing in regulation of energy and body weight balance.

Nutrient-sensing hypothalamic TXNIP links nutrient excess to energy imbalance in mice

Nutrient excess in obesity and diabetes is emerging as a common putative cause for multiple deleterious effects across diverse cell types, responsible for a variety of metabolic dysfunctions. The hypothalamus is acknowledged as an important regulator of whole-body energy homeostasis, through both detection of nutrient availability and coordination of effectors that determine nutrient intake and utilization, thus preventing cellular and whole-body nutrient excess. However, the mechanisms underlying hypothalamic nutrient detection and its impact on peripheral nutrient utilization remain poorly understood. Recent data suggest a role for thioredoxin-interacting protein (TXNIP) as a molecular nutrient sensor important in the regulation of energy metabolism, but the role of hypothalamic TXNIP in the regulation of energy balance has not been evaluated. Here we show in mice that TXNIP is expressed in nutrient-sensing neurons of the mediobasal hypothalamus, responds to hormonal and nutrient signals, and regulates adipose tissue metabolism, fuel partitioning, and glucose homeostasis. Hypothalamic expression of TXNIP is induced by acute nutrient excess and in mouse models of obesity and diabetes, and downregulation of mediobasal hypothalamic TXNIP expression prevents diet-induced obesity and insulin resistance. Thus, mediobasal hypothalamic TXNIP plays a critical role in nutrient sensing and the regulation of fuel utilization.

Modulation of the central melanocortin system by leptin, insulin, and serotonin: co-ordinated actions in a dispersed neuronal network

Over the past century, prevalent models of energy and glucose homeostasis have been developed from a better understanding of the neural circuits underlying obesity and diabetes. From the early hypothalamic lesion reports to the more recent pharmacological and molecular/genetic studies, the hypothalamic melanocortin system has been shown to play a critical role in the regulation of metabolism. This review attempts to highlight contributions to our current understanding of how numerous neuromodulators (leptin, insulin, and serotonin) integrate with the central melanocortin system to coordinate alterations in energy and glucose balance.

mTOR couples cellular nutrient sensing to organismal metabolic homeostasis

The mammalian target of rapamycin complex 1 (mTORC1) has the ability to sense a variety of essential nutrients and respond by altering cellular metabolic processes. Hence, this protein kinase complex is poised to influence adaptive changes to nutrient fluctuations toward the maintenance of whole-body metabolic homeostasis. Defects in mTORC1 regulation, arising from either physiological or genetic conditions, are believed to contribute to the metabolic dysfunction underlying a variety of human diseases, including type 2 diabetes. We are just now beginning to gain insights into the complex tissue-specific functions of mTORC1. In this review, we detail the current knowledge of the physiological functions of mTORC1 in controlling systemic metabolism, with a focus on advances obtained through genetic mouse models.

An oscillatory switch in mTOR kinase activity sets regulatory T cell responsiveness

There is a discrepancy between the in vitro anergic state of CD4(+)CD25(hi)FoxP3(+) regulatory T (Treg) cells and their in vivo proliferative capability. The underlying mechanism of this paradox is unknown. Here we show that the anergic state of Treg cells depends on the elevated activity of the mammalian target of rapamycin (mTOR) pathway induced by leptin: a transient inhibition of mTOR with rapamycin, before T cell receptor (TCR) stimulation, made Treg cells highly proliferative in the absence of exogenous interleukin-2 (IL-2). This was a dynamic and oscillatory phenomenon characterized by an early downregulation of the leptin-mTOR pathway followed by an increase in mTOR activation necessary for Treg cell expansion to occur. These data suggest that energy metabolism, through the leptin-mTOR-axis, sets responsiveness of Treg cells that use this information to control immune tolerance and autoimmunity.

Mammalian target of rapamycin inhibition abrogates insulin-mediated mammary tumor progression in type 2 diabetes

Type 2 diabetes increases breast cancer risk and mortality, and hyperinsulinemia is a major mediator of this effect. The mammalian target of rapamycin (mTOR) is activated by insulin and is a key regulator of mammary tumor progression. Pharmacological mTOR inhibition suppresses tumor growth in numerous mammary tumor models in the non-diabetic setting. However, the role of the mTOR pathway in type 2 diabetes-induced tumor growth remains elusive. Herein, we investigated whether the mTOR pathway is implicated in insulin-induced mammary tumor progression in a transgenic mouse model of type 2 diabetes (MKR mice) and evaluated the impact of mTOR inhibition on the diabetic state. Mammary tumor progression was studied in the double transgenic MMTV-Polyoma Virus middle T antigen (PyVmT)/MKR mice and by orthotopic inoculation of PyVmT- and Neu/ErbB2-driven mammary tumor cells (Met-1 and MCNeuA cells respectively). mTOR inhibition by rapamycin markedly suppressed tumor growth in both wild-type and MKR mice. In diabetic animals, however, the promoting action of insulin on tumor growth was completely blunted by rapamycin, despite a worsening of the carbohydrate and lipid metabolism. Taken together, pharmacological mTOR blockade is sufficient to abrogate mammary tumor progression in the setting of hyperinsulinemia, and thus mTOR inhibitors may be an attractive therapeutic modality for breast cancer patients with type 2 diabetes. Careful monitoring of the metabolic state, however, is important as dose adaptations of glucose- and/or lipid-lowering therapy might be necessary.

Central nervous system nutrient signaling: the regulation of energy balance and the future of dietary therapies

The mammalian target of rapamycin (mTOR) pathway coordinates cell growth in response to nutrient availability. Increasing evidence points to a role for mTOR to also direct whole-body energy balance in response to micronutrient as well as hormonal cues. This positions mTOR as a key central integrator of acute and chronic changes in fuel status. Energy balance is affected by mTOR in several organ systems, including the hypothalamus, where the pathway can modulate feeding. We propose that a greater understanding of this nutrient-sensitive pathway may open the door to more intelligent, effective diet design based on the effects of micronutrients on specific signaling pathways.

TOR on the brain

Signaling by target of rapamycin (mTOR in mammals) has been shown to modulate lifespan in several model organisms ranging from yeast to mice. In mice, reduced mTOR signaling by chronic rapamycin treatment leads to life span extension, raising the possibility that rapamycin and its analogs may benefit the aging brain and serve as effective treatments of age-related neurodegenerative diseases. Here, we review mTOR signaling and how neurons utilize mTOR to regulate brain function, including regulation of feeding, synaptic plasticity and memory formation. Additionally, we discuss recent findings that evaluate the mechanisms by which reduced mTOR activity might benefit the aging brain in normal and pathological states. We will focus on recent studies investigating mTOR and Alzheimer's disease, Parkinson's disease, and polyglutamine expansion syndromes such as Huntington's disease.

Glucocorticoids increase NPY gene expression in the arcuate nucleus by inhibiting mTOR signaling in rat hypothalamic organotypic cultures

The mammalian target of rapamycin (mTOR) has been implicated in the regulation of physiological functions such as cell growth and proliferation, and glucocorticoids reportedly inhibit mTOR signaling in peripheral tissues. Recent studies suggest that the mTOR signaling in the hypothalamus plays a critical role in maintaining energy homeostasis. In this study, we examined whether the mTOR signaling in the hypothalamus is involved in the regulation of neuropeptide Y (Npy) gene expression in the arcuate nucleus by glucocorticoids. In the hypothalamic organotypic cultures, the incubation with rapamycin significantly inhibited the mTOR signaling which was shown by decreases in the levels of phosphorylated p70S6K1 and S6. Similar to the action of the mTOR inhibitor rapamycin, dexamethasone (DEX), a synthetic glucocorticoid, also inhibited the mTOR signaling in the hypothalamic explants. Analyses of the explants with in situ hybridization demonstrated that the DEX or rapamycin alone significantly increased Npy gene expression in the arcuate nucleus, but that there were no additive effects of DEX and rapamycin on the expression. These data suggest that glucocorticoids upregulate the Npy gene expression in the arcuate nucleus by inhibiting mTOR signaling, at least in part.

Hypothalamic nutrient sensing in the control of energy homeostasis

The hypothalamus is a center of convergence and integration of multiple nutrient-related signals. It can sense changes in circulating adiposity hormones, gastric hormones and nutrients, and receives neuroanatomical projections from other nutrient sensors, mainly within the brainstem. The hypothalamus also integrates these signals with various cognitive forebrain-descending information and reward/motivation-related signals coming from the midbrain-dopamine system, to coordinate neuroendocrine, behavioral and metabolic effectors of energy balance. Some of the key nutrient-sensing hypothalamic neurons have been identified in the arcuate, the ventro-medial and the lateral nuclei of the hypothalamus, and the molecular mechanisms underlying intracellular integration of nutrient-related signals in these neurons are currently under intensive investigation. However, little is known about the neural pathways downstream from hypothalamic nutrient sensors, and how they drive effectors of energy homeostasis under physiological conditions. This manuscript will review recent progress from molecular, genetic and neurophysiological studies that identify and characterize the critical intracellular signalling pathways and neurocircuits involved in determining hypothalamic nutrient detection, and link these circuits to behavioral and metabolic effectors of energy balance. We will provide a critical analysis of current data to identify ongoing challenges for future research in this field.

Hypothalamic Sirt1 regulates food intake in a rodent model system

Sirt1 is an evolutionarily conserved NAD(+) dependent deacetylase involved in a wide range of processes including cellular differentiation, apoptosis, as well as metabolism, and aging. In this study, we investigated the role of hypothalamic Sirt1 in energy balance. Pharmacological inhibition or siRNA mediated knock down of hypothalamic Sirt1 showed to decrease food intake and body weight gain. Central administration of a specific melanocortin antagonist, SHU9119, reversed the anorectic effect of hypothalamic Sirt1 inhibition, suggesting that Sirt1 regulates food intake through the central melanocortin signaling. We also showed that fasting increases hypothalamic Sirt1 expression and decreases FoxO1 (Forkhead transcription factor) acetylation suggesting that Sirt1 regulates the central melanocortin system in a FoxO1 dependent manner. In addition, hypothalamic Sirt1 showed to regulate S6K signaling such that inhibition of the fasting induced Sirt1 activity results in up-regulation of the S6K pathway. Thus, this is the first study providing a novel role for the hypothalamic Sirt1 in the regulation of food intake and body weight. Given the role of Sirt1 in several peripheral tissues and hypothalamus, potential therapies centered on Sirt1 regulation might provide promising therapies in the treatment of metabolic diseases including obesity.

Recent advances in understanding leptin signaling and leptin resistance

The brain controls energy homeostasis and body weight by integrating various metabolic signals. Leptin, an adipose-derived hormone, conveys critical information about peripheral energy storage and availability to the brain. Leptin decreases body weight by both suppressing appetite and promoting energy expenditure. Leptin directly targets hypothalamic neurons, including AgRP and POMC neurons. These leptin-responsive neurons widely connect to other neurons in the brain, forming a sophisticated neurocircuitry that controls energy intake and expenditure. The anorexigenic actions of leptin are mediated by LEPRb, the long form of the leptin receptor, in the hypothalamus. LEPRb activates both JAK2-dependent and -independent pathways, including the STAT3, PI 3-kinase, MAPK, AMPK, and mTOR pathways. These pathways act coordinately to form a network that fully mediates leptin response. LEPRb signaling is regulated by both positive (e.g., SH2B1) and negative (e.g., SOCS3 and PTP1B) regulators and by endoplasmic reticulum stress. Leptin resistance, a primary risk factor for obesity, likely results from impairment in leptin transport, LEPRb signaling, and/or the neurocircuitry of energy balance.

Central leptin receptor action and resistance in obesity

The discovery of leptin in 1994 has led to remarkable advances in obesity research. We now know that leptin is a cytokinelike hormone that is produced in adipose tissue and plays a pivotal role in regulation of energy balance and in a variety of additional processes via actions in the central nervous system. This symposium review covers current understandings of neuronal leptin receptor signaling and mechanisms of obesity-related leptin resistance in the central nervous system and provides recent insights into the regulation of peripheral glucose balance by central leptin action in rodents.

Mediobasal hypothalamic leucine sensing regulates food intake through activation of a hypothalamus-brainstem circuit

In response to nutrient stimuli, the mediobasal hypothalamus (MBH) drives multiple neuroendocrine and behavioral mechanisms to regulate energy balance. While central leucine reduces food intake and body weight, the specific neuroanatomical sites of leucine sensing, downstream neural substrates, and neurochemical effectors involved in this regulation remain largely unknown. Here we demonstrate that MBH leucine engages a neural energy regulatory circuit by stimulating POMC (proopiomelanocortin) neurons of the MBH, oxytocin neurons of the paraventricular hypothalamus, and neurons within the brainstem nucleus of the solitary tract to acutely suppress food intake by reducing meal size. We identify central p70 S6 kinase and Erk1/2 pathways as intracellular effectors required for this response. Activation of endogenous leucine intracellular metabolism produced longer-term reductions in meal number. Our data identify a novel, specific hypothalamus-brainstem circuit that links amino acid availability and nutrient sensing to the control of food intake.

Mammalian target of rapamycin complex 1 (mTORC1) signaling in energy balance and obesity

Energy balance is guaranteed by a complex circuitry that in the brain, and in the hypothalamus in particular, integrates and coordinates several types of signals, including hormones and nutrients, so to match energy expenditure to energy needs. Similar to individual cells, the hypothalamus also profits from intracellular pathways known to work as fuel sensors to maintain energy balance. The mammalian target of rapamycin complex 1 (mTORC1) pathway has been recently implicated in such function, due to its ability to integrate nutrient and hormonal signals to control food intake and body weight. This review therefore describes recent advances made in understanding the role of the hypothalamic mTORC1 pathway in energy balance regulation and its possible contribution to the metabolic dysregulation associated with diet-induced obesity.