Hypothalamic mTOR signaling regulates food intake

Leptin deficiency and diet-induced obesity reduce hypothalamic kisspeptin expression in mice

The hormone leptin modulates a diverse range of biological functions, including energy homeostasis and reproduction. Leptin promotes GnRH function via an indirect action on forebrain neurons. We tested whether leptin deficiency or leptin resistance due to a high-fat diet (HFD) can regulate the potent reproductive neuropeptide kisspeptin. In mice with normalized levels of estradiol, leptin deficiency markedly reduced kisspeptin gene expression, particularly in the arcuate nucleus (ARC), and kisspeptin immunoreactive cell numbers in the rostral periventricular region of the third ventricle (RP3V). The HFD model was used to determine the effects of diet-induced obesity and central leptin resistance on kisspeptin cell number and gene expression. DBA/2J mice, which are prone to HFD-induced infertility, showed a marked decrease in kisspeptin expression in both the RP3V and ARC and cell numbers in the RP3V after HFD. This is the first evidence that kisspeptin can be regulated by HFD and/or increased body weight. Next we demonstrated that leptin does not signal (via signal transducer and activator of transcription 3 or 5, or mammalian target of rapamycin) directly on kisspeptin-expressing neurons in the RP3V. Lastly, in leptin receptor-deficient mice, neither GnRH nor kisspeptin neurons were activated during a preovulatory-like GnRH/LH surge induction regime, indicating that leptin's actions on GnRH may be upstream of kisspeptin neurons. These data provide evidence that leptin's effects on reproductive function are regulated by kisspeptin neurons in both the ARC and RP3V, although in the latter site the effects are likely to be indirect.

The effects of an amino acid supplement on glucose homeostasis, inflammatory markers, and incretins after laparoscopic gastric bypass

BACKGROUND

Protein supplements are routinely used after a laparoscopic gastric bypass (LGB). The aim of this study was to evaluate the impact of an amino acid supplement on glucose homeostasis and hormonal and inflammatory markers after LGB.

STUDY DESIGN

Thirty patients undergoing LGB were randomized to receive or not 24 g of an oral supplement containing a leucine metabolite, glutamine, and arginine twice daily. Changes in weight, glucose, insulin, C-peptide, insulin sensitivity, interleukin (IL) 6, C-reactive protein (CRP), leptin, insulin-like growth factor (IGF) 1, ghrelin, and incretins were assessed preoperatively and 2 weeks and 8 weeks postoperatively.

RESULTS

Thirty patients (96.7% female, age 46.9 ± 8.4 years, body mass index 43.3 ± 4.1 kg/m(2)) were randomized. The experimental (n = 14) and control (n = 16) groups were not significantly different at baseline. Weight loss was similar for the 2 groups. Fasting glucose decreased significantly at 2 and 8 weeks compared with base line (p < 0.0001) with no difference between the experimental and control groups (p = 0.8), but insulin and calculated insulin sensitivity, which were similar at baseline, became significantly worse in the experimental group 8 weeks after surgery (p = 0.02 for insulin; p = 0.04 for the homeostasis model assessment of insulin resistance). CRP and IL-6, which were similar at baseline, were found to be significantly higher at 8 weeks in the experimental group (p = 0.018 and p = 0.05, respectively). Leptin and IGF-1 levels decreased significantly from baseline at 2 and 8 weeks (p < 0.0001), but there was no difference between the 2 groups. No significant changes in GLP-1, ghrelin, or gastric inhibitory polypeptide were noticed after 8 weeks.

CONCLUSIONS

An amino acid supplement had no effect on the early postoperative incretins after LGB. It may have a negative influence on glucose kinetics and degree of inflammation. Future studies are needed to clarify these effects.

Fluorescence-based fixative and vital staining of lipid droplets in Caenorhabditis elegans reveal fat stores using microscopy and flow cytometry approaches

The proportions of body fat and fat-free mass are determining factors of adiposity-associated diseases. Work in Caenorhabditis elegans has revealed evolutionarily conserved pathways of fat metabolism. Nevertheless, analysis of body composition and fat distribution in the nematodes has only been partially unraveled because of methodological difficulties. We characterized metabolic C. elegans mutants by using novel and feasible BODIPY 493/503-based fat staining and flow cytometry approaches. Fixative as well as vital BODIPY staining procedures visualize major fat stores, preserve native lipid droplet morphology, and allow quantification of fat content per body volume of individual worms. Colocalization studies using coherent anti-Stokes Raman scattering microscopy, Raman microspectroscopy, and imaging of lysosome-related organelles as well as biochemical measurement confirm our approaches. We found that the fat-to-volume ratio of dietary restriction, TGF-β, and germline mutants are specific for each strain. In contrast, the proportion of fat-free mass is constant between the mutants, although their volumes differ by a factor of 3. Our approaches enable sensitive, accurate, and high-throughput assessment of adiposity in large C. elegans populations at a single-worm level.

Hyperphagia and increased fat accumulation in two models of chronic CNS glucagon-like peptide-1 loss of function

Central administration of glucagon-like peptide-1 (GLP-1) causes a dose-dependent reduction in food intake, but the role of endogenous CNS GLP-1 in the regulation of energy balance remains unclear. Here, we tested the hypothesis that CNS GLP-1 activity is required for normal energy balance by using two independent methods to achieve chronic CNS GLP-1 loss of function in rats. Specifically, lentiviral-mediated expression of RNA interference was used to knock down nucleus of the solitary tract (NTS) preproglucagon (PPG), and chronic intracerebroventricular (ICV) infusion of the GLP-1 receptor (GLP-1r) antagonist exendin (9-39) (Ex9) was used to block CNS GLP-1r. NTS PPG knockdown caused hyperphagia and exacerbated high-fat diet (HFD)-induced fat accumulation and glucose intolerance. Moreover, in control virus-treated rats fed the HFD, NTS PPG expression levels correlated positively with fat mass. Chronic ICV Ex9 also caused hyperphagia; however, increased fat accumulation and glucose intolerance occurred regardless of diet. Collectively, these data provide the strongest evidence to date that CNS GLP-1 plays a physiologic role in the long-term regulation of energy balance. Moreover, they suggest that this role is distinct from that of circulating GLP-1 as a short-term satiation signal. Therefore, it may be possible to tailor GLP-1-based therapies for the prevention and/or treatment of obesity.

Altered hypothalamic function in diet-induced obesity

Energy homeostasis involves a complex network of hypothalamic and extra-hypothalamic neurons that transduce hormonal, nutrient and neuronal signals into responses that ultimately match caloric intake to energy expenditure and thereby promote stability of body fat stores. Growing evidence suggests that rather than reflecting a failure to regulate caloric intake, common forms of obesity involve fundamental changes to this homeostatic system that favor the defense of an elevated level of body adiposity. This article reviews emerging evidence that during high-fat feeding, obesity pathogenesis involves fundamental alteration of hypothalamic systems that regulate food intake and energy expenditure.

Induction of leptin resistance by activation of cAMP-Epac signaling

Leptin regulates energy balance and glucose homeostasis. Shortly after leptin was identified, it was established that obesity is commonly associated with leptin resistance, though the molecular mechanisms remain to be identified. To explore potential mechanisms of leptin resistance, we employed organotypic brain slices to identify candidate signaling pathways that negatively regulate leptin sensitivity. We found that elevation of adenosine 3', 5'-monophosphate (cAMP) levels impairs multiple signaling cascades activated by leptin within the hypothalamus. Notably, this effect is independent of protein kinase A activation. In contrast, activation of Epac, a cAMP-regulated guanine nucleotide exchange factor for the small G protein Rap1, was sufficient to impair leptin signaling with concomitant induction of SOCS-3 expression. Epac activation also blunted leptin-induced depolarization of hypothalamic POMC neurons. Finally, central infusion of an Epac activator blunted the anorexigenic actions of leptin. Thus, activation of hypothalamic cAMP-Epac pathway is sufficient to induce multiple indices of leptin resistance.

Dietary fat sensing via fatty acid oxidation in enterocytes: possible role in the control of eating

Various mechanisms detect the presence of dietary triacylglycerols (TAG) in the digestive tract and link TAG ingestion to the regulation of energy homeostasis. We here propose a novel sensing mechanism with the potential to encode dietary TAG-derived energy by translating enterocyte fatty acid oxidation (FAO) into vagal afferent signals controlling eating. Peripheral FAO has long been implicated in the control of eating ( 141 ). The prevailing view was that mercaptoacetate (MA) and other FAO inhibitors stimulate eating by modulating vagal afferent signaling from the liver. This concept has been challenged because hepatic parenchymal vagal afferent innervation is scarce and because experimentally induced changes in hepatic FAO often fail to affect eating. Nevertheless, intraperitoneally administered MA acts in the abdomen to stimulate eating because this effect was blocked by subdiaphragmatic vagal deafferentation ( 21 ), a surgical technique that eliminates all vagal afferents from the upper gut. These and other data support a role of the small intestine rather than the liver as a FAO sensor that can influence eating. After intrajejunal infusions, MA also stimulated eating in rats through vagal afferent signaling, and after infusion into the superior mesenteric artery, MA increased the activity of celiac vagal afferent fibers originating in the proximal small intestine. Also, pharmacological interference with TAG synthesis targeting the small intestine induced a metabolic profile indicative of increased FAO and inhibited eating in rats on a high-fat diet but not on chow. Finally, cell culture studies indicate that enterocytes oxidize fatty acids, which can be modified pharmacologically. Thus enterocytes may sense dietary TAG-derived fatty acids via FAO and influence eating through changes in intestinal vagal afferent activity. Further studies are necessary to identify the link between enterocyte FAO and vagal afferents and to examine the specificity and potential physiological relevance of such a mechanism.

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.

Leucine deprivation increases hepatic insulin sensitivity via GCN2/mTOR/S6K1 and AMPK pathways

OBJECTIVE

We have previously shown that serum insulin levels decrease threefold and blood glucose levels remain normal in mice fed a leucine-deficient diet, suggesting increased insulin sensitivity. The goal of the current study is to investigate this possibility and elucidate the underlying cellular mechanisms.

RESEARCH DESIGN AND METHODS

Changes in metabolic parameters and expression of genes and proteins involved in regulation of insulin sensitivity were analyzed in mice, human HepG2 cells, and mouse primary hepatocytes under leucine deprivation.

RESULTS

We show that leucine deprivation improves hepatic insulin sensitivity by sequentially activating general control nonderepressible (GCN)2 and decreasing mammalian target of rapamycin/S6K1 signaling. In addition, we show that activation of AMP-activated protein kinase also contributes to leucine deprivation-increased hepatic insulin sensitivity. Finally, we show that leucine deprivation improves insulin sensitivity under insulin-resistant conditions.

CONCLUSIONS

This study describes mechanisms underlying increased hepatic insulin sensitivity under leucine deprivation. Furthermore, we demonstrate a novel function for GCN2 in the regulation of insulin sensitivity. These observations provide a rationale for short-term dietary restriction of leucine for the treatment of insulin resistance and associated metabolic diseases.

Genetic polymorphisms in the hypothalamic pathway in relation to subsequent weight change--the DiOGenes study

BACKGROUND

Single nucleotide polymorphisms (SNPs) in genes encoding the components involved in the hypothalamic pathway may influence weight gain and dietary factors may modify their effects.

AIM

We conducted a case-cohort study to investigate the associations of SNPs in candidate genes with weight change during an average of 6.8 years of follow-up and to examine the potential effect modification by glycemic index (GI) and protein intake.

METHODS AND FINDINGS

Participants, aged 20-60 years at baseline, came from five European countries. Cases ('weight gainers') were selected from the total eligible cohort (n = 50,293) as those with the greatest unexplained annual weight gain (n = 5,584). A random subcohort (n = 6,566) was drawn with the intention to obtain an equal number of cases and noncases (n = 5,507). We genotyped 134 SNPs that captured all common genetic variation across the 15 candidate genes; 123 met the quality control criteria. Each SNP was tested for association with the risk of being a 'weight gainer' (logistic regression models) in the case-noncase data and with weight gain (linear regression models) in the random subcohort data. After accounting for multiple testing, none of the SNPs was significantly associated with weight change. Furthermore, we observed no significant effect modification by dietary factors, except for SNP rs7180849 in the neuromedin β gene (NMB). Carriers of the minor allele had a more pronounced weight gain at a higher GI (P = 2 x 10⁻⁷).

CONCLUSIONS

We found no evidence of association between SNPs in the studied hypothalamic genes with weight change. The interaction between GI and NMB SNP rs7180849 needs further confirmation.

Neuronal suppressor of cytokine signaling-3 deficiency enhances hypothalamic leptin-dependent phosphatidylinositol 3-kinase signaling

Suppressor of cytokine signaling-3 (SOCS3) is thought to be involved in the development of central leptin resistance and obesity by inhibiting STAT3 pathway. Because phosphatidylinositol 3-kinase (PI3K) pathway plays an important role in transducing leptin action in the hypothalamus, we examined whether SOCS3 exerted an inhibition on this pathway. We first determined whether leptin sensitivity in the hypothalamic PI3K pathway was increased in brain-specific Socs3-deficient (NesKO) mice. In NesKO mice, hypothalamic insulin receptor substrate-1 (IRS1)-associated PI3K activity was significantly increased at 30 min and remained elevated up to 2 h after leptin intraperitoneal injection, but in wild-type (WT) littermates, the significant increase was only at 30 min. Hypothalamic p-STAT3 levels were increased up to 5 h in NesKO as opposed to 2 h in WT mice. In food-restricted WT mice with reduced body weight, leptin increased hypothalamic PI3K activity only at 30 min, and p-STAT3 levels at 30-120 min postinjection. These results suggest increased leptin sensitivity in both PI3K and STAT3 pathways in the hypothalamus of NesKO mice, which was not due to a lean phenotype. In the next experiment with a clonal hypothalamic neuronal cell line expressing proopiomelanocortin, we observed that whereas leptin significantly increased IRS1-associated PI3K activity and p-JAK2 levels in cells transfected with control vector, it failed to do so in SOCS3-overexpressed cells. Altogether, these results imply a SOCS3 inhibition of the PI3K pathway of leptin signaling in the hypothalamus, which may be one of the mechanisms behind the development of central leptin resistance and obesity.

Neuropeptide exocytosis involving synaptotagmin-4 and oxytocin in hypothalamic programming of body weight and energy balance

Hypothalamic neuropeptides play essential roles in regulating energy and body weight balance. Energy imbalance and obesity have been linked to hypothalamic signaling defects in regulating neuropeptide genes; however, it is unknown whether dysregulation of neuropeptide exocytosis could be critically involved. This study discovered that synaptotagmin-4, an atypical modulator of synaptic exocytosis, is expressed most abundantly in oxytocin neurons of the hypothalamus. Synaptotagmin-4 negatively regulates oxytocin exocytosis, and dietary obesity is associated with increased vesicle binding of synaptotagmin-4 and thus enhanced negative regulation of oxytocin release. Overexpressing synaptotagmin-4 in hypothalamic oxytocin neurons and centrally antagonizing oxytocin in mice are similarly obesogenic. Synaptotagmin-4 inhibition prevents against dietary obesity by normalizing oxytocin release and energy balance under chronic nutritional excess. In conclusion, the negative regulation of synaptotagmin-4 on oxytocin release represents a hypothalamic basis of neuropeptide exocytosis in controlling obesity and related diseases.

Higher branched-chain amino acid intake is associated with a lower prevalence of being overweight or obese in middle-aged East Asian and Western adults

Beneficial effects on body weight of supplementation with BCAA, including leucine, isoleucine, and valine, have been observed in animal and human studies. However, population-based studies on dietary BCAA intake and body weight are lacking. The objective of this study was to examine the association between dietary BCAA intake and risk of overweight status/obesity among multi-ethnic populations. The International Study of Macro-/Micronutrients and Blood Pressure is a cross-sectional epidemiological investigation in China, Japan, the UK, and the US. The study cohort included 4429 men and women ages 40-59 y who were free of diabetes. Diet was assessed by 4 multi-pass 24-h recalls; data on nutrients including BCAA were derived from country-specific food tables. Overweight status and obesity were defined as BMI ≥ 25 and BMI ≥ 30 kg/m(2), respectively. Multivariable-adjusted OR of overweight status/obesity and 95% CI by quartiles of BCAA intake were estimated by logistic regression. Mean BCAA intake was 2.6 ± 0.6% energy; intake was significantly lower among Chinese participants and similar among participants from the other 3 countries. Compared with those in the first quartile, the multivariable-adjusted OR (95% CI) of overweight status from the 2nd to 4th quartiles of BCAA intake were 0.97 (0.80-1.17), 0.91 (0.75-1.11), and 0.70 (0.57-0.86), respectively (P-trend < 0.01). BCAA intake and obesity were also inversely associated (P-trend = 0.03). In conclusion, higher dietary BCAA intake is associated with lower prevalence of overweight status/obesity among apparently healthy middle-aged adults from East Asian and Western countries.

Nutritional regulation of division of labor in honey bees: toward a systems biology perspective

Organisms adapt their behavior and physiology to environmental conditions through processes of phenotypic plasticity. In one well-studied example, the division of labor among worker honey bees involves a stereotyped yet plastic pattern of behavioral and physiological maturation. Early in life, workers perform brood care and other in-hive tasks and have large internal nutrient stores; later in life, they forage for nectar and pollen outside the hive and have small nutrient stores. The pace of maturation depends on colony conditions, and the environmental, physiological, and genomic mechanisms by which this occurs are being actively investigated. Here we review current knowledge of the mechanisms by which a key environmental variable, nutritional status, influences worker honey bee division of labor. These studies demonstrate that changes in individual nutritional status and conserved food-related molecular and hormonal pathways regulate the age at which individual bees begin to forage. We then outline ways in which systems biology approaches, enabled by the sequencing of the honey bee genome, will allow researchers to gain deeper insight into nutritional regulation of honey bee behavior, and phenotypic plasticity in general.

The chronic ingestion of diets containing different proteins produces marked variations in brain tryptophan levels and serotonin synthesis in the rat

Serotonin (5HT) synthesis in brain is influenced by precursor (tryptophan (TRP)) concentrations, which are modified by food ingestion. Hence, in rats, a carbohydrate meal raises brain TRP and 5HT; a protein-containing meal does not, but little attention has focused on differences among dietary proteins. Recently, single meals containing different proteins have been shown to produce marked changes in TRP and 5HT. The present studies evaluate if such differences persist when rats ingest such diets chronically. Male rats were studied that ingested diets for 9 days containing zein, wheat gluten, soy protein, casein, or α-lactalbumin (17% dry weight). Brain TRP varied up to eightfold, and 5HT synthesis fivefold among the different protein groups. TYR and LEU concentrations, and catecholamine synthesis rate in brain varied much less. The effects of dietary protein on brain TRP and 5HT previously noted after single meals thus continue undiminished when such diets are consumed chronically.

Studies of different female rat models of hypothalamic obesity

Hypothalamic obesity (HO) is a major and unsolved problem in patients with medial hypothalamic lesions and is associated with hyperinsulinemia and hyperleptinemia. The purpose of this study was to create a rodent model that mimics metabolic changes in HO for use in therapeutic testing. Female Sprague-Dawley rats were used to test the individual and combined effects of two types of medial hypothalamic lesions: arcuate nucleus (ARC) lesions by injection of monosodium glutamate at neonatal age, and ventromedial nucleus (VMN) lesions by passing an anodal current through an electrode placed in the VMN at age 80 days. Adiposity in ARC-lesioned animals was associated with decreased food intake and stunted growth, while VMN lesions were associated with hyperphagia but not reduced growth. The greatest weight gain (weight at age 200 days 712 +/- 65 vs. 451 +/- 19 g in controls), hyperphagia (food intake 10 days following surgery 33 +/- 0.8 vs. 18.5 +/- 0.7 g/day in sham-treated rats), hyperinsulinemia and hyperleptinemia occurred in rats that received both ARC and VMN lesions. Thus, the combined medial hypothalamic lesions result in an obesity phenotype similar to that of patients that suffer from HO and are consequently more suitable for testing potential therapeutics for this disorder than lesions of single hypothalamic nuclei.

Targeting type 2 diabetes

The evolving concept of how nutrient excess and inflammation modulate metabolism provides new opportunities for strategies to correct the detrimental health consequences of obesity. In this review, we focus on the complex interplay among lipid overload, immune response, proinflammatory pathways and organelle dysfunction through which excess adiposity might lead to type 2 diabetes. We then consider evidence linking dysregulated CNS circuits to insulin resistance and results on nutrient-sensing pathways emerging from studies with calorie restriction. Subsequently, recent recommendations for the management of type 2 diabetes are discussed with emphasis on prevailing current therapeutic classes of biguanides, thiazolidinediones and incretin-based approaches.

Overcoming insulin resistance with ciliary neurotrophic factor

The incidence of obesity and related co-morbidities such as insulin resistance, dyslipidemia and hypertension are increasing at an alarming rate worldwide. Current interventions seem ineffective to halt this progression. With the failure of leptin as an anti-obesity therapeutic, ciliary neurotrophic factor (CNTF) has proven efficacious in models of obesity and leptin resistance, where leptin proved ineffective. CNTF is a gp130 ligand that has been found to act centrally and peripherally to promote weight loss and insulin sensitivity in both human and rodent models. Future research into novel gp130 ligands may offer new candidates for obesity-related drug therapy.

Gustatory and reward brain circuits in the control of food intake

Gustation is a multisensory process allowing for the selection of nutrients and the rejection of irritating and/or toxic compounds. Since obesity is a highly prevalent condition that is critically dependent on food intake and energy expenditure, a deeper understanding of gustatory processing is an important objective in biomedical research. Recent findings have provided evidence that central gustatory processes are distributed across several cortical and subcortical brain areas. Furthermore, these gustatory sensory circuits are closely related to the circuits that process reward. Here, we present an overview of the activation and connectivity between central gustatory and reward areas. Moreover, and given the limitations in number and effectiveness of treatments currently available for overweight patients, we discuss the possibility of modulating neuronal activity in these circuits as an alternative in the treatment of obesity.

Hypothalamic obesity in patients with craniopharyngioma: profound changes of several weight regulatory circuits

One of the most striking examples of dysfunctional hypothalamic signaling of energy homeostasis is observed in patients with hypothalamic lesions leading to hypothalamic obesity (HO). This drastic condition is frequently seen in patients with craniopharyngioma (CP), an embryological tumor located in the hypothalamic and/or pituitary region, frequently causing not only hypopituitarism, but also leading to damage of medial hypothalamic nuclei due to the tumor and its treatment. HO syndrome in CP patients is characterized by fatigue, decreased physical activity, uncontrolled appetite, and morbid obesity, and is associated with insulin and leptin resistance. Mechanisms leading to the profoundly disturbed energy homeostasis are complex. This review summarizes different aspects of important clinical studies as well as data obtained in rodent studies. In addition a model is provided describing how medial hypothalamic lesion can interact simultaneously with several weight-regulating circuitries.

Sulfated cholecystokinin-8 activates phospho-mTOR immunoreactive neurons of the paraventricular nucleus in rats

The serin/threonin-kinase, mammalian target of rapamycin (mTOR) was detected in the arcuate nucleus (ARC) and paraventricular nucleus of the hypothalamus (PVN) and suggested to play a role in the integration of satiety signals. Since cholecystokinin (CCK) plays a role in the short-term inhibition of food intake and induces c-Fos in PVN neurons, the aim was to determine whether intraperitoneally injected CCK-8S affects the neuronal activity in cells immunoreactive for phospho-mTOR in the PVN. Ad libitum fed male Sprague-Dawley rats received 6 or 10 μg/kg CCK-8S or 0.15M NaCl ip (n=4/group). The number of c-Fos-immunoreactive (ir) neurons was assessed in the PVN, ARC and in the nucleus of the solitary tract (NTS). CCK-8S increased the number of c-Fos-ir neurons in the PVN (6 μg: 103 ± 13 vs. 10 μg: 165 ± 14 neurons/section; p<0.05) compared to vehicle treated rats (4 ± 1, p<0.05), but not in the ARC. CCK-8S also dose-dependently increased the number of c-Fos neurons in the NTS. Staining for phospho-mTOR and c-Fos in the PVN showed a dose-dependent increase of activated phospho-mTOR neurons (17 ± 3 vs. 38 ± 2 neurons/section; p<0.05), while no activated phospho-mTOR neurons were observed in the vehicle group. Triple staining in the PVN showed activation of phospho-mTOR neurons co-localized with oxytocin, corresponding to 9.8 ± 3.6% and 19.5 ± 3.3% of oxytocin neurons respectively. Our observations indicate that peripheral CCK-8S activates phospho-mTOR neurons in the PVN and suggest that phospho-mTOR plays a role in the mediation of CCK-8S's anorexigenic effects.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

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.

Inositol pyrophosphates inhibit Akt signaling, thereby regulating insulin sensitivity and weight gain

The inositol pyrophosphate IP7 (5-diphosphoinositolpentakisphosphate), formed by a family of three inositol hexakisphosphate kinases (IP6Ks), modulates diverse cellular activities. We now report that IP7 is a physiologic inhibitor of Akt, a serine/threonine kinase that regulates glucose homeostasis and protein translation, respectively, via the GSK3β and mTOR pathways. Thus, Akt and mTOR signaling are dramatically augmented and GSK3β signaling reduced in skeletal muscle, white adipose tissue, and liver of mice with targeted deletion of IP6K1. IP7 affects this pathway by potently inhibiting the PDK1 phosphorylation of Akt, preventing its activation and thereby affecting insulin signaling. IP6K1 knockout mice manifest insulin sensitivity and are resistant to obesity elicited by high-fat diet or aging. Inhibition of IP6K1 may afford a therapeutic approach to obesity and diabetes.

Effect of a high-protein diet on food intake and liver metabolism during pregnancy, lactation and after weaning in mice

Major hepatic metabolic pathways are involved in the control of food intake but how dietary proteins affect global metabolism to adjust food intake is incompletely understood, particularly under physiological challenging conditions such as lactation. In order to identify these molecular events, mice were fed a high-protein (HP) diet from pregnancy, during lactation until after weaning and compared with control fed counterparts. Liver specimens were analyzed for regulated proteins using 2-DE and MALDI-TOF-MS and plasma samples for metabolites. Based on the 26 differentially expressed proteins associated with depleted liver glycogen content, elevated urea and citrulline plasma concentrations, we conclude that HP feeding during lactation leads to an activated amino acid, carbohydrate and fatty acid catabolism while it activates gluconeogenesis. From pregnancy to lactation, plasma arginine, tryptophan, serine, glutamine and cysteine decreased, whereas urea concentrations increased in both groups. Concomitantly, hepatic glycogen content decreased while total fat content remained unaltered in both groups. Consideration of 59 proteins differentially expressed between pregnancy and lactation highlights different strategies of HP and control fed mice to meet energy requirements for lactation by adjusting amino acid degradation, carbohydrate and fat metabolism, citrate cycle, but also ATP-turnover, protein folding, secretion of proteins and (de)activation of transcription factors.

Regulation of gastric hormones by systemic rapamycin

The mammalian target of rapamycin (mTOR), an evolutionarily conserved serine-threonine kinase, is an intracellular fuel sensor critical for cellular energy homeostasis. Gastrointestinal endocrine cells play a vital role in the regulation of energy balance by secreting hormones that inform the brain about energy supply. Here we showed the localization of mTOR signaling molecules in more than 90% of gastric ghrelin cells and 36±3% of gastrin cells, while no somatostatin-positive cell showed phospho-S6K1 immunoreactivity. Inhibition of mTOR significantly stimulated expression of gastric ghrelin mRNA and protein, and the concentration of plasma ghrelin (2.06±0.34 ng/ml vs. 12.53±3.9 ng/ml, p<0.05), inhibited gastrin synthesis and secretion (75.01±6.71 pg/ml vs. 54.04±3.65 pg/ml, p<0.05), but had no effect on somatostatin production (165.2±25.07 pg/ml vs. 178.9±29.14 pg/ml, p=0.73). Gastric mTOR is a gastric sensor whose activity is linked to the differential regulation of gastric hormone production and release.

Ectopic expression of human mTOR increases viability, robustness, cell size, proliferation, and antibody production of chinese hamster ovary cells

Engineering of mammalian production cell lines to improve titer and quality of biopharmaceuticals is a top priority of the biopharmaceutical manufacturing industry providing protein therapeutics to patients worldwide. While many engineering strategies have been successful in the past decade they were often based on the over-expression of a single transgene and therefore limited to addressing a single bottleneck in the cell's production capacity. We provide evidence that ectopic expression of the global metabolic sensor and processing protein mammalian target of rapamycin (mTOR), simultaneously improves key bioprocess-relevant characteristics of Chinese hamster ovary (CHO) cell-derived production cell lines such as cell growth (increased cell size and protein content), proliferation (increased cell-cycle progression), viability (decreased apoptosis), robustness (decreased sensitivity to sub-optimal growth factor and oxygen supplies) and specific productivity of secreted human glycoproteins. Cultivation of mTOR-transgenic CHO-derived cell lines engineered for secretion of a therapeutic IgG resulted in antibody titers of up to 50 pg/cell/day, which represents a four-fold increase compared to the parental production cell line. mTOR-based engineering of mammalian production cell lines may therefore have a promising future in biopharmaceutical manufacturing of human therapeutic proteins.

Obesity and leptin resistance: distinguishing cause from effect

Because leptin reduces food intake and body weight, the coexistence of elevated leptin levels with obesity is widely interpreted as evidence of 'leptin resistance.' Indeed, obesity promotes a number of cellular processes that attenuate leptin signaling (referred to here as 'cellular leptin resistance') and amplify the extent of weight gain induced by genetic and environmental factors. As commonly used, however, the term 'leptin resistance' embraces a range of phenomena that are distinct in underlying mechanisms and pathophysiological implications. Moreover, the induction of cellular leptin resistance by obesity complicates efforts to distinguish the mechanisms that predispose to weight gain from those that result from it. We suggest a framework for approaching these issues and important avenues for future investigation.

Mammalian target of rapamycin: hitting the bull's-eye for neurological disorders

The mammalian target of rapamycin (mTOR) and its associated cell signaling pathways have garnered significant attention for their roles in cell biology and oncology. Interestingly,the explosion of information in this field has linked mTOR to neurological diseases with promising initial studies. mTOR, a 289 kDa serine/threonine protein kinase, plays an important role in cell growth and proliferation and is activated through phosphorylation in response to growth factors, mitogens and hormones. Growth factors, amino acids, cellular nutrients and oxygen deficiency can downregulate mTOR activity. The function of mTOR signaling is mediated primarily through two mTOR complexes: mTORC1 and mTORC2. mTORC1 initiates cap-dependent protein translation, a rate-limiting step of protein synthesis, through the phosphorylation of the targets eukaryotic initiation factor 4E-binding protein 1 (4EBP1) and p70 ribosomal S6 kinase (p70S6K). In contrast, mTORC2 regulates development of the cytoskeleton and also controls cell survival. Although closely tied to tumorigenesis, mTOR and the downstream signaling pathways are significantly involved in the central nervous system (CNS) with synaptic plasticity, memory retention, neuroendocrine regulation associated with food intake and puberty and modulation of neuronal repair following injury. The signaling pathways of mTOR also are believed to be a significant component in a number of neurological diseases, such as Alzheimer disease, Parkinson disease and Huntington disease, tuberous sclerosis, neurofibromatosis, fragile X syndrome, epilepsy, traumatic brain injury and ischemic stroke. Here we describe the role of mTOR in the CNS and illustrate the potential for new strategies directed against neurological disorders.

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.

Recent advances in the discovery of small molecule mTOR inhibitors

Mammalian target of rapamycin (mTOR) belongs to the atypical kinase family of phosphatidylinositol-3-kinase-related kinase and function as a master regulators of the switch between catabolic and anabolic metabolism. In the last decade mTOR has emerged as a therapeutic target for various diseases such as cancer, inflammation and metabolic disorders. mTOR plays a crucial role in the PI3K/AKT/PDK1 pathway. In this review we will provide an overview of both selective and nonselective mTOR inhibitors. Since rapamycin and rapalogs have been reviewed before, more emphasis has been placed on nonrapamycin-based small-molecule inhibitors and their modulation of mTOR selectivity. Recent efforts in obtaining mTOR-selective inhibitors have produced a range of compounds with more than 1000-fold selectivity over PI3K, but it is still a matter of debate whether an mTOR-selective inhibitor will be of more clinical significance over a PI3K/AKT/mTOR inhibitor.

Melanocortin-4 receptor activation promotes insulin-stimulated mTOR signaling

The melanocortin signaling system is integral in regulating energy homeostasis. The melanocortin-4 receptor (MC4R) activates several signaling pathways in performance of this function. The effect of MC4R on insulin-stimulated mammalian target of rapamycin (mTOR), a cellular energy sensor, signaling was investigated. The GT1-1 cell line which expresses MC4R expression was utilized. mTOR signaling was measured by Western blotting analysis using Phospho-mTOR (Ser2448) antibody. NDP-MSH dose-dependently enhanced insulin-stimulated mTOR phosphorylation. The MC4R antagonist SHU9119 blocked this effect, demonstrating specificity. The protein kinase A - cyclic AMP pathway and the MAP kinase pathway were not involved in NDP-MSH actions on insulin-stimulated mTOR phosphorylation. In contrast, the AMP-activated protein kinase agonist, AICAR, attenuated this effect. MC4R activation potentiates insulin-stimulated mTOR signaling via the AMPK pathway.

β-Amyloid regulates leptin expression and tau phosphorylation through the mTORC1 signaling pathway

High levels of the adipocytokine leptin are associated with reduced risk of Alzheimer's disease. Leptin treatment also reduces β-amyloid (Aβ) levels in in vivo and in vitro models of Alzheimer's disease. Aβ and leptin interact with the Akt/mammalian target of rapamycin complex 1 (mTORC1) signaling pathway. Akt/mTORC1 activation reduces tau phosphorylation through the inhibition of the downstream enzyme GSK-3β. mTORC1 also regulates translation of many proteins including leptin. While Aβ has been shown to inactivate Akt, inhibit mTORC1, and facilitate the phosphorylation of tau, leptin activates both Akt and mTORC1 and reduces tau phosphorylation. However, the extent to which Aβ may modulate leptin expression and increase tau phosphorylation involving Akt/mTORC1 has not been determined. In this study, we show that incubation of organotypic slices from rabbit hippocampus with Aβ down-regulates leptin expression, inhibits Akt, activates GSK-3β, increases tau phosphorylation, and inactivates mTORC1. Leptin treatment reverses Aβ effects by alleviating Akt inhibition, preventing GSK-3β activation, reducing tau phosphorylation, and activating mTORC1. On the other hand, Rapamycin, an allosteric inhibitor of mTORC1, down-regulates leptin expression, increases tau phosphorylation, and does not affect Akt and GSK-3β. Our results demonstrate for the first time that Aβ regulates leptin expression and tau phosphorylation through mTORC1.

Sensing the fuels: glucose and lipid signaling in the CNS controlling energy homeostasis

The central nervous system (CNS) is capable of gathering information on the body's nutritional state and it implements appropriate behavioral and metabolic responses to changes in fuel availability. This feedback signaling of peripheral tissues ensures the maintenance of energy homeostasis. The hypothalamus is a primary site of convergence and integration for these nutrient-related feedback signals, which include central and peripheral neuronal inputs as well as hormonal signals. Increasing evidence indicates that glucose and lipids are detected by specialized fuel-sensing neurons that are integrated in these hypothalamic neuronal circuits. The purpose of this review is to outline the current understanding of fuel-sensing mechanisms in the hypothalamus, to integrate the recent findings in this field, and to address the potential role of dysregulation in these pathways in the development of obesity and type 2 diabetes mellitus.

Molecular neuroendocrine targets for obesity therapy

PURPOSE OF REVIEW

Although energy balance is tightly regulated in order to maintain a specific level of adiposity, the incidence of obesity continues to increase. Consequently, it is essential that effective therapeutics for the treatment and prevention of obesity be developed. This review provides a brief update on some recent advances in the characterization of neuroendocrine targets for obesity therapy.

RECENT FINDINGS

During the review period, considerable progress occurred in the understanding of previously described neuroendocrine regulators of energy balance, and several novel targets have been identified. Moreover, the understanding of the neural circuitry and molecular mechanisms of the neuroendocrine regulation of energy homeostasis has been expanded.

SUMMARY

Energy balance is maintained by neuroendocrine signals arising from many tissues including the gastrointestinal tract and adipose tissue. These signals are integral to the cessation of meals and to the ability of the brain to monitor energy status and respond accordingly. Many current targets for obesity therapy are based on manipulating the activity of these signals and their receptors; however, to date, clinical-weight loss based on this strategy has been minimal and alternative approaches such as combinatorial therapies are emerging.

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.

A small-molecule scaffold induces autophagy in primary neurons and protects against toxicity in a Huntington disease model

Autophagy is an intracellular turnover pathway. It has special relevance for neurodegenerative proteinopathies, such as Alzheimer disease, Parkinson disease, and Huntington disease (HD), which are characterized by the accumulation of misfolded proteins. Although induction of autophagy enhances clearance of misfolded protein and has therefore been suggested as a therapy for proteinopathies, neurons appear to be less responsive to classic autophagy inducers than nonneuronal cells. Searching for improved inducers of neuronal autophagy, we discovered an N(10)-substituted phenoxazine that, at proper doses, potently and safely up-regulated autophagy in neurons in an Akt- and mTOR-independent fashion. In a neuron model of HD, this compound was neuroprotective and decreased the accumulation of diffuse and aggregated misfolded protein. A structure/activity analysis with structurally similar compounds approved by the US Food and Drug Administration revealed a defined pharmacophore for inducing neuronal autophagy. This pharmacophore should prove useful in studying autophagy in neurons and in developing therapies for neurodegenerative proteinopathies.

Energy balance and puberty onset: emerging role of central mTOR signaling

The onset of puberty is gated by body energy reserves and nutritional cues. The adipose hormone leptin is an essential signal for the metabolic control of puberty, through mechanisms that are yet to be fully characterized. Mammalian target of rapamycin (mTOR), an energetic cell sensor, operates at specific hypothalamic nuclei as a transducer for leptin effects on feeding and energy homeostasis. This review summarizes recent experimental evidence supporting a role for central mTOR signaling in puberty onset. These findings are discussed in the context of topical developments in the field, such as recognition of the roles of the cAMP responsive element-binding protein regulated transcription coactivator-1 (Crtc1) and kisspeptins in the metabolic control of reproduction, thus highlighting novel mechanisms responsible for coupling puberty and energy homeostasis.

Kisspeptins: bridging energy homeostasis and reproduction

Body energy reserves and metabolic state are relevant modifiers of puberty onset and fertility; forms of metabolic stress ranging from persistent energy insufficiency to morbid obesity are frequently linked to reproductive disorders. The mechanisms for such a close connection between energy balance and reproduction have been the subject of considerable attention; however, our understanding of the neurobiological basis for this phenomenon is still incomplete. In mid 1990s, the adipose-hormone, leptin, was proven as an essential signal for transmitting metabolic information onto the centers governing puberty and reproduction; yet, the ultimate mode of action of leptin on GnRH neurons has remained contentious for years. More recently, kisspeptins, a family of neuropeptides encoded by the Kiss1 gene, have emerged as conduits for the metabolic regulation of reproduction and putative effectors of leptin actions on GnRH neurons. This review recapitulates the experimental evidence obtained to date, mostly in laboratory rodents, supporting the function of kisspeptins in bridging energy balance and reproduction, with special emphasis on recent developments in this field, such as the recognition of mTOR (mammalian target of rapamycin) and Crtc1 (Creb1-regulated transcription coactivator-1) as putative mediators for leptin regulation of Kiss1 expression, as well as the identification of other potential metabolic modulators of kisspeptin signaling, such as ghrelin, neuropeptide Y (NPY) and melanin-concentrating hormone (MCH).

Ketogenic essential amino acids modulate lipid synthetic pathways and prevent hepatic steatosis in mice

Background

Although dietary ketogenic essential amino acid (KAA) content modifies accumulation of hepatic lipids, the molecular interactions between KAAs and lipid metabolism are yet to be fully elucidated.

Methodology/Principal Findings

We designed a diet with a high ratio (E/N) of essential amino acids (EAAs) to non-EAAs by partially replacing dietary protein with 5 major free KAAs (Leu, Ile, Val, Lys and Thr) without altering carbohydrate and fat content. This high-KAA diet was assessed for its preventive effects on diet-induced hepatic steatosis and whole-animal insulin resistance. C57B6 mice were fed with a high-fat diet, and hyperinsulinemic ob/ob mice were fed with a high-fat or high-sucrose diet. The high-KAA diet improved hepatic steatosis with decreased de novo lipogensis (DNL) fluxes as well as reduced expressions of lipogenic genes. In C57B6 mice, the high-KAA diet lowered postprandial insulin secretion and improved glucose tolerance, in association with restored expression of muscle insulin signaling proteins repressed by the high-fat diet. Lipotoxic metabolites and their synthetic fluxes were also evaluated with reference to insulin resistance. The high-KAA diet lowered muscle and liver ceramides, both by reducing dietary lipid incorporation into muscular ceramides and preventing incorporation of DNL-derived fatty acids into hepatic ceramides.

Conclusion

Our results indicate that dietary KAA intake improves hepatic steatosis and insulin resistance by modulating lipid synthetic pathways.

Metabolic control of puberty onset: new players, new mechanisms

Puberty, as the end-point of a complex series of maturational events affecting the components of the hypothalamic-pituitary-gonadal (HPG) axis, is gated by the state of body energy reserves and sensitive to different metabolic cues; conditions of severe metabolic stress and energy unbalance (from anorexia to morbid obesity) being commonly linked to perturbation of the onset of puberty. In the last two decades, the neuroendocrine mechanisms responsible for the tight coupling between energy homeostasis and puberty onset have begun to be deciphered. These seemingly involve a plethora of metabolic hormones and neuropeptides, which impinge and integrate (mostly) at the hypothalamic centers governing reproduction. Yet, characterization of the mechanisms of action of such regulators (and even their nature and physiological relevance) still remains incomplete. In this review, we will summarize some recent developments in our knowledge of the effects and mechanisms of action of two key metabolic hormones, leptin and ghrelin, in the control of puberty onset. In addition, the roles of the hypothalamic Kiss1 system in the metabolic gating of puberty will be reviewed, with special attention to its regulation by leptin and the recent identification of the putative roles of Crtc1 and mTOR signaling as molecular conduits for the metabolic control of Kiss1 expression. Elucidation of these novel players and regulatory mechanisms will help for a better understanding of the determinants of the timing of puberty, and its eventual alterations in adverse metabolic conditions.

A role of phosphodiesterase-3B pathway in mediating leptin action on proopiomelanocortin and neurotensin neurons in the hypothalamus

Leptin signaling in the hypothalamus is required for normal food intake and body weight homeostasis. Recent evidence suggests that besides the signal transducer and activator of transcription-3 (STAT3) pathway, several non-STAT3 pathways mediate leptin signaling in the hypothalamus. We have previously demonstrated that leptin stimulates phosphodiesterase-3B (PDE3B) activity in the hypothalamus, and PDE3 inhibitor cilostamide reverses anorectic and body weight reducing effects of leptin. To establish the physiological role of PDE3B signaling in the hypothalamus, we examined if leptin signaling through the PDE3B pathway is responsible for the activation of proopiomelanocortin (POMC) and neurotensin (NT) neurons, which are known to play a critical role in energy homeostasis. To this end, we assessed the effect of cilostamide on leptin-induced POMC and NT gene expression in the rat hypothalamus. Results showed that while central injection of leptin significantly increased both POMC and NT mRNA levels in the medial basal hypothalamus, cilostamide completely reversed this effect of leptin suggesting a PDE3B-activation dependent induction of POMC and NT gene expression by leptin. This result further suggests that the PDE3B pathway plays an important role in mediating leptin action in the hypothalamus.

Role of the glucagon-like-peptide-1 receptor in the control of energy balance

The peripheral and central glucagon-like-peptide-1 (GLP-1) systems play an essential role in glycemic and energy balance regulation. Thus, pharmacological targeting of peripheral and/or central GLP-1 receptors (GLP-1R) may represent a potential long-term treatment option for both obesity and type-II diabetes mellitus (T2DM). Uncovering and understanding the neural pathways, physiological mechanisms, specific GLP-1R populations, and intracellular signaling cascades that mediate the food intake inhibitory and incretin effects produced by GLP-1R activation are vital to the development of these potential successful therapeutics. Particular focus will be given to the essential role of the nucleus tractus solitarius (NTS) in the caudal brainstem, as well as the gut-to-brain communication by vagal afferent fibers in mediating the physiological and behavioral responses following GLP-1R activation. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Chronic leucine supplementation improves glycemic control in etiologically distinct mouse models of obesity and diabetes mellitus

BACKGROUND

Leucine may function as a signaling molecule to regulate metabolism. We have previously shown that dietary leucine supplementation significantly improves glucose and energy metabolism in diet-induced obese mice, suggesting that leucine supplementation could potentially be a useful adjuvant therapy for obesity and type 2 diabetes. Since the underlying cause for obesity and type 2 diabetes is multifold, we further investigated metabolic effects of leucine supplementation in obese/diabetes mouse models with different etiologies, and explored the underlying molecular mechanisms.

METHODS

Leucine supplementation was carried out in NONcNZO10/LtJ (RCS10) - a polygenic model predisposed to beta cell failure and type 2 diabetes, and in B6.Cg-Ay/J (Ay) - a monogenic model for impaired central melanocortin receptor signaling, obesity, and severe insulin resistance. Mice in the treatment group received the drinking water containing 1.5% leucine for up to 8 months; control mice received the tap water. Body weight, body composition, blood HbA1c levels, and plasma glucose and insulin levels were monitored throughout and/or at the end of the study period. Indirect calorimetry, skeletal muscle gene expression, and adipose tissue inflammation were also assessed in Ay mice.

RESULTS

Leucine supplementation significantly reduced HbA1c levels throughout the study period in both RCS10 and Ay mice. However, the treatment had no long term effect on body weight or adiposity. The improvement in glycemic control was associated with an increased insulin response to food challenge in RCS10 mice and decreased plasma insulin levels in Ay mice. In leucine-treated Ay mice, energy expenditure was increased by ~10% (p < 0.05) in both dark and light cycles while the physical activity level was unchanged. The expression levels of UCP3, CrAT, PPAR-alpha, and NRF-1, which are known to regulate mitochondrial oxidative function, were significantly increased in the soleus muscle of leucine-treated Ay mice whereas the expression levels of MCP-1 and TNF-alpha and macrophage infiltration in adipose tissue were significantly reduced.

CONCLUSIONS

Chronic leucine supplementation significantly improves glycemic control in multiple mouse models of obesity and diabetes with distinct etiologies. The metabolic benefits of leucine supplementation are likely mediated via multiple mechanisms in different tissues, but are not necessarily dependent of weight reduction.

SIRT1 deacetylase in POMC neurons is required for homeostatic defenses against diet-induced obesity

Feeding on high-calorie (HC) diets induces serious metabolic imbalances, including obesity. Understanding the mechanisms against excessive body weight gain is critical for developing effective antiobesity strategies. Here we show that lack of nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylase SIRT1 in pro-opiomelanocortin (POMC) neurons causes hypersensitivity to diet-induced obesity due to reduced energy expenditure. The ability of leptin to properly engage the phosphoinositide 3-kinase (PI3K) signaling in POMC neurons and elicit remodeling of perigonadal white adipose tissue (WAT) is severely compromised in mutant mice. Also, electrophysiological and histomorphomolecular analyses indicate a selective reduction in sympathetic nerve activity and brown-fat-like characteristics in perigonadal WAT of mutant mice, suggesting a physiologically important role for POMC neurons in controlling this visceral fat depot. In summary, our results provide direct genetic evidence that SIRT1 in POMC neurons is required for normal autonomic adaptations against diet-induced obesity.

Protein metabolic roles in treatment of obesity

PURPOSE OF REVIEW

To understand the potential benefits of increased dietary protein during weight loss and the importance of distribution of high-quality protein at each meal.

RECENT FINDINGS

Popular weight loss diets emphasize use of protein as a substitute for carbohydrates or fat to reduce insulin and minimize hunger and food cravings. These diets produce short-term weight loss, but long-term benefits remain obscured by failure to differentiate between outcomes of subject compliance and diet effectiveness. New molecular mechanisms have defined the benefits of protein as a meal threshold for the branched-chain amino acid leucine, which has been characterized as a unique signal regulator of muscle protein synthesis. Leucine consumed at 2.5 g triggers a postmeal anabolic response that protects metabolic active tissues during weight loss and increases loss of body fat.

SUMMARY

Balanced daily distribution of protein with increased intake at breakfast and lunch protects metabolically active tissues including skeletal muscle during weight loss.

Leptin in human physiology and therapeutics

Leptin regulates energy homeostasis and reproductive, neuroendocrine, immune, and metabolic functions. In this review, we describe the role of leptin in human physiology and review evidence from recent "proof of concept" clinical trials using recombinant human leptin in subjects with congenital leptin deficiency, hypoleptinemia associated with energy-deficient states, and hyperleptinemia associated with garden-variety obesity. Since most obese individuals are largely leptin-tolerant or -resistant, therapeutic uses of leptin are currently limited to patients with complete or partial leptin deficiency, including hypothalamic amenorrhea and lipoatrophy. Leptin administration in these energy-deficient states may help restore associated neuroendocrine, metabolic, and immune function and bone metabolism. Leptin treatment is currently available for individuals with congenital leptin deficiency and congenital lipoatrophy. The long-term efficacy and safety of leptin treatment in hypothalamic amenorrhea and acquired lipoatrophy are currently under investigation. Whether combination therapy with leptin and potential leptin sensitizers will prove effective in the treatment of garden-variety obesity and whether leptin may have a role in weight loss maintenance is being greatly anticipated.