Hypothalamic sensing of fatty acids

Deregulation of hepatic insulin sensitivity induced by central lipid infusion in rats is mediated by nitric oxide

Background

Deregulation of hypothalamic fatty acid sensing lead to hepatic insulin-resistance which may partly contribute to further impairment of glucose homeostasis.

Methodology

We investigated here whether hypothalamic nitric oxide (NO) could mediate deleterious peripheral effect of central lipid overload. Thus we infused rats for 24 hours into carotid artery towards brain, either with heparinized triglyceride emulsion (Intralipid, IL) or heparinized saline (control rats).

Principal Findings

Lipids infusion led to hepatic insulin-resistance partly related to a decreased parasympathetic activity in the liver assessed by an increased acetylcholinesterase activity. Hypothalamic nitric oxide synthases (NOS) activities were significantly increased in IL rats, as the catalytically active neuronal NOS (nNOS) dimers compared to controls. This was related to a decrease in expression of protein inhibitor of nNOS (PIN). Effect of IL infusion on deregulated hepatic insulin-sensitivity was reversed by carotid injection of non selective NOS inhibitor NG-monomethyl-L-arginine (L-NMMA) and also by a selective inhibitor of the nNOS isoform, 7-Nitro-Indazole (7-Ni). In addition, NO donor injection (L-arginine and SNP) within carotid in control rats mimicked lipid effects onto impaired hepatic insulin sensitivity. In parallel we showed that cultured VMH neurons produce NO in response to fatty acid (oleic acid).

Conclusions/Significance

We conclude that cerebral fatty acid overload induces an enhancement of nNOS activity within hypothalamus which is, at least in part, responsible fatty acid increased hepatic glucose production.

Gastric mammalian target of rapamycin signaling regulates ghrelin production and food intake

Ghrelin, a gastric hormone, provides a hunger signal to the central nervous system to stimulate food intake. Mammalian target of rapamycin (mTOR) is an intracellular fuel sensor critical for cellular energy homeostasis. Here we showed the reciprocal relationship of gastric mTOR signaling and ghrelin during changes in energy status. mTOR activity was down-regulated, whereas gastric preproghrelin and circulating ghrelin were increased by fasting. In db/db mice, gastric mTOR signaling was enhanced, whereas gastric preproghrelin and circulating ghrelin were decreased. Inhibition of the gastric mTOR signaling by rapamycin stimulated the expression of gastric preproghrelin and ghrelin mRNA and increased plasma ghrelin in both wild-type and db/db mice. Activation of the gastric mTOR signaling by l-leucine decreased the expression of gastric preproghrelin and the level of plasma ghrelin. Overexpression of mTOR attenuated ghrelin promoter activity, whereas inhibition of mTOR activity by overexpression of TSC1 or TSC2 increased its activity. Ghrelin receptor antagonist d-Lys-3-GH-releasing peptide-6 abolished the rapamycin-induced increment in food intake despite that plasma ghrelin remained elevated. mTOR is therefore a gastric fuel sensor whose activity is linked to the regulation of energy intake through ghrelin.

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.

Diet-derived nutrients mediate the inhibition of hypothalamic NPY neurons in the arcuate nucleus of mice during refeeding

Fasting activates orexigenic neuropeptide Y neurons in the hypothalamic arcuate nucleus (ARC) of mice, which is reversed by 2 h refeeding with standard chow. Here, we investigated the contribution of diet-derived macronutrients and anorectic hormones to the reversal of the fasting-induced ARC activation during 2 h refeeding. Refeeding of 12-h-fasted mice with a cellulose-based, noncaloric mash induced only a small reduction in c-Fos expression. Refeeding with diets, containing carbohydrates, protein, or fat alone reversed it similar to chow; however, this effect depended on the amount of intake. The fasting-induced ARC activation was unchanged by subcutaneously injected amylin, CCK (both 20 μg/kg), insulin (0.2 U/kg and 0.05 U/kg) or leptin (2.6 mg/kg). Insulin and leptin had no effect on c-Fos expression in neuropeptide Y or proopiomelanocortin-containing ARC neurons. Interestingly, CCK but not amylin reduced the ghrelin-induced c-Fos expression in the ARC in ad libitum-fed mice, suggesting that CCK may inhibit orexigenic ARC neurons when acting together with other feeding-related signals. We conclude that all three macronutrients and also non-nutritive, ingestion-dependent signals contribute to an inhibition of orexigenic ARC neurons after refeeding. Similar to the previously demonstrated inhibitory in vivo action of peptide YY, CCK may be a postprandial mediator of ARC inhibition.

Characteristics and mechanisms of hypothalamic neuronal fatty acid sensing

We assessed the mechanisms by which specialized hypothalamic ventromedial nucleus (VMN) neurons utilize both glucose and long-chain fatty acids as signaling molecules to alter their activity as a potential means of regulating energy homeostasis. Fura-2 calcium (Ca(2+)) and membrane potential dye imaging, together with pharmacological agents, were used to assess the mechanisms by which oleic acid (OA) alters the activity of dissociated VMN neurons from 3- to 4-wk-old rats. OA excited up to 43% and inhibited up to 29% of all VMN neurons independently of glucose concentrations. In those neurons excited by both 2.5 mM glucose and OA, OA had a concentration-dependent effective excitatory concentration (EC(50)) of 13.1 nM. Neurons inhibited by both 2.5 mM glucose and OA had an effective inhibitory concentration (IC(50)) of 93 nM. At 0.5 mM glucose, OA had markedly different effects on these same neurons. Inhibition of carnitine palmitoyltransferase, reactive oxygen species formation, long-chain acetyl-CoA synthetase and ATP-sensitive K(+) channel activity or activation of uncoupling protein 2 (UCP2) accounted for only approximately 20% of OA's excitatory effects and approximately 40% of its inhibitory effects. Inhibition of CD36, a fatty acid transporter that can alter cell function independently of intracellular fatty acid metabolism, reduced the effects of OA by up to 45%. Thus OA affects VMN neuronal activity through multiple pathways. In glucosensing neurons, its effects are glucose dependent. This glucose-OA interaction provides a potential mechanism whereby such "metabolic sensing" neurons can respond to differences in the metabolic states associated with fasting and feeding.

Targeting the CNS to treat type 2 diabetes

Research on the role of peripheral organs in the regulation of glucose homeostasis has led to the development of various monotherapies that aim to improve glucose uptake and insulin action in these organs for the treatment of type 2 diabetes. It is now clear that the central nervous system (CNS) also plays an important part in orchestrating appropriate glucose metabolism, with accumulating evidence linking dysregulated CNS circuits to the failure of normal glucoregulatory mechanisms. There is evidence that there is substantial overlap between the CNS circuits that regulate energy balance and those that regulate glucose levels, suggesting that their dysregulation could link obesity and diabetes. These findings present new targets for therapies that may be capable of both inducing weight loss and improving glucose regulation.

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.

Oleic acid directly regulates POMC neuron excitability in the hypothalamus

The mammalian CNS relies on a constant supply of external glucose for its undisturbed operation. However, neurons can readily switch to using fatty acids and ketones as alternative fuels. Here, we show that oleic acid (OA) excites pro-opiomelanocortin (POMC) neurons by inhibition of ATP-activated potassium (K(ATP)) channels. The involvement of K(ATP) channels is further supported by experiments in SUR1 KO animals. Inhibition of beta-oxidation using carnitine palmitoyltransferase-1 inhibitors blocks OA-induced depolarization. The depolarizing effect of OA is specific because it is not mimicked by octanoic acid. Furthermore, OA does not regulate the excitability of agouti-related peptide neurons. High-fat feeding alters POMC neuron excitability, but not its response to OA. Thus beta-oxidation in POMC neurons may mediate the appetite-suppressing (anorexigenic) effects of OA.

Fuel utilization by hypothalamic neurons: roles for ROS

The hypothalamus plays a major part in regulating energy homeostasis by integrating hormonal and nutritional signals. Increasing evidence shows that specific neurons in the hypothalamus respond to changing glucose, lipid and amino acid levels. However, the intracellular substrate for such 'fuel sensing' and its integration into the neuronal doctrine as it relates to energy homeostasis remains elusive. Evidence points to differential fuel utilization in response to nutrient availability and free radical formation as crucial components in regulating neuronal functions. This review places these components in the context of neurobiological aspects of circuit-specific hypothalamic output, focusing on the melanocortin system. The effects of glucose and fatty acids are discussed with emphasis on free radical production in orexigenic and anorexigenic neurons of the arcuate nucleus.

Regulation of pancreatic beta cell mass by neuronal signals from the liver

Metabolic regulation in mammals requires communication between multiple organs and tissues. The rise in the incidence of obesity and associated metabolic disorders, including type 2 diabetes, has renewed interest in interorgan communication. We used mouse models to explore the mechanism whereby obesity enhances pancreatic beta cell mass, pathophysiological compensation for insulin resistance. We found that hepatic activation of extracellular regulated kinase (ERK) signaling induced pancreatic beta cell proliferation through a neuronal-mediated relay of metabolic signals. This metabolic relay from the liver to the pancreas is involved in obesity-induced islet expansion. In mouse models of insulin-deficient diabetes, liver-selective activation of ERK signaling increased beta cell mass and normalized serum glucose levels. Thus, interorgan metabolic relay systems may serve as valuable targets in regenerative treatments for diabetes.

Fatty acid synthase inhibitors modulate energy balance via mammalian target of rapamycin complex 1 signaling in the central nervous system

OBJECTIVE

Evidence links the hypothalamic fatty acid synthase (FAS) pathway to the regulation of food intake and body weight. This includes pharmacological inhibitors that potently reduce feeding and body weight. The mammalian target of rapamycin (mTOR) is an intracellular fuel sensor whose activity in the hypothalamus is also linked to the regulation of energy balance. The purpose of these experiments was to determine whether hypothalamic mTOR complex 1 (mTORC1) signaling is involved in mediating the effects of FAS inhibitors.

RESEARCH DESIGN AND METHODS

We measured the hypothalamic phosphorylation of two downstream targets of mTORC1, S6 kinase 1 (S6K1) and S6 ribosomal protein (S6), after administration of the FAS inhibitors C75 and cerulenin in rats. We evaluated food intake in response to FAS inhibitors in rats pretreated with the mTOR inhibitor rapamycin and in mice lacking functional S6K1 (S6K1(-/-)). Food intake and phosphorylation of S6K1 and S6 were also determined after C75 injection in rats maintained on a ketogenic diet.

RESULTS

C75 and cerulenin increased phosphorylation of S6K1 and S6, and their anorexic action was reduced in rapamycin-treated rats and in S6K1(-/-) mice. Consistent with our previous findings, C75 was ineffective at reducing caloric intake in ketotic rats. Under ketosis, C75 was also less efficient at stimulating mTORC1 signaling.

CONCLUSIONS

These findings collectively indicate an important interaction between the FAS and mTORC1 pathways in the central nervous system for regulating energy balance, possibly via modulation of neuronal glucose utilization.

Molecular physiology of weight regulation in mice and humans

Evolutionary considerations relating to efficiency in reproduction, and survival in hostile environments, suggest that body energy stores are sensed and actively regulated, with stronger physiological and behavioral responses to loss than gain of stored energy. Many physiological studies support this inference, and suggest that a critical axis runs between body fat and the hypothalamus. The molecular cloning of leptin and its receptor-projects based explicitly on the search for elements in this axis-confirmed the existence of this axis and provided important tools with which to understand its molecular physiology. Demonstration of the importance of this soma-brain reciprocal connection in body weight regulation in humans has been pursued using both classical genetic approaches and studies of physiological responses to experimental weight perturbation. This paper reviews the history of the rationale and methodology of the cloning of leptin (Lep) and the leptin receptor (Lepr), and describes some of the clinical investigation characterizing this axis.

Novel leptin-regulated genes revealed by transcriptional profiling of the hypothalamic paraventricular nucleus

Leptin plays a major role in coordinating the integrated response of the CNS to changes in nutritional state. Neurons within the paraventricular nucleus (PVN) of the hypothalamus express leptin receptors and receive dense innervation from leptin receptor-expressing neurons in the arcuate nucleus. To obtain new insights into the effects of circulating leptin on PVN function, we compared global transcriptional profiles of laser-captured PVN from ad libitum fed mice versus 48 h fasted mice receiving either sham or leptin treatment intraperitoneally. Five hundred twenty-seven PVN-expressed genes were altered by fasting in a manner that was at least partially reversible by leptin. Consistent with previous reports, thyrotrophin releasing hormone mRNA levels were decreased by fasting but restored to fed levels with leptin treatment. mRNA levels of oxytocin, vasopressin, and somatostatin were also reduced by fasting and restored by leptin. Given the known effects of leptin on synaptic remodeling, it is notable that, among the top 15 genes that were positively regulated by leptin, five have been implicated in synaptic function and/or plasticity (basigin, apolipoprotein E, Gap43, GABA(A) receptor-associated protein, and synuclein-gamma). Pathway analysis identified oxidative phosphorylation, in particular, genes encoding complex 1 proteins that play a role in ubiquinone biosynthesis, to be the predominant gene set that was significantly regulated in a leptin-dependent manner. Thus, in addition to its effects on the expression of a broad range of neuropeptides, leptin may also exert more general influences on synaptic function in, and the bioenergetic state of, the PVN.

The long-chain fatty acid receptor, GPR40, and glucolipotoxicity: investigations using GPR40-knockout mice

GPR40 (G-protein-coupled receptor 40) has been shown to be a physiologically relevant receptor for long-chain fatty acids. It is a family A G-protein-coupled receptor highly expressed in the beta-cell where it increases insulin secretion by signalling via Gq and phospholipase C. Fatty acids are well known to mediate both acute stimulatory effects and chronic detrimental effects on the beta-cell. GPR40-transgenic and GPR40-/- animals have been important tools in studies of the metabolic effects of GPR40. In the present article, we review the literature on transgenic GPR40 models and present some of our own studies on the effects of a high-fat diet on the metabolic phenotype of GPR40-/- mice. GPR40 ligands represent interesting novel therapies for Type 2 diabetes but it is presently unclear whether agonists or antagonists represent the best therapeutic approach.

Hypothalamus transcriptome profile suggests an anorexia-cachexia syndrome in the anx/anx mouse model

The anx/anx mouse displays poor appetite and lean appearance and is considered a good model for the study of anorexia nervosa. To identify new genes involved in feeding behavior and body weight regulation we performed an expression profiling in the hypothalamus of the anx/anx mice. Using commercial microarrays we detected 156 differentially expressed genes and validated 92 of those using TaqMan low-density arrays. The expression of a set of 87 candidate genes selected based on literature evidences was also quantified by TaqMan low-density arrays. Our results showed enrichment in deregulated genes involved in cell death, cell morphology, and cancer, as well as an alteration of several signaling circuits involved in energy balance including neuropeptide Y and melanocortin signaling. The expression profile along with the phenotype led us to conclude that anx/anx mice resemble the anorexia-cachexia syndrome typically observed in cancer, infection with human immunodeficiency virus or chronic diseases, rather than starvation, and that anx/anx mice could be considered a good model for the treatment and investigation of this condition.

Hypothalamic IKKbeta/NF-kappaB and ER stress link overnutrition to energy imbalance and obesity

Overnutrition is associated with chronic inflammation in metabolic tissues. Whether metabolic inflammation compromises the neural regulatory systems and therefore promotes overnutrition-associated diseases remains unexplored. Here we show that a mediator of metabolic inflammation, IKKbeta/NF-kappaB, normally remains inactive although enriched in hypothalamic neurons. Overnutrition atypically activates hypothalamic IKKbeta/NF-kappaB at least in part through elevated endoplasmic reticulum stress in the hypothalamus. While forced activation of hypothalamic IKKbeta/NF-kappaB interrupts central insulin/leptin signaling and actions, site- or cell-specific suppression of IKKbeta either broadly across the brain or locally within the mediobasal hypothalamus, or specifically in hypothalamic AGRP neurons significantly protects against obesity and glucose intolerance. The molecular mechanisms involved include regulation by IKKbeta/NF-kappaB of SOCS3, a core inhibitor of insulin and leptin signaling. Our results show that the hypothalamic IKKbeta/NF-kappaB program is a general neural mechanism for energy imbalance underlying obesity and suggest that suppressing hypothalamic IKKbeta/NF-kappaB may represent a strategy to combat obesity and related diseases.

Glycerolipid metabolism and signaling in health and disease

Maintenance of body temperature is achieved partly by modulating lipolysis by a network of complex regulatory mechanisms. Lipolysis is an integral part of the glycerolipid/free fatty acid (GL/FFA) cycle, which is the focus of this review, and we discuss the significance of this pathway in the regulation of many physiological processes besides thermogenesis. GL/FFA cycle is referred to as a "futile" cycle because it involves continuous formation and hydrolysis of GL with the release of heat, at the expense of ATP. However, we present evidence underscoring the "vital" cellular signaling roles of the GL/FFA cycle for many biological processes. Probably because of its importance in many cellular functions, GL/FFA cycling is under stringent control and is organized as several composite short substrate/product cycles where forward and backward reactions are catalyzed by separate enzymes. We believe that the renaissance of the GL/FFA cycle is timely, considering the emerging view that many of the neutral lipids are in fact key signaling molecules whose production is closely linked to GL/FFA cycling processes. The evidence supporting the view that alterations in GL/FFA cycling are involved in the pathogenesis of "fatal" conditions such as obesity, type 2 diabetes, and cancer is discussed. We also review the different enzymatic and transport steps that encompass the GL/FFA cycle leading to the generation of several metabolic signals possibly implicated in the regulation of biological processes ranging from energy homeostasis, insulin secretion and appetite control to aging and longevity. Finally, we present a perspective of the possible therapeutic implications of targeting this cycling.

Central resistin regulates hypothalamic and peripheral lipid metabolism in a nutritional-dependent fashion

Evidence suggests that the adipocyte-derived hormone resistin (RSTN) directly regulates both feeding and peripheral metabolism through, so far, undefined hypothalamic-mediated mechanisms. Here, we demonstrate that the anorectic effect of RSTN is associated with inappropriately decreased mRNA expression of orexigenic (agouti-related protein and neuropeptide Y) and increased mRNA expression of anorexigenic (cocaine and amphetamine-regulated transcript) neuropeptides in the arcuate nucleus of the hypothalamus. Of interest, RSTN also exerts a profound nutrition-dependent inhibitory effect on hypothalamic fatty acid metabolism, as indicated by increased phosphorylation levels of both AMP-activated protein kinase and its downstream target acetyl-coenzyme A carboxylase, associated with decreased expression of fatty acid synthase in the ventromedial nucleus of the hypothalamus. In addition, we also demonstrate that chronic central RSTN infusion results in decreased body weight and major changes in peripheral expression of lipogenic enzymes, in a tissue-specific and nutrition-dependent manner. Thus, in the fed state central RSTN is associated with induced expression of fatty acid synthesis enzymes and proinflammatory cytokines in liver, whereas its administration in the fasted state does so in white adipose tissue. Overall, our results indicate that RSTN controls feeding and peripheral lipid metabolism and suggest that hepatic RSTN-induced insulin resistance may be mediated by central activation of de novo lipogenesis in liver.

Loss-of-function mutation of the GPR40 gene associates with abnormal stimulated insulin secretion by acting on intracellular calcium mobilization

BACKGROUND

Free fatty acids (FFAs) acutely stimulate but chronically impair glucose-stimulated insulin secretion from beta-cells. The G protein-coupled transmembrane receptor 40 (GPR40) mediates both acute and chronic effects of FFAs on insulin secretion and plays a role in glucose homeostasis. Limited information is available on the effect of GPR40 genetic abnormalities on insulin secretion and metabolic regulation in human subjects.

STUDY DESIGN AND RESULTS

For in vivo studies, we screened 734 subjects for the coding region of GPR40 and identified a new single-nucleotide mutation (Gly180Ser). The mean allele frequency was 0.75%, which progressively increased (P < 0.05) from nonobese subjects (0.42%) to moderately obese (body mass index = 30-39.9 kg/m2, 1.07%) and severely obese patients (body mass index > or = 40 kg/m2, 2.60%). The relationship between the GPR40 mutation, insulin secretion, and metabolic alterations was studied in 11 Gly/Ser mutation carriers. In these subjects, insulin secretion (insulinogenic index derived from oral glucose tolerance test) was significantly lower than in 692 Gly/Gly carriers (86.0 +/- 48.2 vs. 183.7 +/- 134.4, P < 0.005). Moreover, a case-control study indicated that plasma insulin and C-peptide responses to a lipid load were significantly (P < 0.05) lower in six Gly/Ser than in 12 Gly/Gly carriers. In vitro experiments in HeLa cells cotransfected with aequorin and the mutated Gly/Ser GPR40 indicated that intracellular Ca2+ concentration increase after oleic acid was significantly lower than in Gly/Gly GPR40-transfected cells. This fact was confirmed using fura-2 acetoxymethyl ester.

CONCLUSIONS

This newly identified GPR40 variant results in a loss of function that prevents the beta-cell ability to adequately sense lipids as an insulin secretory stimulus because of impaired intracellular Ca2+ concentration increase.

UCP2 mediates ghrelin's action on NPY/AgRP neurons by lowering free radicals

The gut-derived hormone ghrelin exerts its effect on the brain by regulating neuronal activity. Ghrelin-induced feeding behaviour is controlled by arcuate nucleus neurons that co-express neuropeptide Y and agouti-related protein (NPY/AgRP neurons). However, the intracellular mechanisms triggered by ghrelin to alter NPY/AgRP neuronal activity are poorly understood. Here we show that ghrelin initiates robust changes in hypothalamic mitochondrial respiration in mice that are dependent on uncoupling protein 2 (UCP2). Activation of this mitochondrial mechanism is critical for ghrelin-induced mitochondrial proliferation and electric activation of NPY/AgRP neurons, for ghrelin-triggered synaptic plasticity of pro-opiomelanocortin-expressing neurons, and for ghrelin-induced food intake. The UCP2-dependent action of ghrelin on NPY/AgRP neurons is driven by a hypothalamic fatty acid oxidation pathway involving AMPK, CPT1 and free radicals that are scavenged by UCP2. These results reveal a signalling modality connecting mitochondria-mediated effects of G-protein-coupled receptors on neuronal function and associated behaviour.

Trafficking of dietary fat and resistance to obesity

The task of maintaining energy balance involves not only making sure that the number of calories ingested equals the number of calories burned but also involves ensuring nutrient balance. This means that over time, the quantity of carbohydrate, fat and protein consumed equals the amount of each oxidized. While the body has the ability to convert protein to carbohydrate and carbohydrate to fat, over long periods of time the body establishes nutrient balance with a high degree of accuracy storing excess nutrients as fat. To make decisions about food intake, the brain must assimilate information about the quantity of nutrients ingested and their disposition through the body over time. This is a very complex time ordered process as different tissues may be in different states of energy balance at different intervals following food ingestion. The fundamental task for the brain is to assess the influx of nutrients relative to stored pools of those nutrients and the rate at which they are being oxidized. It has been suggested that this task is particularly difficult for dietary fat because the stored pool of lipid is quite large compared to either the stored pools of carbohydrate and protein or the quantity of fat ingested per day. It is clear that some organisms resist weight gain even in the face of highly palatable diets. In fact most individuals eat less on any given day than they could given their maximal capacity for consumption. A central question then is: what restrains food intake in the setting of widely available highly palatable food? In this paper we will discuss the evidence that the movement of dietary fat between tissues may play an important role in the fidelity of nutrient sensing and as a result, resistance or susceptibility to obesity. In particular, the relative metabolism of dietary fat favoring oxidation over storage may be associated with more robust signaling of positive energy balance and resistance to dietary induced obesity in both humans and rats.

Central lactate metabolism regulates food intake

The central nervous system regulates food intake (FI) and body weight (BW), but the associated mechanisms remain to be elucidated. Here we report that central injections of lactate reduced FI and BW in rodents. Inhibition of central lactate metabolism to pyruvate with the lactate dehydrogenase inhibitor oxamate abolished the central effects of lactate on FI and BW. Conversely, central injections of pyruvate recapitulated the effects of lactate. Finally, inhibition of central lactate metabolism prevented the ability of circulating lactate to lower FI and BW. Together, the data indicate that activation of central lactate metabolism lowers FI and BW.

Brain lipogenesis and regulation of energy metabolism

PURPOSE OF REVIEW

To revise current available information related to the role of brain lipogenic pathways in the regulation of energy homeostasis.

RECENT FINDINGS

The 'classical' hypothalamic neuropeptide view of feeding regulation has been extensively reviewed and revised during the past few years. Accumulating evidence indicates that the modulation of lipogenesis de novo in the hypothalamus, through selective pharmacologic and genetic manipulation of acetyl-CoA carboxylase, AMP-activated protein kinase, carnitine palmitoyltransferase 1, fatty acid synthase and malonyl-CoA decarboxylase enzymes, has a severe impact on food intake and body weight homeostasis. Furthermore, as these manipulations alter the hypothalamic pool of lipids, such as malonyl-CoA or long chain fatty acyl-CoA or both, the concept of lipids as signals of nutrient abundance able to modulate feeding in the hypothalamus has recently re-emerged.

SUMMARY

In this review, we summarize what is known about brain lipogenesis and energy balance and propose further avenues of research. Defining these novel mechanisms could offer new targets for the treatment of obesity and metabolic syndrome.

Silencing of hepatic fatty acid transporter protein 5 in vivo reverses diet-induced non-alcoholic fatty liver disease and improves hyperglycemia

Non-alcoholic fatty liver disease is a serious health problem linked to obesity and type 2 diabetes. To investigate the biological outcome and therapeutic potential of hepatic fatty acid uptake inhibition, we utilized an adeno-associated virus-mediated RNA interference technique to knock down the expression of hepatic fatty acid transport protein 5 in vivo prior to or after establishing non-alcoholic fatty liver disease in mice. Using this approach, we demonstrate here the ability to achieve specific, non-toxic, and persistent knockdown of fatty acid transport protein 5 in mouse livers from a single adeno-associated virus injection, resulting in a marked reduction of hepatic dietary fatty acid uptake, reduced caloric uptake, and concomitant protection from diet-induced non-alcoholic fatty liver disease. Importantly, knockdown of fatty acid transport protein 5 was also able to reverse already established non-alcoholic fatty liver disease, resulting in significantly improved whole-body glucose homeostasis. Thus, continued activity of hepatic fatty acid transport protein 5 is required to sustain caloric uptake and fatty acid flux into the liver during high fat feeding and may present a novel avenue for the treatment of non-alcoholic fatty liver disease.

Serotonin regulates C. elegans fat and feeding through independent molecular mechanisms

We investigated serotonin signaling in C. elegans as a paradigm for neural regulation of energy balance and found that serotonergic regulation of fat is molecularly distinct from feeding regulation. Serotonergic feeding regulation is mediated by receptors whose functions are not required for fat regulation. Serotonergic fat regulation is dependent on a neurally expressed channel and a G protein-coupled receptor that initiate signaling cascades that ultimately promote lipid breakdown at peripheral sites of fat storage. In turn, intermediates of lipid metabolism generated in the periphery modulate feeding behavior. These findings suggest that, as in mammals, C. elegans feeding behavior is regulated by extrinsic and intrinsic cues. Moreover, obesity and thinness are not solely determined by feeding behavior. Rather, feeding behavior and fat metabolism are coordinated but independent responses of the nervous system to the perception of nutrient availability.

Differential leptin access into the brain--a hierarchical organization of hypothalamic leptin target sites?

Leptin target sites are found in several hypothalamic areas with dense populations of long form leptin receptor (LepRb) expressing neurons which mediate important leptin actions. Leptin action has been most intensely investigated in the arcuate nucleus of the hypothalamus (ARC), which represents an important leptin target site. Recent data have shown that non-ARC leptin target sites mediate important aspects of leptin action, however, including the regulation of energy balance. Therefore, the investigation of discrete leptin signaling systems and their interactions will be an important step to understand the homeostatic action of leptin. In this review I discuss our recent data investigating important differences in leptin accessibility to ARC neurons in contrast to other hypothalamic sites like the dorsomedial hypothalamus (DMH) and discuss their importance for the leptin signaling system.

Hypothalamic substrates of metabolic imprinting

The mammalian brain develops according to intrinsic genetic programs that are influenced by a variety of environmental factors. Developing neural circuits take shape in two major environments: one in utero and a second during postnatal life. Although an abundance of epidemiological and experimental evidence indicates that nutritional variables during perinatal life have a lasting effect on metabolic phenotype, the underlying mechanisms remain unclear. Peripheral hormones are widely regarded as effective signals that reflect the state of peripheral environments and can directly influence the development of a variety of functional neural systems. Recent findings suggest that the adipocyte-derived hormone leptin may play an important role in directing formation of hypothalamic neural pathways that control body weight. The arcuate nucleus of the hypothalamus (ARH) is a key site for the regulatory actions of leptin in adults, and this same hormone is required for the normal development of ARH projections to other parts of the hypothalamus. In this review, the neurobiological role of leptin is considered within the context of hypothalamic development and the possibility that variations in both prenatal and postnatal nutritional environments may impact development of neural circuits that control energy metabolism through an indirect action on leptin secretion, or signaling, during key developmental critical periods.

Upper intestinal lipids trigger a gut-brain-liver axis to regulate glucose production

Energy and glucose homeostasis are regulated by food intake and liver glucose production, respectively. The upper intestine has a critical role in nutrient digestion and absorption. However, studies indicate that upper intestinal lipids inhibit food intake as well in rodents and humans by the activation of an intestine-brain axis. In parallel, a brain-liver axis has recently been proposed to detect blood lipids to inhibit glucose production in rodents. Thus, we tested the hypothesis that upper intestinal lipids activate an intestine-brain-liver neural axis to regulate glucose homeostasis. Here we demonstrate that direct administration of lipids into the upper intestine increased upper intestinal long-chain fatty acyl-coenzyme A (LCFA-CoA) levels and suppressed glucose production. Co-infusion of the acyl-CoA synthase inhibitor triacsin C or the anaesthetic tetracaine with duodenal lipids abolished the inhibition of glucose production, indicating that upper intestinal LCFA-CoAs regulate glucose production in the preabsorptive state. Subdiaphragmatic vagotomy or gut vagal deafferentation interrupts the neural connection between the gut and the brain, and blocks the ability of upper intestinal lipids to inhibit glucose production. Direct administration of the N-methyl-d-aspartate ion channel blocker MK-801 into the fourth ventricle or the nucleus of the solitary tract where gut sensory fibres terminate abolished the upper-intestinal-lipid-induced inhibition of glucose production. Finally, hepatic vagotomy negated the inhibitory effects of upper intestinal lipids on glucose production. These findings indicate that upper intestinal lipids activate an intestine-brain-liver neural axis to inhibit glucose production, and thereby reveal a previously unappreciated pathway that regulates glucose homeostasis.

A malonyl-CoA-dependent switch in the bacterial response to a dysfunction of lipid metabolism

Bacteria stringently regulate the synthesis of their membrane phospholipids, but the responsible regulatory mechanisms are incompletely understood. Bacillus subtilis FabF, the target of the mycotoxin cerulenin, catalyses the condensation of malonyl-ACP with acyl-ACP to extend the growing acyl chain by two carbons. Here we show that B. subtilis strains containing the fabF1 allele, which codes for the cerulenin-insensitive protein FabF[I108F], overexpressed several genes involved in fatty acid and phospholipid biosynthesis (the fap regulon) and had significantly elevated levels of malonyl-CoA. These results pinpointed FabF[I108F] as responsible for the increased malonyl-CoA production, which in turn acts as an inducer of the fap regulon by impairing the binding of the FapR repressor to its DNA targets. Synthesis of acyl-ACPs by a cell-free fatty acid system prepared from fabF1 cells showed the accumulation of short- and medium-chain acyl-ACPs. These results indicate that the acyl-ACP chain length acceptance of FabF[I108F] is biased towards shorter acyl-ACPs. We also provide evidence that upregulation of FabF[I108F] is essential for survival and for resistance to cerulenin of fabF1 cells. These findings indicate that malonyl-CoA is a key molecule to monitor lipid metabolism functioning and trigger appropriate genetic and biochemical adjustments to relieve dysfunctions of this essential metabolic pathway.

Central nervous system regulation of adipocyte metabolism

The white adipose tissue was initially largely known only as an energy storage tissue. It is now well recognized that white adipose tissue is a major endocrine and secretory organ, which releases a wide range of protein signals and factors termed adipokines. The regulation of adipocyte metabolism is an important factor for the understanding of obesity, and some mechanisms are still unknown. Many homeostatic processes, including appetite and food intake, are controlled by neuroendocrine circuits involving the central nervous system. There is substantial evidence demonstrating that the central nervous system also directly regulates adipocyte metabolism. In this review, we discuss the central actions of some peptides with an important role in energy balance regulation on adipocyte metabolism and the physiological relevance of these actions.

Activation of central lactate metabolism lowers glucose production in uncontrolled diabetes and diet-induced insulin resistance

OBJECTIVE

Hypothalamic lactate metabolism lowers hepatic glucose production and plasma glucose levels in normal rodents. However, it remains unknown whether activation of hypothalamic lactate metabolism lowers glucose production and plasma glucose levels in rodents with diabetes and obesity.

RESEARCH DESIGN AND METHODS

We performed intracerebroventricular (ICV) administration of lactate to enhance central lactate metabolism in 1) early-onset streptozotocin-induced uncontrolled diabetic rodents, 2) experimentally induced hypoinsulinemic normal rodents, and 3) early-onset diet-induced insulin-resistant rodents. Tracer-dilution methodology was used to assess the impact of ICV lactate on the rate of glucose production in all three models.

RESULTS

We first report that in the absence of insulin treatment, ICV lactate administration lowered glucose production and glucose levels in rodents with uncontrolled diabetes. Second, ICV lactate administration lowered glucose production and glucose levels in normal rodents with experimentally induced hypoinsulinemia. Third, and finally, ICV lactate administration lowered glucose production in normal rodents with diet-induced insulin resistance.

CONCLUSIONS

Central lactate metabolism lowered glucose production in uncontrolled diabetic and normal rodents with hypoinsulinemia and in rodents with diet-induced insulin resistance. These data suggest that insulin signaling is not required for central lactate to lower glucose production and that the activation of hypothalamic lactate metabolism could consequently bypass insulin resistance and lower glucose levels in early-onset diabetes and obesity.

Forkhead transcription factor FoxO1 in adipose tissue regulates energy storage and expenditure

OBJECTIVE

Adipose tissue serves as an integrator of various physiological pathways, energy balance, and glucose homeostasis. Forkhead box-containing protein O subfamily (FoxO) 1 mediates insulin action at the transcriptional level. However, physiological roles of FoxO1 in adipose tissue remain unclear.

RESEARCH DESIGN AND METHODS

In the present study, we generated adipose tissue-specific FoxO1 transgenic mice (adipocyte protein 2 [aP(2)]-FLAG-Delta 256) using an aP(2) promoter/enhancer and a mutant FoxO1 (FLAG Delta 256) in which the carboxyl terminal transactivation domain was deleted. Using these mice, we analyzed the effects of the overexpression of FLAG Delta 256 on glucose metabolism and energy homeostasis.

RESULTS

The aP(2)-FLAG-Delta 256 mice showed improved glucose tolerance and insulin sensitivity accompanied with smaller-sized adipocytes and increased adiponectin (adipoq) and Glut 4 (Slc2a4) and decreased tumor necrosis factor alpha (Tnf) and chemokine (C-C motif) receptor 2 (Ccr2) gene expression levels in white adipose tissue (WAT) under a high-fat diet. Furthermore, the aP(2)-FLAG-Delta 256 mice had increased oxygen consumption accompanied with increased expression of peroxisome proliferator-activated receptor gamma coactivator (PGC)-1 alpha protein and uncoupling protein (UCP)-1 (Ucp1), UCP-2 (Ucp2), and beta 3-AR (Adrb3) in brown adipose tissue (BAT). Overexpression of FLAG Delta 256 in T37i cells, which are derived from the hibernoma of SV40 large T antigen transgenic mice, increased expression of PGC-1 alpha protein and Ucp1. Furthermore, knockdown of endogenous FoxO1 in T37i cells increased Pgc1 alpha (Ppargc1a), Pgc1 beta (Ppargc1b), Ucp1, and Adrb3 gene expression.

CONCLUSIONS

These data suggest that FoxO1 modulates energy homeostasis in WAT and BAT through regulation of adipocyte size and adipose tissue-specific gene expression in response to excessive calorie intake.

A call for higher standards of evidence for dietary guidelines

Dietary guidelines, especially those designed to prevent the diseases of dietary excess, are a relatively new phenomenon in the United States. National dietary guidelines have been promulgated based on scientific reasoning and indirect evidence. In general, weak evidentiary support has been accepted as adequate justification for these guidelines. This low standard of evidence is based on several misconceptions, most importantly the belief that such guidelines could not cause harm. Using guidelines against dietary fat as a case in point, an analysis is provided that suggests that harm indeed may have been caused by the widespread dissemination of and adherence to these guidelines, through their contribution to the current epidemic of obesity and overweight in the U.S. An explanation is provided of what may have gone wrong in the development of dietary guidelines, and an alternative and more rigorous standard is proposed for evidentiary support, including the recommendation that when adequate evidence is not available, the best option may be to issue no guideline.

Brain insulin, energy and glucose homeostasis; genes, environment and metabolic pathologies

The central nervous system is essential in maintaining energy and glucose homeostasis. In both animals and humans, efficient cerebral insulin signalling is a pivotal control element in these pathophysiological processes. The action of insulin in the brain is under a multilevel control via metabolic, endocrine and neural signals induced by nutrients, integrated mainly by the hypothalamus. Of particular interest is the interaction of insulin with the anabolic and catabolic neuroregulators. The anorexic peptides insulin, leptin and the neurotransmitter serotonin share common signalling pathways involved in food intake, in particular the insulin receptor substrate, phosphatidylinositol-3-kinase (PI3K) pathway. The dialogue of neurotransmitters and peptides via this signalling pathway is potentially of major importance in the pathophysiology of the brain in general and specifically in the regulation of feeding behaviour. At this time, a new concept in the aetiopathology of type 2 diabetes is immerging. This concept proposes that the combination of defective pancreatic beta-cell function and insulin resistance not only in classical insulin target tissues but in every tissue, contributes to the onset of the disease. It highlights the importance of the disruption of cerebral insulin signal transmission and its direct relation to metabolic diseases. Impaired brain insulin signalling, a link coupling obesity to diabetes, may be related to either genetic factors, or environmental factors such as stress, over or under-feeding and unbalanced diets: such factors may work either independently or in concert. Current approaches used for the prevention and treatment of type 2 diabetes are not adequately effective. Most of the anti-diabetic therapies induce many adverse effects, in particular obesity, and thus may initiate a vicious cycle of problems. In order to develop new, more efficient, preventive and therapeutic strategies for metabolic pathologies, there is an urgent need for increased understanding of the complexity of insulin signalling in the brain and on the interactive, central and peripheral effects of insulin.

Glucokinase activators in diabetes management

Type 2 diabetes is a chronic metabolic disease that adversely affects both the quality and longevity of life. The disease is characterised by elevated blood glucose (hyperglycaemia) that is associated with microvascular complications and increased macrovascular risk. Existing oral agents, either alone or in combination, do not exhibit adequate or sustained glucose lowering efficacy in Type 2 diabetics. Consequently, there is an unmet medical need for improved antidiabetic agents which are both more effective at lowering glucose and which display sustained efficacy over a number of years. Such agents would allow present glycaemic treatment targets to be achieved with greater success. Glucokinase activators (GKAs) represent a novel class of glucose-lowering agents. Preclinical data supports the notion that these agents act to lower blood glucose through effects in both the liver and pancreas. It is predicted that this dual compartment mechanism of action of GKAs will translate to robust glucose lowering in diabetic patients. The potential benefits and risks associated with the pharmacological activation of glucokinase are evaluated. The status of GKAs in preclinical and clinical development is assessed are the future prospects of these agents.

Neuronal control of energy homeostasis

Neuronal control of body energy homeostasis is the key mechanism by which animals and humans regulate their long-term energy balance. Various hypothalamic neuronal circuits (which include the hypothalamic melanocortin, midbrain dopamine reward and caudal brainstem autonomic feeding systems) control energy intake and expenditure to maintain body weight within a narrow range for long periods of a life span. Numerous peripheral metabolic hormones and nutrients target these structures providing feedback signals that modify the default "settings" of neuronal activity to accomplish this balance. A number of molecular genetic tools for manipulating individual components of brain energy homeostatic machineries, in combination with anatomical, electrophysiological, pharmacological and behavioral techniques, have been developed, which provide a means for elucidating the complex molecular and cellular mechanisms of feeding behavior and metabolism. This review will highlight some of these advancements and focus on the neuronal circuitries of energy homeostasis.

Alterations in plasma and tissue lipids associated with obesity and metabolic syndrome

The MS (metabolic syndrome) is a cluster of clinical and biochemical abnormalities characterized by central obesity, dyslipidaemia [hypertriglyceridaemia and decreased HDL-C (high-density lipoprotein cholesterol)], glucose intolerance and hypertension. Insulin resistance, hyperleptinaemia and low plasma levels of adiponectin are also widely related to features of the MS. This review focuses on lipid metabolism alterations associated with the MS, paying special attention to changes in plasma lipids and cellular fatty acid oxidation. Lipid metabolism alterations in liver and peripheral tissues are addressed, with particular reference to adipose and muscle tissues, and the mechanisms by which some adipokines, namely leptin and adiponectin, mediate the regulation of fatty acid oxidation in those tissues. Activation of the AMPK (AMP-dependent kinase) pathway, together with a subsequent increase in fatty acid oxidation, appear to constitute the main mechanism of action of these hormones in the regulation of lipid metabolism. Decreased activation of AMPK appears to have a role in the development of features of the MS. In addition, alteration of AMPK signalling in the hypothalamus, which may function as a sensor of nutrient availability, integrating multiple nutritional and hormonal signals, may have a key role in the appearance of the MS.

Pharmacological stimulation of brain carnitine palmitoyl-transferase-1 decreases food intake and body weight

Inhibition of brain carnitine palmitoyl-transferase-1 (CPT-1) is reported to decrease food intake and body weight in rats. Yet, the fatty acid synthase (FAS) inhibitor and CPT-1 stimulator C75 produces hypophagia and weight loss when given to rodents intracerebroventricularly (icv). Thus roles and relative contributions of altered brain CPT-1 activity and fatty acid oxidation in these phenomena remain unclarified. We administered compounds that target FAS or CPT-1 to mice by single icv bolus and examined acute and prolonged effects on feeding and body weight. C75 decreased food intake rapidly and potently at all doses (1-56 nmol) and dose dependently inhibited intake on day 1. Dose-dependent weight loss on day 1 persisted through 4 days of postinjection monitoring. The FAS inhibitor cerulenin produced dose-dependent (560 nmol) hypophagia for 1 day, weight loss for 2 days, and weight regain to vehicle control by day 3. The CPT-1 inhibitor etomoxir (32, 320 nmol) did not alter overall day 1 feeding. However, etomoxir attenuated the hypophagia produced by C75, indicating that CPT-1 stimulation is important for C75's effect. A novel compound, C89b, was characterized in vitro as a selective stimulator of CPT-1 that does not affect fatty acid synthesis. C89b (100, 320 nmol) decreased feeding in mice for 3 days and produced persistent weight loss for 6 days without producing conditioned taste aversion. Similarly, intraperitoneal administration decreased feeding and body weight without producing conditioned taste aversion. These results suggest a role for brain CPT-1 in the regulation of energy balance and implicate CPT-1 stimulation as a pharmacological approach to weight loss.

Lack of starvation-induced activation of AMP-activated protein kinase in the hypothalamus of the Lou/C rats resistant to obesity

OBJECTIVE

The AMP-activated protein kinase (AMPK) is involved in the control of food intake by the hypothalamus. The aim of this work was to investigate if modification of hypothalamic AMPK regulation could be related to the spontaneous food restriction of Lou/C rats, a strain resistant to obesity exhibiting a 40% reduction in caloric intake compared with their lean Wistar counterparts.

DESIGN

Three-month-old male Lou/C rats were compared with age-matched male Wistar rats in both fed ad libitum and 24-h food deprivation state.

MEASUREMENTS AND RESULTS

We first confirmed that starvation activated both isoforms of AMPK catalytic alpha subunits and enhanced the phosphorylation state of its downstream targets acetyl-CoA carboxylase and elongation factor 2 in the hypothalamus of Wistar rats. These changes were not observed in the hypothalamus of Lou/C rats. Interestingly, the starvation-induced changes in hypothalamic mRNA levels of the main orexigenic and anorexigenic neuropeptides were also blunted in the Lou/C rats. Analysis of the concentrations of circulating substrates and hormones known to regulate hypothalamic AMPK indicated that the starvation-induced changes in ghrelin, adiponectin and leptin were not observed in Lou/C rats. Furthermore, an increased phosphorylation state of signal transducer and activator of transcription 3 (STAT3), which admittedly mediates leptin signaling, was evidenced in the hypothalamus of the starved Lou/C rats, as well as modifications of expression of the leptin-sensitive genes suppressor of cytokine signaling-3 and stearoyl-coenzyme A desaturase 1. In addition, despite reduced leptin level in fed Lou/C rats, the phosphorylation state of hypothalamic STAT3 remained similar to that found in fed Wistar rats, an adaptation that could be explained by the concomitant increase in ObRb leptin receptor mRNA expression.

CONCLUSION

Activation of hypothalamic AMPK by starvation, which stimulates food intake through changes in (an)orexigenic neuropeptides in the normal rats, was not observed in the spontaneously hypophagic Lou/C rats.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

Regulation of hypothalamic malonyl-CoA by central glucose and leptin

Hypothalamic malonyl-CoA has been shown to function in global energy homeostasis by modulating food intake and energy expenditure. Little is known, however, about the regulation of malonyl-CoA concentration in the central nervous system. To address this issue we investigated the response of putative intermediates in the malonyl-CoA pathway to metabolic and endocrine cues, notably those provoked by glucose and leptin. Hypothalamic malonyl-CoA rises in proportion to the carbohydrate content of the diet consumed after food deprivation. Malonyl-CoA concentration peaks 1 h after refeeding or after peripheral glucose administration. This response depends on the dose of glucose administered and is blocked by the i.c.v. administration of an inhibitor of glucose metabolism, 2-deoxyglucose (2-DG). The kinetics of change in hypothalamic malonyl-CoA after glucose administration is coincident with the suppression of phosphorylation of AMP kinase and acetyl-CoA carboxylase. Blockade of glucose utilization in the CNS by i.c.v. 2-DG prevented the effects of glucose on 5'AMP-activated protein kinase, malonyl-CoA, hypothalamic neuropeptide expression, and food intake. Finally, we showed that leptin can increase hypothalamic malonyl-CoA and that the increase is additive with glucose administration. Leptin-deficient ob/ob mice, however, showed no defect in the glucose- or refeeding-induced rise in hypothalamic malonyl-CoA after food deprivation, demonstrating that leptin was not required for this effect. These studies show that hypothalamic malonyl-CoA responds to the level of circulating glucose and leptin, both of which affect energy homeostasis.

Genetic links between diet and lifespan: shared mechanisms from yeast to humans

Caloric restriction is the only known non-genetic intervention that robustly extends lifespan in mammals. This regimen also attenuates the incidence and progression of many age-dependent pathologies. Understanding the genetic mechanisms that underlie dietary-restriction-induced longevity would therefore have profound implications for future medical treatments aimed at tackling conditions that are associated with the ageing process. Until recently, however, almost nothing was known about these mechanisms in metazoans. Recent advances in our understanding of the genetic bases of energy sensing and lifespan control in yeast, invertebrates and mammals have begun to solve this puzzle. Evidence is mounting that the brain has a crucial role in sensing dietary restriction and promoting longevity in metazoans.

The central melanocortin system directly controls peripheral lipid metabolism

Disruptions of the melanocortin signaling system have been linked to obesity. We investigated a possible role of the central nervous melanocortin system (CNS-Mcr) in the control of adiposity through effects on nutrient partitioning and cellular lipid metabolism independent of nutrient intake. We report that pharmacological inhibition of melanocortin receptors (Mcr) in rats and genetic disruption of Mc4r in mice directly and potently promoted lipid uptake, triglyceride synthesis, and fat accumulation in white adipose tissue (WAT), while increased CNS-Mcr signaling triggered lipid mobilization. These effects were independent of food intake and preceded changes in adiposity. In addition, decreased CNS-Mcr signaling promoted increased insulin sensitivity and glucose uptake in WAT while decreasing glucose utilization in muscle and brown adipose tissue. Such CNS control of peripheral nutrient partitioning depended on sympathetic nervous system function and was enhanced by synergistic effects on liver triglyceride synthesis. Our findings offer an explanation for enhanced adiposity resulting from decreased melanocortin signaling, even in the absence of hyperphagia, and are consistent with feeding-independent changes in substrate utilization as reflected by respiratory quotient, which is increased with chronic Mcr blockade in rodents and in humans with loss-of-function mutations in MC4R. We also reveal molecular underpinnings for direct control of the CNS-Mcr over lipid metabolism. These results suggest ways to design more efficient pharmacological methods for controlling adiposity.

The role of the vgf gene and VGF-derived peptides in nutrition and metabolism

Energy homeostasis is a complex physiological function coordinated at multiple levels. The issue of genetic regulation of nutrition and metabolism is attracting increasing interest and new energy homeostasis-regulatory genes are continuously identified. Among these genes, vgf is gaining increasing interest following two observations: (1) VGF-/- mice have a lean and hypermetabolic phenotype; (2) the first VGF-derived peptide involved in energy homeostasis, named TLQP-21, has been identified. The aim of this review will be to discuss the role of the vgf gene and VGF derived peptides in metabolic and nutritional functions. In particular we will: (1) provide a brief overview on the central systems regulating energy homeostasis and nutrition particularly focusing on the melanocortin system; (2) introduce the structure and molecular characteristic of vgf; (3) describe the phenotype of VGF deficient mice; (4) present recent data on the metabolic role of VGF-derived peptides, particularly focusing on one peptide named TLQP-21.