Ablation of ghrelin improves the diabetic but not obese phenotype of ob/ob mice

Modulation of ingestive behavior and gastrointestinal motility by ghrelin in diabetic animals and humans

Acyl ghrelin, a 28-amino acid peptide hormone, is the endogenous cognate ligand for the growth hormone secretagogue receptor. Ghrelin is involved in stimulating growth hormone release, eliciting feeding behavior, inducing adiposity and stimulating gastrointestinal motility. Ghrelin is unique for its post-translational modification of O-n-octanoylation at serine 3 through ghrelin O-acyltransferase, and is the only peripheral signal to enhance food intake. Plasma ghrelin levels manifest "biphasic changes" in diabetes mellitus (DM). In the early stage of DM, the stomach significantly increases the secretion of ghrelin into the plasma, and elevated plasma ghrelin levels are correlated with diabetic hyperphagic feeding and accelerated gastrointestinal motility. In the late stage of DM, plasma ghrelin levels may be lower, which might be linked with anorexia/muscle wasting, delayed gastrointestinal transit, and even gastroparesis. Therefore, the unique ghrelin system may be the most important player compared to the other hindgut hormones participating in the "entero-insular axis". Further studies using either knockdown or knockout of ghrelin gene products and ghrelin O-acyltransferase may unravel the pathogenesis of DM, and show benefits in combating this disease and metabolic syndrome.

Uncoupling protein-2 decreases the lipogenic actions of ghrelin

The exact mechanisms through which ghrelin promotes lipogenesis are unknown. Uncoupling protein (UCP)-2 is a mitochondrial protein important in regulating reactive oxygen species; however, recent research shows that it may play an important role fat metabolism. Given that ghrelin increases UCP2 mRNA in white adipose tissue, we examined whether the lipogenic actions of ghrelin are modulated by UCP2 using ucp2(+/+) and ucp2(-/-) mice. Chronic ghrelin treatment either via osmotic minipumps or daily ip injections induced body weight gain in both ucp2(+/+) and ucp2(-/-) mice; however, body weight gain was potentiated in ucp2(-/-) mice. Increased body weight gain was completely due to increased body fat as a result of decreased fat oxidation in ucp2(-/-) mice. Ghrelin treatment of ucp2(-/-) mice resulted in a gene expression profile favoring lipogenesis. In a calorie-restriction model of negative energy balance, ghrelin to ucp2(+/+) mice did not increase body weight; however, ghrelin to ucp2(-/-) mice still induced body weight. These results show that UCP2 plays an important role in fat metabolism by promoting fat oxidation and restricts ghrelin-induced lipogenesis.

Ghrelin O-acyltransferase (GOAT) is essential for growth hormone-mediated survival of calorie-restricted mice

Ghrelin O-acyltransferase (GOAT) attaches octanoate to proghrelin, which is processed to ghrelin, an octanoylated peptide hormone that stimulates release of growth hormone (GH) from pituitary cells. Elimination of the gene encoding ghrelin or its receptor produces only mild phenotypes in mice. Thus, the essential function of ghrelin is obscure. Here, we eliminate the Goat gene in mice, thereby eliminating all octanoylated ghrelin from blood. On normal or high fat diets, Goat(-/-) mice grew and maintained the same weights as wild-type (WT) littermates. When subjected to 60% calorie restriction, WT and Goat(-/-) mice both lost 30% of body weight and 75% of body fat within 4 days. In both lines, fasting blood glucose initially declined equally. After 4 days, glucose stabilized in WT mice at 58-76 mg/dL. In Goat(-/-) mice, glucose continued to decline, reaching 12-36 mg/dL on day 7. At this point, WT mice showed normal physical activity, whereas Goat(-/-) mice were moribund. GH rose progressively in calorie-restricted WT mice and less in Goat(-/-) mice. Infusion of either ghrelin or GH normalized blood glucose in Goat(-/-) mice and prevented death. Thus, an essential function of ghrelin in mice is elevation of GH levels during severe calorie restriction, thereby preserving blood glucose and preventing death.

Ghrelin inhibits insulin secretion through the AMPK-UCP2 pathway in beta cells

Ghrelin inhibits insulin secretion partly via induction of IA-2beta. However, the orexigenic effect of ghrelin is mediated by the AMP-activated protein kinase (AMPK)-uncoupling protein 2 (UCP2) pathway. Here, we demonstrate that ghrelin's inhibitory effect on insulin secretion also occurs through the AMPK-UCP2 pathway. Ghrelin increased AMPK phosphorylation and UCP2 mRNA expression in MIN6 insulinoma cells. Overexpression or downregulation of UCP2 attenuated or enhanced insulin secretion, respectively. Furthermore, AMPK activator had a similar effect to ghrelin on UCP2 and insulin secretion in MIN6 cells. In conclusion, ghrelin's inhibitory effect on insulin secretion is partly mediated by the AMPK-UCP2 pathway, which is independent of the IA-2beta pathway.

Regulation of lipid metabolism by energy availability: a role for the central nervous system

The central nervous system (CNS) is crucial in the regulation of energy homeostasis. Many neuroanatomical studies have shown that the white adipose tissue (WAT) is innervated by the sympathetic nervous system, which plays a critical role in adipocyte lipid metabolism. Therefore, there are currently numerous reports indicating that signals from the CNS control the amount of fat by modulating the storage or oxidation of fatty acids. Importantly, some CNS pathways regulate adipocyte metabolism independently of food intake, suggesting that some signals possess alternative mechanisms to regulate energy homeostasis. In this review, we mainly focus on how neuronal circuits within the hypothalamus, such as leptin- ghrelin-and resistin-responsive neurons, as well as melanocortins, neuropeptide Y, and the cannabinoid system exert their actions on lipid metabolism in peripheral tissues such as WAT, liver or muscle. Dissecting the complicated interactions between peripheral signals and neuronal circuits regulating lipid metabolism might open new avenues for the development of new therapies preventing and treating obesity and its associated cardiometabolic sequelae.

Nutritional and hormonal regulation of uncoupling protein 2

Uncoupling proteins (UCPs) belong to a family of mitochondrial carrier proteins that are present in the mitochondrial inner membrane. Genetic and experimental studies have shown that UCP dysfunction can be involved in metabolic disorders and in obesity. Uncoupling protein-1 (UCP1; also known as thermogenin) was identified in 1988 and found to be highly expressed in brown adipose tissue. UCP1 allows the leak of protons in respiring mitochondria, dissipating the energy as heat; the enzyme has an important role in nonshivering heat production induced by cold exposure or food intake. In 1997, two homologs of UCP1 were identified and named UCP2 and UCP3. These novel proteins also lower mitochondrial membrane potential, but whether they can dissipate metabolic energy as heat as efficiently as UCP1 is open to dispute. Even after a decade of study, the physiological roles of these novel proteins have still not been completely elucidated. This review aims to shed light on the nutritional and hormonal regulation of UCP2 and on its physiological roles.

Effect of ghrelin on glucose-insulin homeostasis: therapeutic implications

Ghrelin is a 28-amino-acid peptide that displays a strong growth hormone- (GH-) releasing activity through the activation of the growth hormone secretagogue receptor (GHSR). The first studies about role of ghrelin were focused on its orexigenic ability, but despite indisputable pharmacological data, the evidence for a physiological role for ghrelin in the control of appetite is much less clear. Mice with targeted deletion of either ghrelin or the GHSR exhibit an essentially normal metabolic phenotype when fed a regular chow diet, suggesting that ghrelin may have a redundant role in the regulation of food intake. RNAs for ghrelin as well as GHSR are expressed in the pancreas of rats and humans and several studies propose that ghrelin could have an important function in glucose homeostasis and insulin release, independent of GH secretion. Low plasma ghrelin levels are associated with elevated fasting insulin levels and insulin resistance, suggesting both physiological and pathophysiological roles for ghrelin. For this reason, at least theoretically, ghrelin and/or its signalling manipulation could be useful for the treatment or prevention of diseases of glucose homeostasis such as type 2 diabetes.

Interactions of gastrointestinal peptides: ghrelin and its anorexigenic antagonists

Food intake behaviour and energy homeostasis are strongly regulated by a complex system of humoral factors and nerval structures constituting the brain-gut-axis. To date the only known peripherally produced and centrally acting peptide that stimulates food intake is ghrelin, which is mainly synthesized in the stomach. Recent data indicate that the orexigenic effect of ghrelin might be influenced by other gastrointestinal peptides such as cholecystokinin (CCK), bombesin, desacyl ghrelin, peptide YY (PYY), as well as glucagon-like peptide (GLP). Therefore, we will review on the interactions of ghrelin with several gastrointestinal factors known to be involved in appetite regulation in order to elucidate the interdependency of peripheral orexigenic and anorexigenic peptides in the control of appetite.

Ghrelin in the regulation of body weight and metabolism

Ghrelin, a peptide hormone predominantly produced by the stomach, was isolated as the endogenous ligand for the growth hormone secretagogue receptor. Ghrelin is a potent stimulator of growth hormone (GH) secretion and is the only circulatory hormone known to potently enhance feeding and weight gain and to regulate energy homeostasis following central and systemic administration. Therapeutic intervention with ghrelin in catabolic situations may induce a combination of enhanced food intake, increased gastric emptying and nutrient storage, coupled with an increase in GH thereby linking nutrient partitioning with growth and repair processes. These qualities have fostered the idea that ghrelin-based compounds may have therapeutic utility in treating malnutrition and wasting induced by various sub-acute and chronic disorders. Conversely, compounds that inhibit ghrelin action may be useful for the prevention or treatment of metabolic syndrome components such as obesity, impaired lipid metabolism or insulin resistance. In recent years, the effects of ghrelin on glucose homeostasis, memory function and gastrointestinal motility have attracted considerable amount of attention and revealed novel therapeutic targets in treating a wide range of pathologic conditions. Furthermore, discovery of ghrelin O-acyltransferase has also opened new research opportunities that could lead to major understanding of ghrelin physiology. This review summarizes the current knowledge on ghrelin synthesis, secretion, mechanism of action and biological functions with an additional focus on potential for ghrelin-based pharmacotherapies.

Ghrelin gene products and the regulation of food intake and gut motility

A breakthrough using "reverse pharmacology" identified and characterized acyl ghrelin from the stomach as the endogenous cognate ligand for the growth hormone (GH) secretagogue receptor (GHS-R) 1a. The unique post-translational modification of O-n-octanoylation at serine 3 is the first in peptide discovery history and is essential for GH-releasing ability. Des-acyl ghrelin, lacking O-n-octanoylation at serine 3, is also produced in the stomach and remains the major molecular form secreted into the circulation. The third ghrelin gene product, obestatin, a novel 23-amino acid peptide identified from rat stomach, was found by comparative genomic analysis. Three ghrelin gene products actively participate in modulating appetite, adipogenesis, gut motility, glucose metabolism, cell proliferation, immune, sleep, memory, anxiety, cognition, and stress. Knockdown or knockout of acyl ghrelin and/or GHS-R1a, and overexpression of des-acyl ghrelin show benefits in the therapy of obesity and metabolic syndrome. By contrast, agonism of acyl ghrelin and/or GHS-R1a could combat human anorexia-cachexia, including anorexia nervosa, chronic heart failure, chronic obstructive pulmonary disease, liver cirrhosis, chronic kidney disease, burn, and postsurgery recovery, as well as restore gut dysmotility, such as diabetic or neurogenic gastroparesis, and postoperative ileus. The ghrelin acyl-modifying enzyme, ghrelin O-Acyltransferase (GOAT), which attaches octanoate to serine-3 of ghrelin, has been identified and characterized also from the stomach. To date, ghrelin is the only protein to be octanylated, and inhibition of GOAT may have effects only on the stomach and is unlikely to affect the synthesis of other proteins. GOAT may provide a critical molecular target in developing novel therapeutics for obesity and type 2 diabetes.

Central ghrelin regulates peripheral lipid metabolism in a growth hormone-independent fashion

GH plays a major role in the regulation of lipid metabolism and alterations in GH axis elicit major changes in fat distribution and mobilization. For example, in patients with GH deficiency (GHD) or in mice lacking the GH receptor, the percentage of fat is increased. In addition to the direct actions of GH on lipid metabolism, current evidence indicates that ghrelin, a stomach-derived peptide hormone with potent GH secretagogue action, increases lipogenesis in white adipose tissue (WAT) through a hypothalamic-mediated mechanism. Still, the mechanism by which GH tone modulates ghrelin actions on WAT remains unclear. Here we investigated the effect of central ghrelin administration on lipid metabolism in lipogenic tissues (liver and WAT) in the absence of GH, by using a model for the study of GHD, namely the spontaneous dwarf rat, which shows increased body fat. Our data demonstrate that central chronic ghrelin administration regulates adipose lipid metabolism, mainly in a GH-independent fashion, as a result of increased mRNA, protein expression, and activity levels of fatty acid metabolism enzymes. On the contrary, central ghrelin regulates hepatic lipogenesis de novo in a GH-independent fashion but lipid mobilization in a GH-dependent fashion because carnitine palmitoyltransferase 1 was decreased only in wild-type Lewis rats. These findings suggest the existence of a new central nervous system-based neuroendocrine circuit, regulating metabolic homeostasis of adipose tissue. Understanding the molecular mechanism underlying the interplay between GH and ghrelin and their effects on lipid metabolism will provide new strategies for the design and development of suitable drugs for the treatment of GHD, obesity, and its comorbidities.

Novel factors as therapeutic targets to treat diabetes. Focus on leptin and ghrelin

BACKGROUND

Obesity is the major cause of type 2 diabetes. In the mid 1990s interest in adipose tissue was revived by the discovery of leptin. The association of obesity and diabetes emphasizes their shared physiopathological features. At the end of the 1990s, ghrelin, a potent gastric orexigenic factor, was found to be involved in obesity. Leptin and ghrelin have opposite actions in several tissues including the regulation of feeding in the brain.

OBJECTIVE/METHODS

To survey the role of leptin and ghrelin in glucose metabolism. We summarize the current state of research and discuss the roles of ghrelin and leptin in glucose homeostases and the potential application of drugs targeting leptin and ghrelin signalling to prevent and treat diabetes.

RESULTS/CONCLUSIONS

A pressing challenge is to determine how leptin, ghrelin and other adipokines or gastric factors are involved in metabolic disorders. Answering these questions will require the development of new pharmacological tools that target specific adipokine systems. Hopefully, new therapeutic targets will be identified.

Colocalization of ghrelin O-acyltransferase and ghrelin in gastric mucosal cells

Ghrelin is a peptide hormone with many known functions, including orexigenic, blood glucose-regulatory, and antidepressant actions, among others. Mature ghrelin is unique in that it is the only known naturally occurring peptide to be posttranslationally modified by O-acylation with octanoate. This acylation is required for many of ghrelin's actions, including its effects on promoting increases in food intake and body weight. GOAT (ghrelin O-acyltransferase), one of 16 members of the MBOAT family of membrane-bound O-acyltransferases, has recently been identified as the enzyme responsible for catalyzing the addition of the octanoyl group to ghrelin. Although the initial reports of GOAT have localized its encoding mRNA to tissues known to contain ghrelin, it is as yet unclear whether the octanoylation occurs within ghrelin-producing cells or in neighboring cells. Here, we have performed dual-label histochemical analysis on mouse stomach sections and quantitative PCR on mRNAs from highly enriched pools of mouse gastric ghrelin cells to demonstrate a high degree of GOAT mRNA expression within ghrelin-producing cells of the gastric oxyntic mucosa. We also demonstrate that GOAT is the only member of the MBOAT family whose expression is highly enriched within gastric ghrelin cells and whose whole body distribution mirrors that of ghrelin.

Characterization of a novel ghrelin cell reporter mouse

Ghrelin is a hormone that influences many physiological processes and behaviors, such as food intake, insulin and growth hormone release, and a coordinated response to chronic stress. However, little is known about the molecular pathways governing ghrelin release and ghrelin cell function. To better study ghrelin cell physiology, we have generated several transgenic mouse lines expressing humanized Renilla reniformis green fluorescent protein (hrGFP) under the control of the mouse ghrelin promoter. hrGFP expression was especially abundant in the gastric oxyntic mucosa, in a pattern mirroring that of ghrelin immunoreactivity and ghrelin mRNA. hrGFP expression also was observed in the duodenum, but not in the brain, pancreatic islet, or testis. In addition, we used fluorescent activated cell sorting (FACS) to collect and partially characterize highly enriched populations of gastric ghrelin cells. We suggest that these novel ghrelin-hrGFP transgenic mice will serve as useful tools to better understand ghrelin cell physiology.

Actions and therapeutic pathways of ghrelin for gastrointestinal disorders

Ghrelin is a peptide hormone that possesses unique orexigenic properties. By acting on the growth-hormone secretagogue receptor 1a, ghrelin induces a short-term increase in food consumption, which ultimately induces a positive energy balance and increases fat deposition. Reduced ghrelin levels have been observed in obese patients and after bariatric surgery. In particular, bariatric procedures that involve gastric resection or bypass lead to reduced ghrelin levels. Administration of physiological doses of exogenous ghrelin to humans does not significantly alter gastric motility; however, administration of high doses stimulates gastric motility, with increased gastric tone and emptying, and increased activity of migrating motor complexes in the small bowel. The potential of ghrelin agonists to be used as prokinetics is being tested in patients with gastroparesis and postoperative ileus. Ghrelin acts directly on pancreatic islet cells to reduce insulin production. Findings from studies in animals have revealed that small-molecule ghrelin antagonists favorably influence glucose tolerance, appetite suppression and weight loss. Other studies have demonstrated that ghrelin antagonists retard gastric emptying only at very high doses, which suggests that these agents will probably not induce upper gastrointestinal symptoms. The potential of this new class of therapeutic agents to influence appetite and glycemic control strongly indicates that they should be tested in clinical trials.

G protein-coupled receptor 39 deficiency is associated with pancreatic islet dysfunction

G protein-coupled receptor (GPR)-39 is a seven-transmembrane receptor expressed mainly in endocrine and metabolic tissues that acts as a Zn(++) sensor signaling mainly through the G(q) and G(12/13) pathways. The expression of GPR39 is regulated by hepatocyte nuclear factor (HNF)-1alpha and HNF-4alpha, and in the present study, we addressed the importance of GPR39 for glucose homeostasis and pancreatic islets function. The expression and localization of GPR39 were characterized in the endocrine pancreas and pancreatic cell lines. Gpr39(-/-) mice were studied in vivo, especially in respect of glucose tolerance and insulin sensitivity, and in vitro in respect of islet architecture, gene expression, and insulin secretion. Gpr39 was down-regulated on differentiation of the pluripotent pancreatic cell line AR42J cells toward the exocrine phenotype but was along with Pdx-1 strongly up-regulated on differentiation toward the endocrine phenotype. Immunohistochemistry demonstrated that GRP39 is localized selectively in the insulin-storing cells of the pancreatic islets as well as in the duct cells of the exocrine pancreas. Gpr39(-/-) mice displayed normal insulin sensitivity but moderately impaired glucose tolerance both during oral and iv glucose tolerance tests, and Gpr39(-/-) mice had decreased plasma insulin response to oral glucose. Islet architecture was normal in the Gpr39 null mice, but expression of Pdx-1 and Hnf-1alpha was reduced. Isolated, perifused islets from Gpr39 null mice secreted less insulin in response to glucose stimulation than islets from wild-type littermates. It is concluded that GPR39 is involved in the control of endocrine pancreatic function, and it is suggested that this receptor could be a novel potential target for the treatment of diabetes.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

Mice with hyperghrelinemia are hyperphagic and glucose intolerant and have reduced leptin sensitivity

OBJECTIVE

Ghrelin is the only known peripheral hormone to increase ingestive behavior. However, its role in the physiological regulation of energy homeostasis is unclear because deletion of ghrelin or its receptor does not alter food intake or body weight in mice fed a normal chow diet. We hypothesized that overexpression of ghrelin in its physiological tissues would increase food intake and body weight.

RESEARCH DESIGN AND METHODS

We used bacterial artificial chromosome transgenesis to generate a mouse model with increased ghrelin expression and production in the stomach and brain. We investigated the effect of ghrelin overexpression on food intake and body weight. We also measured energy expenditure and determined glucose tolerance, glucose stimulated insulin release, and peripheral insulin sensitivity.

RESULTS

Ghrelin transgenic (Tg) mice exhibited increased circulating bioactive ghrelin, which was associated with hyperphagia, increased energy expenditure, glucose intolerance, decreased glucose stimulated insulin secretion, and reduced leptin sensitivity.

CONCLUSIONS

This is the first report of a Tg approach suggesting that ghrelin regulates appetite under normal feeding conditions and provides evidence that ghrelin plays a fundamental role in regulating beta-cell function.

Association studies on ghrelin and ghrelin receptor gene polymorphisms with obesity

Ghrelin exerts a stimulatory effect on appetite and regulates energy homeostasis. Ghrelin gene variants have been shown to be associated with metabolic traits, although there is evidence suggesting linkage and association with obesity and the ghrelin receptor (GHSR). We hypothesized that these genes are good candidates for susceptibility to obesity. Direct sequencing identified 12 ghrelin single-nucleotide polymorphisms (SNPs) and 8 GHSR SNPs. The 10 common SNPs were genotyped in 1,275 obese subjects and in 1,059 subjects from a general population cohort of European origin. In the obesity case-control study, the GHSR SNP rs572169 was found to be associated with obesity (P = 0.007 in additive model, P = 0.001 in dominant model, odds ratio (OR) 1.73, 95% confidence interval (1.23-2.44)). The ghrelin variant, g.A265T (rs4684677), showed an association with obesity (P = 0.009, BMI adjusted for age and sex) in obese families. The ghrelin variant, g.A-604G (rs27647), showed an association with insulin levels at 2-h post-oral glucose tolerance test (OGTT) (P = 0.009) in obese families. We found an association between the eating behavior "overeating" and the GHSR SNP rs2232169 (P = 0.02) in obese subjects. However, none of these associations remained significant when corrected for multiple comparisons. Replication of the nominal associations with obesity could not be confirmed in a German genome-wide association (GWA) study for rs4684677 and rs572169 polymorphisms. Our data suggest that common polymorphisms in ghrelin and its receptor genes are not major contributors to the development of polygenic obesity, although common variants may alter body weight and eating behavior and contribute to insulin resistance, in particular in the context of early-onset obesity.

Ghrelin-producing epsilon cells in the developing and adult human pancreas

AIMS/HYPOTHESIS

While the mechanisms of specification and the reciprocal relationships of the four types of endocrine cell (alpha, beta, delta and pancreatic polypeptide cells) within the human endocrine pancreas are well described in adults and during fetal development, ghrelin-immunoreactive cells (epsilon cells) remain poorly understood.

METHODS

We studied epsilon cells in 24 human fetal pancreases between 11 and 39 weeks of development and in 32 pancreases from adult organ donors.

RESULTS

We observed single epsilon cells scattered in primitive exocrine tissue from gestational week 13 in developing pancreas. Later in the developmental process, epsilon cells started to aggregate into clusters. From gestational week 21, epsilon cells were observed located around developing islets, forming an almost continuous layer at the peripheral rim of the islets. They remain localised on the mantle of the islets, although at different amounts, in the adult pancreas. Co-production of ghrelin with insulin, glucagon or somatostatin was not detected during fetal development. Co-production with pancreatic polypeptide was evident sporadically. Epsilon cells co-produced NK2 homeobox 2 and ISL LIM homeobox 1, but not NK6 homeobox 1 and paired box 6. A quantitative analysis was performed in the adult pancreas: there was an average of 1.17 + 1.17 epsilon cells per islet, the relative epsilon cell volume was 0.14 + 0.16% and the epsilon cell mass was 0.13 + 0.15 g. Neither sex nor age affected the epsilon cell mass, although there was a significant inverse correlation with BMI.

CONCLUSIONS/INTERPRETATION

During fetal development epsilon cells show an ontogenetic and morphogenetic pattern that is distinct from that of alpha and beta cells.

Slow and steady is the key to beta-cell replication

The beta-cells of the pancreas are responsible for insulin production and their destruction results in type I diabetes. beta-cell maintenance, growth and regenerative repair is thought to occur predominately, if not exclusively, through the replication of existing beta-cells, not via an adult stem cell. It was recently found that all beta-cells contribute equally to islet growth and maintenance. The fact that all beta-cells replicate homogeneously makes it possible to set up straightforward screens for factors that increase beta-cell replication either In vitro or in vivo. It is possible that a circulating factor may be capable of increasing beta-cell replication or that intrinsic cell cycle regulators may affect beta-cell growth. An improved understanding of the in vivo maintenance and growth of beta-cells will facilitate efforts to expand beta-cells In vitro and may lead to new treatments for diabetes.

Do Incretins play a role in the remission of type 2 diabetes after gastric bypass surgery: What are the evidence?

Gastric bypass surgery (GBP), in addition to weight loss, results in dramatic remission of type 2 diabetes (T2DM). The mechanisms by which this remission occurs are unclear. Besides weight loss and caloric restriction, the changes in gut hormones that occur after GBP are increasingly gaining recognition as key players in glucose control. Incretins are gut peptides that stimulate insulin secretion postprandially; the levels of these hormones, particularly glucagon-like peptide-1, increase after GBP in response to nutrient stimulation. Whether these changes are causal to changes in glucose homeostasis remain to be determined. The purpose of this review is to assess the evidence on incretin changes and T2DM remission after GBP, and the possible mechanisms by which these changes occur. Our goals are to provide a thorough update on this field of research so that recommendations for future research and criteria for bariatric surgery can be evaluated.

Ghrelin infusion in humans induces acute insulin resistance and lipolysis independent of growth hormone signaling

OBJECTIVE

Ghrelin is a gut-derived peptide and an endogenous ligand for the growth hormone (GH) secretagogue receptor. Exogenous ghrelin stimulates the release of GH (potently) and adrenocorticotropic hormone (ACTH) (moderately). Ghrelin is also orexigenic, but its impact on substrate metabolism is controversial. We aimed to study direct effects of ghrelin on substrate metabolism and insulin sensitivity in human subjects.

RESEARCH DESIGN AND METHODS

Six healthy men underwent ghrelin (5 pmol . kg(-1) . min(-1)) and saline infusions in a double-blind, cross-over study to study GH signaling proteins in muscle. To circumvent effects of endogenous GH and ACTH, we performed a similar study in eight hypopituitary adults but replaced with GH and hydrocortisone. The methods included a hyperinsulinemic-euglycemic clamp, muscle biopsies, microdialysis, and indirect calorimetry.

RESULTS

In healthy subjects, ghrelin-induced GH secretion translated into acute GH receptor signaling in muscle. In the absence of GH and cortisol secretion, ghrelin acutely decreased peripheral, but not hepatic, insulin sensitivity together with stimulation of lipolysis. These effects occurred without detectable suppression of AMP-activated protein kinase phosphorylation (an alleged second messenger for ghrelin) in skeletal muscle.

CONCLUSIONS

Ghrelin infusion acutely induces lipolysis and insulin resistance independently of GH and cortisol. We hypothesize that the metabolic effects of ghrelin provide a means to partition glucose to glucose-dependent tissues during conditions of energy shortage.

Brain regulation of appetite and satiety

Interest in the control of feeding has increased as a result of the obesity epidemic and rising incidence of metabolic diseases. The brain detects alterations in energy stores and triggers metabolic and behavioral responses designed to maintain energy balance. Energy homeostasis is controlled mainly by neuronal circuits in the hypothalamus and brainstem, whereas reward and motivation aspects of eating behavior are controlled by neurons in limbic regions and the cerebral cortex. This article provides an integrated perspective on how metabolic signals emanating from the gastrointestinal tract, adipose tissue, and other peripheral organs target the brain to regulate feeding, energy expenditure, and hormones. The pathogenesis and treatment of obesity and abnormalities of glucose and lipid metabolism are discussed.

Ghrelin and ghrelin receptor inhibitors: agents in the treatment of obesity

BACKGROUND

Current medical treatment of obesity is highly ineffective. Soon after its discovery as the endogenous ligand for the growth hormone secretagogue-receptor (GHS-R), ghrelin was shown to stimulate food intake (including in humans) and promote body weight gain and adipogenesis.

OBJECTIVES

This review discusses the role of the ghrelin/GHS-R pathway in energy homeostasis regulation and its role as a novel molecular target for the treatment of obesity.

METHODS

Medline was searched for relevant articles published in English.

RESULTS/CONCLUSION

A large series of animal studies shows that inhibition of the ghrelin/GHS-R pathway reduces food intake, body weight and adiposity, through reduction of appetite and augmentation of energy expenditure and fat catabolism. This suggests that inhibition of this novel pathway may be used to treat/prevent obesity and its complications.

Ghrelin receptor gene polymorphisms and body size in children and adults

BACKGROUND

The GH secretagogue receptor type 1a gene (GHSR) encodes the cognate receptor of ghrelin, a gut hormone that regulates food intake and pituitary GH secretion. Previous studies in U.S. families and a German population suggested GHSR to be a candidate quantitative locus for association with human obesity and growth.

AIM

The aim of the study was to test common genetic variation in GHSR for association with body size in children and adults.

METHODS

Sequencing was performed to systematically identify novel single nucleotide polymorphisms (SNPs) in GHSR. A set of three haplotype-tagging SNPs that captured all the genetic variation in GHSR was identified. These three haplotype-tagging SNPs were then genotyped in three large population-based U.K. cohort studies (two adult and one childhood cohort) comprising 5807 adults and 843 children.

RESULTS

No significant genotype or haplotype associations were found with adult or childhood height, weight, or body mass index.

CONCLUSION

Common variation in GHSR is not associated with body size in U.K. adults or children.

The continuous infusion of acylated ghrelin enhances growth hormone secretion and worsens glucose metabolism in humans

CONTEXT

Acylated ghrelin (AG) has been discovered as a natural ligand of the GH secretagogue receptor type 1a and is now recognized as an important orexigenic factor. Besides stimulation of GH secretion and appetite, it exerts other central and peripheral actions including modulation of insulin secretion, glucose and lipid metabolism.

OBJECTIVE

To define the effects of the continuous iv infusion of AG in humans with particular attention to metabolic parameters.

MATERIALS AND METHODS

We studied the effects of 16- h (from 21:00 to 13:00 h) infusion of AG (0.5 microg/kg/h) or saline in 8 young volunteers who were provided with isocaloric balanced meals. GH, cortisol, insulin, glucose, free fatty acid (FFA), and ghrelin levels were assayed every 20 min.

RESULTS

AG infusion increased circulating total ghrelin to a steady state that was maintained over 16 h infusion of the peptide. With respect to saline, AG infusion significantly modified GH, cortisol, insulin, and glucose profiles and decreased FFA area under the curve (p<0.01). AG increased GH pulse frequency and approximate entropy (p<0.05). AG enhanced the glucose response to both dinner (p<0.02) and breakfast (p<0.03). AG infusion blunted the early insulin response to dinner (p<0.03) but enhanced the second-phase insulin response to dinner and breakfast (p<0.05).

CONCLUSIONS

The continuous exposure to AG in humans enhances somatotroph secretion but also worsens glucose metabolism, although it inhibits lipolysis. These findings in normal young volunteers are consistent with data from studies in animals and suggest that acylated ghrelin is likely to play a negative role in glucose metabolism.

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.

Introducing GOAT: a target for obesity and anti-diabetic drugs?

The acyltransferase that catalyzes ghrelin octanoylation has recently been identified as ghrelin O-acyltransferase (GOAT). GOAT belongs to a family of membrane-bound O-acyltransferases (MBOATs). GOAT covalently links a medium fatty-acid chain, typically octanoate, to the hydroxyl group of the third serine of ghrelin, a potent orexigenic peptide characterized by this unique post-translational modification. The discovery of GOAT raises important questions and reveals several therapeutical possibilities. Indeed, drugs that inhibit GOAT might be able to prevent diet-induced obesity and might be an effective therapy for type-2 diabetes, increasing insulin secretion and enhancing peripheral insulin sensitivity. Furthermore, research on GOAT is providing new insights into the pathophysiology of energy homeostasis and might lead to the identification of further therapeutic targets. Here, we review what is currently known about the regulatory role of GOAT and discuss the potential of this novel approach for treating obesity and type-2 diabetes.

Ghrelin in pathological conditions

The recently identified gastric hormone ghrelin was initially described as a natural Growth Hormone Secretagogue Receptor ligand. Apart from ghrelin's first discovered action, which was the stimulation of Growth Hormone release, implications for many other functions have been reported. It seems that ghrelin exhibits an important role in conditions related to processes regulating nutrition, body composition and growth, as well as heart, liver, thyroid or kidney dysfunction. In this review, current available knowledge about ghrelin's role in various pathological conditions is presented.

Transgenic mice overexpressing neuropeptide Y in noradrenergic neurons: a novel model of increased adiposity and impaired glucose tolerance

OBJECTIVE

A functional polymorphism leucine 7 proline in the human neuropeptide Y (NPY) gene leading to increased NPY release from sympathetic nerves is associated with traits of metabolic syndrome. Although hypothalamic NPY neurons play an established role in promoting positive energy balance, the role of NPY colocalized with norepinephrine in sympathetic nervous system and brain noradrenergic neurons remains obscure.

RESEARCH DESIGN AND METHODS

To clarify the role of NPY in noradrenergic neurons, we generated a transgenic mouse overexpressing NPY under dopamine-beta-hydroxylase promoter and characterized the metabolic phenotype of the OE-NPY(DbetaH) mouse.

RESULTS

NPY levels are increased by 1.3-fold in adrenal glands and 1.8-fold in the brainstem but not in the hypothalamus in OE-NPY(DbetaH) mice. They display increased white adipose tissue mass and cellularity and liver triglyceride accumulation without hyperphagia or increased body weight. Hyperinsulinemia and impaired glucose tolerance develop by the age of 6 months in the OE-NPY(DbetaH) mice. Furthermore, circulating ghrelin is significantly increased in comparison with wild-type mice.

CONCLUSIONS

The present study shows that even a moderate increase in NPY levels in noradrenergic neurons leads to disturbances in glucose and lipid metabolism. The OE-NPY(DbetaH) mouse is an interesting new model to investigate the pathophysiology of some key components of the cluster of abnormalities characterizing the metabolic syndrome.

Ghrelin octanoylation mediated by an orphan lipid transferase

The peptide hormone ghrelin is the only known protein modified with an O-linked octanoyl side group, which occurs on its third serine residue. This modification is crucial for ghrelin's physiological effects including regulation of feeding, adiposity, and insulin secretion. Despite the crucial role for octanoylation in the physiology of ghrelin, the lipid transferase that mediates this novel modification has remained unknown. Here we report the identification and characterization of human GOAT, the ghrelin O-acyl transferase. GOAT is a conserved orphan membrane-bound O-acyl transferase (MBOAT) that specifically octanoylates serine-3 of the ghrelin peptide. Transcripts for both GOAT and ghrelin occur predominantly in stomach and pancreas. GOAT is conserved across vertebrates, and genetic disruption of the GOAT gene in mice leads to complete absence of acylated ghrelin in circulation. The occurrence of ghrelin and GOAT in stomach and pancreas tissues demonstrates the relevance of GOAT in the acylation of ghrelin and further implicates acylated ghrelin in pancreatic function.

Central nervous system regulation of energy metabolism: ghrelin versus leptin

In this brief review, we introduce some major themes in the regulation of energy, lipid, and glucose metabolism by the central nervous system (CNS). Rather than comprehensively discussing the field, we instead will discuss some of the key findings made regarding the interaction of the hormones ghrelin and leptin with the CNS.

Ghrelin is a physiological regulator of insulin release in pancreatic islets and glucose homeostasis

Ghrelin, an acylated 28-amino acid peptide, was isolated from the stomach as the endogenous ligand for the growth hormone (GH) secretagogue receptor (GHS-R). Circulating ghrelin is produced predominantly in the oxyntic mucosa of stomach. Ghrelin potently stimulates GH release and feeding, and exhibits positive cardiovascular effects, suggesting a possible clinical application. Low plasma ghrelin levels are associated with elevated fasting insulin levels and insulin resistance, suggesting both physiological and pathophysiological roles for ghrelin in glucose metabolism. Here, we review the physiological role of ghrelin in the regulation of insulin release and glucose metabolism, and a potential therapeutic avenue to treat type 2 diabetes by manipulating ghrelin and/or its signaling. Ghrelin inhibits insulin release in mice, rats and humans. The signal transduction mechanisms of ghrelin in islet beta-cells are distinct from those utilized in GH-releasing and/or GHS-R-expressing cells. Ghrelin is expressed in pancreatic islets and released into pancreatic microcirculations. Pharmacological and genetic blockades of islet-derived ghrelin markedly augment glucose-induced insulin release in vitro. In high-fat diet-induced mildly obese mice, ghrelin-deficiency enhances insulin release and prevents impaired glucose tolerance. Thus, manipulation of insulinostatic function of ghrelin--GHS-R system, particularly that in islets, could optimize the amount of insulin release to meet the systemic demand, providing a potential therapeutic application to prevent type 2 diabetes.

Simultaneous deletion of ghrelin and its receptor increases motor activity and energy expenditure

Administration of chemically synthesized ghrelin (Ghr) peptide has been shown to increase food intake and body adiposity in most species. However, the biological role of endogenous Ghr in the molecular control of energy metabolism is far less understood. Mice deficient for either Ghr or its receptor (the growth hormone secretagogue receptor, GHS-R1a) seem to exhibit enhanced protection against high-fat diet-induced obesity but do not show a substantial metabolic phenotype on a standard diet. Here we present the first mouse mutant lacking both Ghr and the Ghr receptor. We demonstrate that simultaneous genetic disruption of both genes of the Ghr system leads to an enhanced energy metabolism phenotype. Ghr/Ghr receptor double knockout (dKO) mice exhibit decreased body weight, increased energy expenditure, and increased motor activity on a standard diet without exposure to a high caloric environment. Mice on the same genetic background lacking either the Ghr or the Ghr receptor gene did not exhibit such a phenotype on standard chow, thereby confirming earlier reports. No differences in food intake, meal pattern, or lean mass were observed between dKO, Ghr-deficient, Ghr receptor-deficient, and wild-type (WT) control mice. Only dKO showed a slight decrease in body length. In summary, simultaneous deletion of Ghr and its receptor enhances the metabolic phenotype of single gene-deficient mice compared with WT mice, possibly suggesting the existence of additional, as of yet unknown, molecular components of the endogenous Ghr system.

Characterization of adult ghrelin and ghrelin receptor knockout mice under positive and negative energy balance

Ghrelin and the ghrelin receptor (GH secretagogue receptor, GHS-R), are believed to have important roles in energy homeostasis. We describe results from the first studies to be conducted in congenic (N10) adult ghrelin(-/-) and Ghsr(-/-) mice under conditions of both positive (high-fat diet) and negative (caloric restriction) energy balance. In contrast to results from young N2 mutant mice, changes in body weight and energy expenditure are not clearly distinguishable across genotypes. Although respiratory quotient was lower in mice fed a high-fat diet, no differences were evident between littermate wild-type and null genotypes. With normal chow, a modest decrease trend in respiratory quotient was detected in ghrelin(-/-) mice but not in Ghsr(-/-) mice. Under caloric restriction, the weight loss of ghrelin(-/-) and Ghsr(-/-) mice was identical to wild-type littermates, but blood glucose levels were significantly lower. We conclude that adult congenic ghrelin(-/-) and Ghsr(-/-) mice are not resistant to diet-induced obesity but under conditions of negative energy balance show impairment in maintaining glucose homeostasis. These results support our hypothesis that the primary metabolic function of ghrelin in adult mice is to modulate glucose sensing and insulin sensitivity, rather than directly regulate energy intake and energy expenditure.

Gastrointestinal peptides controlling body weight homeostasis

Obesity has become an international public health problem. Unfortunately, effective treatment options are limited. In the last 20 years, research in obesity and associated pathologies has derived in a significant increase in the knowledge of the physiological and molecular mechanism regulating body mass, such as gastrointestinal-neuroendocrine communications. Gut-brain peptides may provide attractive therapeutic targets against this disease. This review summarizes research into energy balance through gastrointestinal tract peptides. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.

Small-molecule ghrelin receptor antagonists improve glucose tolerance, suppress appetite, and promote weight loss

Ghrelin, through action on its receptor, GH secretagogue receptor type 1a (GHS-R1a), exerts a variety of metabolic functions including stimulation of appetite and weight gain and suppression of insulin secretion. In the present study, we examined the effects of novel small-molecule GHS-R1a antagonists on insulin secretion, glucose tolerance, and weight loss. Ghrelin dose-dependently suppressed insulin secretion from dispersed rat islets. This effect was fully blocked by a GHS-R1a antagonist. Consistent with this observation, a single oral dose of a GHS-R1a antagonist improved glucose homeostasis in an ip glucose tolerance test in rat. Improvement in glucose tolerance was attributed to increased insulin secretion. Daily oral administration of a GHS-R1a antagonist to diet-induced obese mice led to reduced food intake and weight loss (up to 15%) due to selective loss of fat mass. Pair-feeding experiments indicated that weight loss was largely a consequence of reduced food intake. The impact of a GHS-R1a antagonist on gastric emptying was also examined. Although the GHS-R1a antagonist modestly delayed gastric emptying at the highest dose tested (10 mg/kg), delayed gastric emptying does not appear to be a requirement for weight loss because lower doses produced weight loss without an effect on gastric emptying. Consistent with the hypothesis that ghrelin regulates feeding centrally, the anorexigenic effects of potent GHS-R1a antagonists in mice appeared to correspond with their brain exposure. These observations demonstrate that GHS-R1a antagonists have the potential to improve the diabetic condition by promoting glucose-dependent insulin secretion and promoting weight loss.

Genetically engineered mice: a new paradigm to study gastric physiology

PURPOSE OF REVIEW

To summarize key aspects from recent research as well as review articles on the topic of genetic mouse models, particularly in knockout mice, that have considerably contributed to understanding the pathways and mechanisms underlying gastric physiology.

RECENT FINDINGS

A series of knockout mouse models has proven to be invaluable in elucidating the mechanism and validating the current model of acid secretion. The interaction between the gastrin-histamine and cholecystokinin-somatostatin pathways was identified using the genetic approach as being critical in regulating acid secretion. Curiously, neither ghrelin nor ghrelin receptor knockout mice displayed the expected lean phenotype. Importantly, the study of obestatin in GPR39 knockout mice could be misleading, as zinc rather than obestatin is the endogenous ligand for GPR39. The physiological roles of ghrelin and obestatin have yet to be confirmed using knockout mouse models.

SUMMARY

The knockout mouse continues to serve as an excellent model to dissect the complexity of the mechanism of gastric acid secretion and to study the physiological importance of gastric ghrelin.

Ghrelin, obesity and diabetes

The high prevalence of obesity and diabetes will lead to higher rates of morbidity and mortality. The search for drugs to treat these metabolic disorders has, therefore, intensified. The stomach-derived peptide ghrelin regulates food intake and body weight. Recent work suggests that ghrelin also controls glucose metabolism. In addition, current evidence suggests that most of the actions of ghrelin could contribute to the metabolic syndrome. The ghrelin signaling system is, therefore, a promising target for the development of new drugs for the treatment of obesity and diabetes. Agents that block the ghrelin signaling system might be especially useful targets. This Review summarizes the potential and the limitations of ghrelin as a tool to better understand, prevent and treat obesity and diabetes.

Influence of ghrelin on food intake and energy homeostasis

PURPOSE OF REVIEW

The purpose of this review is to provide updated information on the role of ghrelin in food intake and energy homeostasis, and on its mechanism of action. Moreover, the potential of ghrelin as a target for drugs to treat cachexia and obesity will be discussed.

RECENT FINDINGS

Whereas the effects of ghrelin in the regulation of appetite, food intake and energy homeostasis have been fairly well documented, the pathways responsible for the effects of ghrelin are now increasingly being understood. As a consequence, clinical applications of ghrelin are now being developed.

SUMMARY

Ghrelin is an endogenous orexigenic peptide recently discovered in the stomach. Ghrelin is involved in short-term regulation of food intake since its plasma levels increase before meals and decrease strongly postprandially. Ghrelin is also involved in long-term body-weight regulation by inducing adiposity. Ghrelin might be useful for cachexia and obesity treatment.

Unacylated ghrelin acts as a potent insulin secretagogue in glucose-stimulated conditions

Acylated and unacylated ghrelin (AG and UAG) are gut hormones that exert pleiotropic actions, including regulation of insulin secretion and glucose metabolism. In this study, we investigated whether AG and UAG differentially regulate portal and systemic insulin levels after a glucose load. We studied the effects of the administration of AG (30 nmol/kg), UAG (3 and 30 nmol/kg), the ghrelin receptor antagonist [D-Lys(3)]GHRP-6 (1 micromol/kg), or various combinations of these compounds on portal and systemic levels of glucose and insulin after an intravenous glucose tolerance test (IVGTT, d-glucose 1 g/kg) in anesthetized fasted Wistar rats. UAG administration potently and dose-dependently enhanced the rise of insulin concentration induced by IVGTT in the portal and, to a lesser extent, the systemic circulation. This UAG-induced effect was completely blocked by the coadministration of exogenous AG at equimolar concentrations. Similarly to UAG, [D-Lys(3)]GHRP-6, alone or in combination with AG and UAG, strongly enhanced the portal insulin response to IVGTT, whereas exogenous AG alone did not exert any further effect. Our data demonstrate that, in glucose-stimulated conditions, exogenous UAG acts as a potent insulin secretagogue, whereas endogenous AG exerts a maximal tonic inhibition on glucose-induced insulin release.

Lack of effect of the ghrelin gene-derived peptide obestatin on cardiomyocyte viability and metabolism

Obestatin is a recently discovered peptide encoded by the ghrelin gene that opposes ghrelin effects on food intake and gastrointestinal function. The biological activity of obestatin depends on amidation at its carboxyl terminus and on its postulated binding to the orphan G protein-coupled receptor 39 (GPR39). We have previously demonstrated that ghrelin is synthesized by cardiomyocytes and has direct effects on its viability. Our aim was to know if obestatin, derived from the same gene as ghrelin, also affects cardiomyocyte physiology. By RT-PCR and immunocytochemistry we have demonstrated that murine cardiomyocytes cultured in vitro and human atrial tissue express GPR39 receptor. Competitive binding studies with radioiodine 125I-labeled obestatin recognized specific binding sites for this peptide in the murine cardiomyocyte cell line HL-1. However, obestatin did not modify the cell cycle or viability of these cells, and it was not able to prevent the cytosine arabinoside-induced apoptosis of HL-1 cardiomyocytes, as assessed by Hoechst dye vital staining, flow cytometry analysis and determination of lactate dehydrogenase in the culture media. Finally, treatment with obestatin did not affect fatty acid or glucose uptake by HL-1 cardiomyocytes. In conclusion, obestatin is not a relevant metabolic or viability modifier for cardiomyocytes.

Peripheral tissue-brain interactions in the regulation of food intake

More than 70 years ago the glucostatic, lipostatic and aminostatic hypotheses proposed that the central nervous system sensed circulating levels of different metabolites, changing feeding behaviour in response to the levels of those molecules. In the last 20 years the rapid increase in obesity and associated pathologies in developed countries has involved a substantial increase in the knowledge of the physiological and molecular mechanism regulating body mass. This effort has resulted in the recent discovery of new peripheral signals, such as leptin and ghrelin, as well as new neuropeptides, such as orexins, involved in body-weight homeostasis. The present review summarises research into energy balance, starting from the original classical hypotheses proposing metabolite sensing, through peripheral tissue-brain interactions and coming full circle to the recently-discovered role of hypothalamic fatty acid synthase in feeding regulation. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.