Peptides (v.32, #11)

Editorial by Yvette Taché; Andreas Stengel; Hubert Vaudry (2151-2152).

The discovery of ghrelin: With a little luck and great passion by Masayasu Kojima; Kenji Kangawa (2153-2154).

Recent advances in the phylogenetic study of ghrelin by Hiroyuki Kaiya; Mikiya Miyazato; Kenji Kangawa (2155-2174).
► Ghrelin is a multifunctional hormone with more features than we have originally postulated. ► To understand what is ghrelin, it is also important to know the evolutional process. ► Ghrelin is present in non-mammalian vertebrates and plays crucial roles. ► Comparative study between mammalian and non-mammalian vertebrates let us know the generality and difference. ► In this review, we summarized recent literatures on the ghrelin system of non-mammalian vertebrates.To understand fully the biology of ghrelin, it is important to know the evolutionary history of ghrelin and its receptor. Phylogenetic and comparative genomic studies of mammalian and non-mammalian vertebrates are a useful approach to that end. Ghrelin is a hormone that has apparently evaded natural selection during a long evolutionary history. Surely ghrelin plays crucial physiological roles in living animals. Phylogenetic studies reveal the nature and evolutionary history of this important signaling system.
Keywords: Ghrelin; GHS-R; Non-mammalian vertebrates; Phylogeny;

Structures and molecular forms of the ghrelin-family peptides by Yoshihiro Nishi; Junko Yoh; Hiroshi Hiejima; Masayasu Kojima (2175-2182).
► This review surveys structures of ghrelin peptides and molecular forms of ghrelin gene-derived products. ► A knowledge about structures and functions of these peptides is summarized with an emphasis on those of acyl-forms.Ghrelin is an acylated peptide hormone produced mainly from the stomach. The major active products of the ghrelin gene in the stomach of rats, mice and humans are 28-amino acid peptides acylated at the serine-3 position with an n-octanoyl group (C8:0), called simply ghrelin. However, recent studies have revealed that the ghrelin gene can generate a variety of bioactive molecules besides ghrelin. These include acyl forms of ghrelin other than C8:0-ghrelin (i.e., n-decanoyl ghrelin or n-decenoyl ghrelin), des-acyl ghrelin, obestatin and ghrelin-associated peptides originated from the ghrelin gene. This review surveys the structures of the ghrelin peptides and molecular forms of ghrelin gene-derived products, and summarizes the knowledge about the functions of these peptides, with an emphasis on the acyl forms of the ghrelin peptide.
Keywords: Acyl ghrelin; Decanoyl ghrelin; Ghrelin-associated peptides; Ghrelin gene-derived products; Posttranslational processing and acylation;

Understanding the functional significance of ghrelin processing and degradation by Motoyasu Satou; Yusuke Nakamura; Hiromi Ando; Hiroyuki Sugimoto (2183-2190).
► Processing and degradation of ghrelin and responsible enzymes in blood. ► Deacylation and truncation of ghrelin and responsible enzymes in blood are introduced. ► Acyl protein thioesterase 1 is an important ghrelin deacylation enzyme in serum. ► Significance of ghrelin-processing peptidases in plasma is proposed from MS analysis.Post-translational modification, cleavage and processing of circulating hormones are common themes in the control of hormone activities. Full-length ghrelin is a 28 amino acid protein that exists in several modified and processed forms, including addition of an acyl moiety at the third serine of the N-terminus. When modified with octanoic acid, the first five N-terminal residues of ghrelin can modulate a signaling pathway via the ghrelin receptor GHSR1a. Although modification via a lipid moiety is essential for binding and activation of GHSR1a by ghrelin, many reports suggest that a desacyl form of ghrelin exists and has synergistic, opposing and distinct properties as compared to the acyl form. Therefore, it is important to clarify the physiological relevance of ghrelin derivatives. Based on lines of evidence from various studies, we propose that a larger proportion of secreted ghrelin is present in the deacylated form and furthermore, that circulating acyl and desacyl forms of ghrelin may be hydrolyzed to form short peptide fragments. Here, we summarize the results of studies aimed at understanding ghrelin processing and its implications for physiological function, as well as our recent findings regarding enzymes in the blood capable of generating processed forms of ghrelin.
Keywords: Ghrelin; Lysophospholipase; APT1; Serum; MALDI-TOF MS; Processing peptidases;

► Ghrelin gene polymorphisms interact with environmental weight determinants. ► Ghrelin gene polymorphisms influence hypertension risk and reward seeking behavior. ► Ghrelin receptor gene polymorphisms are important determinants of stature. ► Ghrelin-axis gene polymorphisms are unlikely to influence risk of type 2 diabetes.Ghrelin is a 28 amino acid peptide hormone that is produced both centrally and peripherally. Regulated by the ghrelin O-acyl transferase enzyme, ghrelin exerts its action through the growth hormone secretagogue receptor, and is implicated in a diverse range of physiological processes. These implications have placed the ghrelin signaling pathway at the center of a large number of candidate gene and genome-wide studies which aim to identify the genetic basis of human heterogeneity. In this review we summarize the available data on the genetic variability of ghrelin, its receptor and its regulatory enzyme, and their association with obesity, stature, type 2 diabetes, cardiovascular disease, eating disorders, and reward seeking behavior.
Keywords: Ghrelin; GHSR; Ghrelin receptor; Growth hormone secretagogue receptor; Ghrelin O-acyl transferase (GOAT); Genetic variability; Candidate gene studies; Genome-wide studies;

► The major circulating forms of ghrelin are acyl and desacyl ghrelin. ► Recently, the ghrelin acylating and de-acylating enzymes have been identified. ► The stress-induced alteration of gastrointestinal motility and food intake strongly depends on the stressor. ► The stress-induced alterations of ghrelin signaling are reviewed here. ► A better understanding of these mechanisms could lead to new treatment strategies for e.g. improving food intake and motility under conditions of stress.Ghrelin is the only known peripherally produced and centrally acting peptide hormone that stimulates food intake and digestive functions. Ghrelin circulates as acylated and desacylated forms and recently the acylating enzyme, ghrelin-O-acyltransferase (GOAT) and the de-acylating enzyme, thioesterase 1/lysophospholipase 1 have been identified adding new layers of complexity to the regulation of ghrelin. Stress is known to alter gastrointestinal motility and food intake and was recently shown to modify circulating ghrelin and GOAT levels with differential responses related to the type of stressors including a reduction induced by physical stressors (abdominal surgery and immunological/endotoxin injection, exercise) and elevation by metabolic (cold exposure, acute fasting and caloric restriction) and psychological stressors. However, the pathways underlying the alterations of ghrelin under these various stress conditions are still largely to be defined and may relate to stress-associated autonomic changes. There is evidence that alterations of circulating ghrelin may contribute to the neuroendocrine and behavioral responses along with sustaining the energetic requirement needed upon repeated exposure to stressors. A better understanding of these mechanisms will allow targeting components of ghrelin signaling that may improve food intake and gastric motility alterations induced by stress.
Keywords: Abdominal surgery; Food intake; Ghrelin-O-acyltransferase (GOAT); Motility; Somatostatin 2 receptor;

Sulfated gastrin stimulates ghrelin and growth hormone release but inhibits insulin secretion in cattle by Hongqiong Zhao; Swe YanNaing; Sint ThanThan; Hideto Kuwayama (2218-2224).
► We examined the effects of gastrin on plasma levels of hormones in cattle. ► Gastrin increased plasma ghrelin and GH levels, but decreased insulin levels. ► The GHS-R1a is not involved in the gastrin-induced GH release. ► Gastrin had no effect on plasma glucagon and glucose levels. ► Gastrin maintains the homeostasis of blood glucose via down-regulation of insulin.This study was designed to determine the effects of gastrin on the circulating levels of ghrelin, growth hormone (GH), insulin, glucagon and glucose in ruminants. Two experiments were done in eight Holstein steers. Animals were randomly assigned to receive intravenous bolus injections: (1) 0.1% bovine serum albumin in saline as vehicle, 0.8, 4.0 and 20.0 μg/kg body weight (BW) of bovine sulfated gastrin-34; (2) vehicle, 0.53 μg/kg BW of bovine sulfated gastrin-17 alone or combined with 20.0 μg/kg BW of [d-Lys3]-GHRP-6, the selective antagonist of GHS-R1a. Blood samples were collected from −10 to 150 min relative to injection time. Concentrations of acyl and total ghrelin in response to gastrin-34 injection were significantly increased in a dose-dependent manner. Concentrations of GH were also markedly elevated by gastrin-34 injection; however, the effect of 20.0 μg/kg was weaker than that of 4.0 μg/kg. The three doses of gastrin-34 equally decreased insulin levels within 15 min and maintained the level until the time of last sampling. Gastrin-34 had no effect (P  > 0.05) on the levels of glucagon and glucose. Levels of acyl ghrelin increased after administration of gastrin-17 alone or combined with [d-Lys3]-GHRP-6; however, [d-Lys3]-GHRP-6 did not block the elevation of GH by gastrin-17. The present results indicate that sulfated gastrin stimulates both ghrelin and GH release, but the GHS-R1a may not contribute to the release of GH by gastrin. Moreover, sulfated gastrin seems to indirectly maintain the homeostasis of blood glucose through the down-regulation of insulin in ruminants.
Keywords: Gastrin; Ghrelin; Growth hormone; Insulin; Glucagon; Cattle;

Role of ghrelin system in neuroprotection and cognitive functions: Implications in Alzheimer's disease by Manuel D. Gahete; José Córdoba-Chacón; Rhonda D. Kineman; Raúl M. Luque; Justo P. Castaño (2225-2228).
► AD is a progressive neurodegenerative disorder influenced by the metabolic status. ► Ghrelin system is linked to obesity, metabolic syndrome, neuromodulation and memory. ► Ghrelin system acts autocrinely/paracrinely and its components are expressed CNS. ► We review the role of ghrelin system in neuroprotection, memory and learning. ► We discuss the influence of ghrelin system in AD, and its expression in AD patients.Alzheimer's disease (AD) is a multifactorial progressive neurodegenerative disorder characterized by loss of memory and cognitive deficits, strongly influenced by the metabolic status, in which the impairment of neuropeptides/neurotransmitters systems has been previously observed. Ghrelin is a multifunctional hormone produced in a wide variety of tissues, which has been associated with the progression of obesity and metabolic syndrome, but has been also linked to neuromodulation, neuroprotection and memory and learning processes. In addition, ghrelin system also acts in an autocrine/paracrine fashion where the majority of its components [ghrelin variants (native ghrelin, In1-ghrelin), acylation enzyme (GOAT) and receptors (GHS-Rs)] are expressed in the different regions of central nervous system. In spite of all these pieces of information strongly suggesting a close association between ghrelin system and AD, which could be of pathophysiological relevance, few studies have been addressed to clarify this relationship. In this work, the role of ghrelin system in neuroprotection, memory consolidation and learning is reviewed, and its influence in AD, as well as the regulation of its expression in the brain of AD patients, is discussed.
Keywords: Ghrelin; Alzheimer's disease; Neuroprotection; Memory; Learning;

Physiological roles revealed by ghrelin and ghrelin receptor deficient mice by Rosie G. Albarran-Zeckler; Yuxiang Sun; Roy G. Smith (2229-2235).
► Ghrelin and ghrelin receptor knockout mice exhibit improve insulin sensitivity. ► Ghrelin and ghrelin receptor knockout mice show accelerating aging of the immune system. ► Ghrelin is neuroprotective.Ghrelin is a hormone made in the stomach and known primarily for its growth hormone releasing and orexigenic properties. Nevertheless, ghrelin through its receptor, the GHS-R1a, has been shown to exert many roles including regulation of glucose homeostasis, memory & learning, food addiction and neuroprotection. Furthermore, ghrelin could promote overall health and longevity by acting directly in the immune system and promoting an extended antigen repertoire. The development of mice lacking either ghrelin (ghrelin−/−) or its receptor (ghsr−/−) have provided a valuable tool for determining the relevance of ghrelin and its receptor in these multiple and diverse roles. In this review, we summarize the most important findings and lessons learned from the ghrelin−/− and ghsr−/− mice.
Keywords: Ghrelin; Ghrelin receptor (GHS-R1a); Ghrein knockout mice; Ghrelin receptor knockout mice;

Physiological role of ghrelin as revealed by the ghrelin and GOAT knockout mice by Kihwa Kang; Erik Zmuda; Mark W. Sleeman (2236-2241).
► Similarities and difference in the phenotypes of genetic deletion ghrelin and GOAT knockout mouse lines are reported. ► Overlapping expression patterns are discussed. ► Expression pattern of ghrelin and GOAT suggests that additional peptides may be acylated by GOAT.Ghrelin is a gastric hormone that has been shown to regulate food intake and energy metabolism. One unique feature of ghrelin is that its activity is regulated post transcriptionally by ghrelin O-acyltransferase (GOAT) through the addition of fatty acid to the serine residue in the N terminal region. Despite much biochemical characterization, to date no other proteins have been shown to be specifically octonylated by GOAT, suggesting a unique matching of the acyl transferase for a single ligand, ghrelin. If this is indeed correct, then genetic deletion of ghrelin or GOAT should produce near identical phenotypes and there should be extensive overlap in expression patterns. This review summarizes the similarities and differences in the phenotypes with the genetic deletion of ghrelin and GOAT in the various knockout mouse lines reported to date. While there is considerable overlap in expression pattern between ghrelin and GOAT, the latter does exhibit some unique tissue expression that could suggest that additional peptides may be acylated and await discovery and characterization.
Keywords: Ghrelin; Ghrelin O-acyltransferase; Null phenotype; GHS-R; Glycemia;

► Ghrelin is a multifunctional peptide that is involved in the regulation of energy homeostasis and food intake. ► This mini-review focuses on the effects of ghrelin on energy balance, locomotor activity and lipid metabolism in fish.Ghrelin was first identified and characterized from rat stomach as an endogenous ligand for the growth hormone secretagogue receptor. Ghrelin and its receptor system are present not only in peripheral tissues such as stomach and intestine, but also in the central nervous system of mammals. Interestingly, administration of ghrelin induces an orexigenic effect and also modifies locomotor activity, suggesting its involvement in feeding control and the regulation of energy balance, in addition to the regulation of growth hormone release. Information about ghrelin in non-mammals, such as teleost fish, has also been increasing, and important data have been obtained. An understanding of the evolutionary background of the energy regulation system and the central and peripheral roles of ghrelin in teleost fish could provide indications as to their roles in mammals, particularly humans. In this review, we overview the central and peripheral effects of ghrelin on energy balance, locomotor activity, and lipid metabolism in teleost fish.
Keywords: Ghrelin; Teleost fish; Goldfish; Feeding regulation; Locomotor activity; Energy balance;

Ghrelin is a stomach hormone, secreted into the bloodstream, that initiates food intake by activating NPY/AgRP neurons in the hypothalamic acruate nucleus. This review focuses on recent evidence that details the mechanisms through which ghrelin activate receptors on NPY neurons and downstream signaling within NPY neurons. The downstream signaling involves a novel CaMKK-AMPK-CPT1-UCP2 pathway that enhances mitochondrial efficiency and buffers reactive oxygen species in order to maintain an appropriate firing response in NPY. Recent evidence that shows metabolic status affects ghrelin signaling in NPY is also described. In particular, ghrelin does not activate NPY neurons in diet-induced obese mice and ghrelin does not increase food intake. The potential mechanisms and implications of ghrelin resistance are discussed.
Keywords: NPY; AgRP; Ghrelin; Ghrelin resistance; Intracellular; CaMKK; AMPK; CPT1; UCP;

Ghrelin-mediated appetite regulation in the central nervous system by Katarzyna Kirsz; Dorota A. Zieba (2256-2264).
► Ghrelin is a gut hormone and neuropeptide which acts in CNS on the hypothalamus. ► Best-known biological function of ghrelin is the regulation of appetite. ► The activity of ghrelin is also connected with circardian and annual fluctuation. ► Ghrelin plays an important role in adaptation of animals to energetic requirements.The gut hormone and neuropeptide ghrelin was initially identified in the periphery as a compound released in the bloodstream in response to a negative energetic status. In the central nervous system (CNS), ghrelin mainly acts on the hypothalamus and the limbic system, with its best-known biological role being the regulation of appetitive functions. Recent research has shown that ghrelin is not an indispensable factor in the regulation of food intake. However, it plays a key role in the metabolic changes of lipids, mainly those involving hypothalamic NOS, AMPK, CaMKK2, CPT1 and UCP2 proteins. Ghrelin participates in the regulation of memory processes and the feeling of pleasure resulting from eating, both of which are metabolism-dependent and may be essential for the successful achievement of adaptive appetitive behavior. Ghrelin exerts its biological effect through a complicated network of neuroendocrine links, including the melanocortin and endocannabinoid systems. The activity of ghrelin is connected with circadian and annual fluctuations, which depend on seasons and food availability.
Keywords: Ghrelin; Central nervous system; Metabolism; Biological rhythm;

Ghrelin and food reward: The story of potential underlying substrates by Karolina P. Skibicka; Suzanne L. Dickson (2265-2273).
► Ghrelin impacts on a wide range of food reward behaviors. ► Ghrelin also plays a role in alcohol and drug reward. ► Ghrelin interacts with dopamine to alter food reward. ► Neurotransmitters other than DA are also likely involved in reward effects of ghrelin. ► Widely distributed neurocircuitry likely contributes to ghrelin's food reward effect.The incidence of obesity is increasing at an alarming rate and this worldwide epidemic represents a significant decrease in life span and quality of life of a large part of the affected population. Therefore an understanding of mechanisms underlying food overconsumption and obesity development is urgent and essential to find potential treatments. Research investigating mechanisms underlying obesity and the control of food intake has recently experienced a major shift in focus, from the brain's hypothalamus to additional important neural circuits controlling emotion, cognition and motivated behavior. Among them, the mesolimbic system, and the changes in reward and motivated behavior for food, emerge as new promising treatment targets. Furthermore, there is also growing appreciation of the impact of peripheral hormones that signal nutrition status to the mesolimbic areas, and especially the only known circulating orexigenic hormone, ghrelin. This review article provides a synthesis of recent evidence concerning the impact of manipulation of ghrelin and its receptor on models of food reward/food motivation behavior and the mesolimbic circuitry. Particular attention is given to the potential neurocircuitry and neurotransmitter systems downstream of ghrelin's effects on food reward.
Keywords: Ghrelin; Food reward; Food motivation; Interaction; Neurocircuitry; Obesity; Food intake control;

Physiological roles of preproghrelin-derived peptides in GH secretion and feeding by Philippe Zizzari; Rim Hassouna; Dominique Grouselle; Jacques Epelbaum; Virginie Tolle (2274-2282).
► Acyl, desacyl ghrelin and obestatin are gastrointestinal hormones obtained by post-translational processing of a common precursor, preproghrelin. ► Acyl ghrelin is a powerful stimulator of GH secretion and food intake while desacyl ghrelin and obestatin may modulate these actions. ► The function of obestatin remains controversial because several studies failed to reproduce its anorexigenic effect and its ability to bind and activate GPR-39. ► All preproghrelin-derived peptides are secreted in a pulsatile manner. While variations in acyl ghrelin have been correlated to feeding episodes, variations of these peptides in relation with GH secretion still needs to be fully characterized. ► More sensitive and selective assays to detect the different preproghrelin-derived peptides are being developed and may be the key to a better understanding of their roles in different physiological and pathological conditions.Among the factors playing a crucial role in the regulation of energy metabolism, gastro-intestinal peptides are essential signals to maintain energy homeostasis as they relay to the central nervous system the informations about the nutritional status of the body. Among these factors, preproghrelin is a unique prohormone as it encodes ghrelin, a powerful GH secretagogue and the only orexigenic signal from the gastrointestinal tract and obestatin, a proposed functional ghrelin antagonist. These preproghrelin-derived peptides may contribute to balance energy intake, metabolism and body composition by regulating the activity of the GH/IGF-1 axis and appetite. Whereas the contribution of ghrelin has been well characterized, the role of the more recently identified obestatin, in this regulatory process is still controversial. In this chapter, we describe the contribution of these different preproghrelin-derived peptides and their receptors in the regulation of GH secretion and feeding. Data obtained from pharmacological approaches, mutant models and evaluation of the hormones in animal and human models are discussed.
Keywords: Ghrelin; Obestatin; Growth hormone (GH); Food intake; Pulsatile secretion;

Ghrelin, appetite and gastric electrical stimulation by Syrine Gallas; Sergueï O. Fetissov (2283-2289).
► Gastric electrical stimulation (GES) improves appetite in functional dyspepsia. ► Experimental GES with Enterra parameters increases ghrelin production in rats. ► Ghrelin may underlie improved appetite after GES. ► GES may be proposed to treat chronic anorexia.Ghrelin is a peptide hormone produced mainly by the stomach and has widespread physiological functions including increase in appetite. The stimulation of the ghrelin system represents a potential therapeutic approach in various disorders characterized by deficient ghrelin signaling or by low appetite. This stimulation may be achieved via pharmacological targeting of the ghrelin receptor with synthetic ghrelin or ghrelin mimetics or via increased endogenous ghrelin production. Recently, it was demonstrated that gastric electrical stimulation (GES) with Enterra parameters results in increased ghrelin production in rats. Furthermore, recent data revealed putative role of ghrelin-reactive immunoglobulins in the modulation of the ghrelin signaling which can be also stimulated by GES. Here, we review the links between GES and ghrelin in existing GES experimental and clinical applications for treatment of gastroparesis, functional dyspepsia or obesity and discuss if GES can be proposed as a non-pharmacological approach to improve ghrelin secretion in several pathological conditions characterized by low appetite, such as anorexia nervosa or anorexia–cachexia syndrome.
Keywords: Gut–brain axis; Peptide hormones; Food intake; Hunger; Satiety; Eating disorders; Natural autoantibodies;

Ghrelin and appetite control in humans—Potential application in the treatment of obesity by Michael Patterson; Stephen R. Bloom; James V. Gardiner (2290-2294).
► Ghrelin is a peptide hormone thought to act as a signal of hunger. ► We review the role of ghrelin in appetite regulation in lean and obese humans. ► We then examine ghrelin's potential as an anti-obesity treatment. We discuss different approaches to inhibiting the ghrelin system to treat obesity. ► We conclude that despite early promise inhibiting the ghrelin system might not result in a successful anti-obesity treatment.Ghrelin is a peptide hormone secreted into circulation from the stomach. It has been postulated to act as a signal of hunger. Ghrelin administration acutely increases energy intake in lean and obese humans and chronically induces weight gain and adiposity in rodents. Circulating ghrelin levels are elevated by fasting and suppressed following a meal. Inhibiting ghrelin signaling therefore appears an attractive target for anti-obesity therapies. A number of different approaches to inhibiting the ghrelin system to treat obesity have been explored. Despite this, over a decade after its discovery, no ghrelin based anti-obesity therapies are close to reaching the market. This article discusses the role of ghrelin in appetite control in humans, examines different approaches to inhibiting the ghrelin system and assesses their potential as anti-obesity therapies.
Keywords: Ghrelin; Obesity; Appetite; GOAT; GHRS-1a;

► Ghrelin stimulates growth hormone secretion, food intake and promotes adiposity. ► GH, which regulates IGF-1, is an anabolic hormone that spares protein stores. ► Ghrelin inhibits the production of anorectic proinflammatory cytokines. ► The combination of these actions may have benefits for the treatment of cachexia. ► Trials to treat cachexia of different etiologies with ghrelin have been attempted.Ghrelin, which is a natural ligand for the growth hormone (GH)-secretagogue receptor (GHS-R), stimulates food intake in both animals and humans. Ghrelin is the only circulating hormone known to stimulate appetite in humans. Ghrelin also stimulates GH secretion and inhibits the production of anorectic proinflammatory cytokines. As GH is an anabolic hormone, protein stores are spared at the expense of fat during conditions of caloric restriction. Thus, ghrelin exhibits anti-cachectic actions via both GH-dependent and -independent mechanisms. Several studies are evaluating the efficacy of ghrelin in the treatment of cachexia caused by a variety of diseases, including congestive heart failure, chronic obstructive pulmonary disease, cancer, and end-stage renal disease. These studies will hopefully lead to the development of novel therapeutic applications for ghrelin in the future. This review summarizes the recent advances in this area of research.
Keywords: Anorexia; Sarcopenia; Catabolism; Anabolism; Growth hormone; GHS;

Ghrelin acylation and metabolic control by O. Al Massadi; M.H. Tschöp; J. Tong (2301-2308).
► Ghrelin acylation depends on the function of GOAT and the availability of substrates. ► GOAT plays a distinct role in the regulation of energy and glucose homeostasis. ► GOAT may function as a modifier for the adaptive response to feeding and fasting. ► GOAT inhibition is a promising therapeutic target for weight and glucose control.Since its discovery, many physiologic functions have been ascribed to ghrelin, a gut derived hormone. The presence of a median fatty acid side chain on the ghrelin peptide is required for the binding and activation of the classical ghrelin receptor, the growth hormone secretagogue receptor (GHSR)-1a. Ghrelin O-acyl transferase (GOAT) was recently discovered as the enzyme responsible for this acylation process. GOAT is expressed in all tissues that have been found to express ghrelin and has demonstrated actions on several complex endocrine organ systems such as the hypothalamus–pituitary–gonadal, insular and adrenal axis as well as the gastrointestinal (GI) tract, bone and gustatory system. Ghrelin acylation is dependent on the function of GOAT and the availability of substrates such as proghrelin and short- to medium-chain fatty acids (MCFAs). This process is governed by GOAT activity and has been shown to be modified by dietary lipids. In this review, we provided evidence that support an important role of GOAT in the regulation of energy homeostasis and glucose metabolism by modulating acyl ghrelin (AG) production. The relevance of GOAT and AG during periods of starvation remains to be defined. In addition, we summarized the recent literature on the metabolic effects of GOAT specific inhibitors and shared our view on the potential of targeting GOAT for the treatment of metabolic disorders such as obesity and type 2 diabetes.
Keywords: Acyl ghrelin; GOAT; MCFAs; Metabolism;

Ghrelin and glucose homeostasis by P.J.D. Delhanty; A.J. van der Lely (2309-2318).
► Ghrelin has direct effects on pancreatic islet function. ► Available data indicate the potential of ghrelin blockade as a means to prevent its diabetogenic effects. ► Direct antagonism of the receptor does not always have the desired effects, however, since it can cause increased body weight gain. ► Pharmacological suppression of the ghrelin/des-acyl ghrelin ratio by treatment with des-acyl ghrelin may also be a viable alternative approach which appears to improve insulin sensitivity. ► A promising new approach appears to be the blockade of GOAT activity.Ghrelin plays an important physiological role in modulating GH secretion, insulin secretion and glucose metabolism. Ghrelin has direct effects on pancreatic islet function. Also, ghrelin is part of a mechanism that integrates the physiological response to fasting. However, pharmacologic studies indicate the important obesogenic/diabetogenic properties of ghrelin. This is very likely of physiological relevance, deriving from a requirement to protect against seasonal periods of food scarcity by building energy reserves, predominantly in the form of fat. Available data indicate the potential of ghrelin blockade as a means to prevent its diabetogenic effects. Several studies indicate a negative correlation between ghrelin levels and the incidence of type 2 diabetes and insulin resistance. However, it is unclear if low ghrelin levels are a risk factor or a compensatory response. Direct antagonism of the receptor does not always have the desired effects, however, since it can cause increased body weight gain. Pharmacological suppression of the ghrelin/des-acyl ghrelin ratio by treatment with des-acyl ghrelin may also be a viable alternative approach which appears to improve insulin sensitivity. A promising recently developed approach appears to be through the blockade of GOAT activity, although the longer term effects of this treatment remain to be investigated.
Keywords: Ghrelin; Obesity; Metabolism; Diabetes;

► Some genetic polymorphisms of ghrelin have been associated with obesity-associated diseases. ► Population-specific differences in reported associations exist. ► The role of one particular gene, such as gene coding for ghrelin, in the determination of BMI is likely to be very small.An increasing understanding of the role of genes in the development of obesity may reveal genetic variants that, in combination with conventional risk factors, may help to predict an individual's risk for developing metabolic disorders. Accumulating evidence indicates that ghrelin plays a role in regulating food intake and energy homeostasis and it is a reasonable candidate gene for obesity-related co-morbidities. In cross-sectional studies low total ghrelin concentrations and some genetic polymorphisms of ghrelin have been associated with obesity-associated diseases. The present review highlights many of the important problems in association studies of genetic variants and complex diseases. It is known that population-specific differences in reported associations exist. We therefore conclude that more studies on variants of ghrelin gene are needed to perform in different populations to get deeper understanding on the relationship of ghrelin gene and its variants to obesity.
Keywords: Ghrelin; Gene; Obesity; Diabetes;

Neuroendocrine and metabolic activities of ghrelin gene products by Alessandra Baragli; Fabio Lanfranco; Stefano Allasia; Riccarda Granata; Ezio Ghigo (2323-2332).
► AG modulates GH, ACTH, PRL and LH secretion. ► AG, UAG and Ob modulate glucose–lipid homeostasis and cardiovascular functions. ► AG, UAG and Ob share similar or antagonistic properties depending on the targeted metabolic tissue. ► Imbalance of AG, UAG and Ob relative ratios is often found in metabolically altered settings.Acylated ghrelin (AG) is a 28 amino acid gastric peptide a natural ligand for the growth hormone secretagogue (GHS) receptor type 1a (GHS-R1a), endowed with GH-secreting and orexigenic properties. Besides, ghrelin exerts several peripheral metabolic actions, including modulation of glucose homeostasis and stimulation of adipogenesis. Notably, AG administration causes hyperglycemia in rodents as in humans. Ghrelin pleiotropy is supported by a widespread expression of the ghrelin gene, of GHS-R1a and other unknown ghrelin binding sites. The existence of alternative receptors for AG, of several natural ligands for GHS-R1a and of acylation-independent ghrelin non-neuroendocrine activities, suggests that there might be a complex ‘ghrelin system’ not yet completely explored. Moreover, the patho-physiological implications of unacylated ghrelin (UAG), and obestatin (Ob), the other two ghrelin gene-derived peptides, need to be clarified. Within the next few years, we may better understand the ‘ghrelin system’, where we might envisage clinical applications.
Keywords: Acylated ghrelin; Unacylated ghrelin; Obestatin; Liver; Pancreas; Adipose tissue; Neuroendocrine axis;

Ghrelin potentiates TSH-induced expression of the thyroid tissue-specific genes thyroglobulin, thyroperoxidase and sodium-iodine symporter, in rat PC-Cl3 Cells by J. Morillo-Bernal; J.M. Fernández-Santos; M. De Miguel; R. García-Marín; F. Gordillo-Martínez; E. Díaz-Parrado; J.C. Utrilla; I. Martín-Lacave (2333-2339).
► Ghrelin is expressed in the CA77 C-cell line and in normal and neoplastic rat thyroid C-cells. ► Ghrelin receptor 1α is functionally expressed in normal and neoplastic C-cells and thyrocytes. ► Ghrelin enhances TSH-induced proliferation rate in PC-Cl3 rat thyrocytes. ► Ghrelin potentiates TSH-induced expression of thyroglobulin, thyroperoxidase and sodium–iodine symporter.Ghrelin is a 28-amino-acid peptide that stimulates pituitary growth-hormone secretion and modulates food-intake and energy metabolism in mammals. It is mainly secreted by the stomach, but it is also expressed in many other tissues such as cartilage or the thyroid gland. In the present study we have analyzed by RT-PCR and using immunohistochemistry and immunofluorescence the expression and tissue distribution of ghrelin and its functional receptor (GHS-R type 1α) in thyroid cell-lines and in normal and pathological rat thyroid tissue. Additionally, by measuring the incorporation of BrdU, we have investigated if, as previously noted for FRTL-5 cells, ghrelin enhances the proliferation rate in the PC-Cl3 rat-thyrocyte cell-line. Finally, we have determined the stimulatory effect of ghrelin on TSH-induced expression of the tissue-specific key genes involved in the synthesis of thyroid hormone: thyroglobulin, thyroperoxidase and sodium–iodine symporter. Our data provide direct evidence that C-cell secreted ghrelin may be involved in the paracrine regulation of the thyroid follicular cell function.
Keywords: Ghrelin; PC-Cl3; Thyroglobulin; NIS; TPO; Thyroid;

Ghrelin in neuroendocrine tumors by John P. Vu; Hank S. Wang; Patrizia M. Germano; Joseph R. Pisegna (2340-2347).
► We examine ghrelin expression in neuroendocrine tumors (NETs). ► We review ghrelin's expression in NETs patients with carcinoids, type A chronic atrophic gastritis (CAG), with or without MEN-1, and with and without liver metastases. ► We present a review ghrelin's physiological role in NETs. ► Increasing the significance of ghrelin in NETs will provide a useful guide for future research in this field.Ghrelin is a 28 amino acid peptide, primarily produced by the oxyntic mucosa X/A like neuroendocrine cells in the stomach. It is also found in the small intestine, hypothalamus, pituitary gland, pancreas, heart, adipose tissue, and immune system. In gastrointestinal neuroendocrine tumors (NETs) ghrelin release has been well documented. Ghrelin is a brain-gut circuit peptide with an important role in the physiological regulation of appetite, response to hunger and starvation, metabolic and endocrine functions as energy expenditure, gastric motility and acid secretion, insulin secretion and glucose homeostasis, as well as in the potential connection to the central nervous system. Recently, there has been a significant interest in the biological effects of ghrelin in NETs. In this article, we present a comprehensive review of ghrelin's expression and a brief summary of ghrelin's physiological role in NETs patients with carcinoids, type A chronic atrophic gastritis (CAG), with or without MEN-1, and with and without liver metastases. We hope, with the research reviewed here, to offer compelling evidence of the potential significance of ghrelin in NETs, as well as to provide a useful guide to the future work in this area.
Keywords: Ghrelin; Metastatic neuroendocrine tumors; Endogenous; Orexigenic; Cancer cachexia;

Ghrelin, des-acyl ghrelin and obestatin on the gastrointestinal motility by Mineko Fujimiya; Koji Ataka; Akihiro Asakawa; Chih-Yen Chen; Ikuo Kato; Akio Inui (2348-2351).
► We compare the effects of ghrelin, des-acyl ghrelin and obestatin on the GI motility. ► To examine the brain–gut interaction we used conscious rat model for manometric measurement of GI motility. ► Involvement of hypothalamic peptides was also examined. ► Ghrelin, des-acyl ghrelin and obestatin exert different effects on the GI motility. ► In their actions, different hypothalamic peptides are involved.Ghrelin, des-acyl ghrelin and obestatin are derived from a common prohormone, preproghrelin by posttranslational processing, originating from endocrine cells in the stomach. Ghrelin exerts stimulatory effects on the motility of antrum and duodenum in both fed and fasted state of animals. On the other hand, des-acyl ghrelin exerts inhibitory effects on the motility of antrum but not on the motility of duodenum in the fasted state of animals. Obestatin exerts inhibitory effects on the motility of antrum and duodenum in the fed state but not in the fasted state of animals. NPY Y2 and Y4 receptors in the brain may mediate the action of ghrelin, CRF type 2 receptor in the brain may mediate the action of des-acyl ghrelin, whereas CRF type 1 and type 2 receptors in the brain may mediate the action of obestatin.
Keywords: Ghrelin; Des-acyl ghrelin; Obestatin; Gatrointestinal motility; Hypothalamus;

Ghrelin as a target for gastrointestinal motility disorders by Beverley Greenwood-Van Meerveld; Michael Kriegsman; Richard Nelson (2352-2356).
► Ghrelin receptor agonists act as prokinetics in the gastrointestinal (GI) tract. ► Ghrelin mimetics may be useful in treating GI dysmotility by accelerating transit. ► Evidence supports ghrelin agonists in treating gastroparesis and postoperative ileus. ► Further investigation may support ghrelin agonists for functional bowel disorders.The therapeutic potential of ghrelin and synthetic ghrelin receptor (GRLN-R) agonists for the treatment of gastrointestinal (GI) motility disorders is based on their ability to stimulate coordinated patterns of propulsive GI motility. This review focuses on the latest findings that support the therapeutic potential of GRLN-R agonists for the treatment of GI motility disorders. The review highlights the preclinical and clinical prokinetic effects of ghrelin and a series of novel ghrelin mimetics to exert prokinetic effects on the GI tract. We build upon a series of excellent reviews to critically discuss the evidence that supports the potential of GRLN-R agonists to normalize GI motility in patients with GI hypomotility disorders such as gastroparesis, post-operative ileus (POI), idiopathic chronic constipation and functional bowel disorders.
Keywords: Ghrelin; Motility; Gastrointestinal tract;

Effects of ghrelin on Cx43 regulation and electrical remodeling after myocardial infarction in rats by Ming-Jie Yuan; He Huang; Yan-Hong Tang; Gang Wu; Yong-Wei Gu; Yong-Jun Chen; Cong-Xin Huang (2357-2361).
► Ghrelin showed the protective effect on ventricular arrhythmias after myocardial infarction in rats. ► Ghrelin drastically reduce the incidence of VT and prevent the loss of phosphorylated Cx43 during MI. ► Ghrelin may increase Cx43 expression in the infarcted heart through the inhibition of ET-1. ► Ghrelin can consider as a new strategy for the prevention of postinfarction ventricular arrhythmias.Ghrelin is a novel growth hormone-releasing peptide, which has been shown to exert beneficial effects on ventricular remodeling. In this study, we investigated whether ghrelin could decrease vulnerability to ventricular arrhythmias in rats with myocardial infarction and the possible mechanism. Twenty-four hours after ligation of the anterior descending artery, adult male Sprague-Dawley rats were randomized to ghrelin (100 μg/kg) and saline (control group) for 4 weeks. Sham animals underwent thoracotomy and pericardiotomy, but not LAD ligation. Myocardial endothelin-1 (ET-1) levels were significantly elevated in saline-treated rats at the border zone compared with sham-operated rats. Myocardial connexin43 (Cx43) expression at the border zone was significantly decreased in saline-treated infarcted rats compared with sham-operated rats. Ghrelin significantly decreased the inducibility of ventricular tachyarrhythmias compared with control group. Arrhythmias sores during programmed stimulation in saline-treated rats were significantly higher than scores in those treated with ghrelin. The electrophysiological improvement of fatal ventricular tachyarrhythmias was accompanied with increased immunofluorescence-stained Cx43, myocardial Cx43 protein and mRNA levels in ghrelin treated rats. We also shown that ghrelin significantly decreased tissue ET-1 levels at the infarcted border zone. Thus, ghrelin showed the protective effect on ventricular arrhythmias after myocardial infarction. Although the precise mechanism by which ghrelin modulates the dephosphorylation of Cx43 remains unknown, it is most likely that the ghrelin increased expression of Cx43 through the inhibition of ET-1.
Keywords: Ghrelin; Myocardial infarction; Ventricular arrhythmias; Connexin43;

Developmental effects of ghrelin by Sophie M. Steculorum; Sebastien G. Bouret (2362-2366).
• Ghrelin and GHSRs are expressed in the developing embryo and fetus. • Ghrelin influences perinatal growth. • Alterations in perinatal ghrelin levels cause structural differences in the pancreas, gastrointestinal tract, and brain.Ghrelin is a pleiotropic hormone that was originally described as promoting feeding and stimulating growth hormone release in adults. A growing body of evidence suggests that ghrelin may also exert developmental and organizational effects during perinatal life. The perinatal actions of ghrelin include the regulation of early developmental events such as blastocyst development and perinatal growth. Moreover, alterations in perinatal ghrelin levels result in structural differences in various peripheral organs, such as the pancreas and gastrointestinal tract. Recent data have also suggested that ghrelin acts on appetite-related brain centers in early life. Together, these observations indicate that exposure to factors that alter how ghrelin impacts development may induce lasting effects on physiological regulation.
Keywords: Hormone; GHSR; Brain; Development; Hypothalamus; Proliferation; Pancreas;

Ghrelin inhibited serotonin release from hippocampal slices by Marisa Soledad Ghersi; Sebastian M. Casas; Carla Escudero; Valeria P. Carlini; Florencia Buteler; Ricardo J. Cabrera; Helgi B. Schiöth; Susana R. de Barioglio (2367-2371).
► We studied the effect of Ghrelin (Ghr) upon the release of 5-HT in hippocampus. ► We used hippocampal slices pre-changed with [3H]5-HT. ► The presence of Ghr in the superfusion medium decreases the release of 5-HT. ► The in vivo intra-hippocampal administration of Ghr decreases the neurotransmitters release. ► Ghr as well as other peptides could modulate the release of neurotransmitters.Ghrelin (Ghr) is a peptide produced peripherally and centrally. It participates in the modulation of different biological processes. In our laboratory we have shown that (a) Ghr administration, either intracerebroventricular or directly into the hippocampus enhanced memory consolidation in a step down test in rats (b) the effect of Ghr upon memory decreases in animals pretreated with a serotonin (5-HT) reuptake inhibitor, Fluoxetine, suggesting that Ghr effects in the hippocampus could be related to the availability of 5-HT. It has been demonstrated that Ghr inhibits 5-HT release from rat hypothalamic synaptosomes. Taking in mint these evidences, we studied the release of radioactive 5-HT to the superfusion medium from hippocampal slices treated with two doses of Ghr (0.3 and 3 nm/μl). Ghr inhibited significantly the 5-HT release in relation to those superfused with artificial cerebrospinal fluid (ACSF) (H  = 9.48, df = 2, p  ≤ 0.05). In another set of experiments, Ghr was infused into the CA1 area of hippocampus of the rats immediately after training in the step down test and the 5-HT release from slices was studied 24 h after Ghr injection showing that in this condition also the 5-HT release was inhibited (H  = 11.72, df = 1, p  ≤ 0.05). In conclusion, results provide additional evidence about the neurobiological bases of Ghr action in hippocampus.

Exogenous ghrelin attenuates endotoxin fever in rats by Renato N. Soriano; Lelis G. Nicoli; Evelin C. Carnio; Luiz G.S. Branco (2372-2376).
► Ghrelin administration does not elicit any thermoregulatory effect during euthermia. ► LPS fever is accompanied by increased plasma corticosterone and preoptic PGE2 levels. ► Ghrelin attenuates LPS fever, further increases corticosterone and decreases PGE2. ► Ghrelin effect on PGE2 seems to be directly mediated (independent of corticosterone). ► Ghrelin modulates mechanisms underlying immune-inflammatory challenge.Ghrelin is a gut-derived peptide that plays a role in energy homeostasis. Recent studies have implicated ghrelin in systemic inflammation, showing increased plasma ghrelin levels after endotoxin (lipopolysaccharide, LPS) administration. The aims of this study were (1) to test the hypothesis that ghrelin administration affects LPS-induced fever; and (2) to assess the putative effects of ghrelin on plasma corticosterone secretion and preoptic region prostaglandin (PG) E2 levels in euthermic and febrile rats. Rats were implanted with a temperature datalogger capsule in the peritoneal cavity to record body core temperature. One week later, they were challenged with LPS (50 μg/kg, intraperitoneal, i.p.) alone or combined with ghrelin (0.1 mg/kg, i.p.). In another group of rats, plasma corticosterone and preoptic region PGE2 levels were measured 2 h after injections. In euthermic animals, systemic administration of ghrelin failed to elicit any thermoregulatory effect, and caused no significant changes in basal plasma corticosterone and preoptic region PGE2 levels. LPS caused a typical febrile response, accompanied by increased plasma corticosterone and preoptic PGE2 levels. When LPS administration was combined with ghrelin fever was attenuated, corticosterone secretion further increased, and the elevated preoptic PGE2 levels were relatively reduced, but a correlation between these two variables (corticosterone and PGE2) failed to exist. The present data add ghrelin to the neurochemical milieu controlling the immune/thermoregulatory system acting as an antipyretic molecule. Moreover, our findings also support the notion that ghrelin attenuates fever by means of a direct effect of the peptide reducing PGE2 production in the preoptic region.
Keywords: Lipopolysaccharide; Body temperature; Cyclooxygenase; Systemic inflammation;