Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake

The change in ghrelin and obestatin levels in obese children after weight reduction

AIM

To investigate the role of ghrelin and obestatin in obesity mechanisms.

METHODS

A total of 88 obese children and 25 normal children were enrolled. Moreover, 46 obese children took part in a summer camp for weight reduction. Fasting ghrelin, obestatin and other biochemical parameters were measured in all subjects and re-measured in 45 obese children finishing the camp.

RESULTS

The ghrelin levels in the control and obese groups were 67.26 +/- 23.41 pmol/L and 56.53 +/- 15.97 pmol/L with a significant difference (p = 0.039), while the obestatin levels (89.41 +/- 23.63 vs. 83.13 +/- 17.21 pmol/L) were not significantly different (p = 0.083). The ghrelin/obestatin ratio in the controls was significantly higher than that in the obese group (p = 0.014). In the latter, fasting insulin and alanine aminotransferase were independent factors for ghrelin; fasting insulin, weight and gender were independent factors for obestatin and alanine aminotransferase was an independent factor for ghrelin/obestatin. Moreover, ghrelin, obestatin and ghrelin/obestatin increased after weight reduction (p < 0.05, respectively), and the increment in ghrelin and obestatin was associated with a decrement in insulin resistance.

CONCLUSION

These data suggest that ghrelin, obestatin and/or the ghrelin/obestatin ratio are associated with obesity in childhood.

Ontogeny of ghrelin, obestatin, preproghrelin, and prohormone convertases in rat pancreas and stomach

The processing of preproghrelin in the stomach by prohormone convertase (PC) 1/3 produces ghrelin and possibly obestatin. In the neonate, the pancreas is also a major source of ghrelin. We compared the ontogeny of preproghrelin, ghrelin, obestatin, and PCs in the stomach and pancreas from rat embryos (day 21) and neonates (days 1, 6, 13, 21, and 28) by immunohistochemistry. In stomach, preproghrelin positive cells were present from embryonic day 21 and were in excess of ghrelin cells. The number of ghrelin positive cells progressively increased with age. When preproghrelin cells were immunoreactive for ghrelin, they were also immunoreactive for obestatin and PC1/3. In pancreas, we only found 0 to 2 preproghrelin positive cells per islet and each of these cells was also positive for ghrelin and obestatin. None of the ghrelin positive cells stained for insulin, but we observed ghrelin positive/glucagon negative and ghrelin positive/glucagon positive cells. Ghrelin positive cells contained PC1/3 or PC2. In summary, in stomach, an excess of preproghrelin positive cells compared with ghrelin/PC1/3 positive cells suggests that PC1/3 determines preproghrelin processing to ghrelin. In pancreas, the colocalization of PC1/3 or PC2 in ghrelin positive cells points to a role for both PCs in preproghrelin processing.

Ghrelin and obestatin: different role in fetal lung development?

Ghrelin and obestatin are two proteins that originate from post-translational processing of the preproghrelin peptide. Various authors claim an opposed role of ghrelin and obestatin in several systems. Preproghrelin mRNA is significantly expressed in airway epithelium throughout lung development, predominantly during the earliest stages. The aim of this study was to evaluate the role of ghrelin and obestatin in fetal lung development in vitro. Immunohistochemistry studies were performed at different gestational ages in order to clarify the expression pattern of ghrelin, GHS-R1a, obestatin and GPR39 during fetal lung development. Fetal rat lung explants were harvested at 13.5 days post-conception (dpc) and cultured during 4 days with increasing doses of total ghrelin, acylated ghrelin, desacyl-ghrelin, ghrelin antagonist (D-Lys(3)-GHRP-6) or obestatin. Immunohistochemistry studies demonstrated that ghrelin, GHS-R1a, obestatin and GPR39 proteins were expressed in primitive rat lung epithelium throughout all studied gestational ages. Total and acylated ghrelin supplementation significantly increased the total number of peripheral airway buds, whereas desacyl-ghrelin induced no effect. Moreover, GHS-R1a antagonist significantly decreased lung branching. Finally, obestatin supplementation induced no significant effect in the measured parameters. The present study showed that ghrelin has a positive effect in fetal lung development through its GHS-R1a receptor, whereas obestatin has no effect on lung branching.

Deranged secretion of ghrelin and obestatin in the cephalic phase of vagal stimulation in women with anorexia nervosa

BACKGROUND

Vagal activation in the cephalic phase response to food ingestion promotes ghrelin secretion. Because underweight individuals with anorexia nervosa (AN) are characterized by increased vagal tone, we hypothesized an enhanced ghrelin production in the cephalic phase of vagal stimulation. Therefore, we investigated the responses of ghrelin and its recently discovered sibling peptide obestatin to modified sham feeding (MSF) in both AN and healthy women.

METHODS

Eight AN women and eight age-matched healthy female subjects underwent MSF, with initially seeing and smelling a meal and then chewing the food without swallowing it. Blood samples were drawn before and after MSF for hormone assays.

RESULTS

Circulating ghrelin increased, whereas obestatin decreased after MSF. Compared with healthy women, AN individuals exhibited enhanced ghrelin and obestatin baseline plasma levels and amplified MSF-induced ghrelin increase and obestatin drop. Ghrelin secretion positively correlated with subjects' eating behavior as assessed by the Three-Factor Eating Questionnaire.

CONCLUSIONS

Opposite changes in circulating ghrelin and obestatin occur in the cephalic phase of vagal stimulation, and these changes are amplified in symptomatic AN patients. Given the opposite effects of ghrelin and obestatin on food intake, these findings may have pathophysiologic implications for the dysregulated eating behavior of AN individuals.

Bolus administration of obestatin does not change glucose and insulin levels neither in the systemic nor in the portal circulation of the rat

Obestatin is a second peptide derived from the preproghrelin polypeptide. It was originally thought to have anorexigenic effects, thereby functioning as an antagonist of ghrelin. However, this has been a subject of debate ever since. Since acylated ghrelin strongly induces insulin resistance, it could be hypothesized that obestatin plays a role in glucose homeostasis as well. In the present study we evaluated the effect of obestatin on glucose and insulin metabolism in the systemic and portal circulation. Obestatin 200 nmol/kg was administered systemically as a single intravenous bolus injection to fasted pentobarbital anesthetized adult male Wistar rats. Up to 50 min after administration, blood samples were taken to measure glucose and insulin concentrations, both in the portal and in the systemic circulation. The effect of obestatin was evaluated in fasted and in glucose-stimulated conditions (IVGTT) and compared to control groups treated with saline or IVGTT, respectively. Intravenous administration of obestatin did not have any effect on glucose and insulin concentrations, neither systemic nor portal, when compared to the control groups. Only the glucose peak 1 min after administration of IVGTT was slightly higher in the obestatin treated rats: 605.8+/-106.3% vs. 522.2+/-47.1% in the portal circulation, respectively (NS), and 800.7+/-78.7% vs. 549.6+/-37.0% in the systemic circulation, respectively (P<0.02), but it can be debated whether this has any clinical relevance. In the present study, we demonstrated that intravenously administered obestatin does not influence glucose and insulin concentrations, neither in the portal nor in the systemic circulation.

Different effects of ghrelin, des-acyl ghrelin and obestatin on gastroduodenal motility in conscious rats

Three peptides, ghrelin, des-acyl ghrelin and obestatin are derived from a common prohormone, preproghrelin by posttranslational processing, originating from endocrine cells in the stomach. To examine the effects of these peptides, we applied the manometric measurement of gastrointestinal motility in freely moving conscious rat models. Ghrelin exerts stimulatory effects on the motility of antrum and duodenum in both fed and fasted state of animals. 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 or Y4 receptors in the brain may mediate the action of ghrelin, CRF type 2 receptors in the brain mediate the action of des-acyl ghrelin, whereas CRF type 1 and type 2 receptors in the brain mediate the action of obestatin. Vagal afferent pathways might be involved in the action of ghrelin, but not involved in the action of des-acyl ghrelin, whereas vagal afferent pathways might be partially involved in the action of obestatin.

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.

Copeptin immunoreactivity and calcium mobilisation in hypothalamic neurones of the rat

Copeptin is cleaved from the C-terminus of vasopressin (VP) prohormone. Immunohistochemical studies have revealed intense copeptin-immunoreactivity (irCOPT) in neurones of the rat hypothalamic nuclei, including paraventricular, supraoptic, suprachiasmatic, periventricular, and accessory secretory. Varicose cell processes emanated from irCOPT neurones, some of which projected caudally and traversed the internal layer of the median eminence, and terminated in the posterior pituitary. Double-labelling hypothalamic sections with copeptin antiserum and VP or oxytocin antiserum revealed an extensive overlapping of irCOPT and irVP neurones. The biological activity of human synthetic nonglycosylated copeptin or VP was evaluated in vivo and in vitro. Copeptin (1, 10, and 20 nmol/kg) injected i.v. caused no significant changes in the mean arterial pressure (MAP) and heart rate of urethane-anaesthetised rats. VP (0.1 nmol/kg) increased MAP, which was accompanied by a small decrease of the heart rate. The ratiometric fluorescence method was employed to assess changes in intracellular Ca2+ concentrations [Ca2+](i) which served as an index of the biological activity of peptides. VP (1 microM) markedly increased [Ca2+](i) of rat hypothalamic neurones or vascular smooth muscle cells, whereas copeptin (100 nm to 1 microM) caused a low amplitude, sustained increase of [Ca2+](i) in a population of hypothalamic neurones, but not in any of the vascular smooth muscle cells tested. The results obtained demonstrate that copeptin is expressed in VP neurones and that the peptide in the concentrations tested, although causing little or no detectable changes of blood pressure and heart rate in anaesthetised rats nor changes in [Ca2+](i) of cultured aortic smooth muscle cells, increases [Ca2+](i) in a small population (< 2%) of hypothalamic neurones tested, indicating that copeptin is biologically active in mammalian neurones.

Plasma obestatin, ghrelin, and ghrelin/obestatin ratio are increased in underweight patients with anorexia nervosa but not in symptomatic patients with bulimia nervosa

INTRODUCTION

Peptides of the gut-brain axis have a pivotal role in the regulation of energy homeostasis. Obestatin, a sibling of ghrelin derived from preproghrelin, is thought to oppose ghrelin effects on food intake. Because changes in ghrelin levels have been associated with anorexia nervosa (AN) and bulimia nervosa (BN), the investigation of obestatin production may further contribute to understanding the role of peripheral peptides in patients with eating disorders.

METHODS

In the present study, we measured circulating blood levels of obestatin and ghrelin and assessed their relationships with anthropometric and clinical measures in 20 AN patients, 21 BN patients, and 20 appropriate healthy controls.

RESULTS

Compared with healthy women, patients with BN showed no significant differences in plasma obestatin and ghrelin concentrations and in the ghrelin/obestatin ratio, whereas underweight AN patients displayed significantly increased circulating levels of both obestatin (P<0.009) and ghrelin (P<0.002) and an increased ghrelin/obestatin ratio (P<0.04). Moreover, in AN women, positive correlations emerged between the ghrelin/obestatin ratio and current body weight and body mass index.

CONCLUSIONS

Underweight AN patients are characterized by increased concentrations of ghrelin and obestatin and a higher ghrelin to obestatin ratio. No changes in circulating ghrelin or obestatin as well as in ghrelin to obestatin ratio seem to occur in acutely ill patients with BN. Although those changes likely reflect the physiological state of symptomatic AN individuals, they may also contribute to the pathophysiology of the disorder.

The effect of obestatin on porcine ovarian granulosa cells

The aim of our in vitro experiments was to investigate the role of obestatin, a newly discovered metabolic hormone produced in the stomach and other tissues, in the direct control of ovarian cell proliferation, apoptosis and secretion. Porcine granulosa cells were cultured in the presence of obestatin (0, 1, 10 and 100ng/ml medium). The expression of intracellular peptides associated with proliferation (PCNA, cyclin B1, MAP kinase), as well as markers of apoptosis (Bax, p53, Caspase 3), were detected using immunocytochemistry and Western immunoblotting. Secretion of progesterone (P4), testosterone (T) and estradiol (E2) was measured by EIA. Addition of obestatin (1-100ng/ml) to the culture medium significantly stimulated the expression of PCNA and resulted in an increase in expression of cyclin B1 and MAPK. It also significantly increased the percentage of cells containing the apoptotic and anti-proliferating peptides p53, Caspase 3 and Bax. At 10 and 100ng/ml, obestatin promoted the secretion of P4, but not T or E2. Our results are the first demonstration that obestatin directly controls porcine ovarian cell functions: it can stimulate proliferation (accumulation of rPCNA, cyclin B1 and MAPK), apoptosis (expression of p53, Caspase 3 and Bax) and the secretion of progesterone.

Plasma obestatin levels in men with chronic atrophic gastritis

Obestatin is a recently discovered active peptide isolated from the stomach. The purpose of the present study was to investigate the modification of plasma obestatin levels in men with chronic atrophic gastritis. Men older than 65 years undergoing upper gastrointestinal endoscopy were included. All patients with chronic atrophic gastritis underwent multiple biopsies. Fasting plasma obestatin and ghrelin levels were examined in 50 men with chronic atrophic gastritis and 50 healthy men. Plasma obestatin levels were significantly lower in patients with chronic atrophic gastritis than in healthy subjects. Plasma ghrelin levels and ghrelin to obestatin ratio was decreased in men with chronic atrophic gastritis. There was a significant relationship between atrophy and decreased obestatin. A negative correlation was found between circulating obestatin levels and body mass index (BMI) in healthy subjects, but not in patients with chronic atrophic gastritis. The data indicated that chronic atrophic gastritis influenced plasma obestatin levels as well as ghrelin to obestatin ratio in elderly men.

In vitro metabolic stability of obestatin: kinetics and identification of cleavage products

The in vitro metabolic stability testing on synthetic obestatin peptides from two different species (human hOb and mouse mOb) using HPLC analysis is described. A reversed-phase C(18) column of 300A pore size was used, with a gradient system based on aqueous formic acid and acetonitrile. Electrospray ionization (ESI) ion trap mass spectrometry was used for identification of the chromatographic eluting peptide metabolic products, while UV (DAD) and fluorescence served quantitative purposes. Differences in the metabolic degradation kinetics of hOb and mOb were found in plasma, liver and kidney homogenate, with half-lives ranging between 12.6 and 138.0min. Proteolytic hydrolysis at the N-terminal Phe residue and cleavage at Pro(4)-Phe(5) were found to be two major metabolic pathways, accounting for more than 50% of the metabolic degradation. Several other labile peptide bonds were located. The influence of a standard protease inhibitor cocktail was investigated, as well as the metabolism of iodinated human obestatin in liver homogenate. Our results indicate that the major instability of obestatin peptides, as currently used in biomedical investigations, should be taken into account in the interpretation of the obtained results.

Roles of obestatin and its receptor in regulation of gastrointestinal motility

Obestatin, a novel 23-amino acid amidated brain/gut peptide synthesized in the stomach, was initially reported to reduce food intake, body weight gain and gastric emptying and suppress intestinal motility through an interaction with the orphan G-protein coupled receptor GPR39. Obestatin is derived from the same gene product as ghrelin by differential posttranslational processing and modification, which exerts effects opposite to those of ghrelin. However, recent reports have shown that the above findings had been questioned by several groups. According to the controversy that obestatin is unlikely to be the endogenous ligand for GPR39 and obestatin has no impacts on gastrointestinal motility, this paper reviews the studies related to obestatin and GPR39 and its impacts on gastrointestinal motility.

Vagotomy and accompanying pyloroplasty down-regulates ghrelin mRNA but does not affect ghrelin secretion

In this study, we have examined how the lack of vagus activity affects the long-term secretion of total and active ghrelin. We subjected mice to sham-operation, pyloroplasty or vagotomy and pyloroplasty. The study lasted for 2 weeks, during which body weight development and daily food intake was monitored. At the end of the study, the mice were sacrificed, and serum and fundus were collected. Measurements of total and active serum ghrelin revealed no difference between the surgical groups and sham-operated mice, despite the fact that fundic ghrelin mRNA was down-regulated. The results presented here suggest that the vagus activity is not required for the long-term secretion of neither total nor active ghrelin in mice. They also suggest that fundic ghrelin mRNA expression is affected by pyloroplasty and vagotomy but that this effect does not translate into changes in ghrelin levels in the circulation.

Impacts of peripheral obestatin on colonic motility and secretion in conscious fed rats

Obestatin, a novel putative 23-amino acid peptide, was found to be derived from a mammalian preproghrelin gene by using a bioinformatics approach. Although the effects of obestatin on food intake and upper gut motility remain controversial, no studies have been carried out to explore its influence on lower gut motility and secretion. We investigated the impacts of intravenous (IV) injection of obestatin on rat colonic motor and secretory functions. Colonic transit time, fecal pellet output, and fecal content were measured in freely fed, conscious rats, which were chronically implanted with IV and colonic catheters. To test the validity of this animal model, human/rat corticotropin-releasing factor (h/rCRF) served as a stimulatory inducer of colonic motility and secretion. IV injection of obestatin (45, 100, and 300 nmol/kg) did not affect the colonic transit time, whereas IV injection of h/rCRF (30 nmol/kg) effectively accelerated colonic transit time. IV obestatin, in every dose we tested, also did not modify fecal pellet output, frequency of watery diarrhea, total fecal weight, fecal dried solid weight, or fecal fluid weight in the first hour after injection. On the other hand, IV injection of h/rCRF significantly enhanced fecal pellet output, as well as increased the frequency of watery diarrhea, total fecal weight, fecal dried solid weight, and fecal fluid weight during the first hour after injection compared with IV saline controls. In conclusion, peripheral obestatin administration has no impact on colonic motility and secretion in conscious fed rats.

Effect of des-acyl ghrelin on adiposity and glucose metabolism

Ghrelin, a gastric peptide hormone, has been reported to regulate GH secretion and energy homeostasis. Here, we examined the effect of des-acyl ghrelin driven from the fatty acid-binding protein-4 (FABP4) promoter on adiposity and glucose metabolism. A high level of expression of des-acyl ghrelin (692 +/- 293 fmol/g fat) in adipose tissue was detected in FABP4-ghrelin transgenic mice, but not in wild-type littermates. Circulating des-acyl ghrelin was significantly higher in FABP4-ghrelin transgenic mice (8409 +/- 3390 pm) compared with wild-type mice (513 +/- 58 pm). No significant change was observed for plasma acylated ghrelin and obestatin. Epididymal and perirenal fat masses decreased 35 +/- 9 and 52 +/- 9%, respectively, in FABP4-ghrelin transgenic mice. FABP4-ghrelin transgenic mice are resistant to obesity induced by high-fat diet. Brown fat mass was not affected by overexpression of ghrelin in adipose tissue. Glucose tolerance tests showed glucose levels to be significantly lower in FABP4-ghrelin transgenic mice than in controls after glucose administration. Insulin sensitivity testing showed that FABP4-ghrelin transgenic mice had a 28 +/- 5% greater hypoglycemic response to insulin. Our study demonstrates that overexpression of ghrelin from the FABP4 promoter impairs the development of white adipose tissues, and alters glucose tolerance and insulin sensitivity in mice.

Structure and function of ghrelin

The endogenous ligand for growth-hormone secretagogue receptor (GHS-R) was purified from the stomach and we named it "ghrelin", after a word root ("ghre") in Proto-Indo-European languages meaning "grow", since ghrelin has potent growth hormone (GH) releasing activity. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the blood stream under fasting conditions, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining growth hormone release and energy homeostasis in vertebrates. The diverse functions of ghrelin raise the possibility of its clinical application for GH deficiency, eating disorder, gastrointestinal disease, cardiovascular disease, osteoporosis and aging, etc.

Plasma obestatin concentrations are negatively correlated with body mass index, insulin resistance index, and plasma leptin concentrations in obesity and anorexia nervosa

BACKGROUND

Obestatin is a recently identified ghrelin gene product that was reported to inhibit appetite and gastric motility in contrast to ghrelin. We investigated fasting obestatin and ghrelin levels in patients with obesity and anorexia nervosa.

METHODS

Fasting plasma obestatin, acyl-ghrelin, desacyl-ghrelin, leptin, glucose serum adiponectin, and insulin were measured in 10 obese subjects, 11 restricting-type anorexics, and 11 control subjects.

RESULTS

Obese group had significantly lower levels of obestatin (p < .01), while anorexic group had significantly higher levels (p < .01). Obestatin was negatively correlated with body mass index (BMI) (r = -.74), glucose (r = -.56), insulin (r = -.55), leptin (r = -.66), and also with the homeostasis model assessment of insulin resistance (HOMA-R) (r = -.49) and was positively correlated with acyl-ghrelin (r = .65) and desacyl-ghrelin (r = .60). No correlation was seen between obestatin and adiponectin, but the latter was negatively correlated with both acyl-ghrelin and desacyl-ghrelin. Desacyl-ghrelin to acyl-ghrelin ratio was significantly different between anorexic and control groups (p < .05), while no difference was seen between obese and control groups.

CONCLUSIONS

Both obestatin and ghrelin are increased in anorexic and decreased in obesity. We suggest that obestatin is a nutritional marker reflecting body adiposity and insulin resistance.

Metabolic effects of chronic obestatin infusion in rats

Obestatin is purported to be a peptide hormone encoded in preproghrelin. We studied the metabolic effects of continuous infusion of obestatin via subcutaneously implanted osmotic mini-pumps. Administration of up to 500nmol/kg body weight/day obestatin did not change 24h cumulative food intake or body weight in rats. Similarly, no effects were observed when obestatin was infused at 1000nmol/kg body weight/day for seven days. This dose of obestatin infused during a 24h fast did not alter weight loss, suggesting that obestatin has no effect on energy expenditure, and this dose did not alter glucose or insulin responses during an IPGTT. Obestatin was originally proposed to interact with GPR39 and subsequently the receptor for GLP-1. While both receptors are expressed in pancreatic islets, incubation with obestatin did not alter insulin release from islets in vitro. Moreover, obestatin did not bind to INS-1 beta-cells or HEK cells overexpressing GLP-1 receptors or displace GLP-1 binding to these cells. Our findings do not support the concept that obestatin is a hormone with metabolic actions.

Appetite-Related Gut Peptides in Obesity and Binge Eating Disorder

The worldwide increase in obesity prevalence is a result of positive energy balance, with energy intake exceeding expenditure. The eating behavior in obesity ranges from mild passive overconsumption to excessive overeating with loss of control observed in binge eating disorder (BED). The signaling systems that underlie appetite control in BED are complex and, at this point, not well understood. The present review highlights the current knowledge of key components of the gut peptide system and examines evidence of defects in signaling that differentiate obese binge eaters from obese non-binge eaters. The signaling network underlying hunger, satiety, and metabolic status includes leptin and insulin from energy stores and cholecystokinin, glucagon-like peptide-1, peptide YY(3-36), and ghrelin from the gastrointestinal tract. Of the many gastrointestinal peptides, ghrelin is the only established appetite-stimulating one, whereas cholecystokinin, glucagon-like peptide-1, and peptide YY(3-36) promote satiety. Adipose tissue provides hormonal signals via leptin and insulin to the brain about energy stores and likely from adiponectin and resistin. Binge eating has been related to a dysfunction in the ghrelin signaling system. Moreover, the larger gastric capacity observed in BED may further reduce satiety signals and contribute to overeating.

Ghrelin, des-acyl ghrelin and obestatin: three pieces of the same puzzle

The major active product of ghrelin gene is a 28-amino acid peptide acylated at the serine 3 position with an octanoyl group, called simply ghrelin. Ghrelin has a multiplicity of physiological functions, affecting GH release, food intake, energy and glucose homeostasis, gastrointestinal, cardiovascular, pulmonary and immune function, cell proliferation and differentiation and bone physiology. Nevertheless, recent developments have shown that ghrelin gene can generate various bioactive molecules besides ghrelin, mainly des-acyl ghrelin and obestatin, obtained from alternative splicing or from extensive post-translational modification. Although their receptors have not yet been identified, they have already proven to be active, having intriguingly subtle but opposite physiological actions to ghrelin. This suggests the existence of a novel endocrine system with multiple effector elements which not only may have opposite actions but may regulate the action of each other. In this review, we summarize the steps which lead to the production of the different ghrelin gene products and examine the most significant differences between them in terms of structure and actions.

Different responses of circulating ghrelin, obestatin levels to fasting, re-feeding and different food compositions, and their local expressions in rats

Obestatin, a sibling of ghrelin derived from preproghrelin, opposes several physiological actions of ghrelin. Our previous study has demonstrated that both plasma ghrelin and obestatin levels were decreased significantly 2h after food intake in human. To further expand current knowledge, we investigated the temporal profiles of their levels in ad libitum fed rats, 48h fasted rats and 48h fasted rats refed 2h with a standard chow, crude fiber, 50% glucose or water, and their expressions in stomach, liver and pancreatic islets immunohistochemically. Plasma ghrelin and obestatin levels were measured by EIA. Plasma leptin, insulin and glucose levels were also evaluated. Both plasma ghrelin and obestatin levels increased significantly in fasted rats compared with ad libitum fed rats. The ingestion of standard chow produced a profound and sustained suppression of ghrelin levels, whereas plasma obestatin levels decreased significantly but recovered quickly. Intake of crude fiber or 50% glucose, however, produced a more profound and sustained suppression of obestatin levels, though they had relatively less impact on ghrelin levels. Plasma glucose was the only independent predictor of ghrelin levels, obestatin levels, and ghrelin to obestatin ratios. Obestatin immunoreactivity was detected in the fundus of stomach, liver and pancreatic islets, with roughly similar patterns of distribution to ghrelin. These data show quantitative and qualitative differences in circulating ghrelin and obestatin responses to the short-term feeding status and nutrient composition, and may support a role for obestatin in regulating metabolism and energy homeostasis.

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.

Molecular mechanism of Zn2+ agonism in the extracellular domain of GPR39

Ala substitution of potential metal-ion binding residues in the main ligand-binding pocket of the Zn2+-activated G protein-coupled receptor 39 (GPR39) receptor did not decrease Zn2+ potency. In contrast, Zn2+ stimulation was eliminated by combined substitution of His17 and His19, located in the N-terminal segment. Surprisingly, substitution of Asp313 located in extracellular loop 3 greatly increased ligand-independent signaling and apparently eliminated Zn2+-induced activation. It is proposed that Zn2+ acts as an agonist for GPR39, not in the classical manner by directly stabilizing an active conformation of the transmembrane domain, but instead by binding to His17 and His19 in the extracellular domain and potentially by diverting Asp313 from functioning as a tethered inverse agonist through engaging this residue in a tridentate metal-ion binding site.

Ghrelin and obestatin levels in severely obese women before and after weight loss after Roux-en-Y gastric bypass surgery

BACKGROUND

Ghrelin and obestatin are derived from the same gene but have different effects: Ghrelin stimulates appetite, and previous-albeit inconsistent-data show that obestatin may be involved in satiety. The present study was designed to test the hypothesis that Roux-en-Y gastric bypass (RYGB) surgery and/or the weight loss that reliably results from this procedure would alter levels of ghrelin and obestatin and ghrelin/obestatin ratios in a cohort of morbidly obese women.

METHODS

This is a longitudinal follow-up study in 18 morbidly obese women (mean weight 131.2 kg, mean body mass index [BMI] 47.4). Clinical parameters and fasting serum concentrations of ghrelin, obestatin, triglycerides, low-density lipoprotein cholesterol, glucose, and insulin were measured before and 2 years after RYGB surgery, which was associated with body weight reductions of 41.5 +/- 11.6 kg (mean 62.5% excess weight loss).

RESULTS

Ghrelin concentrations (-12%, p = 0.022) and ghrelin/obestatin ratios (-14%, p = 0.017) were lower after surgery than before, while obestatin levels did not change. Changes in ghrelin concentrations correlated with changes in insulin levels (r = 0.45, p = 0.011). Most cardiovascular risk factors studied improved postsurgically (p < 0.01).

CONCLUSION

In contrast to previous weight loss studies involving gastric banding, ghrelin levels decreased and obestatin levels remained stable after massive weight loss in long-term follow-up. The favorable gastrointestinal hormone profiles observed are likely to contribute to the long-term weight loss success rate attributed to RYGB.

Treadmill exercise reduces obestatin concentrations in rat fundus and small intestine

Ghrelin and obestatin both are orexigenic/anorexigenic peptides which are secreted from gastrointestinal tracts (fundus submucosa cells). Obestatin is a 23 amino acid peptide recently isolated from rat stomach, is encoded by the same gene that encodes ghrelin. It has been suggested that ghrelin/obestatin stimulate growth hormone release and have opposite actions on food intake. Distribution and biological activity of obestatin and its role in energy balance were studied in rodents. The purpose of the present study was to investigate fundus and intestine obestatin concentrations and selected hormonal responses to a treadmill exercise running program. Fourteen adult Wistar male rats (12-14 weeks old, 235-250 g) were used for this study. Animals were divided into control (n=7) and training (n=7) groups. Training group was given exercise on a motor-driven treadmill at 25 m/min (0% grade) for 60 min/day, 5 days/week for 6 weeks. Rats were sacrificed 48 h after the last session of exercise fundus, small intestine, and liver were excised, immediately washed in ice-cold saline, and frozen in liquid nitrogen for determination of obestatin and ATP concentrations and liver glycogen content. Plasma was collected for glucose, growth hormone (GH), insulin, and cortisol measurements. Total obestatin concentrations were significantly (P<0.045, P<0.032, respectively) low in trained rat fundus and intestine at rest. Fundus and intestine ATP content remained unchanged. Liver glycogen content was significantly (P<0.039) higher in trained rats. Changes in plasma total obestatin, glucose, insulin, cortisol levels were not significant. Plasma GH concentrations was significantly (P<0.001) higher in trained animals when compared with control rats. The data indicate that moderate treadmill exercise was able to reduce fundus and small intestine total obestatin concentrations and this reduction was accompanied with a higher plasma GH and liver glycogen content in trained rats. Exercise training might modulate fundus and intestine total obestatin levels via an improvement of energy source and a negative feedback action of GH on this peptide.

Regulation of ghrelin structure and membrane binding by phosphorylation

The peptide hormone ghrelin requires Ser-3 acylation for receptor binding, orexigenic and anti-inflammatory effects. Functions of desacylghrelin are less well understood. In vitro kinase assays reveal that the evolutionarily conserved Ser-18 in the basic C-terminus is an excellent substrate for protein kinase C. Circular dichroism reveals that desacylghrelin is approximately 12% helical in aqueous solution and approximately 50% helical in trifluoroethanol. Ser-18-phosphorylation, Ser-18-Ala substitution, or Ser-3-acylation reduces the helical character in trifluoroethanol to approximately 24%. Both ghrelin and desacylghrelin bind to phosphatidylcholine:phosphatidylserine sucrose-loaded vesicles in a phosphatidylserine-dependent manner. Phosphoghrelin and phosphodesacylghrelin show greatly diminished phosphatidylserine-dependent binding. These results are consistent with binding of ghrelin and desacylghrelin to acidic lipids via the basic face of an amphipathic helix with Ser-18 phosphorylation disrupting both helical character and membrane binding.

Peripheral obestatin has no effect on feeding behavior and brain Fos expression in rodents

Obestatin is produced in the stomach from proghrelin by post-translational cleavage. The initial report claimed anorexigenic effects of obestatin in mice. Contrasting studies indicated no effect of obestatin on food intake (FI). We investigated influences of metabolic state (fed/fasted), environmental factors (dark/light phase) and brain Fos response to intraperitoneal (ip) obestatin in rats, and used the protocol from the original study assessing obestatin effects in mice. FI was determined in male rats injected ip before onset of dark or light phase, with obestatin (1 or 5 micromol/kg), CCK8S (3.5 nmol/kg) or 0.15 M NaCl, after fasting (16 h, n=8/group) or ad libitum (n=10-14/group) food intake. Fos expression in hypothalamic and brainstem nuclei was examined in freely fed rats 90 min after obestatin (5 micromol/kg), CCK8S (1.75 nmol/kg) or 0.15 M NaCl (n=4/group). Additionally, fasted mice were injected ip with obestatin (1 micromol/kg) or urocortin 1 (2 nmol/kg) 15 min before food presentation. No effect on FI was observed after obestatin administration during the light and dark phase under both metabolic conditions while CCK8S reduced FI irrespectively of the conditions. The number of Fos positive neurons was not modified by obestatin while CCK8S increased Fos expression in selective brain nuclei. Obestatin did not influence the refeeding response to a fast in mice, while urocortin was effective. Therefore, peripheral obestatin has no effect on FI under various experimental conditions and did not induce Fos in relevant central neuronal circuitries modulating feeding in rodents.

Presence of obestatin in breast milk: relationship among obestatin, ghrelin, and leptin in lactating women

OBJECTIVE

The peptide hormones ghrelin and leptin have been found in blood and breast milk. This study was undertaken to investigate whether breast milk also contains obestatin, which is derived from the same gene as ghrelin but has opposite actions, and to characterize the relations among serum and milk ghrelin, obestatin, and leptin levels in lactating mothers.

METHODS

Venous blood, colostrum, and mature milk were obtained from healthy lactating women (n = 31) just before suckling. The ghrelin and obestatin concentrations were determined by radioimmunoassay. Leptin levels were measured by enzyme-amplified sensitivity immunoassay.

RESULTS

Obestatin levels in colostrum (538.9 pg/mL) and mature milk (528.5 pg/mL) were more than twice the corresponding blood levels (270.3 and 289.4 pg/mL, respectively). In contrast, leptin levels in colostrum (2.01 ng/mL) and mature milk (2.04 ng/mL) were more than five-fold lower than the corresponding blood levels (11.54 ng/mL). There was no correlation between breast milk ghrelin levels and leptin (r = -0.18, P > 0.05). However, there was a positive correlation between leptin levels in breast milk and blood (r = 0.369, P < 0.05).

CONCLUSION

The origin of milk obestatin is not currently known, but it comes from the blood or breast and may drain through the mammary glands into the milk. Ghrelin, obestatin, and leptin in the milk may directly affect appetite and their levels may be related to the regulation of energy balance and the pathogenesis of obesity.

Distribution of obestatin and ghrelin in human tissues: immunoreactive cells in the gastrointestinal tract, pancreas, and mammary glands

Obestatin and ghrelin are two peptides derived from the same prohormone. It is well established that ghrelin is produced by endocrine cells in the gastric mucosa. However, the distribution of human obestatin immunoreactive cells is not thoroughly characterized. A polyclonal antibody that specifically recognizes human obestatin was produced. Using this antibody and a commercial antibody vs ghrelin, the distribution of obestatin and ghrelin immunoreactive cells was determined in a panel of human tissues using immunohistochemistry. The two peptides were detected in the mucosa of the gastrointestinal tract, from cardia to ileum, and in the pancreatic islets. Interestingly, epithelial cells in the ducts of mammary glands showed distinct immunoreactivity for both ghrelin and obestatin. By double immunofluorescence microscopy, it was shown that all detected cells were immunoreactive for both peptides. Furthermore, the subcellular localization of obestatin and ghrelin was essentially identical, indicating that obestatin and ghrelin are stored in the same secretory vesicles.

Prothyrotropin-releasing hormone targets its processing products to different vesicles of the secretory pathway

Prothyrotropin-releasing hormone (pro-TRH) is initially cleaved by the prohormone convertase-1/3 (PC1/3) in the trans-Golgi network generating N- and C-terminal intermediate forms that are then packed into secretory vesicles. However, it is not known whether these peptides are differentially sorted within the secretory pathway. This is of key importance because the processing products of several prohormones fulfill different biological functions. Using AtT20 cells stably transfected with prepro-TRH cDNA, we found that two specific N- and C-terminal peptides were located in different vesicles. Furthermore, the C-terminal pro-TRH-derived peptides were more efficiently released in response to KCl and norepinephrine, a natural secretagogue of TRH. Similar sorting and secretion of N- and C-terminal peptides occurs in vivo. When we blocked the initial proteolytic processing by a mutagenic approach, the differential sorting and secretion of these peptides were prevented. In summary, our data show that pro-TRH-derived peptides are differentially sorted within the secretory pathway and that the initial cleavage in the trans-Golgi network is key to this process. This could be a common mechanism used by neuroendocrine cells to regulate independently the secretion of different bioactive peptides derived from the same gene product.

Obestatin inhibits motor activity in the antrum and duodenum in the fed state of conscious rats

Obestatin is a novel peptide encoded by the ghrelin precursor gene; however, its effects on gastrointestinal motility remain controversial. Here we have examined the effects of obestatin on fed and fasted motor activities in the stomach and duodenum of freely moving conscious rats. We examined the effects of intravenous (IV) injection of obestatin on the percentage motor index (%MI) and phase III-like contractions in the antrum and duodenum. The brain mechanism mediating the action of obestatin on gastroduodenal motility and the involvement of vagal afferent pathway were also examined. Between 30 and 90 min after IV injection, obestatin decreased the %MI in the antrum and prolonged the time taken to return to fasted motility in the duodenum in fed rats given 3 g of chow after 18 h of fasting. Immunohistochemical analysis demonstrated that corticotropin-releasing factor- and urocortin-2-containing neurons in the paraventricular nucleus in the hypothalamus were activated by IV injection of obestatin. Intracerebroventricular injection of CRF type 1 and type 2 receptor antagonists prevented the effects of obestatin on gastroduodenal motility. Capsaicin treatment blocked the effects of obestatin on duodenal motility but not on antral motility. Obestatin failed to antagonize ghrelin-induced stimulation of gastroduodenal motility. These results suggest that, in the fed state, obestatin inhibits motor activity in the antrum and duodenum and that CRF type 1 and type 2 receptors in the brain might be involved in these effects of obestatin on gastroduodenal motility.

Obestatin is present in saliva: alterations in obestatin and ghrelin levels of saliva and serum in ischemic heart disease

Ghrelin and obestatin are a single gene products and are a multiple functional peptides that regulates energy homeostasis, and food intake. In the present work, we studied the secretion of ghrelin and its co-secreted peptide obestatin in 44 patients with ischemic heart disease with that of 27 healthy matched controls. Here we first conducted using an immunohistochemistry assay to screen whether human salivary glands have any obestatin immunoreactivity. Then, serum and saliva obestatin and acylated ghrelin levels were determined by using Radioimmunoassay. Our immunohistochemical analysis demonstrated that obestatin was localized in the striated and excretory duct of human salivary gland. We also report for the first time that obestatin, like ghrelin, is present in human salivary gland and saliva. No evidence of the role of obestatin or ghrelin saliva levels in the context of ischemic heart disease was found. Salivary ghrelin and obestatin levels are correlated in controls with the blood levels. Determination of salivary values could represent a non-invasive alternative to serum ones that can be useful in clinical practice.

The obestatin/ghrelin system as a novel regulatory mechanism of iris muscle contraction

PURPOSE

To evaluate obestatin and ghrelin effects on iris muscle contraction.

MATERIALS AND METHODS

Obestatin (10(-5) M) or ghrelin (10(-5) M) were tested on two consecutive carbachol-or epinephrine-elicited contractions of iris rabbit sphincter or dilator muscles. Ghrelin and obestatin effects on iris muscles basal tension were also tested, and their effects on iris sphincter EFS-elicited contraction were evaluated.

RESULTS

Compared with the first, tension of the second carbachol-induced contraction of the iris sphincter decreased 11.5+/-5.5% in the vehicle group, increased 19.0+/-10.2% in presence of obestatin, and remained unchanged by ghrelin. Epinephrine-induced contractions were not affected by obestatin or ghrelin. EFS-elicited contractions were decreased 9.3+/-3.2% by ghrelin. Basal tension of the iris sphincter decreased 21.7+/-3.7% in presence of ghrelin (10(-5) M), while that of the dilator decreased 14.1+/-5.0% in presence of obestatin (10(-5) M).

CONCLUSION

This study suggests that obestatin potentiates the cholinergic contraction of the iris sphincter and relaxes the iris dilator muscles.

Obestatin promotes survival of pancreatic beta-cells and human islets and induces expression of genes involved in the regulation of beta-cell mass and function

OBJECTIVE

Obestatin is a newly discovered peptide encoded by the ghrelin gene whose biological functions are poorly understood. We investigated obestatin effect on survival of beta-cells and human pancreatic islets and the underlying signaling pathways.

RESEARCH DESIGN AND METHODS

beta-Cells and human islets were used to assess obestatin effect on cell proliferation, survival, apoptosis, intracellular signaling, and gene expression.

RESULTS

Obestatin showed specific binding on HIT-T15 and INS-1E beta-cells, bound to glucagon-like peptide-1 receptor (GLP-1R), and recognized ghrelin binding sites. Obestatin exerted proliferative, survival, and antiapoptotic effects under serum-deprived conditions and interferon-gamma/tumor necrosis factor-alpha/interleukin-1 beta treatment, particularly at pharmacological concentrations. Ghrelin receptor antagonist [D-Lys(3)]-growth hormone releasing peptide-6 and anti-ghrelin antibody prevented obestatin-induced survival in beta-cells and human islets. beta-Cells and islet cells released obestatin, and addition of anti-obestatin antibody reduced their viability. Obestatin increased beta-cell cAMP and activated extracellular signal-related kinase 1/2 (ERK1/2) and phosphatidylinositol 3-kinase (PI 3-kinase)/Akt; its antiapoptotic effect was blocked by inhibition of adenylyl cyclase/cAMP/protein kinase A (PKA), PI 3-kinase/Akt, and ERK1/2 signaling. Moreover, obestatin upregulated GLP-1R mRNA and insulin receptor substrate-2 (IRS-2) expression and phosphorylation. The GLP-1R antagonist exendin-(9-39) reduced obestatin effect on beta-cell survival. In human islets, obestatin, whose immunoreactivity colocalized with that of ghrelin, promoted cell survival and blocked cytokine-induced apoptosis through cAMP increase and involvement of adenylyl cyclase/cAMP/PKA signaling. Moreover, obestatin 1) induced PI 3-kinase/Akt, ERK1/2, and also cAMP response element-binding protein phosphorylation; 2) stimulated insulin secretion and gene expression; and 3) upregulated GLP-1R, IRS-2, pancreatic and duodenal homeobox-1, and glucokinase mRNA.

CONCLUSIONS

These results indicate that obestatin promotes beta-cell and human islet cell survival and stimulates the expression of main regulatory beta-cell genes, identifying a new role for this peptide within the endocrine pancreas.

Mice with chronically increased circulating ghrelin develop age-related glucose intolerance

Ghrelin is a gut peptide that stimulates food intake and increases body fat mass when administered centrally or peripherally. In this study, ghrelin was overexpressed in neurons using the neuron-specific enolase (NSE) promoter sequences and mouse ghrelin cDNA (NSE-Ghr). Ghrelin expression in NSE-Ghr brain tissues was increased compared with wild-type mice. Ghrelin expression was also increased to a much smaller extent in liver of these mice, but mRNA levels in stomach or duodenum did not differ from wild-type mice. Body weight and composition was analyzed in two lines of NSE-Ghr mice, one line with increased circulating bioactive ghrelin (L43) and one line without (L73). No increases in body weight, food intake, or fat mass were found. Energy expenditure was measured in L43 mice and did not differ from wild-type controls, whereas locomotor activity was increased in NSE-Ghr mice. Young NSE-Ghr mice had normal glucose tolerance; however, L43 NSE-Ghr mice, but not L73 mice, developed glucose intolerance at 32 wk of age. Despite the impaired glucose tolerance in L43 mice, insulin levels did not differ from those of wild-type mice. These findings suggest a role for ghrelin in age-associated impairments of glucose homeostasis.

Mechanisms of disease: the role of gastrointestinal hormones in appetite and obesity

The obesity epidemic is fast becoming one of the leading causes of mortality and morbidity worldwide. Over the past 30 years, gastrointestinal hormones have been increasingly understood to have an important role as regulators of appetite and energy balance in obese individuals. The levels of these hormones are modulated by bariatric surgery, and understanding how they are affected by such procedures can contribute to our comprehension of the underlying mechanisms by which these hormones affect obesity and its treatment. In this Review, we consider several gastrointestinal hormones that can contribute to obesity by modulating the activity of the gut-brain axis, and examine their specific effects on appetite, hunger and energy balance. Better understanding of the mechanisms by which these peptides exert their effects may enable the development of improved weight-loss medications and new treatments for obesity.

Obestatin: its physicochemical characteristics and physiological functions

Obestatin, a novel 23 amino acid amidated peptide encoded by the same gene with ghrelin, was initially reported to reduce food intake, body weight gain, gastric emptying and suppress intestinal motility through an interaction with the orphan receptor GPR39. However, recently reports have shown that above findings had been questioned by several groups. Further studies explained that obestatin was involved in inhibiting thirst and anxiety, improving memory, regulating sleep, affecting cell proliferation, and increasing the secretion of pancreatic juice enzymes. We also identified that obestatin could stimulate piglet liver and adipose cell proliferation, and inhibit the secretion of IGF-I. According to the controversy over the effects and the cognate ligand of obestatin, here we provide the latest review on the structure, distribution and physiological functions of obestatin.

A diffusion barrier between the area postrema and nucleus tractus solitarius

The blood-brain barrier (BBB) is a structural and functional barrier that prevents free exchange of circulating substances with the brain, where the endothelial cells of microvessels are joined by tight junctions. The circumventricular organs (CVOs), by contrast, lack tight junctions and exhibit more direct communication with the circulating blood and cerebrospinal fluid. Despite many outstanding morphological studies at the electron microscopic level, there remain misconceptions that the CVOs provide direct passage of blood-borne substances to the rest of the brain. This study will show the structure of the anatomical borders of the dorsal vagal complex in the brainstem. A distinct diffusion barrier between the area postrema (AP, a CVO) and the nucleus tractus solitarius (NTS) was illustrated by immunohistochemistry at both the light and electron microscopic levels. The border zone between the AP and NTS was underlined by a continuous monolayer of columnar cells that were immunopositive for both the tight junction protein zona occludin-1 and the astrocyte marker glial fibrillary acidic protein. This observation of a diffusion barrier between the AP and NTS resolves a long-standing dispute about whether the NTS is a structural extension of the AP with a leaky BBB.