BIOMEDICINE: Separation of Conjoined Hormones Yields Appetite Rivals

What is the real relevance of endogenous ghrelin?

Ghrelin is a pleiotropic and ubiquitous gastric hormone implicated in body physiology. Ghrelin exhibits potent orexigenic actions and increases body weight and adiposity. Ghrelin is also involved in other metabolic functions among which we can highlight the GH releasing activity and the regulation of glucose homeostasis. Ghrelin needs the enzyme GOAT to be acylated, a step essential for binding to the GHSR1a receptor to exert its functions. Genetic animal models emerge as important tools to delineate the physiological relevance of ghrelin on energy balance. Despite the numerous reports using different genetically engineered mouse models targeting the ghrelin system, its endogenous relevance in metabolism seems to be less important than its pharmaceutical options.

Hypothalamic ceramide levels regulated by CPT1C mediate the orexigenic effect of ghrelin

Recent data suggest that ghrelin exerts its orexigenic action through regulation of hypothalamic AMP-activated protein kinase pathway, leading to a decline in malonyl-CoA levels and desinhibition of carnitine palmitoyltransferase 1A (CPT1A), which increases mitochondrial fatty acid oxidation and ultimately enhances the expression of the orexigenic neuropeptides agouti-related protein (AgRP) and neuropeptide Y (NPY). However, it is unclear whether the brain-specific isoform CPT1C, which is located in the endoplasmic reticulum of neurons, may play a role in this action. Here, we demonstrate that the orexigenic action of ghrelin is totally blunted in CPT1C knockout (KO) mice, despite having the canonical ghrelin signaling pathway activated. We also demonstrate that ghrelin elicits a marked upregulation of hypothalamic C18:0 ceramide levels mediated by CPT1C. Notably, central inhibition of ceramide synthesis with myriocin negated the orexigenic action of ghrelin and normalized the levels of AgRP and NPY, as well as their key transcription factors phosphorylated cAMP-response element-binding protein and forkhead box O1. Finally, central treatment with ceramide induced food intake and orexigenic neuropeptides expression in CPT1C KO mice. Overall, these data indicate that, in addition to formerly reported mechanisms, ghrelin also induces food intake through regulation of hypothalamic CPT1C and ceramide metabolism, a finding of potential importance for the understanding and treatment of obesity.

Obestatin receptor in energy homeostasis and obesity pathogenesis

Based on the bioinformatic prediction, Zhang and colleagues discovered obestatin, a new 23-amino acid hormone from rat stomach extract encoded by the ghrelin gene. Obestatin is present not only in the gastrointestinal tract, but also in the spleen, mammary gland, breast milk, and plasma. Obestatin appears to function as part of a complex gut-brain network whereby hormones and substances from the stomach, intestine and the brain about satiety or hunger. Given the current research regarding the effects of obestatin and its possible cognate receptor(s), this chapter provides the latest review of the physiological and pathological characteristics of this hormone and its possible receptor(s) in energy homeostasis and obesity.

The hungry stomach: physiology, disease, and drug development opportunities

During hunger, a series of high-amplitude contractions of the stomach and small intestine (phase III), which form part of a cycle of quiescence and contractions (known as the migrating motor complex, MMC), play a "housekeeping" role prior to the next meal, and may contribute toward the development of hunger. Several gastrointestinal (GI) hormones are associated with phase III MMC activity, but currently the most prominent is motilin, thought to at least partly mediate phase III contractions of the gastric MMC. Additional GI endocrine and neuronal systems play even more powerful roles in the development of hunger. In particular, the ghrelin-precursor gene is proving to have a complex physiology, giving rise to three different products: ghrelin itself, which is formed from a post-translational modification of des-acyl-ghrelin, and obestatin. The receptors acted on by des-acyl-ghrelin and by obestatin are currently unknown but both these peptides seem able to exert actions which oppose that of ghrelin, either indirectly or directly. An increased understanding of the actions of these peptides is helping to unravel a number of different eating disorders and providing opportunities for the discovery of new drugs to regulate dysfunctional gastric behaviors and appetite. To date, ghrelin and motilin receptor agonists and antagonists have been described. The most advanced are compounds which activate the ghrelin and motilin receptors which are being progressed for disorders associated with gastric hypomotility.

Proghrelin peptides: Desacyl ghrelin is a powerful inhibitor of acylated ghrelin, likely to impair physiological effects of acyl ghrelin but not of obestatin A study of pancreatic polypeptide secretion from mouse islets

BACKGROUND

Proghrelin, produced by the ghrelin (A-like) cells of the gastric mucosa, gives rise to cleavage products, including desacyl ghrelin, acyl ghrelin and obestatin. The products are thought to be secreted concomitantly. In an earlier study we found acyl ghrelin and obestatin, but not desacyl ghrelin, to suppress the release of hormones from isolated islets of mouse and rat pancreas.

RESULTS

Using isolated mouse pancreatic islets to study the suppression of the spontaneous secretion of pancreatic polypeptide (PP) by acyl ghrelin and obestatin, we determined the EC(50) values for the two peptides. For acyl ghrelin it was 2 x 10(-13)M (ranging from 1.7 to 2.8 x 10(-13)M), for obestatin it was 10(-13)M (ranging from 0.3 to 1.1 x 10(-13)M). The Hill coefficient (i.e. the midpoint slope) for the acyl ghrelin dose-response curve was 0.30 (ranging from 0.21 to 0.35); the corresponding value for obestatin was 0.35 (ranging from 0.21 to 0.35). The PP-releasing effect of acyl ghrelin, but not that of obestatin, was counteracted by desacyl ghrelin. The acyl ghrelin dose-response curve was shifted to the right in a parallel manner by increasing concentrations of desacyl ghrelin. A Schild plot was constructed with a slope of 0.78, giving an apparent pA(2) value of 14.

CONCLUSIONS

The results favour the view that acyl ghrelin and obestatin suppress spontaneous PP secretion at physiologically relevant concentrations and that they act on separate receptors. However, we conclude also that desacyl ghrelin acts as a competitive, surmountable (and quite potent) inhibitor of acyl ghrelin. In view of the allegedly high circulating concentrations of desacyl ghrelin it is to be expected that the effect of acyl ghrelin - but not that of obestatin - will be impaired, in fact probably severely blunted by desacyl ghrelin, thereby compromising the functional significance of circulating acyl ghrelin. In addition, we suggest that isolated pancreatic islets are well suited for studies of receptors to acyl ghrelin and obestatin, and that suppression of PP secretion represents a convenient way to measure the effect of both these peptides.

Long-term dietary restriction influences plasma ghrelin and GOAT mRNA level in rats

The objective of this study was to examine the effect of chronic dietary restriction on the physical characteristics of the intestine and gut-derived satiety hormone production. Male Wistar rats (8 weeks) were randomized to ad libitum (AL) or 35% dietary restriction (DR) for 5 months. At the end of the study, physical measurements were made on the intestine and satiety hormone secretion and mRNA expression determined. A comparison group of young, growing AL rats (5 weeks) was also examined. The adult DR rats gained less weight over 5 months and had lower fat mass than adult AL rats (p<0.05). The weight of the small intestine as a percentage of total body weight was greater in adult DR compared to adult AL but lower than young AL rats. Compared to AL, DR down-regulated proglucagon and cholecystokinin mRNA in the duodenum and ghrelin mRNA in the stomach of adult rats but was not different from young AL. Ghrelin-O-acyltransferase (GOAT) mRNA in the stomach was up-regulated 21-fold in adult AL rats compared to young AL and 14-fold compared to adult DR rats. Total and des-acyl ghrelin was approximately 50% higher in adult DR and young AL rats compared to adult AL. Plasma leptin and insulin were lower in adult DR and young AL rats compared to adult AL. Our findings suggest that long-term energy deficits continue to drive up ghrelin levels which may have profound implications for practical implementation of DR as an anti-aging or anti-obesity strategy in humans.

[Gastrointestinal hormones in food intake control]

The discovery of gut hormones regulating the energy balance has aroused great interest in the scientific community. Some of these hormones modulate appetite and satiety, acting on the hypothalamus or the solitary tract nucleus in the brainstem. In general, the endocrine signals generated in the gut have direct or indirect (through the autonomous nervous system) anorexigenic effects. Only ghrelin, a gastric hormone, has been consistently associated with the initiation of food intake and is regarded as the main orexigenic signal both in animal models and humans. In this review, we provide a brief description of the major gastrointestinal hormones implicated in the regulation of food intake. Given the increased importance of food intake disturbances, especially obesity, a better understanding of the underlying mechanisms of action of the gastrointestinal hormones might contribute to the development of new molecules that could increase the therapeutic arsenal for treating obesity and its associated comorbidities.

Appetite and gastrointestinal motility: role of ghrelin-family peptides

Eating disorders, obesity and cachexia endanger the lives of millions of people worldwide. Fortunately, in last decade, there has been a rapid and substantial progress toward uncovering the molecular and neural mechanisms by which energy imbalance develops. In 1999, ghrelin was identified as the first orexigenic gut-derived peptide. It stimulates appetite and controls the gastric motility and the acid secretion through the activation of the growth hormone secretagogue-receptor. After the discovery of ghrelin, other forms of ghrelin-related proteins were isolated from the rat stomach. The unmodified des-n-octanoyl form (des-acyl ghrelin) and the recent obestatin act through distinct receptors and contrarily to acyl ghrelin, show an anorexigenic activity. The finding that these three peptide hormones derive from the same precursor exert opposing physiological actions, highlights the importance of post-translational regulatory mechanisms. Further investigations are required to highlight the complexity of ghrelin physiology in order to better understand the mechanisms regulating the energy balance and provide a successful treatment of eating disorders, obesity and cachexia.

Neuroendocrine and physiological regulation of intake with particular reference to domesticated ruminant animals

The central nervous system undertakes the homeostatic role of sensing nutrient intake and body reserves, integrating the information, and regulating energy intake and/or energy expenditure. Few tasks regulated by the brain hold greater survival value, particularly important in farmed ruminant species, where the demands of pregnancy, lactation and/or growth are not easily met by often bulky plant-based and sometimes nutrient-sparse diets. Information regarding metabolic state can be transmitted to the appetite control centres of the brain by a diverse array of signals, such as stimulation of the vagus nerve, or metabolic 'feedback' factors derived from the pituitary gland, adipose tissue, stomach/abomasum, intestine, pancreas and/or muscle. These signals act directly on the neurons located in the arcuate nucleus of the medio-basal hypothalamus, a key integration, and hunger (orexigenic) and satiety (anorexigenic) control centre of the brain. Interest in human obesity and associated disorders has fuelled considerable research effort in this area, resulting in increased understanding of chronic and acute factors influencing feed intake. In recent years, research has demonstrated that these results have relevance to animal production, with genetic selection for production found to affect orexigenic hormones, feeding found to reduce the concentration of acute controllers of orexigenic signals, and exogenous administration of orexigenic hormones (i.e. growth hormone or ghrelin) reportedly increasing DM intake in ruminant animals as well as single-stomached species. The current state of knowledge on factors influencing the hypothalamic orexigenic and anorexigenic control centres is reviewed, particularly as it relates to domesticated ruminant animals, and potential avenues for future research are identified.

Is GPR39 the natural receptor of obestatin?

GPR39, an orphan receptor belonging to the family of G protein-coupled receptors, was originally reported to be the receptor of obestatin. However recently, numerous reports have questioned this conclusion. In mammals, GPR39 was reported to be involved in the regulation of gastrointestinal and the metabolic functions. In this article, a latest and brief review on the receptor family, structure, distribution and physiological functions of GPR39 has been reported.

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.

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.

Effect of antipsychotics on peptides involved in energy balance in drug-naive psychotic patients after 1 year of treatment

Weight gain has become one of the most common and concerning side effects of antipsychotic treatment. The mechanisms whereby antipsychotics induce weight gain are not known. It has been suggested that peptides related to food intake and energy balance could play a role in weight gain secondary to antipsychotic therapy. To better understand the pathophysiology of antipsychotic-induced weight gain, we studied the effects of 3 antipsychotic drugs (haloperidol, olanzapine, and risperidone) on peptides involved in energy balance (insulin, ghrelin, leptin, adiponectin, visfatin, and resistin) in a population of drug-naive patients with first episode of psychosis.A significant increase in weight (10.16 kg [SD, 8.30 kg]; P < 0.001), body mass index (3.56 kg/m [SD, 2.89 kg/m]; P < 0.001), and fasting insulin (3.93 muU/mL [SD, 3.93 muU/mL]; P = 0.028), leptin (6.76 ng/mL [SD, 7.21 ng/mL]; P < 0.001), and ghrelin (15.47 fmol/mL [SD, 47.90 fmol/mL]; P = 0.009) plasma levels were observed. The increments in insulin and leptin concentrations were highly correlated with the increment in weight and body mass index and seem to be a consequence of the higher fat stores. The unexpected increase in ghrelin levels might be related with the causal mechanism of weight gain induced by antipsychotics. Finally, the 3 antipsychotics had similar effects in all parameters evaluated.

Obestatin and ghrelin levels in obese children and adolescents before and after reduction of overweight

OBJECTIVES

Obestatin and ghrelin, which are derived from the same gene, are observed to have opposite effects on weight status. The aims of this study were to compare obestatin concentrations in obese and normal-weight children and to analyse the effect of weight loss on obestatin and ghrelin levels.

METHODS

We examined anthropometrical markers and fasting serum obestatin, ghrelin, leptin, glucose and insulin concentrations in 44 obese children (mean age 11.2 years) before and after participating in a 1-year outpatient obesity intervention programme based on a high-carbohydrate, fat-reduced diet and increased physical activity. Additionally, total ghrelin, obestatin and leptin levels were determined in 22 normal-weight healthy children of similar age, gender and pubertal stage.

RESULTS

Obestatin and leptin concentrations were significantly (P < 0.001) higher and ghrelin concentrations were significantly (P < 0.001) lower in obese children compared to nonobese children. In contrast to the 13 children without weight loss, substantial weight loss in 31 children led to a significant (P = 0.007) increase in obestatin and to a significant (P < 0.05) decrease in leptin and insulin concentrations, while ghrelin concentrations did not change significantly. Children with substantial weight loss demonstrated significantly (P = 0.009) lower obestatin and a tendency (P = 0.064) to higher ghrelin concentrations at baseline. Changes in insulin were not related to changes in ghrelin or obestatin.

CONCLUSION

The increase in obestatin and the decrease in ghrelin in obese children point towards an adaptation process of weight status. Weight reduction due to a long-term lifestyle intervention resulted in an increase in obestatin levels.

Obesity: the hormonal milieu

PURPOSE OF REVIEW

Obesity has reached epidemic proportions throughout the world and poses significant health and economic burdens to both developed and developing societies. Most recent data from the NHANES study (2003-2004) report that 17.1% of US children are overweight and 32.2% of adults are obese, a significant increase compared with data obtained only 6 years earlier.

RECENT FINDINGS

The neurohormonal control of appetite, body composition, and glucose homeostasis is mediated by hormones secreted from adipose tissue, endocrine glands, and enteroendocrine cells, which converge at the vagus nerve, brainstem and hypothalamus to modulate complex interactions of neurotransmitters and central appetite-regulating peptides. These hormonal signals are tightly regulated to maintain body weight/adiposity within a narrow, individually defined range that may be further impacted by variables such as ingested calories, meal composition, and lifestyle.

SUMMARY

Clinical manifestations of obesity, the metabolic syndrome and impaired glucose tolerance reflect biochemical alterations in a complex hormonal milieu. Elucidation of these hormonal perturbations in obese patients has already provided novel pharmacologic treatments to improve weight management and address the metabolic sequelae of obesity. The remarkable redundancy of these hormones, however, and their interactions make a monopharmaceutical approach unlikely to be successful.

Proghrelin-derived peptides influence the secretion of insulin, glucagon, pancreatic polypeptide and somatostatin: a study on isolated islets from mouse and rat pancreas

Proghrelin, the precursor of the orexigenic and adipogenic peptide hormone ghrelin, is synthetized in endocrine (A-like) cells in the gastric mucosa. During its cellular processing, proghrelin gives rise to the 28-amino acid peptide desacyl ghrelin, which after octanoylation becomes active acyl ghrelin, and to the 23-amino acid peptide obestatin, claimed to be a physiological opponent of acyl ghrelin. This study examines the effects of the proghrelin products, alone and in combinations, on the secretion of insulin, glucagon, pancreatic polypeptide (PP) and somatostatin from isolated islets of mice and rats. Surprisingly, acyl ghrelin and obestatin had almost identical effects in that they stimulated the secretion of glucagon and inhibited that of PP and somatostatin from both mouse and rat islets. Obestatin inhibited insulin secretion more effectively than acyl ghrelin. In mouse islets, acyl ghrelin inhibited insulin secretion at low doses and stimulated at high. In rat islets, acyl ghrelin inhibited insulin secretion in a dose-dependent manner but the IC(50) for the acyl ghrelin-induced inhibition of insulin release was 7.5 x 10(-8) M, while the EC(50) and IC(50) values, with respect to stimulation of glucagon release and to inhibition of PP and somatostatin release, were in the 3 x 10(-12)-15 x 10(-12) M range. The corresponding EC(50) and IC(50) values for obestatin ranged from 5 x 10(-12) to 20 x 10(-12) M. Desacyl ghrelin per se did not affect islet hormone secretion. However, at a ten times higher concentration than acyl ghrelin (corresponding to the ratio of the two peptides in circulation), desacyl ghrelin abolished the effects of acyl ghrelin but not those of obestatin. Acyl ghrelin and obestatin affected the secretion of glucagon, PP and somatostatin at physiologically relevant concentrations; with obestatin this was the case also for insulin secretion. The combination of obestatin, acyl ghrelin and desacyl ghrelin in concentrations and proportions similar to those found in plasma resulted in effects that were indistinguishable from those induced by obestatin alone. From the data it seems that the effects of endogenous, circulating acyl ghrelin may be overshadowed by obestatin or blunted by desacyl ghrelin.

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

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

Peripheral tissue-brain interactions in the regulation of food intake

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

Obestatin-mediated proliferation of human retinal pigment epithelial cells: regulatory mechanisms

In this work, we have evaluated the effect of the new discovered peptide obestatin on cell proliferation in primary cultures of human retinal epithelial cells (hRPE cells). The results showed that this peptide induced, in a dose-dependent manner, cell proliferation by MEK/ERK 1/2 phosphorylation. A sequential analysis of the obestatin transmembrane signaling pathway showed that the ERK 1/2 activity is partially blocked after preincubation of the cells with pertussis toxin (PTX), as well as by wortmannin (an inhibitor of PI3K), claphostin C (an inhibitor of PKC), and PP2 (which inhibits the non receptor tyrosine kinase Src). Upon administration of obestatin, the intracellular levels of phospho-PKCepsilon-, theta-, and micro-isoenzymes rise with different time courses, from which PKCepsilon might be responsible for ERK 1/2 response. Based on the experimental data, a signaling pathway involving the consecutive activation of Gi, PI3K, novel PKC (probably PKCepsilon), and Src for ERK 1/2 activation is proposed. These results incorporate a new mitogenic factor to the group of factors that regulate proliferation of hRPE cells.

The obestatin receptor (GPR39) is expressed in human adipose tissue and is down-regulated in obesity-associated type 2 diabetes mellitus

OBJECTIVE

The G protein-coupled receptor 39 (GPR39) has recently been identified as the receptor for obestatin, a peptidic hormone involved in energy homeostasis. However, the expression levels of this receptor in human adipose tissue in obesity and obesity-associated type 2 diabetes mellitus (T2DM) remain unknown. Therefore, we evaluated the actual presence of GPR39 mRNA in human adipose tissue and whether GPR39 expression levels are altered in obesity and obesity-associated T2DM.

DESIGN

Omental adipose tissue biopsies obtained from 15 women were used in the study. Patients were classified as lean (body mass index 20.8 +/- 1.0 kg/m(2)), obese normoglycaemic (body mass index 48.4 +/- 2.1 kg/m(2)) and obese T2DM patients (body mass index 52.6 +/- 4.9 kg/m(2)). Anthropometric measurements and biochemical profiles were assessed for each subject. Real-time RT-PCR analyses were performed to quantify transcript levels of GPR39 and adiponectin.

RESULTS

Obese T2DM patients exhibited significantly lower GPR39 expression levels compared to lean (P = 0.016) and obese normoglycaemic subjects (P = 0.008), while no differences between lean and obese normoglycaemic patients were observed. The mRNA expression levels of GPR39 were negatively correlated to fasting glucose concentrations (r = -0.581, P = 0.023), while exhibiting a positive correlation to adiponectin mRNA expression levels (r = 0.674, P = 0.006).

CONCLUSION

GPR39 is expressed in human adipose tissue. The reduced expression levels of GPR39 in omental adipose tissue observed in obese patients with T2DM suggest an involvement of obestatin signalling in glucose homeostasis and T2DM development.

Normal food intake and body weight in mice lacking the G protein-coupled receptor GPR39

It has been recently proposed that obestatin, a peptide encoded by the ghrelin gene, reduces food intake by activating the orphan G protein-coupled receptor GPR39. To gain further insights into the role of GPR39 in body weight homeostasis, we characterized the phenotype of mice with targeted disruption of the GPR39 gene. Body weight, adiposity, and food intake were found to be similar between GPR39(+/+) and GPR39(-/-) mice. Furthermore, fasting glucose and insulin levels were similar between both genotypes. Injection of obestatin peptide (1 micromol/kg, ip) obtained from multiple sources did not consistently inhibit food intake in wild-type mice after an overnight fast, and no difference in food intake was observed between wild-type and GPR39 knockout mice after injection of the peptide. Finally, ectopic expression of GPR39 in HEK293T cells revealed a constitutive activation of the receptor that was unaffected by stimulation with obestatin. Our phenotypic characterization suggests that GPR39 is not a major modulator of food intake in mice, although a more subtle role cannot be excluded. The role of GPR39 in normal physiology requires further study and should be conducted independently of the function of obestatin.

Lack of obestatin effects on food intake: should obestatin be renamed ghrelin-associated peptide (GAP)?

Obestatin is a newly identified ghrelin-associated peptide (GAP) that is derived from post-translational processing of the prepro-ghrelin gene. Obestatin has been reported initially to be the endogenous ligand for the orphan receptor G protein-coupled receptor 39 (GPR39), and to reduce refeeding- and ghrelin-stimulated food intake and gastric transit in fasted mice, and body weight gain upon chronic peripheral injection. However, recent reports indicate that obestatin is unlikely to be the endogenous ligand for GPR39 based on the lack of specific binding on GRP39 receptor expressing cells and the absence of signal transduction pathway activation. In addition, a number of studies provided convergent evidence that ghrelin injected intracerebroventricularly or peripherally did not influence food intake, body weight gain, gastric transit, gastrointestinal motility, and gastric vagal afferent activity, as well as pituitary hormone secretions, in rats or mice. Similarly, obestatin did not alter ghrelin-induced stimulation of food intake or gastric transit. Therefore, the present state-of-knowledge on obestatin and GPR39 is leaving many unanswered questions that deserve further consideration. Those relate not only to redefining the biological action of obestatin that should be renamed GAP, but also the identification of the native ligand for GPR39.

Ghrelin, the peripheral hunger hormone

In the current review we summarize the available data concerning the gastric hormone ghrelin and its receptor. Ghrelin stimulates short-term food intake and long-term body weight regulation via its adipogenic and diabetogenic effects. Ghrelin stimulates gastric emptying, and these effects could be explored from a therapeutic point of view. Ghrelin levels change profoundly in anorexia, in states of insulin resistance, in obesity, and after bariatric surgery, suggesting that this is an important hormone in body weight regulation.

Obestatin improves memory performance and causes anxiolytic effects in rats

Obestatin is a peptide hormone that is derived from the same polypeptide precursor (preprogrelin) as ghrelin, but it acts in opposing way on ingestive behavior. Our previous studies showed that ghrelin affects memory and anxiety. Here, we studied the possible effects of icv obestatin injection in rats upon memory retention (using two different paradigms), anxiety like behavior (plus maze test), and food intake. Obestatin induces an increase in the percentage of open arms entries (Obestatin 3.0nmol/rat: 61.74+/-1.81), and percentage of time spent in open arms (Obestatin 3.0nmol/rat: 72.07+/-4.21) in relation to the control (33.31+/-1.54; 25.82+/-1.68), indicating an anxiolytic effect. The two doses of obestatin increased latency time in a step down test and the percentage time of novel object exploration, suggesting that the peptide influences learning and memory processes that involve the hippocampus and the amygdala. This report provides evidence indicating that obestatin effects are functionally opposite on anxiety and hunger to the ghrelin effects, while both these related peptides increase memory retention.

Altered gastrointestinal and metabolic function in the GPR39-obestatin receptor-knockout mouse

BACKGROUND & AIMS

The G-protein-coupled receptor GPR39 is a member of a family that includes the receptors for ghrelin and motilin. Recently the peptide obestatin was identified as a natural ligand for GPR39. The objective of this study was to gain insight into the biological function of the GPR39 receptor.

METHODS

GPR39(-/-) mice were generated and analyzed.

RESULTS

Endogenous GPR39 expression was detected in the brain (septum-amygdala) and the gastrointestinal system (parietal cells, enterocytes, neurons, and pancreas). Gastric emptying of a solid meal (measured by the (14)C octanoic breath test) in GPR39(-/-) mice was accelerated significantly with a gastric half-emptying time of 49.5 +/- 2.2 minutes compared with 86.9 +/- 8.4 minutes in GPR39(+/+) mice. A more effective expulsion of distally located pellets (30%-75% of length) was observed in the colon of GPR39(-/-) mice. Four hours after pylorus ligation, the volume of gastric secretion was increased significantly (GPR39(-/-): 638 +/- 336 microL; GPR39(+/+): 225 +/- 170 microL), but gastric acid secretion was unchanged. The mature body weight and body fat composition of GPR39(-/-) mice was significantly higher compared with GPR39(+/+) mice, but this was not related to hyperphagia because 24-hour food intake did not differ between both genotypes. In contrast, deficiency of the GPR39 receptor led to reduced hyperphagia after fasting. The cholesterol levels were increased significantly in the GPR39(-/-) mice.

CONCLUSIONS

Our data partially confirm and extend the described in vivo effects of obestatin and suggest that this peptide plays a functional role in the regulation of gastrointestinal and metabolic function through interaction with the GPR39 receptor.

One ancestor, several peptides post-translational modifications of preproghrelin generate several peptides with antithetical effects

Preproghrelin is the polypeptide precursor of ghrelin. First discovered in gastric extract as a growth hormone releasing peptide and food intake modulator, it has more recently been revealed to have other physiological aspects. The fine molecular mechanisms of ghrelin biosynthesis show that this peptide is but one piece of a puzzle which contains many other peptides obtained from alternative splicing of the same gene or from extensive post-translational modifications. Recent developments have shown that pro-ghrelin cleavage generates another active peptide named obestatin with an intriguingly subtle but opposite physiological action to ghrelin. Noteworthy, and similar to ghrelin, which requires post-translational processing close to its amino terminus by acylation, the biological activity of the ghrelin-associated peptide obestatin also depends on modification, but by amidation at its carboxyl terminus. In this review we will summarize the steps which led to the identification of pre-proghrelin gene products and will examine the significance and perspectives of the different peptides generated from the same ancestor gene.

Molecular mechanisms of insulin resistance and associated diseases

Insulin resistance is a state in which higher than normal concentrations of insulin are required for normal response. The most common underlying cause is central obesity, although primary insulin resistance in normal-weight individuals is also possible. Excess abdominal adipose tissue has been shown to release increased amounts of free fatty acids which directly affect insulin signalling, diminish glucose uptake in muscle, drive exaggerated triglyceride synthesis and induce gluconeogenesis in the liver. Other factors presumed to play the role in insulin resistance are tumour necrosis factor alpha, adiponectin, leptin, IL-6 and some other adipokines. Hyperinsulinaemia which accompanies insulin resistance may be implicated in the development of many pathological states, such as hypertension and hyperandrogenaemia. Insulin resistance underlies metabolic syndrome and is further associated with polycystic ovary syndrome and lipodystrophies. When beta-cells fail to secrete the excess insulin needed, diabetes mellitus type 2 emerges, which is, besides coronary heart disease, the main complication of insulin resistance and associated diseases.

Synthesis and SAR of 1,3-disubstituted cyclohexylmethyl urea and amide derivatives as non-peptidic motilin receptor antagonists

A series of 1,3-disubstituted cyclohexylmethyl urea and amide derivatives were synthesized as motilin receptor antagonists. Starting from known motilin antagonists, 1a and 1b, the cyclopentene scaffold was replaced and the four recognition elements optimized to arrive at a potent novel series.