Obestatin and ghrelin in obese and in pregnant women

GPR39: An orphan receptor begging for ligands

Progress in the understanding of the receptor GPR39 is held up by inconsistent pharmacological data. First, the endogenous ligand(s) remain(s) contentious. Data pointing to zinc ions (Zn2+) and/or eicosanoids as endogenous ligands are a matter of debate. Second, there are uncertainties in the specificity of the widely used synthetic ligand (agonist) TC-G 1008. Third, activation of GPR39 has been often proposed as a novel treatment strategy, but new data also support that inhibition might be beneficial in certain disease contexts. Constitutive activity/promiscuous signaling suggests the need for antagonists/inverse agonists in addition to (biased) agonists. Here, we scrutinize data on the signaling and functions of GPR39 and critically assess factors that might have contributed to divergent outcomes and interpretations of investigations on this important receptor.

Development of a sensitive and quantitative HPLC-FLD method for the determination of obestatin in human plasma

Obestatin is a gastrointestinal system peptide. The quantification of this peptide is conventionally performed using immunological techniques. In this study, a selective and sensitive HPLC method coupled with fluorescence detection for the quantitation of obestatin in human plasma was developed and validated. The separation was obtained on a C18 (4.6 × 100 mm, 3.5-μm particles) column using a mobile phase composed of acetonitrile and water, both including 0.1% trifluoroacetic acid. The developed method was found to be linear in the concentration range of 20 to 1000 ng/mL, with a coefficient of determination of 0.9982. The precision results were less than 10%, and the accuracy results were between 92% and 107%. The detection and quantification limit values were obtained as 2.8 and 9.4 ng/mL, respectively. Analyte solutions were found stable for 24 h at room temperature, three freeze-thaw cycles, and 2 weeks at -20°C. The developed method was successfully used for the quantification of obestatin in human plasma samples. In conclusion, the developed method is sensitive and specific for measuring the plasma concentrations of obestatin.

The Zinc-Sensing Receptor GPR39 in Physiology and as a Pharmacological Target

The G-protein coupled receptor GPR39 is abundantly expressed in various tissues and can be activated by changes in extracellular Zn²⁺ in physiological concentrations. Previously, genetically modified rodent models have been able to shed some light on the physiological functions of GPR39, and more recently the utilization of novel synthetic agonists has led to the unraveling of several new functions in the variety of tissues GPR39 is expressed. Indeed, GPR39 seems to be involved in many important metabolic and endocrine functions, but also to play a part in inflammation, cardiovascular diseases, saliva secretion, bone formation, male fertility, addictive and depression disorders and cancer. These new discoveries offer opportunities for the development of novel therapeutic approaches against many diseases where efficient therapeutics are still lacking. This review focuses on Zn²⁺ as an endogenous ligand as well as on the novel synthetic agonists of GPR39, placing special emphasis on the recently discovered physiological functions and discusses their pharmacological potential.

Biochemical properties and biological actions of obestatin and its relevence in type 2 diabetes

Obestatin was initially discovered in rat stomach extract, and although it is principally produced in the gastric mucosa, it can be found throughout the gastrointestinal tract. This 23-amino acid C-terminally amidated peptide is derived from preproghrelin and has been ascribed a wide range of metabolic effects relevant to type 2 diabetes. Obestatin reportedly inhibits gastrointestinal motility, reduces food intake and lowers body weight and improves lipid metabolism. Furthermore, it appears to exert actions on the pancreatic β-cell, most notably increasing β-cell mass and upregulating genes associated with insulin production and β-cell regeneration, with relevance to type 2 diabetes. It is becoming evident that obestatin also exerts pleiotropic effects on the cardiovascular system, possibly modulating blood pressure, endothelial function and triggering cardioprotective mechanisms, which may be important in determining cardiovascular outcomes in type 2 diabetes. Furthermore, it seems that like other gut peptides obestatin has neuroprotective properties. This review examines the biochemical properties of the obestatin peptide (its structure, sequence, stability and distribution) and the candidate receptors through which it may act. It provides a balanced examination of the reported pancreatic and extrapancreatic actions of obestatin and evaluates its potential relevance with respect to diabetes therapy, together with discussion of direct evidence linking alterations in obestatin signalling with obesity/diabetes and other diseases.

Obestatin as a key regulator of metabolism and cardiovascular function with emerging therapeutic potential for diabetes

Obestatin is a 23-amino acid C-terminally amidated gastrointestinal peptide derived from preproghrelin and which forms an α helix. Although obestatin has a short biological half-life and is rapidly degraded, it is proposed to exert wide-ranging pathophysiological actions. Whilst the precise nature of many of its effects is unclear, accumulating evidence supports positive actions on both metabolism and cardiovascular function. For example, obestatin has been reported to inhibit food and water intake, body weight gain and gastrointestinal motility and also to mediate promotion of cell survival and prevention of apoptosis. Obestatin-induced increases in beta cell mass, enhanced adipogenesis and improved lipid metabolism have been noted along with up-regulation of genes associated with beta cell regeneration, insulin production and adipogenesis. Furthermore, human circulating obestatin levels generally demonstrate an inverse association with obesity and diabetes, whilst the peptide has been shown to confer protective metabolic effects in experimental diabetes, suggesting that it may hold therapeutic potential in this setting. Obestatin also appears to be involved in blood pressure regulation and to exert beneficial effects on endothelial function, with experimental studies indicating that it may also promote cardioprotective actions against, for example, ischaemia-reperfusion injury. This review will present a critical appraisal of the expanding obestatin research area and discuss the emerging therapeutic potential of this peptide for both metabolic and cardiovascular complications of diabetes.

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.

Cord blood chemerin and obestatin levels in large for gestational age infants

OBJECTIVE

To investigate possible alterations in cord blood levels of adipokines, chemerin and obestatin (secreted by adipose tissue and associated with later development of insulin resistance/metabolic syndrome), as well as insulin, in large for gestational age (LGA) and appropriate for gestational age (AGA) pregnancies, granted that these groups differ in body fat mass and metabolic/endocrine mechanisms.

METHODS

Cord blood chemerin, obestatin, and insulin concentrations were prospectively measured in 40 LGA (9 born from diabetic and 31 from nondiabetic mothers) and 40 AGA singleton full-term infants.

RESULTS

Cord blood chemerin concentrations were significantly higher in LGA compared with AGA neonates (b = 38.91, SE 9.29, p < 0.001). In contrast, no significant differences in obestatin concentrations were observed between groups. Insulin levels were significantly elevated as customized centiles increased (b = 0.003, SE = 0.001, p = 0.032).

CONCLUSIONS

Higher chemerin concentrations in LGA neonates possibly reflect the increased adipose tissue in this group. Lack of difference between the two groups in circulating levels of obestatin-possibly a sensitive marker of insulin resistance-might be due to development of metabolic disorders later in life.

Effect of desacyl ghrelin, obestatin and related peptides on triglyceride storage, metabolism and GHSR signaling in 3T3-L1 adipocytes

Acyl-ghrelin (AG), desacyl-ghrelin (DAG) and obestatin are all derived from the same gene transcript; however their plasma levels do not necessarily change in parallel. The influence of these peptides towards the development of obesity and their direct effects on adipocyte physiology has not been thoroughly investigated. This study was designed to evaluate the direct effects of peptides of the ghrelin family on preadipocyte proliferation, differentiation and adipocyte lipid and glucose metabolism in 3T3-L1 cells. 3T3 cells were treated with physiological peptide concentrations for 1 h to 9 days, and the relevant assays measured. In preadipocytes, AG, GHRP-6 and DAG stimulated proliferation, measured as (3)H-thymidine incorporation (up to 200%, P < 0.05), while all peptides stimulated differentiation (up to 300%, P < 0.01) as compared to standard differentiation conditions. In adipocytes, FA uptake was increased in a concentration-dependent manner especially with obestatin (three- to fourfold, P < 0.001) and DAG (three- to fivefold, P < 0.001). By contrast, glucose transport was unchanged. DAG and obestatin significantly decreased lipolysis measured as non-esterified fatty acid and glycerol release by 50%, P < 0.05-0.01 and 51%, P < 0.01, respectively. Interestingly, DAG stimulation of FA uptake was blocked with GHSR1 antagonist (D-lys(3))-GHRP-6 (P < 0.05), phospholipase C inhibitor U73122 and phosphatidylinositol-3-kinase inhibitor wortmannin (P < 0.001). Finally, in omental but not subcutaneous human adipose tissue, GHSR1 correlated with BMI (r = 0.549, P < 0.05) and insulin (r = 0.681, P < 0.01). Taken together, these results suggest that ghrelin-related peptides may directly affect adipose tissue metabolism.

Chronic treatment with a stable obestatin analog significantly alters plasma triglyceride levels but fails to influence food intake; fluid intake; body weight; or body composition in rats

Obestatin (OB(1-23) is a 23 amino acid peptide encoded on the preproghrelin gene, originally reported to have metabolic actions related to food intake, gastric emptying and body weight. The biological instability of OB(1-23) has recently been highlighted by studies demonstrating its rapid enzymatic cleavage in a number of biological matrices. We assessed the stability of both OB(1-23) and an N-terminally PEGylated analog (PEG-OB(1-23)) before conducting chronic in vivo studies. Peptides were incubated in rat liver homogenate and degradation monitored by LC-MS. PEG-OB(1-23) was approximately 3-times more stable than OB(1-23). Following a 14 day infusion of Sprague-Dawley rats with 50 nmol/kg/day of OB(1-23) or a N-terminally PEGylated analog (PEG-OB(1-23)), we found no changes in food/fluid intake, body weight and plasma glucose or cholesterol between groups. Furthermore, morphometric liver, muscle and white adipose tissue (WAT) weights and tissue triglyceride concentrations remained unaltered between groups. However, with stabilized PEG-OB(1-23) we observed a 40% reduction in plasma triglycerides. These findings indicate that PEG-OB(1-23) is an OB(1-23) analog with significantly enhanced stability and suggest that obestatin could play a role in modulating physiological lipid metabolism, although it does not appear to be involved in regulation of food/fluid intake, body weight or fat deposition.

Meta-analysis of the relationship between obestatin and ghrelin levels and the ghrelin/obestatin ratio with respect to obesity

INTRODUCTION

The association between gastrointestinal hormones, obestatin and ghrelin, and obesity are controversial. Here, the authors summarize results of published works in this field by using meta-analysis.

METHODS

Studies that evaluated these hormones and were published before December 2009 were collected from PubMed, HighWire Press and ProQuest Health and Medical Complete. Software Review Manager 4.2 was used (Cochrane Collaboration, Oxford, England). Differences between obese and normal weight (NW) groups were compared using fixed or random effects models based on heterogeneity.

RESULTS

In obestatin group, 9 studies with 566 participants were included and had significant heterogeneity (I² = 98.6%, P < 0.01). Obestatin of NW groups was 64.19 pg/mL [95% confidence interval (CI) = 36.11-92.26, P < 0.01] higher than those of obese groups. In total ghrelin group, 21 studies with 1187 participants were included and had significant heterogeneity (I² = 97.9%, P < 0.01). Total ghrelin of NW groups was 145.53 pg/mL (95%CI = 109.59-181.47, P < 0.01) higher than those of obese groups. In active ghrelin group, 6 studies with 265 participants were included and had significant heterogeneity (I² = 98.10%, P < 0.01). Active ghrelin in NW groups was 53.22 pg/mL (95%CI = 25.92-80.53, P < 0.01) higher than those of obese groups. In ghrelin/obestatin group, 5 studies with 259 participants were included and had significant heterogeneity (I² = 94.2, P < 0.01). The ratios in NW groups were 2.49 pg/mL (95%CI = 0.03-4.9, P = 0.05) higher than those of obese groups but without significance.

CONCLUSIONS

Obestatin, total and active ghrelin in NW groups were significantly higher than those of obese groups. Ghrelin/obestatin ratios in NW groups were also higher but without significance.

GPR39: a Zn(2+)-activated G protein-coupled receptor that regulates pancreatic, gastrointestinal and neuronal functions

GPR39 is a vertebrate G protein-coupled receptor related to the ghrelin/neurotensin receptor subfamily. The receptor is expressed in a range of tissues including the pancreas, gut/gastrointestinal tract, liver, kidney and in some regions of the brain. GPR39 was initially thought to be the cognitive receptor for the peptide hormone, obestatin. However, subsequent in vitro studies have failed to demonstrate binding of this peptide to the receptor. Zn(2+) has been shown to be a potent stimulator of GPR39 activity via the Gα(q), Gα(12/13) and Gα(s) pathways. The potency and specificity of Zn(2+) in activating GPR39 suggest it to be a physiologically important agonist. GPR39 is now emerging as an important transducer of autocrine and paracrine Zn(2+) signals, impacting upon cellular processes such as insulin secretion, gastric emptying, neurotransmission and epithelial repair. This review focuses on the molecular, structural and biological properties of GPR39 and its various physiological functions.

Ghrelin in the human myometrium

BACKGROUND

Ghrelin is a 28-amino acid octanolyated peptide, synthesised primarily in the stomach. It stimulates growth hormone release, food intake and exhibits many other diverse effects. Our group have previously determined that ghrelin inhibited human contractility in vitro. The aim of this study therefore, was to investigate the expression of ghrelin, its receptor, the growth hormone secretagogue receptor type 1 (GHS-R1), ghrelin O-acyltransferase (GOAT) which catalyses ghrelin octanoylation, prohormone convertase 1/3 (PC1/3) responsible for pro-ghrelin processing, in human myometrium, during pregnancy prior to labour, during labour and in the non-pregnant state. Modulation of ghrelin and ghrelin receptor expression in cultured myometrial cells was also investigated.

METHODS

mRNA and protein were isolated from human myometrium and the myometrial smooth muscle cell line hTERT-HM; and real-time fluorescence RT-PCR, western blotting and fluorescence microscopy performed. The effects of beta-Estradiol and bacterial lipopolysaccharide (LPS) on hTERT-HM gene expression were evaluated by western blotting.

RESULTS

We have reported for the first time the expression and processing of ghrelin, GHS-R1, GOAT and PC1/3 expression in human myometrium, and also the down-regulation of ghrelin mRNA and protein expression during labour. Furthermore, GHS-R1 protein expression significantly decreased at labour. Myometrial GOAT expression significantly increased during term non-labouring pregnancy in comparison to both non-pregnant and labouring myometrium. Mature PC1/3 protein expression was significantly decreased at term pregnancy and labour in comparison to non-pregnant myometrium. Ghrelin, GHS-R1, GOAT and PC1/3 mRNA and protein expression was also detected in the hTERT-HM cells. Ghrelin protein expression decreased upon LPS treatment in these cells while beta-Estradiol treatment increased GHS-R1 expression.

CONCLUSIONS

Ghrelin processing occurred in the human myometrium at term pregnancy and in the non-pregnant state. GOAT expression which increased during term non-labouring pregnancy demonstrating a similar expression pattern to prepro-ghrelin and GHS-R1, decreased at labour, signifying possible myometrial ghrelin acylation. Moreover, the presence of PC1/3 may contribute to pro-ghrelin processing. These results along with the previous in vitro data suggest that myometrially-produced and processed ghrelin plays a significant autocrine or paracrine role in the maintenance of relaxation in this tissue during pregnancy. Furthermore, the significant uterine modulators LPS and beta-Estradiol are involved in the regulation of ghrelin and ghrelin receptor expression respectively, in the human myometrium.

Circulating obestatin levels in normal and Type 2 diabetic subjects

BACKGROUND

Obestatin has been discovered as a new product of the ghrelin gene. Its physiological actions are still a matter of debate, but it seems that this peptide is likely to be involved in the control of insulin secretion and action as well as of adipocyte function. It has been already shown that obestatin secretion in humans is negatively modulated by food intake.

AIM

To clarify obestatin secretion in normal subjects and in patients with Type 2 diabetes (T2D) in basal conditions and after a standardized meal.

SUBJECTS/METHODS

Five normal subjects and 5 T2D patients were studied during infusion of saline (iv for over 5 h from -120 to +180 min). A standardized lunch was served at 0 min. Obestatin, glucose, and insulin levels were assayed at -120, -90, -60, -45, -30, -15, 0, 15, 30, 45, 60, 90, 120, 150, and 180 min.

RESULTS

From -120 to 0 min, obestatin levels in normal and T2D subjects were similar (area under the curve: 32.3+/-5.6 pg/ml/min vs 31.1+/-1.0 pg/ml/min). After the meal, circulating obestatin levels underwent a clear decrease in normal subjects (0 min: 300.6+/-34.7 pg/ml vs nadir at 60 min: 161.8+/-29.4 pg/ml; p=0.002) but not in diabetic patients (0 min: 267.2+/-16.5 pg/ml vs nadir at 180 min: 226.0+/-10.5 pg/ml).

CONCLUSION

This study shows that normal and diabetic subjects display similar levels of circulating obestatin in fasting condition. However patients with T2D look refractory to the inhibitory effect of meal on obestatin secretion.

Ghrelin/obestatin ratio in two populations with low bodyweight: constitutional thinness and anorexia nervosa

Constitutional thinness (CT) and anorexia nervosa (AN) are two categories of severely underweight subjects. Some appetite-regulating hormones display opposite levels in AN and CT. While levels of ghrelin, an orexigenic hormone, fit with the normal food intake in CT, the lack of efficacy of increased ghrelin levels in AN is not clear. Obestatin is a recently described peptide derived from the preproghrelin gene, reported to inhibit appetite in contrast to ghrelin. The aim of this study was to determine whether the circadian profile of obestatin, total and acylated ghrelin levels is different in CT subjects when compared with AN patients. Six-points circadian profiles of plasma obestatin, acylated ghrelin, total ghrelin and other hormonal and nutritional parameters were evaluated in four groups of young women: 10 CT, 15 restricting-type AN, 7 restored from AN and 9 control subjects. Obestatin circadian levels were significantly higher in AN (p<0.0001) while no difference was found between CT and control subjects. Acylated and total ghrelin were found increased in AN. Acylated ghrelin/obestatin and total ghrelin/obestatin were found decreased in AN compared to CT or C subjects (p<0.05). The percentage of acylated ghrelin was found decreased in CT group (p<0.05). The decreased ghrelin/obestatin ratio found in AN might participate in the restraint in nutriment intake of these patients. In contrast, in CT a lower percentage of acylated over total ghrelin might be considered in the aetiology of this condition.

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.

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.

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.