Acylated and unacylated ghrelin impair skeletal muscle atrophy in mice

Cachexia is a wasting syndrome associated with cancer, AIDS, multiple sclerosis, and several other disease states. It is characterized by weight loss, fatigue, loss of appetite, and skeletal muscle atrophy and is associated with poor patient prognosis, making it an important treatment target. Ghrelin is a peptide hormone that stimulates growth hormone (GH) release and positive energy balance through binding to the receptor GHSR-1a. Only acylated ghrelin (AG), but not the unacylated form (UnAG), can bind GHSR-1a; however, UnAG and AG share several GHSR-1a-independent biological activities. Here we investigated whether UnAG and AG could protect against skeletal muscle atrophy in a GHSR-1a-independent manner. We found that both AG and UnAG inhibited dexamethasone-induced skeletal muscle atrophy and atrogene expression through PI3Kβ-, mTORC2-, and p38-mediated pathways in myotubes. Upregulation of circulating UnAG in mice impaired skeletal muscle atrophy induced by either fasting or denervation without stimulating muscle hypertrophy and GHSR-1a-mediated activation of the GH/IGF-1 axis. In Ghsr-deficient mice, both AG and UnAG induced phosphorylation of Akt in skeletal muscle and impaired fasting-induced atrophy. These results demonstrate that AG and UnAG act on a common, unidentified receptor to block skeletal muscle atrophy in a GH-independent manner.

Ghrelin, cholecystokinin, and peptide YY in Atlantic salmon (Salmo salar): molecular cloning and tissue expression

Gastrointestinal (GI) peptide hormones, ghrelin (GHRL), cholecystokinin (CCK), and peptide YY (PYY) genes were identified in Atlantic salmon, Salmo salar. Full-length cDNAs encoding two isoforms of GHRL (GHRL-1 and GHRL-2), two isoforms of CCK (CCK-L and CCK-N) and peptide YY (PYY) cDNA were obtained. The GHRL-1 and GHRL-2 genes encoded proteins of 111- and 108-amino acids, respectively. Both types of GHRL were mainly expressed in the stomach, but also weakly expressed in the pyloric caeca, mid-gut, adipose tissue, and testis. The CCK-L and CCK-N genes encoded preproproteins of 132- and 140-amino acids, respectively. Both types of CCK were strongly expressed in the brain and comparatively weakly expressed in other tissues, including the digestive tract. In the digestive tract, CCK-L was mainly expressed in the pyloric caeca and hind-gut, while CCK-N was only expressed in the pyloric caeca. The PYY gene encoded for 97-amino acid residues and was mainly expressed in the brain and anterior part of the intestine, including the pyloric caeca. In an experiment, we demonstrated that 6 days starvation led to, increased GHRL-1 mRNA levels in the GI tract (stomach), while there no significant changes in expression levels for the other hormones in the GI tract. This suggests an orexigenic role for GHRL-1 in Atlantic salmon. These data contribute to elucidate the functional relationships among teleost gastrointestinal peptide hormones.

Unacylated ghrelin and obestatin increase islet cell mass and prevent diabetes in streptozotocin-treated newborn rats

The ghrelin gene products, namely acylated ghrelin (AG), unacylated ghrelin (UAG), and obestatin (Ob), were shown to prevent pancreatic beta-cell death and to improve beta-cell function under treatment with cytokines, which are major cause of beta-cell destruction in diabetes. Moreover, AG had been described previously to prevent streptozotocin (STZ)-induced diabetes in rats; however, the effect of either UAG or Ob has never been examined in this context. In the present study, we investigated the potential of UAG and Ob to increase islet beta-cell mass and to reduce diabetes at adult age in STZ-treated neonatal rats. One-day-old rats were injected with STZ and subsequently administered with either AG, UAG or Ob for 7 days. On day 70, plasma glucose levels, plasma and pancreatic insulin levels, pancreatic islet area and number, insulin and pancreatic/duodenal homeobox-1 (Pdx1) gene expression, and antiapoptotic BCL2 protein expression were determined. Similarly to AG, both UAG and Ob counteracted STZ-induced high glucose levels and improved plasma and pancreatic insulin levels, which were reduced by the diabetogenic compound. UAG and Ob increased islet area, islet number, and beta-cell mass with respect to STZ treatment alone. Finally, in STZ-treated animals, UAG and Ob up-regulated insulin and Pdx1 mRNA and increased the expression of BCL2 similarly to AG. Taken together, our results suggest that in STZ-treated newborn rats, UAG and Ob improve glucose metabolism and preserve islet cell mass, granting a therapeutic potential in medical conditions associated with impaired beta-cell function.

Polymersomes conjugated with des-octanoyl ghrelin and folate as a BBB-penetrating cancer cell-targeting delivery system

Chemotherapy for brain cancer tumors remains a big challenge for clinical medicine due to the inability to transport sufficient drug across the blood-brain barrier (BBB) and the poor penetration of drug into the tumors. To effectively treat brain tumors and reduce side effects on normal tissues, both des-octanoyl ghrelin and folate conjugated with polymersomal doxorubicin (GFP-D) was developed in this study to help transport across the BBB and target the tumor as well. The size measurements revealed that this BBB-penetrating cancer cell-targeting GFP-D was about 85 nm. In-vitro experiments with a BBB model and C6 glioma cells demonstrated that GFP-D owned a robust penetrating-targeting function for drug delivery. In C6 cell viability tests, GFP-D exhibited an inhibitory effect significantly different from the unmodified polymersomal doxorubicin (P-D). In-vivo antitumor experiments showed that GFP-D performed a much better anti-glioma effect and presented a significant improvement in the overall survival of the tumor-bearing mice as compared to the treatments with free doxorubicin (Dox), liposomal doxorubicin (L-D), P-D, or single ligand conjugated P-D. In addition, Cy 5.5 was used as a probe to investigate the delivery property of this penetrating-targeting delivery system. The overall experimental results indicate that this BBB-penetrating cancer cell-targeting GFP is a highly potential nanocarrier for the treatment of brain tumors.

Effects of short-term starvation on ghrelin, GH-IGF system, and IGF-binding proteins in Atlantic salmon

The effects of short-time fasting on appetite, growth, and nutrient were studied in Atlantic salmon (Salmo salar) smolts. Feed deprivation did change the energy metabolism with reduced plasma protein and muscle indispensible amino acid levels. Plasma levels of ghrelin were significantly higher in starved salmon compared with fed fish after 2 days, but no differences in circulating ghrelin were found between treatments after 14 days. Two mRNA sequences for ghrelin-1 and ghrelin-2, 430 and 533 bp long, respectively, were detected. In addition, the growth hormone secretagogues-receptor like receptor (GHSR-LR) 1a and 1b were identified. Ghrelin-1 but not ghrelin-2 mRNA levels were affected by starvation in the stomach. Lower ghrelin-1 mRNA levels were detected at day 2 in starved fish compared with fed fish. The mRNA levels of GHSR-LR1a were not affected by starvation. Fasting reduced the phenotypic growth and the transcription of insulin-like growth factor (IGF)-II together with IGF-IIR, but IGF-I mRNA were not regulated in fasted salmon after 14 days. Three IGF-binding proteins (IGFBP) at 23, 32, and 43 kDa were found in salmon, and circulating 23 kDa was significantly increased after 14 days of starvation compared with fed fish, indicating increased catabolism. The levels of IGFBP-1 mRNA were significantly higher in fed and starved fish after 14 days compared to those at the start of the experiment, but no significant difference was observed between the treatments. In conclusion, we have shown that circulating ghrelin and ghrelin-1 mRNA is related to changes in energy metabolism in Atlantic salmon.

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.

Effect of HTST and Holder Pasteurization on the Concentration of Immunoglobulins, Growth Factors, and Hormones in Donor Human Milk

Donor human milk (DHM) is submitted to Holder pasteurization (HoP) to ensure its microbiological safety in human milk banks but this treatment affects some of its bioactive compounds. The objective of this work was to compare the effects of HoP and high temperature short time (HTST) treatments on some bioactive compounds found in DHM. A total of 24 DHM batches were processed in a continuous HTST system (70, 72, and 75°C for 5-25 s) and by HoP (62.5°C for 30 min). The concentrations of immunoglobulins (Igs) A, G, and M, transforming growth factor-beta 2 (TGF-β₂), adiponectine, ghrelin, and leptin were measured using a multiplex system, whereas the concentration of epidermal growth factor (EGF) was determined by ELISA. In relation to Igs, IgG showed the highest preservation rates (87-101%) after HTST treatments, followed by IgA (54-88%) and IgM (25-73%). Ig retention after any of the HTST treatments was higher than after HoP (p < 0.001). Treatment times required to reduce the concentration of IgM by 90% (D-value) were 130, 88, and 49 s at 70, 72, and 75°C, while the number of degrees Celsius required to change the D-value by one factor of 10 (z-value) was 11.79°C. None of the heat treatments had a significant effect on the concentrations of TGF-β₂, EGF, adiponectin, and ghrelin. In contrast, leptin was detected only in 4 of the samples submitted to HoP, whereas it was present in all samples after the different HTST treatments, with retention rates ranging between 34 and 68%. Globally, the concentration of IgA, IgG, IgM, and leptin in DHM was significantly higher after HTST pasteurization performed in a continuous system designed to be used in human milk banks than after the HoP procedure that is routinely applied at present.

Recent advances in the phylogenetic study of ghrelin

To understand fully the biology of ghrelin, it is important to know the evolutionary history of ghrelin and its receptor. Phylogenetic and comparative genomic studies of mammalian and non-mammalian vertebrates are a useful approach to that end. Ghrelin is a hormone that has apparently evaded natural selection during a long evolutionary history. Surely ghrelin plays crucial physiological roles in living animals. Phylogenetic studies reveal the nature and evolutionary history of this important signaling system.

The role of ghrelin in the regulation of glucose homeostasis

Ghrelin is a 28-amino acid (aa) stomach-derived peptide discovered in 1999 as the endogenous ligand for growth hormone secretagogue-receptor (GHS-R). Ghrelin-producing cells constitute a distinct group of endocrine cells dispersed throughout the gastric mucosa and to a lesser extent in the small intestine and the endocrine pancreas. Ghrelin plasma levels rise during fasting and chronic caloric restriction to stimulate food intake and fat storage and to prevent life-threatening falls in blood glucose. Plasma ghrelin levels decrease after a meal is consumed and in conditions of energy surplus (such as obesity). Ghrelin has emerged as a key player in the regulation of appetite and energy homeostasis. Ghrelin achieves these functions through binding the ghrelin receptor GHS-R in appetite-regulating neurons and in peripheral metabolic organs including the endocrine pancreas. Ghrelin levels are negatively correlated with body mass index (BMI) and insulin resistance. In addition, ghrelin secretion is impaired in obesity and insulin resistance. Several studies highlight an important role for ghrelin in glucose homeostasis. Genetic, immunological, and pharmacological blockade of ghrelin signaling resulted in improved glucose tolerance and insulin sensitivity. Furthermore, exogenous ghrelin administration was shown to decrease glucose-induced insulin release and increase glucose level in both humans and rodents. GHS-R was shown to be expressed in pancreatic β-cells and ghrelin suppressed insulin release via a Ca2+-mediated pathway. In this review, we provide a detailed summary of recent advances in the field that focuses on the role of insulin and insulin resistance in the regulation of ghrelin secretion and on the role of ghrelin in glucose-stimulated insulin secretion (GSIS).

Molecular characterization of ghrelin and gastrin-releasing peptide in Atlantic cod (Gadus morhua): cloning, localization, developmental profile and role in food intake regulation

Full-length complementary deoxyribonucleic acid as well as genomic sequences encoding for two gastrointestinal appetite-related peptides, ghrelin and for gastrin-releasing peptide (GRP) were cloned from Atlantic cod (Gadus morhua) stomach using reverse transcription and rapid amplification of complementary deoxyribonucleic acid ends. Semi-quantitative reverse transcriptase polymerase chain reaction shows that both ghrelin and GRP are widely distributed in several peripheral tissues and throughout cod brain, although expression levels are very low. During development, ghrelin was detected at the cleavage stage, with low expression levels persisting until the first-feeding stage, while GRP was detected at the blastula stage, showing increased expression from the pre-hatching stage on. Juvenile cod fed medium rations displayed periprandial changes in gut ghrelin, but not GRP, expression, with higher expression levels at meal time compared to 2h before feeding time. Ghrelin gut mRNA expression was not affected by rations, whereas GRP gut mRNA expression was higher in fish fed high rations as compared to fish fed low rations. Neither ghrelin nor GRP gut mRNA expressions were affected by 30 days starvation or 5 days re-feeding.

A practical guide for the stabilization of acylghrelin in human blood collections

OBJECTIVE AND METHODS

To better understand acylghrelin plasma stability, human synthetic acylghrelin was spiked into plasma and tracked by liquid chromatography tandem mass spectrometry. To investigate the best method for quantifying clinical plasma acylghrelin levels, pre- and postprandial human blood was collected from healthy volunteers (n=6) using various sample collections and treatments. Plasma ghrelin levels from human blood collections were analysed by enzyme-linked immunosorbant assay (ELISA).

RESULTS

Acylghrelin's half-life in plasma was approximately 45 min with the formation of des-acylghrelin approaching 50% before the end of the 60-min incubation. Loss of acylghrelin inversely correlated with an increase in des-acylghrelin (P<0.008; r(2) =0.870). Plasma pretreated with 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF) or protease inhibitor cocktail without acidification resulted in no detectible acylghrelin losses. Acylghrelin measurements with AEBSF-treated blood were minimally 40% higher than sodium citrate/citric acid, K(2) EDTA, aprotinin/HCl and P800 collections. HCl addition to AEBSF-treated plasma did not provide enhanced acylghrelin stability and induced deacylation at and above the 100 mM final concentration. Pre- and postprandial ghrelin attenuation was investigated using aprotinin/HCl, AEBSF, protease inhibitor cocktail and no treatment for blood and plasma preparations. Fasting samples treated with AEBSF and protease inhibitor cocktail were approximately threefold higher than aprotinin/HCl and control treatments (P<0.03). Pre- and postprandial ghrelin attenuation was approximately twofold different (P<0.04) with significant counterintuitive trends in aprotinin/HCl and K(2) EDTA groups.

CONCLUSIONS

Our data suggest that AEBSF addition to K(2) EDTA blood immediately after collection without plasma acidification, processing on ice and 14-day 70 °C storage is the best treatment for accurately quantifying acylghrelin in human plasma.

Is desacyl ghrelin a modulator of food intake?

Desacyl ghrelin is produced in the gastric mucosa and plasma by deacylation of ghrelin. It occurs in considerably larger amounts than ghrelin in various regions in the organisms of rats and mice. It exerts biological activities in vitro as different as stimulating adipogenesis or inhibiting glucose output in hepatocytes. In fasted rats, desacyl ghrelin levels decreased under catabolic metabolic conditions and in mice, high desacyl ghrelin concentrations went along with decreased food intake. These observations suggest an influence of the peptide on food intake and energy homeostasis. Behavioral studies led to controversial results, but several suggest an anorexigenic effect. Studies on desacyl ghrelin-induced modulation of food intake indicate the involvement of central nervous pathways, since it is said to cross the blood-brain barrier and to induce increased neuronal activity hypothalamic nuclei. It is likely to be involved in the regulation of the synthesis of anorexigenic hypothalamic mediators. Quite possibly, there might be means of interaction between desacyl ghrelin and its supposable precursor ghrelin.

Structures and molecular forms of the ghrelin-family peptides

Ghrelin is an acylated peptide hormone produced mainly from the stomach. The major active products of the ghrelin gene in the stomach of rats, mice and humans are 28-amino acid peptides acylated at the serine-3 position with an n-octanoyl group (C8:0), called simply ghrelin. However, recent studies have revealed that the ghrelin gene can generate a variety of bioactive molecules besides ghrelin. These include acyl forms of ghrelin other than C8:0-ghrelin (i.e., n-decanoyl ghrelin or n-decenoyl ghrelin), des-acyl ghrelin, obestatin and ghrelin-associated peptides originated from the ghrelin gene. This review surveys the structures of the ghrelin peptides and molecular forms of ghrelin gene-derived products, and summarizes the knowledge about the functions of these peptides, with an emphasis on the acyl forms of the ghrelin peptide.

The expanding roles of the ghrelin-gene derived peptide obestatin in health and disease

Obestatin is a 23 amino acid, ghrelin gene-derived peptide hormone produced in the stomach and a range of other tissues throughout the body. While it was initially reported that obestatin opposed the actions of ghrelin with regards to appetite and food intake, it is now clear that obestatin is not an endogenous ghrelin antagonist, but it is a multi-functional peptide hormone in its own right. In this review we will discuss the controversies associated with the discovery of obestatin and explore emerging central and peripheral roles of obestatin, which includes adipogenesis, pancreatic homeostasis and cancer.

Ontogenetic and tissue-specific expression of preproghrelin in the Atlantic halibut, Hippoglossus hippoglossus L

Ghrelin is a conserved vertebrate hormone that affects both GH release and appetite. We have cloned and characterized Atlantic halibut preproghrelin cDNA and examined for the first time preproghrelin expression during fish larval development using quantitative real-time PCR. In addition, cellular sites of expression in larvae and tissue-specific expression in 3-year-old halibut were studied. A full-length cDNA for preproghrelin was isolated from halibut stomach tissue. The 899 bp cDNA encodes an open reading frame of 105 amino acids that is comprised of a signal peptide and two peptides with high similarity to ghrelin and obestatin. The deduced amino acid sequence of halibut ghrelin peptide (GSSFLSPSHKPPKGKPPRA) shows significant conservation relative to other teleostean sequences and is identical to human ghrelin for the first seven amino acids of the sequence. The putative obestatin peptide is well-conserved among fishes but shares limited similarity with its human counterpart. Expression of ghrelin was localized to two different cell types in the stomach of larval halibut by in situ hybridization. However, sensitive PCR assays on tissues collected from 3-year-old fish additionally identified ghrelin transcripts in pyloric caecae, intestine, and in immature ovary and testis. Ontogenetic studies detected ghrelin expression prior to exogenous feeding during larval development (hatching and mouth-opening stages) with increased expression occurring through metamorphosis. This increase was pronounced during climax metamorphosis and coincided with stomach differentiation. Patterns of preproghrelin expression suggest that ghrelin has important roles during and after larval development in halibut, and that ghrelin is associated with digestive and gonadal tissues in this teleost.

Effects of Cigarette Smoking on Plasma Concentration of the Appetite-Regulating Peptide Ghrelin

BACKGROUND

Weight gain is a common but only a partially understood consequence of smoking cessation. Existing data suggest modulating effects of the orexigenic peptide ghrelin on food intake. The aim of the present study was to investigate the effect of tobacco withdrawal on plasma concentration of acetylated and total ghrelin.

METHODS

Fifty four normal-weighted smokers and 30 non-smoking healthy controls were enrolled in our study. Concentrations of acetylated and total ghrelin were measured in blood plasma drawn two hours after a standardized meal and three hours after the smokers smoked their last cigarette. The severity of tobacco addiction was assessed based on cotinine plasma concentration, the Fagerström Test for Nicotine Dependence (FTND) and the number of cigarettes smoked per day.

RESULTS

The plasma concentration of acetylated ghrelin, but not total ghrelin, was significantly higher in smokers than in non-smokers. Moreover, we found significant negative correlations between acetylated ghrelin and all measures of the severity of nicotine dependence.

CONCLUSIONS

Early abstinence from tobacco smoking seems to be associated with increased plasma concentration of the orexigenic peptide acetylated ghrelin. This could be one reason for increased food craving during nicotine withdrawal and subsequent weight gain. Smokers might compensate these effects by increasing tobacco intake.

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.

Acylated and unacylated ghrelin administration to blunt muscle wasting

PURPOSE OF REVIEW

Muscle wasting is a comorbidity often associated with a wide range of disorders that severely affects patient prognosis and quality of life. Ghrelin, through its receptor GHSR-1a, stimulates appetite and growth hormone (GH) release. Several studies indicate that ghrelin administration is a valid treatment for cachexia because it improves muscle mass and function, likely by restoring a positive energy balance.

RECENT FINDINGS

In addition to its GHSR-1a-mediated effects on muscle mass, ghrelin acts directly on skeletal muscle, wherein it exerts a protective activity against muscle wasting. This direct activity is independent of GHSR-1a and is shared by the unacylated form of ghrelin, which does not bind GHSR-1a and is devoid of the effects on appetite and GH release.

SUMMARY

Both the acylated and unacylated forms of ghrelin might have therapeutic potential for the treatment of skeletal muscle wasting.

Ghrelin Inhibits Post-Operative Adhesions via Blockage of the TGF-β Signaling Pathway

Post-operative adhesions are a critical problem in pelvic and abdominal surgery despite a multitude of studies dedicated to finding modalities to prevent their occurrence. Ghrelin administration promotes an anti-fibrotic response in a surgical mouse model of adhesion-induction, but the mechanisms mediating this effect have not been established. In the current study, the molecular mechanisms that underlie the anti-adhesion effect of ghrelin were investigated. Post-surgical adhesions were experimentally created in C57BL/6 wild-type mice via a combination of ischemic peritoneal buttons and cecal multiple abrasions. Ghrelin or saline intraperitoneal injections were given twice daily from two days before surgery to selected time points post-surgically to assess the phenotypic and molecular effects of treatment (1 day (n = 20), 4 days (n = 20) and 20 days (n = 40) after surgery). Endpoints included the scoring of adhesions and gene and protein expression analysis of pro-fibrogenic factors conducted on peritoneal ischemic tissue by quantitative PCR and Western blot. Ghrelin administration significantly reduced post-surgical adhesions and down-regulated pro-inflammatory gene and protein expression, including Tgfb3 and Tgfbr2. The up-regulation of inhibitory proteins Smad6 and Smad7 confirmed the ghrelin-induced blockage of TGF-β signaling. Ghrelin is a candidate therapeutic drug for post-operative adhesion prevention, inhibiting inflammatory responses via blockage of the TGF-β signaling pathway at the onset of surgery before the occurrence of the granulation-remodeling phase.

Genetic studies on the ghrelin, growth hormone secretagogue receptor (GHSR) and ghrelin O-acyl transferase (GOAT) genes

Ghrelin is a 28 amino acid peptide hormone that is produced both centrally and peripherally. Regulated by the ghrelin O-acyl transferase enzyme, ghrelin exerts its action through the growth hormone secretagogue receptor, and is implicated in a diverse range of physiological processes. These implications have placed the ghrelin signaling pathway at the center of a large number of candidate gene and genome-wide studies which aim to identify the genetic basis of human heterogeneity. In this review we summarize the available data on the genetic variability of ghrelin, its receptor and its regulatory enzyme, and their association with obesity, stature, type 2 diabetes, cardiovascular disease, eating disorders, and reward seeking behavior.

Ghrelin O-acyltransferase assays and inhibition

Ghrelin O-acyltransferase (GOAT) is responsible for catalyzing the attachment of the eight-carbon fatty acid octanoyl to the Ser3 side chain of the peptide ghrelin to generate the active form of this metabolic hormone. As such, GOAT is viewed as a potential therapeutic target for the treatment of obesity and diabetes mellitus. Here, we review recent progress in the development of cell and in vitro assays to measure GOAT action and the identification of several synthetic GOAT inhibitors. In particular, we discuss the design, synthesis, and characterization of the bisubstrate analog GO-CoA-Tat and its ability to modulate weight and blood glucose in mice. We also highlight current challenges and future research directions in our biomedical understanding of this fascinating ghrelin processing enzyme.

Ghrelin axis genes, peptides and receptors: recent findings and future challenges

The ghrelin axis consists of the gene products of the ghrelin gene (GHRL), and their receptors, including the classical ghrelin receptor GHSR. While it is well-known that the ghrelin gene encodes the 28 amino acid ghrelin peptide hormone, it is now also clear that the locus encodes a range of other bioactive molecules, including novel peptides and non-coding RNAs. For many of these molecules, the physiological functions and cognate receptor(s) remain to be determined. Emerging research techniques, including proteogenomics, are likely to reveal further ghrelin axis-derived molecules. Studies of the role of ghrelin axis genes, peptides and receptors, therefore, promises to be a fruitful area of basic and clinical research in years to come.

Ghrelin and its biological effects on pigs

Ghrelin is a 28 amino acid peptide, which produces its marked effects through binding to the endogenous ligand of the growth hormone secretagogue receptor (GHS-R). Based on the contemporary literatures, it was shown that ghrelin was involved in a series of biological functions including regulation of food intake, body weight, gastrointestinal (GI) motility, hormone secretion, glucose release, cardiovascular functions, enzyme release, cell proliferation and reproduction in pigs through binding to GHS-R 1a or unidentified receptors. It was also observed that ghrelin induced adipocyte and hepatocyte proliferation of primary cultured piglet. In this paper, recent research on ghrelin structure, distribution, GHS-R receptor, biological functions and its regulatory mechanisms for pigs are presented.

Metabolic responses to exogenous ghrelin in obesity and early after Roux-en-Y gastric bypass in humans

AIMS

Ghrelin is a gastric-derived hormone that stimulates growth hormone (GH) secretion and has a multi-faceted role in the regulation of energy homeostasis, including glucose metabolism. Circulating ghrelin concentrations are modulated in response to nutritional status, but responses to ghrelin in altered metabolic states are poorly understood. We investigated the metabolic effects of ghrelin in obesity and early after Roux-en-Y gastric bypass (RYGB).

MATERIALS AND METHODS

We assessed central and peripheral metabolic responses to acyl ghrelin infusion (1 pmol kg-1  min-1 ) in healthy, lean subjects (n = 9) and non-diabetic, obese subjects (n = 9) before and 2 weeks after RYGB. Central responses were assessed by GH and pancreatic polypeptide (surrogate for vagal activity) secretion. Peripheral responses were assessed by hepatic and skeletal muscle insulin sensitivity during a hyperinsulinaemic-euglycaemic clamp.

RESULTS

Ghrelin-stimulated GH secretion was attenuated in obese subjects, but was restored by RYGB to a response similar to that of lean subjects. The heightened pancreatic polypeptide response to ghrelin infusion in the obese was attenuated after RYGB. Hepatic glucose production and hepatic insulin sensitivity were not altered by ghrelin infusion in RYGB subjects. Skeletal muscle insulin sensitivity was impaired to a similar degree in lean, obese and post-RYGB individuals in response to ghrelin infusion.

CONCLUSIONS

These data suggest that obesity is characterized by abnormal central, but not peripheral, responsiveness to ghrelin that can be restored early after RYGB before significant weight loss. Further work is necessary to fully elucidate the role of ghrelin in the metabolic changes that occur in obesity and following RYGB.

The peptide hormone ghrelin binds to membrane-mimetics via its octanoyl chain and an adjacent phenylalanine

The peptide hormone ghrelin, which is the natural ligand of the membrane-bound growth hormone secretagogue receptor (GHS-R), regulates overall body and cell growth, energy homeostasis, carbohydrate, protein and lipid metabolism and water electrolyte balance. It contains an O-acyl linked octanoyl group on Ser3 and is the only peptide known to contain such a modification. Using solution state NMR spectroscopy and ultrafiltration we found that human ghrelin binds to membrane-mimetic environments via its octanoyl group as well as the aromatic moiety of Phe4. Relaxation enhancements in a paramagnetic environment reveal that both the octanoyl group on Ser3 and the aromatic group on Phe4 are inserted deep into the hydrophobic core of phosphocholine assemblies while the remaining peptide is freely mobile in solution. In contrast, no binding was observed for des-octanoyl ghrelin. Thus, the octanoyl chain, together with the Phe4 aromatic group of ghrelin, functions as a membrane anchor. Our results are in parallel with the previous finding that a bulky hydrophobic group on Ser3 and Phe4 of ghrelin are necessary for its function and thus indicate that membrane-binding is essential for ghrelin function.