Gastric Hormone Research Articles

Decreased ghrelin and des-acyl ghrelin plasma levels in patients affected by pharmacoresistant epilepsy and maintained on the ketogenic diet

The gastric hormones ghrelin and des-acyl ghrelin have been found to be altered in patients treated with antiepileptic drugs. However, it is unknown if these hormones could be modified by other antiepileptic treatments, such as the ketogenic diet. Especially, a reduction in ghrelin levels could be relevant in view of the growth retardation observed under ketogenic diet treatment. For this reason we aimed to determine the changes in ghrelin and des-acyl ghrelin plasma levels in children affected by refractory epilepsy and treated with the ketogenic diet up to 90 days. Both peptides were measured by immunoassays in plasma obtained from 16 children. Ghrelin plasma levels were progressively reduced by the ketogenic diet, reaching a minimum corresponding to 42% of basal levels after 90 days of ketogenic diet (P<0.05, Duncan's test). Des-acyl ghrelin plasma levels were similarly affected, reaching minimal levels at 30 days (65% of basal levels), and maintaining a significant reduction until 90 days after the onset of ketogenic diet (P<0.01 for both time intervals). No significant changes in growth were observed during the monitored period of ketogenic diet administration. Ghrelin and des-acyl ghrelin are downregulated by the ketogenic diet in children affected by refractory epilepsy. Although no significant changes in growth were observed during the short time period of our investigation, the reduction in ghrelin availability may explain the reported growth retardation found in children treated with the ketogenic diet in the long-term.

The preterm infant stomach actively degrades milk proteins with increasing breakdown across digestion time.

This study investigated the effect of time post-ingestion on gastric digestion and gastric hormones after feeding preterm infants unfortified and fortified human milk. Human milk and infant gastric samples were collected from 14 preterm (23-32weeks birth gestational age) mother-infant pairs within 7-98days postnatal age. Gastric samples were collected one, two and threehours after beginning of feeding. Samples were analysed for pH, proteolysis, general protease activity and the concentrations of pepsin, gastrin and gastrin-releasing peptide (GRP). One-way ANOVA with repeated measures followed by Tukey's multiple comparisons test was used. Gastric pH was significantly decreased after each hour in the preterm infant stomach from one to threehours postprandial. Proteolysis increased significantly from human milk to gastric contents at one, two and threehours postprandial (by 62, 131% and 181%, p<0.05). General protease activity increased significantly by 58% from human milk to the gastric contents at twohours postprandial. GRP was present in human milk, whereas gastrin was produced in the infant stomach. Although preterm infants may digest human milk proteins to a lesser extent than term infants, we demonstrated that the preterm infant stomach actively degrades milk proteins with increasing breakdown over digestion time.

Effect of rivastigmine on plasma butyrylcholine esterase activity and plasma ghrelin levels in patients with dementia in Alzheimer's disease.

Alzheimer's disease causes loss of appetite, resulting in bodyweight reduction. This, in turn, causes progression of cognitive dysfunction and physical complications that hasten death. Earlier care for loss of appetite is essential in Alzheimer's disease management. Rivastigmine is a therapeutic agent for Alzheimer's disease that has dual inhibition effects on acetylcholine esterase and butyrylcholine esterase. Butyrylcholine esterase is known to degrade the gastric hormone, ghrelin, which regulates appetite; therefore, we considered that rivastigmine might have an effect on appetite. The present study aimed to evaluate the hypothesis that rivastigmine improves appetite in Alzheimer's disease patients. Rivastigmine was given to mild-to-moderate Alzheimer's disease patients for 16 weeks. We evaluated the effects of rivastigmine on food intake, bodyweight, motivation (estimated by the vitality index), cognition function (estimated by the Hasegawa Dementia Scale-Revised), plasma butyrylcholine esterase activity, active ghrelin and inactive ghrelin. Plasma butyrylcholine esterase activity significantly decreased over time (percent change: -18.9 ± 27.0%, P < 0.05 at week 8; percent change: -33.4 ± 45.4%, P < 0.05 at week 16). Negative correlations were detected between percent changes in butyrylcholine esterase activity and active ghrelin (rs = -0.62, P = 0.033) or active/inactive ghrelin ratio (rs = -0.73, P = 0.007). Furthermore, motivation (including appetite) improved significantly (percent change: 17.9 ± 18.6%, P < 0.05 at week 16). The present study suggests that rivastigmine might improve appetite in mild-to-moderate Alzheimer's disease patients by suppressing degradation of plasma active ghrelin through the inhibition of plasma butyrylcholine esterase. Geriatr Gerontol Int 2018; 18: 886-891.

Ghrelin Stimulates Hepatocyte Proliferation via Regulating Cell Cycle Through GSK3β/Β-Catenin Signaling Pathway

Background/Aims: Obesity is associated with a reduction in ghrelin, a 28 aa gastric hormone. Whether reduced ghrelin contributes to the impaired proliferation of hepatocytes associated with obesity-related steatosis remains largely unknown. Here we examined the effects of ghrelin on the proliferation of hepatocytes derived from lean and obese mice. Methods: AML 12 cells or hepatocytes isolated from mice fed normal chow diet (NCD) or high fat diet (HFD) were used. Effects of ghrelin on hepatocyte proliferation were detected with CCK8 assay and EdU staining. Cell cycle was analyzed by flow cytometry. Levels of proliferation markers was examined by Western blot. Results: Growth hormone secretagogue receptor 1a (GHS-R1a) mRNA and protein were present in hepatocytes. Levels of GHS-R1a were increased upon ghrelin treatment. Ghrelin significantly increased hepatocyte proliferation measured by Cell Counting Kit-8(CCK8) assay and EdU staining in a dose- and time-dependent manner. Proportion of cells in S phase was markedly increased upon treatment with ghrelin. Ghrelin significantly increased levels of proliferating cell nuclear antigen (PCNA) and cyclin D1, while reducing p27 in hepatocytes from mice fed NCD or HFD. Deletion of GHS-R1a completely abolished the effects of ghrelin in cultured hepatocytes. Ghrelin stimulated the phosphorylation of glycogen synthase kinase 3 beta (GSK3β), leading to subsequent increase of nuclear β-catenin in hepatocytes derived from lean and obese mice. This effect was dependent on the GHS-R1a. Conclusion: Ghrelin activates GHS-R1a to stimulate hepatocyte proliferation via GSK3/β-catenin signaling pathway.

Effects of growth hormone-releasing hormone gene targeted ablation on ghrelin-induced feeding

Impairment of growth hormone (GH) signaling has been associated with increased feeding and adiposity. The gastric hormone ghrelin, in addition to its GH-secretagogue effects, stimulates food intake after both central and peripheral administration. In the present study we further investigated the feeding regulatory role of the ghrelin-GH axis in a mouse model of isolated GH deficiency due to targeted ablation of the GH-releasing hormone (GHRH) gene [GHRH knockout (GHRHKO)]. We evaluated the effects of intracerebroventricular ghrelin administration on feeding behavior, related hypothalamic neuropeptides and neurotransmitters, and serum ghrelin levels in mice homozygous for GHRHKO allele (−/−) and heterozygous (+/−) control animals. Vehicle-treated GHRHKO mice showed increased food intake compared to heterozygotes, associated with increased circulating ghrelin levels. Moreover, −/− mice showed elevated hypothalamic levels of neuropeptide Y (NPY), agouti-related peptide (AgRP) mRNAs and norepinephrine (NE) and decreased corticotropin-releasing hormone (CRH) mRNA levels. Ghrelin treatment significantly augmented food intake in both genotypes, but the relative increase compared to vehicle-treated animals was higher in −/− than +/− mice. In the hypothalamus, ghrelin increased AgRP and decreased CRH gene expression only in heterozygous mice, while it induced a significant reduction in proopiomelanocortin (POMC) mRNA levels in −/− mice. Ghrelin treatment also decreased hypothalamic serotonin (5-hydroxytriptamine, 5-HT) and dopamine (DA) levels in both genotypes. Additionally, we observed increased DA metabolism induced by ghrelin in both genotypes. In conclusion, dysregulation of the ghrelin-GHRH-GH axis in GHRHKO mice could lead to increased feeding secondary to elevated circulating levels of ghrelin, and the obesogenic phenotype is likely mediated by elevated NPY and AgRP, and decreased CRH gene expression in the hypothalamus.

Acylated ghrelin treatment normalizes skeletal muscle mitochondrial oxidative capacity and AKT phosphorylation in rat chronic heart failure.

BackgroundChronic heart failure (CHF) is associated with skeletal muscle abnormalities contributing to exercise intolerance, muscle loss, and negative impact on patient prognosis. A primary role has been proposed for mitochondrial dysfunction, which may be induced by systemic and tissue inflammation and further contribute to low insulin signalling. The acylated form of the gastric hormone ghrelin (AG) may improve mitochondrial oxidative capacity and insulin signalling in both healthy and diseased rodent models.MethodsWe investigated the impact of AG continuous subcutaneous administration (AG) by osmotic minipump (50 nmol/kg/day for 28 days) compared with placebo (P) on skeletal muscle mitochondrial enzyme activities, mitochondrial biogenesis regulators transcriptional expression and insulin signalling in a rodent post‐myocardial infarction CHF model.ResultsNo statistically significant differences (NS) were observed among the three group in cumulative food intake. Compared with sham‐operated, P had low mitochondrial enzyme activities, mitochondrial biogenesis regulators transcripts, and insulin signalling activation at AKT level (P < 0.05), associated with activating nuclear translocation of pro‐inflammatory transcription factor nuclear factor‐κB. AG completely normalized all alterations (P < 0.05 vs P, P = NS vs sham‐operated). Direct AG activities were strongly supported by in vitro C2C12 myotubes experiments showing AG‐dependent stimulation of mitochondrial enzyme activities. No changes in mitochondrial parameters and insulin signalling were observed in the liver in any group.ConclusionsSustained peripheral AG treatment with preserved food intake normalizes a CHF‐induced tissue‐specific cluster of skeletal muscle mitochondrial dysfunction, pro‐inflammatory changes, and reduced insulin signalling. AG is therefore a potential treatment for CHF‐associated muscle catabolic alterations, with potential positive impact on patient outcome.

Unacylated Ghrelin: A Novel Regulator of Muscle Intermediate Metabolism With Potential Beneficial Effects in Chronic Kidney Disease

In patients with chronic kidney disease (CKD), malnutrition with loss of skeletal muscle mass has a negative impact on morbidity and mortality. Emerging evidence indicates that a cluster of oxidative stress, inflammation, and insulin resistance directly contributes to skeletal muscle catabolism by favoring protein breakdown over synthesis. Ghrelin is a gastric hormone discovered and initially studied in its acylated orexigenic form. More recently, a role of unacylated ghrelin (UnAG) has been described to reduce skeletal muscle mitochondrial reactive oxygen species generation, inflammation, and insulin resistance both in experimental models and in clinical studies. UnAG administration could therefore represent a potential comprehensive therapeutic approach for CKD-related metabolic and nutritional complications. Studies of UnAG administration in experimental and clinical CKD are needed to test the hypothesis that UnAG may chronically improve nutritional status and outcome in CKD patients.

Cell cycle dependent expression of the CCK2 receptor by gastrointestinal myofibroblasts: putative role in determining cell migration.

The well‐known action of the gastric hormone gastrin in stimulating gastric acid secretion is mediated by activation of cholecystokinin‐2 receptors (CCK2R). The latter are expressed by a variety of cell types suggesting that gastrin is implicated in multiple functions. During wound healing in the stomach CCK2R may be expressed by myofibroblasts. We have now characterized CCK2R expression in cultured myofibroblasts. Immunocytochemistry showed that a relatively small proportion (1–6%) of myofibroblasts expressed the receptor regardless of the region of the gut from which they were derived, or whether from cancer or control tissue. Activation of CCK2R by human heptadecapeptide gastrin (hG17) increased intracellular calcium concentrations in a small subset of myofibroblasts indicating the presence of a functional receptor. Unexpectedly, we found over 80% of cells expressing CCK2R were also labeled with 5‐ethynyl‐2′‐deoxyuridine (EdU) which is incorporated into DNA during S‐phase of the cell cycle. hG17 did not stimulate EdU incorporation but increased migration of both EdU‐labeled and unlabelled myofibroblasts; the migratory response was inhibited by a CCK2R antagonist and by an inhibitor of IGF receptor tyrosine kinase; hG17 also increased IGF‐2 transcript abundance. The data suggest myofibroblasts express CCK2R in a restricted period of the cell cycle during S‐phase, and that gastrin accelerates migration of these cells; it also stimulates migration of adjacent cells probably through paracrine release of IGF. Together with previous findings, the results raise the prospect that gastrin controls the position of dividing myofibroblasts which may be relevant in wound healing and cancer progression in the gastrointestinal tract.

Impact of ghrelin on body composition and muscle function in a long-term rodent model of critical illness.

BackgroundPatients with multiple injuries or sepsis requiring intensive care treatment invariably develop a catabolic state with resultant loss of lean body mass, for which there are currently no effective treatments. Recovery can take months and mortality is high. We hypothesise that treatment with the orexigenic and anti-inflammatory gastric hormone, ghrelin may attenuate the loss of body mass following critical illness and improve recovery.MethodsMale Wistar rats received an intraperitoneal injection of the fungal cell wall derivative zymosan to induce a prolonged peritonitis and consequent critical illness. Commencing at 48h after zymosan, animals were randomised to receive a continuous infusion of ghrelin or vehicle control using a pre-implanted subcutaneous osmotic mini-pump, and continued for 10 days.ResultsZymosan peritonitis induced significant weight loss and reduced food intake with a nadir at Day 2 and gradual recovery thereafter. Supra-physiologic plasma ghrelin levels were achieved in the treated animals. Ghrelin-treated rats ate more food and gained more body mass than controls. Ghrelin increased adiposity and promoted carbohydrate over fat metabolism, but did not alter total body protein, muscle strength nor muscle morphology. Muscle mass and strength remained significantly reduced in all zymosan-treated animals, even at ten days post-insult.ConclusionsContinuous infusion of ghrelin increased body mass and food intake, but did not increase muscle mass nor improve muscle function, in a long-term critical illness recovery model. Further studies with pulsatile ghrelin delivery or additional anabolic stimuli may further clarify the utility of ghrelin in survivors of critical illness.

Gender-Specific Association of Desacylated Ghrelin with Subclinical Atherosclerosis in the Metabolic Syndrome

Ghrelin, a gastric hormone with pleiotropic effects modulates vascular function and may influence atherosclerosis. Plasma ghrelin is reduced in the metabolic syndrome (MS), which is also characterized by early atherosclerosis. Ghrelin circulates in acylated (AG) and desacylated (DAG) forms. Their relative impact and that of gender on subclinical atherosclerosis in MS is unknown. To investigate potential associations of total, AG and DAG with carotid atherosclerosis and with gender in the MS. Plasma total ghrelin, AG, DAG and carotid artery IMT (cIMT) were measured in 46 MS patients (NCEP-ATP III criteria, 22M/24F). Compared with males, females had higher (p <0.05) total and DAG. In the association analysis, age and plasma glucose were positively (p <0.05) correlated with cIMT in all MS patients. The positive (p <0.05) association between cIMT and age was also confirmed in males, while that between cIMT and glucose was significant in women. In contrast, neither total ghrelin nor AG and DAG were associated with cIMT in all MS patients nor in the male subgroup. In females, a negative (p <0.05) association between carotid artery IMT, DAG and glucose was detected, but not between cIMT, total ghrelin and AG. In multivariate modeling, DAG remained negatively (p <0.05) associated with cIMT after adjusting for plasma glucose and cardiovascular risk factors. These data indicate a negative independent association between DAG and cIMT in middle-aged women with the MS and suggest a gender-specific modulatory function of DAG in the development of atherosclerosis.

Metabolic and Cardiovascular Actions of Nesfatin-1: Implications in Health and Disease.

Metabolic homeostasis is achieved by proper functioning of a complex endocrine milieu, comprising of signals arising from the brain and peripheral tissues. Our knowledge of the factors regulating such balance is rapidly evolving, as new signaling molecules are being characterized. Of central interest is nesfatin-1, owing to its multifunctional tissue specific actions regulating food intake, body weight, blood glucose and cardiac functions. This review discusses the tissue/system wide distribution and actions of nesfatin-1 in regulating metabolic and cardiovascular functions in healthy conditions and diseases. Nesfatin-1 is an 82 amino acid peptide encoded in the secreted precursor, nucleobinin-2 (NUCB2). It was first reported as a novel anorexigen and a fat reducing orphan ligand. Research to date has established nesfatin-1 in the regulation of food intake via modulation of neuropeptides in the feeding centers of brain. Nesfatin-1 expression was also reported to have a spectrum of peripheral tissue specific actions, which includes insulin secretion (pancreas), fat and glucose modulation (adipocyte), gastric motility and gastric acid secretion (stomach), hormone secretion and transit time (intestine) and mean arterial pressure and cardiac injury (heart). Abnormal levels of nesfatin-1 in circulation and/or variations in tissue specific expression have been observed in obesity, diabetes and cardiovascular diseases. The multifunctional biological actions of nesfatin-1 propelled this peptide as a therapeutic target, and as a potential biomarker of diseases. However, a better and comprehensive understanding of tissue specific effects of nesfatin-1 is critical prior to exploring its possible use in the detection and treatment of diseases.

Complexity of Stomach-Brain Interaction Induced by Molecular Hydrogen in Parkinson's Disease Model Mice.

Molecular hydrogen (H2), as a new medical gas, has protective effects in neurological disorders including Parkinson's disease (PD). In our previous report, the neuroprotective effect of drinking water with saturated H2 (H2 water) in PD mice might be due to stomach-brain interaction via release of gastric hormone, ghrelin. In the present study, we assessed the effect of H2-induced ghrelin more precisely. To confirm the contribution of ghrelin in H2 water-drinking PD model mice, ghrelin-knock out (KO) mice were used. Despite the speculation, the effect of H2 water was still observed in ghrelin-KO PD model mice. To further check the involvement of ghrelin, possible contribution of ghrelin-induced vagal afferent effect was tested by performing subdiaphragmatic vagotomy before treating with H2 water and administration of MPTP(1-methyl- 4-phenyl-1,2,3,6-tetrahydropyridine). The protective effect of H2 water was still observed in the vagotomized mice in substantia nigra, suggesting that stimulation of vagal afferent nerves is not involved in H2-induced neuroprotection. Other neuroprotective substitutes in ghrelin-KO mice were speculated because H2-induced neuroprotection was not cancelled by ghrelin receptor antagonist, D-Lys3 GHRP-6, in ghrelin-KO PD model mice, unlike in wild-type PD model mice. Our results indicate that ghrelin may not be the only factor for H2-induced neuroprotection and other factors can substitute the role of ghrelin when ghrelin is absent, raising intriguing options of research for H2-responsive factors.

Effect of shortened sleep on energy expenditure, core body temperature, and appetite: a human randomised crossover trial

The effects of sleep restriction on energy metabolism and appetite remain controversial. We examined the effects of shortened sleep duration on energy metabolism, core body temperature (CBT), and appetite profiles. Nine healthy men were evaluated in a randomised crossover study under two conditions: a 3.5-h sleep duration and a 7-h sleep duration for three consecutive nights followed by one 7-h recovery sleep night. The subjects’ energy expenditure (EE), substrate utilisation, and CBT were continually measured for 48 h using a whole-room calorimeter. The subjects completed an appetite questionnaire every hour while in the calorimeter. Sleep restriction did not affect total EE or substrate utilisation. The 48-h mean CBT decreased significantly during the 3.5-h sleep condition compared with the 7-h sleep condition (7-h sleep, 36.75 ± 0.11 °C; 3.5-h sleep, 36.68 ± 0.14 °C; p = 0.016). After three consecutive nights of sleep restriction, fasting peptide YY levels and fullness were significantly decreased (p = 0.011), whereas hunger and prospective food consumption were significantly increased, compared to those under the 7-h sleep condition. Shortened sleep increased appetite by decreasing gastric hormone levels, but did not affect EE, suggesting that greater caloric intake during a shortened sleep cycle increases the risk of weight gain.

Serum Leptin, Obestatin, and Ghrelin Levels and Gastric Emptying Rates of Liquid and Solid Meals in Non-obese Rats with Roux-en-Y Bypass Surgery or Prosthesis Placement: Implications for the Role of Vagal Afferents.

The present study aimed to investigate the effects of Roux-en-Y gastric bypass (RYGB) and prosthesis placement on gastric emptying rate in conjunction with serum ghrelin-obestatin-leptin responses in non-obese rats with intact or denervated afferent innervation. Under anesthesia, male Sprague-Dawley rats underwent either sham operation, RYGB, prosthesis, and/or Gregory cannula placement. Three weeks later, liquid or solid gastric emptying tests were performed and serum ghrelin, leptin and obestatin levels were measured. Both prosthesis placement and RYGB surgery delayed non-nutrient liquid emptying; while solid nutrient emptying was delayed only by RYGB. Nutrient-dependent (acid, hyperosmolal and peptone) delay in liquid emptying was abolished in rats with prosthesis. By vagal afferent denervation, delayed liquid emptying was abolished, while solid emptying was further delayed in rats with prosthesis. Ghrelin and obestatin levels were depressed in prosthesis-placed rats, but RYGB surgery had no impact on both levels. Leptin level was elevated in solid-food-given rats with prosthesis, but not changed in RYGB group, while it was reduced following liquid meal. All the changes observed in ghrelin, obestatin, or leptin levels in response to meal ingestion were reversed with vagal afferent denervation. Both RYGB and prosthesis placement had delaying effects on gastric emptying rate of non-obese rats. Our results indicate that the short-term changes in gastric motility and hormone responses induced by volume reduction are reversed by afferent denervation, suggesting that sparing the vagal innervation could be essential for reaching optimum motility and hormone changes expected after bariatric surgery.

PS 13-19 A GASTRIC HORMONE GHRELIN IMPROVES PHYSICAL DECLINE THROUGH COMBINED EFFECTS OF MUSCLE ENHANCEMENT AND MITOCHONDRIAL ACTIVATION IN 5/6Nx CKD MODEL MICE

Objective: Chronic kidney disease (CKD) impairs physical performance. A physical decline correlates with an increased risk of a wide range of disease and morbidity. Our recent study showed that a decrease in muscle mitochondria plays a critical role in the physical decline in CKD, associated with a reduction in the expression of muster regulator of mitochondrial biogenesis, peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). In the present study, we examined the usefulness of a gastric hormone, ghrelin, which has been reported to promote mitochondrial oxidation in the skeletal muscle, for a recovery of physical decline in 5/6 nephrectomized (5/6Nx) CKD model mice. Design and Method: 5/6Nx mice were intraperitoneally administered acylated ghrelin (0.1 nmol/gBW; 3 times per week) or a muscle anabolic factor, IGF-1 (0.1 nmol/gBW), for a month. Muscle strength and exercise endurance were measured by a dynamometer and treadmill, respectively. Mitochondrial DNA copy number was determined by quantitative PCR. The methylation level of the cytosine residue at 260 base pairs upstream (C-260) of initiation point of PGC-1α gene, which was demonstrated to decrease the gene expression, was evaluated by methylation specific PCR (MSP) analysis and bisulfite sequencing. Results: Ghrelin treatment improved both muscle strength and exercise endurance in 5/6Nx mice associated with an increase in muscle mass and muscle mitochondrial amount. Ghrelin treatment decreased methylation ratio of C-260 of PGC-1α gene in the skeletal muscle and increased the expression. IGF-1 treatment improved muscle strength and muscle mass; however, muscle mitochondrial amount was not increased, associated with no alteration of methylation ratio of C-260 and expression of PGC-1α. Conclusions: Ghrelin treatment effectively improved physical decline of 5/6Nx mice through the combined effects of the enhancement of both muscle mass and mitochondrial amount, associated with epigenetic modification of muscle PGC-1α gene.

Exendin-4 decreases ghrelin levels through mTOR signaling

Exendin-4 (EX-4), a long-acting glucagon-like peptide-1 receptor (GLP-1R) agonist, regulates feeding behavior through its ability to inhibit gastric emptying. Ghrelin, a gastric hormone, provides a hunger signal to the central nervous system to stimulate appetite. Here, we report that EX-4 suppresses ghrelin production through the mTORC1-dependent mechanism. Central administration of EX-4 reduces gastric, hypothalamic and plasma ghrelin in both C57BL/6J mice and diet induced obese mice. These changes were associated with a significant increase in mTORC1 activity. Both GLP-1 and EX-4 suppressed the expression and secretion of ghrelin in cultured mHypoE-42 cells, a hypothalamic cell line. These effects were associated with significant changes in mTOR signaling. Inhibition of mTORC1 activity by mTOR siRNA or rapamycin abolished the suppression of ghrelin production induced by GLP-1 and EX-4 in mHypoE-42 cells. Our results identify mTORC1 as a critical signaling pathway for the downregulation of ghrelin induced by activation of GLP-1R.

β1-Adrenergic receptor deficiency in ghrelin-expressing cells causes hypoglycemia in susceptible individuals.

Ghrelin is an orexigenic gastric peptide hormone secreted when caloric intake is limited. Ghrelin also regulates blood glucose, as emphasized by the hypoglycemia that is induced by caloric restriction in mouse models of deficient ghrelin signaling. Here, we hypothesized that activation of β1-adrenergic receptors (β1ARs) localized to ghrelin cells is required for caloric restriction-associated ghrelin release and the ensuing protective glucoregulatory response. In mice lacking the β1AR specifically in ghrelin-expressing cells, ghrelin secretion was markedly blunted, resulting in profound hypoglycemia and prevalent mortality upon severe caloric restriction. Replacement of ghrelin blocked the effects of caloric restriction in β1AR-deficient mice. We also determined that treating calorically restricted juvenile WT mice with beta blockers led to reduced plasma ghrelin and hypoglycemia, the latter of which is similar to the life-threatening, fasting-induced hypoglycemia observed in infants treated with beta blockers. These findings highlight the critical functions of ghrelin in preventing hypoglycemia and promoting survival during severe caloric restriction and the requirement for ghrelin cell-expressed β1ARs in these processes. Moreover, these results indicate a potential role for ghrelin in mediating beta blocker-associated hypoglycemia in susceptible individuals, such as young children.

Ghrelin O-acyltransferase (GOAT), a specific enzyme that modifies ghrelin with a medium-chain fatty acid.

In the gastric peptide hormone ghrelin, serine 3 (threonine 3 in frogs) is modified, primarily by n-octanoic acid; this modification is essential for ghrelin's activity. The enzyme that transfers n-octanoic acid to Ser3 of ghrelin is ghrelin O-acyltransferase (GOAT). GOAT, the only enzyme known to catalyze acyl modification of ghrelin, specifically modifies serine (or threonine) at the third position and does not modify other serine residues in ghrelin peptides. GOAT prefers n-hexanoyl-CoA over n-octanoyl-CoA as the acyl donor, although in the stomach the n-octanoyl form is the predominant form of acyl-modified ghrelin. GOAT is a promising target for drug development to treat metabolic diseases and eating disorders.

Jejunal administration of glucose enhances acyl ghrelin suppression in obese humans.

Ghrelin is a gastric hormone that stimulates hunger and worsens glucose metabolism. Circulating ghrelin is decreased after Roux-en-Y gastric bypass (RYGB) surgery; however, the mechanism(s) underlying this change is unknown. We tested the hypothesis that jejunal nutrient exposure plays a significant role in ghrelin suppression after RYGB. Feeding tubes were placed in the stomach or jejunum in 13 obese subjects to simulate pre-RYGB or post-RYGB glucose exposure to the gastrointestinal (GI) tract, respectively, without the confounding effects of caloric restriction, weight loss, and surgical stress. On separate study days, the plasma glucose curves obtained with either gastric or jejunal administration of glucose were replicated with intravenous (iv) infusions of glucose. These "isoglycemic clamps" enabled us to determine the contribution of the GI tract and postabsorptive plasma glucose to acyl ghrelin suppression. Plasma acyl ghrelin levels were suppressed to a greater degree with jejunal glucose administration compared with gastric glucose administration (P < 0.05). Jejunal administration of glucose also resulted in a greater suppression of acyl ghrelin than the corresponding isoglycemic glucose infusion (P ≤ 0.01). However, gastric and isoglycemic iv glucose infusions resulted in similar degrees of acyl ghrelin suppression (P > 0.05). Direct exposure of the proximal jejunum to glucose increases acyl ghrelin suppression independent of circulating glucose levels. The enhanced suppression of acyl ghrelin after RYGB may be due to a nutrient-initiated signal in the jejunum that regulates ghrelin secretion.

Intravenous lipid infusion and total plasma fatty acids positively modulate plasma acylated ghrelin in vivo

Ghrelin is a gastric orexigenic hormone whose activating acylation plays a relevant role in the regulation of energy balance. Nutritional modulators of ghrelin acylation and plasma acylated ghrelin (AG) concentration remain however largely undefined. We aimed at investigating whether circulating free fatty acids (FFA) contribute to regulate plasma AG and its ratio (AG/TG) to total hormone (TG). Plasma FFA, TG, AG and AG/TG were measured in a primary outpatient care setting in a community-based population cohort of 850 individuals (age 54±10 years, M/F: 408/442) from the North-East Italy MoMa study. 150-min intravenous lipid infusions in rodents (10% lipids, 600μl/h) were used to investigate the potential causal role of FFA in the regulation of plasma ghrelin profile. Plasma FFA were associated positively with AG and AG/TG while negatively with TG (P<0.01). Associations between FFA, AG and AG/TG remained statistically significant (P<0.02) in multiple regression analysis including HOMA insulin resistance and metabolic confounders, and both AG and AG/TG but not TG increased through plasma FFA quartiles (P<0.01). Consistent with these findings, intravenous lipid infusion with plasma FFA elevation caused elevations of AG and AG/TG (P<0.05) with no TG modifications. The current findings demonstrate a novel role for circulating FFA availability to up-regulate plasma AG, which could involve FFA-induced stimulation of ghrelin acylation.