Ghrelin O-acyltransferase Research Articles

Ghrelin, Ghrelin O-Acyltransferase, and Carbohydrate Metabolism During Pregnancy in Calorie-Restricted Mice.

Acylation of ghrelin is mediated by ghrelin O-acyltansferase (GOAT). Exogenous acylated ghrelin (AG) stimulates growth hormone (GH) and food intake. In non-pregnant (NP) animals, the GOAT-ghrelin-GH axis prevents hypoglycemia caused by caloric restriction (CR). In humans, maternal malnutrition challenges glucose metabolism, which is a key determinant of fetal health. To clarify the role of AG and GH, we compared effects of CR on the GOAT-ghrelin-GH axis in pregnant (P) and NP mice. C57BL/6 wild type (WT) and GOAT knock-out (KO) P and NP mice were freely fed (FF) or subjected to 50% CR for one week. CR was started in P mice on Day 10.5 after conception. We measured body composition, blood glucose, plasma ghrelin and GH, stomach, hypothalamus and pituitary GOAT and ghrelin expression, and liver glycogen content and Pck1 expression. GOAT and AG were undetectable in KO. In NP mice, CR did not affect blood glucose (-1.3 mmol/l, p>0.05) in WT but was lowered (-1.8 mmol/l, p<0.0001) in KO. GH and Pck1 mRNA expression increased in WT but not in KO. In P mice, CR markedly lowered glucose (-2.7 mmol/l; p<0.0001) in WT and caused fatal hypoglycemia in KO, despite similarly elevated GH in WT and KO mice. KO animals are more prone to hypoglycemia than WT. GH, which is high in P animals, does not prevent hypoglycemia caused by CR during pregnancy. Our data suggest a specific role of AG in the regulation of gluconeogenesis to maintain euglycemia during pregnancy when energy availability is limited.

Ghrelin O-acyltransferase (GOAT) enzyme is overexpressed in prostate cancer, and its levels are associated with patient's metabolic status: Potential value as a non-invasive biomarker

Ghrelin-O-acyltransferase (GOAT) is the key enzyme regulating ghrelin activity, and has been proposed as a potential therapeutic target for obesity/diabetes and as a biomarker in some endocrine-related cancers. However, GOAT presence and putative role in prostate-cancer (PCa) is largely unknown. Here, we demonstrate, for the first time, that GOAT is overexpressed (mRNA/protein-level) in prostatic tissues (n = 52) and plasma/urine-samples (n = 85) of PCa-patients, compared with matched controls [healthy prostate tissues (n = 12) and plasma/urine-samples from BMI-matched controls (n = 28), respectively]. Interestingly, GOAT levels in PCa-patients correlated with aggressiveness and metabolic conditions (i.e. diabetes). Actually, GOAT expression was regulated by metabolic inputs (i.e. In1-ghrelin, insulin/IGF-I) in cultured normal prostate cells and PCa-cell lines. Importantly, ROC-curve analysis unveiled a valuable diagnostic potential for GOAT to discriminate PCa at the tissue/plasma/urine-level with high sensitivity/specificity, particularly in non-diabetic individuals. Moreover, we discovered that GOAT is secreted by PCa-cells, and that its levels are higher in urine samples from a stimulated post-massage vs. pre-massage prostate-test. In conclusion, plasmatic GOAT levels exhibit high specificity/sensitivity to predict PCa-presence compared with other PCa-biomarkers, especially in non-diabetic individuals, suggesting that GOAT holds potential as a novel non-invasive PCa-biomarker.

Ghrelin Octanoylation Is Completely Stabilized in Biological Samples by Alkyl Fluorophosphonates.

Ghrelin is a peptide hormone involved in multiple physiological processes related to energy homeostasis. This hormone features a unique posttranslational serine octanoylation modification catalyzed by the enzyme ghrelin O-acyltransferase, with serine octanoylation essential for ghrelin to bind and activate its cognate receptor. Ghrelin deacylation rapidly occurs in circulation, with both ghrelin and desacyl ghrelin playing important roles in biological signaling. Understanding the regulation and physiological impact of ghrelin signaling requires the ability to rapidly protect ghrelin from deacylation in biological samples such as blood serum or cell lysates to preserve the relative concentrations of ghrelin and desacyl ghrelin. In in vitro ghrelin O-acyltransferase activity assays using insect microsomal protein fractions and mammalian cell lysate and blood serum, we demonstrate that alkyl fluorophosphonate treatment provides rapid, complete, and long-lasting protection of ghrelin acylation against serine ester hydrolysis without interference in enzyme assay or ELISA analysis. Our results support alkyl fluorophosphonate treatment as a general tool for stabilizing ghrelin and improving measurement of ghrelin and desacyl ghrelin concentrations in biochemical and clinical investigations and suggest current estimates for active ghrelin concentration and the ghrelin to desacyl ghrelin ratio in circulation may underestimate in vivo conditions.

Functions of Ghrelin in Brain, Gut and Liver.

Ghrelin production occurs in the gastrointestinal tract with the greatest concentration in the fundus of the stomach, which accounts for approximately 75- 80% of circulating ghrelin levels. Desacyl ghrelin has been suggested to be involved in the regulation of energy homeostasis because of its ability to cross the blood-brain barrier and to induce increased neuronal activity in the hypothalamic arcuate nucleus. The enzyme called ghrelin O-acyl transferase is able to acylate ghrelin and regulates the physiological functions of the final peptide acyl ghrelin. The acylated ghrelin has an anti-inflammatory function that depends on its direct effect on T lymphocytes and monocytes, in which it inhibits the expression of proinflammatory cytokines such as Interleukin-1β (IL-1β), Interleukin-6 (IL-6), and Tumor Necrosis Factor (TNF-α). Some studies conducted in animal models show that ghrelin treatment reduces the amount of fibrogenic cells. In particular, ghrelin treatment decreases α -SMA protein expression, hepatic content of hydroxyproline and reduces the elevation of serum aspartate aminotransferase levels. Moreover, ghrelin attenuates liver injury and collagen deposition through inhibition of hepatic cell apoptosis and antioxidative activity, at least in part by nitric oxide induction. Several studies suggest that the orexigenic action of ghrelin is mediated via the neuropeptide Y neurons in the arcuate nucleus. Neuropeptide Y overexpression in the dorsomedial hypothalamus can cause hyperphagia and obesity in rat, which in turn causes liver steatosis, inflammation and fibrosis. The aims of this review were to examine and discuss the different functions of this hormone, particularly those that link the brain, gut and liver.

Des-Acyl Ghrelin and Ghrelin O-Acyltransferase Regulate Hypothalamic-Pituitary-Adrenal Axis Activation and Anxiety in Response to Acute Stress.

Ghrelin exists in two forms in circulation, acyl ghrelin and des-acyl ghrelin, both of which have distinct and fundamental roles in a variety of physiological functions. Despite this fact, a large proportion of papers simply measure and refer to plasma ghrelin without specifying the acylation status. It is therefore critical to assess and state the acylation status of plasma ghrelin in all studies. In this study we tested the effect of des-acyl ghrelin administration on the hypothalamic-pituitary-adrenal axis and on anxiety-like behavior of mice lacking endogenous ghrelin and in ghrelin-O-acyltransferase (GOAT) knockout (KO) mice that have no endogenous acyl ghrelin and high endogenous des-acyl ghrelin. Our results show des-acyl ghrelin produces an anxiogenic effect under nonstressed conditions, but this switches to an anxiolytic effect under stress. Des-acyl ghrelin influences plasma corticosterone under both nonstressed and stressed conditions, although c-fos activation in the paraventricular nucleus of the hypothalamus is not different. By contrast, GOAT KO are anxious under both nonstressed and stressed conditions, although this is not due to corticosterone release from the adrenals but rather from impaired feedback actions in the paraventricular nucleus of the hypothalamus, as assessed by c-fos activation. These results reveal des-acyl ghrelin treatment and GOAT deletion have differential effects on the hypothalamic-pituitary-adrenal axis and anxiety-like behavior, suggesting that anxiety-like behavior in GOAT KO mice is not due to high plasma des-acyl ghrelin.

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.

Effects of ghrelin in energy balance and body weight homeostasis.

Ghrelin is a gut peptide composed of 28 amino acids mostly secreted in the gastric fundus mucosa. It was isolated and described in 1999 by Kojima et al. and only three years later its specific receptor, GHSR1a, was also identified. Ghrelin, the endogenous ligand for the GH secretagogue receptor, is the only peripheral orexigenic hormone that activates the receptors to be found especially in the appetite center (hypothalamus and pituitary gland). Ghrelin is present in human plasma in two forms: an inactive form known as deacylated ghrelin, and an active form called acylated ghrelin synthesized under the action of ghrelin O-acyltransferase enzyme (GOAT). The literature even mentions an extremely complex ghrelin/GOAT/GHSR system involved in the regulation of human energy, metabolism and adaptation of energy homeostasis to environmental changes. In humans, there is a preprandial rise and a postprandial fall in plasma ghrelin levels, which strongly suggest that the peptide plays a physiological role in meal initiation and may be employed in determining the amount and quality of ingested food. Besides the stimulation of food intake, ghrelin determines a decrease in energy expenditure and promotes the storage of fatty acids in adipocytes. Thus, in the human body ghrelin induces a positive energy balance, an increased adiposity gain, as well as an increase in caloric storage, seen as an adaptive mechanism to caloric restriction conditions. In the current world context, when we are witnessing an increasing availability of food and a reduction of energy expenditure to a minimum level, these mechanisms have become pathogenic. As a consequence, the hypothesis that ghrelin is involved in the current obesity epidemic has been embraced by many scholars and researchers.

Brain Mapping of Ghrelin O-Acyltransferase in Goldfish (Carassius Auratus): Novel Roles for the Ghrelinergic System in Fish?

Ghrelin O-acyltransferase (GOAT) is the enzyme responsible for acylation of ghrelin, a gut-brain hormone with important roles in many physiological functions in vertebrates. Many aspects of GOAT remain to be elucidated, especially in fish, and particularly its anatomical distribution within the different brain areas has never been reported to date. The present study aimed to characterize the brain mapping of GOAT using RT-qPCR and immunohistochemistry in a teleost, the goldfish (Carassius auratus). Results show that goat transcripts are expressed in different brain areas of the goldfish, with the highest levels in the vagal lobe. Using immunohistochemistry, we also report the presence of GOAT immunoreactive cells in different encephalic areas, including the telencephalon, some hypothalamic nuclei, pineal gland, optic tectum and cerebellum, although they are especially abundant in the hindbrain. Particularly, an important signal is observed in the vagal lobe and some fiber tracts of the brainstem, such as the medial longitudinal fasciculus, Mauthneri fasciculus, secondary gustatory tract and spinothalamic tract. Most of the forebrain areas where GOAT is detected, particularly the hypothalamic nuclei, also express the ghs-r1a ghrelin receptor and other appetite-regulating hormones (e.g., orexin and NPY), supporting the role of ghrelin as a modulator of food intake and energy balance in fish. Present results are the first report on the presence of GOAT in the brain using imaging techniques. The high presence of GOAT in the hindbrain is a novelty, and point to possible new functions for the ghrelinergic system in fish. Anat Rec, 299:748-758, 2016. © 2016 Wiley Periodicals, Inc.

Ghrelin, ghrelin-O-acyl transferase, nucleobindin-2/nesfatin-1 and prohormone convertases in the pancreatic islets of Sprague Dawley rats during development

Ghrelin and nesfatin-1 are regulators of blood glucose and energy balance. Prohormone convertases (PCs) enable processing of ghrelin and nesfatin-1 from its precursors. An acylation, enabled by ghrelin O-acyl transferase (GOAT) is critical for many of the biological actions of ghrelin. To date, there is no research on the developmental expression of GOAT, and the co-expression of both NUCB2/nesfatin-1 and prohormone convertases in the pancreas. The objective of this research was to immunolocalize ghrelin, GOAT, NUCB2/nesfatin-1, PC1/3 and PC2 in the pancreas during fetal and postnatal periods of Sprague Dawley (SD) rats using immunohistochemical analysis. GOAT mRNA in the rat pancreas during development was also determined. In the pancreas, not all islet cells immunopositive for GOAT are immunoreactive for ghrelin on postnatal (P) days 20, 27 and adult. GOAT mRNA expression in the pancreas at P27 was higher than the expression levels in rest of the developmental stages tested. Both PC1/3 and PC2 are co-expressed with NUCB2/nesfatin-1 on embryonic (E) day E21, P13, P20. While similar co-localization was also found in P27 for NUCB2/nesfatin-1 and PC1/3, NUCB2/nesfatin-1 and PC2 were found in distinct populations of cells in P27. Some ghrelin and GOAT positive cells stained for nesfatin-1 as well. Meanwhile, no co-localization of somatostatin and glucaon with nesfatin-1 was found in the pancreas of SD rats. Our findings suggest that the endocrine pancreas can produce and process precursors of ghrelin and nesfatin-1 to make bioactive forms of both peptides.

Periprandial changes and effects of short- and long-term fasting on ghrelin, GOAT, and ghrelin receptors in goldfish (Carassius auratus).

The periprandial profile and effects of short- (7days) and long-term (30days) fasting on the ghrelinergic system were studied in goldfish (Carassius auratus). Plasma levels of acyl-ghrelin, desacyl-ghrelin, and ghrelin O-acyl transferase (GOAT) were analyzed by enzymoimmunoassays, and expression of preproghrelin, goat and growth hormone secretagogue receptors (ghs-r) was quantified by real-time PCR. Circulating levels of acyl-ghrelin and GOAT rise preprandially, supporting the role of acyl-ghrelin as a meal initiator in this teleost. Consistently, preproghrelin and ghs-r1a1 expression increases 1h before feeding time in intestinal bulb, suggesting that this receptor subtype might be involved in the preprandial action of ghrelin in this tissue. Significant postfeeding variations are detected for preproghrelin in telencephalon, goat in telencephalon and hypothalamus, ghs-r1a1 in vagal lobe, ghs-r1a2 and ghs-r2a1 in hypothalamus and ghs-r2a2 in telencephalon and vagal lobe, especially in unfed fish. Short- and long-term fasting significantly increase preproghrelin expression in telencephalon and gut. Goat expression is upregulated by short-term fasting in telencephalon and hypothalamus, and by both short- and long-term fasting in gut. Expression of ghs-r increases by fasting in telencephalon, while an upregulation of type 2, but not type 1, receptors is observed in vagal lobe. In intestinal bulb, ghs-r1a2 transcripts increase after both short- and long-term fasting. These results show a high dependence of the ghrelinergic system on feeding and nutritional status in fish, and demonstrate for the first time a differential implication of the various components of this system suggesting different roles for the four ghrelinergic receptor subtypes.

Diversification and coevolution of the ghrelin/growth hormone secretagogue receptor system in vertebrates.

The gut hormone ghrelin is involved in numerous metabolic functions, such as the stimulation of growth hormone secretion, gastric motility, and food intake. Ghrelin is modified by ghrelin O‐acyltransferase (GOAT) or membrane‐bound O‐acyltransferase domain‐containing 4 (MBOAT4) enabling action through the growth hormone secretagogue receptors (GHS‐R). During the course of evolution, initially strong ligand/receptor specificities can be disrupted by genomic changes, potentially modifying physiological roles of the ligand/receptor system. Here, we investigated the coevolution of ghrelin, GOAT, and GHS‐R in vertebrates. We combined similarity search, conserved synteny analyses, phylogenetic reconstructions, and protein structure comparisons to reconstruct the evolutionary history of the ghrelin system. Ghrelin remained a single‐gene locus in all vertebrate species, and accordingly, a single GHS‐R isoform was identified in all tetrapods. Similar patterns of the nonsynonymous (dN) and synonymous (dS) ratio (dN/dS) in the vertebrate lineage strongly suggest coevolution of the ghrelin and GHS‐R genes, supporting specific functional interactions and common physiological pathways. The selection profiles do not allow confirmation as to whether ghrelin binds specifically to GOAT, but the ghrelin dN/dS patterns are more similar to those of GOAT compared to MBOAT1 and MBOAT2 isoforms. Four GHS‐R isoforms were identified in teleost genomes. This diversification of GHS‐R resulted from successive rounds of duplications, some of which remained specific to the teleost lineage. Coevolution signals are lost in teleosts, presumably due to the diversification of GHS‐R but not the ghrelin gene. The identification of the GHS‐R diversity in teleosts provides a molecular basis for comparative studies on ghrelin's physiological roles and regulation, while the comparative sequence and structure analyses will assist translational medicine to determine structure–function relationships of the ghrelin/GHS‐R system.

Progress in Small Molecule and Biologic Therapeutics Targeting Ghrelin Signaling.

Ghrelin is a circulating peptide hormone involved in regulation of a wide array of physiological processes. As an endogenous ligand for growth hormone secretagogue receptor (GHSR1a), ghrelin is responsible for signaling involved in energy homeostasis, including appetite stimulation, glucose metabolism, insulin signaling, and adiposity. Ghrelin has also been implicated in modulation of several neurological processes. Dysregulation of ghrelin signaling is implicated in diseases related to these pathways, including obesity, type II diabetes, and regulation of appetite and body weight in patients with Prader-Willi syndrome. Multiple steps in the ghrelin signaling pathway are available for targeting in the development of therapeutics for these diseases. Agonists and antagonists of GHS-R1a have been widely studied and have shown varying levels of effectiveness within ghrelin-related physiological pathways. Agents targeting ghrelin directly, either through depletion of ghrelin levels in circulation or inhibitors of ghrelin O-acyltransferase whose action is required for ghrelin to become biologically active, are receiving increasing attention as potential therapeutic options. We discuss the approaches utilized to target ghrelin signaling and highlight the current challenges toward developing small-molecule agents as potential therapeutics for ghrelin-related diseases.

RNAi-mediated Ghrelin affects gastric H+–K+-ATPase activity and expression of GOAT–Ghrelin system in vitro

Ghrelin has been implicated in the regulation of gastric functional development, and its physiological functions are mediated by Ghrelin-O-acyltransferase (GOAT) which is capable of generating the active form of this polypeptide hormone. However, whether and how ghrelin gene silencing may modify gastric acid secretion and GOAT–Ghrelin system is yet to be explored. The study was performed in gastric mucosal cells from weanling piglets in vitro. We evaluated the effect of ghrelin on gastric acid secretion, gene expression of GOAT and ghrelin as well as ghrelin levels by RNA interference assay. shGhrelin triggered the down-regulation of ghrelin mRNA expression (P<0.05) via an RNAi mechanism, as observed by real-time RT-PCR. In addition, shGhrelin showed reduced total ghrelin production and secretion (P<0.05) using ELISA in vitro. We also detected that GOAT mRNA expression was reduced in shGhrelin group (P<0.05), compared with control groups. In accordance with the GOAT expression, acylated ghrelin production and secretion were reduced in gastric mucosal cells and culture medium (P<0.05). Silencing of ghrelin gene achieved by RNAi-mediation inhibited the activity of H+–K+-ATPase and pepsin (P<0.05) in gastric mucosal cells. These results indicated that RNAi of Ghrelin gene inhibited the gastric acid secretion with decreased GOAT mRNA and acylated Ghrelin in gastric mucosal cells.

Tissue-specific expression of ghrelinergic and NUCB2/nesfatin-1 systems in goldfish (Carassius auratus) is modulated by macronutrient composition of diets

The macronutrient composition of diets is a very important factor in the regulation of body weight and metabolism. Several lines of research in mammals have shown that macronutrients differentially regulate metabolic hormones, including ghrelin and nesfatin-1 that have opposing effects on energy balance. This study aimed to determine whether macronutrients modulate the expression of ghrelin and the nucleobindin-2 (NUCB2) encoded nesfatin-1 in goldfish (Carassius auratus). Fish were fed once daily on control, high-carbohydrate, high-protein, high-fat and very high-fat diets for 7 (short-term) or 28 (long-term) days. The expression of preproghrelin, ghrelin O-acyl transferase (goat), growth hormone secretagogue receptor 1 (ghs-r1) and nucb2/nesfatin-1 mRNAs was quantified in the hypothalamus, pituitary, gut and liver. Short-term feeding with fat-enriched diets significantly increased nucb2 mRNA levels in hypothalamus and liver, preproghrelin, goat and ghs-r1 expression in pituitary, and ghs-r1 expression in gut. Fish fed on a high-protein diet exhibited a significant reduction in preproghrelin and ghs-r1 mRNAs in the liver. After long-term feeding, fish fed on high-carbohydrate and very high-fat diets had significantly increased preproghrelin, goat and ghs-r1 expression in pituitary. Feeding on a high-carbohydrate diet also upregulated goat and ghs-r1 transcripts in gut, while feeding on a high-fat diet elicited the same effect only for ghs-r1 in liver. Nucb2 expression increased in pituitary, while it decreased in gut after long-term feeding of a high-protein diet. Collectively, these results show for the first time in fish that macronutrients differentially regulate the expression of ghrelinergic and NUCB2/nesfatin-1 systems in central and peripheral tissues of goldfish.

Gastric ghrelin, GOAT, leptin, and leptinR expression as well as peripheral serotonin are dysregulated in humans with obesity.

Gastrointestinal hormone release and the regulation of appetite and body weight are thought to be dysbalanced in obesity. However, human data investigating the expression of gastrointestinal hormones in the obese are rare. We studied the expression of ghrelin, leptin, and the serotonergic system in stomach tissue and serum of obese and non-obese individuals. Gastric tissue and serum were collected from 29 adult obese (BMI 48.7 ± 10.6 kg/m(2) ; mean ± SD) who underwent laparoscopic sleeve gastrectomy. Gastric biopsies, surgery specimen or serum was obtained from 35 adult non-obese humans (BMI 22.7 ± 1.9 kg/m(2) ). Ghrelin, ghrelin O-acyl transferase (GOAT), leptin, leptin receptor, and tryptophan hydroxylase 1 (TPH1) mRNA expression were measured by qRT-PCR. Serotonin (5HT) and leptin protein concentration were quantified in tissue extracts and serum; GOAT and ghrelin-positive cells were immunohistologically quantified in tissue. Additionally, 21 blood immune markers were analyzed. In gastric tissue, GOAT-positive cells were reduced (p < 0.01), but ghrelin-positive cells and mRNA were increased (both p < 0.05) in obese compared with non-obese individuals. Gastric leptin (p < 0.001) and leptin receptor (p < 0.001) mRNA expression, as well as leptin concentrations in serum (p < 0.001), were increased in obese compared with non-obese individuals. Serum 5HT was reduced (p < 0.05), while tissue 5HT and TPH1 mRNA were reduced only by trend. Interleukin 1 receptor a (IL1Ra), IL-8, IL-12, and monocyte chemoattractant protein 1 (IL1Ra) were increased and IL1Ra correlated negatively with serum leptin. Our data indicate that obesity causes a dysregulation of gastrointestinal hormones at the tissue level and serum, including a negative correlation with an increased marker of subclinical inflammation.

Estradiol and testosterone modulate the tissue-specific expression of ghrelin, ghs-r, goat and nucb2 in goldfish

Ghrelin, and nesfatin-1 (encoded by nucleobindin2/nucb2) are two metabolic peptides with multiple biological effects in vertebrates. While sex steroids are known to regulate endogenous ghrelin and NUCB2 in mammals, such actions by steroids in fish remain unknown. This study aimed to determine whether estradiol (E2) and testosterone (T) affects the expression of preproghrelin, ghrelin/growth hormone secretagogue receptor (GHS-R), ghrelin O-acyl transferase (GOAT) and NUCB2 in goldfish (Carassius auratus). First, a dose–response assay was performed in which fish were intraperitoneally (ip) implanted with pellets containing 25, 50 or 100μg/g body weight (BW) of E2 or T. It was found that sex steroids (100μg/g BW) administered for 2.5days achieved the highest E2 or T in circulation. In a second experiment, fish were ip implanted with pellets containing 100μg/g BW of E2, T or without hormone (control). RT-qPCR analyses at 2.5days post-administration show that gut preproghrelin and GOAT expression was upregulated by both E2 and T treatments, while the same effect was observed for GHS-R only in the pituitary. Both treatments also reduced hypothalamic preproghrelin mRNA expression. NUCB2 expression was increased in the forebrain of T treated group and reduced in the gut and pituitary under both treatments. These results show for the first time a modulation of preproghrelin and nucb2/nesfatin-1 by sex steroids in fish. The interaction between sex steroids and genes implicated in both metabolism and reproduction might help meeting the reproduction dependent energy demands in fish.

Ghrelin O-acyltransferase (GOAT) and energy metabolism.

Ghrelin O-acyltransferase (GOAT), a member of MBOATs family, is essential for octanoylation of ghrelin, which is required for active ghrelin to bind with and activate its receptor. GOAT is expressed mainly in the stomach, pancreas and hypothalamus. Levels of GOAT are altered by energy status. GOAT contains 11 transmembrane helices and one reentrant loop. Its invariant residue His-338 and conserved Asn-307 are located in the endoplasmic reticulum lumen and cytosol respectively. GOAT contributes to the regulation of food intake and energy expenditure, as well as glucose and lipids homeostasis. Deletion of GOAT blocks the acylation of ghrelin leading to subsequent impairment in energy homeostasis and survival when mice are challenged with high energy diet or severe caloric restriction. GO-CoA-Tat, a peptide GOAT inhibitor, attenuates acyl-ghrelin production and prevents weight gain induced by a medium-chain triglycerides-rich high fat diet. Further, GO-CoA-Tat increases glucose- induced insulin secretion. Overall, inhibition of GOAT is a novel strategy for treatment of obesity and related metabolic disorders.

Triterpenes suppress octanoylated ghrelin production in ghrelin-expressing human gastric carcinoma cells.

Ghrelin is an appetite-stimulating peptide hormone with an octanoyl modification at serine 3 that is essential for its orexigenic effect. Ghrelin O-acyltransferase (GOAT) is the enzyme that catalyzes ghrelin acylation using fatty acyl-coenzyme A as a substrate. We previously developed an assay system based on the AGS-GHRL8 cell line that produces octanoylated ghrelin in the presence of octanoic acid, and demonstrated that some fatty acids suppressed octanoylated ghrelin production. Recent studies have reported that triterpenes have anti-obesity effect. Since such triterpenes, like fatty acids, have a carboxyl group, we speculated that they can suppress octanoylated ghrelin production. To test this hypothesis, we investigated the effect of triterpenes on octanoylated ghrelin production. Asiatic acid, corosolic acid, glycyrrhetinic acid, oleanolic acid and ursolic acid suppressed octanoylated ghrelin levels in AGS-GHRL8 cells without decreasing transcript expression of GOAT or furin, a protease required for ghrelin maturation. β-amyrin had no effect on octanoylated ghrelin level, which was only slightly inhibited by uvaol; the fact that both these triterpenes lack a carboxyl group indicates that this group is important for suppressing octanoylated ghrelin production. These results suggest that triterpenes may have the potential as obesity-preventing agents with suppressive effect on octanoylated ghrelin production.

Ghrelin O-acyltransferase knockout mice show resistance to obesity when fed high-sucrose diet.

Ghrelin is an appetite-stimulating hormone secreted from stomach. Since the discovery that acylation of the serine-3 residue by ghrelin O-acyltransferase (GOAT) is essential for exerting its functions, GOAT has been regarded as an therapeutic target for attenuating appetite, and thus for the treatment of obesity and diabetes. However, contrary to the expectations, GOAT-knockout (KO) mice have not shown meaningful body weight reduction, under high-fat diet. Here, in this study, we sought to determine whether GOAT has a role in body weight regulation and glucose metabolism with a focus on dietary sucrose, because macronutrient composition of diet is important for appetite regulation. We found that peripherally administered acylated-ghrelin, but not unacylated one, stimulated sucrose consumption in a two-bottle-drinking test. The role of acylated-ghrelin in sucrose preference was further supported by the finding that GOAT KO mice consumed less sucrose solution compared with WT littermates. Then, we investigated the effect of dietary composition of sucrose on food intake and body weight in GOAT KO and WT mice. As a result, when fed on high-fat diet, food intake and body weight were similar between GOAT KO and WT mice. However, when fed on high-fat, high-sucrose diet, GOAT KO mice showed significantly reduced food intake and marked resistance to obesity, leading to amelioration of glucose metabolism. These results suggest that blockade of acylated-ghrelin production offers therapeutic potential for obesity and metabolic disorders caused by overeating of palatable food.

Relationships Between Insulin Resistance, Acyl Ghrelin, Deacyl Ghrelin and Ghrelin O-Acyltransferase in Middle Aged Men

Relationships Between Insulin Resistance, Acyl Ghrelin, Deacyl Ghrelin and Ghrelin O-Acyltransferase in Middle Aged Men