Novel and conventional receptors for ghrelin, desacyl-ghrelin, and pharmacologically related compounds

GHSR blockade, but not reduction of peripherally circulating ghrelin via β1-adrenergic receptor antagonism, decreases binge-like alcohol drinking in mice

Alcohol use disorder (AUD) and binge drinking are highly prevalent public health issues. The stomach-derived peptide ghrelin, and its receptor, the growth hormone secretagogue receptor (GHSR), both of which are expressed in the brain and periphery, are implicated in alcohol-related outcomes. We previously found that systemic and central administration of GHSR antagonists reduced binge-like alcohol drinking, whereas a ghrelin vaccine did not. Thus, we hypothesized that central GHSR drives binge-like alcohol drinking independently of peripheral ghrelin. To investigate this hypothesis, we antagonized β1-adrenergic receptors (β1ARs), which are required for peripheral ghrelin release, and combined them with GHSR blockers. We found that both systemic β1AR antagonism with atenolol (peripherally restricted) and metoprolol (brain permeable) robustly decreased plasma ghrelin levels. Also, ICV administration of atenolol had no effect on peripheral endogenous ghrelin levels. However, only metoprolol, but not atenolol, decreased binge-like alcohol drinking. The β1AR antagonism also did not prevent the effects of the GHSR blockers JMV2959 and PF-5190457 in decreasing binge-like alcohol drinking. These results suggest that the GHSR rather than peripheral endogenous ghrelin is involved in binge-like alcohol drinking. Thus, GHSRs and β1ARs represent possible targets for therapeutic intervention for AUD, including the potential combination of drugs that target these two systems.

Ghrelin, Neuroinflammation, Oxidative Stress, and Mood Disorders: What Are the Connections?

Ghrelin is a gut peptide hormone associated with feeding behavior and energy homeostasis. Acylated ghrelin binds to the growth hormone secretagogue receptor 1a subtype (GHS-R1a) in the hippocampus, leading to GH release from the anterior pituitary. However, in recent years, ghrelin and its receptor have also been implicated in other processes, including the regulation of cardiomyocyte function, muscle trophism, and bone metabolism. Moreover, GHS-R1a is distributed throughout the brain and is expressed in brain areas that regulate the stress response and emotional behavior. Consistently, a growing body of evidence supports the role of ghrelin in regulating stress response and mood. Stress has consistently been shown to increase ghrelin levels, and despite some inconsistencies, both human and rodent studies suggested antidepressant effects of ghrelin. Nevertheless, the precise mechanism by which ghrelin influences stress response and mood remains largely unknown. Intriguingly, ghrelin and GHS-R1a were consistently reported to exert anti-inflammatory, antioxidant, and neurotrophic effects both in vivo and in vitro, although this has never been directly assessed in relation to psychopathology. In the present review we will discuss available literature linking ghrelin with the stress response and depressive-like behavior in animal models as well as evidence describing the interplay between ghrelin and neuroinflammation/oxidative stress. Although further studies are required to understand the mechanisms involved in the action of ghrelin on mood, we hypothesize that the antiinflammatory and anti-oxidative properties of ghrelin may give a key contribution.

Hypothalamic tanycytes internalize ghrelin from the cerebrospinal fluid: Molecular mechanisms and functional implications

OBJECTIVE

The peptide hormone ghrelin exerts potent effects in the brain, where its receptor is highly expressed. Here, we investigated the role of hypothalamic tanycytes in transporting ghrelin across the blood-cerebrospinal fluid (CSF) interface.

METHODS

We investigated the internalization and transport of fluorescent ghrelin (Fr-ghrelin) in primary cultures of rat hypothalamic tanycytes, mouse hypothalamic explants, and mice. We also tested the impact of inhibiting clathrin-mediated endocytosis of ghrelin in the brain ventricular system on the orexigenic and locomotor effects of the hormone.

RESULTS

In vitro, we found that Fr-ghrelin is selectively and rapidly internalized at the soma of tanycytes, via a GHSR-independent and clathrin-dependent mechanism, and then transported to the endfoot. In hypothalamic explants, we also found that Fr-ghrelin is internalized at the apical pole of tanycytes. In mice, Fr-ghrelin present in the CSF was rapidly internalized by hypothalamic β-type tanycytes in a clathrin-dependent manner, and pharmacological inhibition of clathrin-mediated endocytosis in the brain ventricular system prolonged the ghrelin-induced locomotor effects.

CONCLUSIONS

We propose that tanycyte-mediated transport of ghrelin is functionally relevant, as it may contribute to reduce the concentration of this peptide hormone in the CSF and consequently shortens the duration of its central effects.

Ghrelin in Focus: Dissecting Its Critical Roles in Gastrointestinal Pathologies and Therapies

This review elucidates the critical role of ghrelin, a peptide hormone mainly synthesized in the stomach in various gastrointestinal (GI) diseases. Ghrelin participates in diverse biological functions ranging from appetite regulation to impacting autophagy and apoptosis. In sepsis, it reduces intestinal barrier damage by inhibiting inflammatory responses, enhancing GI blood flow, and modulating cellular processes like autophagy and apoptosis. Notably, in inflammatory bowel disease (IBD), serum ghrelin levels serve as markers for distinguishing between active and remission phases, underscoring its potential in IBD treatment. In gastric cancer, ghrelin acts as an early risk marker, and due to its significant role in increasing the proliferation and migration of gastric cancer cells, the ghrelin-GHS-R axis is poised to become a target for gastric cancer treatment. The role of ghrelin in colorectal cancer (CRC) remains controversial; however, ghrelin analogs have demonstrated substantial benefits in treating cachexia associated with CRC, highlighting the therapeutic potential of ghrelin. Nonetheless, the complex interplay between ghrelin's protective and potential tumorigenic effects necessitates a cautious approach to its therapeutic application. In post-GI surgery scenarios, ghrelin and its analogs could be instrumental in enhancing recovery and reducing complications. This article accentuates ghrelin's multifunctionality, shedding light on its influence on disease mechanisms, including inflammatory responses and cancer progression, and examines its therapeutic potential in GI surgeries and disorders, advocating for continued research in this evolving field.

Molecular and cellular mechanisms underlying the hepatoprotective role of ghrelin against NAFLD progression

The underlying mechanisms for the development and progression of nonalcoholic fatty liver disease (NAFLD) are complex and multifactorial. Within the last years, experimental and clinical evidences support the role of ghrelin in the development of NAFLD. Ghrelin is a gut hormone that plays a major role in the short-term regulation of appetite and long-term regulation of adiposity. The liver constitutes a target for ghrelin, where this gut-derived peptide triggers intracellular pathways regulating lipid metabolism, inflammation, and fibrosis. Interestingly, circulating ghrelin levels are altered in patients with metabolic diseases, such as obesity, type 2 diabetes, and metabolic syndrome, which, in turn, are well-known risk factors for the pathogenesis of NAFLD. This review summarizes the molecular and cellular mechanisms involved in the hepatoprotective action of ghrelin, including the reduction of hepatocyte lipotoxicity via autophagy and fatty acid β-oxidation, mitochondrial dysfunction, endoplasmic reticulum stress and programmed cell death, the reversibility of the proinflammatory phenotype in Kupffer cells, and the inactivation of hepatic stellate cells. Together, the metabolic and inflammatory pathways regulated by ghrelin in the liver support its potential as a therapeutic target to prevent NAFLD in patients with metabolic disorders.

Gut Molecules in Cardiometabolic Diseases: The Mechanisms behind the Story

Atherosclerotic cardiovascular disease is the most common cause of morbidity and mortality worldwide. Diabetes mellitus increases cardiovascular risk. Heart failure and atrial fibrillation are associated comorbidities that share the main cardiovascular risk factors. The use of incretin-based therapies promoted the idea that activation of alternative signaling pathways is effective in reducing the risk of atherosclerosis and heart failure. Gut-derived molecules, gut hormones, and gut microbiota metabolites showed both positive and detrimental effects in cardiometabolic disorders. Although inflammation plays a key role in cardiometabolic disorders, additional intracellular signaling pathways are involved and could explain the observed effects. Revealing the involved molecular mechanisms could provide novel therapeutic strategies and a better understanding of the relationship between the gut, metabolic syndrome, and cardiovascular diseases.

G protein-coupled receptor interactions and modification of signalling involving the ghrelin receptor, GHSR1a

The growth hormone secretagogue receptor 1a (GHSR1a) is intriguing because of its potential as a therapeutic target and its diverse molecular interactions. Initial studies of the receptor focused on the potential therapeutic ability for growth hormone (GH) release to reduce wasting in aging individuals, as well as food intake regulation for treatment of cachexia. Known roles of GHSR1a now extend to regulation of neurogenesis, learning and memory, gastrointestinal motility, glucose/lipid metabolism, the cardiovascular system, neuronal protection, motivational salience, and hedonic feeding. Ghrelin, the endogenous agonist of GHSR1a, is primarily located in the stomach and is absent from the central nervous system (CNS), including the spinal cord. However, ghrelin in the circulation does have access to a small number of CNS sites, including the arcuate nucleus, which is important in feeding control. At some sites, such as at somatotrophs, GHSR1a has high constitutive activity. Typically, ghrelin-dependent and constitutive GHSR1a activation occurs via Gα_q/11 pathways. In vitro and in vivo data suggest that GHSR1a heterodimerises with multiple G protein-coupled receptors (GPCRs), including dopamine D1 and D2, serotonin 2C, orexin, oxytocin and melanocortin 3 receptors (MCR3), as well as the MCR3 accessory protein, MRAP2, providing possible mechanisms for its many physiological effects. In all cases, the receptor interaction changes downstream signalling and the responses to receptor agonists. This review discusses the signalling mechanisms of GHSR1a alone and in combination with other GPCRs, and explores the physiological consequences of GHSR1a coupling with other GPCRs.

Effect of Ghrelin on the Cardiovascular System

Simple Summary

Ghrelin is an octanoylated peptide that was initially isolated from rat and human stomachs in the process of searching for an endogenous ligand to the orphan growth hormone secretagogue receptor (GHS-R), a G-protein-coupled receptor. Exogenous or endogenous ghrelin secreted from the stomach binds to GHS-R on gastric vagal nerve terminals, and the signals are transmitted to the central nervous system via the vagal afferent nerve to facilitate growth hormone (GH) secretion, feeding, sympathetic inhibition, parasympathetic activation, and anabolic effects. Ghrelin also binds directly to the pituitary GHS-R and stimulates GH secretion. Ghrelin has beneficial effects on the cardiovascular system, including cardioprotective effects such as anti-heart failure, anti-arrhythmic, and anti-inflammatory actions, and it enhances vascular activity via GHS-R-dependent stimulation of GH/IGF-1 (insulin-like growth factor-1) and modulation of the autonomic nervous system. The anti-heart failure effects of ghrelin could be useful as a new therapeutic strategy for chronic heart failure.

Abstract

Ghrelin, an n-octanoyl-modified 28-amino-acid-peptide, was first discovered in the human and rat stomach as an endogenous ligand for the growth hormone secretagogue receptor (GHS-R). Ghrelin-GHS-R1a signaling regulates feeding behavior and energy balance, promotes vascular activity and angiogenesis, improves arrhythmia and heart failure, and also protects against cardiovascular disease by suppressing cardiac remodeling after myocardial infarction. Ghrelin’s cardiovascular protective effects are mediated by the suppression of sympathetic activity; activation of parasympathetic activity; alleviation of vascular endothelial dysfunction; and regulation of inflammation, apoptosis, and autophagy. The physiological functions of ghrelin should be clarified to determine its pharmacological potential as a cardiovascular medication.

Multiple roles of ghrelin in breast cancer

Breast cancer is one of the most threatening malignant tumors in women worldwide; hence, investigators are continually performing novel research in this field. However, an accurate prediction of its prognosis and postoperative recovery remains difficult. The severity of breast cancer is patient-specific and affected by several health factors; thus, unknown mechanisms may affect its progression. This article analyzes existing literature on breast cancer, ranging from the discovery of ghrelin to its present use, and aims to provide a reference for future research into breast cancer mechanisms and treatment-plan improvement. Various parts of ghrelin have been associated with breast cancer by direct or indirect evidence. The ghrelin system may encompass the direction of expanding breast cancer treatment methods and prognostic indicators. Therefore, we compiled almost all studies on the relationship between the ghrelin system and breast cancer, including unacylated ghrelin, its GHRL gene, ghrelin O-acyltransferase, the receptor growth hormone secretagogue receptor, and several splice variants of ghrelin to lay the foundation for future research.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

The Efficacy, Safety, and Pharmacology of a Ghrelin O-Acyltransferase Inhibitor for the Treatment of Prader-Willi Syndrome

CONTEXT

Acylated ghrelin (AG) stimulates appetite and is elevated compared to its unacylated (UAG) counterpart in Prader-Willi syndrome (PWS). GLWL-01 is a selective, reversible inhibitor of ghrelin O-acyltransferase (GOAT), the enzyme that converts UAG into AG.

OBJECTIVE

This work aimed to assess the efficacy, pharmacokinetics, pharmacodynamics, and safety of GLWL-01 in the treatment of PWS patients.

METHODS

A double-blind, placebo-controlled, phase 2 crossover study was conducted with 2 active treatment periods of 28 days in 19 patients (aged 16-65 years; body mass index (BMI) ≥ 28) with genetically confirmed PWS. The study took place in 7 hospital-based study centers in the United States and Canada. Patients received placebo or GLWL-01 (450 mg twice daily) orally after lead-in placebo and washout periods. The Hyperphagia Questionnaire for Clinical Trials and Caregiver Global Impression of Change were used to measure reductions in hyperphagia. Plasma concentrations of AG and UAG were evaluated as correlates.

RESULTS

Treatment resulted in statistically significant differences compared to placebo in plasma AG (P = .0002), UAG (P = .0488), and AG/UAG (P = .0003). GLWL-01 did not statistically significantly reduce hyperphagia-related behavior or bring about changes in global clinical end points, as assessed by caregivers. Anthropometric and clinical parameters correlated with obesity did not statistically significantly change in response to treatment. Less than half of patients reported a treatment-emergent adverse event (TEAE). No deaths, serious adverse events, or severe TEAEs were reported.

CONCLUSION

GLWL-01 is safe and well tolerated. Pharmacological parameters confirmed the inhibition of GOAT following administration of GLWL-01. Patients' eating behaviors, BMI, blood glucose, and total cholesterol, among other similar measures, were not modified.

Why is motilin active in some studies with mice, rats, and guinea pigs, but not in others? Implications for functional variability among rodents

The gastrointestinal (GI) hormone motilin helps control human stomach movements during hunger and promotes hunger. Although widely present among mammals, it is generally accepted that in rodents the genes for motilin and/or its receptor have undergone pseudonymization, so exogenous motilin cannot function. However, several publications describe functions of low concentrations of motilin, usually within the GI tract and CNS of mice, rats, and guinea pigs. These animals were from institute-held stocks, simply described with stock names (e.g., "Sprague-Dawley") or were inbred strains. It is speculated that variation in source/type of animal introduces genetic variations to promote motilin-sensitive pathways. Perhaps, in some populations, motilin receptors exist, or a different functionally-active receptor has a good affinity for motilin (indicating evolutionary pressures to retain motilin functions). The ghrelin receptor has the closest sequence homology, yet in non-rodents the receptors have a poor affinity for each other's cognate ligand. In rodents, ghrelin may substitute for certain GI functions of motilin, but no good evidence suggests rodent ghrelin receptors are highly responsive to motilin. It remains unknown if motilin has functional relationships with additional bioactive molecules formed from the ghrelin and motilin genes, or if a 5-TM motilin receptor has influence in rodents (e.g., to dimerize with GPCRs and create different pharmacological profiles). Is the absence/presence of responses to motilin in rodents' characteristic for systems undergoing gene pseudonymization? What are the consequences of rodent supplier-dependent variations in motilin sensitivity (or other ligands for receptors undergoing pseudonymization) on gross physiological functions? These are important questions for understanding animal variation.

Therapeutic potential of GHSR-1A antagonism in alcohol dependence, a review

Growth hormone secretagogue receptor type 1A (GHSR-1A) is a functional receptor of orexigenic peptide ghrelin and is highly expressed in mesolimbic dopaminergic systems that regulate incentive value of artificial reward in substance abuse. Interestingly, GHSR-1A has also shown ligand-independent constitutive activity. Alcohol use disorder (AUD) is one of the growing concerns worldwide as it involves complex neuro-psycho-endocrinological interactions. Positive correlation of acylated ghrelin and alcohol-induced human brain response in the right and left ventral striatum are evident. In the last decade, the beneficial effects of ghrelin receptor (GHSR-1A) antagonism to suppress artificial reward circuitries and induce self-control for alcohol consumption have drawn significant attention from researchers. In this updated review, we summarize the available recent preclinical, clinical, and experimental data to discuss functional, molecular actions of central ghrelin-GHSR-1A signaling in different craving levels for alcohol as well as to promote "GHSR-1A antagonism" as one of the potential therapies in early abstinence.

Effects of a 12-Week Diet versus Diet plus Aerobic and Resistance Exercise Program on Acylated and Desacylated Ghrelin, and Ghrelin O-Acyltransferase in Adolescent Girls with Obesity

This study investigated the effects of a 12-week diet versus diet plus aerobic and resistance exercise programme on acylated ghrelin (AG), desacylated ghrelin (DAG), and ghrelin O-acyltransferase (GOAT) concentrations in girls with obesity. We randomised 30 adolescents with obesity to a 12-week aerobic and resistance exercise group (EG) or a control group (CG). At baseline and at 4, 8, and 12 weeks, we measured their body composition, lipid profile, glucose, AG, DAG, and GOAT concentrations. In the EG, the body fat percentage decreased by 2.37% and was significantly lower than that in the CG. The DAG concentrations significantly increased by 48.3% and 27.4% in the EG and CG, respectively. At 4, 8, and 12 weeks, DAG concentrations were significantly higher in the EG than in the CG. AG concentrations were higher at week 12 than at baseline in both groups. In both groups, the GOAT concentrations increased at weeks 8 and 12; however, no between-group differences were observed in the changes in GOAT concentrations. This study showed increased DAG concentrations and non-significant changes in AG and GOAT concentrations after a 12-week aerobic and resistance exercise programme in girls with obesity. These findings suggest that an aerobic and resistance exercise programme influences appetite-regulating hormones, mainly through changes in DAG concentrations.

The Role of Ghrelin/GHS-R1A Signaling in Nonalcohol Drug Addictions

Drug addiction causes constant serious health, social, and economic burden within the human society. The current drug dependence pharmacotherapies, particularly relapse prevention, remain limited, unsatisfactory, unreliable for opioids and tobacco, and even symptomatic for stimulants and cannabinoids, thus, new more effective treatment strategies are researched. The antagonism of the growth hormone secretagogue receptor type A (GHS-R1A) has been recently proposed as a novel alcohol addiction treatment strategy, and it has been intensively studied in experimental models of other addictive drugs, such as nicotine, stimulants, opioids and cannabinoids. The role of ghrelin signaling in these drugs effects has also been investigated. The present review aims to provide a comprehensive overview of preclinical and clinical studies focused on ghrelin's/GHS-R1A possible involvement in these nonalcohol addictive drugs reinforcing effects and addiction. Although the investigation is still in its early stage, majority of the existing reviewed experimental results from rodents with the addition of few human studies, that searched correlations between the genetic variations of the ghrelin signaling or the ghrelin blood content with the addictive drugs effects, have indicated the importance of the ghrelin's/GHS-R1As involvement in the nonalcohol abused drugs pro-addictive effects. Further research is necessary to elucidate the exact involved mechanisms and to verify the future potential utilization and safety of the GHS-R1A antagonism use for these drug addiction therapies, particularly for reducing the risk of relapse.

Neuropeptide Cortistatin Regulates Dermal and Pulmonary Fibrosis in an Experimental Model of Systemic Sclerosis

INTRODUCTION

Scleroderma, or systemic sclerosis, is a complex connective tissue disorder characterized by autoimmunity, vasculopathy, and progressive fibrosis of the skin and internal organs. Because its aetiology is unknown, the identification of genes/factors involved in disease severity, differential clinical forms, and associated complications is critical for understanding its pathogenesis and designing novel treatments. Neuroendocrine mediators in the skin emerge as potential candidates. We investigated the role played by the neuropeptide cortistatin in a preclinical model of scleroderma.

METHODS

Dermal fibrosis was induced by repetitive intradermal injections of bleomycin in wild-type and cortistatin-deficient mice. The histopathological signs and expression of fibrotic markers were evaluated in the skin and lungs.

RESULTS

An inverse correlation between cortistatin levels and fibrogenic activation exists in the damaged skin and dermal fibroblasts. Bleomycin-challenged skin lesions of mice that are partially and totally deficient in cortistatin showed exacerbated histopathological signs of scleroderma, characterized by thicker and more fibrotic dermal layer, enlarged epidermis, and increased inflammatory infiltration in comparison to those of wild-type mice. Cortistatin deficiency enhanced dermal collagen deposits, connective tissue growth factor expression, loss of microvessels, and predisposition to suffer severe complications that co-occur with dermal exposition to bleomycin, including pulmonary fibrotic disease and increased mortality. Treatment with cortistatin mitigated these pathological processes.

DISCUSSION/CONCLUSION

We identify cortistatin as an endogenous break of skin inflammation and fibrosis. Deficiency in cortistatin could be a marker of poor prognosis of scleroderma and associated complications. Cortistatin-based therapies emerge as attractive candidates to treat severe forms of systemic sclerosis and to manage fibrosis-related side effects of bleomycin chemotherapy in oncologic patients.

Capromorelin, a ghrelin receptor agonist, increases feed intake and body weight gain in broiler chickens (Gallus gallus domesticus)

Ghrelin is a hormone that induces orexigenic effects in mammals. However, in avian species, there is scant and conflictive results on the effect of ghrelin on feed intake (FI). Therefore, we evaluated the effect of a ghrelin receptor agonist (capromorelin) on FI, ADG, water intake (WI), animal behavior and concentrations of ghrelin, glucose, growth hormone (GH) and insulin in broiler chickens. One-day-old male broilers were reared as recommended by the industry. At 4 wk of age (experimental day 0; D0), birds were blocked by weight and randomly assigned to 3 treatments in 2 identical trials. Control birds received a vehicle control solution containing 0 mg/kgBW/d of capromorelin. Birds in treatments 2 and 3 received capromorelin at target doses of 6 or 12 mg/kgBW/d of capromorelin (n = 27). FI and WI were measured 3 times a day at 0700 h (Period 1; P1), 1200 h (P2) and 1700 h (P3), while BW was recorded daily. Blood samples were collected on D-1 and D5. Bird behavior (pecking, sitting and standing) was evaluated for 9 h on D2. Data were analyzed using a randomized complete block design with repeated measures over time. Orthogonal polynomial contrasts were used to determine linear and quadratic effects of increasing levels of capromorelin. Polynomial contrasts showed that capromorelin doses linearly increased FI (P = 0.002) and ADG (P = 0.019). There were no treatment, day or treatment x d interactions on glucose, ghrelin and GH concentrations. However, there was a treatment x d interaction (P = 0.041) on insulin concentrations. Concentrations of insulin were higher on D5 for the 0 and 12 mg/kgBW/d treatments as compared with D-1. Polynomial contrasts showed that capromorelin doses linearly increased number of pecks/h (P = 0.018). Per hour FI and WI was higher during P1 (i.e., 0700-1200) as compared to P2 and P3 (P < 0.001). Our observations suggest that capromorelin linearly increases feed intake; thus, the same effect of that reported in mammalian species.

Ghrelin and its role in gastrointestinal tract tumors (Review)

Ghrelin, an orexigenic hormone, is a peptide that binds to the growth hormone secretagogue receptor; it is secreted mainly by enteroendocrine cells in the oxyntic glands of the stomach. Ghrelin serves a role in both local and systemic physiological processes, and is implicated in various pathologies, including neoplasia, with tissue expression in several types of malignancies in both in vitro and in vivo studies. However, the precise implications of the ghrelin axis in metastasis, invasion and cancer progression regulation has yet to be established. In the case of gastrointestinal (GI) tract malignancies, ghrelin has shown potential to become a prognostic factor or even a therapeutic target, although data in the literature are inconsistent and unsystematic, with reports untailored to a specific histological subtype of cancer or a particular localization. The evaluation of immunohistochemical expression shows a limited outlook owing to the low number of cases analyzed, and in vivo analyses have conflicting data regarding differences in ghrelin serum levels in patients with cancer. The aim of this review was to examine the relationship between ghrelin and GI tract malignancies to demonstrate the inconsistencies in current results and to highlight its clinical significance in the outcome of these patients.

Dopamine and ghrelin receptor co-expression and interaction in the spinal defecation centers

BACKGROUND

Dopamine receptor 2 (DRD2) and ghrelin receptor (GHSR1a) agonists both stimulate defecation by actions at the lumbosacral defecation center. Dopamine is in nerve terminals surrounding autonomic neurons of the defecation center, whereas ghrelin is not present in the spinal cord. Dopamine at D2 receptors generally inhibits neurons, but at the defecation center, its effect is excitatory.

METHODS

In vivo recording of defecation and colorectal propulsion was used to investigate interaction between DRD2 and GHSR1a. Localization studies were used to determine sites of receptor expression in rat and human spinal cord.

KEY RESULTS

Dopamine, and the DRD2 agonist, quinpirole, directly applied to the lumbosacral cord, caused defecation. The effect of intrathecal dopamine was inhibited by the GHSR1a antagonist, YIL781, given systemically, but YIL781 was not an antagonist at DRD2. The DRD2 agonist, pramipexole, administered systemically caused colorectal propulsion that was prevented when the pelvic nerves were cut. Drd2 and Ghsr were expressed together in autonomic preganglionic neurons at the level of the defecation centers in rat and human. Behaviorally induced defecation (caused by water avoidance stress) was reduced by the DRD2 antagonist, sulpiride. We had previously shown it is reduced by YIL781.

CONCLUSIONS AND INFERENCES

Our observations imply that dopamine is a transmitter of the defecation pathways whose actions are exerted through interacting dopamine (D2) and ghrelin receptors on lumbosacral autonomic neurons that project to the colorectum. The results explain the excitation by dopamine agonists and the conservation of GHSR1a in the absence of ghrelin.

Unusual orthologs shed new light on the binding mechanism of ghrelin to its receptor GHSR1a

The gastric peptide ghrelin has important functions in energy metabolism and cellular homeostasis by activating growth hormone secretagogue receptor type 1a (GHSR1a). The N-terminal residues of ghrelin orthologs from all vertebrates are quite conserved; however, in orthologs from Cavia porcellus and Phyllostomus discolor, Ser2 and Leu5 are replaced by a smaller Ala and a positively charged Arg, respectively. In the present study, we first demonstrated that the hydrophobic Leu5 is essential for the function of human ghrelin, because Ala replacement caused an approximately 100-fold decrease in activity. However, replacement of Leu5 by an Arg residue caused much less disruption; further replacement of Ser2 by Ala almost restored full activity, although the [S2A] mutation itself showed slight detriments, implying that the positively charged Arg5 in the [S2A,L5R] mutant might form alternative interactions with certain receptor residues to compensate for the loss of the essential Leu5. To identify the responsible receptor residues, we screened GHSR1a mutants in which all conserved negatively charged residues in the extracellular regions and all aromatic residues in the ligand-binding pocket were mutated separately. According to the decrease in selectivity of the mutant receptors towards [S2A,L5R]ghrelin, we deduced that the positively charged Arg5 of the ghrelin mutant primarily interacts with the essential aromatic Phe286 at the extracellular end of the sixth transmembrane domain of GHSR1a by forming cation-π and π-π interactions. The present study provided new insights into the binding mechanism of ghrelin with its receptor, and thus would facilitate the design of novel ligands for GHSR1a.

Development of a ghrelin receptor inverse agonist for positron emission tomography

Imaging of Ghrelin receptors in vivo provides unique potential to gain deeper understanding on Ghrelin and its receptors in health and disease, in particular, in cancer. Ghrelin, an octanoylated 28-mer peptide hormone activates the constitutively active growth hormone secretagogue receptor type 1a (GHS-R1a) with nanomolar activity. We developed novel compounds, derived from the potent inverse agonist K-(D-1-Nal)-FwLL-NH₂ but structurally varied by lysine conjugation with 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA), palmitic acid and/or diethylene glycol (PEG2) to allow radiolabeling and improve pharmacokinetics, respectively. All compounds were tested for receptor binding, potency and efficacy in vitro, for biodistribution and -kinetics in rats and in preclinical prostate cancer models on mice. Radiolabeling with Cu-64 and Ga-68 was successfully achieved. The Cu-64- or Ga-68-NODAGA-NH-K-K-(D-1-NaI)-F-w-L-L-NH₂ radiotracer were specifically accumulated by the GHS-R1a in xenotransplanted human prostate tumor models (PC-3, DU-145) in mice. The tumors were clearly delineated by PET. The radiotracer uptake was also partially blocked by K-(D-1-Nal)-FwLL-NH₂ in stomach and thyroid. The presence of the GHS-R1a was also confirmed by immunohistology. In the arterial rat blood plasma, only the original compounds were found. The Cu-64 or Ga-68-NODAGA-NH-K-K-(D-1-NaI)-F-w-L-L-NH₂ radiolabeled inverse agonists turned out to be potent and safe. Due to their easy synthesis, high affinity, medium potency, metabolic stability, and the suitable pharmacokinetic profiles, they are excellent tools for imaging and quantitation of GHS-R1a expression in normal and cancer tissues by PET. These compounds can be used as novel biomarkers of the Ghrelin system in precision medicine.

Acyl-Ghrelin Influences Pancreatic β-Cell Function by Interference with KATP Channels

The aim for this study was to elucidate how the hypothalamic hunger-inducing hormone acyl-ghrelin (AG), which is also produced in the pancreas, affects β-cell function, with particular attention to the role of ATP-sensitive K⁺ (K_ATP) channels and the exact site of action of the hormone. AG hyperpolarized the membrane potential and decreased cytoplasmic calcium concentration [Ca²⁺]_c and glucose-stimulated insulin secretion (GSIS). These effects were abolished in β-cells from SUR1-knockout (KO) mice. AG increased K_ATP current but only in a configuration with intact metabolism. Unacylated ghrelin counteracted the effects of AG. The influence of AG on membrane potential and GSIS could only be averted in the combined presence of a ghrelin receptor (GHSR1a) antagonist and an inverse agonist. The inhibition of GSIS by AG could be prevented by dibutyryl cyclic-cAMP or 3-isobutyl-1-methylxanthine and the somatostatin (SST) receptor 2-5 antagonist H6056. These data indicate that AG indirectly opens K_ATP channels probably by interference with the cAMP/cAMP-dependent protein kinase pathway, resulting in a decrease of [Ca²⁺]_c and GSIS. The experiments with SUR1-KO β-cells point to a direct effect of AG on β-cells and not, as earlier suggested, to an exclusive effect by AG-induced SST release from δ-cells. Nevertheless, SST receptors may be involved in the effect of AG, possibly by heteromerization of AG and SST receptors.

Ghrelin uses the GHS-R1a/Gi/cAMP pathway and induces differentiation only in mature osteoblasts. This ghrelin pathway is impaired in AIS patients

We have examined the Acylated Ghrelin (AG)/Gi pathway in different human osteoblastic cell lines. We have found that: 1) AG induces differentiation/mineralization only in mature osteoblasts; 2) the expression of GHS-R1a increases up to the mature cell stage, 3) the action is mediated via the GHS-R/Gi/cAMP pathway only in mature osteoblasts, and 4) osteoblastic cells from adolescent idiopathic scoliosis (AIS) are resistant to the AG/Gi/cAMP pathway. Altogether, these results suggested that AG uses the GHS-R1a/Gi/cAMP pathway to induce differentiation in mature osteoblasts only. This pathway is impaired in AIS osteoblasts. Understanding AG-specific pathways involved in normal and pathological osteoblasts may be useful for developing new treatments for pathologies such as AIS or osteoporosis.

The Good, the Bad and the Unknown Aspects of Ghrelin in Stress Coping and Stress-Related Psychiatric Disorders

Ghrelin is a peptide hormone released by specialized X/A cells in the stomach and activated by acylation. Following its secretion, it binds to ghrelin receptors in the periphery to regulate energy balance, but it also acts on the central nervous system where it induces a potent orexigenic effect. Several types of stressors have been shown to stimulate ghrelin release in rodents, including nutritional stressors like food deprivation, but also physical and psychological stressors such as foot shocks, social defeat, forced immobilization or chronic unpredictable mild stress. The mechanism through which these stressors drive ghrelin release from the stomach lining remains unknown and, to date, the resulting consequences of ghrelin release for stress coping remain poorly understood. Indeed, ghrelin has been proposed to act as a stress hormone that reduces fear, anxiety- and depression-like behaviors in rodents but some studies suggest that ghrelin may - in contrast - promote such behaviors. In this review, we aim to provide a comprehensive overview of the literature on the role of the ghrelin system in stress coping. We discuss whether ghrelin release is more than a byproduct of disrupted energy homeostasis following stress exposure. Furthermore, we explore the notion that ghrelin receptor signaling in the brain may have effects independent of circulating ghrelin and in what way this might influence stress coping in rodents. Finally, we examine how the ghrelin system could be utilized as a therapeutic avenue in stress-related psychiatric disorders (with a focus on anxiety- and trauma-related disorders), for example to develop novel biomarkers for a better diagnosis or new interventions to tackle relapse or treatment resistance in patients.

Heal the heart through gut (hormone) ghrelin: a potential player to combat heart failure

Ghrelin, a small peptide hormone (28 aa), secreted mainly by X/A-like cells of gastric mucosa, is also locally produced in cardiomyocytes. Being an orexigenic factor (appetite stimulant), it promotes release of growth hormone (GH) and exerts diverse physiological functions, viz. regulation of energy balance, glucose, and/or fat metabolism for body weight maintenance. Interestingly, administration of exogenous ghrelin significantly improves cardiac functions in CVD patients as well as experimental animal models of heart failure. Ghrelin ameliorates pathophysiological condition of the heart in myocardial infarction, cardiac hypertrophy, fibrosis, cachexia, and ischemia reperfusion injury. This peptide also exerts significant impact at the level of vasculature leading to lowering high blood pressure and reversal of endothelial dysfunction and atherosclerosis. However, the molecular mechanism of actions elucidating the healing effects of ghrelin on the cardiovascular system is still a matter of conjecture. Some experimental data indicate its beneficial effects via complex cellular cross talks between autonomic nervous system and cardiovascular cells, some other suggest more direct receptor-mediated molecular actions via autophagy or ionotropic regulation and interfering with apoptotic and inflammatory pathways of cardiomyocytes and vascular endothelial cells. Here, in this review, we summarise available recent data to encourage more research to find the missing links of unknown ghrelin receptor-mediated pathways as we see ghrelin as a future novel therapy in cardiovascular protection.

The association of periodontal diseases with metabolic syndrome and obesity

Periodontitis is a multifactorial chronic inflammatory disease associated with dysbiotic plaque biofilms and characterized by progressive destruction of the tooth-supporting apparatus. Globally, it is estimated that 740 million people are affected by its severe form. Periodontitis has been suggested to be linked to obesity and metabolic syndrome. Obesity, defined as excessive fat accumulation, is a complex multifactorial chronic inflammatory disease, with a high and increasing prevalence. Metabolic syndrome is defined as a cluster of obesity, dyslipidemia, hypertension, and dysglycemia. Obesity, metabolic syndrome and periodontitis are among the most common non-communicable diseases and a large body of evidence from epidemiologic studies supports the association between these conditions. Extensive research has established plausible mechanisms to explain how these conditions can negatively impact each other, pointing to a bidirectional adverse relationship. At present there is only limited evidence available from a few intervention studies. Nevertheless, the global burden of periodontitis combined with the obesity epidemic has important clinical and public health implications for the dental team. In accordance with the common risk factor approach for tackling non-communicable diseases, it has been proposed that oral healthcare professionals have an important role in the promotion of periodontal health and general well-being through facilitation of healthy lifestyle behaviours.

Three-dimensional growth of breast cancer cells potentiates the anti-tumor effects of unacylated ghrelin and AZP-531

Breast cancer is the most common type of cancer in women and notwithstanding important therapeutic advances, remains the second leading cause of cancer-related death. Despite extensive research relating to the hormone ghrelin, responsible for the stimulation of growth hormone release and appetite, little is known of the effects of its unacylated form, especially in cancer. The present study aimed to characterize effects of unacylated ghrelin on breast cancer cells, define its mechanism of action, and explore the therapeutic potential of unacylated ghrelin or analog AZP-531. We report potent anti-tumor effects of unacylated ghrelin, dependent on cells being cultured in 3D in a biologically-relevant extracellular matrix. The mechanism of unacylated ghrelin-mediated growth inhibition involves activation of Gαi and suppression of MAPK signaling. AZP-531 also suppresses the growth of breast cancer cells in vitro and in xenografts, and may be a novel approach for the safe and effective treatment of breast cancer.

Established and In-trial GPCR Families in Clinical Trials: A Review for Target Selection

The largest family of drug targets in clinical trials constitute of GPCRs (G-protein coupled receptors) which accounts for about 34% of FDA (Food and Drug Administration) approved drugs acting on 108 unique GPCRs. Factors such as readily identifiable conserved motif in structures, 127 orphan GPCRs despite various de-orphaning techniques, directed functional antibodies for validation as drug targets, etc. has widened their therapeutic windows. The availability of 44 crystal structures of unique receptors, unexplored non-olfactory GPCRs (encoded by 50% of the human genome) and 205 ligand receptor complexes now present a strong foundation for structure-based drug discovery and design. The growing impact of polypharmacology for complex diseases like schizophrenia, cancer etc. warrants the need for novel targets and considering the undiscriminating and selectivity of GPCRs, they can fulfill this purpose. Again, natural genetic variations within the human genome sometimes delude the therapeutic expectations of some drugs, resulting in medication response differences and ADRs (adverse drug reactions). Around ~30 billion US dollars are dumped annually for poor accounting of ADRs in the US alone. To curb such undesirable reactions, the knowledge of established and currently in clinical trials GPCRs families can offer huge understanding towards the drug designing prospects including "off-target" effects reducing economical resource and time. The druggability of GPCR protein families and critical roles played by them in complex diseases are explained. Class A, class B1, class C and class F are generally established family and GPCRs in phase I (19%), phase II(29%), phase III(52%) studies are also reviewed. From the phase I studies, frizzled receptors accounted for the highest in trial targets, neuropeptides in phase II and melanocortin in phase III studies. Also, the bioapplications for nanoparticles along with future prospects for both nanomedicine and GPCR drug industry are discussed. Further, the use of computational techniques and methods employed for different target validations are also reviewed along with their future potential for the GPCR based drug discovery.

The new mechanism of Ghrelin/GHSR-1a on autophagy regulation

Autophagy is associated with several diseases. In recent years, accumulating evidence has suggested that ghrelin pathway exerts a protective effect by regulating autophagy. This review aims to assess the potential role and use of ghrelin as a new treatment for obesity, cardiovascular diseases, nonalcoholic fatty liver disease (NFALD), neurodegenerative diseases, and tissue damage associated with autophagy. Ghrelin reduces the basal expression of autophagy-related genes in obesity-associated type 2 diabetes and ghrelin level changes in obesity, heart failure, and NFALD as well as altered autophagy. Ghrelin and its receptor GHSR-1 activation induce the phosphorylation of ERK1/2 and the induction of PI-3 kinase (PI3 K) and phosphorylation of Akt. In the myocardium and hypothalamic NPY/AgRP neurons, ghrelin increases levels of the intracellular energy sensor AMPK and enhances autophagy, protecting cardiac ischemia and inducing neural stem cells. Nonetheless, ghrelin activates the PI3 K/Akt/Bcl-2 pathway and inhibits the activation of autophagy, such as tissues injured by sepsis or doxorubicin. In conclusion, endogenous ghrelin system could be considered as a new target or treatment for metabolism disorders, cardiac diseases, neurodegenerative diseases, and tissue injuries.

Ghrelin forms in the modulation of energy balance and metabolism

Ghrelin is a gastric hormone circulating in acylated (AG) and unacylated (UnAG) forms. This narrative review aims at presenting current emerging knowledge on the impact of ghrelin forms on energy balance and metabolism. AG represents ~ 10% of total plasma ghrelin, has an appetite-stimulating effect and is the only form for which a receptor has been identified. Moreover, other metabolic AG-induced effects have been reported, including the modulation of glucose homeostasis with stimulation of liver gluconeogenesis, the increase of fat mass and the improvement of skeletal muscle mitochondrial function. On the other hand, UnAG has no orexigenic effects, however recent reports have shown that it is directly involved in the modulation of skeletal muscle energy metabolism by improving a cluster of interlinked functions including mitochondrial redox activities, tissue inflammation and insulin signalling and action. These findings are in agreement with human studies which show that UnAG circulating levels are positively associated with insulin sensitivity both in metabolic syndrome patients and in a large cohort from the general population. Moreover, ghrelin acylation is regulated by a nutrient sensor mechanism, specifically set on fatty acids availability. These recent findings consistently point towards a novel independent role of UnAG as a regulator of muscle metabolic pathways maintaining energy status and tissue anabolism. While a specific receptor for UnAG still needs to be identified, recent evidence strongly supports the hypothesis that the modulation of ghrelin-related molecular pathways, including those involved in its acylation, may be a potential novel target in the treatment of metabolic derangements in disease states characterized by metabolic and nutritional complications.Level of evidence Level V, narrative review.

Ghrelin Aggravates Prostate Enlargement in Rats with Testosterone-Induced Benign Prostatic Hyperplasia, Stromal Cell Proliferation, and Smooth Muscle Contraction in Human Prostate Tissues

Epidemiologic studies revealed a context between lower urinary tract symptoms (LUTS) suggestive of benign prostatic hyperplasia (BPH) and metabolic syndrome. However, molecular mechanisms underlying this relationship are largely unknown. Prostate enlargement and increased prostate smooth muscle tone are important factors in the pathophysiology of LUTS suggestive of BPH. In the present study, we studied effects of the metabolic hormone ghrelin on prostate enlargement in rats with experimentally induced BPH, growth of cultured stromal cells from human prostate (WPMY-1), and smooth muscle contraction of human prostate tissues. Ghrelin (20 nmol/kg daily, p.o., 2 weeks) increased prostate size in rats with testosterone-induced BPH. Microarray identified 114 ghrelin-upregulated genes (2-fold or more) in these prostates, with possible roles in growth, smooth muscle contraction, or metabolism. 12 genes were selected for further analyses. In human prostate tissues, mRNA levels of 11 of them correlated positively with ghrelin receptor (GHSR) expression, but only two with the degree of BPH. Accordingly, no correlation was evident between GHSR expression level and BPH in human prostate tissues. In WPMY-1 cells, the GHRS agonist MK0677 upregulated 11 of the selected genes. MK0677 induced proliferation of WPMY-1 cells, shown by EdU assay, colony formation, proliferation markers, flow cytometry, and viability. In myographic measurements, GHSR agonists enhanced contractions of human prostate strips. Together, ghrelin may aggravate prostate enlargement, stromal cell growth, and prostate smooth muscle contraction in BPH. Ghrelin may deteriorate urethral obstruction independently from BPH, qualifying the ghrelin system as an attractive new target to be tested for LUTS treatment in BPH.

The brain-penetrating, orally bioavailable, ghrelin receptor agonist HM01 ameliorates motion-induced emesis in Suncus murinus (house musk shrew)

BACKGROUND AND PURPOSE

HM01, a novel, orally bioavailable, brain-penetrating agonist of ghrelin receptors, ameliorates emesis in Suncus murinus. This study compared HM01's activity against motion sickness with that of the less brain-penetrating ghrelin receptor agonist, HM02.

EXPERIMENTAL APPROACH

The potential of HM01 and HM02 to relax isolated mesenteric arteries and to increase feeding was investigated. Radio telemetry was used to record gastric slow waves and body temperature. Plethysmography was used to measure respiratory function. HM01 and HM02 were administered p.o. 1 hr prior to provocative motion, and c-Fos expression in brain sections was assessed.

KEY RESULTS

HM01 and HM02 both relaxed precontracted arteries, yielding EC₅₀ values of 2.5 ± 0.5 and 3.5 ± 0.4 nM respectively. HM01 increased feeding, but HM02 did not. Both compounds caused hypothermia and bradygastria. Motion induced 123 ± 24 emetic events. HM01, but not HM02, reduced motion-induced emesis by 67.6%. Motion increased c-Fos expression in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (DMNV), medial vestibular nucleus (MVe), central nucleus of the amygdala, and paraventricular hypothalamic nucleus (PVH). HM01 alone increased c-Fos expression in the area postrema, NTS, DMNV, PVH, and arcuate hypothalamic nucleus; HM02 had a similar pattern except it did not increase c-Fos in the PVH. Both compounds antagonized the motion-induced increases in c-Fos expression in the MVe.

CONCLUSIONS AND IMPLICATIONS

HM01 is more effective than HM02 in preventing motion-induced emesis. The difference in potency may relate to activation of ghrelin receptors in the PVH.

Attribution of Ghrelin to Cancer; Attempts to Unravel an Apparent Controversy

Ghrelin is an endogenous peptide hormone mainly produced in the stomach. It has been known to regulate energy homeostasis, stimulate secretion of growth hormone, and mediate many other physiologic effects. Various effects attributed to ghrelin contribute to many aspects of cancer development and progression. Accordingly, a large body of evidence has emerged about the association of ghrelin with several types of cancer in scales of cell-line, animal, and human studies. However, existing data are controversial. This controversy occurs in two main domains: one is the controversial results in local effects of ghrelin on different types of human cancer cell-lines; the second is the apparent disagreement in the results of in-vitro and clinical studies that investigated ghrelin association to one type of cancer. These inconsistencies have hampered the indications to consider ghrelin as a potential tumor biomarker or therapeutic agent in cancer patients. Previous studies have reviewed different parts of current literature about the ghrelin-cancer relationship. Although they have highlighted these controversial results in various ways, no specific recommendations have been given to address it. In this study, we comprehensively reviewed in-vitro, in-vivo, and clinical studies and attempted to use the following approaches to unravel the inconsistencies detected: (a) to distinguish local and systemic effects of ghrelin in interpreting its summary clinical role in each cancer; (b) scrutinizing factors that regulate local effects of ghrelin and could justify different effects of ghrelin on different cancer cell-lines. These approaches could have notable implications for future in-vitro and clinical studies.

The Use and Abuse of Growth Hormone in Sports

GH is banned by the World Anti-Doping Agency as a performance-enhancing anabolic agent. Doping with GH likely began in the early 1980s and became more prevalent with the advent of recombinant technology well before any scientific evidence of benefit. The expectation that GH improves physical function stems from its anabolic and lipolytic properties. Athletic performance depends on muscle strength and the energy required to power muscle function. In recreational athletes, GH selectively improves anaerobic sprint capacity but has not been proven to significantly enhance muscle strength, power, or maximum rate of oxygen consumption. GH is secreted as a family of isoform peptides in a pulsatile manner reflecting intermittent secretion and rapid clearance. Its anabolic actions are largely mediated by IGF-I, which stimulates whole-body protein synthesis, including skeletal muscle and collagen proteins. Two methods have been validated for detecting GH abuse in athletes. The first (the isoform method) is based on distinguishing pure recombinant 22-kDa GH from the heterogeneous isoforms secreted from the pituitary. The second (the marker method) is based on measuring blood levels of GH-responsive proteins, specifically IGF-I and the N-terminal propeptide of type III collagen (P-III-NP). Only a handful of athletes have been caught since the implementation of GH doping tests in 2004. The low rate likely reflects the limitation of in-competition testing using current methods. Improved detection rates may be achieved by more out-of-competition testing, introducing athletes' biological passports, and the development of novel methods. Governance, operational, technical, and political factors influence the effectiveness of an anti-doping program.

Targeting Peripheral CB1 Receptors Reduces Ethanol Intake via a Gut-Brain Axis

Endocannabinoids acting on the cannabinoid-1 receptor (CB₁R) or ghrelin acting on its receptor (GHS-R_1A) both promote alcohol-seeking behavior, but an interaction between the two signaling systems has not been explored. Here, we report that the peripheral CB₁R inverse agonist JD5037 reduces ethanol drinking in wild-type mice but not in mice lacking CB₁R, ghrelin peptide or GHS-R_1A. JD5037 treatment of alcohol-drinking mice inhibits the formation of biologically active octanoyl-ghrelin without affecting its inactive precursor desacyl-ghrelin. In ghrelin-producing stomach cells, JD5037 reduced the level of the substrate octanoyl-carnitine generated from palmitoyl-carnitine by increasing fatty acid β-oxidation. Blocking gastric vagal afferents abrogated the ability of either CB₁R or GHS-R_1A blockade to reduce ethanol drinking. We conclude that blocking CB₁R in ghrelin-producing cells reduces alcohol drinking by inhibiting the formation of active ghrelin and its signaling via gastric vagal afferents. Thus, peripheral CB₁R blockade may have therapeutic potential in the treatment of alcoholism.

Ghrelin Receptor Influence on Cocaine Reward is Not Directly Dependent on Peripheral Acyl-Ghrelin

The peptide hormone acyl-ghrelin and its receptor, GHSR1a, represent intriguing therapeutic targets due to their actions in metabolic homeostasis and reward activity. However, this pleotropic activity makes it difficult to intervene in this system without inducing unwanted effects. Thus, it is desirable to identify passive and active regulatory mechanisms that allow differentiation between functional domains. Anatomical restriction by the blood brain barrier represents one major passive regulatory mechanism. However, it is likely that the ghrelin system is subject to additional passive mechanisms that promote independent regulation of orexigenic behavior and reward processing. By applying acyl-ghrelin sequestering antibodies, it was determined that peripheral sequestration of acyl-ghrelin is sufficient to blunt weight gain, but not cocaine rewarding effects. However, both weight gain and reward-associated behaviors were shown to be blocked by direct antagonism of GHSR1a. Overall, these data indicate that GHSR1a effects on reward are independent from peripheral acyl-ghrelin binding, whereas centrally-mediated alteration of energy storage requires peripheral acyl-ghrelin binding. This demonstration of variable ligand-dependence amongst functionally-distinct GHSR1a populations is used to generate a regulatory model for functional manipulation of specific effects when attempting to therapeutically target the ghrelin system.

No effect of unacylated ghrelin administration on subcutaneous PC3 xenograft growth or metabolic parameters in a Rag1-/- mouse model of metabolic dysfunction

Ghrelin is a peptide hormone which, when acylated, regulates appetite, energy balance and a range of other biological processes. Ghrelin predominately circulates in its unacylated form (unacylated ghrelin; UAG). UAG has a number of functions independent of acylated ghrelin, including modulation of metabolic parameters and cancer progression. UAG has also been postulated to antagonise some of the metabolic effects of acyl-ghrelin, including its effects on glucose and insulin regulation. In this study, Rag1-/- mice with high-fat diet-induced obesity and hyperinsulinaemia were subcutaneously implanted with PC3 prostate cancer xenografts to investigate the effect of UAG treatment on metabolic parameters and xenograft growth. Daily intraperitoneal injection of 100 μg/kg UAG had no effect on xenograft tumour growth in mice fed normal rodent chow or 23% high-fat diet. UAG significantly improved glucose tolerance in host Rag1-/- mice on a high-fat diet, but did not significantly improve other metabolic parameters. We propose that UAG is not likely to be an effective treatment for prostate cancer, with or without associated metabolic syndrome.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST2 and SST5 receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST2 is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

The CD36-PPARγ Pathway in Metabolic Disorders

Uncovering the biological role of nuclear receptor peroxisome proliferator-activated receptors (PPARs) has greatly advanced our knowledge of the transcriptional control of glucose and energy metabolism. As such, pharmacological activation of PPARγ has emerged as an efficient approach for treating metabolic disorders with the current use of thiazolidinediones to improve insulin resistance in diabetic patients. The recent identification of growth hormone releasing peptides (GHRP) as potent inducers of PPARγ through activation of the scavenger receptor CD36 has defined a novel alternative to regulate essential aspects of lipid and energy metabolism. Recent advances on the emerging role of CD36 and GHRP hexarelin in regulating PPARγ downstream actions with benefits on atherosclerosis, hepatic cholesterol biosynthesis and fat mitochondrial biogenesis are summarized here. The response of PPARγ coactivator PGC-1 is also discussed in these effects. The identification of the GHRP-CD36-PPARγ pathway in controlling various tissue metabolic functions provides an interesting option for metabolic disorders.

Effect of interleukin-1β on ghrelin receptor in periodontal cells

OBJECTIVES

Periodontopathogens induce immunoinflammatory responses characterized by the release of inflammatory mediators, e.g., interleukin (IL)-1β, IL-6, and IL-8. Ghrelin (GHRL) is an appetite hormone which mediates its effect via the functional receptor GHS-R1a. This study was to examine the effect of an inflammatory insult on GHS-R1a in human periodontal cells.

MATERIALS AND METHODS

Periodontal ligament (PDL) cells and gingival fibroblasts (HGFs) were exposed to IL-1β in the presence and absence of GHRL. Cells were also pre-incubated with specific inhibitors of NF-κB or MEK1/MEK2 signaling. Gene expression of GHS-R1a and proinflammatory mediators was assessed by real-time PCR, GHS-R1 protein level by immunocytochemistry, and NF-κB nuclear translocation by immunofluorescence.

RESULTS

IL-1β increased significantly the GHS-R1a expression in both cell types in a dose-dependent manner. The stimulatory effect of IL-1β involved the NF-κB and MAPK pathways. Exposure of cells to IL-1β also resulted in an increased production of GHS-R1 protein in both cell types. Furthermore, GHRL counteracted significantly the stimulatory actions of IL-1β on IL-6 and IL-8 in PDL cells.

CONCLUSIONS

This study demonstrates for the first time that IL-1β upregulates the functional ghrelin receptor in periodontal fibroblastic cells. Moreover, these results further support the assumption that the GHRL/GHS-R system exerts anti-inflammatory effects. Therefore, the upregulation of ghrelin receptor in periodontal cells in response to an inflammatory stimulus may represent a negative feedback mechanism to attenuate the initial inflammatory process in periodontal diseases.

CLINICAL RELEVANCE

The anti-inflammatory GHRL/GHS-R system may serve as a promising target for the prevention and therapy of periodontal diseases.

Desacyl Ghrelin Decreases Anxiety-like Behavior in Male Mice

Ghrelin is a 28-amino acid polypeptide that regulates feeding, glucose metabolism, and emotionality (stress, anxiety, and depression). Plasma ghrelin circulates as desacyl ghrelin (DAG) or, in an acylated form, acyl ghrelin (AG), through the actions of ghrelin O-acyltransferase (GOAT), exhibiting low or high affinity, respectively, for the growth hormone secretagogue receptor (GHSR) 1a. We investigated the role of endogenous AG, DAG, and GHSR1a signaling on anxiety and stress responses using ghrelin knockout (Ghr KO), GOAT KO, and Ghsr stop-floxed (Ghsr null) mice. Behavioral and hormonal responses were tested in the elevated plus maze and light/dark (LD) box. Mice lacking both AG and DAG (Ghr KO) increased anxiety-like behaviors across tests, whereas anxiety reactions were attenuated in DAG-treated Ghr KO mice and in mice lacking AG (GOAT KO). Notably, loss of GHSR1a (Ghsr null) did not affect anxiety-like behavior in any test. Administration of AG and DAG to Ghr KO mice with lifelong ghrelin deficiency reduced anxiety-like behavior and decreased phospho-extracellular signal-regulated kinase phosphorylation in the Edinger-Westphal nucleus in wild-type mice, a site normally expressing GHSR1a and involved in stress- and anxiety-related behavior. Collectively, our data demonstrate distinct roles for endogenous AG and DAG in regulation of anxiety responses and suggest that the behavioral impact of ghrelin may be context dependent.

A Linear Fragment of Unacylated Ghrelin (UAG6-13) Protects Against Myocardial Ischemia/Reperfusion Injury in Mice in a Growth Hormone Secretagogue Receptor-Independent Manner

Unacylated ghrelin (UAG), the most abundant form of ghrelin in circulation, has been shown to exert cardioprotective effect in experimental cardiopathies. The present study aimed to investigate the cardioprotective effect of a linear bioactive fragment of UAG against myocardial ischemia-induced injury and dysfunction in C57BL/6 wild type mice and the mechanisms involved. Treatments were administered at doses of 100 (UAG), 1,000 and 3,000 (UAG_6-13) nmol/kg at 12 h interval during 14 days prior to 30 min left coronary artery ligation and reperfusion for a period of 6 or 48 h. The infarct area was decreased in a dose-dependent manner at 48 h of reperfusion, with a reduction of 54% at the highest dose of UAG_6-13 tested. Myocardial hemodynamics were improved as demonstrated by an increase in cardiac output, maximum first derivative of left ventricular pressure, and preload recruitable stroke work, a load-independent contractility index. Six hours after reperfusion, circulating levels of IL-6 and TNF-α pro-inflammatory cytokines were reduced, and the effect was maintained at 48 h for TNF-α. 5' AMP-activated protein kinase (AMPK) was activated, while acetyl-CoA carboxylase (ACC) activity was inhibited, along with a decrease in apoptotic protein levels. In isolated hearts, the effect of UAG_6-13 was unaffected by the presence of D-Lys³-GHRP-6, a ghrelin receptor (GHSR1a) antagonist, suggesting that the peptide acted through a GHSR1a-independent pathway. The results support the therapeutic application of UAG bioactive peptide fragments against myocardial ischemia/reperfusion injury.

Regulation of Ghrelin Receptor by Periodontal Bacteria In Vitro and In Vivo

Ghrelin plays a major role in obesity-related diseases which have been shown to be associated with periodontitis. This study sought to analyze the expression of the functional receptor for ghrelin (GHS-R1a) in periodontal cells and tissues under microbial conditions in vitro and in vivo. The GHS-R1a expression in human periodontal cells challenged with the periodontopathogen Fusobacterium nucleatum, in gingival biopsies from periodontally healthy and diseased individuals, and from rats with and without ligature-induced periodontitis was analyzed by real-time PCR, immunocytochemistry, and immunofluorescence. F. nucleatum induced an initial upregulation and subsequent downregulation of GHS-R1a in periodontal cells. In rat experimental periodontitis, the GHS-R1a expression at periodontitis sites was increased during the early stage of periodontitis, but significantly reduced afterwards, when compared with healthy sites. In human gingival biopsies, periodontally diseased sites showed a significantly lower GHS-R1a expression than the healthy sites. The expression of the functional ghrelin receptor in periodontal cells and tissues is modulated by periodontal bacteria. Due to the downregulation of the functional ghrelin receptor by long-term exposure to periodontal bacteria, the anti-inflammatory actions of ghrelin may be diminished in chronic periodontal infections, which could lead to an enhanced periodontal inflammation and tissue destruction.

The octanoylated energy regulating hormone ghrelin: An expanded view of ghrelin's biological interactions and avenues for controlling ghrelin signaling

Ghrelin is a small peptide hormone that requires a unique post-translational modification, serine octanoylation, to bind and activate the GHS-R1a receptor. Initially demonstrated to stimulate hunger and appetite, ghrelin-dependent signaling is implicated in a variety of neurological and physiological processes influencing diseases such as diabetes, obesity, and Prader-Willi syndrome. In addition to its cognate receptor, recent studies have revealed ghrelin interacts with a range of binding partners within the bloodstream. Defining the scope of ghrelin's interactions within the body, understanding how these interactions work in concert to modulate ghrelin signaling, and developing molecular tools for controlling ghrelin signaling are essential for exploiting ghrelin for therapeutic effect. In this review, we discuss recent findings regarding the biological effects of ghrelin signaling, outline binding partners that control ghrelin trafficking and stability in circulation, and summarize the current landscape of inhibitors targeting ghrelin octanoylation.

Ghrelin and Motilin Control Systems in GI Physiology and Therapeutics

Ghrelin and motilin are released from gastrointestinal endocrine cells during hunger, to act through G protein-coupled receptors that have closely related amino acid sequences. The actions of ghrelin are more complex than motilin because ghrelin also exists outside the GI tract, it is processed to des-acyl ghrelin which has activity, ghrelin can exist in truncated forms and retain activity, the ghrelin receptor can have constitutive activity and is subject to biased agonism and finally additional ghrelin-like and des-acyl ghrelin receptors are proposed. Both ghrelin and motilin can stimulate gastric emptying, acting via different pathways, perhaps influenced by biased agonism at the receptors, but research is revealing additional pathways of activity. For example, it is becoming apparent that reduction of nausea may be a key therapeutic target for ghrelin receptor agonists and perhaps for compounds that modulate the constitutive activity of the ghrelin receptor. Reduction of nausea may be the mechanism through which gastroparesis symptoms are reduced. Intriguingly, a potential ability of motilin to influence nausea is also becoming apparent. Ghrelin interacts with digestive function through its effects on appetite, and ghrelin antagonists may have a place in treating Prader-Willi syndrome. Unlike motilin, ghrelin receptor agonists also have the potential to treat constipation by acting at the lumbosacral defecation centres. In conclusion, agonists of both ghrelin and motilin receptors hold potential as treatments for specific subsets of digestive system disorders.

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

OBJECTIVE

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.

AIM OF THE STUDY

To investigate potential associations of total, AG and DAG with carotid atherosclerosis and with gender in the MS.

METHODS

Plasma total ghrelin, AG, DAG and carotid artery IMT (cIMT) were measured in 46 MS patients (NCEP-ATP III criteria, 22M/24F).

RESULTS

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.

CONCLUSIONS

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.

Des-acyl ghrelin inhibits the capacity of macrophages to stimulate the expression of aromatase in breast adipose stromal cells

Des-acyl ghrelin is the unacylated form of the well-characterized appetite-stimulating hormone ghrelin. It affects a number of physiological processes, including increasing adipose lipid accumulation and inhibiting adipose tissue inflammation. Breast adipose tissue inflammation in obesity is associated with an increase in the expression of the estrogen biosynthetic enzyme, aromatase, and is hypothesized to create a hormonal milieu conducive to tumor growth. We previously reported that des-acyl ghrelin inhibits the expression and activity of aromatase in isolated human adipose stromal cells (ASCs), the main site of aromatase expression in the adipose tissue. The current study aimed to examine the effect of des-acyl ghrelin on the capacity of mouse macrophages (RAW264.7 cells) and human adipose tissue macrophages (ATMs) to stimulate aromatase expression in primary human breast ASCs. RAW264.7 cells were treated with 0, 10 and 100pM des-acyl ghrelin following activation with phorbol 12-myristate 13-acetate, and cells and conditioned media were collected after 6 and 24h. The effect of des-acyl ghrelin on macrophage polarization was examined by assessing mRNA expression of pro-inflammatory M1-specific marker Cd11c and anti-inflammatory M2-specific marker Cd206, as well as expression of Tnf and Ptgs2, known mediators of the macrophage-dependent stimulation of aromatase. TNF protein in conditioned media was assessed by ELISA. The effect of RAW264.7 and ATM-conditioned media on aromatase expression in ASCs was assessed after 6h. Results demonstrate des-acyl ghrelin significantly increases the expression of Cd206 and suppresses the expression of Cd11c, Tnf and Ptgs2 in activated RAW264.7 cells. Treatment of RAW264.7 and ATMs with des-acyl ghrelin also significantly reduces the capacity of these cells to stimulate aromatase transcript expression in human breast ASCs. Overall, these findings suggest that in addition to direct effects on aromatase in ASCs, des-acyl ghrelin also has the capacity to inhibit the macrophage-dependent induction of aromatase, and provides a novel mechanism for potential effects of des-acyl ghrelin to break the linkage between obesity and breast cancer.