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

Prostanoids in the pathophysiology of human coronary artery

Coronary artery disease is one of the leading causes of death in wordwide. There is growing evidence that prostanoids are involved in the physiology and pathophysiology of the human coronary artery by controlling vascular tone, remodelling of the vascular wall or angiogenesis. In this review, the production of prostanoids and the expression of prostanoid receptors in human coronary artery in health or disease are described. In addition, the interactions between sex hormones and prostanoids, their participations in the development of coronary artery diseases have been addressed. Globally, most of the studies performed in human coronary artery preparations have shown that prostacyclin (PGI₂) has beneficial effects by inducing vasodilatation and promoting angiogenesis while reverse effects are confirmed by thromboxane A₂ (TxA₂). More studies are needed to determine the roles of the other prostanoids (PGE₂, PGD₂ and PGF_2α) in vascular functions of the human coronary artery. Finally, in addition to the in vitro data about the human coronary artery, myocardial infarction induced by cyclooxygenase-2 (COX-2) inhibitor and the protective effects of aspirin after coronary artery bypass surgery suggest that prostanoids are key mediators in coronary homeostasis.

[Ghrelin: a gastric hormone at the crossroad between growth and appetite regulation]

Ghrelin is a 28 amino acid peptide hormone synthesized within the gastrointestinal tract. Initially identified as the endogenous ligand of the GHS-R1a (Growth Hormone Secretagogue Receptor 1a), ghrelin is a powerful stimulator of growth hormone (GH) secretion. At the crossroad between nutrition, growth and long-term energy metabolism, ghrelin also plays a unique role as the first identified gastric hormone increasing appetite and adiposity. However, the role of the ghrelin/GHS-R system in the physiology of growth, feeding behaviour and energy homeostasis needs to be better understood. Utilization of pharmacological tools and complementary animal models with deficiency in preproghrelin, ghrelin-O-acyl-transferase (GOAT - the enzyme that acylates ghrelin -) or GHS-R in situations of chronic undernutrition or high fat diet gives a more precise overview of the role of ghrelin in the pathophysiology of eating and metabolic disorders.

Is Ghrelin Synthesized in the Central Nervous System?

Ghrelin is an octanoylated peptide that acts via its specific receptor, the growth hormone secretagogue receptor type 1a (GHSR-1a), and regulates a vast variety of physiological functions. It is well established that ghrelin is predominantly synthesized by a distinct population of endocrine cells located within the gastric oxyntic mucosa. In addition, some studies have reported that ghrelin could also be synthesized in some brain regions, such as the hypothalamus. However, evidences of neuronal production of ghrelin have been inconsistent and, as a consequence, it is still as a matter of debate if ghrelin can be centrally produced. Here, we provide a comprehensive review and discussion of the data supporting, or not, the notion that the mammalian central nervous system can synthetize ghrelin. We conclude that no irrefutable and reproducible evidence exists supporting the notion that ghrelin is synthetized, at physiologically relevant levels, in the central nervous system of adult mammals.

Ghrelin and Breast Cancer: Emerging Roles in Obesity, Estrogen Regulation, and Cancer

Local and systemic factors have been shown to drive the growth of breast cancer cells in postmenopausal obese women, who have increased risk of estrogen receptor-positive breast cancer. Estrogens, produced locally in the breast fat by the enzyme aromatase, have an important role in promoting cancer cell proliferation. Ghrelin, a 28-amino acid peptide hormone, may also influence cancer growth. This peptide is produced in the stomach and acts centrally to regulate appetite and growth hormone release. Circulating levels of ghrelin, and its unacylated form, des-acyl ghrelin, are almost always inversely correlated with obesity, and these peptide hormones have recently been shown to inhibit adipose tissue aromatase expression. Ghrelin and des-acyl ghrelin have also been shown to be produced by some tumor cells and influence tumor growth. The ghrelin/des-acyl ghrelin–cancer axis is complex, one reason being that tumor cells have been shown to express splice variants of ghrelin, and ghrelin and des-acyl ghrelin might act at receptors other than the cognate ghrelin receptor, growth hormone secretagogue receptor 1a, in tumors. Effects of ghrelin and des-acyl ghrelin on energy homeostasis may also affect tumor development and growth. This review will summarize our current understanding of the role of ghrelin and des-acyl ghrelin in hormone-dependent cancers, breast cancer in particular.

Neuropeptides and Microglial Activation in Inflammation, Pain, and Neurodegenerative Diseases

Microglial cells are responsible for immune surveillance within the CNS. They respond to noxious stimuli by releasing inflammatory mediators and mounting an effective inflammatory response. This is followed by release of anti-inflammatory mediators and resolution of the inflammatory response. Alterations to this delicate process may lead to tissue damage, neuroinflammation, and neurodegeneration. Chronic pain, such as inflammatory or neuropathic pain, is accompanied by neuroimmune activation, and the role of glial cells in the initiation and maintenance of chronic pain has been the subject of increasing research over the last two decades. Neuropeptides are small amino acidic molecules with the ability to regulate neuronal activity and thereby affect various functions such as thermoregulation, reproductive behavior, food and water intake, and circadian rhythms. Neuropeptides can also affect inflammatory responses and pain sensitivity by modulating the activity of glial cells. The last decade has witnessed growing interest in the study of microglial activation and its modulation by neuropeptides in the hope of developing new therapeutics for treating neurodegenerative diseases and chronic pain. This review summarizes the current literature on the way in which several neuropeptides modulate microglial activity and response to tissue damage and how this modulation may affect pain sensitivity.

Progressive Seizure Aggravation in the Repeated 6-Hz Corneal Stimulation Model Is Accompanied by Marked Increase in Hippocampal p-ERK1/2 Immunoreactivity in Neurons

The 6-Hz corneal stimulation test is used to screen novel antiepileptic molecules to overcome the problem of drug refractoriness. Although recognized as a standard test, it has been evaluated only recently in the attempt to characterize the putative neuronal networks involved in seizures caused by corneal stimulation. In particular, by recording from the CA1 region we previously established that the hippocampus participates to propagation of seizure activity. However, these findings were not corroborated by using markers of neuronal activation such as FosB/ΔFosB antigens. In view of this discrepancy, we performed new experiments to characterize the changes in levels of phosphorylated extracellular signal-regulated kinases1/2 (p-ERK1/2), which are also used as markers of neuronal activation. To this aim, mice underwent corneal stimulation up to three different times, in three sessions separated by an interval of 3 days. To characterize a group in which seizures could be prevented by pharmacological treatment, we also considered pretreatment with the ghrelin receptor antagonist EP-80317 (330 μg/kg). Control mice were sham-treated. Video electrocorticographic (ECoG) recordings were obtained from mice belonging to each group of treatment. Animals were finally used to characterize the immunoreactivity for FosB/ΔFosB and p-ERK1/2 in the hippocampus. As previously shown, FosB/ΔFosB levels were highly increased throughout the hippocampus by the first induced seizure but, in spite of the progressively increased seizure severity, they were restored to control levels after the third stimulation. At variance, corneal stimulation caused a progressive increase in p-ERK1/2 immunoreactivity all over the hippocampus, especially in CA1, peaking in the third session. Predictably, EP-80317 administration reduced both duration and severity of seizures, prevented the increase in FosB/ΔFosB levels in the first session, and partially counteracted the increase in p-ERK1/2 levels in the third session. The vast majority of p-ERK1/2 immunopositive cells were co-labeled with FosB/ΔFosB antibodies, suggesting the existence of a relationship between the investigated markers in a subpopulation of neurons activated by seizures. These findings suggest that p-ERK1/2 are useful markers to define the aggravation of seizures and the response to anticonvulsant treatments. In particular, p-ERK1/2 expression clearly identified the involvement of hippocampal regions during seizure aggravation in the 6-Hz model.

Hypothalamic regulation of body growth and appetite by ghrelin-derived peptides during balanced nutrition or undernutrition

Among the gastrointestinal hormones that regulate food intake and energy homeostasis, ghrelin plays a unique role as the first one identified to increases appetite and stimulate GH secretion. This review highlights the latest mechanism by which ghrelin modulates body growth, appetite and energy metabolism by exploring pharmacological actions of the hormone and consequences of genetic or pharmacological blockade of the ghrelin/GHS-R (Growth Hormone Secretagogue Receptor) system on physiological responses in specific nutritional situations. Within the hypothalamus, novel mechanisms of action of this hormone involve its interaction with other ghrelin-derived peptides, such as desacyl ghrelin and obestatin, which are thought to act as functional ghrelin antagonists, and possible modulation of the GHS-R with other G-protein coupled receptors. During chronic undernutrition such as anorexia nervosa, variations of ghrelin-derived peptides may be an adaptative metabolic response to maintain normal glycemic control. Interestingly, some of ghrelin's metabolic actions are thought to be relayed through modulation of GH, an anabolic and hyperglycemic agent.

Fasting modulates GH/IGF-I axis and its regulatory systems in the mammary gland of female mice: Influence of endogenous cortistatin

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are essential factors in mammary-gland (MG) development and are altered during fasting. However, no studies have investigated the alterations in the expression of GH/IGF-I and its regulatory systems (somatostatin/cortistatin and ghrelin) in MG during fasting. Therefore, this study was aimed at characterizing the regulation of GH/IGF-I/somatostatin/cortistatin/ghrelin-systems expression in MG of fasted female-mice (compared to fed-controls) and the influence of endogenous-cortistatin (using cortistatin-knockouts). Fasting decreased IGF-I while increased IGF-I/Insulin-receptors expression in MGs. Fasting provoked an increase in GH expression that might be associated to enhanced ghrelin-variants/ghrelin-O-acyl-transferase enzyme expression, while an upregulation of somatostatin-receptors was observed. However, cortistatin-knockouts mice showed a decrease in GH and somatostatin receptor-subtypes expression. Altogether, we demonstrate that GH/IGF-I, somatostatin/cortistatin and ghrelin systems expression is altered in MG during fasting, suggesting a relevant role in coordinating its response to metabolic stress, wherein endogenous cortistatin might be essential for an appropriate response.

Is there an effect of ghrelin/ghrelin analogs on cancer? A systematic review

Ghrelin is a hormone with multiple physiologic functions, including promotion of growth hormone release, stimulation of appetite and regulation of energy homeostasis. Treatment with ghrelin/ghrelin-receptor agonists is a prospective therapy for disease-related cachexia and malnutrition. In vitro studies have shown high expression of ghrelin in cancer tissue, although its role including its impact in cancer risk and progression has not been established. We performed a systematic literature review to identify peer-reviewed human or animal in vivo original research studies of ghrelin, ghrelin-receptor agonists, or ghrelin genetic variants and the risk, presence, or growth of cancer using structured searches in PubMed database as well as secondary searches of article reference lists, additional reviews and meta-analyses. Overall, 45 (73.8%) of the 61 studies reviewed, including all 11 involving exogenous ghrelin/ghrelin-receptor agonist treatment, reported either a null (no statistically significant difference) or inverse association of ghrelin/ghrelin-receptor agonists or ghrelin genetic variants with cancer risk, presence or growth; 10 (16.7%) studies reported positive associations; and 6 (10.0%) reported both negative or null and positive associations. Differences in serum ghrelin levels in cancer cases vs controls (typically lower) were reported for some but not all cancers. The majority of in vivo studies showed a null or inverse association of ghrelin with risk and progression of most cancers, suggesting that ghrelin/ghrelin-receptor agonist treatment may have a favorable safety profile to use for cancer cachexia. Additional large-scale prospective clinical trials as well as basic bioscientific research are warranted to further evaluate the safety and benefits of ghrelin treatment in patients with cancer.

Lentivirus-mediated shRNA interference of ghrelin receptor blocks proliferation in the colorectal cancer cells

Ghrelin, an orexigenic peptide, acts via the growth hormone secretagogue receptor (GHSR) to stimulate the release of growth hormone. Moreover, it has a range of biological actions, including the stimulation of food intake, modulation of insulin signaling and cardiovascular effects. Recently, it has been demonstrated that ghrelin has a proliferative and antiapoptotic effects in cancers, suggesting a potential role in promoting tumor growth. However, it remains unknown whether GHSR contributes to colorectal cancer proliferation. In this study, the therapeutic effect of lentivirus‐mediated short hairpin RNA (shRNA) targeting ghrelin receptor 1a (GHSR1a) was analyzed in colorectal cancer cell line SW480 both in vitro and in vivo. Our study demonstrated that ghrelin and GHSR1a are significantly upregulated in cancerous colorectal tissue samples and cell lines. In vitro, human colorectal cancer cell line SW480 with downregulation of GHSR1a by shRNA showed significant inhibition of cell viability compared with blank control (BC) or scrambled control (SC) regardless of the application of exogenous ghrelin. Furthermore, GHSR1a silencing by target specific shRNA was shown capable of increasing PTEN, inhibiting AKT phosphorylation and promoting the release of p53 in SW480 cells. In addition, the effects of GHSR1a knockdown were further explored in vivo using colorectal tumor xenograft mouse model. The tumor weights were decreased markedly in GHSR1α knockdown SW480 mouse xenograft tumors compared with blank control or negative control tumors. Our results suggested that the expression of GHSR1a is significantly correlated with the growth of colorectal cancer cells, and the GHSR1a knockdown approach may be a potential therapy for the treatment of colorectal cancer.

Protective actions of des-acylated ghrelin on brain injury and blood–brain barrier disruption after stroke in mice

Stroke is a leading cause of death, but treatments are limited. This experimental study reveals that the hormone ghrelin powerfully protects the brain and its blood vessels against injury after stroke, raising the possibility that it could be exploited therapeutically.

Association of cord blood des-acyl ghrelin with birth weight, and placental GHS-R1 receptor expression in SGA, AGA, and LGA newborns

Although ghrelin in cord blood has been associated to birth weight, its role in fetal and postnatal growth has not been elucidated. The aim of this study was to analyze total ghrelin, acyl ghrelin (AG), and des-acyl ghrelin (DAG) in cord blood of newborns with idiopathic birth weight alterations, and to evaluate protein expression of placental GHS-R1, in order to investigate their correlation with birth weight and placental weight. We performed a cross-sectional comparative study in umbilical cord blood and placentas from healthy mothers of SGA, AGA, and LGA (small, adequate and large for gestational age) term newborns (n = 20 per group). Cord blood total ghrelin, AG, and DAG were measured by ELISA, and placental GHS-R1 expression was evaluated by Western blot. Cord blood DAG was higher in SGA compared to AGA newborns (902.1 ± 109.1 and 597.4 ± 58.2 pg/ml, respectively, p = 0.01) while LGA and AGA showed similar values (627.2 ± 76.4 pg/ml for LGA, p = 0.80). DAG negatively correlated with birthweight (r = -0.31, p = 0.02) and placental weight (r = -0.33, p = 0.02). No differences in AG or total ghrelin were found. GHS-R1 protein in placenta was not differentially expressed among SGA, AGA, and LGA. Our results suggest a role of DAG in intrauterine growth. Further studies are needed in order to elucidate the mechanisms by which DAG participates in fetal growth.

Ghrelin Receptor Mutations and Human Obesity

Growth hormone secretagogue receptor (GHSR) was originally identified as an orphan receptor in porcine and rat anterior pituitary membranes. In 1999, GHSR was deorphanized and shown to be a receptor for ghrelin, a peptide hormone secreted from the stomach. Therefore, GHSR is also called ghrelin receptor. In addition to regulating growth hormone secretion, ghrelin receptor regulates various physiological processes, including food intake and energy expenditure, glucose metabolism, cardiovascular functions, gastric acid secretion and motility, and immune function. Several human genetic studies conducted in populations originated from Europe, Africa, South America, and East Asia identified rare mutations and single nucleotide polymorphisms that might be associated with human obesity and short stature. Functional analyses of mutant GHSRs reveal multiple defects, including cell surface expression, ligand binding, and basal and stimulated signaling. With growing understanding in the functionality of naturally occurring GHSR mutations, potential therapeutic strategies including pharmacological chaperones and novel ligands could be used to correct the GHSR mutants.

Energy homeostasis and appetite regulating hormones as predictors of weight loss in men and women

Sex differences in weight loss are often seen despite using the same weight loss program. There has been relatively little investigation of physiological influences on weight loss success in males and females, such as energy homeostasis and appetite regulating hormones. The aims were to 1) characterise baseline plasma leptin, ghrelin and adiponectin concentrations in overweight and obese males and females, and 2) determine whether baseline concentrations of these hormones predict weight loss in males and females. Subjects were overweight or obese (BMI 25-40 kg/m(2)) adults aged 18-60 years. Weight was measured at baseline, and after three and six months participation in a weight loss program. Baseline concentrations of leptin, adiponectin and ghrelin were determined by enzyme-linked immunosorbent assay (ELISA). An independent t-test or non-parametric equivalent was used to determine any differences between sex. Linear regression determined whether baseline hormone concentrations were predictors of six-month weight change. Females had significantly higher baseline concentrations of leptin, adiponectin and unacylated ghrelin as well as ratios of leptin:adiponectin and leptin:ghrelin. The ratio of acylated:unacylated ghrelin was significantly higher in males. In males and females, a higher baseline concentration of unacylated ghrelin predicted greater weight loss at six months. Additionally in females, higher baseline total ghrelin predicted greater weight loss and a higher ratio of leptin:ghrelin predicted weight gain at six months. A higher pre-weight-loss plasma concentration of unacylated ghrelin is a modest predictor of weight loss success in males and females, while a higher leptin:ghrelin ratio is a predictor of weight loss failure in females. Further investigation is required into what combinations and concentrations of these hormones are optimal for weight loss success.

Des-Acyl Ghrelin Directly Targets the Arcuate Nucleus in a Ghrelin-Receptor Independent Manner and Impairs the Orexigenic Effect of Ghrelin

Ghrelin is a stomach-derived octanoylated peptide hormone that plays a variety of well-established biological roles acting via its specific receptor known as growth hormone secretagogue receptor (GHSR). In plasma, a des-octanoylated form of ghrelin, named des-acyl ghrelin (DAG), also exists. DAG is suggested to be a signalling molecule that has specific targets, including the brain, and regulates some physiological functions. However, no specific receptor for DAG has been reported until now, and, consequently, the potential role of DAG as a hormone has remained a matter of debate. In the present study, we show that DAG specifically binds to and acts on a subset of arcuate nucleus (ARC) cells in a GHSR-independent manner. ARC cells labelled by a DAG fluorescent tracer include the neuropeptide Y (NPY) and non-NPY neurones. Given the well-established role of the ARC in appetite regulation, we tested the effect of centrally administered DAG on food intake. We found that DAG failed to affect dark phase feeding, as well as food intake, after a starvation period; however, it impaired the orexigenic actions of peripherally administered ghrelin. Thus, we conclude that DAG directly targets ARC neurones and antagonises the orexigenic effects of peripherally administered ghrelin.

Serum ghrelin and prediction of metabolic parameters in over 20-year follow-up

Ghrelin is a peptide hormone from the stomach, with an ability to release growth-hormone from the pituitary. Numerous cross-sectional studies indicate that ghrelin also has a role in metabolic abnormalities, such as metabolic syndrome and type 2 diabetes, but evidence for long-term effect is scarce. We investigated, whether ghrelin concentration measured in middle age would predict the development or absence of metabolic disturbances subsequently. Study population consisted of 600 middle-aged persons, and the follow-up time was approximately 21 years. Plasma total ghrelin concentration was measured at the baseline, and divided to tertiles. Numerous anthropometric and other clinical measurements (including blood pressure), and laboratory test were made both at the baseline and at the follow-up. After the follow-up the prevalence of high systolic blood pressure according to MetS IDF-criteria was the lowest in the highest ghrelin tertile, and the highest in the first (p<0.03). When only subjects free of hypertension medication at baseline were considered, subjects belonging to the highest ghrelin tertile developed less new hypertension and high blood pressure according to IDF-criteria as well as medication for it during the follow-up (p<0.05). Although serum insulin levels were negatively correlated to ghrelin levels at both points in time (p<0.001 at baseline and p=0.003 at follow-up), plasma ghrelin concentration did not predict the development of abnormalities in glucose tolerance. The association with ghrelin and metabolic syndrome was lost during the follow-up. In conclusion, our results suggest high ghrelin to be protective against the development of hypertension in the long-term follow-up.

Structure-activity relationship for peptídic growth hormone secretagogues

Growth hormone releasing peptides (GHRPs) could be widely used by cheating athletes because they produce growth hormone (GH) secretion, so may generate an ergogenic effect in the body. Knowledge of the essential amino acids needed in GHRP structure for interaction with the target biological receptor GHSR1a, the absorption through different administration routes, and the maintenance of pharmacological activity of potential biotransformation products may help in the fight against their abuse in sport. Several GHRPs and truncated analogues with the common core Ala-Trp-(D-Phe)-Lys have been studied with a radio-competitive assay for the GHSR1a receptor against the radioactive natural ligand ghrelin. Relevant chemical modifications influencing the activity for positions 1, 2, 3, and 7 based on the structure aa-aa-aa-Ala-Trp-(D-Phe)-Lys have been obtained. To test in vivo the applicability of the activities observed, the receptor assay activity in samples from excretion studies performed after nasal administration of GHRP-1, GHRP-2, GHRP-6, Hexarelin, and Ipamorelin was confirmed. Overall results obtained allow to infer structure-activity information for those GHRPs and to detect GHSR1a binding (intact GHRPs plus active metabolites) in excreted urines. Copyright © 2016 John Wiley & Sons, Ltd.

Ghrelin and motilin receptors as drug targets for gastrointestinal disorders

The gastrointestinal tract is the major source of the related hormones ghrelin and motilin, which act on structurally similar G protein-coupled receptors. Nevertheless, selective receptor agonists are available. The primary roles of endogenous ghrelin and motilin in the digestive system are to increase appetite or hedonic eating (ghrelin) and initiate phase III of gastric migrating myoelectric complexes (motilin). Ghrelin and motilin also both inhibit nausea. In clinical trials, the motilin receptor agonist camicinal increased gastric emptying, but at lower doses reduced gastroparesis symptoms and improved appetite. Ghrelin receptor agonists have been trialled for the treatment of diabetic gastroparesis because of their ability to increase gastric emptying, but with mixed results; however, relamorelin, a ghrelin agonist, reduced nausea and vomiting in patients with this disorder. Treatment of postoperative ileus with a ghrelin receptor agonist proved unsuccessful. Centrally penetrant ghrelin receptor agonists stimulate defecation in animals and humans, although ghrelin itself does not seem to control colorectal function. Thus, the most promising uses of motilin receptor agonists are the treatment of gastroparesis or conditions with slow gastric emptying, and ghrelin receptor agonists hold potential for the reduction of nausea and vomiting, and the treatment of constipation. Therapeutic, gastrointestinal roles for receptor antagonists or inverse agonists have not been identified.

Site and mechanism of the colokinetic action of the ghrelin receptor agonist, HM01

BACKGROUND

It has been recently demonstrated that the ghrelin receptor agonist, HM01, caused defecation in rats that were treated to provide a model for the constipation of Parkinson's disease. HM01 significantly increased fecal output and increased Fos activity in neurons of the hypothalamus and hindbrain, but not in the spinal defecation center. Other ghrelin agonists act on the defecation center.

METHODS

Receptor pharmacology was examined in ghrelin receptor (GHSR1a) transfected cells. Anesthetized rats were used to investigate sites and mechanisms of action.

KEY RESULTS

HM01 activated rat GHSR1a at nanomolar concentrations and was antagonized by the GHSR1a antagonist, YIL781. HM01, intravenous, was potent to activate propulsive colorectal contractions. This was prevented by pelvic nerve section and by intravenous YIL781, but not by spinal cord section rostral to the defecation centers. Direct intrathecal application of HM01 to the defecation center at spinal level L6-S1 initiated propulsive contractions of the colorectum.

CONCLUSIONS & INFERENCES

HM01 stimulates GHSR1a receptors on neurons in the lumbosacral defecation centers to cause propulsive contractions and emptying of the colorectum. It has greater potency when given systemically, compared with other GHSR1a agonists.

Altered gut and adipose tissue hormones in overweight and obese individuals: cause or consequence?

The aim of this article is to review the research into the main peripheral appetite signals altered in human obesity, together with their modifications after body weight loss with diet and exercise and after bariatric surgery, which may be relevant to strategies for obesity treatment. Body weight homeostasis involves the gut–brain axis, a complex and highly coordinated system of peripheral appetite hormones and centrally mediated neuronal regulation. The list of peripheral anorexigenic and orexigenic physiological factors in both animals and humans is intimidating and expanding, but anorexigenic glucagon-like peptide 1 (GLP-1), cholecystokinin (CCK), peptide YY (PYY) and orexigenic ghrelin from the gastrointestinal tract, pancreatic polypeptide (PP) from the pancreas and anorexigenic leptin from adiposites remain the most widely studied hormones. Homeostatic control of food intake occurs in humans, although its relative importance for eating behaviour is uncertain, compared with social and environmental influences. There are perturbations in the gut–brain axis in obese compared with lean individuals, as well as in weight-reduced obese individuals. Fasting and postprandial levels of gut hormones change when obese individuals lose weight, either with surgical or with dietary and/or exercise interventions. Diet-induced weight loss results in long-term changes in appetite gut hormones, postulated to favour increased appetite and weight regain while exercise programmes modify responses in a direction expected to enhance satiety and permit weight loss and/or maintenance. Sustained weight loss achieved by bariatric surgery may in part be mediated via favourable changes to gut hormones. Future work will be necessary to fully elucidate the role of each element of the axis, and whether modifying these signals can reduce the risk of obesity.

Agonism, Antagonism, and Inverse Agonism Bias at the Ghrelin Receptor Signaling

The G protein-coupled receptor GHS-R1a mediates ghrelin-induced growth hormone secretion, food intake, and reward-seeking behaviors. GHS-R1a signals through Gq, Gi/o, G13, and arrestin. Biasing GHS-R1a signaling with specific ligands may lead to the development of more selective drugs to treat obesity or addiction with minimal side effects. To delineate ligand selectivity at GHS-R1a signaling, we analyzed in detail the efficacy of a panel of synthetic ligands activating the different pathways associated with GHS-R1a in HEK293T cells. Besides β-arrestin2 recruitment and ERK1/2 phosphorylation, we monitored activation of a large panel of G protein subtypes using a bioluminescence resonance energy transfer-based assay with G protein-activation biosensors. We first found that unlike full agonists, Gq partial agonists were unable to trigger β-arrestin2 recruitment and ERK1/2 phosphorylation. Using G protein-activation biosensors, we then demonstrated that ghrelin promoted activation of Gq, Gi1, Gi2, Gi3, Goa, Gob, and G13 but not Gs and G12. Besides, we identified some GHS-R1a ligands that preferentially activated Gq and antagonized ghrelin-mediated Gi/Go activation. Finally, we unambiguously demonstrated that in addition to Gq, GHS-R1a also promoted constitutive activation of G13. Importantly, we identified some ligands that were selective inverse agonists toward Gq but not of G13. This demonstrates that bias at GHS-R1a signaling can occur not only with regard to agonism but also to inverse agonism. Our data, combined with other in vivo studies, may facilitate the design of drugs selectively targeting individual signaling pathways to treat only the therapeutically relevant function.

Ghrelin and atherosclerosis

PURPOSE OF REVIEW

The increasing prevalence of metabolic syndrome and the consequent cardiovascular diseases has stimulated an active search for novel risk factors. The hormones regulating energy balance are of special interest as potential risk factor indicators of cardiovascular diseases. Ghrelin provides an attractive target for studies of atherosclerosis being a gastric peptide hormone having multiple functions including orexigenic and growth hormone-releasing effects and is also involved in the regulation of cardiovascular and immunomodulatory system.

RECENT FINDINGS

The aim of the present article is to review recent studies on the role of ghrelin in the atherosclerosis. Ghrelin seems to influence known risk factors of atherosclerosis, endothelial dysfunction, inflammation and oxidation. These themes will be covered by the present article and a summary of clinical studies of ghrelin in atherosclerosis will be given.

SUMMARY

Ghrelin plays a beneficial role in multiple processes of atherogenesis and vascular function. However, ghrelin's effects are not as strong as those of traditional known risk factors.

Ghrelin receptors as targets for novel motility drugs

Constipation arises from a multitude of causes, including aging, spinal cord injury (SCI), and dietary issues. The heterogeneity of inciting factors has made the treatment of constipation particularly challenging. Agonists of ghrelin receptors have beneficial effects on delayed gastric emptying, but less is known about their ability to improve colorectal motility. Recent publications indicate that the activation of the ghrelin receptors in the spinal cord can alleviate constipation due to dietary causes, Parkinsonism, and SCI in rodents. Ghrelin-responsive neurons in the intermediolateral cell column of the lumbosacral spinal cord can activate enteric microcircuits that coordinate propulsive colorectal contractions, leading to defecation. Learning more about the properties of neurons in the spinal defecation center and the roles of ghrelin receptors in the defecation reflex will accelerate the development of improved treatments of constipation.

Membrane bound O-acyltransferases and their inhibitors

Since the identification of the membrane-bound O-acyltransferase (MBOATs) protein family in the early 2000s, three distinct members [porcupine (PORCN), hedgehog (Hh) acyltransferase (HHAT) and ghrelin O-acyltransferase (GOAT)] have been shown to acylate specific proteins or peptides. In this review, topology determination, development of assays to measure enzymatic activities and discovery of small molecule inhibitors are compared and discussed for each of these enzymes.

In1-ghrelin splicing variant is overexpressed in pituitary adenomas and increases their aggressive features

Pituitary adenomas comprise a heterogeneous subset of pathologies causing serious comorbidities, which would benefit from identification of novel, common molecular/cellular biomarkers and therapeutic targets. The ghrelin system has been linked to development of certain endocrine-related cancers. Systematic analysis of the presence and functional implications of some components of the ghrelin system, including native ghrelin, receptors and the recently discovered splicing variant In1-ghrelin, in human normal pituitaries (n = 11) and pituitary adenomas (n = 169) revealed that expression pattern of ghrelin system suffers a clear alteration in pituitary adenomasas comparedwith normal pituitary, where In1-ghrelin is markedly overexpressed. Interestingly, in cultured pituitary adenoma cells In1-ghrelin treatment (acylated peptides at 100 nM; 24–72 h) increased GH and ACTH secretion, Ca²⁺ and ERK1/2 signaling and cell viability, whereas In1-ghrelin silencing (using a specific siRNA; 100 nM) reduced cell viability. These results indicate that an alteration of the ghrelin system, specially its In1-ghrelin variant, could contribute to pathogenesis of different pituitary adenomas types, and suggest that this variant and its related ghrelin system could provide new tools to identify novel, more general diagnostic, prognostic and potential therapeutic targets in pituitary tumors.

Analysis of the ghrelin receptor-independent vascular actions of ulimorelin

Ulimorelin (TZP101) is a ghrelin receptor agonist that stimulates intestinal motility, but also reduces blood pressure in rodents and humans and dilates blood vessels. It has been proposed as a treatment for intestinal motility disorders. Here we investigated the mechanisms through which ulimorelin affects vascular diameter. Actions of ulimorelin on wall tension of rodent arteries were investigated and compared with other ghrelin receptor agonists. Saphenous, mesenteric and basilar arteries were obtained from Sprague-Dawley rats (male, 8 weeks) and saphenous arteries were obtained from wild type or ghrelin receptor null mice. These were mounted in myography chambers to record artery wall tension. Ulimorelin (0.03-30µM) inhibited phenylephrine-induced contractions of rat saphenous (IC50=0.6µM; Imax=66±5%; n=3-6) and mesenteric arteries (IC50=5µM, Imax=113±16%; n=3-4), but not those contracted by U46619, ET-1 or 60mM [K(+)]. Relaxation of phenylephrine-constricted arteries was not observed with ghrelin receptor agonists TZP102, capromorelin or AZP-531. In rat saphenous and basilar arteries, ulimorelin (10-100µM) and TZP102 (10-100µM) constricted arteries (EC50=9.9µM; Emax=50±7% and EC50=8µM; Emax=99±16% respectively), an effect not attenuated by the ghrelin receptor antagonist YIL 781 3µM or mimicked by capromorelin or AZP-531. In mesenteric arteries, ulimorelin, 1-10µM, caused a surmountable rightward shift in the response to phenylephrine (0.01-1000µM; pA2=5.7; n=3-4). Ulimorelin had similar actions in mouse saphenous artery from both wild type and ghrelin receptor null mice. We conclude that ulimorelin causes vasorelaxation through competitive antagonist action at α1-adrenoceptors and a constrictor action not mediated via the ghrelin receptor.

Ghrelin-related peptides exert protective effects in the cerebral circulation of male mice through a nonclassical ghrelin receptor(s)

The ghrelin-related peptides, acylated ghrelin, des-acylated ghrelin, and obestatin, are novel gastrointestinal hormones. We firstly investigated whether the ghrelin gene, ghrelin O-acyltransferase, and the ghrelin receptor (GH secretagogue receptor 1a [GHSR1a]) are expressed in mouse cerebral arteries. Secondly, we assessed the cerebrovascular actions of ghrelin-related peptides by examining their effects on vasodilator nitric oxide (NO) and superoxide production. Using RT-PCR, we found the ghrelin gene and ghrelin O-acyltransferase to be expressed at negligible levels in cerebral arteries from male wild-type mice. mRNA expression of GHSR1a was also found to be low in cerebral arteries, and GHSR protein was undetectable in GHSR-enhanced green fluorescent protein mice. We next found that exogenous acylated ghrelin had no effect on the tone of perfused cerebral arteries or superoxide production. By contrast, exogenous des-acylated ghrelin or obestatin elicited powerful vasodilator responses (EC50 < 10 pmol/L) that were abolished by the NO synthase inhibitor N(ω)-nitro-L-arginine methyl ester. Furthermore, exogenous des-acylated ghrelin suppressed superoxide production in cerebral arteries. Consistent with our GHSR expression data, vasodilator effects of des-acylated ghrelin or obestatin were sustained in the presence of YIL-781 (GHSR1a antagonist) and in arteries from Ghsr-deficient mice. Using ghrelin-deficient (Ghrl(-/-)) mice, we also found that endogenous production of ghrelin-related peptides regulates NO bioactivity and superoxide levels in the cerebral circulation. Specifically, we show that NO bioactivity was markedly reduced in Ghrl(-/-) vs wild-type mice, and superoxide levels were elevated. These findings reveal protective actions of exogenous and endogenous ghrelin-related peptides in the cerebral circulation and show the existence of a novel ghrelin receptor(s) in the cerebral endothelium.