Expressions of ghrelin and GHSR-1a in the corpus luteum and the stimulatory effect of ghrelin on luteal function of pregnant sows

Studies have shown that ghrelin played direct actions in ovarian function, but the direct role of ghrelin in corpus luteum (CL) of pregnant sows has remained obscure. The study aimed to examine the expressions of ghrelin and its functional receptor (GHSR-1a) in the CL of sows during pregnancy, and evaluate the role of ghrelin in CL function of pregnant sows. Immunohistochemistry analysis showed that ghrelin and GHSR-1a are both predominantly localized in the luteal cells of pregnant sows CL. Strong immunoreactivity for ghrelin and GHSR-1a is detected at days 20 (early) and 50 (middle), but weak immunoreactivity is observed at days 90 (late) post mating. Similarly, there is a significant effect of pregnant phase on the expression (mRNA and protein) of ghrelin and GHSR-1a in the CL, with higher levels at days 20 (early) and 50 (middle), and lower values at 90 (late) post mating. In vitro, treatments of luteal cells with ghrelin (from 0.01 to 10 ng/mL) are promoted cell viability and P4 secretion in a dose-dependent manner. Ghrelin is also accelerated the LH-induced P4 secretion in luteal cells. Moreover, ghrelin is induced the release and mRNA expression of LH, and increased the release of prostaglandin (PG)E₂, but reduced the secretion of PGF_2α in luteal cells. In conclusion, the presences of ghrelin and GHSR-1a in the porcine CL during pregnancy, and the stimulatory effect of ghrelin on luteal cells suggest positive regulation by ghrelin in CL function of pregnant sows.

Increased growth hormone secretagogue receptor-1a (GHSR-1a) in hypothalamus during olanzapine treatment in rats

Weight gain is the one of the most important factors which increases global burden of psychiatric disorder. Second-generation antipsychotics, olanzapine (Olz) and valproic acid (Vpa) in particular, are held responsible for weight gain. However, it is still uncertain how these drugs cause this. Thus, the rats selected for the experiment were randomly divided into 3 groups. The 1st group received only 0.5 ml saline solution intraperitoneally (n = 20, control group); the second group was given 200 mg / kg Vpa intraperitoneally (n = 20, Vpa group) and 2 mg / kg Olz was given intraperitoneally to the 3rd group (n = 20, Olz group) between 8 and 10 am for 30 days. We examined serum leptin, adiponectin, resistin, TNF-α, IL-6, ghrelin level and, the amount of ghrelin secreting cells in the stomach and growth hormone secretagogue receptor-1a (GHSR-1a, ghrelin receptor) expression in the hypothalamus. The hypothalamic GHS-1a receptor index was significantly higher in the Olz group compared with the control group and Vpa group (p = 0.036 and p = 0.016 respectively). Ghrelin immune positive cell index in stomach was statistically significantly lower in the Vpa group compared with the control and Olz groups (p = 0.028 and p = 0.013 respectively) There was no difference between the groups in terms of serum leptin, resistin, IL-6 and ghrelin levels. In the Vpa group, a statistically significant increase was found in serum adiponectin level compared with both the control group and the Olz group (p = 0009 and p = 0024 respectively) and, significant decrease was found in serum TNF-α level compared to Olz group (p = 0007). In conclusion, we found that the main cause of weight gain in Olz use was the increase in the number of hypothalamic ghrelin receptors. Investigating the mechanism by which Olz increases the number of ghrelin receptors may help to develop effective treatment strategies in preventing obesity in psychiatric patients.

Design, synthesis and characterization of a fluorescently labeled functional analog of full-length human ghrelin

Human ghrelin receptor (GHSR) is a recognized prospective target in the diagnosis and therapy of multiple cancer types. To gain a better understanding of this receptor signaling system, we have synthesized a novel full-length ghrelin analog that is fluorescently labeled at the side-chain of a C-terminal cysteine extension. This analog exhibited nanomolar affinity and potency for the ghrelin receptor. It shows comparable efficacy with that of endogenous ghrelin. The fluorescently-labeled ghrelin analog is a valuable tool for in vitro imaging of cell lines that express ghrelin receptor.

Differential functional selectivity and downstream signaling bias of ghrelin receptor antagonists and inverse agonists

The ghrelin receptor [growth hormone secretagogue receptor (GHSR)-1a] represents a promising pharmacologic target for the treatment of metabolic disorders, including obesity and cachexia, via central appetite modulation. The GHSR-1a has a complex pharmacology, highlighted by G-protein-dependent and -independent downstream signaling pathways and high basal constitutive activity. The functional selectivity and signaling bias of many GHSR-1a-specific ligands has not been fully characterized. In this study, we investigated the pharmacologic properties of ghrelin, MK-0677, L692,585, and [d-Lys3]-growth hormone-releasing peptide-6 (Dlys), JMV2959, and [d-Arg(1),d-Phe(5),d-Trp(7, 9),Leu(11)]-substance P (SP-analog). We investigated their effect on basal GHSR-1a constitutive signaling, ligand-directed downstream GHSR-1a signaling, functional selectivity, and signaling bias. Dlys behaved as a partial antagonist with a strong bias toward GHSR-1a-β-arrestin signaling, whereas JMV2959 acted as a full unbiased GHSR-1a antagonist. Moreover, the SP-analog behaved as an inverse agonist increasing G-protein-dependent signaling, but only at high concentrations, whereas, at low concentrations, the SP-analog attenuated β-arrestin-dependent signaling. Considering the limited success in the clinical development of GHSR-1a-targeted drugs so far, these findings provide a novel insight into the pharmacologic characteristics of GHSR-1a ligands and their signaling bias, which has important implications in the design of novel, more selective GHSR-1a ligands with predictable functional outcome and selectivity for preclinical and clinical drug development.-Ramirez, V. T., van Oeffelen, W. E. P. A., Torres-Fuentes, C., Chruścicka, B., Druelle, C., Golubeva, A. V., van de Wouw, M., Dinan, T. G., Cryan, J. F., Schellekens, H. Differential functional selectivity and downstream signaling bias of ghrelin receptor antagonists and inverse agonists.

Ghrelin is expressed in the pregnant mammary glands of dairy goats and promotes the cell proliferation of mammary epithelial cells

Little is known about ghrelin's effects on cell proliferation in pregnant mammary epithelial cells (MECs) even though it is known to be a mitogen for a variety of other cell types. The objectives of this study were to evaluate the expression and localization of ghrelin and its functional receptor, GHSR-1a, in the mammary glands of dairy goats during pregnancy and to investigate the direct role of ghrelin in cell proliferation of primary cultured MECs. Compared to the early stage (days 30) of pregnancy, the abundance of transcripts and protein of ghrelin and GHSR-1a were significantly greater in mid- and late-phases (between days 90 and days 120) of pregnancy (p < .05). Immunohistochemistry analysis showed that ghrelin and GHSR-1a were predominantly localized in the alveolar and ductal mammary epithelial cells at various stages of pregnancy. In our in vitro experiments, ghrelin induced a dose- and time-dependent promotory effect on cell proliferation of MECs. At the dose of 10³ pg/mL treatment 24 h, ghrelin augmented the expression of proliferation-related peptides (PCNA and cyclin B1). Furthermore, ghrelin promoted the expression of prolactin (PRL) and GHSR-1a in cultured MECs. Additionally, the stimulatory effects of ghrelin were blocked by d-Lys3-GHRP6, a selective antagonist of GHSR-1a. As the temporal changes in ghrelin and GHSR-1a expression in pregnant goat mammary glands coincided with the mammary growth and development during the pregnancy, activation of GHSR-1a signal transduction pathways by ghrelin may play a direct role in the regulation of mammary growth in dairy goats.

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Expression of ghrelin and its receptor in rats after coronary artery ligation

Ghrelin is a novel growth hormone-releasing peptide, which has been shown to exert beneficial effects on cardiac function and ventricular remodeling. The present study aimed to investigate the expression of ghrelin and the growth hormone (GH) secretagogue receptor 1a (GHSR-1a), and the association with cardiac remodeling in rats with myocardial infarction (MI). Twenty-four hours after ligation of the anterior descending artery (LAD), adult male Sprague-Dawley rats were randomized to 3 d, 7 d and 28 d group. Sham animals underwent thoracotomy and pericardiotomy, but not LAD ligation. Expression of both ghrelin and GHSR-1a was assessed by means of immunohistochemistry and real-time PCR. Plasma ghrelin levels were measured by ELISA kit. In addition, cardiac remodeling was assessed by echocardiographic and hemodynamic measurements. Plasma and cardiac expression of ghrelin decreased on days 3, 7 and 28 compared with the sham group (P<0.05). In contrast the GHSR-1a mRNA levels increased during the same days (P<0.05). Decreased positive immunoreaction for ghrelin and increased positive GHSR-1a were also observed in the infarcted heart. Interestingly, plasma ghrelin correlated negatively with left ventricular end-diastolic pressure (r=-0.59, P=0.002) and left ventricular end-diastolic dimension (r=-0.73, P<0.01). The ghrelin system may play an important role regulating cardiac remodeling after MI and present as a potential significant target for pharmacological modulation and treating cardiac remodeling.

Identification of the ghrelin-GHSR 1 system and its influence in the modulation of induced ocular hypertension in rabbit and rat eyes

Recent studies evidenced a decrease in ghrelin's aqueous humor levels in patients with glaucoma. The goal of our investigation was to study the effect of the ghrelin-GHSR-1a system in the modulation of intraocular pressure in acute ocular hypertension models and its expression and distribution in ocular tissues. Two animal models of acute ocular hypertension were used to study the effect of the ghrelin-GHSR-1a system in the modulation of intraocular pressure: the rabbit and the rat. Ocular hypertension was induced by an intravitreal injection of 20% NaCl. Ghrelin or des-acyl ghrelin were delivered subconjunctivally and the intraocular pressure was assessed by a rebound tonometer that was calibrated for each species. In addition, we have studied the influence of nitric oxide and prostaglandins on ghrelin's effect in the rabbit animal model. Finally, we determined by immunofluorescence the expression of ghrelin and GHSR-1 in the rat's ocular tissue. Ghrelin decreased the intraocular pressure in both animal models (maximum decrease: 43.8±12.0% in the rabbit and 29.0±7.46% in the rat). In the rabbit, this effect was blunted in the presence of l-NAME and ketorolac. Des-acyl ghrelin only decreased the intraocular pressure in the rat (maximum decrease: 34.9±8.15%). Ghrelin expression was detected in the ciliary processes and GHSR-1 expression was detected in the trabecular meshwork and ciliary body. The ghrelin-GHSR-1 system is expressed in the anterior segment of the eye. Ghrelin and des-acyl ghrelin are responsible for a hypotensive effect in acute ocular hypertension animal models.

Potential new role of the GHSR-1a-mediated signaling pathway in cardiac remodeling after myocardial infarction (Review)

The gastrointestinal hormone ghrelin has important cardiovascular protective effects, however, its specific mechanisms are not yet completely understood. Recent studies have shown that the ghrelin receptor, growth hormone secretagogue receptor type 1a (GHSR-1a), regulates cell proliferation, apoptosis and inflammation-related signaling pathways. In human aortic endothelial cells, ghrelin activates NO production through AMP-activated protein kinase (AMPK) and Akt activation, and these effects can be blocked by knockdown of GHSR-1a. Obese mice have been found to exhibit an increased GHSR-1a content and expression in the heart, associated with an increase in phosphatidylinositol 3-kinase (PI3K) content and an increase AKT content and phosphorylation. Furthermore, GHSR-1a expression was observed to be increased in heart failure after myocardial infarction (MI) in rats. Given such complexity in GHSR-1a signaling and crosstalk with the AMPK and PI3K/Akt signaling pathways, both of which are well-known factors involved in cardiac remodeling after MI, we speculate that GHSR-1a signaling may play a regulatory role in cardiac protection and hope to identify new drugs targets. However, to date, no direct association between GHSR-1a and cardiac remodeling has been found. Therefore, further studies are required.

Ghrelin in ocular pathophysiology: from the anterior to the posterior segment

Ghrelin is a 28 amino acid acylated peptide produced in several organs that binds the growth hormone secretagogues receptor type 1a (GHSR-1a). It acts over a wide range of systems, e.g. the endocrine, cardiovascular, musculoskeletal and immune systems and the eye. The aim of this work is to review the physiologic and pathologic implications of the ghrelin-GHSR-1a in the eye. A systematic revision of studies published between 2000 and 2013 in English, Spanish or Portuguese in MEDLINE, EMBASE and Scopus was performed. Search words used included: ghrelin, GHSR-1a, ocular production, iris muscular kinetics, ciliary body, glaucoma, retinopathy and uvea. The production of ghrelin by the ocular tissue has been detected both in the anterior and posterior segments, as well as the presence of GHSR-1a. This peptide promotes the relaxation of the iris sphincter and dilator muscles, being this effect independent from GHSR-1a and dependent on prostaglandins release in the first case and dependent on GHSR-1a in the second. Regarding ocular pathology, ghrelin levels in the aqueous humor appear to be decreased in individuals with glaucoma. Moreover, ghrelin has been shown to decrease the intraocular pressure in animal models of ocular hypertension through GHSR-1a. In the posterior segment, the ghrelin-GHSR-1a system interferes with the development of oxygen-induced retinopathy, being protective in the vaso-obliterative phase and deleterious in the vaso-proliferative stage of the disease. Thus, the ghrelin-GHSR-1a system presents as a possible local regulatory mechanism in the eye, with pathophysiological implications, constituting a target for future clinical and therapeutic research and interventions.

Mechanism of the inhibitory effect of ghrelin in sepsis

Sepsis and septic shock are the leading causes of death in intensive care units. Approximately 40%-70% of the mortality is associated with severe sepsis and septic shock. Systemic antibiotic usage, surgical intervention, aggressive fluid resuscitation and careful monitoring are common measures currently used to treat sepsis. Despite the advances in the understanding of the pathophysiology of sepsis, very little progress has been made towards therapeutic interventions. Recently we have shown that ghrelin, a stomach-derived peptide which is an endogenous ligand for the growth hormone secretagogue receptor (GHSR-1a), is beneficial in attenuating the inflammatory response, organ injury and mortality in an experimental model of polymicrobial sepsis induced by cecal ligation and puncture (CLP). In this review, we describe the mechanism of action of ghrelin in sepsis, highlight the role ghrelin plays in attenuating the hepatic dysfunction induced by sepsis and septic shock and suggest in developing ghrelin as a potential therapy for sepsis.