Identification of ghrelin receptor blocker, D-[Lys3] GHRP-6 as a CXCR4 receptor antagonist

Ghrelin receptor antagonism and satiety attenuate Pavlovian-instrumental transfer

Animals rely on learned cues to guide their behaviour for rewards such as food. The Pavlovian-instrumental transfer (PIT) task can be used to investigate the influence of Pavlovian stimuli on instrumental responding. Ghrelin, an orexigenic peptide, and its receptor, growth hormone secretagogue receptor 1A (GHS-R1A), has received growing interest for its role in reward-motivated learning and behaviours. A significant population of GHS-R1A have been identified within the ventral tegmental area (VTA), a critical node in the mesolimbic reward circuit that is necessary for the expression of PIT. As ghrelin has been found to increase dopaminergic activity in the VTA, we predicted that GHS-R1A antagonism with JMV-2959 would attenuate PIT. Further, given the relationship between hunger levels and changes in ghrelin signalling, we sought to compare the effects GHS-R1A antagonism with those of satiety, hypothesizing parallel effects, with each attenuating PIT. Rats received daily sessions of Pavlovian and then instrumental training over 3 weeks. Across three experiments, we examined the effects of a shift to satiety, or treatment with the GHS-R1A antagonist JMV-2959, either peripherally or directly into the VTA. We found that presentations of a stimulus paired with food reward enhanced responding for food across all conditions, thus demonstrating the expected PIT effect. Further, GHS-R1A antagonism, both peripherally and within the VTA, as well as satiety significantly reduced the magnitude of the PIT effect compared to control conditions. These results clarify our understanding of ghrelin signalling in PIT and begin to elucidate the role of feeding-related peptides in the modulation of reward-related responding.

Inhibition of ghrelin activity by the receptor antagonist [D-Lys3]-GHRP-6 enhances hepatic fatty acid oxidation and gluconeogenesis in a growing pig model

Despite its central role in regulating energy intake and metabolism, ghrelin is little understood when it comes to its effects on hepatic lipid and glucose metabolism. Growing pigs were intravenously injected with ghrelin receptor antagonist [D-Lys3]-GHRP-6 (DLys; 6 mg/kg body weight) for seven days to determine whether ghrelin plays a role in glucose and lipid metabolism. DLys treatment significantly reduced body weight gain and adipose histopathology found that DLys treatment dramatically reduced adipocyte size. DLys treatment significantly increased serum NEFA and insulin levels, hepatic glucose level and HOMA-IR, and significantly decreased serum TBA level of growing pigs after fasting. Moreover, DLys treatment changed the dynamics of serum metabolic parameters, including glucose, NEFA, TBA, insulin, GH, leptin, and cortisol. Liver transcriptome showed that DLys treatment affected the metabolism-related pathways. Compared with the control group, adipose tissue lipolysis (the adipose triglyceride lipase level was significantly increased), hepatic gluconeogenesis (the G6PC protein level was significantly increased) and fatty acid oxidation (the CPT1A protein level was significantly increased) were promoted in the DLys group. DLys treatment expanded degrees of oxidative phosphorylation in the liver, coming about in a higher NAD+ /NADH proportion and enactment of the SIRT1 signaling pathway. Additionally, the liver protein levels of the DLys group were significantly higher than those of the control group for GHSR, PPAR alpha, and PGC-1. To summarize, inhibition of ghrelin activity can significantly affect metabolism and alter energy levels by enhancing fat mobilization, hepatic fatty acid oxidation and gluconeogenesis without affecting fatty acid uptake and synthesis in the liver.

Ghrelin Represses Thymic Stromal Lymphopoietin Gene Expression through Activation of Glucocorticoid Receptor and Protein Kinase C Delta in Inflamed Skin Keratinocytes

Ghrelin, a peptide hormone secreted from enteroendocrine cells of the gastrointestinal tract, has anti-inflammatory activity in skin diseases, including dermatitis and psoriasis. However, the molecular mechanism underlying the beneficial effect of ghrelin on skin inflammation is not clear. In this study, we found that ghrelin alleviates atopic dermatitis (AD)-phenotypes through suppression of thymic stromal lymphopoietin (TSLP) gene activation. Knockdown or antagonist treatment of growth hormone secretagogue receptor 1a (GHSR1a), the receptor for ghrelin, suppressed ghrelin-induced alleviation of AD-like phenotypes and suppression of TSLP gene activation. We further found that ghrelin induces activation of the glucocorticoid receptor (GR), leading to the binding of GR with histone deacetylase 3 (HDAC3) and nuclear receptor corepressor (NCoR) NCoR corepressor to negative glucocorticoid response element (nGRE) on the TSLP gene promoter. In addition, ghrelin-induced protein kinase C δ (PKCδ)-mediated phosphorylation of p300 at serine 89 (S89), which decreased the acetylation and DNA binding activity of nuclear factor- κB (NF-κB) p65 to the TSLP gene promoter. Knockdown of PKCδ abolished ghrelin-induced suppression of TSLP gene activation. Our study suggests that ghrelin may help to reduce skin inflammation through GR and PKCδ-p300-NF-κB-mediated suppression of TSLP gene activation.

Relationship between Orexigenic Peptide Ghrelin Signal, Gender Difference and Disease

Growth hormone secretagogue receptor 1a (GHS-R1a), which is one of the G protein-coupled receptors (GPCRs), is involved in various physiological actions such as energy consumption, growth hormone secretion promoting action, and cardiovascular protective action. The ligand was searched for as an orphan receptor for a while, but the ligand was found to be acylated ghrelin (ghrelin) discovered by Kangawa and Kojima et al. in 1999. Recently, it has also been reported that dysregulation of GHS-R1a mediates reduced feeding in various diseases. On the other hand, since the physiological effects of ghrelin have been studied exclusively in male mice, few studies have been conducted on gender differences in ghrelin reactivity. In this review, we describe (1) the characteristics of GHS-R1a, (2) the role of ghrelin in hypophagia due to stress or anticancer drugs, and (3) the gender differences in the physiological effects of GHS-R1a and the influence of stress on it.

Ghrelin Inhibits Intestinal Epithelial Cell Apoptosis Through the Unfolded Protein Response Pathway in Ulcerative Colitis

Ulcerative colitis (UC) is a type of inflammatory bowel disease (IBD) that occurs in the lining of the rectum and colon. Apoptosis of the intestinal epithelial cells (IECs) is common in active UC patients. Ghrelin is reported to be downregulated in apoptosis of IECs induced by tumor necrosis factor-α (TNF-α). Therefore, we hypothesized that ghrelin might play an antiapoptotic role in UC progression, which was investigated using in vitro and in vivo studies. The TNF-α-treated Caco-2 cell model and mouse colitis model induced by dextran sulfate sodium (DSS) or 2,4,6-trinitrobenzenesulfonic acid (TNBS) were established and employed. We found that ghrelin could inhibit the apoptosis of Caco-2 cells induced by TNF-α, which could be disturbed by [D-lys3]-GHRP-6, the antagonist of ghrelin receptor GHS-R1a. Similarly, in the DSS- and TNBS-induced mouse colitis models, ghrelin could also protect intestinal tissues from apoptosis in DSS- and TNBS-induced colitis depending on GHS-R1a. Furthermore, ghrelin modulated the unfolded protein response (UPR) pathway and regulated the expressions of caspase-3, BAX, and Bcl-2, which contributed to the inhibition of cell apoptosis. In conclusion, ghrelin protects IECs from apoptosis during the pathogenesis of colitis by regulating the UPR pathway.

Acylated ghrelin protects against doxorubicin-induced nephropathy by activating silent information regulator 1

This study investigated the nephroprotective role of acylated ghrelin (AG) against DOX-induced nephropathy and examined whether the protection involves silent information regulator 1 (SIRT1). Rats were divided into control, control + AG, DOX, DOX + AG, DOX + AG + [D-Lys3]-GHRP-6 (a ghrelin receptor antagonist), and DOX + AG + EX-527 (a sirt1 inhibitor). DOX was given over the first 2 weeks. AG (10 ng/kg) and both inhibitors were given as 3 doses/wk for 5 weeks. AG improved the structure and the function of the kidneys; down-regulated the renal expression of TGF-β1, collagen 1A1 and α-SMA; and inhibited the renal collagen deposition in the kidneys of DOX-treated rats. Concomitantly, it reduced the renal levels of ROS, MDA, TNF-α, and IL-6 and protein levels of cytochrome-c, TGF-β1, Smad3 and α-SMA in these rats. In both the control and DOX-treated rats, AG significantly increased the renal levels of SOD and GSH, decreased the expression of cleaved caspase-3 and Bax, increased the total levels and the nuclear activity of SIRT1 and reduced the deacetylation of p53, NF-κB and FOXO-31. All the effects were abolished by the concurrent administration of EX-527 and [D-Lys3]-GHRP-6. In conclusion, AG prevents DOX-induced nephropathy in SIRT1 and GSHRa1-dependent mechanism.

A new understanding of GHSR1a--independent of ghrelin activation

Growth hormone secretagogue receptor 1a (GHSR1a), a member of the G protein-coupled receptor (GPCR) family, is a functional receptor of ghrelin. The expression levels and activities of GHSR1a are affected by various factors. In past years, it has been found that the ghrelin-GHSR1a system can perform biological functions such as anti-inflammation, anti-apoptosis, and anti-oxidative stress. In addition to mediating the effect of ghrelin, GHSR1a also has abnormally high constitutive activity; that is, it can still transmit intracellular signals without activation of the ghrelin ligand. This constitutive activity affects brain functions, growth and development of the body; therefore, it has profound impacts on neurodegenerative diseases and some other age-related diseases. In addition, GHSR1a can also form homodimers or heterodimers with other GPCRs, affecting the release of neurotransmitters, appetite regulation, cell proliferation and insulin release. Therefore, further understanding of the constitutive activities and dimerization of GHSR1a will enable us to better clarify the characteristics of GHSR1a and provide more therapeutic targets for drug development. Here, we focus on the roles of GHSR1a in various biological functions and provide a comprehensive summary of the current research on GHSR1a to provide broader therapeutic prospects for age-related disease treatment.

Phytochemical Screening and Evaluation of the Antiarthritic Potentialof Ammoides pusilla Aqueous Extract on Freund's Adjuvant-Induced Rheumatoid Arthritis

Background: Ammoides pusilla plant is a species of therapeutic interest which used in traditional medicines. This work aims to valorize this plant by the characterization of their bioactive components and the evaluation of the in vivo anti-inflammatory potential against a severe disease affecting the bone structure and the stability of the articular cartilage. Methods: First, the phytochemical screening of the polyphenolic extracts of A. pusilla was carried out. The second part of our study is devoted to the evaluation of the in vivo anti-inflammatory activity of the aqueous extract of A. pusilla based on the method of Freund's adjuvant-induced rheumatoid arthritis. Results: Phytochemical tests demonstrated the richness of extract with flavonoids, tannins, coumarins, anthocyanins and triterpenes. Whereas, the quantitative determination reveals that the aqueous extract of A. pusilla is the richest with bioactive components with contents of total polyphenols, flavonoids and tannins equal to 9.52 ± 0.11 mg gallic acid/g, 4.75 ± 0.05 mg quercetin/g and 8.64 ± 0.02 mg catechin/g respectively. The results of the anti-inflammatory activity showed that the aqueous infused extract of A. pusilla has an interesting antiarthritic potential on Freund's adjuvant-induced rheumatoid arthritis in mice. It is manifested by a weight gain; a normal arthritic index and biochemical parameters close to those of Diclofenac®. Conclusion: The aqueous infused extract of A. pusilla is therefore of considerable therapeutic interest as an alternative compound for the prevention of inflammation and for the improvement in bone structure.

Ghrelin antagonist overrides the mRNA expression of NPY in hypothalamus in feed restricted ewes

A negative energy balance (NEB) is detrimental to reproduction in animals. A suggested link between NEB and reproductive failure is the gastrointestinal hormone ghrelin, because of the association between ghrelin and the hypothalamo-pituitary-gonadal axis. The [D-Lys3]-Growth Hormone Releasing Peptide-6 ([D-Lys3]-GHRP-6) is a ghrelin antagonist that acts on ghrelin receptors (GHS-R1). The objective of this study was to evaluate the effect of [D-Lys3]-GHRP-6 on reproduction variables in feed restricted ewes. Two experiments were conducted. Experiment I was conducted for 30 days; and Experiment II for 13 days. In both experiments the ewes (n = 18) were randomly assigned to: Control (CO): fed to meet maintenance requirements; Feed restriction (FR): 80% of maintenance restriction; or Ghrelin antagonist (GA): feed restricted and daily subcutaneous of 7.5μg/kg of [D-Lys3]-GHRP-6. Plasma was collected to measure hormones and metabolite concentration. In Experiment II, the hypothalamus and ovaries were collected on day 13. In both Experiments, sheep allocated to the FR and GA treatments decreased their body weight compared with sheep in the CO group (P < 0.06); progesterone however, did not differ between treatments (P > 0.10). Experiment I: Plasma ghrelin concentration was greater (P < 0.01) in FR and GA compared with CO ewes. Plasma non-esterified fatty acids concentration was greater (P < 0.01) in GA and FR than CO. Experiment II: Kisspeptin1-Receptor (Kiss1-R) mRNA expression was greater in FR (P < 0.01) and tended to be greater in GA (P = 0.10) compared with CO ewes. The neuro peptide-Y (NPY) mRNA expression was greater (P = 0.03) in FR than CO; and tended to be greater (P = 0.06) compared with GA ewes. Growth hormone releasing hormone (GhRH) mRNA expression was greater in GA (P = 0.04) and tended to be greater in FR (P = 0.07) compared with CO ewes. Feed restriction increased GhRH, NPY, and Kiss-R mRNA expression in hypothalamus without affecting reproductive variables.Ghrelin antagonist may prevent an increase inNPY expression in ewes.

Inhibition of Ghrelin Activity by Receptor Antagonist [d-Lys-3] GHRP-6 Attenuates Alcohol-Induced Hepatic Steatosis by Regulating Hepatic Lipid Metabolism

Alcoholic steatosis, characterized by an accumulation of triglycerides in hepatocytes, is one of the earliest pathological changes in the progression of alcoholic liver disease. In our previous study, we showed that alcohol-induced increase in serum ghrelin levels impair insulin secretion from pancreatic β-cells. The consequent reduction in the circulating insulin levels promote adipose-derived fatty acid mobilization to ultimately contribute to hepatic steatosis. In this study, we determined whether inhibition of ghrelin activity in chronic alcohol-fed rats could improve hepatic lipid homeostasis at the pancreas-adipose-liver axis. Adult Wistar rats were fed Lieber-DeCarli control or an ethanol liquid diet for 7 weeks. At 6 weeks, a subset of rats in each group were injected with either saline or ghrelin receptor antagonist, [d-Lys-3] GHRP-6 (DLys; 9 mg/kg body weight) for 5 days and all rats were sacrificed 2 days later. DLys treatment of ethanol rats improved pancreatic insulin secretion, normalized serum insulin levels, and the adipose lipid metabolism, as evidenced by the decreased serum free fatty acids (FFA). DLys treatment of ethanol rats also significantly decreased the circulating FFA uptake, de novo hepatic fatty acid synthesis ultimately attenuating alcoholic steatosis. To summarize, inhibition of ghrelin activity reduced alcoholic steatosis by improving insulin secretion, normalizing serum insulin levels, inhibiting adipose lipolysis, and preventing fatty acid uptake and synthesis in the liver. Our studies provided new insights on the important role of ghrelin in modulating the pancreas-adipose-liver, and promoting adipocyte lipolysis and hepatic steatosis. The findings offer a therapeutic approach of not only preventing alcoholic liver injury but also treating it.

Limited short-term effects on human prostate cancer xenograft growth and epidermal growth factor receptor gene expression by the ghrelin receptor antagonist [D-Lys3]-GHRP-6

PURPOSE

The ghrelin axis regulates many physiological functions (including appetite, metabolism, and energy balance) and plays a role in disease processes. As ghrelin stimulates prostate cancer proliferation, the ghrelin receptor antagonist [D-Lys³]-GHRP-6 is a potential treatment for castrate-resistant prostate cancer and for preventing the metabolic consequences of androgen-targeted therapies. We therefore explored the effect of [D-Lys³]-GHRP-6 on PC3 prostate cancer xenograft growth.

METHODS

NOD/SCID mice with PC3 prostate cancer xenografts were administered 20 nmoles/mouse [D-Lys³]-GHRP-6 daily by intraperitoneal injection for 14 days and tumour volume and weight were measured. RNA sequencing of tumours was conducted to investigate expression changes following [D-Lys³]-GHRP-6 treatment. A second experiment, extending treatment time to 18 days and including a higher dose of [D-Lys³]-GHRP-6 (200 nmoles/mouse/day), was undertaken to ensure repeatability.

RESULTS

We demonstrate here that daily intraperitoneal injection of 20 nmoles/mouse [D-Lys³]-GHRP-6 reduces PC3 prostate cancer xenograft tumour volume and weight in NOD/SCID mice at two weeks post treatment initiation. RNA-sequencing revealed reduced expression of epidermal growth factor receptor (EGFR) in these tumours. Further experiments demonstrated that the effects of [D-Lys³]-GHRP-6 are transitory and lost after 18 days of treatment.

CONCLUSIONS

We show that [D-Lys³]-GHRP-6 has transitory effects on prostate xenograft tumours in mice, which rapidly develop an apparent resistance to the antagonist. Although further studies on [D-Lys³]-GHRP-6 are warranted, we suggest that daily treatment with the antagonist is not a suitable treatment for advanced prostate cancer.

New Metabolic Influencer on Oxytocin Release: The Ghrelin

Background: The hypothalamic–pituitary axis by secreting neuropeptides plays a key role in metabolic homeostasis. In light of the metabolic regulation, oxytocin is a potential neuropeptide for therapies against obesity and related disorders. The aim of our study is to measure ghrelin-induced oxytocin secretion in rats and to detect the changes after administration of ghrelin antagonist. Methods: Ghrelin was administrated centrally (intracerebroventricular, i.c.v., 1.0, 10.0, and 100.0 pmol) or systemically (intravenous, i.v., 1.0, and 10.0 nmol). [d-Lys³]-GHRP-6 ghrelin antagonist was injected 15 min before ghrelin injection in a dose of 10.0 pmol i.c.v. and 10.0 nmol i.v. Results: Either i.c.v. or i.v. administration of ghrelin dose-dependently increased the plasma oxytocin concentration. Following pretreatment with the ghrelin antagonist [d-Lys³]-GHRP-6, the high plasma oxytocin level induced by ghrelin was significantly reduced. Conclusion: The results indicate that the release of oxytocin is influenced directly by the ghrelin system. Examination of the mechanism of ghrelin-induced oxytocin secretion is a new horizon for potential therapeutic options.

Centrally Administered Cortistation-14 Induces Antidepressant-Like Effects in Mice via Mediating Ghrelin and GABAA Receptor Signaling Pathway

Cortistatin-14 (CST-14), a recently discovered cyclic neuropeptide, can bind to all five cloned somatostatin receptors (SSTRs) and ghrelin receptor to exert its biological activities and co-exists with GABA within the cortex and hippocampus. However, the role of CST-14 in the control of depression processes is not still clarified. Here, we tested the behavioral effects of CST-14 in the in a variety of classical rodent models of depression [forced swimming test (FST), tail suspension test (TST) and novelty-suppressed feeding test]. In the models of depression, CST-14 produced antidepressant-like effects, and does not altered locomotor activity levels. And, we found that CST-14 mRNA and BDNF mRNA were significantly decreased in the hippocampus and cortex after mice exposed to stress. Further data show that i.c.v. administration of CST-14 produce rapid antidepressant effects, and does not altered locomotor activity levels. Then these antidepressant-like effects were significantly reversed by [D-Lys₃]GHRP-6 (ghrelin receptor antagonist), but not c-SOM (SSTRs antagonist). Meanwhile, the effects of some neurotransmitter blockers indicates that only GABA_A system, but not CRF1 receptor, α/β-adrenergic receptor, is involved in the antidepressant effect of CST-14. The effects of the mTOR inhibitor (rapamycin), the PI3K inhibitor (LY294002) and the p-ERK1/2 inhibitor (U0126) suggesting that the ERK/mTOR or PI3K/Akt/mTOR signaling pathway is not involved in the antidepressant effects of CST-14. Interestingly, intranasal administration of CST-14 led to reducing depressive-like behavior, and near-infrared fluorescent experiments showed the real-time in vivo bio-distribution in brain after intranasal infusion of Cy7.5-CST-14. Taken all together, the results of present study point to a role for CST-14 in the modulation of depression processes via the ghrelin and GABA_A receptor, and suggest cortistation may represent a novel strategy for the treatment of depression disorders.

Highlights:-

CST-14 and BDNF mRNA are decreased in hippocampus and cortex once mice exposed to stress.

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i.c.v. or intranasal administration of CST-14 produce rapid antidepressant effects.

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NIR fluorescence imaging detected the brain uptake and distribution after intranasal CST-14.

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Antidepressant effects of CST-14 were only related to ghrelin and GABA_A system.

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Co-injection of CST-14 and NPS produce antidepressant effect, and do not impair memory.

Ghrelin is a persistent biomarker for chronic stress exposure in adolescent rats and humans

Prolonged stressor exposure in adolescence enhances the risk of developing stress-sensitive mental illnesses, including posttraumatic stress disorder (PTSD), for many years following exposure cessation, but the biological underpinnings of this long-term vulnerability are unknown. We show that severe stressor exposure increased circulating levels of the hormone acyl-ghrelin in adolescent rats for at least 130 days and in adolescent humans for at least 4.5 years. Using a rodent model of longitudinal PTSD vulnerability in which rodents with a history of stressor exposure during adolescence display enhanced fear in response to fear conditioning administered weeks after stressor exposure ends, we show that systemic delivery of a ghrelin receptor antagonist for 4 weeks surrounding stressor exposure (2 weeks during and 2 weeks following) prevented stress-enhanced fear memory. These data suggest that protracted exposure to elevated acyl-ghrelin levels mediates a persistent vulnerability to stress-enhanced fear after stressor exposure ends.

Ghrelin accelerates wound healing in combined radiation and wound injury in mice

Impaired wound healing caused by radiation happens frequently in clinical practice, and the exact mechanisms remain partly unclear. Various countermeasures have been taken to tackle with this issue. Ghrelin was considered as a potent endogenous growth hormone-releasing peptide, and its role in enhancing wound repair and regeneration was firstly investigated in whole-body irradiated (γ-ray) mice in this study. Collagen deposition and neovascularization were mostly discussed. The results demonstrated that ghrelin administration promoted cutaneous wound healing in irradiated mice, followed with reduced average wound closure time, increased spleen index (SI) and improved haematopoiesis. After isolation and analysis of granulation tissues in combined radiation and wound injury (CRWI) mice treated with and without ghrelin, a phenomenon of increased DNA, hexosamine, nitrate and nitrite synthesis, elevated collagen content and enhanced neovascularization was observed after ghrelin treatment. Western blotting indicated that ghrelin also increased the expression of vascular endothelial growth factor (VEGF) and transforming growth factor-β (TGF-β), both responsible for wound healing. However, previous administration of growth hormone secretagogue receptor 1a (GHS-R1a) blocker blunted these therapeutic effects of ghrelin on CRWI mice. Our results identify ghrelin as a novel peptide that could be used for radiation-induced impaired wound healing.

The ventromedial hypothalamic nucleus plays an important role in anxiolytic-like effect of neuropeptide S

Neuropeptide S (NPS), the endogenous neuropeptide ligand of NPSR, has been reported to regulate anxiety-related behavior involved in multiple brain regions, including amygdale, locus coeruleus and Barrington's nucleus. However, little research has been conducted on the anxiolytic-like behaviors of NPS on the hypothalamus, which was an important area in defensive behavior. Here, we investigated a role of hypothalamus in anxiolytic-like behaviors of NPS. We found that NPSR protein of mouse distributed mainly in the ventromedial hypothalamus (VMH). And in the single prolonged stress model (SPS), the results showed that NPS mRNA of the mice exposed to SPS was significantly higher than control, while NPSR mRNA was remarkable lower than control in hypothalamus. Further studies found that NPS intra-VMH infusion dose-dependently (1, 10 and 100pmol) induced anxiolytic effects, using elevated plus maze and open field tests. These anxiolytic effects could be blocked by NPSR antagonist (SHA68), but not by picrotoxin (a GABA_A receptor antagonist) and sacolfen (a GABA_B receptor antagonist). Meanwhile, our data showed that the expression of c-Fos was significantly increased in VMH after NPS delivered into the lateral ventricles. These results cast a new light on the hypothalamic nucleus in the anxiolytic-like effect of NPS system.

Long-term treatment with the ghrelin receptor antagonist [d-Lys3]-GHRP-6 does not improve glucose homeostasis in nonobese diabetic MKR mice

Long-term treatment with the ghrelin receptor antagonist [d-Lys3]-GHRP-6 does not improve glucose homeostasis in nonobese diabetic MKR mice. Am J Physiol Regul Integr Comp Physiol 314: R71-R83, 2018. First published September 13, 2017; doi: 10.1152/ajpregu.00157.2017 .-Ghrelin secretion has been associated with increased caloric intake and adiposity. The expressions of ghrelin and its receptor (GHS-R1a) in the pancreas has raised the interest about the role of ghrelin in glucose homeostasis. Most of the studies showed that ghrelin promoted hyperglycemia and inhibited insulin secretion. This raised the interest in using GHS-R1a antagonists as therapeutic targets for type 2 diabetes. Available data of GHS-R antagonists are on a short-term basis. Moreover, the complexity of GHS-R1a signaling makes it difficult to understand the mechanism of action of GHS-R1a antagonists. This study examined the possible effects of long-term treatment with a GHS-R1a antagonist, [d-Lys3]-growth hormone-releasing peptide (GHRP)-6, on glucose homeostasis, food intake, and indirect calorimetric parameters in nonobese diabetic MKR mice. Our results showed that [d-Lys3]-GHRP-6 (200 nmol/mouse) reduced pulsatile growth hormone secretion and body fat mass as expected but worsened glucose and insulin intolerances and increased cumulative food intake unexpectedly. In addition, a significant increase in blood glucose and decreases in plasma insulin and C-peptide levels were observed in MKR mice following long-term [d-Lys3]-GHRP-6 treatment, suggesting a direct inhibition of insulin secretion. Immunofluorescence staining of pancreatic islets showed a proportional increase in somatostatin-positive cells and a decrease in insulin-positive cells in [d-Lys3]-GHRP-6-treated mice. Furthermore, [d-Lys3]-GHRP-6 stimulated food intake on long-term treatment via reduction of proopiomelanocortin gene expression and antagonized GH secretion via reduced growth hormone-releasing hormone gene expression in hypothalamus. These results demonstrate that [d-Lys3]-GHRP-6 is not completely opposite to ghrelin and may not be a treatment option for type 2 diabetes.

Hypoglycemic Effect of Combined Ghrelin and Glucagon Receptor Blockade

Glucagon receptor (GcgR) blockade has been proposed as an alternative to insulin monotherapy for treating type 1 diabetes since deletion or inhibition of GcgRs corrects hyperglycemia in models of diabetes. The factors regulating glycemia in a setting devoid of insulin and glucagon function remain unclear but may include the hormone ghrelin. Not only is ghrelin release controlled by glucose but also ghrelin has many actions that can raise or reduce falls in blood glucose level. Here, we tested the hypothesis that ghrelin rises to prevent hypoglycemia in the absence of glucagon function. Both GcgR knockout (Gcgr^-/-) mice and db/db mice that were administered GcgR monoclonal antibody displayed lower blood glucose levels accompanied by elevated plasma ghrelin levels. Although treatment with the pancreatic β-cell toxin streptozotocin induced hyperglycemia and raised plasma ghrelin levels in wild-type mice, hyperglycemia was averted in similarly treated Gcgr^-/- mice and the plasma ghrelin level was further increased. Notably, administration of a ghrelin receptor antagonist further reduced blood glucose levels into the markedly hypoglycemic range in overnight-fasted, streptozotocin-treated Gcgr^-/- mice. A lowered blood glucose level also was observed in overnight-fasted, streptozotocin-treated ghrelin receptor-null mice that were administered GcgR monoclonal antibody. These data suggest that when glucagon activity is blocked in the setting of type 1 diabetes, the plasma ghrelin level rises, preventing hypoglycemia.

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

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

Pharmacological characterization of the first in class clinical candidate PF-05190457: a selective ghrelin receptor competitive antagonist with inverse agonism that increases vagal afferent firing and glucose-dependent insulin secretion ex vivo

BACKGROUND AND PURPOSE

Ghrelin increases growth hormone secretion, gastric acid secretion, gastric motility and hunger but decreases glucose-dependent insulin secretion and insulin sensitivity in humans. Antagonizing the ghrelin receptor has potential as a therapeutic approach in the treatment of obesity and type 2 diabetes. Therefore, the aim was to pharmacologically characterize the novel small-molecule antagonist PF-05190457 and assess translational pharmacology ex vivo.

EXPERIMENTAL APPROACH

Radioligand binding in filter and scintillation proximity assay formats were used to evaluate affinity, and europium-labelled GTP to assess functional activity. Rat vagal afferent firing and calcium imaging in dispersed islets were used as native tissues underlying food intake and insulin secretion respectively.

KEY RESULTS

PF-05190457 was a potent and selective inverse agonist on constitutively active ghrelin receptors and acted as a competitive antagonist of ghrelin action, with a human Kd of 3 nM requiring 4 h to achieve equilibrium. Potency of PF-05190457 was similar across different species. PF-05190457 increased intracellular calcium within dispersed islets and increased vagal afferent firing in a concentration-dependent manner with similar potency but was threefold less potent as compared with the in vitro Ki in recombinant overexpressing cells. The effect of PF-05190457 on rodent islets was comparable with glibenclamide, but glucose-dependent and additive with the insulin secretagogue glucagon-like peptide-1.

CONCLUSIONS AND IMPLICATIONS

Together, these data provide the pharmacological in vitro and ex vivo characterization of the first ghrelin receptor inverse agonist, which has advanced into clinical trials to evaluate the therapeutic potential of blocking ghrelin receptors in obesity and type 2 diabetes.

Effects of peripheral administration of ghrelin antagonist [D-Lys³]-GHRP-6 on growth performance and blood biochemical indices in broiler chickens

Abstract. In the present study, possible effects of peripheral administration of ghrelin antagonist [D-Lys3]-GHRP-6 on chicken performance, thyroid hormones level and serum biochemical parameters were investigated. Broiler chicks divided into five experimental groups were reared up to day 42. On day 21, a treatment was assigned to the five groups: group 1 (control), chickens without any administration of peptide or solution; group 2 (G50), chickens with intraperitoneal (IP) injection of 50 ng per 100 g body weight (BW) of D-Lys3 peptide on day 21; group 3 (G100), chickens with IP injection of 100 ng per 100 g BW of D-Lys3 peptide on day 21; group 4 (G150), chickens with IP injection of 150 ng per 100 g BW of D-Lys3 peptide on day 21; and group 5 (G200), chickens with IP injection of 200 ng per 100 g BW of D-Lys3 peptide on day 21. On days 21 (post-injection) and 42 (post-rearing), blood samples were obtained from the animals for laboratory analyses. Experimental groups administered the GHS-R antagonist showed less feed intake – i.e., administration of greater doses led to less feed intake (P < 0.01). Daily weight gains within groups G150 and G200 decreased (P < 0.01) in comparison with the control. The feed conversion ratio (FCR) did not differ among the groups. There was a significant difference between control and experimental groups for glucose, total cholesterol and phosphorus levels (P < 0.01) in post-injection samples. In post-injection and post-rearing blood samples, the thyroid hormone (T3 and T4) in 6 h increased in treated groups in comparison with the control (P < 0.01). The infusion of ghrelin antagonist [D-Lys3]-GHRP-6 reduces feed intake and body weight. With regard to increase in T4 level, it can be inferred that [D-Lys3]-GHRP-6 may increase metabolic rate, lipolysis and weight loss, which is similar to results obtained in mammalian species.

Ghrelin Inhibition Restores Glucose Homeostasis in Hepatocyte Nuclear Factor-1α (MODY3)-Deficient Mice

Hepatocyte nuclear factor-1α (HNF1α) is a transcription factor expressed in tissues of endoderm origin. Mutations in HNF1A are associated with maturity-onset diabetes of the young 3 (MODY3). Mice deficient for Hnf1α are hyperglycemic, with their pancreatic β-cells being defective in glucose-sensing insulin secretion. The specific mechanisms involved in this defect are unclear. Gut hormones control glucose homeostasis. Our objective was to explore whether changes in these hormones play a role in glucose homeostasis in the absence of Hnf1α. An increase in ghrelin gene transcript and a decrease in glucose-dependent insulinotropic polypeptide (GIP) gene transcripts were observed in the gut of Hnf1α-null mice. These changes correlated with an increase of ghrelin and a decrease of GIP-labeled cells. Ghrelin serological levels were significantly induced in Hnf1α-null mice. Paradoxically, GIP levels were also induced in these mice. Treatment of Hnf1α-null mice with a ghrelin antagonist led to a recovery of the diabetic symptoms. We conclude that upregulation of ghrelin in the absence of Hnf1α impairs insulin secretion and can be reversed by pharmacological inhibition of ghrelin/GHS-R interaction. These observations open up on future strategies to counteract ghrelin action in a program that could become beneficial in controlling non-insulin-dependent diabetes.

[D-Lys3]-GHRP-6 exhibits pro-autophagic effects on skeletal muscle

[D-Lys3]-GHRP-6 is regarded as a highly selective growth-hormone secretagogue receptor (GHSR) antagonist and has been widely used to investigate the dependency of GHSR-1a signalling mediated by acylated ghrelin. However, [D-Lys3]-GHRP-6 has been reported to influence other cellular processes which are unrelated to GHSR-1a. This study aimed to examine the effects of [D-Lys3]-GHRP-6 on autophagic and apoptotic cellular signalling in skeletal muscle. [D-Lys3]-GHRP-6 enhanced the autophagic signalling demonstrated by the increases in protein abundances of beclin-1 and LC3 II-to-LC3 1 ratio in both normal muscle and doxorubicin-injured muscle. [D-Lys3]-GHRP-6 reduced the activation of muscle apoptosis induced by doxorubicin. No histological abnormalities were observed in the [D-Lys3]-GHRP-6-treated muscle. Intriguingly, the doxorubicin-induced increase in centronucleated muscle fibres was not observed in muscle treated with [D-Lys3]-GHRP-6, suggesting the myoprotective effects of [D-Lys3]-GHRP-6 against doxorubicin injury. The [D-Lys3]-GHRP-6-induced activation of autophagy was found to be abolished by the co-treatment of CXCR4 antagonist, suggesting that the pro-autophagic effects of [D-Lys3]-GHRP-6 might be mediated through CXCR4. In conclusion, [D-Lys3]-GHRP-6 exhibits pro-autophagic effects on skeletal muscle under both normal and doxorubicin-injured conditions.

Ghrelin gene products in acute and chronic inflammation

Ghrelin gene products--the peptides ghrelin, unacylated ghrelin, and obestatin--have several actions on the immune system, opening new perspectives within neuroendocrinology, metabolism and inflammation. The aim of this review is to summarize the available evidence regarding the less known role of these peptides in the machinery of inflammation and autoimmunity, outlining some of their most promising therapeutic applications.

Effects of peptidic growth hormone secretagogue receptor (GHS-R) antagonist [D-Lys3] on some of serum hormonal and biochemical parameters in Wistar rat model

OBJECTIVE

The present study investigated the effects of different dosages of a GHS-R antagonist [D-Lys3] on some serum hormonal (cortisol, T3 and T4) and biochemical parameters in a rat.

MATERIALS AND METHODS

Thirty-six 60-day-old male rats were assigned to four treatments. [D-Lys3]-GHRP-6 solutions were infused via intraperitoneal injections. Blood was collected and analyzed.

RESULTS

The large dosages of a GHS-R antagonist (200 ng/kg BW) caused increases in cortisol, whereas no significant changes occurred when low dosages were injected. There were no significant changes in T3 and T4 following the administration of the GHS-R antagonist, but a considerable increase was observed in blood glucose levels of the groups (G50, G100, and G200 ng/kg BW). There was a significant increase in total protein when the greatest dose was administrated (G200 ng/kg BW). However, total cholesterol, triglycerides, and albumin showed no significant changes.

CONCLUSIONS

Exogenous GHS-R antagonist can cause an increase in glucose and moderate increases in cortisol and total protein, yet it has no significant effect on T3 and T4 levels or on the concentrations of serum lipids. The effect of GHS-R antagonist is not completely adverse to the effects of ghrelin. Further molecular studies are necessary to identify the physiological effects of the peptidic GHS-R antagonist.

Growth hormone secretagogue receptor deficiency in mice protects against obesity-induced hypertension

Abstract Growth hormone secretagogue receptor (GHS-R) signaling has been associated with growth hormone release, increases in food intake and pleiotropic cardiovascular effects. Recent data demonstrated that acute GHS-R antagonism leads to increases in mean arterial pressure mediated by the sympathetic nervous system in rats; a highly undesirable effect if GHS-R antagonism was to be used as a therapeutic approach to reducing food intake in an already obese, hypertensive patient population. However, our data in conscious, freely moving GHS-R deficient mice demonstrate that chronic absence of GHS-R signaling is protective against obesity-induced hypertension. GHS-R deficiency leads to reduced systolic blood pressure variability (SBPV); in response to acute high-fat diet (HFD)-feeding, increases in the sympathetic control of SBPV are suppressed in GHS-R KO mice. Our data further suggest that GHS-R signaling dampens the immediate HFD-mediated increase in spontaneous baroreflex sensitivity. In diet-induced obesity, absence of GHS-R signaling leads to reductions in obesity-mediated hypertension and tachycardia. Collectively, our findings thus suggest that chronic blockade of GHS-R signaling may not result in adverse cardiovascular effects in obesity.

Central ghrelin signaling mediates the metabolic response of C57BL/6 male mice to chronic social defeat stress

Chronic stressors promote metabolic disturbances, including obesity and metabolic syndrome. Ghrelin, a peptide that promotes appetite and the accumulation of adipose tissue, is also secreted in response to stressors to protect the brain and peripheral tissues from the effects of these stressors. Here we demonstrate that elevated ghrelin levels produced by chronic exposure to social stress are associated with increased caloric intake and body weight gain in male C57BL mice. In contrast, stressed mice lacking ghrelin receptors (GHSR KO mice) or C57BL mice receiving chronic intracerebroventricular delivery of the ghrelin receptor antagonist [d-Lys(3)]-GHRP-6 show attenuated weight gain and feeding responses under the same social stress paradigm. Interestingly, stressed GHSR KO mice showed depleted sc and intrascapular brown fat depots, whereas stressed young wild-type mice did not. In old wild-type mice, chronic social defeat increased visceral and intrascapular brown fat depots in association with increases in obesity markers like hyperleptinemia and hyperinsulinemia along with increased hypothalamic expression of neuropeptide Y and Agouti related peptide. Importantly, the elevated expression of these peptides persisted least for 2 weeks after cessation of the stressor regimen. In contrast, old GHSR KO mice did not show these alterations after chronic social defeat. These results suggest that ghrelin plays an important role in the metabolic adaptations necessary to meet the energetic demands posed by stressors, but chronic exposure to stress-induced ghrelin elevations ultimately could lead to long lasting metabolic dysfunctions.

Small molecule inhibitors of CXCR4

CXCR4 is a G-protein-coupled receptor involved in a number of physiological processes in the hematopoietic and immune systems. The SDF-1/CXCR4 axis is significantly associated with several diseases, such as HIV, cancer, WHIM syndrome, rheumatoid arthritis, pulmonary fibrosis and lupus. For example, CXCR4 is one of the major co-receptors for HIV entry into target cells, while in cancer it plays an important role in tumor cell metastasis. Several promising CXCR4 antagonists have been developed to block SDF-1/CXCR4 interactions that are currently under different stages of development. The first in class CXCR4 antagonist, plerixafor, was approved by the FDA in 2008 for the mobilization of hematopoietic stem cells and several other drugs are currently in clinical trials for cancer, HIV, and WHIM syndrome. While the long-term safety data for the first generation CXCR4 antagonists are not yet available, several new compounds are under preclinical development in an attempt to provide safer and more efficient treatment options for HIV and cancer patients.

Exogenous and endogenous ghrelin counteracts GLP-1 action to stimulate cAMP signaling and insulin secretion in islet β-cells

We studied interactive effects of insulinotropic GLP-1 and insulinostatic ghrelin on rat pancreatic islets. GLP-1 potentiated glucose-induced insulin release and cAMP production in isolated islets and [Ca(2+)](i) increases in single β-cells, and these potentiations were attenuated by ghrelin. Ghrelin suppressed [Ca(2+)](i) responses to an adenylate cyclase activator forskolin. Moreover, GLP-1-induced insulin release and cAMP production were markedly enhanced by [D-lys(3)]-GHRP-6, a ghrelin receptor antagonist, in isolated islets. These results indicate that both exogenous and endogenous islet-derived ghrelin counteracts glucose-dependent GLP-1 action to increase cAMP production, [Ca(2+)](i) and insulin release in islet β-cells, positioning ghrelin as a modulator of insulinotropic GLP-1.