Altered myocardial expression of ghrelin and its receptor (GHSR-1a) in patients with severe heart failure

Ultrastructural study of neuronal cells and localization of ghrelin-like peptide and its receptor in the ganglia of the golden apple snail (Pomacea canaliculata)

Pomacea canaliculata is an invasive snail species causing major problems in agriculture. The snail biology was then investigated. The main objective of the present study was to investigate the nervous system of the snail. The nervous system comprises pairs of cerebral, buccal, pedal, pleural, parietal ganglia and an unpaired visceral ganglion. Most neurons were concentrated at the periphery of the ganglia. The neurons were classified into four types: NR1, NR2, NR3, and NR4. The percentages of the NR3 and NR4 in the pleural and pedal ganglia were significantly higher than those of other ganglia. Ultrastructural study revealed that nuclei of all neuronal types exhibited mostly euchromatins. Many organelles including ribosomes and endoplasmic reticulum were found in their cytoplasm. However, various mitochondria were found in the NR2 and NR3. The immunohistochemistry revealed immunoreactivity of ghrelin-like peptide in the neurons of the cerebral, pleural and pedal ganglia. However, immunoreactivity of GHS-R1a-like peptide existed only in the neurons of the pleural and pedal ganglia. The present study is the first to demonstrate the existence of ghrelin-like peptide and its receptor in P. canaliculata nervous system.

Insights on discovery, efficacy, safety and clinical applications of ghrelin receptor agonist capromorelin in veterinary medicine

Growth hormone and insulin like growth factor-1 plays an important role in the regulation of body composition and metabolism. Growth Hormone is released from the pituitary through a specific G-protein coupled receptor (GPCR) called growth hormone secretagogue receptor 1a expressed in the hypothalamus. Ghrelin is a peptide hormone released from the cells in the stomach, which stimulates appetite and food intake in mammals, regulates gut motility, gastric acid secretion, taste sensation, circadian rhythm, learning and memory, oxidative stress, autophagy, glucose metabolism etc. When the release of the endogenous ligand GHSR-1a, i.e., ghrelin is malfunctioned or stopped, external substitutes are administrated to induce the stimulation of growth hormone and appetite. A class of compound known as ghrelin receptor agonists are developed as an external substitute of ghrelin for regulation and stimulation of growth hormone in frailty, for body weight gain, muscle mass gain, prevention of cachexia and for the treatment of chronic fatigue syndromes. Capromorelin [Entyce™ (Aratana Therapeutics, Leawood, KS, USA)] is the only FDA (Food and Drug Administration) approved (May 2016) drug used for stimulating appetite in dogs and was marketed in the fall of 2017. In 2020, USFDA approved Capromorelin [Elura™ (Elanco US Inc.)] for the management of weight loss in chronic kidney disease of cats. This article reviews the discovery of the ghrelin receptor agonist capromorelin, its efficacy, safety, clinical applications and aims to delineate its further scope of use in veterinary practice.

Nutrient-sensing growth hormone secretagogue receptor in macrophage programming and meta-inflammation

OBJECTIVE

Obesity-associated chronic inflammation, aka meta-inflammation, is a key pathogenic driver for obesity-associated comorbidity. Growth hormone secretagogue receptor (GHSR) is known to mediate the effects of nutrient-sensing hormone ghrelin in food intake and fat deposition. We previously reported that global Ghsr ablation protects against diet-induced inflammation and insulin resistance, but the site(s) of action and mechanism are unknown. Macrophages are key drivers of meta-inflammation. To unravel the role of GHSR in macrophages, we generated myeloid-specific Ghsr knockout mice (LysM-Cre;Ghsrf/f).

METHODS

LysM-Cre;Ghsrf/f and control Ghsrf/f mice were subjected to 5 months of high-fat diet (HFD) feeding to induce obesity. In vivo, metabolic profiling of food intake, physical activity, and energy expenditure, as well as glucose and insulin tolerance tests (GTT and ITT) were performed. At termination, peritoneal macrophages (PMs), epididymal white adipose tissue (eWAT), and liver were analyzed by flow cytometry and histology. For ex vivo studies, bone marrow-derived macrophages (BMDMs) were generated from the mice and treated with palmitic acid (PA) or lipopolysaccharide (LPS). For in vitro studies, macrophage RAW264.7 cells with Ghsr overexpression or Insulin receptor substrate 2 (Irs2) knockdown were studied.

RESULTS

We found that Ghsr expression in PMs was increased under HFD feeding. In vivo, HFD-fed LysM-Cre;Ghsrf/f mice exhibited significantly attenuated systemic inflammation and insulin resistance without affecting food intake or body weight. Tissue analysis showed that HFD-fed LysM-Cre;Ghsrf/f mice have significantly decreased monocyte/macrophage infiltration, pro-inflammatory activation, and lipid accumulation, showing elevated lipid-associated macrophages (LAMs) in eWAT and liver. Ex vivo, Ghsr-deficient macrophages protected against PA- or LPS-induced pro-inflammatory polarization, showing reduced glycolysis, increased fatty acid oxidation, and decreased NF-κB nuclear translocation. At molecular level, GHSR metabolically programs macrophage polarization through PKA-CREB-IRS2-AKT2 signaling pathway.

CONCLUSIONS

These novel results demonstrate that macrophage GHSR plays a key role in the pathogenesis of meta-inflammation, and macrophage GHSR promotes macrophage infiltration and induces pro-inflammatory polarization. These exciting findings suggest that GHSR may serve as a novel immunotherapeutic target for the treatment of obesity and its associated comorbidity.

Ghrelin/GHSR Axis Induced M2 Macrophage and Alleviated Intestinal Barrier Dysfunction in a Sepsis Rat Model by Inactivating E2F1/NF-κB Signaling

Sepsis is an inflammatory reaction disorder state that is induced by infection. The activation and regulation of the immune system play an essential role in the development of sepsis. Our previous studies have shown that ghrelin ameliorates intestinal dysfunction in sepsis. Very little is known about the mechanism of ghrelin and its receptor (GHSR) on the intestinal barrier and the immune function of macrophage regulation. Our research is to investigate the regulatory effect and molecular mechanism of the ghrelin/GHSR axis on intestinal dysfunction and macrophage polarization in septic rats. A rat model of sepsis was established by cecal ligation and puncture (CLP) operation. Then, the sepsis rats were treated with a ghrelin receptor agonist (TZP-101) or ghrelin inhibitor (obestatin). The results suggested that TZP-101 further enhanced ghrelin and GHSR expressions in the colon and spleen of septic rats and obestatin showed the opposite results. Ghrelin/GHSR axis ameliorated colonic structural destruction and intestinal epithelial tight junction injury in septic rats. In addition, the ghrelin/GHSR axis promoted M2-type polarization of macrophages, which was characterized by the decreases of IL-1β, IL-6, and TNF-α, as well as the increase of IL-10. Mechanistically, the ghrelin/GHSR axis promoted E2F2 expression and suppressed the activation of the NF-κB signaling pathway in septic rats. Collectively, targeting ghrelin/GHSR during sepsis may represent a novel therapeutic approach for the treatment of intestinal barrier injury.

Gut Molecules in Cardiometabolic Diseases: The Mechanisms behind the Story

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

Radiofluorination of non-activated aromatic prosthetic groups for synthesis and evaluation of fluorine-18 labelled ghrelin(1-8) analogues

The growth hormone secretagogue receptor 1a (GHSR) is differentially expressed in various disease states compared to healthy tissues and thus is a target for molecular imaging. The endogenous ligand for the GHSR is ghrelin, a 28 amino acid peptide with a unique octanoyl group on the serine-3 residue. A recently reported ghrelin analogue revealed the successful use of fluorine-containing, polycyclic aromatic groups in place of the octanoyl side chain, thereby providing potential access to new ¹⁸F-PET imaging probes. The peptide [Inp¹,Dpr³(6-FN),1Nal⁴,Thr⁸]ghrelin(1-8) amide (1) showed sub-nanomolar receptor affinity (IC₅₀ = 0.11 nM) toward the GHSR making it the strongest affinity ghrelin analogue reported to date. However, attempts to label such non-activated aromatic groups with fluoride-18 through conventional substitution methods resulted in low radiochemical yields, impractical for use in vivo. Since larger, non-activated aromatic groups appear to be of value for incorporating fluorine into ghrelin(1-8) analogues, an additional peptide bearing a 4'-fluorobiphenyl-4-carboxyl (4'-FBC) group in place of the octanoyl side chain was also of interest. Herein, we describe the radiosynthesis of [Inp¹,Dpr³([¹⁸F]6-FN),1Nal⁴,Thr⁸]ghrelin(1-8) amide ([18F]1) and [Inp¹,Dpr³([¹⁸F]4'-FBC),1Nal⁴,Thr⁸]ghrelin(1-8) amide ([18F]2) using a prosthetic group approach from iodonium ylide precursors as well as initial in vitro and in vivo evaluation of [18F]1 as a potential PET tracer for targeted imaging of the GHSR.

Temporal changes and prognostic value of plasma ghrelin level in patients with acute heart failure: a prospective study

BACKGROUND

Plasma ghrelin levels can be elevated in patients with acute heart failure (AHF). This study aimed to analyze the temporal changes and prognostic value of ghrelin levels in patients with AHF.

METHODS

This prospective study included patients with AHF at the Cardiology Department, Weifang People's Hospital (May 2018-October 2019), and age- and sex-matched healthy controls. Plasma ghrelin levels were measured. Multivariable logistic regression and receiver operating characteristic (ROC) curve analyses were used to evaluate whether ghrelin levels could predict major cardiac adverse events (MACEs) during a 1-year follow-up.

RESULTS

Finally, 92 patients with AHF and 50 healthy controls were enrolled. Ghrelin levels were higher in patients with AHF at 1, 3, 12, and 24 h compared with controls (all P < 0.01). Ghrelin levels in the AHF group were higher at 3 and 12 h than at 1 and 24 h (P < 0.001). Ghrelin level at 3 h in patients with AHF was negatively correlated with the left ventricular end-diastolic diameter and left ventricular ejection fraction (both P < 0.05). MACEs occurred in 48 patients with AHF. Ghrelin levels were higher in the MACE group than in the non-MACE group at 1 (P = 0.011) and 3 h (P = 0.034). Multivariable regression showed that ghrelin level at 3 h was independently associated with MACEs [OR = 0.629, 95% confidence interval (CI): 0.515-0.742, P = 0.010], but the area under the ROC curve was only 0.629 (95% CI 0.515-0.742).

CONCLUSIONS

Plasma ghrelin levels are elevated in AHF and patients with MACEs during follow-up.

GHSR deficiency exacerbates cardiac fibrosis: role in macrophage inflammasome activation and myofibroblast differentiation

AIMS

Sustained activation of β-adrenergic signalling induces cardiac fibrosis, which marks progression to heart failure. GHSR (growth hormone secretagogue receptor) is the receptor for ghrelin, which is an orexigenic gastric hormone with newly defined cardiovascular effects. The present study determined the effects of GHSR deficiency in a mouse model of isoproterenol (ISO)-induced cardiac fibrosis and examined the underlying mechanism.

METHODS AND RESULTS

Histochemical studies showed that GHSR deficiency exacerbated cardiac fibrosis. Quantitative RT-PCR, western blotting, and immunofluorescence staining demonstrated that cardiac fibroblasts isolated from GHSR-/- mice exhibited increased expression of marker genes for myofibroblast trans-differentiation (α-SMA, SM22, and calponin) upon transforming growth factor-β treatment compared to wild-type mice. RNA-sequencing of heart transcriptomes revealed that differentially expressed genes in GHSR-/- hearts were enriched in such biological processes as extracellular matrix organization, inflammatory response, lipid metabolism, cell cycle, migration, and adhesion. Particularly, GHSR deficiency increased Wnt/β-catenin pathway activation in ISO-induced myocardial fibrosis. In addition, loss of GHSR in macrophages instigated inflammasome activation with increased cleavage and release of interleukin-18.

CONCLUSION

These results for the first time demonstrated that GHSR deficiency aggravated ISO-induced cardiac fibrosis, suggesting that GHSR was a potential target for the intervention of cardiac fibrosis.

Ghrelin Based Therapy of Metabolic Diseases

BACKGROUND

Ghrelin, a unique 28 amino acid peptide hormone secreted by the gastric X/A like cells, is an endogenous ligand of the growth hormone secretagogue receptor (GHSR). Ghrelin-GHSR signaling has been found to exert various physiological functions, including stimulation of appetite, regulation of body weight, lipid and glucose metabolism, and increase of gut motility and secretion. This system is thus critical for energy homeostasis.

OBJECTIVE

The objective of this review is to highlight the strategies of ghrelin-GHSR based intervention for therapy of obesity and its related metabolic diseases.

RESULTS

Therapeutic strategies of metabolic disorders targeting the ghrelin-GHSR pathway involve neutralization of circulating ghrelin by antibodies and RNA spiegelmers, antagonism of ghrelin receptor by its antagonists and inverse agonists, inhibition of ghrelin O-acyltransferase (GOAT), as well as potential pharmacological approach to decrease ghrelin synthesis and secretion.

CONCLUSION

Various compounds targeting the ghrelin-GHSR system have shown promising efficacy for the intervention of obesity and relevant metabolic disorders in animals and in vitro. Further clinical trials to validate their efficacy in human beings are urgently needed.

The Complex World of Regulation of Pituitary Growth Hormone Secretion: The Role of Ghrelin, Klotho, and Nesfatins in It

The classic concept of how pituitary GH is regulated by somatostatin and GHRH has changed in recent years, following the discovery of peripheral hormones involved in the regulation of energy homeostasis and mineral homeostasis. These hormones are ghrelin, nesfatins, and klotho. Ghrelin is an orexigenic hormone, released primarily by the gastric mucosa, although it is widely expressed in many different tissues, including the central nervous system and the pituitary. To be active, ghrelin must bind to an n-octanoyl group (n = 8, generally) on serine 3, forming acyl ghrelin which can then bind and activate a G-protein-coupled receptor leading to phospholipase C activation that induces the formation of inositol 1,4,5-triphosphate and diacylglycerol that produce an increase in cytosolic calcium that allows the release of GH. In addition to its direct action on somatotrophs, ghrelin co-localizes with GHRH in several neurons, facilitating its release by inhibiting somatostatin, and acts synergistically with GHRH stimulating the synthesis and secretion of pituitary GH. Gastric ghrelin production declines with age, as does GH. Klotho is an anti-aging agent, produced mainly in the kidneys, whose soluble circulating form directly induces GH secretion through the activation of ERK1/2 and inhibits the inhibitory effect that IGF-I exerts on GH. Children and adults with untreated GH-deficiency show reduced plasma levels of klotho, but treatment with GH restores them to normal values. Deletions or mutations of the Klotho gene affect GH production. Nesfatins 1 and 2 are satiety hormones, they inhibit food intake. They have been found in GH3 cell cultures where they significantly reduce the expression of gh mRNA and that of pituitary-specific positive transcription factor 1, consequently acting as inhibitors of GH production. This is a consequence of the down-regulation of the cAMP/PKA/CREB signaling pathway. Interestingly, nesfatins eliminate the strong positive effect that ghrelin has on GH synthesis and secretion. Throughout this review, we will attempt to broadly analyze the role of these hormones in the complex world of GH regulation, a world in which these hormones already play a very important role.

Research progress of ghrelin on cardiovascular disease

Ghrelin, a 28-aminoacid peptide, was isolated from the human and rat stomach and identified in 1999 as an endogenous ligand for the growth hormone secretagogue-receptor (GHS-R). In addition to stimulating appetite and regulating energy balance, ghrelin and its receptor GHS-R1a have a direct effect on the cardiovascular system. In recent years, it has been shown that ghrelin exerts cardioprotective effects, including the modulation of sympathetic activity and hypertension, enhancement of the vascular activity and angiogenesis, inhibition of arrhythmias, reduction in heart failure and inhibition of cardiac remodeling after myocardial infarction (MI). The cardiovascular protective effect of ghrelin may be associated with anti-inflammation, anti-apoptosis, inhibited sympathetic nerve activation, regulated autophagy, and endothelial dysfunction. However, the molecular mechanisms underlying the effects of ghrelin on the cardiovascular system have not been fully elucidated, and no specific therapeutic agent has been established. It is important to further explore the pharmacological potential of ghrelin pathway modulation for the treatment of cardiovascular diseases.

Regional Differences in the Ghrelin-Growth Hormone Secretagogue Receptor Signalling System in Human Heart Disease

Background

The hormone ghrelin and its receptor, the growth hormone secretagogue receptor (GHSR) are expressed in myocardium. GHSR binding activates signalling pathways coupled to cardiomyocyte survival and contractility. These properties have made the ghrelin-GHSR axis a candidate for a biomarker of cardiac function. The dynamics of ghrelin-GHSR are altered significantly in late stages of heart failure (HF) and cardiomyopathy, when left ventricular (LV) function is failing. We examined the relationship of GHSR with ghrelin in cardiac tissue from patients with valvular disease with no detectable changes in LV function.

Methods

Biopsy samples from the left ventricle and left atrium were obtained from 25 patients with valvular disease (of whom 13 also had coronary artery disease) and preserved LV ejection fraction, and compared to control samples obtained via autopsy. Using quantitative confocal fluorescence microscopy, levels of GHSR were determined using [Dpr³(n-octanoyl),Lys¹⁹(sulfo-Cy5)]ghrelin(1-19), and immunofluorescence determined ghrelin, the heart failure marker natriuretic peptide type-B (BNP), and contractility marker sarcoplasmic reticulum ATPase pump (SERCA2a).

Results

A positive correlation between GHSR and ghrelin was apparent in only diseased tissue. Ghrelin and BNP significantly correlated in the left ventricle and strongly colocalized to the same intracellular compartment in diseased and control tissue. GHSR, ghrelin, and BNP all strongly and significantly correlated with SERCA2a in the left ventricle of diseased tissue only.

Conclusions

Our results suggest that the dynamics of the myocardial ghrelin-GHSR axis is altered in cardiovascular disease in the absence of measurable changes in heart function, and might accompany a regional shift in endocrine programming.

A Decade's Progress in the Development of Molecular Imaging Agents Targeting the Growth Hormone Secretagogue Receptor

The growth hormone secretagogue receptor 1a (GHSR), also called the ghrelin receptor, is a G protein-coupled receptor known to play an important metabolic role in the regulation of various physiological processes, including energy expenditure, growth hormone secretion, and cell proliferation. This receptor has been implicated in numerous health issues including obesity, gastrointestinal disorders, type II diabetes, and regulation of body weight in patients with Prader-Willi syndrome, and there has been growing interest in studying its mechanism of behavior to unlock further applications of GHSR-targeted therapeutics. In addition, the GHSR is expressed in various types of cancer including prostate, breast, and testicular cancers, while aberrant expression has been reported in cardiac disease. Targeted molecular imaging of the GHSR could provide insights into its role in biological processes related to these disease states. Over the past decade, imaging probes targeting this receptor have been discovered for the imaging modalities PET, SPECT, and optical imaging. High-affinity analogues of ghrelin, the endogenous ligand for the GHSR, as well as small molecule inhibitors have been developed and evaluated both in vitro and in pre-clinical models. This review provides a comprehensive overview of the molecular imaging agents targeting the GHSR reported to the end of 2019.

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

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

The Additional Prognostic Value of Ghrelin for Mortality and Readmission in Elderly Patients with Acute Heart Failure

Purpose

To evaluate the prognostic value of ghrelin, a growth hormone-releasing peptide, for mortality and readmission in elderly patients with acute heart failure (AHF).

Patients and Methods

We measured plasma ghrelin and pro B-type natriuretic peptide (NT-proBNP) levels upon emergency admission in 241 prospectively recruited elderly AHF patients (61.0% men). The outcomes were all-cause mortality and/or readmission due to heart failure (HF). Multivariate Cox proportional hazards regression analyses were used to evaluate the prognostic value of ghrelin. Discrimination, calibration, and reclassification indices were compared between models, with or without ghrelin.

Results

During 1.2 years of follow-up, we observed 90 events (57 deaths and 33 readmissions due to HF). Plasma ghrelin levels were significantly elevated in elderly AHF patients, when compared to healthy control subjects (P < 0.001). Patients with events had significantly higher baseline ghrelin levels, when compared to those without (P < 0.001). Ghrelin levels were positively correlated with NT-proBNP levels and HF severity, whereas they were negatively correlated with nutritional status (all P < 0.05). Log transformed ghrelin levels were independently associated with AHF events (hazard ratio = 2.64, 95% confidence interval = 1.11–6.25, P = 0.028). The incorporation of ghrelin into the reference model, or reference with the NT-proBNP model, both improved C-statistics (from 0.742–0.780 and 0.836–0.857; P = 0.074 and 0.044, respectively), resulting in an improvement in net reclassification index (14.42% and 10.45%, P = 0.020 and 0.025, respectively), and integrated discrimination index (5.64% and 3.60%, both P < 0.001). Patients who displayed the above NT-proBNP and ghrelin median levels had a markedly higher risk of AHF adverse events (P < 0.001).

Conclusion

Plasma ghrelin is an independent predictor of adverse events in elderly AHF patients. Ghrelin may provide additional value to clinical parameters or NT-proBNP for prognostic risk stratification in AHF.

Erythromycin inhibits neutrophilic inflammation and mucosal disease by upregulating DEL-1

Macrolide antibiotics exert antiinflammatory effects; however, little is known regarding their immunomodulatory mechanisms. In this study, using 2 distinct mouse models of mucosal inflammatory disease (LPS-induced acute lung injury and ligature-induced periodontitis), we demonstrated that the antiinflammatory action of erythromycin (ERM) is mediated through upregulation of the secreted homeostatic protein developmental endothelial locus-1 (DEL-1). Consistent with the anti-neutrophil recruitment action of endothelial cell-derived DEL-1, ERM inhibited neutrophil infiltration in the lungs and the periodontium in a DEL-1-dependent manner. Whereas ERM (but not other antibiotics, such as josamycin and penicillin) protected against lethal pulmonary inflammation and inflammatory periodontal bone loss, these protective effects of ERM were abolished in Del1-deficient mice. By interacting with the growth hormone secretagogue receptor and activating JAK2 in human lung microvascular endothelial cells, ERM induced DEL-1 transcription that was mediated by MAPK p38 and was CCAAT/enhancer binding protein-β dependent. Moreover, ERM reversed IL-17-induced inhibition of DEL-1 transcription, in a manner that was dependent not only on JAK2 but also on PI3K/AKT signaling. Because DEL-1 levels are severely reduced in inflammatory conditions and with aging, the ability of ERM to upregulate DEL-1 may lead to a novel approach for the treatment of inflammatory and aging-related diseases.

A traditional herbal medicine rikkunshito prevents angiotensin II-Induced atrial fibrosis and fibrillation

BACKGROUND

Rikkunshito (RKT), a traditional herbal medicine, has been demonstrated to exert anti-inflammatory, anti-apoptotic, and anti-fibrotic effects in several organs. This study tested the hypothesis that RKT can suppress angiotensin II (AngII)-induced inflammatory atrial fibrosis and ameliorate enhanced vulnerability to atrial fibrillation (AF).

METHODS

Eight-week-old male C57BL/6 mice were subcutaneously infused with either vehicle or AngII (2.0 mg/kg/day) for 2 weeks. Water or RKT at a dose of 1000 mg/kg/day were orally administered once daily for 2 weeks. Morphological, histological, and biochemical analyses were performed. AF was induced either by transesophageal burst pacing in vivo or by burst/extrastimuli in isolated perfused hearts using a Langendorff apparatus.

RESULTS

RKT at a dose of 1000 mg/kg/day for 2 weeks attenuated atrial interstitial fibrosis and profibrotic and proinflammatory signals induced by continuous infusion of AngII. RKT attenuated AngII-induced enhanced vulnerability to AF in in vivo experiments and in isolated perfused hearts. Atractylodin, an active component of RKT, exhibited antifibrotic activity comparable to that of RKT. RKT reversed AngII-induced suppression of sirtuin 1 (Sirt1) translocation to the nuclei. RKT suppressed AngII-induced phosphorylation of IκB, overexpression of p53, and cellular apoptotic signals and apoptosis. All of the antagonizing effects of RKT against AngII were attenuated by a concomitant treatment with a growth hormone secretagogue receptor (GHSR)-inhibitor.

CONCLUSION

Our results demonstrated that RKT prevented atrial fibrosis and attenuated enhanced vulnerability to AF induced by AngII. The results also suggested that potentiating the GHSR-Sirt1 pathway is involved in these processes.

Pretreatment of ghrelin protects H9c2 cells against hypoxia/reoxygenation-induced cell death via PI3K/AKT and AMPK pathways

Ghrelin has been widely recognized as a key peptide in the cardiovascular system. This study detected the potential of ghrelin in MI management and tried to decode one of the possible underlying mechanisms. H9c2 cells were pretreated with ghrelin and were subjected to hypoxia/reoxygenation (H/R). CCK-8, flow cytometry, Western blot and LDH analysis were conducted to assess the changes in cell survival. LY294002 and Compound C were used to treat H9c2 cells for blocking PI3K/AKT and AMPK pathways, respectively. Ghrelin expression in H9c2 cells was suppressed by siRNA-mediated silencing to see the effects of endogenous ghrelin. We found that, following H/R, H9c2 cells viability was decreased, CyclinD1 and CDK4 were down-regulated, apoptosis was induced, the release of LDH was enhanced, and the expression levels of Cox-2 and iNOS were up-regulated. Ghrelin protected H9c2 cells against H/R induced these alterations. Besides, ghrelin activated PI3K/AKT and AMPK pathways even in H/R-stimulated cells. The protective effects of ghrelin against H/R-induced cell damage were all attenuated by the addition of LY294002 or Compound C. Moreover, endogenous inhibition of ghrelin significantly induced cell death of H9c2 cells. In conclusion, this study demonstrated that ghrelin pretreatment protected H9c2 cells against H/R-induced cell damage, possibly via PI3K/AKT and AMPK pathways.

Ghrelin Derangements in Idiopathic Dilated Cardiomyopathy: Impact of Myocardial Disease Duration and Left Ventricular Ejection Fraction

Background: Ghrelin may exert positive effects on cardiac structure and function in heart failure (HF) patients. Methods: We assessed ghrelin levels in 266 dilated cardiomyopathy (DCM) patients and in 200 age, gender and body mass index (BMI) matched controls. Further, we evaluated the expression of ghrelin and growth hormone secretagogue-receptor (GHSR) in the myocardium of 41 DCM patients and in 11 controls. Results: DCM patients had significantly lower levels of total, acylated and unacylated ghrelin when compared to controls (p < 0.05 for all). In controls, we observed a negative correlation of ghrelin with age, male gender and BMI. These correlations were lost in the DCM group, except for male gender. Total ghrelin was higher in patients with more recent diagnosis when compared to patients with longer duration of the DCM (p = 0.033). Further, total ghrelin was higher in patients with lower left ventricular systolic function (<40% LVEF, vs. 40% ≤ LVEF < 49% vs. LVEF ≥ 50%: 480.8, vs. 429.7, vs. 329.5 pg/mL, respectively, p = 0.05). Ghrelin prepropeptide was expressed more in DCM patients than in controls (p = 0.0293) while GHSR was expressed less in DCM patients (p < 0.001). Furthermore, ghrelin showed an inverse correlation with its receptor (ρ = −0.406, p = 0.009), and this receptor showed a significant inverse correlation with Interleukin-1β (ρ = −0.422, p = 0.0103). Conclusion: DCM duration and severity are accompanied by alterations in the ghrelin–GHSR system.

Development and Characteristics of Pancreatic Epsilon Cells

Pancreatic endocrine cells expressing the ghrelin gene and producing the ghrelin hormone were first identified in 2002. These cells, named ε cells, were recognized as the fifth type of endocrine cells. Differentiation of ε cells is induced by various transcription factors, including Nk2 homeobox 2, paired box proteins Pax-4 and Pax6, and the aristaless-related homeobox. Ghrelin is generally considered to be a "hunger hormone" that stimulates the appetite and is produced mainly by the stomach. Although the population of ε cells is small in adults, they play important roles in regulating other endocrine cells, especially β cells, by releasing ghrelin. However, the roles of ghrelin in β cells are complex. Ghrelin contributes to increased blood glucose levels by suppressing insulin release from β cells and is also involved in the growth and proliferation of β cells and the prevention of β cell apoptosis. Despite increasing evidence and clarification of the mechanisms of ε cells over the last 20 years, many questions remain to be answered. In this review, we present the current evidence for the participation of ε cells in differentiation and clarify their characteristics by focusing on the roles of ghrelin.

Dynamics of the Ghrelin/Growth Hormone Secretagogue Receptor System in the Human Heart Before and After Cardiac Transplantation

Currently, the early preclinical detection of left ventricular dysfunction is difficult because biomarkers are not specific for the cardiomyopathic process. The underlying molecular mechanisms leading to heart failure remain elusive, highlighting the need for identification of cardiac-specific markers. The growth hormone secretagogue receptor (GHSR) and its ligand ghrelin are present in cardiac tissue and are known to contribute to myocardial energetics. Here, we examined tissue ghrelin-GHSR levels as specific markers of cardiac dysfunction in patients who underwent cardiac transplantation. Samples of cardiac tissue were obtained from 10 patients undergoing cardiac transplant at the time of organ harvesting and during serial posttransplant biopsies. Quantitative fluorescence microscopy using a fluorescent ghrelin analog was used to measure levels of GHSR, and immunofluorescence was used to measure levels of ghrelin, B-type natriuretic peptide (BNP), and tissue markers of cardiomyocyte contractility and growth. GHSR and ghrelin expression levels were highly variable in the explanted heart, less in the grafted heart biopsies. GHSR and ghrelin were strongly positively correlated, and both markers were negatively correlated with left ventricular ejection fraction. Ghrelin had stronger positive correlations than BNP with the signaling markers for contractility and growth. These data suggest that GHSR-ghrelin have potential use as an integrated marker of cardiac dysfunction. Interestingly, tissue ghrelin appeared to be a more sensitive indicator than BNP to the biochemical processes that are characteristic of heart failure. This work allows for further use of ghrelin-GHSR to interrogate cardiac-specific biochemical mechanisms in preclinical stages of heart failure (HF).

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

Development and Characterization of an 18F-labeled Ghrelin Peptidomimetic for Imaging the Cardiac Growth Hormone Secretagogue Receptor

One-third of patients with heart disease develop heart failure, which is diagnosed through imaging and detection of circulating biomarkers. Imaging strategies reveal morphologic and functional changes but fall short of detecting molecular abnormalities that can lead to heart failure, and circulating biomarkers are not cardiac specific. Thus, there is critical need for biomarkers that are endogenous to myocardial tissues. The cardiac growth hormone secretagogue receptor 1a (GHSR1a), which binds the hormone ghrelin, is a potential biomarker for heart failure. We have synthesized and characterized a novel ghrelin peptidomimetic tracer, an ¹⁸F-labeled analogue of G-7039, for positron emission tomography (PET) imaging of cardiac GHSR1a. In vitro analysis showed enhanced serum stability compared to natural ghrelin and significantly increased cellular uptake in GHSR1a-expressing OVCAR cells. Biodistribution studies in mice showed that tissue uptake of the tracer was independent of circulating ghrelin levels, and there was negligible cardiac uptake and high uptake in the liver, intestines, and kidneys. Specificity of tracer uptake was assessed using ghsr ^−/− mice; both static and dynamic PET imaging revealed no difference in cardiac uptake, and there was no significant correlation between cardiac standardized uptake values and GHSR1a expression. Our study lays the groundwork for further refinement of peptidomimetic PET tracers targeting cardiac GHSR1a.

Posttraumatic Stress Disorder Augments Plasma Triglycerides in TT Homozygotes of rs495225 at Growth Hormone Secretagogue Receptor Gene

Posttraumatic stress disorder (PTSD) and growth hormone secretagogue receptor (GHSR) were reported to be associated with plasma lipid and glucose levels. However, interplays of PTSD with GHSR on plasma lipid and glucose levels have not been explored yet. This study was to investigate the interplays of PTSD and GHSR rs495225 on plasma glucose and lipid profiles. A total of 709 high school students were recruited at 6 months after the 2008 Wenchuan earthquake. Variants of GHSR rs495225 were identified by polymerase chain reaction-restriction fragment length polymorphism analyses and verified by DNA sequencing. The PTSD Checklist Civilian Version (PCL-C) was used to assess PTSD. There was no significant difference of PTSD prevalence between the TT homozygotes and the C allele carriers. However, the students with PTSD had significantly lower levels of glucose, insulin and homeostasis model assessment of insulin resistance (HOMA-IR) than the students without PTSD in the C allele carriers of GHSR rs495225 after the adjustment for age, gender and body mass index (BMI), but higher levels of TG and TG/HDL-C in the TT homozygotes. Meanwhile, the TT homozygotes had lower levels of HDL-C than the C allele carriers in the students without PTSD, but higher levels of insulin and HOMA-IR in the subjects with PTSD. After the adjustment of age and gender, and additional adjustment for BMI, the results were not changed except the difference of insulin was only a tendency (p = 0.054) after the additional adjustment for BMI. PTSD may augment TG levels and the related lipid ratio TG/HDL-C in the TT homozygotes of GHSR rs495225 but decrease the levels of glucose, insulin and HOMA-IR in the C allele carriers.

Role of ghrelin in pancreatic development and function

Ghrelin is a gastric peptide with anabolic functions. It acutely stimulates growth hormone (GH) secretion from the anterior pituitary glands and modulates hypothalamic circuits that control food intake and energy expenditure. Besides its central activity, ghrelin is also involved in the regulation of pancreatic development and physiology. Particularly, several studies highlighted the ability of ghrelin to sustain β-cell viability and proliferation. Furthermore, ghrelin seems to exert inhibitory effects on pancreatic acinar and endocrine secretory functions. Due to its pleiotropic activity on energy metabolism, ghrelin has become a topic of great interest for experimental research focused on type II diabetes and obesity. The aim of this review is to illustrate the complex and not fully understood interplay between ghrelin, pancreas and glucose homeostasis.

Peptidomimetic growth hormone secretagogue derivatives for positron emission tomography imaging of the ghrelin receptor

The ghrelin receptor is a seven-transmembrane (7-TM) receptor known to have an increased level of expression in human carcinoma and heart failure. Recent work has focused on the synthesis of positron emission tomography (PET) probes designed to target and image this receptor for disease diagnosis and staging. However, these probes have been restricted to small-molecule quinalizonones and peptide derivatives of the endogenous ligand ghrelin. We describe the design, synthesis and biological evaluation of a series of 4-fluorobenzoylated growth hormone secretagogues (GHSs) derived from peptidic (GHRP-1, GHPR-2 and GHRP-6) and peptidomimetic (G-7039, [1-Nal⁴]G-7039 and ipamorelin) families in order to test locations for the insertion of fluorine-18 for PET imaging. The peptidomimetic G-7039 was found to be the most suitable for ¹⁸F-radiolabelling as its non-radioactive 4-fluorobenzoylated analogue ([1-Nal⁴,Lys⁵(4-FB)]G-7039), had both a high binding affinity (IC₅₀ = 69 nM) and promising in vitro efficacy (EC₅₀ = 1.1 nM). Prosthetic group radiolabelling of the precursor compound [1-Nal⁴]G-7039 using N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) delivered the PET probe [1-Nal⁴,Lys⁵(4-[¹⁸F]-FB)]G-7039 in an average decay-corrected radiochemical yield of 48%, a radio-purity ≥ 99% and an average molar activity of >34 GBq/μmol. This compound could be investigated as a PET probe for the detection of diseases that are characterised by overexpression of the ghrelin receptor.

Ghrelin Ameliorates Angiotensin II-Induced Myocardial Fibrosis by Upregulating Peroxisome Proliferator-Activated Receptor Gamma in Young Male Rats

Angiotensin (Ang) II contributes to the formation and development of myocardial fibrosis. Ghrelin, a gut peptide, has demonstrated beneficial effects against cardiovascular disease. In the present study, we explored the effect and related mechanism of Ghrelin on myocardial fibrosis in Ang II-infused rats. Adult Sprague-Dawley (SD) rats were divided into 6 groups: Control, Ang II (200ng/kg/min, microinfusion), Ang II+Ghrelin (100 μg/kg, subcutaneously twice daily), Ang II+Ghrelin+GW9662 (a specific PPAR-γ inhibitor, 1 mg/kg/d, orally), Ang II+GW9662, and Ghrelin for 4 wks. In vitro, adult rat cardiac fibroblasts (CFs) were pretreated with or without Ghrelin, Ghrelin+GW9662, or anti-Transforming growth factor (TGF)-β1 antibody and then stimulated with or without Ang II (100 nmol/L) for 24 h. Ang II infusion significantly increased myocardial fibrosis, expression of collagen I, collagen III, and TGF-β1, as well as TGF-β1 downstream proteins p-Smad2, p-Smad3, TRAF6, and p-TAK1 (all p<0.05). Ghrelin attenuated these effects. Similar results were seen in Ang II-stimulated rat cardiac fibroblasts in vitro. In addition, Ghrelin upregulated PPAR-γ expression in vivo and in vitro, and treatment with GW9662 counteracted the effects of Ghrelin. In conclusion, Ghrelin ameliorated Ang II-induced myocardial fibrosis by upregulating PPAR-γ and in turn inhibiting TGF-β1signaling.

Changes in the Cardiac GHSR1a-Ghrelin System Correlate With Myocardial Dysfunction in Diabetic Cardiomyopathy in Mice

Ghrelin and its receptor, the growth hormone secretagogue receptor 1a (GHSR1a), are present in cardiac tissue. Activation of GHSR1a by ghrelin promotes cardiomyocyte contractility and survival, and changes in myocardial GHSR1a and circulating ghrelin track with end-stage heart failure, leading to the hypothesis that GHSR1a is a biomarker for heart failure. We hypothesized that GHSR1a could also be a biomarker for diabetic cardiomyopathy (DCM). We used two models of streptozotocin (STZ)-induced DCM: group 1, adult mice treated with 35 mg/kg STZ for 3 days; and group 2, neonatal mice treated with 70 mg/kg STZ at days 2 and 5 after birth. In group 1, mild fasting hyperglycemia (11 mM) was first detected 8 weeks after the last injection, and in group 2, severe fasting hyperglycemia (20 mM) was first detected 1 to 3 weeks after the last injection. In group 1, left ventricular function was slightly impaired as measured by echocardiography, and Western blot analysis showed a significant decrease in myocardial GHSR1a. In group 2, GHSR1a levels were also decreased as assessed by Cy5-ghrelin(1–19) fluorescence microscopy, and there was a significant negative correlation between GHSR1a levels and glucose tolerance. There were significant positive correlations between GHSR1a and ghrelin and between GHSR1a and sarcoplasmic reticulum Ca²⁺-ATPase 2a (SERCA2a), a marker for contractility, but not between GHSR1a and B-type natriuretic peptide, a marker for heart failure. We conclude that the subclinical stage of DCM is accompanied by alterations in the myocardial ghrelin-GHSR1a system, suggesting the possibility of a biomarker for DCM.

Characterization of a Monoclonal Antibody Specific for the Growth Hormone Secretagogue Receptor

Ghrelin is an orexigenic peptide hormone that primarily regulates growth hormone secretion, food intake, and energy homeostasis. It has been shown to also play a role in numerous higher brain functions, such as the regulation of inflammation and cell proliferation. Ghrelin is the endogenous ligand of the growth hormone secretagogue receptor (GHSR), a G-protein-coupled receptor highly expressed in brain and detectable in some peripheral tissues. The wide distribution of ghrelin receptor and the number of tissues and cell types known to respond to ghrelin suggest that a number of systems may be affected by treatment with this hormone or its analogues. In this study, we characterized a new GHSR specific monoclonal antibody recognizing specifically the ghrelin receptor. This could be a useful tool for immunoassays aimed at obtaining insights into the physiological and pathological significance of the GHSR/ghrelin system.

Therapeutic Potential of Targeting the Ghrelin Pathway

Ghrelin was discovered in 1999 as the endogenous ligand of the growth-hormone secretagogue receptor 1a (GHSR1a). Since then, ghrelin has been found to exert a plethora of physiological effects that go far beyond its initial characterization as a growth hormone (GH) secretagogue. Among the numerous well-established effects of ghrelin are the stimulation of appetite and lipid accumulation, the modulation of immunity and inflammation, the stimulation of gastric motility, the improvement of cardiac performance, the modulation of stress, anxiety, taste sensation and reward-seeking behavior, as well as the regulation of glucose metabolism and thermogenesis. Due to a variety of beneficial effects on systems’ metabolism, pharmacological targeting of the endogenous ghrelin system is widely considered a valuable approach to treat metabolic complications, such as chronic inflammation, gastroparesis or cancer-associated anorexia and cachexia. The aim of this review is to discuss and highlight the broad pharmacological potential of ghrelin pathway modulation for the treatment of anorexia, cachexia, sarcopenia, cardiopathy, neurodegenerative disorders, renal and pulmonary disease, gastrointestinal (GI) disorders, inflammatory disorders and metabolic syndrome.

Ghrelin and the Cardiovascular System

Ghrelin is a small peptide released primarily from the stomach. It is a potent stimulator of growth hormone secretion from the pituitary gland and is well known for its regulation of metabolism and appetite. There is also a strong relationship between ghrelin and the cardiovascular system. Ghrelin receptors are present throughout the heart and vasculature and have been linked with molecular pathways, including, but not limited to, the regulation of intracellular calcium concentration, inhibition of proapoptotic cascades, and protection against oxidative damage. Ghrelin shows robust cardioprotective effects including enhancing endothelial and vascular function, preventing atherosclerosis, inhibiting sympathetic drive, and decreasing blood pressure. After myocardial infarction, exogenous administration of ghrelin preserves cardiac function, reduces the incidence of fatal arrhythmias, and attenuates apoptosis and ventricular remodeling, leading to improvements in heart failure. It ameliorates cachexia in end-stage congestive heart failure patients and has shown clinical benefit in pulmonary hypertension. Nonetheless, since ghrelin's discovery is relatively recent, there remains a substantial amount of research needed to fully understand its clinical significance in cardiovascular disease.

Cardioprotection Effects of Sevoflurane by Regulating the Pathway of Neuroactive Ligand-Receptor Interaction in Patients Undergoing Coronary Artery Bypass Graft Surgery

This study was designed to identify attractor modules and further reveal the potential biological processes involving in sevoflurane-induced anesthesia in patients treated with coronary artery bypass graft (CABG) surgery. Microarray profile data (ID: E-GEOD-4386) on atrial samples obtained from patients receiving anesthetic gas sevoflurane prior to and following CABG procedure were downloaded from EMBL-EBI database for further analysis. Protein-protein interaction (PPI) networks of baseline and sevoflurane groups were inferred and reweighted according to Spearman correlation coefficient (SCC), followed by systematic modules inference using clique-merging approach. Subsequently, attract method was utilized to explore attractor modules. Finally, pathway enrichment analyses for genes in the attractor modules were implemented to illuminate the biological processes in sevoflurane group. Using clique-merging approach, 27 and 36 modules were obtained from the PPI networks of baseline and sevoflurane-treated samples, respectively. By comparing with the baseline condition, 5 module pairs with the same gene composition were identified. Subsequently, 1 out of 5 modules was identified as an attractor based on attract method. Additionally, pathway analysis indicated that genes in the attractor module were associated with neuroactive ligand-receptor interaction. Accordingly, sevoflurane might exert important functions in cardioprotection in patients following CABG, partially through regulating the pathway of neuroactive ligand-receptor interaction.

Effects of Growth Hormone on Cardiac Remodeling During Resistance Training in Rats

Background

Although the beneficial effects of resistance training (RT) on the cardiovascular system are well established, few studies have investigated the effects of the chronic growth hormone (GH) administration on cardiac remodeling during an RT program.

Objective

To evaluate the effects of GH on the morphological features of cardiac remodeling and Ca2+ transport gene expression in rats submitted to RT.

Methods

Male Wistar rats were divided into 4 groups (n = 7 per group): control (CT), GH, RT and RT with GH (RTGH). The dose of GH was 0.2 IU/kg every other day for 30 days. The RT model used was the vertical jump in water (4 sets of 10 jumps, 3 bouts/wk) for 30 consecutive days. After the experimental period, the following variables were analyzed: final body weight (FBW), left ventricular weight (LVW), LVW/FBW ratio, cardiomyocyte cross-sectional area (CSA), collagen fraction, creatine kinase muscle-brain fraction (CK-MB) and gene expressions of SERCA2a, phospholamban (PLB) and ryanodine (RyR).

Results

There was no significant (p > 0.05) difference among groups for FBW, LVW, LVW/FBW ratio, cardiomyocyte CSA, and SERCA2a, PLB and RyR gene expressions. The RT group showed a significant (p < 0.05) increase in collagen fraction compared to the other groups. Additionally, the trained groups (RT and RTGH) had greater CK-MB levels compared to the untrained groups (CT and GH).

Conclusion

GH may attenuate the negative effects of RT on cardiac remodeling by counteracting the increased collagen synthesis, without affecting the gene expression that regulates cardiac Ca²⁺ transport.

New perspectives of incretin research

Incretins are a group of hormones released into the blood stream by gastrointestinal cells after food stimulation, including glucagon like peptide and glucose-dependent insulinotropic polypeptide (GIP), which can promote insulin secretion and regulate blood sugar. GLP-1 is secreted by intestinal L cells, and fulfills its function through the specific GLP-1 receptor (GLP-1R). GLP-1R is widely distributed in the pancreas and non-pancreatic tissues such as central nervous system, gastrointestinal tract, cardiovascular system, lungs, and kidneys. In recent years, GLP-1 drugs have been used for the treatment of diabetes, but it has attracted more interest because of its beneficiary effects on β cell function, weight reduction, endothelial function, and Alzheimer's disease. This article will review the recent progress in research of GLP-1 with regards to its synthesis and secretion, its effects on taste and Alzheimer's disease, and its relationship with other gastrointestinal hormones, with an aim to illuminate the future clinical use and research of GLP-1.

Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

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.

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.

The gut hormone ghrelin partially reverses energy substrate metabolic alterations in the failing heart

BACKGROUND

The gut-derived hormone ghrelin, especially its acylated form, plays a major role in the regulation of systemic metabolism and exerts also relevant cardioprotective effects; hence, it has been proposed for the treatment of heart failure (HF). We tested the hypothesis that ghrelin can directly modulate cardiac energy substrate metabolism.

METHODS AND RESULTS

We used chronically instrumented dogs, 8 with pacing-induced HF and 6 normal controls. Human des-acyl ghrelin [1.2 nmol/kg per hour] was infused intravenously for 15 minutes, followed by washout (rebaseline) and infusion of acyl ghrelin at the same dose. (3)H-oleate and (14)C-glucose were coinfused and arterial and coronary sinus blood sampled to measure cardiac free fatty acid and glucose oxidation and lactate uptake. As expected, cardiac substrate metabolism was profoundly altered in HF because baseline oxidation levels of free fatty acids and glucose were, respectively, >70% lower and >160% higher compared with control. Neither des-acyl ghrelin nor acyl ghrelin significantly affected function and metabolism in normal hearts. However, in HF, des-acyl and acyl ghrelin enhanced myocardial oxygen consumption by 10.2±3.5% and 9.9±3.7%, respectively (P<0.05), and cardiac mechanical efficiency was not significantly altered. This was associated, respectively, with a 41.3±6.7% and 32.5±10.9% increase in free fatty acid oxidation and a 31.3±9.2% and 41.4±8.9% decrease in glucose oxidation (all P<0.05).

CONCLUSIONS

Acute increases in des-acyl or acyl ghrelin do not interfere with cardiac metabolism in normal dogs, whereas they enhance free fatty acid oxidation and reduce glucose oxidation in HF dogs, thus partially correcting metabolic alterations in HF. This novel mechanism might contribute to the cardioprotective effects of ghrelin in HF.

Characterization of a far-red analog of ghrelin for imaging GHS-R in P19-derived cardiomyocytes

Ghrelin and its receptor, the growth hormone secretagogue receptor (GHS-R), are expressed in the heart, and may function to promote cardiomyocyte survival, differentiation and contractility. Previously, we had generated a truncated analog of ghrelin conjugated to fluorescein isothiocyanate for the purposes of determining GHS-R expression in situ. We now report the generation and characterization of a far-red ghrelin analog, [Dpr(3)(octanoyl), Lys(19)(Cy5)]ghrelin (1-19), and show that it can be used to image changes in GHS-R in developing cardiomyocytes. We also generated the des-acyl analog, des-acyl [Lys(19)(Cy5)]ghrelin (1-19) and characterized its binding to mouse heart sections. Receptor binding affinity of Cy5-ghrelin as measured in HEK293 cells overexpressing GHS-R1a was within an order of magnitude of that of fluorescein-ghrelin and native human ghrelin, while the des-acyl Cy5-ghrelin did not bind GHS-R1a. Live cell imaging in HEK293/GHS-R1a cells showed cell surface labeling that was displaced by excess ghrelin. Interestingly, Cy5-ghrelin, but not the des-acyl analog, showed concentration-dependent binding in mouse heart tissue sections. We then used Cy5-ghrelin to track GHS-R expression in P19-derived cardiomyocytes. Live cell imaging at different time points after DMSO-induced differentiation showed that GHS-R expression preceded that of the differentiation marker aMHC and tracked with the contractility marker SERCA 2a. Our far-red analog of ghrelin adds to the tools we are developing to map GHS-R in developing and diseased cardiac tissues.

The growth hormone secretagogue receptor: its intracellular signaling and regulation

The growth hormone secretagogue receptor (GHSR), also known as the ghrelin receptor, is involved in mediating a wide variety of biological effects of ghrelin, including: stimulation of growth hormone release, increase of food intake and body weight, modulation of glucose and lipid metabolism, regulation of gastrointestinal motility and secretion, protection of neuronal and cardiovascular cells, and regulation of immune function. Dependent on the tissues and cells, activation of GHSR may trigger a diversity of signaling mechanisms and subsequent distinct physiological responses. Distinct regulation of GHSR occurs at levels of transcription, receptor interaction and internalization. Here we review the current understanding on the intracellular signaling pathways of GHSR and its modulation. An overview of the molecular structure of GHSR is presented first, followed by the discussion on its signaling mechanisms. Finally, potential mechanisms regulating GHSR are reviewed.

Novel molecular aspects of ghrelin and leptin in the control of adipobiology and the cardiovascular system

Ghrelin and leptin show opposite effects on energy balance. Ghrelin constitutes a gut hormone that is secreted to the bloodstream in two major forms, acylated and desacyl ghrelin. The isoforms of ghrelin not only promote adiposity by the activation of hypothalamic orexigenic neurons but also directly stimulate the expression of several fat storage-related proteins in adipocytes, including ACC, FAS, LPL and perilipin, thereby stimulating intracytoplasmic lipid accumulation. Moreover, both acylated and desacyl ghrelin reduce TNF-α-induced apoptosis and autophagy in adipocytes, suggesting an anti-inflammatory role of ghrelin in human adipose tissue. On the other hand, leptin is an adipokine with lipolytic effects. In this sense, leptin modulates via PI3K/Akt/mTOR the expression of aquaglyceroporins such as AQP3 and AQP7 that facilitate glycerol efflux from adipocytes in response to the lipolytic stimuli via its translocation from the cytosolic fraction (AQP3) or lipid droplets (AQP7) to the plasma membrane. Ghrelin and leptin also participate in the homeostasis of the cardiovascular system. Ghrelin operates as a cardioprotective factor with increased circulating acylated ghrelin concentrations in patients with left ventricular hypertrophy (LVH) causally related to LV remodeling during the progression to LVH. Additionally, leptin induces vasodilation by inducible NO synthase expression (iNOS) in the vascular wall. In this sense, leptin inhibits the angiotensin II-induced Ca(2+) increase, contraction and proliferation of VSMC through NO-dependent mechanisms. Together, dysregulation of circulating ghrelin isoforms and leptin resistance associated to obesity, type 2 diabetes, or the metabolic syndrome contribute to cardiometabolic derangements observed in these pathologies.

Characterization of pericardial and plasma ghrelin levels in patients with ischemic and non-ischemic heart disease

Ghrelin is an endocrine regulatory peptide with multiple functions including cardioprotective effects. It is produced in various tissues among others in the myocardium. Pericardial fluid has been proven to be a biologically active compartment of the heart that communicates with the myocardial interstitium. Thus, pericardial level of certain agents may reflect their concentration in the myocardium well. In our study we measured acylated (active) and total (acylated and non-acylated) pericardial and plasma ghrelin levels of patients with ischemic and non-ischemic heart disease. Pericardial fluid and plasma samples were obtained from patients with coronary artery disease (ISCH, n=54) or valvular heart disease (VHD, n=41) undergoing cardiac surgery. Acylated pericardial ghrelin concentrations were found to be significantly higher in patients with ischemic heart disease (ISCH vs. VHD, 32±3 vs. 16±2pg/ml, p<0.01), whereas plasma levels of the peptide showed no difference between patient groups. Pericardial-to-plasma ratio, an index abolishing systemic effects on local ghrelin level was also significantly higher in ISCH group for both acylated and total ghrelin. Plasma total ghrelin showed negative correlation to BMI, plasma insulin and insulin resistance index HOMA-A. Pericardial acylated and total ghrelin concentrations were negatively correlated with posterior wall thickness (R=-0.31, p<0.05 and R=-0.35, p<0.01, respectively). Plasma insulin concentration and HOMA-A showed significant negative correlation with pericardial ghrelin levels. In conclusion, increased pericardial active ghrelin content and higher pericardial-to-plasma ghrelin ratio were found in ischemic heart disease as compared to non-ischemic patients suggesting an increased ghrelin production of the chronically ischemic myocardium. According to our results, pericardial ghrelin content is negatively influenced by left ventricular hypertrophy and insulin resistance.

Ghrelin signaling in heart remodeling of adult obese mice

Ghrelin, an endogenous ligand of the growth hormone secretagogue receptor (GHS-R), has been suggested to be associated to obesity, insulin secretion, cardiovascular growth and homeostasis. GHS-R has been found in most of the tissues, and among the hormone action it is included the regulation of heart energy metabolism. Therefore, hypernutrition during early life leads to obesity, induces cardiac hypertrophy, compromises myocardial function, inducing heart failure in adulthood. We examined ghrelin signaling process in cardiac remodeling in these obese adult mice. The cardiomyocytes (cmy) of left ventricle were analyzed by light microscopy and stereology, content and phosphorilation of cardiac proteins: ghrelin receptor (growth hormone secretagogue receptor 1a, GHSR-1a), protein kinase B (AKT and pAKT), phosphatidil inositol 3 kinase (PI3K), AMP-activated protein kinase (AMPK and pAMPK) and actin were achieved by Western blotting. GHSR-1a gene expression was analyzed by Real Time-PCR. We observed hyperglycemia and higher liver and visceral fat weight in obese when compared to control group. Obese mice presented a marked increase in heart weight/tibia length, indicating an enlarged heart size or a remodeling process. Obese mice had increased GHSR-1a content and expression in the heart associated to PI3K content and increased AKT content and phosphorylation. In contrast, AMPK content and phosphorylation in heart was not different between experimental groups. Ghrelin plasma levels in obese group were decreased when compared to control group. Our data suggest that remodeled myocardial in adult obese mice overnourished in early life are associated with higher phosphorylation of GHSR-1a, PI3K and AKT but not with AMPK.

Ghrelin - a pleiotropic hormone secreted from endocrine x/a-like cells of the stomach

The gastric X/A-like endocrine cell receives growing attention due to its peptide products with ghrelin being the best characterized. This peptide hormone was identified a decade ago as a stimulator of food intake and to date remains the only known peripherally produced and centrally acting orexigenic hormone. In addition, subsequent studies identified numerous other functions of this peptide including the stimulation of gastrointestinal motility, the maintenance of energy homeostasis and an impact on reproduction. Moreover, ghrelin is also involved in the response to stress and assumed to play a role in coping functions and exert a modulatory action on immune pathways. Our knowledge on the regulation of ghrelin has markedly advanced during the past years by the identification of the ghrelin acylating enzyme, ghrelin-O-acyltransferase, and by the description of changes in expression, activation, and release under different metabolic as well as physically and psychically challenging conditions. However, our insight on regulatory processes of ghrelin at the cellular and subcellular levels is still very limited and warrants further investigation.