Ghrelin and growth hormone (GH) secretagogue receptor (GHSR) mRNA expression in human pituitary adenomas

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.

Diverse and Complementary Effects of Ghrelin and Obestatin

Ghrelin and obestatin are two “sibling proteins” encoded by the same preproghrelin gene but possess an array of diverse and complex functions. While there are ample literature documenting ghrelin’s functions, the roles of obestatin are less clear and controversial. Ghrelin and obestatin have been perceived to be antagonistic initially; however, recent studies challenge this dogma. While they have opposing effects in some systems, they function synergistically in other systems, with many functions remaining debatable. In this review, we discuss their functional relationship under three “C” categories, namely complex, complementary, and contradictory. Their functions in food intake, weight regulation, hydration, gastrointestinal motility, inflammation, and insulin secretion are complex. Their functions in pancreatic beta cells, cardiovascular, muscle, neuroprotection, cancer, and digestive system are complementary. Their functions in white adipose tissue, thermogenesis, and sleep regulation are contradictory. Overall, this review accumulates the multifaceted functions of ghrelin and obestatin under both physiological and pathological conditions, with the intent of contributing to a better understanding of these two important gut hormones.

Targeting the Ghrelin Receptor as a Novel Therapeutic Option for Epilepsy

Epilepsy is a neurological disease affecting more than 50 million individuals worldwide. Notwithstanding the availability of a broad array of antiseizure drugs (ASDs), 30% of patients suffer from pharmacoresistant epilepsy. This highlights the urgent need for novel therapeutic options, preferably with an emphasis on new targets, since “me too” drugs have been shown to be of no avail. One of the appealing novel targets for ASDs is the ghrelin receptor (ghrelin-R). In epilepsy patients, alterations in the plasma levels of its endogenous ligand, ghrelin, have been described, and various ghrelin-R ligands are anticonvulsant in preclinical seizure and epilepsy models. Up until now, the exact mechanism-of-action of ghrelin-R-mediated anticonvulsant effects has remained poorly understood and is further complicated by multiple downstream signaling pathways and the heteromerization properties of the receptor. This review compiles current knowledge, and discusses the potential mechanisms-of-action of the anticonvulsant effects mediated by the ghrelin-R.

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.

Rater Reliability of the Hardy Classification for Pituitary Adenomas in the Magnetic Resonance Imaging Era

Objectives  The Hardy classification is used to classify pituitary tumors for clinical and research purposes. The scale was developed using lateral skull radiographs and encephalograms, and its reliability has not been evaluated in the magnetic resonance imaging (MRI) era. Design  Fifty preoperative MRI scans of biopsy-proven pituitary adenomas using the sellar invasion and suprasellar extension components of the Hardy scale were reviewed. Setting  This study was a cohort study set at a single institution. Participants  There were six independent raters. Main Outcome Measures  The main outcome measures of this study were interrater reliability, intrarater reliability, and percent agreement. Results  Overall interrater reliability of both Hardy subscales on MRI was strong. However, reliability of the intermediate scores was weak, and percent agreement among raters was poor (12-16%) using the full scales. Dichotomizing the scale into clinically useful groups maintained strong interrater reliability for the sellar invasion scale and increased the percent agreement for both scales. Conclusion  This study raises important questions about the reliability of the original Hardy classification. Editing the measure to a clinically relevant dichotomous scale simplifies the rating process and may be useful for preoperative tumor characterization in the MRI era. Future research studies should use the dichotomized Hardy scale (sellar invasion Grades 0-III versus Grade IV, suprasellar extension Types 0-C versus Type D).

Combined Administration of Ghrelin and Corticotropin-Releasing Hormone in the Diagnosis of Cushing's Disease

BACKGROUND/AIMS

Exaggerated adrenocorticotropic hormone (ACTH) and cortisol responses to ghrelin in Cushing's disease (CD) have previously been reported, similarly to responses to corticotropin-releasing hormone (CRH). We assessed the ability of ghrelin to enhance ACTH and cortisol responses when added to CRH stimulation in CD patients.

METHODS

In 21 CD patients (18 females, 3 males; age 49.8 ± 10.2 years; BMI 29.8 ± 0.8) and 8 healthy subjects (7 females, 1 male; age 40.6 ± 5.3 years; BMI 29.9 ± 1.2), we administered (1) ghrelin 100 µg i.v. bolus, (2) CRH 100 µg i.v. bolus, and (3) ghrelin + CRH combination. ACTH and cortisol were analyzed by commercially available kits from samples taken at 0, 15, 30, 45, 60, 90 and 120 min. ACTH and cortisol responses were calculated as peak and area under the curve (AUC0-120 min).

RESULTS

ACTH and cortisol at baseline and stimulated with ghrelin and/or CRH (peak and AUC0-120 min) were significantly higher in CD patients compared to controls (p < 0.01). ACTH and cortisol responses to ghrelin or CRH were similar in CD patients. Combined ghrelin + CRH administration in CD patients produced the highest ACTH response (peak and AUC0-120 min) compared to ghrelin or CRH alone (p < 0.01). Cortisol responses after ghrelin + CRH were uncoupled with ACTH responses and similar to the response to ghrelin or CRH alone in both groups. ACTH and cortisol responses, during all three tests, were similar in CD patients with micro- or macroadenomas.

CONCLUSION

Ghrelin administration causes exaggerated ACTH and cortisol responses in CD patients compared to healthy controls. In combination with CRH, it additionally enhances ACTH secretion without further additive effect on cortisol output.

Microarray technology reveals potentially novel genes and pathways involved in non-functioning pituitary adenomas

Microarray data of non-functioning pituitary adenomas (NFPAs) were analyzed to disclose novel genes and pathways involved in NFPA tumorigenesis. Raw microarray data were downloaded from Gene Expression Omnibus. Data pre-treatment and differential analysis were conducted using packages in R. Functional and pathway enrichment analyses were performed using package GOs-tats. A protein-protein interaction (PPI) network was constructed using server STRING and Cytoscape. Known genes involved in pituitary adenomas (PAs), were obtained from the Comparative Toxicogenomics Database. A total of 604 differentially expressed genes (DEGs) were identifed between NFPAs and controls, including 177 up- and 427 down-regulated genes. Jak-STAT and p53 signaling pathways were significantly enriched by DEGs. The PPI network of DEGs was constructed, containing 99 up- and 288 down-regulated known disease genes (e.g. EGFR and ESR1) as well as 16 up- and 17 down-regulated potential novel NFPAs-related genes (e.g. COL4A5, LHX3, MSN, and GHSR). Genes like COL4A5, LHX3, MSN, and GHSR and pathways such as p53 signaling and Jak-STAT signaling, might participate in NFPA development. Although further validations are required, these findings might provide guidance for future basic and therapy researches.

Ghrelin Expression and Actions: A Novel Peptide for an Old Cell Type of the Diffuse Endocrine System

Ghrelin is a gastric peptide involved in food intake control and growth hormone release. Its cell localization has been defined in distinct ghrelin cells of the gastric mucosa in humans and other mammals. Ghrelin production was also described in a number of other sites of the diffuse endocrine system, including the pituitary, thyroid, lung, pancreas, adrenal gland, and intestine. In addition, ghrelin cells were identified early during fetal life and in the placenta and gonads. Finally, endocrine growths and tumors of the diffuse endocrine system may present ghrelin-producing cells, and in a few cases high levels of circulating ghrelin were reported. Besides its well-defined orexigenic role, ghrelin is likely to exert a local paracrine role similar to other brain-gut axis hormones. This review aims to summarize recent data on ghrelin cell distribution in the diffuse endocrine system and discuss local and general ghrelin function during development, adulthood, and endocrine tumor development.

Implications of ghrelin and hexarelin in diabetes and diabetes-associated heart diseases

Ghrelin and its synthetic analog hexarelin are specific ligands of growth hormone secretagogue (GHS) receptor. GHS have strong growth hormone-releasing effect and other neuroendocrine activities such as stimulatory effects on prolactin and adrenocorticotropic hormone secretion. Recently, several studies have reported other beneficial functions of GHS that are independent of GH. Ghrelin and hexarelin, for examples, have been shown to exert GH-independent cardiovascular activity. Hexarelin has been reported to regulate peroxisome proliferator-activated receptor gamma (PPAR-γ) in macrophages and adipocytes. PPAR-γ is an important regulator of adipogenesis, lipid metabolism, and insulin sensitization. Ghrelin also shows protective effects on beta cells against lipotoxicity through activation of phosphatidylinositol-3 kinase/protein kinase B, c-Jun N-terminal kinase (JNK) inhibition, and nuclear exclusion of forkhead box protein O1. Acylated ghrelin (AG) and unacylated ghrelin (UAG) administration reduces glucose levels and increases insulin-producing beta cell number, and insulin secretion in pancreatectomized rats and in newborn rats treated with streptozotocin, suggesting a possible role of GHS in pancreatic regeneration. Therefore, the discovery of GHS has opened many new perspectives in endocrine, metabolic, and cardiovascular research areas, suggesting the possible therapeutic application in diabetes and diabetic complications especially diabetic cardiomyopathy. Here, we review the physiological roles of ghrelin and hexarelin in the protection and regeneration of beta cells and their roles in the regulation of insulin release, glucose, and fat metabolism and present their potential therapeutic effects in the treatment of diabetes and diabetic-associated heart diseases.

Correlation of ghrelin and growth hormone secretagogue receptor expression with clinical features in human pituitary adenomas

Ghrelin, as a brain-gut peptide, has growth hormone (GH)-releasing and appetite-inducing activities and a widespread tissue distribution. Furthermore, ghrelin is an endogenous ligand of the GH secretagogue receptor (GHSR), and both ghrelin and GHSR are expressed in the pituitary; however, the data regarding the expression of ghrelin and GHSR in pituitary adenomas are divergent and conflicting. In the present study, therefore, the expression of ghrelin and GHSR was examined in the full spectrum of human pituitary adenoma subtypes (n=34) and in normal pituitary tissue (n=3). The mRNA and protein expression levels were quantified using a competitive reverse transcription-polymerase chain reaction and western blotting and the correlation of the results with the clinical parameters was assessed. mRNA and protein expression of ghrelin and GHSR was detected in all samples with the highest mean level in GH adenomas, a moderate level in clinically non-functioning adenomas and the lowest level in adrenocorticotropin adenomas. A significant correlation between the ghrelin and GHSR mRNA expression levels was observed in the GH adenomas (n=12) (r=0.8435, P=0.0006). The ghrelin mRNA expression level in the GH adenomas correlated positively with the basic serum GH level (n=12) (r=0.6488, P=0.0225). Furthermore, the mean level of ghrelin mRNA expression was significantly higher in invasive adenomas than in noninvasive adenomas (P<0.01). Collectively, the results of the study provided evidence that ghrelin and GHSR are expressed in the various subtypes of pituitary adenoma, with specific overexpression in GH adenomas. The study suggests that the binding of ghrelin to GHSR promotes the secretion of GH and plays an important role in the development of GH adenomas via autocrine and/or paracrine effects.

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

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

Ghrelin gene products, receptors, and GOAT enzyme: biological and pathophysiological insight

Ghrelin is a 28-amino acid acylated hormone, highly expressed in the stomach, which binds to its cognate receptor (GHSR1a) to regulate a plethora of relevant biological processes, including food intake, energy balance, hormonal secretions, learning, inflammation, etc. However, ghrelin is, in fact, the most notorious component of a complex, intricate regulatory system comprised of a growing number of alternative peptides (e.g. obestatin, unacylated ghrelin, and In1-ghrelin, etc.), known (GHSRs) and, necessarily unknown receptors, as well as modifying enzymes (e.g. ghrelin-O-acyl-transferase), which interact among them as well as with other regulatory systems in order to tightly modulate key (patho)-physiological processes. This multiplicity of functions and versatility of the ghrelin system arise from a dual, genetic and functional, complexity. Importantly, a growing body of evidence suggests that dysregulation in some of the components of the ghrelin system can lead to or influence the development and/or progression of highly concerning pathologies such as endocrine-related tumors, inflammatory/cardiovascular diseases, and neurodegeneration, wherein these altered components could be used as diagnostic, prognostic, or therapeutic targets. In this context, the aim of this review is to integrate and comprehensively analyze the multiple components and functions of the ghrelin system described to date in order to define and understand its biological and (patho)-physiological significance.

Ghrelin: much more than a hunger hormone

PURPOSE OF REVIEW

Ghrelin is a multifaceted gut hormone that activates its receptor, growth hormone secretagogue receptor (GHS-R). Ghrelin's hallmark functions are its stimulatory effects on growth hormone release, food intake and fat deposition. Ghrelin is famously known as the 'hunger hormone'. However, ample recent literature indicates that the functions of ghrelin go well beyond its role as an orexigenic signal. Here, we have reviewed some of the most recent findings on ghrelin and its signalling in animals and humans.

RECENT FINDINGS

Ghrelin regulates glucose homeostasis by inhibiting insulin secretion and regulating gluconeogenesis/glycogenolysis. Ghrelin signalling decreases thermogenesis to regulate energy expenditure. Ghrelin improves the survival prognosis of myocardial infarction by reducing sympathetic nerve activity. Ghrelin prevents muscle atrophy by inducing muscle differentiation and fusion. Ghrelin regulates bone formation and metabolism by modulating proliferation and differentiation of osteoblasts.

SUMMARY

In addition to ghrelin's effects on appetite and adiposity, ghrelin signalling also plays crucial roles in glucose and energy homeostasis, cardioprotection, muscle atrophy and bone metabolism. These multifaceted roles of ghrelin make ghrelin and GHS-R highly attractive targets for drug development. Ghrelin mimetics may be used to treat heart diseases, muscular dystrophy/sarcopenia and osteoporosis; GHS-R antagonists may be used to treat obesity and insulin resistance.

The ghrelin axis--does it have an appetite for cancer progression?

Ghrelin, the endogenous ligand for the GH secretagogue receptor (GHSR), is a peptide hormone with diverse physiological roles. Ghrelin regulates GH release, appetite and feeding, gut motility, and energy balance and also has roles in the cardiovascular, immune, and reproductive systems. Ghrelin and the GHSR are expressed in a wide range of normal and tumor tissues, and a fluorescein-labeled, truncated form of ghrelin is showing promise as a biomarker for prostate cancer. Plasma ghrelin levels are generally inversely related to body mass index and are unlikely to be useful as a biomarker for cancer, but may be useful as a marker for cancer cachexia. Some single nucleotide polymorphisms in the ghrelin and GHSR genes have shown associations with cancer risk; however, larger studies are required. Ghrelin regulates processes associated with cancer, including cell proliferation, apoptosis, cell migration, cell invasion, inflammation, and angiogenesis; however, the role of ghrelin in cancer is currently unclear. Ghrelin has predominantly antiinflammatory effects and may play a role in protecting against cancer-related inflammation. Ghrelin and its analogs show promise as treatments for cancer-related cachexia. Further studies using in vivo models are required to determine whether ghrelin has a role in cancer progression.

A role of ghrelin in cancerogenesis

Ghrelin is a 28 amino-acid multi-functional peptide hormone, which was identified as a natural ligand of the growth hormone secretagogue receptor (GHS-R). Pituitary growth hormone-releasing activity in both animals and humans has been well documented. It has various biological functions, including regulation of appetite and body weight, control of energy homeostasis, modulation of cardiovascular and gastrointestinal system and anti-inflammatory effect. However, both ghrelin and its receptor (GHS-R) are widely distributed in various tumors, which strongly implies their role in neoplastic cell growth trough autocrine/paracrine mechanism. Multiple studies have demonstrated the role of ghrelin in cancer cells proliferation, differentiation, invasiveness and apoptosis inhibition. The ghrelin axis is more complex than it was originally thought and consist of several compounds that might interact with each other and affect ghrelin activities. Here, we provide an overview of the ghrelin and its receptor role in tumor progression.

Ghrelin expression in esophageal adenocarcinoma and Barrett's esophagus

AIM: To investigate the expression of Ghrelin in the esophageal mucosa of patients with esophageal adenocarcinoma and those with Barrett's esophagus (BE).

METHODS: Thirty patients with esophageal adenocarcinoma, 35 patients with BE, and 35 normal controls were enrolled in the study. The expression of Ghrelin in specimens taken from the above subjects was detected by immunohistochemistry.

RESULTS: The expression of Ghrelin in esophageal adenocarcinoma was lower than that in normal controls and patients with BE. The expression of Ghrelin in the BE group was higher than that in the control group (1.34 ± 0.51 vs 4.86 ± 0.82 vs 3.54 ± 0.79, F = 27.21, P < 0.05). In the esophageal adenocarcinoma group, the expression of Ghrelin in moderately and well differentiated specimens were higher than that in poorly differentiated specimens (Z = 4.60, P < 0.05).

CONCLUSION: The expression of Ghrelin in the esophageal adenocarcinoma is different from that in BE. The level of Ghrelin changes during the evolution of esophageal cancer. Disruption of the esophageal Ghrelin-producing mechanism may occur during esophageal carcinogenesis. There is an association between the degree of esophageal carcinoma differentiation and Ghrelin production.

Ghrelin and cancer

Ghrelin is a peptide hormone that was originally isolated from the stomach as the endogenous ligand for the growth hormone secretagogue receptor (GHSR). Ghrelin has many functions, including the regulation of appetite and gut motility, growth hormone release from the anterior pituitary and roles in the cardiovascular and immune systems. Ghrelin and its receptor are expressed in a number of cancers and cancer cell lines and may play a role in processes associated with cancer progression, including cell proliferation, apoptosis, and cell invasion and migration.

Ghrelin and its potential in the treatment of eating/wasting disorders and cachexia

The gastrointestinal "hunger" hormone ghrelin is the only known circulating peripheral molecule with the ability to decrease body fat utilization and to increase body weight gain. Accordingly, due to ghrelin's effects to promote food intake while decreasing energy expenditure ghrelin may offer potential as a drug for treatment of eating/wasting disorders and cachexia. Therapeutic potential of ghrelin and ghrelin analogues to promote food intake and body weight gain was recently indicated in several clinical studies. The recent discovery of the ghrelin O-acyltransferase as the key enzyme responsible for ghrelin acylation has further deepened our understanding of ghrelin activation, thereby paving the way for more efficient targeting of the ghrelin pathway. Here, we summarize the current knowledge pertaining to the potential of the endogenous ghrelin system as a drug target for the treatment of eating/wasting disorders and cachexia.

Ghrelin's role as a major regulator of appetite and its other functions in neuroendocrinology

Ghrelin is a circulating growth-hormone-releasing and appetite-inducing brain-gut peptide. It is a known natural ligand of the growth hormone secretagogue receptor (GHS-R). Ghrelin is acylated on its serine 3 residue by ghrelin O-acyltransferase (GOAT). The acylation is essential for its orexigenic and adipogenic effects. Ghrelin exerts its central orexigenic effect through activation of various hypothalamic and brain stem neurons. Several new intracellular targets/mediators of the appetite-inducing effect of ghrelin in the hypothalamus have recently been identified, including the AMP-activated protein kinase, its upstream kinase calmodulin kinase kinase 2, components of the fatty acid pathway and the uncoupling protein 2. The ghrelin/GOAT/GHS-R system is now recognised as a potential target for the development of anti-obesity treatment. Ghrelin regulates the function of the anterior pituitary through stimulation of secretion not only of growth hormone, but also of adrenocorticotrophin and prolactin. The implication of ghrelin and its receptor in the pathogenesis of the neuroendocrine tumors will also be discussed in this review.

ACTH and cortisol responses to ghrelin and desmopressin in patients with Cushing's disease and adrenal enlargement

BACKGROUND

Overexpression of ghrelin and vasopressin (V3) receptors demonstrated on corticotrophe adenomas accounts for exaggerated ACTH and cortisol responses to ghrelin and desmopressin (DDAVP) in patients with Cushing's disease (CD).

AIM

In this study we have compared ACTH and cortisol responsiveness to DDAVP and ghrelin in CD patients with and without adrenal enlargement.

SUBJECTS AND METHODS

Ghrelin and DDAVP tests were performed in 15 patients with CD (7 with and 8 without signs of adrenal enlargement) with CRH test in 8 patients. In 7 age and sex-matched healthy subjects, ghrelin test was performed. Plasma ACTH and serum cortisol concentrations were measured after ghrelin, DDAVP and CRH. Growth hormone was measured after stimulation with ghrelin.

RESULTS

Significantly higher baseline and peak ACTH and cortisol concentrations after ghrelin were observed in all patients with CD compared to healthy control subjects. Patients with CD and adrenal enlargement had significantly lower baseline and peak ACTH concentrations after stimulation with ghrelin compared to CD patients without adrenal enlargement, while cortisol levels at baseline and after ghrelin administration were similar. Three out of seven patients with CD and adrenal enlargement did not respond to DDAVP while they responded well to CRH and ghrelin.

CONCLUSION

Patients with CD and adrenal enlargement pose special diagnostic problems. They may have lower baseline ACTH levels and may not respond to DDAVP while they respond to ghrelin and CRH. Despite increased endogenous cortisol levels in CD, cortisol responses to ghrelin and CRH are preserved in patients with CD and adrenal enlargement.

Ghrelin: a potential therapeutic target for cancer

Ghrelin is a recently identified 28-amino-acid peptide, capable of stimulating pituitary growth hormone release in humans and other mammals. It is mainly secreted from the gastric mucosa, but it is also widely expressed in a variety of tissues, in both normal and malignant conditions. Ghrelin has a multiplicity of physiological functions in gastrointestinal, cardiovascular, pulmonary and immune system, and also exerts a variety of roles, from increasing food intake (orexigenic effect) to affecting cell proliferation. The actions of ghrelin are mediated by the ghrelin receptor, also known as the growth hormone secretagogue receptor (GHS-R). The purpose of this review is to provide an overview of the expression and putative role of ghrelin and its receptor in cancer. Ghrelin and its receptor are detected in tumor tissues, and evidence is emerging that ghrelin plays an autocrine/paracrine role in cancer and could serve as a diagnostic or prognostic tool or as therapeutic target.

Ghrelin gene products and the regulation of food intake and gut motility

A breakthrough using "reverse pharmacology" identified and characterized acyl ghrelin from the stomach as the endogenous cognate ligand for the growth hormone (GH) secretagogue receptor (GHS-R) 1a. The unique post-translational modification of O-n-octanoylation at serine 3 is the first in peptide discovery history and is essential for GH-releasing ability. Des-acyl ghrelin, lacking O-n-octanoylation at serine 3, is also produced in the stomach and remains the major molecular form secreted into the circulation. The third ghrelin gene product, obestatin, a novel 23-amino acid peptide identified from rat stomach, was found by comparative genomic analysis. Three ghrelin gene products actively participate in modulating appetite, adipogenesis, gut motility, glucose metabolism, cell proliferation, immune, sleep, memory, anxiety, cognition, and stress. Knockdown or knockout of acyl ghrelin and/or GHS-R1a, and overexpression of des-acyl ghrelin show benefits in the therapy of obesity and metabolic syndrome. By contrast, agonism of acyl ghrelin and/or GHS-R1a could combat human anorexia-cachexia, including anorexia nervosa, chronic heart failure, chronic obstructive pulmonary disease, liver cirrhosis, chronic kidney disease, burn, and postsurgery recovery, as well as restore gut dysmotility, such as diabetic or neurogenic gastroparesis, and postoperative ileus. The ghrelin acyl-modifying enzyme, ghrelin O-Acyltransferase (GOAT), which attaches octanoate to serine-3 of ghrelin, has been identified and characterized also from the stomach. To date, ghrelin is the only protein to be octanylated, and inhibition of GOAT may have effects only on the stomach and is unlikely to affect the synthesis of other proteins. GOAT may provide a critical molecular target in developing novel therapeutics for obesity and type 2 diabetes.

Ghrelin's role on gastrointestinal tract cancer

Ghrelin is a recently identified 28-amino-acid peptide, with pituitary growth hormone releasing activities in humans and other mammals. In mammals, ghrelin plays a variety of roles, including influence on food intake, gastric motility, and acid secretion of the gastrointestinal tract. It is mainly secreted from the stomach mucosa, but it is also expressed widely in other tissues - in normal and malignant conditions - and, therefore, ghrelin may exert such variable endocrine and paracrine effects, as autocrine and/or paracrine function in cancer. Ghrelin's actions are mediated via its receptor, known as growth hormone secretagogue receptor (GHS-R), type 1a and 1b. Several endocrine and non-endocrine cancers, such as gastro-entero-pancreatic carcinoids, colorectal neoplasms, pituitary adenomas, pulmonary and thyroid tumours, as well as lung, breast, and pancreatic carcinomas express ghrelin at both mRNA and protein levels. In the current review, we summarise the available so far data with regard to: (a) the structure of the ghrelin molecule and its receptor; (b) its tissue contribution in physiologic and neoplasmatic conditions; and (c) ghrelin's possible role in carcinogenesis; specifically, in the area of gastrointestinal tract cancer. The aim of the present study is to determine whether or not ghrelin promotes the proliferation rate of the gastrointestinal tract (GIT) tumours.

The neuromedin U-growth hormone secretagogue receptor 1b/neurotensin receptor 1 oncogenic signaling pathway as a therapeutic target for lung cancer

Using a genome-wide cDNA microarray to search for genes that were specifically up-regulated in non-small cell lung cancers (NSCLC), we identified an abundant expression of neuromedin U (NMU) in the great majority of lung cancers. Immunohistochemical analysis showed a significant association of NMU expression with poorer prognosis of patients with NSCLC. Treatment of NSCLC cells with short interfering RNA against NMU suppressed its expression and inhibited the growth of the cells; on the other hand, the induction of exogenous expression of NMU conferred growth-promoting activity and enhanced cell mobility in vitro. We found that two G protein-coupled receptors, growth hormone secretagogue receptor 1b and neurotensin receptor 1, were also overexpressed in NSCLC cells, and that a heterodimer complex of these receptors functioned as an NMU receptor. The NMU-receptor interaction subsequently induced the generation of a second messenger, cyclic AMP, to activate its downstream genes including transcription factors and cell cycle regulators. Treatment of NSCLC cells with short interfering RNAs for growth hormone secretagogue receptor or neurotensin receptor 1 suppressed the expression of those genes and the growth of NSCLC cells. These data strongly implied that targeting the NMU signaling pathway would be a promising therapeutic strategy for the treatment of lung cancers.

Ghrelin in neuroendocrine organs and tumours

Ghrelin is a 28 amino-acid hormone with multiple functions. It is predominantly produced by the stomach but has also been detected in other organs, including the small intestine, pancreas, hypothalamus and pituitary, as well as in the immune system and almost every other normal human tissue examined. It is also present in neuroendocrine tumours, pituitary adenomas, endocrine tumours of the pancreas, breast tumours, and thyroid and medullary thyroid carcinomas. Ghrelin is a brain-gut peptide with growth hormone-releasing and appetite-inducing activities, and is the endogenous ligand of the G protein-coupled growth hormone secretagogue receptor (GHS-R). In this review we comprehensively summarize the available data regarding (a) the expression of ghrelin and the GHS-R in normal endocrine tissues and in pituitary adenomas and neuroendocrine tumours, (b) the levels of circulating ghrelin in patients with pituitary adenomas and neuroendocrine tumours and (c) the effects of ghrelin administration in these patients on the levels of other hormones and on the rate of proliferation of the tumour. It is clear that ghrelin has many more functions and is involved in many more processes than was initially postulated, and its endocrine, paracrine and autocrine effects play a role in its physiological and pathophysiological functions.

Ghrelin and cortistatin in lung cancer: expression of peptides and related receptors in human primary tumors and in vitro effect on the H345 small cell carcinoma cell line

Ghrelin, a natural GH secretagogue (GHS) acylated peptide, and cortistatin (CST), a natural SRIF-like peptide, interfere with neoplastic growth in different cancers. We tested forty-one lung carcinomas and the H345 small cell lung carcinoma (SCLC) cell line by RT-PCR to investigate the presence of ghrelin and CST and related receptors, including type 1a GHS receptor (GHS-R1a), all SRIF-receptor subtypes (sst 1-5) and MRGX2. Moreover, the presence of ghrelin and CST peptides was studied in both tumors and H345 cells. Ghrelin and CST mRNA were present in the majority of tested tumors, but ghrelin and CST proteins were revealed only in tumors with a neuroendocrine phenotype. All the receptors mRNA had a heterogeneous expression without correlation between ghrelin (or CST) and their receptor distribution. All the transcripts, but not GHS-R1a, were expressed in H345 cells. However, ghrelin and desacyl ghrelin induced in vitro a dose-dependent inhibition on the H345 cell proliferation and increased apoptosis. Conversely, neither CST nor SRIF affected H345 cell growth, despite the presence of their specific receptors. The anti-proliferative and the pro-apoptotic effects of ghrelin were consistent with binding experiments on H345 cell, where either acylated or des-acylated ghrelin recognized a common binding site. In conclusion, the present study indicates that: a) ghrelin and CST mRNAs are expressed in lung cancers, although some neuroendocrine tumors contain detectable amounts of the peptides; b) GHSR-1a mRNA is present exclusively in neuroendocrine tumors, whereas MRGX2 mRNA (but not peptide) is expressed in all histological types; c) both ghrelin forms inhibit H345 cell proliferation, both directly and enhancing apoptosis, despite the absence of GHS-R1a, whereas CST and its receptors do not interfere with cell growth.

Ghrelin and the growth hormone secretagogue receptor constitute a novel autocrine pathway in astrocytoma motility

Originally thought of as a stomach-derived endocrine peptide acting via its receptors in the central nervous system to stimulate food intake and growth hormone expression, ghrelin and its receptor (growth hormone secretagogue receptor (GHS-R)) are widely expressed in a number of organ systems, including cancer cells. However, the direct functional role of ghrelin and its receptor in tumors of central nervous system origin remains to be defined. Here, we demonstrate that the human astrocytoma cell lines U-118, U-87, CCF-STTG1, and SW1088 express 6-, 11-, 15-, and 29-fold higher levels of GHS-R compared with primary normal human astrocytes. The ligation of GHS-R by ghrelin on these cells resulted in an increase in intracellular calcium mobilization, protein kinase C activation, actin polymerization, matrix metalloproteinase-2 activity, and astrocytoma motility. In addition, ghrelin led to actin polymerization and membrane ruffling on cells, with the specific co-localization of the small GTPase Rac1 with GHS-R on the leading edge of the astrocytoma cells and imparting the tumor cells with a motile phenotype. Disruption of the endogenous ghrelin/GHS-R pathway by RNA interference resulted in diminished motility, matrix metalloproteinase activity, and Rac expression, whereas tumor cells stably overexpressing GHS-R exhibited increased cell motility. The relevance of ghrelin and GHS-R expression was verified in clinically relevant tissues from 20 patients with oligodendrogliomas and grade II-IV astrocytomas. Analysis of a central nervous system tumor tissue microarray revealed that strong GHS-R and ghrelin expression was significantly more common in high grade tumors compared with low grade ones. Together, these findings suggest a novel role for the ghrelin/GHS-R axis in astrocytoma cell migration and invasiveness of cancers of central nervous system origin.

Cell biology of the ghrelin receptor

Ghrelin, a gastric peptide involved in growth hormone release and energy homeostasis, is the endogenous ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a), a G-protein coupled receptor mainly expressed in the pituitary and hypothalamus. This receptor mediates the main ghrelin-stimulated endocrine actions and some of the nonendocrine actions. However, a number of nonendocrine actions associated with ghrelin appear to be mediated by various GHS-R1a-related receptor subtypes, which are widely distributed in the central and peripheral tissues. This review summarises data concerning the localisation, regulation and function of GHS-R1a, as well as related receptors.

Loss of growth hormone secretagogue receptor 1a and overexpression of type 1b receptor transcripts in human adrenocortical tumors

OBJECTIVE AND METHODS

Quantitative analysis of mRNA expression of ghrelin and its receptors GHS-R1a and -R1b in a large series of normal and neoplastic human adrenocortical tissues. Evaluation of the effects of ghrelin on GHS-R expression and proliferation of human adrenocortical carcinoma (ACC) cell lines.

RESULTS

Ghrelin and GHS-R transcripts are expressed in normal adrenal cortex, with GHS-R1b mRNA levels being 5- to 10-fold higher than GHS-R1amRNA. A significant increase in ghrelin expression was observed in adrenocortical adenomas, but not in carcinomas. GHS-R1a was undetectable in about 60% of both benign and malignant tumor samples, except for cortisol-producing adenomas, which showed increased receptor expression. At variance, GHS-R1b was overexpressed in both benign and malignant adrenocortical tumors. In vitro studies in human ACC cell lines demonstrated that GHS-R1a is downregulated and GHS-R1bmRNA expression is upregulated by ghrelin, while inhibiting cell proliferation.

CONCLUSION

Downregulation ofGHS-R1a in adrenal tumors and the presence of high levels of GHS-R1b transcripts in adrenocortical tissue suggest a role for these receptors in adrenal function and growth. In this regard, ghrelin inhibits cell proliferation and modulates GHS-R expression in ACC cells in vitro.

Characterization and regulation of the rat and human ghrelin promoters

Ghrelin is a recently discovered stomach hormone and endogenous ligand for the GH secretagogue receptor. The aim of these studies is to elucidate molecular mechanisms underlying regulation of the ghrelin gene. Distal and proximal transcription initiation sites are present. A short transcript, a product of the proximal site, showed a more widespread distribution. Two sets of 5'-upstream segments of the rat and human ghrelin genes were cloned and sequenced. Rat promoter segments upstream of the distal site showed highest activity in kidney (COS-7) and stomach (AGS) cells, whereas human promoter segments upstream of the proximal site showed highest activity in AGS and pituitary (GH3) cells in transient transfection assays. For the human, the core promoter spanned -667 to -468 bp, including the noncoding exon 1 and a short 5' sequence of intron 1. For the rat, the core promoter spanned -581 to -469 bp, and inclusion of exon 1 and a short 5'-sequence of intron 1 reduced activity by 67%. Mutation of initiator-like elements in the rat lowered activity by 20-50%, whereas in the human, all activity was abolished. Overexpression of upstream stimulatory factors increased ghrelin core promoter activity. Fasting increases stomach ghrelin expression, glucagon-a fasting-induced hormone, increased ghrelin expression in vivo in rats, and promoter activity by approximately 25-50%. Together, these findings indicate that structural differences between the rat and human ghrelin core promoters may account in part for the differences in their transcriptional regulation. Nonetheless, upstream stimulatory factor and glucagon exert similar effects on regulation of rat and human ghrelin promoters.

Expression of hypoxia-inducible factor 1alpha and vascular endothelial growth factor in pituitary adenomas

Hypoxia-inducible factor (HIF)-1alpha is a transcription factor in hypoxia adaptation mechanisms. In malignant tumors, HIF-1alpha upregulates vascular endothelial growth factor (VEGF) expression to induce tumor angiogenesis. Although VEGF and HIF-1alpha are expressed in pituitary adenomas, the relationships of these factors remain unclear. Therefore, we examined the expression of HIF-1alpha and VEGF using real-time RT-PCR and immunohistochemistry to clarify the relationship of these factors in pituitary adenomas. HIF-1alpha mRNA and VEGF mRNA levels in pituitary adenoma tissues from 25 operated patients were quantified using real-time RT-PCR. Some tissues were also studied by double fluorescent immunohistochemical methods. HIF-1alpha mRNA and protein were expressed in all pituitary adenomas examined. Their expression tended to be higher in GH-producing and lower in ACTH-producing tumors. VEGF mRNA and protein were also expressed in all pituitary adenomas. There was no significant correlation in the expression levels of HIF-1alpha and VEGF mRNA. The mutual expression of HIF-1alpha and VEGF was of no significance; in only a few cells were HIF-1alpha and VEGF co-localized. Our results suggest that in pituitary adenomas VEGF expression may not depend strongly on HIF-1alpha expression.

Preoperative identification of clearly separated double pituitary adenomas

OBJECTIVE

Double pituitary adenomas are extremely rare. They can be divided into contiguous and clearly separated types. Most contiguous tumours are surgically removed as one tumour and the co-existence of different adenoma types can be confirmed by histological methods. In contrast, detailed preoperative neuroimaging studies can suggest the co-existence of separated multiple adenomas. In patients with multiple adenomas, surgical failure may result when one adenoma is missed during surgery. Among 600 surgical cases we encountered four patients with clearly separated double pituitary adenomas; all were highly suspect on preoperative MRI studies.

PATIENTS AND RESULTS

All four patients manifested acromegalic symptoms; one patient also exhibited hyperprolactinemia and two had familial pituitary adenomas unrelated to multiple endocrine neoplasia type I (MEN-1). All underwent transsphenoidal surgery and histology confirmed the diagnosis of GH-producing plus gonadotroph adenoma in two cases and of two GH-producing adenomas each in the other two patients.

CONCLUSION

Although the pathogenesis of our double adenomas remains unknown, genetic abnormalities may be involved because two patients had familial pituitary adenomas unrelated to MEN-1. When preoperative MRI is suggestive of double adenomas, careful surgical exploration is necessary to avoid missing the other adenoma because the risk of surgical failure is high, especially in patients with functioning adenomas.

Ghrelin: integrative neuroendocrine peptide in health and disease

OBJECTIVE

Ghrelin is a novel gastric hormone recognized in 1999 as a mediator of growth hormone release. Since growth hormone is anabolic, an important function of ghrelin may be to coordinate energy needs with the growth process. Newly discovered biologic roles of ghrelin imply that it may have other important physiological functions as well. This is a review of recent clinically relevant, yet less well-known, physiologic actions of ghrelin.

SUMMARY BACKGROUND DATA

Ghrelin has profound orexigenic, adipogenic, and somatotrophic properties, increasing food intake and body weight. Secreted predominantly from the stomach, ghrelin is the natural ligand for the growth hormone secretagogue receptor in the pituitary gland, thus fulfilling criteria of a brain-gut peptide. The brain-gut axis is the effector of anabolism by regulating growth, feeding, and metabolism via vagal afferents mediating ghrelin signaling. However, the wide tissue distribution of ghrelin suggests that it may have other functions as well.

METHODS

Systematic literature review of all PubMed citations between 1999 and August 2003 focusing on clinically relevant biochemical and physiological characteristics of ghrelin.

RESULTS

Ghrelin is an important component of an integrated regulatory system of growth and metabolism acting via the vagus nerve, and is implicated in a variety of altered energy states such as obesity, eating disorders, neoplasia, and cachexia. It also enhances immune responses and potentially down-regulates anti-inflammatory molecules. Ghrelin's role as a brain-gut peptide emphasizes the significance of afferent vagal fibers as a major pathway to the brain, serving the purpose of maintaining physiologic homeostasis.

CONCLUSIONS

The discovery of ghrelin has increased our understanding of feeding regulation, nutritional homeostasis, and metabolic processes. Further characterization of ghrelin's functions will likely generate new pharmacological approaches to diagnose and treat different disease entities including those related to the over-nutrition of obesity and the catabolic response to surgical trauma.

Ghrelin mRNA and GH secretagogue receptor mRNA in human GH-producing pituitary adenomas is affected by mutations in the alpha subunit of G protein

OBJECTIVE

Ghrelin and its receptor, growth hormone secretagogue (GHS) receptor (GHSR), are expressed in the normal pituitary gland and various types of pituitary adenoma. Somatic mutations in the subunit of Gs alpha protein (gsp), which led to a constitutive activation of adenylyl cyclase, are reported in GH-producing pituitary adenomas. We analysed the relationship between ghrelin mRNA and GHSR mRNA expression levels in gsp mutation-positive and -negative GH-producing pituitary adenomas.

PATIENTS

Pituitary adenoma tissue was obtained at surgery from 20 patients with acromegaly.

METHODS

The expression levels of human ghrelin mRNA and GHSR mRNA were quantified using a competitive RT-PCR method. To detect the gsp mutations, amplified Gs alpha subunit cDNA fragments were sequenced directly using RT-PCR method.

RESULTS

There was no significant difference in the expression of ghrelin mRNA between mutation-positive and -negative adenomas. The expression of GHSR mRNA was significantly lower in gsp mutation-positive than -negative adenomas. There was a significant negative correlation between the levels of ghrelin mRNA and GHSR mRNA expression in mutation-negative adenomas; no such correlation was found in mutation-positive adenomas.

CONCLUSION

These results suggest that GHSR mRNA expression is downregulated by ghrelin in gsp mutation-negative GH-producing pituitary adenomas, and that changes in intracellular signalling pathways in gsp mutation-positive GH-producing pituitary adenomas affect the expression of G protein-coupled receptors such as GHSR. The absence of negative correlation between ghrelin and GHSR expression might be induced by lowered GHSR expression in gsp mutation-positive GH-producing adenomas.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

The potential autocrine/paracrine roles of ghrelin and its receptor in hormone-dependent cancer

Ghrelin is a recently identified 28 amino acid peptide capable of stimulating pituitary growth hormone release in humans. The actions of ghrelin are mediated via the naturally occurring ghrelin receptor, also known as the growth hormone secretagogue receptor (GHS-R). Ghrelin and its receptors are now being recognized as components of the growth hormone axis and are therefore potentially involved in tissue growth and development. As is the case for other members of this axis, evidence is rapidly emerging to indicate that ghrelin/GHS-R may play an important autocrine/paracrine role in some cancers. This review highlights the evidence for the expression, regulation and potential functional role of ghrelin and its receptor in hormone-dependent cancers, such as prostate and breast cancer.

Ghrelin expression in fetal, infant, and adult human lung

Ghrelin is a recently identified hormone with potent growth hormone (GH)-releasing activity. It is produced by rat and human gastric endocrine cells and by the pituitary, hypothalamus, placenta, and by gastroenteropancreatic tumors. No evidence of ghrelin production by foregut-derived organs other than stomach has been provided to date. The aim of the present study was to investigate ghrelin expression by human fetal (20 cases), infant (13 cases), and adult (seven cases) lungs by immunohistochemistry, in situ hybridization, and RT-PCR. Expression of the GH secretagogue receptor, the endogenous receptor for ghrelin, was also investigated by RT-PCR. Ghrelin protein was found in the endocrine cells of the fetal lung in decreasing amounts from embryonic to late fetal periods. Its expression was maintained in newborns and children under 2 years but was virtually absent in older individuals. Scattered positive cells were also found in the trachea and the esophagus. Ghrelin mRNA was detected in adult lung by the more sensitive RT-PCR technique. GHS receptor mRNA was detected in nine cases of infant and adult lungs, possibly indicating the existence of local autocrine circuits. We conclude that the fetal lung is an additional source of circulating ghrelin, whose functions at the respiratory tract level remain to be clarified.

Cytochemical and molecular biological aspects of the pituitary and pituitary adenomas--cell differentiation and transcription factors

The anterior pituitary is composed of several cell types, each responsible for the production of specific hormones. Each hormone secreting cells is defined by the activation of its respective hormone genes in a temporally and spatially regulated manner. Recent development in cytochemistry and molecular biology have provided various aspects of human pituitary adenomas, i.e., functional differentiation and classification. The molecular factors that determine hormone production have now been identified as transcription factors. Many novel transcription factors that play a role in anterior pituitary development are implicated. In this review, we focus on the transcriptional factors roles on functional differentiation of the pituitary cells and adenomas and the contribution of cytochemistry and recent development in molecular biological techniques.

Ghrelin: a novel peptide for growth hormone release and feeding regulation

PURPOSE OF REVIEW

A novel peptide hormone, ghrelin, has been identified from the stomach and recognized as an important regulator of growth hormone release and energy homeostasis. It is interesting to note that the stomach may play an important role in not only digestion but also pituitary growth hormone release and central feeding regulation. Thus, we summarize the recent findings on the mechanism of these effects induced by ghrelin.

RECENT FINDINGS

The coadministration of ghrelin and growth hormone releasing hormone was found to have a synergistical effect on pituitary growth hormone secretion. The infusion of growth hormone releasing hormone in rats resulted in a significant increase in pituitary gene expression of ghrelin and its receptor system, suggesting that this system in the pituitary gland could modulate the regulation of growth hormone secretion by growth hormone releasing hormone. Ghrelin promoted the production of orexigenic neuropeptides (neuropeptide Y and agouti-related protein) in the hypothalamic arcuate nuclei and activated the neurons which produce these orexigenic neuropeptides, resulting in an increase in feeding and body weight. Gastric acid release and pancreatic protein secretions were also regulated by ghrelin through vagal and intrapancreatic neuronal activation, respectively. It is possible that ghrelin may participate in the regulation of cell proliferation, glucose homeostasis, and the immune system.

SUMMARY

Ghrelin, secreted from the stomach, modulates growth hormone release and feeding promotion. Further elucidation of the mechanisms of ghrelin effects will help to improve the diagnosis and treatment of eating disorders and disturbed conditions of nutritional homeostasis.

Neuroendocrine and peripheral activities of ghrelin: implications in metabolism and obesity

Ghrelin, a 28-amino acid acylated peptide predominantly produced by the stomach, displays strong growth hormone (GH)-releasing activity mediated by the hypothalamus-pituitary GH secretagogue (GHS)-receptors specific for synthetic GHS. The discovery of ghrelin definitely changes our understanding of GH regulation but it is also already clear that ghrelin is much more than simply a natural GHS. Ghrelin acts also on other central and peripheral receptors and shows other actions including stimulation of lactotroph and corticotroph secretion, orexia, influence on gastro-entero-pancreatic functions, metabolic, cardiovascular and anti-proliferative effects. GHS were born more than 20 years ago as synthetic molecules suggesting the option that GH deficiency could be treated by orally active GHS as an alternative to recombinant human GH (rhGH). Up to now, this has not been the case and also their usefulness as anabolic anti-aging intervention restoring GH/insulin-like growth factor-I axis in somatopause is still unclear. We are now confronted with the theoretical possibility that GHS analogues could become candidate drugs for treatment of pathophysiological conditions in internal medicine totally unrelated to disorders of GH secretion. Particularly, GHS receptor agonists or antagonists acting on appetite could represent new drug intervention in eating disorders.