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

Neuronal ablation of GHSR mitigates diet-induced depression and memory impairment via AMPK-autophagy signaling-mediated inflammation

Obesity is associated with chronic inflammation in the central nervous system (CNS), and neuroinflammation has been shown to have detrimental effects on mood and cognition. The growth hormone secretagogue receptor (GHSR), the biologically relevant receptor of the orexigenic hormone ghrelin, is primarily expressed in the brain. Our previous study showed that neuronal GHSR deletion prevents high-fat diet-induced obesity (DIO). Here, we investigated the effect of neuronal GHSR deletion on emotional and cognitive functions in DIO. The neuron-specific GHSR-deficient mice exhibited reduced depression and improved spatial memory compared to littermate controls under DIO. We further examined the cortex and hippocampus, the major regions regulating cognitive and emotional behaviors, and found that the neuronal deletion of GHSR reduced DIO-induced neuroinflammation by suppressing proinflammatory chemokines/cytokines and decreasing microglial activation. Furthermore, our data showed that neuronal GHSR deletion suppresses neuroinflammation by downregulating AMPK-autophagy signaling in neurons. In conclusion, our data reveal that neuronal GHSR inhibition protects against DIO-induced depressive-like behavior and spatial cognitive dysfunction, at least in part, through AMPK-autophagy signaling-mediated neuroinflammation.

Sarcopenia and Ghrelin System in the Clinical Outcome and Prognosis of Gastroenteropancreatic Neuroendocrine Neoplasms

Simple Summary

Malnutrition and sarcopenia affect clinical outcomes in cancer patients. Nutritional evaluation in patients with neuroendocrine neoplasms (NENs) is not routinely performed. Currently, the evaluation of sarcopenia using CT scans is the gold standard in cancer patients, additionally, anthropometric, biochemical and molecular analysis of patients with gastroenteropancreatic NENs at diagnosis was perfomed. The expression levels of key ghrelin system components were assessed in 63 tumor samples. Results: Nutritional parameters were similar in GEP-NEN tumors of different origin. Relapsed disease was associated with decreased BMI. Patients who presented with weight loss at diagnosis had significantly lower overall survival (108 (25–302) vs. 263 (79–136) months). Ghrelin O-acyltransferase (GOAT) enzyme expression was higher in these patients. The prevalence of sarcopenia using CT images reached 87.2%. Mortality was observed only in patients with sarcopenia. Muscle evaluation was correlated with biochemical parameters but not with the expression of ghrelin system components. Conclusion: Survival is related to the nutritional status of patients with GEP-NENs and also to the molecular expression of some relevant ghrelin system components. Routine nutritional evaluation should be performed in these patients, in order to prescribe appropriate nutritional support, when necessary, for increasing quality of life and improving clinical outcomes.

Abstract

Background: Malnutrition and sarcopenia affect clinical outcomes and treatment response in cancer patients. Patients with neuroendocrine neoplasms (NENs) may present with additional symptoms related to tumor localization in the gastrointestinal tract and hormone secretion, increasing the risk and effects of sarcopenia. Aim: To explore the presence of malnutrition and sarcopenia in gastroenteropancreatic (GEP)-NEN patients, their relation to tumor characteristics, patient outcomes, survival and the molecular expression of ghrelin system components in the tumor. Patients and methods: One-hundred-and-four patients were included. Anthropometric, biochemical and CT-scans at diagnosis were evaluated. The expression levels of key ghrelin system components were assessed in 63 tumor samples. Results: Nutritional parameters were similar in GEP-NEN tumors of different origin. Relapsed disease was associated with decreased BMI. Patients who presented with weight loss at diagnosis had significantly lower overall survival (108 (25–302) vs. 263 (79–136) months). Ghrelin O-acyltransferase (GOAT) enzyme expression was higher in these patients. The prevalence of sarcopenia using CT images reached 87.2%. Mortality was observed only in patients with sarcopenia. Muscle evaluation was correlated with biochemical parameters but not with the expression of ghrelin system components. Conclusion: Survival is related to the nutritional status of patients with GEP-NENs and also to the molecular expression of some relevant ghrelin system components. Routine nutritional evaluation should be performed in these patients, in order to prescribe appropriate nutritional support, when necessary, for increasing quality of life and improving clinical outcomes.

GHSR methylation-dependent expression of a variant ligand and receptor of the ghrelin system induces thymoma tumorigenesis

Our previous study reported that the DNA methylation of growth hormone secretagogue receptor (GHSR) was significantly higher in thymoma or thymic carcinoma (TC) than in normal thymic tissue samples. Thymic epithelial tumors (TETs) with higher GHSR DNA methylation were associated with significantly worse prognosis than those with lower levels of DNA methylation. Diversified components of the ghrelin-GHSR axis may exert opposing effects in cancer progression, depending on the cancer type in question. However, the precise function of the axis remains unclear. In the present study, the mRNA expression of five key components of the ghrelin system [native ligand ghrelin, variant ligand In-1 ghrelin, native receptor GHSR1a, variant receptor GHSR1b and acylation enzyme ghrelin O-acyltransferase (GOAT)] were examined in 58 TET samples by reverse transcription-quantitative PCR, and protein expression of GHSR1a and GHSR1b was assessed in 20 TETs using immunohistochemistry. The results revealed that In-1 ghrelin, GHSR1b (variant forms) and GOAT were more strongly expressed in thymoma compared with thymic-adjacent tissue. By contrast, no significant differences were observed in the expression of ghrelin and GHSR1a (native forms) between thymoma and thymic tissue. The mRNA expression of In-1 ghrelin and GHSR1b (variant forms) was positively associated with GHSR methylation in thymoma tissue samples. However, a relationship was not found between ghrelin, GHSR1a or GOAT expression (native forms) and GHSR methylation in thymoma. Immunohistochemical analysis revealed that mRNA expression of GHSR1a and GHSR1b generally correlated with expression of the corresponding protein, and that the expression of GHSR1b was increased in advanced-stage TETs. These results indicate that the DNA methylation of GHSR is associated with a shift from native expression (ghrelin and GHSR1a) to variant expression (In-1 ghrelin and GHSR1b), which induces the tumorigenesis of thymoma, but not TC.

Protective and Healing Effects of Ghrelin and Risk of Cancer in the Digestive System

Ghrelin is an endogenous ligand for the ghrelin receptor, previously known as the growth hormone secretagogue receptor. This hormone is mainly produced by endocrine cells present in the gastric mucosa. The ghrelin-producing cells are also present in other organs of the body, mainly in the digestive system, but in much smaller amount. Ghrelin exhibits a broad spectrum of physiological effects, such as stimulation of growth hormone secretion, gastric secretion, gastrointestinal motility, and food intake, as well as regulation of glucose homeostasis and bone formation, and inhibition of inflammatory processes. This review summarizes the recent findings concerning animal and human data showing protective and therapeutic effects of ghrelin in the gut, and also presents the role of growth hormone and insulin-like growth factor-1 in these effects. In addition, the current data on the possible influence of ghrelin on the carcinogenesis, its importance in predicting the risk of developing gastrointestinal malignances, as well as the potential usefulness of ghrelin in the treatment of cancer, have been presented.

Rikkunshito attenuates induction of epithelial-mesenchymal switch via activation of Sirtuin1 in ovarian cancer cells

Rikkunshito, a traditional Japanese herbal medicine, improves appetite via activation of gastrointestinal hormone ghrelin pathway. The function of ghrelin is mediated by growth hormone secretagogue receptor (GHSR1a), and ghrelin has been known to possess diverse physiological functions including growth suppression of some cancer cells. Considering that increased ghrelin signaling by Rikkunshito could enhance sirtuin1 (SIRT1) activity in nervous system, we aimed to investigate the effect of Rikkunshito in ovarian cancer cells. Ovarian cancer cell lines were treated with Rikkunshito, and cellular viability, gene expressions and epithelial-mesenchymal transition (EMT) status were investigated. To investigate the involvement of SIRT1 by Rikkunshito in SKOV3 cancer cells, endogenous expression of SIRT1 was depleted using small interfering RNA (siRNA). Treatment with Rikkunshito elevated ghrelin, GHSR1a and SIRT1, while cellular viability was decreased. The treatment of Rikkunshito also inhibited cellular migration and invasion status in a dose-dependent manner, and these effects were translated to the enhanced EMT status, although the role of SIRT1 was not determined. Our study revealed a novel function of Rikkunshito in enhancing EMT status of ovarian cancer cells. Therefore, we would like to propose that Rikkunshito may be used as a novel adjunctive therapy in chemotherapy of ovarian cancer because platinum-based chemotherapy frequently used for the treatment of ovarian cancer inevitably impairs appetite.

Ghrelin promotes midbrain neural stem cells differentiation to dopaminergic neurons through Wnt/β-catenin pathway

Ghrelin plays a neuroprotective role in the process of dopaminergic (DAergic) neurons degeneration in Parkinson's disease (PD). However, it still largely unknown whether ghrelin could affect the midbrain neural stem cells (mbNSCs) from which DAergic neurons are originated. In the present study, we observed that ghrelin enhanced mbNSCs proliferation, and promoted neuronal differentiation especially DAergic neuron differentiation both in vitro and ex vivo. The messenger RNA levels of Wnt1, Wnt3a, and glial cell line-derived neurotrophic factor were increased in response to the ghrelin treatment. Results showed that Wnt/β-catenin pathway was relevant to this DAergic neuron differentiation induced by ghrelin. Our finding gave a new evidence that ghrelin may enable clinical therapies for PD by its neurogenesis role.

Expression of the ghrelin/growth hormone secretagogue receptor axis and its functional role in promoting tumor growth in primary central nervous system lymphomas

Ghrelin and its receptor, growth hormone secretagogue receptor (GHS-R), have been found in a variety of malignant tumor tissues, suggesting a biological function of the ghrelin/GHS-R axis in tumor growth and progression. Among central nervous system tumors, primary central nervous system lymphomas (PCNSLs) are relatively rare and characterized by a rapid progression and poor prognosis. In order to clarify ghrelin expression and its functional role in promoting tumor growth and progression in PCNSLs, we undertook an immunohistochemical investigation for ghrelin and GHS-R expression in 43 patients and tested the effect of ghrelin inhibition on lymphoma cells. Furthermore, we investigated the expression of CD105, a marker for tumor angiogenesis, to explore its association with the ghrelin/GHS-R axis. The Kaplan-Meier method and Cox's proportional hazards regression model were used to determine the association of ghrelin/GHS-R expression with overall survival rate. The immunohistochemical study showed moderate/strong immunostaining of cells for ghrelin and GHS-R in 40 patients (93.0%) and 39 patients (90.7%), respectively. A ghrelin inhibitor did not affect tumor cell proliferation in vitro. Expression levels of ghrelin and GHS-R were divided into high and low groups by the rate of moderate-strong staining cells to tumor cells. The survival rate was significantly lower in patients with high GHS-R expression (P = 0.0368 by log-rank test; P = 0.0219 by Wilcoxon test). In addition, multivariate analysis of overall survival using Cox's proportional hazards regression model indicated that GHS-R was a significant independent prognostic factor (P = 0.0426). CD105 expression on tumor vessels was positive in 33 patients (33/37, 89.2%). There was a positive correlation between the moderate-strong staining rate of ghrelin and CD105-positive vessel count. These results indicated that the ghrelin/GHS-R axis plays a potential role in promoting tumor growth and progression through neoangiogenesis, rather than the proliferation of tumor cells.

Single Amino Acid Replacement in G-7039 Leads to a 70-fold Increase in Binding toward GHS-R1a

The growth hormone secretagogue receptor type 1a (GHS-R1a) is a class A rhodopsin-like G protein coupled receptor (GPCR) that is expressed in a variety of human tissues and is differentially expressed in benign and malignant prostate cancer. Previously, the peptidomimetic [1-Nal⁴ ,Lys⁵ (4-fluorobenzoyl)]G-7039 was designed as a molecular imaging tool for positron emission tomography (PET). However, this candidate was a poor binder (IC₅₀ =69 nm), required a lengthy four-step radiosynthesis, and had a cLogP above 8. To address these challenges, we now report on changes targeted at the 4^th position of G-7039. A 2-fluoropropionic acid (2-FPA) group was added on to Lys⁵ to determine the potential binding affinity of the [¹⁸ F]-2-FP radiolabeled analogue, which could be prepared by simplified radiochemistry. Lead candidate [Tyr⁴ ,Lys⁵ (2-fluoropropionyl)]G-7039 exhibited an IC₅₀ of 0.28 nm and low picomolar activity toward GHS-R1a. Molecular docking revealed a molecular basis for this picomolar affinity.

Attribution of Ghrelin to Cancer; Attempts to Unravel an Apparent Controversy

Ghrelin is an endogenous peptide hormone mainly produced in the stomach. It has been known to regulate energy homeostasis, stimulate secretion of growth hormone, and mediate many other physiologic effects. Various effects attributed to ghrelin contribute to many aspects of cancer development and progression. Accordingly, a large body of evidence has emerged about the association of ghrelin with several types of cancer in scales of cell-line, animal, and human studies. However, existing data are controversial. This controversy occurs in two main domains: one is the controversial results in local effects of ghrelin on different types of human cancer cell-lines; the second is the apparent disagreement in the results of in-vitro and clinical studies that investigated ghrelin association to one type of cancer. These inconsistencies have hampered the indications to consider ghrelin as a potential tumor biomarker or therapeutic agent in cancer patients. Previous studies have reviewed different parts of current literature about the ghrelin-cancer relationship. Although they have highlighted these controversial results in various ways, no specific recommendations have been given to address it. In this study, we comprehensively reviewed in-vitro, in-vivo, and clinical studies and attempted to use the following approaches to unravel the inconsistencies detected: (a) to distinguish local and systemic effects of ghrelin in interpreting its summary clinical role in each cancer; (b) scrutinizing factors that regulate local effects of ghrelin and could justify different effects of ghrelin on different cancer cell-lines. These approaches could have notable implications for future in-vitro and clinical studies.

Temporospatial effects of acyl-ghrelin on activation of astrocytes after ischaemic brain injury

The protective mechanisms of astrocyte signalling are based on the release of neurotrophic factors and the clearing of toxic substances in the early stages of cerebral ischaemia. However, astrocytes are also responsible for the detrimental effects that occur during the later stages of ischaemia, in which glial scars are formed, thereby impeding neural recovery. Acyl-ghrelin has been found to be neuroprotective after stroke, although the influence of acyl-ghrelin on astrocytes after ischaemic injury is yet to be clarified. In the present study, we used permanent middle cerebral arterial occlusion to establish a brain ischaemia model in vivo, as well as oxygen and glucose deprivation (OGD) to mimic ischaemic insults in vitro. We found that acyl-ghrelin injection significantly increased the number of activated astrocytes in the peri-infarct area at day 3 after brain ischaemia and decreased the number of activated astrocytes after day 9. Moreover, the expression of fibroblast growth factor 2 (FGF2) in the ischaemic hemisphere increased markedly after day 3, and i.c.v. injection of SU5402, an inhibitor of FGF2 signalling, abolished the suppression effects of acyl-ghrelin on astrocyte activation in the peri-infarct region during the later stages of ischaemia. The results from in vitro studies also showed the dual effect of acyl-ghrelin on astrocyte viability. Acyl-ghrelin increased the viability of uninjured astrocytes in an indirect way by stimulating the secretion from OGD-injured astrocytes. It also inhibited the astrocyte viability in the presence of FGF2 in a dose-dependent manner. Furthermore, the expression of acyl-ghrelin receptors on astrocytes was increased after acyl-ghrelin and FGF2 co-treatment. In conclusion, acyl-ghrelin promoted astrocyte activation in the early stages of ischaemia but suppressed the activation in later stages of ischaemic injury. These later effects were likely to be triggered by the increased expression of endogenous FGF2 after brain ischaemia.

Ras suppressor-1 (RSU-1) promotes cell invasion in aggressive glioma cells and inhibits it in non-aggressive cells through STAT6 phospho-regulation

Most gliomas are invasive tumors formed from glial cells and associated with high mortality rates. In this study, we characterized four glioma cell lines of varying degree of aggressiveness (H4, SW1088, A172 and U87-MG) in terms of morphology, cytoskeleton organization and stiffness, and evaluated their invasive potential by performing invasion, colony forming and spheroid invasion assays. Cells were divided into two distinct groups: aggressive cell lines (A172 and U87-MG) with more elongated, softer and highly invasive cells and less aggressive cells (H4 and SW088). Interestingly, we found that Ras Suppressor-1 (RSU-1), a cell-matrix adhesion protein involved in cancer cell invasion, was significantly upregulated in more aggressive glioma cells compared to less aggressive. Importantly, RSU-1 silencing had opposing effects on glioma cell invasion depending on their aggressiveness, inhibiting migration and invasion of aggressive cells and promoting those of less aggressive cells. Finally, we found that RSU-1 silencing in aggressive cells led to decreased Signal Transducer and Activator of Transcription6 (STAT6) phosphorylation and Matrix Metalloproteinase13 (MMP13) expression in contrast to less invasive cells. Our study demonstrates that RSU-1 promotes invasion of aggressive glioma cells and inhibits it in the non-aggressive cells, indicating that it could serve as a predictor of gliomas progression.

Ghrelin Upregulates Oncogenic Aurora A to Promote Renal Cell Carcinoma Invasion

Ghrelin is a peptide hormone, originally identified from the stomach, that functions as an endogenous ligand of the growth hormone secretagogue receptor (GHSR) and promotes growth hormone (GH) release and food intake. Increasing reports point out ghrelin’s role in cancer progression. We previously characterized ghrelin’s prognostic significance in the clear cell subtype of renal cell carcinoma (ccRCC), and its pro-metastatic ability via Snail-dependent cell migration. However, ghrelin’s activity in promoting cell invasion remains obscure. In this study, an Ingenuity Pathway Analysis (IPA)-based investigation of differentially expressed genes in Cancer Cell Line Encyclopedia (CCLE) dataset indicated the potential association of Aurora A with ghrelin in ccRCC metastasis. In addition, a significant correlation between ghrelin and Aurora A expression level in 15 ccRCC cell line was confirmed by variant probes. ccRCC patients with high ghrelin and Aurora A status were clinically associated with poor outcome. We further observed that ghrelin upregulated Aurora A at the protein and RNA levels and that ghrelin-induced ccRCC in vitro invasion and in vivo metastasis occurred in an Aurora A-dependent manner. Furthermore, MMP1, 2, 9 and 10 expressions are associated with poor outcome. In particular, MMP10 is significantly upregulated and required for the ghrelin-Aurora A axis to promote ccRCC invasion. The results of this study indicated a novel signaling mechanism in ccRCC metastasis.

No effect of unacylated ghrelin administration on subcutaneous PC3 xenograft growth or metabolic parameters in a Rag1-/- mouse model of metabolic dysfunction

Ghrelin is a peptide hormone which, when acylated, regulates appetite, energy balance and a range of other biological processes. Ghrelin predominately circulates in its unacylated form (unacylated ghrelin; UAG). UAG has a number of functions independent of acylated ghrelin, including modulation of metabolic parameters and cancer progression. UAG has also been postulated to antagonise some of the metabolic effects of acyl-ghrelin, including its effects on glucose and insulin regulation. In this study, Rag1-/- mice with high-fat diet-induced obesity and hyperinsulinaemia were subcutaneously implanted with PC3 prostate cancer xenografts to investigate the effect of UAG treatment on metabolic parameters and xenograft growth. Daily intraperitoneal injection of 100 μg/kg UAG had no effect on xenograft tumour growth in mice fed normal rodent chow or 23% high-fat diet. UAG significantly improved glucose tolerance in host Rag1-/- mice on a high-fat diet, but did not significantly improve other metabolic parameters. We propose that UAG is not likely to be an effective treatment for prostate cancer, with or without associated metabolic syndrome.

A stable meta-carborane enables the generation of boron-rich peptide agonists targeting the ghrelin receptor

Boron neutron capture therapy (BNCT) is a binary cancer therapy, which combines the biochemical targeting of a boron-containing drug with the regional localization of radiation treatment. Although the concept of BNCT has been known for decades, the selective delivery of boron into tumor cells remains challenging. G protein-coupled receptors that are overexpressed on cancer cells in combination with peptidic ligands can be potentially used as shuttle system for a tumor-directed boron uptake. In this study, we present the generation of short, boron-rich peptide conjugates that target the ghrelin receptor. Expression of the ghrelin receptor on various cancer cells makes it a viable target for BNCT. We designed a novel hexapeptide super-agonist that was modified with different specifically synthesized carborane monoclusters and tested for ghrelin receptor activation. A meta-carborane building block with a mercaptoacetic acid linker was found to be optimal for peptide modification, owing to its chemical stability and a suitable activation efficacy of the conjugate. The versatility of this carborane for the development of peptidic boron delivery agents was further demonstrated by the generation of highly potent, boron-loaded conjugates using the backbone of the known ghrelin receptor ligands growth hormone releasing peptide 6 and Ipamorelin.

Ghrelin promotes human non-small cell lung cancer A549 cell proliferation through PI3K/Akt/mTOR/P70S6K and ERK signaling pathways

Ghrelin is a gastric acyl-peptide that plays an important role in cell proliferation. In the present study, we explored the role of ghrelin in A549 cell proliferation and the possible molecular mechanisms. We found that ghrelin promotes A549 cell proliferation, knockdown of the growth hormone secretagogue receptor (GHSR) attenuated A549 cell proliferation caused by ghrelin. Ghrelin induced the rapid phosphorylation of phosphatidylinositol 3-kinase (PI3K), Akt, ERK, mammalian target of rapamycin (mTOR) and P70S6K. PI3K inhibitor (LY 294002), ERK inhibitor (PD98059) and mTOR inhibitor (Rapamycin) inhibited ghrelin-induced A549 cell proliferation. Moreover, GHSR siRNA inhibited phosphorylation of PI3K, Akt, ERK, mTOR and P70S6K induced by ghrelin. Akt and mTOR/P70S6K phosphorylation was inhibited by LY 294002 but not by PD98059. These results indicate that ghrelin promotes A549 cell proliferation via GHSR-dependent PI3K/Akt/mTOR/P70S6K and ERK signaling pathways.

Serum ghrelin levels in papillary thyroid carcinoma

OBJECTIVE

Ghrelin plays a role in several processes of cancer progression, and numerous cancer types express ghrelin and its receptor. We aimed to investigate serum levels of ghrelin in patients with papillary thyroid carcinoma (PTC) and its association with the prognostic factors in PTC.

MATERIALS AND METHODS

We enrolled 54 patients with thyroid cancer (7 male, 47 female) and 24 healthy controls (6 male, 18 female) in the study. We compared demographic, anthropometric, and biochemical data, and serum ghrelin levels between the groups. Serum ghrelin levels were measured using as enzyme-linked immunosorbent assay.

RESULTS

Ghrelin levels were similar between the groups, but plasma ghrelin levels were significantly higher in tumors larger than 1 cm diameter compared with papillary microcarcinomas. Serum ghrelin levels also correlated with tumor size (r = 0.499; p < 0.001). Body mass index, thyroid-stimulating hormone, and HOMA-IR levels were similar between the groups. There were no statistically significant differences regarding average age and other prognostic parameters including lymph node invasion, capsule invasion, multifocality and surgical border invasion between patients with microcarcinoma and tumors larger than 1 cm.

CONCLUSION

In our study, no significant difference in serum ghrelin levels was determined between patients with papillary thyroid cancer and healthy controls however, serum ghrelin levels were higher in tumors larger than 1 cm compared to in those with thyroid papillary microcarcinoma.

The Ghrelin/GOAT System Regulates Obesity-Induced Inflammation in Male Mice

Ghrelin plays a key role in appetite, energy homeostasis, and glucose regulation. Recent evidence suggests ghrelin suppresses inflammation in obesity; however, whether this is modulated by the acylated and/or des-acylated peptide is unclear. We used mice deficient in acylated ghrelin [ghrelin octanoyl-acyltransferase (GOAT) knockout (KO) mice], wild-type (WT) littermates, and C57BL/6 mice to examine the endogenous and exogenous effects of acyl and des-acyl ghrelin on inflammatory profiles under nonobese and obese conditions. We demonstrate that in the spleen, both ghrelin and GOAT are localized primarily in the red pulp. Importantly, in the thymus, ghrelin was predominantly localized to the medulla, whereas GOAT was found in the cortex, implying differing roles in T cell development. Acute exogenous treatment with acyl/des-acyl ghrelin suppressed macrophage numbers in spleen and thymus in obese mice, whereas only acyl ghrelin increased CD3+ T cells in the thymus in mice fed both chow and a high-fat-diet (HFD). Consistent with this result, macrophages were increased in the spleen of KO mice on a HFD. Whereas there was no difference in CD3+ T cells in the plasma, spleen, or thymus of WT vs KO mice, KO chow and HFD-fed mice displayed decreased leukocytes. Our results suggest that the acylation status affects the anti-inflammatory properties of ghrelin under chow and HFD conditions.

Involvement of Astrocytes in Mediating the Central Effects of Ghrelin

Although astrocytes are the most abundant cells in the mammalian brain, much remains to be learned about their molecular and functional features. Astrocytes express receptors for numerous hormones and metabolic factors, including the appetite-promoting hormone ghrelin. The metabolic effects of ghrelin are largely opposite to those of leptin, as it stimulates food intake and decreases energy expenditure. Ghrelin is also involved in glucose-sensing and glucose homeostasis. The widespread expression of the ghrelin receptor in the central nervous system suggests that this hormone is not only involved in metabolism, but also in other essential functions in the brain. In fact, ghrelin has been shown to promote cell survival and neuroprotection, with some studies exploring the use of ghrelin as a therapeutic agent against metabolic and neurodegenerative diseases. In this review, we highlight the possible role of glial cells as mediators of ghrelin's actions within the brain.

Ghrelin and cancer progression

Ghrelin is a small peptide with 28 amino acids, and has been characterized as the ligand of the growth hormone secretagogue receptor (GHSR). In addition to its original function in stimulating pituitary growth hormone release, ghrelin is multifunctional and plays a role in the regulation of energy balance, gastric acid release, appetite, insulin secretion, gastric motility and the turnover of gastric and intestinal mucosa. The discovery of ghrelin and GHSR expression beyond normal tissues suggests its role other than physiological function. Emerging evidences have revealed ghrelin's function in regulating several processes related to cancer progression, especially in metastasis and proliferation. We further show the relative GHRL and GHSR expression in pan-cancers from The Cancer Genome Atlas (TCGA), suggesting the potential pathological role of the axis in cancers. This review focuses on ghrelin's biological function in cancer progression, and reveals its clinical significance especially the impact on cancer patient outcome.

The neurological effects of ghrelin in brain diseases: Beyond metabolic functions

Ghrelin, a peptide released by the stomach that plays a major role in regulating energy metabolism, has recently been shown to have effects on neurobiological behaviors. Ghrelin enhances neuronal survival by reducing apoptosis, alleviating inflammation and oxidative stress, and accordingly improving mitochondrial function. Ghrelin also stimulates the proliferation, differentiation and migration of neural stem/progenitor cells (NS/PCs). Additionally, the ghrelin is benefit for the recovery of memory, mood and cognitive dysfunction after stroke or traumatic brain injury. Because of its neuroprotective and neurogenic roles, ghrelin may be used as a therapeutic agent in the brain to combat neurodegenerative disease. In this review, we highlight the pre-clinical evidence and the proposed mechanisms underlying the role of ghrelin in physiological and pathological brain function.

Multi-species sequence comparison reveals conservation of ghrelin gene-derived splice variants encoding a truncated ghrelin peptide

The peptide hormone ghrelin is a potent orexigen produced predominantly in the stomach. It has a number of other biological actions, including roles in appetite stimulation, energy balance, the stimulation of growth hormone release and the regulation of cell proliferation. Recently, several ghrelin gene splice variants have been described. Here, we attempted to identify conserved alternative splicing of the ghrelin gene by cross-species sequence comparisons. We identified a novel human exon 2-deleted variant and provide preliminary evidence that this splice variant and in1-ghrelin encode a C-terminally truncated form of the ghrelin peptide, termed minighrelin. These variants are expressed in humans and mice, demonstrating conservation of alternative splicing spanning 90 million years. Minighrelin appears to have similar actions to full-length ghrelin, as treatment with exogenous minighrelin peptide stimulates appetite and feeding in mice. Forced expression of the exon 2-deleted preproghrelin variant mirrors the effect of the canonical preproghrelin, stimulating cell proliferation and migration in the PC3 prostate cancer cell line. This is the first study to characterise an exon 2-deleted preproghrelin variant and to demonstrate sequence conservation of ghrelin gene-derived splice variants that encode a truncated ghrelin peptide. This adds further impetus for studies into the alternative splicing of the ghrelin gene and the function of novel ghrelin peptides in vertebrates.

The role of astrocytes in the hypothalamic response and adaptation to metabolic signals

The hypothalamus is crucial in the regulation of homeostatic functions in mammals, with the disruption of hypothalamic circuits contributing to chronic conditions such as obesity, diabetes mellitus, hypertension, and infertility. Metabolic signals and hormonal inputs drive functional and morphological changes in the hypothalamus in attempt to maintain metabolic homeostasis. However, the dramatic increase in the incidence of obesity and its secondary complications, such as type 2 diabetes, have evidenced the need to better understand how this system functions and how it can go awry. Growing evidence points to a critical role of astrocytes in orchestrating the hypothalamic response to metabolic cues by participating in processes of synaptic transmission, synaptic plasticity and nutrient sensing. These glial cells express receptors for important metabolic signals, such as the anorexigenic hormone leptin, and determine the type and quantity of nutrients reaching their neighboring neurons. Understanding the mechanisms by which astrocytes participate in hypothalamic adaptations to changes in dietary and metabolic signals is fundamental for understanding the neuroendocrine control of metabolism and key in the search for adequate treatments of metabolic diseases.

Hypothalamic innate immune reaction in obesity

Findings from rodent and human studies show that the presence of inflammatory factors is positively correlated with obesity and the metabolic syndrome. Obesity-associated inflammatory responses take place not only in the periphery but also in the brain. The hypothalamus contains a range of resident glial cells including microglia, macrophages and astrocytes, which are embedded in highly heterogenic groups of neurons that control metabolic homeostasis. This complex neural-glia network can receive information directly from blood-borne factors, positioning it as a metabolic sensor. Following hypercaloric challenge, mediobasal hypothalamic microglia and astrocytes enter a reactive state, which persists during diet-induced obesity. In established mouse models of diet-induced obesity, the hypothalamic vasculature displays angiogenic alterations. Moreover, proopiomelanocortin neurons, which regulate food intake and energy expenditure, are impaired in the arcuate nucleus, where there is an increase in local inflammatory signals. The sum total of these events is a hypothalamic innate immune reactivity, which includes temporal and spatial changes to each cell population. Although the exact role of each participant of the neural-glial-vascular network is still under exploration, therapeutic targets for treating obesity should probably be linked to individual cell types and their specific signalling pathways to address each dysfunction with cell-selective compounds.

Ghrelin promotes renal cell carcinoma metastasis via Snail activation and is associated with poor prognosis

Ghrelin is an appetite-regulating molecule that promotes growth hormone (GH) release and food intake through growth hormone secretagogue receptor (GHS-R). Recently, high ghrelin levels have been detected in various types of human cancer. Ghrelin expression is observed in proximal and distal renal tubules, where renal cell carcinoma (RCC) arises. However, whether ghrelin is up-regulated and promotes renal cell carcinogenesis remains obscure. In this study, we observed that ghrelin was highly expressed in renal tumours, especially in metastatic RCC. In addition, high ghrelin levels correlated with poor outcome, lymph node and distant metastasis. The addition of ghrelin promoted the migration ability of RCC cell lines 786-0, ACHN and A-498. Furthermore, knockdown of ghrelin expression reduced in vitro migration and in vivo metastasis, suggesting a requirement for ghrelin accumulation in the microenvironment for RCC metastasis. Analysis of microarray signatures using Ingenuity Pathway Analysis (IPA) and MetaCore pointed to the potential regulation by ghrelin of Snail, a transcriptional repressor of E-cadherin. We further observed that Ghrelin increased the expression, nuclear translocation and promoter-binding activity of Snail. Snail silencing blocked the ghrelin-mediated effects on E-cadherin repression and cell migration. Snail-E-cadherin regulation was mediated by GHS-R-triggered Akt phosphorylation at Ser473 and Thr308. Pretreatment with PI3K inhibitors, LY294002 and wortmannin, as well as Akt siRNA, decreased ghrelin-induced Akt phosphorylation, Snail promoter binding activity and migration. Taken together, our findings indicate that ghrelin can activate Snail function via the GHS-R-PI3K-Akt axis, which may contribute to RCC metastasis. The microarray raw data were retrieved from the Cancer Genome Atlas (TCGA) [KIRC gene expression (IlluminaHiSeq) dataset].

Silencing of ghrelin receptor expression inhibits endometrial cancer cell growth in vitro and in vivo

Ghrelin is a 28-amino acid peptide hormone produced predominantly in the stomach but also in a range of normal cell types and tumors, where it has endocrine, paracrine, and autocrine roles. Previously, we have demonstrated that ghrelin has proliferative and antiapoptotic effects in endometrial cancer cell lines, suggesting a potential role in promoting tumor growth. In the present study, we investigated the effect of ghrelin receptor, GHSR, and gene silencing in vitro and in vivo and characterized ghrelin and GHSR1a protein expression in human endometrial tumors. GHSR gene silencing was achieved in the Ishikawa and KLE endometrial cancer cell lines, using a lentiviral short-hairpin RNA targeting GHSR. The effects of GHSR1a knockdown were further analyzed in vivo using the Ishikawa cell line in a NOD/SCID xenograft model. Cell proliferation was reduced in cultured GHSR1a knockdown Ishikawa and KLE cells compared with scrambled controls in the absence of exogenously applied ghrelin and in response to exogenous ghrelin (1,000 nM). The tumor volumes were reduced significantly in GHSR1a knockdown Ishikawa mouse xenograft tumors compared with scrambled control tumours. Using immunohistochemistry, we demonstrated that ghrelin and GHSR1a are expressed in benign and cancerous glands in human endometrial tissue specimens, although there was no correlation between the intensity of staining and cancer grade. These data indicate that downregulation of GHSR expression significantly inhibits endometrial cancer cell line and mouse xenograft tumour growth. This is the first preclinical evidence that downregulation of GHSR may be therapeutic in endometrial cancer.

Ghrelin induces gastric cancer cell proliferation, migration, and invasion through GHS-R/NF-κB signaling pathway

This study aims to investigate the roles of ghrelin signaling in human gastric carcinoma cell lines AGS and SGC7901. Effects of ghrelin signaling on CDK6, P53, NF-κB/P65 and MMP2 mRNA and/or protein expression were determined by real-time PCR and western blot. MTT method and flow cytometry were performed to assess the gastric cancer cell proliferation. The SGC7901 cells overexpressing ghrelin were inoculated into nude mice to produce tumors which were measured later. The wound-healing assay and cell invasion assay were used to test the cell migration and invasive ability of gastric cancer. Ghrelin signaling promotes the oncogene CDK6 gene expression and represses the tumor suppressor gene P53 gene expression in gastric cancer. Ghrelin activates NF-κB/P65 signaling pathway through GHS-R in gastric cancer. Ghrelin upregulates the metastasis factor MMP2 expression via GHS-R/NF-κB signaling pathway in gastric cancer cells and promotes tumor cells migration and invasion, suggesting that ghrelin signaling is a critical pathway in cancer metastasis. Ghrelin induces cell proliferation, migration and invasion via GHS-R/NF-κB signaling pathway in gastric cancer cells. Ghrelin treatment must be avoided for gastric cancer patients.

Sensitivity of edge detection methods for quantifying cell migration assays

Quantitative imaging methods to analyze cell migration assays are not standardized. Here we present a suite of two-dimensional barrier assays describing the collective spreading of an initially-confined population of 3T3 fibroblast cells. To quantify the motility rate we apply two different automatic image detection methods to locate the position of the leading edge of the spreading population after , and hours. These results are compared with a manual edge detection method where we systematically vary the detection threshold. Our results indicate that the observed spreading rates are very sensitive to the choice of image analysis tools and we show that a standard measure of cell migration can vary by as much as 25% for the same experimental images depending on the details of the image analysis tools. Our results imply that it is very difficult, if not impossible, to meaningfully compare previously published measures of cell migration since previous results have been obtained using different image analysis techniques and the details of these techniques are not always reported. Using a mathematical model, we provide a physical interpretation of our edge detection results. The physical interpretation is important since edge detection algorithms alone do not specify any physical measure, or physical definition, of the leading edge of the spreading population. Our modeling indicates that variations in the image threshold parameter correspond to a consistent variation in the local cell density. This means that varying the threshold parameter is equivalent to varying the location of the leading edge in the range of approximately 1–5% of the maximum cell density.

Modulation of ghrelin axis influences the growth of colonic and prostatic cancer cells in vitro

BACKGROUND

The risk of different cancers seems to be associated with obesity. Moreover, low ghrelin levels observed in obese people may be implicated in cancer development and progression. The aim of this study was to examine the direct effects of both forms of ghrelin (acylated and unacylated) and ghrelin receptor type 1a antagonist (D-Lys-GHRP-6) on the growth of murine colon cancer MC38 and human prostate cancer DU145 cell lines in vitro.

METHODS

The cells were cultured for 72 h in the presence of rat or human acylated ghrelin (rG, hG), human unacylated ghrelin (hUAG), D-Lys-GHRP-6 (GHS-RA) applied either alone or jointly. The cell line growth was assessed by the colorimetric Mosmann method.

RESULTS

hUAG (10(-6), 10(-7) and 10(-10) M) inhibited MC38 cancer cell growth and, at some concentrations (10(-8), 10(-9), 10(-10) M), enhanced the antineoplastic effect of GHS-RA(10(-4) M). In turn, GHS-RA evoked a biphasic effect on MC38 cancer growth: inhibitory at 10(-4) M and stimulatory at 10(-5) and 10(-6) M. Moreover, GHS-RA at the highest examined concentration (10(-4) M) enhanced the cytostatic effect of FU. Human acylated and unacylated ghrelin and GHS-RA inhibited DU145 cancer growth with moderate and different potencies. A dose-response effect was observed for the inhibitory action of hG together with the synergistic effect of hUAG and GHS-RA.

CONCLUSION

The obtained results indicate an involvement of the ghrelin axis in the growth regulation of colon and prostate cancers and may suggest new therapeutic options for these neoplasms.

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 canine mammary carcinoma cells proliferation, apoptosis and migration

Background

Ghrelin is a natural ligand of the growth hormone secretagogue receptor (GHS-R). They are often co-expressed in multiple human tumors and related cancer cell lines what can indicate that the ghrelin/GHS-R axis may have an important role in tumor growth and progression. However, a role of ghrelin in canine tumors remains unknown. Thus, the aim of our study was two-fold: (1) to assess expression of ghrelin and its receptor in canine mammary cancer and (2) to examine the effect of ghrelin on carcinoma cells proliferation, apoptosis, migration and invasion. The expression of ghrelin and its receptor in canine mammary cancer tissues and cell lines (isolated from primary tumors and their metastases) was examined using Real-time qPCR and immunohistochemistry. For apoptosis analysis the Annexin V and propidium iodide dual staining was applied whereas cell proliferation was evaluated by MTT assay and BrdU incorporation test. The influence of ghrelin on cancer cells migration and invasion was assessed using Boyden chamber assays and wound healing assay.

Results

The highest expression of ghrelin was observed in metastatic cancers whereas the lowest expression of ghrelin receptor was detected in tumors of the 3^rd grade of malignancy. Higher expression of ghrelin and its receptor was detected in cancer cell lines isolated from metastases than in cell lines isolated from primary tumors. In vitro experiments demonstrated that exposure to low doses of ghrelin stimulates cellular proliferation, inhibits apoptosis and promotes motility and invasion of canine mammary cancer cells. Growth hormone secretagogue receptor inhibitor ([D-Lys³]-GHRP6) as well as RNA interference enhances early apoptosis.

Conclusion

The presence of ghrelin and GHS-R in all of the examined canine mammary tumors may indicate their biological role in cancer growth and development. Our experiments conducted in vitro confirmed that ghrelin promotes cancer development and metastasis.

Emerging role of glial cells in the control of body weight

Glia are the most abundant cell type in the brain and are indispensible for the normal execution of neuronal actions. They protect neurons from noxious insults and modulate synaptic transmission through affectation of synaptic inputs, release of glial transmitters and uptake of neurotransmitters from the synaptic cleft. They also transport nutrients and other circulating factors into the brain thus controlling the energy sources and signals reaching neurons. Moreover, glia express receptors for metabolic hormones, such as leptin and insulin, and can be activated in response to increased weight gain and dietary challenges. However, chronic glial activation can be detrimental to neurons, with hypothalamic astrocyte activation or gliosis suggested to be involved in the perpetuation of obesity and the onset of secondary complications. It is now accepted that glia may be a very important participant in metabolic control and a possible therapeutical target. Here we briefly review this rapidly advancing field.

Ghrelin inhibits insulin release by regulating the expression of inwardly rectifying potassium channel 6.2 in islets

INTRODUCTION

The objective is to investigate the influence of ghrelin administration on both the insulin secretion and the expression of ATP-sensitive K(+) channels in islet.

METHODS

Ghrelin and [D-Lys] growth hormone releasing peptide-6 were administered via intraperitoneal injection in Wistar rats at the doses 10 and 10 μmol/kg/d for 2 weeks, respectively. Then glucose tolerance tests were performed and plasma insulin concentrations were measured. Islets were isolated for insulin release experiments. Single β cells were isolated for electrophysiological experiments. Determination of the Kir6.2 and SUR1 mRNA and protein expression levels in islets was performed by polymerase chain reaction and western blotting.

RESULTS

Intraperitoneal administration of exogenous ghrelin significantly (P < 0.05) increased blood glucose concentrations, attenuated insulin responses during glucose tolerance tests, reduced insulin release from the isolated islets induced by 11.1 and 16.7 mmol/L glucose, hyperpolarized the resting membrane potential and increased the Kir6.2 mRNA and protein expression levels. In contrast, counteraction of ghrelin by intraperitoneal injection of [D-Lys(3)] growth hormone releasing peptide-6 significantly (P < 0.05) attenuated the aforementioned changes. SUR1 expression levels were not altered in this study.

CONCLUSIONS

Ghrelin via pancreatic growth hormone secretagogue receptor up-regulates the Kir6.2 expression in islet by hyperpolarizing the resting membrane potential which results in the inhibition of insulin release.

Ghrelin stimulates angiogenesis via GHSR1a-dependent MEK/ERK and PI3K/Akt signal pathways in rat cardiac microvascular endothelial cells

Ghrelin, an endogenous ligand of the growth hormone secretagogue receptor (GHSR), is thought to exert a protective effect on the cardiovascular system, specifically by promoting vascular endothelial cell function such as cell proliferation, migration, survival and angiogenesis. However, the effect of ghrelin on angiogenesis and the corresponding mechanisms have not yet been extensively studied in cardiac microvascular endothelial cells (CMECs) isolated from left ventricular myocardium of adult Sprague-Dawley (SD) rats. In our study, we found that ghrelin and GHSR are constitutively expressed in CMECs. Ghrelin significantly increases CMECs proliferation, migration, and in vitro angiogenesis. The ghrelin-induced angiogenic process was accompanied by phosphorylation of ERK and Akt. MEK inhibitor PD98059 abolished ghrelin-induced phosphorylation of ERK, but had no effect on Akt phosphorylation. PI3K inhibitor LY294002 abolished ghrelin-induced phosphorylation of Akt, but had no effect on ERK phosphorylation. Ghrelin-induced angiogenesis was partially blocked by treatment with PD98059 or LY294002. In addition, this angiogenic effect was almost completely inhibited by PD98059+LY294002. Pretreatment with GHSR1a blocker [D-Lys3]-GHRP-6 abolished ghrelin-induced phosphorylation of ERK, Akt and in vitro angiogenesis. In conclusion, this is the first demonstration that ghrelin stimulates CMECs angiogenesis through GHSR1a-mediated MEK/ERK and PI3K/Akt signal pathways, indicating that two pathways are required for full angiogenic activity of ghrelin. This study suggests that ghrelin may play an important role in myocardial angiogenesis.

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.

The effects of ghrelin on inflammation and the immune system

A number of hormones and metabolic mediators signal the brain of changes in the body's energy status and when an imbalance occurs; the brain coordinates the appropriate changes in energy intake and utilization via the control of appetite and food consumption. Under conditions of chronic inflammation and immune activation, there is often a significant loss of body mass and appetite suggesting the presence of shared ligands and signaling pathways mediating "crosstalk" between the immune and neuroendocrine systems. Ghrelin, the endogenous ligand for growth hormone secretagogue receptor (GHS-R), is produced primarily by cells in the stomach and serves as a potent circulating orexigenic hormone controlling food intake, energy expenditure, adiposity and GH secretion. The functional roles of ghrelin and other growth hormone secretagogues (GHS) within the immune system and under states of inflammatory stress and injury are only now coming to light. A number of reports over the past decade have described ghrelin to be a potent anti-inflammatory mediator both in vitro and in vivo and a promising therapeutic agent in the treatment of inflammatory diseases and injury. Moreover, ghrelin has also been shown to promote lymphocyte development in the primary lymphoid organs (bone marrow and thymus) and to ablate age-associated thymic involution. In the current report, we review the literature supporting a role for ghrelin as an anti-inflammatory agent and immunoregulatory hormone/cytokine and its potential use in the treatment of inflammatory diseases and injury.

Ghrelin and obestatin promote the allergic action in rat peritoneal mast cells as basic secretagogues

Ghrelin is an endogenous ligand of the type 1a growth hormone secretagogue receptor (GHSR1a) that regulates energy balance. Ghrelin and obestatin, derived from the post-translational processing of preproghrelin, are involved in a diverse range of biological activities, yet their effect on the immune system is not fully understood. In the present study, we investigated the roles of ghrelin and obestatin on mast cell degranulation and found that both ghrelin and obestatin induce the release of histamine from rat peritoneal mast cells. This induced histamine release was inhibited by the pertussis toxin, an inhibitor of Gα(i) protein, and extracellular Ca(2+). Rat peritoneal mast cells and rat basophilic leukemia (RBL-2H3) cells did not express the ghrelin receptor GHSR1a, suggesting that histamine release induced by ghrelin occurs via a receptor-independent mechanism. We report here that ghrelin and obestatin, belonging to the family of basic secretagogues, stimulate mast cells independent of a receptor, and this may play a crucial role at the site of allergy or inflammation.

Rejuvenation of the aging thymus: growth hormone-mediated and ghrelin-mediated signaling pathways

One of the major fundamental causes for the aging of the immune system is the structural and functional involution of the thymus, and the associated decline in de novo naïve T-lymphocyte output. This loss of naïve T-cell production weakens the ability of the adaptive immune system to respond to new antigenic stimuli and eventually leads to a peripheral T-cell bias to the memory phenotype. While the precise mechanisms responsible for age-associated thymic involution remain unknown, a variety of theories have been forwarded including the loss of expression of various growth factors and hormones that influence the lymphoid compartment and promote thymic function. Extensive studies examining two hormones, namely growth hormone (GH) and ghrelin (GRL), have demonstrated their contributions to thymus biology. In the current review, we discuss the literature supporting a role for these hormones in thymic physiology and age-associated thymic involution and their potential use in the restoration of thymic function in aged and immunocompromised individuals.

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 attenuates acute pancreatitis-induced lung injury and inhibits substance P expression

OBJECTIVE

To investigate the effect of ghrelin administration on the severity of acute lung injury and on the production of proinflammatory cytokines and Substance P (SP) in rats with acute pancreatitis (AP).

METHODS

AP was induced in rats by sodium taurocholate injection through pancreaticobiliary duct. Ghrelin 20 nmol/kg was given before and after the treatment. Tumor necrosis factor-alpha, interleukin-1beta, and -6 levels in the serum were measured using the radioimmunoassay method. Morphological signs of lung injury, pulmonary water content, microvascular permeability, and myeloperoxidase activity were measured. Meanwhile, the determination of pulmonary SP mRNA level and its expression were performed by reverse transcriptase polymerase chain reaction and immunohistochemistry.

RESULTS

The serum proinflammatory cytokines, pulmonary water content, microvascular permeability, and myeloperoxidase activity were increased, and morphological damages were observed in the lung of AP rats. SP mRNA level and its expression were significantly higher in sham-operated rats (P < 0.05). Morphological damages were attenuated and serum cytokines and pulmonary parameters were reduced by pre- and posttreatment with ghrelin. Pulmonary SP expression was also significantly down-regulated by ghrelin (P < 0.05).

CONCLUSIONS

Ghrelin attenuates the severity of acute lung injury induced by AP. The reduction of neutrophil sequestration, limitation of proinflammatory cytokines release, and inhibition of pulmonary SP expression may be the mechanisms involved in the therapeutic effect of ghrelin.

Ghrelin inhibits the development of acute pancreatitis and nuclear factor kappaB activation in pancreas and liver

OBJECTIVES

To investigate the influence of ghrelin on the development of severe acute pancreatitis (SAP) and the expression of nuclear factor kappaB (NF-kappaB) p65 in the pancreas and liver.

METHODS

Severe acute pancreatitis was induced in rat by sodium taurocholate injection in the pancreaticobiliary duct. Ghrelin was administrated twice at the dose 10 or 20 nmol/kg per injection, respectively. Then, serum amylase activity; serum tumor necrosis factor alpha, interleukin 1beta, and interleukin 6 concentrations; and morphological signs of pancreatitis and hepatic damage were measured. Meanwhile, determination of pancreatic and hepatic NF-kappaB p65 expression was performed by Western blotting and immunohistochemistry.

RESULTS

The serumal parameters increased, and morphological damages were observed in the pancreas and liver in SAP rats. Nuclear factor kappaB p65 expression was significantly higher in the pancreas and liver than sham-operated rats (P < 0.05). Treatment with ghrelin attenuated the morphological damages, and reduced the serumal parameters. Nuclear factor kappaB p65 expression was also significantly reduced by ghrelin (P < 0.05), both in the pancreas and liver.

CONCLUSIONS

Ghrelin inhibits the development of acute pancreatitis induced by sodium taurocholate. It exerts the therapeutic effects through inhibiting NF-kappaB expression, thereby blocks the inflammatory signal transduction pathway and reduces the release of inflammatory media and cytokines.

Inhibition of thymic adipogenesis by caloric restriction is coupled with reduction in age-related thymic involution

Aging of thymus is characterized by reduction in naive T cell output together with progressive replacement of lymphostromal thymic zones with adipocytes. Determining how calorie restriction (CR), a prolongevity metabolic intervention, regulates thymic aging may allow identification of relevant mechanisms to prevent immunosenescence. Using a mouse model of chronic CR, we found that a reduction in age-related thymic adipogenic mechanism is coupled with maintenance of thymic function. The CR increased cellular density in the thymic cortex and medulla and preserved the epithelial signatures. Interestingly, CR prevented the age-related increase in epithelial-mesenchymal transition (EMT) regulators, FoxC2, and fibroblast-specific protein-1 (FSP-1), together with reduction in lipid-laden thymic fibroblasts. Additionally, CR specifically blocked the age-related elevation of thymic proadipogenic master regulator, peroxisome proliferator activated receptor gamma (PPARgamma), and its upstream activator xanthine-oxidoreductase (XOR). Furthermore, we found that specific inhibition of PPARgamma in thymic stromal cells prevented their adipogenic transformation in an XOR-dependent mechanism. Activation of PPARgamma-driven adipogenesis in OP9-DL1 stromal cells compromised their ability to support T cell development. Conversely, CR-induced reduction in EMT and thymic adipogenesis were coupled with elevated thymic output. Compared with 26-mo-old ad libitum fed mice, the T cells derived from age-matched CR animals displayed greater proliferation and higher IL-2 expression. Furthermore, CR prevented the deterioration of the peripheral TCR repertoire diversity in older animals. Collectively, our findings demonstrate that reducing proadipogenic signaling in thymus via CR may promote thymopoiesis during aging.

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.

Reduction of T cell-derived ghrelin enhances proinflammatory cytokine expression: implications for age-associated increases in inflammation

Ghrelin (Grln) is a peptide hormone that is predominantly produced in the stomach and stimulates appetite and induces growth hormone (GH) release. We have previously reported that ghrelin is also expressed in T cells and exerts prothymic and anti-inflammatory effects. However, the biologic relevance of T cell-derived ghrelin remains to be determined. Here, we report that acylated-bioactive ghrelin is expressed in human T cells and preferentially segregates within the lipid raft domains upon TCR ligation. The RNA interference (RNAi)-mediated down-regulation of ghrelin in primary human T cells activates IkB, and increases Th1 cytokines and IL-17 secretion. Ghrelin expression declines with increasing age in spleen and T cells and exogenous ghrelin administration in old mice reduces proinflammatory cytokines. These findings demonstrate that ghrelin functions in an autocrine and paracrine capacity to regulate proinflammatory cytokine expression in human and murine T cells and may contribute in regulating "inflamm-aging."

Ghrelin promotes thymopoiesis during aging

The decline in adaptive immunity, T lymphocyte output, and the contraction of the TCR repertoire with age is largely attributable to thymic involution. The loss of thymic function with age may be due to diminished numbers of progenitors and the loss of critical cytokines and hormones from the thymic microenvironment. We have previously demonstrated that the orexigenic hormone ghrelin is expressed by immune cells and regulates T cell activation and inflammation. Here we report that ghrelin and ghrelin receptor expression within the thymus diminished with progressive aging. Infusion of ghrelin into 14-month-old mice significantly improved the age-associated changes in thymic architecture and thymocyte numbers, increasing recent thymic emigrants and improving TCR diversity of peripheral T cell subsets. Ghrelin-induced thymopoiesis during aging was associated with enhanced early thymocyte progenitors and bone marrow-derived Lin(-)Sca1(+)cKit(+) cells, while ghrelin- and growth hormone secretagogue receptor-deficient (GHS-R-deficient) mice displayed enhanced age-associated thymic involution. Leptin also enhanced thymopoiesis in aged but not young mice. Our findings demonstrate what we believe to be a novel role for ghrelin and its receptor in thymic biology and suggest a possible therapeutic benefit of harnessing this pathway in the reconstitution of thymic function in immunocompromised subjects.