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

Multiple roles of ghrelin in breast cancer

Breast cancer is one of the most threatening malignant tumors in women worldwide; hence, investigators are continually performing novel research in this field. However, an accurate prediction of its prognosis and postoperative recovery remains difficult. The severity of breast cancer is patient-specific and affected by several health factors; thus, unknown mechanisms may affect its progression. This article analyzes existing literature on breast cancer, ranging from the discovery of ghrelin to its present use, and aims to provide a reference for future research into breast cancer mechanisms and treatment-plan improvement. Various parts of ghrelin have been associated with breast cancer by direct or indirect evidence. The ghrelin system may encompass the direction of expanding breast cancer treatment methods and prognostic indicators. Therefore, we compiled almost all studies on the relationship between the ghrelin system and breast cancer, including unacylated ghrelin, its GHRL gene, ghrelin O-acyltransferase, the receptor growth hormone secretagogue receptor, and several splice variants of ghrelin to lay the foundation for future research.

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.

Ghrelin mediated regulation of neurosynaptic transmitters in depressive disorders

Ghrelin is a peptide released by the endocrine cells of the stomach and the neurons in the arcuate nucleus of the hypothalamus. It modulates both peripheral and central functions. Although ghrelin has emerged as a potent stimulator of growth hormone release and as an orexigenic neuropeptide, the wealth of literature suggests its involvement in the pathophysiology of affective disorders including depression. Ghrelin exhibits a dual role through the advancement and reduction of depressive behavior with nervousness in the experimental animals. It modulates depression-related signals by forming neuronal networks with various neuropeptides and classical neurotransmitter systems. The present review emphasizes the integration and signaling of ghrelin with other neuromodulatory systems concerning depressive disorders. The role of ghrelin in the regulation of neurosynaptic transmission and depressive illnesses implies that the ghrelin system modulation can yield promising antidepressive therapies.

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Ghrelin is the orexigenic type of neuropeptide.

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It binds with the growth hormone secretagogue receptor (GHSR).

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GHSR is ubiquitously present in the various brain regions.

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Ghrelin is involved in the regulation of depression-related behavior.

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The review focuses on the neurotransmission and signaling of ghrelin in neuropsychiatric and depressive disorders.

Ghrelin-Mediated Regeneration and Plasticity After Nervous System Injury

The nervous system is highly vulnerable to different factors which may cause injury followed by an acute or chronic neurodegeneration. Injury involves a loss of extracellular matrix integrity, neuronal circuitry disintegration, and impairment of synaptic activity and plasticity. Application of pleiotropic molecules initiating extracellular matrix reorganization and stimulating neuronal plasticity could prevent propagation of the degeneration into the tissue surrounding the injury. To find an omnipotent therapeutic molecule, however, seems to be a fairly ambitious task, given the complex demands of the regenerating nervous system that need to be fulfilled. Among the vast number of candidates examined so far, the neuropeptide and hormone ghrelin holds within a very promising therapeutic potential with its ability to cross the blood-brain barrier, to balance metabolic processes, and to stimulate neurorepair and neuroactivity. Compared with its well-established systemic effects in treatment of metabolism-related disorders, the therapeutic potential of ghrelin on neuroregeneration upon injury has received lesser appreciation though. Here, we discuss emerging concepts of ghrelin as an omnipotent player unleashing developmentally related molecular cues and morphogenic cascades, which could attenuate and/or counteract acute and chronic neurodegeneration.

Ghrelin and oral diseases

Eating food is one of the most complicated behaviours in mammals, especially humans. The primary function of ghrelin is regulation of the appetite level and its stimulation. It is also responsible for the body's energy balance and glucose homeostasis. Ghrelin has been shown to affect many brain structures, which confirms the presence of ghrelin receptors in the brain. Studies are also conducted to assess the possible role of ghrelin in anxiety states and in memory disorders and motor dysfunctions. Ghrelin has been found in saliva and salivary glands, teeth and gums, and in the taste buds of the tongue epithelium; it is also secreted by mucosal cells and gingival fibroblasts. The presence of ghrelin in developmental enamel, especially in odontoblasts and ameloblasts, may suggest its regulatory role in the development of teeth. Patients with chronic periodontitis have significantly higher concentrations of ghrelin in the peripheral blood serum, as compared to the control group. Ghrelin plays a special role in the proliferation of cancer cells and in the development of neoplastic metastases. The abundant presence of ghrelin receptors in cancer cells is considered an important target in the treatment of neoplasms. Ghrelin is a hormone whose multidirectional mechanism of action has not yet been fully understood. However, its ubiquitous occurrence in the human body and its very diverse participation in metabolic processes may prove to be a significant obstacle in achieving the expected clinical effect of ghrelin as an effective drug in selected disease units.

Can Estradiol and Ghrelin Play a Protective Role in Epithelial Ovarian Cancer Incidence in Postmenopausal Women?

OBJECTIVE

The present study aimed to investigate the association between estradiol, n-octanoylated, des-octanoylated, total ghrelin, and ghrelin/des-octanoylated ghrelin ratio levels along with pathological parameters and epithelial ovarian cancer (EOC) odds in postmenopausal women.

MATERIALS AND METHODS

A case-control study was carried out on 45 patients with EOC and 33 age-matched postmenopausal women as the control group. Plasma levels of estradiol, n-octanoylated, des-octanoylated, and total ghrelin were measured by ELISA method.

RESULTS

Estradiol's plasma levels were significantly higher in patients with EOC than in control women (p <0.001). Although the ratio levels of n-octanoylated, des-octanoylated, total ghrelin, and ghrelin/des-octanoylated ghrelin were not associated with EOC in logistic regression models, estradiol levels were significantly related to the increase in EOC odds (OR: 1.083, 95% CI: 1.037-1.13, p <0.001). However, estradiol levels in the two first quartiles (Q₁, Q₂) were associated with decreased odds of EOC (OR: 0.011, 95% CI: 0.001-0.118, p <0.001, and OR: 0.030, 95% CI: 0.003-0.284, p = 0.002, respectively). For those patients in the third quartile of plasma des-octanoylated and total ghrelin compared to those in the highest (Q4), the multivariate odds ratios of EOC were respectively 0.192 and 0.25.

CONCLUSION

In conclusion, higher concentrations of des-octanoylated and total ghrelin might be associated with the decreased EOC odds. Furthermore, the findings suggest that high levels of estradiol might be a potential odds factor in EOC, however, lower estradiol levels may have a protective effect on EOC development.

Recognition of Invasive Prostate Cancer Using a GHRL Polypeptide Probe Targeting GHSR in a Mouse Model In Vivo

BACKGROUND

Ghrelin (GHRL) is a polypeptide that can specifically bind to the growth hormone secretagogue receptor (GHSR). The expression of GHSR is significantly different in normal and prostate cancer (PC) tissues in humans. It is important to find an effective diagnostic method for the diagnosis and prognosis of invasive PC/neuroendocrine prostate cancer (NEPC).

METHODS

GHRL and GHSR mRNA levels were determined by a quantitative real-time polymerase chain reaction in PC tissues. The expression of GHRL and GHSR proteins was assessed by Western blot assay and immunohistochemistry. A GHRL polypeptide probe was synthesized by standard solid-phase polypeptide synthesis, and labeled with Alexa Fluor 660. Confocal microscopy was used to capture fluorescence images. Living imaging analysis showed tumor areas of different invasiveness in mice models.

RESULTS

The levels of GHRL and GHSR copy number amplification and mRNA expression were increased in invasive PC/NEPC, and the protein expression levels of GHRL and GHSR were similarly increased in NEPC. The GHRL polypeptide probe could effectively bind to GHSR. In PC3 cells, it was found that the GHRL probe specifically binds to GHSR on the cell membrane and accumulates in the cells through internalization after binding. Live imaging in mice models showed that there were different signal intensities in tumor areas with different invasiveness.

CONCLUSION

GHSR and GHRL might be used in molecular imaging diagnosis for invasive PC/NEPC in the future.

Plasma levels of acylated ghrelin in patients with insulinoma and expression of ghrelin and its receptor in insulinomas

BACKGROUND

Insulinoma is a subtype of pancreatic neuroendocrine tumors. Many patients with insulinoma are obese due to frequent food intake. Ghrelin is associated with obesity and blood levels of insulin. It is not clear if plasma levels of ghrelin in insulinoma patients correlate with hyperinsulinemia and obesity. Expression of ghrelin and its receptor has not been well demonstrated in insulinoma.

OBJECTIVE

To study if plasma levels of ghrelin is associated with obesity and hyperinsulinemia or hyperproinsulinemia in patients with insulinoma, and to detect the expression of ghrelin and its receptor in insulinoma.

METHODS

Plasma levels of acylated ghrelin, insulin, and proinsulin were measured in 37 patients with insulinoma and 25 controls by ELISA. Expression of ghrelin and its receptor GHS-R1A was examined in 20 insulinoma and paired pancreatic specimens by immunostaining. P ≤ 0.05 was considered significant.

RESULTS

The plasma levels of acylated ghrelin in patients with insulinoma were significantly lower than that in the controls (median 15 pg/ml vs. 19 pg/ml, respectively, P = 0.016). The reduced plasma levels of acylated ghrelin in patients were significantly correlated with obesity, hyperinsulinemia, and hyperproinsulinemia (P = 0.029 and P = 0.028, respectively). Expression of ghrelin and its receptor GHS-R1A was shown in the majority of insulinoma specimens. The expression of GHS-R1A was positively correlated with ghrelin expression in insulinoma (P = 0.014).

CONCLUSIONS

Plasma levels of acylated ghrelin decreased in patients with insulinoma, probably due to the hyperinsulinemia and obesity in the patients. Expression of both ghrelin and its receptor is common in insulinoma.

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.

Development of a [68Ga]-ghrelin analogue for PET imaging of the ghrelin receptor (GHS-R1a)

The ghrelin receptor is a member of the growth hormone secretagogue receptor (GHS-R) family and is present at low concentrations in tissues such as the brain, kidney, cardiovascular system, and prostate. The ghrelin receptor plays an important role in cellular proliferation, apoptosis, invasion, and migration associated with the progression of many cancers, including prostate, breast, ovarian, testicular, and intestinal carcinomas. Ghrelin, the endogenous ligand, is a 28 amino acid peptide (IC₅₀ = 3.1 nM) known to have poor in vivo stability. Herein, we report the synthesis and evaluation of [Dpr³(octanoyl),Lys¹⁹(Ga-DOTA)]ghrelin(1-19). This new ghrelin analogue has a binding affinity (IC₅₀ = 5.9 nM) comparable to that of natural ghrelin. Preliminary in vivo evaluation shows higher uptake of [Dpr³(octanoyl),Lys¹⁹(⁶⁸Ga-DOTA)]ghrelin(1-19) in HT1080/GHSR-1a xenografts than the non-transfected HT1080 xenografts in NOD-SCID mice, although considerable uptake is observed in the kidneys. This is the first example of ghrelin receptor PET imaging in a xenograft model using a peptide derived directly from the endogenous ligand and serves as motivation for developing more effective ghrelin-based radiopeptides.

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.

In1-ghrelin splicing variant is associated with reduced disease-free survival of breast cancer patients and increases malignancy of breast cancer cells lines

Ghrelin gene generates several variants that regulate multiple pathophysiological functions, including tumor-related processes. In1-ghrelin is a splicing variant that was previously shown to be overexpressed in breast cancer (BCa), where it correlated with proliferation markers; however, its possible association with clinical outcome of BCa patients and underlying mechanisms are still unknown. To address this issue, expression levels and clinical associations of In1-ghrelin were analyzed in a cohort of 117 BCa samples. Additionally, a battery of cellular and molecular assays was implemented using two BCa cell lines (MCF-7 and MDA-MB-231), wherein the role of In1-ghrelin on proliferation, migration, dedifferentiation and signaling pathways was explored. The results generated revealed that high expression of In1-ghrelin in BCa samples was associated with lymph node metastasis and reduced disease-free survival. Indeed, In1-ghrelin overexpression stimulated proliferation and migration in MCF-7 and MDA-MB-231 cells. Similar results were found by treating MDA-MB-231 and MCF-7 with In1-ghrelin-derived peptides. Conversely, In1-ghrelin silencing decreased proliferation and migration capacities of MDA-MB-231. Furthermore, In1-ghrelin (but not ghrelin) overexpression increased the capacity to form mammospheres in both cell lines. These effects could be associated with activation of MAPK-ERK, Jag1/Notch, Wnt/β-catenin and/or TGF-β1 pathways. Altogether, our data indicate that In1-ghrelin could play relevant functional roles in the regulation of BCa development and progression and may provide insights to identify novel biomarkers and new therapeutic approaches for this pathology.

Receptor of ghrelin is expressed in cutaneous neurofibromas of individuals with neurofibromatosis 1

BACKGROUND

Multiple cutaneous neurofibromas are a hallmark of neurofibromatosis 1 (NF1). They begin to appear during puberty and increase in number and volume during pregnancy, suggesting a hormonal influence. Ghrelin is a hormone that acts via growth hormone secretagogue receptor (GHS-R), which is overexpressed in many neoplasms and is involved in tumorigenesis. We aimed to investigate GHS-R expression in NF1 cutaneous neurofibromas and its relationship with tumors volume, and patient's age and gender.

RESULTS

Sample comprised 108 cutaneous neurofibromas (55 large and 53 small tumors) from 55 NF1 individuals. GHS-R expression was investigated by immunohistochemistry in tissue micro and macroarrays and quantified using a digital computer-assisted method. All neurofibromas expressed GHS-R, with a percentage of positive cells ranging from 4.9% to 76.1%. Large neurofibromas expressed more GHS-R than the small ones. The percentage of GHS-R-positive cells and intensity of GHS-R expression were positively correlated with neurofibromas volume. GHS-R expression was more common in female gender.

CONCLUSIONS

GHS-R is expressed in cutaneous neurofibromas. Larger neurofibromas have a higher percentage of positive cells and higher GHS-R intensity. Based on our results we speculate that ghrelin may have an action on the tumorigenesis of cutaneous neurofibromas. Future studies are required to understand the role of ghrelin in the pathogenesis of NF1-associated cutaneous neurofibroma.

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 emerging role of anamorelin hydrochloride in the management of patients with cancer anorexia-cachexia

Cancer cachexia affects many patients with advanced cancer. This multifactorial syndrome, which involves loss of muscle mass and body weight, profoundly affects patients' physical functioning and quality of life. Pharmacologic interventions that target weight loss and also improve patient-reported measures are required. Anamorelin hydrochloride is an oral ghrelin receptor agonist for the treatment of cancer anorexia-cachexia that stimulates release of growth hormone and insulin-like growth factor 1, and improves food intake and body weight. Phase II and III trials have demonstrated that anamorelin increases body muscle and fat composition, and improves patient-reported appetite and quality of life. Anamorelin shows promise as an anabolic agent with benefits maintained over time, without the virilizing side effects of other anabolic medications.

Obesity-associated digestive cancers: A review of mechanisms and interventions

The prevalence of obesity has steadily increased over the past few decades. Previous studies suggest that obesity is an oncogenic factor and that over 20% of all cancers are obesity-related. Among such cancers, digestive system malignancies (including esophageal adenocarcinomas, colorectal cancers, and cancers of the gastric cardia, liver, and pancreas) are reported most frequently. While the 5-year survival rates of cancers of the breast and prostate are 90%, that rate is only 45% for digestive cancers. In this review, the mechanisms of obesity-associated digestive cancers are discussed, with an emphasis on obesity-related gene mutations, insulin and insulin-like growth factor signaling pathways, chronic inflammation, and altered adipokine levels. Evidence that these factors often function interdependently rather than independently in carcinogenesis is presented. Recommended interventions that may reduce the burden of obesity-associated digestive cancers, such as participation in physical activity, diet modulation, and calorie restriction, are also described.

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.

Prognostic Significance of Preoperative and Postoperative Plasma Levels of Ghrelin in Gastric Cancer: 3-Year Survival Study

OBJECTIVES:

We aimed to investigate prognostic effects of plasma levels of ghrelin before and after gastrectomy in gastric cancer (GC).

METHODS:

We followed 81 GC patients up to 3 years in this study. They were candidates for curative gastrectomy with or without neoadjuvant chemotherapy. Plasma levels of total and active ghrelins before and after the operation were assessed. Association of plasma levels of ghrelin with survival were assessed and adjusted for other potential prognostic factors using Cox regression analyses.

RESULTS:

Both total and active ghrelins dropped after gastrectomy (P<0.001 for both). Multiple Cox models revealed worse survival for patients with postoperative total ghrelins below median (hazards ratio (HR)=2.33, 95% confidence interval (CI): 1.01–5.41) or 25th percentile (HR=4.29, 95% CI: 1.48–12.44) compared with patients with higher ghrelin levels. In case of preoperative total ghrelin, patients with either second or third quartiles of plasma ghrelin showed worse survival compared with patients with the lowest quartile (HR=2.67, 95% CI: 1.11–6.38 for second quartile, and HR=2.32, 95% CI: 1.01–5.35 for third quartile vs. the lowest quartile). However, there was no difference between patients with the highest and lowest quartiles (HR=0.78, 95% CI: 0.22–2.73). Similar pattern was observed for preoperative active ghrelin (HR=4.92, 95% CI: 1.80–13.54 for second quartile, and HR=2.87, 95% CI: 1.11–7.38 for third quartile vs. the lowest quartile). Advanced TNM stage (HR=4.88, 95% CI: 1.10–21.77), cachexia (HR=2.99, 95% CI: 1.35–6.63), and receiving no neoadjuvant chemotherapy (HR=2.02, 95% CI: 1.04–3.92) were other poor prognostic factors.

CONCLUSIONS:

Preoperative and postoperative plasma levels of ghrelin could predict survival of GC patients with different patterns. This prognostic effect was independent of stage and cachexia. Measurement of plasma ghrelin in GC patients could complement conventional staging for more precise risk-stratification of the patients. Extrinsic admirations of ghrelin after total gastrectomy has potentials to improve survival of GC patients.

Is there an effect of ghrelin/ghrelin analogs on cancer? A systematic review

Ghrelin is a hormone with multiple physiologic functions, including promotion of growth hormone release, stimulation of appetite and regulation of energy homeostasis. Treatment with ghrelin/ghrelin-receptor agonists is a prospective therapy for disease-related cachexia and malnutrition. In vitro studies have shown high expression of ghrelin in cancer tissue, although its role including its impact in cancer risk and progression has not been established. We performed a systematic literature review to identify peer-reviewed human or animal in vivo original research studies of ghrelin, ghrelin-receptor agonists, or ghrelin genetic variants and the risk, presence, or growth of cancer using structured searches in PubMed database as well as secondary searches of article reference lists, additional reviews and meta-analyses. Overall, 45 (73.8%) of the 61 studies reviewed, including all 11 involving exogenous ghrelin/ghrelin-receptor agonist treatment, reported either a null (no statistically significant difference) or inverse association of ghrelin/ghrelin-receptor agonists or ghrelin genetic variants with cancer risk, presence or growth; 10 (16.7%) studies reported positive associations; and 6 (10.0%) reported both negative or null and positive associations. Differences in serum ghrelin levels in cancer cases vs controls (typically lower) were reported for some but not all cancers. The majority of in vivo studies showed a null or inverse association of ghrelin with risk and progression of most cancers, suggesting that ghrelin/ghrelin-receptor agonist treatment may have a favorable safety profile to use for cancer cachexia. Additional large-scale prospective clinical trials as well as basic bioscientific research are warranted to further evaluate the safety and benefits of ghrelin treatment in patients with cancer.

Ghrelin promotes oral tumor cell proliferation by modifying GLUT1 expression

In our study, ghrelin was investigated with respect to its capacity on proliferative effects and molecular correlations on oral tumor cells. The presence of all molecular components of the ghrelin system, i.e., ghrelin and its receptors, was analyzed and could be detected using real-time PCR and immunohistochemistry. To examine cellular effects caused by ghrelin and to clarify downstream-regulatory mechanisms, two different oral tumor cell lines (BHY and HN) were used in cell culture experiments. Stimulation of either cell line with ghrelin led to a significantly increased proliferation. Signal transduction occurred through phosphorylation of GSK-3β and nuclear translocation of β-catenin. This effect could be inhibited by blocking protein kinase A. Glucose transporter1 (GLUT1), as an important factor for delivering sufficient amounts of glucose to tumor cells having high requirements for this carbohydrate (Warburg effect) was up-regulated by exogenous and endogenous ghrelin. Silencing intracellular ghrelin concentrations using siRNA led to a significant decreased expression of GLUT1 and proliferation. In conclusion, our study describes the role for the appetite-stimulating peptide hormone ghrelin in oral cancer proliferation under the particular aspect of glucose uptake: (1) tumor cells are a source of ghrelin. (2) Ghrelin affects tumor cell proliferation through autocrine and/or paracrine activity. (3) Ghrelin modulates GLUT1 expression and thus indirectly enhances tumor cell proliferation. These findings are of major relevance, because glucose uptake is assumed to be a promising target for cancer treatment.

Ghrelin receptor modulates T helper cells during intestinal inflammation

BACKGROUND

The orexigenic peptide ghrelin has anti-inflammatory properties in colitis, however, the mechanism of action and the immune cells targeted remain still to be elucidated. Here, we assessed the possible effect of ghrelin on T helper (Th) cells in a T cell transfer model of chronic colitis.

METHODS

Disease was induced in the recombination activating gene 1 knockout mice (Rag1(-/-) ) by adoptive transfer of naïve Th cells from ghrelin receptor knockout mice (GRLN-R(-/-) ) or littermate wild-type (WT) mice. The course and severity of colitis was assessed by monitoring body weight, diarrhea score, histological analysis, gene expression, and flow cytometry analysis. The possible effects of ghrelin on Th cell proliferation, polarization, and apoptosis was examined in vitro.

KEY RESULTS

Our data showed that Rag1(-/-) mice injected with GRLN-R(-/-) Th cells displayed increased severity of colitis compared to mice injected with WT Th cells. In addition, Rag1(-/-) mice injected with GRLN-R(-/-) Th cells had significantly higher intestinal inflammation and increased accumulation of Th1 and Th17 cells in the colon. In vitro, ghrelin directly affected proliferation of Th cells and induced apoptosis whereas it did not influence Th cell polarization.

CONCLUSION & INFERENCES

Our observations suggest that ghrelin modulates Th effector cells in the gut controlling proliferation and inducing apoptosis. Our findings further support the use of ghrelin as a novel therapeutic option to treat intestinal inflammatory diseases.

Ghrelin administration suppresses inflammation-associated colorectal carcinogenesis in mice

Ghrelin is a 28-amino-acid peptide that stimulates the release of pituitary growth hormone. Because of its orexigenic effects, ghrelin is being developed as a therapeutic option for postoperative support and treatment of anorexia-cachexia syndrome of cancer patients. However, ghrelin has a multiplicity of physiological functions, and it also affects cell proliferation. Therefore, the effects of ghrelin administration on carcinogenesis and cancer progression in patients susceptible to cancer should be clarified. In this study, we examined the effects of ghrelin on cancer promotion in vivo using murine intestinal carcinogenesis models. Intestinal tumorigenesis was examined to determine the effects of either exogenous ghrelin administration or ghrelin deficiency following deletion of the Ghrl gene. Two murine intestinal tumorigenesis models were used. The first was the azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced inflammation-associated colon carcinogenesis model and the second was the Apc^Min/+ genetic cancer susceptibility model. In AOM/DSS-treated mice, administration of ghrelin significantly suppressed tumor formation in the colon. In contrast, ghrelin administration did not affect the number of intestinal tumors formed in Apc^Min/+ mice. The absence of endogenous ghrelin did not affect the incidence of intestinal tumors in either AOM/DSS-treated mice or Apc^Min/+ mice, though tumor size tended to be larger in Ghrl^−/− colons in the AOM/DSS model. No tumor-promoting effect was observed by ghrelin administration in either tumorigenesis model. In summary, this study provides in vivo experimental evidence for the usefulness of ghrelin administration in the chemoprevention of inflammation-associated colorectal carcinogenesis and may suggest its safety in patients under colitis-associated cancer susceptibility conditions.

Ghrelin induces apoptosis in colon adenocarcinoma cells via proteasome inhibition and autophagy induction

Ghrelin is a metabolism-regulating hormone recently investigated for its role in cancer survival and progression. Controversially, ghrelin may act as either anti-apoptotic or pro-apoptotic factor in different cancer cells, suggesting that the effects are cell type dependent. Limited data are currently available on the effects exerted by ghrelin on intracellular proteolytic pathways in cancer. Both the lysosomal and the proteasomal systems are fundamental in cellular proliferation and apoptosis regulation. With the aim of exploring if the proteasome and autophagy may be possible targets of ghrelin in cancer, we exposed human colorectal adenocarcinoma cells to ghrelin. Preliminary in vitro fluorimetric assays evidenced for the first time a direct inhibition of 20S proteasomes by ghrelin, particularly evident for the trypsin-like activity. Moreover, 1 μM ghrelin induced apoptosis in colorectal adenocarcinoma cells by inhibiting the ubiquitin-proteasome system and by activating autophagy, with p53 having an "interactive" role.

Ghrelin O-acyltransferase (GOAT) is expressed in prostate cancer tissues and cell lines and expression is differentially regulated in vitro by ghrelin

BACKGROUND

Ghrelin is a 28 amino acid peptide hormone that is expressed in the stomach and a range of peripheral tissues, where it frequently acts as an autocrine/paracrine growth factor. Ghrelin is modified by a unique acylation required for it to activate its cognate receptor, the growth hormone secretagogue receptor (GHSR), which mediates many of the actions of ghrelin. Recently, the enzyme responsible for adding the fatty acid residue (octanoyl/acyl group) to the third amino acid of ghrelin, GOAT (ghrelin O-acyltransferase), was identified.

METHODS

We used cell culture, quantitative real-time reverse transcription (RT)-PCR and immunohistochemistry to demonstrate the expression of GOAT in prostate cancer cell lines and tissues from patients. Real-time RT-PCR was used to demonstrate the expression of prohormone convertase (PC)1/3, PC2 and furin in prostate cancer cell lines. Prostate-derived cell lines were treated with ghrelin and desacyl ghrelin and the effect on GOAT expression was measured using quantitative RT-PCR.

RESULTS

We have demonstrated that GOAT mRNA and protein are expressed in the normal prostate and human prostate cancer tissue samples. The RWPE-1 and RWPE-2 normal prostate-derived cell lines and the LNCaP, DU145, and PC3 prostate cancer cell lines express GOAT and at least one other enzyme that is necessary to produce mature, acylated ghrelin from proghrelin (PC1/3, PC2 or furin). Finally, ghrelin, but not desacyl ghrelin (unacylated ghrelin), can directly regulate the expression of GOAT in the RWPE-1 normal prostate derived cell line and the PC3 prostate cancer cell line. Ghrelin treatment (100nM) for 6 hours significantly decreased GOAT mRNA expression two-fold (P < 0.05) in the PC3 prostate cancer cell line, however, ghrelin did not regulate GOAT expression in the DU145 and LNCaP prostate cancer cell lines.

CONCLUSIONS

This study demonstrates that GOAT is expressed in prostate cancer specimens and cell lines. Ghrelin regulates GOAT expression, however, this is likely to be cell-type specific. The expression of GOAT in prostate cancer supports the hypothesis that the ghrelin axis has autocrine/paracrine roles. We propose that the RWPE-1 prostate cell line and the PC3 prostate cancer cell line may be useful for investigating GOAT regulation and function.

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 receptor as a novel imaging target for prostatic neoplasms

BACKGROUND

Ghrelin is a natural growth hormone secretagogue (GHS) that is co-expressed with its receptor GHSR in human prostate cancer (PCa) cells. Imaging probes that target receptors for ghrelin may delineate PCas from benign disease. The specificity of a novel ghrelin-imaging probe for PCa over normal tissue or benign disease was assessed.

METHODS

A fluorescein-bearing ghrelin analogue was synthesized (fluorescein-ghrelin(1-18)), and its application for imaging was evaluated in a panel of PCa cell lines and human prostate tissue. Prostate core biopsy samples were collected from fresh surgery specimens of 13 patients undergoing radical prostatectomy. Ghrelin probe signal was detected and quantified in each sample using a hapten amplification technique and associated with pathological features.

RESULTS

The ghrelin probe was taken up by GHSR-expressing LNCaP and PC-3 cells, and not in BPH cells that express low levels of GHSR. Binding was blocked by competition with excess unlabeled probe. The ghrelin probe signal was 4.7 times higher in PCa compared to benign hyperplasia tissue (P = 0.0027) and normal tissue (P = 0.0093). Furthermore, while the ghrelin probe signal was 1.9-fold higher in PIN compared to benign hyperplasia (P = 0.0022) and normal tissue (P = 0.0047), there was no significant difference in the signal of benign hyperplasia compared to normal tissue.

CONCLUSION

The imaging probe fluorescein-ghrelin(1-18) is specific for PCa, and did not associate significantly with benign hyperplasia or normal prostate tissue. This data suggests that ghrelin analogues may be useful as molecular imaging probes for prostatic neoplasms in both localized and metastatic disease.

Ghrelin signaling in heart remodeling of adult obese mice

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

Ghrelin immunoexpression in pituitary adenomas

Ghrelin, an orexigenic hormone, is normally produced mainly in stomach. In addition, it has been demonstrated in gastric carcinoid tumors and less often in other neuroendocrine tumors. We investigated ghrelin expression by immunohistochemistry (streptavidin-biotin-peroxidase complex method) in the full spectrum of resected pituitary adenoma subtypes. Quantification of staining considered both the frequency of ghrelin-reactive tumor cells as well as their staining intensity. Cytoplasmic ghrelin immunopositivity was identified in several adenoma subtypes. Cellular staining varied considerably. In addition, the intensity of cell staining differed within the same tumor and between adenoma subtypes. The highest scores were noted in GH producing adenomas exposed to long-acting somatostatin analogs. In decreasing order, lower scores were encountered in ACTH adenomas in Cushing disease, silent subtype 3 adenomas, untreated GH adenomas, silent corticotroph adenomas of subtypes 1 and 2, dopamine agonist-treated PRL adenomas, ACTH adenomas in Nelson syndrome, and gonadotroph adenomas. No significant immunoreactivity was noted in TSH, untreated PRL, and null cell adenomas. The high immunoexpression of ghrelin in GH adenomas exposed to long-acting somatostatin analogs remains unexplained, but may be due to either increased ghrelin production or to suppression of its release. Based on our findings, it appears that ghrelin immunopositivity does not serve as a biomarker of biologic behavior, prognosis and therapeutic responsiveness in pituitary adenomas.

A novel human ghrelin variant (In1-ghrelin) and ghrelin-O-acyltransferase are overexpressed in breast cancer: potential pathophysiological relevance

The human ghrelin gene, which encodes the ghrelin and obestatin peptides, contains 5 exons (Ex), with Ex1-Ex4 encoding a 117 amino-acid (aa) preproprotein that is known to be processed to yield a 28-aa (ghrelin) and/or a 23-aa (obestatin) mature peptides, which possess biological activities in multiple tissues. However, the ghrelin gene also encodes additional peptides through alternative splicing or post-translational modifications. Indeed, we previously identified a spliced mRNA ghrelin variant in mouse (In2-ghrelin-variant), which is regulated in a tissue-dependent manner by metabolic status and may thus be of biological relevance. Here, we have characterized a new human ghrelin variant that contains Ex0-1, intron (In) 1, and Ex2 and lacks Ex3-4. This human In1-ghrelin variant would encode a new prepropeptide that conserves the first 12aa of native-ghrelin (including the Ser3-potential octanoylation site) but has a different C-terminal tail. Expression of In1-variant was detected in 22 human tissues and its levels were positively correlated with those of ghrelin-O-acyltransferase (GOAT; p = 0.0001) but not with native-ghrelin expression, suggesting that In1-ghrelin could be a primary substrate for GOAT in human tissues. Interestingly, levels of In1-ghrelin variant expression in breast cancer samples were 8-times higher than those of normal mammary tissue, and showed a strong correlation in breast tumors with GOAT (p = 0.0001), ghrelin receptor-type 1b (GHSR1b; p = 0.049) and cyclin-D3 (a cell-cycle inducer/proliferation marker; p = 0.009), but not with native-ghrelin or GHSR1a expression. Interestingly, In1-ghrelin variant overexpression increased basal proliferation of MDA-MB-231 breast cancer cells. Taken together, our results provide evidence that In1-ghrelin is a novel element of the ghrelin family with a potential pathophysiological role in breast cancer.

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 axis genes, peptides and receptors: recent findings and future challenges

The ghrelin axis consists of the gene products of the ghrelin gene (GHRL), and their receptors, including the classical ghrelin receptor GHSR. While it is well-known that the ghrelin gene encodes the 28 amino acid ghrelin peptide hormone, it is now also clear that the locus encodes a range of other bioactive molecules, including novel peptides and non-coding RNAs. For many of these molecules, the physiological functions and cognate receptor(s) remain to be determined. Emerging research techniques, including proteogenomics, are likely to reveal further ghrelin axis-derived molecules. Studies of the role of ghrelin axis genes, peptides and receptors, therefore, promises to be a fruitful area of basic and clinical research in years to come.

Ghrelin inhibits insulin resistance induced by glucotoxicity and lipotoxicity in cardiomyocyte

Ghrelin has wide effects on cardiovascular and endocrine system. The aims of this study are to investigate the direct damage effect of high glucose and high palmitate on cardiomyocyte, and to study the effect of ghrelin on insulin resistance induced by glucotoxicity/lipotoxicity in cardiomyocyte and the possible mechanism underlying the cardioprotective activities of ghrelin. The changes of [(3)H]-2-deoxy-d-glucose ((3)H-G) intake rates were detected by isotope tracer method and the gene expressions in insulin signal transduction pathway were detected by real-time PCR and Western blot assay. The (3)H-G intake rate significantly reduced in high glucose (25mmol/l) or high palmitate (0.5mmol/l) treated primary rat ventricular myocytes. After the treatment of ghrelin (10(-7)mol/l), the (3)H-G intake rate recovered to the normal level. In addition, the phosphorylation of AKT occurred in 10min and was the highest in 30min after the stimulation with ghrelin, which can be blocked by phosphoinositide 3-kinase (PI3K) inhibitor, LY2940002. Ghrelin also increased the mRNA levels of glucose transporter 4 (GLUT4), peroxisome proliferators (PPARr) and AMP activated protein kinase (AMPK) genes in insulin signal transduction pathway. These results indicate that the direct damage of high glucose and high palmitate on cardiomyocyte might be through insulin resistance (IR). Ghrelin can inhibit gluco/lipotoxicity induced insulin resistance by PI3K/AKT pathway. This may provide a clue for therapy for myocardial disease in diabetes mellitus.

Polymorphisms of genes coding for ghrelin and its receptor in relation to colorectal cancer risk: a two-step gene-wide case-control study

Background

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHSR), has two major functions: the stimulation of the growth hormone production and the stimulation of food intake. Accumulating evidence also indicates a role of ghrelin in cancer development.

Methods

We conducted a case-control study to examine the association of common genetic variants in the genes coding for ghrelin (GHRL) and its receptor (GHSR) with colorectal cancer risk. Pairwise tagging was used to select the 11 polymorphisms included in the study. The selected polymorphisms were genotyped in 680 cases and 593 controls from the Czech Republic.

Results

We found two SNPs associated with lower risk of colorectal cancer, namely SNPs rs27647 and rs35683. We replicated the two hits, in additional 569 cases and 726 controls from Germany.

Conclusion

A joint analysis of the two populations indicated that the T allele of rs27647 SNP exerted a protective borderline effect (P_trend = 0.004).

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 in the regulation of body weight and metabolism

Ghrelin, a peptide hormone predominantly produced by the stomach, was isolated as the endogenous ligand for the growth hormone secretagogue receptor. Ghrelin is a potent stimulator of growth hormone (GH) secretion and is the only circulatory hormone known to potently enhance feeding and weight gain and to regulate energy homeostasis following central and systemic administration. Therapeutic intervention with ghrelin in catabolic situations may induce a combination of enhanced food intake, increased gastric emptying and nutrient storage, coupled with an increase in GH thereby linking nutrient partitioning with growth and repair processes. These qualities have fostered the idea that ghrelin-based compounds may have therapeutic utility in treating malnutrition and wasting induced by various sub-acute and chronic disorders. Conversely, compounds that inhibit ghrelin action may be useful for the prevention or treatment of metabolic syndrome components such as obesity, impaired lipid metabolism or insulin resistance. In recent years, the effects of ghrelin on glucose homeostasis, memory function and gastrointestinal motility have attracted considerable amount of attention and revealed novel therapeutic targets in treating a wide range of pathologic conditions. Furthermore, discovery of ghrelin O-acyltransferase has also opened new research opportunities that could lead to major understanding of ghrelin physiology. This review summarizes the current knowledge on ghrelin synthesis, secretion, mechanism of action and biological functions with an additional focus on potential for ghrelin-based pharmacotherapies.

Fluorine and rhenium substituted ghrelin analogues as potential imaging probes for the growth hormone secretagogue receptor

In our effort to create imaging probes targeting the growth hormone secretagogue receptor (GHSR), we now report on the design and synthesis of fluorine and rhenium containing ghrelin analogues through modification of the n-octanoyl Ser-3 side chain. Fluorine analogues were designed whereby the fluorine atom is situated at the terminus of an aliphatic chain using diaminopropionic acid (Dpr) as residue-3. Truncated ghrelin(1-5) and ghrelin(1-14) fluorine-bearing analogues were prepared, the best of which had a 28 nM IC(50) for GHSR. Ghrelin(1-14) analogues were also prepared containing rhenium, as a surrogate metal for technetium-99m, with a cyclopentadienylrhenium tricarbonyl being situated at the terminus of the residue-3 side chain, yielding compounds the best of which had a 35 nM IC(50). This represents a rare case of incorporating rhenium into a peptide structure where the metal complex is required for biological activity. These fluorine and rhenium derivatives demonstrate the ability to modify the Ser-3 side chain of ghrelin in order to create imaging probes for the GHSR.

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.

Ghrelin is dispensable for embryonic pancreatic islet development and differentiation

Ghrelin is a peptide hormone that has been implicated in the regulation of food intake and energy homeostasis. Ghrelin is predominantly produced in the stomach, but is also expressed in many other tissues where its functions are not well characterized. In the rodent and human pancreas, ghrelin levels peak at late gestation and gradually decline postnatally. Several studies have suggested that ghrelin regulates beta cell function during embryonic development and in the adult. In addition, in a number of mouse models, ghrelin cells appear to replace insulin- and glucagon-producing cells in the islet. In this analysis, we investigated whether the absence or overexpression of ghrelin influenced the development and differentiation of the pancreatic islet during embryonic development. These studies revealed that ghrelin is dispensable for normal pancreas development during gestation. Conversely, we demonstrated that elevated ghrelin in the Nkx2.2 null islets is not responsible for the absence of insulin- and glucagon-producing cells. Finally, we have also determined that in the absence of insulin, ghrelin cells form in their normal numbers and ghrelin is expressed at wild type levels.

The constitutive activity of the ghrelin receptor attenuates apoptosis via a protein kinase C-dependent pathway

The ghrelin receptor (GHS-R1a) displays a high level of constitutive signaling through a phospholipase C/protein kinase C-dependent pathway. Therefore, we have investigated the role of agonist-dependent and agonist-independent signaling of GHS-R1a in apoptosis using the seabream GHS-R1a stably expressed in human embryonic kidney 293 cells (HEK-sbGHS-R1a cells). Cadmium-induced activation of caspase-3 was significantly attenuated in HEK-sbGHS-R1a cells compared to wild-type HEK293 cells, while the apoptotic responses to the protein kinase C inhibitor staurosporine were similar. GHS-R1a ligands had no effect on caspase-3 activation or on cell proliferation. Concentrations of the inverse agonist [d-Arg(1),d-Phe(5),d-Trp(7,9),Leu(11)]-substance P sufficient to inhibit constitutive inositol phosphate generation did not enhance caspase-3 activity, suggesting a possible role of phosphatidylcholine-specific phospholipase C in the anti-apoptotic activity of GHS-R1a. In conclusion, our data suggests that the constitutive activity of sbGHS-R1a may be sufficient alone to attenuate apoptosis via a protein kinase C-dependent pathway.

The role of ghrelin and ghrelin analogues in wasting disease

PURPOSE OF REVIEW

The purpose of this review is to summarize recent studies that investigated the role of ghrelin and ghrelin analogs in wasting conditions.

RECENT FINDINGS

Numerous studies have demonstrated potential beneficial effects exerted by ghrelin in a number of diseases associated with wasting. Besides ghrelin's orexigenic effect, anabolic as well as anti-inflammatory activity mediated by ghrelin have been investigated in wasting conditions such as cancer, diabetes mellitus, malabsorptive diseases, chronic obstructive pulmonary disease, anorexia nervosa, renal failure, liver failure, and chronic heart failure. Encouraging results have been obtained from experimental studies and a few clinical trials using subcutaneous administration of ghrelin and ghrelin agonists in cachexia. In-vitro studies have shown ghrelin and des-acyl ghrelin biological activities on proliferation of tumor cells and abnormal tissues.

SUMMARY

The recent studies support the possible positive effects of ghrelin in therapeutic approaches and adjunct treatment of a number of diseases associated with wasting. Utilization of agonists of the ghrelin receptor growth hormone secretagogue-1a is a promising approach for clinical use. Randomized and placebo-controlled studies, including large number of patients are further required.