The endogenous growth hormone secretagogue (ghrelin) is synthesized and secreted by chondrocytes

Decreased memory for novel object recognition in chronically food-restricted mice is reversed by acute ghrelin administration

It has been demonstrated, in normal and aged rats and mice, that acute i.c.v. ghrelin (Ghr) administration increases memory retention. In order to evaluate if this treatment, restores memory retention in animals exhibiting impaired memory, in the present work we selected a chronic food restriction mouse model (since undernutrition prejudices higher nervous functions). We employed adult female mice with 28 days of 50% food restriction and evaluated: a) behavioral performance using novel object recognition test for memory, and plus maze for anxiety-like behavior, b) some morphometric parameters as body and hepatic weights and c) plasma Ghr levels. The animals with 50% food restriction showed an increase in plasma Ghr levels and a decrease in morphometric parameters and in the percentage of novel object recognition time. When the peptide was i.c.v. injected in food-restricted animals (0.03, 0.3 or 3.0 nmol/microl), memory increases in relation to food-restricted mice injected with vehicle, reaching a performance similar to controls.

The influence of ghrelin, adiponectin, and leptin on bone mineral density in healthy postmenopausal women

The association of body fat mass (FM) with bone mineral mass (BMC) and bone mineral density (BMD) has been attributed to a mechanical load exerted on the skeleton by FM and by the effect of different hormones. The aim of the present study was to determine whether there is a relationship between ghrelin, adiponectin, and leptin with BMC and BMD in healthy postmenopausal women (n = 88; age, 68.9 +/- 6.8 years; body mass index, 27.4 +/- 3.6 kg/m(2)). Body composition, BMC, and BMD were derived by dual-energy X-ray absorptiometry. Waist-to-hip (WHR) and waist-to-thigh (WTR) ratios were also obtained. Ghrelin was associated with total BMC (beta = -0.945; P = 0.0001), total BMD (beta = -0.959; P = 0.0001), lumbar spine BMD (beta = -0.945; P = 0.0001), and femoral neck BMD (beta = -0.957; P = 0.0001), and remained associated (P < 0.041) in different analyses that controlled for measured body composition and hormonal and insulin resistance values. However, the associations between ghrelin and measured bone mineral values were no longer significant (P > 0.149) when adjusted for body fat distribution values (WHR, WTR). Adiponectin was significantly related to total BMC (beta = -0.931; P = 0.0001), total BMD (beta = -0.940; P = 0.0001), lumbar spine BMD (beta = -0.937; P = 0.0001), and femoral neck BMD (beta = -0.940; P = 0.0001) values, and these relationships remained significant (P < 0.019) after adjusting for measured body fat, hormonal, and insulin resistance values but not when adjusted for fat-free mass (FFM; P > 0.106). In addition, significant associations of leptin with total BMC (beta = 0.912; P = 0.0001), total BMD (beta = 0.907; P = 0.0001), lumbar spine BMD (beta = 0.899; P = 0.0001), and femoral neck BMD (beta = 0.906; P = 0.0001) were found. These associations remained significant (P < 0.010) in different analyses that controlled for hormonal and insulin resistance values, but the associations between leptin and bone mineral values were no longer significant (P > 0.145) when adjusted for specific body composition values (WHR, WTR, FM, and FFM). In conclusion, it appears that the influence of plasma ghrelin, adiponectin, and leptin levels on BMC and BMD values is mediated or confounded by the specific body composition parameters in healthy postmenopausal women.

The ghrelin system in acinar cells: localization, expression, and regulation in the exocrine pancreas

OBJECTIVES

Ghrelin and its receptor are expressed abundantly in the stomach and pituitary. Recently, a ghrelin system, consisting of both ligand and receptor, has also been found to exist in the endocrine cells of pancreatic islets. This ghrelin system may play a role in regulating insulin secretion and glucose homeostasis. The aim of the present study was to investigate whether a functional ghrelin system also exists in the exocrine pancreas.

METHODS

Precise localization and expression of ghrelin and its receptor in rat pancreatic acinar cells were examined by immunocytochemistry and Western blot, whereas messenger RNA levels were examined by semiquantitative reverse transcription-polymerase chain reaction. The roles of physiological and pathophysiological conditions, such as gastric acid inhibition, starvation, and acute pancreatitis, in regulation of ghrelin and its receptor were also examined.

RESULTS

Both ghrelin and its receptor were detected, at both protein and messenger RNA levels, in the acinar cells of the exocrine pancreas. Ghrelin receptor expression was up-regulated by gastric acid inhibition and down-regulated by acute pancreatitis, whereas levels remained unchanged after food deprivation. In contrast, ghrelin expression did not exhibit significant changes in any condition.

CONCLUSIONS

Our data indicate that a ghrelin system exists in the acinar cells of the exocrine pancreas. This system is subject to regulation by physiological and pathophysiological stimuli and may thus regulate exocrine functions by paracrine and/or autocrine mechanisms.

Response of plasma CNP forms to acute anabolic and catabolic interventions in growing lambs

Using a novel marker of C-type natriuretic peptide (CNP) synthesis [amino-terminal pro-CNP (NT-proCNP)], we have recently shown that plasma NT-proCNP is strongly correlated with skeletal growth and markers of bone formation and is reversibly reduced by glucocorticoids. The effects on CNP of other catabolic or anabolic factors, known to affect skeletal growth, are unknown. Accordingly, we have studied the response of plasma CNP forms to acute catabolic (caloric restriction) and anabolic [growth hormone (GH) stimulation] interventions in lambs and related the findings to circulating IGF-I levels, growth velocity, and markers of bone formation. Lambs fed a reduced caloric intake (25% of normal) for 6 days exhibited reduced live weight, plasma urea, and IGF-I (P < 0.001 for all) compared with control lambs. Basal levels of NT-proCNP (40.1 +/- 0.9 pmol/l) fell promptly to a nadir (28.1 +/- 0.8 pmol/l, P < 0.001) on day 6, returning rapidly to basal levels upon refeeding. Although plasma alkaline phosphatase (ALP) fell (P < 0.001), reductions in metacarpal growth velocity were not significant within the 12-day period of study. In contrast to caloric restriction, long-acting bovine recombinant GH (2.5 mg/kg on days 0 and 6), as expected, increased plasma IGF-I more than twofold above control for 12 days (P < 0.001). Growth velocity did not differ during the 30 days of observation, and, consistent with unchanged growth velocity, plasma NT-proCNP and ALP were also unaffected. In conclusion, CNP synthesis and markers of bone formation are acutely sensitive to catabolism but unaffected by doses of GH that fail to stimulate skeletal growth.

Contractile effect of ghrelin on isolated guinea-pig renal arteries

Ghrelin, a 28-amino acid peptide, known to exist in both acylated and des-acylated varieties, was identified as the first endogenous ligand of growth hormone secretagogue receptor in 1999. Various arteries are known to express ghrelin receptors, but the direct action of ghrelin on blood vessels has been unclear. In the present study we show that ghrelin concentration-dependently potentiates endothelin-1 (ET-1) induced tension development of guinea-pig renal artery, as measured using a wire-type isometric myography of vascular segments. In vascular smooth muscle cells (SMC) ghrelin caused activation of potassium outward currents via phospholipase C (PLC)-->inositol-1,4,5-trisphosphate (IP3) and PLC-->protein kinase C (PKC) signalling cascade, resulting in hyperpolarizaton of the cell membrane. On a tissue level ghrelin by itself had no effect on isometric tone, but augmented ET-1 induced contraction by a mechanism, involving PLC, Rho-kinase and intracellular IP3 -sensitive Ca2+ release, and not nucleotide-sensitive protein kinases or PKC. Together with our previous findings the data in this study suggest that ghrelin exerts its contractile activity on guinea-pig renal artery by facilitation of ET-1 triggered intracellular signalling in SMC, and/or by stimulating the release of a yet unknown contractile mediator from endothelium.

Exercise-induced suppression of acylated ghrelin in humans

Ghrelin is an orexigenic hormone secreted from endocrine cells in the stomach and other tissues. Acylation of ghrelin is essential for appetite regulation. Vigorous exercise induces appetite suppression, but this does not appear to be related to suppressed concentrations of total ghrelin. This study examined the effect of exercise and feeding on plasma acylated ghrelin and appetite. Nine male subjects aged 19-25 yr participated in two, 9-h trials (exercise and control) in a random crossover design. Trials began at 0800 in the morning after an overnight fast. In the exercise trial, subjects ran for 60 min at 72% of maximum oxygen uptake between 0800 and 0900. After this, they rested for 8 h and consumed a test meal at 1100. In the control trial, subjects rested for 9 h and consumed a test meal at 1100. Area under the curve values for plasma acylated ghrelin concentration (assessed from venous blood samples) were lower over the first 3 h and the full 9 h of the exercise trial compared with the control trial: 317+/-135 vs. 510+/-186 pg.ml(-1).3 h and 917+/-342 vs. 1,401+/-521 pg.ml(-1).9 h (means+/-SE) respectively (P<0.05). Area under the curve values for hunger (assessed using a visual scale) were lower over the first 3 h of the exercise trial compared with the control trial (P=0.013). These findings demonstrate that plasma acylated ghrelin concentration and hunger are suppressed during running.

Physiological, pathological and potential therapeutic roles of ghrelin

Ghrelin, a hormone that is produced mainly by the stomach, was identified originally as the endogenous ligand of the growth hormone secretagogue (GHS) receptor. Ghrelin might also be synthesized in other organs, where it might have autocrine or paracrine effects. GHS receptors are present in tissues other than the hypothalamus and pituitary, which indicates that ghrelin has other effects in addition to stimulating the release of growth hormone. Recently, it has been suggested that ghrelin might be involved in the pathogenesis of many diseases and be a therapeutic target in these diseases. Here, we provide an overview of the physiological effects of ghrelin and of its pathological and potential therapeutic roles.

Osteoprotegerin and RANKL serum levels and their relationship with serum ghrelin in children with chronic renal failure and on dialysis

BACKGROUND

Osteoprotegerin (OPG) and receptor activator of the nuclear factor kappaB ligand (RANKL) constitute a complex system of mediators involved in the regulation of bone resorption process. Ghrelin, a growth hormone secretagogue, has been shown to modulate proliferation and differentiation of osteoblasts. The present study was carried out to evaluate the serum concentrations of OPG and sRANKL in children with chronic renal impairment (CRI) and on dialysis, and to establish a possible relationship between their serum levels and that of ghrelin.

METHODS

33 patients including 10 patients with CRI, 12 peritoneal dialysis (PD) and 11 hemodialysis (HD) patients and 22 healthy controls were enrolled into the study. OPG, sRANKL and ghrelin levels were studied with radioimmunoassay.

RESULTS

Serum OPG levels in CRI, PD and HD groups were significantly higher than the healthy controls (p = 0.002, p < 0.001, p < 0.001, respectively) whereas sRANKL levels were significantly lower than the healthy controls (p = 0.03, p = 0.01, p = 0.001, respectively). Ghrelin levels were significantly higher in CRI, PD and HD groups compared to healthy controls (p = 0.001, p < 0.001, p < 0.001, respectively). We observed a negative correlation between the sRANKL and OPG levels (r = -0.27, p = 0.04) as well as between sRANKL and ghrelin levels (r = -0.31, p = 0.02). OPG levels showed a positive correlation with ghrelin levels (r = 0.63, p < 0.001).

CONCLUSION

We found a lower RANKL bioactivity index in children with CRI and on dialysis. The mechanism and the role of elevated OPG and low sRANKL in uremia are unclear, but they might partly represent a compensatory mechanism to the negative balance of bone remodeling in renal bone disease in children. Additionally, we demonstrated for the first time that ghrelin and the RANKL/OPG system have a close relationship in CRF. Therefore, ghrelin may be of importance in mediating the effects of the RANKL/OPG system in renal bone disease.

Ghrelin, the peripheral hunger hormone

In the current review we summarize the available data concerning the gastric hormone ghrelin and its receptor. Ghrelin stimulates short-term food intake and long-term body weight regulation via its adipogenic and diabetogenic effects. Ghrelin stimulates gastric emptying, and these effects could be explored from a therapeutic point of view. Ghrelin levels change profoundly in anorexia, in states of insulin resistance, in obesity, and after bariatric surgery, suggesting that this is an important hormone in body weight regulation.

Glucocorticoid inhibition of growth in rats: partial reversal with the full-length ghrelin analog BIM-28125

Glucocorticoids are important immunosuppressive hormones; these steroids also inhibit somatic growth by decreased growth hormone (GH) secretion and induced protein catabolism. The ability of ghrelin, the endogenous ligand for the GHS-1a receptor, to increase body weight is attributed to a combination of enhanced food intake, increased gastric emptying and increased food assimilation, coupled with potent GH releasing activity. The aim of the present study was to evaluate the ability of a full-length, metabolically stabilized ghrelin agonist, BIM-28125, to reverse the dexamethasone-induced decrease of growth rate of prepubertal Sprague-Dawley male rats. Twenty-one days old rats were randomly assigned to two treatment groups. Beginning on day 23 of age, 16 animals were treated ip either with saline or DEX (40 microg/kg/day). On day 33 after birth, these two groups were further subdivided and treated sc with either vehicle or BIM-28125 (80 nmol/kg, t.i.d.). On day 47 after birth, rats were killed and trunk blood was collected for hormone determinations. DEX significantly reduced final body weight and nose-anal length; BIM-28125 increased linear growth in saline-treated rats and reversed growth inhibition in DEX-treated rats. The inhibitory effects of DEX on somatic growth was paralleled by decreased 24 h food intake (FI), decreased food efficiency (FE) and lower plasma IGF-1 levels versus vehicle-treated rats. BIM-28125 induced an increase of FI, FE and plasma IGF-1 in saline-treated rats, and reversed the inhibitory effects of DEX. These preclinical results leads to the conclusion that BIM-28125 may represent a good tool to reverse the catabolic effects induced by glucocorticoids.

Unacylated ghrelin is active on the INS-1E rat insulinoma cell line independently of the growth hormone secretagogue receptor type 1a and the corticotropin releasing factor 2 receptor

Both unacylated ghrelin (UAG) and acylated ghrelin (AG) exert metabolic effects. To investigate the interactions between AG and UAG on ghrelin receptors we evaluated the effects of AG and UAG on INS-1E rat insulinoma cells, using insulin secretion after 30min static incubation as a read-out. A possible involvement of the growth hormone secretagogue receptor type 1a (GHS-R1a) or the corticotropin-releasing factor 2 (CRF2) receptor (CRF2R), as a putative receptor for UAG, was also studied determining their mRNA expression and the functional effects of receptor antagonists on insulin release. Both UAG and AG stimulated insulin release dose-dependently in the nanomolar range. The AG-induced insulin output was antagonized by two GHS-R1a antagonists ([d-Lys(3)]GHRP-6 and BIM28163), which did not block UAG actions. These effects occurred in the presence of low levels of GHS-R1a mRNA. Neither CRF2R expression nor effects of the CRF2R antagonist (astressin(2)B) on insulin output were observed. In conclusion, we provide a sensitive and reproducible assay for specific effects of UAG, which in this study is responsible for insulin release by INS-1E cells. Our data support the existence of a specific receptor for UAG, other than the CRF2R and GHS-R1a. The stimulatory effect on insulin secretion by AG in this cell line is mediated by the GHS-R1a.

Cell biology of the ghrelin receptor

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

Ghrelin, the same peptide for different functions: player or bystander?

Ghrelin is a recently discovered brain-gut peptide with two main physiological actions: growth hormone secretagogue activity and food intake inducer. Although its production is prevalently gastric, ghrelin is widely expressed in several tissues, where it might therefore act as a paracrine or autocrine factor. It is becoming clear that ghrelin is much more than a simple growth hormone secretagogue. In addition to its formerly envisaged role, ghrelin has other activities including stimulation of pituitary hormones secretion, modulation of food intake and control of energy metabolism, regulation of gastric and pancreatic activity, and cardiovascular and hemodynamic activities. In addition, modulation of cartilage and bone homeostasis, sleep and behavioral influences, and modulation of the immune system, as well as effects on cell proliferation, are other relevant actions of ghrelin. Thus, the peptide appears to be an important component of an integrated multifaceted regulatory system. In this review, we summarize several aspects of ghrelin biology and attempt to inform the reader with information regarding unexpected functions of this gastric peptide.

Ghrelin and bone metabolism in adolescent girls with anorexia nervosa and healthy adolescents

CONTEXT

Anorexia nervosa (AN) in adolescents is associated with low bone mineral density (BMD) and increases in ghrelin secretion, an orexigenic GH secretagogue that stimulates osteoblast proliferation in vitro.

OBJECTIVE

We hypothesized that ghrelin may have independent effects on bone in AN adolescents. STUDY DESIGN, SUBJECTS, AND OUTCOME MEASURES: Frequent sampling was performed overnight every 30 min for 12 h in 23 adolescent AN girls aged 12-18 yr and 21 controls of comparable maturity. Ghrelin, leptin, cortisol, and GH secretion were examined using Cluster and deconvolution. We measured BMD and body composition (dual-energy x-ray absorptiometry) and carboxy-terminal peptide of type I procollagen and N-telopeptide levels.

RESULTS

In healthy adolescents, ghrelin secretion strongly predicted BMD; secretory burst mass being the strongest predictor of lumbar spine (LS) bone mineral apparent density (BMAD) (r = 0.66, P = 0.003), LS BMAD z-scores (BMAD-z) (r = 0.59, P = 0.01), hip BMD (r = 0.55, P = 0.02), and hip BMD-z (r = 0.52, P = 0.03). When body composition measures (body mass index, lean and fat mass), and hormonal predictors (GH, IGF-I, cortisol, leptin, and estradiol) were entered into a regression model with ghrelin secretion to determine independent BMD predictors, ghrelin was the strongest predictor of LS BMAD, BMAD-z, hip BMD, and hip BMD-z, contributing to 43, 30, 26, and 19% of the variability, respectively, independent of GH or cortisol effects. Conversely, in AN, ghrelin secretion did not predict LS BMAD or hip-z and weakly predicted LS BMAD-z and hip BMD. Ghrelin did not predict carboxy-terminal peptide of type I procollagen or N-telopeptide/creatinine, which were predicted by GH and cortisol.

CONCLUSION

Ghrelin secretion predicts bone density independent of body composition, the GH-IGF-I axis, cortisol, or estradiol in healthy girls but not in those with AN.