The negative association between total ghrelin levels, body mass and insulin secretion is lost in hypercortisolemic patients with Cushing's disease

Ghrelin

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

Advances in the epidemiology, pathogenesis, and management of Cushing's syndrome complications

Cushing's syndrome (CS) is a clinical condition resulting from chronic exposure to glucocorticoid excess. As a consequence, hypercortisolism contributes significantly to the early development of systemic disorders by direct and/or indirect effects. Complications such as obesity, hypertension, diabetes, dyslipidemia, and hypercoagulability cause premature atherosclerosis and increase cardiovascular mortality. Impairment of the skeletal system is a relevant cause of morbidity and disability in these patients especially due to the high prevalence of vertebral fractures. In addition, muscle weakness, emotional lability, depression, and impairment of quality of life are very common. Clinical management of these patients is complex and should be particularly careful in identifying global cardiovascular risks and aim at controlling all complications. Although the primary goal in the prevention and treatment of complications is the correction of hypercortisolism, treatment does not completely eliminate these comorbidities. Given that cardiovascular risk and fracture risk can persist after cure, early detection of each morbidity could prevent the development of irreversible damage. In this review we present the various complications of CS and their pathogenetic mechanisms. We also suggest the clinical management of these patients based on our extensive clinical experience and on the available literature.

Adipokines and cardiovascular risk in Cushing's syndrome

Cushing's syndrome (CS) is associated with increased cardiovascular morbidity and mortality. Recent evidence also suggests that increased cardiovascular risk may persist even after long-term remission of CS. Increased central obesity, a typical feature of CS, is associated with altered production of adipokines, which contributes to the pathogenesis of several metabolic and cardiovascular complications observed in this condition. In vitro and in vivo studies have shown a relationship between cortisol and adipokines in several experimental settings. In patients with either active or 'cured' CS, an increase in leptin and resistin levels as well as the release of pro-inflammatory cytokines, such as tumor necrosis factor-α and interleukin-6, may be associated with increased cardiovascular risk. For other adipokines, including adiponectin, results are inconclusive. Studies are needed to further elucidate the interactions between clinical and subclinical increases in cortisol production and altered adipokine release in CS.

Changes in total ghrelin within the somatotropic axis in severe burn patients: comparison of those with inhalation injury and those without inhalation injury

OBJECTIVE

The aims of this study were to clarify changes in total ghrelin within the somatotropic axis in severe burn subjects with or without inhalation injury as well as the responsiveness of GH, IGF-1, and IGFBP-3 to the different severity of burn injuries.

DESIGN

Twenty-three patients with severe burn injuries (>30% of 2nd degree burns or >10% of 3rd degree burns) were classified into 2 groups according to inhalation injury: group I with inhalation injury (n=9) and group II without inhalation injury (n=14). The evaluations of serum GH, IGF-1, IGFBP-3, and total ghrelin were done on post-burn injury days 3, 7, 14, 21, and 40. Cortisol levels were measured from 24-h urine collections on post-burn injury days 7 and 21.

RESULTS

In all subjects, the levels of GH fluctuated throughout the observation period whereas IGF-1 showed an initial decline with nadir on day 7 and a subsequent increase through day 40. The levels of IGFBP-3 and total ghrelin showed a progressive increase with nadir on day 3. Compared with the group II, the GH levels were increased in the group I on post-burn days 3, 7, and 14, of which day 7 showed statistical significance (p<0.05). The levels of IGF-1 (days 7 and 21; p<0.05) and IGFBP-3 (days 7, 14, 21, and 40; p<0.05, p<0.01, p<0.05, p<0.05, respectively) were lower in the group I than in the group II throughout the study period. On post-burn injury days 3, 7, 14, and 21, total ghrelin levels were lower in the group I than in the group II with statistical significance on post-burn day 7 (p<0.001).

CONCLUSIONS

Our present data show a concurrence of elevated GH levels and decreased IGF-I, IGFBP-3, and total ghrelin levels during the early burn injury period, in addition to more GH burst amplitude as well as greater falling of IGF-I, IGFBP-3 and total ghrelin levels proportional to the severity of burn injury. Further studies are needed to ascertain whether acyl- and desacyl-ghrelin instead of total ghrelin are completely independent of increased GH or other stress mediators, and whether GH-releasing hormone (GHRH) mainly stimulates the production and release of GH in acute critical conditions.

Acute ghrelin response to intravenous dexamethasone administration in idiopathic short stature or isolated idiopathic growth hormone-deficient children

Acylated ghrelin has been originally described for its potent GH-releasing activity mediated by the activation of the GH secretagogue receptor type 1a. More recently, ghrelin has been reported to exert several other GH-independent biological actions, among which in the modulation of metabolic functions. Glucocorticoids are well known to exert important metabolic functions but also to modulate GH secretion, although through mechanisms that have not been fully clarified so far. Interestingly, the existence of a feedback link between glucocorticoids and ghrelin system has already been reported. The aim of our study was to evaluate the acute GH and ghrelin responses to dexamethasone (DEX) administration in children with idiopathic short stature (ISS) or isolated idiopathic GH deficiency (GHD). Eight children with ISS (age: 9.5+/-1.2 yr) and 7 with GHD (12.1+/-1.4 yr) underwent iv DEX administration (0.3 mg/body surface area at 0 min). IGF-I, GH, and ghrelin levels were assayed at baseline and every 30 min from 120 up to 240 min after DEX. Compared to baseline levels DEX decreased ghrelin in ISS at 120 min and 240 min (p<0.04). On the other hand DEX did not modify ghrelin levels in GHD. After DEX, ghrelin was reduced in ISS compared to GHD (p<0.02). DEX increased GH in ISS but not in GHD (peak: 11.1+/-1.2 vs 7.6+/-0.9 microg/l). Basal, as well as after-DEX ghrelin levels negatively correlated with IGF-I in GHD (p<0.03) and with height SD score (HSDS) in ISS (p<0.02). Acute DEX administration is able to decrease ghrelin in ISS, but not in GHD children. Both basal and after-DEX ghrelin levels negatively correlate with IGF-I and HSDS. All these data suggest the existence of a feedback link among ghrelin, glucocorticoids and the GH/IGF-I axis.

Effect of one month ketoconazole treatment on GH, cortisol and ACTH release after ghrelin, GHRP-6 and GHRH administration in patients with cushing’s disease

GH responses to ghrelin, GHRP-6, and GHRH in Cushing’s disease (CD) are markedly blunted. There is no data about the effect of reduction of cortisol levels with steroidogenesis inhibitors, like ketoconazole, on GH secretion in CD. ACTH levels during ketoconazole treatment are controversial. The aims of this study were to compare the GH response to ghrelin, GHRP-6, and GHRH, and the ACTH and cortisol responses to ghrelin and GHRP-6 before and after one month of ketoconazole treatment in 6 untreated patients with CD. Before treatment peak GH (mg/L; mean ± SEM) after ghrelin, GHRP-6, and GHRH administration was 10.0 ± 4.5; 3.8 ± 1.6, and 0.6 ± 0.2, respectively. After one month of ketoconazole there was a significant decrease in urinary cortisol values (mean reduction: 75%), but GH responses did not change (7.0 ± 2.0; 3.1 ± 0.8; 0.9 ± 0.2, respectively). After treatment, there was a significant reduction in cortisol (mg/dL) responses to ghrelin (before: 30.6 ± 5.2; after: 24.2 ± 5.1). No significant changes in ACTH (pg/mL) responses before (ghrelin: 210.9 ± 69.9; GHRP-6: 199.8 ± 88.8) and after treatment (ghrelin: 159.7 ± 40.3; GHRP-6: 227 ± 127.2) were observed. In conclusion, after short-term ketoconazole treatment there are no changes in GH or ACTH responses, despite a major decrease of cortisol levels. A longer period of treatment might be necessary for the recovery of pituitary function.

Growth hormone and ghrelin secretion in severely obese women before and after bariatric surgery

OBJECTIVE

The objective was to evaluate ghrelin and growth hormone (GH) interactions and responses to a growth hormone-releasing hormone (GHRH)/arginine test in severe obesity before and after surgically-induced weight loss.

RESEARCH METHODS AND PROCEDURES

Our study population included 11 severely obese women 39 +/- 12 years of age, with a mean BMI of 48.6 +/- 2.4 kg/m2, re-studied in a phase of stabilized body weight, with a BMI of 33.4 +/- 1.2 kg/m2, 18 months after having successfully undergone biliopancreatic diversion (BPD). A GHRH/arginine test was performed before and 18 months after BPD to evaluate ghrelin and GH interactions. Active ghrelin, measured by radioimmunoassay (RIA), and GH, measured by chemiluminescence assay, were assayed before and after the GHRH/arginine test.

RESULTS

Fasting serum GH levels and GH area under the curve (AUC) significantly increased from 0.2 +/- 0.05 ng/mL to 1 +/- 0.3 ng/mL (p < 0.05) and from 514.76 +/- 98.7 ng/mL for 120 minutes to 1957.3 +/- 665.1 ng/mL for 120 minutes after bariatric surgery (p < 0.05), respectively. Although no significant change in fasting ghrelin levels was observed (573 +/- 77.9 before BPD vs. 574.1 +/- 32.7 after BPD), ghrelin AUC significantly increased from -3253.9 +/- 2180.9 pg/mL for 120 minutes to 1142.3 +/- 916.4 pg/mL for 120 minutes after BPD (p < 0.05). Fasting serum insulin-like growth factor (IGF)-1 concentration did not change significantly (133.6 +/- 9.9 ng/mL before vs. 153.3 +/- 25.2 ng/mL after BPD).

DISCUSSION

Our study demonstrates that the mechanisms involved in ghrelin and GH secretion after the secretagogue stimulus (GHRH/arginine) are consistent with patterns observed in other populations.

Ghrelin in neuroendocrine organs and tumours

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

Decreased ghrelin-induced GH release in thyrotoxicosis: comparison with GH-releasing peptide-6 (GHRP-6) and GHRH

In thyrotoxicosis GH response to several stimuli is impaired, but there is no data on ghrelin-induced GH release in these patients. Ghrelin is a potent GH secretagogue and it also increases glucose levels in men. The aim of this study was to evaluate the effects of ghrelin (1 microg/kg), GHRP-6 (1 mug/kg) and GHRH (100 microg), i.v., on GH levels in 10 hyperthyroid patients and in 8 controls. Glucose levels were also measured during ghrelin and GHRP-6 administration. In control subjects and hyperthyroid patients peak GH (microg/l; mean +/- SE) values after ghrelin injection (controls: 66.7 +/- 13.6; hyper: 19.3 +/- 2.4) were significantly higher than those obtained after GHRP-6 (controls: 26.7 +/- 5.1; hyper: 12.6 +/- 1.3) and GHRH (controls: 13.5 +/- 4.3; hyper: 5.3 +/- 1.3). There was a significant decrease in GH responsiveness to ghrelin, GHRP-6 and GHRH in the hyperthyroid group compared to controls. In control subjects and hyperthyroid patients basal glucose (mmol/l) values were 4.5 +/- 0.1 and 4.7 +/- 0.2, respectively. There was a significant increase in glucose levels 30 min after ghrelin injection (controls: 4.9 +/- 0.1; hyper: 5.2 +/- 0.2), which remained elevated up to 120 min. When the two groups were compared no differences in glucose values were observed. GHRP-6 administration was not able to increase glucose levels in both groups. Our data shows that GH release after ghrelin, GHRP-6 and GHRH administration is decreased in thyrotoxicosis. This suggests that thyroid hormone excess interferes with GH-releasing pathways activated by these peptides. Our results also suggest that ghrelin's ability to increase glucose levels is not altered in thyrotoxicosis.

Decreased GH secretion and enhanced ACTH and cortisol release after ghrelin administration in Cushing's disease: comparison with GH-releasing peptide-6 (GHRP-6) and GHRH

GH responsiveness to GH secretagogues (GHS) is blunted in Cushing's disease (CD), while ACTH/cortisol responses are enhanced, by mechanisms still unclear. Ghrelin, the endogenous ligand for GHS-receptors (GHS-R), increases GH, ACTH, cortisol and glucose levels in humans. This study evaluated the GH, ACTH, cortisol and glucose-releasing effects of ghrelin in CD in comparison with GHRP-6. GHRH-induced GH release was also studied. Ten patients with CD (BMI 26.9+/-1.0 kg/m(2)) and ten controls (BMI 24.4+/-1.1 kg/m(2)) received ghrelin (1 microg/kg), GHRP-6 (1 microg/kg) and GHRH (100 microg) separately. GH, ACTH, cortisol and glucose levels were measured. In CD ghrelin-induced GH (microg/L; mean +/- SE) release (peak: 7.2+/-3.0) was higher than seen with GHRP-6 (2.7+/-1.0) and GHRH (0.7+/-0.2), but lower than in controls (ghrelin: 58.3+/-12.1; GHRP-6: 22.9+/-4.8; GHRH: 11.3+/-3.7). In controls ACTH (pg/mL) release after ghrelin (79.2+/-26.8) was higher than after GHRP-6 (23.6+/-5.7). In CD these responses (ghrelin: 192+/-43; GHRP-6: 185+/-56) were similar, and enhanced compared to controls. The same was observed with cortisol. Glucose levels failed to increase after ghrelin in CD, differently than in controls. Our data suggests that hypothalamic and pituitary pathways of GH release activated by ghrelin, GHRP-6 and GHRH are deranged in chronic hypercortisolism. The increased ACTH/cortisol responses to ghrelin and GHRP-6 in CD could be mediated by overexpression of GHS-R in ACTH-secreting adenomas. Hypercortisolism apparently impairs the ability of ghrelin to increase glucose levels.