Effects of free fatty acids, growth hormone and growth hormone receptor blockade on serum ghrelin levels in humans

Perceptions of appetite do not match hormonal measures of appetite in trained competitive cyclists and triathletes following a ketogenic diet compared to a high-carbohydrate or habitual diet: A randomized crossover trial

Endurance athletes may implement rigid dietary strategies, such as the ketogenic diet (KD), to improve performance. The effect of the KD on appetite remains unclear in endurance athletes. This study analyzed the effects of a KD, a high-carbohydrate diet (HCD), and habitual diet (HD) on objective and subjective measures of appetite in trained cyclists and triathletes, and hypothesized that the KD would result in greater objective and subjective appetite suppression. Six participants consumed the KD and HCD for 2-weeks each, in a random order, following their HD. Fasting appetite measures were collected after 2-weeks on each diet. Postprandial appetite measures were collected following consumption of a ketogenic meal after the KD, high-carbohydrate meal after the HCD, and standard American/Western meal after the HD. Fasting total ghrelin (GHR) was lower and glucagon-like peptide-1 (GLP-1) and hunger were higher following the KD versus HD and HCD. Fasting insulin was not different. Mixed-effects model repeated measures analysis and effect sizes and 95% confidence intervals showed that postprandial GHR and insulin were lower and GLP-1 was higher following the ketogenic versus the standard and high-carbohydrate meals. Postprandial appetite ratings were not different across test meals. In conclusion, both fasting and postprandial concentrations of GHR were lower and GLP-1 were higher following the KD than the HC and HD, and postprandial insulin was lower on the KD. Subjective ratings of appetite did not correspond with the objective measures of appetite in trained competitive endurance athlete. More research is needed to confirm our findings.

Acipimox Administration With Exercise Induces a Co-feedback Action of the GH, PP, and PYY on Ghrelin Associated With a Reduction of Peripheral Lipolysis in Bulimic and Healthy-Weight Czech Women: A Randomized Study

Objective: Anti-lipolytic drugs and exercise are enhancers of growth hormone (GH) secretion. Decreased circulating free fatty acids (FFA) have been proposed to exert ghrelin-GH feedback loop after administration of an anti-lipolytic longer-acting analog of nicotinic acid, Acipimox (OLB, 5-Methylpyrazine-2-carboxylic acid 4-oxide, molecular weight of 154.1 Da). OLB administration strongly suppresses plasma FFA during exercise. Neuroendocrine perturbations of the adipose tissue (AT), gut, and brain peptides may be involved in the etiopathogenesis of eating disorders including bulimia nervosa (BN) and anorexia nervosa. BN is characterized by binge eating, self-induced vomiting or excessive exercise. Approach: To test the hypothesis that treatment with OLB together with exercise vs. exercise alone would induce feedback action of GH, pancreatic polypeptide (PP), peptide tyrosine tyrosine (PYY), and leptin on ghrelin in Czech women with BN and in healthy-weight Czech women (HW). The lipolysis rate (as glycerol release) in subcutaneous abdominal AT was assessed with microdialysis. At an academic medical center, 12 BN and 12 HW (the control group) were randomized to OLB 500 mg 1 h before a single exercise bout (45 min, 2 W/kg of lean body mass [LBM]) once a week vs. identical placebo over a total of 2 weeks. Blood plasma concentrations of GH, PP, PYY, leptin, ghrelin, FFA, glycerol, and concentrations of AT interstitial glycerol were estimated during the test by RIA utilizing ¹²⁵I-labeled tracer, the electrochemiluminescence technique (ECLIA) or colorimetric kits. Results: OLB administration together with short-term exercise significantly increased plasma GH (P < 0.0001), PP (P < 0.0001), PYY, and leptin concentrations and significantly decreased plasma ghrelin (P < 0.01) concentrations in both groups, whereas short-term exercise with placebo resulted in plasma ghrelin (P < 0.05) decrease exclusively in BN. OLB administration together with short-term exercise significantly lowered local subcutaneous abdominal AT interstitial glycerol (P < 0.0001) to a greater extent in BN. Conclusion: OLB-induced suppression of plasma ghrelin concentrations together with short-term exercise and after the post-exercise recovering phase suggests a potential negative co-feedback of GH, PP, PYY, and leptin on ghrelin secretion to a greater extent in BN. Simultaneously, the exercise-induced elevation in AT interstitial glycerol leading to a higher inhibition of peripheral lipolysis by OLB in BN. Clinical Trial Registration: www.ClinicalTrials.gov, identifier NCT03338387.

Effect of 24-h severe energy restriction on appetite regulation and ad libitum energy intake in lean men and women

BACKGROUND

Intermittent severe energy restriction (SER) can induce substantial weight loss, but the appetite regulatory responses to SER are unknown and may dictate long-term dietary adherence.

OBJECTIVE

We determined the effect of 24-h SER on appetite regulation, metabolism, and energy intake.

DESIGN

Eighteen lean men and women completed two 3-d trials in randomized, counterbalanced order. On day 1 subjects consumed standardized diets containing 100% (mean ± SD: 9.3 ± 1.3 MJ; energy balance) or 25% [2.3 ± 0.3 MJ; energy restriction (ER)] of energy requirements. On day 2, a standardized breakfast was consumed, with plasma concentrations of acylated ghrelin, glucagon-like peptide 1, insulin, glucose, and nonesterified fatty acids determined for 4 h. Ad libitum energy intake was assessed at lunch and dinner with subjective appetite and resting metabolism assessed throughout. On day 3, ad libitum energy intake was assessed at breakfast and by weighed food records.

RESULTS

Energy intake was 7% greater on day 2 (P < 0.05) during ER but not significantly different on day 3 (P = 0.557). Subjective appetite was greater during ER on the morning of day 2 (P < 0.05) but was not significantly different thereafter (P > 0.145). During ER, postprandial concentrations of acylated ghrelin were lower (P < 0.05), whereas glucose (P < 0.05) and nonesterified fatty acids (P < 0.0001) were higher. Postprandial glucagon-like peptide 1_7-36 (P = 0.784) and insulin (P = 0.06) concentrations were not significantly different between trials. Energy expenditure was lower during ER in the morning (P < 0.01).

CONCLUSIONS

In lean young adults, 24-h SER transiently elevated subjective appetite and marginally increased energy intake, but hormonal appetite markers did not respond in a manner indicative of hyperphagia. These results suggest that intermittent SER might be useful to attenuate energy intake and control body weight in this population. This trial was registered at www.clinicaltrials.gov.uk as NCT02696772.

Intravenous lipid infusion and total plasma fatty acids positively modulate plasma acylated ghrelin in vivo

BACKGROUND & AIMS

Ghrelin is a gastric orexigenic hormone whose activating acylation plays a relevant role in the regulation of energy balance. Nutritional modulators of ghrelin acylation and plasma acylated ghrelin (AG) concentration remain however largely undefined. We aimed at investigating whether circulating free fatty acids (FFA) contribute to regulate plasma AG and its ratio (AG/TG) to total hormone (TG).

METHODS

Plasma FFA, TG, AG and AG/TG were measured in a primary outpatient care setting in a community-based population cohort of 850 individuals (age 54 ± 10 years, M/F: 408/442) from the North-East Italy MoMa study. 150-min intravenous lipid infusions in rodents (10% lipids, 600 μl/h) were used to investigate the potential causal role of FFA in the regulation of plasma ghrelin profile.

RESULTS

Plasma FFA were associated positively with AG and AG/TG while negatively with TG (P < 0.01). Associations between FFA, AG and AG/TG remained statistically significant (P < 0.02) in multiple regression analysis including HOMA insulin resistance and metabolic confounders, and both AG and AG/TG but not TG increased through plasma FFA quartiles (P < 0.01). Consistent with these findings, intravenous lipid infusion with plasma FFA elevation caused elevations of AG and AG/TG (P < 0.05) with no TG modifications.

CONCLUSIONS

The current findings demonstrate a novel role for circulating FFA availability to up-regulate plasma AG, which could involve FFA-induced stimulation of ghrelin acylation.

Circulating acylghrelin levels are suppressed by insulin and increase in response to hypoglycemia in healthy adult volunteers

OBJECTIVE

Ghrelin has glucoregulatory and orexigenic actions, but its role in acute hypoglycemia remains uncertain. We aimed to investigate circulating levels of acylghrelin (AG) and unacylated ghrelin (UAG) in response to hyperinsulinemia and to hypoglycemia.

DESIGN

A randomized, single-blind, placebo-controlled crossover study including 3 study days was performed at a university hospital clinical research center.

METHODS

Nine healthy men completed 3 study days: i) saline control (CTR), ii) hyperinsulinemic euglycemia (HE) (bolus insulin 0.1 IE/kg i.v. and glucose 20% i.v. for 105 min, plasma glucose ≈5 mmol/l), and iii) hyperinsulinemic hypoglycemia (HH) (bolus insulin 0.1 IE/kg i.v.).

RESULTS

HH and HE suppressed AG concentrations at t=45-60 min as compared with CTR (P<0.05). At t=90 min, a rebound increase in AG was observed in response to HH as compared with both HE and CTR (P<0.05). UAG also decreased during HH and HE at t=45 min (P<0.05), whereas the AG-to-UAG ratio remained unaffected.

CONCLUSIONS

This study demonstrates that AG and UAG are directly suppressed by hyperinsulinemia and that AG concentrations increase after a latency of ≈1 h in response to hypoglycemia, suggesting a potential counterregulatory role of AG.

The regulation of circulating ghrelin - with recent updates from cell-based assays

Ghrelin is a stomach-derived orexigenic hormone with a wide range of physiological functions. Elucidation of the regulation of the circulating ghrelin level would lead to a better understanding of appetite control in body energy homeostasis. Earlier studies revealed that circulating ghrelin levels are under the control of both acute and chronic energy status: at the acute scale, ghrelin levels are increased by fasting and decreased by feeding, whereas at the chronic scale, they are high in obese subjects and low in lean subjects. Subsequent studies revealed that nutrients, hormones, or neural activities can influence circulating ghrelin levels in vivo. Recently developed in vitro assay systems for ghrelin secretion can assess whether and how individual factors affect ghrelin secretion from cells. In this review, on the basis of numerous human, animal, and cell-based studies, we summarize current knowledge on the regulation of circulating ghrelin levels and enumerate the factors that influence ghrelin levels.

Prospective study of surgical treatment of acromegaly: effects on ghrelin, weight, adiposity, and markers of CV risk

CONTEXT

Although epidemiological studies have found that GH and IGF-1 normalization reduce the excess mortality of active acromegaly to expected rates, cross-sectional data report some cardiovascular (CV) risk markers to be less favorable in remission than active acromegaly.

OBJECTIVE

The objective of the study was to test the hypothesis that remission of acromegaly after surgical therapy increases weight and adiposity and some CV risk markers and these changes are paralleled by a rise in ghrelin.

DESIGN

Forty-two adults with untreated, active acromegaly were studied prospectively. Changes in outcome measures from before to after surgery were assessed in 26 subjects achieving remission (normal IGF-1) and 16 with persistent active acromegaly (elevated IGF-1) after surgery.

SETTING

The study was conducted at tertiary referral centers for pituitary tumors.

MAIN OUTCOME MEASURES

Endocrine, metabolic, and CV risk parameters, anthropometrics, and body composition by dual-energy X-ray absorptiometry were measured.

RESULTS

Remission increased total ghrelin, body weight, waist circumference, C-reactive protein, homocysteine, high-density lipoprotein, and leptin and reduced systolic blood pressure, homeostasis model assessment score, triglycerides, and lipoprotein (a) by 6 months and for 32 ± 4 months after surgery. The ghrelin rise correlated with the fall in the levels of GH, IGF-1, and insulin and insulin resistance. Weight, waist circumference, and ghrelin did not increase significantly in the persistent active acromegaly group. Total body fat, trunk fat, and perentage total body fat increased by 1 year after surgery in 15 remission subjects: the increase in body fat correlated with the rise in total ghrelin.

CONCLUSIONS

Although most markers of CV risk improve with acromegaly remission after surgery, some markers and adiposity increase and are paralleled by a rise in total ghrelin, suggesting that these changes may be related. Understanding the mechanisms and long-term implications of the changes that accompany treatment of acromegaly is important to optimizing management because some aspects of the postoperative profile associate with the increased metabolic and CV risk in other populations.

Seven transmembrane G protein-coupled receptor repertoire of gastric ghrelin cells

The molecular mechanisms regulating secretion of the orexigenic-glucoregulatory hormone ghrelin remain unclear. Based on qPCR analysis of FACS-purified gastric ghrelin cells, highly expressed and enriched 7TM receptors were comprehensively identified and functionally characterized using in vitro, ex vivo and in vivo methods. Five Gαs-coupled receptors efficiently stimulated ghrelin secretion: as expected the β1-adrenergic, the GIP and the secretin receptors but surprisingly also the composite receptor for the sensory neuropeptide CGRP and the melanocortin 4 receptor. A number of Gαi/o-coupled receptors inhibited ghrelin secretion including somatostatin receptors SSTR1, SSTR2 and SSTR3 and unexpectedly the highly enriched lactate receptor, GPR81. Three other metabolite receptors known to be both Gαi/o- and Gαq/11-coupled all inhibited ghrelin secretion through a pertussis toxin-sensitive Gαi/o pathway: FFAR2 (short chain fatty acid receptor; GPR43), FFAR4 (long chain fatty acid receptor; GPR120) and CasR (calcium sensing receptor). In addition to the common Gα subunits three non-common Gαi/o subunits were highly enriched in ghrelin cells: GαoA, GαoB and Gαz. Inhibition of Gαi/o signaling via ghrelin cell-selective pertussis toxin expression markedly enhanced circulating ghrelin. These 7TM receptors and associated Gα subunits constitute a major part of the molecular machinery directly mediating neuronal and endocrine stimulation versus metabolite and somatostatin inhibition of ghrelin secretion including a series of novel receptor targets not previously identified on the ghrelin cell.

Postprandial inhibition of gastric ghrelin secretion by long-chain fatty acid through GPR120 in isolated gastric ghrelin cells and mice

Ghrelin is a gastric peptide hormone that controls appetite and energy homeostasis. Plasma ghrelin levels rise before a meal and fall quickly thereafter. Elucidation of the regulation of ghrelin secretion has been hampered by the difficulty of directly interrogating ghrelin cells diffusely scattered within the complex gastric mucosa. Therefore, we generated transgenic mice with ghrelin cell expression of green fluorescent protein (GFP) to enable characterization of ghrelin secretion in a pure population of isolated gastric ghrelin-expressing GFP (Ghr-GFP) cells. Using quantitative RT-PCR and immunofluorescence staining, we detected a high level of expression of the long-chain fatty acid (LCFA) receptor GPR120, while the other LCFA receptor, GPR40, was undetectable. In short-term-cultured pure Ghr-GFP cells, the LCFAs docosadienoic acid, linolenic acid, and palmitoleic acid significantly suppressed ghrelin secretion. The physiological mechanism of LCFA inhibition on ghrelin secretion was studied in mice. Serum ghrelin levels were transiently suppressed after gastric gavage of LCFA-rich lipid in mice with pylorus ligation, indicating that the ghrelin cell may directly sense increased gastric LCFA derived from ingested intraluminal lipids. Meal-induced increase in gastric mucosal LCFA was assessed by measuring the transcripts of markers for tissue uptake of LCFA, lipoprotein lipase (LPL), fatty acid translocase (CD36), glycosylphosphatidylinositol-anchored HDL-binding protein 1, and nuclear fatty acid receptor peroxisome proliferator-activated receptor-γ. Quantitative RT-PCR studies indicate significantly increased mRNA levels of lipoprotein lipase, glycosylphosphatidylinositol-anchored HDL-binding protein 1, and peroxisome proliferator-activated receptor-γ in postprandial gastric mucosa. These results suggest that meal-related increases in gastric mucosal LCFA interact with GPR120 on ghrelin cells to inhibit ghrelin secretion.

Metabolomic linkage reveals functional interaction between glucose-dependent insulinotropic polypeptide and ghrelin in humans

The gastric peptide ghrelin promotes energy storage, appetite, and food intake. Nutrient intake strongly suppresses circulating ghrelin via molecular mechanisms possibly involving insulin and gastrointestinal hormones. On the basis of the growing evidence that glucose-dependent insulinotropic polypeptide (GIP) is involved in the control of fuel metabolism, we hypothesized that GIP and/or insulin, directly or via changes in plasma metabolites, might affect circulating ghrelin. Fourteen obese subjects were infused with GIP (2.0 pmol·kg(-1)·min(-1)) or placebo in the fasting state during either euglycemic hyperinsulinemic (EC) or hyperglycemic hyperinsulinemic clamps (HC). Apart from analysis of plasma ghrelin and insulin levels, GC-TOF/MS analysis was applied to create a hormone-metabolite network for each experiment. The GIP and insulin effects on circulating ghrelin were analyzed within the framework of those networks. In the HC, ghrelin levels decreased in the absence (19.2% vs. baseline, P = 0.028) as well as in the presence of GIP (33.8%, P = 0.018). Ghrelin levels were significantly lower during HC with GIP than with placebo, despite insulin levels not differing significantly. In the GIP network combining data on GIP-infusion, EC+GIP and HC+GIP experiments, ghrelin was integrated into hormone-metabolite networks through a connection to a group of long-chain fatty acids. In contrast, ghrelin was excluded from the network of experiments without GIP. GIP decreased circulating ghrelin and might have affected the ghrelin system via modification of long-chain fatty acid pools. These observations were independent of insulin and offer potential mechanistic underpinnings for the involvement of GIP in systemic control of energy metabolism.

Influence of pegvisomant on serum ghrelin and leptin levels in acromegalic patients

INTRODUCTION

Pegvisomant (peg) is a GH receptor antagonist. In de novo acromegalic patients with high GH levels, ghrelin and leptin levels are reduced, suggesting a direct GH-mediated effect. The aim of our study was to evaluate whether peg treatment in acromegalic patients may abolish the GH impact on ghrelin and leptin levels.

METHODS

Ghrelin, leptin and endogenous GH were measured in ten peg-treated acromegalic patients (three females/seven males, 47 years (28-57)), ten patients with active (act) and ten patients with inactive disease (inact) as well as in ten gender-, age- and body mass index (BMI)-matched healthy volunteers (controls). Endogenous GH was measured using a special in-house assay without interference by peg; total ghrelin and leptin were determined using a commercial RIA and an immunofluorometric in-house assay respectively.

RESULTS

Age and BMI did not differ significantly between groups. Endogenous GH was significantly higher in peg (6.3 μg/l (1.5-41)) and act (9.3 μg/l (1.7-70)) compared with controls (0.1 μg/l (0.1-3.1)) and inact (0.35 μg/l (0.1-2.0), P<0.001). Ghrelin was significantly higher in peg (232  ng/l (96-351)) compared with act (102 ng/l (33-232), P<0.01), whereas ghrelin was not significantly different between the other groups. Leptin was highest in controls (19 μg/l (4-57)) and lowest in act (6 μg/l (2-21)), but this difference did not reach significance.

CONCLUSION

Treatment with peg seems to disrupt the feedback loop of ghrelin and GH, leading to elevated ghrelin levels. Furthermore, peg therapy appears not to have a strong impact on leptin levels, as acromegalic patients with and without peg treatment showed similar leptin levels.

Integrating GHS into the Ghrelin System

Oligopeptide derivatives of metenkephalin were found to stimulate growth-hormone (GH) release directly by pituitary somatotrope cells in vitro in 1977. Members of this class of peptides and nonpeptidyl mimetics are referred to as GH secretagogues (GHSs). A specific guanosine triphosphatate-binding protein-associated heptahelical transmembrane receptor for GHS was cloned in 1996. An endogenous ligand for the GHS receptor, acylghrelin, was identified in 1999. Expression of ghrelin and homonymous receptor occurs in the brain, pituitary gland, stomach, endothelium/vascular smooth muscle, pancreas, placenta, intestine, heart, bone, and other tissues. Principal actions of this peptidergic system include stimulation of GH release via combined hypothalamopituitary mechanisms, orexigenesis (appetitive enhancement), insulinostasis (inhibition of insulin secretion), cardiovascular effects (decreased mean arterial pressure and vasodilation), stimulation of gastric motility and acid secretion, adipogenesis with repression of fat oxidation, and antiapoptosis (antagonism of endothelial, neuronal, and cardiomyocyte death). The array of known and proposed interactions of ghrelin with key metabolic signals makes ghrelin and its receptor prime targets for drug development.

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

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

No increased risk of hypoglycaemic episodes during 48 h of subcutaneous glucagon-like-peptide-1 administration in fasting healthy subjects

OBJECTIVE

It is uncertain whether the ability to avoid hypoglycaemia during fasting is preserved, and the risk of reactive hypoglycaemia after an oral glucose stimulus following a prolonged fasting period is increased at augmented glucagon-like peptide-1 (GLP-1) levels.

DESIGN

A randomized, double-blind placebo-controlled cross-over study in eight healthy men to assess the safety, in terms of hypoglycaemia, of a continuously infused pharmacological dose of native GLP-1 during long-term fasting. After an overnight fast the fasting period continued for 48 h and was followed by a 3-h oral glucose tolerance test (OGTT). GLP-1(7-36 amide) or placebo was continuously infused subcutaneously and titrated to a dose of 4.8 pmol/kg per min.

RESULTS

Two subjects in the GLP-1 group and one subject in the placebo group were withdrawn due to protocol specified plasma glucose (PG) < or = 2.8 mm and neuroglycopaenic symptoms. The infusion of GLP-1 resulted in pharmacological levels of intact GLP-1. During the fasting period PG, insulin and C-peptide levels declined and glucagon, GH and free fatty acid (FFA) levels increased with no differences between GLP-1 and placebo. During OGTT circulating levels of insulin and C-peptide were higher with GLP-1 infusion. However, PG was similar during GLP-1 vs. placebo infusions. GLP-1 infusion increased norepinephrine and cortisol levels during OGTT.

CONCLUSION

The counter-regulatory response during 48 h of subcutaneous GLP-1 infusion was preserved despite long-term fasting with no apparent increased risk of hypoglycaemic episodes. No reactive hypoglycaemia was observed when the fast was followed by an OGTT. Thus use of long-acting GLP-1 analogues may not increase the risk of hypoglycaemia.

Comparison of the effects of food versus protein restriction on selected nutritional and inflammatory markers in rats

We previously demonstrated that feeding a 2% protein AIN-76 diet ad libitum for 14 days resulted in substantial clinical and biochemical changes including weight loss, hypoglycemia, hypoalbuminemia, higher levels of plasma cytokines, oxidative stress in the liver, and activation of inflammatory signaling to interleukin (IL)-6, as compared with a 20% protein diet. In the present study, 54 rats were randomly given a standard rat chow diet ad libitum, or a 25% or 50% reduction of this intake for 14 days. The results showed that weight gain was less in the 25% food-restricted group and halted in the 50% group as compared with the control group. Unlike protein restriction, neither level of food restriction altered plasma levels of albumin and glucose, the hepatic protein abundance of signal transducers and activators of transcriptions and of mitogen-activated protein kinases, or the hepatic contents of total glutathione and malondialdehyde. The intracellular signaling in response to IL-6 stimulation was also well maintained. However, both levels of food restriction elevated IL-1 and corticosterone in plasma, did not alter ghrelin, and decreased plasma levels of free fatty acids. Because these latter 3 markers were not examined previously, 20 rats were fed an AIN-76 diet, either with 20% or 2% protein, ad libitum for 14 days. The 2% protein diet significantly decreased plasma levels of free fatty acids and increased ghrelin and corticosterone as compared with the 20% protein diet. Thus, food restriction, where all essential nutrients are reduced in proportion, is a physiologic stress that, while limiting growth, does not activate or impair the systemic inflammatory response, whereas a very low protein diet with little change in energy intake has a substantial impact on systemic inflammation, body composition, and growth.

Visceral fat mass is a strong predictor of circulating ghrelin levels in premenopausal women

OBJECTIVE

A well known inverse relationship exists between obesity and circulating ghrelin concentrations. However, obesity is a heterogeneous disease entity and upper-body obesity (UBO) is associated with more profound metabolic disturbances than lower-body obesity (LBO). We therefore aimed to investigate the impact of body composition on circulating ghrelin levels in women spanning a wide range of body composition phenotypes.

SUBJECTS AND METHODS

Ten (UBO; waist-to-hip ratio (WHR) >0.85, body mass index (BMI) >28 kg/m(2)), ten LBO (WHR <0.80, BMI >28 kg/m(2)) and ten lean women (BMI<25 kg/m(2)) were studied. Total ghrelin levels were measured under basal and hyperinsulinemic (0.6 mU/kg per min) conditions. Body fat distribution was determined by dual X-ray absorptiometry in combination with computed tomography at the L2-L3 level.

RESULTS

As expected, an inverse correlation existed between basal ghrelin concentration and BMI (r=-0.40, P=0.03) and total fat mass (r=-0.39, P=0.04). Visceral fat mass was a strong predictor (r=-0.56, P=0.003) of circulating ghrelin levels, even when adjusted for BMI (P=0.02) or body composition group (P=0.04). The suppressive effect of insulin on ghrelin concentration was significantly diminished in the UBO compared with the lean controls (P=0.012) and a highly significant inverse correlation existed with visceral fat mass (r=-0.52, P=0.004).

CONCLUSIONS

Visceral fat mass is a strong predictor of basal ghrelin concentrations and also attenuates the suppressive effect of insulin on ghrelin concentrations. These data provide further evidence that the UBO phenotype is associated with more profound metabolic abnormalities than obesity per se.

Estrogen elevates the peak overnight production rate of acylated ghrelin

CONTEXT

Acylated ghrelin is the putatively bioactive GH secretagogue.

HYPOTHESIS

Estradiol (E2) stimulates the synthesis rather than inhibits the metabolic clearance of acylated ghrelin.

SETTING

The study took place at an academic medical center.

SUBJECTS

Healthy postmenopausal women participated.

INTERVENTIONS

Interventions included prospectively randomized, double-blind separate-day iv infusions of saline or five graded doses of ghrelin in estrogen-deficient (n=12) and E2-supplemented (n=8) women.

OUTCOMES

Metabolic clearance rate (MCR), volume of distribution, half-life, and secretion rate of acylated ghrelin were assessed.

RESULTS

In pilot iv bolus ghrelin infusions, the median half-lives of acylated and total ghrelin were 21 and 36 min (P<0.01), MCRs 58 and 8.1 liters/kg.d (P<0.01), and volumes of distribution of 1.0 and 0.32 liters/kg (P<0.01), respectively. Transdermal E2 supplementation for 3 wk increased peak nighttime acylated ghrelin concentrations from 99+/-12 to 141+/-34 pg/ml (P=0.039). Exposure to E2 did not alter the linear relationships between 1) plasma acylated ghrelin concentration and ghrelin infusion rate (638+/-12 slope units), 2) MCR of acylated ghrelin and ghrelin infusion rate (10+/-2.5 slope units), and 3) MCR and plasma concentration of acylated ghrelin (0.017+/-0.004 slope units). These data predict peak nighttime production rates of acylated ghrelin of 3.8+/-0.9 (E2) and 1.9+/-0.2 (no E2) ng/kg.min (P=0.039).

CONCLUSION

Acylated ghrelin has a multifold larger distribution volume and MCR than total ghrelin. An estrogenic milieu augments synthesis and/or acylation of ghrelin peptide without altering its MCR.

Growth hormone signaling in vivo in human muscle and adipose tissue: impact of insulin, substrate background, and growth hormone receptor blockade

CONTEXT

GH induces insulin resistance in muscle and fat, and in vitro data indicate that this may involve cross-talk between the signaling pathways of the two hormones.

OBJECTIVE

Our objective was to investigate GH and insulin signaling in vivo in human muscle and fat tissue in response to GH, GH receptor blockade, and insulin stimulation.

DESIGN

We conducted two randomized crossover studies.

PARTICIPANTS

Sixteen healthy males participated.

INTERVENTION

GH was administered as a bolus (n = 8) and constant infusion (n = 8). The bolus study included three arms: 1) control (saline), 2) GH (0.5 mg iv), and 3) GH blockade (pegvisomant 30 mg sc), each combined with a hyperinsulinemic glucose clamp. The infusion study included two arms: 1) GH infusion (45 ng/.kg.min, 5.5 h) and 2) saline infusion (5.5 h) combined with a hyperinsulinemic glucose clamp during the final 2.5 h.

MAIN OUTCOME MEASURES

Muscle and fat biopsies were subjected to Western blotting for expression of Stat5/p-Stat5, Akt/p-Akt, and ERK1/2/p-ERK1/2 and to real-time RT-PCR for expression of SOCS1-3 and IGF-I mRNA.

RESULTS

GH significantly reduced insulin sensitivity. The GH bolus as well as GH infusion induced phosphorylation of Stat5 in muscle and fat, and SOCS3 and IGF-I mRNA expression increased after GH infusion. Hyperinsulinemia induced Akt phosphorylation in both tissues, irrespective of GH status. In muscle, ERK1/2 phosphorylation was increased by insulin, but insulin per se did not induce phosphorylation of Stat5.

CONCLUSIONS

GH exposure associated with insulin resistance acutely translates into GH receptor signaling in human muscle and fat without evidence of cross-talk with insulin signaling pathways. The molecular mechanisms subserving GH-induced insulin resistance in humans remain unclarified.

Evidence for acyl-ghrelin modulation of growth hormone release in the fed state

CONTEXT

The timing and frequency of GH secretory episodes is regulated by GHRH and somatostatin. This study provides evidence for amplification of these GH pulses by endogenous acyl-ghrelin.

DESIGN

Blood was sampled every 10 min for 26.5 h during a fed admission with standardized meals and also during the final 24 h of a 61.5-h fast. GH secretion profiles were derived from deconvolution of 10-min sampling data, and full-length acyl-ghrelin levels were measured using a newly developed two-site sandwich assay.

SETTING

The study was conducted at a university hospital general clinical research center.

PARTICIPANTS

Participants included eight men with mean (+/- sd) age 24.5 +/- 3.7 yr (body mass index 24 +/- 2.1 kg/m(2)).

RESULTS

Correlations were computed between amplitudes of individual GH secretory events and the average acyl-ghrelin concentration in the 60-min interval preceding each GH burst. In the fed state, the peak correlations were positive for all subjects and significantly higher than in the fasting state when acyl-ghrelin levels declined [mean (+/- sem): 0.7 (0.04) vs. 0.29 (0.08), P = 0.017]. In addition, long-term fasting was associated with an increase in the GH secretory pulse mass and amplitude but not frequency [fed vs. fasting pulse mass: 0.22 (0.05) vs. 0.44 (0.06) microg/liter, P = 0.002; amplitude: 5.2 (1.3) vs. 11.8 (1.9) microg/liter/min, P = 0.034; pulses per 24 h: 19.4 (0.5) vs. 22.0 (1.4), P = 0.1].

CONCLUSION

Our data support the hypothesis that under normal conditions in subjects given regular meals endogenous acyl-ghrelin acts to increase the amplitude of GH pulses.

Glucagon-like peptide-1 inhibits blood-brain glucose transfer in humans

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) has many effects on glucose homeostasis, and GLP-1 receptors are broadly represented in many tissues including the brain. Recent research in rodents suggests a protective effect of GLP-1 on brain tissue. The mechanism is unknown. We therefore tested whether these neuroprotective effects could relate to changes of glucose transport and consumption.

RESEARCH DESIGN AND METHODS

We studied 10 healthy men in a randomized, double-blinded, placebo-controlled cross-over experiment. We used positron emission tomography to determine the acute insulin-independent effect of GLP-1 on unidirectional glucose transport into the brain during a pituitary-pancreatic normoglycemic (plasma glucose approximately 4.5 mmol/l) clamp with 18-fluoro-deoxy-glucose as tracer.

RESULTS

On average, GLP-1 reduced cerebral glucose transport by 27% in total cerebral gray matter (P = 0.05) and by 25-30% in individual gray matter regions (P = 0.02-0.06). The same regions revealed a uniform trend toward similarly reduced cerebral glucose metabolism. Consequently, the intracerebral glucose concentration remained constant in all regions, with and without GLP-1.

CONCLUSIONS

We have demonstrated that a hormone involved in postprandial glucose regulation also limits glucose delivery to brain tissue and hence provides a possible regulatory mechanism for the link between plasma glucose and brain glucose. Because GLP-1 reduces glucose uptake across the intact blood-brain barrier at normal glycemia, GLP-1 may also protect the brain by limiting intracerebral glucose fluctuation when plasma glucose is increased.

Bacterial endotoxin induces biphasic changes in plasma ghrelin in healthy humans

CONTEXT

Ghrelin is a gut hormone with a highly preserved biological activity, which seems not to be restricted to the regulation of food intake, body composition, and growth. Continuous research is unraveling new properties of ghrelin, among others cardiovascular and antiinflammatory activities. Ghrelin is recently implicated in the host response to bacterial endotoxin in rodents and suggested as a possible therapeutic tool in sepsis.

OBJECTIVE

This study aimed to investigate plasma ghrelin levels during human bacterial endotoxemia.

DESIGN AND SETTING

We conducted a randomized, placebocontrolled, crossover clinical trial at a university medical center.

STUDY PARTICIPANTS

Participants included 10 healthy men.

INTERVENTION

After an overnight fast, study subjects were randomized to 2 ng/kg Escherichia coli endotoxin [lipopolysaccharide (LPS)] or placebo and monitored for 6 h.

MAIN OUTCOME MEASURES

We measured ghrelin, GH, ACTH, cortisol, glucose, free fatty acids, TNF-alpha, IL-6, and IL-1 receptor antagonist.

RESULTS

LPS administration induced a rapid ghrelin surge at 120 min (Delta ghrelin 100.2 +/- 30.3 vs. 7.2 +/- 26.4 pg/ml on the placebo day, P = 0.042). This ghrelin peak occurred 30 min after the TNF-alpha peak and corresponded with IL-6, GH, and ACTH peaks. Starting from 120 min and thereafter, ghrelin continuously decreased, reaching a nadir at 5 h after LPS administration (Delta ghrelin, -43.8 +/- 28.4 compared with 70.3 +/- 38.2 pg/ml on the control days, P = 0.038).

CONCLUSIONS

Ghrelin is one of the first hormones rapidly increasing in the human physiological response to bacterial endotoxic shock. Plasma ghrelin might be part of the complex immuno-neuroendocrine mechanisms activated by systemic infection and inflammation in humans.