Central ghrelin regulates peripheral lipid metabolism in a growth hormone-independent fashion

The Impact of Gastrointestinal Hormones on Human Adipose Tissue Function

BACKGROUND

Obesity is a global issue, the development of which depends on many interacting factors. Among these, hormones secreted in the gastrointestinal tract play an important role. The aim of this review was to assess the impact of these hormones on the functions of adipose tissue.

METHODS

The analysis was based on the latest research concerning both adipose tissue and gastrointestinal hormones.

RESULTS

It was found that these hormones can significantly affect adipose tissue, both directly and indirectly. Some hormones, when secreted in excess, can stimulate adipose tissue formation processes, while others can inhibit them. The impact of hormones depends on the location and type of adipose tissue as well as the physiological state of the body. It should also be noted that no hormone acts in isolation but in close cooperation with other factors.

CONCLUSIONS

The relationship between gastrointestinal hormones and adipose tissue, and their role in obesity, is a complex and evolving field of study. Further research is necessary, particularly into the interactions between hormones and other factors, as well as their mutual interactions.

Ghrelin regulating liver activity and its potential effects on liver fibrosis and Echinococcosis

Ghrelin widely exists in the central nervous system and peripheral organs, and has biological activities such as maintaining energy homeostasis, regulating lipid metabolism, cell proliferation, immune response, gastrointestinal physiological activities, cognition, memory, circadian rhythm and reward effects. In many benign liver diseases, it may play a hepatoprotective role against steatosis, chronic inflammation, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress and apoptosis, and improve liver cell autophagy and immune response to improve disease progression. However, the role of Ghrelin in liver Echinococcosis is currently unclear. This review systematically summarizes the molecular mechanisms by which Ghrelin regulates liver growth metabolism, immune-inflammation, fibrogenesis, proliferation and apoptosis, as well as its protective effects in liver fibrosis diseases, and further proposes the role of Ghrelin in liver Echinococcosis infection. During the infectious process, it may promote the parasitism and survival of parasites on the host by improving the immune-inflammatory microenvironment and fibrosis state, thereby accelerating disease progression. However, there is currently a lack of targeted in vitro and in vivo experimental evidence for this viewpoint.

Molecular and cellular mechanisms underlying the hepatoprotective role of ghrelin against NAFLD progression

The underlying mechanisms for the development and progression of nonalcoholic fatty liver disease (NAFLD) are complex and multifactorial. Within the last years, experimental and clinical evidences support the role of ghrelin in the development of NAFLD. Ghrelin is a gut hormone that plays a major role in the short-term regulation of appetite and long-term regulation of adiposity. The liver constitutes a target for ghrelin, where this gut-derived peptide triggers intracellular pathways regulating lipid metabolism, inflammation, and fibrosis. Interestingly, circulating ghrelin levels are altered in patients with metabolic diseases, such as obesity, type 2 diabetes, and metabolic syndrome, which, in turn, are well-known risk factors for the pathogenesis of NAFLD. This review summarizes the molecular and cellular mechanisms involved in the hepatoprotective action of ghrelin, including the reduction of hepatocyte lipotoxicity via autophagy and fatty acid β-oxidation, mitochondrial dysfunction, endoplasmic reticulum stress and programmed cell death, the reversibility of the proinflammatory phenotype in Kupffer cells, and the inactivation of hepatic stellate cells. Together, the metabolic and inflammatory pathways regulated by ghrelin in the liver support its potential as a therapeutic target to prevent NAFLD in patients with metabolic disorders.

Hunger & satiety signals: another key mechanism involved in the NAFLD pathway

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent metabolic disease, although prevalence could change according to region, nowadays is considered a public health problem whose real impact on the health system is unknown. NAFLD has a multifactorial and complex pathophysiology, due to this, developing a unique and effective pharmacological treatment has not been successful in reverting or avoiding the progression of this liver disease. Even though NAFLD pathophysiology is known, all actual treatments are focused on modifying or regulating the metabolic pathways, some of which interplay with obesity. It has been known that impairments in hunger and satiety signals are associated with obesity, however, abnormalities in these signals in patients with NAFLD and obesity are not fully elucidated. To describe these mechanisms opens an additional option as a therapeutic target sharing metabolic pathways with NAFLD, therefore, this review aims to describe the hormones and peptides implicated in both hunger-satiety in NAFLD. It has been established that NAFLD pharmacological treatment cannot be focused on a single purpose; hence, identifying interplays that lead to adding or modifying current treatment options could also have an impact on another related outcome such as hunger or satiety signals.

Role of the ghrelin system in alcohol use disorder and alcohol-associated liver disease: A narrative review

Unhealthy alcohol consumption is a global health problem. Adverse individual, public health, and socioeconomic consequences are attributable to harmful alcohol use. Epidemiological studies have shown that alcohol use disorder (AUD) and alcohol-associated liver disease (ALD) are the top two pathologies among alcohol-related diseases. Consistent with the major role that the liver plays in alcohol metabolism, uncontrolled drinking may cause significant damage to the liver. This damage is initiated by excessive fat accumulation in the liver, which can further progress to advanced liver disease. The only effective therapeutic strategies currently available for ALD are alcohol abstinence or liver transplantation. Any molecule with dual-pronged effects at the central and peripheral organs controlling addictive behaviors and associated metabolic pathways are a potentially important therapeutic target for treating AUD and ALD. Ghrelin, a hormone primarily derived from the stomach, has such properties, and regulates both behavioral and metabolic functions. In this review, we highlight recent advances in understanding the peripheral and central functions of the ghrelin system and its role in AUD and ALD pathogenesis. We first discuss the correlation between blood ghrelin concentrations and alcohol use or abstinence. Next, we discuss the role of ghrelin in alcohol-seeking behaviors and finally its role in the development of fatty liver by metabolic regulations and organ crosstalk. We propose that a better understanding of the ghrelin system could open an innovative avenue for improved treatments for AUD and associated medical consequences, including ALD.

Exogenous administration of unacylated ghrelin attenuates hepatic steatosis in high-fat diet-fed rats by modulating glucose homeostasis, lipogenesis, oxidative stress, and endoplasmic reticulum stress

Low levels of unacylated ghrelin (UAG) and a higher ratio of acylated ghrelin (AG)/UAG in obesity are associated with non-alcoholic fatty liver disease (NAFLD). This study tested the potential protective effect of increased circulatory levels of UAG by exogenous UAG administration on hepatic steatosis in high-fat diet (HFD)-fed rats and investigated some possible mechanisms. Rats were divided (n = 6/group) as low fat diet (LFD), LFD + UAG (200 mg/kg), HFD, HFD + UAG (50, 100, or 200 mg/kg). Treatments were given for 8 weeks. Increasing the dose of UAG increased circulatory levels of UAG and normalized the ratio of AG/UAG at the dose of 200 mg/kg. With no change in insulin levels, and in a dose-dependent manner, treatment with UAG to HFD rats attenuated the gain in food intake, body weights, and liver weights, lowered fasting glucose levels, prevented hepatic cytoplasmic vacuolization, and reduced serum and hepatic levels of cholesterol, triglycerides, and free fatty acids. They also progressively reduced levels of reactive oxygen species, lipid peroxides, tumor necrosis factor-α, and interleukin-6, as well as mRNA levels of Bax and caspase-3 but increased levels of glutathione and superoxide dismutase and mRNA levels of Bcl2. In concomitant, UAG, in a dose-response manner, significantly reduced hepatic mRNA levels of SREBP1, SREBP2, ATF-6, IRE-1, and eIF-2α but increased those of PPARα. In conclusion, reducing the circulatory ratio of AG/UAG ratio by exogenous administration of UAG attenuates HFD-induced hepatic steatosis by suppressing lipogenesis, stimulating FAs oxidation, preventing oxidative stress, inflammation, ER stress, and apoptosis.

MECHANISMS IN ENDOCRINOLOGY: The gut-brain axis: regulating energy balance independent of food intake

Obesity is a global pandemic with a large health and economic burden worldwide. Bodyweight is regulated by the ability of the CNS, and especially the hypothalamus, to orchestrate the function of peripheral organs that play a key role in metabolism. Gut hormones play a fundamental role in the regulation of energy balance, as they modulate not only feeding behavior but also energy expenditure and nutrient partitioning. This review examines the recent discoveries about hormones produced in the stomach and gut, which have been reported to regulate food intake and energy expenditure in preclinical models. Some of these hormones act on the hypothalamus to modulate thermogenesis and adiposity in a food intake-independent fashion. Finally, the association of these gut hormones to eating, energy expenditure, and weight loss after bariatric surgery in humans is discussed.

Increased lipolysis after infusion of acylated ghrelin: a randomized, double-blinded placebo-controlled trial in hypopituitary patients

CONTEXT

Acylated ghrelin increases growth hormone (GH) and adrenocorticotrophic hormone (ACTH) secretion from the anterior pituitary gland. Additionally, it increases free fatty acid levels independently of GH and ACTH, but the impact of ghrelin on fatty acid turnover has not been determined. This study was designed to test whether acylated ghrelin directly increases the turnover rate of fatty acids.

DESIGN

Eight hypopituitary patients on stable replacement with GH and hydrocortisone were included in a randomized, double-blinded, placebo-controlled crossover study including two study days: (a) infusion of acylated ghrelin and (b) infusion of saline. The study day comprised a basal period (t = 0-120 minutes) and a hyperinsulinaemic-euglycemic clamp period (t = 120-300 minutes). Whole-body lipolysis was estimated at t = 90-120 and t = 270-300 minutes with a palmitate isotope dilution technique.

RESULTS

Infusion of acylated ghrelin resulted in 10 times increased total ghrelin area under the curve (AUC) levels in the basal period and 15 times increased AUC levels in the clamp period compared with saline infusion (P < .001). GH_AUC levels were largely unaffected by ghrelin compared to saline infusion during both the basal and clamp period, but cortisol_AUC levels increased by 15% after ghrelin compared to saline infusion in the basal period (P = .03). Palmitate turnover was increased by 43% in the basal period (difference: 77 (20) µmol/min, P = .01) and unchanged in the clamp period (difference 0.9 (17) µmol/min, P = 1.0) after ghrelin compared to saline infusion.

CONCLUSIONS

Our results support the hypothesis that pharmacological levels of acylated ghrelin directly activate lipolysis at the whole-body level.

Pathophysiology of NAFLD and NASH in Experimental Models: The Role of Food Intake Regulating Peptides

Obesity, diabetes, insulin resistance, sedentary lifestyle, and Western diet are the key factors underlying non-alcoholic fatty liver disease (NAFLD), one of the most common liver diseases in developed countries. In many cases, NAFLD further progresses to non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and to hepatocellular carcinoma. The hepatic lipotoxicity and non-liver factors, such as adipose tissue inflammation and gastrointestinal imbalances were linked to evolution of NAFLD. Nowadays, the degree of adipose tissue inflammation was shown to directly correlate with the severity of NAFLD. Consumption of higher caloric intake is increasingly emerging as a fuel of metabolic inflammation not only in obesity-related disorders but also NAFLD. However, multiple causes of NAFLD are the reason why the mechanisms of NAFLD progression to NASH are still not well understood. In this review, we explore the role of food intake regulating peptides in NAFLD and NASH mouse models. Leptin, an anorexigenic peptide, is involved in hepatic metabolism, and has an effect on NAFLD experimental models. Glucagon-like peptide-1 (GLP-1), another anorexigenic peptide, and GLP-1 receptor agonists (GLP-1R), represent potential therapeutic agents to prevent NAFLD progression to NASH. On the other hand, the deletion of ghrelin, an orexigenic peptide, prevents age-associated hepatic steatosis in mice. Because of the increasing incidence of NAFLD and NASH worldwide, the selection of appropriate animal models is important to clarify aspects of pathogenesis and progression in this field.

Ghrelin and liver disease

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are two of the most common liver diseases associated with obesity, type 2 diabetes and metabolic syndrome. The prevalence of these conditions are increasingly rising and presently there is not a pharmacological option available in the market. Elucidation of the mechanism of action and the molecular underpinnings behind liver disease could help to better understand the pathophysiology of these illnesses. In this sense, in the last years modulation of the ghrelin system in preclinical animal models emerge as a promising therapeutic tool. In this review, we compile the latest knowledge of the modulation of ghrelin system and its intracellular pathways that regulates lipid metabolism, hepatic inflammation and liver fibrosis. We also describe novel processes implicated in the regulation of liver disease by ghrelin, such as autophagy or dysregulated circadian rhythms. In conclusion, the information displayed in this review support that the ghrelin system could be an appealing strategy for the treatment of liver disease.

Physiological Effect of Ghrelin on Body Systems

Ghrelin is a relatively novel multifaceted hormone that has been found to exert a plethora of physiological effects. In this review, we found/confirmed that ghrelin has effect on all body systems. It induces appetite; promotes the use of carbohydrates as a source of fuel while sparing fat; inhibits lipid oxidation and promotes lipogenesis; stimulates the gastric acid secretion and motility; improves cardiac performance; decreases blood pressure; and protects the kidneys, heart, and brain. Ghrelin is important for learning, memory, cognition, reward, sleep, taste sensation, olfaction, and sniffing. It has sympatholytic, analgesic, antimicrobial, antifibrotic, and osteogenic effects. Moreover, ghrelin makes the skeletal muscle more excitable and stimulates its regeneration following injury; delays puberty; promotes fetal lung development; decreases thyroid hormone and testosterone; stimulates release of growth hormone, prolactin, glucagon, adrenocorticotropic hormone, cortisol, vasopressin, and oxytocin; inhibits insulin release; and promotes wound healing. Ghrelin protects the body by different mechanisms including inhibition of unwanted inflammation and induction of autophagy. Having a clear understanding of the ghrelin effect in each system has therapeutic implications. Future studies are necessary to elucidate the molecular mechanisms of ghrelin actions as well as its application as a GHSR agonist to treat most common diseases in each system without any paradoxical outcomes on the other systems.

Acylated Ghrelin and The Regulation of Lipid Metabolism in The Intestine

Acylated ghrelin (AG) is a gastrointestinal (GI) peptide mainly secreted by the stomach that promotes cytosolic lipid droplets (CLD) hypertrophy in adipose tissues and liver. However, the role of AG in the regulation of lipid metabolism in the intestine remains unexplored. This study aimed at determining whether AG influences CLD production and chylomicron (CM) secretion in the intestine. The effects of AG and oleic acid on CLD accumulation and CM secretion were first investigated in cultured Caco-2/15 enterocytes. Intestinal lipid metabolism was also studied in Syrian Golden Hamsters submitted to conventional (CD) or Western (WD) diets for 8 weeks and continuously administered with AG or physiological saline for the ultimate 2 weeks. In cultured Caco-2/15 enterocytes, CLD accumulation influenced CM secretion while AG reduced fatty acid uptake. In WD hamsters, continuous AG treatment amplified chylomicron output while reducing postprandial CLD accumulation in the intestine. The present study supports the intimate relationship between CLD accumulation and CM secretion in the intestine and it underlines the importance of further characterizing the mechanisms through which AG exerts its effects on lipid metabolism in the intestine.

Role of ghrelin isoforms in the mitigation of hepatic inflammation, mitochondrial dysfunction, and endoplasmic reticulum stress after bariatric surgery in rats

BACKGROUND/OBJECTIVES

Bariatric surgery improves nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH), but the underlying mechanisms remain elusive. We evaluated the potential role of ghrelin isoforms in the amelioration of hepatic inflammation after sleeve gastrectomy and Roux-en-Y gastric bypass (RYGB).

SUBJECTS/METHODS

Plasma ghrelin isoforms were measured in male Wistar rats (n = 129) subjected to surgical (sham operation, sleeve gastrectomy, or RYGB) or dietary interventions [fed ad libitum a normal (ND) or a high-fat diet (HFD) or pair-fed diet]. The effect of acylated and desacyl ghrelin on markers of inflammation, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress in primary rat hepatocytes under palmitate-induced lipotoxic conditions was assessed.

RESULTS

Plasma desacyl ghrelin was decreased after sleeve gastrectomy and RYGB, whereas the acylated/desacyl ghrelin ratio was augmented. Both surgeries diminished obesity-associated hepatic steatosis, CD68⁺- and apoptotic cells, proinflammatory JNK activation, and Crp, Tnf, and Il6 transcripts. Moreover, a postsurgical amelioration in the mitochondrial DNA content, oxidative phosphorylation (OXPHOS) complexes I and II, and ER stress markers was observed. Specifically, following bariatric surgery GRP78, spliced XBP-1, ATF4, and CHOP levels were reduced, as were phosphorylated eIF2α. Interestingly, acylated and desacyl ghrelin inhibited steatosis and inflammation of palmitate-treated hepatocytes in parallel to an upregulation of OXPHOS complexes II, III, and V, and a downregulation of ER stress transducers IRE1α, PERK, ATF6, their downstream effectors, ATF4 and CHOP, as well as chaperone GRP78.

CONCLUSIONS

Our data suggest that the increased relative acylated ghrelin levels after bariatric surgery might contribute to mitigate obesity-associated hepatic inflammation, mitochondrial dysfunction, and ER stress.

Soy Isoflavones Ameliorate Metabolic and Immunological Alterations of Ovariectomy in Female Wistar Rats: Antioxidant and Estrogen Sparing Potential

Hormone replacement therapy (HRT) can alleviate estrogen deficiency symptoms especially during menopause. The present study aimed at investigating the effect of soy isoflavones as HRT on immunological and bone health-related parameters with a special focus on the interactions between immunological status and metabolism. Thirty healthy cyclic female Wistar rats were used in this experiment. Ten females were sham-operated, and 20 females were subjected to ovariectomy. Overiectomized (OVX) female rats were randomly divided into 2 groups: the control group (G1, OVX/casein) was fed a casein-based diet, and the second group (G2, OVX/soy) was fed a high soy isoflavone diet. Both groups were compared to a sham-operated group (G3, sham/casein). Treatments continued for 7 weeks. Feed intake, weight gain, and lymphoid organ relative weights were recorded. Some metabolic, immunological, and bone health-related parameters were measured. Moreover, nitric oxide (NO), malondialdehyde (MDA), and total antioxidant capacity (TAC) were determined. Bone histopathology and immunohistochemistry to estrogen receptor alpha (ERα) were done. Feeding soy to OVX females reduced feed intake, weight gain, relative lymphoid organ weight, and T-lymphocytes transformation. Soy isoflavone administration normalized nearly all metabolic and immunological parameters to a level comparable to the sham group via oxidative stress amelioration and bone ERα promotion. Soy isoflavones seemed to be good HRT in estrogen deprivation which modulated the appetite, weight gain, lipid profile, proinflammation, and bone turnover.

The Homeostatic Force of Ghrelin

Ghrelin, a gastric-derived acylated peptide, regulates energy homeostasis by transmitting information about peripheral nutritional status to the brain, and is essential for protecting organisms against famine. Ghrelin operates brain circuits to regulate homeostatic and hedonic feeding. Recent research advances have shed new light on ghrelin's multifaceted roles in cellular homeostasis, which could maintain the internal environment and overcome metaflammation in metabolic organs. Here, we highlight our current understanding of the regulatory mechanisms of the ghrelin system in energy metabolism and cellular homeostasis and its clinical trials. Future studies of ghrelin will further elucidate how the stomach regulates systemic homeostasis.

Gut Microbiome-Induced Shift of Acetate to Butyrate Positively Manages Dysbiosis in High Fat Diet

SCOPE

A recent study revealed that the accumulation of gut microbiota-produced acetate (GMPA) led to insulin over-secretion and obesity symptom. To further develop this scientific point, the effect of resistant starch (RS) or exogenous acetate carried by RS (RSA) in the gut on metabolic syndrome is investigated using diet-induced obese rats.

METHODS AND RESULTS

The metabonomics analysis shows that the gut of rats in the RSA group generate more butyrate in both serum and feces rather than acetate compared to the rats in RS group, indicating the conversion among metabolites, in particular from acetate to butyrate via gut microbiota. Consistently, the gut microbiome uses acetate as a substrate to produce butyrate, such as Coprococcus, Faecalibacterium, Roseburia, and Eubacterium and was highly promoted in RSA group, which further supports the metabolic conversion. This is the first report to reveal the accumulation of gut microbiota-produced butyrate (GMPB) but not GMPA significantly enriched AMPK signaling pathway with reduced expression of lipogenesis-associated genes for suppressing sphingosines and ceramides biosynthesis to trigger insulin sensitivity.

CONCLUSION

Gut microbiome profile and lipogenesis pathway are regulated by GMPB, which substantially influences energy harvesting in the gut from patterns opposed to GMPA.

Epistatic effects between pairs of the growth hormone secretagogue receptor 1a, growth hormone, growth hormone receptor, non-SMC condensin I complex, subunit G and stearoyl-CoA desaturase genes on carcass, price-related and fatty acid composition traits in Japanese Black cattle

Growth hormone secretagogue receptor 1a (GHSR1a), growth hormone (GH), growth hormone receptor (GHR), non-SMC condensin I complex, subunit G (NCAPG) and stearoyl-CoA desaturase (SCD), are known to play important roles in growth and lipid metabolisms. Single and epistatic effects of the five genes on carcass, price-related and fatty acid (FA) composition traits were analyzed in a commercial Japanese Black cattle population of Ibaraki Prefecture. A total of 650 steers and 116 heifers for carcass and price-related traits, and 158 steers for FA composition traits were used in this study. Epistatic effects between pairs of the five genes were found in several traits. Alleles showing strain-specific differences in the five genes had significant single and epistatic effects in some traits. The data suggest that a TG-repeat polymorphism of the GHSR1a.5'UTR-(TG)_n locus plays a central role in gene-gene epistatic interaction of FA composition traits in the adipose tissue of Japanese Black cattle.

The cellular and molecular bases of leptin and ghrelin resistance in obesity

Obesity, a major risk factor for the development of diabetes mellitus, cardiovascular diseases and certain types of cancer, arises from a chronic positive energy balance that is often due to unlimited access to food and an increasingly sedentary lifestyle on the background of a genetic and epigenetic vulnerability. Our understanding of the humoral and neuronal systems that mediate the control of energy homeostasis has improved dramatically in the past few decades. However, our ability to develop effective strategies to slow the current epidemic of obesity has been hampered, largely owing to the limited knowledge of the mechanisms underlying resistance to the action of metabolic hormones such as leptin and ghrelin. The development of resistance to leptin and ghrelin, hormones that are crucial for the neuroendocrine control of energy homeostasis, is a hallmark of obesity. Intensive research over the past several years has yielded tremendous progress in our understanding of the cellular pathways that disrupt the action of leptin and ghrelin. In this Review, we discuss the molecular mechanisms underpinning resistance to leptin and ghrelin and how they can be exploited as targets for pharmacological management of obesity.

Relationship between IGF-I Concentration and Metabolic Profile in Children with Growth Hormone Deficiency: The Influence of Children's Nutritional State as well as the Ghrelin, Leptin, Adiponectin, and Resistin Serum Concentrations

BACKGROUND

Some, however not all, children with growth hormone deficiency (GHD) reveal a tendency towards metabolic disorders. Insulin-like growth factor I (IGF-I) is the main mediator of GH anabolic effects.

OBJECTIVE

The aim of the study was to compare ghrelin, adiponectin, leptin, resistin, lipid, glucose, and insulin concentrations in GHD children, depending on the IGF-I bioavailability.

METHODS

The analysis comprised 26 children with GHD, aged 5.7-15.3 yrs. Fasting serum concentrations of IGF-I, IGFBP-3, ghrelin, leptin, adiponectin, resistin, lipids, glucose, and insulin were measured. The GHD children were divided into two subgroups: (1) with lower IGF-I/IGFBP-3 molar ratio and (2) with higher IGF-I/IGFBP-3 molar ratio. The control group consisted of 39 healthy children, aged 5.1-16.6 yrs, of normal height and body mass.

RESULTS

GHD children with lower IGF-I/IGFBP-3 molar ratio were found to have a significantly lower body mass and insulin and triglyceride concentrations, as well as significantly higher ghrelin and adiponectin concentrations than GHD children with higher IGF-I/IGFBP-3.

CONCLUSIONS

A better metabolic profile characterised GHD children with low IGF-I bioavailability. This phenomenon may be the result of high adiponectin and ghrelin concentrations in those children and their influence on adipose tissue, glucose uptake, and orexigenic axis.

Nutritional Status in Short Stature Children Is Related to Both Ghrelin and Insulin-like Growth Factor I Concentrations

OBJECTIVES

Ghrelin plays an important role in the growth processes in children. In addition, it regulates appetite. The aim of the study was to assess ghrelin and insulin-like growth factor type I (IGF-I) concentrations in children with idiopathic short stature, dependent on nutritional status.

METHODS

The study group included 116 children, ages 10.6 ± 3.5 years (mean ± standard deviation), with idiopathic short stature (height <-2.0 standard deviation scores [SDS], maximal growth hormone [GH] secretion during 2 GH-stimulating tests->10 ng/mL). In each child, fasting ghrelin, IGF-I, insulin-like growth factor binding protein 3 (IGFBP-3), glucose, insulin, lipids, leptin, adiponectin, and resistin concentrations were assessed. The IGF-I/IGFBP-3 molar ratio was calculated to determine the IGF-I bioavailability. According to body mass index SDS calculated for height age, the children were divided into 3 groups: poorly nourished (thin), normal, and obese. The control group consisted of 19 healthy children, ages 11.0 ± 3.5 years, with normal body weight and height.

RESULTS

Ghrelin concentration was significantly higher in short, thin children than in short, obese children (1458.3 ± 798.5 vs 917.2 ± 303.0 pg/mL; P < 0.005). In turn, IGF-I/IGFBP-3 molar ratio was significantly lower in short, thin children than in short, obese children (0.16 ± 0.06 vs 0.28 ± 0.15; P < 0.005).

CONCLUSIONS

In short, thin children, despite elevated ghrelin production, the low IGF-I concentration is observed, probably due to undernutrition and worse IGF-I formation. In short, normal-weight children and in short, obese ones, ghrelin and IGF-I production is normal, and it seems that mechanisms responsible for their short stature are other than low IGF-I.

Estradiol effects on hypothalamic AMPK and BAT thermogenesis: A gateway for obesity treatment?

In addition to their prominent roles in the control of reproduction, estrogens are important modulators of energy balance, as evident in conditions of deficiency of estrogens, which are characterized by increased feeding and decreased energy expenditure, leading to obesity. AMP-activated protein kinase (AMPK) is a ubiquitous cellular energy gauge that is activated under conditions of low energy, increasing energy production and reducing energy wasting. Centrally, the AMPK pathway is a canonical route regulating energy homeostasis, by integrating peripheral signals, such as hormones and metabolites, with neuronal networks. As a result of those actions, hypothalamic AMPK modulates feeding, as well as brown adipose tissue (BAT) thermogenesis and browning of white adipose tissue (WAT). Here, we will review the central actions of estrogens on energy balance, with particular focus on hypothalamic AMPK. The relevance of this interaction is noteworthy, because some agents with known actions on metabolic homeostasis, such as nicotine, metformin, liraglutide, olanzapine and also natural molecules, such as resveratrol and flavonoids, exert their actions by modulating AMPK. This evidence highlights the possibility that hypothalamic AMPK might be a potential target for the treatment of obesity.

Current Understanding of the Hypothalamic Ghrelin Pathways Inducing Appetite and Adiposity

Ghrelin is a multifaceted regulator of metabolism. Ghrelin regulates energy balance in the short term via induction of appetite and in the long term via increased body weight and adiposity. Recently, several central pathways modulating the metabolic actions of ghrelin were unmasked, and it was shown to act through different hypothalamic nuclei to induce feeding. Ghrelin also modulates glucose homeostasis, but the central mechanisms responsible for this action have not been studied in detail. Although ghrelin also acts through extrahypothalamic areas to promote feeding, this review specifically dissects hypothalamic control of ghrelin's orexigenic and adipogenic actions and presents current understanding of the intracellular ghrelin orexigenic pathways, including their dependence on other relevant systems implicated in energy balance.

Acylated and desacyl ghrelin are associated with hepatic lipogenesis, β-oxidation and autophagy: role in NAFLD amelioration after sleeve gastrectomy in obese rats

Bariatric surgery improves non-alcoholic fatty liver disease (NAFLD). Our aim was to investigate the potential role of ghrelin isoforms in the resolution of hepatic steatosis after sleeve gastrectomy, a restrictive bariatric surgery procedure, in diet-induced obese rats. Male Wistar rats (n = 161) were subjected to surgical (sham operation and sleeve gastrectomy) or dietary interventions [fed ad libitum a normal (ND) or a high-fat (HFD) diet or pair-fed]. Obese rats developed hepatosteatosis and showed decreased circulating desacyl ghrelin without changes in acylated ghrelin. Sleeve gastrectomy induced a dramatic decrease of desacyl ghrelin, but increased the acylated/desacyl ghrelin ratio. Moreover, sleeve gastrectomy reduced hepatic triglyceride content and lipogenic enzymes Mogat2 and Dgat1, increased mitochondrial DNA amount and induced AMPK-activated mitochondrial FFA β-oxidation and autophagy to a higher extent than caloric restriction. In primary rat hepatocytes, the incubation with both acylated and desacyl ghrelin (10, 100 and 1,000 pmol/L) significantly increased TG content, triggered AMPK-activated mitochondrial FFA β-oxidation and autophagy. Our data suggest that the decrease in the most abundant isoform, desacyl ghrelin, after sleeve gastrectomy contributes to the reduction of lipogenesis, whereas the increased relative acylated ghrelin levels activate factors involved in mitochondrial FFA β-oxidation and autophagy in obese rats, thereby ameliorating NAFLD.

Intracerebroventricular ghrelin treatment affects lipid metabolism in liver of rainbow trout (Oncorhynchus mykiss)

We aimed to elucidate in rainbow trout (Oncorhynchus mykiss) the effects of central ghrelin (GHRL) treatment on the regulation of liver lipid metabolism, and the possible modulatory effect of central GHRL treatment on the simultaneous effects of raised levels of oleate. Thus, we injected intracerebroventricularly (ICV) rainbow trout GHRL in the presence or absence of oleate and evaluated in liver variables related to lipid metabolism. Oleate treatment elicited in liver of rainbow trout decreased lipogenesis and increased oxidative capacity in agreement with previous studies. Moreover, as demonstrated for the first time in fish in the present study, GHRL also acts centrally modulating lipid metabolism in liver, resulting in increased potential for lipogenesis and decreased potential for fatty acid oxidation, i.e. the converse effects to those elicited by central oleate treatment. The simultaneous treatment of GHRL and oleate confirmed these counteractive effects. Thus, the nutrient sensing mechanisms present in hypothalamus, particularly those involved in sensing of fatty acid, are involved in the control of liver energy metabolism in fish, and this control is modulated by the central action of GHRL. These results give support to the notion of hypothalamus as an integrative place for the regulation of peripheral energy metabolism in fish.

The Role of Ghrelin in Senescence: A Mini-Review

Ghrelin, a 28-amino acid hormone produced mainly by the X/A-like endocrine cells in gastric mucosa, has a widespread tissue distribution and diverse physiological functions such as hormonal, orexigenic, metabolic, cardiovascular, neurological, and immunological activities. Considerable evidence has suggested that ghrelin plays an important role in organism senescence or aging. The present review provides a comprehensive picture of this new development. We first reviewed the aging (senescence)-dependent reduction of ghrelin signaling, and then highlighted its relationship with the aging-associated alteration in food intake, energy metabolism, cardiovascular function, neurological activity, and adaptive immunity. Our literature review suggests that ghrelin is an innovative and promising agent in the treatment of these pathophysiological conditions associated with senescence.

Ghrelin promotes hepatic lipogenesis by activation of mTOR-PPARγ signaling pathway

Although ghrelin has been demonstrated to stimulate energy intake and storage through a central mechanism, its effect on hepatic lipid metabolism remains largely uncharacterized. Ghrelin receptor antagonism or gene deletion significantly decreased obesity-associated hepatic steatosis by suppression of de novo lipogenesis, whereas exogenous ghrelin stimulated lipogenesis, leading to hepatic lipid accumulation in mice. The effects of ghrelin were mediated by direct activation of its receptor on hepatocytes. Cultured hepatocytes responded to ghrelin with increased lipid content and expression of lipogenesis-related genes. Ghrelin increased phosphorylation of S6, the downstream target of mammalian target of rapamycin (mTOR) signaling in cultured hepatocytes, whereas ghrelin receptor antagonism reduced hepatic phosphorylation of S6 in db/db mice. Inhibition of mTOR signaling by rapamycin markedly attenuated ghrelin-induced up-regulation of lipogenesis in hepatocytes, whereas activation of hepatic mTOR signaling by deletion of TSC1 increased hepatic lipogenesis. By interacting with peroxisome proliferator-activated receptor-γ (PPARγ), mTOR mediates the ghrelin-induced up-regulation of lipogenesis in hepatocytes. The stimulatory effect of ghrelin on hepatic lipogenesis was significantly attenuated by PPARγ antagonism in cultured hepatocytes and in PPARγ gene-deficient mice. Our study indicates that ghrelin activates its receptor on hepatocytes to promote lipogenesis via a mechanism involving the mTOR-PPARγ signaling pathway.

Peripheral targets in obesity treatment: a comprehensive update

Obesity is a major epidemic of our time and is associated with diseases such as metabolic syndrome, type 2 diabetes mellitus and atherosclerotic cardiovascular disease. Although weight loss drugs, when accompanied by diet and exercise, could be a very helpful medical tool in treating obese or overweight patients, their usefulness has been questioned due to the complexity of this type of medication, which regards a plethora of issues such as efficacy and safety of the drug and also risks and benefits among different patients. In general, obesity drugs that target peripheral pathophysiological mechanisms can be divided into two main categories. The first category includes anti-obesity agents able to reduce or limit energy absorption, such as pancreatic lipase and microsomal triglyceride transfer protein inhibitors. The second category consists of a heterogeneous group of compounds aiming to decrease fat mass by increasing energy expenditure or by redistributing adipose tissue. Angiogenesis inhibitors, beta-3 receptor agonists, sirtuin-I activators, diazoxide and other molecules belong to this group. The glucagon-like peptide-1 receptor agonists consist the third category of peripheral anti-obesity agents discussed therein. This review aims to provide a general overview of the molecules and substances that are already or could potentially be used as peripheral anti-obesity drugs, the molecular mechanisms by which they act, as well as their current stage of development, production and/or availability.

Long-term increased carnitine palmitoyltransferase 1A expression in ventromedial hypotalamus causes hyperphagia and alters the hypothalamic lipidomic profile

Lipid metabolism in the ventromedial hypothalamus (VMH) has emerged as a crucial pathway in the regulation of feeding and energy homeostasis. Carnitine palmitoyltransferase (CPT) 1A is the rate-limiting enzyme in mitochondrial fatty acid β-oxidation and it has been proposed as a crucial mediator of fasting and ghrelin orexigenic signalling. However, the relationship between changes in CPT1A activity and the intracellular downstream effectors in the VMH that contribute to appetite modulation is not fully understood. To this end, we examined the effect of long-term expression of a permanently activated CPT1A isoform by using an adeno-associated viral vector injected into the VMH of rats. Peripherally, this procedure provoked hyperghrelinemia and hyperphagia, which led to overweight, hyperglycemia and insulin resistance. In the mediobasal hypothalamus (MBH), long-term CPT1AM expression in the VMH did not modify acyl-CoA or malonyl-CoA levels. However, it altered the MBH lipidomic profile since ceramides and sphingolipids increased and phospholipids decreased. Furthermore, we detected increased vesicular γ-aminobutyric acid transporter (VGAT) and reduced vesicular glutamate transporter 2 (VGLUT2) expressions, both transporters involved in this orexigenic signal. Taken together, these observations indicate that CPT1A contributes to the regulation of feeding by modulating the expression of neurotransmitter transporters and lipid components that influence the orexigenic pathways in VMH.

Neonatal events, such as androgenization and postnatal overfeeding, modify the response to ghrelin

It is currently accepted that ambient, non-genetic factors influence perinatal development and evoke structural and functional changes that may persist throughout life. Overfeeding and androgenization after birth are two of these key factors that could result in “metabolic imprinting” of neuronal circuits early in life and, thereby, increase the body weight homeostatic “set point”, stimulate appetite, and result in obesity. Our aim was to determine the influence of these obesogenic factors on the response to ghrelin. We observed the expected orexigenic effect of ghrelin regardless of the nutritional or hormonal manipulations to which the animals were subjected to at early postnatal development and this effect remained intact at later stages of development. In fact, ghrelin responses increased significantly when the animals were subjected to one of the two manipulations, but not when both were combined. An increased response to ghrelin could explain the obese phenotype displayed by individuals with modified perinatal environment.

Nicotine improves obesity and hepatic steatosis and ER stress in diet-induced obese male rats

Nicotine, the main addictive component of tobacco, promotes body weight reduction in humans and rodents. Recent evidence has suggested that nicotine acts in the central nervous system to modulate energy balance. Specifically, nicotine modulates hypothalamic AMP-activated protein kinase to decrease feeding and to increase brown adipose tissue thermogenesis through the sympathetic nervous system, leading to weight loss. Of note, most of this evidence has been obtained in animal models fed with normal diet or low-fat diet (LFD). However, its effectiveness in obese models remains elusive. Because obesity causes resistance towards many factors involved in energy homeostasis, the aim of this study has been to compare the effect of nicotine in a diet-induced obese (DIO) model, namely rats fed a high-fat diet, with rats fed a LFD. Our data show that chronic peripheral nicotine treatment reduced body weight by decreasing food intake and increasing brown adipose tissue thermogenesis in both LFD and DIO rats. This overall negative energy balance was associated to decreased activation of hypothalamic AMP-activated protein kinase in both models. Furthermore, nicotine improved serum lipid profile, decreased insulin serum levels, as well as reduced steatosis, inflammation, and endoplasmic reticulum stress in the liver of DIO rats but not in LFD rats. Overall, this evidence suggests that nicotine diminishes body weight and improves metabolic disorders linked to DIO and might offer a clear-cut strategy to develop new therapeutic approaches against obesity and its metabolic complications.

Structure and physiological actions of ghrelin

Ghrelin is a gastric peptide hormone, discovered as being the endogenous ligand of growth hormone secretagogue receptor. Ghrelin is a 28 amino acid peptide presenting a unique n-octanoylation modification on its serine in position 3, catalyzed by ghrelin O-acyl transferase. Ghrelin is mainly produced by a subset of stomach cells and also by the hypothalamus, the pituitary, and other tissues. Transcriptional, translational, and posttranslational processes generate ghrelin and ghrelin-related peptides. Homo- and heterodimers of growth hormone secretagogue receptor, and as yet unidentified receptors, are assumed to mediate the biological effects of acyl ghrelin and desacyl ghrelin, respectively. Ghrelin exerts wide physiological actions throughout the body, including growth hormone secretion, appetite and food intake, gastric secretion and gastrointestinal motility, glucose homeostasis, cardiovascular functions, anti-inflammatory functions, reproductive functions, and bone formation. This review focuses on presenting the current understanding of ghrelin and growth hormone secretagogue receptor biology, as well as the main physiological effects of ghrelin.

The orexigenic effect of orexin-A revisited: dependence of an intact growth hormone axis

Fifteen years ago orexins were identified as central regulators of energy homeostasis. Since then, that concept has evolved considerably and orexins are currently considered, besides orexigenic neuropeptides, key modulators of sleep-wake cycle and neuroendocrine function. Little is known, however, about the effect of the neuroendocrine milieu on orexins' effects on energy balance. We therefore investigated whether hypothalamic-pituitary axes have a role in the central orexigenic action of orexin A (OX-A) by centrally injecting hypophysectomized, adrenalectomized, gonadectomized (male and female), hypothyroid, and GH-deficient dwarf rats with OX-A. Our data showed that the orexigenic effect of OX-A is fully maintained in adrenalectomized and gonadectomized (females and males) rats, slightly reduced in hypothyroid rats, and totally abolished in hypophysectomized and dwarf rats when compared with their respective vehicle-treated controls. Of note, loss of the OX-A effect on feeding was associated with a blunted OX-A-induced increase in the expression of either neuropeptide Y or its putative regulator, the transcription factor cAMP response-element binding protein, as well as its phosphorylated form, in the arcuate nucleus of the hypothalamus of hypophysectomized and dwarf rats. Overall, this evidence suggests that the orexigenic action of OX-A depends on an intact GH axis and that this neuroendocrine feedback loop may be of interest in the understanding of orexins action on energy balance and GH deficiency.

Ghrelin requires p53 to stimulate lipid storage in fat and liver

Ghrelin, a stomach-derived peptide, stimulates feeding behavior and adiposity. For its orexigenic action, ghrelin triggers a central SIRT1/p53/AMPK pathway. The tumor suppressor p53 also plays an important role in white adipose tissue (WAT), where it is up-regulated in the adipocytes of obese mice. It is not known, however, whether p53 has any role in mediating the peripheral action of ghrelin. In the present study, chronic peripheral ghrelin treatment resulted in increased body weight and fat-mass gain in wild-type mice. Correspondingly, mRNA levels of several adipogenic and fat-storage-promoting enzymes were up-regulated in WAT, whereas hepatic triglyceride content and lipogenic enzymes were also increased in wild-type mice following ghrelin treatment. In contrast, mice lacking p53 failed to respond to ghrelin treatment, with their body weight, fat mass, and adipocyte and hepatic metabolism remaining unchanged. Thus, our results show that p53 is necessary for the actions of ghrelin on WAT and liver, leading to changes in expression levels of lipogenic and adipogenic genes, and modifying body weight.

The CB1 receptor mediates the peripheral effects of ghrelin on AMPK activity but not on growth hormone release

This study aimed to investigate whether the growth hormone release and metabolic effects of ghrelin on AMPK activity of peripheral tissues are mediated by cannabinoid receptor type 1 (CB1) and the central nervous system. CB1-knockout (KO) and/or wild-type mice were injected peripherally or intracerebroventricularly with ghrelin and CB1 antagonist rimonabant to study tissue AMPK activity and gene expression (transcription factors SREBP1c, transmembrane protein FAS, enzyme PEPCK, and protein HSL). Growth hormone levels were studied both in vivo and in vitro. Peripherally administered ghrelin in liver, heart, and adipose tissue AMPK activity cannot be observed in CB1-KO or CB1 antagonist-treated mice. Intracerebroventricular ghrelin treatment can influence peripheral AMPK activity. This effect is abolished in CB1-KO mice and by intracerebroventricular rimonabant treatment, suggesting that central CB1 receptors also participate in the signaling pathway that mediates the effects of ghrelin on peripheral tissues. Interestingly, in vivo or in vitro growth hormone release is intact in response to ghrelin in CB1-KO animals. Our data suggest that the metabolic effects of ghrelin on AMPK in peripheral tissues are abolished by the lack of functional CB1 receptor via direct peripheral effect and partially through the central nervous system, thus supporting the existence of a possible ghrelin-cannabinoid-CB1-AMPK pathway.

Chronic central administration of Ghrelin increases bone mass through a mechanism independent of appetite regulation

Leptin plays a critical role in the central regulation of bone mass. Ghrelin counteracts leptin. In this study, we investigated the effect of chronic intracerebroventricular administration of ghrelin on bone mass in Sprague-Dawley rats (1.5 μg/day for 21 days). Rats were divided into control, ghrelin ad libitum-fed (ghrelin ad lib-fed), and ghrelin pair-fed groups. Ghrelin intracerebroventricular infusion significantly increased body weight in ghrelin ad lib-fed rats but not in ghrelin pair-fed rats, as compared with control rats. Chronic intracerebroventricular ghrelin infusion significantly increased bone mass in the ghrelin pair-fed group compared with control as indicated by increased bone volume percentage, trabecular thickness, trabecular number and volumetric bone mineral density in tibia trabecular bone. There was no significant difference in trabecular bone mass between the control group and the ghrelin ad-lib fed group. Chronic intracerebroventricular ghrelin infusion significantly increased the mineral apposition rate in the ghrelin pair-fed group as compared with control. In conclusion, chronic central administration of ghrelin increases bone mass through a mechanism that is independent of body weight, suggesting that ghrelin may have a bone anabolic effect through the central nervous system.

Energy balance regulation by thyroid hormones at central level

Classically, medical textbooks taught that most effects of thyroid hormones (THs) on energy homeostasis are directly exerted in peripheral tissues. However, current evidence is changing (and challenging) our perspective about the role of THs from a 'peripheral' to a 'central' vision, implying that they affect food intake, energy expenditure, and metabolism by acting, to a large extent, at the central level. Interestingly, effects of THs are interrelated with global energy sensors in the central nervous system (CNS), such as uncoupling protein 2 (UCP2), AMP-activated protein kinase (AMPK; the 'AMPK-BAT axis'), and mechanistic target of rapamycin (mTOR). Here, we review what is currently known about THs and their regulation of energy balance and metabolism in both peripheral and central tissues.

Ghrelin and cannabinoids require the ghrelin receptor to affect cellular energy metabolism

INTRODUCTION

Ghrelin is a potent orexigenic brain-gut peptide with lipogenic and diabetogenic effects, possibly mediated by growth hormone secretagogue receptor (GHS-R1a). Cannabinoids also have orexigenic and lipogenic effects. AMPK is a regulator of energy homeostasis and we have previously shown that ghrelin and cannabinoids stimulate hypothalamic AMPK activity while inhibiting it in the liver and adipose tissue, suggesting that AMPK mediates both the central appetite-inducing and peripheral effects of ghrelin and cannabinoids.

AIMS

Using GHS-R KO mice, we investigated whether the known ghrelin receptor GHS-R1a is required for the tissue-specific effects of ghrelin on AMPK activity, and if an intact ghrelin signalling pathway is necessary for the effects of cannabinoids on AMPK activity.

METHODS

Wild-type and GHS-R KO mice were treated intraperitoneally with ghrelin 500 ng/g bodyweight or CB1 agonist HU210 20 ng/g and hypothalamic, hepatic and adipose AMPK activity was studied using a functional kinase assay.

RESULTS

Ghrelin and HU210 significantly stimulated hypothalamic AMPK activity in wild-type animals (mean±SEM, 122.5±5.2% and 128±11.6% of control, p<0.05) and inhibited it in liver (55.1±4.8% and 62.2±14.5%, p<0.01) and visceral fat (mesenteric fat (MF): 54.6±16% and 52.0±9.3%, p<0.05; epididymal fat (EF): 47.9±12.1% and 45.6±1.7%, p<0.05). The effects of ghrelin, and interestingly also HU210, on hypothalamic, visceral fat and liver AMPK activity were abolished in the GHS-R KO mice (hypothalamus: 107.9±7.7% and 87.4±13.3%, liver: 100.5±11.6% and 116.7±5.4%, MF: 132.1±29.9% and 107.1±32.7%, EF: 89.8±7.3% and 91.7±18.3%, p>0.05).

CONCLUSIONS

Ghrelin requires GHS-R1a for its effect on hypothalamic, liver and adipose tissue AMPK activity. An intact ghrelin signalling pathway is necessary for the effects of cannabinoids on AMPK activity.

Ghrelin-induced hippocampal neurogenesis and enhancement of cognitive function are mediated independently of GH/IGF-1 axis: lessons from the spontaneous dwarf rats

We recently have reported that ghrelin modulates adult hippocampal neurogenesis. However, there is a possibility that the action of ghrelin on hippocampal neurogenesis could be, in part, due to the ability of ghrelin to stimulate the GH/insulin-like growth factor (IGF)-1 axis, where both GH and IGF-1 infusions are known to increase hippocampal neurogenesis. To explore this possibility, we assessed the impact of ghrelin on progenitor cell proliferation and differentiation in the dentate gyrus (DG) of spontaneous dwarf rats (SDRs), a dwarf strain with a mutation of the GH gene resulting in total loss of GH. Double immunohistochemical staining revealed that Ki-67-positive progenitor cells and doublecortin (DCX)-positive neuroblasts in the DG of the SDRs expressed ghrelin receptors. We found that ghrelin treatment in the SDRs significantly increased the number of proliferating cell nuclear antigen- and BrdU-labeled cells in the DG. The number of DCX-labeled cells in the DG of ghrelin-treated SDRs was also significantly increased compared with the vehicle-treated controls. To test whether ghrelin has a direct effect on cognitive performance independently of somatotropic axis, hippocampus-dependent learning and memory were assessed using the Y-maze and novel object recognition (NOR) test in the SDRs. Ghrelin treatment for 4 weeks by subcutaneous osmotic pump significantly increased alternation rates in the Y-maze and exploration time for novel object in the NOR test compared to vehicle-treated controls. Our results indicate that ghrelin-induced adult hippocampal neurogenesis and enhancement of cognitive function are mediated independently of somatotropic axis.

Hypothalamic mTOR signaling mediates the orexigenic action of ghrelin

Current evidence suggests that ghrelin, a stomach derived peptide, exerts its orexigenic action through specific modulation of Sirtuin1 (SIRT1)/p53 and AMP-activated protein kinase (AMPK) pathways, which ultimately increase the expression of agouti-related protein (AgRP) and neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus (ARC). However, there is a paucity of data about the possible action of ghrelin on alternative metabolic pathways at this level. Here, we demonstrate that ghrelin elicits a marked upregulation of the hypothalamic mammalian target of rapamycin (mTOR) signaling pathway. Of note, central inhibition of mTOR signaling with rapamycin decreased ghrelin’s orexigenic action and normalized the mRNA expression of AgRP and NPY, as well as their key downstream transcription factors, namely cAMP response-element binding protein (pCREB) and forkhead box O1 (FoxO1, total and phosphorylated). Taken together, these data indicate that, in addition to previous reported mechanisms, ghrelin also promotes feeding through modulation of hypothalamic mTOR pathway.

Points of integration between the intracellular energy sensor AMP-activated protein kinase (AMPK) activity and the somatotroph axis function

AMP-activated protein kinase (AMPK), an enzyme functioning as a cellular sensor of low energy, stores and promotes adaptive changes in growth, differentiation, and metabolism. While AMPK is primarily thought of as a regulator of systemic metabolism, it has been clearly established that it also has a role in the regulation of cell growth and may be a therapeutic target for proliferative disorders. Growth hormone (GH) secretion from the anterior pituitary and GH-induced synthesis and release of insulin-like-growth-factor-1 (IGF-1) from the liver determine linear growth before puberty. Actually, GH and IGF-1 are potent growth factors affecting cell growth and differentiation in different tissues, and still have anabolic functions and serve as essential regulators of fuel metabolism in adulthood, as well. A variety of peripheral hormonal and metabolic signals regulate GH secretion either by acting directly on the anterior pituitary and/or modulating GH-releasing hormone or somatostatin release from the hypothalamus. Actually, intracellular transduction of endocrine and metabolic signals regulating somatotroph function is still debated. Based on the previously summarized contents, the aim of the present work has been to review currently available data suggesting a role of AMPK in the interplay between GH axis activity and metabolic functions.

Changes in expression of hepatic genes involved in lipid metabolism during prehibernation period in captive adult female Japanese black bears (Ursus thibetanus japonicus)

Body fat accumulation in the prehibernation period is crucial for survival and reproduction during hibernation for bears. Bear body mass increases rapidly during their autumnal hyperphagia phase, which is attributed not only to an increase in food availability, but also to physiological changes in lipid metabolism. To test this hypothesis, we investigated changes in blood biochemical values and mRNA expression levels of hepatic genes involved in lipid metabolism during the active period (June, August, October, and November) in Japanese black bears ( Ursus thibetanus japonicus Schlegel, 1857), which were fed a constant ration throughout this period. Blood biochemical analysis revealed that plasma triglyceride concentrations decreased in October and November, implying that peripheral triglyceride uptake was accelerated in autumn. The liver was sampled by needle biopsy. Real-time polymerase chain reaction (PCR) analysis revealed that mRNA expressions of enzymes involved in glycolysis (glucokinase), as well as fatty acid and triglyceride synthesis (ATP-citrate lyase, acetyl-CoA carboxykinase 1, fatty acid synthase, and diacylglycerol O-acyltransferase 2), increased in November, which suggests that hepatic lipogenesis becomes accelerated during the hyperphagia phase. These results suggest that lipid metabolism is seasonally controlled even without changes in food intake. These physiological changes seen in the prehibernation period would contribute to the rapid mass gain necessary for hibernation.

Underlying mechanism of aconitum lizhong acting on experimental hypothermia with indigestion in rats: role of ghrelin

This study is aimed to investigate the Aconitum Lizhong pill (ALZ) pharmacological actions on hypothermia with indigestion, especially the ghrelin roles. The littermate-matched rats were randomly divided into four groups. Control did sham operation or standard diet, Model carried out interscapular brown adipose (IBA) removal with standard diet, Fat-diet did IBA removal with fat-diet, and ALZ did IBA removal and fat-diet with 4.536 g/kg/d ALZ. The potency of adaptive thermogenesis, ghrelin levels in plasma or gastric mucosa, thyroid hormones and metabolite in sera, expression of ghrelin mRNA, and protein in gastric mucous membrane were determined. ALZ relieved the hypothermia processes with indigestion, via inhibiting ghrelin expression and increasing ghrelin secretion; the dynamics from the therapy is supported with the energy changes as less body weight loss, less plasma lipid decrease, more plasma T₃ or T₄ increase with TSH decrease, and more compensation of thermogenic AUC decrease. Ghrelin played key roles in the actions of ALZ on the hypothermia with indigestion. The pharmacological mechanisms of ALZ involved the homeostasis of ghrelin expression and secretion.

Ghrelin - a pleiotropic hormone secreted from endocrine x/a-like cells of the stomach

The gastric X/A-like endocrine cell receives growing attention due to its peptide products with ghrelin being the best characterized. This peptide hormone was identified a decade ago as a stimulator of food intake and to date remains the only known peripherally produced and centrally acting orexigenic hormone. In addition, subsequent studies identified numerous other functions of this peptide including the stimulation of gastrointestinal motility, the maintenance of energy homeostasis and an impact on reproduction. Moreover, ghrelin is also involved in the response to stress and assumed to play a role in coping functions and exert a modulatory action on immune pathways. Our knowledge on the regulation of ghrelin has markedly advanced during the past years by the identification of the ghrelin acylating enzyme, ghrelin-O-acyltransferase, and by the description of changes in expression, activation, and release under different metabolic as well as physically and psychically challenging conditions. However, our insight on regulatory processes of ghrelin at the cellular and subcellular levels is still very limited and warrants further investigation.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

The role of ghrelin in the control of energy balance

Ghrelin is the only potent orexigenic peptide in circulation. It stimulates food intake and leads to positive energy balance, adipogenesis, and body weight gain. However, the physiological significance of ghrelin in the regulation of energy homeostasis is controversial, since loss of ghrelin function in rodents does not necessarily lead to anorexia and weight loss. In this chapter, we discuss the metabolic function of ghrelin and are highlighting recent findings including the discovery and function of ghrelin-acylating enzyme ghrelin O-acyltransferase (GOAT). Based on available published data, we conclude that ghrelin is a principally important endogenous regulator of energy balance, which however may affect both food intake and systemic metabolism via independent mechanisms. Importantly, ghrelin, when acylated by GOAT, might represent a key molecular link between the sensing of consumed calories and the neuroendocrine control of energy homeostasis. Thus, agents antagonizing the action of ghrelin may have therapeutic potential in the therapy of obesity.