Neuroendocrine and metabolic activities of ghrelin gene products

Dysregulation of IGF-1/GLP-1 signaling in the progression of ALS: potential target activators and influences on neurological dysfunctions

The prominent causes for motor neuron diseases like ALS are demyelination, immune dysregulation, and neuroinflammation. Numerous research studies indicate that the downregulation of IGF-1 and GLP-1 signaling pathways plays a significant role in the progression of ALS pathogenesis and other neurological disorders. In the current review, we discussed the dysregulation of IGF-1/GLP-1 signaling in neurodegenerative manifestations of ALS like a genetic anomaly, oligodendrocyte degradation, demyelination, glial overactivation, immune deregulation, and neuroexcitation. In addition, the current review reveals the IGF-1 and GLP-1 activators based on the premise that the restoration of abnormal IGF-1/GLP-1 signaling could result in neuroprotection and neurotrophic effects for the clinical-pathological presentation of ALS and other brain diseases. Thus, the potential benefits of IGF-1/GLP-1 signal upregulation in the development of disease-modifying therapeutic strategies may prevent ALS and associated neurocomplications.

How treatments with endocrine and metabolic drugs influence pituitary cell function

A variety of endocrine and metabolic signals regulate pituitary cell function acting through the hypothalamus-pituitary neuroendocrine axes or directly at the pituitary level. The underlying intracellular transduction mechanisms in pituitary cells are still debated. AMP-activated protein kinase (AMPK) functions as a cellular sensor of low energy stores in all mammalian cells and promotes adaptive changes in response to calorie restriction. It is also regarded as a target for therapy of proliferative disorders. Various hormones and drugs can promote tissue-specific activation or inhibition of AMPK by enhancing or inhibiting AMPK phosphorylation, respectively. This review explores the preclinical studies published in the last decade that investigate the role of AMP-activated protein kinase in the intracellular transduction pathways downstream of endocrine and metabolic signals or drugs affecting pituitary cell function, and its role as a target for drug therapy of pituitary proliferative disorders. The effects of the hypoglycemic agent metformin, which is an indirect AMPK activator, are discussed. The multiple effects of metformin on cell metabolism and cell signalling and ultimately on cell function may be either dependent or independent of AMPK. The in vitro effects of metformin may also help highlighting differences in metabolic requirements between pituitary adenomatous cells and normal cells.

Ghrelin forms in the modulation of energy balance and metabolism

Ghrelin is a gastric hormone circulating in acylated (AG) and unacylated (UnAG) forms. This narrative review aims at presenting current emerging knowledge on the impact of ghrelin forms on energy balance and metabolism. AG represents ~ 10% of total plasma ghrelin, has an appetite-stimulating effect and is the only form for which a receptor has been identified. Moreover, other metabolic AG-induced effects have been reported, including the modulation of glucose homeostasis with stimulation of liver gluconeogenesis, the increase of fat mass and the improvement of skeletal muscle mitochondrial function. On the other hand, UnAG has no orexigenic effects, however recent reports have shown that it is directly involved in the modulation of skeletal muscle energy metabolism by improving a cluster of interlinked functions including mitochondrial redox activities, tissue inflammation and insulin signalling and action. These findings are in agreement with human studies which show that UnAG circulating levels are positively associated with insulin sensitivity both in metabolic syndrome patients and in a large cohort from the general population. Moreover, ghrelin acylation is regulated by a nutrient sensor mechanism, specifically set on fatty acids availability. These recent findings consistently point towards a novel independent role of UnAG as a regulator of muscle metabolic pathways maintaining energy status and tissue anabolism. While a specific receptor for UnAG still needs to be identified, recent evidence strongly supports the hypothesis that the modulation of ghrelin-related molecular pathways, including those involved in its acylation, may be a potential novel target in the treatment of metabolic derangements in disease states characterized by metabolic and nutritional complications.Level of evidence Level V, narrative review.

Growth Hormone Response to Oral Glucose Load: From Normal to Pathological Conditions

The exact physiological basis of acute growth hormone (GH) suppression by oral glucose is not fully understood. Glucose-mediated increase in hypothalamic somatostatin seems to be the most plausible explanation. Attempts to better understand its underlying mechanisms are compromised by species disparities in the response of GH to glucose load. While in humans, glucose inhibits GH release, the acute elevation of circulating glucose levels in rats has either no effect on GH secretion or may be stimulatory. Likewise, chronic hyperglycemia alters GH release in both humans and rats nonetheless in opposite directions. Several factors influence nadir GH concentrations including, age, gender, body mass index, pubertal age, and the type of assay used. Besides the classical suppressive effects of glucose on GH release, a paradoxical GH increase to oral glucose may be observed in around one third of patients with acromegaly as well as in various other disorders. Though its pathophysiology is poorly characterized, an altered interplay between somatostatin and GH-releasing hormone has been suggested and a link with pituitary ectopic expression of glucose-dependent insulinotropic polypeptide receptor has been recently demonstrated. A better understanding of the dynamics mediating GH response to glucose may allow a more optimal use of the OGTT as a diagnostic tool in various conditions, especially acromegaly.

Multiple signaling pathways convey central and peripheral signals to regulate pituitary function: Lessons from human and non-human primate models

The anterior pituitary gland is a key organ involved in the control of multiple physiological functions including growth, reproduction, metabolism and stress. These functions are controlled by five distinct hormone-producing pituitary cell types that produce growth hormone (somatotropes), prolactin (lactotropes), adrenocorticotropin (corticotropes), thyrotropin (thyrotropes) and follicle stimulating hormone/luteinizing hormone (gonadotropes). Classically, the synthesis and release of pituitary hormones was thought to be primarily regulated by central (neuroendocrine) signals. However, it is now becoming apparent that factors produced by pituitary hormone targets (endocrine and non-endocrine organs) can feedback directly to the pituitary to adjust pituitary hormone synthesis and release. Therefore, pituitary cells serve as sensors to integrate central and peripheral signals in order to fine-tune whole-body homeostasis, although it is clear that pituitary cell regulation is species-, age- and sex-dependent. The purpose of this review is to provide a comprehensive, general overview of our current knowledge of both central and peripheral regulators of pituitary cell function and associated intracellular mechanisms, focusing on human and non-human primates.

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

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

[Ghrelin: a gastric hormone at the crossroad between growth and appetite regulation]

Ghrelin is a 28 amino acid peptide hormone synthesized within the gastrointestinal tract. Initially identified as the endogenous ligand of the GHS-R1a (Growth Hormone Secretagogue Receptor 1a), ghrelin is a powerful stimulator of growth hormone (GH) secretion. At the crossroad between nutrition, growth and long-term energy metabolism, ghrelin also plays a unique role as the first identified gastric hormone increasing appetite and adiposity. However, the role of the ghrelin/GHS-R system in the physiology of growth, feeding behaviour and energy homeostasis needs to be better understood. Utilization of pharmacological tools and complementary animal models with deficiency in preproghrelin, ghrelin-O-acyl-transferase (GOAT - the enzyme that acylates ghrelin -) or GHS-R in situations of chronic undernutrition or high fat diet gives a more precise overview of the role of ghrelin in the pathophysiology of eating and metabolic disorders.

The role of ghrelin in the regulation of glucose homeostasis

Ghrelin is a 28-amino acid (aa) stomach-derived peptide discovered in 1999 as the endogenous ligand for growth hormone secretagogue-receptor (GHS-R). Ghrelin-producing cells constitute a distinct group of endocrine cells dispersed throughout the gastric mucosa and to a lesser extent in the small intestine and the endocrine pancreas. Ghrelin plasma levels rise during fasting and chronic caloric restriction to stimulate food intake and fat storage and to prevent life-threatening falls in blood glucose. Plasma ghrelin levels decrease after a meal is consumed and in conditions of energy surplus (such as obesity). Ghrelin has emerged as a key player in the regulation of appetite and energy homeostasis. Ghrelin achieves these functions through binding the ghrelin receptor GHS-R in appetite-regulating neurons and in peripheral metabolic organs including the endocrine pancreas. Ghrelin levels are negatively correlated with body mass index (BMI) and insulin resistance. In addition, ghrelin secretion is impaired in obesity and insulin resistance. Several studies highlight an important role for ghrelin in glucose homeostasis. Genetic, immunological, and pharmacological blockade of ghrelin signaling resulted in improved glucose tolerance and insulin sensitivity. Furthermore, exogenous ghrelin administration was shown to decrease glucose-induced insulin release and increase glucose level in both humans and rodents. GHS-R was shown to be expressed in pancreatic β-cells and ghrelin suppressed insulin release via a Ca2+-mediated pathway. In this review, we provide a detailed summary of recent advances in the field that focuses on the role of insulin and insulin resistance in the regulation of ghrelin secretion and on the role of ghrelin in glucose-stimulated insulin secretion (GSIS).

Ghrelin Actions on Somatotropic and Gonadotropic Function in Humans

Ghrelin, a 28 amino-acid octanoylated peptide predominantly produced by the stomach, was discovered to be the natural ligand of the type 1a GH secretagogue receptor (GHS-R1a). It was thus considered as a natural GHS additional to GHRH, although later on ghrelin has mostly been considered a major orexigenic factor. The GH-releasing action of ghrelin takes place both directly on pituitary cells and through modulation of GHRH from the hypothalamus; some functional antisomatostatin action has also been shown. However, ghrelin is much more than a natural GH secretagogue. In fact, it also modulates lactotroph and corticotroph secretion in humans as well as in animals and plays a relevant role in the modulation of the hypothalamic-pituitary-gonadal function. Several studies have indicated that ghrelin plays an inhibitory effect on gonadotropin pulsatility, is involved in the regulation of puberty onset in animals, and may regulate spermatogenesis, follicular development and ovarian cell functions in humans. In this chapter ghrelin actions on the GH/IGF-I and the gonadal axes will be revised. The potential therapeutic role of ghrelin as a treatment of catabolic conditions will also be discussed.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-GH axis: the past 60 years

At the time of the publication of Geoffrey Harris's monograph on 'Neural control of the pituitary gland' 60 years ago, the pituitary was recognised to produce a growth factor, and extracts administered to children with hypopituitarism could accelerate growth. Since then our understanding of the neuroendocrinology of the GH axis has included identification of the key central components of the GH axis: GH-releasing hormone and somatostatin (SST) in the 1970s and 1980s and ghrelin in the 1990s. Characterisation of the physiological control of the axis was significantly advanced by frequent blood sampling studies in the 1980s and 1990s; the pulsatile pattern of GH secretion and the factors that influenced the frequency and amplitude of the pulses have been defined. Over the same time, spontaneously occurring and targeted mutations in the GH axis in rodents combined with the recognition of genetic causes of familial hypopituitarism demonstrated the key factors controlling pituitary development. As the understanding of the control of GH secretion advanced, developments of treatments for GH axis disorders have evolved. Administration of pituitary-derived human GH was followed by the introduction of recombinant human GH in the 1980s, and, more recently, by long-acting GH preparations. For GH excess disorders, dopamine agonists were used first followed by SST analogues, and in 2005 the GH receptor blocker pegvisomant was introduced. This review will cover the evolution of these discoveries and build a picture of our current understanding of the hypothalamo-GH axis.

Obesity and colorectal cancer: Role of adipokines in tumor initiation and progression

Obesity-associated diseases account for a large portion of public health challenges. Among obesity-related disorders, a direct and independent relationship has been ascertained for colorectal cancer (CRC). The evidence that adipocyte hypertrophy and excessive adipose tissue accumulation (mainly visceral) can promote pathogenic adipocyte and adipose tissue-related diseases, has led to formulate the concept of “adiposopathy”, defined as adipocyte and adipose tissue dysfunction that contributes to metabolic syndrome. Adipose tissue can, indeed, be regarded as an important and highly active player of the innate immune response, in which cytokine/adipokine secretion is responsible for a paracrine loop between adipocytes and macrophages, thus contributing to the systemic chronic low-grade inflammation associated with visceral obesity, which represents a favorable niche for tumor development. The adipocyte itself participates as a central mediator of this inflammatory response in obese individuals by secreting hormones, growth factors and proinflammatory cytokines, which are of particular relevance for the pathogenesis of CRC. Among adipocyte-secreted hormones, the most relevant to colorectal tumorigenesis are adiponectin, leptin, resistin and ghrelin. All these molecules have been involved in cell growth and proliferation, as well as tumor angiogenesis and it has been demonstrated that their expression changes from normal colonic mucosa to adenoma and adenocarcinoma, suggesting their involvement in multistep colorectal carcinogenesis. These findings have led to the hypothesis that an unfavorable adipokine profile, with a reduction of those with an anti-inflammatory and anti-cancerous activity, might serve as a prognostic factor in CRC patients and that adipokines or their analogues/antagonists might become useful agents in the management or chemoprevention of CRC.

Central but not systemic administration of ghrelin induces wakefulness in mice

Ghrelin is a brain-gut peptide hormone widely known for its orexigenic and growth hormone-releasing activities. Findings from our and other laboratories indicate a role of ghrelin in sleep regulation. The effects of exogenous ghrelin on sleep-wake activity in mice are, however, unknown. The aim of the present study was to determine the sleep-modulating effects of ghrelin after central and systemic administrations in mice. Sleep-wake activity after intracerebroventricular (i.c.v.) administration of 0.2, 1 and 5 µg ghrelin and intraperitoneal injections of 40, 100, and 400 µg/kg ghrelin prior to light onset were determined in C57BL/6 mice. In addition, body temperature, motor activity and 1-hour food intake was measured after the systemic injections. Sleep effects of systemic ghrelin (40 and 400 µg/kg) injected before dark onset were also determined. I.c.v. injection of ghrelin increased wakefulness and suppressed non-rapid-eye-movement sleep and electroencephalographic slow-wave activity in the first hour after injections. Rapid-eye-movement sleep was decreased for 2-4 hours after each dose of ghrelin. Sytemic administration of ghrelin did not induce changes in sleep-wake activity in mice at dark or light onset. Motor activity and body temperature remained unaltered and food intake was significantly increased after systemic injections of ghrelin given prior the light period. These findings indicate that the activation of central, but not peripheral, ghrelin-sensitive mechanisms elicits arousal in mice. The results are consistent with the hypothesis that the activation of the hypothalamic neuronal circuit formed by ghrelin, orexin, and neuropeptide Y neurons triggers behavioral sequence characterized by increased wakefulness, motor activity and feeding in nocturnal rodents.

Role of SST, CORT and ghrelin and its receptors at the endocrine pancreas

Somatostatin (SST), cortistatin (CORT), and its receptors (sst1-5), and ghrelin and its receptors (GHS-R) are two highly interrelated neuropeptide systems with a broad range of overlapping biological actions at central, cardiovascular, and immune levels among others. Besides their potent regulatory role on GH release, its endocrine actions are highlighted by SST/CORT and ghrelin influence on insulin secretion, glucose homeostasis, and insulin resistance. Interestingly, most components of these systems are expressed at the endocrine pancreas and are actively involved in the modulation of pancreatic islet function and, consequently influence glucose homeostasis. In addition, some of them also participate in islet survival and regeneration. Furthermore, under severe metabolic condition as well as in endocrine pathologies, their expression profile is severely deregulated. These findings suggest that SST/CORT and ghrelin systems could play a relevant role in pancreatic function under metabolic and endocrine pathologies. Accordingly, these systems have been therapeutically targeted for the prevention or amelioration of certain metabolic conditions (obesity) as well as for tumor growth inhibition and/or hormonal regulation in endocrine pathologies (neuroendocrine tumors). This review focuses on the interrelationship between SST/CORT and ghrelin systems and their role in severe metabolic conditions and some endocrine disorders.