Ghrelin inhibits insulin secretion through the AMPK-UCP2 pathway in beta cells

Fenofibrate reduces cardiac remodeling by mitochondrial dynamics preservation in a renovascular model of cardiac hypertrophy

Fenofibrate, a PPAR-α agonist clinically used to lower serum lipid levels, reduces cardiac remodeling and improves cardiac function. However, its mechanism of action is not completely elucidated. In this study we examined the effect of fenofibrate on mitochondria in a rat model of renovascular hypertension, focusing on mediators controlling mitochondrial dynamics and autophagy. Rats with two-kidney one-clip (2K1C) hypertension were treated with fenofibrate 150 mg/kg/day (2K1C-FFB) or vehicle (2K1C-VEH) for 8 weeks. Systolic blood pressure and cardiac functional were in-vivo assessed, while cardiomyocyte size and protein expression of mediators of cardiac hypertrophy and mitochondrial dynamics were ex-vivo examined by histological and Western blot analyses. Fenofibrate treatment counteracted the development of hypertension and the increase of left ventricular mass, relative wall thickness and cross-sectional area of cardiomyocytes. Furthermore, fenofibrate re-balanced the expression Mfn2, Drp1 and Parkin, regulators of fusion, fission, mitophagy respectively. Regarding autophagy, the LC3-II/LC3-I ratio was increased in 2K1C-VEH and 2K1C-FFB, whereas the autophagy was increased only in 2K1C-FFB. In cultured H9C2 cardiomyoblasts, fenofibrate reversed the Ang II-induced mRNA up-regulation of hypertrophy markers Nppa and Myh7, accumulation of reactive oxygen species and depolarization of the mitochondrial membrane exerting protection mediated by up-regulation of the Uncoupling protein 2. Our results indicate that fenofibrate acts directly on cardiomyocytes and counteracts the pressure overload-induced cardiac maladaptive remodeling. This study reveals a so far hidden mechanism involving mitochondrial dynamics in the beneficial effects of fenofibrate, support its repurposing for the treatment of cardiac hypertrophy and provide new potential targets for its pharmacological function.

Allyl isothiocyanate, a TRPA1 agonist, protects against olanzapine-induced hypothalamic and hepatic metabolic aberrations in female mice

Olanzapine, a widely prescribed atypical antipsychotic, poses a great risk to the patient's health by fabricating a plethora of severe metabolic and cardiovascular adverse effects eventually reducing life expectancy and patient compliance. Its heterogenous receptor binding profile has made it difficult to point out a specific cause or treatment for the related side effects. Growing body of evidence suggest that transient receptor potential (TRP) channel subfamily Ankyrin 1 (TRPA1) has pivotal role in pathogenesis of type 2 diabetes and obesity. With this background, we aimed to investigate the role of pharmacological manipulations of TRPA1 channels in antipsychotic (olanzapine)-induced metabolic alterations in female mice using allyl isothiocyanate (AITC) and HC-030031 (TRPA1 agonist and antagonist, respectively). It was found that after 6 weeks of treatment, AITC prevented olanzapine-induced alterations in body weight and adiposity; serum, and liver inflammatory markers; glucose and lipid metabolism; and hypothalamic appetite regulation, nutrient sensing, inflammatory and TRPA1 channel signaling regulating genes. Furthermore, several of these effects were absent in the presence of HC-030031 (TRPA1 antagonist) indicating protective role of TRPA1 agonism in attenuating olanzapine-induced metabolic alterations. Supplementary in-depth studies are required to study TRPA1 channel effect on other aspects of olanzapine-induced metabolic alterations.

The effect of a pharmaceutical ghrelin agonist on lifespan in C57BL/6J male mice: A controlled experiment

Interventions for animal lifespan extension like caloric restriction (CR) have identified physiologic and biochemical pathways related to hunger and energy-sensing status as possible contributors, but mechanisms have not been fully elucidated. Prior studies using ghrelin agonists show greater food intake but no effect on lifespan in rodent models. This experiment in male C57BL/6J mice tested the influence of ghrelin agonism for perceived hunger, in the absence of CR, on longevity. Mice aged 4 weeks were allowed to acclimate for 2 weeks prior to being assigned (N = 60/group). Prior to lights off daily (12:12 cycle), animals were fed a ghrelin agonist pill (LY444711; Eli Lilly) or a placebo control (Ctrl) until death. Treatment (GhrAg) animals were pair-fed daily based on the group mean food intake consumed by Ctrl (ad libitum feeding) the prior week. Results indicate an increased lifespan effect (log-rank p = 0.0032) for GhrAg versus placebo Ctrl, which weighed significantly more than GhrAg (adjusted for baseline weight). Further studies are needed to determine the full scope of effects of this ghrelin agonist, either directly via increased ghrelin receptor signaling or indirectly via other hypothalamic, systemic, or tissue-specific mechanisms.

Cross-Talk between Obesity and Diabetes: Introducing Polyphenols as an Effective Phytomedicine to Combat the Dual Sword Diabesity

: Obesity-associated diabetes mellitus, a chronic metabolic affliction accounting for 90% of all diabetic patients, has been affecting humanity extremely badly and escalating the risk of developing other serious disorders. It is observed that 0.4 billion people globally have diabetes, whose major cause is obesity. Currently, innumerable synthetic drugs like alogliptin and rosiglitazone are being used to get through diabetes, but they have certain complications, restrictions with severe side effects, and toxicity issues. Recently, the frequency of plant-derived phytochemicals as advantageous substitutes against diabesity is increasing progressively due to their unparalleled benefit of producing less side effects and toxicity. Of these phytochemicals, dietary polyphenols have been accepted as potent agents against the dual sword "diabesity". These polyphenols target certain genes and molecular pathways through dual mechanisms such as adiponectin upregulation, cannabinoid receptor antagonism, free fatty acid oxidation, ghrelin antagonism, glucocorticoid inhibition, sodium-glucose cotransporter inhibition, oxidative stress and inflammation inhibition etc. which sequentially help to combat both diabetes and obesity. In this review, we have summarized the most beneficial natural polyphenols along with their complex molecular pathways during diabesity.

From "Hunger Hormone" to "It's Complicated": Ghrelin Beyond Feeding Control

Discovered as a peptide involved in releasing growth hormone, ghrelin was initially characterized as the "hunger hormone." However, emerging research indicates that ghrelin appears to play an important part in relaying information regarding nutrient availability and value and adjusting physiological and motivational processes accordingly. These functions make ghrelin an interesting therapeutic candidate for metabolic and neuropsychiatric diseases involving disrupted nutrition that can further potentiate the rewarding effect of maladaptive behaviors.

Early postnatal stress impairs insulin secretion in response to psychological stress in adult rats

PURPOSE

Adversity in early life can induce metabolic defects in exposure to stress in adulthood. Therefore, the exploration of involving mechanisms can be helpful in the treatment of metabolic disorders. So, the present study was conducted in terms of exploring the effects of interaction between early postnatal stress and young adulthood psychological stress on insulin secretion and pancreatic GLUT-2 levels in male rats.

METHODS

Footshock as a model of early life stress (at 2 weeks of age) and psychological stress induced by communication box as a model of young adulthood stress (at 8-10 weeks of age) were induced in male Wistar rats for five consecutive days (2 times/day). Blood samples were drawn to measure glucose, insulin, homeostatic model assessment of insulin resistance (HOMA-IR) and homeostasis model assessment of β-cell dysfunction (HOMA-B), before and after stress protocol in young adult rats. Corticosterone was measured on days 1 and 5 of stress induction. The day after the stress period, factors including glucose tolerance, TNF-alpha, isolated islets' insulin output and levels of pancreatic GLUT-2 protein via western blotting were determined.

RESULTS

The combination of early footshock exposure and psychological stress during adulthood did not affect plasma corticosterone, but increased plasma insulin, HOMA-IR, HOMA-B and TNF-alpha levels. Plasma TNF was not only increased by the combination of both stressors, but also after only E STR exposure. HOMA-IR was increased in both Psy STR and E + Psy-STR groups. Plasma glucose just increased in Psy STR group. The combination of these two life stressors further increased the in vitro insulin secretion from isolated islets in response to 16.7-mM glucose. The level of Glut2 was increased in Psy STR and decreased in both E STR and E + Psy STR groups. Finally, glucose tolerance was impaired and glucose-stimulated insulin secretion was increased in E + Psy STR group.

CONCLUSIONS

In conclusion, inducing stress in early life makes the organism more susceptible to metabolic defects in exposure to psychological stress later in life.

Chronic stress and adipose tissue in the anorexic state: endocrine and epigenetic mechanisms

Although adipose tissue metabolism in obesity has been widely studied, there is limited research on the anorexic state, where the endocrine system is disrupted by reduced adipose tissue mass and there are depot-specific changes in adipocyte type and function. Stress exposure at different stages of life can alter the balance between energy intake and expenditure and thereby contribute to the pathogenesis of anorexia nervosa. This review integrates information from human clinical trials to describe endocrine, genetic and epigenetic aspects of adipose tissue physiology in the anorexic condition. Changes in the hypothalamus-pituitary-thyroid, -adrenal, and -gonadal axes and their relationships to appetite regulation and adipocyte function are discussed. Because of the role of stress in triggering or magnifying anorexia, and the dynamic but also persistent nature of environmentally-induced epigenetic modifications, epigenetics is likely the link between stress and long-term changes in the endocrine system that disrupt homoeostatic food intake and adipose tissue metabolism. Herein, we focus on the adipocyte and changes in its function, including alterations reinforced by endocrine disturbance and dysfunctional adipokine regulation. This information is critical because of the poor understanding of anorexic pathophysiology, due to the lack of suitable research models, and the complexity of genetic and environmental interactions.

The Potential Roles of Ghrelin in Metabolic Syndrome and Secondary Symptoms of Alzheimer's Disease

Although the major causative factors of Alzheimer's disease (AD) are the accumulation of amyloid β and hyperphosphorylated tau, AD can also be caused by metabolic dysfunction. The major clinical symptom of AD is cognitive dysfunction. However, AD is also accompanied by various secondary symptoms such as depression, sleep-wake disturbances, and abnormal eating behaviors. Interestingly, the orexigenic hormone ghrelin has been suggested to have beneficial effects on AD-related metabolic syndrome and secondary symptoms. Ghrelin improves lipid distribution and alters insulin sensitivity, effects that are hypothesized to delay the progression of AD. Furthermore, ghrelin can relieve depression by enhancing the secretion of hormones such as serotonin, noradrenaline, and orexin. Moreover, ghrelin can upregulate the expression of neurotrophic factors such as brain-derived neurotrophic factor and modulate the release of proinflammatory cytokines such as tumor necrosis factor α and interleukin 1β. Ghrelin alleviates sleep-wake disturbances by increasing the levels of melatonin, melanin-concentrating hormone. Ghrelin reduces the risk of abnormal eating behaviors by increasing neuropeptide Y and γ-aminobutyric acid. In addition, ghrelin increases food intake by inhibiting fatty acid biosynthesis. However, despite the numerous studies on the role of ghrelin in the AD-related pathology and metabolic disorders, there are only a few studies that investigate the effects of ghrelin on secondary symptoms associated with AD. In this mini review, our purpose is to provide the insights of future study by organizing the previous studies for the role of ghrelin in AD-related pathology and metabolic disorders.

Plasma levels of acylated ghrelin in patients with insulinoma and expression of ghrelin and its receptor in insulinomas

BACKGROUND

Insulinoma is a subtype of pancreatic neuroendocrine tumors. Many patients with insulinoma are obese due to frequent food intake. Ghrelin is associated with obesity and blood levels of insulin. It is not clear if plasma levels of ghrelin in insulinoma patients correlate with hyperinsulinemia and obesity. Expression of ghrelin and its receptor has not been well demonstrated in insulinoma.

OBJECTIVE

To study if plasma levels of ghrelin is associated with obesity and hyperinsulinemia or hyperproinsulinemia in patients with insulinoma, and to detect the expression of ghrelin and its receptor in insulinoma.

METHODS

Plasma levels of acylated ghrelin, insulin, and proinsulin were measured in 37 patients with insulinoma and 25 controls by ELISA. Expression of ghrelin and its receptor GHS-R1A was examined in 20 insulinoma and paired pancreatic specimens by immunostaining. P ≤ 0.05 was considered significant.

RESULTS

The plasma levels of acylated ghrelin in patients with insulinoma were significantly lower than that in the controls (median 15 pg/ml vs. 19 pg/ml, respectively, P = 0.016). The reduced plasma levels of acylated ghrelin in patients were significantly correlated with obesity, hyperinsulinemia, and hyperproinsulinemia (P = 0.029 and P = 0.028, respectively). Expression of ghrelin and its receptor GHS-R1A was shown in the majority of insulinoma specimens. The expression of GHS-R1A was positively correlated with ghrelin expression in insulinoma (P = 0.014).

CONCLUSIONS

Plasma levels of acylated ghrelin decreased in patients with insulinoma, probably due to the hyperinsulinemia and obesity in the patients. Expression of both ghrelin and its receptor is common in insulinoma.

Matrine attenuates oxidative stress and cardiomyocyte apoptosis in doxorubicin-induced cardiotoxicity via maintaining AMPKα/UCP2 pathway

Oxidative stress and cardiomyocyte apoptosis are involved in the pathogenesis of doxorubicin (DOX)-induced cardiotoxicity. Matrine is well-known for its powerful anti-oxidant and anti-apoptotic capacities. Our present study aimed to investigate the effect of matrine on DOX-induced cardiotoxicity and try to unearth the underlying mechanisms. Mice were exposed with DOX to generate DOX-induced cardiotoxicity or normal saline as control. H9C2 cells were used to verify the effect of matrine in vitro. DOX injection triggered increased generation of reactive oxygen species (ROS) and excessive cardiomyocyte apoptosis, which were significantly mitigated by matrine. Mechanistically, we found that matrine ameliorated DOX-induced uncoupling protein 2 (UCP2) downregulation, and UCP2 inhibition by genipin could blunt the protective effect of matrine on DOX-induced oxidative stress and cardiomyocyte apoptosis. Besides, 5'-AMP-activated protein kinase α2 (Ampkα2) deficiency inhibited matrine-mediated UCP2 preservation and abolished the beneficial effect of matrine in mice. Besides, we observed that matrine incubation alleviated DOX-induced H9C2 cells apoptosis and oxidative stress level via activating AMPKα/UCP2, which were blunted by either AMPKα or UCP2 inhibition with genetic or pharmacological methods. Matrine attenuated oxidative stress and cardiomyocyte apoptosis in DOX-induced cardiotoxicity via maintaining AMPKα/UCP2 pathway, and it might be a promising therapeutic agent for the treatment of DOX-induced cardiotoxicity.

Selenium attenuates apoptosis and p-AMPK expressions in fluoride-induced NRK-52E cells

Fluoride is widely distributed in the environment, and excessive fluoride intake can induce cytotoxicity, DNA damage, and cell cycle changes in many tissues and organs, including the kidney. Accumulating evidence demonstrates that selenium (Se) administration ameliorates sodium fluoride (NaF)-induced kidney damage. However, the potentially beneficial effects of Se against NaF-induced cytotoxicity of the kidney and the underlying molecular mechanisms of this protection are not fully understood. At present, in this study, the normal rat kidney cell (NRK-52E) was used to investigate the potentially protective mechanism of Se against NaF-induced apoptosis, by using the methods of pathology, colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, flow cytometry, and Western blot. The experiment was designed with a control group, two NaF-treated groups (NaF, 5, 20 mg/L), two sodium selenite-treated groups (Na₂SeO₃, 17.1, 34.2 μg/L), and four Se + NaF-treated groups (Na₂SeO₃, 17.1, 34.2 μg/L; NaF, 5, 20 mg/L). The results indicate that selenium can attenuate apoptosis and AMPK phosphorylation in the NRK-52E cell induced with fluoride. These results imply that selenium is capable to modulate fluoride-induced NRK-52E cell apoptosis via regulating the expression levels of the proteins involved in mitochondrial pathway and changes in p-AMPK expressions may also be a key process in preventing fluorosis.

Uncoupling Protein 2 in Cardiovascular Health and Disease

Uncoupling protein 2 (UCP2) belongs to the family of mitochondrial anion carrier proteins. It uncouples oxygen consumption from ATP synthesis. UCP2 is ubiquitously expressed in most cell types to reduce oxidative stress. It is tightly regulated at the transcriptional, translational, and post-translational levels. UCP2 in the cardiovascular system is being increasingly recognized as an important molecule to defend against various stress signals such as oxidative stress in the pathology of vascular dysfunction, atherosclerosis, hypertension, and cardiac injuries. UCP2 protects against cellular dysfunction through reducing mitochondrial oxidative stress and modulation of mitochondrial function. In view of the different functions of UCP2 in various cell types that contribute to whole body homeostasis, cell type-specific modification of UCP2 expression may offer a better approach to help understanding how UCP2 governs mitochondrial function, reactive oxygen species production and transmembrane proton leak and how dysfunction of UCP2 participates in the development of cardiovascular diseases. This review article provided an update on the physiological regulation of UCP2 in the cardiovascular system, and also discussed the involvement of UCP2 deficiency and associated oxidative stress in the pathogenesis of several common cardiovascular diseases. Drugs targeting UCP2 expression and activity might serve another effective strategy to ameliorate cardiovascular dysfunction. However, more detailed mechanistic study will be needed to dissect the role of UCP2, the regulation of UCP2 expression, and the cellular responses to the changes of UCP2 expression in normal and stressed situations at different stages of cardiovascular diseases.

The Stomach as an Endocrine Organ: Expression of Key Modulatory Genes and Their Contribution to Obesity and Non-alcoholic Fatty Liver Disease (NAFLD)

PURPOSE OF REVIEW

Obesity is currently seen in epidemic proportions globally and is one of the largest contributors to the development of NAFLD. The spectrum of NAFLD, particularly the progressive forms of NASH, is likely to become the leading cause of liver disease in the next decade.

RECENT FINDINGS

Soluble molecules, encoded by the stomach tissue, have been shown to have pleiotropic effects in both central and peripheral systems involved in energy homeostasis and obesity regulation. As such, the stomach is one of the important players in the complex, multi-system deregulation leading to obesity and NAFLD. The understanding of the stomach tissue as an active endocrine organ that contributes to the signaling milieu leading to the development of obesity and NAFLD is crucial.

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.

The Role of Ghrelin and Ghrelin Signaling in Aging

With our aging society, more people hope for a long and healthy life. In recent years, researchers have focused on healthy longevity factors. In particular, calorie restriction delays aging, reduces mortality, and extends life. Ghrelin, which is secreted during fasting, is well known as an orexigenic peptide. Because ghrelin is increased by caloric restriction, ghrelin may play an important role in the mechanism of longevity mediated by calorie restriction. In this review, we will discuss the role of orexigenic peptides with a particular focus on ghrelin. We conclude that the ghrelin-growth hormone secretagogue-R signaling pathway may play an important role in the anti-aging mechanism.

Sex-related differences in the association of ghrelin levels with obesity in adolescents

BACKGROUND

The utility of ghrelin as a biomarker may be different depending on gender. The aim of this study was to assess ghrelin levels in a population-based sample of adolescents, and to evaluate their association with obesity and obesity-related parameters depending on sex.

METHODS

The studied population included 601 randomly selected 14-to 16-year-old children. Anthropometrical data were measured and body mass index (BMI) and waist to hip ratio calculated. Body composition was assessed using an impedance body composition analyzer. Total serum ghrelin levels were determined using a multiplexed bead immunoassay. Serum leptin and adiponectin levels were determined by ELISA and insulin by RIA.

RESULTS

Ghrelin levels were significantly higher in girls than in boys. Serum ghrelin concentrations were significantly lower (p<0.01) in obese than in normal weight (NW) girls, but showed no differences by weight category in boys. Ghrelin showed a significant negative relationship with waist circumference (WC), waist to hip ratio and fat mass (p<0.05) in both genders, and with weight and BMI (p<0.01) in girls, and insulin (p<0.01) and HOMA (p<0.05) in boys. Ghrelin also correlated negatively with leptin levels in girls (p<0.01).

CONCLUSIONS

Our study describes serum ghrelin levels in adolescents, showing a sexual dimorphism in ghrelin levels in these 14-to 16-year-old children, and a different association of ghrelin levels with obesity by gender that suggests a different appetite and energy expenditure control depending on sex at this age.

Increased ghrelin signaling prolongs survival in mouse models of human aging through activation of sirtuin1

Caloric restriction (CR) is known to retard aging and delay functional decline as well as the onset of diseases in most organisms. Ghrelin is secreted from the stomach in response to CR and regulates energy metabolism. We hypothesized that in CR ghrelin has a role in protecting aging-related diseases. We examined the physiological mechanisms underlying the ghrelin system during the aging process in three mouse strains with different genetic and biochemical backgrounds as animal models of accelerated or normal human aging. The elevated plasma ghrelin concentration was observed in both klotho-deficient and senescence-accelerated mouse prone/8 (SAMP8) mice. Ghrelin treatment failed to stimulate appetite and prolong survival in klotho-deficient mice, suggesting the existence of ghrelin resistance in the process of aging. However, ghrelin antagonist hastened death and ghrelin signaling potentiators rikkunshito and atractylodin ameliorated several age-related diseases with decreased microglial activation in the brain and prolonged survival in klotho-deficient, SAMP8 and aged ICR mice. In vitro experiments, the elevated sirtuin1 (SIRT1) activity and protein expression through the cAMP-CREB pathway was observed after ghrelin and ghrelin potentiator treatment in ghrelin receptor 1a-expressing cells and human umbilical vein endothelial cells. Furthermore, rikkunshito increased hypothalamic SIRT1 activity and SIRT1 protein expression of the heart in the all three mouse models of aging. Pericarditis, myocardial calcification and atrophy of myocardial and muscle fiber were improved by treatment with rikkunshito. Ghrelin signaling may represent one of the mechanisms activated by CR, and potentiating ghrelin signaling may be useful to extend health and lifespan.

Chronic administration of ghrelin regulates plasma glucose and normalizes insulin levels following fasting hyperglycemia and hyperinsulinemia

Ghrelin is an endogenous ligand for the growth hormone secretagogue receptor. The majority of the previous studies have shown that the short-term ghrelin treatment induces hyperglycemia and hypoinsulinemia in healthy humans and rodents. However, the results obtained from long-term treatment with ghrelin are not clear enough. In this study, we assessed acute (1 day) and chronic (21 days) effects of intraperitoneally administered ghrelin (at different doses of 1, 10 and 20 μg/kg) during a 12-h fasting period in rats using glucose oxidase method and direct sandwich ELISA (the Enzyme-Linked Immunosorbent Assay) and then compared the effects of exogenous ghrelin on blood glucose and insulin levels on day 21 with those on day 1. The results showed that acute ghrelin administration markedly increased fasting plasma glucose at doses of 1 and 10 μg/kg as well as insulin levels at 1 μg/kg in comparison to control values. Ghrelin (at 1 μg/kg) altered plasma glucose but not insulin levels on the 21st day compared to control values. In addition, the comparison of the influence of ghrelin administration on plasma glucose and insulin levels on day 21 with those on the first day revealed that the chronic administration of ghrelin notably decreased plasma glucose and insulin levels relative to the acute ghrelin treatment. These findings indicate that hyperglycemia and hyperinsulinemia caused by the exogenous ghrelin during acute treatment are temporary and prolonged treatment with ghrelin regulates plasma glucose and restores insulin to normal levels, suggesting a possible role for ghrelin in improving insulin resistance.

Ghrelin regulates GLP-1 production through mTOR signaling in L cells

Glucagon-like peptide (GLP-1), an intestinal incretin produced in L-cells and released in response to meal intake, functions to promote insulin secretion and to decrease plasma glucose. Ghrelin is an orexigenic hormone critical for glucose homeostasis. The molecular mechanism by which ghrelin alters GLP-1 production remains largely unknown. Here we showed that ghrelin attenuates GLP-1 production through mTOR signaling. In GHSR1a null mice, ileal mTOR signaling, proglucagon and circulating GLP-1 were significantly increased. Antagonism of the GHSR1a by D-Lys-3-GHRP-6 increased GLP-1 synthesis and release in STC-1 cells. Treatment of STC-1 cells with ghrelin decreased the production of GLP-1. This effect was associated with a significant inhibition of mTOR signaling. Overexpression of ghrelin inhibited proglucagon promoter activity and GLP-1 production. Inhibition of mTOR activity by mTOR siRNA blocked D-Lys-3-GHRP-6 induced GLP-1 production in STC-1 cells. Our results suggest that mTOR signaling mediates the inhibitory effect of ghrelin on GLP-1 production.

Activation of AMPK Stimulates Neurotensin Secretion in Neuroendocrine Cells

AMP-activated protein kinase (AMPK), a critical fuel-sensing enzyme, regulates the metabolic effects of various hormones. Neurotensin (NT) is a 13-amino acid peptide predominantly localized in enteroendocrine cells of the small bowel and released by fat ingestion. Increased fasting plasma levels of pro-NT (a stable NT precursor fragment produced in equimolar amounts relative to NT) are associated with an increased risk of diabetes, cardiovascular disease, and mortality; however, the mechanisms regulating NT release are not fully defined. We previously reported that inhibition of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) increases NT secretion and gene expression through activation of the MEK/ERK pathway. Here, we show that activation of AMPK increases NT secretion from endocrine cell lines (BON and QGP-1) and isolated mouse crypt cells enriched for NT-positive cells. In addition, plasma levels of NT increase in mice treated with 5-aminoimidazole-4-carboxamide riboside, a pharmacologic AMPK activator. Small interfering RNA-mediated knockdown of AMPKα decrease, whereas overexpression of the subunit significantly enhances, NT secretion from BON cells treated with AMPK activators or oleic acid. Similarly, small interfering RNA knockdown of the upstream AMPK kinases, liver kinase B1 and Ca(2+) calmodulin-dependent protein kinase kinase 2, also attenuate NT release and AMPK phosphorylation. Moreover, AMPK activation increases NT secretion through inhibition of mTORC1 signaling. Together, our findings show that AMPK activation enhances NT release through inhibition of mTORC1 signaling, thus demonstrating an important cross talk regulation for NT secretion.

New perspectives of incretin research

Incretins are a group of hormones released into the blood stream by gastrointestinal cells after food stimulation, including glucagon like peptide and glucose-dependent insulinotropic polypeptide (GIP), which can promote insulin secretion and regulate blood sugar. GLP-1 is secreted by intestinal L cells, and fulfills its function through the specific GLP-1 receptor (GLP-1R). GLP-1R is widely distributed in the pancreas and non-pancreatic tissues such as central nervous system, gastrointestinal tract, cardiovascular system, lungs, and kidneys. In recent years, GLP-1 drugs have been used for the treatment of diabetes, but it has attracted more interest because of its beneficiary effects on β cell function, weight reduction, endothelial function, and Alzheimer's disease. This article will review the recent progress in research of GLP-1 with regards to its synthesis and secretion, its effects on taste and Alzheimer's disease, and its relationship with other gastrointestinal hormones, with an aim to illuminate the future clinical use and research of GLP-1.

Ghrelin - Physiological Functions and Regulation

Ghrelin is an orexigenic peptide predominantly secreted from the stomach and stimulates appetite and growth hormone (GH) release. Studies have provided evidence that ghrelin exercises a wide range of functions, including regulation of food intake and energy metabolism, modulation of cardiovascular function, stimulation of osteoblast proliferation and bone formation and stimulation of neurogenesis and myogenesis. In the gastrointestinal system, ghrelin affects multiple functions, including secretion of gastric acid, gastric motility and pancreatic protein output. Most of these functions have been attributed to the actions of acylated ghrelin. The balance among its secretion rate, degradation rate and clearance rate determines the circulating level of ghrelin. This review explains what ghrelin is, its physiological functions and the factors that influence its level.

Implications of ghrelin and hexarelin in diabetes and diabetes-associated heart diseases

Ghrelin and its synthetic analog hexarelin are specific ligands of growth hormone secretagogue (GHS) receptor. GHS have strong growth hormone-releasing effect and other neuroendocrine activities such as stimulatory effects on prolactin and adrenocorticotropic hormone secretion. Recently, several studies have reported other beneficial functions of GHS that are independent of GH. Ghrelin and hexarelin, for examples, have been shown to exert GH-independent cardiovascular activity. Hexarelin has been reported to regulate peroxisome proliferator-activated receptor gamma (PPAR-γ) in macrophages and adipocytes. PPAR-γ is an important regulator of adipogenesis, lipid metabolism, and insulin sensitization. Ghrelin also shows protective effects on beta cells against lipotoxicity through activation of phosphatidylinositol-3 kinase/protein kinase B, c-Jun N-terminal kinase (JNK) inhibition, and nuclear exclusion of forkhead box protein O1. Acylated ghrelin (AG) and unacylated ghrelin (UAG) administration reduces glucose levels and increases insulin-producing beta cell number, and insulin secretion in pancreatectomized rats and in newborn rats treated with streptozotocin, suggesting a possible role of GHS in pancreatic regeneration. Therefore, the discovery of GHS has opened many new perspectives in endocrine, metabolic, and cardiovascular research areas, suggesting the possible therapeutic application in diabetes and diabetic complications especially diabetic cardiomyopathy. Here, we review the physiological roles of ghrelin and hexarelin in the protection and regeneration of beta cells and their roles in the regulation of insulin release, glucose, and fat metabolism and present their potential therapeutic effects in the treatment of diabetes and diabetic-associated heart diseases.

Effect of early life stress on pancreatic isolated islets' insulin secretion in young adult male rats subjected to chronic stress

Early stressful experiences may predispose organisms to certain disorders, including those of metabolic defects. This study aimed to explore the effects of early life stress on pancreatic insulin secretion and glucose transporter 2 (GLUT2) protein levels in stressed young adult male rats. Foot shock stress was induced in early life (at 2 weeks of age) and/or in young adulthood (at 8-10 weeks of age) for five consecutive days. Blood samples were taken before and after stress exposure in young adult rats. At the end of the experiment, glucose tolerance, isolated islets' insulin secretion, and pancreatic amounts of GLUT2 protein were measured. Our results show that early life stress has no effect on basal plasma corticosterone levels and adrenal weight, either alone or combined with young adulthood stress, but that early life + young adulthood stress could prevent weight gain, and cause an increase in basal plasma glucose and insulin. The homeostasis model assessment of insulin resistance index did not increase, when the rats were subjected to early life stress alone, but increased when combined with young adulthood stress. Moreover, glucose tolerance was impaired by the combination of early life + young adult stress. There was a decrease in islet's insulin secretion in rats subjected to early life stress in response to 5.6 mM glucose concentration, but an increase with a concentration of 16.7 mM glucose. However, in rats subjected to early life + young adulthood stress, islet's insulin secretion increased in response to both the levels of glucose concentrations. GLUT2 protein levels decreased in response to early life stress and early life + young adulthood stress, but there was a greater decrease in the early life stress group. In conclusion, perhaps early life stress sensitizes the body to stressors later in life, making it more susceptible to metabolic syndrome only when the two are in combination.

Endocrine determinants of changes in insulin sensitivity and insulin secretion during a weight cycle in healthy men

Objective

Changes in insulin sensitivity (IS) and insulin secretion occur with perturbations in energy balance and glycemic load (GL) of the diet that may precede the development of insulin resistance and hyperinsulinemia. Determinants of changes in IS and insulin secretion with weight cycling in non-obese healthy subjects remain unclear.

Methods

In a 6wk controlled 2-stage randomized dietary intervention 32 healthy men (26±4y, BMI: 24±2kg/m²) followed 1wk of overfeeding (OF), 3wks of caloric restriction (CR) containing either 50% or 65% carbohydrate (CHO) and 2wks of refeeding (RF) with the same amount of CHO but either low or high glycaemic index at ±50% energy requirement. Measures of IS (basal: HOMA-index, postprandial: Matsuda-ISI), insulin secretion (early: Stumvoll-index, total: tAUC-insulin/tAUC-glucose) and potential endocrine determinants (ghrelin, leptin, adiponectin, thyroid hormone levels, 24h-urinary catecholamine excretion) were assessed.

Results

IS improved and insulin secretion decreased due to CR and normalized upon RF. Weight loss-induced improvements in basal and postprandial IS were associated with decreases in leptin and increases in ghrelin levels, respectively (r = 0.36 and r = 0.62, p<0.05). Weight regain-induced decrease in postprandial IS correlated with increases in adiponectin, fT3, TSH, GL of the diet and a decrease in ghrelin levels (r-values between -0.40 and 0.83, p<0.05) whereas increases in early and total insulin secretion were associated with a decrease in leptin/adiponectin-ratio (r = -0.52 and r = -0.46, p<0.05) and a decrease in fT4 (r = -0.38, p<0.05 for total insulin secretion only). After controlling for GL associations between RF-induced decrease in postprandial IS and increases in fT3 and TSH levels were no longer significant.

Conclusion

Weight cycling induced changes in IS and insulin secretion were associated with changes in all measured hormones, except for catecholamine excretion. While leptin, adiponectin and ghrelin seem to be the major endocrine determinants of IS, leptin/adiponectin-ratio and fT4 levels may impact changes in insulin secretion with weight cycling.

Trial Registration

ClinicalTrials.gov NCT01737034

Uncoupling protein-2 mediates DPP-4 inhibitor-induced restoration of endothelial function in hypertension through reducing oxidative stress

AIMS

Although uncoupling protein 2 (UCP2) negatively regulates intracellular reactive oxygen species (ROS) production and protects vascular function, its participation in vascular benefits of drugs used to treat cardiometabolic diseases is largely unknown. This study investigated whether UCP2 and associated oxidative stress reduction contribute to the improvement of endothelial function by a dipeptidyl peptidase-4 inhibitor, sitagliptin, in hypertension.

RESULTS

Pharmacological inhibition of cyclooxygenase-2 (COX-2) but not COX-1 prevented endothelial dysfunction, and ROS scavengers reduced COX-2 mRNA and protein expression in spontaneously hypertensive rats (SHR) renal arteries. Angiotensin II (Ang II) evoked endothelium-dependent contractions (EDCs) in C57BL/6 and UCP2 knockout (UCP2KO) mouse aortae. Chronic sitagliptin administration attenuated EDCs in SHR arteries and Ang II-infused C57BL/6 mouse aortae and eliminated ROS overproduction in SHR arteries, which were reversed by glucagon-like peptide 1 receptor (GLP-1R) antagonist exendin 9-39, AMP-activated protein kinase (AMPK)α inhibitor compound C, and UCP2 inhibitor genipin. By contrast, sitagliptin unaffected EDCs in Ang II-infused UCP2KO mice. Sitagliptin increased AMPKα phosphorylation, upregulated UCP2, and downregulated COX-2 expression in arteries from SHR and Ang II-infused C57BL/6 mice. Importantly, exendin 9-39, compound C, and genipin reversed the inhibitory effect of GLP-1R agonist exendin-4 on Ang II-stimulated mitochondrial ROS rises in SHR endothelial cells. Moreover, exendin-4 improved the endothelial function of renal arteries from SHR and hypertensive patients.

INNOVATION

We elucidate for the first time that UCP2 serves as an important signal molecule in endothelial protection conferred by GLP-1-related agents. UCP2 could be a useful target in treating hypertension-related vascular events.

CONCLUSIONS

UCP2 inhibits oxidative stress and downregulates COX-2 expression through GLP-1/GLP-1R/AMPKα cascade.

Chronic aerobic exercise training attenuates aortic stiffening and endothelial dysfunction through preserving aortic mitochondrial function in aged rats

Aging leads to large vessel arterial stiffening and endothelial dysfunction, which are important determinants of cardiovascular risk. The aim of present work was to assess the effects of chronic aerobic exercise training on aortic stiffening and endothelial dysfunction in aged rats and investigate the underlying mechanism about mitochondrial function. Chronic aerobic exercise training attenuated aortic stiffening with age marked by reduced collagen concentration, increased elastin concentration and reduced pulse wave velocity (PWV), and prevented aging-related endothelial dysfunction marked by improved endothelium-mediated vascular relaxation of aortas in response to acetylcholine. Chronic aerobic exercise training abated oxidative stress and nitrosative stress in aortas of aged rats. More importantly, we found that chronic aerobic exercise training in old rats preserved aortic mitochondrial function marked by reduced reactive oxygen species (ROS) formation and mitochondrial swelling, increased ATP formation and mitochondrial DNA content, and restored activities of complexes I and III and electron-coupling capacity between complexes I and III and between complexes II and III. In addition, it was found that chronic aerobic exercise training in old rats enhanced protein expression of uncoupling protein 2 (UCP-2), peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α), manganese superoxide dismutase (Mn-SOD), aldehyde dehydrogenase 2 (ALDH-2), prohibitin (PHB) and AMP-activated kinase (AMPK) phosphorylation in aortas. In conclusion, chronic aerobic exercise training preserved mitochondrial function in aortas, which, at least in part, explained the aorta-protecting effects of exercise training in aging.

Hepatic overexpression of ATP synthase β subunit activates PI3K/Akt pathway to ameliorate hyperglycemia of diabetic mice

ATP synthase β subunit (ATPSβ) had been previously shown to play an important role in controlling ATP synthesis in pancreatic β-cells. This study aimed to investigate the role of ATPSβ in regulation of hepatic ATP content and glucose metabolism in diabetic mice. ATPSβ expression and ATP content were both reduced in the livers of type 1 and type 2 diabetic mice. Hepatic overexpression of ATPSβ elevated cellular ATP content and ameliorated hyperglycemia of streptozocin-induced diabetic mice and db/db mice. ATPSβ overexpression increased phosphorylated Akt (pAkt) levels and reduced PEPCK and G6pase expression levels in the livers. Consistently, ATPSβ overexpression repressed hepatic glucose production in db/db mice. In cultured hepatocytes, ATPSβ overexpression increased intracellular and extracellular ATP content, elevated the cytosolic free calcium level, and activated Akt independent of insulin. The ATPSβ-induced increase in cytosolic free calcium and pAkt levels was attenuated by inhibition of P2 receptors. Notably, inhibition of calmodulin (CaM) completely abolished ATPSβ-induced Akt activation in liver cells. Inhibition of P2 receptors or CaM blocked ATPSβ-induced nuclear exclusion of forkhead box O1 in liver cells. In conclusion, a decrease in hepatic ATPSβ expression in the liver, leading to the attenuation of ATP-P2 receptor-CaM-Akt pathway, may play an important role in the progression of diabetes.

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.

Changes in energy-regulated molecules in the trophocytes and fat cells of young and old worker honeybees (Apis mellifera)

Trophocytes and fat cells of honeybees (Apis mellifera) have been used for cellular senescence studies, but the changes in the expression, concentration, and activity of cellular energy-regulated molecules that occur with aging in worker bees is unknown. In this study, energy-regulated molecules were evaluated in the trophocytes and fat cells of young and old workers. The results showed that (i) adenosine monophosphate-activated protein kinase-α2 (AMPK-α2) expression increased with aging, whereas phosphorylated AMPK-α2 expression, the phosphorylated AMPK/AMPK ratio, and AMPK activity decreased with aging; (ii) adenosine diphosphate and adenosine triphosphate concentrations decreased with aging, the AMP concentration was unchanged, the adenosine diphosphate/adenosine triphosphate ratio did not change with aging, and the AMP/adenosine triphosphate ratio increased with aging; (iii) the cyclic AMP concentration decreased with aging, and cyclic AMP-specific phosphodiesterases activity increased with aging; (iv) silent information regulator 2 (Sir2) expression increased with aging, whereas its activity decreased with aging; and (v) peroxisome proliferator-activated receptor-α expression decreased with aging. These results show that the trophocytes and fat cells of young workers have higher cellular energy status and express higher levels of energy-regulated molecules than those of old workers and that aging results in a decline in the energy status of trophocytes and fat cells in worker honeybees.

TRPV1-mediated UCP2 upregulation ameliorates hyperglycemia-induced endothelial dysfunction

Background

Diabetic cardiovascular complications are characterised by oxidative stress-induced endothelial dysfunction. Uncoupling protein 2 (UCP2) is a regulator of mitochondrial reactive oxygen species (ROS) generation and can antagonise oxidative stress, but approaches that enhance the activity of UCP2 to inhibit ROS are scarce. Our previous studies show that activation of transient receptor potential vanilloid 1 (TRPV1) by capsaicin can prevent cardiometabolic disorders. In this study, we conducted experiments in vitro and in vivo to investigate the effect of capsaicin treatment on endothelial UCP2 and oxidative stress. We hypothesised that TRPV1 activation by capsaicin attenuates hyperglycemia-induced endothelial dysfunction through a UCP2-mediated antioxidant effect.

Methods

TRPV1^-/-, UCP2 ^-/- and db/db mice, as well as matched wild type (WT) control mice, were included in this study. Some mice were subjected to dietary capsaicin for 14 weeks. Arteries isolated from mice and endothelial cells were cultured. Endothelial function was examined, and immunohistological and molecular analyses were performed.

Results

Under high-glucose conditions, TRPV1 expression and protein kinase A (PKA) phosphorylation were found to be decreased in the cultured endothelial cells, and the effects of high-glucose on these molecules were reversed by the administration of capsaicin. Furthermore, high-glucose exposure increased ROS production and reduced nitric oxide (NO) levels both in endothelial cells and in arteries that were evaluated respectively by dihydroethidium (DHE) and DAF-2 DA fluorescence. Capsaicin administration decreased the production of ROS, restored high-glucose-induced endothelial dysfunction through the activation of TRPV1 and acted in a UCP2-dependent manner in vivo. Administration of dietary capsaicin for 14 weeks increased the levels of PKA phosphorylation and UCP2 expression, ameliorated the vascular oxidative stress and increased NO levels observed in diabetic mice. Prolonged dietary administration of capsaicin promoted endothelium-dependent relaxation in diabetic mice. However, the beneficial effect of capsaicin on vasorelaxation was absent in the aortas of UCP2 ^-/- mice exposed to high-glucose levels.

Conclusion

TRPV1 activation by capsaicin might protect against hyperglycemia-induced endothelial dysfunction through a mechanism involving the PKA/UCP2 pathway.

AMPK modulates glucose-sensing in insulin-secreting cells by altered phosphotransfer to KATP channels

Glucose-sensing (GS) behaviour in pancreatic β-cells is dependent on ATP-sensitive K(+) channel (KATP) activity, which is controlled by the relative levels of the KATP ligands ATP and ADP, responsible for closing and opening KATP, respectively. However, the mechanism by which β-cells transfer energy status from mitochondria to KATP, and hence to altered electrical excitability and insulin secretion, is presently unclear. Recent work has demonstrated a critical role for AMP-activated protein kinase (AMPK) in GS behaviour of cells. Electrophysiological recordings, coupled with measurements of gene and protein expression were made from rat insulinoma cells to investigate whether AMPK activity regulates this energy transfer process. Using the whole-cell recording configuration with sufficient intracellular ATP to keep KATP closed, raised AMPK activity induced GS electrical behaviour. This effect was prevented by the AMPK inhibitor, compound C and required a phosphotransfer process. Indeed, high levels of intracellular phosphocreatine or the presence of the adenylate kinase (AK) inhibitor AP5A blocked this action of AMPK. Using conditions that maximised AMPK-induced KATP opening, there was a significant increase in AK1, AK2 and UCP2 mRNA expression. Thus we propose that KATP opening in response to lowered glucose concentration requires AMPK activity, perhaps in concert with increased AK and UCP2 to enable mitochondrial-derived ADP signals to be transferred to plasma membrane KATP by phosphotransfer cascades.

AMP-activated protein kinase: implications on ischemic diseases

Ischemia is a blockage of blood supply due to an embolism or a hemorrhage in a blood vessel. When an organ cannot receive oxygenated blood and can therefore no longer replenish its blood supply due to ischemia, stresses, such as the disruption of blood glucose homeostasis, hypoglycemia and hypoxia, activate the AMPK complex. LKB1 and CaMKKβ are essential activators of the AMPK signaling pathway. AMPK triggers proangiogenic effects through the eNOS protein in tissues with ischemic conditions, where cells are vulnerable to apoptosis, autophagy and necrosis. The AMPK complex acts to restore blood glucose levels and ATP levels back to homeostasis. This review will discuss AMPK, as well as its key activators (LKB1 and CaMKKβ), as a central energy regulator and evaluate the upstream and downstream regulating pathways of AMPK. We will also discuss how we can control this important enzyme in ischemic conditions to prevent harmful effects in patients with vascular damage.

Mouse hypothalamic GT1-7 cells demonstrate AMPK-dependent intrinsic glucose-sensing behaviour

AIMS/HYPOTHESIS

Hypothalamic glucose-excited (GE) neurons contribute to whole-body glucose homeostasis and participate in the detection of hypoglycaemia. This system appears defective in type 1 diabetes, in which hypoglycaemia commonly occurs. Unfortunately, it is at present unclear which molecular components required for glucose sensing are produced in individual neurons and how these are functionally linked. We used the GT1-7 mouse hypothalamic cell line to address these issues.

METHODS

Electrophysiological recordings, coupled with measurements of gene expression and protein levels and activity, were made from unmodified GT1-7 cells and cells in which AMP-activated protein kinase (AMPK) catalytic subunit gene expression and activity were reduced.

RESULTS

Hypothalamic GT1-7 neurons express the genes encoding glucokinase and ATP-sensitive K(+) channel (K(ATP)) subunits K ( ir ) 6.2 and Sur1 and exhibit GE-type glucose-sensing behaviour. Lowered extracellular glucose concentration hyperpolarised the cells in a concentration-dependent manner, an outcome that was reversed by tolbutamide. Inhibition of glucose uptake or metabolism hyperpolarised cells, showing that energy metabolism is required to maintain their resting membrane potential. Short hairpin (sh)RNA directed to Ampkα2 (also known as Prkaa2) reduced GT1-7 cell AMPKα2, but not AMPKα1, activity and lowered the threshold for hypoglycaemia-induced hyperpolarisation. shAmpkα1 (also known as Prkaa1) had no effect on glucose-sensing or AMPKα2 activity. Decreased uncoupling protein 2 (Ucp2) mRNA was detected in AMPKα2-reduced cells, suggesting that AMPKα2 regulates UCP2 levels.

CONCLUSIONS/INTERPRETATION

We have demonstrated that GT1-7 cells closely mimic GE neuron glucose-sensing behaviour, and reducing AMPKα2 blunts their responsiveness to hypoglycaemic challenge, possibly by altering UCP2 activity. These results show that suppression of AMPKα2 activity inhibits normal glucose-sensing behaviour and may contribute to defective detection of hypoglycaemia.

Ghrelin's second life: From appetite stimulator to glucose regulator

Ghrelin, a 28 amino acid peptide hormone produced by the stomach, was the first orexigenic hormone to be discovered from the periphery. The octanoyl modification at Ser³, mediated by ghrelin O-acyltransferase (GOAT), is essential for ghrelin’s biological activity. Ghrelin stimulates food intake through binding to its receptor (GRLN-R) on neurons in the arcuate nucleus of the hypothalamus. Ghrelin is widely expressed throughout the body; accordingly, it is implicated in several other physiological functions, which include growth hormone release, gastric emptying, and body weight regulation. Ghrelin and GRLN-R expression are also found in the pancreas, suggesting a local physiological role. Accordingly, several recent studies now point towards an important role for ghrelin and its receptor in the regulation of blood glucose homeostasis, which is the main focus of this review. Several mechanisms of this regulation by ghrelin have been proposed, and one possibility is through the regulation of insulin secretion. Despite some controversy, most studies suggest that ghrelin exerts an inhibitory effect on insulin secretion, resulting in increased circulating glucose levels. Ghrelin may thus be a diabetogenic factor. Obesity-related type 2 diabetes has become an increasingly important health problem, almost reaching epidemic proportions in the world; therefore, antagonists of the ghrelin-GOAT signaling pathway, which will tackle both energy- and glucose homeostasis, may be considered as promising new therapies for this disease.

Ghrelin contributes to derangements of glucose metabolism induced by rapamycin in mice

AIMS/HYPOTHESIS

Rapamycin impairs glucose tolerance and insulin sensitivity. Our previous study demonstrated that rapamycin significantly increases the production of gastric ghrelin, which is critical in the regulation of glucose metabolism. Here, we investigated whether ghrelin contributes to derangements of glucose metabolism induced by rapamycin.

METHODS

The effects of rapamycin on glucose metabolism were examined in mice receiving ghrelin receptor antagonist or with Ghsr1a gene knockout. Changes in GLUT4, c-Jun N-terminal kinase (JNK) and phosphorylated ribosomal protein S6 (pS6) were investigated by immunofluorescent staining or western blotting. Related hormones were detected by radioimmunoassay kits.

RESULTS

Rapamycin impaired glucose metabolism and insulin sensitivity not only in normal C57BL/6J mice but also in both obese mice induced by a high fat diet and db/db mice. This was accompanied by elevation of plasma acylated ghrelin. Rapamycin significantly increased the levels of plasma acylated ghrelin in normal C57BL/6J mice, high-fat-diet-induced obese mice and db/db mice. Elevation in plasma acylated ghrelin and derangements of glucose metabolism upon administration of rapamycin were significantly correlated. The deterioration in glucose homeostasis induced by rapamycin was blocked by D: -Lys3-GHRP-6, a ghrelin receptor antagonist, or by deletion of the Ghsr1a gene. Ghrelin receptor antagonism and Ghsr1a knockout blocked the upregulation of JNK activity and downregulation of GLUT4 levels and translocation in the gastrocnemius muscle induced by rapamycin.

CONCLUSIONS/INTERPRETATION

The current study demonstrates that ghrelin contributes to derangements of glucose metabolism induced by rapamycin via altering the content and translocation of GLUT4 in muscles.

AMP-activated protein kinase, stress responses and cardiovascular diseases

AMPK (AMP-activated protein kinase) is one of the key players in maintaining intracellular homoeostasis. AMPK is well known as an energy sensor and can be activated by increased intracellular AMP levels. Generally, the activation of AMPK turns on catabolic pathways that generate ATP, while inhibiting cell proliferation and biosynthetic processes that consume ATP. In recent years, intensive investigations on the regulation and the function of AMPK indicates that AMPK not only functions as an intracellular energy sensor and regulator, but is also a general stress sensor that is important in maintaining intracellular homoeostasis during many kinds of stress challenges. In the present paper, we will review recent literature showing that AMPK functions far beyond its proposed energy sensor and regulator function. AMPK regulates ROS (reactive oxygen species)/redox balance, autophagy, cell proliferation, cell apoptosis, cellular polarity, mitochondrial function and genotoxic response, either directly or indirectly via numerous downstream pathways under physiological and pathological conditions.

Leptin boosts cellular metabolism by activating AMPK and the sirtuins to reduce tau phosphorylation and β-amyloid in neurons

Leptin is a pleiotropic hormone primarily secreted by adipocytes. A high density of functional Leptin receptors has been reported to be expressed in the hippocampus and other cortical regions of the brain, the physiological significance of which has not been explored extensively. Alzheimer's disease (AD) is marked by impaired brain metabolism with decreased glucose utilization in those regions which often precede pathological changes. Recent epidemiological studies suggest that plasma Leptin is protective against AD. Specifically, elderly with plasma Leptin levels in the lowest quartile were found to be four times more likely to develop AD than those in the highest quartile. We have previously reported that Leptin modulates AD pathological pathways in vitro through a mechanism involving the energy sensor, AMP-activated protein kinase (AMPK). To this end, we investigated the extent to which activation of AMPK as well as another class of sensors linking energy availability to cellular metabolism, the sirtuins (SIRT), mediate Leptin's biological activity. Leptin directly activated neuronal AMPK and SIRT in cell lines. Additionally, the ability of Leptin to reduce tau phosphorylation and β-amyloid production was sensitive to the AMPK and sirtuin inhibitors, compound C and nicotinamide, respectively. These findings implicate that Leptin normally acts as a signal for energy homeostasis in neurons. Perhaps Leptin deficiency in AD contributes to a neuronal imbalance in handling energy requirements, leading to higher Aβ and phospho-tau, which can be restored by replenishing low Leptin levels. This may also be a legitimate strategy for therapy.

Recent advances in pancreatic endocrine and exocrine secretion

PURPOSE OF REVIEW

This review presents recent advancements in the mechanisms by which integrated signaling mechanisms elicit and regulate pancreatic endocrine and exocrine secretion.

RECENT FINDINGS

Cholecystokinin (CCK) can stimulate exocrine secretion by acting directly on neurons located in the dorsal motor of the vagus or indirectly by acting on pancreatic stellate cells. The importance of small GTPases such as RhoA and Rac1 in CCK-induced pancreatic secretion is also described. Ghrelin attenuates insulin secretion through the AMP-activated protein kinase-uncoupling protein 2 pathway. An exciting new report describes that leptin can influence insulin release by osteoclastin, a hormone produced by osteoblasts. This finding adds a new layer of complexity in the regulation of insulin secretion with implications for glucose and energy homeostasis. In addition, leptin also mediates insulin secretion through the sympathetic system and via pro-opiomelanocortin neurons, which could serve as the cross-road for leptin and melanocortin signaling pathways. Recent reports on the action of numerous other regulators such as atrial natriuretic peptide, neurotensin, and orexin B are also discussed.

SUMMARY

The pancreas is an extremely complex gland. Elucidation of the secretory and regulatory pathways that control pancreatic secretion will aid in the development of treatment for diseases such as pancreatitis, diabetes, and obesity.

Ghrelin directly stimulates glucagon secretion from pancreatic alpha-cells

Previous work has demonstrated that the peptide hormone ghrelin raises blood glucose. Such has been attributed to ghrelin's ability to enhance GH secretion, restrict insulin release, and/or reduce insulin sensitivity. Ghrelin's reported effects on glucagon have been inconsistent. Here, both animal- and cell-based systems were used to determine the role of glucagon in mediating ghrelin's effects on blood glucose. The tissue and cell distribution of ghrelin receptors (GHSR) was evaluated by quantitative PCR and histochemistry. Plasma glucagon levels were determined following acute acyl-ghrelin injections and in pharmacological and/or transgenic mouse models of ghrelin overexpression and GHSR deletion. Isolated mouse islets and the α-cell lines αTC1 and InR1G9 were used to evaluate ghrelin's effects on glucagon secretion and the role of calcium and ERK in this activity. GHSR mRNA was abundantly expressed in mouse islets and colocalized with glucagon in α-cells. Elevation of acyl-ghrelin acutely (after sc administration, such that physiologically relevant plasma ghrelin levels were achieved) and chronically (by slow-releasing osmotic pumps and as observed in transgenic mice harboring ghrelinomas) led to higher plasma glucagon and increased blood glucose. Conversely, genetic GHSR deletion was associated with lower plasma glucagon and reduced fasting blood glucose. Acyl-ghrelin increased glucagon secretion in a dose-dependent manner from mouse islets and α-cell lines, in a manner requiring elevation of intracellular calcium and phosphorylation of ERK. Our study shows that ghrelin's regulation of blood glucose involves direct stimulation of glucagon secretion from α-cells and introduces the ghrelin-glucagon axis as an important mechanism controlling glycemia under fasting conditions.

Ghrelin in obesity and endocrine diseases

Ghrelin shows orexigenic effect through its action on the hypothalamic appetite-regulating pathways, while in the periphery ghrelin increases adipose tissue accumulation and has a diabetogenic effect on the liver and pancreas. Adenosine monophosphate-activated protein kinase (AMPK) has been suggested as one of the mediators of ghrelin's effects. Plasma ghrelin levels are dependent on body mass index as well as food intake patterns. Ghrelin levels are in general reduced in obese individuals and in subjects with insulin resistance. In contrast to other forms of obesity, patients with Prader-Willi syndrome (PWS) display high levels of ghrelin, reduced visceral adiposity and relative hypoinsulinemia. Relationships between obesity and common genomic variants of GHRL and GHS-R genes have been studied. Ghrelin may have a role in the weight-reducing effect of bariatric surgery; however, this is a much debated issue. Altered ghrelin levels have also been observed in Cushing's syndrome and thyroid disease probably due to the secondary insulin resistance in these subjects.

WITHDRAWN: Ghrelin in obesity and endocrine diseases

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.mce.2011.02.011. The duplicate article has therefore been withdrawn.

Exercise training enhances rat pancreatic islets anaplerotic enzymes content despite reduced insulin secretion

Endurance exercise has been shown to reduce pancreatic islets glucose-stimulated insulin secretion (GSIS). Anaplerotic/cataplerotic pathways are directly related to GSIS signaling. However, the effect of endurance training upon pancreatic islets anaplerotic enzymes is still unknown. In this sense, we tested the hypothesis that endurance exercise decreases GSIS by reducing anaplerotic/cataplerotic enzymes content. Male Wistar rats were randomly assigned to one of the four experimental groups as follows: control sedentary group (CTL), trained 1 day per week (TRE1×), trained 3 days per week (TRE3×) and trained 5 days per week (TRE5x) and submitted to an 8 weeks endurance-training protocol. After the training protocol, pancreatic islets were isolated and incubated with basal (2.8 mM) and stimulating (16.7 mM) glucose concentrations for GSIS measurement by radioimmunoassay. In addition, pyruvate carboxylase (PYC), pyruvate dehydrogenase (PDH), pyruvate dehydrogenase kinase 4 (PDK4), ATP-citrate lyase (ACL) and glutamate dehydrogenase (GDH) content were quantified by western blotting. Our data showed that 8 weeks of chronic endurance exercise reduced GSIS by 50% in a dose-response manner according to weekly exercise frequency. PYC showed significant twofold increase in TRE3×. PYC enhancement was even higher in TRE5× (p < 0.0001). PDH and PDK4 reached significant 25 and 50% enhancement, respectively compared with CTL. ACL and GDH also reported significant 50 and 75% increase, respectively. The absence of exercise-induced correlations among GSIS and anaplerotic/cataplerotic enzymes suggests that exercise may control insulin release by activating other signaling pathways. The observed anaplerotic and cataplerotic enzymes enhancement might be related to β-cell surviving rather than insulin secretion.

The ghrelin O-acyltransferase-ghrelin system: a novel regulator of glucose metabolism

PURPOSE OF REVIEW

Ghrelin, an orexigenic hormone secreted from the stomach, exists in the circulation in two isoforms: des-acyl and acyl ghrelin. Acylation by the enzyme ghrelin O-acyl-transferase (GOAT) enables ghrelin to activate the ghrelin receptor. This review discusses recent findings illustrating the role of acyl ghrelin, des-acyl ghrelin and GOAT in regulating glucose homeostasis.

RECENT FINDINGS

Recent publications support a role of ghrelin in modulating glucose homeostasis. Novel cellular mechanisms have been proposed to explain these effects. Controversy on this topic continues to exist owing to inconsistent observations made in both rodents and humans. Many recent studies are uncovering a role of des-acyl ghrelin in glucose metabolism specifically in modulating insulin sensitivity and glucose uptake into adipocytes. A novel role of ghrelin acylation by the enzyme GOAT in regulating glucose metabolism during caloric deprivation has newly been discovered.

SUMMARY

Ghrelin plays a role in regulating glucose homeostasis through the modulation of insulin secretion and insulin sensitivity. Acyl ghrelin and des-acyl ghrelin appear to have opposing glucoregulatory effects and regulation of acylation by the enzyme GOAT appears to play a role in mediating glucose metabolism. Modulation of GOAT or ghrelin signaling may be a clinically relevant strategy to treat metabolic diseases such as type II diabetes.