The β-cell GHSR and downstream cAMP/TRPM2 signaling account for insulinostatic and glycemic effects of ghrelin

GLP-1 and ghrelin inversely regulate insulin secretion and action in pancreatic islets, vagal afferents, and hypothalamus for controlling glycemia and feeding

Glucagon-like peptide-1 (GLP-1) was discovered as an incretin hormone, which is released from the intestine upon nutrient intake and stimulates insulin secretion from the pancreatic islet β-cells. Subsequently, its ability to suppress appetite was recognized. Ghrelin, discovered as the ligand for growth hormone secretagogue-receptor (GHS-R), is released from the stomach and produces appetite. Later, its ability to inhibit insulin secretion and elevate blood glucose was found. Thus, GLP-1 and ghrelin regulate insulin secretion and appetite toward opposite directions. The receptor agonists for GLP-1 and ghrelin have been developed and are now used to treat metabolic diseases, in which insulin plays a key role. However, underlying action mechanism and possible interplay of these hormones have remained elusive. Here, we describe that GLP-1 and ghrelin reciprocally regulate the insulin system. GLP-1 enhances and ghrelin suppresses insulin secretion in pancreatic β-cells. Moreover, GLP-1 cooperates with and ghrelin counteracts insulin action in the vagal afferent and hypothalamic arcuate nucleus (ARC) neurons, the interfaces between the peripheral metabolism and brain. Notably, ghrelin rises and works preprandially and GLP-1 rises and works postprandially. The interplay of ghrelin, GLP-1, and insulin leads to optimal circadian control of feeding, glycemia, and metabolism.

Ghrelin-LEAP2 interactions along the stomach-liver axis

Ghrelin produced in the stomach promotes food intake and GH secretion, and acts as an anabolic peptide during starvation. Ghrelin binds to the growth hormone secretagogue receptor, a G protein-coupled receptor (GPCR), whose high-resolution complex structures have been determined in the apo state and when bound to an antagonist. Anamorelin, a low-molecular-weight ghrelin agonist, has been launched in Japan for the treatment of cancer cachexia, and its therapeutic potential has attracted attention due to the various biological activities of ghrelin. In 2019, liver-expressed antimicrobial peptide (LEAP2), initially discovered as an antimicrobial peptide produced in the liver, was identified to be upregulated in the stomach of diet-induced obese mice after vertical sleeve gastrectomy. LEAP2 binds to the GHSR and antagonizes ghrelin's activities. The serum concentrations of human LEAP2 are positively correlated with body mass index, body fat accumulation, and fasting serum concentrations of glucose and triglyceride. Serum LEAP2 elevated and ghrelin reduced in obesity. Ghrelin and LEAP2 regulate body weight, food intake, and GH and blood glucose concentrations, and other physiological phenomena through their interactions with the same receptor, GHSR.

The antidiabetic effect of safflower yellow by regulating the GOAT/ghrelin/GHS-R1a/cAMP/TRPM2 pathway

Safflower yellow (SY), derived from Carthamus tinctorius L., is a valuable natural edible pigment that exhibits anti-type 2 diabetes mellitus (T2DM) efficacy; however, its mechanism of action is unclear, which hinders its effective use. In this study, we examined the impact of SY on glucose metabolism and insulin secretion both in vivo and in vitro and elucidated the possible underlying mechanism. First, molecular docking demonstrated a strong binding affinity between SY and ghrelin O-acyltransferase (GOAT) protein, which was validated by a cell heat transfer assay (CETSA) and drug affinity response target stability (DARTS) in MIN6 cells. In MIN6 cells, SY increased insulin secretion and showed time- and dose-dependent inhibition of GOAT expression and acyl ghrelin (AG) secretion without affecting the overall levels of ghrelin. Furthermore, ELISA revealed that SY enhanced high glucose (HG)-induced insulin secretion, and immunofluorescence revealed the co-localization of GOAT and ghrelin in MIN6 cells, which was suppressed by SY treatment. The mechanism analysis by Western blot demonstrated that SY downregulated the protein levels of GOAT and GHS-R1a in MIN6 cells while increasing HG-stimulated cAMP and activation of transient receptor potential melastatin 2 (TRPM2). In in vivo experiments, the intraperitoneal injection of SY significantly improved pathological damage to the pancreas, glucose tolerance, and insulin resistance in a mouse model of high-fat diet (HFD)/streptozotocin (STZ)-induced T2DM in a dose-dependent manner. SY enhanced insulin secretion by inhibiting the GOAT/ghrelin system in vivo. In conclusion, we demonstrated that SY exhibits an observable protective effect on diabetes through the GOAT/ghrelin/GHS-R1a/cAMP/TRPM2 pathway. Our findings provide a basis for further investigation of the hypoglycemic mechanism of SY and its potential for further development and utilization.

GHSR Deletion in β-Cells of Male Mice: Ineffective in Obesity, but Effective in Protecting against Streptozotocin-Induced β-Cell Injury in Aging

Insulin secretion from pancreatic β cells is a key pillar of glucose homeostasis, which is impaired under obesity and aging. Growth hormone secretagogue receptor (GHSR) is the receptor of nutrient-sensing hormone ghrelin. Previously, we showed that β-cell GHSR regulated glucose-stimulated insulin secretion (GSIS) in young mice. In the current study, we further investigated the effects of GHSR on insulin secretion in male mice under diet-induced obesity (DIO) and streptozotocin (STZ)-induced β-cell injury in aging. β-cell-specific-Ghsr-deficient (Ghsr-βKO) mice exhibited no glycemic phenotype under DIO but showed significantly improved ex vivo GSIS in aging. We also detected reduced insulin sensitivity and impaired insulin secretion during aging both in vivo and ex vivo. Accordingly, there were age-related alterations in expression of glucose transporter, insulin signaling pathway, and inflammatory genes. To further determine whether GHSR deficiency affected β-cell susceptibility to acute injury, young, middle-aged, and old Ghsr-βKO mice were subjected to STZ. We found that middle-aged and old Ghsr-βKO mice were protected from STZ-induced hyperglycemia and impaired insulin secretion, correlated with increased expression of insulin signaling regulators but decreased pro-inflammatory cytokines in pancreatic islets. Collectively, our findings indicate that β-cell GHSR has a major impact on insulin secretion in aging but not obesity, and GHSR deficiency protects against STZ-induced β-cell injury in aging.

Regulation of peripheral tissue substrate metabolism by the gut-derived hormone ghrelin

Ghrelin increases in the circulation prior to entrained mealtimes, with the acylated (AG) form functioning to stimulate food intake and growth hormone release. Acutely, AG induces whole-body insulin resistance, potentially to maintain glycemia between meals. Alternatively, chronic administration of both AG and the unacylated isoform of ghrelin (unAG) is associated with improved skeletal muscle insulin sensitivity as well as reduced intramuscular lipids and inflammation. This may be due to effects on lipid metabolism, with ghrelin promoting storage of fat in adipose and liver while stimulating oxidation in skeletal muscle, preventing ectopic lipid accumulation. This is of specific relevance in the handling of meal-derived lipids, as ghrelin rises preprandially with effects persisting for 2-3 h following exposure in skeletal muscle, coinciding with elevated plasma FFAs. We hypothesize that ghrelin acts as a preparatory signal for incoming lipids, as well as a regulatory hormone for their use and storage. The effects of ghrelin on skeletal muscle are lost with high fat diet feeding and physical inactivity, potentially being implicated in the pathogenesis of metabolic disease. This review summarizes the metabolic effects of both ghrelin isoforms on peripheral tissues including the pancreas, adipose, liver, and skeletal muscle. Additionally, we speculate on the physiological relevance of these effects in vivo and suggest that ghrelin may be a key regulatory hormone for nutrient handling in the postprandial state.

Role of TRP Channels in Metabolism-Related Diseases

Metabolic syndrome (MetS), with its high prevalence and significant impact on cardiovascular disease, poses a substantial threat to human health. The early identification of pathological abnormalities related to MetS and prevention of the risk of associated diseases is of paramount importance. Transient Receptor Potential (TRP) channels, a type of nonselective cation channel, are expressed in a variety of tissues and have been implicated in the onset and progression of numerous metabolism-related diseases. This study aims to review and discuss the expression and function of TRP channels in metabolism-related tissues and blood vessels, and to elucidate the interactions and mechanisms between TRP channels and metabolism-related diseases. A comprehensive literature search was conducted using keywords such as TRP channels, metabolic syndrome, pancreas, liver, oxidative stress, diabetes, hypertension, and atherosclerosis across various academic databases including PubMed, Google Scholar, Elsevier, Web of Science, and CNKI. Our review of the current research suggests that TRP channels may be involved in the development of metabolism-related diseases by regulating insulin secretion and release, lipid metabolism, vascular functional activity, oxidative stress, and inflammatory response. TRP channels, as nonselective cation channels, play pivotal roles in sensing various intra- and extracellular stimuli and regulating ion homeostasis by osmosis. They present potential new targets for the diagnosis or treatment of metabolism-related diseases.

Germline and conditional ghrelin knockout increases islet size

Conflicting studies in recent years report that genetic or pharmacological increases or decreases in ghrelin either increase or have no effect on islet size. In this issue of the JCI, Gupta, Burstein, and colleagues applied a rigorous approach to determine the effects of reducing ghrelin on islet size in germline and conditional ghrelin-knockout mice as well as across varying ages and weight. Both germline and conditional ghrelin-knockout mice associated with increased islet size, which was further exacerbated by older age and diet-induced obesity. These findings suggest that modulation of ghrelin may open a therapeutic window to prevent or treat diabetes.

Ghrelin deletion and conditional ghrelin cell ablation increase pancreatic islet size in mice

Ghrelin exerts key effects on islet hormone secretion to regulate blood glucose levels. Here, we sought to determine whether ghrelin's effects on islets extend to the alteration of islet size and β cell mass. We demonstrate that reducing ghrelin - by ghrelin gene knockout (GKO), conditional ghrelin cell ablation, or high-fat diet (HFD) feeding - was associated with increased mean islet size (up to 62%), percentage of large islets (up to 854%), and β cell cross-sectional area (up to 51%). In GKO mice, these effects were more apparent in 10- to 12-week-old mice than in 4-week-old mice. Higher β cell numbers from decreased β cell apoptosis drove the increase in β cell cross-sectional area. Conditional ghrelin cell ablation in adult mice increased the β cell number per islet by 40% within 4 weeks. A negative correlation between islet size and plasma ghrelin in HFD-fed plus chow-fed WT mice, together with even larger islet sizes in HFD-fed GKO mice than in HFD-fed WT mice, suggests that reduced ghrelin was not solely responsible for diet-induced obesity-associated islet enlargement. Single-cell transcriptomics revealed changes in gene expression in several GKO islet cell types, including upregulation of Manf, Dnajc3, and Gnas expression in β cells, which supports decreased β cell apoptosis and/or increased β cell proliferation. These effects of ghrelin reduction on islet morphology might prove useful when designing new therapies for diabetes.

Mechanistic Investigation of GHS-R Mediated Glucose-Stimulated Insulin Secretion in Pancreatic Islets

Ghrelin receptor, a growth hormone secretagogue receptor (GHS-R), is expressed in the pancreas. Emerging evidence indicates that GHS-R is involved in the regulation of glucose-stimulated insulin secretion (GSIS), but the mechanism by which GHS-R regulates GSIS in the pancreas is unclear. In this study, we investigated the role of GHS-R on GSIS in detail using global Ghsr^−/− mice (in vivo) and Ghsr-ablated pancreatic islets (ex vivo). GSIS was attenuated in both Ghsr^−/− mice and Ghsr-ablated islets, while the islet morphology was similar between WT and Ghsr^−/− mice. To elucidate the mechanism underpinning Ghsr-mediated GSIS, we investigated the key steps of the GSIS signaling cascade. The gene expression of glucose transporter 2 (Glut2) and the glucose-metabolic intermediate—glucose-6-phosphate (G6P) were reduced in Ghsr-ablated islets, supporting decreased glucose uptake. There was no difference in mitochondrial DNA content in the islets of WT and Ghsr^−/− mice, but the ATP/ADP ratio in Ghsr^−/− islets was significantly lower than that of WT islets. Moreover, the expression of pancreatic and duodenal homeobox 1 (Pdx1), as well as insulin signaling genes of insulin receptor (IR) and insulin receptor substrates 1 and 2 (IRS1/IRS2), was downregulated in Ghsr^−/− islets. Akt is the key mediator of the insulin signaling cascade. Concurrently, Akt phosphorylation was reduced in the pancreas of Ghsr^−/− mice under both insulin-stimulated and homeostatic conditions. These findings demonstrate that GHS-R ablation affects key components of the insulin signaling pathway in the pancreas, suggesting the existence of a cross-talk between GHS-R and the insulin signaling pathway in pancreatic islets, and GHS-R likely regulates GSIS via the Akt-Pdx1-GLUT2 pathway.

Status of ghrelin as an islet hormone and paracrine/autocrine regulator of insulin secretion

Ghrelin is expressed in the pancreatic islet cells as well as the stomach. In the perfused pancreas and isolated islets, GHS-R antagonism, ghrelin immunoneutralization and ghrelin-knockout (Ghr-KO) all increase glucose-induced insulin release. Thus, pharmacological, immunological and genetic blockades of ghrelin in the pancreatic islets all markedly augment glucose-induced insulin release, showing that islet-derived ghrelin physiologically restricts insulin release in rodents. In this review, we focus on the current understanding of the following key questions: 1) from which islet cells ghrelin is released, 2) on which islet cells ghrelin acts, and 3) mechanisms by which the islet-derived ghrelin inhibits insulin secretion.

Large-Scale Functional Genomics Screen to Identify Modulators of Human β-Cell Insulin Secretion

Type 2 diabetes (T2D) is a chronic metabolic disorder affecting almost half a billion people worldwide. Impaired function of pancreatic β-cells is both a hallmark of T2D and an underlying factor in the pathophysiology of the disease. Understanding the cellular mechanisms regulating appropriate insulin secretion has been of long-standing interest in the scientific and clinical communities. To identify novel genes regulating insulin secretion we developed a robust arrayed siRNA screen measuring basal, glucose-stimulated, and augmented insulin secretion by EndoC-βH1 cells, a human β-cell line, in a 384-well plate format. We screened 521 candidate genes selected by text mining for relevance to T2D biology and identified 23 positive and 68 negative regulators of insulin secretion. Among these, we validated ghrelin receptor (GHSR), and two genes implicated in endoplasmic reticulum stress, ATF4 and HSPA5. Thus, we have demonstrated the feasibility of using EndoC-βH1 cells for large-scale siRNA screening to identify candidate genes regulating β-cell insulin secretion as potential novel drug targets. Furthermore, this screening format can be adapted to other disease-relevant functional endpoints to enable large-scale screening for targets regulating cellular mechanisms contributing to the progressive loss of functional β-cell mass occurring in T2D.

In Search of Small Molecules That Selectively Inhibit MBOAT4

Ghrelin is a 28-residue peptide hormone produced by stomach P/D1 cells located in oxyntic glands of the fundus mucosa. Post-translational octanoylation of its Ser-3 residue, catalyzed by MBOAT4 (aka ghrelin O-acyl transferase (GOAT)), is essential for the binding of the hormone to its receptor in target tissues. Physiological roles of acyl ghrelin include the regulation of food intake, growth hormone secretion from the pituitary, and inhibition of insulin secretion from the pancreas. Here, we describe a medicinal chemistry campaign that led to the identification of small lipopeptidomimetics that inhibit GOAT in vitro. These molecules compete directly for substrate binding. We further describe the synthesis of heterocyclic inhibitors that compete at the acyl coenzyme A binding site.

High Coexpression of the Ghrelin and LEAP2 Receptor GHSR With Pancreatic Polypeptide in Mouse and Human Islets

Islets represent an important site of direct action of the hormone ghrelin, with expression of the ghrelin receptor (growth hormone secretagogue receptor; GHSR) having been localized variably to alpha cells, beta cells, and/or somatostatin (SST)-secreting delta cells. To our knowledge, GHSR expression by pancreatic polypeptide (PP)-expressing gamma cells has not been specifically investigated. Here, histochemical analyses of Ghsr-IRES-Cre × Cre-dependent ROSA26-yellow fluorescent protein (YFP) reporter mice showed 85% of GHSR-expressing islet cells coexpress PP, 50% coexpress SST, and 47% coexpress PP + SST. Analysis of single-cell transcriptomic data from mouse pancreas revealed 95% of Ghsr-expressing cells coexpress Ppy, 100% coexpress Sst, and 95% coexpress Ppy + Sst. This expression was restricted to gamma-cell and delta-cell clusters. Analysis of several single-cell human pancreatic transcriptome data sets revealed 59% of GHSR-expressing cells coexpress PPY, 95% coexpress SST, and 57% coexpress PPY + SST. This expression was prominent in delta-cell and beta-cell clusters, also occurring in other clusters including gamma cells and alpha cells. GHSR expression levels were upregulated by type 2 diabetes mellitus in beta cells. In mice, plasma PP positively correlated with fat mass and with plasma levels of the endogenous GHSR antagonist/inverse agonist LEAP2. Plasma PP also elevated on LEAP2 and synthetic GHSR antagonist administration. These data suggest that in addition to delta cells, beta cells, and alpha cells, PP-expressing pancreatic cells likely represent important direct targets for LEAP2 and/or ghrelin both in mice and humans.

Protective and Healing Effects of Ghrelin and Risk of Cancer in the Digestive System

Ghrelin is an endogenous ligand for the ghrelin receptor, previously known as the growth hormone secretagogue receptor. This hormone is mainly produced by endocrine cells present in the gastric mucosa. The ghrelin-producing cells are also present in other organs of the body, mainly in the digestive system, but in much smaller amount. Ghrelin exhibits a broad spectrum of physiological effects, such as stimulation of growth hormone secretion, gastric secretion, gastrointestinal motility, and food intake, as well as regulation of glucose homeostasis and bone formation, and inhibition of inflammatory processes. This review summarizes the recent findings concerning animal and human data showing protective and therapeutic effects of ghrelin in the gut, and also presents the role of growth hormone and insulin-like growth factor-1 in these effects. In addition, the current data on the possible influence of ghrelin on the carcinogenesis, its importance in predicting the risk of developing gastrointestinal malignances, as well as the potential usefulness of ghrelin in the treatment of cancer, have been presented.

The Role of cAMP in Beta Cell Stimulus-Secretion and Intercellular Coupling

Pancreatic beta cells secrete insulin in response to stimulation with glucose and other nutrients, and impaired insulin secretion plays a central role in development of diabetes mellitus. Pharmacological management of diabetes includes various antidiabetic drugs, including incretins. The incretin hormones, glucagon-like peptide-1 and gastric inhibitory polypeptide, potentiate glucose-stimulated insulin secretion by binding to G protein-coupled receptors, resulting in stimulation of adenylate cyclase and production of the secondary messenger cAMP, which exerts its intracellular effects through activation of protein kinase A or the guanine nucleotide exchange protein 2A. The molecular mechanisms behind these two downstream signaling arms are still not fully elucidated and involve many steps in the stimulus-secretion coupling cascade, ranging from the proximal regulation of ion channel activity to the central Ca2+ signal and the most distal exocytosis. In addition to modifying intracellular coupling, the effect of cAMP on insulin secretion could also be at least partly explained by the impact on intercellular coupling. In this review, we systematically describe the possible roles of cAMP at these intra- and inter-cellular signaling nodes, keeping in mind the relevance for the whole organism and translation to humans.

Oestrogen receptor α in T cells controls the T cell immune profile and glucose metabolism in mouse models of gestational diabetes mellitus

AIMS/HYPOTHESIS

The imbalance between maternal insulin resistance and a relative lack of insulin secretion underlies the pathogenesis of gestational diabetes mellitus (GDM). Alterations in T cell subtypes and increased levels of circulating proinflammatory cytokines have been proposed as potential mechanisms underlying the pathophysiology of insulin resistance in GDM. Since oestrogen modulates T cell immunity, we hypothesised that oestrogen plays a homeostatic role in visceral adipose tissue by coordinating T cell immunity through oestrogen receptor α (ERα) in T cells to prevent GDM.

METHODS

Female CD4-cre ERα^fl/fl (KO) mice on a C57BL/6 background with ERα ablation specifically in T cells, and ERα^fl/fl (ERα-floxed [FL]) mice were fed 60 kJ% high-fat diet (HFD) for 4 weeks. Female mice mated with male BALB/c mice to achieve allogenic pregnancy and were maintained on an HFD to generate the GDM model. Mice were divided into four experimental groups: non-pregnant FL, non-pregnant KO, pregnant FL (FL-GDM) and pregnant KO (KO-GDM). GTTs and ITTs were performed on day 12.5 or 13.5 and 16.5 after breeding, respectively. On day 18.5 after breeding, mice were killed and T cell subsets in the gonadal white adipose tissue (gWAT) and spleen were analysed using flow cytometry. Histological examination was also conducted and proinflammatory gene expression in gWAT and the liver was evaluated.

RESULTS

KO mice that mated with BALB/c mice showed normal fertility rates and fetal weights as compared with FL mice. Body and tissue weights were similar between FL and KO mice. When compared with FL-GDM mice, KO-GDM mice showed decreased insulin secretion (serum insulin concentration 15 min after glucose loading: 137.3 ± 18.3 pmol/l and 40.1 ± 36.5 pmol/l, respectively; p < 0.05), impaired glucose tolerance (glucose AUC in GTT: 2308.3 ± 54.0 mmol/l × min and 2620.9 ± 122.1 mmol/l × min, respectively; p < 0.05) and increased numbers of T helper (Th)17 cells in gWAT (0.4 ± 0.0% vs 0.8 ± 0.1%; p < 0.05). However, the contents of Th1 and regulatory T cells (Tregs) in gWAT remained similar between FL-GDM and KO-GDM. Glucose-stimulated insulin secretion was similar between isolated islets derived from FL and KO mice, but was reduced by IL-17A treatment. Moreover, the levels of proinflammatory gene expression, including expression of Emr1 and Tnfa in gWAT, were significantly higher in KO-GDM mice than in FL-GDM mice (5.1-fold and 2.7-fold, respectively; p < 0.01 for both). Furthermore, KO-GDM mice showed increased expression of genes encoding hepatokines, Ahsg and Fgf21 (both were 2.4-fold higher vs FL-GDM mice; p < 0.05 and p = 0.09, respectively), with no changes in inflammatory gene expression (e.g., Tnfa and Ifng) in the liver compared with FL-GDM mice.

CONCLUSIONS/INTERPRETATION

Deletion of ERα in T cells caused impaired maternal adaptation of insulin secretion, changes in hepatokine profiles, and enhanced chronic inflammation in gWAT alongside an abnormal increase in Th17 cells. These results suggest that the ERα-mediated oestrogen signalling effects in T cells regulate T cell immunity and contribute to glucose homeostasis in pregnancy.

Stimulation of endogenous pulsatile growth hormone secretion by activation of growth hormone secretagogue receptor reduces the fat accumulation and improves the insulin sensitivity in obese mice

Obese individuals often show low growth hormone (GH) secretion, which leads to reduced lipid mobilization and further fat accumulation. Pharmacological approaches to increase GH levels in obese individuals by GH injection or GH-releasing hormone receptor agonist showed promising effects on fat reduction. However, side effects on glucose metabolism and the heavy costs on making large peptides hindered their clinical application. Here, we tested whether stimulation of endogenous GH secretion by a synthetic GH secretagogue receptor (GHSR) agonist, hexarelin, improved the metabolism in a hyperphagic obese mouse model. Male melanocortin 4 receptor knockout mice (MC4RKO) were pair-fed and received continuous hexarelin (10.56 μg/day) or vehicle infusion by an osmotic pump for 3-4 weeks. Hexarelin treatment significantly increased the pulsatile GH secretion without detectable alteration on basal GH secretion in MC4RKO mice. The treated mice showed increased lipolysis and lipid oxidation in the adipose tissue, and reduced de novo lipogenesis in the liver, leading to reduced visceral fat mass, reduced triglyceride content in liver, and unchanged circulating free fatty acid levels. Importantly, hexarelin treatment improved the whole-body insulin sensitivity but did not alter glucose tolerance, insulin levels, or insulin-like growth factor 1 (IGF-1) levels. The metabolic effects of hexarelin were likely through the direct action of GH, as indicated by the increased expression level of genes involved in GH signaling pathways in visceral adipose tissues and liver. In conclusion, hexarelin treatment stimulated the pulsatile GH secretion and reduced the fat accumulation in visceral depots and liver in obese MC4RKO mice with improved insulin sensitivity without altered levels of insulin or IGF-1. It provides evidence for managing obesity by enhancing pulsatile GH secretion through activation of GHSR in the pituitary gland.

Disrupting the ghrelin-growth hormone axis limits ghrelin's orexigenic but not glucoregulatory actions

Objective

Acyl-ghrelin regulates eating, body weight, blood glucose, and GH secretion upon binding to its receptor GHSR (growth hormone secretagogue receptor; ghrelin receptor). GHSR is distributed in several brain regions and some peripheral cell-types including pituitary somatotrophs. The objective of the current study was to determine the functional significance of acyl-ghrelin's action on GHSR-expressing somatotrophs in mediating GH secretion and several of acyl-ghrelin's metabolic actions.

Methods

GH-IRES-Cre mice and loxP-flanked (floxed) GHSR mice were newly developed and then crossed to one another to generate mice that lacked GHSR selectively from somatotrophs. Following validation of mice with somatotroph-selective GHSR deletion, metabolic responses of these mice and control littermates were assessed following both acute and chronic acyl-ghrelin administration, a 24-h fast, and a prolonged 60% chronic caloric restriction protocol modeling starvation.

Results

In mice with somatotroph-selective GHSR deletion, a single peripheral injection of acyl-ghrelin failed to induce GH secretion or increase food intake, unlike wild-type and other littermate control groups. However, the usual acute blood glucose increase in response to the acyl-ghrelin bolus was preserved. Similarly, chronic s.c. acyl-ghrelin administration to mice with somatotroph-selective GHSR deletion failed to increase plasma GH, food intake, or body weight. Physiologically elevating plasma acyl-ghrelin via a 24-h fast also failed to raise plasma GH and resulted in a limited hyperphagic response upon food reintroduction in mice with somatotroph-selective GHSR deletion, although those mice nonetheless did not exhibit an exaggerated reduction in blood glucose. Physiologically elevating plasma acyl-ghrelin via a 15-day caloric restriction protocol which provided only 40% of usual daily calories failed to raise plasma GH in mice with somatotroph-selective GHSR deletion, although those mice did not exhibit life-threatening hypoglycemia.

Conclusions

These results reveal that direct engagement of GHSR-expressing somatotrophs is required for a peripheral ghrelin bolus to acutely stimulate GH secretion and the actions of chronic acyl-ghrelin delivery and physiological plasma acyl-ghrelin elevations to increase plasma GH. These results also suggest that actions of acyl-ghrelin to increase food intake and body weight are reliant on direct activation of GHSRs expressed on somatotrophs. Furthermore, these results suggest that the glucoregulatory actions of acyl-ghrelin – in particular, its actions to raise blood glucose when acutely administered, prevent small blood glucose drops following a 24-h fast, and avert life-threatening hypoglycemia during an acute-on-chronic caloric restriction protocol – do not depend on GHSR expression by somatotrophs.

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Mice with pituitary somatotroph-selective GHSR deletion were generated.

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Somatotroph-expressed GHSRs mediate GH secretion and food intake after acute ghrelin.

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Body weight effects of chronic ghrelin infusion require somatotroph-expressed GHSRs.

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Somatotroph-expressed GHSRs enable GH to increase upon chronic caloric restriction.

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Mice lacking somatotroph GHSRs maintain euglycemia upon chronic caloric restriction.

β Cell GHS-R Regulates Insulin Secretion and Sensitivity

Growth hormone secretagogue receptor (GHS-R) is widely known to regulate food intake and adiposity, but its role in glucose homeostasis is unclear. In this study, we investigated the expression of GHS-R in mouse pancreatic islets and its role in glycemic regulation. We used Ghsr-IRES-tauGFP mice, with Green Fluorescent Protein (GFP) as a surrogate for GHS-R, to demonstrate the GFP co-localization with insulin and glucagon expression in pancreatic islets, confirming GHS-R expression in β and α cells. We then generated β-cell-specific GHSR-deleted mice with MIP-Cre/ERT and validated that GHS-R suppression was restricted to the pancreatic islets. MIP-Cre/ERT;Ghsr^f/f mice showed normal energy homeostasis with similar body weight, body composition, and indirect calorimetry profile. Interestingly, MIP-Cre/ERT;Ghsr^f/f mice exhibited an impressive phenotype in glucose homeostasis. Compared to controls, MIP-Cre/ERT;Ghsr^f/f mice showed lower fasting blood glucose and insulin; reduced first-phase insulin secretion during a glucose tolerance test (GTT) and glucose-stimulated insulin secretion (GSIS) test in vivo. The isolated pancreatic islets of MIP-Cre/ERT;Ghsr^f/f mice also showed reduced insulin secretion during GSIS ex vivo. Further, MIP-Cre/ERT;Ghsr^f/f mice exhibited improved insulin sensitivity during insulin tolerance tests (ITT). Overall, our results confirmed GHS-R expression in pancreatic β and α cells; GHS-R cell-autonomously regulated GSIS and modulated systemic insulin sensitivity. In conclusion, β cell GHS-R was an important regulator of glucose homeostasis, and GHS-R antagonists may have therapeutic potential for Type 2 Diabetes.

Acyl-Ghrelin Influences Pancreatic β-Cell Function by Interference with KATP Channels

The aim for this study was to elucidate how the hypothalamic hunger-inducing hormone acyl-ghrelin (AG), which is also produced in the pancreas, affects β-cell function, with particular attention to the role of ATP-sensitive K⁺ (K_ATP) channels and the exact site of action of the hormone. AG hyperpolarized the membrane potential and decreased cytoplasmic calcium concentration [Ca²⁺]_c and glucose-stimulated insulin secretion (GSIS). These effects were abolished in β-cells from SUR1-knockout (KO) mice. AG increased K_ATP current but only in a configuration with intact metabolism. Unacylated ghrelin counteracted the effects of AG. The influence of AG on membrane potential and GSIS could only be averted in the combined presence of a ghrelin receptor (GHSR1a) antagonist and an inverse agonist. The inhibition of GSIS by AG could be prevented by dibutyryl cyclic-cAMP or 3-isobutyl-1-methylxanthine and the somatostatin (SST) receptor 2-5 antagonist H6056. These data indicate that AG indirectly opens K_ATP channels probably by interference with the cAMP/cAMP-dependent protein kinase pathway, resulting in a decrease of [Ca²⁺]_c and GSIS. The experiments with SUR1-KO β-cells point to a direct effect of AG on β-cells and not, as earlier suggested, to an exclusive effect by AG-induced SST release from δ-cells. Nevertheless, SST receptors may be involved in the effect of AG, possibly by heteromerization of AG and SST receptors.

Identification and Metabolic Profiling of a Novel Human Gut-derived LEAP2 Fragment

CONTEXT

The mechanisms underlying Roux-en-Y gastric bypass (RYGB) surgery-induced weight loss and the immediate postoperative beneficial metabolic effects associated with the operation remain uncertain. Enteroendocrine cell (EEC) secretory function has been proposed as a key factor in the marked metabolic benefits from RYGB surgery.

OBJECTIVE

To identify novel gut-derived peptides with therapeutic potential in obesity and/or diabetes by profiling EEC-specific molecular changes in obese patients following RYGB-induced weight loss.

SUBJECTS AND METHODS

Genome-wide expression analysis was performed in isolated human small intestinal EECs obtained from 20 gut-biopsied obese subjects before and after RYGB. Targets of interest were profiled for preclinical and clinical metabolic effects.

RESULTS

Roux-en-Y gastric bypass consistently increased expression levels of the inverse ghrelin receptor agonist, liver-expressed antimicrobial peptide 2 (LEAP2). A secreted endogenous LEAP2 fragment (LEAP238-47) demonstrated robust insulinotropic properties, stimulating insulin release in human pancreatic islets comparable to the gut hormone glucagon-like peptide-1. LEAP238-47 showed reciprocal effects on growth hormone secretagogue receptor (GHSR) activity, suggesting that the insulinotropic action of the peptide may be directly linked to attenuation of tonic GHSR activity. The fragment was infused in healthy human individuals (n = 10), but no glucoregulatory effect was observed in the chosen dose as compared to placebo.

CONCLUSIONS

Small intestinal LEAP2 expression was upregulated after RYGB. The corresponding circulating LEAP238-47 fragment demonstrated strong insulinotropic action in vitro but failed to elicit glucoregulatory effects in healthy human subjects.

Thermosensation involving thermo-TRPs

The transient receptor potential (TRP) channels constitute a superfamily of large ion channels that are activated by a wide range of chemical, mechanical and thermal stimuli. TRP channels with temperature sensitivity are called thermo-TRPs. They are involved in diverse physiological functions through their detection of external environmental temperature and internal body temperature. Each thermo-TRP has its own characteristic temperature threshold for activation. As a group, they cover temperatures ranging from cold to nociceptive high temperatures. Recently, many studies have identified the functions of thermo-TRPs residing in deep organs where they are exposed to body temperature. Importantly, temperature thresholds of thermo-TRPs can be regulated by physiological factors enabling their function at relatively constant body temperature. Moreover, several thermo-TRPs are reportedly engaged in body temperature regulation. This review will summarize the current understanding of thermo-TRPs, including their roles in thermosensation and functional regulation of physiological responses at body temperature and the regulation of body temperature.

LEAP-2: An Emerging Endogenous Ghrelin Receptor Antagonist in the Pathophysiology of Obesity

Liver-expressed antimicrobial peptide 2 (LEAP-2), originally described as an antimicrobial peptide, has recently been recognized as an endogenous blocker of growth hormone secretagogue receptor 1a (GHS-R1a). GHS-R1a, also known as ghrelin receptor, is a G protein-coupled receptor (GPCR) widely distributed on the hypothalamus and pituitary gland where it exerts its major functions of regulating appetite and growth hormone (GH) secretion. The activity of GHS-R1a is controlled by two counter-regulatory endogenous ligands: Ghrelin (activation) and LEAP-2 (inhibition). Ghrelin activates GHS-R1a on the neuropeptide Y/Agouti-related protein (NPY/AgRP) neurons at the arcuate nucleus (ARC) to promote appetite, and on the pituitary somatotrophs to stimulate GH release. On the flip side, LEAP-2, acts both as an endogenous competitive antagonist of ghrelin and an inverse agonist of constitutive GHS-R1a activity. Such a biological property of LEAP-2 vigorously blocks ghrelin's effects on food intake and hormonal secretion. In circulation, LEAP-2 displays an inverse pattern as to ghrelin; it increases with food intake and obesity (positive energy balance), whereas decreases upon fasting and weight loss (negative energy balance). Thus, the LEAP-2/ghrelin molar ratio fluctuates in response to energy status and modulation of this ratio conversely influences energy intake. Inhibiting ghrelin's activity has shown beneficial effects on obesity in preclinical experiments, which sheds light on LEAP-2's anti-obesity potential. In this review, we will analyze LEAP-2's effects from a metabolic point of view with a focus on metabolic hormones (e.g., ghrelin, GH, and insulin), and discuss LEAP-2's potential as a promising therapeutic target for obesity.

β-Cell-Specific Deletion of HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) Reductase Causes Overt Diabetes due to Reduction of β-Cell Mass and Impaired Insulin Secretion

Inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), statins, which are used to prevent cardiovascular diseases, are associated with a modest increase in the risk of new-onset diabetes. To investigate the role of HMGCR in the development of β-cells and glucose homeostasis, we deleted Hmgcr in a β-cell-specific manner by using the Cre-loxP technique. Mice lacking Hmgcr in β-cells (β-KO) exhibited hypoinsulinemic hyperglycemia as early as postnatal day 9 (P9) due to decreases in both β-cell mass and insulin secretion. Ki67-positive cells were reduced in β-KO mice at P9; thus, β-cell mass reduction was caused by proliferation disorder immediately after birth. The mRNA expression of neurogenin3 (Ngn3), which is transiently expressed in endocrine progenitors of the embryonic pancreas, was maintained despite a striking reduction in the expression of β-cell-associated genes, such as insulin, pancreatic and duodenal homeobox 1 (Pdx1), and MAF BZIP transcription factor A (Mafa) in the islets from β-KO mice. Histological analyses revealed dysmorphic islets with markedly reduced numbers of β-cells, some of which were also positive for glucagon. In conclusion, HMGCR plays critical roles not only in insulin secretion but also in the development of β-cells in mice.

Ghrelin suppresses insulin secretion in human islets and type 2 diabetes patients have diminished islet ghrelin cell number and lower plasma ghrelin levels

It is not known how ghrelin affects insulin secretion in human islets from patients with type 2 diabetes (T2D) or whether islet ghrelin expression or circulating ghrelin levels are altered in T2D. Here we sought out to identify the effect of ghrelin on insulin secretion in human islets and the impact of T2D on circulating ghrelin levels and on islet ghrelin cells. The effect of ghrelin on insulin secretion was assessed in human T2D and non-T2D islets. Ghrelin expression was assessed with RNA-sequencing (n = 191) and immunohistochemistry (n = 21). Plasma ghrelin was measured with ELISA in 40 T2D and 40 non-T2D subjects. Ghrelin exerted a glucose-dependent insulin-suppressing effect in islets from both T2D and non-T2D donors. Compared with non-T2D donors, T2D donors had reduced ghrelin mRNA expression and 75% less islet ghrelin cells, and ghrelin mRNA expression correlated negatively with HbA1c. T2D subjects had 25% lower fasting plasma ghrelin levels than matched controls. Thus, ghrelin has direct insulin-suppressing effects in human islets and T2D patients have lower fasting ghrelin levels, likely as a result of reduced number of islet ghrelin cells. These findings support inhibition of ghrelin signaling as a potential therapeutic avenue for stimulation of insulin secretion in T2D patients.

Molecular Regulations and Functions of the Transient Receptor Potential Channels of the Islets of Langerhans and Insulinoma Cells

Insulin secretion from the β-cells of the islets of Langerhans is triggered mainly by nutrients such as glucose, and incretin hormones such as glucagon-like peptide-1 (GLP-1). The mechanisms of the stimulus-secretion coupling involve the participation of the key enzymes that metabolize the nutrients, and numerous ion channels that mediate the electrical activity. Several members of the transient receptor potential (TRP) channels participate in the processes that mediate the electrical activities and Ca²⁺ oscillations in these cells. Human β-cells express TRPC1, TRPM2, TRPM3, TRPM4, TRPM7, TRPP1, TRPML1, and TRPML3 channels. Some of these channels have been reported to mediate background depolarizing currents, store-operated Ca²⁺ entry (SOCE), electrical activity, Ca²⁺ oscillations, gene transcription, cell-death, and insulin secretion in response to stimulation by glucose and GLP1. Different channels of the TRP family are regulated by one or more of the following mechanisms: activation of G protein-coupled receptors, the filling state of the endoplasmic reticulum Ca²⁺ store, heat, oxidative stress, or some second messengers. This review briefly compiles our current knowledge about the molecular mechanisms of regulations, and functions of the TRP channels in the β-cells, the α-cells, and some insulinoma cell lines.

Combined Loss of Ghrelin Receptor and Cannabinoid CB1 Receptor in Mice Decreases Survival but does not Additively Reduce Body Weight or Eating

Ghrelin administration increases food intake, body weight (BW), adiposity, and blood glucose. In contrast, although mouse models lacking ghrelin or its receptor (Growth Hormone Secretagogue Receptor (GHSR)) exhibit life-threatening hypoglycemia in starvation-like states, they do not exhibit appreciable reductions in food intake, BW, adiposity, blood glucose, or survival when food availability is unrestricted. This suggests the existence of a parallel neuromodulatory system that can compensate for disruptions in the ghrelin system in certain settings. Here, we hypothesized that the cannabinoid CB1 receptor (CB1R) may encode this putative redundancy, and as such, that genetic deletion of both GHSR and CB1R would exaggerate the metabolic deficits associated with deletion of GHSR alone. To test this hypothesis, we assessed food intake, BW, blood glucose, survival, and plasma acyl-ghrelin in ad libitum-fed male wild-type mice and those that genetically lack GHSR (GHSR-nulls), CB1R (CB1R-nulls), or both GHSR and CB1R (double-nulls). BW, fat mass, and lean mass were similar in GHSR-nulls and wild-types, lower in CB1R-nulls, but not further reduced in double-nulls. Food intake, plasma acyl-ghrelin, and blood glucose were similar among genotypes. Deletion of either GHSR or CB1R alone did not have a statistically-significant effect on survival, but double-nulls demonstrated a statistical trend towards decreased survival (p = 0.07). We conclude that CB1R is not responsible for the normal BW, adiposity, food intake, and blood glucose observed in GHSR-null mice in the setting of unrestricted food availability. Nor is CB1R required for plasma acyl-ghrelin secretion in that setting. However, GHSR may be protective against exaggerated mortality associated with CB1R deletion.

Ghrelin in rat pancreatic islets decreases islet blood flow

The peptide ghrelin is mainly produced in some of the epithelial cells in the stomach, but also, during starvation, by the ε-cells in the endocrine pancreas. Ghrelin, as an endogenous ligand for the growth hormone secretagogue receptor (GHS-R1α), exerts a variety of metabolic functions including stimulation of appetite and weight gain. Its complete role is not yet fully understood, including whether it has any vascular functions. The present study evaluated if ghrelin affects pancreatic and islet blood flow. Ghrelin and the GHS-R1α receptor antagonist GHRP-6 were injected intravenously in rats followed by blood flow measurements using a microsphere technique. Ghrelin decreased, while GHRP-6 in fasted, but not fed, rats selectively increased islet blood flow fourfold. GHS-R1α was identified not only on glucagon-producing cells but also seemed to be present in the islet arterioles. GHRP-6 in fasted rats, only, also improved the peak insulin response to glucose in vivo, thereby substantially blunting the hyperglycemia. GHRP-6 doubled glucose-stimulated insulin release in vitro of both islets obtained from fed and fasted rats. Our results indicate a novel role for endogenous ghrelin acting directly or indirectly as a local vasoconstrictor in the islets during fasting, thereby restricting the insulin response to hyperglycemia. This is to the best of our knowledge the first report that shows this physiological mechanism to restrict insulin delivery from the islets by acting on the vasculature; a mode of action that can be envisaged to complement the previously well-described mechanisms of ghrelin acting directly on the islet endocrine cells.

Redox TRPs in diabetes and diabetic complications: Mechanisms and pharmacological modulation

Transient receptor potential (TRP) channels have shown to be involved in a wide variety of physiological functions and pathophysiological conditions. Modulation of TRP channels reported to play a major role in number of disorders starting from central nervous system related disorders to cardiovascular, inflammatory, cancer, gastrointestinal and metabolic diseases. Recently, a subset of TRP ion channels called redox TRPs gained importance on account of their ability to sense the cellular redox environment and respond accordingly to such redox stimuli. Diabetes, the silent epidemic of the world is increasing at an alarming rate in spite of novel therapeutic interventions. Moreover, diabetes and its associated complications are reported to arise due to a change in oxidative status of cell induced by hyperglycemia. Such a change in cellular oxidative status can modulate the activities of various redox TRP channels (TRPA1, TRPC5, TRPMs and TRPV1). Targeting redox TRPs have potential in diabetes and diabetic complications like neuropathy, cardiomyopathy, retinopathy, cystopathy, and encephalopathy. Thus in this review, we have discussed the activities of different redox sensing TRPs in diabetes and diabetic complications and how they can be modulated pharmacologically, so as to consider them a potential novel therapeutic target in treating diabetes and its comorbidity.

Ghrelin's Relationship to Blood Glucose

Much effort has been directed at studying the orexigenic actions of administered ghrelin and the potential effects of the endogenous ghrelin system on food intake, food reward, body weight, adiposity, and energy expenditure. Although endogenous ghrelin's actions on some of these processes remain ambiguous, its glucoregulatory actions have emerged as well-recognized features during extreme metabolic conditions. The blood glucose-raising actions of ghrelin are beneficial during starvation-like conditions, defending against life-threatening falls in blood glucose, but they are seemingly detrimental in obese states and in certain monogenic forms of diabetes, contributing to hyperglycemia. Also of interest, blood glucose negatively regulates ghrelin secretion. This article reviews the literature suggesting the existence of a blood glucose-ghrelin axis and highlights the factors that mediate the glucoregulatory actions of ghrelin, especially during metabolic extremes such as starvation and diabetes.

Capromorelin: a ghrelin receptor agonist and novel therapy for stimulation of appetite in dogs

Ghrelin is a hormone, secreted from cells in the stomach, which is important in the regulation of appetite and food intake in mammals. It exerts its action by binding to a specific G-protein-coupled receptor, the growth hormone secretagogue receptor 1a (GHS-R1a) which is found in areas of the brain associated with the regulation of food intake. Ghrelin causes a release of growth hormone (GH) through binding to GHS-R1a in the hypothalamus and pituitary gland. A class of compounds known as growth hormone secretagogues, or ghrelin receptor agonists, were developed for therapeutic use in humans for the stimulation of GH in the frail elderly, and have subsequently been studied for their effects on increasing appetite and food intake, increasing body weight, building lean muscle mass, and treating cachexia. Subsequent research has shown that ghrelin has anti-inflammatory and immunomodulatory effects. This article reviews the basic physiology of ghrelin and the ghrelin receptor agonists, including the available evidence of these effects in vitro and in vivo in rodent models, humans, dogs and cats. One of these compounds, capromorelin, has been FDA-approved for the stimulation of appetite in dogs (ENTYCE ®). The data available on the safety and effectiveness of capromorelin is reviewed, along with a discussion of the potential clinical applications for ghrelin receptor agonists in both human and veterinary medicine.

Ghrelin Receptor Is Required for the Effect of Nesfatin-1 on Glucose Metabolism

Studies of nesfatin-1 in glucose metabolism have become a topic of interest recently, however, the specific receptor for nesfatin-1 has not yet been identified. Some studies hinted at a connection between nesfatin-1 and the ghrelin receptor, growth hormone secretagogue receptor. Therefore, we aimed to study the role of GHSR in the glycemic effects of nesfatin-1 as well as its downstream pathways. We employed C57/BL6 mice (wild type and GHSR knockout mice) eating a normal chow diet and a high fat diet in this study, and the experimental technique included western blot, real-time PCR, immunofluorescence and ELISA. We found that in mice fed a normal chow diet (NCD), nesfatin-1 improved glucose tolerance, up-regulated AKT kinase (AKT) mRNA levels and phosphorylation and GLUT4 membrane translocation in skeletal muscle. These effects were blocked by co-injection of GHSR antagonist [D-Lys3]-GHRP-6 and were attenuated in GHSR knockout mice. In mice fed high-fat diet (HFD), nesfatin-1 not only exerted the effects observed in NCD mice, but also suppressed appetite and raised AKT levels in liver tissues that also required GHSR. Peripheral nesfatin-1 suppressed c-fos expression of GHSR immunoreactive neurons induced by fasting in hypothalamic nuclei, indicating that nesfatin-1 inhibited the activation of central GHSR. We concluded that the effects of nesfatin-1 on food intake and glucose metabolism were GHSR-dependent, and that the glycemic effect was associated with AKT and GLUT4. This study should stimulate further exploration of the nesfatin-1 receptor.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Involvement of thermosensitive TRP channels in energy metabolism

To date, 11 thermosensitive transient receptor potential (thermo-TRP) channels have been identified. Recent studies have characterized the mechanism of thermosensing by thermo-TRPs and the physiological role of thermo-TRPs in energy metabolism. In this review, we highlight the role of various thermo-TRPs in energy metabolism and hormone secretion. In the pancreas, TRPM2 and other TRPs regulate insulin secretion. TRPV2 expressed in brown adipocytes contributes to differentiation and/or thermogenesis. Sensory nerves that express TRPV1 promote increased energy expenditure by activating sympathetic nerves and adrenaline secretion. Here, we first show that capsaicin-induced adrenaline secretion is completely impaired in TRPV1 knockout mice. The thermogenic effects of TRPV1 agonists are attributable to brown adipose tissue (BAT) activation in mice and humans. Moreover, TRPA1- and TRPM8-expressing sensory nerves also contribute to potentiation of BAT thermogenesis and energy expenditure in mice. Together, thermo-TRPs are promising targets for combating obesity and metabolic disorders.

Ghrelin protects against nucleus pulposus degeneration through inhibition of NF-κB signaling pathway and activation of Akt signaling pathway

The objective of the present study was to examine the potential role of ghrelin in degeneration of nucleus pulposus (NP). Lower expression levels of ghrelin were found in human NP cells stimulated with interleukin-1β (IL-1β). Moreover, exogenous ghrelin suppressed IL-1β induced degeneration and inflammation associated biomarkers in human NP cells, including matrix metalloproteinase-13, a disintegrin and metalloproteinase with thrombospondin motifs-5, tumor necrosis factor-α and iNOS, which was possibly mediated by antagonization of NF-κB signaling. Moreover, ghrelin enhanced production of critical extracellular matrix of NP cells, including collagen 2, aggrecan, and Sox-9 in NP cells. Ghrelin also promoted NP tissue regeneration in a rabbit IVD degeneration model, which seems to be associated with growth hormone secretagogue receptor. Additionally, the protective role of ghrelin in anabolism potentially relies on activation of Akt signaling pathway. Taken together, ghrelin may represent a molecular target for prevention and treatment of intervertebral disc degeneration.

Endogenous α2A-Adrenoceptor-Operated Sympathoadrenergic Tones Attenuate Insulin Secretion via cAMP/TRPM2 Signaling

In pancreatic β-cells, pharmacological concentrations of catecholamines, including adrenaline, have been used to inhibit insulin release and explore the multiple mechanisms involved. However, the significance of these signaling pathways for physiological adrenergic functions in β-cells is largely unknown. In the process of glucose-induced insulin secretion, opening of background current through nonselective cation channels (NSCCs) might facilitate membrane depolarization by closure of the ATP-sensitive K⁺ channels. Here, we examined whether physiological insulinostatic adrenaline action is mediated via the transient receptor potential melastatin 2 (TRPM2) channel, a type of NSCC, in β-cells. Results showed that physiological concentrations of adrenaline strongly suppressed glucose-induced and incretin-potentiated cAMP production and insulin secretion and inhibited NSCCs current and membrane excitability via the α2A-adrenoceptor in wild-type mice; however, insulin secretion was not attenuated in TRPM2-knockout (KO) mice. Administration of yohimbine, an α2-adrenoceptor antagonist, failed to affect glucose tolerance in TRPM2-KO mice, in contrast to an improved glucose tolerance in wild-type mice receiving the antagonist. The current study demonstrated that a physiological concentration of adrenaline attenuates insulin release via coupling of α2A-adrenoceptor to cAMP/TRPM2 signaling, thereby providing a potential therapeutic tool to treat patients with type 2 diabetes.

Ghrelin Is a Regulator of Glucagon-Like Peptide 1 Secretion and Transcription in Mice

The gut hormones ghrelin, glucagon-like peptide 1 (GLP-1), and glucose-dependent insulinotropic peptide (GIP) have been intensively studied for their role in metabolism. It is, however, not well known whether the hormones interplay and regulate the secretion of each other. In this study, we studied the effect of ghrelin on GLP-1, GIP, and insulin secretion during an oral glucose tolerance test (OGTT) in mice. Intravenous administration of ghrelin caused increased GLP-1 secretion during the OGTT. On the other hand, ghrelin had no effect on circulating levels of glucose, insulin, and GIP. Furthermore, ghrelin treatment reduced proglucagon mRNA expression in GLUTag cells. The effect of ghrelin on GLP-1 secretion and proglucagon transcription was reinforced by the presence of GHS-R1a in human and mouse ileal L-cells, as well as in GLUTag cells. In summary, ghrelin is a regulator of GLP-1 secretion and transcription, and interfering with GHS-R1a signaling may be a way forward to enhance endogenous GLP-1 secretion in subjects with type 2 diabetes.

Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB

The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.

Glucose and GTP-binding protein-coupled receptor cooperatively regulate transient receptor potential-channels to stimulate insulin secretion [Review]

In pancreatic β-cells, glucose-induced closure of the ATP-sensitive K⁺ (KATP) channel is an initial process triggering glucose-stimulated insulin secretion (GSIS). This KATP-channel dependent pathway has been believed to be a central mechanism for GSIS. However, since the resting membrane potential of cells is determined by the balance of the net result of current amplitudes in outward and inward directions, it must be taken into consideration that not only KATP channel inhibition but also inward current via the basal opening of non-selective cation channels (NSCCs) plays a crucial role in membrane potential regulation. The basal activity of NSCCs is essential to effectively evoke depolarization in concert with KATP channel closure that is dependent on glucose metabolism. The present study summarizes recent findings regarding the roles of NSCCs in GSIS and GTP-binding protein coupled receptor-(GPCR) operated potentiation of GSIS.

Transcriptomic profiling of pancreatic alpha, beta and delta cell populations identifies delta cells as a principal target for ghrelin in mouse islets

AIMS/HYPOTHESIS

Intra-islet and gut-islet crosstalk are critical in orchestrating basal and postprandial metabolism. The aim of this study was to identify regulatory proteins and receptors underlying somatostatin secretion though the use of transcriptomic comparison of purified murine alpha, beta and delta cells.

METHODS

Sst-Cre mice crossed with fluorescent reporters were used to identify delta cells, while Glu-Venus (with Venus reported under the control of the Glu [also known as Gcg] promoter) mice were used to identify alpha and beta cells. Alpha, beta and delta cells were purified using flow cytometry and analysed by RNA sequencing. The role of the ghrelin receptor was validated by imaging delta cell calcium concentrations using islets with delta cell restricted expression of the calcium reporter GCaMP3, and in perfused mouse pancreases.

RESULTS

A database was constructed of all genes expressed in alpha, beta and delta cells. The gene encoding the ghrelin receptor, Ghsr, was highlighted as being highly expressed and enriched in delta cells. Activation of the ghrelin receptor raised cytosolic calcium levels in primary pancreatic delta cells and enhanced somatostatin secretion in perfused pancreases, correlating with a decrease in insulin and glucagon release. The inhibition of insulin secretion by ghrelin was prevented by somatostatin receptor antagonism.

CONCLUSIONS/INTERPRETATION

Our transcriptomic database of genes expressed in the principal islet cell populations will facilitate rational drug design to target specific islet cell types. The present study indicates that ghrelin acts specifically on delta cells within pancreatic islets to elicit somatostatin secretion, which in turn inhibits insulin and glucagon release. This highlights a potential role for ghrelin in the control of glucose metabolism.

β1-Adrenergic receptor deficiency in ghrelin-expressing cells causes hypoglycemia in susceptible individuals

Ghrelin is an orexigenic gastric peptide hormone secreted when caloric intake is limited. Ghrelin also regulates blood glucose, as emphasized by the hypoglycemia that is induced by caloric restriction in mouse models of deficient ghrelin signaling. Here, we hypothesized that activation of β1-adrenergic receptors (β1ARs) localized to ghrelin cells is required for caloric restriction-associated ghrelin release and the ensuing protective glucoregulatory response. In mice lacking the β1AR specifically in ghrelin-expressing cells, ghrelin secretion was markedly blunted, resulting in profound hypoglycemia and prevalent mortality upon severe caloric restriction. Replacement of ghrelin blocked the effects of caloric restriction in β1AR-deficient mice. We also determined that treating calorically restricted juvenile WT mice with beta blockers led to reduced plasma ghrelin and hypoglycemia, the latter of which is similar to the life-threatening, fasting-induced hypoglycemia observed in infants treated with beta blockers. These findings highlight the critical functions of ghrelin in preventing hypoglycemia and promoting survival during severe caloric restriction and the requirement for ghrelin cell-expressed β1ARs in these processes. Moreover, these results indicate a potential role for ghrelin in mediating beta blocker-associated hypoglycemia in susceptible individuals, such as young children.

Lentivirus-mediated shRNA interference of ghrelin receptor blocks proliferation in the colorectal cancer cells

Ghrelin, an orexigenic peptide, acts via the growth hormone secretagogue receptor (GHSR) to stimulate the release of growth hormone. Moreover, it has a range of biological actions, including the stimulation of food intake, modulation of insulin signaling and cardiovascular effects. Recently, it has been demonstrated that ghrelin has a proliferative and antiapoptotic effects in cancers, suggesting a potential role in promoting tumor growth. However, it remains unknown whether GHSR contributes to colorectal cancer proliferation. In this study, the therapeutic effect of lentivirus‐mediated short hairpin RNA (shRNA) targeting ghrelin receptor 1a (GHSR1a) was analyzed in colorectal cancer cell line SW480 both in vitro and in vivo. Our study demonstrated that ghrelin and GHSR1a are significantly upregulated in cancerous colorectal tissue samples and cell lines. In vitro, human colorectal cancer cell line SW480 with downregulation of GHSR1a by shRNA showed significant inhibition of cell viability compared with blank control (BC) or scrambled control (SC) regardless of the application of exogenous ghrelin. Furthermore, GHSR1a silencing by target specific shRNA was shown capable of increasing PTEN, inhibiting AKT phosphorylation and promoting the release of p53 in SW480 cells. In addition, the effects of GHSR1a knockdown were further explored in vivo using colorectal tumor xenograft mouse model. The tumor weights were decreased markedly in GHSR1α knockdown SW480 mouse xenograft tumors compared with blank control or negative control tumors. Our results suggested that the expression of GHSR1a is significantly correlated with the growth of colorectal cancer cells, and the GHSR1a knockdown approach may be a potential therapy for the treatment of colorectal cancer.

Comprehensive alpha, beta and delta cell transcriptomes reveal that ghrelin selectively activates delta cells and promotes somatostatin release from pancreatic islets

OBJECTIVE

Complex local crosstalk amongst endocrine cells within the islet ensures tight coordination of their endocrine output. This is illustrated by the recent demonstration that the negative feedback control by delta cells within pancreatic islets determines the homeostatic set-point for plasma glucose during mouse postnatal development. However, the close association of islet endocrine cells that facilitates paracrine crosstalk also complicates the distinction between effects mediated directly on beta cells from indirect effects mediated via local intermediates, such as somatostatin from delta cells.

METHODS

To resolve this problem, we generated reporter mice that allow collection of pure pancreatic delta cells along with alpha and beta cells from the same islets and generated comprehensive transcriptomes for each islet endocrine cell type. These transcriptomes afford an unparalleled view of the receptors expressed by delta, alpha and beta cells, and allow the prediction of which signal targets which endocrine cell type with great accuracy.

RESULTS

From these transcriptomes, we discovered that the ghrelin receptor is expressed exclusively by delta cells within the islet, which was confirmed by fluorescent in situ hybridization and qPCR. Indeed, ghrelin increases intracellular calcium in delta cells in intact mouse islets, measured by GCaMP6 and robustly potentiates glucose-stimulated somatostatin secretion on mouse and human islets in both static and perfusion assays. In contrast, des-acyl-ghrelin at the same dose had no effect on somatostatin secretion and did not block the actions of ghrelin.

CONCLUSIONS

These results offer a straightforward explanation for the well-known insulinostatic actions of ghrelin. Rather than engaging beta cells directly, ghrelin engages delta cells to promote local inhibitory feedback that attenuates insulin release. These findings illustrate the power of our approach to resolve some of the long-standing conundrums with regard to the rich feedback that occurs within the islet that is integral to islet physiology and therefore highly relevant to diabetes.