Pancreatic GLP-1 receptor activation is sufficient for incretin control of glucose metabolism in mice

The signalling association of glucagon-like peptide-1 and its receptors in the gastrointestinal tract and GPR40 and insulin receptor in the pancreas of sheep

The present study was aimed at gaining insight into the signalling relationship between glucagon-like peptide-1 (GLP-1) and its receptor (GLP-1R) in the regulation of glucose metabolism. Further, to assess the role of G-protein-coupled receptor 40 (GPR40) and insulin receptor (INSR) in the pancreas of sheep that were supplemented with calcium salts of long-chain fatty acids (CSFAs). An experiment was carried out over a period of 60 days with eighteen sheep, and they were fed with a standard basal diet. The sheep were divided into three groups: CSFA0 (without CSFAs), while CSFA3 and CSFA5 were supplemented with 3 % and 5 % of CSFAs, respectively. Plasma concentrations of GLP-1, insulin, glucagon, and glucose were assessed every two weeks. At the end of the experiment, sheep were slaughtered, and samples of gastrointestinal tract (GIT) epithelial tissues and pancreas were collected to assess the relative expression of mRNA of GPR40, GLP-1R, and INSR. Postprandial GLP-1 and insulin were increased by 3.7-4.1 and 1.45-1.5 times, respectively, in the CSFAs-supplemented groups compared to CSFA0. Post-feeding, glucagon and glucose levels decreased in CSFA3 and CSFA5 compared to CSFA0. The results indicated that the supplementation of LCFAs increased the expression of GLP-1R in the GIT and pancreas, as well as the mRNA of GPR40 and INSR in the pancreas. Chemosensing of LCFAs by GPR40 in the pancreas triggers signalling transduction, and enhanced GLP-1 and GLP-1R resulted in moderately increased insulin secretion and reduced glucagon levels. These combined effects, along with the glucose-lowering effect of GLP-1, effectively lowered glucose levels in normoglycemic sheep.

Impaired brain glucose metabolism in glucagon-like peptide-1 receptor knockout mice

BACKGROUND

Quantitative mapping of the brain's metabolism is a critical tool in studying and diagnosing many conditions, from obesity to neurodegenerative diseases. In particular, noninvasive approaches are urgently required. Recently, there have been promising drug development approaches for the treatment of disorders related to glucose metabolism in the brain and, therefore, against obesity-associated diseases. One of the most important drug targets to emerge has been the Glucagon-like peptide-1 (GLP-1) and its receptor (GLP-1R). GLP and GLP-1R play an important role in regulating blood sugar and maintaining energy homeostasis. However, the macroscopic effects on brain metabolism and function due to the presence of GLP-1R are unclear.

METHODS

To explore the physiological role of GLP-1R in mouse brain glucose metabolism, and its relationship to brain function, we used three methods. We used deuterium magnetic resonance spectroscopy (DMRS) to provide quantitative information about metabolic flux, fluorodeoxyglucose positron emission tomography (FDG-PET) to measure brain glucose metabolism, and resting state-functional MRI (rs-fMRI) to measure brain functional connectivity. We used these methods in both mice with complete GLP-1R knockout (GLP-1R KO) and wild-type C57BL/6N (WT) mice.

RESULTS

The metabolic rate of GLP-1R KO mice was significantly slower than that of WT mice (p = 0.0345, WT mice 0.02335 ± 0.057 mM/min, GLP-1R KO mice 0.01998 ± 0.07 mM/min). Quantification of the mean [18F]FDG signal in the whole brain also showed significantly reduced glucose uptake in GLP-1R KO mice versus control mice (p = 0.0314). Observing rs-fMRI, the functional brain connectivity in GLP-1R KO mice was significantly lower than that in the WT group (p = 0.0032 for gFCD, p = 0.0002 for whole-brain correlation, p < 0.0001 for ALFF).

CONCLUSIONS

GLP-1R KO mice exhibit impaired brain glucose metabolism to high doses of exogenous glucose, and they also have reduced functional connectivity. This suggests that the GLP-1R KO mouse model may serve as a model for correlated metabolic and functional connectivity loss.

High frequency of germline recombination in Nestin-Cre transgenic mice crossed with Glucagon-like peptide 1 receptor floxed mice

The Cre-loxP strategy for tissue-specific gene inactivation has become a widely employed tool in several research studies. Conversely, inadequate breeding and genotyping without considering the potential for non-specific Cre-recombinase expression may lead to misinterpretations of results. Nestin-Cre transgenic mice, widely used for the selective deletion of genes in neurons, have been observed to have an incidence of Cre-line germline recombination. In this study, we attempted to generate neuron-specific Glucagon-like peptide 1 receptor (Glp1r) knock-out mice by crossing mice harboring the Nestin-Cre transgene with mice harboring the Glp1r gene modified with loxP insertion, in order to elucidate the role of Glp1r signaling in the nervous system. Surprisingly, during this breeding process, we discovered that the null allele emerged in the offspring irrespective of the presence or absence of the Nestin-Cre transgene, with a high probability of occurrence (93.6%). To elucidate the cause of this null allele, we conducted breeding experiments between mice carrying the heterozygous Glp1r null allele but lacking the Nestin-Cre transgene. We confirmed that the null allele was inherited by the offspring independently of the Nestin-Cre transgene. Furthermore, we assessed the gene expression, protein expression, and phenotype of mice carrying the homozygous Glp1r null allele generated from the aforementioned breeding, thereby confirming that the null allele indeed caused a global knock-out of Glp1r. These findings suggest that the null allele in the NestinCre-Glp1r floxed breeding arose due to germline recombination. Moreover, we demonstrated the possibility that germline recombination may occur not only during the spermatogenesis at testis but also during epididymal sperm maturation. The striking frequency of germline recombination in the Nestin-Cre driver underscores the necessity for caution when implementing precise breeding strategies and employing suitable genotyping methods.

Endogenous glucagon-like peptide (GLP)-1 as alternative for GLP-1 receptor agonists: Could this work and how?

In recent years, we have witnessed the many beneficial effects of glucagon-like peptide (GLP)-1 receptor agonists, including the reduction in cardiovascular risk in patients with type 2 diabetes, and the reduction of body weight in those with obesity. Increasing evidence suggests that these agents differ considerably from endogenous GLP-1 when it comes to their routes of action, although their clinical effects appear to be the same. Given the limitations of the GLP-1 receptor agonists, could it be useful to develop agents which stimulate GLP-1 release? Here we will discuss the differences and similarities between GLP-1 receptor agonists and endogenous GLP-1, and will detail how endogenous GLP-1-when stimulated appropriately-could have clinically relevant effects.

RISING STARS: Endocrine regulation of metabolic homeostasis via the intestine and gut microbiome

The gastrointestinal system is now considered the largest endocrine organ, highlighting the importance of gut-derived peptides and metabolites in metabolic homeostasis. Gut peptides are secreted from intestinal enteroendocrine cells in response to nutrients, microbial metabolites, and neural and hormonal factors, and they regulate systemic metabolism via multiple mechanisms. While extensive research is focused on the neuroendocrine effects of gut peptides, evidence suggests that several of these hormones act as endocrine signaling molecules with direct effects on the target organ, especially in a therapeutic setting. Additionally, the gut microbiota metabolizes ingested nutrients and fiber to produce compounds that impact host metabolism indirectly, through gut peptide secretion, and directly, acting as endocrine factors. This review will provide an overview of the role of endogenous gut peptides in metabolic homeostasis and disease, as well as the potential endocrine impact of microbial metabolites on host metabolic tissue function.

Efficacy and safety of subcutaneous semaglutide in adults with overweight or obese: a subgroup meta-analysis of randomized controlled trials

Introduction

Semaglutide shows significant performance on weight reduction in several clinical trials. However, it is not clear what kind of administration frequency or dosage will achieve better effects. This study aims to explore the different therapeutic effect of semaglutide on weight control under the diverse administration circumstances.

Methods

The PubMed, Embase, Web of Science, Cochrane Library, and the Clinical Trials.gov were searched from inception until 6 June, 2022 to include randomized controlled trials evaluating the Efficacy and safety of subcutaneous semaglutide in overweight or obese adults. Random effects or fixed effects model was conducted based on the heterogeneity among trials. Subgroup analysis was performed to identify the detailed effects under different intervention situations.

Results and discussion

Our study included 13 RCTs involving 5,838 participants with 3,794 ones in semaglutide group and 2,044 in placebo group. Semaglutide was associated with a significant reduction on weight loss related outcomes, including the absolute value of weight loss (WMD -8·97, 95% CI -10·73 to -7·21), percentage of weight loss (WMD -10·00, 95% CI -11·99 to -8·00), body mass index (WMD-3·19, 95% CI -4·02 to -2·37) and waist circumference (WMD -7·21,95% CI -8·87 to -5·56). Subgroup analyses illustrated participants with high weekly dosage, long-term treatment duration and severe baseline BMI (Class II obesity) had a more remarkably decreasing on the main outcomes of weight loss (P for interaction<0·05). Total adverse reactions occurred more frequently in the daily administration group than that in the weekly group (P for interaction =0·01). During the treatment, the incidence rate of hypoglycemia was higher in the group without lifestyle intervention compared with that with lifestyle intervention (P for interaction =0·04). Interpretation Subcutaneous semaglutide had significant benefits on weight loss with reasonable safety in overweight or obese adults. Moreover, additional benefits on cardiometabolic profiles were also seen. We recommended semaglutide treatment to be coupled with lifestyle interventions, and target dose of 2·0 mg or more subcutaneously once weekly. Clinicians can choose suitable treatment schemes based on diverse individual situations.

Systematic review registration

https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=337099, identifier PROSPERO (CRD42022337099).

Exenatide regulates Th17/Treg balance via PI3K/Akt/FoxO1 pathway in db/db mice

BACKGROUND

The T helper 17 (Th17)/T regulatory (Treg) cell imbalance is involved in the course of obesity and type 2 diabetes mellitus (T2DM). In the current study, the exact role of glucagon-like peptide-1 receptor agonist (GLP-1RA) exenatide on regulating the Th17/Treg balance and the underlying molecular mechanisms are investigated in obese diabetic mice model.

METHODS

Metabolic parameters were monitored in db/db mice treated with/without exenatide during 8-week study period. The frequencies of Th17 and Treg cells from peripheral blood and pancreas in db/db mice were assessed. The phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt)/Forkhead box O1 (FoxO1) pathway in Th17 and Treg cells from the spleens of male C57BL/6J mice was detected by western blotting. In addition, the expression of glucagon-like peptide-1 receptor (GLP-1R) in peripheral blood mononuclear cells (PBMCs) of male C57BL/6J mice was analyzed.

RESULTS

Exenatide treatment improved β-cell function and insulitis in addition to glucose, insulin sensitivity and weight. Increased Th17 and decreased Treg cells in peripheral blood were present as diabetes progressed while exenatide corrected this imbalance. Progressive IL-17 + T cell infiltration of pancreatic islets was alleviated by exenatide intervention. In vitro study showed no significant difference in the level of GLP-1R expression in PBMCs between control and palmitate (PA) groups. In addition, PA could promote Th17 but suppress Treg differentiation along with down-regulating the phosphorylation of PI3K/Akt/FoxO1, which was reversed by exenatide intervention. FoxO1 inhibitor AS1842856 could abrogate all these effects of exenatide against lipid stress.

CONCLUSIONS

Exenatide could restore systemic Th17/Treg balance via regulating FoxO1 pathway with the progression of diabetes in db/db mice. The protection of pancreatic β-cell function may be partially mediated by inhibiting Th17 cell infiltration into pancreatic islets, and the resultant alleviation of islet inflammation.

miRNAs: Regulators of immune system in diabetes

Diabetes, one of the most common multifactorial metabolic disorders, is a jeopardizing cause of human health worldwide. MicroRNAs (miRNAs) are a group of small non-coding RNAs that have been contributed to the regulation of gene expression through post-transcriptional mechanisms. The potential role of miRNAs has been studied in the most of biological processes and mechanisms underlying the progression of variety diseases including diabetes. In this review, we focus on the role of miRNAs in regulating pivotal molecular and cellular mechanisms associated with immune system that progress diabetic disorders.

Role of the gut-brain axis in energy and glucose metabolism

The gastrointestinal tract plays a role in the development and treatment of metabolic diseases. During a meal, the gut provides crucial information to the brain regarding incoming nutrients to allow proper maintenance of energy and glucose homeostasis. This gut-brain communication is regulated by various peptides or hormones that are secreted from the gut in response to nutrients; these signaling molecules can enter the circulation and act directly on the brain, or they can act indirectly via paracrine action on local vagal and spinal afferent neurons that innervate the gut. In addition, the enteric nervous system can act as a relay from the gut to the brain. The current review will outline the different gut-brain signaling mechanisms that contribute to metabolic homeostasis, highlighting the recent advances in understanding these complex hormonal and neural pathways. Furthermore, the impact of the gut microbiota on various components of the gut-brain axis that regulates energy and glucose homeostasis will be discussed. A better understanding of the gut-brain axis and its complex relationship with the gut microbiome is crucial for the development of successful pharmacological therapies to combat obesity and diabetes.

Interorgan crosstalk in pancreatic islet function and pathology

Pancreatic β cells secrete insulin in response to glucose, a process that is regulated at multiple levels, including a network of input signals from other organ systems. Impaired islet function contributes to the pathogenesis of type 2 diabetes mellitus (T2DM), and targeting inter-organ communications, such as GLP-1 signalling, to enhance β-cell function has been proven to be a successful therapeutic strategy in the last decade. In this review, we will discuss recent advances in inter-organ communication from the metabolic, immune and neural system to pancreatic islets, their biological implication in normal pancreas endocrine function and their role in the (mal)adaptive responses of islet to nutrition-induced stress.

Identification of a regulatory pathway of L-phenylalanine-induced GLP-1 secretion in the enteroendocrine L cells

Glucagon like peptide-1 (GLP-1) is one of incretin hormone and is secreted when enteroendocrine L cells sense saccharides, amino acids, and fatty acids. Some amino acids have been shown to promote GLP-1 secretion from small intestinal enteroendocrine L cells. However, the molecular mechanisms that L-phenylalanine, a potent trigger of GLP-1 secretion, causes GLP-1 secretion from enteroendocrine L cells has not been elucidated. In this study, we used live-cell imaging to clarify the pathway by which L-phenylalanine activates enteroendocrine L cells. The results showed that L-phenylalanine was sensed by G_q-coupled receptor GPR142 and caused an increase in intracellular Ca²⁺ concentration. In addition, L-phenylalanine was taken up directly into the cell via Na⁺-dependent amino acid transporter, causing membrane depolarization and enhancing GLP-1 secretion. In summary, enteroendocrine L cells may regulate blood glucose levels in the body by detecting L-phenylalanine in the lumen and secreting GLP-1 via GPR142 and Na⁺-dependent amino acid transporters.

E. coli Nissle 1917 modulates host glucose metabolism without directly acting on glucose

Managing postprandial glycemic response, or the increase in blood sugar following a meal, is a crucial component to maintaining healthy blood sugar in patients with diabetes. To test whether oral probiotics can impact postprandial glycemic response, E. coli Nissle 1917 (EcN) was evaluated in an oral glucose tolerance test. Oral gavage of EcN concurrent with a glucose bolus reduced the post-gavage glycemic response in mice. However, there was no difference in glycemic response when comparing EcN to a mutant deficient in glucose metabolism. This suggests that while EcN can alter glycemic response to a glucose bolus, this effect is not mediated by direct uptake of glucose. Of the possible indirect effects EcN could have, gastric emptying rate was highlighted as a likely cause, but EcN had no effect on gastric emptying rate in mice. This leaves many more possible indirect explanations for the interaction between EcN and host glucose metabolism to be explored in future work.

Distinct Identity of GLP-1R, GLP-2R, and GIPR Expressing Cells and Signaling Circuits Within the Gastrointestinal Tract

Enteroendocrine cells directly integrate signals of nutrient content within the gut lumen with distant hormonal responses and nutrient disposal via the production and secretion of peptides, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). Given their direct and indirect control of post-prandial nutrient uptake and demonstrated translational relevance for the treatment of type 2 diabetes, malabsorption and cardiometabolic disease, there is significant interest in the locally engaged circuits mediating these metabolic effects. Although several specific populations of cells in the intestine have been identified to express endocrine receptors, including intraepithelial lymphocytes (IELs) and αβ and γδ T-cells (Glp1r+) and smooth muscle cells (Glp2r+), the definitive cellular localization and co-expression, particularly in regards to the Gipr remain elusive. Here we review the current state of the literature and evaluate the identity of Glp1r, Glp2r, and Gipr expressing cells within preclinical and clinical models. Further elaboration of our understanding of the initiating G-protein coupled receptor (GPCR) circuits engaged locally within the intestine and how they become altered with high-fat diet feeding can offer insight into the dysregulation observed in obesity and diabetes.

Novel Approaches to Restore Pancreatic Beta-Cell Mass and Function

Beta-cell dysfunction and beta-cell death are critical events in the development of type 2 diabetes mellitus (T2DM). Therefore, the goals of modern T2DM management have shifted from merely restoring normoglycemia to maintaining or regenerating beta-cell mass and function. In this review we summarize current and novel approaches to achieve these goals, ranging from lifestyle interventions to N-methyl-D-aspartate receptor (NMDAR) antagonism, and discuss the mechanisms underlying their effects on beta-cell physiology and glycemic control. Notably, timely intervention seems critical, but not always strictly required, to maximize the effect of any approach on beta-cell recovery and disease progression. Conventional antidiabetic medications are not disease-modifying in the sense that the disease does not progress or reoccur while on treatment or thereafter. More invasive approaches, such as bariatric surgery, are highly effective in restoring normoglycemia, but are reserved for a rather small proportion of obese individuals and sometimes associated with serious adverse events. Finally, we recapitulate the broad range of effects mediated by peripheral NMDARs and discuss recent evidence on the potential of NMDAR antagonists to be developed as a novel class of antidiabetic drugs. In the future, a more refined assessment of disease risk or disease subtype might enable more targeted therapies to prevent or treat diabetes.

β-Arrestin-1 is required for adaptive β-cell mass expansion during obesity

Obesity is the key driver of peripheral insulin resistance, one of the key features of type 2 diabetes (T2D). In insulin-resistant individuals, the expansion of beta-cell mass is able to delay or even prevent the onset of overt T2D. Here, we report that beta-arrestin-1 (barr1), an intracellular protein known to regulate signaling through G protein-coupled receptors, is essential for beta-cell replication and function in insulin-resistant mice maintained on an obesogenic diet. Specifically, insulin-resistant beta-cell-specific barr1 knockout mice display marked reductions in beta-cell mass and the rate of beta-cell proliferation, associated with pronounced impairments in glucose homeostasis. Mechanistic studies suggest that the observed metabolic deficits are due to reduced Pdx1 expression levels caused by beta-cell barr1 deficiency. These findings indicate that strategies aimed at enhancing barr1 activity and/or expression in beta-cells may prove useful to restore proper glucose homeostasis in T2D.

Corrination of a GLP-1 Receptor Agonist for Glycemic Control without Emesis

Glucagon-like peptide-1 receptor (GLP-1R) agonists used to treat type 2 diabetes mellitus often produce nausea, vomiting, and in some patients, undesired anorexia. Notably, these behavioral effects are caused by direct central GLP-1R activation. Herein, we describe the creation of a GLP-1R agonist conjugate with modified brain penetrance that enhances GLP-1R-mediated glycemic control without inducing vomiting. Covalent attachment of the GLP-1R agonist exendin-4 (Ex4) to dicyanocobinamide (Cbi), a corrin ring containing precursor of vitamin B12, produces a "corrinated" Ex4 construct (Cbi-Ex4). Data collected in the musk shrew (Suncus murinus), an emetic mammal, reveal beneficial effects of Cbi-Ex4 relative to Ex4, as evidenced by improvements in glycemic responses in glucose tolerance tests and a profound reduction of emetic events. Our findings highlight the potential for clinical use of Cbi-Ex4 for millions of patients seeking improved glycemic control without common side effects (e.g., emesis) characteristic of current GLP-1 therapeutics.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

TCF7 is not essential for glucose homeostasis in mice

Objective

Glucose-dependent insulinotropic polypeptide (GIP) and Glucagon-like peptide-1 (GLP-1) are incretin hormones that exert overlapping yet distinct actions on islet β-cells. We recently observed that GIP, but not GLP-1, upregulated islet expression of Transcription Factor 7 (TCF7), a gene expressed in immune cells and associated with the risk of developing type 1 diabetes. TCF7 has also been associated with glucose homeostasis control in the liver. Herein we studied the relative metabolic importance of TCF7 expression in hepatocytes vs. islet β-cells in mice.

Methods

Tcf7 expression was selectively inactivated in adult mouse hepatocytes using adenoviral Cre expression and targeted in β-cells using two different lines of insulin promoter-Cre mice. Glucose homeostasis, plasma insulin and triglyceride responses, islet histology, hepatic and islet gene expression, and body weight gain were evaluated in mice fed regular chow or high fat diets. Tcf7 expression within pancreatic islets and immune cells was evaluated using published single cell RNA-seq (scRNA-seq) data, and in islet RNA from immunodeficient Rag2^−/−Il2rg^−/− mice.

Results

Reduction of hepatocyte Tcf7 expression did not impair glucose homeostasis, lipid tolerance or hepatic gene expression profiles linked to control of metabolic or immune pathways. Similarly, oral and intraperitoneal glucose tolerance, plasma insulin responses, islet histology, body weight gain, and insulin tolerance were not different in mice with targeted recombination of Tcf7 in insulin-positive β-cells. Surprisingly, islet Tcf7 mRNA transcripts were not reduced in total islet RNA containing endocrine and associated non-endocrine cell types from Tcf7^{βcell−/−} mice, despite Cre-mediated recombination of islet genomic DNA. Furthermore, glucose tolerance was normal in whole body Tcf7^−/− mice. Analysis of scRNA-seq datasets localized pancreatic Tcf7 expression to islet progenitors during development, and immune cells, but not within differentiated islet β-cells or endocrine lineages within mature islets. Moreover, the expression of Tcf7 was extremely low in islet RNA from Rag2^−/−Il2rg^−/− mice and, consistent with expression within immune cells, Tcf7 was highly correlated with levels of Cd3g mRNA transcripts in RNA from wild type mouse islets.

Conclusions

These findings demonstrate that Tcf7 expression is not a critical determinant of glucose homeostasis in mice. Moreover, the detection of Tcf7 expression within islet mRNA is attributable to the expression of Tcf7 RNA in islet-associated murine immune cells, and not in islet β-cells.

•Reduction of hepatocyte Tcf7 does not impair glucose homeostasis.

•Targeting beta cell Tcf7 using insulin-promoter-Cre does not reduce islet Tcf7 expression.

•RNA-seq localizes pancreatic Tcf7 to islet progenitors and lymphocytes.

•Tcf7 expression is markedly reduced in islet RNA from Rag2^−/−Il2rg^−/− mice.

Revisiting the Complexity of GLP-1 Action from Sites of Synthesis to Receptor Activation

Glucagon-like peptide-1 (GLP-1) is produced in gut endocrine cells and in the brain, and acts through hormonal and neural pathways to regulate islet function, satiety, and gut motility, supporting development of GLP-1 receptor (GLP-1R) agonists for the treatment of diabetes and obesity. Classic notions of GLP-1 acting as a meal-stimulated hormone from the distal gut are challenged by data supporting production of GLP-1 in the endocrine pancreas, and by the importance of brain-derived GLP-1 in the control of neural activity. Moreover, attribution of direct vs indirect actions of GLP-1 is difficult, as many tissue and cellular targets of GLP-1 action do not exhibit robust or detectable GLP-1R expression. Furthermore, reliable detection of the GLP-1R is technically challenging, highly method dependent, and subject to misinterpretation. Here we revisit the actions of GLP-1, scrutinizing key concepts supporting gut vs extra-intestinal GLP-1 synthesis and secretion. We discuss new insights refining cellular localization of GLP-1R expression and integrate recent data to refine our understanding of how and where GLP-1 acts to control inflammation, cardiovascular function, islet hormone secretion, gastric emptying, appetite, and body weight. These findings update our knowledge of cell types and mechanisms linking endogenous vs pharmacological GLP-1 action to activation of the canonical GLP-1R, and the control of metabolic activity in multiple organs.

The metabolic impact of small intestinal nutrient sensing

The gastrointestinal tract maintains energy and glucose homeostasis, in part through nutrient-sensing and subsequent signaling to the brain and other tissues. In this review, we highlight the role of small intestinal nutrient-sensing in metabolic homeostasis, and link high-fat feeding, obesity, and diabetes with perturbations in these gut-brain signaling pathways. We identify how lipids, carbohydrates, and proteins, initiate gut peptide release from the enteroendocrine cells through small intestinal sensing pathways, and how these peptides regulate food intake, glucose tolerance, and hepatic glucose production. Lastly, we highlight how the gut microbiota impact small intestinal nutrient-sensing in normal physiology, and in disease, pharmacological and surgical settings. Emerging evidence indicates that the molecular mechanisms of small intestinal nutrient sensing in metabolic homeostasis have physiological and pathological impact as well as therapeutic potential in obesity and diabetes.

The gastrointestinal tract participates in maintaining metabolic homeostasis in part through nutrient-sensing and subsequent gut-brain signalling. Here the authors review the role of small intestinal nutrient-sensing in regulation of energy intake and systemic glucose metabolism, and link high-fat diet, obesity and diabetes with perturbations in these pathways.

Beinaglutide shows significantly beneficial effects in diabetes/obesity-induced nonalcoholic steatohepatitis in ob/ob mouse model

AIMS

Beinaglutide has been approved for glucose lowering in type 2 diabetes mellitus (T2DM) in China. In addition to glycemic control, significant weight loss is observed from real world data. This study is designed to investigate the pharmacological and pharmacokinetic profiles of beinaglutide in different models.

METHODS

The pharmacological efficacy of beinaglutide was evaluated in C57BL/6 and ob/ob mice after single administration. Pharmacokinetic profiles in mice were investigated after single or multiple administration. Sub-chronic pharmacological efficacy was investigated in ob/ob mice for two weeks treatment and diet-induced ob/ob mice model of nonalcoholic steatohepatitis (NASH) for four weeks treatment.

KEY FINDINGS

Beinaglutide could dose-dependently reduce the glucose levels and improve insulin secretion in glucose tolerance tests, inhibit food intake and gastric emptying after single administration. At higher doses, beinaglutide could inhibit food intake over 4 h, which results in weight loss in ob/ob mice after about two weeks treatment. No tachyphylaxis is observed for beinaglutide in food intake with repeated administration. In NASH model, beinaglutide could reduce liver weight and hepatic steatosis and improve insulin sensitivity. Signiant changes of gene levels were observed in fatty acid β-oxidation (Ppara, Acadl, Acox1), mitochondrial function (Mfn1, Mfn2), antioxidation (Sod2), Sirt1, and et al. SIGNIFICANCE: Our results characterize the pharmacological and pharmacokinetic profiles of beinaglutide in mice and supported that chronic use of beinaglutde could lead to weight loss and reduce hepatic steatosis, which suggest beinaglutide may be effective therapy for the treatment of obesity and NASH.

A second-generation glucagon-like peptide-1 receptor agonist mitigates vomiting and anorexia while retaining glucoregulatory potency in lean diabetic and emetic mammalian models

AIM

To develop a conjugate of vitamin B12 bound to the glucagon-like peptide-1 receptor (GLP-1R) agonist exendin-4 (Ex4) that shows reduced penetrance into the central nervous system while maintaining peripheral glucoregulatory function.

METHODS

We evaluated whether a vitamin B12 conjugate of Ex4 (B12-Ex4) improves glucose tolerance without inducing anorexia in Goto-Kakizaki (GK) rats, a lean type 2 diabetes model of an understudied but medically compromised population of patients requiring the glucoregulatory effects of GLP-1R agonists without anorexia. We also utilized the musk shrew (Suncus murinus), a mammalian model capable of emesis, to test B12-Ex4 on glycaemic profile, feeding and emesis.

RESULTS

In both models, native Ex4 and B12-Ex4 equivalently blunted the rise in blood glucose levels during a glucose tolerance test. In both GK rats and shrews, acute Ex4 administration decreased food intake, leading to weight loss; by contrast, equimolar administration of B12-Ex4 had no effect on feeding and body weight. There was a near absence of emesis in shrews given systemic B12-Ex4, in contrast to reliable emesis produced by Ex4. When administered centrally, both B12-Ex4 and Ex4 induced similar potency of emesis, suggesting that brain penetrance of B12-Ex4 is required for induction of emesis.

CONCLUSIONS

These findings highlight the potential therapeutic value of B12-Ex4 as a novel treatment for type 2 diabetes devoid of weight loss and with reduced adverse effects and better tolerance, but similar glucoregulation to current GLP-1R agonists.

β-Cell-specific ablation of sirtuin 4 does not affect nutrient-stimulated insulin secretion in mice

Sirtuins are a family of proteins that regulate biological processes such as cellular stress and aging by removing posttranslational modifications (PTMs). We recently identified several novel PTMs that can be removed by sirtuin 4 (SIRT4), which is found in mitochondria. We showed that mice with a global loss of SIRT4 [SIRT4-knockout (KO) mice] developed an increase in glucose- and leucine-stimulated insulin secretion, and this was followed by accelerated age-induced glucose intolerance and insulin resistance. Because whole body SIRT4-KO mice had alterations to nutrient-stimulated insulin secretion, we hypothesized that SIRT4 plays a direct role in regulating pancreatic β-cell function. Thus, we tested whether β-cell-specific ablation of SIRT4 would recapitulate the elevated insulin secretion seen in mice with a global loss of SIRT4. Tamoxifen-inducible β-cell-specific SIRT4-KO mice were generated, and their glucose tolerance and glucose- and leucine-stimulated insulin secretion were measured over time. These mice exhibited normal glucose- and leucine-stimulated insulin secretion and maintained normal glucose tolerance even as they aged. Furthermore, 832/13 β-cells with a CRISPR/Cas9n-mediated loss of SIRT4 did not show any alterations in nutrient-stimulated insulin secretion. Despite the fact that whole body SIRT4-KO mice demonstrated an age-induced increase in glucose- and leucine-stimulated insulin secretion, our current data indicate that the loss of SIRT4 specifically in pancreatic β-cells, both in vivo and in vitro, does not have a significant impact on nutrient-stimulated insulin secretion. These data suggest that SIRT4 controls nutrient-stimulated insulin secretion during aging by acting on tissues external to the β-cell, which warrants further study.

Analysis of long noncoding RNA-associated competing endogenous RNA network in glucagon-like peptide-1 receptor agonist-mediated protection in β cells

BACKGROUND

Long noncoding RNAs (lncRNAs) and mRNAs are widely involved in various physiological and pathological processes. The use of glucagon-like peptide-1 receptor agonists (GLP-1RAs) is a novel therapeutic strategy that could promote insulin secretion and decrease the rate of β-cell apoptosis in type 2 diabetes mellitus (T2DM) patients. However, the specific lncRNAs and mRNAs and their functions in these processes have not been fully identified and elucidated.

AIM

To identify the lncRNAs and mRNAs that are involved in the protective effect of GLP-1RA in β cells, and their roles.

METHODS

Rat gene microarray was used to screen differentially expressed (DE) lncRNAs and mRNAs in β cells treated with geniposide, a GLP-1RA. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to assess the underlying functions of DE mRNAs. Hub mRNAs were filtered using the STRING database and the Cytoscape plugin, CytoHubba. In order to reveal the regulatory relationship between lncRNAs and hub mRNAs, their co-expression network was constructed based on the Pearson coefficient of DE lncRNAs and mRNAs, and competing endogenous RNA (ceRNA) mechanism was explored through miRanda and TargetScan databases.

RESULTS

We identified 308 DE lncRNAs and 128 DE mRNAs with a fold change filter of ≥ 1.5 and P value < 0.05. GO and KEGG pathway enrichment analyses indicated that the most enriched terms were G-protein coupled receptor signaling pathway, inflammatory response, calcium signaling pathway, positive regulation of cell proliferation, and ERK1 and ERK2 cascade. Pomc, Htr2a, and Agtr1a were screened as hub mRNAs using the STRING database and the Cytoscape plugin, CytoHubba. This result was further verified using SwissTargetPrediction tool. Through the co-expression network and competing endogenous (ceRNA) mechanism, we identified seven lncRNAs (NONRATT027738, NONRATT027888, NONRATT030038, etc.) co-expressed with the three hub mRNAs (Pomc, Htr2a, and Agtr1a) based on the Pearson coefficient of the expression levels. These lncRNAs regulated hub mRNA functions by competing with six miRNAs (rno-miR-5132-3p, rno-miR-344g, rno-miR-3075, etc.) via the ceRNA mechanism. Further analysis indicated that lncRNA NONRATT027738 interacts with all the three hub mRNAs, suggesting that it is at a core position within the ceRNA network.

CONCLUSION

We have identified key lncRNAs and mRNAs, and highlighted here how they interact through the ceRNA mechanism to mediate the protective effect of GLP-1RA in β cells.

Intestinal glucagon-like peptide-1 effects on food intake: Physiological relevance and emerging mechanisms

The gut-brain hormone glucagon-like peptide-1 (GLP-1) has received immense attention over the last couple of decades for its widespread metabolic effects. Notably, intestinal GLP-1 has been recognized as an endogenous satiation signal. Yet, the underlying mechanisms and the pathophysiological relevance of intestinal GLP-1 in obesity remain unclear. This review first recapitulates early findings indicating that intestinal GLP-1 is an endogenous satiation signal, whose eating effects are primarily mediated by vagal afferents. Second, on the basis of recent findings challenging a paracrine action of intestinal GLP-1, a new model for the mediation of GLP-1 effects on eating by two discrete vagal afferent subsets will be proposed. The central mechanisms processing the vagal anorexigenic signals need however to be further delineated. Finally, the idea that intestinal GLP-1 secretion and/or effects on eating are altered in obesity and play a pathophysiological role in the development of obesity will be discussed. In summary, despite the successful therapeutic use of GLP-1 receptor agonists as anti-obesity drugs, the eating effects of intestinal GLP-1 still remain to be elucidated. Specifically, the findings presented here call for a further evaluation of the vago-central neuronal substrates activated by intestinal GLP-1 and for further investigation of its pathophysiological role in obesity.

miR-7 Regulates GLP-1-Mediated Insulin Release by Targeting β-Arrestin 1

Glucagon-like peptide-1 (GLP-1) has been shown to potentiate glucose-stimulated insulin secretion binding GLP-1 receptor on pancreatic β cells. β-arrestin 1 (βARR1) is known to regulate the desensitization of GLP-1 receptor. Mounting evidence indicates that microRNAs (miRNAs, miRs) are fundamental in the regulation of β cell function and insulin release. However, the regulation of GLP-1/βARR1 pathways by miRs has never been explored. Our hypothesis is that specific miRs can modulate the GLP-1/βARR1 axis in β cells. To test this hypothesis, we applied a bioinformatic approach to detect miRs that could target βARR1; we identified hsa-miR-7-5p (miR-7) and we validated the specific interaction of this miR with βARR1. Then, we verified that GLP-1 was indeed able to regulate the transcription of miR-7 and βARR1, and that miR-7 significantly regulated GLP-1-induced insulin release and cyclic AMP (cAMP) production in β cells. Taken together, our findings indicate, for the first time, that miR-7 plays a functional role in the regulation of GLP-1-mediated insulin release by targeting βARR1. These results have a decisive clinical impact given the importance of drugs modulating GLP-1 signaling in the treatment of patients with type 2 diabetes mellitus.

Gastric submucosal alleviated pro-inflammation cytokines mediated initial dysfunction of islets allografts

BACKGROUND

The liver and renal capsule are the most common site for experimental pancreatic islet transplantation, but it is not optimal. Gastric submucosa space may be an ideal site for islet transplantation; however, whether pro-inflammation factors mediated islet dysfunction could be avoided or alleviated is still unclear.

METHODS

Islets of Sprague Dawley (SD) rat were transplanted into the streptozotocin-induced diabetic SD rats. Transplantation sites included gastric submucosa (GS), intraportal vein (PV) and kidney capsule (KC), and the efficiency of glycemic control and site-specific differences of islet grafts were compared.

RESULTS

With limited number of islets (800 IEQ) transplanted, improvement of recipient glycometabolism was superior in the GS group. When transplanted with 1200 IEQ islets, the survival of islet grafts were significantly prolonged in the GS group (25.87 ± 4.08 days, compared to 15.97 ± 0.83 days and 17.33 ± 1.41 days in PV and KC groups, respectively, P < .05). Compared with the PV group, the levels of IL-1β and TNF-α were significantly depressed in GS group after 12 h transplantation (15.5 ± 0.70 pg/mL and 13.28 ± 2.80 pg/mL vs. 262.26 ± 53.37 pg/mL and 138.51 ± 39.58 pg/mL, P < .05).

CONCLUSIONS

Gastric submucosal would be a potential ideal site for islet transplantation in rat. Gastric submucosal might alleviate the early islet dysfunction triggered by the IL-1β and TNF-α, and which requires a low number of transplanted islets and have a good glycemic control in return.

Dipeptidyl Peptidase-4 at the Interface Between Inflammation and Metabolism

Dipeptidyl peptidase-4 (DPP4) is a serine protease that rapidly inactivates the incretin peptides, glucagon-like peptide-1, and glucose-dependent insulinotropic polypeptide to modulate postprandial islet hormone secretion and glycemia. Dipeptidyl peptidase-4 also has nonglycemic effects by controlling the progression of inflammation, which may be mediated more through direct protein-protein interactions than catalytic activity in the context of nonalcoholic fatty liver disease (NAFLD), obesity, and type 2 diabetes (T2D). Failure to resolve inflammation resulting in chronic subclinical activation of the immune system may influence the development of metabolic dysregulation. Thus, through both its cleavage and regulation of the bioactivity of peptide hormones and its influence on inflammation, DPP4 exhibits a diverse array of effects that can influence the progression of metabolic disease. Here, we highlight our current understanding of the complex biology of DPP4 at the intersection of inflammation, obesity, T2D, and NAFLD. We compare and review new mechanisms identified in basic laboratory and clinical studies, which may have therapeutic application and relevance to the pathogenesis of obesity and T2D.

Major miRNA Involved in Insulin Secretion and Production in Beta-Cells

Insulin is implicated as a leading factor in glucose homeostasis and an important theme in diabetes mellitus (DM). Numerous proteins are involved in insulin signaling pathway and their dysregulation contributes to DM. microRNAs (miRNAs) as single-strand molecules have a critical effect on gene expression at post-transcriptional levels. Intensive investigation done by DM researchers disclosed that miRNAs have a significant role in insulin secretion by direct targeting numerous proteins engaged in insulin signaling pathway; so, their dysregulation contributes to DM. In this review, we presented some major miRNAs engaged in the insulin production and secretion.

Remodeling of the Acetylproteome by SIRT3 Manipulation Fails to Affect Insulin Secretion or β Cell Metabolism in the Absence of Overnutrition

SIRT3 is a nicotinamide adenine dinucleotide (NAD⁺)- dependent mitochondrial protein deacetylase purported to influence metabolism through post-translational modification of metabolic enzymes. Fuel-stimulated insulin secretion, which involves mitochondrial metabolism, could be susceptible to SIRT3-mediated effects. We used CRISPR/Cas9 technology to manipulate SIRT3 expression in β cells, resulting in widespread SIRT3-dependent changes in acetylation of key metabolic enzymes but no appreciable changes in glucose- or pyruvate-stimulated insulin secretion or metabolomic profile during glucose stimulation. Moreover, these broad changes in the SIRT3-targeted acetylproteome did not affect responses to nutritional or ER stress. We also studied mice with global SIRT3 knockout fed either standard chow (STD) or high-fat and high-sucrose (HFHS) diets. Only when chronically fed HFHS diet do SIRT3 KO animals exhibit a modest reduction in insulin secretion. We conclude that broad changes in mitochondrial protein acetylation in response to manipulation of SIRT3 are not sufficient to cause changes in islet function or metabolism.

Peterson et al. report that ablation of SIRT3 in 832/13 β cells dramatically alters the mitochondrial acetylproteome but does not affect insulin secretion, metabolomic profile, or β cell survival. Moreover, SIRT3 knockout causes a modest reduction in insulin secretion in mice fed a high-fat and high-sucrose but not a standard chow diet.

Effects of Duodenal Infusion of Lauric Acid and L-Tryptophan, Alone and Combined, on Fasting Glucose, Insulin and Glucagon in Healthy Men

The fatty acid, lauric acid ('C12'), and the amino acid, tryptophan ('Trp'), when given intraduodenally at loads that individually do not affect energy intake, have recently been shown to stimulate plasma cholecystokinin, suppress ghrelin and reduce energy intake much more markedly when combined. Both fatty acids and amino acids stimulate insulin secretion by distinct mechanisms; fatty acids enhance glucose-stimulated insulin secretion, while amino acids may have a direct effect on pancreatic β cells. Therefore, it is possible that, by combining these nutrients, their effects to lower blood glucose may be enhanced. We have investigated the potential for the combination of C12 and Trp to have additive effects to reduce blood glucose. To address this question, plasma concentrations of glucose, insulin and glucagon were measured in 16 healthy, lean males during duodenal infusions of saline (control), C12 (0.3 kcal/min), Trp (0.1 kcal/min), or C12+Trp (0.4 kcal/min), for 90 min. Both C12 and C12+Trp moderately reduced plasma glucose compared with control (p < 0.05). C12+Trp, but not C12 or Trp, stimulated insulin and increased the insulin-to-glucose ratio (p < 0.05). There was no effect on plasma glucagon. In conclusion, combined intraduodenal administration of C12 and Trp reduced fasting glucose in healthy men, and this decrease was driven primarily by C12. The effects of these nutrients on postprandial blood glucose and elevated fasting blood glucose in type 2 diabetes warrant evaluation.

Glucagon-like peptide-1 increased the vascular relaxation response via AMPK/Akt signaling in diabetic mice aortas

The incretin glucagon-like peptide-1 (GLP-1) elicits direct favorable effects on the cardiovascular system. This study aimed to evaluate the acute effects of GLP-1 on improving aortic endothelial dysfunction in diabetic mice. Additionally, we examined whether GLP-1 elucidated the underlying mechanisms. Using the diabetic mouse models induced by nicotinamide and streptozotocin, we investigated the functional changes in the aorta caused by GLP-1. Organ baths were performed for vascular reactivity in isolated aortic rings, and western blotting was used for protein analysis. The diabetic aortas showed enhanced GLP-1-induced relaxation response and nitric oxide (NO) production. However, the pretreatment of GLP-1 did not significantly change the endothelial-dependent relaxation response to acetylcholine and -independent relaxation response to sodium nitroprusside. On the other hand, the GLP-1-induced relaxation response and NO production were abolished by the endothelial NO synthase inhibitor, GLP-1 receptor antagonist, Akt inhibitor, and AMP-activated protein kinase (AMPK) inhibitor. Finally, in diabetic mice, considerable increases in phosphorylation of Akt and AMPK were found in aortas stimulated with GLP-1, both of which were decreased by pretreatment with the AMPK inhibitor. GLP-1 significantly enhanced endothelial-dependent relaxation in diabetic aortas. The effect may be mediated through activation of the AMPK/Akt pathway via a GLP-1 receptor-dependent mechanism.

Localization of Glucagon-Like Peptide-2 Receptor Expression in the Mouse

Glucagon-like peptide-2 (GLP-2), secreted from enteroendocrine cells, attenuates gut motility, enhances barrier function, and augments nutrient absorption, actions mediated by a single GLP-2 receptor (GLP-2R). Despite extensive analyses, the precise distribution and cellular localization of GLP-2R expression remains controversial, confounded by the lack of suitable GLP-2R antisera. Here, we reassessed murine Glp2r expression using regular and real-time quantitative PCR (qPCR), in situ hybridization (ISH), and a Glp2rLacZ reporter mouse. Glp2r mRNA expression was detected from the stomach to the rectum and most abundant in the jejunum. Glp2r transcripts were also detected in cerebral cortex, mesenteric lymph nodes, gallbladder, urinary bladder, and mesenteric fat. Surprisingly, Glp2r mRNA was found in testis by qPCR at levels similar to jejunum. However, the testis Glp2r transcripts, detected by different primer pairs and qPCR, lacked 5' mRNA coding sequences, and only a minute proportion of them corresponded to full-length Glp2r mRNA. Within the gut, Glp2r-driven LacZ expression was localized to enteric neurons and lamina propria stromal cells, findings confirmed by ISH analysis of the endogenous Glp2r mRNA. Unexpectedly, vascular Glp2rLacZ expression was localized to mesenteric veins and not arteries. Moreover, mesenteric fat Glp2rLacZ expression was detected within blood vessels and not adipocytes. Reporter LacZ expression was not detected in all tissues expressing an endogenous Glp2r transcript, such as gallbladder, urinary bladder, and mesenteric lymph nodes. Collectively, these findings extend our understanding of the cellular domains of Glp2r expression and highlight limitations inherent in application of commonly used technologies to infer analysis of gene expression.

Glucagon lowers glycemia when β-cells are active

Glucagon and insulin are commonly believed to have counteracting effects on blood glucose levels. However, recent studies have demonstrated that glucagon has a physiologic role to activate β-cells and enhance insulin secretion. To date, the actions of glucagon have been studied mostly in fasting or hypoglycemic states, yet it is clear that mixed-nutrient meals elicit secretion of both glucagon and insulin, suggesting that glucagon also contributes to glucose regulation in the postprandial state. We hypothesized that the elevated glycemia seen in the fed state would allow glucagon to stimulate insulin secretion and reduce blood glucose. In fact, exogenous glucagon given under fed conditions did robustly stimulate insulin secretion and lower glycemia. Exogenous glucagon given to fed Gcgr:Glp1rβcell-/- mice failed to stimulate insulin secretion or reduce glycemia, demonstrating the importance of an insulinotropic glucagon effect. The action of endogenous glucagon to reduce glycemia in the fed state was tested with administration of alanine, a potent glucagon secretagogue. Alanine raised blood glucose in fasted WT mice or fed Gcgr:Glp1rβcell-/- mice, conditions where glucagon is unable to stimulate β-cell activity. However, alanine given to fed WT mice produced a decrease in glycemia, along with elevated insulin and glucagon levels. Overall, our data support a model in which glucagon serves as an insulinotropic hormone in the fed state and complements rather than opposes insulin action to maintain euglycemia.

β-Cell-intrinsic β-arrestin 1 signaling enhances sulfonylurea-induced insulin secretion

Beta-arrestin-1 and -2 (Barr1 and Barr2, respectively) are intracellular signaling molecules that regulate many important metabolic functions. We previously demonstrated that mice lacking Barr2 selectively in pancreatic beta-cells showed pronounced metabolic impairments. Here we investigated whether Barr1 plays a similar role in regulating beta-cell function and whole body glucose homeostasis. Initially, we inactivated the Barr1 gene in beta-cells of adult mice (beta-barr1-KO mice). Beta-barr1-KO mice did not display any obvious phenotypes in a series of in vivo and in vitro metabolic tests. However, glibenclamide and tolbutamide, two widely used antidiabetic drugs of the sulfonylurea (SU) family, showed greatly reduced efficacy in stimulating insulin secretion in the KO mice in vivo and in perifused KO islets in vitro. Additional in vivo and in vitro studies demonstrated that Barr1 enhanced SU-stimulated insulin secretion by promoting SU-mediated activation of Epac2. Pull-down and co-immunoprecipitation experiments showed that Barr1 can directly interact with Epac2 and that SUs such as glibenclamide promote Barr1/Epac2 complex formation, triggering enhanced Rap1 signaling and insulin secretion. These findings suggest that strategies aimed at promoting Barr1 signaling in beta-cells may prove useful for the development of efficacious antidiabetic drugs.

β Cell tone is defined by proglucagon peptides through cAMP signaling

Paracrine interactions between pancreatic islet cells have been proposed as a mechanism to regulate hormone secretion and glucose homeostasis. Here, we demonstrate the importance of proglucagon-derived peptides (PGDPs) for α to β cell communication and control of insulin secretion. Signaling through this system occurs through both the glucagon-like peptide receptor (Glp1r) and glucagon receptor (Gcgr). Loss of PGDPs, or blockade of their receptors, decreases insulin secretion in response to both metabolic and nonmetabolic stimulation of mouse and human islets. This effect is due to reduced β cell cAMP and affects the quantity but not dynamics of insulin release, indicating that PGDPs dictate the magnitude of insulin output in an isolated islet. In healthy mice, additional factors that stimulate cAMP can compensate for loss of PGDP signaling; however, input from α cells is essential to maintain glucose tolerance during the metabolic stress induced by high-fat feeding. These findings demonstrate an essential role for α cell regulation of β cells, raising the possibility that abnormal paracrine signaling contributes to impaired insulin secretion in diabetes. Moreover, these findings support reconsideration of the role for α cells in postprandial glucose control.

The islet-expressed Lhx1 transcription factor interacts with Islet-1 and contributes to glucose homeostasis

The LIM-homeodomain (LIM-HD) transcription factor Islet-1 (Isl1) interacts with the LIM domain-binding protein 1 (Ldb1) coregulator to control expression of key pancreatic β-cell genes. However, Ldb1 also has Isl1-independent effects, supporting that another LIM-HD factor interacts with Ldb1 to impact β-cell development and/or function. LIM homeobox 1 (Lhx1) is an Isl1-related LIM-HD transcription factor that appears to be expressed in the developing mouse pancreas and in adult islets. However, roles for this factor in the pancreas are unknown. This study aimed to determine Lhx1 interactions and elucidate gene regulatory and physiological roles in the pancreas. Co-immunoprecipitation using β-cell extracts demonstrated an interaction between Lhx1 and Isl1, and thus we hypothesized that Lhx1 and Isl1 regulate similar target genes. To test this, we employed siRNA-mediated Lhx1 knockdown in β-cell lines and discovered reduced Glp1R mRNA. Chromatin immunoprecipitation revealed Lhx1 occupancy at a domain also known to be occupied by Isl1 and Ldb1. Through development of a pancreas-wide knockout mouse model ( Lhx1^∆Panc), we demonstrate that aged Lhx1^∆Panc mice have elevated fasting blood glucose levels, altered intraperitoneal and oral glucose tolerance, and significantly upregulated glucagon, somatostatin, pancreatic polypeptide, MafB, and Arx islet mRNAs. Additionally, Lhx1^∆Panc mice exhibit significantly reduced Glp1R, an mRNA encoding the insulinotropic receptor for glucagon-like peptide 1 along with a concomitant dampened Glp1 response and mild glucose intolerance in mice challenged with oral glucose. These data are the first to reveal that the Lhx1 transcription factor contributes to normal glucose homeostasis and Glp1 responses.

β-Cell Receptor Tyrosine Kinases in Controlling Insulin Secretion and Exocytotic Machinery: c-Kit and Insulin Receptor

Insulin secretion from pancreatic β-cells is initiated through channel-mediated depolarization, cytoskeletal remodeling, and vesicle tethering at the cell membrane, all of which can be regulated through cell surface receptors. Receptor tyrosine kinases (RTKs) promote β-cell development and postnatal signaling to improve β-cell mass and function, yet their activation has also been shown to initiate exocytotic events in β-cells. This review examines the role of RTK signaling in insulin secretion, with a focus on RTKs c-Kit and insulin receptor (IR). Pathways that control insulin release and the potential interplay between c-Kit and IR signaling are discussed, along with clinical implications of RTK therapy on insulin secretion.

Lixisenatide requires a functional gut-vagus nerve-brain axis to trigger insulin secretion in controls and type 2 diabetic mice

Endogenous glucagon-like peptide-1 (GLP-1) regulates glucose-induced insulin secretion through both direct β-cell-dependent and indirect gut-brain axis-dependent pathways. However, little is known about the mode of action of the GLP-1 receptor agonist lixisenatide. We studied the effects of lixisenatide (intraperitoneal injection) on insulin secretion, gastric emptying, vagus nerve activity, and brain c-Fos activation in naive, chronically vagotomized, GLP-1 receptor knockout (KO), high-fat diet-fed diabetic mice, or db/db mice. Lixisenatide dose-dependently increased oral glucose-induced insulin secretion that is correlated with a decrease of glycemia. In addition, lixisenatide inhibited gastric emptying. These effects of lixisenatide were abolished in vagotomized mice, characterized by a delay of gastric emptying and in GLP-1 receptor KO mice. Intraperitoneal administration of lixisenatide also increased the vagus nerve firing rate and the number of c-Fos-labeled neurons in the nucleus tractus solitarius (NTS) of the brainstem. In diabetic mouse models, lixisenatide increased the firing rate of the vagus nerve when administrated simultaneously to an intraduodenal glucose. It increased also insulin secretion and c-Fos activation in the NTS. Altogether, our findings show that lixisenatide requires a functional vagus nerve and neuronal gut-brain-islets axis as well as the GLP-1 receptor to regulate glucose-induced insulin secretion in healthy and diabetic mice. NEW & NOTEWORTHY Lixisenatide is an agonist of the glucagon-like protein (GLP)-1 receptor, modified from exendin 4, used to treat type 2 diabetic patients. However, whereas the mode of action of endogenous GLP-1 is extensively studied, the mode of action of the GLP-1 analog lixisenatide is poorly understood. Here, we demonstrated that lixisenatide activates the vagus nerve and recruits the gut-brain axis through the GLP-1 receptor to decrease gastric emptying and stimulate insulin secretion to improve glycemia.

Dual Therapy with Liraglutide and Ghrelin Promotes Brain and Peripheral Energy Metabolism in the R6/2 Mouse Model of Huntington's Disease

Neuronal loss alongside altered energy metabolism, are key features of Huntington’s disease (HD) pathology. The orexigenic gut-peptide hormone ghrelin is known to stimulate appetite and affect whole body energy metabolism. Liraglutide is an efficient anti-type 2 diabetes incretin drug, with neuroprotective effects alongside anorectic properties. Combining liraglutide with the orexigenic peptide ghrelin may potentially promote brain/cognitive function in HD. The R6/2 mouse model of HD exhibits progressive central pathology, weight loss, deranged glucose metabolism, skeletal muscle atrophy and altered body composition. In this study, we targeted energy metabolism in R6/2 mice using a co-administration of liraglutide and ghrelin. We investigated their effect on brain cortical hormone-mediated intracellular signalling pathways, metabolic and apoptotic markers, and the impact on motor function in HD. We here demonstrate that liraglutide, alone or together with ghrelin (subcutaneous daily injections of 150 µg/kg (ghrelin) and 0.2 mg/kg (liraglutide), for 2 weeks), normalized glucose homeostatic features in the R6/2 mouse, without substantially affecting body weight or body composition. Liraglutide alone decreased brain cortical active GLP-1 and IGF-1 levels in R6/2 mice, alongside higher ADP levels. Liraglutide plus ghrelin decreased brain insulin, lactate, AMP and cholesterol levels in R6/2 mice. Taken together, our findings encourage further studies targeting energy metabolism in HD.

A vitamin B12 conjugate of exendin-4 improves glucose tolerance without associated nausea or hypophagia in rodents

AIMS

While pharmacological glucagon-like peptide-1 receptor (GLP-1R) agonists are FDA-approved for treating type 2 diabetes mellitus (T2DM) and obesity, a major side effect is nausea/malaise. We recently developed a conjugate of vitamin B12 (B12) bound to the GLP-1R agonist exendin-4 (Ex4), which displays enhanced proteolytic stability and retention of GLP-1R agonism. Here, we evaluate whether the conjugate (B12-Ex4) can improve glucose tolerance without producing anorexia and malaise.

MATERIALS AND METHODS

We evaluated the effects of systemic B12-Ex4 and unconjugated Ex4 on food intake and body weight change, oral glucose tolerance and nausea/malaise in male rats, and on intraperitoneal glucose tolerance in mice. To evaluate whether differences in the profile of effects of B12-Ex4 vs unconjugated Ex4 are the result of altered CNS penetrance, rats received systemic injections of fluorescein-Ex4 (Flex), Cy5-B12 or Cy5-B12-Ex4 and brain penetrance was evaluated using confocal microscopy. Uptake of systemically administered Cy5-B12-Ex4 in insulin-containing pancreatic beta cells was also examined.

RESULTS

B12-Ex4 conjugate improves glucose tolerance, but does not elicit the malaise and anorexia produced by unconjugated Ex4. While Flex robustly penetrates into the brain (dorsal vagal complex, paraventricular hypothalamus), Cy5-B12 and Cy5-B12-Ex4 fluorescence were not observed centrally, supporting an absence of CNS penetrance, in line with observed reduction in CNS-associated Ex4 side effects. Cy5-B12-Ex4 colocalizes with insulin in the pancreas, suggesting direct pancreatic action as a potential mechanism underlying the hypoglycaemic effects of B12-Ex4.

CONCLUSION

These novel findings highlight the potential clinical utility of B12-Ex4 conjugates as possible future T2DM therapeutics with reduced incidence of adverse effects.

Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1

Glucagon-like peptide-1 (GLP-1) released from gut enteroendocrine cells controls meal-related glycemic excursions through augmentation of insulin and inhibition of glucagon secretion. GLP-1 also inhibits gastric emptying and food intake, actions maximizing nutrient absorption while limiting weight gain. Here I review the circuits engaged by endogenous versus pharmacological GLP-1 action, highlighting key GLP-1 receptor (GLP-1R)-positive cell types and pathways transducing metabolic and non-glycemic GLP-1 signals. The role(s) of GLP-1 in the benefits and side effects associated with bariatric surgery are discussed and actions of GLP-1 controlling islet function, appetite, inflammation, and cardiovascular pathophysiology are highlighted. Refinement of the risk-versus-benefit profile of GLP-1-based therapies for the treatment of diabetes and obesity has stimulated development of orally bioavailable agonists, allosteric modulators, and unimolecular multi-agonists, all targeting the GLP-1R. This review highlights established and emerging concepts, unanswered questions, and future challenges for development and optimization of GLP-1R agonists in the treatment of metabolic disease.

Regulation of intestinal lipid and lipoprotein metabolism by the proglucagon-derived peptides glucagon like peptide 1 and glucagon like peptide 2

PURPOSE OF REVIEW

The intestine is highly efficient at absorbing and packaging dietary lipids onto the structural protein apoB48 for distribution throughout the body. Here, we summarize recent advances into understanding the physiological and pharmacological actions of the proglucagon-derived peptides: glucagon like peptide 1 (GLP-1) and glucagon like peptide 2 (GLP-2) on intestinal lipoprotein secretion.

RECENT FINDINGS

Several recent studies have elucidated mechanisms underlying the paradoxical effects of GLP-1 and GLP-2 on intestinal production of triglyceride-rich lipoproteins (TRLs). Both gut-derived peptides are secreted on an equimolar basis in response to the same nutrient stimulus. Despite neither receptor demonstrating clear localization to enterocytes, a single injection of a GLP-1R agonist rapidly decreases delivery of intestinally packaged fatty acids into the plasma, while conversely GLP-2 receptor (GLP-2R) activation acutely increases TRL concentrations in plasma.

SUMMARY

The regulation of TRL secretion is dependent on the coordination of many processes: fatty acid availability uptake, assembly onto the apoB48 polypeptide backbone, secretion and reuptake, which the hormonal, neural, inflammatory and metabolic milieu can all strongly influence. Understanding of how GLP-1 and GLP-2 receptor agonists control TRL production has clinical importance given that GLP1R agonists were recently demonstrated not only to provide glycemic control but also to prevent major adverse cardiovascular events in patients with T2DM and the success of GLP-2R agonists in treating short bowel disease.

ACOT1 expression is associated with poor prognosis in gastric adenocarcinoma

Acyl-CoA thioesterase 1 (ACOT1) is an important isoform of the ACOT family that catalyzes the reaction of fatty acyl-CoAs to CoA-SH and free fatty acids. Recent studies of gastrointestinal tumor metabolism suggest that there is abnormal metabolism of lipids and fatty acids during tumor progression. However, the function and contribution of ACOT1 in gastric cancer development are still poorly understood. In addition, GLI3 is a major transcription factor in the regulation of hedgehog signaling. GLI3 mutations induce glandular expansion and intestinal metaplasia in gastric cancer, which indicates a role for GLI3 in the preneoplastic process. Thus, we investigated the relationship between ACOT1 expression and GLI3 in gastric adenocarcinoma. A tissue microarray was constructed from 280 cases of gastric adenocarcinoma. The immunohistochemistry method was performed on tissue sections of 4 μm from each tissue microarray block. We found a significant correlation between ACOT1 expression and poor histologic grade, a lower T category, TNM stage, and increased GLI3 expression. In addition, the survival analysis revealed that the ACOT1-positive group had significantly decreased overall survival rates compared with the ACOT1-negative group. Furthermore, GLI3 expression had a significant positive correlation with ACOT1 expression in gastric adenocarcinoma cells. In summary, these findings demonstrate that increased expression of ACOT1 is correlated with pivotal clinicopathological parameters and poor prognosis in gastric adenocarcinoma through increased expression of the potential tumor-promoting protein GLI3.

Mechanism of cardiovascular disease benefit of glucagon-like peptide 1 agonists

Glucagon-like peptide 1 (GLP-1)-based therapies reduce hyperglycaemia in type 2 diabetes. Diabetes cardiovascular comorbidity remains prevalent, although current treatments are effective at reducing hyperglycaemia. GLP-1 exerts specific actions on the cardiovascular system in both healthy individuals and patients with cardiovascular pathology, and GLP-1 therapies have improved the cardiovascular profile of diabetic patients. GLP-1 exerts its action by binding to its receptor (GLP-1 receptor) at the cell surface. Mechanistically, it is not clear how GLP-1 therapies exert beneficial effects on the cardiovascular system. It is difficult to arrive at any conclusions on the ability of GLP-1 receptor agonism to reduce cardiovascular disease from animal/human studies because of varying experimental designs. This review highlights recent findings from long-term human GLP-1 therapy studies, and summarizes postulated mechanisms as to how GLP-1 receptor agonism may alleviate cardiovascular disease.