Glucagon-like Peptide-1 Production Research Articles

Mechano-regulation of GLP-1 production by Piezo1 in intestinal L cells.

Glucagon-like peptide 1 (GLP-1) is a gut-derived hormone secreted by intestinal L cells and vital for postprandial glycemic control. As open-type enteroendocrine cells, whether L cells can sense mechanical stimuli caused by chyme and thus regulate GLP-1 synthesis and secretion is unexplored. Molecular biology techniques revealed the expression of Piezo1 in intestinal L cells. Its level varied in different energy status and correlates with blood glucose and GLP-1 levels. Mice with L cell-specific loss of Piezo1 (Piezo1 IntL-CKO) exhibited impaired glucose tolerance, increased body weight, reduced GLP-1 production and decreased CaMKKβ/CaMKIV-mTORC1 signaling pathway under normal chow diet or high-fat diet. Activation of the intestinal Piezo1 by its agonist Yoda1 or intestinal bead implantation increased the synthesis and secretion of GLP-1, thus alleviated glucose intolerance in diet-induced-diabetic mice. Overexpression of Piezo1, Yoda1 treatment or stretching stimulated GLP-1 production and CaMKKβ/CaMKIV-mTORC1 signaling pathway, which could be abolished by knockdown or blockage of Piezo1 in primary cultured mouse L cells and STC-1 cells. These experimental results suggest a previously unknown regulatory mechanism for GLP-1 production in L cells, which could offer new insights into diabetes treatments.

Mechano-regulation of GLP-1 production by Piezo1 in intestinal L cells

Glucagon-like peptide 1 (GLP-1) is a gut-derived hormone secreted by intestinal L cells and vital for postprandial glycemic control. As open-type enteroendocrine cells, whether L cells can sense mechanical stimuli caused by chyme and thus regulate GLP-1 synthesis and secretion is unexplored. Molecular biology techniques revealed the expression of Piezo1 in intestinal L cells. Its level varied in different energy status and correlates with blood glucose and GLP-1 levels. Mice with L cell-specific loss of Piezo1 (Piezo1 IntL-CKO) exhibited impaired glucose tolerance, increased body weight, reduced GLP-1 production and decreased CaMKKβ/CaMKIV-mTORC1 signaling pathway under normal chow diet or high-fat diet. Activation of the intestinal Piezo1 by its agonist Yoda1 or intestinal bead implantation increased the synthesis and secretion of GLP-1, thus alleviated glucose intolerance in diet-induced-diabetic mice. Overexpression of Piezo1, Yoda1 treatment or stretching stimulated GLP-1 production and CaMKKβ/CaMKIV-mTORC1 signaling pathway, which could be abolished by knockdown or blockage of Piezo1 in primary cultured mouse L cells and STC-1 cells. These experimental results suggest a previously unknown regulatory mechanism for GLP-1 production in L cells, which could offer new insights into diabetes treatments.

Fam3a-mediated prohormone convertase switch in α-cells regulates pancreatic GLP-1 production in an Nr4a2-Foxa2-dependent manner

BackgroundFam3a has been demonstrated to regulate pancreatic β-cell function and glucose homeostasis. However, the role and mechanism of Fam3a in regulating α-cell function remain unexplored. MethodsGlucagon and glucagon-like peptide-1 (GLP-1) levels in pancreas and plasma were measured in global Fam3a knockout (Fam3a−/−) mice. Human islet single-cell RNA sequencing (scRNA-seq) datasets were utilized to analyze gene expression correlations between FAM3A and PCSK1 (encoding PC1/3, which processes proglucagon into GLP-1). Mouse pancreatic α-cell line αTC1.9 cells were transfected with Fam3a siRNA or plasmid for Fam3a knockdown or overexpression to explore the effects of Fam3a on PC1/3 expression and GLP-1 production. The downstream mediator (including Nr4a2) was identified by transcriptomic analysis, and its role was confirmed by Fam3a knockdown or overexpression in αTC1.9 cells. Based on the interacted protein of Nr4a2 and the direct binding to Pcsk1 promoter, the transcription factor Foxa2 was selected for further verification. Nuclear translocation assay and dual-luciferase reporter assay were used to clarify the involvement of Fam3a-Nr4a2-Foxa2 pathway in PC1/3 expression and GLP-1 production. Moreover, α-cell-specific Fam3a knockout (Fam3aα−/−) mice were constructed to evaluate the metabolic variables and hormone levels under normoglycemic, high-fat diet (HFD)-fed and streptozotocin (STZ)-induced diabetic conditions. Exendin 9–39 (Ex9), a GLP-1 receptor antagonist, was used to investigate GLP-1 paracrine effects in Fam3aα−/− mice and in their primary islets. ResultsCompared with wild-type mice, pancreatic and plasma active GLP-1 levels were increased in Fam3a−/− mice. Analysis of human islet scRNA-seq datasets showed a significant negative correction between FAM3A and PCSK1 in α-cells. Fam3a knockdown upregulated PC1/3 expression and GLP-1 production in αTC1.9 cells, while Fam3a overexpression displayed inverse effects. Transcriptomic analysis identified Nr4a2 as a key downstream mediator of Fam3a, and Nr4a2 expression in αTC1.9 cells was downregulated and upregulated by Fam3a knockdown and overexpression, respectively. Nr4a2 silencing increased PC1/3 expression, albeit Nr4a2 did not directly bind to Pcsk1 promoter. Instead, Nr4a2 formed a complex with Foxa2 to facilitate Fam3a-mediated Foxa2 nuclear translocation. Foxa2 negatively regulated PC1/3 expression and GLP-1 production. Besides, Foxa2 inhibited the transcriptional activity of Pcsk1 promoter at specific binding sites 10 and 6, and this inhibition was intensified by Nr4a2 in αTC1.9 cells. Compared with Flox/cre littermates, improved glucose tolerance, increased active GLP-1 level in pancreas and plasma, upregulated plasma insulin level in response to glucose, and decreased plasma glucagon level were observed in Fam3aα−/− mice. Primary islets isolated from Fam3aα−/− mice also showed an increase in active GLP-1 and insulin release. In addition, the insulinotropic effect of intra-islet GLP-1 was blocked by Ex9 in Fam3aα−/− mice and in their primary islets. Similarly, HFD-fed Fam3aα−/− mice also exhibited an improved glucose tolerance. Both HFD-fed and STZ-induced diabetic Fam3aα−/− mice showed an increased pancreatic active GLP-1 level, an elevated plasma insulin level and a reduced plasma glucagon level. ConclusionsFam3a deficiency in α-cells enhances pancreatic GLP-1 production to improve β-cell function via paracrine signaling in an Nr4a2-Foxa2-PC1/3-dependent manner. Our study unveils a novel strategy for reprogramming α-cell proglucagon processing output from glucagon to GLP-1 and deepen the understanding of crosstalk between α-cells and β-cells.

Near-infrared remote triggering of bio-enzyme activation to control intestinal colonization by orally administered microorganisms

Oral biotherapeutics hold significant promise, but their lack of controllability and targeting poses a major challenge, particularly for intestinal bacterial biotherapeutics. In response, we have developed a nanoencapsulation approach that responds to the release of enzyme activity in the organism and activates the enzyme in situ, allowing for controlled colonization of microbes in the gut. The nano-coating comprises a two-layer structure: an inner layer of polydopamine with photothermal and adhesive properties, and an outer layer of gelatin–sodium carboxymethylcellulose, which is hydrolyzed by cellulases in the gut following photothermal interaction with dopamine. We have successfully achieved controlled colonization of a wide range of microorganisms. Furthermore, in a diabetes model, this approach has had a profound impact on regulating glucagon-like peptide-1 (GLP-1) production, β-cell physiology, and promoting insulin secretion. This nanocoating is achieved by in situ activation of cellulase without the need for genetic or targeted molecular modification, representing a new paradigm and alternative strategy for microbial therapy. It not only enables precise and controlled colonization of probiotics but also demonstrates great potential for broader application in the field of oral biotherapy. Statement of significanceWe have developed a nano-encapsulation method that triggers enzyme activity in response to enzymatic activity, resulting in the controlled release and adhesion of a wide range of microorganisms in the gut. The nano coating comprises two layers: an inner layer of polydopamine with photothermal and adhesion properties, and an outer layer of a gelatin-sodium carboxymethylcellulose polymer, which can be hydrolyzed by cellulases in the intestine. Additionally, this method allows for the preparation of various microbial coatings. This approach holds significant promise for regulating GLP-1 production, the physiological function of pancreatic β-cells, and promoting insulin secretion in diabetes models.

A Controversy Regarding the Identity of the Enzyme That Mediates Glucagon-Like Peptide 1 Synthesis in Human Alpha Cells.

Processing of proglucagon into glucagon-like peptide-1 (GLP-1) and GLP-2 in intestinal L cells is mediated by the prohormone convertase 1/3 (PC1/3) while PC2 is responsible for the synthesis of glucagon in pancreatic alpha cells. While GLP-1 is also produced by alpha cells, the identity of the convertase involved in its synthesis is still unsettled. It also remains to be determined whether all alpha cells produce the incretin. The aims of this study were first, to elucidate the identity of the proconvertase responsible for GLP-1 production in human alpha cells, and second, to ascertain whether the number of glucagon cells expressing GLP-1 increase during diabetes. To answer these questions, sections of pancreas from donors' non-diabetic controls, type 1 and type 2 diabetes were processed for double-labelled immunostaining of glucagon and GLP-1 and of each hormone and either PC1 or PC2. Stained sections were examined by confocal microscopy. It was found that all alpha cells of islets from those three groups expressed GLP-1 and PC2 but not PC1/3. This observation supports the view that PC2 is the convertase involved in GLP-1 synthesis in all human glucagon cells and suggests that the regulation of its activity may have important clinical application in diabetes.

Lactobacillus plantarum NCHBL-004 modulates high-fat diet–induced weight gain and enhances GLP-1 production for blood glucose regulation

ObjectivesThis study investigated the therapeutic potential of Lactobacillus plantarum NCHBL-004 (NCHBL-004) in the treatment of obesity and associated metabolic disorders. MethodsMice were fed either a normal diet (ND) or a high-fat diet (HFD) with oral administration of NCHBL-004. After euthanasia, blood, liver and adipose tissue were collected. Furthermore, the microbiome and short-chain fatty acids (SCFAs) were analyzed from feces. ResultsOral administration of live NCHBL-004 to mice fed a HFD resulted in notable reductions in weight gain, improvements in glucose metabolism, and maintenance of balanced lipid levels. A comparative analysis with other Lactobacillus strains highlighted the superior efficacy of NCHBL-004. Moreover, heat-killed NCHBL-004 demonstrated beneficial effects similar to those of live NCHBL-004. Additionally, administration of live NCHBL-004 induced glucagon-like peptide 1 (GLP-1) production and increased the levels of short-chain fatty acids (SCFAs), including acetate and propionate, in feces, positively influencing liver lipid metabolism and mitigating inflammation. Consistent with this, analysis of the gut microbiome following NCHBL-004 administration showed increases in SCFA-producing microbes with increased proportions of Lactobacillus spp. and a significant increase in the proportion of microbes capable of promoting GLP-1 secretion. ConclusionsThese findings underscore the potential of both live and inactivated NCHBL-004 as potential therapeutic approaches to managing obesity and metabolic disorders, suggesting avenues for further investigation and clinical applications.

Intestinal Nogo-B reduces GLP1 levels by binding to proglucagon on the endoplasmic reticulum to inhibit PCSK1 cleavage

Glucagon-like peptide 1 (GLP1), which is mainly processed and cleaved from proglucagon in enteroendocrine cells (EECs) of the intestinal tract, acts on the GLP1 receptor in pancreatic cells to stimulate insulin secretion and to inhibit glucagon secretion. However, GLP1 processing is not fully understood. Here, we show that reticulon 4B (Nogo-B), an endoplasmic reticulum (ER)-resident protein, interacts with the major proglucagon fragment of proglucagon to retain proglucagon on the ER, thereby inhibiting PCSK1-mediated cleavage of proglucagon in the Golgi. Intestinal Nogo-B knockout in male type 2 diabetes mellitus (T2DM) mice increases GLP1 and insulin levels and decreases glucagon levels, thereby alleviating pancreatic injury and insulin resistance. Finally, we identify aberrantly elevated Nogo-B expression and inhibited proglucagon cleavage in EECs from diabetic patients. Our study reveals the subcellular regulatory processes involving Nogo-B during GLP1 production and suggests intestinal Nogo-B as a potential therapeutic target for T2DM.

Hydrogen sulfide produced by the gut microbiota impairs host metabolism via reducing GLP-1 levels in male mice.

Dysbiosis of the gut microbiota has been implicated in the pathogenesis of metabolic syndrome (MetS) and may impair host metabolism through harmful metabolites. Here, we show that Desulfovibrio, an intestinal symbiont enriched in patients with MetS, suppresses the production of the gut hormone glucagon-like peptide 1 (GLP-1) through the production of hydrogen sulfide (H2S) in male mice. Desulfovibrio-derived H2S is found to inhibit mitochondrial respiration and induce the unfolded protein response in intestinal L cells, thereby hindering GLP-1 secretion and gene expression. Remarkably, blocking Desulfovibrio and H2S with an over-the-counter drug, bismuth subsalicylate, improves GLP-1 production and ameliorates diet-induced metabolic disorder in male mice. Together, our study uncovers that Desulfovibrio-derived H2S compromises GLP-1 production, shedding light on the gut-relayed mechanisms by which harmful microbiota-derived metabolites impair host metabolism in MetS and suggesting new possibilities for treating MetS.

Heterologous expression of P9 from Akkermansia muciniphila increases the GLP-1 secretion of intestinal L cells.

Glucagon-like peptide-1(GLP-1) is an incretin hormone secreted primarily from the intestinal L-cells in response to meals. GLP-1 is a key regulator of energy metabolism and food intake. It has been proven that P9 protein from A. muciniphila could increase GLP-1 release and improve glucose homeostasis in HFD-induced mice. To obtain an engineered Lactococcus lactis which produced P9 protein, mature polypeptide chain of P9 was codon-optimized, fused with N-terminal signal peptide Usp45, and expressed in L. lactis NZ9000. Heterologous secretion of P9 by recombinant L. lactis NZP9 were successfully detected by SDS-PAGE and western blotting. Notably, the supernatant of L. lactis NZP9 stimulated GLP-1 production of NCI-H716 cells. The relative expression level of GLP-1 biosynthesis gene GCG and PCSK1 were upregulated by 1.63 and 1.53 folds, respectively. To our knowledge, this is the first report on the secretory expression of carboxyl-terminal processing protease P9 from A. muciniphila in L. lactis. Our results suggest that genetically engineered L. lactis which expressed P9 may have therapeutic potential for the treatment of diabetes, obesity and other metabolic disorders.

Citrus pectin protects mice from burn injury by modulating intestinal microbiota, GLP-1 secretion and immune response

Water-soluble rhamnogalacturonan-I enriched citrus pectin (WRP) has promising effect on antimicrobial defense. We aim to determine whether the modified acidic (A) or neutral (B) WRP solutions can improve intestinal microbial dysbiosis in burn-injured mice. Male Balb/c mice were gavaged with WRPs at 80, 160, 320 mg/kg. Body weight daily for 21 days before exposed to thermal injury of 15 % total body surface area and mortality was monitored. Mice with 80 mg/kg WRPs were also subjected to fecal DNAs and T cell metabonomics analysis, intestinal and plasma glucagon-like peptide 1 (GLP-1) detection, plasma defensin, immunoglobin and intestinal barrier examinations at 1 and 3d postburn (p.b.). Burn-induced mortality was only improved by low dose WRP-A (P = 0.039). Both WRPs could prevent the dysbiosis of gut microbiota in burn injury by reducing the expansion of inflammation-promoting bacteria. Both WRPs suppressed ileum GLP-1 production at 1d p.b. (P = 0.002) and plasma GLP-1 levels at 3d p.b. (P = 0.013). Plasma GLP-1 level correlated closely with ileum GLP-1 production (P = 0.019) but negatively with microbiota diversity at 1d p.b. (P = 0.003). Intestinal T cell number was increased by both WRPs in jejunum at 3d p.b. However, the exaggerated splenic T cell metabolism in burn injury was reversed by both WRPs at 1d p.b. The burn-increased plasma defensin β1 level was only reduced by WRP-B. Similarly, the intestinal barrier permeability was only rescued by WRP-B at 1d p.b. WRP-A rather than WRP-B could reduce burn-induced mortality in mice by suppressing intestinal GLP-1 secretion, restoring gut microbiota dysbiosis and improving adaptive immune response.

Identification of glucagon like peptide-1 (GLP-1) in mice stomach

Glucagon like peptide-1 (GLP-1) is a peptide hormone encoded by the pre-proglucagon gene that serves multiple physiological functions, including incretin action. While GLP-1 is primarily synthesized in the L cells of the lower intestine, recent findings indicate its presence in the stomachs of both rats and humans. However, the role of gastric GLP-1 in other species remains unclear. In this study, we aimed to identify GLP-1-producing cells and examine the localization of GLP-1 production in the mouse stomach. We found that pre-proglucagon mRNA was higher in the corpus than that in the antrum of the stomach. In addition, GLP-1 immunoreactive cells were found in the gastric mucosa, and their cell number was higher in the corpus than that in the antrum. Double immunofluorescence showed that some GLP-1 immunoreactive cells displayed somatostatin immunoreactivity, whereas did not co-localize with ghrelin and gastrin. Moreover, transmembrane G protein-coupled Receptor 5 (TGR5) agonist decreased pre-proglucagon mRNA expression in SG-1 cells in a concentration-dependent manner, and in vivo experiments showed a decrease in its mRNA levels in the gastric corpus but not in the antrum. This study marks the first report of GLP-1 production in the mouse stomach. Our findings suggest that gastric pre-proglucagon mRNA expression is regulated by a distinct mechanism compared to the L cells of the lower intestine.

RNA sequencing unravels novel L cell constituents and mechanisms of GLP-1 secretion in human gastric bypass-operated intestine

Aims/hypothesisRoux-en-Y gastric bypass surgery (RYGB) frequently results in remission of type 2 diabetes as well as exaggerated secretion of glucagon-like peptide-1 (GLP-1). Here, we assessed RYGB-induced transcriptomic alterations in the small intestine and investigated how they were related to the regulation of GLP-1 production and secretion in vitro and in vivo.MethodsHuman jejunal samples taken perisurgically and 1 year post RYGB (n=13) were analysed by RNA-seq. Guided by bioinformatics analysis we targeted four genes involved in cholesterol biosynthesis, which we confirmed to be expressed in human L cells, for potential involvement in GLP-1 regulation using siRNAs in GLUTag and STC-1 cells. Gene expression analyses, GLP-1 secretion measurements, intracellular calcium imaging and RNA-seq were performed in vitro. OGTTs were performed in C57BL/6j and iScd1−/− mice and immunohistochemistry and gene expression analyses were performed ex vivo.ResultsGene Ontology (GO) analysis identified cholesterol biosynthesis as being most affected by RYGB. Silencing or chemical inhibition of stearoyl-CoA desaturase 1 (SCD1), a key enzyme in the synthesis of monounsaturated fatty acids, was found to reduce Gcg expression and secretion of GLP-1 by GLUTag and STC-1 cells. Scd1 knockdown also reduced intracellular Ca2+ signalling and membrane depolarisation. Furthermore, Scd1 mRNA expression was found to be regulated by NEFAs but not glucose. RNA-seq of SCD1 inhibitor-treated GLUTag cells identified altered expression of genes implicated in ATP generation and glycolysis. Finally, gene expression and immunohistochemical analysis of the jejunum of the intestine-specific Scd1 knockout mouse model, iScd1−/−, revealed a twofold higher L cell density and a twofold increase in Gcg mRNA expression.Conclusions/interpretationRYGB caused robust alterations in the jejunal transcriptome, with genes involved in cholesterol biosynthesis being most affected. Our data highlight SCD as an RYGB-regulated L cell constituent that regulates the production and secretion of GLP-1.Graphical

Neuroprotective properties of GABA and its derivatives in diabetic encephalopathy in old animals

The aim of the work was to evaluate the GABA neuroprotective properties and its structural analogues in old animals after seven months of hyperglycemia.Materials and methods. Diabetes mellitus was modeled in white outbred male rats (12 months old) by the administration of a streptozotocin (65 mg/kg) and nicotinamide (230 mg/kg) combination. After 6 months, the animals with a postprandial glycemia level between 11 and 18 mmol/l were selected for the study. After the groups had been formed, the animals were administrated with GABA and GABAergic compounds (Compositions МРВАand PPC), respectively, for 1 month, the control group animals were administrated with saline. After the treatment, an oral glucose tolerance test and a set of behavioral tests aimed at studying sensory-motor (Open Field, Adhesion test, Rotarod) and cognitive functions (New Object Recognition and Morris Water Maze), as well as the functional state evaluation of the endothelium were performed. Further on, sampling of blood and brain tissues for a biochemical and enzyme immunoassay (the level of glucagon-like peptide-1 (GLP-1) and TNF-α in serum and the level of Klotho protein, BDNF, Nrf2, NF-κB and malondialdehyd (MDA) in brain homogenates), as well as a morphological analysis of changes in CA1 and CA3 neurons of the hippocampus and somatosensory cortex, was carried out.Results. GABA and compositions with its derivatives had a pronounced neuroprotective effect in old animals with prolonged hyperglycemia. The hypoglycemic effect of the studied compositions was accompanied by an increase in the production of GLP-1. In the animals with DM, after 6 weeks of the test substances administration, higher rates of sensory-motor and cognitive functions and a less structural damage to the sensory-motor cortex and the brain hippocampus were recorded. These effects may be due to higher levels of the Klotho proteins, Nrf2 and BDNF, as well as lower levels of NF-κB, which may underlie the suppression of the oxidative stress, the reduction of MDA and inflammation (TNF-α).Conclusion. After 6 weeks of the administration, GABA and its compositions in old animals (19 months old) significantly improved sensory-motor and cognitive functions, reduced negative structural changes in the hippocampus and somatosensory cerebral cortex.

1775-P: Liraglutide Corrects Impaired Insulin Secretion in 14-3-3-ζ Overexpressing Mice

Crosstalk between α-cells and β-cells is an important regulator of healthy islet function. α-cells display heterogeneity in their hormone profile. α-cells typically produce glucagon but, under certain conditions, can produce glucagon-like peptide-1 (GLP-1). However, the mechanisms regulating α-cell GLP-1 production are poorly understood. We found that increased β-cell GLP-1R signaling via the GLP-1 analog, liraglutide, activates α-cell GLP-1 production through a paracrine signaling pathway. We recently identified the protein 14-3-3-ζ as a key downstream mediator of this pathway. Specifically, liraglutide downregulates β-cell 14-3-3-ζ expression, which activates α-cell GLP-1 production. Consistent with this, we found that β-cell 14-3-3-ζ ablation enhances glucose-stimulated insulin secretion (GSIS). Furthermore, mice with 14-3-3-ζ overexpression (OE) have impaired GSIS. Therefore, we tested the hypothesis that liraglutide can correct the effect of 14-3-3-ζ OE to impair GSIS. Male and female wild-type (WT) or 14-3-3-ζ OE mice maintained on high-fat diet underwent an oral glucose tolerance test (OGTT) after 2 weeks of liraglutide (200 μg/kg sc twice daily) or saline treatment. Liraglutide improved glucose tolerance in WT mice and tended to improve glucose tolerance in OE mice. Saline-treated 14-3-3-ζ OE mice exhibited lower GSIS compared with WT (P<0.05). Liraglutide treatment of 14-3-3-ζ OE mice normalized GSIS to match that of liraglutide-treated WT mice (Insulin secretion index [Insulin AUC/Glucose AUC] *10-9: Saline WT = 38 ± 4, Lira WT = 37 ± 5, Saline OE = 25 ± 4, Lira OE = 39 ± 7; n=7-8 per group, P<0.05 Saline WT vs. Saline OE, P=0.06 Saline OE vs. Lira OE). Overall, these data suggest that liraglutide can rescue deficits in GSIS induced by 14-3-3-ζ OE. Disclosure R.Shishani: None. M.Tucker: None. R.Moreno: None. J.M.Bustamante: None. G.E.Lim: None. B.Cummings: None. Funding Hartwell Foundation; National Institute of Diabetes and Digestive and Kidney Diseases (R56DK124853)

Interactions between Intestinal Homeostasis and NAD+ Biology in Regulating Incretin Production and Postprandial Glucose Metabolism

The intestine has garnered attention as a target organ for developing new therapies for impaired glucose tolerance. The intestine, which produces incretin hormones, is the central regulator of glucose metabolism. Glucagon-like peptide-1 (GLP-1) production, which determines postprandial glucose levels, is regulated by intestinal homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD+) biosynthesis in major metabolic organs such as the liver, adipose tissue, and skeletal muscle plays a crucial role in obesity- and aging-associated organ derangements. Furthermore, NAMPT-mediated NAD+ biosynthesis in the intestines and its upstream and downstream mediators, adenosine monophosphate-activated protein kinase (AMPK) and NAD+-dependent deacetylase sirtuins (SIRTs), respectively, are critical for intestinal homeostasis, including gut microbiota composition and bile acid metabolism, and GLP-1 production. Thus, boosting the intestinal AMPK-NAMPT-NAD+-SIRT pathway to improve intestinal homeostasis, GLP-1 production, and postprandial glucose metabolism has gained significant attention as a novel strategy to improve impaired glucose tolerance. Herein, we aimed to review in detail the regulatory mechanisms and importance of intestinal NAMPT-mediated NAD+ biosynthesis in regulating intestinal homeostasis and GLP-1 secretion in obesity and aging. Furthermore, dietary and molecular factors regulating intestinal NAMPT-mediated NAD+ biosynthesis were critically explored to facilitate the development of new therapeutic strategies for postprandial glucose dysregulation.

PS-B11-3: THE ROLE OF INTESTINAL NAMPT-MEDIATED NAD+ BIOSYNTHESIS IN WHOLE-BODY ENERGY AND GLUCOSE METABOLISM

The purpose of the present study was to test our hypothesis that intestinal nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD+) biosynthesis plays a pivotal role in whole-body energy and glucose metabolism. To this end, we generated a new mouse model, namely intestinal epithelial cell-specific Nampt knockout (INKO) mice. Under regular chow diet, deletion of intestinal epithelial NAMPT did not affect body weight gain, food intake, or whole-body energy expenditure, suggesting that intestinal epithelial NAMPT deficiency had no significant impacts on overall energy intake or expenditure. However, INKO mice displayed reduced early-phase insulin secretion and postprandial hyperglycemia, at least partly due to diminished glucagon-like peptide-1 (GLP-1) production. In addition, we found that diet-induced obese mice had impaired intestinal NAMPT-mediated NAD+ biosynthesis, associated with impaired GLP-1 production and whole-body glucose metabolism, similar to the INKO mice. Finally, administration of a key NAD+ intermediate, nicotinamide mononucleotide, to INKO and diet-induced obese mice, restored ileal NAD+ levels and obesity-associated metabolic derangements, manifested by an impairment in GLP-1 production as well as postprandial hyperglycemia. Taken together, our study provides a new understanding of the role of intestinal NAMPT-mediated NAD+ biosynthesis in energy and glucose metabolism, and therapeutic insights into intestinal NAD+ biology related to obesity-associated dysregulation of GLP-1 production and postprandial hyperglycemia (Nagahisa, Yamaguchi et al. Endocrinology. 2022). A future study using INKO mice to investigate if diminished GLP-1 production is involved in obesity-associated hypertension is warranted.

Altered chronic glycemic control in a clinically relevant model of rat thoracic spinal contusion.

The lifetime risk for Type 2 diabetes mellitus remains higher in people with spinal cord injuries (SCIs) than in the able-bodied population. However, the mechanisms driving this disparity remain poorly understood. The goal of the present study was to evaluate the impact of a palatable high-fat diet (HFD) on glycemic regulation using a rodent model of moderate thoracic contusion. Animals were placed on either Chow or HFD and tolerance to glucose, insulin, and ENSURE mixed meal were investigated. Important targets in the gut-brain axis were investigated. HFD consumption equally induced weight gain in SCI and naïve rats over chow (CH) rats. Elevated blood glucose was observed during intraperitoneal glucose tolerance test in HFD-fed rats over CH-fed rats. Insulin tolerance test (ITT) was unremarkable among the three groups. Gavage of ENSURE resulted in high glucagon-like peptide 1 (GLP-1) release from SCI rats over naïve controls. An elevation in terminal total GLP-1 was measured, with a marked reduction in circulating dipeptidyl peptidase 4, the GLP-1 cleaving enzyme, in SCI rats, compared with naïve. Increased glucagon mRNA in the pancreas and reduced immunoreactive glucagon-positive staining in the pancreas in SCI rats compared with controls suggested increased glucagon turnover. Finally, GLP-1 receptor gene expression in the ileum, the primary source of GLP-1 production and release, in SCI rats suggests the responsivity of the gut to altered circulating GLP-1 in the body. In conclusion, the actions of GLP-1 and its preprohormone, glucagon, are markedly uncoupled from their actions on glucose control in the SCI rat. More work is required to understand GLP-1 in the human.

Circulating GLP-1 Levels in Patients with Pheochromocytoma/Paraganglioma.

The sympathoadrenal system has been shown to stimulate the secretory activity of enteroendocrine cells, although the response is transient. Our aim was to investigate the effects of long-term catecholamine excess on circulating glucagon-likepeptide-1 (GLP-1) levels in patients with pheochromocytoma/paraganglioma (PPGL). Thirty patients diagnosed with PPGL were analyzed. A significant negative association was observed between fasting plasma GLP-1 levels and elevated plasma-free metanephrine (r = -0.407, p = 0.026). After adjustment for age, sex, body mass index (BMI), serum creatinine, and the presence of hyperglycemia, the negative association between plasma GLP-1 and metanephrine persisted by multiple linear regression analysis (β = -0.493, p = 0.013). Positive correlations between fasting glucose and plasma metanephrine (r = 0.380, p = 0.038) and normetanephrine levels (r = 0.450, p = 0.013) were also found. Mean fasting levels of total GLP-1 increased significantly from 25.81 to 39.01 pmol/L (p = 0.017) after PPGL resection. In conclusion, long-term overproduction of catecholamines appears to induce suppression of GLP-1 production compared to an acute response to a stress stimulus. Further studies are required to elucidate the mechanism of GLP-1 secretion with chronic exposure to catecholamine.

Hepatic thyroid hormone signalling modulates glucose homeostasis through the regulation of GLP-1 production via bile acid-mediated FXR antagonism

Thyroid hormones (TH) regulate systemic glucose metabolism through incompletely understood mechanisms. Here, we show that improved glucose metabolism in hypothyroid mice after T3 treatment is accompanied with increased glucagon-like peptide-1 (GLP-1) production and insulin secretion, while co-treatment with a GLP-1 receptor antagonist attenuates the effects of T3 on insulin and glucose levels. By using mice lacking hepatic TH receptor β (TRβ) and a liver-specific TRβ-selective agonist, we demonstrate that TRβ-mediated hepatic TH signalling is required for both the regulation of GLP-1 production and the insulinotropic and glucose-lowering effects of T3. Moreover, administration of a liver-targeted TRβ-selective agonist increases GLP-1 and insulin levels and alleviates hyperglycemia in diet-induced obesity. Mechanistically, T3 suppresses Cyp8b1 expression, resulting in increased the levels of Farnesoid X receptor (FXR)-antagonistic bile acids, thereby potentiating GLP-1 production and insulin secretion by repressing intestinal FXR signalling. T3 correlates with both plasma GLP-1 and fecal FXR-antagonistic bile acid levels in people with normal thyroid function. Thus, our study reveals a role for hepatic TH signalling in glucose homeostasis through the regulation of GLP-1 production via bile acid-mediated FXR antagonism.

Alpha-cell paracrine signaling in the regulation of beta-cell insulin secretion.

As an incretin hormone, glucagon-like peptide 1 (GLP-1) lowers blood glucose levels by enhancing glucose-stimulated insulin secretion from pancreatic beta-cells. Therapies targeting the GLP-1 receptor (GLP-1R) use the classical incretin model as a physiological framework in which GLP-1 secreted from enteroendocrine L-cells acts on the beta-cell GLP-1R. However, this model has come into question, as evidence demonstrating local, intra-islet GLP-1 production has advanced the competing hypothesis that the incretin activity of GLP-1 may reflect paracrine signaling of GLP-1 from alpha-cells on GLP-1Rs on beta-cells. Additionally, recent studies suggest that alpha-cell-derived glucagon can serve as an additional, albeit less potent, ligand for the beta-cell GLP-1R, thereby expanding the role of alpha-cells beyond that of a counterregulatory cell type. Efforts to understand the role of the alpha-cell in the regulation of islet function have revealed both transcriptional and functional heterogeneity within the alpha-cell population. Further analysis of this heterogeneity suggests that functionally distinct alpha-cell subpopulations display alterations in islet hormone profile. Thus, the role of the alpha-cell in glucose homeostasis has evolved in recent years, such that alpha-cell to beta-cell communication now presents a critical axis regulating the functional capacity of beta-cells. Herein, we describe and integrate recent advances in our understanding of the impact of alpha-cell paracrine signaling on insulin secretory dynamics and how this intra-islet crosstalk more broadly contributes to whole-body glucose regulation in health and under metabolic stress. Moreover, we explore how these conceptual changes in our understanding of intra-islet GLP-1 biology may impact our understanding of the mechanisms of incretin-based therapeutics.