Molecular mechanisms underlying nutrient-stimulated incretin secretion

Impact of Some Natural and Artificial Sweeteners Consumption on Different Hormonal Levels and Inflammatory Cytokines in Male Rats: In Vivo and In Silico Studies

Substituting sugar with noncaloric sweeteners prevents overweight and diabetes development. They come in two types: artificial, like aspartame and sucralose, and natural, such as sorbitol. This research aimed to assess the effects of sucrose and these sweeteners on nutritional parameters, hematological parameters, hormones, and anti- and pro-inflammatory cytokines in male rats. Thirty rats had been separated into five groups. The results showed the highest significant increase in body weight gain, total food intake, and feed efficiency noticed in the aspartame group followed by sucralose, sucrose, and sorbitol, respectively. In contrast to RBCs and platelets, all sweeteners significantly reduced the hemoglobin level, Hct %, and WBC count. The aspartame group showed the highest decline in glycoproteins, steroids, and T3, and T4 hormones and a dramatic elevation in thyroid stimulating hormone, eicosanoid, and amine hormones compared with the control group. A vigorous elevation in anti- and proinflammatory cytokine levels was observed in the aspartame group, followed by sucralose, sucrose, and sorbitol groups. Aspartame has the highest docking scores when studying the interactions of sweeteners and a target protein associated with hormones or cytokines using in silico molecular docking, with the best absorption, distribution, metabolism, elimination, and toxicity properties compared to the remaining sweeteners.

A Comprehensive Review on Weight Gain following Discontinuation of Glucagon-Like Peptide-1 Receptor Agonists for Obesity

Obesity is considered the leading public health problem in the medical sector. The phenotype includes overweight conditions that lead to several other comorbidities that drastically decrease health. Glucagon-like receptor agonists (GLP-1RAs) initially designed for treating type 2 diabetes mellitus (T2DM) had demonstrated weight loss benefits in several clinical trials. In vivo studies showed that GLP-1RA encourages reduced food consumption and consequent weight reduction by stimulating brown fat and enhancing energy outlay through the action of the sympathetic nervous system (SNS) pathways. Additionally, GLP-1RAs were found to regulate food intake through stimulation of sensory neurons in the vagus, interaction with the hypothalamus and hindbrain, and through inflammation and intestinal microbiota. However, the main concern with the use of GLP-1RA treatment was weight gain after withdrawal or discontinuation. We could identify three different ways that could lead to weight gain. Potential factors might include temporary hormonal adjustment in response to weight reduction, the central nervous system's (CNS) incompetence in regulating weight augmentation owing to the lack of GLP-1RA, and β-cell malfunction due to sustained exposure to GLP-1RA. Here, we also review the data from clinical studies that reported withdrawal symptoms. Although the use of GLP-1RA could be beneficial in multiple ways, withdrawal after years has the symptoms reversed. Clinical studies should emphasize the downside of these views we highlighted, and mechanistic studies must be carried out for a better outcome with GLP-1RA from the laboratory to the bedside.

Protective Effects of Home T2DM Treatment with Glucagon-Like Peptide-1 Receptor Agonists and Sodium-Glucose Co-transporter-2 Inhibitors Against Intensive Care Unit Admission and Mortality in the Acute Phase of the COVID-19 Pandemic: A Retrospective Observational Study in Italy

INTRODUCTION

Type 2 diabetes mellitus (T2DM) is a relevant risk factor for severe forms of COVID-19 (SARS coronavrus 2 [SARS-CoV-2] disease 2019), and calls for caution because of the high prevalence of T2DM worldwide and the high mortality rates observed in patients with T2DM who are infected with SARS-CoV-2. People with T2DM often take dipeptidyl peptidase-4 inhibitors (DPP-4is), glucagon-like peptide-1 receptor agonists (GLP-1ras), or sodium-glucose co-transporter-2 inhibitors (SGLT-2is), all of which have clear anti-inflammatory effects. The study aimed to compare (i) the severity and duration of hospital stay between patients with T2DM categorized by pre-hospitalization drug class utilization and (ii) the COVID-19-related death rates of those three groups.

METHODS

We designed an observational, retrospective, multi-center, population-based study and extracted the hospital admission data from the health care records of 1916 T2DM patients over 18 years old who were previously on GLP-1ra, SGLT-2i, or DPP-4i monotherapy and were hospitalized for COVID-19 (diagnosis based on ICD.9/10 codes) between January 2020 and December 2021 in 14 hospitals throughout Italy. We analyzed general data, pre-admission treatment schedules, date of admission or transfer to the intensive care unit (ICU) (i.e., the index date; taken as a marker of increased COVID-19 disease severity), and death (if it had occurred). Statistics analyzed the impact of drug classes on in-hospital mortality using propensity score logistic regressions for (i) those admitted to intensive care and (ii) those not admitted to intensive care, with a random match procedure used to generate a 1:1 comparison without diabetes cohort replacement for each drug therapy group by applying the nearest neighbor method. After propensity score matching, we checked the balance achieved across selected variables if a balance was ever achieved. We then used propensity score matching between the three drug classes to assemble a sample in which each patient receiving an SGLT-2i was matched to one on a GLP-1ra, and each patient on a DPP-4i was matched to one on a GLP-1ra, adjusting for covariates. We finally used GLP-1ras as references in the logistic regression.

RESULTS

The overall mortality rate (MR) of the patients was 14.29%. The MR in patients with COVID was 53.62%, and it was as high as 42.42% in the case of associated T2DM, regardless of any glucose-lowering therapy. In those on DPP-4is, there was excess mortality; in those treated with GLP-1ras and SGLT-2is, the death rate was significantly lower, i.e., almost a quarter of the overall mortality observed in COVID-19 patients with T2DM. Indeed, the odds ratio (OR) in the logistic regression resulted in an extremely high risk of in-hospital death in individuals previously treated with DPP-4is [incidence rate (IR) 4.02, 95% confidence interval (CI) 2.2-5.7) and only a slight, nonsignificantly higher risk in those previously treated with SGLT-2is (IR 1.42, 95% CI 0.6-2.1) compared to those on GLP-1ras. Moreover, the longer the stay, the higher the death rate, which ranged from 22.3% for ≤ 3-day stays to 40.3% for 4- to 14-day stays (p < 0.01 vs. the former) and 77.4% for over-14-day stays (p < 0.001 vs. both the others).

DISCUSSION

Our data do not support a protective role of DPP-4is; indeed, this role has already been questioned due to previous observations. However, the data do show a strong protective effect of SGLT-2is and GLP-1ras. Beyond lowering circulating glucose levels, those two drug classes were found to exert marked anti-phlogistic effects: SGLT-2is increased adiponectin and reduced urate, leptin, and insulin concentrations, thus positively affecting overall low-grade inflammation, and GLP-1ras may also greatly help at the lung tissue level, meaning that their extra-glycemic effects extend well beyond those acknowledged in the cardiovascular and renal fields.

CONCLUSIONS

The aforedescribed observational clinical data relating to a population of Italian inpatients with T2DM suggest that GLP-1ras and SGLT-2is can be considered antidiabetic drugs of choice against COVID-19, and might even prove beneficial in the event of any upcoming pandemic that has life-threatening effects on the pulmonary and cardiovascular systems.

Wheat Alkylresorcinols Modulate Glucose Homeostasis through Improving GLP-1 Secretion in High-Fat-Diet-Induced Obese Mice

Wheat alkylresorcinols (ARs) consumption has been evidenced to improve obesity and its associated insulin resistance. However, the effect of ARs on glucagon-like peptide 1 (GLP-1) secretion and the underlying mechanism of action are still unclear. In this study, C57BL/6J mice were fed low-fat diet (LFD), high-fat diet (HFD), and HFD supplemented with 0.4% (w/w) ARs separately for 9 weeks. The results showed that ARs intervention significantly improved glucose homeostasis and restored the serum level of GLP-1 compared with the HFD control group. Moreover, ARs treatment alleviated HFD-induced ileal epithelium damage according to TUNEL staining, immunofluorescence, and transmission electron microscopy observation. The alleviative effect was further verified by apoptosis analysis and mitochondrial function evaluation. Furthermore, palmitic acid (PA) was administered to the intestinal secretin tumor cell line (STC-1) to clarify the protective effect of ARs on GLP-1 secretion in vitro. In consistence with the results of animal studies, ARs treatment could significantly improve GLP-1 secretion in STC-1 cells compared with PA treatment alone in a dose-dependent manner, accompanied by a reduction in apoptosis and mitochondrial dysfunction. In addition, ARs treatment notably enhanced the abundance of SCFA (short-chain fatty acid)-producing bacteria, such as Bacteroides, Bifidobacterium, and Akkermansia. The increased levels of intestinal SCFAs, such as acetic acid, propionic acid, and butyric acid, improved the expression of short-chain fatty acid receptors (FFAR3) and glucagon-like peptide-1 receptor (GLP-1R), enhancing the secretion of the intestinal hormones GLP-1. Thus, this study provides potential clinical implications of whole wheat as a dietary strategy to improve glucose homeostasis for obese populations.

The Impact of GLP-1 RAs and DPP-4is on Hospitalisation and Mortality in the COVID-19 Era: A Two-Year Observational Study

Novel antidiabetic drugs have the ability to produce anti-inflammatory effects regardless of their glucose-lowering action. For this reason, these molecules (including GLP-1 RAs and DPP-4is) were hypothesized to be effective against COVID-19, which is characterized by cytokines hyperactivity and multiorgan inflammation. The aim of our work is to explore the potential protective role of GLP-1 RAs and DPP-4is in COVID-19 (with the disease intended to be a model of an acute stressor) and non-COVID-19 patients over a two-year observation period. Retrospective and one-versus-one analyses were conducted to assess the impact of antidiabetic drugs on the need for hospitalization (in both COVID-19- and non-COVID-19-related cases), in-hospital mortality, and two-year mortality. Logistic regression analyses were conducted to identify the variables associated with these outcomes. Additionally, log-rank tests were used to plot survival curves for each group of subjects, based on their antidiabetic treatment. The performed analyses revealed that despite similar hospitalization rates, subjects undergoing home therapy with GLP-1 RAs exhibited significantly lower mortality rates, even over a two-year period. These individuals demonstrated improved survival estimates both within hospital and non-hospital settings, even during a longer observation period.

DPP-4 inhibitors for treating T2DM - hype or hope? an analysis based on the current literature

DPP-4 inhibition is an interesting line of therapy for treating Type 2 Diabetes Mellitus (T2DM) and is based on promoting the incretin effect. Here, the authors have presented a brief appraisal of DPP-4 inhibitors, their modes of action, and the clinical efficiency of currently available drugs based on DPP-4 inhibitors. The safety profiles as well as future directions including their potential application in improving COVID-19 patient outcomes have also been discussed in detail. This review also highlights the existing queries and evidence gaps in DPP-4 inhibitor research. Authors have concluded that the excitement surrounding DPP-4 inhibitors is justified because in addition to controlling blood glucose level, they are good at managing risk factors associated with diabetes.

Riceberry rice (Oryza sativa L.) slows gastric emptying and improves the postprandial glycaemic response

Postprandial glycaemia is a key determinant of overall glycaemic control. One mechanism by which dietary strategies can reduce postprandial glycaemic excursions is by slowing gastric emptying. This study aimed to evaluate the acute effect of ingesting riceberry rice (RR) compared with that of ingesting white rice (WR) on gastric emptying rate (GER), plasma glucose and glucose-regulating hormones, including insulin, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1), in healthy subjects. A randomised, open-label, within-subject, crossover study was performed in six healthy men. GER was measured by scintigraphy over 240 min, and plasma concentrations of glucose, insulin, GLP-1 and GIP were measured at multiple time points over 180 min. This study revealed that RR slows GER with a reduction in postprandial plasma glucose concentrations compared with WR. Plasma insulin and GLP-1 concentrations did not differ between RR and WR. However, plasma GIP concentrations were markedly increased after WR ingesting v. after RR ingestion. We conclude that RR attenuates postprandial glycaemia by slowing GER without altering plasma insulin or GLP-1. Plasma GIP concentrations are likely related to differences in GER and carbohydrate absorption. We propose that dietary fibre-enriched foods, including RR, could contribute to improvement in postprandial glycaemia via delayed gastric emptying.

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.

Colonic Lactulose Fermentation Has No Impact on Glucagon-like Peptide-1 and Peptide-YY Secretion in Healthy Young Men

CONTEXT

The colon houses most of humans' gut microbiota, which ferments indigestible carbohydrates. The products of fermentation have been proposed to influence the secretion of glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) from the many endocrine cells in the colonic epithelium. However, little is known about the colonic contribution to fasting or postprandial plasma levels of L-cell products.

OBJECTIVE

To determine the impact of colonic lactulose fermentation on gut peptide secretion and to evaluate whether colonic endocrine secretion contributes to gut hormone concentrations measurable in the fasting state.

METHODS

Ten healthy young men were studied on 3 occasions after an overnight fast. On 2 study days, lactulose (20 g) was given orally and compared to water intake on a third study day. For 1 of the lactulose visits, participants underwent a full colonic evacuation. Over a 6-h study protocol, lactulose fermentation was assessed by measuring exhaled hydrogen, and gut peptide secretion, paracetamol, and short-chain fatty acid levels were measured in plasma.

RESULTS

Colonic evacuation markedly reduced hydrogen exhalation after lactulose intake (P = 0.013). Our analysis suggests that the colon does not account for the measurable amounts of GLP-1 and PYY present in the circulation during fasting and that fermentation and peptide secretion are not acutely related.

CONCLUSION

Whether colonic luminal contents affect colonic L-cell secretion sufficiently to influence circulating concentrations requires further investigation. Colonic evacuation markedly reduced lactulose fermentation, but hormone releases were unchanged in the present study.

Incretin Hormones in Obesity and Related Cardiometabolic Disorders: The Clinical Perspective

The prevalence of obesity continues to grow rapidly worldwide, posing many public health challenges of the 21st century. Obese subjects are at major risk for serious diet-related noncommunicable diseases, including type 2 diabetes mellitus, cardiovascular disease, and non-alcoholic fatty liver disease. Understanding the mechanisms underlying obesity pathogenesis is needed for the development of effective treatment strategies. Dysregulation of incretin secretion and actions has been observed in obesity and related metabolic disorders; therefore, incretin-based therapies have been developed to provide new therapeutic options. Incretin mimetics present glucose-lowering properties, together with a reduction of appetite and food intake, resulting in weight loss. In this review, we describe the physiology of two known incretins—glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and their role in obesity and related cardiometabolic disorders. We also focus on the available and incoming incretin-based medications that can be used in the treatment of the above-mentioned conditions.

S1P Signaling Pathways in Pathogenesis of Type 2 Diabetes

The pathogenesis of type 2 diabetes mellitus (T2DM) is very complicated. The currently well-accepted etiology is the "Ominous Octet" theory proposed by Professor Defronzo. Since presently used drugs for T2DM have limitations and harmful side effects, studies regarding alternative treatments are being conducted. Analyzing the pharmacological mechanism of biomolecules in view of pathogenesis is an effective way to assess new drugs. Sphingosine 1 phosphate (S1P), an endogenous lipid substance in the human body, has attracted increasing attention in the T2DM research field. This article reviews recent study updates of S1P, summarizing its effects on T2DM with respect to pathogenesis, promoting β cell proliferation and inhibiting apoptosis, reducing insulin resistance, protecting the liver and pancreas from lipotoxic damage, improving intestinal incretin effects, lowering basal glucagon levels, etc. With increasing research, S1P may help treat and prevent T2DM in the future.

The role of diet in the pathophysiology and management of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic gastrointestinal (GI) disorder that reportedly affects 5% to 20% of the world population. The etiology of IBS is not completely understood, but diet appears to play an important role in its pathophysiology. Asian diets differ considerably from those in Western countries, which might explain differences in the prevalence, sex, and clinical presentation seen between patients with IBS in Asian and Western countries. Dietary regimes such as a low-fermentable oligo-, di-, monosaccharides, and polyols (FODMAP) diet and the modified National Institute for Health and Care Excellence (NICE) diet improve both symptoms and the quality of life in a considerable proportion of IBS patients. It has been speculated that diet is a prebiotic for the intestinal microbiota and favors the growth of certain bacteria. These bacteria ferment the dietary components, and the products of fermentation act upon intestinal stem cells to influence their differentiation into enteroendocrine cells. The resulting low density of enteroendocrine cells accompanied by low levels of certain hormones gives rise to intestinal dysmotility, visceral hypersensitivity, and abnormal secretion. This hypothesis is supported by the finding that changing to a low-FODMAP diet restores the density of GI cells to the levels in healthy subjects. These changes in gut endocrine cells caused by low-FODMAP diet are also accompanied by improvements in symptoms and the quality of life.

Gut Hormone GIP Induces Inflammation and Insulin Resistance in the Hypothalamus

The hypothalamus plays a critical role in controlling energy balance. High-fat diet (HFD) feeding increases the gene expression of proinflammatory mediators and decreases insulin actions in the hypothalamus. Here, we show that a gut-derived hormone, glucose-dependent insulinotropic polypeptide (GIP), whose levels are elevated during diet-induced obesity, promotes and mediates hypothalamic inflammation and insulin resistance during HFD-induced obesity. Unbiased ribonucleic acid sequencing of GIP-stimulated hypothalami revealed that hypothalamic pathways most affected by intracerebroventricular (ICV) GIP stimulation were related to inflammatory-related responses. Subsequent analysis demonstrated that GIP administered either peripherally or centrally, increased proinflammatory-related factors such as Il-6 and Socs3 in the hypothalamus, but not in the cortex of C57BL/6J male mice. Consistently, hypothalamic activation of IκB kinase-β inflammatory signaling was induced by ICV GIP. Further, hypothalamic levels of proinflammatory cytokines and Socs3 were significantly reduced by an antagonistic GIP receptor (GIPR) antibody and by GIPR deficiency. Additionally, centrally administered GIP reduced anorectic actions of insulin in the brain and diminished insulin-induced phosphorylation of Protein kinase B and Glycogen synthase kinase 3β in the hypothalamus. Collectively, these findings reveal a previously unrecognized role for brain GIP signaling in diet-induced inflammation and insulin resistance in the hypothalamus.

Alleviation of Depression by Glucagon-Like Peptide 1 Through the Regulation of Neuroinflammation, Neurotransmitters, Neurogenesis, and Synaptic Function

Depression has emerged as a major cause of mortality globally. Many studies have reported risk factors and mechanisms associated with depression, but it is as yet unclear how these findings can be applied to the treatment and prevention of this disorder. The onset and recurrence of depression have been linked to diverse metabolic factors, including hyperglycemia, dyslipidemia, and insulin resistance. Recent studies have suggested that depression is accompanied by memory loss as well as depressive mood. Thus, many researchers have highlighted the relationship between depressive behavior and metabolic alterations from various perspectives. Glucagon-like peptide-1 (GLP-1), which is secreted from gut cells and hindbrain areas, has been studied in metabolic diseases such as obesity and diabetes, and was shown to control glucose metabolism and insulin resistance. Recently, GLP-1 was highlighted as a regulator of diverse pathways, but its potential as the therapeutic target of depressive disorder was not described comprehensively. Therefore, in this review, we focused on the potential of GLP-1 modulation in depression.

Gut-Pancreas-Liver Axis as a Target for Treatment of NAFLD/NASH

Non-alcoholic fatty liver disease (NAFLD) represents the most common form of chronic liver disease worldwide. Due to its association with obesity and diabetes and the fall in hepatitis C virus morbidity, cirrhosis in NAFLD is becoming the most frequent indication to liver transplantation, but the pathogenetic mechanisms are still not completely understood. The so-called gut-liver axis has gained enormous interest when data showed that its alteration can lead to NAFLD development and might favor the occurrence of non-alcoholic steatohepatitis (NASH). Moreover, several therapeutic approaches targeting the gut-pancreas-liver axis, e.g., incretins, showed promising results in NASH treatment. In this review, we describe the role of incretin hormones in NAFLD/NASH pathogenesis and treatment and how metagenomic/metabolomic alterations in the gut microbiota can lead to NASH in the presence of gut barrier modifications favoring the passage of bacteria or bacterial products in the portal circulation, i.e., bacterial translocation.

Possible role of intestinal stem cells in the pathophysiology of irritable bowel syndrome

The pathophysiology of irritable bowel syndrome (IBS) is not completely understood. However, several factors are known to play a role in pathophysiology of IBS such as genetics, diet, gut microbiota, gut endocrine cells, stress and low-grade inflammation. Understanding the pathophysiology of IBS may open the way for new treatment approaches. Low density of intestinal stem cells and low differentiation toward enteroendocrine cells has been reported recently in patients with IBS. These abnormalities are believed to be the cause of the low density of enteroendocrine cells seen in patients with IBS. Enteroendocrine cells regulate gastrointestinal motility, secretion, absorption and visceral sensitivity. Gastrointestinal dysmotility, abnormal absorption/secretion and visceral hypersensitivity are all seen in patients with IBS and haven been attributed to the low density the intestinal enteroendocrine cells in these patients. The present review conducted a literature search in Medline (PubMed) covering the last ten years until November 2019, where articles in English were included. Articles about the intestinal stem cells and their possible role in the pathophysiology of IBS are discussed in the present review. The present review discusses the assumption that intestinal stem cells play a central role in the pathophysiology of IBS and that the other factors known to contribute to the pathophysiology of IBS such as genetics, diet gut microbiota, stress, and low-grade inflammation exert their effects through affecting the intestinal stem cells. It reports further the data that support this assumption on genetics, diet, gut microbiota, stress with depletion of glutamine, and inflammation.

The Race to Bash NASH: Emerging Targets and Drug Development in a Complex Liver Disease

Nonalcoholic steatohepatitis (NASH) is a severe form of nonalcoholic fatty liver disease (NAFLD) characterized by liver steatosis, inflammation, and hepatocellular damage. NASH is a serious condition that can progress to cirrhosis, liver failure, and hepatocellular carcinoma. The association of NASH with obesity, type 2 diabetes mellitus, and dyslipidemia has led to an emerging picture of NASH as the liver manifestation of metabolic syndrome. Although diet and exercise can dramatically improve NASH outcomes, significant lifestyle changes can be challenging to sustain. Pharmaceutical therapies could be an important addition to care, but currently none are approved for NASH. Here, we review the most promising targets for NASH treatment, along with the most advanced therapeutics in development. These include targets involved in metabolism (e.g., sugar, lipid, and cholesterol metabolism), inflammation, and fibrosis. Ultimately, combination therapies addressing multiple aspects of NASH pathogenesis are expected to provide benefit for patients.

Intestinal Fructose and Glucose Metabolism in Health and Disease

The worldwide epidemics of obesity and diabetes have been linked to increased sugar consumption in humans. Here, we review fructose and glucose metabolism, as well as potential molecular mechanisms by which excessive sugar consumption is associated to metabolic diseases and insulin resistance in humans. To this end, we focus on understanding molecular and cellular mechanisms of fructose and glucose transport and sensing in the intestine, the intracellular signaling effects of dietary sugar metabolism, and its impact on glucose homeostasis in health and disease. Finally, the peripheral and central effects of dietary sugars on the gut-brain axis will be reviewed.

Incretin Hormones: The Link between Glycemic Index and Cardiometabolic Diseases

This review aimed to describe the potential mechanisms by which incretin hormones could mediate the relationship between glycemic index and cardiometabolic diseases. A body of evidence from many studies suggests that low glycemic index (GI) diets reduces the risk for type 2 diabetes and coronary heart disease. In fact, despite the extensive literature on this topic, the mechanisms underlying unfavorable effects of high GI foods on health remain not well defined. The postprandial and hormonal milieu could play a key role in the relationship between GI and cardiovascular risk. Incretin hormones, glucagon-like peptide1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), are important regulators of postprandial homeostasis by amplifying insulin secretory responses. Response of GIP and GLP-1 to GI have been studied more in depth, also by several studies on isomaltulose, which have been taken as an ideal model to investigate the kinetics of incretin secretion in response to foods’ GI. In addition, extrapancreatic effects of these incretin hormones were also recently observed. Emerging from this have been exciting effects on several targets, such as body weight regulation, lipid metabolism, white adipose tissue, cardiovascular system, kidney, and liver, which may importantly affect the health status.

Gut-derived GIP activates central Rap1 to impair neural leptin sensitivity during overnutrition

Nutrient excess, a major driver of obesity, diminishes hypothalamic responses to exogenously administered leptin, a critical hormone of energy balance. Here, we aimed to identify a physiological signal that arises from excess caloric intake and negatively controls hypothalamic leptin action. We found that deficiency of the gastric inhibitory polypeptide receptor (Gipr) for the gut-derived incretin hormone GIP protected against diet-induced neural leptin resistance. Furthermore, a centrally administered antibody that neutralizes GIPR had remarkable antiobesity effects in diet-induced obese mice, including reduced body weight and adiposity, and a decreased hypothalamic level of SOCS3, an inhibitor of leptin actions. In contrast, centrally administered GIP diminished hypothalamic sensitivity to leptin and increased hypothalamic levels of Socs3. Finally, we show that GIP increased the active form of the small GTPase Rap1 in the brain and that its activation was required for the central actions of GIP. Altogether, our results identify GIPR/Rap1 signaling in the brain as a molecular pathway linking overnutrition to the control of neural leptin actions.

Diet in Irritable Bowel Syndrome (IBS): Interaction with Gut Microbiota and Gut Hormones

Diet plays an important role not only in the pathophysiology of irritable bowel syndrome (IBS), but also as a tool that improves symptoms and quality of life. The effects of diet seem to be a result of an interaction with the gut bacteria and the gut endocrine cells. The density of gut endocrine cells is low in IBS patients, and it is believed that this abnormality is the direct cause of the symptoms seen in IBS patients. The low density of gut endocrine cells is probably caused by a low number of stem cells and low differentiation progeny toward endocrine cells. A low fermentable oligo-, di-, monosaccharide, and polyol (FODMAP) diet and fecal microbiota transplantation (FMT) restore the gut endocrine cells to the level of healthy subjects. It has been suggested that our diet acts as a prebiotic that favors the growth of a certain types of bacteria. Diet also acts as a substrate for gut bacteria fermentation, which results in several by-products. These by-products might act on the stem cells in such a way that the gut stem cells decrease, and consequently, endocrine cell numbers decrease. Changing to a low-FODMAP diet or changing the gut bacteria through FMT improves IBS symptoms and restores the density of endocrine cells.

Bile acids drive colonic secretion of glucagon-like-peptide 1 and peptide-YY in rodents

A large number of glucagon-like-peptide-1 (GLP-1)- and peptide-YY (PYY)-producing L cells are located in the colon, but little is known about their contribution to whole body metabolism. Since bile acids (BAs) increase GLP-1 and PYY release, and since BAs spill over from the ileum to the colon, we decided to investigate the ability of BAs to stimulate colonic GLP-1 and PYY secretion. Using isolated perfused rat/mouse colon as well as stimulation of the rat colon in vivo, we demonstrate that BAs significantly enhance secretion of GLP-1 and PYY from the colon with average increases of 3.5- and 2.9-fold, respectively. Furthermore, we find that responses depend on BA absorption followed by basolateral activation of the BA-receptor Takeda-G protein-coupled-receptor 5. Surprisingly, the apical sodium-dependent BA transporter, which serves to absorb conjugated BAs, was not required for colonic conjugated BA absorption or conjugated BA-induced peptide secretion. In conclusion, we demonstrate that BAs represent a major physiological stimulus for colonic L-cell secretion. NEW & NOTEWORTHY By the use of isolated perfused rodent colon preparations we show that bile acids are potent and direct promoters of colonic glucagon-like-peptide 1 and peptide-YY secretion. The study provides convincing evidence that basolateral Takeda-G protein-coupled-receptor 5 activation is mediating the effects of bile acids in the colon and thus add to the existing literature described for L cells in the ileum.

Sensing of L-Arginine by Gut-Expressed Calcium Sensing Receptor Stimulates Gut Satiety Hormones Cholecystokinin and Glucose-Dependent Insulinotropic Peptide Secretion in Pig Model

Nutrients regulate the secretion of gut satiety hormones, which is related to the modulation of food intake and blood glucose levels. Calcium-sensing receptor (CaSR) is involved in regulating gut hormone secretion in response to l-amino acids and multivalent cations. Rodents are often used to investigate the effect of nutrients on these hormonal release. However, results obtained using rodent models are difficult to be applied in humans, we used pigs as a model in this study because their physiology is similar to that of humans. In this study, we investigated whether l-Arginine (l-Arg) could induce gut hormones cholecystokinin (CCK) and glucose-dependent insulinotropic peptide (GIP) secretion in the porcine duodenum and if so, whether CaSR mediated l-Arg-regulated gut satiety hormone secretion. Our data showed that treatment with 20 and 50 mM l-Arg induced CCK and GIP secretion compared with 0 mM l-Arg. However, treatment with d-Arg (an inactive isomer) failed to elicit this response. The potency of l-Arg to induce CCK and GIP secretion was enhanced in the presence of extracellular Ca²⁺ and CaSR agonist cinacalcet. However, the effect of Arg on CCK and GIP secretion was attenuated by blocking CaSR and its downstream signaling molecules adenylate cyclase (AC) and phospholipase C (PLC). Taken all together, pig duodenum provides an appropriate model to explore the effects of l-Arg on the secretion of the satiety-related gut hormones CCK and GIP and the role of CaSR in this effect. Further investigations are needed to verify the effect of l-Arg on food intake and blood glucose in human study. PRACTICAL APPLICATION: l-Arginine is able to modulate cholecystokinin and glucose-dependent insulinotropic peptide secretion through the CaSR in pig model, which has a potential role in regulating food intake and blood glucose levels.

The impact of short-chain fatty acids on GLP-1 and PYY secretion from the isolated perfused rat colon

The colonic epithelium harbors a large number of endocrine cells, but little is known about the endocrine functions of the colon. However, the high density of glucagon like peptide-1 (GLP-1)- and peptide-YY (PYY)-secreting L cells is of great interest because of the potential antidiabetic and antiobesity effects of GLP-1 and PYY. Short-chain fatty acids (SCFAs) produced by local bacterial fermentation are suggested to activate the colonic free fatty acid receptors FFAR2 (GPR43) and FFAR3 (GPR41), stimulating the colonic L cells. We used the isolated perfused rat colon as a model of colonic endocrine secretion and studied the effects of the predominant SCFAs formed: acetate, propionate, and butyrate. We show that luminal and especially vascular infusion of acetate and butyrate significantly increases colonic GLP-1 secretion, and to a minor extent also PYY secretion, but only after enhancement of intracellular cAMP. Propionate neither affected GLP-1 nor PYY secretion whether administered luminally or vascularly. A FFAR2- and FFAR3-specific agonist [( S)-2-(4-chlorophenyl)-3,3-dimethyl- N-(5-phenylthiazol-2-yl)butamide (CFMB)/ AR420626 ] had no effect on colonic GLP-1 output, and a FFAR3 antagonist ( AR399519 ) did not decrease the SCFA-induced GLP-1 response. However, the voltage-gated Ca²⁺-channel blocker nifedipine, the K_ATP-channel opener diazoxide, and the ATP synthesis inhibitor 2,4-dinitrophenol completely abolished the responses. FFAR2 receptor studies confirmed low-potent partial agonism of acetate, propionate, and butyrate, compared with CFMB, which is a full agonist with ~750-fold higher potency than the SCFAs. In conclusion, SCFAs may increase colonic GLP-1/PYY secretion, but FFAR2/FFAR3 do not seem to be involved. Rather, SCFAs are metabolized and appear to function as a colonocyte energy source. NEW & NOTEWORTHY By the use of in situ isolated perfused rat colon we show that short-chain fatty acids (SCFAs) primarily are used as a colonocyte energy source in the rat, subsequently triggering glucagon like peptide-1 (GLP-1) secretion independent of the free fatty acid receptors FFAR2 and FFAR3. Opposite many previous studies on SCFAs and FFAR2/FFAR3 and GLP-1 secretion, this experimental model allows investigation of the physiological interactions between luminal nutrients and secretion from cells whose function depend critically on their blood supply as well as nerve and paracrine interactions.

Chronic Consumption of Sweeteners and Its Effect on Glycaemia, Cytokines, Hormones, and Lymphocytes of GALT in CD1 Mice

BACKGROUND

The consumption of sweeteners has increased in recent years, being used to control body weight and blood glucose. However, they can cause increased appetite, modification of immune function, and secretion of hormones in the GALT.

OBJECTIVE

To assess the effect of chronic sweetener consumption on glycaemia, cytokines, hormones, and GALT lymphocytes in CD1 mice.

MATERIAL AND METHODS

72 CD1 mice divided into 3 groups were used: (a) baseline, (b) middle, and (c) final. Groups (b) and (c) were divided into 4 subgroups: (i) Control, (ii) Sucrose, (iii) Sucralose, and (iv) Stevia. The following were determined: body weight, hormones (GIP, insulin, and leptin), lymphocytes CD3+T cells and CD19+B cells, IgA+ plasma cells, and cytokines (IL-4, IL-5, IFN-γ, and TNF-α).

RESULTS

Sucralose reduces secretion of GIP and glycaemia but does not modify insulin concentration, increases body weight, and reduces food intake. Stevia increases the secretion of GIP, insulin, leptin, body weight, and glycaemia but keeps food consumption normal. Sucralose and Stevia showed a higher percentage of CD3+T cells, CD19+B cells, and IgA+ plasma cells in Peyer's patches, but only Stevia in lamina propria.

CONCLUSION

Sweeteners modulate the hormonal response of cytokines and the proliferation of lymphocytes in the intestinal mucosa.

Incretin hormones: Their role in health and disease

Incretin hormones are gut peptides that are secreted after nutrient intake and stimulate insulin secretion together with hyperglycaemia. GIP (glucose-dependent insulinotropic polypeptide) und GLP-1 (glucagon-like peptide-1) are the known incretin hormones from the upper (GIP, K cells) and lower (GLP-1, L cells) gut. Together, they are responsible for the incretin effect: a two- to three-fold higher insulin secretory response to oral as compared to intravenous glucose administration. In subjects with type 2 diabetes, this incretin effect is diminished or no longer present. This is the consequence of a substantially reduced effectiveness of GIP on the diabetic endocrine pancreas, and of the negligible physiological role of GLP-1 in mediating the incretin effect even in healthy subjects. However, the insulinotropic and glucagonostatic effects of GLP-1 are preserved in subjects with type 2 diabetes to the degree that pharmacological stimulation of GLP-1 receptors significantly reduces plasma glucose and improves glycaemic control. Thus, it has become a parent compound of incretin-based glucose-lowering medications (GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase-4 or DPP-4). GLP-1, in addition, has multiple effects on various organ systems. Most relevant are a reduction in appetite and food intake, leading to weight loss in the long term. Since GLP-1 secretion from the gut seems to be impaired in obese subjects, this may even indicate a role in the pathophysiology of obesity. Along these lines, an increased secretion of GLP-1 induced by delivering nutrients to lower parts of the small intestines (rich in L cells) may be one factor (among others like peptide YY) explaining weight loss and improvements in glycaemic control after bariatric surgery (e.g., Roux-en-Y gastric bypass). GIP and GLP-1, originally characterized as incretin hormones, have additional effects in adipose cells, bone, and the cardiovascular system. Especially, the latter have received attention based on recent findings that GLP-1 receptor agonists such as liraglutide reduce cardiovascular events and prolong life in high-risk patients with type 2 diabetes. Thus, incretin hormones have an important role physiologically, namely they are involved in the pathophysiology of obesity and type 2 diabetes, and they have therapeutic potential that can be traced to well-characterized physiological effects.

Effect of diet and individual dietary guidance on gastrointestinal endocrine cells in patients with irritable bowel syndrome (Review)

Irritable bowel syndrome (IBS) is a common chronic gastrointestinal (GI) disorder that is characterized by a combination of abdominal pain or discomfort, bloating and alterations in bowel movements. This review presents recent developments concerning the roles of diet and GI endocrine cells in the pathophysiology of IBS and of individual dietary guidance in the management of IBS. Patients with IBS typically report that food aggravates their IBS symptoms. The interactions between specific types of foodstuffs rich in fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAPs) and GI endocrine cells induce changes in cell densities. Providing individual dietary guidance about a low FODMAP intake, high soluble-fiber intake, and changing the proportions of protein, fat and carbohydrates helps to reduce the symptoms experienced by patients with IBS and to improve their quality of life. These improvements are due to restoring the densities of the GI endocrine cells back to normal. The reported observations emphasize the role of GI endocrine cells in the pathophysiology of IBS and support the provision of dietary guidance as a first-line treatment for managing IBS.

Role of the Gut on Glucose Homeostasis: Lesson Learned from Metabolic Surgery

Purpose of Review

Bariatric surgery was initially intended to reduce weight, and only subsequently was the remission of type two diabetes (T2D) observed as a collateral event. At the moment, the term “metabolic surgery” is used to underline the fact that this type of surgery is performed specifically to treat diabetes and its metabolic complications, such as hyperlipidemia.

Recent Findings

Randomized, controlled studies have recently supported the use of bariatric surgery, and in particular of Roux-en-Y gastric bypass (RYGB) and biliopancreatic diversion (BPD) as an effective treatment for decompensated T2D. The lesson learned from these randomized and many other non-randomized clinical studies is that the stomach and the small intestine play a central role in glucose homeostasis. Bypassing the duodenum and parts of the jejunum exerts a substantial effect on insulin sensitivity and secretion. In fact, with BPD, nutrient transit bypasses duodenum, the entire jejunum and a small portion of the ileum, resulting in reversal of insulin sensitivity back to normal and reduction of insulin secretion, whereas RYGB has little effect on insulin resistance but increases insulin secretion. Hypotheses concerning the mechanism of action of metabolic surgery for diabetes remission vary from theories focusing on jejunal nutrient sensing, to incretin action, to the blunted secretion of putative insulin resistance hormone(s), to changes in the microbiota.

Summary

Whatever the mechanism, metabolic surgery has the undoubted merit of exposing the central role of the small intestine in insulin sensitivity and glucose homeostasis.

Gastrointestinal neuroendocrine peptides/amines in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic recurrent condition whose etiology is unknown, and it includes ulcerative colitis, Crohn’s disease, and microscopic colitis. These three diseases differ in clinical manifestations, courses, and prognoses. IBD reduces the patients’ quality of life and is an economic burden to both the patients and society. Interactions between the gastrointestinal (GI) neuroendocrine peptides/amines (NEPA) and the immune system are believed to play an important role in the pathophysiology of IBD. Moreover, the interaction between GI NEPA and intestinal microbiota appears to play also a pivotal role in the pathophysiology of IBD. This review summarizes the available data on GI NEPA in IBD, and speculates on their possible role in the pathophysiology and the potential use of this information when developing treatments. GI NEPA serotonin, the neuropeptide Y family, and substance P are proinflammatory, while the chromogranin/secretogranin family, vasoactive intestinal peptide, somatostatin, and ghrelin are anti-inflammatory. Several innate and adaptive immune cells express these NEPA and/or have receptors to them. The GI NEPA are affected in patients with IBD and in animal models of human IBD. The GI NEPA are potentially useful for the diagnosis and follow-up of the activity of IBD, and are candidate targets for treatments of this disease.

Dietary fiber in irritable bowel syndrome (Review)

Irritable bowel syndrome (IBS) is a common chronic gastrointestinal disorder. It is widely believed that IBS is caused by a deficient intake of dietary fiber, and most physicians recommend that patients with IBS increase their intake of dietary fiber in order to relieve their symptoms. However, different types of dietary fiber exhibit marked differences in physical and chemical properties, and the associated health benefits are specific for each fiber type. Short-chain soluble and highly fermentable dietary fiber, such as oligosaccharides results in rapid gas production that can cause abdominal pain/discomfort, abdominal bloating/distension and flatulence in patients with IBS. By contrast, long-chain, intermediate viscous, soluble and moderately fermentable dietary fiber, such as psyllium results in a low gas production and the absence of the symptoms related to excessive gas production. The effects of type of fiber have been documented in the management of IBS, and it is known to improve the overall symptoms in patients with IBS. Dietary fiber acts on the gastrointestinal tract through several mechanisms, including increased fecal mass with mechanical stimulation/irritation of the colonic mucosa with increasing secretion and peristalsis, and the actions of fermentation byproducts, particularly short-chain fatty acids, on the intestinal microbiota, immune system and the neuroendocrine system of the gastrointestinal tract. Fiber supplementation, particularly psyllium, is both safe and effective in improving IBS symptoms globally. Dietary fiber also has other health benefits, such as lowering blood cholesterol levels, improving glycemic control and body weight management.

Changes in duodenal enteroendocrine cells in patients with irritable bowel syndrome following dietary guidance

The densities of enteroendocrine cells are abnormal in patients with irritable bowel syndrome (IBS); however, they tend to change toward normal levels in stomach, ileum, and colon following dietary guidance. The aim was to identify the types of duodenal enteroendocrine cells affected after receiving dietary guidance in the same group of patients with IBS. Fourteen patients with IBS and 14 control subjects were included. The patients received three sessions of dietary guidance. Both groups underwent gastroscopies at baseline, and again for the patients after 3–9 months (median, four months) from receiving dietary guidance. Tissue biopsies were collected from the descending part of the duodenum and were immunostained for all the types of enteroendocrine cells and were then quantified by using computerized image analysis. Using the Kruskal–Wallis non-parametric test with Dunn’s test as a post-test, the results showed a significant difference in the secretin cell densities between control subjects and patients with IBS prior to and following dietary guidance (P = 0.0001 and 0.011, respectively). The corresponding P values for cholecystokinin (CCK) cell densities were 0.03 and 0.42, respectively; gastric inhibitory peptide (GIP) cell densities were 0.06 and 0.43, respectively; serotonin cell densities were <0.0001 and 0.002, respectively; and for somatostatin cell densities were <0.0001 and 0.052, respectively. The Paired t-test showed a significant difference only in the serotonin (P = 0.03) and somatostatin (P < 0.0001) cell densities between IBS patients prior to and following dietary guidance. The changes in the cell densities of secretin, CCK, and GIP were not significant between IBS patients prior to and following dietary guidance. In conclusion, the densities of several duodenal enteroendocrine cells in IBS patients changed toward the values measured in control subjects following dietary guidance. The changes in serotonin and somatostatin cell densities may have contributed to the improvements in IBS symptoms, particularly pain and diarrhea.

The possible role of gastrointestinal endocrine cells in the pathophysiology of irritable bowel syndrome

The etiology of irritable bowel syndrome (IBS) is unknown, but several factors appear to play a role in its pathophysiology, including abnormalities of the gastrointestinal endocrine cells. The present review illuminates the possible role of gastrointestinal hormones in the pathophysiology of IBS and the possibility of utilizing the current knowledge in treating the disease. Areas covered: Research into the intestinal endocrine cells and their possible role in the pathophysiology of IBS is discussed. Furthermore, the mechanisms underlying the abnormalities in the gastrointestinal endocrine cells in IBS patients are revealed. Expert commentary: The abnormalities observed in the gastrointestinal endocrine cells in IBS patients explains their visceral hypersensitivity, gastrointestinal dysmotility, and abnormal intestinal secretion, as well as the interchangeability of symptoms over time. Clarifying the role of the intestinal stem cells in the pathophysiology of IBS may lead to new treatment methods for IBS.

Spatiotemporal Modeling of Triggering and Amplifying Pathways in GLP-1 Secreting Intestinal L Cells

Glucagon-like peptide 1 (GLP-1) is secreted by intestinal L-cells, and augments glucose-induced insulin secretion, thus playing an important role in glucose control. The stimulus-secretion pathway in L-cells is still incompletely understood and a topic of debate. It is known that GLP-1 secreting cells can sense glucose to promote electrical activity either by the electrogenic sodium-glucose cotransporter SGLT1, or by closure of ATP-sensitive potassium channels after glucose metabolism. Glucose also has an effect on GLP-1 secretion downstream of electrical activity. An important aspect to take into account is the spatial organization of the cell. Indeed, the glucose transporter GLUT2 is located at the basolateral, vascular side, while SGLT1 is exposed to luminal glucose at the apical side of the cell, suggesting that the two types of transporters play different roles in glucose sensing. Here, we extend our recent model of electrical activity in primary L-cells to include spatiotemporal glucose and Ca2+ dynamics, and GLP-1 secretion. The model confirmed that glucose transportation into the cell through SGLT1 cotransporters can induce Ca2+ influx and release of GLP-1 as a result of electrical activity, while glucose metabolism alone is insufficient to depolarize the cell and evoke GLP-1 secretion in the model, suggesting a crucial role for SGLT1 in triggering GLP-1 release in agreement with experimental studies. We suggest a secondary, but participating, role of GLUT2 and glucose metabolism for GLP-1 secretion via an amplifying pathway that increases the secretion rate at a given Ca2+ level.

Dietary guidance and ileal enteroendocrine cells in patients with irritable bowel syndrome

The enteroendocrine cells of the ileum are stimulated by the luminal contents to release specific hormones that regulate its functions. The density of ileal enteroendocrine cells is abnormal in patients with irritable bowel syndrome (IBS), and the majority of patients with IBS associate their symptoms to the consumption of certain foodstuffs. The present study investigated the effect of dietary guidance on the enteroendocrine cells of the ileum in 11 patients with IBS. A total of 10 control subjects were also included. Each patient received three sessions of dietary guidance. Colonoscopies were performed on both controls and patients with IBS (at baseline and 3-9 months after the patients had received dietary guidance). Biopsy samples from the ileum were immunostained for all enteroendocrine cells and quantified by computerized image analysis. The densities of serotonin-immunoreactive cells in controls and in patients with IBS prior to and following dietary guidance were 35.5±5.7, 38.7±7.1 and 22.3±2.6 cells/mm², respectively (mean ± standard error of the mean; P=0.046); the corresponding values for PYY-immunoreactive cells were 16.7±2.8, 20.2±5.1 and 21.3±2.7 cells/mm² (P=0.86). These results suggest that changes in enteroendocrine cell densities in the ileum along with changes in enteroendocrine cells throughout the gastrointestinal tract may contribute to the improvement in IBS symptoms following dietary guidance.

Allosteric Optical Control of a Class B G-Protein-Coupled Receptor

Allosteric regulation promises to open up new therapeutic avenues by increasing drug specificity at G-protein-coupled receptors (GPCRs). However, drug discovery efforts are at present hampered by an inability to precisely control the allosteric site. Herein, we describe the design, synthesis, and testing of PhotoETP, a light-activated positive allosteric modulator of the glucagon-like peptide-1 receptor (GLP-1R), a class B GPCR involved in the maintenance of glucose homeostasis in humans. PhotoETP potentiates Ca(2+) , cAMP, and insulin responses to glucagon-like peptide-1 and its metabolites following illumination of cells with blue light. PhotoETP thus provides a blueprint for the production of small-molecule class B GPCR allosteric photoswitches, and may represent a useful tool for understanding positive cooperativity at the GLP-1R.

Interaction between diet and gastrointestinal endocrine cells

The gastrointestinal endocrine cells are essential for life. They regulate the gastrointestinal motility, secretion, visceral sensitivity, absorption, local immune defense, cell proliferation and appetite. These cells act as sensory cells with specialized microvilli that project into the lumen that sense the gut contents (mostly nutrients and/or bacteria byproducts), and respond to luminal stimuli by releasing hormones into the lamina propria. These released hormones exert their actions by entering the circulating blood and reaching distant targets (endocrine mode), nearby structures (paracrine mode) or via afferent and efferent synaptic transmission. The mature intestinal endocrine cells are capable of expressing several hormones. A change in diet not only affects the release of gastrointestinal hormones, but also alters the densities of the gut endocrine cells. The interaction between ingested foodstuffs and the gastrointestinal endocrine cells can be utilized for the clinical management of gastrointestinal and metabolic diseases, such as irritable bowel syndrome, obesity and diabetes.

Altered expression of uncoupling protein 2 in GLP-1-producing cells after chronic high glucose exposure: implications for the pathogenesis of diabetes mellitus

Glucagon-like peptide-1 (GLP-1) is a gut L-cell hormone that enhances glucose-stimulated insulin secretion. Several approaches that prevent GLP-1 degradation or activate the GLP-1 receptor are being used to treat type 2 diabetes mellitus (T2DM) patients. In T2DM, GLP-1 secretion has been suggested to be impaired, and this defect appears to be a consequence rather than a cause of impaired glucose homeostasis. However, although defective GLP-1 secretion has been correlated with insulin resistance, little is known about the direct effects of chronic high glucose concentrations, which are typical in diabetes patients, on GLP-1-secreting cell function. In the present study, we demonstrate that glucotoxicity directly affects GLP-1 secretion in GLUTag cells chronically exposed to high glucose. Our results indicate that this abnormality is associated with a decrease in ATP production due to the elevated expression of mitochondrial uncoupling protein 2 (UCP2). Furthermore, UCP2 inhibition using small interfering RNA (siRNA) and the application of glibenclamide, an ATP-sensitive potassium (KATP+) channel blocker, reverse the GLP-1 secretion defect induced by chronic high-glucose treatment. These results show that glucotoxicity diminishes the secretory responsiveness of GLP-1-secreting cells to acute glucose stimulation. We conclude that the loss of the incretin effect, as observed in T2DM patients, could at least partially depend on hyperglycemia, which is typical in diabetes patients. Such an understanding may not only provide new insight into diabetes complications but also ultimately contribute to the identification of novel molecular targets within intestinal L-cells for controlling and improving endogenous GLP-1 secretion.

Changes in small intestinal chromogranin A-immunoreactive cell densities in patients with irritable bowel syndrome after receiving dietary guidance

Chromogranin A (CgA) is a common marker for enteroendocrine cells in the gut, and CgA-immunoreactive cell densities are abnormal in patients with irritable bowel syndrome (IBS). The majority of patients with IBS report that their symptoms develop after consuming certain foodstuffs. In the present study, we investigated the effects of dietary guidance on the total enteroendocrine cell densities in the small intestine, as detected by CgA. A total of 14 patients with IBS underwent a gastroscopy with duodenal biopsies and 11 of them also underwent a colonoscopy, with biopsy samples obtained from the ileum. Fourteen control subjects were also included. Each patient received 3 sessions of dietary guidance. Gastroscopies and colonoscopies were performed on both the controls and patients with IBS (at baseline and at 3–9 months after receiving guidance). Biopsy samples obtained from the duodenum and ileum were immunostained for CgA using the avidin-biotin complex (ABC) method and were quantified using computerized image analysis. The density of CgA-immunoreactive cells in the duodenum (mean ± SEM values) in the control subjects was 235.9±31.9 cells/mm²; in the patients with IBS, the density was 36.9±9.8 and 103.7±16.9 cells/mm² before and after they received dietary guidance, respectively (P=0.007). The density of CgA-immunoreactive cells in the ileum in the control subjects was 47.4±8.3 cells/mm²; in the patients with IBS, the density was 48.4±8.1 and 17.9±4.4 cells/mm², before and after they received dietary guidance, respectively (P=0.0006). These data indicate that changes in CgA-immunoreactive cell densities in patients with IBS after receiving dietary guidance may reflect a change in the densities of the small intestinal enteroendocrine cells, which may contribute to an improvement in the IBS symptoms.

GLP-1 is both anxiogenic and antidepressant; divergent effects of acute and chronic GLP-1 on emotionality

Glucagon-like peptide 1 (GLP-1), produced in the intestine and hindbrain, is known for its glucoregulatory and appetite suppressing effects. GLP-1 agonists are in clinical use for treatment of type 2 diabetes and obesity. GLP-1, however, may also affect brain areas associated with emotionality regulation. Here we aimed to characterize acute and chronic impact of GLP-1 on anxiety and depression-like behavior. Rats were subjected to anxiety and depression behavior tests following acute or chronic intracerebroventricular or intra-dorsal raphe (DR) application of GLP-1 receptor agonists. Serotonin or serotonin-related genes were also measured in the amygdala, DR and the hippocampus. We demonstrate that both GLP-1 and its long lasting analog, Exendin-4, induce anxiety-like behavior in three rodent tests of this behavior: black and white box, elevated plus maze and open field test when acutely administered intraperitoneally, into the lateral ventricle, or directly into the DR. Acute central GLP-1 receptor stimulation also altered serotonin signaling in the amygdala. In contrast, chronic central administration of Exendin-4 did not alter anxiety-like behavior but significantly reduced depression-like behavior in the forced swim test. Importantly, this positive effect of Exendin-4 was not due to significant body weight loss and reduced food intake, since rats pair-fed to Exendin-4 rats did not show altered mood. Collectively we show a striking impact of central GLP-1 on emotionality and the amygdala serotonin signaling that is divergent under acute versus chronic GLP-1 activation conditions. We also find a novel role for the DR GLP-1 receptors in regulation of behavior. These results may have direct relevance to the clinic, and indicate that Exendin-4 may be especially useful for obese patients manifesting with comorbid depression.

Regulation of gut hormone secretion. Studies using isolated perfused intestines

The incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted from enteroendocrine cells in the intestine along with other gut hormones (PYY, CCK and neurotensin) shown to affect metabolism and/or appetite. The secretion of many gut hormones is highly increased after gastric bypass operations, which have turned out to be an effective therapy of not only obesity but also type 2 diabetes. These effects are likely to be due, at least in part, to increases in the secretion of these gut hormones (except GIP). Therefore, stimulation of the endogenous hormone represents an appealing therapeutic strategy, which has spurred an interest in understanding the regulation of gut hormone secretion and a search for particularly GLP-1 and PYY secretagogues. The secretion of the gut hormones is stimulated by oral intake of nutrients often including carbohydrate, protein and lipid. This review focuses on stimulators of gut hormone secretion, the mechanisms involved, and in particular models used to investigate secretion. A major break-through in this field was the development of methods to identify and isolate specific hormone producing cells, which allow detailed mapping of the expression profiles of these cells, whereas they are less suitable for physiological studies of secretion. Isolated perfused preparations of mouse and rat intestines have proven to be reliable models for dynamic hormone secretion and should be able to bridge the gap between the molecular details derived from the single cells to the integrated patterns observed in the intact animals.

Pharmacokinetic and Pharmacodynamic Effects of Multiple-dose Administration of Omarigliptin, a Once-weekly Dipeptidyl Peptidase-4 Inhibitor, in Obese Participants With and Without Type 2 Diabetes Mellitus

PURPOSE

Omarigliptin (MK-3102) is a potent, oral, long-acting dipeptidyl peptidase (DPP)-4 inhibitor approved in Japan and in global development as a once-weekly treatment for type 2 diabetes mellitus (T2DM). The aim of this study was to investigate the pharmacokinetic (PK) and pharmacodynamic (PD) effects of omarigliptin in obese participants with and without T2DM.

METHODS

This was a Phase I, randomized, double-blind, placebo-controlled, multiple-dose study of 50-mg omarigliptin administered once weekly for 4 weeks. Participants included 24 obese but otherwise healthy subjects (panel A; omarigliptin, n = 18; placebo, n = 6) and 8 obese patients with T2DM (treatment naive, hemoglobin A1c ≥ 6.5% and ≤ 10.0% [panel B]; omarigliptin, n = 6; placebo, n = 2). Participants were 45 to 65 years of age with a body mass index of ≥ 30 and ≤ 40 kg/m(2). Blood sampling occurred at select time points, depending on the study panel, to evaluate the PK properties of omarigliptin, DPP-4 activity, active glucagon-like peptide 1 levels, and plasma glucose concentrations. Body weight was an exploratory end point. Due to sparse sampling in panel A, a thorough PK analysis was performed in obese patients with T2DM (panel B) only. PD analyses were performed in the overall study population (pooled panels A and B).

FINDINGS

PK profiles in obese participants with and without T2DM were similar to those observed in nonobese reference subjects (historical data). Steady state was achieved after 1 or 2 weekly doses in obese participants with and without T2DM. In obese patients with T2DM, omarigliptin was rapidly absorbed, with a median Tmax of 1 to 2.5 hours (days 1 and 22). Compared with those in reference subjects, the geometric mean ratios (95% CI) (Obese T2DM/reference) for steady-state plasma AUC0-168h, Cmax, and C168h were 0.80 (0.65-0.98), 0.86 (0.53-1.41), and 1.08 (0.88-1.33), respectively. Trough DPP-4 activity was inhibited by ~90%; postprandial (PP) 4-hour weighted mean active GLP-1 concentrations were increased ~2-fold; and PP glucose was significantly reduced with omarigliptin versus placebo in the pooled population. Omarigliptin was generally well-tolerated in the pooled population, and there were no hypoglycemic events. Consistent with other DPP-4 inhibitors, omarigliptin had no effect on body weight in this short-duration study.

IMPLICATIONS

The administration of omarigliptin was generally well-tolerated in obese participants with and without T2DM, and the favorable PK and PD profiles support once-weekly dosing. Omarigliptin may provide an important once-weekly treatment option for patients with T2DM. ClinicalTrials.gov identifier: NCT01088711.

Mathematical Modeling of Interacting Glucose-Sensing Mechanisms and Electrical Activity Underlying Glucagon-Like Peptide 1 Secretion

Intestinal L-cells sense glucose and other nutrients, and in response release glucagon-like peptide 1 (GLP-1), peptide YY and other hormones with anti-diabetic and weight-reducing effects. The stimulus-secretion pathway in L-cells is still poorly understood, although it is known that GLP-1 secreting cells use sodium-glucose co-transporters (SGLT) and ATP-sensitive K+-channels (K(ATP)-channels) to sense intestinal glucose levels. Electrical activity then transduces glucose sensing to Ca2+-stimulated exocytosis. This particular glucose-sensing arrangement with glucose triggering both a depolarizing SGLT current as well as leading to closure of the hyperpolarizing K(ATP) current is of more general interest for our understanding of glucose-sensing cells. To dissect the interactions of these two glucose-sensing mechanisms, we build a mathematical model of electrical activity underlying GLP-1 secretion. Two sets of model parameters are presented: one set represents primary mouse colonic L-cells; the other set is based on data from the GLP-1 secreting GLUTag cell line. The model is then used to obtain insight into the differences in glucose-sensing between primary L-cells and GLUTag cells. Our results illuminate how the two glucose-sensing mechanisms interact, and suggest that the depolarizing effect of SGLT currents is modulated by K(ATP)-channel activity. Based on our simulations, we propose that primary L-cells encode the glucose signal as changes in action potential amplitude, whereas GLUTag cells rely mainly on frequency modulation. The model should be useful for further basic, pharmacological and theoretical investigations of the cellular signals underlying endogenous GLP-1 and peptide YY release.

Dietary guidance normalizes large intestinal endocrine cell densities in patients with irritable bowel syndrome

Background/Objectives:

To determine the large intestinal endocrine cell types affected following dietary guidance in patients with irritable bowel syndrome (IBS).

Subjects/Methods:

The study included 13 IBS patients and 13 control subjects. The patients received three sessions of individualized dietary guidance. Both the control subjects and the patients were scheduled for colonoscopies at baseline and again for the patients at 3–9 months after dietary guidance. Biopsy samples were taken from the colon and rectum and were immunostained for all types of large intestinal endocrine cells. The endocrine cells were quantified using computerized image analysis.

Results:

The daily total consumption (mean±s.e.m. values) of fruits and vegetables rich in FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols) decreased significantly from 16.2±5.3 g before receiving dietary guidance to 9.2±3.2 g after receiving dietary guidance (P=0.02). In the total colon, the densities of serotonin cells were 46.8±8.9, 10.5±2.1 and 22.6±3.2 cells/mm² in control subjects and in IBS patients before and after receiving dietary guidance, respectively (P=0.007); the corresponding densities of peptide YY cells were 11.6±1.8, 10.8±1.7 and 16.8±2.1 cells/mm², respectively (P=0.06). The cell densities for both serotonin and peptide YY did not change significantly in the rectum. The densities of somatostatin cells in the rectum were 13.5±3.0, 13.2±3.0, and 22.3±3.2 cells/mm² for control subjects and for IBS patients before and after receiving dietary guidance, respectively (P=0.01).

Conclusions:

The densities of the large intestinal endocrine cells tend to normalize following dietary guidance that may have contributed to the improvement of the patients with IBS symptoms.

Effects on Clinical Outcomes of Adding Dipeptidyl Peptidase-4 Inhibitors Versus Sulfonylureas to Metformin Therapy in Patients With Type 2 Diabetes Mellitus

BACKGROUND

Recent studies concluded that dipeptidyl peptidase-4 (DPP-4) inhibitors provide glycemic control but also raised concerns about the risk for heart failure in patients with type 2 diabetes mellitus (T2DM). However, large-scale studies of the effects on cardiovascular outcomes of adding DPP-4 inhibitors versus sulfonylureas to metformin therapy remain scarce.

OBJECTIVE

To compare clinical outcomes of adding DPP-4 inhibitors versus sulfonylureas to metformin therapy in patients with T2DM.

DESIGN

Nationwide study using Taiwan's National Health Insurance Research Database.

SETTING

Taiwan.

PATIENTS

All patients with T2DM aged 20 years or older between 2009 and 2012. A total of 10,089 propensity score-matched pairs of DPP-4 inhibitor users and sulfonylurea users were examined.

MEASUREMENTS

Cox models with exposure to sulfonylureas and DPP-4 inhibitors included as time-varying covariates were used to compare outcomes. The following outcomes were considered: all-cause mortality, major adverse cardiovascular events (MACEs) (including ischemic stroke and myocardial infarction), hospitalization for heart failure, and hypoglycemia. Patients were followed until death or 31 December 2013.

RESULTS

DPP-4 inhibitors were associated with lower risks for all-cause death (hazard ratio [HR], 0.63 [95% CI, 0.55 to 0.72]), MACEs (HR, 0.68 [CI, 0.55 to 0.83]), ischemic stroke (HR, 0.64 [CI, 0.51 to 0.81]), and hypoglycemia (HR, 0.43 [CI, 0.33 to 0.56]) compared with sulfonylureas as add-on therapy to metformin but had no effect on risks for myocardial infarction and hospitalization for heart failure.

LIMITATION

Observational study design.

CONCLUSION

Compared with sulfonylureas, DPP-4 inhibitors were associated with lower risks for all-cause death, MACEs, ischemic stroke, and hypoglycemia when used as add-ons to metformin therapy.

PRIMARY FUNDING SOURCE

None.

Recent developments in the pathophysiology of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder, the pathophysiology of which is not completely known, although it has been shown that genetic/social learning factors, diet, intestinal microbiota, intestinal low-grade inflammation, and abnormal gastrointestinal endocrine cells play a major role. Studies of familial aggregation and on twins have confirmed the heritability of IBS. However, the proposed IBS risk genes are thus far nonvalidated hits rather than true predisposing factors. There is no convincing evidence that IBS patients suffer from food allergy/intolerance, with the effect exerted by diet seemingly caused by intake of poorly absorbed carbohydrates and fiber. Obesity is a possible comorbidity of IBS. Differences in the microbiota between IBS patients and healthy controls have been reported, but the association between IBS symptoms and specific bacterial species is uncertain. Low-grade inflammation appears to play a role in the pathophysiology of a major subset of IBS, namely postinfectious IBS. The density of intestinal endocrine cells is reduced in patients with IBS, possibly as a result of genetic factors, diet, intestinal microbiota, and low-grade inflammation interfering with the regulatory signals controlling the intestinal stem-cell clonogenic and differentiation activities. Furthermore, there is speculation that this decreased number of endocrine cells is responsible for the visceral hypersensitivity, disturbed gastrointestinal motility, and abnormal gut secretion seen in IBS patients.

Molecular mechanisms of incretin hormone secretion

Incretin is a kind of intestinal hormone secreted by the enteroendocrine cells in the intestinal epithelium. There has been plenty of research to explore the molecular mechanisms of incretin hormone secretion, including secretion-promoting factors such as glucose, lipid, protein and other nutrients in enteroendocrine cells. This review aims to discuss the signal pathways related to incretin hormone secretion.