Glucagon‐Like Peptide‐1: Actions and Influence on Pancreatic Hormone Function

Incretin and glucagon receptor polypharmacology in chronic kidney disease

Chronic kidney disease is a debilitating condition associated with significant morbidity and mortality. In recent years, the kidney effects of incretin-based therapies, particularly glucagon-like peptide-1 receptor agonists (GLP-1RAs), have garnered substantial interest in the management of type 2 diabetes and obesity. This review delves into the intricate interactions between the kidney, GLP-1RAs, and glucagon, shedding light on their mechanisms of action and potential kidney benefits. Both GLP-1 and glucagon, known for their opposing roles in regulating glucose homeostasis, improve systemic risk factors affecting the kidney, including adiposity, inflammation, oxidative stress, and endothelial function. Additionally, these hormones and their pharmaceutical mimetics may have a direct impact on the kidney. Clinical studies have provided evidence that incretins, including those incorporating glucagon receptor agonism, are likely to exhibit improved kidney outcomes. Although further research is necessary, receptor polypharmacology holds promise for preserving kidney function through eliciting vasodilatory effects, influencing volume and electrolyte handling, and improving systemic risk factors.

High temperature and humidity in the environment disrupt bile acid metabolism, the gut microbiome, and GLP-1 secretion in mice

High temperature and humidity in the environment are known to be associated with discomfort and disease, yet the underlying mechanisms remain unclear. We observed a decrease in plasma glucagon-like peptide-1 levels in response to high-temperature and humidity conditions. Through 16S rRNA gene sequencing, alterations in the gut microbiota composition were identified following exposure to high temperature and humidity conditions. Notably, changes in the gut microbiota have been implicated in bile acid synthesis. Further analysis revealed a decrease in lithocholic acid levels in high-temperature and humidity conditions. Subsequent in vitro experiments demonstrated that lithocholic acid increases glucagon-like peptide-1 secretion in NCI-H716 cells. Proteomic analysis indicated upregulation of farnesoid X receptor expression in the ileum. In vitro experiments revealed that the combination of lithocholic acid with farnesoid X receptor inhibitors resulted in a significant increase in GLP-1 levels compared to lithocholic acid alone. In this study, we elucidate the mechanism by which reduced lithocholic acid suppresses glucagon-like peptide 1 via farnesoid X receptor activation under high-temperature and humidity condition.

The Regulation Role of the Gut-Islets Axis in Diabetes

The gut-islets axis is an important endocrine signaling axis that regulates the function of islets by modulating the gut micro-environment and its endocrine metabolism. The discovery of intestinal hormones, such as GLP-1 and GIP, has established a preliminary link between the gut and the islet, paving the way for the development of GLP-1 receptor agonists based on the regulation theory of the gut-islets axis for diabetes treatment. This discovery has created a new paradigm for diabetes management and rapidly made the regulation theory of the gut-islets axis a focal point of research attention. Recent years, with in-depth study on gut microbiota and the discovery of intestinal-derived extracellular vesicles, the concept of gut endocrine and the regulation theory of the gut-islets axis have been further expanded and updated, offering tremendous research opportunities. The gut-islets axis refers to the complex interplay between the gut and the islet, which plays a crucial role in regulating glucose homeostasis and maintaining metabolic health. The axis involves various components, including gut microbiota, intestinal hormones, amino acids and ACE2, which contribute to the communication and coordination between the gut and the islet.

Safety, tolerability, pharmacokinetics and pharmacokinetic-pharmacodynamic modeling of cetagliptin in patients with type 2 diabetes mellitus

Aims

To evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of cetagliptin (CAS number:2243737-33-7) in Chinese patients with type 2 diabetes mellitus (T2DM). A population PK/PD model was developed to quantify the PK and PD characteristics of cetagliptin in patients.

Materials and methods

32 Chinese adults with T2DM were enrolled in this study. The subjects were randomly assigned to receive either cetagliptin (50 mg or 100 mg), placebo, or sitagliptin (100 mg) once daily for 14 days. Blood samples were collected for PK and PD analysis. Effects on glucose, insulin, C-peptide, and glucagon were evaluated following an oral glucose tolerance test (OGTT) (day15). Effects on HbA1c and glycated albumin (GA), and safety assessments were also conducted. Meanwhile, a population PK/PD model was developed by a sequential two-step analysis approach using Phoenix.

Results

Following multiple oral doses, cetagliptin was rapidly absorbed and the mean half-life were 34.9-41.9 h. Steady-state conditions were achieved after 1 week of daily dosing and the accumulation was modest. The intensity and duration of DPP-4 inhibition induced by 50 mg cetagliptin were comparable with those induced by sitagliptin, and 100 mg cetagliptin showed a much longer sustained DPP-4 inhibition (≥80%) than sitagliptin. Compared with placebo group, plasma active GLP-1 AUEC0-24h increased by 2.20- and 3.36-fold in the 50 mg and 100 mg cetagliptin groups. A decrease of plasma glucose and increase of insulin and C-peptide were observed following OGTT in cetagliptin groups. Meanwhile, a tendency of reduced GA was observed, whereas no decreasing trend was observed in HbA1c. All adverse events related to cetagliptin and sitagliptin were assessed as mild. A population PK/PD model was successfully established. The two-compartment model and Sigmoid-Emax model could fit the observed data well. Total bilirubin (TBIL) was a covariate of volume of peripheral compartment distribution (V2), and V2 increased with the increase of TBIL.

Conclusions

Cetagliptin was well tolerated, inhibited plasma DPP-4 activity, increased plasma active GLP-1 levels, and exhibited a certain trend of glucose-lowering effect in patients with T2DM. The established population PK/PD model adequately described the PK and PD characteristics of cetagliptin.

GLP-1/GLP-1RAs: New Options for the Drug Treatment of NAFLD

Non-alcoholic fatty liver disease (NAFLD) has recently emerged as a global public health concern. Currently, the cornerstone of NAFLD treatment is lifestyle modification and, if necessary, weight loss. However, compliance is a challenge, and this approach alone may not be sufficient to halt and treat the more serious disease development, so medication is urgently needed. Nevertheless, no medicines are approved to treat NAFLD. Glucagon-like peptide-1 (GLP-1) is an enteropeptide hormone that inhibits glucagon synthesis, promotes insulin secretion, and delays gastric emptying. GLP-1 has been found in recent studies to be beneficial for the management of NAFLD, and the marketed GLP-1 agonist drugs have different degrees of effectiveness for NAFLD while lowering blood glucose. In this article, we review GLP-1 and its physiological roles, the pathogenesis of NAFLD, the correlation between NAFLD and GLP-1 signaling, and potential strategies for GLP-1 treatment of NAFLD.

A New Dietary Fiber Can Enhance Satiety and Reduce Postprandial Blood Glucose in Healthy Adults: A Randomized Cross-Over Trial

BACKGROUND

Dietary fiber plays a potential role in regulating energy intake and stabilizing postprandial blood glucose levels. Soluble dietary fiber has become an important entry point for nutritional research on the regulation of satiety.

METHODS

this was a double-blind, randomized cross-over trial enrolling 12 healthy subjects to compare the effects of RPG (R+PolyGly) dietary fiber products (bread, powder, and capsule) and pectin administered with a standard meal on satiety, blood glucose, and serum insulin level.

RESULTS

Adding 3.8% RPG dietary fiber to bread significantly increased the volume, water content, hardness, and chewiness of bread compared to 3.8% pectin bread and white bread and significantly improved the sensory quality of bread. RPG bread had better appetite suppression effects at some time points than the other two groups and the best postprandial blood glucose lowering effects among the three groups. Administration of RPG capsules containing 5.6 g of RPG dietary fiber with meals improved satiety and reduced hunger compared to 6 g of RPG powder and 6 g of pectin, which had the greatest effect on suppressing appetite and reducing prospective food consumption. The peak level of serum glucagon-like peptide-1 (GLP-1) in the RPG capsule group (578.17 ± 19.93 pg/mL) was significantly higher than that in other groups at 0 min and 30 min after eating. RPG powder had the best effect in reducing postprandial blood glucose and increasing serum insulin levels; the total area under the curve (AUC) of serum insulin with RPG powder was higher than other groups (5960 ± 252.46 μU min/mL).

CONCLUSION

RPG dietary fiber products can improve the sensory properties of food, reduce postprandial blood glucose, and enhance satiety, especially in capsule and powder forms. Further research on the physiological effects of RPG dietary fiber is required to facilitate its use as a functional ingredient in food products.

An Association between Decreased Small Intestinal RNA Modification and Disturbed Glucagon-like Peptide-1 Secretion under High-Fat Diet Stress

Unhealthy diets rich in fats and/or sugar are considered as the major external cause of the obesity epidemic, which is often accompanied by a significant decrease in gut hormone glucagon-like peptide-1 (GLP1) levels. Numerous studies have demonstrated notable contributions of the gut microbiota in this process. Nevertheless, the underlying mechanism still needs further investigation. The role of epigenetic modifications in gene expression and metabolism has been well demonstrated, with m6A methylation on RNAs being the most prevalent modification throughout their metabolism. In the present study, we found that the expressions of small intestinal Gcg and Pc3, two key genes regulating GLP1 expression, were significantly downregulated in obese mice, associated with reduced GLP1 level. Immunohistochemistry analysis indicated that a high-fat diet slightly increased the density of enteroendocrine L cells in the small intestine, implying that decreased GLP1 levels were not caused by the changes in L cell intensity. Instead, the small intestinal m6A level as well as the expression of known "writers", mettl3/14 and wtap, were found to be positively correlated with the expression of Gcg and Pc3. Fecal microbiota transplantation with feces from normal and obese mice daily to antibiotic-treated mice revealed that dysbiosis in diet-induced obesity was sufficient to reduce serum GLP1, small intestinal m6A level, and intestinal expressions of Gcg, Pc3, and writer genes (mettl3/14, wtap). However, as the most direct and universal methyl donor, the production of fecal S-adenosylmethionine was neither affected by the different dietary patterns nor their shaped microbiota. These results suggested that microbial modulation of the epitranscriptome may be involved in regulating GLP1 expression, and highlighted epitranscriptomic modifications as an additional level of interaction between diet and individual health.

Lactation from the inside out: Maternal homeorhetic gastrointestinal adaptations regulating energy and nutrient flow into milk production

Lactation invokes homeorhetic processes to ramp up and supply milk synthesis components to fulfil nutritional, immunological and microbiological requirements of developing offspring, overseen by complex neuroendocrine networks. The maternal gut meets these intense metabolic demands, supported by hyperphagia and rapid adjustments to process larger food quantities. Enteroplasticity describes an inherent ability of the gastrointestinal tract to harness metabolic and structural adaptations that increase nutrient absorption. Most shifts in response to increased demands are transitory and by secreting milk, the continuous energetic drain out of the maternal body avoids development of pathological metabolic diseases. Lactation has various positive benefits for long-term maternal health but many females do not lactate for long post pregnancy and younger women are increasingly pre-disposed to excessive body mass and/or metabolic complications prior to reproducing. Inadvertently invoking intestinal adaptations to harvest and store excess nutrients has negative health implications with increased risks for both mother and offspring.

Proceedings from the Albert Charitable Trust Inaugural Workshop on 'Understanding the Acute Effects of Exercise on the Brain'

An inaugural workshop supported by "The Leo and Anne Albert Charitable Trust," was held October 4-7, 2019 in Scottsdale, Arizona, to focus on the effects of exercise on the brain and to discuss how physical activity may prevent or delay the onset of aging-related neurodegenerative conditions. The Scientific Program Committee (led by Dr. Jeff Burns) assembled translational, clinical, and basic scientists who research various aspects of the effects of exercise on the body and brain, with the overall goal of gaining a better understanding as to how to delay or prevent neurodegenerative diseases. In particular, research topics included the links between cardiorespiratory fitness, the cerebrovasculature, energy metabolism, peripheral organs, and cognitive function, which are all highly relevant to understanding the effects of acute and chronic exercise on the brain. The Albert Trust workshop participants addressed these and related topics, as well as how other lifestyle interventions, such as diet, affect age-related cognitive decline associated with Alzheimer's and other neurodegenerative diseases. This report provides a synopsis of the presentations and discussions by the participants, and a delineation of the next steps towards advancing our understanding of the effects of exercise on the aging brain.

Mof acetyltransferase inhibition ameliorates glucose intolerance and islet dysfunction of type 2 diabetes via targeting pancreatic α-cells

BACKGROUND

Previously, we reported that Mof was highly expressed in α-cells, and its knockdown led to ameliorated fasting blood glucose (FBG) and glucose tolerance in non-diabetic mice, attributed by reduced total α-cell but enhanced prohormone convertase (PC)1/3-positive α-cell mass. However, how Mof and histone 4 lysine 16 acetylation (H4K16ac) control α-cell and whether Mof inhibition improves glucose handling in type 2 diabetes (T2DM) mice remain unknown.

METHODS

Mof overexpression and chromatin immunoprecipitation sequence (ChIP-seq) based on H4K16ac were applied to determine the effect of Mof on α-cell transcriptional factors and underlying mechanism. Then we administrated mg149 to α-TC1-6 cell line, wild type, db/db and diet-induced obesity (DIO) mice to observe the impact of Mof inhibition in vitro and in vivo. In vitro, western blotting and TUNEL staining were used to examine α-cell apoptosis and function. In vivo, glucose tolerance, hormone levels, islet population, α-cell ratio and the co-staining of glucagon and PC1/3 or PC2 were examined.

RESULTS

Mof activated α-cell-specific transcriptional network. ChIP-seq results indicated that H4K16ac targeted essential genes regulating α-cell differentiation and function. Mof activity inhibition in vitro caused impaired α-cell function and enhanced apoptosis. In vivo, it contributed to ameliorated glucose intolerance and islet dysfunction, characterized by decreased fasting glucagon and elevated post-challenge insulin levels in T2DM mice.

CONCLUSION

Mof regulates α-cell differentiation and function via acetylating H4K16ac and H4K16ac binding to Pax6 and Foxa2 promoters. Mof inhibition may be a potential interventional target for T2DM, which led to decreased α-cell ratio but increased PC1/3-positive α-cells.

Preproglucagon Products and Their Respective Roles Regulating Insulin Secretion

Historically, intracellular function and metabolic adaptation within the α-cell has been understudied, with most of the attention being placed on the insulin-producing β-cells due to their role in the pathophysiology of type 2 diabetes mellitus. However, there is a growing interest in understanding the function of other endocrine cell types within the islet and their paracrine role in regulating insulin secretion. For example, there is greater appreciation for α-cell products and their contributions to overall glucose homeostasis. Several recent studies have addressed a paracrine role for α-cell-derived glucagon-like peptide-1 (GLP-1) in regulating glucose homeostasis and responses to metabolic stress. Further, other studies have demonstrated the ability of glucagon to impact insulin secretion by acting through the GLP-1 receptor. These studies challenge the central dogma surrounding α-cell biology describing glucagon's primary role in glucose counterregulation to one where glucagon is critical in regulating both hyper- and hypoglycemic responses. Herein, this review will update the current understanding of the role of glucagon and α-cell-derived GLP-1, placing emphasis on their roles in regulating glucose homeostasis, insulin secretion, and β-cell mass.

GLP-1 receptor signaling increases PCSK1 and β cell features in human α cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that potentiates glucose-stimulated insulin secretion. GLP-1 is classically produced by gut L cells; however, under certain circumstances α cells can express the prohormone convertase required for proglucagon processing to GLP-1, prohormone convertase 1/3 (PC1/3), and can produce GLP-1. However, the mechanisms through which this occurs are poorly defined. Understanding the mechanisms by which α cell PC1/3 expression can be activated may reveal new targets for diabetes treatment. Here, we demonstrate that the GLP-1 receptor (GLP-1R) agonist, liraglutide, increased α cell GLP-1 expression in a β cell GLP-1R-dependent manner. We demonstrate that this effect of liraglutide was translationally relevant in human islets through application of a new scRNA-seq technology, DART-Seq. We found that the effect of liraglutide to increase α cell PC1/3 mRNA expression occurred in a subcluster of α cells and was associated with increased expression of other β cell-like genes, which we confirmed by IHC. Finally, we found that the effect of liraglutide to increase bihormonal insulin+ glucagon+ cells was mediated by the β cell GLP-1R in mice. Together, our data validate a high-sensitivity method for scRNA-seq in human islets and identify a potentially novel GLP-1-mediated pathway regulating human α cell function.

An Overview of Similarities and Differences in Metabolic Actions and Effects of Central Nervous System Between Glucagon-Like Peptide-1 Receptor Agonists (GLP-1RAs) and Sodium Glucose Co-Transporter-2 Inhibitors (SGLT-2is)

GLP-1 receptor agonists (GLP-1RAs) and SGLT-2 inhibitors (SGLT-2is) are novel antidiabetic medications associated with considerable cardiovascular benefits therapying treatment of diabetic patients. GLP-1 exhibits atherosclerosis resistance, whereas SGLT-2i acts to ameliorate the neuroendocrine state in the patients with chronic heart failure. Despite their distinct modes of action, both factors share pathways by regulating the central nervous system (CNS). While numerous preclinical and clinical studies have demonstrated that GLP-1 can access various nuclei associated with energy homeostasis and hedonic eating in the CNS via blood-brain barrier (BBB), research on the activity of SGLT-2is remains limited. In our previous studies, we demonstrated that both GLP-1 receptor agonists (GLP-1RAs) liraglutide and exenatide, as well as an SGLT-2i, dapagliflozin, could activate various nuclei and pathways in the CNS of Sprague Dawley (SD) rats and C57BL/6 mice, respectively. Moreover, our results revealed similarities and differences in neural pathways, which possibly regulated different metabolic effects of GLP-1RA and SGLT-2i via sympathetic and parasympathetic systems in the CNS, such as feeding, blood glucose regulation and cardiovascular activities (arterial blood pressure and heart rate control). In the present article, we extensively discuss recent preclinical studies on the effects of GLP-1RAs and SGLT-2is on the CNS actions, with the aim of providing a theoretical explanation on their mechanism of action in improvement of the macro-cardiovascular risk and reducing incidence of diabetic complications. Overall, these findings are expected to guide future drug design approaches.

Updating the Role of α-Cell Preproglucagon Products on GLP-1 Receptor-Mediated Insulin Secretion

While the field of islet biology has historically focused its attention on understanding β-cell function and the mechanisms by which these cells become dysfunctional with diabetes, there has been a scientific shift toward greater understanding of other endocrine cells of the islet and their paracrine role in regulating the β-cell. In recent years, many questions and new data have come forward regarding the paracrine role of the α-cell and specifically preproglucagon peptides in regulating insulin secretion. The role of intestinally secreted glucagon-like peptide 1 (GLP-1) in regulation of insulin secretion has been questioned, and a physiological role of pancreatic GLP-1 in regulation of insulin secretion has been proposed. In addition, in the last 2 years, a series of studies demonstrated a physiological role for glucagon, acting via the GLP-1 receptor, in paracrine regulation of insulin secretion. Altogether, this work challenges the textbook physiology of both GLP-1 and glucagon and presents a critical paradigm shift for the field. This article addresses these new findings surrounding α-cell preproglucagon products, with a particular focus on GLP-1, in the context of their roles in insulin secretion and consequently glucose metabolism.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

Mof regulates glucose level via altering different α-cell subset mass and intra-islet glucagon-like peptide-1, glucagon secretion

BACKGROUND

Males absent on the first (Mof) is implicated in gene control of diverse biological processes, such as cell growth, differentiation, apoptosis and autophagy. However, the relationship between glucose regulation and Mof-mediated transcription events remains unexplored. We aimed to unravel the role of Mof in glucose regulation by using global and pancreatic α-cell-specific Mof-deficient mice in vivo and α-TC1-6 cell line in vitro.

METHODS

We used tamoxifen-induced temporal Mof-deficient mice first to show Mof regulate glucose homeostasis, islet cell proportions and hormone secretion. Then we used α-cell-specific Mof-deficient mice to clarify how α-cell subsets and β-cell mass were regulated and corresponding hormone level alterations. Ultimately, we used small interfering RNA (siRNA) to knockdown Mof in α-TC1-6 and unravel the mechanism regulating α-cell mass and glucagon secretion.

RESULTS

Mof was mainly expressed in α-cells. Global Mof deficiency led to lower glucose levels, attributed by decreased α/β-cell ratio and glucagon secretion. α-cell-specific Mof-deficient mice exhibited similar alterations, with more reduced prohormone convertase 2 (PC2)-positive α-cell mass, responsible for less glucagon, and enhanced prohormone convertase 1 (PC1/3)-positive α-cell mass, leading to more glucagon-like peptide-1 (GLP-1) secretion, thus increased β-cell mass and insulin secretion. In vitro, increased DNA damage, dysregulated autophagy, enhanced apoptosis and altered cell fate factors expressions upon Mof knockdown were observed. Genes and pathways linked to impaired glucagon secretion were uncovered through transcriptome sequencing.

CONCLUSION

Mof is a potential interventional target for glucose regulation, from the aspects of both α-cell subset mass and glucagon, intra-islet GLP-1 secretion. Upon Mof deficiency, Up-regulated PC1/3 but down-regulated PC2-positive α-cell mass, leads to more GLP-1 and insulin but less glucagon secretion, and contributed to lower glucose level.

Long-Term Liraglutide Administration Induces Pancreas Neogenesis in Adult T2DM Mice

In vivo beta-cell neogenesis may be one way to treat diabetes. We aimed to investigate the effect of glucagon-like peptide-1 (GLP-1) on beta-cell neogenesis in type 2 diabetes mellitus (T2DM). Male C57BL/6J mice, 6 wk old, were randomly divided into three groups: Control, T2DM, and T2DM + Lira. T2DM was induced using high-fat diet and intraperitoneal injection of streptozotocin (40 mg/kg/d for 3 d). At 8 wk after streptozotocin injection, T2DM + Lira group was injected intraperitoneally with GLP-1 analog liraglutide (0.8 mg/kg/d) for 4 wk. Apparently for the first time, we report the appearance of a primitive bud connected to pancreas in all adult mice from each group. The primitive bud was characterized by scattered single monohormonal cells expressing insulin, GLP-1, somatostatin, or pancreatic polypeptide, and four-hormonal cells, but no acinar cells and ductal epithelial cells. Monohormonal cells in it were small, newborn, immature cells that rapidly proliferated and expressed cell markers indicative of immaturity. In parallel, Ngn3+ endocrine progenitors and Nestin+ cells existed in the primitive bud. Liraglutide facilitated neogenesis and rapid growth of acinar cells, pancreatic ducts, and blood vessels in the primitive bud. Meanwhile, scattered hormonal cells aggregated into cell clusters and grew into larger islets; polyhormonal cells differentiated into monohormonal cells. Extensive growth of exocrine and endocrine glands resulted in the neogenesis of immature pancreatic lobes in adult mice of T2DM + Lira group. Contrary to predominant acinar cells in mature pancreatic lobes, there were still a substantial number of mesenchymal cells around acinar cells in immature pancreatic lobes, which resulted in the loose appearance. Our results suggest that adult mice preserve the capacity of pancreatic neogenesis from the primitive bud, which liraglutide facilitates in adult T2DM mice. To our knowledge, this is the first time such a phenomenon has been reported.