Glucose-induced Insulin Secretion Research Articles

6494 GLP-1/GIP/Glucagon Triagonist Improves Glycemic Control via Gαq Signaling Pathway and TRPM5 Activation

Abstract Disclosure: N. Khajavi: None. P.C. Schreier: None. P. Beyerle: None. P.S. Reinach: None. A. Breit: None. I. Boekhoff: None. T.D. Müller: Grant Recipient; Self; Novo Nordisk. Speaker; Self; Novo Nordisk. Stock Owner; Self; Novo Nordisk. T. Gudermann: None. Cutting-edge research on obesity and diabetes strives to pioneer innovative drug combinations targeting various dysfunctional metabolic regulatory pathways. The single-molecule triagonist has been developed to directly interact with the glucagon-like peptide-1 receptor (GLP-1R), glucose-dependent insulinotropic polypeptide receptor (GIPR), and the glucagon receptor (GcgR). It provides a breakthrough in the treatment of obesity and type 2 diabetes because of greater efficacy compared to conventional single-agonist therapy. While the triagonist is already undergoing phase two clinical trials, the downstream mechanism requires further study. Previously, we showed that triagonist elicits transient rises in intracellular Ca2+ concentration that augment glucose-induced rises in insulin secretion in murine (MIN6) and human (1.1B4) pancreatic β cell lines. Although Gαs signaling is known as the main mediator of both GLP-1 and GIP signaling, our research indicates that the superior effect of the triagonist on insulin secretion is not solely dependent on the activation of the classical Gαs pathway and the exchange factor activated by cAMP (EPAC) signaling. Instead, we report that the triagonist enhances glucose-induced rises in insulin secretion via Gαq signaling and its downstream effector, the TRPM5 channel. This involvement is evident, as blocking Gαq signaling with a specific blocker, YM254890, reduced triagonist-induced rises in insulin secretion in mouse pancreatic islets. Similarly, blocking TRPM5 with the TPPO blocker attenuated triagonist-induced increases in insulin secretion and Ca2+ signaling in mouse pancreatic islets. Strikingly, both blockers had no remarkable impact on GLP-1- and GIP-induced rises in insulin secretion. Our in vivo analysis demonstrates that the triagonist improves glucose tolerance in diet-induced obese wild-type mice, but this effect was absent in TRPM5 knockout mice. Notably, the triagonist augments glucose-induced insulin secretion in isolated islets of wild-type mice more than treatment with GLP1 and GIP mono-agonists. Furthermore, this enhanced response is absent in TRPM5 knockout islets. Recent studies propose that persistent membrane depolarization of β cells leads to a switch from Gαs to Gαq signaling, which is a major amplifying pathway of insulin secretion under diabetic conditions. Our study shows that the enhancing effect of the triagonist, acting as a biased agonist, on insulin secretion is dependent on TRPM5 activation through the Gαq signaling pathway. Presentation: 6/2/2024

Gut microbiota mediates ambient PM2.5 exposure-induced abnormal glucose metabolism via short-chain fatty acids

PM2.5 exposure has been found to cause gut dysbiosis and impair glucose homeostasis in human and animals, yet their underlying biological connection remain unclear. In the present study, we aim to investigate the biological significance of gut microbiota in PM2.5-induced glucose metabolic abnormalities. Our results showed that microbiota depletion by antibiotics treatment significantly alleviated PM2.5-induced glucose intolerance and insulin resistance, as indicated by the intraperitoneal glucose tolerance test, glucose-induced insulin secretion, insulin tolerance test, insulin-induced phosphorylation levels of Akt and GSK-3β in insulin sensitive tissues. In addition, faecal microbiota transplantation (FMT) from PM2.5-exposed donor mice successfully remodeled the glucose metabolism abnormalities in recipient mice, while the transplantation of autoclaved faecal materials did not. Faecal microbiota analysis demonstrated that the composition and alpha diversity of the gut bacterial community were altered by PM2.5 exposure and in FMT recipient mice. Furthermore, short-chain fatty acids levels analysis showed that the circulating acetate was significantly decreased in PM2.5-exposed donor and FMT recipient mice, and supplementation of sodium acetate for 3 months successfully improved the glucose metabolism abnormalities induced by PM2.5 exposure. These results indicate that manipulating gut microbiota or its metabolites could be a potential strategy for preventing the adverse health effects of ambient PM2.5.

Apigenin potentiates glucose-stimulated insulin secretion through the PKA-MEK kinase signaling pathway independent of K-ATP channels

AimApigenin, a natural bioflavonoid, is reported as an anti-diabetic agent since it possesses the ability to inhibit α-glucosidase activity, cause stimulation of insulin action and secretion, manage ROS, and prevent diabetes complications. Apigenin was identified as a new insulin secretagogue that enhances glucose-stimulated insulin secretion and seems like a better antidiabetic drug candidate. Here we explored the insulinotropic mechanism(s) of apigenin in vitro in mice islets and in vivo in diabetic rats. MethodsSize-matched pancreatic islets were divided into groups and incubated in the presence or absence of apigenin and agonists or antagonists of major insulin signaling pathways. The secreted insulin was measured by ELISA. The intracellular cAMP was estimated by cAMP acetylation assay. The acute and chronic effects of apigenin were evaluated in diabetic rats. Resultsapigenin dose-dependently enhanced insulin secretion in isolated mice islets, and its insulinotropic effect was exerted at high glucose concentrations distinctly different from glibenclamide. Furthermore, apigenin amplified glucose-induced insulin secretion in depolarized and glibenclamide-treated islets. Apigenin showed no effect on intracellular cAMP concentration; however, an additive effect was observed by apigenin in both forskolin and IBMX-induced insulin secretion. Interestingly, H89, a PKA inhibitor, and U0126, a MEK kinase inhibitor, significantly inhibited apigenin-induced insulin secretion; however, no significant effect was observed by using ESI-05, an epac2 inhibitor. Apigenin improved glucose tolerance and increased glucose-stimulated plasma insulin levels in diabetic rats. Apigenin also lowered blood glucose in diabetic rats upon chronic treatment. ConclusionApigenin exerts glucose-stimulated insulin secretion by modulating the PKA-MEK kinase signaling cascade independent of K-ATP channels.

1729-P: Hypertonicity Mediates Glucose-Induced First-Phase Insulin Secretion

1729-P: Hypertonicity Mediates Glucose-Induced First-Phase Insulin Secretion

1743-P: STING Mediates Glucose-Induced Insulin Secretion through Pax6

1743-P: STING Mediates Glucose-Induced Insulin Secretion through Pax6

A Novel Peptide COX52-69 Inhibits High Glucose-induced Insulin Secretion by Modulating BK Channel Activity.

Excessive insulin is the leading cause of metabolic syndromes besides hyperinsulinemia. Insulin-lowering therapeutic peptides have been poorly studied and warrant urgent attention. The main purpose of this study, was to introduce a novel peptide COX52-69 that was initially isolated from the porcine small intestine and possessed the ability to inhibit insulin secretion under high-glucose conditions by modulating large conductance Ca2+-activated K+ channels (BK channels) activity. Enzyme-linked immunosorbent assay results indicate that COX52-69 supressed insulin release induced by high glucose levels in pancreatic islets and animal models. Furthermore, electrophysiological data demonstrated that COX52-69 can increase BK channel currents and hyperpolarize cell membranes. Thus, cell excitability decreased, corresponding to a reduction in insulin secretion. Our study provides a novel approach to modulate high glucose-stimulated insulin secretion in patients with hyperinsulinemia.

Regulatory Role of Fatty Acid Metabolism on Glucose-Induced Changes in Insulin and Glucagon Secretion by Pancreatic Islet Cells.

A detailed study of palmitate metabolism in pancreatic islets subject to different experimental conditions, like varying concentrations of glucose, as well as fed or starved conditions, has allowed us to explore the interaction between the two main plasma nutrients and its consequences on hormone secretion. Palmitate potentiates glucose-induced insulin secretion in a concentration-dependent manner, in a physiological range of both palmitate (0-2 mM) and glucose (6-20 mM) concentrations; at glucose concentrations lower than 6 mM, no metabolic interaction with palmitate was apparent. Starvation (48 h) increased islet palmitate oxidation two-fold, and the effect was resistant to its inhibition by glucose (6-20 mM). Consequently, labelled palmitate and glucose incorporation into complex lipids were strongly suppressed, as well as glucose-induced insulin secretion and its potentiation by palmitate. 2-bromostearate, a palmitate oxidation inhibitor, fully recovered the synthesis of complex lipids and insulin secretion. We concluded that palmitate potentiation of the insulin response to glucose is not attributable to its catabolic mitochondrial oxidation but to its anabolism to complex lipids: islet lipid biosynthesis is dependent on the uptake of plasma fatty acids and the supply of α-glycerol phosphate from glycolysis. Islet secretion of glucagon and somatostatin showed a similar dependence on palmitate anabolism as insulin. The possible mechanisms implicated in the metabolic coupling between glucose and palmitate were commented on. Moreover, possible mechanisms responsible for islet gluco- or lipotoxicity after a long-term stimulation of insulin secretion were also discussed. Our own data on the simultaneous stimulation of insulin, glucagon, and somatostatin by glucose, as well as their modification by 2-bromostearate in perifused rat islets, give support to the conclusion that increased FFA anabolism, rather than its mitochondrial oxidation, results in a potentiation of their stimulated release. Starvation, besides suppressing glucose stimulation of insulin secretion, also blocks the inhibitory effect of glucose on glucagon secretion: this suggests that glucagon inhibition might be an indirect or direct effect of insulin, but not of glucose. In summary, there seems to exist three mechanisms of glucagon secretion stimulation: 1. glucagon stimulation through the same secretion coupling mechanism as insulin, but in a different range of glucose concentrations (0 to 5 mM). 2. Direct or indirect inhibition by secreted insulin in response to glucose (5-20 mM). 3. Stimulation by increased FFA anabolism in glucose intolerance or diabetes in the context of hyperlipidemia, hyperglycemia, and hypo-insulinemia. These conclusions were discussed and compared with previous published data in the literature. Specially, we discussed the mechanism for inhibition of glucagon release by glucose, which was apparently contradictory with the secretion coupling mechanism of its stimulation.

Reductions of food intake and body weight in diet-induced obese rats following chronic treatment with a monomeric peptide multiagonist

While therapies based on endogenous gut peptides such as glucagon-like peptide-1 (GLP-1) receptor agonists (GLP-1RAs) have been compelling therapeutic agents for obesity and type 2 diabetes (T2D), only a few have achieved long-term weight loss and all have shown significant side-effects, including nausea/malaise and gastrointestinal ailments. As the pathophysiology of obesity is driven by dysregulation of multiple, inter-related, pathways, we tested a novel peptide targeting multiple receptors of complementary neurocircuits regulating the controls of energy balance. Response to daily injections of GEP44, a GLP-1R and neuropeptide Y1R and Y2R receptor (Y1R/Y2R) triple agonist was tested vs. the GLP-1R agonist liraglutide (LIRA) in diet-induced obese (DIO) male and female rats. Glucose tolerance tests after intraperitoneal injection of glucose (IPGTT) were performed at baseline and after 14-d of treatment in GEP44 treated rats. Other metabolic parameters were assessed in blood at the end of a 28-d intervention. Upon conclusion at 28-d, body weight reduction compared to vehicle was-15.6%/-11.9% in response to GEP44, vs.-9.7%/-5.1% after LIRA, males, and females, respectively. Significant reductions of cumulative food intake occurred over 28-d in female rats treated with GEP44 (-30%; p<0.0001), vs. LIRA (-10%), and in male rats GEP44 (-39%; p<0.0001), vs. LIRA (-20%; p=0.003). In IPGTTs, a similar stimulation glucose induced insulin secretion was noted in rats treated with GEP44 and LIRA. The strong reductions of body weight in response to long-term applications of the triple agonist GEP44 confirms the therapeutic potential of targeting multiple receptors for achieving more robust and potentially more sustained improvement of energy balance.

Dipeptidyl Peptidase-4 (DPP-4) Inhibitors: New Oral Medications for the Treatment of Type 2 Diabetes

Type 2 diabetes (T2D) is a metabolic disorder characterized by hyperglycaemia, insulin resistance at peripheral target tissues, and pancreatic β-cell dysfunction. It is a global health problem with epidemic proportions and a huge economic burden. The dipeptidyl peptidase (DPP-4) inhibitors are a new class of antihyperglycaemic agents that are developed for the treatment of T2D. Many clinical studies have suggested that DPP-4 inhibitors (gliptins) are safe from cardiovascular perspective, and may also possess cardioprotective effect. These are regulators of inflammation and metabolism, and also prevent the proteolytic breakdown. These reduce postprandial glucose with minimal risk of hypoglycaemia and gastrointestinal adverse effects. These increase biologically intact forms of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), both of which enhance glucose-induced insulin secretion from pancreatic β-cells. These medications become popular and are widely used worldwide for their efficacy, safety, weight neutrality, and tolerability. This article reviews basic studies of the DPP-4 inhibitors in briefly for the management of T2D.

Comprehensive data on the relationship between KCNJ11 polymorphisms and gestational diabetes mellitus predisposition: a meta-analysis.

The genetic aspect of gestational diabetes mellitus (GDM) is influenced by multiple causal genetic variants, each with different effect sizes. The KCNJ11 gene is particularly noteworthy as a potential contributor to the risk of GDM due to its role in regulating glucose-induced insulin secretion. To evaluate the association between KCNJ11 polymorphisms and GDM, a comprehensive meta-analysis was conducted to review the existing literature and quantitatively assess the correlation. A thorough search was performed on the PubMed, EMBASE, Scopus, and CNKI databases until December 25, 2023, using precise terms and keywords related to Gestational Diabetes, KCNJ11 gene, and polymorphism. Odds ratios and 95% confidence intervals were used to evaluate the relationships. The statistical analysis was conducted using Comprehensive Meta-Analysis software, and the Cochrane risk of bias assessment tool was used to determine bias presence. The meta-analysis comprised 9 studies with 3108 GDM cases and 5374 controls for the rs5219 polymorphism, and 3 studies with 1209 GDM cases and 1438 controls for the rs5210 polymorphism. The pooled data indicated a noteworthy link between the rs5219 polymorphism and GDM globally and among various ethnic groups, notably in Caucasian and Asian populations. However, no substantial association was observed between the rs5210 polymorphism and GDM. Pooled data showed a correlation between the KCNJ11 rs5219 polymorphism and GDM susceptibility, but no association was found for the rs5210 polymorphism. Future research with larger sample sizes and more diverse populations is needed to improve result generalizability. The online version contains supplementary material available at 10.1007/s40200-024-01428-0.

Exploring the potential of semi-synthetic Swertiamarin analogues for GLUT facilitation and insulin secretion in NIT-1 cell lines: a molecular docking and in-vitro study

Synthesis, characterisation, and anti-diabetic potential of swertiamarin analogues against DPP-4 enzymatic inhibition was done prior to this study. However, swertiamarin and its analogues inhibited DPP-4 enzyme significantly. Semisynthetic swertiamarin analogues have been studied for antidiabetic potential and mechanism of action utilising molecular docking and in-vitro techniques. The mechanism of action for swertiamarin analogues was determined by in-silico molecular docking studies using glucose-transporters, GLUT-1 (PDB ID: 4PYP), GLUT-3 (PDB ID: 7SPS), and GLUT-4 (PDB ID: 7WSM) along with in-vitro glucose uptake and glucose-induced insulin secretion assays. These studies found that synthesised swertiamarin analogues SNIPERSV3, SNIPERSV4, and SNIPERSV7 shown better docking score against different GLUTs and better anti-diabetic effects on glucose uptake and insulin secretion in NIT-1 cell line than standard glibenclamide and swertiamarin. Thus, swertiamarin analogues might be studied for diabetes therapy in the future.

Simultaneous TIRF imaging of subplasmalemmal Ca2+ dynamics and granule fusions in insulin-secreting INS-1 cells reveals coexistent synchronized and asynchronous release

The basal and glucose-induced insulin secretion from pancreatic beta cells is a tightly regulated process that is triggered in a Ca2+-dependent fashion and further positively modulated by substances that raise intracellular levels of adenosine 3′,5′-cyclic monophosphate (cAMP) or by certain antidiabetic drugs. In a previous study, we have temporally resolved the subplasmalemmal [Ca2+]i dynamics in beta cells that are characterized by trains of sharply delimited spikes, reaching peak values up to 5 µM. Applying total internal reflection fluorescence (TIRF) microscopy and synaptopHluorin to visualize fusion events of individual granules, we found that several fusion events can coincide within 50 to 150 ms. To test whether subplasmalemmal [Ca2+]i microdomains around single or clustered Ca2+ channels may cause a synchronized release of insulin-containing vesicles, we applied simultaneous dual-color TIRF microscopy and monitored Ca2+ fluctuations and exocytotic events in INS-1 cells at high frame rates. The results indicate that fusions can be triggered by subplasmalemmal Ca2+ spiking. This, however, does account for a minority of fusion events. About 90 %-95 % of fusion events either happen between Ca2+ spikes or incidentally overlap with subplasmalemmal Ca2+ spikes. We conclude that only a fraction of exocytotic events in glucose-induced and tolbutamide- or forskolin-enhanced insulin release from INS-1 cells is tightly coupled to Ca2+ microdomains around voltage-gated Ca2+ channels.

Pharmacological and phytochemical insights on the pancreatic β-cell modulation by Angelica L. roots

Ethnopharmacological relevanceAngelica roots are a significant source of traditional medicines for various cultures around the northern hemisphere, from indigenous communities in North America to Japan. Among its many applications, the roots are used to treat type 2 diabetes mellitus; however, this application is not mentioned often. Ethnopharmacological studies have reported the use of A. japonica var. hirsutiflora, A. furcijuga, A. shikokiana, and A. keiskei to treat diabetes symptoms, and further reports have demonstrated the three angelica roots, i.e., A. japonica var. hirsutiflora, A. reflexa, and A. dahurica, exhibit insulin secretagogue activity. Aim of the studyThis study aimed to phytochemically characterize and compare angelica roots monographed in the European Pharmacopeia 11th, isolate major plant metabolites, and assess extracts and isolates' capability to modulate pancreatic β-cell function. Materials and methodsRoot extracts of Angelica archangelica, Angelica dahurica, Angelica biserrata, and Angelica sinensis were phytochemically profiled using liquid chromatography method coupled with mass spectrometry. Based on this analysis, simple and furanocoumarins were isolated using chromatography techniques. Extracts (1.6–50 μg/mL) and isolated compounds (5–40 μmol/L) were studied for their ability to modulate insulin secretion in the rat insulinoma INS-1 pancreatic β-cell model. Insulin was quantified by the homogeneous time-resolved fluorescence method. ResultsForty-one secondary metabolites, mostly coumarins, were identified in angelica root extracts. A. archangelica, A. dahurica, and A. biserrata root extracts at concentration of 12.5–50 μg/mL potentiated glucose-induced insulin secretion, which correlated with their high coumarin content. Subsequently, 23 coumarins were isolated from these roots and screened using the same protocol. Coumarins substituted with the isoprenyl group were found to be responsible for the extracts' insulinotropic effect. ConclusionsInsulinotropic effects of three pharmacopeial angelica roots were found, the metabolite profiles and pharmacological activities of the roots were correlated, and key structures responsible for the modulation of pancreatic β-cell function were identified. These findings may have implications for the traditional use of angelica roots in treating diabetes. Active plant metabolites may also become lead structures in the search for new antidiabetic treatments.

Impact of the timing of metformin administration on glycaemic and glucagon-like peptide-1 responses to intraduodenal glucose infusion in type 2 diabetes: a double-blind, randomised, placebo-controlled, crossover study

Aims/hypothesisMetformin lowers postprandial glycaemic excursions in individuals with type 2 diabetes by modulating gastrointestinal function, including the stimulation of glucagon-like peptide-1 (GLP-1). The impact of varying the timing of metformin administration on postprandial glucose metabolism is poorly defined. We evaluated the effects of metformin, administered at different intervals before an intraduodenal glucose infusion, on the subsequent glycaemic, insulinaemic and GLP-1 responses in metformin-treated type 2 diabetes.MethodsSixteen participants with type 2 diabetes that was relatively well-controlled by metformin monotherapy were studied on four separate days in a crossover design. On each day, participants were randomised to receive a bolus infusion of metformin (1000 mg in 50 ml 0.9% saline) via a nasoduodenal catheter at t = −60, −30 or 0 min (and saline at the other timepoints) or saline at all timepoints (control), followed by an intraduodenal glucose infusion of 12.56 kJ/min (3 kcal/min) at t = 0–60 min. The treatments were blinded to both participants and investigators involved in the study procedures. Plasma glucose, insulin and total GLP-1 levels were measured every 30 min between t = −60 min and t = 120 min.ResultsThere was a treatment-by-time interaction for metformin in reducing plasma glucose levels and increasing plasma GLP-1 and insulin levels (p<0.05 for each). The reduction in plasma glucose levels was greater when metformin was administered at t = −60 or −30 min vs t = 0 min (p<0.05 for each), and the increases in plasma GLP-1 levels were evident only when metformin was administered at t = −60 or −30 min (p<0.05 for each). Although metformin did not influence insulin sensitivity, it enhanced glucose-induced insulin secretion (p<0.05), and the increases in plasma insulin levels were comparable on the 3 days when metformin was given.Conclusions/interpretationIn well-controlled metformin-treated type 2 diabetes, glucose-lowering by metformin is greater when it is given before, rather than with, enteral glucose, and this is associated with a greater GLP-1 response. These observations suggest that administration of metformin before meals may optimise its effect in improving postprandial glycaemic control.Trial registrationwww.anzctr.org.au ACTRN12621000878875FundingThe study was not funded by a specific research grant.Graphical

Antidiabetic Potential of a Trimeric Anthranilic Acid Peptide Isolated from Malbranchea flocciformis.

Compound 3, a trimeric anthranilic acid peptide, and another three metabolites were isolated from an organic extract from the culture medium of Malbranchea flocciformis ATCC 34530. The chemical structure proposed previously for 3 was unequivocally assigned via synthesis and X-ray diffraction analysis. Tripeptide 3 showed insulinotropic properties by decreasing the postprandial peak in healthy and hyperglycemic mice. It also increased glucose-induced insulin secretion in INS-1E at 5 μM, specifically at higher glucose concentrations. These results revealed that 3 might act as an insulin sensitizer and a non-classical insulin secretagogue. Altogether, these findings are in harmony with the in vivo oral glucose tolerance test and acute oral hypoglycemic assay. Finally, the chemical composition of the extract was established by the Global Natural Products Social Molecular Network platform. Phylogenetic analysis using the internal transcribed spacer region revealed that M. flocciformis ATCC 34530 is related to the Malbrancheaceae.

Mouse islet-derived stellate cells are similar to, but distinct from, mesenchymal stromal cells and influence the beta cell function.

Evidence is accumulating of the therapeutic benefits of mesenchymal stromal cells (MSCs) in diabetes-related conditions. We have identified a novel population of stromal cells within islets of Langerhans - islet stellate cells (ISCs) - which have a similar morphology to MSCs. In this study we characterize mouse ISCs and compare their morphology and function to MSCs to determine whether ISCs may also have therapeutic potential in diabetes. ISCs isolated from mouse islets were compared to mouse bone marrow MSCs by analysis of cell morphology; expression of cell-surface markers and extracellular matrix (ECM) components; proliferation; apoptosis; paracrine activity; and differentiation into adipocytes, chondrocytes and osteocytes. We also assessed the effects of co-culture with ISCs or MSCs on the insulin secretory capacity of islet beta cells. Although morphological similar, ISCs were functionally distinct from MSCs. Thus, ISCs were less proliferative and more apoptotic; they had different expression levels of important paracrine factors; and they were less efficient at differentiation down multiple lineages. Co-culture of mouse islets with ISCs enhanced glucose induced insulin secretion more effectively than co-culture with MSCs. ISCs are a specific sub-type of islet-derived stromal cells that possess biological behaviors distinct from MSCs. The enhanced beneficial effects of ISCs on islet beta cell function suggests that they may offer a therapeutic target for enhancing beta cell functional survival in diabetes.

Stimulation of insulin secretion induced by low 4-cresol dose involves the RPS6KA3 signalling pathway.

4-cresol (4-methylphenol, p-cresol) is a xenobiotic substance negatively correlated with type 2 diabetes and associated with health improvement in preclinical models of diabetes. We aimed at refining our understanding of the physiological role of this metabolite and identifying potential signalling mechanisms. Functional studies revealed that 4-cresol does not deteriorate insulin sensitivity in human primary adipocytes and exhibits an additive effect to that of insulin on insulin sensitivity in mouse C2C12 myoblasts. Experiments in mouse isolated islets showed that 4-cresol potentiates glucose induced insulin secretion. We demonstrated the absence of off target effects of 4-cresol on a panel of 44 pharmacological compounds. Screening large panels of 241 G protein-coupled receptors (GPCRs) and 468 kinases identified binding of 4-cresol only to TNK1, EIF2AK4 (GCN2) and RPS6KA3 (RSK2), a kinase strongly expressed in human and rat pancreatic islets. Islet expression of RPS6KA3 is reduced in spontaneously diabetic rats chronically treated with 4-cresol and Rps6ka3 deficient mice exhibit reduction in both body weight and fasting glycemia, modest improvement in glycemic control and enhanced insulin release in vivo. Similar to low doses of 4-cresol, incubation of isolated rat islets with low concentrations of the RPS6KA3 inhibitor BIX 02565 stimulates both glucose induced insulin secretion and β-cell proliferation. These results provide further information on the role of low 4-cresol doses in the regulation of insulin secretion.

NPC1 is required for postnatal islet β cell differentiation by maintaining mitochondria turnover.

Rationale: NPC1 is a protein localized on the lysosome membrane regulating intracellular cholesterol transportation and maintaining normal lysosome function. GWAS studies have found that NPC1 variants in T2D was a pancreatic islet expression quantitative trait locus, suggesting a potential role of NPC1 in T2D islet pathophysiology. Methods: Two-week-old Npc1-/- mice and wild type littermates were employed to examine pancreatic β cell morphology and functional changes induced by loss of Npc1. Single cell RNA sequencing was conducted on primary islets. Npc1-/- Min6 cell line was generated using CRISPR/Cas9 gene editing. Seahorse XF24 was used to analyze primary islet and Min6 cell mitochondria respiration. Ultra-high-resolution cell imaging with Lattice SIM2 and electron microscope imaging were used to observe mitochondria and lysosome in primary islet β and Min6 cells. Mitophagy Dye and mt-Keima were used to measure β cell mitophagy. Results: In Npc1-/- mice, we found that β cell survival and pancreatic β cell mass expansion as well as islet glucose induced insulin secretion in 2-week-old mice were reduced. Npc1 loss retarded postnatal β cell differentiation and growth as well as impaired mitochondria oxidative phosphorylation (OXPHOS) function to increase mitochondrial superoxide production, which might be attributed to impaired autophagy flux particularly mitochondria autophagy (mitophagy) induced by dysfunctional lysosome in Npc1 null β cells. Conclusion: Our study revealed that NPC1 played an important role in maintaining normal lysosome function and mitochondria turnover, which ensured establishment of sufficient mitochondria OXPHOS for islet β cells differentiation and maturation.

Insulin granule morphology and crinosome formation in mice lacking the pancreatic β cell-specific phogrin (PTPRN2) gene.

We recently reported that phogrin, also known as IA-2β or PTPRN2, forms a complex with the insulin receptor in pancreatic β cells upon glucose stimulation and stabilizes insulin receptor substrate 2. In β cells of systemic phogrin gene knockout (IA-2β-/-) mice, impaired glucose-induced insulin secretion, decreased insulin granule density, and an increase in the number and size of lysosomes have been reported. Since phogrin is expressed not only in β cells but also in various neuroendocrine cells, the precise impact of phogrin expressed in β cells on these cells remains unclear. In this study, we performed a comprehensive analysis of morphological changes in RIP-Cre+/-Phogrinflox/flox (βKO) mice with β cell-specific phogrin gene knockout. Compared to control RIP-Cre+/- Phogrin+/+ (Ctrl) mice, aged βKO mice exhibited a decreased density of insulin granules, which can be categorized into three subtypes. While no differences were observed in the density and size of lysosomes and crinosomes, organelles involved in insulin granule reduction, significant alterations in the regions of lysosomes responding positively to carbohydrate labeling were evident in young βKO mice. These alterations differed from those in Ctrl mice and continued to change with age. These electron microscopic findings suggest that phogrin expression in pancreatic β cells plays a role in insulin granule homeostasis and crinophagy during aging, potentially through insulin autocrine signaling and other mechanisms.

Glucocorticoid receptor-NECAB1 axis can negatively regulate insulin secretion in pancreatic β-cells

The mechanisms of impaired glucose-induced insulin secretion from the pancreatic β-cells in obesity have not yet been completely elucidated. Here, we aimed to assess the effects of adipocyte-derived factors on the functioning of pancreatic β-cells. We prepared a conditioned medium using 3T3-L1 cell culture supernatant collected at day eight (D8CM) and then exposed the rat pancreatic β-cell line, INS-1D. We found that D8CM suppressed insulin secretion in INS-1D cells due to reduced intracellular calcium levels. This was mediated by the induction of a negative regulator of insulin secretion—NECAB1. LC–MS/MS analysis results revealed that D8CM possessed steroid hormones (cortisol, corticosterone, and cortisone). INS-1D cell exposure to cortisol or corticosterone increased Necab1 mRNA expression and significantly reduced insulin secretion. The increased expression of Necab1 and reduced insulin secretion effects from exposure to these hormones were completely abolished by inhibition of the glucocorticoid receptor (GR). NECAB1 expression was also increased in the pancreatic islets of db/db mice. We demonstrated that the upregulation of NECAB1 was dependent on GR activation, and that binding of the GR to the upstream regions of Necab1 was essential for this effect. NECAB1 may play a novel role in the adipoinsular axis and could be potentially involved in the pathophysiology of obesity-related diabetes mellitus.