Insulin Secretion In Pancreatic Β-cells Research Articles

Diabetes mellitus: Plasticizers and nanomaterials acting as endocrine-disrupting chemicals (Review).

Various plasticizers and nanomaterials have been linked to endocrine disruptors or endocrine-disrupting chemicals (EDCs) which represent a large, heterogeneous, yet incompletely understood group of structures acting on normal and pathological body pathways such as hormonal production, secretion, transport and receptor binding. By contrast, various applications of nanoparticles are currently under investigation since the delivery of useful drugs, particularly insulin in diabetes mellitus, is essential in case of insulin deficiency. The aim of the present review was to introduce and examine different plasticizers and nanomaterials with potential applications for diabetic patients (such as selenium or gold-based nanoparticles that help the oral delivery of insulin) or plasticizers/nanomaterials acting similarly to EDCs in relation to the human and animal body, particularly glucose metabolism impairment such as diabetes mellitus (DM). Bisphenol A is a chemical used worldwide; however, the effect of exposure varies with regard to the source, environment, time of exposure and the age of the organism. Daily exposure is mostly related to food and drinks stored in polycarbonate plastics. However, exposure may also be through the skin or through the maternal placenta or breast milk which are risk factors for the fetus and for the newborn. It exerts an estrogen-like profile, but it also induces insulin resistance by impairing peripheral insulin receptors or it decreases insulin secretion by acting at the level of insulin-secreting pancreatic β-cells. Phthalates, compounds of flexible plastics, act as EDCs via their human metabolites such as diethyl phthalate and derivative monoethyl phthalate. Their role in inducing gestational DM and weight gain/obesity during pregnancy has been showcased. The vast field of plasticizers and nanomolecules acting as endocrine disruptors is widely linked to clinical aspects of DM, a serious condition with a major population impact. The importance of understanding and using these agents and applications is reflected in saving numerous human lives.

Glucose-6-phosphatase catalytic subunit 2 negatively regulates glucose oxidation and insulin secretion in pancreatic β-cells

Elevated fasting blood glucose (FBG) is associated with increased risks of developing type 2 diabetes (T2D) and cardiovascular-associated mortality. G6PC2 is predominantly expressed in islets, encodes a glucose-6-phosphatase catalytic subunit that converts glucose-6-phosphate (G6P) to glucose, and has been linked with variations in FBG in genome-wide association studies. Deletion of G6pc2 in mice has been shown to lower FBG without affecting fasting plasma insulin levels in vivo. At 5 mM glucose, pancreatic islets from G6pc2 knockout (KO) mice exhibit no glucose cycling, increased glycolytic flux, and enhanced glucose-stimulated insulin secretion (GSIS). However, the broader effects of G6pc2 KO on β-cell metabolism and redox regulation are unknown. Here we used CRISPR/Cas9 gene editing and metabolic flux analysis in βTC3 cells, a murine pancreatic β-cell line, to examine the role of G6pc2 in regulating glycolytic and mitochondrial fluxes. We found that deletion of G6pc2 led to ∼60% increases in glycolytic and citric acid cycle (CAC) fluxes at both 5 and 11 mM glucose concentrations. Furthermore, intracellular insulin content and GSIS were enhanced by approximately two-fold, along with increased cytosolic redox potential and reductive carboxylation flux. Normalization of fluxes relative to net glucose uptake revealed upregulation in two NADPH-producing pathways in the CAC. These results demonstrate that G6pc2 regulates GSIS by modulating not only glycolysis but also, independently, citric acid cycle activity in β-cells. Overall, our findings implicate G6PC2 as a potential therapeutic target for enhancing insulin secretion and lowering FBG, which could benefit individuals with prediabetes, T2D, and obesity.

In Silico Searching for Alternative Lead Compounds to Treat Type 2 Diabetes through a QSAR and Molecular Dynamics Study.

Free fatty acid receptor 1 (FFA1) stimulates insulin secretion in pancreatic β-cells. An advantage of therapies that target FFA1 is their reduced risk of hypoglycemia relative to common type 2 diabetes treatments. In this work, quantitative structure–activity relationship (QSAR) approach was used to construct models to identify possible FFA1 agonists by applying four different machine-learning algorithms. The best model (M2) meets the Tropsha’s test requirements and has the statistics parameters R2 = 0.843, Q2CV = 0.785, and Q2ext = 0.855. Also, coverage of 100% of the test set based on the applicability domain analysis was obtained. Furthermore, a deep analysis based on the ADME predictions, molecular docking, and molecular dynamics simulations was performed. The lipophilicity and the residue interactions were used as relevant criteria for selecting a candidate from the screening of the DiaNat and DrugBank databases. Finally, the FDA-approved drugs bilastine, bromfenac, and fenofibric acid are suggested as potential and lead FFA1 agonists.

The Taurine-Conjugated Bile Acid (TUDCA) Normalizes Insulin Secretion in Pancreatic β-Cells Exposed to Fatty Acids: The Role of Mitochondrial Metabolism.

Bile acid tauroursodeoxycholic (TUDCA), formed from the association of ursodeoxycholic acid (UDCA) with taurine, has already been shown to increase mitochondrial biogenesis and cell survival, in addition to reduce reticulum stress markers in different cell types. However, its mechanism of action upon insulin secretion control in obesity is still unknown. In this sense, we seek to clarify whether taurine, associated with bile acid, could improve the function of the pancreatic β-cells exposed to fatty acids through the regulation of mitochondrial metabolism. To test this idea, insulin-producing cells (INS1-E) were exposed to a fatty acid mix containing 500μM of each palmitate and oleate for 48hours treated or not with 300μM of TUDCA. After that, glucose-stimulated insulin secretion and markers of mitochondrial metabolism were evaluated. Our results showed that the fatty acid mix was efficient in inducing hyperfunction of INS1-E cells as observed by the increase in insulin secretion, protein expression of citrate synthase, and mitochondrial density, without altering cell viability. The treatment with TUDCA normalized insulin secretion, reducing the protein expression of citrate synthase, mitochondrial mass, and the mitochondrial membrane potential. This effect was associated with a decrease in the generation of mitochondrial superoxide and c-Jun N-terminal kinase (JNK) protein content. The findings are also consistent with the hypothesis that TUDCA normalizes insulin secretion by improving mitochondrial metabolism and redox balance. Thus, it highlights likely mechanisms of the action of this bile acid on the glycemic homeostasis reestablishment in obesity.

ATP is an essential autocrine factor for pancreatic β-cell signaling and insulin secretion.

ATP has been previously identified as an autocrine signaling factor that is co‐released with insulin to modulate and propagate β‐cell activity within islets of Langerhans. Here, we show that β‐cell activity and insulin secretion essentially rely on the presence of extracellular ATP. For this, we monitored changes of the intracellular Ca2+ concentration ([Ca2+]i oscillations) as an immediate read‐out for insulin secretion in live cell experiments. Extensive washing of cells or depletion of extracellular ATP levels by recombinant apyrase reduced [Ca2+]i oscillations and insulin secretion in pancreatic cell lines and primary β‐cells. Following ATP depletion, [Ca2+]i oscillations were stimulated by the replenishment of ATP in a concentration‐dependent manner. Inhibition of endogenous ecto‐ATP nucleotidases increased extracellular ATP levels, along with [Ca2+]i oscillations and insulin secretion, indicating that there is a constant supply of ATP to the extracellular space. Our combined results demonstrate that extracellular ATP is essential for β‐cell activity. The presented work suggests extracellular ATPases as potential drug targets for the modulation of insulin release. We further found that exogenous fatty acids compensated for depleted extracellular ATP levels by the recovery of [Ca2+]i oscillations, indicating that autocrine factors mutually compensate for the loss of others. Thereby, our results contribute to a more detailed and complete understanding of the general role of autocrine signaling factors as a fundamental regulatory mechanism of β‐cell activity and insulin secretion.

Effect of vitamin D supplementation on HbA1c level in a cohort of type 1 diabetic patients

Background and objective: Glycemic control is essential to halt the progression of diabetic complications. Some studies previously showed the effect of vitamin D on pancreatic β-cell function and insulin secretion. This study aimed to assess the effect of vitamin D supplementation on HbA1c in patients with type 1 diabetes and vitamin D deficiency. Methods: This prospective quasi-experimental study was conducted in Erbil city from 1st September to 1st December 2019. In this study, 76 patients attending Layla Qasim diabetic center were tested. Only 50 patients were recruited that had vitamin D deficiency (25OHD: <20) and were aged less than 18 years. They received vitamin D supplementation for 12 weeks. HbA1c and vitamin D level was measured before and after supplementation. Results: Before vitamin D supplementation, the mean level of serum 25(OH)D among females (8.67 ± 2.56) was lower than males (14.31 ± 3.19). However, no association was found between the initial HbA1c level and gender. HbA1c level (mean = 8.24 ± 0.49) improved significantly after vitamin D supplementation to the level of 7.93 ± 0.67 (P = 0.032). There was a significant difference between groups of glycosylated hemoglobin levels of (first tertile<7.5%, second tertile 7.6%-9.9%, third tertile ≥10%) after 12 weeks of supplementation with vitamin D (P <0.001). Conclusion: Vitamin D supplementation in type 1 diabetic patients with vitamin D deficiency lead to significant improvement in HbA1c level. Keywords: Diabetes; Deficiency; Vitamin D.

SGC-CK2-1 Is an Efficient Inducer of Insulin Production and Secretion in Pancreatic β-Cells.

The pyrazolopyrimidine based compound SGC-CK2-1 is a potent and highly specific CK2 inhibitor and a new tool to study the biological functions of protein kinase CK2 irrespective from off-target effects. We used this compound in comparison with the well-established CK2 inhibitor CX-4945 to analyze the importance of CK2 for insulin production and secretion from pancreatic β-cells. Both inhibitors affected the proliferation and viability of MIN6 cells only marginally and downregulated the endogenous CK2 activity to a similar level. Furthermore, both inhibitors increased the message for insulin and boosted the secretion of insulin from storage vesicles. Thus, regarding the high specificity of SGC-CK2-1, we can clearly attribute the observed effects to biological functions of protein kinase CK2.

Potential Mechanisms Underlying Hypoxia-Induced Diabetes in a Rodent Model: Implications for COVID-19

Previous studies reported that repetitive hypoxia in rat pups reduces insulin secretion and elevates fasting blood glucose levels; these sequelae persisted for several months. This report describes how episodic hypoxic events elevate a chloride ion exporter, K+-Cl− cotransporter-2 (KCC2), in the plasma membrane of insulin-secreting pancreatic β-cells. We assume that acute diabetic symptoms observed in rat pups with periodic oxygen desaturation could result from a lack of blood insulin levels due to disturbed β-cell function. This acute hypo-insulinemia may result from a disruption in chloride balance in β-cells arising from an imbalanced KCC2-NKCC1 (chloride exporter-importer) density as a consequence of periodic oxygen desaturation. Mechanistically, we postulate that a reduced insulin secretion due to the KCC2-NKCC1 imbalance subsequent to acute oxygen desaturation could result in hyperglycemia in rat pups, paralleling symptoms shown in patients with COVID-19 who experienced acute respiratory distress.

High bicarbonate concentration increases glucose-induced insulin secretion in pancreatic β-cells

Low serum bicarbonate is closely related to type 2 diabetes mellitus. However, the precise role of bicarbonate on glucose homeostasis and insulin secretion remains unknown. In this study, we investigated the effects of bicarbonate concentration on pancreatic β-cells. It was observed that the high bicarbonate concentration of the cell culture medium significantly increased the glucose-induced insulin secretion (GSIS) levels in mouse islets, MIN6, and the INS-1E β cells. MIN6 cells presented an impaired GSIS; the cells produced a lower bicarbonate concentration when co-cultured with Capan-1 than when with CFPAC-1. NBCe1, a major bicarbonate transporter was observed to block the increasing insulin secretions, which were promoted by a high concentration of bicarbonate. In addition, higher extracellular bicarbonate concentration significantly increased the intracellular cAMP level, pHi, and calcium concentration with a 16.7 mM of glucose stimulation. Further study demonstrated that a low concentration of extracellular bicarbonate significantly impaired the functioning of pancreatic β cells by reducing coupling Ca2+ influx, whose process may be modulated by NBCe1. Taken together, our results conclude that bicarbonate may serve as a novel target in diabetes prevention-related research.

Structure-activity relationships of agonists for the orphan G protein-coupled receptor GPR27.

GPR27 belongs, with GPR85 and GPR173, to a small subfamily of three receptors called "Super-Conserved Receptors Expressed in the Brain" (SREB). It has been postulated to participate in key physiological processes such as neuronal plasticity, energy metabolism, and pancreatic β-cell insulin secretion and regulation. Recently, we reported the first selective GPR27 agonist, 2,4-dichloro-N-(4-(N-phenylsulfamoyl)phenyl)benzamide (I, pEC50 6.34, Emax 100%). Here, we describe the synthesis and structure-activity relationships of a series of new derivatives and analogs of I. All products were evaluated for their ability to activate GPR27 in an arrestin recruitment assay. As a result, agonists were identified with a broad range of efficacies including partial and full agonists, showing higher efficacies than the lead compound I. The most potent agonist was 4-chloro-2,5-difluoro-N-(4-(N-phenylsulfamoyl)phenyl)benzamide (7y, pEC50 6.85, Emax 37%), and the agonists with higher efficacies were 4-chloro-2-methyl-N-(4-(N-phenylsulfamoyl)phenyl)benzamide (7p, pEC50 6.04, Emax 123%), and 2-bromo-4-chloro-N-(4-(N-phenylsulfamoyl)phenyl)benzamide (7r, pEC50 5.99, Emax 123%). Docking studies predicted the putative binding site and interactions of agonist 7p with GPR27. Selected potent agonists were found to be soluble and devoid of cellular toxicity within the range of their pharmacological activity. Therefore, they represent important new tools to further characterize the (patho)physiological roles of GPR27.

Angiotensin(1–7) activates MAS-1 and upregulates CFTR to promote insulin secretion in pancreatic β-cells: the association with type 2 diabetes

ObjectiveThe beneficial effect of angiotensin(1–7) (Ang(1–7)), via the activation of its receptor, MAS-1, has been noted in diabetes treatment; however, how Ang(1–7) or MAS-1 affects insulin secretion remains elusive and whether the endogenous level of Ang(1–7) or MAS-1 is altered in diabetic individuals remains unexplored. We recently identified an important role of cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl− channel, in the regulation of insulin secretion. Here, we tested the possible involvement of CFTR in mediating Ang(1–7)’s effect on insulin secretion and measured the level of Ang(1–7), MAS-1 as well as CFTR in the blood of individuals with or without type 2 diabetes.MethodsAng(1–7)/MAS-1/CFTR pathway was determined by specific inhibitors, gene manipulation, Western blotting as well as insulin ELISA in a pancreatic β-cell line, RINm5F. Human blood samples were collected from 333 individuals with (n = 197) and without (n = 136) type 2 diabetes. Ang(1–7), MAS-1 and CFTR levels in the human blood were determined by ELISA.ResultsIn RINm5F cells, Ang(1–7) induced intracellular cAMP increase, cAMP-response element binding protein (CREB) activation, enhanced CFTR expression and potentiated glucose-stimulated insulin secretion, which were abolished by a selective CFTR inhibitor, RNAi-knockdown of CFTR, or inhibition of MAS-1. In human subjects, the blood levels of MAS-1 and CFTR, but not Ang(1–7), were significantly higher in individuals with type 2 diabetes as compared to those in non-diabetic healthy subjects. In addition, blood levels of MAS-1 and CFTR were in significant positive correlation in type-2 diabetic but not non-diabetic subjects.ConclusionThese results suggested that MAS-1 and CFTR as key players in mediating Ang(1–7)-promoted insulin secretion in pancreatic β-cells; MAS-1 and CFTR are positively correlated and both upregulated in type 2 diabetes.

Protective Effect of Siegesbeckia orientalis on Pancreatic β-Cells under High Glucose-Induced Glucotoxicity

The glucotoxicity caused by long-term exposure of β-cells to high glucose (HG) conditions may lead to the generation of more reactive oxygen species (ROS), reduce the activity of antioxidant enzymes, cause cell damage and apoptosis, and induce insulin secretion dysfunction. Siegesbeckia orientalis linne is a traditional folk herbal medicine used to treat snake bites, rheumatoid arthritis, allergies, and immune deficiencies. In this study, we evaluated the protective effect of S. orientalis ethanol extract (SOE) on cell death and oxidative stress in RIN-m5f pancreatic β-cells stimulated by two HG concentrations (50–100 mM). In the cell viability assay, SOE could significantly increase the survival rate of pancreatic β-cells under HG-induced conditions. For the oxidative stress induced by HG condition, the treatment of SOE effectively reduced the ROS formation, increased the content of intracellular glutathione, and up-regulated the expression of antioxidant enzymes, catalase, superoxide dismutase, and glutathione peroxidase. As a result, the SOE treatment could decrease the glucotoxicity-mediated oxidative damage on RIN-m5F β-cells. Moreover, SOE had the function of regulating insulin secretion in pancreatic β-cells under different HG-mediated conditions. It could decrease the increasing intracellular insulin secretion under the low glucose concentration to normal level; while increase the decreasing intracellular insulin secretion under the relatively high glucose concentration to normal level. Taken together, this study suggests that SOE has a protective effect on pancreatic β-cells under the HG-stimulated glucotoxic environment.

Let7b-5p inhibits insulin secretion and decreases pancreatic β-cell mass in mice

MicroRNAs are crucial regulators for the development, mass and function of pancreatic β-cells. MiRNA dysregulation is associated with β-cell dysfunction and development of diabetes. The members of let7 family are important players in regulating cellular growth and metabolism. In this study we investigated the functional role of let7b-5p in the mouse pancreatic β-cells. We generated pancreatic β-cell-specific let7b-5p transgenic mouse model and analyzed the glucose metabolic phenotype, β-cells mass and insulin secretion in vivo. Luciferase reporter assay, immunofluorescence staining and western blot were carried out to study the target genes of let7b-5p in β-cells. Let7b-5p overexpression impaired the insulin production and secretion of β-cells and resulted impaired glucose tolerance in mice. The overexpressed let7b-5p inhibited pancreatic β-cell proliferation and decreased the expression of cyclin D1 and cyclin D2. Our findings demonstrated that let7b-5p was critical in regulating the proliferation and insulin secretion of pancreatic β-cells.

Long Non-Coding RNA Small Nucleolar RNA Host Gene 14 Enhances Pancreatic-β-Cell Function by Sponging MicroRNA-206 and Thereby Upregulating Insulin-Like Growth Factor-1

Diabetes mellitus (DM) characterised by pancreatic β-cell dysfunction and insulin resistance is already prevalent worldwide, and the complications of DM increased the mortality of diabetic patients. More and more reports showed that long non-coding RNA (lncRNA) plays key roles in DM, and in this study we explored the effect of long non-coding RNA SNHG14 (SNHG14) in DM. Quantitative reverse transcription PCR (qRT-PCR) was used to analyze the expression of SNHG14 and microRNA (miR)-206 in peripheral blood from 60 pairs of healthy and DM samples. Then the effect of SNHG14 on the insulin secretion of pancreatic β-cells was investigated. Moreover, the effects of SNHG14 on the viability and apoptosis of pancreatic β-cells were determined by 3-(4, 5)-dimethylthiahiazo (-z-y1)-3, 5-di- phenytetrazoliumromide (MTT) assay and flow cytometry (FCM) respectively using INS-1 cells transfected with SNHG14-plasmid and/or miR-206 mimic. Bioinformatics and Daul luciferase reporter assay were used to confirm the direct target of miR-206. Our data showed that SNHG14 level was decreased but miR-206 was increased significantly in the peripheral blood of DM patients. Furthermore, over-expression of SNHG14 enhanced insulin secretion and cell viability but inhibited apoptosis of pancreatic β-cells, whereas, miR-206 over-expression reversed these effects. In addition, insulin-like growth factor-1 (IGF1) was proved to be regulated by miR-206 negatively and over-expression of SNHG14 increased the expression of IGF1 and IGF1R in pancreatic β-cells. In conclusion, we found that SNHG14 was significantly down-regulated in DM patients, and SNHG14/miR-206/IGF1 may serve as a potential candidate for the clinical target of DM.

Tentonin 3/TMEM150C regulates glucose-stimulated insulin secretion in pancreatic β-cells.

Glucose homeostasis is initially regulated by the pancreatic hormone insulin. Glucose-stimulated insulin secretion in β-cells is composed of two cellular mechanisms: a high glucose concentration not only depolarizes the membrane potential of the β-cells by ATP-sensitive K+ channels but also induces cell inflation, which is sufficient to release insulin granules. However, the molecular identity of the stretch-activated cation channel responsible for the latter pathway remains unknown. Here, we demonstrate that Tentonin 3/TMEM150C (TTN3), a mechanosensitive channel, contributes to glucose-stimulated insulin secretion by mediating cation influx. TTN3 is expressed specifically in β-cells and mediates cation currents to glucose and hypotonic stimulations. The glucose-induced depolarization, firing activity, and Ca2+ influx of β-cells were significantly lower in Ttn3-/- mice. More importantly, Ttn3-/- mice show impaired glucose tolerance with decreased insulin secretion invivo. We propose that TTN3, as a stretch-activated cation channel, contributes to glucose-stimulated insulin secretion.

Schisandrin C Affects Glucose-Stimulated Insulin Secretion in Pancreatic β-Cells and Glucose Uptake in Skeletal Muscle Cells.

The aim of our study was to investigate the effect of three lignans (schisandrol A, schisandrol B, and schisandrin C) on insulin secretion in rat INS-1 pancreatic β-cells and glucose uptake in mouse C2C12 skeletal muscle cells. Schisandrol A and schisandrin C enhanced insulin secretion in response to high glucose levels with no toxic effects on INS-1 cells. The effect of schisandrin C was superior to that of gliclazide (positive control), a drug commonly used to treat type 2 diabetes (T2D). In addition, western blot analysis showed that the expression of associated proteins, including peroxisome proliferator-activated receptor γ (PPARγ), pancreatic and duodenal homeobox 1 (PDX-1), phosphatidylinositol 3-kinase (PI3K), Akt, and insulin receptor substrate-2 (IRS-2), was increased in INS-1 cells after treatment with schisandrin C. In addition, insulin secretion effect of schisandrin C were enhanced by the Bay K 8644 (L-type Ca2+ channel agonist) and glibenclamide (K+ channel blocker), were abolished by the nifedipine (L-type Ca2+ channel blocker) and diazoxide (K+ channel activator). Moreover, schisandrin C enhanced glucose uptake with no toxic effects on C2C12 cells. Western blot analysis showed that the expression of associated proteins, including insulin receptor substrate-1 (IRS-1), AMP-activated protein kinase (AMPK), PI3K, Akt, glucose transporter type 4 (GLUT-4), was increased in C2C12 cells after treatment with schisandrin C. Schisandrin C may improve hyperglycemia by enhancing insulin secretion in pancreatic β-cells and improving glucose uptake into skeletal muscle cells. Our findings may provide evidence that schisandrin C may be beneficial in devising novel anti-T2D strategies.

Pancreatic β-Cell O-GlcNAc Transferase Overexpression Increases Susceptibility to Metabolic Stressors in Female Mice.

The nutrient-sensor O-GlcNAc transferase (Ogt), the sole enzyme that adds an O-GlcNAc-modification onto proteins, plays a critical role for pancreatic β-cell survival and insulin secretion. We hypothesized that β-cell Ogt overexpression would confer protection from β-cell failure in response to metabolic stressors, such as high-fat diet (HFD) and streptozocin (STZ). Here, we generated a β-cell-specific Ogt in overexpressing (βOgtOE) mice, where a significant increase in Ogt protein level and O-GlcNAc-modification of proteins were observed in islets under a normal chow diet. We uncovered that βOgtOE mice show normal peripheral insulin sensitivity and glucose tolerance with a regular chow diet. However, when challenged with an HFD, only female βOgtOE (homozygous) Hz mice developed a mild glucose intolerance, despite increased insulin secretion and normal β-cell mass. While female mice are normally resistant to low-dose STZ treatments, the βOgtOE Hz mice developed hyperglycemia and glucose intolerance post-STZ treatment. Transcriptome analysis between islets with loss or gain of Ogt by RNA sequencing shows common altered pathways involving pro-survival Erk and Akt and inflammatory regulators IL1β and NFkβ. Together, these data show a possible gene dosage effect of Ogt and the importance O-GlcNAc cycling in β-cell survival and function to regulate glucose homeostasis.

MafA, NeuroD1, and HNF1β synergistically activate the Slc2a2 (Glut2) gene in β-cells.

Glucose transporter type 2 (GLUT2), encoded by the SLC2A2 gene, is an essential component of glucose-stimulated insulin secretion in pancreatic islet β-cells. Like that of the gene encoding insulin, expression of the SLC2A2 gene expression is closely linked to β-cell functionality in rodents, but the mechanism by which β-cell-specific expression of SLC2A2 is controlled remains unclear. In this report, to identify putative enhancer elements of the mouse Slc2a2 gene, we examined evolutional conservation of the nucleotide sequence of its genomic locus, together with ChIP-seq data of histone modifications and various transcription factors published in previous studies. Using luciferase reporter assays, we found that an evolutionarily conserved region (ECR) located approximately 40 kbp downstream of the transcription start site of Slc2a2 functions as an active enhancer in the MIN6 β-cell line. We also found that three β-cell-enriched transcription factors, MafA, NeuroD1, and HNF1β, synergistically activate transcription through this 3' downstream distal enhancer (ECR3') and the proximal promoter region of the gene. Our data also indicate that the simultaneous binding of HNF1β to its target sites within the promoter and ECR3' of Slc2a2 is indispensable for transcriptional activation, and that binding of MafA and NeuroD1 to their respective target sites within the ECR3' enhances transcription. Co-immunoprecipitation experiments suggested that MafA, NeuroD1, and HNF1β interact with each other. Overall, these results suggest that promoter-enhancer communication through MafA, NeuroD1, and HNF1β is critical for Slc2a2 gene expression. These findings provide clues to help elucidate the mechanism of regulation of Slc2a2 gene expression in β-cells.

Nicotinamide-cinnamic acid cocktail exerts pancreatic β-cells survival coupled with insulin secretion through ERK1/2 signaling pathway in an animal model of apoptosis.

Pancreatic β-cells protection is integral to insulin secretion in diabetic conditions. In this context, we investigated cinnamic acid in combination with nicotinamide on the regulation of insulin secretion and apoptosis in pancreatic β-cells using streptozotocin (STZ)-induced apoptotic model in vivo. The pancreata of nicotinamide (NA)-cinnamic acid (CA) treated rats were studied using histopathological, immunofluorescence, molecular docking, and RT-PCR analyses, supported by serum glucose and insulin levels. The biochemical data revealed that the acute treatment of NA and CA in combination significantly increased serum insulin, thereby lowering blood glucose level in vivo. From histological findings, NA-CA pre-treatment displayed significant protection against STZ-apoptotic trends, improved insulin secretion, and recapitulated the STZ-induced morphology to normal control. The upregulated expressions of caspases, caused by STZ-treatments, were significantly downregulated with NA-CA in immunofluorescent detection and their translational levels, respectively. We found dense ERK½-insulin staining and p-ERK½ expression, which was further supported by strong ERK½ residues-ligands interactions based on in silico analysis. From the pre-clinical data, we thus conclude that NA-CA cocktail exerts dual insulin releasing and survival effects in pancreatic β-cells by targeting ERK½ pathway.

Contribution of Mitochondria to Insulin Secretion by Various Secretagogues

Significance:Mitochondria determine glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells by elevating ATP synthesis. As the metabolic and redox hub, mitochondria provide numerous links to the plasma membrane channels, insulin granule vesicles (IGVs), cell redox, NADH, NADPH, and Ca2+ homeostasis, all affecting insulin secretion.Recent Advances:Mitochondrial redox signaling was implicated in several modes of insulin secretion (branched-chain ketoacid [BCKA]-, fatty acid [FA]-stimulated). Mitochondrial Ca2+ influx was found to enhance GSIS, reflecting cytosolic Ca2+ oscillations induced by action potential spikes (intermittent opening of voltage-dependent Ca2+ and K+ channels) or the superimposed Ca2+ release from the endoplasmic reticulum (ER). The ATPase inhibitory factor 1 (IF1) was reported to tune the glucose sensitivity range for GSIS. Mitochondrial protein kinase A was implicated in preventing the IF1-mediated inhibition of the ATP synthase.Critical Issues:It is unknown how the redox signal spreads up to the plasma membrane and what its targets are, what the differences in metabolic, redox, NADH/NADPH, and Ca2+ signaling, and homeostasis are between the first and second GSIS phase, and whether mitochondria can replace ER in the amplification of IGV exocytosis.Future Directions:Metabolomics studies performed to distinguish between the mitochondrial matrix and cytosolic metabolites will elucidate further details. Identifying the targets of cell signaling into mitochondria and of mitochondrial retrograde metabolic and redox signals to the cell will uncover further molecular mechanisms for insulin secretion stimulated by glucose, BCKAs, and FAs, and the amplification of secretion by glucagon-like peptide (GLP-1) and metabotropic receptors. They will identify the distinction between the hub β-cells and their followers in intact and diabetic states. Antioxid. Redox Signal. 36, 920–952.