Microarray analysis of novel candidate genes responsible for glucose-stimulated insulin secretion in mouse pancreatic β cell line MIN6

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

BACKGROUND

Excessive insulin is the leading cause of metabolic syndromes besides hyperinsulinemia. Insulin-lowering therapeutic peptides have been poorly studied and warrant urgent attention.

OBJECTIVES

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.

METHODS AND RESULTS

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.

CONCLUSION

Our study provides a novel approach to modulate high glucose-stimulated insulin secretion in patients with hyperinsulinemia.

A large-scale functional analysis of genes expressed differentially in insulin secreting MIN6 sublines with high versus mildly reduced glucose-responsiveness

Molecular mechanisms of glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells are not fully understood. GSIS deteriorations are believed to underlie the pathogenesis of type 2 diabetes mellitus. By comparing transcript levels of 3 insulin secreting MIN6 cell sublines with strong glucose-responsiveness and 3 with mildly reduced responsiveness, we identified 630 differentially expressed genes. Using our recently developed system based on recombinase-mediated cassette exchange, we conducted large-scale generation of stable clones overexpressing such genes in the doxycycline-regulated manner. We found that overexpressions of 18, out of 83, genes altered GSIS. Sox11 ((sex determining region Y)-box 11) was selected to confirm its roles in regulating insulin secretion, and the gene was subjected to shRNA-mediated suppression. While Sox11 overexpression decreased GSIS, its suppression increased GSIS, confirming the role of Sox11 as a negative regulator of insulin secretion. Furthermore, metabolic experiments using radiolabelled glucose showed Sox11 to participate in regulating glucose metabolism. Our data suggested that overexpression screening is a feasible option for systemic functional testing to identify important genes in GSIS.

Aberrant Expression of Rest4 Gene in Low-Functioning Pancreatic Beta Cell Line

Repressor element-1 silencing transcription factor (Rest) is not expressed in pancreatic beta cells and neuronal cells. However, Rest4, a truncated form of Rest, is expressed in high passaged MIN6 (HP-MIN6) cells, a pancreatic beta cell line that lost glucose-responsive insulin secretion. Rest4 is also expressed in injured MIN6 cells and isolated islets. Herein, the forced expression of dominant negative form of Rest in HP-MIN6 cells was subjected to microarray analysis of gene expression to investigate the role of Rest4 gene in MIN6 cells. Furthermore, the forced expression of Rest4 gene in MIN6 cells was subjected to microarray analysis of gene expression to investigate the function of Rest4 in normal insulin-producing cells. The results showed that Rest4 inhibits cell proliferation and DNA and RNA metabolism and stimulates secretory mechanisms and nervous system gene expression. These findings suggest that Rest4 may act defensively against cellular injury in pancreatic beta cells.

Application of In Vitro Models for Studying the Mechanisms Underlying the Obesogenic Action of Endocrine-Disrupting Chemicals (EDCs) as Food Contaminants-A Review

Obesogenic endocrine-disrupting chemicals (EDCs) belong to the group of environmental contaminants, which can adversely affect human health. A growing body of evidence supports that chronic exposure to EDCs can contribute to a rapid increase in obesity among adults and children, especially in wealthy industrialized countries with a high production of widely used industrial chemicals such as plasticizers (bisphenols and phthalates), parabens, flame retardants, and pesticides. The main source of human exposure to obesogenic EDCs is through diet, particularly with the consumption of contaminated food such as meat, fish, fruit, vegetables, milk, and dairy products. EDCs can promote obesity by stimulating adipo- and lipogenesis of target cells such as adipocytes and hepatocytes, disrupting glucose metabolism and insulin secretion, and impacting hormonal appetite/satiety regulation. In vitro models still play an essential role in investigating potential environmental obesogens. The review aimed to provide information on currently available two-dimensional (2D) in vitro animal and human cell models applied for studying the mechanisms of obesogenic action of various industrial chemicals such as food contaminants. The advantages and limitations of in vitro models representing the crucial endocrine tissue (adipose tissue) and organs (liver and pancreas) involved in the etiology of obesity and metabolic diseases, which are applied to evaluate the effects of obesogenic EDCs and their disruption activity, were thoroughly and critically discussed.

DYRK1A inhibitors leucettines and TGF-β inhibitor additively stimulate insulin production in beta cells, organoids, and isolated mouse islets

The decreased β-cell mass and impaired β-cell functionality are the primary causes of diabetes mellitus (DM). Nevertheless, the underlying molecular mechanisms by which β-cell growth and function are controlled are not fully understood. In this work, we show that leucettines, known to be DYRK1A kinase inhibitors, can improve glucose-stimulated insulin secretion (GSIS) in rodent β-cells and isolated islets, as well as in hiPSC-derived β-cells islets. We confirm that DYRK1A is expressed in murine insulinoma cells MIN6. In addition, we found that treatment with selected leucettines stimulates proliferation of β-cells and promotes MIN6 cell cycle progression to the G2/M phase. This effect is also confirmed by increased levels of cyclin D1, which is highly responsive to proliferative signals. Among other leucettines, leucettine L43 had a negligible impact on β-cell proliferation, but markedly impair GSIS. However, leucettine L41, in combination with LY364947, a, a potent and selective TGF-β type-I receptor, significantly promotes GSIS in various cellular diabetic models, including MIN6 and INS1E cells in 2D and 3D culture, iPSC-derived β-cell islets derived from iPSC, and isolated mouse islets, by increased insulin secretion and decreased glucagon level. Our findings confirm an important role of DYRK1A inhibitors as modulators of β-cells function and suggested a new potential target for antidiabetic therapy. Moreover, we show in detail that leucettine derivatives represent promising antidiabetic agents and are worth further evaluation, especially in vivo.

Perfluorooctanoic acid promotes pancreatic β cell dysfunction and apoptosis through ER stress and the ATF4/CHOP/TRIB3 pathway

Perfluorooctanoic acid (PFOA), a widely used chemical substance, causes an increased risk of human type 2 diabetes (T2D), but its underlying mechanism is not well elucidated. The aim of the present study was to investigate whether PFOA regulates the functions of pancreatic β cells, which are specialized for the biosynthesis and secretion of insulin. The treatment of the mouse pancreatic β cell line (MIN6 cells) with PFOA caused a time- and dose-dependent inhibition of cell viability in CCK-8 assays. Annexin V/PI and TUNEL staining results confirmed that exposure to a high PFOA dose (500 μM) promoted apoptosis of β cells, while a low dose (300 μM) had no effects on β cell survival. PFOA treatment, even at a low dose, diminished glucose-stimulated insulin secretion (GSIS) in both primary islet perfusion and MIN6 cell experiments. RNA-sequencing data showed significantly increased expression of endoplasmic reticulum (ER) stress-associated genes, with tribbles homolog 3 (Trib3) ranking first among the altered genes. The activation of ER stress pathways was verified by qRT-PCR assays, and the ATF4/CHOP/TRIB3 pathway contributed to PFOA-induced β cell damage. The inhibition of TRIB3 expression significantly protected MIN6 cells from PFOA-induced GSIS defects and apoptosis by ameliorating ER stress. These findings reveal a link between ER stress and PFOA-induced β cell defects, opening up a new set of questions about the pathogenesis of T2D due to environmental chemicals.

Importance of Heparan Sulfate Proteoglycans in Pancreatic Islets and β-Cells

β-cells in the islets of Langerhans of the pancreas secrete insulin in response to the glucose concentration in the blood. When these pancreatic β-cells are damaged, diabetes develops through glucose intolerance caused by insufficient insulin secretion. High molecular weight polysaccharides, such as heparin and heparan sulfate (HS) proteoglycans, and HS-degrading enzymes, such as heparinase, participate in the protection, maintenance, and enhancement of the functions of pancreatic islets and β-cells, and the demand for studies on glycobiology within the field of diabetes research has increased. This review introduces the roles of complex glycoconjugates containing high molecular weight polysaccharides and their degrading enzymes in pancreatic islets and β-cells, including those obtained in studies conducted by us earlier. In addition, from the perspective of glycobiology, this study proposes the possibility of application to diabetes medicine.

MiR-574-3p inhibits glucose toxicity-induced pancreatic β-cell dysfunction by suppressing PRMT1

BACKGROUND

Pancreatic β-cell dysfunction is commonly observed in patients with type 2 diabetes mellitus. Protein arginine methyltransferase 1 (PRMT1) plays an important role in pancreatic β-cell dysfunction. However, the detailed mechanisms remain largely unknown.

METHODS

RT-qPCR, western blotting, and immunofluorescence assays were used to evaluate PRMT1 and miR-574-3p levels. Cell Counting Kit-8, Advanced Dlycation End products (AGEs), Reactive Oxygen Species (ROS), and glucose-stimulated insulin secretion were assayed, and flow cytometry and RT-qPCR were performed to detect the role of PRMT1 and miR-574-3p in MIN6 cells. Luciferase reporter assays were performed to determine the interactions between PRMT1 and miR-574-3p.

RESULTS

High-glucose treatment resulted in the high expression of PRMT1. PRMT1 silencing could alleviate the reduced proliferation, insulin secretion, and GLUT1 level, in addition to suppressing the induced apoptosis, and AGEs and ROS levels, under high glucose conditions. MiR-574-3p was established as an upstream regulator of PRMT1 using luciferase reporter assays. More importantly, miR-574-3p reversed the effect of PRMT1 silencing in MIN6 cells.

CONCLUSIONS

miR-574-3p suppresses glucose toxicity-induced pancreatic β-cell dysfunction by targeting PRMT1.

Using recombinase-mediated cassette exchange to engineer MIN6 insulin-secreting cells based on a newly identified safe harbor locus

AIMS/INTRODUCTION

Recent studies have identified genomic and transcript level changes along with alterations in insulin secretion in patients with diabetes and in rodent models of diabetes. It is important to establish an efficient system for testing functional consequences of these changes. We aimed to generate such a system using insulin-secreting MIN6 cells.

MATERIALS AND METHODS

MIN6 cells were first engineered to have a tetracycline-regulated expression system. Then, we used the recombination-mediated cassette exchange strategy to explore the silencing-resistant site in the genome and generated a master cell line based on this site.

RESULTS

We identified a site 10.5 kbps upstream from the Zxdb gene as a locus that allows homogenous transgene expression from a tetracycline responsible promoter. Placing the Flip/Frt-based platform on this locus using CRISPR/Cas9 technology generated modified MIN6 cells applicable to achieving cassette exchange on the genome. Using this cell line, we generated MIN6 subclones with over- or underexpression of glucokinase. By analyzing a mixed population of these cells, we obtained an initial estimate of effects on insulin secretion within 6 weeks. Furthermore, we generated six MIN6 cell sublines simultaneously harboring genes of inducible overexpression with unknown functions in insulin secretion, and found that Cited4 and Arhgef3 overexpressions increased and decreased insulin secretion, respectively.

CONCLUSIONS

We engineered MIN6 cells, which can serve as a powerful tool for testing genetic alterations associated with diabetes, and studied the molecular mechanisms of insulin secretion.

MEK/ERK Signaling in β-Cells Bifunctionally Regulates β-Cell Mass and Glucose-Stimulated Insulin Secretion Response to Maintain Glucose Homeostasis

In diabetic pathology, insufficiency in β-cell mass, unable to meet peripheral insulin demand, and functional defects of individual β-cells in production of insulin are often concurrently observed, collectively causing hyperglycemia. Here we show that the phosphorylation of ERK1/2 is significantly decreased in the islets of db/db mice as well as in those of a cohort of subjects with type 2 diabetes. In mice with abrogation of ERK signaling in pancreatic β-cells through deletion of Mek1 and Mek2, glucose intolerance aggravates under high-fat diet-feeding conditions due to insufficient insulin production with lower β-cell proliferation and reduced β-cell mass, while in individual β-cells dampening of the number of insulin exocytosis events is observed, with the molecules involved in insulin exocytosis being less phosphorylated. These data reveal bifunctional roles for MEK/ERK signaling in β-cells for glucose homeostasis, i.e., in regulating β-cell mass as well as in controlling insulin exocytosis in individual β-cells, thus providing not only a novel perspective for the understanding of diabetes pathophysiology but also a potential clue for new drug development for diabetes treatment.

ZIGIR, a Granule-Specific Zn2+ Indicator, Reveals Human Islet α Cell Heterogeneity

Numerous mammalian cells contain abundant Zn²⁺ in their secretory granules, yet available Zn²⁺ sensors lack the desired specificity and sensitivity for imaging granular Zn²⁺. We developed a fluorescent zinc granule indicator, ZIGIR, that possesses numerous desired properties for live cell imaging, including >100-fold fluorescence enhancement, membrane permeability, and selective enrichment to acidic granules. The combined advantages endow ZIGIR with superior sensitivity and specificity for imaging granular Zn²⁺. ZIGIR enables separation of heterogenous β cells based on their insulin content and sorting of mouse islets into pure α cells and β cells. In human islets, ZIGIR facilitates sorting of endocrine cells into highly enriched α cells and β cells, reveals unexpectedly high Zn²⁺ activity in the somatostatin granule of some δ cells, and uncovers variation in the glucagon content among human α cells. We expect broad applications of ZIGIR for studying Zn²⁺ biology and Zn²⁺-rich secretory granules and for engineering β cells with high insulin content for treating diabetes.

Identification of potential lncRNAs and co-expressed mRNAs in gestational diabetes mellitus by RNA sequencing

AIM

Gestational diabetes mellitus is common during pregnancy, impacting maternal health and fetal development. The aim of this study was to identify potential long non-coding RNAs (lncRNAs) and mRNAs in gestational diabetes mellitus.

METHODS

The placenta tissues from four women patients with gestational diabetes mellitus and three healthy pregnant women were used for RNA sequencing. Differentially expressed lncRNAs and mRNAs were obtained. Then, interaction networks of lncRNA-nearby targeted mRNA and lncRNA-co-expressed mRNA were constructed, followed by functional annotation of co-expressed mRNAs. Third, GSE51546 dataset was utilized to validate the expression of selected co-expressed mRNAs. In addition, in vitro experiment was applied to expression validation of lncRNAs and mRNAs. Finally, GSE70493 dataset was utilized for diagnostic analysis of selected co-expressed mRNAs.

RESULTS

A total of 78 differentially expressed lncRNAs and 647 differentially expressed mRNAs in gestational diabetes mellitus were obtained. Several interaction pairs of lncRNA-co-expressed mRNA including LINC01504-CASP8, FUT8-AS1-TLR5/GDF15, GATA2-AS1-PQLC3/KIAA2026, and EGFR-AS1-HLA-G were identified. Endocytosis (involved HLA-G) and toll-like receptor signaling pathway (involved TLR5 and CASP8) were remarkably enriched signaling pathways of co-expressed mRNAs. It is noted that CASP8, TLR5, and PQLC3 had a significant prognosis value for gestational diabetes mellitus.

CONCLUSIONS

Our study identified several differentially expressed lncRNAs and mRNAs, and their interactions, especially co-expression, may be associated with gestational diabetes mellitus.

Importance of Both Imprinted Genes and Functional Heterogeneity in Pancreatic Beta Cells: Is There a Link?

Diabetes mellitus now affects more than 400 million individuals worldwide, with significant impacts on the lives of those affected and associated socio-economic costs. Although defects in insulin secretion underlie all forms of the disease, the molecular mechanisms which drive them are still poorly understood. Subsets of specialised beta cells have, in recent years, been suggested to play critical roles in "pacing" overall islet activity. The molecular nature of these cells, the means through which their identity is established and the changes which may contribute to their functional demise and "loss of influence" in both type 1 and type 2 diabetes are largely unknown. Genomic imprinting involves the selective silencing of one of the two parental alleles through DNA methylation and modified imprinted gene expression is involved in a number of diseases. Loss of expression, or loss of imprinting, can be shown in mouse models to lead to defects in beta cell function and abnormal insulin secretion. In the present review we survey the evidence that altered expression of imprinted genes contribute to loss of beta cell function, the importance of beta cell heterogeneity in normal and disease states, and hypothesise whether there is a direct link between the two.

Establishment of a long-term stable β-cell line and its application to analyze the effect of Gcg expression on insulin secretion

A pancreatic β-cell line MIN6 was previously established in our lab from an insulinoma developed in an IT6 transgenic mouse expressing the SV40 T antigen in β-cells. This cell line has been widely used for in vitro analysis of β-cell function, but tends to lose the mature β-cell features, including glucose-stimulated insulin secretion (GSIS), in long-term culture. The aim of this study was to develop a stable β-cell line that retains the characteristics of mature β-cells. Considering that mice derived from a cross between C3H and C57BL/6 strains are known to exhibit higher insulin secretory capacity than C57BL/6 mice, an IT6 male mouse of this hybrid background was used to isolate insulinomas, which were independently cultured. After 7 months of continuous culturing, we obtained the MIN6-CB4 β-cell line, which stably maintains its GSIS. It has been noted that β-cell lines express the glucagon (Gcg) gene at certain levels. MIN6-CB4 cells were utilized to assess the effects of differential Gcg expression on β-cell function. Our data show the functional importance of Gcg expression and resulting basal activation of the GLP-1 receptor in β-cells. MIN6-CB4 cells can serve as an invaluable tool for studying the regulatory mechanisms of insulin secretion, such as the GLP-1/cAMP signaling, in β-cells.

Imprinted Genes Impact Upon Beta Cell Function in the Current (and Potentially Next) Generation

Beta cell failure lies at the centre of the aetiology and pathogenesis of type 2 diabetes and the epigenetic control of the expression of critical beta cell genes appears to play a major role in this decline. One such group of epigenetically-controlled genes, termed 'imprinted' genes, are characterised by transgenerational monoallelic expression due to differential allelic DNA methylation and play key functional roles within beta cells. Here, we review the evidence for this functional importance of imprinted genes in beta cells as well as their nutritional regulation by the diet and their altered methylation and/or expression in rodent models of diabetes and in type 2 diabetic islets. We also discuss imprinted genes in the context of the next generation, where dietary overnutrition in the parents can lead to their deregulation in the offspring, alongside beta cell dysfunction and defective glucose handling. Both the modulation of imprinted gene expression and the likelihood of developing type 2 diabetes in adulthood are susceptible to the impact of nutritional status in early life. Imprinted loci, therefore, represent an excellent opportunity with which to assess epigenomic changes in beta cells due to the diet in both the current and next generation.

New Role for Growth/Differentiation Factor 15 in the Survival of Transplanted Brown Adipose Tissues in Cooperation with Interleukin-6

To identify factors involved in the earliest phase of the differentiation of human embryonic stem cells (hESCs) into brown adipocytes (BAs), we performed multi-time point microarray analyses. We found that growth/differentiation factor 15 (GDF15) expressions were specifically upregulated within three days of differentiation, when expressions of immature hESC markers were sustained. Although GDF15 expressions continued to increase in the subsequent differentiation phases, GDF15-deficient hESCs differentiated into mature BAs (Day 10) without apparent abnormalities. In addition, GDF15-deficient mice had normal brown adipose tissue (BAT) and were metabolically healthy. Unexpectedly, we found that interleukin-6 (IL6) expression was significantly lowered in the BAT of GDF15^-/- mice. In addition, GDF15^-/- hESCs showed abortive IL6 expressions in the later phase (>Day 6) of the differentiation. Interestingly, GDF15 expression was markedly repressed throughout the whole course of the differentiation of IL6^-/- hESCs into BAs, indicating IL6 is essential for the induction of GDF15 in the differentiation of hESCs. Finally, intraperitoneally transplanted BAT grafts of GDF15^-/- donor mice, but not those of wild-type (WT) mice, failed in the long-term survival (12 weeks) in GDF15^-/- recipient mice. Collectively, GDF15 is required for long-term survival of BAT grafts by creating a mutual gene induction loop with IL6.

Insulin sensor cells for the analysis of insulin secretion responses in single living pancreatic β cells

Investigation of the functions of insulin-secreting cells in response to glucose in single-living cells is essential for improving our knowledge on the pathogenesis of diabetes. Therefore, it is desired to develop a new convenient method that enables the direct detection of insulin secreted from single-living cells. Here, insulin-sensor-cells expressing a protein-based insulin-detecting probe immobilized on the extracellular membrane were developed to evaluate the insulin-secretion response in single-living pancreatic β cells. The protein-based insulin-detecting probe (NαLY) was composed of a bioluminescent protein (nano-luc), the αCT segment of the insulin receptor, L1 and CR domains of the insulin receptor, and a fluorescent protein (YPet). NαLY exhibited a bioluminescence resonance energy transfer (BRET) signal in response to insulin; thus, cells of Hepa1-6 line were genetically engineered to express NαLY on the extracellular membrane. The cells were found to act as insulin-sensor-cells, exhibiting a BRET signal in response to insulin. When the insulin-sensor-cells and pancreatic β cells (MIN6 cell line) were cocultured and stimulated with glucose, insulin-sensor-cells nearby pancreatic β cells showed the spike-shaped BRET signal response, whereas the insulin-sensor-cells close to one pancreatic β cell did not exhibit such signal response. However, all the insulin-sensor-cells showed a gradual increase in BRET signals, which were presumably attributed to the increase in insulin concentrations in the culture dish, confirming the function of these insulin-sensor-cells. Therefore, we demonstrated that heterogenetic insulin secretion in single-living pancreatic β cells could be measured directly using the insulin sensor cells.

Exogenous Cytokine-Free Differentiation of Human Pluripotent Stem Cells into Classical Brown Adipocytes

We previously established a method for a directed differentiation of human pluripotent stem cells into classical brown adipocytes (BA) by forming aggregates via massive floating culture in the presence of a specific cytokine cocktail. However, use of recombinant cytokines requires significant cost. Moreover, an enforced differentiation by exogenously added cytokines may amend skewed differentiation propensity of patient’s pluripotent stem cells, providing unsatisfactory disease models. Therefore, an exogenous cytokine-free method, where cytokines required for differentiation are provided in an auto/paracrine manner mimicking natural developmental process, is beneficial. Here we show that, if human pluripotent stem cells are cultured as size-controlled spheroids (100–120 µm radius, 2000–2500 cells/spheroid) in a mutually segregated manner with half-change of the medium every other day, they differentiate into classical BA via an authentic MYF5-positive myoblast route in the absence of exogenous cytokines. Differentiated BA exerted thermogenic activity in transplanted mice in response to beta-adrenergic receptor agonist stimuli. The cytokine-free differentiation method has further advantages in exploring BATokines, BA-derived physiologically active substances. Indeed, we have found that BA produces an unknown small (<1000 Da), highly hydrophilic molecule that augments insulin secretion from pancreatic beta cells. Our upgraded technique will contribute to an advancement of stem cell study for diverse purposes.

A multi-method evaluation of the effects of Inflammatory cytokines (IL-1β, IFN-γ, TNF-α) on pancreatic β-cells

We aimed to explore the effects of Inflammatory cytokines (IL-1β, IFN-γ, TNF-α) on pancreatic β-cells. CCK-8 assay showed that the cell viability decreased after 24 hr treatment of TNF-α, 48 hr of IFN-γ, and 84 hr of IL-1β. EdU assay illustrated that after 24 hr treatment, there were significantly reduced EdU-labeled red fluorescence cells in TNF-α group while not in IFN-γ and IL-1β groups. Flow Cytometry results displayed that TNF-α and IFN-γ groups increased apoptosis while IL-1β group did not. Cell apoptosis results found that there was an increase in the S-phase population of IL-1β and TNF-α groups, however, there was no significant difference in cell cycle between IFN-γ group and the control. TEM images showed that there were reduction in the number of granules and mitochondria in IL-1β and IFN-γ groups, in particular paucity of insulin granules and mitochondria in TNF-α group. Radioimmunoassay results presented that TNF-α inhibited glucose-induced insulin secretion, while there were no significant changes in IL-1β and IFN-γ groups when compared with the control. Metabolomic analysis found amino acid metabolism and Krebs cycle were the most robust altered metabolism pathways after inflammatory cytokines treatments. Overall, the altered amino acid metabolism and Krebs cycle metabolism might be important mechanisms of TNF-α induced mouse pancreatic β-cells dysfuction.

High dose of histone deacetylase inhibitors affects insulin secretory mechanism of pancreatic beta cell line

Objective. Histone deacytylase inhibitors (HDACis) inhibit the deacetylation of the lysine residue of proteins, including histones, and regulate the transcription of a variety of genes. Recently, HDACis have been used clinically as anti-cancer drugs and possible anti-diabetic drugs. Even though HDACis have been proven to protect the cytokine-induced damage of pancreatic beta cells, evidence also shows that high doses of HDACis are cytotoxic. In the present study, we, therefore, investigated the eff ect of HDACis on insulin secretion in a pancreatic beta cell line. Methods. Pancreatic beta cells MIN6 were treated with selected HDACis (trichostatin A, TSA; valproic acid, VPA; and sodium butyrate, NaB) in medium supplemented with 25 mM glucose and 13% heat-inactivated fetal bovine serum (FBS) for indicated time intervals. Protein expression of Pdx1 and Mafa in MIN6 cells was demonstrated by immunohistochemistry and immunocytochemistry, expression of Pdx1 and Mafa genes was measured by quantitative RT-PCR method. Insulin release from MIN6 cells and insulin cell content were estimated by ELISA kit. Superoxide production in MIN6 cells was measured using a Total ROS/Superoxide Detection System. Results. TSA, VPA, and NaB inhibited the expression of Pdx1 and Mafa genes and their products. TSA treatment led to beta cell malfunction, characterized by enhanced insulin secretion at 3 and 9 mM glucose, but impaired insulin secretion at 15 and 25 mM glucose. Th us, TSA induced dysregulation of the insulin secretion mechanism. TSA also enhanced reactive oxygen species production in pancreatic beta cells. Conclusions. Our results showed that HDACis caused failure to suppress insulin secretion at low glucose concentrations and enhance insulin secretion at high glucose concentrations. In other words, when these HDACis are used clinically, high doses of HDACis may cause hypoglycemia in the fasting state and hyperglycemia in the fed state. When using HDACis, physicians should, therefore, be aware of the capacity of these drugs to modulate the insulin secretory capacity of pancreatic beta cells.

Histone deacetylase regulates insulin signaling via two pathways in pancreatic β cells

Recent studies demonstrated that insulin signaling plays important roles in the regulation of pancreatic β cell mass, the reduction of which is known to be involved in the development of diabetes. However, the mechanism underlying the alteration of insulin signaling in pancreatic β cells remains unclear. The involvement of epigenetic control in the onset of diabetes has also been reported. Thus, we analyzed the epigenetic control of insulin receptor substrate 2 (IRS2) expression in the MIN6 mouse insulinoma cell line. We found concomitant IRS2 up-regulation and enhanced insulin signaling in MIN6 cells, which resulted in an increase in cell proliferation. The H3K9 acetylation status of the Irs2 promoter was positively associated with IRS2 expression. Treatment of MIN6 cells with histone deacetylase inhibitors led to increased IRS2 expression, but this occurred in concert with low insulin signaling. We observed increased IRS2 lysine acetylation as a consequence of histone deacetylase inhibition, a modification that was coupled with a decrease in IRS2 tyrosine phosphorylation. These results suggest that insulin signaling in pancreatic β cells is regulated by histone deacetylases through two novel pathways affecting IRS2: the epigenetic control of IRS2 expression by H3K9 promoter acetylation, and the regulation of IRS2 activity through protein modification. The identification of the histone deacetylase isoform(s) involved in these mechanisms would be a valuable approach for the treatment of type 2 diabetes.

Androgen receptor-deficient islet β-cells exhibit alteration in genetic markers of insulin secretion and inflammation. A transcriptome analysis in the male mouse

AIMS

Testosterone action is mediated via the androgen receptor (AR). We have reported that male mice lacking AR selectively in β-cells (βARKO^-/y) develop decreased glucose-stimulated insulin secretion (GSIS), producing glucose intolerance. We showed that testosterone action on AR in β-cells amplifies the insulinotropic action of GLP-1 on its receptor via a cAMP-dependent protein kinase-A pathway.

METHODS

To investigate AR-dependent gene networks in β-cells, we performed a high throughput whole transcriptome sequencing (RNA-Seq) in islets from male βARKO^-/y and control mice.

RESULTS

We identified 214 differentially expressed genes (DEGs) (158 up- and 56 down-regulated) with a false discovery rate (FDR) < 0.05 and a fold change (FC) > 2. Our analysis of individual transcripts revealed alterations in β-cell genes involved in cellular inflammation/stress and insulin secretion. Based on 312 DEGs with an FDR < 0.05, the pathway analysis revealed 23 significantly enriched pathways, including cytokine-cytokine receptor interaction, Jak-STAT signaling, insulin signaling, MAPK signaling, type 2 diabetes (T2D) and pancreatic secretion. The gene ontology analysis confirmed the results of the individual DEGs and the pathway analysis in showing enriched biological processes encompassing inflammation, ion transport, exocytosis and insulin secretion.

CONCLUSIONS

AR-deficient islets exhibit altered expression of genes involved in inflammation and insulin secretion demonstrating the importance of androgen action in β-cell health in the male with implications for T2D development in men.

Phosphorylation of MafA enhances interaction with Beta2/NeuroD1

AIMS

MafA is a critical regulator of insulin expression and mature β-cell function. MafA binds to the insulin promoter through its carboxyl-terminal basic domain-leucine zipper (bZip) region and activates transcription synergistically with the β-cell-enriched transactivators Beta2 (NeuroD1) and Pdx1. MafA protein is highly phosphorylated in β-cells, and phosphorylation at multiple sites within its amino-terminal region is critical for its DNA-binding and transactivating abilities, as well as for regulation of its degradation. Here, we investigated whether phosphorylation of MafA affects its interaction with Beta2.

METHODS

By mutational analysis, we identified interaction domains of MafA and Beta2. Using in situ proximity ligation assay (PLA), we explored mechanism of phosphorylation-dependent binding of MafA with Beta2. We also searched for a pathophysiological condition that would induce lower levels of MafA phosphorylation.

RESULTS

Mutational analysis revealed that the phosphorylation sites within the amino-terminal region of MafA were not necessary for interaction with Beta2. In situ PLA suggested that phosphorylation induces conformational or configurational changes in MafA, thereby regulating the interaction with Beta2. We also found that long-term culture of the MIN6 insulinoma cell line under high-glucose conditions resulted in a decrease in β-cell-specific transcripts including insulin, along with a decrease in MafA phosphorylation and DNA binding.

CONCLUSION

Phosphorylation of MafA plays a critical role in β-cell function by regulating multiple functionalities, including binding to DNA, interaction with Beta2, and transactivation.

Functional Analysis of Novel Candidate Regulators of Insulin Secretion in the MIN6 Mouse Pancreatic β Cell Line

Elucidating the regulation of glucose-stimulated insulin secretion (GSIS) in pancreatic β cells is important for understanding and treating diabetes. The pancreatic β cell line, MIN6, retains GSIS but gradually loses it in long-term culture. The MIN6 subclone, MIN6c4, exhibits well-regulated GSIS even after prolonged culture. We previously used DNA microarray analysis to compare gene expression in the parental MIN6 cells and MIN6c4 cells and identified several differentially regulated genes that may be involved in maintaining GSIS. Here we investigated the potential roles of six of these genes in GSIS: Tmem59l (Transmembrane protein 59 like), Scgn (Secretagogin), Gucy2c (Guanylate cyclase 2c), Slc29a4 (Solute carrier family 29, member 4), Cdhr1 (Cadherin-related family member 1), and Celsr2 (Cadherin EGF LAG seven-pass G-type receptor 2). These genes were knocked down in MIN6c4 cells using lentivirus vectors expressing gene-specific short hairpin RNAs (shRNAs), and the effects of the knockdown on insulin expression and secretion were analyzed. Suppression of Tmem59l, Scgn, and Gucy2c expression resulted in significantly decreased glucose- and/or KCl-stimulated insulin secretion from MIN6c4 cells, while the suppression of Slc29a4 expression resulted in increased insulin secretion. Tmem59l overexpression rescued the phenotype of the Tmem59l knockdown MIN6c4 cells, and immunostaining analysis indicated that the TMEM59L protein colocalized with insulin and GM130, a Golgi complex marker, in MIN6 cells. Collectively, our findings suggested that the proteins encoded by Tmem59l, Scgn, Gucy2c, and Slc29a4 play important roles in regulating GSIS. Detailed studies of these proteins and their functions are expected to provide new insights into the molecular mechanisms involved in insulin secretion.

Osteoprotegerin Regulates Pancreatic β-Cell Homeostasis upon Microbial Invasion

Osteoprotegerin (OPG), a decoy receptor for receptor activator of NF-κB ligand (RANKL), antagonizes RANKL’s osteoclastogenic function in bone. We previously demonstrated that systemic administration of lipopolysaccharide (LPS) to mice elevates OPG levels and reduces RANKL levels in peripheral blood. Here, we show that mice infected with Salmonella, Staphylococcus, Mycobacteria or influenza virus also show elevated serum OPG levels. We then asked whether OPG upregulation following microbial invasion had an effect outside of bone. To do so, we treated mice with LPS and observed OPG production in pancreas, especially in β-cells of pancreatic islets. Insulin release following LPS administration was enhanced in mice lacking OPG, suggesting that OPG inhibits insulin secretion under acute inflammatory conditions. Consistently, treatment of MIN6 pancreatic β-cells with OPG decreased their insulin secretion following glucose stimulation in the presence of LPS. Finally, our findings suggest that LPS-induced OPG upregulation is mediated in part by activator protein (AP)-1 family transcription factors, particularly Fos proteins. Overall, we report that acute microbial infection elevates serum OPG, which maintains β-cell homeostasis by restricting glucose-stimulated insulin secretion, possibly preventing microbe-induced exhaustion of β-cell secretory capacity.

Mio acts in the Drosophila brain to control nutrient storage and feeding

Animals recognize the availability of nutrients and regulate the intake and storage of these nutrients accordingly. However, the molecular mechanisms underlying nutrient sensing and subsequent changes in behavior and metabolism are not fully understood. Mlx interactor (Mio), the Drosophila homolog of carbohydrate response element binding protein (ChREBP), functions as a transcription factor in the fat body of the fly to control triglyceride storage as well as feeding, suggesting that Mio may act in a nutrient-sensing pathway to coordinate food consumption and metabolism. Here, we show that Mio functions in neurons in Drosophila to regulate feeding and nutrient storage. Pan-neuronal disruption of Mio function leads to increased triglyceride and glycogen storage, and this phenotype is not due to increased food consumption. Interestingly, targeted disruption of Mio specifically in the insulin-producing cells (IPCs) has little effect on nutrient storage, but increases food consumption suggesting that Mio acts in these neurons to control feeding behavior. Since Mio is a transcription factor, one possible way Mio may act in the IPCs to control feeding is through regulating the expression of Drosophila insulin-like peptides (dilps) or drosulfakinin (dsk), neuropeptides produced in the IPCs. Consistent with this hypothesis, IPC-specific knockdown of Mio leads to an increase in dilp3 expression, while not affecting dilp2, 5 or dsk levels. Together, this study indicates a new function for Mio in the Drosophila brain and specifically in the IPCs, controlling neuropeptide gene expression, feeding and metabolism in accordance with nutrient availability.

A BRET-based homogeneous insulin assay using interacting domains in the primary binding site of the insulin receptor

A new homogeneous insulin assay requiring no chemical modification of an insulin recognition domain, which can be applied to continuous monitoring of the time-dependent cellular response in vitro, was developed. The carboxy-terminal α-chain (αCT) segment and first leucine-rich-repeat (L1) domain in the primary binding site on the insulin receptor were genetically fused with a bioluminescent protein (Nanoluc, Nluc) and a fluorescent protein (yellow fluorescent protein, YPet) to produce the insulin-sensing probe proteins Nluc-αCT and L1-YPet. The BRET signal was observed on simple mixing of insulin with these protein probes, in a so-called homogeneous assay. The BRET signal was proportional to the insulin concentration, and the lower detection limit was 0.8 μM. Time-dependent insulin secretion from drug-stimulated MIN6 cells was also successfully monitored continuously with the probe proteins. This BRET-based homogeneous insulin assay method is thus expected to be applicable to drug development by high-throughput screening.

Hypomethylation of the promoter of the catalytic subunit of protein phosphatase 2A in response to hyperglycemia

In order to identify epigenetic mechanisms through which hyperglycemia can affect gene expression durably in β cells, we screened DNA methylation changes induced by high glucose concentrations (25 mmol/L) in the BTC3 murine cell line, using an epigenome‐wide approach. Exposure of BTC3 cells to high glucose modified the expression of 1612 transcripts while inducing significant methylation changes in 173 regions. Among these 173 glucose‐sensitive differentially methylated regions (DMRs), 14 were associated with changes in gene expression, suggesting an epigenetic effect of high glucose on gene transcription at these loci. Among these 14 DMRs, we selected for further study Pp2ac, a gene previously suspected to play a role in β‐cell physiology and type 2 diabetes. Using RT‐qPCR and bisulfite pyrosequencing, we confirmed our previous observations in BTC3 cells and found that this gene was significantly demethylated in the whole blood cells (WBCs) of type 2 diabetic patients compared to controls.

In order to identify epigenetic mechanisms through which hyperglycemia can affect gene expression durably in β cells, we screened DNA methylation changes induced by high glucose concentration in the BTC3 murine cell line. We identified one interesting gene, PP2AC, and confirmed it in type 2 diabetic patients.