Regulation of the pdx1 gene promoter in pancreatic beta-cells

Single-cell RNA sequencing identifies endothelial-derived HBEGF as promoting pancreatic beta cell proliferation in mice via the EGFR-Kmt5a-H4K20me pathway

AIMS/HYPOTHESIS

Pancreatic beta cell mass is dynamically regulated in response to increased physiological and pathological demands. Understanding the mechanisms that control physiological beta cell proliferation could provide valuable insights into novel therapeutic approaches to diabetes. Here, we aimed to analyse the intracellular and extracellular signalling pathways involved in regulating the physiological proliferation of beta cells using single-cell RNA-seq (scRNA-seq) and in vitro functional assays.

METHODS

Islets isolated from nulliparous mice, mice at different time points of gestation and mice at day 4 after delivery were analysed using scRNA-seq. Bioinformatics analyses of scRNA-seq data were performed to determine the heterogeneous transcriptomic characteristics of beta cells and to identify the proliferating subpopulation. CellChat was used to analyse cell-cell communication and identify the ligand-receptor pairs between beta cell subclusters as well as between non-beta cells and proliferating beta cells. In vitro functional assays were conducted in mouse and rat beta cell lines and isolated mouse primary islets to validate the role of Kmt5a- mono-methylation of histone H4 at lysine 20 (H4K20me) signalling and endothelial-derived heparin-binding EGF-like growth factor (HBEGF) in beta cell proliferation.

RESULTS

Of 43,724 endocrine and non-endocrine cells within islets analysed by scRNA-seq, 15,569 beta cells were clustered into eight distinct populations, each exhibiting unique heterogeneity. A proliferating beta cell subcluster was identified that highly expressed the histone methyltransferase Kmt5a. Activation of Kmt5a-H4K20me signalling upregulated the expression of Cdk1 and promoted beta cell proliferation. The crosstalk between endothelial cells and the proliferating beta cell subcluster, mediated by the HBEGF-EGF receptor (EGFR) ligand-receptor interaction, increased as beta cell mass expanded. HBEGF increased the expression levels of genes involved in the cell cycle and promoted beta cell proliferation by regulating the Kmt5a-H4K20me signalling pathway.

CONCLUSIONS/INTERPRETATION

Our study demonstrates that, under physiological conditions, endothelial-derived HBEGF regulates beta cell proliferation through the Kmt5a-H4K20me signalling pathway, which may serve as a potential target to promote beta cell expansion and treat diabetes.

DATA AVAILABILITY

The scRNA-seq and RNA-seq datasets are available from the Gene Expression Omnibus (GEO) using the accession numbers GSE278860 and GSE278861, respectively.

DNA methylation in the pathogenesis of type 2 diabetes

Type 2 diabetes (T2D) is a metabolic disease characterized by the development of β-cell dysfunction with hepatic, muscular and adipose tissue insulin resistance. Although the molecular mechanisms leading to its development are not entirely known, investigations of its causes reveal a multifactorial contribution to its development and progression in most cases. In addition, regulatory interactions mediated by epigenetic modifications such as DNA methylation, histone tail modifications and regulatory RNAs have been found to play a significant role in the etiology of T2D. In this chapter, we discuss the role of DNA methylation and its dynamics in the development of the pathological features of T2D.

PDX1 is the cornerstone of pancreatic β-cell functions and identity

Diabetes has been a worldwide healthcare problem for many years. Current methods of treating diabetes are still largely directed at symptoms, aiming to control the manifestations of the pathology. This creates an overall need to find alternative measures that can impact on the causes of the disease, reverse diabetes, or make it more manageable. Understanding the role of key players in the pathogenesis of diabetes and the related β-cell functions is of great importance in combating diabetes. PDX1 is a master regulator in pancreas organogenesis, the maturation and identity preservation of β-cells, and of their role in normal insulin function. Mutations in the PDX1 gene are correlated with many pancreatic dysfunctions, including pancreatic agenesis (homozygous mutation) and MODY4 (heterozygous mutation), while in other types of diabetes, PDX1 expression is reduced. Therefore, alternative approaches to treat diabetes largely depend on knowledge of PDX1 regulation, its interaction with other transcription factors, and its role in obtaining β-cells through differentiation and transdifferentiation protocols. In this article, we review the basic functions of PDX1 and its regulation by genetic and epigenetic factors. Lastly, we summarize different variations of the differentiation protocols used to obtain β-cells from alternative cell sources, using PDX1 alone or in combination with various transcription factors and modified culture conditions. This review shows the unique position of PDX1 as a potential target in the genetic and cellular treatment of diabetes.

Rice Bran Phenolic Extracts Modulate Insulin Secretion and Gene Expression Associated with β-Cell Function

Oxidative stress is known to modulate insulin secretion and initiate gene alterations resulting in impairment of β-cell function and type 2 diabetes mellitus (T2DM). Rice bran (RB) phenolic extracts contain bioactive properties that may target metabolic pathways associated with the pathogenesis of T2DM. This study aimed to examine the effect of stabilized RB phenolic extracts on the expression of genes associated with β-cell function such as glucose transporter 2 (Glut2), pancreatic and duodenal homeobox 1 (Pdx1), sirtuin 1 (Sirt1), mitochondrial transcription factor A (Tfam), and insulin 1 (Ins1) in addition to evaluating its impact on glucose-stimulated insulin secretion. It was observed that treatment with different concentrations of RB phenolic extracts (25-250 µg/mL) significantly increased the expression of Glut2, Pdx1, Sirt1, Tfam, and Ins1 genes and glucose-stimulated insulin secretion under both normal and high glucose conditions. RB phenolic extracts favourably modulated the expression of genes involved in β-cell dysfunction and insulin secretion via several mechanisms such as synergistic action of polyphenols targeting signalling molecules, decreasing free radical damage by its antioxidant activity, and stimulation of effectors or survival factors of insulin secretion.

Nanotopography Promotes Pancreatic Differentiation of Human Embryonic Stem Cells and Induced Pluripotent Stem Cells

Although previous studies suggest that nanotopographical features influence properties and behaviors of stem cells, only a few studies have attempted to derive clinically useful somatic cells from human pluripotent stem cells using nanopatterned surfaces. In the present study, we report that polystyrene nanopore-patterned surfaces significantly promote the pancreatic differentiation of human embryonic and induced pluripotent stem cells. We compared different diameters of nanopores and showed that 200 nm nanopore-patterned surfaces highly upregulated the expression of PDX1, a critical transcription factor for pancreatic development, leading to an approximately 3-fold increase in the percentage of differentiating PDX1(+) pancreatic progenitors compared with control flat surfaces. Furthermore, in the presence of biochemical factors, 200 nm nanopore-patterned surfaces profoundly enhanced the derivation of pancreatic endocrine cells producing insulin, glucagon, or somatostatin. We also demonstrate that nanopore-patterned surface-induced upregulation of PDX1 is associated with downregulation of TAZ, suggesting the potential role of TAZ in nanopore-patterned surface-mediated mechanotransduction. Our study suggests that appropriate cytokine treatments combined with nanotopographical stimulation could be a powerful tool for deriving a high purity of desired cells from human pluripotent stem cells.

Quantitative assessment of Pdx1 promoter activity in vivo using a secreted luciferase reporter system

The luciferase reporter system is useful for the assessment of various biological processes in vivo. The transcription factor pancreatic and duodenal homeobox 1 (Pdx1) is critical for the formation and the function of pancreatic β-cells. A novel reporter system using secreted Gaussia princeps luciferase (GLuc) under the control of a Pdx1 promoter was generated and activated in rat and mouse β-cell lines. This Pdx1-GLuc construct was used as a transgene for the generation of reporter mice to monitor Pdx1 promoter activity in vivo via the measurement of secreted GLuc activity in a small aliquot of blood. Significantly increased plasma GLuc activity was observed in Pdx1-GLuc mice. Analysis of Pdx1-GLuc mice by bioluminescence imaging, GLuc reporter assays using homogenates of various organs, and immunohistochemistry revealed that GLuc expression and activity were exponentially higher in pancreatic β-cells than in pancreatic non-β-cells, the duodenum, and other organs. In addition, GLuc activity secreted into the culture medium from islets isolated from Pdx1-GLuc mice correlated with the number of islets. The transplantation of Pdx1-GLuc islets into severe combined immunodeficiency mice elevated their plasma GLuc activity. Conversely, a partial pancreatectomy in Pdx1-GLuc mice reduced plasma GLuc activity. These results suggest that a secreted luciferase reporter system in vivo enables not only the monitoring of promoter activity but also a quantitative and minimally invasive assessment of physiological and pathological changes in small cell masses, such as pancreatic β-cells.

In vitro differentiation into insulin-producing β-cells of stem cells isolated from human amniotic fluid and dental pulp

AIM

To investigate the ability of human amniotic fluid stem cells and human dental pulp stem cells to differentiate into insulin-producing cells.

METHODS

Human amniotic fluid stem cells and human dental pulp stem cells were induced to differentiate into pancreatic β-cells by a multistep protocol. Islet-like structures were assessed in differentiated human amniotic fluid stem cells and human dental pulp stem cells after 21 days of culture by dithizone staining. Pancreatic and duodenal homebox-1, insulin and Glut-2 expression were detected by immunofluorescence and confocal microscopy. Insulin secreted from differentiated cells was tested with SELDI-TOF MS and by enzyme-linked immunosorbent assay.

RESULTS

Human amniotic fluid stem cells and human dental pulp stem cells, after 7 days of differentiation started to form islet-like structures that became evident after 14 days of induction. SELDI-TOF MS analysis, revealed the presence of insulin in the media of differentiated cells at day 14, further confirmed by enzyme-linked immunosorbent assay after 7, 14 and 21 days. Both stem cell types expressed, after differentiation, pancreatic and duodenal homebox-1, insulin and Glut-2 and were positively stained by dithizone. Either the cytosol to nucleus translocation of pancreatic and duodenal homebox-1, either the expression of insulin, are regulated by glucose concentration changes. Day 21 islet-like structures derived from both human amniotic fluid stem cells and human dental pulp stem cell release insulin in a glucose-dependent manner.

CONCLUSION

The present study demonstrates the ability of human amniotic fluid stem cells and human dental pulp stem cell to differentiate into insulin-producing cells, offering a non-pancreatic, low-invasive source of cells for islet regeneration.

Characterization of insulin-producing cells derived from PDX-1-transfected neural stem cells

Islet cell transplantation is a promising treatment strategy for type-1 diabetes. However, functional islet cells are hard to obtain for transplantation and are in short supply. Directing the differentiation of stem cells into insulin‑producing cells, which serve as islet cells, would overcome this shortage. Bone marrow contains hematopoietic stem cells and mesenchymal stem cells. The present study used bone marrow cells isolated from rats and neural stem cells (NSCs) that were derived from bone marrow cells in culture. Strong nestin staining was detected in NSCs, but not in bone marrow stromal cells (BMSCs). In vitro transfection of the pancreatic duodenal homeobox-1 (PDX-1) gene into NSCs generated insulin‑producing cells. Reverse transcription polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) analysis confirmed that PDX-1-transfected NSCs expressed insulin mRNA and released insulin protein. However, insulin release from PDX-1-transfected NSCs did not respond to the challenge of glucose and glucagon-like peptide-1. These results support the use of bone marrow-derived NSCs as a renewable source of insulin-producing cells for autologous transplantation to treat type-1 diabetes.

Intestinal Pdx1 mediates nutrient metabolism gene networks and maternal expression is essential for perinatal growth in mice

The homeodomain transcription factor Pdx1 is essential for pancreas formation and functions in pancreatic islets cells to regulate genes involved in maintenance of glucose homeostasis. In order to investigate a role for Pdx1 in intestinal cells, we analyzed the functions and networks associated with genes differentially expressed by Pdx1 overexpression in human Caco-2 cells. In agreement with previous results for intestine isolated from mice with Pdx1 inactivation, functional analysis of genes differentially expressed with Pdx1 overexpression revealed functions significantly associated with nutrient metabolism. Similarly, network analysis examining the interactions among the differentially expressed genes revealed gene networks involved in lipid metabolism. Consistent with defects in maternal nutrient metabolism, mouse pups born to dams with intestine-specific Pdx1 inactivation are underweight and fail to thrive in the neonatal period compared to pups born to control dams. We conclude that Pdx1 mediates lipid metabolism gene networks in intestinal cells and that maternal expression is essential for perinatal growth in mice.

Reprogramming of pancreatic exocrine cells towards a beta (β) cell character using Pdx1, Ngn3 and MafA

Pdx1 (pancreatic and duodenal homeobox 1), Ngn3 (neurogenin 3) and MafA (v-maf musculoaponeurotic fibrosarcoma oncogene family, protein A) have been reported to bring about the transdifferentiation of pancreatic exocrine cells to beta (β) cells in vivo. We have investigated the mechanism of this process using a standard in vitro model of pancreatic exocrine cells, the rat AR42j-B13 cell line. We constructed a new adenoviral vector encoding all three genes, called Ad-PNM (adenoviral Pdx1, Ngn3, MafA construct). When introduced into AR42j-B13 cells, Ad-PNM caused a rapid change to a flattened morphology and a cessation of cell division. The expression of exocrine markers is suppressed. Both insulin genes are up-regulated as well as a number of transcription factors normally characteristic of beta cells. At the chromatin level, histone tail modifications of the Pdx1, Ins1 (insulin 1) and Ins2 (insulin 2) gene promoters are shifted in a direction associated with gene activity, and the level of DNA CpG methylation is reduced at the Ins1 promoter. The transformed cells secrete insulin and are capable of relieving diabetes in streptozotocin-treated NOD-SCID (non-obese diabetic severe combined immunodeficiency) mice. However the transformation is not complete. The cells lack expression of several genes important for beta cell function and they do not show glucose-sensitive insulin secretion. We conclude that, for this exocrine cell model, although the transformation is dramatic, the reprogramming is not complete and lacks critical aspects of the beta cell phenotype.

Upregulation of β-cell genes and improved function in rodent islets following chronic glucokinase activation

Glucokinase (GK) plays a critical role in controlling blood glucose; GK activators have been shown to stimulate insulin secretion acutely both in vitro and in vivo. Sustained stimulation of insulin secretion could potentially lead to β-cell exhaustion; this study examines the effect of chronic GK activation on β-cells. Gene expression and insulin secretion were measured in rodent islets treated in vitro with GKA71 for 72 h. Key β-cell gene expression was measured in rat, mouse and global GK heterozygous knockout mouse islets (gk(del/wt)). Insulin secretion, after chronic exposure to GKA71, was measured in perifused rat islets. GKA71 acutely increased insulin secretion in rat islets in a glucose-dependent manner. Chronic culture of mouse islets with GKA71 in 5 mmol/l glucose significantly increased the expression of insulin, IAPP, GLUT2, PDX1 and PC1 and decreased the expression of C/EBPβ compared with 5 mmol/l glucose alone. Similar increases were shown for insulin, GLUT2, IAPP and PC1 in chronically treated rat islets. Insulin mRNA was also increased in GKA71-treated gk(del/wt) islets. No changes in GK mRNA were observed. Glucose-stimulated insulin secretion was improved in perifused rat islets following chronic treatment with GKA71. This was associated with a greater insulin content and GK protein level. Chronic treatment of rodent islets with GKA71 showed an upregulation of key β-cell genes including insulin and an increase in insulin content and GK protein compared with glucose alone.

Hybrid of 1-deoxynojirimycin and polysaccharide from mulberry leaves treat diabetes mellitus by activating PDX-1/insulin-1 signaling pathway and regulating the expression of glucokinase, phosphoenolpyruvate carboxykinase and glucose-6-phosphatase in alloxan-induced diabetic mice

ETHNOPHARMACOLOGICAL RELEVANCE

1-Deoxynojirimycin (DNJ) discovered from mulberry trees has been reported to be a potent inhibitor of intestinal α-glycosidases (sucrase, maltase, glucoamylase), and many polysaccharides were useful in protecting against alloxan-induced pancreatic islets damage through their scavenging ability. This study was aimed to evaluate the therapeutic effect and potential mechanism(s) of the hybrid of DNJ and polysaccharide (HDP) from mulberry leaves on alloxan-induced diabetic mice.

MATERIALS AND METHODS

Daily oral treatment with HDP (150 mg/kg body weight) to diabetic mice for 12 weeks, body weight and blood glucose were determined every week, oral glucose tolerance test was performed after 4 and 8 weeks, biochemical values were measured using assay kits and gene expressions were investigated by RT-PCR.

RESULTS

A significant decline in blood glucose, glycosylated hemoglobin, triglyceride, aspartate transaminase and alanine transaminase levels and an evident increase in body weight, plasma insulin level and high density lipoprotein were observed in HDP treated diabetic mice. The polysaccharide (P1) showed a significant scavenging hydroxyl radicals and superoxide anion radical effects in vitro, which indicated that P1 could protect alloxan-induced pancreatic islets from damage by scavenging the free radicals and repaired the destroyed pancreatic β-cells. Pharmacokinetics assay showed that DNJ could be absorbed from the gastrointestinal mucosa and diffused rapidly into the liver, resulted in postprandial blood glucose decrease and alleviated the toxicity caused by sustained supra-physiological glucose to pancreatic β-cells. RT-PCR results indicated that HDP could modulate the hepatic glucose metabolism and gluconeogenesis by up/down-regulating the expression of rate-limiting enzymes (glucokinase, phosphoenolpyruvate carboxykinase and glucose-6-phosphatase) in liver and up-regulating the pancreatic and duodenal homeobox factor-1 (PDX-1), insulin-1 and insulin-2 expressions in pancreas.

CONCLUSION

These findings suggested that HDP has complimentary potency to develop an antihyperglycemic agent for treatment of diabetes mellitus.

New potential adjuncts to treatment of children with type 1 diabetes mellitus

Insulin administration is the primary therapy for type 1 diabetes mellitus (T1DM). Current available insulin therapies do not successfully enable children with T1DM to reach glycemic goals without side effects such as hypoglycemia and weight gain. Pramlintide is a synthetic analog of human amylin that acts in conjunction with insulin to delay gastric emptying and inhibit the release of glucagon and is indicated for use in patients with type 1 and type 2 diabetes. Recent studies in adult patients have examined the role of glucagon-like peptide 1 (GLP-1) and agents that bind to its receptor in type 1 diabetes. It is hypothesized that a major component of the glycemic effect is attributable to the known action of GLP-1 to delay gastric emptying and to inhibit glucagon secretion. Further studies with the use of amylin analogs and long-acting GLP-1 agonists as congeners with insulin in T1DM are indicated in children. In recent years, our better understanding of the pathophysiology of diabetes has led to the development of new therapies for diabetes. This article reviews the potential use of these newer pharmacologic agents as adjunctive therapy in T1DM in children and adolescents.

ParaHox genes in pancreatic cell cultures: effects on the insulin promoter regulation

The gene encoding PDX1 (pancreatic duodenum homeobox 1), the main transcription factor regulating the glucose-dependent transactivation of the insulin promoter in pancreatic β-cells, clusters with two closely related homeobox genes (Gsh1 and Cdx2/3), all of them belonging to the ParaHox gene family. The ParaHox gene evolutionary history in the vertebrate lineage involved duplications of the cluster and subsequent loss of some members, so that eventually, the human and murine genomes contain only 6 ParaHox genes. The crucial role of PDX1 in pancreas development, beta-cell formation and insulin transcription regulation has long been established. There is some data on CDX2/3 function in α-cells, but remarkably, nothing is known on the role of the other ParaHox genes, which are also expressed in the endocrine pancreas. Homeobox transcription factors that belong to the same family show high conservation of the homeodomain and share similar target sites and oligomeric partners, and thus may act redundantly, synergistically or antagonistically on the same promoters. Therefore, we explored the effects of the Parahox proteins (GSH1, GSH2, CDX1, CDX2/3 and CDX4) on the regulation of the insulin promoter in transfected α- and β- cultured cell lines at different glucose concentrations and compared them to those of PDX1. Noticeably, several ParaHox transcription factors are able to transactivate or inhibit the insulin promoter, depending on the cell type and glucose concentration, thus suggesting their possible participation in the regulation of similar target genes, such as insulin, either by silencing or activating them, in the absence of PDX1.

Effects of rosiglitazone and metformin on pancreatic beta cell gene expression

AIMS/HYPOTHESIS

Rosiglitazone and metformin are two oral antihyperglycaemic drugs used to treat type 2 diabetes. While both drugs have been shown to improve insulin-sensitive glucose uptake, the direct effects of these drugs on pancreatic beta cells is only now beginning to be clarified. The aim of the present study was to determine the direct effects of these agents on beta cell gene expression.

METHODS

We used reporter gene analysis to examine the effects of rosiglitazone and metformin on the activity of the proinsulin and insulin promoter factor 1 (IPF1) gene promoters in the glucose-responsive mouse beta cell line Min6. Western blot and gel retardation analyses were used to examine the effects of both drugs on the regulation of IPF1 protein production, nuclear accumulation and DNA binding activity in both Min6 cells and isolated rat islets of Langerhans.

RESULTS

Over 24 h, rosiglitazone promoted the nuclear accumulation of IPF1 and forkhead homeobox A2 (FOXA2), independently of glucose concentration, and stimulated a two-fold increase in the activity of the Ipf1 gene promoter (p<0.01). Stimulation of the Ipf1 promoter by rosiglitazone was unaffected by the presence of the peroxisome proliferator activated receptor gamma antagonist GW9662. No effect of either rosiglitazone or metformin was observed on proinsulin promoter activity. Metformin stimulated IPF1 nuclear accumulation and DNA binding activity in a time-dependent manner, with maximal effects observed after 2 h.

CONCLUSIONS/INTERPRETATION

Metformin and rosiglitazone have direct effects on beta cell gene expression, suggesting that these agents may play a previously unrecognised role in the direct regulation of pancreatic beta cell function.

Palmitate inhibits insulin gene expression by altering PDX-1 nuclear localization and reducing MafA expression in isolated rat islets of Langerhans

Abnormalities in lipid metabolism have been proposed as contributing factors to both defective insulin secretion from the pancreatic beta cell and peripheral insulin resistance in type 2 diabetes. Previously, we have shown that prolonged exposure of isolated rat islets of Langerhans to excessive fatty acid levels impairs insulin gene transcription. This study was designed to assess whether palmitate alters the expression and binding activity of the key regulatory factors pancreas-duodenum homeobox-1 (PDX-1), MafA, and Beta2, which respectively bind to the A3, C1, and E1 elements in the proximal region of the insulin promoter. Nuclear extracts of isolated rat islets cultured with 0.5 mm palmitate exhibited reduced binding activity to the A3 and C1 elements but not the E1 element. Palmitate did not affect the overall expression of PDX-1 but reduced its nuclear localization. In contrast, palmitate blocked the stimulation of MafA mRNA and protein expression by glucose. Combined adenovirus-mediated overexpression of PDX-1 and MafA in islets completely prevented the inhibition of insulin gene expression by palmitate. These results demonstrate that prolonged exposure of islets to palmitate inhibits insulin gene transcription by impairing nuclear localization of PDX-1 and cellular expression of MafA.

Regenerative and therapeutic effects of heparin-binding epidermal growth factor-like growth factor on diabetes by gene transduction through retrograde pancreatic duct injection of adenovirus vector

OBJECTIVES

In the adult pancreas, pre-existing beta cells, stem cells, and endocrine progenitor cells residing in the duct lining are considered important sources for beta-cell regeneration. A member of the epidermal growth factor (EGF) family, heparin binding (HB)-EGF, may promote this process. We examined whether HB-EGF gene transduction into duct cells could promote beta-cell regeneration.

METHODS

We administered an HB-EGF adenovirus vector construct to male Institute of Cancer Research mice by retrograde injection through the pancreatic duct. We also performed HB-EGF gene transduction into cultured duct cells.

RESULTS

On immunohistochemical and histomorphometric analysis of the experimental group, insulin-positive cells differentiated from duct cells, and the 5-bromo-2-deoxyuridine labeling index of beta cells was significantly increased. beta-cell mass was also increased, and the glucose tolerance of diabetic mice was improved at 12 weeks after injection. Using cultured pancreatic duct cells, we confirmed that HB-EGF gene transduction induced both insulin gene expression and insulin production by these cells.

CONCLUSIONS

These results indicate that HB-EGF gene transduction into adult pancreatic duct cells not only promotes the proliferation of pre-existing beta cells but also leads to beta-cell differentiation from duct cells, and the resulting increase in beta-cell mass improves glucose tolerance.

Transcriptional regulation of the IAPP gene in pancreatic beta-cells

Islet amyloid polypeptide (IAPP or amylin) is co-secreted with insulin from the pancreatic beta-cells. Transcription of the IAPP gene is controlled by a complex promoter region, spanning from -2798 to +450 relative to the transcriptional start site. In the present study, we have used reporter gene analysis and semi-quantitative RT-PCR to establish that insulin, glucagon, glucagon-like peptide-1 (GLP-1) and the GLP-1 derivatives GLP(7-36)Amide and Exendin-4 all stimulate IAPP promoter activity, as well as endogenous IAPP mRNA levels in isolated islets of Langerhans. In contrast, somatostatin had no effect, and whilst the inflammatory cytokines TNF-alpha, IL-1alpha and IL-1beta had no effect on promoter activity, they all decreased IAPP mRNA levels in isolated islets. Finally, utilising a series of deletion reporter gene constructs of the human IAPP gene promoter, we used overexpression studies to establish that HNF-3beta (FoxA2) negatively regulates the IAPP promoter, whilst the MODY3 transcription factor HNF-1alpha positively regulates promoter activity.

Distinct roles for insulin and insulin-like growth factor-1 receptors in pancreatic beta-cell glucose sensing revealed by RNA silencing

The importance of the insulin receptor (IR) and the insulin-like growth factor-1 receptor (IGF-1R) for glucose-regulated insulin secretion and gene expression in pancreatic islet beta-cells is at present unresolved. Here, we have used small interfering RNAs (siRNAs) to silence the expression of each receptor selectively in clonal MIN6 beta-cells. Reduction of IR levels by >90% completely inhibited glucose (30 mM compared with 3 mM)-induced insulin secretion, but had no effect on depolarization-stimulated secretion. IR depletion also blocked the accumulation of preproinsulin (PPI), pancreatic duodenum homoeobox-1 (PDX-1) and glucokinase (GK) mRNAs at elevated glucose concentrations, as assessed by quantitative real-time PCR analysis (TaqMan). Similarly, depletion of IGF-1R inhibited glucose-induced insulin secretion but, in contrast with the effects of IR silencing, had little impact on the regulation of gene expression by glucose. Moreover, loss of IGF-1R, but not IR, markedly inhibited glucose-stimulated increases in cytosolic and mitochondrial ATP, suggesting a role for IGF-1R in the maintenance of oxidative metabolism and in the generation of mitochondrial coupling factors. RNA silencing thus represents a useful tool for the efficient and selective inactivation of receptor tyrosine kinases in isolated beta-cells. By inhibiting glucose-stimulated insulin secretion through the inactivation of IGF-1R, this approach also demonstrates the existence of insulin-independent mechanisms whereby elevated glucose concentrations regulate PPI, PDX-1 and GK gene expression in beta-cells.