A novel glucose-responsive element in the human insulin gene functions uniquely in primary cultured islets

The Diabetes Gene JAZF1 Is Essential for the Homeostatic Control of Ribosome Biogenesis and Function in Metabolic Stress

The ability of pancreatic β-cells to respond to increased demands for insulin during metabolic stress critically depends on proper ribosome homeostasis and function. Excessive and long-lasting stimulation of insulin secretion can elicit endoplasmic reticulum (ER) stress, unfolded protein response, and β-cell apoptosis. Here we show that the diabetes susceptibility gene JAZF1 is a key transcriptional regulator of ribosome biogenesis, global protein, and insulin translation. JAZF1 is excluded from the nucleus, and its expression levels are reduced upon metabolic stress and in diabetes. Genetic deletion of Jazf1 results in global impairment of protein synthesis that is mediated by defects in ribosomal protein synthesis, ribosomal RNA processing, and aminoacyl-synthetase expression, thereby inducing ER stress and increasing β-cell susceptibility to apoptosis. Importantly, JAZF1 function and its pleiotropic actions are impaired in islets of murine T2D and in human islets exposed to metabolic stress. Our study identifies JAZF1 as a central mediator of metabolic stress in β-cells.

TRIB3 mediates glucose-induced insulin resistance via a mechanism that requires the hexosamine biosynthetic pathway

In the current study, we investigated the role of tribbles homolog 3 (TRIB3) in glucose-induced insulin resistance and whether the induction of TRIB3 by glucose is dependent on the nutrient-sensing hexosamine biosynthetic pathway (HBP) known to mediate glucose toxicity in diabetes. In diabetic rats, TRIB3 expression in skeletal muscle was increased after 10 days of hyperglycemia, and glycemia and muscle TRIB3 were both restored toward normal by insulin therapy. In L6 myocytes, the induction of TRIB3 by high glucose or glucosamine was reversible upon removal of these substrates. To assess the role of HBP in the induction of TRIB3, we demonstrated that the ability of high glucose to augment TRIB3 expression was prevented by azaserine, an inhibitor of glutamine: fructose-6-phosphate amidotransferase (GFAT), which is the rate-limiting enzyme in the HBP pathway. TRIB3 expression was also substantially stimulated by glucosamine, which bypasses GFAT, accompanied by a decrease in the insulin-stimulated glucose transport rate, and neither response was affected by azaserine. Further, knockdown of TRIB3 inhibited, and TRIB3 overexpression enhanced, the ability of both high glucose and glucosamine to induce insulin resistance. These data provide the mechanistic link between the HBP flux and insulin resistance and point to TRIB3 as a novel target for treatment of glucose-induced insulin resistance.

Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in β-cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5' flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and β-2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in β-cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5' flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and β-2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

An siRNA screen in pancreatic beta cells reveals a role for Gpr27 in insulin production

The prevalence of type 2 diabetes in the United States is projected to double or triple by 2050. We reasoned that the genes that modulate insulin production might be new targets for diabetes therapeutics. Therefore, we developed an siRNA screening system to identify genes important for the activity of the insulin promoter in beta cells. We created a subclone of the MIN6 mouse pancreatic beta cell line that expresses destabilized GFP under the control of a 362 base pair fragment of the human insulin promoter and the mCherry red fluorescent protein under the control of the constitutively active rous sarcoma virus promoter. The ratio of the GFP to mCherry fluorescence of a cell indicates its insulin promoter activity. As G protein coupled receptors (GPCRs) have emerged as novel targets for diabetes therapies, we used this cell line to screen an siRNA library targeting all known mouse GPCRs. We identified several known GPCR regulators of insulin secretion as regulators of the insulin promoter. One of the top positive regulators was Gpr27, an orphan GPCR with no known role in beta cell function. We show that knockdown of Gpr27 reduces endogenous mouse insulin promoter activity and glucose stimulated insulin secretion. Furthermore, we show that Pdx1 is important for Gpr27's effect on the insulin promoter and insulin secretion. Finally, the over-expression of Gpr27 in 293T cells increases inositol phosphate levels, while knockdown of Gpr27 in MIN6 cells reduces inositol phosphate levels, suggesting this orphan GPCR might couple to Gq/11. In summary, we demonstrate a MIN6-based siRNA screening system that allows rapid identification of novel positive and negative regulators of the insulin promoter. Using this system, we identify Gpr27 as a positive regulator of insulin production.

Pancreatic beta cells are the only physiologic source of insulin. When these cells are destroyed in type 1 diabetics, there is uncontrolled hyperglycemia from complete insulin deficiency. In type 2 diabetes, these same cells fail to increase insulin secretion to compensate for peripheral insulin resistance leading to relative insulin deficiency. We constructed a novel screening system to find new regulators of insulin production in this critical cell type. Here, we describe a screen of the G protein coupled receptors (GPCRs) and show a role for orphan GPCR, Gpr27, in insulin promoter activity and insulin secretion. We propose that Gpr27 is a novel target for diabetes therapeutics.

Signal control through Raf: in sickness and in health

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is the prototype mammalian mitogen-activated protein kinase (MAPK) signaling cascade that regulates a number of processes, including proliferation, differentiation, survival, migration, stress responses and apoptosis. How this seemingly linear cascade is modulated to achieve a specific cellular function has been a main focus of the field. In this review, we describe new as well as old findings in the regulation of the ERK1/2 pathway in normal and disease states via MAP3Ks.

Mapping of INS promoter interactions reveals its role in long-range regulation of SYT8 transcription

Insulin (INS) synthesis and secretion from pancreatic β cells are tightly regulated; their deregulation causes diabetes. Here we map INS-associated loci in human pancreatic islets by 4C and 3C techniques and show that the INS gene physically interacts with the SYT8 gene, located over 300 kb away. This interaction is elevated by glucose and accompanied by increases in SYT8 expression. Inactivation of the INS promoter by promoter-targeting siRNA reduces SYT8 gene expression. SYT8-INS interaction and SYT8 transcription are attenuated by CTCF depletion. Furthermore, SYT8 knockdown decreases insulin secretion in islets. These results reveal a non-redundant role for SYT8 in insulin secretion and indicate that the INS promoter acts from a distance to stimulate SYT8 transcription. This suggests a function for the INS promoter in coordinating insulin transcription and secretion through long-range regulation of SYT8 expression in human islets.

The role of small molecule GPR119 agonist, AS1535907, in glucose-stimulated insulin secretion and pancreatic β-cell function

AIM

AS1535907, a small molecule agonist of GPR119, was assessed for its glucose-stimulated insulin secretory activity and pancreatic β-cell function in type 2 diabetes.

METHODS

Both in vitro and in vivo tests were conducted using NIT-1 and HEK293 cell lines, male normal and db/db mice and isolated perfused rat pancreas preparations.

RESULTS

AS1535907 had an EC₅₀ value of 1.5 µM for human GPR119 transfected in HEK293 cells. AS1535907 enhanced insulin secretion in NIT-1 cells and in the perfused rat pancreas. A transient increase in the human insulin promoter activity was also observed in NIT-1 cells. First-phase insulin secretion was particularly more evident in the AS1535907-treated perfused rat pancreas than that in the nateglinide or glibenclamide-treated group. Oral glucose tolerance improved following a single dose of AS1535907 in normal and db/db mice. Subsequently, 2 weeks of multiple dosing significantly increased plasma insulin levels and decreased blood glucose levels in db/db mice. After 3 weeks of treatment in db/db mice, the numbers of insulin and proliferation cell nuclear antigen-positive cells and the islet area were significantly higher than those in the vehicle-treated mice. As compared with the vehicle, gene expression analysis revealed that AS1535907 significantly upregulated transcription factors (Nkx 2.2, Nkx 6.1, NeuroD and activin A), responsible for β-cell regulation and prohormone-converting enzyme 1 responsible for insulin biosynthesis.

CONCLUSION

These results suggest that AS1535907 can potentially regulate first-phase insulin secretion and exert a protective effect on pancreatic β-cell function via regulation of transcription factors.

Sex steroids effects on the endocrine pancreas

The endocrine pancreas is central in the physiopathology of diabetes mellitus. Nutrients and hormones control endocrine pancreatic function and the secretion of insulin and other pancreatic islet hormones. Although the pancreas is not usually considered as a target of steroids, increasing evidence indicates that sex steroid hormones modify pancreatic islet function. The biological effects of steroid hormones are transduced by both, classical and non-classical steroid receptors that in turn produce slow genomic and rapid non-genomic responses. In this review, we focused on the effects of sex steroid hormones on endocrine pancreatic function, with special emphasis in animal studies.

Recessive mutations in the INS gene result in neonatal diabetes through reduced insulin biosynthesis

Heterozygous coding mutations in the INS gene that encodes preproinsulin were recently shown to be an important cause of permanent neonatal diabetes. These dominantly acting mutations prevent normal folding of proinsulin, which leads to beta-cell death through endoplasmic reticulum stress and apoptosis. We now report 10 different recessive INS mutations in 15 probands with neonatal diabetes. Functional studies showed that recessive mutations resulted in diabetes because of decreased insulin biosynthesis through distinct mechanisms, including gene deletion, lack of the translation initiation signal, and altered mRNA stability because of the disruption of a polyadenylation signal. A subset of recessive mutations caused abnormal INS transcription, including the deletion of the C1 and E1 cis regulatory elements, or three different single base-pair substitutions in a CC dinucleotide sequence located between E1 and A1 elements. In keeping with an earlier and more severe beta-cell defect, patients with recessive INS mutations had a lower birth weight (-3.2 SD score vs. -2.0 SD score) and were diagnosed earlier (median 1 week vs. 10 weeks) compared to those with dominant INS mutations. Mutations in the insulin gene can therefore result in neonatal diabetes as a result of two contrasting pathogenic mechanisms. Moreover, the recessively inherited mutations provide a genetic demonstration of the essential role of multiple sequence elements that regulate the biosynthesis of insulin in man.

The Krüppel-like zinc finger protein Glis3 directly and indirectly activates insulin gene transcription

Glis3 is a member of the Krüppel-like family of transcription factors and is highly expressed in islet β cells. Mutations in GLIS3 cause the syndrome of neonatal diabetes and congenital hypothyroidism (NDH). Our aim was to examine the role of Glis3 in β cells, specifically with regard to regulation of insulin gene transcription. We demonstrate that insulin 2 (Ins2) mRNA expression in rat insulinoma 832/13 cells is markedly increased by wild-type Glis3 overexpression, but not by the NDH1 mutant. Furthermore, expression of both Ins1 and Ins2 mRNA is downregulated when Glis3 is knocked down by siRNA. Glis3 binds to the Ins2 promoter in the cell, detected by chromatin immunoprecipitation. Deletion analysis of Ins2 promoter identifies a sequence (5′-GTCCCCTGCTGTGAA-3′) from −255 to −241 as the Glis3 response element and binding occur specifically via the Glis3 zinc finger region as revealed by mobility shift assays. Moreover, Glis3 physically and functionally interacts with Pdx1, MafA and NeuroD1 to modulate Ins2 promoter activity. Glis3 also may indirectly affect insulin promoter activity through upregulation of MafA and downregulation of Nkx6-1. This study uncovers a role of Glis3 for regulation of insulin gene expression and expands our understanding of its role in the β cell.

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.

Glucose regulation of insulin gene transcription and pre-mRNA processing in human islets

Glucose is the primary regulator of insulin granule release from pancreatic islets. In rodent islets, the role of glucose in the acute regulation of insulin gene transcription has remained unclear, primarily because the abundance and long half-life of insulin mRNA confounds analysis of transcription by traditional methods that measure steady-state mRNA levels. To investigate the nature of glucose-regulated insulin gene transcription in human islets, we first quantitated the abundance and half-lives of insulin mRNA and pre-mRNAs after addition of actinomycin D (to stop transcription). Our results indicated that intron 1-and intron 2-containing pre-mRNAs were approximately 150- and 2,000-fold less abundant, respectively, than mature mRNA. 5' intron 2-containing pre-mRNAs displayed half-lives of only approximately 60 min, whereas all other transcripts displayed more extended lifetimes. In response to elevated glucose, pre-mRNA species increased within 60 min, whereas increases in mature mRNA did not occur until 48 h, suggesting that measurement of mature mRNA species does not accurately reflect the acute transcriptional response of the insulin gene to glucose. The acute increase in pre-mRNA species was preceded by a sixfold increase in histone H4 acetylation and a twofold increase in RNA polymerase II recruitment at the insulin promoter. Taken together, our data suggest that pre-mRNA species may be a more reliable reflection of acute changes to human insulin gene transcriptional rates and that glucose acutely enhances insulin transcription by a mechanism that enhances chromatin accessibility and leads to recruitment of basal transcriptional machinery.

Comparative analysis of insulin gene promoters: implications for diabetes research

DNA sequences that regulate expression of the insulin gene are located within a region spanning approximately 400 bp that flank the transcription start site. This region, the insulin promoter, contains a number of cis-acting elements that bind transcription factors, some of which are expressed only in the beta-cell and a few other endocrine or neural cell types, while others have a widespread tissue distribution. The sequencing of the genome of a number of species has allowed us to examine the manner in which the insulin promoter has evolved over a 450 million-year period. The major findings are that the A-box sites that bind PDX-1 are among the most highly conserved regulatory sequences, and that the conservation of the C1, E1, and CRE sequences emphasize the importance of MafA, E47/beta2, and cAMP-associated regulation. The review also reveals that of all the insulin gene promoters studied, the rodent insulin promoters are considerably dissimilar to the human, leading to the conclusion that extreme care should be taken when extrapolating rodent-based data on the insulin gene to humans.

Regulation of the insulin gene by glucose and fatty acids

The insulin gene is expressed almost exclusively in pancreatic beta-cells. Metabolic regulation of insulin gene expression enables the beta-cell to maintain adequate stores of intracellular insulin to sustain the secretory demand. Glucose is the major physiologic regulator of insulin gene expression; it coordinately controls the recruitment of transcription factors [e.g., pancreatic/duodenal homeobox-1 (PDX-1), mammalian homologue of avian MafA/L-Maf (MafA), Beta2/Neuro D (B2), the rate of transcription, and the stability of insulin mRNA. However, chronically elevated levels of glucose (glucotoxicity) and lipids (lipotoxicity) also contribute to the worsening of beta-cell function in type 2 diabetes, in part via inhibition of insulin gene expression. The mechanisms of glucotoxicity, which involve decreased binding activities of PDX-1 and MafA and increased activity of C/EBPbeta, are mediated by high-glucose-induced generation of oxidative stress. On the other hand, lipotoxicity is mediated by de novo ceramide synthesis and involves inhibition of PDX-1 nuclear translocation and MafA gene expression. Glucotoxicity and lipotoxicity have common targets, which makes their combination particularly harmful to insulin gene expression and beta-cell function in type 2 diabetes.

Specific regulation of IRS-2 expression by glucose in rat primary pancreatic islet beta-cells

Insulin receptor substrate 2 (IRS-2) plays a critical role in pancreatic beta-cells. Increased IRS-2 expression promotes beta-cell growth and survival, whereas decreased IRS-2 levels lead to apoptosis. It was found that IRS-2 turnover in rat islet beta-cells was rapid, with mRNA and protein half-lives of approximately 90 min and approximately 2 h, respectively. However, this was countered by specific glucose-regulated IRS-2 expression mediated at the transcriptional level. Glucose (> or = 6 mM) increased IRS-2 mRNA and protein levels in a dose-dependent manner, reaching a maximum 4-fold increase in IRS-2 mRNA and a 5-6-fold increase in IRS-2 protein levels at > or = 12 mM glucose (p < or = 0.01). Glucose (15 mM) regulation of islet beta-cell IRS-2 gene expression was rapid, with a significant increase in IRS-2 mRNA levels within 2 h that reached a maximum 4-fold increase by 4 h. IRS-2 protein expression in beta-cells followed that of IRS-2 mRNA. Glucose metabolism was necessary for increased IRS-2 expression in beta-cells. Moreover, inhibition of a glucose-induced rise in islet beta-cell cytosolic [Ca2+]i prevented an increase in IRS-2 expression, indicating this was Ca2+-dependent. The glucose-induced rise in IRS-2 levels correlated with increased IRS-2 tyrosine phosphorylation and downstream activation of protein kinase B. These data indicate that fluctuations of glucose in the normal physiological range (5-15 mM) promote beta-cell survival via regulation of IRS-2 expression and a subsequent parallel protein kinase B activation. Given that the onset of type-2 diabetes is marked by loss of beta-cells, these data further the idea that controlled IRS-2 expression in beta-cells could be a therapeutic means to promote beta-cell survival and delay the onset of the disease.

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.

Transcription factors, pancreatic development, and β-cell maintenance

Transcription factors play an important role during pancreatic development ensuring normal differentiation of the islet endocrine cells. In mature beta-cells, expression of specific transcription factors is essential in maintaining normal beta-cell function.

Neurogenin3 and hepatic nuclear factor 1 cooperate in activating pancreatic expression of Pax4

During fetal development, paired/homeodomain transcription factor Pax4 controls the formation of the insulin-producing beta cells and the somatostatin-producing delta cells in the islets of Langerhans in the pancreas. Targeting of Pax4 expression to the islet lineage in the fetal pancreas depends on a short sequence located approximately 2 kb upstream of the transcription initiation site of the PAX4 gene. This short sequence contains binding sites for homeodomain transcription factors PDX1 and hepatic nuclear factor (HNF)1, nuclear receptor HNF4alpha, and basic helix-loop-helix factor Neurogenin3. In the current study we demonstrate that the HNF1alpha and Neurogenin3 binding sites are critical for activity of the region through synergy between the two proteins. Synergy involves a physical interaction between the factors and requires the activation domains of both factors. Furthermore, exogenous expression of Neurogenin3 is sufficient to induce expression of the endogenous pax4 gene in the mouse pancreatic ductal cell line mPAC, which already expresses HNF1alpha, whereas expression of both Neurogenin3 and HNF1alpha are necessary to activate the pax4 gene in the fibroblast cell line NIH3T3. These data demonstrate how Neurogenin3 and HNF1alpha activate the pax4 gene during the cascade of gene expression events that control pancreatic endocrine cell development.

Regulation of insulin gene transcription by ERK1 and ERK2 in pancreatic beta cells

We show that the mitogen-activated protein kinases ERK1/2 are components of the mechanism by which glucose stimulates insulin gene expression. ERK1/2 activity is required for glucose-dependent transcription from both the full-length rat insulin I promoter and the glucose-sensitive isolated E2A3/4 promoter element in intact islets and beta cell lines. Dominant negative ERK2 and MEK inhibitors suppress glucose stimulation of the rat insulin I promoter and the E2A3/4 element. Overexpression of ERK2 is sufficient to stimulate transcription from the E2A3/4 element. The glucose-induced response is dependent upon ERK1/2 phosphorylation of a subset of transcription factors that include Beta2 (also known as NeuroD1) and PDX-1. Phosphorylation increases their functional activity and results in a cumulative transactivation of the promoter. Thus, ERK1/2 act at multiple points to transduce a glucose signal to insulin gene transcription.

The transcription factor PDX-1 is post-translationally modified by O-linked N-acetylglucosamine and this modification is correlated with its DNA binding activity and insulin secretion in min6 beta-cells

The pancreatic/duodenal homeobox-1 protein (PDX-1, also called STF-1, IPF-1) is a transcription factor that plays an important role in pancreatic function and development. Here, we have overexpressed and purified PDX-1 from baculovirus/sf-9 cells, transiently transfected Cos-7 cells and native Min6 cells and demonstrated that the protein is posttranslationally modified by O-linked N-acetylglucosamine (O-GlcNAc). The approaches we used include binding of the protein to the lectin WGA, labeling with galactosyltransferase and UDP-[(3)H]gal and probing with the O-GlcNAc-specific antibody, RL-2. PNGase F treatment and structural analysis indicate that the carbohydrate is beta-linked O-GlcNAc. Mapping of [(3)H]gal-labeled tryptic peptides indicates that PDX-1 has two major sites for O-GlcNAcylation. In Min6 cells, elevated glucose concentration leads to an increase in protein O-GlcNAcylation and this hyperglycosylation correlates with an increase in DNA binding activity of PDX-1 and insulin secretion. On the other hand, the GFAT inhibitor azaserine reduces intracellular O-GlcNAc levels and profoundly attenuates glucose-stimulated insulin secretion. These data suggest that O-GlcNAcylation may be involved in the regulation of PDX-1 DNA binding activity and in glucose-stimulated insulin secretion in beta-cells.

Glucose regulates insulin gene transcription by hyperacetylation of histone h4

Induction of insulin gene expression in response to high blood glucose levels is essential for maintaining glucose homeostasis. Although several transcription factors including Beta-2, Ribe3b1, and Pdx-1 have been shown to play a role in glucose stimulation of insulin gene expression, the exact molecular mechanism(s) by which this regulation occurs is unknown. Previous data demonstrate that the transcription factors Beta-2/NeuroD1 and Pdx-1, which are involved in glucose-stimulated insulin gene expression, interact with the histone acetylase p300, suggesting a role for histone acetylation in glucose regulation of the insulin gene expression. We report that exposure of mouse insulinoma 6 cells to high concentrations of glucose results in hyperacetylation of histone H4 at the insulin gene promoter, which correlates with the increased level of insulin gene transcription. In addition, we demonstrate that hyperacetylation of histone H4 in response to high concentrations of glucose also occurs at the glucose transporter-2 gene promoter. Using histone deacetylase inhibitors, we show that increases in histone H4 acetylation cause stimulation of insulin gene transcription even in the absence of high concentrations of glucose. Furthermore, we show that fibroblasts, which lack insulin gene expression, also lack histone acetylation at the insulin gene promoter. In summary, our data support the idea that high concentrations of glucose stimulate insulin gene expression by causing hyperacetylation of histone H4 at the insulin gene promoter.

The mif gene is transcriptionally regulated by glucose in insulin-secreting cells

Macrophage migration inhibitory factor (MIF) is an important regulator of glucose homeostasis. In pancreatic beta-cells, MIF expression is regulated by glucose and its secretion potentiates the glucose-induced insulin secretion. The molecular mechanisms by which glucose mediates its effect on MIF expression are not elucidated. Herein, we report that incubating the differentiated insulin-secreting cell line INS-1 in high glucose concentration increases MIF transcriptional activity as well as the reporter gene activity driven by the -1033 to +63 bp fragment of the MIF promoter. A minimal region located between -187 and -98 bp of this promoter sequence contributes both to basal activity and glucose-responsiveness of the gene. Within this promoter region, two cis-binding sequences were identified by mobility shift assays and footprinting experiments. Both cis-elements interact with nuclear proteins expressed specifically in insulin-secreting cells. In conclusion, we identified a minimal region of the MIF promoter which contributes to the glucose stimulation of the mif gene in insulin-secreting cells.

Induction of c-Myc expression suppresses insulin gene transcription by inhibiting NeuroD/BETA2-mediated transcriptional activation

Insulin biosynthesis and secretion are critical for pancreatic beta-cell function, but both are impaired under diabetic conditions. We have found that hyperglycemia induces the expression of the basic helix-loop-helix transcription factor c-Myc in islets in several different diabetic models. To examine the possible implication of c-Myc in beta-cell dysfunction, c-Myc was overexpressed in isolated rat islets using adenovirus. Adenovirus-mediated c-Myc overexpression suppressed both insulin gene transcription and glucose-stimulated insulin secretion. Insulin protein content, determined by immunostaining, was markedly decreased in c-Myc-overexpressing cells. In gel-shift assays c-Myc bound to the E-box in the insulin gene promoter region. Furthermore, in betaTC1, MIN6, and HIT-T15 cells and primary rat islets, wild type insulin gene promoter activity was dramatically decreased by c-Myc overexpression, whereas the activity of an E-box mutated insulin promoter was not affected. In HeLa and HepG2 cells c-Myc exerted a suppressive effect on the insulin promoter activity only in the presence of NeuroD/BETA2 but not PDX-1. Both c-Myc and NeuroD can bind the E-box element in the insulin promoter, but unlike NeuroD, the c-Myc transactivation domain lacked the ability to activate insulin gene expression. Additionally p300, a co-activator of NeuroD, did not function as a co-activator of c-Myc. In conclusion, increased expression of c-Myc in beta-cells suppresses the insulin gene transcription by inhibiting NeuroD-mediated transcriptional activation. This mechanism may explain some of the beta-cell dysfunction found in diabetes.

Functional analysis of the glucose response element of the rat glucagon receptor gene in insulin-producing INS-1 cells

Glucose stimulates the transcription of the glucagon receptor gene in hepatocytes and in pancreatic beta cells. We recently identified a glucose response element in the immediate upstream non-coding region of the rat glucagon receptor gene. We previously showed that this DNA element is centered on a palindromic sequence of 19 nucleotides (termed as G box), containing two E boxes separated by three nucleotides. In the present study, we further characterized the DNA sequence requirements for the glucose induced expression of the rat glucagon receptor gene. Transfection study realized in the insulin-producing INS-1 cells revealed that a fragment of 49 nucleotides, centered on the G box, bears all the features required for the glucose activation. Mutations performed in the 5'-E box totally abolished the glucose responsiveness, whereas mutations or deletion of the 3'-E box only had a limited effect. Deletions performed upstream from the G box revealed that an accessory factor binding site, located in the region just upstream from the G box, is required for full stimulation by glucose. Finally, by using G box based probes in gel shift experiments, we demonstrated that USF1/USF2 transcription factors are part of the proteinic complex that binds to the glucose response element of the rat glucagon receptor gene promoter. In conclusion, in contrast to many other glucose regulated genes, only the 5'-E box appears to be a crucial DNA element for the glucose transcriptional effect. However, an accessory factor binding site located in the region just upstream from the G box is required for a complete stimulation by glucose.

Prolactin induction of insulin gene transcription: roles of glucose and signal transducer and activator of transcription 5

GH and PRL stimulate insulin production in pancreatic beta-cells through induction of insulin gene transcription. The transcriptional effects of GH are mediated through the binding of signal transducer and activator of transcription-5 (STAT5) to a consensus recognition sequence (TTCnnnGAA) in the rat insulin-1 promoter. In this study we demonstrate that PRL also induces the binding of STAT5 proteins to the rat insulin-1 STAT5 motif. However, the magnitude of binding of STAT5 nuclear proteins, as assessed by electrophoretic mobility shift assays, was only 1/30th that of the binding of the same STAT5 proteins to the beta-casein STAT5 site. The differences in the affinities of the rat insulin-1 and beta-casein STAT5 motifs are explained in part by differences in promoter sequences flanking the STAT5 sites. To assess the importance of the STAT motif in PRL induction of insulin gene transcription, we deleted the STAT5 consensus sequence in the rat insulin 1 promoter, cloned the truncated promoter upstream of the luciferase reporter gene, and transfected the construct into rat insulinoma (INS-1) cells. The transcriptional activity of this construct was compared with that of the wild-type promoter. Although deletion of the STAT5 site in the promoter reduced the basal luciferase activity, the response to PRL was unaffected. PRL also induced transcription of constructs containing the wild-type human insulin promoter or the rat insulin-2 promoter, which contain no classic STAT5 sequences. The transcriptional effect of PRL was manifest even when cells were incubated in glucose-free medium, indicating that the action of the hormone is not mediated solely through changes in glucose uptake or glucose metabolism. To identify PRL-responsive regions of the rat and human insulin promoters, we constructed a series of promoter truncations and assessed their responsiveness to PRL. A PRL-responsive region of the rat insulin-1 promoter was localized between nucleotides -165 and -109. A PRL-responsive region of the human insulin promoter was localized between nucleotides -346 and -250. Additional regions of the human and rat insulin-1 promoters were required for PRL induction of a heterologous, minimal thymidine kinase promoter, suggesting that there are multiple PRL-responsive elements in the insulin genes. These observations suggest a glucose- and STAT5-independent pathway by which PRL may induce insulin gene transcription.

Testosterone effect on insulin content, messenger ribonucleic acid levels, promoter activity, and secretion in the rat

Coexistence of hyperinsulinemia and hyperandrogenism in women has been frequently described. Most of the studies addressing this issue have focused on the mechanisms by which insulin produces hyperandrogenism. In the present study, we analyzed the effects of testosterone in vivo and in vitro upon insulin gene expression and release in the rat. Our studies demonstrate that testosterone increases insulin messenger RNA (mRNA) levels in vitro as well as in vivo. In both prepuberal and intact adult rats, serum testosterone concentrations were positively correlated with insulin mRNA levels and insulin concentration in serum. Testosterone deprivation after gonadectomy decreased both insulin gene expression and serum insulin concentration. Insulin mRNA levels were partially restored after 3 days of testosterone administration and serum insulin was 80% and 27% above baseline values at 5 and 7 days posttreatment. Primary cultured pancreatic islets treated with the sexual steroid increased about 80% insulin mRNA, as well as protein, and release. In transfected islets, testosterone increased the activity of the -410 bp rat insulin promoter I by 154%. These data demonstrate that testosterone has a direct effect upon pancreatic islet function by favoring insulin gene expression and release.

Activin A increases Pax4 gene expression in pancreatic beta cell lines

Activin A, a member of the TGFbeta superfamily, has many physiological and developmental functions. In pancreatic beta cell cultures, activin promotes cell differentiation and insulin production. The author has found activin increases gene expression of the PAX4, one of the major transcription factors determining pancreatic beta cell differentiation. This effect was mediated, at least in part, by the type IB activin receptor (ALK4). Moreover, the activity of human insulin promoter-reporter system was controlled by PAX4 and its isoform PAX4 delta(G239-P251) in a unique fashion; positively by low concentrations, and negatively by high concentrations of these proteins. And the repression activities were different between these proteins. These findings confirm the importance of activin signal transduction in pancreatic beta cell development and function.

Regulation of insulin gene transcription

Recent studies of the insulin gene promoter and the transcription factors that regulate it have expanded our understanding of both how the production of insulin is restricted to the pancreatic beta -cell, and how that production is regulated by physiologic signals such as glucose. A picture is emerging in which an elaborate set of transcription factors binds to specific sequences along the promoter and recruits additional transcriptional co-activators to build a functional transcriptional activation complex that is unique to beta -cells. Surprisingly, however, genetic experiments in mice have demonstrated an unexpected degree of redundancy in the factors that control insulin gene expression, and have revealed the presence of a network of transcription factors that coordinate the expression of factors forming the insulin gene activation complex.

Insulin stimulates pancreatic-duodenal homoeobox factor-1 (PDX1) DNA-binding activity and insulin promoter activity in pancreatic beta cells

Pancreatic-duodenal homoeobox factor-1 (PDX1) is a homoeodomain transcription factor that plays an important role in linking glucose metabolism in pancreatic beta cells to the regulation of insulin gene transcription. Our previous results indicated that glucose activates PDX1 DNA-binding activity and insulin promoter activity via a stress-activated signalling pathway involving phosphatidylinositol 3-kinase (PtdIns 3-kinase) and stress-activated protein kinase 2 (SAPK2/p38). The present study was undertaken to determine the effects of other metabolizable and non-metabolizable nutrients. The results indicate that non-metabolizable nutrients, with the exception of 2-deoxyglucose, had no effect. Metabolizable nutrients that could stimulate calcium uptake and insulin release were shown to activate both PDX1 and the insulin promoter. The possible role of insulin acting via an autoregulatory loop was therefore examined. Insulin was shown to potently activate PDX1 DNA-binding activity and insulin promoter activity. The effects of insulin were inhibited by the PtdIns 3-kinase inhibitors wortmannin and LY294002 and by the SAPK2 inhibitor SB203580, suggesting that its effects were mediated via activation of PtdIns 3-kinase and SAPK2. Further support for the insulin-mediated activation of SAPK2 came from the observation that both glucose and insulin stimulated the phosphorylation of SAPK2. These results suggest that both glucose and insulin stimulate PDX1 DNA-binding activity and insulin promoter activity via a pathway involving PtdIns 3-kinase and SAPK2.

Paired-homeodomain transcription factor PAX4 acts as a transcriptional repressor in early pancreatic development

The paired-homeodomain transcription factor PAX4 is expressed in the developing pancreas and along with PAX6 is required for normal development of the endocrine cells. In the absence of PAX4, the numbers of insulin-producing beta cells and somatostatin-producing delta cells are drastically reduced, while the numbers of glucagon-producing alpha cells are increased. To gain insight into PAX4 function, we cloned a full-length Pax4 cDNA from a beta-cell cDNA library and identified a bipartite consensus DNA binding sequence consisting of a homeodomain binding site separated from a paired domain binding site by 15 nucleotides. The paired half of this consensus sequence has similarities to the PAX6 paired domain consensus binding site, and the two proteins bind to common sequences in several islet genes, although with different relative affinities. When expressed in an alpha-cell line, PAX4 represses transcription through the glucagon or insulin promoters or through an isolated PAX4 binding site. This repression is not simply due to competition with the PAX6 transcriptional activator for the same binding site, since PAX4 fused to the unrelated yeast GAL4 DNA binding domain also represses transcription through the GAL4 binding site in the alpha-cell line and to a lesser degree in beta-cell lines and NIH 3T3 cells. Repressor activity maps to more than one domain within the molecule, although the homeodomain and carboxyl terminus give the strongest repression. PAX4 transcriptional regulation apparently plays a role only early in islet development, since Pax4 mRNA as determined by reverse transcriptase PCR peaks at embryonic day 13.5 in the fetal mouse pancreas and is undetectable in adult islets. In summary, PAX4 can function as a transcriptional repressor and is expressed early in pancreatic development, which may allow it to suppress alpha-cell differentiation and permit beta-cell differentiation.

Glucose regulation of the expression of the glucagon receptor gene

The glucagon receptor gene is a member of a gene family, the expression of which is strongly upregulated by glucose. This review deals with the structure of both the glucagon receptor gene and its promoter. Attention is focused on the glucose regulatory element that we discovered in the promoter of this gene. Regulation by glucose of genes implicated in glucose homeostasis represents one mechanism contributing to the control of fuel utilization. Its deficiency or imbalance could potentially lead to or participate in pathological situations such as diabetes mellitus. On the other hand, the regulatory element of the glucagon receptor gene promoter could be used as a tool for the glucose-regulated expression of other genes. Indeed, an analysis of the glucagon receptor gene promoter demonstrated that only a short fragment of the genomic DNA, easy to subclone, contains all required elements for activation by glucose. Its potential use for gene therapy is also considered, therefore, in this report.

Biotin regulation of pancreatic glucokinase and insulin in primary cultured rat islets and in biotin-deficient rats

Biotin has been reported to affect glucose homeostasis; however, its role on pancreatic islets of Langerhans has not been assessed. In this report, we demonstrate that physiologic concentrations of biotin stimulate glucokinase activity in rat islets in culture. Using the branched DNA (bDNA) assay, a sensitive signal amplification technique, we detected relative increases in glucokinase mRNA levels of 41.5 +/- 13% and 81.3 +/- 19% at 12 and 24 h respectively in islets treated with [10(-6) M] biotin. Because glucokinase activity controls insulin secretion, we also investigated the effect of biotin on insulin release. Treatment with [10(-6) M] biotin for 24 h increased insulin secretion. We extended our studies by analyzing the effect of biotin deficiency on pancreatic islet glucokinase expression and activity, as well as insulin secretion. Our results show that islet glucokinase activity and mRNA are reduced by 50% in the biotin deficient rat. Insulin secretion in response to glucose was also impaired in islets isolated from the deficient rat. These data show that biotin affects pancreatic islet glucokinase activity and expression and insulin secretion in cultured islets.

Differential regulation of the human insulin gene transcription by GG1 and GG2 elements with GG- and C1-binding factors

Using a human growth hormone reporter system, the introduced mutations in GG1 alone or both GG elements of GG1 and GG2 in the human insulin promoter abolished 94 or 96% of the beta-cell-specific transcriptional activity in a pancreatic islet beta-cell line of MIN6, while the mutations in GG2 or its total deletion abolished 85 or 86% of the transcriptional activity. When linked to the thymidine kinase promoter, mutations in GG1 or both GG elements abolished 74% of the transcriptional activity in MIN6 cells, while the mutations in GG2 or its total deletion abolished 55 or 54%. In the electrophoretic mobility shift assay (EMSA), one nuclear factor was shown to interact with two GG elements, and another C1-binding factor with GG1 and C1. The differential effects of deletions or selective mutations in the GG2 or GG1 sequence in the oligonucleotide probes on the binding activity of GG- or C1-binding factors in EMSA proved the requirement of both GG1 and GG2 or both GG1 and C1, respectively, for the transaction of these two factors. The molecular size of the GG-binding factor was estimated about 30 kDa. Based on these, we conclude that two GG elements contribute, with GG1 more critically than GG2, to the beta-cell-specific transcription of the human insulin gene through transaction with the GG- and C1-binding factors.