Cell-specific and ubiquitous factors are responsible for the enhancer activity of the rat insulin II gene

In vivo evaluation of GG2-GG1/A2 element activity in the insulin promoter region using the CRISPR-Cas9 system

The insulin promoter is regulated by ubiquitous as well as pancreatic β-cell-specific transcription factors. In the insulin promoter, GG2-GG1/A2-C1 (bases - 149 to - 116 in the human insulin promoter) play important roles in regulating β-cell-specific expression of the insulin gene. However, these events were identified through in vitro studies, and we are unaware of comparable in vivo studies. In this study, we evaluated the activity of GG2-GG1/A2 elements in the insulin promoter region in vivo. We generated homozygous mice with mutations in the GG2-GG1/A2 elements in each of the Ins1 and Ins2 promoters by CRISPR-Cas9 technology. The mice with homozygous mutations in the GG2-GG1/A2 elements in both Ins1 and Ins2 were diabetic. These data suggest that the GG2-GG1/A2 element in mice is important for Ins transcription in vivo.

Mutations in the C1 element of the insulin promoter lead to diabetic phenotypes in homozygous mice

Genome editing technologies such as CRISPR-Cas9 are widely used to establish causal associations between mutations and phenotypes. However, CRISPR-Cas9 is rarely used to analyze promoter regions. The insulin promoter region (approximately 1,000 bp) directs β cell-specific expression of insulin, which in vitro studies show is regulated by ubiquitous, as well as pancreatic, β cell-specific transcription factors. However, we are unaware of any confirmatory in vivo studies. Here, we used CRISPR-Cas9 technology to generate mice with mutations in the promoter regions of the insulin I (Ins1) and II (Ins2) genes. We generated 4 homozygous diabetic mice with 2 distinct mutations in the highly conserved C1 elements in each of the Ins1 and Ins2 promoters (3 deletions and 1 replacement in total). Remarkably, all mice with homozygous or heterozygous mutations in other loci were not diabetic. Thus, the C1 element in mice is required for Ins transcription in vivo.

Kindlin-2 modulates MafA and β-catenin expression to regulate β-cell function and mass in mice

β-Cell dysfunction and reduction in β-cell mass are hallmark events of diabetes mellitus. Here we show that β-cells express abundant Kindlin-2 and deleting its expression causes severe diabetes-like phenotypes without markedly causing peripheral insulin resistance. Kindlin-2, through its C-terminal region, binds to and stabilizes MafA, which activates insulin expression. Kindlin-2 loss impairs insulin secretion in primary human and mouse islets in vitro and in mice by reducing, at least in part, Ca²⁺ release in β-cells. Kindlin-2 loss activates GSK-3β and downregulates β-catenin, leading to reduced β-cell proliferation and mass. Kindlin-2 loss reduces the percentage of β-cells and concomitantly increases that of α-cells during early pancreatic development. Genetic activation of β-catenin in β-cells restores the diabetes-like phenotypes induced by Kindlin-2 loss. Finally, the inducible deletion of β-cell Kindlin-2 causes diabetic phenotypes in adult mice. Collectively, our results establish an important function of Kindlin-2 and provide a potential therapeutic target for diabetes.

Beta cell dysfunction and reduction in beta cell mass are hallmark events in the pathogenesis of diabetes mellitus. We identify focal adhesion protein Kindlin-2 as a key factor that controls insulin synthesis and secretion and beta cell mass by modulating MafA and beta-catenin proteins in pancreatic beta cells.

Glucose regulates MafA transcription factor abundance and insulin gene expression by inhibiting AMP-activated protein kinase in pancreatic β-cells

Insulin mRNA expression in pancreatic islet β-cells is up-regulated by extracellular glucose concentration, but the underlying mechanism remains incompletely understood. MafA is a transcriptional activator specifically enriched in β-cells that binds to the insulin gene promoter. Its expression is transcriptionally and posttranscriptionally regulated by glucose. Moreover, AMP-activated protein kinase (AMPK), a regulator of cellular energy homeostasis, is inhibited by high glucose, and this inhibition is essential for the up-regulation of insulin gene expression and glucose-stimulated insulin secretion (GSIS). Here we mutagenized the insulin promoter and found that the MafA-binding element C1/RIPE3b is required for glucose- or AMPK-induced alterations in insulin gene promoter activity. Under high-glucose conditions, pharmacological activation of AMPK in isolated mouse islets or MIN6 cells by metformin or 5-aminoimidazole-4-carboxamide riboside decreased MafA protein levels and mRNA expression of insulin and GSIS-related genes (i.e. glut2 and sur1). Overexpression of constitutively active AMPK also reduced MafA and insulin expression. Conversely, pharmacological AMPK inhibition by dorsomorphin (compound C) or expression of a dominant-negative form of AMPK increased MafA and insulin expression under low-glucose conditions. However, AMPK activation or inhibition did not change the expression levels of the β-cell-enriched transcription factors Pdx1 and Beta2/NeuroD1. AMPK activation accelerated MafA protein degradation, which is not dependent on the proteasome. We also noted that MafA overexpression prevents metformin-induced decreases in insulin and GSIS-related gene expression. These findings indicate that high glucose concentrations inhibit AMPK, thereby increasing MafA protein levels and activating the insulin promoter.

MafA and MafB activity in pancreatic β cells

Analyses in mouse models have revealed crucial roles for MafA (musculoaponeurotic fibrosarcoma oncogene family A) and MafB in islet β cells, with MafB being required during development and MafA in adults. These two closely related transcription factors regulate many genes essential for glucose sensing and insulin secretion in a cooperative and sequential manner. Significantly, the switch from MafB to MafA expression also appears to be vital for functional maturation of β cells produced by human embryonic stem (hES) cell differentiation. This review summarizes the discovery, distribution, and function of MafA and MafB in rodent pancreatic β cells, and describes some key questions regarding their importance to β cells.

TAT-mediated transduction of MafA protein in utero results in enhanced pancreatic insulin expression and changes in islet morphology

Alongside Pdx1 and Beta2/NeuroD, the transcription factor MafA has been shown to be instrumental in the maintenance of the beta cell phenotype. Indeed, a combination of MafA, Pdx1 and Ngn3 (an upstream regulator of Beta2/NeuroD) was recently reported to lead to the effective reprogramming of acinar cells into insulin-producing beta cells. These experiments set the stage for the development of new strategies to address the impairment of glycemic control in diabetic patients. However, the clinical applicability of reprogramming in this context is deemed to be poor due to the need to use viral vehicles for the delivery of the above factors. Here we describe a recombinant transducible version of the MafA protein (TAT-MafA) that penetrates across cell membranes with an efficiency of 100% and binds to the insulin promoter in vitro. When injected in utero into living mouse embryos, TAT-MafA significantly up-regulates target genes and induces enhanced insulin production as well as cytoarchitectural changes consistent with faster islet maturation. As the latest addition to our armamentarium of transducible proteins (which already includes Pdx1 and Ngn3), the purification and characterization of a functional TAT-MafA protein opens the door to prospective therapeutic uses that circumvent the use of viral delivery. To our knowledge, this is also the first report on the use of protein transduction in utero.

ATF2 interacts with beta-cell-enriched transcription factors, MafA, Pdx1, and beta2, and activates insulin gene transcription

Pancreatic β-cell-restricted expression of insulin is established through several critical cis-regulatory elements located in the insulin gene promoter region. The principal cis elements are A-boxes, E1, and C1/RIPE3b. The β-cell-enriched transcription factors Pdx1 and Beta2 bind to the A-boxes and E1 element, respectively. A β-cell-specific trans-acting factor binding to C1/RIPE3b (termed RIPE3b1 activator) was detected by electrophoretic mobility shift assay and has been identified as MafA, a member of the Maf family of basic leucine zipper (bZip) proteins. Here, ATF2, a member of the ATF/CREB family of basic leucine zipper proteins, was identified as a component of the RIPE3b1 activator. ATF2 alone was unable to bind to the C1/RIPE3b element but acquired binding capacity upon complex formation with MafA. ATF2 also interacted with Pdx1 and Beta2, and co-expression of ATF2, MafA, Pdx1, and Beta2 resulted in a synergistic activation of the insulin promoter. Immunohistochemical analysis of mouse pancreas tissue sections showed that ATF2 is enriched in islet endocrine cells, including β-cells. RNAi-mediated knockdown of MafA or ATF2 in the MIN6 β-cell line resulted in a significant decrease in endogenous levels of insulin mRNA. These data indicate that ATF2 is an essential component of the positive regulators of the insulin gene expression.

Activin receptor-like kinase and the insulin gene

The biological responses of the transforming growth factor-β (TGF-β) superfamily, which includes Activins and Nodal, are induced by activation of a receptor complex and Smads. A type I receptor, which is a component of the complex, is known as an activin receptor-like kinase (ALK); currently seven ALKs (ALK1-ALK7) have been identified in humans. Activins signaling, which is mediated by ALK4 and 7 together with ActRIIA and IIB, plays a critical role in glucose-stimulated insulin secretion, development/neogenesis, and glucose homeostatic control of pancreatic endocrine cells; the insulin gene is regulated by these signaling pathways via ALK7, which is a receptor for Activins AB and B and Nodal. This review discusses signal transduction of ALKs in pancreatic endocrine cells and the role of ALKs in insulin gene regulation.

p38 MAPK is a major regulator of MafA protein stability under oxidative stress

Mammalian MafA/RIPE3b1 is an important glucose-responsive transcription factor that regulates function, maturation, and survival of beta-cells. Increased expression of MafA results in improved glucose-stimulated insulin secretion and beta-cell function. Because MafA is a highly phosphorylated protein, we examined whether regulating activity of protein kinases can increase MafA expression by enhancing its stability. We demonstrate that MafA protein stability in MIN6 cells and isolated mouse islets is regulated by both p38 MAPK and glycogen synthase kinase 3. Inhibiting p38 MAPK enhanced MafA stability in cells grown under both low and high concentrations of glucose. We also show that the N-terminal domain of MafA plays a major role in p38 MAPK-mediated degradation; simultaneous mutation of both threonines 57 and 134 into alanines in MafA was sufficient to prevent this degradation. Under oxidative stress, a condition detrimental to beta-cell function, a decrease in MafA stability was associated with a concomitant increase in active p38 MAPK. Interestingly, inhibiting p38 MAPK but not glycogen synthase kinase 3 prevented oxidative stress-dependent degradation of MafA. These results suggest that the p38 MAPK pathway may represent a common mechanism for regulating MafA levels under oxidative stress and basal and stimulatory glucose concentrations. Therefore, preventing p38 MAPK-mediated degradation of MafA represents a novel approach to improve beta-cell function.

Glucose-responsive gene expression system for gene therapy

Regulation of gene expression by glucose is an important mechanism for mammals in adapting to their nutritional environment. Glucose, the primary fuel for most cells, modulates gene expression that is crucial in the cellular adaptation to glycemic variation. Transcription of the genes for insulin and glycolytic and lipogenic enzymes is stimulated by glucose in pancreatic beta-cells and liver. Recent findings further support the key role of the carbohydrate-responsive element binding protein in the regulation of glycolytic and lipogenic genes by glucose and dietary carbohydrates. Herein, we review the transcriptional regulation of glucose-responsive genes, and recent advances in the gene therapy using glucose-responsive gene expression for diabetes.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

The protein kinases ERK1/2 and their roles in pancreatic beta cells

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) activities are modulated in a manner that reflects the secretory demand on beta cells to integrate long- and short-term nutrient sensing information. Our studies have focused on the mechanisms of ERK1/2 activation in beta cells and on the actions of ERK1/2 that regulate beta cell function. Insulin and growth factors regulate ERK1/2 in beta cells in a largely calcium-independent manner. Nutrients and anticipatory hormones, in contrast, activate ERK1/2 in a calcium-dependent manner in these cells. We are exploring the key intermediates in these distinct activation pathways and find that calcineurin is essential for the nutrient pathway but is not essential for the growth factor pathway. Using reporter assays, heterologous reconstitution, electrophoretic mobility shift assays, Northern analysis, Q-PCR and chromatin immunoprecipitation, we have examined several genes that are regulated by ERK1/2, primarily the insulin gene and the apoptotic factor C/EBP-homologous protein (CHOP)-10 (GADD153/DDIT-3), a bZIP protein. ERK1/2-sensitive transcriptional regulators common to these two genes are C/EBP-beta and MafA. The insulin promoter is both positively and negatively regulated by glucose and other nutrients. Exposure to glucose for minutes to hours causes an increase in the rate of insulin gene transcription. In contrast, exposure to elevated glucose for 48 h or more results in inhibition of the insulin gene promoter. Both of these processes depend on ERK1/2 activity. Expression of CHOP is induced by stresses including nutrient deprivation and endoplasmic reticulum stress. CHOP gene expression, especially that regulated by nutrients, is also ERK1/2-dependent in beta cells, These studies support the hypothesis that the genes regulated by ERK1/2 and the mechanisms employed are key to maintaining normal beta cell function.

Roles and regulation of transcription factor MafA in islet beta-cells

Insulin is a critical hormone in the regulation of blood glucose levels. It is produced exclusively by pancreatic islet beta-cells. beta-cell-enriched transcription factors, such as Pdx1 and Beta2, have dual roles in the activation of the insulin gene promoter establishing beta-cell-specific insulin expression, and in the regulation of beta-cell differentiation. It was shown that MafA, a beta-cell-specific member of the Maf family of transcription factors, binds to the conserved C1/RIPE3b element of the insulin promoter. The Maf family proteins regulate tissue-specific gene expression and cell differentiation in a wide variety of tissues. MafA acts synergistically with Pdx1 and Beta2 to activate the insulin gene promoter, and mice with a targeted deletion of mafA develop age-dependent diabetes. MafA also regulates genes involved in beta-cell function such as Glucose transporter 2, Glucagons-like peptide 1 receptor, and Prohormone convertase 1/3. The abundance of MafA in beta-cells is regulated at both the transcriptional and post-translational levels by glucose and oxidative stress. This review summarizes recent progress in determining the functions and roles of MafA in the regulation of insulin gene transcription in beta-cells.

MafA stability in pancreatic beta cells is regulated by glucose and is dependent on its constitutive phosphorylation at multiple sites by glycogen synthase kinase 3

Regulation of insulin gene expression by glucose in pancreatic beta cells is largely dependent on a cis-regulatory element, termed RIPE3b/C1, in the insulin gene promoter. MafA, a member of the Maf family of basic leucine zipper (bZip) proteins, is a beta-cell-specific transcriptional activator that binds to the C1 element. Based on increased C1-binding activity, MafA protein levels appear to be up-regulated in response to glucose, but the underlying molecular mechanism for this is not well understood. In this study, we show evidence supporting that the amino-terminal region of MafA is phosphorylated at multiple sites by glycogen synthase kinase 3 (GSK3) in beta cells. Mutational analysis of MafA and pharmacological inhibition of GSK3 in MIN6 beta cells strongly suggest that the rate of MafA protein degradation is regulated by glucose, that MafA is constitutively phosphorylated by GSK3, and that phosphorylation is a prerequisite for rapid degradation of MafA under low-glucose conditions. Our data suggest a new glucose-sensing signaling pathway in islet beta cells that regulates insulin gene expression through the regulation of MafA protein stability.

MafA regulates expression of genes important to islet beta-cell function

Insulin transcription factor MafA is unique in being exclusively expressed at the secondary and principal phase of insulin-expressing cell production during pancreas organogenesis and is the only transcriptional activator present exclusively in islet beta-cells. Here we show that ectopic expression of MafA is sufficient to induce a small amount of endogenous insulin expression in a variety of non-beta-cell lines. Insulin mRNA and protein expression was induced to a much higher level when MafA was provided with two other key insulin activators, pancreatic and duodenal homeobox (PDX-1) and BETA2. Potentiation by PDX-1 and BETA2 was entirely dependent upon MafA, and MafA binding to the insulin enhancer region was increased by PDX-1 and BETA2. Treatment with activin A and hepatocyte growth factor induced even larger amounts of insulin in AR42J pancreatic acinar cells, compared with other non-beta endodermal cells. The combination of PDX-1, BETA2, and MafA also induced the expression of other important regulators of islet beta-cell activity. These results support a critical role of MafA in islet beta-cell function.

Differential regulation of CHOP-10/GADD153 gene expression by MAPK signaling in pancreatic beta-cells

CHOP-10 (GADD153/DDIT-3) is a bZIP protein involved in differentiation and apoptosis. Its expression is induced in response to stresses such as nutrient deprivation, perturbation of the endoplasmic reticulum, redox imbalance, and UV exposure. Here we show that CHOP expression is induced in cultured pancreatic beta-cells maintained in a basal glucose concentration of 5.5 mM and repressed by stimulatory glucose (>or=11 mM). Both induction and repression of CHOP are dependent on the MAPKs ERK1 and ERK2. Two regulatory composite sites containing overlapping MafA response elements (MARE) and CAAT enhancer binding (CEB) elements regulate transcription in an ERK1/2-dependent manner. One site (MARE-CEB), from -320 to -300 bp in the promoter, represses transcription. The other site (CEB-MARE), from +2,628 to +2,641 bp in the first intron of the CHOP gene, activates it. MafA can influence transcription of both sites. The MARE-CEB is repressed by MafA, whereas the CEB-MARE site, which is homologous to the A2C1 component of the glucose-sensitive RIPE3b region of the insulin gene promoter, is activated by MafA. These results indicate that ERK1/2 have dual roles in regulating CHOP gene expression via both promoter and intronic regions, depending on environmental and metabolic stresses imposed on pancreatic beta-cells.

A switch from MafB to MafA expression accompanies differentiation to pancreatic beta-cells

Major insulin gene transcription factors, such as PDX-1 or NeuroD1, have equally important roles in pancreatic development and the differentiation of pancreatic endocrine cells. Previously, we identified and cloned another critical insulin gene transcription factor MafA (RIPE3b1) and reported that other Maf factors were expressed in pancreatic endocrine cells. Maf factors are important regulators of cellular differentiation; to understand their role in differentiation of pancreatic endocrine cells, we analyzed the expression pattern of large-Maf factors in the pancreas of embryonic and adult mice. Ectopically expressed large-Maf factors, MafA, MafB, or cMaf, induced expression from insulin and glucagon reporter constructs, demonstrating a redundancy in their function. Yet in adult pancreas, cMaf was expressed in both alpha- and beta-cells, and MafA and MafB showed selective expression in the beta- and alpha-cells, respectively. Interestingly, during embryonic development, a significant proportion of MafB-expressing cells also expressed insulin. In embryos, MafB is expressed before MafA, and our results suggest that the differentiation of beta-cells proceeds through a MafB+ MafA- Ins+ intermediate cell to MafB- MafA+ Ins+ cells. Furthermore, the MafB to MafA transition follows induction of PDX-1 expression (Pdx-1(high)) in MafB+ Ins+ cells. We suggest that MafB may have a dual role in regulating embryonic differentiation of both beta- and alpha-cells while MafA may regulate replication/survival and function of beta-cells after birth. Thus, this redundancy in the function and expression of the large-Maf factors may explain the normal islet morphology observed in the MafA knockout mice at birth.

Regulation of insulin gene expression by overlapping DNA-binding elements

The transcription factor MafA/RIPE3b1 is an important regulator of insulin gene expression. MafA binds to the insulin enhancer element RIPE3b (C1-A2), now designated as insulin MARE (Maf response element). The insulin MARE element shares an overlapping DNA-binding region with another insulin enhancer element A2. A2.2, a beta-cell-specific activator, like the MARE-binding factor MafA, binds to the overlapping A2 element. Our previous results demonstrated that two nucleotides in the overlapping region are required for the binding of both factors. Surprisingly, instead of interfering with each other's binding activity, the MafA and the A2-binding factors co-operatively activated insulin gene expression. To understand the molecular mechanisms responsible for this functional co-operation, we have determined the nucleotides essential for the binding of the A2.2 factor. Using this information, we have constructed non-overlapping DNA-binding elements and their derivatives, and subsequently analysed the effect of these modifications on insulin gene expression. Our results demonstrate that the overlapping binding site is essential for maximal insulin gene expression. Furthermore, the overlapping organization is critical for MafA-mediated transcriptional activation, but has a minor effect on the activity of A2-binding factors. Interestingly, the binding affinities of both MafA and A2.2 to the overlapping or non-overlapping binding sites were not significantly different, implying that the overlapping binding organization may increase the activation potential of MafA by physical/functional interactions with A2-binding factors. Thus our results demonstrate a novel mechanism for the regulation of MafA activity, and in turn beta-cell function, by altering expression and/or binding of the A2.2 factor. Our results further suggest that the major downstream targets of MafA will in addition to the MARE element have a binding site for the A2.2 factor.

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.

ERK1/2-dependent Activation of Transcription Factors Required for Acute and Chronic Effects of Glucose on the Insulin Gene Promoter

The insulin promoter is both positively and negatively regulated in response to conditions to which pancreatic beta-cells are exposed. Exposure of intact rat islets and INS-1 pancreatic beta-cells to 11 mm glucose for minutes to hours results in an enhancement in the rate of insulin gene transcription assessed with a reporter linked to the insulin gene promoter. In contrast, chronic exposure of rat islets or beta-cells to 11 mm glucose results in loss of the glucose responsiveness of the insulin gene promoter. By 48 h, glucose inhibits insulin gene promoter activity. Here we show that not only the acute effect of elevated glucose to stimulate the insulin gene promoter but also the chronic effect of elevated glucose to inhibit the insulin gene promoter depend on ERK1/2 mitogen-activated protein kinase activity. In examining the underlying mechanism, we found that acute exposure to 11 mm glucose resulted in the binding of the transcription factors NFAT and Maf to the glucose-responsive A2C1 element of the insulin gene promoter. An NFAT and C/EBP-beta complex was observed in cells chronically exposed to 11 mm glucose. Formation of NFAT-Maf and NFAT-C/EBP-beta complexes was sensitive to inhibitors of ERK1/2 and calcineurin, consistent with our previous finding that activation of ERK1/2 by glucose required calcineurin activity and the well documented regulation of NFAT by calcineurin. These results indicate that the ERK1/2 pathway modulates partners of NFAT, which may either stimulate or repress insulin gene transcription during stimulatory and chronic exposure to elevated glucose.

Oxidative stress-mediated, post-translational loss of MafA protein as a contributing mechanism to loss of insulin gene expression in glucotoxic beta cells

Glucose toxicity in pancreatic islet beta cells causes loss of insulin gene expression, content, and secretion due to loss of binding of transcription factors, most notably PDX-1 and RIPE-3b1 activator, to the promoter region of the insulin gene. Recently, RIPE-3b1 activator was cloned and identified as the mammalian homologue of avian MafA/Maf-L (MafA). This enabled us to carry out more extensive studies of the role of MafA in glucotoxicity than were hitherto possible. Northern analysis of glucotoxic HIT-T15 cells revealed normal amounts of MafA mRNA, but Western analysis demonstrated a 97 +/- 1% reduction in MafA protein (p < 0.0001). The proteasome is a likely site for MafA degradation as lactacystin, an irreversible proteasome inhibitor, caused an accumulation of MafA protein. Antioxidants have previously been shown to prevent the adverse effects of glucose toxicity on beta cell function both in vivo and in vitro. In the current study, chronic culturing of HIT-T15 cells with the antioxidant N-acetylcysteine (NAC) prevented loss of MafA protein (late passage = 18.9 +/- 10.4% of early passage, p < 0.001; late passage with NAC = 68.7 +/- 19.7% of early passage, p = not significant) and loss of DNA binding (late passage = 63.7 +/- 9% of early passage, p < 0.02; late passage with NAC = 116 +/- 10% of early passage, p = not significant). Additionally, transient transfection of PDX-1 or MafA cDNA into glucotoxic cells increased PDX-1 and MafA protein levels and individually increased insulin promoter activity (untreated = 34%, PDX-1 = 70%, MafA = 78%; percentage of activity of early passage cells), whereas the combined transfection of MafA and PDX-1 completely restored insulin promoter activity. This recovery of promoter activity following transient transfection had no effect on endogenous insulin mRNA. However, adenoviral infection of MafA and PDX-1 significantly increased endogenous insulin mRNA levels by 93% (121 +/- 9 versus 233 +/- 18 density light units; n = 5, p < 0.001). We conclude that the absence of MafA protein from beta cells via chronic oxidative stress contributes importantly to the loss of endogenous insulin gene expression as glucose toxicity develops.

The islet beta cell-enriched MafA activator is a key regulator of insulin gene transcription

The islet-enriched MafA, PDX-1, and BETA2 activators contribute to both beta cell-specific and glucose-responsive insulin gene transcription. To investigate how these factors impart activation, their combined impact upon insulin enhancer-driven expression was first examined in non-beta cell line transfection assays. Individual expression of PDX-1 and BETA2 led to little or no activation, whereas MafA alone did so modestly. MafA together with PDX-1 or BETA2 produced synergistic activation, with even higher insulin promoter activity found when all three proteins were present. Stimulation was attenuated upon compromising either MafA transactivation or DNA-binding activity. MafA interacted with endogenous PDX-1 and BETA2 in coimmunoprecipitation and in vitro GST pull-down assays, suggesting that regulation involved direct binding. Dominant-negative acting and small interfering RNAs of MafA also profoundly reduced insulin promoter activity in beta cell lines. In addition, MafA was induced in parallel with insulin mRNA expression in glucose-stimulated rat islets. Insulin mRNA levels were also elevated in rat islets by adenoviral-mediated expression of MafA. Collectively, these results suggest that MafA plays a key role in coordinating and controlling the level of insulin gene expression in islet beta cells.

The role of calcium/calmodulin-dependent protein kinase cascade in glucose upregulation of insulin gene expression

A number of factors have been reported to affect insulin synthesis in beta-cells. Although glucose is the most important regulator of insulin gene expression in pancreatic beta-cells, the mechanisms whereby glucose stimulates insulin gene transcription in response to changes in glucose concentration have not been clarified yet. In this study, we examined the role of the Ca(2+)/calmodulin (CaM)-dependent protein kinase (CaM-K) cascade in transcriptional activation of insulin. RT-PCR, Western blotting, and immunohistochemical staining analysis revealed that CaM-K kinase-alpha (CaM-KKalpha) and CaM-KIV were localized in rat pancreatic beta-cells and their cell line, INS-1. Exposure of INS-1 cells to 11.2 mmol/l glucose elicited an increase of insulin promoter activity as well as upregulation of CaM-KIV activity within 2 min after stimulation. We investigated the influence on insulin promoter activity of the constitutively active form (CaM-KIVc) or dominant-negative mutant (CaM-KIVdn) of CaM-KIV in transfected INS-1 cells. CaM-KIVc alone was sufficient, and the upstream kinase, CaM-KK, was enhanced to upregulate the insulin promoter activity in INS-1 cells. Furthermore, cotransfection of CaM-KIVdn suppressed to a significant degree the glucose-upregulated activity of the insulin promoter. Taken together, these results indicated that the CaM-KK/CaM-KIV cascade might play an important role in glucose-upregulated transcriptional activation of the insulin gene.

Mouse MafA, homologue of zebrafish somite Maf 1, contributes to the specific transcriptional activity through the insulin promoter

Large Maf transcription factors, which are members of the basic leucine zipper (b-Zip) superfamily, have been reported to be involved in embryonic development and cell differentiation. Previously, we isolated a novel zebrafish large Maf cDNA, somite Maf1 (SMaf1), which possesses transactivational activity within its N-terminus domain. To elucidate SMaf1 function in mammals, we tried to isolate the mouse homologue of zebrafish SMaf1. We isolated the mouse homologue of zebrafish SMaf1, which is the same molecule as the recently reported MafA. MafA mRNA was detected in formed somites, head neural tube, and liver cells in the embryos. In the adult mouse, MafA transcript was amplified in the brain, lung, spleen, and kidney by RT-PCR. MafA mRNA was also detectable in beta-cell line. Next, we analyzed the transcriptional activity of MafA using rat insulin promoters I and II (RIPI and II), since a part of RIP sequence was similar to the Maf recognition element (MARE) and MafA was expressed in pancreatic beta cells. MafA was able to activate transcription from RIPII, but not RIPI, in a dose dependent manner and the activity was dependent on RIPE3b/C1 sequences. In addition, the amount of MafA protein was regulated by glucose concentration. These results indicate that MafA is the homologue of zebrafish SMaf1 and acts as a transcriptional activator of the insulin gene promoter through the RIPE3b element.

The MafA transcription factor appears to be responsible for tissue-specific expression of insulin

Insulin gene expression is regulated by several islet-enriched transcription factors. However, MafA is the only beta cell-specific activator. Here, we show that MafA selectively induces endogenous insulin transcription in non-beta cells. MafA was also first detected in the insulin-producing cells formed during the second and predominant phase of beta cell differentiation, and absent in the few insulin-positive cells found in Nkx6.1(-/-) pancreata, which lack the majority of second-phase beta cells. These results demonstrate that MafA is a potent insulin activator that is likely to function downstream of Nkx6.1 during islet insulin-producing cell development.

Acetylation of the BETA2 transcription factor by p300-associated factor is important in insulin gene expression

The BETA2 transcription factor influences islet beta cell development and function. Activation of insulin gene transcription by this member of the basic helix-loop-helix gene family is mediated by p300 through the ability of this coactivator to form a functional bridge between the basal transcriptional apparatus, BETA2, and PDX-1, another key transcription factor. In this report, we examined whether BETA2-mediated stimulation was also directly influenced by the acetyltransferase activities of p300 or the p300-associated factor. BETA2 was specifically and selectively acetylated by p300-associated factor in beta cells. Sites of BETA2 acetylation were found within the loop region of the basic helix-loop-helix DNA binding/dimerization domain and a more C-terminal region involved in activation. Insulin gene transcription was decreased by blocking acetylation of BETA2 because of effects on DNA binding and activation potential. These findings suggest that acetylation of BETA2 plays a role in controlling the activation state of this islet regulatory factor.

Members of the large Maf transcription family regulate insulin gene transcription in islet beta cells

The C1/RIPE3b1 (-118/-107 bp) binding factor regulates pancreatic-beta-cell-specific and glucose-regulated transcription of the insulin gene. In the present study, the C1/RIPE3b1 activator from mouse beta TC-3 cell nuclear extracts was purified by DNA affinity chromatography and two-dimensional gel electrophoresis. C1/RIPE3b1 binding activity was found in the roughly 46-kDa fraction at pH 7.0 and pH 4.5, and each contained N- and C-terminal peptides to mouse MafA as determined by peptide mass mapping and tandem spectrometry. MafA was detected in the C1/RIPE3b1 binding complex by using MafA peptide-specific antisera. In addition, MafA was shown to bind within the enhancer region (-340/-91 bp) of the endogenous insulin gene in beta TC-3 cells in the chromatin immunoprecipitation assay. These results strongly suggested that MafA was the beta-cell-enriched component of the RIPE3b1 activator. However, reverse transcription-PCR analysis demonstrated that mouse islets express not only MafA but also other members of the large Maf family, specifically c-Maf and MafB. Furthermore, immunohistochemical studies revealed that at least MafA and MafB were present within the nuclei of islet beta cells and not within pancreas acinar cells. Because MafA, MafB, and c-Maf were each capable of specifically binding to and activating insulin C1 element-mediated expression, our results suggest that all of these factors play a role in islet beta-cell function.

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.

Covalent histone modifications underlie the developmental regulation of insulin gene transcription in pancreatic beta cells

Histone modifying enzymes contribute to the activation or inactivation of transcription by ultimately catalyzing the unfolding or further compaction, respectively, of chromatin structure. Actively transcribed genes are typically hyperacetylated at Lys residues of histones H3 and H4 and hypermethylated at Lys-4 of histone H3 (H3-K4). To determine whether covalent histone modifications play a role in the beta cell-specific expression of the insulin gene, we performed chromatin immunoprecipitation assays using anti-histone antibodies and extracts from beta cell lines, non-beta cell lines, and ES cells, and quantitated specific histone modifications at the insulin promoter by real-time PCR. Our studies reveal that the proximal insulin promoter is hyperacetylated at histone H3 only in beta cells. This hyperacetylation is highly correlated to recruitment of the histone acetyltransferase p300 to the proximal promoter in beta cells, and is consistent with the role of hyperacetylation in promoting euchromatin formation. We also observed that the proximal insulin promoter of beta cells is hypermethylated at H3-K4, and that this modification is correlated to the recruitment of the histone methyltransferase SET7/9 to the promoter. ES cells demonstrate a histone modification pattern intermediate between that of beta cells and non-beta cells, and is consistent with their potential to express the insulin gene. We therefore propose a model in which insulin transcription in the beta cell is facilitated by a unique combination of transcription factors that acts in the setting of an open, euchromatic structure of the insulin gene.

The islet beta cell-enriched RIPE3b1/Maf transcription factor regulates pdx-1 expression

Pancreatic duodenal homeobox factor-1, PDX-1, is required for pancreas development, islet cell differentiation, and the maintenance of beta cell function. Selective expression in the pancreas appears to be principally regulated by Area II, one of four conserved regulatory sequence domains found within the 5'-flanking region of the pdx-1 gene. Detailed mutagenesis studies have identified potential sites of interaction for both positive- and negative-acting factors within the conserved sequence blocks of Area II. The islet beta cell-enriched RIPE3b1 transcription factor, the activator of insulin C1 element-driven expression, was shown here to also stimulate Area II by binding to sequence blocks 4 and 5 (termed B4/5). Accordingly, B4/5 DNA-binding protein's molecular mass (i.e. 46 kDa), binding specificity, and islet beta cell-enriched distribution were identical to RIPE3b1. Area II-mediated activation was also unaffected upon replacing B4/5 with the insulin C1/RIPE3b1 binding site. In addition, the chromatin immunoprecipitation assay showed that the Area II region of the endogenous pdx-1 gene was precipitated by an antiserum that recognizes the large Maf protein that comprises the RIPE3b1 transcription factor. These results strongly suggest that RIPE3b1/Maf has an important role in generating and maintaining physiologically functional beta cells.

BETA2 activates transcription from the upstream glucokinase gene promoter in islet beta-cells and gut endocrine cells

Glucokinase (GK) gene transcription initiates in the islet (beta-cell), gut, and brain from promoter sequences residing approximately 35 kbp upstream from those used in liver. Expression of betaGK is controlled in beta-cells by cell-enriched (i.e. pancreatic duodenal homeobox 1 [PDX-1]) and ubiquitously (i.e., Pal) distributed factors that bind to and activate from conserved sequence motifs within the upstream promoter region (termed betaGK). Here, we show that a conserved E-box element also contributes to control in the islet and gut. betaGK promoter-driven reporter gene activity was diminished by mutating the specific sequences involved in E-box-mediated basic helix-loop-helix factor activator binding in islet beta-cells and enteroendocrine cells. Gel shift assays demonstrated that the betaGK and insulin gene E-box elements formed the same cell-enriched (BETA2:E47) and generally distributed (upstream stimulatory factor [USF]) protein-DNA complexes. betaGK E-box-driven activity was stimulated in cotransfection assays performed in baby hamster kidney (BHK) cells with BETA2 and E47, but not USF. Chromatin immunoprecipitation assays performed with BETA2 antisera showed that BETA2 occupies the upstream promoter region of the endogenous betaGK gene in beta-cells. We propose that BETA2 (also termed NeuroD1) regulates betaGK promoter activity.

MafA is a glucose-regulated and pancreatic beta-cell-specific transcriptional activator for the insulin gene

The insulin gene is specifically expressed in beta-cells of the Langerhans islets of the pancreas, and its transcription is regulated by the circulating glucose level. Previous reports have shown that an unidentified beta-cell-specific nuclear factor binds to a conserved cis-regulatory element called RIPE3b and is critical for its glucose-regulated expression. Based on the sequence similarity of the RIPE3b element and the consensus binding sequence of the Maf family of basic leucine zipper transcription factors, we here identified mammalian homologue of avian MafA/L-Maf, an eye-specific member of the Maf family, as the RIPE3b-binding transcriptional activator. Reverse transcription-PCR analysis showed that mafA mRNA is detected only in the eyes and in pancreatic beta-cells and not in alpha-cells. MafA protein as well as its mRNA is up-regulated by glucose, consistent with the glucose-regulated binding of MafA to the RIPE3b element in beta-cell nuclear extracts. In transient luciferase assays, we also showed that expression of MafA greatly enhanced insulin promoter activity and that a dominant-negative form of MafA inhibited it. Therefore, MafA is a beta-cell-specific and glucose-regulated transcriptional activator for insulin gene expression and thus may be involved in the function and development of beta-cells as well as in the pathogenesis of diabetes.

Glucose induced MAPK signalling influences NeuroD1-mediated activation and nuclear localization

The helix-loop-helix transcription factor NeuroD1 (also known as Beta2) is involved in beta-cell survival during development and insulin gene transcription in adults. Here we show NeuroD1 is primarily cytoplasmic at non-stimulating glucose concentrations (i.e. 3 mM) in MIN6 beta-cells and nuclear under stimulating conditions (i.e. 20 mM). Quantification revealed that NeuroD1 was in 40-45% of the nuclei at 3 mM and 80-90% at 20 mM. Treatment with the MEK inhibitor PD98059 or substitution of a serine for an alanine at a potential mitogen-activated protein kinase phosphorylation site (S274) in NeuroD1 significantly increased the cytoplasmic level at 20 mM glucose. The rise in NeuroD1-mediated transcription in response to glucose also correlated with the change in sub-cellular localization, a response attenuated by PD98059. The data strongly suggest that glucose-stimulation of the MEK-ERK signalling pathway influences NeuroD1 activity at least partially through effects on sub-cellular localization.

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.

Identification of beta-cell-specific insulin gene transcription factor RIPE3b1 as mammalian MafA

Of the three critical enhancer elements that mediate beta-cell-specific and glucose-responsive expression of the insulin gene, only the identity of the transcription factor binding to the RIPE3b element (RIPE3b1) has remained elusive. Using a biochemical purification approach, we have identified the RIPE3b1 factor as a mammalian homologue of avian MafA/L-Maf (mMafA). The avian MafA is a cell-type determination factor that expressed ectopically can trigger lens differentiation program, but no mammalian homologue of avian MafA has previously been identified. Here, we report cloning of the human mafA (hMafA) and demonstrate that it can specifically bind the insulin enhancer element RIPE3b and activate insulin-gene expression. In addition, mMafA has a very restrictive cellular distribution and is selectively expressed in pancreatic beta but not in alpha cells. We suggest that mMafA has an essential role in the function and differentiation of beta-cells and thus may be associated with the pathophysiological origins of diabetes.

Insulin gene transcription is mediated by interactions between the p300 coactivator and PDX-1, BETA2, and E47

Pancreatic beta-cell-type-specific expression of the insulin gene requires both ubiquitous and cell-enriched activators, which are organized within the enhancer region into a network of protein-protein and protein-DNA interactions to promote transcriptional synergy. Protein-protein-mediated communication between DNA-bound activators and the RNA polymerase II transcriptional machinery is inhibited by the adenovirus E1A protein as a result of E1A's binding to the p300 coactivator. E1A disrupts signaling between the non-DNA-binding p300 protein and the basic helix-loop-helix DNA-binding factors of insulin's E-element activator (i.e., the islet-enriched BETA2 and generally distributed E47 proteins), as well as a distinct but unidentified enhancer factor. In the present report, we show that E1A binding to p300 prevents activation by insulin's beta-cell-enriched PDX-1 activator. p300 interacts directly with the N-terminal region of the PDX-1 homeodomain protein, which contains conserved amino acid sequences essential for activation. The unique combination of PDX-1, BETA2, E47, and p300 was shown to promote synergistic activation from a transfected insulin enhancer-driven reporter construct in non-beta cells, a process inhibited by E1A. In addition, E1A inhibited the level of PDX-1 and BETA2 complex formation in beta cells. These results indicate that E1A inhibits insulin gene transcription by preventing communication between the p300 coactivator and key DNA-bound activators, like PDX-1 and BETA2:E47.

The DNA binding activity of the RIPE3b1 transcription factor of insulin appears to be influenced by tyrosine phosphorylation

The RIPE3b1 DNA binding factor plays a critical role in pancreatic islet beta cell-specific and glucose-regulated transcription of the insulin gene. Recently it was shown that RIPE3b1 binding activity in beta cell nuclear extracts is reduced by treatment with either calf intestinal alkaline phosphatase (CIAP) or a brain-enriched phosphatase preparation (BPP) (Zhao, L., Cissell, M. A., Henderson, E., Colbran, R., and Stein, R. (2000) J. Biol. Chem. 275, 10532-10537). Evidence is presented here suggesting that a tyrosine phosphatase(s) influences the ability of RIPE3b1 to bind to the insulin C1 element in beta cells. We found that RIPE3b1 binding was inhibited upon incubating beta cell nuclear extracts at 30 degrees C. In contrast, PDX-1 and MLTF-1 transcription factor binding activity was unaffected under these conditions. The loss in RIPE3b1 binding activity was prevented by inhibitors of tyrosine phosphatases (sodium orthovanadate and sodium molybdate) but not by inhibitors of serine/threonine phosphatases (sodium fluoride, okadaic acid, and microcystin LR). CIAP- and BPP-catalyzed inhibition of RIPE3b1 binding was also blocked by these tyrosine phosphatase inhibitors. Collectively, the data suggested that removal of a tyrosine(s) within RIPE3b1 prevented activator binding to insulin C1 control element sequences. The presence of a key phosphorylated tyrosine(s) within this transcription factor was further supported by the ability of the 4G10 anti-phosphotyrosine monoclonal antibody to immunoprecipitate RIPE3b1 DNA binding activity. We discuss how tyrosine phosphorylation, a very rare and highly significant regulatory modification, may control RIPE3b1 activator function.

Transcription factors recognizing overlapping C1-A2 binding sites positively regulate insulin gene expression

Transcription factors binding the insulin enhancer region, RIPE3b, mediate beta-cell type-specific and glucose-responsive expression of the insulin gene. Earlier studies demonstrate that activator present in the beta-cell-specific RIPE3b1-binding complex is critical for these actions. The DNA binding activity of the RIPE3b1 activator is induced in response to glucose stimulation and is inhibited under glucotoxic conditions. The C1 element within the RIPE3b region has been implicated as the binding site for RIPE3b1 activator. The RIPE3b region also contains an additional element, A2, which shares homology with the A elements in the insulin enhancer. Transcription factors (PDX-1 and HNF-1 alpha) binding to A elements are critical regulators of insulin gene expression and/or pancreatic development. Hence, to understand the roles of C1 and A2 elements in regulating insulin gene expression, we have systematically mutated the RIPE3b region and analyzed the effect of these mutations on gene expression. Our results demonstrate that both C1 and A2 elements together constitute the binding site for the RIPE3b1 activator. In addition to C1-A2 (RIPE3b) binding complexes, three binding complexes that specifically recognize A2 elements are found in nuclear extracts from insulinoma cell lines; the A2.2 complex is detected only in insulin-producing cell lines. Furthermore, two base pairs in the A2 element were critical for binding of both RIPE3b1 and A2.2 activators. Transient transfection results indicate that both C1-A2 and A2-specific binding activators cooperatively activate insulin gene expression. In addition, RIPE3b1- and A2-specific activators respond differently to glucose, suggesting that their overlapping binding specificity and functional cooperation may play an important role in regulating insulin gene expression.

Autoregulation and maturity onset diabetes of the young transcription factors control the human PAX4 promoter

During pancreatic development, the paired homeodomain transcription factor PAX4 is required for the differentiation of the insulin-producing beta cells and somatostatin-producing delta cells. To establish the position of PAX4 in the hierarchy of factors controlling islet cell development, we examined the control of the human PAX4 gene promoter. In both cell lines and transgenic animals, a 4.9-kilobase pair region directly upstream of the human PAX4 gene transcriptional start site acts as a potent pancreas-specific promoter. Deletion mapping experiments demonstrate that a 118-base pair region lying approximately 1.9 kilobase pairs upstream of the transcription start site is both necessary and sufficient to direct pancreas-specific expression. Serial deletions through this region reveal the presence of positive elements that bind several pancreatic transcription factors as follows: the POU homeodomain factor HNF1alpha, the orphan nuclear receptor HNF4alpha, the homeodomain factor PDX1, and a heterodimer composed of two basic helix-loop-helix factors. Interestingly, mutations in the genes encoding four of these factors cause a dominantly inherited form of human diabetes called Maturity Onset Diabetes of the Young. In addition, PAX4 itself has at least two high affinity binding sites within the promoter through which it exerts a strong negative autoregulatory effect. Together, these results suggest a model in which PAX4 expression is activated during pancreatic development by a combination of pancreas-specific factors but is then switched off once PAX4 protein reaches sufficient levels.

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.

Human homologue of maid: A dominant inhibitory helix-loop-helix protein associated with liver-specific gene expression

The helix-loop-helix (HLH) family of transcriptional regulatory proteins are key regulators in numerous developmental processes. The class I HLH proteins, such as E12 are ubiquitously expressed. Class II HLH proteins, such as MyoD, are expressed in a tissue-specific manner. Class I and II heterodimers can bind to E-boxes (CANNTG) and regulate lineage commitments of embryonic cells. In an attempt to identify partners for the E12 protein that may exert control during liver development, we performed the yeast 2-hybrid screen using an expression complementary DNA library from human fetal liver. A novel dominant inhibitory HLH factor, designated HHM (human homologue of maid), was isolated and characterized. HHM is structurally related to the Id family and was highly expressed in brain, pituitary gland, lung, heart, placenta, fetal liver, and bone marrow. HHM physically interacted with E12 in vitro and in mammalian cells. Comparison of the dominant inhibitory effects of HHM and Id2 on the binding of E12/MyoD dimer to an E-box element revealed a weaker inhibition by HHM. However, HHM but not Id2 specifically inhibited the luciferase gene activation induced by hepatic nuclear factor 4 (HNF4) promoter. The HHM was transiently expressed during stem-cell-driven regeneration of the liver at the stage in which the early basophilic foci of hepatocytes started to appear. These results suggest that HHM is a novel type of dominant inhibitory HLH protein that might modulate liver-specific gene expression.

The RIPE3b1 activator of the insulin gene is composed of a protein(s) of approximately 43 kDa, whose DNA binding activity is inhibited by protein phosphatase treatment

Glucose-stimulated and pancreatic islet beta cell-specific expression of the insulin gene is mediated in part by the C1 DNA-element binding complex, termed RIPE3b1. In this report, we define the molecular weight range of the protein(s) that compose this beta cell-enriched activator complex and show that protein phosphatase treatment inhibits RIPE3b1 DNA binding activity. Fractionation of beta cell nuclear extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that RIPE3b1 binding was mediated by a protein(s) within the 37-49-kDa ranges. Direct analysis of the proteins within the RIPE3b1 complex by ultraviolet light cross-linking analysis identified three binding species of approximately 51, 45, and 38 kDa. Incubating beta cell nuclear extracts with either calf alkaline phosphatase or a rat brain phosphatase preparation dramatically reduced RIPE3b1 DNA complex formation. Phosphatase inhibition of RIPE3b1 binding was prevented by sodium pyrophosphate, a general phosphatase inhibitor. We discuss how changes in the phosphorylation status of the RIPE3b1 activator may influence its DNA binding activity.

Review article: transcriptional events controlling the terminal differentiation of intestinal endocrine cells

Secretin-producing enteroendocrine cells arise from a multipotential endocrine progenitor in the crypts of the small intestine. As these cells migrate up the crypt-villus axis, they produce secretin and stop dividing as they terminally differentiate and die. Transcription of the secretin gene is controlled by a complex enhancer binding to multiple transcription factors. The basic helix-loop-helix protein, BETA2, binds to an E box sequence and associates with the p300 coactivator to activate transcription of the secretin gene. Basic helix-loop-helix proteins appear to play a pivotal role in the control of cellular differentiation. BETA2 induces cell cycle arrest and apoptosis in addition to activating secretin gene expression. Thus BETA2 may function as a master regulatory gene to coordinate terminal differentiation of secretin cells.

Transforming growth factor beta induction of insulin gene expression is mediated by pancreatic and duodenal homeobox gene-1 in rat insulinoma cells

Although transforming growth factor-beta (TGF-beta) stimulates pancreatic islet cells to synthesize and secret insulin, the mechanism underlying this effect is not known. To investigate this question, we examined the insulin promoter activity focusing on a transcription factor, pancreatic and duodenal homeobox gene-1 (PDX-1) that binds to the A3 element of the rat insulin promoter. Studies performed using the rat insulinoma cell line, INS-1 showed that TGF-beta stimulation of endogenous insulin mRNA expression correlated with increased activity of a reporter construct containing the insulin promoter. A potential mechanism for this increase arose from, electrophoretic mobility shift assay showing that the nuclear extract from TGF-beta treated cells contained higher levels of A3 binding activity. Western blot analysis confirmed that PDX-1 was increased in the nuclear extract from INS-1 cells treated with TGF-beta. As expected, a mutant insulin promoter that lacked the PDX-1 binding site was not stimulated by TGF-beta. In summary, the results of these studies show that TGF-beta stimulates the transcription of insulin gene and this action is mediated by the transcription factor, PDX-1.

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.

The nuclear localization signal (NLS) of PDX-1 is part of the homeodomain and represents a novel type of NLS

The beta-cell homeodomain transcription factor PDX-1 has vital functions both in controlling the expression of pancreatic polypeptide hormones and in the development of the pancreas. The transactivating and DNA-binding properties of PDX-1 have been well characterized, but nuclear transport is still undefined. Here we show that PDX-1 bears a nuclear localization signal (NLS) that is part of helix 3 of the homeodomain. PDX-1 deletion mutants were tagged with enhanced green fluorescent protein (EGFP) and expressed in COS-7 cells. Subcellular localization of the respective PDX-1-EGFP fusion proteins was analyzed by direct fluorescence microscopy and Western immunoblotting using an anti-(GFP). As a result we were able to demonstrate that the homeodomain or helix 3 alone was sufficient and necessary for transport into the nucleus. Point mutations of basic amino acid residues within helix 3 led to identification of an NLS with six amino acids being crucial for nuclear transport of PDX-1. Because this NLS does not match known examples of NLSs, the PDX-1 NLS may represent a novel class of NLS.