Pancreatic beta cell-specific transcription of the pdx-1 gene. The role of conserved upstream control regions and their hepatic nuclear factor 3beta sites

Neonatal exendin-4 prevents the development of diabetes in the intrauterine growth retarded rat

Uteroplacental insufficiency resulting in fetal growth retardation is a common complication of pregnancy and a significant cause of perinatal morbidity and mortality. Epidemiological studies show an increased incidence of type 2 diabetes in humans who were growth retarded at birth. The mechanisms by which an abnormal intrauterine milieu leads to the development of diabetes in adulthood are not known. Therefore, a rat model of uteroplacental insufficiency was developed; intrauterine growth-retarded (IUGR) rats develop diabetes with a phenotype similar to that observed in the human with type 2 diabetes. We show here that administration of a pancreatic beta-cell trophic factor, exendin-4 (Ex-4), during the prediabetic neonatal period dramatically prevents the development of diabetes in this model. This occurs because neonatal Ex-4 prevents the progressive reduction in insulin-producing beta-cell mass that is observed in IUGR rats over time. Expression of PDX, a critical regulator of pancreas development and islet differentiation, is restored to normal levels, and islet beta-cell proliferation rates are normalized by the neonatal Ex-4 treatment. These results indicate that exposure to Ex-4 in the newborn period reverses the adverse consequences of fetal programming and prevents the development of diabetes in adulthood.

Gene expression cascades in pancreatic development

The specialized endocrine and exocrine cells of the pancreas originally derive from a pool of apparently identical cells in the early gut endoderm. Serial changes in their gene expression program, controlled by a hierarchy of pancreatic transcription factors, direct this progression from multipotent progenitor cell to mature pancreatic cell. When the cells differentiate, this hierarchy of factors coalesces into a network of factors that maintain the differentiated phenotype of the cells. As we develop an understanding of the pancreatic transcription factors, we are also acquiring the tools with which we can ultimately control pancreatic cell differentiation.

Experimental models of transcription factor-associated maturity-onset diabetes of the young

Six monogenic forms of maturity-onset diabetes of the young (MODY) have been identified to date. Except for MODY2 (glucokinase), all other MODY subtypes have been linked to transcription factors. We have established a MODY3 transgenic model through the beta-cell-targeted expression of dominant-negative HNF-1alpha either constitutively (rat insulin II promoter) or conditionally (Tet-On system). The animals display either overt diabetes or glucose intolerance. Decreased insulin secretion and reduced pancreatic insulin content contribute to the hyperglycemic state. The conditional approach in INS-1 cells helped to define new molecular targets of hepatocyte nuclear factor (HNF)-1alpha. In the cellular system, nutrient-induced insulin secretion was abolished because of impaired glucose metabolism. Conditional suppression of HNF-4alpha, the MODY1 gene, showed a similar phenotype in INS-1 cells to HNF-1alpha. The existence of a regulatory circuit between HNF-4alpha and HNF-1alpha is confirmed in these cell models. The MODY4 gene, IPF-1 (insulin promoter factor-1)/PDX-1 (pancreas duodenum homeobox-1), controls not only the transcription of insulin but also expression of enzymes involved in its processing. Suppression of Pdx-1 function in INS-1 cells does not alter glucose metabolism but rather inhibits insulin release by impairing steps distal to the generation of mitochondrial coupling factors. The presented experimental models are important tools for the elucidation of the beta-cell pathogenesis in MODY syndromes.

Regulation of pdx-1 gene expression

The homeodomain-containing transcription factor pancreatic duodenal homeobox 1 (PDX-1) plays a key role in pancreas development and in beta-cell function. Upstream sequences of the gene up to about -6 kb show islet-specific activity in transgenic mice. Attempts to identify functional regulatory elements involved in the controlled expression of the pdx-1 gene led to the identification of distinct distal beta-cell-specific enhancers in human and rat genes. Three additional sequences, conserved between the mouse and the human 5'-flanking regions, two of which are also found in the chicken gene, conferred beta-cell-specific expression on a reporter gene, albeit to different extents. A number of transcription factors binding to and modulating the transcriptional activity of the regulatory elements were identified, such as hepatocyte nuclear factor (HNF)-3beta, HNF-1alpha, SP1/3, and, interestingly, PDX-1 itself. A fourth conserved region was localized to the proximal promoter around an E-box motif and was found to bind members of the upstream stimulatory factor (USF) family of transcription factors. We postulate that disruption of pdx-1 cis-acting regulatory sequences and/or mutations or functional impairment of transcription factors controlling the expression of the gene can lead to diabetes.

Regulation of the pdx1 gene promoter in pancreatic beta-cells

In the adult pancreas the expression of the transcription factor PDX1 is mainly restricted to the beta-cells of the islets of Langerhans. In this study we have identified a region of the pdx1 promoter between -2715 and -1960 which was essential to direct pancreatic islet-cell-specific expression of PDX1. We have also begun for the first time to understand the complex nutritional and hormonal regulation controlling PDX1 expression. The current study has established the fact that glucose, GLP-1, insulin, T(3), HB-EGF, and TNF-alpha all positively regulate the PDX1 gene promoter in pancreatic beta-cells. This study represents the first detailed exploration of the nutritional and hormonal regulation of this vital beta-cell gene.

Exendin-4 differentiation of a human pancreatic duct cell line into endocrine cells: involvement of PDX-1 and HNF3beta transcription factors

Exendin-4 (EX-4), a long acting agonist of GLP-1, induces an endocrine phenotype in Capan-1 cells. Under culture conditions which include serum, approximately 10% of the cells contain insulin and glucagon. When exposed to EX-4 (0.1 nM, up to 5 days), the number of cells containing insulin and glucagon increased to approximately 40%. Western blot analysis detected a progressive increase in protein levels of glucokinase and GLUT2 over 3 days of EX-4 treatment. We explored the sequence of activation of certain transcription factors known to be essential for the beta cell phenotype: PDX-1, Beta2/NeuroD, and hepatocyte nuclear factor 3beta (HNF3beta). Double immunostaining showed that PDX-1 coexisted with insulin and glucagon in EX-4-treated cells. Treatment caused an increase in PDX-1 protein levels by 24 h and induced its nuclear translocation. Beta2/NeuroD protein levels also increased progressively over 24 h. HNF3beta protein level increased twofold as early as 6 h after EX-4 treatment. EMSA results indicated that EX-4 caused a 12-fold increase in HNF3beta binding to PDX-1 promoter area II. Beta2/NeuroD protein levels progressively increased after 24 h treatment. Differentiation to insulin-producing cells was also seen when Capan-1 cells were transfected with pdx-1, with 80% of these cells expressing insulin 3 days after transfection. PDX-1 antisense totally inhibited such conversion. During the differentiation of duct cells to endocrine cells, cAMP levels (EX-4 is a ligand for the GLP-1, G-protein coupled receptor) and MAP kinase activity increased. Our results indicate that EX-4 activates adenylyl cyclase and MAP kinase which, in turn, may lead to activation of transcription factors necessary for an endocrine phenotype.

PDX-1 induces differentiation of intestinal epithelioid IEC-6 into insulin-producing cells

A homeodomain containing transcription factor PDX-1 can induce beta-cell-specific gene expressions in some non-beta-cells and may therefore be useful for future diabetes gene/cell therapy. Among the potential target organs or tissues for transcription factor-mediated induction of beta-cell-like differentiation are the intestinal epithelial cells. They have certain merits over other tissues and organs in terms of accessibility for gene delivery and of similarity in developmental background to the pancreatic primordium. In this study, we used an intestinal epithelium-derived cell line, IEC-6 cells, and investigated the possible effects of PDX-1 expression in those cells. By exogenous expression of the PDX-1 gene, IEC-6 cells started expressing multiple beta-cell-specific genes such as amylin, glucokinase, and Nkx6.1, which were not found in the original IEC-6 cells. Insulin gene expression, which was missing initially even in the PDX-1-transfected IEC-6 cells, became detectable when the cells were transplanted under the renal capsule of a rat. When the PDX-1(+) IEC-6 cells were kept in vitro, treatment with betacellulin could also confer insulin gene expression to them. Although insulin secretory granules became visible by electron microscopy, they were secreted regardless of glucose concentration. The in vivo or in vitro inductions of the insulin gene expression were not observed in the PDX-1(-) IEC-6 cells. Thus, our present observations demonstrate the potency of intestinal epithelial cells as a tool for diabetes gene/cell therapy and provide further support for the potency of PDX-1 in driving beta-cell-like differentiation in non-beta-cells.

Conserved sequences in a tissue-specific regulatory region of the pdx-1 gene mediate transcription in Pancreatic beta cells: role for hepatocyte nuclear factor 3 beta and Pax6

Pancreas duodenum homeobox 1 (PDX-1) is absolutely required for pancreas development and the maintenance of islet beta-cell function. Temporal and cell-type-specific transcription of the pdx-1 gene is controlled by factors acting upon sequences found within its 5'-flanking region. Critical cis-acting transcriptional control elements are located within a nuclease hypersensitive site that contains three conserved subdomains, termed areas I, II, and III. We show that area II acts as a tissue-specific regulatory region of the pdx-1 gene, directing transgene expression to a subpopulation of islet cells. Mutation of the area II hepatocyte nuclear factor 3 (HNF3) binding element in the larger area I- and area II- containing PstBst fragment also decreases PB(hsplacZ) transgene penetrance. These two results indicate possible ontogenetic and/or functional heterogeneity of the beta-cell population. Several other potential positive- and negative-acting control elements were identified in area II after mutation of the highly conserved sequence blocks within this subdomain. Pax6, a factor essential for islet alpha-cell development and islet hormone gene expression, was shown to bind in area II in vitro. Pax6 and HNF3 beta were also found to bind to this region in vivo by using the chromatin immunoprecipitation assay. Collectively, these data suggest an important role for both HNF3 beta and Pax6 in regulating pdx-1 expression in beta cells.

Foxa2 (HNF3beta ) controls multiple genes implicated in metabolism-secretion coupling of glucose-induced insulin release

The transcription factor Foxa2 is implicated in blood glucose homeostasis. Conditional expression of Foxa2 or its dominant-negative mutant DN-Foxa2 in INS-1 cells reveals that Foxa2 regulates the expression of genes important for glucose sensing in pancreatic beta-cells. Overexpression of Foxa2 results in blunted glucose-stimulated insulin secretion, whereas induction of DN-Foxa2 causes a left shift of glucose-induced insulin release. The mRNA levels of GLUT2 and glucokinase are drastically decreased after induction of Foxa2. In contrast, loss of Foxa2 function leads to up-regulation of hexokinase (HK) I and II and glucokinase (HK-IV) mRNA expression. The glucokinase and the low K(m) hexokinase activities as well as glycolysis are increased proportionally. In addition, induction of DN-Foxa2 also reduces the expression of beta-cell K(ATP) channel subunits Sur1 and Kir6.2 by 70%. Furthermore, in contrast to previous reports, induction of Foxa2 causes pronounced decreases in the HNF4alpha and HNF1alpha mRNA levels. Foxa2 fails to regulate the expression of Pdx1 transcripts. The expression of insulin and islet amyloid polypeptide is markedly suppressed after induction of Foxa2, while the glucagon mRNA levels are significantly increased. Conversely, Foxa2 is required for glucagon expression in these INS-1-derived cells. These results suggest that Foxa2 is a vital transcription factor evolved to control the expression of genes essential for maintaining beta-cell glucose sensing and glucose homeostasis.

Pdx1 restores beta cell function in Irs2 knockout mice

The homeodomain transcription factor Pdx1 is required for pancreas development, including the differentiation and function of beta cells. Mutations in Pdx1 or upstream hepatocyte nuclear factors cause autosomal forms of early-onset diabetes (maturity-onset diabetes of the young [MODY]). In mice, the Irs2 branch of the insulin/Igf signaling system mediates peripheral insulin action and pancreatic beta cell growth and function. To investigate whether beta cell failure in Irs2(-/-) mice might be related to dysfunction of MODY-related transcription factors, we measured the expression of Pdx1 in islets from young Irs2(-/-) mice. Before the onset of diabetes, Pdx1 was reduced in islets from Irs2(-/-) mice, whereas it was expressed normally in islets from wild-type or Irs1(-/-) mice, which do not develop diabetes. Whereas male Irs2(-/-)Pdx1(+/+) mice developed diabetes between 8 and 10 weeks of age, haploinsufficiency for Pdx1 caused diabetes in newborn Irs2(-/-) mice. By contrast, transgenic expression of Pdx1 restored beta cell mass and function in Irs2(-/-) mice and promoted glucose tolerance throughout life, as these mice survived for at least 20 months without diabetes. Our results suggest that dysregulation of Pdx1 might represent a common link between ordinary type 2 diabetes and MODY.

Quantitative assessment of gene targeting in vitro and in vivo by the pancreatic transcription factor, Pdx1. Importance of chromatin structure in directing promoter binding

The transcription factor Pdx1 is expressed in the pancreatic beta-cell, where it is believed to regulate several beta-cell-specific genes. Whereas binding by Pdx1 to elements of beta-cell genes has been demonstrated in vitro, almost none of these genes has been demonstrated to be a direct binding target for Pdx1 within cells (where complex chromatin structure exists). To determine which beta-cell promoters are bound by Pdx1 in vivo, we performed chromatin immunoprecipitation assays using Pdx1 antiserum and chromatin from beta-TC3 cells and Pdx1-transfected NIH3T3 cells and subsequently quantitated co-immunoprecipitated promoters using real-time PCR. We compared these in vivo findings to parallel immunoprecipitations in which Pdx1 was allowed to bind to promoter fragments in in vitro reactions. Our results show that in all cells Pdx1 binds strongly to the insulin, islet amyloid polypeptide, glucagon, Pdx1, and Pax4 promoters, whereas it does not bind to either the glucose transporter type 2 or albumin promoters. In addition, no binding by Pdx1 to the glucokinase promoter was observed in beta-cells. In contrast, in in vitro immunoprecipitations, Pdx1 bound all promoters to an extent approximately proportional to the number of Pdx1 binding sites. Our findings suggest a critical role for chromatin structure in directing the promoter binding selectivity of Pdx1 in beta-cells and non-beta-cells.

Molecular etiologies of MODY and other early-onset forms of diabetes

Maturity-onset diabetes of the young (MODY) are monogenic forms of type 2 diabetes that are characterized by an early disease onset, autosomal-dominant inheritance, and defects in insulin secretion. Genetic studies have identified mutations in at least eight genes associated with different forms of MODY. The majority of the MODY subtypes are caused by mutations in transcription factors that include hepatocyte nuclear factor (HNF)-4 alpha, HNF-1 alpha, PDX-1, HNF-1 beta, and NEURO-DI/BETA-2. In addition, genetic defects in the glucokinase gene, the glucose sensor of the pancreatic beta cells, and the insulin gene also lead to impaired glucose tolerance. Biochemical and genetic studies have demonstrated that the MODY genes are functionally related and form an integrated transcriptional network that is important for many metabolic pathways.

Reduction in pancreatic transcription factor PDX-1 impairs glucose-stimulated insulin secretion

Complete lack of transcription factor PDX-1 leads to pancreatic agenesis, whereas heterozygosity for PDX-1 mutations has been recently noted in some individuals with maturity-onset diabetes of the young (MODY) and in some individuals with type 2 diabetes. To determine how alterations in PDX-1 affect islet function, we examined insulin secretion and islet physiology in mice with one PDX-1 allele inactivated. PDX-1(+/-) mice had a normal fasting blood glucose and pancreatic insulin content but had impaired glucose tolerance and secreted less insulin during glucose tolerance testing. The expression of PDX-1 and glucose transporter 2 in islets from PDX-1(+/-) mice was reduced to 68 and 55%, respectively, whereas glucokinase expression was not significantly altered. NAD(P)H generation in response to glucose was reduced by 30% in PDX-1(+/-) mice. The in situ perfused pancreas of PDX-1(+/-) mice secreted about 45% less insulin when stimulated with 16.7 mm glucose. The K(m) for insulin release was similar in wild type and PDX-1(+/-) mice. Insulin secretion in response to 20 mm arginine was unchanged; the response to 10 nm glucagon-like peptide-1 was slightly increased. However, insulin secretory responses to 10 mm 2-ketoisocaproate and 20 mm KCl were significantly reduced (by 61 and 66%, respectively). These results indicate that a modest reduction in PDX-1 impairs several events in glucose-stimulated insulin secretion (such as NAD(P)H generation, mitochondrial function, and/or mobilization of intracellular Ca(2+)) and that PDX-1 is important for normal function of adult pancreatic islets.

Profound defects in pancreatic beta-cell function in mice with combined heterozygous mutations in Pdx-1, Hnf-1alpha, and Hnf-3beta

Defects in pancreatic beta-cell function contribute to the development of type 2 diabetes, a polygenic disease that is characterized by insulin resistance and compromised insulin secretion. Hepatocyte nuclear factors (HNFs) -1alpha, -3beta, -4alpha, and Pdx-1 contribute in the complex transcriptional circuits within the pancreas that are involved in beta-cell development and function. In mice, a heterozygous mutation in Pdx-1 alone, but not Hnf-1alpha(+/-), Hnf-3beta(+/-), or Hnf-4alpha(+/-), causes impaired glucose-stimulated insulin secretion in mice. To investigate the possible functional relationships between these transcription factors on beta-cell activity in vivo, we generated mice with the following combined heterozygous mutations: Pdx-1(+/-)/Hnf-1alpha(+/-), Pdx-1(+/-)/Hnf-3beta(+/-), Pdx-1(+/-)/Hnf-4alpha(+/-), Hnf-1alpha(+/-)/Hnf-4alpha(+/-), and Hnf-3beta(+/-)/Hnf-4alpha(+/-). The greatest loss in function was in combined heterozygous null alleles of Pdx-1 and Hnf-1alpha (Pdx-1(+/-)/Hnf-1alpha(+/-)), or Pdx-1 and Hnf-3beta (Pdx-1(+/-)/Hnf-3beta(+/-)). Both double mutants develop progressively impaired glucose tolerance and acquire a compromised first- and second-phase insulin secretion profile in response to glucose compared with Pdx-1(+/-) mice alone. The loss in beta-cell function in Pdx-1(+/-)/Hnf-3beta(+/-) mice was associated with decreased expression of Nkx-6.1, glucokinase (Gck), aldolase B (aldo-B), and insulin, whereas Nkx2.2, Nkx-6.1, Glut-2, Gck, aldo-B, the liver isoform of pyruvate kinase, and insulin expression was reduced in Pdx-1(+/-)/Hnf-1alpha(+/-) mice. The islet cell architecture was also abnormal in Pdx-1(+/-)/Hnf-3beta(+/-) and Pdx-1(+/-)/Hnf-1alpha(+/-) mice, with glucagon-expressing cells scattered throughout the islet, a defect that may be connected to decreased E-cadherin expression. Our data suggest that functional interactions between key islet regulatory factors play an important role in maintaining islet architecture and beta-cell function. These studies also established polygenic mouse models for investigating the mechanisms contributing to beta-cell dysfunction in diabetes.

Regulation of pancreatic cell differentiation and morphogenesis

Organogenesis requires tissue interactions to initiate the cascade of inductive and repressive signals necessary for normal organ development. Tissue interactions initiate the pancreatic lineage within the primitive foregut endodermal epithelium and continue to direct the morphogenesis and differentiation of the endocrine, exocrine and ductal portions of the pancreas. An understanding of the mechanisms controlling pancreatic growth would enable the development of alternative therapies for diseases such as type 1 diabetes.

The role of hepatic nuclear factor 1 alpha and PDX-1 in transcriptional regulation of the pdx-1 gene

The PDX-1 homeodomain transcription factor regulates pancreatic development and adult islet beta cell function. Expression of the pdx-1 gene is almost exclusively localized to beta cells within the adult endocrine pancreas. Islet beta cell-selective transcription is controlled by evolutionarily conserved subdomain sequences (termed Areas I (-2839 to -2520 base pairs (bp)), II (-2252 to -2023 bp), and III (-1939 to -1664 bp)) found within the 5'-flanking region of the pdx-1 gene. Areas I and II are independently capable of directing beta cell-selective reporter gene activity in transfection assays, with Area I-mediated stimulation dependent upon binding of hepatic nuclear factor 3 beta (HNF3 beta), a key regulator of islet beta cell function. To identify other transactivators of Area I, highly conserved sequence segments within this subdomain were mutagenized, and their effect on activation was determined. Several of the sensitive sites were found by transcription factor data base analysis to potentially bind endodermally expressed transcription factors, including HNF1 alpha (-2758 to -2746 bp, Segment 2), HNF4 (-2742 to -2730 bp, Segment 4; -2683 to -2671 bp, Segment 7-8), and HNF6 (-2727 to -2715 bp, Segment 5). HNF1 alpha, but not HNF4 and HNF6, binds specifically to Area I sequences in vitro. HNF1 alpha was also shown to specifically activate Area I-driven transcription through Segment 2. In addition, PDX-1 itself was found to stimulate Area I activation. The chromatin immunoprecipitation assay performed with PDX-1 antisera also demonstrated that this factor bound to Area I within the endogenous pdx-1 gene in beta cells. Our results indicate that regulatory factors binding to Area I conserved sequences contribute to the selective transcription pattern of the pdx-1 gene and that control is mediated by endodermal regulators like HNF1 alpha, HNF3 beta, and PDX-1.

Loss of HNF-1alpha function in mice leads to abnormal expression of genes involved in pancreatic islet development and metabolism

Mutations in hepatocyte nuclear factor 1alpha (HNF-1alpha) lead to maturity-onset diabetes of the young type 3 as a result of impaired insulin secretory response in pancreatic beta-cells. The expression of 50 genes essential for normal beta-cell function was studied to better define the molecular mechanism underlying the insulin secretion defect in Hnf-1alpha(-/-) mice. We found decreased steady-state mRNA levels of genes encoding glucose transporter 2 (Glut2), neutral and basic amino acid transporter, liver pyruvate kinase (L-Pk), and insulin in Hnf-1alpha(-/-) mice. In addition, we determined that the expression of several islet-enriched transcription factors, including Pdx-1, Hnf-4alpha, and Neuro-D1/Beta-2, was reduced in Hnf-1alpha(-/-) mice. These changes in pancreatic islet mRNA levels were already apparent in newborn animals, suggesting that loss of Hnf-1alpha function rather than chronic hyperglycemia is the primary cause of the altered gene expression. This expression profile was pancreatic islet-specific and distinct from hepatocytes, where we found normal expression of Glut2, L-Pk, and Hnf-4alpha in the liver of Hnf-1alpha(-/-) mice. The expression of small heterodimer partner (Shp-1), an orphan receptor that can heterodimerize with Hnf-4alpha and inhibit its transcriptional activity, was also reduced in Hnf-1alpha(-/-) islets. We characterized a 0.58-kb Shp-1 promoter and determined that the decreased expression of Shp-1 may be indirectly mediated by a downregulation of Hnf-4alpha. We further showed that Shp-1 can repress its own transcriptional activation by inhibiting Hnf-4alpha function, thereby establishing a feedback autoregulatory loop. Our results indicate that loss of Hnf-1alpha function leads to altered expression of genes involved in glucose-stimulated insulin secretion, insulin synthesis, and beta-cell differentiation.

Regulatory regions driving developmental and tissue-specific expression of the essential pancreatic gene pdx1

pdx1 (pancreatic and duodenal homeobox gene-1), which is expressed broadly in the embryonic pancreas and, later, in a more restricted manner in the mature beta cells in the islets of Langerhans, is essential both for organ formation and beta cell gene expression and function. We carried out a transgenic reporter gene analysis to identify region- and cell type-specific regulatory regions in pdx1. A 14.5-kb pdx1 genomic fragment corrected the glucose intolerance of pdx1(+/-) animals but, moreover, fully rescued the severe gut and pancreas defects in pdx1(-/-) embryos. Sequences sufficient to direct reporter expression to the entire endogenous pdx1 expression domain lie within 4.3 kb of 5' flanking DNA. In this region, we identified two distinct fragments that drive reporter gene expression to different sets of islet neuroendocrine cells. One shows pan-endocrine cell specificity, the other is selectively activated in insulin-producing beta cells. The endocrine-specific regulatory regions overlap a localized region of 5' flanking DNA that is remarkably conserved in sequence between vertebrate pdx1 genes, and which has been associated with beta cell-selective expression in cultured cell lines. This region contains potential binding sites for several transcription factors implicated in endodermal development and the pathogenesis of some forms of type-2 diabetes. These results are consistent with our previous proposal that conserved upstream pdx1 sequences exert control over pdx1 during embryonic organogenesis and islet endocrine cell differentiation. We propose that mutations affecting the expression and/or activity of transcription factors operating via these sequences may predispose towards diabetes, at least in part by direct effects on endocrine pdx1 expression.

Differentiation of embryonic stem cells to insulin-secreting structures similar to pancreatic islets

Although the source of embryonic stem (ES) cells presents ethical concerns, their use may lead to many clinical benefits if differentiated cell types can be derived from them and used to assemble functional organs. In pancreas, insulin is produced and secreted by specialized structures, islets of Langerhans. Diabetes, which affects 16 million people in the United States, results from abnormal function of pancreatic islets. We have generated cells expressing insulin and other pancreatic endocrine hormones from mouse ES cells. The cells self-assemble to form three-dimensional clusters similar in topology to normal pancreatic islets where pancreatic cell types are in close association with neurons. Glucose triggers insulin release from these cell clusters by mechanisms similar to those employed in vivo. When injected into diabetic mice, the insulin-producing cells undergo rapid vascularization and maintain a clustered, islet-like organization.

A pancreatic beta -cell-specific enhancer in the human PDX-1 gene is regulated by hepatocyte nuclear factor 3beta (HNF-3beta ), HNF-1alpha, and SPs transcription factors

The PDX-1 transcription factor plays a key role in pancreas development. Although expressed in all cells at the early stages, in the adult it is mainly restricted to the beta-cell. To characterize the regulatory elements and potential transcription factors necessary for human PDX-1 gene expression in beta-cells, we constructed a series of 5' and 3' deletion fragments of the 5'-flanking region of the gene, fused to the luciferase reporter gene. In this report, we identify by transient transfections in beta- and non-beta-cells a novel beta-cell-specific distal enhancer element located between -3.7 and -3.45 kilobases. DNase I footprinting analysis revealed two protected regions, one binding the transcription factors SP1 and SP3 and the other hepatocyte nuclear factor 3beta (HNF-3beta) and HNF-1alpha. Cotransfection experiments suggest that HNF-3beta, HNF-1alpha, and SP1 are positive regulators of the herein-described human PDX-1 enhancer element. Furthermore, mutations within each motif abolished the binding of the corresponding factor(s) and dramatically impaired the enhancer activity, therefore suggesting cooperativity between these factors.

Regulation of the pancreatic pro-endocrine gene neurogenin3

Neurogenin3 (ngn3), a basic helix-loop-helix (bHLH) transcription factor, functions as a pro-endocrine factor in the developing pancreas: by itself, it is sufficient to force undifferentiated pancreatic epithelial cells to become islet cells. Because ngn3 expression determines which precursor cells will differentiate into islet cells, the signals that regulate ngn3 expression control islet cell formation. To investigate the factors that control ngn3 gene expression, we mapped the human and mouse ngn3 promoters and delineated transcriptionally active sequences within the human promoter. Surprisingly, the human ngn3 promoter drives transcription in all cell lines tested, including fibroblast cell lines. In contrast, in transgenic animals the promoter drives expression specifically in regions of ngn3 expression in the developing pancreas and gut; and the addition of distal sequences greatly enhances transgene expression. Within the distal enhancer, binding sites for several pancreatic transcription factors, including hepatocyte nuclear factor (HNF)-1 and HNF-3, form a tight cluster. HES1, an inhibitory bHLH factor activated by Notch signaling, binds to the proximal promoter and specifically blocks promoter activity. Together with previous genetic data, these results suggest a model in which the ngn3 gene is activated by the coordinated activities of several pancreatic transcription factors and inhibited by Notch signaling through HES1.

Characterization of the mouse islet-specific glucose-6-phosphatase catalytic subunit-related protein gene promoter by in situ footprinting: correlation with fusion gene expression in the islet-derived betaTC-3 and hamster insulinoma tumor cell lines

Glucose-6-phosphatase (G6Pase) is a multicomponent system located in the endoplasmic reticulum comprising a catalytic subunit and transporters for glucose-6-phosphate, inorganic phosphate, and glucose. We have recently cloned a novel gene that encodes an islet-specific G6Pase catalytic subunit-related protein (IGRP) (Ebert et al., Diabetes 48:543-551, 1999). To begin to investigate the molecular basis for the islet-specific expression of the IGRP gene, a series of truncated IGRP-chloramphenicol acetyltransferase (CAT) fusion genes were transiently transfected into the islet-derived mouse betaTC-3 and hamster insulinoma tumor cell lines. In both cell lines, basal fusion gene expression decreased upon progressive deletion of the IGRP promoter sequence between -306 and -66, indicating that multiple promoter regions are required for maximal IGRP-CAT expression. The ligation-mediated polymerase chain reaction footprinting technique was then used to compare trans-acting factor binding to the IGRP promoter in situ in betaTC-3 cells, which express the endogenous IGRP gene, and adrenocortical Y1 cells, which do not. Multiple trans-acting factor binding sites were selectively identified in betaTC-3 cells that correlate with regions of the IGRP promoter identified as being required for basal IGRP-CAT fusion gene expression. The data suggest that hepatocyte nuclear factor 3 may be important for basal IGRP gene expression, as it is for glucagon, GLUT2, and Pdx-1 gene expression. In addition, binding sites for several trans-acting factors not previously associated with islet gene expression, as well as binding sites for potentially novel proteins, were identified.

Functional conservation of regulatory elements in the pdx-1 gene: PDX-1 and hepatocyte nuclear factor 3beta transcription factors mediate beta-cell-specific expression

The PDX-1 transcription factor plays a key role in pancreatic development and in the regulation of the insulin gene in the adult beta cell. As its functions appear to be similar in humans and mice, we analyzed the functional conservation of homologous sequences important for the maintenance and the cell-specific regulation of the pdx-1 gene. Apart from the proximal promoter region, three highly homologous (PH1 to PH3) sequences were apparent in the human and mouse 5' flanking regions of the gene. By transient transfections in beta and non-beta cells, we show that mainly PH1 and PH2 preferentially confer beta-cell-specific activation on a heterologous promoter. DNase I footprinting and binding analyses revealed that both bind to and are transactivated by hepatocyte nuclear factor 3beta (HNF-3beta). Furthermore, the PH1 enhancer element also binds the PDX-1 transcription factor itself, which acts cooperatively with adjacent HNF-3beta to regulate its transcriptional potency. This finding suggests a possible autoregulatory loop as a mechanism for PDX-1 to control its own expression.

The hepatocyte nuclear factor 3 (HNF3 or FOXA) family in metabolism

The genes encoding hepatocyte nuclear factor 3 (HNF3) proteins play a pivotal role in the regulation of metabolism and in the differentiation of metabolic tissues such as the pancreas and liver. HNF3 transcription factors bind to cis-regulatory elements in hundreds of genes encoding gluconeogenic and glycolytic enzymes, serum proteins and hormones. Genetic analysis in mice has shown that HNF3 beta is necessary for the development of the foregut endoderm, from which the liver and pancreas arise. HNF3 alpha is required for the full activation of glucagon in the pancreas, whereas HNF3 gamma induces the activation of gluconeogenic enzymes to prevent hypoglycemia during fasting.

Expression of neurogenin3 reveals an islet cell precursor population in the pancreas

Differentiation of early gut endoderm cells into the endocrine cells forming the pancreatic islets of Langerhans depends on a cascade of gene activation events controlled by transcription factors including the basic helix-loop-helix (bHLH) proteins. To delineate this cascade, we began by establishing the position of neurogenin3, a bHLH factor found in the pancreas during fetal development. We detect neurogenin3 immunoreactivity transiently in scattered ductal cells in the fetal mouse pancreas, peaking at embryonic day 15.5. Although not detected in cells expressing islet hormones or the islet transcription factors Isl1, Brn4, Pax6 or PDX1, neurogenin3 is detected along with early islet differentiation factors Nkx6.1 and Nkx2.2, establishing that it is expressed in immature cells in the islet lineage. Analysis of transcription factor-deficient mice demonstrates that neurogenin3 expression is not dependent on neuroD1/BETA2, Mash1, Nkx2.2, Nkx6.1, or Pax6. Furthermore, early expression of neurogenin3 under control of the Pdx1 promoter is alone sufficient to drive early and ectopic differentiation of islet cells, a capability shared by the pancreatic bHLH factor, neuroD1/BETA2, but not by the muscle bHLH factor, MyoD. However, the islet cells produced in these transgenic experiments are overwhelmingly (alpha) cells, suggesting that factors other than the bHLH factors are required to deviate from a default * cell fate. These data support a model in which neurogenin3 acts upstream of other islet differentiation factors, initiating the differentiation of endocrine cells, but switching off prior to final differentiation. The ability to uniquely identify islet cell precursors by neurogenin3 expression allows us to determine the position of other islet transcription factors in the differentiation cascade and to propose a map for the islet cell differentiation pathway.