Differential regulation of the glucagon and insulin I gene promoters by the basic helix-loop-helix transcription factors E47 and BETA2

LINC MIR503HG Controls SC-β Cell Differentiation and Insulin Production by Targeting CDH1 and HES1

Stem cell-derived pancreatic progenitors (SC-PPs), as an unlimited source of SC-derived β (SC-β) cells, offers a robust tool for diabetes treatment in stem cell-based transplantation, disease modeling, and drug screening. Whereas, PDX1+/NKX6.1+ PPs enhances the subsequent endocrine lineage specification and gives rise to glucose-responsive SC-β cells in vivo and in vitro. To identify the regulators that promote induction efficiency and cellular function maturation, single-cell RNA-sequencing is performed to decipher the transcriptional landscape during PPs differentiation. The comprehensive evaluation of functionality demonstrated that manipulating LINC MIR503HG using CRISPR in PP cell fate decision can improve insulin synthesis and secretion in mature SC-β cells, without effects on liver lineage specification. Importantly, transplantation of MIR503HG-/- SC-β cells in recipients significantly restored blood glucose homeostasis, accompanied by serum C-peptide release and an increase in body weight. Mechanistically, by releasing CtBP1 occupying the CDH1 and HES1 promoters, the decrease in MIR503HG expression levels provided an excellent extracellular niche and appropriate Notch signaling activation for PPs following differentiation. Furthermore, this exhibited higher crucial transcription factors and mature epithelial markers in CDH1High expressed clusters. Altogether, these findings highlighted MIR503HG as an essential and exclusive PP cell fate specification regulator with promising therapeutic potential for patients with diabetes.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

MafB Is Critical for Glucagon Production and Secretion in Mouse Pancreatic α Cells In Vivo

The MafB transcription factor is expressed in pancreatic α and β cells during development but becomes exclusive to α cells in adult rodents. Mafb-null (Mafb^-/- ) mice were reported to have reduced α- and β-cell numbers throughout embryonic development. To further analyze the postnatal function of MafB in the pancreas, we generated endocrine cell-specific (Mafb^ΔEndo ) and tamoxifen-dependent (Mafb^ΔTAM ) Mafb knockout mice. Mafb^ΔEndo mice exhibited reduced populations of insulin-positive (insulin⁺) and glucagon⁺ cells at postnatal day 0, but the insulin⁺ cell population recovered by 8 weeks of age. In contrast, the Arx⁺ glucagon⁺ cell fraction and glucagon expression remained decreased even in adulthood. Mafb^ΔTAM mice, with Mafb deleted after pancreas maturation, also demonstrated diminished glucagon⁺ cells and glucagon content without affecting β cells. A decreased Arx⁺ glucagon⁺ cell population in Mafb^ΔEndo mice was compensated for by an increased Arx⁺ pancreatic polypeptide⁺ cell population. Furthermore, gene expression analyses from both Mafb^ΔEndo and Mafb^ΔTAM islets revealed that MafB is a key regulator of glucagon expression in α cells. Finally, both mutants failed to respond to arginine, likely due to impaired arginine transporter gene expression and glucagon production ability. Taken together, our findings reveal that MafB is critical for the functional maintenance of mouse α cells in vivo, including glucagon production and secretion, as well as in development.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Role of NeuroD1 on the negative regulation of Pomc expression by glucocorticoid

The mechanism of the negative regulation of proopiomelanocortin gene (Pomc) by glucocorticoids (Gcs) is still unclear in many points. Here, we demonstrated the involvement of neurogenic differentiation factor 1 (NeuroD1) in the Gc-mediated negative regulation of Pomc. Murine pituitary adrenocorticotropic hormone (ACTH) producing corticotroph tumor-derived AtT20 cells were treated with dexamethasone (DEX) (1-100 nM) and cultured for 24 hrs. Thereafter, Pomc mRNA expression was studied by quantitative real-time PCR and rat Pomc promoter (-703/+58) activity was examined by luciferase assay. Both Pomc mRNA expression and Pomc promoter activity were inhibited by DEX in a dose-dependent manner. Deletion and point mutant analyses of Pomc promoter suggested that the DEX-mediated transcriptional repression was mediated via E-box that exists at -376/-371 in the promoter. Since NeuroD1 is known to bind to and activate E-box of the Pomc promoter, we next examined the effect of DEX on NeuroD1 expression. Interestingly, DEX dose-dependently inhibited NeuroD1 mRNA expression, mouse NeuroD1 promoter (-2.2-kb) activity, and NeuroD1 protein expression in AtT20 cells. In addition, we confirmed the inhibitory effect of DEX on the interaction of NeuroD1 and E-box on Pomc promoter by chromatin immunoprecipitation (ChIP) assay. Finally, overexpression of mouse NeuroD1 could rescue the DEX-mediated inhibition of Pomc mRNA expression and Pomc promoter activity. Taken together, it is suggested that the suppression of NeuroD1 expression and the inhibition of NeuroD1/E-box interaction may play an important role in the Gc-mediated negative regulation of Pomc.

Protein Inhibitor of Activated STAT Y (PIASy) Regulates Insulin Secretion by Interacting with LIM Homeodomain Transcription Factor Isl1

It is known that the LIM homeodomain transcription factor Isl1 is highly expressed in all pancreatic endocrine cells and functions in regulating pancreatic development and insulin secretion. The Isl1 mutation has been found to be associated with type 2 diabetes, but the mechanism responsible for Isl1 regulation of insulin synthesis and secretion still needs to be elucidated. In the present study, the protein inhibitor of activated STAT Y (PIASy) was identified as a novel Isl1-interacting protein with a yeast two-hybrid system, and its interaction with Isl1 was further confirmed by a co-immunoprecipitation experiment. PIASy and Isl1 colocalize in human and mouse pancreas and NIT beta cells. Furthermore, PIASy and Isl1 upregulate insulin gene expression and insulin secretion in a dose-dependent manner by activating the insulin promoter. PIASy and Isl1 mRNA expression levels were also increased in type 2 diabetic db/db mice. In addition, our results demonstrate that PIASy and Isl1 cooperate to activate the insulin promoter through the Isl1 homeodomain and PIASy ring domain. These data suggest that that PIASy regulates insulin synthesis and secretion by interacting with Isl1 and provide new insight into insulin regulation, although the detailed molecular mechanism needs to be clarified in future studies.

Glucose-dependent downregulation of glucagon gene expression mediated by selective interactions between ALX3 and PAX6 in mouse alpha cells

AIMS/HYPOTHESIS

The stimulation of glucagon secretion in response to decreased glucose levels has been studied extensively. In contrast, little is known about the regulation of glucagon gene expression in response to fluctuations in glucose concentration. Paired box 6 (PAX6) is a key transcription factor that regulates the glucagon promoter by binding to the G1 and G3 elements. Here, we investigated the role of the transcription factor aristaless-like homeobox 3 (ALX3) as a glucose-dependent modulator of PAX6 activity in alpha cells.

METHODS

Experiments were performed in wild-type or Alx3-deficient islets and alphaTC1 cells. We used chromatin immunoprecipitations and electrophoretic mobility shift assays for DNA binding, immunoprecipitations and pull-down assays for protein interactions, transfected cells for promoter activity, and small interfering RNA and quantitative RT-PCR for gene expression.

RESULTS

Elevated glucose concentration resulted in stimulated expression of Alx3 and decreased glucagon gene expression in wild-type islets. In ALX3-deficient islets, basal glucagon levels were non-responsive to changes in glucose concentration. In basal conditions ALX3 bound to the glucagon promoter at G3, but not at G1. ALX3 could form heterodimers with PAX6 that were permissive for binding to G3 but not to G1. Thus, increasing the levels of ALX3 in response to glucose resulted in the sequestration of PAX6 by ALX3 for binding to G1, thus reducing glucagon promoter activation and glucagon gene expression.

CONCLUSIONS/INTERPRETATION

Glucose-stimulated expression of ALX3 in alpha cells provides a regulatory mechanism for the downregulation of glucagon gene expression by interfering with PAX6-mediated transactivation on the glucagon G1 promoter element.

Role of transcription factors in the transdifferentiation of pancreatic islet cells

The α and β cells act in concert to maintain blood glucose. The α cells release glucagon in response to low levels of glucose to stimulate glycogenolysis in the liver. In contrast, β cells release insulin in response to elevated levels of glucose to stimulate peripheral glucose disposal. Despite these opposing roles in glucose homeostasis, α and β cells are derived from a common progenitor and share many proteins important for glucose sensing and hormone secretion. Results from recent work have underlined these similarities between the two cell types by revealing that β-to-α as well as α-to-β transdifferentiation can take place under certain experimental circumstances. These exciting findings highlight unexpected plasticity of adult islets and offer hope of novel therapeutic paths to replenish β cells in diabetes. In this review, we focus on the transcription factor networks that establish and maintain pancreatic endocrine cell identity and how they may be perturbed to facilitate transdifferentiation.

Melatonin-receptor-1-deficiency affects neurogenic differentiation factor immunoreaction in pancreatic islets and enteroendocrine cells of mice

Neurogenic differentiation factor (NeuroD) is a transcription factor involved in the differentiation of neurons and in the control of energy balance and metabolism. It plays a key role in type 1 and type 2 diabetes. Melatonin is an important rhythmic endocrine signal within the circadian system of mammals and modulates insulin secretion and glucose metabolism. In the mouse pars tuberalis, NeuroD mRNA levels show day/night variation, which is independent of the molecular clock gene mPER1 but depends on the functional melatonin receptor 1 (MT1). So far, little is known about the effect of melatonin on NeuroD synthesis in the gastrointestinal tract. Thus, NeuroD protein levels and cellular localization were analyzed by immunohistochemistry in pancreatic islets and duodenal enteroendocrine cells of MT1- and mPER1-deficienct mice. In addition, the localization of NeuroD-positive cells was analyzed by double-immunofluorescence and confocal laser microscopy. In duodenal enteroendocrine cells and pancreatic islets of WT and PER1-deficient mice, NeuroD immunoreaction showed a peak during the early subjective night. In contrast, this peak was absent in MT1-deficent mice. These data suggest that melatonin, by acting on MT1 receptors, affects NeuroD expression in the gastrointestinal tract and thus might contribute to circadian regulation in metabolic functions.

Glucagon gene expression in the endocrine pancreas: the role of the transcription factor Pax6 in α-cell differentiation, glucagon biosynthesis and secretion

The glucagon gene is expressed in α-cells of the pancreas, L cells of the intestine and the hypothalamus. The determinants of the α-cell-specific expression of the glucagon gene are not fully characterized, although Arx, Pax6 and Foxa2 are critical for α-cell differentiation and glucagon gene expression; in addition, the absence of the β-cell-specific transcription factors Pdx1, Pax4 and Nkx6.1 may allow for the glucagon gene to be expressed. Pax6, along with cMaf and MafB, binds to the DNA control element G(1) which confers α-cell specificity to the promoter and to G(3) and potently activates glucagon gene transcription. In addition, to its direct role on the transcription of the glucagon gene, Pax6 controls several transcription factors involved in the activation of the glucagon gene such as cMaf, MafB and NeuroD1/Beta2 as well as different steps of glucagon biosynthesis and secretion. We conclude that Pax6 independently of Arx and Foxa2 is critical for α-cell function by coordinating glucagon gene expression as well as glucagon biosynthesis and secretion.

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.

The activation of the rat insulin gene II by BETA2 and PDX-1 in rat insulinoma cells is repressed by Pax6

The transcriptional transactivator Pax6 binds the pancreatic islet cell-specific enhancer sequence (PISCES) of the rat insulin I gene. However the human, mouse, and rat insulin gene II promoters do not contain a PISCES element. To analyze the role of Pax6 in those PISCES-less promoters, we investigated its influence on rat insulin gene II expression and included in our studies the main activators: pancreatic and duodenal homeobox protein-1 (PDX-1) and BETA2/E47. Luciferase assays, Northern blots, and RIA were used to study effects of Pax6 overexpression, gel shift and chromatin precipitation assays to study its binding to the DNA, and yeast two-hybrid assays and glutathione S transferase capture assays to investigate its interactions with PDX-1 and BETA2. Finally, glucose-dependent intracellular transport of Pax6 was demonstrated by fluorescence microscopy. Overexpression of Pax6 prevents activation of the rat insulin II gene by BETA2 and PDX-1 and hence suppresses insulin synthesis and secretion. In vitro, Pax6 binds to the A-boxes, thereby blocking binding of PDX-1, and at the same time, its paired domain interacts with BETA2. Fluorescence microscopy demonstrated that the nuclear-cytoplasmic localization of Pax6 and PDX-1 are oppositely regulated by glucose. From the results, it is suggested that at low concentrations of glucose, Pax6 is localized in the nucleus and prevents the activation of the insulin gene by occupying the PDX-1 binding site and by interacting with BETA2.

Pax6 controls the expression of critical genes involved in pancreatic {alpha} cell differentiation and function

The paired box homeodomain Pax6 is crucial for endocrine cell development and function and plays an essential role in glucose homeostasis. Indeed, mutations of Pax6 are associated with diabetic phenotype. Importantly, homozygous mutant mice for Pax6 are characterized by markedly decreased β and δ cells and absent α cells. To better understand the critical role that Pax6 exerts in glucagon-producing cells, we developed a model of primary rat α cells. To study the transcriptional network of Pax6 in adult and differentiated α cells, we generated Pax6-deficient primary rat α cells and glucagon-producing cells, using either specific siRNA or cells expressing constitutively a dominant-negative form of Pax6. In primary rat α cells, we confirm that Pax6 controls the transcription of the Proglucagon and processing enzyme PC2 genes and identify three new target genes coding for MafB, cMaf, and NeuroD1/Beta2, which are all critical for Glucagon gene transcription and α cell differentiation. Furthermore, we demonstrate that Pax6 directly binds and activates the promoter region of the three genes through specific binding sites and that constitutive expression of a dominant-negative form of Pax6 in glucagon-producing cells (InR1G9) inhibits the activities of the promoters. Finally our results suggest that the critical role of Pax6 action on α cell differentiation is independent of those of Arx and Foxa2, two transcription factors that are necessary for α cell development. We conclude that Pax6 is critical for α cell function and differentiation through the transcriptional control of key genes involved in glucagon gene transcription, proglucagon processing, and α cell differentiation.

Pdx-1 regulation of the INGAP promoter involves sequestration of NeuroD into a non-DNA-binding complex

OBJECTIVE

Islet neogenesis-associated protein (INGAP) can enhance beta-cell mass to offset progression of diabetes. Identifying how transcription factors regulate INGAP gene expression could reveal key checkpoints governing islet neogenesis.

METHODS

Protein complex interactions at the INGAP promoter were detected using a beta-galactosidase reporter, these protein-DNA complexes being validated in competitive electrophoresis mobility shift assays. The relevance of the revealed promoter interactions was confirmed in small interfering RNA (siRNA) gene knockdown studies.

RESULTS

Pdx-1 negatively regulates stimulation of the INGAP promoter by Pan-1/NeuroD. Independently, Pdx-1, Pan-1, and NeuroD bind to the INGAP promoter as revealed by electrophoresis mobility shift assay studies. In combination, Pdx-1 selectively displaces NeuroD from a DNA-binding complex with Pan-1 to form a non-DNA-binding unit. The importance of this interaction is shown in HIT cells that have a forced reduction of Pdx-1 expression. In siRNA/Pdx-1-depleted HIT cells, the interaction of Pan-1/NeuroD with the INGAP promoter is increased 6-fold. Furthermore, endogenous INGAP expression is detected in Pdx-1-depleted cells.

CONCLUSIONS

These data reveal a dynamic interaction between Pdx-1, NeuroD, and Pan-1 for the regulation of INGAP promoter activity. Modulating molecular regulators of DNA expression may be a consideration in diabetic therapies that translate exogenous stimuli into new endogenous beta-cell mass.

The LIM-homeodomain protein ISL1 activates insulin gene promoter directly through synergy with BETA2

The LIM-homeodomain transcription factor ISL1 (islet factor 1) is essential for pancreatic islet cell and dorsal mesenchyme development. Mutations in ISL1 are associated with maturity-onset diabetes of the young and type 2 diabetes. Whether ISL1 plays a role in the insulin gene expression has not been fully elucidated. In the present study, we show that ISL1 can synergistically activate insulin gene transcription with BETA2 in pancreatic beta cells. The protein-protein interactions of ISL1 and BETA2 are directly mediated by the LIM domains of ISL1 and the basic helix-loop-helix domain of BETA2. Deletion of the two LIM domains of ISL1 enhances the transcriptional activation of the insulin gene, indicating a key role for the homeodomain in activating the insulin promoter. Furthermore, ISL1 can bind with the A3/4 box in the rat insulin gene capital I, Ukrainian promoter through its homeodomain. ISL1 expression is up-regulated at the mRNA level in type 2 diabetes (db/db mouse model) but down-regulated by dexamethasone in rat insulinoma cells. These results suggest that ISL1 is a transcriptional activator for insulin gene expression, and the interactions of ISL1 with BETA2 are required for the transcriptional activity of the insulin gene. Reduction in Isl1 gene expression appears to be involved in the impairment of insulin expression mediated by dexamethasone.

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

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

Pax6 regulates the proglucagon processing enzyme PC2 and its chaperone 7B2

Pax6 is important in the development of the pancreas and was previously shown to regulate pancreatic endocrine differentiation, as well as the insulin, glucagon, and somatostatin genes. Prohormone convertase 2 (PC2) is the main processing enzyme in pancreatic alpha cells, where it processes proglucagon to produce glucagon under the spatial and temporal control of 7B2, which functions as a molecular chaperone. To investigate the role of Pax6 in glucagon biosynthesis, we studied potential target genes in InR1G9 alpha cells transfected with Pax6 small interfering RNA and in InR1G9 clones expressing a dominant-negative form of Pax6. We now report that Pax6 controls the expression of the PC2 and 7B2 genes. By binding and transactivation studies, we found that Pax6 indirectly regulates PC2 gene transcription through cMaf and Beta2/NeuroD1 while it activates the 7B2 gene both directly and indirectly through the same transcription factors, cMaf and Beta2/NeuroD1. We conclude that Pax6 is critical for glucagon biosynthesis and processing by directly and indirectly activating the glucagon gene through cMaf and Beta2/NeuroD1, as well as the PC2 and 7B2 genes.

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.

MafA is a dedicated activator of the insulin gene in vivo

As successful generation of insulin-producing cells could be used for diabetes treatment, a concerted effort is being made to understand the molecular programs underlying islet beta-cell formation and function. The closely related MafA and MafB transcription factors are both key mammalian beta-cell regulators. MafA and MafB are co-expressed in insulin+beta-cells during embryogenesis, while in the adult pancreas only MafA is produced in beta-cells and MafB in glucagon+alpha-cells. MafB-/- animals are also deficient in insulin+ and glucagon+ cell production during embryogenesis. However, only MafA over-expression selectively induced endogenous Insulin mRNA production in cell line-based assays, while MafB specifically promoted Glucagon expression. Here, we analyzed whether these factors were sufficient to induce insulin+ and/or glucagon+ cell formation within embryonic endoderm using the chick in ovo electroporation assay. Ectopic expression of MafA, but not MafB, promoted Insulin production; however, neither MafA nor MafB were capable of inducing Glucagon. Co-electroporation of MafA with the Ngn3 transcription factor resulted in the development of more organized cell clusters containing both insulin- and glucagon-producing cells. Analysis of chimeric proteins of MafA and MafB demonstrated that chick Insulin activation depended on sequences within the MafA C-terminal DNA-binding domain. MafA was also bound to Insulin and Glucagon transcriptional control sequences in mouse embryonic pancreas and beta-cell lines. Collectively, these results demonstrate a unique ability for MafA to independently activate Insulin transcription.

The basic helix-loop-helix transcription factor NeuroD1 facilitates interaction of Sp1 with the secretin gene enhancer

The basic helix-loop-helix transcription factor NeuroD1 is required for late events in neuronal differentiation, for maturation of pancreatic beta cells, and for terminal differentiation of enteroendocrine cells expressing the hormone secretin. NeuroD1-null mice demonstrated that this protein is essential for expression of the secretin gene in the murine intestine, and yet it is a relatively weak transcriptional activator by itself. The present study shows that Sp1 and NeuroD1 synergistically activate transcription of the secretin gene. NeuroD1, but not its widely expressed dimerization partner E12, physically interacts with the C-terminal 167 amino acids of Sp1, which include its DNA binding zinc fingers. NeuroD1 stabilizes Sp1 DNA binding to an adjacent Sp1 binding site on the promoter to generate a higher-order DNA-protein complex containing both proteins and facilitates Sp1 occupancy of the secretin promoter in vivo. NeuroD-dependent transcription of the genes encoding the hormones insulin and proopiomelanocortin is potentiated by lineage-specific homeodomain proteins. The stabilization of binding of the widely expressed transcription factor Sp1 to the secretin promoter by NeuroD represents a distinct mechanism from other NeuroD target genes for increasing NeuroD-dependent transcription.

Effective ablation of pancreatic cancer cells in SCID mice using systemic adenoviral RIP-TK/GCV gene therapy

BACKGROUND

Studies have demonstrated that adenovirus subtype 5 mediated rat insulin promoter directed thymidine kinase (A-5-RIP-TK)/ganciclovir (GCV) gene therapy resulted in significant enhanced cytotoxicity to both PANC-1 and MIA PaCa2 pancreatic cancer cells in vitro. However, little is known about the effect in vivo. In this study we examine the in vivo safety and efficacy of intravenous A-5-RIP-TK/GCV gene therapy.

MATERIALS AND METHODS

1 x 10(6) Mia PaCa2 cells were injected intraperitoneally (i.p.) into SCID mice to create a mouse model of human pancreatic cancer. A-5-RIP-TK gene construct was administered intravenously (i.v.), followed by i.p. GCV administration. Intravenous injection of A-5-RIP-lacZ reporter gene constructs was used for evaluation of Ad-RIP-gene expression in tumors and other tissues. Optimal adenoviral and GCV doses and treatment duration were determined. Tumor volume, serum insulin, and glucose levels were measured. Immunohistochemical staining of pancreata and tumors were performed to assess morphology and hormone expression and apoptotic rates were determined.

RESULTS

All A-5-RIP-TK/GCV-treated mice had reduced tumor volume compared with controls, but maximal tumor volume reduction was observed with 10(8) vp followed by GCV treatment for 4 wk. A-5-RIP-TK/GCV gene therapy contributed to significant survival advantage in MIA PaCa2 bearing mice, and the greatest survival benefit was observed with 10(8) vp and was not affected by length of treatment of GCV. A-5-RIP-TK/GCV therapy increased PDX-1 expression and tumor cells apoptosis, and altered islet morphology. However, A-5-RIP-TK/GCV gene therapy caused diabetes associated with islet cell apoptosis, increased delta-cells and reduced pancreatic polypeptide (PP)-cell numbers.

CONCLUSIONS

Systemically administered A-5-RIP-TK/GCV is an effective treatment of pancreatic cancer. A-5-RIP-TK/GCV cytotoxicity to malignant cells varies with adenoviral dose and length of GCV treatment. However, A-5-RIP-TK/GCV is associated with islet cell toxicity and diabetogenesis. The type of diabetes observed is distinct from Types 1 and 2 and is associated with islet cell apoptosis and reduced delta- and PP-cells.

The beta-cell specific transcription factor Nkx6.1 inhibits glucagon gene transcription by interfering with Pax6

The transcription factor Nkx6.1 is required for the establishment of functional insulin-producing beta-cells in the endocrine pancreas. Overexpression of Nkx6.1 has been shown to inhibit glucagon gene expression while favouring insulin gene activation. Down-regulation resulted in the opposite effect, suggesting that absence of Nkx6.1 favours glucagon gene expression. To understand the mechanism by which Nkx6.1 suppresses glucagon gene expression, we studied its effect on the glucagon gene promoter activity in non-islet cells using transient transfections and gel-shift analyses. In glucagonoma cells transfected with an Nkx6.1-encoding vector, the glucagon promoter activity was reduced by 65%. In BHK21 cells, Nkx6.1 inhibited by 93% Pax6-mediated activation of the glucagon promoter, whereas Cdx2/3 and Maf stimulations were unaltered. Although Nkx6.1 could interact with both the G1 and G3 element, only the former displayed specificity for Nkx6.1. Mutagenesis of the three potential AT-rich motifs within the G1 revealed that only the Pax6-binding site preferentially interacted with Nkx6.1. Chromatin immunoprecipitation confirmed interaction of Nkx6.1 with the glucagon promoter and revealed a direct competition for binding between Pax6 and Nkx6.1. A weak physical interaction between Pax6 and Nkx6.1 was detected in vitro and in vivo suggesting that Nkx6.1 predominantly inhibits glucagon gene transcription through G1-binding competition. We suggest that cell-specific expression of the glucagon gene may only proceed when Nkx6.1, in combination with Pdx1 and Pax4, are silenced in early alpha-cell precursors.

Neurogenin 3 recruits CBP co-activator to facilitate histone H3/H4 acetylation in the target gene INSM1

INSM1 is a downstream target gene of neurogenin 3 (ngn3). A promoter construct containing the -426/+40bp region transiently co-transfected into NIH-3T3 cells with a ngn3 expression plasmid resulted in a 12-fold increase in promoter activity. The ngn3/E47 heterodimer selectively binds and activates the E-box3 of the INSM1 promoter. The endogenous ngn3 and CREB-binding protein (CBP) co-activator occupy the INSM1 promoter, resulting in hyper-acetylation of histone H3/H4 chromatin in a human neuroblastoma cell line, IMR-32. Additionally, adenoviral ngn3 can induce endogenous INSM-1 expression in pancreatic ductal carcinoma-1 cells through the recruitment of CBP to the INSM1 promoter and increase the acetylation of the INSM1 promoter region.

Induction by NeuroD of the components required for regulated exocytosis

NeuroD is a transcriptional factor critical in differentiation of neuronal cells, enteroendocrine cells, and pancreatic endocrine cells. However, little is known of its roles in cellular functions. We show here that introduction of NeuroD into human fetal epithelial cell line Intestine 407 cells induces neuron-like morphology. In addition, multiple genes associated with vesicular trafficking and exocytotic machinery, including Sec24D, carboxypeptidase E, myosin Va, SNAP25, syntaxin 1A, Rab, Rims, Munc18-1, and adenylyl cyclase, were up-regulated by NeuroD gene transfer. Moreover, low osmotic pressure-induced exocytosis monitored by FM1-43 was enhanced by overexpression of NeuroD. These results suggest that NeuroD plays an important role in regulated exocytosis by inducing expressions of various components required in the process.

Islet cell differentiation in liver by combinatorial expression of transcription factors neurogenin-3, BETA2, and RIPE3b1

Transcription factors, such as PDX-1, that normally mediate pancreatic development are capable of inducing hepatic progenitor cells to differentiate into cells with pancreatic islet characteristics. We hypothesized that simultaneous expression of multiple transcription factors involved in islet development might enhance the differentiation of hepatic progenitor cells. Bi- or tri-cistronic constructs were generated in hybrid adenovirus/adeno-associated virus (Ad/AAV) vectors containing neurogenin 3 (NGN3), BETA2 (NeuroD), and RIPE3b1 (MafA), each of which plays a role in islet cell differentiation. These vectors efficiently express multiple transcription factors and stimulate insulin promoter activity in a combinatorial manner. When these multi-cistronic constructs were administered in vivo, they induce hepatic expression of islet-specific markers, including PDX-1, insulin, glucagon, somatostatin, and islet-amyloid peptide. Administration of the Ad/AAV hybrid vectors to streptozotocin-induced diabetic mice reversed hyperglycemia, consistent the differentiation of functional hepatic insulin-secreting cells. These results indicate that Ad/AAV hybrid vectors can be used to administer combinations of factors that induce islet cell differentiation in hepatic progenitor cells.

Transcriptional regulation of human MAP2 gene in melanoma: role of neuronal bHLH factors and Notch1 signaling

Microtubule-associated protein 2 (MAP2), a neuron-specific protein, stabilizes microtubules and is critical for neurite outgrowth and dendrite development. Although MAP2 is widely used as a marker of neuronal differentiation, regulation of its transcription has not been investigated. We showed that MAP2 is frequently activated in human cutaneous melanoma. Here, we identified a 2.2 kb region that is sufficient for neuronal-specific expression in vitro and in vivo. Comparative analysis of the mouse, rat and human MAP2 promoter sequences showed the presence of a conserved bHLH factor binding sites. Electrophoretic mobility shift analysis, promoter mutagenesis and co-transfection experiments showed that NeuroD, a pro-neuronal differentiation factor, and Hairy and Enhancer of Split (HES1), a transcription repressor, are involved in the regulation of MAP2 promoter activity. Melanoma cells express both NeuroD and HES1. Chromatin immunoprecipitation showed that in metastatic melanoma cells N-box region of the MAP2 promoter is occupied by endogenous HES1. We show that the inhibition of Notch signaling, a regulator of HES1 gene expression, and/or shRNA knockdown of HES1 results in the upregulation of MAP2 promoter activity. Thus, our data suggest that Notch signaling, which is implicated in melanoma progression, and HES1 play a role in MAP2 gene regulation during melanoma progression.

MafB: an activator of the glucagon gene expressed in developing islet alpha- and beta-cells

The large Maf family of basic leucine-zipper-containing transcription factors are known regulators of key developmental and functional processes in various cell types, including pancreatic islets. Here, we demonstrate that within the adult pancreas, MafB is only expressed in islet alpha-cells and contributes to cell type-specific expression of the glucagon gene through activation of a conserved control element found between nucleotides -77 to -51. MafB was also shown to be expressed in developing alpha- and beta-cells as well as in proliferating hormone-negative cells during pancreatogenesis. In addition, MafB expression is maintained in the insulin(+) and glucagon(+) cells remaining in mice lacking either the Pax4 or Pax6 developmental regulators, implicating a potentially early role for MafB in gene regulation during islet cell development. These results indicate that MafB is not only important to islet alpha-cell function but may also be involved in regulating genes required in both endocrine alpha- and beta-cell differentiation.

Neuronatin, a downstream target of BETA2/NeuroD1 in the pancreas, is involved in glucose-mediated insulin secretion

BETA2 (NeuroD1) is a member of the basic helix-loop-helix transcription factor family. BETA2 plays an important role in the development of the pancreas and the nervous system. Using microarray technology, we identified neuronatin (Nnat) as differentially expressed between wild-type (WT) and knockout (KO) pancreatic RNA from embryonic day 14 (e14.5). NNAT is a member of the proteolipid family of amphipathic polypeptides and is believed to be involved in ion channel transport or channel modulation. Northern blot and in situ hybridization analysis of WT and KO samples confirmed the downregulation of Nnat in pancreas of mutant BETA2 embryos. Chromatin immunoprecipitation and gel shift assays were performed and demonstrated the presence of BETA2 on the Nnat promoter, thus confirming the direct transcriptional regulation of Nnat by BETA2. To assess NNAT potential function, we performed knockdown studies by siRNA in NIT cells and observed a reduction in the ability of the NIT cells to respond to glucose. These results suggest for the first time an important role for NNAT in insulin secretion and for proper beta-cell function.

NeuroD1 in the endocrine pancreas: Localization and dual function as an activator and repressor

The basic helix-loop-helix transcription factor NeuroD1 regulates cell fate in the nervous system but previously has not been considered to function similarly in the endocrine pancreas due to its reported expression in all islet cell types in the newborn mouse. Because we found that NeuroD1 potently represses somatostatin expression in vitro, its pattern of expression was examined in both strains of mice in which lacZ has been introduced into the NeuroD1 locus by homologous recombination. Analysis of adult transgenic mice revealed that NeuroD1 is predominantly expressed in beta-cells and either absent or expressed below the limit of lacZ detection in mature alpha-, delta-, or PP cells. Consistent with a previous report, NeuroD1 colocalizes with glucagon as well as insulin in immature islets of the newborn mouse. However, no colocalization of NeuroD1with somatostatin was detected in the newborn. In vitro, ectopic expression of NeuroD1 in TRM-6/PDX-1, a human pancreatic delta-cell line, resulted in potent repression of somatostatin concomitant with induction of the beta-cell hormones insulin and islet amyloid polypeptide. Additionally, NeuroD1 induced expression of Nkx2.2, a transcription factor expressed in beta- but not delta-cells. Transfection studies using insulin and somatostatin promoters confirm the ability of NeuroD1 to act as both a transcriptional repressor and activator in the same cell, suggesting a more complex role for NeuroD1 in the establishment and/or maintenance of mature endocrine cells than has been recognized previously.

The Pal elements in the upstream glucokinase promoter exhibit dyad symmetry and display cell-specific enhancer activity when multimerised

AIMS/HYPOTHESIS

The upstream glucokinase (betaGK) promoter has previously been shown to contain two 9-bp sequences, termed the Pal motifs, that are conserved in humans, rats and mice. These motifs are necessary for expression of the betaGK promoter in pancreatic beta cell lines and pituitary corticotrope cell lines. DNA probes containing the Pal motifs bind cell-type-specific protein complexes, but these motifs have not been completely characterised.

METHODS

Methylation interference and ultraviolet-crosslinking analysis were utilised to characterise, at the single nucleotide level, sites of protein binding within the elements themselves. To determine the function of these elements, mutational analysis of the betaGK promoter and of multimerised GK promoter sequences was performed.

RESULTS

Both Pal elements are 14 bp in length and have dyad symmetry. However, while the Pal-1 element is a perfect inverted repeat (GTCACCA-TGGTGAC), the Pal-2 element (GTCACCA-TAGAAAC) is an imperfect repeat. Ultraviolet-crosslinking analysis using nuclear extracts prepared from hamster insulinoma tumour (HIT) cells revealed that the three resolvable complexes that bind to the Pal-1 and Pal-2 elements contain different ratios of three proteins of different size (approximately 90, 110 and 150 M(r)). Mutation of a single nucleotide binding site abrogates betaGK promoter activity. Multimerised repeats of the Pal-1 element augment transcription in HIT cells, but not in baby hamster kidney (BHK) cells.

CONCLUSIONS/INTERPRETATION

These results suggest that different combinations of three proteins of different size bind to the Pal elements, probably as homodimers and heterodimers. Together, these results indicate that the betaGK promoter contains two novel 14-bp elements that, when multimerised, exhibit enhancer activity specific to neuroendocrine cells.

Identification of a novel PDX-1 binding site in the human insulin gene enhancer

Islet beta cell type-specific transcription of the insulin gene is regulated by a number of cis-acting elements found within the proximal 5'-flanking region. The control sequences conserved between mammalian insulin genes are acted upon by transcription factors, like PDX-1 and BETA-2, that are also involved in islet beta cell function and formation. In the current study, we investigated the contribution to human insulin expression of the GG2 motif found between nucleotides -145 and -140 relative to the transcription start site. Site-specific mutants were generated within GG2 that displayed a parallel increase (i.e. -144 base pair) or decrease (i.e. -141 base pair) in insulin enhancer-driven reporter and gel shift binding activity in beta cells consistent with human GG2 being under positive regulatory control. In contrast, the corresponding site in the rodent insulin gene, which only differs from the human at nucleotides -144 and -141, is negatively regulated by the Nkx2.2 transcription factor (Cissell, M. A., Zhao, L., Sussel, L., Henderson, E., and Stein, R. (2003) J. Biol. Chem. 278, 751-756). Human GG2 activator binding activity was present in nuclear extracts prepared from human islets and enriched in those from rodent beta cell lines. The human GG2 activator binding factor(s) was shown to be approximately 38-40 kDa and distinct from other size-matched islet-enriched transcription factors, including Nkx2.2, Pax-4, Cdx2/3, and Isl-1. Combined DNA chromatographic purification and mass spectrometry analysis revealed that the GG2 activator was PDX-1. These results demonstrate that the GG2 element, despite its divergence from the core homeodomain consensus binding motif, is a site for PDX-1 activation in the human insulin gene.

Orphan nuclear receptor small heterodimer partner, a novel corepressor for a basic helix-loop-helix transcription factor BETA2/neuroD

Small heterodimer partner (SHP; NR0B2) is an atypical orphan nuclear receptor that lacks a conventional DNA binding domain (DBD) and represses the transcriptional activity of various nuclear receptors. In this study, we examined the novel cross talk between SHP and BETA2/NeuroD, a basic helix-loop-helix transcription factor. In vitro and in vivo protein interaction studies showed that SHP physically interacts with BETA2/NeuroD, but not its heterodimer partner E47. Moreover, confocal microscopic study and immunostaining results demonstrated that SHP colocalized with BETA2 in islets of mouse pancreas. SHP inhibited BETA2/NeuroD-dependent transactivation of an E-box reporter, whereas SHP was unable to repress the E47-mediated transactivation and the E-box mutant reporter activity. In addition, SHP repressed the BETA2-dependent activity of glucokinase and cyclin-dependent kinase inhibitor p21 gene promoters. Gel shift and in vitro protein competition assays indicated that SHP inhibits neither dimerization nor DNA binding of BETA2 and E47. Rather, SHP directly repressed BETA2 transcriptional activity and p300-enhanced BETA2/NeuroD transcriptional activity by inhibiting interaction between BETA2 and coactivator p300. We also showed that C-terminal repression domain within SHP is also required for BETA2 repression. However, inhibition of BETA2 activity was not observed by naturally occurring human SHP mutants that cannot interact with BETA2/NeuroD. Taken together, these results suggest that SHP acts as a novel corepressor for basic helix-loop-helix transcription factor BETA2/NeuroD by competing with coactivator p300 for binding to BETA2/NeuroD and by its direct transcriptional repression function.

Discovery of gene networks regulating cytokine-induced dysfunction and apoptosis in insulin-producing INS-1 cells

Locally released cytokines contribute to beta-cell dysfunction and apoptosis in type 1 diabetes. In vitro exposure of insulin-producing INS-1E cells to the cytokines interleukin (IL)-1beta + interferon (IFN)-gamma leads to a significant increase in apoptosis. To characterize the genetic networks implicated in beta-cell dysfunction and apoptosis and its dependence on nitric oxide (NO) production, we performed a time-course microarray analysis of cytokine-induced genes in insulin-producing INS-1E cells. INS-1E cells were exposed in duplicate to IL-1beta + IFN-gamma for six different time points (1, 2, 4, 8, 12, and 24 h) with or without the inducible NO synthase (iNOS) blocker N(G)-monomethyl-L-arginine (NMA). The microarray analysis identified 698 genes as cytokine modified (>or=2.5-fold change compared with control) in at least one time point. Based on their temporal pattern of variation, the cytokine-regulated genes were classified into 15 clusters by the k-means method. These genes were further classified into 14 different groups according to their putative function. Changes in the expression of genes related to metabolism, signal transduction, and transcription factors at all time points studied indicate beta-cell attempts to adapt to the effects of continuous cytokine exposure. Notably, several apoptosis-related genes were modified at early time points (2-4 h) preceding iNOS expression. On the other hand, 46% of the genes modified by cytokines after 8-24 h were NO dependent, indicating the important role of this radical for the late effects of cytokines. The present results increase by more than twofold the number of known cytokine-modified genes in insulin-producing cells and yield comprehensive information on the role of NO for these modifications in gene expression. These data provide novel and detailed insights into the gene networks activated in beta-cells facing a prolonged immune assault.

Contrasting patterns of expression of transcription factors in pancreatic alpha and beta cells

Pancreatic alpha and beta cells are derived from the same progenitors but play opposing roles in the control of glucose homeostasis. Disturbances in their function are associated with diabetes mellitus. To identify many of the proteins that define their unique pathways of differentiation and functional features, we have analyzed patterns of gene expression in alphaTC1.6 vs. MIN6 cell lines by using oligonucleotide microarrays. Approximately 9-10% of >11,000 transcripts examined showed significant differences between the two cell types. Of >700 known transcripts enriched in either cell type, transcription factors and their regulators (TFR) was one of the most significantly different categories. Ninety-six members of the basic zipper, basic helix-loop-helix, homeodomain, zinc finger, high mobility group, and other transcription factor families were enriched in alpha cells; in contrast, homeodomain proteins accounted for 51% of a total of 45 TFRs enriched in beta cells. Our analysis thus highlights fundamental differences in expression of TFR subtypes within these functionally distinct islet cell types. Interestingly, the alpha cells appear to express a large proportion of factors associated with progenitor or stem-type cells, perhaps reflecting their earlier appearance during pancreatic development. The implications of these findings for a better understanding of alpha and beta cell dysfunction in diabetes mellitus are also considered.

NeuroD1/E47 regulates the E-box element of a novel zinc finger transcription factor, IA-1, in developing nervous system

IA-1 is a novel zinc finger transcription factor with a restricted tissue distribution in the embryonic nervous system and tumors of neuroendocrine origin. The 1.7-kilobase 5'-upstream DNA sequence of the human IA-1 gene directed transgene expression predominantly in the developing nervous system including forebrain, midbrain, hindbrain, spinal cord, retina, olfactory bulb, and cerebellum, which recapitulated the expression patterns of neuroendocrine tissues and childhood brain tumors. The IA-1 promoter deletion reporter gene constructs revealed that the sequence between -426 and -65 bp containing three putative E-boxes (approximately 361 bp) upstream of the transcription start site was sufficient to confer tissue-specific transcriptional activity. Further mutation analysis revealed that the proximal E-box (E3) closest to the start site is critical to confer transcriptional activity. Electrophoretic mobility shift assay and transient transfection studies demonstrated that the NeuroD1 and E47 heterodimer are the key transcription factors that regulate the proximal E-box of the IA-1 promoter. Therefore, we concluded that the IA-1 gene is developmentally expressed in the nervous system and the NeuroD1/E47 transcription factors up-regulate IA-1 gene expression through the proximal E-box element of the IA-1 promoter.

The cellular Pax–Hox–Helix connection

Basic helix-loop-helix (bHLH) transcription factors are important regulators of lineage determination during embryogenesis. Initial experiments in Drosophila showed that early neural selection and specification are dependent on atonal (ato) and members of the achaete-scute complex (as-c). In mammals, transcription factors homologous to as-c and ato are causally involved during development of organs throughout the body. Development of subsets of lineages in intestine, stomach, pancreas, lung, thyroid and placenta have been shown to be regulated by members of the as-c and ato families. These functional studies show that an individual bHLH transcription factor can regulate multiple developmental processes throughout the mammalian body, which implicates that extant as-c and ato transcription factors play a distinct function dependent on their cellular context. Based on the synergistic activation of the insulin, POMC and Pax4 promotors by bHLH and homeobox (Hox) protein complexes, we hypothesize that the underlying cellular function-modulating factors include members of the Hox and paired box (Pax) multigene families. These examples indicate that unique combinations of bHLH and Hox proteins, mediated by protein-protein interactions, might be responsible for activating cell-specific sets of target genes.

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.

Upstream stimulatory factor (USF) and neurogenic differentiation/beta-cell E box transactivator 2 (NeuroD/BETA2) contribute to islet-specific glucose-6-phosphatase catalytic-subunit-related protein (IGRP) gene expression

Islet-specific glucose-6-phosphatase (G6Pase) catalytic-subunit-related protein (IGRP) is a homologue of the catalytic subunit of G6Pase, the enzyme that catalyses the final step of the gluconeogenic pathway. The analysis of IGRP-chloramphenicol acetyltransferase (CAT) fusion-gene expression through transient transfection of islet-derived beta TC-3 cells revealed that multiple promoter regions, located between -306 and -97, are required for maximal IGRP-CAT fusion-gene expression. These regions correlated with trans -acting factor-binding sites in the IGRP promoter that were identified in beta TC-3 cells in situ using the ligation-mediated PCR (LMPCR) footprinting technique. However, the LMPCR data also revealed additional trans -acting factor-binding sites located between -97 and +1 that overlap two E-box motifs, even though this region by itself conferred minimal fusion-gene expression. The data presented here show that these E-box motifs are important for IGRP promoter activity, but that their action is only manifest in the presence of distal promoter elements. Thus mutation of either E-box motif in the context of the -306 to +3 IGRP promoter region reduces fusion-gene expression. These two E-box motifs have distinct sequences and preferentially bind NeuroD/BETA2 (neurogenic differentiation/beta-cell E box transactivator 2) and upstream stimulatory factor (USF) in vitro, consistent with the binding of both factors to the IGRP promoter in situ, as determined using the chromatin-immunoprecipitation (ChIP) assay. Based on experiments using mutated IGRP promoter constructs, we propose a model to explain how the ubiquitously expressed USF could contribute to islet-specific IGRP gene expression.

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.

Novel transcriptional potentiation of BETA2/NeuroD on the secretin gene promoter by the DNA-binding protein Finb/RREB-1

The basic helix-loop-helix protein BETA2/NeuroD activates transcription of the secretin gene and is essential for terminal differentiation of secretin-producing enteroendocrine cells. However, in heterodimeric complexes with its partner basic helix-loop-helix proteins, BETA2 does not appear to be a strong activator of transcription by itself. Mutational analysis of a proximal enhancer in the secretin gene identified several cis-acting elements in addition to the E-box binding site for BETA2. We identified by expression cloning the zinc finger protein RREB-1, also known to exist as a longer form, Finb, as the protein binding to one of the mutationally sensitive elements. Finb/RREB-1 lacks an intrinsic activation domain and by itself did not activate secretin gene transcription. Here we show that Finb/RREB-1 can associate with BETA2 to enhance its transcription-activating function. Both DNA binding and physical interaction of Finb/RREB-1 with BETA2 are required to potentiate transcription. Thus, Finb/RREB-1 does not function as a classical activator of transcription that recruits an activation domain to a DNA-protein complex. Finb/RREB-1 may be distinguished from coactivators, which increase transcription without sequence-specific DNA binding. We suggest that Finb/RREB-1 should be considered a potentiator of transcription, representing a distinct category of transcription-regulating proteins.

Pax-2 activates the proglucagon gene promoter but is not essential for proglucagon gene expression or development of proglucagon-producing cell lineages in the murine pancreas or intestine

Tissue-specific proglucagon gene transcription is achieved through combinations of transcription factors expressed in pancreatic A cells and enteroendocrine L cells of the small and large intestine. Cell transfection and electrophoretic mobility shift assay experiments previously identified Pax-2 as a regulator of islet proglucagon gene expression. We examined whether Pax-2 regulates gut proglucagon gene expression using enteroendocrine cell lines and Pax2(1NEU) mutant mice. Immunoreactive Pax-2 was detected in STC-1 enteroendocrine cells, and Pax-2 activated proglucagon promoter activity in transfected baby hamster kidney and GLUTag cells. Pax-2 antisera diminished the formation of a Pax-2-G3 complex in electrophoretic mobility shift assay studies using nuclear extracts from islet and enteroendocrine cell lines. Surprisingly, Pax-2 mRNA transcripts were not detected by RT-PCR in RNA isolated from adult rat pancreas, rat islets, embryonic d 19 or adult murine pancreas and gastrointestinal tract. Furthermore, embryonic d 19 or neonatal d 1 Pax2(1NEU) mice exhibited normal islet A cells and gut endocrine L cells, and no decrement in pancreatic or intestinal glucagon gene expression. These findings demonstrate that Pax-2 is not essential for the developmental formation of islet A or gut L cells and does not play a role in the physiological control of proglucagon gene expression in vivo.

Transactivation of the mouse sulfonylurea receptor I gene by BETA2/NeuroD

The sulfonylurea receptor 1 (SUR1) plays a key role in regulation of insulin secretion in pancreatic beta-cells. In this study we investigated the mechanism for tissue-specific expression of the SUR1 gene. A -138/-20 fragment exhibited basal promoter activity while the -660/-20 fragment contained a regulatory element for tissue-specific expression of the mouse SUR1 gene. A pancreatic beta-cell-specific transcription factor, BETA2 (beta-cell E box transcription factor)/NeuroD, enhanced the promoter activity of the -660/-20 fragment in cooperation with E47. Coexpression of a dominant negative mutant of BETA2/NeuroD, BETA2(1-233), repressed the promoter activity of the -660/-20 fragment. BETA2/NeuroD bound specifically to the E3 element located at -141. The E3 sequence in a heterologous context conferred transactivation by BETA2/NeuroD in HeLa and HIT cells. Mutation of E3 eliminated the stimulatory effect of BETA2/NeuroD. Unlike BETA2/NeuroD, neurogenin 3 (ngn3) could not activate the E3 element in HeLa cells. Overexpression of ngn3 concomitantly increased expression of BETA2/NeuroD and SUR1 in HIT cells but not in HeLa cells. These results indicate that BETA2/NeuroD induces tissue-specific expression of the SUR1 gene through the E3 element. These results also suggest that E3 is specific for BETA2/NeuroD, and the stimulatory effect of ngn3 in HIT cells may require factors specifically expressed in HIT cells.

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.