A tissue-specific nuclear factor binds to multiple sites in the human insulin-gene enhancer

Biophysical insights into glucose-dependent transcriptional regulation by PDX1

The pancreatic and duodenal homeobox 1 (PDX1) is a central regulator of glucose-dependent transcription of insulin in pancreatic β cells. PDX1 transcription factor activity is integral to the development and sustained health of the pancreas; accordingly, deciphering the complex network of cellular cues that lead to PDX1 activation or inactivation is an important step toward understanding the etiopathologies of pancreatic diseases and the development of novel therapeutics. Despite nearly 3 decades of research into PDX1 control of Insulin expression, the molecular mechanisms that dictate the function of PDX1 in response to glucose are still elusive. The transcriptional activation functions of PDX1 are regulated, in part, by its two intrinsically disordered regions, which pose a barrier to its structural and biophysical characterization. Indeed, many studies of PDX1 interactions, clinical mutations, and posttranslational modifications lack molecular level detail. Emerging methods for the quantitative study of intrinsically disordered regions and refined models for transactivation now enable us to validate and interrogate the biochemical and biophysical features of PDX1 that dictate its function. The goal of this review is to summarize existing PDX1 studies and, further, to generate a comprehensive resource for future studies of transcriptional control via PDX1.

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

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

Intrinsically disordered substrates dictate SPOP subnuclear localization and ubiquitination activity

Speckle-type POZ protein (SPOP) is a ubiquitin ligase adaptor that binds substrate proteins and facilitates their proteasomal degradation. Most SPOP substrates present multiple SPOP-binding (SB) motifs and undergo liquid-liquid phase separation with SPOP. Pancreatic and duodenal homeobox 1 (Pdx1), an insulin transcription factor, is downregulated by interaction with SPOP. Unlike other substrates, only one SB motif has previously been reported within the Pdx1 C-terminal intrinsically disordered region (Pdx1-C). Given this difference, we aimed to determine the specific mode of interaction of Pdx1 with SPOP and how it is similar or different to that of other SPOP substrates. Here, we identify a second SB motif in Pdx1-C, but still find that the resulting moderate valency is insufficient to support phase separation with SPOP in cells. Although Pdx1 does not phase separate with SPOP, Pdx1 and SPOP interaction prompts SPOP relocalization from nuclear speckles to the diffuse nucleoplasm. Accordingly, we find that SPOP-mediated ubiquitination activity of Pdx1 occurs in the nucleoplasm and that highly efficient Pdx1 turnover requires both SB motifs. Our results suggest that the subnuclear localization of SPOP-substrate interactions and substrate ubiquitination may be directed by the properties of the substrate itself.

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

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

A novel synthetic human insulin super promoter for targeting PDX-1-expressing pancreatic cancer

Our previous studies have shown that a rat insulin promoter II fragment (RIP) was used to effectively target pancreatic adenocarcinoma (PDAC) and insulinoma that over-express pancreatic and duodenal homeobox-1 (PDX-1). To enhance the activity and specificity of the human insulin promoter, we engineered a synthetic human insulin super-promoter (SHIP). Reporter assay demonstrated that SHIP1 was the most powerful promoter among all of the SHIPs and had far greater activity than the endogenous human insulin promoters and RIP in PDAC expressing PDX-1. Over-expression, knockdown and competitive inhibition of PDX-1 expression assay proved that PDX-1 is a critical transcript factor to regulate the activity of SHIP1. SHIP1-driven viral thymidine kinase followed by ganciclovir (SHIP1-TK/GCV) resulted in cytotoxicity to PDAC cells in vitro. Systemic delivery of SHIP1-TK/GCV in PDAC xenograft mice significantly suppressed PANC-1 tumor growth in vivo greater than RIP-TK/GCV and CMV-TK/GCV controls (p < .05). These preclinical data suggest that SHIP1 is a powerful novel promoter that can be used to target human PDAC expressing PDX-1 in clinical trials. Furthermore, this novel strategy of engineering synthetic super-promoters could be used for other cancer targets.

Transcriptomic analysis of the lesser spotted catshark (Scyliorhinus canicula) pancreas, liver and brain reveals molecular level conservation of vertebrate pancreas function

Background

Understanding the evolution of the vertebrate pancreas is key to understanding its functions. The chondrichthyes (cartilaginous fish such as sharks and rays) have often been suggested to possess the most ancient example of a distinct pancreas with both hormonal (endocrine) and digestive (exocrine) roles. The lack of genetic, genomic and transcriptomic data for cartilaginous fish has hindered a more thorough understanding of the molecular-level functions of the chondrichthyan pancreas, particularly with respect to their “unusual” energy metabolism (where ketone bodies and amino acids are the main oxidative fuel source) and their paradoxical ability to both maintain stable blood glucose levels and tolerate extensive periods of hypoglycemia. In order to shed light on some of these processes, we carried out the first large-scale comparative transcriptomic survey of multiple cartilaginous fish tissues: the pancreas, brain and liver of the lesser spotted catshark, Scyliorhinus canicula.

Results

We generated a mutli-tissue assembly comprising 86,006 contigs, of which 44,794 were assigned to a particular tissue or combination of tissues based on mapping of sequencing reads. We have characterised transcripts encoding genes involved in insulin regulation, glucose sensing, transcriptional regulation, signaling and digestion, as well as many peptide hormone precursors and their receptors for the first time. Comparisons to mammalian pancreas transcriptomes reveals that mechanisms of glucose sensing and insulin regulation used to establish and maintain a stable internal environment are conserved across jawed vertebrates and likely pre-date the vertebrate radiation. Conservation of pancreatic hormones and genes encoding digestive proteins support the single, early evolution of a distinct pancreatic gland with endocrine and exocrine functions in jawed vertebrates. In addition, we demonstrate that chondrichthyes lack pancreatic polypeptide (PP) and that reports of PP in the literature are likely due cross-reaction with PYY and/or NPY in the pancreas. A three hormone islet organ is therefore the ancestral jawed vertebrate condition, later elaborated upon only in the tetrapod lineage.

Conclusions

The cartilaginous fish are a great untapped resource for the reconstruction of patterns and processes of vertebrate evolution and new approaches such as those described in this paper will greatly facilitate their incorporation into the rank of “model organism”.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1074) contains supplementary material, which is available to authorized users.

The transcription factors islet and Lim3 combinatorially regulate ion channel gene expression

Expression of appropriate ion channels is essential to allow developing neurons to form functional networks. Our previous studies have identified LIM-homeodomain (HD) transcription factors (TFs), expressed by developing neurons, that are specifically able to regulate ion channel gene expression. In this study, we use the technique of DNA adenine methyltransferase identification (DamID) to identify putative gene targets of four such TFs that are differentially expressed in Drosophila motoneurons. Analysis of targets for Islet (Isl), Lim3, Hb9, and Even-skipped (Eve) identifies both ion channel genes and genes predicted to regulate aspects of dendritic and axonal morphology. Significantly, some ion channel genes are bound by more than one TF, consistent with the possibility of combinatorial regulation. One such gene is Shaker (Sh), which encodes a voltage-dependent fast K(+) channel (Kv1.1). DamID reveals that Sh is bound by both Isl and Lim3. We used body wall muscle as a test tissue because in conditions of low Ca(2+), the fast K(+) current is carried solely by Sh channels (unlike neurons in which a second fast K(+) current, Shal, also contributes). Ectopic expression of isl, but not Lim3, is sufficient to reduce both Sh transcript and Sh current level. By contrast, coexpression of both TFs is additive, resulting in a significantly greater reduction in both Sh transcript and current compared with isl expression alone. These observations provide evidence for combinatorial activity of Isl and Lim3 in regulating ion channel gene expression.

Cooperation between HMGA1, PDX-1, and MafA is Essential for Glucose-Induced Insulin Transcription in Pancreatic Beta Cells

The high-mobility group AT-hook 1 (HMGA1) protein is a nuclear architectural factor that can organize chromatin structures. It regulates gene expression by controlling the formation of stereospecific multiprotein complexes called "enhanceosomes" on the AT-rich regions of target gene promoters. Previously, we reported that defects in HMGA1 caused decreased insulin receptor expression and increased susceptibility to type 2 diabetes mellitus in humans and mice. Interestingly, mice with disrupted HMGA1 gene had significantly smaller islets and decreased insulin content in their pancreata, suggesting that HMGA1 may have a direct role in insulin transcription and secretion. Herein, we investigate the regulatory roles of HMGA1 in insulin transcription. We provide evidence that HMGA1 physically interacts with PDX-1 and MafA, two critical transcription factors for insulin gene expression and beta-cell function, both in vitro and in vivo. We then show that the overexpression of HMGA1 significantly improves the transactivating activity of PDX-1 and MafA on human and mouse insulin promoters, while HMGA1 knockdown considerably decreased this transactivating activity. Lastly, we demonstrate that high glucose stimulus significantly increases the binding of HMGA1 to the insulin (INS) gene promoter, suggesting that HMGA1 may act as a glucose-sensitive element controlling the transcription of the INS gene. Together, our findings provide evidence that HMGA1, by regulating PDX-1- and MafA-induced transactivation of the INS gene promoter, plays a critical role in pancreatic beta-cell function and insulin production.

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.

Efficient, glucose responsive and islet-specific transgene expression by a modified rat insulin promoter

This study was done to improve efficiency and islet specificity of the rat insulin promoter (RIP). Various RIP lengths were prepared and tested in vitro to drive luciferase reporter gene expression in INS1-cells, alpha-cells, acinar cells, ductal cells and fibroblasts. The CMV promoter was used as a positive control. In addition, the DsRed reporter gene was administered in vivo to rat pancreas by ultrasound-targeted microbubble destruction (UTMD). Confocal microscopy was used to detect the presence and distribution of DsRed within the pancreas after UTMD. A modified RIP3.1 promoter, which includes portions of the insulin gene after its transcription start site is fivefold more active in INS-1 cells than the full-length RIP promoter or the CMV promoter. RIP3.1 is regulated by glucose level and various islet transcription factors in vitro, and exhibits activity in alpha-cells, but not in exocrine cells. In vivo delivery of RIP3.1-DsRed resulted in expression of DsRed protein in beta-cells, and to a lesser extent in alpha-cells under normal glucose conditions. No DsRed signal was present in exocrine pancreas under RIP3.1. A modified RIP, RIP3.1, efficiently and specifically directs gene expression to endocrine pancreas.

Glucose-responsive gene expression system for gene therapy

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

ATF-2 stimulates the human insulin promoter through the conserved CRE2 sequence

The insulin promoter contains a number of dissimilar cis-acting regulatory elements that bind a range of tissue specific and ubiquitous transcription factors. Of the regulatory elements within the insulin promoter, the cyclic AMP responsive element (CRE) binds by far the most diverse array of transcription factors. Rodent insulin promoters have a single CRE site, whereas there are four CREs within the human insulin gene, of which CRE2 is the only one conserved between species. The aim of this study was to characterise the human CRE2 site and to investigate the effects of the two principal CRE-associated transcription factors; CREB-1 and ATF-2. Co-transfection of INS-1 pancreatic beta-cells with promoter constructs containing the human insulin gene promoter placed upstream of the firefly luciferase reporter gene and expression plasmids for ATF-2 or CREB-1 showed that ATF-2 stimulated transcriptional activity while CREB-1 elicited an inhibitory effect. Mutagenesis of CRE2 diminished the effect of ATF-2 but not that of CREB-1. ATF-2 was shown to bind to the CRE2 site by electrophoretic mobility shift assay and by chromatin immunoprecipitation, while siRNA mediated knockdown of ATF-2 diminished the stimulatory effects of cAMP related signalling on promoter activity. These results suggest that ATF-2 may be a key regulator of the human insulin promoter possibly stimulating activity in response to extracellular signals.

Comparative analysis of insulin gene promoters: implications for diabetes research

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

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

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

Regulation of insulin gene expression by overlapping DNA-binding elements

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

The Drosophila LIM-homeodomain protein Islet antagonizes proneural cell specification in the peripheral nervous system

The pattern of the external sensory organs (SO) in Drosophila depends on the activity of the basic helix-loop-helix (bHLH) transcriptional activators Achaete/Scute (Ac/Sc) that are expressed in clusters of cells (pro-neural clusters) and provide the cells with the potential to develop a neural fate. In the mesothorax, the GATA1 transcription factor Pannier (Pnr), together with its cofactor Chip, activates ac/sc genes directly through binding to the dorso-central enhancer (DC) of ac/sc. We identify the LIM-homeo domain (LIM-HD) transcription factor Islet (Isl) by genetic screening and investigate its role in the thoracic pre-patterning. We show that isl loss-of-function mutations result in expanded Ac expression in DC and scutellar (SC) pro-neural clusters and formation of ectopic sensory organs. Overexpression of Isl decreases pro-neural expression and suppresses bristle development. Moreover, Isl is coexpressed with Pnr in the posterior region of the mesothorax. In the DC pro-neural cluster, Isl antagonizes Pnr activity both by dimerization with the DNA-binding domain of Pnr and via competitive inhibition of the Chip-bHLH interaction. We propose that sensory organ pre-patterning relies on the antagonistic activity of individual Chip-binding factors. The differential affinities of these binding-factors and their precise stoichiometry are crucial in specifying pre-patterns within the different pro-neural clusters.

Relative contribution of PDX-1, MafA and E47/beta2 to the regulation of the human insulin promoter

The insulin promoter binds a number of tissue-specific and ubiquitous transcription factors. Of these, the homoeodomain protein PDX-1 (pancreatic duodenal homeobox factor-1), the basic leucine zipper protein MafA and the basic helix-loop-helix heterodimer E47/BETA2 (beta-cell E box transactivator 2; referred to here as beta2) bind to important regulatory sites. Previous studies have shown that PDX-1 can interact synergistically with E47 and beta2 to activate the rat insulin 1 promoter. The aim of the present study was to determine the relative contribution of PDX-1, MafA and E47/beta2 in regulating the human insulin promoter, and whether these factors could interact synergistically in the context of the human promoter. Mutagenesis of the PDX-1, MafA and E47/beta2 binding sites reduced promoter activity by 60, 74 and 94% respectively, in INS-1 beta-cells. In the islet glucagonoma cell line alphaTC1.6, overexpression of PDX-1 and MafA separately increased promoter activity approx. 2.5-3-fold, and in combination approx. 6-fold, indicating that their overall effect was additive. Overexpression of E47 and beta2 had no effect. In HeLa cells, PDX-1 stimulated the basal promoter by approx. 40-fold, whereas MafA, E47 and beta2 each increased activity by less than 2-fold. There was no indication of any synergistic effects on the human insulin promoter. On the other hand, the rat insulin 1 promoter and a mutated version of the human insulin promoter, in which the relevant regulatory elements were separated by the same distances as in the rat insulin 1 promoter, did exhibit synergy. PDX-1 was shown further to activate the endogenous insulin 1 gene in alphaTC1.6 cells, whereas MafA activated the insulin 2 gene. In combination, PDX-1 and MafA activated both insulin genes. Chromatin immunoprecipitation assays confirmed that PDX-1 increased the association of acetylated histones H3 and H4 with the insulin 1 gene and MafA increased the association of acetylated histone H3 with the insulin 2 gene.

Specification of Drosophila motoneuron identity by the combinatorial action of POU and LIM-HD factors

In both vertebrates and invertebrates, members of the LIM-homeodomain (LIM-HD) family of transcription factors act in combinatorial codes to specify motoneuron subclass identities. In the developing Drosophila embryo, the LIM-HD factors Islet (Tailup) and Lim3, specify the set of motoneuron subclasses that innervate ventral muscle targets. However, as several subclasses express both Islet and Lim3, this combinatorial code alone cannot explain how these motoneuron groups are further differentiated. To identify additional factors that may act to refine this LIM-HD code, we have analyzed the expression of POU genes in the Drosophila embryonic nerve cord. We find that the class III POU protein, Drifter (Ventral veinless), is co-expressed with Islet and Lim3 specifically in the ISNb motoneuron subclass. Loss-of-function and misexpression studies demonstrate that the LIM-HD combinatorial code requires Drifter to confer target specificity between the ISNb and TN motoneuron subclasses. To begin to elucidate molecules downstream of the LIM-HD code, we examined the involvement of the Beaten path (Beat) family of immunoglobulin-containing cell-adhesion molecules. We find that beat Ic genetically interacts with islet and Lim3 in the TN motoneuron subclass and can also rescue the TN fasciculation defects observed in islet and Lim3 mutants. These results suggest that in the TN motoneuron context, Islet and Lim3 may specify axon target selection through the actions of IgSF call-adhesion molecules.

Reversal of diabetes in the rat by injection of hematopoietic stem cells infected with recombinant adeno-associated virus containing the preproinsulin II gene

AIM

To study the effect of injecting hematopoietic stem cells containing the preproinsulin gene II (rI2) via recombinant adeno-associated virus (rAAV) into normal and streptozotocin-diabetic rats.

METHODS

rI2 was transfected into rat hematopoietic stem cells using rAAV vector. Stem cells were injected by intravenous route into normal and STZ-induced diabetic rats to study blood sugar and expression of rI2 in various tissues. The pLP-1 recombinant plasmid containing rI2 (vLP-1) was engineered as previously described. Bone marrow from female Wistar-Furth rats was enriched for stem cells by using plastic adherence and monoclonal antirat CD3 and CD45 RA to deplete T and B cells. The remaining cells were exposed to vLP-1 (multiplicity of infection MOI =50:1 or 100:1) for 2 h. Approximately ten million exposed stem cells were injected by intravenous route into each animal; there were four groups: normal animals at MOI 50:1 (group 1) or MOI 100:1 (group 2); group 3 animals (n = 9) were streptozotocin-induced diabetic animals at MOI 100:1. Animals that showed reversal of diabetes from group 3 were sacrificed for study of gene expression at weeks 1, 2, and 6, respectively. Control diabetic animals did not receive stem cells or virus constituted group 4. Expression of rI2 was analyzed by RT-PCR and Southern analyses.

RESULTS

Despite introduction of insulin gene, groups 1 and 2 had blood sugar concentrations that remained within normal levels, while 3 of 9 animals in group 3 showed reversal of diabetes; using RT-PCR,group 1 expressed rI2 in liver, spleen, thymus, brain, and heart at week 1 only. In group 2, rI2 was seen in the thymus up to 6 weeks; in diabetic animals (group 3) rI2 was seen in liver, bone marrow, spleen, thymus, and peripheral blood lymphocytes at week 2 and in thymus and lymphocytes at week 6.

CONCLUSIONS

We have shown that (1) rAAV is a useful vector for transferring rI2 into rat hematopoietic stem cells; (2) normal animals remained euglycemic after injection of stem cells containing rI2 despite identification in various tissues suggesting autoregulation, and (3) short-term reversal of diabetes was achieved in some animals by injection of stem cells containing rI2.

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

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

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

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

Pdx-1 is required for activation in vivo from a duodenum-specific enhancer

The purine metabolic gene adenosine deaminase (ADA) is expressed along a defined spatiotemporal pattern in the developing mammalian small intestine, where high-level expression is limited to the villous epithelium of the duodenum. This activation is observed in rodents as the intestine completes the final maturation resulting in adult crypt-villus structures at 2-3 weeks postpartum. A regulatory module responsible for this pattern of expression has been identified in the second intron of the human ADA gene. Of the multiple duodenal proteins that can interact with this small duodenal enhancer region, the studies contained in this work describe the identification of five of these proteins as the dispersed homeobox protein PDX-1. This transcription factor exhibits a profile of expression in the small intestine similar to that observed for ADA, making it an ideal candidate factor for the duodenum-specific ADA enhancer. Loss of PDX-1 binding, via a PDX-1 mutated enhancer transgenic construction, resulted in complete loss of high-level activation in the duodenum, demonstrating the absolute requirement for this factor in vivo. However, co-transfection experiments suggest that other proteins that bind the enhancer are also required for enhancer function because PDX-1 alone was incapable of significant transactivation.

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

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

A duodenum-specific enhancer regulates expression along three axes in the small intestine

Adenosine deaminase (ADA) is expressed at high levels in the epithelium of proximal small intestine. Transgenic mice were used to characterize the regulatory region governing this activation. A duodenum-specific enhancer is located in intron 2 of the human ADA gene at the central site among a cluster of seven DNase I-hypersensitive sites present in duodenal DNA. Flanking DNA, including the remaining hypersensitive sites, is required for consistent high-level enhancer function. The enhancer activates expression in a pattern identical to endogenous ADA along both the anterior-posterior axis of the small intestine and the crypt-villus differentiation axis of the intestinal epithelium. Timing of activation by the central enhancer mimics endogenous mouse ADA activation, occurring at 2-3 wk of age. However, two upstream DNA segments, one proximal and one distal, collaborate to change enhancer activation to a perinatal time point. Studies with duodenal nuclear extracts identified five distinct DNase I footprints within the enhancer. Protected regions encompass six putative binding sites for the transcription factor PDX-1, as well as proposed CDX, hepatocyte nuclear factor-4, and GATA-type sites.

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

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

Upstream stimulatory factor regulates Pdx-1 gene expression in differentiated pancreatic beta-cells

The homeobox gene Pdx-1 plays a key role in the development of the pancreas. In the adult, however, expression of the Pdx-1 gene is restricted to pancreatic beta-cells and endocrine cells of duodenal epithelium. Recently, the transcription factor, upstream stimulatory factor (USF), has been shown to bind in vitro to a mutationally sensitive E-box motif within the 5'-flanking region of the Pdx-1 gene [Sharma, Leonard, Lee, Chapman, Leiter and Montminy (1996) J. Biol. Chem. 271, 2294-2299]. In the present study, we show that USF not only binds to the Pdx-1 gene promoter but also functionally regulates the expression of the Pdx-1 gene in differentiated pancreatic beta-cells. Adenovirus-mediated overexpression of a dominant negative form of USF2 decreased binding of endogenous USF to the E-box element by approximately 90%. This reduction in endogenous USF binding led to a greater than 50% decrease in Pdx-1 gene promoter activity, which, in turn, resulted in marked reductions in Pdx-1 mRNA and protein levels. Importantly, the lower Pdx-1 protein levels led to a greater than 50% reduction in Pdx-1 binding activity to the A3 element on the insulin gene promoter, and a significant reduction in insulin mRNA levels. Overall, our results show that USF functionally regulates Pdx-1 gene expression in differentiated pancreatic beta-cells and provide the first functional data for a role of USF in the regulation of a normal cellular gene.

Glucose stimulates translocation of the homeodomain transcription factor PDX1 from the cytoplasm to the nucleus in pancreatic beta-cells

One of the mechanisms whereby glucose stimulates insulin gene transcription in pancreatic beta-cells involves activation of the homeodomain transcription factor PDX1 (pancreatic/duodenal homeobox-1) via a stress-activated pathway involving stress-activated protein kinase 2 (SAPK2, also termed RK/p38, CSBP, and Mxi2). In the present study we show, by Western blotting and electrophoretic mobility shift assay, that in human islets of Langerhans incubated in low glucose (3 mM) PDX1 exists as an inactive 31-kDa protein localized exclusively in the cytoplasm. Transfer of the islets to high (16 mM) glucose results in rapid (within 10 min) conversion of PDX1 to an active 46-kDa form that was present predominantly in the nucleus. Activation of PDX1 appears to involve phosphorylation, as shown by incorporation of 32Pi into the 46-kDa form of the protein. These effects of glucose could be mimicked by chemical stress (sodium arsenite), or by overexpression of SAPK2 in the beta-cell line MIN6. Overexpression of SAPK2 also stimulated PDX1-dependent transcription of a -50 to -250 region of the human insulin gene promoter linked to a firefly luciferase reporter gene. The effects of glucose were inhibited by the SAPK2 inhibitor SB 203580, and by wortmannin and LY 294002, which inhibit phosphatidylinositol 3-kinase, although the effects of stress (arsenite) were inhibited only by SB 203580. These results demonstrate that glucose regulates the insulin gene promoter through activation and nuclear translocation of PDX1 via the SAPK2 pathway.

Short-term regulation of insulin gene transcription by glucose

Whereas short-term regulation of insulin biosynthesis at the level of translation is well accepted, glucose-dependent transcriptional control is still believed to be a long-term effect occurring after more than 2 hr of glucose stimulation. Because pancreatic beta cells are exposed to elevated glucose levels for minutes rather than hours after food uptake, we hypothesized the existence of a short-term transcriptional control. By studying the dynamics of newly synthesized (prepro)insulin RNA and by employing on-line monitoring of gene expression in single, insulin-producing cells, we were able to provide convincing evidence that insulin gene transcription indeed is affected by glucose within minutes. Exposure of insulinoma cells and isolated pancreatic islets to elevated glucose for only 15 min resulted in a 2- to 5-fold elevation in (prepro)insulin mRNA levels within 60-90 min. Similarly, insulin promoter-driven green fluorescent protein expression in single insulin-producing cells was significantly enhanced after transient glucose stimulation. Thus, short-term signaling, such as that involved in insulin secretion, also may regulate insulin gene transcription.

Identification and characterization of a glucose-responsiveness region upstream of human insulin gene in transfected HIT-T 15 cells

To determine possible regulation of full-length human insulin gene promoter activity by glucose, we examined a 2-kilobase pair (kbp) 5'-flanking region of the human insulin gene and characterized the DNA elements in transfected HIT-T 15 cells. The expression of the 2-kilobase pair 5'-flanking region human insulin gene fused to the luciferase reporter gene occurred by transfection. In 0.8 mM glucose of the F-12 K medium, the element mediating the negative regulatory region was localized from -1782 to -1295 base pairs (bp) and stimulatory element from -1295 to -1138 bp. The elements from -1138 to -880 bp and from -356 to +252 bp possessed the elements dose-dependently responsive to 0.8 mM, 7.0 and 22.2 mM glucose. In fragment D, cotransfection of oligonucleotide that confers RIPE3b1 activator decreased the glucose-stimulated promoter activity, but the other oligonucleotide that confers STF-1 did not. The present data indicated that 2 kbp possesses glucose-responsive region in the element from -1138 to -880 bp, in addition to the previously reported element from -356 to initiation site. There may exist a RIPE3b1 activator binding site in the glucose-responsive element from -1138 to -880 bp. In addition, negatively regulatory region may exist from -1782 to -1295 bp.

Fatty acids decrease IDX-1 expression in rat pancreatic islets and reduce GLUT2, glucokinase, insulin, and somatostatin levels

IDX-1 (islet/duodenum homeobox-1) is a transcription factor expressed in the duodenum and pancreatic beta and delta cells. It is required for embryonic development of the pancreas and transactivates the Glut2, glucokinase, insulin, and somatostatin genes. Here we show that exposure of isolated rat pancreatic islets to palmitic acid induced a approximately 70% decrease in IDX-1 mRNA and protein expression as well as 40 and 65% decreases in the binding activity of IDX-1 for its cognate cis-regulatory elements of the Glut2 and insulin promoters, respectively. The inhibitory effect of palmitic acid required its mitochondrial oxidation since it was prevented by the carnitine palmitoyltransferase I inhibitor bromopalmitic acid. The palmitic acid effect on IDX-1 was correlated with decreases in GLUT2 and glucokinase expression of 40 and 25%, respectively, at both the mRNA and protein levels. Insulin and somatostatin mRNA expression was also decreased by 40 and 60%, whereas glucagon mRNA expression was not modified. After 48 h of exposure to fatty acids, total islet insulin, somatostatin, and glucagon contents were decreased by 85, 55, and 65%, respectively. At the same time, total hormone release was strongly stimulated (13-fold) for glucagon, whereas its was only marginally increased for insulin and somatostatin (1.5- and 1.7-fold, respectively). These results indicate that elevated fatty acid levels 1) negatively regulate Idx-1 expression; 2) decrease the expression of genes transactivated by IDX-1 such as those for GLUT2, glucokinase, insulin, and somatostatin; and 3) lead to an important increase in glucagon synthesis and secretion. Fatty acids thus have pleiotropic effects on pancreatic islet gene expression, and the negative control of Idx-1 expression may be an initial event in the development of these multiple defects.

Impaired expression of transcription factor IUF1 in a pancreatic beta-cell line derived from a patient with persistent hyperinsulinaemic hypoglycaemia of infancy (nesidioblastosis)

Persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI), or nesidioblastosis, is a rare disorder which may be familial or sporadic, and which is characterized by unregulated secretion of insulin and profound hypoglycaemia in the neonate. The defect has been linked in some patients to mutations in the sulphonyl urea receptor gene (SUR). The present study investigated potential defects in the regulation of the insulin gene by glucose in a beta-cell line (NES 2Y) derived from a patient with PHHI. The results show that the insulin promoter is unresponsive to glucose in PHHI, and that this defect can be attributed to impaired expression of the transcription factor IUF1. Because IUF1 is involved not only in linking glucose metabolism to the control of the insulin, but is also a major regulator of beta-cell differentiation during embryogenesis, we propose that impaired expression of IUF1 contributes to beta-cell dysfunction in PHHI by leading to abnormal beta-cell differentiation.

The p38/reactivating kinase mitogen-activated protein kinase cascade mediates the activation of the transcription factor insulin upstream factor 1 and insulin gene transcription by high glucose in pancreatic beta-cells

Insulin upstream factor 1 (IUF1), a transcription factor present in pancreatic beta-cells, binds to the sequence C(C/T)TAATG present at several sites within the human insulin promoter. Here we isolated and sequenced cDNA encoding human IUF1 and exploited it to identify the signal transduction pathway by which glucose triggers its activation. In human islets, or in the mouse beta-cell line MIN6, high glucose induced the binding of IUF1 to DNA, an effect mimicked by serine/threonine phosphatase inhibitors, indicating that DNA binding was induced by a phosphorylation mechanism. The glucose-stimulated binding of IUF1 to DNA and IUF1-dependent gene transcription were both prevented by SB 203580, a specific inhibitor of stress-activated protein kinase 2 (SAPK2, also termed p38 mitogen-activated protein kinase, reactivating kinase, CSBP, and Mxi2) but not by several other protein kinase inhibitors. Consistent with this finding, high glucose activated mitogen-activated protein kinase-activated protein kinase 2 (MAPKAP kinase-2) (a downstream target of SAPK2) in MIN6 cells, an effect that was also blocked by SB 203580. Cellular stresses that trigger the activation of SAPK2 and MAPKAP kinase-2 (arsenite, heat shock) also stimulated IUF1 binding to DNA and IUF1-dependent gene transcription, and these effects were also prevented by SB 203580. IUF1 expressed in Escherichia coli was unable to bind to DNA, but binding was induced by incubation with MgATP, SAPK2, and a MIN6 cell extract, which resulted in the conversion of IUF1 to a slower migrating form. SAPK2 could not be replaced by p42 MAP kinase, MAPKAP kinase-2, or MAPKAP kinase-3. The glucose-stimulated activation of IUF1 DNA binding and MAPKAP kinase-2 (but not the arsenite-induced activation of these proteins) was prevented by wortmannin and LY 294002 at concentrations similar to those that inhibit phosphatidylinositide 3-kinase. Our results indicate that high glucose (a cellular stress) activates SAPK2 by a novel mechanism in which a wortmannin/LY 294002-sensitive component plays an essential role. SAPK2 then activates IUF1 indirectly by activating a novel IUF1-activating enzyme.

Human insulin gene expression in transgenic mice: mutational analysis of the regulatory region

A mini-human insulin gene and four derivatives mutated at several regions potentially involved in the regulation of gene expression were used to generate transgenic mouse lines. The effect of these mutations on the efficiency of gene expression and cell specificity was studied using three approaches: (1) Northern blot analysis using total RNA from pancreas and other organs, (2) radioimmunoassay to detect the human C-peptide in urine samples, and (3) immunocytochemistry of pancreas sections to examine whether expression of the transgene was still specifically expressed in beta-cells. Mutation of the cis-acting elements located between -238 and -206 (GCII and CTII motifs) resulted in a strong decrease of gene expression in the pancreas of transgenic mice, but it did not lead to complete extinction of the transgene expression. This region alone (-255/-202), when linked to the minimal Herpes simplex virus thymidine kinase gene (tk) promoter, failed to activate chloramphenicol acetyltransferase (CAT) gene expression in transfected insulinoma cells, while it was activated by the equivalent region of the rat insulin I gene. On the contrary, mutation of the DNA motifs located between -109 and -75 (GCI and CTI) or between -323 and -297 (CTIII) did not significantly affect the level of the human insulin gene expression in transgenic mice. Replacement of the insulin promoter (-58/+l) by the tk promoter did not alter its level of expression in transgenic mice. In all instances, expression of the different transgenes remained localized in the islet beta-cells. Altogether, these results indicate that the GCII-CTII motif is an important regulatory element for efficient expression of the human insulin gene in vivo, although it alone does not allow gene expression as it would require the association of other elements.

Regulation of glucose transporters (GLUT-1 and GLUT-3) in human retinal endothelial cells

The regulation of glucose transporters (GLUT-1 and GLUT-3), in terms of both mRNA and protein, in human retinal endothelial cells was investigated. The cells responded within 1 h of exposure to 5 mM glucose with an increase in the level of GLUT-3 mRNA that was due to an increase in the transcription of the 4.1 kb mRNA of the gene for GLUT-3. In the absence of glucose, the gene for GLUT-1 was not transcribed but the level of GLUT-3 mRNA was increased in these conditions and this was the result of an increase in the transcription of the 4.1 kb mRNA. The level of GLUT-1 and GLUT-3 mRNA was maximal when the cells were exposed to 15 mM glucose. These results are discussed in the light of the glucose regulatory potential of the retinal microvasculature and the implications that this may have for the mechanisms of diabetic retinopathy.

Chronic exposure of betaTC-6 cells to supraphysiologic concentrations of glucose decreases binding of the RIPE3b1 insulin gene transcription activator

We have shown previously that chronic exposure of HIT-T15 cells to supraphysiologic glucose concentrations causes decreased insulin gene transcription and decreased binding activities of two beta-cell specific transcription factors, STF-1 and the RIPE3b1 activator, and have suggested that these events may provide a mechanism for glucose toxicity on beta-cell function. However, this contention can be criticized because it is not clear whether these observations are unique to the HIT-T15 cell or generalizable to other beta-cell lines and the islet. Therefore, we cultured betaTC-6 cells for up to 41 wk in either 11.1 or 0.8 mM glucose. We observed a passage-dependent decrease in insulin content and insulin mRNA levels in betaTC-6 cells chronically cultured in 11.1 mM glucose. Cells chronically cultured in 0.8 mM glucose had higher insulin mRNA levels than cells chronically cultured in 11.1 mM glucose. The relative activity of a chloramphenicol acetyl transferase (CAT) reporter gene controlled by the 5' regulatory region of the human insulin gene was decreased in late passage betaTC-6 cells chronically cultured in 11.1 mM glucose, but was preserved in late passages of cells chronically cultured in 0.8 mM glucose. Electromobility shift assays demonstrated that binding of a specific nuclear protein that recognizes the RIPE3b1 binding site of the insulin gene was markedly diminished in late passage cells chronically exposed to 11.1 mM glucose, whereas binding activities of STF-1 and ICE activators were unchanged. RIPE3b1 binding activity was preserved in late passage cells chronically exposed to 0.8 mM glucose. Mutation of the RIPE3b1 binding site almost completely abolished insulin gene transcription as well as binding activity. We conclude that chronic exposure of betaTC-6 cells to high glucose concentrations paradoxically decreases insulin gene transcription, in part, by decreasing activity of the trans-activating factor which binds to the RIPE3b1 sequence. This study uniquely demonstrates that altered binding to the RIPE3b1 sequence mediates glucose toxicity in betaTC-6 cells, thus reinforcing the importance of this cis-acting element in the regulation of insulin gene transcription. We conclude that the phenomenon of glucose toxicity decreasing binding of transcription factors and thereby reducing insulin gene expression is not a feature solely of HIT-T15 cells and may be demonstrable generally in beta-cell lines.

Function of the human insulin promoter in primary cultured islet cells

Pancreatic islet beta cells regulate the rate of insulin gene transcription in response to a number of nutrients, the most potent of which is glucose. To test for its regulation by glucose, the promoter sequence was isolated from the human insulin gene. When linked to chloramphenicol acetyltransferase and transfected into primary islet cultures, the human insulin promoter is activated by glucose. In parallel islet transfections, glucose also activates the L-pyruvate kinase and islet amyloid chain ketoacid dehydrogenase E1a promoter, but it does not affect the beta cell glucose kinase promoter. Using deletion and substitution mutations of the proximal human insulin promoter, we mapped a metabolic response element to the E box, E1, at -100 base pairs relative to the transcription start site. Although the isolated E1 element responds to glucose, inclusion of either of two AT-rich sequences, A1 or A2/C1 on either side of E1, results in dramatic synergistic activation. Inclusion of A2/C1 also increases the response to glucose. The A2-E1-A1 region alone, however, does not explain all of the activity of the human insulin promoter in cultured islets, and other transcriptionally important elements likely to contribute to the glucose response as well.

Insulin upstream factor 1 and a novel ubiquitous factor bind to the human islet amyloid polypeptide/amylin gene promoter

The islet amyloid polypeptide (IAPP) gene is expressed primarily in the islet beta-cell and the peptide is co-secreted with insulin. To investigate mechanisms important in its regulation, we have used the electrophoretic mobility-shift assay and methylation interference to determine systematically sites of DNA-protein interactions in the human IAPP promoter. We identified beta-cell-specific DNA-protein complexes at three sites, each of which contained a consensus binding site for insulin upstream factor I (IUF-I). This complex was displaced with an antiserum to IUF-1, confirming that IUF-1 binds to the human IAPP promoter in vitro. We have also identified a DNA-protein complex within the region -220/-250 in both beta- and non-beta-cell lines. This region contains a motif with partial identity with the binding site for the ubiquitous transcription factor upstream stimulatory factor (USF), which binds to the human insulin promoter. However, purified USF was not able to bind to this putative site in the IAPP promoter and an oligonucleotide containing a functional USF-binding site was unable to displace binding from the IAPP oligonucleotide. Methylation interference revealed that the DNA-protein complex binds to a sequence that overlaps the USE-like sequence, and may therefore be a novel helix-loop-helix protein. These results suggest that, although both IAPP and insulin are beta-cell peptides, IAPP contains regulatory regions both common to and distinct from insulin.

Role of glucose regulatory mechanisms in diabetic retinopathy

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Reduction of insulin gene transcription in HIT-T15 beta cells chronically exposed to a supraphysiologic glucose concentration is associated with loss of STF-1 transcription factor expression

Chronic exposure of HIT-T15 beta cells to elevated glucose concentrations leads to decreased insulin gene transcription. The reduction in expression is accompanied by diminished binding of a glucose-sensitive transcription factor (termed GSTF) that interacts with two (A+T)-rich elements within the 5' flanking control region of the insulin gene. In this study we examined whether GSTF corresponds to the recently cloned insulin gene transcription factor STF-1, a homeodomain protein whose expression is restricted to the nucleus of endodermal cells of the duodenum and pancreas. We found that an affinity-purified antibody recognizing STF-1 supershifted the GSTF activator complex formed from HIT-T15 extracts. In addition, we demonstrated a reduction in STF-1 mRNA and protein levels that closely correlated with the change in GSTF binding in HIT-T15 cells chronically cultured under supraphysiologic glucose concentrations. The reduction in STF-1 expression in these cells could be accounted for by a change in the rate of STF-1 gene transcription, suggesting a posttranscriptional control mechanism. In support of this hypothesis, no STF-1 mRNA accumulated in HIT-T15 cells passaged in 11.1 mM glucose. The only RNA species detected was a 6.4-kb STF-1 RNA species that hybridized with 5' and 3' STF-1-specific cDNA probes. We suggest that the 6.4-kb RNA represents an STF-1 mRNA precursor and that splicing of this RNA is defective in these cells. Overall, this study suggests that reduced expression of a key transcriptional regulatory factor, STF-1, contributes to the decrease in insulin gene transcription in HIT-T15 cells chronically cultured in supraphysiologic glucose concentration.

Identification and characterization of a functional retinoic acid/thyroid hormone-response element upstream of the human insulin gene enhancer

A deletion analysis of the human insulin gene extending to 2 kb upstream of the transcription start site provided evidence of regulatory sequences located upstream of the insulin-linked polymorphic region (ILPR). Within this ILPR-distal region is a sequence (Ink, for insulin kilobase upstream) which contains three potential nuclear hormone-receptor half-sites, closely matching the consensus sequence AGGTCA. These sequences are arranged as a palindromic element with zero spacing over-lapping a direct repeat with 2 bp spacing. The Ink sequence was used in electrophoretic mobility-shift assays within nuclear extracts from COS-7 cells overexpressing the vitamin D, thyroid hormone or retinoic acid receptors, or from an insulin-expressing hamster cell line, HIT-T15. These studies suggest that the insulin-expressing cell line contains thyroid hormone and retinoic acid receptors at least, and that these receptors are able to recognize the Ink sequence. Three copies of the Ink sequence were placed upstream of the thymidine kinase promoter and firefly luciferase reporter gene. In COS-7 cells expressing the appropriate nuclear hormone receptor, this construct was responsive to both thyroid hormone (18-fold) and all-trans-retinoic acid (31-fold). In HIT-T15 cells the same construct responded to all-trans-retinoic acid, but not to thyroid hormone. Within the context of a 2 kb insulin gene fragment, the Ink sequence was shown to be activated by retinoic acid and by the retinoic acid receptor, but acted as a negative element in the presence of both retinoic acid and the retinoic acid receptor. Mutagenesis studies demonstrated that the palindromic sequence was important for the retinoic acid response, and for binding of complexes containing retinoic acid receptor. In human islets of Langerhans, retinoic acid was shown to stimulate insulin mRNA levels. These results demonstrate that a functional nuclear hormone-receptor-response element is located upstream of the human ILPR. As retinoic acid and thyroid hormone are frequently involved in developmental regulatory processes, it is possible that this element may be important in the process of islet cell differentiation.

Functional analysis of a newly identified TAAT-box of the rat insulin-II gene promoter

Transcriptional regulation of insulin gene expression is achieved by an interplay of tissue-specific and ubiquitous cis- and trans-acting elements. E-box like motifs and TAAT-motifs were shown to play a crucial role in initiating insulin gene transcription. Studying the AT-rich region of the rat insulin-II promoter between nucleotides -212 and -196, we observed a base difference at -211, an adenosine instead of a cytidine, compared to the previously reported sequence (EMBL Accession No. J00748). Sequence analysis of promoter fragments from different rat strains showed that adenosine at position -211 represents the wild type (EMBL Accession No. X82162). This base exchange leads to the formation of an additional TAAT-motif, i.e. TAAT3, at the complementary DNA strand directly upstream of the previously studied TAAT2 motif, formerly named CT-2. Here we show that the newly identified motif TAAT3 is involved in (i) transcriptional control in vivo, (ii) in vitro DNA/protein interactions, and that (iii) TAAT1, TAAT2 and TAAT3 are binding sites for the homeodomain-containing factor IPF-1.

Tissue-specific expression of the rat insulin 1 gene in vivo requires both the enhancer and promoter regions

The tissue specificity conferred by cis-acting regulatory elements of the rat insulin 1 gene was examined in both cultured cells and transgenic mice. The enhancer region (-346/-103) coupled to a ubiquitous promoter activated expression of a reporter gene in insulinoma cells but not in fibroblasts, in agreement with our previous work, and the specific expression was limited to a subregion containing the FAR and FLAT elements (-252/-199). In transgenic mice, however, this FAR-FLAT minienhancer alone failed to activate a reporter gene. Under the same conditions, in vivo, the enhancer (-346/-103) activated gene expression, but did not confer complete pancreatic specificity. The transgene, in this case, was expressed in pancreas and also in brain. Reassociation of the rat insulin 1 promoter (-102/+9) with the enhancer (-346/-103) prevented expression in brain and thus restored pancreatic specificity. All of these observations indicate that tissue-specific expression of the rat insulin 1 gene, in vivo, results from interaction of multiple sequence elements and not from any single minimal sequence.

Expression of a human insulin transgene in cholinergic neurons of the mouse medial habenula

We explored the possibility that an insulin gene deleted in its 5'-flanking region is expressed in adult mouse brain. We used three independent lines of mice carrying a human insulin transgene which included the insulin gene transcription unit flanked by 168 base pairs upstream and 5.5 kb downstream. Using a reverse transcription-polymerase chain reaction assay, human insulin mRNAs were detected in whole brain extracts. In all three lines, human insulin mRNAs were localized by in situ hybridization in a single cerebral site, the medial habenula. With a monoclonal antibody specific for human C-peptide and human proinsulin, labelling was restricted to a subset of habenular cholinergic neurons, with rare immunostained fibers. No labelling was observed in the projection fibers of the retroflexus fasciculus or in their axon terminals in the interpeduncular nucleus. Electron microscope studies suggested that the transgene expressing cells. These findings demonstrate that the human insulin transgene tested here includes a habenula specific promoter which could be useful for physiological and molecular studies on the habenula.

Pancreatic beta cells express a diverse set of homeobox genes

Homeobox genes, which are found in all eukaryotic organisms, encode transcriptional regulators involved in cell-type differentiation and development. Several homeobox genes encoding homeodomain proteins that bind and activate the insulin gene promoter have been described. In an attempt to identify additional beta-cell homeodomain proteins, we designed primers based on the sequences of beta-cell homeobox genes cdx3 and lmx1 and the Drosophila homeodomain protein Antennapedia and used these primers to amplify inserts by PCR from an insulinoma cDNA library. The resulting amplification products include sequences encoding 10 distinct homeodomain proteins; 3 of these proteins have not been described previously. In addition, an insert was obtained encoding a splice variant of engrailed-2, a homeodomain protein previously identified in the central nervous system. Northern analysis revealed a distinct pattern of expression for each homeobox gene. Interestingly, the PCR-derived clones do not represent a complete sampling of the beta-cell library because no inserts encoding cdx3 or lmx1 protein were obtained. Beta cells probably express additional homeobox genes. The abundance and diversity of homeodomain proteins found in beta cells illustrate the remarkable complexity and redundancy of the machinery controlling beta-cell development and differentiation.

Transcriptional regulation of the human insulin gene is dependent on the homeodomain protein STF1/IPF1 acting through the CT boxes

Insulin gene transcription is a unique feature of the pancreatic beta cells and is increased in response to glucose. The recent cloning of insulin promoter factor 1 (IPF1) and somatostatin transcription factor 1 (STF1) unexpectedly revealed that these are mouse and rat homologues of the same protein mediating transactivation through binding of CT box-like elements in rat insulin 1 and somatostatin promoter/enhancer regions, respectively. By using oligonucleotides representing each of the three CT boxes of the human insulin (HI) gene enhancer and nuclear extracts from the mouse islet tumor cell lines beta TC3 and alpha TC1, we have identified a beta-cell-specific binding activity as reported for IPF1, which has maximal affinity toward the CT2 box. However, in pluripotent, HI-transfected rat islet tumor cells, NHI-6F, this binding activity is present prior to induction of (human) insulin gene transcription. Its migration is identical to that of in vitro translated STF1 in electrophoretic mobility-shift assays; it is specifically recognized by anti-STF1 antibodies and has an apparent molecular mass of 46 kDa. Mutation of the CT2 box decreases transcriptional activity of a HI reporter plasmid by approximately 65% in beta TC3 cells and blocks the glucose response in isolated newborn rat islet cells. Furthermore, cotransfection with STF1 cDNA into the glucagon-producing alpha TC1 cells increases the activity of the HI enhancer 4- to 5-fold, suggesting that STF1/IPF1 can confer on alpha TC1 cells the ability to transcribe the HI gene. We conclude that STF1/IPF1 is a necessary but not sufficient key regulator of insulin gene activity, possibly also involved in glucose-regulated transcription.