Insulin enhancer binding protein has helix-loop-helix structure

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.

Molecular characterization of the rat insulin enhancer-binding complex 3b2. Cloning of a binding factor with putative helicase motifs

Cell-specific expression of the rat insulin II gene is in part mediated through an element located in the 5'-flanking region. The element, termed RIPE3b (-126 to -101), confers beta-cell-specific expression in conjunction with an adjacent element RIPE3a (-110 to -86). Here we report the characterization of one of the RIPE3b-binding complexes, 3b2. UV cross-linking analysis demonstrated that it is composed of at least three polypeptides: p58, p62, and p110. Furthermore, a cDNA was isolated via expression screening for binding to RIPE3b. Sequence analysis reveals that the encoded protein, designated Rip-1, possessed putative helicase motifs and a potential transcription activation domain. Overexpression of Rip-1 in cells greatly enhances the 3b2 binding complex, suggesting that Rip-1 is involved in the binding of 3b2.

The insulin enhancer binding site 2 (IEB2; FAR) box of the insulin gene regulatory region binds at least three factors that can be distinguished by their DNA binding characteristics

Located at approximately 230 bp upstream from the transcription start site, the insulin enhancer binding site 2 (IEB2) or FAR region of the insulin gene is one of several important sequences involved in regulating transcription of the gene. The present study was undertaken to characterize the transcription factors binding at the IEB2/FAR region of the rat insulin II gene and to compare these with factors known to bind to the equivalent sequence in the rat I and human insulin genes. An endocrine-enriched factor, EFD3, was identified, which bound to the sequence CAGGAG. A second factor (D4) was identified as the widely expressed factor USF (upstream stimulating factor), while a third factor (D5) remained largely uncharacterized. The binding affinities of these three factors differed in the three genes, suggesting that the role of the IEB2/FAR sequence may vary subtly between the rat insulin II, rat insulin I and human insulin genes.

Glucose modulates the binding activity of the beta-cell transcription factor IUF1 in a phosphorylation-dependent manner

In the human insulin gene, three regulatory sequences upstream of the transcription start site at -77 (the CT1 box), -210 (the CT2 box), and -315 (the CT3 box) bind a beta-cell-specific transcription factor, IUF1. Recent studies have mapped a glucose response element to a CT-like sequence in the rat insulin I gene. The present study was therefore undertaken to ascertain the role of IUF1 in glucose-stimulated insulin gene transcription. IUF1-binding activity was measured by electrophoretic mobility shift assay using the CT2 box as probe. When freshly isolated rat islets of Langerhans were incubated in medium containing low concentrations (3 mM) of glucose IUF1 activity fell to undetectable levels within 6 h. In high (20 mM) glucose IUF1 activity remained constant over a 24 h period. The loss of IUF1 activity was reversible. Thus when islets were incubated for 4 h in low glucose and transferred to high glucose, IUF1 levels recovered within 15 min. This effect was dependent on glucose metabolism as it was inhibited by mannoheptulose. Incubation of islets for 4 h in low concentrations of glucose supplemented with phosphatase inhibitors prevented the fall in IUF1 activity. No recovery in IUF1 activity was observed when islets were treated for 4 h with low glucose and then for a further 1 h with low glucose and dibutyryl cyclic AMP, or forskolin, or the phorbol ester phorbol 12-myristate 13-acetate. These results demonstrate that the IUF1-binding activity in islets of Langerhans is modulated by glucose in a phosphorylation-dependent manner, and that protein kinase A or protein kinase C are not involved. Finally, IUF1 was shown to be immunologically related to a recently cloned factor, IPF1, that binds to a CT-like sequence in the rat insulin I gene promoter.

Transcription of the insulin gene: towards defining the glucose-sensitive cis-element and trans-acting factors

Previous work has shown that the sequence -196 to -247 of the rat insulin I gene mediates the stimulatory effect of glucose in fetal islets. We have used adult rat and human islets to delineate the glucose-sensitive cis-element to the sequence -193 to -227. In electrophoretic mobility shift assays, a 22 bp nucleotide corresponding to the sequence -206 to -227 bound all the nuclear proteins that could be bound by the entire minienhancer sequence -196 to -247. The rat insulin I sequence -206 to -227 formed three major complexes; in contrast, the corresponding human insulin sequence formed one single band with human and rat islet nuclear extracts, corresponding to the complex C1 of the rat insulin gene. Incubation of islets with varying glucose levels resulted in a dose-dependent increase in the intensity of the C1 band, while the other nuclear complexes formed with the insulin sequence, or the AP1 and SP1 binding activities used as control, were glucose insensitive. This is thus the first demonstration of a physiologic glucose-sensitive trans-acting factor for the insulin gene, whose further study may markedly enhance our understanding of the regulation of insulin biosynthesis in normal and diabetic beta cells. Furthermore, once cloned, the introduction of this glucose sensitive factor may enable the construction of truly physiologic artificial beta cells.

Positive and negative regulatory elements are involved in transcriptional control of the rat glucokinase gene in the insulin producing cell line HIT M2.2.2

Nested deletion mutants of the 5' flanking region of the beta-cell transcription unit of the rat glucokinase gene (r beta GK) were fused to the CAT-reporter gene. Transient expression studies in HIT M2.2.2 and BHK21 cells revealed a distal (upstream of -359) and a proximal promoter region (between -278/-49) harbouring positive and negative regulatory elements. DNaseI footprinting revealed three protected areas between nucleotides -190 and -60. DNA-elements playing a crucial role in transcriptional control of the insulin genes (IEB- and CT-motifs) have been detected within the proximal promoter region and contribute to beta-cell specific gene regulation. 3' deletion analysis revealed that DNA-elements located downstream from transcription initiation sites (up to +123) contribute to transcriptional regulation.

The helix-loop-helix transcription factor USF (upstream stimulating factor) binds to a regulatory sequence of the human insulin gene enhancer

Two important sequence elements, designated insulin enhancer binding site 1 (IEB1) or NIR and IEB2 or FAR, are involved in regulating expression of the rat insulin I gene. These elements bind a helix-loop-helix transcription factor, insulin enhancer factor 1 (IEF1). The IEB1 site is highly conserved among insulin genes but the IEB2 site is not conserved. To investigate the factors binding at the equivalent IEB1 and IEB2 sites in the human insulin gene enhancer, electrophoretic mobility shift assays were performed using a variety of cell extracts and probes specific for the homologous IEB1 and IEB2 sites. The results indicate that a factor with similar tissue distribution and binding characteristics to those of IEF1 binds to the IEB1 site in the human insulin gene, but that a separate factor, identified as the adenovirus major late transcription factor [MLTF, or upstream stimulating factor (USF)] binds to the IEB2 site.

Human insulin gene enhancer-binding proteins in pancreatic alpha and beta cell lines

Electrophoretic mobility shift assays were performed using oligonucleotides corresponding to known protein binding sites within the human insulin gene enhancer and nuclear extracts from mouse pancreatic alpha and beta cell lines. The results demonstrate that a previously described factor, IUF-1, binds to three sites at -82 (the CT1 box), -215 (the CT2 box), and -319 (the CT3 box) in the human insulin gene enhancer. IUF-1 was present only in beta but not in alpha cells, while all other DNA-binding proteins were present in both cell lines. IUF-1 may therefore be an important determinant of insulin gene beta cell-specific expression.

Chronic exposure of HIT cells to high glucose concentrations paradoxically decreases insulin gene transcription and alters binding of insulin gene regulatory protein

Chronically culturing HIT-T15 cells in media containing high glucose concentrations leads to decreased insulin mRNA levels, insulin content, and insulin secretion. These changes can be prevented by culturing the cells in media containing lower glucose levels (Robertson, R. P., H.-J. Zhang, K. L. Pyzdrowski, and T. F. Walseth. 1992. J. Clin. Invest. 90:320-325). The mechanism of this seemingly paradoxical phenomenon was examined by transiently transfecting HIT cells with a chloramphenicol acetyl transferase (CAT) reporter gene controlled by the 5'-regulatory domain of the human insulin gene (INSCAT). Early passages of HIT cells readily expressed INSCAT, whereas late passages of cells chronically cultured in 11.1 mM glucose expressed only 28.7 +/- 2.3% (mean +/- SEM) of the CAT activity expressed in early passages. In contrast, late passages of HIT cells chronically cultured in 0.8 mM glucose retained the ability to express the INSCAT reporter gene to 69.6 +/- 10.0% of the CAT activity observed in early passages. The decrease in INSCAT expression in late passages of cells serially cultured in 11.1 mM glucose was associated with the inability to form a specific nuclear protein-DNA complex with the CT motifs of the human insulin promoter. Formation of this specific protein-DNA complex was preserved in late passages of HIT cells when serially cultured in 0.8 mM glucose. Mutations of the CT motifs caused markedly diminished CAT activity in all passages examined. These data indicate that chronic exposure of the beta cell to high glucose concentrations can paradoxically decrease insulin gene transcription, in part, by altering the ability of a regulatory protein (GSTF) to interact with the insulin gene promoter. This provides a potential mechanism for glucotoxic effects on the beta cell at the level of the insulin gene.

An insulinoma nuclear factor binding to GGGCCC motifs in human insulin gene

Cell specific expression of the insulin gene is achieved through transcriptional mechanisms operating on 5' flanking DNA elements. In the enhancer of rat I insulin gene, two elements, the Nir and Far boxes, located at positions -104 and -233 respectively and containing the same octameric motif are essential for B cell specific transcription activity. Homologous sequences are present in the human insulin gene. While studying the binding of nuclear proteins from insulinoma cells to the -258/+241 region of the human insulin gene, we observed a previously undetected protein binding site in the intron I region between nucleotides +160 and +175. The binding activity was present in insulin producing cells such as RIN and HIT insulinoma cells but not in fibroblasts or insulin negative fibroblast x RIN hybrid cells. DNAse I footprinting and gel retardation/methylation interference experiments allowed us to define the core binding site of the intron binding factor as a GGGCCC hexamer. This factor is also capable to bind to a related sequence, contiguous to the Far-like element in rat and human insulin genes. The binding of the GGGCCC binding factor in this critical region of the insulin gene enhancer may participate in the regulation of insulin gene expression.

Regulatory regions of rat insulin I gene necessary for expression in transgenic mice

Ten transgenic mouse lines harboring the -346/-103 fragment of the rat insulin I enhancer linked to a heterologous promoter and a reporter gene (Eins-Ptk-CAT construct) were produced. Expression of the hybrid transgene was essentially observed in pancreas and to a lesser extent in brain. These results indicate that the rat insulin I promoter is dispensable for pancreatic expression. This insulin gene sequence is the shortest fragment described as conferring tissue-specific expression in transgenic mice. Two short homologous sequences in the rat insulin I enhancer fragment used, IEB2 and IEB1, have been described as playing a dominant role in the regulation of HIT hamster insulinoma cell-specific transcription of the insulin gene (1). We investigated whether the combination of IEB2 and IEB1 sequences is sufficient to confer specific expression in transgenic mice to a IEB2-IEB1-Ptk-CAT gene construct. No CAT activity was observed neither in pancreas nor in any other organ examined in 19 different transgenic mice. Moreover in transient expression experiments in RIN2A rat insulinoma cells, the IEB sequences had a very weak or no enhancer activity. These observations contribute to the conclusion that DNA regulatory elements other than the IEB sequences are necessary for gene expression in vivo.

Two proteins act as the IUF1 insulin gene enhancer binding factor

IUF1 is a pancreatic beta cell-specific factor which binds to the sequence 5'-CPyCTAATG-3' (CT box) within the human insulin gene enhancer. Here we show that IUF1 is composed of 2 binding activities that can be separated by DEAE ion exchange chromatography. South Western blot analysis indicates that these distinct binding activities have apparent molecular weights of 115 kDa and 46 kDa.

Insulin gene expression in nonexpressing cells appears to be regulated by multiple distinct negative-acting control elements

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located between nucleotides -340 and -91 relative to the transcription start site. Transcription from the enhancer is controlled by both positive- and negative-acting cellular factors. Cell-type-specific expression is mediated principally by a single cis-acting enhancer element located between -100 and -91 in the rat insulin II gene (referred to as the insulin control element [ICE]), which is acted upon by both of these cellular activities. Analysis of the effect of 5' deletions within the insulin enhancer has identified a region between nucleotides -217 and -197 that is also a site of negative control. Deletion of these sequences from the 5' end of the enhancer leads to transcription of the enhancer in non-insulin-producing cells, even though the ICE is intact. Derepression of this ICE-mediated effect was shown to be due to the binding of a ubiquitously distributed cellular factor to a sequence element which resides just upstream of the ICE (i.e., between nucleotides -110 and -100). We discuss the possible relationship of these results to cell-type-specific regulation of the insulin gene.

Insulin gene expression in nonexpressing cells appears to be regulated by multiple distinct negative-acting control elements

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located between nucleotides -340 and -91 relative to the transcription start site. Transcription from the enhancer is controlled by both positive- and negative-acting cellular factors. Cell-type-specific expression is mediated principally by a single cis-acting enhancer element located between -100 and -91 in the rat insulin II gene (referred to as the insulin control element [ICE]), which is acted upon by both of these cellular activities. Analysis of the effect of 5' deletions within the insulin enhancer has identified a region between nucleotides -217 and -197 that is also a site of negative control. Deletion of these sequences from the 5' end of the enhancer leads to transcription of the enhancer in non-insulin-producing cells, even though the ICE is intact. Derepression of this ICE-mediated effect was shown to be due to the binding of a ubiquitously distributed cellular factor to a sequence element which resides just upstream of the ICE (i.e., between nucleotides -110 and -100). We discuss the possible relationship of these results to cell-type-specific regulation of the insulin gene.

Pancreatic beta-cell-type-specific transcription of the insulin gene is mediated by basic helix-loop-helix DNA-binding proteins

The pancreatic beta-cell-specific expression of the insulin gene is mediated, at least in part, by the interaction of unique trans-acting beta-cell factors with a cis-acting DNA element found within the insulin enhancer (5'-GC CATCTG-3'; referred to as the insulin control element [ICE]) present in the rat insulin II gene between positions -100 and -91. This sequence element contains the consensus binding site for a group of DNA-binding transcription factors called basic helix-loop-helix proteins (B-HLH). As a consequence of the similarity of the ICE with the DNA sequence motif associated with the cis-acting elements of the B-HLH class of binding proteins (CANNTG), the ability of this class of proteins to regulate cell-type-specific expression of the insulin gene was addressed. Cotransfection experiments indicated that overexpression of Id, a negative regulator of B-HLH protein function, inhibits ICE-mediated activity. Antibody to the E12/E47 B-HLH proteins attenuated the formation, in vitro, of a previously described (J. Whelan, S. R. Cordle, E. Henderson, P. A. Weil, and R. Stein, Mol. Cell. Biol. 10:1564-1572, 1990) beta-cell-specific activator factor(s)-ICE DNA complex. Both of these B-HLH proteins (E12 and E47) bound efficiently and specifically to the ICE sequences. The role of B-HLH proteins in mediating pancreatic beta-cell-specific transcription of the insulin gene is discussed.

Pancreatic beta-cell-type-specific transcription of the insulin gene is mediated by basic helix-loop-helix DNA-binding proteins

The pancreatic beta-cell-specific expression of the insulin gene is mediated, at least in part, by the interaction of unique trans-acting beta-cell factors with a cis-acting DNA element found within the insulin enhancer (5'-GC CATCTG-3'; referred to as the insulin control element [ICE]) present in the rat insulin II gene between positions -100 and -91. This sequence element contains the consensus binding site for a group of DNA-binding transcription factors called basic helix-loop-helix proteins (B-HLH). As a consequence of the similarity of the ICE with the DNA sequence motif associated with the cis-acting elements of the B-HLH class of binding proteins (CANNTG), the ability of this class of proteins to regulate cell-type-specific expression of the insulin gene was addressed. Cotransfection experiments indicated that overexpression of Id, a negative regulator of B-HLH protein function, inhibits ICE-mediated activity. Antibody to the E12/E47 B-HLH proteins attenuated the formation, in vitro, of a previously described (J. Whelan, S. R. Cordle, E. Henderson, P. A. Weil, and R. Stein, Mol. Cell. Biol. 10:1564-1572, 1990) beta-cell-specific activator factor(s)-ICE DNA complex. Both of these B-HLH proteins (E12 and E47) bound efficiently and specifically to the ICE sequences. The role of B-HLH proteins in mediating pancreatic beta-cell-specific transcription of the insulin gene is discussed.