Expression of murine STF-1, a putative insulin gene transcription factor, in beta cells of pancreas, duodenal epithelium and pancreatic exocrine and endocrine progenitors during ontogeny

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

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

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

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

In vitro cultivation of human islets from expanded ductal tissue

A major obstacle to successful islet transplantation for both type 1 and 2 diabetes is an inadequate supply of insulin-producing tissue. This need for transplantable human islets has stimulated efforts to expand existing pancreatic islets and/or grow new ones. To test the hypothesis that human adult duct tissue could be expanded and differentiated in vitro to form islet cells, digested pancreatic tissue that is normally discarded from eight human islet isolations was cultured under conditions that allowed expansion of the ductal cells as a monolayer whereupon the cells were overlaid with a thin layer of Matrigel. With this manipulation, the monolayer of epithelial cells formed three-dimensional structures of ductal cysts from which 50-to 150- micrometer diameter islet-like clusters of pancreatic endocrine cells budded. Over 3-4 weeks culture the insulin content per flask increased 10- to 15-fold as the DNA content increased up to 7-fold. The cultivated human islet buds were shown by immunofluorescence to consist of cytokeratin 19-positive duct cells and hormone-positive islet cells. Double staining of insulin and non-beta cell hormones in occasional cells indicated immature cells still in the process of differentiation. Insulin secretion studies were done over 24 h in culture. Compared with their basal secretion at 5 mM glucose, cysts/cultivated human islet buds exposed to stimulatory 20 mM glucose had a 2.3-fold increase in secreted insulin. Thus, duct tissue from human pancreas can be expanded in culture and then be directed to differentiate into glucose responsive islet tissue in vitro. This approach may provide a potential new source of pancreatic islet cells for transplantation.

Adult insulin- and glucagon-producing cells differentiate from two independent cell lineages

To analyze cell lineage in the pancreatic islets, we have irreversibly tagged all the progeny of cells through the activity of Cre recombinase. Adult glucagon alpha and insulin beta cells are shown to derive from cells that have never transcribed insulin or glucagon, respectively. Also, the beta-cell progenitors, but not alpha-cell progenitors, transcribe the pancreatic polypeptide (PP) gene. Finally, the homeodomain gene PDX1, which is expressed by adult beta-cells, is also expressed by alpha-cell progenitors. Thus the islet alpha- and beta-cell lineages appear to arise independently during ontogeny, probably from a common precursor.

Homeobox genes and gut development

The gut of vertebrates exhibits a common anteroposterior regional differentiation. The role of homeobox genes in establishing this pattern is inferred by their sites of expression. It is suggested that the primary source of positional information is in the endoderm, which subsequently establishes a 'dialogue' with the surrounding visceral layer of the lateral plate mesoderm. This results in the anatomical and physiological specialization of the adult gut.

Pax4 represses pancreatic glucagon gene expression

The paired box and homeodomain containing transcription factors Pax4 and Pax6 are known to be essential for development of the pancreatic endocrine cells. In this report we demonstrate that stable expression of Pax4 in a rat glucagon-producing cell line inhibits the endogenously expressed glucagon gene completely. Furthermore, Pax4 represses Pax6 independent transcription of the insulin promoter, suggesting that Pax4 can actively repress transcription in addition to acting by competition with the transcriptional activator Pax6.

Pax genes and the differentiation of hormone-producing endocrine cells in the pancreas

Despite the pivotal role of the pancreas in hormonally-regulated pathways in the body, e.g. glucose homeostasis, the genetic mechanisms defining it have for many years remained largely enigmatic. After years out of the spotlight, pancreas development has once again come to centre stage. To a large extent, this is due to recent advances made through the detailed analysis of transgenic mice which have been engineered to carry mutations in specific developmental control genes. This review specifically focuses on the specification of the endocrine pancreas lineage and in particular on the role of the developmental control genes Pax4 and Pax6 in the generation of specific endocrine cell types. The comparison of various phenotypes of different mouse mutants affecting endocrine development supports a model in which Pax4 and Pax6 are required for the differentiation of certain endocrine cell lineages and implies a potential for acting at different levels of endocrine development.

Multiple polypeptide hormone expression in pancreatic islet cell carcinomas derived from phosphoenolpyruvatecarboxykinase-SV40 T antigen transgenic rats

Transgenic rats carrying a PEPCK-SV40 large T-antigen (TAg) transgene rapidly develop numerous pancreatic islet cell neoplasms, the cells of which express TAg. Although many of the larger neoplasms contain relatively undifferentiated cells, many tumors contain areas of well-differentiated cells with abundant endoplasmic reticulum (ER) and secretory granules for endocrine hormones like those observed in normal pancreatic islets. In the well-differentiated lesions, glucagon-producing alpha-cells, insulin-producing beta-cells, and somatostatin-producing delta-cells are readily identifiable morphologically under the electron microscope. Beta-cells were observed in all normal and hyperplastic islets, and nests of these cells were scattered throughout the larger neoplasms. These nests varied from small clusters of epithelium-like cells that stain intensely for insulin, to sheets of small, basophilic cells that stain more diffusely for the hormone. Alpha-cells were also present in all of the normal and hyperplastic islets, but in larger hyperplastic islets, the peripheral localization was absent. Larger neoplasms contained many nests of glucagon-expressing cells, as well as scattered glucagon-producing single cells. Delta-cells were rarely observed in the hyperplastic islets and in the neoplasms. Blood-glucose levels were unaltered in the transgenic animals relative to their nontransgenic litter mates. Thus although these islet cell neoplasms express several polypeptide hormones, there is no obvious clinical effect of such expression in vivo.

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

To identify potential transactivators of pdx-1, we sequenced approximately 4.5 kilobases of the 5' promoter region of the human and chicken homologs, assuming that sequences conserved with the mouse gene would contain critical cis-regulatory elements. The sequences associated with hypersensitive site 1 (HSS1) represented the principal area of homology within which three conserved subdomains were apparent: area I (-2694 to -2561 base pairs (bp)), area II (-2139 to -1958 bp), and area III (-1879 to -1799 bp). The identities between the mouse and chicken/human genes are very high, ranging from 78 to 89%, although only areas I and III are present within this region in chicken. Pancreatic beta cell-selective expression was shown to be controlled by mouse and human area I or area II, but not area III, from an analysis of pdx-1-driven reporter activity in transfected beta- and non-beta cells. Mutational and functional analyses of conserved hepatic nuclear factor 3 (HNF3)-like sites located within area I and area II demonstrated that activation by these regions was mediated by HNF3beta. To determine if a similar regulatory relationship might exist within the context of the endogenous gene, pdx-1 expression was measured in embryonic stem cells in which one or both alleles of HNF3beta were inactivated. pdx-1 mRNA levels induced upon differentiation to embryoid bodies were down-regulated in homozygous null HNF3beta cells. Together, these results suggest that the conserved sequences represented by areas I and II define the binding sites for factors such as HNF3beta, which control islet beta cell-selective expression of the pdx-1 gene.

Expansion of Pdx1-expressing pancreatic epithelium and islet neogenesis in transgenic mice overexpressing transforming growth factor alpha

BACKGROUND & AIMS

The progenitor cells responsible for transforming growth factor (TGF)-alpha-induced pancreatic ductal metaplasia and neoplasia remain uncharacterized. During pancreatic development, differentiated cell types arise from ductal progenitor cells expressing the Pdx1 homeodomain transcription factor. The aims of this study were, first, to evaluate the role of Pdx1-expressing stem cells in MT-TGFalpha transgenic mice, and second, to further characterize cell proliferation and differentiation in this model.

METHODS

To assess Pdx1 gene expression in normal and metaplastic epithelium, we performed in vivo reporter gene analysis using heterozygous Pdx1(lacZ/+) and bigenic Pdx1(lacZ/+)/MT-TGFalpha mice.

RESULTS

Pdx1(lacZ/+)/MT-TGFalpha bigenics showed up-regulated Pdx1 expression in premalignant metaplastic ductal epithelium. In addition to Pdx1 gene activation, TGF-alpha-induced metaplastic epithelium demonstrated a pluripotent differentiation capacity, as evidenced by focal expression of Pax6 and initiation of islet cell neogenesis. The majority of Pdx1-positive epithelial cells showed no expression of insulin, similar to the pattern observed during embryonic development.

CONCLUSIONS

Overexpression of TGF-alpha induces expansion of a Pdx1-expressing epithelium characterized by focal expression of Pax6 and initiation of islet neogenesis. These findings suggest that premalignant events induced by TGF-alpha in mouse pancreas may recapitulate a developmental program active during embryogenesis.

Pbx-Hox heterodimers recruit coactivator-corepressor complexes in an isoform-specific manner

Homeobox (hox) proteins have been shown to regulate cell fate and segment identity by promoting the expression of specific genetic programs. In contrast to their restricted biological action in vivo, however, most homeodomain factors exhibit promiscuous DNA binding properties in vitro, suggesting a requirement for additional cofactors that enhance target site selectivity. In this regard, the pbx family of homeobox genes has been found to heterodimerize with and thereby augment the DNA binding activity of certain hox proteins on a subset of potential target sites. Here we examine the transcriptional properties of a forced hox-pbx heterodimer containing the pancreas-specific orphan homeobox factor pdx fused to pbx-1a. Compared to the pdx monomer, the forced pdx-pbx1a dimer, displayed 10- to 20-fold-higher affinity for a consensus hox-pbx binding site but was completely unable to bind a hox monomer recognition site. The pdx-pbx dimer stimulated target gene expression via an N-terminal trans-activation domain in pdx that interacts with the coactivator CREB binding protein. The pdx-pbx dimer was also found to repress transcription via a C-terminal domain in pbx-1a that associates with the corepressors SMRT and NCoR. The transcriptional properties of the pdx-pbx1 complex appear to be regulated at the level of alternative splicing; a pdx-pbx polypeptide containing the pbx1b isoform, which lacks the C-terminal extension in pbx1a, was unable to repress target gene expression via NCoR-SMRT. Since pbx1a and pbx1b are differentially expressed in endocrine versus exocrine compartments of the adult pancreas, our results illustrate a novel mechanism by which pbx proteins may modulate the expression of specific genetic programs, either positively or negatively, during development.

Developmental expression of the homeodomain protein IDX-1 in mice transgenic for an IDX-1 promoter/lacZ transcriptional reporter

Expression of the homeodomain transcription factor IDX-1 (also known as IPF-1, STF-1, and PDX-1) is required for pancreas development, because disruption of the gene in mice and humans results in pancreatic agenesis. During embryonic development the idx-1 gene is first expressed in a localized region of foregut endoderm from which the duodenum and pancreas later develop. To more fully understand the role of IDX-1 in pancreas development, transgenic mice expressing the Escherichia coli lacZ gene under control of the 5'-proximal 4.6 kb of the idx-1 promoter were created as a reporter for the developmental expression of IDX-1. Here we show that the determinants for the developmental and tissue-specific expression of the endogenous idx-1 gene are faithfully reproduced by the 4.6-kb region of the idx-1 promoter. Expression of lacZ is detected in the development of the exocrine and endocrine pancreas in pancreatic ducts, common bile and cystic ducts, pyloric glands of the distal stomach, Brunner's glands, the intestinal epithelium of the duodenum, and the spleen. The observed spatial and temporal pattern of lacZ expression directed by the IDX-1 promoter further supports an important role of IDX-1 in specifying the development of several endodermal structures within the midsegment of the body. An unexpected finding is that IDX-1 promoter-driven (transcriptional) lacZ activity does not always coincide with the localization of IDX-1 messenger RNA by in situ hybridization and IDX-1 protein by immunocytochemistry in adult rat duodenum, suggesting the existence of regulation of IDX-1 expression at the posttranscriptional level of expression of the idx-1 gene.

Gut specific expression using mammalian promoters in transgenic Xenopus laevis

The recent development of transgenic methods for the frog Xenopus laevis provides the opportunity to study later developmental events, such as organogenesis, at the molecular level. Our studies have focused on the development of the tadpole gut, where tissue specific promoters have yet to be identified. We have used mammalian promoters, for the genes elastase, pancreatic duodenal homeobox-1, transthyretin, and intestinal fatty acid binding protein to drive green fluorescent protein expression in live tadpoles. All of these were shown to drive appropriate tissue specific expression, suggesting that the molecular mechanisms organising the gut are similar in amphibians and mammals. Furthermore, expression from the elastase promoter is initiated in the pancreatic buds before morphological definition becomes possible, making it a powerful tool for the study of pancreatic determination.

Developmental expression of proprotein convertase 1/3 in the rat

We have isolated a clone that has 3' end sequence identity with prohormone convertase 1/3 (PC1/3) from a rat islet cDNA library. Northern blot analysis and immunocytochemical studies have confirmed its presence in the endocrine pancreas. Analysis of poly A mRNA from various adult tissues demonstrated that it was relatively abundant in whole brain, lung and spleen, but not detectable in kidney, testis and heart. Using probes consisting of either the coding region or the 3' end sequences, the mRNA transcripts identified were 5.0, 3.0 and 8.5 kb. The 8.5 kb transcript detected has not been described previously. RT-PCR of RNA isolated from rat embryonic tissues using a primer set corresponding to the 3' end of the PC1/3 sequence showed a steady increase of expression in fetal pancreas and intestine during the course of development. In contrast, comparatively high and constant levels of PC1/3 expression were detected in fetal lung, whereas low and constant expression was detected in fetal liver. Double immuno-staining showed that PC1/3 was co-localised with insulin throughout development, and at mid-gestation, PC1/3 immunoreactivity could also be detected within glucagon-producing cells in the developing pancreas. Thus, we have identified a novel PC1/3 mRNA transcript in the rat by using sequence-specific probes and have demonstrated that the developmental expression of prohormone convertase PC1/3 is confined primarily to pancreas and intestine, suggesting that it may play a possible role in regulating growth and differentiation of these tissues.

Early appearance of pancreatic hormone-expressing cells in the zebrafish embryo

Adult pancreatic islets comprise four cell types, alpha, beta, delta and PP, expressing glucagon, insulin, somatostatin and pancreatic-polypeptide, respectively, arising from cell lineages whose relationships during endocrine pancreas differentiation are still uncertain [Edlund, 1998. Diabetes 47, 1817-1823]. As zebrafish (Danio rerio) represents an attractive vertebrate model to study mutants affecting pancreatic organogenesis [Pack et al., 1996. Development 123, 321-328], we have investigated the expression patterns of islet hormones in zebrafish embryos, from the 16-somite (17 h) to 48-h stages, by whole-mount in situ hybridization and immunofluorescence. Results showed that in the zebrafish pancreatic primordium (a) insulin is the first hormone gene to be expressed, and (b) somatostatin colocalizes with insulin while glucagon-expressing cells, since their appearance, are distinct from insulin- or insulin/somatostatin-expressing cells. Notably, both somatostatin and glucagon, but not insulin, are first expressed in extrapancreatic regions.

Irs-2 coordinates Igf-1 receptor-mediated beta-cell development and peripheral insulin signalling

Insulin receptor substrates (Irs proteins) mediate the pleiotropic effects of insulin and Igf-1 (insulin-like growth factor-1), including regulation of glucose homeostasis and cell growth and survival. We intercrossed mice heterozygous for two null alleles (Irs1+/- and Irs2+/-) and investigated growth and glucose metabolism in mice with viable genotypes. Our experiments revealed that Irs-1 and Irs-2 are critical for embryonic and post-natal growth, with Irs-1 having the predominant role. By contrast, both Irs-1 and Irs-2 function in peripheral carbohydrate metabolism, but Irs-2 has the major role in beta-cell development and compensation for peripheral insulin resistance. To establish a role for the Igf-1 receptor in beta-cells, we intercrossed mice heterozygous for null alleles of Igf1r and Irs2. Our results reveal that Igf-1 receptors promote beta-cell development and survival through the Irs-2 signalling pathway. Thus, Irs-2 integrates the effects of insulin in peripheral target tissues with Igf-1 in pancreatic beta-cells to maintain glucose homeostasis.

Distribution and function of the adhesion molecule BEN during rat development

It is well established that the notochord influences the development of adjacent neural and mesodermal tissue. Involvement of the notochord in the differentiation of the dorsal pancreas has been demonstrated. However, our knowledge of the signals involved in pancreatic development is still incomplete. In order to identify proteins potentially implicated during pancreatic differentiation, we raised and characterized monoclonal antibodies against previously established embryonic pancreatic ductal epithelial cell lines (BUD and RED). Using the MAb 2117, the cell surface antigen 2117 (Ag 2117) was cloned. The predicted sequence for Ag 2117 is the rat homologue of BEN. Initially reported as a protein expressed on epithelial cells of the chicken bursa of Fabricius, BEN is expressed in a variety of tissues during development and described as a marker for the developing central and peripheral chicken nervous systems. A role has been suggested for BEN in the adhesion of stem cells and progenitor cells to the blood-forming tissue microenvironment. In this study, we demonstrate that BEN, initially expressed exclusively in the notochord during the early development of rat, is implicated in pancreatic development. We show that Ag 2117 regulates the pancreatic epithelial cell growth through the ras and Jun kinase pathways. In addition, we demonstrate that Ag 2117 is able to regulate the expression of the transcription factor PDX1, required for insulin gene expression, in embryonic pancreas organ cultures.

Increase in PDX-1 levels suppresses insulin gene expression in RIN 1046-38 cells

RIN1046-38 cells (RIN-38) exhibit a passage-dependent reduction in both basal and glucose-regulated insulin secretion, accompanied by decreased insulin content. In an attempt to explain the mechanism of the gradual decrease in insulin production in cultured cells, we analyzed the insulin promoter activity and the levels of an important trans-activator of the insulin gene, PDX-1, as a function of aging in culture. We demonstrate that the decrease in insulin content and secretion is reflected in decreased promoter activity and is associated with a decrease in E47 and BETA2 nuclear factors, but with a paradoxical 3-fold increase in PDX-1 protein levels. To dissect the effect of increased PDX-1 from the decrease in the additional transcription factors on insulin promoter activity, we overexpressed PDX-1 protein in low passage RIN-38 cells by recombinant adenovirus technology. PDX-1 overexpression did not reduce E47 and BETA2 levels, but was sufficient to suppress rat insulin promoter activity in a dose-dependent manner. The fact that PDX-1 levels participate in trans-activation of insulin promoter activity was demonstrated in HIT-T15 cells. Treating HIT-T15 cells with 1-2 multiplicity of infection of AdCMV-PDX-1 increased rat insulin promoter activity, whereas higher doses repressed insulin promoter activity in these cells as in RIN-38 cells. Our data demonstrate that PDX-1 regulates transcription of the insulin gene in a dose-dependent manner. Depending on its nuclear dosage and the levels of additional cooperating transcription factors, PDX-1 may act as an activator or a repressor of insulin gene expression, such that low as well as high doses may be deleterious to insulin production.

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

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

Gene expression in pancreatic adenocarcinoma

Both acinar and duct cell-specific gene products are expressed by pancreatic adenocarcinoma. In order to begin to understand the mechanisms by which genes of both cell types are expressed in pancreatic adenocarcinoma, an understanding of the underlying transcription factors is important. PDX1 plays an important role in the development of the pancreas and is also expressed in the adult pancreas; it is known to be involved in the regulation of expression of both acinar and islet cell-specific gene products. We have examined pancreatic adenocarcinoma cell lines and have determined that they also express PDX1, making it a candidate transcription factor for the abnormal regulation of these acinar and duc cell gene products.

Differential mRNA display analysis of two related but functionally distinct rat insulinoma (RIN) cell lines: identification of CD24 and its expression in the developing pancreas

To identify genes that might play a role in growth and differentiation of pancreatic beta-cells, we have applied the technique of differential mRNA display to the lineage-related, but functionally distinct rat insulinoma (RIN) cell lines RIN-5AH and RIN-A12. Direct comparison of PCR-generated RIN-5AH and RIN-A12 cDNAs on DNA sequencing gels revealed 31 differentially expressed bands. By Northern blot hybridization, authentic differential expression was confirmed for three cDNAs derived from RIN-5AH cells and four cDNAs from RIN-A12 cells. Nucleotide sequences were determined for these cDNAs and database searches identified one known gene that encoded heat stable antigen CD24. Of the remaining six genes, three matched with established sequence tags from fetal tissue, and three were potentially novel. By RT-PCR analysis, five of the seven genes were expressed in normal fetal and/or adult pancreas. In a detailed survey of CD24 protein expression in the pancreas using the CD24-specific monoclonal antibody J11d, CD24 was predominantly expressed in ductal epithelial cells (E13.5-15.5), developing endocrine (alpha, beta and delta) and exocrine cells (E15.5-20.5) and mature exocrine and peripheral islet delta-cells post E20.5. The retention of CD24 expression in a large proportion of delta-cells but only in a minority of alpha- and beta-cells leads us to hypothesize that CD24 may mark a pool of precursor endocrine cells within adult islets.

Role of Genes in Endoderm-derived Organs

Pax genes, which encode a family of transcription factors, are essentially required for the formation of several tissues from all germ layers in the mammalian embryo. Specifically, in organogenesis, they are involved in triggering early events of cell differentiation. The differentiation of endoderm-derived endocrine pancreas is mediated through Pax4 and Pax6. In the thyroid gland, Pax8 is essential for the formation of thyroxine-producing follicular cells, also of endodermal origin. The analysis of loss-of-function mutants revealed a common function of Pax genes in organogenesis.

Pax6 and Pdx1 form a functional complex on the rat somatostatin gene upstream enhancer

The somatostatin upstream enhancer (SMS-UE) is a highly complex enhancer element. The distal A-element contains overlapping Pdx1 and Pbx binding sites. However, a point mutation in the A-element that abolishes both Pdxl and Pbx binding does not impair promoter activity. In contrast, a point mutation that selectively eliminates Pdx1 binding to a proximal B-element reduces the promoter activity. The B-element completely overlaps with a Pax6 binding site, the C-element. A point mutation in the C-element demonstrates that Pax6 binding is essential for promoter activity. Interestingly, a block mutation in the A-element reduces both Pax6 binding and promoter activity. In heterologous cells, Pdx1 potentiated Pax6 mediated activation of a somatostatin reporter. We conclude that the beta/delta-cell-specific activity of the SMS-UE is achieved through simultaneous binding of Pdx1 and Pax6 to the B- and C-elements, respectively. Furthermore, the A-element appears to stabilise Pax6 binding.

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.

The NeuroD1/BETA2 sequences essential for insulin gene transcription colocalize with those necessary for neurogenesis and p300/CREB binding protein binding

NeuroD1/BETA2 is a key regulator of pancreatic islet morphogenesis and insulin hormone gene transcription in islet beta cells. This factor also appears to be involved in neurogenic differentiation, because NeuroD1/BETA2 is able to induce premature differentiation of neuronal precursors and convert ectoderm into fully differentiated neurons upon ectopic expression in Xenopus embryos. We have identified amino acid sequences in mammalian and Xenopus NeuroD1/BETA2 that are necessary for insulin gene expression and ectopic neurogenesis. Our results indicate that evolutionarily conserved sequences spanning the basic helix-loop-helix (amino acids [aa] 100 to 155) and C-terminal (aa 156 to 355) regions are important for both of these processes. The transactivation domains (AD1, aa 189 to 299; AD2, aa 300 to 355) were within the carboxy-terminal region, as analyzed by using GAL4:NeuroD1/BETA2 chimeras. Selective activation of mammalian insulin gene enhancer-driven expression and ectopic neurogenesis in Xenopus embryos was regulated by two independent and separable domains of NeuroD1/BETA2, located between aa 156 to 251 and aa 252 to 355. GAL4:NeuroD1/BETA2 constructs spanning these sequences demonstrated that only aa 252 to 355 contained activation domain function, although both aa 156 to 251 and 300 to 355 were found to interact with the p300/CREB binding protein (CBP) coactivator. These results implicate p300/CBP in NeuroD1/BETA2 function and further suggest that comparable mechanisms are utilized to direct target gene transcription during differentiation and in adult islet beta cells.

The bHLH protein PTF1-p48 is essential for the formation of the exocrine and the correct spatial organization of the endocrine pancreas

We have generated a mouse bearing a null allele of the gene encoding basic helix-loop-helix (bHLH) protein p48, the cell-specific DNA-binding subunit of hetero-oligomeric transcription factor PTF1 that directs the expression of genes in the exocrine pancreas. The null mutation, which establishes a lethal condition shortly after birth, leads to a complete absence of exocrine pancreatic tissue and its specific products, indicating that p48 is required for differentiation and/or proliferation of the exocrine cell lineage. p48 is so far the only developmental regulator known to be required exclusively for committing cells to an exocrine fate. The hormone secreting cells of all four endocrine lineages are present in the mesentery that normally harbors the pancreatic organ until day 16 of gestation. Toward the end of embryonic life, cells expressing endocrine functions are no longer detected at their original location but are now found to colonize the spleen, where they persist in a functional state until postnatal death of the organism occurs. These findings suggest that the presence of the exocrine pancreas is required for the correct spatial assembly of the endocrine pancreas and that, in its absence, endocrine cells are directed by default to the spleen, a site that, in some reptiles, harbors part of this particular cellular compartment.

On the development of the islets of Langerhans

Studies of pancreatic development have suggested that the islet cells develop through multihormonal stages. Abundant data have confirmed that multihormonal cells are common during pancreatic development. A number of transcription factors and homeotic proteins have also been found to be important to pancreatic and islet cell development. While one of these factors (Isl1) is important for the development of the dorsal pancreatic bud and mesenchyme, another factor (Pdx1) is needed for growth and branching of both pancreatic buds. Studies of the expression patterns of pancreatic hormones and transcription factors and other marker proteins seem at present to be most compatible with the view that early glucagon and glucagon + insulin expressing cells are precursors to the glucagon cells of the islets while mature B cells arise through differentiation from glucagon-negative precursor cells. Recent data also point to possibilities of local paracrine interactions between islet cell types and the parenchymal tissue during development.

An endocrine-exocrine switch in the activity of the pancreatic homeodomain protein PDX1 through formation of a trimeric complex with PBX1b and MRG1 (MEIS2)

HOX proteins and some orphan homeodomain proteins form complexes with either PBX or MEIS subclasses of homeodomain proteins. This interaction can increase the binding specificity and transcriptional effectiveness of the HOX partner. Here we show that specific members of both PBX and MEIS subclasses form a multimeric complex with the pancreatic homeodomain protein PDX1 and switch the nature of its transcriptional activity. The two activities of PDX1 are exhibited through the 10-bp B element of the transcriptional enhancer of the pancreatic elastase I gene (ELA1). In pancreatic acinar cells the activity of the B element requires other elements of the ELA1 enhancer; in beta-cells the B element can activate a promoter in the absence of other enhancer elements. In acinar cell lines the activity is mediated by a complex comprising PDX1, PBX1b, and MRG1 (MEIS2). In contrast, beta-cell lines are devoid of PBX1b and MRG1, so that a trimeric complex does not form, and the beta-cell-type activity is mediated by PDX1 without PBX1b and MRG1. The presence of specific nuclear isoforms of PBX and MEIS is precisely regulated in a cell-type-specific manner. The beta-cell-type activity can be detected in acinar cells if the B element is altered to retain binding of PDX1 but prevent binding of the PDX1-PBX1b-MRG1 complex. These observations suggest that association with PBX and MEIS partners controls the nature of the transcriptional activity of the organ-specific PDX1 transcription factor in exocrine versus endocrine cells.

Targeted oncogenesis of hormone-negative pancreatic islet progenitor cells

Transgenic mice containing an upstream glucokinase (betaGK) promoter- simian virus 40 T antigen (Tag) fusion gene develop neuroendocrine tumors primarily in the pancreas, gut, and pituitary. Pancreatic tumors from a line with delayed tumorigenesis were of two different types: insulinomas and noninsulinomas. The noninsulinomas are often periductal in location, express none of the four major islet peptide hormones, Glut-2, Pdx1, tyrosine hydroxylase, Pax4, Pax6, or Nkx6.1, but do express glucokinase, Sur1, Isl1, Hnf3beta, Hnf6, Beta2/NeuroD, and Nkx2.2. Cells from two different noninsulinoma tumors, when adapted to culture, began to express either insulin, glucagon, or somatostatin. Given the partial gene expression repertoire of the noninsulinoma tumors, their apparent periductal origin, and the ability of these cells to partially cytodifferentiate in culture, we suggest that these tumors are derived from islet progenitor cells. Thus, betaGK-Tag transgenic mice provide a new model system for studying the events that occur during both islet cell neogenesis and normal embryonic development.

beta-cell-specific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the beta-cell phenotype and maturity onset diabetes

To study the late beta-cell-specific function of the homeodomain protein IPF1/PDX1 we have generated mice in which the Ipf1/Pdx1 gene has been disrupted specifically in beta cells. These mice develop diabetes with age, and we show that IPF1/PDX1 is required for maintaining the beta cell identity by positively regulating insulin and islet amyloid polypeptide expression and by repressing glucagon expression. We also provide evidence that IPF1/PDX1 regulates the expression of Glut2 in a dosage-dependent manner suggesting that lowered IPF1/PDX1 activity may contribute to the development of type II diabetes by causing impaired expression of both Glut2 and insulin.

Mice lacking the homeodomain transcription factor Nkx2.2 have diabetes due to arrested differentiation of pancreatic beta cells

The endocrine pancreas is organized into clusters of cells called islets of Langerhans comprising four well-defined cell types: alpha beta, delta and PP cells. While recent genetic studies indicate that islet development depends on the function of an integrated network of transcription factors, the specific roles of these factors in early cell-type specification and differentiation remain elusive. Nkx2.2 is a member of the mammalian NK2 homeobox transcription factor family that is expressed in the ventral CNS and the pancreas. Within the pancreas, we demonstrate that Nkx2.2 is expressed in alpha, beta and PP cells, but not in delta cells. In addition, we show that mice homozygous for a null mutation of Nkx2.2 develop severe hyperglycemia and die shortly after birth. Immunohistochemical analysis reveals that the mutant embryos lack insulin-producing beta cells and have fewer glucagon-producing alpha cells and PP cells. Remarkably, in the mutants there remains a large population of islet cells that do not produce any of the four endocrine hormones. These cells express some beta cell markers, such as islet amyloid polypeptide and Pdx1, but lack other definitive beta cell markers including glucose transporter 2 and Nkx6.1. We propose that Nkx2.2 is required for the final differentiation of pancreatic beta cells, and in its absence, beta cells are trapped in an incompletely differentiated state.

Rat endocrine pancreatic development in relation to two homeobox gene products (Pdx-1 and Nkx 6.1)

We studied the distribution of the homeodomain proteins Pdx-1 and Nkx 6.1 in the developing rat pancreas. During early development, nuclear staining for both Pdx-1 and Nkx 6.1 occurred in most epithelial cells of the pancreatic anlage. Subsequently, Nkx 6.1 became more beta-cell-restricted, and Pdx-1 also occurred in other islet cell types and in the duodenal epithelium. During early pancreatic development, cells co-storing insulin and glucagon were regularly detected. The vast majority of these did not possess nuclear staining for either Pdx-1 or Nkx 6.1. Subsequently, cells storing insulin only appeared. Such cells displayed strongly Pdx-1- and Nkx 6.1-positive nuclei. Therefore, Nkx 6.1, like Pdx-1, may be an important factor in pancreatic development and in mature insulin cell function.

p300 mediates transcriptional stimulation by the basic helix-loop-helix activators of the insulin gene

Pancreatic beta-cell-type-specific and glucose-inducible transcription of the insulin gene is mediated by the basic helix-loop-helix factors that bind to and activate expression from an E-box element within its enhancer. The E-box activator is a heteromeric complex composed of a beta-cell-enriched factor, BETA2/NeuroD, and ubiquitously distributed proteins encoded by the E2A and HEB genes. Previously, we demonstrated that the adenovirus type 5 E1A proteins repressed stimulation by the E-box activator in beta cells. In this study, our objective was to determine how E1A repressed activator function. The results indicate that E1A reduces activation by binding to and sequestering the p300 cellular coactivator protein. Thus, we show that expression of p300 in beta cells can relieve inhibition by E1A, as well as potentiate activation by the endogenous insulin E-box transcription factors. p300 stimulated activation from GAL4 (amino acids 1 to 147) fusion constructs of either BETA2/NeuroD or the E2A-encoded E47 protein. The sequences spanning the activation domains of BETA2/NeuroD (amino acids 156 to 355) and E47 (amino acids 1 to 99 and 325 to 432) were required for this response. The same region of BETA2/NeuroD was shown to be important for binding to p300 in vitro. The sequences of p300 involved in E47 and BETA2/NeuroD association resided between amino acids 1 and 1257 and 1945 and 2377, respectively. A mutation in p300 that abolished binding to BETA2/NeuroD also destroyed the ability of p300 to activate insulin E-box-directed transcription in beta cells. Our results indicate that physical and functional interactions between p300 and the E-box activator factors play an important role in insulin gene transcription.

Conservation of PDX-1 structure, function, and expression in zebrafish

Development of the mammalian pancreas has been studied extensively in mice. The stages from budding of the pancreatic anlaga through endocrine and exocrine cell differentiation and islet formation have been described in detail. Recently, the homeodomain transcription factor PDX-1 has been identified as an important factor in the proliferation and differentiation of the pancreatic buds to form a mature pancreas. To evaluate the possibility of using zebrafish as a model for the genetic analysis of pancreas development, we have cloned and characterized PDX-1 from this organism. The deduced sequence of zebrafish PDX-1 contains 246 amino acids and is 95% identical to mammalian PDX-1 in the homeodomain. We also cloned zebrafish preproinsulin complementary DNA as a marker for islet tissue. By in situ hybridization we demonstrate that PDX-1 and insulin are coexpressed during embryonic development and in adults, although PDX-1 expression appears to be biphasic. Insulin expression apparently begins before 44 hpf, the earliest stage examined in this study. Additionally, very high levels of PDX-1 expression were observed in the pyloric caeca, the accessory digestive organs that also are derived from the proximal region of the intestine in teleosts. Finally, our data show that the evolutionary conservation of zebrafish PDX-1 extends to its DNA binding properties. Zebrafish PDX-1 was equally as effective as mouse PDX-1 in stimulating insulin gene transcription, and maximum promoter activation was dependent on the presence of four intact A elements. The demonstration of this capability suggests that transcriptional regulatory mechanisms that control pancreatic development and insulin gene expression have been conserved among vertebrates.

Isolation of full-length cDNA of mouse PAX4 gene and identification of its human homologue

Recent genetic studies have suggested that PAX4, a member of the paired box (PAX) gene family, is involved in the mechanism regulating the fate of pancreatic islet endocrine progenitor cells. Murine PAX4 was originally identified by genomic screening and, to date, only a partial sequence of PAX4 has been reported. In this study, we cloned the full-length cDNA of mouse PAX4 by RACE (rapid amplification of cDNA ends) using RNA from MIN6 cells, a mouse insulinoma cell line. The full length of cDNA was 1.38 kb, consistent with the estimated size of the transcript by Northern blot. The deduced mouse PAX4 protein was 349 amino acids and had the predicted molecular weight of 38 kDa. Two DNA binding motifs, a 128-amino acid paired domain and a 61-amino acid paired-type homeodomain exhibit the highest amino acid homology with PAX6 (71.2%, 65.0%, respectively), another member of the PAX gene family. However, the sequence of the C-terminal segment of PAX4 diverged and showed no significant homology with any other known PAX genes. As to the genomic DNA, the coding region of the mouse PAX4 gene spanned approximately 5.5 kb and was composed of 10 exons. In the public DNA database, a human cosmid (g1572c264), which was localized on human chromosome 7q31.3, was found to contain a gene homologous to PAX4. The nucleotide and protein sequence homologies between mouse PAX4 and its human homologue were 83.1% and 80.0%, respectively. Interestingly, the ARP5 (ADP-ribosylation factor 5) gene was also found in the same cosmid g1572c264, suggesting the ARP5 gene to be adjacent to the human PAX4 homologue. The human cosmid g1572c264 contains at least four SSRPs (simple sequence repeat polymorphism), which could be used for genetic linkage studies of the locus. The results of this study, i.e. isolation of the full-length cDNA sequence of PAX4 and identification of the homologous human gene, will facilitate further functional and genetic studies of the PAX4 gene.

The cut-homeodomain transcriptional activator HNF-6 is coexpressed with its target gene HNF-3 beta in the developing murine liver and pancreas

Murine hepatocyte nuclear factor-3 beta (HNF-3 beta) protein is a member of a large family of developmentally regulated transcription factors that share homology in the winged helix/fork head DNA binding domain and that participate in embryonic pattern formation. HNF-3 beta also mediates cell-specific transcription of genes important for the function of hepatocytes, intestinal and bronchiolar epithelial, and pancreatic acinar cells. We have previously identified a liver-enriched transcription factor, HNF-6, which is required for HNF-3 beta promoter activity and also recognizes the regulatory region of numerous hepatocyte-specific genes. In this study we used the yeast one-hybrid system to isolate the HNF-6 cDNA, which encodes a cut-homeodomain-containing transcription factor that binds with the same specificity as the liver HNF-6 protein. Cotransfection assays demonstrate that HNF-6 activates expression of a reporter gene driven by the HNF-6 binding site from either the HNF-3 beta or transthyretin (TTR) promoter regions. We used interspecific backcross analysis to determine that murine Hnf6 gene is located in the middle of mouse chromosome 9. In situ hybridization studies of staged specific embryos demonstrate that HNF-6 and its potential target gene, HNF-3 beta, are coexpressed in the pancreatic and hepatic diverticulum. More detailed analysis of HNF-6 and HNF-3 beta's developmental expression patterns provides evidence of colocalization in hepatocytes, intestinal epithelial, and in the pancreatic ductal epithelial and exocrine acinar cells. The expression patterns of these two transcription factors do not overlap in other endoderm-derived tissues or the neurotube. We also found that HNF-6 is also abundantly expressed in the dorsal root ganglia, the marginal layer, and the midbrain. At day 18 of gestation and in the adult pancreas, HNF-6 and HNF-3 beta transcripts colocalize in the exocrine acinar cells, but their expression patterns diverge in other pancreatic epithelium. HNF-6, but not HNF-3 beta, expression continues in the pancreatic ductal epithelium, whereas only HNF-3 beta becomes restricted to the endocrine cells of the islets of Langerhans. We discuss these expression patterns with respect to specification of hepatocytes and differentiation of the endocrine and exocrine pancreas.

Expression of heparin-binding epidermal growth factor-like growth factor during pancreas development. A potential role of PDX-1 in transcriptional activation

The development of the pancreas appears to be regulated by various growth factors. We report here the expression of heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) in the developing pancreas. Immunostaining of fetal and neonatal rat pancreata, in which endocrine cells are visible as cell clusters often associated with primitive ducts or ductular cells, revealed that most of the cluster-forming cells and primitive ducts or ductular cells express HB-EGF protein. In contrast, the exocrine pancreas lacked HB-EGF expression. Based on findings that the expression pattern was similar to that of the homeodomain-containing transcription factor PDX-1 (IDX-1/STF-1/IPF1) and that the regulatory region of the HB-EGF gene contained sequences similar to the PDX-1-binding A element, we examined whether PDX-1 could be a potential activator of HB-EGF gene expression. The results of reporter gene analyses suggested that the HB-EGF gene promoter is PDX-1-responsive and that the activity of the promoter in pancreatic beta cell-derived betaTC1 cells depends on the PDX-1 binding site-like sequences. Gel-mobility shift analyses using an anti-PDX-1 antibody indicated that PDX-1 is a specific and dominant binding factor for an A element-like sequence in the HB-EGF gene. These observations suggest the possible involvement of HB-EGF in pancreas development. While PDX-1 is essential for pancreas development, HB-EGF may function as a mediator of PDX-1 and thus be involved in the development of the endocrine pancreas.

Establishment and immunocharacterization of an immortalized pancreatic cell line derived from the H-2Kb-tsA58 transgenic mouse

This study describes the establishment and characterization of an immortalized cell line derived from the pancreas of an adult H-2Kb-tsA58 transgenic mouse. These cells, designated IMPAN for IMmortalized PANcreatic cells, displayed a cobblestone appearance typical of confluent epithelial cells and a distinct polarity in the organization of their cytoplasmic organelles. Immunocytochemical studies revealed that all IMPAN cells stained positively for a wide range of markers characteristic of pancreatic acinar cells, namely the secretory products alpha-amylase, chymotrypsinogen, DNAse, the lectinlike secretory protein PAP (pancreatitis associated protein), and the zymogen granule membrane proteins GP-2 and gp300. They also stained positively for carbonic anhydrase II and cytokeratin 19, two proteins characteristic of pancreatic duct cells, as well as for rab3A, a small GTP-binding protein specifically localized in pancreatic islet cells. No reactivity was ever obtained with insulin antibodies. Taken together, these results show that the IMPAN cells exhibit a phenotype comparable to exocrine pancreatic acinar cells. However the expression of some proteins more specific to duct and islet cells make them similar to in vivo or in vitro growing acinar cells. The cell line should be a valuable model to study the mechanisms of growth, differentiation, and transformation of the exocrine pancreatic acinar cell.

Expression of homeobox genes, including an insulin promoting factor, in the murine yolk sac at the time of hematopoietic initiation

The visceral yolk sac (YS), a simple bilayer structure formed during gastrulation, supplies blood cells and intestine- and liver-like functions to support embryonic growth. To better understand gene regulation in extraembryonic tissues, we examined the early murine YS for expression of the homeobox family of developmental transcription regulators. We identified a subset of known homeobox sequences (Hox 1l, b1, a9, c9, a7, b7, b8, a10, cdx-1, and PDX-1), as well as two novel homeodomains consisting of a fourth labial class Hox genes and one that matches the Antennapedia class on the amino acid level. The two most frequently isolated YS Hox genes, a9 and c9, are initially expressed only in the YS (E.5) and subsequently expressed in both the embryo and YS (E8.5). Another of the identified genes, PDX-1, is involved in pancreatic development and insulin regulation. Whereas the4 rodent YS is known to produce insulin from mid to late gestation, YS insulin expression had not been examined earlier in development . We detected insulin mRNA in the YS at both E7.5 and E8.5, prior to expression in the embryo proper or formation of the pancreas. However, other pancreatic products, such as glucagon, somatostatin, and carboxypeptidase A, are not expressed in the YS. In situ analysis indicates insulin is produced in YS mesothelial cells and endoderm cells, but not in blood cells. We hypothesize the early expression of insulin in the YS is required for the expansion of insulin responsive cells including primitive erythroblasts.

Analysis of the role of E2A-encoded proteins in insulin gene transcription

Pancreatic beta-cell type-specific transcription of the insulin gene is mediated, in part, by factors in the basic helix-loop-helix (bHLH) family that act on a site within the insulin enhancer, termed the E1-box. Expression from this element is regulated by a heteromeric protein complex containing ubiquitous (i.e. the E2A- and HEB-encoded proteins) and islet-enriched members of the bHLH family. Recent studies indicate that the E2A- and HEB-encoded proteins contain a transactivation domain, termed AD2, that functions more efficiently in transfected beta-cell lines. In the present report, we extend this observation by demonstrating that expression of full-length E2A proteins (E47, E12, and E2/5) activates insulin E element-directed transcription in a beta-cell line-selective manner. Stimulation required functional interactions with other key insulin gene transcription factors, including its islet bHLH partner as well as those that act on the RIPE3b1 and RIPE3a2 elements of the insulin gene enhancer. The conserved AD2 domain in the E2A proteins was essential in this process. The effect of the E2A- and HEB-encoded proteins on insulin gene expression was also analyzed in mice lacking a functional E2A or HEB gene. There was no apparent difference in insulin production between wild type, heterozygote, and homozygous mutant E2A or HEB mice. These results suggest that neither the E2A- or HEB-encoded proteins are essential for insulin transcription and that one factor can substitute for the other to impart normal insulin E1 activator function in mutant animals.

SNAP-25 is phosphorylated by glucose and GLP-1 in RIN 1046-38 cells

We investigated the possibility that tyrosine phosphorylation might play a role in insulin secretion in the insulinoma cell line, RIN 1046-38 cells. At least 4 proteins of 18, 25, 35, and 46 kDa size were found to be tyrosine phosphorylated in the presence of glucose and an insulin secretagogue, glucagon-like peptide-1 (GLP-1). The addition of glucose and GLP-1 to cells that were exposed to the tyrosine kinase inhibitor genistein resulted in a decrease in the extent of phosphorylation of the 18, 25, and 35 kDa proteins and a concomitant reduction in insulin secretion, whereas treatment with vanadate, a tyrosine phosphatase inhibitor, led to enhanced responses. Immunoprecipitation of cellular proteins with an anti-phosphotyrosine antibody followed by immunoblotting with a specific monoclonal antibody to SNAP-25 (synaptosome-associated protein of 25 kDa) revealed that the 25 kDa protein is SNAP-25. These results suggest that tyrosine phosphorylation of SNAP-25 may be involved in the regulation of insulin secretion in RIN 1046-38 cells.

Diabetes, defective pancreatic morphogenesis, and abnormal enteroendocrine differentiation in BETA2/neuroD-deficient mice

Candidate transcription factors involved in pancreatic endocrine development have been isolated using insulin gene regulation as a paradigm. The cell-type restricted basic helix-loop-helix (bHLH) gene, BETA2/NeuroD, expressed in pancreatic endocrine cells, the intestine, and the brain, activates insulin gene transcription and can induce neurons to differentiate. To understand the importance of BETA2 in pancreatic endocrine cell differentiation, mice lacking a functional BETA2 gene were generated by gene targeting experiments. Mice carrying a targeted disruption of the BETA2 gene developed severe diabetes and died perinatally. Homozygous BETA2 null mice had a striking reduction in the number of insulin-producing beta cells and failed to develop mature islets. Islet morphogenesis appeared to be arrested between E14.5 and E17.5, a period characterized by major expansion of the beta cell population. The presence of severe diabetes in these mice suggests that proper islet structure plays an important role in blood glucose homeostasis. In addition, secretin- and cholecystokinin-producing enteroendocrine cells failed to develop in the absence of BETA2. The absence of these two pancreatic secretagogs may explain the abnormal cellular polarity and inability to secrete zymogen granules in pancreatic acinar exocrine cells. The nervous system appeared to develop normally, despite abundant expression of BETA2 in differentiating neurons. Thus, BETA2 is critical for the normal development of several specialized cell types arising from the gut endoderm.

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.

Molecular analysis of a novel winged helix protein, WIN. Expression pattern, DNA binding property, and alternative splicing within the DNA binding domain

We have cloned a novel winged helix factor, WIN, from the rat insulinoma cell line, INS-1. Northern blot analysis demonstrated that WIN is highly expressed in a variety of insulinoma cell lines and rat embryonic pancreas and liver. In adults, WIN expression was detected in thymus, testis, lung, and several intestinal regions. We determined the DNA sequences bound in vitro by baculovirus-expressed WIN protein in a polymerase chain reaction-based selection procedure. WIN was found to bind with high affinity to the selected sequence 5'-AGATTGAGTA-3', which is similar to the recently identified HNF-6 binding sequence 5'-DHWATTGAYTWWD-3' (where W = A or T, Y = T or C, H is not G, and D is not C). We have isolated human WIN cDNAs by library screening and 5'-rapid amplification of cDNA ends. Sequence analysis indicates that the carboxyl terminus of human WIN has been previously isolated as a putative phosphorylation substrate, MPM2-reactive phosphoprotein 2 (MPP2); WIN may be regulated by phosphorylation. Alignment of the rat and human WIN cDNAs and their comparison with mouse genomic sequence revealed that the WIN DNA binding domain is encoded by four exons, two of which (exons 4 and 6) are alternatively spliced to generate at least three classes of mRNA transcripts. These transcripts were shown by RNase protection assay to be differentially expressed in different tissues. Alternative splicing within the winged helix DNA binding domain might result in modulation of DNA binding specificity.

Exploring the role of homeobox and zinc finger proteins in pancreatic cell proliferation, differentiation, and apoptosis

Transcription factors are DNA binding proteins that regulate gene expression in response to a large variety of extracellular stimuli, and thereby act as key molecular switches for controlling cell differentiation, proliferation, and apoptosis. During the last decade, a myriad of these proteins have been identified and classified into different structural families, including homeobox, zinc finger, leucine zipper, and helix-loop-helix transcription factors. Members of the homeobox and zinc finger superfamilies are among the best-characterized transcription factors known to act as potent regulators of normal development in organisms ranging from insects to humans. In addition, mutations or aberrant expression in genes encoding these proteins can result in neoplastic transformation in several different cell types, further supporting their role as "guardians" of normal cell growth and differentiation. Therefore, the purpose of this article is to review this field of research with a particular emphasis on the role of homeobox- and zinc finger-containing transcription factors in pancreatic cell growth, cell differentiation, and apoptosis. The potential participation of these proteins in neoplastic transformation is also discussed.

Genetic analysis reveals that PAX6 is required for normal transcription of pancreatic hormone genes and islet development

We present genetic and biochemical evidence that PAX6 is a key regulator of pancreatic islet hormone gene transcription and is required for normal islet development. In embryos homozygous for a mutant allele of the Pax6 gene, Small eye (Sey(Neu)), the numbers of all four types of endocrine cells in the pancreas are decreased significantly, and islet morphology is abnormal. In the remaining islet cells, hormone production, particularly glucagon production, is markedly reduced because of decreased gene transcription. These effects appear to result from a lack of PAX6 protein in the mutant embryos. Biochemical studies identify wild-type PAX6 protein as the transcription factor that binds to a common element in the glucagon, insulin, and somatostatin promoters, and show that PAX6 transactivates the glucagon and insulin promoters.