Pancreatic duodenal homeobox-1 (PDX-1) originally appeared in the literature under several guises, namely IUF-1, IPF-1, IDX-1, STF-1 and GSF. It was discovered independently by a number of laboratories working on the regulation of hormone gene expression and development in the islets of Langerhans and in the developmental biology of the frog. The early studies on the insulin gene promoter, which were predominantly on the rat insulin I gene, led to mapping two major regulatory sequences of the regions ±104 to ±112 and ±233 to ±241 [1]. Termed initially the IEB1 and IEB2 boxes but now known as the E1 and E2 boxes [2], these sequences bound a single protein, IEF1, that was expressed specifically in beta cells [3]. IEF1 was found to be a heterodimer comprising two basic helix loop helix (bHLH) proteins, E47 and NeuroD1 also known as BETA2 [4, 5]. E47, one of two splice products of the E2A gene, has a widespread distribution whereas NeuroD1 is restricted to neuroendocrine cells. Mutagenesis of the E boxes completely abolished the activity of the insulin promoter.Subsequently a beta cell-specific factor, named IUF-1, was identified by electrophoretic mobility shift assay (EMSA). It bound to three sites located between ±77 and ±84, ±210 and ±217, and ±313 and ±320 in the human insulin gene promoter [6]. Termed the CT boxes, and now known as the A boxes [2], one of these sites (the CT2 box) was closely related to a
AbstractPancreatic duodenal homeobox ±1 is a transcription factor that is expressed in beta and d cells of the islets of Langerhans and in dispersed endocrine cells of the duodenum. It is involved in regulating the expression of a number of key beta-cell genes as well as somatostatin. It also plays a pivotal part in the development of the pancreas and islet cell ontogeny. Thus homozygous disruption of the gene in mice and humans results in pancreatic agenesis. Heterozygous mutations in the gene result in impaired glucose tolerance and symptoms of diabetes as seen in MODY4 and late-onset Type II (non-insulin-dependent) diabetes mellitus. In adults pancreatic duodenal homeobox-1 expression is increased in duct cells of the pancreas that have been induced to proliferate and differentiate to form new islets. Defects in pancreatic duodenal homeobox-1 could therefore contribute to Type II diabetes by affecting compensatory mechanisms that increase the rate of beta-cell neogenesis to meet the increased insulin secretory demand. It could also be a pharmacological target for beta-cell defects in Type II diabetes, while its role as a regulator of islet stem cell activity is being exploited to produce a replenishable source of islet tissue for transplantation in Type I (insulin-dependent) diabetes mellitus. [Diabetologia (2001)
One of the mechanisms whereby glucose stimulates insulin gene transcription in pancreatic -cells involves activation of the homeodomain transcription factor PDX1 (pancreatic/duodenal homeobox-1) via a stressactivated 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 (
Insulin upstream factor 1 (IUF1), a transcription factor present in pancreatic -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 -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 MAP-KAP 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, MAP-KAP 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.
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