The Islet ॆ Cell-enriched RIPE3b1/Maf Transcription Factor Regulates pdx-1 Expression

Abstract

Pancreatic duodenal homeobox factor-1, PDX-1, is required for pancreas development, islet cell differentiation, and the maintenance of ␤ cell function. Selective expression in the pancreas appears to be principally regulated by Area II, one of four conserved regulatory sequence domains found within the 5-flanking region of the pdx-1 gene. Detailed mutagenesis studies have identified potential sites of interaction for both positive-and negative-acting factors within the conserved sequence blocks of Area II. The islet ␤ cell-enriched RIPE3b1 transcription factor, the activator of insulin C1 elementdriven expression, was shown here to also stimulate Area II by binding to sequence blocks 4 and 5 (termed B4/5). Accordingly, B4/5 DNA-binding protein's molecular mass (i.e. 46 kDa), binding specificity, and islet ␤ cell-enriched distribution were identical to RIPE3b1. Area II-mediated activation was also unaffected upon replacing B4/5 with the insulin C1/RIPE3b1 binding site. In addition, the chromatin immunoprecipitation assay showed that the Area II region of the endogenous pdx-1 gene was precipitated by an antiserum that recognizes the large Maf protein that comprises the RIPE3b1 transcription factor. These results strongly suggest that RIPE3b1/Maf has an important role in generating and maintaining physiologically functional ␤ cells.Targeting of the pancreatic duodenal homeobox factor-1 (pdx-1) 1 gene in mice has established that expression in a common progenitor cell population is essential for the development of both the endocrine and exocrine compartments of the pancreas. PDX-1 acts by stimulating proliferation, branching, and differentiation of the pancreatic epithelium (1-3). In contrast, all other characterized islet endocrine-(e.g. PAX6 (4, 5), Ngn3 (6), BETA2 (7), and exocrine (PTF1-p48 (8, 9)-enriched transcription factors act downstream of PDX-1 and are principally involved in islet or exocrine cell differentiation. Selective elimination of PDX-1 in mouse ␤ cells in vivo also results in a reduction in both insulin secretion and islet ␤ cell numbers (1). These animals become glucose intolerant and diabetic, largely because of their inability to synthesize appropriate amounts of PDX-1-regulated gene products that are involved in maintaining glucose homeostasis (1) (e.g. insulin (10, 11), GLUT2 (12), and glucokinase (13)). Moreover, mutations in pdx-1 cause pancreatic agenesis (2, 3) and a form of maturity onset diabetes of the young in humans (14, 15). These data have established an essential role for PDX-1 in islet ␤ cell development and function.The recent success in reversing type 1 diabetes by islet transplantation has led to renewed optimism for this form of treatment (16). However, the availability of human islets is limited and will never be sufficient to treat all patients. Because islet-enriched transcription factors are essential for islet cell development, information valuable for generating transplantable cells will likely be gained by understanding how their expression is regulated. Therefore, e...

Oxidative Stress-mediated, Post-translational Loss of MafA Protein as a Contributing Mechanism to Loss of Insulin Gene Expression in Glucotoxic Beta Cells

“…We also reported that loss of RIPE-3b1 binding precedes the loss of PDX-1 binding to the insulin promoter as glucotoxicity develops (8). The chronology of these losses is likely to be important in light of a recent report that RIPE-3b1/MafA directly activates PDX-1 transcription (9).…”

Pancreas organogenesis: From bud to plexus to gland

“…Areas I-II impart islet-specific Pdx1 expression, while Area III working alone can confer b-cell-specific expression . Areas I-II-III harbor binding sites for many TFs such as HNF1a, Pdx1 (Marshak et al, 2000;Gerrish et al, 2001), Foxa2 (Gerrish et al, 2000;BenShushan et al, 2001), Pax6 (Samaras et al, 2002), HNF6 (Jacquemin et al, 2003), and MafA (Samaras et al, 2003). Deletion of the Area I-II-III cis-regulatory region creates a Pdx1 hypomorphic allele (Pdx1 DI-II-III ), and revealed that proper pancreas specification and outgrowth/differentiation from the foregut endoderm requires high Pdx1 levels.…”

Transcription factor regulation of pancreatic organogenesis, differentiation and maturation

“…93 The Pdx1 gene has 4 highly conserved regions viz., Areas I-II-III (the proximal enhancer region) 94 and Area IV (the distal enhancer). 95 Hnf1a, 96 Foxa2, 94 Hnf6, 69 Pax6, 97 and MafA 98 have binding sites within Areas I-II-III whereas Foxa1 and Foxa2 regulate Pdx1 expression via Area IV. 79 Pdx1 regulates b-cell identity by repressing the a-cell program via a shift in transcription profiles.…”