Relative contribution of PDX-1, MafA and E47/β2 to the regulation of the human insulin promoter

Abstract

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

Chronic exposure to leucine in vitro induces β-cell dysfunction in INS-1E cells and mouse islets

“…There was no change in total insulin content or output in INS-1E cells. Pdx1 and MafA are potent stimulators of the transcription of insulin genes (Docherty et al 2005), and the increased Pdx1 mRNA expression found in this study ( Fig. 2A) is very interesting; it is in accordance with increased mRNA expression of Pdx1 in isolated islets from T2D patients, despite less insulin release than control islets in response to glucose (Del Guerra et al 2005); furthermore, it has previously been shown that glutamine upregulates Pdx1 expression in BRIN-BD11 cells (Corless et al 2006).…”

HOX D13 expression across 79 tumor tissue types

“…42 In physical contiguity with PCGEM1 is located the gene Neuro D1, whose ectopic expression causes conversion of epithelial cells into neurons 43 and is actively expressed in advanced prostate cancer. 30 Neuro D1 is further able to interact with the insulin promoter 44 and is between major players of pancreas organogenesis as is expressed in all differentiated and postmitotic pancreatic endocrine cells. It has been postulated that the involvement of Neuro D1 in promoting cell cycle exit inducing pancreatic progenitor cells to differentiate.…”

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

“…These interacting factors include bHLH proteins that bind to Eboxes (BETA2/NeuroD1 and E2A proteins) [122,123,140,141], the basic leucine zipper factor MafA that binds to the C-box [73,142], the homeodomain proteins Pbx1 and MEIS2 [47,48,143,144], and high mobility group (HMG) proteins that bind to the DNA backbone [123]. The free energy gained from these interactions may increase the likelihood that Pdx1 targets genes with binding sites for these factors [123,144].…”