Regulation of insulin gene transcription by the immediate-early growth response gene Egr-1.
Changes in extracellular glucose levels regulate the expression of the immediate-early response gene and zinc finger transcription factor early growth response-1 (Egr-1) in insulin-producing pancreatic beta-cells, but key target genes of Egr-1 in the endocrine pancreas have not been identified. We found that overexpression of Egr-1 in clonal (INS-1) beta-cells increased transcriptional activation of the rat insulin I promoter. In contrast, reductions in Egr-1 expression levels or function with the introduction of either small interfering RNA targeted to Egr-1 (siEgr-1) or a dominant-negative form of Egr-1 decreased insulin promoter activation, and siEgr-1 suppressed insulin gene expression. Egr-1 did not directly interact with insulin promoter sequences, and mutagenesis of a potential G box recognition sequence for Egr-1 did not impair the Egr-1 responsiveness of the insulin promoter, suggesting that regulation of insulin gene expression by Egr-1 is probably mediated through additional transcription factors. Overexpression of Egr-1 increased, and reduction of Egr-1 expression decreased, transcriptional activation of the glucose-responsive FarFlat minienhancer within the rat insulin I promoter despite the absence of demonstrable Egr-1-binding activity to FarFlat sequences. Notably, augmenting Egr-1 expression levels in insulin-producing cells increased the mRNA and protein expression levels of pancreas duodenum homeobox-1 (PDX-1), a major transcriptional regulator of glucose-responsive activation of the insulin gene. Increasing Egr-1 expression levels enhanced PDX-1 binding to insulin promoter sequences, whereas mutagenesis of PDX-1-binding sites reduced the capacity of Egr-1 to activate the insulin promoter. We propose that changes in Egr-1 expression levels in response to extracellular signals, including glucose, can regulate PDX-1 expression and insulin production in pancreatic beta-cells.
Eto
Kazuhiro
K
Laboratory of Molecular Endocrinology and Diabetes Unit, Massachusetts General Hospital, Boston, Massachusetts 02114, USA.
Kaur
Varinderpal
V
Thomas
Melissa K
MK
D000818:Animals; D001665:Binding Sites; D004247:DNA / Q000378:metabolism; D051766:Early Growth Response Protein 1 / Q000235:genetics*; D004742:Enhancer Elements, Genetic; D005786:Gene Expression Regulation; D018398:Homeodomain Proteins / Q000235:genetics; D007328:Insulin / Q000235:genetics*; D051379:Mice; D011401:Promoter Regions, Genetic; D034741:RNA, Small Interfering / Q000494:pharmacology; D015534:Trans-Activators / Q000235:genetics; D015533:Transcriptional Activation
D051766:Early Growth Response Protein 1; C486109:Egr1 protein, mouse; D018398:Homeodomain Proteins; D007328:Insulin; D034741:RNA, Small Interfering; D015534:Trans-Activators; C084010:pancreatic and duodenal homeobox 1 protein; D004247:DNA