Pancreatic β-Cell-specific Repression of Insulin Gene Transcription by CCAAT/Enhancer-binding Protein β

Abstract

Chronic exposure of ␤-cells to supraphysiologic glucose concentrations results in decreased insulin gene transcription. Here we identify the basic leucine zipper transcription factor, CCAAT/enhancer-binding protein ␤ (C/EBP␤), as a repressor of insulin gene transcription in conditions of supraphysiological glucose levels. C/EBP␤ is expressed in primary rat islets. Moreover, after exposure to high glucose concentrations the ␤-cell lines HIT-T15 and INS-1 express increased levels of C/ EBP␤. The rat insulin I gene promoter contains a consensus binding motif for C/EBP␤ (CEB box) that binds C/EBP␤. In non-␤-cells C/EBP␤ stimulates the activity of the rat insulin I gene promoter through the CEB box. Paradoxically, in ␤-cells C/EBP␤ inhibits transcription, directed by the promoter of the rat insulin I gene by direct protein-protein interaction with a heptad leucine repeat sequence within activation domain 2 of the basic helix-loop-helix transcription factor E47. This interaction leads to the inhibition of both dimerization and DNA binding of E47 to the E-elements of the insulin promoter, thereby reducing functionally the transactivation potential of E47 on insulin gene transcription. We suggest that the induction of C/EBP␤ in pancreatic ␤-cells by chronically elevated glucose levels may contribute to the impaired insulin secretion in severe type II diabetes mellitus.Insulin is a hormone essential for the control of mammalian glucose homeostasis and is produced predominantly in pancreatic ␤-cells of adult animals (1). The expression of the insulin gene occurs to a large extent at the level of transcription. Control elements residing in the 5Ј 350-base pair sequence flanking exon 1 of the rat insulin I gene are sufficient to direct ␤-cell-specific expression (2) (Fig. 1A). Arrays of A and E elements 1 (Far-FLAT, Nir-P1) constitute symmetrical enhansons that cooperatively account for Ͼ90% of the transcriptional activity of the insulin gene promoter (3). The E elements are recognition motifs for transcription factors in the basic helixloop-helix (bHLH) 2 family, such as E12 and E47, which activate the insulin promoter in close synergism with A element binding homeobox transcription factors, such as IDX-1.Chronic hyperglycemia may contribute to the pancreatic ␤-cell dysfunction observed in patients with type II diabetes, a phenomenon attributed to the concept of glucose toxicity (4). Studies using in vivo animal models and in vitro ␤-cell lines have demonstrated that a reduction of insulin gene transcription by glucose toxicity is associated with the loss of transactivator proteins such as IDX-1/IPF-1/STF-1 and RIPE3b1-binding protein (5-10). Because insulin gene transcription is both positively and negatively regulated, we sought to identify repressors that might also mediate the effects of glucose toxicity on insulin gene transcription. In this report we describe CCAAT/enhancer binding protein-␤ (C/EBP␤) as a glucoseinduced repressor of insulin gene transcription.C/EBPs are a family of transcription factors that regulate...

Pancreatic and duodenal homeobox gene 1 induces hepatic dedifferentiation by suppressing the expression of CCAAT/enhancer-binding protein β

“…Although it has been previously suggested that C/EBP␤-PDX-1 interplay may affect the activation of insulin gene expression in pancreatic tissues, 5,23,30 our data raise the possibility that these proteins have a much wider reciprocal impact on the cellular phenotype. Although PDX-1 inhibits hepatic differentiation through C/EBP␤ suppression, C/EBP␤ not only inhibits insulin gene expression but also restricts pancreatic differentiation in the liver.…”

“…The inhibition of C/EBP␤ activity by ectopic LIP expression modestly activated the expression of the INS gene in adult human liver cells ( in agreement with the documented inhibitory effect of C/EBP␤ on the activation of INS gene expression in ␤-cells. 23 Unlike ectopic PDX-1 expression, 4 LIP expression alone did not induce differentiated characteristics specific to ␤-cells, such as glucose-stimulated C-peptide secretion (Fig. 7B) or the expression of pancreatic transcription factors that characterizes mature pancreatic endocrine differentiation.…”

Induction of c-Myc Expression Suppresses Insulin Gene Transcription by Inhibiting NeuroD/BETA2-mediated Transcriptional Activation

“…In addition, C/EBP␤ expression is increased in islets from Zucker diabetic fatty rats and partial pancreatectomy rats (49). Because it has been reported that C/EBP␤ functions as a negative regulatory factor of insulin gene transcription (51), the increase of C/EBP␤ expression may also contribute to the suppression of insulin gene expression in diabetes.…”

Multiple chromatin-bound protein kinases assemble factors that regulate insulin gene transcription

“…An array of factors can antagonize glucose-stimulated insulin gene transcription directly or indirectly and contribute to ␤ cell failure (9,(24)(25)(26)(27)(28). Notable among these are C/EBP-␤ and c-Myc (13,20,28,29), reduced expression of PDX-1 and MafA (30)(31)(32)(33), export of PDX-1 from the nucleus (31,34), and activation of c-Jun (35). Here, we explored signaling activated by IL-1␤ under normal and hyperglycemic conditions and effects on interactions of stimulatory and inhibitory factors with the insulin gene promoter in ␤ cell lines and human islets.…”

“…PDX-1, BETA2 (NeuroD1), and MafA activate the insulin gene promoter synergistically and are considered essential for glucose-stimulated insulin gene transcription (13)(14)(15)(16). Mutations in two of these factors have been associated with maturity onset diabetes of the young (MODY) (17,18).…”