Covalent Histone Modifications Underlie the Developmental Regulation of Insulin Gene Transcription in Pancreatic β Cells

Suzanne M Ziesmann,Raghavendra G Mirmira,Joshua Francis,James C Garmey,Swarup K Chakrabarti

doi:10.1074/jbc.m303423200

Suzanne M Ziesmann, Raghavendra G Mirmira + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m303423200

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Abstract

Histone modifying enzymes contribute to the activation or inactivation of transcription by ultimately catalyzing the unfolding or further compaction, respectively, of chromatin structure. Actively transcribed genes are typically hyperacetylated at Lys residues of histones H3 and H4 and hypermethylated at Lys-4 of histone H3 (H3-K4). To determine whether covalent histone modifications play a role in the beta cell-specific expression of the insulin gene, we performed chromatin immunoprecipitation assays using anti-histone antibodies and extracts from beta cell lines, non-beta cell lines, and ES cells, and quantitated specific histone modifications at the insulin promoter by real-time PCR. Our studies reveal that the proximal insulin promoter is hyperacetylated at histone H3 only in beta cells. This hyperacetylation is highly correlated to recruitment of the histone acetyltransferase p300 to the proximal promoter in beta cells, and is consistent with the role of hyperacetylation in promoting euchromatin formation. We also observed that the proximal insulin promoter of beta cells is hypermethylated at H3-K4, and that this modification is correlated to the recruitment of the histone methyltransferase SET7/9 to the promoter. ES cells demonstrate a histone modification pattern intermediate between that of beta cells and non-beta cells, and is consistent with their potential to express the insulin gene. We therefore propose a model in which insulin transcription in the beta cell is facilitated by a unique combination of transcription factors that acts in the setting of an open, euchromatic structure of the insulin gene.

Highlights

The pancreatic ␤ cell is exclusively responsible for the synthesis and secretion of insulin
We propose a model in which insulin transcription in the ␤ cell is facilitated by a unique combination of transcription factors that acts in the setting of an open, euchromatic structure of the insulin gene
This hypothesis predicts that the various covalent histone modifications at any given gene are interrelated, and that in combination they contribute to a mechanism that can alter the chromatin structure and transcriptional rate of that gene

Summary

Introduction

The pancreatic ␤ cell is exclusively responsible for the synthesis and secretion of insulin. To explain Ins gene transcription in the ␤ cell more fully, we hypothesized that ␤ cells contain a characteristic set of histone modifications that lend a state of euchromatin in the region of the Ins gene. To test this hypothesis, we performed quantitative ChIP assays using anti-histone antisera and extracts from ␤ cell lines (␤TC3 and MIN6) and several non-␤-cell lines (␣TC1.6, mPAC, NIH3T3, and ES cells) to examine the chromatin structure of the proximal and distal mouse Ins promoter. We propose a model in which Ins transcription in the ␤ cell is facilitated by a unique combination of transcription factors that acts in the setting of an open, euchromatic structure of the Ins gene

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