Effects Of Chromatin Structure
Efficient transcription is the outcome of coordinated dynamic arrangements upon the promoter. ChIP assays using MIN6 ß-cells have shown that PDX-1, MafA, E47, and ß2 bind to the mouse insulin 2 promoter in a cyclical manner with a periodicity of ~10–15 min. Insulin gene regulation is also influenced by epigenic factors that include DNA methylation and alterations in histone modifications, which affect the packaging of DNA within chromatin. There are a number of studies on the role of histone acetylation and methylation in the control of insulin gene expression. A key role for histone acetyl transferase (HAT) p300 in insulin promoter regulation has been demonstrated by the observations that PDX-1 and ß2 mediate their effects on the rat insulin 2 gene through an interaction with p300,[31,126,127] while activation of a rat insulin 1 promoter construct in HeLa cells by PDX-1 requires interactions with p300. It has also been shown that the effects of glucose on a rat insulin 1 promoter construct in the mouse MIN6 ß-cell line involved the recruitment by PDX-1 of HAT and histone deacetylase activities (HDAC) activities. Thus, under low-glucose conditions, PDX-1 associated with HDACs to repress transcription, whereas under high glucose conditions PDX-1 recruited the HAT p300 to activate transcription. PDX-1 has also been linked to the presence of methylated histone H3, i.e., H3K4me (nomenclature as per), at the proximal promoter and coding regions of the insulin gene in rodent cells. More recently, the histone methyl transferase set9 has been localized to ß-cells in association with the insulin gene.
Investigations into the role of chromatin accessibility in insulin expression have revealed that PDX-1 shows preferential binding to open chromatin (euchromatin) over condensed chromatin (heterochromatin). In particular, PDX-1 occupies the endogenous insulin promoter in mouse ßTC3 ß-cells but not in mPAC ductal cells, which do not express insulin. Furthermore, the binding affinity of PDX-1 is strongly influenced by the position of nucleosomes relative to its regulatory element. Even within euchromatin, the degree of openness varies as the A3/A4 region (–126 to –296) to which PDX-1 can bind contained the most open chromatin structure based on micrococcal nuclease digestion, whereas the adjacent region (–297 to –460), which is not as crucial for ß-cell–specific insulin transcription, was more condensed. Although it is likely that the insulin gene is embedded in euchromatin in ß-cells and in more condensed heterochromatin in non-ß-cells, it may be of relevance that the synteny studies (see INSULIN GENES) show that the human insulin gene lies only 2 kbp from the transcriptionally active TH gene, whereas this distance is >100-fold greater in rodents. Thus, the diverse efforts to induce insulin expression in non-ß-cells may be less problematic in humans than in rodents.
Diabetes. 2006;55(12):3201-3213. © 2006 American Diabetes Association, Inc.
Cite this: Comparative Analysis of Insulin Gene Promoters: Implications for Diabetes Research - Medscape - Dec 01, 2006.