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Abstract and Introduction
Abstract
DNA sequences that regulate expression of the insulin gene are located within a region spanning ~400 bp that flank the transcription start site. This region, the insulin promoter, contains a number of cis-acting elements that bind transcription factors, some of which are expressed only in the ß-cell and a few other endocrine or neural cell types, while others have a widespread tissue distribution. The sequencing of the genome of a number of species has allowed us to examine the manner in which the insulin promoter has evolved over a 450 millionyear period. The major findings are that the A-box sites that bind PDX-1 are among the most highly conserved regulatory sequences, and that the conservation of the C1, E1, and CRE sequences emphasize the importance of MafA, E47/ß2, and cAMP-associated regulation. The review also reveals that of all the insulin gene promoters studied, the rodent insulin promoters are considerably dissimilar to the human, leading to the conclusion that extreme care should be taken when extrapolating rodent-based data on the insulin gene to humans.
Introduction
The cloning and sequencing of the insulin gene in 1980 was a landmark breakthrough that opened up a new field of research on the mechanisms controlling expression of the gene. This in turn led to the discovery of transcription factors that, in addition to regulating the insulin gene in a tissue-specific and temporal manner, participated in the development of the endocrine pancreas and in the maintenance of islet cell function. Some of these transcription factors have been identified as maturity-onset diabetes of the young (MODY) genes, and at least one has been associated with type 2 diabetes. Their use in the development of novel therapies for diabetes based on the differentiation of embryonic or adult stem cells toward a ß-cell–like phenotype and the forced expression of endogenous insulin genes in nonislet cells has also been exploited.
The early work on characterizing the DNA sequences involved in regulating insulin gene expression focused on the rat insulin 1 gene. The reason for this was that at the time there were no available human ß-cell lines and it was felt important to correlate data from transfected promoter constructs with effects on the endogenous insulin gene. As it turned out, most of the studies involved transfecting the rat insulin gene constructs in the Syrian hamster HITm2.2.5 cell line, which transfected much more efficiently using available techniques than the rat RINm5F cell line. There was also a perception that human insulin promoter constructs would not function in transfected rodent cells. However, these worries proved to be unfounded after it was later shown that there is a very high degree of sequence and functional conservation within the transcription factors that regulate the gene (e.g., 89% identity between rat and human PDX-1) and the human insulin promoter exhibited the expected pattern of activity in transgenic mice. As a result of the decision to concentrate on a detailed analysis of the rat insulin promoter, most of the literature on the insulin promoter pertains to this promoter.
The structure and evolution of the insulin gene has been previously reviewed. In this article, we focus on the sequences that lie upstream of, or flank, the transcription start site and are known to affect transcription of the gene. One major conclusion is that the rodent promoters are markedly different from the human promoter, and we urge caution in extrapolating data from rodent promoter studies to the etiology and therapy of diabetes.
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