[Regulation of human insulin gene expression by cAMP]

Various hormones and neurotransmitters as well as glucose are known to increase the cAMP concentration in pancreatic beta cells. To determine the mechanism by which cAMP augments insulin gene expression, we first identified the cAMP response elements (CREs) of the human insulin gene. In DNase I footprint analysis, the bacterially synthesized CRE-binding protein, CRE-BP1, protected four sites: two sites in the region upstream from the insulin core promoter, one site in the first exon, and one site in the first intron. To examine the roles of those four sites, we constructed a series of DNA plasmids in which the wild-type and mutant insulin promoters were linked to the chloramphenicol acetyltranferase (CAT) gene. Studies of the transcriptional activity of these plasmids after transfection into hamster insulinoma cells (HIT) showed that these four sites contributed additively to the cAMP inducibility of the insulin promoter.