MUTUALLY EXCLUSIVE INTERACTIONS BETWEEN FACTORS BINDING TO ADJACENT SP1 AND AT-RICH ELEMENTS REGULATE GASTRIN GENE-TRANSCRIPTION IN INSULINOMA CELLS

The gastrin gene is transiently expressed in fetal pancreatic islets d uring islet neogenesis but then switched off after birth when islet ce lls become fully differentiated. Previous studies identified a cis-reg ulatory sequence between -109 and -75 in the human gastrin promoter wh ich binds islet cell-specific activators and a nonspecific repressor a nd thus may act as a molecular switch. The present study identified an other cis-regulatory sequence ((-163)ACACTAAATGAAAGGGCGGGGCAG(-140)) w hich bound two islet nuclear proteins in a mutually exclusive manner, as defined by gel shift competition, methylation interference, and DNa se I footprinting assays. The general transactivator Sp1 recognized th e downstream GGGCGGGG sequence, but Sp1 binding was prevented when ano ther islet factor bound to the adjacent AT-rich sequence (CTAAATGA). T his gastrin AT-rich element is nearly identical to the binding site (A TAAATGA) for the islet-specific transcription factor beta TF-1. Howeve r, the gastrin AT-binding factor appeared to differ from beta TF-1 in its gel mobility shift pattern. Transfections of rat insulinoma cells revealed that mutations which blocked binding to the AT-rich element b ut allowed Sp1 binding up-regulated transcriptional activity. These re sults suggest that the gastrin AT-binding factor blocks transactivatio n by Sp1 and may have a role in the repression of gastrin transcriptio n seen at the end of islet differentiation.