TRANSCRIPTIONAL REGULATION OF THE HUMAN INSULIN GENE IS DEPENDENT ON THE HOMEODOMAIN PROTEIN STF1 IPF1 ACTING THROUGH THE CT BOXES/

Insulin gene transcription is a unique feature of the pancreatic beta cells and is increased in response to glucose. The recent cloning of i nsulin promoter factor 1 (IPF1) and somatostatin transcription factor 1 (STF1) unexpectedly revealed that these are mouse and rat homologues of the same protein mediating transactivation through binding of CT b ox-like elements in rat insulin 1 and somatostatin promoter/enhancer r egions, respectively. By using oligonucleotides representing each of t he three CT boxes of the human insulin (HI) gene enhancer and nuclear extracts from the mouse islet tumor cell lines beta TC3 and alpha TC1, we have identified a beta-cell-specific binding activity as reported for IPF1, which has maximal affinity toward the CT2 box. However, in p luripotent, HI-transfected rat islet tumor cells, NHI-6F, this binding activity is present prior to induction of (human) insulin gene transc ription. Its migration is identical to that of in vitro translated STF 1 in electrophoretic mobility-shift assays; it is specifically recogni zed by anti-STF1 antibodies and has an apparent molecular mass of 46 k Da. Mutation of the CT2 box decreases transcriptional activity of a HI reporter plasmid by approximate to 65% in beta TC3 cells and blocks t he glucose response in isolated newborn rat islet cells. Furthermore, cotransfection with STP1 cDNA into the glucagon-producing alpha TC1 ce lls increases the activity of the HI enhancer 4- to 5-fold, suggesting that STF1/IPF1 can confer on alpha TC1 cells the ability to transcrib e the HT gene. We conclude that STF1/IPF1 is a necessary but not suffi cient key regulator of insulin gene activity, possibly also involved i n glucose-regulated transcription.