AN E2F BINDING SEQUENCE NEGATIVELY REGULATES THE RESPONSE OF THE INSULIN-LIKE GROWTH-FACTOR-1 (IGF-I) PROMOTER TO SIMIAN-VIRUS-40T ANTIGEN AND TO SERUM

The promoter of the Insulin-like growth factor I (IGF-I) gene is activ ated by the Simian Virus 40 large T antigen (SVLT), and one of the ele ments responding to SVLT activation has been localized to a short 124 bp immediately upstream of the first initiation of transcription site. This short promoter contains an E2F binding site, that, in gel shifts , binds a protein complex, but only when the promoter activity is redu ced or absent. A mutation in the E2F binding site deregulates the acti vity of the promoter, which becomes active even in those conditions in which the wild type promoter is inactive. By using antibodies in gel retardation analyses, we can show that the different protein complexes include, at least, the following proteins: E2F, cyclin A and p107. We conclude that the short IGF-I promoter is negatively regulated by an E2F binding site that complexes with several proteins. Our data sugges t that disaggregation of these complexes by the action of SVLT (or oth er activators) increases expression from the promoter, thus establishi ng a link between the regulation of cell proliferation by growth facto rs and the E2F-associated proteins.