REGULATION OF INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR GENE-EXPRESSION BY SP1 - PHYSICAL AND FUNCTIONAL INTERACTIONS OF SP1 AT GC BOXES AND AT A CT ELEMENT

The insulin-like growth factor I (IGF-I) receptor mediates signal transduction by the IGFs and plays a critical role in growth and development, The proximal promoter region of the rat IGF-I receptor gene contains multiple Sp1 consensus-binding sites (GC boxes), Various promoter fragments fused to a luciferase reporter gene were transiently cotransfected together with an Sp1 expression vector into Drosophila Schneider cells, which lack endogenous Sp1. A proximal promoter fragment containing 476 nucleotides of 5'-flanking region and 640 nucleotides of 5'-untranslated region was strongly activated by Spl (an average of 116-fold), and progressive 5'-deletions of the promoter that sequentially removed GC boxes reduced Sp1 activation to 15-fold over basal promoter activity, DNase I footprinting studies with purified Sp1 protein revealed four GC boxes in the 5'-flanking region of the promoter and one homopurine/homopyrimidine motif (CT element) in the 5'-untranslated region that bound Sp1. Mutation of the CT element reduced Spl activation by 70%, Taken together, these results demonstrate that Spl can regulate expression of the IGF-I receptor promoter by acting both on GC boxes in the 5'-flanking region of the promoter and on a CT element in the 5'-untranslated region.