PROXIMAL ENHANCER OF THE HUMAN INSULIN-RECEPTOR GENE BINDS THE TRANSCRIPTION FACTOR SP1

The insulin receptor is a growth regulator present on the surface of m ost cells that transmits a mitogenic signal in response to insulin. Th us, the gene for the insulin receptor is constitutively expressed at l ow levels in all cells. We characterize a constitutive enhancer elemen t that is present in the proximal promoter of the human insulin recept or gene. We have localized the enhancer to a 26- base-pair (26-bp) seq uence from -528 to -503. When this sequence is inserted into the proxi mal promoter, a three- to fourfold increase in promoter activity is ob served, and when two copies are inserted, a five- to sixfold increase is seen. Electrophoretic mobility shift analysis demonstrates that nuc lear factors binding to this sequence are found in many different cell types. At least two proteins with different specificities bind within this 26-bp sequence. The identity of the predominant binding protein is Sp1, because an oligonucleotide composed of an Sp1 consensus bindin g sequence can compete for several of the DNA-protein complexes. in ad dition, we demonstrate that purified Sp1 can bind to the 26-bp oligonu cleotide and that this complex comigrates with a DNA-protein complex f ormed with a HeLa nuclear extract. Finally, an antibody to human Sp1 p rotein is able to bind to the enhancer DNA/HeLa protein complex and su pershift this complex. These findings suggest that this sequence corre sponds to a general element that may contribute to the ubiquitous expr ession of the human insulin receptor gene.