INSULIN-LIKE GROWTH-FACTOR-I REGULATES TRANSCRIPTION OF THE ELASTIN GENE THROUGH A PUTATIVE RETINOBLASTOMA CONTROL ELEMENT - A ROLE FOR SP3ACTING AS A REPRESSOR OF ELASTIN GENE-TRANSCRIPTION

Previous studies have demonstrated that insulin-like growth factor-I ( IGF-I) increases elastin gene transcription in aortic smooth muscle ce lls and that this up-regulation is accompanied by a loss of protein bi nding to the proximal promoter. Sp1 has been identified as one of the factors whose binding is lost, and in the present study we show that S p3 binding is also abrogated by IGF-I, but in a selected manner. In fu nctional analyses using Drosophila SL-2 cells, Sp1 expression can driv e transcription from the elastin proximal promoter, while co expressio n of Sp3 results in a repression of Sp1 activity. Footprint and gel sh ift analyses position the IGF-I responsive sequences to a putative ret inoblastoma control element (RCE). Mutation of the putative RCE sequen ce as assessed by transient transfection of smooth muscle cells result s in an increase in reporter activity equal in magnitude to that confe rred by IGF-I on the wild type promoter. Together these results suppor t the hypothesis that IGF-I-mediated increase in elastin transcription occurs via a mechanism of derepression involving the abrogation of a repressor that appears to be Sp3 binding to the RCE.