Insulin-like Growth Factor-I Regulates Transcription of the Elastin Gene through a Putative Retinoblastoma Control Element: A ROLE FOR Sp3 ACTING AS A REPRESSOR OF ELASTIN GENE TRANSCRIPTION

Kelly J Conn,Celeste B Rich,Donna E Jensen,Marta R Fontanilla,Muhammad M Bashir,Joel Rosenbloom,Judith Ann Foster

doi:10.1074/jbc.271.46.28853

Abstract

Previous studies have demonstrated that insulin-like growth factor-I (IGF-I) increases elastin gene transcription in aortic smooth muscle cells and that this up-regulation is accompanied by a loss of protein binding 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 Sp3 binding is also abrogated by IGF-I, but in a selected manner. In functional analyses using Drosophila SL-2 cells, Sp1 expression can drive transcription from the elastin proximal promoter, while co-expression of Sp3 results in a repression of Sp1 activity. Footprint and gel shift analyses position the IGF-I responsive sequences to a putative retinoblastoma control element (RCE). Mutation of the putative RCE sequence as assessed by transient transfection of smooth muscle cells results in an increase in reporter activity equal in magnitude to that conferred by IGF-I on the wild type promoter. Together these results support 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.

Highlights

Previous studies have demonstrated that insulin-like growth factor-I (IGF-I) increases elastin gene transcription in aortic smooth muscle cells and that this up-regulation is accompanied by a loss of protein binding to the proximal promoter
What was somewhat puzzling to us was why the loss of Sp1 binding could result in an activation of elastin gene transcription, since our data suggested that IGF-I increased transcription of the elastin gene through a mechanism of derepression
Using neonatal SMC as a model system, we have shown that IGF-I is capable of increasing levels of elastin gene transcription in cells rendered quiescent

Summary

Introduction

Previous studies have demonstrated that insulin-like growth factor-I (IGF-I) increases elastin gene transcription in aortic smooth muscle cells and that this up-regulation is accompanied by a loss of protein binding to the proximal promoter. Our laboratory has shown that the increase in elastin gene transcription elicited by IGF-I in aortic SMC is accompanied by a loss of DNA-protein binding to an inhibitor sequence in the proximal promoter [9].

Results

Conclusion