Induction of Vascular Smooth Muscle α-Actin Gene Transcription in Transforming Growth Factor β1-Activated Myofibroblasts Mediated by Dynamic Interplay between the Pur Repressor Proteins and Sp1/Smad Coactivators

摘要

The mouse vascular smooth muscle α-actin (SMA) gene enhancer is activated in fibroblasts by transforming growth factor β1 (TGFβ1), a potent mediator of myofibroblast differentiation and wound healing. The SMA enhancer contains tandem sites for the Sp1 transcriptional activator protein and Purα and β repressor proteins. We have examined dynamic interplay between these divergent proteins to identify checkpoints for possible control of myofibroblast differentiation during chronic inflammatory disease. A novel element in the SMA enhancer named SPUR was responsible for both basal and TGFβ1-dependent transcriptional activation in fibroblasts and capable of binding Sp1 and Pur proteins. A novel Sp1:Pur:SPUR complex was dissociated when SMA enhancer activity was increased by TGFβ1 or Smad protein overexpression. Physical association of Pur proteins with Smad2/3 was observed as was binding of Smads to an upstream enhancer region that undergoes DNA duplex unwinding in TGFβ1-activated myofibroblasts. Purβ repression of the SMA enhancer could not be relieved by TGFβ1, whereas repression mediated by Purα was partially rescued by TGFβ1 or overexpression of Smad proteins. Interplay between Pur repressor isoforms and Sp1 and Smad coactivators may regulate SMA enhancer output in TGFβ1-activated myofibroblasts during episodes of wound repair and tissue remodeling.