Induction of Vascular Smooth Muscle ␣-actin Gene Transcription in Transforming Growth Factor ␤1-activated Myofibroblasts Mediated by Dynamic Interplay between the Pur Repressor Proteins and Sp1/smad Coactivators the Mouse Vascular Smooth Muscle ␣-actin (sma) Gene Enhancer Is Activated in Fibroblasts