The Purα/Purβ Single-Strand DNA-Binding Proteins Attenuate Smooth-Muscle Actin Gene Transactivation in Myofibroblasts

Expression of smooth muscle alpha-actin (SMA) is essential for myofibroblast-mediated wound contraction following tissue injury. The Pur / and YB-1 transcriptional repressors govern the DNA-binding activity of serum response factor (SRF) and phosphorylated Smad3 (pSmad3) transcriptional activators during induction of SMA gene expression in human pulmonary myofibroblasts. In quiescent fibroblasts, Pur exhibited a novel function in enhancing stability of pre-existing SRF complexes with SMA core promoter DNA, whereas Pur was more effective in disrupting SRF-DNA interaction. Pur proteins were less efficient competitors of pre-existing, core-promoter complexes containing both SRF and pSmad3 in nuclear extracts from TGF1-activated myofibroblasts. TGF1 signaling dissociated a SRF/Pur protein complex with concurrent formation of a transient pSmad3/MRTF-A/Pur complex during early phase myofibroblast differentiation. Pur was replaced by Pur in the pSmad3/MRTF-A complex in mature myofibroblasts. Combining all three repressors potently inhibited SRF and pSmad3 binding to promoter DNA in quiescent fibroblasts and TGF1-activated myofibroblasts, respectively. The results point to dynamic interplay between transcriptional activators and repressors in regulating SMA gene output during myofibroblast differentiation. Therapeutic targeting of nucleoprotein complexes regulating the SMA promoter may prevent excessive myofibroblast accumulation associated with chronic cardiopulmonary fibrosis and dysfunctional tissue remodeling. J. Cell. Physiol. 229: 1256-1271, 2014. (c) 2014 Wiley Periodicals, Inc.