Serum Response Factor Neutralizes Purα- and Purβ-mediated Repression of the Fetal Vascular Smooth Muscle α-actin Gene in Stressed Adult Cardiomyocytes

Mouse hearts subjected to repeated transplant surgery and ischemia/reperfusion injury develop substantial interstitial and perivascular fibrosis that was spatially associated with dysfunctional activation of fetal SMαA gene expression in graft ventricular cardiomyocytes. Compared to cardiac fibroblasts where nuclear levels of the Sp1 and Smad 2/3 transcriptional activating proteins increased markedly after transplant injury, the most abundant SMαA gene activating protein in cardiomyocyte nuclei was serum response factor (SRF). Additionally, cardiac intercalated discs in heart grafts contained substantial deposits of Purα, a mRNA-binding protein and known negative modulator of SRF-activated SMαA gene transcription. Activation of fetal SMαA gene expression in perfusion-isolated adult cardiomyocytes was linked to elevated binding of a novel protein complex consisting of SRF and Purα to a purine-rich DNA element in the SMαA promoter called SPUR previously shown to be required for induction of SMαA gene transcription in injury-activated myofibroblasts. Increased SRF binding to SPUR DNA plus one of two nearby CARG box consensus elements was observed in SMαA-positive cardiomyocytes in parallel with enhanced Purα:SPUR protein:protein interaction. The data suggest that de novo activation of the normally silent SMαA gene in reprogrammed adult cardiomyocytes is linked to elevated interaction of SRF with fetal-specific CARG and injury-activated SPUR elements in the SMαA promoter as well as the appearance of novel Purα protein complexes in both the nuclear and cytosolic compartments of these cells. Page 2 of 56