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 smooth muscle α-actin (SMαA) gene expression in graft ventricular cardiomyocytes. Compared with cardiac fibroblasts in which 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, an 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.