Myocardin Is Sufficient for a Smooth Muscle–Like

Background—Myocardin (Myocd) is a strong coactivator that binds the serum response factor (SRF) transcription factor over CArG elements embedded within smooth muscle cell (SMC) and cardiac muscle cyto-contractile genes. Here, we sought to ascertain whether Myocd-mediated gene expression confers a structural and physiological cardiac or SMC phenotype. Methods and Results—Adenoviral-mediated expression of Myocd in the BC3H1 cell line induces cardiac and SMC genes while suppressing both skeletal muscle markers and cell growth. Immunofluorescence microscopy shows that SRF and a SMC-like cyto-contractile apparatus are elevated with Myocd overexpression. A short hairpin RNA to Srf impairs BC3H1 cyto-architecture; however, cotransduction with Myocd results in complete restoration of the cyto-architecture. Electron microscopic studies demonstrate a SMC ultrastructural phenotype with no evidence for cardiac sarcomerogenesis. Biochemical and time-lapsed videomicroscopy assays reveal clear evidence for Myocd-induced SMC-like contraction. Conclusion—Myocd is sufficient for the establishment of a SMC-like contractile phenotype. (Arterioscler Thromb Vasc Biol. 2008;28:1505-1510) Key Words: smooth muscle serum response factor myocardin contraction knockdown Differentiated vascular smooth muscle cells (SMCs) have 2important phenotypic characteristics. First, they replicate infrequently within the normal vessel wall.1 Second, they ex