Myocardin Is Sufficient for a Smooth Muscle-Like Contractile Phenotype

Abstract

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-induc... Though Myocd activates cardiac and smooth muscle genes, which cell type is conferred physiologically is unclear. We show Myocd overexpression is sufficient for structural and functional attributes of the smooth muscle contractile phenotype. Such studies have implications for understanding and treating a variety of smooth muscle-associated diseases where the normal contractile phenotype is destabilized.