Key Words: vascular smooth muscle cells Ⅲ microRNA Ⅲ miR-133 Ⅲ smooth muscle differentiation Ⅲ vascular remodeling V ascular smooth muscle cells (VSMCs) within adult blood vessels proliferate at a very low rate, exhibit very low synthetic activity, and express a unique repertoire of contractile proteins, ion channels, and signaling molecules. 1 Unlike skeletal muscle and cardiac muscle, which consist of terminally differentiated cells, adult VSMCs retain remarkable plasticity and can undergo rather profound and reversible changes in phenotype and growth properties in response to changes in local environmental cues. Salient examples of VSMC plasticity can be seen in response to vascular injury when VSMCs dramatically increase their proliferation, migration, and synthetic capacity, playing a critical role in vascular repair. 1,2 A detrimental consequence of the high degree of plasticity exhibited by adult VSMCs is that it can lead to an adverse phenotypic switch and acquisition of characteristics that can contribute to development or progression of vascular disease in humans, including atherosclerosis, restenosis, cancer, and hypertension. [1][2][3] VSMC phenotypic modulation is characterized by significant changes in gene expression patterns, matrix and cytokine production, contractility, and growth state, ultimately leading to their switch from a synthetic to a proliferative phenotype.Original received January 3, 2011; revision received August 8, 2011; accepted August 9, 2011. In July 2011, the average time from submission to first decision for all original research papers submitted to Circulation Research was 13.5 days.From Thus, understanding the regulatory mechanisms underlying the VSMC phenotypic switch is of paramount importance. [1][2][3] One of the key breakthroughs for the study of gene expression regulation has recently been the discovery of microRNAs (miRNAs or miRs) and their role in gene silencing through mRNA degradation or translational inhibition. 4,5 Increasing evidence indicates that miRNAs regulate key genetic programs in cardiovascular biology, physiology, and disease. 4,5 In particular, miR-21, -143, -145, -221, -222 have all been implicated to play a role in VSMC function and phenotypic plasticity. 6 -11 More recently, 2 articles demonstrated that miR-1 is induced by myocardin overexpression in human SMCs, contributing to myocardin-dependent reduction of human SMC growth in vitro. 12,13 miR-133a-1/miR-1-2 and miR133a-2/miR-1-1 are 2 bicistronic miRNA clusters reported to be specifically expressed in cardiac and skeletal muscle. 4,5 A third bicistronic miRNA cluster, comprising miR-206 and miR-133b, is expressed specifically in skeletal muscle but not in the heart. 4,5 miR-1 (miR-1-1/miR-1-2) and miR-133 (miR133a-1/miR-133a-2) play essential roles in cardiac and skeletal muscle development, physiology, and disease 4,5 ; however, their functions in VSMCs and vascular disease are largely unknown. Thus, the aim of the present study was to evaluate the role, if any, of miR-1 and miR-133 in V...