Myocardin-Related Transcription Factors A and B Are Key Regulators of TGF-β1-Induced Fibroblast to Myofibroblast Differentiation

Myofibroblasts are contractile, smooth muscle-like cells that are characterized by the de novo expression of smooth muscle alpha-actin (SM alpha A) and normally function to assist in wound closure, but have been implicated in pathological contractures. Transforming growth factor beta-1 (TGF-beta 1) helps facilitate the differentiation of fibroblasts into myofibroblasts, but the exact mechanism by which this differentiation occurs, in response to TGF-beta 1, remains unclear. Myocardin-related transcription factors A and B (MRTFs, MRTF-A/B) are transcriptional co-activators that regulate the expression of smooth muscle-specific cytoskeletal proteins, including SM alpha A, in smooth muscle cells and fibroblasts. In this study, we demonstrate that TGF-beta 1 mediates myofibroblast differentiation and the expression of a contractile gene program through the actions of the MRTFs. Transient transfection of a constitutively active MRTF-A induced an increase in the expression of SM alpha A and other smooth muscle-specific cytoskeletal proteins, and an increase in myofibroblast contractility, even in the absence of TGF-beta 1. MRTF-A/B knockdown, in TGF-beta 1-differentiated myofibroblasts, resulted in decreased smooth muscle-specific cytoskeletal protein expression levels and reduced contractile force generation, as well as a decrease in focal adhesion size and number. These results provide direct evidence that the MRTFs are mediators of myofibroblast differentiation in response to TGF-beta 1.