Force activates smooth muscle α-actin promoter activity through the Rho signaling pathway

• Published in: Journal of cell science Vol. 120; no. 10; pp. 1801 - 1809
• Main Authors: Zhao, Xiao-Han, Laschinger, Carol, Arora, Pam, Szászi, Katalin, Kapus, Andras, McCulloch, Christopher A
• Format: Journal Article
• Language: English
• Published: England The Company of Biologists Limited 15.05.2007
• Subjects: Actin Cytoskeleton - metabolism; Actins - genetics; Actins - metabolism; Animals; Cardiomegaly - genetics; Cardiomegaly - metabolism; Cardiomegaly - physiopathology; Cell Culture Techniques; Cell Differentiation - physiology; Cofilin 1 - metabolism; Enzyme Inhibitors - pharmacology; Fibroblasts - metabolism; Lim Kinases; Mechanotransduction, Cellular - physiology; Myoblasts, Cardiac - metabolism; Phosphorylation; Pressure - adverse effects; Promoter Regions, Genetic - genetics; Protein Kinases - metabolism; Rats; rho GTP-Binding Proteins - metabolism; rhoA GTP-Binding Protein - metabolism; Signal Transduction - physiology; Stress, Mechanical; Tensile Strength - physiology; Trans-Activators - metabolism; Transcriptional Activation - genetics
• DOI: 10.1242/jcs.001586
• ISSN: 0021-9533

In pressure or volume overload, hypertrophic growth of the myocardium is associated with myofibroblast differentiation, a process in which cardiac fibroblasts express smooth muscle α-actin (SMA). The signaling mechanisms that mediate force-induced myofibroblast differentiation and SMA expression are not defined. We examined the role of the Rho-Rho-kinase pathway in force-induced SMA expression in fibroblasts using an in vitro model system that applies static tensile forces (0.65 pN/μm²) to integrins via collagen-coated magnetite beads. Force maximally induced RhoA activation at 10 minutes that was localized to force application sites and required intact actin filaments. Force application induced phosphorylation of LIM kinase (5-10 minutes) and an early dephosphorylation of cofilin (5 minutes) that was followed by prolonged cofilin phosphorylation. These responses were blocked by Y27632, an inhibitor of Rho kinase. Force promoted actin filament assembly at force application sites (10-20 minutes), a process that required Rho kinase and cofilin. Force application induced nuclear translocation of the transcriptional co-activator MRTF-A but not MRTF-B. Nuclear translocation of MRTF-A required Rho kinase and intact actin filaments. Force caused 3.5-fold increases of SMA promoter activity that were completely blocked by transfection of cells with dominant-negative MRTF-A or by inhibition of Rho kinase or by actin filament disassembly. These data indicate that mechanical forces mediate actin assembly through the Rho-Rho-kinase-LIMK cofilin pathway. Force-mediated actin filament assembly promotes nuclear translocation of MRTF and subsequent activation of the SMA promoter to enhance SMA expression.