Myocardin and myocardin-related transcription factor-A synergistically mediate actin cytoskeletal-dependent inhibition of liver fibrogenesis.

Activation of hepatic stellate cells (HSCs), characterized by development of a robust actin cytoskeleton and expression of abundant extracellular matrix (ECM) proteins, such as type 1 collagen (COL.1), is a central cellular and molecular event in liver fibrosis. It has been demonstrated that HSCs express both myocardin and myocardin-related transcription factor-A (MRTF-A). However, the biological effects of myocardin and MRTF-A on HSC activation and liver fibrosis, as well as the molecular mechanism under the process, remain unclear. Here, we report that myocardin and MRTF-A's expression and nuclear accumulation are prominently increased during the HSC activation process, accompanied by robust activation of actin cytoskeleton dynamics. Targeting myocardin and MRTF-A binding and function with a novel small molecule, CCG-203971, led to dose-dependent inhibition of HSC actin cytoskeleton dynamics and abrogated multiple functional features of HSC activation (i.e., HSC contraction, migration and proliferation) and decreased COL.1 expression in vitro and liver fibrosis in vivo. Mechanistically, blocking the myocardin and MRTF-A nuclear translocation pathway with CCG-203971 directly inhibited myocardin/MRTF-A-mediated serum response factor (SRF), and Smad2/3 activation in the COL.1α2 promoter and indirectly abrogated actin cytoskeleton-dependent regulation of Smad2/3 and Erk1/2 phosphorylation and their nuclear accumulation. Finally, there was no effect of CCG-203971 on markers of inflammation, suggesting a direct effect of the compound on HSCs and liver fibrosis. These data reveal that myocardin and MRTF-A are two important cotranscriptional factors in HSCs and represent entirely novel therapeutic pathways that might be targeted to treat liver fibrosis. NEW & NOTEWORTHY Myocardin and myocardin-related transcription factor-A (MRTF-A) are upregulated in activated hepatic stellate cells (HSCs) in vitro and in vivo, closely associated with robustly increased actin cytoskeleton remodeling. Targeting myocardin and MRTF-A by CCG-203971 leads to actin cytoskeleton-dependent inhibition of HSC activation, reduced cell contractility, impeded cell migration and proliferation, and decreased COL.1 expression in vitro and in vivo. Dual expression of myocardin and MRTF-A in HSCs may represent novel therapeutic targets in liver fibrosis.