Regulation of Kidney Fibroblast Functions by Palladin-MRTF Circuit in Response to TGF-β1

Background: Kidney fibrosis is the common mechanism resulting in chronic kidney disease. We previously clarified myocardin-related transcription factor (MRTF) – serum response factor (SRF) signaling drives renal fibroblast activation and focal adhesion formation consisting of integrins through actin cytoskeleton reorganization. Thus far, it has been reported that actin-associated proteins (AAPs) regulate actin filament assembly and disassembly. However, it has yet to be understood whether AAPs relates to the pathogenesis of the fibrosis. Here, we investigated whether AAPs contributed to the renal fibroblasts (RFBs) activation and focal adhesion formation through actin cytoskeleton reorganization. Methods: Cultured renal fibroblasts were used to examine transforming growth factor (TGF)-β1-dependent AAPs. Of these AAPs, palladin was selected for further investigation. The effects of palladin on actin polymerization and MRTF-SRF signaling were evaluated in TGF-β1-stimulated renal fibroblasts (RFBs) treated with paladin siRNA. In addition, palladin expression was evaluated with siRNA targeting MRTF-A/-B. Furthermore, expressions of α1 chain of type I procollagen (COLIAI), α-smooth muscle actin (αSMA), and integrin α11/β1 were explored with siRNA targeting palladin. Results: Palladin was significantly upregulated in RFBs with TGF-β1 stimulation. Palladin knockdown suppressed actin polymerization and MRTF-SRF signaling induced by TGF-β1 whereas palladin expression was suppressed by siRNA targeting MRTF-A/-B. These results suggested that the amplification loop was created between palladin and MRTF-SRF signaling. In addition, the treatment of RFBs with siRNA targeting palladin attenuated COLIAI and αSMA. In addition, integrin α11/β1 expression induced by the stimulation with TGF-β1 was also suppressed by palladin knockdown. Finally, the inhibition of integrin α11/β1 expression reduced MRTF-SRF signaling and expressions of COLIAI and αSMA. Conclusion: TGF-β1-induced amplification loop between palladin and MRTF-SRF signaling mediates renal fibroblast activation and focal adhesion formation, resulting in the upregulation of extracellular matrix expression.