Cdc42 regulates myocardin-related transcription factor nuclear shuttling and α-smooth muscle actin promoter activity during renal tubular epithelial-mesenchymal transition

Background/Aims: Epithelial–mesenchymal transition of tubular cells into α-smooth muscle actin (SMA)-expressing myofibroblasts is a central mechanism in tubulointerstitial fibrosis. Previously, a ‘two-hit’ model was proposed for epithelial–mesenchymal transition wherein an initial injury of the intercellular contacts and TGF-β1 are both required for SMA protein expression in LLC-PK1 cells. The Rho-Rho kinase-myosin light chain-myocardin-related transcription factor (MRTF)-serum response factor (SRF) pathway and Rac1, p21-activated kinase (PAK) and p38 were described as important regulators of MRTF localization and SMA expression. Cdc42 is another small G protein situated upstream of PAK and p38, and is activated upon cell contact disassembly. Here, we investigated its potential role in the regulation of MRTF nuclear shuttling and in the regulation of the SMA promoter. Results: Transfection of a constitutive active (CA) Cdc42 construct alone induced the activation of the SMA promoter. The dominant negative (DN) Cdc42 construct prevented the activation of the promoter induced by cell contact disassembly, and reduced the combined effect of cell contact disruption and TGF-β1. SRF showed a marked nuclear accumulation in CA Cdc42-transfected cells. Cdc42 induced the nuclear translocation of MRTF, while DN Cdc42 inhibited its nuclear translocation induced by cell contact disassembly. Blocking PAK, MRTF and p38 by the corresponding DN constructs blunted the effects of CA Cdc42 on the SMA promoter. Conclusion: Cdc42 is involved in the regulation of SMA promoter activation through PAK, p38, MRTF and SRF. Cdc42 may be an important regulator of MRTF cellular localization.