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We recently demonstrated that the cardiotonic steroid marinobufagenin (MBG) induced fibrosis in rat hearts through direct stimulation of collagen I secretion by cardiac fibroblasts. This stimulation was also responsible for the cardiac fibrosis seen in experimental renal failure. In this study, the effect of MBG on the development of renal fibrosis in rats was investigated. Four weeks of MBG infusion triggered mild periglomerular and peritubular fibrosis in the cortex and the appearance of fibrotic scars in the corticomedullary junction of the kidney. MBG also significantly increased the protein levels and nuclear localization of the transcription factor Snail in the tubular epithelia. It is known that activation of Snail is associated with epithelial-to-mesenchymal transition (EMT) during renal fibrosis. To examine whether MBG alone can trigger EMT, we used the porcine proximal tubular cell line LLC-PK1. MBG (100 nM) caused LLC-PK1 cells grown to confluence to acquire a fibroblast-like shape and have an invasive motility. The expressions of the mesenchymal proteins collagen I, fibronectin, and vimentin were increased twofold. However, the total level of E-cadherin remained unchanged. These alterations in LLC-PK1 cells in the presence of MBG were accompanied by elevated expression and nuclear translocation of Snail. During the time course of EMT, MBG did not have measurable inhibitory effects on the ion pumping activity of its natural ligand, [Na.sup.+]-[K.sup.+]-ATPase. Our data suggest that the MBG may be an important factor in inducing EMT and, through this mechanism, elevated levels of MBG in chronic renal failure may play a role in the progressive fibrosis. cardiotonic steroid hormone; ouabain; [Na.sup.+]-[K.sup.+]-ATPase; transcription factor Snail