TDAG51 mediates epithelial-to-mesenchymal transition in human proximal tubular epithelium

Carlisle RE, Heffernan A, Brimble E, Liu L, Jerome D, Collins CA, Mohammed-Ali Z, Margetts PJ, Austin RC, Dickhout JG. TDAG51 mediates epithelial-to-mesenchymal transition in human proximal tubular epithelium. Am J Physiol Renal Physiol 303: F467-F481, 2012. First published May 16, 2012; doi:10.1152/ajprenal.00481.2011.Epithelial-to-mesenchymal transition (EMT) contributes to renal fibrosis in chronic kidney disease. Endoplasmic reticulum (ER) stress, a feature of many forms of kidney disease, results from the accumulation of mis-folded proteins in the ER and leads to the unfolded protein response (UPR). We hypothesized that ER stress mediates EMT in human renal proximal tubules. ER stress is induced by a variety of stressors differing in their mechanism of action, including tunicamycin, thapsigargin, and the calcineurin inhibitor cyclosporine A. These ER stressors increased the UPR markers GRP78, GRP94, and phospho-eIF2 alpha in human proximal tubular cells. Thapsigargin and cyclosporine A also increased cytosolic Ca2+ concentration and T cell death-associated gene 51 (TDAG51) expression, whereas tunicamycin did not. Thapsigargin was also shown to increase levels of active transforming growth factor (TGF)-beta 1 in the media of cultured human proximal tubular cells. Thapsigargin induced cytoskeletal rearrangement, beta-catenin nuclear translocation, and alpha-smooth muscle actin and vinculin expression in proximal tubular cells, indicating an EMT response. Subconfluent primary human proximal tubular cells were induced to undergo EMT by TGF-beta 1 treatment. In contrast, tunicamycin treatment did not produce an EMT response. Plasmid-mediated overexpression of TDAG51 resulted in cell shape change and beta-catenin nuclear translocation. These results allowed us to develop a two-hit model of ER stress-induced EMT, where Ca2+ dysregulation-mediated TDAG51 upregulation primes the cell for mesenchymal transformation via Wnt signaling and then TGF-beta 1 activation leads to a complete EMT response. Thus the release of Ca2+ from ER stores mediates EMT in human proximal tubular epithelium via the induction of TDAG51.