54778

RNF111

Transforming growth factor-beta (TGF-beta) signaling has been linked with tubular epithelial to mesenchymal cell transition. In this study, we examined the role of Arkadia, an E3 ubiquitin ligase that is critically required for TGF-beta signaling during epithelial to mesenchymal cell transition. We found that when normal human renal tubular epithelial cells in culture were stimulated with TGF-beta1, which increased their levels of Arkadia, Smurf2, TGF-beta type I receptor (TbetaRI), and Smad7 mRNA, but had low levels of Smad7 protein. When these cells were preincubated with Arkadia siRNA (small interfering RNA) and lactacystin (an inhibitor of proteasomal degradation), the TGF-beta(1) induced expression of Smad7, alpha-smooth muscle actin, and E-cadherin was partly reversed, but the expression of TbetaRI protein and Smad7 mRNA was not affected. In contrast, Smurf2 siRNA had no influence on the expression of these targets. Our studies suggest that Arkadia stimulates renal tubular epithelial to mesenchymal cell transition through degradation of Smad7.

mutations in TRPS1 cause tricho-rhino-pharyngeal syndrome (TRPS). Trps1 is essential for nephron development, acting downstream of Bmp7. Because Bmp7 counteracts epithelial-to-mesenchymal transition (EMT) and reverses chronic renal injury, we examined the function of Trps1 in renal fibrosis. Immunohistochemistry revealed Trps1 expression in proximal tubular epithelial cells of mice. Unilateral ureteral obstruction reduced mRNA and protein expression of Trps1 in wild-type and heterozygous Trps1-knockout (Trps1(+/-)) mice. Trps1 haploinsufficiency promoted tubulointerstitial fibrosis via increased phosphorylation of Smad3 and decreased Smad7 protein. In primary culture, Trps1 deficiency promoted TGF-beta1-mediated EMT in proximal tubule cells. Trps1(+/-)-derived cells had higher levels of phosphorylated Smad3, and TGF-beta1 induced a time-dependent decrease in Smad7 protein in wild-type and Trps1(+/-) kidneys. In addition, compared with wild-type cells, Trps1(+/-) cells had double the amount of the E3 ubiquitin ligase Arkadia, and TGF-beta1 induced further Arkadia expression. Furthermore, knockdown of Arkadia inhibited TGF-beta1-induced EMT in Trps1(+/-) cells. Collectively, these data suggest that Trps1 haploinsufficiency enhances TGF-beta1-induced EMT and tubulointerstitial fibrosis by modulating the amount of Smad7 through Arkadia/ubiquitin-mediated degradation.