Arkadia regulates TGF-β signaling during renal tubular epithelial to mesenchymal cell transition

Transforming growth factor-β (TGF-β) 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-β signaling during epithelial to mesenchymal cell transition. We found that when normal human renal tubular epithelial cells in culture were stimulated with TGF-β1, which increased their levels of Arkadia, Smurf2, TGF-β type I receptor (TβRI), 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-β1 induced expression of Smad7, α-smooth muscle actin, and E-cadherin was partly reversed, but the expression of TβRI 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.