3.3. Involvement of Smurf1 in Nrf2-mediated Smad7 elevationObtained results showed that protein levels of Smad7 were upregulated without the elevation in transcript levels in shKeap1 cells. In our previous study, we identified that Smurf1 protein was differentially expressed in Keap1 knockdown human renal tubular epithelial cells [39]. In this context, we investigated the presence of negative feedback regulation of Smurf1 and Smad7 in Keap1 knockdown mesangial cells. As shown in Fig. 3A, protein levels of Smurf1, an E3 ubiquitin ligase for Smad7 degradation, were significantly decreased in shKeap1 cells, whereas Smurf2 levels were not affected by Keap1 deletion. In addition, decrease in Smurf1 mRNA levels was confirmed in shKeap1 cells (Fig. 3B). Smurf1 level was found to marginally elevated by TGF-β1 treatment, which is accompanied by Smad7 elevation (Fig. 3C). This suggests that Smurf1 level is also dependent on TGF-β1 signaling in our cell system. Unfortunately, the cause of Smurf1 changes in shKeap1 cells has not yet been identified; however, we observed that the protein levels of Smad7 were increased by si-Smurf1 transfection in the control MES-13 cells, indicating the direct link between Smurf1 and Smad7 reduction (Fig. 3D). In line with this, overexpression of Smurf1 in shKeap1 cells repressed Smad7 levels to that of the control cells (Fig. 3E). These observations indicate that the Nrf2 pathway leads to Smad7 elevation through Smurf1 reduction in MES-13 cells.