Ubiquitin-dependent degradation of SnoN and Ski is increased in renal fibrosis induced by obstructive injury

Transforming growth factor-beta (TGF-beta) plays a critical role in the progression of renal fibrosis. The activity of TGF-beta is tightly controlled by various mechanisms, among which antagonizing Smad-mediated gene transcription by co-repressors represents one of the important components. We investigated the expression, degradation, and ubiquitination of Smad transcriptional co-repressors SnoN (ski-related novel gene N) and Ski (Sloan-Kettering Institute proto-oncogene) in renal fibrogenesis. We also studied the involvement of Smad-ubiquitination regulatory factor 2 (Smurf2) in ubiquitination of SnoN protein. The kidneys of mice with unilateral ureteral obstruction (UUO) and those of sham-operated mice were used. Renal lesions and the expression of TGF-beta1, type I collagen, SnoN, Ski, and Smurf2 were examined by immunohistochemistry, Western blot, and/or real-time reverse transcriptase-polymerase chain reaction. Degradation and ubiquitination of SnoN/Ski proteins were also investigated. The obstructed kidneys of UUO mice showed progressive tubulointerstitial fibrosis, high expression levels of TGF-beta1, type I collagen, SnoN and Ski mRNAs, and low levels of SnoN and Ski proteins. Both degradation and ubiquitination of SnoN/Ski proteins were markedly increased in the obstructed kidneys, in which Smurf2 expression was increased. Smurf2 immunodepletion in extracts of obstructed kidneys resulted in reduced ubiquitination of SnoN. Our results suggest that the reduction of SnoN/Ski proteins resulting from increased ubiquitin-dependent degradation is involved in the progression of tubulointerstitial fibrosis.