Ubiquitin-dependent degradation of Smad2 is increased in the glomeruli of rats with anti-thymocyte serum nephritis.

The overexpression of transforming growth factor (TGF)-beta and Smad-mediated intracellular TGF-beta signaling in the kidney underlies the development of renal scarring from pathological matrix accumulation. However, changes in the Smad proteins during the progression of kidney disease are unclear. In this study, we investigated the regulation of Smad proteins in the glomeruli of rats with anti-thymocyte serum nephritis. We found that Smad2 protein decreased markedly in nephritic glomeruli, whereas no significant changes were observed in the levels of Smad3 and Smad4 proteins. In contrast, the level of Smad2 mRNA in nephritic glomeruli did not differ significantly from that in control glomeruli. Based on recent reports of the ubiquitin-mediated degradation of Smad2, we investigated the degradation and ubiquitination activity directed against Smad2 in glomerular extracts. Both the degradation and ubiquitination of Smad2 were markedly increased in glomerular extracts from rats with nephritis. We also found that Smurf2, a ubiquitin ligase for Smad2, was increased in the nephritic glomerular extracts. These data suggest that the decrease in Smad2 resulted from enhanced ubiquitin-dependent degradation of Smad2 mediated by Smurf2, and is involved in the regulation of Smad2-mediated TGF-beta signaling in nephritic glomeruli.