Up-regulated Transcriptional Repressors SnoN and Ski Bind Smad Proteins to Antagonize Transforming Growth Factor-β Signals during Liver Regeneration*

Abstract

Transforming growth factor-β (TGF-β) functions as an antiproliferative factor for hepatocytes. However, for unexplained reasons, hepatocytes become resistant to TGF-β signals and can proliferate despite the presence of TGF-β during liver regeneration. TGF-β is up-regulated during liver regeneration, although it is not known whether it is active or latent. TGF-β activity may be examined by assessing Smad activation, a downstream signaling pathway. Smad pathway activation during liver regeneration induced by partial hepatectomy or CCl₄ injury was examined by assessing the levels of phospho-Smad2 and Smad2-Smad4 complexes. We found that Smad proteins were slightly activated in quiescent liver, but that their activation was further enhanced in regenerating liver. Interestingly, TGF-β/Smad pathway inhibitors (SnoN and Ski) were up-regulated during regeneration, and notably, SnoN was induced mainly in hepatocytes. SnoN and Ski are transcriptional repressors that may render some cells resistant to TGF-β via binding Smad proteins. Complexes between SnoN, Ski, and the activated Smad proteins were detected from 2 to 120 h during the major proliferative phase in regenerating liver. Inhibitory complexes decreased after liver mass restitution (5–15 days), suggesting that persistently activated Smad proteins might participate in returning the liver to a quiescent state. Our data show that active TGF-β/Smad signals are present during regeneration and suggest that SnoN/Ski induction might explain hepatocyte resistance to TGF-β during the proliferative phase.