Contribution of the constitutive and inducible degradation of Smad3 by the ubiquitin-proteasome pathway to transforming growth factor-beta signaling.
Smad proteins are crucial for the intracellular signaling of transforming growth factor-beta (TGF-beta). After receptor-induced activation, Smad proteins are phosphorylated and translocated to the nucleus to activate transcription of a select set of target genes. Here, we investigated the turnover of Smad3, positively regulating Smad for TGF-beta signaling. In a steady state, the inhibition of proteasome activity leads to stabilization of Smad3 protein. Smad proteins are multi-ubiquitinated and degraded independently of the phosphorylation induced by the TGF-beta receptors. Moreover, the degradation of Smad3 was enhanced by treatment with TGF-beta, and phosphorylated Smad3 was accumulated on proteasome inhibition. Ubiquitination of phosphorylated Smad3 but not Smad3(3SA), a receptor-mediated phosphorylation-incompetent mutant, was observed in the nucleus after treatment with TGF-beta. These findings suggest that, in a steady state, Smad3 is constitutively degraded via the ubiquitin-proteasome pathway in the cytoplasm and that, in response to TGF-beta, it is phosphorylated and translocated into the nucleus, where it is degraded through the ubiquitin-proteasome pathway.
Department of Molecular Health Sciences, Graduate School of Pharmaceutical Sciences, Nagoya City University, Mizuho, Nagoya 467-8603, Japan.
Yasumichi
Y
Inoue
Masatoshi
M
Kitagawa
Kikuo
K
Onozaki
Hidetoshi
H
Hayashi
D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D020125:Mutation, Missense / Q000235:genetics; D009687:Nuclear Proteins; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D017434:Protein Structure, Tertiary; D018125:Receptors, Transforming Growth Factor beta; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D051900:Smad3 Protein; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D016212:Transforming Growth Factor beta / Q000378:metabolism* / Q000494:pharmacology; D025801:Ubiquitin / Q000378:metabolism*
D004268:DNA-Binding Proteins; D007976:Leupeptins; D009097:Multienzyme Complexes; D009687:Nuclear Proteins; D018125:Receptors, Transforming Growth Factor beta; D011994:Recombinant Proteins; C497790:SMAD3 protein, human; D051900:Smad3 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D025801:Ubiquitin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde