Smad4 protein stability is regulated by ubiquitin ligase SCF beta-TrCP1

SMAD4 is a key intracellular mediator for the transforming growth factor-beta (TGF-beta) superfamily of growth factors and is also an important tumor suppressor. The receptor-regulated Smad (R-Smad) proteins are regulated by ubiquitin-mediated degradation, yet the precise control of SMAD4 protein stability is unclear. We have identified SCF(beta-TrCP1), a ubiquitin (E3) Ligase, as a critical determinant for the protein degradation of SMAD4 protein. F-box protein beta-TrCP1 in this E3 Ligase interacts with SMAD4 both in yeast and in mammalian cells, but has no interaction with SMAD2 and has weak interaction with SMAD3. The beta-TrCP1/SMAD3 interaction was abolished by SMAD4 gene silencing, indicating the interaction is indirect and is through SMAD4. Ectopic expression of SCF complex containing beta-TrCP1 is sufficient to induce the ubiquitination and degradation of SMAD4. Furthermore, small interfering RNA-triggered endogenous beta-TrCP1 suppression increases the expression of SMAD4 protein. Consistent with these results, cells that overexpress the SCF complex display an inhibited TGF-beta-dependent transcriptional activity and an impaired cell cycle arrest function. Thus, SCF(beta-TrCP1) abrogates TGF-beta function in vivo by decreasing SMAD4 stability.