Transforming growth factor β facilitates β-TrCP-mediated degradation of Cdc25A in a Smad3-dependent manner

Ubiquitin-dependent degradation of Cdc25A is a major mechanism for damage-induced S-phase checkpoint. Two ubiquitin ligases, the Skp1-cullin-β-TrCP (SCF^β-TrCP) complex and the anaphase-promoting complex (APC^Cdh1), are involved in Cdc25A degradation. Here we demonstrate that the transforming growth factor β (TGF-β)-Smad3 pathway promotes SCF^β-TrCP-mediated Cdc25A ubiquitination. Cells treated with TGF-β, as well as cells transfected with Smad3 or a constitutively active type I TGF-β receptor, exhibit increased ubiquitination and markedly shortened half-lives of Cdc25A. Furthermore, Cdc25A is stabilized in cells transfected with Smad3 small interfering RNA (siRNA) and cells from Smad3-null mice. TGF-β-induced ubiquitination is associated with Cdc25A phosphorylation at the β-TrCP docking site (DS⁸²G motif) and physical association of Cdc25A with Smad3 and β-TrCP. Cdc25A mutant proteins deficient in DS⁸²G phosphorylation are resistant to TGF-β-Smad3-induced degradation, whereas a Cdc25A mutant protein defective in APC^Cdh1 recognition undergoes efficient degradation. Smad3 siRNA inhibits β-TrCP-Cdc25A interaction and Cdc25A degradation in response to TGF-β. β-TrCP2 siRNA also inhibits Smad3-induced Cdc25A degradation. In contrast, Cdh1 siRNA had no effect on Cdc25A down-regulation by Smad3. These data suggest that Smad3 plays a key role in the regulation of Cdc25A ubiquitination by SCF^β-TrCP and that Cdc25A stabilization observed in various cancers could be associated with defects in the TGF-β-Smad3 pathway.