Transforming Growth Factor β Facilitates β-TrCP-Mediated Degradation of Cdc25A in a Smad3-Dependent Manner

Abstract

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.

ACKNOWLEDGMENTS

We thank Michele Pagano for sharing the sequences of β-TrCP1/2 siRNAs prior to publication; Chuxia Deng for Smad3^−/− MEFs; Peter Jackson for the Emi1 and Cdh1 plasmids; Pradip Raychaudhuri for cullin plasmids and technical suggestions; and Helen Piwnica-Worms, Antonio Iavarone, Siwanon Jirawatnotai, and Evan Osmundson for helpful discussions.

This work was supported in part by funds provided to H.K. by the National Institutes of Health (R01HD38085 and R01CA100204) and the Department of Defense (DAMD 17-02-1-0413).