Mechanism for Mutational Inactivation of the Tumor Suppressor Smad2

Abstract

Transforming growth factor β (TGF-β) is a potent natural antiproliferative agent that plays an important role in suppressing tumorigenicity. In numerous tumors, loss of TGF-β responsiveness is associated with inactivating mutations that can occur in components of this signaling pathway, such as the tumor suppressor Smad2. Although a general framework for how Smads transduce TGF-β signals has been proposed, the physiological relevance of alterations of Smad2 functions in promoting tumorigenesis is still unknown. Here, we show that expression of Smad2.P445H, a tumor-derived mutation of Smad2 found in human cancer, suppresses the ability of the Smads to mediate TGF-β-induced growth arrest and transcriptional responses. Smad2.P445H is phosphorylated by the activated TGF-β receptor at the carboxy-terminal serine residues and associates with Smad3 and Smad4 but is unable to dissociate from the receptor. Upon ligand-induced phosphorylation, Smad2.P445H interacts stably with wild-type Smad2, thereby blocking TGF-β-induced nuclear accumulation of wild-type Smad2 and Smad2-dependent transcription. The ability of the Smad2.P445H to block the nuclear accumulation of wild-type Smad2 protein reveals a new mechanism for loss of sensitivity to the growth-inhibitory functions of TGF-β in tumor development.

ACKNOWLEDGMENTS

C. Prunier and N. Ferrand contributed equally to this work.

We thank G. Cherqui for helpful discussions and P. Fontange for assistance with immunofluorescence and confocal microscopy.

This work was supported by INSERM (Institut National de la Santéet de la Recherche Médicale), Centre National de la Recherche Scientifique (CNRS), la Ligue contre le Cancer Comité de Paris, and ARC (Association pour la Recherche sur le Cancer).