The dual role of Smad7 in the control of cancer growth and metastasis

Abstract

Smad7 was initially identified as an inhibitor of Transforming growth factor (TGF)-β due mainly to its ability to bind TGF-β receptor type I and prevent TGF-β-associated Smad signaling. More recently, it has been demonstrated that Smad7 can interact with other intracellular proteins and regulate also TGF-β-independent signaling pathways thus making a valid contribution to the neoplastic processes in various organs. In particular, data emerging from experimental studies indicate that Smad7 may differently modulate the course of various tumors depending on the context analyzed. These observations, together with the demonstration that Smad7 expression is deregulated in many cancers, suggest that therapeutic interventions around Smad7 can help interfere with the development/progression of human cancers. In this article we review and discuss the available data supporting the role of Smad7 in the modulation of cancer growth and progression.

Figure 1.

Schematic overview of the transforming growth factor (TGF)-β signaling pathway. Binding of TGF-β to its type II receptor (TGF-β RII) (1) attracts the TGF-β type I receptor (TGF-β RI) (2) and leads to formation of a receptor complex (3) and phosphorylation of TGF-β RI (4). Thus activated, TGF-β RI in turn phosphorylates a receptor-activated Smad (R-Smad) (4), allowing this protein to associate with Smad4 and move into the nucleus (5). Once in the nucleus, this Smad complex associates with DNA-binding proteins (6) to activate the transcription of specific target genes (7). Negative regulators of this signaling pathway are indicated in red. * and ** indicate mechanisms by which Smad7 antagonizes TGF-β signaling in the cytoplasm and in the nucleus respectively.

Figure 2.

Schematic overview depicting the pathogenic roles of a deregulated Smad7 protein expression. Abbreviations: CNS, central nervous system; HP, Helicobacter-pylori.