Physical and Functional Interaction of Murine and Xenopus Smad7 with Bone Morphogenetic Protein Receptors and Transforming Growth Factor-β Receptors

Abstract

Members of the transforming growth factor-␤ (TGF-␤)family transmit signals from membrane to nucleus via intracellular proteins known as Smads. A subclass of Smad proteins has recently been identified that antagonize, rather than transduce, TGF-␤ family signals. Smad7, for example, binds to and inhibits signaling downstream of TGF-␤ receptors. Here we report that the C-terminal MAD homology domain of murine Smad7 (mSmad7) is sufficient for both of these activities. In addition, we show that mSmad7 interacts with activated bone morphogenetic protein (BMP) type I receptors (BMPR-Is), inhibits BMPR-I-mediated Smad phosphorylation, and phenocopies the effect of known BMP antagonists when overexpressed in ventral cells of Xenopus embryos. Xenopus Smad7 (XSmad7, previously termed Smad8) and mSmad7 are nearly identical within their bioactive C-domain, but have quite distinct Ndomains. We found that XSmad7, similar to mSmad7, interacted with BMP and TGF-␤ type I receptors and inhibited receptor-mediated phosphorylation of downstream signal-transducing Smads. However, XSmad7 is a less efficient inhibitor of T␤R-I-mediated responses in mammalian cells than is mSmad7. Furthermore, overexpression of XSmad7 in Xenopus embryos produces patterning defects that are not observed following overexpression of mSmad7, suggesting that mSmad7 and XSmad7 may preferentially target distinct signaling pathways. Our results are consistent with the possibility that the C-domain of antagonistic Smads is an effector domain whereas the N-domain may confer specificity for distinct signaling pathways.

Downregulation of ZEB1 and overexpression of Smad7 contribute to resistance to TGF-β1-mediated growth suppression in adult T-cell leukemia/lymphoma

“…Several reports have described the mechanism by which I-Smads inhibit TGF-b1 signaling. I-Smads located in the cytoplasm have been reported to bind TGF-b type I receptors, recruit E3 ubiquitin ligases (Smurfs) to type I receptors or bind to phosphorylated Smad1 and inhibit the formation of the R-Smad/Co-Smad complex (Hata et al, 1998;Souchelnytskyi et al, 1998;Kavsak et al, 2000;Ebisawa et al, 2001). Conversely, I-Smads Figure 6 ZEB1 regulates the duration of Smad-dependent transcription.…”

Regulation of cell proliferation by Smad proteins

“…One way by which this occurs is through the action of inhibitory (I-) Smads (Fig. 1) CELL PROLIFERATION CONTROL BY SMADS (Hayashi et al, 1997;Imamura et al, 1997;Nakao et al, 1997;Hata et al, 1998a;Ishisaki et al, 1998Ishisaki et al, , 1999Souchelnytskyi et al, 1998;Lebrun et al, 1999). Whereas Smad7 acts as a general inhibitor of TGF-b family member signaling pathway, Smad6 preferentially blocks BMP signaling Ishisaki et al, 1999).…”

“…Whereas Smad7 acts as a general inhibitor of TGF-b family member signaling pathway, Smad6 preferentially blocks BMP signaling Ishisaki et al, 1999). The ®rst described mechanism for I-Smads antagonism was through their competition with R-Smads for the interaction with the activated type I receptor (Hayashi et al, 1997;Imamura et al, 1997;Nakao et al, 1997;Souchelnytskyi et al, 1998). More recently, another mechanism by which I-Smads inhibit TGF-b/ Smad signaling has been reported.…”

Transforming Growth Factor β1 Induces Nuclear Export of Inhibitory Smad7

“…Cross-linking was performed as described previously (29), except that incubation with 125 I-TGF-␤1 was performed at room temperature for 2 h. The transfected cells lines were stimulated for 24 h with 100 M zinc chloride to induce the expression of Smad7. Complexes of Smad7 and affinity-labeled receptors were immunoprecipitated with antibody directed against the Flag in F-Smad7.…”