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Transforming growth factor beta1 induces nuclear export of inhibitory Smad7
S Itóh1, M Landström, A Hermansson
1Ludwig Institute for Cancer Research, Biomedical Center Box 595, S-751 24 Uppsala, Sweden.
Abstract:
Transforming growth factor beta (TGF-beta) signals from membrane to nucleus through serine/threonine kinase receptors and their downstream effector molecules, termed Smad proteins. Recently, Smad6 and Smad7 were identified, which antagonize TGF-beta family signaling by preventing the activation of signal-transducing Smad complexes. Here we report that Smad7, but not Smad6, inhibits TGF-beta1-induced growth inhibition and the expression of immediate early response genes, including Smad7. Interestingly, in the absence of ligand, Smad7 was found to be predominantly localized in the nucleus, whereas Smad7 accumulated in the cytoplasm upon TGF-beta receptor activation. The latter is in accordance with the physical association of Smad7 with the ligand-activated TGF-beta receptor complex in the cell membrane. Whereas the ectopically expressed C-terminal domain of Smad7 was also exported from the nucleus to the cytoplasm upon TGF-beta challenge, a Smad7 mutant with a small deletion at the C terminus or only the N-terminal domain of Smad7 was localized mainly in the cytoplasm in the absence or presence of ligand. This suggests that an intact Mad homology 2 domain is important for nuclear localization of Smad7. The nuclear localization of Smad7 suggests a functional role distinct from its antagonistic effect in receptor-mediated Smad activation.
Insights
Smad7 antagonizes transforming growth factor beta (TGF-beta) signaling by inhibiting growth and gene expression. Smad7
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Biochemistry
Background:
- Transforming growth factor beta (TGF-beta) signaling is crucial for cellular processes.
- Smad proteins act as downstream effectors in TGF-beta signaling.
- Smad6 and Smad7 are known antagonists of TGF-beta family signaling.
Purpose of the Study:
- To investigate the inhibitory role of Smad7 in TGF-beta1 signaling.
- To elucidate the subcellular localization dynamics of Smad7 in response to TGF-beta.
- To identify domains of Smad7 critical for its nuclear localization and function.
Main Methods:
- Cell-based assays to assess growth inhibition and gene expression.
- Immunofluorescence microscopy to determine Smad7 localization.
- Expression of wild-type and mutant Smad7 constructs.
Main Results:
- Smad7, but not Smad6, inhibits TGF-beta1-induced growth inhibition and immediate early gene expression.
- Smad7 is predominantly nuclear in the absence of ligand but translocates to the cytoplasm upon TGF-beta receptor activation.
- An intact Mad homology 2 (MH2) domain is essential for Smad7 nuclear localization.
Conclusions:
- Smad7 plays a significant role in antagonizing TGF-beta1 signaling pathways.
- Ligand-dependent nuclear-cytoplasmic shuttling of Smad7 is mediated by TGF-beta receptor activation.
- The nuclear localization of Smad7 suggests functions beyond its role in receptor-mediated Smad activation.