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Evidence that Smad2 is a tumor suppressor implicated in the control of cellular invasion
1INSERM U 482, Hôpital Saint-Antoine, 184 Rue du Faubourg Saint-Antoine, 75571, Paris Cedex 12, France.
Abstract:
The Smad2 protein plays an essential role in the transforming growth factor-beta (TGF-beta) signaling pathway. This pathway mediates growth inhibitory signals from the cell surface to the nucleus. Although Smad2 protein is significantly mutated in human cancers, there is no definitive evidence implicating Smad2 as a tumor-suppressor gene. Here we show that overexpression of the tumor-derived missense mutation Smad2.D450E, an unphosphorylable form of Smad2 found in colorectal and lung cancers, did not abolish the TGF-beta-mediated growth arrest, suggesting that resistance to the growth-inhibiting effects of TGF-beta exhibited by human tumors cannot be linked to the inactivation of Smad2 protein. In contrast, overexpression of Smad2.D450E induces cellular invasion, and this effect was enhanced by TGF-beta. A similar invasive phenotype was obtained in cells expressing another inactivating mutation in Smad2 (Smad2.P445H) found in colorectal cancer. These findings indicate that genetic defects in Smad2 are sufficient to confer the invasion-promoting effect of TGF-beta and reveal that TGF-beta acts through Smad2 to induce cellular invasion by a novel mechanism that is independent of Smad2 phosphorylation by the activated TGF-beta type I receptor.
Insights
Mutations in Smad2 (a key protein in transforming growth factor-beta signaling) do not stop TGF-beta
Area of Science:
- Molecular Biology
- Cancer Research
- Cell Signaling
Background:
- Smad2 protein is crucial for transforming growth factor-beta (TGF-beta) signaling, mediating growth inhibition.
- Smad2 mutations are frequent in human cancers, but its role as a tumor suppressor is unconfirmed.
- Tumors often show resistance to TGF-beta's growth-inhibiting effects.
Purpose of the Study:
- To investigate the functional consequences of Smad2 mutations in cancer.
- To determine if Smad2 inactivation explains TGF-beta resistance in tumors.
- To explore Smad2's role in TGF-beta-induced cellular invasion.
Main Methods:
- Overexpression of Smad2 missense mutations (Smad2.D450E, Smad2.P445H) in cancer cells.
- Assessment of TGF-beta-mediated growth arrest.
- Evaluation of TGF-beta's effect on cellular invasion.
Main Results:
- Overexpression of Smad2.D450E did not abolish TGF-beta-induced growth arrest.
- Smad2.D450E and Smad2.P445H mutations promoted cellular invasion, an effect amplified by TGF-beta.
- TGF-beta enhances invasion via Smad2 through a non-phosphorylatable mechanism.
Conclusions:
- Smad2 inactivation is not the cause of TGF-beta resistance in human cancers.
- Smad2 defects are sufficient to promote cellular invasion, potentiated by TGF-beta.
- TGF-beta induces invasion via Smad2 through a novel, phosphorylation-independent pathway.