Smad family members are essential intracellular signaling components of the transforming growth factor-beta (TGF-β) superfamily involved in a range of biological activities. Two highly homologous molecules, Smad2 and Smad3, have so far been identified as receptor-activated Smads for TGF-β signaling and have become the focus of intensive studies. However, no definite differences in regulation or function have been established between these TGF-β signaling molecules. In the present study, we show that the expression of Smad3, but not its dose relative, Smad2, is down-regulated by TGF-β mediated signals themselves in human lung epithelial cells. This down-regulation of Smad3 by TGF-β treatment did not appear to result from shortening of the half-life of Smad3 mRNA. Constitutive expression of Smad3 in the presence of TGF-β induced apoptotic cell death, with an adverse effect on the cell growth of human lung epithelial cells. Apoptotic cell death could also be induced by forced expression of Smad2 in the presence of TGF-β, but less efficiently than by that of Smad3. These findings clearly define the distinctions between Smad2 and Smad3 for the first time in that a qualitative difference was observed with regard to the regulation of their expression in response to TGF-β, while Smad2 and Smad3 appeared to have quantitatively different capabilities regarding the induction of apoptotic cell death in human lung epithelial cells.
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