Smad7 is a TGF-beta-inducible attenuator of Smad2/3-mediated inhibition ofembryonic lung morphogenesis

Smad7 was recently shown to antagonize TGF-beta-induced activation of signa l-transducing Smad2 and Smad3 proteins. However, the biological function of Smad7 in the process of lung organogenesis is not known. Since Smad2/3-med iated TGF-beta signaling is known to inhibit embryonic lung branching morph ogenesis, we tested the hypothesis that smad7 regulates early lung developm ent by modulating TGF-beta signal transduction. An antisense oligodeoxynucl eotide (ODN) was designed to specifically block endogenous Smad7 gene expre ssion at both transcriptional and translational levels in embryonic mouse l ungs in culture. TGF-beta-mediated inhibition of lung branching morphogenes is was significantly potentiated in cultured embryonic lungs in the absence of Smad7 gene expression: abrogation of Smad7 potentiated TGF-beta-mediate d inhibition of lung branching morphogenesis from 76 to 52% of the basal le vel in lungs cultured in the presence of 5 ng/ml TGF-beta 1 ligand. Likewis e, TGF-beta 1 EC50 (concentration of TGF-beta 1 that induced half maximal b ranching inhibition) was reduced from 5 to 1 ng/ml when Smad7 gene expressi on was abrogated in lung culture, indicating an enhanced level of TGF-beta signaling in lung tissue with abolished Smad7 gene expression. By immunocyt ochemistry, Smad7 protein was co-localized with both Smad2 and Smad3 in dis tal bronchial epithelial cells, supporting the concept that Smad7 inhibits TGF-beta signaling by competing locally with Smad2 and Smad3 for TGF-beta r eceptor complex binding during lung morphogenesis. Furthermore, antisense S mad7 ODN increased the negative effect of TGF-beta 1 on epithelial cell gro wth in developing lungs in culture. We also demonstrated that Smad7 mRNA le vels were rapidly and potently induced upon TGF-beta 1 stimulation of lungs in culture, suggesting that Smad7 regulates TGF-beta responses in a negati ve feedback loop. These studies define a novel function for Smad7 as an int racellular antagonist of TGF-beta-induced, Smad2/3-mediated inhibition of m urine embryonic lung growth and branching morphogenesis in culture. The opt imization of TGF-beta signaling during early lung development therefore req uires a finely-regulated competitive balance between both permissive and in hibitory members of the Smad family. (C) 2000 Elsevier Science ireland Ltd. All rights reserved.