The Regulation Of TGF-β Signal Transduction Pathway By SMAD7 Gene In The Lung Carcinogenisis

Lung cancer is one of the most common cancers throughout the world. Radon and its progeny are the second most important risk factor for lung cancer after smoking. So it is important to study the mechanism of lung cancer induced by alpha particles emitted by radon and its progeny. Previous studies suggest that the expression of Smad7 is higher in tumorigenic BERP35T2 cell line transformed by alpha particles than that in its parental BEP2D cells, a human papillomavirus -18 immortalized human bronchial epithelial cell line. BERP35T2 cells with higher expression level of Smad7 take more superiority in proliferation and less responsiveness to TGF-β1-mediated growth suppression than BEP2D cells. Stable transfection of BEP2D and BERP35T2 cells with the full length Smad7 cDNA construct results in marked increase of the proliferation of cells and loss of the growth inhibitory response to TGF-β1. The expression of Smad7 is increased in tumor tissues of human squamous cell carcinoma. These findings indicate that Smad7 overexpression may facilitate cells proliferation and contribute to the process of cells carcinogenesis. In this study, western blot were performed to investigate the expression of TβR, Smads and STRAP in the process of malignant transformation of immortalized human bronchial epithelial cells BEP2D. Transient transfection was performed to investigate the effects of Smad7 on transforming growth factor-β (TGF-β) -mediated nuclear translocation of Smad2/3,Smad4.We also cotransfected three vectors into a mammalian cell line: full-length Smad7 cDNA construct pCISmad7.neo or Smad7 siRNA, the transactivator vector (pTet-ELK, -JUN) and the reporter vector pTRE-Luc. Then performed a standard luciferase assay to investigate the regulation of Smad7 on MAPK signal transduction pathway. The results were as follows: 1. Effects of Smad7 on expression of TβR,Smads and STRAP: the expression of Smad7 was increased and the expression of TβRⅠor TβRⅡwas decreased in tumor tissues of human squamous cell carcinoma as compare to adjacent normal tissues. Stable transfection of pCISmad7.neo leads to decreased expression of TβRⅠand/or TβR Ⅱ and increased expression of Smad2 and STRAP in both BEP2D and BERP35T2 cells. There were no changes in expression of Smad3 and Smad4. 2. Effects of Smad7 overexpression on transforming growth factor-β (TGF-β) -mediated nuclei transposition of Smad2/3,Smad4: Both in BEP2D and BERP35T2 cells, Smad3 demonstrated obvious nuclear translocation with stimulation by TGF- β. The nuclear translocation of Smad3 and Smad4 had been inhibited after transfect of pCISmad7.neo. There were no changes in nuclear translocation of Smad2. 3. Regulation of Smad7 gene on MAPK signal transduction pathway: In BEP2D cells, when treated with TGF-β1, activities of Elk and Jun were up-regulated; Transient cotransfect of pCISmad7.neo lead to increased activity of Elk and decreased activity of Jun. In BERP35T2 cells, when treated with TGF-β1, activity of Elk was up-regulated; Transient cotransfect of Smad7-SiRNA lead to decreased activity of Elk; but activity of Jun couldn't be assessed in BERP35T2 cells. These findings indicate that the overexpression of Smad7 may facilitate cells proliferation and contribute to the process of cells carcinogenesis by antagonizing TGF-β-mediated activation of Smads signaltransduction pathway and by promoting TGF-β-mediated activation of ERK signal transduction pathway.