The Effects And Related Mechanisms Of Smad7 In The Process Of Human Bronchial Epithelial Cells Malignant Transformation Induced By α-particles

Lung cancer is one of the most common cancers throughout the world, and its incidence is still increasing. In China, the adjusted mortality rate of this disease was 9.49/100 000 between 1973 and 1974, but increased to 49.7/100 000 in 2002. Radon and its progeny is the second most important risk factor for lung cancer after smoking. The contribution of domestic radon exposure to lung cancer has been equal to or exceeded to that of uranium mine exposure. Even though the exposure level of domestic radon is lower, that could increase the rate of lung cancer incidence. The study confirms the incidence rate of lung cancer should have increased 19%~31% if the average radon concentration increased by 100Bq/m3. So it is important to study the mechanism of lung cancer induced by alpha particles emitted by radon and its progeny. In this study, northern blot and immunohistochemistry were performed to investigate the expression of Smad7 in cell models and in non-small cell lung cancer samples; Touch-down PCR was performed to clone the full code region of Smad7, and stable gene transfection was performed to investigate the effect of Smad7 on cells proliferation and on transforming growth factor-β (TGF-β) -mediated growth suppression in human bronchial carcinogenesis induced by alpha particles; we also performed transient transfection and RT-PCR to further investigate the mechanism of Smad7 on cells proliferation. The results are as follow:1. The expression of Smad7 in cell models and in non-small cell lung cancer samples and its responsiveness to TGF-β1: the expression of Smad7 was 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 cellline, we also found that the expression of SMAD7 was increased in tumor tissues of human squamous cell carcinoma as compare to adjacent normal tissues (55% vs 15 %, p<0.01) but no difference between adenocarcinoma and matched adjacent normal tissues (30.8% vs 11.5%, p>0.05). The responsiveness of Smad? to TGF-P1 was normal in BEP2D cells but disordered in BERP35T2 cells.2. Effects of Smad7 overexpression on proliferation and TGF-P1- mediated growth suppression in BEP2D and BERP35T2 cells: BERP35T2 cells which with higher expression level of Smad7 have more superiority in proliferation and less responsiveness to TGF-J31 -mediated growth suppression than BEP2D cells. Stable transfection of BEP2D and BERP35T2 cells with the full length Smad? cDNA construct resulted in markedly increasing the proliferation of cells and loss of the growth inhibitory response to TGF-βl.3. The mechanism of Smad7 overexpression on cells proliferation: Transient transfection of SBE4-SEAP reporter vector into BEP2D and BERP35T2 cells showed that the basal activity of reporter was higher in BEP2D cells than that in BERP35T2 cells. Transient co-transfection Smad7 with SBE4-SEAP reporter vector leads to the decreased activity of reporter. Transient co-transfection Smad7 with pMyc-SEAP reporter vector showed the activity of c-myc cis-acting enhancer element was significantly up-regulated; the results of RT-PCR showed that the expression of c-myc was up-regulated and the expression of pl5Ink4B and p21Clpl was down-regulated in Smad7 stable transfectants of BEP2D and BERP35T2 cells.These findings indicate that in the process of cells malignant transformation induced by alpha particles, the expression of Smad7 was increased. Its overexpression may facilitate cells proliferation and contribute to the process of cells carcinogenesis by antagonizing TGF-β-mediated growth inhibition.