BACKGROUND & OBJECTIVE: Smad7 is an inhibitor of transforming growth factor-beta (TGF-beta) signal pathway. TGF-beta could induce the expression of several genes through activating SMAD and ras/MEK/ERK pathways. This study was to determine whether Smad7 is involved in regulating mitogen-activated protein kinase (MAPK) signal pathway with TGF-beta in malignant transformation of human bronchial epithelial BEP2D cells. METHODS: Immortalized BEP2D cells and malignant BERP35T2 cells were co-transfected with full-length Smad7 cDNA constructed pCISmad7.neo or Smad7 siRNA, transactivator vector pTet-Elk or pTet-Jun, and reporter vector pTRE-Luc, and stimulated with TGF-beta. The regulatory effect of Smad7 on MAPK signal pathway was investigated by standard luciferase assay. RESULTS: In BEP2D cells, when treated with TGF-beta1, phosphorylated activities of Elk and Jun were up-regulated (P(Elk)=0.033, P(Jun)=0.016); after co-transfection of Elk or Jun with pCISmad7.neo, phosphorylated activity of Elk was increased, and that of Jun was decreased (P(Elk)=0.017, P(Jun)=0.028); after co-transfection of Elk or Jun with Smad7 siRNA, phosphorylated activity of Elk was decreased, and that of Jun was increased (P(Elk)=0.018, P(Jun)=0.005). In BERP35T2 cells, when treated with TGF-beta1, phosphorylated activity of Elk was up-regulated (P=0.006); after co-transfection of Elk and Smad7 siRNA, phosphorylated activity of Elk was decreased (P=0.000); no activity of Jun was detected in BERP35T2 cells. CONCLUSIONS: In the process of malignant transformation of BEP2D cells, the intervention of Smad7 in MAPK signal pathway leads to the activity imbalance between extracellular signal-related protein kinase (ERK) and c-Jun NH2-terminal kinase (JNK), which in turn promotes cell proliferation. All these could contribute to further malignant transformation of these cells.