[Effect of Smad7 on transforming growth factor-beta1-induced alveolar epithelial to mesenchymal transition]

To investigate whether transforming growth factor beta1 (TGF-beta1) can induce in vitro alveolar epithelial-mesenchymal transition (EMT), and whether Smad7 gene transfer can block this transition and the possible signaling mechanism.

Rat alveolar type II epithelial cells of the line RLE-6TN were cultured. TGF beta1 (3 ng/mL) was added into the culture fluid. Lipofectamine 2000 was used to transfect Smad7 gene to the RLE-6TN cells. The expression of the markers of the epithelial cells, including E-cadherin and cytokeratin-19 (CK19), and markers of mesenchymal cells, including fibronectin (FN), vimentin, and alpha-smooth muscle actin (alpha-SMA) were assayed using Western blotting and real-time PCR. The morphological alterations were examined by phase-contrast microscope while the ultrastructure changes were examined by electron microscope.

Smad7 was successfully transfected to the RLE-6TN cells. Before transfection TGF-beta1 treatment could lead to the expression upregulation of the mesenchymal markers and downregulation of the epithelial markers at the levels of both mRNA and protein, and after transfection, the mesenchymal makers were downregulated while the epithelial markers were upregulated. Before the transfection, TGF-beta1 treatment could lead to the expression upregulation of phosphorylated Smad2/3 which did not obvious change following transfection. TGF beta1 treatment could induce the EMT process of the RLE-6TN cells. Smad7 gene transfected into the RLE-6TN cells could block the process of EMT. Exposed of the RLE-6TN cells to TGF beta1 resulted in degeneration, tumefaction, and gradual disappearance of the osmiophilic multilamellar bodies, markers of type II alveolar epithelial cells.

Under TGF beta1 treatment, RLE-6TN undergoes a conversion process into myofibroblasts in vitro with the conversion mechanism related to Smad signaling pathway, and transfection of Smad7 gene can partly reverse this process.