Downregulation of gangliotetraosylceramide and β1,3-galactosyltransferase-4 gene expression by Smads during transforming growth factor β-induced epithelial-mesenchymal transition.

The process of epithelial‑mesenchymal transition (EMT) is essential for the proliferation and metastasis of tumor cells. Our previous study demonstrated that the expression of gangliotetraosylceramide (Gg4) and the transcription of UDP‑Gal: β1,3-galactosyltransferase‑4 (β3GalT4), a gene which controls the expression of Gg4, are reduced during transforming growth factor (TGF)‑β‑induced EMT in normal murine mammary gland (NMuMG) cells. The present study revealed that the transcription level of β3GalT4 in patients with breast cancer was decreased compared with healthy control subjects. However, the molecular basis underlying these effects remains to be elucidated. Analysis of the β3GalT4 promoter sequence revealed a putative Smad‑binding element (SBE) of Smad4, which is a transcriptional factor in TGFβ responses and forms a complex with Smad3. To clarify the association between the Smad3/4 complex and the β3GalT4 gene during EMT in NMuMG cells, an electrophoretic mobility shift assay and a chromatin immunoprecipitation assay were performed. The Smad3/4 complex was found to bind directly to the SBE of the β3GalT4 promoter. Overexpression of Smad3 and Smad4 through stable transfection had no notable effect on cell phenotype, but did alter the expression of various EMT protein markers. Treatment with TGFβ reduced the expression of Gg4 and the mRNA levels of the β3GalT4 gene in Smad3‑ and Smad4‑overexpressing cells compared with vector‑transfected cells. Expression of the epithelial markers E‑cadherin and β‑catenin decreased in parallel with the reduction in Gg4. These findings suggested that the activated Smad3/4 complex downregulated the expression of Gg4 and the β3GalT4 gene through translocation into the nucleus and binding to the β3GalT4 promoter.