The metastasis suppressor CD82/KAI1 represses the TGF-β1 and Wnt signalings inducing epithelial-to-mesenchymal transition linked to invasiveness of prostate cancer cells

Background The epithelial-to-mesenchymal transition (EMT) is closely associated with cancer invasion and metastasis. Since the transforming growth factor beta (TGF-beta) and Wnt signals induce EMT in various epithelial cell types, we examined whether and how the CD82/KAI1 metastasis suppressor affects the TGF-beta and Wnt signal-dependent EMT in human prostate cancer cells. Methods The invasiveness of cancer cells was evaluated by examining their ability to pass through the basement membrane matrigel. The subcellular localizations of Smad4 and beta-catenin proteins were respectively examined by confocal microscopy following immunofluorescence antibody staining and immunoblotting analysis following subcellular fractionation. The transcriptional activities of the TGF-beta(1)-responsive TRE and Wnt-responsive Tcf/Lef promoters were determined by a luciferase reporter assay following transfection of the recombinant reporter vector into the cell. Results TGF-beta(1) and Wnt3a treatments of human prostate cancer cells without CD82 expression resulted in not only increased invasiveness but also EMT involving the development of motile structures, downregulation of E-cadherin, and upregulation of the mesenchymal proteins. However, in the cells with high levels of CD82, the TGF-beta(1) and Wnt3a stimulations neither elevated invasiveness nor induced EMT. Furthermore, the TGF-beta(1) signaling events occurring in the CD82-deficient cells, such as phosphorylation of Smad2, nuclear translocation of Smad4, and transactivation of the TRE promoter, did not take place in the high CD82-expressing cells. Further, high CD82 expression interfered with the Wnt signal-dependent alterations in the phosphorylation pattern of glycogen synthase kinase 3 beta (GSK-3 beta) in prostate cancer cells, which allowed GSK-3 beta to continue phosphorylating beta-catenin, thereby attenuating the Wnt signaling effects on the nuclear translocation of beta-catenin and subsequent transactivation of the Tcf/Lef promoter. Conclusions The results of the present study suggest that CD82/KAI1 functions in suppressing TGF-beta(1)- and Wnt-induced EMT in prostate cancer cells by inhibiting the TGF-beta(1)/Smad and Wnt/beta-catenin pathways. Therefore, loss or decrease of CD82 expression is likely to render prostate cancer cells prone to respond to the TGF-beta(1) and Wnt signals with EMT, resulting in the development of a motile and invasive mesenchymal phenotype related to the initiation of the metastatic cascade.