Differential expression of cell-cell adhesion proteins and cyclin D in MEK1-transdifferentiated MDCK cells

Overexpression of a constitutively active mutant of the mitogen-activated p rotein kinase kinase MEK1 (caMEK1) in epithelial Madin-Darby canine kidney (MDCK)-C7 cells disrupts morphogenesis, induces an invasive phenotype, and is associated with a reduced rate of cell proliferation. The role of cell-c ell adhesion molecules and cell cycle proteins in these processes, however, has not been investigated. We now report loss of E-cadherin expression as well as a marked reduction of beta- and alpha-catenin expression in transdi fferentiated MDCK-C7 cells stably expressing caMEK1 (C7caMEK1) compared wit h epithelial mock-transfected MDCK-C7 (C7Mock1) cells. At least part of the remaining alpha-catenin was coimmunoprecipitated with beta-catenin, wherea s no E-cadherin was detected in beta-catenin immunoprecipitates. In both ce ll types, the proteasome-specific protease inhibitors N-acetyl-Leu-Leu-norl eucinal (ALLN) and lactacystin led to a time-dependent accumulation of beta -catenin, including the appearance of high-molecular-weight beta-catenin sp ecies. Quiescent as well as serum-stimulated C7caMEK1 cells showed a higher cyclin D expression than epithelial C7Mock1 cells. The MEK inhibitor U-012 6 inhibited extracellular signal-regulated kinase phosphorylation and cycli n D expression in C7caMEK1 cells and almost abolished their already reduced cell proliferation rate. We conclude that the transdifferentiated and inva sive phenotype of C7caMEK1 cells is associated with a diminished expression of proteins involved in cell-cell adhesion. Although beta-catenin expressi on is reduced, C7caMEK1 cells show a higher expression of U-0126-sensitive cyclin D protein.