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 protein 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-cell 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 transdifferentiated MDCK-C7 cells stably expressing caMEK1 (C7caMEK1) compared with epithelial mock-transfected MDCK-C7 (C7Mock1) cells. At least part of the remaining alpha-catenin was coimmunoprecipitated with beta-catenin, whereas no E-cadherin was detected in beta-catenin immunoprecipitates. In both cell types, the proteasome-specific protease inhibitors N-acetyl-Leu-Leu-norleucinal (ALLN) and lactacystin led to a time-dependent accumulation of beta-catenin, including the appearance of high-molecular-weight beta-catenin species. Quiescent as well as serum-stimulated C7caMEK1 cells showed a higher cyclin D expression than epithelial C7Mock1 cells. The MEK inhibitor U-0126 inhibited extracellular signal-regulated kinase phosphorylation and cyclin D expression in C7caMEK1 cells and almost abolished their already reduced cell proliferation rate. We conclude that the transdifferentiated and invasive phenotype of C7caMEK1 cells is associated with a diminished expression of proteins involved in cell-cell adhesion. Although beta-catenin expression is reduced, C7caMEK1 cells show a higher expression of U-0126-sensitive cyclin D protein.

Ochratoxin A induces JNK activation and apoptosis in MDCK-C7 cells at nanomolar concentrations.

Ochratoxin A (OTA) is a ubiquitous fungal metabolite with nephritogenic, carcinogenic, and teratogenic action. Epidemiological studies indicate that OTA may be involved in the pathogenesis of different forms of human nephropathies. Previously we have shown that OTA activates extracellular signal-regulated kinases 1 and 2, members of the mitogen-activated protein kinases (MAPK) family, in the C7-clone but not in the C11-clone of renal epithelial Madin-Darby canine kidney (MDCK) cells. Here we show that nanomolar concentrations of OTA lead to activation of a second member of the MAPK family, namely, c-jun amino-terminal-kinase (JNK) in MDCK-C7 cells but virtually not in MDCK-C11 cells, as determined by kinase assay and Western blot. Furthermore, OTA potentiated the effect of tumor necrosis factor-alpha on JNK activation. In parallel to its effects on JNK, nanomolar OTA induced apoptosis in MDCK-C7 cells but not in MDCK-C11 cells, as determined by DNA fragmentation, DNA ladder formation, and caspase activation. In addition, OTA potentiated the proapoptotic action of tumor necrosis factor-alpha. Our data provide additional evidence that OTA interacts in a cell type-specific way with distinct members of the MAPK family at concentrations where no acute toxic effect can be observed. Induction of apoptosis via the JNK pathway can explain some of the OTA-induced changes in renal function as well as part of its teratogenic action.