MDCK-C7 cells dedifferentiated either by transient alkaline stress (C7F cells) or by transfection with a constitutively active mutant of the mitogen-activated protein kinase kinase MEK1 (C7caMEK1 cells) were analyzed by western blot and immunofluorescence microscopy to compare the expression of different cytokeratins, vimentin, and alpha-smooth muscle actin. Expression of all cytokeratins tested, the type II-neutral and basic cytokeratins CK5, CK7, CK8 as well as the type I-acidic keratins CK17 and CK19, was substantially reduced in dedifferentiated cell lines C7F and C7caMEK1 when compared with epithelial wild-type MDCK-C7 cells or mock-transfected MDCK-C7 cells. While vimentin expression was detected in all of the four MDCK-C7 cell lines examined, only the dedifferentiated cell lines C7F and C7caMEK1, which have been reported to express highly active ERK2, exhibited formation of alpha-smooth muscle actin-containing stress fibers. Taken together our results show that, associated with an increase in ERK2 activity, an epithelial to mesenchymal dedifferentiation occurred in both MDCK-C7F cells and caMEK1-transfected MDCK-C7 cells.