Constitutively active mitogen-activated protein kinase kinase MEK1 disrupts morphogenesis and induces an invasive phenotype in Madin-Darby canine kidney epithelial cells

During certain developmental processes, as well as during tumor progression , polarized epithelial cells integrated within multicellular structures con vert into scattered, freely migrating fibroblast-like cells. Despite the bi ological and clinical importance of this phenomenon, the intracellular bioc hemical cascades that control the switch between the epithelial and mesench ymal phenotypes have not been elucidated, Using Madin-Darby canine kidney ( MDCK) cells (clone C7) as a model system, we have assessed the potential ro le of the mitogen-activated protein kinase (MAPK)/ extracellular signal-reg ulated kinase (ERK) cascade in the modulation of epithelial plasticity. Whe n grown in three-dimensional collagen gels, MDCK-C7 cells form spherical cy sts composed of polarized epithelial cells circumscribing a central lumen. This morphogenetic behavior is profoundly subverted in MDCK-C7 cells expres sing a constitutively active MAPK/ERK kinase 1 (caMEK1) mutant (C7-caMEK1 c ells). When suspended in collagen gels, C7-caMEK1 cells assume an elongated fibroblastoid shape and are unable to generate multicellular cysts. In add ition, when seeded onto the surface of a collagen gel, C7-caMEK1 cells pene trate extensively into the underlying matrix, unlike wild-type and mock-tra nsfected MDCK-C7 cells, which remain confined to the surface of the gel. Si milar changes in morphogenetic and invasive properties are observed in MDCK -C7F cells, a nontransfected, stably dedifferentiated derivative of MDCK-C7 cells that expresses substantially increased ERK2 activity. Both C7-caMEK1 and MDCK-C7F cells but not wild-type or mock-transfected MDCK-C7 cells exp ress activated M-r 72,000 gelatinase A [matrix metalloproteinase (MIMP)-2] as well as elevated levels of membrane type-1 MMP, Synthetic MMP inhibitors as well as recombinant tissue inhibitor of metalloproteinases 2 and 3 supp ress the invasion of collagen gels and restore the capacity of C7-caMEK1 ce lls to form cysts, thereby implicating the membrane type-1 MMP/MMP-2 proteo lytic system in epithelial cell invasiveness and loss of multicellular orga nization. Taken together, our data demonstrate that increased activity of t he MEK1-ERK2 signaling module in MDCK-C7 cells is associated with failure o f morphogenesis and expression of a highly invasive phenotype, Sustained ac tivation of the MAPK cascade therefore results in the destabilization of th e three-dimensional architecture and the conversion of polarized epithelial cells into migrating mesenchymal-like cells.