Expression of activated MEK1 or MEK2 morphologically transforms intestinal epithelial cells. IEC-6 cells were infected with retroviruses encoding the indicated MEK constructs and populations of puromycine-resistant cells were selected for further analysis. (A) Expression of HA-tagged MEK1 and MEK2 was analyzed by immunoblotting with anti-HA antibody. (B) Expression of MEK1 and MEK2 was analyzed by immunoblotting with specific antibodies. (C) The ectopically expressed MEK proteins were immunoprecipitated with anti-HA antibody, and phosphotransferase activity was measured using an ERK2 reactivation assay. Results are representative of at least three independent experiments. (D) Morphology of exponentially proliferating IEC-6 cells stably expressing the indicated MEK constructs was examined by phase-contrast microscopy. (E) Immunofluorescence analysis of E-cadherin and vimentin expression. (F) Expression of E-cadherin, cytokeratins, vimentin and smooth muscle α-actin was analyzed by immunoblotting in the indicated cell lines. α-Tubulin was used as loading control. (G) Quantitative PCR analysis of E-cadherin mRNA levels. Expression levels are expressed as fold-increase relative to vector-infected cells.