Oncogenic Raf-1 regulates epithelial to mesenchymal transition via distinct signal transduction pathways in an immortalized mouse hepatic cell line

The epithelial to mesenchymal transition (EMT) is considered to be an important event during malignant tumor progression and metastasis. Although Raf/MEK/ERK signaling causes EMT, the mechanisms, including the signaling pathways, are as yet unclear. In the present study we have examined the effects of signal transduction pathways on oncogenic Raf-1-induced EMT, using an immortalized mouse hepatic cell line. Oncogenic Raf-1-induced EMT is characterized by down-regulation of adherens and tight junctions and the reorganization of actin. An active Raf-1 gene was introduced into a mouse hepatic cell line which was then treated with the MAP kinase inhibitor PD98059, the p38 MAP kinase inhibitor SB203580, the PI3 kinase inhibitor LY294002 or the c-Src tyrosine kinase inhibitor PP2. The expression and localization of the adherens and tight junction proteins E-cadherin, occludin, ZO-1, claudin-1 and claudin-2 were determined by western blotting, RT-PCR and immunocytochemistry. The barrier function of tight junctions was assessed by measurements of transepithelial electric resistance (TER) and permeability in terms of fluxes of [C-14]mannitol and [C-14]inulin. In Raf-1-transfected cells expression of occludin and claudin-2 was markedly down-regulated at the protein and mRNA levels and the TER value was decreased, while the permeability was increased. The distribution of ZO-1, pancadherin and F-actin was changed from linear to zipper-like structures at cell borders. In Raf-1-transfected cells treated with PD98059 and SB203580, but not LY294002, expression and localization of claudin-2, but not occludin, recovered, together with barrier function, measured as the TER value. The distributions of ZO-1, pancadherin and F-actin also recovered on treatment with PD98059 and SB203580, but not LY294002. Expression and localization of occludin recovered slightly on treatment with PP2. Thus, oncogenic Raf-1 regulates EMT via distinct MAP kinase, p38 MAP kinase and c-Src tyrosine kinase signal pathways in the mouse hepatic cell line.