Fra‐1 regulates vimentin during Ha‐RAS‐induced epithelial mesenchymal transition in human colon carcinoma cells

Abstract

The process of epithelial mesenchymal transition, whereby cells acquire molecular alterations and fibroblastic features, is a fundamental process of embryogenesis and cancer invasion/metastasis. The mechanisms responsible for epithelial mesenchymal transition remain elusive. Human tumors frequently establish constitutively activated RAS signaling, which contributes to the malignant phenotype. In an effort to dissect distinct RAS isoform specific functions, we previously established human colon cell lines stably overexpressing activated Harvey-RAS (Ha-RAS) and Kirsten-RAS (Ki-RAS). Using these, we observed that only oncogenic Ha-RAS overexpression resulted in morphologic and molecular changes suggestive of epithelial to mesenchymal transition. We showed that vimentin, a key molecule of epithelial mesenchymal transition, was differentially regulated between Ha-RAS and Ki-RAS leading to a Ha-RAS specific induction of a migrative phenotype and eventually epithelial to mesenchymal transition. We demonstrated that the AP-1 sites in vimentin promoter could be involved in this regulation. A potential role of FRA-1 was suggested in the regulation of vimentin during the Ha-RAS-induced epithelial to mesenchymal transition, in association with colon cell migration. Our results therefore propose that in colon cells, the induction of epithelial mesenchymal transition by oncogenic Ha-RAS could occur through the overexpression of proteins like FRA-1 and vimentin. ' 2007 Wiley-Liss, Inc.Key words: Ras; vimentin; FRA-1; epithelial-mesenchymal transition RAS proteins (comprising Ha-RAS, Ki-RAS and N-RAS) are very important molecular switches for a wide variety of signal pathways that control proliferation, cell adhesion, apoptosis and cell migration. RAS proteins are often deregulated in cancer, leading to increased invasion and metastasis and decreased apoptosis. Mutations in the RAS family of proto-oncogenes are very common, found in 30% of all human tumors and in 50% of colon tumors in particular. 1 The most common mutations are found on residues 12 and 61. The glycine to valine mutation on residue 12 renders RAS insensitive to inactivation. RAS protein functions within a signal transducing cascade of reactions. Among them, the mitogen activated protein (MAP) kinases that transmit signals downstream to other protein kinases and gene regulatory proteins, are of leading importance. 2 Epithelial to mesenchymal transition (EMT) is a highly conserved and fundamental process that not only governs morphogenesis but also cancer invasion and metastasis in multicellular organisms. There is good evidence that EMT gives rise to the dissemination of single carcinoma cells from the site of primary tumors. In addition, increasing evidence suggests that EMT could play a specific role in the migration of cells from a primary tumor into the blood circulation. EMT can require the cooperation of oncogenic RAS or tyrosine kinase receptor, with endogenous signaling molecules. It involves the transition from an epithelial to a fibroblastic or mesench...

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