Role of Epithelial-Mesenchymal Transition in Tumor Progression

Epithelial-mesenchymal transition (EMT) is a fundamental process of morphogenesis whereby epithelial cells acquire the mesenchymal phenotype. Multiple data suggest a critical role of EMT in tumor progression. In carcinomas, EMT can be initiated and promoted by many oncogenic signaling pathways, hypoxia, and signals of tumor microenvironment resulting in epithelial cells losing their cell polarity and cell-cell adhesion and gaining the migratory and invasive properties. Downregulation of expression of the cell adhesion protein E-cadherin is considered a poor prognostic factor in cancer. Many tumors are characterized by incomplete EMT, where tumor cells acquire mesenchymal characteristics but retain their epithelial markers, in particular, E-cadherin. In cells with the hybrid epithelial-mesenchymal phenotype, E-cadherin is accumulated in adherens junctions which are less stable than adherens junctions in normal epithelial cells. E-cadherin-based adherens junctions are essential for efficient collective migration and invasion of carcinoma cells, and their survival in metastases. The plasticity of the hybrid epithelial-mesenchymal phenotype improves adaptive capabilities of cancer cells. By undergoing EMT, carcinoma cells become resistant to chemotherapy and acquire the ability to suppress immune response. Emergence of cancer stem cells after EMT activation has been observed in many types of carcinoma.