Cancer Res Treat. 2012;44(3):151-156 through chronic and repeated exposure to gefitinib. Phenotypic changes, including a spindle-cell shape and increased pseudopodia formation, suggesting EMT, were observed in A549/GR cells. These changes were accompanied by a decrease of E-cadherin and an increase of vimentin, a mesenchymal marker. In addition, A549/GR cells showed 7.7-fold greater resistance to gefitinib and demonstrated cross-resistance against other EGFR-TKI, including CL-387,758, erlotinib, and ZD6478. E M T - M e d i a t e d M e t a s t a s i s L u n g C a n c e r Metastasis occurs when genetically unstable cancer cells adapt to a tissue microenvironment that is distant from the primary tumor [41]. Origins of invasive and metastatic phenotypes of carcinoma cells have been the subject of intensive investigation. Tumor metastasis involves a sequential series of processes that promote and regulate the escape of migratory cancer cells for generation of metastatic lesions at distant sites [42]. Mounting evidence has suggested an association of metastasis of primary epithelial tumor cells, the primary cause of death in cancer patients, with loss of E-cadherin-mediated cell-cell contacts and gain of a mesenchymal phenotype, potentially facilitating cellular motility and invasion [43]. Molecular technologies, including DNA, antibody, and proteomic arrays, and short-hairpin RNA libraries, have been used for identification of multiple molecules that contribute to the metastatic process, including growth factors, cytokines and chemokines, pro-angiogenic factors, extracellular matrix-remodeling molecules, and, most recently, transcription factors that may regulate cellular changes during tumor invasion-metas- tasis [44]. One view proposes that metastatic traits are acquired through exposure of epithelial cancer cells to paracrine signals received from mesenchymal cell types within the tumour-associated stroma [45]. Metastatic colonization involves lodgment or adherence of cancer cells in the microvasculature and subsequent migration of those cells across the endothelium into a secondary organ site [46]. E M T L e a d s t o C a n c e r S t e m C e l l s Cancer stem cells have the capacity for division and expansion of the cancer stem cell pool and for differentiation into the heterogeneous non-tumorigenic cancer cell types that, in most cases, appear to constitute the bulk of cancer cells within the tumor [47]. A four-week period of exposure of immortalized human bronchial epithelial cells to tobacco carcinogens has been shown to induce a persistent, irreversible, and multifaceted dedifferentiation program marked by EMT and the emergence of stem cell-like properties [48]. Induction of EMT in immortalized HMLEs has been reported to result 154 CANCER RESEARCH AND TREATMENT in acquisition of mesenchymal traits, as well as expression of stem-cell markers [49]. In addition, these cells have an increased ability to form mammospheres, a property associated with mammary epithelial stem cells [49,50]. Cancer cells in EMT, that is, EMT cells, share many properties with the classical so-called ‘cancer stem cells (CSCs).’ In fact, there are many indications that CSC-tumor propagating cells present characteristics of EMT cells, and, conversely, that EMT cells acquire properties of the so-called ‘CSCs’(for example, markers CD44+/CD24+/- , dormancy, etc.) and that these are reversible [51]. Ding et al. [52] investigated the question of whether CSC markers are associated with EMT. For Capan1M9, a highly migratory cell subclone established from human pancreatic cancer cell line Capan-1, expression, migration, and invasion of CD133 were greater than that for the parent cells. Up-regulation of EMT-related transcription factors Slug and Snail, as well as a substantial increase in N-cadherin and fibronectin, were observed in Capan1M9 cells. By contrast, occludin and desmoplakin were suppressed. Knockdown of endogenous CD133 in Capan1M9 cells led to suppression of Slug and reduction of migration and invasion. Taken together, CD133 has an important role in migration and invasion through facilitation of EMT in pancreatic cancer cells. On the basis of their studies, Brabletz et al. [53] postulated that EMT and stem-cell properties are combined in invasive cancer cells–which often co-express EMT and stem-cell markers–and proposed the concept of the ‘migrating cancer stem cell.’ This concept links two features: 1) classical EMT-associated features that support tumor cell motility and dissemination, and allow for survival of cells under the stressful con- ditions of a new environment; and 2) the associated stem-cell properties, including prevention of senescence, allowing for unlimited reconstitu- tion of the tumor at a distant site, leading ultimately to metastasis [53]. C l i n i c a l P r o g n o s t i c o f E M T a n d C u r r e n t E M T - T a r g e t e d T h e r a p y Soltermann et al. [54] investigated the prognostic significance of the EMT indicator proteins periostin and vimentin in comparison with versican, a putative indicator of the opposite mechanism, MET, and the desmoplasia proteins collagen and elastin in NSCLC. Tumors of 533 patients with surgically resected NSCLC were used for analysis of stromal and epithelial protein expression by immunohistochemistry. High expression of versican in either stroma or epithelia as well as of stromal collagen showed fewer but concordant associations with advanced tumor and periostin, respectively. High expression of elastin showed an opposite association with less advanced disease. Loss of epithelial proteins and/or acquisition of mesenchymal proteins are associated with poorly differentiated histology, advanced stage, and poor outcome [55]. However, findings from the study also revealed that EMT status in the primary tumor is not predictive of postoperative recur- rence or disease-free survival in patients with lung cancer [56]. EMT has been established as a mechanism that confers tumor cells