**3. EMT and cancer pathogenesis**

**2. EMT: a naturally occurring transdifferentiation program**

change the state of cells in nature rather than forced to ectopic gene expression.

Snail, Twist, and Zeb families [4, 5].

relative to their epithelial counterparts.

formation of weak cell adhesion between adjacent cells.

to EMT are insignificant.

218 Tumor Metastasis

Normal adult tissues in terminally differentiated cells have been reprogrammed into pluripo‐ tent stem cells in the past 10 years [1, 2]. This process has resulted in the wide acceptance of the initial hypothesis that nearly any type of dedifferentiation or transdifferentiation is possible if the ectopic expression of a transcription factor is properly combined into adult cells. The successful experiments on reprogramming have led to the exploration of the factors that

EMT is the most important cell biology program among naturally occurring transdifferentia‐ tion programs. This process converts epithelial cells into mesenchymal derivatives, which is the reverse process of mesenchymal-epithelial transformation (MET) [3]. Accumulated evidence for the past two decades has suggested that EMT occurs during development to ensure the interconversions of cells utilized in the formation of different types of cells, thereby forming the organs of organization and complex multicellular organisms [3, 4]. This cell biological program is orchestrated by a group of transcription factors (EMT-TFs), such as the

Two other aspects of EMT are worthy to be discussed in detail. The EMT program in some epithelial tissues is apparently correlated with the residence of cells in stem cell-like states. Moreover, versions of the EMT program are adopted by cancer cells to obtain a series of processes associated with higher levels of malignancy. EMT exhibits the presence of mechan‐ ical connections between an individual and the pathogenesis of cancer. These processes prior

EMT governs changes in cell states along the epithelial versus mesenchymal axes and converts epithelial cells to mesenchymal cells when this program is fully executed. Weinberg described the extreme poles of the epithelial versus mesenchymal axes. Epithelial cells, frequently with polygonal shapes in monolayer culture, are polarized along their apical-basal axis and are tightly connected with one another laterally via adherens and tight junctions in vivo. These lateral ties can ensure the structural integrity of epithelial cell sheets. By contrast, full mesen‐ chymal cells exhibit spindle-like morphology with no sign of apical-basal polarity. These cells are loosely attached to the surrounding extracellular matrix (ECM) through focal adhesions. These features can help improve motility and explain the invasion of mesenchymal cells

The deep layer of biological contact between epithelial and mesenchymal cells is determined by the differences in their respective transcription programs. These programs also control the expression of other gene products and key structural proteins, including those involved in the maintenance of the cytoskeleton and the strengthening of cell-cell adhesion [3–5]. Thus, epithelial cells express different types of keratin to form intermediate filaments, whereas vimentin constitutes the intermediate filament protein of mesenchymal cells. The expression of cell adhesion molecules and polarized complexes in mesenchymal cells is generally inhibited. EMT is marked by the replacement of E-cadherin by N-cadherin, which leads to the

Nearly 80% of malignant tumors are derived from epithelial tissues, which produce common cancers, such as tumors of the lung, colon, breast, pancreas, prostate, bladder, ovary, kidney, and liver. The epithelial states of the corresponding normal cells of origin in each determined case sustain the expression of cytokeratin and E-cadherin, which are signs of the epithelial states of early tumor tissues. In addition, tumor cells in early tumors remain the key biological phenotypes of epithelial cells, such as a lack of motility and capability to form a continuous cell sheet. These qualities exhibit a sharp contrast with those of advanced cancer cells, which are products of a complex succession process that is frequently referred to as "tumor progres‐ sion." Highly invasive tumor cells present mesenchymal features, such as motility and invasion, and the latter is associated with metastasis [10–12]. The acquisition of these malignant features at the mechanism level can be explained by activating EMT, which is previously dormant in tumor cells during tumor progression.

The acquisition of mesenchymal features in breast cancer is positively correlated with tumor progression and an aggressive subtype of the disease [13, 14]. Investigating a large body of loss-of-function and gain-of-function in xenograft tumor models is a direct means to describe the link between the activation of EMT and the degree of malignancy of a tumor. The con‐ sumption of EMT-TFS, such as Twist, Snail, and Zinc finger E-box binding homeobox 1 (ZEB1), in both human and mouse breast cancer cell lines significantly inhibits the metastatic dissem‐ ination, whether the site is the primary site of tumor formation (e.g., mammary fat pad mass) or after experimental introduction of cancer cells into venous circulation (i.e., tail-vein injection). By contrast, the ectopic activation of EMT can enhance metastatic dissemination of orthotopically implanted human breast cancer cells by forcing the expression of EMT-TFS [15– 17].

EMT in patients with breast cancer does not only promote systemic dissemination but also function as a major factor of drug resistance and disease recurrence [18, 19]. A similar phe‐ nomenon has been observed in a mouse model of Her2-induced tumors in which Snail EMT-TF can be autoactivated in recurrent malignancy in vivo and make the tumors highly mesenchymal phenotype [20]. The results evidently show an association between EMT activation and tumor relapse. Moreover, the link between EMT activation and enhanced tumorigenicity has been verified in various human cancer cell lines [21]. The inhibition of epithelial-mesenchymal plasticity suppresses the valid transition of carcinoma cells from a weakly tumorigenic into a highly tumorigenic state by blocking the activation of Zeb1 EMT-TF; that is, a state where tumor-initiating cells demonstrate their improved capabilities [22].

Recent studies have associated EMT with the acquisition of immunosuppressive capabilities in various types of cancers. The expression of Snail EMT-TF in melanoma can simultaneously inhibit the differentiation of cytotoxic T cells and induce the immune inhibition of the forma‐ tion of regulatory T cells; the latter effect is mediated through the production of platelets [23]. Meanwhile, EMT in breast cancer cells enhances the resistance of tumor cells to cytotoxic T cell-mediated lysis, which at least partially induces autophagy [24, 25]. The activation of EMT-TF ZEB1 in lung cancer cells has been linked to the upregulation of programmed death ligand 1 (PD-L1), which is an immunosuppressive molecule that can block tumor-infiltrating lymphocyte attack [26].

EMT activation has a pleiotropic function in driving cancer progression, and an increasing number of reports confirm that invasive cancer is related to various types of aggressive carcinoma cells. Thus, we believe that all carcinomas essentially develop traits that are related with malignancy through the activation of an EMT program in their constituent neoplastic cells. However, the EMT program associated with common cancers has not yet been deter‐ mined because many clinical pathologists doubt the existence of this program and its role in the production of high-grade cancer [27]. Such reluctance primarily originates from the fact that all markers of clinical biopsy are difficult to score. Although scoring will be possible, cancer cells that underwent EMT are also difficult to distinguish from normal host tissues and adjacent tumor stromal cells. Many fibroblasts and myofibroblasts self-express EMT-related markers. Clear evidence of this program during tumor development can be obtained from cell detection that co-express both mesenchymal and epithelial traits with certain retained epithelial markers inherited from their fully epithelial precursors because cancer cells frequently experience the only part of the EMT program. Recent analysis suggests that all subtypes of invasive breast cancer tumor cells exhibit both epithelial and mesenchymal characteristics as shown by the in situ hybridization in human breast cancer specimens of pooled epithelial and mesenchymal markers [28]. In addition, a part of the circulating tumor cells is isolated from the peripheral blood of patients with advanced prostate and breast cancers, and they also co-express both epithelial and mesenchymal markers [28–30].
