**4. EMT and tumor metastasis**

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–

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

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

17].

220 Tumor Metastasis

lymphocyte attack [26].

epithelial and mesenchymal markers [28–30].

Through the blood or lymphatic system, tumor cells can migrate far from the primary site and then settle and grow in a remote site to complete tumor metastasis. This process plays a key role in tumor disease, malignance level, and death of over 90% of tumor patients. Several questionnaires have shown the low efficiency of tumor metastasis. The majority of tumor cells can fall off into the blood or lymphatic system, such that only a small portion can form micro metastases, and even fewer can achieve actual metastases and present specific organ affinity. Moreover, the metastatic times of different tumors are not the same [31]. Thus, understanding this process is significant, and the importance of EMT should be discussed from different aspects.

EMT is a characteristic of most metastatic cells [32]. In particular, highly differentiated epithelial cells are converted into undifferentiated and isolated ectomesenchymal cells with migration and invasive properties. Migration is one of the four basic steps in tumor cell metastasis. Various substances, such as secretion factors, growth factors, and ECM compo‐ nents, can stimulate the migration of tumor cells. Cell migration stimulated by these substances can be divided into random and directed migrations. Thus, tumor cells can migrate and metastasize. The migration capability of tumor cells is related to their potential to metastasize. The density of the negative charges that separate from the surface of the tumor cells increases, which enhances electrostatic repulsion between cells. This process facilitates the removal of tumor cells into a free state from the tumor tissue. The adhesion of molecules on the surface of tumor cells is mediated by cell adhesion molecules, namely selectins, integrins, Ig super‐ family, and cadherin [33]. Intercellular adhesion capability decreases in the same type, which leads to the detachment of tumor cells from the primary tumor and the abnormal intercellular adhesion contact with the implantation of tumor cells in the vascular wall. Invasion is the important biological characteristic of malignant tumors. Every organization or organ has its own structure. Tumor cells that invade an organ should respond to environmental stress, such as the lack of oxygen and nutrients, low pH, active oxygen free radicals, and inflammation regulatory factor. When the invasive capability is strong, the degree of tumor malignancy will be high. Epithelial cells lose the characteristics of the epithelial cells of ectomesenchymal cells, and the phenotype EMT process is the molecular basis of cancer stem cell invasion and metastasis. In addition, during tumor development, many tumor cells exhibit changes in good plasticity through morphology and phenotype transformation, such as collective amoeboid transition (CAT) and mesenchymal-to-amoeboid transition (MAT) [34]. EMT is a transient dynamic process that is influenced by the microenvironment. Furthermore, in vitro studies are necessary to build an improved genetic mouse model and reliable marker for EMT to realize real-time monitoring of the body, understand the mechanism of tumor evolution and EMT, establish new EMT signs, and to explore the role of transcription factors in the induction of EMT [35]. The loss of epithelial cell polarity during EMT reduces contact between the envi‐ ronment and stromal cells. This process enhances cell migration and mobility, which results in mesenchymal phenotypes. Moreover, changes in cell phenotype, coupled with an alteration in the expression levels of E-cadherin, vimentin, N-cadherin, and α-SMA, among others, in particular, a drop in E-cadherin level, can reduce the adhesion of cells, which facilitates the invasion and metastasis of cells. The loss of E-cadherin expression has been considered the most notable feature of EMT [36]. EMT presents increased opportunities for cell metastasis during tumorigenesis, probably because of its loose cell characteristics. EMT can promote the transfer of various tumor cells. Tumor metastasis includes several steps, such as attacks that are the precondition for cell transfer. EMT plays an important role during tumor invasion. Non-invasive tumors are turned into highly invasive tumors when the E-cadherin protein of tumor cells is cut. Experiments have established EMT marks in some tumor cells in metastases.

Snail, Twist, and ZEBl transcription factors closely linked with EMT can enhance invasion and promote the degradation of E-cadherin. EMT is the interaction between tumor cells and adjacent tumor-associated stromal cells caused by the induction of the transcription factor in the tumor cells. The activation of tumor EMT typically occurs during signal swaps between tumor cells and adjacent stromal cells. The progression of primary tumor cells can raise each model into the surrounding stroma. The recruitment of cells form a "reactive" matrix, induce the release of EMT signals, and start tumor cells by activating the EMT transcription factors [37]. **Figure 1** shows the tumor metastasis and EMT.

**Figure 1.** In EMT processes, tumor cells change from epithelial-like cells to mesenchymal-like cells and get the ability to metastasis.
