**5. EMT-activating transcription factors in cancer**

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

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

[37]. **Figure 1** shows the tumor metastasis and EMT.

to metastasis.

222 Tumor Metastasis

Many transcription factors can induce EMT. Molecular reprogramming during an EMT is caused by three groups of transcription factors, namely the Snail, Twist, and ZEB families [38].

The Snail family includes Snail1, Snail2 (Slug), and Snail3 (Smuc). These factors regulate epithelial and mesenchymal markers [39, 40]. Snail1 induces signal to initiate EMT [41, 42]. These factors inhibit other epithelial markers that affect E-cadherin and bind to the E-cadherin promoter to inhibit its transcription. The Snail factors activate the expression of mesenchymallike and pro-invasive genes that promote cell migration [43].

Snail factors are absent in normal epithelial cells. Snail1 is expressed higher than Snail2 and Snail3. An upregulated nuclear Snail1 expression is associated with tumor progression and can be found in the cytoplasm of several carcinomas. Snail1 staining is found among fibroblastlike cells, endothelial cells at the peritumoral stroma, and inflammation of colorectal carcino‐ mas [44]. Snail1 promotes the recurrence of Her2/neu-induced breast tumors in mice, and its mesenchymal-like characteristics are exhibited in recurrent human carcinomas [45]. Therefore, recurrent breast carcinomas are induced by Snail1 spontaneously. High level of Snail1 is an independent predictor for reduced relapse-free survival in breast cancer patients. This factor is considered an independent prognostic factor for worst evolution and poor survival in many carcinomas [43].

Twist factor induces EMT by influencing other EMT-ATFs. Twist1 represses E-cadherin by inducing Snail1 or Snail2 and then binding to its promoter [46–48]. The knockdown of Twist1 in breast cancer cells represses the metastasis in xenograft models, but does not influence the formation of primary tumors [49]. Twist1 induces N-cadherin by driving its transcription and the mechanisms of post-transcription [50, 51]. Twist1 promotes the expression of mesenchymal markers without eliciting an N-cadherin/E-cadherin switch in glioblastoma cells [52]. In cell motility, the excessive expression of Twist1 upregulates the expression of cytoskeletal and ECM genes.

Twist1 and Twist2 are upregulated at the invasive front of carcinomas in cancer and stromal cells [53–55]. These factors are absent in normal epithelium but are induced in many human carcinomas, such as those of the digestive tract, liver, breast, ovary endometrium, and prostate [43]. Twist factors are upregulated in the cytoplasm and nuclei of cancer cells. Twist factors are independent prognostic factors for increased tumor recurrence, tumor aggressiveness, and the low survival rate of patients [49, 53, 56]. Twist and Snail factors play distinct but collabo‐ rative roles among EMT-ATFs.

The ZEB family includes zinc finger/homeodomain proteins, namely ZEB1 and ZEB2. The expression of ZEB factors drives an EMT by activating mesenchymal properties and repressing epithelial markers [43].

ZEB1 and ZEB2 bind to E-box sequences in the E-cadherin promoter but recruit different sets of co-repressors, namely SWI/SNF and CtBP for ZEB1 and NuRD and CtBP for ZEB2. ZEB proteins bind and repress the promoters of epithelial markers, such as R- and P-cadherins, gap junctions (connexins 26 and 31), cell polarity markers (Crumbs3, Pals1-associated tight junction protein, and lethal giant larvae homologue 2), desmosomes (plakophilin 3, desmoplakin), and components of tight junctions (claudin 7, occludin, junctional adhesion molecule 1, and zonula occludens protein 3). ZEB proteins activate mesenchymal markers, such as N-cadherin and vimentin [43]. ZEB1 and ZEB2 repress epithelial splicing regulatory proteins-1 and 2, the overexpression of which inhibits EMT [57].

ZEB1 inhibits epithelial phenotype, although this factor is found in isolated fibroblasts and immune cells in the interstitial matrix. This factor is not expressed in normal epithelium and well-differentiated carcinomas that express E-cadherin [58, 59]. ZEB1 is highly expressed in invading dedifferentiated cancer cells of many tumors, such as colorectal, breast, liver, endometrial, lung, prostate, and pancreatic carcinomas. ZEB1 and ZEB2 are expressed by stromal cells in epithelial tissues and organs of normal E-cadherin-positive epithelial cells [60]. ZEB-dependent paracrine signaling from the stroma can cooperate in E-cadherin repression in other parts of the tumor [61].

**Figure 2.** The main signaling pathways involved in EMT.

Other transcription factors also induce EMT and tumor invasiveness. The homeobox factor goosecoid induces EMT by activating mesenchymal genes and repressing epithelial markers [62]. TGF-β induces goosecoid in breast epithelial cells, and goosecoid is overexpressed in ductal breast carcinomas and atypical ductal hyperplasia [62]. **Figure 2** shows the main signaling pathways involved in EMT.
