**8. Regulatory role of microRNAs in EMT**

explore the inhibition approach for SDH- and FH-associated cancers are worthy to be explored

EMT is a comprehensive reprogramming during tumor development. This process involves metabolism, epigenetics, and differentiation. In a specific tumor microenvironment, EMTdedifferentiated cells escape the primary tumor after they acquire migration and invasion capabilities, invade the surrounding tissues, enter into the blood or lymphatic vessels, and settle in distant organs. EMT converts differentiated epithelial cancer cells to an undifferenti‐ ated state, thereby expressing stem cell markers and acquiring stem cell-like functions. This process is reversible, and mesenchymal cells can differentiate into epithelial phenotypes. Thus,

an important process is developed in the macroscopic metastases in different organs.

DNA methylation, histone modification, and microRNA are the three types of epigenetic modification. Many studies have shown that epigenetic modifications play a key role in tumor metastasis [81]. The downregulation of E-cadherin (a cell adhesion molecule) expression during EMT is an important feature. Therefore, the precise regulation of E-cadherin expression via epigenetic modifications is extremely important to the occurrence of EMT. Several EMTrelated transcription factors are recruited as E-cadherin gene promoter, which inhibits transcription [4]. Studies have shown that E-cadherin can be inhibited by the synergy of various histone modification enzymes. The E-cadherin gene promoter is inhibited to different

The characteristics of EMT are reversible in the type of stem cells and malignant features. Tumor stem cells are unique undifferentiated cells, rather than increasing diffusion, compared with most differentiated epithelial cells. The diffusion of anabolic needs is related to the maintenance of an undifferentiated state, which may be metabolic alterations of the links between EMT and tumor [83]. A change in cell metabolism is an important sign of cancer. The best metabolic phenotype in tumor cell is characterized by the Warburg effect, which proves that ATP is not the only metabolite of tumor cells [84]. Further studies have shown that aerobic glycolysis can better satisfy the basic needs of cell division, known as the post-Warburg model. This process is not only associated with cancer and normal cell proliferation, but also inhibits mitotic cell differentiation. In the same inducers, bunah and inactivated tumor suppressor genes, even oxygen glycolysis capability increased in the EMT, can be attributed to cell undifferentiated state. The appropriate energy level is sufficient for biosynthesis precursor,

Epigenetic modifications are complex, dynamic, and connected with the extracellular envi‐ ronment and nuclear transcription. Energy availability is extremely important. Energy-rich substances, such as carbohydrates and fats, in the human body translate into ATP, along with a large number of metabolites, such as glycolysis and fatty acid oxidation. These metabolites can also drive epigenetic modifications in gene expression. A change in the intermediate metabolites of EMT may not be simple. Metabolic reprogramming plays a role in the energy

[80].

226 Tumor Metastasis

**7. Epigenetic modification and EMT**

extents to silence E-cadherin expression [82].

balancing normal state, and maintaining an undifferentiated state.

MicroRNAs (miRNAs) are expressed endogenously as small, non-coding RNAs that regulate various biological processes by modulating gene expression at the post-transcriptional level [86]. MiRNAs play an important role in controlling tumor growth and progression. Some miRNAs function as oncogenes and tumor suppressors. Moreover, miRNAs are also master regulators of EMT and dynamically regulate balance between EMT and MET.

The miR-200 family consists of five miRNA sequences, namely miR-200a, miR-200b, miR-200c, miR-141, and miR-429. Aggregated miRNAs that are expressed as two independent polycis‐ tronic pri-miRNA transcripts are as follows: miR-200a, miR-200b, and miR-429 (chromosome 1); and miR-200c and miR-141 (chromosome 12) [87].

The autocrine TGF-β/ZEB/miR-200 signaling regulatory networks that control epithelial and mesenchymal states change in the cells. ZEB1/2 and TGF-β exhibit strong correlation, and negative correlations are detected between TGF-β and miR-200, as well as between ZEB1/2 and miR-200, in invasive ductal carcinomas [88]. ZEB1/2 can induce EMT by inhibiting various epithelial genes [89].

Other miRNAs can directly target EMT transcription factors. MiR-205 in mammary cells maintains epithelial differentiation [90–92]. MiR-29b in prostate cancer inhibits metastasis by regulating the EMT signal. MiR-148a in hepatocellular carcinoma (HCC) cells can negatively regulate Met/Snail signaling and prevent EMT and metastasis [93]. Snail and miR-34 form another double feedback loop. EMT is induced by TGF-β, and an increase in Snail expression can be inhibited by miR-34. A novel miR-203/SNAI1 feedback loop has also been reported in breast cancer. These double feedback loops can enhance the balance between EMT activation and control of the two states of the cell (epithelial and mesenchymal). A novel EMT network that integrates the negative feedback loops, miR-203/Snai1, and miR-200/ZEB has been proposed recently as a control epithelial cell plasticity switches during differentiation and cancer [94]. The expression of miR-10b in metastatic breast cancer cells is shown to be induced by the transcription factor Twist, which binds directly to the putative promoter of miR-10b. Twist-induced miR-10b inhibits the translation of mRNA encoding homeobox D10, which results in an increased expression of RHOC, which is a well-characterized prometastatic gene [95].

Many miRNAs interfere with EMT by targeting the structures of cell components [96–98]. MiR-155 is the direct transcriptional target of TGF-β/Smad 4 signaling [99]. MiR-155 ectopic expression can reduce the RhoA (Ras homolog gene families, member A) protein, a small GTPase protein, to modulate the formation of tight junctions in the formation of stress fibers of the actin cytoskeleton during expression and destruction [100]. The activation of miR-31 in establishing metastases results in the regression of metastasis and the enhancement of the survival of patients. In addition, the induction of miR-31 can reduce the metastatic potential of cancer cells by targeting the RhoA [101]. The upregulation of miR-9 and direct repression of E-cadherin-1 (CDH1) in human breast cancer cells are involved in the regulation of cell-cell adhesion, migration, and the epithelial cell proliferation mechanism of calcium-dependent protein. CDH1 repression results in increased cell motility and invasion [102].
