*Molecular Mechanisms of Breast Cancer Metastasis DOI: http://dx.doi.org/10.5772/intechopen.108424*

detoxification (glutathione S-transferase pi 1-*GSTP1*), apoptosis (Homeobox protein Hox-A5—*HOXA5*), the target of methylation-induced silencing (*TMS1*), cell adhesion and invasion (Twist-related protein—*TWIST*), Cadherin-1 (*CDH1*), metalloproteinase 3 (*TIMP3*), hormone receptors (*ESR1* and progesterone—*PGR*) are among the genes that methylated and thus are silenced [35]. The most important genes for breast cancer, *BRCA1*, and *BRCA2* are tumor suppressor genes that maintain genomic stability by participating in homologous recombination repair and gene conversion of double-stranded DNA breaks. Mutations in BRCA1 and BRCA2 tend to develop breast cancer. Loss of BRCA function due to pathogenic mutations in BRCA causes a lack of homologous recombination. *BRCA1* tumors are high-grade and negative for hormone receptors as well as have a high proliferation rate. Also, *BRCA1* tumors are positive for some cell cycle promoter genes. *BRCA2* tumors, on the other hand, present an opposite phenotype to *BRCA1* tumors but are very similar to sporadic tumors except for *BRCA2*. A research proposed that BRCA1 carriers may be more likely to develop triple-negative cancers and also develop invasive ductal carcinomas of high nuclear, histological grade, and hormone receptor-positive tumors are more common in *BRCA2* mutation carriers [35].

A total of 220 different DNA methylation sites in malignancies were examined. It is demonstrated that with these loci, normal and benign tissues of BC are distinct [36]. Genome-wide researches on breast tumors demonstrated that large number of genes have hypermethylation patterns, known as the "CpG island methylator" phenotype. This phenotype has some advantages [37]. For instance, it is protective, with a specific epigenomic profile linked to reduced metastatic risk and longevity. In contrast, a significant risk of metastatic disease and mortality is observed in the absence of this phenotype. In addition, DNA methylation patterns can be different in BC subtypes [38]. Luminal B tumors are more commonly methylated than basal-like or TNBC [39]. As a result, it is clear that methylation has a substantial role in distinct subgroups of BC and it will be crucial to elucidate the mechanisms in the methylation states. In this way, BC may be targeted therapeutically. Last, but not least, the DNA methylation pattern in endocrine-resistant cancer might give precise indicators to identify and predict the response to therapy. Thus, drugs that target particular enzymes that have crucial roles in epigenetic alterations are being developed and evaluated [38].

Ubiquitination, phosphorylation, and SUMOylation are all examples of posttranslational modifications of histone tails. However, acetylation/deacetylation and methylation are well-studied modifications to the expression of genes. The acetyl groups from ε-amino groups of lysine residues are removed by histone deacetylases (HDACs). In this way, chromatins are compacted into well-ordered nucleosomes, preventing transcription factors from accessing DNA. Histone acetyltransferases (HATs) acetylate the lysines, loosening chromatin and facilitating transcription factor binding. When histones are methylated, the genes are generally turned off. On the other hand, when histones are demethylated, the genes are turned on by loosening histone tails. In a summary, histone methylations prevent DNA to be bound by transcription factors, therefore controlling the activity of genes. HDACs and HATs are divided into various groups, each of which catalyzes a different biological process [40].

Based on their structure, HDACs are divided into two groups: zinc-dependent class I, IIa, IIb, and IV, and zinc-independent class III. According to their chemical structure, HDAC inhibitors are classified into four classes: hydroxamic acids, cyclic peptides, short-chain FAs, and benzamides. Some of them can inhibit cancer cell proliferation and promote apoptosis by repressing silenced genes. Vorinostat and

other HDAC inhibitors including entinostat and panobinostat (LBH-589) are being studied in several Phase I and II clinical trials for the treatment of BC. Moreover, their use in combination with standard cytotoxic (paclitaxel) and endocrine (tamoxifen) therapies, as well as therapies targeting HER2 (Herceptin; trastuzumab) or Vascular endothelial growth factor (VEGF), (Avastin; bevacizumab). A combination therapy that uses HDAC inhibitors and DNMT inhibitors works together to re-express suppressed genes, causing apoptosis and reducing tumor metastasis [41].

Lysine (K) and arginine (R) residues restrict histone methylation, with lysins being the most prevalent. Lysine methyltransferases and demethylases reverse the process. Active transcription is linked to methylation of histone H3 lysine 4 (H3K4), H3K36, or H3K79, while gene silencing is linked to methylation of H3K9, H3K20, or H3K27 [42]. Enhancer of zeste homolog 2 (EZH2) is a highly conserved histone methyltransferase that acts as a transcriptional repressor and methylates H3K27. Overexpression of the *EZH2* is linked to aggressive and metastatic BC tumors. The EZH2 inhibitor 3-Deazaneplanocin (DZNep) promotes apoptosis in BC cells although this is not the case in normal ones. Tanshindiols are EZH2 inhibitors that also have an anticancer effect in a variety of tumor cell lines. Last but not least, inhibitory EZH2 peptides have been developed, one of which, SQ037, has been verified and found to have significant anti-EZH2 potency. These reagents show how specificity may be tailored to create medications that specifically target epigenomic enzymes and have the desired effect with minimum adverse effects [43].

The methyltransferase *SMYD3*, which is overexpressed in various tumors, including BCs, targets H3K4. The use of short interfering RNAs to silence *SMYD3* decreases the development of cancer cells. Novobiocin suppresses the proliferation and migration of MDA-MB-231 BC cells via inhibiting *SMYD3* expression. Tranylcypromine is another powerful H3K4 methylase. This tiny chemical demethylation inhibitor inhibits the transcription of key target genes, including the pluripotent stem cell marker *OCT4* [44]. LSD1 demethylates H3K4, as well as nonhistone proteins including p53 and DNMT1. This indicates that it has a wide range of biological roles. When histonemodifying enzymes like LSD1 and EZH2 are overexpressed, they silence essential genes like tumor suppressor genes. Inactivation of these proteins is suspected to have a role in the development of BC and other cancers. However, because LSD1 is abundantly expressed in ER-breast tumors and is a hallmark of aggressiveness, its control in malignancies needs further investigation [45].

In metastasis, miRNAs perform a unique role: while overexpression of a few miRNAs leads to metastasis, the expression of some miRNAs suppresses metastasis. Inflammation and BC metastasis suppressor 1 (*BRMS1*)-mediated metastasis suppression are both controlled by miR-146. Overexpression of miR-146a/b in MDA-MB-231 cells resulted in a substantial drop in epidermal growth factor receptor (*EGFR*) expression, as well as decreased migration, invasion, and metastasis to the lungs [46]. Additionally, in human BC cells, expression levels of miR-335 and miR-206 decreased as the metastatic potential increased. Although a decrease in the expression of these miRs in cancer cells reduced lung and bone metastases, the initial tumor size had no effect. miR-335 inhibits metastasis via regulating the expression of *SOX4* [47].

In BC cells, studies show a negative association between miR-142-3p and the migration of cells. When the miR-142-3p expression is suppressed, the expression of proteins such as zinc finger E-box binding homeobox 1 (*ZEB1*) and Ras-related C3 botulinum toxin substrate 1 (*RAC1*), that allow for the development of an invasive phenotype increases. Additionally, recent research has indicated that overexpression of miR-142-3p has been linked to the suppression of BACH-1, MMP9, chemokine
