MicroRNAs regulate the epithelial–mesenchymal transition and influence breast cancer invasion and metastasis

Abstract

MicroRNAs are small RNA molecules that play a major role in the post-transcriptional regulation of genes and influence the development, differentiation, proliferation, and apoptosis of cells and the development and progression of tumors. The epithelial–mesenchymal transition is a process by which epithelial cells morphologically transform into cells with a mesenchymal phenotype. The epithelial–mesenchymal transition plays a highly important role in tumor invasion and metastasis. Increasing evidence indicates that microRNAs are tightly associated with epithelial–mesenchymal transition regulation in tumor cells. In breast cancer, various microRNA molecules have been identified as epithelial–mesenchymal transition inducers or inhibitors, which, through different mechanisms and signaling pathways, participate in the regulation of breast cancer invasion and metastasis among various biological behaviors. The epithelial–mesenchymal transition–related microRNAs in breast cancer provide valuable molecules for researching cell invasion and metastasis, and they also provide candidate targets that may be significant for the targeted therapy of breast cancer.

Keywords

MicroRNA epithelial–mesenchymal transition breast cancer signal transduction pathway molecular mechanism

Introduction

MicroRNAs (miRNAs) are non-coding single-stranded RNAs approximately 22 nucleotides in length in eukaryotes. They inhibit the translation of target genes or degrade the target messenger RNA (mRNA) through specific base-pairing to the mRNA of target genes, thereby regulating post-transcriptional gene expression and participating in the growth and development of the body; the differentiation, proliferation, apoptosis, and metabolism of cells; and the formation of tumors.^1,2 Recent studies have shown that anomalies in the post-transcriptional regulation of mRNA by miRNAs can lead to cell proliferation, suppress cell apoptosis, and enhance cell invasion ability, leading to tumor development and progression.³ Tumor invasion and migration is a multi-step, multi-factor, multi-stage, cascade complex process based on the weakening of tumor cell adhesion and the strengthening of cell motility.⁴ The epithelial–mesenchymal transition (EMT) is a process by which polarized epithelial cells morphologically transform into cells with a mesenchymal phenotype and acquire the ability to migrate. After the occurrence of EMT, tumor cells with epithelial characteristics exhibit significantly enhanced invasion and migration abilities. Therefore, EMT plays an essential role in the process of tumor invasion and migration.⁵ Multiple signaling pathways that participate in the EMT process in tumor cells have been found, for example, Wnt, Hedgehog, phosphatidylinositol 3-kinase (PI3K), nuclear factor kappa B (NF-κB), transforming growth factor-β (TGF-β), and Notch signaling.⁶ A hot topic in recent studies is that miRNAs also regulate the above signaling pathways to participate in the tumor EMT process and influence tumor progression.^7–9

Changes can be found in a series of miRNAs related to the regulation of the epithelial cell phenotype during the invasion and metastasis of breast cancer cells.¹⁰ Reduced expression levels of the miR-200 family (miR-200a, miR-200b, miR-200c, miR-141, and miR-429) in breast cancer lead to elevated expression levels of their target gene transcription inhibitors ZEB1/ZEB2, thereby activating TGF-β/bone morphogenetic protein (BMP) signal transduction and promoting the occurrence of EMT and the invasion and metastasis of cancer cells.¹¹ Moreover, numerous other miRNAs, for example, miR-21,¹² miR-7,¹³ miR-10b,¹⁴ miR-125b,¹⁵ miR-155,¹⁶ miR-9,¹⁷ miR-497,^18,19 and miR-5003-3p,²⁰ act as inducers or inhibitors of EMT; through different mechanisms and signaling pathways, these miRNAs participate in the regulation of breast cancer EMT and affect breast cancer invasion and metastasis in addition to patient prognosis. This article reviews the advances in the research on the miRNA regulation of the EMT in terms of breast cancer invasion and migration.

miRNAs and EMT

miRNAs are a class of endogenous small single-stranded RNA molecules approximately 18–24 nucleotides in length; they were initially discovered in Caenorhabditis elegans in 1993 and play a major role in the post-transcriptional regulation of genes. Originally, miRNAs are generated in the form of primary transcription products (pri-miRNAs) through transcription by RNA polymerase II; then, miRNA precursors (pre-miRNAs) with a hairpin structure are formed in the nucleus through cleavage by the microprocessor complex comprising the enzyme Drosha and its cofactor DGCR8 (also known as Pasha); pre-miRNAs are transported out of the nucleus by the nuclear export factor exportin 5 and further cleaved and processed into mature miRNAs in the cytoplasm by the enzyme Dicer. Mature miRNAs and RNA-induced silencing complex (RISC) form the miRISC complex and play a role in gene silencing.

The concept of EMT was initially proposed by Greenburg and Hay²¹ in 1982. EMT refers to the transition from epithelial to mesenchymal cells in particular physiological or pathological conditions. After the occurrence of EMT, cells show decreased expression of epithelial phenotype molecules, such as E-cadherin and keratin, while the expression of mesenchymal molecules, such as vimentin, fibronectin, and N-cadherin, is increased. Epithelial cells lose polarity, and their contact with the surrounding cells and matrix is reduced. The connections between cells become loose, resulting in reduced cell adhesion capabilities and enhanced cell migration and movement capabilities; thus, the cells can more easily move away from their original position and undergo invasion and metastasis.

The occurrence and regulation of EMT are highly complex during tumor development and progression. Numerous transcription factors, cytokines, and miRNAs are involved in regulating the EMT process, and a number of signal transduction pathways are also involved.²² Multiple miRNAs may target the EMT regulatory factors to inhibit or promote EMT. A group of miRNAs including miR-187, miR-34a, miR-506, miRNA-138, miR-30c, miR-30d, miR-30e-3p, miR-370, and miR-106a were found to either enhance or suppress the ovarian carcinoma-associated EMT.²³ Recent evidence suggests that a growing list of miRNAs is also implicated in EMT process in breast cancer. The miR-200 family, including miR-200a, miR-200b, miR-429, miR-200c, and miR-141, can suppress ZEB 1/2 and β-catenin to interrupt EMT signaling.²⁴ Conversely, miR-221 and miR-222 target TRPS1, a ZEB repressor, resulting in E-cadherin reduction to promote EMT.²⁵ These miRNA molecules and signaling pathways generate complex cross-conduction or mutual antagonism, jointly regulating the process of tumor invasion and metastasis.

miRNAs related to EMT in breast cancer

When regulating the biological behavior of tumor cells, miRNAs indirectly perform the function of oncogenes and tumor suppressor genes (anti-oncogenes), thus playing an important role in tumor development and progression. The miRNA expression profiles vary in different types of cancer. Iorio et al.²⁶ were the first to identify abnormal miRNA expression profiles in breast cancer in 2005. This group analyzed 76 cases of breast cancer tissues and 10 cases of normal breast tissue by microarray analysis. Significant expression changes were found in 29 types of miRNA, including significantly upregulated miR-21 and miR-155 and significantly downregulated miR-10b, miR-125b, and miR-145. Additionally, Yu et al.²⁷ screened two breast cancer cell lines, A549 and HeLa, in which 19 types of miRNAs were found to be associated with cancer cell proliferation. The most significant effect was observed with the let-7 family, whose downregulation could promote the growth of breast cancer cells. Moreover, Si et al.²⁸ reported that knockdown of miR-21 in breast cancer MCF-7 cells results in an increased proportion of apoptotic cells and a decreased rate of cell proliferation. Further study on the molecular mechanism of miR-21 showed that programmed cell death 4 (PDCD4) is the target gene.²⁹ MiR-21 can inhibit PDCD4 expression and induce cell apoptosis, thereby playing a critical role in regulating the transition to proliferation and apoptosis in breast cancer cells.³⁰ Camps et al.³¹ showed that the production of miR-210 is induced in a hypoxic environment; the expression of miR-210 is negatively correlated with the overall survival and disease-free survival rates of patients, making miR-210 an effective marker of poor prognosis in breast cancer. Further research in breast cancer cell lines, large-sample-size clinical specimens, and animal xenograft models found that an increasing number of miRNAs is closely associated with the regulation of the biological behavior of breast cancer, particularly those regulating the EMT in breast cancer cells (Table 1).

Table 1.

Key miRNAs associated with EMT regulation in breast cancer.

miRNA	Effect on EMT	Target gene	Reference
miR-10b	Promotion	PTEN, HOXD10	Han et al.,¹⁴ Zadeh et al.,²⁹ Chen et al.³⁰ and Camps et al.³¹
miR-21	Promotion	PDCD4, tropomyosin, PTEN	Stinson et al.,²⁵ Huang et al.,³² Bahena-Ocampo et al.,³³ Liang et al.,³⁴ Ma et al.,³⁵ Petrovic,³⁶ Yan et al.³⁷ and Zhu et al.³⁸
miR-155	Promotion	FOXO3a, RhoA	Han et al.^39,40 and Wu et al.⁴¹
miR-9	Promotion	E-cadherin	De Mattos-Arruda et al.⁴² and Yu et al.⁴³
miR-29a	Promotion	Tristetraprolin	Kong et al.⁴⁴
miR-103/107	Promotion	Dicer	Kong et al.⁴⁵
miR-5003-3p	Promotion	MDM2	Kwak et al.²⁰
miR-181b-3p	Promotion	YWHAG	Khew-Goodall et al.⁴⁶
miR-221/222	Promotion	TRPS1	Ma et al.⁴⁷
miR-183/96/182 cluster	Promotion	BRMS1L, GHR	Gebeshuber et al.⁴⁸
miR-373	Promotion	TRPS1	Martello et al.⁴⁹
miR-100	Promotion	SMARCA5	Yoo et al.⁵⁰
miR-200 family	Inhibition	ZEB1, ZEB2	Chen et al.,⁵¹ Rasheed et al.,⁵² Tavazoie et al.,⁵³ Chou et al.,⁵⁴ Zhang et al.⁵⁵ and Bracken et al.⁵⁶
miR-34 family	Inhibition	Snail, TPD52, Notch4	Lu et al.,⁵⁷ Kong et al.⁵⁸ and Wang and Ruan⁵⁹
miR-497	Inhibition	Smad7	Liu et al.¹⁸
miR-125b	Inhibition	MAP2K7	Hong et al.¹⁵
miR-206	Inhibition	TGF-β, NRP1, Smad2	Eger et al.⁶⁰
miR-30a	Inhibition	Slug	Siemens et al.⁶¹
miR-138	Inhibition	Vimentin	Li et al.⁶²
miR-195	Inhibition	FASN, HMGCR, ACACA, CYP27B1	Yu et al.⁶³
miR-143	Inhibition	ERK5	Yin et al.⁶⁴
miR-671-5p	Inhibition	FOXM1	Chang et al.⁶⁵
miR-153	Inhibition	Metadherin	Zhang et al.⁶⁶
miR-300	Inhibition	Twist	Singh et al.⁶⁷

EMT: epithelial–mesenchymal transition; YWHAG: tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein gamma, 14-3-3γ; TRPS1: trichorhinophalangeal 1; BRMS1L: breast cancer metastasis suppressor 1-like; GHR: growth hormone receptor; ZEB: transcription factor zinc finger E-box-binding homeobox; TPD52: tumor protein D52; MAP2K7: mitogen-activated protein kinase kinase 7; NRP1: neuropilin-1; FASN: fatty-acid synthase; HMGCR: 3-hydroxy-3-methylglutaryl CoA reductase; ACACA: acetyl-CoA carboxylase; CYP27B1: cytochrome P450 family 27, subfamily B, polypeptide 1; FOXM1: forkhead box protein M1.

miRNAs promoting breast cancer invasion and metastasis

Invasion and metastasis are the basic properties of tumor cells, the pathological basis of tumor recurrence and metastasis, and an important cause of death in patients. Thus, inhibition of tumor cell invasion and migration plays a pivotal role in treatment. The EMT is considered as an essential step in the early stage of tumor metastasis, which plays an essential role in the process of tumor invasion and migration. In breast cancer, some overexpressed miRNAs negatively regulate the expression of particular anti-oncogenes to promote cancer invasion and metastasis. For instance, Huang et al.³² transduced the breast cancer cell line MCF-7 with miR-373 and miR-520c and found that both molecules can enhance the invasion and migration ability of cancer cells. This group further showed that miR-373 and miR-520c promote breast cancer invasion and migration by inhibiting the expression of the metastasis-related gene CD44. Thus far, multiple miRNAs have been found to promote breast cancer invasion and migration. The main types of these miRNAs that function by regulating EMT are summarized as follows:

miR-10b: miR-10b is located on chromosome 2q31.1, between the homeobox D4 (HOXD4) and HOXD8 genes encoding transcription factors. Overexpression of miR-10b in breast cancer cells can inhibit the expression of phosphatase and tensin homolog (PTEN), activate the activity of AKT signaling, and facilitate the EMT and stemness marker expression in cells, thereby promoting breast cancer invasion and metastasis.³³ MiR-10b is overexpressed in metastatic breast cancer cells. When non-metastatic breast cancer cells are transduced with miR-10b, the invasion and migration ability of cancer cells is enhanced.³⁴ Moreover, miR-10b can inhibit the mRNA translation of the homeobox D10 (HOXD10) gene and thereby promote the invasion and migration of breast cancer cells. Orthotopic implantation of the non-metastatic breast cancer cell lines SUM149 and SUM159 into the breast of mice showed that the miR-10b expression group exhibited more cancer cell invasion and some presented lung metastasis; cancer cell invasion and metastasis were markedly inhibited in the miR-10b antagonism group.³⁵ Furthermore, a study reported that the expression of miR-10b, a target gene of TGF-β1, can promote the occurrence of the TGF-β1-induced EMT in breast cancer cells, whereas the inhibition of miR-10b expression can partially reverse the EMT and attenuate the proliferation and invasion ability of cancer cells.¹⁴

miR-21: miR-21 expression is upregulated in various solid tumors, including breast cancer. miR-21 is an important miRNA associated with the invasion and metastasis of breast cancer cells, and it plays a role in promoting tumor progression.³⁶ A study by Yan et al.³⁷ demonstrated in a breast cancer cell line that miR-21 can enhance the invasion and migration ability of cancer cells, whereas the invasion ability of the cells is inhibited after miR-21 knockout. The mechanism of miR-21 is highly complex. The known targets of miR-21 mainly include PDCD4,²⁹ tropomyosin,³⁸ and PTEN genes.³⁹ Additionally, miR-21 can inhibit PTEN gene expression and activate AKT signaling, thereby inducing the EMT in cancer cells, promoting the invasion and migration activity of breast cancer cells, and reducing the sensitivity of cancer cells to gemcitabine.^39–41 Furthermore, miR-21 can trigger the IL-6/STAT3/NF-κB-mediated signaling loop in human epidermal growth factor receptor 2 (HER2)-positive breast cancer cells, maintain the EMT phenotype of cancer cells, and build an immune microenvironment suitable for the growth of cancer cells.⁴²

miR-155: miR-155 is an miRNA exhibiting oncogene properties. The expression of miR-155 is markedly increased in breast cancer cells. miR-155 can inactivate forkhead box O3a (FOXO3a) to induce EMT, activate mitogen-activated protein kinase (MAPK) signaling, and downregulate ras homolog gene family, member A (RhoA) expression, thus enhancing drug resistance in cancer cells.⁴³ A study showed that the upregulation of miR-155 can facilitate tumor angiogenesis, while it is associated with a poor prognosis for triple-negative breast cancer patients.⁴⁴ Kong et al.⁴⁵ reported that miR-155 can act on TGF-β/Smad4 signaling through RhoA, thereby promoting the EMT; in contrast, knockdown of miR-155 can significantly inhibit the TGF-β/Smad4-induced EMT.

Other miRNAs, miR-9, a target gene of Myc protein, can act directly on E-cadherin and promote the EMT and mesenchymal angiogenesis in breast cancer cells.⁴⁶ It was found that miR-9 can downregulate E-cadherin expression in the breast cancer cell line SUM149, while it activates Wnt/β-catenin signaling to induce the EMT and promote the invasion and metastasis of cancer cells.⁴⁷ Overexpression of miR-9 is directly correlated with the malignant progression of breast cancer, suggesting a poor prognosis.¹⁷ miR-29a can induce the EMT in breast cancer and facilitate the metastasis of cancer cells by inhibiting the expression of a zinc finger protein.⁴⁸ miR-103/107 induces the EMT in breast cancer cells by downregulating the expression of the miR-200 family, suggesting that different miRNA molecules may cross-regulate the tumor EMT process.⁴⁹ Snail is an EMT-promoting transcription factor and E-cadherin transcriptional repressor. miR-5003-3p can downregulate the expression of the direct target gene murine double minute 2 (MDM2) and stabilize the Snail protein, thereby promoting the EMT in breast cancer cells and the metastasis of cancer cells.²⁰ Other known miRNAs that promote EMT in breast cancer cells include miR-181b-3p,⁵⁰ miR-221/222,⁶⁸ miR-183/96/182 cluster,⁶⁹ and miR-373.⁷⁰ However, sometimes the occurrence of the EMT is not entirely consistent with tumor invasion and metastasis. miR-100 can downregulate E-cadherin expression and induce the EMT through targeted inhibition of SMARCA5, a regulator of CDH1 promoter methylation. Furthermore, miR-100 can inhibit the formation of breast cancer and the invasion and migration ability of cancer cells through direct, targeted regulation of HOXA1 expression.⁵¹

miRNAs inhibiting breast cancer invasion and metastasis

Tumor suppressor miRNAs generally show low or no expression in tumors. Restoring the expression of tumor suppressor miRNAs can play a role in inhibiting tumor cell invasion and metastasis. In vitro experiments showed that miR-31 can inhibit the invasion and metastasis of breast cancer cells.⁵² Moreover, miR-126, miR-206, and miR-335 can suppress breast cancer metastasis.⁵³ Chou et al.⁵⁴ demonstrated that the transcription factor GATA3 can suppress breast cancer metastasis by upregulating miR-29b expression. To date, multiple miRNAs have been found to inhibit breast cancer invasion and metastasis. The main types of miRNAs that function by regulating the EMT are summarized as follows:

miR-200 family: the miR-200 family comprises miR-200a, miR-200b, miR-200c, miR-141, and miR-429. All miR-200 family members exhibit low expression in breast cancer cells with high metastatic activity. The miR-200 family can directly act on the transcription factor zinc finger E-box-binding homeobox 1 (ZEB1) and ZEB2 to upregulate E-cadherin expression, thereby decreasing the occurrence of the EMT in tumor; as a negative regulator of EMT, miR-200 can suppress tumor invasion and metastasis.⁵⁵ Additionally, ZEB1 and ZEB2 negatively regulate the miR-200 family and form a negative feedback loop, which regulates tumor invasion and metastasis through a combined action.⁵⁶ Among the miR-200 family members, miR-200a/b is regulated by Restin. Overexpressed Restin interacts with the p53 family member p73 to increase miR-200a/b expression levels, downregulate ZEB1/2 expression and activity, activate epithelial marker expression in breast cancer cells, and inhibit the expression of mesenchymal cell markers, thereby reducing the mobility of cancer cells.⁵⁷ Both miR-200b and miR-429 are modulated by p53 binding protein 1 to upregulate E-cadherin expression, downregulate vimentin expression, and inhibit EMT process.⁶⁸ Wang et al.⁵⁹ found that one of the targets of miR-200c is the tumor elongation factor TCF8. When the breast cancer cell line MDA-MB-231 exhibiting low miR-200c expression is transduced with miR-200c, downregulated TCF8 levels and upregulated E-cadherin levels decrease the occurrence of the EMT and thereby inhibit the invasion and metastasis of breast cancer cells.⁶⁰

miR-34 family: the miR-34 family comprises three homologous miRNA molecules (miR-34a, miR-34b, and miR-34c), which are broad-spectrum anti-oncogenes. It is reported that the activation of the miR-34a/b/c gene can result in the activation of the P53 gene, thereby downregulating the EMT-promoting transcription factor Snail and decreasing the occurrence of the EMT.⁶¹ In contrast, inhibiting the miR-34a/b/c gene can upregulate the expression of mesenchymal markers, Snail, and the EMT and thus promote tumor invasion and metastasis. The transcription factors Snail and ZEB1 can also bind to the E-box region in the miR-34a/b/c promoters, thereby inhibiting the gene expression. There is a loss of miR-34a expression in breast cancer tissue with lymph node metastasis and breast cancer cell lines. Elevating the expression of miR-34a can inactivate the oncoprotein TPD52 and inhibit both the EMT process in cancer cells and the progression and metastasis of breast cancer.⁶² miR-34c can regulate the expression of the target gene Notch4 to reduce the self-renewal ability of breast cancer–initiating cells, suppress the EMT process, and weaken the migration ability of cancer cells.⁶³

Other miRNAs: SMAD7 is an inhibitor of TGF-β signaling. miR-497 can inhibit the expression of the target gene SMAD7 to play a role in suppressing the breast cancer EMT and inhibiting cancer cell proliferation, invasion, and survival.¹⁸ miR-125b can suppress the EMT through the target gene MAP2K7 and result in anticancer activity against triple-negative breast cancer.¹⁵ Moreover, other miRNAs, such as miR-206,⁶⁴ miR-30a,⁶⁵ miR-138,⁶⁶ miR-195,⁶⁷ miR-143,⁷¹ miR-671-5p,⁷² miR-153,⁷³ and miR-300,⁷⁴ inhibit the breast cancer EMT and exhibit anticancer activity through multiple target genes and signaling pathways.

Potential value of EMT-related miRNAs in the treatment of breast cancer

Surgical therapy in combination with adjuvant drug therapy, radiation therapy, and endocrine therapy, among various combined therapies, can reduce the recurrence and mortality rates in early breast cancer; however, there is no obvious advantage for metastatic breast cancer. Treatment modalities designed accordingly based on the miRNA regulation of the EMT may become a new direction of research on the treatment of breast cancer invasion and metastasis. This treatment is expected to achieve the therapeutic effect of inhibiting cancer cell proliferation, invasion, and migration by virus- or liposome-based transfection of breast cancer cells with miRNAs showing an anti-tumor effect or an antisense oligonucleotide (ASO) of miRNA showing a pro-cancer effect to regulate gene expression. Transfection of breast cancer cell line MCF-7 with an ASO of miR-21 significantly inhibited cell growth.²⁸ Furthermore, this group inoculated the ASO into rats, and in vivo observation showed that tumor growth was significantly inhibited. Herceptin has achieved great success in the early treatment and metastatic treatment of HER2-positive breast cancer; however, Herceptin resistance and the resulting potential metastasis cannot be solved. Breast cancer–induced Herceptin resistance is associated with the EMT and enhanced invasion ability, among various malignant phenotypes.⁷⁵ Further studies showed that miR-200c can regulate TGF-β signals by targeting the zinc finger protein 217 and ZEB1, thereby reversing Herceptin resistance and inhibiting metastasis in breast cancer. This study provided a new idea for the treatment of Herceptin resistance or metastatic breast cancer. miRNA-targeting drugs have also entered the clinical trial stage; for example, the newly developed miR-34 liposome is in a stage-I clinical trial. Further investigation is required to determine whether these drugs can be extensively used in the treatment of breast cancer. The ASO strategy, such as anti-miRNA oligonucleotides (AMOs), miRNA antagomirs, and multiple-target anti-miRNA antisense oligodeoxyribonucleotides (MTg-AMOs), could inhibit the functions of oncogenic miRNA and be applied in breast cancer therapy.⁷⁶ Transfection of ASOs AS-miR-221 and AS-miR-222 into breast cancer cells dramatically inhibited the expression of miR-221 and miR-222, respectively, and the suppression of miRNA-221/222 increased the sensitivity of ER-positive MCF-7 breast cancer cells to tamoxifen.⁷⁷ A recent study reported a tumor treatment strategy that simultaneously disturbs the function of multiple oncogenic miRNAs. The strategy uses a viral vector to mediate the expression of an artificially designed interfering long non-coding RNA (lncRNA). The interfering lncRNA comprises the binding sites of multiple oncogenic miRNAs (miR-21, miR-221/222, miR-224, miR-17-5p/20a, miR-10b, miR-106b, miR-151-5p, miR-155, miR-181a/181b, miR-184, miR-501-5p, miR-125a-5p/125b, miR-146a/146b-5p, miR-17, and miR-19a/19b) in series and thus neutralizes oncogenic miRNAs in cancer cells, fully protecting the function of target anti-oncogenes and exhibiting an effective anti-tumor effect.^78,79 This technique can provide a useful reference for the treatment of breast cancer.

Conclusion

Numerous factors are related to the regulation of the EMT during the development, progression, invasion, and metastasis of breast cancer, and the mechanism by which miRNAs regulate the EMT is highly complex. In addition to the above-mentioned miRNAs, other new, undiscovered miRNAs or signaling pathways must exist. Further research will identify an increasing number of miRNAs that participate in the regulation of breast cancer invasion and metastasis by regulating the EMT, and their functions and mechanisms will be gradually clarified. However, numerous issues remain to be further studied. Does the heterogeneity of cancer cells affect the coordination or antagonism among various miRNAs or signaling pathways that regulate the EMT process? Can we find molecular markers of greater significance, in addition to the currently used classic EMT marker molecules, such as E-cadherin and vimentin? More in-depth studies are needed to address these questions. Along with research on the function of EMT-related miRNAs and their target genes, new targeted therapeutic strategies targeting miRNAs should be developed, which will provide a more effective means for treating patients with breast cancer.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research was funded by a grant from the Indigenous Innovation Policies Program of Hefei (No. 2014(71)-7).

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