Xiaobo Zheng
Fuzhen Dai2
Lei Feng- 1Department of Liver Surgery, West China Hospital, Sichuan University, Chengdu, China
- 2Department of General Surgery, The First People's Hospital of Longquanyi District, Chengdu, China
- 3Department of Biliary Surgery, West China Hospital, Sichuan University, Chengdu, China
- 4General Surgery Center of PLA, General Hospital of Western Theater Command, Chengdu, China
- 5Department of General Surgery, Hospital of Chengdu University of Traditional Chinese Medicine, Chengdu, China
- 6Department of Hepatopancreatobiliary Surgery, Meishan City People's Hospital, Meishan Hospital of West China Hospital, Sichuan University, Meishan, China
The epithelial–mesenchymal transition (EMT) is closely associated with the acquisition of aggressive traits by carcinoma cells and is considered responsible for metastasis, relapse, and chemoresistance. Molecular links between the EMT and cancer stem cells (CSCs) have indicated that EMT processes play important roles in the expression of CSC-like properties. It is generally thought that EMT-related transcription factors (EMT-TFs) need to be downregulated to confer an epithelial phenotype to mesenchymal cells and increase cell proliferation, thereby promoting metastasis formation. However, the genetic and epigenetic mechanisms that regulate EMT and CSC activation are contradictory. Emerging evidence suggests that EMT need not be a binary model and instead a hybrid epithelial/mesenchymal state. This dynamic process correlates with epithelial–mesenchymal plasticity, which indicates a contradictory role of EMT during cancer progression. Recent studies have linked the epithelial–mesenchymal plasticity and stem cell-like traits, providing new insights into the conflicting relationship between EMT and CSCs. In this review, we examine the current knowledge about the interplay between epithelial–mesenchymal plasticity and CSCs in cancer biology and evaluate the controversies and future perspectives. Understanding the biology of epithelial–mesenchymal plasticity and CSCs and their implications in therapeutic treatment may provide new opportunities for targeted intervention.
Introduction
Epithelial–mesenchymal transition (EMT) is the process through which epithelial cells alter their phenotype, enabling them to lose their main epithelial cell traits and convert into cells expressing mesenchymal cell markers (1, 2). Following the EMT, cells switch from polygonal to spindle-like fusiform shape, lose cell polarity, and gain increased resistance to apoptosis and the ability to migrate and invade (3–8). The EMT occurs in various physiological and pathological conditions, including embryonic processes essential for normal development, tissue morphogenesis and repair, tissue reconstruction, fibrogenesis, and tumorigenesis (9–12).
EMT has been associated with cancer stemness, characterized by an increase in the number of cancer stem cells (CSCs) (13–16). CSCs are subclone cells in the tumor tissue that obtain stem cell traits (17, 18). CSCs exhibit self-renewal, maintain tumor formation ability, and can differentiate into various cells to support the tumor; therefore, these cells are considered a source of tumorigenesis, metastasis, and relapse (19–21). In addition, CSCs are highly related to chemoresistance (22). Even if most tumor cells are eliminated by chemotherapy, if CSCs are not eradicated, relapse, metastasis, and chemoresistance still commonly occur (23). Similar to normal stem cells, most CSCs are quiescent and slow growing, which is why they are resistant to anticancer drugs (24).
Previous studies have suggested that cells undergoing EMT possess stem cell traits and that tumor cells retaining stemness express markers of the EMT (25). However, several studies have shown that stemness is coupled with the mesenchymal–epithelial transition (MET) rather than the EMT and that regulation of the EMT and stemness is distinct (26–30). Thus, the correlation between the EMT and stemness is not clear. Recently, the EMT was defined as a dynamic, hybrid epithelial/mesenchymal state (31–33). Of note, this hybrid state is a coexistence of epithelial and mesenchymal phenotypes, rather than the junction of separate phenotypes; epithelial–mesenchymal plasticity was used to describe this hybrid epithelial/mesenchymal state during EMT. Importantly, epithelial–mesenchymal plasticity is involved in cancer progression and associated with stem cell-like traits, which may help explain the conflicting relationship between EMT and CSCs (34, 35). In this review, we discuss the relationship between EMT/MET/epithelial–mesenchymal plasticity and CSCs.
EMT Confers Tumor Cells With Traits of CSCS
Stemness acquired after the induction of the EMT provides cells with the traits of increased migratory ability and antitumor drug resistance and promotes tumor metastasis and recurrence (36–38). Dang et al. (39) found that the EMT induced by transforming growth factor (TGF) correlates with acquisition of tumor-initiating stem cells (TISCs) in breast cancer. SNAIL directly regulates the expression of Nanog in mesenchymal cells generated by the EMT. Deletion of SNAIL influences the growth but not the initial formation of the tumor. Kim et al. (40) found that CD13+ liver CSCs can survive in a hypoxic environment after chemotherapy and that EMT enhances cell stemness by suppressing the activity of reactive oxygen species. Garg suggested that CSCs could be classified into two distinct functional transition states, one of which is cyclic CSCs with predominant epithelial phenotype that can self-renew and differentiate into mature cancer cells. The other subset is autophagic/non-cyclic CSCs with predominant mesenchymal phenotype that have the capacity to invade and metastasize and that are majorly responsible for cancer mortality (41). The EMT seems to work together with the microenvironment to promote proliferation and homing of CSCs. Cytokines are critical for regulating the microenvironment and are necessary for initiating the EMT (42). It was reported that the TGF-β/BMP signaling pathways regulate primary tumor and metastasis microenvironments in colorectal and breast cancer (43). Collectively, these studies point to the fact that CSCs can be induced by EMT and exhibit a mesenchymal phenotype with greater metastatic potential.
EMT and acquisition of CSC-like characteristics are key steps in the metastasis and recurrence of a tumor after radical resection. A study by Mani and colleagues (25) showed that EMT induction in immortalized human mammary epithelial cells led to increased mammosphere formation in vitro and tumorigenicity in vivo, suggesting that the EMT process can stimulate the acquisition of cell stemness. Moreover, the EMT confers stem cell-like properties, consistent with the migratory CSC concept. Several lines of evidence have supported the relationship between EMT and stemness. For example, Morel et al. (44) showed that CD44−CD24+ breast epithelial cells, which are non-tumorigenic, can induce the EMT after activating the RAS/mitogen-activated protein kinase pathway and acquire the CD44+CD24− phenotype and stem cell traits. CD24+ cells treated with TGF do undergo the EMT, as demonstrated by the downregulation of E-cadherin and the upregulation of vimentin, accompanied by acquisition of CD24− features. It is thus tempting to speculate that the EMT may be an important step in controlling the transition of CD44−CD24+ cells to CD44+CD24− cells. In CSCs isolated from colorectal cancer surgery samples and analyzed using gene chips, Hwang et al. (45) found a high expression of CD44 and CD166, stem cell markers that regulate the EMT-associated transcription factor (TF) SNAIL. Subclones of basement-like breast cancer cells have a high proportion of CD44+CD24− stem cell-like cells and overexpress EMT-correlated genes (46). Another study found that Fox2, an EMT-related TF, is highly expressed in breast cancer and is closely linked with basement-like subclones (47). Immunohistochemical analysis of 479 infiltrated breast cancer samples revealed that basement-like breast cancers express high levels of EMT-associated factors but low levels of E-cadherin (48). Taken together, these studies confirmed the fact that stemness and EMT are indeed intricately linked.
MET in Recovering Epithelial Traits for Enhancing Stemness to Facilitate Distal Colonization
Several results have linked the MET and stem cell-like traits, challenging the view on the relationship between the EMT and CSCs (31, 49–51). The general view suggests that EMT-related transcription factors (EMT-TFs) must be downregulated in order to convert mesenchymal cells into epithelial cells and to increase proliferation, thereby promoting tumor metastasis formation (52). In order to form clones, malignant tumor cells need to assume an epithelial phenotype and maintain a state of stemness (53). Interestingly, Padmanaban et al. revealed that the expression of E-cadherin needs to be rescued via the inhibition of TGFβ-receptor signaling during the detachment, systemic dissemination, and seeding phases of metastasis in invasive breast ductal carcinomas (54). Fibroblasts must experience the MET to complete their progression to induced pluripotent stem cells (29). A study by Tsai et al. (27) clearly supported the role of the EMT in dissemination and the subsequent MET for colonization and macrometastasis. Additionally, a study by Ocaña et al. (28) also supported the role of the EMT in dissemination and the necessity of reversing the EMT for metastasis.
The mechanism underlying MET in recovering cancer cells stemness is still complicated. Several studies have reported that downregulation of traditional EMT-TFs cannot induce stemness because the regulation of stem cell properties is independent from that of epithelial plasticity (55). However, stem cell properties can be acquired through inhibition of Prrx1 (28, 56). Prrx1 and TWIST can both induce EMT properties alone; however, while deletion of Prrx1 induces lung metastasis, ablation of TWIST does not have the same effect. Intriguingly, deletion of Prrx1 in BT-549 cells enhances stemness, accompanied with increased mammosphere formation, self-renewal ability, and CD24−/CD44+ CSC proportion (28). Instead, downregulation of TWIST does not induce stemness, suggesting that downregulation of traditional EMT-TFs is not related to the occurrence of stemness and that regulation of the EMT and CSCs is distinct. In addition, Prrx1 expression predicts better prognosis and higher metastasis-free survival (57). Celià-Terrassa et al. (58) illustrated that the EMT can inhibit TISCs, suggesting that there are different subpopulations of EMT-TFs. In addition, the EMT must be reversed to allow growth and clonal expansion because invasive dedifferentiated tumor cells from the EMT were found to be quiescent, whereas proliferation was detected in redifferentiated metastatic tumor cells, suggesting that the EMT should reverse to the MET.
It has been reported that the mesenchymal state is related to early events in metastasis, such as dissemination, invasion, and intravascular infiltration. EMT-TFs initiate the invasion–metastasis cascade when aberrantly activated in tumors. A recent study showed that CSC formation is an early and frequent event in LSC progression (59). It has also been suggested that the epithelial state with stemness is correlated with later phases of metastasis (55). Moreover, the observations that CSC plasticity is elevated in advanced cancers and that regulation of the epithelial–mesenchymal states is increased are highly relevant (35). Mesenchymal state-associated invasion and dissemination are necessary, but not sufficient, to induce metastasis, and additional epithelial state with stemness is required to complete the full metastasis cascade (60). Therefore, epithelial–mesenchymal heterogeneity with stemness plasticity is involved in the entire process of invasion and metastasis.
Emergence of Epithelial–Mesenchymal Plasticity
Recently, epithelial–mesenchymal plasticity was recommended as unified nomenclature by the EMT international association and was termed as the ability of cells to adopt mixed epithelial/mesenchymal (E/M) features and transit between EMT and MET states (50, 61). This epithelial–mesenchymal plasticity has been variably referred to as partial EMT, hybrid E/M status, intermediate EMT, a metastable EMT state, EMT continuum, and EMT spectrum, which were widely used in past studies (62). The cells undergoing epithelial–mesenchymal plasticity express a mixture of epithelial and mesenchymal features and express both epithelial and mesenchymal markers (63). Epithelial–mesenchymal plasticity also helps to account for the reversibility of the EMT process. Epithelial cells going through EMT give rise to cell populations that may enter reversibly into states with various proportions of epithelial and mesenchymal features (64, 65). Epithelial–mesenchymal plasticity is thought to provide cells with the fitness and flexibility to fulfill the diverse requirements during the course of either developmental or pathological processes. These cell transitions allow them to migrate from the primary tumor and invade the secondary site, playing a fundamental role in cancer metastasis. Epithelial–mesenchymal plasticity is associated with tumor cell migration, invasion, colonization, stemness, and drug resistance (66).
Epithelial–mesenchymal plasticity has been reported in many studies. The hybrid E/M phenotypes have been confirmed both in vitro and in vivo. Huang et al. systematically analyzed the protein levels of the epithelial and mesenchymal markers in 42 ovarian carcinoma cell lines. Among these 42 cell lines, 9 have been characterized as epithelial cells, 7 as mesenchymal cells, and the remaining 26 cell lines were characterized as hybrid E/M phenotypes (67). The existence of hybrid E/M states has also been observed in animal models. Pastushenko and colleagues screened a large panel of cell surface markers, such as EpCAM, vimentin, CD106, CD61, and CD51, in genetic mouse models of skin and mammary primary tumors. They identified the existence of multiple tumor subpopulations associated with different EMT stages: from completely epithelial to completely mesenchymal states, passing through intermediate hybrid states (68). The hybrid E/M phenotypes were also detected in clinical samples. Metastatic breast cancers were categorized as either having an epithelial or hybrid phenotype using a prediction algorithm, where the VIM:CDH1 gene expression ratio was combined with the expression of CLDN7 (69). A partial EMT process including the upregulation of mesenchymal genes in conjunction with the downregulation of certain epithelial genes was confirmed in a subset of HNSCC cells through single-cell RNA sequencing (70). Taken together, the epithelial–mesenchymal plasticity in cancer cells describes the presence of both epithelial and mesenchymal markers in the same cancer cells. It might reflect a stable state of cancer type or a transition phase of cancer cells while they are switching their phenotype. Its correlation with aggressiveness and metastasis further enforces the crucial role of epithelial–mesenchymal plasticity in cancer progression.
The Interplay Between Epithelial–Mesenchymal Plasticity and CSCS
Distal tumor subclone formation is thought to be a multistep and long-term process, which also explains why various subclones have distinct proliferative abilities (71). Stemness is also a state of plasticity in tumor progression, allowing static and migratory CSCs to coexist (72). These ideas are consistent with the concept of epithelial–mesenchymal plasticity, a process that is thought to be a hybrid state during metastasis. The shift among the hybrid states of EMT may orchestrate the entire process of distant metastasis formation, from acquisition of invasive ability from primary tumor and dissemination via the bloodstream, to seeding in distant organs, stemness recovery for clonal expansion, and macrometastasis (73). Emerging evidence from theoretical and experimental studies has revealed the association of epithelial–mesenchymal plasticity with CSCs (74–77). Pastushenko et al. reported that the earliest EMT state already exhibits increased CSC frequency, and tumor stemness does not increase further in later hybrid epithelial–mesenchymal states (68). Francescangeli et al. reported that a pre-existing population of ZEB2+ quiescent cells in colorectal cancer showed both stemness and mesenchymal features and dictated chemotherapy resistance (78). Co-expression of stem cell and both epithelial and mesenchymal characters was also observed in circulating tumor cells of bladder cancer patients (79). Epithelial–mesenchymal plasticity was associated with miRNA let-7, which was an important factor affecting the CSC phenotype in high-grade serous ovarian carcinoma samples and could be correlated with tumor growth and metastasis (80). Quan et al. reported that ~60% of the leader CSCs in collective invasion co-existed with hybrid epithelial–mesenchymal states, indicating that CSCs with epithelial–mesenchymal plasticity play a key role in cancer cell collective invasion (81). Moreover, a previous study reported that in response to microenvironmental signals, lung cancer cells converted to CSC state through regulation of the balance between epithelial and mesenchymal transition (82). Collectively, the above results indicate that epithelial–mesenchymal plasticity confers cancer cells with the traits of stemness.
The mechanism underlying epithelial–mesenchymal plasticity and CSCs is still largely unknown. Recently, some factors that regulate the epithelial–mesenchymal plasticity and stemness of CSCs were reported. OvoL/Shavenbaby factors are a family of key epithelial stabilizers and are critical for adult stem cell homeostasis. Stemness and epithelial–mesenchymal plasticity could be regulated by interaction of EMT transcription factors and OvoL/Shavenbaby (83). A study reported that the non-coding RNAs expressed on the DLK1-DIO3 locus regulate the epithelial–mesenchymal plasticity in breast epithelial progenitor cells, providing evidence of the interplay of epithelial–mesenchymal plasticity and stemness (84). miRNAs, which are important factors in tumorigenesis and progression of cancers, are also involved in mediating interactions between epithelial–mesenchymal plasticity and CSCs (85–88). Jiang et al. reported that Prrx1 promotes epithelial–mesenchymal plasticity and activates cell dormancy in head and neck squamous cell carcinoma and that miR-642b-3p restoration rescues PRRX1-induced phenotype and cell dormancy (85). Furthermore, You et al. observed that miRNA-495 confers inhibitory effects on CSCs, as well as EMT, in oral squamous cell carcinoma through HOXC6-mediated TGF-β signaling pathway (86). The long non-coding RNA H19 mediates epithelial–mesenchymal plasticity by differentially sponging miR-200b/c and let-7b, wherein the latter is a CSC regulator in colon cancer (88, 89). Several studies have shown that cells with hybrid E/M states and CSC phenotypes are spatially segregated in the primary tumor (90). Bocci et al. observed through a mechanism-based dynamical model that the diffusion of EMT-inducing signals such as TGF-β, together with non-cell autonomous control of EMT and CSC decision-making via the Notch signaling pathway, can explain the experimentally observed disparate localization of subsets of CSCs with varying EMT phenotypes in the tumor (74). These results offer insights into the principles of spatiotemporal patterning in epithelial–mesenchymal plasticity and identify a relevant target during hybrid E/M states to alleviate multiple CSC subsets. Using a mechanism-based model, Bocci et al. explained how metformin can both inhibit EMT and blunt the aggressive potential of CSCs simultaneously by driving the cells out of a hybrid E/M stem-like state with enhanced Notch-Jagged signaling (91).
The expression levels of EMT-TFs, such as SNAIL and TWIST, in primary tumors are also fluctuant associated with cancer cell stemness. The consecutive expression of SNAIL, Prrx1, and TWIST also inhibits the formation of metastasis because EMT-TFs must be downregulated to facilitate stemness recovery and tumor formation (58). This is not contradictory, but simply reflects epithelial–mesenchymal plasticity and the dynamic process resembling the migration of embryonic cell populations to distant organs/sites. Thus, EMT-TFs are related to cell behavior rather than to cell fate, and hence, their expression is dynamic. Accordingly, as recommended by the EMT international association, EMT-TFs alone cannot be used as markers of differentiated cell populations, the equivalent of differentiated distant metastases (92). Further investigation of the downregulation of EMT-TFs in signaling pathways associated with the formation of CSCs is needed, particularly with regard to the interplay among the epithelial–mesenchymal plasticity, invasion of the primary tumor, and stemness recovery for tumor metastatic colonization. The complex interplay between epithelial–mesenchymal plasticity, CSCs, and tumor microenvironment gives rise to tumor heterogeneity that still represents the major challenge hampering therapy for metastasis and chemoresistance.
Conclusion and Perspectives
This review suggests that epithelial–mesenchymal plasticity is involved in the process of CSC development. The coexistence of epithelial–mesenchymal plasticity and CSCs correlates with poor prognosis and resistance to therapy (93). Furthermore, emerging evidence has shown that targeting epithelial–mesenchymal plasticity-induced CSCs can effectively regulate tumor progression and drug resistance. Liu et al. reported that metformin inhibits prostate cancer resistant to enzalutamide by reducing the cells with hybrid E/M status and, thereby, restricting the formation of CSCs (94). Nevertheless, the mechanism underlying the relationship between epithelial–mesenchymal plasticity and CSCs still remains poorly understood. Additional research is required at the molecular level to clarify, for example, how TWIST and Prrx1 interact, and how Prrx1 inhibits the induction of stemness while not inhibiting the EMT-promoting function of TWIST, as well as to elucidate the roles of other potential EMT-related factors, such as SNAIL and ZEB1. This will help to unveil the mechanisms underlying CSC initiation and tumor metastasis. In addition, it is plausible that non-CSCs can transition to CSCs during the dynamic process of epithelial–mesenchymal plasticity. Therefore, the plasticity of CSCs needs to be considered to explore therapeutic strategies aimed at overcoming tumor heterogeneity and chemoresistance by targeting CSCs. In all, understanding these molecular mechanisms can help improve the efficiency of the ongoing and planned therapeutic trials to control cancer progression, treatment resistance, and disease recurrence.
Author Contributions
MX proposed the research. XZ, FD, and LeF collected the references. XZ, HZ, and LiF analyzed the references. XZ and MX wrote the paper. All authors contributed to the design and interpretation of the study and to the writing of the drafts.
Funding
This study was supported by the grants from the National Natural Science Foundation of China (No. 81803574), China Postdoctoral Science Foundation (2019M653430), and Post-Doctor Research Project, West China Hospital, Sichuan University (2018HXBH003)), and Key Technology Research and Development Program of the Sichuan Province (Nos. 2019YFS0208, 2021YFSY0009).
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
References
1. Thiery JP, Acloque H, Huang RY, Nieto MA. Epithelial-mesenchymal transitions in development and disease. Cell. (2009) 139:871–90. doi: 10.1016/j.cell.2009.11.007
2. Kalluri R, Weinberg RA. The basics of epithelial-mesenchymal transition. J Clin Invest. (2009) 119:1420–8. doi: 10.1172/JCI39104
3. Abdullah A, Akhand SS, Paez JSP, Brown W, Pan L, Libring S. Epigenetic targeting of neuropilin-1 prevents bypass signaling in drug-resistant breast cancer. Oncogene. (2020) 40:322–33. doi: 10.1038/s41388-020-01530-6
4. Ji R, Zhu XJ, Wang ZR, Huang LQ. Cortactin in epithelial-mesenchymal transition. Front Cell Dev Biol. (2020) 8:585619. doi: 10.3389/fcell.2020.585619
5. Coban B, Bergonzini C, Zweemer AJM, Danen EHJ. Metastasis: crosstalk between tissue mechanics and tumour cell plasticity. Br J Cancer. (2020) 124:49–57. doi: 10.1038/s41416-020-01150-7
6. Babaei G, Aziz SG, Jaghi NZZ. EMT, cancer stem cells and autophagy; the three main axes of metastasis. Biomed Pharmacotherap. (2020) 133:110909. doi: 10.1016/j.biopha.2020.110909
7. Peyre L, Meyer M, Hofman P, Roux J. TRAIL receptor-induced features of epithelial-to-mesenchymal transition increase tumour phenotypic heterogeneity: potential cell survival mechanisms. Br J Cancer. (2020) 124:91–101. doi: 10.1038/s41416-020-01177-w
8. Sinha D, Saha P, Samanta A, Bishayee A. Emerging concepts of hybrid epithelial-to-mesenchymal transition in cancer progression. Biomolecules. (2020) 10:1561. doi: 10.3390/biom10111561
9. Brabletz T. EMT and MET in metastasis: where are the cancer stem cells? Cancer Cell. (2012) 22:699–701. doi: 10.1016/j.ccr.2012.11.009
10. Lamouille S, Xu J, Derynck R. Molecular mechanisms of epithelial-mesenchymal transition. Nat Rev Mol Cell Biol. (2014) 15:178–96. doi: 10.1038/nrm3758
11. De Craene B, Berx G. Regulatory networks defining EMT during cancer initiation and progression. Nat Rev Cancer. (2013) 13:97–110. doi: 10.1038/nrc3447
12. Kim DH, Xing T, Yang Z, Dudek R, Lu Q, Chen YH. Epithelial Mesenchymal transition in embryonic development, tissue repair and cancer: a comprehensive overview. J Clin Med. (2017) 7:1. doi: 10.3390/jcm7010001
13. Castagnoli L, Tagliabue E. Inhibition of the wnt signalling pathway: an avenue to control breast cancer aggressiveness. Int J Mol Sci. (2020) 21:9069. doi: 10.3390/ijms21239069
14. Dong X, Bai X. Exosomes and breast cancer drug resistance. Cell Death Dis. (2020) 11:987. doi: 10.1038/s41419-020-03189-z
15. Roy S, Sunkara RR, Parmar MY, Shaikh S, Waghmare SK. EMT imparts cancer stemness and plasticity: new perspectives and therapeutic potential. Front Biosci. (2021) 26:238–65. doi: 10.2741/4893
16. Tanabe S, Quader S, Cabral H, Ono R. Interplay of EMT and CSC in cancer and the potential therapeutic strategies. Front Pharmacol. (2020) 11:904. doi: 10.3389/fphar.2020.00904
17. Kreso A, Dick JE. Evolution of the cancer stem cell model. Cell Stem Cell. (2014) 14:275–91. doi: 10.1016/j.stem.2014.02.006
18. Chaffer CL, Weinberg RA. A perspective on cancer cell metastasis. Science. (2011) 331:1559–64. doi: 10.1126/science.1203543
19. Clevers H. The cancer stem cell: premises, promises and challenges. Nat Med. (2011) 17:313–9. doi: 10.1038/nm.2304
20. Eppert K, Takenaka K, Lechman ER, Waldron L, Nilsson B, van Galen P, et al. Stem cell gene expression programs influence clinical outcome in human leukemia. Nat Med. (2011) 17:1086–93. doi: 10.1038/nm.2415
21. Huang T, Song X, Xu D, Tiek D, Goenka A, Wu B, et al. Stem cell programs in cancer initiation, progression, and therapy resistance. Theranostics. (2020) 10:8721–43. doi: 10.7150/thno.41648
22. Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J, et al. Identification of a cancer stem cell in human brain tumors. Cancer Res. (2003) 63:5821–8.
23. Xiang L, Semenza GL. Hypoxia-inducible factors promote breast cancer stem cell specification and maintenance in response to hypoxia or cytotoxic chemotherapy. Adv Cancer Res. (2019) 141:175–212. doi: 10.1016/bs.acr.2018.11.001
24. Visvader JE, Lindeman GJ. Cancer stem cells: current status and evolving complexities. Cell Stem Cell. (2012) 10:717–28. doi: 10.1016/j.stem.2012.05.007
25. Mani SA, Guo W, Liao MJ, Eaton EN, Ayyanan A, Zhou AY, et al. The epithelial-mesenchymal transition generates cells with properties of stem cells. Cell. (2008) 133:704–15. doi: 10.1016/j.cell.2008.03.027
26. Pinzani M. Epithelial-mesenchymal transition in chronic liver disease: fibrogenesis or escape from death? J Hepatol. (2011) 55:459–65. doi: 10.1016/j.jhep.2011.02.001
27. Tsai JH, Donaher JL, Murphy DA, Chau S, Yang J. Spatiotemporal regulation of epithelial-mesenchymal transition is essential for squamous cell carcinoma metastasis. Cancer Cell. (2012) 22:725–36. doi: 10.1016/j.ccr.2012.09.022
28. Ocaña OH, Córcoles R, Fabra A, Moreno-Bueno G, Acloque H, Vega S, et al. Metastatic colonization requires the repression of the epithelial-mesenchymal transition inducer Prrx1. Cancer Cell. (2012) 22:709–24. doi: 10.1016/j.ccr.2012.10.012
29. Li R, Liang J, Ni S, Zhou T, Qing X, Li H, et al. A mesenchymal-to-epithelial transition initiates and is required for the nuclear reprogramming of mouse fibroblasts. Cell Stem Cell. (2010) 7:51–63. doi: 10.1016/j.stem.2010.04.014
30. Sannino G, Marchetto A, Kirchner T, Grünewald TGP. Epithelial-to-mesenchymal and mesenchymal-to-epithelial transition in mesenchymal tumors: a paradox in sarcomas? Cancer Res. (2017) 77:4556–61. doi: 10.1158/0008-5472.CAN-17-0032
31. Williams ED, Gao D. Controversies around epithelial-mesenchymal plasticity in cancer metastasis. Nat Rev Cancer. (2019) 19:716–32. doi: 10.1038/s41568-019-0213-x
32. Genna A, Vanwynsberghe AM, Villard AV, Pottier C, Ancel J, Polette M, et al. EMT-associated heterogeneity in circulating tumor cells: sticky friends on the road to metastasis. Cancers. (2020) 12:1632. doi: 10.3390/cancers12061632
33. Jia D, Li X, Bocci F. Quantifying cancer epithelial-mesenchymal plasticity and its association with stemness and immune response. J Clin Med. (2019) 8:725. doi: 10.3390/jcm8050725
34. Santamaria PG, Moreno-Bueno G, Portillo F, Cano A. EMT: present and future in clinical oncology. Mol Oncol. (2017) 11:718–38. doi: 10.1002/1878-0261.12091
35. Liao TT, Yang MH. Revisiting epithelial-mesenchymal transition in cancer metastasis: the connection between epithelial plasticity and stemness. Mol Oncol. (2017) 11:792–804. doi: 10.1002/1878-0261.12096
36. Pradella D, Naro C, Sette C, Ghigna C. EMT and stemness: flexible processes tuned by alternative splicing in development and cancer progression. Mol Cancer. (2017) 16:8. doi: 10.1186/s12943-016-0579-2
37. Srivastava C, Irshad K, Dikshit B, Chattopadhyay P, Sarkar C, Gupta DK, et al. FAT1 modulates EMT and stemness genes expression in hypoxic glioblastoma. Int J Cancer. (2018) 142:805–12. doi: 10.1002/ijc.31092
38. Rodriguez-Aznar E, Wiesmüller L, Sainz B Jr, Hermann PC. EMT and stemness-key players in pancreatic cancer stem cells. Cancers. (2019) 11:1136. doi: 10.3390/cancers11081136
39. Dang H, Ding W, Emerson D, Rountree CB. Snail1 induces epithelial-to-mesenchymal transition and tumor initiating stem cell characteristics. BMC Cancer. (2011) 11:396. doi: 10.1186/1471-2407-11-396
40. Kim HM, Haraguchi N, Ishii H, Ohkuma M, Okano M, Mimori K, et al. Increased CD13 expression reduces reactive oxygen species, promoting survival of liver cancer stem cells via an epithelial-mesenchymal transition-like phenomenon. Ann Surg Oncol. (2012) 19(Suppl. 3):S539–48. doi: 10.1245/s10434-011-2040-5
41. Garg M. Epithelial plasticity, autophagy and metastasis: potential modifiers of the crosstalk to overcome therapeutic resistance. Stem Cell Rev Rep. (2020) 16:503–10. doi: 10.1007/s12015-019-09945-9
42. Weng YS, Tseng HY, Chen YA, Shen PC, Al Haq AT, Chen LM, et al. MCT-1/miR-34a/IL-6/IL-6R signaling axis promotes EMT progression, cancer stemness and M2 macrophage polarization in triple-negative breast cancer. Mol Cancer. (2019) 18:42. doi: 10.1186/s12943-019-0988-0
43. Shibue T, Brooks MW, Weinberg RA. An integrin-linked machinery of cytoskeletal regulation that enables experimental tumor initiation and metastatic colonization. Cancer Cell. (2013) 24:481–98. doi: 10.1016/j.ccr.2013.08.012
44. Morel AP, Lièvre M, Thomas C, Hinkal G, Ansieau S, Puisieux A. Generation of breast cancer stem cells through epithelial-mesenchymal transition. PLoS ONE. (2008) 3:e2888. doi: 10.1371/journal.pone.0002888
45. Hwang WL, Yang MH, Tsai ML, Lan HY, Su SH, Chang SC, et al. SNAIL regulates interleukin-8 expression, stem cell-like activity, and tumorigenicity of human colorectal carcinoma cells. Gastroenterology. (2011) 141:279–91:91.e1-5. doi: 10.1053/j.gastro.2011.04.008
46. Alkatout I, Wiedermann M, Bauer M, Wenners A, Jonat W, Klapper W. Transcription factors associated with epithelial-mesenchymal transition and cancer stem cells in the tumor centre and margin of invasive breast cancer. Exp Mol Pathol. (2013) 94:168–73. doi: 10.1016/j.yexmp.2012.09.003
47. Hollier BG, Tinnirello AA, Werden SJ, Evans KW, Taube JH, Sarkar TR, et al. FOXC2 expression links epithelial-mesenchymal transition and stem cell properties in breast cancer. Cancer Res. (2013) 73:1981–92. doi: 10.1158/0008-5472.CAN-12-2962
48. Sigurdsson V, Hilmarsdottir B, Sigmundsdottir H, Fridriksdottir AJ, Ringnér M, Villadsen R, et al. Endothelial induced EMT in breast epithelial cells with stem cell properties. PLoS ONE. (2011) 6:e23833. doi: 10.1371/journal.pone.0023833
49. Dongre A, Weinberg RA. New insights into the mechanisms of epithelial-mesenchymal transition and implications for cancer. Nat Rev Mol Cell Biol. (2019) 20:69–84. doi: 10.1038/s41580-018-0080-4
50. Lu W, Kang Y. Epithelial-mesenchymal plasticity in cancer progression and metastasis. Dev Cell. (2019) 49:361–74. doi: 10.1016/j.devcel.2019.04.010
51. Markopoulos GS, Roupakia E, Marcu KB, Kolettas E. Epigenetic regulation of inflammatory cytokine-induced epithelial-to-mesenchymal cell transition and cancer stem cell generation. Cells. (2019) 8:1143. doi: 10.3390/cells8101143
52. Ye X, Weinberg RA. Epithelial-mesenchymal plasticity: a central regulator of cancer progression. Trends Cell Biol. (2015) 25:675–86. doi: 10.1016/j.tcb.2015.07.012
53. van Denderen BJ, Thompson EW. Cancer: the to and fro of tumour spread. Nature. (2013) 493:487–8. doi: 10.1038/493487a
54. Padmanaban V, Krol I, Suhail Y, Szczerba BM, Aceto N, Bader JS, et al. E-cadherin is required for metastasis in multiple models of breast cancer. Nature. (2019) 573:439–44. doi: 10.1038/s41586-019-1526-3
55. Krebs AM, Mitschke J, Lasierra Losada M, Schmalhofer O, Boerries M, Busch H, et al. The EMT-activator Zeb1 is a key factor for cell plasticity and promotes metastasis in pancreatic cancer. Nat Cell Biol. (2017) 19:518–29. doi: 10.1038/ncb3513
56. Shi L, Tang X. A SIRT1-centered circuitry regulates breast cancer stemness and metastasis. Oncogene. (2018) 37:6299–315. doi: 10.1038/s41388-018-0370-5
57. Hirata H, Sugimachi K, Takahashi Y, Ueda M, Sakimura S, Uchi R, et al. Downregulation of PRRX1 confers cancer stem cell-like properties and predicts poor prognosis in hepatocellular carcinoma. Ann Surg Oncol. (2015) 22(Suppl. 3):S1402–9. doi: 10.1245/s10434-014-4242-0
58. Celià-Terrassa T, Meca-Cortés O, Mateo F, Martínez de Paz A, Rubio N, Arnal-Estapé A, et al. Epithelial-mesenchymal transition can suppress major attributes of human epithelial tumor-initiating cells. J Clin Invest. (2012) 122:1849–68. doi: 10.1172/JCI59218
59. Garg S, Reyes-Palomares A. Hepatic leukemia factor is a novel leukemic stem cell regulator in DNMT3A, NPM1, and FLT3-ITD triple-mutated AML. Blood. (2019) 134:263–76. doi: 10.1182/blood.2018862383
60. Castañón E, Soltermann A, López I, Román M, Ecay M, Collantes M, et al. The inhibitor of differentiation-1 (Id1) enables lung cancer liver colonization through activation of an EMT program in tumor cells and establishment of the pre-metastatic niche. Cancer Lett. (2017) 402:43–51. doi: 10.1016/j.canlet.2017.05.012
61. Hass R, von der Ohe J, Ungefroren H. The intimate relationship among EMT, MET and TME: A T(ransdifferentiation) E(nhancing) M(ix) to be exploited for therapeutic purposes. Cancers. (2020) 12:3674. doi: 10.3390/cancers12123674
62. Simeone P, Trerotola M, Franck J, Cardon T, Marchisio M, Fournier I, et al. The multiverse nature of epithelial to mesenchymal transition. Semin Cancer Biol. (2019) 58:1–10. doi: 10.1016/j.semcancer.2018.11.004
63. Zhang Y, Weinberg RA. Epithelial-to-mesenchymal transition in cancer: complexity and opportunities. Front Med. (2018) 12:361–73. doi: 10.1007/s11684-018-0656-6
64. Chen C, Zhao S, Karnad A, Freeman JW. The biology and role of CD44 in cancer progression: therapeutic implications. J Hematol Oncol. (2018) 11:64. doi: 10.1186/s13045-018-0605-5
65. Saitoh M. Involvement of partial EMT in cancer progression. J Biochem. (2018) 164:257–64. doi: 10.1093/jb/mvy047
66. Puré E, Hingorani SR. Mesenchymal cell plasticity and perfidy in epithelial malignancy. Trends Cancer. (2018) 4:273–7. doi: 10.1016/j.trecan.2018.02.007
67. Huang RY, Wong MK, Tan TZ, Kuay KT, Ng AH, Chung VY, et al. An EMT spectrum defines an anoikis-resistant and spheroidogenic intermediate mesenchymal state that is sensitive to e-cadherin restoration by a src-kinase inhibitor, saracatinib (AZD0530). Cell Death Dis. (2013) 4:e915. doi: 10.1038/cddis.2013.442
68. Pastushenko I, Brisebarre A, Sifrim A, Fioramonti M, Revenco T, Boumahdi S, et al. Identification of the tumour transition states occurring during EMT. Nature. (2018) 556:463–8. doi: 10.1038/s41586-018-0040-3
69. George JT, Jolly MK, Xu S, Somarelli JA, Levine H. Survival outcomes in cancer patients predicted by a partial EMT gene expression scoring metric. Cancer Res. (2017) 77:6415–28. doi: 10.1158/0008-5472.CAN-16-3521
70. Puram SV, Tirosh I, Parikh AS, Patel AP, Yizhak K, Gillespie S, et al. Single-cell transcriptomic analysis of primary and metastatic tumor ecosystems in head and neck cancer. Cell. (2017) 171:1611–24.e24. doi: 10.1016/j.cell.2017.10.044
71. Nguyen DX, Bos PD, Massagué J. Metastasis: from dissemination to organ-specific colonization. Nat Rev Cancer. (2009) 9:274–84. doi: 10.1038/nrc2622
72. Massagué J, Obenauf AC. Metastatic colonization by circulating tumour cells. Nature. (2016) 529:298–306. doi: 10.1038/nature17038
74. Bocci F, Gearhart-Serna L, Boareto M. Toward understanding cancer stem cell heterogeneity in the tumor microenvironment. Proc. Natl Acad Sci USA. (2019) 116:148–57. doi: 10.1073/pnas.1815345116
75. Shibue T, Weinberg RA. EMT, CSCs, and drug resistance: the mechanistic link and clinical implications. Nat Rev Clin Oncol. (2017) 14:611–29. doi: 10.1038/nrclinonc.2017.44
76. Varga J, Greten FR. Cell plasticity in epithelial homeostasis and tumorigenesis. Nat Cell Biol. (2017) 19:1133–41. doi: 10.1038/ncb3611
77. Garg M. Epithelial plasticity and cancer stem cells: Major mechanisms of cancer pathogenesis and therapy resistance. World J Stem Cells. (2017) 9:118–26. doi: 10.4252/wjsc.v9.i8.118
78. Francescangeli F, Contavalli P, De Angelis ML, Careccia S, Signore M, Haas TL, et al. A pre-existing population of ZEB2(+) quiescent cells with stemness and mesenchymal features dictate chemoresistance in colorectal cancer. J Exp Clin Cancer Res. (2020) 39:2. doi: 10.1186/s13046-019-1505-4
79. Zhang R, Xia J, Wang Y, Cao M, Jin D, Xue W, et al. Co-expression of stem cell and epithelial mesenchymal transition markers in circulating tumor cells of bladder cancer patients. Onco Targets Ther. (2020) 13:10739–48. doi: 10.2147/OTT.S259240
80. Chirshev E, Hojo N, Bertucci A, Sanderman L, Nguyen A, Wang H, et al. Epithelial/mesenchymal heterogeneity of high-grade serous ovarian carcinoma samples correlates with miRNA let-7 levels and predicts tumor growth and metastasis. Mol Oncol. (2020) 14:2796–813. doi: 10.1002/1878-0261.12762
81. Quan Q, Wang X, Lu C, Ma W, Wang Y, Xia G, et al. Cancer stem-like cells with hybrid epithelial/mesenchymal phenotype leading the collective invasion. Cancer Sci. (2020) 111:467–76. doi: 10.1111/cas.14285
82. Andriani F, Bertolini G, Facchinetti F, Baldoli E, Moro M, Casalini P, et al. Conversion to stem-cell state in response to microenvironmental cues is regulated by balance between epithelial and mesenchymal features in lung cancer cells. Mol Oncol. (2016) 10:253–71. doi: 10.1016/j.molonc.2015.10.002
83. Mancheno-Ferris A, Polesello C, Payre F. [OvoL factors: a family of key regulators of epithelium mesenchyme plasticity and stem cells]. Med Sci. (2020) 36 Hors série n° 1:61–6. doi: 10.1051/medsci/2020193
84. Budkova Z, Sigurdardottir AK, Briem E, Bergthorsson JT, Sigurdsson S, Magnusson MK, et al. Expression of ncRNAs on the DLK1-DIO3 locus is associated with basal and mesenchymal phenotype in breast epithelial progenitor cells. Front Cell Dev Biol. (2020) 8:461. doi: 10.3389/fcell.2020.00461
85. Jiang J, Zheng M, Zhang M, Yang X, Li L, Wang S-S, et al. PRRX1 regulates cellular phenotype plasticity and dormancy of head and neck squamous cell carcinoma through miR-642b-3p. Neoplasia. (2019) 21:216–29. doi: 10.1016/j.neo.2018.12.001
86. You X, Zhou Z, Chen W, Wei X, Zhou H, Luo W. MicroRNA-495 confers inhibitory effects on cancer stem cells in oral squamous cell carcinoma through the HOXC6-mediated TGF-β signaling pathway. Stem Cell Res Therap. (2020) 11:117. doi: 10.1186/s13287-020-1576-3
87. Selth LA, Das R, Townley SL, Coutinho I, Hanson AR, Centenera MM, et al. A ZEB1-miR-375-YAP1 pathway regulates epithelial plasticity in prostate cancer. Oncogene. (2017) 36:24–34. doi: 10.1038/onc.2016.185
88. Zhou W, Ye X-L, Xu J, Cao M-G, Fang Z-Y, Li L-Y, et al. The lncRNA H19 mediates breast cancer cell plasticity during EMT and MET plasticity by differentially sponging miR-200b/c and let-7b. Sci Signal. (2017) 10:eaak9557. doi: 10.1126/scisignal.aak9557
89. Cha ST, Tan CT, Chang CC, Chu CY, Lee WJ, Lin BZ, et al. G9a/RelB regulates self-renewal and function of colon-cancer-initiating cells by silencing Let-7b and activating the K-RAS/β-catenin pathway. Nat Cell Biol. (2016) 18:993–1005. doi: 10.1038/ncb3395
90. Liu S, Cong Y, Wang D, Sun Y, Deng L, Liu Y, et al. Breast cancer stem cells transition between epithelial and mesenchymal states reflective of their normal counterparts. Stem Cell Rep. (2014) 2:78–91. doi: 10.1016/j.stemcr.2013.11.009
91. Bocci F, Jolly MK, George JT, Levine H, Onuchic JN. A mechanism-based computational model to capture the interconnections among epithelial-mesenchymal transition, cancer stem cells and Notch-Jagged signaling. Oncotarget. (2018) 9:29906–20. doi: 10.18632/oncotarget.25692
92. Yang J, Antin P, Berx G, Blanpain C, Brabletz T, Bronner M, et al. Guidelines and definitions for research on epithelial-mesenchymal transition. Nat Rev Mol Cell Biol. (2020) 21:341–52. doi: 10.1038/s41580-020-0237-9
93. Jolly MK, Somarelli JA, Sheth M, Biddle A, Tripathi SC, Armstrong AJ, et al. Hybrid epithelial/mesenchymal phenotypes promote metastasis and therapy resistance across carcinomas. Pharmacol Therap. (2019) 194:161–84. doi: 10.1016/j.pharmthera.2018.09.007
Keywords: epithelial-mesenchymal plasticity, cancer stem cells, epithelial-mesenchymal transition, mesenchymal-epithelial transition, metastasis, stemness
Citation: Zheng X, Dai F, Feng L, Zou H, Feng L and Xu M (2021) Communication Between Epithelial–Mesenchymal Plasticity and Cancer Stem Cells: New Insights Into Cancer Progression. Front. Oncol. 11:617597. doi: 10.3389/fonc.2021.617597
Received: 15 October 2020; Accepted: 23 February 2021;
Published: 21 April 2021.
Edited by:
Lorenzo Gerratana, University of Udine, ItalyReviewed by:
Federico Bocci, University of California, Irvine, United StatesMinal Garg, University of Lucknow, India
Copyright © 2021 Zheng, Dai, Feng, Zou, Feng and Xu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Mingqing Xu, xumingqing0018@163.com