Mechanism of the mesenchymal-epithelial transition and its relationship with metastatic tumor formation

Cancer metastasis consists of a sequential series of events, and the epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) are recognized as critical events for metastasis of carcinomas. A current area of focus is the histopathological similarity between primary and metastatic tumors, and MET at sites of metastases has been postulated to be part of the process of metastatic tumor formation. Here, we summarize accumulating evidence from experimental studies that directly supports the role of MET in cancer metastasis, and we analyze the main mechanisms that regulate MET or reverse EMT in carcinomas. Given the critical role of MET in metastatic tumor formation, the potential to effectively target the MET process at sites of metastasis offers new hope for inhibiting metastatic tumor formation.     

Autophagy and EMT in cancer and metastasis: Who controls whom?

Metastasis consists of hallmark events, including Epithelial-Mesenchymal Transition (EMT), angiogenesis, initiation of inflammatory tumor microenvironment, and malfunctions in apoptosis. Autophagy is known to play a pivotal role in the metastatic process. Autophagy has pulled researchers towards it in recent times because of its dual role in the maintenance of cancer cells. Evidence states that cells undergoing EMT need autophagy in order to survive during migration and dissemination. Additionally, it orchestrates EMT markers in certain cancers. On the other side of the coin, autophagy plays an oncosuppressive role in impeding early metastasis. This review aims to project the interrelationship between autophagy and EMT. Targeting EMT via autophagy as a useful strategy is discussed in this review. Furthermore, for the first time, we have covered the possible reciprocating roles of EMT and autophagy and its consequences in cancer metastasis.     

Epithelial-Mesenchymal Transition in Cancer: A Historical Overview

Epithelial-mesenchymal transitions (EMTs), the acquisition of mesenchymal features from epithelial cells, occur during some biological processes and are classified into three types: the first type occurs during embryonic development, the second type is associated with adult tissue regeneration, and the third type occurs in cancer progression. EMT occurring during embryonic development in gastrulation, renal development, and the origin and fate of the neural crest is a highly regulated process, while EMT occurring during tumor progression is highly deregulated. EMT allows the solid tumors to become more malignant, increasing their invasiveness and metastatic activity. Secondary tumors frequently maintain the typical histologic characteristics of the primary tumor. These histologic features connecting the secondary metastatic tumors to the primary is due to a process called mesenchymal-epithelial transition (MET). MET has been demonstrated in different mesenchymal tumors and is the expression of the reversibility of EMT. EMT modulation could constitute an approach to avoid metastasis. Some of the targeted small molecules utilized as antiproliferative agents have revealed to inhibit EMT initiation or maintenance because EMT is regulated through signaling pathways for which these molecules have been designed.     

High-content siRNA screen of the kinome identifies kinases involved in Git2-induced mesenchymal-epithelial transition

Epithelial-mesenchymal transition (EMT) and its reverse process mesenchymal-epithelial transition (MET) programs are involced in the metastatic process. More and more evidence confirms that EMT is vital for the initiation and dissemination of cancer cells whereas MET is critical for successful metastatic colonization of a secondary organ. The regulating mechanism of EMT mediated cancer progression and metastasis has been deeply investigated. However, what processes are dependent on MET in metastatic cascades remains unclear. Here, we created a cell based high-content siRNA screen using the breast cancer cell line 4TO7 to search for kinases that were involved in Git2-induced MET. Our results revealed that 58 kinases including transferase, phosphorylation regulators, ATP/nucleotide partners potentially participate in Git2-induced MET. Our preliminary data is expected to facilitate elucidation of the mechanism on how MET is initiated during cancer metastasis.     

间质-上皮转化在肿瘤转移中的作用

肿瘤转移是一个多步骤、多因素参与的复杂过程,是目前临床上绝大多数肿瘤患者的致死因素.上皮-间质转化(epithelial-mesenchymal transition, EMT)过程已被证实可促使肿瘤细胞发生转移.近年来许多研究表明,间质-上皮转化(mesenchymal-epithelial transition, MET)即EMT的逆过程,与肿瘤也密切相关,特别是肿瘤转移即形成继发性的肿瘤转移灶.深入研究肿瘤MET有望为肿瘤转移的预防和诊治提供新思路.     

HOX cluster-embedded lncRNAs and epithelial-mesenchymal transition in cancer: Molecular mechanisms and therapeutic opportunities

Although there has been substantial improvement in the treatment modalities, cancer remains the major cause of fatality worldwide. Metastasis, recurrence, and resistance to oncological therapies are the leading causes of cancer mortality. Epithelial-mesenchymal transition (EMT) is a complex biological process that allows cancer cells to undergo morphological transformation into a mesenchymal phenotype to acquire invasive potential. It encompasses reversible and dynamic ontogenesis by neoplastic cells during metastatic dissemination. Hence, understanding the molecular landscape of EMT is imperative to identify a reliable clinical biomarker to combat metastatic spread. Accumulating evidence reveals the role of HOX (homeobox) cluster-embedded long non-coding RNAs (lncRNAs) in EMT and cancer metastasis. They play a crucial role in the induction of EMT, modulating diverse biological targets. The present review emphasizes the involvement of HOX cluster-embedded lncRNAs in EMT as a molecular sponge, chromatin remodeler, signaling regulator, and immune system modulator. Furthermore, the molecular mechanisms behind therapy resistance and the potential use of novel drugs targeting HOX cluster-embedded lncRNAs in the clinical management of distant metastasis will be discussed.     

The tumor microenvironment: An irreplaceable element of tumor budding and epithelial-mesenchymal transition-mediated cancer metastasis

Tumor budding occurs at the invasive front of cancer; the tumor cells involved have metastatic and stemness features, indicating a poor prognosis. Tumor budding is partly responsible for cancer metastasis, and its initiation is based on the epithelial-mesenchymal transition (EMT) process. The EMT process involves the conversion of epithelial cells into migratory and invasive cells, and is a profound event in tumorigenesis. The EMT, associated with the formation of cancer stem cells (CSCs) and resistance to therapy, results from a combination of gene mutation, epigenetic regulation, and microenvironmental control. Tumor budding can be taken to represent the EMT in vivo. The EMT process is under the influence of the tumor microenvironment as well as tumor cells themselves. Here, we demonstrate that the tumor microenvironment dominates EMT development and impacts cancer metastasis, as well as promotes CSC formation and mediates drug resistance. In this review, we mainly discuss components of the microenvironment, such as the extracellular matrix (ECM), inflammatory cytokines, metabolic products, and hypoxia, that are involved in and impact on the acquisition of tumor-cell motility and dissemination, the EMT, metastatic tumor-cell formation, tumor budding and CSCs, and cancer metastasis, including subsequent chemo-resistance. From our point of view, the tumor microenvironment now constitutes a promising target for cancer therapy.     

MicroRNAs: critical regulators of epithelial to mesenchymal (EMT) and mesenchymal to epithelial transition (MET) in cancer progression

MicroRNAs (miRNAs) are a class of small highly conserved RNAs that provide widespread expressional control through the translational repression of mRNA. MiRNAs have fundamental roles in the regulation of intracellular processes, and their importance during malignant transformation and metastasis is becoming increasingly well recognized. An important event in the metastatic cascade is epithelial to mesenchymal transition (EMT), a reversible phenotypic switch over, which endows malignant epithelial cells with the capacity to break free from one another and invade the surrounding stroma. Our understanding of EMT has been significantly improved by the characterization of miRNAs that influence the signalling pathways and downstream events that define EMT on a molecular level. Here, we detail the role of miRNAs in EMT, and in doing so demonstrate their importance in the early stages of the metastatic cascade; we discuss a significant body of data that suggest new opportunities for drug development, and we highlight critical knowledge gaps that remain to be addressed.     

Establishment of a Chinese bladder cancer cell line (T921) with high metastatic activity

Muscle-invasive bladder cancer is prone to metastasis without a standard organ preference. The current cell lines used to study bladder cancer have primarily been derived from individuals in Western populations, and no human bladder cancer cell line has been established from the Chinese population. A bladder cancer cell line was derived from a female Chinese patient with muscle-invasive bladder cancer, and these cells were then xenografted into the bladders of three nude mice. Five weeks later, these mice were killed to observe local invasion and distant metastasis. The metastatic tumors were also removed and analyzed to assess the metastatic mechanism. This bladder cancer cell line, named T921, was successfully established, as evidenced by karyotype and immunohistochemistry analyses. Multi-organ metastases were observed in all three of the nude mice 5 wk after the orthotopic transfer of the cell line. In addition, epithelial–mesenchymal transition (EMT)-related genes were involved in the tumor metastases. The T921 bladder cancer cell line was successfully established, and EMT was observed to play a role in bladder cancer metastasis.     

Ubiquitin C-terminal hydrolase-L3 regulates EMT process and cancer metastasis in prostate cell lines

Ubiquitin C-terminal hydrolase-L3 (UCH-L3) is among the deubiquitinating enzymes (DUBs) that cleave ubiquitin (Ub) from Ub precursors or protein substrates. Many DUBs have been shown to participate in cancer progression in various tissues. However, the mechanism and role of UCH-L3 in carcinogenesis has largely been unknown until recently. Here we investigated the implication of UCH-L3 in prostate cancer progression. Interestingly, UCH-L3 is upregulated in normal or non-metastatic prostate cancer cells and is downregulated in metastatic prostate cancer cell lines. Notably, knockdown of UCH-L3 in normal prostate cell line RWPE1 promotes epithelial-to-mesenchymal transition (EMT), an important process for cancer cell invasion and metastasis. The induction of EMT by UCH-L3 knockdown results in an increase of cell migration and invasion. Yet, to the contrary, overexpression of UCH-L3 in highly metastatic prostate cancer cell line PC3 reverses EMT but the active site mutant UCH-L3 did not. Collectively, our findings identify UCH-L3 as a novel EMT regulator in prostate cells and highlight UCH-L3 as a potential therapeutic target for preventing metastatic prostate cancer.     

In vitro and ex vivo vanadium antitumor activity in (TGF-β)-induced EMT. Synergistic activity with carboplatin and correlation with tumor metastasis in cancer patients

Epithelial to mesenchymal transition (EMT) plays a key role in tumor progression and metastasis as a crucial event for cancer cells to trigger the metastatic niche. Transforming growth factor-β (TGF-β) has been shown to play an important role as an EMT inducer in various stages of carcinogenesis. Previous reports had shown that antitumor vanadium inhibits the metastatic potential of tumor cells by reducing MMP-2 expression and inducing ROS-dependent apoptosis. However, the role of vanadium in (TGF-β)-induced EMT remains unclear. In the present study, we report for the first time on the inhibitory effects of vanadium on (TGF-β)-mediated EMT followed by down-regulation of ex vivo cancer stem cell markers. The results demonstrate blockage of (TGF-β)-mediated EMT by vanadium and reduction in the mitochondrial potential of tumor cells linked to EMT and cancer metabolism. Furthermore, combination of vanadium and carboplatin (a) resulted in synergistic antitumor activity in ex vivo cell cultures, and (b) prompted G₀/G₁ cell cycle arrest and sensitization of tumor cells to carboplatin-induced apoptosis. Overall, the findings highlight the multifaceted antitumor action of vanadium and its synergistic antitumor efficacy with current chemotherapy drugs, knowledge that could be valuable for targeting cancer cell metabolism and cancer stem cell-mediated metastasis in aggressive chemoresistant tumors.     

MicroRNA-197 induces epithelial–mesenchymal transition and invasion through the downregulation of HIPK2 in lung adenocarcinoma

The major cause of cancer-related deaths in patients with lung adenocarcinoma (LAD) is due to distant metastasis. Many reports have indicated that miRNA plays a key role in tumour metastasis. The expression of miR-197 is correlated with LAD progression, however, the mechanism of miR-197 is still unknown in the processing of LAD. A Boyden chamber migration/invasion assay was used for the metastatic function study in vitro. Real-time PCR and Western blot assays were employed to analyse the EMT hallmark changes in both the mRNA and protein levels. \(3^{\prime }\)-UTR reporter luciferase assay was used to show that HIPK2 is a direct target of miR-197. miR-197 enhances LAD cell migration and invasion miR-197. The downregulation of miR-197 suppresses the EMT and migration ability. HIPK2 is a direct functional target of miR-197 in LAD metastasis. In summary, miR-197 controls EMT and metastasis by directly silencing HIPK2. The findings suggest that interfering with the miR-197-dependent regulation of HIPK2 could be a useful approach for the treatment of patients with late stage metastatic LAD.     

Elevation of YAP promotes the epithelial-mesenchymal transition and tumor aggressiveness in colorectal cancer

Tumor metastasis is the leading cause of death in cancer patients. Identifying metastatic biomarkers in tumor cells would help cancer diagnoses and the development of therapeutic targets. Yes-associated protein (YAP) plays an important role in organ development and has gained much attention in tumorigenesis. However, the role of YAP and the underlying mechanism in tumor metastasis of colorectal cancer (CRC) is still unclear. In this study, we generated metastatic 116-LM cells from the HCT116 CRC cell line. We found that the capacity for tumor aggressiveness was elevated in 116-LM cells and identified that YAP and its mRNA level were upregulated in 116-LM cells. Moreover, expression of YAP was found to correlate with epithelial-mesenchymal transition (EMT) marker expressions, whereas suppression of YAP decreased EMT marker expressions and impeded tumor migration and invasion. Additionally, upregulation of YAP was identified in colon cancer patients, and it was correlated with EMT gene expressions. Furthermore, we identified LBH589, a histone deacetylase inhibitor, that was capable of inhibiting tumor growth and aggressiveness in both HCT116 and 116-LM cells. LBH589 potentially inhibited YAP and its mRNA expression, accompanied by diminished expressions of YAP downstream genes and EMT markers. Together, YAP plays a crucial role in aggressiveness and metastasis of CRC, and YAP may be an attractive therapeutic target.     

Role of Bmi-1 in Regulation of Ionizing Irradiation-Induced Epithelial-Mesenchymal Transition and Migration of Breast Cancer Cells

Radiotherapy is a widely used treatment for cancer. However, recent studies suggest that ionizing radiation (IR) can promote tumor invasion and metastasis. Bmi-1, a member of the polycomb group protein family, has been observed as a regulator of oxidative stress and promotes metastasis in some tumors. But, its potential role in the metastasis induced by IR of breast cancer has not been explored. In our study, we found that increased levels of Bmi-1 were correlated to EMT of breast cancer cells. Through analyzing the EMT state and metastasis of breast cancer induced by IR, we found the metastatic potential of breast cancer cells can either be inhibited or accelerated by IR following a time-dependent pattern. Silencing Bmi-1 completely abolished the ability of the IR to alter, reduce or increase, the migration of breast cancer cells. Also, when Bmi-1 was knocked down, the effect of inhibition of PI3K/AKT signaling on EMT affected by IR was blocked. These results suggest that Bmi-1 is a key gene in regulation of EMT and migration of breast cancer cells induced by IR through activation of PI3K/AKT signaling; therefore, Bmi-1 could be a new target for inhibiting metastasis caused by IR.     

Significance of PELP1 in ER-negative breast cancer metastasis

Breast cancer metastasis is a major clinical problem. The molecular basis of breast cancer progression to metastasis remains poorly understood. PELP1 is an estrogen receptor (ER) coregulator that has been implicated as a proto-oncogene whose expression is deregulated in metastatic breast tumors and whose expression is retained in ER-negative tumors. We examined the mechanism and significance of PELP1-mediated signaling in ER-negative breast cancer progression using two ER-negative model cells (MDA-MB-231 and 4T1 cells) that stably express PELP1-shRNA. These model cells had reduced PELP1 expression (75% of endogenous levels) and exhibited less propensity to proliferate in growth assays in vitro. PELP1 downregulation substantially affected migration of ER-negative cells in Boyden chamber and invasion assays. Using mechanistic studies, we found that PELP1 modulated expression of several genes involved in the epithelial mesenchymal transition (EMT), including MMPs, SNAIL, TWIST, and ZEB. In addition, PELP1 knockdown reduced the in vivo metastatic potential of ER-negative breast cancer cells and significantly reduced lung metastatic nodules in a xenograft assay. These results implicate PELP1 as having a role in ER-negative breast cancer metastasis, reveal novel mechanism of coregulator regulation of metastasis via promoting cell motility/EMT by modulating expression of genes, and suggest PELP1 may be a potential therapeutic target for metastatic ER-negative breast cancer.     

miR-195 Inhibits EMT by Targeting FGF2 in Prostate Cancer Cells

Prostate cancer (PCa) is one of the leading causes of deaths in America. The major cause of mortality can be attributed to metastasis. Cancer metastasis involves sequential and interrelated events. miRNAs and epithelial-mesenchymal transition (EMT) are implicated in this process. miR-195 is downregulated in many human cancers. However, the roles of miR-195 in PCa metastasis and EMT remain unclear. In this study, data from Memorial Sloan Kettering Cancer Center (MSKCC) prostate cancer database were re-analysed to detect miR-195 expression and its roles in PCa. miR-195 was then overexpressed in castration-resistant PCa cell lines, DU-145 and PC-3. The role of miR-195 in migration and invasion in vitro was also investigated, and common markers in EMT were evaluated through Western blot analysis. A luciferase reporter assay was conducted to confirm the target gene of miR-195; were validated in PCa cells. In MSKCC data re-analyses, miR-195 was poorly expressed in metastatic PCa; miR-195 could be used to diagnose metastatic PCa by measuring the corresponding expression. Area under the receiver operating characteristic curve (AUC-ROC) was 0.705 (P = 0.017). Low miR-195 expression was characterised with a shorter relapse-free survival (RFS) time. miR-195 overexpression suppressed cell migration, invasion and EMT. Fibroblast growth factor 2 (FGF2) was confirmed as a direct target of miR-195. FGF2 knockdown also suppressed migration, invasion and EMT; by contrast, increased FGF2 partially reversed the suppressive effect of miR-195. And data from ONCOMINE prostate cancer database showed that PCa patients with high FGF2 expression showed shorter RFS time (P = 0.046). Overall, this study demonstrated that miR-195 suppressed PCa cell metastasis by downregulating FGF2. miR-195 restoration may be considered as a new therapeutic method to treat metastatic PCa.     

Cancer metastasis facilitated by developmental pathways: Sonic hedgehog, Notch, and bone morphogenic proteins

This review will highlight the significance of three critical pathways in developmental biology and our emerging understanding of their roles in regulating tumor metastasis: Bone morphogenic protein (BMP), Notch and Sonic hedgehog (SHH). We will discuss parallels between their known roles in development and how these processes can be used by tumor cells to create microenvironments that enhance tumor metastasis. That tumor cells usurp pathways critical to the developing embryo is not surprising, as many of the normal developmental programs include processes that are also seen during tumor progression to a metastatic phenotype, including epithelial to mesenchymal transition (EMT), tissue specific morphogenesis, cellular motility and invasion. BMPs are involved in EMT, contribute to tissue specific morphogenesis, and are expressed in highly-metastatic tumor cells. BMPs have also been hypothesized to have a role in the establishment of a pre-neoplastic niche. Notch and SHH facilitate neovascularization, angiogenesis, EMT and can contribute to the maintenance of highly-metastatic tumor stem cells.     

Reprogramming during epithelial to mesenchymal transition under the control of TGFβ

Epithelial-mesenchymal transition (EMT) refers to plastic changes in epithelial tissue architecture. Breast cancer stromal cells provide secreted molecules, such as transforming growth factor β (TGFβ), that promote EMT on tumor cells to facilitate breast cancer cell invasion, stemness and metastasis. TGFβ signaling is considered to be abnormal in the context of cancer development; however, TGFβ acting on breast cancer EMT resembles physiological signaling during embryonic development, when EMT generates or patterns new tissues. Interestingly, while EMT promotes metastatic fate, successful metastatic colonization seems to require the inverse process of mesenchymal-epithelial transition (MET). EMT and MET are interconnected in a time-dependent and tissue context-dependent manner and are coordinated by TGFβ, other extracellular proteins, intracellular signaling cascades, non-coding RNAs and chromatin-based molecular alterations. Research on breast cancer EMT/MET aims at delivering biomolecules that can be used diagnostically in cancer pathology and possibly provide ideas for how to improve breast cancer therapy.     

Downregulation of CFTR promotes epithelial-to-mesenchymal transition and is associated with poor prognosis of breast cancer

The epithelial-to-mesenchymal transition (EMT), a process involving the breakdown of cell–cell junctions and loss of epithelial polarity, is closely related to cancer development and metastatic progression. While the cystic fibrosis transmembrane conductance regulator (CFTR), a Cl^? and HCO₃^? conducting anion channel expressed in a wide variety of epithelial cells, has been implicated in the regulation of epithelial polarity, the exact role of CFTR in the pathogenesis of cancer and its possible involvement in EMT process have not been elucidated. Here we report that interfering with CFTR function either by its specific inhibitor or lentiviral miRNA-mediated knockdown mimics TGF-β₁-induced EMT and enhances cell migration and invasion in MCF-7. Ectopic overexpression of CFTR in a highly metastatic MDA-231 breast cancer cell line downregulates EMT markers and suppresses cell invasion and migration in vitro, as well as metastasis in vivo. The EMT-suppressing effect of CFTR is found to be associated with its ability to inhibit NFκB targeting urokinase-type plasminogen activator (uPA), known to be involved in the regulation of EMT. More importantly, CFTR expression is found significantly downregulated in primary human breast cancer samples, and is closely associated with poor prognosis in different cohorts of breast cancer patients. Taken together, the present study has demonstrated a previously undefined role of CFTR as an EMT suppressor and its potential as a prognostic indicator in breast cancer.