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ZEB1 plays a pivotal role in epithelial-to-mesenchymal transition (EMT), (cancer) cell stemness and cancer therapy resistance. The M13HS tumor hybrids, which were derived from spontaneous fusion events between the M13SV1-EGFP-Neo breast epithelial cells and HS578T-Hyg breast cancer cells, express ZEB1 and exhibit prospective cancer stem cell properties. To explore a possible correlation between the ZEB1 and stemness/ EMT-related properties in M13HS tumor hybrids, ZEB1 was knocked-out by CRISPR/Cas9. Colony formation, mammosphere formation, cell migration, invasion assays, flow cytometry and Western blot analyses were performed for the characterization of ZEB1 knock-out cells. The ZEB1 knock-out in M13HS tumor cells was not correlated with the down-regulation of the EMT-related markers N-CADHERIN (CDH2) and VIMENTIN and up-regulation of miR-200c-3p. Nonetheless, both the colony formation and mammosphere formation capacities of the M13HS ZEB1 knock-out cells were markedly reduced. Interestingly, the M13HS-2 ZEB1-KO cells harbored a markedly higher fraction of ALDH1-positive cells. The Transwell/ Boyden chamber migration assay data indicated a reduced migratory activity of the M13HS ZEB1-knock-out tumor hybrids, whereas in scratch/ wound-healing assays only the M13SH-8 ZEB1-knock-out cells possessed a reduced locomotory activity. Similarly, only the M13HS-8 ZEB1-knock-out tumor hybrids showed a reduced invasion capacity. Although the ZEB1 knock-out resulted in only moderate phenotypic changes, our data support the role of ZEB1 in EMT and stemness. [ABSTRACT FROM AUTHOR]

CS/ICs have raised great expectations in cancer research and therapy, as eradication of this key cancer cell type is expected to lead to a complete cure. Unfortunately, the biology of CS/ICs is rather complex, since no common CS/IC marker has yet been identified. Certain surface markers or ALDH1 expression can be used for detection, but some studies indicated that cancer cells exhibit a certain plasticity, so CS/ICs can also arise from non-CS/ICs. Another problem is intratumoral heterogeneity, from which it can be inferred that different CS/IC subclones must be present in the tumor. Cell–cell fusion between cancer cells and normal cells, such as macrophages and stem cells, has been associated with the generation of tumor hybrids that can exhibit novel properties, such as an enhanced metastatic capacity and even CS/IC properties. Moreover, cell–cell fusion is a complex process in which parental chromosomes are mixed and randomly distributed among daughter cells, resulting in multiple, unique tumor hybrids. These, if they have CS/IC properties, may contribute to the heterogeneity of the CS/IC pool. In this review, we will discuss whether cell–cell fusion could also lead to the origin of different CS/ICs that may expand the overall CS/IC pool in a primary tumor. [ABSTRACT FROM AUTHOR]

Background: The biological phenomenon of cell fusion has been associated with cancer progression since it was determined that normal cell × tumor cell fusion-derived hybrid cells could exhibit novel properties, such as enhanced metastatogenic capacity or increased drug resistance, and even as a mechanism that could give rise to cancer stem/initiating cells (CS/ICs). CS/ICs have been proposed as cancer cells that exhibit stem cell properties, including the ability to (re)initiate tumor growth.Methods: Five M13HS hybrid clone cells, which originated from spontaneous cell fusion events between M13SV1-EGFP-Neo human breast epithelial cells and HS578T-Hyg human breast cancer cells, and their parental cells were analyzed for expression of stemness and EMT-related marker proteins by Western blot analysis and confocal laser scanning microscopy. The frequency of ALDH1-positive cells was determined by flow cytometry using AldeRed fluorescent dye. Concurrently, the cells' colony forming capabilities as well as the cells' abilities to form mammospheres were investigated. The migratory activity of the cells was analyzed using a 3D collagen matrix migration assay.Results: M13HS hybrid clone cells co-expressed SOX9, SLUG, CK8 and CK14, which were differently expressed in parental cells. A variation in the ALDH1-positive putative stem cell population was observed among the five hybrids ranging from 1.44% (M13HS-7) to 13.68% (M13HS-2). In comparison to the parental cells, all five hybrid clone cells possessed increased but also unique colony formation and mammosphere formation capabilities. M13HS-4 hybrid clone cells exhibited the highest colony formation capacity and second highest mammosphere formation capacity of all hybrids, whereby the mean diameter of the mammospheres was comparable to the parental cells. In contrast, the largest mammospheres originated from the M13HS-2 hybrid clone cells, whereas these cells' mammosphere formation capacity was comparable to the parental breast cancer cells. All M13HS hybrid clones exhibited a mesenchymal phenotype and, with the exception of one hybrid clone, responded to EGF with an increased migratory activity.Conclusion: Fusion of human breast epithelial cells and human breast cancer cells can give rise to hybrid clone cells that possess certain CS/IC properties, suggesting that cell fusion might be a mechanism underlying how tumor cells exhibiting a CS/IC phenotype could originate. [ABSTRACT FROM AUTHOR]

Cancer is one of the most common diseases worldwide, and treatment bears many challenges such as drug and radioresistance and formation of metastases. These difficulties are due to tumor heterogeneity, which has many origins. One may be cell fusion, a process that is relevant in both physiological (e.g., wound healing) and pathophysiological (cancer and viral infection) processes. In this study, we examined if cell fusion between mesenchymal stem/stromal cells (MSCs) and breast cancer (BC) cells occurs and if newly generated hybrid cells may exhibit cancer stem/initiating cell (CS/IC) characteristics. Therefore, several methods such as mammosphere assay, AldeRed assay, flow cytometry (CD24, CD44, CD104) and Western blot analysis (of epithelial to mesenchymal transition markers such as SNAIL, SLUG and Twist) were applied. In short, four different hybrid clones, verified by short tandem repeat (STR) analysis, were analyzed; each expressed an individual phenotype that seemed not to be explicitly related to either a more stem cell or cancer cell phenotype. These results show that cancer cells and MSCs are able to fuse spontaneously in vitro, thereby giving rise to hybrid cells with new properties, which likely indicate that cell fusion may be a trigger for tumor heterogeneity. [ABSTRACT FROM AUTHOR]

Cell fusion is a cell-biological phenomenon, which was originally described as an event of development and homeostasis. Moreover, recent discoveries give rise to the theory that fusion events between stem cells and tumor cells could act as malignant and crucial originators in cancer stem cell (CSC) generation, besides other theories such as CSCs deriving from tissue stem cells that have accumulated genetic aberrations or CSCs deriving from differentiated progenitor cells that have regained self- renewing capacity. By now CSCs are known as cancer-initiating cells with characteristic features of stem cells, such as self-renewing, differentiation, regeneration of tissues at low cell numbers and drug resistance. This rare population of cancer-initiating cells attracts attention as a target in cancer therapies and, therefore, there is an urgent need for further investigation on CSCs is present. After 24 h co-cultivation of M13SV1 enhanced green fluorescent protein (EGFP)-neo breast stem cells and invasive HS578T-Hyg breast cancer cells under selective conditions, cell clones that emerged from spontaneous fusion events were isolated and cultivated separately. Short tandem repeat analysis of hybrid cells showed an overlap of the parental alleles, indicating that hybrid cells have been originated from real cell fusion events. Despite some varieties concerning the data of the further hybrid cell characterization due to different genetic profile among them, one can detect clear tendencies of similarity between all hybrid cell lines compared with their parental cell lines. XTT-proliferation assays, for example, show a significantly higher proliferation rate of the hybrid cell lines compared with their parental cell lines. Dependent on the hybrid cell line a 5-10-times higher proliferation rate could be detected in comparison with the breast stem cell line M13SV1 EGFP-neo, whereas, in contrast to the breast cancer cell line HS578T Hyg, the proliferation shows an up to four-times higher rate. Moreover, expression pattern analysis by real-time PCR data revealed interesting results in multiple up- and down-regulation of cancer and drug metabolism genes compared with the HS578T cell line and the M13SV1 cell line, for example, upregulation of cell cycle regulators as p16 and p53 in all hybrids, drug resistance transporters as ABCC6, ABCB1 and major vault protein (MVP) in all hybrid cell lines as well as upregulation of androgen receptor in three of four hybrid cell lines, which stands in a reciprocal relationship to downregulation of estrogen receptor in some poor prognosis breast cancers, probably indicating that fusion might cause a switch to an estrogen-independent tumor growth. The real-time PCR data have been confirmed by Western blot analysis. On the basis of spontaneous fusion between M13SV1 breast stem cells and invasive HS578T breast cancer cells we could give a promising insight into characterization of fusion cells as a possible model for CSCs. [ABSTRACT FROM AUTHOR]