Breast tumor cell hybrids form spontaneously in vivo and contribute to breast tumor metastases

Mechanisms of Cell Fusion in Cancer

Cell-cell fusion is a normal physiological mechanism that requires a well-orchestrated regulation of intracellular and extracellular factors. Dysregulation of this process could lead to diseases such as osteoporosis, malformation of muscles, difficulties in pregnancy, and cancer. Extensive literature demonstrates that fusion occurs between cancer cells and other cell types to potentially promote cancer progression and metastasis. However, the mechanisms governing this process in cancer initiation, promotion, and progression are less well-studied. Fusogens involved in normal physiological processes such as syncytins and associated factors such as phosphatidylserine and annexins have been observed to be critical in cancer cell fusion as well. Some of the extracellular factors associated with cancer cell fusion include chronic inflammation and inflammatory cytokines, hypoxia, and viral infection. The interaction between these extracellular factors and cell's intrinsic factors potentially modulates actin dynamics to drive the fusion of cancer cells. In this review, we have discussed the different mechanisms that have been identified or postulated to drive cancer cell fusion.

Heterogeneity of Circulating Epithelial Cells in Breast Cancer at Single-Cell Resolution: Identifying Tumor and Hybrid Cells

Circulating tumor cells and hybrid cells formed by the fusion of tumor cells with normal cells are leading players in metastasis and have prognostic relevance. This study applies single-cell RNA sequencing to profile CD45-negative and CD45-positive circulating epithelial cells (CECs) in nonmetastatic breast cancer patients. CECs are represented by transcriptionally-distinct populations that include both aneuploid and diploid cells. CD45^- CECs are predominantly aneuploid, but one population contained more diploid than aneuploid cells. CD45⁺ CECs mostly diploid: only two populations have aneuploid cells. Diploid CD45⁺ CECs annotated as different immune cells, surprisingly harbored many copy number aberrations, and positively correlated to tumor grade. It is noteworthy that cancer-associated signaling pathways areabundant only in one aneuploid CD45^- CEC population, which may represent an aggressive subset of circulating tumor cells. Thus, CD45^- and CD45⁺ CECs are highly heterogeneous in breast cancer patients and include aneuploid cells, which are most likely circulating tumor and hybrid cells, respectively, and diploid cells. DNA ploidy analysis can be an effective instrument for identifying tumor and hybrid cells among CECs. Further follow-up study is needed to determine which subsets of circulating tumor and hybrid cells contribute to breast cancer metastasis.

Tumor Hybrid Cells: Nature and Biological Significance

Metastasis is the leading cause of cancer death and can be realized through the phenomenon of tumor cell fusion. The fusion of tumor cells with other tumor or normal cells leads to the appearance of tumor hybrid cells (THCs) exhibiting novel properties such as increased proliferation and migration, drug resistance, decreased apoptosis rate, and avoiding immune surveillance. Experimental studies showed the association of THCs with a high frequency of cancer metastasis; however, the underlying mechanisms remain unclear. Many other questions also remain to be answered: the role of genetic alterations in tumor cell fusion, the molecular landscape of cells after fusion, the lifetime and fate of different THCs, and the specific markers of THCs, and their correlation with various cancers and clinicopathological parameters. In this review, we discuss the factors and potential mechanisms involved in the occurrence of THCs, the types of THCs, and their role in cancer drug resistance and metastasis, as well as potential therapeutic approaches for the prevention, and targeting of tumor cell fusion. In conclusion, we emphasize the current knowledge gaps in the biology of THCs that should be addressed to develop highly effective therapeutics and strategies for metastasis suppression.

Cell-Cell Fusion Mediated by Viruses and HERV-Derived Fusogens in Cancer Initiation and Progression

Cell fusion is a well-known, but still scarcely understood biological phenomenon, which might play a role in cancer initiation, progression and formation of metastases. Although the merging of two (cancer) cells appears simple, the entire process is highly complex, energy-dependent and tightly regulated. Among cell fusion-inducing and -regulating factors, so-called fusogens have been identified as a specific type of proteins that are indispensable for overcoming fusion-associated energetic barriers and final merging of plasma membranes. About 8% of the human genome is of retroviral origin and some well-known fusogens, such as syncytin-1, are expressed by human (cancer) cells. Likewise, enveloped viruses can enable and facilitate cell fusion due to evolutionarily optimized fusogens, and are also capable to induce bi- and multinucleation underlining their fusion capacity. Moreover, multinucleated giant cancer cells have been found in tumors derived from oncogenic viruses. Accordingly, a potential correlation between viruses and fusogens of human endogenous retroviral origin in cancer cell fusion will be summarized in this review.

Stem cells part of the innate and adaptive immune systems as a therapeutic for Covid-19

Some stem cell types not only release molecules that reduce viral replication, but also reduce the hypercytokinemia and inflammation induced by the immune system, and have been found to be part of the innate and adaptive immune systems. An important component of the stem cell's ability to ameliorate viral diseases, especially the complications post-clearance of the pathogen, is the ability of adult stem cells to reset the innate and adaptive immune systems from an inflammatory state to a repair state. Thus, the molecules released from certain stem cell types found to be safe and efficacious, may be an important new means for therapeutic development in Covid-19, especially for late-stage inflammation and tissue damage once the virus has cleared, particularly in the aged population.

Spontaneous Fusion of MSC with Breast Cancer Cells Can Generate Tumor Dormancy

Direct cellular interactions of MDA-MB-231^cherry breast cancer cells with GFP-transduced human mesenchymal stroma/stem-like cells (MSC^GFP) in a co-culture model resulted in spontaneous cell fusion by the generation of MDA-MSC-hyb5^{cherry GFP} breast cancer hybrid cells. The proliferative capacity of MDA-MSC-hyb5 cells was enhanced about 1.8-fold when compared to the parental MDA-MB-231^cherry breast cancer cells. In contrast to a spontaneous MDA-MB-231^cherry induced tumor development in vivo within 18.8 days, the MDA-MSC-hyb5 cells initially remained quiescent in a dormancy-like state. At distinct time points after injection, NODscid mice started to develop MDA-MSC-hyb5 cell-induced tumors up to about a half year later. Following tumor initiation, however, tumor growth and formation of metastases in various different organs occurred rapidly within about 10.5 days. Changes in gene expression levels were evaluated by RNA-microarray analysis and revealed certain increase in dormancy-associated transcripts in MDA-MSC-hyb5. Chemotherapeutic responsiveness of MDA-MSC-hyb5 cells was partially enhanced when compared to MDA-MB-231 cells. However, some resistance, e.g., for taxol was detectable in cancer hybrid cells. Moreover, drug response partially changed during the tumor development of MDA-MSC-hyb5 cells; this suggests the presence of unstable in vivo phenotypes of MDA-hyb5 cells with increased tumor heterogeneity.

Impact of the Tumor Microenvironment on Tumor Heterogeneity and Consequences for Cancer Cell Plasticity and Stemness

Tumor heterogeneity is considered the major cause of treatment failure in current cancer therapies. This feature of solid tumors is not only the result of clonal outgrowth of cells with genetic mutations, but also of epigenetic alterations induced by physical and chemical signals from the tumor microenvironment (TME). Besides fibroblasts, endothelial and immune cells, mesenchymal stroma/stem-like cells (MSCs) and tumor-associated macrophages (TAMs) intimately crosstalk with cancer cells and can exhibit both anti- and pro-tumorigenic effects. MSCs can alter cancer cellular phenotypes to increase cancer cell plasticity, eventually resulting in the generation of cancer stem cells (CSCs). The shift between different phenotypic states (phenotype switching) of CSCs is controlled via both genetic programs, such as epithelial-mesenchymal transdifferentiation or retrodifferentiation, and epigenetic alterations triggered by signals from the TME, like hypoxia, spatial heterogeneity or stromal cell-derived chemokines. Finally, we highlight the role of spontaneous cancer cell fusion with various types of stromal cells. i.e., MSCs in shaping CSC plasticity. A better understanding of cell plasticity and phenotype shifting in CSCs is a prerequisite for exploiting this phenomenon to reduce tumor heterogeneity, thereby improving the chance for therapy success.

Altered Tumor Plasticity after Different Cancer Cell Fusions with MSC

While cell fusion demonstrates an important pathway during tissue development and regeneration of distinct organs, this process can also contribute to pathophysiological phenotypes during tumor progression. Hybrid cell formation after heterofusion between cancer cells and various other cell types within the tumor microenvironment is observed in vitro and in vivo. In particular, mesenchymal stroma/stem-like cells (MSC) perform diverse levels of communication with cancer cells by exhibiting anti- and pro-tumorigenic effects. During these cellular interactions, MSC can eventually fuse with cancer cells. Thereby, the newly generated disparate hybrid populations display aneuploidy associated with chromosomal instability. Based upon a subsequent post-hybrid selection process (PHSP), fused cancer cells can undergo apoptosis/necroptosis, senescence, dormancy, or a proliferative state by acquisition of new properties. Consequently, PHSP-surviving hybrid cancer cells demonstrate altered functionalities within the tumor tissue. This is accompanied by changes in therapeutic responsiveness and a different metastatic behavior. Accordingly, enhanced tumor plasticity interferes with successful therapeutic interventions and aggravates patient prognoses. The present review article focusses on fusion of MSC with different human cancer cells, in particular breast cancer populations and resulting characteristics of various cancer hybrid cells. Moreover, some mechanisms of cancer cell fusion are discussed together with multiple PHSP pathways.

Computational models of migration modes improve our understanding of metastasis

Tumor cells migrate through changing microenvironments of diseased and healthy tissue, making their migration particularly challenging to describe. To better understand this process, computational models have been developed for both the ameboid and mesenchymal modes of cell migration. Here, we review various approaches that have been used to account for the physical environment's effect on cell migration in computational models, with a focus on their application to understanding cancer metastasis and the related phenomenon of durotaxis. We then discuss how mesenchymal migration models typically simulate complex cell–extracellular matrix (ECM) interactions, while ameboid migration models use a cell-focused approach that largely ignores ECM when not acting as a physical barrier. This approach greatly simplifies or ignores the mechanosensing ability of ameboid migrating cells and should be reevaluated in future models. We conclude by describing future model elements that have not been included to date but would enhance model accuracy.

Anti-Tumor Effects of Exosomes Derived from Drug-Incubated Permanently Growing Human MSC

Similar to growth-limited human primary cultures of mesenchymal stroma/stem-like cells (MSC), the continuously proliferating human MSC544 cell line produced extracellular vesicles as characterized by expression of the tetraspanin molecules CD9, CD63, and CD81. Release of these particles was predominantly detectable during continuous cell growth of MSC544 in contrast to confluency-mediated transient growth arrest. For therapeutic use, these particles were isolated from proliferating MSC544 after taxol treatment and applied to different cancer cell cultures. A pronounced cytotoxicity of lung, ovarian, and breast cancer cells was observed primarily with taxol-loaded exosomes, similar to the effects displayed by application of taxol substance. While these findings suggested pronounced cancer cell targeting of MSC544 exosomes, a tumor therapeutic approach was performed using a mouse in vivo breast cancer model. Thus, intravenous injection of taxol-loaded MSC544 exosomes displayed superior tumor-reducing capabilities as compared to application of taxol exosomes by oral gavage. To broaden this therapeutic spectrum, epirubicin was applied to MSC544, and the derived exosomes likewise exhibited significant cytotoxic effects in different cancer cell cultures. These findings suggest an unlimited source for large-scale exosome production with reproducible quality to enable variable drug targeting of tumors or other diseases.

Role of MSC in the Tumor Microenvironment

The tumor microenvironment represents a dynamically composed matrix in which tissue-associated cancer cells are embedded together with a variety of further cell types to form a more or less separate organ-like structure. Constantly mutual interactions between cells of the tumor microenvironment promote continuous restructuring and growth in the tumor. A distinct organization of the tumor stroma also facilitates the formation of transient cancer stem cell niches, thereby contributing to progressive and dynamic tumor development. An important but heterogeneous mixture of cells that communicates among the cancer cells and the different tumor-associated cell types is represented by mesenchymal stroma-/stem-like cells (MSC). Following recruitment to tumor sites, MSC can change their functionalities, adapt to the tumor's metabolism, undergo differentiation and synergize with cancer cells. Vice versa, cancer cells can alter therapeutic sensitivities and change metastatic behavior depending on the type and intensity of this MSC crosstalk. Thus, close cellular interactions between MSC and cancer cells can eventually promote cell fusion by forming new cancer hybrid cells. Consequently, newly acquired cancer cell functions or new hybrid cancer populations enlarge the plasticity of the tumor and counteract successful interventional strategies. The present review article highlights some important features of MSC within the tumor stroma.

Potential Role of MSC/Cancer Cell Fusion and EMT for Breast Cancer Stem Cell Formation

Solid tumors comprise of maturated cancer cells and self-renewing cancer stem-like cells (CSCs), which are associated with various other nontumorigenic cell populations in the tumor microenvironment. In addition to immune cells, endothelial cells, fibroblasts, and further cell types, mesenchymal stroma/stem-like cells (MSC) represent an important cell population recruited to tumor sites and predominantly interacting with the different cancer cells. Breast cancer models were among the first to reveal distinct properties of CSCs, however, the cellular process(es) through which these cells are generated, maintained, and expanded within neoplastic tissues remains incompletely understood. Here, we discuss several possible scenarios that are not mutually exclusive but may even act synergistically: fusion of cancer cells with MSC to yield hybrid cells and/or the induction of epithelial-mesenchymal transition (EMT) in breast cancer cells by MSC, which can relay signals for retrodifferentiation and eventually, the generation of breast CSCs (BCSCs). In either case, the consequences may be promotion of self-renewal capacity, tumor cell plasticity and heterogeneity, an increase in the cancer cells’ invasive and metastatic potential, and the acquisition of resistance mechanisms towards chemo- or radiotherapy. While specific signaling mechanisms involved in each of these properties remain to be elucidated, the present review article focusses on a potential involvement of cancer cell fusion and EMT in the development of breast cancer stem cells.

Conserved pathway activation following xenogeneic, heterotypic fusion

Fusion between cells of different organisms (i.e., xenogeneic hybrids) can occur, and for humans this may occur in the course of tissue transplantation, animal handling, and food production. Previous work shows that conferred advantages are rare in xenogeneic hybrids, whereas risks of cellular dysregulation are high. Here, we explore the transcriptome of individual xenogeneic hybrids of human mesenchymal stem cells and murine cardiomyocytes soon after fusion and ask whether the process is stochastic or involves conserved pathway activation. Toward this end, single-cell RNA sequencing was used to analyze the transcriptomes of hybrid cells with respect to the human and mouse genomes. Consistent with previous work, hybrids possessed a unique transcriptome distinct from either fusion partner but were dominated by the cardiomyocyte transcriptome. New in this work is the documentation that a few genes that were latent in both fusion partners were consistently expressed in hybrids. Specifically, human growth hormone 1, murine ribosomal protein S27, and murine ATP synthase H⁺ transporting, mitochondrial Fo complex subunit C2 were expressed in nearly all hybrids. The consistent activation of latent genes between hybrids suggests conserved signaling mechanisms that either cause or are the consequence of fusion of these 2 cell types and might serve as a target for limiting unwanted xenogeneic fusion in the future.-Yuan, C., Freeman, B. T., McArdle, T. J., Jung, J. P., Ogle, B. M. Conserved pathway activation following xenogeneic, heterotypic fusion.

Involvement of Actin Cytoskeletal Components in Breast Cancer Cell Fusion with Human Mesenchymal Stroma/Stem-Like Cells

Cell fusion as a rare event was observed following the co-culture of human MDA-MB-231cherry breast cancer cells or benign neoplastic MCF10Acherry breast epithelial cells together with different mesenchymal stroma/stem-like cells (MSCGFP) cultures, respectively, resulting in the generation of double-fluorescing hybrid cells. Analysis of potential molecular mechanisms for the formation of cancer hybrid cells revealed cytoskeletal components, including F-actin. Thus, a sub-lethal concentration of cytochalasin D, which blocks elongation of actin filaments, was able to significantly reduce cancer hybrid cell formation. Simultaneously, cell cycle progression of the different co-cultures remained unaffected following treatment with cytochalasin D, indicating continued proliferation. Moreover, exposure to 50 nM cytochalasin D revealed little if any effect on the expression of various integrins and cell adhesion molecules in the different co-cultures. However, LC-MS proteome analysis of the different control co-cultures compared to corresponding cytochalasin-treated co-cultures demonstrated predominant differences in the expression of actin-associated cytoskeletal proteins. In addition, the requirement of structured actin to provide an appropriate cytoskeletal network for enabling subsequent fusion processes was also substantiated by the actin filament disrupting latrunculin B, which inhibits the fusion process between the breast cancer populations and mesenchymal stroma/stem-like cells (MSC). Together, these findings suggest an important role of distinct actin structures and associated cytoskeletal components during cell fusion and the formation of breast cancer hybrid cells.

In Vivo Cell Fusion between Mesenchymal Stroma/Stem-Like Cells and Breast Cancer Cells

Cellular communication within the tumor microenvironment enables important interactions between cancer cells and recruited adjacent populations including mesenchymal stroma/stem-like cells (MSC). These interactions were monitored in vivo following co-injection of GFP-labeled human MSC together with mcherry-labeled MDA-MB-231 breast cancer cells in NODscid mice. Within 14 days of tumor development the number of initially co-injected MSC had significantly declined and spontaneous formation of breast cancer/MSC hybrid cells was observed by the appearance of double fluorescing cells. This in vivo fusion displayed a rare event and occurred in less than 0.5% of the tumor cell population. Similar findings were observed in a parallel in vitro co-culture. Characterization of the new cell fusion products obtained after two consecutive flow cytometry cell sorting and single cell cloning revealed two populations, termed MDA-hyb3 and MDA-hyb4. The breast cancer fusion cells expressed both, GFP and mcherry and displayed more characteristics of the MDA-MB-231 cells than of the parental MSC. While little if any differences were determined in the proliferative capacity, a significant delay of MDA-hyb3 cells in tumor formation was observed when compared to the parental MDA-MB-231 cells. Moreover, MDA-hyb3 cells developed an altered pattern of distant organ metastases. These findings demonstrated dynamic tumor changes by in vivo and in vitro fusion with the development of new breast cancer hybrid cells carrying altered tumorigenic properties. Consequently, cancer cell fusion contributes to progressively increasing tumor heterogeneity which complicates a therapeutic regimen.

Rationally engineered advances in cancer research

The physical and engineering sciences have much to offer in understanding, diagnosing, and even treating cancer. Microfluidics, imaging, materials, and diverse measurement devices are all helping to shift paradigms of tumorigenesis and dissemination. Using materials and micro-probes of elasticity, for example, epithelia have been shown to transform into mesenchymal cells when the elasticity of adjacent tissue increases. Approaches common in engineering science enable such discoveries, and further application of such tools and principles will likely improve existing cancer models in vivo and also create better models for high throughput analyses in vitro. As profiled in this special topic issue composed of more than a dozen manuscripts, opportunities abound for the creativity and analytics of engineering and the physical sciences to make advances in and against cancer.