Inflammation and proliferation act together to mediate intestinal cell fusion

Fusion of tumor cells and mesenchymal stem/stroma cells: a source of tumor heterogeneity, evolution and recurrence

The heterogeneity and evolution of tumors remain significant obstacles in cancer treatment, contributing to both therapy resistance and relapse. Mesenchymal stem/stromal cells (MSCs) are multipotent stromal cells within the tumor microenvironment that interact with tumor cells through various mechanisms, including cell fusion. While previous research has largely focused on the effects of MSC-tumor cell fusion on tumor proliferation, migration, and tumorigenicity, emerging evidence indicates that its role in tumor maintenance, evolution, and recurrence, particularly under stress conditions, may be even more pivotal. This review examines the connection between MSC-tumor cell fusion and several critical factors like tumor heterogeneity, cancer stem cells, and therapy resistance, highlighting the crucial role of cell fusion in tumor survival, evolution, and recurrence. Additionally, we explore potential therapeutic strategies aimed at targeting this process.

Monocyte-cancer cell fusion is mediated by phosphatidylserine-CD36 receptor interaction and induced by ionizing radiation

Emerging evidence suggests that fusion of cancer cells with leucocytes, such as macrophages, plays a significant role in cancer metastasis and results in tumor hybrid cells that acquire resistance to chemo- and radiation therapy. However, the precise mechanisms behind the leukocyte-cancer cell fusion remain unclear. The present in vitro study explores the presence of fusion between the monocyte cell line (THP-1) and the breast cancer cell line (MCF-7) in relation to the expression of CD36 and phosphatidylserine with and without treatment of these cells with ionizing radiation. The study reveals that spontaneous THP-1/MCF-7 cell fusion increases significantly from 2.8% to 6% after irradiation. The interaction between CD36 and phosphatidylserine plays a pivotal role in THP-1/MCF-7 cell fusion, as inhibiting this interaction using anti-CD36 antibodies significantly reduces cell fusion. While irradiation leads to a dose-dependent escalation in phosphatidylserine expression in MCF-7 cells, it does not impact the expression of CD36 in either THP-1 or MCF-7 cells. To the best of our knowledge, this is the first study to demonstrate the involvement of the CD36-phosphatidylserine interaction in the fusion between monocytes and cancer cells, shedding light on a novel explanatory mechanism for the roles of CD36 and phosphatidylserine in tumor progression.

Circulating Neoplastic-Immune Hybrid Cells Are Biomarkers of Occult Metastasis and Treatment Response in Pancreatic Cancer

BACKGROUND/OBJECTIVES

Pancreatic ductal adenocarcinoma (PDAC) presents significant diagnostic and prognostic challenges, as current biomarkers frequently fail to accurately stage disease, predict rapid metastatic recurrence (rPDAC), or assess response to neoadjuvant therapy (NAT). We investigated the potential for circulating neoplastic-immune hybrid cells (CHCs) as a non-invasive, multifunctional biomarker for PDAC.

METHODS

Peripheral blood specimens were obtained from patients diagnosed with PDAC. CHCs were detected by co-expression of pan-cytokeratin and CD45, normalized to 50,000 peripheral blood mononuclear cells. rPDAC was defined as metastatic recurrence within six months of margin-negative pancreatectomy. Cyclic immunofluorescence (CyCIF) analyses compared hybrid phenotypes in blood and tumors.

RESULTS

Blood samples were collected from 42 patients with PDAC prior to resection. Those with radiographically occult metastatic disease and rPDAC had higher preoperative CHC numbers compared to patients who did not (65.0 and 74.4, vs. 11.52 CHCs; p < 0.001). Patients with complete or near-complete pathologic responses to NAT had lower preoperative CHC numbers than partial and/or non-responders (1.7 vs. 13.1 CHCs; p = 0.008). When assessed longitudinally, those with partial pathologic response saw CHC levels become undetectable while on treatment but increase in the interval between NAT completion and resection. In contrast, patients with poor responses or development of metastatic disease experienced persistent CHC detection during therapy or rising levels prior to radiographic evidence of metastases. Further, in metastatic PDAC patients, treatment-induced phenotypic changes in hybrid cells mirrored those in paired metastatic tumor samples.

CONCLUSIONS

CHC enumeration and phenotyping display promise as a real-time indicator of disease burden, recurrence risk, and treatment response in PDAC. CHCs have great potential as tumor-derived biomarkers to optimize therapeutic strategies and improve survival in patients with PDAC.

Uveal Melanoma: Comprehensive Review of Its Pathophysiology, Diagnosis, Treatment, and Future Perspectives

Uveal melanoma (UM) is the most common intraocular malignancy in adults. Recent advances highlight the role of tumor-derived extracellular vesicles (TEV) and circulating hybrid cells (CHC) in UM tumorigenesis. Bridged with liquid biopsies, a novel technology that has shown incredible performance in detecting cancer cells or products derived from tumors in bodily fluids, it can significantly impact disease management and outcome. The aim of this comprehensive literature review is to provide a summary of current knowledge and ongoing advances in posterior UM pathophysiology, diagnosis, and treatment. The first section of the manuscript discusses the complex and intricate role of TEVs and CHCs. The second part of this review delves into the epidemiology, etiology and risk factors, clinical presentation, and prognosis of UM. Third, current diagnostic methods, ensued by novel diagnostic tools for the early detection of UM, such as liquid biopsies and artificial intelligence-based technologies, are of paramount importance in this review. The fundamental principles, limits, and challenges associated with these diagnostic tools, as well as their potential as a tracker for disease progression, are discussed. Finally, a summary of current treatment modalities is provided, followed by an overview of ongoing preclinical and clinical research studies to provide further insights on potential biomolecular pathway alterations and therapeutic targets for the management of UM. This review is thus an important resource for all healthcare professionals, clinicians, and researchers working in the field of ocular oncology.

Genetic reprogramming with stem cells regenerates glomerular epithelial podocytes in Alport syndrome

Glomerular filtration relies on the type IV collagen (ColIV) network of the glomerular basement membrane, namely, in the triple helical molecules containing the α3, α4, and α5 chains of ColIV. Loss of function mutations in the genes encoding these chains (Col4a3, Col4a4, and Col4a5) is associated with the loss of renal function observed in Alport syndrome (AS). Precise understanding of the cellular basis for the patho-mechanism remains unknown and a specific therapy for this disease does not currently exist. Here, we generated a novel allele for the conditional deletion of Col4a3 in different glomerular cell types in mice. We found that podocytes specifically generate α3 chains in the developing glomerular basement membrane, and that its absence is sufficient to impair glomerular filtration as seen in AS. Next, we show that horizontal gene transfer, enhanced by TGFβ1 and using allogenic bone marrow-derived mesenchymal stem cells and induced pluripotent stem cells, rescues Col4a3 expression and revive kidney function in Col4a3-deficient AS mice. Our proof-of-concept study supports that horizontal gene transfer such as cell fusion enables cell-based therapy in Alport syndrome.

How Much Do You Fuse? A Comparison of Cell Fusion Assays in a Breast Cancer Model

Cell fusion is a biological process that is crucial for the development and homeostasis of different tissues, but it is also pathophysiologically associated with tumor progression and malignancy. The investigation of cell fusion processes is difficult because there is no standardized marker. Many studies therefore use different systems to observe and quantify cell fusion in vitro and in vivo. The comparability of the results must be critically questioned, because both the experimental procedure and the assays differ between studies. The comparability of the fluorescence-based fluorescence double reporter (FDR) and dual split protein (DSP) assay was investigated as part of this study, in which general conditions were kept largely constant. In order to be able to induce both a high and a low cell fusion rate, M13SV1 breast epithelial cells were modified with regard to the expression level of the fusogenic protein Syncytin-1 and its receptor ASCT2 and were co-cultivated for 72 h with different breast cancer cell lines. A high number of fused cells was found in co-cultures with Syncytin-1-overexpressing M13SV1 cells, but differences between the assays were also observed. This shows that the quantification of cell fusion events in particular is highly dependent on the assay selected, but the influence of fusogenic proteins can be visualized very well.

The Hallmarks of Circulating Hybrid Cells

While tumor metastases represent the primary driver of cancer-related mortality, our understanding of the mechanisms that underlie metastatic initiation and progression remains incomplete. Recent work identified a novel tumor-macrophage hybrid cell population, generated through the fusion between neoplastic and immune cells. These hybrid cells are detected in primary tumor tissue, peripheral blood, and in metastatic sites. In-depth analyses of hybrid cell biology indicate that they can exploit phenotypic properties of both parental tumor and immune cells, in order to intravasate into circulation, evade the immune response, and seed tumors at distant sites. Thus, it has become increasingly evident that the development and dissemination of tumor-immune hybrid cells play an intricate and fundamental role in the metastatic cascade and can provide invaluable information regarding tumor characteristics and patient prognostication. In this chapter, we review the current understanding of this novel hybrid cell population, the specific hallmarks of cancer that these cells exploit to promote cancer progression and metastasis, and discuss exciting new frontiers that remain to be explored.

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.

Cell Fusion and Syncytia Formation in Cancer

The natural phenomenon of cell-cell fusion does not only take place in physiological processes, such as placentation, myogenesis, or osteoclastogenesis, but also in pathophysiological processes, such as cancer. More than a century ago postulated, today the hypothesis that the fusion of cancer cells with normal cells leads to the formation of cancer hybrid cells with altered properties is in scientific consensus. Some studies that have investigated the mechanisms and conditions for the fusion of cancer cells with other cells, as well as studies that have characterized the resulting cancer hybrid cells, are presented in this review. Hypoxia and the cytokine TNFα, for example, have been found to promote cell fusion. In addition, it has been found that both the protein Syncytin-1, which normally plays a role in placentation, and phosphatidylserine signaling on the cell membrane are involved in the fusion of cancer cells with other cells. In human cancer, cancer hybrid cells were detected not only in the primary tumor, but also in the circulation of patients as so-called circulating hybrid cells, where they often correlated with a worse outcome. Although some data are available, the questions of how and especially why cancer cells fuse with other cells are still not fully answered.

Cardiac and intestinal tissue conduct developmental and reparative processes in response to lymphangiocrine signaling

Lymphatic vessels conduct a diverse range of activities to sustain the integrity of surrounding tissue. Besides facilitating the movement of lymph and its associated factors, lymphatic vessels are capable of producing tissue-specific responses to changes within their microenvironment. Lymphatic endothelial cells (LECs) secrete paracrine signals that bind to neighboring cell-receptors, commencing an intracellular signaling cascade that preludes modifications to the organ tissue's structure and function. While the lymphangiocrine factors and the molecular and cellular mechanisms themselves are specific to the organ tissue, the crosstalk action between LECs and adjacent cells has been highlighted as a commonality in augmenting tissue regeneration within animal models of cardiac and intestinal disease. Lymphangiocrine secretions have been owed for subsequent improvements in organ function by optimizing the clearance of excess tissue fluid and immune cells and stimulating favorable tissue growth, whereas perturbations in lymphatic performance bring about the opposite. Newly published landmark studies have filled gaps in our understanding of cardiac and intestinal maintenance by revealing key players for lymphangiocrine processes. Here, we will expand upon those findings and review the nature of lymphangiocrine factors in the heart and intestine, emphasizing its involvement within an interconnected network that supports daily homeostasis and self-renewal following injury.

Wound-Induced Syncytia Outpace Mononucleate Neighbors during Drosophila Wound Repair

All organisms have evolved to respond to injury. Cell behaviors like proliferation, migration, and invasion replace missing cells and close wounds. However, the role of other wound-induced cell behaviors is not understood, including the formation of syncytia (multinucleated cells). Wound-induced epithelial syncytia were first reported around puncture wounds in post-mitotic Drosophila epidermal tissues, but have more recently been reported in mitotically competent tissues such as the Drosophila pupal epidermis and zebrafish epicardium. The presence of wound-induced syncytia in mitotically active tissues suggests that syncytia offer adaptive benefits, but it is unknown what those benefits are. Here, we use in vivo live imaging to analyze wound-induced syncytia in mitotically competent Drosophila pupae. We find that almost half the epithelial cells near a wound fuse to form large syncytia. These syncytia use several routes to speed wound repair: they outpace diploid cells to complete wound closure; they reduce cell intercalation during wound closure; and they pool the resources of their component cells to concentrate them toward the wound. In addition to wound healing, these properties of syncytia are likely to contribute to their roles in development and pathology.

Why do certain cancer cells alter functionality and fuse?

Cancer cell fusion represents a rare event. However, the surviving cancer hybrid cells after a post-hybrid selection process (PHSP) can overgrow other cancer cells by exhibiting a proliferation advantage and/or expression of cancer stem-like properties. Addition of new tumor properties during hetero-fusion of cancer cells e.g. with mesenchymal stroma-/stem-like cells (MSC) contribute to enhanced tumor plasticity via acquisition of new/altered functionalities. This provides new avenues for tumor development and metastatic behavior. Consequently, the present review article will also address the question as to whether cancer cell fusion represents a general and possibly evolutionary-conserved program or rather a random process?

You complete me: tumor cell-myeloid cell nuclear fusion as a facilitator of organ-specific metastasis

Every cancer genome is unique, resulting in potentially near infinite cancer cell phenotypes and an inability to predict clinical outcomes in most cases. Despite this profound genomic heterogeneity, many cancer types and subtypes display a non-random distribution of metastasis to distant organs, a phenomenon known as organotropism. Proposed factors in metastatic organotropism include hematogenous versus lymphatic dissemination, the circulation pattern of the tissue of origin, tumor-intrinsic factors, compatibility with established organ-specific niches, long-range induction of premetastatic niche formation, and so-called "prometastatic niches" that facilitate successful colonization of the secondary site following extravasation. To successfully complete the steps required for distant metastasis, cancer cells must evade immunosurveillance and survive in multiple new and hostile environments. Despite substantial advances in our understanding of the biology underlying malignancy, many of the mechanisms used by cancer cells to survive the metastatic journey remain a mystery. This review synthesizes the rapidly growing body of literature demonstrating the relevance of an unusual cell type known as "fusion hybrid" cells to many of the hallmarks of cancer, including tumor heterogeneity, metastatic conversion, survival in circulation, and metastatic organotropism. Whereas the concept of fusion between tumor cells and blood cells was initially proposed over a century ago, only recently have technological advancements allowed for detection of cells containing components of both immune and neoplastic cells within primary and metastatic lesions as well as among circulating malignant cells. Specifically, heterotypic fusion of cancer cells with monocytes and macrophages results in a highly heterogeneous population of hybrid daughter cells with enhanced malignant potential. Proposed mechanisms behind these findings include rapid, massive genome rearrangement during nuclear fusion and/or acquisition of monocyte/macrophage features such as migratory and invasive capability, immune privilege, immune cell trafficking and homing, and others. Rapid acquisition of these cellular traits may increase the likelihood of both escape from the primary tumor site and extravasation of hybrid cells at a secondary location that is amenable to colonization by that particular hybrid phenotype, providing a partial explanation for the patterns observed in some cancers with regard to sites of distant metastases.

The impact and outcomes of cancer-macrophage fusion

BACKGROUND

Cancer's hallmark feature is its ability to evolve, leading to metastasis and recurrence. Although genetic mutations and epigenetic changes have been implicated, they don't fully explain the leukocytic traits that many cancers develop. Cell fusion between cancer and somatic cells, particularly macrophages, has been suggested as an alternative pathway for cancer cells to obtain new traits by acquiring exogenous genetic material.

METHODS

This study aims to investigate the potential biological outcomes of tumor-myeloid cell fusion by generating tumor-macrophage hybrid cells. Two clones with markedly different tumorigenicity were selected, and RNA-seq was used to compare their RNA expressions with that of the control cells. Based on the results that the hybrid cells showed differential activation in several upstream regulator pathways that impact their biological behaviors, the hybrid cells' abilities to recruit stromal cells and establish angiogenesis as well as their cell cycle distributions were investigated through in vitro and in vivo studies.

RESULTS

Although both hybrid clones demonstrated p53 activation and reduced growth rates, they exhibited distinct cell cycle distributions and ability to grow in vivo. Notably, while one clone was highly tumorigenic, the other showed little tumorigenicity. Despite these differences, both hybrid clones were potent environmental modifiers, exhibiting significant abilities to recruit stromal and immune cells and establish angiogenesis.

CONCLUSIONS

The study revealed that tumor-somatic cell fusion is a potent environmental modifier that can modulate tumor survival and evolution, despite its relatively low occurrence. These findings suggest that tumor-somatic cell fusion could be a promising target for developing new cancer therapies. Furthermore, this study provides an experimental animal platform to investigate cancer-myeloid fusion and highlights the potential role of tumor-somatic cell fusion in modulating the tumor environment.

Intrinsic signalling factors associated with cancer cell-cell fusion

Cellular fusion e.g. between cancer cells and normal cells represents a stepwise process that is tightly regulated. During a pre-hybrid preparation program somatic cells and/or cancer cells are promoted to a pro-fusogenic state as a prerequisite to prepare a fusion process. A pro-fusogenic state requires significant changes including restructure of the cytoskeleton, e.g., by the formation of F-actin. Moreover, distinct plasma membrane lipids such as phosphatidylserine play an important role during cell fusion. In addition, the expression of distinct fusogenic factors such as syncytins and corresponding receptors are of fundamental importance to enable cellular mergers. Subsequent hybrid formation and fusion are followed by a post-hybrid selection process. Fusion among normal cells is important and often required during organismal development. Cancer cells fusion appears more rarely and is associated with the generation of new cancer hybrid cell populations. These cancer hybrid cells contribute to an elevated tumour plasticity by altered metastatic behaviour, changes in therapeutic and apoptotic responses, and even in the formation of cancer stem/ initiating cells. While many parts within this multi-step cascade are still poorly understood, this review article predominantly focusses on the intracellular necessities for fusion among cancer cells or with other cell populations of the tumour microenvironment. Video Abstract.

Stem cell derived therapies to preserve and repair the developing intestine

Stem cell research and the use of stem cells in therapy have seen tremendous growth in the last two decades. Neonatal intestinal disorders such as necrotizing enterocolitis, Hirschsprung disease, and gastroschisis have high morbidity and mortality and limited treatment options with varying success rates. Stem cells have been used in several pre-clinical studies to address various neonatal disorders with promising results. Stem cell and patient population selection, timing of therapy, as well as safety and quality control are some of the challenges that must be addressed prior to the widespread clinical application of stem cells. Further research and technological advances such as the use of cell delivery technology can address these challenges and allow for continued progress towards clinical translation.

Extracellular Events Involved in Cancer Cell-Cell Fusion

Fusion among different cell populations represents a rare process that is mediated by both intrinsic and extracellular events. Cellular hybrid formation is relayed by orchestrating tightly regulated signaling pathways that can involve both normal and neoplastic cells. Certain important cell merger processes are often required during distinct organismal and tissue development, including placenta and skeletal muscle. In a neoplastic environment, however, cancer cell fusion can generate new cancer hybrid cells. Following survival during a subsequent post-hybrid selection process (PHSP), the new cancer hybrid cells express different tumorigenic properties. These can include elevated proliferative capacity, increased metastatic potential, resistance to certain therapeutic compounds, and formation of cancer stem-like cells, all of which characterize significantly enhanced tumor plasticity. However, many parts within this multi-step cascade are still poorly understood. Aside from intrinsic factors, cell fusion is particularly affected by extracellular conditions, including an inflammatory microenvironment, viruses, pH and ionic stress, hypoxia, and exosome signaling. Accordingly, the present review article will primarily highlight the influence of extracellular events that contribute to cell fusion in normal and tumorigenic tissues.

Metabolic management of microenvironment acidity in glioblastoma

Glioblastoma (GBM), similar to most cancers, is dependent on fermentation metabolism for the synthesis of biomass and energy (ATP) regardless of the cellular or genetic heterogeneity seen within the tumor. The transition from respiration to fermentation arises from the documented defects in the number, the structure, and the function of mitochondria and mitochondrial-associated membranes in GBM tissue. Glucose and glutamine are the major fermentable fuels that drive GBM growth. The major waste products of GBM cell fermentation (lactic acid, glutamic acid, and succinic acid) will acidify the microenvironment and are largely responsible for drug resistance, enhanced invasion, immunosuppression, and metastasis. Besides surgical debulking, therapies used for GBM management (radiation, chemotherapy, and steroids) enhance microenvironment acidification and, although often providing a time-limited disease control, will thus favor tumor recurrence and complications. The simultaneous restriction of glucose and glutamine, while elevating non-fermentable, anti-inflammatory ketone bodies, can help restore the pH balance of the microenvironment while, at the same time, providing a non-toxic therapeutic strategy for killing most of the neoplastic cells.

Circulating Cells with Macrophage-like Characteristics in Cancer: The Importance of Circulating Neoplastic-Immune Hybrid Cells in Cancer

Simple Summary

In cancer, disseminated neoplastic cells circulating in blood are a source of tumor DNA, RNA, and protein, which can be harnessed to diagnose, monitor, and better understand the biology of the tumor from which they are derived. Historically, circulating tumor cells (CTCs) have dominated this field of study. While CTCs are shed directly into circulation from a primary tumor, they remain relatively rare, particularly in early stages of disease, and thus are difficult to utilize as a reliable cancer biomarker. Neoplastic-immune hybrid cells represent a novel subpopulation of circulating cells that are more reliably attainable as compared to their CTC counterparts. Here, we review two recently identified circulating cell populations in cancer—cancer-associated macrophage-like cells and circulating hybrid cells—and discuss the future impact for the exciting area of disseminated hybrid cells.

Abstract

Cancer remains a significant cause of mortality in developed countries, due in part to difficulties in early detection, understanding disease biology, and assessing treatment response. If effectively harnessed, circulating biomarkers promise to fulfill these needs through non-invasive “liquid” biopsy. While tumors disseminate genetic material and cellular debris into circulation, identifying clinically relevant information from these analytes has proven difficult. In contrast, cell-based circulating biomarkers have multiple advantages, including a source for tumor DNA and protein, and as a cellular reflection of the evolving tumor. While circulating tumor cells (CTCs) have dominated the circulating cell biomarker field, their clinical utility beyond that of prognostication has remained elusive, due to their rarity. Recently, two novel populations of circulating tumor-immune hybrid cells in cancer have been characterized: cancer-associated macrophage-like cells (CAMLs) and circulating hybrid cells (CHCs). CAMLs are macrophage-like cells containing phagocytosed tumor material, while CHCs can result from cell fusion between cancer and immune cells and play a role in the metastatic cascade. Both are detected in higher numbers than CTCs in peripheral blood and demonstrate utility in prognostication and assessing treatment response. Additionally, both cell populations are heterogeneous in their genetic, transcriptomic, and proteomic signatures, and thus have the potential to inform on heterogeneity within tumors. Herein, we review the advances in this exciting field.

Macrophage orchestration of epithelial and stromal cell homeostasis in the intestine

The intestinal tract is a complex ecosystem where numerous cell types of epithelial, immune, neuronal, and endothelial origin coexist in an intertwined, highly organized manner. The functional equilibrium of the intestine relies heavily on the proper crosstalk and cooperation among each cell population. Furthermore, macrophages are versatile, innate immune cells that participate widely in the modulation of inflammation and tissue remodeling. Emerging evidence suggest that macrophages are central in orchestrating tissue homeostasis. Herein, we describe how macrophages interact with epithelial cells, neurons, and other types of mesenchymal cells under the context of intestinal inflammation, followed by the therapeutic implications of cellular crosstalk pertaining to the treatment of inflammatory bowel disease.

Tumour Stem Cells in Breast Cancer

Tumour stem cells (CSCs) are a self-renewing population that plays important roles in tumour initiation, recurrence, and metastasis. Although the medical literature is extensive, problems with CSC identification and cancer therapy remain. This review provides the main mechanisms of CSC action in breast cancer (BC): CSC markers and signalling pathways, heterogeneity, plasticity, and ecological behaviour. The dynamic heterogeneity of CSCs and the dynamic transitions of CSC⁻ non-CSCs and their significance for metastasis are considered.

Generation of Cancer Stem/Initiating Cells by Cell-Cell Fusion

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.

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.

Hybrid Formation and Fusion of Cancer Cells In Vitro and In Vivo

The generation of cancer hybrid cells by intra-tumoral cell fusion opens new avenues for tumor plasticity to develop cancer stem cells with altered properties, to escape from immune surveillance, to change metastatic behavior, and to broaden drug responsiveness/resistance. Genomic instability and chromosomal rearrangements in bi- or multinucleated aneuploid cancer hybrid cells contribute to these new functions. However, the significance of cell fusion in tumorigenesis is controversial with respect to the low frequency of cancer cell fusion events and a clonal advantage of surviving cancer hybrid cells following a post-hybrid selection process. This review highlights alternative processes of cancer hybrid cell development such as entosis, emperipolesis, cannibalism, therapy-induced polyploidization/endoreduplication, horizontal or lateral gene transfer, and focusses on the predominant mechanisms of cell fusion. Based upon new properties of cancer hybrid cells the arising clinical consequences of the subsequent tumor heterogeneity after cancer cell fusion represent a major therapeutic challenge.

Can the Mitochondrial Metabolic Theory Explain Better the Origin and Management of Cancer than Can the Somatic Mutation Theory?

A theory that can best explain the facts of a phenomenon is more likely to advance knowledge than a theory that is less able to explain the facts. Cancer is generally considered a genetic disease based on the somatic mutation theory (SMT) where mutations in proto-oncogenes and tumor suppressor genes cause dysregulated cell growth. Evidence is reviewed showing that the mitochondrial metabolic theory (MMT) can better account for the hallmarks of cancer than can the SMT. Proliferating cancer cells cannot survive or grow without carbons and nitrogen for the synthesis of metabolites and ATP (Adenosine Triphosphate). Glucose carbons are essential for metabolite synthesis through the glycolysis and pentose phosphate pathways while glutamine nitrogen and carbons are essential for the synthesis of nitrogen-containing metabolites and ATP through the glutaminolysis pathway. Glutamine-dependent mitochondrial substrate level phosphorylation becomes essential for ATP synthesis in cancer cells that over-express the glycolytic pyruvate kinase M2 isoform (PKM2), that have deficient OxPhos, and that can grow in either hypoxia (0.1% oxygen) or in cyanide. The simultaneous targeting of glucose and glutamine, while elevating levels of non-fermentable ketone bodies, offers a simple and parsimonious therapeutic strategy for managing most cancers.

Relevance of circulating hybrid cells as a non-invasive biomarker for myriad solid tumors

Metastatic progression defines the final stages of tumor evolution and underlies the majority of cancer-related deaths. The heterogeneity in disseminated tumor cell populations capable of seeding and growing in distant organ sites contributes to the development of treatment resistant disease. We recently reported the identification of a novel tumor-derived cell population, circulating hybrid cells (CHCs), harboring attributes from both macrophages and neoplastic cells, including functional characteristics important to metastatic spread. These disseminated hybrids outnumber conventionally defined circulating tumor cells (CTCs) in cancer patients. It is unknown if CHCs represent a generalized cancer mechanism for cell dissemination, or if this population is relevant to the metastatic cascade. Herein, we detect CHCs in the peripheral blood of patients with cancer in myriad disease sites encompassing epithelial and non-epithelial malignancies. Further, we demonstrate that in vivo-derived hybrid cells harbor tumor-initiating capacity in murine cancer models and that CHCs from human breast cancer patients express stem cell antigens, features consistent with the potential to seed and grow at metastatic sites. Finally, we reveal heterogeneity of CHC phenotypes reflect key tumor features, including oncogenic mutations and functional protein expression. Importantly, this novel population of disseminated neoplastic cells opens a new area in cancer biology and renewed opportunity for battling metastatic disease.

Cell-Cell Fusion and the Roads to Novel Properties of Tumor Hybrid Cells

The phenomenon of cancer cell–cell fusion is commonly associated with the origin of more malignant tumor cells exhibiting novel properties, such as increased drug resistance or an enhanced metastatic capacity. However, the whole process of cell–cell fusion is still not well understood and seems to be rather inefficient since only a certain number of (cancer) cells are capable of fusing and only a rather small population of fused tumor hybrids will survive at all. The low survivability of tumor hybrids is attributed to post-fusion processes, which are characterized by the random segregation of mixed parental chromosomes, the induction of aneuploidy and further random chromosomal aberrations and genetic/epigenetic alterations in daughter cells. As post-fusion processes also run in a unique manner in surviving tumor hybrids, the occurrence of novel properties could thus also be a random event, whereby it might be speculated that the tumor microenvironment and its spatial habitats could direct evolving tumor hybrids towards a specific phenotype.

Cell fusion in cancer hallmarks: Current research status and future indications

Cell fusion is involved in several physiological processes, such as reproduction, development and immunity. Although cell fusion in tumors was reported 130 years ago, it has recently attracted great interest, with recent progress in tumorigenesis research. However, the role of cell fusion in tumor progression remains unclear. The pattern of cell fusion and its role under physiological conditions are the basis for our understanding of the pathological role of cell fusion. However, the role of cell fusion in tumors and its functions are complicated. Cell fusion can directly increase tumor heterogeneity by forming polyploids or aneuploidies. Several studies have reported that cell fusion is associated with tumorigenesis, metastasis, recurrence, drug resistance and the formation of cancer stem cells. Given the diverse roles cell fusion plays in different tumor phenotypes, methods based on targeted cell fusion have been designed to treat tumors. Research on cell fusion in tumors may provide novel ideas for further treatment.

Could gastrointestinal tumor-initiating cells originate from cell-cell fusion in vivo?

Tumor-initiating cells (TICs) or cancer stem cells are believed to be responsible for gastrointestinal tumor initiation, progression, metastasis, and drug resistance. It is hypothesized that gastrointestinal TICs (giTICs) might originate from cell-cell fusion. Here, we systemically evaluate the evidence that supports or opposes the hypothesis of giTIC generation from cell-cell fusion both in vitro and in vivo. We review giTICs that are capable of initiating tumors in vivo with 5000 or fewer in vivo fused cells. Under this restriction, there is currently little evidence demonstrating that giTICs originate from cell-cell fusion in vivo. However, there are many reports showing that tumor generation in vitro occurs with more than 5000 fused cells. In addition, the mechanisms of giTIC generation via cell-cell fusion are poorly understood, and thus, we propose its potential mechanisms of action. We suggest that future research should focus on giTIC origination from cell-cell fusion in vivo, isolation or enrichment of giTICs that have tumor-initiating capabilities with 5000 or less in vivo fused cells, and further clarification of the underlying mechanisms. Our review of the current advances in our understanding of giTIC origination from cell-cell fusion may have significant implications for the understanding of carcinogenesis and future cancer therapeutic strategies targeting giTICs.

Alcohol stimulates the proliferation of mouse small intestinal epithelial cells via Wnt signaling

The intestinal epithelium is one of the fastest renewing tissues in mammals and is a barrier against toxic substances such as alcohol. Excessive alcohol can induce intestinal damage leading to intestinal bowel diseases. Thus, the control of small intestinal epithelial cell (IEC) regeneration is thought to be important for homeostasis in response to epithelium damage. However, reports on how epithelial cells respond to small intestinal damage are scarce. We investigated the effects of alcohol consumption on small intestinal epithelial cells of mice. To verify that alcohol altered the small intestinal epithelium, we used 8-10 weeks old male C57BL/6J mice for models of chronic and binge alcohol consumption (the NIAAA model) in addition to an organoid model. Alcohol promoted the proliferative activity of IECs and intestinal stem cells (ISCs) in intestinal crypts. Alcohol consumption increased expression of the proliferation marker cyclin D1 and activated the p44/42 MAPK (Erk1/2) signaling pathway in small intestinal epithelial cells. The Wnt target genes were markedly increased in IECs from alcohol-treated mice. In the small intestinal organoid model exposed to alcohol, the organoid area and numbers of buds increased with alcohol concentrations up to 0.5% similar to in vivo observations. These results suggest that alcohol consumption stimulates the proliferation of small intestinal epithelial cells via Wnt signaling.

Characterization of cell fusion in an experimental mouse model of endometriosis†

Cell fusion is involved in the development of some adult organs, is implicated in the pathogenesis of specific types of cancer, and is known to participate in repair/regeneration processes mediated by bone-marrow-derived cells (BMDCs). Endometriosis is a disease characterized by growth of functional endometrial tissue outside of the uterine cavity. Endometriosis shares some molecular properties with cancer and BMDCs home to endometriosis lesions in a mouse model. Our objective was to determine if cell fusion can occur in endometriosis and establish whether bone-marrow-derived cells participate in cell fusion events in lesions. We employed a Cre-Lox system to identify cell fusion events in a mouse model of endometriosis. Fused cells were detected in endometriotic lesions, albeit at a low frequency (∼1 in 400 cells), localized to the stromal compartment, and displayed restricted proliferation. Using 5-fluorouracil-based nongonadotoxic bone marrow transplantation model, we demonstrate that bone marrow cells represent a principal cell source for fusion events in lesions. Cell fusion progeny uniformly lacked expression of selected markers of hematopoietic, endothelial, and epithelial markers, though they expressed the mesenchymal/stromal markers Sca-1 and CD29. This study is the first to describe the phenomenon of cell fusion in endometriosis and points to a mesenchymal population derived from cell fusion events with limited proliferative activity, properties previously attributed to endometrial stem cells. Their putative role in the pathogenesis of the disease remains to be elucidated.

Roles of cell fusion, hybridization and polyploid cell formation in cancer metastasis

Cell-cell fusion is a normal biological process playing essential roles in organ formation and tissue differentiation, repair and regeneration. Through cell fusion somatic cells undergo rapid nuclear reprogramming and epigenetic modifications to form hybrid cells with new genetic and phenotypic properties at a rate exceeding that achievable by random mutations. Factors that stimulate cell fusion are inflammation and hypoxia. Fusion of cancer cells with non-neoplastic cells facilitates several malignancy-related cell phenotypes, e.g., reprogramming of somatic cell into induced pluripotent stem cells and epithelial to mesenchymal transition. There is now considerable in vitro, in vivo and clinical evidence that fusion of cancer cells with motile leucocytes such as macrophages plays a major role in cancer metastasis. Of the many changes in cancer cells after hybridizing with leucocytes, it is notable that hybrids acquire resistance to chemo- and radiation therapy. One phenomenon that has been largely overlooked yet plays a role in these processes is polyploidization. Regardless of the mechanism of polyploid cell formation, it happens in response to genotoxic stresses and enhances a cancer cell’s ability to survive. Here we summarize the recent progress in research of cell fusion and with a focus on an important role for polyploid cells in cancer metastasis. In addition, we discuss the clinical evidence and the importance of cell fusion and polyploidization in solid tumors.

Cell Fusion-Mediated Tissue Regeneration as an Inducer of Polyploidy and Aneuploidy

The biological phenomenon of cell fusion plays a crucial role in several physiological processes, including wound healing and tissue regeneration. Here, it is assumed that bone marrow-derived stem cells (BMSCs) could adopt the specific properties of a different organ by cell fusion, thereby restoring organ function. Cell fusion first results in the production of bi- or multinucleated hybrid cells, which either remain as heterokaryons or undergo ploidy reduction/heterokaryon-to-synkaryon transition (HST), thereby giving rise to mononucleated daughter cells. This process is characterized by a merging of the chromosomes from the previously discrete nuclei and their subsequent random segregation into daughter cells. Due to extra centrosomes concomitant with multipolar spindles, the ploidy reduction/HST could also be associated with chromosome missegregation and, hence, induction of aneuploidy, genomic instability, and even putative chromothripsis. However, while the majority of such hybrids die or become senescent, aneuploidy and genomic instability appear to be tolerated in hepatocytes, possibly for stress-related adaption processes. Likewise, cell fusion-induced aneuploidy and genomic instability could also lead to a malignant conversion of hybrid cells. This can occur during tissue regeneration mediated by BMSC fusion in chronically inflamed tissue, which is a cell fusion-friendly environment, but is also enriched for mutagenic reactive oxygen and nitrogen species.

Lymphatic Endothelial Cell Progenitors in the Tumor Microenvironment

Tumor lymphatics play a key role in cancer progression as they are solely responsible for transporting malignant cells to regional lymph nodes (LNs), a process that precedes and promotes systemic lethal spread. It is broadly accepted that tumor lymphatic sprouting is induced mainly by soluble factors derived from tumor-associated macrophages (TAMs) and malignant cells. However, emerging evidence strongly suggests that a subset of TAMs, myeloid-lymphatic endothelial cell progenitors (M-LECP), also contribute to the expansion of lymphatics through both secretion of paracrine factors and a self-autonomous mode. M-LECP are derived from bone marrow (BM) precursors of the monocyte-macrophage lineage and characterized by unique co-expression of markers identifying lymphatic endothelial cells (LEC), stem cells, M2-type macrophages, and myeloid-derived immunosuppressive cells. This review describes current evidence for the origin of M-LECP in the bone marrow, their recruitment tumors and intratumoral trafficking, similarities to other TAM subsets, and mechanisms promoting tumor lymphatics. We also describe M-LECP integration into preexisting lymphatic vessels and discuss potential mechanisms and significance of this event. We conclude that improved mechanistic understanding of M-LECP functions within the tumor environment may lead to new therapeutic approaches to suppress tumor lymphangiogenesis and metastasis to lymph nodes.

The Potential of Mesenchymal Stromal Cell as Therapy in Neonatal Diseases

Mesenchymal stromal cells (MSCs) can be derived from various tissue sources, such as the bone marrow (BMSCs), adipose tissue (ADSCs), umbilical cord (UC-MSCs) and umbilical cord blood (UCB-MSCs). Clinical trials have been conducted to investigate the potential of MSCs in ameliorating neonatal diseases, including bronchopulmonary dysplasia (BPD), intraventricular hemorrhage (IVH) and necrotizing enterocolitis (NEC). In preclinical studies, MSC therapy has been tested for the treatment of various neonatal diseases affecting the heart, eye, gut, and brain as well as sepsis. Up to date, the number of clinical trials using MSCs to treat neonatal diseases is still limited. The data reported thus far positioned MSC therapy as safe with positive outcomes. However, most of these trials are still preliminary and generally smaller in scale. Larger trials with more appropriate controls and a longer follow-up period need to be conducted to prove the safety and efficacy of the therapy more conclusively. This review discusses the current application of MSCs in treating neonatal diseases, its mechanism of action and future direction of this novel therapy, including the potential of using MSC-derived extracellular vesicles instead of the cells to treat various clinical conditions in the newborn.

Circulating Hybrid Cells Join the Fray of Circulating Cellular Biomarkers

Gastrointestinal cancers account for more cancer-related deaths than any other organ system, owing in part to difficulties in early detection, treatment response assessment, and post-treatment surveillance. Circulating biomarkers hold the promise for noninvasive liquid biopsy platforms to overcome these obstacles. Although tumors shed detectable levels of degraded genetic material and cellular debris into peripheral blood, identifying reproducible and clinically relevant information from these analytes (eg, cell-free nucleotides, exosomes, proteins) has proven difficult. Cell-based circulating biomarkers also present challenges, but have multiple advantages including allowing for a more comprehensive tumor analysis, and communicating the risk of metastatic spread. Circulating tumor cells have dominated the cancer cell biomarker field with robust evidence in extraintestinal cancers; however, establishing their clinical utility beyond that of prognostication in colorectal and pancreatic cancers has remained elusive. Recently identified novel populations of tumor-derived cells bring renewed potential to this area of investigation. Cancer-associated macrophage-like cells, immune cells with phagocytosed tumor material, also show utility in prognostication and assessing treatment responsiveness. In addition, circulating hybrid cells are the result of tumor-macrophage fusion, with mounting evidence for a role in the metastatic cascade. Because of their relative abundance in circulation, circulating hybrid cells have great potential as a liquid biomarker for early detection, prognostication, and surveillance. In all, the power of the cell reaches beyond enumeration by providing a cellular source of tumor DNA, RNA, and protein, which can be harnessed to impact overall survival.

Tumor Microenvironment and Cell Fusion

Cell fusion is a highly regulated biological process that occurs under both physiological and pathological conditions. The cellular and extracellular environment is critical for the induction of the cell-cell fusion. Aberrant cell fusion is initiated during tumor progression. Tumor microenvironment is a complex dynamic system formed by the interaction between tumor cells and their surrounding cells. Cell-cell fusion mediates direct interaction between tumor cells and their surrounding cells and is associated with tumor initiation and progression. Various microenvironmental factors affect cell fusion in tumor microenvironment and generate hybrids that acquire genomes of both parental cells and exhibit novel characteristics, such as tumor stem cell-like properties, radioresistance, drug resistance, immune evasion, and enhanced migration and invasion abilities, which are closely related to the initiation, invasion, and metastasis of tumor. The phenotypic characteristics of hybrids are based on the phenotypes of parental cells, and the fusion of tumor cells with diverse types of microenvironmental fusogenic cells is concomitant with phenotypic heterogeneity. This review highlights the types of fusogenic cells in tumor microenvironment that can fuse with tumor cells and their specific significance and summarizes the various microenvironmental factors affecting tumor cell fusion. This review may be used as a reference to develop strategies for future research on tumor cell fusion and the exploration of cell fusion-based antitumor therapies.

Minocycline impairs TNF-α-induced cell fusion of M13SV1-Cre cells with MDA-MB-435-pFDR1 cells by suppressing NF-κB transcriptional activity and its induction of target-gene expression of fusion-relevant factors

Background

To date, several studies have confirmed that driving forces of the inflammatory tumour microenvironment trigger spontaneous cancer cell fusion. However, less is known about the underlying factors and mechanisms that facilitate inflammation-induced cell fusion of a cancer cell with a normal cell. Recently, we demonstrated that minocycline, a tetracycline antibiotic, successfully inhibited the TNF-α-induced fusion of MDA-MB-435 cancer cells with M13SV1 breast epithelial cells. Here, we investigated how minocycline interferes with the TNF-α induced signal transduction pathway.

Methods

A Cre-LoxP recombination system was used to quantify the fusion of MDA-MB-435-pFDR1 cancer cells and M13SV1-Cre breast epithelial cells. The impact of minocycline on the TNF-α signalling pathway was determined by western blotting. The transcriptional activity of NF-κB was characterised by immunocytochemistry, western blot and ChIP analyses. An NF-κB-luciferase reporter assay was indicative of NF-κB activity.

Results

Minocycline treatment successfully inhibited the TNFR1-TRAF2 interaction in both cell types, while minocycline abrogated the phosphorylation of IκBα and NF-κB-p65 to suppress nuclear NF-κB and its promotor activity only in M13SV1-Cre cells, which attenuated the expression of MMP9 and ICAM1. In MDA-MB-435-pFDR1 cells, minocycline increased the activity of NF-κB, leading to greater nuclear accumulation of NF-κB-p65, thus increasing promoter activity to stimulate the expression of ICAM1. Even though TNF-α also activated all MAPKs (ERK1/2, p38 and JNK), minocycline differentially affected these kinases to either inhibit or stimulate their activation. Moreover, SRC activation was analysed as an upstream activator of MAPKs, but no activation by TNF-α was revealed. The addition of several specific inhibitors that block the activation of SRC, MAPKs, AP-1 and NF-κB confirmed that only NF-κB inhibition was successful in inhibiting the TNF-α-induced cell fusion process.

Conclusion

Minocycline is a potent inhibitor in the TNF-α-induced cell fusion process by targeting the NF-κB pathway. Thus, minocycline prevented NF-κB activation and nuclear translocation to abolish the target-gene expression of MMP9 and ICAM1 in M13SV1-Cre cells, resulting in reduced cell fusion frequency.

Cancer cell fusion: a potential target to tackle drug-resistant and metastatic cancer cells

Cell fusion is an integral, established phenomenon underlying various physiological processes in the cell cycle. Although research in cancer metastasis has hypothesised numerous molecular mechanisms and signalling pathways responsible for invasion and metastasis, the origin and progression of metastatic cells within primary tumours remains unclear. Recently, the role of cancer cell fusion in cancer metastasis and development of multidrug resistance (MDR) in tumours has gained prominence. However, evidence remains lacking to justify the role of cell fusion in cancer metastasis and drug resistance. Here, we highlight plausible mechanisms governing cell fusion with different cell types in the tumour microenvironment (TME), the clinical relevance of cancer cell fusion, its potential as a target for overcoming MDR and inhibiting metastasis, and putative modes of treatment.

Provocative Question: Should Ketogenic Metabolic Therapy Become the Standard of Care for Glioblastoma?

No major advances have been made in improving overall survival for glioblastoma (GBM) in almost 100 years. The current standard of care (SOC) for GBM involves immediate surgical resection followed by radiotherapy with concomitant temozolomide chemotherapy. Corticosteroid (dexamethasone) is often prescribed to GBM patients to reduce tumor edema and inflammation. The SOC disrupts the glutamate-glutamine cycle thus increasing availability of glucose and glutamine in the tumor microenvironment. Glucose and glutamine are the prime fermentable fuels that underlie therapy resistance and drive GBM growth through substrate level phosphorylation in the cytoplasm and the mitochondria, respectively. Emerging evidence indicates that ketogenic metabolic therapy (KMT) can reduce glucose availability while elevating ketone bodies that are neuroprotective and non-fermentable. Information is presented from preclinical and case report studies showing how KMT could target tumor cells without causing neurochemical damage thus improving progression free and overall survival for patients with GBM.

Cell Fusion in Human Cancer: The Dark Matter Hypothesis

Current strategies to determine tumor × normal (TN)-hybrid cells among human cancer cells include the detection of hematopoietic markers and other mesodermal markers on tumor cells or the presence of donor DNA in cancer samples from patients who had previously received an allogenic bone marrow transplant. By doing so, several studies have demonstrated that TN-hybrid cells could be found in human cancers. However, a prerequisite of this cell fusion search strategy is that such markers are stably expressed by TN-hybrid cells over time. However, cell fusion is a potent inducer of genomic instability, and TN-hybrid cells may lose these cell fusion markers, thereby becoming indistinguishable from nonfused tumor cells. In addition, hybrid cells can evolve from homotypic fusion events between tumor cells or from heterotypic fusion events between tumor cells and normal cells possessing similar markers, which would also be indistinguishable from nonfused tumor cells. Such indistinguishable or invisible hybrid cells will be referred to as dark matter hybrids, which cannot as yet be detected and quantified, but which contribute to tumor growth and progression.

Cell fusion potentiates tumor heterogeneity and reveals circulating hybrid cells that correlate with stage and survival

High lethality rates associated with metastatic cancer highlight an urgent medical need for improved understanding of biologic mechanisms driving metastatic spread and identification of biomarkers predicting late-stage progression. Numerous neoplastic cell intrinsic and extrinsic mechanisms fuel tumor progression; however, mechanisms driving heterogeneity of neoplastic cells in solid tumors remain obscure. Increased mutational rates of neoplastic cells in stressed environments are implicated but cannot explain all aspects of tumor heterogeneity. We present evidence that fusion of neoplastic cells with leukocytes (for example, macrophages) contributes to tumor heterogeneity, resulting in cells exhibiting increased metastatic behavior. Fusion hybrids (cells harboring hematopoietic and epithelial properties) are readily detectible in cell culture and tumor-bearing mice. Further, hybrids enumerated in peripheral blood of human cancer patients correlate with disease stage and predict overall survival. This unique population of neoplastic cells provides a novel biomarker for tumor staging, as well as a potential therapeutic target for intervention.

Matrix metalloproteinase-9 (MMP9) is involved in the TNF-α-induced fusion of human M13SV1-Cre breast epithelial cells and human MDA-MB-435-pFDR1 cancer cells

Background

In addition to physiological events such as fertilisation, placentation, osteoclastogenesis, or tissue regeneration/wound healing, cell fusion is involved in pathophysiological conditions such as cancer. Cell fusion, which applies to both the proteins and conditions that induce the merging of two or more cells, is not a fully understood process. Inflammation/pro-inflammatory cytokines might be a positive trigger for cell fusion. Using a Cre-LoxP-based cell fusion assay we demonstrated that the fusion between human M13SV1-Cre breast epithelial cells and human MDA-MB-435-pFDR1 cancer cells was induced by the pro-inflammatory cytokine tumour necrosis factor-α (TNF-α).

Methods

The gene expression profile of the cells in the presence of TNF-α and under normoxic and hypoxic conditions was analysed by cDNA microarray analysis. cDNA microarray data were verified by qPCR, PCR, Western blot and zymography. Quantification of cell fusion events was determined by flow cytometry. Proteins of interest were either blocked or knocked-down using a specific inhibitor, siRNA or a blocking antibody.

Results

The data showed an up-regulation of various genes, including claudin-1 (CLDN1), ICAM1, CCL2 and MMP9 in M13SV1-Cre and/or MDA-MB-435-pFDR1 cells. Inhibition of these proteins using a blocking ICAM1 antibody, CLDN1 siRNA or an MMP9 inhibitor showed that only the blockage of MMP9 was correlated with a decreased fusion rate of the cells. Likewise, the tetracycline-based antibiotic minocycline, which exhibits anti-inflammatory properties, was also effective in both inhibiting the TNF-α-induced MMP9 expression in M13SV1-Cre cells and blocking the TNF-α-induced fusion frequency of human M13SV1-Cre breast epithelial cells and human MDA-MB-435-pFDR1 cancer cells.

Conclusions

The matrix metalloproteinase-9 (MMP9) is most likely involved in the TNF-α-mediated fusion of human M13SV1-Cre breast epithelial cells and human MDA-MB-435-pFDR1 cancer cells. Likewise, our data indicate that the tetracycline-based antibiotic minocycline might exhibit anti-fusogenic properties because it inhibits a cell fusion-related mechanism.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0226-1) contains supplementary material, which is available to authorized users.

Endogenous Mobilization of Bone-Marrow Cells Into the Murine Retina Induces Fusion-Mediated Reprogramming of Müller Glia Cells

Müller glial cells (MGCs) represent the most plastic cell type found in the retina. Following injury, zebrafish and avian MGCs can efficiently re-enter the cell cycle, proliferate and generate new functional neurons. The regenerative potential of mammalian MGCs, however, is very limited. Here, we showed that N-methyl-d-aspartate (NMDA) damage stimulates murine MGCs to re-enter the cell cycle and de-differentiate back to a progenitor-like stage. These events are dependent on the recruitment of endogenous bone marrow cells (BMCs), which, in turn, is regulated by the stromal cell-derived factor 1 (SDF1)-C-X-C motif chemokine receptor type 4 (CXCR4) pathway. BMCs mobilized into the damaged retina can fuse with resident MGCs, and the resulting hybrids undergo reprogramming followed by re-differentiation into cells expressing markers of ganglion and amacrine neurons. Our findings constitute an important proof-of-principle that mammalian MGCs retain their regenerative potential, and that such potential can be activated via cell fusion with recruited BMCs. In this perspective, our study could contribute to the development of therapeutic strategies based on the enhancement of mammalian endogenous repair capabilities.

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Endogenous bone marrow cells migrate into NMDA-damaged murine retinae and fuse with retinal Müller glial cells (MGCs).

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MGCs can be reprogrammed to retinal progenitors to then differentiate into ganglion and amacrine neurons.

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Modulation of the SDF1/CXCR4 pathway regulates BMC migration, BMC-MGC fusion, and MGC reprogramming.

Retinal degeneration is present in a large and heterogeneous group of debilitating diseases, often not curable. Cell therapy represents an interesting approach to regenerate injured retinal tissue. However, it comes with some hurdles in terms of engraftment and differentiation of the transplanted cells. Here, we reported that murine Müller glia cells can be converted into retinal neurons after fusion with endogenous bone marrow cells. The efficiency of this mechanism can be enhanced by perturbation of the SDF1/CXCR4 signaling pathway. Our study provides an important proof-of-principle that the limited endogenous regeneration capability of mammals can be enhanced by modulation of specific signaling pathways.

Stem cell therapy in necrotizing enterocolitis: Current state and future directions

Stem cell therapy is a promising treatment modality for necrotizing enterocolitis. Among the many promising stem cells identified to date, it is likely that mesenchymal stem cells will be the most useful and practical cell-based therapies for this condition. Using acellular components such as exosomes or other paracrine mediators are promising as well. Multiple mechanisms are likely at play in the positive effects provided by these cells, and further research is underway to further elucidate these effects.

Fusion between M2-macrophages and cancer cells results in a subpopulation of radioresistant cells with enhanced DNA-repair capacity

Cell fusion is a natural biological process in normal development and tissue regeneration. Fusion between cancer cells and macrophages results in hybrids that acquire genetic and phenotypic characteristics from both maternal cells. There is a growing body of in vitro and in vivo data indicating that this process also occurs in solid tumors and may play a significant role in tumor progression. However, investigations of the response of macrophage:cancer cell hybrids to radiotherapy have been lacking. In this study, macrophage:MCF-7 hybrids were generated by spontaneous in vitro cell fusion. After irradiation, both hybrids and their maternal MCF-7 cells were treated with 0 Gy, 2.5 Gy and 5 Gy γ-radiation and examined by clonogenic survival and comet assays at three time points (0 h, 24 h, and 48 h). Compared to maternal MCF-7 cells, the hybrids showed increased survival fraction and plating efficiency (colony formation ability) after radiation. The hybrids developed less DNA-damage, expressed significantly lower residual DNA-damage, and after higher radiation dose showed less heterogeneity in DNA-damage compared to their maternal MCF-7 cells. To our knowledge this is the first study that demonstrates that macrophage:cancer cell fusion generates a subpopulation of radioresistant cells with enhanced DNA-repair capacity. These findings provide new insight into how the cell fusion process may contribute to clonal expansion and tumor heterogeneity. Furthermore, our results provide support for cell fusion as a mechanism behind the development of radioresistance and tumor recurrence.