Polyethylene Glycol-Mediated Fusion between Primary Mouse Mesenchymal Stem Cells and Mouse Fibroblasts Generates Hybrid Cells with Increased Proliferation and Altered Differentiation

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.

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.

Mesenchymal stromal cells for cartilage repair in osteoarthritis

Treatment for articular cartilage damage is quite challenging as it shows limited repair and regeneration following injury. Non-operative and classical surgical techniques are inefficient in restoring normal anatomy and function of cartilage in osteoarthritis (OA). Thus, investigating new and effective strategies for OA are necessary to establish feasible therapeutic solutions. The emergence of the new discipline of regenerative medicine, having cell-based therapy as its primary focus, may enable us to achieve repair and restore the damaged articular cartilage. This review describes progress and development of employing mesenchymal stromal cell (MSC)-based therapy as a promising alternative for OA treatment. The objective of this review is to first, discuss how in vitro MSC chondrogenic differentiation mimics in vivo embryonic cartilage development, secondly, to describe various chondrogenic differentiation strategies followed by pre-clinical and clinical studies demonstrating their feasibility and efficacy. However, several challenges need to be tackled before this research can be translated to the clinics. In particular, better understanding of the post-transplanted cell behaviour and learning to enhance their potency in the disease microenvironment is essential. Final objective is to underscore the importance of isolation, storage, cell shipment, route of administration, optimum dosage and control batch to batch variations to realise the full potential of MSCs in OA clinical trials.

Lipopolysaccharide (LPS) Promotes Apoptosis in Human Breast Epithelial × Breast Cancer Hybrids, but Not in Parental Cells

Toll-like receptors (TLRs) belong to the group of pathogen recognition receptors known to play a crucial role in the innate immune system. In cancer, TLR expression is still debated controversially due to contradictory results reporting that both induction of apoptosis as well as tumor progression could depend on TLR signaling, whereby recent data rather indicate a pro-tumorigenic effect. The biological phenomenon of cell fusion has been associated with cancer progression due to findings revealing that fusion-derived hybrid cells could exhibit properties like an increased metastatogenic capacity and an increased drug resistance. Thus, M13MDA435 hybrid cell lines, which derived from spontaneous fusion events between human M13SV1-EGFP-Neo breast epithelial cells and human MDA-MB-435-Hyg breast cancer cells, were investigated. Cultivation of cells in the presence of the TLR4 ligand LPS potently induced apoptosis in all hybrid clones, but not in parental cells, which was most likely attributed to differential kinetics of the TLR4 signal transduction cascade. Activation of this pathway concomitant with NF-κB nuclear translocation and TNF-α expression was solely observed in hybrid cells. However, induction of LPS mediated apoptosis was not TNF-α dependent since TNF-α neutralization was not correlated to a decreased amount of dead cells. In addition to TNF-α, LPS also caused IFN-β expression in hybrid clones 1 and 3. Interestingly, hybrid clones differ in the mode of LPS induced apoptosis. While neutralization of IFN-β was sufficient to impair the LPS induced apoptosis in M13MDA435-1 and -3 hybrids, the amount of apoptotic M13MDA435-2 and -4 hybrid cells remained unchanged in the presence of neutralizing IFN-β antibodies. In summary, the fusion of non-LPS susceptible parental human breast epithelial cells and human breast cancer cells gave rise to LPS susceptible hybrid cells, which is in view with the cell fusion hypothesis that hybrid cells could exhibit novel properties.

Cardiac Stem Cell Hybrids Enhance Myocardial Repair

RATIONALE

Dual cell transplantation of cardiac progenitor cells (CPCs) and mesenchymal stem cells (MSCs) after infarction improves myocardial repair and performance in large animal models relative to delivery of either cell population.

OBJECTIVE

To demonstrate that CardioChimeras (CCs) formed by fusion between CPCs and MSCs have enhanced reparative potential in a mouse model of myocardial infarction relative to individual stem cells or combined cell delivery.

METHODS AND RESULTS

Two distinct and clonally derived CCs, CC1 and CC2, were used for this study. CCs improved left ventricular anterior wall thickness at 4 weeks post injury, but only CC1 treatment preserved anterior wall thickness at 18 weeks. Ejection fraction was enhanced at 6 weeks in CCs, and functional improvements were maintained in CCs and CPC+MSC groups at 18 weeks. Infarct size was decreased in CCs, whereas CPC+MSC and CPC parent groups remained unchanged at 12 weeks. CCs exhibited increased persistence, engraftment, and expression of early commitment markers within the border zone relative to combinatorial and individual cell population-injected groups. CCs increased capillary density and preserved cardiomyocyte size in the infarcted regions suggesting CCs role in protective paracrine secretion.

CONCLUSIONS

CCs merge the application of distinct cells into a single entity for cellular therapeutic intervention in the progression of heart failure. CCs are a novel cell therapy that improves on combinatorial cell approaches to support myocardial regeneration.

A biotin-streptavidin-biotin bridge dramatically enhances cell fusion

Although the generation of hybrid cells by cell fusion plays a significant role in biotechnology and biomedicine, the low cell-fusion rates and the limitation of large-scale cell fusion for clinical applications of the two widely used approaches, polyethylene-glycol (PEG)-mediated cell fusion and electrofusion, hinder the application of this critical technology in certain key areas, including cancer immunotherapy. In the present study, a simple procedure that can not only significantly increase the heterologous cell fusion but is also capable of producing fused cells on a large scale is reported. A biotin-streptavidin-biotin (BSB) bridge was created by coating one to-be-fused cell with biotin and the other with biotin-streptavidin. The BSB bridge enhances cell-fusion rates induced with PEG fusion or electrofusion by 10–30% depending on the cell types when compared with cell fusions without the bridge. The procedure described increases heterologous cell pairing and eliminates the alignment step required for the majority of electrofusions. Notably, it can be used to make large-scale cell fusions for clinical applications.

Mechanical and In Vitro Biocompatibility of Brushite Cement Modified by Polyethylene Glycol

Brushite (dicalcium phosphate dihydrate, DCPD) cement, owing to its high solubility in physiological condition and ability to guide new bone formation, is widely used to treat bone defects. In the present study, we have evaluated the effects of poly ethylene glycol (PEG) addition on the setting time, compressive strength and in vitro biocompatibility of brushite cement. The brushite cements were prepared by mixing β-tricalcium phosphate [β-TCP, Ca(3)(PO(4))(2)] and monocalcium phosphate monohydrate [MCPM, Ca(H(2)PO(4))(2). H(2)O]. PEG was introduced at 2.0 and 5.0 wt% with the liquid. Introduction of PEG resulted in marginal increase in both initial and final setting time; however, significantly affected the compressive strength. Effects of PEG incorporation on in vitro biocompatibility of brushite cements were studied by using human fetal osteoblast cells (hFOB) cells. Field emission scanning electron microscope (FESEM) images and immunohistochemical analysis indicated that pure and PEG incorporated brushite cement facilitates cell adhesion, proliferation and differentiation. Fewer cells expressed vinculin protein with increased PEG content in the cement. Cell proliferation was found to decrease with increased PEG concentration while the cell differentiation increased with PEG content. Our results provide a better understanding of in vitro biocompatibility of PEG added brushite cements that can be used to customize the cement compositions based on application need.

Phenotypic characteristics of hybrid cells generated by transferring neuronal nuclei into bone marrow stromal cell cytoplasts

Bone marrow stromal cells (BMSCs) are promising donor cells for transplantation therapies for a variety of diseases. However, there still lack efficient ways to induce directional differentiation of BMSCs to promote their practical use in transplantation therapy. In this study, we constructed hybrid cells by transferring neuronal nuclei into BMSC cytoplasts and investigated the proliferative capacity and phenotypic characteristics of the hybrid cells. The neuronal nuclei were labeled with Hoechst 33342 before the transfer process, and the cell membrane antigen CD71 was used as a marker of BMSC cytoplasts. The BMSC cytoplasts and neuronal karyoplasts were separated by Ficoll density gradient ultracentrifugation. The hybrid cells were generated by the polyethylene glycol-mediated fusion of BMSC cytoplasts with neuronal karyoplasts. The hybrid cells exhibited Hoechst 33342 staining in their nuclei and CD71 staining on their cytomembranes, which confirmed the success of cell fusion. The hybrid cells were positive for BrdU immunostaining. Viability analysis of the cultured hybrid cells by the MTT assay demonstrated their proliferative ability. Immunocytochemical staining revealed the expression of the neuron-specific markers NeuN and MAP2 in the third passage hybrid cells, which indicated their neuronal phenotypic characteristics. The results demonstrated that the hybrid cells produced by fusing neuronal karyoplasts with BMSC cytoplasts had proliferative capability and expressed the neuron-specific markers. Further study is required to investigate the phenotype of the hybrid cells both structurally and functionally.

Pancreatic ductal cells acquire mesenchymal characteristics through cell fusion with bone marrow-derived mesenchymal stem cells and SIRT1 attenuates the apoptosis of hybrid cells

Bone marrow-derived mesenchymal stem cells (bMSCs) contribute to tissue repair and regeneration. Cell fusion between somatic cells and bMSCs to form hybrid cells may have an important role in tissue repair through the subsequent reprogramming of the somatic cell nucleus. Few studies have assessed the mesenchymal characteristics of fusion-induced hybrid cells and their survival mechanisms. In this study, we investigated the effect of cell fusion on the biological characteristics of pancreatic ductal cells (PDCs) and on the survival mechanism of hybrid cells. To this end, we generated mouse-mouse hybrid cells in vitro by polyethylene glycol-mediated fusion of primary mouse bMSCs with primary mouse PDCs. Hybrid cells showed an enhanced capacity for proliferation and self-renewal compared with PDCs. No PDC had the capacity for anchorage-independent growth or invasion into Matrigel, but some hybrid cells were able to form colonies in soft agar and invade Matrigel. Expression of the tumor suppressor protein p53, which initiates apoptosis, was detected in hybrid cells but not in PDCs or bMSCs. However, the p53 deacetylase, sirtuin 1 (SIRT1), was also detected in hybrid cells, and the level of acetylated p53, the active form, was low. The addition of nicotinamide (Nam) inhibited the deacetylation activity of SIRT1 on p53 and induced cell apoptosis in hybrid cells. This study demonstrated that PDCs could obtain high proliferation rates, self-renewal capabilities, and mesenchymal characteristics by fusion with bMSCs. SIRT1 expression in the hybrid cells attenuated their apoptosis.

Cell Fusion, Drug Resistance and Recurrence CSCs

Cancer stem cells (CSCs) are a rare population of cancer cells exhibiting stem cell properties, such as self-renewal, differentiation and tissue restoration. Beside the initiation of the primary tumor, CSCs have also been associated with metastasis formation and cancer relapses. In the context of cancer relapses, we have recently postulated the existence of so-called recurrence CSCs (rCSCs). These specific CSC subtype will initiate relapses exhibiting an "oncogenic resistance" phenotype, which are characterized by a markedly increased malignancy concomitant with a drug resistance towards first line therapy. In the present chapter we will discuss the necessity of rCSCs as a distinct CSC subtype and that cell fusion could be one mechanism how rCSCs could originate.

Adipogenic differentiation of adult equine mesenchymal stromal cells

Equine adipose tissue-derived mesenchymal stem cells (ASCs) have only recently been investigated for their adipogenic, chondrogenic, and osteogenic differentiation potential. This chapter will briefly outline the molecular mechanisms leading to adipogenesis and the methods of equine adipose tissue harvest, ASC isolation, and adipogenic differentiation. The reader is also directed to other reported methods of adipogenesis for ASCs and mesenchymal stem cells (MSCs) from other tissues.

BMP-2 and FGF-2 synergistically facilitate adoption of a cardiac phenotype in somatic bone marrow c-kit+/Sca-1+ stem cells

The aim of this study was to explore the effect of bone morphogenetic protein-2 (BMP-2) and fibroblast growth factor-2 (FGF-2)- paracrine factors implicated in both cardiac embryogenesis and cardiac repair following myocardial infarction (MI)-on murine bone marrow stem cell (mBMSC) differentiation in an ex vivo cardiac microenvironment. For this purpose, green fluorescent protein (GFP) expressing hematopoietic lineage negative (lin-) c-kit ligand (c-kit) and stem cell antigen-1 (Sca-1) positive (GFP-lin-/c-kit+/sca+) mBMSC were co-cultured with neonatal rat ventricular cardiomyocytes (NVCMs). GFP+ mBMSC significantly induced the expression of BMP-2 and FGF-2 in NVCMs, and approximately 4% GFP+ mBMSCs could be recovered from the co-culture at day 10. The addition of BMP-2 in concert with FGF-2 significantly enhanced the amount of integrated GFP+ mBMSCs by 5-fold ( approximately 20%), whereas the addition of anti-BMP-2 and/or anti-FGF-2 antibodies completely abolished this effect. An analysis of calcium cycling revealed robust calcium transients in GFP+ mBMSCs treated with BMP-2/FGF-2 compared to untreated co-cultures. BMP-2 and FGF-2 addition led to a significant induction of early (NK2 transcription factor related, locus 5; Nkx2.5, GATA binding protein 4; GATA-4) and late (myosin light chain kinase [MLC-2v], connexin 43 [Cx43]) cardiac marker mRNA expression in mBMSCs following co-culture. In addition, re-cultured fluorescence-activated cell sorting (FACS)-purified BMP-2/FGF-2-treated mBMSCs revealed robust calcium transients in response to electrical field stimulation which were inhibited by the L-type calcium channel (LTCC) inhibitor, nifedipine, and displayed caffeine-sensitive intracellular calcium stores. In summary, our results show that mBMSCs can adopt a functional cardiac phenotype through treatment with factors essential to embryonic cardiogenesis that are induced after cardiac ischemia. This study provides the first evidence that mBMSCs with long-term self-renewal potential possess the capability to serve as a functional cardiomyocyte precursor through the appropriate paracrine input and cross-talk within an appropriate cardiac microenvironment.

Recurrence cancer stem cells--made by cell fusion?

Within the past 10-15 years our knowledge about cancer and how cancer cells might originate has changed dramatically. It is now generally believed that a tumor has its origin in cancer stem cells (CSCs), which originated either from transformed tissue stem cells or transformed progenitor cells that have regained self-renewal activity. CSCs share several characteristics of normal stem cells, such as self-renewal capacity, slow cell cycle activity, differentiation capacity, possessing an enhanced resistance towards cytotoxic agents and radiation, as well as tissue restoration capacity. Due to the increased drug and radiation resistance and slow cell cycle activity concomitant with tumor initiation capacity it is generally assumed that recurrent cancers originate from first line therapy surviving CSCs. But how does the CSC hypothesis explain "oncogenic resistance", which describes the phenomenon that most recurrent cancers are characterized by both an increased malignancy as well as resistance towards first line cancer therapy. To us, "oncogenic resistance" can not be simply attributed to the resistance properties of normal CSCs. If so, the recurring tumor should be treatable by first line therapy, which is mostly not the case. Thus, we conclude that "oncogenic resistance" demands a new type of tumor initiating cells, the so-called recurrence CSCs (rCSCs). This type of tumor initiating cell originates during first line therapy and is characterized by giving rise to first line therapy resistant and highly malignant progenies. Because several characteristics of "oncogenic resistance", such as increased drug resistance, increased resistance to apoptosis and an enhanced malignancy have been linked to cell fusion we further conclude that rCSCs might originate from this cellular event. However, which cell types have to fuse with each other to ultimately give rise to rCSCs is not clear. In any case, tumor tissues, particularly those being destructed by first line therapy comprise of a variety of fusogenic cells including tumor cells and CSCs as well as recruited monocytes/macrophages and bone marrow-derived stem cells. The fusogenic properties of these cells concomitant with phenotypic heterogeneity, which is also a property of cell fusion, will then lead to the origin of rCSCs. In accordance with Darwinian evolution only those cells will survive that can resist best to the selection pressure first line therapy.

Cell fusion: from hybridoma to dendritic cell-based vaccine

The deployment of dendritic cell (DC) and tumor cell fusions is increasing in tumor immunotherapy. In animal and human studies, fusion cell vaccines have been shown to possess the elements essential for processing and presenting tumor antigens to host immune cells, for inducing effective immune response and for breaking T-cell tolerance to tumor-associated antigens. Moreover, fusion cell vaccines provide protection against challenge with tumor cells and mediate regression of established tumors. Despite these unique features of fusion cell vaccines and the observation of tumor eradication in animal studies, limited success has occurred in clinical trials. This article reviews the methods used for optimizing the preparation and selection of DC-tumor fusion cells and analyzes factors influencing the success or failure of fusion cell-mediated immunotherapy. In addition, we discuss the challenges facing effective fusion cell vaccine production, including factors in preparation, selection and quality control of fusion cell vaccines, as well as approaches for enhancing anti-tumor immunity.

Epigenetic reprogramming of nonreplicating somatic cells for long-term proliferation by temporary cell-cell contact

Embryonic stem (ES) cells are potential sources of tissue regeneration; however, transplanted ES cells produce tumors in the host tissues. In addition, transplantation between genetically unrelated individuals often results in graft rejection. Although the development of patient specific stem cell lines by somatic cell nuclear transfer (SCNT) represents a means of overcoming the problem of rejection, its human application has ethical dilemmas. Adult stem (AS) cells can also differentiate into specialized cells and may provide an alternative source of cells for human applications. In common with other somatic cells, AS cells have limited capacity for proliferation and cannot be produced in large quantities without genetic manipulation. We demonstrate here that nonreplicating mammalian cells can be reprogrammed for long-term proliferation by temporary cell-cell contact through coculture of AS cells with the GM05267-derived F7 mouse cell line. Subsequent elimination of F7 cells from the co-culture allows proliferation of previously nonreplicating cells, colonies of which can be isolated to produce cell lines. We also demonstrate that the epigenetically reprogrammed AS cells, without the physical transfer of either nuclear or cytoplasmic material from other cells, are capable of long-term proliferation and able to relay signals to other nonreplicating cells to reinitiate proliferation with no addition of recombinant factors. The reported cell amplification procedure is methodologically simple and can be easily reproduced. This procedure allows the production of an unlimited number of cells from a limited number of AS cells, making them an ideal source of cells for applications involving autologous cell transplantation.