Protective effects of EVs/exosomes derived from permanently growing human MSC on primary murine ALS motor neurons

In recent years, the neuroprotective potential of mesenchymal stroma-/stem-like cells (MSC) as well as of MSC-derived extracellular vesicles (EVs) like exosomes has been intensively explored. This included preclinical evaluation regarding treatment of neurodegenerative disorders such as the fatal motor neuron disease amyotrophic Lateral Sclerosis (ALS). Several studies have reported that MSC-derived exosomes can stimulate tissue regeneration and reduce inflammation. MSC release EVs and trophic factors and thereby modify cell-to-cell communication. These cell-free products may protect degenerating motor neurons (MNs) and represent a potential therapeutic approach for ALS. In the present study we investigated the effects of exosomes derived from a permanently growing MSC line on both, wild type and ALS (SOD1G93A transgenic) primary motor neurons. Following application in a normal and stressed environment we could demonstrate beneficial effects of MSC exosomes on neurite growth and morphology indicating the potential for further preclinical evaluation and clinical therapeutic development. Investigation of gene expression profiles detected transcripts of several antioxidant and anti-inflammatory genes in MSC exosomes. Characterization of their microRNA (miRNA) content revealed miRNAs capable of regulating antioxidant and anti-apoptotic pathways.

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?

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.

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.

Human MSC-Derived Exosomes Reduce Cellular Senescence in Renal Epithelial Cells

Cellular senescence of renal tubular cells is associated with chronic diseases and age-related kidney disorders. Therapies to antagonize senescence are, therefore, explored as novel approaches in nephropathy. Exosomes derived from human mesenchymal stroma-/stem-like cells (MSC) entail the transfer of multiple bioactive molecules, exhibiting profound regenerative potential in various tissues, including therapeutic effects in kidney diseases. Here, we first demonstrate that exosomes promote proliferation and reduce senescence in aged MSC cultures. For potential therapeutic perspectives in organ rejuvenation, we used MSC-derived exosomes to antagonize senescence in murine kidney primary tubular epithelial cells (PTEC). Exosome treatment efficiently reduced senescence while diminishing the transcription of senescence markers and senescence-associated secretory phenotype (SASP) factors. Concomitantly, we observed less DNA damage foci and more proliferating cells. These data provide new information regarding the therapeutic property of MSC exosomes in the development of renal senescence, suggesting a contribution to a new chapter of regenerative vehicles in senotherapy.

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.

MSC-Derived Extracellular Vesicles in Tumors and Therapy

Simple Summary

Therapeutic functions of mesenchymal stroma-/stem-like cells (MSCs) are mediated predominantly through paracrine effects by the release of various different components. Upon recruitment of MSCs to damaged tissue sites or tumors, several bioactive molecules and organelles that are secreted by MSCs among others are cytokines, chemokines, metabolites, and extracellular vesicles including exosomes. The MSC-mediated cargo of released exosomes contains specific proteins and nucleic acids with varying regulatory microRNAs according to the tissue origin and the MSC microenvironment. These MSC-released exosomes are taken up by different target cells in damaged tissues to promote a regulatory network of tissue repair, including immune modulation and induction of angiogenesis. Conversely, in tumors, MSC-derived exosomes can confer predominant signals to suppress neovascularization and to relay further tumor-inhibitory effects. However, MSCs that adapted to the tumor tissue by mutual interaction with cancer cells progressively alter to an aberrant phenotype with the release of exosomes carrying tumor-supportive material.

Abstract

Exosomes derived from mesenchymal stroma-/stem-like cells (MSCs) as part of extracellular vesicles are considered cell-free biocompatible nanovesicles that promote repair activities of damaged tissues or organs by exhibiting low immunogenic and cytotoxic effects. Contributions to regenerative activities include wound healing, maintenance of stem cell niches, beneficial regenerative effects in various diseases, and reduction of senescence. However, the mode of action in MSC-derived exosomes strongly depends on the biological content like different regulatory microRNAs that are determined by the tissue origin of MSCs. In tumors, MSCs use indirect and direct pathways in a communication network to interact with cancer cells. This leads to mutual functional changes with the acquisition of an aberrant tumor-associated MSC phenotype accompanied by altered cargo in the exosomes. Consequently, MSC-derived exosomes either from normal tissue-originating MSCs or from aberrant tumor-associated MSCs can confer different actions on tumor development. These processes exhibiting tumor-inhibitory and tumor-supportive effects with a focus on exosome microRNA content will be discriminated and discussed within this review.

Cancer Cell Fusion and Post-Hybrid Selection Process (PHSP)

Fusion of cancer cells either with other cancer cells (homotypic fusion) in local vicinity of the tumor tissue or with other cell types (e.g., macrophages, cancer-associated fibroblasts (CAFs), mesenchymal stromal-/stem-like cells (MSC)) (heterotypic fusion) represents a rare event. Accordingly, the clinical relevance of cancer-cell fusion events appears questionable. However, enhanced tumor growth and/or development of certain metastases can originate from cancer-cell fusion. Formation of hybrid cells after cancer-cell fusion requires a post-hybrid selection process (PHSP) to cope with genomic instability of the parental nuclei and reorganize survival and metabolic functionality. The present review dissects mechanisms that contribute to a PHSP and resulting functional alterations of the cancer hybrids. 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. However, cellular partners during cancer-cell fusion such as MSC within the tumor microenvironment or MSC-derived exosomes may provide a suitable vehicle to specifically address and deliver anti-tumor cargo to cancer cells.

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

Simple Summary

Cell fusion as a fundamental biological process is required for various physiological processes, including fertilization, placentation, myogenesis, osteoclastogenesis, and wound healing/tissue regeneration. However, cell fusion is also observed during pathophysiological processes like tumor development. Mesenchymal stroma/stem-like cells (MSC) which play an important role within the tumor microenvironment like other cell types such as macrophages can closely interact and hybridize with cancer cells. The formation of cancer hybrid cells can involve various different mechanisms whereby the genomic parts of the hybrid cells require rearrangement to form a new functional hybrid cell. The fusion of cancer cells with neighboring cell types may represent an important mechanism during tumor development since cancer hybrid cells are detectable in various tumor tissues. During this rare event with resulting genomic instability the cancer hybrid cells undergo a post-hybrid selection process (PHSP) to reorganize chromosomes of the two parental nuclei whereby the majority of the hybrid population undergoes cell death. The remaining cancer hybrid cells survive by displaying altered properties within the tumor tissue.

Abstract

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.

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

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

Effect of SSRI exposure on the proliferation rate and glucose uptake in breast and ovary cancer cell lines

Breast cancer is the most prevalent malignancy amongst women worldwide while ovarian cancer represents the leading cause of death among gynecological malignancies. Women suffering from these cancers displayed heightened rates of major depressive disorder, and antidepressant treatment with selective serotonin reuptake inhibitors (SSRIs) is frequently recommended. Recently, narrative reviews and meta-analyses showed increased recurrence risks and mortality rates in SSRI-treated women with breast and ovarian cancer. We therefore examined whether three commonly prescribed SSRIs, fluoxetine, sertraline and citalopram, affect proliferation or glucose uptake of human breast and ovarian cancer cell lines characterized by different malignancies and metastatic potential. SSRI treatment or serotonin stimulation with therapeutically relevant concentrations over various time periods revealed no consistent dose- or time-dependent effect on proliferation rates. A marginal, but significant increase in glucose uptake was observed in SK-OV-3 ovarian cancer cells upon fluoxetine or sertraline, but not citalopram treatment. In three breast cancer cell lines and in two additional ovarian cancer cell lines no significant effect of SSRIs on glucose uptake was observed. Our data suggest that the observed increase in recurrence- and mortality rates in SSRI-treated cancer patients is unlikely to be linked to antidepressant therapies.

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

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

The Intimate Relationship Among EMT, MET and TME: A T(ransdifferentiation) E(nhancing) M(ix) to Be Exploited for Therapeutic Purposes

Simple Summary

Intratumoral heterogeneity is considered the major cause of drug resistance and hence treatment failure in cancer patients. Tumor cells are known for their phenotypic plasticity that is the ability of a cell to reprogram and change its identity to eventually adopt multiple phenotypes. Tumor cell plasticity involves the reactivation of developmental programs, the acquisition of cancer stem cell properties and an enhanced potential for retro- or transdifferentiation. A well-known transdifferentiation mechanism is the process of epithelial-mesenchymal transition (EMT). Current evidence suggests a complex interplay between EMT, genetic and epigenetic alterations, and various signals from the tumor microenvironment (TME) in shaping a tumor cell’s plasticity. The vulnerabilities exposed by cancer cells when residing in a plastic or stem-like state have the potential to be exploited therapeutically, i.e., by converting highly metastatic cells into less aggressive or even harmless postmitotic ones.

Abstract

Intratumoral heterogeneity is considered the major cause of drug unresponsiveness in cancer and accumulating evidence implicates non-mutational resistance mechanisms rather than genetic mutations in its development. These non-mutational processes are largely driven by phenotypic plasticity, which is defined as the ability of a cell to reprogram and change its identity (phenotype switching). Tumor cell plasticity is characterized by the reactivation of developmental programs that are closely correlated with the acquisition of cancer stem cell properties and an enhanced potential for retrodifferentiation or transdifferentiation. A well-studied mechanism of phenotypic plasticity is the epithelial-mesenchymal transition (EMT). Current evidence suggests a complex interplay between EMT, genetic and epigenetic alterations, and clues from the tumor microenvironment in cell reprogramming. A deeper understanding of the connections between stem cell, epithelial–mesenchymal, and tumor-associated reprogramming events is crucial to develop novel therapies that mitigate cell plasticity and minimize the evolution of tumor heterogeneity, and hence drug resistance. Alternatively, vulnerabilities exposed by tumor cells when residing in a plastic or stem-like state may be exploited therapeutically, i.e., by converting them into less aggressive or even postmitotic cells. Tumor cell plasticity thus presents a new paradigm for understanding a cancer’s resistance to therapy and deciphering its underlying mechanisms.

Altered Tumor Plasticity after Different Cancer Cell Fusions with MSC

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

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

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

Cell culture media notably influence properties of human mesenchymal stroma/stem-like cells from different tissues

BACKGROUND AIMS

Mesenchymal stroma/stem-like cells (MSCs) are a popular cell source and hold huge therapeutic promise for a broad range of possible clinical applications. However, to harness their full potential, current limitations in harvesting, expansion and characterization have to be overcome. These limitations are related to the heterogeneity of MSCs in general as well as to inconsistent experimental protocols. Here we aim to compare in vitro methods to facilitate comparison of MSCs generated from various tissues.

METHODS

MSCs from 3 different tissues (bone marrow, dental pulp, adipose tissue), exemplified by cells from 3 randomly chosen donors per tissue, were systematically compared with respect to their in vitro properties after propagation in specific in-house standard media, as established in the individual laboratories, or in the same commercially available medium.

RESULTS

Large differences were documented with respect to the expression of cell surface antigens, population doubling times, basal expression levels of 5 selected genes and osteogenic differentiation. The commercial medium reduced differences in these parameters with respect to individual human donors within tissue and between tissues. The extent, size and tetraspanin composition of extracellular vesicles were also affected.

CONCLUSIONS

The results clearly demonstrate the extreme heterogeneity of MSCs, which confirms the problem of reproducibility of results, even when harmonizing experimental conditions, and questions the significance of common parameters for MSCs from different tissues in vitro.

Role of MSC in the Tumor Microenvironment

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

Reversible Growth-Arrest of a Spontaneously-Derived Human MSC-Like Cell Line

Life cycle limitation hampers the production of high amounts of primary human mesenchymal stroma-/stem-like cells (MSC) and limits cell source reproducibility for clinical applications. The characterization of permanently growing MSC544 revealed some differentiation capacity and the simultaneous presence of known MSC markers CD73, CD90, and CD105 even after continuous long-term culture for more than one year and 32 passages. The expression of CD13, CD29, CD44, and CD166 were identified as further surface proteins, all of which were also simultaneously detectable in various other types of primary MSC populations derived from the umbilical cord, bone marrow, and placenta suggesting MSC-like properties in the cell line. Proliferating steady state MSC544 exhibited immune-modulatory activity similar to a subpopulation of long-term growth-inhibited MSC544 after 189d of continuous culture in confluency. This confluent connective cell layer with fibroblast-like morphology can spontaneously contract and the generated space is subsequently occupied by new cells with regained proliferative capacity. Accordingly, the confluent and senescence-associated beta-galactosidase-positive MSC544 culture with about 95% G0/G1 growth-arrest resumed re-entry into the proliferative cell cycle within 3d after sub-confluent culture. The MSC544 cells remained viable during confluency and throughout this transition which was accompanied by marked changes in the release of proteins. Thus, expression of proliferation-associated genes was down-modulated in confluent MSC544 and re-expressed following sub-confluent conditions whilst telomerase (hTERT) transcripts remained detectable at similar levels in both, confluent growth-arrested and proliferating MSC544. Together with the capability of connective cell layer formation for potential therapeutic approaches, MSC544 provide a long term reproducible human cell source with constant properties.

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

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

Taxol-Loaded MSC-Derived Exosomes Provide a Therapeutic Vehicle to Target Metastatic Breast Cancer and Other Carcinoma Cells

MSC-derived exosomes display, among others, an efficient biocompatibility and a reduced intrinsic immunogenicity, representing a valuable vehicle for drug delivery in a tumor-therapeutic approach. Following treatment of several human mesenchymal stroma/stem-like cell (MSC) populations with sub-lethal concentrations of taxol for 24 h, exosomes were isolated and applied to different human cancer populations including A549 lung cancer, SK-OV-3 ovarian cancer, and MDA-hyb1 breast cancer cells. While MSC control exosomes revealed little if any growth inhibition on the tumor cells, exposure to taxol-loaded MSC-derived exosomes was associated with 80–90% cytotoxicity. A similar application of taxol-loaded exosomes from HuVEC displayed much fewer effects. Quantification by LC-MS/MS analysis demonstrated a 7.6-fold reduced taxol concentration in MSC exosomes when compared to equivalent cytotoxic in vitro effects achieved with taxol substances, indicating a specific and more efficient tumor-targeting property. Consequently, MSC-derived taxol exosomes were tested in vivo. Highly metastatic MDA-hyb1 breast tumors were induced in NODscid mice, and systemic intravenous application of MSC-derived taxol exosomes revealed a more than 60% reduction of subcutaneous primary tumors. Moreover, the amount of distant organ metastases observed at least in lung, liver, spleen, and kidney was reduced by 50% with MSC taxol exosomes, similar to the effects observed with taxol, although the concentration of taxol in exosomes was about 1000-fold reduced. Together, these findings in different cancer cell populations and in vivo provide promising future perspectives for drug-loaded MSC-derived exosomes in efficiently targeting primary tumors and metastases by reducing side effects.

Changes in uPA, PAI-1, and TGF-β Production during Breast Cancer Cell Interaction with Human Mesenchymal Stroma/Stem-Like Cells (MSC)

The interactions of cancer cells with neighboring non-malignant cells in the microenvironment play an important role for progressive neoplastic development and metastasis. Long-term direct co-culture of human MDA-MB-231^cherry breast cancer cells with benign human mesenchymal stroma/stem-like cells (MSC) MSC544^GFP stably expressing mCherry and eGFP fluorescence proteins, respectively, was associated with the formation of three-dimensional (3D) tumor spheroids in vitro. The quantification of the breast tumor marker urokinase plasminogen activator (uPA) in mono-cultured MDA-MB-231 cells revealed an approximately 14-fold enhanced expression when compared to five different normal human MSC mono-cultures. Moreover, uPA levels in 3D tumor spheroids remained elevated 9.4-fold above the average of five different human MSC cultures. In contrast, the expression of the corresponding plasminogen activator inhibitor type-1 (PAI-1) declined by 2.6-fold in the breast cancer cells and was even further reduced by 3.2-fold in the MDA-MB-231^cherry/MSC544^GFP 3D co-culture spheroids when compared to the various MSC populations. The supportive data were obtained for the production of TGF-β1, which is an important growth factor in the regulation of tumor growth and metastasis formation. Whereas, TGF-β1 release in MDA-MB-231^cherry/MSC544^GFP co-cultures was elevated by 1.56-fold as compared to MSC544 mono-cultures after 24 h; this ratio further increased to 2.19-fold after 72 h. Quantitative PCR analyses in MSC544 and MDA-MB-231 cells revealed that MSC, rather than the breast cancer cells, are responsible for TGF-β1 synthesis and that TGF-β1 contributes to its own synthesis in these cells. These findings suggested potential synergistic effects in the expression/secretion of uPA, PAI-1, and TGF-β during the co-culture of breast cancer cells with MSC.

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

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

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

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

CDK4/6 inhibitors target SMARCA4-determined cyclin D1 deficiency in hypercalcemic small cell carcinoma of the ovary

Inactivating mutations in SMARCA4 (BRG1), a key SWI/SNF chromatin remodelling gene, underlie small cell carcinoma of the ovary, hypercalcemic type (SCCOHT). To reveal its druggable vulnerabilities, we perform kinase-focused RNAi screens and uncover that SMARCA4-deficient SCCOHT cells are highly sensitive to the inhibition of cyclin-dependent kinase 4/6 (CDK4/6). SMARCA4 loss causes profound downregulation of cyclin D1, which limits CDK4/6 kinase activity in SCCOHT cells and leads to in vitro and in vivo susceptibility to CDK4/6 inhibitors. SCCOHT patient tumors are deficient in cyclin D1 yet retain the retinoblastoma-proficient/p16INK4a-deficient profile associated with positive responses to CDK4/6 inhibitors. Thus, our findings indicate that CDK4/6 inhibitors, approved for a breast cancer subtype addicted to CDK4/6 activation, could be repurposed to treat SCCOHT. Moreover, our study suggests a novel paradigm whereby critically low oncogene levels, caused by loss of a driver tumor suppressor, may also be exploited therapeutically.

RAC1B: A Rho GTPase with Versatile Functions in Malignant Transformation and Tumor Progression

RAC1B is an alternatively spliced isoform of the monomeric GTPase RAC1. It differs from RAC1 by a 19 amino acid in frame insertion, termed exon 3b, resulting in an accelerated GDP/GTP-exchange and an impaired GTP-hydrolysis. Although RAC1B has been ascribed several protumorigenic functions such as cell cycle progression and apoptosis resistance, its role in malignant transformation, and other functions driving tumor progression like epithelial-mesenchymal transition, migration/invasion and metastasis are less clear. Insertion of exon 3b endows RAC1B with specific biochemical properties that, when compared to RAC1, encompass both loss-of-functions and gain-of-functions with respect to the type of upstream activators, downstream targets, and binding partners. In its extreme, this may result in RAC1B and RAC1 acting in an antagonistic fashion in regulating a specific cellular response with RAC1B behaving as an endogenous inhibitor of RAC1. In this review, we strive to provide the reader with a comprehensive overview, rather than critical discussions, on various aspects of RAC1B biology in eukaryotic cells.

MSC stimulate ovarian tumor growth during intercellular communication but reduce tumorigenicity after fusion with ovarian cancer cells

The tumor microenvironment enables important cellular interactions between cancer cells and recruited adjacent populations including mesenchymal stroma/stem cells (MSC). In vivo cellular interactions of primary human MSC in co-culture with human SK-OV-3 ovarian cancer cells revealed an increased tumor growth as compared to mono-cultures of the ovarian cancer cells. Moreover, the presence of MSC stimulated formation of liver metastases. Further interactions of MSC with the ovarian cancer cells resulted in the formation of hybrid cells by cell fusion. Isolation and single cell cloning of these hybrid cells revealed two differentially fused ovarian cancer cell populations termed SK-hyb1 and SK-hyb2. RNA microarray analysis demonstrated expression profiles from both parental partners whereby SK-hyb1 were attributed with more SK-OV-3 like properties and SK-hyb2 cells displayed more similarities to MSC. Both ovarian cancer hybrid populations exhibited reduced proliferative capacity compared to the parental SK-OV-3 cells. Moreover, the fused populations failed to develop tumors in NODscid mice. Together, these data suggested certain stimulatory effects on ovarian tumor growth in the presence of MSC. Conversely, fusion of MSC with SK-OV-3 cells contributed to the generation of new cancer hybrid populations displaying a significantly reduced tumorigenicity.

Histone Deacetylase Inhibitors Synergize with Catalytic Inhibitors of EZH2 to Exhibit Antitumor Activity in Small Cell Carcinoma of the Ovary, Hypercalcemic Type

Small cell carcinoma of the ovary, hypercalcemic type (SCCOHT) is a rare but extremely lethal malignancy that mainly impacts young women. SCCOHT is characterized by a diploid genome with loss of SMARCA4 and lack of SMARCA2 expression, two mutually exclusive ATPases of the SWI/SNF chromatin-remodeling complex. We and others have identified the histone methyltransferase EZH2 as a promising therapeutic target for SCCOHT, suggesting that SCCOHT cells depend on the alternation of epigenetic pathways for survival. In this study, we found that SCCOHT cells were more sensitive to pan-HDAC inhibitors compared with other ovarian cancer lines or immortalized cell lines tested. Pan-HDAC inhibitors, such as quisinostat, reversed the expression of a group of proteins that were deregulated in SCCOHT cells due to SMARCA4 loss, leading to growth arrest, apoptosis, and differentiation in vitro and suppressed tumor growth of xenografted tumors of SCCOHT cells. Moreover, combined treatment of HDAC inhibitors and EZH2 inhibitors at sublethal doses synergistically induced histone H3K27 acetylation and target gene expression, leading to rapid induction of apoptosis and growth suppression of SCCOHT cells and xenografted tumors. Therefore, our preclinical study highlighted the therapeutic potential of combined treatment of HDAC inhibitors with EZH2 catalytic inhibitors to treat SCCOHT.

Concise Review: Crosstalk of Mesenchymal Stroma/Stem-Like Cells with Cancer Cells Provides Therapeutic Potential

Various direct and indirect cellular interactions between multi-functional mesenchymal stroma/stem-like cells (MSCs) and cancer cells contribute to increasing plasticity within the tumor tissue and its microenvironment. Direct and tight communication between MSC and cancer cells is based on membrane protein interactions and the exchange of large plasma membrane fragments also known as trogocytosis. An ultimate but rare direct interaction resumes in fusion of these two cellular partners resulting in the formation of new cancer hybrid cell populations. Alternatively, indirect interactions are displayed by the release of membranous vesicle-encapsulated microRNAs and proteins or soluble components such as molecular growth factors, hormones, chemo-/cytokines, and metabolites. Released single molecules as well as multivesicular bodies including exosomes and microvesicles can form local concentration gradients within the tumor microenvironment and are incorporated not only by adjacent neighboring cells but also affect distant target cells. The present Review will focus on vesicle-mediated indirect communication and on cancer cell fusion with direct contact between MSC and cancer cells. These different types of interaction are accompanied by functional interference and mutual acquisition of new cellular properties. Consequently, alterations in cancer cell functionalities paralleled by the capability to reorganize the tumor stroma can trigger changes in metastatic behavior and promote retrodifferentiation to develop new cancer stem-like cells. However, exosomes and microvesicles acting over long distances may also provide a tool with therapeutic potential when loaded with anti-tumor cargo. Stem Cells 2018;36:951-968.

In Vitro Fusion of Normal and Neoplastic Breast Epithelial Cells with Human Mesenchymal Stroma/Stem Cells Partially Involves Tumor Necrosis Factor Receptor Signaling

Formation of hybrid cells by "accidental cell fusion" of normal and neoplastic breast epithelial cells with local tissue-associated mesenchymal stroma/stem-like cells (MSC) in an inflammatory microenvironment can generate new cancer cell populations whereby molecular signaling mechanisms of this process remain unclear. Fusions of lentiviral enhanced green fluorescent protein-labeled MSC with mcherry-labeled breast epithelial cells were quantified and effects of tumor necrosis factor alpha (TNF-α) and receptor downstream signaling were investigated. Cocultures of MSC with normal human mammary epithelial cells, with neoplastic MCF10A, or with MDA-MB-231 or MCF7 breast cancer cells demonstrated hybrid cell formation between 0.1% and about 2% of the populations within 72 hours, whereby the fusion process occurred in less than 5 minutes. Addition of the pro-inflammatory cytokine TNF-α significantly enhanced MCF10A-MSC cell fusion. Small-interfering RNA (siRNA) knockdown experiments revealed an involvement of tumor necrosis factor (TNF) receptor-1 and -2 in this process. This was also substantiated by siRNA knockdown of tumor necrosis factor receptor type 1-associated death domain which abolished TNF-α-stimulated fusion. While TNF receptor signaling can be relayed via the Mitogen-activated protein kinase 8 (MAPK8), NF-κB or cell death pathways, examination of further downstream signaling exhibited little if any effects of MAPK8 or RelA (p65) on TNF-α-mediated cell fusion, respectively. These data suggested that cell fusion between MSC and MCF10A breast epithelial cells can be stimulated by TNF-α involving TNF receptor-activated cell death pathways or additional NF-κB signaling. Stem Cells 2018;36:977-989.

Ponatinib Shows Potent Antitumor Activity in Small Cell Carcinoma of the Ovary Hypercalcemic Type (SCCOHT) through Multikinase Inhibition

Purpose: Small cell carcinoma of the ovary, hypercalcemic type (SCCOHT) is a rare, aggressive ovarian cancer in young women that is universally driven by loss of the SWI/SNF ATPase subunits SMARCA4 and SMARCA2. A great need exists for effective targeted therapies for SCCOHT.Experimental Design: To identify underlying therapeutic vulnerabilities in SCCOHT, we conducted high-throughput siRNA and drug screens. Complementary proteomics approaches profiled kinases inhibited by ponatinib. Ponatinib was tested for efficacy in two patient-derived xenograft (PDX) models and one cell-line xenograft model of SCCOHT.Results: The receptor tyrosine kinase (RTK) family was enriched in siRNA screen hits, with FGFRs and PDGFRs being overlapping hits between drug and siRNA screens. Of multiple potent drug classes in SCCOHT cell lines, RTK inhibitors were only one of two classes with selectivity in SCCOHT relative to three SWI/SNF wild-type ovarian cancer cell lines. We further identified ponatinib as the most effective clinically approved RTK inhibitor. Reexpression of SMARCA4 was shown to confer a 1.7-fold increase in resistance to ponatinib. Subsequent proteomic assessment of ponatinib target modulation in SCCOHT cell models confirmed inhibition of nine known ponatinib target kinases alongside 77 noncanonical ponatinib targets in SCCOHT. Finally, ponatinib delayed tumor doubling time 4-fold in SCCOHT-1 xenografts while reducing final tumor volumes in SCCOHT PDX models by 58.6% and 42.5%.Conclusions: Ponatinib is an effective agent for SMARCA4-mutant SCCOHT in both in vitro and in vivo preclinical models through its inhibition of multiple kinases. Clinical investigation of this FDA-approved oncology drug in SCCOHT is warranted. Clin Cancer Res; 24(8); 1932-43. ©2018 AACR.

Enhanced metastatic capacity of breast cancer cells after interaction and hybrid formation with mesenchymal stroma/stem cells (MSC)

Background

Fusion of breast cancer cells with tumor-associated populations of the microenvironment including mesenchymal stroma/stem-like cells (MSC) represents a rare event in cell communication whereby the metastatic capacity of those hybrid cells remains unclear.

Methods

Functional changes were investigated in vitro and in vivo following spontaneous fusion and hybrid cell formation between primary human MSC and human MDA-MB-231 breast cancer cells. Thus, lentiviral eGFP-labeled MSC and breast cancer cells labeled with mcherry resulted in dual-fluorescing hybrid cells after co-culture.

Results

Double FACS sorting and single cell cloning revealed two different aneuploid male hybrid populations (MDA-hyb1 and MDA-hyb2) with different STR profiles, pronounced telomerase activities, and enhanced proliferative capacities as compared to the parental cells. Microarray-based mRNA profiling demonstrated marked regulation of genes involved in epithelial-mesenchymal transition and increased expression of metastasis-associated genes including S100A4. In vivo studies following subcutaneous injection of the breast cancer and the two hybrid populations substantiated the in vitro findings by a significantly elevated tumor growth of the hybrid cells. Moreover, both hybrid populations developed various distant organ metastases in a much shorter period of time than the parental breast cancer cells.

Conclusion

Together, these data demonstrate spontaneous development of new tumor cell populations exhibiting different parental properties after close interaction and subsequent fusion of MSC with breast cancer cells. This formation of tumor hybrids contributes to continuously increasing tumor heterogeneity and elevated metastatic capacities.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0215-4) contains supplementary material, which is available to authorized users.

TGF-β-Dependent Growth Arrest and Cell Migration in Benign and Malignant Breast Epithelial Cells Are Antagonistically Controlled by Rac1 and Rac1b

Despite improvements in diagnosis and treatment, breast cancer is still the most common cancer type among non-smoking females. TGF-β can inhibit breast cancer development by inducing cell cycle arrest in both, cancer cells and, as part of a senescence program in normal human mammary epithelial cells (HMEC). Moreover, TGF-β also drives cell migration and invasion, in part through the small GTPases Rac1 and Rac1b. Depletion of Rac1b or Rac1 and Rac1b in MDA-MB-231 or MDA-MB-435s breast cancer cells by RNA interference enhanced or suppressed, respectively, TGF-β1-induced migration/invasion. Rac1b depletion in MDA-MB-231 cells also increased TGF-β-induced p21^WAF1 expression and ERK1/2 phosphorylation. Senescent HMEC (P15/P16), when compared to their non-senescent counterparts (P11/P12), presented with dramatically increased migratory activity. These effects were paralleled by elevated expression of genes associated with TGF-β signaling and metastasis, downregulated Rac1b, and upregulated Rac1. Our data suggest that acquisition of a motile phenotype in HMEC resulted from enhanced autocrine TGF-β signaling, invasion/metastasis-associated gene expression, and a shift in the ratio of antimigratory Rac1b to promigratory Rac1. We conclude that although enhanced TGF-β signaling is considered antioncogenic in HMEC by suppressing oncogene-induced transformation, this occurs at the expense of a higher migration and invasion potential.

Breast Carcinoma: From Initial Tumor Cell Detachment to Settlement at Secondary Sites

Metastasis represents a multistep cascade of cancer cell alterations accompanied by structural and functional changes within the tumor microenvironment which may involve the induction of a retrodifferentiation program. Major steps in metastatic developments include (A) cell detachment from the primary tumor site involving epithelial-mesenchymal transition (EMT), (B) migration and invasion into surrounding tissue, (C) transendothelial intravasation into the vasculature of blood and/or lymphatic vessels as circulating tumor cells (CTCs), (D) dissemination to distant organs, and (E) extravasation of CTCs to secondary sites as disseminated tumor cells (DTCs). This article highlights some aspects of the metastatic cascade with a focus on breast cancer cells. Metastatic steps critically depend on the capability of cancer cells to adapt to distant tissues and the corresponding new microenvironment. As a consequence, increasing plasticity and developmental changes paralleled by acquisition of new cancer cell functionalities challenge a successful therapeutic approach.

The role of TGF-β and its crosstalk with RAC1/RAC1b signaling in breast and pancreas carcinoma

This article focusses on the role of TGF-β and its signaling crosstalk with the RHO family GTPases RAC1 and RAC1b in the progression of breast and pancreatic carcinoma. The aggressive nature of these tumor types is mainly due to metastatic dissemination. Metastasis is facilitated by desmoplasia, a peculiar tumor microenvironment and the ability of the tumor cells to undergo epithelial-mesenchymal transition (EMT) and to adopt a motile and invasive phenotype. These processes are controlled entirely or in part by TGF-β and the small RHO GTPase RAC1 with both proteins acting as tumor promoters in late-stage cancers. Data from our and other studies point to signaling crosstalk between TGF-β and RAC1 and the related isoform, RAC1b, in pancreatic and mammary carcinoma cells. Based on the exciting observation that RAC1b functions as an endogenous inhibitor of RAC1, we propose a model on how the relative abundance or activity of RAC1 and RAC1b in the tumor cells may determine their responses to TGF-β and, ultimately, the metastatic capacity of the tumor.

c-Met expression and activity in urogenital cancers - novel aspects of signal transduction and medical implications

C-Met is a receptor tyrosine kinase with multiple functions throughout embryonic development, organogenesis and wound healing and is expressed in various epithelia. The ligand of c-Met is Hepatocyte Growth Factor (HGF) which is secreted among others by mesenchymal stroma/stem (MSC) cells.

Physiological c-Met functions are centred around processes that underly cellular motility and invasive growth. Aberrant c-Met expression and activity is observed in numerous cancers and makes major contributions to cell malignancy. Importantly, HGF/c-Met signaling is crucial in the context of communication between cancer cells and the the tumor stroma.

Here, we review recent findings on roles of dysregulated c-Met in urogenital tumors such as cancers of the urinary bladder, prostate, and ovary. We put emphasis on novel aspects of cancer-associated c-Met expression regulation on both, HGF-dependent and HGF-independent non-canonical mechanisms. Moreover, this review focusses on c-Met-triggered signalling with potential relevance for urogenital oncogenesis, and on strategies to specifically inhibit c-Met activity.

Cancer stem cell niche models and contribution by mesenchymal stroma/stem cells

Background

The initiation and progression of malignant tumors is driven by distinct subsets of tumor-initiating or cancer stem-like cells (CSCs) which develop therapy/apoptosis resistance and self-renewal capacity. In order to be able to eradicate these CSCs with novel classes of anti-cancer therapeutics, a better understanding of their biology and clinically-relevant traits is mandatory.

Main body

Several requirements and functions of a CSC niche physiology are combined with current concepts for CSC generation such as development in a hierarchical tumor model, by stochastic processes, or via a retrodifferentiation program. Moreover, progressive adaptation of endothelial cells and recruited immune and stromal cells to the tumor site substantially contribute to generate a tumor growth-permissive environment resembling a CSC niche. Particular emphasis is put on the pivotal role of multipotent mesenchymal stroma/stem cells (MSCs) in supporting CSC development by various kinds of interaction and cell fusion to form hybrid tumor cells.

Conclusion

A better knowledge of CSC niche physiology may increase the chances that cancer stemness-depleting interventions ultimately result in arrest of tumor growth and metastasis.

SCCOHT tumors acquire chemoresistance and protection by interacting mesenchymal stroma/stem cells within the tumor microenvironment

Chemotherapeutic drug testing of SCCOHT-1 and BIN-67 tumor cells revealed synergistic growth-inhibition of >95% in vitro with a combination of foretinib and FK228. Application of this drug combination in vivo in NODscid mice-induced SCCOHT-1GFP tumors was associated with ~6-fold reduction in tumor mass within 10 days, whereby synergistic effects of the two compounds remained undetectable compared to previous results with foretinib treatment alone. Histopathologic evaluation revealed a reduced vascularization and a lower amount of proliferating cells in the treated tumors. Surprisingly, a simultaneous significant accumulation of extracellular matrix structures with positive elastin-van Gieson staining was observed following foretinib/FK228 exposure. Expression analysis of treated animal tumors exhibited various changes including increased mouse transcript levels of elastin, laminin, and fibronectin. In parallel, markers for mesenchymal stroma/stem cells (MSC) including CD73 and CD90 were detectable in all mouse tumors suggesting a possible involvement of these cells in extracellular matrix restructure. Indeed, incubation of MSC with FK228 or foretinib/FK228 demonstrated morphologic alterations and enhanced expression of laminin and fibronectin. Moreover, a co-culture of MSC with lentiviral-labeled SCCOHT-1GFP cells contributed to protection of the tumor cells against FK228-mediated cytotoxicity. Furthermore, explant cultures of SCCOHT-1GFP-induced tumors acquired an increased resistance to FK228 and a combination of foretinib/FK228 in contrast to foretinib alone. Together, these data suggested that FK228-mediated extracellular matrix protein expression by MSC contributes to increased protection and enhanced resistance of SCCOHT tumors which could represent a more general mechanism of MSC during drug-induced alterations of a tumor microenvironment.

Interaction of MSC with tumor cells

Tumor development and tumor progression is not only determined by the corresponding tumor cells but also by the tumor microenvironment. This includes an orchestrated network of interacting cell types (e.g. immune cells, endothelial cells, fibroblasts, and mesenchymal stroma/stem cells (MSC)) via the extracellular matrix and soluble factors such as cytokines, chemokines, growth factors and various metabolites. Cell populations of the tumor microenvironment can interact directly and indirectly with cancer cells by mutually altering properties and functions of the involved partners. Particularly, mesenchymal stroma/stem cells (MSC) play an important role during carcinogenesis exhibiting different types of intercellular communication. Accordingly, this work focusses on diverse mechanisms of interaction between MSC and cancer cells. Moreover, some functional changes and consequences for both cell types are summarized which can eventually result in the establishment of a carcinoma stem cell niche (CSCN) or the generation of new tumor cell populations by MSC-tumor cell fusion.

c-Met inhibitors attenuate tumor growth of small cell hypercalcemic ovarian carcinoma (SCCOHT) populations

A cellular model (SCCOHT-1) of the aggressive small cell hypercalcemic ovarian carcinoma demonstrated constitutive chemokine and growth factor production including HGF. A simultaneous presence of c-Met in 41% SCCOHT-1 cells suggested an autocrine growth mechanism. Expression of c-Met was also observed at low levels in the corresponding BIN-67 cell line (6.5%) and at high levels in ovarian adenocarcinoma cells (NIH:OVCAR-3 (84.4%) and SK-OV-3 (99.3%)). Immunohistochemistry of c-Met expression in SCCOHT tumors revealed a heterogeneous distribution between undetectable levels and 80%. Further characterization of SCCOHT-1 and BIN-67 cells by cell surface markers including CD90 and EpCAM demonstrated similar patterns with differences to the ovarian adenocarcinoma cells. HGF stimulation of SCCOHT-1 cells was associated with c-Met phosphorylation at Tyr1349 and downstream Thr202/Tyr204 phosphorylation of p44/42 MAP kinase. This HGF-induced signaling cascade was abolished by the c-Met inhibitor foretinib. Cell cycle analysis after foretinib treatment demonstrated enhanced G2 accumulation and increasing apoptosis within 72 h. Moreover, the IC50 of foretinib revealed 12.4 nM in SCCOHT-1 cells compared to 411 nM and 481 nM in NIH:OVCAR-3 and SK-OV-3 cells, respectively, suggesting potential therapeutic effects. Indeed, SCCOHT-1 and BIN-67 tumor xenografts in NODscid mice exhibited an approximately 10-fold and 5-fold reduced tumor size following systemic application of foretinib, respectively. Furthermore, foretinib-treated tumors revealed a significantly reduced vascularization and little if any c-Met-mediated signal transduction. Similar findings of reduced proliferative capacity and declined tumor size were observed after siRNA-mediated c-Met knock-down in SCCOHT-1 cells demonstrating that in vivo inhibition of these pathways contributed to an attenuation of SCCOHT tumor growth.

Comparison of in vitro-cultivation of human mesenchymal stroma/stem cells derived from bone marrow and umbilical cord

Cell-mediated therapy is currently considered as a novel approach for many human diseases. Potential uses range from topic applications with the regeneration of confined tissue areas to systemic applications. Stem cells including mesenchymal stroma/stem cells (MSCs) represent a highly attractive option. Their potential to cure or alleviate human diseases is investigated in a number of clinical trials. A wide variety of methods has been established in the past years for isolation, cultivation and characterization of human MSCs as expansion is presently deemed a prerequisite for clinical application with high numbers of cells carrying reproducible properties. MSCs have been retrieved from various tissues and used in a multitude of settings whereby numerous experimental protocols are available for expansion of MSCs in vitro. Accordingly, different isolation, culture and upscaling techniques contribute to the heterogeneity of MSC characteristics and the, sometimes, controversial results. Therefore, this review discusses and summarizes certain experimental conditions for MSC in vitro culture focusing on adult bone marrow-derived and neonatal umbilical cord-derived MSCs in order to enhance our understanding for MSC tissue sources and to stratify different procedures. Copyright © 2016 John Wiley & Sons, Ltd.

Massive Clonal Selection and Transiently Contributing Clones During Expansion of Mesenchymal Stem Cell Cultures Revealed by Lentiviral RGB-Barcode Technology

Mesenchymal stem (or stromal) cells (MSCs) have been used in more than 400 clinical trials for the treatment of various diseases. The clinical benefit and reproducibility of results, however, remain extremely variable. During the in vitro expansion phase, which is necessary to achieve clinically relevant cell numbers, MSCs show signs of aging accompanied by different contributions of single clones to the mass culture. Here we used multicolor lentiviral barcode labeling to follow the clonal dynamics during in vitro MSC expansion from whole umbilical cord pieces (UCPs). The clonal composition was analyzed by a combination of flow cytometry, fluorescence microscopy, and deep sequencing. Starting with highly complex cell populations, we observed a massive reduction in diversity, transiently dominating populations, and a selection of single clones over time. Importantly, the first wave of clonal constriction already occurred in the early passages during MSC expansion. Consecutive MSC cultures from the same UCP implied the existence of more primitive, MSC culture-initiating cells. Our results show that microscopically homogenous MSC mass cultures consist of many subpopulations, which undergo clonal selection and have different capabilities. Among other factors, the clonal composition of the graft might have an impact on the functional properties of MSCs in experimental and clinical settings.

SIGNIFICANCE

Mesenchymal stem cells (MSCs) can easily be obtained from various adult or embryonal tissues and are frequently used in clinical trials. For their clinical application, MSCs have to be expanded in vitro. This unavoidable step influences the features of MSCs, so that clinical benefit and experimental results are often highly variable. Despite a homogenous appearance under the microscope, MSC cultures undergo massive clonal selection over time. Multicolor fluorescence labeling and deep sequencing were used to demonstrate the dynamic clonal composition of MSC cultures, which might ultimately explain the variable clinical performance of the cells.

Conditioned umbilical cord tissue provides a natural three-dimensional storage compartment as in vitro stem cell niche for human mesenchymal stroma/stem cells

Background

The use of large amounts of human multipotent mesenchymal stroma/stem cells (MSC) for cell therapies represents a desirable property in tissue engineering and banking in the field of regenerative medicine.

Methods and results

Whereas cryo-storage of umbilical cord (UC) tissue pieces in liquid nitrogen without ingredients was associated with predominant appearance of apoptotic cells after thawing and re-culture, progressive growth of MSC was observed following use of cryo-medium. Moreover, conditioning of UC tissue pieces by initial explant culture and subsequent cryo-storage with cryo-medium accelerated a further MSC culture after thawing. These findings suggested that conditioning of UC tissue pieces provides an in vitro stem cell niche by maintenance of a 3-dimensional natural microenvironment for continuous MSC outgrowth and expansion. Indeed, culture of GFP-labeled UC tissue pieces was accompanied by increased outgrowth of GFP-labeled cells which was accelerated in conditioned UC tissue after cryo-storage. Moreover, cryopreserved conditioned UC tissue pieces in cryo-medium after thawing and explant culture could be cryopreserved again demonstrating renewed MSC outgrowth after repeated thawing with similar population doublings compared to the initial explant culture. Flow cytometry analysis of outgrowing cells revealed expression of the typical MSC markers CD73, CD90, and CD105. Furthermore, these cells demonstrated little if any senescence and cultures revealed stem cell-like characteristics by differentiation along the adipogenic, chondrogenic and osteogenic lineages.

Conclusions

Expression of MSC markers was maintained for at least 10 freeze/thaw/explant culture cycles demonstrating that repeated cryopreservation of conditioned UC tissue pieces provided a reproducible and enriched stem cell source.

Acquisition of new tumor cell properties by MSC-derived exosomes

Interaction between multi-functional mesenchymal stroma/stem cells (MSC) and human tumor cells involves the exchange of biological material via extracellular vesicles including exosomes. Protein analysis of MSC-derived exosomes demonstrated the presence of MMP-2 and MSC-specific markers including CD90 and ecto-5'-nucleotidase (CD73). Incubation of tumor cells with these membranous particles revealed a rapid uptake of MSC-released microvesicles whereby breast cancer cells incorporated ~19% and SCCOHT-1 cells representing a rare type of small cell ovarian cancer assimilated ~28% of available exosomes within 24 h. This interaction was accompanied by functional alterations of tumor cell properties during integration of exosomal content from MSC. Indeed, exosome-associated MMP-2 exhibited functional enzyme activity and MCF-7 breast cancer cells with undetectable MMP-2 protein acquired expression of this enzyme and corresponding gelatinase functionality after stimulation with MSC-derived exosomes. Similar effects were observed in SCCOHT-1 cells during culture in the presence of MSC-derived exosomes which enabled new metabolic activities in this tumor cell type. Together, these findings demonstrated that the internalization of MSC-derived exosomes was associated with the acquisition of new tumor cell properties by altering cellular functionalities and providing the capability to re-organize the tumor microenvironment.

Human mesenchymal stroma/stem cells exchange membrane proteins and alter functionality during interaction with different tumor cell lines

To analyze effects of cellular interaction between human mesenchymal stroma/stem cells (MSC) and different cancer cells, direct co-cultures were performed and revealed significant growth stimulation of the tumor populations and a variety of protein exchanges. More than 90% of MCF-7 and primary human HBCEC699 breast cancer cells as well as NIH:OVCAR-3 ovarian adenocarcinoma cells acquired CD90 proteins during MSC co-culture, respectively. Furthermore, SK-OV-3 ovarian cancer cells progressively elevated CD105 and CD90 proteins in co-culture with MSC. Primary small cell hypercalcemic ovarian carcinoma cells (SCCOHT-1) demonstrated undetectable levels of CD73 and CD105; however, both proteins were significantly increased in the presence of MSC. This co-culture-mediated protein induction was also observed at transcriptional levels and changed functionality of SCCOHT-1 cells by an acquired capability to metabolize 5'cAMP. Moreover, exchange between tumor cells and MSC worked bidirectional, as undetectable expression of epithelial cell adhesion molecule (EpCAM) in MSC significantly increased after co-culture with SK-OV-3 or NIH:OVCAR-3 cells. In addition, a small population of chimeric/hybrid cells appeared in each MSC/tumor cell co-culture by spontaneous cell fusion. Immune fluorescence demonstrated nanotube structures and exosomes between MSC and tumor cells, whereas cytochalasin-D partially abolished the intercellular protein transfer. More detailed functional analysis of FACS-separated MSC and NIH:OVCAR-3 cells after co-culture revealed the acquisition of epithelial cell-specific properties by MSC, including increased gene expression for cytokeratins and epithelial-like differentiation factors. Vice versa, a variety of transcriptional regulatory genes were down-modulated in NIH:OVCAR-3 cells after co-culture with MSC. Together, these mutual cellular interactions contributed to functional alterations in MSC and tumor cells.

Human Umbilical Cord-Derived Mesenchymal Stem Cells Utilize Activin-A to Suppress Interferon-γ Production by Natural Killer Cells

Following allogeneic hematopoietic stem cell transplantation (HSCT), interferon (IFN)-γ levels in the recipient's body can strongly influence the clinical outcome. Human umbilical cord-derived mesenchymal stem cells (UC-MSCs) are lucrative as biological tolerance-inducers in HSCT settings. Hence, we studied the molecular mechanism of how UC-MSCs influence natural killer (NK) cell-mediated IFN-γ production. Allogeneic NK cells were cultured in direct contact with UC-MSCs or cell-free supernatants from mesenchymal stem cell (MSC) cultures (MSC-conditioned media). We found that soluble factors secreted by UC-MSCs strongly suppressed interleukin (IL)-12/IL-18-induced IFN-γ production by NK cells by reducing phosphorylation of STAT4, NF-κB, as well as T-bet activity. UC-MSCs secreted considerable amounts of activin-A, which could suppress IFN-γ production by NK cells. Neutralization of activin-A in MSC-conditioned media significantly abrogated their suppressive abilities. Till date, multiple groups have reported that prostaglandin (PG)-E2 produced by MSCs can suppress NK cell functions. Indeed, we found that inhibition of PGE2 production by MSCs could also significantly restore IFN-γ production. However, the effects of activin-A and PGE2 were not cumulative. To the best of our knowledge, we are first to report the role of activin-A in MSC-mediated suppression of IFN-γ production by NK cells.

Role of gamma-secretase in human umbilical-cord derived mesenchymal stem cell mediated suppression of NK cell cytotoxicity

Background

Mesenchymal stem cells (MSCs) are increasingly considered to be used as biological immunosuppressants in hematopoietic stem cell transplantation (HSCT). In the early reconstitution phase following HSCT, natural killer (NK) cells represent the major lymphocyte population in peripheral blood and display graft-vs-leukemia (GvL) effects. The functional interactions between NK cells and MSCs have the potential to influence the leukemia relapse rate after HSCT. Until date, MSC-NK cell interaction studies are largely focussed on bone marrow derived (BM)-MSCs. Umbilical cord derived (UC)-MSCs might be an alternative source of therapeutic MSCs. Thus, we studied the interaction of UC-MSCs with unstimulated allogeneic NK cells.

Results

UC-MSCs could potently suppress NK cell cytotoxicity in overnight cultures via soluble factors. The main soluble immunosuppressant was identified as prostaglandin (PG)-E2. Maximal PGE2 release involved IL-1β priming of MSCs after close contact between the NK cells and UC-MSCs. Interestingly, blocking gamma-secretase activation alleviated the immunosuppression by controlling PGE2 production. IL-1 receptor activation and subsequent downstream signalling events were found to require gamma-secretase activity.

Conclusion

Although the role of PGE2 in NK cell-MSC has been reported, the requirement of cell-cell contact for PGE2 induced immunosuppression remained unexplained. Our findings shed light on this puzzling observation and identify new players in the NK cell-MSC crosstalk.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-014-0063-9) contains supplementary material, which is available to authorized users.

In vitro and in vivo therapeutic approach for a small cell carcinoma of the ovary hypercalcaemic type using a SCCOHT-1 cellular model

Background

The small cell ovarian carcinoma of the hypercalcemic type (SCCOHT) which preferably affects young women during regenerative age represents a rare and aggressive form of ovarian tumors with poor prognosis and lacks an efficient therapy.

Methods and results

In vitro chemotherapy testing in a fluorescence assay using a recently developed cellular model from a recurrent SCCOHT revealed sensitivity for certain epothilones, methotrexate and topotecan whereas little if any cytotoxicity was observed with other chemotherapeutics including platin-based compounds. In particular, epothilone B demonstrated a high sensitivity in contrast to ixabepilone with only little detectable effects. Western blot and cell cycle analysis revealed that the epothilone B sensitivity was associated with increased Ser¹⁵ phosphorylation of p53, a significant G₁ and G₂ cell cycle accumulation and subsequent cell death in subG₁ phase. Moreover, tubulinβ3 expression in SMARCA4/BRG1-defective SCCOHT-1 in contrast to other ovarian cancer cells was also affected during chemotherapy treatment. Increased extracellular Ca²⁺ levels further enhanced the epothilone B cytotoxicity in SCCOHT-1 cells. These in vitro effects were also confirmed in vivo in NOD/scid mouse xenografts demonstrating an attenuated tumor growth in epothilone B / Ca²⁺-treated mice. After 4d of subsequent treatment, the tumor sizes were reduced by about 90% as compared to continuously growing control tumors. In parallel, a hypercalcemia in control tumor-carrying mice was reverted to normal serum Ca²⁺ levels after epothilone B / Ca²⁺ therapy.

Conclusions

Taken together, these data demonstrated anti-tumorigenic effects of epothilone B / Ca²⁺ in SCCOHT providing a focused therapeutic approach against this rare disease and arising recurrent tumors.

Interference of Ca²⁺ with the proliferation of SCCOHT-1 and ovarian adenocarcinoma cells

A recently established cellular model for the rare small cell carcinoma of the ovary hypercalcemic type (SCCOHT-1) was characterized in comparison to ovarian adenocarcinoma cells (NIH:OVCAR-3 and SK-OV-3). The different cancer populations exhibited a common sensitivity in acidic pH milieu and a continuous proliferation in alkaline medium of pH 8.0-9.0. In the presence of elevated Ca2+ concentrations, the ovarian cancer cells demonstrated a progressively reduced proliferation within 72 h in contrast to other tumor types such as breast cancer cells. This significant growth inhibition was calcium-specific since the proliferation was unaffected after culture of the ovarian cancer cells in the presence of similar concentrations of other cations. The Ca2+ effects on the ovarian cancer cells were associated with marked differences in the activation of intracellular signaling pathways including enhanced phosphorylation of the p42/44 MAP kinase (Thr202/Tyr204). Further analysis of the signaling pathway revealed a significantly enhanced Ca2+-dependent and p42/44 MAP kinase activation-mediated prostaglandin E2 (PGE2) production in SK-OV-3 and SCCOHT-1 and to a lesser extent in NIH:OVCAR-3 cells. Vice versa, exogenous PGE2 did not affect the proliferative capacity of the ovarian cancer cells and inhibition of the Ca2+-mediated MAP kinase activation did not abolish the Ca2+-mediated cytotoxicity. Collectively, these data suggest that multiple pathways are activated by exogenous Ca2+ in the different ovarian cancer cells, including a specific MAP kinase signaling cascade with subsequent PGE2 production and a parallel pathway for the induction of cell death.

Mesenchymal stromal cells derived from human umbilical cord tissues: primitive cells with potential for clinical and tissue engineering applications

Mesenchymal stem or stromal cells (MSCs) have a high potential for cell-based therapies as well as for tissue engineering applications. Since Friedenstein first isolated stem or precursor cells from the human bone marrow (BM) stroma that were capable of osteogenesis, BM is currently the most common source for MSCs. However, BM presents several disadvantages, namely low frequency of MSCs, high donor-dependent variations in quality, and painful invasive intervention. Thus, tremendous research efforts have been observed during recent years to find alternative sources for MSCs.In this context, the human umbilical cord (UC) has gained more and more attention. Since the UC is discarded after birth, the cells are easily accessible without ethical concerns. This postnatal organ was found to be rich in primitive stromal cells showing typical characteristics of bone-marrow MSCs (BMSCs), e.g., they grow as plastic-adherent cells with a fibroblastic morphology, express a set of typical surface markers, and can be directly differentiated at least along mesodermal lineages. Compared to BM, the UC tissue bears a higher frequency of stromal cells with a higher in vitro expansion potential. Furthermore, immune-privileged and immune-modulatory properties are reported for UC-derived cells, which open highly interesting perspectives for clinical applications.