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Mesenchymal stem cells raise great expectations in regenerative medicine due to their capacity to regenerate damaged tissues, thereby restoring organ tissue integrity and functionality. Even though it is not yet clear how mesenchymal stem cells are guided to injured tissue it is generally assumed that the directed migration of these cells is facilitated by the same soluble factors that also recruit immune competent cells to inflamed tissue areas. Tumor tissue represents another type of (chronically) inflamed tissue and because of that mesenchymal stem cells are highly attracted. Although some data indicate that esenchymal stem cells might have a beneficial effect on tumor growth due to anti-tumor effects the plethora of data suggest that tumor tissue recruited mesenchymal stem cells rather promote tumor growth and metastasis formation. Nonetheless, the enhanced tumor tropism of mesenchymal stem cells makes them ideal candidates for novel anti-cancer strategies. Like Trojan Horses genetically modified mesenchymal stem cells will deliver their deadly cargo, such as anti-tumor cytokines or oncolytic viruses, into cancerous tissues, thereby destroying the tumor form within. In this chapter we will summarize the current concepts of genetic modification of mesenchymal stem cells for future anti-cancer therapies.

Addition of the inflammatory cytokine interleukin (IL)-6 to the culture medium of human cord blood haematopoietic stem and progenitor cells (HSPCs) has been shown to lead to an altered stromal cell-derived factor-1alpha-dependent migratory phenotype. This study investigated whether this effect was attributed to a differential engagement of protein kinase C (PKC) isotypes. The migratory activity of both Flt3-ligand and Flt3-ligand/IL-6 cultured cord blood HSPCs was PKC-alpha dependent on day 1, but PKC-alpha independent after 5 d of cultivation. PKC-alpha expression was not down-regulated in cells cultured for 5 d indicating a switch of signalling molecules directing cell migration.

Stromal cell-derived factor-1alpha (SDF-1alpha) is a strong migratory stimulant for hematopoietic stem and progenitor cells (HSPCs). The hematopoietic cytokines thrombopoietin (TPO), Flt3-ligand (FL), stem cell factor (SCF) and interleukin 11 (IL-11) are able to stimulate amplification of primitive murine hematopoietic stem cells (HSCs) in vitro. The effects of these cytokines on SDF-1alpha-induced migratory activity of murine Lin(-)c-kit+ HSPC were analyzed by cultivation of these cells in the presence of 12 combinations of FL, TPO, SCF and IL-11. Migratory activity was measured in a three-dimensional collagen matrix using time-lapse video microscopy. Each cytokine combination had a distinct effect on SDF-1alpha-stimulated migratory activity. For instance, FL- and SCF-cultivated cells showed a high migratory SDF-1alpha response, while cells cultivated with SCF, TPO and IL-11 did not react to SDF-1alpha stimulation with an elevated migration rate. Our data indicate that the differences in the migratory SDF-1alpha response are not related to different CXCR4 expression levels, but rather to the differential engagement of the CXCR4-dependent MAPK p42/44 and PI3K signal transduction pathways. This indicates that hematopoietic cytokines can have a significant impact on SDF-1alpha-stimulated migratory activity and the underlying intracellular signaling processes in cultivated HSPCs.

Migration is an innate and fundamental cellular function that enables hematopoietic stem cells (HSCs) and endothelial progenitors (EPCs) to leave the bone marrow, relocate to distant tissue, and to return to the bone marrow. An increasing number of studies demonstrate the widening scope of the therapeutic potential of both HSCs and endothelial cells. Therapeutic success however not only relies upon their ability to repair damaged tissue, but is also fundamentally dependent on the migration to these areas. Extensive in vivo and in vitro research efforts have shown that the most significant effects seen on HSC migration are initiated by the chemokine SDF-1alpha. In this review we will elucidate the many cellular and systemic factors of HSC and EPC cell migration and their modi operandi.

The overall mechanism of bone marrow-derived stem cell (BMDC) trans-differentiation seems to be simple: BMDCs trans-differentiate as referred to the blueprint, which is given by the tissue itself. Thereby, the blueprint can be the local tissue micro-environment (defined by the tissue-specific cytokine, chemokine, adhesion molecule pattern, etc.), it can be a single cell (cell fusion), or it can be a combination of both. In fact stem cell trans-differentiation is a complex not yet fully understood process. In between the start- and stop-points of transdifferentiation several gene reprogramming steps have to occur in a sequential step-by-step manner, for which a defined set of instructions is a prerequisite to ensure an accurate transdifferentiation. However, a recent study indicated that the ability of BMDCs - to adopt tissue function by reading its blueprint - seems to be a double-edged sword since BMDCs that have received a faulty blueprint, provided by chronically inflamed tissue, trans-differentiated into a neoplastic phenoytpe. Here, we review the importance of an accurate blueprint for BMDC trans-differentiation and discuss a model showing that BMDCs might contribute to overall tumor development due to recruitment to tumor tissue.

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

Periodontium-derived stem cells (pdSCs) can be cultured as dentospheres and differentiated into various cells of the neuronal lineage such as glial cells, thereby demonstrating their stem cell state. This study investigated whether pdSCs could be differentiated into the osteogenic lineage and, if so, whether these cells are able to regenerate periodontal tissue in vivo in an athymic rat model. Human adult pdSCs were isolated during minimally invasive periodontal surgery and expanded in vitro. To induce osteogenic differentiation, expanded pdSCs were cultured for 3 weeks in osteogenic differentiation media. Staining for alkaline phosphatase expression was positive, suggesting osteogenic differentiation. For in vivo studies, pdSCs were delivered onto suitable collagen sponges and implanted into periodontal defects on the right buccal cortex of the mandible in 16 immunodeficient nude rats. Histologic analysis of samples from the test side revealed reformation of periodontal ligament-like tissue, collagen fibers, and elements of bone, but no functional periodontal tissue regeneration. The data show that human adult pdSCs are capable of regenerating elements of bone and collagen fibers in an in vivo animal model. [ABSTRACT FROM AUTHOR]

There is a growing list of data indicating that cancer (stem) cells could functionally adapt foreign tissue features, such as endothelial-like cells or neuroendocrine cells, express lineage markers or could differentiate into various lineages in response to appropriate differentiation criteria. The finding that cancer (stem) cells may possess some kind of differentiation capacity poses the question whether this might be an inherent or acquired property. Cancer stem cells share stem cell characteristics and may thus possess an inherent differentiation capacity enabling the cells to respond to various differentiation stimuli. Considering the plasticity of cancer (stem) cells, even non-tumorigenic (and putatively non-differentiable) tumor cells could give rise to tumorigenic tumor stem cells, exhibiting stem cell characteristics including an inherent differentiation capacity. On the contrary, cancer (stem) cells may have acquired differentiation capacity as a consequence of a previous cell fusion event with cell types exhibiting differentiation potential and being fusogenic, such as macrophages or stem cells. Of pivotal interest in a tumor context are macrophages, which chiefly foster the chronically inflamed tumor microenvironment. Because chronically inflamed tissue is a well-known trigger for cell fusion and both macrophages and stem cells are highly fusogenic we conclude that cell fusion events between these cell types and cancer (stem) cells should frequently occur, thereby giving rise to hybrid cells exhibiting not only novel properties, like an enhanced metastatogenic phenotype, but also parental characteristics, such as differentiation capacity. Conceivably, the combination of both properties might be advantageous for metastasizing cancer (stem) cells to adapt better and faster to a foreign organ tissue environment. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study has been to characterize adult human somatic periodontium-derived stem cells (PDSCS) isolated from human periodontium and to follow their differentiation after cell culture. PDSCS were isolated from human periodontal tissue and cultured as spheres in serum-free medium. After 10 days the primary spheres were dissociated and the secondary spheres sub-cultured for another 1–2 weeks. Cells from different time points were analyzed, and immunohistochemical and electron microscopic investigations carried out. Histological analysis showed differentiation of spheres deriving from the PDSCS with central production of extracellular matrix beginning 3 days after sub-culturing. Isolated PDSCS developed pseudopodia which contained actin. Tubulin was found in the central portion of the cells. Pseudopodia between different cells anastomosed, indicating intercellular transport. Immunostaining for osteopontin demonstrated a positive reaction in primary spheres and within extracellular matrix vesicles after sub-culturing. In cell culture under serum-free conditions human PDSCS form spheres which are capable of producing extracellular matrix. Further investigations have do be carried out to investigate the capability of these cells to differentiate into osteogenic progenitor cells. [ABSTRACT FROM AUTHOR]

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

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

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