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Background: Several physiological (fertilization, placentation, wound healing) and pathophysiological processes (infection with enveloped viruses, cancer) depend on cell fusion. In cancer it was postulated that the fusion of cancer cells with normal cells such as macrophages or stem cells may not only give rise to hybrid cells exhibiting novel properties, such as an increased metastatic capacity and drug resistance, but possibly also cancer stem/ initiating cell properties. Hence, hybrid clone cells (M13HS, M13MDA435 and M13MDA231) that were derived from spontaneous fusion events of human M13SV1-EGFP-Neo breast epithelial cells and HS578T-Hyg, MDA-MB-435-Hyg and MDA-MB-231-Hyg cancer cells were investigated regarding potential in vitro cancer stem/ initiating cell properties.Methods: CD44/CD24 expression pattern and ALDH1 activity of parental cells and hybrid clones was determined by flow cytometry. A colony formation and mammosphere formation assay was applied to determine the cells' capability to form colonies and mammospheres. Sox9, Slug and Snail expression levels were determined by Western blot analysis.Results: Flow cytometry revealed that all hybrid clone cells were CD44+/CD24-/low, but differed markedly among each other regarding ALDH1 activity. Likewise, each hybrid clone possessed a unique colony formation and mammosphere capacity as well as unique Snail, Slug and Sox9 expression patterns. Nonetheless, comparison of hybrid clones revealed that M13HS hybrids exhibited more in vitro cancer stem/ initiating cell properties than M13MDA231 and M13MDA435 hybrids, such as more ALDH1 positive cells or an increased capacity to form colonies and mammospheres.Conclusion: The fate whether cancer stem/ initiating cells may originate from cell fusion events likely depends on the specific characteristics of the parental cells. [ABSTRACT FROM AUTHOR]

The biological phenomenon of cell fusion has been linked to tumor progression because several data provided evidence that fusion of tumor cells and normal cells gave rise to hybrid cell lines exhibiting novel properties, such as increased metastatogenic capacity and an enhanced drug resistance. Here we investigated M13HS hybrid cell lines, derived from spontaneous fusion events between M13SV1-EGFP-Neo breast epithelial cells exhibiting stem cell characteristics and HS578T-Hyg breast cancer cells, concerning CCL21/CCR7 signaling. Western Blot analysis showed that all cell lines varied in their CCR7 expression levels as well as differed in the induction and kinetics of CCR7 specific signal transduction cascades. Flow cytometry-based calcium measurements revealed that a CCL21 induced calcium influx was solely detected in M13HS hybrid cell lines. Cell migration demonstrated that only M13HS hybrid cell lines, but not parental derivatives, responded to CCL21 stimulation with an increased migratory activity. Knockdown of CCR7 expression by siRNA completely abrogated the CCL21 induced migration of hybrid cell lines indicating the necessity of CCL21/CCR7 signaling. Because the CCL21/CCR7 axis has been linked to metastatic spreading of breast cancer to lymph nodes we conclude from our data that cell fusion could be a mechanism explaining the origin of metastatic cancer (hybrid) cells. [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]