Background: A variety of different tumors including breast cancer cells can closely interact with mesenchymal stroma/stem-like cells (MSC) in the tumor microenvironment eventually resulting in cell fusion and formation of new hybrid cancer cell populations displaying altered properties.
Methods: Lentiviral-transduced MDA-MB-231 cherry breast cancer cells and MSC GFP were co-cultured and a resulting hybrid cancer cell population (MDA-MSC-hyb5) was isolated. Characterization was performed for marker expression and short tandem repeat (STR) fragment analysis compared to the parental cells. Moreover, in vivo tumor development and metastatic capacity of MDA-MSC-hyb5 was studied and unique properties were analyzed by RNA microarray expression analyses compared to other breast cancer hybrid populations. Potential chemotherapeutic sensitivity was carried out in tumor explant cultures of MDA-MSC-hyb5 cells.
Results: Direct cellular interactions of MDA-MB-231 cherry breast cancer cells with human MSC GFP in a co-culture model resulted in spontaneous cell fusion by generation of MDA-MSC-hyb5 cherry GFP breast cancer hybrid cells. Proliferative capacity of MDA-MSC-hyb5 cells was about 1.8-fold enhanced 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 MDA-MSC-hyb5 cells initially remained quiescent in a dormancy-like state. At distinct time points up to about a half year later after injection NODscid mice started to develop MDA-MSC-hyb5 cell-induced tumors. Following tumor initiation, 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 as 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 tumor development of MDA-MSC-hyb5 cells suggesting unstable in vivo phenotypes of MDA-hyb5 cells with increased tumor heterogeneity.
Conclusions: The spontaneous formation of cancer hybrid cell populations like MDA-MSC-hyb5 by cell fusion contributes to tumorigenic diversification by acquisition of new properties such as altered chemotherapeutic responsiveness. The unique tumor dormancy of MDA-MSC-hyb5 cells not observed in other breast cancer hybrid cells so far markedly increases tumor heterogeneity.