Breast stem cells spontaneously fuse with breast cancer cells: Impacts on cancer stem cell formation?

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA 5007 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. By now CSCs are known as cancer initiating cells with characteristic features of stem cells like self-renewing, differentiation, tissue restoration at low cell numbers and drug resistance. This rare population of cancer initiating cells attracts attention as a target in cancer therapies. M13SV1-EGFP-Neo breast stem cells were co-cultured with HS578T-Hyg and MDA-MB-435-Hyg, respectively, breast cancer cells. After 24h cell were cultured under selective conditions (+ G418, + Hyg) and cell clones 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. Characterization of hybrid cells revealed variations among them as well as in comparison to their parental cell lines. Proliferation studies revealed higher proliferation rates of distinct hybrid cell clones of up to 1.5-fold in comparison to the appropriate parental breast cancer cells and up to 10-fold as compared to the breast stem cell line, respectively. RealTime-PCR-based gene expression pattern analyses, which have also been validated by Western-Blot analysis, revealed multiple up- and down-regulation of cancer and drug metabolism genes in hybrid cell clones as compared to their appropriate parental relatives. For instance, distinct M13HS and M13MDA hybrid cells, derived from M13SV1-EGFP-Neo and HS578T-Hyg and M13SV1-EGFP-Neo and MDA-MB-435-Hyg fusion events, respectively, showed an up-regulation of drug resistance transporters such as ABCC6, ABCG2 and Major Vault Protein as well as EGFR up-regulation. We further observed androgen receptor up-regulation, but estrogen receptor-1 down-regulation in distinct M13SV1 and M13MDA hybrid cell lines suggesting that fusion of breast stem cells with breast cancer cells might cause a switch to an estrogen-independent tumor growth. FACS analysis of adhesion molecules revealed differential expression of β1-integrin expression levels in M13MDA hybrid cells, which correlated with differential adhesive properties of these cells. Interestingly, cell migration studies using the 3D-collagen matrix migration assay revealed that both the diverse hybrid cell clones and the parental cells possess similar migratory activities and susceptibilities toward stimulatory agents. Nevertheless, the finding that breast stem cell/ breast cancer cell hybrids exhibit a higher proliferatory activity and increased expression levels of drug resistance transporters indicate that such fusion events might contribute to tumor progression and possible to the evolution of a CSC phenotype.