Abstract
Because of its semi-solid character in dissemination and growth, advanced ovarian cancer appears to be an excellent in vivo model for studying the cancer stem cell hypothesis. The exact nature of these tumor-initiating cells endowed with the properties of anoikis-resistance and unlimited self-renewal is not clear at present, however.
Until now, no universal marker(s) has been found to faithfully isolate (ovarian) cancer stem cells. As these multipotent cells are known to possess highly elaborated efflux systems for cytotoxic agents, these pump systems have been exploited to outline putative stem cells as a side-population (SP) via dye exclusion analysis.
An alterantive is to isolate stem cells based on surface markers. In the Vienna variant of the ovarian cancer cell line A2780 a proof-of-principle model with both a stable SP and a stable ALDH1A1+ cell population was established.
Double staining clearly revealed that both cell fractions were not identical. Of note, A2780V cells were negative for expression of surface markers CD44 and CD117 (c-kit).
Cultured on monolayers of human mesothelial cells, GFP- transfected SP of A2780V exhibited spheroid-formation, whereas non-side-population (NSP) developed a spare monolayer growing over the mesothelium. A2780V SP was found to be partially resistant to platinum, but this resistance could not be explained by over-expression of the "excision repair cross-complementation group 1" (ERCC1) gene, which is essentially involved in the repair of platinated DNA damage.
ERCC1 was, nonetheless, over-expressed in A2780V cells grown as spheres under stem cell-selective conditions. The same was true for the primary ovarian cancer cells B-57.In summary our investigations indicate that even in multi-passaged cancer cell lines hierarchic government of growth and differentiation is conserved and that the key cancer stem cell population may be composed of small overlapping cell fractions defined by various arbitrary markers.