Investigators first determined that stem cells from rhesus monkey bone marrow, mouse skeletal muscle and murine embryonic stem cells, which constitute the 'side population' of effluxing cells, expressed ABCG2 (Zhou et al, 2001).
Thus, side population or drug efflux status and sphere forming efficiency may be related at least partially via CD133 expression.
In G-2 cell cultures and primary cells purified from WAP-T tumors, for example, no "side population" as measured by efflux of the Hoechst dye could been observed (data not shown), and no correlation was found between metabolic Aldefluor conversion and tumorigenic activity.
Here, we investigated the 'side population' phenotype based on efflux properties of ATP-binding cassette transporters in freshly isolated human glioblastoma samples and intracranial xenografts derived thereof.
Using fluorescence-activated cell sorting (FACS) analysis of irradiated animals stained with Hoechst, two cell populations can be discerned that are lost compared to non-irradiated animals: the 'X1' gate, which represents cells in the cell cycle with >2C DNA; and the 'X2' gate, which registers as a <2C DNA population due to Hoechst efflux.
Fairly simple phenotypes have so far been used as stem cell markers, defined by the presence of a maximum of 10 to 12 antigens or by the presence of a particular cell function, such as transmembrane efflux (SP population) [ 6] or enzymatic activity (aldehyde dehydrogenase) [ 7].
All the paclitaxel- and doxorubicin-selected variants and SI populations displayed increased doxorubicin efflux, although this is not associated with significant increases in resistance in the doxorubicin-selected variants.
Importantly, the level of expression between stromal and brain side populations was very similar, confirming similar efflux properties in these two populations.
Population II represents the CO2 efflux values from a deep source of a magmatic origin and Population III may represent gases resulting from mixing of diffuse and bubbling degassing.
