Efflux pumps may mediate relative chemoresistance of ALDH^high cells.

<p>(A) ALDH^high and ALDH^low TC71 cells were incubated with Hoechst 33342 for 45 minutes, followed by a 45 minute wash out period. Average cellular fluorescence was quantified as relative fluorescence units (RFU). Assay was performed in triplicate at least three times. Bars = SEM. (B) Unsorted TC71 cells, as well as ALDH^high and ALDH^low cells, were incubated for 48 hours with 500 nM doxorubicin, 10 µM verapamil, or both. Surviving cells were quantified based on Trypan blue exclusion. Assays were performed in triplicate at least three times. Bars = SEM. (C) Expression of EWS-FLI1 in ALDH^high cells as detected by RT-PCR. RNA was isolated from ALDH^high, ALDH^low, and unsorted TC71 cells. RT-PCR was performed using primers specific for EWS-FLI1 and for ribosomal RNA 36B4 as a positive control. Human myelomonocytic leukemia cell line U937 serves as a negative control.