The Proportion of ALDEFLUOR-Positive Cancer Stem Cells Changes with Cell Culture Density Due to the Expression of Different ALDH Isoforms

Abstract

A significant number of discrepancies exist within the literature regarding ALDEFLUOR-
positive stem cell populations in cell lines. We hypothesized that these inconsistencies
resulted from differences in culture conditions, particularly cell density. We cultured several
colon cancer cell lines (N=8) at high and low densities and found a significant decrease in
ALDEFLUOR-positive cell populations at high density. However, we found no changes in the
CD166-positive stem cell population, self-renewal, or cell cycle distribution of cells cultured
at different densities. Interestingly, when we sorted both ALDEFLUOR positive and negative
populations from the different density cultures, we identified a significant number of Aldehyde
dehydrogenase (ALDH) isoforms whose expression was decreased in ALDEFLUOR-positive
stem cells cultured at high density. This novel finding suggests that multiple ALDH isoforms
contribute to ALDEFLUOR activity in colon cancer stem cells and decreases in ALDEFLUOR-
positive stem cells at high cell density are due to decreased expression of multiple ALDH
isoforms. Thus, designing therapeutics to target ALDEFLUOR-positive cancer stem cells may
require inhibition of multiple ALDH isoforms.