BREAST CANCER RESISTANCE PROTEIN - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 342

(a)

(b)

(c)

(d)

Hoechst Blue

FIGURE 12.2. The stem cell phenotype does not always identify cancer stem cells. NCI-H460
cells ( a , b ), Pgp-overexpressing SW620 Ad300 cells ( c ), or ABCG2-overexpressing MCF-7
FLV100 cells ( d ) were incubated in 2.5 g/mL Hoechst 33342 for 30 minutes, washed, then
incubated in Hoechst-free medium for an additional 60 minutes. For ( b ), cells were incubated
in Hoechst in the presence of 10 M FTC to prevent ABCG2-mediated Hoechst transport.

this is because the entire population, rather than a subset of cells, expresses ABCG2.
As shown in Figure 12.2 a , when NCI-H460 cells are incubated with Hoechst 33342,
half of the population of cells is in the SP gate, and this phenomenon is reversed
by the addition of the ABCG2-specific inhibitor FTC (Figure 12.2 b ). When Pgp-
positive SW620 Ad300 cells are incubated with Hoechst, generating the plot shown
in Figure 12.2 c , they give the appearance of an SP population, as is the case for
ABCG2-overexpressing MCF-7 FLV500 cells (Figure 12.2 d ). Drug-resistant cells
that overexpress ABCG2 are not considered stem cells. Although ABCG2 may be
expressed in stem cells as a protection from xenobiotics, it cannot define a stem cell
in the normal or malignant state.

12.12. CONCLUSIONS

Identification of ABCG2 in drug-resistant cancer cells opened a line of investigation
that has led to multiple new insights into normal human physiology and xenobiotic
protection. However, the original hope—that a new therapeutic target for drug resis-
tance reversal has been identified—has not yet been tested or validated in clinical