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. 2003 Apr 11;100(9):5205–5210. doi: 10.1073/pnas.0935978100

Figure 4.

Figure 4

Acidic pH cytotoxicity is reduced in TOP2-deficient cells. Cytotoxicity was measured by using the clonogenic assay as described in Materials and Methods. (A) Acidic pH cytotoxicity is reduced in TOP2-deficient MDA-MB-231/3000 cells. MDA-MB-231 and TOP2-deficient MDA-MB-231/3000 (attached cells) were treated with 50 μM VP-16, 1 μM CPT, pH 4.5 medium, or pH 7.5 medium for 30 min. Cells were then replenished with drug-free neutral medium and incubated for ≈2 wk. Clonogenic survival was measured by counting colonies on plates. (B) Acidic pH cytotoxicity is reduced in TOP2-deficient T47D/VP1 cells. T47D and TOP2-deficient T47D/VP1 cells (attached cells) were treated with 50 μM VP-16 or pH 4.5 medium for 30 min. Cells were then incubated in drug-free neutral medium for 2 wk. Clonogenic survival was also measured as described in A. (C) Acidic pH cytotoxicity is reduced in TOP2-deficient HL-60/MX2 cells. HL-60 and TOP2-deficient HL-60/MX2 cells (suspension cells) were treated with 2 μM VM-26 or pH 4.5 medium for 30 min. Cells were then resuspended in soft agar (in drug-free neutral medium), and clonogenic survival in soft agar was measured after ≈2 wk as described in Materials and Methods.