Fig. 4.

Forced expression of TOP2α/90 in K562 cells reduces etoposide sensitivity. (A) Representative immunoassay using cellular lysates isolated from empty vector or pcDNA/TOP2a/90-transfected K562 cells. (B) Etoposide (1 and 2 μM)-induced DNA damage in empty vector and pcDNA/TOP2α/90-transfected K562 cells was determined by neutral comet assays (assessing DNA double-strand breaks) after a 1-hour incubation and subtraction of DMSO vehicle controls, as described in Materials and Methods. Results shown are the mean ± S.E.M. for three experiments run on separate days. For all experimental conditions in each experiment, greater than 100 cells were evaluated by OpenComet software. *P < 0.025, comparing pcDNA/TOP2α/90-transfected to empty vector-transfected K562 cells. (C) Etoposide (10 and 25 µM)-induced cytotoxicity in empty vector- and pcDNA/TOP2α-transfected K562 cells was assessed by clonogenic assays. Results shown are the mean ± S.E.M. for six experiments run on separate days. *P < 0.025, comparing pcDNA/TOP2α/90-transfected to empty vector-transfected K562 cells.