Establishment and characterization of a mouse FM3A cell mutant resistant to topoisomerase II-inhibitor NC-190.

We established an NC-190-resistant cell line, FM/NC-R, from the murine mammary carcinoma cell line FM3A and examined some of its characteristics. FM/NC-R cells were prepared by mutagen treatment followed by exposure to NC-190 in the culture medium. FM/NC-R cells were 76.5 times more resistant against NC-190 than FM3A cells as measured by their growth in vitro. FM/NC-R cells also showed cross-resistance to etoposide with NC-190. Neither NC-190 nor etoposide increased the lifespan of FM/NC-R-bearing mice at doses that prolonged the lifespan of FM3A-bearing mice more than four times. This resistance was not due to the change in the concentration of NC-190 in the cells, and there was no change in the expression of P-glycoprotein, a drug efflux pump in the cells. NC-190 and etoposide are inhibitors of DNA topoisomerase II, but there was no difference in cellular content of DNA topoisomerase II between the two cell lines as determined by Western blot analysis. The stabilization of DNA-DNA topoisomerase II cleavable complexes induced by NC-190 was lost in FM/NC-R cells. It was found that Gly881, which is located in the ATP binding site, was replaced by Arg in topoisomerase IIα of FM/NC-R cells. These results indicate that the NC-190-resistant cell line FM/NC-R contains a mutated DNA topoisomerase IIα.