DIFFERENTIAL EXPRESSION OF DNA TOPOISOMERASE-II-ALPHA AND TOPOISOMERASE-BETA IN P-GP AND MRP-NEGATIVE VM26, MAMSA AND MITOXANTRONE-RESISTANT SUBLINES OF THE HUMAN SCLC CELL-LINE GLC(4)

Sublines of the human small-cell lung carcinoma (SCLC) cell line GLC(4 ) with acquired resistance to teniposide, amsacrine and mitoxantrone ( GLC(4)/VM(20x), GLC(4)/AM(3x) and GLC(4)/MIT(60x), respectively) were derived to study the contribution of DNA topoisomerase II alpha and -b eta (TopuII alpha and -beta) to resistance for TopoII-targeting drugs. The cell lines did not overexpress P-glycoprotein or the multidrug re sistance-associated protein but were cross-resistant to other TopoII d rugs. GLC(4)/VM(20x) showed a major decrease in TopoII alpha protein ( 54%; for all assays presented in this paper the GLC(4) level was defin ed to be 100%) without reduction in TopoII beta protein; GLC(4)/AM(3x) showed only a major decrease in TopoII beta protein (to 18%) and not in TopoII alpha. In GLC(4)/MIT(60x), the TopoII alpha and -beta protei n levels were both decreased (TopoII alpha to 31%:TopoII beta protein was undetectable). The decrease in TopoII alpha protein in GLC(4)/VM(2 0x) and GLC(4)/MIT(60x), was mediated by decreased TopoII alpha mRNA l evels. Loss of TopoII alpha gene copies contributed to the mRNA decrea se in these cell lines. Only in the GLC(4)/MIT(60x) cell line was an a ccumulation defect observed for the drug against which the cell line w as made resistant. In conclusion, TopoII alpha and -beta levels were d ecreased differentially in the resistant cell lines, suggesting that r esistance to these drugs may be mediated by a decrease in a specific i sozyme.