Differential expression of DNA topoisomerase II alpha and -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 -beta (TopuII alpha and -beta) to resistance for TopoII-targeting drugs. The cell lines did not overexpress P-glycoprotein or the multidrug resistance-associated protein but were cross-resistant to other TopoII drugs. 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 defined 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 protein levels were both decreased (TopoII alpha to 31%:TopoII beta protein was undetectable). The decrease in TopoII alpha protein in GLC(4)/VM(20x) and GLC(4)/MIT(60x), was mediated by decreased TopoII alpha mRNA levels. Loss of TopoII alpha gene copies contributed to the mRNA decrease in these cell lines. Only in the GLC(4)/MIT(60x) cell line was an accumulation defect observed for the drug against which the cell line was made resistant. In conclusion, TopoII alpha and -beta levels were decreased differentially in the resistant cell lines, suggesting that resistance to these drugs may be mediated by a decrease in a specific isozyme.