Reduced expression of DNA topoisomerase II confers resistance to etoposide (VP-16) in small cell lung cancer cell lines established from a refractory tumor of a patient and by in vitro selection.

Cell lines, LC-5 and LC-172, were established from tumors of a small cell lung cancer patient prior to and after combination chemotherapy including etoposide (VP-16), when drug-resistant tumors developed in relapse. A VP-16-resistant cell line, LC-172/VP, was selected from the LC-172 cells in culture in multiple steps with VP-16. LC-172 cells were 3.5-fold resistant to VP-16 in growth inhibition, and 3.3-fold resistant to adriamycin as compared with LC-5 cells. LC-172/VP cells showed large differences in cross-resistance to topoisomerase II-targeting drugs such as VP-16, 200-fold, adriamycin, 10-fold, and MST-16, 4.3-fold; the cells were moderately refractory, 5.5-fold, to vincristine. VP-16 accumulation in the cells was similar in three cell lines. Topoisomerase II unknotting activity was reduced 7- to 10-fold in LC-172/VP and 1.5- to 2-fold in LC-172 cells compared with LC-5 cells, while relaxing activity of topoisomerase I appeared to be unchanged. Topoisomerase II protein was also reduced 5- to 10-fold in LC-172/VP and marginally so in LC-172 cells. Topoisomerase II alpha and II beta were each reduced 10-fold and 2-fold, respectively, in LC-172/VP cells, while they were both slightly decreased (-1.5-fold), respectively, in LC-172 cells compared with LC-5 cells. No apparent alteration in ATP requirement for catalytic activity and in sensitivity to VP-16 was observed for topoisomerase II from the three cell lines. Taken together, these results suggested that resistance to VP-16 in LC-172 and LC-172/VP is associated with a quantitative reduction in expression of topoisomerase II alpha of the parental type.