DETERMINANTS OF RESPONSE TO THE DNA TOPOISOMERASE-II INHIBITORS DOXORUBICIN AND ETOPOSIDE IN HUMAN LUNG-CANCER CELL-LINES

Background: Small-cell lung cancer (SCLC) is more sensitive to anticancer agents than non-small-cell lung cancer (NSCLC), but few studies have analyzed the mechanisms of natural drug resistance responsible for this difference. Purpose: To elucidate these mechanisms, we determined drug sensitivity and evaluated the biochemical parameters affecting response to the DNA topoisomerase II inhibitors doxorubicin and etoposide in both types of cancer cell lines, in particular the activity and content of DNA topoisomerase II, as well as etoposide uptake and cell doubling time. Methods: Drug sensitivity and cellular uptake of etoposide were determined by clonogenic assay and accumulation of radiolabeled drug, respectively. The topoisomerase II activity was assayed by decatenation of kinetoplast DNA to minicircle DNA using nuclear protein, and the content was determined by immunoblot analysis of nuclear extracts. We also compared the topoisomerase II content in parent cell lines with that in lines with cisplatin resistance acquired in vitro. Results: Sensitivities to doxorubicin and etoposide were higher in SCLC cell lines than in NSCLC lines, and the difference was statistically significant. Etoposide uptake in SCLC cells was higher than in NSCLC cells; the difference was statistically significant, but this difference may not be sufficient to account for the variation in sensitivities of the cell lines. Topoisomerase II activities of nuclear protein from SCLC cell lines were reproducibly twofold higher than those for NSCLC cell lines. The topoisomerase II content in nuclear protein appeared to be higher in SCLC cell lines than in NSCLC cell lines and corresponded to the sensitivities to doxorubicin and etoposide. In the cisplatin-resistant NSCLC cell lines PC-7/CDDP and PC-14/CDDP, the topoisomerase II content was increased compared with that in the parent lines, but the topoisomerase II content in other cisplatin-sensitive parent lines was similar to that in resistant sublines. Conclusions: These findings suggest that the topoisomerase II activity and content may be major factors in determining sensitivity to topoisomerase II inhibitors.