EXPRESSION OF TOPOISOMERASE-II-ALPHA AND TOPOISOMERASE-II-BETA IN CHINESE-HAMSTER LUNG-CELLS RESISTANT TO TOPOISOMERASE-II INHIBITORS

Chinese hamster lung cells resistant to 9-OH-ellipticine, i.e., DC-3F/ 9-OH-E cells, are several hundredfold resistant to DNA topoisomerase I I inhibitors. According to previous studies, this resistance is associ ated with reduced topoisomerase II activity (about 4-fold) and decreas ed capacity of the topoisomerase II inhibitors to induce stabilization of the cleavable complex (about 10-fold). In the present work, an ant ibody was raised against a fragment of human topoisomerase II alpha. T his antibody, which recognizes both isoforms, was used to determine th e amounts of topoisomerases II alpha and beta in the sensitive and res istant cells. Northern and immunoblot analyses showed that (i) in the parental DC-3F cells the alpha enzyme is about 20-fold more abundant t han the beta enzyme and the enzyme isoforms undergo reciprocal regulat ion during the cell growth phases, with the expression of the alpha en zyme dropping at the plateau phase while the expression of the beta en zyme increases, and (ii) in the resistant cells the amount of alpha en zyme is about 4-5-fold smaller than that in the sensitive cells, where as the beta enzyme is almost undetectable. Analysis of DNA restriction sites in several independently selected resistant subclones revealed some rearrangements in the beta gene in two clones. However, these gen e alterations did not correlate with changes in the resistance level. The relative contribution of these different changes to the resistance phenotype is discussed.