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DNA topoisomerase II is an enzyme that affects nuclear structure and function and is the target of a number of anticancer drugs in clinical use, including teniposide (VM-26). We have used our polyclonal antisera that recognize both the M(r) 170,000 and 180,000 forms of topoisomerase II to examine the nuclear distribution of topoisomerase II in cytospin preparations of drug-sensitive (CEM) and VM-26-resistant (CEM/VM-1 and CEM/VM-1-5) human leukemic lymphoblasts. We have also examined the nuclear distribution of topoisomerase II in monolayer cultures of a human rhabdomyosarcoma (Rh30) cell line. In the absence of drug, we observed a focal "patchy" staining of nuclear topoisomerase II in all cell lines, that was especially notable in the lymphoblastic cells. Treatment of CEM and Rh30 cells with VM-26 under conditions that increase the number of covalent topoisomerase II-DNA complexes increased both the intensity and the homogeneity of nuclear topoisomerase II staining in a subpopulation of cells; focal staining was less evident after treatment with drug. These responses were roughly proportional to the concentration of VM-26 used and required only brief (approximately 25-min) incubation with drug. We also found that treatment of CEM cells with 4'-(9-acridinylamino)methanesulfon-m-anisidide similarly increased the intensity and homogeneity of nuclear topoisomerase II immunostaining. In contrast, 4'-(9-acridinylamino)methanesulfon-o-anisidide and 1-beta-D-arabinofuranosylcytosine, agents that do not inhibit topoisomerase II, did not produce this effect. Finally, the VM-26-mediated alteration in topoisomerase II staining intensity and distribution was attenuated in proportion to the degree of VM-26 resistance in the CEM/VM-1 and CEM/VM-1-5 sublines. These results appear to be related to the ability of the drug to stabilize DNA-topoisomerase covalent ("cleavable") complexes in intact cells. Our findings indicate that anti-topoisomerase II drugs, such as VM-26, have profound effects on the ability to detect topoisomerase II in the nucleus and provide a novel way of examining drug-stabilized DNA topoisomerase II complexes in intact single tumor cells.

Our human T-cell leukemia line, CEM/VM-1, selected for resistance to VM-26 (teniposide), is cross-resistant to several drugs that interact with topoisomerase II, including VP-16 (etoposide), 4'-(9-acridinylamino)methanesulphon-m-anisidide, daunorubicin, and mitoxantrone. However, in contrast to cell lines exhibiting multidrug resistance (MDR) associated with overexpression of P-glycoprotein, this line is not cross-resistant to the Vinca alkaloids, is not impaired in drug accumulation, and does not overexpress the mdrl gene (Cancer Res., 47: 1297, 5455, 1987). More recently we found that nuclear extracts of these cells exhibit decreased topoisomerase II catalytic and cleavage activity, compared to the drug-sensitive line (Biochemistry, 1988). These results suggest that an alteration in topoisomerase II or a modulator of this enzyme may be responsible for this altered topoisomerase II-form of multidrug resistance (at-MDR). In the present work, we studied the somatic cell genetics of at-MDR. We produced hybrid cell lines by polyethylene glycol-mediated fusion of the CEM/VM-1 line with a hypoxanthine-guanine phosphoribosyl transferase-deficient, ouabain-resistant CEM line (CEM.AG1.OU1.5) that exhibits VM-26 sensitivity. Ten of the hybrid lines that grew in selective medium were randomly chosen for expansion and four were analyzed for both DNA content by flow cytometry and VM-26 sensitivity in a 72-h growth inhibition assay. The hybrid lines all contained approximately 2x DNA compared to unfused controls, indicating that the fusions were successful. The IC50 for VM-26 in 3 of the 4 lines was the same as that of the sensitive controls, ranging from 4.7 to 7.4 x 10(-8) M, and another was 76 x 10(-8) M. These data indicate that drug sensitivity was reconstituted by the hybridization procedure. By comparison, the VM-26 IC50 values in the CEM/VM-1 cells and CEM/VM-1 x CEM/VM-1 control "fusions" were 360 and 750 x 10(-8) M, respectively. To determine whether a topoisomerase II-mediated function was reconstituted in the hybrids, we measured drug-stimulated DNA cleavage ("cleavable complex formation"). Using 32P-labeled pBR322 DNA as substrate with nuclear extracts from drug sensitive cells, 100 microM VM-26 maximally stimulated DNA cleavage by approximately 11-fold compared to no-drug controls.(ABSTRACT TRUNCATED AT 400 WORDS)