Hyperthermia, thermotolerance and topoisomerase II inhibitors

The cytoxicity of both intercalating (m-AMSA) and non-intercalating (VP16, VM26) topoisomerase II-targeting drugs is thought to occur via trapping DNA topoisomerase II on DNA in the form of cleavable complexes. First, analysis of cleavable complexes (detected as DNA double-strand breaks) by pulsed-field gel electrophoresis confirmed the correlation between cleavable complex formation and cytotoxicity of three topoisomerase-targeting drugs in HeLa S3 cells (the order of effects being VM26 > m-AMSA > VP16). In contrast to many antineoplastic agents, hyperthermic treatments were found to protect cells against the toxicity of all three topoisomerase II drugs. Hyperthermia treatment does not alter drug accumulation but reduces the ability of the drug-topoisomerase II complex to form the cleavable complexes. Nuclear protein aggregation induced by heat at the sites of topoisomerase II-DNA interaction may explain such an effect. In thermotolerant cells, the toxic effects of VP16 but not m-AMSA were reduced. For both drugs, however, the status of thermotolerance did not affect cleavable complex formation by the drugs. Thus, protection against VP-16 toxicity seems not to be associated with heat-induced activation of the P-gp 170 pump or altered topoisomerase II-DNA interactions. Rather, a protective (heat shock protein mediated?) mechanism against non-intercalating topoisomerase II drugs seems to occur at a stage after DNA-drug interaction. Finally, heat treatment before topoisomerase II drug treatment reduced toxicity and cleavable complex formation in thermotolerant cells to about the same extent as in non-tolerant cells, consistent with the presumption of nuclear protein aggregation being responsible for this effect.

Analysis of eukaryotic topoisomerase II cleavage sites in the presence of the quinolone CP-115,953 reveals drug-dependent and -independent recognition elements

The quinolone derivative CP-115,953 [6,8-difluoro-7-(4-hydroxyphenyl)-1-cyclopropyl-4-quinolone-3-carboxylic acid] has been shown to induce eukaryotic topoisomerase II-mediated breaks in DNA, producing cleavage patterns that are distinct from those induced by the anticancer drugs amsacrine, etoposide, and teniposide. High levels of the quinolone have been found to inhibit topoisomerase II activity via an interaction with the enzyme and not by DNA unwinding. Topoisomerase II cleavage sites were analyzed on nine DNA fragments, and 85 quinolone-induced sites were sequenced, as well as 86 amsacrine and 134 teniposide sites. A consensus sequence was derived for the quinolone sites that is different from those reported for other drugs; however, because topoisomerase II cleavage sites are double-stranded but not palindromic, different consensus sequences are not easily compared. For this reason, a new, double-stranded, consensus sequence method, the "unique-base analysis," was developed; this was applied to the quinolone sites as well as six other large sets of topoisomerase II sites determined in the absence or presence of drugs. For each of the seven sets of sites, conserved bases were found in the 16-base region spanning positions -6 to +10, relative to the enzyme cleavage site (DNA breakage between -1 and +1). The conserved bases were virtually identical in the regions flanking the cleavage site for all seven data sets. In contrast, the base preferences identified proximal to the cleavage sites were unique to the drug tested. These observations suggest that the selection of cleavage sites by topoisomerase II involves both enzyme-dependent and drug-dependent recognition elements. The single most preferred base in the quinolone sites was a cytosine at -1; the same preference was found with teniposide, and 60 of the 85 quinolone sites co-localized with teniposide sites.

Expression, domain structure, and enzymatic properties of an active recombinant human DNA topoisomerase II beta

Human cells express two genetically distinct isoforms of DNA topoisomerase II, alpha and beta, which catalyze ATP-dependent DNA strand passage and are an important antitumor drug target. Here we report for the first time the successful overexpression of human topoisomerase II beta in yeast by cloning a topoisomerase II beta cDNA in a yeast shuttle vector under the control of a galactose-inducible promoter. Recombinant human topoisomerase II beta (residues 46-1621 fused to the first 5 residues of yeast topoisomerase II) was purified to homogeneity, yielding an enzymatically active polypeptide in sufficient quantity to allow analysis of its domain structure and comparison with that of recombinant human topoisomerase II alpha. Partial digestion of beta with either trypsin or protease SV8 generated fragments of approximately 130, 90, 62, and 45-50 kDa, arising from cleavage at three limited and discrete regions of the protein (A, B, and C) indicating the presence of at least four structural domains. Recombinant human topoisomerase II alpha and beta induced DNA breakage which was promoted by a variety of agents. Isoform differences in drug-induced DNA breakage were observed. These studies of human topoisomerase II beta in concert with alpha should aid the determination of their individual roles in cancer chemotherapy and should facilitate the design, targeting, and testing of cytotoxic antitumor agents.

The influence of antibiotics and antitumor agents on the relaxation activity of Pisum sativum leaf chloroplast topoisomerase I

DNA topoisomerase was isolated from pea leaf chloroplasts. The relaxation activity of this topoisomerase was Mg2+ dependent and sensitive to ethidium bromide and novobiocin, a gyrase inhibitor. Chloroplast topoisomerase (Topo I) was ATP independent, as shown by the characteristic gel distribution of topoisomers. Topoisomerase, compared with the known eucaryotic topoisomerase I, was not stimulated by polyamines as are spermidine, spermine, and cadaverine. Ethidium bromide, DAPI, heparin, nalidixic acid, and m-AMSA (but not camptothecin) were able to inhibit the relaxation activity of chloroplast topo I. Nalidixic acid, novobiocin, m-AMSA, camptothecin, and amiloride were tested for their effects on the topoisomerase-catalyzed "cleavage complex" between DNA and chloroplast DNA topoisomerase I.

DNA topoisomerase II is the molecular target of bisdioxopiperazine derivatives ICRF-159 and ICRF-193 in Saccharomyces cerevisiae

Bisdioxopiperazines such as ICRF-159 and ICRF-193 have been shown to inhibit DNA topoisomerase II. To determine the molecular target of these compounds in vivo, we utilized a yeast genetic system in which the topoisomerase II activity can be modulated. To reduce topoisomerase II activity, we used top2-1 mutant yeast cells that have normal DNA topoisomerase II activity at 25 degrees C but greatly reduced enzyme activity at 30 degrees C, a temperature that is semipermissive for growth. At 25 degrees C top2-1 cells are as sensitive to the ICRF compounds as the wild-type strain; at 30 degrees C the cells became hypersensitive to these agents. In contrast, top2-1 strains become very resistant to the class of topoisomerase II inhibitors such as amsacrine and etoposide, which stabilize the covalent enzyme-DNA intermediate of the enzyme reaction. Overexpression of topoisomerase II from a plasmid-born TOP2 gene results in lower susceptibility to ICRF compounds and higher susceptibility to amsacrine than the parental strain exhibits. These results support the hypothesis that the main cellular target of ICRF compounds is DNA topoisomerase II, and that these compounds, unlike amsacrine and etoposide, inhibit topoisomerase II activity without stabilizing an enzyme-DNA covalent complex.

Distinct patterns of cell cycle disturbance elicited by compounds interfering with DNA topoisomerase I and II activity

DNA topoisomerases are enzymes governing the multitude of conformational changes DNA undergoes during the cell cycle. Several compounds are likely to interfere with specific steps of the catalytic cycle of these enzymes. Camptothecin arrests the activity of DNA topoisomerase I by provoking the formation of a single-stranded DNA break with the enzyme molecule covalently attached to the DNA. Exposure to m-AMSA arrests DNA topoisomerase II by the formation of a ternary complex involving the drug, the enzyme, and DNA carrying a double-stranded break. Netropsin, distamycin A, and berenil inhibit DNA topoisomerase-mediated relaxation of supercoiled DNA by an as-yet unknown mechanism. Here, we analyze the cell cycle kinetic effects of exposure to camptothecin, m-AMSA, netropsin, distamycin A, and berenil by using continuous bromodeoxyuridine labeling followed by bivariate Hoechst 33258/ethidium bromide flow cytometry. Camptothecin elicits an accumulation of cells in all compartments of the cell cycle, while exposure to m-AMSA leads mainly to retention of cells in the G0/G1 compartment and to accumulation in the G2 phase. Neither camptothecin nor m-AMSA shows a synergism with bromodeoxyuridine incorporation into the DNA. These results point toward distinct functions of the two DNA topoisomerases in the process of cell cycle traverse. The compounds binding to the minor groove of DNA interfere with all phases of the cell cycle, but with a relative emphasis on the G2 phase. Neither camptothecin nor m-AMSA exhibits a synergistic effect in combination with berenil. Hence, at the level of perturbed cell cycle kinetics a distinction can be made between compounds provoking an abortive inhibition of the catalytic cycle of DNA topoisomerases (e.g., camptothecin, m-AMSA) and those interfering with the activity of the enzyme by a distinct mechanism.

Altered topoisomerase I and II activities in suramin-resistant lung fibrosarcoma cells

To better understand the molecular basis for the cytotoxic effects of suramin, we have developed suramin-resistant DC-3F/SU 1000 cells by continuous exposure of fibrosarcoma cells to increasing concentrations of suramin. The suramin resistance (approximately 10-fold) is not associated with changes in uptake or intracellular distribution of the drug. The sensitivity to actinomycin D, cytarabine, aphidicolin, hydroxyurea, vincristine, and 5-fluorouracil is unaltered. In contrast, DC-3F/SU 1000 cells are about 2-fold resistant to classical DNA topoisomerase II inhibitors such as doxorubicin, amsacrine, and etoposide, whereas the cells are 1.5-fold more sensitive to the topoisomerase I inhibitor camptothecin. The cross-resistance to topoisomerase II inhibitors occurred earlier than the collateral sensitivity to camptothecin. Amsacrine- and etoposide-induced DNA-protein complex formation is reduced about 2-fold in DC-3F/SU 1000 cells, compared with DC-3F cells, whereas camptothecin-induced DNA-protein complex formation is increased 1.5-fold. Western blot analysis of cellular lysates from the two cell lines shows no significant differences in the level of topoisomerase II, whereas the level of topoisomerase I is increased 2.5-fold in DC-3F/SU 1000 cells. The catalytic activities of topoisomerases I and II in nuclear extracts from DC-3F/SU 1000 cells are both about 2-fold higher than those in extracts from DC-3F cells, whereas amsacrine- and etoposide-induced DNA-protein complex formation is comparable between the two cell lines. Taken together, our results support the involvement of DNA topoisomerases in the cytotoxic activity of suramin. We further believe that the DC-3F/SU 1000 cells may be a useful model for the elucidation of factors that lead to low, clinically relevant, levels of resistance to topoisomerase II inhibitors.

DNA sequence preferences at sites cleaved by human DNA topoisomerase II in response to novel quinolone derivatives

We have examined the DNA cleavage site specificity of human type II DNA topoisomerase in the presence of each of five novel quinolone derivatives. Each quinolone derivative inhibited the human enzyme, inducing double-strand breaks with a four-base stagger. Break sites generated in response to each derivative had a predominance of C in the 3'-terminal position. Consensus sequences derived for cleavage sites induced by each derivative were strikingly similar, not only at the 3'-terminal position, but also at additional positions on either side of the broken phosphodiester bond. Analysis of these consensus sequences yielded information about possible interactions of specific substituents on the quinolone derivatives with DNA and/or topoisomerase. Comparison of the quinolone-based consensus sequences with those derived for cleavage sites generated by the human type II topoisomerase in the presence of either m-AMSA or VM-26, or in the absence of drug, provided compelling evidence that DNA cleavage sites include two domains: one which interacts with drug and a second, larger domain which interacts with topoisomerase.

An etoposide-induced block in vaccinia virus telomere resolution is dependent on the virus-encoded DNA ligase

Etoposide, an inhibitor of the breakage-reunion reaction associated with cellular type II DNA topoisomerases, was shown to inhibit plaque formation of vaccinia virus. This drug had a major effect on the segregation of newly replicated DNA concatemers. Gene expression and the initiation and elongation phases of viral DNA replication were essentially unaffected. Pulsed-field gel electrophoresis of viral DNA replicated in the presence of etoposide revealed two major classes of DNA: the mature monomeric linear genome and DNA that failed to enter the gel (the relative proportions depending on the concentrations of drug). Restriction enzyme analysis showed a severe defect in telomere resolution. In addition, slowly migrating restriction fragments were suggestive of a general recombination defect. We have isolated several etoposide-resistant mutants and used marker rescue and DNA sequencing to localize the resistance-causing mutation to the amino terminus of the viral DNA ligase gene. Inactivation of the DNA ligase also resulted in an etoposide-resistant phenotype, but to a lesser extent. The telomere resolution and segregation defects were corrected both in the drug-resistant mutants and in the DNA ligase knockout mutants. Reinsertion of wild-type or mutant DNA ligase in the viral thymidine kinase locus confirmed the role of the viral DNA ligase in conferring sensitivity not only to etoposide but also to another topoisomerase II inhibitor, 4'-(9-acridinylamino) methanesulphon-m-anisidide (mAMSA). The data suggest that the nonessential DNA ligase is involved in telomere resolution, possibly as part of a general recombinase.

Correlation between DNA topoisomerase II activity and cytotoxicity in pMC540 and merodantoin sensitive and resistant human breast cancer cells

We have shown previously that preactivated merocyanine 540 (pMC540) and merodantoin appear to mediate their cytotoxic effects via interaction with Topo II. Now, we demonstrate a correlation between DNA Topo II activity and drug-sensitive (MCF-7) and -insensitive (MDA-MB-231) breast cancer cell lines. Further studies indicate that MDA-MB-231 cells are insensitive to the cytotoxic and DNA cleavage effects of pMC540 and merodantoin. This loss of sensitivity is not associated with M(r) 170,000 P-glycoprotein over expression. However, in drug insensitive cells, the Topo II catalytic activity in crude nuclear extract was reduced two- to-three-fold and in cellular extracts was virtually absent as determined by decatenation of kDNA. Topoisomerase I activities appeared similar in extracts from MCF-7 and MDA-MB-231 cell lines. Drug-induced DNA cleavage was reduced two-to-threefold in nuclear extracts from MDA-MB-231. m-AMSA was more effective in inhibiting the decatenation activity in the nuclear extracts from MDA-MB-231 as compared to MCF-7 cells. Western blot analysis of whole-cell lysates revealed undetectable immunoreactivity of Topo II in the drug-insensitive cells. These data indicate that insensitivity of MDA-MB-231 to pMC540 and merodantoin is in part due to the reduced drug-induced formation of the cleavage complex and Topo II (170 kD) enzyme content.

Aurintricarboxylic acid, a putative inhibitor of apoptosis, is a potent inhibitor of DNA topoisomerase II in vitro and in Chinese hamster fibrosarcoma cells

Aurintricarboxylic acid (ATA) is a polyanionic, polyaromatic compound which has been shown to inhibit apoptotic cell death in various cell types induced by a variety of factors. Since ATA is known to be a general inhibitor of nuclease activities in vitro (ID50S ranging from 2 to 50 microM), the in vivo effects are usually attributed to inhibition of endogenous endonuclease activities. We show herein that ATA is a potent inhibitor of the nuclear enzyme DNA topoisomerase II. ATA inhibits the catalytic activity of purified yeast topoisomerase II with an ID50 of approx. 75nM as measured by relaxation assays. ATA does not stabilize the covalent DNA-topoisomerase II reaction intermediate ("cleavable complex") as do other inhibitors of this enzyme such as 4'-(9-acridinylamino)-methane sulfon-m-anisidide (amsacrime), 4'-demethyl-epipodophyllotoxin-9-(4,6-O-ethylidine-beta-D-gluco pyr anoside) (etoposide) and ellipticines. In contrast, cleavable complex formation induced by amsacrine and etoposide is trongly inhibited in the presence of ATA. ATA also prevents the binding of topoisomerase II to DNA and inhibits topoisomerase II-catalysed ATP hydrolysis. The ability of ATA to interfere with more than one step in t he catalytic cycle of DNA topoisomerase II may explain its unusual potency as an inhibitor of this enzyme. ATA reduces the number of amsacrine-induced DNA-protein complexes in intact DC-3F Chinese hamster fibrosarcoma cells and protects these cells from the cytotoxic action of amsacrine. The effects of ATA on DNA-protein complex formation in living cells appear to be due to the direct interaction of the drug with topoisomerase II, since similar results are found when nuclei from untreated DC-3F cells are exposed to amsacrine after a short preincubation with ATA. Cells resistant to 9-hydroxyellipticine, which have been shown to possess altered topoisomerase II activity, are approx. 5-fold more resistant to ATA than the sensitive parental cells as shown by colony formation essays. We conclude that ATA is a potent inhibitor of topoisomerase II and that the drug interacts with topoisomerase II in living cells. Our findings raise the possibility that the protective effects of ATA towards apoptotic cell death might, at least in part, involve DNA topoisomerase II.

Different topoisomerase II antitumor drugs direct similar specific long-range fragmentation of an amplified c-MYC gene locus in living cells and in high-salt-extracted nuclei

We have analyzed the long-range distribution of topoisomerase II-mediated cleavages induced in an amplified human c-MYC gene locus in the presence of several antitumor agents. The long-range cleavage patterns were found to be nonrandom and similar for all antitumor drugs tested. Cleavages occurred within several kilobase-long areas (approximately 5 kb) highly accessible to topoisomerase II and separated by extended regions (approximately 70-100 kb) of less accessibility, possibly reflecting the mode of DNA organization into loops along the chromosome. Within the cleavage areas, the patterns of cleavage sites showed a certain dependence on the type of drug used for entrapment of topoisomerase II-DNA complexes. Importantly, distribution of cleavage areas in native chromatin and histone-depleted nuclei was very similar, if not identical, suggesting that the primary target of antitumor agents in vivo is topoisomerase II associated with the high-salt-insoluble nuclear matrix. These data show that matrix-attached DNA is preferentially damaged by topoisomerase II-targeting agents, which may be an important cellular event contributing to drug-induced cell death.

Induction of apoptotic cell death by DNA topoisomerase II inhibitors

We have analyzed the interference of antitumoral drugs acting through the inhibition of DNA topoisomerase II on the human HeLa cell metabolism. Different compounds characterized by a diverse mechanism of action have been used, namely m-amsacrine, an intercalative drug, etoposide, which does not intercalate DNA, and suramin, which exerts its effect through an unknown mechanism. In HeLa cells treated with increasing doses of these drugs, we have examined cell viability and DNA synthesis capacity, and we have evaluated topoisomerase II activity. Cellular morphology and DNA integrity have been studied in order to characterize the mechanism of cell death. The results we have obtained clearly indicate that topoisomerase II poisons induce cell death by apoptosis. These observations suggest a role of the inhibition of topoisomerase II activity in the apoptotic program.

Reduced topoisomerase II activity in multidrug-resistant human non-small cell lung cancer cell lines

Multidrug-resistant (MDR) cell lines often have a compound phenotype, combining reduced drug accumulation with a decrease in topoisomerase II. We have analysed alterations in topoisomerase II in MDR derivatives of the human lung cancer cell line SW-1573. Selection with doxorubicin frequently resulted in reduced topo II alpha mRNA and protein levels, whereas clones selected with vincristine showed normal levels of topo II alpha. No alterations of topo II beta levels were detected. To determine the contribution of topo II alterations to drug resistance, topo II activity was analysed by the determination of DNA breaks induced by the topo II-inhibiting drug 4'-(9-acridinylamino)methane-sulphon-m-anisidide (m-AMSA) in living cells, as m-AMSA is not affected by the drug efflux mechanism in the SW-1573 cells. The number of m-AMSA-induced DNA breaks correlated well (r = 0.96) with in vitro m-AMSA sensitivity. Drug sensitivity, however, did not always correlate with reduced topo II mRNA or protein levels. In one of the five doxorubicin-selected clones m-AMSA resistance and a reduction in m-AMSA-induced DNA breaks were found in the absence of reduced topo II protein levels. Therefore, we assume that post-translational modifications of topo II also contribute to drug resistance in SW-1573 cells. These results suggest that methods that detect quantitative as well as qualitative alterations of topo II should be used to predict the responsiveness of tumours to cytotoxic agents. The assay we used, which measures DNA breaks as an end point of topo II activity, could be a good candidate.

DNA topoisomerases I & II cleavage sites in the type 1 human immunodeficiency virus (HIV-1) DNA promoter region

Topoisomerase sites were mapped in the 5'-long terminal repeat of HIV-1 DNA by agarose and sequencing gel electrophoresis. Topoisomerase II sites were observed in the absence and presence of teniposide and amsacrine in the transcription initiation region and the TATA box, consistent with a possible role of topoisomerase II in transcription. The NF-kB and Sp1 regions were poorly cleaved. Topoisomerase I sites were relatively unfrequent even in the presence of camptothecin. They were absent in the core promoter and were concentrated in the TAR and the upstream region near the junction with the host DNA.

Subcellular distribution of the alpha and beta topoisomerase II-DNA complexes stabilized by VM-26

Studies were done to determine (a) the subcellular distribution of the alpha (170 kDa) and beta (180 kDa) isozymes of topoisomerase II, and (b) the extent to which each isozyme forms complexes with DNA in tumor cells incubated with and without VM-26. Western blotting revealed that topoisomerase II beta was highly unstable during cell fractionation. However, preincubation of human CEM leukemia cells with 5-100 microM VM-26 for 30 min protected the beta isozyme from degradation by progressively increasing the amount of this isoform bound to DNA. The amount of topoisomerase II beta detected in nuclei of CEM cells incubated for 30 min with 25 microM VM-26 was 7-fold greater than in nuclei from untreated control cells. VM-26 also had a protective effect on topoisomerase II beta in HL-60 leukemia and WiDR colon carcinoma cells. In contrast, the intercalating agents mitoxantrone and m-AMSA did not protect topoisomerase II beta from degradation during cell fractionation. The stabilization of topoisomerase II beta by VM-26 allowed subsequent studies of the subcellular distribution of the topoisomerase II isozymes. Both isozymes were detected in the nonmatrix (high salt-soluble) fraction of nuclei from CEM cells, but only topoisomerase II alpha was present in the nuclear matrix. VM-26 stabilized binding of the alpha and beta topoisomerase II isoenzymes to nonmatrix DNA and topoisomerase II alpha to matrix DNA. The differences observed in the subnuclear distribution and DNA binding pattern of the topoisomerase II isozymes support the hypotheses that each isozyme has a distinct cellular function, and that both the alpha and beta isozymes are potential targets for VM-26 in intact cells. In addition, the results demonstrated that pretreatment of various cell lines with VM-26 is a useful way to stabilize topoisomerase II beta during cell fractionation.

Unique sequence specificity of topoisomerase II DNA cleavage stimulation and DNA binding mode of streptonigrin

Streptonigrin stimulated unique intensity patterns of topoisomerase II-mediated DNA cleavage in agarose and sequencing gels with no similarity to those of doxorubicin, VM-26,4'(9-acridinylamino)-methanesulfon-m-anisidide, genistein, and mitoxantrone. Surprisingly, a statistical analysis of 60 sites stimulated by streptonigrin in SV40 and pBR322 DNAs showed that the drug required the dinucleotide 5'-TA-3' from 2- to 3-positions at the DNA cleavage site. Streptonigrin did not intercalate into the double helix; however, a positive value of the reduced linear dichroism indicated that indeed the drug interacted with the DNA. An angle of 45 degrees was found between the major drug and local DNA axes, suggesting a minor groove binding mode. Moreover, a DNA winding assay showed that streptonigrin may tighten the helical twist of DNA, similar to the known minor groove binder distamycin. Drug competition for receptor site binding was then evaluated by drug combination in the cleavage reaction. DNA cleavage intensity patterns were altered only with the streptonigrin/mitoxantrone combination, suggesting that the two compounds may compete for ternary complex formation. The results indicate that streptonigrin may bind to the DNA in a manner similar to that of minor groove binders and that its pharmacophore, possibly different from other topoisomerase II inhibitors, may be an important determinant of its unique sequence position specificity.

Mutations in the gyrB domain of eukaryotic topoisomerase II can lead to partially dominant resistance to etoposide and amsacrine

Anti-topoisomerase II agents represent a major class of anticancer therapeutic agents. Resistance to this class of agents can be mediated by several possible mechanisms. One mechanism may involve mutations in the structural gene(s) for topoisomerases, altering the drug sensitivity of the enzymes. Several mutations have been described in mammalian cell lines that were selected for resistance to topoisomerase II-targeting drugs such as Adriamycin, etoposide, or amsacrine. The difficulty of performing genetic analysis in mammalian cell lines has complicated the determination of whether the observed mutations are responsible for drug resistance. We have reconstructed, in the yeast topoisomerase II gene, the arginine to glutamine mutation at position 450 of human topoisomerase II alpha that was originally identified by Bugg et al. [Proc. Natl. Acad. Sci. USA 88:7654-7658 (1991)]. Mutation of Lys439, the equivalent amino acid in the yeast protein, to either glutamine or glutamic acid confers resistance to etoposide and amsacrine. Interestingly, in diploid yeast cells the heterozygous mutation can still confer partial drug resistance, compared with a diploid strain that is homozygous for wild-type topoisomerase II. Because mutations in the topoisomerase II gene that can confer dominant resistance to anti-topoisomerase II agents are relatively rare, mutations in the gyrB region may be important in the development of clinical drug resistance.

Interactions between cladribine (2-chlorodeoxyadenosine) and standard antileukemic drugs in primary cultures of human tumor cells from patients with acute myelocytic leukemia

The semiautomated fluorometric microculture cytotoxicity assay (FMCA), based on the measurement of fluorescence generated from cellular hydrolysis of fluorescein diacetate (FDA) to fluorescein in microtiter plates, was used for in vitro evaluation of Cladribine (2-chlorodeoxyadenosine, CdA) interactions with five standard antileukemic drugs: amsacrine (Am), etoposide (VP16), daunorubicin (Dnr), cytosine arabinoside (AraC), and mitoxantrone (Mit). Samples from 31 patients with acute myelocytic leukemia (AML) were tested with continuous drug exposure. A large heterogeneity with respect to cell kill was observed for all combinations tested. An additive model provided a significantly better fit of the data compared to the effect of the most active single agent of the combination (Dmax) only for CdA+AraC. When the frequency of additive and synergistic interactions were calculated according to the multiplicative concept for drug interactions, the highest frequencies were observed for CdA+AraC and CdA+Dnr. This interaction pattern was confirmed by isobologram analysis. Cross-resistance analysis revealed high correlations between CdA and AraC whereas the correlations were weaker between CdA and the other drugs. The highest frequency of synergistic interactions was obtained for AraC+CdA, despite their cross-resistance. Of the non-cross-resistant drugs tested, Dnr appears to be the most effective adjunct to CdA in terms of interactions at the cellular level.

A cisplatin-resistant murine leukemia cell line exhibits increased topoisomerase II activity

cis-Dichlorodiammineplatinum(II) (CDDP) resistance in L1210/10 murine leukemia cells is multifactorial and involves decreased drug uptake, increased glutathione content, and enhanced DNA repair activity. We show here that 0.35 M NaCl nuclear extracts from L1210/10 cells possess an approximately 3-fold increase in DNA topoisomerase II activity, compared with parental L1210 cells, as measured by decatenation of kinetoplast DNA. No difference in topoisomerase I activity is observed between the two cell lines. Immunoblot analysis of topoisomerase II protein in resistant and sensitive cells suggests that the observed differences in topoisomerase II activity cannot be explained by differences in the level of protein expressed. L1210/10 cells are 2.5-fold more sensitive than L1210 cells to the cytotoxic effects of the topoisomerase II inhibitor 4'-(9-acridylamino)methane-sulfon-m-anisidide. Sequential treatment with 4'-(9-acridyl-amino)methanesulfon-m-anisidide and CDDP leads to an additive cytotoxic effect of the two drugs in sensitive L1210 cells, as determined by colony formation in semi-solid medium. In contrast, the same treatment leads to a supra-additive effect in L1210/10 cells, which strongly suggests a role for topoisomerase II in the CDDP resistance of this cell line.

A human small cell lung carcinoma cell line, resistant to 4'-(9-acridinylamino)-methanesulfon-m-anisidide and cross-resistant to camptothecin with a high level of topoisomerase I

N417/AMSA cells, about 80-fold resistant to mAMSA [4'-(9-acridinylamino)-methanesulfon-m-anisidide], were obtained by serial passages of the parental human small cell lung carcinoma NCI-N417 (N417/p) in stepwise drug concentrations. The N417/AMSA cells were found to be 114-, 100-, and 9-fold cross-resistant to the topoisomerase II (Topo II) inhibitors VM26, VP16 and Doxorubicin (DXR); they showed a 2-fold decrease in Topo II activity. Interestingly, N417/AMSA cells which exhibited a 3-fold increase in topoisomerase I (Topo I) activity were 28-fold cross-resistant to camptothecin (CPT), a specific inhibitor of Topo I. In order to investigate the cellular mechanisms leading to the development of resistance, the effects of mAMSA and CPT on parental and resistant cell lines were analysed by alkaline elution. A decrease in DNA single-strand breaks (DNA-SSB) was observed in N417/AMSA cells treated with mAMSA or CPT compared to parental cells. Similar differences were obtained in isolated nuclei, suggesting that no modification of mAMSA and CPT accumulation occurred in resistant cells. Topo I was purified from N417/p (Topo I/p) and N417/AMSA (Topo I/AMSA) cells in the exponential phase of growth, and the inhibitory effects of CPT on relaxation activities were determined. Topo I/AMSA was found to be about 7-fold less sensitive to CPT than Topo I/p, suggesting the possible involvement of a mutation outside the gene region sequenced (codons 420 to 642) or post-translational modifications of the Topo I enzyme. These data indicate that increased Topo I activity cannot be related to CPT resistance, and suggest that mAMSA can generate multiple cellular modifications which may be involved in resistance to various drugs.

Mechanistic studies of amsacrine-resistant derivatives of DNA topoisomerase II. Implications in resistance to multiple antitumor drugs targeting the enzyme

Wild-type yeast DNA topoisomerase II and three of its amsacrine-resistant derivatives L475A/L480P, L475A/R476G, and A642G, named according to amino acid changes at the codons specified, were overexpressed and purified. Because cells expressing several mutant enzymes missing portions of the carboxyl-terminal domain of the wild-type enzyme were previously found to exhibit amsacrine resistance, a carboxyl-terminal truncation protein Top2(1-1166), which lacks the last 263 amino acids of the wild-type enzyme, was also overexpressed and purified. These purified enzymes were used in the measurement of the turnover numbers of the DNA-dependent hydrolysis of ATP, the transport of one DNA segment through another, and the effects of amsacrine, teniposide or Ca(II) on the formation of the enzyme-DNA covalent intermediate. The results of these studies indicate that mutations leading to cellular resistance to amsacrine may occur by several different mechanisms, including reduction of the nuclear concentration and attenuation of the intrinsic catalytic steps of the enzyme. The significance of this underpinning mechanistic diversity of drug resistance and its relation to the simultaneous development of cellular resistance to chemically distinct drugs that target DNA topoisomerase II are discussed.

The action of the DNA intercalating agents 4'-(9-acridinylamino) methanesulphon-m-anisidide and 1,4-bis(butylamino) benzo[g]phthalazine in U-937 human promonocytic cells: relationship between cell cycle and differentiation

The action of two structurally related DNA intercalating agents has been studied and compared, namely 4'-(9-acridinylamino) methanesulphon-m-anisidide (amsacrine, mAMSA) and 1,4-bis(butylamino)benzo[g]phthalazine (ABP) on the cell cycle and differentiation of U-937 human promonocytic leukemia cells. mAMSA (0.1 microM) and ABP (4 microM) reduced the proliferation activity to a similar extent and caused little cell mortality. At these subcytotoxic concentrations mAMSA induced the cells to accumulate at the G2 phase of the cycle, while cycle inhibition provoked by ABP was not phase specific. In addition, mAMSA caused an increase in the cell mass while ABP provoked cell shrinkage. This was consistent with the fact that ABP considerably inhibited protein synthesis, while mAMSA did not significantly affect this activity. SDS/K+DNA precipitation assays indicated that mAMSA, but not ABP, stimulated protein-DNA covalent complex formation. Finally, it was found that mAMSA, but not ABP, elicited the expression of differentiation markers, namely nitroblue tetrazolium reduction, activation of vimentin and leukocyte integrin (CD11b/CD18 and CD11c/CD18) expression, and downregulation of c-myc expression. The DNA intercalators doxorubicin and mitoxantrone, which like mAMSA induced the cells to accumulate at the G2 phase and increased the cell mass, induced the expression of differentiation markers. In contrast, the intercalators aclarubicin and caffeine and the non-intercalator novobiocin, which produced minor alterations on cell-cycle distribution and caused cell shrinkage, did not significantly elicit differentiation. These results support the conclusion that differentiation of myeloid leukemia cells by cytostatic drugs depends on the perturbations of the cell cycle, leading to disproportionate increases in cell mass.

Yeast topoisomerase II mutants resistant to anti-topoisomerase agents: identification and characterization of new yeast topoisomerase II mutants selected for resistance to etoposide

We describe a system that allows us to easily isolate and characterize mutants in yeast topoisomerase II that are resistant to antitumor agents that target this enzyme. The system uses yeast strains that are sensitive to those agents and that carry temperature-sensitive top2 mutations. The temperature-sensitive mutation allows the isolation of recessive drug-resistant mutations. The mutagenized TOP2 gene we have used is under the control of the yeast DED1 promoter; this overexpression of TOP2 is designed to avoid isolating mutants that are drug resistant solely because the mutated topoisomerase II has low enzymatic activity. We describe three mutants that we isolated using this system. Two of the three mutants show resistance to etoposide and amsacrine, while the third mutant is partially resistant to etoposide and fluoroquinolones but not to amsacrine. DNA sequence changes have been identified in all of these mutant TOP2 genes. The mutant with partial resistance to etoposide and fluoroquinolones has an amino acid change at position 738 of TOP2, which is three amino acids from the site homologous to Ser83 of E. coli gyrA, an amino acid which had previously been shown to be an important target for resistance to quinolones in bacteria. One of the alleles that confers resistance to both etoposide and amsacrine, top2-103, has changes in amino acid 824 and amino acid 1186 of TOP2. Reconstruction of the mutations by oligonucleotide-directed mutagenesis demonstrates that the change at amino acid 824 is responsible for the drug resistance of this allele.

Heat sensitivity of HeLa S3 cell DNA topoisomerase II

The sensitivity of HeLa DNA topoisomerase II to 45 degrees C heat shock was measured both in the intact cell and in vitro. In the intact cell, DNA topoisomerase II activity was estimated by measuring the formation and reversal of enzyme-DNA cleavable complexes by alkaline filter elution of cells exposed to the enzyme poison 4'-(9-acridinylamino)methanesulfon-m-anisidide). In vitro enzymatic activity was estimated by measuring changes in the topological state of plasmid and kinetoplast DNA produced by sonicates of nuclei from previously heated cells. The capacity of the enzyme to form, or reverse, enzyme-DNA cleavable complexes was inactivated during 45 degrees C heating with a reciprocal slope of 120 or 15 min, respectively. In vitro estimates of the activity of the enzyme from previously heated cells indicated that the enzyme was inactivated with a reciprocal slope of 99, 45, and 21 min after 45, 46 and 47 degrees C heating, respectively. DNA topoisomerase I activity was inactivated with a reciprocal slope of 130 min at 45 degrees C. The cumulative results indicate that during 45 degrees C heat shock, thermal inactivation of neither DNA topoisomerase I nor II is rate limiting for either cell survival or for DNA replication. While DNA topoisomerase II is resistant in situ to heat inactivation, in vivo assays indicate that the enzyme's capacity to function in the intact cell may be compromised by hyperthermic changes in the enzyme's environment.