Mutational alteration of the breakage/resealing subunit of bacteriophage T4 DNA topoisomerase confers resistance to antitumor agent m-AMSA

Bacteriophage T4 provides a simple model system in which to examine the mechanism of action of antitumor agents that have been proposed to attack type II DNA topoisomerases. Prior results demonstrated that T4 type II DNA topoisomerase is the target of antitumor agent 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) in phage-infected Escherichia coli: a point mutation in topoisomerase structural gene 39 was shown to confer both m-AMSA-resistant phage growth and m-AMSA-insensitive topoisomerase activity. We report here that a point mutation in T4 topoisomerase structural gene 52 can also independently render both phage growth and topoisomerase activity resistant to m-AMSA. The DNA relaxation and DNA cleavage activities of this newly isolated mutant topoisomerase were significantly insensitive to m-AMSA. The drug-resistance mutation in gene 52, as well as that in gene 39, alters the DNA cleavage site specificity of wild-type T4 topoisomerase. This finding is consistent with a mechanism of drug action in which both topoisomerase and DNA participate in formation of the drug-binding site.

Stimulation by gamma-carboline derivatives (simplified analogues of antitumor ellipticines) of site specific DNA cleavage by calf DNA topoisomerase II

gamma-Carbolines are tricyclic aromatic compounds which intercalate into DNA base pairs and exhibit significant cytotoxic and antitumor activities. These compounds which are structurally related to ellipticine by deletion of an aromatic ring, induce DNA breaks in cultured L1210 cells. Since the mechanism of cytotoxic activity of ellipticines involves DNA topoisomerase II, this enzyme might also be a target for gamma-Carbolines. We have tested this hypothesis using an in vitro system containing purified enzyme and pBR322 DNA. The ability of nine derivatives to stabilize the DNA-enzyme covalent complex was studied and compared to their cytotoxicity. The four less cytotoxic compounds do not induce cleavable complex to a significant extent. In contrast, the two most cytotoxic gamma-Carbolines are the most efficient stabilizers of the cleavable complex. The last three compounds exhibit an intermediate cytotoxicity and cleavage activity. In the presence of gamma-Carbolines, cleavage occurs predominantly at a single site in pBR322 which is one of the cleavage sites observed with ellipticines. The cleavage position was determined at the nucleotide level. The increased DNA cleavage specificity observed with gamma-Carbolines suggests that a tricyclic system is as efficient as ellipticines for DNA topoisomerase II cleavage at DNA sequences involved specifically in cytotoxic response. The data presented support the hypothesis that DNA topoisomerase II is a target involved in the mechanisms of action of antitumor gamma-Carbolines.

Induction of mammalian topoisomerase II dependent DNA cleavage by nonintercalative flavonoids, genistein and orobol

Two isoflavones, genistein (4',5,7-trihydroxyisoflavone) (1) and orobol (5,7,3',4'-tetrahydroxyisoflavone) (2) induced mammalian topoisomerase II dependent DNA cleavage in vitro. The cleavage activities of 1 and 2 were comparable to those of known antitumor agents with topoisomerase II dependent DNA cleavage activity such as 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and demethylepipodophyllotoxin ethylidene-beta-D-glucoside (VP-16). Two flavones, fisetin (3,7,3',4'-tetrahydroxyflavone) (3) and quercetin (3,5,7,3',4'-pentahydroxyflavone) (4) showed topoisomerase II dependent DNA cleavage activity with similar potentials to that of Adriamycin. Addition of salt (0.5 M NaCl) to the reaction mixture containing genistein and topoisomerase II resulted in a great reduction of DNA cleavage, suggesting that the mechanism of the topoisomerase II dependent DNA cleavage induced by flavonoids is through the cleavable complex formation as seen with m-AMSA and VP-16. DNA unwinding assay using mammalian topoisomerase I showed that both 1 and 2 did not intercalate into DNA but both 3 and 4 intercalated like m-AMSA. Other structurally related flavonoids could not induce topoisomerase II dependent DNA cleavage, indicating that the restricted structures of flavonoids were required for the cleavage activity.

Inhibition of replicon cluster ligation into chromosomal DNA at elevated temperatures

The rate-limiting enzymatic step for DNA replication in HeLa cells incubated at 43.5 degrees C was the ligation of clusters of replicons into the cell's genome. At 43.5 degrees C the reciprocal slope for inhibition of DNA chain (replicon) initiation, or of the ligation of replicon clusters into the genome, was 18 or 7 min, respectively. The failure of replicon clusters to be ligated into chromosomal DNA was not a consequence of the failure of histone proteins to be deposited onto replicating DNA, or of chromatin replicated at 43.5 degrees C to be organized into fully condensed chromatin. In addition it was not due to the failure of fully active topoisomerase II to be deposited at a normal frequency along replicating chromatin DNA. The failure of replicon clusters to be ligated into the genome resulted in the persistence of single, but not double, DNA strand breaks in the cell's genome 24 hours after cell heating.

Sequence-selective topoisomerase II inhibition by anthracycline derivatives in SV40 DNA: relationship with DNA binding affinity and cytotoxicity

Topoisomerase II mediated double-strand breaks produced by anthracycline analogues were studied in SV40 DNA. The compounds included doxorubicin, daunorubicin, two doxorubicin stereoisomers (4'-epimer and beta-anomer), and five chromophore-modified derivatives, with a wide range of cytotoxic activity and DNA binding affinity. Cleavage of 32P-end-labeled DNA fragments was visualized by autoradiography of agarose and polyacrylamide gels. Structure-activity relationships indicated that alterations in the chromophore structure greatly affected drug action on topoisomerase II. In particular, removal of substituents on position 4 of the D ring resulted in more active inducers of cleavage with lower DNA binding affinity. The stereochemistry between the sugar and the chromophore was also essential for activity. All the active anthracyclines induced a single region of prominent cleavage in the entire SV40 DNA, which resulted from a cluster of sites between nucleotides 4237 and 4294. DNA cleavage intensity patterns exhibited differences among analogues and were also dependent upon drug concentration. Intensity at a given site depended on both stimulatory and suppressive effects depending upon drug concentration and DNA sequence. A good correlation was found between cytotoxicity and intensity of topoisomerase II mediated DNA breakage.

Reduced DNA topoisomerase II activity and drug-induced DNA cleavage activity in an adriamycin-resistant human small cell lung carcinoma cell line

In a previous study we suggested that, in addition to the reduced Adriamycin accumulation, part of the resistance in an Adriamycin-resistant human small cell lung carcinoma cell line (GLC4/ADR) could be explained by supposing a changed Adriamycin-DNA-topoisomerase II (Topo II) interaction. The present study showed that the Mr 170,000 P-glycoprotein was not overexpressed in GLC4/ADR and that verapamil did not reverse the Adriamycin resistance. GLC4/ADR expressed cross-resistance to teniposide, etoposide, 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA), and mitoxantrone. Further investigations of the drug-Topo II interaction revealed that the decatenation activity of Topo II was two- to threefold reduced in both cellular and nuclear extracts from GLC4/ADR. Topo I activities appeared similar in extracts from GLC4/ADR and the parental sensitive cell line (GLC4). The slight increase in doubling time from 15 to 18 h, while the cell cycle distribution remained unchanged, could not account for the reduced Topo II activity in GLC4/ADR. Etoposide and m-AMSA-induced DNA cleavage was 5-fold reduced in cellular extracts from GLC4/ADR. Inhibition of the decatenation activity of Topo II in the presence of VP-16 and m-AMSA was increased twofold in the cellular extracts from GLC4/ADR. Therefore, these results suggest that resistance of GLC4/ADR to Adriamycin was in part due to the reduced drug-induced formation of the cleavage complex.

DNA topoisomerase I as a site of action for 10-hydroxycamptothecin in human promyelocytic leukemia cells

We investigated the antiproliferative effect of 10-hydroxycamptothecin (HCPT), an alkaloid isolated from Camptotheca acuminata, on the human promyelocytic leukemia cell line, HL-60, and a 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA)-resistant mutant, HL-60/m-AMSA. Using trypan blue dye exclusion and colony formation, doses of HCPT ranging from 0.01 to 1 microM progressively inhibited growth in both cell lines in a concentration-dependent manner. A minimal cross-resistance, approximately five-fold, between the wild-type and resistant cells was observed. Using the technique of alkaline elution, HCPT produced DNA single-strand breaks and protein-associated DNA strand cleavage in HL-60 and HL-60/m-AMSA cells. Quantitative analysis of drug-induced protein-DNA complexes was performed using sodium dodecyl sulfate-potassium chloride precipitation. In both cell lines, a good correlation with HCPT-induced cytotoxicity was observed. Similar results were achieved in wild-type cells treated with m-AMSA. Enzyme activity was measured in nuclei isolated from HL-60 and HL-60/m-AMSA cells, and in each case HCPT inhibited topoisomerase I activity to the same extent. The data suggest that the principle mechanisms for HCPT-induced cytotoxicity in HL-60 and HL-60/m-AMSA cells are inhibition of DNA topoisomerase I and production of protein-associated DNA strand breaks.

Chemoprotection by 9-aminoacridine derivatives against the cytotoxicity of topoisomerase II-directed drugs

The effect of three acridine derivatives, 9-aminoacridine (9AA), 4'-(9-acridinylamino)-methanesulphon-O-anisidide (O-AMSA) and quinacrine were compared in their ability to protect against the cytotoxicity of amsacrine, 9-[[2-methoxy-4-[(methylsulfonyl)amino]phenyl]amino)-N,5-dimethyl-4- acridine-carboxamide (CI-921), N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (AC), etoposide, mitoxantrone and doxorubicin. Cytotoxicity was measured in vitro by clonogenic survival assay and in vivo by life extension assays. All three acridine derivatives protected a Lewis lung cell line in vitro against CI-921, with 9AA having the highest activity. Cellular uptake of [14C] CI-921 by cultured Lewis lung cells was unaffected by 9AA, and slightly stimulated by O-AMSA and quinacrine. 9AA protected Lewis lung cells in vitro against the cytotoxicity of amsacrine, CI-921, AC and etoposide, partially against mitoxantrone but not against doxorubicin. A similar result was obtained with the human melanoma cell line MM96, where 9AA protected against CI-921 but not against doxorubicin toxicity. 9AA protected P388 leukaemia in vivo against amsacrine, CI-921 and AC cytotoxicity, partially against etoposide but not against mitoxantrone or doxorubicin. 9AA also protected against animal toxicity caused by high dose amsacrine and partially against CI-921 toxicity. It is hypothesized that DNA intercalating chemoprotectors act by restricting the conformational flexibility of the DNA and thus the ability of topoisomerase II to form a 'cleavable complex' in which the DNA is covalently linked to the enzyme.

A rapid and quantitative microtiter assay for eukaryotic topoisomerase II

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Induction of DNA-protein crosslinks by antitumor 1-nitro-9-aminoacridines in L1210 leukemia cells

Ledakrin [1-nitro-9-(3'-dimethylamino-N-propylamino)acridine], an antitumor drug of the 1-nitro-9-aminoacridine family, was able to induce DNA-protein crosslinks in intact L1210 leukemia cells, as demonstrated by the potassium-dodecyl sulfate precipitation technique. Ledakrin-induced DNA-protein crosslinks were not readily reversible nor were they accompanied by DNA double-strand breaks. Also, ledakrin produced virtually no crosslinks in isolated nuclei. Ledakrin-induced DNA-protein crosslinks seemed not to be mediated by topoisomerase II, unlike well-established effects of a chemically related antitumor drug, 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA). Four ledakrin analogs of divergent cytotoxic potencies also induced DNA-protein crosslinks but not DNA double-strand breaks in intact L1210 cells. A significant positive correlation existed between the ability of ledakrin and its 1-nitro analogs to induced DNA-protein crosslinks and the antiproliferative effects of these drugs. The results are consistent with the previously shown ability of 1-nitro-9-aminoacridines to covalently bind to macromolecules after metabolic activation in the cell. In addition to previously demonstrated DNA interstrand crosslinks and monofunctional adducts, DNA-protein crosslinks constitute another type of DNA lesion induced by 1-nitro-9-aminoacridines.

Differential requirement of DNA replication for the cytotoxicity of DNA topoisomerase I and II inhibitors in Chinese hamster DC3F cells

The cytotoxicity of topoisomerase inhibitors is thought to result from the induction of enzyme-mediated DNA breaks. The fact that these breaks reverse rapidly in cells programmed to die, led us to investigate further the cytotoxic mechanisms of topoisomerase I (camptothecin) and topoisomerase II inhibitors (VP-16 and amsacrine) in Chinese Hamster lung fibroblasts (DC3F). Exposures (30 min) to camptothecin produced limited cell killing with approximately 20% of the cells naturally resistant. This resistance was overcome by increasing the drug exposure time. Inhibition of DNA synthesis by 5-min pretreatments with aphidicolin or hydroxyurea abolished the cytotoxicity of camptothecin without changing the level of camptothecin-induced DNA breaks. A good correlation was found between the degree of DNA synthesis inhibition by aphidicolin and the reduction of camptothecin cytotoxicity. In similar experiments performed with topoisomerase II inhibitors, aphidicolin prevented only partially against VP-16- and amsacrine-induced cytotoxicities, yet had no effect upon drug-induced DNA breaks. These results indicate that the production of topoisomerase-mediated DNA breaks by antitumor drugs is not sufficient for cell killing. Instead, an interference of moving DNA replication forks with drug-stabilized topoisomerase-DNA complexes is critical for cell death. The cytotoxicity of camptothecin seemed to be completely related to this process, while that of topoisomerase II inhibitors seemed to involve additional mechanisms in DC3F cells.

Nonproductive rearrangement of DNA topoisomerase I and II genes: correlation with resistance to topoisomerase inhibitors

Topoisomerase inhibitors comprise an important group of agents that is used in cancer treatment. Because the development of resistance to cancer chemotherapeutic agents represents a major limitation of cancer chemotherapy, we investigated the mechanism of resistance by murine P388 leukemia to camptothecin (topoisomerase I inhibitor) or amsacrine (topoisomerase II inhibitor). The resistant cells contained reduced levels of topoisomerase activity and messenger RNA. The topoisomerase gene of these cells was rearranged (only in one allele) and hypermethylated. These topoisomerase gene alterations probably resulted in reduced transcription and, thus, enzyme production, which was correlated with resistance to the topoisomerase inhibitor.

Stimulation of the topoisomerase II induced DNA cleavage sites in the c-myc protooncogene by antitumor drugs is associated with gene expression

The antitumor drugs mAMSA and VM26 were shown to stimulate the topoisomerase II (Topo II) cleavage activity on the c-myc protooncogene in several human tumor cell lines (N417, HL60, EJ, H146, CaSki, A431, IGROV1, and CAL18A) and human peripheral lymphocytes. The mAMSA-induced gene cleavage was found to increase with the steady-state levels of c-myc transcripts in cell lines while no cleavage could be evidenced in the other genes so far tested. In mAMSA-treated N417 cells, the overall genomic DNA cleavage detected by alkaline elution was found to be about 20 times lower than the c-myc gene cleavage. Topo II mRNA levels were associated with the nuclear Topo II decatenating activity in cell lines and increased with c-myc cleavage. Topo II decatenating activity was found to be 3 times lower in quiescent than in exponentially growing N417 cells, but the c-myc cleavage induced by mAMSA was found as intense in quiescent as in growing cells. Thus, our data seem to indicate that c-myc gene cleavage is not related to cellular Topo II content but rather to c-myc gene transcription. Therefore, we suggest that only a small fraction of the Topo II is able to react with drug on the c-myc gene in relation to its transcriptional accessibility. Since c-myc overexpression is frequently found to be related to human cancer progression, we suggest that this gene could be an important target for Topo II related antitumor drugs.

Characterization of an amsacrine-resistant line of human leukemia cells. Evidence for a drug-resistant form of topoisomerase II

HL-60/AMSA is a human leukemia cell line that is 100 times more resistant to the cytotoxic actions of the antineoplastic, topoisomerase II-reactive DNA intercalating acridine derivative amsacrine (m-AMSA) than is its parent HL-60 line. HL-60/AMSA cells are minimally resistant to etoposide, a topoisomerase II-reactive drug that does not intercalate. Previously we showed that HL-60 topoisomerase II activity in cells, nuclei, or nuclear extracts was sensitive to m-AMSA and etoposide, while HL-60/AMSA topoisomerase II was resistant to m-AMSA but sensitive to etoposide. Now we show that purified topoisomerase II from the two cell lines exhibits the same drug sensitivity or resistance as that in the nuclear extracts although the magnitude of the m-AMSA resistance of HL-60/AMSA topoisomerase II in vitro is not as great as the resistance of the intact HL-60/AMSA cells. In addition HL-60/AMSA cells are cross-resistant to topoisomerase II-reactive intercalators from the anthracycline and ellipticine families and the pattern of sensitivity or resistance to the cytotoxic actions of the various topoisomerase II-reactive drugs is paralleled by topoisomerase II-reactive drug-induced DNA cleavage and protein cross-link production in cells and the production of drug-induced, topoisomerase II-mediated DNA cleavage and protein cross-linking in isolated biochemical systems. In addition to its lowered sensitivity to intercalators, HL-60/AMSA differed from HL-60 in 1) the susceptibility of its topoisomerase II to stimulation of DNA topoisomerase II complex formation by ATP, 2) the catalytic activity of its topoisomerase II in an ionic environment chosen to reproduce the environment found within the living cell, and 3) the observed restriction enzyme pattern on a Southern blot probed with a cDNA for human topoisomerase II. These data indicate that an m-AMSA-resistant form of topoisomerase II contributes to the resistance of HL-60/AMSA to m-AMSA and to other topoisomerase II-reactive DNA intercalating agents. The drug resistance is associated with additional biochemical and molecular alterations that may be important determinants of cellular sensitivity or resistance to topoisomerase II-reactive drugs.

Evaluation of an amsacrine analog in a human tumor cloning system

A human tumor-cloning system was used to compare the antitumor activity of CI-921, a new amsacrine analog with that of its parent compound (amsacrine gluconate). A total of 48 specimens of 9 histologically different types of human malignancy were evaluable for a direct comparison of the cytotoxic activity. Both compounds were tested simultaneously at 10 micrograms/ml final concentration under continuous exposure. The overall activity was similar for both drugs, but the degree of cross-resistance was low. We concluded that in patients, CI-921 might have a different spectrum of antitumor activity from that of its parent compound.

A study of drug-induced topoisomerase II-mediated DNA lesions on episomal chromatin

A well defined extrachromosomal DNA element, referred to as an episome (Ostrowski, M., Richard-Foy, H., Wolford, R., Berard, D., and Hager, G. (1983) Mol. Cell. Biol. 3, 2045-2057), was employed as a target for the topoisomerase II inhibitors amsacrine and teniposide. Both drugs have distinct mechanisms of action in cleaving the episome, as defined by topological forms conversion assays. The concentration ranges required to measure episomal cleavage are similar. The onset of damage induced by amsacrine begins within 1 min and is maintained at that level for at least 1 h. Teniposide induces damage that peaks between 30 and 60 min. The amsacrine-induced damage is only partially reversible, whereas teniposide-induced damage is almost completely reversible. Sites of specific cleavage are quite dissimilar. Multiple cleavage sites are formed in the episomal regulatory regions after amsacrine treatment, whereas a single cleavage in the regulatory region and one outside this region are found after teniposide treatment. Transcriptional activation using dexamethasone does not change the amount or site preference of episomal cleavage induced by either agent. Damage to the episome was quantitatively compared with damage produced in genomic DNA between 500 and 24,000 rad equivalents. The study showed that amsacrine has a significant (33-38-fold) preference for episomal DNA over genomic DNA.

Novobiocin, nalidixic acid, etoposide, and 4'-(9-acridinylamino)methanesulfon-m-anisidide effects on G2 and mitotic Chinese hamster ovary cell progression

Exponentially growing Chinese hamster ovary cells, exposed to inhibitors of topoisomerase II (novobiocin, nalidixic acid, etoposide, and 4'-(9-acridinylamino)methanesulfon-m-anisidide were blocked in progression through G2. The manner of recovery from the novobiocin-induced block, following drug removal, indicated that the blockade was at and before a specific point in G2 (a transition point). The transition point for novobiocin and putative transition points for nalidixic acid and 4'-(9-acridinylamino)methanesulfon-m-anisidide were about 30 min before metaphase. The transition point for nalidixic acid varied with concentration from about 70 min before metaphase, at 1 microgram/ml, to 24 min before metaphase at 15 micrograms/ml and above. The novobiocin- and nalidixic acid-induced G2 block could not be accounted for by cytotoxicity or DNA damage (detected by neutral elution). The novobiocin-induced G2 block could not be attributed to gross RNA synthesis inhibition. Progress beyond metaphase was blocked by novobiocin but not by nalidixic acid, when cells were exposed to drug concentrations which inhibited G2 cell progression. It is suggested that the progression of Chinese hamster ovary cells into but not through mitosis may require topoisomerase II.

Application of a degenerate consensus sequence to quantify recognition sites by vertebrate DNA topoisomerase II

A consensus sequence has been derived for vertebrate topoisomerase II cleavage of DNA (Spitzner, J. R. and Muller, M. T. (1988) Nucleic Acid. Res. 16, 5533-5556). An independent sample of 65 topoisomerase II sites (obtained in the absence of topoisomerase II inhibitors) was analyzed and found to match the consensus sequence as well as enzyme sites determined in the presence of the anti-tumor drug 4'-(9-acridinyl-amino)-methanesulfon-m-anisidide (m-AMSA). As originally described, conventional application of the consensus sequence afforded accuracy in the prediction of the locations but not the frequencies of topoisomerase II cleavages. In the present report, we describe a new method which quantitatively discriminates sites from nonsites, called the 'matrix mean' method (the mean match of a site to the matrix of base proportions from the original consensus sequence derivation). Furthermore, we derived a second method, called the 'unique score' model, which predicts frequency of topoisomerase II activity at a cleavage site. In the unique score method both DNA strands of a site are examined to determine the total number of the consensus positions that match on at least one strand of a potential site. From the new data base of 65 topoisomerase II sites, cleavages were scored for relative cleavage strength. Linear regression analysis showed a significant (p less than 0.01) correlation between the unique score and cleavage strength. The study was extended to show that the unique score model accurately and quantitatively predicts topoisomerase II sites either in the absence or presence of m-AMSA using the same consensus sequence.

Topoisomerase II-mediated DNA cleavage activity induced by ellipticines on the human tumor cell line N417

Ellipticine derivatives have been shown to induce DNA strand breaks by trapping DNA-topoisomerase II (Topo II) in an intermediary covalent complex between Topo II and DNA which could be related to their cytotoxic effects. We report here that Celiptium and Detalliptinium, two ellipticine derivatives clinically used for their antitumoral properties against breast cancer, exhibit the highest in vitro activity on Topo II DNA cleavage reaction and decatenation among a series of 14 ellipticine derivatives. The in vitro cleavage site specificity in pBR 322 plasmid DNA and in a human c-myc gene inserted in a lambda phage DNA is identical for both ellipticines, but different from m-AMSA, another Topo II related antitumoral agent. Recently, it has been shown that the ellipticine derivative Celiptium presents a strong cytotoxic activity in vitro on different human tumors including small cell lung carcinoma (SCLC). However, the studies that involved Topo II as a target for ellipticine derivatives have been performed only by using animal tumor cell lines. Therefore we have studied the in vivo DNA cleavage activity of Celiptium and Detalliptinium on a human SCLC cell line, NCI N417, comparatively to that obtained with m-AMSA. The respective IC50 on cell growth are 9, 8 and 1 microM for Celiptium, Detalliptinium and m-AMSA, respectively. Using the alkaline elution technique, we have observed that Celiptium and Detalliptinium exhibit a weak cleavage activity on genomic DNA from whole cells. The ellipticines are about 50 times less potent than m-AMSA in inducing DNA strand breaks. Analysis of in vivo c-myc gene cleavage by Southern blot hybridization also demonstrates a lack of activity of the ellipticine derivatives as no gene cleavage could be detected up to 50 microM of the drug. With m-AMSA, c-myc gene cleavage is detected at a concentration of 0.2 microM, which indicates that this methodology is less sensitive in detecting DNA strand breaks than is the alkaline elution. Further studies of the drug effect on isolated nuclei by alkaline elution also show that the DNA cleavage activity of Celiptium and Detalliptinium is increased when compared to whole cells. Our data indicate that these two drugs have a weaker cytotoxic effect than m-AMSA on NCI N417 cell line, due to a limited access to the cell nucleus rather than to a lack of activity on Topo II as assessed by in vitro and isolated nuclei experiments.

Synthesis, biological activity and DNA interaction of anilinoacridine and bithiazole peptide derivatives related to the anti-tumor drugs m-AMSA and bleomycin

The synthesis of two depsipeptides including a peptide metal-chelating moiety (Gly-His-Lys) and a moiety with DNA affinity, namely either glycyl-anilino-9-aminoacridine 1 or 2'-(2-aminoethyl)-4-methoxycarbonyl-2",4'-bithiazole 2, has been carried out. The goal was to introduce separately on the same molecule the two factors contributing to the biological activity of many anti-tumor drugs. The interaction of both drugs with DNA has been studied and the acridine ring of 1 was found to intercalate in the double helix. The production of free radicals has been evidenced by spin-trapping for 1 although both compounds were revealed to be good copper-chelating agents. In vitro cytostatic activity and inhibition of [3H]-thymidine incorporation were obtained for 1 while 2 exhibited no activity in both tests. In view of these results, it can be pointed out that the anti-tumor properties of such drugs rely (1) on their ability to reach and to bind DNA and (2) on redox mechanisms involving interactions between the drugs, metals and molecular oxygen. The latter phenomenon leads to the formation of active radical species, able to degrade the DNA.

In vitro and intracellular inhibition of topoisomerase II by the antitumor agent merbarone

Merbarone has previously been shown to have antitumor activity of unknown mechanism in P388 and L1210 tumor models (A. D. Brewer et al., Biochem. Pharmacol., 34:2047-2050, 1985) and is currently undergoing Phase I clinical trials. Here we report that merbarone is an inhibitor of topoisomerase II. Merbarone inhibited purified mammalian topoisomerase II with a 50% inhibitory concentration of 20 microM, as assessed by ATP-dependent unknotting of P4 phage DNA or relaxation of supercoiled pBR322 plasmid. In contrast to the type II enzyme, inhibition of catalytic activity of topoisomerase I required about 10-fold higher concentrations of merbarone, with a 50% inhibitory concentration of approximately 200 microM. Unlike epipodophyllotoxin analogues and certain DNA intercalative agents which stabilize the topoisomerase II-DNA "cleavable complex," merbarone did not cause detectable topoisomerase II-induced DNA cleavage. Furthermore, merbarone inhibited the production by amsacrine or teniposide of topoisomerase II-associated DNA strand breaks; under identical conditions novobiocin did not decrease these breaks, setting merbarone apart from a novobiocin-like class of topoisomerase II inhibitor. In L1210 cells, merbarone produced only small numbers of protein-associated DNA strand breaks, and only at very high concentrations. Merbarone reduced in a concentration-dependent manner the number of amsacrine- or teniposide-stimulated protein-associated DNA strand breaks in L1210 cells or their isolated nuclei. The data suggest that merbarone represents a novel type of topoisomerase II inhibitor.

DNA minor groove binding agents interfere with topoisomerase II mediated lesions induced by epipodophyllotoxin derivative VM-26 and acridine derivative m-AMSA in nuclei from L1210 cells

This study demonstrated that agents capable of interacting with the minor groove in nuclear DNA interfere with topoisomerase II mediated effects of antitumor drugs such as VM-26 and m-AMSA. Distamycin, Hoechst 33258, and DAPI were used as agents capable of AT-specific binding in the minor groove of DNA while producing no profound long-range distortion of DNA structure. In intact nuclei from L1210 cells, these minor groove binders inhibited the induction of topoisomerase II mediated DNA damage (DNA-protein cross-links and DNA double-strand breaks) by VM-26 and m-AMSA. The inhibitory effects of distamycin reflected prevention of formation of new lesions but not reversal of preexisting damage. The minor groove binders did not differentiate between lesions induced by an intercalator, m-AMSA, or by a DNA-nonbinding drug, VM-26. All three groove binders inhibited DNA breaks more strongly than DNA-protein cross-links. The inhibitory potency correlated with the size of minor groove binders and the size of their DNA-binding sites: distamycin (5 bp) greater than Hoechst 33258 (4 bp) greater than DAPI (3 bp). The results showed that DNA minor groove binders are a new type of modulators of the action of topoisomerase II targeted drugs.

Interrelationship between affinity for DNA, cytotoxicity and induction of DNA-breaks in cultured L1210 cells for two series of tricyclic intercalators. Simplified analogues of ellipticine derivatives

The interrelationship between affinity for DNA, cytotoxicity and induction of single-strand DNA breaks in cultured L1210 cells was studied for 21 compounds belonging to two series of tricyclic intercalators: 1-amino-substituted 4-methyl-5H-pyrido[4,3-b]indoles (gamma CARB) and 1-amino-substituted 4-methyl-5H-pyrido[3',4':4,5]pyrrolo[2,3-c]pyridines (PPP), which are simplified analogues of Ellipticine derivatives obtained by deletion of one cycle. Adriamycin, m-AMSA (4'-(9-acridinylamino) methanesulfon-m-anisidide), PZE (10-[diethylaminopropyl amino]-6-methyl-5H-pyrido[3',4':4,5]-pyrrolo[2,3-g] isoquinoline and RTE [( 1-(3-diethylaminopropylamino)-9-methoxy ellipticine, bimaleate) are used as reference compounds. The intercalation of these compounds into DNA was strongly suggested by three experimental observations: (i) the competitive inhibition of ethidium bromide intercalation, (ii) bathochromic and hypochromic effects on absorption spectra induced by DNA, and (iii) drug-induced increase of the DNA length, measured by viscosimetry. PPP derivatives are generally less cytotoxic and induce DNA breaks less efficiently than the gamma CARB ones, both in terms of maximum breakage frequencies and required drug concentrations. The most active compounds induced SSB in the DNA of L1210 cells, in a bell-shaped manner: the SSB frequency increased, rose to a maximum and then decreased as the drug concentrations increased. The maximum SSB frequencies induced by the most active compounds are of the same order as those of reference compounds Adriamycin and PZE. The structurally important requirements are essentially the same for both DNA breakage activity and cytotoxicity: (i) a N-CH3 in the 5-position, (ii) a CH3 in the 4-position, (iii) a hydroxy in the 8-position and (iv) the presence of an (aminoalkyl)amino side chain with preferentially a 3 carbon unit. There is no direct relationship between DNA affinity in vitro and induction of DNA breaks in cells, although a relatively high affinity seemed to be a necessary condition, since the most active compounds have the highest affinities and compounds having a very low affinity are totally inactive. The close correlation between cytotoxicity and extent of induction of DNA breaks suggests that these breaks may be in fact the lethal lesions responsible for cell death and thereby for the antitumor properties of these tricyclic intercalators.

Modulation of 4'-(9-acridinylamino)methanesulfon-m-anisidide-induced, topoisomerase II-mediated DNA cleavage by gossypol

Our earlier studies have shown that gossypol [1,1',6,6',7,7'-hexahydroxy-5,5-diisopropyl - 3,3'-dimethyl - (2,2'- binaphthalene)-8,8'-dicarboxyaldehyde], a male contraceptive, inhibits DNA synthesis by decreasing the activities of DNA polymerase alpha and beta, resulting in the arrest of cells in mid-S phase [L.J. Rosenberg, R.C. Adlakha, D.M. Desai, and P.N. Rao, Biochim. Biophys. Acta, 866: 258-267, 1986]. Now we have examined the effects of gossypol on another enzyme of importance to cellular functions, topoisomerase II (topo II). We have determined the consequences of gossypol treatment on 4'-(9-acridinylamino)methane-sulfon-m anisidide (m-AMSA)-induced topoisomerase II-mediated, protein-associated DNA cleavage using the alkaline elution technique. In HeLa cells pretreated with gossypol (3.4-17.5 microM) for 8-16 h we observed a dose- and time-dependent decrease (50-75%) in DNA cleavage compared to that quantified in cells treated with m-AMSA alone. Gossypol by itself did not induce more than 25 rad-equivalents of DNA single-strand breaks even at the highest dose tested (26 microM). [14C]m-AMSA uptake was identical in treated and untreated cells. Pretreatment of cells with another inhibitor of DNA synthesis, thymidine, which blocks cells at G1/S boundary increased the m-AMSA-induced DNA cleavage by 25%, suggesting that the effect of gossypol might be due to the arrest of cells in mid-S phase. In contrast to gossypol's effects on m-AMSA-induced DNA cleavage, m-AMSA-induced cytotoxicity was actually increased in gossypol pretreated cells. Gossypol blocked topo II strand passing activity (decatenation of kinetoplast DNA) of cellular extracts from HeLa cells. The inhibition of this activity by gossypol was synergistic with the inhibition produced by m-AMSA or etoposide. These data suggest that gossypol can both inhibit topo II catalytic activity and interfere with the stabilization of topo II-DNA complex formation by m-AMSA. These data indicate that the magnitude of m-AMSA-induced DNA cleavage may not necessarily parallel the magnitude of m-AMSA-induced cytotoxicity. The cytotoxicity data may rather be explained by an action of gossypol and m-AMSA to block topo II catalytic activity at a point in the enzyme's strand passing cycle prior to cleavage complex formation that might be particularly toxic to cells in S phase. Gossypol should therefore be useful in improving our understanding of the cellular role of topo II and the consequences of interference with topo II activity by active antineoplastic agents.