Absence of swiveling at sites of intercalator-induced protein-associated deoxyribonucleic acid strand breaks in mammalian cell nucleoids

Synchronous block in DNA synthesis initiation with change in chromatin topology mediated by VP16

The impact of chromatin topology on the DNA synthetic process was studied in the human squamous-cell carcinoma cell line SQ-20B. A 1-h exposure < or = 10 microM VP16 produced an increase in DNA supercoil tension, measured by recording laser light scatter from salt-extracted nuclei. This change was precisely paralleled by a decrease in DNA synthesis. The effects on both DNA supercoiling and DNA synthesis were suppressed at VP16 concentrations between 10 and 20 microM. The changes in DNA supercoiling and synthesis at VP16 concentrations -10 microM were eliminated by coincubation with mimosine, a DNA synthesis initiator poison. We conclude that brief exposure to low concentrations of VP16 disturbs the balance of torsional energy within discrete replicon domains by affecting normal topoisomerase II activity at sites of replication initiation. The resultant increase in negative supercoil tension mediates a topologic checkpoint, limiting the initiation of DNA synthesis. Such a checkpoint may be a common pathway for control, both during the normal replicative cycle and subsequent to DNA damage.

Topologically constrained domains of supercoiled DNA in eukaryotic cells

The size of supercoiled, topologically constrained DNA domains within the squamous carcinoma cell line SQ-20B were determined by direct comparison with a panel of irradiated supercoiled plasmid DNAs. Loss of supercoiling in plasmids was determined by gel electrophoresis and in cells by nucleoid flow cytometry. Comparison of dose-response data for plasmid relaxation with that obtained from SQ-20B cells enabled a direct estimation of supercoil target size in these cells. Plasmids pUCD9P (3.9 kbp), pXT-1 (10.1 kbp), pdBPV-MMT-neo (14.6 kbp), pRK290 (20.0 kbp), and R6K (38 kbp) were used and analyzed under the same exposure conditions as nucleoid DNA. Two sizes of topologically closed domains were found in nucleoids of 0.51+/-0.17Mbp and 1.34+/-0.3 Mbp. In an attempt to relate these large-scale organizations of DNA with function, cells were exposed to the DNA topoisomerase II inhibitor, VP16 and the G1/S cell cycle blocking agent mimosine. A 1 h exposure to VP16 was effective in reducing DNA synthesis which was associated with a parallel increase in nucleoid supercoiling. Addition of the G1 > S inhibitor mimosine enhanced both responses. It is concluded that chromosomes and interphase nuclei are organized into at least two sizes of topologically constrained domains of DNA which may have functional relevance to the control and execution of DNA synthesis.

Study of potential in vitro and in vivo genotoxicity in hepatocytes of quinolone antibiotics

The genotoxicity of quinolone antibiotics has been evaluated in hepatocytes following in vitro and in vivo exposure. Unscheduled DNA synthesis (UDS) was induced in vitro in rat hepatocytes by norfloxacin, ofloxacin, pefloxacin, and ciprofloxacin but not by nalidixic acid. In vivo UDS was not observed in hepatocytes isolated 4 to 24 hr after exposure of adult male F344 rats to either a single dose (30 to 190 mg/kg) or repeated doses (40 mg/kg) of ciprofloxacin. Using the 32P-postlabeling technique, no modified bases were detected in hepatocytes exposed in vitro to ciprofloxacin. In summary, UDS was induced in hepatocytes by in vitro exposure to high concentrations of norfloxacin, ofloxacin, pefloxacin, or ciprofloxacin. There was no evidence of in vitro DNA adduct formation by ciprofloxacin or in vivo DNA damage under the conditions tested. These findings suggest that ciprofloxacin is not DNA reactive, but it induces in vitro UDS as a consequence of some indirect action.

Inhibition of replicon initiation in human cells following stabilization of topoisomerase-DNA cleavable complexes

Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.

Inhibition of replicon initiation in human cells following stabilization of topoisomerase-DNA cleavable complexes

Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.

Mitoxantrone-DNA binding and the induction of topoisomerase II associated DNA damage in multi-drug resistant small cell lung cancer cells

The cytotoxicity anti-tumour intercalating agents such as the anthraquinone mitoxantrone is thought to relate to DNA binding and the trapping of DNA topoisomerase II complexes on cellular DNA. We have studied the uptake, nuclear location, DNA binding mode and DNA damaging capacity of mitoxantrone in a small cell lung carcinoma cell line (NCI-H69) compared with an in vitro-derived variant subline (NCI-H69/LX4) that exhibits "classical" multi-drug resistance (MDR). Variant cells maintained under doxorubicin selection showed reduced RNA levels that returned to control values within 7 days of growth under non-selective conditions. Variant cells released from selection stress showed resistance to DNA cleavage by doxorubicin, mitoxantrone, 4'-epidoxorubicin, 4'-deoxy-doxorubicin but reduced resistance to aclacinomycin A and a 9-alkyl substituted anthracycline in broad agreement with the cross-resistance patterns for cytotoxicity. Mitoxantrone treated NCI-H69 cells were found to accumulate DNA-protein crosslinks during a 4 hr post-treatment incubation period whereas variant cells maintained depressed levels of crosslinking. There was no apparent abnormality in the availability or drug sensitivity of topoisomerase II assayed in crude nuclear extracts of NCI-H69/LX4 cells. Whole cell uptake of radiolabelled mitoxantrone was depressed (50%) in NCI-H69/LX4 compared with NCI-H69, whereas assessment of nuclear-bound drug in individual cells by a fluorescence quenching technique showed at least a 10-fold greater level of target protection. The quenching results provide evidence of a high affinity, saturable mode of drug binding, favoured at low drug concentrations, that correlated with DNA cleavage capacity. We propose that the cytotoxic action of mitoxantrone is dependent upon a restricted and persistent form of binding to DNA that favours the long-term or progressive trapping of topoisomerase II complexes.

Tests for the genotoxicity of m-AMSA, etoposide, teniposide and ellipticine in Neurospora crassa

The antitumor agents m-AMSA, etoposide, teniposide and ellipticine have been reported to be potent clastogens in mammalian cells but non- or weakly mutagenic in bacteria; these observations have been correlated to the interference of these chemicals with DNA topoisomerase II activity in the former, but not in the latter, organisms. The genotoxicity of these 4 agents was evaluated using ad-3 reverse- and forward-mutation tests in Neurospora crassa. These agents (up to 0.8 mumole/plate) did not cause reversion in conidia of the ad-3A frameshift strains N24 and 12-9-26 using the overlay plate test, as contrasted to the positive control frameshift mutagen ICR-170. Heterokaryon 12 (H-12) of N. crassa permits the recovery of all classes of forward mutation at the ad-3+ region, including multilocus deletions. Using resting conidia of H-12 in a suspension assay, ellipticine was moderately mutagenic but no increase in ad-3 mutants was noted with the other 3 agents at a dose of 100 micrograms/ml. In vegetative cultures of H-12 grown in the presence of these agents, all 4 agents were nonmutagenic at a dose of 100 micrograms/ml. The positive control mutagen ICR-170 was mutagenic in both resting conidia and growing cultures of H-12. A similarity between the topoisomerase II of N. crassa and DNA gyrase of bacteria is suggested.

Use of in vitro topoisomerase II assays for studying quinolone antibacterial agents

Several quinolones and antitumor compounds were tested as inhibitors of purified calf thymus topoisomerase II in unknotting, catenation, radiolabeled DNA cleavage, and quantitative nonradiolabeled cleavage assays. The antitumor agents VP-16 (demethylepipodophyllotoxin ethylio-beta-D-glucoside) and ellipticine demonstrated drug-enhanced topoisomerase II DNA cleavage (the concentration of drug that induced 50% of the maximal DNA cleavage in the test system [CC50]) at levels of less than or equal to 5 micrograms/ml. Nalidixic acid, norfloxacin, and oxolinic acid did not induce significant topoisomerase II DNA cleavage, whereas ciprofloxacin did induce some cleavage above background levels. CP-67,015, a new 6,8-difluoro-7-pyridyl 4-quinolone which possesses potent antibacterial activity, inhibited bacterial DNA gyrase at 0.125 micrograms/ml in a nonradioactive DNA cleavage assay. Unlike other quinolones characterized to date, CP-67,015 was shown to strongly enhance topoisomerase II-induced radiolabeled DNA cleavage with a CC50 of 33 micrograms/ml and demonstrated cleavage in a nonradiolabeled DNA cleavage assay with a CC50 of 73 micrograms/ml. The topoisomerase II-mediated cleavage of DNA by CP-67,015 is consistent with its reported clastogenic effect on DNA in cell culture and its positive mutagenic response in mouse lymphoma cells. In vitro topoisomerase II catalytic and cleavage assays are useful for gaining preliminary information concerning the possible interaction(s) of some quinolones with eucaryotic topoisomerase II which may relate directly to their safety (mutagenicity, clastogenicity, or both) in human and veterinary medicinal usage.

Cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells, xrs 5 and xrs 6. IV. Study of chromosomal aberrations and sister-chromatid exchanges by restriction endonucleases and inhibitors of DNA topoisomerase II

Induction of chromosomal aberrations and sister-chromatid exchanges (SCEs) was studied in wild-type Chinese hamster ovary (CHO-K1) cells and its 2 X-ray-sensitive mutants xrs 5 and xrs 6 (known to be deficient in repair of DNA double-strand breaks (DSBs] by restriction endonucleases (REs) and inhibitors of DNA topoisomerase II known to induce DNA strand breaks. Five different types of REs, namely CfoI, EcoRI, HpaII (which induce cohesive DSBs), HaeIII and AluI (which induce blunt DSBs) were employed. REs that induce blunt-end DNA DSBs were found to be more efficient in inducing chromosomal aberrations than those inducing cohesive breaks. xrs 5 and xrs 6 mutants responded with higher sensitivity (50-100% increase in the frequency of aberrations per aberrant cell) to these REs than wild-type CHO-K1 cells. All these REs were also tested for their ability to induce SCEs. The frequency of SCEs increased in wild-type as well as mutant CHO cells, the induced frequency being about 2-fold higher in xrs mutants than in the wild-type cells. We also studied the effect of inhibitors of DNA topoisomerase II, namely 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and etoposid (VP 16), at different stages of the cell cycle of these 3 types of cells. Both drugs increased the frequency of chromosomal aberrations in G2 cells. The mutants showed increased sensitivity to m-AMSA and VP 16, xrs 6 cells being 10- and 2-fold more sensitive than wild-type CHO-K1 cells respectively, and xrs 5 responding with 2-fold higher sensitivity than xrs 6 cells. G1 treatment of CHO cells with m-AMSA increased both chromosome- and chromatid-type aberrations, xrs mutants being about 3-fold more sensitive than CHO-K1 cells. The frequency of SCEs increased also after treatment of exponentially growing and S-phase CHO cells with m-AMSA and the higher sensitivity of xrs mutants (2-fold) was evident. The S-phase appeared to be a specific stage which is most prone for the induction of SCEs by m-AMSA. The results indicate that DNA DSBs induced by REs and inhibitors of DNA topoisomerase II correlate closely with induced chromosomal aberrations and SCEs in these cell lines, indicating that DSBs are responsible for the production of these 2 genetic endpoints.

Effect of combination of m-AMSA and doxorubicin on their redox properties and on DNA cleavage

In vitro studies with the drug combination m-AMSA and doxorubicin were carried out in order to point out whether they can form a redox-system. Indeed, while doxorubicin is known to be bioactivated by NADPH cytochrome P-450 reductase, m-AMSA is readily and reversibly oxidized either chemically or microsomally, to give a quinone diimine with electrophile properties. This redox chemistry has been shown to play a major role in the antineoplastic properties of both drugs. The oxidation of m-AMSA was followed by absorption spectroscopy and the reduction of doxorubicin was observed by circular dichroism. It has been found that both drugs may form a redox-couple and that their association enhances their ability to cut DNA in the absence of cupric ions. Indeed, doxorubicin catalyses the oxidation of m-AMSA in sodium borate buffer (pH 9.25) and conversely the chemical reduction of doxorubicin by m-AMSA induces single and double strand breaks in pBR 322 DNA. This chemical activation may be of importance in vivo, and perhaps the combination of both drugs may lead to a therapeutic advantage.

Reduced DNA topoisomerase II activity and drug-stimulated DNA cleavage in 9-hydroxyellipticine resistant cells

We have isolated a Chinese hamster lung cell line resistant to 9-hydroxyellipticine (DC-3F/9-OH-E) which is also cross-resistant to topoisomerase II inhibitors such as amsacrine and etoposide. In this work we have studied quantitatively both DNA topoisomerase II activity by decatenation of kinetoplast DNA and drug-stimulated DNA cleavage of pBR 322. DNA topoisomerase II activity of DC-3F/9-OH-E nuclear extract was reduced by 3.5-fold as compared to that from DC-3F (sensitive parent) nuclear extract. We also found that DC-3F/9-OH-E nuclear extracts have a reduced capacity to induce in vitro topoisomerase II-mediated DNA cleavage upon stimulation by etoposide and amsacrine (7- and 10-fold respectively). Besides, mixing nuclear extracts from both sensitive and resistant cells indicates that either the enzyme in resistant cells is modified or a modulating factor is associated to it. Our results suggest that the resistance of the DC-3F/9-OH-E cell line to topoisomerase II inhibitors might be due to both a reduced amount of the enzyme and its reduced ability to form the cleavable complex in the presence of drugs.

Effect of adriamycin and hyperthermia on the sedimentation of nucleoids from L1210 cells

We report on the individual and combined effects of adriamycin (ADR) and hyperthermia (HYP) on the sedimentation behavior of L1210 mouse leukemia cell nucleoids in neutral sucrose gradients. Nucleoid sedimentation profiles obtained from cells incubated with ADR (1-10 microM; 30 min; 37 degrees C) exhibited an increased sedimentation rate associated with an increased protein content of these subnuclear units. Exposure of cells to HYP (1-3 h; 42 degrees C) produced similar results. Simultaneous exposure of L1210 cells to conditions of HYP and ADR which resulted in minimal changes in nucleoid sedimentation when used singly, produced an enhanced effect. A similar enhancement was observed with other intercalating antineoplastic agents believed to exert their effect, at least partially, via free radicals (daunorubicin, amsacrine, bisantrene, mitoxantrone). However, enhancement with HYP was not observed with (a) the classic intercalating agent, ethidium bromide; (b) non-intercalating DNA-breaking agents (bleomycin, lithocholic acid, etoposide); (c) inhibitors of poly (ADP-ribose) polymerase (m-methoxybenzamide, benzamide); or (d) non-intercalating antineoplastic agents capable of causing free radical formation (bleomycin). The results suggest that DNA intercalating agents capable of initiating free radical processes may show an enhanced toxicity with simultaneous HYP treatment, and that the nucleoid assay may be a means of screening agents with these biological properties for potential clinical usefulness in combination with HYP.

Effect of 1-beta-D-arabinofuranosylcytosine (ara-C) on nuclear topoisomerase II activity and on the DNA cleavage and cytotoxicity produced by 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and etoposide in m-AMSA-sensitive and -resistant human leukemia cells

The ability of a noncytotoxic dose of ara-C to modulate the amount of 4'-(9-acridinylamino)-methanesulfon-m-anisidide (m-AMSA)- or etoposide-induced topoisomerase II-mediated DNA cleavage and cytotoxicity was examined in m-AMSA-sensitive and -resistant HL-60 human leukemia cells. Ara-C pretreatment (0.1 microM x 48 hr) sensitized m-AMSA-sensitive cells to the cytotoxicity and DNA cleavage produced by both m-AMSA and etoposide. The actions of m-AMSA in the m-AMSA-resistant cells were affected minimally by ara-C. By contrast, ara-C enhanced etoposide-induced DNA cleavage and, to an even greater extent, etoposide-induced cytotoxicity in m-AMSA-resistant cells. These cells were only minimally cross-resistant to etoposide. Ara-C did not affect the cellular uptake of m-AMSA or etoposide, the amount of 0.35 M NaCl-extractable nuclear topoisomerase II activity from either cell line, or the ability of this enzyme activity to covalently bind to DNA in the presence of the drugs, m-AMSA- and etoposide-induced DNA cleavage is thought to result from drug-induced stabilization of a topoisomerase II-DNA complex. The ability of ara-C to modulate this effect and associated cytotoxicity appears to be mediated by the effects of ara-C on cellular targets other than topoisomerase II but which are important to topoisomerase II-mediated events, such as protein-associated DNA cleavage. A good candidate for such a target may be cellular chromatin.

DNA topoisomerase II: a primer on the enzyme and its unique role as a multidrug target in cancer chemotherapy

Based on the weight of evidence accrued in the past eight years, there is little question that the nuclear enzyme, topoisomerase II, serves as a common intracellular target for the cytotoxic effect of drugs of widely varying structure. The enzyme appears to be unique as a chemotherapy target in that it is recruited into a lethal process under the influence of drug. Its role contrasts sharply with other more classical chemotherapy targets, such as dihydrofolate reductase, whose activity must be successfully inhibited for the expression of cytotoxicity. Resistance to inhibitors of this enzyme frequently results from marked elevations in intracellular enzyme content. In contrast, the presence of topoisomerase is required for drug effect, and, in general, the greater the cellular content of the enzyme, the more sensitive the cell will be to these agents. However, important issues remain unresolved. The biochemical events that are initiated by cleavable complex formation and result in cell death must be more fully defined. It is likely a better understanding of the drug-enzyme interaction will be required for rational drug development. Finally, those aspects of the drug-topoisomerase interaction that confer therapeutic selectivity and/or clinical resistance are of paramount importance if the phenomenon is ever to be fully exploited.

Characterization of the topoisomerase II-induced cleavage sites in the c-myc proto-oncogene. In vitro stimulation by the antitumoral intercalating drug mAMSA

In an attempt to get an insight into the activity of mAMSA (a DNA topoisomerase II-mediated drug) on the human proto-oncogene c-myc, an in vitro system consisting of purified calf thymus DNA topoisomerase II and a c-myc DNA inserted in lambda phage was utilized. The occurrence of discrete bands, detected by hybridization of Southern blots with appropriate c-myc probes, indicated the presence of cleavage sites in the sole presence of DNA topoisomerase II. The band intensity increased in the presence of mAMSA, while no significant difference occurred in the cleavage pattern. The location of the cleavage sites along the c-myc locus revealed a striking correspondence with that of some DNase hypersensitive sites. These results indicate that DNA topoisomerase II is most certainly implicated in the mAMSA activity and that the drug stimulates the topoisomerase II cleaving activity at specific sites, which may be involved in the biological activity of the drug.

A simple model for DNA elution from filters

DNA chain scission, induced both in vitro and in vivo by various agents, is an event of great biological relevance. The damage is currently evaluated by empirical membrane separation techniques; the results are quite reproducible and the sensitivity higher than 1 single strand break per 10(9) Daltons. We outline a simple theory of the filtration of coiled macrosolutes, having a random size distribution, through porous membranes, considered as being in quasi-steady flow. The basic transport equation Jj = cj (1 - sigma)Jv is solved by considering that the value of sigma j, the reflection coefficient of component j, (1 less than or equal to j less than or equal to N), is given by (1 - KjRj), where Kj is the partition constant between pore and solution, a function of the conformational entropy loss of the coil, and Rj accounts for the frictional force experienced by a particle moving along the pore. The problem of evaluating the volume Vs filled up with solute has been approached according to a simplified theory of the excluded volume for flexible polymers; the result is Vs = sigma nj4/3 pi(rGj)3 where rGj is the jth radius of gyration. The solution of the resulting set of N differential equations gives nj, the number of molecules of component j remaining on the filter, as a function of the elution volume V. The theory demonstrates that the process is governed by the average dimensions of the coil, so affording a universal calibration of filter elution methods, in excellent agreement with the experiments.

Isolation of intercalator-dependent protein-linked DNA strand cleavage activity from cell nuclei and identification as topoisomerase II

DNA intercalating agents such as 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) have previously been found to induce in mammalian cells the formation of protein-associated DNA single- and double-strand breaks. In the current work, an activity characterized by the production of DNA-protein links associated with DNA strand breaks and by stimulation by m-AMSA was isolated from L1210 cell nuclei and was shown to be due to topoisomerase II. Nuclei were extracted with 0.35 M NaCl, and the extract was fractionated by gel filtration, DNA-cellulose chromatography, and glycerol gradient centrifugation. A rapid filter binding assay was devised to monitor the fractionation procedure on the basis of DNA-protein linking activity. The active DNA-cellulose fraction contained both topoisomerase I and topoisomerase II whereas the glycerol gradient purified material contained only topoisomerase II activity. The properties of the active material were studied at both stages of purification. m-AMSA enhanced the formation of complexes between purified topoisomerase II and SV40 DNA in which the DNA sustained a single- or double-strand cut and the enzyme was covalently linked to the 5' terminus of the DNA. This action was further enhanced by ATP, as well as by nonhydrolyzable ATP analogues. m-AMSA inhibited the topoisomerization and catenation reactions of topoisomerase II, probably because of trapping of the enzyme-DNA complexes. The activity showed a dependence on the type of DNA intercalators used, analogous to what was previously observed in intact cells. m-AMSA had no effect on topoisomerase I.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of 4'-(9-acridinylamino)methanesulphon-m-anisidine) action by verapamil

Verapamil was shown to increase growth inhibition and decrease viability of PY815 mastocytoma cells treated with the anti-cancer drug mAMSA 4'-(9-acridinylamino)methanesulphon-m-anisidide (mAMSA) or its normally inactive congener, 4'-(9-acridinylamino)methanesulphon-o-anisidide (oAMSA). Verapamil also potentiated the effect of sub-optimal concentrations of mAMSA or oAMSA on DNA scission in intact cells. Uptake of [14C]mAMSA by PY815 cells was considerably enhanced, while efflux of [14C]mAMSA from precharged cells was inhibited by verapamil. It is concluded that verapamil potentiates the action of mAMSA on PY815 cells in culture by reducing efflux of drug from the cells. The possibility that verapamil may affect systems that sequester or metabolize AMSA drugs is suggested.

A type-II DNA topoisomerase and a catenating protein from the transplantable VX2 carcinoma

It has recently been suggested that topoisomerases could be important targets for several DNA intercalating drugs used in cancer therapy. This prompted us to purify and characterize a type II topoisomerase in a highly tumorigenic transplantable rabbit tumor isolated from a skin carcinoma associated with cottontail rabbit papillomavirus. We have found that the decatenating activity present in tumor cells was 40-100 times higher than that in the rabbit liver, while no activity could be found in skin extracts. The type II topoisomerases purified from tumor and liver cells consist of two subunits with molecular masses of about 160 kDa. The conditions of the reactions of relaxation, unknotting and decatenation catalyzed by these topoisomerases II were found to be similar to those observed with enzymes of other eukaryotic cells. In the course of the purification of the VX2 enzyme, we isolated and characterized a protein of about 30 kDa in whose presence the topoisomerase II was able to catenate very efficiently supercoiled DNA molecules. This protein has the same electrophoretic mobility as an H1-2 histone, and cross-reacts with an anti-H1 antiserum. The VX2 topoisomerase II as well as the VX2 tumor should constitute useful models for assays of antitumoral drugs.

DNA topoisomerase II as a target of antineoplastic drug therapy

A major goal of cancer therapy research is identification of critical biochemical targets that mediate the ability of effective cancer chemotherapy to kill tumor cells while allowing the maintenance of normal cell function. A candidate for such a target is DNA topoisomerase II, a ubiquitous enzyme that alters three-dimensional conformation of supercoiled DNA. DNA intercalating agents and epipodophyllotoxins stabilize a DNA and topoisomerase II complex. The process of stabilization probably represents the poisoning of an intermediate state in the normal functioning of the enzyme. This stabilized intermediate state can be measured in whole cells using the filter elution method of Kohn to quantify protein-associated DNA cleavage produced when the cells are exposed to intercalators or epipodophyllotoxins. By altering cell populations in quantifiable ways, four factors appear to influence the magnitude of drug-induced, topoisomerase II-mediated DNA cleavage and cytotoxicity: the proliferative state of the cell (proliferating cells are more sensitive than quiescent ones); the cell cycle state (cells pharmacologically recruited into G1-S are more sensitive than asynchronously growing cells); the chromatin conformation (DNA methylation, polyamine depletion, and other chromosomal changes can alter the magnitude of topoisomerase II-mediated effects); the cellular phenotype (in an as yet uncharacterized manner, malignant cells apparently are more sensitive to topoisomerase II-mediated events than normal cells). These data suggest that the biochemical basis of the therapeutic index of drugs such as the intercalating agents or epipodophyllotoxins may be the intrinsic hypersensitivity of the topoisomerase II in malignant cells to poisoning by these drugs.