A novel mechanism of cell killing by anti-topoisomerase II bisdioxopiperazines

DNA strand breaks induced by the anti-topoisomerase II bis-dioxopiperazine ICRF-193

The bis-dioxopiperazine ICRF-193 has long time been considered as a pure topoisomerase II catalytic inhibitor able to exert its inhibitory effect on the enzyme without stabilization of the so-called cleavable complex formed by DNA covalently bound to topoisomerase II. In recent years, however, this concept has been challenged, as a number of reports have shown that ICRF-193 really "poisons" the enzyme, most likely through a different mechanism from that shown by the classical topoisomerase II poisons used in cancer chemotherapy. In the present investigation, we have carried out a study of the capacity of ICRF-193 to induce DNA strand breaks, as classical poisons do, in cultured V79 and irs-2 Chinese hamster lung fibroblasts using the comet assay and pulsed-field gel electrophoresis (PFGE). Our results clearly show that ICRF-193 readily induces breakage in DNA through a mechanism as yet poorly understood.

A dual mechanism of action of the anticancer agent F 11782 on human topoisomerase II alpha

F 11782 is a novel epipodophyllotoxin that targets eukaryotic topoisomerases and inhibits enzyme binding to DNA. While F 11782 has not been found to stabilize either topoisomerase I or topoisomerase II covalent complexes, drug treatment appears to result in DNA damage. F 11782 has also been shown to inhibit the DNA nucleotide excision repair (NER) pathway. Bisdioxopiperazine-resistant small cell lung cancer (SCLC) OC-NYH/Y165S and Chinese hamster ovary (CHO) CHO/159-1 cells having functional Y49F and Y165S mutations in the topoisomerase II alpha isoform were both resistant to F 11782. The catalytic activity of purified human Y50F and Y165S mutant topoisomerase II alpha (Y50F in the human protein corresponds to Y49F in the CHO protein) was likewise resistant to the inhibitory action of F 11782. F 11782 was also found to induce a non-covalent salt-stable complex of human topoisomerase II with DNA that was ATP-independent. F 11782 thus displays a dual mechanism of action on human topoisomerase II alpha, reducing its affinity for DNA while also stabilizing the protein bound in the form of a salt-stable complex. Our results suggest that topoisomerase II alpha is a target of F 11782 in vivo, and that F 11782 may act as a novel topoisomerase II poison.

Catalytic topoisomerase II inhibitors in cancer therapy

The nuclear enzyme DNA topoisomerase II is a major target for antineoplastic agents. All topoisomerase II-directed agents are able to interfere with at least one step of the catalytic cycle. Agents able to stabilize the covalent DNA topoisomerase II complex (also known as the cleavable complex) are traditionally called topoisomerase II poisons, while agents acting on any of the other steps in the catalytic cycle are called catalytic inhibitors. Thus, catalytic topoisomerase II inhibitors are a heterogeneous group of compounds that might interfere with the binding between DNA and topoisomerase II (aclarubicin and suramin), stabilize noncovalent DNA topoisomerase II complexes (merbarone, ICRF-187, and structurally related bisdioxopiperazine derivatives), or inhibit ATP binding (novobiocin). Some, such as fostriecin, may also have alternative biological targets. Whereas topoisomerase II poisons are used solely for their antitumor activities, catalytic inhibitors are utilized for a variety of reasons, including their activity as antineoplastic agents (aclarubicin and MST-16), cardioprotectors (ICRF-187), or modulators in order to increase the efficacy of other agents (suramin and novobiocin). In this review, the mechanism and biological activity of different catalytic inhibitors is described, with emphasis on therapeutically used compounds. We will then discuss future development and applications of this interesting class of compounds.

Probing the role of linker substituents in bisdioxopiperazine analogs for activity against wild-type and mutant human topoisomerase II alpha

The bisdioxopiperazines are catalytic inhibitors of eukaryotic type II DNA topoisomerases capable of trapping these enzymes as a salt-stable closed-clamp complex on circular DNA. The various bisdioxopiperazine analogs differ from each other because of structural differences in the linker connecting the two dioxopiperazine rings. Although the composition of this linker region has been found to be important for potency, the structural basis for this is largely unknown. To elucidate the role of the linker region in drug action, we have analyzed the effect of different linker substituents in otherwise identical analogs by studying their interaction with wild-type and mutant human topoisomerase II alpha. Two mutations, L169I and R162Q, displayed differential sensitivity toward closely related analogs, suggesting that the linker region in these compounds plays a highly specific role in protein drug interaction. The finding that the L169I mutation, which probably represents a subtle structural change, was sufficient to confer resistance further emphases the importance of this region of the protein for bisdioxopiperazine inhibition of topoisomerase II. Comparing the sensitivity profiles of different bisdioxopiperazines against wild-type and mutant proteins with that of mitindomide, we observed a spectrum of sensitivity closely resembling that of ICRF-154, a bisdioxopiperazine with no linker substituents. We discuss the implications of these observations for the understanding of the mechanism of bisdioxopiperazine action on topoisomerase II.

The topoisomerase IIbeta circular clamp arrests transcription and signals a 26S proteasome pathway

It has been proposed that the topoisomerase II (TOP2)beta-DNA covalent complex arrests transcription and triggers 26S proteasome-mediated degradation of TOP2beta. It is unclear whether the initial trigger for proteasomal degradation is due to DNA damage or transcriptional arrest. In the current study we show that the TOP2 catalytic inhibitor 4,4-(2,3-butanediyl)-bis(2,6-piperazinedione) (ICRF-193), which traps TOP2 into a circular clamp rather than the TOP2-DNA covalent complex, can also arrest transcription. Arrest of transcription, which is TOP2beta-dependent, is accompanied by proteasomal degradation of TOP2beta. Different from TOP2 poisons and other DNA-damaging agents, ICRF-193 did not induce proteasomal degradation of the large subunit of RNA polymerase II. These results suggest that proteasomal degradation of TOP2beta induced by the TOP2-DNA covalent complex or the TOP2 circular clamp is due to transcriptional arrest but not DNA damage. By contrast, degradation of the large subunit of RNA polymerase II is due to a DNA-damage signal.

Induction of endoreduplication by topoisomerase II catalytic inhibitors

The striking phenomenon of endoreduplication has long attracted attention from cytogeneticists and researchers into cell cycle enzymology and dynamics alike. Because of the variety of agents able to induce endoreduplication and the various cell types where it has been described, until now no clear or unique mechanism of induction of this phenomenon, rare in animals but otherwise quite common in plants, has been proposed. Recent years, however, have witnessed the unfolding of a number of essential physiological roles for DNA topoisomerase II, with special emphasis on its major role in mitotic chromosome segregation after DNA replication. In spite of the lack of mammalian mutants defective in topoisomerase II as compared with yeast, experiments with inhibitors of the enzyme have supported the hypothesis that this crucial untangling of daughter DNA molecules by passing an intact helix through a transient double-stranded break carried out by the enzyme, when it fails, leads to aberrant mitosis that results in endoreduplication, polyploidy and eventually cell death. Anticancer drugs that interfere with topoisomerase II can be classified into two groups. The classical poisons act by stabilizing the enzyme in the so-called cleavable complex and result in DNA damage, which represents a problem in the study of endoreduplication. The true catalytic inhibitors, which are not cleavable complex stabilizers, allow us to use doses efficient in the induction of endoreduplication while eliminating high levels of DNA and chromosome damage. This review will discuss the basic and applied aspects of this as yet scarcely explored field.

Evidence from studies with intact mammalian cells that merbarone and bis(dioxopiperazine)s are topoisomerase II poisons

A Chinese hamster V79 cell-based assay for detection of topoisomerase II (topo II) poisons and catalytic inhibitors has been applied to study two bis(dioxopiperazine)s (ICRF-187 and ICRF-154) and a structurally distinct but related compound, merbarone. All three compounds have been previously characterized as being catalytic inhibitors of DNA topo II based primarily on in vitro studies with purified enzymes. The present studies indicate, to the contrary, that all three compounds are very potent DNA clastogens in V79 cells, by virtue of their ability to produce micronuclei, the formation of which is strongly antagonized under conditions in which DNA topo II is rendered catalytically inactive. None of the compounds could be demonstrated to possess catalytic inhibitory activity in intact V79 cells under the conditions tested. These studies provide biological evidence that bis(dioxopiperazine)s are capable of functional topo II poisoning in intact mammalian cells.

Roles of DNA topoisomerases in chromosome segregation and mitosis

DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the DNA molecule in a very precise and unique fashion. Taking into account their fundamental roles in many events during DNA metabolism such as replication, transcription, recombination, condensation or segregation, it is no wonder that the last decade has witnessed an exponential interest on topoisomerases, mainly after the discovery of their potential role as targets in novel antitumor therapy. The difficulty of the lack of topoisomerase II mutants in higher eukaryotes has been partly overcome by the availability of drugs that act as either poisons or true catalytic inhibitors of the enzyme. These chemical tools have provided strong evidence that accurate performance of topoisomerase II is essential for chromosome segregation before anaphase, and this in turn constitutes a prerequisite for the development of normal mitosis. In the absence of cytokinesis, cells become polyploid or endoreduplicated.

ATR enforces the topoisomerase II-dependent G2 checkpoint through inhibition of Plk1 kinase

An ATR-dependent G(2) checkpoint responds to inhibition of topoisomerase II and delays entry into mitosis by sustaining nuclear exclusion of cyclin B1-Cdk1 complexes. Here we report that induction of this checkpoint with ICRF-193, a topoisomerase II catalytic inhibitor that does not cause DNA damage, was associated with an ATR-dependent inhibition of polo-like kinase 1 (Plk1) kinase activity and a decrease in cyclin B1 phosphorylation. Expression of constitutively active Plk1 but not wild type Plk1 reversed ICRF-193-induced mitotic delay in HeLa cells, suggesting that Plk1 kinase activity is important for the checkpoint response to ICRF-193. G(2)/M synchronized normal human fibroblasts, when treated with ICRF-193, showed a decrease in cyclin B1 phosphorylation and Plk1 kinase activity despite high cyclin B1-Cdk1 kinase activity. G(2) fibroblasts that were treated with caffeine to override the checkpoint response to ICRF-193 displayed a high incidence of chromosomal aberrations. Taken together, these results suggest that ATR-dependent inhibition of Plk1 kinase activity may be one mechanism to regulate cyclin B1 phosphorylation and sustain nuclear exclusion during the G(2) checkpoint response to topoisomerase II inhibition. Moreover, the results demonstrate an important role for the topoisomerase II-dependent G(2) checkpoint in the preservation of human genomic stability.

In vitro search for synergy and antagonism: evaluation of docetaxel combinations in breast cancer cell lines

The use of combination chemotherapy is the accepted standard for most human malignancies but little attention has been paid to drug interactions. A combination of drugs may be synergistic, additive, or antagonistic in cytotoxic activity. This study evaluated combinations of agents with docetaxel, one of the most active agents in human breast cancer, using a median effects model to look at synergy or antagonism in vitro as a potential predictor of clinical outcome. Three human breast cancer cell lines, MCF7/wt, MCF7/adr (multiply drug resistant), and BT474 were grown to confluence, plated into 96 well dishes, and incubated with combinations of drugs for 72h. Cytotoxic effect was measured by the MTT assay. Median effect analysis was used to calculate the combination index (CI) with values less than 1 indicating synergism, 1 additive effects, and greater than 1 antagonism. Potentially useful combinations for clinical study which were identified included docetaxel with vinorelbine, docetaxel with dexrazoxane, docetaxel with cis-retinoic acid, docetaxel with disulfiram and either doxorubicin or epirubicin, and docetaxel with dexrazoxane and epirubicin.

Human small cell lung cancer NYH cells resistant to the bisdioxopiperazine ICRF-187 exhibit a functional dominant Tyr165Ser mutation in the Walker A ATP binding site of topoisomerase II alpha

Bisdioxopiperazine anti-cancer agents are catalytic inhibitors of topoisomerase II which by unknown means lock the enzyme in a closed clamp form and inhibit its ATPase activity. In order to demarcate a putative pharmacophore, we here describe a novel Tyr165Ser mutation in the enzyme's Walker A ATP binding site leading to specific bisdioxopiperazine resistance when transformed into a temperature-conditional yeast system. The Tyr165Ser mutation differed from a previously described Arg162Gln by being heterozygous and by purified Tyr165Ser enzyme being drug-resistant in a kinetoplast DNA decatenation enzymatic assay. This suggested dominant nature of Tyr165Ser was supported by co-transformation studies in yeast of plasmids carrying wild type and mutant genes. These results enable a model of the bisdioxopiperazine pharmacophore using the proposed asymmetric ATP hydrolysis of the enzyme.

Formation and longevity of idarubicin-induced DNA topoisomerase II cleavable complexes in K562 human leukaemia cells

Idarubicin (IDA) is an anthracycline used during treatment of acute myelogenous leukaemia (AML) and is clinically important because of its potency and lipophilicity (compared to the related compounds daunorubicin and doxorubicin). These drugs target DNA topoisomerase II (topo II), a nuclear enzyme that regulates DNA topology. Topo II poisoning leads to the trapping of an intermediate in the enzyme's cycle termed the "cleavable complex." This study aims to increase understanding of drug interactions by use of the "TARDIS" (trapped in agarose DNA immunostaining) assay to measure drug-induced topo II cleavable complexes in individual cells treated with anthracyclines. Mammalian cells contain two isoforms of topo II (alpha and beta) and the TARDIS assay enables visualisation of isoform-specific complexes. Drug-treated cells were embedded in agarose, lysed and incubated with anti-topo II antibodies to microscopically detect topo IIalpha or beta complexes. Results for K562 cells (at clinically relevant concentrations) showed that IDA and idarubicinol, its metabolite, formed mainly topo IIalpha cleavable complexes, the level of which decreases at doses > 1 microM for IDA. Our data suggest that this decrease is due to catalytic inhibition by IDA at these doses. Doxorubicin formed low levels of topo IIalpha complexes and negligible topo IIbeta complexes. In cytotoxicity studies, IDA and idarubicinol were equipotent, but both were more potent than daunorubicin and doxorubicin. We showed for the first time that there was a persistent increase in levels of topo IIalpha cleavable complexes after removal of IDA, suggesting that its greater effectiveness may be associated with both the longevity and high levels of these complexes.

Maleimide is a potent inhibitor of topoisomerase II in vitro and in vivo: a new mode of catalytic inhibition

Maleimide, N-ethyl-maleimide (NEM), and N-methyl-maleimide (NMM) were identified as potent catalytic inhibitors of purified human topoisomerase IIalpha, whereas the ring-saturated analog succinimide was completely inactive. Catalytic inhibition was not abrogated by topoisomerase II mutations that totally abolish the effect of bisdioxopiperazine compounds on catalytic inhibition, suggesting a different mode of action by these maleimides. Furthermore, in DNA cleavage assay maleimide and NEM could antagonize etoposide-induced DNA double-strand breaks. Consistently, maleimide could antagonize the effect of topoisomerase II poisons in three different in vivo assays: 1) In an alkaline elution assay maleimide protected against etoposide-induced DNA damage. 2) In a band depletion assay maleimide reduced etoposide-induced trapping of topoisomerase IIalpha and beta on DNA. 3) In a clonogenic assay maleimide antagonized the cytotoxicity of etoposide and daunorubicin on four different cell lines of human and murine origin. at-MDR cell lines with reduced nuclear topoisomerase IIalpha content are fully sensitive to maleimide, indicating that it is not a topoisomerase II poison in vivo. Our finding that topoisomerase II is sensitive to maleimide, NMM, and NEM but insensitive to succinimide demonstrates a strict requirement for the unsaturated ring bond for activity. We suggest that the observed antagonism in vitro and in vivo is caused by covalent modification of topoisomerase II cysteine residues reducing the amount of catalytically active enzyme sensitive to the action of topoisomerase II poisons.

High yield of endoreduplication induced by ICRF-193: a topoisomerase II catalytic inhibitor

An uncommonly high yield of spontaneous endoreduplication is a feature of the CHO mutant EM9, besides its defective repair of single, as well as double-DNA strand-breaks and its extraordinarily elevated yield of sister chromatid exchanges (SCEs) after bromodeoxyuridine (BrdU) incorporation into DNA. Since the nuclear enzyme topoisomerase II (topo II) has been reported to be responsible for the segregation of daughter chromosomes during mitosis, in the present investigation we have made use of the bisdioxopiperazine ICRF-193, a topo II catalytic inhibitor that interferes with the normal turnover of the enzyme. In order to see whether both EM9 cells and its parental cell line AA8, which show differences in the spontaneous frequency of endoreduplicated cells are or not equally sensitive to the topo II catalytic inhibitor, both cell lines have been treated with a range of doses of the bisdioxopiperazine. Our results show that both cell lines respond to the treatment entering in an endoreduplication cycle, but the EM9 cells are extremely sensitive to the inhibition of topo II.

ATPase domain of eukaryotic DNA topoisomerase II. Inhibition of ATPase activity by the anti-cancer drug bisdioxopiperazine and ATP/ADP-induced dimerization

We have prepared full-length Drosophila and human topoisomerase II and truncation constructs containing the amino-terminal ATPase domain, and we have analyzed their biochemical properties. The ATPase activity of the truncation proteins, similar to that of the full-length proteins, is greatly stimulated by the presence of DNA. This activity of the truncation proteins is also sensitive to the inhibition by the drug bisdioxopiperazine, ICRF-193, albeit at a much lower level than the full-length protein. Therefore, bisdioxopiperazine can directly interact with the NH(2)-terminal ATPase domain, but the drug-enzyme interaction may involve other domains as well. The ATPase activity of the ATPase domain protein showed a quadratic dependence on enzyme concentration, suggesting that dimerization of the NH(2)-terminal domain is a rate-limiting step. Using both protein cross-linking and sedimentation equilibrium analysis, we showed that the ATPase domain exists as a monomer in the absence of cofactors but can readily dimerize in the presence of a nonhydrolyzable analog of ATP, 5'-adenylyl-beta,gamma-imidodiphosphate. More interestingly, both ATP and ADP can also promote protein dimerization. This result thus suggests that the protein clamp, mediated through the dimerization of ATPase domain, remains closed after ATP hydrolysis and opens upon the dissociation of ADP.

Topoisomerase II poisoning by ICRF-193

Antineoplastic bis(dioxopiperazine)s, such as meso-2,3-bis(2,6-dioxopiperazin-4-yl)butane (ICRF-193), are widely believed to be only catalytic inhibitors of topoisomerase II. However, topoisomerase inhibitors have little or no antineoplastic activity unless they are topoisomerase poisons, a special subclass of topoisomerase-targeting drugs that stabilize topoisomerase-DNA strand passing intermediates and thus cause the topoisomerase to become a cytotoxic DNA-damaging agent. Here we report that ICRF-193 is a very significant topoisomerase II poison. Detection of topoisomerase II poisoning by ICRF-193 required the use of a chaotropic protein denaturant in the topoisomerase poisoning assays. ICRF-193 caused dose-dependent cross-linking of human topoisomerase IIbeta to DNA and stimulated topoisomerase IIbeta-mediated DNA cleavage at specific sites on (32)P-end-labeled DNA. Human topoisomerase IIalpha-mediated DNA cleavage was stimulated to a lesser extent by ICRF-193. In vivo experiments with MCF-7 cells also showed the requirement of a chaotropic protein denaturant in the assays and selectivity for the beta-isozyme of human topoisomerase II. Studies with two topoisomerase IIbeta-negative cell model systems confirmed significant topoisomerase II poisoning by ICRF-193 in the wild type cells and were consistent with beta-isozyme selectivity. Common use of only the detergent, SDS, in assays may have led to failure to detect topoisomerase II poisoning by ICRF-193 in earlier studies.

In vitro effects of dexrazoxane (Zinecard) and classical acute leukemia therapy: time to consider expanded clinical trials?

Anthracyclines have been the backbone of acute leukemia therapy in the adult for many years, but little attention has been paid to the long-term toxicity of these agents in this disease because of the poor survival of this population of patients. Recent studies have examined dose-intensified daunorubicin with dosages as high as 95 mg/m2 daily x 3 in this population with the attendant concerns of both acute and chronic toxicity. We have examined three human leukemia cell lines in vitro, treated with either daunorubicin, mitoxantrone, with or without cytosine arabinoside in the presence of dexrazoxane to determine whether such treatment would be synergistic or antagonistic. AML-193, CRF-SB, and Molt-4 cell lines were grown to confluence, plated into microtiter dishes and incubated for 72 h with varying concentrations of the above drugs. Cytotoxicity was determined by the MTT assay, and synergy or antagonism by median effect analysis. Dexrazoxane demonstrated additive or synergistic cytotoxic effects (CI <1) under most conditions. The triplet of daunorubicin, cytosine arabinoside, and dexrazoxane showed profound synergy in all three cell lines. These effects occurred at clinically achievable levels. If high dosages of anthracyclines are contemplated in this population, these preclinical data suggest that the addition of dexrazoxane to classical therapy is not antagonistic and thus may allow an investigation of the role of dexrazoxane as a cardiac protectant.

Decreased topoisomerase IIalpha expression confers increased resistance to ICRF-193 as well as VP-16 in mouse embryonic stem cells

To elucidate the relationship between topoisomerase (topo) II expression and sensitivity to anti-topo II drugs in mammalian cells, we generated mouse embryonic stem cell mutants heterozygous for the topo IIalpha gene by gene targeting. The level of topo IIalpha in the heterozygous cells reduced to one-half of that found in wild-type cells, while topo IIbeta levels were similar in both cell types. Importantly, the heterozygous cells exhibited an increased resistance to ICRF-193 as well as VP-16, suggesting that ICRF-193, like VP-16, exerts its cytotoxicity through converting topo II to a poison.

Ku antigen is required to relieve G2 arrest caused by inhibition of DNA topoisomerase II activity by the bisdioxopiperazine ICRF-193

Ku antigen is necessary for DNA double-strand break (DSB) repair through its ability to bind DNA ends with high affinity and to recruit the catalytic subunit of DNA-PK to the DSBs. Ku-deficient cells are hypersensitive to agents causing DSBs in DNA but also to the DNA topoisomerase II (topo II) inhibitor ICRF-193, which does not induce DSBs. This suggests a new role of Ku antigen, that is independent of DSB repair by DNA-PK. Here we characterize the basis for the hypersensitivity of Ku-deficient cells to ICRF-193. Chromosome condensation and segregation, which are dependent on topo II, but also the catalytic activity of topo II in late S-G2 were inhibited to a comparable extent when ICRF-193 was applied to Ku-deficient cells or wild-type cells. However, mutant cells arrested in G2 by ICRF-193 treatment were unable to progress into M phase upon drug removal, although drug-trapped topo II complexes were removed from DNA and the two isoforms of topo II recovered their catalytic activity as in wild-type cells. The reversibility of G2 arrest was recovered by complementation of mutant cells with a human Ku86 cDNA. Notably, chromosome condensation was abnormal in Ku-deficient cells after suppression of the G2 arrest by caffeine, even in the absence of ICRF-193. These results reflect the involvement of Ku-antigen in the cellular response to topo II inhibition, more particularly in relieving G2 arrest caused by topo II inhibition in late S/G2 and the subsequent recovery of chromosome condensation.

Role of an inverted CCAAT element in human topoisomerase IIalpha gene expression in ICRF-187-sensitive and -resistant CEM leukemic cells

DNA topoisomerase (topo) IIalpha gene expression or activity is altered in tumor cells selected for resistance to inhibitors of topoII. To better understand the mechanisms by which topoIIalpha expression levels are modulated, we examined topoIIalpha transcriptional regulation in ICRF-187-sensitive and ICRF-187-resistant human leukemic cell lines that express an increased amount of topoIIalpha protein and mRNA. Transient transfections of luciferase reporter plasmids containing either the full-length human topoIIalpha promoter or fragments of it revealed that topoIIalpha transcriptional activity was significantly increased in the drug-resistant CEM/ICRF-8 cells, compared with CEM cells. Specifically, the transcriptional activity of the full-length topoIIalpha promoter (nucleotides -557 to +90) was doubled in CEM/ICRF-8 compared with CEM cells. Serial deletion of the topoIIalpha promoter permitted localization of the region responsible for its up-regulation in the drug-resistant cells between nucleotides -557 and -162, which includes the last three inverted CCAAT elements (ICE) 3 to 5. Note that construction of a point mutation in ICE3 resulted in a significant increase in transcriptional activity of the topoIIalpha promoter in the drug-sensitive CEM cells. In addition, by electrophoretic mobility shift assay, ICE3 was recognized by a protein complex containing NF-YB that was present at reduced levels in the topoIIalpha-overexpressing CEM/ICRF-8 extracts, suggesting that ICE3 plays a negative regulatory role in human topoIIalpha gene expression. This is the first study to show that topoIIalpha transcriptional up-regulation in ICRF-187-resistant cells is mediated in part by altered regulation of the third inverted CCAAT box in the topoIIalpha promoter.

Anti-topoisomerase drugs as potent inducers of chromosomal aberrations

DNA topoisomerases catalyze topological changes in DNA that are essential for normal cell cycle progression and therefore they are a preferential target for the development of anticancer drugs. Anti-topoisomerase drugs can be divided into two main classes: "cleavable complex" poisons and catalytic inhibitors. The "cleavable complex" poisons are very effective as anticancer drugs but are also potent inducers of chromosome aberrations so they can cause secondary malignancies. Catalytic inhibitors are cytotoxic but they do not induce chromosome aberrations. Knowledge about the mechanism of action of topoisomerase inhibitors is important to determine the best anti-topoisomerase combinations, with a reduced risk of induction of secondary malignancies.

Characterization of the biological and biochemical activities of F 11782 and the bisdioxopiperazines, ICRF-187 and ICRF-193, two types of topoisomerase II catalytic inhibitors with distinctive mechanisms of action

F 11782 is a newly identified catalytic inhibitor of topoisomerases I and II, without any detectable interaction with DNA. This study aimed to establish whether its catalytic inhibition of topoisomerase II was mediated by mechanisms similar to those identified for the bisdioxopiperazines. In vitro combinations of F 11782 with etoposide resulted in greater than additive cytotoxicity in L1210 cells, contrasting with marked antagonism for combinations of etoposide with either ICRF-187 or ICRF-193. All three compounds caused a G2/M blockade of P388 cells after an 18-h incubation, but by 40 h polyploidization was evident only with the bisdioxopiperazines. Gel retardation data revealed that only F 11782, and not the bisdioxopiperazines, was capable of completely inhibiting the DNA-binding activity of topoisomerase II, confirming its novel mechanism of action. Furthermore, unlike ICRF-187 and ICRF-193, the cytotoxicity of F 11782 appeared mediated, at least partially, by DNA damage induction in cultured GCT27 human teratoma cells, as judged by a fluorescence-enhancement assay and monitoring p53 activation. Finally, the major in vivo antitumor activity of F 11782 against the murine P388 leukemia (i.v. implanted) and the B16 melanoma (s.c. grafted) contrasted with the bisdioxopiperazines' general lack of activity. Overall, F 11782 and the bisdioxopiperazines appear to function as quite distinctive catalytic topoisomerase II inhibitors.

Probing the interaction of the cytotoxic bisdioxopiperazine ICRF-193 with the closed enzyme clamp of human topoisomerase IIalpha

Topoisomerase II is an ATP-operated protein clamp that captures a DNA helix and transports it through another DNA duplex, allowing chromosome segregation at mitosis. A number of cytotoxic bisdioxopiperazines such as ICRF-193 target topoisomerase II by binding and trapping the closed enzyme clamp. To investigate this unusual mode of action, we have used yeast to select plasmid-borne human topoisomerase IIalpha alleles resistant to ICRF-193. Mutations in topoisomerase IIalpha of Leu-169 to Phe (L169F) (in the N-terminal ATPase domain) and Ala-648 to Pro (A648P) (in the core domain) were identified as conferring >50-fold and 5-fold resistance to ICRF-193 in vivo, respectively. The L169F mutation, located next to the Walker A box ATP-binding sequence, resulted in a mutant enzyme displaying ICRF-193-resistant topoisomerase and ATPase activities and whose closed clamp was refractory to ICRF-193-mediated trapping as an annulus on closed circular DNA. These data imply that the mutation interferes directly with ICRF-193 binding to the N-terminal ATPase gate. In contrast, the A648P enzyme displayed topoisomerase activities exhibiting wild-type sensitivity to ICRF-193. We suggest that the inefficient trapping of the A648P closed clamp results either from the observed increased ATP requirement, or more likely, from lowered salt stability, perhaps involving destabilization of ICRF-193 interactions with the B'-B' interface in the core domain. These results provide evidence for at least two different phenotypic classes of ICRF-193 resistance mutations and suggest that bisdioxopiperazine action involves the interplay of both the ATPase and core domains of topoisomerase IIalpha.

N-terminal and core-domain random mutations in human topoisomerase II alpha conferring bisdioxopiperazine resistance

Random mutagenesis of human topoisomerase II alpha cDNA followed by functional expression in yeast cells lacking endogenous topoisomerase II activity in the presence of ICRF-187, identified five functional mutations conferring cellular bisdioxopiperazine resistance. The mutations L169F, G551S, P592L, D645N, and T996L confer > 37, 37, 18, 14, and 19 fold resistance towards ICRF-187 in a 24 h clonogenic assay, respectively. Purified recombinant L169F protein is highly resistant towards catalytic inhibition by ICRF-187 in vitro while G551S, D645N, and T996L proteins are not. This demonstrates that cellular bisdioxopiperazine resistance can result from at least two classes of mutations in topoisomerase II; one class renders the protein non-responsive to bisdioxopiperazine compounds, while an other class does not appear to affect the catalytic sensitivity towards these drugs. In addition, our results indicate that different protein domains are involved in mediating the effect of bisdioxopiperazine compounds.