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

Etoposide promotes DNA loop trapping and barrier formation by topoisomerase II

Etoposide is a broadly employed chemotherapeutic and eukaryotic topoisomerase II poison that stabilizes cleaved DNA intermediates to promote DNA breakage and cytotoxicity. How etoposide perturbs topoisomerase dynamics is not known. Here we investigated the action of etoposide on yeast topoisomerase II, human topoisomerase IIα and human topoisomerase IIβ using several sensitive single-molecule detection methods. Unexpectedly, we found that etoposide induces topoisomerase to trap DNA loops, compacting DNA and restructuring DNA topology. Loop trapping occurs after ATP hydrolysis but before strand ejection from the enzyme. Although etoposide decreases the innate stability of topoisomerase dimers, it increases the ability of the enzyme to act as a stable roadblock. Interestingly, the three topoisomerases show similar etoposide-mediated resistance to dimer separation and sliding along DNA but different abilities to compact DNA and chirally relax DNA supercoils. These data provide unique mechanistic insights into the functional consequences of etoposide on topoisomerase II dynamics.

Prodrug of ICRF-193 provides promising protective effects against chronic anthracycline cardiotoxicity in a rabbit model in vivo

The anthracycline (ANT) anticancer drugs such as doxorubicin or daunorubicin (DAU) can cause serious myocardial injury and chronic cardiac dysfunction in cancer survivors. A bisdioxopiperazine agent dexrazoxane (DEX) has been developed as a cardioprotective drug to prevent these adverse events, but it is uncertain whether it is the best representative of the class. The present study used a rabbit model of chronic ANT cardiotoxicity to examine another bisdioxopiperazine compound called GK-667 (meso-(butane-2,3-diylbis(2,6-dioxopiperazine-4,1-diyl))bis(methylene)-bis(2-aminoacetate) hydrochloride), a water-soluble prodrug of ICRF-193 (meso-4,4'-(butan-2,3-diyl)bis(piperazine-2,6-dione)), as a potential cardioprotectant. The cardiotoxicity was induced by DAU (3 mg/kg, intravenously, weekly, 10 weeks), and GK-667 (1 or 5 mg/kg, intravenously) was administered before each DAU dose. The treatment with GK-667 was well tolerated and provided full protection against DAU-induced mortality and left ventricular (LV) dysfunction (determined by echocardiography and LV catheterization). Markers of cardiac damage/dysfunction revealed minor cardiac damage in the group co-treated with GK-667 in the lower dose, whereas almost full protection was achieved with the higher dose. This was associated with similar prevention of DAU-induced dysregulation of redox and calcium homeostasis proteins. GK-667 dose-dependently prevented tumor suppressor p53 (p53)-mediated DNA damage response in the LV myocardium not only in the chronic experiment but also after single DAU administration. These effects appear essential for cardioprotection, presumably because of the topoisomerase IIβ (TOP2B) inhibition provided by its active metabolite ICRF-193. In addition, GK-667 administration did not alter the plasma pharmacokinetics of DAU and its main metabolite daunorubicinol (DAUol) in rabbits in vivo. Hence, GK-667 merits further investigation as a promising drug candidate for cardioprotection against chronic ANT cardiotoxicity.

Structure-Activity Relationship Study of Dexrazoxane Analogues Reveals ICRF-193 as the Most Potent Bisdioxopiperazine against Anthracycline Toxicity to Cardiomyocytes Due to Its Strong Topoisomerase IIβ Interactions

Cardioprotective activity of dexrazoxane (ICRF-187), the only clinically approved drug against anthracycline-induced cardiotoxicity, has traditionally been attributed to its iron-chelating metabolite. However, recent experimental evidence suggested that the inhibition and/or depletion of topoisomerase IIβ (TOP2B) by dexrazoxane could be cardioprotective. Hence, we evaluated a series of dexrazoxane analogues and found that their cardioprotective activity strongly correlated with their interaction with TOP2B in cardiomyocytes, but was independent of their iron chelation ability. Very tight structure-activity relationships were demonstrated on stereoisomeric forms of 4,4'-(butane-2,3-diyl)bis(piperazine-2,6-dione). In contrast to its rac-form 12, meso-derivative 11 (ICRF-193) showed a favorable binding mode to topoisomerase II in silico, inhibited and depleted TOP2B in cardiomyocytes more efficiently than dexrazoxane, and showed the highest cardioprotective efficiency. Importantly, the observed ICRF-193 cardioprotection did not interfere with the antiproliferative activity of anthracycline. Hence, this study identifies ICRF-193 as the new lead compound in the development of efficient cardioprotective agents.

DNA topoisomerases as molecular targets for anticancer drugs

The significant role of topoisomerases in the control of DNA chain topology has been confirmed in numerous research conducted worldwide. The prevalence of these enzymes, as well as the key importance of topoisomerase in the proper functioning of cells, have made them the target of many scientific studies conducted all over the world. This article is a comprehensive review of knowledge about topoisomerases and their inhibitors collected over the years. Studies on the structure-activity relationship and molecular docking are one of the key elements driving drug development. In addition to information on molecular targets, this article contains details on the structure-activity relationship of described classes of compounds. Moreover, the work also includes details about the structure of the compounds that drive the mode of action of topoisomerase inhibitors. Finally, selected topoisomerases inhibitors at the stage of clinical trials and their potential application in the chemotherapy of various cancers are described.

Cell Cycle-Dependent Control and Roles of DNA Topoisomerase II

Type II topoisomerases are ubiquitous enzymes in all branches of life that can alter DNA superhelicity and unlink double-stranded DNA segments during processes such as replication and transcription. In cells, type II topoisomerases are particularly useful for their ability to disentangle newly-replicated sister chromosomes. Growing lines of evidence indicate that eukaryotic topoisomerase II (topo II) activity is monitored and regulated throughout the cell cycle. Here, we discuss the various roles of topo II throughout the cell cycle, as well as mechanisms that have been found to govern and/or respond to topo II function and dysfunction. Knowledge of how topo II activity is controlled during cell cycle progression is important for understanding how its misregulation can contribute to genetic instability and how modulatory pathways may be exploited to advance chemotherapeutic development.

DNA Damage by an essential enzyme: A delicate balance act on the tightrope

DNA topoisomerases are essential for DNA metabolic processes such as replication and transcription. Since DNA is double stranded, the unwinding needed for these processes results in DNA supercoiling and catenation of replicated molecules. Changing the topology of DNA molecules to relieve supercoiling or resolve catenanes requires that DNA be transiently cut. While topoisomerases carry out these processes in ways that minimize the likelihood of genome instability, there are several ways that topoisomerases may fail. Topoisomerases can be induced to fail by therapeutic small molecules such as by fluoroquinolones that target bacterial topoisomerases, or a variety of anti-cancer agents that target the eukaryotic enzymes. Increasingly, there have been a large number of agents and processes, including natural products and their metabolites, DNA damage, and the intrinsic properties of the enzymes that can lead to long-lasting DNA breaks that subsequently lead to genome instability, cancer, and other diseases. Understanding the processes that can interfere with topoisomerases and how cells respond when topoisomerases fail will be important in minimizing the consequences when enzymes need to transiently interfere with DNA integrity.

Effects of an unusual poison identify a lifespan role for Topoisomerase 2 in Saccharomyces cerevisiae

A progressive loss of genome maintenance has been implicated as both a cause and consequence of aging. Here we present evidence supporting the hypothesis that an age-associated decay in genome maintenance promotes aging in Saccharomyces cerevisiae (yeast) due to an inability to sense or repair DNA damage by topoisomerase 2 (yTop2). We describe the characterization of LS1, identified in a high throughput screen for small molecules that shorten the replicative lifespan of yeast. LS1 accelerates aging without affecting proliferative growth or viability. Genetic and biochemical criteria reveal LS1 to be a weak Top2 poison. Top2 poisons induce the accumulation of covalent Top2-linked DNA double strand breaks that, if left unrepaired, lead to genome instability and death. LS1 is toxic to cells deficient in homologous recombination, suggesting that the damage it induces is normally mitigated by genome maintenance systems. The essential roles of yTop2 in proliferating cells may come with a fitness trade-off in older cells that are less able to sense or repair yTop2-mediated DNA damage. Consistent with this idea, cells live longer when yTop2 expression levels are reduced. These results identify intrinsic yTop2-mediated DNA damage as a potentially manageable cause of aging.

Resveratrol: A novel type of topoisomerase II inhibitor

Resveratrol, a polyphenol found in various plant sources, has gained attention as a possible agent responsible for the purported health benefits of certain foods, such as red wine. Despite annual multi-million dollar market sales as a nutriceutical, there is little consensus about the physiological roles of resveratrol. One suggested molecular target of resveratrol is eukaryotic topoisomerase II (topo II), an enzyme essential for chromosome segregation and DNA supercoiling homeostasis. Interestingly, resveratrol is chemically similar to ICRF-187, a clinically approved chemotherapeutic that stabilizes an ATP-dependent dimerization interface in topo II to block enzyme activity. Based on this similarity, we hypothesized that resveratrol may antagonize topo II by a similar mechanism. Using a variety of biochemical assays, we find that resveratrol indeed acts through the ICRF-187 binding locus, but that it inhibits topo II by preventing ATPase domain dimerization rather than stabilizing it. This work presents the first comprehensive analysis of the biochemical effects of both ICRF-187 and resveratrol on the human isoforms of topo II, and reveals a new mode for the allosteric regulation of topo II through modulation of ATPase status. Natural polyphenols related to resveratrol that have been shown to impact topo II function may operate in a similar manner.

Novel xanthone-polyamine conjugates as catalytic inhibitors of human topoisomerase IIα

It has been proposed that xanthone derivatives with anticancer potential act as topoisomerase II inhibitors because they interfere with the ability of the enzyme to bind its ATP cofactor. In order to further characterize xanthone mechanism and generate compounds with potential as anticancer drugs, we synthesized a series of derivatives in which position 3 was substituted with different polyamine chains. As determined by DNA relaxation and decatenation assays, the resulting compounds are potent topoisomerase IIα inhibitors. Although xanthone derivatives inhibit topoisomerase IIα-catalyzed ATP hydrolysis, mechanistic studies indicate that they do not act at the ATPase site. Rather, they appear to function by blocking the ability of DNA to stimulate ATP hydrolysis. On the basis of activity, competition, and modeling studies, we propose that xanthones interact with the DNA cleavage/ligation active site of topoisomerase IIα and inhibit the catalytic activity of the enzyme by interfering with the DNA strand passage step.

Single-molecule Förster resonance energy transfer (FRET) analysis discloses the dynamics of the DNA-topoisomerase II (Top2) interaction in the presence of TOP2-targeting agents

Topoisomerases play crucial roles in DNA replication, transcription, and recombination. For instance, topoisomerase II (Top2) is critically important for resolving DNA tangles during cell division, and as such, it is a broad anticancer drug target. Top2 regulates DNA topology by transiently breaking one double-stranded DNA molecule (cleavage), allowing a second double strand to pass through the opened DNA gate (opening), and then closing the gate by rejoining the broken ends. Drugs that modulate Top2 catalysis may therefore affect enzymatic activity at several different steps. Previous studies have focused on examining DNA cleavage and ligation; however, the dynamic opening and closing of the DNA gate has been less explored. Here, we used the single-molecule Förster resonance energy transfer (smFRET) method to observe the open and closed state of the DNA gate and to measure dwell times in each state. Our results show that Top2 binds and bends DNA to increase the energy transfer efficiency (E_FRET), and ATP treatment further induces the fluctuation of E_FRET, representing the gate opening and closing. Additionally, our results demonstrate that both types of Top2-targeting anticancer drugs, the catalytic inhibitor dexrazoxane (ICRF187) and mechanistic poison teniposide (VM26), can interfere with DNA gate dynamics and shorten the dwell time in the closed state. Moreover, Top2 bound to the nonhydrolyzable ATP analog 5'-adenylyl-β,γ-imidodiphosphate exhibits altered DNA gate dynamics, but the DNA gate appears to open and close even after N-gate closure. In summary, we have utilized single-molecule detection to unravel Top2 DNA gate dynamics and reveal previously unknown effects of Top2 drugs on these dynamics.

EVALUATION OF ANTICANCER POTENTIAL OF EIGHT VEGETAL SPECIES FROM THE STATE OF OAXACA

Background:

Eight plant species from Oaxaca, some of them used in traditional medicine, were subjected to screening of several biological activities to provide data regarding their anticancer potential, although no scientific information is available about their pharmacological effects.

Materials and methods:

Methanol extracts from stems or roots of the eight plants were tested for antioxidant activity by the DPPH- method. Antimicrobial activity was determined using the agar diffusion method and the minimal inhibitory concentration (MIC) was obtained by broth dilution method. Antitopoisomerase activity was assessed using mutant strains of Saccharomyces cerevisiae JN362a, JN394, JN394t_-1, JN394t₂.₄ and JN394t_2-5. The mutagenic activity was evaluated using the Ames test (Salmonella typhimurium TA1535).

Results:

No extract showed significant antioxidant activity. The best antimicrobial activity was observed for Salpianthus arenarius (MIC 56.25 μg/mL) and Lantana achyranthifolia (MIC 78.12 μg/mL) against Staphylococcus aureus. Extracts of Acalypha cuspidata, Alloispermum integrifolium and L. achyranthifolia stems showed antitopoisomerase II activity with JN394_t-1 growth of -30.88±0.0%, -38.11±4.95%, and -70.97±12.02% respectively. Galium mexicanum stem extract showed antitopoisomerase I activity with growth of 35.31±6.36% on the same mutant strain. All plant extracts were non-mutagenic. Fractionation of A. cuspidata extract led to identification of two subfractions with antitopoisomerase I and II activity at 154μg/mL (Positive controls 50 and 100μg/mL).

Conclusion:

Methanol extracts of A. cuspidata, A. integrifolium, G. mexicanum, and L. achyranthifolia stems showed antitopoisomerase and non-mutagenic activities, and consequently could be promising as a source of anticancer drugs.

Cell-based high-throughput compound screening reveals functional interaction between oncofetal HMGA2 and topoisomerase I

HMGA2 is an important chromatin factor that interacts with DNA via three AT-hook domains, thereby regulating chromatin architecture and transcription during embryonic and fetal development. The protein is absent from differentiated somatic cells, but aberrantly re-expressed in most aggressive human neoplasias where it is causally linked to cell transformation and metastasis. DNA-binding also enables HMGA2 to protect cancer cells from DNA-damaging agents. HMGA2 therefore is considered to be a prime drug target for many aggressive malignancies. Here, we have developed a broadly applicable cell-based reporter system which can identify HMGA2 antagonists targeting functionally important protein domains, as validated with the known AT-hook competitor netropsin. In addition, high-throughput screening can uncover functional links between HMGA2 and cellular factors important for cell transformation. This is demonstrated with the discovery that HMGA2 potentiates the clinically important topoisomerase I inhibitor irinotecan/SN-38 in trapping the enzyme in covalent DNA-complexes, thereby attenuating transcription.

The catalytic topoisomerase II inhibitor dexrazoxane induces DNA breaks, ATF3 and the DNA damage response in cancer cells

BACKGROUND AND PURPOSE

The catalytic topoisomerase II inhibitor dexrazoxane has been associated not only with improved cancer patient survival but also with secondary malignancies and reduced tumour response.

EXPERIMENTAL APPROACH

We investigated the DNA damage response and the role of the activating transcription factor 3 (ATF3) accumulation in tumour cells exposed to dexrazoxane.

KEY RESULTS

Dexrazoxane exposure induced topoisomerase IIα (TOP2A)-dependent cell death, γ-H2AX accumulation and increased tail moment in neutral comet assays. Dexrazoxane induced DNA damage responses, shown by enhanced levels of γ-H2AX/53BP1 foci, ATM (ataxia telangiectasia mutated), ATR (ATM and Rad3-related), Chk1 and Chk2 phosphorylation, and by p53 accumulation. Dexrazoxane-induced γ-H2AX accumulation was dependent on ATM. ATF3 protein was induced by dexrazoxane in a concentration- and time-dependent manner, which was abolished in TOP2A-depleted cells and in cells pre-incubated with ATM inhibitor. Knockdown of ATF3 gene expression by siRNA triggered apoptosis in control cells and diminished the p53 protein level in both control and dexrazoxane -treated cells. This was accompanied by increased γ-H2AX accumulation. ATF3 knockdown also delayed the repair of dexrazoxane -induced DNA double-strand breaks.

CONCLUSIONS AND IMPLICATIONS

As with other TOP2A poisons, dexrazoxane induced DNA double-strand breaks followed by activation of the DNA damage response. The DNA damage-triggered ATF3 controlled p53 accumulation and generation of double-strand breaks and is proposed to serve as a switch between DNA damage and cell death following dexrazoxane treatment. These findings suggest a mechanistic explanation for the diverse clinical observations associated with dexrazoxane.

An improved high yield total synthesis and cytotoxicity study of the marine alkaloid neoamphimedine: an ATP-competitive inhibitor of topoisomerase IIα and potent anticancer agent

Recently, we characterized neoamphimedine (neo) as an ATP-competitive inhibitor of the ATPase domain of human Topoisomerase IIα. Thus far, neo is the only pyridoacridine with this mechanism of action. One limiting factor in the development of neo as a therapeutic agent has been access to sufficient amounts of material for biological testing. Although there are two reported syntheses of neo, both require 12 steps with low overall yields (≤6%). In this article, we report an improved total synthesis of neo achieved in 10 steps with a 25% overall yield. In addition, we report an expanded cytotoxicity study using a panel of human cancer cell lines, including: breast, colorectal, lung, and leukemia. Neo displays potent cytotoxicity (nM IC50 values) in all, with significant potency against colorectal cancer (lowest IC50 = 6 nM). We show that neo is cytotoxic not cytostatic, and that neo exerts cytotoxicity by inducing G2-M cell cycle arrest and apoptosis.

Isobenzofuranone derivatives exhibit antileishmanial effect by inhibiting type II DNA topoisomerase and inducing host response

Leishmania, a protozoan parasite, causes a wide range of human diseases ranging from the localized self-healing cutaneous lesions to fatal visceral leishmaniasis. Toxicity of traditional first line drugs and emergence of drug-resistant strains have worsened the situation. DNA topoisomerase II in kinetoplastid protozoan parasites are of immense interest as drug target because they take part in replication of unusual kinetoplast DNA network. In this study, we have taken target-based therapeutic approaches to combat leishmaniasis. Two isobenzofuranone compounds, viz., (1) 3,5-bis(4-chlorophenyl)-7-hydroxyisobenzofuran-1(3H)-one (JVPH3) and (2) (4-bromo)-3'-hydroxy-5'-(4-bromophenyl)-benzophenone(JVPH4) were synthesized chemically and characterized by NMR and mass spectrometry analysis. Activity of type II DNA topoisomerase of leishmania (LdTOPII) was monitored by decatenation assay and plasmid cleavage assay. The antiparasitic activity of these compounds was checked in experimental BALB/c mice model of visceral leishmaniasis. Isobenzofuranone derivatives exhibited potent antileishmanial effect on both antimony (Sb) sensitive and resistant parasites. Treatment with isobenzofuranone derivatives on promastigotes caused induction of reactive oxygen species (ROS)-mediated apoptosis like cell death in leishmania. Both the compounds inhibited the decatenation activity of LdTOPII but have no effect on bi-subunit topoisomerase IB. Treatment of LdTOPII with isobenzofuranone derivatives did not stabilize cleavage complex formation both in vitro and in vivo. Moreover, treatment with isobenzofuranone derivatives on Leishmania donovani-infected mice resulted in clearance of parasites in liver and spleen by induction of Th1 cytokines. Taken together, our data suggest that these compounds can be exploited as potential antileishmanial agents targeted to DNA topoisomerase II of the parasite.

Type IA topoisomerase inhibition by clamp closure

Bacterial DNA topoisomerase I (topoI) catalyzes relaxation of negatively supercoiled DNA. The enzyme alters DNA topology through protein-operated DNA gate, switching between open and closed conformations during its reaction. We describe the mechanism of inhibition of Mycobacterium smegmatis and Mycobacterium tuberculosis topoI by monoclonal antibodies (mAbs) that bind with high affinity and inhibit at 10-50 nM concentration. Unlike other inhibitors of topoisomerases, the mAbs inhibited several steps of relaxation reaction, namely DNA binding, cleavage, strand passage, and enzyme-DNA dissociation. The enhanced religation of the cleaved DNA in presence of the mAb indicated closing of the enzyme DNA gate. The formation of enzyme-DNA heterocatenane in the presence of the mAbs as a result of closing the gate could be inferred by the salt resistance of the complex, visualized by atomic force microscopy and confirmed by fluorescence measurements. Locking the enzyme-DNA complex as a closed clamp restricted the movements of the DNA gate, affecting all of the major steps of the relaxation reaction. Enzyme trapped on DNA in closed clamp conformation formed roadblock for the elongating DNA polymerase. The unusual multistep inhibition of mycobacterial topoisomerases may facilitate lead molecule development, and the mAbs would also serve as valuable tools to probe the enzyme mechanism.

Topoisomerase assays

Topoisomerases are nuclear enzymes that play essential roles in DNA replication, transcription, chromosome segregation, and recombination. All cells have two major forms of topoisomerases: type I enzymes, which make single-stranded cuts in DNA, and type II enzymes, which cut and pass double-stranded DNA. DNA topoisomerases are important targets of approved and experimental anti-cancer agents. The protocols described in this unit are for assays used to assess new chemical entities for their ability to inhibit both forms of DNA topoisomerase. Included are an in vitro assay for topoisomerase I activity based on relaxation of supercoiled DNA, and an assay for topoisomerase II based on the decatenation of double-stranded DNA. The preparation of mammalian cell extracts for assaying topoisomerase activity is described, along with a protocol for an ICE assay to examine topoisomerase covalent complexes in vivo, and an assay for measuring DNA cleavage in vitro.

Destroying the ring: Freeing DNA from Ku with ubiquitin

The Ku heterodimer, consisting of the proteins Ku70 and Ku80, is the central component of the non-homologous end joining (NHEJ) pathway of double strand break (DSB) repair. Ku is able to recognize and bind a DSB by virtue of its ring-like structure. Both pre-repair and topologically trapped post-repair Ku heterodimers are thought to be inhibitory to multiple cellular processes. Thus, a regulated mechanism for the removal of Ku from chromatin was predicted to exist. Recent evidence shows that Ku80 is removed from DNA through a ubiquitin-mediated process. Similar processes have been shown to be involved in the regulated dissociation of a host of other proteins from chromatin, and this appears to be a general and conserved mechanism for the regulation of chromatin-associated factors. A potential mechanism for this pathway is discussed.

Topoisomerase IIalpha maintains genomic stability through decatenation G(2) checkpoint signaling

Topoisomerase IIalpha (topoIIalpha) is an essential mammalian enzyme that topologically modifies DNA and is required for chromosome segregation during mitosis. Previous research suggests that inhibition of topoII decatenatory activity triggers a G(2) checkpoint response, which delays mitotic entry because of insufficient decatenation of daughter chromatids. Here we examine the effects of both topoIIalpha and topoIIbeta on decatenatory activity in cell extracts, DNA damage and decatenation G(2) checkpoint function, and the frequencies of p16(INK4A) allele loss and gain. In diploid human fibroblast lines, depletion of topoIIalpha by small-interfering RNA was associated with severely reduced decatenatory activity, delayed progression from G(2) into mitosis and insensitivity to G(2) arrest induced by the topoII catalytic inhibitor ICRF-193. Furthermore, interphase nuclei of topoIIalpha-depleted cells showed increased frequencies of losses and gains of the tumor suppressor genetic locus p16(INK4A). This study shows that the topoIIalpha protein is required for decatenation G(2) checkpoint function, and inactivation of decatenation and the decatenation G(2) checkpoint leads to abnormal chromosome segregation and genomic instability.

Revised genetic requirements for the decatenation G2 checkpoint: the role of ATM

The decatenation G2 checkpoint is proposed to delay cellular progression from G2 into mitosis when intertwined daughter chromatids are insufficiently decatenated. Previous studies indicated that the ATM- and Rad3-related (ATR) checkpoint kinase, but not the ataxia telangiectasia-mutated (ATM) kinase, was required for decatenation G2 checkpoint function. Here, we show that the method used to quantify decatenation G2 checkpoint function can influence the identification of genetic requirements for the checkpoint. Normal human diploid fibroblast (NHDF) lines responded to the topoisomerase II (topo II) catalytic inhibitor ICRF-193 with a stringent G2 arrest and a reduction in the mitotic index. While siRNA-mediated depletion of ATR and CHEK1 increased the mitotic index in ICRF-193 treated NHDF lines, depletion of these proteins did not affect the mitotic entry rate, indicating that the decatenation G2 checkpoint was functional. These results suggest that ATR and CHEK1 are not required for the decatenation G2 checkpoint, but may influence mitotic exit after inhibition of topo II. A re-evaluation of ataxia telangiectasia (AT) cell lines using the mitotic entry assay indicated that ATM was required for the decatenation G2 checkpoint. Three NHDF cell lines responded to ICRF-193 with a mean 98% inhibition of the mitotic entry rate. Examination of the mitotic entry rates in AT fibroblasts upon treatment with ICRF-193 revealed a significantly attenuated decatenation G2 checkpoint response, with a mean 59% inhibition of the mitotic entry rate. In addition, a normal lymphoblastoid line exhibited a 95% inhibition of the mitotic entry rate after incubation with ICRF-193, whereas two AT lymphoblastoid lines displayed only 36% and 20% inhibition of the mitotic entry rate. Stable depletion of ATM in normal human fibroblasts with short hairpin RNA also attenuated decatenation G2 checkpoint function by an average of 40%. Western immunoblot analysis demonstrated that treatment with ICRF-193 induced ATM autophosphorylation and ATM-dependent phosphorylation of Ser15-p53 and Thr68 in Chk2, but no appreciable phosphorylation of Ser139-H2AX or Ser345-Chk1. The results suggest that inhibition of topo II induces ATM to phosphorylate selected targets that contribute to a G2 arrest independently of DNA damage.

Targeting DNA topoisomerase II in cancer chemotherapy

Recent molecular studies have expanded the biological contexts in which topoisomerase II (TOP2) has crucial functions, including DNA replication, transcription and chromosome segregation. Although the biological functions of TOP2 are important for ensuring genomic integrity, the ability to interfere with TOP2 and generate enzyme-mediated DNA damage is an effective strategy for cancer chemotherapy. The molecular tools that have allowed an understanding of the biological functions of TOP2 are also being applied to understanding the details of drug action. These studies promise refined targeting of TOP2 as an effective anticancer strategy.

The requirement of Artemis in double-strand break repair depends on the type of DNA damage

Artemis is a recently identified factor involved in V(D)J recombination and nonhomologous end joining (NHEJ) of DNA double-strand break (DSB) repair. Here, we performed targeted disruption of the Artemis gene (ARTEMIS) in the human pre-B cell line Nalm-6. Unexpectedly, we found that cells lacking Artemis exhibit increased sensitivity to low doses, but not high doses, of ionizing radiation. We also show that ARTEMIS-deficient cells are hypersensitive to the topoisomerase II inhibitor etoposide, but to a much lesser extent than cells lacking DNA ligase IV, a critical component of NHEJ. Unlike DNA ligase IV-deficient cells, ARTEMIS-deficient cells were not hypersensitive to ICRF-193, a topoisomerase II inhibitor that does not stabilize topoisomerase II-DNA cleavable complexes. Collectively, our results suggest that Artemis only partially participates in the NHEJ pathway to repair DSBs in human somatic cells.

Effects of dexrazoxane and amifostine on evolution of Doxorubicin cardiomyopathy in vivo

Doxorubicin is one of the most active drugs in oncology, with cardiotoxicity as a serious side effect of its application. The aim of this study was to investigate dexrazoxane and amifostine impact on the evolution of myocardial changes induced by doxorubicin. BalbC female mice were treated with doxorubicin only (10 mg/kg, single intravenous push), or with dexrazoxane (200 mg/kg, intraperitoneal [ip]) or amifostine (200 mg/kg, ip) 60 mins or 30 mins prior to treatment with doxorubicin, respectively. Blood sampling for determination of conventional serum-marker activity was performed 48 hrs later. The grade of histopathology changes was evaluated by light microscopy 1.5 and 3 months after treatments using the Billingham scoring method. Control groups consisted of nontreated mice. After doxorubicin-only treatment, the grade of heart tissue damage was found to increase in the period between 1.5 and 3 months. A similar but less intense progression was also detected in amifostine-pretreated animals, with significant difference among median Billingham scores between the two time points. The pretreatment with dexrazoxane suspended expansion of tissue lesions in time. Changes in serum enzyme activity revealed two correlations: the greater reduction in alpha-hydroxybutyrate dehydrogenase (alpha-HBDH) leakage is associated with a lower percentage of damaged tissue, and the creatine kinase to alpha-HBDH percent of difference ratio being greater than one is correlated with limited spreading of pathological lesions. Our results indicate that the development of doxorubicin-induced heart failure is based on a slow and persistent expansion of pathological process even long after the completion of the treatment. Dexrazoxane has proved to be successful and superior over amifostine against such an evolution of doxorubicin cardiomyopathy.

A mouse model for studying the interaction of bisdioxopiperazines with topoisomerase IIalpha in vivo

The bisdioxopiperazines such as (+)-(S)-4,4'-propylenedi-2,6-piperazinedione (dexrazoxane; ICRF-187), 1,2-bis(3,5-dioxopiperazin-1-yl)ethane (ICRF-154), and 4,4'-(1,2-dimethyl-1,2-ethanediyl)bis-2,6-piperazinedione (ICRF-193) are agents that inhibit eukaryotic topoisomerase II, whereas their ring-opened hydrolysis products are strong iron chelator. The clinically approved analog ICRF-187 is a pharmacological modulator of topoisomerase II poisons such as etoposide in preclinical animal models. ICRF-187 is also used to protect against anthracycline-induced cardiomyopathy and has recently been approved as an antidote for alleviating tissue damage and necrosis after accidental anthracycline extravasation. This dual modality of bisdioxopiperazines, including ICRF-187, raises the question of whether their pharmacological in vivo effects are mediated through interaction with topoisomerase II or via their intracellular iron chelating activity. In an attempt to distinguish between these possibilities, we here present a transgenic mouse model aimed at identifying the contribution of topoisomerase IIalpha to the effects of bisdioxopiperazines. A tyrosine 165 to serine mutation (Y165S) in topoisomerase IIalpha, demonstrated previously to render the human ortholog of this enzyme highly resistant toward bisdioxopiperazines, was introduced at the TOP2A locus in mouse embryonic stem cells by targeted homologous recombination. These cells were used for the generation of transgenic TOP2A(Y165S/+) mice, which were demonstrated to be resistant toward the general toxicity of both ICRF-187 and ICRF-193. Hematological measurements indicate that this is most likely caused by a decreased ability of these agents to induce myelosuppression in TOP2A(Y165S/+) mice, highlighting the role of topoisomerase IIalpha in this process. The biological and pharmacological implications of these findings are discussed, and areas for further investigations are proposed.

The topoisomerase II-Hsp90 complex: a new chemotherapeutic target?

The modulation of DNA topology by topoisomerase II plays a crucial role during chromosome condensation and segregation in mitosis and has thus become a highly attractive target for chemotherapeutic drugs. However, these drugs are highly toxic, and so new approaches are required. One such strategy is to target topoisomerase II-interacting proteins. Here we report the identification of potential topoisomerase II-associated proteins using immunoprecipitation, followed by 1-D and 2-D gel electrophoresis and MALDI-TOF mass spectrometry. A total of 23 proteins were identified and, of these, 17 were further validated as topoisomerase IIalpha-associated proteins by coimmunoprecipitation and Western blot. Six of the interacting proteins were cellular chaperones, including 3 members of the heat shock protein-90 (Hsp90) family, and so the effect of Hsp90 modulation on the antitumor activity of topoisomerase II drugs was tested using the sulforhodamine B assay, clonogenic assays and a xenograft model. The Hsp90 inhibitors geldanamycin, 17-AAG (17-allylamino-17-demethoxygeldanamycin) and radicicol significantly enhanced the activity of the topoisomerase II poisons etoposide and mitoxantrone in vitro and in vivo. Thus, our method of identifying topoisomerase II-interacting proteins appears to be effective, and at least 1 novel topoisomerase IIalpha-associated protein, Hsp90, may represent a valid drug target in the context of topoisomerase II-directed chemotherapy.

Cell cycle-dependent DNA damage signaling induced by ICRF-193 involves ATM, ATR, CHK2, and BRCA1

Topoisomerase II is essential for cell proliferation and survival and has been a target of various anticancer drugs. ICRF-193 has long been used as a catalytic inhibitor to study the function of topoisomerase II. Here, we show that ICRF-193 treatment induces DNA damage signaling. Treatment with ICRF-193 induced G2 arrest and DNA damage signaling involving gamma-H2AX foci formation and CHK2 phosphorylation. DNA damage by ICRF-193 was further demonstrated by formation of the nuclear foci of 53BP1, NBS1, BRCA1, MDC1, and FANCD2 and increased comet tail moment. The DNA damage signaling induced by ICRF-193 was mediated by ATM and ATR and was restricted to cells in specific cell cycle stages such as S, G2, and mitosis including late and early G1 phases. Downstream signaling of ATM and ATR involved the phosphorylation of CHK2 and BRCA1. Altogether, our results demonstrate that ICRF-193 induces DNA damage signaling in a cell cycle-dependent manner and suggest that topoisomerase II might be essential for the progression of the cell cycle at several stages including DNA decondensation.

DNA damage responses after exposure to DNA-based products

BACKGROUND

The development of DNA-based therapies holds great promise for the treatment of diseases that remain difficult to manage using conventional pharmaceuticals. Whilst there are considerable data regarding chemical-induced DNA damage, there are limited reports published studying the potential of exogenous DNA to damage genomic DNA.

METHODS

To investigate this problem, the differential gene expression (DGE) of DNA repair genes was examined to identify biomarkers, based on the hypothesis that DNA damage, including double-strand breaks (DSBs) and insertional mutagenesis, would be expected to induce biological pathways associated with repair. Human HepG2 cells were exposed to the chemical genotoxins, etoposide (ETOP) and methylmethanesulphonate (MMS), as positive controls, or biological agents (i.e. exogenous DNA with and without the use of transfection complexes or via various viral vectors). Following transfection (6-72 h) the cells were harvested for RNA and DGE was determined by quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

The expression of genes involved in the repair of DSBs were significantly increased after treatment with ETOP (>4-fold) or MMS (>5-fold). Transfection using Effectene and ExGen 500 resulted in no significant changes; however, transfection with ExGen 500 resulted in an increase in the expression levels of GADD45 mRNA, consistent with global cellular stress. Viral vectors increased (3-6-fold) expression of genes associated with DSBs and cellular stress responses and, as expected, the effect was the most marked with the retroviral vector.

CONCLUSIONS

The DGE profiles observed in HepG2 cells following transduction/transfection suggest that a subset of DNA repair genes may provide novel biomarkers to rapidly detect DNA damage induced by DNA products at the level of the genome, rather than at selected genes.

Inhibition of cellular proliferation by diterpenes, topoisomerase II inhibitor

We examined the effects of 12 terpene compounds derived from the roots of Euphorbia kansui on the proliferative activity of Xenopus embryo cells. Eight of these compounds showed significant inhibition of cellular proliferation even at low concentrations, while four of them needed to be present at higher concentrations to inhibit cellular proliferation. In order to define the mechanism of inhibition of cellular proliferation by these compounds, the effects of diterpene compounds on the activity of topoisomerase II were measured. Most of the diterpene compounds that inhibited cellular proliferation also inhibited topoisomerase II activity.

Topoisomerase II-DNA complexes trapped by ICRF-193 perturb chromatin structure

DNA topoisomerase II (topo II) is involved in unlinking replicating DNA and organizing mitotic chromosomes. Topo II is the target of many antitumour drugs. Topo II inhibition results in extensive catenation of newly replicated DNA and may potentially perturb chromatin assembly. Here, we show that the topo II inhibitor ICRF-193 does not prevent the bulk of nucleosome deposition, but perturbs nucleosome spacing in Xenopus egg extracts. This is due to the trapping of topo II-closed clamps on the DNA rather than increased DNA catenation. Inhibition of replicative DNA decatenation has in itself little or no effect on nucleosome deposition and spacing, suggesting that DNA can easily accommodate the sharp bending constraints imposed by the co-habitation of nucleosomes and catenane nodes. Chromatin perturbation by topo II clamps may explain some dominant cellular effects of ICRF-193. Nucleosome-driven bending of precatenane nodes may facilitate their unlinking by topo II during unperturbed replication.

Induction of genotoxic and cytotoxic damage by aclarubicin, a dual topoisomerase inhibitor

The anthracycline aclarubicin (ACLA) is an intercalative antibiotic and antineoplastic agent that efficiently binds to DNA, leading to a secondary inhibition of the catalytic activity of topoisomerase II (topo II) on DNA. Besides this activity, ACLA has been reported to exert a concomitant poisoning effect on topo I, in a fashion similar to that of the antitumor drug camptothecin and its derivatives. As a consequence of this dual (topo II catalytic inhibiting/topo I poisoning) activity of ACLA, the picture is somewhat confusing with regards to DNA damage and cytotoxicity. We studied the capacity of ACLA to induce catalytic inhibition of topo II as well as cytotoxic effects and DNA damage in cultured Chinese hamster V79 cells and their radiosensitive counterparts irs-2. The ultimate purpose was to find out whether differences could be observed between the two cell lines in their response to ACLA, as has been widely reported for radiosensitive cells treated with topo poisons. Our results seem to agree with the view that the radiosensitive irs-2 cells appear as hypersensitive ACLA as compared with radiation repair-proficient V79 cells. The recovery after ACLA treatment was also followed-up, and the irs-2 mutant was found to be less proficient than V79 to repair DNA strand breaks induced by ACLA.

Stability of the topoisomerase II closed clamp conformation may influence DNA-stimulated ATP hydrolysis

Type II DNA topoisomerases catalyze changes in DNA topology and use nucleotide binding and hydrolysis to control conformational changes required for the enzyme reaction. We examined the ATP hydrolysis activity of a bisdioxopiperazine-resistant mutant of human topoisomerase II alpha with phenylalanine substituted for tyrosine at residue 50 in the ATP hydrolysis domain of the enzyme. This substitution reduced the DNA-dependent ATP hydrolysis activity of the mutant protein without affecting the relaxation activity of the enzyme. A similar but stronger effect was seen when the homologous mutation (Tyr28 --> Phe) was introduced in yeast Top2. The ATPase activities of human TOP2alpha(Tyr50 --> Phe) and yeast Top2(Tyr28 --> Phe) were resistant to both bisdioxopiperazines and the ATPase inhibitor sodium orthovanadate. Like bisdioxopiperazines, vanadate traps the enzyme in a salt-stable closed conformation termed the closed clamp, which can be detected in the presence of circular DNA substrates. Consistent with the vanadate-resistant ATPase activity, salt-stable closed clamps were not detected in reactions containing the yeast or human mutant protein, vanadate, and ATP. Similarly, ADP trapped wild-type topoisomerase II as a closed clamp, but could not trap either the human or yeast mutant enzymes. Our results demonstrate that bisdioxopiperazine-resistant mutants exhibit a difference in the stability of the closed clamp formed by the enzyme and that this difference in stability may lead to a loss of DNA-stimulated ATPase. We suggest that the DNA-stimulated ATPase of topoisomerase II is intimately connected with steps that occur while the N-terminal domain of the enzyme is dimerized.

Toward a comprehensive model for induced endoreduplication

Both the biological significance and the molecular mechanism of endoreduplication (END) have been debated for a long time by cytogeneticists and researchers into cell cycle enzymology and dynamics alike. Mainly due to the fact that a wide variety of agents have been reported as able to induce endoreduplication and the diversity of 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. DNA topoisomerase II (topo II), plays a major role in mitotic chromosome segregation after DNA replication. The classical topo II poisons act by stabilizing the enzyme in the so-called cleavable complex and result in DNA damage as well as END, while the true catalytic inhibitors, which are not cleavable-complex-stabilizers, do induce END without concomitant DNA and chromosome damage. Taking into account these observations on the induction of END by drugs that interfere with topo II, together with our recently obtained evidence that the nature of DNA plays an important role for chromosome segregation [Cortes, F., Pastor, N., Mateos, S., Dominguez, I., 2003. The nature of DNA plays a role in chromosome segregation: endoreduplication in halogen-substituted chromosomes. DNA Repair 2, 719-726.], a straightforward model is proposed in which the different mechanisms leading to induced END are considered.

Enhanced oligonucleotide-directed gene targeting in mammalian cells following treatment with DNA damaging agents

Targeted gene repair, a form of oligonucleotide-directed mutagenesis, employs end-modified single-stranded DNA oligonucleotides to mediate single-base changes in chromosomal DNA. In this work, we use a specific 72-mer to direct the repair of a mutated eGFP gene stably integrated in the genome of DLD-1 cells. Corrected cells express eGFP that can be identified and quantitated by FACS. The repair of this mutant gene is dependent on the presence of a specifically designed oligonucleotide and the frequency with which the mutation is reversed is affected by the induction of DNA damage. We used hydroxyurea, VP16 (etoposide), and thymidine to modulate the rate of DNA replication through the stalling of the replication forks or the introduction of lesions. Addition of hydroxyurea or VP16 before the electroporation of the oligonucleotide, results in an accumulation of double-strand breaks (DSB) whose repair is facilitated by either nonhomologous end joining (NHEJ) or homologous recombination (HR). The addition of thymidine results in DNA damage within replication forks, damage that is repaired through the process of homologous recombination. Our data suggest that gene repair activity is elevated when DNA damage induces or activates the homologous recombination pathway.

Characterisation of cytotoxicity and DNA damage induced by the topoisomerase II-directed bisdioxopiperazine anti-cancer agent ICRF-187 (dexrazoxane) in yeast and mammalian cells

Background

Bisdioxopiperazine anti-cancer agents are inhibitors of eukaryotic DNA topoisomerase II, sequestering this protein as a non-covalent protein clamp on DNA. It has been suggested that such complexes on DNA represents a novel form of DNA damage to cells. In this report, we characterise the cytotoxicity and DNA damage induced by the bisdioxopiperazine ICRF-187 by a combination of genetic and molecular approaches. In addition, the well-established topoisomerase II poison m-AMSA is used for comparison.

Results

By utilizing a panel of Saccharomyces cerevisiae single-gene deletion strains, homologous recombination was identified as the most important DNA repair pathway determining the sensitivity towards ICRF-187. However, sensitivity towards m-AMSA depended much more on this pathway. In contrast, disrupting the post replication repair pathway only affected sensitivity towards m-AMSA. Homologous recombination (HR) defective irs1SF chinese hamster ovary (CHO) cells showed increased sensitivity towards ICRF-187, while their sensitivity towards m-AMSA was increased even more. Furthermore, complementation of the XRCC3 deficiency in irs1SF cells fully abrogated hypersensitivity towards both drugs. DNA-PK_cs deficient V3-3 CHO cells having reduced levels of non-homologous end joining (NHEJ) showed slightly increased sensitivity to both drugs. While exposure of human small cell lung cancer (SCLC) OC-NYH cells to m-AMSA strongly induced γH2AX, exposure to ICRF-187 resulted in much less induction, showing that ICRF-187 generates fewer DNA double strand breaks than m-AMSA. Accordingly, when yeast cells were exposed to equitoxic concentrations of ICRF-187 and m-AMSA, the expression of DNA damage-inducible genes showed higher levels of induction after exposure to m-AMSA as compared to ICRF-187. Most importantly, ICRF-187 stimulated homologous recombination in SPD8 hamster lung fibroblast cells to lower levels than m-AMSA at all cytotoxicity levels tested, showing that the mechanism of action of bisdioxopiperazines differs from that of classical topoisomerase II poisons in mammalian cells.

Conclusion

Our results point to important differences in the mechanism of cytotoxicity induced by bisdioxopiperazines and topoisomerase II poisons, and suggest that bisdioxopiperazines kill cells by a combination of DNA break-related and DNA break-unrelated mechanisms.

In vitro chemopreventive activity of Camellia sinensis, Ilex paraguariensis and Ardisia compressa tea extracts and selected polyphenols

Several herbal teas contain bioactive compounds that have been associated with a lower risk of chronic diseases including cancer. The aim of this study was to evaluate the chemopreventive activity of tea aqueous extracts and selected constituent pure polyphenols using a battery of in vitro marker systems relevant for the prevention of cancer. The effects of (-) epigallocatechin gallate (EGCG), quercetin (Q), gallic acid (GA), green tea (GT, Camellia sinensis), ardisia tea (AT, Ardisia compressa) and mate tea (MT, Ilex paraguariensis) extracts were tested. Cytotoxicity, TPA-induced ornithine decarboxylase (ODC) and quinone reductase (QR) activities were evaluated in vitro using HepG2 cells. The topoisomerase inhibitory activity was also tested, using the Saccharomyces cerevisiae yeast system. Results suggest that MT, AT and GT are cytotoxic to the HepG2 cells, with MT demonstrating dominant cytotoxicity. EGCG showed greater cytotoxicity than Q and GA against HepG2 cells. The greatest inhibition (82%) of TPA-induced ODC activity was shown by Q, with 25 microM (IC50 = 11.90 microM). Topoisomerase II, but not topoisomerase I, was the cellular target of MT, AT, EGCG, Q and GA, which acted mainly as true catalytic inhibitors. The cytotoxic activity and the inhibition of topoisomerase II may contribute to the overall chemopreventive activity of AT and MT extracts. Ardisia and mate teas may thus share a public health potential as chemopreventive agents.

Anthracyclines: Molecular Advances and Pharmacologic Developments in Antitumor Activity and Cardiotoxicity

The clinical use of anthracyclines like doxorubicin and daunorubicin can be viewed as a sort of double-edged sword. On the one hand, anthracyclines play an undisputed key role in the treatment of many neoplastic diseases; on the other hand, chronic administration of anthracyclines induces cardiomyopathy and congestive heart failure usually refractory to common medications. Second-generation analogs like epirubicin or idarubicin exhibit improvements in their therapeutic index, but the risk of inducing cardiomyopathy is not abated. It is because of their janus behavior (activity in tumors vis-à-vis toxicity in cardiomyocytes) that anthracyclines continue to attract the interest of preclinical and clinical investigations despite their longer-than-40-year record of longevity. Here we review recent progresses that may serve as a framework for reappraising the activity and toxicity of anthracyclines on basic and clinical pharmacology grounds. We review 1) new aspects of anthracycline-induced DNA damage in cancer cells; 2) the role of iron and free radicals as causative factors of apoptosis or other forms of cardiac damage; 3) molecular mechanisms of cardiotoxic synergism between anthracyclines and other anticancer agents; 4) the pharmacologic rationale and clinical recommendations for using cardioprotectants while not interfering with tumor response; 5) the development of tumor-targeted anthracycline formulations; and 6) the designing of third-generation analogs and their assessment in preclinical or clinical settings. An overview of these issues confirms that anthracyclines remain "evergreen" drugs with broad clinical indications but have still an improvable therapeutic index.

Dissecting the cell-killing mechanism of the topoisomerase II-targeting drug ICRF-193

Topoisomerase II is an essential enzyme that is targeted by a number of clinically valuable anticancer drugs. One class referred to as topoisomerase II poisons works by increasing the cellular level of topoisomerase II-mediated DNA breaks, resulting in apoptosis. Another class of topoisomerase II-directed drugs, the bis-dioxopiperazines, stabilizes the conformation of the enzyme where it attains an inactive salt-stable closed clamp structure. Bis-dioxopiperazines, similar to topoisomerase II poisons, induce cell killing, but the underlying mechanism is presently unclear. In this study, we use three different biochemically well characterized human topoisomerase IIalpha mutant enzymes to dissect the catalytic requirements needed for the enzyme to cause dominant sensitivity in yeast to the bis-dioxopirazine ICRF-193 and the topoisomerase II poison m-AMSA. We find that the clamp-closing activity, the DNA cleavage activity, and even both activities together are insufficient for topoisomerase II to cause dominant sensitivity to ICRF-193 in yeast. Rather, the strand passage event per se is an absolute requirement, most probably because this involves a simultaneous interaction of the enzyme with two DNA segments. Furthermore, we show that the ability of human topoisomerase IIalpha to cause dominant sensitivity to m-AMSA in yeast does not depend on clamp closure or strand passage but is directly related to the capability of the enzyme to respond to m-AMSA with increased DNA cleavage complex formation.

A mutation in human topoisomerase II alpha whose expression is lethal in DNA repair-deficient yeast cells

Type II DNA topoisomerases are ATP-dependent enzymes that catalyze alterations in DNA topology. These enzymes are important targets of a variety of anti-bacterial and anti-cancer agents. We identified a mutation in human topoisomerase II alpha, changing aspartic acid 48 to asparagine, that has the unique property of failing to transform yeast cells deficient in recombinational repair. In repair-proficient yeast strains, the Asp-48 --> Asn mutant can be expressed and complements a temperature-sensitive top2 mutation. Purified Asp-48 --> Asn Top2alpha has relaxation and decatenation activity similar to the wild type enzyme, but the purified protein exhibits several biochemical alterations compared with the wild type enzyme. The mutant enzyme binds both covalently closed and linear DNA with greater avidity than the wild type enzyme. hTop2alpha(Asp-48 --> Asn) also exhibited elevated levels of drug-independent cleavage compared with the wild type enzyme. The enzyme did not show altered sensitivity to bisdioxopiperazines nor did it form stable closed clamps in the absence of ATP, although the enzyme did form elevated levels of closed clamps in the presence of a non-hydrolyzable ATP analog compared with the wild type enzyme. We suggest that the lethality exhibited by the mutant is likely because of its enhanced drug-independent cleavage, and we propose that alterations in the ATP binding domain of the enzyme are capable of altering the interactions of the enzyme with DNA. This mutant enzyme also serves as a new model for understanding the action of drugs targeting topoisomerase II.

Small Molecule Modulation of the Human Chromatid Decatenation Checkpoint

After chromosome replication, the intertwined sister chromatids are disentangled by topoisomerases. The integrity of this process is monitored by the chromatid decatenation checkpoint. Here, we describe small molecule modulators of the human chromatid decatenation checkpoint identified using a cell-based, chemical genetic modifier screen. Similar to 1,2,7-trimethylyxanthine (caffeine), these small molecules suppress the G(2)-phase arrest caused by ICRF-193, a small molecule inhibitor of the enzymatic activity of topoisomerase II. Analysis of specific suppressors, here named suptopins for suppressor of Topoisomerase II inhibition, revealed distinct effects on cell cycle progression, microtubule stability, nucleocytoplasmic transport of cyclin B1, and no effect on the chromatin deacetylation checkpoint induced by trichostatin A. The suptopins provide new molecular tools for dissecting the role of topoisomerases in maintaining genomic stability and determining whether inhibiting the chromatid decatenation checkpoint sensitizes tumor cells to chemotherapeutics.

Bufalin influences the repair of X-ray-induced DNA breaks in Chinese hamster cells

The bufadienolide bufalin, a component of the Chinese medicine chan'su, has been reported to selectively inhibit the growth of various lines of human cancer cells, due at least in part to its specific effect on topoisomerase (topo) II. We have treated Chinese hamster ovary (CHO) cells with doses of bufalin that result in a dramatic reduction in both the level and catalytic activity of topo II without any concomitant induction of DNA damage, as assessed by the comet assay. When cells were pre-treated with bufalin and then irradiated with X-rays, a follow-up study revealed that the kinetics of DNA repair was clearly affected, with a general delay in the restoration of DNA to the situation observed in non-irradiated controls. The possible involvement of topo II in radiation damage repair is discussed.

Hypersensitivity of nonhomologous DNA end-joining mutants to VP-16 and ICRF-193: implications for the repair of topoisomerase II-mediated DNA damage

A number of clinically useful anticancer drugs, including etoposide (VP-16), target DNA topoisomerase (topo) II. These drugs, referred to as topo II poisons, stabilize cleavable complexes, thereby generating DNA double-strand breaks. Bis-2,6-dioxopiperazines such as ICRF-193 also inhibit topo II by inducing a distinct type of DNA damage, termed topo II clamps, which has been believed to be devoid of double-strand breaks. Despite the biological and clinical importance, the molecular mechanisms for the repair of topo II-mediated DNA damage remain largely unknown. Here, we perform genetic analyses using the chicken DT40 cell line to investigate how DNA lesions caused by topo II inhibitors are repaired. Notably, we show that LIG4-/- and KU70-/- cells, which are defective in nonhomologous DNA end-joining (NHEJ), are extremely sensitive to both VP-16 and ICRF-193. In contrast, RAD54-/- cells (defective in homologous recombination) are much less hypersensitive to VP-16 than the NHEJ mutants and, more importantly, are not hypersensitive to ICRF-193. Our results provide the first evidence that NHEJ is the predominant pathway for the repair of topo II-mediated DNA damage; that is, cleavable complexes and topo II clamps. The outstandingly increased cytotoxicity of topo II inhibitors in the absence of NHEJ suggests that simultaneous inhibition of topo II and NHEJ would provide a powerful protocol in cancer chemotherapy involving topo II inhibitors.

Elongation by RNA polymerase II on chromatin templates requires topoisomerase activity

Transcription on chromatin by RNA polymerase II (pol II) is repressed as compared with transcription on histone-free DNA. In this study, we show that human topoisomerase I (topo I) and yeast topoisomerase II (topo II), each of which relax both positive and negative superhelical tension, reverse the transcriptional repression by chromatin. In the presence of bacterial topo I, which can relax only negative superhelical tension, the transcription is repressed on chromatin templates. The data together show that the relaxation of positive superhelical tension by these enzymes was the key property required for RNA synthesis from chromatin templates. In the absence of topoisomerase, transcriptional repression on chromatin depended on RNA length. The synthesis of transcripts of 100 nt or shorter was unaffected by chromatin, but repression was apparent when the RNA transcript was 200 nt or longer. These findings suggest that transcription on chromatin templates results in the accumulation of positive superhelical tension by the elongating polymerase, which in turn inhibits further elongation in the absence of topoisomerase activity.