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

DNA-Stimulated Liquid-Liquid phase separation by eukaryotic topoisomerase ii modulates catalytic function

Type II topoisomerases modulate chromosome supercoiling, condensation, and catenation by moving one double-stranded DNA segment through a transient break in a second duplex. How DNA strands are chosen and selectively passed to yield appropriate topological outcomes - for example, decatenation vs. catenation - is poorly understood. Here, we show that at physiological enzyme concentrations, eukaryotic type IIA topoisomerases (topo IIs) readily coalesce into condensed bodies. DNA stimulates condensation and fluidizes these assemblies to impart liquid-like behavior. Condensation induces both budding yeast and human topo IIs to switch from DNA unlinking to active DNA catenation, and depends on an unstructured C-terminal region, the loss of which leads to high levels of knotting and reduced catenation. Our findings establish that local protein concentration and phase separation can regulate how topo II creates or dissolves DNA links, behaviors that can account for the varied roles of the enzyme in supporting transcription, replication, and chromosome compaction.

Lack of modification of rat hepatocarcinogenesis by fernane-type triterpenoids, isolated from a Euphorbia genus

Inhibitors of topoisomerases, enzymes that produce an unusual type of DNA damage, are considered as antitumor agents. Recently it has been reported that the fernane-type triterpenoid EC-2 and its hydroxyl derivative, isolated from Euphorbia, are potent topoisomerase II inhibitors. In this study, the modifying effects of EC-2 and EC-4 on the development of putative preneoplastic lesions, glutathione S-transferase placental form (GST-P)-positive foci, in the liver of rats were investigated using a medium-term bioassay system. Fisher 344 male, 6-week-old rats were given a single intraperitoneal injection (200 mg/kg b.w.) of diethylnitrosamine or saline at the beginning of the experiment and subjected to 2/3 partial hepatectomy at the 3rd week. The test compounds were administered five times/week by i.g. gavage at a dose of 1 mg/kg b.w. from 2 to 8 weeks. Quantitation of the numbers and areas per cm(2) of induced GST-P positive foci did not demonstrated any significant differences among the groups and no variation in cell proliferation as indicated by 5-bromo- 2'-deoxyuridine (BrdU) labeling. Our results suggest that EC-2 and EC-4 have no modifying effects on rat hepatocarcinogenesis.

Differential expression of human topoisomerase IIIalpha during the cell cycle progression in HL-60 leukemia cells and human peripheral blood lymphocytes

Human topoisomerase IIIalpha (huTop IIIalpha) has been demonstrated to belong to type IA subfamily. In this study, we found that huTop IIIalpha expressed constitutively and remained at high levels throughout the cell cycle in HL-60 cells when compared to the cell-cycle-dependent expression of huTop IIIalpha in phytohemagglutinin-activated peripheral blood lymphocytes. During the cell cycle progression, this protein remained accentuated in the nucleolus without significant translocation from the nucleolus to the nucleoplasm. In addition, during the course of granulocytic maturation in DMSO-treated HL-60 cells, huTop IIIalpha levels decreased when cells stopped proliferation and nucleoli diminished in size. However, its level remained unchanged during the course of monocytic maturation of vitamin D(3)-treated HL-60 cells which still retained its proliferative capacity and did not change the size of the nucleolus. The data suggested that huTop IIIalpha is involved in rDNA metabolism, such as rDNA transcription. Its cellular level appeared to be under control during the cell cycle progression of normal lymphocytes, but was found to be deregulated in HL-60 cells which may be associated with the tumor transformed cell phenotypes.

The catalytic activities of DNA topoisomerase II are most closely associated with the DNA cleavage/religation steps

DNA topoisomerase II regulates the three-dimensional organisation of DNA and is the principal target of many important anticancer and antimicrobial agents. These drugs usually act on the DNA cleavage/religation steps of the catalytic cycle resulting in accumulation of covalent DNA-topoisomerase II complexes. We have studied the different steps of the catalytic cycle as a function of salt concentration, which is a classical way to evaluate the biochemical properties of proteins. The results show that the catalytic activity of topoisomerase II follows a bell-shaped curve with optimum between 100 and 225 mM KCl. No straight-forward correlation exists between DNA binding and catalytic activity. The highest levels of drug-induced covalent DNA-topoisomerase II complexes are observed between 100 and 150 mM KCl. Remarkably, at salt concentrations between 150 mM and 225 mM KCl, topoisomerase II is converted into a drug-resistant form with greatly reduced levels of drug-induced DNA-topoisomerase II complexes. This is due to efficient religation rather than to absence of DNA cleavage as witnessed by relaxation of the supercoiled DNA substrate. In the absence of DNA, ATP hydrolysis is strongest at low salt concentrations. Unexpectedly, the addition of DNA stimulates ATP hydrolysis at 100 and 150 mM KCl, but has little or no effect below 100 mM KCl in spite of strong non-covalent DNA binding at these salt concentrations. Therefore, DNA-stimulated ATP hydrolysis appears to be associated with covalent rather than non-covalent binding of DNA to topoisomerase II. Taken together, the results suggest that it is the DNA cleavage/religation steps that are most closely associated with the catalytic activities of topoisomerase II providing a unifying theme for the biological and pharmacological modulation of this enzyme.

Inhibition of DNA topoisomerase II alpha gene expression by the p53 tumor suppressor

DNA topoisomerase II (topo II) is an essential nuclear enzyme involved in major cellular functions such as DNA replication, transcription, recombination, and mitosis. While an elevated level of topo II alpha is associated with cell proliferation, wild-type (wt) p53 inhibits the expression of various growth-stimulatory genes. To determine if p53 downregulates topo II alpha gene expression, a murine cell line, (10)1val, that expresses a temperature-sensitive p53 was utilized. The (10)1val cells had significantly lower levels of topo II alpha mRNA and protein following incubation for 24 h at 32 degrees C (p53 with wt conformation) than at 39 degrees C (p53 with mutant conformation). The effect of p53 on the human topo II alpha gene promoter was determined by using luciferase reporter plasmids containing varying lengths of the topo II alpha promoter transiently cotransfected into p53-deficient (10)1 cells together with wt or mutant p53 expression plasmids. Transcription from the full-length (bp -557 to +90) topo II alpha promoter was decreased 15-fold by wt p53 in a concentration-dependent manner, whereas mutant p53 exerted much weaker inhibition. Consecutive deletion of the five inverted CCAAT elements (ICEs) from the topo II alpha promoter reduced both the basal promoter activity and wt p53-induced suppression. Transcription of the minimal promoter (-32 to +90), which contains no ICE, was slightly stimulated by wt or mutant p53 expression. When point mutations were introduced into the most proximal ICE (-68), the inhibitory effect of wt p53 was alleviated and stimulation of topo II alpha expression resulted. Our study suggests that wt p53 functions as a transcriptional repressor of topo II alpha gene expression, possibly through a functional interaction with specific ICEs. Inactivation of wt p53 may reduce normal regulatory suppression of topo II alpha and contribute to abortive cell cycle checkpoints, accelerated cell proliferation, and alterations in genomic stability associated with neoplasia.

Mechanisms of resistance to topoisomerases poisons

1. Drug resistance remains a major obstacle to cancer treatment. Resistance to chemotherapy can be intrinsic, characterised by the nonresponsiveness of the tumour to the initial treatment. Alternatively, cancers that initially respond to chemotherapy can relapse after various times because of acquired resistance. 2. Resistance to drugs used as single agents is generally accompanied by the development of resistance to other drugs that can be structurally and functionally different. 3. Among the drugs commonly used in cancer treatment there are compounds that have been shown to inhibit DNA topoisomerases (Topos). These critical enzymes regulate the topological conformation of the DNA and participate in essential cellular processes. 4. This paper reviews the Topos' cellular functions, their catalytic activities and the mechanisms of resistance to inhibitors of Topos, with particular attention to the atypical multidrug resistance phenotype.

A study of the expression of four chemoresistance-related genes in human primary and metastatic brain tumours

We investigated four mechanisms of intrinsic chemoresistance in a series of 67 human brain tumours including 31 gliomas (one grade I ganglioglioma, nine grade II and 10 grade III astrocytomas, 11 glioblastomas), 13 cerebral metastases, one medulloblastoma, one malignant teratoma, three ependymomas and 18 meningiomas. We studied four genes by northern blotting: multidrug-resistance (MDR 1), glutathione-s transferase (GST pi), dihydrofolate reductase (DHFR), and topoisomerase II (Topo II). The Topo II gene was absent in the normal adult brain (100%) and in 64% of the tumour samples tested. A second gene, GST pi, was found to be overexpressed in 38% of brain tumours. The two other chemoresistance-related genes were occasionally overexpressed in brain tumours (2% for MDR1, 9% for DHFR). Our results provide evidence that chemoresistance is intrinsic to the brain tissue and seems likely to be a multifactorial process.

Study of molecular markers of resistance to m-AMSA in a human breast cancer cell line. Decrease of topoisomerase II and increase of both topoisomerase I and acidic glutathione S transferase

Resistance to 0.8 microM 4'-(9-acridinylamino)methanesulphon-m-anisidide (m-AMSA) was induced by stepwise increases of drug concentration in the human tumor cell line CALc18 originating from a breast adenocarcinoma. The resistant cell line CALc18/AMSA exhibited a resistance index of 10 and a cross-resistance to other topoisomerase II inhibitors. A 3-fold decrease in the levels of topoisomerase II decatenating activity was found in CALc18/AMSA cells. By contrast, topoisomerase I activity was increased by about 3-fold in resistant cells. Interestingly this line was hypersensitive to camptothecin, a specific inhibitor of topoisomerase I. Restriction endonuclease patterns of the topoisomerase I and topoisomerase II loci were found to be identical in CALc18/AMSA and CALc18 with no evidence of gene amplification and rearrangements. Alkaline elution of m-AMSA-treated cells showed that DNA single strand breaks and DNA-protein crosslinks were decreased in CALc18/AMSA. The DNA lesions also obtained in m-AMSA-treated nuclei indicated that no drug uptake modification occurred in both cells. Moreover, the in vitro m-AMSA-induced DNA cleavage per unit of decatenating activity and the inhibitory effects of antitumoral drugs on decatenation were not found to be different with topoisomerase II from sensitive or resistant cells. However the specific cleavage induced by m-AMSA/per mg of crude protein from resistant cells was 2 to 3 times decreased. Multidrug resistance gene transcripts were not detected while levels of acidic glutathione S transferase mRNA were found to be 8 to 10-fold greater in resistant than in sensitive cell line with no amplification of the gene. In conclusion, the diminution of topoisomerase II activity and the increase of both topoisomerase I and acidic glutathione S transferase transcripts could contribute to the resistant phenotype of these breast cancer cells.

In vitro assays used to measure the activity of topoisomerases

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

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

Biochemical basis for the interactions of type I and type II topoisomerases with DNA

DNA topoisomerases are complex and unique enzymes which alter the topological state of DNA without changing its chemical structure. Between the type I and II enzymes, topoisomerases carry out a multitude of reactions, including DNA binding, site specific DNA cleavage/religation, relaxation, catenation/decatenation, and knotting/unknotting of nucleic acid substrates, DNA strand transfer, and ATP hydrolysis. In vivo, topoisomerases are involved in many aspects of nucleic acid metabolism and play critical roles in maintaining chromosome and nuclear structure. Finally, these enzymes are of clinical relevance, as they appear to be the primary cellular targets for many varied classes of antineoplastic agents. Considering the importance of the topoisomerases, it is distressing that we know so little about their enzymatic mechanisms. Many major questions remain. Just a few include, "How do topoisomerases recognize their nucleic acid interaction sites?"; "What amino acid residues comprise the enzymes' active sites?"; "What are the conformational changes that accompany DNA strand passage?"; "How does phosphorylation stimulate enzyme activity?"; "How does topoisomerase function when it is part of an immobilized structure such as the nuclear matrix or the mitotic chromosome scaffold?"; and "How do antineoplastic agents interact with their topoisomerase targets and stabilize covalent enzyme.DNA cleavage products?" Clearly, before the physiological functions of the topoisomerases can be fully described, these and similar issues will have to be addressed. Hopefully, the next several years will produce answers for at least some of these important questions.

Abrogation of etoposide-mediated cytotoxicity by cycloheximide

The antitumor agent etoposide interacts with DNA topoisomerase II to produce a unique form of DNA-enzyme intermediate referred to as a "cleavable complex". These drug-induced DNA strand breaks initiate poorly defined cell processes which result in lethality. To explore the mechanism of etoposide cytotoxicity, we studied the effect of protein synthesis inhibitor on Balb/C 3T3 fibroblasts and CCRF-CEM and L1210 leukemia cells by exposing these cell lines to cycloheximide for various periods of time prior to etoposide challenge. Cycloheximide alone during these periods of exposure was not cytotoxic; however, it conferred increasing cytoprotection from etoposide in a time-dependent fashion when it preceded etoposide. Although cycloheximide did cause a decrease in enzyme content and in etoposide-induced DNA cleavage of Balb/C 3T3 and the CCRF-CEM cell lines, cytoprotection by cycloheximide could not be accounted for completely by these phenomena since, in L1210 cells, cytoprotection was observed without significant change in DNA cleavage or enzyme content. Cycloheximide diminished DNA synthesis as well as protein synthesis. However, DNA synthesis resumed within 6 hr after removal of cycloheximide, in spite of the fact that cytoprotection persisted. Cycloheximide did not alter cell cycle distribution as measured by flow cytometry. Our data, therefore, clearly demonstrate that cycloheximide can diminish the cytotoxic potential of etoposide-mediated topoisomerase-DNA complexes. Elucidation of the mechanism by which cytoprotection occurs should shed light on the basis for the cytotoxic effect of topoisomerase II-active drugs.

In vivo exposure to four ellipticine derivatives with topoisomerase inhibitory activity results in chromosome clumping and sister chromatid exchange in murine bone marrow cells

A single dose of 9-hydroxy-ellipticine, 2-N-methyl-9-hydroxy-ellipticine, 9-fluoro-ellipticine, and 9-amino-ellipticine (5 to 10 mg/kg body wt, ip) resulted in murine bone marrow toxicity as shown by chromosome clumping, chromatid aberrations, and micronuclei formation. An increase in sister chromatid exchanges (SCE) was also observed. These effects are most likely directly related to the topoisomerase inhibitory effect of these drugs since topoisomerase II is involved in the separation of intertwined chromosomal DNA molecules during mitosis as well as being a mediator of DNA exchanges. The two antitumor drugs 2-N-methyl-9-hydroxy-ellipticine and 9-hydroxy-ellipticine were most genotoxic with chromosome abnormalities occurring in 33-95% of the cells and SCE on the order of 12.3 to 19.2 events per cell. Both of these drugs show high topoisomerase II inhibitory activity in vitro. In contrast, 9-amino-ellipticine and 9-fluoro-ellipticine were less genotoxic with chromosomal abnormalities occurring in 14-17% of the cells and SCE on the order of 7.1 to 7.7 events per cell. These two derivatives are both inactive toward experimental tumors and show less topoisomerase II inhibitory activity in vitro. Our results suggest that the ellipticine drugs owe at least some of their cytotoxicity to their genotoxic effects, which seem to be mediated through interaction with topoisomerase II.

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

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

ATP-independent type II topoisomerase from trypanosomes

We have characterized in Trypanosoma cruzi a DNA topoisomerase capable of decatenating complex trypanosomal kinetoplast DNA networks in the absence of ATP. The enzymatic activity requires Mg2+ and K+. Using a defined DNA topoisomer we showed that the linking number changes by steps of 2, which characterizes the enzyme as a type II topoisomerase. The enzyme can catenate supercoiled DNA molecules, unknot DNA, and cleave double-stranded DNA. The enzyme has no ATPase activity. The native enzyme has an Mr of about 200,000. Crude extracts and partially purified fractions contain an aggregating factor that can substitute spermidine in catenating reactions. Because of the presence of this factor, the kinetoplast DNA can only be decatenated by purified fractions. The enzyme is inhibited by certain drugs and provides a potential target for chemotherapy. Such an enzyme was also characterized in Trypanosoma equiperdum.

The in vitro involvement of topoisomerase II in the activity of aza-ellipticine analogues is not correlated with drug activity on isolated nuclei

Aza-ellipticines are DNA intercalative ellipticine analogues with antitumor activity that induce protein-linked DNA breaks in NIH 3T3 cells in culture. The effects of two aza-ellipticine congeners (BD-40 and BR-76) on the activity of purified Calf Thymus type II topoisomerase were studied using pUC13 DNA as substrate. DNA cleavage was stimulated by both molecules at those doses required for inducing lethal effects in cells (DE5O). This effect was reversed by high salt treatment, indicating that it was actually mediated by Topo II. Mapping of cleavage sites on linearized and 3' end-labelled pUC13 DNA showed that ellipticine and aza-ellipticines stimulated the same sites, which differed from those stimulated by m-AMSA. Decatenating activity of Topo II on Trypanosoma cruzi kDNA was both inhibited by ellipticine and BD-40 at concentrations much higher than DE50 concentrations. Activity of aza-ellipticines was also investigated on isolated nuclei. Unlike ellipticine which promoted DNA-breaking activity, BD-40 and BR-76 were repeatedly inactive. Prior treatment of DNA by Proteinase K did not reveal hidden breaks which are formed in intact cells treated with BD-40 (Vilarem et al., 1984, Nucleic Ac. Res. 12, 8653). Concordant with these data, BD-40 did not impair DNA-synthetic activity in isolated nuclei, while Ellipticine largely decreased it. These results indicate that lesions induced in DNA by Aza-ellipticines are mediated by Topo II. The absence of effect of these drugs on isolated nuclei compared to that of Ellipticine may be due to some specific features of the association between Topo II and Aza-ellipticines or reflect a bioactivation step as a prerequisite for in vivo activity.

In vivo stimulation by antitumor drugs of the topoisomerase II induced cleavage sites in c-myc protooncogene

Several antitumor drugs including DNA intercalative and non intercalative agents induce in vitro and in vivo double-stranded DNA breaks by stabilization of a topoisomerase II-DNA complex. In order to locate cleavage sites in an actively transcribed oncogene, N417 cells, originating from a human small cell lung carcinoma and containing 45-50 copies of c-myc oncogene, were treated with mAMSA, 9 hydroxyellipticine and VM 26. The presence of DNA lesions in c-myc was investigated by Southern blot hybridization with a human c-myc probe. In addition to normal bands, DNA patterns of drug treated-cells revealed the presence of new bands most likely corresponding to topoisomerase II-mediated cleavage as these bands were not found in untreated control DNA and in DNA treated with oAMSA, a biologically inactive stereoisomer of mAMSA. Major cleavage sites induced by drugs in the N417 cell c-myc locus were located in the 5' end of the c-myc exon 1 closely to some DNAse I hypersensitive sites which are assumed to reflect an activity of the gene. Therefore our data suggest that TopoII-mediated drug activity correlates with gene activity.

Isolation and characterization of DNA topoisomerase II from cauliflower inflorescences

Type II DNA topoisomerase has been isolated from inflorescences of cauliflower (Brassica oleracea var. botrytis) through a sequence of polyethylene glycol fractionation, ammonium sulfate precipitation, and column chromatography on CM-Sephadex, hydroxyapatite and phosphocellulose. The molecular weight of the native enzyme, based on sedimentation coefficient (9S) and gel filtration analysis (Stokes radius, 60 Å), was estimated to be 223 000. This enzyme was able to catalyze fully the relaxation of supercoiled DNA by breaking and then rejoining the double-stranded DNA. The breaking reaction was reversible by a change in salt concentrations. When an antitumor drug, 4'-(9-acridinylamino)-methanesulfon-m-anisidide, was added to the topoisomerase reaction, DNA cleavage fragments were accumulated; and this suggested that the drug interfered with the reaction at the rejoining step. This enzyme also catalyzed the formation of DNA catenanes in the presence of 8% polyethylene glycol or histone H1, while few catenanes were formed in the presence of spermidine, which was highly effective on a bacterial enzyme.

Purification and characterization of Plasmodium berghei DNA topoisomerases I and II: drug action, inhibition of decatenation and relaxation, and stimulation of DNA cleavage

It has recently been suggested that topoisomerases could be important targets for drugs used in several diseases. This prompted us to purify and characterize the topoisomerases I and II present in the erythrocytes of protozoan parasites of the genus Plasmodium, the causative agent of malaria, in order to later use these enzymatic systems in antimalarial drug assays. The topoisomerases were purified from Plasmodium berghei, a parasite of mouse red cells. The Plasmodium topoisomerase II consists of two subunits with a molecular weight of about 160K. The enzyme is ATP- and Mg2+-dependent. The conditions for the reactions of relaxation, unknotting, decatenation, and catenation were found to be similar to those observed with enzymes from other eukaryotic cells. The Plasmodium topoisomerase I is a monomeric enzyme with a Mr of 70K-100K. It is ATP-independent and K+- or Na-dependent. Mg2+ is not required for relaxation but stimulates the reaction. Topoisomerase II was more sensitive to drug action than topoisomerase I. The most active drugs were the ellipticine derivatives. The antimalarial drugs, currently used in human clinical therapy, were poor inhibitors. Some antitumoral drugs stimulated the double-stranded DNA cleavage activity of Plasmodium topoisomerase II, like that of mammalian topoisomerases II. Antimalarial drugs had no stimulating activity. It is therefore suggested that Plasmodium topoisomerases are not good targets for antimalarial drugs.