Arrest of replication fork progression at sites of topoisomerase II-mediated DNA cleavage in human leukemia CEM cells incubated with VM-26

Targeting DNA Topoisomerase II in Antifungal Chemotherapy

Topoisomerase inhibitors have been in use clinically for the treatment of several diseases for decades. Although those enzymes are significant molecular targets in antibacterial and anticancer chemotherapy very little is known about the possibilities to target fungal topoisomerase II (topo II). Raising concern for the fungal infections, lack of effective drugs and a phenomenon of multidrug resistance underlie a strong need to expand the range of therapeutic options. In this review paper, we discussed the usefulness of fungal topo II as a molecular target for new drug discovery. On the basis of previously published data, we described structural and biochemical differences between fungal and human enzymes as well as a molecular basis of differential sensitivity to known anticancer drugs targeting the latter. This review focuses especially on highlighting the differences that may underlie the selectivity of action of new inhibitors. Distinct sites within fungal topo II in comparison with human counterparts are observed and should be further studied to understand the significance of those sites and their possible usage in design of new drugs.

Broken by the Cut: A Journey into the Role of Topoisomerase II in DNA Fragility

DNA topoisomerase II (TOP2) plays a critical role in many processes such as replication and transcription, where it resolves DNA structures and relieves torsional stress. Recent evidence demonstrated the association of TOP2 with topologically associated domains (TAD) boundaries and CCCTC-binding factor (CTCF) binding sites. At these sites, TOP2 promotes interactions between enhancers and gene promoters, and relieves torsional stress that accumulates at these physical barriers. Interestingly, in executing its enzymatic function, TOP2 contributes to DNA fragility through re-ligation failure, which results in persistent DNA breaks when unrepaired or illegitimately repaired. Here, we discuss the biological processes for which TOP2 is required and the steps at which it can introduce DNA breaks. We describe the repair processes that follow removal of TOP2 adducts and the resultant broken DNA ends, and present how these processes can contribute to disease-associated mutations. Furthermore, we examine the involvement of TOP2-induced breaks in the formation of oncogenic translocations of leukemia and papillary thyroid cancer, as well as the role of TOP2 and proteins which repair TOP2 adducts in other diseases. The participation of TOP2 in generating persistent DNA breaks and leading to diseases such as cancer, could have an impact on disease treatment and prevention.

Mitoxantrone, More than Just Another Topoisomerase II Poison

Mitoxantrone is a synthetic anthracenedione originally developed to improve the therapeutic profile of the anthracyclines and is commonly applied in the treatment of breast and prostate cancers, lymphomas, and leukemias. A comprehensive overview of the drug's molecular, biochemical, and cellular pharmacology is presented here, beginning with the cardiotoxic nature of its predecessor doxorubicin and how these properties shaped the pharmacology of mitoxantrone itself. Although mitoxantrone is firmly established as a DNA topoisomerase II poison within mammalian cells, it is now clear that the drug interacts with a much broader range of biological macromolecules both covalently and noncovalently. Here, we consider each of these interactions in the context of their wider biological relevance to cancer therapy and highlight how they may be exploited to further enhance the therapeutic value of mitoxantrone. In doing so, it is now clear that mitoxantrone is more than just another topoisomerase II poison.

Homologous recombination repair is essential for repair of vosaroxin-induced DNA double-strand breaks

Vosaroxin (formerly voreloxin) is a first-in-class anticancer quinolone derivative that intercalates DNA and inhibits topoisomerase II, inducing site-selective double-strand breaks (DSB), G2 arrest and apoptosis. Objective responses and complete remissions were observed in phase 2 studies of vosaroxin in patients with solid and hematologic malignancies, and responses were seen in patients whose cancers were resistant to anthracyclines. The quinolone-based scaffold differentiates vosaroxin from the anthracyclines and anthracenediones, broadly used DNA intercalating topoisomerase II poisons. Here we report that vosaroxin induces a cell cycle specific pattern of DNA damage and repair that is distinct from the anthracycline, doxorubicin. Both drugs stall replication and preferentially induce DNA damage in replicating cells, with damage in G2 / M > S >> G1. However, detectable replication fork collapse, as evidenced by DNA fragmentation and long tract recombination during S phase, is induced only by doxorubicin. Furthermore, vosaroxin induces less overall DNA fragmentation. Homologous recombination repair (HRR) is critical for recovery from DNA damage induced by both agents, identifying the potential to clinically exploit synthetic lethality.

Homologous recombination repair is essential for repair of vosaroxin-induced DNA double-strand breaks

Vosaroxin (formerly voreloxin) is a first-in-class anticancer quinolone derivative that intercalates DNA and inhibits topoisomerase II, inducing site-selective double-strand breaks (DSB), G2 arrest and apoptosis. Objective responses and complete remissions were observed in phase 2 studies of vosaroxin in patients with solid and hematologic malignancies, and responses were seen in patients whose cancers were resistant to anthracyclines. The quinolone-based scaffold differentiates vosaroxin from the anthracyclines and anthracenediones, broadly used DNA intercalating topoisomerase II poisons. Here we report that vosaroxin induces a cell cycle specific pattern of DNA damage and repair that is distinct from the anthracycline, doxorubicin. Both drugs stall replication and preferentially induce DNA damage in replicating cells, with damage in G2 / M > S >> G1. However, detectable replication fork collapse, as evidenced by DNA fragmentation and long tract recombination during S phase, is induced only by doxorubicin. Furthermore, vosaroxin induces less overall DNA fragmentation. Homologous recombination repair (HRR) is critical for recovery from DNA damage induced by both agents, identifying the potential to clinically exploit synthetic lethality.

Repair of idarubicin-induced DNA damage: A cause of resistance?

Idarubicin, a widely used anticancer drug inhibits topoisomerase (topo) IIalpha and induces DNA double strand breaks. The finding that idarubicin-induced DNA damage is repaired before cell death is initiated encouraged us to examine the role of DNA repair for the cytotoxicity of idarubicin in human promyelocytic HL60 leukaemia cells. We found that DNA double strand breaks induced by a 90 min transient exposure to 0.5 microgml(-1) idarubicin were rapidly repaired throughout the whole population, while topo IIalpha itself was degraded. In spite of DNA repair, the vast majority of cells died within 40 h. Using differential staining of the chromatids and microscopic evaluation of DNA break points, we found evidence for a high number of false ligations of loose DNA strands arising from the inhibition of topo IIalpha action by idarubicin. If mainly actively transcribed genes are affected, this results in a disruption of vital genetic information, of regulatory sequences and, ultimately, in induction of the cell death pathway. Our results confirm the hypothesis that misrepair of DNA damage is a decisive event in idarubicin-induced cell death. They are discussed in the context of topo IIalpha-function and the currently known mechanisms of DNA double strand break repair.

Differential cell cycle-specificity for chromosomal damage induced by merbarone and etoposide in V79 cells

Merbarone, a topoisomerase II (topo II) inhibitor which, in contrast to etoposide, does not stabilize topo II-DNA cleavable complexes, was previously shown to be a potent clastogen in vitro and in vivo. To investigate the possible mechanisms, we compared the cell cycle-specificity of the clastogenic effects of merbarone and etoposide in V79 cells. Using flow cytometry and BrdU labeling techniques, etoposide was shown to cause a rapid and persistent G2 delay while merbarone was shown to cause a prolonged S-phase followed by a G2 delay. To identify the stages which are susceptible to DNA damage, we performed the micronucleus (MN) assay with synchronized cells or utilized a combination of BrdU pulse labeling and the cytokinesis-blocked MN assay with non-synchronized cells. Treatment of M phase cells with either agent did not result in increased MN formation. Etoposide but not merbarone caused a significant increase in MN when cells were treated during G2 phase. When treated during S-phase, both chemicals induced highly significant increases in MN. However, the relative proportion of MN induced by merbarone was substantially higher than that induced by etoposide. Both chemicals also caused significant increases in MN in cells that were treated during G1 phase. To confirm the observations in the MN assay, first division metaphases were evaluated in the chromosome aberration assay. The chromosomes of cells treated with merbarone and etoposide showed increased frequencies of both chromatid- and chromosome-type of aberrations. Our findings indicate that while etoposide causes DNA damage more evenly throughout the G1, S and G2 phases of the cell cycle, an outcome which may be closely associated with topo II-mediated DNA strand cleavage, merbarone induces DNA breakage primarily during S-phase, an effect which is likely due to the stalling of replication forks by inhibition of topo II activity.

Sequential oral 9-nitrocamptothecin and etoposide: a pharmacodynamic- and pharmacokinetic-based phase I trial

PURPOSE

Resistance to topoisomerase (topo) I inhibitors has been related to down-regulation of nuclear target enzyme, whereas sensitization to topo II inhibitors may result from induction of topo II by topo I inhibitors. Here, we evaluated a sequence-specific administration of a topo I inhibitor followed by a topo II inhibitor.

EXPERIMENTAL DESIGN

Twenty-five patients with advanced or metastatic malignancies were treated with increasing doses (0.75, 1.0, 1.25, 1.5, 1.75, or 2.0 mg/m(2)) of 9-nitrocamptothecin (9-NC) on days 1 to 3, followed by etoposide (100 or 150 mg/d) on days 4 and 5. At the maximally tolerated dose, 20 additional patients were enrolled. The median age was 60 years (range, 40-84 years). Endpoints included pharmacokinetic analyses of 9-NC and etoposide, and treatment-induced modulations of topo I and II expression in peripheral blood mononuclear cells.

RESULTS

Neutropenia, thrombocytopenia, nausea, vomiting, diarrhea, and fatigue were dose-limiting toxicities and occurred in six patients. Despite a median number of four prior regimens (range 1-12), 2 (4%) patients had an objective response and 13 (29%) patients had stable disease. In contrast to the expected modulation in topo I and IIalpha levels, we observed a decrease in topo IIalpha levels, whereas topo I levels were not significantly altered by 9-NC treatment.

CONCLUSIONS

Sequence-specific administration of 9-NC and etoposide is tolerable and active. However, peripheral blood mononuclear cells may not be a predictive biological surrogate for drug-induced modulation of topo levels in tumor tissues and should be further explored in larger studies.

Merbarone Induces Activation of Caspase-Activated DNase and Excision of Chromosomal DNA Loops from the Nuclear Matrix

Studies were carried out to address possible cellular mechanisms by which merbarone, a catalytic inhibitor of DNA topoisomerase II, can block tumor cell growth without inducing extensive DNA cleavage. Merbarone induced the release of high molecular weight DNA fragments from the nuclear matrix of HL-60 leukemia cells, which preceded the internucleosomalsize DNA fragmentation characteristic of late-stage apoptosis. The chromatin fragments were enriched in a matrix attachment region (MAR) sequence compared with a non-MAR sequence and were similar in size to DNA loops extracted from nuclear matrices. However, merbarone did not directly induce the excision of high molecular weight DNA fragments from the nuclear matrix by promoting topoisomerase II-catalyzed DNA cleavage, because the drug inhibited topoisomerase II-mediated cleavage in isolated nuclear matrix preparations. Instead, merbarone induced rapid activation of the mitochondrial apoptosis pathway, which included the following temporal sequence of events: dissipation of the mitochondrial transmembrane potential within 30 min, release of mitochondrial cytochrome c, and activation of caspase-activated DNase (CAD) by its inhibitor ICAD. The excision of high molecular weight DNA was inhibited at least 80% in merbarone-treated cells preincubated with the pan-caspase inhibitor z-VAD-fmk [Z-Val-Ala-Asp(OMe)-fluoromethyl ketone] and in caspase-resistant Jurkat cells (ICAD/double-mutated) that express a mutant form of ICAD. These results provide evidence that merbarone can induce rapid disorganization of DNA in tumor cells that have a functional mitochondrial apoptosis pathway without inducing extensive DNA cleavage.

In vitro activity of cyclin-dependent kinase inhibitor CYC202 (Seliciclib, R-roscovitine) in mantle cell lymphomas

BACKGROUND

Mantle cell lymphoma (MCL) has the worst prognosis of all B-cell lymphomas and has poor response to conventional therapy. It is characterized by the presence of a chromosomal translocation t(11:14) (q13;q32) which results in deregulated cyclin D1 expression. Since defects in cell cycle regulation and apoptosis are primary events in MCL, small-molecule inhibitors of cdks-cyclins may play an important role in the therapy of this disorder. CYC202 (Seliciclib, R-roscovitine; Cyclacel Ltd., Dundee, UK) is a purine analogue and a selective inhibitor of the cdk2-cyclin E as well as cdk7-cyclin H and cdk9-cyclin T.

MATERIALS AND METHODS

The activity of CYC202 was tested in four human MCL cell lines: REC, Granta-519, JeKo-1 and NCEB-1. The effect of CYC202 on the cell cycle and on apoptosis-, cell-cycle- and transcription-regulation-related proteins was assessed.

RESULTS

The IC50 was 25 microM for REC, Granta-519 and JeKo-1 cells and 50 microM for NCEB-1 cells. CYC202 caused an accumulation of cells in the G2-M phase of the cell cycle and apoptosis. CYC202 caused down-regulation of cyclin D1 and Mcl-1 protein levels, possibly because of the inhibition of transcription elongation.

CONCLUSIONS

Our data suggest that CYC202 is an active agent in MCL. The concomitant decrease of the phosphorylated and total forms of RNA polymerase II suggests that this could be the main mechanism mediating the biological effects of CYC202 in MCL cells. The drug might represent a new therapeutic agent in this lymphoma subtype.

Antitumor activity of irofulven monotherapy and in combination with mitoxantrone or docetaxel against human prostate cancer models

BACKGROUND

Irofulven (6-hydroxymethylacylfulvene, HMAF, MGI 114) is a novel antitumor agent currently undergoing clinical trials in hormone-refractory prostate cancer. This report examines the efficacy of irofulven alone or in combination with mitoxantrone or docetaxel against androgen-independent prostate cancer cell lines.

METHODS

To elucidate the activity of irofulven monotherapy and in combination, PC-3 and DU-145 cell lines were utilized in cellular viability assessments and tumor growth inhibition studies.

RESULTS

Viability assays with irofulven and mitoxantrone show additive to synergistic activity. Furthermore, irofulven and mitoxantrone in combination exhibit enhanced antitumor activity against PC-3 and DU-145 xenografts. Additive combination effects are also observed when irofulven and docetaxel were tested against PC-3 xenografts and curative activity (8/10 CR) is observed in DU-145 xenografts.

CONCLUSIONS

These studies demonstrate that irofulven displays strong activity as monotherapy and in combination with mitoxantrone or docetaxel against androgen-independent prostate cancer in vitro and in vivo; thus, supporting the clinical investigation of irofulven against hormone-refractory prostate cancer.

The role of RAD51 in etoposide (VP16) resistance in small cell lung cancer

Etoposide (VP16) is a potent inducer of DNA double-strand breaks (DSBs) and is efficiently used in small cell lung cancer (SCLC) therapy. However, acquired VP16 resistance remains an important barrier to effective treatment. To understand the underlying mechanisms for VP16 resistance in SCLC, we investigated DSB repair and cellular VP16 sensitivity of SCLC cells. VP16 sensitivity and RAD51, DNA-PK(cs), topoisomerase IIalpha and P-glycoprotein protein levels were determined in 17 SCLC cell lines. In order to unravel the role of RAD51 in VP16 resistance, we cloned the human RAD51 gene, transfected SCLC cells with RAD51 sense or antisense constructs and measured the VP16 resistance. Finally, we measured VP16-induced DSBs in the 17 SCLC cell lines. Two cell lines exhibited a multidrug-resistant phenotype. In the other SCLC cell lines, the cellular VP16 resistance was positively correlated with the RAD51 protein level. In addition, downregulation or overexpression of the RAD51 gene altered the VP16 sensitivity. Furthermore, the levels of the RAD51 and DNA-PK(cs) proteins were related to VP16-induced DSBs. The results suggest that repair of VP16-induced DSBs is mediated through both RAD51-dependent homologous recombination and DNA-PK(cs)-dependent nonhomologous end-joining and may be a determinant of the variation in clinical treatment effect observed in human SCLC tumors of identical histologic subtype. Finally, we propose RAD51 as a potential target to improve VP16 efficacy and predict tumor resistance in the treatment of SCLC patients.

DNA sequence specificity for topoisomerase II poisoning by the quinoxaline anticancer drugs XK469 and CQS

The two known antineoplastic quinoxaline topoisomerase II poisons, XK469 (NSC 697887) and CQS (chloroquinoxaline sulfonamide, NSC 339004), were compared for DNA cleavage site specificity, using purified human topoisomerase IIalpha and human topoisomerase IIbeta. The DNA cleavage intensity pattern for topoisomerase IIalpha poisoning by CQS closely resembled that of VM-26, despite the lack of any apparent common pharmacophore. In contrast, the topoisomerase IIalpha DNA cleavage intensity patterns of XK469 and CQS were very different from one another despite the similar overall structures of the two drugs. This suggests that the differences in DNA site specificity of topoisomerase II poisoning by XK469 and CQS may be caused by differences in their geometry, side chains, or electronic structure. The topoisomerase IIbeta-mediated DNA cleavage sites of CQS and XK469 were also very different from one another, adding further support to this idea. Earlier work has demonstrated that a number of specific topoisomerase II poisons show very similar patterns of DNA cleavage with either topoisomerase IIalpha or topoisomerase IIbeta, suggesting that the topoisomerase II isozymes play only a minor role in choices of DNA cleavage sites. However, both of the quinoxaline topoisomerase II poisons in this study showed distinctly different and unique DNA cleavage intensity patterns with each topoisomerase II isozyme. This indicates that topoisomerase II isozymes can play a major role in DNA cleavage site selection for some classes of topoisomerase II poisons.

Quantitation of gene-specific DNA damage by competitive PCR

A sensitive assay for quantitating DNA damage within individual genes would be a valuable tool for identifying the molecular mechanisms of disease and the sites of action of various carcinogens and anticancer drugs. This report describes a competitive PCR assay that was used to quantitate DNA damage induced by anticancer drugs within a 683-bp region of the c-myc gene in human CEM leukemia cells. Absolute quantitation of gene-specific DNA damage (attomoles or molecules of damaged DNA sequences) was achieved by coamplification of a homologous internal standard that has the same primer binding sites and PCR amplification efficiency as c-myc. The variability (standard error) associated with four separate determinations of the amount of c-myc sequence in 300 ng of DNA from untreated cells (6.80 +/- 0.05 SE amol) was less than 1% of the mean. The assay was capable of quantitating direct DNA damage that was induced by therapeutic concentrations of VM-26 and cisplatin prior to the onset of cellular apoptosis or necrosis. Both VM-26 (1-10 microM) and cisplatin (25-100 microM) induced a dose-dependent decrease in the amount of intact c-myc sequence. This assay should be readily adaptable to current real-time PCR protocols.

DNA topoisomerase II is essential for preimplantation mouse development

Topoisomerase II (topo II) is an essential enzyme that alters DNA topology. This activity is important for a variety of chromosome functions including DNA replication, transcription, recombination, and chromosome condensation and segregation. Previously we localized topo II in mouse gametes and preimplantation embryos using isoform-specific antibodies demonstrating the presence of the enzyme in oocytes and embryos, but not sperm. To probe functions of topo II during preimplantation development, we treated mouse zygotes with 100 nM teniposide, and assessed embryo morphology and DNA replication. Teniposide blocked cleavage in 69% embryos; the remainder cleaved once but had abnormal nuclei. Teniposide-treated embryos were devoid of topo II immunofluorescence. Teniposide also prevented DNA replication, implicating topo II in this process. Embryos treated with a 2 hr pulse of teniposide recovered and developed to the blastocyst stage, indicating 100 nM teniposide did not induce apoptosis. To more specifically analyze topo IIalpha function, we treated zygotes with topo IIalpha-targeted antisense oligodeoxynucleotides. Most zygotes arrested at the 2-cell stage while controls developed into blastocysts indicating topo IIalpha is essential for preimplantation development. The absence of topo IIalpha, but not beta immunofluorescence in antisense-treated embryos confirms the specificity and impact of the treatment. In addition, topo IIalpha is newly synthesized at the 2-cell stage. These results establish an essential function for topo II in mouse preimplantation embryonic development.

Suppression of c-myc expression and c-Myc function in response to sustained DNA damage in MCF-7 breast tumor cells

The topoisomerase II inhibitors teniposide (VM-26), doxorubicin, and amsacrine (m-AMSA), as well as ionizing radiation, induce a transient suppression of c-myc mRNA, which correlates with growth inhibition of MCF-7 breast tumor cells. To further assess the involvement of c-mvc in the DNA damage-induced signal transduction pathways of the breast tumor cell, we determined the influence of sustained DNA damage on c-myc expression, c-Myc protein levels and c-Myc function. Continuous exposure of MCF-7 breast tumor cells to VM-26 induced DNA strand breaks that were sustained for at least 9 hr. DNA strand breakage was accompanied by a decline in c-myc transcripts and c-Myc protein levels by >90% after VM-26 exposure for 24 hr. The activity of a transcriptional target of the c-Myc protein, ornithine decarboxylase, was reduced by approximately 75% within 9 hr of DNA damage, in parallel to the declines in c-myc mRNA and protein levels. Extended exposure to VM-26 resulted in an initial loss of approximately 35% of the cell population followed by the death of additional cells such that by 72 hr only 50% of the cells were viable. Although apoptosis was evident 72 hr after initiating drug exposure [based on cell cycle analysis, terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assays, and an assessment of cell morphology], the primary phase of cell killing, which occurred during the first 24 hr was non-apoptotic. These studies indicate that non-apoptotic pathways can also mediate cell death in the breast tumor cell and support the role of c-myc expression, c-Myc protein, and c-Myc function as elements of the DNA damage response pathway in the breast tumor cell.

Imbalanced DNA synthesis induced by cytosine arabinoside and fludarabine in human leukemia cells11Abbreviations: araC, 1-β-d-arabinofuranosylcytosine (cytosine arabinoside); araA, 1-β-d-arabinofuranosyladenine; BrdUrd, 5-bromo-2′-deoxyuridine; FaraA, 1-β-d-arabinofuranosyl-2-fluoroadenine (fludarabine); ic50, concentration that reduces cloning efficiency by 50%; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PALA, N-(phosphonacetyl)-l-aspartate; and SSC, standard saline citrate

Previous studies have demonstrated that cytosine arabinoside (araC) induces an accumulation of Okazaki fragments, while fludarabine (FaraA) inhibits Okazaki fragment synthesis. We extended these observations in the present study to provide insights into various mechanisms by which these anticancer drugs affect DNA replication and induce genomic instability in human CEM leukemia cells. Neither araC nor FaraA induced a detectable amount of re-replicated DNA in S-phase cells, which indicated that drug-induced alterations in Okazaki fragment synthesis were not accompanied by DNA re-replication. Synthesis on both leading and lagging DNA strands within the c-myc locus was measured in cells incubated with equitoxic concentrations of araC or FaraA. In araC-treated cells, nascent DNA from the lagging strand was enriched about 5-fold compared with the leading strand. In contrast, FaraA did not induce any replication imbalance. AraC- and FaraA induced changes in the frequency of N-(phosphonacetyl)-l-aspartate (PALA) resistance and the extent of CAD gene amplification were monitored as markers of drug-induced genomic instability. At concentrations that reduced cloning efficiency by 50% (IC(50)), araC increased the frequency of PALA resistance about 4-fold, while FaraA did not have a significant effect on the frequency of PALA resistance. Pretreatment with araC also increased the extent of CAD gene amplification. We propose that the imbalanced DNA synthesis induced by araC leads to the accumulation of Okazaki fragments on the lagging arms and single-stranded DNA regions on the leading arms of replication forks. The formation of these abnormal replication structures was associated with the generation of genomic instability.

The complex of DNA gyrase and quinolone drugs on DNA forms a barrier to the T7 DNA polymerase replication complex

Quinolone drugs can inhibit bacterial DNA replication, via interaction with the type II topoisomerase DNA gyrase. Using a DNA template containing a preferred site for quinolone-induced gyrase cleavage, we have demonstrated that the passage of the bacteriophage T7 replication complex is blocked in vitro by the formation of a gyrase-drug-DNA complex. The majority of the polymerase is arrested approximately 10 bp upstream of this preferred site, although other minor sites of blocking have been observed. The ability of mutant gyrase proteins to arrest DNA replication in vitro has been investigated. Gyrase containing mutations in the A subunit at either the active-site tyrosine (Tyr122) or Ser83 (a residue known to be involved in quinolone interaction) failed to halt the progress of the polymerase. A low-level, quinolone-resistant mutation in the B subunit of gyrase showed reduced blocking compared to wild-type. We have demonstrated that DNA cleavage and replication blocking occur on similar time-scales and we conclude that formation of the cleavable complex is a prerequisite for polymerase blocking. Additionally, we have shown that collision of the replication proteins with the gyrase-drug-DNA complex is not sufficient to render this complex irreversible and that further factors must be involved in processing this stalled complex.

Topoisomerase II cleavable complex formation within DNA loop domains

The distribution of VM-26 (Teniposide)-stabilized cleavable complexes within DNA loops bound to the nuclear matrix was determined to provide further insights into the mode of DNA synthesis inhibition by VM-26. Covalent binding of [(3)H]VM-26 was 9-fold greater per milligram of nuclear matrix protein compared with high salt-soluble nonmatrix protein of CEM cells. The ratio declined from 9-fold in CEM cells to 4-fold in drug-resistant VM-1/C2 cells, which have decreased nuclear matrix DNA topoisomerase IIalpha. VM-26 induced a concentration-dependent increase in the frequency of cleavable complex formation with actively replicating matrix DNA. At 25 microM VM-26, the frequency was 32 +/- 2 (SEM) complexes per 10(6) bp of replicating matrix DNA compared with 13 +/- 2 (SEM) complexes per 10(6) bp of nonreplicating DNA in the matrix fraction. VM-26 at concentrations as high as 25 microM stabilized less than 3 complexes per 10(6) bp in the various nonmatrix DNA domains, since the nonmatrix DNA comprises the DNA loop domains that are distal to the matrix-bound replication sites. A negligible frequency of cleavable complex formation was detected in both the matrix and nonmatrix DNA domains of drug-resistant VM-1/C2 cells. Compared with untreated control cells, VM-26 induced an accumulation of nascent DNA in the nuclear matrix fraction of CEM cells but decreased the amount of nascent DNA in the nonmatrix fraction. The extensive cleavable complex formation on matrix replicating DNA stalled most of the replication forks within 1 kb of the replication sites on the nuclear matrix. The results provide evidence that nascent DNA bound to the nuclear matrix is an important site of VM-26 cleavable complex formation, and that these complexes inhibit DNA synthesis by blocking the movement of nascent DNA away from replication sites on the nuclear matrix.

Topoisomerase II as a target for anticancer drugs: when enzymes stop being nice

Topoisomerase II is an essential enzyme that plays a role in virtually every cellular DNA process. This enzyme interconverts different topological forms of DNA by passing one nucleic acid segment through a transient double-stranded break generated in a second segment. By virtue of its double-stranded DNA passage reaction, topoisomerase II is able to regulate DNA over- and underwinding, and can resolve knots and tangles in the genetic material. Beyond the critical physiological functions of the eukaryotic enzyme, topoisomerase II is the target for some of the most successful anticancer drugs used to treat human malignancies. These agents are referred to as topoisomerase II poisons, because they transform the enzyme into a potent cellular toxin. Topoisomerase II poisons act by increasing the concentration of covalent enzyme-cleaved DNA complexes that normally are fleeting intermediates in the catalytic cycle of topoisomerase II. As a result of their action, these drugs generate high levels of enzyme-mediated breaks in the genetic material of treated cells and ultimately trigger cell death pathways. Topoisomerase II is also the target for a second category of drugs referred to as catalytic inhibitors. Compounds in this category prevent topoisomerase II from carrying out its required physiological functions. Drugs from both categories vary widely in their mechanisms of actions. This review focuses on topoisomerase II function and how drugs alter the catalytic cycle of this important enzyme.

An antitumor drug-induced topoisomerase cleavage complex blocks a bacteriophage T4 replication fork in vivo

Many antitumor and antibacterial drugs inhibit DNA topoisomerases by trapping covalent enzyme-DNA cleavage complexes. Formation of cleavage complexes is important for cytotoxicity, but evidence suggests that cleavage complexes themselves are not sufficient to cause cell death. Rather, active cellular processes such as transcription and/or replication are probably necessary to transform cleavage complexes into cytotoxic lesions. Using defined plasmid substrates and two-dimensional agarose gel analysis, we examined the collision of an active replication fork with an antitumor drug-trapped cleavage complex. Discrete DNA molecules accumulated on the simple Y arc, with branch points very close to the topoisomerase cleavage site. Accumulation of the Y-form DNA required the presence of a topoisomerase cleavage site, the antitumor drug, the type II topoisomerase, and a T4 replication origin on the plasmid. Furthermore, all three arms of the Y-form DNA were replicated, arguing strongly that these are trapped replication intermediates. The Y-form DNA appeared even in the absence of two important phage recombination proteins, implying that Y-form DNA is the result of replication rather than recombination. This is the first direct evidence that a drug-induced topoisomerase cleavage complex blocks the replication fork in vivo. Surprisingly, these blocked replication forks do not contain DNA breaks at the topoisomerase cleavage site, implying that the replication complex was inactivated (at least temporarily) and that topoisomerase resealed the drug-induced DNA breaks. The replication fork may behave similarly at other types of DNA lesions, and thus cleavage complexes could represent a useful (site-specific) model for chemical- and radiation-induced DNA damage.

DNA Topoisomerase II-alpha as a marker of cell proliferation in endocrine and other neoplasms

DNA topoisomerases are enzymes that are able to link and unlink DNA strands. They are classified as type I or type II topoisomerase if they catalyze transient single-strand (topo I) or double-strand (topo II) DNA breaks. Topo II-alpha has been used as a proliferation marker and it can also serve as a molecular target for a variety of anticancer drugs that are used clinically.Topo II-alpha expression is similar to MIB1 immunoreactivity in breast, ovarian, cervix, gastric, endometrial, adrenocortical, and hematological malignancies. In a study of adrenocortical tumors with metastases topo II was significantly higher than in tumors without metastases.Studies of topo II-alpha expression may provide information about the biological behavior of specific tumors and may also provide insights into the role that this enzyme plays in the response of human cancers to topo II-targeted anticancer drugs.