A rapid and quantitative microtiter assay for eukaryotic topoisomerase II

A novel indeno[1,2-b]pyridinone derivative, a DNA intercalative human topoisomerase IIα catalytic inhibitor, for caspase 3-independent anticancer activity

A novel 2-(furan-2-yl)-4-(pyridin-2-yl)-5H-indeno[1,2-b]pyridin-5-one (TI-1-190) was synthesized using a simple microwave-assisted method and its mode of action was systematically characterized.

Biophysical and molecular docking studies of naphthoquinone derivatives on the ATPase domain of human topoisomerase II

Numerous naphthoquinone derivatives, such as rhinacanthins function as anticancer drugs, which target hTopoII. The structure of hTopoII contains both an ATPase domain and a DNA binding domain. Several drugs bind to either one or both of these domains, thus modifying the activity of hTopoII. The naphthoquinone esters and amides used in this study showed that their hTopoIIα inhibitory activity was inversely proportional to ATP concentration. In order to better characterize the inhibitory action of these compounds, sufficient quantities of soluble functional hTopoII-ATPase domain were required. Therefore, both the alpha and beta isoforms of the hTopoII-ATPase domain were over-expressed in Escherichia coli. The hTopoIIα-ATPase activity was reduced in the presence of naphthoquinone derivatives. Additionally, a molecular docking study revealed that the selected naphthoquinone ester and amide bind to the ATP-binding domain of hTopoIIα. Collectively, the results here provide for the first time a novel insight into the interaction between naphthoquinone esters and amides, and the ATP-binding domain of hTopoIIα. The further elucidation of the mechanism of action of the naphthoquinone esters and amides inhibitory activity is essential.

Topoisomerase II beta expression level correlates with doxorubicin-induced apoptosis in peripheral blood cells

Anthracyclines are widely used in oncology. Both the response and side-effects of anthracyclines are individually variable, but determinants or predictive markers of this variability are not available. We investigated the variability in the expression of the anthracycline targets topoisomerases II (topo II) alpha and beta and its significance for the apoptotic response following exposure to the anthracycline doxorubicin. Only topo II beta protein expression was detected in peripheral blood cells. Usually considered a constitutively expressed protein, topo II beta varied 3-, 18-, and 16-fold on the mRNA, protein and activity levels, respectively, among the volunteers tested. In addition, the expression of topo II beta was modified by several mitogens, suggesting a role in the regulation of cell cycle. Strikingly, topo II beta activity correlated statistically significantly with the apoptotic response in peripheral blood leukocytes exposed to 1 microM doxorubicin. A longitudinal study in a subset of study subjects demonstrated that 30% of the topo II expression variability may be inherited. However, resequencing of the TOP2B gene in 48 unrelated individuals revealed only 8 gene variants, none of them with obvious effects on the expression or protein sequence of topo II beta. Taken together, the apoptotic response to doxorubicin in peripheral blood cells may be mediated by topo II beta. The expression level of topo II beta is intra- and inter-individually variable, and may in part determine the apoptotic response to doxorubicin and other anthracyclines.

Mechanism targeted discovery of antitumor marine natural products

Antitumor drug discovery programs aim to identify chemical entities for use in the treatment of cancer. Many strategies have been used to achieve this objective. Natural products have always played a major role in anticancer medicine and the unique metabolites produced by marine organisms have increasingly become major players in antitumor drug discovery. Rapid advances have occurred in the understanding of tumor biology and molecular medicine. New insights into mechanisms responsible for neoplastic disease are significantly changing the general philosophical approach towards cancer treatment. Recently identified molecular targets have created exciting new means for disrupting tumor-specific cell signaling, cell division, energy metabolism, gene expression, drug resistance and blood supply. Such tumor-specific treatments could someday decrease our reliance on traditional cytotoxicity-based chemotherapy and provide new less toxic treatment options with significantly fewer side effects. Novel molecular targets and state-of-the-art, molecular mechanism-based screening methods have revitalized antitumor research and these changes are becoming an ever-increasing component of modern antitumor marine natural products research. This review describes marine natural products identified using tumor-specific mechanism-based assays for regulators of angiogenesis, apoptosis, cell cycle, macromolecule synthesis, mitochondrial respiration, mitosis, multidrug efflux and signal transduction. Special emphasis is placed on natural products directly discovered using molecular mechanism-based screening.

Antitumor triptycene bisquinones: a novel synthetic class of dual inhibitors of DNA topoisomerase I and II activities

Synthetic triptycene analogs (TT code number) mimic the antitumor effects of daunorubicin in the nanomolar range in vitro, but have the advantage of blocking nucleoside transport and retaining their efficacy in multidrug-resistant (MDR) tumor cells. Since TT bisquinones induce poly(ADP-ribose) polymerase-1 cleavage at 6 h and internucleosomal DNA fragmentation at 24 h, which are, respectively, early and late markers of apoptosis, these lead antitumor drugs were tested for their ability to trigger the DNA topoisomerase (Topo) inhibitions responsible for the initial and massive high-molecular-weight cleavage of DNA required for tumor cells to commit apoptosis. Interestingly, antitumor TTs have the unusual ability to inhibit, in a concentration-dependent manner, the relaxation of supercoiled plasmid DNA catalyzed by both purified human Topo I and II enzymes. However, if there is a relationship between the ability of TT analogs to inhibit Topo activities and their quinone functionality and cytotoxicity, it is far from perfect, suggesting that other molecular targets may be involved in the mechanism of action of these antitumor drugs. Moreover, one of the most cytotoxic TT bisquinone, 6-bromo-7-methoxy- or 7-bromo-6-methoxy-2-N-methylamino-1 H,4 H,5 H,8H-9,10-dihydro-9,10-[1',2']benzenoanthracene-1,4,5,8-tetraone (TT24), inhibits Topo II activity more effectively than amsacrine (m-AMSA) and matches the Topo I inhibitory effect of camptothecin (CPT). The dual inhibitory activity of TT24 is substantiated by the findings that TT24 mimics the action of m-AMSA in the Topo II assay, where the Topo I inhibitor CPT is ineffective, and also mimics the action of CPT in the Topo I assay, where the Topo II inhibitor etoposide is ineffective. Because of their ability to target nucleoside transport and topoisomerase activities, synthetic TT bisquinones might represent a novel class of bifunctional drugs valuable to develop new means of polychemotherapy and circumvent MDR.

Influence of polyamine architecture on the transport and topoisomerase II inhibitory properties of polyamine DNA-intercalator conjugates

An efficient five-step synthetic method was developed to access a series of spermine derivatives containing appended acridine, anthracene, and 7-chloroquinoline motifs. The derivatives were composed of a spermine fragment covalently tethered at its N4 and N9 positions to an aromatic nucleus via an aliphatic chain (e.g., 8: acridine -[C4 aliphatic tether]-spermine-[C4 aliphatic tether]-acridine). The distance separating the spermine and aromatic nuclei was altered via different tethers composed of four or five methylene units. These bis ligands (8, 9, 12, and 13) were shown to inhibit human DNA topoisomerase II (topo II) activity at 5 microM. Enzymatic activity was assessed as the ability to unknot (decatenate) and cleave kinetoplast DNA (kDNA). Polyamine conjugation did not disrupt the ability of the acridine-spermine conjugates 8 and 9 to inhibit topo II activity as compared with the 9-aminoacridine and 9-(N-butyl)aminoacridine controls (at 5 microM). The parent polyamines, spermine (5 microM) and spermidine (10 microM), had little effect on topo II activity. In general, the bis-substituted spermine derivatives (8, 9, 12, and 13) were more efficient topo II inhibitors at 5 microM than their monosubstituted spermidine counterparts (22-25) at 10 microM. Within the bisintercalator spermine series, insertion of an additional methylene unit (i.e., C5 tethers) increased potency 2-fold (8, bis-C4-acridine, 47 h IC(50) = 40 microM; 9, bis-C5-acridine, IC(50) = 17 microM). Comparison of the bis- and monoacridine spermine motifs (8 and 17) revealed a 4-fold increase in potency for the latter architecture (94 h IC(50) for 8, 74 microM; for 17, 17 microM). In general the bisintercalators (8, 9, 12, and 13) behaved as cytostatic agents, while the monosubstituted acridine and anthracene derivatives (22-25) were cytotoxic. Anthracene-containing conjugates were generally more toxic than their acridine counterparts in an L1210 (murine leukemia) cell assay. Of the conjugates tested the (monointercalator)-spermine motif (e.g., 17) had the highest affinity for the L1210 polyamine transporter as revealed by spermidine protection experiments.

The effect of polyamine homologation on the transport and cytotoxicity properties of polyamine-(DNA-intercalator) conjugates

An efficient five-step synthetic method was developed to access a homologous series of spermidine-acridine and spermidine-anthracene conjugates. The derivatives were comprised of a spermidine fragment covalently tethered at its N4 position to either an acridine or anthracene nucleus via an aliphatic chain (e.g., spermidine-[aliphatic tether]-acridine). The distance separating the spermidine and aromatic nucleus was altered by using different tethers comprised of four or five methylene units, respectively. These ligands (2-5) were shown to inhibit human DNA topoisomerase-II (TOPO-II) activity at 10 microM. Enzymatic activity was assessed as the ability to unknot (decatenate) and cleave kinetoplast DNA (kDNA). Polyamine conjugation did not disrupt the ability of the acridine-spermidine conjugates 2 and 3 to inhibit TOPO-II activity as compared with the 9-aminoacridine and 9-(N-butyl)aminoacridine controls (at 10 microM). In general, the acridine derivatives (2 and 3) showed higher TOPO-II inhibitory activity than their anthracene counterparts (4 and 5). However, this trend was reversed in a whole cell assay with L1210 (murine leukemia) cells, wherein the anthracene analogues were more potent than their acridine counterparts. In this regard the qualitative enzyme-based assay did not predict the trends in the corresponding IC(50) values. Within either series insertion of an additional methylene unit did not significantly alter activity. While the appended spermidine unit did not disrupt TOPO II inhibition by the tethered DNA intercalator, it did provide an alternative mode of entry into the cell as demonstrated by spermidine protection assays. These results were compared with a spermine-intercalator analogue. Of all the conjugates tested the N(4)-(4-(9-aminoacridinyl)butyl)spermine hexahydrochloride (conjugate 16)resulted in the highest degree of L1210 cell rescue upon cotreatment of the cells with exogenous spermidine. It was concluded that the monoalkylated spermine motif present in 16 holds promise as a better vector than its N4 monoalkylated spermidine counterpart.

Inhibition of topoisomerase II by liriodenine

The cytotoxic oxoaporphine alkaloid liriodenine, isolated from Cananga odorata, was found to be a potent inhibitor of topoisomerase II (EC 5.99.1.3) both in vivo and in vitro. Liriodenine treatment of SV40 (simian virus 40)-infected CV-1 cells caused highly catenated SV40 daughter chromosomes, a signature of topoisomerase II inhibition. Strong catalytic inhibition of topoisomerase II by liriodenine was confirmed by in vitro assays with purified human topoisomerase II and kinetoplast DNA. Liriodenine also caused low-level protein-DNA cross-links to pulse-labeled SV40 chromosomes in vivo, suggesting that it may be a weak topoisomerase II poison. This was supported by the finding that liriodenine caused topoisomerase II-DNA cross-links in an in vitro assay for topoisomerase II poisons. Verapamil did not increase either liriodenine-induced protein-DNA cross-links or catalytic inhibition of topoisomerase II in SV40-infected cells. This indicates that liriodenine is not a substrate for the verapamil-sensitive drug efflux pump in CV-1 cells.

Regulation of DNA metabolic enzymes upon induction of preB cell development and V(D)J recombination: up-regulation of DNA polymerase delta

Withdrawal of interleukin-7 from cultured murine preB lymphocytes induces cell differentiation including V(D)J immunoglobulin gene rearrangements and cell cycle arrest. Advanced steps of the V(D)J recombination reaction involve processing of coding ends by several largely unidentified DNA metabolic enzymes. We have analyzed expression and activity of DNA polymerases alpha, beta, delta and epsilon, proliferating cell nuclear antigen (PCNA), topoisomerases I and II, terminal deoxynucleotidyl transferase (TdT) and DNA ligases I, III and IV upon induction of preB cell differentiation. Despite the immediate arrest of cell proliferation, DNA polymerase delta protein levels remained unchanged for approximately 2 days and its activity was up-regulated several-fold, while PCNA was continuously present. Activity of DNA polymerases alpha,beta and epsilon decreased. Expression and activity of DNA ligase I were drastically reduced, while those of DNA ligases III and IV remained virtually constant. No changes in DNA topoisomerases I or II expression and activity occurred and TdT expression was moderately increased early after induction. Our results render DNA polymerase delta a likely candidate acting in DNA synthesis related to V(D)J recombination in lymphocytes.

Inhibition of topoisomerase II by ICRF-193, the meso isomer of 2,3-bis(2,6-dioxopiperazin-4-yl)butane. Critical dependence on 2,3-butanediyl linker absolute configuration

The bis(2,6-dioxopiperazine)s are a structurally and mechanistically unique class of topoisomerase II inhibitors that do not bind DNA and that do not stabilize topoisomerase II-DNA strand passing intermediates ("cleavable complexes"). The most effective topoisomerase II inhibitor in the bis(2,6-dioxopiperazine) series is ICRF-193 (meso or S*, R* isomer), with a meso 2,3-butanediyl linker connecting the dioxopiperazine rings. The two enantiomeric diastereomers, (R,R) and (S,S), of ICRF-193 possessing the two optically active 2,3-butanediyl linkers have been prepared from their respective optically pure 2,4-diaminobutanes via 2,3-diaminobutane-N,N,N',N'-tetraacetic acid, esterification, and imide formation. Both in vivo and in vitro assays for catalytic inhibition of topoisomerase II were employed to show that the (S,S)- and (R,R)-isomers are almost inactive as topoisomerase II inhibitors. The data indicate that the meso stereochemistry of the alkanediyl linker is crucial for activity and provides additional evidence that the cytotoxicity of the bis(2,6-dioxopiperazine)s is due to their ability to inhibit topoisomerase II.

Quantitative immunocytochemical assays of topoisomerase II in lung adenocarcinoma cell lines. Correlation to topoisomerase II alpha content and topoisomerase II catalytic activity

The examination of topoisomerase II alpha content by Western blot analysis or topoisomerase II catalytic activity by decatenation of kDNA requires a large number of cells, but it is difficult to collect sufficient cells for these biochemical analyses from lung cancer patients by transbronchial brushing or aspiration. In this study, we explored the relationship between these biochemical analyses and topoisomerase II immunostaining in cytospin preparations of three lung adenocarcinoma cell lines. The levels of topoisomerase II alpha content were about 8.4 for A549, 2.9 for PC-3 and 1 for RERF-LC-MS, and the levels of topoisomerase II catalytic activity were about 4, 2, and 1, respectively. The percentages of strongly positive cells for topoisomerase II immunostaining were 60.9% for A549, 33.3% for PC-3, and 14.3% for RERF-LC-MS, and these were compatible with the levels of topoisomerase II alpha content or topoisomerase II catalytic activity. Our results indicate that topoisomerase II immunostaining can be utilized in place of biochemical analysis.

Association between the p170 form of human topoisomerase II and progeny viral DNA in cells infected with herpes simplex virus type 1

Endogenous host topoisomerase II acts upon herpes simplex virus type 1 (HSV-1) DNA in infected cells (S.N. Ebert, S.S. Shtrom, and M.T. Muller, J. Virol. 56:4059-4066, 1990), and cleavage is directed exclusively at progeny viral DNA while parental DNA is resistant. To evaluate the possibility that HSV-1 induces topoisomerase II activity which could account for the preferential cleavage of progeny viral DNA, we assessed topoisomerase II cleavage activity on cellular and viral DNA substrates before and after the initiation of viral DNA replication. We show that cleavage of a host gene in mock-infected cells was similar to that observed in HSV-1-infected cells, regardless of whether viral DNA replication had occurred. In addition, quantitative measurements revealed comparable amounts of topoisomerase II activity in infected and mock-infected cells; thus, HSV-1 neither induces nor encodes its own type II topoisomerase and cleavages in vivo are due to a preexisting host topoisomerase. Human cells contain two isozymes of topoisomerase II (p170 and p180), encoded by separate genes. Through the use of isozyme-specific antibodies, we demonstrate that only p170 was found to be cross-linked to HSV-1 DNA even though both forms were present at nearly constant levels in HSV-1-infected cells. Immunofluorescence revealed that by 6 h postinfection, p170 becomes redistributed and localized to sites of active viral DNA synthesis. The data suggest that p170 gains preferential access to replicated viral DNA molecules, which explains why topoisomerase II activity is concentrated on progeny DNA.