Antagonistic effect of aclarubicin on camptothecin induced cytotoxicity: role of topoisomerase I

Natural Compounds as Modulators of Cell Cycle Arrest: Application for Anticancer Chemotherapies

Natural compounds from various plants, microorganisms and marine species play an important role in the discovery novel components that can be successfully used in numerous biomedical applications, including anticancer therapeutics. Since uncontrolled and rapid cell division is a hallmark of cancer, unraveling the molecular mechanisms underlying mitosis is key to understanding how various natural compounds might function as inhibitors of cell cycle progression. A number of natural compounds that inhibit the cell cycle arrest have proven effective for killing cancer cells in vitro, in vivo and in clinical settings. Significant advances that have been recently made in the understanding of molecular mechanisms underlying the cell cycle regulation using the chemotherapeutic agents is of great importance for improving the efficacy of targeted therapeutics and overcoming resistance to anticancer drugs, especially of natural origin, which inhibit the activities of cyclins and cyclin-dependent kinases, as well as other proteins and enzymes involved in proper regulation of cell cycle leading to controlled cell proliferation.

Dual topoisomerase I/II inhibitors

Topoisomerase (topo) I and II are nuclear enzymes, which play a major role in the topological rearrangement of DNA during replication and transcription processes. In the course of years, many different agents have been found which can inhibit the topos and thereby exploit cytotoxicity, also against tumour cells. Selective inhibition of the topo I enzyme can, however, induce a reactive increase in topo II levels, and vice versa. This mechanism is associated with the development of drug resistance. Dual inhibition of both topo I and II may, theoretically, overcome this resistance problem. In this review, the most important and promising dual topo I/II inhibitors designed as anticancer agents will be discussed. Thus far, only the indolyl quinoline derivative TAS-103, the 7 H-benzo [ e] pyrido [4,3- b] indole derivative intoplicine, and the acridine derivative PZA have been shown to be dual topo inhibitors with high cytotoxicity.

Physicochemical Mechanisms of Synergistic Biological Action of Combinations of Aromatic Heterocyclic Compounds

The mechanisms of synergistic biological effects observed in the simultaneous use of aromatic heterocyclic compounds in combination are reviewed, and the specific biological role of heteroassociation of aromatic molecules is discussed.

Therapeutic intervention by the simultaneous inhibition of DNA repair and type I or type II DNA topoisomerases: one strategy, many outcomes

Many anticancer drugs reduce the integrity of DNA, forming strand breaks. This can cause mutations and cancer or cell death if the lesions are not repaired. Interestingly, DNA repair-deficient cancer cells (e.g., those with BRCA1/2 mutations) have been shown to exhibit increased sensitivity to chemotherapy. Based on this observation, a new therapeutic approach termed 'synthetic lethality' has been developed, in which radiation therapy or cytotoxic anticancer agents are employed in conjunction with selective inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1). Such combinations can cause severe genomic instability in transformed cells resulting in cell death. The synergistic effects of combining PARP-1 inhibition with anticancer drugs have been demonstrated. However, the outcome of this therapeutic strategy varies significantly between cancer types, suggesting that synthetic lethality may be influenced by additional cellular factors. This review focuses on the outcomes of the combined action of PARP-1 inhibitors and agents that affect the activity of DNA topoisomerases.

Catalytic topoisomerase II inhibitors in cancer therapy

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

Inhibition of DNA topoisomerases II and/or I by pyrazolo[1,5-a]indole derivatives and their growth inhibitory activities

DNA topoisomerases (topos) I and II are molecular targets of several potent anticancer agents. Thus inhibitors of these enzymes are potential candidates or model compounds for anticancer drugs. We found some of the totally synthetic pyrazolo[1,5-a]indole derivatives, GS-2, -3, and -4, to be strong inhibitors of topo II, and GS-5 was found to be a dual inhibitor of topos I and II (IC(50) values were in the range of 10-30 microM). Because of the DNA-intercalating activity of these compounds affecting supercoil structure of closed circular DNA, the method of evaluation of topo I inhibition designed for such compounds by Pommier et al. (Nucleic Acids Res 15:6713-6731, 1987) was employed. Results showed that only GS-5 with a hydroxyl group at position C-6 was found to be a strong inhibitor of topo I with an IC(50) of approximately 10 microM. Inhibition of topo I and/or topo II by these compounds does not involve significant accumulation of DNA-topo I/II cleavable complexes, demonstrating that they are not topo poisons but catalytic inhibitors. In the "band depletion" analysis for in vivo targeting of topo I and II, these compounds were shown to suppress depletion of intracellular free enzymes by the topo poisons etoposide and/or camptothecin, indicating that they do target topo I and/or II in living cells. These compounds also exhibit moderate to strong growth-inhibitory activity in panels of human cancer cell lines. This study shows pyrazolo[1,5-a]indole derivatives to be a novel group of anticancer chemotherapeutic agents with single or dual catalytic inhibitory activities against topo I and topo II.

Synergistic cytotoxicity, apoptosis and protein-linked DNA breakage by etoposide and camptothecin in human U87 glioma cells: dependence on tyrosine phosphorylation

In this study, simultaneous administration of certain inhibitors of topoisomerase I and topoisomerase II produced synergistic cytotoxicity in a series of human glioma cell lines. Camptothecin (CPT) and etoposide (VP-16) produced combination indices (CI) <1.0 in all glioma cell lines tested, including those that were relatively resistant to the two topoisomerase inhibitors individually. In contrast, CPT and VP-16 produced additive cytotoxicity in HT-29 and SW-620 colon carcinoma cell lines. To explore the molecular basis for synergy in glioma cells, we focused on one glioma cell line (U87) in which even sub-cytotoxic doses of CPT potentiated the action of VP-16. Except for genistein (a topo II agent with tyrosine kinase inhibitory function), all topo II inhibitors tested (doxorubicin, ellipticine, and m-AMSA) were synergistic with CPT. While CPT and VP-16 produced cytotoxicity and protein-linked DNA breaks (PLDB) that were supra-additive in U87 glioma cells, CPT and genistein produced additive results. Pretreatment of U87 cells with the tyrosine kinase inhibitor tyrphostin-A23 or the tyrosine phosphatase activator O-phospho-L-tyrosine (OPLT) reduced combination PLDB from synergistic to additive levels, but had no effect on the formation of PLDB induced by either CPT or VP-16 alone. CPT and VP-16 also produced a synergistic accumulation of sub-G0 (apoptotic) cells which was blocked by tyrphostin-A23. No significant increase in topoisomerase protein levels could be detected in response to combination treatment. Thus, synergistic effects between topoisomerase I and topoisomerase II inhibitors in U87 glioma cells may depend upon phosphorylation of cellular proteins other than the topoisomerases themselves.

Mechanism of action of eukaryotic DNA topoisomerase I and drugs targeted to the enzyme

DNA topoisomerase I is essential for cellular metabolism and survival. It is also the target of a novel class of anticancer drugs active against previously refractory solid tumors, the camptothecins. The present review describes the topoisomerase I catalytic mechanisms with particular emphasis on the cleavage complex that represents the enzyme's catalytic intermediate and the site of action for camptothecins. Roles of topoisomerase I in DNA replication, transcription and recombination are also reviewed. Because of the importance of topoisomerase I as a chemotherapeutic target, we review the mechanisms of action of camptothecins and the other topoisomerase I inhibitors identified to date.

Diversity of DNA topoisomerases I and inhibitors

The present review first describes the different type I topoisomerases found in eukaryotic cells: nuclear topoisomerase I (top1), topoisomerase 3 (top3), mitochondrial topoisomerase I and viral topoisomerases I. The second part of the review provides extensive information on the topoisomerase I inhibitors identified to date. These drugs can be grouped in two categories: top1 poisons and top1 suppressors. Both inhibit enzyme catalytic activity but top1 poisons trap the top1 catalytic intermediates ('cleavage complexes') while top1 suppressors prevent or reverse top1 cleavage complexes. The molecular interactions of camptothecin with the top1 cleavage complexes are discussed as well as the mechanisms of selective killing of cancer cells.

pH-dependent regulation of camptothecin-induced cytotoxicity and cleavable complex formation by the antimalarial agent chloroquine

Two classes of drugs interact with DNA topoisomerase (topo) I, namely topoI poisons such as the camptothecins, which create DNA single-strand breaks and the catalytic inhibitors, which do not. Here, we demonstrate that the antimalarial agent chloroquine is a catalytic inhibitor of eukaryote topoI, as the drug inhibited topoI-mediated DNA relaxation. Chloroquine is known to be a topoII catalytic inhibitor and as such is able to inhibit the activity of a topoII poison, i.e. etoposide. We now show that chloroquine also inhibits the topol poison camptothecin as camptothecin-stimulated nicking of plasmid DNA was inhibited by chloroquine. These observations also apply to endogenous topoI in whole cells. Accordingly, camptothecin-induced single-strand breaks as well as cytotoxicity were antagonised by chloroquine. Further, in a band depletion assay in whole cells, chloroquine prevented camptothecin-mediated topoI trapping, indicating that chloroquine inhibits topoI by interfering with the DNA binding step of the enzyme. In contrast to camptothecin, chloroquine is a weak base and therefore does not enter the cell if the extracellular fluid is acidic, as is the case in most solid tumors. This leads to the possibility of directing cytotoxicity to solid tumors with low extracellular pH by combining a neutral anticancer agent, i.e. camptothecin with a weak base antagonist, i.e. chloroquine. To test the feasibility of this principle, we investigated the drug combination at varying extracellular pH. We found that the antagonising effect of chloroquine on camptothecin-mediated trapping of topoI and DNA single-strand break formation was abolished at acidic extracellular pH. In a clonogenic assay, camptothecin in combination with chloroquine selectively killed cells at low pH (6.2), while camptothecin cytotoxicity was antagonised by chloroquine at normal pH (7.2). In conclusion, we show that the topoI catalytic inhibitor chloroquine inhibits camptothecin and that chloroquine can target the cytotoxic effect of camptothecin to tumor cells in acidic environments.

Effects of bifunctional netropsin-related minor groove-binding ligands on mammalian type I DNA topoisomerase

We investigated the effects of compounds with two covalently linked netropsin moieties (bis-netropsin) on the function of mammalian type I DNA topoisomerase (topo I) in vitro. We initiated these studies because earlier studies had shown that certain bis-netropsins possess a several-fold higher antitumor and antiviral activity than netropsin. We confirmed that the parent compound netropsin, but not its bifunctional derivatives, induce supercoils in closed DNA. We determined that bis-netropsins inhibit the binding of topo I to DNA more efficiently than netropsin and that bis-netropsins but not netropsin induce specific DNA strand cleavage in the presence of topo I. We discuss a model explaining the different effects of netropsin and bis-netropsins on topo I.

Separation and functional analysis of eukaryotic DNA topoisomerases by chromatography and electrophoresis

DNA topoisomerases are enzymes that control DNA topology by cleaving and rejoining DNA strands and passing other DNA strands through the transient gaps. Consequently, these enzymes play a crucial role in the regulation of the physiological function of the genome. Beyond their normal functions, topoisomerases are important cellular targets in the treatment of human cancers. In this review we summarize current protocols for extracting and purifying DNA topoisomerases, and for separating subtypes and isoforms of these enzymes. Furthermore, we discuss methods for measuring the catalytic activity of topoisomerases and for monitoring the molecular effects of topoisomerase-directed antitumor drugs in cell-free assays.

Mutagenicity of anticancer drugs that inhibit topoisomerase enzymes

Topoisomerases are enzymes that catalyse the transient breakage and rejoining of either one (topo I) or two (topo II) DNA strands, to allow one strand to pass through another and prevent unresolvable tangles during processes such as DNA replication. A number of important clinical antitumour agents act through inhibition of topo II enzymes, while some topo I inhibitors appear likely to enter clinical use. Although these chemicals do not covalently interact with DNA, they have strong mutagenic potential, generally causing events at the level of the chromosome rather than that of the gene. Most are recombinogens, may affect gene expression and can also lead to aneuploidy through effects on chromosome segregation. Most topo I and topo II inhibitors primarily cause mutagenic events associated with the replication fork. However, at least in mitotic chromosomes, topo II enzymes are located at the base of chromosome loops, and topo II inhibitors may facilitate subunit exchanges, leading to major deletions and illegitimate recombinational events. There is evidence that programmed cell death provides an alternative pathway to mutagenesis following treatment by either topo I or topo II inhibitors. The final fate of the cell will result from a balance between these two processes.

From amsacrine to DACA (N-[2-(dimethylamino)ethyl]acridine-4-carboxamide): selectivity for topoisomerases I and II among acridine derivatives

A number of acridine derivatives, including the clinical antileukaemia agent amsacrine and the experimental agent DACA (N-[2-(dimethylamino)ethyl]acridine-4-carboxamide), target the enzyme topoisomerase II. We demonstrate here that DACA induces DNA cleavage in the presence of topoisomerase I as well as of topoisomerase II. We also investigate a series of acridine derivatives which link amsacrine to DACA in terms of DNA binding, topoisomerase poisoning and biological activity. The presence of an acridine 4-linked N-2-(dimethylamino)ethyl group provides both a pronounced G-C preference for DNA binding and activity towards topoisomerase I. The removal of the anilino side chain of amsacrine, in combination with the presence of the N-2-(dimethylamino)ethyl group, provides in vitro biological activity against "atypical" multidrug resistant leukaemia lines with low topoisomerase II activity. Among these compounds, suppression of the ionisation of the acridine nitrogen to produce the compound DACA is associated with experimental activity against solid tumours. The addition of an acridine 2-chloro substituent to DACA suppresses the stimulation of topoisomerase II-dependent DNA cleavage but increases stimulation of topoisomerase I cleavage. 2-Substitution also increases activity against the "atypical" multidrug resistant cell lines. Overall, the results suggest that augmentation of topoisomerase I-dependent activity in this series by appropriate chemical substitution in this series leads to circumvention of topoisomerase II-mediated multidrug resistance.

Postincubation with aclarubicin reverses topoisomerase II mediated DNA cleavage, strand breaks, and cytotoxicity induced by VP-16

In previous studies, we found that VP-16 (etoposide) induced cytotoxicity and protein-concealed strand break formation was prevented in a small cell lung cancer (SCLC) cell line, when the cells were incubated with aclarubicin prior to treatment with VP-16. In the present work, we studied the effect of adding aclarubicin to the cell suspension after VP-16. In a clonogenic assay, we found that the cytotoxicity induced by VP-16 in SCLC cells was inhibited when cells were postincubated with aclarubicin. The addition of aclarubicin at any time in relation to VP-16 was able to stop further cytotoxicity induced by the topoisomerase II (topo-II) targeting drug. Aclarubicin was also found to antagonize the cytotoxicity induced by VM-26 (teniposide), and m-AMSA. With the alkaline elution technique we found that postincubating the cells with aclarubicin inhibited VP-16-induced DNA strand break formation. In an in vitro system with purified topo-II and naked DNA we likewise found, that postincubation with aclarubicin prevented VP-16 induced cleavage. In the same in vitro system, also baseline cleavage induced by topo-II was inhibited when aclarubicin was present. Importantly, aclarubicin exerted the antagonism to topo-II targeting drugs both when administered prior to and after the topo-II targeting agents. Thus, our data suggest that sequential rather than simultaneous administration of aclarubicin and topo-II targeting agents may be superior with respect to net-cytotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)